A method for improving the uniformity of mechanical properties of phase transformation strengthened ultra-high strength steel plates

Abstract

The present invention discloses a method for improving the uniformity of mechanical properties of phase-change strengthened ultra-high strength steel plates, and belongs to the field of steel plate production. It has the following steps: 1. Rolling, using 2-frame rough rolling and 7-frame finishing hot rolling mills, the pressure measurement of the pressure measuring machine edge is 0-50mm; the rough rolling final rolling temperature is 1100-1170℃, and the tail temperature of the slab at the rough rolling outlet is 30-50℃ higher than the head temperature; the edge heater is used at the finishing entrance, the edge heating setting temperature is 40-60℃, and the finishing rolling final rolling temperature is 870-910℃; 2. Cooling, after the slab is rolled, the slab is cooled in three stages, which are ultra-fast cooling, air cooling and water cooling in sequence; 3. Coiling, the target coiling temperature is ≤150℃; 4. Leveling, the working roll of the leveling machine adopts a concave roll, and the convexity is 40-60μm. The present invention can significantly improve the uniformity of mechanical properties of thin-gauge phase-change strengthened hot-rolled ultra-high strength steel without adding new equipment investment.

Description

Technical Field

[0001] The present invention belongs to the field of steel plate production, and more specifically, relates to a method for improving the uniformity of mechanical properties of phase transformation strengthened ultra-high strength steel plates. Background Art

[0002] With the improvement of process technology and the upgrading of rolling cooling equipment, hot-rolled ultra-high strength steel (tensile strength R m ≥1000MPa) has become a research hotspot for major steel companies and scientific research institutions. The product strengthening method is mainly achieved through online direct quenching through the ultra-fast cooling system after rolling, and the product strengthening method is mainly phase transformation strengthening.

[0003] Phase change strengthening can effectively solve the problem that ultra-high strength steel is difficult to achieve both high strength and easy forming. However, due to factors such as high ultra-fast cooling strength, thin thickness specifications, narrow cooling process window, and large influence of temperature fluctuations on phase change transformation, the performance of phase change strengthened hot-rolled ultra-high strength steel fluctuates greatly in width and length, which is specifically manifested as: high in the middle and low on both sides in the width direction, low at the head and high at the tail in the length direction, and the fluctuation range of yield strength and tensile strength is as high as about 200MPa. Therefore, the control of performance uniformity of phase change strengthened hot-rolled ultra-high strength steel has always been a difficult problem in the industry. Uneven performance will directly lead to inconsistent deformation degrees at different positions during the subsequent product processing and forming process, affecting the appearance, quality and service safety performance of the final product. Therefore, downstream users have higher and higher requirements for the performance uniformity of hot-rolled ultra-high strength steel.

[0004] About phase transformation strengthened hot rolled ultra-high strength steel (R m ≥1000MPa), such as CN103233161B, CN104532126B, CN108411203B, CN107904509A, CN110331326A, CN110578095A, CN104532126A, CN110760752A, CN111334720A, etc. However, the main technical problem solved by the above patents is how to achieve coordinated control of the three properties of ultra-high strength, high wear resistance and easy forming by reducing the alloy composition design and combining the optimization of rolling and cooling processes, and the technical problem of how to improve the uniformity of its mechanical properties is not involved.

[0005] Regarding the patent application for improving the uniformity of the mechanical properties of steel strips, the patent with publication number CN107824619A discloses a method for improving the uniformity of the performance of steel strips, which mainly adopts a step cooling method in the width direction of the steel strips to reduce the cooling water flow on both sides of the steel strips, improve the temperature uniformity in the width direction of the steel strips, and thus improve the uniformity of the performance in the width direction of the steel strips. However, this method is mainly used to improve the uniformity of the performance in the width direction of the steel strips, and the uniformity of the performance in the length direction of the steel strips is not involved, and the steel types targeted by the embodiments are all R m For low-strength steel plates with a strength of ≤615MPa, the main strengthening method of the material is fine grain strengthening.

[0006] The two patents with publication numbers CN107983784B and CN112246910A use a segmented head-to-tail shielding method to improve the uniformity of the performance of the steel plate in the length direction, but the uniformity of the performance in the width direction of the strip is not involved, and the method is only for R m Medium and thick plates with a strength level of ≤605MPa are not suitable for thin specifications (thickness ≤6mm) and hot-rolled ultra-high strength steel with Rm≥1000MPa. At the same time, this method only involves the cooling process and does not involve other processes. The uniformity of strip performance (under certain chemical composition conditions) is closely related to the processes involving temperature such as heating, rolling, cooling, and coiling, and needs to be analyzed and optimized from the whole process.

[0007] The patent with publication number CN109576467B mainly uses hot rolling + annealing process to optimize and improve the uniformity of performance of cold-rolled strip in the length direction. The targeted steel types are precipitation-strengthened cold-rolled high-strength steel containing Nb and non-phase transformation-strengthened hot-rolled ultra-high-strength steel, and the uniformity of performance in the width direction is not involved.

[0008] The patent with publication number CN113106208A mainly improves the performance uniformity of 780MPa grade galvanized dual-phase steel in the length direction by optimizing the continuous annealing process. The performance uniformity in the width direction is not involved, and it is a cold-rolled strip steel.

[0009] The two patents with publication numbers CN103031419B and CN113249557A mainly use U-shaped cooling process to improve the uniformity of the performance of the strip in the longitudinal direction. On the one hand, this method is only applicable to Ti precipitation-strengthened high-strength steel (R m ≤700MPa) or SPHC low carbon steel, is not applicable to Rm≥1000MPa level phase transformation strengthened hot rolled ultra-high strength steel. On the other hand, its width direction performance uniformity is not involved.

[0010] The two patents with publication numbers CN112063815A and CN110614280A mainly improve the temperature uniformity and performance uniformity of the strip in the length and width directions by adding an online heat preservation cover device after the coiler. However, this method is only applicable to Ti precipitation-strengthened hot-rolled high-strength steel and requires additional equipment investment after the coiler, which is costly and not applicable to R m ≥1000MPa level phase transformation strengthened hot-rolled ultra-high strength steel.

[0011] In summary, the existing technologies for improving the uniformity of mechanical properties of steel strip can be summarized as follows: (1) The research objects focus on fine-grained or precipitation-hardened hot-rolled high-strength steel (R m ≤700MPa) or thicker medium and thick plates, rather than thinner (≤6mm) phase change strengthened hot rolled ultra-high strength steel (R m ≥1000MPa); (2) The research object is 780MPa-grade phase transformation strengthened cold-rolled high-strength steel. The product strength level is relatively low and the performance uniformity of the strip in the length direction is mainly improved by optimizing the subsequent annealing process. The hot rolling process parameter optimization and the width direction are not involved; (3) The performance uniformity of the strip in the length direction is improved by segmented head and tail shielding or U-shaped cooling process, and the performance uniformity in the width direction is not involved; (4) By adding an online insulation cover device after the coiler, this method is only applicable to the improvement of the performance uniformity of precipitation strengthened high-strength steel, not applicable to phase transformation strengthened ultra-high strength steel, and requires additional equipment investment. Therefore, the above methods are not suitable for improving the performance uniformity of thin-gauge (≤6mm) phase transformation strengthened hot-rolled ultra-high strength steel (R m ≥1000MPa) uniformity of mechanical properties. Summary of the invention

[0012] 1. Problems to be solved

[0013] The existing technology has not yet perfected the thin-gauge (≤6mm) phase transformation strengthened hot-rolled ultra-high strength steel (R m ≥1000MPa), the present invention provides a method for improving the uniformity of mechanical properties of phase transformation strengthened ultra-high strength steel plates, which can significantly improve the uniformity of mechanical properties of thin-gauge phase transformation strengthened hot-rolled ultra-high strength steel through reasonable coordination of the entire process of rolling, cooling, coiling and flattening without adding additional equipment investment.

[0014] 2. Technical solution

[0015] To solve the above problems, the present invention adopts the following technical solutions.

[0016] A method for improving the uniformity of mechanical properties of phase transformation strengthened ultra-high strength steel plates comprises the following steps:

[0017] 1. Rolling

[0018] A 2-stand rough rolling and 7-stand finishing hot rolling mill is used, and the edge pressure measurement of the pressure measuring machine is 0-50mm; the rough rolling and final rolling temperature is 1100-1170℃, and the tail temperature of the slab at the rough rolling outlet is 30-50℃ higher than the head temperature; an edge heater is used at the finishing rolling entrance, and the edge heating setting temperature is 40-60℃, and the finishing rolling temperature is 870-910℃;

[0019] 2. Cooling

[0020] After the slab is rolled, it is cooled in three stages, which are ultra-fast cooling, air cooling and water cooling;

[0021] 3. Coil

[0022] Target coiling temperature ≤150℃;

[0023] 4. Leveling

[0024] The working roll of the leveling machine adopts a concave roll with a convexity of -40 to -60μm.

[0025] As a further improvement of the technical solution, in step 1, the rough rolling adopts 3+5 rolling passes, and the rough rolling dephosphorization strategy is the first pass of the R1 stand and the second pass of the R2 stand.

[0026] As a further improvement of the technical solution, in step 1, the outlet convexity C of the finishing rolling F7 stand is 40 =20~40μm, the rolling center line is -20~+20mm, among which C40 is the difference between the thickness at the midpoint of the strip cross section and the average thickness at a distance of 40mm from the two side edges.

[0027] As a further improvement of the technical solution, in step 2, the ultra-fast cooling section adopts centralized cooling, the cooling rate is ≥80°C / s, the cooling water flow ratio of the upper and lower surfaces of the strip is (1:1.15) to (1:1.30), and the side spraying is symmetrical cross-side spraying.

[0028] As a further improvement of the technical solution, in step 2, the air cooling period is 4 to 8 seconds.

[0029] As a further improvement of the technical solution, in step 2, the water cooling section adopts a sparse cooling method, the cooling water flow ratio of the upper and lower surfaces of the strip is (1:1.05) to (1:1.15), the side spraying is symmetrical cross-side spraying, and the cooling rate is 50 to 80°C / s.

[0030] As a further improvement of the technical solution, in step 2, the water cooling section uses an edge shielding process, and the edge shielding distance is 100 to 150 mm.

[0031] As a further improvement of the technical solution, in step three, the coiled steel coils are separately stacked in a low-temperature finished product warehouse.

[0032] As a further improvement of the technical solution, in step 4, the flattening rolling force is 700-900t, the bending roll force is 80-100t, and the flattening rate is 50-100m / min.

[0033] 3. Beneficial effects

[0034] Compared with the prior art, the method of improving the uniformity of mechanical properties of phase transformation strengthened ultra-high strength steel plate in the present invention solves the existing thin specification (≤6mm) phase transformation strengthened hot-rolled ultra-high strength steel (R m ≥1000MPa) has large performance fluctuations, and the uniformity of mechanical properties is good. The yield strength fluctuation range of hot-rolled strip in width and length is ≤90MPa, and the tensile strength fluctuation range is ≤70MPa, which better meets the requirements of subsequent processing and forming procedures of the material, and the plate quality is significantly improved, which improves user satisfaction with the product. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 The following are the fluctuation line graphs of the mechanical properties of the steel strip in the width and length directions of Example 1;

[0036] Figure 2 The following are the fluctuation line graphs of the mechanical properties of the steel strip in the width and length directions of Example 2;

[0037] Figure 3 The mechanical property fluctuation line graphs of the steel strip of Example 3 in the width and length directions respectively;

[0038] Figure 4 The following are the fluctuation line graphs of the mechanical properties of the steel strip of Comparative Example 1 in the width and length directions respectively;

[0039] Figure 5 The following are the fluctuation line graphs of the mechanical properties of the steel strip of Comparative Example 2 in the width and length directions respectively;

[0040] Figure 6 The following are the fluctuation line graphs of the mechanical properties of the steel strip of Comparative Example 3 in the width and length directions respectively;

[0041] Figure 7 The rough rolling temperature distribution diagram of the intermediate billet in the length direction after rough rolling in Example 1 and Comparative Example 1;

[0042] Figure 8 The rough rolling temperature distribution diagram of the intermediate billet in the length direction after rough rolling in Example 2 and Comparative Example 2;

[0043] Fig. 9 The rough rolling temperature distribution diagram of the intermediate billet in the length direction after rough rolling of Example 3 and Comparative Example 3. DETAILED DESCRIPTION

[0044] Exemplary embodiments of the present invention are described in detail below. Although these exemplary embodiments are described in sufficient detail to enable those skilled in the art to implement the present invention, it should be understood that other embodiments may be implemented and various changes may be made to the present invention without departing from the spirit and scope of the present invention. The more detailed description of the embodiments of the present invention below is not intended to limit the scope of the claimed invention, but is merely for illustration and does not limit the description of the features and characteristics of the present invention, in order to propose the best mode for performing the present invention, and is sufficient to enable those skilled in the art to implement the present invention. Therefore, the scope of the present invention is limited only by the appended claims.

[0045] The present invention provides a method for improving the uniformity of mechanical properties of phase transformation strengthened ultra-high strength steel plates, for thickness ≤ 6 mm, tensile strength R m Phase transformation strengthened ultra-high strength steel plate with a strength of ≥1000MPa can comprehensively improve the overall uniformity of its mechanical properties. Among them, the chemical composition and mass percentage of the steel plate are: C: 0.12-0.16%, Si: 0.80-1.30%, Mn: 1.2-2.0%, P: ≤0.010%, S: ≤0.005%, Als: 0.40-0.60%, Ti: 0.015-0.050%, and the rest is Fe and unavoidable inclusions. The specific process and principle are described in detail below.

[0046] The method comprises the following steps:

[0047] 1. Rolling

[0048] A 2-stand rough rolling and 7-stand finishing hot rolling mill is used. The rough rolling adopts 3+5 rolling passes, and the edge pressure measurement of the pressure measuring machine is 0-50mm. The rough rolling dephosphorization strategy is the 1st pass of the R1 frame and the 2nd pass of the R2 frame. In this embodiment, RX represents the Xth frame in the rolling direction. The rough rolling final rolling temperature R2DT=1100-1170℃, and the tail temperature of the slab at the rough rolling outlet is 30~50℃ higher than the head temperature. The insulation covers between the rough rolling and finishing rolling units are all put into use. The edge heater is put into use at the finishing entrance, and the edge heating setting temperature is 40~60℃. The finishing rolling adopts a small convexity and constant speed rolling control strategy. The F7 frame outlet convexity C 40 =20~40μm, C40 is the difference between the thickness of the midpoint of the strip cross section and the average thickness at a position 40mm away from the two side edges. The rolling center line is controlled at -20~+20mm, and the finishing temperature FDT=870~910℃.

[0049] In this step, the side pressure of the side press before rough rolling is 0-50mm. This is mainly based on the consideration that excessive side pressure will increase the temperature gradient between the edges on both sides of the strip and the middle position, and aggravate the temperature non-uniformity in the width direction of the strip. Using a small side pressure can reduce the impact of the measured pressure on the temperature drop on the edges on both sides of the strip.

[0050] The rough rolling dephosphorization passes are the first pass of the R1 stand and the second pass of the R2 stand, mainly based on the need for shot blasting on the product surface, low requirements for surface quality, and minimizing the impact of high-pressure descaling water on the temperature uniformity of the strip width and length.

[0051] The R2DT corresponding to the tail of the intermediate billet is 30-50℃ higher than that of the head, mainly based on constant speed rolling, increasing the temperature and performance uniformity of the strip in the length direction. The use of insulation cover and edge heater is to improve the temperature and performance uniformity of the strip in the width direction.

[0052] F7 frame outlet convexity C 40 =20-40μm. The main purpose is to improve the thickness accuracy and stability in the width direction, and avoid the occurrence of double-sided waves after the strip exits the F7 stand due to thickness fluctuations in the width direction, which is not conducive to the temperature uniformity and performance uniformity in the width direction of subsequent laminar cooling. Controlling the rolling center line at -20~+20mm is mainly based on controlling the same amount of cooling water shielding on both sides of the plate width direction in the laminar cooling section, and improving the edge shielding for the temperature and performance uniformity in the width direction. Constant speed rolling is adopted, the main purpose of which is to increase the air cooling time of the strip in the length direction of the air cooling section of the cooling process, and the stability of the end temperature of the air cooling section, so as to lay the foundation for improving the coiling temperature uniformity and performance uniformity in the length direction after the strip is cooled.

[0053] 2. Cooling

[0054] After the steel plate leaves the F7 finishing mill, it is cooled in three stages: ultra-fast cooling, air cooling, and water cooling. The ultra-fast cooling stage uses centralized cooling, with a cooling rate of ≥80℃ / s. The cooling water flow ratio of the upper and lower surfaces of the strip is (1:1.15) to (1:1.30), and the side spraying is symmetrical cross-side spraying. After ultra-fast cooling, air cooling is performed, and the side spraying of the air cooling stage is closed. After 4 to 8 seconds of air cooling, water cooling is performed. The water cooling stage uses sparse cooling and uses the edge shielding process. The edge shielding distance is 100 to 150 mm. The cooling water flow ratio of the upper and lower surfaces of the strip is (1:1.05) to (1:1.15), and the side spraying is symmetrical cross-side spraying. The cooling rate is 50 to 80℃ / s, and coiling is performed after water cooling.

[0055] In this step, the three-stage cooling mode of ultra-fast cooling-air cooling-water cooling is adopted mainly to obtain a ferrite + martensite dual-phase structure.

[0056] Among them, the ultra-fast cooling section adopts centralized cooling and a cooling rate of ≥80℃. The main purpose is to reduce the cooling time of the strip, quickly reduce the temperature of the strip from the austenite phase to the ferrite phase, and improve the consistency of phase transformation and uniformity of performance in the width and length directions of the strip. In order to control the temperature uniformity of the strip in the ultra-fast cooling section and the water cooling section in the width direction, measures such as controlling the upper and lower cooling water ratio, cross-symmetrical side spraying, and edge shielding are mainly adopted. The purpose of opening less cooling water in the ultra-fast cooling section and the water cooling section than in the lower cooling water is mainly to avoid excessive upper cooling water remaining and agglomerating on the upper surface of the strip, resulting in uneven temperature distribution in the width direction of the strip, which is not conducive to the uniformity of performance in the width direction of the strip. The edge shielding of the water cooling section is mainly to improve the uniformity of temperature and performance in the width direction of the strip. The sparse cooling and water cooling cooling rate of 50-80℃ / s are mainly to reduce the cooling rate as much as possible to avoid the generation of organizational stress and phase change stress due to excessive cooling rate, which leads to cooling single-side waves and double-side waves in the strip, which is not conducive to the uniformity of temperature and performance in the width and length directions. The air cooling time of the air cooling section is 4-8s, which is mainly based on the need to control the material structure and performance to ensure that the material obtains a certain proportion of ferrite structure.

[0057] 3. Coil

[0058] Low-temperature coiling is adopted, and the target coiling temperature CT≤150℃. The coiled steel coils are stacked separately in the low-temperature finished product warehouse to avoid mixed stacking with other high-temperature hot-rolled finished coils.

[0059] In this step, the low-temperature coiling strategy is mainly used to control the uniformity of the coiling temperature in the width and length directions of the strip. Because the existing cooling model does not fully consider the influence of the internal heat conduction generated by the phase change during the cooling process of the strip on the surface heat transfer coefficient, the coiling temperature in the width and length directions of the strip fluctuates greatly when the coiling temperature is 200-400°C, and the uniformity is difficult to control. The coiling temperature fluctuation will significantly affect the yield strength and yield strength ratio of the ferrite and martensite phase transformation strengthening types, and thus the uniformity of the material properties is difficult to control. After coiling, the hot coil is stacked separately in the low-temperature finished product warehouse to avoid mixed stacking with other high-temperature hot-rolled finished coils, mainly to avoid the high-temperature coil causing the temperature of the outer ring of the tail of the hot coil of the strip to increase through heat conduction, thereby increasing the yield strength and yield strength ratio of the strip, which is not conducive to the uniformity of the performance in the length direction of the strip.

[0060] 4. Leveling

[0061] A high-strength leveling mill is used for leveling. The working roll of the leveling mill adopts a concave roll with a convexity of -40 to -60 μm, a leveling rolling force of 700 to 900 tons, a bending roll force of 80 to 100 tons, and a leveling rate of 50 to 100 m / min.

[0062] In this step, the working roll of the leveling machine adopts a concave roll profile with a specific convexity, combined with the coordination of the leveling rolling force and the bending roll force, which can increase the deformation of the two side edge positions in the width direction of the strip and reduce the deformation of the middle position of the strip. The degree of work hardening is cleverly used to affect the mechanical properties of the strip, especially the yield strength, so that the degree of work hardening in the middle position of the strip is smaller, the increase in yield strength is reduced, and the degree of work hardening at the two side edge positions of the strip is larger, and the increase in yield strength is increased, thereby reducing the performance difference between the middle position and the two side edge positions in the width direction of the strip, and improving the uniformity of performance in the width direction of the strip. The leveling rate is 50-100m / min, which should not be too fast, mainly based on the stability of the leveling process and the improvement of plate quality.

[0063] In summary, this method solves the existing thin-gauge (≤6mm) phase change strengthened hot-rolled ultra-high strength steel (R m ≥1000MPa) has large performance fluctuations, and the uniformity of mechanical properties is good. The yield strength fluctuation range of hot-rolled strip in width and length is ≤90MPa, and the tensile strength fluctuation range is ≤70MPa, which better meets the requirements of subsequent processing and forming procedures of the material, and the plate quality is significantly improved, which improves user satisfaction with the product.

[0064] It must be noted that this method emphasizes the control of the uniformity of the mechanical properties of phase-change strengthened ultra-high-strength steel plates from the perspective of the entire process of rolling-cooling-coiling-flattening, rather than optimizing and controlling a certain process parameter of a certain process in rolling, cooling, coiling, and flattening. Under the premise of a certain chemical composition of the material, the uniformity of the mechanical properties of the steel plate is closely related to the uniformity of the organization, and the uniformity of the organization is closely related to the uniformity of the process temperature and cooling rate of the rolling-cooling-coiling process. The uniformity of the temperature and cooling rate of the previous and subsequent processes are closely linked, and the abnormal temperature and cooling rate of a certain process will affect the final organization of the steel plate. In addition, the uniformity of the mechanical properties of the steel plate that is rolled and cooled to room temperature is also related to the degree of hardness of the subsequent finishing process. The greater the degree of hardening, the greater the increase in the strength of the strip. The influence of the finishing process on the mechanical properties of the material must be considered. Therefore, it is necessary to control the uniformity of the mechanical properties of phase-change strengthened ultra-high-strength steel plates from the perspective of the entire process of rolling-cooling-coiling-flattening in order to obtain a steel coil with the required performance.

[0065] The technical solution of the present invention is described below through specific embodiments and comparative examples.

[0066] The tensile strength level of the corresponding steel grades of Examples 1-3 and Comparative Examples 1-3 is 1000 MPa, and their chemical compositions and weight percentage contents are: 0.14% C, 1.0% Si, 1.65% Mn, 0.14% Cr, 0.009% P, 0.003% S, 0.46% Als, 0.025% Ti, and the rest is Fe and unavoidable inclusions.

[0067] Through the rolling, cooling, coiling and leveling process, the R2DT distribution in the length direction of the intermediate billet after rough rolling in the embodiment and the comparative example is shown in FIG. Figures 7 to 9 The specific process parameters are shown in Tables 1 and 2.

[0068] The evaluation method for uniformity of mechanical properties is as follows: 3 full-plate width specimens are taken from the head, middle and tail of the hot-rolled strip in the longitudinal direction, and 10 longitudinal tensile specimens are cut from each full-plate width specimen at equal intervals in the width direction, totaling 30 specimens, and room temperature tensile tests are carried out according to GB / T228.1 to obtain the yield strength and tensile strength indicators of the material. The uniformity of mechanical properties is based on the fluctuation range and fluctuation average of yield strength and tensile strength.

[0069] Among them, the fluctuation range of yield strength in the width direction = Max (yield strength of 10 tensile specimens) - Min (yield strength of 10 tensile specimens), the fluctuation range of tensile strength in the width direction = Max (tensile strength of 10 tensile specimens) - Min (tensile strength of 10 tensile specimens), and the average fluctuation value of yield strength or tensile strength in the width direction is the average value of the fluctuation range of yield strength or tensile strength corresponding to the specimens at the three positions of the head, middle and tail in the width direction.

[0070] The fluctuation range of yield strength in the length direction = Max (yield strength of the head, middle and tail of the hot coil corresponding to the same width position) - Min (yield strength of the head, middle and tail of the hot coil corresponding to the same width position), the fluctuation range of tensile strength in the length direction = Max (tensile strength of the head, middle and tail of the hot coil corresponding to the same width position) - Min (tensile strength of the head, middle and tail of the hot coil corresponding to the same width position), the average value of yield strength or tensile strength fluctuation in the length direction is the average value of the fluctuation range of yield strength or tensile strength of tensile specimens at 10 positions in the width direction.

[0071] The performance distribution of the hot-rolled strip in the width and length directions corresponding to the embodiment and the comparative example is shown in Figure 1-6 As shown, the yield strength and tensile strength fluctuation ranges of the strip width and length are shown in Table 3. It can be seen that the yield strength fluctuation range of the strip width and length corresponding to the embodiment is ≤90MPa, and the tensile strength fluctuation range is ≤70MPa. Compared with the comparative example, the performance uniformity is significantly improved.

[0072] Table 1 Hot-rolled strip rolling and cooling process parameters of the embodiment and comparative example

[0073]

[0074] Table 2 Hot-rolled strip leveling process parameters of the embodiments and comparative examples

[0075]

[0076] Table 3 Hot rolled strip width and length performance fluctuation values ​​corresponding to the embodiments and comparative examples

[0077]

[0078] The examples described in the present invention are merely descriptions of the preferred implementation modes of the present invention, and are not intended to limit the concept and scope of the present invention. Without departing from the design concept of the present invention, various modifications and improvements made to the technical solutions of the present invention by engineers and technicians in this field should all fall within the protection scope of the present invention.

Claims

1. A method for improving the uniformity of mechanical properties of phase transformation strengthened ultra-high strength steel plate, characterized in that: The following steps are involved:

1. Rolling A 2-stand rough rolling and 7-stand finishing hot rolling mill is used, and the edge pressure measurement of the pressure measuring machine is 0-50mm; the rough rolling and final rolling temperature is 1100-1170℃, and the tail temperature of the slab at the rough rolling outlet is 30-50℃ higher than the head temperature; an edge heater is used at the finishing rolling entrance, and the edge heating setting temperature is 40-60℃, and the finishing rolling temperature is 870-910℃; 2. Cooling After the slab is rolled, it is cooled in three stages, which are ultra-fast cooling, air cooling and water cooling; 3. Coil Target coiling temperature ≤150℃; 4. Leveling The working roll of the leveling machine adopts a concave roll with a convexity of -40 to -60 μm; In the step 1, the outlet convexity C of the finishing rolling F7 stand 40 =20~40μm, the rolling center line is -20~+20mm, where C40 is the difference between the thickness at the midpoint of the strip cross section and the average thickness at a distance of 40mm from the two sides; In the step 2, the ultra-fast cooling section adopts centralized cooling, the cooling rate is ≥80°C / s, the cooling water flow ratio of the upper and lower surfaces of the strip is (1:1.15) to (1:1.30), and the side spraying is symmetrical cross-side spraying; In the step 2, the air cooling period is 4 to 8 seconds; In the step 2, the water cooling section adopts a sparse cooling method, the cooling water flow ratio of the upper and lower surfaces of the strip is (1:1.05) to (1:1.15), the side spraying is symmetrical cross-side spraying, and the cooling rate is 50 to 80°C / s; In the step 4, the flattening rolling force is 700-900t, the bending roll force is 80-100t, and the flattening rate is 50-100m / min; The thickness of the steel plate is ≤6mm, and the tensile strength is R m ≥1000MPa, its chemical composition and its mass percentage are: C: 0.12-0.16%, Si: 0.80-1.30%, Mn: 1.2-2.0%, P: ≤0.010%, S: ≤0.005%, Als: 0.40-0.60%, Ti: 0.015-0.050%, and the rest are Fe and unavoidable inclusions.

2. A method for improving the uniformity of mechanical properties of phase transformation strengthened ultra-high strength steel plate according to claim 1, characterized in that: In the step 1, the rough rolling adopts 3+5 rolling passes, and the rough rolling dephosphorization strategy is the first pass of the R1 stand and the second pass of the R2 stand.

3. The method for improving the uniformity of mechanical properties of phase transformation strengthened ultra-high strength steel plate according to claim 1, characterized in that: In the step 2, the water cooling section uses an edge shielding process, and the edge shielding distance is 100 to 150 mm.

4. A method for improving the uniformity of mechanical properties of phase transformation strengthened ultra-high strength steel plate according to claim 3, characterized in that: In the step three, the coiled steel coils are individually stacked in a low-temperature finished product warehouse.

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