CN1258413C - Cast steel piece and steel product excellent in forming characteristics and method for treatment of molted steel therefor and method for production thereof - Google Patents

Abstract

A cast steel piece excellent in forming characteristics, characterized in that 60 % or more of the total cross section is constituted by equiaxial grains satisfying the following formula: D < 1.2 X<1/3> + 0.75 wherein D represents a diameter(mm) of an equiaxial grain as a structure having same orientation of crystal and X represents a distance from the surface (mm) of the cast steel piece. The cast steel piece and a steel product produced by forming the cast steel piece is extremely reduced in the amount of surface and internal defects.

Description

The processing method of molten steel

Technical field

The present invention relates to have the solidified structure of crystal grain diameter homogeneous, blemish and internal flaw are few, the slab that processing characteristics and quality characteristic are good, and process this slab and the steel made.

The invention still further relates to use ingot casting method or continuous casting process with Decarburising and refining after molten steel when manufacturing square billet and slab, thereby promote nucleus generation, make the solidified structure refinement can improve the molten steel processing method of quality characteristic and processing characteristics.

The chrome-bearing steel water casting method that the present invention also relates to that solidified structure is fine, blemish and internal flaw is few and with the seamless steel pipe of its manufacturing.

Background technology

In the past, slab adopts ingot mould ingot casting method, or with the continuous manufacture of mould oscillation, belt caster or strip caster etc., and molten steel is cast as slab ingot, shaped blooms or middle-size and small-size steel billet or thin-walled slab greatly, and then cuts into the preliminary dimension manufacturing and form.

Above-mentioned slab is processed into steel such as steel plate and shaped steel again through breaking down and finish rolling etc. after heating such as heating furnace.

Make the slab that seamless steel pipe is used, make after with ingot casting method and continuous casting process molten steel being cast as big shaped blooms or middle-size and small-size steel billet equally.After slab heats, be transported to the tubulation operation with steel through just rolling into tubulation in heating furnace.Then, it is rectangle or circle that this steel are added post forming again, makes seamless steel pipe with top (プ ラ グ) perforation.

Except the condition of rolling processing etc., the solidified structure of slab all has a significant impact the material and the quality of steel before the processing.

The tissue of slab, as shown in Figure 7, more tiny quenching crystal that is become through the rapid cooled and solidified of mold by the top layer, and the big column crystal of side formation within it usually and the equiax crystal that forms in the centre are formed, and column crystal can reach the centre sometimes.

Therefore, slab top layer part exists under the situation of thick column crystal, copper etc. are sneaked into the crystal boundary place segregation at this thick column crystal of element and compound thereof, this position is become fragile, make the slab top layer crackle occur like this and because of inhomogeneous blemish that forms flaw such as pit and so on such as coolings, thereby increase phenomenons such as finishing operation such as grinding or slab fragmentation, reduce yield rate.

Add man-hour with this slab is rolling, the crystal grain diameter heterogeneity strengthens each side's opposite sex of distortion, cause deformational behavior difference on the length and width direction, be easy to generate defectives such as folding flaw of squamous and crackle, and also variation of r value processing characteristics such as (section contraction processing indexs), produce wrinkle flaw blemish such as (especially unidirectional wrinkle on the corrosion resistant plate (リ ジ Application グ) and streaks).

Particularly pay attention to the stainless steel of outward appearance, the appearance of blemish such as edge crack flaw and streak causes bad order and end trim amount to increase.

Use this slab to make seamless steel pipe, on the steel pipe with blemish such as remaining scale that comes from slab and crackles, perhaps there are internal flaws such as underbead crack, hole, center segregation. in addition during tubulation, above-mentioned defective also can be aggravated because of moulding and perforation procedure, and steel pipe internal-surface is cracked and flaws such as scale.Cause increasing finishing operations such as grinding, perhaps reduce because of phenomenons such as the fragmentation that takes place frequently make yield rate.

For containing chrome ferritic class stainless-steel seamless pipe, this tendency shows significantly especially.

In addition, slab inside exists under the situation of thick column crystal or big equiax crystal, projection to slab is corrected processing etc. and can be produced strain with crooked the work, this strain causes inside crackle to occur, the center porosity crack (porous) that molten steel solidification shrink to produce, and solidify the later stage and do not produce internal flaws such as center segregation because of solidifying molten steel flow.

Therefore, the blemish that produces on the slab causes increasing finishing operations such as grinding, and reduces because of phenomenons such as the fragmentation that takes place frequently make yield rate.When directly this slab being carried out breaking down and finish rolling, except slab produces blemish, steel inside is also with internal flaws such as remaining crackle, porous and center segregations, causes that UST is defective, intensity reduces or outward appearance worsens, and increases problems such as the steel finishing operation and the fragmentation that takes place frequently.

The generation of this blemish and internal flaw in the slab can be inhibited by the solidified structure that improves slab.

The generation of blemish such as face crack that slab causes because of inhomogeneous cooling and solidification shrinkage inequality and pit flaw can be suppressed by the method that makes the slab solidified structure evenly form fine solidified structure.

In addition, the slab solidified inside is shunk and the underbead crack of not solidified steel current movable property life and the generation of internal flaws such as center porosity (porous) and center segregation, can be suppressed with the method for the equiaxial crystal ratio that improves slab inside.

Therefore, the steel that suppress slab and use this slab to make produce blemish, and quality characteristics such as the processing characteristics of raising slab and toughness importantly suppress column crystal alligatoring in the slab top layer, improve the inner equiaxial crystal ratio of slab simultaneously, making all becomes the fine solidified structure of homogeneous.

Various tests have been done as the countermeasure people, promptly control the form of field trash in the molten steel, control process of setting simultaneously, make solidified structure become fine equiaxed grain structure, observe the situation prevent that blemish and internal flaw produce in the steel that slab and processing slab obtain.

Yet improve the method for equiaxial crystal ratio in the slab solidified structure in the past, known have: the low temperature casting that 1) reduces liquid steel temperature, 2) to the electromagnetic stirring method of process of setting molten steel, 3) when molten steel solidification, in molten steel, add oxide nucleus and field trash itself, perhaps add adding ingredient, make its method that in molten steel, generates, perhaps with said method 1)～3) method that is used in combination.

Above-mentioned 1) example of low temperature casting, can enumerate the method for putting down in writing in the special fair 7-84617 communique: when the continuous casting molten steel, overtemperature (deduct from the temperature of actual molten steel this molten steel liquidus temperature after temperature) is controlled at below 40 ℃, in mold, stretch under the cooling on one side, the equiaxial crystal ratio that solidifies slab is reached more than 70%, prevent that unidirectional wrinkle from appearring in ferrite stainless steel.

Yet the method for special fair 7-84617 communique record, reduce overtemperature, will in casting process, make spray nozzle clogging admittedly by the invar water-setting, perhaps the adhewsive action because of base bullion makes the casting difficulty, and the invar water viscosity increases the obstruction inclusion floating, because of the field trash that remains in the molten steel causes defective etc.Therefore in the said method, it is difficult overtemperature being reduced to the slab that can obtain to have enough equiaxial crystal ratios.

For preventing blemish and internal flaw, produce the good slab of processing characteristics, it be unclear that so far and should make top layer to inner equiax crystal have much crystal grain diameters, and and how homogenization of slab solidified structure.

The spy opens that disclosed method is in the clear 57-62804 communique, exists in inside under the state of coagulated substance not, depresses slab, with near the compacting center, internal flaws such as center porosity occur to stop slab.

But above-mentioned spy opens the method for clear 57-62804 communique record, owing to be to utilize the method for depressing to make near the compacting slab center, so big as if not solidifying the part amount, fragile solidification layer is applied big pressure, will cause underbead crack and center segregation.Otherwise if insufficient pressure, internal flaw such as remaining center porosity still then like this during the middle punch of tubulation operation, can crack and internal flaw such as scale, makes steel pipe quality reduction etc.

Therefore the chrome-bearing steel material slab that known method is difficult to produce that solidified structure is fine, blemish and internal flaw are suppressed, and difficult under no breaking down (heavy reduction) condition with the slab tubulation that casts continuously.And, in order stably to make chrome-bearing steel (ferritic stainless steel) steel pipe in the zero defect mode, how to cast and how slab such as is handled at problem industrial, it be unclear that so far.

Above-mentioned 2) molten steel is carried out the method for electromagnetic agitation, the for example special method of putting down in writing in clear 49-52725 and the flat 2-151354 communique of Te Kai of opening, be in mold or to the molten steel in the process of setting in mold downstream, to carry out electromagnetic agitation, promote floating foreign, suppress the column crystals growth, improve the solidified structure of slab.

Yet, the spy opens the method for clear 49-52725 and the flat 2-151354 record of Te Kai, with electromagnetic agitation near the molten steel the mold is given under the situation that stirs stream, the slab surface portion can form fine solidified structure, but the miniaturization of slab solidified inside tissue is insufficient.On the other hand, the mold downstream is given under the situation that stirs stream, though the solidified structure of slab inside can miniaturization, slab top layer part but forms thick column crystal, so the top layer part of slab and inside all miniaturizations simultaneously.

But only the molten steel of giving process of setting with electromagnetic agitation to be stirring stream, difficultly obtains the slab that existing predetermined crystal grain diameter has fine solidified structure again, and it is limited utilizing electromagnetic agitation to make the solidified structure miniaturization.

The electromagnetic agitation method of molten steel, open as the spy and to put down in writing in the clear 50-16616 communique, prevent that this method that wrinkle occurs from being that the molten steel in the process of setting is carried out electromagnetic agitation, the column crystal end of growing up is cut off, as freezing nuclei, make that equiaxial crystal ratio reaches more than 60% in the slab solidified structure with the part of this column crystal.

The spy opens the method for clear 50-16616 communique record, be that the slab that takes out from mold is carried out electromagnetic agitation, but there is column crystal in part on the top layer of slab, this column crystal makes the slab surface blemish such as crackle and pit occur, perhaps make the rolling processing back steel that wait except that blemish such as scale and crackle, also produce blemish such as unidirectional wrinkle.

In addition, the spy opens the method for putting down in writing in the clear 52-47522 communique and is, 1.5～3.0 meters positions of molten steel surface are provided with electromagnetic mixing apparatus in the continuous casting mold, stirs under 60 millimetres of mercury thrusts, makes the fine slab of solidified structure.The method that the spy opens clear 52-60231 communique record is, be to cast under 10～50 ℃ the condition at molten steel overheat, and the not solidification layer of slab in the casting carried out electromagnetic agitation, make the solidified structure of slab become the micro organization that forms by equiax crystal, produce the steel of internal flaws such as no center segregation and center porosity.

The spy opens the method for clear 52-47522 record, be that the molten steel that is solidifying in the mold is stirred, so can suppress column crystal (ingotism) growth, though form fine solidified structure near making the electromagnetic agitation position to a certain extent, but make all miniaturizations of solidified structure in the slab, multistage electromagnetic mixing apparatus must be arranged, and cost of equipment is big.In addition, from the space that device is provided with, it is also extremely difficult that multistage electromagnetic mixing apparatus is set.Therefore use the special said method of opening clear 52-47522 communique record, the slab of making the equal miniaturization of all solidified structures has limitation.

The method that the spy opens clear 52-60231 communique record is to carry out the low temperature casting, so field trash can be attached on the surface in the dipping spray nozzle, perhaps plug nozzle perhaps makes the molten steel skinning because of temperature in the mold descends, sometimes have to interrupt casting, the problem of fluctuation of service is arranged.

Therefore, in low temperature when casting, is owing to can reduce the casting temperature of molten steel, so nozzle produces to stop up and makes the states of affairs such as the reduction generation with the minimizing of molten steel injection rate of casting interruption or casting speed during the interior injection of oriented mold molten steel, therefore is difficult to make casting temperature to be reduced to and can makes the slab solidified structure stablize the degree of miniaturization.

Adopt in addition under the situation of electromagnetic mixing apparatus, even in the molten steel solidification process, carry out local electromagnetic agitation, the top layer of slab or inside also can generate column crystal and thick equiax crystal, it is the reason that produces blemish or internal flaw, perhaps increase because of trim amount and broken phenomenon takes place frequently and makes the yield rate reduction, the quality of steel suffers damage because of the existence of internal flaws such as underbead crack and center porosity, center segregation.

On the other hand, someone proposes in the downstream that comprises meniscus a plurality of electromagnetic mixing apparatus to be set, make the method for the whole section solidified structure of slab miniaturization, but because the degree of miniaturization is different with stirring the position, so can not all can both stablize and obtain fine solidified structure spreading all over slab.And to stablize the fine solidified structure of acquisition, will increase the electromagnetic mixing apparatus number.And the number that is provided with of electromagnetic mixing apparatus is subjected to installation cost and continuous casting apparatus structural limitations, so be difficult to be provided with necessary number agitating device.In any case but, even a plurality of electromagnetic mixing apparatus are set, can not make the abundant miniaturization of solidified structure.

Said method 3) instantiation is, in molten steel, add the oxide and the field trash itself that can form freezing nuclei, perhaps utilize adding ingredient in molten steel, to generate the method for these materials, the for example special method of the record in the clear 53-90129 communique of opening, promptly in molten steel, add the wire wrapped up iron powder and Co, B, oxides such as W, Mo, give stirring stream in this dissolving wiry position with electromagnetic agitation, make slab all form the solidified structure that equiax crystal is formed.But in this method, the additive dissolving that is in the wire is unstable, produces dissolved residue sometimes.When dissolved residue occurring, this dissolved residue just becomes the origin cause of formation of product defects.In addition, even all dissolvings of the additive in the wire, the also extremely difficult all homogeneous dispersions of top layer to inside from slab of this additive cause the solidified structure size uneven, inadvisable.The equiax crystal effect is owing to the influence that is subjected to the electromagnetic agitation position and stirs thrust, so restricted by appointed condition in addition.Though the spy opens the method for having added the TiN particulate when clear 63-140061 communique has been put down in writing casting, but finds when implementing the method to exist with the spy and opens clear 53-90129 communique same disadvantages.

About add the effect that required composition makes it to generate the field trash freezing nuclei in molten steel, 4-S79 in 1974 goes up record as " iron と steel ", generally knows that to utilize to make in the ferritic stainless steel molten steel to generate TiN, realizes the solidified structure equiax crystalization.But to generate TiN obtaining abundant equiax crystal effect with said method, in above-mentioned " iron と steel " record, must make that Ti concentration reaches more than the 0.15 weight % in the molten steel.

Yet, will obtain abundant equiax crystal effect with the method that above-mentioned TiN generates, just should increase the addition of expensive Ti alloy, the result not only makes manufacturing cost improve, but also can in casting, produce the nozzle undergauge that forms thick TiN inducement, perhaps make board product produce scale flaw etc.With regard to it and add with regard to the relation between the TiN amount, owing to be subjected to the restriction that composition of steel is formed, so the steel grade that can be suitable for is limited.

By adding certain composition of trace as far as possible, as the means of effective acquisition equiax crystal micro organization slab, the someone proposes to add Mg in molten steel.

But 1107 ℃ of Mg boiling points are lower than liquid steel temperature, and the solubility in molten steel is almost nil, so drop into metal M g in molten steel, most of Mg becomes the steam volatilization.Therefore it is extremely low to add the Mg yield according to usual method, so the Mg additive process also must be done further research.

The inventor etc. just add in the research of Mg and find, the yield of Mg and Mg add the composition that the back generates oxide and influenced by molten steel composition, but also influenced by slag composition.That is to say, only have been found that and add Mg, be difficult to generate field trash in the molten steel with effective freezing nuclei effect composition to molten steel.

For example specially open flat 7-48616 communique and put down in writing a kind of method that improves steel quality, this method will cover the slag of the interior molten steel surface of container such as ladle, and furnishing contains MgO 3～15 weight %, contains FeO, Fe ₂O ₃With MnO be CaOSiO below the 5 weight % ₂Al ₂O ₃Be slag, run through this slag and add the Mg alloy that this method can improve Mg utilization rate in the molten steel, and owing to generate fine MgO and MgOAl ₂O ₃Oxide and can improve steel quality.

The method of Te Kaiping 7-48616 communique record is because molten steel is by CaOSiO ₂Al ₂O ₃Be that slag covers, thereby advantage is can suppress the Mg evaporation and increase operation rate.But the special method of opening flat 7-48616 communique record, only FeO, Fe in the slag of regulation covering molten steel ₂O ₃With the MnO total amount less than 5 weight %, do not stipulate SiO ₂Amount.Therefore as if SiO in the slag ₂The content height is then when adding metal M g and Mg alloy, because contained SiO in Mg and the slag ₂Reaction and the Mg utilization rate is reduced.The Mg utilization rate is in a single day low, just can not make Al in the molten steel ₂O ₃Contain the MgO oxide Deng becoming, the result causes remaining Al in the molten steel ₂O ₃Be thick oxide, make slab and steel produce defective.

Al ₂O ₃Be that oxide is little as the effect of freezing nuclei, can make thickization of slab solidified structure, crack and defectives such as center segregation and center porosity, cause the yield rate reduction of slab etc. in the surface or the inside of slab.

Even this slab is processed into steel, also can produce the blemish and the internal flaw that cause because of solidified structure is thick, problems such as yield rate and quality are low appear.

In addition, because to not in addition any restriction of Ca concentration in CaO concentration or the molten steel in the slag, so can not generate the low melting point complex chemical compound (CaO-Al of freezing nuclei effect sometimes ₂O ₃-MgO is an oxide), rather than high-melting-point MgO etc.

The flat 10-296409 communique of Te Kaiping 10-102131 and Te Kai proposes to improve the method for slab solidified structure, this method contains 0.001～0.015 weight %Mg by making molten steel, form the oxide of fine and favorable dispersibility, it is all at slab that this oxide is distributed.

But the special method of opening flat 10-102131 communique and the flat 10-296409 record of Te Kai is because oxide is with 50/millimeter ²High density is disperseed from the inside portion of top layer part homogeneous in slab, so in the steel that slab in slab, process or processing slab obtain, occur defectives such as crackle that oxide causes and scale sometimes.Must carry out finishing such as surfacing in this case and handle, and the invar material is broken and reduce the goods yield rate sometimes.

In addition, near steel surface exposes oxide or its top layer, exist under the situation of oxide, with acid when salt solution etc. contacts, oxide (oxide that contains Mg) will dissolve, so there are problems such as corrosion resistance of steel is low.

The inventor etc. add Mg to realize the optimum condition of equiax crystalization in order to find out in molten steel, carry out various tests, the result has a new discovery, even the interpolation of deoxidant elements such as promptly for example molten steel composition and/or slag composition are identical, Al and Mg order all can have an immense impact on to the equiax crystal effect.

That is to say following discovery: in a single day add Al after in molten steel, adding Mg, because the MgO surface that generates behind the interpolation Mg is by Al ₂O ₃The MgO that generates covers, so can not play the freezing nuclei effect effectively.

As a result, can not obtain the solidified structure micronized effect that produced by MgO, thickization of solidified structure such as cracks at blemish and internal flaws such as center segregation and center porosity.Its result perhaps increases the finishing operation of slab and steel, and perhaps the fragmentation because of slab and steel reduces the yield rate of goods.

In sum, the oxide that in molten steel, added as freezing nuclei for the past and the method for field trash itself, perhaps add required composition and make the method that generates freezing nuclei in the molten steel, all be difficult to obtain the zero defect slab of solidified structure homogeneous, thereby can not obtain the good slab of processing characteristics such as steel rolling, so problem is to obtain the few colory steel of defective.

Present present situation is, stably makes for the good slab of zero defect and processing characteristics for industrial, it be unclear that to form which kind of solidified structure up to now.

Therefore, present present situation is, by low temperature casting and electromagnetic agitation, perhaps make the existing method of slab equiax crystalization by the oxide that adds the formation freezing nuclei, all can not be in suppressing slab under the condition that produces of crackle and pit defect, center segregation and center porosity blemish such as (porous) and internal flaw, obtain the solidified structure of homogeneous crystal grain diameter, make flawless slab, improve the processing characteristics of this slab, at the industrial few and colory steel of defective that stably produce.

Disclosure of an invention

The present invention makes in light of this situation, purpose is to provide a kind of solidified structure that makes to become fine and solidified structure homogeneous, make blemish such as can suppressing crackle and center porosity, center segregation and internal flaw and produce, and processing characteristics and/or the good slab of quality characteristic.

Another object of the present invention is to provide that this slab processing of a kind of usefulness obtains, do not have blemish and the good steel of internal flaw, processing characteristics and/or quality characteristic.

Another purpose of the present invention is to provide a kind of processing method of molten steel, and this method can promote high-melting-point in the molten steel to contain the generation of MgO oxide, makes it to work with the freezing nuclei form, makes the solidified structure miniaturization of slab.

The other purpose of the present invention is to provide a kind of continuous casing of slab, this method can make the solidified structure of slab be transformed into fine solidified structure, suppress the generation of blemish such as crackle and segregation and internal flaw, the steel defective was few when slab was processed into steel, and the quality characteristics such as corrosion resistance of slab are also good.

In addition, other purpose of the present invention is to provide a kind of casting method of chrome-bearing steel slab and seamless steel pipe made from this slab cast, described method can make the solidified structure of slab become fine solidified structure, suppress the generation of blemish such as crackle and segregation and internal flaw, steel tube defect was few when slab was made seamless steel pipe, goods yield rate height.

Satisfy the slab of the present invention (below be called " slab A ") of above-mentioned purpose, the full section that it is characterized in that casting slab is the equiax crystal that satisfies following formula more than 60%.

D＜1.2X ^1/3+0.75

D is the equiax crystal diameter (mm) of crystallization direction homologue in the formula, and X is apart from the distance (mm) between the slab surface.

Satisfy the solidified structure of following formula in the slab by acquisition, the column crystal width that minimizing is remaining on the slab top layer, suppress the small segregation that the solid-liquid distribution of molten steel composition produces when solidifying, strengthen crack resistance, the slab crack defect that inhibition produces because of distortion and slab bulging and detorsion machining stress in the process of setting, and can prevent the generation of internal flaws such as the center porosity that occurs because of the molten steel solidification contraction of mid-depth part and MOLTEN STEEL FLOW and center segregation.

Slab A with the solidified structure that satisfies following formula rollingly adds man-hour by owing to the distortion homogeneous has good processing characteristics, so the generation of blemish and internal flaw can suppress in the steel of processing.

In addition, can make above-mentioned equiax crystal be full of whole sections of slab among the slab A.

If make whole sections of slab all be the fine solidified structure of the homogeneous that does not have column crystal, reduce slab top layer and inner microsegregation, then can further strengthen the crack resistance that distortion and stress in the process of setting are cracked.Its result can prevent the generation of blemish and internal flaw in the slab, improve add man-hour from the top layer of slab the homogeneity to internal modification, thereby improve processing characteristics.

Can satisfy the good another kind of slab of above-mentioned purpose of the present invention and processing characteristics (following note is made " slab B "), it is characterized in that and the equidistant crystal grain diameter maximum of slab case depth of casting, be below three times of this depth-averaged crystal grain diameter.

By obtaining the solidified structure that crystal grain diameter satisfies above-mentioned relation, can make the crystal grain diameter homogeneous that exists apart from predetermined temperature place, slab top layer.Its result can suppress the local cyrystal boundary segregation that copper etc. is sneaked into element, and the grain-boundary crack at position, top layer.In addition,, suppress distortion and concentrate on the specific crystal grain, shrink the r value of processing index and can also stop blemish such as wrinkle flaw, unidirectional wrinkle and streak so can improve as section owing to add the distortion homogeneous that can make crystal grain man-hour.

In addition, can also make among the slab B and to be equiax crystal more than 60% along the thickness direction section.

Be equiax crystal by making among the slab B more than 60% along the thickness direction section, can make the solidified structure of slab become the solidified structure that suppresses the column crystals growth.Its result, slab top layer and inner cyrystal boundary segregation further are inhibited, the distortion of process of setting and the crack resistance of crackle that stress causes are improved, the blemish and the internal flaw that not only suppress slab produce, and can also improve the isotropism that adds the deformational behavior in the man-hour elongation of width and length direction (when the depressing), thereby can improve processing characteristics.That is to say, can stop in the steel generation of the blemish such as wrinkle flaw that the inhomogeneities because of crackle and scale and machining deformation causes.

In addition, can also make among the slab B and be equiax crystal along the whole sections of slab thickness direction.

In this solidified structure, because microsegregation further is inhibited, and solidified structure homogeneous more, so the inhibition to crackle etc. in the slab can further strengthen, the obtaining of blemish and internal flaw more positively prevents, and can add man-hour from the top layer homogeneity to internal modification, further improve processing characteristics, r value and toughness.

Can satisfy the good slab of the quality characteristic of the present invention of above-mentioned purpose and processing characteristics (below be called " slab C "), it is characterized in that wherein containing 100/centimetre ²Form when being taken in molten steel solidification with the non-coherency of delta ferrite lattice less than 6% field trash.

The field trash little with the non-coherency of delta ferrite lattice plays effectively to form most nuclear species nucleus that solidify.If form most freezing nucleis, then solidified structure miniaturization, its result can suppress slab top layer and inner small segregation, causes the crack resistance of crackle to improve to inhomogeneous cooling and contraction distortion etc.Blocking action (growth of back crystal grain is solidified in inhibition) is arranged after freezing nuclei solidifies, suppress thickization of solidified structure, can obtain finer solidified structure.

Slab with this solidified structure, rolling grade add is depressing the direction distortion man-hour easily.That is to say that the processing characteristics of this slab is fabulous.

In case contained field trash number is less than 100/centimetre in the slab ², then forming the decreased number of freezing nuclei, the wad act after solidifying simultaneously is also insufficient, so the solidified structure of slab becomes thick, its result will produce blemish and internal flaw on slab.

For slab C, can also make above-mentioned field trash contain 100/centimetre in addition ²More than be of a size of the field trash below 10 microns.

Field trash is tiny, can effectively form most freezing nucleis, and because of improving blocking action, so can obtain finer uniform solidified tissue.The slab of this solidified structure is rolled adds man-hour, good processability, steel can not produce scale flaw and blemish and internal flaws such as face crack and wrinkle.

In case the field trash size is greater than 10 microns, though the freezing nuclei effect is then arranged when molten steel solidification, problem is to be easy to generate scale flaw and layering flaw.

In addition, slab C also can be that the primary crystallization that solidifies is the delta ferrite steel grade.

Slab produces phase transformation in process of setting, even solidify and become the steel grade of organizing beyond the ferrite in back or the cooling procedure, the field trash of slab C also plays kind of a nucleus effect, owing to promote the delta ferrite freezing nuclei to generate solidified structure homogeneous so can obtain fine.Its result makes cooling back slab crystalline structure become fine.

According to the good slab of above-mentioned purpose quality characteristic of the present invention (below be called " slab D "), in the slab that it is characterized in that casting when molten steel adds molten steel solidification, forming required metal of freezing nuclei or metallic compound, the inner contained number that is of a size of metallic compound below 10 microns is that the slab skin section is divided more than 1.3 times of contained size metallic compound number below 10 microns on the slab top layer.

Therefore, in this slab D, in metallic compound that in molten steel, adds the metal generation or the direct metallic compound that in molten steel, adds,, more than slab top layer part at the number of the inner contained size metallic compound below 10 microns of slab.This metallic compound plays the freezing nuclei effect when molten steel solidification, the equi-axed crystal diameter of solidified structure is reduced, and its result will suppress cyrystal boundary segregation.And this metallic compound also has blocking action, can suppress thickization of equiax crystal.

So, can prevent among the slab C crackle and pit flaw that distortion and stress because of process of setting cause, and the blemish that causes because of field trash produces, distortion when enhancing is corrected slab projection and bending machining causes the crack resistance of underbead crack, and can suppress to solidify latter stage invar water-setting and shrink internal flaw generations such as the center porosity (porous) that causes with MOLTEN STEEL FLOW and center segregation admittedly.

For slab D,,, thereby can improve quality characteristic and processing characteristics such as corrosion resistance so slab rolling adding reduces because of the blemish that field trash produces man-hour because top layer metallic compound number lacks than interior metal compound number.

The top layer part of so-called slab D is the span top layer greater than the scope between 10% to 25%.Outside this scope, the top layer part is thin excessively, and metallic compound inside and top layer how is approaching, and interior metal compound number increases, and skin section is divided form fine solidified structure, and slab adds the defective that easy generation in man-hour metallic compound causes.

Wherein, the non-coherency of the plain lattice of δ iron that forms when contained metallic compound of molten steel and molten steel solidification can be in below 6%.

Therefore, freezing nuclei formation ability improves during molten steel solidification, can obtain finer solidified structure, can make the small segregation of top layer part and inside minimum.And it is easier to depress direction distortion, can stablize and make processing characteristics and the good slab of quality characteristic.

In addition, slab D can be made the ferrite-group stainless steel slab.

For ferrite-group stainless steel slab D, can easily the solidified structure that is easy to thickization be become fine equiax crystal.

Above-mentioned slab of the present invention, can make it to contain in the oriented molten steel add Mg or Mg alloy and generate contain the MgO oxide.

By making it to contain the MgO oxide, can suppress oxide cohesion in the molten steel, improve the dispersiveness of oxide, increased the oxide number of freezing nuclei effect.Its result, the solidified structure of slab is more stable, forms fine solidified structure.

After the above-mentioned slab heating of the present invention, for example after 1100～1350 ℃ of heating, the rolling steel of being processed into, owing to have above-mentioned characteristic, so rolling crack resistance height in man-hour that adds, can prevent from rollingly to add man-hour distortion and concentrate on the specific die, obtain the distortion (isotropism of deformational behavior) of crystal grain homogeneous.

Therefore, distortion homogeneous when above-mentioned slab of the present invention is depressed on width and length direction, so process the steel of the present invention that this slab obtains, blemish such as those scale flaws that in common steel, take place and crackle, and internal flaws such as center porosity and center segregation are few.And steel of the present invention are also few because of blemish and internal flaw that field trash causes, so have quality characteristics such as good anti-corrosion.

Below explanation is made the above-mentioned slab of the present invention with the processing method of molten steel (below be called " processing method of the present invention ").

One of processing method of the present invention (below be called " processing method I ") is characterized in that total Ca amount in the molten steel of refining furnace refining is adjusted to below the 0.0010 quality %, adds the Mg of scheduled volume then in this molten steel.

According to this processing method I, can suppress calcium aluminate (12CaOAl in the molten steel ₂O ₃Etc. the low melting point field trash) generation.The result forms CaO-Al in the time of preventing to add Mg oxide (MgO) in calcium aluminate ₂O ₃-MgO ternary compound oxides can form the MgO or the MgOAl of freezing nuclei effect ₂O ₃Deng the high-melting-point oxide.

Total Ca amount described here is meant that the Ca that exists and CaO etc. contain the total amount of Ca composition in the Ca compound in molten steel; The content of stipulating among the processing method I is meant not contain Ca in the molten steel fully, perhaps contains the content of the following Ca situation of 0.0010 quality %.

In addition, among the processing method I of the present invention, also can make not contain the calcium aluminate composite oxides in the molten steel.

Therefore, when having oxide (magnesia) in the molten steel, can stably prevent from usually to form CaO-Al by calcium aluminate and oxide (magnesia) ₂O ₃-MgO ternary compound oxides, the result can make more certain formation MgO and MgOAl in the molten steel ₂O ₃Deng high-melting-point oxide (below be called " containing the MgO oxide "), make the solidified structure miniaturization of slab, prevent that slab from producing blemish and internal flaw.

The addition of magnesium in the molten steel is preferably 0.0010～0.10 quality %.

When the magnesium addition was lower than 0.0010 quality %, the freezing nuclei quantity that contains the generation of MgO oxide in the molten steel reduced, and can not make the solidified structure miniaturization.On the other hand, in case the addition of magnesium surpasses 0.10 weight %, because of the micronized effect of solidified structure is saturated, the Mg of interpolation or Mg alloy are inoperative, perhaps usually produce defective because of comprising MgO with the oxide increase that contains the MgO oxide.

The molten steel casting that processing method I of the present invention is handled is in mold, slab of the present invention through the cooling manufacturing, because of fine MgO and/or contain the MgO oxide and make the solidified structure miniaturization, thereby can suppress the generation of blemish such as crackle that the slab surface produces and pit and internal flaws such as center porosity (porous) and center segregation.Therefore, when the rolling grade of this slab is processed into steel, can prevent that steel from producing blemish and internal flaw, not have finishing and broken, finished product rate and quality of materials are all high.

Another processing method of the present invention (below be called " processing method II ") is characterized in that adding in molten steel before the Mg of scheduled volume, adds scheduled volume and contain the Al alloy and carry out deoxidation treatment in molten steel.

This processing method II at first adds and contains the Al alloy, makes this contain oxygen, MnO, SiO in Al alloy and the molten steel ₂, reaction such as FeO generates Al ₂O ₃, make Al by adding scheduled volume Mg then ₂O ₃The oxidized generation of surface Mg MgO perhaps forms MgOAl ₂O ₃Al ₂O ₃The MgO of last existence or MgOAl ₂O ₃, be in below 6% because of it and as the non-coherence degree that solidifies lattice between the delta ferrite of primary crystallization, so when molten steel solidification, have the freezing nuclei effect.Its result makes the solidified structure miniaturization, can suppress the generation of blemish such as crackle and internal flaws such as center segregation and center porosity, and can suppress processing characteristics and corrosion resistance reduces.

Described Al alloy is meant the alloy of Al such as containing metal A l and Fe-Al alloy, and the Mg of interpolation is meant that metal M g and Fe-Si-Mg alloy, Ni-Mg alloy etc. contain the Mg alloy.

In addition, in processing method II of the present invention, in molten steel, add Mg before, except scheduled volume contains the Al alloy, can also add scheduled volume and contain the Ti alloy and carry out deoxidation treatment.

By adding the above-mentioned Ti of containing alloy, make Ti solid solution in molten steel, make wherein a part of TiN of generation play the freezing nuclei effect, and the Al that deoxidation is generated ₂O ₃Form MgO or MgOAl on the surface ₂O ₃, can play the freezing nuclei effect simultaneously.The wherein said Ti alloy that contains is meant that metal Ti and Fe-Ti alloy etc. contain the Ti alloy.

Among the processing method II of the present invention, preferably make the addition of Mg be in 0.0005～0.10 quality %.

Be in this scope the Al that deoxidation is formed by the addition that makes Mg ₂O ₃Fully form MgO or MgOAl on the surface ₂O ₃This MgO or MgOAl ₂O ₃, when solidifying in molten steel, the freezing nuclei effect is abundant, will make solidified structure finer.

When the addition of Mg is lower than 0.0005 quality %, have and the non-coherency of delta ferrite lattice oxide number deficiency less than 6% surface, can not make solidified structure fine.On the other hand, when the addition of Mg surpasses 0.10 quality %,, add the required cost of Mg simultaneously and also improve because of oxide makes the effect of solidified structure miniaturization saturated.

Among the processing method II of the present invention, molten steel can be made the molten steel of ferrite-group stainless steel.

According to processing method II of the present invention, can make the solidified structure miniaturization of the ferrite-group stainless steel of the easy alligatoring of solidified structure, its result can suppress the crackle and the defectives such as pit defect, underbead crack, center porosity and center segregation that produce on the slab surface.

In processing method I of the present invention and processing method II, more preferably add Mg, make slag contained in the molten steel and deoxidation product non-oxidized substance, and the oxide that generates when adding Mg in molten steel satisfies following formula (1) and (2):

17.4(kAl ₂O ₃)+3.9(kMgO)+0.3(kMgAl ₂O ₄)

+18.7(kCaO)≤500 …(1)

(kAl ₂O ₃)+(kMgO)+(kMgAl ₂O ₄)+(kCaO)

≥95 …(2)

In the formula, k represents the mole % of oxide.

Utilize the method for this interpolation Mg, can generate CaOAl ₂O ₃MgO, MgOAl ₂O ₃, composite oxides such as MgO, the non-coherency of these oxides and delta ferrite lattice is the oxide that effectively plays the freezing nuclei effect less than 6%.During molten steel solidification, these composite oxides play the freezing nuclei effect, make it to generate equiax crystal, thereby make the solidified structure miniaturization of slab.

This Mg additive process also is applicable to the molten steel of ferrite-group stainless steel.

That is to say that above-mentioned Mg additive process can make the solidified structure of the ferrite-group stainless steel of easy thickization of solidified structure form finer solidified structure, suppresses the underbead crack and center segregation, the center porosity etc. that produce in the slab.And, can prevent to process this slab and make streak and the edge crack defective that produces because of thick solidified structure in the steel.

A processing method more of the present invention (following note do " processing method III ") is characterized in that satisfying in the molten steel of solubility product that TiN separates out crystallization to Ti concentration and N concentration more than the molten steel liquidus temperature, adds the Mg of scheduled volume.

According to this processing method III, under the high temperature of not separating out TiN, generate the MgO or the MgOAl of favorable dispersibility ₂O ₃Deng the oxide that contains MgO, along with the reduction of liquid steel temperature, on this MgO of containing oxide, separate out TiN, be dispersed in the molten steel, play the freezing nuclei effect, make the solidified structure miniaturization of slab.Here the interpolation of Mg, the mode that also can utilize input metal M g and Fe-Si-Mg alloy, Ni-Mg alloy etc. to contain magnesium alloy is carried out.

Wherein, above-mentioned Ti concentration [%Ti] and N concentration [%N] preferably can satisfy the following formula requirement:

[％Ti]×[％N]≥([％Cr] ^2.5+150)×10 ^-6

In the formula, [%Ti] is the quality % of Ti in the molten steel, the quality % of N in [%N] molten steel, and [%Cr] is the quality % of Cr in the molten steel.

In processing method III of the present invention because Ti contained in the molten steel and N concentration are remained in the preset range, and add scheduled volume Mg, so the TiN that can make generation along with dispersiveness high contain the MgO oxide, stably be dispersed among the molten steel.This TiN plays the freezing nuclei effect when molten steel solidification, thereby makes the solidified structure of slab finer.

Even processing method III of the present invention contains for the Cr ferrite-group stainless steel for easy thickization of solidified structure, also can bring into play the effect that makes the solidified structure miniaturization, and can prevent that slab and steel from producing blemish and internal flaw.

Processing method III of the present invention is specially adapted to contain the casting of the ferrite-group stainless steel molten steel of 10～23 quality %Cr.

If Cr content is lower than 10 weight %, then the corrosion resistance of steel reduces, and can not obtain required micronized effect simultaneously.On the other hand, if Cr content surpasses 23 weight %, even then add the corrosion resistance that the Cr ferroalloy can not improve steel, and the increasing of ferroalloy addition causes cost to increase.

Another processing method of the present invention (below be called " processing method IV ") is characterized in that making and contains the oxide that 1～30 quality % can be reduced by Mg in the slag that covers molten steel.

According to this processing method IV, owing to make oxide gross mass contained in the slag remain on predetermined value, so the Mg that adds in molten steel can improve MgO and contain the generation ratio (yield rate) of MgO oxide, its result can make fine MgO and contain the oxide of MgO (below be called " containing the MgO oxide ") and be dispersed among the molten steel.

So this MgO and contain the MgO oxide and play the freezing nuclei effect makes the solidified structure miniaturization of slab.The result, can suppress the slab surface cracks and pit, suppress inside and crack and defectives such as center segregation, center porosity,, slab such as is rolled at the quality of being processed into steel thereby can make perhaps because of need not to repair slab or prevent that fragmentation from can improve the yield rate of slab.

Wherein, the oxide in the described slag is meant FeO, Fe ₂O ₃, MnO and SiO ₂In one or more oxides.

By the oxide in the suitable selection slag, can suppress in the slag oxide to the consumption of Mg, improve the rate that retains of Mg, Mg is added among the molten steel effectively.

In addition, in processing method IV of the present invention, preferably make Al contained in the molten steel ₂O ₃Be between 0.005～0.10 quality %.

Can make the high Al of fusing point like this ₂O ₃Be transformed into MgOAl ₂O ₃Deng composite oxides, and utilize the dispersiveness of MgO that these composite oxides are dispersed in the molten steel, improved the ratio that contains the MgO oxide of freezing nuclei effect.

Another processing method of the present invention (below be called " processing method V ") is characterized in that adding before the scheduled volume Mg in molten steel, makes that CaO activity is in below 0.3 in the slag that covers molten steel.

According to this processing method, by add Mg in molten steel, the oxide that can make good MgO of delta ferrite lattice coherency and high-melting-point contain MgO generates finely, and is dispersed among the molten steel.

So when molten steel solidification, this MgO and contain the MgO oxide owing to play the freezing nuclei effect will be so that the solidified structure of slab will become will be fine.

In case the CaO activity in the slag surpasses 0.3, can not contain the freezing nuclei effect CaO low melting point oxide or surpass 6% oxide with the non-coherence degree of delta ferrite lattice and will increase.

In processing method V of the present invention, the basicity of slag is in below 10.

If the basicity of slag is adjusted to below 10, then can stably suppresses the activity of CaO in the slag, thereby can prevent to contain the MgO oxide and be transformed into low melting point oxide or surpass 6% oxide with the non-coherence degree of delta ferrite lattice.

In addition, processing method V of the present invention can be applicable to the molten steel of ferrite-group stainless steel well.

If use processing method V of the present invention to handle the molten steel of ferrite-group stainless steel, when molten steel solidification, can make the solidified structure miniaturization of easy thickization, thereby the formation blemish and the internal flaws such as steel that can prevent slab and be processed into.

Above-mentioned slab of the present invention can enough continuous casting process manufacturings, and this continuous casting process is characterised in that the molten steel casting that will contain MgO or contain the MgO oxide in mold, casts while stir molten steel with electromagnetic mixing apparatus.

According to this continuous casing, by in molten steel, forming dispersed high MgO and/or containing the MgO oxide, because this oxide can make the solidified structure miniaturization of slab to facilitation and blocking action (growth of back tissue is solidified in inhibition) that freezing nuclei forms.

By the stirring of electromagnetic mixing apparatus, can reduce the oxide that slab surface exists, prevent from slab and the steel to produce defectives such as scale and crackle, but also can improve corrosion resistance because of oxide.

In continuous casing of the present invention, preferably electromagnetic mixing apparatus is arranged in the mold within 2.5 meters in the meniscus downstream.

When being arranged on electromagnetic mixing apparatus in the above-mentioned scope, the top layer part that flushing was on one side solidified by the initial stage is pounced on the oxide of grasping, make the solidified structure miniaturization of top layer part on one side, make slab inside contain many MgO and/or contain the MgO oxide, can make solidified structure become finer solidified structure.Its result can prevent appearance produces because of oxide in slab and the steel scale and crack defect, and can also improve corrosion resistance.

The mixing position of electromagnetic mixing apparatus, when being in meniscus (molten steel surface) top molten steel is formed and stir stream, solidified shell becomes blocked up when surpassing 2.5 meters downstream and be in, and the solidified shell inner oxide that becomes the top layer part is increased, and can produce the problem that corrosion resistance reduces.

Continuous casing of the present invention is preferably given the stirring stream of molten steel with the above flow velocity of 10 cels with electromagnetic mixing apparatus.

Can utilize the molten steel flow washing to remove like this and pounce on the oxide of catching by the solidified shell of slab.

When the flow velocity that stirs stream is lower than 10 cels, can not wash near the oxide of removing the solidified shell, and if to stir the stream flow velocity too high, then owing to can be involved in the powder that molten steel surface covers, make the meniscus in the mold produce disturbance, be defined as 50 cels so preferably will stir on the flow velocity that flows.

And preferably electromagnetic mixing apparatus is provided with to such an extent that make the in-mold molten steel surface form the stirring stream of horizontal direction rotation.

Stirring stream by means of the horizontal direction rotation can wash effectively to remove by slab top layer part and pounce on the oxide of catching, and makes slab inside have many fine oxides.

Continuous casing of the present invention also is applicable to the situation with ferrite-group stainless steel molten steel casting slab.

Above-mentioned molten steel particularly contains the molten steel of 10～23 quality %Cr, 0.0005～0.010 quality %Mg.

Facilitation and filling effect (growth of back tissue is solidified in inhibition) that nucleus is generated utilize this method to form dispersed high MgO in the molten steel and/or contain the oxide of MgO, owing to can make the solidified structure of slab become fine solidified structure.

Thereby can suppress the defectives such as blemish, the inner crackle that produces and center porosity that slab top layer part produces.

To slab when perforation after the processing, can be suppressed at internal surface of hole and crack appearance with defective such as scale, improve the quality of steel pipe.

In a single day Mg content be lower than 0.0005 quality %, and the MgO in the molten steel will reduce, and freezing nuclei can not fully form, and blocking action also weakens, and can not make the solidified structure miniaturization.On the other hand, in case MgO content surpasses 0.010 weight %, the micronized effect of solidified structure will be saturated and do not have a remarkable result, and Mg and contain consumption such as Mg alloy and increase increases manufacturing cost simultaneously.Chromium content is if be lower than 10 quality %, and then the corrosion resistance of steel pipe reduces, and the micronized effect of solidified structure also reduces simultaneously.When chromium content surpassed 23 quality %, the addition of evanohm increased, and manufacturing cost rises.

When using continuous casing of the present invention to cast the ferrite-group stainless steel molten steel continuously, cast while can stir this molten steel with electromagnetic mixing apparatus.

By above-mentioned stirring the column crystal tip that generates when solidifying is fractureed, can suppress the growth of column crystal, the interaction of part and freezing nuclei can make the solidified structure of slab finer.

In addition, under the situation of above-mentioned application, preferably in being in a ratio of 0.2～0.7 scope, consolidating of slab begin slab is depressed gently.

The center porosity that produces is shunk in the inner remaining partial coagulation of not solidifying of slab, can gently be pressed institute's compacting by this, thereby can prevent center segregation that solidified steel current movable property is not given birth to etc.

Press than beginning in less than 0.2 scope is light in solid phase, because solidification zone is not too much, so even depress and can not obtain consolidation effect, and fragile solidified shell also can crack.If depress than in solid phase, then often can't depress center porosity greater than 0.7 scope.Therefore,, must adopt and depress power more greatly, thereby screwdown gear is maximized for the center porosity compacting.

The seamless steel pipe of the present invention that meets above-mentioned purpose, be to have added the molten steel casting of 10～23 quality % chromium and 0.0005～0.010 quality % magnesium in mold, rely on the cooling of this mold, and, in the tubulation operation, the slab perforation that obtains is made while the method continuous casting that utilizes the cooling water nozzles spray water cooling that is arranged on hold segment to make it to solidify.

This steel pipe is owing to adopt the fine slab of solidified structure, so when the middle punch of tubulation operation, surface and inner surface that can killer tube crack and the scale defective, nor needs finishing such as grinding, is colory steel pipe.

Brief description of drawings

Accompanying drawing 1 is to make the sectional drawing of slab of the present invention with continuous casting apparatus.

Accompanying drawing 2 is near sectional drawings the mold in the continuous casting apparatus shown in the accompanying drawing 1.

Accompanying drawing 3 is B-B sectional drawings of mold shown in the accompanying drawing 2.

Accompanying drawing 4 is sectional drawings of A-A section in the continuous casting apparatus shown in the accompanying drawing 1.

Accompanying drawing 5 is molten steel processing method of the present invention sectional drawings with treating apparatus.

Accompanying drawing 6 is processing method of the present invention sectional drawings with another treating apparatus.

Accompanying drawing 7 is ideographs of traditional slab thickness direction solidified structure.

Accompanying drawing 8 is slab of the present invention distances apart from the top layer, and the schematic diagram that concerns between equiax crystal diameter and the column crystal width.

Accompanying drawing 9 is ideographs of solidified structure on the slab thickness direction of the present invention.

Accompanying drawing 10 is schematic diagrames of other relation between the distance on slab middle distance of the present invention top layer and the equiax crystal diameter.

Accompanying drawing 11 is slab of the present invention distances apart from the top layer, and the schematic diagram of other relation between equiax crystal diameter and the column crystal width.

Accompanying drawing 12 is slab of the present invention distances apart from the top layer, and the schematic diagram of other relation between the equiax crystal diameter.

Accompanying drawing 13 is sectional drawings of slab thickness direction of the present invention.

Accompanying drawing 14 is slab of the present invention distances apart from the top layer, and the schematic diagram that concerns between maximum crystal grain diameter/average crystal grain diameter in the crystal grain diameter.

Accompanying drawing 15 is traditional slab distances apart from the top layer, and the schematic diagram that concerns between maximum crystal grain diameter/average crystal grain diameter in the crystal grain diameter.

Accompanying drawing 16 be field trash number below 10 microns in the slab (individual/centimetre ²) and proportion of equiaxed grain (%) between the relation schematic diagram.

Accompanying drawing 17 is CaO-Al ₂O ₃Among-MgO system status the figure, belong to the curve map of compositing area of the present invention.

Accompanying drawing 18 is in the molten steel processing method of the present invention, the schematic diagram of relation between the solubility product of Ti concentration and N concentration in the molten steel: [%Ti] * [%N] and the Cr concentration [%Cr].

Accompanying drawing 19 is in the molten steel processing method of the present invention, adds before the Mg FeO, Fe in the slag ₂O ₃, MnO and SiO ₂Gross mass % and Mg handle the schematic diagram that concerns between the Mg retention rate in the molten steel of back.

Accompanying drawing 20 is in the molten steel processing method of the present invention, the schematic diagram that concerns between the basicity of slag and the CaO activity.

The best mode that carries out an invention

1) below, describes with regard to the present invention and specific embodiment with reference to accompanying drawing, for understanding the present invention.

As depicted in figs. 1 and 2, make the continuous casting apparatus 10 that slab of the present invention is used, have the pans 12 that store molten steel, be provided with to mold 13, the cast dipping spray nozzle 15 of molten steel 11 usefulness outlet 14 of pans 12, stir the electromagnetic mixing apparatus 16 of molten steel 11 in the mold 13, by the hold segment 17 that the watering from the unshowned cooling water nozzle of accompanying drawing is solidified molten steel 11, the middle body of slab 18 is depressed with reduction section 19, and back slab 18 usefulness withdrawing rolls 20 and 21. are depressed in drawing

Electromagnetic mixing apparatus 16 as shown in Figure 3, is arranged on the outside of mold 13 longwell 13a and 13b, and longwell 13a and longwell 13b are respectively arranged with solenoid 16a and 16b, and 16c and 16d.

Wherein can use this electromagnetic mixing apparatus 16. in case of necessity

Reduction section 19 as shown in Figure 4, by the support roller 22 that remains on below the slab 18, and contacts with the upper surface side of slab 18 and is made up of the depress roller 24 of bossing 23.This depress roller 24 is depressed by the oil gear that accompanying drawing does not illustrate, and bossing 23 can be pressed into the desired depth position, and the part 18b that do not solidify of slab 18 is depressed.In the accompanying drawing 2, symbol 18a is the solidified shell of slab.

Slab 18 is transported in the subsequent handling after being cut into preliminary dimension thereafter, after heating such as not shown heating furnace, soaking pit, makes steel through pressure processing.

Processing method of the present invention is shown in accompanying drawing 5 and 6 accompanying drawings 5 with treating apparatus.Treating apparatus 25 shown in the accompanying drawing 5, has the ladle 26 that bears molten steel 11, the Al alloy that contains that is arranged on ladle 26 tops is stored hopper 27, and N alloys such as Ti alloy such as storage sponge Ti, Fe-Ti alloy or Fe-N alloy, N-Mn alloy, N-Cr alloy are with hopper 28, as required from the chute 29. of the molten steel 11 interpolations above-mentioned alloy of these storage hoppers 27,28 to ladle 26 in

In addition, treating apparatus 25 also has under conduit 32 guiding, will be processed into the wire 30 of wire by the metal M g that iron pipe 29 covers, and runs through the feedway 31 that slag 33 is supplied with in the molten steel 11.

Symbol 34 is to supply with the inert gas porous plug in ladle 26 in the accompanying drawing 5.

Treating apparatus 35 shown in the accompanying drawing 6 has ladle 11 and winding-up Mg or Mg alloy powder spray gun 36.Spray gun 36 is housed in the ladle 26, and dipping is formed with among the molten steel 11 of slag 33 from the teeth outwards, utilizes inert gas through this spray gun 36, sprays into Mg or the Mg alloy powder of 0.0005～0.010 quality % that for example is equivalent to the Mg amount.

Generally speaking, the solidified structure of slab as shown in Figure 7, is carefully brilliant through the Quench of the tiny crystalline structure of the rapid cooled and solidified of mold by top layer (surface layer part), and forms at the brilliant column crystal of organizing of big burl that the thin intracrystalline side of this Quench forms.

In addition, also form equiax crystal in slab inside sometimes, column crystal extends to core sometimes.

Column crystal is thick texture, and the anisotropy of being out of shape when carrying out pressure processing is strong, and is different with the deformational behavior on the length direction at width.

Therefore, the steel that use slab with the big solidified structure of column crystal proportion to make are compared with the steel of the slab manufacturing with fine equiax crystal, and quality of materials is poor, often generates blemish such as wrinkle easily.

Exist when the slab top layer under the situation of thick column crystal, because the crisp microsegregation of thick column crystal crystal boundary existence,, will crack on the slab top layer and blemish such as pit so it exists the position to become fragile.

In addition, exist in slab inside under the situation of column crystal or big grain equiax crystal, the underbead crack (crack) and the center porosity (porous) that cause because of the microsegregation that exists in the solidified structure and solidification shrinkage, and solidify the internal flaws such as center segregation that produce because of MOLTEN STEEL FLOW before stopping and be easy to generate, thereby will damage the quality of slab and the quality of steel.

2) (1) can prevent the generation of above-mentioned blemish and internal flaw by making the solidified structure that has the equiax crystal that satisfies following formula more than 60% of the total section of slab.

D＜1.2×X ^1/3+0.75

In the formula, D is the equiax crystal diameter (millimeter) as the identical tissue of crystallization direction, and X is apart from the distance on slab surface (millimeter).

That is to say, by the slab that the solidified structure with the equiax crystal that satisfies following formula is formed, be slab A of the present invention.

The diameter of this equiax crystal is on thickness direction whole section to be corroded after molten steel solidification becomes slab, when light shines in its surface, and definite solidified structure size according to the catoptrical light and shade of macrostructure's crystallization direction reflection.

Detecting of this equiax crystal diameter is slab to be cut off the section expose thickness direction and after grinding, and for example makes it to corrode with the method for hydrochloric acid or nital (Nital, the mixed liquor of nitric acid and ethanol) reaction to carry out.

Average equiax crystal diameter can be with the enlarged photograph of macrostructure being taken into 1～100 times of amplification, this enlarged photograph is made image handle the equiax crystal diameter (mm) that obtains and obtain.Maximum in this equiax crystal diameter is maximum equiax crystal diameter.

The curve map that concerns between the distance on accompanying drawing 8 expressions slab A middle distance of the present invention top layer and the equiax crystal diameter.When the solidified structure that forms has such characteristics, promptly 60% above equiax crystal satisfies following formula in the total section of slab, so not only can suppress the growth of column crystal in the top layer, and can make the equiax crystal miniaturization of inside.

Among this slab A, as shown in Figure 9, owing to can suppress the growth of top layer part column crystal, so it is few to be present in the fragility microsegregation of crystal boundary, even and have also minimum.Therefore, even this slab A shrinks with mold cooling with when solidifying and heterogeneity appears in stress etc., also can suppress with the microsegregation is the generation of blemish such as the crackle of starting point generation and pit.

In addition, as shown in Figure 9,,, can improve crack resistance so the microsegregation that produces at crystal boundary also reduces equally with the top layer part because inner equiax crystal diameter also reduces, and the underbead crack that inhibition slab bulging is processed and the straightening distortion causes etc.

Therefore, because slab A processing characteristics and quality of materials are all good, so during with this slab A manufacturing steel, can obtain not have the steel of blemish such as wrinkle.

Satisfy the equiax crystal of following formula, in case it is full face 60% to be lower than slab, not only the scope of column crystal can enlarge, and inner equiax crystal diameter also increases, and cracks on slab and defectives such as pit.Its result, or must repair slab, or just take place broken; And slab added on steel surface, will produce blemish and internal flaw man-hour, steel quality is reduced.

The solidified structure of slab A of the present invention, as shown in figure 10, by the full section that makes slab is the equiax crystal that satisfies following formula, can make solidified structure all become the solidified structure of homogeneous at slab, and can in all scopes of slab, reduce to exist with crystal boundary on the fragility microsegregation.Its result, the crack resistance of slab is improved, even shrink and unbalanced stress utilizing mold when cooling or when solidifying to produce, also can suppress with microsegregation really partly is blemish such as the crackle of starting point and pit, and defectives such as the underbead crack generation that causes because of the distortion of bulging processing and straightening processing.

Making it with the freezing nuclei is starting point when solidifying, and can reduce the equiax crystal diameter, and molten steel flow improved before its result was solidified and stopped, and can prevent that molten steel from shrinking defectives such as the center porosity (porous) that causes and center segregation, can cast the zero defect slab.

In addition, slab A of the present invention is not more than three times of average equiax crystal diameters by making maximum equiax crystal diameter, can also obtain the preferred result that makes solidified structure finer.

This is because the change of equiax crystal diameter reduces in the solidified structure, can obtain to have the slab of high homogeneity solidified structure, can suppress the microsegregation that equiax crystal circle forms littler, thereby can prevent the cause that blemish and internal flaw produce.

In addition, because the equiax crystal diameter is little, so the rolling homogeneity that adds the deformational behavior in man-hour will further improve.

If maximum equiax crystal diameter is greater than three times of average equiax crystal diameter, machining deformation that then should the part can become uneven, produces striated wrinkle defective etc. in steel sometimes.

And in slab A of the present invention, handle the equiax crystal diameter that obtains if be conceived to image, then as shown in figure 11, can make the equiax crystal that satisfies following formula account for more than 60% of the whole sections of slab, thereby can access preferred solidified structure.

D＜0.08X ^0.78+0.5

In the formula, X is apart from the distance on slab surface (millimeter), and D is to be that X is to equiax crystal diameter (millimeter) apart from the slab surface distance.

In addition, slab A of the present invention as shown in figure 12, can make the full section of slab form the equiax crystal that satisfies following formula, thereby can access preferred solidified structure.

Use under the occasion of the continuous casting apparatus continuous casting slab of the present invention shown in attached Fig. 1 and 2, Mg or Mg alloy are added in the molten steel 11 in the pans 12, make the composite oxides that form the MgO monomer in the molten steel 11 or contain MgO (below be called " containing the MgO oxide ").

MgO evenly disperses in molten steel after becoming the particulate of favorable dispersibility, plays the freezing nuclei effect, and above-mentioned oxide itself plays obstruction (thickization of after coagulation tissue solidified in inhibition) simultaneously and acts on, suppress thickization of solidified structure, form equiax crystal, make the miniaturization of equiax crystal own simultaneously, the slab homogeneous.

The Mg or the Mg alloy that add, the addition in molten steel is enough to be equivalent to 0.0005～0.10 quality %Mg, can add in the molten steel Mg of interpolation and the oxygen in the molten steel and by FeO, SiO to ₂, the oxygen reaction that provides of oxide such as MnO, can form MgO or contain the MgO oxide.

The adding method of Mg or Mg alloy can directly add Mg or Mg alloy in the molten steel, perhaps supplies with continuously with the wire of making wire behind thin steel covering Mg or the Mg alloy.

When the Mg addition was lower than 0.0005 quality %, the freezing nuclei lazy weight was because of the nuclear deficiency that generates is difficult to obtain fine solidified structure.

And if the Mg addition surpasses 0.10 quality %, then the generation effect of equiax crystal is saturated, and slab subscale total amount increases simultaneously, will reduce corrosion resistance.In addition, cost of alloy also can rise.

The slab that this method is made, the fine and homogeneous of solidified structure, blemish and internal flaw are few, have good processing characteristics.

Slab A of the present invention except continuous casting process, can also use casting method castings such as ingot casting method, belt continuous metal cast process, double roller therapy.

Below describe with regard to the steel that use slab A of the present invention to make.

Steel of the present invention are with the unshowned heating furnace of accompanying drawing and soaking pit etc., the slab A that has the equiax crystal solidified structure that satisfies following formula more than 60% in the whole sections of solidified structure is heated to 1150～1250 ℃ after, rolling forming (for example steel plate, shaped steel).

D＜1.2X ^1/3+0.75

D is the equiax crystal diameter (millimeter) of crystallization direction homologue in the formula, and X is apart from the distance on slab surface (millimeter).

This steel, improve microsegregation part crack resistance so the fragility microsegregation that crystal boundary exists is little owing to manufacture with the slab A with above-mentioned solidified structure, are the steel of few surface defects such as crackle and scale.

In addition, the center segregation that forms etc. all is inhibited because slab underbead crack and the center porosity (porous) that produces because of the solidification shrinkage of not solidifying molten steel and invar water flow, thus steel inner that exist, because of the internal flaw of internal flaw generation few.

Slab A of the present invention is because the rolling homogeneity that adds man-hour distortion is high and processing characteristics is good, so material such as toughness is good in steel, and few surface defects such as wrinkle defective and crackle.

Particularly use full section to satisfy the equiax crystal slab of following formula, be rolled the steel of being processed into after the heating, owing to used slab with uniform solidified tissue, so blemish and internal flaw are few, and the homogeneity that adds the distortion in man-hour is better, so processing characteristics and material etc. are all good.

Be in by the maximum equiax crystal diameter that makes slab in three times of average equiax crystal diameter, can be suppressed at the microsegregation size that equiax crystal diameter crystal boundary forms, can obtain to have the steel of uniform material characteristic more.

(2) slab B of the present invention is characterized in that, apart from slab surface even depth place crystal grain diameter maximum, is in three times of this depth-averaged crystal grain diameter.

Among the above-mentioned slab B, as shown in figure 13, because apart from slab 18 surperficial even depth a millimeters, for example 2～10 millimeters position crystal grain diameter maximums are in three times of this degree of depth a millimeter place average crystal grain diameter value, form thick column crystal so can suppress the top layer, Cu etc. sneak into cyrystal boundary segregation minimizings such as element.Its result can prevent in the slab pit defect and crack defect that the inequality because of cooling and solidification shrinkage produces, can make the slab tissue tissue high to the resistance of crackle.

In addition, because slab surface and the inner crackle that produces reduce, so the situation of finishing such as grinding and slab fragmentation that slab carried out reduces, the yield rate of slab improves.

Processing characteristics when in addition, slab being made pressure processing also improves.

Numerical value apart from even depth a millimeter place, slab surface crystal grain diameter adopts for example with behind millimeter position, slab surfacing to 2～10, the measured value that the crystal grain diameter of exposing surface is measured.And this grinding also can proceed near the slab centre.

Apart from same degree of depth place, slab surface crystal grain diameter maximum, in case above three times of this depth-averaged crystal grain diameter value, the fluctuation of crystal grain diameter will increase, its result adds distortion heterogeneity when man-hour, distortional stress concentrated on the specific die, produce blemish such as wrinkle, yield rate is reduced.

And be easy to generate the high position of cyrystal boundary segregation, be that starting point produces face crack and underbead crack often with this position.Its result will produce blemish and internal flaw, reduce because of situation increases such as the finishing of slab and fragmentation make yield rate, and the quality of steel also can reduce.

Among the slab B of the present invention, as shown in figure 14, by the crystal grain diameter maximum is in three times of this same degree of depth place average crystal grain diameter value, and make in the full section of slab and be equiax crystal at least more than 60%, as shown in Figure 9, can suppress the top layer and form thick column crystal, and can form the tissue of homogeneous in all scopes.

In the traditional slab of accompanying drawing 15 expression apart from the relation between the maximum crystal grain diameter/average crystal grain diameter of the distance on top layer and crystal grain diameter.

Slab B of the present invention is added man-hour, owing to can suppress distortional stress and concentrate on the specific die, can guarantee the isotropism (at width and length direction because of depressing the elongation that causes) of deformational behavior, so slab B processing characteristics of the present invention is higher.

Therefore, when the processing slab is made steel, except can preventing that defectives such as crackle and scale from producing, can also prevent that wrinkle defective defectives such as (particularly wrinkle on the corrosion resistant plate and streaks) from producing.

In addition, can also make cyrystal boundary segregation that tramp elements such as Cu that crystal boundary forms etc. produce still less, further improve the crack resistance to crackle etc. when depressing processing such as steel rolling, thereby defective such as can prevent to crack in slab and the steel.

When equiax crystal does not reach full face 60% the time, because of the column crystal scope increases,, increase the finishing and the broken number of times of slab so will crack and defective such as pit, steel after the processing produce blemish and internal flaw, thereby often make yield rate reduce to reduce with quality.

Based on same reason, by making the full section of slab all form equiax crystal, make and be organized in the crystal grain that all has homogeneous in all scopes, therefore also can reduce cyrystal boundary segregation, improve top layer part and inner crack resistance, suppress generations such as pit and crackle, further improve each side's same sex of machining deformation, improve the quality and the materials such as toughness of r value (section contraction processing index) and steel.

Wherein, crystal grain diameter is the crystal grain diameter (millimeter) of the identical tissue of crystallization direction, is with after the slab surface corrosion, the size of definite solidified structure according to the catoptrical light and shade of the crystallization direction of macrostructure reflection.

Detecting in such a way of this crystal grain diameter carried out: along predetermined length it is cut off, make it to expose the section that solidifies back slab thickness direction, be ground to desired depth apart from its periphery, after grinding this surface of exposing again, make it to corrode with the mode of for example hydrochloric acid or the reactions such as (mixed liquors of nitric acid and ethanol) of nitric acid alcohol mixeding liquid.

To the enlarged photograph of 1～100 times of macrostructure's picked-up, make image and handle, measure crystal grain diameter, obtain maximum and mean value.

When casting slab B of the present invention continuously, add Mg or Mg alloy in the molten steel 11 in pans 12 (referring to attached Fig. 1 and 2), make to form the MgO monomer in the molten steel or contain the MgO oxide.

The addition of Mg, action effect and adding method are identical with the situation of slab A of the present invention.

In addition, slab B of the present invention, same with slab A of the present invention, except continuous casting process, can also use casting method castings such as belt continuous metal cast process, double roller therapy.

Slab B of the present invention is after unshowned heating furnace of accompanying drawing and soaking pit etc. are heated to 1150～1250 ℃, again through the rolling steel such as steel plate and shaped steel of being processed into.

In this steel, internal flaw such as blemish such as crackle and scale and underbead crack is few, is the good steel of processing characteristics.

Particularly use the slab thickness direction to be at least 60% slab that is equiax crystal as the slab or the whole face of equiax crystal in the section, can access defective still less, processing characteristics, for example section shrinks the good steel of processing characteristics.

(3) slab C of the present invention is characterized in that wherein containing 100/centimetre ²More than when molten steel solidification, generate be in field trash below 6% with the non-coherency of delta ferrite lattice.

Dipping spray nozzle 15 from pans 12 settings, the primary crystallization that will solidify (phase of separating out at first when molten steel 11 solidifies) is cast in the mold 13 (referring to attached Fig. 1 and 2) for the molten steel of delta ferrite steel grade (the ferrite-group stainless steel molten steel that contains chromium 13 quality %) 11, the cooling back forms solidified shell 18a limit and becomes slab 18, enter the below of hold segment 17 subsequently, by the cooling water under spilling heat is taken away, solidified shell 18a thickness edges increases later on, limit section of being depressed 19 in the way of advancing is depressed (referring to accompanying drawing 4), solidifies fully.

Solidified structure on the existing slab thickness direction section as shown in Figure 7, is that the top layer (top layer part) of slab is carefully brilliant by the Quench that the rapid cooled and solidified of mold forms fine tissue, and forms big columanar structure in the thin intracrystalline side of this Quench.

There is the microsegregation on the column crystal crystal boundary in this top layer part, enbrittles in this microsegregation position, so the inhomogeneity of cooling off and shrinking because of mold will become the reason that cracks on the slab top layer with pit defect.

In addition, inner at slab owing to cool off slowlyer than top layer part, so will generate column crystal or big grain equiax crystal, have the microsegregation same with the top layer part at the crystal boundary place of solidified structure.

This microsegregation has and the same fragility of top layer part, and mechanical stresses such as the thermal contraction when becoming solidified inside and processing of the bulging of slab and detorsion cause the starting point of underbead crack.

On the other hand, under the big situation of the inner equi-axed crystal diameter of slab,, will produce the center porosity that invar water undersupply causes in slab inside along with the propelling of solidifying, and because of solidify stop before the internal flaws such as center segregation that cause of MOLTEN STEEL FLOW, thereby impair the slab quality.

Therefore, in order to prevent above-mentioned blemish and internal flaw, when molten steel solidification, must make in the molten steel to have 100/centimetre ²More than be in field trash below 6% with the non-coherency of delta ferrite lattice.

The method that this field trash is existed in molten steel is, interpolation can with contained O, C, N, S and SiO in the molten steel 12 ₂On the metal of oxide reaction formation field trash, perhaps in molten steel, add field trash itself.

O, C, N, S, SiO in above-mentioned metal and the molten steel ₂Deng the field trash that reaction forms, the field trash that perhaps adds in molten steel forms the field trash below 10 microns in the molten steel.This field trash plays freezing nuclei when molten steel solidification, become the starting point that begins to solidify.

In addition, utilize the blocking action of above-mentioned field trash can suppress the solidified structure growth, can access the slab of fine solidified structure.

By using dispersed good field trash, rely on the stirring action of the discharge logistics of molten steel 11 in the mold 13, and the stirring action of electromagnetic mixing apparatus 16, when forming 100/centimetre 2 above field trashes below 10 microns, above-mentioned freezing nuclei and blocking action thereof are more obvious, as shown in figure 16, can access have equiaxial crystal ratio be more than 60% the tissue slab.

Accompanying drawing 9 shows the solidified structure in the slab thickness direction section, can form fine equiaxed grain structure in slab inside, and has suppressed the growth of column crystal in the top layer part.

Owing to increased the field trash below 10 microns, so the solidified structure that slab is assigned to inner whole section from skin section all can become the equiax crystal of fine homogeneous.

Slab C of the present invention with fine equiax crystal is because crack resistance is strong, so blemish such as crackle that occurs on the slab surface and pit are difficult to produce.

The inner fragility microsegregation of slab C of the present invention is partly few, even produce thermal contraction and stress, it is also few that underbead crack etc. produce, and the generation of solidifying every defectives such as the center porosity that causes because of the molten steel undersupply before stopping and center segregation also can prevent.

When slab was carried out pressure processing, the fine equiax crystal among the slab C of the present invention was because easily depressing the direction distortion, so slab C of the present invention has higher processing characteristics.

Because processing characteristics is good, thus carry out can not producing wrinkle blemish such as (streak, wrinkle, edge cracks) after the pressure processing, and the internal flaws such as crackle that exist internal flaw to cause because of slab can eliminate steel rolling the time.

In order to form the field trash (this field trash is a metallic compound) that the ferrite steel grade uses, use the compound of metal such as Mg, Mg alloy, Ti, Ce, Ca, Zr and metal, make it with molten steel in O, C, N, S, SiO ₂React Deng oxide.

The field trash that adds in the molten steel uses MgO, MgAl ₂O ₄, TiN, CeS, Ce ₂O ₃, CaS, ZrO ₂, TiC, VN etc. and the non-coherency of delta ferrite lattice be below 6%.From the dispersiveness when molten steel adds with solidify caryogenic stability, preferred especially MgO, MgAl ₂O ₄, TiN.

Wherein with the non-coherency of delta ferrite lattice, be meant lattice paprmeter poor of the lattice paprmeter metallizing thing of the delta ferrite that molten steel solidification generates, divided by the numerical value that the lattice paprmeter of molten steel solidification nuclear obtains, the more little freezing nuclei of this value generates well more.

In order to measure the number of field trash in the slab, use SEM SEM (Scanning Electron Microscope) and slip (ス ラ イ system) method etc., number goes out to be equivalent to the field trash number below 10 microns in the unit are.

The size of metallic compound is with the field trash on the full section of electron microscope observations such as SEM, with the mean value of the maximum gauge of single field trash and the minimum diameter size as this field trash.

When adopting slurry process, the full face part of slab is downcut, dissolve this section, then the field trash classification is taken out, determine its size, obtain number with this size by the maximum of working as a field trash and the mean value that minimum of a value is obtained.

In order to cast the slab that contains this field trash continuously, the molten steel 11 in pans 12 (referring to accompanying drawing 1 and 3) adds metal, make it with molten steel in oxygen or FeO, SiO ₂, reactions such as MnO, N, C, form MgO, MgAl ₂O ₄, field trashes such as TiN, TiC, perhaps directly in molten steel, add these field trashes.

Especially in molten steel, add Mg or Mg alloy, make it in molten steel, to form MgO itself or by containing the field trash that the MgO oxide is formed, in this case owing to improving the dispersiveness of field trash in molten steel, so can obtain better result.

For example, in molten steel, add Mg or Mg alloy, make the addition of Mg be equivalent to 0.0005～0.10 quality % of molten steel.

Mg or Mg alloy are directly added in the adding method employing in molten steel, perhaps Mg or Mg alloy are coated and are processed into stalloy in the continuous supply in the thread back molten steel (with reference to accompanying drawing 5 and 6).

When the addition of Mg is lower than 0.0005 quality %, be difficult to obtain fine solidified structure because of freezing nuclei is not enough.And the inhibitory action of solidified structure being grown because the blockage effect of field trash itself weakens reduces, and can not obtain fine solidified structure.

Otherwise if the Mg addition surpasses 0.10 quality %, then the generation of freezing nuclei reaches capacity, and slab subscale total amount increases simultaneously, and corrosion resistance is reduced.But also cost of alloy is risen.

The primary crystallization that solidifies is the molten steel of delta ferrite steel grade, and SUS stainless steel that contains 11～17 weight % chromium etc. is for example arranged.

Therefore, slab C of the present invention, solidified structure homogeneous and fine can suppress the generation of blemish and internal flaw, has good processing characteristics.

In addition, slab C of the present invention except continuous casting process, can also cast with castings such as ingot casting method and Strip casting method, double roller therapies.

Slab C of the present invention after unshowned cutter cuts into preliminary dimension in accompanying drawing, is transported to subsequent handlings such as steel rolling with withdrawing roll 20 and 21 (referring to accompanying drawing 1) traction.

After above-mentioned conveying, slab C of the present invention is heated to 1150～1250 ℃ with unshowned heating furnace and soaking pit in the accompanying drawing, carries out pressure processing then, makes steel such as slab, thin plate, shaped steel.

This steel are to organize crack resistance strong, the steel of few surface defects such as crackle that processing neutralization processing back produces and scale.

This steel also lack because of the internal flaw that the slab internal flaw produces so add man-hour owing to can suppress the center segregation of slab inside.

Have the slab C of the present invention of fine and uniform solidified tissue, processing characteristics such as r value are good, easily slab is processed, and the toughness of processing back welding portion are also good.

Be formed with numerous good dispersions, size slab less than 10 microns field trashes, become steel through rolling processing and manufacturing, this steel are gone up the defectives such as producing scale and crackle, because of the characteristics of depressing the direction easy deformation have higher processing characteristics such as extension except preventing the surface really.

(4) slab D of the present invention, be to form the metal of freezing nuclei or metallic compound when in molten steel, being added on molten steel solidification and the slab made, it is characterized in that being of a size of metallic compound number below 10 microns in the top layer of described slab part with interior part, is that skin section is divided contained more than 1.3 times of metallic compound number below 10 microns that are of a size of.

Slab D of the present invention, in order to prevent that blemish and internal flaw from producing, in molten steel, add the metal that can form metallic compound, perhaps in molten steel, add this metallic compound itself, form freezing nuclei when making molten steel solidification with the reactions such as O, C, N and oxide in the molten steel.

In case but the metallic compound of formation various sizes in the molten steel, and the size of metallic compound is above 10 microns, just be difficult to become freezing nuclei, and thickization of the equiax crystal inhibitory action that is produced by the blocking action of metallic compound itself can not fully occur, and can not realize the miniaturization of solidified structure.

Therefore, metal that adds in the molten steel or metallic compound, the use good dispersion, it is very important making it to form the metallic compound this point of numerous sizes below 10 microns.

In addition, must there be 1.3 times of number in this metallic compound below 10 microns greater than slab top layer part in the inner number that exists of slab.

This is because part cooling in slab top layer is carried out rapidly, even it is less to form the metallic compound of freezing nuclei, also can obtain the cause of fine equiax crystal solidified structure.

Be in the top layer part more than 1.3 times the time in the number of the inner metallic compound below 10 microns of slab, owing to play freezing nuclei effect and blocking action, so can promote the equiax crystal miniaturization, suppress thickization of equiax crystal simultaneously, thereby can access and have homogeneous and the solidified structure of fine equiax crystal.

As shown in Figure 9, can obtain a kind of slab with following solidified structure, the solidified structure on slab thickness direction section is fine equiax crystal more than 60%, and the column crystal of top layer part also is suppressed very for a short time.

And can also obtain having a kind of like this slab of solidified structure, the solidified structure from slab top layer part to inner whole section is by fine and equiax crystal homogeneous is formed.

Slab D of the present invention, the crackle and the pit that produce because of the distortion and the stress of process of setting, and the blemish that causes because of field trash can both be inhibited, and be enhanced to process the crack resistance that iso-stress produces underbead crack because of slab bulging processing and detorsion, owing to can guarantee molten steel flow, produce in addition so can suppress internal flaws such as center porosity and center segregation.

Slab D of the present invention, especially few and inner many because form the metallic compound number of freezing nuclei in the top layer part, so when slab being processed into steel such as thin plate and shaped steel, can suppress because of blemish such as surperficial scale that field trash produces, crackles, and can also prevent to be exposed to thin plate and shaped steel surface or near being present in the top layer, to cause corrosion resistance reduction etc. because of metallic compound.

In case the number of slab inside is less than 1.3 times of slab top layer part number, the required freezing nuclei of solidified structure miniaturization is just not enough, and wad act is also poor, so cause thickization of solidified structure, can not obtain the solidified structure of homogeneous, the stress that the cooling of the heterogeneity of cooling and process of setting produces during casting, blemish such as crackle that internal contraction causes and pit, and the processing characteristics when impairing pressure processing all can take place in internal flaws such as center porosity and center segregation.

Contained metallic compound in the molten steel uses MgO, MgAl ₂O ₄, TiN, CeS, Ce ₂O ₃, CaS, ZrO ₂, TiC, VN etc. and the non-coherency of delta ferrite lattice be below 6%.From dispersed when molten steel adds and solidify caryogenic stability, more preferably MgO, MgAl ₂O ₄, TiN.

The metal that adds in molten steel uses Mg, Mg alloy, Ti, Ce, metals such as Ca, Zr.Use can with O in the molten steel and C, N, SiO ₂Deng oxide reaction, form above-mentioned metallic compound, but also can use the metallic compound that contains these metals etc.

Particularly interpolation can form with the metal of the non-coherency 6% following metallic compound of delta ferrite lattice or add in molten steel under the situation of metallic compound in molten steel, owing to can promote the freezing nuclei of useful effect to form, and wad act shows significantly, so can obtain having the slab of the solidified structure of being made up of finer equiax crystal.This slab is out of shape depressing on the direction easily, so processing characteristicies such as calendering are good especially.

When casting this slab that contains metallic compound continuously, in the molten steel 11 in pans 12 (referring to attached Fig. 1 and 2), add Mg, Mg alloy, Ti, Ce, Ca, Zr etc., make it with molten steel in O or FeO, SiO ₂, reactions such as MnO, nitrogen, carbon, form MgO, MgAl ₂O ₄, metallic compounds such as TiN, TiC.Particularly in molten steel, add Mg or Mg alloy, make it to generate MgO or contain under the situation of MgO oxide, because the dispersiveness of metallic compound improves in the molten steel, so can access better result.For example in molten steel, add Mg or Mg alloy, make the Mg that contains 0.0005～0.010 quality % in the molten steel.

Mg or Mg alloy are directly added in the adding method employing in molten steel, perhaps Mg or Mg alloy are coated and are processed into stalloy in the continuous supply in the thread back molten steel (with reference to accompanying drawing 5 and 6).

When the Mg addition is lower than 0.0005 quality %, freezing nuclei absolute magnitude deficiency, freezing nuclei and blockage effect reduce, and are difficult to obtain fine solidified structure.

On the other hand, in case the Mg addition surpasses 0.010 quality %, freezing nuclei forms will be saturated, and slab subscale total amount increases simultaneously, and corrosion resistance is reduced.But also cost of alloy is risen.

Adopt the slab D of the present invention of this method casting, the solidified structure homogeneous, being inhibited of blemish and internal flaw has good processing characteristics.

Slab D of the present invention, except continuous casting process, though can also cast with castings such as ingot casting method and belt continuous metal cast process, double roller therapies, but when thickness reaches more than 100 millimeters, the adjustment that field trash (metallic compound) distributes becomes easy, equiax crystal adjusting from the top layer to the solidified inside tissue easily is so can access good result.In the casting, for example adopt two ends to connect the product of the vertical or crooked continuous casting process casting of mold, its micronized effect increases, and can access good result.

Slab D of the present invention carries out pressure processing after unshowned heating furnace and soaking pit are heated to 1150～1250 ℃ in the accompanying drawing, be processed into steel such as thin plate, shaped steel.

This steel, because the crack resistance of slab interior microscopic segregation part crackle is strengthened, so be the steel of few surface defects such as crackle and scale.

What in addition, the internal flaw that causes because of the slab internal flaw of steel inside and the internal flaws such as underbead crack that cause because of pressure processing produced is also few.Slab D of the present invention is because processing characteristics and corrosion resistance are also good, so the steel of being processed into by slab D also have good processing characteristics and corrosion resistance.

When 3) making slab of the present invention, must do certain processing to molten steel.Below processing method (the processing method I of the present invention～V) describe of molten steel of the present invention just.

(1) processing method I of the present invention is characterized in that making that total calcium content drops to below the 0.0010 quality % in the molten steel, adds Mg then in this molten steel.

In the treating apparatus shown in accompanying drawing 5 and 6, adjust the TC that institute's calcic or calcium oxide etc. amount in the molten steel 11 in the ladle 26, make it to reach 0.0010 quality % following (comprising 0 situation).And make Al ₂O ₃With the low-melting compound (composite oxides) of CaO be calcium aluminate (12 CaO7Al ₂O ₃) do not generate.

In case contained TC surpasses 0.0010 quality % in the molten steel, will form calcium oxide as the calcium of strong deoxidier, with original calcium oxide together, with Al ₂O ₃In conjunction with forming low-melting compound.

And, because of adding MgO and the CaOAl that Mg or Mg alloy generate ₂O ₃The composite oxides combination generates CaO-Al ₂O ₃This ternary compound oxides of-MgO.This composite oxides can melt in the temperature range of molten steel, so can not play freezing nuclei, its result can not obtain fine solidified structure.Even perhaps above-mentioned composite oxides are the higher field trashes of fusing point, but owing to contain calcium oxide, so low with delta ferrite lattice coherency, there is not the effect of freezing nuclei.

In order to adjust the generation of this TC and calcium aluminate, during with refining furnace or ladle 26 deoxidations, perhaps make deoxidation of molten steel, perhaps with the alloyed iron or the few ferroalloy of calcium content of calcic do not make deoxidation of molten steel without calcium and calcium alloy.

The addition of Mg or Mg alloy is equivalent to 0.0005～0.10 quality %.This is that the freezing nuclei deficiency of generation is difficult to obtain micro organization when being lower than 0.0005 quality % because of the Mg addition.And if surpass 0.10 quality %, then the generation effect of equiax crystal is saturated, and slab subscale total amount increases simultaneously, and corrosion resistance is reduced.But also cost of alloy is risen.

In processing method I of the present invention, so contained TC relies on contained oxygen or FeO, SiO in the molten steel in the molten steel owing to having reduced ₂, oxide such as MnO supplies with oxygen, can form magnesia itself and MgOAl ₂O ₃Deng composite oxides, homogeneous is dispersed among the molten steel after these oxide fine particleizations.

During this molten steel solidification, form the freezing nuclei of numerous quantity, above-mentioned in addition oxide itself is owing to have blockage effect (thickization of back tissue solidified in inhibition), thereby can suppress thickization of slab tissue, and the while can also make the miniaturization of equiax crystal own and homogenize.

Can in treating apparatus 25,35 (referring to accompanying drawing 5 and 6), adjust contained TC in Mg addition and the molten steel, preferably adjust to and to suppress calcium aluminate (12CaO7Al ₂O ₃Deng low-melting compound) generation.

By means of oxygen contained in the molten steel or FeO, SiO ₂, oxide such as MnO supplies with oxygen, forms MgO itself and MgOAl ₂O ₃Deng containing the MgO oxide, the oxide homogeneous of grain refined is dispersed among the molten steel.

Molten steel after will handling with processing method I of the present invention is continuously cast into slab, and its solidified structure as shown in Figure 9, will become the solidified structure that the equiax crystal fine by homogeneous formed.

To handle like this back fluid origin slab, cut into preliminary dimension after, be transported in the subsequent handling, with carrying out pressure processing after the heating such as unshowned heating furnace and soaking pit in the accompanying drawing, make steel.This slab is improved significantly owing to processability, so have good shrink finishing quality energy and toughness with the steel of this slab manufacturing.

In addition, except continuous casting process, can also use casting method casting slabs such as ingot casting method, belt continuous metal cast process, double roller therapy.If for example with continuous casting process cast thickness slab more than 100 millimeters, the equiax crystal diameter then adjusting from the top layer to interior tissue easily can obtain good result because of micronized effect is bigger.

(2) processing method II of the present invention is characterized in that adding in molten steel before the scheduled volume Mg, adds in this molten steel and carries out deoxidation treatment after scheduled volume contains the Al alloy.

In the treating apparatus shown in the accompanying drawing 5 25, to carry out molten steel 11 (150 tons) behind the Decarburising and refining is housed in the ladle 26 and adjusts composition, add 70 kilograms of Al from storage funnel 27 then, add with chute 29, supply with argon gas from the porous plug 34 that is arranged on the ladle bottom simultaneously, the limit is stirred the molten steel limit and is added Al, in this way fully deoxidation.

After the Al deoxidation, continue to supply with argon gas, start unshowned rotary drum in the feedway 31, supply with silk material 30, pass slag 33, in molten steel 11, supply with 0.75～15 kilogram metal M g (0.0005～0.010 quality %) through conduit 32 guidings through porous plug 34.

Like this, before adding scheduled volume Mg, add scheduled volume Al, make it with molten steel in oxygen, MnO, SiO ₂, reaction such as FeO generates Al ₂O ₃, add Mg then, do not play the Al of freezing nuclei effect when making with the non-coherency of delta ferrite lattice greater than 6% molten steel solidification ₂O ₃On the surface, generate MgO, MgOAl ₂O ₃Deng containing the MgO oxide.Utilize that this method makes field trash in the molten steel with the non-coherency of delta ferrite lattice less than 6%, make this field trash when molten steel solidification, play freezing nuclei.

Its result owing to contain the MgO of numerous dispersions and/or contain the MgO oxide, be that starting point begins to solidify in many places with these oxides when solidifying, so the solidified structure of slab will become fine in the molten steel.

According to processing method III of the present invention, do not crack and pit defect on the slab surface, can suppress inner center segregation and the center porosity etc. of producing, slab can be controlled and the finishing and the fragmentation of the steel be processed into, thereby quality can be improved.

In molten steel 11, add before the Mg, promptly carry out the Al deoxidation after, also can emit 50 kilograms of Fe-Ti alloys, in the molten steel 11 in chute 29 adds ladle 26 to from storage hopper 28.

At first in molten steel, add Al, utilize deoxygenation to generate Al ₂O ₃So even add the Fe-Ti alloy, Ti does not wherein generate TiO yet ₂,, perhaps combine and generate TiN with N in the molten steel in the form solid solution of molten steel with Ti.

Then, start the rotary drum in the feedway 31, under the guiding of conduit 32, in molten steel, infeed silk material 30, in case in molten steel, infeed 0.75～15 kilogram of Mg, will be at Al ₂O ₃The surface generates MgO and contains MgO oxide (MgOAl ₂O ₃).

Cover Al ₂O ₃The MgO and/or the MgOAl on surface ₂O ₃Since with the non-coherency of delta ferrite lattice less than 6%, so when molten steel solidification, will play the freezing nuclei effect.

In addition, above-mentioned TiN has the freezing nuclei effect too, utilization and MgO and/or MgOAl ₂O ₃Synergy, can make the solidified structure miniaturization.Particularly the interpolation of Al and Ti order except above-mentioned interpolation order, also can be added Ti earlier and make it to generate TiO ₂, adding Al then and make the Ti reduction, the Ti after the reduction is solid-solubilized in the molten steel.

And no matter under which kind of situation, Ti perhaps forms TiN separately with the MgO oxide, can both further improve the freezing nuclei effect.So add a small amount of Ti, can reduce cost of alloy, and can also prevent that the defective that TiN causes from producing.

Molten steel after the processing method II processing of the present invention is taken out a part as sample, studied the composition that contains the MgO oxide with EPMA (electron probe microanalysis (EPMA)) method of electron microscope.

Its result formerly adds under the situation of adding Mg behind the Al, plays the field trash of freezing nuclei effect, can verify that inside is Al ₂O ₃, be MgO around it, or by MgOAl ₂O ₃The material of forming that contains the coating of MgO oxide.

In addition, add Ti behind the interpolation Al, and then add under the situation of Mg, the structure of observing field trash is: contain the MgO oxide with Al ₂O ₃The surface coats, and part is coated by TiN around it; This field trash since with the non-coherency of delta ferrite lattice less than 6%, so can play the freezing nuclei effect effectively.

With regard to the interpolation order of Ti, add according to Ti, Al order (or Al, Ti order), add then under the situation of Mg, perhaps after adding Al, add Mg, add at last under the situation of Ti, the clad structure of field trash all is Al ₂O ₃The surface is by MgO or MgOAl ₂O ₃Coat, wherein part or all is coated by TiN, can play the freezing nuclei effect effectively.

Therefore, the fluid origin slab of molten steel after handling with processing method II of the present invention, as shown in Figure 9, and no matter under which kind of situation, the top layer of slab section and inside, solidified structure is all very fine.

(3) in processing method I of the present invention and processing method II, preferably in molten steel, add scheduled volume Mg, the oxide that generates when making oxide such as slag contained in the molten steel and deoxidation product and adding Mg in molten steel satisfies following formula (1) and (2);

17.4(k Al ₂O ₃)+3.9(kMgO)+0.3(kMgAl ₂O ₄)

+18.7(kCaO)≤500 …(1)

(k Al ₂O ₃)+(kMgO)+(kMgAl ₂O ₄)+(kCaO)

≥95 …(2)

In the formula, k represents the mole % of oxide.

In molten steel, add Mg and make it to generate oxide, and make under the situation of solidified structure miniaturization of slab, can form MgOAl according to other composition etc. that adds element and slag ₂O ₃CaO is that oxide or MgOCaO are refractory oxide etc.

But because MgOAl ₂O ₃CaO is that oxide is low-melting, so do not play the freezing nuclei effect during molten steel solidification.On the other hand,, MgOCaO exists with solid state shape because of fusing point is high though being oxide, and poor with the delta ferrite lattice coherency of the primary crystallization that solidifies, do not play the freezing nuclei effect.

Therefore, the inventor is with regard to these MgOAl ₂O ₃CaO is that oxide or MgOCaO are that oxide is furtherd investigate, found that, if the composition of these oxides is in the proper range, then can suppresses the eutectic of these oxides and reveal, and can improve the non-coherency of lattice of the delta ferrite of the primary crystallization that solidifies with conduct.

In the treating apparatus shown in the accompanying drawing 5, after the refining furnace decarburization and removing impurity such as dephosphorization, sulphur, 150 tons of molten steel 11 are packed in the ladle 26.

The limit is blown into hydrogen by porous plug 34 then, and 50～double centner Al is added with hopper 27 in the limit, and molten steel is mixed, and carries out deoxidation.

Then sampling from molten steel 11 is analyzed the tissue of oxide with EPMA, with following formula (3) calculate as oxide and with the α value of the non-coherency index of delta ferrite lattice.

For this value is in below 500, consider the rate of recovery, obtain the Mg addition, start feedway 31, under conduit 32 is guided, the Mg silk 30 that is equivalent to this value is added among the molten steel 11.

α＝17.4(k Al ₂O ₃)+3.9(kMgO)+0.3(kMgAl ₂O ₄)

+18.7(kCaO)≤500 …(3)

K represents the mole % of oxide in the formula.

Accompanying drawing 7 expression CaO-Al ₂O ₃If-MgO ternary state diagram is CaO-Al ₂O ₃-MgO system complex oxide is among the figure that satisfies following formula (3) in the zone (being enclosed fixed oblique line scope by symbol zero), then can effectively play the freezing nuclei effect.

The α value is in case surpass 500, and composite oxides will be revealed by eutectic, even perhaps high-melting-pointization, the MgO oxide that contains on capping oxide surface also reduces, thereby does not play the freezing nuclei effect.

In addition, can obtain the β value by following formula (4).This β value is if be lower than 95, SiO ₂, other oxide such as FeO will increase, will hinder those composite oxides that will become freezing nuclei and form.

β＝(k Al ₂O ₃)+(kMgO)+(kMg Al ₂O ₄)+(kCaO)

≥95 …(4)

In the formula, k represents the mole % of oxide.

Therefore, be in below 500, and the β value is lower than under 95 the situation, consider the rate of recovery, obtain the addition of Mg for making the α value.

Start feedway 31, under the guiding of conduit 32, the Mg silk 30 that is equivalent to obtain like this Mg value is added in the molten steel 11.

Its result is added to Al except forming majority with MgO ₂O ₃Go up the CaOAl that forms with CaO ₂O ₃Outside the MgO ternary oxide, can also generate Al ₂O ₃MgO and MgO are dispersed in the molten steel these divided oxides, along with the reduction of temperature is that starting point makes molten steel 11 begin to solidify with these materials, generates equiax crystal, thereby can produce the slab with fine solidified structure.

So, molten steel 11 solidifies the solidified structure of back slab, will become the sort of fine solidified structure shown in the accompanying drawing 9.

By making the solidified structure miniaturization, can prevent that the internal flaws such as underbead crack, center segregation, center porosity of slab from producing.In addition, the steel that fine slab processing obtains to solidified structure, drawability improves, and the generation of blemish such as while edge crack and striped etc. also can stably obtain preventing.

The addition of Mg is preferably adjusted to and is equivalent to 0.0005～0.010 quality % concentration range.

Mg concentration is if be lower than 0.0005 quality %, then can not generate with the non-coherency of delta ferrite lattice less than 5% composite oxides, can not make micro organization's miniaturization of slab.Otherwise if Mg concentration surpasses 0.010 quality %, then the micronized effect of solidified structure is saturated, and the interpolation cost of Mg rises.

(4) processing method III of the present invention is characterized in that adding the Mg of scheduled volume in Ti concentration and N concentration satisfy the molten steel of the solubility product of separating out the TiN crystal more than the molten steel liquidus temperature.

Therefore, processing method III of the present invention is under the situation of ferrite-group stainless steel molten steel at molten steel, preferably makes above-mentioned Ti concentration [%Ti] and N concentration [%N] satisfy the following formula requirement:

[％Ti]×[％N]≥([％Cr] ^2.5+150)×10 ^-6

In the formula, [%Ti] is the quality % of Ti in the molten steel, the quality % of N in [%N] molten steel, and [%Cr] is the quality % of Cr in the molten steel.

And, in processing method III of the present invention, make contained Al in the molten steel ₂O ₃Reach 0.005～0.10 quality %.

The non-coherency of TiN and delta ferrite lattice (lattice paprmeter of TiN and delta ferrite lattice paprmeter poor, the numerical value that obtains divided by the delta ferrite lattice paprmeter) is 4%, though fine, this TiN condenses easily.Therefore, thick TiN causes that easily dipping spray nozzle stops up, and perhaps has the problem that makes the steel layering.

Processing method III of the present invention, except TiN when the molten steel solidification can effectively play the freezing nuclei effect, other characteristics also have: add the MgO oxide that contains that generates behind the MgO in the molten steel and disperse property fabulous, and TiN preferentially separates out crystallization containing on the MgO oxide.

The inventor is conceived to this point, in processing method III of the present invention, utilization contains the MgO oxide, improves the dispersiveness of separating out crystallization on the MgO oxide and playing the TiN of freezing nuclei effect containing, and effective freezing nuclei majority with the solidified structure miniaturization is dispersed in the molten steel.

If add Ti and N in molten steel, then the TiN temperature of separating out crystallization depends on that Ti concentration and N concentration are long-pending, i.e. solubility product [%Ti] * [%N].

For example, after adding Ti and N in the molten steel, by controlling its addition, about 1500 ℃ of liquidus temperatures can be higher than, and be higher than under 1506 ℃ of temperature of temperature that TiN separates out crystallization, make it former state and be solid-solubilized in the molten steel, about below 1505 ℃ the time when it is cooled to, begin to separate out crystallization with the TiN form.

The inventor tests with regard to the solubility product of Ti concentration and N concentration and the relation between the Cr concentration in order to make the solidified structure miniaturization of the ferrite-group stainless steel that contains aequum Cr, has obtained the result shown in 18 that interprets blueprints.Following formula is obtained by the result shown in the accompanying drawing 18.

In the accompanying drawing 18, * be the not example of miniaturization of solidified structure, the 0th, the example of the abundant miniaturization of solidified structure, though and △ be the solidified structure miniaturization, produce the example of spray nozzle clogging when casting.

In treating apparatus shown in Figure 5,150 tons of molten steel, the 11 injection ladles 26 with the refining furnace decarburization and after removing impurity such as dephosphorization, sulphur.This molten steel 11 is ferrite-group stainless steel molten steel, contains 10～23 quality %Cr.

Then, added 150 kilograms of Fe-Ti alloys, added 30 kilograms of N-Mn alloys, mixed while stirring with hopper 28 through hopper 27.

Therefore, when above-mentioned interpolation Fe-Ti alloy and N-Mn alloy, addition makes Ti contained in the molten steel, N concentration satisfy following formula; And under the situation of 10 quality %Cr, make Ti and N concentration reach 0.020 quality % and 0.024 quality % respectively.

When the non-coherency of TiN and delta ferrite lattice is low when reaching 4%, form the delta ferrite freezing nuclei easily.Therefore, generate equiax crystal, the micronized effect of solidified structure thereby good during molten steel solidification easily.

In order to make TiN play the freezing nuclei effect, must more than the liquidus temperature that molten steel begins to solidify, for example begin to separate out the TiN crystal more than 1500 ℃, even separate out crystallization being lower than under the liquidus temperature, can not make solidified structure obtain micronized effect.

Therefore, liquidus temperature be must determine, in solubility product satisfies above-mentioned scope, Ti and N added.

In order to improve the micronized effect that this TiN produces, can consider to adopt the method that increases Ti and N addition, improve the TiN crystallization amount of separating out under the same temperature.But Ti amount and N amount but are subjected to the restriction of steel grade.For example, even increasing under the occasion that Ti amount and N measure, separate out after the crystallization As time goes on TiN because of thickization of cohesion, can find the phenomenon that the number of freezing nuclei may not increase, occur nozzle on the contrary even stopped up, and steel produce defectives such as scale by thick TiN.

Therefore, even the Ti amount is identical with the N amount, but start feedway 31, under conduit 32 guidings, infeed Mg silk 30 (referring to accompanying drawing 5), the Mg amount of the supply in the molten steel reaches 75 kilograms, this moment, Mg concentration was equivalent to 0.0005～0.010 quality %, contained the MgO oxide owing to generate, so the TiN crystallization of separating out is dispersed in the molten steel with micro situation.

That is to say, add before Ti and the N, perhaps after adding Ti, add Mg, make to contain the MgO oxide and generate in being higher than under the high temperature that TiN separates out temperature.

Therefore, though TiN separates out crystallization along with the reduction of liquid steel temperature, but owing to the non-coherency of the lattice that contains MgO oxide and TiN is close, so TiN preferentially separates out crystallization in containing on the MgO oxide of fine dispersion, compare when not adding Mg, most crystallizations of separating out are dispersed in the molten steel more efficiently.

For the Mg that adds in molten steel is remained on the high-recovery, add Mg behind the interpolation Ti, and shorten the time before the casting, can access good result.

Its result can prevent to add the result in blockage unstability of operations such as nozzle of the thick TiN that produces under the situation of Ti and N (not adding Mg), even and the addition of Ti also can make molten steel solidification become the solidified structure miniaturization of slab less, as shown in Figure 9.

By making the solidified structure miniaturization, can prevent the generation of the internal flaws such as underbead crack, center segregation and center porosity that cause because of solidification shrinkage and thick tissue.

Therefore, the steel of being processed into by the fine slab of solidified structure are because solidified structure is fine, so can stably suppress the generation of the blemish etc. of goods such as scale, edge crack, striped.

(5) processing method IV of the present invention is characterized in that making and contains the oxide that 1～30 quality % can be reduced by Mg in the slag that covers molten steel in advance.

Therefore, in processing method IV of the present invention, the oxide that can be reduced by Mg is FeO, Fe ₂O ₃, MnO and SiO ₂In one or more oxides.

And in processing method IV of the present invention, should make Al contained in the molten steel ₂O ₃Reach 0.005～0.10 quality %.

In the treating apparatus shown in the accompanying drawing 5, with behind the Decarburising and refining again the molten steel 11 through vacuum double refining (double refining) inject ladles 26.

In molten steel 11, add the aluminum or aluminum alloy deoxidier, make it contain 0.005～0.10 quality %Al in advance ₂O ₃

This is because by promoting MgOAl ₂O ₃Deng the generation of composite oxides and form the dystectic MgO of containing oxide, the Al of cohesion easily by making bad dispersibility in addition ₂O ₃Combine with MgO and improve miniaturization and dispersiveness, the effect that improves freezing nuclei makes the cause of organizing miniaturization of slab and steel.

In case contained Al in the molten steel ₂O ₃Be lower than 0.005 quality %, the MgO of generation will with Fe ₂O ₃, SiO ₂Form low melting point oxide Deng combination, as the effect reduction of freezing nuclei.On the other hand, as if contained Al in the molten steel ₂O ₃Surpass 0.10 quality %, because of the Al that condenses easily ₂O ₃Too much, often make in slab and the steel to produce and result from the defective of oxide.

When molten steel 11 is injected ladles 26, also can sneak into from converter or when flowing into double refining because of adding the slag 33 that flux etc. generates, with the surface coverage of molten steel 11 in the ladle 26.

Then start feedway 31,, make Mg or Mg alloy silk 30 pass slag and enter in the molten steel 11, in molten steel, add Mg and Mg alloy in this way via the speed of conduit 32 with 2～50 meters/minute.

In the past, though the slag of covering molten steel surface is with CaO, SiO ₂, Al ₂O ₃, FeO, Fe ₂O ₃With MnO etc. be main component, but in case add Mg to being covered in the molten steel by this slag, the oxide on molten steel and slag interface in the slag will with Mg and Mg alloy reaction, the MgO of generation will enter in the slag.Its result can not improve the Mg concentration in the molten steel, has reduced the rate of recovery of Mg in the molten steel.

The inventor furthers investigate this phenomenon, found that: the free energy of formation of oxide should be bigger than the free energy of formation of MgO, in other words, has important relationship between the rate of recovery of Mg in unsettled oxide gross weight and the molten steel on the thermodynamics.

That is to say, as shown in figure 19, add unsettled oxide (FeO, Fe on the thermodynamics that exists in the preceding slag of Mg ₂O ₃, MnO, SiO ₂) total amount, be in 1～30 quality % scope, in molten steel, supply with Mg and Mg alloy silk in case pass slag, the rate of recovery of Mg is reached more than 10%.

The wherein this Mg rate of recovery is Mg contained in the molten steel and contain the rate of recovery that obtains when the MgO oxide all is converted into the Mg amount.In fact, in the molten steel Mg have a form, almost all be MgO itself or MgOAl ₂O ₃Composite oxides.

It is believed that in case add Mg in molten steel, the oxide in the above-mentioned slag will be pressed the chemical reaction reduction shown in following formula (1)～(4) by Mg.

…(1)

…(2)

…(3)

…(4)

The Mg that promptly is added in the molten steel is consumed according to chemical equation shown in following formula (1)～(4), and the MgO of generation moves among the slag.

At this moment, in case FeO, Fe in the slag ₂O ₃, MnO, SiO ₂Total amount is less than 1 quality %, though the Mg and the Mg in the Mg alloy that have suppressed to add react slag and also can be reduced by the quantity of the dissolved oxygen in the molten steel that balance determined on the molten steel thermodynamics with slag.

Its result is not in case the Mg that adds in molten steel itself forms MgO or MgOAl ₂O ₃Deng composite oxides, As time goes on Mg will evaporate, and the rate of recovery is reduced.

In addition, if oxide aggregate surpasses 30 weight % in the above-mentioned slag, the Mg in Mg that adds in the molten steel and the Mg alloy and the reaction of slag will become violent, manyly among the Mg that adds carry out chemical reaction according to following formula (1)～(4) and generate MgO, and transfer in the slag, reduce so in molten steel, formed the fine quantity that contains the MgO oxide of freezing nuclei effect, the rate of recovery of adding Mg is reduced, can not realize the miniaturization of slab tissue.

In order to reach the required Mg concentration of miniaturization, must increase addition, thereby cause manufacturing cost to rise, add Mg and Mg alloy and cause temperature to reduce, and cause operating barrier because of the variation of slag proterties.

In sum, in order to improve the rate of recovery of adding Mg in the molten steel, make it to form MgO, MgOAl ₂O ₃Deng the high-melting-point composite oxides, and more stably generate fine freezing nuclei, the oxide in the slag is in the scope shown in the following formula, further make it to be in 2～20 weight % scopes, can obtain good result.

1 quality %≤FeO+Fe ₂O ₃+ MnO+SiO ₂≤ 30 quality %

For oxide concentration in the slag that will cover molten steel is adjusted in the scope shown in the following formula, add Mg before with the slag sucking-off, reduce slag amount, easy of like this reduction of the reduction composition in the molten steel, also can adopt universal method in slag, to add reducing agent and handle.

The Mg alloy that adds in molten steel can use alloys such as Si-Mg alloy, Fe-Si-Mg alloy, Al-Mg alloy, Fe-Si-Mn-Mg alloy.

(6) processing method V of the present invention is characterized in that adding before the scheduled volume Mg in molten steel, and the CaO activity of the slag that covers molten steel is in below 0.3.

And then in processing method V of the present invention, the basicity of slag is in below 10.

In the treating apparatus shown in the accompanying drawing 5, will inject ladle 26 through ferrite-group stainless steel molten steel 11 vacuum double refining (double refining), that contain 0.01～0.05 quality % carbon, 0.10～0.50 quality % manganese and 10～20 quality % chromium again behind the Decarburising and refining.

When molten steel 11 is injected ladles 26, also can sneak into from converter or add the slag 33 that flux etc. generates when flowing into double refining, with the surface coverage of molten steel 11 in the ladle 26.

This slag 33, its thickness are 50～100 millimeters, add activity that flux etc. adjusts CaO in the slag 33 then below 0.3, and basicity (CaO/SiO ₂) reach below 10.

Then, start feedway 31,,, make Mg or Mg alloy silk 30 pass slag 33 and enter in the molten steel 11, in molten steel, add Mg and Mg alloy in this way with 2～50 meters/minute speed via conduit 32 guidings.

Cover the slag of molten steel surface, contain CaO or SiO in the past ₂.Al ₂O ₃, oxide such as FeO because good, so often can improve CaO concentration in the slag with the desulfurization and the dephosphorization effect of converter and double refining.

In this case, be shown below, the Ca concentration in the molten steel increases because of the balanced reaction between slag and the molten steel.

In case in this molten steel, add Mg or Mg alloy, will generate CaO-Al in the molten steel ₂O ₃Low melting point composite oxides such as-MgO perhaps generate and the big oxide of the non-coherency of delta ferrite lattice.

These oxides do not play the freezing nuclei effect when molten steel solidification, and do not have blockage effect (solidifying the growth effect that the back suppresses equiax crystal), so solidified structure is thick.Its result on slab and the steel be processed into it, can crack and blemish and internal flaws such as scale and center porosity.

Therefore, for effect and the blockage effect that improves freezing nuclei, as shown in figure 20, the CaO activity (aCaO) in the slag is in below 0.3, described activity (aCaO) can be obtained by the basicity of slag according to following formula:

aCaO＝0.027(CaO/SiO ₂) ^0.8+0.13

By the CaO activity (aCaO) in the slag is in below 0.3, contained Mg will become MgO or contain MgO-Al in Mg and Mg alloy ₂O ₃Deng high-melting-point, and with the non-coherency of delta ferrite lattice little contain the MgO oxide, when molten steel solidification, will fully work as freezing nuclei.In addition, find that also this MgO of containing oxide has sufficient blockage effect, thus can make the solidified structure miniaturization of slab, and can suppress the generation of the blemish and the internal flaw of slab.

When this CaO activity is in 0.2 when following, can improve the fusing point that contains the MgO oxide of generation, and can also make the freezing nuclei effect stronger.

And replace the activity of CaO in the slag with the basicity of slag, and basicity is in below 10, also can make it to generate MgO or MgO-Al ₂O ₃Contain the MgO oxide Deng high-melting-point.

By regulating the slag thickness that covers molten steel, perhaps utilize in slag, to add and contain Al ₂O ₃Or the flux of MgO, can adjust this CaO activity and basicity.

Basicity surpasses under 10 the situation, and the Mg or the Mg in the Mg alloy that are included in interpolation form CaO-Al ₂O ₃Low melting point composite oxides such as-MgO not only can not play the freezing nuclei effect, but also become the generation of defects point, influence the quality of slab and steel.

In case CaO activity less than 0.2 or basicity less than 6, can promote to contain the generation of MgO oxide (playing the freezing nuclei effect), simultaneously its blockage effect also further improves, so can make the solidified structure miniaturization of slab really.

Wherein, as the alloy that in molten steel, adds, can use alloys such as Si-Mg alloy, Fe-Si-Mg alloy, Al-Mg alloy, Fe-Si-Mn-Mg alloy, Ni-Mg alloy.

So after in mold, making the molten steel solidification that has added 0.0005～0.010 quality %Mg, make slab.

4) below, the manufacture method of slab A～D of the present invention is described.Slab A～D of the present invention, can be in order to the below manufactured: the molten steel casting that will contain the MgO oxide utilizes the electromagnetic agitation limit to stir this molten steel limit continuous casting in mold.

When casting slab of the present invention continuously, electromagnetic mixing apparatus is arranged in the mold within 2.5 meters of meniscus downstream side.

When casting slab of the present invention continuously, make molten steel with the stream of the stirring more than 10 cels flow rate with electromagnetic mixing apparatus.

In the continuous casting apparatus shown in the accompanying drawing 1～4, the molten steel 11 that will contain 16.5 quality % chromium, discharge gate 14 from dipping spray nozzle 15 casts in the mold 13, by mold cooling and the watering cooling that is arranged on hold segment 17 places cooling water nozzles ejection, form solidified shell 18a, continue to solidify the back and form slab 18, it is pulled out by withdrawing roll 20 and 21.

Contain 0.0005～0.010 quality %Mg in the molten steel 11, oxygen, SiO in these Mg and the molten steel 11 ₂, the reaction of oxide such as MnO, generate MgO, MgOAl ₂O ₃Deng oxide.

In a single day this Mg content be lower than 0.0005 quality %, and the MgO in the molten steel just reduces, and the growing amount of freezing nuclei and wad act degree will reduce, and can not make the solidified structure miniaturization.On the other hand, if Mg content surpasses 0.010 quality %, then the solidified structure micronized effect is saturated, can not find significant effect, and interpolation cost such as Mg also can increase.

And, electromagnetic mixing apparatus 16 is arranged on 500 millimeters positions, liquid steel level (meniscus) downstream in the mold 13.

The mode that stirs is, utilizes solenoid 16a, 16b, makes it along mold 13 longwell 13a inboards, produce stirring stream from shortwall 13d to shortwall 13c direction, utilize solenoid 16c, 16d simultaneously, make it, produce to shortwall 13d direction from shortwall 13c and stir stream along longwell 13b inboard.Make molten steel 11 all shown in the arrow among Fig. 3, form stirring stream with the horizontal direction rotation.

So, cooled off by mold 13 from the molten steel 11 of discharge gate 14 casting, wash out near the oxide that exists the solidified shell 18a, can prevent that oxide is solidified shell 18a and pounces on and catch, and can form the few top layer part of oxide.

This top layer part is owing to cooled off by mold 13 and the watering that is set at the cooling water nozzle ejection of hold segment 17 is cooled off rapidly, so form fine crystallization easily.And, stir stream the column crystal head is fractureed, perhaps formed cold (solute distributes denseization with solid-liquid on the freezing interface, and local fusing point is reduced) and obtain relaxing and promoting equiax crystalization, so even oxide also can obtain fine solidified structure less because of so-called.

From near the solidified shell 18a by the oxide that washed out, part come-up back is pounced on by the not shown powder that goes out in meniscus surface and is caught, slab is inner to play freezing nuclei effect and wad act and almost all remain in, so slab inside can form fine solidified structure.

Stirring in the molten steel 11 stream is to feed the different three-phase alternating current of phase place to solenoid 16a～16d, makes in the molten steel 11 by known Fleming's rule to form the shifting magnetic field, and (5～90mmFe) give for the shifting magnetic field thrust that produces thus.

The power of thrust is adjusted to 10～40 cels with the current value adjustment that flows into solenoid 16a～16d with its flow velocity.

Its result, can both form fine solidified structure to inner more than 60% from the top layer of slab 18, can suppress the generation that processing of blemish such as crackle and pit and bulging and detorsion add the underbead crack in man-hour, guarantee not solidify the flowability of molten steel, produce the high-quality slab 18 that is inhibited of center porosity (porous) and center segregation.

The steel that this slab 18 is made through pressure processing, because being inhibited of blemish such as crackle and scale, center porosity (porous) and center segregation and internal flaw, so contraction processing characteristics and material characteristic good.

If the fine solidified structure of slab 18 is lower than 60%, then crystal grain increases, and produces blemish and internal flaw, shrinks material characteristic degradations such as processing.

In addition,, the homogeneity of solidified structure is further improved, prevent that more really the surface of slab and steel and internal flaw from occurring, and further stablizes material because above-mentioned reason all becomes fine solidified structure by making the full section of slab 18 thickness directions.

The slab that particularly utilizes this method to make is because skin section divides contained oxide few, so can reduce oxide through surface such as the thin plate of pressure processing and shaped steel and near existence thereof.

The oxide of surface and near surface is in case reduce, and the oxide (containing the MgO oxide) of stripping is measured and just can be inhibited during with releasing such as acid and salt solution, thereby to prevent to produce with it be the corrosion of steel of starting point.Therefore, with the slab that continuous casing of the present invention is made, the steel that obtain through processing also have superior corrosion resistance.

(8) continuous casing of the present invention can be used in the continuous casting of ferritic stainless steel molten steel.

Be particularly suitable for containing the casting of the ferritic stainless steel molten steel of 10～23 quality %Cr and 0.0005～0.010 quality %Mg.

In the continuous casting apparatus shown in the accompanying drawing 1～4, the molten steel 11 that will contain 10～23 quality %Cr is cast in the mold 13 through the discharge gate 14 of dipping spray nozzle 15, stir with electromagnetic mixing apparatus 16 on one side, the watering cooling of cooling off and utilizing the cooling water nozzle ejection that is arranged on hold segment 17 on one side by mold 13, form solidified shell 18a, constantly continue to solidify the slab 18 of formation, pull it with withdrawing roll 20 and 21.

Make molten steel 11 contain 0.0005～0.010 quality %Mg, contained O, SiO in these Mg and the molten steel 11 ₂, the reaction of oxide such as MnO, generate MgO or MgOAl ₂O ₃Deng the high-melting-point oxide.

These MgO or MgOAl ₂O ₃Deng oxide, will promote the solidified structure equiax crystalization of freezing nuclei effect, and can also bring into play the so-called wad act that suppresses to solidify the back tissue growth.In addition, by promoting the equiax crystal growth, can make full section 60% become fine solidified structure (equiax crystal) with upper surface.

If the fine solidified structure (equiax crystal) of slab is lower than 60%, then full face crystal grain diameter increases, and is easy to generate surface and internal flaw.

If Mg content is lower than 0.0005 quality %, MgO and/or contain the MgO oxide and reduce in the molten steel then, the generation of solidified shell and wad act deficiency can not make the solidified structure miniaturization.On the other hand, Mg content surpasses 0.010 quality %, and the micronized effect of solidified structure will be saturated, remarkable result can not occur, thereby increases the interpolation cost of Mg.

Electromagnetic mixing apparatus 16 is arranged on 500 millimeters places in liquid level (meniscus) 25 downstreams in the mold 13 in advance, molten steel 11 in the mold 13 is formed along the inwall rotation of mold 13 stir stream.

The flow velocity and the action effect of this stirring stream, Xiang Zhongyi illustrates in advance in above-mentioned (7).

The slab that obtains, as shown in Figure 9, the top layer part that stirs the stream effect becomes superfine equiax crystal, and inside is the solidified structure with fine equiax crystal.

And, fine equiax crystal solidified structure, can improve slab inside and not solidify molten steel flow among the part 18b, so can suppress the generation of center porosity (porous) and center segregation, can make slab and do not crack and blemish and internal flaws such as scale with the steel pipe of slab manufacturing.

Produce in order to suppress the center porosity phenomenon, sometimes slab is implemented gently to depress processing.That is to say, with reduction section 19 make slab 18 below remain on the support roller 22, gently press by means of the bossing on the depress roller 24 23, make center upper portion produce 3～10 millimeter impressions.Utilize this operation of gently depressing, can really slab 18 inside not solidified the center porosity compacting of part 18b and formation.

Gently depress operation, begin when reaching in 0.2～0.7 scope than (concreting thickness/slab thickness) in the solid phase of slab 18.

Wherein the solid phase ratio is to use drift is squeezed into slab, judges the method for its most advanced and sophisticated melting loss state, measure slab solidify (solid phase) district and not solidification zone obtain.

To this slab 18, needn't depress than the breaking down (depressing greatly) that surpasses 0.90, can omit with general breaking down operation etc. with the steel rolling process that milling train carries out, can reduce manufacturing cost significantly.

Then, the slab of casting is like this cut into predetermined length, heat shaping again by the tubulation operation after, with top perforation, make seamless steel pipe.

Make the slab that this steel pipe uses, except that organize fine owing to gently depress operation, so easy machining deformation when boring a hole with the top reaming with compacting really such as center porosity, can prevent really that inner face from cracking and defectives such as scale, obtains colory steel pipe.

In addition, there is no need to carry out the finishing operations such as grinding behind the tubulation, can prevent the fragmentation that defective causes, improve the yield rate and the productivity ratio of product.

Be used under near the situation of carrying out the slab tubulation made after the electromagnetic agitation mold especially, because the slab skin section divides contained oxide few, also can reduce through the steel tube surface of tubulation operation perforation and near the oxide of existence thereof, so the oxide (containing the MgO oxide) of stripping is measured in the time of can suppressing surface and acid and salt solution etc. contacts, and to suppress with it be the pipe corrosion of starting point, improves corrosion resistance.

5) following explanation embodiments of the invention.

Wherein, the present invention is not subjected to the restriction of embodiment, and in the scope that does not exceed the object of the invention and main points, the change of any condition and the change of embodiment etc. all are within the scope of the present invention.

Embodiment 1-1

Present embodiment relates to the embodiment of slab A of the present invention.

After adding 0.005 quality %Mg in the molten steel in pans, it is cast in the mold that is of a size of 1200 mm wides and 250 millimeters deep,, make it to be frozen into slab through mold cooling and hold segment watering cooling, after depressing 3～7 millimeters with reduction section, it is pulled out with withdrawing roll.

After slab cut off, observe solidified structure (state of equiax crystal) and slab top layer and inner defective along the thickness direction section, again this slab is heated to 1250 ℃ in addition after hot rolling, and examination steel top layer and inner defective and processing characteristics.Shown in the table 1 is its result.

Table 1

Project		Embodiment	1	Embodiment 2	Embodiment 3
						Slab macrostructure		Top layer: column crystal inside: equiax crystal (60%)	On the full section is equiax crystal	On the full section is equiax crystal, and maximum equiax crystal diameter is in 3 times of average diameter such as scape such as axle such as grade
The slab quality		○	○	○
					Steel quality	Blemish	○	◎	◎
Internal flaw	○	◎	◎
				The processability of steel		○	○	◎

Table 2

Project		Reference examples 1	Reference examples 2
				Slab macrostructure		Top layer: column crystal (50%) inside: equiax crystal (50%)	Though be equiax crystal on the full section, the equiax crystal on top layer is discontented with unabridged version invention formula
The slab quality		×	△
				The steel quality	Blemish	×	△
Internal flaw	×	△
			The processability of steel		×	△

Embodiment 1 in the table 1 relates to a kind of like this slab, in the solidified structure on the full section of its thickness direction, have 60% to be the equiax crystal (1～5.2 millimeter equiax crystal) that satisfies following formula, though can see in the column crystal scope of this slab some crackles are arranged, but crackle and internal flaws such as porous and center segregation as internal flaw but are suppressed, and obtain the result of overall good (representing with symbol zero).

D＜1.2X ^1/3+0.75

In the formula, D is the equiax crystal diameter (millimeter) of the identical tissue of crystallization direction.X is apart from the distance on slab surface (millimeter).

In addition, with this slab rolled steel, top layer scale and crack defect are few, and internal flaws such as crackle, porous and center segregation are also few, belong to good (representing) with symbol zero, solidified structure is fine and microsegregation is little, so depressing the direction distortion easily, the toughness after the processing is good (representing with symbol zero) also.

Embodiment 2 includes the slab that the full section of slab thickness direction all satisfies equiax crystal (1.0～4.5 millimeters the equiax crystal) composition of following formula, and this slab top layer does not have column crystal, and the defective of top layer and inside is few, quality good (representing with symbol zero).

In addition, with this slab rolled steel, scale and crackle seldom take place on the top layer, and internal flaws such as crackle, porous and center segregation are also few, thereby good (◎ represents with symbol).Microsegregation is few because solidified structure is fine for this steel, so depress the direction easy deformation, the toughness after the processing etc. good (representing with symbol zero).

The slab that embodiment 3 relates to satisfies all along the solidified structure in the full section of its thickness direction that the equiax crystal (0.9～2.6 millimeter of equiax crystal diameter) of following formula forms, and maximum equiax crystal diameter is less than three times of average equiax crystal diameter simultaneously.In this slab, skin section divides the microsegregation that forms little, and can suppress its fluctuation, so scale and crackle take place still less, inside does not have axle internal flaws (representing with symbol zero) such as porous and center segregation yet.

In addition, with this slab rolled steel, cosmetic bug such as scale and crackle on the top layer, and internal flaws such as crackle, porous and center segregation still less, very excellent (◎ represents with symbol), depress the direction easy deformation, also excellent (◎ represent with symbol) such as the toughness after the processing.

In contrast to this, as shown in table 2, the slab that reference examples 1 relates to, equiax crystal accounts for 50% of section at the slab thickness direction, and there is column crystal 50% in the top layer.In this slab, column crystal partly cracks on the top layer, and internal flaw also takes place, and is cited as poor (with symbol * expression).

In addition, with this slab rolled steel, scale and crackle on the top layer, and internal flaws such as crackle, porous and center segregation all produce (with symbol * expression), and the toughness after processing characteristics and the processing etc. also is chosen as poor (usefulness symbol * expression).

The slab that reference examples 2 relates to, though be equiax crystal along the full section of slab thickness direction, the equiax crystal of its top layer (account for all 40%) does not satisfy the following formula requirement.Blemish such as scale in this slab top layer and crackle and internal flaws such as center porosity and center segregation all are cited as poor slightly (△ represents with symbol).With this slab rolled steel, top layer scale and crackle have generation slightly, and internal flaws such as porous and center segregation also have generation (△ represents with symbol) slightly, also poor slightly (△ represent with symbol) such as toughness after processing characteristics and the processing.

Embodiment 1-2

The present embodiment explanation, among the slab A of the present invention, equiax crystal diameter D (millimeter) satisfies D＜0.08X ^0.78The situation of+0.5 (in the formula, X is apart from the distance on slab surface (millimeter), and D represents to be in the equiax crystal diameter (millimeter) of surperficial X distance).

After adding 0.1 quality %Mg in the molten steel in pans, it is cast in the mold of wide 1200 millimeters and dark 250 mm sizes, utilize mold cooling and slab solidified from the watering cooling of hold segment, utilize reduction section to depress 3～7 millimeters after, with withdrawing roll it is pulled out.

Then cut off slab, the solidified structure (situation of equiax crystal diameter) of investigation thickness direction section and slab top layer and inner defective.The top layer of steel and the defective and the processing characteristics of inner existence have been studied in steel rolling after then this slab being heated to 1250 ℃.The results are shown among the table 3.

Table 3

Project		Embodiment	1	Embodiment 2	Embodiment 3	Reference examples 1	Reference examples 2
								The slab quality	The blemish internal flaw	△ ○	○ ○	○ ◎	× ×	△ ×
Steel quality	Blemish internal flaw processing characteristics	△ ○ ○	○ ○ ○	○ ◎ ◎	× × ×	△ × ×

Symbolic representation quality grade in the table 3: ◎ is excellent, and zero is good, and △ is good slightly, * poor.

The steel that embodiment 1 relates to a kind of slab and makes with it in the table 3, the solidified structure in the full section of this slab are the equiax crystal (1.5～3.2 millimeters in equiax crystal footpath) that satisfies following formula more than 60%.The quality of slab is, the crackle of generation is less, and internal flaws such as crackle, porous and center line segregation are also few, thereby are good.

In addition, be with this slab rolled steel quality, the less generation of scale and crackle on the top layer, internal flaws such as crackle, porous and center segregation are also few, thereby good, and the toughness after the processing etc. are also good.

The steel that embodiment 2 relates to a kind of slab and makes with it, the full section of this slab are the equiax crystal (0.3～2.9 millimeter of equiax crystal diameter) that satisfies following formula.What crackle produced in this slab is less, does not also have internal flaws such as crackle, porous and center line segregation, and quality is good.

In addition, with the quality of this slab rolled steel, scale and crackle take place to lack on the top layer, and internal flaws such as crackle, porous and center segregation are also few, thereby good, and the toughness after the processing etc. are also good.

The steel that embodiment 3 relates to a kind of slab and makes with it, the equiax crystal diameter on the full section of this slab is 0.5～1.4 millimeter, maximum equiax crystal diameter is less than three times of average equiax crystal diameter.Crackle produces still less in this slab, and internal flaws such as crackle, porous and center line segregation are also few, best in quality.

In addition, with this slab rolled steel, blemish such as scale and crackle on the top layer, and the generation of internal flaws such as crackle, porous and center segregation greatly suppressed, and the toughness after the processing etc. are also good.

In contrast to this, reference examples 1 relates to a kind of like this slab and by its steel of making, all there is column crystal in the solidified structure on this slab thickness direction section in the 40% above scope of top layer, and the equiax crystal diameter in the solidified inside tissue is 2.0～3.1 millimeters.In this slab and steel, the microsegregation in the top layer is big, produces the crackle that comes from casting process and mold cooling procedure, and axle internal flaws such as crackle, porous and center segregation also produce.

In addition, with the steel that this slab rolls into, blemish such as scale and crackle and internal flaws such as crackle, porous and center segregation all occur, and the toughness after processability and the processing is also poor.

Reference examples 2 relates to a kind of like this slab and by its steel that roll into, it is the equiax crystal (2.8～5.7 millimeters of equiax crystal diameters) that satisfies following formula that the solidified structure of this slab thickness direction section has more than 40%.Crackle in this bolt for fastening a door from outside base and the steel top layer etc. is quite suppressed, but internal flaws such as crackle, porous and center segregation but occur.

In addition, with the steel that this slab rolls into, scale and crackle take place to some extent, and axle internal flaws such as crackle, porous and center segregation also occur, and the toughness after processability and the processing is also poor.

Embodiment 2

Present embodiment relates to slab B of the present invention.

After adding 0.005 quality %Mg in the molten steel in pans, carry out direct casting with wide 1200 millimeters molds with dark 250 mm sizes, utilize mold cooling and slab solidified from the watering cooling of hold segment, depress 3～7 millimeters with reduction section after, with withdrawing roll it is pulled out.

Then cut off slab, check the equiax crystal of thickness direction fractography, after 2 millimeters of each grindings of slab surface, measure the lip-deep crystal grain diameter of same thickness position, and investigated slab top layer and inner defective.And then steel rolling after this slab is heated to 1250 ℃, studied surface blemish and the wrinkle and the processing characteristics thereof of this steel.It the results are shown among the table 34.

Table 4

Project	Slab		Steel
						Face crack	Underbead crack	Surface blemish	The wrinkle flaw	Processing characteristics
	Embodiment
1 embodiment 2		○ ◎	○ ◎	○ ◎	○ ◎	○ ◎
	Reference examples						×	×	×	×	×

Embodiment 1 relates to a kind of slab in the table 4, and 30% forms equiax crystal in the full section of this slab, and on the surface of same thickness position, maximum crystal grain diameter/average crystal grain diameter ratio is in 2～2.7.This slab face crack and underbead crack all do not have (representing with symbol zero).By the steel that this slab steel rolling is made, surface blemish and crackle flaw produce slightly (representing with symbol zero), and processing characteristics is good (representing with symbol zero) also.

Embodiment 2 is the slabs shown in the solid line of accompanying drawing 14, its inner equiax crystal that forms 60% or more, and crystal grain diameter/the average crystal grain diameter ratio is 1.7～2.5 in the surface of same thickness position maximum.This slab face crack and underbead crack all do not have (◎ represents with symbol).By the steel that this slab steel rolling is made, do not produce surface blemish and crackle flaw (◎ represents with symbol), processing characteristics fabulous (◎ represents with symbol) yet.

In contrast to this, reference examples 1 is the slab shown in the solid line of accompanying drawing 15, and the equiaxial crystal ratio of this slab inside is low to moderate about 20%, and core is thick equiax crystal, in the crystal grain diameter of same thickness position, the ratio of a maximum crystal grain diameter/average crystal grain diameter part surpasses three times (2.5～4.7).Can find that this slab has face crack and underbead crack (with symbol * expression).In the steel that this slab steel rolling is made, produce surface blemish and wrinkle flaws (with symbol * expression) such as face crack, processing characteristics also poor (with symbol * expression).

Embodiment 3

Present embodiment relates to slab C of the present invention.

After adding 0.005 quality %Mg in the molten steel in being contained in pans, carry out direct casting with wide 1200 millimeters molds with dark 250 mm sizes, rely on the mold cooling and slab solidified from the watering cooling of hold segment, depress 3～7 millimeters with reduction section after, with withdrawing roll it is pulled out.

Then cut off slab, investigated the equiaxial crystal ratio of thickness direction section solidified structure, average equi-axed crystal diameter (millimeter) and top layer and inner defective.And steel rolling after this slab is heated to 1250 ℃, studied at this steel surface and inner defective and the processing characteristics that exists.It the results are shown among the table 5.

Table 5

Project	The field trash number is (individual/cm 2)	The size of field trash (micron)	Equiaxial crystal ratio (%)	Average equiax crystal diameter (millimeter)	The blemish of slab and steel	The internal flaw of slab and steel	The r value of steel	The toughness of steel welding position
									Embodiment
1	104	More than 10	62	1.8	○	○	○	○
									Embodiment 2	141	Below 10	81	1.3	◎	◎	◎	○
Reference examples 1	70	Below 10	27	2.5	×	×	×	×
									Reference examples 2	45	Below 10	15	4.7	×	×	×	×

The slab that embodiment 1 in the table 5 relates to is, contained and the non-coherency delta ferrite lattice are 104/centimetre less than 6% field trash number in the ferritic steel slab ², this field trash is of a size of more than 10 microns, and equiaxial crystal ratio is 62%, and average equi-axed crystal diameter is 1.8 millimeters.In this slab, blemish such as crackle and pit produce fewly (representing with symbol zero), also lack (representing with symbol zero) as the internal flaws such as crackle, porous and center segregation of internal flaw.

In addition, the steel that roll into this slab, unidirectional wrinkle and edge crack few (representing with symbol zero) on the top layer, internal flaws such as crackle, porous and center segregation also few (representing with symbol zero) are as the r value of processability index etc. also very (representing with symbol zero).

The slab that relates among the embodiment 2 is, contained and the non-coherency delta ferrite lattice are 141/centimetre less than 6% field trash number in the ferritic steel slab ², this field trash is of a size of below 10 microns, and equiaxial crystal ratio is 81%, and average equi-axed crystal diameter is 1.3 millimeters.In this slab, blemish such as crackle and pit produce few (◎ represents with symbol), also lack (◎ represents with symbol) as the internal flaws such as crackle, porous and center segregation of internal flaw.

In addition, the steel that roll into this slab, unidirectional wrinkle and edge crack few (◎ represents with symbol) on the top layer, internal flaws such as crackle, porous and center segregation be few (◎ represents with symbol) also, as also excellent (◎ represent with symbol) such as r values of processability index.

Relative therewith, the slab that reference examples 1 relates to is that contained field trash number is 70/centimetre in the slab ², this field trash is of a size of below 10 microns, and equiaxial crystal ratio is 27%, and average equi-axed crystal diameter is 2.5 millimeters.In this slab, blemish such as crackle and pit all produce (with symbol * expression), and slab inside cracks and internal flaws such as center porosity and center segregation (with symbol * expression).

In addition, the steel that roll into this slab, scale, unidirectional wrinkle and edge crack all produce (with symbol * expression) on the top layer, and internal flaws such as crackle, hole and center segregation not good (with symbol * expression) are as also poor (with symbol * expression) such as r values of processability index.

The slab that reference examples 2 relates to is, in the metallic compound that in the slab unit are, exists, and the metallic compound number below 10 microns: 45/centimetre on top layer ², inner 45/centimetre ², the maximum equi-axed crystal diameter of top layer part and inner maximum equi-axed crystal diameter all become big.In this slab, blemish such as crackle and pit, and internal flaws such as crackle, porous and center segregation all produce (with symbol * expression).

In addition, with the steel that this slab rolls into, the blemish of scale and crackle, and internal flaws such as crackle, porous and center segregation all produce (with symbol * expression), as also poor (the usefulness symbol * expressions) such as r values of processability index.

Embodiment 4

Present embodiment relates to slab D of the present invention.

After adding 0.005 quality %Mg in the molten steel in being contained in pans, carry out direct casting with wide 1200 millimeters molds with dark 250 mm sizes, rely on the mold cooling and slab solidified from the watering cooling of hold segment, depress 3～7 millimeters with reduction section after, pull out by withdrawing roll.

Then cut off slab, investigated equiax crystal size and the top layer and the inner defective that exists of thickness direction section solidified structure.And then steel rolling after this slab is heated to 1250 ℃, studied at this steel top layer and inner defective and the processing characteristics that exists.It the results are shown among the table 6.

Table 6

	The metallic compound number is (individual/cm 2)			Maximum equi-axed crystal diameter (mm)		Slab or internal defects of steel and blemish	The r value of steel
								(a) top layer part	(b) inside	(b)/(a)	The top layer part	Inner
	Embodiment
1		50	66	1.32	1.7	4.9	○	○
	Embodiment 2								95	130	1.37	1.1	3.1	○	○
Reference examples 1		45	46	1.02	1.8	5.5	×	×
	Reference examples 2								97	116	1.19	1.2	4.2	○	×

In the table 6, the slab characteristics that embodiment 1 relates to: in the contained metallic compound of slab, the metallic compound number below 10 microns: 50/centimetre on top layer ², inner 66/centimetre ², formed good equiax crystal.In this slab, crackle and pit, and few surface defects such as unidirectional wrinkle and edge crack, internal flaws such as crackle, porous and center segregation are also few.With the steel that this slab rolls into, flaws such as the unidirectional wrinkle on top layer and edge crack etc., and internal flaws such as crackle, porous and center segregation also few (representing with symbol zero) are as the r value of processability index etc. also well (representing with symbol zero).

The slab characteristics that embodiment 2 relates to: in the metallic compound that on the slab unit are, exists, the metallic compound number below 10 microns: 95/centimetre of top layer parts ², inner 130/centimetre ², formed good equiax crystal.In this slab, the generation of crackle and pit, and few surface defects such as unidirectional wrinkle and edge crack, internal flaws such as crackle, porous and center segregation are also few.With the steel that this bolt for fastening a door from outside base rolls into, flaws such as the unidirectional wrinkle on top layer and edge crack etc., and internal flaws such as crackle, porous and center segregation also few (representing), as the r value of processability index etc. also well (representing) with symbol zero with symbol zero.

In contrast to this, the slab that reference examples 1 relates to is, in the metallic compound that on the slab unit are, exists, and the metallic compound number below 10 microns: 45/centimetre on top layer ², inner 46/centimetre ²The maximum crystal grain diameter of top layer part and inner maximum crystal grain diameter all become big.In this slab, blemish such as crackle and pit, and internal flaws such as crackle, porous and center segregation all produce.The steel that this slab rolls into, blemish such as scale and crackle, and internal flaws such as crackle, porous and center segregation all produce (with symbol * expression), r value etc. also poor (with symbol * expression).

The slab that reference examples 2 relates to is, in the metallic compound that on the slab unit are, exists, and the metallic compound number below 10 microns: 97/centimetre on top layer ², inner 116/centimetre ²Top layer part and inner equi-axed crystal diameter all reduce.The steel that this slab and this slab roll into, blemish and internal flaw are good, (representing with symbol zero), but r value difference (with symbol * expression).

Wherein, than identical with embodiment 2, added MgO, the MgAl of 0.06 quality % for metallic compound number below 10 microns as metallic compound with embodiment 1 ₂O ₄, TiN, TiC slab, and the steel that roll into this slab, equiax crystal size and the top layer and the inner defective that exists of solidified structure have also been investigated, and this slab is heated to 1250 ℃, steel rolling, investigate steel top layer and inner defective and the processing characteristics that exists, also obtained good result.

Embodiment 5

Present embodiment relates to processing method I of the present invention.

Do not contain Ca at molten steel, and molten steel contains under several situations such as total Ca 0.0002 quality %, 0.0005 quality %, 0.0006 quality % and 0.0010 quality %, add 0.005 quality %Mg respectively in the molten steel in being contained in pans, carry out direct casting with wide 1200 millimeters molds then with dark 250 mm sizes, rely on the mold cooling and slab is solidified from the watering cooling of hold segment, after depressing 3～7 millimeters with reduction section, draw through withdrawing roll.

And then investigated oxide main component in the preceding molten steel of interpolation Mg, oxide main component in the molten steel behind the interpolation Mg, and the miniaturization situation of slab tissue.It the results are shown among the table 7.

Table 7

		The gross mass % of Ca in the molten steel before Mg adds	Field trash before Mg adds in the molten steel	Mg adds the field trash in the molten steel of back	The miniaturization situation of slab solidified structure	Overall merit
							Embodiment
	1	0.0000％	Al 2O 3	Al 2O 3·MgO，MgO	Atomic thin (particle diameter＜1 millimeter)	◎
								2	0.0002％	Al 2O 3	Al 2O 3·MgO，MgO	Atomic thin (particle diameter＜1 millimeter)	◎
	3	0.0005％	Al 2O 3	Al 2O 3·MgO，MgO	Atomic thin (particle diameter＜1 millimeter)	◎
								4	0.0006％	Al 2O 3CaO (CaO is below several %)	Al 2O 3MgOCaO, MgOCaO (CaO is below several %)	Fine (particle diameter＜3 millimeter)	○
	5	0.0010％	Al 2O 3CaO (CaO is below several %)	Al 2O 3MgOCaO, MgOCaO (CaO is below several %)	Fine (particle diameter＜3 millimeter)	○
							Reference examples	1	0.0012％	Al 2O 3·CaO	Al 2O 3·MgO·CaO	Thick	×
2	0.0015％	Al 2O 3·CaO	Al 2O 3·MgO·CaO	Thick	×
						3		0.0023％	Al 2O 3·CaO	Al 2O 3·MgO·CaO	Thick	×

In the table 7, embodiment 1 relates to the situation that molten steel does not contain Ca, and the field trash that promptly adds in the preceding molten steel of Mg is with Al ₂O ₃Be the oxide of main component, the field trash behind the interpolation Mg in the molten steel is Al ₂O ₃MgO and MgO are the situation of the oxide of main component.The slab that this molten steel casting obtains, its solidified structure be by the extreme miniaturization, overall merit fabulous (◎ represents with symbol).

Embodiment 2 relates to the situation that molten steel contains 0.0002 quality %Ca, and the field trash that promptly adds in the preceding molten steel of Mg is with Al ₂O ₃Be the oxide of main component, the field trash behind the interpolation Mg in the molten steel is Al ₂O ₃MgO and MgO are the situation of the oxide of main component.Do not generate calcium aluminate in this molten steel, the slab that this molten steel casting obtains, its solidified structure be by the extreme miniaturization, overall merit fabulous (◎ represents with symbol).

Embodiment 3 relates to the situation that molten steel contains 0.0005 quality %Ca, and the field trash that promptly adds in the preceding molten steel of Mg is with Al ₂O ₃Be the oxide of main component, the field trash behind the interpolation Mg in the molten steel is Al ₂O ₃MgO and MgO are the situation of the oxide of main component.Do not generate calcium aluminate in this molten steel, the slab that this molten steel casting obtains, its solidified structure be by the extreme miniaturization, overall merit fabulous (◎ represents with symbol).

Embodiment 4 relates to the situation that molten steel contains 0.0006 quality %Ca, promptly adds before the Mg in the molten steel oxide as field trash, except main component Al ₂O ₃Outside, also contain the CaO below several percentage points, and the field trash in the molten steel is the Al that contains CaO below several percentage points behind the interpolation Mg ₂O ₃MgOCaO and MgOCaO are the situation of the oxide of main component.

Though can detect the CaO in the field trash in this molten steel before and after Mg adds, its content is below several percentage points, so during molten steel solidification, can find the brilliant effect of kind.Therefore, with the slab that this molten steel casting obtains, the miniaturization that becomes of its solidified structure, overall merit is good (representing with symbol zero).

Embodiment 5 relates to the situation that molten steel contains 0.0010 quality %Ca, promptly adds before the Mg in the molten steel oxide as field trash, except main component Al ₂O ₃Outside, also contain the CaO below several percentage points, and the field trash in the molten steel is to contain the Al of CaO below several percentage points behind the interpolation Mg ₂O ₃MgOCaO and MgOCaO are as the situation of the oxide of main component.

Though can detect the CaO in the field trash in this molten steel before and after Mg adds, its content is below several percentage points, so during molten steel solidification, can find the brilliant effect of kind.Therefore, with the slab that this molten steel casting obtains, the miniaturization that becomes of its solidified structure, overall merit is good (representing with symbol zero).

In contrast to this, the Ca that reference examples 1 relates in molten steel reaches under the situation of 0.0012 quality %, and the field trash that adds in the preceding molten steel of Mg is with Al ₂O ₃-CaO (calcium aluminate) is as the oxide of main component, and the field trash behind the interpolation Mg in the molten steel is with CaO-Al ₂O ₃-MgO is as the situation of the oxide of main component.The slab that this molten steel obtains through casting, its thickization of solidified structure, overall merit is poor (with symbol * expression).

The Ca that reference examples 2 relates in molten steel is under the situation of 0.015 quality %, and the field trash that adds in the preceding molten steel of Mg is following CaOAl ₂O ₃(calcium aluminate) as the oxide of main component, the field trash behind the interpolation Mg in the molten steel is with CaO-Al ₂O ₃-MgO is as the situation of the oxide of main component.The slab that this molten steel obtains through casting, its thickization of solidified structure, overall merit is poor (with symbol * expression).

The Ca that reference examples 3 relates in molten steel is under the situation of 0.023 quality %, and the field trash that adds in the preceding molten steel of Mg is with Al ₂O ₃-CaO (calcium aluminate) is as the oxide of main component, and the field trash behind the interpolation Mg in the molten steel is with CaO-Al ₂O ₃-MgO is as the situation of the oxide of main component.The slab that this molten steel obtains through casting, its thickization of solidified structure, overall merit is poor (with symbol * expression).

Embodiment 6

Present embodiment relates to processing method II of the present invention.

Decarburising and refining and the adjusted 150 tons of molten steel of composition are injected ladle, change adding conditional, in this molten steel, add Al and Ti, under the porous plug through being provided with on the ladle infeeds the stirring of argon gas, carry out deoxidation treatment simultaneously, in molten steel, infeed 0.75～15 kilogram of Mg then.Then investigated with the slab top layer of this molten steel continuous casting and having or not of internal flaw, and the quality of solidified structure miniaturization.It the results are shown among the table 8.

Table 8

Project		Embodiment			Reference examples
							1	2	3	1	2
		Molten steel amount (ton)		150	150	150						150	150
Deoxygenation conditions	The amount of deoxidier (kilogram)						Metal A l 50	Metal A l 75 Fe-Ti 50	Fe-Ti 50 metal A l 75	Metal A l 75 and metal M g 0.75 add simultaneously	After Fe-Ti 50 metal M g 15 add simultaneously, add metal A l 75
		After the amount (kilogram) of the metal M g that adds	Metal M g 0.75	Metal M g 15	Metal M g 15
	There is zero defect slab top layer and inside					Do not have	Do not have	Do not have	Have			Have
Whether solidified structure is good			Very	Very	Very					Not	Not
		Overall merit				○	○	○	×			×

In the table 8, after embodiment 1 relates to 50 kilograms of Al deoxidations of interpolation, the test situation of adding 0.75 kilogram of Mg again.The top layer of slab and inner all zero defects, the fully miniaturization of solidified structure quilt, overall merit good (representing) with symbol zero.

Embodiment 2 relate to add add the deoxidation of 50 kilograms of Fe-Ti alloys again behind 75 kilograms of Al after, the test situation of adding 15 kilograms of Mg.The top layer of slab and inner all zero defects, the fully miniaturization of solidified structure quilt, overall merit good (representing) with symbol zero.

Embodiment 3 adds 75 kilograms of Al and carries out deoxidation after relating to 50 kilograms of Fe-Ti alloys of interpolation, adds the test situation of 15 kilograms of Mg then.The top layer of slab and inner all zero defects, the fully miniaturization of solidified structure quilt, overall merit good (representing) with symbol zero.

Wherein, under embodiment 1～3 any situation, as shown in Figure 9, the solidified structure of slab all forms equiax crystal in inside, and all miniaturizations.

In contrast to this, reference examples 1 is to add simultaneously the test of carrying out deoxidation behind 75 kilograms of Al and the 0.75 kilogram of Mg in molten steel.Though formed MgO and Al in the molten steel ₂O ₃Composite oxides, but contain in the surface texture of MgO oxide, the ratio of MgO is all below 10%, and is poor with the coherency of delta ferrite lattice, is not suitable as freezing nuclei.As a result, in slab top layer and all defectiveness generations of inside, as shown in Figure 7, it is thick that solidified structure also becomes, and overall merit is poor (with symbol * expression).

Reference examples 2 is to add 15 kilograms of Mg add 50 kilograms of Fe-Ti alloys in molten steel after, and then adds the test that 75 kilograms of Al carry out deoxidation.Oxide in the molten steel is though core is MgO, owing to formed Al on the surface ₂O ₃So, do not play the freezing nuclei effect.As a result, in slab top layer and all defectiveness generations of inside, it is thick that solidified structure also becomes, and overall merit is poor (with symbol * expression).

Embodiment 7

The processing method that present embodiment relates to is, in processing method I of the present invention and II, in molten steel, add scheduled volume Mg, the oxide that generates when making oxide such as slag contained in the molten steel and deoxidation product and adding Mg in molten steel, satisfy following formula (1) and (2):

α＝17.4(kAl ₂O ₃)+3.9(kMgO)+0.3(kMgAl ₂O ₄)

+18.7(kCaO)≤500 …(1)

β＝(kAl ₂O ₃)+(kMgO)+(kMgAl ₂O ₄)+(kCaO)

≥95 …(2)

Use combined blown converter, 150 tons of molten steel that will contain chromium 10～23 quality % inject ladle, on one side through porous plug winding-up argon gas, add double centner Al on one side from hopper, evenly mix under molten steel stirs, and carry out deoxidation.

Then, take a sample from molten steel, measure oxide with EPMA and form, adjust the Mg amount of adding, the requirement that makes it to satisfy following formula (1) and (2) is the formation composite oxides down.Then cast molten steel continuously and make slab.

So examined or check the underbead crack of slab and having or not of internal flaws such as center segregation, center porosity, whether solidified structure is good, and the surface texture and the processing characteristics of processing back steel.The results are shown among the table 9.

Table 9

Project	Mg addition (kg)	Oxide is formed (mole %)					The α value of oxide	The slab internal flaw	The solidified structure of slab	The steel surface proterties	The processability of steel	Overall merit
													Al 2O 3	MgO	MgAl 2O 4	CaO	Other
		Embodiment
1	125		5.1	37.2	52.4	4.1	1.2	326	Do not have	Very	Very	Very	○
		Embodiment 2												30	7.4	22.3	51.2	14.2	4.9	497	Do not have	Very	Very	Very	○
Reference examples 1	85		3.3	46.8	29.3	16.8	3.8	563	Have	Difference	Difference	Difference	×
		Reference examples 2												30	15.9	30.8	37.2	12.3	11.2	638	Have	Difference	Difference	Difference	×

In the table 9, embodiment 1 stirs molten steel add 125 kilograms of Mg in molten steel after, make α value (the equal sign left side in the following formula (1) of contained composite oxides in the molten steel, be the index of the non-coherency of lattice of oxide and delta ferrite) be 326 o'clock test situation, do not produce internal flaw in slab inside, solidified structure is also by miniaturization, and the surface texture of steel and processing characteristics are also good, overall merit good (representing with symbol zero).

Embodiment 2 stirs molten steel add 30 kilograms of Mg in molten steel after, the α value that makes contained composite oxides in the molten steel is 497 o'clock a test situation, surface and inside at slab do not produce internal flaw, as shown in Figure 9, the solidified structure miniaturization, the surface texture and the processing characteristics of steel are good, overall merit good (representing with symbol zero).

In contrast to this, reference examples 1 and reference examples 2 are that the composition that adds the molten steel institute oxycompound before the Mg is not added under the situation of consideration, the test situation when adding stirring molten steel behind 85 kilograms and the 30 kilograms of Mg respectively.As a result, the α value of contained composite oxides surpasses 500 in the molten steel, the inner defective that produces of slab, and as shown in Figure 7, the solidified structure in two examples is all because of thickization worsens, and overall merit is poor (with symbol * expression).

Embodiment 8

Present embodiment relates to the test situation of processing method III of the present invention.

Use combined blown converter, ladle is injected in 150 tons of decarburizations and the molten steel that contains chromium 0～23 quality % that removes impurity such as dephosphorization, sulphur, on one side through porous plug winding-up argon gas, add Fe-Ti alloy and N-Mn alloy on one side, make the Ti concentration in the molten steel reach 0.013～0.125 quality %, after N concentration reaches 0.0012～0.024 quality %, then add Mg and cast continuously, make slab.Whether operation stable when having investigated casting then, and whether slab is organized good, and the having or not of the blemish of the internal flaw of slab and steel.The results are shown among the table 10.

Table 10

Project		Molten steel amount (ton)	Cr concentration (quality %)	Ti concentration (quality %)	N concentration (quality %)	Mg concentration (quality %)	Whether stable operation	Whether the solidified structure miniaturization	There is zero defect slab inside	Steel surface has zero defect	Overall merit
												Embodiment
	1	150	0	0.013	0.012	0.0035	Very	Very	Do not have	Do not have	○
													2	150	10	0.020	0.024	0.0015	Very	Very	Do not have	Do not have	○
	3	150	23	0.125	0.022	0.0025	Very	Very	Do not have	Do not have	○
												Reference examples	1	150	10	0.021	0.023	Do not add	Not	Not	Have	Have	×
2	150	23	0.198	0.038	Do not add	Not	Very	Do not have	Have	△ (spray nozzle clogging)

In the table 10, embodiment 1 relates to that to make Cr concentration be that Ti concentration reaches 0.013 quality % in 0% the molten steel, and N concentration is added the test of 0.0035 quality %Mg after reaching 0.012 quality %.Stable operation during casting, the solidified structure of slab be by miniaturization, the equal zero defect of slab and steel, overall merit good (representing with symbol zero).

Embodiment 2 relates to that to make Cr concentration be that Ti concentration reaches 0.020 quality % in the molten steel of 10 quality %, and N concentration is added the test of 0.0015 quality %Mg after reaching 0.024 quality %.Stable operation during casting, the solidified structure of slab be by miniaturization, the equal zero defect of slab and steel, overall merit good (representing with symbol zero).

Embodiment 3 relates to that to make Cr concentration be that Ti concentration reaches 0.125 quality % in the 23 quality % molten steel, and N concentration is added the test of 0.0025 quality %Mg after reaching 0.022 quality %.Stable operation during casting, the solidified structure of slab be by miniaturization, the equal zero defect of slab and steel, overall merit good (representing with symbol zero).

Relative therewith, reference examples 1 relates to that to make molten steel Cr concentration be 10 quality %, and Ti concentration is 0.021 quality %, and N concentration is not added the test of Mg after reaching 0.023 quality %.Produce fluctuation of services such as spray nozzle clogging during casting, the solidified structure of slab, thickization as shown in Figure 7, the equal defectiveness of slab and steel produces, overall merit poor (with symbol * expression).

Reference examples 2 relates to that to make molten steel Cr concentration be 23 quality %, and Ti concentration is 0.198 quality %, and N concentration reaches 0.038 quality %, and the solubility product ([%Ti] * [%N]) of two elements is in the scope that TiN do not separate out, and the test of not adding Mg.Under the occasion of reference examples 2, though the solidified structure miniaturization, spray nozzle clogging makes operation become unstable when casting, and the defective that causes because of thick TiN produces on steel surface, so comprehensively roughly be evaluated as poor (△ represents with symbol).

Embodiment 9

Present embodiment relates to the situation of processing method IV of the present invention.

150 tons of molten steel are injected ladle, and making the slag thickness that covers this molten steel is 100 millimeters, with FeO, Fe ₂O ₃, MnO, SiO ₂Gross mass adjust in the preset range, run through this slag layer and supply with Mg alloy silk, make that pure Mg composition reaches 50 kilograms (0.0333 quality %) in the molten steel.

After this, use mold inside dimension wide 1200 millimeters and dark 250 millimeters continuous casting apparatus, cast this molten steel with 0.6 meter/minute speed.

Examine or check Mg then and handled Mg quality % in Mg quality % in the molten steel of back, the slab, and the miniaturization situation of slab solidified structure.It the results are shown among the table 11.

Table 11

Project		FeO+Fe in the slag before Mg adds 2O 3+ MnO+SiO 2Gross mass %	Mg handles the Mg quality % in the molten steel of back	Mg quality % in the slab	Solidified structure miniaturization situation
						Embodiment
	1	2.5	0.0041	0.0015	Fine
							2	11.3	0.0061	0.0020	Fine
	3	16.1	0.0065	0.0035	Fine
							4	22.4	0.0063	0.0031	Fine
	5	28.5	0.0036	0.0019	Fine
						Reference examples	1	0.5	0.0025	0.0009	A part is thick
2	36.3	0.0028	0.0008	A part is thick

In the table 11, before embodiment 1 relates to interpolation Mg, with FeO, Fe in the slag ₂O ₃, MnO, SiO ₂The test situation of the 2.5 quality % that adjust to of total amount.Can with in the molten steel and the Mg in the slab be adjusted to 0.0041 quality % and 0.0015 quality %, the solidified structure of slab is realized miniaturization.

Before embodiment 2,3 and 4 relates to interpolation Mg, respectively with FeO, Fe in the slag ₂O ₃, MnO, SiO ₂The 11.3 quality % that total amount is adjusted to, the test situation of 16.1 quality %, 22.4 quality %.Mg is respectively 0.0061 quality %, 0.0065 quality % and 0.0063 quality % in molten steel, and when Mg is respectively 0.0020 quality %, 0.0035 quality % and 0.0031 quality % in the slab, and yield rate is high and stablize, and the solidified structure of slab is also by miniaturization.

Before embodiment 5 relates to interpolation Mg, FeO, Fe in the slag ₂O ₃, MnO and SiO ₂Test situation when total amount is adjusted to 28.5 quality %.In the molten steel and slab in Mg can reach 0.0036 quality % and 0.0019 quality %, the solidified structure miniaturization of slab respectively.

In contrast to this, before reference examples 1 relates to interpolation Mg, FeO, Fe in the slag ₂O ₃, MnO and SiO ₂Test situation when total amount is adjusted to 0.5 quality %.Though Mg is 0.0025 quality % in the molten steel, the Mg in the slab but becomes 0.0009 quality %, and the rate of recovery of Mg is low, a part of thickization in the solidified structure of slab.

Reference examples 2 relates to adds FeO, Fe in the preceding slag of Mg ₂O ₃, MnO and SiO ₂Test situation during 36.6 quality % that total amount is adjusted to.Though Mg is 0.0028 quality % in the molten steel, the Mg in the slab but becomes 0.0008 quality %, and the rate of recovery of Mg is low, a part of thickization in the solidified structure of slab.

Embodiment 10

Present embodiment relates to the situation of processing method V of the present invention.

150 tons of molten steel are injected ladle, making the slag thickness that covers this molten steel is 100 millimeters, adjust the basicity of Ca activity and slag in the slag, run through this slag layer and supplies with Mg alloy silk make it to be melted in the molten steel in molten steel, measuring in the Mg of pure Mg composition interpolation is 50 kilograms.

After this, use mold inside dimension dark 250 millimeters and wide 1200 millimeters continuous casting apparatus, this molten steel is cast with 0.6 meter/minute speed.

Investigated the miniaturization state that Mg handles Mg quality % and slab solidified structure in the molten steel of back then.It the results are shown among the table 12.

Table 12

Project		CaO activity in the slag	Basicity (the CaO/SiO of slag 2)	Mg concentration (quality %) in the molten steel	The solidified structure of slab	Overall merit
							Embodiment
	1	0.20	3	0.0010	◎	◎
								2	0.25	7	0.0020	◎	◎
	3	0.30	10	0.0020	◎	◎
							Reference examples	1	0.36	15	0.0050	×	×
2	0.42	20	0.0100	×	×

Embodiment 1 relate to CaO activity in the slag be adjusted to 0.2, basicity is adjusted to the test situation that Mg alloy silks are added in 3 backs.Mg concentration is 0.0010 quality % in the molten steel of Mg processing back, can make the solidified structure miniaturization (◎ represents with symbol) of slab, overall merit excellent (◎ represents with symbol).

Embodiment 2 and 3 relates to CaO activity in the slag is adjusted to 0.25 and 0.30 respectively, and basicity of slag is adjusted to 7 and 10 test situation respectively.It is also high that Mg handles in the molten steel of back Mg concentration, and the solidified structure of slab is also by miniaturization (◎ represents with symbol), overall merit excellent (◎ represents with symbol).

In contrast to this, reference examples 1 relate to CaO activity in the slag be adjusted to 0.36, basicity is adjusted to 15 backs and adds Mg alloy silks, the test situation when making Mg handle Mg in the molten steel of back to be 0.0050 quality %.The solidified structure of slab becomes thick (with symbol * expression), overall merit poor (with symbol * expression).

Reference examples 2 relate to CaO activity in the slag be adjusted to 0.42, basicity is adjusted to 20 backs and adds Mg alloy silks, the test situation when making Mg handle Mg in the molten steel of back to be 0.0100 quality %.The solidified structure of slab becomes thick (with symbol * expression), overall merit poor (with symbol * expression).

Embodiment 11

Present embodiment relates to the continuous casing of making slab A～D of the present invention.

In the molten steel that contains Cr16.5 quality %, add 0.005 quality %Mg, carry out direct casting with wide 1200 millimeters and dark 250 millimeters mould oscillation then, rely on the watering of mold cooling and hold segment to cool off slab is solidified, pull it with withdrawing roll.

Investigated slab top layer and inner number of defects and field trash number then, and solidified structure.Then investigated and this slab is heated to steel rolling after 1250 ℃ has made the situation that steel surface corrosion resistance and wrinkle produce.It the results are shown among the table 13.

Table 13

Project			Embodiment	Reference examples 1	Reference examples 2
						Mg adds			Have	Have	Do not have
Electromagnetic agitation			Have	Do not have	Have
						Slab	The top layer	Field trash	Few	Many	Do not have
Solidified structure	Fine	Fine	Fine
				Face crack	Do not have			Do not have	Do not have
Inner	Field trash	Many	Many							Do not have
				Solidified structure	Fine		Fine	Thick
	Underbead crack	Do not have	Do not have						Have
				Center segregation	Slightly		Slightly	Significantly
	Steel	The surface corrosion resistance							Well	Bad	Well
Wrinkle flaw during steel rolling				Well	Well	Bad

Embodiment in the table 13 relates on one side with being arranged on 500 millimeters in mold meniscus downstream and locates electromagnetic mixing apparatus and stir core is rotated, Yi Bian the test that casts continuously.In this embodiment, MgO oxide (field trash) number that contains on slab top layer is reduced, the surface-layer solidification structure miniaturization prevents defective generations such as face crack.And in slab inside, containing MgO oxide (field trash) number increases, and can obtain fine equiax crystal, and its result can reduce underbead crack, makes center segregation slight.

In addition, with this slab rolled steel, the corrosion resistance on surface is good, and the wrinkle flaw that causes because of thickization of solidified structure does not produce yet.

In contrast to this, reference examples 1 relates to the test situation that stirs molten steel with electromagnetic mixing apparatus.In the top layer and the inside of slab, containing MgO oxide (field trash) number increases, though top layer and inner solidified structure can miniaturizations, can find on the surface of steel to contain the Staining that the MgO oxide is a starting point rolling into.This steel practicality is bad.

Reference examples 2 relates to does not add Mg, and stirs the test situation of molten steel with electromagnetic mixing apparatus.The solidified structure of slab inside becomes thick, and underbead crack and center segregation all have generation, processes in the steel that this slab makes, and has produced the wrinkle flaw that causes because of thickization of solidified structure etc.

Embodiment 12

Present embodiment relates to above-mentioned continuous casting process casting ferrite-group stainless steel molten steel of the present invention, and the test situation of making seamless steel pipe with the slab of casting.

In the molten steel that contains chromium 13.0 quality %, add 0.0010 quality %Mg, cast continuously with wide 600 millimeters mould oscillation then with dark 250 mm sizes, rely on the watering cooling of mold cooling and hold segment that slab is solidified, with withdrawing roll it is pulled out.

Solidified structure and the seamless steel pipe surface through boring a hole and the production of internal flaw of slab have been investigated then.It the results are shown among the table 14.

Table 14

Project		Mg addition (quality %) in the molten steel	The electromagnetic agitation condition		Gently depress condition		The solidified structure of slab	The steel duct blemish	Overall merit
										Have or not	Mixing position	The beginning solid rate	Drafts (millimeter)
			Embodiment	1	0.0010	Do not have								-	-	-	○	○	○
2	0.0010	Have					Meniscus downstream 500mm	0.5	6	◎	◎	◎
				3	0.0010	Do not have							-	0.4	7	○	◎	◎
Reference examples	1	Do not add					Have	Meniscus downstream 500mm	-	-	×	×							×
			2	Do not add	Do not have	-							0.4	7	×	×	×

In the table 14, embodiment 1 relates in molten steel and to cast after adding 0.0010 quality %Mg, makes the test situation of seamless steel pipe.The solidified structure miniaturization of slab (representing with symbol zero), the surface of steel pipe and inside do not have crackle and scale (representing with symbol zero) during perforation, overall merit good (representing with symbol zero).

Embodiment 2 relates on one side and stirs to such an extent that make the core rotation with the electromagnetic mixing apparatus that is arranged on 500 millimeters places, mold meniscus downstream, Yi Bian cast continuously, is the test situation that 0.5 position begins gently to depress by solid rate.The number that contains the MgO oxide on the slab top layer reduces, all energy miniaturizations of solidified structure that slab is all, (◎ represents with symbol), the surface of steel pipe and inside do not have crackle and scale (◎ represents with symbol) fully during perforation, overall merit excellent (◎ represents with symbol).

Embodiment 3 relates in molten steel and to cast after adding 0.0010 quality %Mg, is 0.4 the position test situation when beginning all gently to depress in the scope till solidifying 7 millimeters of the degree of depth by solid rate.The solidified structure miniaturization of slab (representing with symbol zero), the surface of steel pipe and inside do not have crackle and scale (◎ represents with symbol) during perforation, overall merit excellent (◎ represents with symbol).

In contrast to this, reference examples 1 relates under the situation of not adding Mg in the molten steel casts, and electromagnetic agitation is carried out in 500 millimeters positions from the meniscus downstream, the test situation after the perforation.The solidified structure of slab becomes thick (with symbol * expression), and the surface of perforation back steel pipe and inside crack and scale flaw (with symbol * expression), overall merit poor (with symbol * expression).

Reference examples 2 relates under the situation of not adding Mg in the molten steel casts, and is the test situation when all gently depressing 7 millimeters of the degree of depth in 0.4 the scope of position till solidifying from solid rate.The solidified structure of slab becomes thick (with symbol * expression), and the surface of perforation back steel pipe and inside crack and scale flaw (with symbol * expression), overall merit poor (with symbol * expression).

The possibility of utilizing on the industry

Slab of the present invention, make the blemish such as crackle that slab produces and pit because of the distortion of process of setting and stress, and the generation of the internal flaws such as the blemish that causes of field trash and underbead crack and center porosity (porous), center segregation can be inhibited all.

Therefore, slab processing characteristics of the present invention and quality characteristic are good, do not need finishing processing operation and the Fragmentation Phenomena of slabs such as grinding also few, so yield rate is high.

Processing method of the present invention by adjusting the form of molten steel characteristic and inclusion content in melting steel when the molten steel solidification, realizes the solidified structure miniaturization, can obtain slab of the present invention, is extremely useful steel treatment method.

In addition, make the continuous casing of slab of the present invention, can further improve the action effect that processing method of the present invention is given molten steel during continuous casting.

And process the steel such as steel plate that slab of the present invention makes, steel pipe, and can suppress equally the generation of blemish and internal flaw with slab, processing characteristics and quality characteristic are also good.

Claims (9)

1, a kind of processing method of molten steel, it is characterized in that handling in the processing method of molten steel for the solidified structure miniaturization that makes slab, after total Ca amount was adjusted to below the 0.0010 quality % in the molten steel, adding Mg in described molten steel was 0.0005～0.10 quality %.

2, a kind of processing method of molten steel, it is characterized in that handling in the processing method of molten steel for the solidified structure miniaturization that makes slab, add in molten steel before the Mg, add in described molten steel and contain the Al alloy and carry out deoxidation treatment, the addition of Mg is 0.0005～0.10 quality %.

According to the processing method of the described molten steel of claim 2, it is characterized in that 3,, in molten steel, add before the Mg that except that containing the Al alloy, also interpolation contains the Ti alloy and carries out deoxidation treatment.

4, according to the processing method of claim 1 or 2 described molten steel, it is characterized in that, make and oxide such as contained slag or deoxidation product in the molten steel and the oxide that generates when in molten steel, adding Mg satisfy following formula (1) and (2):

17.4(kAl ₂O ₃)+3.9(kMgO)+0.3(kMgAl ₂O ₄)

+18.7(kCaO)≤500 …(1)

(kAl ₂O ₃)+(kMgO)+(kMgAl ₂O ₄)+(kCaO)

≥95 …(2)

In the formula, k represents the mole % of oxide.

According to the processing method of claim 1 or 2 described molten steel, it is characterized in that 5, described molten steel is the ferrite-group stainless steel molten steel.

6, a kind of processing method of molten steel, it is characterized in that making the solidified structure miniaturization of slab with in the processing method at the processing molten steel, add in molten steel before the Mg, make that CaO activity is below 0.3 in the slag that covers molten steel, the addition of Mg is 0.0005～0.10 quality %.

7, according to the processing method of the described molten steel of claim 6, it is characterized in that, the basicity of slag is in below 10.

According to the processing method of claim 6 or 7 described molten steel, it is characterized in that 8, described molten steel is the ferrite-group stainless steel molten steel.

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