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. An integrated casting-rolling plant for producing a ferritic hot-rolled strip by carrying out the process 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; 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.

2. The integrated casting-rolling plant as claimed in claim 1, 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.

3. The integrated casting-rolling plant as claimed in claim 1, 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.

Description

BRIEF DESCRIPTION OF THE DRAWING(S)

[0034] 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:

[0035] FIG. 1 a schematic depiction of an integrated casting-rolling plant according to the invention for carrying out the process of the invention,

[0036] FIG. 2 a temperature profile of the process of the invention, and

[0037] FIG. 3 a thickness profile for the process of the invention.

DESCRIPTION OF THE EMBODIMENTS

[0038] 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 VVVElement % 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 ? 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

[0039] 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

[0040] 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.

[0041] 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.

[0042] 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

[0043] 1 Integrated casting-rolling plant [0044] 2 Continuous casting plant [0045] 3 Strip [0046] 4 Intermediate strip [0047] 5 Roughing stand [0048] 6 Hot-rolled strip or finished strip [0049] 7 Surface heating module [0050] 8 Finishing stand [0051] 9 Cooling section [0052] 10 Cutter [0053] 14 Insulation panel [0054] 15, DC Coiler [0055] D Descaling apparatus [0056] F1 . . . F5 First to fifth set of the finishing stand [0057] IH Induction furnace [0058] In Entry of an apparatus [0059] Out Exit of an apparatus [0060] R1 . . . R3 First to third set of the roughing stand [0061] T.sub.act Actual surface temperature [0062] T.sub.int Intended surface temperature