Energy-efficient production of hot-rolled strip from ferritic steel in a combination casting and rolling plant

Abstract

The invention relates to the energy-efficient production of ferritic hot-rolled strip (6) in a cast-rolling complex (1). The task of the invention is to change the known method for producing ferritic hot-rolled strip (6) in a cast-rolling complex (1) in such a way that the ferritic hot-rolled strip (6) can be produced significantly more energy-efficiently, but nevertheless still has good metallurgical properties and a good surface quality. This task is solved by a method according to claim 1.

Description

Technical Field

[0001] This invention relates to the technical field of steel metallurgy, and more particularly to the particularly energy-efficient manufacture of ferritic hot-rolled strip in a casting-rolling composite equipment.

[0002] On one hand, the present invention relates to a method for producing ferrite hot-rolled strip in a casting-rolling composite equipment, comprising the following steps: continuously casting liquid steel into a billet having a slab cross-section or a thin slab cross-section in a continuous casting equipment; rough rolling the billet into an intermediate strip in a multi-stand roughing mill; descaling the wide side of the heated intermediate strip in a descaling device; finishing rolling the descaled intermediate strip into a hot-rolled strip in a multi-stand finishing mill, wherein at least one last rolling pass is performed in the finishing mill within the temperature range of ferrite in the steel; adjusting the hot-rolled strip to a winding temperature; and winding the hot-rolled strip in a winding device.

[0003] On the other hand, the present invention relates to a casting-rolling composite apparatus particularly well suited for manufacturing ferritic hot-rolled strip, comprising: a continuous casting apparatus for continuously casting liquid steel into a slab having a slab cross-section or a thin slab cross-section; a multi-stand roughing mill for rough rolling the slab into an intermediate strip; a descaling device for descaling the wide side of the heated intermediate strip; a multi-stand finishing mill for finishing rolling the descaled intermediate strip into hot-rolled strip, wherein at least one rolling pass is performed in the finishing mill within the temperature range of ferritic steel; a cooling section for adjusting the hot-rolled strip to a winding temperature; and a winding apparatus for winding the hot-rolled strip. Background Technology

[0004] As known from application WO 2021 / 013488 A1, ferritic hot-rolled strip is manufactured in a casting-rolling complex by the following steps: continuously casting a slab having a slab cross-section or a thin slab cross-section; rough rolling the slab into an intermediate strip in a multi-stand roughing mill; heating the intermediate strip to an average temperature ≥1070°C; descaling the heated intermediate strip; finishing the descaled intermediate strip into a hot-rolled strip in a multi-stand finishing mill, wherein at least one last rolling pass is performed in the ferritic temperature range in the finishing mill; cooling the hot-rolled strip to the winding temperature; and winding the hot-rolled strip in a winding device.

[0005] Although the hot-rolled ferritic strip produced has good metallurgical properties and surface quality, this method is very energy-intensive because it first requires raising the average temperature of the intermediate strip to a high temperature of ≥1070°C, then descaling the intermediate strip, and finally cooling the average temperature of the intermediate strip to <900°C in an intensified cooling step. How to modify this method so that the hot-rolled strip achieves the same good metallurgical properties and surface quality while significantly reducing energy consumption is not discussed in this literature. Summary of the Invention

[0006] The objective of this invention is to modify a method for producing ferrite hot-rolled strip in a cast-roll composite apparatus such that the hot-rolled ferrite strip can be produced significantly more energy-efficiently, while still maintaining good metallurgical properties and surface quality. Furthermore, a cast-roll composite apparatus particularly well suited to this purpose should be described.

[0007] The methodological aspect of this task is solved by a method. The invention also relates to advantageous improvements.

[0008] Specifically, the task is solved by a method for producing ferrite hot-rolled strip in a casting-rolling composite equipment, the method comprising the steps of: continuously casting molten steel into a slab having a slab cross-section or a thin slab cross-section in a continuous casting equipment; rough rolling the slab into an intermediate strip in a multi-stand roughing mill; and heating the wide side of the intermediate strip to a surface temperature of ≥1000°C, preferably ≥1050°C, by one or more induction surface heating modules, wherein the surface heating modules operate with alternating current having a first frequency f1 and for which: f1 ≥ 20 kHz, preferably f1 ≥ 50 kHz, particularly preferably f1 ≥ 100 kHz. kHz; Descaling the wide side of the heated intermediate strip in a descaling device; Finish rolling the descaled intermediate strip into the hot-rolled strip in a multi-stand finishing mill, wherein the descaled intermediate strip enters the first stand of the finishing mill at an average temperature of 775-900°C without further cooling after descaling, and at least one last rolling pass is performed in the finishing mill within the temperature range of the ferrite of the steel; Adjusting the hot-rolled strip to the winding temperature; and winding the hot-rolled strip in a winding device.

[0009] "Average temperature" (also referred to as average temperature) should refer to the temperature corresponding to the average temperature of the different layers of the intermediate zone along the thickness direction. Therefore, it is not typically the temperature that the intermediate zone has at the center (i.e., in the central region) along the thickness direction.

[0010] When adjusting the hot-rolled strip to the winding temperature, it is typically insulated in the area between the last stand of the finishing mill and the winding equipment, so that the average temperature of the hot-rolled strip drops only slightly. This achieves a high winding temperature without the need for active heating or reheating of the hot-rolled strip. Alternatively, the hot-rolled strip can be actively cooled or even heated by a heating device. It is also possible to combine the heating device after the last stand of the finishing mill with a cooling section for active cooling of the hot-rolled strip before winding, which is advantageous for certain steel qualities.

[0011] According to the invention, the intermediate strip is heated to a surface temperature of ≥1000°C by at least one surface heating module. Because the one or more surface heating modules operate with AC power having a first frequency f1 and applicable to the first frequency f1: f1 ≥ 20 kHz, only the layer near the surface of the wide side is heated, while the temperature of the core of the intermediate strip changes only slightly. In other words, the surface temperature on the wide side of the intermediate strip is increased significantly beyond the average temperature of the intermediate strip by the one or more surface heating modules. Subsequently, the wide side of the hot intermediate strip is descaled, for example, by a so-called pinch roll descaler. The descaled intermediate strip immediately after descaling, i.e., without further cooling, enters the first stand of the finishing mill at an average temperature of 775-900°C, and is finished rolled into hot-rolled strip in a multi-stand finishing mill. To directly produce ferritic hot-rolled strip in a cast-roll composite equipment, at least one last rolling pass is performed in the finishing mill within the ferritic temperature range of the steel. The temperature of the hot-rolled ferrite strip is then adjusted to the winding temperature and it is wound into coils in a winding machine.

[0012] This results in several differences compared to existing technologies: Firstly, the inductive surface heating module heats only the near-surface layer of the wide side and not uniformly all layers of the intermediate strip. Because the wide side has a surface temperature of ≥1000°C before descaling, the descaling is very thorough, achieving high surface quality of the hot-rolled strip. Secondly, the descaled intermediate strip enters the first stand of the finishing mill directly at an average temperature of 775-900°C, without requiring a separate enhanced cooling step after descaling. Therefore, energy is saved because only the near-surface layer of the wide side of the intermediate strip needs to be heated to a relatively high temperature before descaling, rather than heating the entire intermediate strip. Furthermore, the average temperature of the intermediate strip before descaling can be very low (e.g., between 875 and 990°C), which is highly advantageous for the energy efficiency of the manufacturing method.

[0013] Preferably, the ratio between the thickness s of the intermediate strip and the penetration depth d into the heated intermediate strip is: s / d ≤ 6, preferably s / d ≤ 10, particularly preferably s / d ≤ 14, and very particularly preferably s / d ≤ 16. The penetration depth δ (also known as current penetration rate) refers to the region in the intermediate strip where the current density decreases to 37% compared to the outer edge of the wide side. In this region of penetration depth, 86% of the induced energy is converted into heat, with only 14% heating the deeper regions. Specifically, this means that, for example, for an intermediate strip thickness of 24 mm, the penetration depth can be a maximum of 4 mm when s / d ≤ 6. The penetration depth can be determined by the formula...

[0014]

[0015] To estimate, where μ0 represents the magnetic field constant, μ r Let f represent the relative electromagnetic permeability of the steel, f represent the frequency of the alternating current, and κ represent the conductivity. All the parameters mentioned can be used in SI units. In particular, κ also includes μ. r The height depends on the temperature, and these values ​​must be used at the current temperature when heating.

[0016] Preferably, the surface heating module uses lateral field heating to heat the intermediate band. However, it is also possible that the heating is performed using longitudinal field heating. In lateral field heating, it is advantageous that a first sensor heats the upper wide side of the intermediate band, and a second sensor, positioned vertically opposite the first sensor, heats the lower wide side of the intermediate band.

[0017] Advantageously, the so-called coupling gap, which is the vertical distance between the upper sensor and the upper wide side of the intermediate strip, can be adjusted or kept constant according to the thickness of the intermediate strip. This adjustment is made, for example, by a linear motor.

[0018] To thoroughly descale the intermediate band along its width, it is advantageous to descale each wide side of the intermediate band using at least one row of nozzles, each equipped with multiple nozzles. The nozzles are either fixedly arranged or mounted on a rotating rotor.

[0019] Good descaling is achieved when descaling is performed using a liquid descaling agent, such as water, wherein the descaling agent is applied to the nozzle at a pressure of 450 bar > p > 100 bar.

[0020] In order to retain the descaling agent in the descaling device, it is advantageous to arrange a pair of drive rollers adjusted to the intermediate belt along the material flow direction before the first row of nozzles and after the last row of nozzles.

[0021] Depending on the steel quality, operating mode (continuous, semi-continuous, or batch operation), or casting speed, it is advantageous to increase the average temperature of the intermediate zone in the induction furnace using multiple induction heat-through modules before heating the wide side. The average temperature of the intermediate zone is increased by one or more heat-through modules to approximately the same extent as the surface temperature. Advantageously, the induction surface heating module operates at a first frequency f1, and the induction heat-through module operates at a second frequency f2, wherein f1 > f2, preferably f1 ≥ 2 * f2, and particularly preferably f1 ≥ 5 * f2. The heat-through modules preferably heat the intermediate zone via longitudinal field heating. However, it is also possible that the heating is performed via transverse field heating.

[0022] To regulate the entry temperature of the descaled intermediate strip into the finishing mill, it is advantageous to measure the surface temperature T of the partially finished intermediate strip between the first and second finishing stands or between the second and third finishing stands using a pyrometer. Ist The temperature regulator takes T into account Ist In the case of target surface temperature T Soll The adjustment is output to at least one, preferably multiple, inductive heat-penetrating modules, and these modules heat the intermediate zone so intensely that the measured surface temperature T... Ist As close as possible to the target surface temperature T Soll .

[0023] This method is based on the understanding that the temperature of the heated and descaled intermediate strip can be measured only imprecisely before the finishing mill train, and that temperature measurement in one of the intermediate stand areas of the first three stands is much more accurate. The inductive heat penetration module is adjusted by the temperature controller according to the measured actual temperature, taking into account the target temperature, so that the actual temperature corresponds as well as possible to the target temperature.

[0024] The apparatus aspect of the described technical task is explained by way of a casting-rolling composite equipment. The invention also relates to advantageous improvements.

[0025] Specifically, the task is solved by a hot-rolled strip for producing ferrite, particularly a casting-rolling composite apparatus for implementing the method according to the invention, the casting-rolling composite apparatus having:

[0026] Continuous casting equipment used to continuously cast liquid steel into billets with slab cross-sections or thin slab cross-sections;

[0027] A multi-stand roughing mill train for roughing the billet into an intermediate strip;

[0028] One or more inductive surface heating modules are used to heat the wide side of the intermediate band to a surface temperature of ≥1000°C, wherein the surface heating modules are operated by alternating current having a first frequency f1, and the first frequency f1 is applicable as follows: f1≥20 kHz, preferably f1≥50 kHz, particularly preferably f1≥100 kHz;

[0029] A descaling device for descaling the wide sides of a heated intermediate zone;

[0030] A multi-stand finishing mill for finishing descaling intermediate strip into hot-rolled strip, wherein the descaled intermediate strip enters the first stand of the finishing mill at an average temperature of 775-900°C without further cooling after descaling and undergoes at least one last rolling pass in the finishing mill within the temperature range of the ferrite of the steel.

[0031] A cooling section for adjusting the hot-rolled strip to the winding temperature; and

[0032] Winding equipment for winding the hot-rolled strip.

[0033] Preferably, an induction furnace with multiple induction heat-through modules is arranged between the roughing mill train and the induction surface heating module along the material flow direction, wherein the induction furnace increases the average temperature of the intermediate zone.

[0034] More preferably, a surface temperature T for partially finished strip is arranged between the first and second finishing mill stands or between the second and third finishing mill stands of the finishing mill train. Ist A pyrometer is used for measurement, the pyrometer being signal-connected to a temperature regulator, and the temperature regulator being signal-connected to at least one inductive heat-transmitting module. The temperature regulator is capable of taking into account T... Ist In the case of target surface temperature T Soll The adjustment amount is output to at least one inductive heat-penetrating module, which is capable of heating the intermediate zone so intensely that the measured surface temperature T... Ist As close as possible to the target surface temperature T Soll . Attached Figure Description

[0035] The features, characteristics, and advantages of the present invention described above, as well as the ways and methods of achieving these features, characteristics, and advantages, will become clearer and more readily understood in conjunction with the following description of embodiments, which will be explained in detail with reference to the accompanying drawings. Hereinafter:

[0036] Figure 1 A schematic diagram of the casting-rolling composite apparatus according to the invention for implementing the method according to the invention is shown.

[0037] Figure 2 The temperature distribution for use according to the method of the present invention is shown, and

[0038] Figure 3 The thickness distribution for use according to the method of the present invention is shown. Detailed Implementation

[0039] exist Figure 1 In the casting-rolling composite equipment 1, the continuous casting equipment 2 will have the following chemical composition.

[0040] element %(weight) C <0.004 Mn <0.2 P <0.01 Ti+Nb 0.03 Fe The remaining part

[0041] Table 1: Chemical Composition of Steel

[0042] Liquid steel is continuously cast into a slab 3 with a slab cross-section. The slab 3 exits the continuous casting equipment 2 with a thickness of 90 mm and a speed of 6 m / min. Preferably, the partially solidified slab 3 undergoes soft-core or liquid-core thinning (LCR) in an arc-shaped slab guide mechanism. This reduces the thickness of the slab and improves its internal quality. The slab 3 enters the three-stand roughing mill 5 without cutting and is thinned there into an intermediate strip 4 with a thickness of 12.4 mm. The last rolling pass in stand R3 of the roughing mill 5 is performed at a final rolling temperature of 1050°C within the austenitic temperature range. Subsequently, the average temperature of the intermediate strip 4 is increased from 900°C to 950°C by six heat-through modules of the induction furnace IH. Immediately thereafter, the surface temperature of the wide side of the heat-through intermediate strip 4 is heated to 1070°C by two surface heating modules 7. The surface heating module operates at a frequency of 50 kHz and heats the intermediate strip via transverse field heating. Through heating the wide side, the average temperature of the intermediate strip rises to 960°C. After heating, the wide side of the intermediate strip 4 is descaled in a descaling device D, specifically a pinch roll descaler. Here, the average temperature of the intermediate strip drops to 850°C. After descaling, the descaled intermediate strip 3 enters a five-stand finishing mill 8 and is finished rolled in five passes into a hot-rolled strip 6 with a thickness of 1.7 mm. Because the last rolling pass in stand F5 is performed at an average temperature of 760°C, a hot-rolled strip with a ferritic structure exists at least after the last rolling pass. Preferably, the last three rolling passes (particularly preferably all rolling passes) are performed in stands F3, F4, and F5 of the finishing mill 8 using roll gap lubrication. Here, mineral oil is sprayed between the work rolls and the workpiece of the finishing mill stand, reducing the coefficient of friction in the roll gap to a value μ < 0.15. This prevents the formation of shear bands in the hot-rolled strip after finishing, which would result in undesirable Gaussian texture. The hot-rolled strip 6 leaves the finishing mill stand 8 at a surface temperature of 760°C. To achieve a high winding temperature, the hot-rolled strip is not actively cooled in the region of the cooling section 9 (shown in dashed lines), but is instead insulated by a partition plate 14. The winding temperature is 700°C. Shortly before the coil reaches its target weight, the continuous hot-rolled strip is transversely cut by a shearing machine 10 and continues winding to another (…). Figure 1 On a winding device (not shown), the ferrite in the hot-rolled strip 6 at least partially constitutes a {1 1 1} texture. The average temperature in each unit of the casting-rolling composite equipment 1 is either determined by... Figure 2 Alternatively, it can be derived from the following table:

[0043] 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

[0044] Table 2: Temperature Control.

[0045] The thinning rate in each of the stands R1...R3 and F1...F5, and the thickness of the slab 2, intermediate strip 4, and hot-rolled strip 6, are either determined by... Figure 3 Alternatively, it can be derived from the following table:

[0046]

[0047] Table 3: Thickness and thinning rate.

[0048] To ensure the continuous operation of the casting and rolling composite equipment 1, the hot-rolled strip 6 is cut directly before the winding device and alternately wound through at least two winding devices DC.

[0049] By applying the method according to the invention in the casting and rolling composite equipment 1, the wound hot-rolled strip 6 has good deep-drawing capability, and it is not necessary to cold-roll or anneal the hot-rolled strip 6 after hot rolling.

[0050] Although the invention has been illustrated and described in detail through preferred embodiments, the invention is not limited to the disclosed examples and other variations can be derived by those skilled in the art without departing from the scope of protection of the invention.

[0051] List of reference numerals in the attached diagram:

[0052] 1. Casting and rolling composite equipment

[0053] 2. Continuous casting equipment

[0054] 3. Casting billet

[0055] 4. Intermediate zone

[0056] 5. Roughing mill train

[0057] 6. Hot-rolled strip or finished strip

[0058] 7 Surface heating module

[0059] 8 Finishing Mill Train

[0060] 9 Cooling Section

[0061] 10 Shearing machine

[0062] 14 Insulation Board

[0063] 15. DC winding equipment

[0064] D Descaling device

[0065] F1...F5 Finishing mill stands, first to fifth stands

[0066] IH Induction Furnace

[0067] In the unit entrance

[0068] Out unit outlet

[0069] R1...R3 Roughing Mill Stands 1 to 3

[0070] T Ist Actual surface temperature

[0071] T Soll Target surface temperature.

Claims

1. A method for producing ferrite hot-rolled strip (6) in a casting-rolling composite apparatus (1), comprising the following steps: Liquid steel is continuously cast into a billet with a slab cross-section or a thin slab cross-section in a continuous casting equipment (2, CCM) (3). The billet (3) is rough rolled into an intermediate strip (4) in a multi-stand roughing mill (5). The wide side of the intermediate band (4) is heated to a surface temperature of ≥1000°C by one or more inductive surface heating modules (7), wherein the surface heating modules (7) are operated with AC power having a first frequency f1 and for the first frequency f1: f1≥20 kHz; Descaling is performed on the wide side of the heated intermediate zone (4) in the descaling device (D); The descaled intermediate strip (4) is finished rolled into hot-rolled strip (6) in a multi-stand finishing mill (8), wherein the descaled intermediate strip (4) enters the first stand (F1) of the finishing mill (8) at an average temperature of 775-900°C without further cooling after descaling, and at least one last rolling pass (F5) is performed in the finishing mill within the temperature range of the ferrite of the steel. The hot-rolled strip (6) is adjusted to the winding temperature; and The hot-rolled strip (6) is wound in a winding device (15, DC). The average temperature of the intermediate strip (4) is increased from 900°C to 950°C by an induction furnace (IH), and then the surface temperature of the wide side of the heat-through intermediate strip (4) is heated to 1070°C by a surface heating module (7).

2. The method of claim 1, wherein, For the ratio between the thickness s of the intermediate band (4) and the depth d of penetration into the heated intermediate band (4), the following applies: s / d ≤ 6.

3. The method according to claim 1 or 2, characterized in that, The inductive surface heating module (7) heats the intermediate zone (4) through transverse field heating.

4. The method of claim 3, wherein, The first sensor heats the upper wide side of the intermediate band (4), and the second sensor heats the lower wide side of the intermediate band (4).

5. The method of claim 4, wherein, The vertical distance between the first sensor and the upper wide side is kept constant based on the thickness of the middle band.

6. The method of claim 1 or 2, wherein, In the descaling device (D), each wide side of the intermediate zone (4) is descaled by at least one row of nozzles.

7. The method of claim 6, wherein, A row of nozzles is either fixed or arranged on a rotating rotor.

8. The method of claim 6, wherein, The descaling is performed using a liquid descaling agent, which is applied to the nozzle at a pressure of 450 bar > p > 100 bar.

9. The method of claim 8, wherein, A pair of drive rollers, adjusted to the intermediate belt (4), are arranged along the material flow direction before the first row of nozzles and after the last row of nozzles, so that the descaling agent cannot leave the descaling device.

10. The method of claim 1 or 2, wherein, Before heating the wide side of the intermediate strip, the average temperature of the intermediate strip is increased in an induction furnace (IH) by a plurality of induction heat-through modules, wherein the average temperature is increased to approximately the same extent as the surface temperature of the intermediate strip.

11. The method of claim 10, wherein, The inductive surface heating module operates at a first frequency f1, and the inductive heat penetration module operates at a second frequency f2, wherein f1 > f2.

12. The method of claim 10, wherein, The surface temperature T of the intermediate strip (4) partially finished between the first finishing stand (F1) and the second finishing stand (F2) or between the second finishing stand (F2) and the third finishing stand (F3) in the finishing mill line (8) is measured using a pyrometer. Ist The temperature regulator takes T into account Ist In the case of target surface temperature T Soll The adjustment is output to at least one sensing heat-penetrating module, and the heat-penetrating module heats the intermediate zone so intensely that the measured surface temperature T... Ist As close as possible to the target surface temperature T Soll .

13. A casting-rolling composite apparatus (1) for producing ferrite hot-rolled strip (6) in the method according to any one of claims 1 to 12, comprising: Continuous casting equipment (2, CCM), which is used to continuously cast liquid steel into slabs with slab cross-section or thin slab cross-section (3). A multi-stand roughing mill (5) is used to rough roll the billet (3) into an intermediate strip (4). One or more inductive surface heating modules (7) for heating the wide side of the intermediate band (4) to a surface temperature of ≥1000°C, wherein the surface heating modules (7) are operated with alternating current having a first frequency f1, and for the first frequency f1: f1≥20 kHz; Descaling device (D), which is used to descale the wide side of the heated intermediate zone (4); A multi-stand finishing mill (8) for finishing rolling a descaled intermediate strip (4) into a hot-rolled strip (6), wherein the descaled intermediate strip (4) enters the first stand (F1) of the finishing mill (8) at an average temperature of 775-900°C without further cooling after descaling, and the last rolling pass (F5) is performed in the finishing mill (8) at least within the temperature range of the ferrite of the steel. Cooling section (9) for adjusting the hot-rolled strip (6) to the winding temperature; and Winding equipment (15, DC) for winding the hot-rolled strip (6).

14. The cast and roll complex of claim 13, wherein, An induction furnace (IH) with multiple induction heat penetration modules is arranged between the roughing mill column (5) and the induction surface heating module (7) along the material flow direction, wherein the induction furnace (IH) increases the average temperature of the intermediate zone.

15. The cast and roll complex according to claim 13 or 14, characterized in that A surface temperature T of the intermediate strip (4) for partial finishing is arranged between the first finishing mill stand (F1) and the second finishing mill stand (F2) or between the second finishing mill stand (F2) and the third finishing mill stand (F3) of the finishing mill train (8). Ist A pyrometer is used for measurement, the pyrometer being signal-connected to a temperature regulator, and the temperature regulator being signal-connected to at least one inductive heat-transmitting module. The temperature regulator is capable of taking into account T... Ist In the case of target surface temperature T Soll The adjustment is fed to at least one inductive heat-penetrating module, which is capable of heating the intermediate zone so intensely that the measured surface temperature T... Ist As close as possible to the target surface temperature T Soll .

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