Method for improving shape control of cold rolled high strength steel sheet

Abstract

The application discloses a control method for improving the shape of a cold-rolled high-strength steel plate, which comprises the following steps: taking a high-strength steel casting blank to carry out rough rolling, finish rolling, laminar cooling, coiling, pickling and continuous annealing. The high-strength steel obtained by the control method comprises the following components in terms of mass fraction: C 0.05-0.20%, Mn 1.30-3.0%, Si 0.20-1.30%, Als 0.010-0.030%, P≤0.016%, S≤0.0025%, N≤0.004%, Cr 0.010-0.80%, Nb 0.003-0.060%, Ti 0.003-0.080%, Mo 0.002-0.50%, and the rest is Fe and inevitable impurities. The application provides a control method for improving the shape of a cold-rolled high-strength steel plate, the product has good shape by comprehensively controlling the hot rolling, pickling and subsequent continuous annealing process, the quality defects in production are reduced, and the shape before use is ensured. The control method is simple and easy to implement, does not need to increase the equipment by thickening, and can be implemented on the existing conventional production line.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of cold-rolled high-strength steel manufacturing, in particular to a method for improving the shape control of cold-rolled high-strength steel. BACKGROUND

[0002] In the production of cold-rolled high-strength steel, as the thickness of the strip steel becomes thinner and the strength increases, the shape control of the product becomes more and more difficult. Common shape defects include edge waves, intermediate waves, rib waves, and compound waves, which are difficult to eliminate in the subsequent production process and directly affect the quality of the strip steel and the normal use of customers.

[0003] Most of the existing shape control methods for cold-rolled high-strength steel are through the back tension control method of the pickling mill. For example, a cold-rolled thin strip steel shape control method is disclosed in Chinese Patent Application No. CN201710757095.5, which adjusts the back tension distribution of the strip steel in real time to eliminate the uneven plastic extension strain difference of the strip steel along the width direction, thereby improving the shape defects. For example, a high-strength cold-rolled strip steel rolling control method is disclosed in Chinese Patent Application No. CN200910046535.1, which provides a high-strength cold-rolled strip steel rolling process with the optimal shape as the target. In the cold rolling stage, the shape of the cold-rolled plate is controlled through the following five methods: (1) the proportion of the incoming material convexity is less than 0.05, and the ratio of the wedge shape to the plate thickness is less than 0.04; (2) special support roller shape technology is adopted; (3) the working roll change convexity of the five cold rolling stands is set; (4) the fifth stand adopts a constant rolling force mode; and (5) the shape target curve is set to a micro-intermediate wave mode, and the elongation rate difference is 10±2 IU, etc. to control the shape of the cold-rolled high-strength steel, achieving good control effect. Chinese Patent Application No. CN201811233342.2 discloses a method for controlling the double-edge wave and intermediate wave shape of a five-stand cold continuous rolling high-strength steel plate strip, which adjusts the bending roller force in time according to the difference between the actual shape and the preset target to achieve the purpose of controlling the shape.

[0004] The above existing technologies mainly focus on the pickling stage, and control the shape of the strip steel through tension, bending roller force, and roller cooling of the pickling mill, achieving good results. These methods are universal, but for some special steel grades, such as steel with high Si content, the shape is greatly affected by the organization change of each process, and only using the above methods cannot achieve good results.

[0005] Therefore, the applicant considers controlling the shape through comprehensive control of the whole process of hot rolling, pickling, and continuous annealing. SUMMARY

[0006] In order to overcome the above technical deficiencies, the purpose of the present application is to provide a cold rolled high strength steel shape control method, which can obtain good shape by comprehensive control of hot rolling material, pickling process and subsequent annealing process.

[0007] In order to achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0008] A cold rolled high strength steel shape control method, comprising the following steps: taking high strength steel billet for heating, rough rolling, finishing rolling, laminar cooling, coiling, pickling and continuous annealing.

[0009] Preferably, the cold rolled high strength steel shape control method comprises the following steps: steelmaking, continuous casting to obtain high strength steel billet, and then heating, rough rolling, finishing rolling, laminar cooling, coiling, pickling and continuous annealing.

[0010] Preferably, the finishing rolling step adopts F1-F7 finishing mill; wherein the finishing rolling F7 rolling reduction is 3-15%, and the finishing rolling final rolling temperature is 900-960℃. If the final rolling temperature is too low, the hot rolling deformation resistance increases, which easily leads to uneven deformation in the length and width directions, thus leading to poor shape. If the final rolling temperature is too high, it easily leads to uneven oxidation of the plate surface, which affects the cooling uniformity in the width and length directions during the subsequent cooling process, and easily leads to uneven structure and poor shape.

[0011] Preferably, the laminar cooling step adopts three-stage cooling. The three-stage cooling can accurately control the hot rolling structure transformation, improve the structure control precision, and thus improve the structure uniformity in the full length and full width directions of the strip, which is beneficial to the uniform deformation of the strip in the full length and full width directions.

[0012] Preferably, in the three-stage cooling process, the first-stage cooling rate is 60-200℃ / s, the cooling time is 0.5-5s, the second-stage cooling rate is 2-20℃ / s, the cooling time is 3-15s, and the third-stage cooling rate is 10-60℃ / s, the cooling time is 3-20s. Too low or too high first-stage cooling rate and too long or too short cooling time easily lead to uneven structure, thus leading to uneven shape. Accurate control of the second-stage cooling rate and time is beneficial to improving the structure uniformity and producing certain soft phase structure, which creates conditions for the uniform deformation of the subsequent strip. Too high or too low cooling rate easily leads to uneven structure, too much or too little soft phase structure, thus leading to uneven deformation and poor shape. Too high third-stage cooling rate and too long time easily produce too much hard phase structure, which is not conducive to uniform deformation. Too low third-stage cooling rate and too short time easily lead to uneven structure in the width and length directions, thus leading to uneven deformation and poor shape.

[0013] Preferably, in the coiling step, the coiling temperature is 460-600℃. Too low or too high coiling temperature can result in uneven microstructure in the head and tail of the strip and in the width direction, leading to uneven cold rolling deformation and poor plate shape.

[0014] Preferably, in the pickling step, the pickling speed is 200-600m / min, the pickling reduction is 45-70%, the total reduction of the first and second stands is 35-60%, and the reduction of the last stand is 3-10%. Too low pickling speed can affect production efficiency, and too high pickling speed can result in too high deformation speed, uneven hardness in the length and width directions of the strip, and uneven deformation, leading to poor plate shape. Too low pickling reduction can not effectively eliminate the poor plate shape caused by hot rolling, and too high pickling reduction can exacerbate uneven deformation in the length and width directions of the strip, leading to poor plate shape. Too low total reduction of the first and second stands can increase the deformation and hardness of the plate in the subsequent process, leading to uneven deformation in the length and width directions and poor plate shape. Too high deformation rate can result in too high work hardening, making the strip difficult to deform and leading to poor plate shape. Too low reduction of the last stand can result in insufficient plastic deformation in the length and width directions of the strip, leading to poor plate shape, and too high reduction can result in severe work hardening of the strip, also leading to poor plate shape.

[0015] Preferably, in the continuous annealing step, the continuous annealing temperature is 820-880℃. Too low or too high continuous annealing temperature can result in uneven microstructure in the length and width directions of the strip, leading to poor plate shape.

[0016] Preferably, the control method further comprises obtaining a high-strength steel strip by performing skin passing or tempering after the continuous annealing step; and in the skin passing or tempering process, the reduction is 0.2%-1.0%. Too low reduction can not ensure plastic deformation in the length and width directions, which is not conducive to correcting poor plate shape. Too high reduction can result in too much work hardening, which is also not conducive to plate shape control.

[0017] Preferably, the thickness of the strip is 0.7-2.5mm, the width is 900-1800mm, and the width-thickness ratio is 500-2000.

[0018] The high-strength steel prepared by the above control method comprises, by mass fraction: C 0.05%-0.20%, Mn 1.30%-3.0%, Si 0.20%-1.30%, Als 0.010%-0.030%, P≤0.016%, S≤0.0025%, N≤0.004%, Cr 0.010%-0.80%, Nb 0.003%-0.060%, Ti 0.003%-0.080%, Mo 0.002%-0.50%, and the balance being Fe and unavoidable impurities.

[0019] Compared with the prior art, the present application has the following advantages:

[0020] The present application provides a method for improving the shape control of cold-rolled high-strength steel, which controls the hot rolling, pickling and subsequent continuous annealing processes to make the product have good shape, reduce quality defects in production, and ensure the shape before use. The method is simple and easy to implement, does not require additional thickening of the equipment, and can be implemented on existing conventional production lines. BRIEF DESCRIPTION OF DRAWINGS

[0021] Figure 1 The figure is the flatness curve after pickling in Example 1 of the present application. In the figure, the vertical coordinate represents flatness in μm, and the horizontal coordinate represents the length of the strip in m.

[0022] Figure 2 The figure is the shape after continuous annealing in Example 1 of the present application.

[0023] Figure 3 The figure is the flatness curve after pickling in Comparative Example 1 of the present application. In the figure, the vertical coordinate represents flatness in μm, and the horizontal coordinate represents the length of the strip in m. DETAILED DESCRIPTION

[0024] In order to better explain the present application, the main content of the present application is further illustrated below in combination with the drawings, Examples 1-5 and Comparative Examples 1-6, but the content of the present application is not limited to the following examples.

[0025] The main components of the high-strength steel in Examples 1-5 and Comparative Examples 1-6 are shown in Table 1. The main components of the high-strength steel in Examples 1-3 and Comparative Examples 1-4 are shown in Component 1 in Table 1, and the main components of the high-strength steel in Examples 4-5 and Comparative Examples 5-6 are shown in Component 2 in Table 1.

[0026] Table 1: Main components of high-strength steel in Examples 1-5 and Comparative Examples 1-6

[0027] Components C Si Mn Als Nb Ti N P S Cr Mo 1 0.09 0.45 2.1 0.020 0.005 0.010 0.0015 0.016 0.0020 0.015 0.03 2 0.13 0.72 1.70 0.015 0.012 0.012 0.0035 0.012 0.002 0.20 0.10

[0028] Table 2: Partial process parameters of high-strength steel in Examples 1-5 and Comparative Examples 1-6

[0029]

[0030] Examples 1-5 and Comparative Examples 1-6 are produced by the following steps, the difference being in the different process parameters of the relevant steps, wherein the process parameters are shown in Tables 2 and 3. The production steps include: steelmaking, continuous casting, heating, rough rolling, finishing rolling, laminar cooling, coiling, pickling, continuous annealing, and flattening to obtain high-strength steel strip. Among them, F1-F7 finishing rolling mills are used in the finishing rolling step.

[0031] Table 3: Partial process parameters of high-strength steel in Examples 1-5 and Comparative Examples 1-6

[0032]

[0033] Depend on Figures 1-2 As shown in the flatness curve and strip shape diagram after pickling and rolling in Example 1, within a total length of 2000m, the flatness of the strip after pickling and rolling in Example 1 is less than 5μm, and its strip shape is well controlled.

[0034] Examples 1-5 have process parameter control and microstructure within the scope of this invention, thus their plate shape is well controlled.

[0035] like Figure 3 As shown, within the entire 1750m length, the straightness of the strip after pickling in Comparative Example 1 was greater than 5μm, especially at the tail end where the straightness reached 20μm. Its low final rolling temperature and single-stage laminar cooling resulted in poor hot-rolled coil shape, ultimately leading to poor shape after pickling and continuous annealing. In Comparative Example 2, the single-stage laminar cooling and low coiling temperature also resulted in poor hot-rolled coil shape, ultimately leading to poor shape after pickling and continuous annealing. In Comparative Example 3, the single-stage laminar cooling and high reduction rate on the final pickling stand resulted in poor shape after pickling and continuous annealing, ultimately leading to poor shape after continuous annealing. In Comparative Example 4, the excessively high pickling reduction rate, excessively high rolling speed, and low continuous annealing temperature resulted in poor shape after both pickling and continuous annealing. In Comparative Example 5, the poor sheet shape was caused by excessively low reduction rates in the first and second pickling passes, excessively high reduction rates in the last pickling stand, high continuous annealing temperatures, and low flatness. In Comparative Example 6, the poor sheet shape was caused by excessively high hot rolling temperatures, excessively high reduction rates in the last pickling stand, and high continuous annealing temperatures.

[0036] In summary, this invention provides a method for controlling the shape of cold-rolled high-strength steel sheets. By comprehensively controlling the hot rolling, pickling, and subsequent continuous annealing processes, the product achieves a good sheet shape, reduces quality defects during production, and ensures the sheet shape before use by the user. This method is simple and easy to implement, requires no additional equipment, and can be carried out on existing conventional production lines.

Claims

1. A method for controlling the shape of cold-rolled high-strength steel sheets, characterized in that: Includes the following steps: High-strength steel billets are heated, rough rolled, finish rolled, laminar flow cooled, coiled, pickled, and continuously annealed. The finishing rolling temperature is 900~960℃; The laminar flow cooling process adopts a three-stage cooling method; the first stage has a cooling rate of 60~200℃ / s and a cooling time of 0.5~5s, the second stage has a cooling rate of 2~20℃ / s and a cooling time of 3~15s, and the third stage has a cooling rate of 10~60℃ / s and a cooling time of 3~20s. The winding temperature is 460~600℃; The pickling and rolling speed is 200~600m / min, the pickling and rolling reduction rate is 45~70%, the total reduction rate of the first and second pickling and rolling stands is 35~60%, and the reduction rate of the last stand is 3~10%. The continuous annealing temperature is 820~880℃; The high-strength steel comprises the following components by mass fraction: C 0.05%~0.20%, Mn 1.30%~3.0%, Si 0.20%~1.30%, Als 0.010%~0.030%, P≤0.016%, S≤0.0025%, N≤0.004%, Cr 0.010%~0.80%, Nb 0.003%~0.060%, Ti 0.003%~0.080%, Mo 0.002%~0.50%, with the remainder being Fe and unavoidable impurities.

2. The control method according to claim 1, characterized in that: The finishing rolling process uses F1-F7 finishing mills; among them, the finishing rolling reduction rate of F7 is 3~15%.

3. The control method according to any one of claims 1 to 2, characterized in that: It also includes finishing or leveling after the continuous annealing step to obtain high-strength steel strip; during the finishing or leveling process, the reduction rate is 0.2%~1.0%.

4. The control method according to claim 3, characterized in that: The strip has a thickness of 0.7~2.5mm, a width of 900~1800mm, and a width-to-thickness ratio of 500~2000.

5. High-strength steel prepared by the control method according to any one of claims 1 to 4, characterized in that: Based on mass fraction, it includes the following components: C 0.05%~0.20%, Mn 1.30%~3.0%, Si 0.20%~1.30%, Als 0.010%~0.030%, P≤0.016%, S≤0.0025%, N≤0.004%, Cr 0.010%~0.80%, Nb 0.003%~0.060%, Ti 0.003%~0.080%, Mo 0.002%~0.50%, with the remainder being Fe and unavoidable impurities.

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