CN112522568A

CN112522568A - Fire-resistant weather-resistant steel plate/belt and manufacturing method thereof

Info

Publication number: CN112522568A
Application number: CN201910888757.1A
Authority: CN
Inventors: 吴建春; 方园; 陈惠明
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2019-09-19
Filing date: 2019-09-19
Publication date: 2021-03-19

Abstract

A fire-resistant weather-resistant steel plate/belt and a manufacturing method thereof are disclosed, wherein the fire-resistant weather-resistant steel plate/belt comprises the following chemical components in percentage by weight: c: 0.02 to 0.06%, Si: 0.1-0.55%, Mn: 0.4-1.7%, P is less than or equal to 0.03%, S is less than or equal to 0.007%, Cr: 0.30-0.80%, Cu: 0.10-0.60%, Mo: 0.20-0.60%, N: 0.004-0.010%, Als:<0.001%, B: 0.001-0.006% of total oxygen [ O ]]_T: 0.007-0.020%, and the balance of Fe and inevitable impurities; and, simultaneously: Mn/S>250 of (a); pcm is less than or equal to 0.27 percent. In the invention, microalloy elements such as Mo/Cr and the like and B elements are selectively added into steel; controlling the alkalinity of slag, the type and melting point of inclusions in steel, the content of free oxygen in molten steel and the content of acid-soluble aluminum Als in the smelting process; adopting double-roller thin strip continuous casting, directly entering a lower closed chamber with non-oxidizing atmosphere after the cast strip is discharged from a crystallization roller, and then entering online hot rolling; and cooling the strip steel by adopting an air atomization cooling mode after rolling.

Description

Fire-resistant weather-resistant steel plate/belt and manufacturing method thereof

Technical Field

The invention belongs to a continuous casting process in the metallurgical industry, and particularly relates to a fire-resistant weather-resistant steel plate/belt and a manufacturing method thereof.

Background

The traditional thin strip steel is produced by casting blanks with the thickness of 70-200mm through multi-pass continuous rolling, and the traditional hot rolling process flow is as follows: continuous casting, casting blank reheating and heat preservation, rough rolling, finish rolling, cooling and coiling, namely firstly, a casting blank with the thickness of about 200mm is obtained through continuous casting, the casting blank is reheated and heat preserved, then rough rolling and finish rolling are carried out to obtain a steel strip with the thickness generally larger than 2mm, and finally laminar cooling and coiling are carried out to the steel strip to complete the whole hot rolling production process. The difficulty is relatively great if steel strip with a thickness of less than 1.5mm inclusive is to be produced, which is usually done by subsequent cold rolling and annealing of the hot rolled strip. And the process flow is long, the energy consumption is high, the number of unit equipment is large, the capital construction cost is high, and the production cost is high.

The thin slab continuous casting and rolling process flow is as follows: continuous casting, heat preservation and soaking of a casting blank, hot continuous rolling, cooling and coiling. The main differences between the process and the traditional process are as follows: the thickness of a casting blank in the thin slab process is greatly reduced to 50-90mm, and the casting blank is thin, so that the casting blank can be reduced to the required specification before finish rolling only by carrying out 1-2 times of rough rolling (when the thickness of the casting blank is 70-90 mm) or not carrying out the rough rolling (when the thickness of the casting blank is 50 mm) on the casting blank in the traditional process through repeated multi-pass rolling; and the casting blank of the thin slab process directly enters a soaking pit furnace for soaking and heat preservation without cooling or supplementing a small amount of heat, so that the thin slab process greatly shortens the process flow, reduces the energy consumption and the investment, thereby reducing the production cost. However, the higher cooling rate of the continuous casting and rolling of the sheet bar leads to the improvement of the strength of steel and the yield ratio, thereby increasing the rolling load, so that the thickness specification of the hot rolled product which can be economically produced cannot be too thin, generally more than or equal to 1.5mm, see Chinese patents CN200610123458.1, CN200610035800.2 and CN 200710031548.2.

The ESP realizes the continuous casting of the slab continuously, cancels the slab flame cutting and the heating furnace with the functions of heat preservation, soaking and slab transition, and greatly shortens the length of the whole production line to about 190 meters. The thickness of the continuously cast plate blank of the continuous casting machine is 90-110mm, the width of the continuously cast plate blank is 1100-1600mm, the continuously cast plate blank plays a role in heat preservation and uniform heating on the plate blank through a section of induction heating roller way, and then the continuously cast plate blank sequentially enters the working procedures of rough rolling, finish rolling, layer cooling and coiling to obtain a hot rolled plate. The process realizes endless rolling, can obtain the hot rolled plate with the thinnest thickness of 0.8mm, expands the specification range of the hot rolled plate, and has the output of a single production line reaching 220 kiloton per year. At present, the process is rapidly developed and popularized, and a plurality of ESP production lines are operated and produced in the world at present.

The process flow shorter than the thin slab continuous casting and rolling is a thin strip continuous casting and rolling process, the thin strip continuous casting technology is a leading-edge technology in the fields of metallurgy and material research, the emergence of the technology brings a revolution to the steel industry, the production process of steel strips in the traditional metallurgy industry is changed, continuous casting, rolling, even heat treatment and the like are integrated into a whole, the produced thin slab is subjected to online hot rolling once to form a thin steel strip, the production process is greatly simplified, the production period is shortened, and the length of the process line is only about 50 m; the equipment investment is correspondingly reduced, the product cost is obviously reduced, and the method is a low-carbon and environment-friendly hot-rolled thin strip production process. The twin-roll thin strip continuous casting process is a main form of the thin strip continuous casting process and is the only thin strip continuous casting process for realizing industrialization in the world.

A typical process flow of twin roll strip casting is shown in FIG. 1, molten steel in a ladle 1 is directly poured into a molten pool 7 surrounded by two relatively rotating and rapidly cooled crystallizing

rollers

8a, 8b and side sealing devices 6a, 6b through a ladle nozzle 2, a tundish 3, a submerged nozzle 4 and a distributor 5, the molten steel is solidified on the rotating circumferential surfaces of the crystallizing

rollers

8a, 8b to form solidified shells and gradually grows, a cast strip 11 with the thickness of 1-5mm is formed at the position (nip point) where the roll gaps between the two crystallizing

rollers

8a, 8b are the smallest, the cast strip 11 is fed into a rolling mill 13 through a guide plate 9 to be rolled into a thin strip with the thickness of 0.7-2.5mm, then is cooled through a cooling device 14, is cut by a flying shear device 16, and is finally fed into a coiler 19 to be coiled.

In recent years, with the continuous emergence of large buildings and high-rise buildings at home and abroad, in order to avoid the collapse of the buildings caused by the sudden decrease of the strength of the building steel due to the heating when the buildings are in fire, and great loss of lives and properties of residents, the fire protection process design of the building steel is widely regarded by people internationally, and the fire protection process design of the building steel becomes one of necessary measures for ensuring the building safety. The strength and the bearing capacity of the steel for the common building are rapidly reduced when the steel is heated, and the requirement of fire safety cannot be met. In order to improve the fire resistance of buildings constructed by using common construction steel, the conventional method is to adopt measures such as spraying a fire-resistant coating or covering a fire-proof plate, and the like, but now a great amount of fire-resistant steel is used, so that the fire-resistant coating or the fire-proof plate can be thinned or omitted, the steel can be ensured to keep higher strength level at high temperature, the construction period can be shortened, the quality of the buildings can be reduced, the safety of the buildings can be improved, the construction cost can be reduced, and the economic benefit and the social benefit are obvious. Meanwhile, most of the constructional steel is exposed in the air, and the service life is long, so that the steel has weather resistance on the basis of fire resistance, and a brand-new fire-resistant weather-resistant steel product is derived. Under the large application background, the existing fire-resistant weather-resistant steel plate/belt is increasingly applied to industries and fields of building steel with fire protection requirements, tower and mast columns, engineering machinery, industrial structures and the like, and is particularly widely applied to places with high fire safety requirements, such as high-rise buildings, large public buildings, high-grade houses and the like.

The fire-resistant weathering steel is required to have good high-temperature performance, but is different from the heat-resistant steel which is used at high temperature for a long time, and the heat-resistant steel is required to have good high-temperature strength and high-temperature stability, and generally adopts high alloy steel. The fire-resistant weathering steel is loaded at normal temperature, but is required to maintain higher yield strength under the short-time (usually 1-3h) high-temperature condition of fire, and can gain precious time for the safe evacuation of personnel, so that the fire-resistant weathering steel belongs to the category of low-alloy structural steel, and excessive expensive alloy elements are not suitable to be added in the component design.

The safety design specifications of the steel structure of Japan specify: 2/3, the yield strength of steel at normal temperature, corresponds to the long-term allowable stress value of the material, and when a fire breaks out, if the yield point of the fire-resistant weathering steel can still maintain the value, the building will not collapse. Therefore, the yield strength of the fire-resistant weathering steel is required to be not lower than 2/3 which is the yield strength at room temperature at a certain high temperature. In order to improve the seismic resistance of steel structures, it is always desirable to improve the seismic energy absorbing capacity of steel as much as possible. If the yield ratio (σ) of the steel_s/σ_b) Higher, local stress concentration and local large deformation can be generated when an earthquake occurs, and at the moment, the steel structure can only absorb less energy, so the low yield ratio is favorable for improving the energy absorbed by the building structure when the earthquake occurs, and the anti-seismic fire-resistant weathering steel is generally requiredThe yield ratio is less than or equal to 0.8.

One of the keys to the development of fire resistant weathering steels is to set the fire temperature at which it is desired to achieve high temperature strength. Mo-containing steel resisting the high temperature of 900-plus 1000 ℃ has been studied in European Creusot-Loire steel works, but the cost is too high and the Mo-containing steel is not popularized and applied. In order to determine the fire-resistant temperature which is optimally matched with the reduction of the production cost and the reduction of the consumption of the fire-resistant materials, the high-temperature tensile test is carried out on several typical steels with the tensile strength of 400-780MPa in Xinri iron, and the test shows that: yield strength (σ) of all test steels_s) Sharply decreases in the range of 500-600 ℃, and sigma is at 700 ℃ or above 700 DEG C_sReducing the pressure to below 50 MPa. If the temperature for ensuring the strength is set at 700 ℃, a large amount of alloy elements must be added, which makes it difficult to ensure good weldability and greatly increases the cost. If the temperature for ensuring the strength is set at a lower temperature of 500 ℃, the thickness of the refractory coating can be only slightly reduced, which is not in accordance with the original purpose of using refractory weathering steel. Therefore, the fire resistance temperature of the fire resistant weathering steel was set to 600 ℃.

In conclusion, according to the general requirements of the construction steel, the performance indexes of the fire-resistant weathering steel are determined as follows:

(1) fire resistance: sigma_s,600℃≥(2/3)σ_s,20℃；

(2) The room temperature mechanical property and other quality indexes meet the requirements of the standard of common construction steel;

(3) shock resistance: sigma at room temperature_s/σ_bLess than or equal to 0.8, and the fluctuation range of the yield strength is as small as possible;

(4) the weldability is good, equal to or better than that of common construction steel;

(5) weather resistance: compared with Q345B, the relative corrosion rate is less than or equal to 60 percent by adopting the standard TB/T2375-one 1993 for determination.

The high-strength fire-resistant weather-resistant steel plate/strip is produced by adopting a strip continuous casting process, and due to the sub-rapid solidification characteristic of the strip continuous casting process, a fine crystal layer with a certain thickness can be naturally formed on the surface of the strip steel, so that the produced steel has weather resistance; meanwhile, the thin-gauge cable has certain manufacturing and cost advantages in thin-gauge use occasions. The specification characteristic thickness of the thin-specification high-strength fire-resistant weather-resistant steel plate/strip is 1.2-2.0mm, and because the product thickness is thin, if the production is difficult by adopting a traditional continuous casting and hot continuous rolling production line, the roll consumption of a roll is large even if the thin slab continuous casting and rolling production is adopted, and the production cost of the thin-specification high-strength fire-resistant weather-resistant steel plate/strip is increased by the production process.

When the hot-rolled strip steel is used as a thin hot-rolled plate product, the requirement on the surface quality of the strip steel is high. It is generally required that the thinner the scale on the surface of the strip, the better the scale formation is, and it is required to control the scale formation in the subsequent stages of the strip casting, for example, in the twin roll strip casting process, the oxidation of the strip is prevented by using a closed chamber device in the crystallizing roll up to the entrance of the rolling mill, and the scale formation on the surface of the strip can be controlled by adding hydrogen gas in the closed chamber device such as US6920912 and controlling the oxygen content to be less than 5% in US 20060182989. However, there are few patents on how to control the thickness of the scale during the transport from the rolling mill to the coiling, and particularly, in the cooling of the strip by laminar cooling or spray cooling, the strip at high temperature is in contact with cooling water, and the scale thickness on the surface of the cast strip increases rapidly. Meanwhile, the contact between the high-temperature strip steel and the cooling water also brings about a plurality of problems: firstly, water spots (rusts) are formed on the surface of strip steel, which affects the surface quality; secondly, cooling water for laminar cooling or spray cooling easily causes uneven local cooling on the surface of the strip steel and uneven microstructure inside the strip steel, thereby causing uneven performance of the strip steel and influencing the product quality; thirdly, the local cooling of the surface of the strip steel is uneven, which causes the deterioration of the plate shape and influences the quality of the plate shape.

However, the continuous strip casting has the fast solidification process characteristics, so that the produced steel generally has the problems of uneven structure, low elongation, high yield ratio and poor formability; meanwhile, the austenite grains of the cast strip have obvious nonuniformity, which can cause the structure of a final product obtained after austenite phase transformation to be nonuniform, thereby causing the instability of the performance, particularly the forming performance, of the product. Therefore, the method for producing the fire-resistant weather-resistant steel plate/strip product by adopting the thin-strip continuous casting process has certain difficulty and challenge, and needs to make a breakthrough in composition and process.

Chinese patent CN200610123458.1 discloses a method for producing 700MPa grade high-strength weathering steel by adopting Ti microalloying process based on thin slab continuous casting and rolling process, the chemical components of the weathering steel plate manufactured by the method are as follows: c: 0.03 to 0.07%, Si: 0.3-0.5%, Mn: 1.2-1.5, P: less than or equal to 0.04 percent, S: less than or equal to 0.008 percent, Al: 0.025-0.05%, Cr: 0.3-0.7%, Ni: 0.15-0.35%, Cu: 0.2-0.5%, Ti: 0.08-0.14%, N: less than or equal to 0.008 percent, and the balance of Fe and inevitable impurities. The yield strength of the steel plate is more than or equal to 700MPa, the tensile strength is more than or equal to 775MPa, and the elongation is more than or equal to 21 percent. In the patent, the phosphorus is controlled according to impurity elements, the content is less than or equal to 0.04 percent, and is less than or equal to 0.025 percent compared with the traditional process, so that the phosphorus content is relaxed.

Chinese patent CN200610035800.2 discloses a method for producing 700MPa grade V-N microalloyed weathering steel based on a thin slab continuous casting and rolling process, and the chemical components of the weathering steel plate manufactured by the method are as follows: c: less than or equal to 0.08 percent, Si: 0.25 to 0.75%, Mn: 0.8-2, P: less than or equal to 0.07-0.15 percent, S: less than or equal to 0.04 percent, Cr: 0.3-1.25%, Ni: less than or equal to 0.65%, Cu: 0.25-0.6%, V: 0.05-0.2%, N: 0.015 to 0.03 percent, and the balance of Fe and inevitable impurities. The yield strength of the steel plate is more than or equal to 700MPa, the tensile strength is more than or equal to 785MPa, and the elongation is more than or equal to 21%. In this patent, phosphorus is controlled according to elements for improving weather resistance, and the content is 0.07-0.15%; the copper content is 0.25-0.6%, and the lower limit and the upper limit are respectively 0.2% and 0.55% higher than the lower limit and the upper limit of the copper content of the traditional process.

Chinese patent CN1633509A discloses a method for producing copper-containing carbon steel products by strip casting, which emphasizes that the strip is subjected to heat treatment processes such as annealing, tempering and the like at the temperature range of 400-700 ℃ to precipitate or recrystallize copper elements in the strip. The manufacturing method of the high-copper low-alloy thin strip mentioned in the patent US2008264525/CN200580009354.1 is technically characterized in that the strip steel is cooled to be lower than 1080 ℃ in a non-oxidizing atmosphere before entering a rolling mill so as to prevent the strip steel from generating the phenomenon of hot brittleness.

These patents all relate to the weather resistance of steel, but do not relate to the effect of fire resistance, and the present invention is a steel grade specifically proposed for the fire and weather resistance characteristics.

Disclosure of Invention

The invention aims to provide a fire-resistant weather-resistant steel plate/belt and a manufacturing method thereof, the produced fire-resistant weather-resistant steel plate/belt can ensure that steel keeps higher strength level at high temperature, the quality of a building can be reduced, the safety of the building is improved, and the construction cost is reduced; meanwhile, by utilizing the technological characteristics of thin strip continuous casting, the produced fire-resistant weather-resistant steel plate/strip naturally forms a fine crystal layer with a certain thickness on the surface of the strip steel on the basis of the fire resistance, so that the product has weather resistance. The product has excellent earthquake resistance and excellent fire resistance, is an ideal material in the field of steel for earthquake resistance and fire prevention, can be widely applied to industries and fields of construction steel with fireproof and earthquake resistance requirements, tower and mast column, engineering machinery, industrial structures and the like, and is particularly applied to places with higher fire safety requirements such as high-rise buildings, large public buildings, high-grade houses and the like.

In order to achieve the purpose, the technical scheme of the invention is as follows:

in the invention, microalloy elements such as Mo/Cr and the like and B elements are selectively added into steel; controlling the alkalinity of slag, the type and melting point of inclusions in steel, the content of free oxygen in molten steel and the content of acid-soluble aluminum Als in the smelting process; then, performing double-roller thin-strip continuous casting to cast strip steel with the thickness of 1.5-3mm, directly entering a lower closed chamber with non-oxidizing atmosphere after the strip steel exits from a crystallization roller, and entering an online rolling mill for hot rolling under the closed condition; the rolled strip steel is cooled by adopting an air atomization cooling mode, and the air atomization cooling mode can effectively reduce the thickness of oxide skins on the surface of the strip steel, improve the temperature uniformity of the strip steel and improve the surface quality of the strip steel. The finally produced steel coil can be directly used as a hot rolled plate/strip, and can also be used as a finishing plate/strip after edge cutting and flattening.

Specifically, the fire-resistant weather-resistant steel plate/belt comprises the following chemical components in percentage by weight: c: 0.02 to 0.06%, Si: 0.1-0.55%, Mn:0.4-1.7％，P≤0.03％，S≤0.007％，Cr：0.30-0.80％，Cu：0.10-0.60％，Mo：0.20-0.60％，N：0.004-0.010％，Als：<0.001%, B: 0.001-0.006% of total oxygen [ O ]]_T: 0.007-0.020%, and the balance of Fe and inevitable impurities; and, simultaneously:

Mn/S>250；

Pcm≤0.27％，Pcm＝C+Si/30+Mn/20+Cu/20+Cr/20+Mo/15+5B。

the microstructure of the fire-resistant weather-resistant steel plate/belt is a mixed microstructure of block ferrite, pearlite and acicular ferrite or a mixed microstructure of block ferrite, pearlite and lower bainite.

The room temperature yield strength of the fire-resistant weather-resistant steel plate/belt is not less than 345MPa, the tensile strength is not less than 490MPa, and the elongation is not less than 17%; yield ratio sigma_s/σ_bLess than or equal to 0.8; the yield strength at 600 ℃ is more than or equal to 232MPa and sigma_s,600℃/σ_s,20℃Not less than 0.67; the relative corrosion rate is less than or equal to 60 percent.

In the chemical composition design of the fire-resistant weather-resistant steel plate/belt, the following steps are carried out:

c: c is the most economical and basic strengthening element in steel, and enhances the strength of steel by solid solution strengthening and precipitation strengthening. C is an essential element for precipitating cementite during austenite transformation, so the strength level of the steel is determined to a great extent by the content of C, namely, higher C content corresponds to higher strength level. However, since interstitial solid solution and precipitation of C are harmful to the plasticity and toughness of steel, and an excessively high C content is disadvantageous to welding performance, the C content cannot be excessively high, and the strength of steel is compensated by appropriate addition of alloying elements. Meanwhile, for conventional slab continuous casting, casting in a peritectic reaction region is easy to generate surface cracks of a casting blank, and steel leakage accidents can happen in severe cases. The same is true for thin strip casting, where casting a cast strip in the peritectic reaction zone is prone to surface cracking and, in severe cases, strip breakage. Therefore, strip casting of Fe — C alloys also requires avoidance of the peritectic reaction zone. Therefore, the content range of C adopted by the invention is 0.02-0.06%.

Si: si acts as a solid solution strengthening in steel, and addition of Si to steel improves steel purity and deoxidation, but too high a Si content leads to deterioration of weldability and toughness in weld heat affected zone. Therefore, the Si content range adopted by the invention is 0.1-0.55%.

Mn: mn is one of the cheapest alloy elements, can improve the hardenability of steel, has considerable solid solubility in the steel, improves the strength of the steel through solid solution strengthening, basically has no damage to the plasticity and the toughness of the steel, is the most main strengthening element for improving the strength of the steel, and can play a role in deoxidizing in the steel. However, too high Mn content results in deterioration of weldability and toughness of weld heat-affected zone. Therefore, the Mn content range adopted by the invention is 0.4-1.7%.

P: high content of P is easy to be segregated in grain boundary, increases cold brittleness of steel, deteriorates welding performance, reduces plasticity and deteriorates cold bending performance. In the thin strip continuous casting process, the solidification and cooling rates of the cast strip are extremely high, and the segregation of P can be effectively inhibited, so that the disadvantages of P can be effectively avoided, and the advantages of P can be fully exerted. Therefore, in the invention, the P content is higher than that in the traditional process, the content of the P element is properly relaxed, the dephosphorization procedure is eliminated in the steelmaking procedure, in the actual operation, the dephosphorization procedure is not required to be carried out intentionally, and no extra phosphorus is required to be added, and the range of the P content is less than or equal to 0.03 percent.

S: in general, S is a harmful element in steel, causes hot brittleness of steel, reduces ductility and toughness of steel, and causes cracks during rolling. S also reduces weldability and corrosion resistance. Therefore, in the present invention, S is also controlled as an impurity element, and the content thereof is in the range of 0.007% or less. And Mn/S > 250.

And Als: in order to control inclusions in steel, the invention requires that Al cannot be used for deoxidation, and in the use of refractory materials, the additional introduction of Al is avoided as much as possible, and the content of acid-soluble aluminum Als is strictly controlled: < 0.001%.

N: similar to C element, N element can improve the strength of steel through interstitial solid solution, and the invention needs to generate a BN precipitated phase by utilizing the action of N and B in the steel and needs a certain content of N in the steel. However, the interstitial solid solution of N has great harm to the plasticity and toughness of the steel, and the existence of free N can improve the yield ratio of the steel, so that the content of N cannot be too high. The content range of N adopted by the invention is 0.004-0.010%.

Cr: the Cr is added into the fire-resistant weathering steel, so that the high-temperature strength and creep strength of the steel are improved, the weather resistance of the steel is improved, and the corrosion resistance of the steel can be obviously improved. However, the welding property is seriously deteriorated by too high a content thereof, and the Cr content is limited to 0.30 to 0.80% in the present invention.

Mo: the solid solution in the ferrite strengthens the ferrite matrix, and the diffusion speed of Mo in the ferrite is slow at high temperature, so that the high-temperature strength and creep strength of the steel can be obviously improved. Research shows that the solid-dissolved Mo is easy to segregate on the grain boundary, and plays a role in improving the high-temperature strength of the steel; mo can also increase the stability of super-cooled austenite, and the volume fraction of bainite in steel is increased after Mo is added; the bainite structure with high dislocation density enables the fire-resistant weathering steel to obtain good high-temperature performance. Mo precipitates in steel to form carbide, which improves the high-temperature strength of the steel. Too high Mo increases the raw material smelting cost of the steel. In the present invention, the Mo content is limited to 0.20 to 0.60%.

Cu: the Cu is also an element for improving weather resistance, and the Cu is an easily segregated element, so that the Cu content is generally strictly controlled in the traditional process flow. The invention improves the upper limit of Cu to 0.60% by applying the rapid solidification effect of thin strip continuous casting. The improvement of the Cu content can fully utilize the scrap steel in a certain sense, and screening is not needed when the scrap steel raw material is prepared, so that the smelting operation rate is improved, the cost is reduced, the recycling of steel is promoted, and the purpose of sustainable development is realized; and the effective utilization of copper in poor-quality ore resources (such as high copper ore) can be realized.

B: the significant role of B in steel is: the extremely trace boron can multiply the hardenability of the steel, and B can preferentially precipitate coarse BN particles in high-temperature austenite to inhibit the precipitation of fine AlN, weaken the pinning effect of the fine AlN on grain boundaries, improve the growth capability of grains, coarsen and homogenize austenite grains, facilitate the recrystallization after rolling and coarsen and homogenize the austenite grainsAfter homogenization, the yield ratio sigma of the product is favorably reduced_s/σ_bThe fluctuation range of the yield strength is reduced, and the anti-seismic performance is improved; in addition, the combination of B and N can effectively prevent the grain boundary low melting point phase B₂O₃Is present.

B is an active segregation-prone element and is easy to segregate in a grain boundary, and the content of B is generally controlled to be very strict and is generally about 0.001-0.003% when B-containing steel is produced by a traditional process; in the thin-strip continuous casting process, the solidification and cooling rates are high, the segregation of B can be effectively inhibited, and more B content is dissolved, so that the content of B can be properly widened; and coarse BN particles can be generated through reasonable process control, and the precipitation of fine AlN is inhibited, so that the nitrogen fixation effect is realized. It is also shown that B, when added in combination with Nb and Mo, produces better effects, reduces the tendency of segregation of C atoms and avoids grain boundary Fe₂₃(C,B)₆So that more B can be added. Therefore, in the present invention, a higher B content is used than in the conventional process, in the range of 0.001-0.006%.

In order to ensure the welding performance of the steel grade, the following relational expression is satisfied when the elements are designed:

Pcm≤0.27％，Pcm＝C+Si/30+Mn/20+Cu/20+Cr/20+Mo/15+5B。

the invention relates to a method for manufacturing a fire-resistant weather-resistant steel plate/belt, which comprises the following steps:

a) smelting

Smelting according to the requirements of the chemical components, wherein the basicity a of slagging in the steelmaking process is CaO/SiO₂Control in a<1.5, preferably a<1.2, or a ═ 0.7-1.0; obtaining low melting point MnO-SiO in molten steel₂-Al₂O₃MnO/SiO in ternary inclusions₂Controlling the concentration to be 0.5-2, preferably 1-1.8; free oxygen [ O ] in molten steel]_FreeThe contents are as follows: 0.0005-0.005%; in the molten steel composition, Mn/S>250；

b) Continuous casting

The continuous casting adopts double-roller thin strip continuous casting, and a casting strip with the thickness of 1.5-3mm is formed at the position with the minimum gap between two crystallizing rollers; the diameter of the crystallization roller is between 500 and 1500mm, and the preferred diameter is 800 mm; cooling the crystallization roller by introducing water, wherein the casting speed of the casting machine is 60-150 m/min; the continuous casting flow distribution adopts a two-stage steel water distribution system, namely a tundish and a flow distributor;

c) lower sealed chamber protection

After the casting strip is taken out of the crystallization roller, the temperature of the casting strip is 1420-1480 ℃, the casting strip directly enters a lower closed chamber, non-oxidizing gas is introduced into the lower closed chamber, and the oxygen concentration in the lower closed chamber is controlled to be less than 5 percent; the temperature of the casting belt at the outlet of the lower closed chamber is 1150-1300 ℃;

d) in-line hot rolling

The cast strip is sent to a rolling mill by a pinch roll in a lower closed chamber to be rolled into strip steel with the thickness of 0.8-2.5mm, the rolling temperature is 1100-1250 ℃, the hot rolling reduction rate is controlled to be 10-50%, preferably 30-50%, the thickness of the rolled strip steel is 0.8-2.5mm, preferably 1.2-2.0 mm;

e) cooling after rolling

Cooling the rolled strip steel, and cooling the strip steel by adopting an air atomization cooling mode, wherein the cooling rate range of air atomization cooling is 20-100 ℃/s;

f) strip steel coiling

And cutting the cooled hot rolled strip steel by a cutting head to remove the head with poor quality, and directly coiling the hot rolled strip steel into coils at the coiling temperature of 580-680 ℃.

Further, the method also comprises a step g) of subsequent treatment, wherein the steel coil is directly used as a hot rolled plate/strip or used as a finishing plate/strip after edge cutting and flattening.

Preferably, in the step a), 100% of all-waste steel can be selected as smelting raw materials, pre-screening is not needed, and electric furnace steelmaking is adopted for molten steel smelting; or, the smelting adopts a converter for steel making, the scrap steel is added into the converter according to the proportion of more than 20 percent of the smelting raw materials, and pre-screening is not needed; then refining in an LF furnace, a VD/VOD furnace or an RH furnace.

Preferably, in step c), the non-oxidizing gas in the lower closed chamber is N₂Or Ar, or CO obtained by sublimation on dry ice₂A gas.

Preferably, in the step e), the gas-water ratio of the gas atomization cooling is 15: 1-10: 1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa.

Preferably, the step f) take-up is in the form of a twin winder or in the form of a carrousel take-up.

In the manufacturing method of the present invention:

in order to improve the castability of thin-strip continuous casting molten steel, the basicity a of slagging in the steel-making process is CaO/SiO₂Control in a<1.5, preferably a<1.2, or a ═ 0.7 to 1.0.

To improve the castability of thin strip continuous casting molten steel, it is necessary to obtain MnO-SiO of low melting point₂-Al₂O₃Ternary inclusions, e.g. red shaded areas, MnO-SiO, FIG. 2₂-Al₂O₃MnO/SiO in ternary inclusions₂The concentration is controlled to be 0.5 to 2, preferably 1 to 1.8.

In order to improve the castability of thin strip continuous casting molten steel, oxygen (O) in the steel is an essential element for forming oxide inclusions, and the present invention requires formation of MnO-SiO with a low melting point₂-Al₂O₃The ternary inclusions of (2) require free oxygen [ O ] in molten steel]_FreeThe range is as follows: 0.0005-0.005%.

In order to improve the castability of the thin strip continuous casting molten steel, the Mn and S of the above components should satisfy the following relation: Mn/S > 250.

The molten steel can be smelted by an electric furnace or a converter and then enters necessary refining processes, such as an LF furnace, a VD/VOD furnace, an RH furnace and the like.

The theoretical basis of BN precipitated phase involved in the lower sealing process of the cast strip is as follows:

the thermodynamic equation of boron with nitrogen, aluminum and nitrogen in gamma-Fe in steel is as follows:

BN＝B+N；Log[B][N]＝-13970/T+5.24 (1)

AlN＝Al+N；Log[Al][N]＝-6770/T+1.03 (2)

as shown in FIG. 3, the initial precipitation temperature of BN in the steel is about 1280 ℃ and the precipitation of BN at 980 ℃ is in equilibrium, but the precipitation of AlN is just started (the precipitation temperature of AlN is about 980 ℃), and the precipitation of BN is thermodynamically preferred to AlN. The invention completes the combination of B and N in the lower closed chamber to generate coarse BN particles, thereby inhibiting the precipitation of fine AlN,weakening the pinning effect of fine AlN on the grain boundary and improving the growth capability of grains, thereby coarsening austenite grains, leading the austenite grains to be more uniform and being beneficial to reducing the yield ratio sigma of the product_s/σ_bThe fluctuation range of the yield strength is reduced, and the anti-seismic performance is improved; in addition, the combination of B and N can effectively prevent the grain boundary low melting point phase B₂O₃Is present.

The strip steel after on-line hot rolling is cooled after rolling, the rolled strip steel is cooled by adopting an air atomization cooling mode, and the air atomization cooling mode can effectively reduce the thickness of oxide skins on the surface of the strip steel, improve the temperature uniformity of the strip steel and improve the surface quality of the strip steel. The gas-water ratio of gas atomization cooling is 15: 1-10: 1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa. High-pressure water mist is formed after gas atomization and sprayed on the surface of the steel strip, so that on one hand, the temperature of the steel strip is reduced, on the other hand, the water mist can form a compact air film to be coated on the surface of the steel strip, the anti-oxidation effect of the steel strip is achieved, and the growth of oxide skin on the surface of the hot-rolled steel strip is effectively controlled. The cooling mode can avoid the problems caused by the traditional spraying or laminar cooling, so that the surface temperature of the strip steel is uniformly reduced, the temperature uniformity of the strip steel is improved, and the effect of homogenizing the internal microstructure is achieved; meanwhile, the cooling is uniform, so that the shape quality and the performance stability of the strip steel can be improved; effectively reducing the thickness of the oxide scale on the surface of the strip steel. The cooling rate of the gas atomization cooling is in the range of 20-100 ℃/s.

And cutting the head of the cooled hot-rolled strip steel by using a cutting head to remove the head with poor quality, and directly coiling the hot-rolled strip steel into coils. The coiling temperature of the hot rolled strip is controlled to be 580-680 ℃, so that the rolled high-temperature austenite structure is transformed into a mixed microstructure of bulk ferrite, pearlite and acicular ferrite or a mixed microstructure of bulk ferrite, pearlite and lower bainite, as shown in figure 4.

The strip steel coiling of the invention adopts a double-coiling machine form and can also adopt a carrousel coiling form, thereby ensuring the continuous production of the strip steel.

The invention has the main advantages that:

the boron (B) -containing fire-resistant and weather-resistant steel is produced by using a thin strip continuous casting technology, and has the following advantages that no report exists so far:

1. the invention omits the complex processes of slab heating, multi-pass repeated hot rolling and the like, and has the advantages of shorter production flow, higher efficiency and greatly reduced production line investment cost and production cost by adopting the working procedures of double-roller thin-strip continuous casting and one-pass online hot rolling.

2. The invention omits a plurality of complex intermediate steps in the production of the traditional process, and compared with the traditional production process, the energy consumption and CO2 emission of the production are greatly reduced, thus the invention is a green and environment-friendly product. 3. The invention adopts the thin strip continuous casting process to produce the hot-rolled thin-specification fire-resistant weather-resistant steel, and because of the sub-rapid solidification characteristic of the thin strip continuous casting process, a fine crystal layer with a certain thickness can be naturally formed on the surface of the strip steel, so that the produced steel has weather resistance; meanwhile, the thickness of the cast strip is thin, and the thin-specification product can be directly supplied to the market for use by online hot rolling to the desired product thickness, so that the purpose of supplying the thin-specification hot rolled plate is achieved, and the cost performance of the plate strip can be obviously improved.

4. According to the invention, trace B element is added, coarse BN particles are preferentially precipitated in high-temperature austenite, so that the precipitation of fine AlN is inhibited, the pinning effect of fine AlN on grain boundaries is weakened, the growth capacity of grains is improved, and therefore, austenite grains are coarsened and homogenized, and the forming performance of products is favorably improved.

5. The smelting adopts the electric furnace for steelmaking, the smelting raw materials can realize 100 percent of all scrap steel smelting in the true sense, pre-screening is not needed, and the raw material cost is greatly reduced; if the steel is smelted by the converter, the scrap steel is added into the converter according to the proportion of more than 20 percent of the smelting raw materials without pre-screening, so that the scrap steel ratio of the converter is improved to the maximum extent, and the smelting cost and the energy consumption are greatly reduced.

6, the improvement of the Cu content can fully utilize the scrap steel in a certain sense, and screening is not needed when the scrap steel raw material is prepared, so that the smelting operation rate is improved, the cost is reduced, the recycling of steel is promoted, and the purpose of sustainable development is realized; and the effective utilization of copper in poor-quality ore resources (such as high copper ore) can be realized.

7. The invention adopts the air atomization cooling mode of the rolled strip steel, can avoid the problems brought by the traditional spray or laminar cooling, uniformly reduces the surface temperature of the strip steel, improves the temperature uniformity of the strip steel, and achieves the effect of homogenizing the internal microstructure; meanwhile, the cooling is uniform, so that the shape quality and the performance stability of the strip steel can be improved; effectively reducing the thickness of the oxide scale on the surface of the strip steel.

8. In the traditional process, alloy elements are separated out in the cooling process of the plate blank, and the utilization rate of the alloy elements is reduced because the re-dissolution of the alloy elements is insufficient when the plate blank is reheated. In the thin strip continuous casting process, the high-temperature cast strip is directly hot-rolled, and the added alloy elements mainly exist in a solid solution state, so that the alloy utilization rate can be improved.

9. According to the invention, the hot rolled steel strip carrousel coiling machine is selected, so that the length of a production line is effectively shortened; meanwhile, the control precision of the coiling temperature can be greatly improved by the co-position coiling, and the stability of the product performance is improved.

10. The most obvious characteristic of the invention which is different from the prior thin strip continuous casting technology is the roller diameter of the crystallization roller and the corresponding distribution mode. The EUROSTRIP technology is characterized in that the crystallization roller with a large roller diameter of 1500mm phi is large, the molten steel capacity of a molten pool is large, the distribution is easy, and the manufacturing cost and the operation cost of the crystallization roller are high. The CASTIP technology is characterized in that a crystallization roller with a small roller diameter of 500mm phi is small, the molten steel capacity of a molten pool is small, the distribution is very difficult, but the manufacturing, operation and maintenance costs of casting machine equipment are low. CASTIP adopts a three-level steel water distribution system (tundish, transition ladle and distributor) to solve the problem of uniform distribution of small molten pools. Because a three-level flow distribution system is adopted, the cost of the refractory material is directly increased; more importantly, the three-stage flow distribution system enables the flowing path of the molten steel to be lengthened, the temperature drop of the molten steel is large, and in order to meet the temperature of molten steel in a molten pool, the tapping temperature needs to be greatly increased. The increase of the tapping temperature causes problems such as increase of steel-making cost, increase of energy consumption, and shortening of the life of refractory.

11. The diameter of the crystallization roller is 500-1500mm, preferably the diameter of the crystallization roller is 800mm, and a two-stage steel water distribution system (a tundish and a flow distributor) is adopted. Molten steel flowing out of the flow distributor forms different flow distribution modes along the roller surface and the two end surfaces, and flows in two paths without mutual interference. Because a two-stage flow distribution system is adopted, compared with a three-stage flow distribution system, the cost of the refractory material is greatly reduced; the reduction of the flow path of the molten steel reduces the temperature drop of the molten steel, and can reduce the tapping temperature by 30-50 ℃ compared with a three-level flow distribution system. The reduction of the tapping temperature can effectively reduce the steel-making cost, save the energy consumption and prolong the service life of refractory materials. The invention is matched with the crystallization roller with the optimal roller diameter of phi 800mm, and adopts a two-stage steel water distribution system, thereby not only realizing the requirement of stable distribution of molten steel, but also realizing the aims of simple structure, convenient operation and low processing cost.

Drawings

FIG. 1 is a schematic layout of a twin roll strip casting process;

FIG. 2 shows MnO-SiO₂-Al₂O₃Ternary phase diagram (shaded area: low melting point region);

FIG. 3 is a schematic diagram of a thermodynamic curve of BN, AlN precipitation;

FIG. 4 is a photograph showing the microstructure of the steel according to the example of the present invention.

Detailed Description

The invention is further illustrated by the following examples, which are not intended to limit the invention in any way. Any variations in the practice of the invention that may occur to those skilled in the art and which are made in the light of the teachings of this specification are intended to be within the scope of the following claims.

Referring to fig. 1, molten steel designed according to the chemical composition of the present invention is directly poured into a molten pool 7 surrounded by two

crystallization rollers

8a, 8b which rotate relatively and can be rapidly cooled and side closing plate devices 6a, 6b through a ladle 1, a tundish 2, a tundish 3, a submerged nozzle 4 and a distributor 5, the molten steel is solidified on the circumferential surfaces of the rotation of the

crystallization rollers

8a, 8b, thereby forming solidified shells and gradually growing, and then a casting strip 11 with the thickness of 1.5-3mm is formed at the minimum clearance (nip point) between the two crystallization rollers; the diameter of the crystallization roller is between 500 and 1500mm, and water is introduced for cooling. The casting speed of the casting machine ranges from 60 to 150m/min according to the thickness of the cast strip.

After the casting strip 11 comes out of the

crystallization rollers

8a and 8b, the casting strip is directly fed into the lower closed chamber 10 at 1420-1480 ℃, the strip steel is protected by the lower closed chamber 10 through non-oxidizing gas, so as to realize the anti-oxidation protection of the strip steel, the atmosphere of the anti-oxidation protection can be N2, can also be Ar, and can also be other non-oxidizing gas, such as CO2 gas obtained by sublimation of dry ice, and the oxygen concentration in the lower closed chamber 10 is controlled to be less than 5%. The lower enclosed chamber 10 protects the cast strip 11 against oxidation to the rolling mill 13 inlet. The temperature of the cast strip at the outlet of the lower closed chamber 10 is 1150-1300 ℃. Then the cast strip is sent to a hot rolling mill 13 through a swinging guide plate 9 and a pinch roll 12, a hot rolled strip with the thickness of 0.8-2.5mm is formed after hot rolling, and the rolled strip steel is cooled by adopting an air atomization cooling mode, so that the temperature uniformity of the strip steel is improved. After the head is cut by the flying shear device 16, the head falls into the flying shear pit 18 along the flying shear guide plate 17, and the hot rolled strip after the head is cut enters the coiler 19 for coiling. And taking the steel coil off the coiling machine, and naturally cooling to room temperature. The finally produced steel coil can be directly used as a hot rolled plate/strip, and can also be used as a finishing plate/strip after edge cutting and flattening.

The process of the invention is further illustrated by the examples. The chemical compositions of the examples of the present invention are shown in table 1, and the balance of the compositions is Fe and other unavoidable impurities. The manufacturing method of the invention has the process parameters shown in the table 2, and the mechanical properties of the finally obtained hot rolled strip are shown in the table 3.

The corrosion resistance of the example steels was tested: a72 h periodic infiltration cycle corrosion test is carried out according to a 72h periodic infiltration corrosion test method (TB/T2375-1993) by taking plain carbon steel Q345B as a comparison sample. And calculating the corrosion weight loss of the sample in unit area to obtain the average corrosion rate and further obtain the relative corrosion rate of the steel grade. The test results are shown in Table 4.

In conclusion, the fire-resistant weather-resistant steel plate/strip manufactured by the thin-strip continuous casting process according to the designed steel grade components has the room-temperature yield strength of more than or equal to 345MPa, the tensile strength of more than or equal to 490MPa and the elongation of more than or equal to 17 percent; yield ratio sigma_s/σ_bLess than or equal to 0.8; the yield strength at 600 ℃ is more than or equal to 232MPa and sigma_s,600℃/σ_s,20℃Not less than 0.67; and the cold-processing bending performance is qualified.

The comparison result of the corrosion resistance also shows that the relative corrosion rate of the steel grade is less than or equal to 60 percent. The material meets and exceeds the performance requirements of fire-resistant and weather-resistant steel, can realize the supply of thin hot-rolled products, greatly reduces the production cost, has low yield ratio of materials, excellent earthquake resistance and excellent fire resistance, and is an ideal material in the field of earthquake-resistant and fire-resistant steel.

Claims

1. A fire-resistant weather-resistant steel plate/belt comprises the following chemical components in percentage by weight: c: 0.02 to 0.06%, Si: 0.1-0.55%, Mn: 0.4-1.7%, P is less than or equal to 0.03%, S is less than or equal to 0.007%, Cr: 0.30-0.80%, Cu: 0.10-0.60%, Mo: 0.20-0.60%, N: 0.004-0.010% of Als<0.001%, B: 0.001-0.006% of total oxygen [ O ]]_T: 0.007-0.020%, and the balance of Fe and inevitable impurities; and, simultaneously:

Mn/S>250；

Pcm≤0.27％，Pcm＝C+Si/30+Mn/20+Cu/20+Cr/20+Mo/15+5B。

2. the steel plate/strip according to claim 1, wherein the microstructure of the steel plate/strip is a mixed microstructure of bulk ferrite + pearlite + acicular ferrite, or a mixed microstructure of bulk ferrite + pearlite + lower bainite.

3. The fire and weather resistant steel plate/belt as claimed in claim 1 or 2, wherein the fire and weather resistant steel plate/belt has a room temperature yield strength of 345MPa or more, a tensile strength of 490MPa or more, and an elongation of 17% or more;

yield ratio sigma_s/σ_bLess than or equal to 0.8; the yield strength at 600 ℃ is more than or equal to 232MPa and sigma_s,600℃/σ_s,20℃≥0.67；

The relative corrosion rate is less than or equal to 60 percent.

4. The method for producing a fire-resistant and weather-resistant steel sheet/strip as claimed in claim 1, 2 or 3, wherein: which comprises the following steps:

a) smelting

Smelting according to the chemical composition requirement of claim 1, wherein the basicity a of slagging in the steelmaking process is CaO/SiO₂Control in a<1.5, preferably a<1.2, or a ═ 0.7-1.0; obtaining low melting point MnO-SiO in molten steel₂-Al₂O₃MnO/SiO in ternary inclusions₂Controlling the concentration to be 0.5-2, preferably 1-1.8; free oxygen [ O ] in molten steel]_FreeThe contents are as follows: 0.0005-0.005%; in the molten steel composition, Mn/S>250；

b) Continuous casting

c) lower sealed chamber protection

d) in-line hot rolling

e) cooling after rolling

f) strip steel coiling

5. The method for producing a fire-resistant and weather-resistant steel sheet/strip as claimed in claim 4, wherein: and g) carrying out subsequent treatment, wherein the steel coil is directly used as a hot rolled plate/strip or used as a finishing plate/strip after edge cutting and flattening.

6. The method for producing a fire-resistant and weather-resistant steel sheet/strip as claimed in claim 4, wherein: in the step a), 100% of all-scrap steel can be selected as smelting raw materials, pre-screening is not needed, and electric furnace steelmaking is adopted for molten steel smelting; or, the smelting adopts a converter for steel making, the scrap steel is added into the converter according to the proportion of more than 20 percent of the smelting raw materials, and pre-screening is not needed; then refining in an LF furnace, a VD/VOD furnace or an RH furnace.

7. The method for producing a fire-resistant and weather-resistant steel sheet/strip as claimed in claim 4, wherein: in step c), the non-oxidizing gas in the lower closed chamber is N₂Or Ar, or CO obtained by sublimation on dry ice₂A gas.

8. The method for producing a fire-resistant and weather-resistant steel sheet/strip as claimed in claim 4, wherein: in the step e), the gas-water ratio of the gas atomization cooling is 15: 1-10: 1, the air pressure is 0.5-0.8 MPa, and the water pressure is 1.0-1.5 MPa.

9. The method for producing a fire-resistant and weather-resistant steel sheet/strip as claimed in claim 4, wherein: and f) coiling in a double coiler mode or a carrousel coiling mode.