WO1998046801A1

WO1998046801A1 - New process for the production at low temperature of grain oriented electrical steel

Info

Publication number: WO1998046801A1
Application number: PCT/EP1997/001887
Authority: WO
Inventors: Laura Appolloni; Giuseppe Abbruzzese; Giancarlo Di Schino
Original assignee: Acciai Speciali Terni S.P.A.
Priority date: 1997-04-16
Filing date: 1997-04-16
Publication date: 1998-10-22
Also published as: AU2698097A

Abstract

Process for the production of grain oriented silicon steel sheets, in which a steel of the following composition, in weight, % C less than 0.040, Al less than 0.020, Si 2.5-4.5, Sn 0.03-0.30, Cu 0.1-0.35, S 0.010-0.025, Mn 0.030-0.30, remaining being iron and minor impurities, is continuously cast in slabs, whose thickness is reduced by 1-10 %, and then hot-rolled with a temperature at the entrance of the finishing stand of between 1050 and 1200 °C, within a temperature interval of 30 °C, and a temperature of between 900 and 1050 °C at the exit from the finishing stand, the strip being forcibly cooled starting after 5-10 s from said exit at a temperature of between 500 and 700 °C; the strip is then cold-rolled in at least two reduction stages, after which the usual final treatments are performed, in which during the decarburizing annealing the heating rate is higher than 10 °C/s.

Description

NEW PROCESS FOR THE PRODUCTION AT LOW TEMPERATURE OF GRAIN ORIENTED ELECTRICAL STEEL

FIELD OF THE INVENTION

The present invention refers to a process for the production at low temperature of grain oriented electrical steel strips and, more precisely, refers to a less than usual complex process which, through a specific combination in cooperation relationship of a careful definition of the steel chemical composition with some well calibrated process specifications, allows to obtain a high quality product. BACKGROUND OF THE INVENTION

Before describing the state of the art referring to this kind of products, it seems appropriate to remind relevant scientific and technical basis. Silicon steel consists of a plurality of separate contiguous grains (or crystals), each having a body-centered cubic lattice, in which the axes corresponding to the cube corners, crystallographically designed with <100>, are directions of easest magnetization.

Considering the structure of the most common product utilizing oriented grain electrical steel strip, i.e. nuclei for electric transformers, which are formed by stacks of silicon steel relatively narrow bands cut parallelely to the rolled strip length and wound in form of torus, and the working scheme of transformers, in which a magnetic field induces in the nucleus a magnetic flux directed along the lines of easiest magnetization of the material forming the nucleus itself, it follows that the <001> axes should preferably lay parallel to the rolling direction of the strip, i.e. to the strip lenght.

Moreover, it is necessary that the lattices of said grains are all oriented in the same way, with the minimum degree of mutual disorientation. Further, it is necessary that number and dimensions of said grains are maintained within given limits, well known to the experts.

Only by keeping to said general conditions it is possible to obtain a material having good magnetization characteristics, i.e. magnetic permeability, expressed as magnetic flux density induced in the nucleus by a magnetic field of given value, and energy loss in operation, usually referred to as core losses at given frequency and permeability and expressed in W/kg.

The correct orientation of grains in the end product is obtained during a termal treatment called secondary recrystallization annealing, in which it is possible to allow the growth only of grains having the desired orientation at the beginning of the annealing.

Number and dimensions of the grains thus obtained somewhat depend from the corresponding starting values.

The selective grain growth process is temperature activated and is due to the fact that some crystals, for kinetic and/or energetic reasons more "charged" than the others, start to grow at the expenses of adjacent crystals at a lesser temperature than the others, thus more quickly reaching the critical dimensions permitting their predominance in growing. However, the production process of such steel strips comprises a number of high-temperature treatments, during some of which a grain growth could start which, should it occur with wrong modalities and timing, will prevent to reach the wanted final results. The secondary recrystallization is controlled by some compounds, such as manganese sulfide, manganese selenide, aluminum nitride and the like, which duly precipitated within the steel inhibit the grain growth up to a temperature, at which are solubilized, thus permitting the secondary recrystallization to start.

As the technological aspect is concerned, modern production of grain oriented silicon steel strips requires preparing a molten steel of controlled composition, with particular reference to the content of silicon, carbon, oxygen, manganese, sulfur, aluminum, nitrogen, and continuously casting it in slabs having a thichness usually comprised between 15 and 25 cm, a width of around a metre and a length of some metres . Such slabs are translated, at a temperature not lesser than 300 °C, and then reheated (possibly with a pre-rolling of no more than 25% at 1100-1200 °C) at high temperature, usually at 1300-1400 °C, hot rolled and the strip, if necessary annealed, cold rolled to the final thickness, usually comprised between 0.18 and 0.35 mm ■ and then subjected to a number of high-temperature final treatments, intended to drastically reduce the carbon content (decarburation annealing) , sulfur and nitrogen, to obtain the desired magnetic properties (secondary recrystallization annealing), to form on the strip surface insulating inorganic coatings, for instance magnesium phosphate and silica based. Each of the above steps is fundamental for the reaching of final characteristics of the product, and thus it must be carefully planned and controlled. For instance, the continuous casting requires a quick initial cooling of the molten steel in the mould, to allow a quick extraction of the slab comprising a solid skin, a soft intermediate mass and a quantity of liquid steel at the centre, which will solidify later. From such initial conditions some consequencies ensue requiring opportune careful control. In fact, the metal undergo two radically different cooling rates, a first very quick at the surface and then a second more slow at the core, and then solidifies in two different structures, at the surface in small equiaxic crystals and at the core in elongated much larger crystals, called columnar. This starting difference, if not amended, induces a non omogeneous structure in the final product, and a lesser quality.

Moreover, the relatively slow cooling rate of the slabs bulk brings both to an abnormal growth of the fraction of columnar crystals with respect to the fraction of the equaxic ones, and to the segregation of some elements as well as to the coagulation of some compounds , such as manganese sulfide, in large lumps not easily dissolved at the reheating temperatures, which then cannot be reprecipitated as finely dispersed particles, necessary to correctly perform as grain growth inhibitors.

Thus, since the very beginning of the production process, it is necessary to accurately control, for instance through a prerolling, a number of variables in order to avoid an excessive dimension difference of grains, and to obtain a sufficiently fine and homogeneous distribution of inhibitors. To obtain the above, the slabs are heated at high temperature, typically above 1330 °C, to dissolve the compounds precipitated during the slab cooling as large lumps, and to allow them to be more homogeneously diffused within the metal. The furnaces usually utilized to reach such a high_.heating temperature have a number of inconveniences , among which some very important are temperature differences found between surface and core of the slabs and the high overheating of the slab surface, necessary to let the core assume the desired temperature within an acceptable period of time, which factors induce an unwanted grain growth, as well as the formation of liquid slag on the slab surface, which requires specific furnaces difficult to manage, thus increasing production costs. During the hot rolling process, the metal undergoes a thickness reduction at such temperature and reduction rates to obtain acceptable grain dimensions and to precipitate in fine particles, due to the cooling, the above mentioned compounds, such as manganese sulfide. To obtain the desired grain dimensions, a pre-rolling is usually utilized, consisting in a first hot rolling pass carried out before the maximum heating temperature is reached; this obviously calls for higher costs, mainly due to the fact that slabs have to be extracted from the furnace , rolled and then put again in the furnace . It is easy to understand now how complex and costly is the production of a good grain oriented silicon steel strip, and hence how important is to utilize in the more efficient way any possible technique to reduce production costs.

A first step in this direction was trying to eliminate the prerolling step and to reduce the slab heating temperature before hot rolling; this last step being particularly costly, essentially due to the high temperature to be reached, the long treating time, the large dimensions of the slabs to be treated and the necessity to utilize specific furnaces, as already mentioned.

To this end, since the dissolution temperature of manganese sulfide in the steel is a function of a number of factors, among which the content of oxygen (and then the internal oxidation level of the steel), manganese and sulfur, by careful controlling such elements it is possible to reduce by many tens of degrees the slab heating temperature.

All the above, though not exhaustive, clearly shows the complexity of the electrical steel sheet production process, which complexity is further enhanced by the fact that different kinds of such electrical steels exist, mainly divided into the kinds (i) conventionally oriented grain having, for a sheet thickness of 0.30 mm, permeability higher than I.78 Tesla (T) and core losses (P17) lesser than 1.55 W/kg at 50 Hz, and (ii) super-oriented grain, with permeability higher than 1.88 T and core losses lower than 1.11 W/kg, each kind of product having its own specific process characteristics.

Thus, for instance, in the composition of conventional grain oriented sheets there is no aluminum, which is deemed to adversely affect the final magnetic properties in that it forms undesired oxide precipitates, while in the super-oriented kind aluminum is specifically utilized, in small percentage, essentially to combine with specific amounts of nitrogen thus forming aluminum nitride, utilized as grain growth inhibitor at temperatures higher than solubilization temperature of manganese sulfide; thus two seemingly similar compositions, being in effect very low the contents of aluminum, nitrogen and axigen, bring to products having sharply different characteristics. STATE OF THE ART

Answering to the above reminded high complexity of the process, further enhanced because some mechanisms are not yet fully understood, many different solutions were proposed, concerning different aspects of the process.

Belgian patent 792.173 refers to the continuous casting of grain oriented electrical steel slabs, in which a steel is cast comprising, in wt % , C 0.025-0.060, Mn > 0.035, S > 0.010, Si 2.0-4.0, acid soluble Al < 0.005, remainig being iron and minor impurities, with a Si0₂/Al₂0 ratio of less than 1.1. French patent 2.158.458 refers to a steel comprising, in wt Ji, C 0.02- 0.05, Mn 0.04-1.12, S 0.015-0.035. Si 2-4, N < 0.01, Al < 0.04, which, after being continuously cast, is heated at a temperature higher than 750 °C but lesser than the grain growth one, hor rolled with at least a 5% reduction, heated at 1350-1400 °C, hot rolled, cold rolled to the final thickness, decarburized and annealed for final grain growth. Published german patent application DE 4.311-1 1 refers to a steel comprising, in wt % , C 0.02-0.10, Si 2.5~5, Mn 0.04-0.15, S 0.010, Al 0.010-0.035, N 0.0045-0.0120, Cu 0.020-0.300, remaining being essentially iron; slabs of this steel are heated at a temperature insufficient to dissolve manganese sulfide but sufficient to dissolve copper sulfide; the slabs are then hot rolled with an end-rolling temperature between 880 and 1000 °C to a thickness of between 1.5 and 7 mm, the strip so obtained is annealed at 880-1150 °C for 100-600 s and then cooled at a rate of 15 °K/s. The secondary recrystallization mechanism is thus controlled by finely precipitated copper sulfide. In JP 04 301 035-A and 05 2 442-A the cooling rate after the last hot-rolling stand is controlled.

In JP 04 289 121-A, the strip is reduced by O.5-I5 % before cold rolling in a rolling stand whose rolls have a diametre of 0 times the strip thickness, and then annealed at 700-1100 °C. In EP-393508 a process is described referring to a silicon steel comprising, in wt % , C 0.021-0.100, Si 2.5~4.5_> one or more elements inhibiting the grain growt such as Al, N, Mn, S, Se, Sb, B, Cu, Bi, Nb, Cr, Sn, Ti . The hot rolled strip is coiled at a temperature comprised between 00 and 700 °C and the coil, of a weight comprised between 5 and 20 t, is cooled in air or preferably in water. The usual cold rolling and annealing follow.

In JP 02 133 5-A, the hot rolling ends at a temperature of at least 900 °C and the strip is cooled at a rate of at least 40 °C/s and coiled at a temperature of between 300 and 500 °C. JP 61 186456 discloses a steel comprising, in wt % , C 0.01-0.06, Si 3.1-4.5, Mn 0.01-0.2, Mo 0.003-0.1, Sb 0.005-0.2, S and/or Se 0.005- 0.1, and at least one between Cr 0.01-0.03, Cu 0.01-0.5, Sn 0.005-0.2. JP 61 79722-A discloses a steel comprising, in wt % , C < O.O85, Si 2- 4, Mn 0.03-0.1, Al_s 0.01-0.05, and moreover Sn O.O3-O.5 and Cu 0.02- 0.3; it is specified that Sn helps reducing the grain dimensions in the secondary recrystallization while Cu enhances adhesion of final glass coatings; moreover, both of said elements act as grain growth inhibitors . In BE 89403 steel is disclosed comprising Sn and Cu in a ratio of between 0.5=1 and 1:1. The hot rolled strip is precipitation annealed at 900-1250 °C for 0.5-30 min and then quickly cooled to precipitate A1N. BE 89 038 discloses a treating process for a silicon steel comprising, in wt % , Cu 0.02-0.2, in which the entering temperature of the strip in the last hot-rolling stand is comprised between 1100-1250 °C, while the exit temperature is of 900-1050 °C for the upper part of the strip and of 950-1100 °C for the middle and lower parts.

JP 01 309924-A discloses a treating process for a silicon steel in which a slab is heated at a maximum temperature of 1270 °C, hot rolled at an exit temperature of 700-900 °C and coiled at less than 600 °C. JP 02 101120-A disclosed a process allowing to eliminate the precipitation annealing, yet permitting to obtain excellent magnetic characteristics. The process comprises finishing the hot rolling at a temperature higher than 900 °C, with the temperature at the beginning and at the end of the strip within 10 % of its entire length, higher by 50-200 °C than the one of the remaining of the strip, which is coiled at a temperature of more than 700 °C, held at this temperature for 5^_6θ min and then water cooled. DESCRIPTION OF THE INVENTION

According to the present invention, it is necessary to carefully choose the steel composition and, depending on this composition, to control the correct realization of some fundamental steps of the production process.

As far as the composition is concerned, it is necessary to keep a relatively low initial content of carbon and acid soluble aluminum (Al_s), respectively lower than 400 and 200 ppm, preferably between 200 and 350 ppm for C amd between 30 and 100 ppm for Al_s> The silicon is controlled within weigth percentages between 2.5 and 4.5, particularly between 2.9 and 3-3- To this composition are added from 0.030 and 0.300 % Sn and from 0.100 and O.35O Cu. As grain growth inhibiting elements can be utilized, together to or instead of elements such as Al, elements chosen between Nb and/or Ti and/or V, in total percentages comprised between 0.01 and 0. 3% . The experience did show that, according to present invention, it is possible to reduce by 1—10% the thickness of the slab as it leaves the continuous casting mould, when it is not completely solidified; particularly, said reduction can be carried out when solidification of 20-40 of the steel did occur if the sum of Sn and Cu percentages is lower than 0.30 , and when 4θ-8θ% of steel did already solidify if said sum is higher than 0.30%.

The steel slabs thus obtained are heated at a temperature lesser than 1300 °C, preferably between 1200 and 1290 °C, and then hot rolled, the entering temperature into the finishing stand being comprised between 1050 and 1200 °C and the exit temperature being comprised between 900 and 1059 °C, preferably between 950 and 1000 °C. During this step, the thermal homogeinity of the steel bar must also be carefully controlled, ensuring that the entrance temperature into the finishing stand is kept within 30 °C. The cooling of the strip is started ^_ 0 s after the exit from the finishing stand, and is preferably carried out in water; the strip is then coiled at a temperature of beteen 00 and 800 °C, preferably lesser than 56O °C.

The thus obtained hot rolled strip undergoes the usual treatments before being cold rolled in at least two rolling steps; the last cold rolling pass must have a reduction rate of 45^_70 if the Al content is lesser than 80 ppm, and of 60-80% if the Al content is higher than 8θ ppm. The cold rolled strip undergoes the usual final treatments of decarburization, secondary recrystallization and formation of an inorganic insulating coating, keeping m mind that during the decarburization annealing the heating rate must be higher than 10 °C/s, preferably between 1 and 20 °C/s.

The present invention will be now illustrated with some examples which are not intended to limit the scope of the invention itself. EXAMPLE 1

The following steel composition, expressed n wt % if not otherwise stated, were prepared:

ELEMENT A B C D E

C 280 ppm 300 ppm 340 ppm 300 ppm 320 ppm

Si 3.10 3-30 3-10 3.25 3.19

Al 50 ppm 65 ppm 70 ppm 20 ppm 30 ppm N N 6 622 ppppmm 511 PpPp^mm 57 PPm 30 ppm 30 ppm

Mn 570 ppm 600 ppm 58θ ppm 750 ppm 720 ppm

S 190 ppm 200 ppm 220 ppm 280 ppm 250 ppm

Cu 0.10 0.29 0.18 0.13 0.17

Sn 0.020 0.018 0.152 0.020 0.030 The remaining being iron and minor impurities.

Composition A is according to the invention but for the Cu and Sn content sum; compositions B and C are according to the invention; compositions D and E are known in the literature. All those steels did undergo the following transformation cycle: A- CASTING

Steels A, B, C and D were continuously cast and the slabs were reduced by 8% (from 222 to 202 mm) during the secondary cooling outside of the casting mould when their cores not fully solidified, with a solidification percentage of between 50 and 60 % .

The extracted slabs were cut and sent to the hot rolling plant before which were heated at 1260 °C. B- HOT ROLLING

Hot rolling was performed in a number of passes, keeping the steel temperature at the entrance of the finishing stand between 1090 and

1100 ° C and the temperature at the exit from the finishing stand between 98O and 1000 A. After the exit from the finishing stand the forced cooling of the strip was delayed by 8 s . The final thickness of the hot rolled strip was 2.1 mm .

The hot rolled strip was then annealed at 1000 °C for 1 min.

C- COLD ROLLING Part of the above strips were cold rolled up to an intermediate thickness of 1 mm, the other strips were rolled at O.76 mm.

All those intermediate strips were annealed at 9δ0 °C for 1 min and then cold rolled to a final thichness of 0.30 mm for the intermediate thickess of 1 mm while for the others the final thickness was 0.23 mm. All the strips were then treated according to a same cycle comprising a decarburization annealing, a coating with MgO and a box-annealing to obtain the so called glass film, a thermal flattening and a final coating with an insulating tensioning film.

The steel E did undergo a fully traditional transformation cycle well known to the experts.

The obtained magnetic characteristics are now reported, expressed as core losses, in W/kg at an induction of 1.5 and 1.7 T (PI.5 and PI.7 W/kg) at 50 Hz, and as permeability in a field of 800 As/m (BβOO) in T.

THICKNESS 0.30 mm

MAGNETIC A B C D CHARACTERISTICS

P 1.5 (W/kg) 0.91 0.83 0.84 ι.4o 0.85

P 1.7 (W/kg) 1.32 1.20 1.21 2.15 1.23

^B800 W 1.806 1.850 1.855 1.606 ι.84o

THICKNESS 0.23 mm

MAGNETIC A B C D CHARACTERISTICS

P 1.5 (W/kg) 0.80 0.68 O.69 1.60 0.71 P 1.7 (W/kg) 1.25 1.06 1.07 2.33 1.13

^B800 <^τ) 1.795 1.860 I.855 1.571 1.830 As t can be seen from the above results, compositions B and C, treated according to the invention, show magnetic characteristics similar to those of composition E, which is traditional and was treated according to the conventional method, while composition A, m which the sum of Sn and Cu contents is out of present invention, and composition D, which is conventional, both treated according to the invention, do not reach satisfactory results. EXAMPLE 2

In this example the effect of delaying forced cooling after hot rolling s verifyed.

The following composition, expressed in wt % unless otherwhise specified, was prepared: C 290 ppm, Si 3-30, Al 70 ppm, N 51 ppm, Mn 0.058%, S 210 ppm, Cu 0.28, Sn 0.030, remaining being iron and minor impurities . This composition was cast according to Example 1, which was followed also for slab heating and hot rolling. At the exit from the finishing stand the cooling was started after:

(i) 0 s; (ii) 5 s; (iii) 7 s; (iv) 13 s.

The transformation cycle according to the invention was then completed, but for some strips which did not undergo annealing after cooling.

The magnetic characteristics obtained on cold rolled strip 0.30 mm thick, were the following:

CYCLE WITH ANNEALING OF HOT ROLLED STRIP

MAGNETIC (i) (ϋ) (iϋ) (iv)

CHARACTERISTICS

P 1.5 (W/kg) 0.89 0.85 0.83 0.91 p 1.7 (w/kg) 1.28 1.21 1.20 1.31

^B800 (^τ) 1.840 1.850 1.855 1.805

CYCLE WITHOUT ANNEALING OF HOT ROLLED STRIP

MAGNETIC (i) (ϋ) (iii) (iv)

CHARACTERISTICS

P 1.5 (W/kg) O.96 0.90 0.87 0.95

P 1.7 (W/kg) 1.33 1.29 1.27 1.32

^B800 (^T) 1.770 1.830 1.840 1.803

As it can be seen, with a delayed cooling according to present invention, (ii) and (iii), good magnetic characteristiscs are obtained, while with insufficient, (i) , or excessive, (iv), delays the magnetic characteristics are rather lesser.

Moreover, even omitting the annealing of the hot rolled and cooled strip, farly acceptable magnetic characteristics can be obtained according to the invention, while in the other cases indufficient results are obtained.

EXAMPLE 3

In this example the effect of the aluminum content and of the cold rolling rates on the final properties of the product was investigated.

A steel was used similar to composition B of Example 1, but with an Al content of 85 ppm.

The transformation cycle was according to the invention, but with the following final cold rolling reduction rates, in % , to obtain a final thickness of 0.23 mm-

% REDUCTION RATE

MAGNETIC 57 65 70 83

CHARACTERISTICS

P 1.5 (W/kg) 0.82 0.69 0.66 1.20 P P 11..77 ((WW//kkgg)) 1 1..2211 1.10 1.07 1.45

^B800 (T) 1.805 1.850 1.860 1.690 As it is apparent, the reduction rates according to present invention, which are a function of Al content, allow to obtain good magnetic characteristics, in line with those of the best products on the market .

Further experiments, exploring the above mentioned composition and process intervals according to the invention gave results comparabvle to those already reported.

Claims

CLAIMS 1. A process for the production of oriented grain electrical steel strips, in which a steel of desired composition is melt, and continuously cast in slabs which are heated at elevated temperature and then sent to the hot rolling plant, hot rolled to a strip of desired thickness which is coiled and then cold rolled to the final thickness, the cold rolled strip thus produced being subjected to final treatmens, such as decarburization annealing and secondary recrystallization annealing, characterized by the combination in cooperation relationship of the following steps: a) chosing a steel composition comprising, in wt JS, C < 0.θ4θ, Al < 0.020, Si 2.5-4.5, Sn 0.03-0.30, Cu 0.1-0.35, S 0.010-0.025, Mn 0.030- O.OδO remaining being iron and minor impurities; b) continuously casting said steel and subjecting the obtained slab, during the secondary cooling and while still not completely solidified, to a thickness reduction comprised between 1 and 10 %; c) heating such slab at a temperature lesser than 1300 °C; d) hot rolling the heated slabs, keeping at the entrance into the finishing stand a temperature comprised between 1050 and 1200 °C, held within 30 °C for the entire length of the strip, while at the exit from said finishing stand the strip temperature id maintained between 900 and 1050 °C; e) cooling the strip leaving the finishing stand to a coliling temperature comprised between 500 and 700 ^CC, starting said cooling after 5^_10 s after the strip leaves the finishing stand; f) cold rolling the hot rolled strip to the final thickness in at least two rolling steps; g) performing the usual finishing treatments, with a heating rate of the strip during the decarburization annealing of at least 10 °C/s. 2. A process according to claim 1, characterized in that the steel has a carbon content of between 0.0200 and 0.0350 wt %. 3- A process according to claim 1, characterized in that the steel has an acid soluble Al content of between 0.0030 and 0.0100 wt %. 4. A process according to claim 1, characterized in that the slab thickness reduction at the exit from the continuous casting mould is performed when 20-40 wt % of the steel already solidified if the sum of Cu and Sn percentages is lesser than 0.30 % while it is performed when 4θ-8θ wt % of steel already solidified if said sum is higher than 0.30 % . 5- A process according to claim 1, characterized in that the slabs are heated at a temperature comprised between 1200 and 1290 °C. 6. A process according to claim 1, characterized in that the strip coming out from the hot rolling finishing stand has a temperature comprised between 950 and 1000 °C. 7- A process according to claim 1, characterized in that the coiling temperature of the hot rolled strip is comprised between 500 and 60 °C 8. A process according to claim 1, characterized in that in the last cold rolling step the strip reduction rate is between 45 and 70 % if the Al content is lesser than 80 ppm and is between 60 and 80 % if the Al content is higher than 80 ppm. 9- A process according to clain 1, characterized in that the heating rate during the decarburization annealing is comprised between 15 and 20 °C/s.