KR100373793B1 - Process and device for producing a steel strip with the properties of a cold-rolled product - Google Patents

Abstract

The present invention relates to a method for producing a cold-rolled strip, comprising the steps of: a) producing a thin slab having a thickness (coagulation thickness) of 30 mm to 10 mm from molten steel by continuous casting, b) C) preparing intermediate strips having a maximum thickness of 20 mm by reducing the thickness by at least 50% by hot rolling the descaled thin slab at a temperature in the range of 1150 DEG C to 900 DEG C D) quenching the semi-finished strip obtained after hot rolling, e) cooling the semi finished strip to a finish finishing train with at least three stands at a temperature between 850 ° C and 600 ° C, F) quenching the isothermal rolled strip to a temperature of less than or equal to 100 캜, and winding it, preferably as a finished strip, It provides a way to. The invention also relates to an apparatus for carrying out the method.

Description

TECHNICAL FIELD [0001] The present invention relates to a method of manufacturing a steel sheet having characteristics of a cold rolled product,

According to the process disclosed in EP 0541 574 B1, finished strips with the characteristics of cold-rolled products are produced directly on the hot-rolling train, using the material cast close to the final dimensions . In the continuous casting apparatus of this process, a continuous thin slab having a thickness of at most 100 mm is first prepared, in which a cast strip including a liquid core and a solid phase core is rolled to a solidification thickness in a rolling apparatus arranged immediately after the continuous casting mold (Casting rolling). The continuous thin slabs are then scaled and hot-rolled to a thickness of 10 mm to 30 mm in a rolling apparatus consisting, for example, of three stages at a temperature of 1100 ° C or higher. The intermediate strips thus hot rolled are divided into partial lengths by strip shears. This strip, cut to partial length, is wound primarily in coils, then loosened again to be able to continue hot rolling later, and scaled back if necessary. Before stripping again, preferably before winding the strip, the strip-shaped material is again heated to a hot rolling temperature of at least 1100 DEG C by induction heating. The second hot rolling is performed at a temperature of Ar ₃ or higher. Immediately after this process the strip is cooled to a temperature below Ar ₃ , preferably in the range of 600 ° C to 250 ° C. The strip thus produced is subsequently finishing rolled and cold rolled in one or several sequential stands to form a coil.

The known method aims at producing a cold-rolled strip while consuming as little energy as possible. For this purpose, a method of casting close to the final dimension (thin slab manufacturing) and a method of casting rolling are used. That is, the thickness is reduced while the hot cast steel sheet still partially has the liquid core. In addition, hot rolling is partially performed using the remaining heat in the continuous casting process. Here, despite the use of heat from continuous casting, the disadvantage is that the strip intermediate product must be heated in an induction manner for the second part of the hot rolling.

The present invention relates to a method for manufacturing a steel sheet having the characteristics of a cold rolled product according to the preamble of claim 1 and an apparatus for carrying out the method.

1 is a schematic view of an apparatus according to the present invention.

It is an object of the present invention to provide a method of separately reheating a strip-shaped intermediate product and avoiding associated energy and equipment costs, and an apparatus for practicing the method. In addition, the properties of the material produced are improved as much as possible to match the properties of the cold rolled product.

This object is achieved by a method according to the features of claim 1. Are disclosed in dependent claims 2 to 14 with respect to advantageous further development. An apparatus for carrying out the method according to the present invention is disclosed in claim 15 and can be further advantageously implemented by the features disclosed in the dependent claims 16 to 25.

Unlike the process disclosed in EP 0 541 574 B1, the present invention requires only a single continuous hot-rolling process. Thus, the present invention does not require a secondary hot rolling step and does not require intermediate induction heating for such a step. Instead, according to the present invention, hot rolling is carried out in a single pass, and quenching proceeds at a temperature in the range of 850 캜 to 600 캜 at the end of the process. When the cooling temperature is reached, a finished steel strip is produced by isothermic rolling in at least three stages of rolling passes, wherein a thickness reduction of at least 35% is achieved in each step of the rolling pass. After the finish rolling, the strip is quenched to a temperature of 100 DEG C or less. On the other hand, in the conventional technique, the finish rolling is performed at a significantly low temperature (about 250 ° C to 600 ° C). During the isothermal rolling according to the present invention, the temperature of the steel sheet is not kept constant in a strict sense, but the tolerance band (for example, ΔT = 0 ° C. to 20 ° C.) . During the isothermal rolling, the temperature must never fall below the threshold, and the inevitable heat loss due to radiation must be compensated for at least by deformation work on the strip. Preferably, the method requires that the heat contribution from a particular deformation operation (" speed up ") be always greater than the expected heat loss due to radiation, while the temperature adjustment is aimed at between rolling steps And proceeds in a manner controlled by cooling. It is almost impossible to raise the temperature to a desired value even if the rolling parameters are changed in any way, if the actual temperature of the strip falls even below the threshold at any time during the rolling process.

The present invention will be described in more detail with reference to the apparatus diagram shown in Fig.

A molten steel, preferably a deep drawing steel, is injected from the ladle 10 into the tundish 11. The tundish 11 causes the molten steel contained therein to flow into the continuous casting mold 12 located under the tundish in a continuous flow state. The continuous casting mold 12 has a liquid cooling device (not shown) and serves to produce a cast strip composed of a strip shell and a liquid core. In this state, the hot cast strip enters the casting rolling apparatus located below the continuous casting mold (12). The casting and rolling apparatus further reduces the thickness of the cast strip partially with the liquid core. As a result, a thin layer continuous slab 1 having a thickness of 30 mm to 100 mm, preferably 40 mm to 70 mm, comes out of the casting rolling apparatus 13. The reduction in thickness during the casting rolling process is at least 10%, preferably at least 30%. The strip then enters the descaler 19, an example of which is a hydromechanical descaler. After the scale is removed, the temperature of the thin slab 1 is in the range of 1150 캜 to 900 캜. In this state, the thin slab 1 is supplied to the hot rolling apparatus 15 provided immediately behind the descaler 19. In the hot rolling apparatus 15, the thickness of the thin slab 1 is reduced by at least 50% to form an intermediate strip having a maximum thickness of 20 mm, preferably 10 mm to 20 mm. In some cases it may also be advantageous to place an equalizing furnace (not shown) directly in front of the hot rolling device 15 to maintain the thin slabs divided in partial length at the desired hot rolling temperature. After the hot rolling apparatus 15 having a two- or three-stage stand or a reverse rolling mill, for example, a strip shear machine 17 is used to divide the produced intermediate strip into partial lengths described above, It is desirable to connect separation aggregates of the form. According to the present invention, the hot rolled intermediate strip is quenched in a temperature range of 850 캜 to 600 캜. The basis for choosing such a particular cooling temperature is, in each case, based on the chemical composition of the input steel, the desired microstructure, and the mechanical and technical properties desired to be obtained in the final strip. Cooling proceeds in the primary cooling device 18 directly attached to the strip shear 17 on the drawing. In many cases, for spatial reasons, the partial length of the intermediate strip (the temperature of which is the temperature required for subsequent final rolling) is coiled in the coil arrangement 20 to form these intermediate strip coils in the homogenizer 21 It is preferable to maintain the temperature at a desired temperature. The intermediate strip is unwound again for subsequent final rolling in a uncoiling device 22 connected directly to the homogenization furnace 21. Prior to the final rolling, it is desirable to perform the descaling process again in the descaling device 23, for example, to avoid quality deterioration due to the newly formed scale. Final rolling is conducted by isothermal rolling at a temperature in the range of 600 캜 to 850 캜 in a rolling apparatus (24) having at least a three-stage stand. In many cases, it is preferred that the rolling apparatus has a four-stage or more five-stage stand. More rolling stands are not desirable. The rolling stand is operated so that the strip thickness is reduced by at least 25% for each rolling pass. The thickness of the finished strip when exiting the rolling apparatus is at most 2 mm, preferably 0.5 to 1.5 mm. In order to ensure isothermal rolling conditions (as approximations) it is necessary to provide a cooling device (not shown) for absorbing excess heat in a controlled manner, for example a spray chiller, between each rolling stand of the rolling device 24 desirable. The actual temperature of the strip in the rolling apparatus 24 is monitored by a temperature sensor (not shown). The strip exiting from the rolling apparatus 24 is immediately quenched in the secondary cooling apparatus 25 to a temperature of 100 DEG C or lower. The quenching process preferably proceeds at a rate of 10 ° C to 25 ° C per second. For this purpose, the finally treated strip may be fed via, for example, a liquid cooling bath. However, a spray cooling device may be used along a roller table with a minimum rolling distance of 250 mm or less in a known manner. Preferably, the final treatment strip produced in this manner must be wound to carry it in coil form. A suitable coil device 26 for this purpose is shown in the device schematic.

Planning the forming step of the intermediate strip coil between the hot rolling device 15 and the rolling device 24 has the advantage of forming a material buffer that allows the rolling device to cause less failures during operation. Furthermore, the homogenization furnace 21 required to maintain the temperature of the buffer material takes up relatively little space.

Process Example

A melt of deep drawn steel having the following composition was cast in a continuous casting apparatus for thin slabs at a temperature of about 1540 DEG C:

0.04% C

0.02% Si

0.21% Mn

0.018% P

0.006% S

0.035% A1

0.05% Cu

0.05% Cr

0.04% Ni

0.0038% N

Residual iron and standard impurities (T _liq → 1520 ° C)

After passing through a continuous casting mold, the cast strip having a thickness of 80 mm and a width of 1300 mm still had a liquid core. The average temperature of the cast strip at the mold exit was about 1310 ° C. In this state, the thin slab strip was fed into the casting rolling apparatus and the thickness was reduced by 25%, resulting in a coagulation thickness of 60 mm. After removing the scale using a high-pressure water jet, the thin slab strip achieved a thickness reduction of about 66% in a three-stage stand hot rolling row (row), resulting in an intermediate strip of 20 mm thickness. The temperature at the time of entering the hot rolling row was 1130 占 폚, and it was 938 占 폚 at a better time. Immediately after the process, the intermediate strip was divided into several pieces and quenched to about 700 ° C. After passing through a homogenization furnace running at about 700 ° C and scaling off, the intermediate strip coils produced in partial split lengths were fed into a final rolling row. The final rolling heat had a total of five stages and a total thickness reduction of 95%. The intermediate strip fed to the primary rolling stand at 650 ° C exhibited a temperature slightly above 658 ° C when it exited the stand and was cooled back to about 650 ° C by a spray chiller installed in front of the secondary rolling stand. Likewise, the outlet temperature after the secondary rolling stand of 664 ° C dropped again to 650 ° C, the temperature dropped to the tertiary rolling stand by the spray chiller in front of the tertiary rolling stand. The same process was applied to the 4th and 5th rolling stands. Immediately after this process, the final strip produced to a thickness of 1.00 mm was cooled to about 90 ° C at a cooling rate of 21 ° C / s in a cooling water bath and wound into a finished coil. The final treated strip thus produced had excellent mechanical properties comparable to cold rolled strips.

The finishing process according to the present invention is particularly advantageous for forming a microstructure consisting of microcrystalline grains and this microstructure is more advantageous than the results obtained according to the known European Patent 0 541 574 B1. In the known method, reheating to 1100 占 폚, which is performed before the second hot rolling step, makes the crystal grains considerably rough. In the method according to the present invention, this phenomenon can not occur because the selected temperature range is from 850 캜 to 600 캜. Additional differences in crystal grain size result from differences in the final rolling mode. In the process according to the invention, dynamic crystal grain refinement such as increased strength and toughness occurs during the isothermal rolling process. The isothermal rolling process is carried out at the recrystallization limit temperature and is also performed under a given total deformation degree of over 90%. Since the strain is clearly small in each rolling pass, this phenomenon does not occur at all in the conventional process. In addition, a high strength that can be achieved by cold-hardening in conventional processes can be achieved with a rolling cycle suitably adjusted by the method according to the invention. In addition, these improvements will obviously result in improved toughness.

As a result, the steel sheet produced by the method according to the present invention has a very high strength, excellent deformation characteristics and toughness.

Claims (25)

a) A thin slab having a thickness (coagulation thickness) of 30 mm to 100 mm is produced from a molten steel by a continuous casting process, and after the cast strip is discharged from the mold of the continuous casting apparatus, Reducing the thickness of the cast strip by at least 10% during the cast rolling of the cast strip, b) scaling the thin slabs prepared according to step a) c) subjecting said unscaled thin slab to hot strip rolling at a temperature in the range of 1150 DEG C to 900 DEG C to reduce the thickness by at least 50% to produce an intermediate strip having a maximum thickness of 20 mm; A method of manufacturing a steel strip having a plurality of strips, d) after hot rolling, accelerating the intermediate strip to a temperature in the range of 850 캜 to 600 캜, c) rolling said cooled intermediate strip into a strip having a maximum thickness of 2 mm by isothermic rolling at a temperature of 850 DEG C to 600 DEG C in a finishing train having at least a three- By at least 25% for each roll pass; and f) Subsequently, the isothermal rolled steel sheet is cooled by an accelerated cooling method to a temperature of 100 ° C or less, and further preferably, the step of winding the thus-cold rolled steel sheet with a coil as a finished strip. The method of claim 1, Wherein the thin slab is manufactured to have a solidification thickness of 40 to 70 mm. 3. The method according to claim 1 or 2, Wherein the thin slab is reduced in thickness by at least 20%, in particular by 30%, during casting rolling. 3. The method according to claim 1 or 2, Wherein the thin slab is maintained at a temperature within an equalizing furnace prior to hot rolling. 3. The method according to claim 1 or 2, Wherein the intermediate strip is manufactured to have a thickness of 10 to 20 mm. 3. The method according to claim 1 or 2, And after said step (d), said intermediate strip is divided into partial lengths and rolled into a coil. 3. The method according to claim 1 or 2, Wherein the intermediate strip cooled in step (d) is maintained at a cooling temperature in the homogenization furnace prior to isothermal rolling. 3. The method according to claim 1 or 2, And the isothermal rolling is performed in four passes or five passes. 3. The method according to claim 1 or 2, Wherein the steel sheet is subjected to isothermal rolling so as to have a thickness of 0.5 to 1.5 mm. 3. The method according to claim 1 or 2, Wherein subsequent cooling of the steel sheet is performed at a cooling rate of 10 ° C / s to 25 ° C / s. 3. The method according to claim 1 or 2, Wherein the intermediate strip is again scaled off immediately before isothermal rolling. 3. The method according to claim 1 or 2, Wherein in each case the descaling is carried out hydraulically. 3. The method according to claim 1 or 2, Wherein during the isothermal rolling, the temperature of the intermediate strip is controlled by spray cooling, in particular between each rolling pass. 3. The method according to claim 1 or 2, Wherein a molten steel of deep drawing quality is used in said step a). A continuous casting apparatus for producing a thin slab 1, A cast-rolling device 13 positioned immediately behind the mold 12 of the continuous casting machine, A scale removing device 19 disposed behind the casting rolling device 13, A method for manufacturing a steel sheet having characteristics of a cold rolled product for carrying out the method according to claim 1 including a hot rolling apparatus (15) including at least two stands or one reverse rotating stand and connected to the descaling apparatus In this case, Behind the hot rolling apparatus 15, there is provided a primary cooling apparatus 18 for accelerated cooling of the intermediate strip produced in the hot rolling apparatus 15, After the primary cooling device 18, there is provided a rolling device 24 for isothermal rolling with at least three rolling stands, And a secondary cooling device (25) for accelerated cooling of the produced steel sheet is provided immediately after the rolling device (24). 16. The method of claim 15, Wherein the scale removing device (19) is designed as a hydrodynamic scale removing device. 16. The method according to claim 15 or 16, And a homogenizing furnace is positioned between the casting rolling apparatus (13) and the hot rolling apparatus (15). 17. The method according to claim 15 or 16, And a separation aggregate (shearing machine 17) for dividing the hot-rolled intermediate strip into partial lengths is provided after the hot rolling apparatus 15. 19. The method of claim 18, Wherein a coil device (20) and an uncoiling device (22) for a middle strip coil are installed behind the primary cooling device (18). The method of claim 18, A heating assembly (homogenizing furnace) for maintaining the temperature of the partial length of the intermediate strip between the primary cooling device 18 and the isothermal rolling device 24, in particular between the coil device 20 and the coil expanding device 22 21) is provided on the surface of the steel plate. 17. The method according to claim 15 or 16, Characterized in that a descale device (23) is installed in front of the isothermal rolling device (24). 17. The method according to claim 15 or 16, The apparatus for manufacturing a steel plate according to claim 1, wherein the hot rolling apparatus (15) comprises three stands. The method according to claim 5 or 6, Wherein the isothermal rolling device (24) comprises four or five stands. 17. The method according to claim 15 or 16, Characterized in that a coil ring device (26) for winding a finished product strip is provided behind the secondary cooling device (25). 17. The method according to claim 15 or 16, Characterized in that a cooling device for temperature control, in particular a spray cooling device, is installed between the rolling stands of the rolling device (24) for isothermal rolling.

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