ENERGY-EFFICIENT PRODUCTION OF A FERRITIC HOT-ROLLED STRIP IN AN INTEGRATED CASTING-ROLLING PLANT

Abstract

Energy-efficient production of a ferritic hot-rolled strip (6) in an integrated casting-rolling plant (1), which modifies the known processes for producing a ferritic hot-rolled strip (6) in an integrated casting-rolling plant (1) so that the ferritic hot-rolled strip (6) can be produced significantly more energy-efficiently but nevertheless has good metallurgical properties and a good surface quality.

Claims

1. A process for producing a ferritic hot-rolled strip in an integrated casting-rolling plant, comprising: continuous casting of a liquid steel to give a strand having a slab or thin slab cross section in a continuous casting plant; prerolling of the strand to give an intermediate strip in a multistand roughing mill; heating of broad sides of the intermediate strip by at least one inductive surface heating module to a surface temperature of 1000° C., wherein the at least one surface heating module is operated using an alternating current having a first frequency f1≥20 kHz; descaling of the broad sides of the heated intermediate strip in a descaling apparatus; rolling of the descaled intermediate strip to give the hot-rolled strip in a multistand finishing mill, where the descaled intermediate strip after descaling and without further cooling enters a first stand of the finishing mill with an average temperature of 775-900° C. and at least a last rolling pass in the finishing mill takes place in the ferritic temperature range of the steel; setting of the hot-rolled strip to coiler temperature; and winding-up of the hot-rolled strip in a coiler.

2. The process as claimed in claim 1, wherein ratio of the thickness s of the intermediate strip and the penetration depth d into the heated intermediate strip obeys: s/d≤6.

3. The process as claimed in claim 1, wherein the at least one inductive surface heating module heats the intermediate strip by transverse field heating.

4. The process as claimed in claim 3, wherein a first inductor heats an upper broad side of the intermediate strip and a second inductor heats a lower broad side of the intermediate strip.

5. The process as claimed in claim 4, wherein a vertical distance between the first inductor and the upper broad side is kept constant as a function of thickness of the intermediate strip.

6. The process as claimed in claim 1, wherein each broad side of the intermediate strip is descaled by at least one respective row of a plurality of spray nozzles in the descaling apparatus.

7. The process as claimed in claim 6, wherein the spray nozzles of at least one of the rows are either stationary or arranged on rotating rotors.

8. The process as claimed in claim 6, wherein the descaling is carried out using a liquid descaling agent having a pressure in a range 450 bar>p>100 bar at the spray nozzles.

9. The process as claimed in claim 6, wherein a pair of pinch rolls next to the intermediate strip is arranged, in the flow direction of the material, before a first row and after a last row of spray nozzles so that the descaling agent cannot leave the descaling apparatus.

10. The process as claimed in claim 1, wherein an average temperature of the intermediate strip is increased using a plurality of inductive volume-heating modules in an induction furnace (IH) before heating of the broad sides of the intermediate strip, and wherein the average temperature is increased to about a temperature of a surface of the intermediate strip.

11. The process as claimed in claim 10, wherein the at least one inductive surface heating module is operated at a first frequency f1 and the inductive volume-heating modules are operated at a second frequency f2, where: f1>f2.

12. The process as claimed in claim 10, wherein surface temperature Tact of a partially finished intermediate strip between the first stand of the finishing mill and a second set of the finishing mill or between the second stand and a third stand of the finishing mill is measured by a pyrometer, a temperature regulator transmits an actuation variable as a function of an intended surface temperature Tint and taking into account Tact to at least one inductive volume-heating module from the plurality of inductive volume-heating modules and the at least one volume-heating module heats the intermediate strip to such a degree that the surface temperature Tact corresponds closely to the intended surface temperature Tint.

13. An integrated casting-rolling plant for producing a ferritic hot-rolled strip by carrying out the process of claim 1, comprising: the continuous casting plant for continuously casting the liquid steel to give the strip having the slab or thin slab cross section; a multipart roughing stand for prerolling the strip to give the intermediate strip; the at least one inductive surface heating module being configured for heating the broad sides of the intermediate strip to a surface temperature of 1000° C., wherein the at least one surface heating module is heated by an alternating current having a first frequency f1≥20 kHz; the descaling apparatus configured for descaling the broad sides of the heated intermediate strip; the multipart finishing stand configured for final rolling of the descaled intermediate strip to give the hot-rolled strip, wherein the descaled intermediate strip after descaling and without further cooling enters the first set of the finishing stand with an average temperature of 775-900° C. and at least the last rolling pass in the finishing stand takes place in the ferritic temperature range of the steel; a cooling section configured for bringing the hot-rolled strip to coiler temperature; and the coiler configured for winding up the hot-rolled strip.

14. The integrated casting-rolling plant as claimed in claim 13, wherein an induction furnace (IH) having a plurality of inductive volume-heating modules is arranged in the flow direction of the material between the multistand roughing mill and the at least one inductive surface heating module, wherein the induction furnace (IH) increases the average temperature of the intermediate strip.

15. The integrated casting-rolling plant as claimed in claim 13, wherein a pyrometer for measuring a surface temperature Tact of the partially finished intermediate strip is arranged between the first stand of the finishing mill and a second stand of the finishing mill or between the second stand and a third stand of the finishing mill, the pyrometer is connected so as to be able to transmit a signal to a temperature regulator and the temperature regulator is connected so as to be able to transmit a signal to the at least one inductive volume-heating module, the temperature regulator can transmit an actuating variable as a function of an intended surface temperature Tint and taking into account the surface temperature Tact to at least one inductive volume-heating module, wherein the volume-heating modules can heat the intermediate strip to such a degree that the surface temperature Tact corresponds closely to the intended surface temperature Tint.

16. The process as claimed in claim 1, wherein f1≥50 kHz.

17. The process as claimed in claim 1, wherein f1≥100 kHz.

18. The process as claimed in claim 1, wherein the surface temperature ≥1050° C.

19. The process as claimed in claim 2, wherein s/d≤10.

20. The process as claimed in claim 2, wherein s/d≤14.

21. The process as claimed in claim 2, wherein s/d≤1.

22. The process as claimed in claim 10, wherein the at least one inductive surface heating module is operated at a first frequency f1 and the inductive volume-heating modules are operated at a second frequency f2, and wherein f1≥2*f2.

23. The process as claimed in claim 10, wherein the at least one inductive surface heating module is operated at a first frequency f1 and the inductive volume-heating modules are operated at a second frequency f2, and wherein f1≥5*f2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The above-described properties, features and advantages of the present invention and the way in which these are achieved will become clearer and more easily understood in connection with the following description of a working example which is explained in more detail in conjunction with the drawings. The drawings show:

[0034] FIG. 1 a schematic depiction of an integrated casting-rolling plant according to the invention for carrying out the process of the invention,

[0035] FIG. 2 a temperature profile of the process of the invention, and

[0036] FIG. 3 a thickness profile for the process of the invention.

DESCRIPTION OF THE EMBODIMENTS

[0037] In the integrated casting-rolling plant 1 of FIG. 1, liquid steel having the following chemical composition

TABLE-US-00001 TABLE 1 Chemical composition of the steel Element % by weight C <0.004 Mn <0.2 P <0.01 Ti + Nb 0.03 Fe Balance
is continuously cast in the continuous casting plant 2 to give a strip 3 having a slab cross section. The strip 3 leaves the continuous casting plant 2 with a thickness of 90 mm and at a speed of 6 m/min. The partially solidified strip 3 is preferably subjected to a soft core reduction or a liquid core reduction (LCR) in the arc-shaped course of the strip. This reduces the thickness of the strip and improves the internal quality thereof. The strip 3 goes uncut into the three-stage finishing stand 5 and is reduced there to an intermediate strip 4 having a thickness of 12.4 mm. The last rolling pass in the set R3 of the finishing stand 5 is carried out in the austenitic temperature range at a final rolling temperature of 1050° C. The average temperature of the intermediate strip 4 is subsequently increased from 900° C. to 950° C. by six volume-heating modules of an induction furnace IH. Subsequently, the surface temperature on the broad sides of the heated intermediate strip 4 is brought to 1070° C. by two surface heating modules 7. The surface heating modules are operated at a frequency of 50 kHz and heat the intermediate strip by transverse field heating. The heating of the broad sides increases the average temperature of the intermediate strip to 960° C. After heating, the broad sides of the intermediate strip 4 are descaled in a descaling apparatus D, specifically a pinch roll descaler. In the step, the average temperature of the intermediate strip decreases to 850° C. After descaling, the descaled intermediate strip 3 enters the five-stage finishing stand 8 and is there subjected to final rolling in 5 rolling passes to give a hot-rolled strip 6 having a thickness of 1.7 mm. Since the last rolling pass in the set F5 takes place at an average temperature of 760° C., a hot-rolled strip having a ferritic microstructure is present at the latest after the last rolling pass. The last three rolling passes in the rolling sets F3, F4 and F5 (particularly preferably all rolling passes) of the finishing stand 8 are preferably carried out using roller gap lubrication. Here, a mineral oil is sprayed in each case between the working rollers of the finishing set and the material being rolled so that the coefficient of friction in the roller gap is reduced to a value μ of <0.15. This prevents shear bands, which lead to development of an undesirable GOSS texture, being formed in the finished hot-rolled strip. The hot-rolled strip 6 leaves the finishing stand 8 with a surface temperature of 760° C. In order to achieve a high coiling temperature, the hot-rolled strip is not actively cooled in the region of the cooling section 9 shown as a broken line but is instead thermally insulated by insulation panels 14. The coiling temperature is 700° C. Shortly before the coil has attained its target weight, the continuous hot-rolled strip is parted transversely by the cutter 10 and the winding-up is continued on a further coiling device (not shown in FIG. 1), where the ferrite in the hot-rolled strip 6 at least partly forms a {1 1 1} texture. The average temperatures in the individual apparatuses of the integrated casting-rolling plant 1 can be seen either from FIG. 2 or the following table:

TABLE-US-00002 TABLE 2 Temperature profile Temperature [° C.] CCM Out 1200 R1 1150 R2 1100 R3 1050 IH In 900 IH Out 950 SHM In 950 SHM Out 1070 D 850 F1 840 F2 820 F3 800 F4 780 F5 760 DC 700

[0038] The degrees of reduction in the individual sets R1 . . . R3 and F1 . . . F5 and also the thicknesses of the thin slab 2, the intermediate strip 4 and the hot-rolled strip 6 can be derived either from FIG. 3 or the following table:

TABLE-US-00003 TABLE 3 Thicknesses and degrees of reduction Thickness Degree of reduction [mm] [%] CCM Out 90.0 R1 In 90.0 50 R1 Out 45.0 R2 In 45.0 50 R2 Out 22.5 R3 In 22.5 45 R3 Out 12.4 IH In 12.4 IH Out 12.4 SHM In 12.4 SHM Out 12.4 D 12.4 F1 In 12.4 45 F1 Out 6.8 F2 In 6.8 40 F2 Out 4.1 F3 In 4.1 35 F3 Out 2.7 F4 In 2.7 25 F4 Out 2.0 F5 In 2.0 15 F5 Out 1.7 DC 1.7

[0039] In order to ensure the continuous operation of the integrated casting-rolling plant 1, the hot-rolled strip 6 is cut immediately before the coiling devices and alternatively wound up by at least two coiling devices DC.

[0040] As a result of the use of the process of the invention in the integrated casting-rolling plant 1, the coiled hot-rolled strip 6 has good deep drawability without the hot-rolled strip 6 having to be additionally cold-rolled or heat treated after the hot rolling.

[0041] Although the invention has been illustrated and described in detail by the preferred working examples, the invention is not restricted by the examples disclosed and other variations can be derived therefrom by a person skilled in the art, without going outside the scope of protection of the invention.

LIST OF REFERENCE SYMBOLS

[0042] 1 Integrated casting-rolling plant

[0043] 2 Continuous casting plant

[0044] 3 Strip

[0045] 4 Intermediate strip

[0046] 5 Roughing stand

[0047] 6 Hot-rolled strip or finished strip

[0048] 7 Surface heating module

[0049] 8 Finishing stand

[0050] 9 Cooling section

[0051] 10 Cutter

[0052] 14 Insulation panel

[0053] 15, DC Coiler

[0054] D Descaling apparatus

[0055] F1 . . . F5 First to fifth set of the finishing stand

[0056] IH Induction furnace

[0057] In Entry of an apparatus

[0058] Out Exit of an apparatus

[0059] R1 . . . R3 First to third set of the roughing stand

[0060] T.sub.act Actual surface temperature

[0061] T.sub.int Intended surface temperature