A 1700 mpa grade martensitic cold rolled high strength steel and method of production thereof

Abstract

The application discloses a 1700MPa-grade martensite cold-rolled high-strength steel and a production method thereof. The chemical composition of the cold-rolled high-strength steel comprises the following components: C: 0.26%-0.33%, Si: 0.2%-0.5%, Mn: 0.5%-0.8%, P: 0.015% or less, S: 0.005% or less, Als: 0.03%-0.07%, Cr: 0.2%-0.4%, Ni: 0.03-0.05%, Nb: 0.02%-0.04%, Ti: 0.02%-0.05%, Cu: 0.12%-0.17%, Ca: 0.003%-0.007%, N: 0.006% or less, and the rest is Fe and inevitable impurities. The production method comprises smelting, continuous casting, hot rolling, acid rolling and continuous annealing processes. The cold-rolled high-strength steel provided by the application has good formability and excellent resistance to delayed cracking.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of advanced high-strength steel production, and particularly relates to a 1700MPa-grade martensitic cold-rolled high-strength steel and a production method thereof. BACKGROUND

[0002] Under the atmosphere of carbon emission reduction, with the in-depth development of automobile lightweight, the application proportion of advanced high-strength steel in the field of automobile processing and manufacturing is also rising. The hot-formed steel can obtain high strength after hot forming and quenching, but it needs to heat the material to the austenitizing temperature again, which requires high energy consumption and does not meet the requirement of carbon emission reduction, so it is only suitable for complex-shaped structural parts and is difficult to be widely promoted and applied. The cold-rolled martensite has high strength and does not need to be heated again, and the high-temperature austenite is usually quenched on line to complete the martensite transformation to realize the requirements of high strength and formability. However, the strength of the developed martensitic steel cold-rolled plate is limited to 1500MPa, and the hot-formed steel is mainly used for more than 1500MPa, and there are few reports on high-grade cold-rolled high-strength steel, or only laboratory development is realized, and industrial production is not realized, and there is no suitable technical solution for the problems in batch production. High-strength steel has high hardenability elements, and the problems of weld breakage and edge crack breakage in the production process are also more prominent. Therefore, it is necessary to develop high-grade martensitic cold-rolled high-strength steel and realize its stable production.

[0003] The patent application with the publication number CN 109321828 A provides a 1600MPa-grade cold-rolled martensitic steel and a production method thereof. The composition of the steel plate is as follows in terms of percentage by weight: C: 0.23%~0.28%, Si: 0.5%~1.0%, Mn: 1.8%~2.3%, Al: 0.02%~0.07%, P≤0.02%, S≤0.005%, N≤0.005%, and the balance is Fe and inevitable impurities; the production method comprises smelting, continuous casting, hot rolling, pickling, cold rolling and continuous annealing. The chemical composition designed by the application is simple, and no rare or noble metal elements are added except C, Si and Mn, which belongs to a low-cost economic product. By using the composition design and production method provided by the application, an ultrahigh-strength cold-rolled steel plate with a tensile strength greater than 1600MPa and an elongation greater than 5% can be manufactured; but the Mn and Si contents are high, MnS is easy to become a crack source of hydrogen-induced delayed cracking, and high Mn content is easy to form banded structure, which is not conducive to processing and forming, and the application does not mention how to improve the uniformity of the coiling in the production process.

[0004] Patent application CN 112522633 A discloses a thin-gauge martensitic steel strip and its manufacturing method. The strip's composition by weight percentage is: C: 0.16-0.26%, Si: 0.1-0.5%, Mn: 0.4-1.7%, P≤0.02%, S≤0.007%, N: 0.004-0.010%, Als: <0.001%, total oxygen [O]: 0.007-0.020%, with the balance being Fe and other unavoidable impurities. It also satisfies the following condition: containing Cu. It contains one or both of the following: 0.1-0.6% or Sn: 0.005-0.04%; contains one or both of the following: Nb: 0.01-0.08% and Mo: 0.1-0.4%; Mn / S>250; it mainly involves hot-rolled martensitic steel, which inevitably has iron oxide scale on the surface. At the same time, it is not tempered online, resulting in excessive stress during the subsequent blanking process, causing the plate to warp and deform, affecting the use effect, and making it difficult to guarantee that hydrogen-induced delayed cracking will not occur during application.

[0005] Compared with the above technical solutions, the present invention relies on existing hot rolling and cold rolling equipment, and at the same time utilizes the ultra-fast cooling (online quenching) and reheating functions of continuous annealing production equipment. The cold-rolled sheet produced not only has high strength and good formability, but also excellent resistance to delayed cracking, and can be used to process high-strength body parts required by the automotive industry. Summary of the Invention

[0006] The purpose of this invention is to provide a 1700MPa grade martensitic cold-rolled high-strength steel and its production method. The provided cold-rolled high-strength steel has good formability and excellent resistance to delayed cracking, and can be used to process high-strength body parts required by the automotive industry.

[0007] To achieve the above objectives, the technical solution provided by the present invention is as follows:

[0008] A 1700MPa grade martensitic cold-rolled high-strength steel has the following chemical composition and mass percentage: C: 0.26%–0.33%, Si: 0.2%–0.5%, Mn: 0.5%–0.8%, P≤0.015%, S≤0.005%, Als: 0.03%–0.07%, Cr: 0.2%–0.4%, Ni: 0.03%–0.05%, Nb: 0.02%–0.04%, Ti: 0.02%–0.05%, Cu: 0.12%–0.17%, Ca: 0.003%–0.007%, N≤0.006%, with the remainder being Fe and unavoidable impurities.

[0009] The rationale for the design of each component of the high-strength steel in this invention is as follows:

[0010] C: C is the main element for solid solution strengthening and hardenability in steel. The higher the content, the greater the martensitic hardenability, and the higher the risk of hydrogen-induced delayed cracking. Therefore, the C content is 0.26% to 0.33%.

[0011] Si and Al: These are elements that inhibit carbide formation. They can reduce the precipitation of cementite, thus decreasing the hardenability and material properties caused by the martensitic transformation. Excessive Si increases the difficulty of removing oxide scale from hot-rolled surfaces and can also cause temper brittleness. Therefore, Si should be controlled within the range of 0.2% to 0.5%. Excessive Al will increase the viscosity of molten steel, making continuous casting more difficult. Therefore, Al should be controlled within the range of 0.03% to 0.07%.

[0012] Mn: Mn improves the hardenability of materials and promotes the formation of martensite, thereby increasing material strength. However, excessive Mn content can easily lead to segregation and the formation of banded structures. Furthermore, MnS readily becomes a hydrogen accumulation site, deteriorating the material's resistance to delayed cracking. Additionally, excessive Mn inhibits the transformation of austenite to martensite during cooling after annealing. Therefore, the Mn content in this invention is 0.5%–0.8%.

[0013] P and S: P and S are residual harmful elements. P easily causes central segregation in the matrix, reducing the toughness of steel and hindering delayed cracking. S readily combines with Mn to form MnS, which easily becomes a hydrogen accumulation site, worsening the material's resistance to delayed cracking. Therefore, both should be controlled at the lowest possible levels, i.e., P ≤ 0.015% and S ≤ 0.005%.

[0014] Cr: An effective element for improving the hardenability of materials, it helps refine grains during hot rolling and improves the strength and plasticity of materials. This invention selects a Cr content of 0.2% to 0.4%.

[0015] Ni: As a hardenability element, it can improve the strength and toughness of the steel of this invention, and helps to improve the material's resistance to hydrogen-induced delayed cracking. Therefore, the Ni content is 0.03% to 0.05%.

[0016] Nb and Ti possess dispersion strengthening and grain refinement strengthening effects, which are beneficial for forming dispersed fine precipitates. This facilitates the localized accumulation of hydrogen and creates hydrogen traps at the matrix-precipitate interface, thus improving delayed cracking and enhancing the material's porosity and edge-flaring properties. The reaction of Ti and N to form large TiN particles is detrimental to delayed cracking performance; therefore, it is necessary to control the N content and TiN size to mitigate its adverse effects. Thus, the Nb content is typically between 0.02% and 0.04%, and the Ti content between 0.02% and 0.05%.

[0017] Cu: This is a key element in this invention. As a non-carbide-forming element, it plays a role in solid solution strengthening and precipitation strengthening. It can precipitate as a copper-rich phase, exerting precipitation strengthening and grain refinement effects, while simultaneously inhibiting surface oxidation and corrosion, thereby improving the bending properties of the strip. It can also improve the material's resistance to hydrogen-induced delayed cracking and its ability to expand and flange holes. Excessive content can cause surface defects or copper embrittlement; therefore, the Cu content is between 0.12% and 0.17%.

[0018] Ca: It can improve the morphology of sulfides such as MnS, making sulfides spheroidized, thereby improving the morphology of inclusions. If the Ca content is too high, it will deteriorate the formability of the matrix structure, so the Ca content is controlled between 0.003% and 0.007%.

[0019] N: In steel, it can react with Ti to precipitate large TiN particles. When large TiN particles are located near the surface of the steel plate, they are prone to become areas of hydrogen accumulation, which can then form crack initiation sites. Therefore, it is required to control the N content to be less than 0.006%.

[0020] The method for producing cold-rolled high-strength steel according to the present invention includes smelting, continuous casting, hot rolling, pickling and continuous annealing processes.

[0021] Furthermore, the smelting and continuous casting process of the present invention is as follows: the smelting process is: converter → LF → RH; wherein, the RH treatment time is 10-25 min, the vacuum degree of the vacuum chamber is ≤120 Pa, and the hydrogen content at the RH endpoint is ≤1.7 ppm; the continuous casting process is dynamically reduced by 5-7 mm, the superheat is controlled at 10-25℃, and the casting speed is 0.9-1.3 m / min, to obtain a slab that meets the composition requirements.

[0022] Furthermore, in the hot rolling process described in this invention: the slab is heated in a heating furnace to 1200-1240℃, held for 170-230 minutes, rough rolled in 3+1 passes, with edge heating temperature compensation of 30-60℃, and the final rolling temperature is 880-920℃. The laminar cooling section adopts front-end centralized cooling, the water volume ratio of the upper and lower cooling manifolds in the laminar flow section is between 0.6 and 0.8, the edge is shielded by 30-100mm, and the surface of the strip is swept with high-pressure air of 0.9-1.1MPa to ensure uniform cooling of the upper and lower surfaces and the width of the strip. U-shaped coiling is adopted, the coiling temperature is 480-560℃, and the temperature compensation of 80℃ is applied to the first and last 60m. The thickness of the hot-rolled plate is between 1.8-5mm, and the hot-rolled plate is slowly cooled for 6-10 hours under a heat preservation cover after leaving the line.

[0023] Furthermore, the pickling and rolling process of this invention involves: laser welding, tension leveling, pickling, and cold rolling of hot-rolled raw materials to obtain cold-rolled strips with a specification of 0.9-2.0mm; wherein: the laser welding output power is 6-8kW, the thickness difference of the coil before and after welding is ±0.6mm, the pre-welding temperature of the welding machine is controlled at 350-500℃ for 5-10s, the post-welding temperature is controlled at 300-500℃ for 5-10s, and the welding speed is 4-6m / min; the tension leveling elongation is 0.5-1.2%; the hydrochloric acid solution temperature used for pickling is 70-90℃, the immersion time of the strip at the same position in the acid solution is 30-60s, and the concentration of the acid solution at its highest concentration range is 120-150g / L; and the cold rolling reduction rate is 40-65%.

[0024] Furthermore, the annealing process of this invention involves annealing the chilled and hardened strip steel obtained from the pickling and rolling process in a continuous annealing process. Specifically, the strip steel is heated to 880℃~910℃ at a heating rate of 1-5℃ / s, held for 40-130s, and then slowly cooled to 790℃~820℃. In the water quenching section, the strip steel is rapidly cooled to 50-120℃ using an ultra-fast water mist cooling mode at 100℃ / s-150℃ / s, with a tension of 15-20KN in the water quenching section. Then, the strip steel is subjected to spray pickling to remove the surface oxide layer formed by the water quenching process, with a hydrochloric acid solution pressure of 0.2-0.4MPa. Finally, the strip steel is heated to 180-250℃ for aging, with an aging time of 200-300s. The strip steel is leveled using a constant elongation mode, with an elongation of 0.3%~0.7%.

[0025] This invention involves reacting the material in 0.1 mol / L HCl for 10... -5 The elongation loss Iε (hydrogen embrittlement index) was calculated using the SSRT slow tensile test at a tensile rate of / s to evaluate the material's resistance to hydrogen-induced delayed cracking. Where εA is the elongation without hydrogen flow, and εE is the elongation after hydrogen flow. The smaller the Iε value, the better the resistance to hydrogen-induced hysteresis cracking.

[0026] The inventive principle and beneficial effects of the technical solution of this invention are as follows:

[0027] This invention controls the RH treatment time to 10-25 min, the vacuum degree of the vacuum chamber to ≤120 Pa, and the hydrogen content at the RH endpoint to ≤1.7 ppm to reduce the gas content in the steel. At the same time, combined with the dynamic light reduction of 5-7 mm in the continuous casting process, the superheat degree is controlled at 10-25℃, and the casting speed is 0.9-1.3 m / min, which helps to improve the uniformity of the billet structure, reduce the formation of defects such as porosity, inclusions, and segregation, thereby improving the steel's resistance to hydrogen-induced delayed cracking and its ability to expand and flange.

[0028] This invention employs edge heating temperature compensation in the hot rolling process, and utilizes U-shaped coiling and optimized water volume in the laminar flow section manifold. This can improve the uniformity of the steel coil structure throughout the coil, promote the uniform and dispersed precipitation of microalloys, and help improve the material's resistance to delayed cracking.

[0029] This invention controls the steel plate coiling temperature at 480-560℃ and uses a heat insulation cover for slow cooling. The objectives are: ① to prevent the formation of pearlite during coiling, thereby reducing flattening defects caused by reheating after hot rolling; ② to form a small amount of F+ bainite, which is beneficial for refining the cold-rolled structure and improving its strength, plasticity, and ability to expand and flanging; ③ to suppress the precipitation of microalloyed carbides, reducing excessive deformation resistance during cold rolling and avoiding fluctuations in microstructure properties caused by uneven precipitation of microalloying elements; ④ to promote the precipitation of Cu-rich phases, which is beneficial for improving the material's resistance to hydrogen-induced delayed cracking and its ability to expand and flanging.

[0030] The pickling and rolling process employs preheating and tempering of the weld microstructure, improving the uniformity of the weld's microstructure and properties, and resolving the issue of high weld breakage rate in martensitic steel. Simultaneously, it leverages the descaling effect of tension leveling, optimizes the pickling time and acid concentration, reduces hydrogen penetration during pickling, and further enhances the martensitic steel's resistance to hydrogen-induced delayed cracking.

[0031] The annealing process employs austenitizing temperature holding + ultra-rapid cooling + reheating, achieving the required high strength of martensite microstructure. Simultaneously, reheating eliminates microstructural stress and promotes the dispersed precipitation of strong carbide microalloys, which is beneficial for the formation of hydrogen traps and the improvement of porosity. Furthermore, the water-quenched strip is pickled using a low-pressure water mist spray process, which reduces surface color difference defects, eliminates the surface oxide layer during water quenching, and improves the surface gloss of the strip.

[0032] The technical solution of this invention enables the stable production of 1700MPa grade martensitic cold-rolled high-strength steel. The produced cold-rolled high-strength steel has a yield strength Rp0.2 of 1300-1500MPa and a tensile strength Rm of 1700-1790MPa. 80 Elongation 3-5%, Ceq≤0.48, porosity λ≥20%, T≥12MPa; where Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15, T=Rm×A 80 ×λ.

[0033] The cold-rolled high-strength steel provided by this invention contains more than 95% tempered martensite, with the remainder being ferrite, carbides, and / or trace amounts of retained austenite; the dispersed precipitated carbide particles are ≤90nm, the hydrogen embrittlement index Iε≤50%, and the porosity λ≥20%, exhibiting good resistance to hydrogen-induced delayed cracking and porosity expansion and flanging properties. Attached Figure Description

[0034] Figure 1 The microstructure of cold-rolled martensitic high-strength steel 1700MS is shown in Example 1. Detailed Implementation

[0035] To further illustrate the content of this invention, the technical solution of this invention will be further described below with reference to the embodiments, but this does not limit the invention to the scope of the embodiments described.

[0036] Examples 1-6

[0037] The chemical composition of the 1700MPa grade martensitic cold-rolled high-strength steel in each embodiment is shown in Table 1.

[0038] Table 1 Chemical composition (wt / %) of each example

[0039] Examples C Si Mn P S Als Cr Ni Ti Nb Cu Ca N Ceq 1 0.26 0.45 0.8 0.015 0.005 0.06 0.22 0.05 0.02 0.04 0.16 0.006 0.006 0.451 2 0.33 0.22 0.51 0.011 0.004 0.07 0.2 0.03 0.045 0.027 0.13 0.004 0.0047 0.466 3 0.28 0.37 0.5 0.013 0.005 0.03 0.35 0.03 0.037 0.035 0.14 0.007 0.0051 0.445 4 0.29 0.5 0.58 0.0014 0.003 0.043 0.4 0.04 0.05 0.02 0.12 0.007 0.0038 0.477 5 0.27 0.2 0.63 0.013 0.005 0.052 0.27 0.042 0.041 0.03 0.17 0.003 0.0032 0.443 6 0.31 0.28 0.54 0.012 0.003 0.066 0.36 0.039 0.024 0.033 0.15 0.005 0.0043 0.485

[0040] The production methods for 1700MPa grade martensitic cold-rolled high-strength steel in each embodiment include smelting, continuous casting, hot rolling, pickling, and continuous annealing processes; the details are as follows:

[0041] (1) Smelting and continuous casting: The smelting process is: converter → LF → RH; wherein, the RH treatment time is 10-25 min, the vacuum degree of the vacuum chamber is ≤120 Pa, and the hydrogen content at the RH endpoint is ≤1.7 ppm; the continuous casting process is dynamically reduced by 5-7 mm, the superheat is controlled at 10-25℃, and the casting speed is 0.9-1.3 m / min to obtain a slab that meets the composition requirements. The smelting and continuous casting process parameters of each embodiment are shown in Table 2.

[0042] Table 2 Smelting and continuous casting process parameters for each embodiment

[0043]

[0044] (2) Hot rolling: The slab is heated in a furnace to 1200-1240℃ and held for 170-230 minutes. Rough rolling is performed in 3+1 passes, with edge heating temperature compensation of 30-60℃. The final rolling temperature is 880-920℃. The laminar flow section uses centralized front-end cooling. The water volume ratio of the upper and lower cooling manifolds in the laminar flow section is between 0.6 and 0.8. Edge shielding is 30-100mm. Side spraying with 0.9-1.1MPa high-pressure air is used to blow away the surface of the strip to ensure uniform cooling of the upper and lower surfaces and the entire width of the strip. U-shaped coiling is used, with a coiling temperature of 480-560℃ and a temperature compensation of 80℃ for the first and last 60m sections. The thickness of the hot-rolled plate is between 1.8 and 5mm. The hot-rolled plate is slowly cooled for 6-10 hours under an insulation cover after being removed from the line. The hot rolling process parameters for each embodiment are shown in Table 3.

[0045] Table 3 Hot rolling process parameters for each embodiment

[0046]

[0047] (3) Pickling and rolling: Hot-rolled raw materials are subjected to laser welding, tension straightening, pickling, and cold rolling to obtain cold-hardened strips with specifications of 0.9-2.0mm. Among them: the laser welding output power is 6-8kW, the thickness difference of the coil before and after welding is ±0.6mm, the preheating temperature of the welding machine is controlled at 350-500℃ for 5-10s, the postheating temperature is controlled at 300-500℃ for 5-10s, and the welding speed is 4-6m / min; the tension straightening elongation is 0.5-1.2%, the acid temperature is 70-90℃, the acid immersion time at the same position of the strip is 30-60s, the highest concentration of the acid is 120-150g / L, and the cold rolling reduction is 40-65%. The pickling and rolling process parameters of each embodiment are shown in Table 4.

[0048] Table 4. Pickling process parameters for each embodiment

[0049]

[0050]

[0051] (4) Annealing: The cold-hardened strip steel obtained from the pickling and rolling process is annealed in continuous annealing, specifically: heated to 880℃~910℃ at a heating rate of 1-5℃ / s, held for 40~130s, and slow-cooled at 790℃~820℃. The water quenching section is rapidly cooled to 50~120℃ using an ultra-fast cooling mode of 100℃ / s-150℃ / s with water mist. The tension in the water quenching section is 15-20kN. Then, the strip is sprayed with pickling to remove the surface oxide layer formed by the water quenching process. The acid pressure is 0.2-0.4MPa. The strip is then heated to 180-250℃ for aging, with an aging time of 200-300s. The strip is leveled using a constant elongation mode with an elongation of 0.3%~0.7%. The annealing process parameters for each embodiment are shown in Table 5.

[0052] Table 5 Annealing process parameters for each embodiment

[0053]

[0054] The microstructure and properties of the high-strength steel in each embodiment are shown in Table 6.

[0055] Table 6 Organization and Performance of Each Embodiment

[0056]

[0057] Example 1: Microstructure of cold-rolled martensitic high-strength steel 1700MS as shown in Figure 1 Figure 1 As shown, the microstructure is mainly martensitic, with relatively fine lath bundles and trace amounts of ferrite and carbides between the lath bundles.

Claims

1. A 1700MPa grade martensitic cold-rolled high-strength steel, characterized in that, The chemical composition and mass percentage of the cold-rolled high-strength steel are as follows: C: 0.26%–0.33%, Si: 0.2%–0.5%, Mn: 0.5%–0.8%, P≤0.015%, S≤0.005%, Als: 0.03%–0.07%, Cr: 0.2%–0.4%, Ni: 0.03%–0.05%, Nb: 0.02%–0.04%, Ti: 0.02%–0.05%, Cu: 0.12%–0.17%, Ca: 0.003%–0.007%, N≤0.006%, with the remainder being Fe and unavoidable impurities; the yield strength Rp0.2 of the cold-rolled high-strength steel is 1300-1500 MPa, the tensile strength Rm is 1700-1790 MPa, and A… 80 Elongation 3-5%, Ceq≤0.48, porosity λ≥20%, T≥12MPa; where Ceq=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15, T=Rm×A 80 ×λ; The microstructure of the cold-rolled steel strip contains more than 95% tempered martensite, with the remainder being ferrite, carbides and / or retained austenite; the dispersed precipitated carbide particles are ≤90nm, and the hydrogen embrittlement index Iε is ≤50%.

2. The method for producing 1700MPa grade martensitic cold-rolled high-strength steel according to claim 1, characterized in that, The process includes smelting, continuous casting, hot rolling, pickling and rolling and continuous annealing. The annealing process has a slow cooling temperature of 790℃~820℃ and the strip is leveled using a constant elongation mode with an elongation of 0.3%~0.7%.

3. The method for producing 1700MPa grade martensitic cold-rolled high-strength steel according to claim 2, characterized in that, The smelting process is as follows: converter → LF → RH; wherein, the RH treatment time is 10-25 min, the vacuum degree of the vacuum chamber is ≤120 Pa, and the hydrogen content at the RH endpoint is ≤1.7 ppm.

4. The method for producing 1700MPa grade martensitic cold-rolled high-strength steel according to claim 2, characterized in that, The continuous casting process involves dynamic light reduction of 5-7 mm, superheat control at 10-25℃, and casting speed of 0.9-1.3 m / min.

5. The method for producing 1700MPa grade martensitic cold-rolled high-strength steel according to claim 2, characterized in that, The hot rolling process is as follows: the slab is heated in a heating furnace to 1200-1240℃ and held for 170-230 minutes. It is rough rolled in 3+1 passes with a temperature compensation of 30-60℃ at the edge. The final rolling temperature is 880-920℃. The laminar flow section adopts centralized cooling at the front. The water volume ratio of the upper and lower cooling manifolds in the laminar flow section is between 0.6 and 0.

8. The edge is shielded by 30-100mm. The surface of the strip is blown by high-pressure air of 0.9-1.1MPa at the side. U-shaped coiling is adopted with a coiling temperature of 480-560℃. The thickness of the hot-rolled plate is between 1.8 and 5mm. The hot-rolled plate is slowly cooled by an insulation cover for 6-10 hours after leaving the line.

6. The method for producing 1700MPa grade martensitic cold-rolled high-strength steel according to claim 2, characterized in that, The pickling and rolling process involves laser welding, tension straightening, pickling, and cold rolling of hot-rolled plates to obtain cold-rolled strips with a diameter of 0.9-2.0 mm. Specifically: the laser welding output power is 6-8 kW; the thickness difference between the coil and the sheet before and after welding is ±0.6 mm; the pre-welding temperature is controlled at 350-500℃ for 5-10 seconds, and the post-welding temperature is controlled at 300-500℃ for 5-10 seconds; the welding speed is 4-6 m / min; the tension straightening elongation is 0.5-1.2%; the pickling acid temperature is 70-90℃; the same position on the strip is immersed in the acid for 30-60 seconds; the highest concentration of the acid is 120-150 g / L; and the cold rolling reduction is 40-65%.

7. The method for producing 1700MPa grade martensitic cold-rolled high-strength steel according to claim 2, characterized in that, The annealing process involves annealing the cold-hardened strip steel obtained from the pickling and rolling process in a continuous annealing process. Specifically, the strip steel is heated to 880℃~910℃ at a rate of 1-5℃ / s and held for 40~130s. In the water quenching section, the strip steel is rapidly cooled to 50-120℃ using an ultra-fast water mist cooling mode at 100℃ / s-150℃ / s with a tension of 15-20KN. Then, the strip steel is subjected to spray pickling to remove the surface oxide layer formed by the water quenching process. The acid pressure is 0.2-0.4MPa. Finally, the strip steel is heated to 180-250℃ for aging, with an aging time of 200-300s.

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