Quenched and partitioned steel for automobiles and method for manufacturing the same by gradient partitioning

Abstract

The application belongs to the technical field of cold-rolled high-strength steel for automobiles, and particularly relates to a quenched and partitioned steel for automobiles and a gradient partitioning preparation method thereof. The chemical composition of the steel comprises, in mass percentage, C: 0.17-0.24%, Mn: 1.60-2.40%, Si: 0.80-1.80%, Al: 0.05-0.80%, Ti: 0.015-0.025%, P: 0.007-0.012%, S: 0.001-0.004%, and the balance of Fe and inevitable impurities. The method comprises the following steps: continuous casting, hot rolling, pickling, cold rolling, continuous annealing / continuous annealing and galvanizing. The application breaks through the action mechanism of QP steel by reasonable component and process design, and proposes a gradient partitioning process idea. The prepared QP steel product exceeds the international leading level compared with the same grade steel plate, and can be applied to more complex automobile structural parts.

Description

Technical Field

[0001] This invention belongs to the technical field of cold-rolled high-strength steel for automobiles, specifically relating to a quenched fractional steel for automobiles and its gradient fractionation preparation method. Background Technology

[0002] Quenching and partitioning (Q&P) is a novel process proposed by Speer et al. in 2003 for preparing high-strength, high-ductility steels with a mixed martensitic and retained austenitic microstructure. The specific process route is as follows: First, the steel is austenitized or partially austenitized and then quenched to a temperature between the start (Ms) and end (Mf) temperatures of the austenitic transformation, followed by a brief holding period to obtain a certain content of martensite and untransformed austenite. Subsequently, the experimental steel undergoes isothermal partitioning treatment at the quenching temperature or a temperature higher than the quenching temperature to achieve the diffusion and enrichment of carbon from supersaturated martensite to untransformed austenite, thereby stabilizing the austenite. Finally, the experimental steel is cooled to room temperature, resulting in a final microstructure of either a mixed martensite and retained austenite microstructure or a mixed ferrite, martensite, and retained austenite microstructure. In third-generation advanced high-strength steels, high-strength steel products using the quenching and partitioning process are considered Q&P steels. The representative steel of the third generation of advanced high-strength steel is Q&P steel. Q&P steel is also the most widely used, most deeply understood, and most industrialized advanced high-strength steel product in the world.

[0003] The Chinese standard GB / T20564.9 discloses the performance of Q&P 980 as follows: yield strength ≥ 550 MPa, tensile strength ≥ 980 MPa, and elongation after fracture ≥ 18%. It can be used in more complex structural components or reinforcements of the car body, such as the inner and outer panels of the B-pillar and bumpers. However, the limited plasticity of Q&P steel makes it difficult to meet the needs of more complex structural applications, such as replacing DP780, 420LA, or even DP590. The Japan Advanced Materials Research Institute (JAMA) points out that the plasticity target for advanced high-strength steel at the 980 MPa level in the future is 35%. At that time, 980 MPa level products can replace almost all existing car body structural components from a formability perspective. Of course, a 35% plasticity target requires new innovative design mechanisms and corresponding upgraded equipment processes. Therefore, under the current circumstances, achieving a plasticity of 25% to 30% is a target that can be achieved in the short term.

[0004] Chinese patent CN202010319605.2 discloses a method for manufacturing 980MPa grade cold-rolled Q&P steel with excellent plasticity. The steel plate has the following weight percentage composition: C: 0.18~0.21%, Mn: 1.8~2.1%, Si: 1.4~1.6%, Al: 0.02~0.06%, P≤0.02%, S≤0.01%, Nb: 0.04~0.06%, with the balance being Fe and other unavoidable impurities. The preparation method includes smelting, hot rolling, annealing, pickling, cold rolling, and continuous annealing, resulting in a steel plate with a tensile strength of 982~1065MPa and an elongation of 18.8~24.9%.

[0005] Chinese patent CN201810144307.7 discloses a 980MPa grade cold-rolled high-strength Q&P steel for automobiles and its production method. The steel's alloy composition is: C: 0.18~0.24%, Si: 0.6~1.3%, Mn: 1.6~2.4%, Nb: 0.04~0.07%, Als: 0.5~1.0%, P: 0.02~0.04%, S≤0.005%, with the balance being Fe and other unavoidable impurities. The preparation method includes smelting, hot rolling, annealing, pickling, cold rolling, and continuous annealing, yielding a steel plate with a yield strength ≥550MPa, tensile strength ≥980MPa, and elongation ≥18%, with a maximum elongation of 23.5% in the examples. Therefore, it is evident that Q&P steel, based on existing composition and process systems, struggles to achieve a plasticity index exceeding 25%. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention provides a quenched and graded steel for automobiles and its preparation method. Based on existing equipment conditions, it achieves higher-dimensional plasticity indicators for Q&P steel products, thereby significantly increasing the possibility of Q&P steel products being used in car bodies.

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

[0008] This invention provides a quenched steel for automobiles, wherein the chemical composition of the steel, by mass percentage, comprises: C: 0.17%~0.24%, Mn: 1.60%~2.40%, Si: 0.80%~1.80%, Al: 0.05%~0.80%, Ti: 0.015%~0.025%, P: 0.007%~0.012%, S: 0.001%~0.004%, with the balance being Fe and unavoidable impurities.

[0009] In the above technical solution, the chemical composition of the steel may further include one or more of Ni, Cr, Mo, and Nb; wherein, by mass percentage, Ni: 0.1%~0.30%, Cr: 0.1%~0.30%, Mo: 0.05%~0.30%, and Mn+Ni+Cr+Mo≤2.50%, Nb: 0.01%~0.025%.

[0010] In the above technical solution, the steel has a yield strength of 600-700 MPa, a tensile strength of 980-1100 MPa, and an elongation of 25%-30%.

[0011] In the above technical solution, further, by volume percentage, the microstructure of the steel consists of 40%~60% ferrite, 20%~30% martensite, 10%~20% bainite and 10%~20% retained austenite, wherein the ferrite is critical zone ferrite and oriented epiphytic ferrite.

[0012] The selection principles and content design rationale for the various chemical components of the steel of this invention are as follows:

[0013] Carbon (C) is the most economical strong element in steel, improving the hardenability of steel plates and thus increasing their strength. In the Q&P steel of this invention, C is the most critical factor, affecting the transformation behavior of supercooled austenite. During the cooling stage, the relatively rich C in the supercooled austenite ensures the amount of martensite transformed during the transformation process. At the same time, the untransformed supercooled austenite relies on the diffusion of C from the surrounding martensite during the isothermal partitioning stage to improve its stability, thus remaining as retained austenite. However, excessively high C content will increase the risk of edge cracking in hot rolling and cold rolling in industrial production. In addition, excessively high C content will lead to the formation of a high proportion of twinned martensite at the weld nugget, deteriorating weldability. Therefore, this invention controls the C content at 0.17%~0.24%.

[0014] Mn: Mn is a common and economical strengthening element in steel, enhancing solid solution strengthening and improving hardenability to increase the overall strength of the steel plate. In the Q&P steel of this invention, Mn mainly reduces the cooling rate in the critical zone and increases the proportion of martensite during rapid cooling; it also works with C to improve the stability of the austenite phase. However, the Mn content should not exceed the range of this invention, considering the C / Mn segregation problem caused by excessive Mn content. Therefore, this invention controls the Mn content to be between 1.60% and 2.40%.

[0015] Si: Si is a common and economical strengthening element, ensuring the matrix strength of ferrite. Simultaneously, the addition of Si increases the AC3 point of the steel sheet, effectively adjusting the annealing process window during continuous annealing and ensuring an appropriate ratio of ferrite and austenite in the critical region at industrial continuous annealing temperatures. In the Q&P steel of this invention, the main role of Si addition is that sufficient Si content can inhibit the formation of carbides during the over-aging stage, preventing the steel sheet from experiencing performance degradation due to carbide precipitation. It is worth noting that in the production of galvanized products, excessive Si content can lead to surface quality issues such as "incomplete galvanizing" on the galvanized surface. Therefore, this invention controls the Si content to 0.80%~1.80%.

[0016] Al: The addition of Al in conventional steel plates is limited, generally used as a deoxidizer in the smelting process. In this invention, a higher content of Al is added to replace Si during the galvanized product manufacturing stage to inhibit carbide precipitation; however, the content of Al replacing Si should not be too high, as excessive addition will lead to difficulties in tapping steel during the continuous casting crystallization stage, an upward shift in the homogenization window of continuous annealing / continuous annealing, and increased production difficulty. In this invention, the Al content is controlled at 0.05%~0.80%.

[0017] Ti: In conventional steel plates, Ti functions as a nitrogen fixation element. In this invention, Ti is appropriately added as a strength supplement. Since some planned compositions cannot meet strength requirements, Ti precipitation refines the original austenite grains, strengthening the grains and supplementing strength through precipitation reinforcement. In this invention, the Ti content is controlled at 0.015~0.025%.

[0018] P: P is an impurity element in steel, which readily agglomerates at grain boundaries. When the P content in steel is high, Fe2P particles are easily formed, reducing the plasticity and toughness of the steel. Therefore, the lower the P content, the better. In this invention, the P content is controlled at 0.070%~0.012%.

[0019] S: S is an impurity element in steel. It easily combines with Mn to form MnS inclusions, which worsens the plasticity of the steel plate. Therefore, the lower its content, the better. In this invention, the S content is controlled at 0.001%~0.004%.

[0020] You can also add elements:

[0021] Ni (Ni) is a solid solution strengthening element, similar to C and Mn, which improves the stability of austenite; at the same time, Ni improves the corrosion resistance of steel plates to a certain extent. It can be added in appropriate amounts to the optional components of this invention to enhance corrosion resistance. In this invention, the Ni content is controlled at 0.10%~0.30%.

[0022] Cr and Mo: Cr and Mo are solid solution strengthening elements, which strengthen the steel plate. In this invention, Cr and Mo can improve the hardenability of the steel plate, delay the formation of pearlite and bainite during the cooling stage, and promote the formation of martensite. At the same time, Cr and Mo can change the type of iron oxide scale during the coiling process, limit the oxidation within the steel plate, and improve the surface quality of the steel plate. In this invention, Cr and Mo are added after Mn addition to balance the problems of edge cracking in hot rolling and edge cracking in cold rolling. Therefore, in this invention, the Cr content is controlled at 0.10%~0.30%, and the Mo content is controlled at 0.05%~0.30%.

[0023] As mentioned earlier, alloying elements such as Ni, Cr, and Mo are all substitutes for Mn, and their main function in this invention is to improve the stability of austenite. However, considering factors such as cost, casting difficulty, hot rolling difficulty, and cold rolling difficulty, the overall addition should meet the integrated goals of low cost, ease of production, and high yield. Therefore, the Mn+Ni+Cr+Mo content in this invention is ≤2.5%.

[0024] Nb: Nb is a microalloying strengthening element that refines grains and improves strength. In this invention, Nb is added in combination with Ti to compensate for situations where the strength of the designed composition is too low. However, the Nb content should not be too high, as this will lead to excessively fine grains in hot rolling, excessively high strength in hot-rolled coils, and increased difficulty in cold rolling. In this invention, the Nb content is controlled between 0.01% and 0.025%.

[0025] Another aspect of the present invention provides a gradient fractionation preparation method for the above-mentioned quenched fractionation steel for automobiles, the method comprising the following steps: continuous annealing or continuous annealing galvanizing;

[0026] The specific steps of the method are as follows:

[0027] Retreat:

[0028] The cold-rolled sheet is heated to 800~830℃ and held at a constant temperature for 80~180s. It is then slowly cooled to 700~740℃ at a cooling rate of 1.2~3.6℃ / s, followed by rapid cooling to 250~280℃ at a rate of 15~25℃ / s. Subsequently, it is heated to 380~410℃ at a rate of 20℃ / s or higher for a first-stage over-aging treatment, with a constant temperature of 120~280s. As the sheet temperature rises, it enters a second-stage over-aging treatment at an aging temperature of 300~380℃, with a constant temperature of 120~280s.

[0029] Or continuous annealing:

[0030] The cold-rolled sheet is heated to 820~860℃ and held at a constant temperature for 60~120s. It is then slowly cooled to 700~740℃ at a cooling rate of 1.2~3.6℃ / s, followed by rapid cooling to 250~280℃ at a rate of 18~25℃ / s. Subsequently, it is heated to 480~510℃ at a rate of 20℃ / s for over-aging treatment, with a constant temperature time of 20~40s. After over-aging treatment, it is placed in a zinc pot at a temperature of 450~470℃ for 2~5s.

[0031] In the above technical solution, the thickness of the cold-rolled sheet is 1.4 / 1.6 / 1.8mm, the 1.4mm sheet thickness corresponds to the 2.8mm hot-rolled steel sheet, and the 1.6mm and 1.8mm sheet thicknesses correspond to the 3.0~3.5mm hot-rolled steel sheet.

[0032] The rationale for the design of the preparation steps in this invention is as follows:

[0033] During the soaking stage of continuous annealing / continuous annealing galvanizing (annealing temperature 800~830℃, isothermal for 80~180s or continuous annealing galvanizing 820~860℃, isothermal for 60~120s), a critical zone ferrite structure of 35%-45% is obtained, balancing the strength of the steel plate and ensuring the C concentration in the austenite under austenitization conditions; slow cooling to 700~740℃ at a rate of 1.2~3.6℃ / s yields 5%~10% oriented epiphytic ferrite, preventing ferrite from containing... Excessive ferrite content leads to reduced strength, while simultaneously preventing excessively low ferrite content and excessively high strength. More importantly, it is crucial to ensure the carbon concentration gradient of the supercooled austenite before rapid cooling and thus determine the transformation of bainite and martensite in subsequent processes. Subsequently, the austenite is cooled at a relatively high cooling rate to 250~280℃ to obtain 25%~30% martensite structure and the remaining untransformed supercooled austenite structure. Excessively low martensite content leads to reduced steel plate strength, while excessively high martensite content leads to reduced retained austenite content.

[0034] The key processes of continuous annealing / continuous annealing galvanizing are: ① Rapid heating rate (above 20℃ / s). Slow heating will inhibit the transformation of residual supercooled austenite in the steel plate into bainite because the carbon concentration required for the bainite transformation termination line T0 gradually decreases during the slow heating process. ② Gradient distribution: This invention adopts a two-stage distribution process, with the first stage temperature higher than the second stage, forming a cooling gradient. This is because the slow cooling process (natural cooling of the steel plate in the furnace) will significantly shift the bainite transformation termination line T0 to the right, increasing the required carbon concentration for termination, delaying the end of the bainite transformation, significantly increasing the residual austenite content of the steel plate to above 15%, and theoretically minimizing the residual secondary martensite content, thus significantly improving the formability of the steel plate. Therefore, after the previous process, 25%~30% of the supercooled austenite forms 12~15% bainite and 15%~18% residual austenite, thereby achieving an elongation after fracture of over 25%.

[0035] In the above technical solution, the method for preparing the cold-rolled sheet further includes the following steps: continuous casting, hot rolling, pickling, and cold rolling;

[0036] The specific steps of the method are as follows:

[0037] (1) Continuous casting: Continuous casting is carried out according to the chemical composition of steel;

[0038] (2) Hot rolling: The heating temperature is 1230~1280℃, the furnace time is 180~240min, the roughing rolling temperature is 1150~1200℃, the intermediate billet thickness is 50~80mm, the finishing rolling temperature is 1070~1130℃, the final rolling temperature is above 920℃, the coiling temperature is 450~520℃, and the thickness of the hot-rolled steel plate is 2.8~3.5mm;

[0039] (3) Pickling and cold rolling: After pickling, cold rolling is carried out with a rolling reduction rate of 46.7%~48.6%.

[0040] In the above technical solution, further, in step (1), the casting temperature is 1580~1620℃ and the billet thickness is 220~280mm.

[0041] The rationale behind the design of each step in the cold-rolled sheet preparation process of this invention is as follows:

[0042] Step (2), the heating temperature is controlled at 1230~1280℃, and the furnace time is 180~240min. The purpose is to promote the full solid solution of the alloy and control the banded structure caused by segregation. The purpose of the two-stage rolling in the finishing rolling stage is to promote the recrystallization behavior of the original austenite grains and inhibit the coarsening of the non-recrystallized austenite grains. The coiling temperature is controlled at 450~520℃. The purpose is to prevent the formation of Si-rich oxide on the surface of the steel plate after the addition of Si content, which in turn leads to the formation of internal oxide layer and grain boundary oxide layer.

[0043] In step (3), an excessively low rolling reduction rate cannot guarantee sufficient cold rolling deformation energy storage, resulting in insufficient ferrite recrystallization during the continuous annealing stage; an excessively high rolling reduction rate significantly increases the load on the cold rolling mill, which cannot guarantee the achievement of the target thickness.

[0044] The beneficial effects of this invention are as follows:

[0045] (1) Through reasonable composition and process design, this invention breaks through the plasticity limitation mechanism of QP steel and proposes a gradient distribution process idea, giving full play to the advantages of bainite-based configuration, achieving the optimal design of residual austenite content and the best microstructure coordination deformation capacity. The QP steel products obtained by this invention have a yield strength of 600~660MPa, a tensile strength of 980~1100MPa, and an elongation of 25~31%, which exceeds the international leading level compared with the same grade of steel plates and can be applied to more complex automotive structural parts;

[0046] (2) Based on existing equipment conditions, the present invention has the advantages of low production cost and stable production process;

[0047] (3) The QP steel product of the present invention can achieve automobile lightweighting, thereby reducing weight and exhaust emissions from the application end. Attached Figure Description

[0048] Figure 1 This is the scanned tissue from Embodiment 1 of the present invention. Detailed Implementation

[0049] The present invention will be described in more detail through embodiments. These embodiments are merely descriptions of the best mode of the invention and do not limit the scope of the invention in any way.

[0050] Examples 1-10

[0051] The chemical composition of the automotive quenched steel provided in Examples 1-10 is shown in Table 1.

[0052] Table 1. Chemical composition of steels in Examples 1-10, wt%.

[0053] The gradient distribution preparation method of the above-mentioned quenched steel for automobiles includes the following steps: continuous casting, hot rolling, pickling, cold rolling, continuous annealing or continuous annealing galvanizing;

[0054] The specific steps are as follows:

[0055] (1) Continuous casting: Continuous casting is carried out according to the chemical composition of steel, the casting temperature is 1580~1620℃, and the billet thickness is 220~280mm;

[0056] (2) Hot rolling: The heating temperature is 1230~1280℃, the furnace time is 180~240min, the roughing rolling temperature is 1150~1200℃, the intermediate billet thickness is 50~80mm, the finishing rolling temperature is 1070~1130℃, the final rolling temperature is above 920℃, the coiling temperature is 450~520℃, and the thickness of the hot-rolled steel plate is 2.8~3.5mm;

[0057] (3) Pickling and cold rolling: After pickling, cold rolling is carried out. The thickness of the cold rolled plate is 1.4 / 1.6 / 1.8mm. The 1.4mm plate thickness corresponds to 2.8mm hot rolled steel plate, and the 1.6 and 1.8mm plate thicknesses correspond to 3.0~3.5mm hot rolled steel plates. The cold rolling reduction rate is 46.7~48.6%.

[0058] (4) Retreat consecutively:

[0059] The cold-rolled sheet is heated to 800~830℃ and held at a constant temperature for 80~180s. It is then slowly cooled to 700~740℃ at a cooling rate of 1.2~3.6℃ / s, followed by rapid cooling to 250~280℃ at a rate of 15~25℃ / s. Subsequently, it is heated to 380~410℃ at a rate of 20℃ / s or higher for a first-stage over-aging treatment, with a constant temperature of 120~280s. As the sheet temperature rises, it enters a second-stage over-aging treatment, with the aging temperature controlled at 300~380℃ and a constant temperature of 120~280s.

[0060] Or continuous annealing:

[0061] The cold-rolled sheet is heated to 820~860℃ and held at a constant temperature for 60~120s. It is then slowly cooled to 700~740℃ at a cooling rate of 1.2~3.6℃ / s, followed by rapid cooling to 250~280℃ at a rate of 18~25℃ / s. Subsequently, it is heated to 480~510℃ at a rate of 20℃ / s for over-aging treatment, with a constant temperature time of 20~40s. After over-aging treatment, it is placed in a zinc pot at a temperature of 450~470℃ for 2~5s.

[0062] Table 2 lists the continuous casting and hot rolling process parameters for the steels of Examples 1-10, and Table 3 lists the cold rolling and continuous annealing / continuous annealing galvanizing process parameters for the steels of Examples.

[0063] Table 2. Continuous casting and hot rolling process parameters for steels in Examples 1-10

[0064]

[0065] Table 3 Cold rolling and continuous annealing / continuous annealing galvanizing process parameters for steels in Examples 1-10

[0066] Table 4 lists the mechanical properties of the steels in Examples 1-10.

[0067] Table 4 Mechanical properties of steels from Examples 1-10

[0068] The above embodiments are merely preferred embodiments of the present invention and are not intended to limit the implementation. The scope of protection of the present invention should be determined by the scope defined in the claims. Other variations or modifications can be made based on the above description. Obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A quenched and partitioned steel for automotive use, characterized in that, The chemical composition of the steel, by mass percentage, comprises: C: 0.17%~0.24%, Mn: 1.60%~2.40%, Si: 0.80%~1.80%, Al: 0.05%~0.80%, Ti: 0.015%~0.025%, P: 0.007%~0.012%, S: 0.001%~0.004%, with the balance being Fe and unavoidable impurities; The method for preparing the steel includes the following steps: continuous annealing or continuous annealing galvanizing; The specific steps of the method are as follows: Retreat: The cold-rolled sheet is heated to 800~830℃ and held at a constant temperature for 80~180s. It is then slowly cooled to 700~740℃ at a cooling rate of 1.2~3.6℃ / s, followed by rapid cooling to 250~280℃ at a rate of 15~25℃ / s. Subsequently, it is heated to 380~410℃ at a rate of 20℃ / s or higher for a first-stage over-aging treatment, with a constant temperature of 120~280s. As the sheet temperature rises, it enters a second-stage over-aging treatment at an aging temperature of 300~380℃, with a constant temperature of 120~280s. Or continuous annealing: The cold-rolled sheet is heated to 820~860℃ and held at a constant temperature for 60~120s. It is then slowly cooled to 700~740℃ at a cooling rate of 1.2~3.6℃ / s, followed by rapid cooling to 250~280℃ at a rate of 18~25℃ / s. Subsequently, it is heated to 480~510℃ at a rate of 20℃ / s for over-aging treatment, with a constant temperature time of 20~40s. After over-aging treatment, it is placed in a zinc pot at a temperature of 450~470℃ for 2~5s.

2. The quenched and partitioned steel for automotive use according to claim 1, characterized by, The chemical composition of the steel may also include one or more of Ni, Cr, Mo, and Nb; wherein, by mass percentage, Ni: 0.10%~0.30%, Cr: 0.10%~0.30%, Mo: 0.05%~0.30%, and Mn+Ni+Cr+Mo≤2.50%, Nb: 0.01%~0.025%.

3. The quenched and partitioned steel for automotive use according to claim 1, characterized by, The steel has a yield strength of 600-700 MPa, a tensile strength of 980-1100 MPa, and an elongation of 25%-30%.

4. The quenched steel for automobiles according to claim 1, characterized in that, By volume percentage, the microstructure of the steel consists of 40%~60% ferrite, 20%~30% martensite, 10%~20% bainite and 10%~20% retained austenite, wherein the ferrite is critical zone ferrite and oriented epiphytic ferrite.

5. A method for preparing gradient partitioning of quenched partitioned steel for automobiles according to any one of claims 1-4, characterized in that, The method includes the following steps: continuous annealing or continuous annealing galvanizing; The specific steps of the method are as follows: Retreat: The cold-rolled sheet is heated to 800~830℃ and held at a constant temperature for 80~180s. It is then slowly cooled to 700~740℃ at a cooling rate of 1.2~3.6℃ / s, followed by rapid cooling to 250~280℃ at a rate of 15~25℃ / s. Subsequently, it is heated to 380~410℃ at a rate of 20℃ / s or higher for a first-stage over-aging treatment, with a constant temperature of 120~280s. As the sheet temperature rises, it enters a second-stage over-aging treatment at an aging temperature of 300~380℃, with a constant temperature of 120~280s. Or continuous annealing: The cold-rolled sheet is heated to 820~860℃ and held at a constant temperature for 60~120s. It is then slowly cooled to 700~740℃ at a cooling rate of 1.2~3.6℃ / s, followed by rapid cooling to 250~280℃ at a rate of 18~25℃ / s. Subsequently, it is heated to 480~510℃ at a rate of 20℃ / s for over-aging treatment, with a constant temperature time of 20~40s. After over-aging treatment, it is placed in a zinc pot at a temperature of 450~470℃ for 2~5s.

6. The preparation method according to claim 5, characterized in that, The cold-rolled sheet thickness is 1.4 / 1.6 / 1.8mm, with 1.4mm thickness corresponding to 2.8mm hot-rolled steel sheet, and 1.6mm and 1.8mm thickness corresponding to 3.0~3.5mm hot-rolled steel sheet.

7. The preparation method according to claim 5, characterized in that, The method for preparing the cold-rolled sheet includes the following steps: continuous casting, hot rolling, pickling, and cold rolling; The specific steps of the method are as follows: (1) Continuous casting: Continuous casting is carried out according to the chemical composition of steel; (2) Hot rolling: The heating temperature is 1230~1280℃, the furnace time is 180~240min, the roughing rolling temperature is 1150~1200℃, the intermediate billet thickness is 50~80mm, the finishing rolling temperature is 1070~1130℃, the final rolling temperature is above 920℃, the coiling temperature is 450~520℃, and the thickness of the hot-rolled steel plate is 2.8~3.5mm; (3) Pickling and cold rolling: After pickling, cold rolling is carried out with a rolling reduction rate of 46.7%~48.6%.

8. The preparation method according to claim 7, characterized in that, In step (1), the casting temperature is 1580~1620℃ and the billet thickness is 220~280mm.

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