A method of producing a 1500ms martensitic steel and a roll formed product

Abstract

The application discloses a production method of 1500MS martensite steel and roll forming product, which comprises the steps of hot rolling, cover annealing, acid rolling and continuous annealing; the hot rolling step is characterized in that the coiling temperature is 530 DEG C + / - 17 DEG C; the cover annealing step is characterized in that the cover annealing is carried out at 480-500 DEG C for 5-6 h; and the continuous annealing step is characterized in that the product is first heated at the soaking section temperature of 870-890 DEG C, then slowly cooled at the slow cooling section temperature of 825-850 DEG C at a speed of 0.1-0.5 DEG C / s, then quickly cooled at the quick cooling section temperature of 360-320 DEG C at a speed of 10-25 DEG C / s, and finally cooled at the aging section temperature at a speed of 1-2 DEG C / s. The method finally obtains a pure and homogeneous structure mainly composed of fine martensite and a small amount of bainite, the yield strength of the finished product is 1210-1231 MPa, the tensile strength is 1509-1544 MPa, the elongation is 7.22%-9.77%, the strength and plasticity match is excellent, and the bending forming performance is greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of martensitic steel technology, and in particular to a production method of 1500MS martensitic steel and roll forming products. Background Technology

[0002] While 1500MPa grade martensitic steel (1500MS) possesses cold forming potential, its roll forming process is limited by the material's ultra-high strength and low plasticity, mainly facing problems such as high risk of cracking, difficulty in controlling springback, large residual stress, and limitations in subsequent processing; as detailed below: Localized cracking and low forming limit: The elongation of 1500MS at room temperature is extremely low (usually <10%). During the rolling process, the edges or rounded corners of the sheet are prone to edge cracks or transverse fractures due to tensile strain exceeding the limit. In particular, when the bending radius is less than a multiple of the material thickness (e.g., <3.5t), the risk of cracking increases exponentially.

[0003] Severe springback leads to poor dimensional accuracy: Due to its extremely high yield strength, the elastic recovery after unloading is large, and the springback angle can reach several degrees or even more. This causes the cross-sectional shape of the part to deviate from the mold surface, requiring correction through complex bending compensation or subsequent shaping processes; otherwise, it is difficult to meet assembly tolerance requirements (deviations often exceed ±1.5mm).

[0004] High residual stress induces delayed cracking: The plastic deformation caused by rolling accumulates a large amount of tensile residual stress inside the material. Combined with the sensitivity of martensite to hydrogen, this easily induces hydrogen-induced delayed cracking. This defect may occur suddenly within hours to days after forming, and often appears in stress concentration areas (such as hole edges and sharp corners).

[0005] Surface damage and die wear: Materials with tensile strength exceeding 1000 MPa require greater forming force and more passes, exacerbating tool wear and increasing the risk of strip breakage. High-hardness materials exhibit high frictional resistance to rollers, easily causing scratches or roughening on the sheet surface; simultaneously, they accelerate die / roller wear, requiring frequent downtime for maintenance and increasing production costs.

[0006] Therefore, there is an urgent need for a multi-pass roll forming method for producing 1500MS martensitic steel that can solve the above-mentioned technical problems. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a method for producing 1500MS martensitic steel; the present invention also provides a method for producing 1500MS martensitic steel roll forming products.

[0008] To solve the above-mentioned technical problems, the technical solution adopted by the present invention includes hot rolling, annealing heat treatment, pickling rolling and continuous annealing steps; The hot rolling step: coiling temperature 530℃±17℃; The heat treatment step of the cover is as follows: heat treatment of the cover at 480-500℃ for 5-6 hours; The continuous cooling process involves: first, homogenizing the temperature in the homogenization zone at 870–890°C; then, slowly cooling the temperature in the slow cooling zone at 0.1–0.5°C / s to 825–850°C; then, rapidly cooling the temperature in the rapid cooling zone at 10–25°C / s to 360–320°C; and finally, cooling the temperature in the aging zone at 1–2°C / s.

[0009] Furthermore, in the continuous annealing step, the aging temperature is 320–280°C.

[0010] Furthermore, in the continuous cooling step, the heat is evenly heated for at least 130 seconds in the heating zone, kept warm for 50-90 seconds in the slow cooling zone, kept warm for 5-12 seconds in the rapid cooling zone, and kept warm for 240-350 seconds in the aging zone.

[0011] Furthermore, the hot rolling step includes rough rolling, pre-finishing compensation, and finishing rolling; in the rough rolling process, the heating temperature is 1250℃±20℃, the soaking time is 40~50min, and the furnace time is ≥200min; in the pre-finishing compensation process, the dry head and tail of the billet are compensated by 35~40℃ based on the finishing rolling inlet temperature; in the finishing rolling process, the finishing rolling inlet temperature is 1030~1050℃, and the final rolling temperature is 890℃±14℃; after finishing rolling, the billet is coiled.

[0012] Furthermore, it also includes hot metal pretreatment, converter smelting, LF refining, RH refining and continuous casting steps; The hot metal pretreatment steps are as follows: the dephosphorization endpoint [P] is controlled to be ≤0.015% and the desulfurization endpoint [S] is controlled to be ≤0.005%; the slag is completely removed in all three processes of desiliconization, dephosphorization and desulfurization, and the slag thickness is <20mm; The RH refining step involves vacuum degassing at a vacuum level ≤100Pa, requiring the finished product to have H≤1.5ppm, P≤0.012%, S≤0.002%, and N≤40ppm.

[0013] To solve the above-mentioned technical problems, the production method of the product of the present invention adopts the above-obtained continuous annealing plate, and the technical solution adopted is as follows: including the following roll forming steps: using 19 to 38 passes of gradient room temperature cold roll forming, with a forming line speed of 6 to 12 m / min; the first 8 passes are pre-bending and leveling passes, with the deformation of the cross section controlled at 1% to 2.5% per pass, and using 0.8° to 1.5° micro-bending compensation; the middle 9 to 24 passes are the main forming passes, with the deformation of the cross section ≤4% per pass, and matched with 2.5° to 6.5° graded over-deformation compensation; the last 2 to 6 passes are pre-finishing passes, with a forming allowance of 0.11 to 0.22 mm reserved.

[0014] Furthermore, it also includes a finishing and sizing closed-loop process and a welding process; the finishing and sizing closed-loop process uses a high-speed laser section detection system to detect the profile section side length, bending angle, outer diameter, straightness and symmetry in real time, with a detection accuracy of ±0.01mm.

[0015] Furthermore, the sampling frequency of the high-speed laser cross-section detection system is 30–50 Hz.

[0016] Furthermore, the welding steps involve: high-frequency induction welding or fiber laser welding; and a closed-loop constraint shaping module is installed at the rear end of the welding station.

[0017] The beneficial effects of adopting the above technical solution are as follows: (1) By using low-temperature winding, carbide dispersion control and precise continuous annealing phase change controlled cooling process, the problems of uneven stress and forming defects caused by microstructure agglomeration and severe banded structure are solved. The traditional 1500MS uses high-temperature winding above 600℃ and conventional annealing process, which is prone to pearlite banded structure, layered carbide agglomeration and C / Mn element enrichment, resulting in uneven microstructure hardness and local stress concentration. The forming process is very prone to local cracking and uneven deformation. This invention innovatively employs a low-temperature coiling process at 530±17℃, transforming the traditional hot-rolled microstructure from "ferrite + pearlite" to a uniform "bainite + a small amount of ferrite," completely eliminating the inherited carbon segregation in pearlite. Following this, a low-temperature annealing treatment at 480–500℃ for 5–6 hours transforms the carbides from a layered segregated state to a dispersed and uniformly precipitated state, avoiding large-sized high-carbide hard spots. Combined with a precise continuous annealing process window established based on static CCT curves, a segmented controlled cooling strategy is employed: high-temperature homogenization at 870–890℃, slow cooling at 0.1–0.5℃ / s, rapid cooling to approach the Ms point at 10–25℃ / s, and low-temperature aging and voltage stabilization. This results in a uniform martensitic microstructure with fine laths and clear grain boundaries, free from significant microstructure segregation and banded defects, significantly improving material isotropy and effectively avoiding localized cracking and non-uniform deformation caused by microstructure inhomogeneity during the forming process.

[0018] (2) Furthermore, by using ultra-low P and S clean smelting and full-process slag removal technology, inclusions and grain boundary embrittlement defects are eliminated from the source, and the problem of easy cracking in cold forming of high-strength steel is completely solved. The sulfur and phosphorus content in traditional process steel is too high, which easily forms MnS long strip inclusions and grain boundary phosphorus segregation, causing local embrittlement of the matrix. When high-strength steel is bent and rolled, it is very easy to generate microcracks and crack failure. This invention employs a refined molten iron pretreatment process, controlling the pre-dephosphorization endpoint [P] ≤ 0.015% and the desulfurization endpoint [S] ≤ 0.005%. After LF and RH deep treatment, the finished product has [S] ≤ 0.002% and [P] ≤ 0.012%. Simultaneously, it strictly controls the residue thickness throughout the entire process to < 20mm, achieving thorough slag removal in the entire desiliconization, dephosphorization, and desulfurization process, avoiding phosphorus and sulfur reversion and foreign inclusions. Combined with ultra-pure control of RH vacuum degree ≤ 100Pa deep degassing, H ≤ 1.5ppm, and N ≤ 40ppm, it significantly reduces gases and harmful inclusions in the steel. The pure and uniform matrix eliminates stress concentration sources, significantly improving the cold deformation limit of martensitic steel, fundamentally reducing the risk of cracking during rolling and bending, enabling the finished product to achieve longitudinal 3T and transverse 4T bending without cracking, improving bending performance by 2.5T compared to traditional processes.

[0019] (3) Furthermore, by using a multi-pass progressive roll forming + sequential springback compensation process, the industry problem of large springback and difficult dimensional accuracy control of ultra-high strength martensitic steel is overcome. 1500MS martensitic steel has a high yield strength ratio and large elastic recovery. Traditional concentrated forming with small passes and large deformation is prone to angular springback, cross-sectional distortion, and dimensional deviation, which cannot meet the requirements of high-precision automotive pipe assembly. This invention abandons the one-time large deformation forming method and adopts a 19-38 pass small deformation progressive roll forming process, which distributes the overall bending deformation evenly to each pass, resulting in small deformation per pass and smooth stress release. At the same time, it is equipped with sequential roller springback pre-compensation, online multi-pass sizing and finishing and closed-loop welding shaping technology to correct cross-sectional errors step by step and offset the elastic springback of high-strength steel in advance. This effectively solves the problems of uncontrollable springback in ultra-high strength steel forming, cross-sectional deviation of pipes, and poor straightness, and significantly improves the dimensional accuracy and batch consistency of tubular components such as seat beams, anti-collision beams, and energy-absorbing boxes.

[0020] (4) Furthermore, by employing a full-process low-stress forming, phase change stabilization, and post-weld finishing process, the residual stress of the components is significantly reduced and homogenized, solving the problems of aging deformation and fatigue failure. Traditional processes involve single-stage high-pressure forming, severe microstructure segregation, and a coarse cooling regime, resulting in high peak values ​​and chaotic distribution of residual stress inside the pipe fittings. This leads to warping, twisting, and aging deformation during subsequent storage, painting, and service, resulting in poor fatigue stability. This invention controls stress from both the material and forming ends: at the material end, long-term stabilization of the low-temperature aging section (240-350s) during continuous annealing fully releases the phase change residual stress; at the hot-rolled end, a hydrogen release and slow cooling regime is adopted for the billet (72-88h) and the windbreak wall after the line is removed to eliminate hydrogen-induced stress and uneven cooling stress; at the forming end, multi-pass smooth flow forming is adopted to avoid local stress stacking, and multi-pass sizing and straightening after welding further homogenizes the overall residual stress. The final finished pipe fittings have low residual stress levels and uniform distribution, with no subsequent aging deformation, warping, or cracking problems, and significantly improved structural dimensional stability and fatigue life.

[0021] (5) By optimizing the entire process to achieve homogenization and controllable stress, the problem of limited subsequent processing and large performance fluctuations of ultra-high strength steel is solved. Traditional 1500MS steel has a segregated microstructure, high residual stress, and many brittle inclusions, which leads to easy chipping, cracking, and deformation during subsequent cutting, drilling, secondary bending, and assembly processing, resulting in poor processing adaptability and low yield. This invention eliminates brittle inclusions through ultra-pure smelting, and obtains a uniform fine lath martensitic microstructure throughout the plate through low-temperature coiling, dispersion annealing, and precise continuous annealing. The mechanical properties of the coil are highly stable, with yield strength fluctuation ≤21 MPa and tensile strength fluctuation ≤35 MPa. The material's plasticity, toughness, and anisotropy are significantly improved, and the residual stress is low and uniform, greatly improving the performance of subsequent cutting, finishing, secondary forming, and welding assembly, and completely solving the technical shortcomings of limited subsequent processing and poor product consistency of ultra-high strength martensitic steel.

[0022] (6) Excellent comprehensive performance, suitable for mass production applications of high-strength safety components in multiple fields. The invention ultimately obtains a pure homogeneous structure with fine martensite as the main component and a very small amount of bainite. The finished product has a yield strength of 1210-1231 MPa, a tensile strength of 1509-1544 MPa, and an elongation of 7.22%-9.77%. It has excellent strength-plasticity matching and significantly improved bending and forming performance. It can be stably applied to high-strength safety tubular components such as the rear crossbeam of the front seat of automobiles, the seat support tube of passenger cars, automobile bumpers, anti-collision beams, and energy-absorbing boxes. While achieving structural lightweighting, it greatly improves the impact resistance, fatigue resistance, and low-temperature brittle fracture resistance of the components. It has good process stability and high mass production consistency, and has broad prospects for industrialization and promotion. Attached Figure Description

[0023] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0024] Figure 1 This is a microstructure diagram of the hot-rolled steel coil obtained in Example 1; Figure 2 This is a microstructure diagram of the degraded steel coil obtained in Example 1; Figure 3 The microstructure of the steel coil in the degraded state obtained in Comparative Example 1 is shown in the diagram. Figure 4 This is a microstructure diagram of longitudinal-edge samples taken from the continuous annealing plate obtained in Example 1; Figure 5 This is a microstructure diagram of the longitudinal-intermediate sample taken from the continuous ejection plate obtained in Example 1; Figure 6 This is a microstructure diagram of the transverse-intermediate sampling of the continuous ejection plate obtained in Example 1; Figure 7 The images show the transverse and longitudinal bending diagrams of the tubular automotive product obtained in Example 1. Detailed Implementation

[0025] The chemical composition and mass percentage of the 1500MS martensitic steel described in this production method are as follows: C 0.22%–0.28%, Si 0.10%–0.15%, Mn 1.6%–2.0%, Cr 0.45%–0.55%, Ti 0.02%–0.03%, V 0.015%–0.025%, Ni 0.13%–0.27%, B 0.0015%–0.0025%, Cu 0.13%–0.27%, Nb 0.015%–0.025%, Mo 0.18%–0.28%, P≤0.012%, S≤0.002%, H≤1.5ppm, N≤40ppm, Ca 15–35ppm, and the balance being Fe and unavoidable impurities.

[0026] The production method of this 1500MS martensitic steel includes hot metal pretreatment, converter smelting, LF refining, RH refining, continuous casting, hot rolling, annealing heat treatment, pickling, and continuous annealing steps; the processes of each step are described below: (1) Hot metal pretreatment: Hot metal is subjected to dephosphorization, desulfurization and desiliconization treatment; the dephosphorization endpoint [P] is controlled to be ≤0.015wt%, the desulfurization endpoint [S] is controlled to be ≤0.005wt%, and the silicon content of hot metal after desiliconization treatment is 0.10wt%~0.15wt%; the total temperature drop of the whole process is <40℃, the desiliconization temperature range is 1290~1330℃, the dephosphorization temperature range is 1300~1350℃, and the desulfurization temperature range is 1300~1380℃; the slag is completely removed in the three processes of dephosphorization, desulfurization and desiliconization, and the slag thickness is <20mm.

[0027] (2) Converter smelting: Add raw materials according to the formula requirements, the converter smelting temperature is 1550~1600℃, the converter smelting time is 1~2h, carry out alloying treatment, and tap the steel after the converter smelting is completed.

[0028] (3) LF refining: The temperature of molten steel entering the station is 1510~1520℃. Argon gas is used for strong stirring at the bottom of the ladle to quickly remove sulfur from the molten steel. The LF refining time is 5~10min.

[0029] (4) RH refining: Refining for 15 to 20 minutes under vacuum of ≤100Pa to perform vacuum degassing, while using electromagnetic stirring, and the temperature of molten steel leaving the station is 1550 to 1560℃.

[0030] (5) Continuous casting: The continuous casting process requires the finished billet to have H content ≤1.5ppm, P ≤0.012%, S ≤0.002%, and N ≤40ppm; the electromagnetic stirring parameters are: current 220~250A, frequency 4.0~5.0HZ, and a cyclical mode of forward rotation → stop → reverse rotation → stop → forward rotation again is adopted for stirring, with each forward rotation time being 10~20s, stop time being 5~8s, and reverse rotation time being 10~20s. It is put into use under light pressure, with a pressure of 6~8mm, and the constant casting speed is controlled at 1.0~1.2m / min. The liquid level fluctuation is controlled within ±3mm to obtain the billet. The hot billet is free of cracks. The billet corner sample and sulfur stamp sample are taken to test the billet quality; the billet sulfur stamp rating is C1.0. In order to avoid hydrogen-induced delayed cracking, the billet is slowly cooled for 72~88h to release hydrogen, and the billet is cold-loaded and cold-inspected.

[0031] (6) Hot rolling: including roughing, pre-finishing compensation, finishing, and coiling processes. In the roughing process, the heating temperature is 1250℃±20℃, the soaking time is 40-50 min, and the furnace time is ≥200 min; the roughing passes are 3+3, and the roughing thickness is ≤36 mm. In the pre-finishing compensation process, a heat insulation cover and edge heaters are used for head and tail compensation. The dry head and tail of the billet are compensated by 35-40℃ based on the finishing inlet temperature, preferably 60m for the dry head and 60m for the tail, using a U-shaped laminar flow cooling process. In the finishing process, the finishing inlet temperature is 1030-1050℃, the final rolling temperature is 890℃±14℃, and the thickness is finished to 2.0-4.0 mm. After finishing, the coiling is performed at a temperature of 530℃±17℃; after coiling, it is slowly cooled inside a baffle wall to obtain a hot-rolled steel coil.

[0032] Compared with the previous high-temperature coiling at 630℃, the low-temperature coiling method at 530±17℃ used in this method exhibits significant differences in microstructure: 1) Microstructure type: It changes from the original "ferrite + pearlite" to "bainite as the main component + ferrite", with no pearlite generated, which avoids the aggregation of carbon elements in the hot-rolled state and the inheritance of carbon segregation after continuous annealing. The distribution of carbon elements in the bainite + ferrite microstructure is more uniform; 2) The microstructure is finer; 3) The banded structure is better improved, and no high carbides (pearlite or martensite) are generated at the 1 / 2 position; 4) For 1500MS, the overall microstructure is better with the low-temperature coiling process.

[0033] The mechanical properties and microstructure of the obtained hot-rolled steel coils are as follows: yield strength 827-854 MPa, tensile strength 1294-1348 MPa, elongation 11.37-13.6%; the microstructure is mainly bainite with a small amount of ferrite, and the banded structure of the hot-rolled state is significantly improved, with no obvious segregation of elements such as C, Mn, and S.

[0034] The following measures are recommended to prevent edge cracking during the hot rolling process: ① Side water spray shutdown: shut off side sprays F6 and F7 inside the stand, shut off the F7 outlet water spray, and shut off the 23rd set of side sprays for laminar flow cooling. ② Enclosure: close the main gate of the plant and the curtain door of the finishing mill roll changing passage, implementing closed management to reduce edge temperature drop. ③ Before finishing milling: use insulation covers and edge heaters. ④ Laminar flow cooling: reduce the number of automatic side sprays from 3 to 2, and shut off the 24th set of horizontal sprays. ⑤ Before coiling: shut off the side sprays in front of the DC1, DC2, and DC3 pinch rolls. ⑥ Warehouse cooling: slow cooling inside the baffle wall after the strip enters the line. Inspection: quality inspection and tracking records the edge quality of the strip.

[0035] (7) Shelf annealing: A bell-type annealing furnace is used to heat-treat the steel coil at 480-500℃ for 5-6 hours to obtain the sheet-annealed steel coil. The mechanical properties and microstructure of the sheet-annealed steel coil are as follows: yield strength 835-848MPa, tensile strength 1189-1191MPa, elongation 11.6-12%; the microstructure consists of ferrite with dispersed carbides and no obvious agglomeration.

[0036] (8) Pickling: The stripped steel coil is rolled from 2.0 to 4.0 mm to 1.0 to 2.0 mm with a rolling force of 22 to 26 MN and a reduction rate of 40 to 50%.

[0037] (9) Continuous annealing: First, perform homogenization in the homogenization zone at a temperature of 870-890℃ for at least 130s; then, slowly cool at 0.1-0.5℃ / s to the slow cooling zone at a temperature of 825-850℃, and hold in the slow cooling zone for 50-90s; then, rapidly cool at 10-25℃ / s to the rapid cooling zone at a temperature of 360-320℃, and hold in the rapid cooling zone for 5-12s; finally, cool at 1-2℃ / s to the aging zone at a temperature of 320-280℃, and hold in the aging zone for 240-350s; thus obtaining the continuously annealed plate. Under the aforementioned process window, the tensile properties of 1500MS are all satisfactory. When using this method for rapid cooling-aging, the bending performance is optimal at this temperature. To balance the actual production process, the stability of the cooling process, and the best tensile properties, the rapid cooling temperature during continuous annealing is selected to be close to the Ms point temperature, i.e., 360–320℃. This ensures stable production while achieving good overall performance. After using this continuous annealing process, 1500MS exhibits the best combination of tensile properties and the best bending limit angle, and it is feasible for on-site production.

[0038] Microstructure and mechanical properties of the obtained continuously annealed steel sheet: The microstructure is mainly martensite with a very small amount of bainite; the martensite laths are fine and the polygonal grain boundaries are clear; there is no large-sized high-carbide segregation. The good microstructure provides a foundation for improved strength and formability. Mechanical properties: Yield strength 1210~1231MPa, tensile strength 1509~1544MPa, elongation 7.22~9.77%, all qualified. The tensile strength fluctuation range of the coil is 35MPa, and the yield strength fluctuation range is 21MPa, with good uniformity of mechanical properties. Bending performance samples were taken to test the transverse and longitudinal bending conditions. After bending performance testing, the bending angle is 180 degrees, the transverse bending of 4T is qualified, and the longitudinal bending of 3T does not crack. Compared with 1500MS produced by conventional process, the transverse and longitudinal bending performance is improved by 2.5T, where T is the thickness of the steel plate.

[0039] The design concept of the aforementioned continuous annealing process is as follows: The inventors conducted research on static CCT continuous phase transition and carried out a large number of continuous annealing thermal simulation experiments. Based on the research results of the static CCT continuous phase transition law and the results of thermal simulation experiments, the continuous annealing process was innovatively improved, resulting in excellent overall simulation performance.

[0040] The phase transformation temperature of the experimental steel was determined using an expansion method combined with the alloy phase-hardness method, and the phase transformation temperature points of the experimental steel were then plotted, leading to the plotting of the continuous cooling phase transformation curve. The specific experimental method is as follows: The steel was heated to 1100℃ at a rate of 10℃ / s under vacuum and held for 3 min to homogenize the austenite composition. The Ac1 and Ac3 values ​​during the heating process at this heating rate were determined using the tangent method. The sample was then cooled to room temperature at cooling rates of 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, and 50℃ / s, and the expansion curves during the cooling process were measured. The phase transformation point was determined by combining the tangent method and the vertex method.

[0041] Metallographic samples were prepared using conventional mechanical grinding and polishing methods. Etching was performed using a 4% nitric acid-alcohol solution. The microstructure was observed using a LEXT OLS4100 confocal optical microscope, and the Vickers hardness was measured using an HMAS-D1000SMZ Vickers hardness tester. The phase transformation temperatures of 1500MS steel at different cooling rates are shown in Table 1 below.

[0042] Table 1: Phase transformation temperatures of 1500MS steel at different cooling rates

[0043] Note: A represents austenite; F represents ferrite; B represents bainite; M represents martensite; Ts and Tf represent the phase transformation start temperature and transformation end temperature, respectively.

[0044] As can be seen from the above, when the cooling rate of 1500MS steel is 0.1℃ / s, the microstructure is a mixture of ferrite and bainite. When the cooling rate is 0.5℃ / s, the microstructure is mainly martensite. When the cooling rate is 1℃ / s, martensite is gradually generated, and after the cooling rate is greater than 2℃ / s, all the transformation products are martensite.

[0045] The production method of this 1500MS martensitic steel roll forming product includes roll forming, finishing and sizing closed-loop process, and welding steps. The process of each step is described below: (1) Roll forming: In view of the characteristics of low plasticity and easy cracking of 1500MS martensitic steel, a gradual cold roll forming at room temperature with 19 to 38 passes is adopted. There is no preheating or high-temperature processing throughout the process. The deformation amount of each stage is strictly controlled to avoid stress concentration. The forming line speed is controlled at 6 to 12 m / min, and a low-speed mode is adopted. The first 8 passes are pre-bending and leveling passes. The deformation amount of the cross section in a single pass is controlled at 1% to 2.5%, and a micro-bending compensation of 0.8° to 1.5° is adopted. This process only completes the stress release of the plate and the basic micro-bending forming. The middle 9 to 24 passes are the main forming passes. The deformation amount of a single pass is ≤4%, and a graded over-deformation compensation of 2.5° to 6.5° is matched. This process completes bending, edge wrapping and closing forming in steps to prevent grain tearing and micro-cracks caused by single large deformation. The last 2 to 6 passes are pre-finishing passes, with a forming allowance of 0.11 to 0.22 mm to ensure that the cross-sectional profile of the parts is close to the design size, providing a reference for springback compensation and finishing sizing.

[0046] The roll forming step described in this method differs from the traditional overall post-compensation process. It adopts a pass-by-pass measurement and calibration + dynamic pre-compensation mechanism to accurately offset the inherent springback characteristics of 1500MS steel. Through batch sample calibration, the springback angle range of 1500MS cold roll forming was determined to be 3.5° to 6.5°, and the springback amount varies linearly with the bending angle and plate thickness. Each forming pass is independently equipped with a servo fine-tuning mechanism, with a single-pass angle compensation accuracy of ±0.1° and a roll forming height fine-tuning accuracy of ±0.01mm. The pre-bending and leveling pass uses 0.8° to 1.5° micro-bending compensation, and the main forming pass uses 2.5° to 6.5° graded over-deformation compensation according to the forming angle to offset elastic springback in real time. Through a full-pass error accumulation algorithm, the forming deviation is corrected pass by pass, and the forming angle deviation of a single pass is controlled within ±0.2°, completely solving the problem of cumulative error in high-strength steel forming.

[0047] (2) Finishing and sizing closed loop: A high-speed laser section detection system is adopted with a sampling frequency of 30-50Hz to detect the side length, bending angle, outer diameter, straightness and symmetry of the profile section in real time with a detection accuracy of ±0.01mm; so as to achieve secondary precise locking of the forming size.

[0048] (3) Welding: A synchronous linkage process of roll forming, welding and shaping is adopted, which is adapted to the welding heat deformation characteristics of 1500MS high-strength steel. After the finishing and sizing closed-loop step is completed, welding is carried out. Welding is carried out by high-frequency induction welding or fiber laser welding. A closed ring constraint shaping module is set at the rear end of the welding station. A multi-directional enclosed rigid limiting structure is used to constrain the cross section of the part in all dimensions, suppressing the expansion deformation and cooling rebound caused by the high temperature of welding; thus, the automotive tubular product can be obtained.

[0049] The automotive tubular products include front seat rear crossbeams, passenger car seat support tubes, car bumpers, anti-collision beams, energy-absorbing boxes, etc.; they can be stably applied to safety-bearing tubular products such as front seat rear crossbeams and passenger car seat support tubular structures. The complete mass production process solution adopts a progressive multi-pass cold roll forming + pass-by-pass springback compensation + online finishing and sizing + closed-loop welding and shaping, which solves the industry pain points of low plasticity, large forming springback, easy bending cracking, and out-of-tolerance tubular cross-section dimensions of 1500MS ultra-high strength martensitic steel, and is suitable for the high precision, high rigidity, and long fatigue life requirements of automotive seat tubular components.

[0050] Example 1: The production method of this 1500MS martensitic steel and roll forming product is as follows.

[0051] (1) Hot metal pretreatment: desiliconization temperature 1310℃, dephosphorization temperature 1330℃, desulfurization temperature 1350℃. All three processes of desiliconization, dephosphorization and desulfurization are completely slag removed, and the residue thickness is <20mm. The dephosphorization endpoint [P] is 0.009%; the desulfurization endpoint [S] is 0.003%; the initial silicon content of the blast furnace hot metal is 0.50%, and after the pre-desiliconization treatment by injection, the silicon content of the hot metal is controlled at 0.12%; the total temperature drop of the whole process is <40℃.

[0052] (2) Converter smelting: The converter smelting temperature is 1580℃ and the converter smelting time is 1.5h. Alloying treatment is carried out, and steel is tapped after the converter smelting is completed.

[0053] (3) LF refining: The temperature of molten steel entering the station is 1515℃. Argon gas is used for strong stirring at the bottom of the ladle to quickly remove sulfur from the molten steel. The LF refining time is 8min.

[0054] (4) RH refining: vacuum degree 100Pa, RH refining time 18min, vacuum degassing is carried out, and electromagnetic stirring is used at the same time. The temperature of molten steel leaving the station is 1555℃.

[0055] (5) Continuous casting: The continuous casting process requires an H content of 1.5 ppm, a finished product P content of 0.012%, an S content of 0.002%, and a nitrogen content of 40 ppm. The electromagnetic stirring parameters are: current 230A, frequency 4.5HZ, forward rotation time 15s, stop time 6s, then reverse rotation 15s, stop time 6s; then forward rotation again, repeating this process. Under light pressure, the process is normal, with a reduction of 7mm. A constant casting speed of 1.1m / min is maintained, and the liquid level fluctuation is controlled within ±3mm to obtain the billet. The billet is then slowly cooled for 80 hours to release hydrogen. The billet sulfur mark rating is C1.0.

[0056] The chemical composition (wt) of the obtained ingot is as follows: C 0.25%, Si 0.12%, Mn 1.8%, Cr 0.50%, Ti 0.025%, V 0.020%, Ni 0.21%, B 0.0020%, Cu 0.20%, Nb 0.020%, Mo 0.23%, P 0.012%, S 0.002%, H 1.5ppm, N 40ppm, Ca 25ppm, and balance Fe.

[0057] (6) Hot rolling: This includes roughing, pre-finishing compensation, and finishing. The roughing temperature is 1250℃, the soaking time is 45min, the furnace time is 200min, the roughing passes are 3+3, and the roughing thickness is 36mm. Pre-finishing compensation: A heat insulation cover and edge heaters are used for head and tail compensation, U-cooling, and an additional 37℃ compensation is added to the dry head and tail based on the finishing inlet temperature. The finishing inlet temperature is 1040℃, the final rolling temperature is 890℃, the finishing thickness is 2.0mm, and the coiling temperature is 530℃. After coiling, the coil is slowly cooled inside the baffle wall to obtain a hot-rolled steel coil. After cooling off the line, it is leveled. Measures to prevent edge cracking are taken.

[0058] The obtained hot-rolled steel coil has the following properties: yield strength 854 MPa, tensile strength 1348 MPa, and elongation 11.37%. Figure 1 This is a microstructure diagram of the hot-rolled steel coil. Figure 1 As shown, the microstructure of the hot-rolled plate is mainly bainite with a small amount of ferrite, and the banded structure in the hot-rolled state is significantly improved, with no obvious segregation of elements such as C, Mn, and S.

[0059] (7) Sheet annealing: The hot-rolled steel coil is annealed at 490℃ for 5.5h to obtain the sheet annealed steel coil. The obtained sheet annealed steel coil has the following properties: yield strength 848MPa, tensile strength 1191MPa, and elongation 11.6%; Figure 2 The microstructure diagram of the delaminated steel coil is shown below. Figure 2 As shown, the microstructure consists of ferrite with dispersed carbides and no obvious segregation.

[0060] (8) Pickling: The steel coil in the shrouded state is transferred to the pickling workshop for pickling. The thickness is rolled from 2.0 mm to 1.0 mm, the rolling force is 25 MN, and the reduction rate is 50%.

[0061] (9) Continuous annealing: First, the temperature of the uniform heating section is 880℃ and uniform heating is carried out for 130s; then, it is slowly cooled to the temperature of the slow cooling section at 0.3℃ / s and held for 60s; then, it is rapidly cooled to the temperature of the rapid cooling section at 15℃ / s and held for 10s; finally, it is cooled to the temperature of the aging section at 1.5℃ / s and held for 300s; thus, the continuous annealing plate is obtained.

[0062] Microstructure and mechanical properties of the obtained continuous annealing plate: Microstructure: Figure 4 , Figure 5 , Figure 6 This is a microstructure diagram of the finished continuous annealing sheet, in which... Figure 4 Microstructure diagram of longitudinal-edge sampling Figure 5 Microstructure diagram of longitudinal-intermediate sampling Figure 6 This is a microstructure diagram of a transverse-intermediate sample. Figure 4 , Figure 5 , Figure 6 As shown, the microstructure of the finished product is mainly martensite with a very small amount of bainite; the martensite laths are fine and the polygonal grain boundaries are clear; the mechanical properties are: yield strength 1231MPa, tensile strength 1544MPa, elongation 7.22%, all of which are qualified; the through-wound tensile strength fluctuates within a range of 35MPa, the yield strength fluctuates within a range of 21MPa, and the mechanical properties are uniform.

[0063] (10) Roll forming: 26-pass gradient room temperature cold roll forming is adopted, the forming line speed is 8m / min, the deformation of the single cross section of the first 8 pre-bending and leveling passes is controlled at 1.5%, and the first 8 pre-bending and leveling passes adopt 1.1° micro-bending compensation; the deformation of the single pass of the middle 15 main forming passes is 4%, and the middle 15 main forming passes are matched with 4.0° graded over-deformation compensation; the last 3 pre-finishing passes reserve 0.18mm forming allowance.

[0064] (11) Finishing and sizing closed loop: An online finishing and sizing closed loop process is set up before welding to achieve secondary precise locking of the forming size; a high-speed laser section detection system is adopted with a sampling frequency of 40Hz to detect the side length, bending angle, outer diameter, straightness and symmetry of the profile section in real time, with a detection accuracy of ±0.01mm.

[0065] (12) Welding: After the online finishing and sizing closed-loop process is completed, welding is performed. High-frequency induction welding or fiber laser welding is used. A closed ring-shaped constraint shaping module is set at the rear end of the welding station to obtain the automotive tubular product. The automotive tubular product in this embodiment includes the rear crossbeam of the front seat, the passenger car seat support tube, the car bumper, the anti-collision beam or the energy absorption box.

[0066] (13) The tubular product obtained in this embodiment underwent bending performance testing and was formed by bending at 180 degrees and 1500 MS. Figure 7As shown, from top to bottom, the bending times are 3.5T (lateral), 4T (lateral), 2.5T (longitudinal), and 3T (longitudinal). The figure shows that the bending angle is 180 degrees. The 4T (lateral) bend is acceptable, and the 3T (longitudinal) bend does not crack. Compared to previously produced 1500MS steel, the bending performance in both directions is improved by 2.5T, where T represents the steel plate thickness. The resulting 1500MS martensitic steel automotive tubular products did not exhibit edge cracking during service.

[0067] Example 2: The production method of this 1500MS martensitic steel and roll forming product is as follows.

[0068] (1) Hot metal pretreatment: desiliconization temperature 1290℃, dephosphorization temperature 1300℃, desulfurization temperature 1300℃. All three processes of desiliconization, dephosphorization and desulfurization are completely slag removed, and the residue thickness is <20mm. The dephosphorization endpoint [P] is 0.012%; the desulfurization endpoint [S] is 0.005%; the initial silicon content of the blast furnace hot metal is 0.40%, and after the pre-desiliconization treatment by injection, the silicon content of the hot metal is controlled to 0.10%; the total temperature drop of the whole process is <40℃.

[0069] (2) Converter smelting: The converter smelting temperature is 1550℃ and the converter smelting time is 1h. Alloying treatment is carried out, and steel is tapped after the converter smelting is completed.

[0070] (3) LF refining: The temperature of molten steel entering the station is 1510℃. Argon gas is used for strong stirring at the bottom of the ladle to quickly remove sulfur from the molten steel. The LF refining time is 5min.

[0071] (4) RH refining: vacuum degree 90Pa, RH refining time 15min, vacuum degassing, electromagnetic stirring at the same time, molten steel outlet temperature 1550℃.

[0072] (5) Continuous casting: The continuous casting process requires H content ≤ 1.5ppm, finished product P content ≤ 0.012%, S content ≤ 0.002%, and nitrogen content ≤ 40ppm. The electromagnetic stirring parameters are: current 220A, frequency 4.0HZ, forward rotation time 10s, stop time 5s, then reverse rotation 10s, stop time 5s; then forward rotation, repeating in sequence. Sulfur mark rating of the billet: C1.0 grade. Normal operation under light pressure, pressure reduction 6mm, constant casting speed 1.0m / min control, liquid level fluctuation controlled within ±3mm to obtain the billet, the billet is slowly cooled for 72h to release hydrogen.

[0073] The chemical composition (wt) of the obtained ingot is as follows: C 0.22%, Si 0.10%, Mn 1.6%, Cr 0.45%, Ti 0.02%, V 0.015%, Ni 0.13%, B 0.0015%, Cu 0.13%, Nb 0.015%, Mo 0.18%, P 0.010%, S 0.001%, H 1.2ppm, N 30ppm, Ca 15ppm and balance Fe.

[0074] (6) Hot rolling: This includes roughing, pre-finishing compensation, and finishing. The roughing temperature is 1230℃, the soaking time is 40 min, the furnace time is 250 min, the roughing passes are 3+3, and the roughing thickness is 30 mm. Pre-finishing compensation: Insulation covers and edge heaters are used for head and tail compensation, U-cooling, and an additional 35℃ compensation is added to the dry head and tail based on the finishing inlet temperature. The finishing inlet temperature is 1030℃, the final rolling temperature is 876℃, the finishing thickness is 2.0 mm, and the coiling temperature is 513℃. After coiling, the coil is slowly cooled inside the baffle wall to obtain a hot-rolled steel coil. After cooling off the line, it is leveled. Measures to prevent edge cracking are taken.

[0075] The obtained hot-rolled steel coil has a yield strength of 827 MPa, a tensile strength of 1294 MPa, and an elongation of 13.6%. The microstructure of the hot-rolled plate is mainly bainite with a small amount of ferrite, and the banded microstructure is significantly improved in the hot-rolled state, with no obvious segregation of elements such as C, Mn, and S.

[0076] (7) Sheet annealing treatment: Hot-rolled steel coils are annealed at 480℃ for 5 hours to obtain annealed steel coils. The obtained annealed steel coils have a yield strength of 835MPa, a tensile strength of 1189MPa, and an elongation of 12%; the microstructure consists of ferrite with dispersed carbides and no obvious agglomeration.

[0077] (8) Pickling: The steel coil in the shrouded state is transferred to the pickling workshop for pickling. The thickness is rolled from 2.0 mm to 1.2 mm, the rolling force is 22 MN, and the reduction rate is 40%.

[0078] (9) Continuous annealing: First, heat the plate at 870℃ in the heat-equalizing section for 150s; then, cool it slowly at 0.1℃ / s to 825℃ in the slow cooling section and hold it for 50s; then, cool it rapidly at 10℃ / s to 360℃ in the rapid cooling section and hold it for 5s; finally, cool it at 1℃ / s to 320℃ in the aging section and hold it for 240s; and the annealed plate is obtained.

[0079] Microstructure and mechanical properties of the obtained continuous annealing sheet: The microstructure is mainly martensite with a very small amount of bainite; the martensite laths are fine and the polygonal grain boundaries are clear; Mechanical properties: Yield strength 1210MPa, tensile strength 1509MPa, elongation 9.77%, all of which are qualified; the tensile strength fluctuation range of the whole roll is 35MPa, the yield strength fluctuation range is 21MPa, and the uniformity of mechanical properties is good; After bending performance test, the bending angle is 180 degrees, the transverse bending of 4T is qualified, and the longitudinal bending of 3T does not crack.

[0080] (10) Roll forming: 19-pass gradient room temperature cold roll forming is adopted, the forming line speed is 6m / min, the deformation of the single cross section of the first 8 pre-bending and leveling passes is controlled at 1%, and the first 8 pre-bending and leveling passes adopt 0.8° micro-bending compensation; the deformation of the single pass of the middle 9 main forming passes is 3%, and the middle 9 main forming passes are matched with 2.5° graded over-deformation compensation; the last 2 pre-finishing passes reserve 0.11mm forming allowance.

[0081] (11) Finishing and sizing closed loop: An online finishing and sizing closed loop process is set up before welding to achieve secondary precise locking of the forming size; a high-speed laser section detection system is adopted with a sampling frequency of 30Hz to detect the side length, bending angle, outer diameter, straightness and symmetry of the profile section in real time, with a detection accuracy of ±0.01mm.

[0082] (12) Welding: After the online finishing and sizing closed-loop process is completed, welding is performed. High-frequency induction welding or fiber laser welding is used. A closed ring-shaped constraint shaping module is set at the rear end of the welding station to obtain the automotive tubular product. The automotive tubular product in this embodiment includes the rear crossbeam of the front seat, the passenger car seat support tube, the car bumper, the anti-collision beam or the energy absorption box.

[0083] Example 3: The production method of this 1500MS martensitic steel and roll forming product is as follows.

[0084] (1) Hot metal pretreatment: desiliconization temperature 1330℃, dephosphorization temperature 1350℃, desulfurization temperature 1380℃. All three processes of desiliconization, dephosphorization and desulfurization are completely slag removed, and the residue thickness is <20mm. The dephosphorization endpoint [P] is 0.015%; the desulfurization endpoint [S] is 0.004%; the initial silicon content of the blast furnace hot metal is 0.70%, and the silicon content of the hot metal is controlled to 0.15% after pre-desiliconization treatment by injection; the total temperature drop of the whole process is <40℃.

[0085] (2) Converter smelting: The converter smelting temperature is 1600℃ and the converter smelting time is 2h. Alloying treatment is carried out, and steel is tapped after the converter smelting is completed.

[0086] (3) LF refining: The temperature of molten steel entering the station is 1520℃. Argon gas is used for strong stirring at the bottom of the ladle to quickly remove sulfur from the molten steel. The LF refining time is 10min.

[0087] (4) RH refining: vacuum degree 95Pa, RH refining time 20min, vacuum degassing, electromagnetic stirring at the same time, molten steel outlet temperature 1560℃.

[0088] (5) Continuous casting: The continuous casting process requires H content ≤ 1.5ppm, finished product P content ≤ 0.012%, S content ≤ 0.002%, and nitrogen content ≤ 40ppm. The electromagnetic stirring parameters are: current 250A, frequency 5.0HZ, forward rotation time 20s, stop time 8s, then reverse rotation 20s, stop time 8s; then forward rotation, repeating in sequence. Sulfur mark rating of the billet: C1.0 grade. Normal operation under light pressure, pressure reduction 8mm, constant casting speed 1.2m / min control, liquid level fluctuation controlled within ±3mm to obtain the billet, the billet is slowly cooled for 88h to release hydrogen.

[0089] The chemical composition of the resulting billet (wt) is as follows: C 0.28%, Si 0.15%, Mn 2.0%, Cr 0.55%, Ti 0.03%, V 0.025%, Ni 0.27%, B 0.0025%, Cu 0.27%, Nb 0.025%, Mo 0.28%, P 0.011%, S 0.0007%, H 1.1ppm, N 35ppm, Ca 35ppm, and balance Fe.

[0090] (6) Hot rolling: This includes roughing, pre-finishing compensation, and finishing. The roughing temperature is 1270℃, the soaking time is 50min, the furnace time is 220min, the roughing passes are 3+3, and the roughing thickness is 35mm. Pre-finishing compensation: A heat insulation cover and edge heaters are used for head and tail compensation, U-cooling, and an additional 40℃ compensation is added to the dry head and tail based on the finishing inlet temperature. The finishing inlet temperature is 1050℃, the final rolling temperature is 904℃, the finishing thickness is 4.0mm, and the coiling temperature is 547℃. After coiling, the coil is slowly cooled inside the baffle wall to obtain a hot-rolled steel coil. After cooling off the line, it is leveled. Measures to prevent edge cracking are taken.

[0091] The obtained hot-rolled steel coil has a yield strength of 838 MPa, a tensile strength of 1315 MPa, and an elongation of 12.4%. The microstructure of the hot-rolled plate is mainly bainite with a small amount of ferrite, and the banded microstructure is significantly improved in the hot-rolled state, with no obvious segregation of elements such as C, Mn, and S.

[0092] (7) Sheet annealing treatment: The hot-rolled steel coil is subjected to sheet annealing treatment at 500℃ for 6 hours to obtain sheet annealed steel coil. The obtained sheet annealed steel coil has a yield strength of 840MPa, a tensile strength of 1190MPa, and an elongation of 11.8%; the microstructure consists of ferrite with dispersed carbides and no obvious agglomeration.

[0093] (8) Pickling: The steel coil in the shrouded state is transferred to the pickling workshop for pickling. The thickness is rolled from 4.0 mm to 2.0 mm, the rolling force is 26 MN, and the reduction rate is 50%.

[0094] (9) Continuous annealing: First, the temperature of the heat-equalizing section is 890℃ and the heat-equalizing section is heated for 140s; then the temperature is slowly cooled to 850℃ in the slow cooling section at 0.5℃ / s and held for 90s; then the temperature is rapidly cooled to 320℃ in the rapid cooling section at 25℃ / s and held for 12s; finally, the temperature is cooled to 280℃ in the aging section at 2℃ / s and held for 350s; thus, the continuous annealing plate is obtained.

[0095] The resulting annealed sheet has the following microstructure: mainly martensite with a very small amount of bainite; the martensite laths are fine and the polygonal grain boundaries are clear; mechanical properties: yield strength 1222MPa, tensile strength 1521MPa, elongation 8.05%, tensile strength fluctuation range of the whole roll 35MPa, yield strength fluctuation range 21MPa; bending angle is 180 degrees, transverse bending 4T is qualified, longitudinal bending 3T does not crack.

[0096] (10) Roll forming: 38-pass gradient room temperature cold roll forming is adopted, the forming line speed is 12m / min, the deformation of the single cross section of the first 8 pre-bending and leveling passes is controlled at 2.5%, and the first 8 pre-bending and leveling passes adopt 1.5° micro-bending compensation; the deformation of the single pass of the middle 24 main forming passes is 3.5%, and the middle 24 main forming passes are matched with 6.5° graded over-deformation compensation; the last 6 pre-finishing passes reserve 0.22mm forming allowance.

[0097] (11) Finishing and sizing closed loop: An online finishing and sizing closed loop process is set up before welding to achieve secondary precise locking of the forming size; a high-speed laser section detection system is adopted with a sampling frequency of 50Hz to detect the side length, bending angle, outer diameter, straightness and symmetry of the profile section in real time, with a detection accuracy of ±0.01mm.

[0098] (12) Welding: After the online finishing and sizing closed-loop process is completed, welding is performed. High-frequency induction welding or fiber laser welding is used. A closed ring-shaped constraint shaping module is set at the rear end of the welding station to obtain the automotive tubular product. The automotive tubular product in this embodiment includes the rear crossbeam of the front seat, the passenger car seat support tube, the car bumper, the anti-collision beam or the energy absorption box. Example 4

[0099] The only difference between this embodiment and Embodiment 1 is the following consecutive unwinding steps: Continuous annealing: First, the plate is heated to 875℃ in the heat-soaking zone for 140 seconds; then, it is slowly cooled to 830℃ in the slow cooling zone at a rate of 0.2℃ / s, and held for 70 seconds in the slow cooling zone; then, it is rapidly cooled to 330℃ in the rapid cooling zone at a rate of 20℃ / s, and held for 8 seconds in the rapid cooling zone; finally, it is cooled to 290℃ in the aging zone at a rate of 2℃ / s, and held for 250 seconds in the aging zone; thus, the plate is continuously annealed.

[0100] The finished annealed sheet obtained in this embodiment has the following microstructure: mainly martensite with a very small amount of bainite; the martensite laths are fine and the polygonal grain boundaries are clear; the mechanical properties are: yield strength 1219MPa, tensile strength 1522MPa, elongation 8.34%, tensile strength fluctuation range of the whole roll is 16MPa, yield strength fluctuation range is 25MPa; the bending angle is 180 degrees, the transverse bending of 4T is qualified, and the longitudinal bending of 3T does not crack. Example 5

[0101] The only difference between this embodiment and Embodiment 1 is the following consecutive unwinding steps: Continuous annealing: First, the plate is heated to 885℃ in the heat-soaking zone for 130 seconds; then, it is slowly cooled to 845℃ in the slow cooling zone at a rate of 0.4℃ / s, and held for 80 seconds in the slow cooling zone; then, it is rapidly cooled to 350℃ in the rapid cooling zone at a rate of 12℃ / s, and held for 11 seconds in the rapid cooling zone; finally, it is cooled to 310℃ in the aging zone at a rate of 1℃ / s, and held for 320 seconds in the aging zone; thus, the plate is annealed continuously.

[0102] The finished continuous annealing sheet obtained in this embodiment has the following microstructure: mainly martensite with a very small amount of bainite; the martensite laths are fine and the polygonal grain boundaries are clear; the mechanical properties are: yield strength 1229MPa, tensile strength 1527MPa, elongation 9.58%, tensile strength fluctuation range of the whole roll is 13MPa, yield strength fluctuation range is 28MPa; the bending angle is 180 degrees, the transverse bending of 4T is qualified, and the longitudinal bending of 3T does not crack.

[0103] Comparative Example 1: The only difference between this comparative example and Example 1 is that the heat treatment process in Example 1, which involves "hot-rolled steel coils undergoing heat treatment at 490°C for 5.5 hours", is replaced with "hot-rolled steel coils undergoing heat treatment at 600°C for 5.5 hours".

[0104] Figure 3 This is a microstructure diagram of the steel coil in the deteriorated state obtained in this comparative example. Figure 3 As shown, layered carbides are distributed on the ferrite matrix in the microstructure, and the carbides show obvious segregation.

[0105] Comparative Example 2: The only difference between this comparative example and Example 1 is that the rapid cooling section is omitted in the continuous cooling step.

[0106] The mechanical properties of the continuously annealed sheet obtained in this comparative example are: yield strength 1056MPa, tensile strength 1325MPa, elongation 9.05%, tensile strength fluctuation range of the whole roll 89MPa, yield strength fluctuation range 52MPa; bending angle is 120 degrees, cracking occurs after 4T of transverse bending and 3T of longitudinal bending.

[0107] Comparative Example 3: The only difference between this comparative example and Example 1 is the use of the following consecutive retraction steps: Continuous annealing: First, the plate is heated to 850℃ in the heat-soaking zone for 100 seconds; then, it is slowly cooled to 820℃ in the slow cooling zone at a rate of 0.1℃ / s, and held for 40 seconds in the slow cooling zone; then, it is rapidly cooled to 370℃ in the rapid cooling zone at a rate of 8℃ / s, and held for 3 seconds in the rapid cooling zone; finally, it is cooled to 310℃ in the aging zone at a rate of 0.5℃ / s, and held for 200 seconds in the aging zone; thus, the plate is continuously annealed.

[0108] The mechanical properties of the continuously annealed sheet obtained in this comparative example are: yield strength 1120MPa, tensile strength 1419MPa, elongation 6.54%, tensile strength fluctuation range of the whole roll 67MPa, yield strength fluctuation range 44MPa; bending angle is 150 degrees, cracking occurs after 4T of transverse bending and 3T of longitudinal bending.

[0109] Comparative Example 4: The only difference between this comparative example and Example 1 is the following consecutive retraction steps: Continuous annealing: First, the plate is heated to 900℃ in the heat-soaking zone for 120 seconds; then, it is slowly cooled to 860℃ in the slow cooling zone at a rate of 0.6℃ / s, and held for 100 seconds in the slow cooling zone; then, it is cooled to 300℃ in the rapid cooling zone at a rate of 30℃ / s, and held for 20 seconds in the rapid cooling zone; finally, it is cooled to 250℃ in the aging zone at a rate of 3℃ / s, and held for 400 seconds in the aging zone; thus, the plate is continuously annealed.

[0110] The mechanical properties of the continuously annealed sheet obtained in this comparative example are: yield strength 1110MPa, tensile strength 1400MPa, elongation 6.78%, tensile strength fluctuation range of the whole roll 69MPa, yield strength fluctuation range 45MPa; bending angle is 150 degrees, cracking occurs after 4T of transverse bending and 3T of longitudinal bending.

[0111] Comparative Example 5: The only difference between this comparative example and Example 1 is that the pre-finishing compensation process is omitted.

[0112] The multi-pass roll-formed martensitic steel automotive tubular product obtained in this comparative example developed edge cracking after being placed for 3 days.

[0113] Comparative Example 6: The only difference between this comparative example and Example 1 is the following roll forming step: Roll forming: 15-pass gradient room temperature cold roll forming is adopted, with a forming line speed of 5m / min. The deformation of the single cross section of the first 8 pre-bending and leveling passes is controlled within 0.5%, and 0.5° micro-bending compensation is adopted for the first 8 pre-bending and leveling passes. The deformation of the single pass of the middle 5 main forming passes is 5%, and 2.0° graded over-deformation compensation is matched for the middle 5 main forming passes. 0.10mm forming allowance is reserved for the last 2 pre-finishing passes.

[0114] The multi-pass roll-formed martensitic steel automotive tubular product obtained in this comparative example developed edge cracking after being placed for 3 days.

[0115] Comparative Example 7: The only difference between this comparative example and Example 1 is the following roll forming step: Roll forming: A 40-pass gradient room temperature cold roll forming process is adopted, with a forming line speed of 15m / min. The deformation of the single cross section of the first 8 pre-bending and leveling passes is controlled at 3.0%, and a 2.0° micro-bending compensation is adopted for the first 8 pre-bending and leveling passes. The deformation of the single pass of the middle 25 main forming passes is 2%, and a 7.0° graded over-deformation compensation is matched for the middle 25 main forming passes. A 0.25mm forming allowance is reserved for the last 7 pre-finishing passes.

[0116] The multi-pass roll-formed martensitic steel automotive tubular product obtained in this comparative example developed edge cracking after being placed for 3 days.

Claims

1. A method for producing 1500MS martensitic steel, characterized in that: This includes hot rolling, annealing heat treatment, pickling, and continuous annealing steps; The hot rolling step: coiling temperature 530℃±17℃; The heat treatment step of the cover is as follows: heat treatment of the cover at 480-500℃ for 5-6 hours; The continuous cooling process involves: first, homogenizing the temperature in the homogenization zone at 870–890°C; then, slowly cooling the temperature in the slow cooling zone at 0.1–0.5°C / s to 825–850°C; then, rapidly cooling the temperature in the rapid cooling zone at 10–25°C / s to 360–320°C; and finally, cooling the temperature in the aging zone at 1–2°C / s.

2. The method for producing 1500MS martensitic steel according to claim 1, characterized in that: The continuous annealing step involves an aging period temperature of 320–280°C.

3. The method for producing 1500MS martensitic steel according to claim 1, characterized in that: The continuous cooling process involves heating for at least 130 seconds in the heating zone, maintaining the temperature for 50-90 seconds in the slow cooling zone, maintaining the temperature for 5-12 seconds in the rapid cooling zone, and maintaining the temperature for 240-350 seconds in the aging zone.

4. The method for producing 1500MS martensitic steel according to claim 1, characterized in that: The hot rolling process includes roughing, pre-finishing compensation, and finishing rolling. In the roughing process, the heating temperature is 1250℃±20℃, the soaking time is 40-50 minutes, and the furnace time is ≥200 minutes. In the pre-finishing compensation process, the dry ends of the billet are compensated by 35-40℃ based on the finishing rolling inlet temperature. In the finishing rolling process, the finishing rolling inlet temperature is 1030-1050℃, and the final rolling temperature is 890℃±14℃. The billet is then coiled after finishing rolling.

5. A method for producing 1500MS martensitic steel according to any one of claims 1-4, characterized in that: It also includes hot metal pretreatment, converter smelting, LF refining, RH refining and continuous casting steps; The hot metal pretreatment steps are as follows: the dephosphorization endpoint [P] is controlled to be ≤0.015% and the desulfurization endpoint [S] is controlled to be ≤0.005%; the slag is completely removed in all three processes of desiliconization, dephosphorization and desulfurization, and the slag thickness is <20mm; The RH refining step involves vacuum degassing at a vacuum level ≤100Pa, requiring the finished product to have H≤1.5ppm, P≤0.012%, S≤0.002%, and N≤40ppm.

6. A method for producing 1500MS martensitic steel roll-formed products, using a continuous annealing plate obtained by any one of claims 1-5, characterized in that, The process includes the following roll forming steps: 19 to 38 passes of gradual cold roll forming at room temperature with a forming line speed of 6 to 12 m / min; the first 8 passes are pre-bending and leveling passes, with the cross-sectional deformation controlled at 1% to 2.5% per pass, and micro-bending compensation of 0.8° to 1.5° is used; the middle 9 to 24 passes are the main forming passes, with the deformation per pass ≤4%, and graded over-deformation compensation of 2.5° to 6.5° is used; the last 2 to 6 passes are pre-finishing passes, with a forming allowance of 0.11 to 0.22 mm reserved.

7. The method for producing a 1500MS martensitic steel roll-formed product according to claim 6, characterized in that: It also includes a finishing and sizing closed-loop process and a welding process; the finishing and sizing closed-loop process uses a high-speed laser section detection system to detect the profile section side length, bending angle, outer diameter, straightness and symmetry in real time, with a detection accuracy of ±0.01mm.

8. The method for producing a 1500MS martensitic steel roll-formed product according to claim 7, characterized in that: The sampling frequency of the high-speed laser cross-section detection system is 30–50 Hz.

9. A method for producing a 1500MS martensitic steel roll-formed product according to claim 7 or 8, characterized in that, The welding steps are as follows: high-frequency induction welding or fiber laser welding is used; a closed ring-shaped constraint shaping module is set at the rear end of the welding station.

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