WO2024244041A1 - Quenched and partitioned steel for automobiles and gradient partitioning preparation method therefor - Google Patents

Abstract

The present invention relates to the technical field of cold-rolled high-strength steel for automobiles, and specifically relates to a quenched and partitioned steel for automobiles and a gradient partitioning preparation method therefor. The chemical composition of the steel comprises, by 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%, and S: 0.001-0.004%, the remainder being Fe and unavoidable impurities. The method comprises the following steps: continuous casting, hot rolling, pickling, cold rolling, and continuous annealing/continuous annealing galvanizing. The present invention makes a breakthrough in the mechanism of action of QP steel by means of reasonable composition and process design, and proposes a gradient partitioning process idea. The prepared QP steel product exceeds the international leading level compared with steel plates of the same grade, and can be applied to more complex automotive structural members.

Description

Quenching and partitioning steel for automobiles and its gradient partitioning preparation method Technical Field

The invention belongs to the technical field of cold-rolled high-strength steel for automobiles, and in particular relates to a quenching and partitioning steel for automobiles and a gradient partitioning preparation method thereof.

Background Art

Quenching and partitioning (Q&P) is a new process proposed by Speer et al. in 2003 for preparing high-strength and high-plasticity steel with a mixed structure of martensite and retained austenite. The specific process route is as follows: First, the steel is austenitized or partially austenitized, then quenched to a temperature between the start (Ms) and end (Mf) temperatures of the martensite phase transformation of austenite and subjected to a short heat preservation treatment to obtain a certain content of martensite and untransformed austenite. Subsequently, the experimental steel is isothermally partitioned at a certain temperature at or above the quenching temperature to achieve the diffusion and enrichment of carbon from supersaturated martensite to untransformed austenite, thereby achieving the stabilization of austenite. Finally, the experimental steel is cooled to room temperature, and the final structure obtained is a mixed structure of martensite + retained austenite or a mixed structure of ferrite + martensite + retained austenite. In the third generation of advanced high-strength steels, high-strength steel products using quenching and partitioning technology are regarded as Q&P steels. With the gradual advancement of global energy conservation and emission reduction, the lightweight process of automobiles has continued to develop, and the proportion of third-generation advanced high-strength steel in white bodies has gradually increased. For example, the proportion of advanced high-strength steel in brands such as BMW, Toyota and Mercedes-Benz has increased from about 5% around 2017 to more than 15% in 2022; the proportion of high-strength steel in new models released by Chinese automakers represented by BYD and Geely has risen to about 30%. The gradual increase in the use of advanced high-strength steel is mainly due to the continuous optimization and improvement of advanced high-strength steel production and manufacturing technology. The representative steel grade of the third-generation advanced high-strength steel is Q&P steel, which is also the most widely used, most deeply recognized, and most maturely industrialized advanced high-strength steel product in the world.

The Chinese standard GB/T20564.9 discloses the performance of Q&P 980 as follows: yield strength ≥550MPa, tensile strength ≥980MPa, elongation after fracture ≥18%, which can be used for more complex structural parts or reinforcements of the car body, such as inner and outer panels of the B-pillar, bumpers, etc. However, it is obvious that the limited plasticity of Q&P steel makes it difficult to meet the application of more complex structural parts, such as replacing DP780, 420LA, and even DP590. The Japanese New Materials Institute pointed out that the plasticity target of 980MPa-level advanced high-strength steel in the future is 35%. At that time, 980MPa-level products can replace almost all existing car body structural parts from the perspective of formability. Of course, the 35% plasticity index requires new innovative design mechanisms and corresponding upgraded equipment processes. Therefore, based on the current situation, achieving a plasticity of more than 25% to about 30% is a goal that can be achieved in the short term.

Chinese patent CN202010319605.2 discloses a method for manufacturing 980MPa grade cold-rolled Q&P steel with excellent plasticity. The weight percentage of each component of the steel plate is: 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%, and the balance is Fe and other unavoidable impurities. The preparation method includes smelting, hot rolling, annealing, pickling, cold rolling, and continuous annealing to obtain a steel plate with a tensile strength of 982-1065MPa and an elongation of 18.8-24.9%.

Chinese patent CN201810144307.7 discloses 980MPa grade cold-rolled high-strength Q&P steel for automobiles and its production method. The steel 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%, P0.02-0.04%, S≤0.005%, and the balance is Fe and other unavoidable impurities. The preparation method includes smelting, hot rolling, annealing, pickling, cold rolling, and continuous annealing to obtain a steel plate with a yield strength of ≥550MPa, a tensile strength of ≥980MPa, and an elongation of ≥18%. The maximum elongation in the embodiment is 23.5%. It can be seen that it is difficult for Q&P steel to achieve a plasticity index of more than 25% based on the existing composition and process system.

Summary of the invention

In order to solve the above technical problems, the present invention provides a quenching and partitioning steel for automobiles and a gradient partitioning preparation method thereof, which, based on the existing equipment conditions, achieves higher-dimensional plasticity indicators of Q&P steel products, thereby greatly improving the possibility of body application of Q&P steel products.

In order to achieve the above object, the technical solution of the present invention is as follows:

On one hand, the present invention provides a quenching and partitioning steel for automobiles, wherein the chemical composition of the steel comprises, by 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 is Fe and unavoidable impurities.

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

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

In the above technical solution, further, the microstructure of the steel is composed of 40% to 60% ferrite, 20% to 30% martensite, 10% to 20% bainite and 10% to 20% residual austenite by volume percentage, wherein the ferrite is intercritical ferrite and oriented epitaxial ferrite.

The selection principles and content design reasons of the chemical components of the steel of the present invention are as follows:

C: C is the most economical strong element in steel, which improves the hardenability of steel plates and thus improves the strength of steel plates. In the Q&P steel of the present invention, C is the most critical factor, affecting the phase transformation behavior of supercooled austenite. The relatively enriched C in the supercooled austenite during the cooling stage ensures the content of martensite transformed during the transformation process. At the same time, the untransformed supercooled austenite relies on the C diffusion of the surrounding martensite during the isothermal partitioning stage to improve stability and thus remain as residual austenite. However, too high a C content will increase the risks of hot-rolled edge cracking and cold-rolled edge cracking in industrial production. In addition, too high a C content will lead to the formation of a high proportion of twin martensite at the spot welding nugget, deteriorating the welding performance. Therefore, the present invention controls the C element content to 0.17% to 0.24%.

Mn: Mn is a common economical strengthening element in steel, which improves the solid solution strengthening effect and the hardenability of the steel plate to improve the overall strength of the steel plate. In the Q&P steel of the present invention, the Mn element mainly plays a role in reducing the cooling rate in the critical zone and increasing the proportion of martensite in the rapid cooling stage; at the same time, it is combined with C addition to improve the stability of the austenite phase. However, the added content of the Mn element should not exceed the scope of the present invention, considering the C/Mn segregation problem caused by excessive Mn content. Therefore, the present invention controls the Mn element content to 1.60% to 2.40%.

Si: Si is a common economical strengthening element, which ensures the matrix strength of ferrite. At the same time, Si addition will increase the AC3 point of the steel plate, effectively adjust the annealing process window in the continuous annealing stage, and ensure the appropriate ferrite and austenite ratio in the critical zone at the industrial continuous annealing temperature. In the Q&P steel of the present invention, the main role of Si addition is that sufficient Si addition can inhibit the formation of carbides in the over-aging stage, and avoid the performance of the steel plate being reduced due to carbide precipitation. It is worth noting that in the case of producing galvanized products, it is considered that too high Si content will cause "leakage plating" on the galvanized surface and other problems that affect the surface quality. Therefore, the present invention controls the Si element content to 0.80% to 1.80%.

Al: Al is added to a limited amount in conventional steel plates and is generally used as a deoxidizer in the smelting process. In the present invention, a relatively high content of Al is added to replace Si during the production and manufacturing stage of galvanized products to inhibit the precipitation of carbides; however, the content of Al replacing Si should not be too high. Too high addition will lead to difficulties in tapping during the continuous casting crystallization stage, upward shifting of the galvanizing soaking window during continuous annealing/continuous annealing, and increased production difficulty. In the present invention, the Al element content is controlled at 0.05% to 0.80%.

Ti: In conventional steel plates, Ti is used to fix nitrogen. In the present invention, Ti is appropriately added as a strength supplement. Some planned components cannot meet the strength requirements. The precipitation of Ti is relied on to refine the original austenite grains for fine grain strengthening, and precipitation strengthening to supplement the strength. In the present invention, the Ti content is controlled to be 0.015-0.025%.

P: P is an impurity element in steel, which is easily concentrated at the grain boundary. When the P content in steel is high, Fe2P particles are easily formed, which reduces the plasticity and toughness of the steel. Therefore, the lower the content, the better. In the present invention, the P content is controlled to 0.070% to 0.012%.

S: S is an impurity element in steel, which is easy to combine with Mn to form MnS inclusions, which deteriorates the plasticity of the steel plate. Therefore, the lower the S content, the better. In the present invention, the S content is controlled to be 0.001% to 0.004%.

You can also add elements:

Ni: It is a solid solution strengthening element. Like C and Mn, it improves the stability of austenite. At the same time, Ni improves the corrosion resistance of the steel plate to a certain extent. It can be added in an appropriate amount in the optional components of the present invention to improve the corrosion resistance. In the present invention, the Ni element content is controlled at 0.10% to 0.30%.

Cr and Mo: Cr and Mo are solid solution strengthening elements themselves, which play a role in strengthening the steel plate. In the present invention, Cr and Mo can improve the hardenability of the steel plate, delay the formation of pearlite and bainite in 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 in the steel plate, and improve the surface quality of the steel plate. In the present invention, Cr and Mo are both added after Mn to balance the problems of hot-rolled edge cracks and cold-rolled edge cracks. Therefore, in the present invention, the Cr element content is controlled at 0.10% to 0.30%, and the Mo element content is controlled at 0.05% to 0.30%.

As mentioned above, alloying elements such as Ni, Cr, and Mo are all substitute elements for supplementing Mn, and their main role in the present invention is to improve the stabilization of austenite and supplement the stability of austenite. However, considering the cost, difficulty of steel casting, difficulty of hot rolling, difficulty of cold rolling and other multi-dimensional considerations, the overall addition should meet the integrated purpose of low cost, easy production, and high yield rate. Therefore, the content of Mn+Ni+Cr+Mo in the present invention is ≤2.5%.

Nb: Nb is a microalloy strengthening element that refines grains and improves strength. In the present invention, Nb is added in combination with Ti to make up for the situation where the strength of some designed components is too low. However, the Nb content should not be added too high, as it will lead to too high a degree of hot-rolled grain refinement, too high a strength of hot-rolled coils, and increased difficulty in cold rolling. In the present invention, the Nb element content is controlled at 0.01% to 0.025%.

Another aspect of the present invention provides a method for preparing the above-mentioned quenching and partitioning steel for automobiles by gradient partitioning, the method comprising the following steps: continuous annealing or continuous annealing galvanizing;

The specific steps of the method are as follows:

Continuous retreat:

The cold-rolled sheet is heated to 800-830°C, isothermal for 80-180s, slowly cooled to 700-740°C at a cooling rate of 1.2-3.6°C/s, and then rapidly cooled to 250-280°C at a rate of 15-25°C/s, and then heated to 380-410°C at a rate of more than 20°C/s for a first-stage over-aging treatment, with an isothermal time of 120-280s; the second-stage over-aging treatment is carried out with the sheet temperature, with an aging temperature of 300-380°C and an isothermal time of 120-280s;

Or continuous galvanizing:

The cold-rolled sheet is heated to 820-860°C, kept isothermal for 60-120s, slowly cooled to 700-740°C at a cooling rate of 1.2-3.6°C/s, and then rapidly cooled to 250-280°C at a rate of 18-25°C/s, and then heated to 480-510°C at a rate of more than 20°C/s for aging treatment. The isothermal time is 20-40s. After aging treatment, it enters the zinc pot at a temperature of 450-470°C and stays in the zinc pot for 2-5s.

In the above technical solution, further, the thickness of the cold-rolled plate is 1.4/1.6/1.8 mm, the 1.4 mm plate thickness corresponds to the 2.8 mm hot-rolled steel plate, and the 1.6 mm and 1.8 mm plate thicknesses correspond to the 3.0-3.5 mm hot-rolled steel plates.

The reasons for designing the preparation steps of the present invention are as follows:

In the soaking stage of continuous annealing/continuous galvanizing (continuous annealing temperature 800-830℃, isothermal 80-180s or continuous annealing galvanizing 820-860℃, isothermal 60-120s), 35%-45% of the critical zone ferrite structure is obtained to balance the strength of the steel plate and ensure the C concentration in the austenite under the austenitization degree; slowly cool to 700-740℃ at a rate of 1.2-3.6℃/s to obtain 5%-10% oriented epitaxial ferrite to prevent ferrite from containing Too high austenite content leads to reduced strength, while too low austenite content leads to too high strength; more importantly, the C concentration gradient of supercooled austenite after slow cooling and before rapid cooling is guaranteed, which determines the transformation amount of bainite and martensite in the subsequent process; then cool to 250-280℃ at a higher cooling rate to obtain 25%-30% martensite structure and the remaining untransformed supercooled austenite structure. Too low austenite content leads to reduced strength of the steel plate, and too high austenite content leads to a decrease in the residual austenite content.

The key process of continuous annealing/continuous galvanizing is: ① Rapid heating rate (above 20℃/s), slow heating will lead to the inhibition of the transformation of the remaining supercooled austenite in the steel plate to bainite, because the C concentration required for the bainite phase transformation termination line _T0 gradually decreases during the slow heating process. ② Gradient partitioning: The present invention adopts a two-stage partitioning treatment, and the temperature of the first stage is higher than that of the second stage, forming a cooling gradient, because the slow cooling process (the steel plate cools naturally in the furnace) will greatly act on the bainite phase transformation termination line _T0 to the right, so that the C concentration at the required termination is increased, the end of bainite transformation is delayed, and the residual austenite content of the steel plate is greatly increased to more than 15%, and the residual secondary martensite content is theoretically the lowest, which greatly improves the formability of the steel plate. As a result, after the previous process treatment, 25% to 30% of the supercooled austenite forms 12 to 15% of bainite and 15% to 18% of residual austenite, thereby achieving an elongation of more than 25%.

In the above technical solution, further, the method for preparing the cold-rolled sheet comprises 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: heating temperature is 1230-1280°C, furnace time is 180-240 min, rough rolling temperature is 1150-1200°C, intermediate billet thickness is 50-80 mm, finishing rolling temperature is 1070-1130°C, final rolling temperature is above 920°C, coiling temperature is 450-520°C, and hot rolled steel plate thickness is 2.8-3.5 mm;

(3) Pickling and cold rolling: After pickling, cold rolling is performed, and the cold rolling reduction rate is 46.7% to 48.6%.

In the above technical solution, further, in step (1), the casting temperature is 1580-1620° C., and the thickness of the ingot is 220-280 mm.

The reasons for designing each step of cold-rolled sheet preparation of the present invention are as follows:

In step (2), the heating temperature is controlled at 1230-1280°C and the furnace time is 180-240 min, in order to promote full solid solution of the alloy and control the banded structure caused by segregation. The finishing rolling stage is divided into two stages to promote the recrystallization behavior of the original austenite grains and inhibit the coarsening of the unrecrystallized austenite grains; the coiling temperature is controlled at 450-520°C to prevent the formation of Si-rich oxides on the surface of the steel plate after adding Si content, thereby causing the formation of inner oxide layer and grain boundary oxide layer.

In step (3), too low a rolling reduction ratio cannot ensure sufficient cold rolling deformation energy storage, resulting in insufficient ferrite recrystallization effect in the continuous annealing stage; too high a rolling reduction ratio greatly increases the load of the cold rolling mill and cannot ensure the target thickness is achieved.

The beneficial effects of the present invention are:

(1) The present invention breaks through the plasticity limitation mechanism of QP steel through reasonable composition and process design, proposes a gradient partitioning process idea, gives full play to the advantages of bainite matrix configuration, realizes the optimal design of retained austenite content and the best coordinated deformation ability of organization, and the QP steel product prepared by the present invention has 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 steel plates and can be applied to more complex automotive structural parts;

(2) The present invention is based on existing equipment conditions and has the advantages of low production cost and stable production process;

(3) The QP steel product of the present invention can achieve lightweighting of automobiles, reduce weight and exhaust emissions from the application end, and meet the dual carbon strategy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a scanning structure of Example 1 of the present invention.

DETAILED DESCRIPTION

The present invention is described in more detail by way of examples. These examples are merely descriptions of the best mode for carrying out the present invention and do not limit the scope of the present invention in any way.

Examples 1-10

The chemical compositions of the automotive quenching and partitioning steels provided in Examples 1-10 are shown in Table 1.

Table 1 Chemical composition of steels of Examples 1-10, wt%

The gradient partitioning preparation method of the quenching partitioning steel for automobiles comprises the following steps: continuous casting, hot rolling, pickling, cold rolling, continuous annealing or continuous annealing galvanizing;

The specific steps are as follows:

(1) Continuous casting: Continuous casting is carried out according to the chemical composition of the steel, the casting temperature is 1580-1620°C, and the thickness of the casting is 220-280 mm;

(2) Hot rolling: heating temperature is 1230-1280°C, furnace time is 180-240 min, rough rolling temperature is 1150-1200°C, intermediate billet thickness is 50-80 mm, finishing rolling temperature is 1070-1130°C, final rolling temperature is above 920°C, coiling temperature is 450-520°C, and hot rolled steel plate thickness is 2.8-3.5 mm;

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

(4) Continuous retreat:

The cold-rolled sheet is heated to 800-830°C, isothermal for 80-180s, slowly cooled to 700-740°C at a cooling rate of 1.2-3.6°C/s, and then rapidly cooled to 250-280°C at a rate of 15-25°C/s, and then heated to 380-410°C at a rate of more than 20°C/s for a first-stage over-aging treatment, with an isothermal time of 120-280s; the second-stage over-aging treatment is carried out with the sheet temperature, and the aging temperature is controlled at 300-380°C, with an isothermal time of 120-280s;

Or continuous galvanizing:

The cold-rolled sheet is heated to 820-860°C, kept isothermal for 60-120s, slowly cooled to 700-740°C at a cooling rate of 1.2-3.6°C/s, and then rapidly cooled to 250-280°C at a rate of 18-25°C/s, and then heated to 480-510°C at a rate of more than 20°C/s for aging treatment. The isothermal time is 20-40s. After aging treatment, it enters the zinc pot at a temperature of 450-470°C and stays in the zinc pot for 2-5s.

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

Table 2 Continuous casting and hot rolling process parameters of steels in Examples 1-10

Table 3 Cold rolling and continuous annealing/continuous annealing galvanizing process parameters of steels in Examples 1-10

Table 4 lists the mechanical properties of the steels of Examples 1-10.

Table 4 Mechanical properties of steels in Examples 1-10

The above embodiments are only preferred embodiments of the present invention and are not intended to limit the implementation methods. The protection scope of the present invention shall be subject to the scope defined by the claims. Other different forms of changes or modifications may be made based on the above description. Obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims (8)

A quenching and partitioning steel for automobiles, characterized in that the chemical composition of the steel includes, by 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 is Fe and unavoidable impurities. The quenching and partitioning steel for automobiles according to claim 1 is characterized in that the chemical composition of the steel can also include one or more of Ni, Cr, Mo, and Nb; wherein, in terms of mass percentage, Ni: 0.10% to 0.30%, Cr: 0.10% to 0.30%, Mo: 0.05% to 0.30%, and Mn+Ni+Cr+Mo≤2.50%, Nb: 0.01% to 0.025%. The quenching and partitioning steel for automobiles according to claim 1 is characterized in that the steel has a yield strength of 600-700 MPa, a tensile strength of 980-1100 MPa, and an elongation of 25%-30%. The quenching and partitioning steel for automobiles according to claim 1 is characterized in that, in terms of volume percentage, the microstructure of the steel consists of 40% to 60% ferrite, 20% to 30% martensite, 10% to 20% bainite and 10% to 20% retained austenite, wherein the ferrite is intercritical ferrite and oriented epitaxial ferrite. A method for preparing a gradient partitioning of a quenching and partitioning steel for automobiles according to any one of claims 1 to 4, characterized in that the method comprises the following steps: continuous annealing or continuous annealing galvanizing; The specific steps of the method are as follows: Continuous retreat: The cold-rolled sheet is heated to 800-830°C, isothermal for 80-180s, slowly cooled to 700-740°C at a cooling rate of 1.2-3.6°C/s, and then rapidly cooled to 250-280°C at a rate of 15-25°C/s, and then heated to 380-410°C at a rate of more than 20°C/s for a first-stage over-aging treatment, with an isothermal time of 120-280s; the second-stage over-aging treatment is carried out with the sheet temperature, with an aging temperature of 300-380°C and an isothermal time of 120-280s; Or continuous galvanizing: The cold-rolled sheet is heated to 820-860°C, kept isothermal for 60-120s, slowly cooled to 700-740°C at a cooling rate of 1.2-3.6°C/s, and then rapidly cooled to 250-280°C at a rate of 18-25°C/s, and then heated to 480-510°C at a rate of more than 20°C/s for aging treatment. The isothermal time is 20-40s. After aging treatment, it enters the zinc pot at a temperature of 450-470°C and stays in the zinc pot for 2-5s. The preparation method according to claim 5 is characterized in that the thickness of the cold-rolled plate is 1.4/1.6/1.8 mm, the plate thickness of 1.4 mm corresponds to the hot-rolled steel plate of 2.8 mm, and the plate thickness of 1.6 mm and 1.8 mm corresponds to the hot-rolled steel plate of 3.0-3.5 mm. The preparation method according to claim 5 is characterized in that the preparation method of the cold-rolled sheet comprises 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: heating temperature is 1230-1280°C, furnace time is 180-240 min, rough rolling temperature is 1150-1200°C, intermediate billet thickness is 50-80 mm, finishing rolling temperature is 1070-1130°C, final rolling temperature is above 920°C, coiling temperature is 450-520°C, and hot rolled steel plate thickness is 2.8-3.5 mm; (3) Pickling and cold rolling: After pickling, cold rolling is performed, and the cold rolling reduction rate is 46.7% to 48.6%. The preparation method according to claim 7 is characterized in that in step (1), the casting temperature is 1580-1620°C and the thickness of the ingot is 220-280 mm.