Method for avoiding warping of hot-rolled high-strength steel plates under high-temperature coiling conditions

Abstract

The present invention discloses a method for preventing warping of hot-rolled high-strength steel plates under high-temperature coiling conditions. The method divides the cooling process of the hot-rolled high-strength steel plates into a temperature differential cooling stage and an asymmetric cooling stage. The stage before coiling after hot rolling is the temperature differential cooling stage, in which a temperature differential exists between the upper and lower surfaces of the high-strength steel plates. The coiling stage is the asymmetric cooling stage, in which the temperature fields and applied stress fields on the upper and lower surfaces of the high-strength steel plates are asymmetric. By introducing a pre-temperature differential before the high-strength steel plates enter the coiling stage, the effect of phase transformation during the asymmetric cooling stage on the phase transformation plastic strain on the upper and lower surfaces of the high-strength steel plates is suppressed. This controls the residual stress distribution along the thickness direction of the high-strength steel plates after phase transformation, thereby preventing warping of the hot-rolled high-strength steel plates under high-temperature coiling conditions. This method prevents warping of hot-rolled high-strength steel plates under high-temperature coiling conditions.

Description

Technical Field

[0001] The present invention relates to the preparation of hot-rolled high-strength steel, and in particular to a method for preventing warping of hot-rolled high-strength steel plates under high-temperature coiling conditions. Background Art

[0002] High-strength steel plates, a mature structural material, are widely used in engineering machinery, transportation, and other high-end manufacturing fields. To reduce material costs and improve welding performance, high-strength steel is generally designed with low alloy compositions. TMCP (Thermo Mechanical Control Process) and rapid cooling processes are used to refine grains and control precipitation phases to improve the material's strength and toughness. However, while rapid cooling improves material strength and toughness, it can also easily lead to flatness defects in high-strength steel plates. To control these flatness defects, many companies add flattening and tempering processes after hot rolling, which increases the production cost per ton of steel by tens or even hundreds of yuan.

[0003] As we all know, residual stress is the main cause of warping and deformation in steel plates, which in turn causes flatness defects. To improve the warping of steel plates, many scholars have focused on the control of residual stress during the phase transformation process of steel materials. For example, Chinese Patent ZL201910791084.8, "Method for Preparing Hot-Rolled High-Strength Strip with Reduced Residual Stress," Chinese Patent ZL202210630360.4, "Method for Reconstructing Intrinsic Strain and Controlling Residual Stress in Quenching Based on Cooling Rate Control," and Chinese Patent ZL202210633180.1, "Method for Reconstructing Intrinsic Strain and Controlling Residual Stress in Quenching Based on Stress Control," all aim to control the phase transformation plastic behavior generated during the phase transformation process during the cooling process, thereby achieving the control goal of reducing the residual stress level in the thickness direction of the steel plate.

[0004] However, while residual stress levels within steel plates have a significant impact on their warpage, simply controlling residual stress levels cannot fundamentally address the issue of flatness control. For example, two steel plates with identical residual stress extremes may have one exhibiting excellent flatness while the other exhibits significant warpage. Alternatively, two plates with identical residual stress extremes may exhibit completely different warpage forms and degrees.

[0005] To fundamentally address the issue of flatness control for steel plate products, the applicant recently proposed a method for preventing warping of hot-rolled high-strength steel plates. The cooling process of the hot-rolled high-strength steel plate is divided into a symmetrical cooling stage and an asymmetrical cooling stage. The symmetrical cooling stage is the pre-coiling stage after hot rolling, where the temperature fields on the upper and lower surfaces of the high-strength steel plate are symmetrically distributed. The coiling stage is the asymmetrical cooling stage, where the temperature fields and applied stress fields on the upper and lower surfaces of the high-strength steel plate are asymmetrically distributed. By constraining the temperature field distribution on both sides of the neutral plane during the phase transformation of the high-strength steel plate, the phase transformation of the high-strength steel plate is completed as much as possible in the symmetrical cooling stage, limiting the phase transformation amount of the high-strength steel plate in the asymmetric cooling stage, and thus controlling the residual stress distribution along the thickness direction of the high-strength steel plate after the phase transformation, thereby preventing warping of the hot-rolled high-strength steel plate. The phase transformation amount of the high-strength steel plate in the asymmetric cooling stage is limited by setting the coiling temperature at the midpoint of the high-strength steel plate.

[0006] However, some steel grades are limited by material performance control requirements or production unit capacity requirements and must maintain high-temperature coiling conditions, which cannot meet the above-mentioned coiling temperature requirements. How to control the flatness of such products is a key problem faced in production. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for avoiding warping of hot-rolled high-strength steel plates under high-temperature coiling conditions. By inserting a pre-temperature difference, the influence of phase transformation in the asymmetric cooling stage on the phase transformation plastic strain of the upper and lower surfaces of the high-strength steel plates is suppressed, thereby avoiding warping of hot-rolled high-strength steel plates under high-temperature coiling conditions.

[0008] The technical solution adopted in the present invention is:

[0009] A method for avoiding warping of hot-rolled high-strength steel plates under high-temperature coiling conditions, wherein the cooling process of the high-strength steel plates after hot rolling is divided into a temperature difference cooling stage and an asymmetric cooling stage, wherein the stage before coiling after hot rolling is the temperature difference cooling stage, in which there is a temperature difference in the temperature field of the upper and lower surfaces of the high-strength steel plates, and the coiling stage is the asymmetric cooling stage, in which the temperature field and the applied stress field on the upper and lower surfaces of the high-strength steel plates are asymmetrically distributed; by inserting a pre-temperature difference before the high-strength steel plates enter the coiling stage, the influence of phase change in the asymmetric cooling stage on the phase change plastic strain of the upper and lower surfaces of the high-strength steel plates is suppressed, thereby controlling the residual stress distribution morphology of the high-strength steel plates along the thickness direction after the phase change, thereby avoiding warping of the hot-rolled high-strength steel plates under high-temperature coiling conditions.

[0010] Furthermore, in the temperature difference cooling stage, the temperature difference between the upper and lower surface temperature fields of the high-strength steel plate is achieved by increasing the cooling rate of the lower surface of the high-strength steel plate or reducing the cooling rate of the upper surface of the high-strength steel plate.

[0011] Furthermore, during the temperature difference cooling stage, the cooling rate of the lower surface of the high-strength steel plate increases Or the cooling rate of the high-strength steel plate surface is reduced The calculation formula is

[0012]

[0013] in, is the thermal conductivity of high-strength steel plate, is the temperature difference between the upper and lower surfaces of the high-strength steel plate, The density of high-strength steel plate, is the specific heat of high-strength steel plate.

[0014] Furthermore, during the temperature difference cooling stage, the temperature difference between the upper and lower surfaces of the high-strength steel plate ,in The temperature difference between the upper and lower surfaces of the high-strength steel plate during the coiling stage, The temperature difference required to produce a temperature stress equivalent to the coiling bending stress.

[0015] Furthermore, the temperature difference required to generate the temperature stress equivalent to the coiling bending stress The calculation formula is

[0016]

[0017] in, is the coiling bending stress, is the thermal expansion coefficient of high-strength steel plate, is the elastic modulus of high-strength steel plate.

[0018] The beneficial effects of the present invention are:

[0019] The present invention suppresses the influence of phase transformation in the asymmetric cooling stage on the phase transformation plastic strain of the upper and lower surfaces of the high-strength steel plate by inserting a pre-temperature difference, thereby avoiding the warping of the hot-rolled high-strength steel plate under high-temperature coiling conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic diagram of the residual stress distribution along the thickness direction of high-strength steel plates after hot rolling, where a) is a "U"-type distribution and b) is an "N"-type distribution.

[0021] Figure 2 Schematic diagram of the cooling process of a high-strength steel plate after hot rolling in an embodiment of the present invention.

[0022] In the figure: 1- finishing mill; 2- outlet temperature measuring point; 3- coiling temperature measuring point; 4- turning roller; 5- coiler. DETAILED DESCRIPTION

[0023] The present invention will be further described below with reference to the accompanying drawings and examples.

[0024] The applicant has found that for hot-rolled high-strength steel plate products, the residual stress along the thickness direction presents two different distribution forms: Figure 1 As shown in (a), it is a "U"-shaped distribution. The phase transformation strain and phase transformation plastic strain formed on the upper and lower surfaces of the high-strength steel plate are basically the same, and its thickness-stress curve is generally U-shaped; Figure 1 As shown in (b), an "N"-shaped distribution is observed. The phase transformation strain and phase transformation plastic strain formed on the upper and lower surfaces of the high-strength steel plate differ, resulting in a generally N-shaped thickness-stress curve. Furthermore, the applicant has discovered that if the residual stress along the thickness direction of the high-strength steel plate after phase transformation exhibits a "U"-shaped distribution, warping defects are less likely to occur. However, if the residual stress along the thickness direction of the high-strength steel plate after phase transformation exhibits an "N"-shaped distribution, warping defects are more likely to occur.

[0025] It is well known that the phase change strain and phase change plastic strain generated by the material during the phase change process are the key to determining the magnitude and distribution of residual stress. The tiny stress acting on the material during the phase change process can change the magnitude and direction of the phase change plastic strain. When the steel plate that has not yet completed the phase change enters the coiling process in the asymmetric cooling stage, the asymmetric temperature stress caused by the difference in cooling modes between the upper and lower surfaces of the steel plate is superimposed on the bending stress formed after the steel plate is wound, making the stress on the upper and lower surfaces of the steel plate different in magnitude and opposite in direction. As a result, the phase change plastic strain of the steel plate along the thickness direction after the phase change is completed is asymmetric, causing the residual stress to be distributed in an "N" shape, thereby inducing the formation of warping defects in the steel plate.

[0026] The present application discloses a method for preventing warping of hot-rolled high-strength steel plates under high-temperature coiling conditions, wherein the cooling process of the hot-rolled high-strength steel plates is divided into a temperature difference cooling stage and an asymmetric cooling stage. Figure 2 As shown, the stage before coiling after hot rolling is a temperature difference cooling stage, and there is a temperature difference in the temperature field of the upper and lower surfaces of the high-strength steel plate. The coiling stage is an asymmetric cooling stage, and the temperature field and the external stress field on the upper and lower surfaces of the high-strength steel plate are asymmetric. By inserting a pre-temperature difference before the high-strength steel plate enters the coiling stage, the influence of the phase change in the asymmetric cooling stage on the phase change plastic strain of the upper and lower surfaces of the high-strength steel plate is suppressed, and then the residual stress distribution morphology of the high-strength steel plate along the thickness direction after the phase change is controlled, thereby avoiding the warping of the hot-rolled high-strength steel plate under high-temperature coiling conditions.

[0027] In the temperature difference cooling stage, the temperature difference between the upper and lower surfaces of the high-strength steel plate is achieved by increasing the cooling rate of the lower surface of the high-strength steel plate or reducing the cooling rate of the upper surface of the high-strength steel plate. (Unit, K / s) or cooling rate of the surface of high-strength steel plate (Unit, K / s) is calculated as follows:

[0028]

[0029] in, is the thermal conductivity of high-strength steel plate (unit: W / (mK)), is the temperature difference between the upper and lower surfaces of the high-strength steel plate (unit, K), is the density of high-strength steel plate (unit: kg / m 3 )、 is the specific heat of high-strength steel plate (unit: J / (kgK)).

[0030] During the temperature difference cooling stage, the temperature difference between the upper and lower surfaces of the high-strength steel plate ,in The temperature difference between the upper and lower surfaces of the high-strength steel plate during the coiling stage, The temperature difference required to generate the temperature stress equivalent to the coiling bending stress. (unit, K) is calculated as

[0031]

[0032] in, is the coiling bending stress (unit, MPa), is the thermal expansion coefficient of high-strength steel plate (unit: 1 / K), is the elastic modulus of high-strength steel plate (unit: MPa).

[0033] The present invention suppresses the influence of phase transformation in the asymmetric cooling stage on the phase transformation plastic strain of the upper and lower surfaces of the high-strength steel plate by inserting a pre-temperature difference, thereby avoiding the warping of the hot-rolled high-strength steel plate under high-temperature coiling conditions.

[0034] The embodiments described above are part of the embodiments of the present application, rather than all of the embodiments. The detailed description of the embodiments of the present application is not intended to limit the scope of the present application for protection, but merely represents selected embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

Claims

1. A method for preventing warping of hot-rolled high-strength steel plates under high-temperature coiling conditions, characterized in that: The cooling process of the high-strength steel plate after hot rolling is divided into a temperature difference cooling stage and an asymmetric cooling stage, wherein the stage before coiling after hot rolling is a temperature difference cooling stage, and there is a temperature difference in the temperature field of the upper and lower surfaces of the high-strength steel plate; the coiling stage is an asymmetric cooling stage, and the temperature field and the external stress field on the upper and lower surfaces of the high-strength steel plate are asymmetricly distributed; by inserting a pre-temperature difference before the high-strength steel plate enters the coiling stage, the influence of the phase change in the asymmetric cooling stage on the phase change plastic strain of the upper and lower surfaces of the high-strength steel plate is suppressed, and then the residual stress distribution morphology of the high-strength steel plate along the thickness direction after the phase change is controlled, thereby avoiding the warping of the hot-rolled high-strength steel plate under high-temperature coiling conditions; in the temperature difference cooling stage, the temperature difference of the temperature field on the upper and lower surfaces of the high-strength steel plate is ,in The temperature difference between the upper and lower surfaces of the high-strength steel plate during the coiling stage, The temperature difference required to produce a temperature stress equivalent to the coiling bending stress.

2. The method for preventing warping of hot-rolled high-strength steel plates under high-temperature coiling conditions according to claim 1, wherein: During the temperature difference cooling stage, the temperature difference between the upper and lower surface temperature fields of the high-strength steel plate is achieved by increasing the cooling rate of the lower surface of the high-strength steel plate or reducing the cooling rate of the upper surface of the high-strength steel plate.

3. The method for preventing warping of hot-rolled high-strength steel plates under high-temperature coiling conditions according to claim 2, wherein: During the temperature difference cooling stage, the cooling rate of the lower surface of the high-strength steel plate increases Or the cooling rate of the high-strength steel plate surface is reduced The calculation formula is in, is the thermal conductivity of high-strength steel plate, is the temperature difference between the upper and lower surfaces of the high-strength steel plate, The density of high-strength steel plate, is the specific heat of high-strength steel plate.

4. The method for preventing warping of hot-rolled high-strength steel plates under high-temperature coiling conditions according to claim 1, wherein: The temperature difference required to produce the temperature stress equivalent to the coiling bending stress The calculation formula is in, is the coiling bending stress, is the thermal expansion coefficient of high-strength steel plate, is the elastic modulus of high-strength steel plate.

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