Title of Invention

A METHOD FOR PRODUCING A STEEL STRIP WITH COLD-ROLLED PROPERTIES AND A MACHINE FOR THE SAME

Abstract

(54): " A METHOD FOR PRODUCING A STEEL STRIP WITH COLD-ROLLED PROPERTIES AND A MACHINE FOR THE SAME". (57) Abstract: The present invention relates a method and a machine for producing a steel strip with cold-rolled properties comprising the sequential steps of: (a) producing a thin slab 30 to 100 mm thick from a steel melt by continuous casting in a continuous casting machine, and after a cast strip emerges from a mold of the continuous casting machine, cast rolling the cast strip with a liquid core to reduce thickness of the cast strip by atleast 10% (b) descaling the thin slab produced in step a); (c) hot rolling the descaled thin slab at temperatures ion a range of 1150° to 900 tfor reducing thickness at least 50% to produce an intermediate strip with a maximum thickness of 20mm; (d) after hot rolling, accelerated cooling of the intermediate strip to a temperature in a range of 850°to600°C- (e) rolling down the cooled intermediate strip by isothermic rolling at 850° to 600°£- on a finishing train with at least three stands into strips with a maximum thickness of 2mm, whereby the strip thickness is reduced by at least 25% per roll pass; and (f) subsequently cooling the isothermic rolled strip in accelerated fashion to a temperature less than 100 Degreee C and preferably coiled as a finished strip.

Full Text

The present invention relates to a method and a machine for producing a steel strip with cold-rolled properties. European Patent Number 541 574 Bl discloses a method of the type in question in which finished strip with cold-rolled properties is produced directly in a hot rolling train from a feed material resulting from near net shape casting. A continuous thin slab of a maximum thickness of 100mm is firstly produced in a continuous casting plant, with the continuous casting mould being followed directly by a rolling device at which the continuous casting with a liquid and solid core is rolled to the solidification thickness (direct strand reduction). The continuous thin slab is then descaled and hot-rolled to a thickness of 10-30 mm at a temperature above 1100°C at a rolling device with, for example, three stands. The intermediate strip which is hot- rolled in this way is divided into sub-lengths by means of strip shears. These sub-lengths are preferably wound into coils, subsequently unwound again for additional hot rolling and descaled again if required. The strip-like material is reheated to a hot rolling temperature exceeding 1100°C by inductive hearing before the additional hot rolling operation, preferably also before being coiled. The second hot rolling operation is carried out at a temperature exceeding Ar3. This is followed immediately by cooling to a temperature below Ar3, preferably to a temperature ranging from 250 to 600°C. The strip material which is thus produced is then finish-rolled by cold rolling at one or more stands disposed in tandem and wound into coils. The aim of the known method is to produce cold-rolled strip with the minimum expenditure of energy. The near net shape casting method (thin slab production) and the direct strand reduction method, i.e. reducing the thickness of the hot continuous casting with a core which is still partly liquid, are on the one hand used for this purpose. Hot rolling is on the other hand carried out in part with the heat remaining from the continuous casting process. The disadvantage in this case lies in the fact that, in spite of the utilisation of the heat from the continuous casting process, inductive heating of the strip-like intermediate product must be provided for the second part of the hot rolling operation. The object of the invention is to provide a method wherein separate reheating of the strip-like intermediate product and the expenditure of energy and on equipment which this entails are avoided, and a plant for carrying out this method. A further aim is to improve the properties of the produced material in the direction of cold-rolled properties. This object is solved in a method of the type according to the preamble by the characterising features of claim 1. Advantageous developments are contained in subclaims 2 to 14. A plant according to the invention for carrying out this method has the features of claim 15 and can be further developed in an advantageous manner by'the characterising features of subclaims 16 to 25. Unlike the method known from EP 0 541 574 Bl, the invention provides just one continuous hot rolling operation and thus dispenses with a second hot rolling operation and the inductive intermediate heating which this requires. According to the invention the hot rolling instead takes place in a single pass, which is followed by accelerated cooling to a temperature ranging from 850 to 600°C. The finished steel strip is then produced at this temperature by isothermal rolling in at least three reduction passes, in each of which the thickness is reduced by at least 35%, and after this finish rolling operation is cooled at an accelerated rate to a maximum temperature of just 100°C. In contrast, the finish -rolling of the known method is carried out at a temperature which is by comparison distinctly lower (approximately 250 to 600°C) . During the isothermal rolling according to the invention the temperature of the steel strip does not remain constant in the strict sense, instead varying within a relatively narrow toleration range (e.g. T = 0 to 20°C). Measures must be taken during the isothermal rolling operation to ensure that the temperature never falls below a critical value and that the inevitable heat loss through radiation is at least balanced by the deformation energy introduced into the steel strip. The method should advantageously be conducted such that the heat input through specially introduced deformation ..- work („speed up") always exceeds the expected heat loss through radiation and that the temperature is regulated by way of specific cooling between the reduction passes. For once the actual temperature of the steel strip has fallen below a critical value during the rolling process, it is hardly practically possible to reach the desired value again by changing the rolling parameters. Accordingly the present invention provides a method for producing a steel strip with cold-rolled properties comprising the sequential steps of: a) producing a thin slab 30 to 100 mm thick from a steel melt by continuous casting in a continuous casting machine, and, after a cast strip emerges from a mold of the continuous casting machine, cast rolling the cast strip with a liquid core to reduce thickness of the cast srtrip by atleast 10%; b) descaling the thin slab produced in step a); c) hot rolling the descaled thin slab at temperatures in a range of 1150° to 900°C for reducing thickness by at least 50% to produce an intermediate strip with a maximum thickness of 20mm; d) after hot rolling, accelerated cooling of the intermediate strip to a temperature in a range of 850° to 600°C; e) rolling down the cooled intermediate strip by isothermic rolling at 850° to 600°C on a finishing train with at least three stands into strips with a maximum thickness of 2 mm, whereby the strip thickness is reduced by at least 25% per roll pass; and f) subsequently cooling the isothermic rolled strip in accelerated fashion to a temperature less than 100°C and preferably coiled as a finished strip. The present invention also provides a machine for producing steel strip with cold-rolled properties by the method as herein above described, said machine comprising: a continuous casting device with a mold for producing thin slabs; a cast rolling device located immediately behind, in a strip producing direction, the mold of the continuous casting device; a first descaling device located behind the cast-rolling device; a hot rolling device, which comprises one of at least two stands and one reversing stand, connected to the descaling device, for producing intermediate strip; first cooling means arranged behind the hot rolling device for accelerated cooling of the intermediate strip produced in the hot rolling device; rolling means arranged behind the first cooling means, the rolling means having at least three roll stands for isothermic rolling of the cooled intermediate strip; and second cooling means immediately behind the rolling means for accelerated cooling of a steel strip produced by the rolling means. The invention is explained in detail in following on the basis of the plant diagram shown in the accompanying drawing. A tundish 11 is filled with a steel melt, preferably a deep drawing steel melt, from a ladle 10. The tundish 11 lets the contained steel melt flow in a continuous stream into a continuous casting mould 12 which is disposed below, provided with liquid cooling (not shown) and results in the formation of a continuous casting consisting of a strand shell and a liquid core. In this state the hot continuous casting enters a direct strand reduction device which in disposed below the continuous casting mould 12 and further reduces the thickness of the continuous casting with the partly liquid core. As a result the continuous thin slab l emerges from the direct strand reduction device 13 with a thickness of 30 to 100 mm, preferably 40 to 70 mm. The thickness is reduced by at least 10%, preferably at least 30%, during direct strand reduction. The strand then enters a descaling device 19, which is preferably formed as a hydromechanical descaling device. Following descaling the continuous thin slab 1 is at a temperature ranging from 1150 to 900°C. In this state the continuous thin slab 1 is delivered to a hot rolling device 15 which is disposed immediately after the descaling device 13 and in which the thickness of the continuous thin slab 1 is reduced by at least 50% to produce an intermediate strip of a maximum thickness of 2 0 mm, preferably 10 to 20 mm. In some cases it may also be advantageous to provide a soaking pit (not shown) immediately before the hot rolling device IS to keep the continuous L'nin clab 1, %'lvich 'is advantageously divided into sub-lengths, at the desired hot rolling temperature. It is normally advisable to insert a dividing unit, e.g. in the form of strip shears 17, after the hot rolling device 15, which advantageously comprises two or more stands or even also a reversing mill, in order to divide the intermediate strip which is produced into the above-mentioned sub-lengths. The hot-rolled intermediate strip is cooled at an accelerated rate according to the invention to a temperature ranging from 850 to 600°C. The cooling temperature which it is advisable to select in each case ;depends on the chemical composition of the steel which is used as well as the desired structural composition and the mechanical-technological properties which are to be achieved in the finished strip. T'ooling takes place in a first cooling device 18, which in the illustrated diagram directly adjoins the strip shears 17. For reasons regarding space it is in many cases advisable for the sub-pieces of the intermediate strip, which are at the temperature desired for subsequent finish rolling, to be wound up in a winding device 20 to form intermediate strip coils and for these to be maintained at the desired temperature in a soaking pit 21. The intermediate atrip is unwound again at an unwinding device 22, which is disposed immediately after this soaking pit 21, in order to carry out the subsequent finish rolling- It is advisable to descale again in a descaling device 23 before finish rolling in order to prevent quality impairments due to scale which may have meanwhile formed. A rolling device 24 comprising at least three stands is provided for finish rolling, which is carried out in the form of isothermal rolling in the temperature range from 600 to 850°C. In many cases it is advisable to use a rolling device with four or a maximum of five stands. A greater number of finish rolling stands is nov generally pr^ctic^.i. The stands are operated such that the strip thickness is reduced by at least 25% per reduction pass. The finished strip leaving the rolling device has a maximum thickness of 2 mm, preferably a thickness of 0.5 to 1.5 mm. In order to ensure that (approximately) isothermal rolling conditions exist, it is advisable to provide cooling devices (not shown), e.g. in the form of spray cooling devices, which remove excess heat in a controlled manner between the individual roll stands of the rolling device 24. The actual temperature of the steel strip in the rolling device 24 is monitored by temperature sensors (not shown). The steel strip emerging from the rolling device 24 is cooled immediately afterwards at an accelerated rate in a second cooling device 25 to a temperature of at most 100°C. The accelerated coolifig is advantageously carried out at a cooling rate ranging from 1 to 2 5°C/s. For this purpose the finished strip may, for example, be conveyed through a liquid cooling bath. It is, however, also possible to use spray cooling devices in a manner known per se along the roller table with the smallest possible roller spacings of less than 250 mm. The finished strip which is thus produced should advantageously be wound up for transport in the form of coils. A suitable winding device 26 is provided for this purpose in the plant diagram. The advantages of forming intermediate strip coils between the hot rolling device 15 and the rolling device 24 lie in the one hand in the creation of an intermediate material buffer, which enables the rolling devices to be operated with less susceptibility to disturbances, and on the other in the fact that the soaking pit 21 required to maintain the temperature of this buffer material only occupies a comparatively small area. Example of the method the remainder iron and usual impurities (Tliq = 1520°C) was cast in a continuous thin slab casting plant at a temperature of approximately 1540°C. Upon emerging from the continuous casting mould the continuous casting, which was 80 mm thick and 1300 mm wide, still had a liquid core. The mean temprature of this continuous- casting at the mould outlet was approximately 1310°C. The continuous thin slab was then introduced in this state into a direct strand reduction device and its thickness reduced by 25%, so as to produce a solidification thickness of 60 mm. Following descaling by means of a high-pressure water jet, the thickness of the continuous thin slab was reduced at a three-stand hot rolling train by approximately 66%, thus producing an intermediate strip of a thickness of 20 mm. The temperature upon entering the hot rolling train was 1130DC and upon leaving 938°C. Immediately afterwards this intermediate strip was divided into sub-pieces and cooled at an accelerated rate to a temperature of approximately 700°C. After passing through a soaking pit, which was also operated at 700°C, and after descaling, the intermediate strip coils produced from the sub-lengths were delivered to the finish rolling train. This had a total of five stands, which were operated to reduce the thickness by 95% overall. Upon leaving the first stand, to which it was delivered at 650°C, the intermediate strip had a slightly raised temperature of 658°C, which was reduced to approximately 650°C again by a spray cooling device disposed before the second roll stand. The exit temperature of 664°C at the second roll stand was reduced in a corresponding manner before the third roll stand by another spray cooling device to an entry temperature of 650°C for the third roll stand. The same applies to the fourth and fifth stands. Immediately afterwards the l.0 mm thick finished strip which was thus produced was cooled in a water cooling bath at a cooling rate of 21°C/s to approximately 90?C and then wound into finished coils. The finished strip which was thus produced exhibited excellent mechanical-technological properties, which were comparable wit:h those of a cold-rolled strip. ~ The production mode according to the invention results in a particularly fine-grained crystalline structure, which is clearly more favourable than the result according to the method known from EP 0 541 574 Bl. The reheating to 1100°C before the second hot rolling operation which is carried out in the known method gives rise to a distinct grain coarsening process, which cannot occur in the method according to the invention on account of the selected temperature range of 850 to 600°C. A further difference with respect to the grain size formation is due to the different type of finish rolling. In the method according to the invention an additional dynamic grain refinement with a simultaneous increase in strength and ductility takes place with the prescribed total forming degree, which clearly exceeds 90%, during the isothermal rolling, which is carried out at temperatures lying at the recrystallisation threshold. The occurrence of this refinement is not nearly as marked in the known method on account of the distinctly lower degress of forming in the individual reduction passes The high strength values which can be achieved in the known method through the strain hardening process are also possible according to the method of the invention through an appropriately adapted rolling cycle and are, moreover, accompanied by distinctly better ductility properties. To summarise, it can therefore be said that steel strip which is produced by the method according to the invention is distinguished by the fact that very high strength values can at the same time be combined with extremely favourable deformation and ductility properties. WE CLAIM: 1. A method for producing a steel strip with cold-rolled properties comprising the sequential steps of: a) producing a thin slab 30 to 100 mm thick from a steel melt by continuous casting in a continuous casting machine, and, after a cast strip emerges from a mold of the continuous casting machine, cast rolling the cast strip with a liquid core to reduce thickness of the cast srtrip by atleast 10%; b) descaling the thin slab produced in step a); c) hot rolling the descaled thin slab at temperatures in a range of 1150° to 900°C for reducing thickness by at least 50% to produce an intermediate strip with a maximum thickness of 20mm; d) after hot rolling, accelerated cooling of the intermediate strip to a temperature in a range of 850° to 600°C; e) rolling down the cooled intermediate strip by isothermic rolling at 850° to 600°C on a finishing train with at least three stands into strips with a maximum thickness of 2 mm, whereby the strip thickness is reduced by at least 25% per roll pass; and f) subsequently cooling the isothermic rolled strip in accelerated fashion to a temperature less than 100°C and preferably coiled as a finished strip. 2. The method according to claim 1, wherein the continuous thin slab is produced with a solidification thickness of 40 - 70 mm. 3. The method according to claim I or 2, wherein the continuous thin slab is reduced during direct strand reduction by at least 20% preferably 30%. 4. The method according to one of claims 1 to 3, wherein the temperature of the continuous thin slab is maintained in a soaking pit before hot rolling. 5. The method according to one of claims 1 to 4, wherein the intermediate strip is produced with a thickness of 10 - 20 mm. 6. The method according to one of claims 1 to 5, wherein the intermediate strip is divided into sub-lengths and wound into coils after step d). 7. The method according to one of claims 1 to 6, wherein the intermediate strip which is cooled in step d) is maintained at the cooling temperature in a soaking pit before carrying out isothermal rolling. 8. The method according to one of claims 1 to 7, wherein the isothermal rolling is carried out in four or five passes. 9. The method according to one of claims 1 to 8, wherein the steel strip is isothermally rolled to a thickness of 0.5- 1.5 mm. 10. The method according to one of claims 1 to 9, wherein the final cooling of the steel strip is carried out at a cooling rate ranging from 10 to 25°C/S. 11. The method according to one of claims 1 to 10, wherein the intermediate strip is descaled again immediately before isothermal rolling. 12. The method according to one of claims 1 to 11, wherein the descaling is in each case carried out by hydromechanical means. 13. The method according to one of claims 1 to 12, wherein the temperature of the intermediate strip is regulated between the individual reduction passes during isothermal rolling by cooling, in preferably by spray cooling. 14. The method according to one of claims 1 to 13, wherein the melt used in step a) is of a deep drawing quality steel; 15. A machine for producing steel strip with cold-rolled properties by the method as claimed in claim 1 to 14, said machine comprising: a continuous casting device with a mold for producing thin slabs; a cast rolling device located immediately behind, in a strip producing direction, the mold of the continuous casting device; a first descaling device located behind the cast-rolling device; a hot rolling device, which comprises one of at least two stands and one reversing stand, connected to the descaling device, for producing intermediate strip; first cooling means arranged behind the hot rolling device for accelerated cooling of the intermediate strip produced in the hot rolling device; rolling means arranged behind the first cooling means, the rolling means having at least three roll stands for isothermic rolling of the cooled intermediate strip; and second cooling means immediately behind the rolling means for accelerated cooling of a steel strip produced by the rolling means. 16. The machine according to claim IS, wherein the descaling device (19) is a hydromechanical descaling device. 17. The machine according to claims 15 or 16, wherein a soaking pit is disposed between the direct strand reduction device (13) and the hot rolling device (15). 18. The machine according to claims 15 to 17, wherein a dividing unit (17) for dividing the hot-rolled intermediate strip into sub-lengths is disposed behind the hot rolling device (15). 19. The machine according to claim 18, wherein a winding device (20) and an unwinding device (22) for intermediate strip coils are disposed behind the first cooling device (18). 20. The machine according to claims 18 or 19, wherein the soaking pit (21) is disposed between the first cooling device (18) and the isothermal rolling device (24) for maintaining the temperature of the sub-lengths of the intermediate strip. 21. The machine according to claims 18 or 19, wherein the soaking pit (21) is disposed between the winding device (20) and the unwinding device (22) for maintaining the temperature of the sub-lengths of the intermediate strip. 22. The machine according to claims 15 to 20, wherein a descaling device (23) is disposed immediately before the isothermal rolling device (24). 23. The machine according to claims 15 to 22, wherein the hot rolling device (15) comprises three stands. 24. The machine according to claims 15 to 23, wherein the isothermal rolling device (24) comprises four or five stands. 25. The machine according to claims 15 to 24, wherein a cooling device (26) for winding up the finished strip is disposed after the second cooling device (25). 26. A method for producing a steel strip with properties of a cold- rolled product, substantially hereinabove described with reference to the accompanying drawings. 27. A machine for producing a steel strip with properties of a cold- rolled product, substantially hereinabove described with the accompanying drawings.

Documents:

1251-mas-1995 abstract.jpg

1251-mas-1995 abstract.pdf

1251-mas-1995 claims.pdf

1251-mas-1995 correspondence others.pdf

1251-mas-1995 correspondence po.pdf

1251-mas-1995 description (complete).pdf

1251-mas-1995 drawings.pdf

1251-mas-1995 form-1.pdf

1251-mas-1995 form-26.pdf

1251-mas-1995 form-4.pdf

1251-mas-1995 form-9.pdf

1251-mas-1995 petition.pdf