A rolling method for Q690 grade longitudinally variable thickness steel plate and the longitudinally variable thickness steel plate

Abstract

This application belongs to the technical field of low-alloy high-strength steel, and particularly relates to a rolling method for Q690 grade longitudinally variable thickness steel plates and the longitudinally variable thickness steel plates themselves. This method, starting from the coupling mechanism of "temperature-deformation-phase transformation-plate shape," solves the problem of "shape-property synergy" in high-strength variable thickness steel plates in one go. Using a complete process chain of "high-temperature homogenization → high-reduction fine grain → constant-force thickening → speed-hydraulic synchronization → micro-stress straightening → fine heat treatment," it obtains high-strength Q690 longitudinally variable thickness steel plates with good plate shape and precise length of the thickened section in one go.

Description

Technical Field

[0001] This application belongs to the field of low alloy high strength steel technology, and particularly relates to a rolling method for Q690 grade longitudinal variable thickness steel plate and the longitudinal variable thickness steel plate. Background Technology

[0002] Traditional medium-thick plate rolling adopts a uniform thickness design. When facing scenarios such as bridges and engineering machinery that require both lightweighting and local high load-bearing capacity, the thickness transition can only be achieved by welding plates or secondary machining, resulting in stress concentration in the weld, high processing costs, and long manufacturing cycles.

[0003] The existing patent CN107716550B's method for producing low-alloy longitudinally variable thickness steel plates is only applicable to grades with a yield strength ≤460 MPa. Its narrow rolling force window cannot suppress edge and center waviness defects in Q690 grade high-strength steel under high temperature and high pressure conditions. Furthermore, its single-point thickness setting method lacks real-time matching with the HGC hydraulic cylinder's pressing speed, resulting in a variable thickness section length fluctuation exceeding ±200 mm, which is insufficient to meet the stringent requirements of high-end equipment for accuracy ≤±50 mm. In addition, this existing technology employs multi-pass reciprocating rolling, leading to a large temperature drop and low final rolling temperature. This results in uneven microstructure in Q690 steel, deterioration of the plate shape after quenching, with flatness exceeding 5 mm / m. Subsequent hot straightening requires multiple tempering processes, resulting in high energy consumption and poor performance stability. Summary of the Invention

[0004] This application provides a rolling method for Q690 grade longitudinal variable thickness steel plate and a longitudinal variable thickness steel plate to solve the following technical problem: how to obtain Q690 grade longitudinal variable thickness steel plate with good plate shape and accurate length of variable thickness section in one rolling process.

[0005] In a first aspect, embodiments of this application provide a rolling method for Q690 grade longitudinally variable thickness steel plates, the method comprising: The 195 mm–200 mm thick billet is heated from room temperature to 1240 ℃–1280 ℃ and held for 180 min–240 min; The heated billet is rolled in eight passes in a single-stand finishing mill. The first five passes are equal-thickness rolling with a maximum single-pass reduction of 25 mm–35 mm, and the last three passes are variable-thickness rolling with the rolling force controlled within the range of 25 MN–35 MN. During the variable thickness rolling stage, the bite speed is set to 0.8 m / s–1.2 m / s, and the bite acceleration is ≤0.2 m / s². 2 Bit-in length 3.5 m–4.5 m, rolling speed 1.8 m / s–2.2 m / s, rolling acceleration 0.3 m / s² 2 –0.7 m / s 2To match the pressing speed of the HGC hydraulic cylinder, thereby synchronously controlling the length of the longitudinal variable thickness section; After rolling, the steel plate is air-cooled and then hot-straightened with a thickness of 18 mm–22 mm. The tilting amount of the hot straightening is set to 1.5 mm–2.5 mm. The straightened steel plate is subjected to quenching and tempering in sequence. The quenching temperature is 890 ℃–910 ℃ and the holding temperature is 15 min–25 min. The tempering temperature is 640 ℃–660 ℃ and the holding temperature is 25 min–35 min, so as to obtain a steel plate with a yield strength ≥690 MPa and a continuous longitudinal thickness.

[0006] Optionally, the thickness of the cast billet is 198 mm.

[0007] Optionally, the heating temperature is 1260 ℃, and the heat preservation time is 220 min.

[0008] Optionally, the bite velocity is 1 m / s and the bite acceleration is 0 m / s². 2 The bite length is 4 m, the rolling speed is 2 m / s, and the rolling acceleration is 0.5 m / s². 2 .

[0009] Optionally, the reduction amounts for the first five passes are 29.7 mm, 28.7 mm, 28.9 mm, 29.1 mm, and 29.3 mm, respectively.

[0010] Optionally, the reduction amounts for the last three passes are 14.8 mm, 10.6 mm, and 7.6 mm, respectively.

[0011] Optionally, the inlet roll gap of the thermal straightening is set to 18 mm, and the outlet roll gap is set to 20 mm.

[0012] Secondly, embodiments of this application provide a Q690 grade longitudinally variable thickness steel plate, the steel plate being prepared by the method described in any one of the first aspects.

[0013] Optionally, the width of the steel plate is 2550 mm–2650 mm; The steel plate has six thickness control points arranged sequentially along its length, dividing it into a head section equal thickness zone, a first variable thickness zone, a middle section equal thickness zone, a second variable thickness zone, and a tail section equal thickness zone. The thickness of the head section equal thickness zone and the tail section equal thickness zone are both 10 mm–14 mm, the thickness of the middle section equal thickness zone is 18 mm–22 mm, the thickness of the first variable thickness zone and the second variable thickness zone transitions continuously between 10 mm and 22 mm, the length of the first variable thickness zone and the second variable thickness zone are both 2700 mm–2800 mm, the length of the middle section equal thickness zone is 12000 mm–12400 mm, and the length of the head section equal thickness zone and the tail section equal thickness zone are both 1450 mm–1550 mm.

[0014] Optionally, the flatness of the entire steel plate is ≤3 mm / m.

[0015] The technical solutions provided in this application have the following advantages compared with the prior art: This application provides a rolling method for Q690 grade longitudinally variable thickness steel plates. This method starts from the coupling mechanism of "temperature-deformation-phase transformation-plate shape" and solves the problem of "shape-property coordination" of high-strength variable thickness steel plates in one go.

[0016] 1. Long-term high-temperature homogenization at 1240–1280 ℃ fully dissolves microalloying elements and homogenizes austenite grains, laying the foundation for high dislocation density energy storage under subsequent high pressure and deformation-induced phase transformation to refine the microstructure.

[0017] 2. In the 8-pass cross rolling, the first 5 passes of equal thickness large reduction (25–35 mm) repeatedly “break” the austenite grains through dynamic / static recrystallization to pre-establish a uniform fine-grained structure; the last 3 passes of variable thickness rolling are fixed at a constant rolling force of 25–35 MN. The rolling mill bounce equation and the reduction speed of the HGC hydraulic cylinder are matched in real time to transform the “force-displacement” coupling into a precise “thickness-length” relationship, realize closed-loop control of the length of the variable thickness zone, and eliminate the abrupt change in end shape caused by the traditional “roll gap adjustment”.

[0018] 3. During the bite-in stage, a low speed (0.8–1.2 m / s) with zero acceleration reduces head impact and ensures smooth metal filling; during the stable rolling stage, a high speed (1.8–2.2 m / s) is superimposed with 0.3–0.7 m / s. 2 Acceleration, synchronized with the HGC response frequency, suppresses the "steps" and "fishtails" of the thickness transition section, allowing for a smooth wedge shape to be obtained in one forming process.

[0019] 4. Post-rolling air cooling + small tilting dynamic hot straightening (1.5–2.5 mm) is carried out in the temperature range where austenite is not fully recrystallized. The longitudinal residual stress is released by micro-plastic deformation, and cooling waves are suppressed. Then, quenching at 890–910 ℃ + tempering at 640–660 ℃ is carried out. The yield strength of more than 690 MPa is achieved by relying on fine grains and dispersed precipitation (Mo, Nb, V carbonitrides). At the same time, the microstructure of the thickened area is uniform and the properties are continuous.

[0020] In summary, this method uses a complete process chain of "high temperature homogenization → high pressure fine grain → constant force thickening → speed-hydraulic synchronization → micro-stress straightening → fine heat treatment" to obtain high-strength Q690 longitudinally variable thickness steel plates with good plate shape and accurate length of the thickened section in one go. Attached Figure Description

[0021] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0022] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0023] Figure 1 This application provides a schematic diagram of a rolling method for a Q690 grade longitudinally variable thickness steel plate. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0025] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within that range. For example, the range descriptions of "1 to 6" or "1~6" cover all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms "including" and "contains" as used herein mean "including but not limited to"; relational terms such as "first" and "second" are used only to distinguish different entities or operations and do not imply an actual order or relationship; "and / or" indicates that multiple situations can exist individually or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. The proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the first and second terms of a proportional formula, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0026] Figure 1 This diagram illustrates a rolling method for a Q690 grade longitudinally variable thickness steel plate, as provided in this application embodiment. Figure 1 As shown: In a first aspect, embodiments of this application provide a rolling method for Q690 grade longitudinally variable thickness steel plates, the method comprising: S1. Heat the 195 mm–200 mm thick billet from room temperature to 1240 ℃–1280 ℃ and hold it for 180 min–240 min. S2. The heated billet is rolled in 8 passes in a single-stand finishing mill. The first 5 passes are equal thickness rolling with a maximum single pass reduction of 25 mm–35 mm. The last 3 passes are variable thickness rolling with the rolling force controlled within the range of 25 MN–35 MN. S3. During the variable thickness rolling stage, the bite speed is set to 0.8 m / s–1.2 m / s, the bite acceleration is ≤0.2 m / s², the bite length is 3.5 m–4.5 m, the rolling speed is 1.8 m / s–2.2 m / s, and the rolling acceleration is 0.3 m / s²–0.7 m / s² to match the pressing speed of the HGC hydraulic cylinder, thereby synchronously controlling the length of the longitudinal variable thickness section. S4. After rolling, the steel plate is air-cooled and then hot-straightened with a thickness of 18 mm–22 mm. The tilting amount of the hot straightening is set to 1.5 mm–2.5 mm. S5. The straightened steel plate is subjected to quenching and tempering in sequence. The quenching heating temperature is 890 ℃–910 ℃ and the holding temperature is 15 min–25 min. The tempering heating temperature is 640 ℃–660 ℃ and the holding temperature is 25 min–35 min, so as to obtain a steel plate with a yield strength ≥690 MPa and a continuous change in longitudinal thickness.

[0027] The 195 mm, 196 mm, 197 mm, 198 mm, 199 mm, and 200 mm billet thicknesses provide a total compression ratio of ≥3.5, thereby ensuring that the austenite grains in the region with a maximum thickness of 22 mm are fully flattened and broken, thus providing a microstructure basis for the formation of fine-grained martensite after quenching, thereby achieving a yield strength of over 690 MPa.

[0028] The combination of heating temperatures of 1240 ℃, 1250 ℃, 1260 ℃, 1270 ℃, and 1280 ℃ with holding times of 180 min, 190 min, 200 min, 210 min, 220 min, 230 min, and 240 min allows the Nb, V, and Ti microalloyed carbonitrides to completely dissolve in austenite, thereby reducing deformation resistance and minimizing rolling force fluctuations. This, in turn, ensures that the rolling force for the subsequent three passes with varying thicknesses can be stabilized within the range of 25 MN, 27 MN, 29 MN, 31 MN, 33 MN, and 35 MN, thus suppressing edge and center waves and obtaining steel plates with good shape.

[0029] The first five passes are rolled to the same thickness with single-pass reductions of 25 mm, 27 mm, 29 mm, 31 mm, 33 mm, and 35 mm, respectively. The accumulated true strain in the austenite recrystallization region is ≥0.8, thereby repeatedly recrystallizing and refining the austenite grains to ≤25 μm. This provides the genetic conditions for obtaining martensite lath bundles with a size of ≤5 μm after quenching, thus ensuring a yield strength ≥690 MPa.

[0030] The subsequent three passes of variable thickness rolling are all controlled within the range of 25 MN, 27 MN, 29 MN, 31 MN, 33 MN, and 35 MN. The force fluctuation is limited to ±2 MN by using the mill bounce equation Δh = F / M, thereby limiting the thickness fluctuation to ±0.15 mm. This allows for continuous wedge thickness to be obtained in one pass, avoiding secondary rolling and thus precisely controlling the length of the longitudinal variable thickness section.

[0031] Bite penetration speeds: 0.8 m / s, 0.9 m / s, 1.0 m / s, 1.1 m / s, 1.2 m / s; bite penetration acceleration: 0 m / s². 2 0.05m / s 2 0.1 m / s 2 0.15 m / s2 0.2 m / s 2 The bite lengths are 3.5 m, 3.8 m, 4.0 m, 4.2 m, and 4.5 m; the rolling speeds are 1.8 m / s, 1.9 m / s, 2.0 m / s, 2.1 m / s, and 2.2 m / s; and the rolling acceleration is 0.3 m / s². 2 0.4 m / s 2 0.5 m / s 2 0.6 m / s 2 0.7 m / s 2 This allows the metal to complete the thickness transition in the low-speed zone, thereby maintaining a fixed ratio of 2.2–2.6 between the HGC hydraulic cylinder pressing speed and the horizontal speed of the rolled piece. This locks the longitudinal thickness change slope at 0.3% / mm–0.4% / mm, ensuring that the length error of the variable thickness section is ≤±50 mm, thus obtaining a variable thickness section with precise length.

[0032] After rolling, the steel plate is air-cooled, so that the surface temperature of the steel plate is uniformly reduced to 550 ℃–600 ℃, thereby avoiding the uneven phase transformation stress generated by water cooling, thus providing a temperature window for hot straightening.

[0033] Hot straightening was performed with process parameters of 18 mm, 19 mm, 20 mm, 21 mm, and 22 mm thickness, and tilting amounts of 1.5 mm, 1.8 mm, 2.0 mm, 2.2 mm, and 2.5 mm, respectively. Tensile strain of 0.05%–0.08% was applied in the steel plate temperature range of 550 ℃–600 ℃, thereby releasing the transverse residual stress to <80 MPa, so that the flatness of the whole plate is ≤3 mm / m, thus obtaining a steel plate with good shape.

[0034] The quenching heating temperatures are 890 ℃, 895 ℃, 900 ℃, 905 ℃, and 910 ℃, and the holding temperatures are 15 min, 17 min, 20 min, 22 min, and 25 min, respectively, to fully austenitize the steel plate, thereby forming ≥95% martensite and ensuring a tensile strength ≥790 MPa.

[0035] Tempering temperatures of 640 ℃, 645 ℃, 650 ℃, 655 ℃, and 660 ℃, with holding times of 25 min, 27 min, 30 min, 32 min, and 35 min, respectively, precipitate (Nb,V)C nanoparticles at the martensitic lath boundaries and reduce dislocation density. This ensures a yield strength ≥690 MPa while increasing the impact toughness at -20 ℃ to ≥80 J, resulting in Q690 grade steel plates with continuously varying longitudinal thickness and meeting performance standards.

[0036] In some embodiments, the thickness of the cast billet is 198 mm.

[0037] The 198 mm billet thickness and the maximum finished product thickness of 22 mm form a compression ratio of 9:1, which ensures central density while minimizing the number of rolling passes. This reduces the temperature drop rate of the steel plate during rolling to ≤15 ℃ / pass, thereby maintaining a final rolling temperature of ≥950 ℃. This ensures that the rolling force for the last three variable thickness passes is stabilized within the range of 25 MN–35 MN, resulting in a steel plate with good shape.

[0038] In some embodiments, the heating temperature is 1260 °C and the holding time is 220 min.

[0039] The combination of a heating temperature of 1260 ℃ and a holding time of 220 min ensures that 0.04 % Nb and 0.08 % V are completely dissolved and uniformly diffused, thereby eliminating the hardness difference of the segregation zone in the center of the billet (≤15 HV). This results in subsequent variable thickness rolling force fluctuations of <±1.5MN, maintains a constant thickness transition slope, and ensures that the length error of the variable thickness section is ≤±30 mm, thus obtaining a variable thickness section with precise length.

[0040] In some embodiments, the bite velocity is 1 m / s and the bite acceleration is 0 m / s². 2 The bite length is 4 m, the rolling speed is 2 m / s, and the rolling acceleration is 0.5 m / s². 2 .

[0041] 1 m / s bite speed, 0 m / s 2 The bite acceleration and 4 m bite length enable the steel plate head to complete the thickness transition from 12 mm to 20 mm within a 4 m distance, thereby controlling the strain gradient to 0.035 mm. -1 This avoids localized stress concentrations exceeding 700 MPa, which could cause head warping and result in a steel plate with a good shape.

[0042] Rolling speed of 2 m / s and 0.5 m / s 2 The rolling acceleration linearly increases the remaining thickness zone within a length of 2700 mm–2800 mm, thus strictly synchronizing it with the slope of the HGC hydraulic cylinder pressing speed curve of 0.22 mm / s. This ensures that the length error of the thickness zone is ≤ ±30 mm, thereby obtaining a thickness zone with precise length.

[0043] In some embodiments, the reduction amounts for the first five passes are 29.7 mm, 28.7 mm, 28.9 mm, 29.1 mm, and 29.3 mm, respectively.

[0044] The reduction sequence of 29.7 mm, 28.7 mm, 28.9 mm, 29.1 mm, and 29.3 mm ensures a uniform distribution of the cumulative true strain of 1.15, thereby refining the austenite grains from 120 μm to ≤20 μm. This provides the genetic conditions for obtaining a 5 μm martensite bundle size after quenching, thus ensuring a yield strength ≥690 MPa.

[0045] A reduction difference of ≤1 mm reduces the rolling force by <1 MN per pass, thereby avoiding sudden changes in mill bounce and ensuring good strip shape.

[0046] In some embodiments, the reduction amounts for the last three passes are 14.8 mm, 10.6 mm, and 7.6 mm, respectively.

[0047] The decreasing pressure of 14.8 mm, 10.6 mm, and 7.6 mm gradually reduces the wedge slope of the variable thickness zone from 1.8% / m to 0.9% / m, thereby matching the nonlinear compensation curve of the rolling mill bounce and locking the thickness deviation within ±0.2 mm. This allows for a precise length of 2700 mm–2800 mm to be obtained in one forming process, resulting in a variable thickness section with accurate length.

[0048] In some embodiments, the inlet roll gap of the thermal straightening is set to 18 mm and the outlet roll gap is set to 20 mm.

[0049] An 18 mm inlet roll gap applies 0.3% compressive deformation to the steel plate, thereby eliminating head warping; an 20 mm outlet roll gap matches the maximum thickness of the finished product, thereby releasing residual stress peak <60 MPa, resulting in an overall plate unevenness ≤3 mm / m, thus obtaining a steel plate with good shape.

[0050] Secondly, embodiments of this application provide a Q690 grade longitudinally variable thickness steel plate, the steel plate being prepared by the method described in any one of the first aspects.

[0051] Because the steel plate is processed through the complete process chain described in the first aspect, it simultaneously possesses a yield strength ≥690 MPa, an impact toughness ≥80 J at -20 ℃, an overall plate unevenness ≤3 mm / m, and a variable thickness zone length of 2700 mm–2800 mm with an error ≤±30 mm. This satisfies the dual requirements of weight reduction and strength for bridges and buildings in one go, resulting in a Q690 grade longitudinal variable thickness steel plate with good plate shape and accurate variable thickness section length.

[0052] In some embodiments, the width of the steel plate is 2550 mm–2650 mm; The steel plate has six thickness control points arranged sequentially along its length, dividing it into a head section equal thickness zone, a first variable thickness zone, a middle section equal thickness zone, a second variable thickness zone, and a tail section equal thickness zone. The thickness of the head section equal thickness zone and the tail section equal thickness zone are both 10 mm–14 mm, the thickness of the middle section equal thickness zone is 18 mm–22 mm, the thickness of the first variable thickness zone and the second variable thickness zone transitions continuously between 10 mm and 22 mm, the length of the first variable thickness zone and the second variable thickness zone are both 2700 mm–2800 mm, the length of the middle section equal thickness zone is 12000 mm–12400 mm, and the length of the head section equal thickness zone and the tail section equal thickness zone are both 1450 mm–1550 mm.

[0053] Widths are 2550 mm, 2560 mm, 2570 mm, 2580 mm, 2590 mm, 2600 mm, 2610 mm, 2620 mm, 2630 mm, 2640 mm, and 2650 mm; the thicknesses of the head and tail sections are 10 mm, 11 mm, 12 mm, 13 mm, and 14 mm respectively; the thicknesses of the middle section are 18 mm, 19 mm, 20 mm, 21 mm, and 22 mm; the thicknesses of the first and second variable thickness sections transition continuously between 10 mm and 22 mm; the lengths of the first and second variable thickness sections are 2700 mm, 2720 mm, 2740 mm, 2760 mm, 2780 mm, and 2800 mm respectively; and the lengths of the middle section are 12000 mm, 12050 mm, 12100 mm, 12150 mm, and 12200 mm respectively. mm, 12250 mm, 12300 mm, 12350 mm, 12400 mm, and the lengths of the equal thickness zones of the head and tail sections are 1450 mm, 1470 mm, 1490 mm, 1500 mm, 1510 mm, 1530 mm, and 1550 mm, respectively.

[0054] A 100-150 mm margin is left between the 2550 mm–2650 mm width and the 2800 mm length of the rolling mill work rolls to avoid excessive temperature drop at the edges, which could lead to uneven hardness and ensure good plate shape. Six thickness control points ensure that the thickness variation slope is ≤0.4% / mm, thus guaranteeing a dynamic impact coefficient ≤1.05 when wheels pass over it, resulting in a steel plate with good performance. The length of the 2700 mm–2800 mm variable thickness zone matches the 3000 mm spacing of the bridge diaphragms, eliminating the need for secondary cutting, reducing manufacturing costs by 10%, and obtaining a precisely measured variable thickness section.

[0055] In some embodiments, the flatness of the entire steel plate is ≤3 mm / m.

[0056] The overall plate unevenness is ≤3 mm / m, ensuring that the maximum warping height of the steel plate within a 12 m length is ≤36 mm. This meets the requirements of GB / T19879-2015 for direct on-site installation of bridge steel plates without straightening, thereby saving 100% of the cost of secondary straightening on-site and obtaining Q690 grade longitudinal variable thickness steel plates with good plate shape.

[0057] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0058] I. Implementation Examples Example 1 A rolling method for Q690 grade longitudinally variable thickness steel plate includes: The 198 mm thick billet was heated from room temperature to 1260 °C and held for 220 min. The heated billet was subjected to eight passes of transverse rolling in a single-stand finishing mill. The first five passes were equal-thickness rolling with reductions of 29.7 mm, 28.7 mm, 28.9 mm, 29.1 mm, and 29.3 mm, respectively. The last three passes were variable-thickness rolling with rolling forces of 27 MN, 28 MN, and 31 MN, respectively. During the variable thickness rolling stage, the bite speed is set to 1 m / s, the bite acceleration to 0 m / s², the bite length to 4 m, the rolling speed to 2 m / s, and the rolling acceleration to 0.5 m / s², in order to match the pressing speed of the HGC hydraulic cylinder and thus synchronously control the length of the longitudinal variable thickness section. After rolling, the steel plate is air-cooled and then hot-straightened with a thickness of 20 mm. The tilting amount of hot straightening is set to 2 mm, the inlet roll gap of hot straightening is set to 18 mm, and the outlet roll gap is set to 20 mm. The straightened steel plate was quenched and tempered sequentially. The quenching temperature was 900 ℃ and held for 20 min, and the tempering temperature was 650 ℃ and held for 30 min, thereby obtaining a steel plate with a yield strength ≥690 MPa and a continuously varying longitudinal thickness.

[0059] Example 2 A 195 mm thick billet was heated from room temperature to 1240 °C and held for 180 min. The maximum single-pass reduction for the first 5 passes is 25 mm, and the rolling force for the last 3 passes is controlled at 25 MN. Biting speed 0.8 m / s, biting acceleration 0 m / s², biting length 3.5 m, rolling speed 1.8 m / s, rolling acceleration 0.3 m / s²; The thickness process parameters for hot straightening are 18 mm and the tilting amount is 1.5 mm. The quenching temperature is 890 ℃ and held for 15 min, and the tempering temperature is 640 ℃ and held for 25 min; the remaining steps are the same as in Example 1.

[0060] Example 3 A 200 mm thick billet was heated from room temperature to 1280 ℃ and held for 240 min. The maximum single-pass reduction for the first 5 passes is 35 mm, and the rolling force for the last 3 passes is controlled at 35 MN. Biting speed 1.2 m / s, biting acceleration 0.2 m / s², biting length 4.5 m, rolling speed 2.2 m / s, rolling acceleration 0.7 m / s²; The thermal straightening process parameters are: thickness 22 mm, tilting amount 2.5 mm; The quenching temperature is 910 ℃ and held for 25 min, and the tempering temperature is 660 ℃ and held for 35 min; the remaining steps are the same as in Example 1.

[0061] Example 4 The 197 mm thick billet was heated from room temperature to 1250 °C and held for 200 min. The maximum single-pass reduction for the first 5 passes is 30 mm, and the rolling force for the last 3 passes is controlled at 30 MN. Biting speed 0.9 m / s, biting acceleration 0.1 m / s², biting length 4.0 m, rolling speed 2.0 m / s, rolling acceleration 0.5 m / s²; The thickness process parameters for hot straightening are 20 mm and the tilting amount is 2.0 mm. The quenching temperature is 900 ℃ and held for 20 min, and the tempering temperature is 650 ℃ and held for 30 min; the remaining steps are the same as in Example 1.

[0062] Example 5 The 196 mm thick billet was heated from room temperature to 1270 °C and held for 210 min. The maximum single-pass reduction in the first 5 passes was 32 mm, and the rolling force in the last 3 passes was controlled at 32 MN. Biting speed 1.1 m / s, biting acceleration 0.15 m / s², biting length 4.2 m, rolling speed 2.1 m / s, rolling acceleration 0.6 m / s²; The thermal straightening process parameters are: thickness 21 mm, tilting amount 2.3 mm; The quenching temperature was 905 ℃ and held for 22 min, and the tempering temperature was 655 ℃ and held for 32 min; the remaining steps were the same as in Example 1.

[0063] Example 6 The 199 mm thick billet was heated from room temperature to 1265 °C and held for 230 min. The maximum single-pass reduction in the first 5 passes was 28 mm, and the rolling force in the last 3 passes was controlled at 29 MN. Biting speed 1.05 m / s, biting acceleration 0.05 m / s², biting length 3.8 m, rolling speed 1.9 m / s, rolling acceleration 0.4 m / s²; The thickness process parameters for hot straightening are 19 mm and the tilting amount is 1.8 mm. The quenching temperature was 895 ℃ and held for 18 min, and the tempering temperature was 645 ℃ and held for 28 min; the remaining steps were the same as in Example 1.

[0064] II. Comparative Examples (4 cases in total) Comparative Example 1 The 198 mm thick billet was heated from room temperature to 1100 ℃ and held for 120 min. The rolling force for the last three passes was controlled at 20 MN. Biting speed 1.5 m / s, biting acceleration 0.5 m / s², biting length 2 m, rolling speed 3 m / s, rolling acceleration 1.5 m / s²; The thermal straightening process parameters are: thickness 20 mm, tilting amount 2 mm; The quenching temperature is 900 ℃ and held for 20 min, and the tempering temperature is 650 ℃ and held for 30 min; the remaining steps are the same as in Example 1.

[0065] Comparative Example 2 The rolling force for the last three passes was controlled at 40 MN. The biting speed is 1.5 m / s, the biting acceleration is 0.5 m / s², the biting length is 2 m, the rolling speed is 3 m / s, and the rolling acceleration is 1.5 m / s²; the remaining heating, straightening, and heat treatment parameters are the same as in Example 1.

[0066] Comparative Example 3 The bite speed is 1.5 m / s, the bite acceleration is 0.5 m / s², the bite length is 2 m, the rolling speed is 3 m / s, and the rolling acceleration is 1.5 m / s²; the other heating, rolling force, straightening, and heat treatment parameters are the same as in Example 1.

[0067] Comparative Example 4 The quenching temperature was 850 ℃ and held for 10 min, and the tempering temperature was 600 ℃ and held for 20 min; the remaining heating, rolling and straightening parameters were the same as in Example 1.

[0068] III. Results Data Experimental methods for evaluating results: Yield strength and tensile strength: According to GB / T 228.1-2021, take a rectangular specimen of full thickness in the transverse direction at the center of the finished steel plate thickness, with a gauge length of 50 mm and a tensile speed of 5 mm / min, and record the yield strength and the tensile strength corresponding to the maximum force.

[0069] -20 ℃ Impact toughness: According to GB / T 229-2020, a 10 mm × 10 mm × 55 mm Charpy V-notch specimen was taken at 1 / 4 thickness of the finished steel plate, the test temperature was -20 ℃, and the impact absorbed energy was recorded.

[0070] Length of variable thickness section: After the steel plate is cooled, the distance from the start point to the end point of the first variable thickness zone is measured along the rolling center line using a laser rangefinder and recorded as the length of the variable thickness section. The difference between this length and the target value of 2760 mm is the length error.

[0071] Unevenness: According to Clause 6.2 of GB / T 19879-2015, the steel plate is laid flat on the platform, and the maximum gap per meter is measured with a 1 m straightedge and feeler gauge, and recorded as the unevenness value.

[0072] Table 1. Results data for both the examples and comparative examples.

[0073] As shown in Table 1, the technological advancements of this application's technical solution include: 1. The yield strength is stably 690 MPa or above, the tensile strength is ≥775 MPa, and the impact toughness at -20 ℃ is ≥80J. It simultaneously meets the dual indicators of high strength and high toughness, thus solving the industry problem of large performance fluctuations in Q690 grade variable thickness steel plates.

[0074] 2. The length control error of the variable thickness section is ≤ ±15 mm, which is reduced by an order of magnitude compared with the maximum error of ±130 mm in the comparative example. Thus, it is the first time that the requirement of 3000 mm spacing between bridge diaphragms can be met in one rolling process without secondary cutting, thereby significantly reducing manufacturing costs.

[0075] 3. The overall flatness of the board is ≤2.5 mm / m, and the board shape rating reaches Grade A. Compared with the comparative Grade C, the flatness is reduced by ≥50%, thereby eliminating the secondary straightening process on site and saving 100% of the installation cost.

[0076] 4. Within the coordinated window of strictly locking the rolling force of the last three passes at 25 MN–35 MN, the bite speed at 0.8–1.2 m / s, the rolling speed at 1.8–2.2 m / s, and the bite length at 3.5–4.5 m, the controllable thickness transition slope of 0.3% / mm–0.4% / mm was achieved for the first time, thereby solving the "step" defect in the variable thickness section and obtaining a smooth wedge shape.

[0077] 5. By using high-temperature long-term homogenization at 1240 ℃–1280 ℃ for 180 min–240 min, the Nb, V and Ti microalloys are completely dissolved, thereby suppressing the rolling force fluctuation within ±1.5 MN. This ensures that the plate shape, thickness accuracy and mechanical properties meet the standards simultaneously, thus breaking through the bottleneck of high-strength steel variable thickness plate shape and performance being unable to be balanced.

[0078] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A rolling method of a Q690 grade longitudinal variable thickness steel plate, characterized in that, The method comprises: heating the casting blank with a thickness of 195 mm-200 mm from room temperature to 1240 ℃-1280 ℃ and maintaining for 180 min-240 min; implementing 8 passes of cross-rolling on the heated casting blank in a single-stand finishing mill, the first 5 passes are equal-thickness rolling with a maximum single-pass reduction of 25 mm-35 mm, and the last 3 passes are variable-thickness rolling with a rolling force controlled in the range of 25 MN-35 MN; In the variable thickness rolling stage, the bite speed is set to 0.8 m / s-1.2 m / s, the bite acceleration is ≤0.2 m / s 2 , the bite length is 3.5 m-4.5 m, the rolling speed is 1.8 m / s-2.2 m / s, and the rolling acceleration is 0.3 m / s 2 -0.7 m / s 2 , to match the HGC hydraulic cylinder speed, thereby synchronously controlling the length of the longitudinal variable thickness section. after rolling, implementing air cooling on the steel plate, and then implementing hot straightening with a thickness process parameter of 18 mm-22 mm, the inclination amount of the hot straightening is set to 1.5 mm-2.5 mm; implementing quenching and tempering on the straightened steel plate in sequence, the quenching heating temperature is 890 ℃-910 ℃ and the holding time is 15 min-25 min, the tempering heating temperature is 640 ℃-660 ℃ and the holding time is 25 min-35 min, so as to obtain a steel plate with a yield strength ≥690 MPa and a longitudinal thickness continuously varying.

2. The method of claim 1, wherein, The casting blank has a thickness of 198 mm.

3. The method of claim 1, wherein, The heating temperature is 1260 ℃, and the holding time is 220 min.

4. The method of claim 1, wherein, The bite speed is 1 m / s, the bite acceleration is 0 m / s 2 The bite length is 4 m, the rolling speed is 2 m / s, the rolling acceleration is 0.5 m / s 2 .

5. The method of claim 1, wherein, The reduction of the first 5 passes is 29.7 mm, 28.7 mm, 28.9 mm, 29.1 mm and 29.3 mm in sequence.

6. The method of claim 1, wherein, The reduction of the last 3 passes is 14.8 mm, 10.6 mm and 7.6 mm in sequence.

7. The method of claim 1, wherein, The entry roll gap of the hot straightening is set to 18 mm, and the exit roll gap is set to 20 mm.

8. A Q690 grade longitudinal variable thickness steel plate characterized by, The steel plate is obtained by the method according to any one of claims 1 to 7.

9. The steel sheet according to claim 8, characterized by The steel plate has a width of 2550 mm-2650 mm. Six thickness control points are arranged in the length direction of the steel plate in sequence, the six thickness control points divide the steel plate into a head equal-thickness zone, a first variable-thickness zone, a middle equal-thickness zone, a second variable-thickness zone and a tail equal-thickness zone, the thickness of the head equal-thickness zone and the tail equal-thickness zone is 10 mm-14 mm, the thickness of the middle equal-thickness zone is 18 mm-22 mm, the thickness of the first variable-thickness zone and the second variable-thickness zone continuously transitions between 10 mm-22 mm, the length of the first variable-thickness zone and the second variable-thickness zone is 2700 mm-2800 mm, the length of the middle equal-thickness zone is 12000 mm-12400 mm, and the length of the head equal-thickness zone and the tail equal-thickness zone is 1450 mm-1550 mm.

10. The steel sheet according to claim 8, characterized by The whole plate unevenness of the steel plate is ≤3 mm / m.

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