980MPa grade galvanized steel sheet for automobiles resistant to embrittlement of spot welded liquid metal and its manufacturing method

Abstract

This invention discloses a galvanized steel sheet for automobiles with a 980MPa grade resistance to embrittlement of spot welded liquid metal and its manufacturing method. The steel sheet is divided into roll-formed steel sheet and stamped steel sheet. The chemical composition of the roll-formed steel sheet, by mass percentage, includes: C: 0.13%–0.16%, Mn: 1.60%–2.30%, Si: 0.80%–1.00%, Al: 0.035%–0.06%, P: 0.008%–0.015%, S≤0.003%, with the balance being Fe and unavoidable impurities. The chemical composition of the stamped steel sheet, by mass percentage, includes: C: 0.18%–0.25%, Mn: 1.80%–2.50%, Si: 0.80%–1.80%, Al: 0.05%–0.80%, P: 0.008%–0.015%, S≤0.003%, with the balance being Fe and unavoidable impurities. The steel plate is prepared using a continuous casting-hot rolling-cold rolling-continuous annealing galvanizing process. This invention offers the advantages of a low-cost, high-performance dual-carbon strategy, producing roll-formed and stamped LME-QP980MPa resistant products.

Description

Technical Field

[0001] This invention relates to the field of cold-rolled and hot-dip galvanized high-strength steel for automobiles, and more particularly to a 980MPa grade galvanized steel sheet for automobiles that is resistant to embrittlement of spot welded liquid metal and its manufacturing method. Background Technology

[0002] High strength and thinner materials are consistently applied in automotive material design, effectively improving fuel efficiency, reducing carbon dioxide emissions, and ensuring passenger safety. High-strength materials are also a major trend in automotive development. In recent years, with increasingly stringent anti-corrosion regulations, Europe has required higher coating ratios. To fully ensure the corrosion resistance of vehicle bodies and components, high-strength alloyed hot-dip galvanized steel sheets are used in addition to high-strength hot-dip galvanized steel sheets. However, during the welding stage of components, cracks known as liquid metal embrittlement (LME) sometimes occur at the spot weld. LME occurs when the zinc in the galvanized layer melts due to the resistance heat during spot welding. The molten zinc penetrates the grain boundaries of the steel sheet structure at the weld, causing cracks under tensile stress. It is noteworthy that LME can occur not only during the spot welding stage of coated steel sheets but also during the spot welding of bare steel sheets with coated steel sheets. Therefore, the application of high-strength coated steel sheets is limited.

[0003] LME (Liquid Metal Embrittlement) occurs under the following conditions: ① the presence of liquid zinc. Liquid metal embrittlement only occurs in the coating state of hot-dip galvanized steel sheets or during the spot welding stage between bare sheets and hot-dip galvanized steel sheets; ② high-strength steel systems. Liquid metal embrittlement mostly occurs in high-strength galvanized steel sheets with a strength of 980 MPa because of the high residual stress acting on the tensile stress state under spot welding conditions. Therefore, the research scope of LME mainly focuses on high-strength galvanized steel sheets with a strength of 980 MPa and above, such as DP980, TRIP980, TWIP980, and QP980 hot-dip galvanized or alloyed galvanized products. Furthermore, for high-strength automotive steels with a strength of 980 MPa and above, only DP and QP are most widely used among the aforementioned products. Therefore, research on LME-resistant products of DP and QP types with a strength of 980 MPa and above is crucial for their application. In addition, meeting higher forming requirements while effectively suppressing LME is equally important.

[0004] 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 martensitic transformation, followed by a brief holding period to obtain a certain content of martensite and untransformed austenite. Subsequently, the experimental steel is subjected to isothermal partitioning treatment at the quenching temperature or at 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 that is either a mixed martensite and retained austenite microstructure or a mixed ferrite, martensite, and retained austenite microstructure, thus achieving an excellent balance between strength and ductility.

[0005] Regarding prior art techniques for suppressing embrittlement cracks in liquid metals, Japanese Patent Application Publication No. 2006-265671 discloses an alloyed hot-dip galvanized high-tensile steel sheet with excellent workability and resistance to embrittlement cracks in liquid metals. This sheet is a coated steel sheet with an alloyed hot-dip galvanized coating applied to its surface. The base steel has the following composition: C: 0.04–0.25 wt%, Si: 0.01–2.0 wt%, Mn: 0.5–3.0 wt%, P: less than 0.1 wt%, and S: 0.03 wt%. Less than wt%, further comprising one or more of the following: Ti: 0.001–0.1wt%, Nb: 0.001–0.1wt%, V: 0.01–0.3wt%, Mo: 0.01–0.5wt%, Zr: 0.01–0.5wt%, with the balance being Fe and unavoidable impurities, and having the following metallographic structure: ferrite with an area fraction of 40–95%, one or more of the following: bainite, pearlite, and martensite, and retained austenite with a volume fraction of 1–10%.

[0006] Japanese Patent Application Publication No. 2008-231493 discloses a method for manufacturing alloyed hot-dip galvanized steel sheet for spot welding. The method involves hot-rolling a base steel containing, by weight, 0.05–0.20% C, 0.5–2.0% Si, 1.0–2.5% Mn, with the balance being Fe and unavoidable impurities. After hot rolling, the steel sheet is cooled at a rate of 30°C / second or higher and coiled at 450–580°C, thereby achieving a grain boundary oxidation depth of less than 5 μm. The hot-rolled steel sheet is then cold-rolled to achieve an adhesion weight of 3 g / m². 2 The above methods are used to perform Fe-based electroplating on cold-rolled steel sheets, followed by alloying hot-dip galvanizing, so that the grain boundary oxidation depth of the alloyed hot-dip galvanized steel sheet reaches below 5μm. Summary of the Invention

[0007] The purpose of this invention is to provide a galvanized steel sheet for automobiles with a 980MPa grade resistance to embrittlement of spot welded liquid metal and its manufacturing method.

[0008] To achieve the above objectives, the technical solution of the present invention is as follows:

[0009] In one aspect, this invention provides a galvanized steel sheet for automobiles with a 980MPa grade resistance to embrittlement of spot welded liquid metal, wherein the steel sheet is divided into roll-formed steel sheet and stamped steel sheet;

[0010] The chemical composition of the roll-formed steel sheet, by mass percentage, includes: C: 0.13%–0.16%, Mn: 1.60%–2.30%, Si: 0.80%–1.00%, Al: 0.035%–0.06%, P: 0.008%–0.015%, S≤0.003%, with the balance being Fe and unavoidable impurities;

[0011] The chemical composition of the stamped steel sheet, by mass percentage, includes: C: 0.18%–0.25%, Mn: 1.80%–2.50%, Si: 0.80%–1.80%, Al: 0.05%–0.80%, P: 0.008%–0.015%, S≤0.003%, with the balance being Fe and unavoidable impurities.

[0012] In the above technical solution, the chemical composition of the steel plate, by mass percentage, further includes: Ti: 0.005% to 0.025%, Nb: 0.005% to 0.025%, and Ti+Nb≤0.03%, B: 0.002% to 0.005%.

[0013] In the above technical solution, the chemical composition of the steel plate further includes at least one of Ni, Cr, and Mo, wherein, by mass percentage: Ni: 0.10% to 0.30%, Cr: 0.10% to 0.30%, Mo: 0.10% to 0.30%, and Mn+Ni+Cr+Mo≤2.60%.

[0014] The rationale for the design of the chemical composition and content of the steel plate of this invention is as follows:

[0015] Carbon (C) is the most economical strengthening element in steel, improving the hardenability of the steel plate and thus increasing its strength. In the Q&P steel of this invention, C is the most critical factor, affecting the behavior of supercooled austenite phase transformation. 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 the weldability.

[0016] Mn: Mn is a common and economical strengthening element in steel, enhancing solid solution strengthening and hardenability to improve 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 limits of this invention, considering the C / Mn segregation problem caused by excessive Mn content.

[0017] Si: Si is a common and economical strengthening element that ensures 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 levels may lead to surface quality issues such as "uncoated areas" on the galvanized surface.

[0018] Al: The addition of Al in conventional steel plates is limited, and it is generally used as a deoxidizer in the smelting process. In this invention, a higher content of Al is added to replace Si during the production of galvanized products, which helps to inhibit the precipitation of carbides; 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 galvanizing, and increased production difficulty.

[0019] Ti: In conventional steel plates, Ti is used for nitrogen fixation. In this invention, Ti is appropriately added as a strength supplement. Some planned compositions cannot meet the strength requirements. The precipitation of Ti can refine the original austenite grains and strengthen them, as well as supplement the strength through precipitation strengthening.

[0020] Nb: Nb is a microalloying strengthening element that refines grains and improves strength. In this invention, Nb is added in combination with Ti to fill the gap where the strength of some designed components is too low. However, the Nb content should not be too high, as too high a content will lead to excessively fine grains in hot rolling, excessively high strength in hot-rolled coils, and increased difficulty in cold rolling.

[0021] 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 steel's plasticity and toughness. Therefore, the lower the P content, the better. In this invention, the P content is controlled at P ≤ 0.0015%.

[0022] 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 S≤0.003%.

[0023] 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.

[0024] 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 as Mn to balance the problems of edge cracking in hot rolling and edge cracking in cold rolling.

[0025] Mn+Ni+Cr+Mo≤2.6%: As mentioned earlier, Ni, Cr, Mo, and other alloying elements are all substitutes for Mn, and their main role 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.

[0026] Boron (B): Boron segregates at austenite grain boundaries during welding, strengthening grain boundaries and improving resistance to embrittlement cracking in liquid metals. Therefore, it can be included as needed. However, if the boron content exceeds 0.005%, carbides and nitrides will form, saturating the above effects and reducing hot workability. If a soft decarburized layer exists on the surface of the base material, stress is reduced, making cracking less likely.

[0027] In the above technical solution, the yield strength of the roll-formed steel sheet is ≥780MPa, the tensile strength is ≥980MPa, the elongation is 12% to 15%, and the hole expansion rate is 40% to 70%.

[0028] The stamped steel sheet has a yield strength ≥600MPa, a tensile strength ≥980MPa, and an elongation of 20% to 23%.

[0029] Another aspect of the present invention provides a method for preparing the above-mentioned 980MPa grade resistant embrittlement of spot welded liquid metal galvanized steel sheet for automobiles, the method comprising the following steps:

[0030] (1) Continuous casting: Continuous casting is carried out according to the chemical composition of steel, and the casting temperature is 1580~1620℃;

[0031] (2) Hot rolling: The heating temperature is 1230~1280℃, the furnace time is 180~240min, the rough rolling temperature is 1150~1200℃, the finishing rolling temperature is 1070~1130℃, the final rolling temperature is ≥920℃, and the coiling temperature is 550~600℃.

[0032] (3) Cold rolling: The rolling reduction rate is 46.7-48.6%;

[0033] (4) Continuous annealing galvanizing:

[0034] The continuous annealing galvanizing process for the roll-formed steel sheet is as follows:

[0035] The cold-rolled steel sheet is heated to 900-950℃, isothermaled for 80-180s, slowly cooled to 750-820℃ at a cooling rate of 1.2-3.6℃ / s, then rapidly cooled to 250-400℃ at a rate of 15-25℃ / s, followed by aging treatment at a rate of 20℃ / s or higher, then galvanized in a zinc pot, and finally alloyed galvanized at a temperature of 480-520℃.

[0036] The continuous annealing galvanizing process for the stamped steel sheet is as follows:

[0037] The cold-rolled steel sheet is heated to 820–860℃ and held at a constant temperature for 60–120 seconds. It is then slowly cooled to 700–740℃ at a cooling rate of 1.2–3.6℃ / s, followed by rapid cooling to 250–350℃ at a rate of 18–25℃ / s. Subsequently, it is heated to 380–420℃ at a rate of 20℃ / s or higher for over-aging treatment, held at a constant temperature for 20–40 seconds, and then heated to 455–470℃ before being placed in a zinc bath. Finally, it is heated to 480–520℃ for alloy galvanizing.

[0038] In the above technical solution, further, in step (1), the thickness of the billet is 220-280 mm.

[0039] In the above technical solution, further, in step (2), the thickness of the intermediate billet is 50-80 mm.

[0040] The beneficial effects of this invention are as follows:

[0041] (1) The roll-formed and stamped steel plates obtained by the present invention have the advantage of resisting the embrittlement of spot welded liquid metal. The roll-formed steel plate has a yield strength ≥780MPa, a tensile strength ≥980MPa, an elongation of 12% to 15%, and a hole expansion rate of 40% to 70%. The stamped steel plate has a yield strength ≥600MPa, a tensile strength ≥980MPa, and an elongation of 20% to 23%, thus obtaining a product resistant to LME-QP980MPa.

[0042] (2) This invention has the advantages of a low-cost, high-performance dual-carbon strategy;

[0043] (3) This invention proposes a gradient partitioning process concept and applies it to industry. Detailed Implementation

[0044] 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.

[0045] Table 1 lists the chemical composition of the steel plates in the embodiments of the present invention.

[0046] Table 1 Chemical composition of the steel in the examples, wt%.

[0047]

[0048] The method for preparing the steel plate in the above embodiments includes the following steps:

[0049] (1) Continuous casting: Continuous casting is carried out according to the chemical composition of steel, and the casting temperature is 1580~1620℃ to obtain a billet with a thickness of 220~280mm;

[0050] (2) Hot rolling: The heating temperature is 1230-1280℃, the furnace time is 180-240min, the rough rolling temperature is 1150-1200℃, the intermediate billet thickness is 50-80mm, the finishing rolling temperature is 1070-1130℃, the final rolling temperature is ≥920℃, and the coiling temperature is 550-600℃. The purpose of controlling the heating temperature at 1230-1280℃ and the furnace time at 180-240min is to promote the full solid solution of the alloy and control the banded structure caused by segregation. The purpose of rolling in two stages during the finishing stage is to promote the recrystallization behavior of the original austenite grains and inhibit the coarsening of the non-recrystallized austenite grains. The purpose of controlling the coiling temperature at 550-600℃ is to prevent the formation of Si-rich oxides on the surface of the steel plate after adding Si content, which would lead to excessive internal oxide layer and grain boundary oxide layer.

[0051] (3) Cold rolling: The thickness of cold-rolled sheet is 1.4 / 1.6 / 1.8mm. 1.4mm sheet thickness corresponds to 2.8mm hot-rolled steel sheet, and 1.6 and 1.8mm sheet thicknesses correspond to 3.0-3.5mm hot-rolled steel sheet. The rolling reduction rate is 46.7-48.6%. Too low a rolling reduction rate cannot guarantee sufficient cold rolling deformation energy storage, resulting in insufficient ferrite recrystallization effect during continuous annealing. Too high a rolling reduction rate greatly increases the load on the cold rolling mill and cannot guarantee the achievement of the target thickness.

[0052] (4) Continuous annealing galvanizing:

[0053] The continuous annealing galvanizing process for roll-formed steel sheets is as follows:

[0054] The cold-rolled steel sheet is heated to 900-950℃, isothermaled for 80-180s, slowly cooled to 750-820℃ at a cooling rate of 1.2-3.6℃ / s, then rapidly cooled to 250-400℃ at a rate of 15-25℃ / s, followed by aging treatment at a rate of 20℃ / s or higher, then galvanized in a zinc pot, and finally alloyed galvanized at a temperature of 480-520℃.

[0055] During the galvanizing soaking stage of the roll-formed steel (heating to 820-860℃, isothermal for 60-120s), a fully austenitic structure is obtained. The steel is then slowly cooled to 750-820℃, controlling the ferrite content to below 5% to prevent excessive ferrite content from reducing strength, and simultaneously preventing excessively low ferrite content from resulting in excessively high strength. More importantly, it ensures the carbon concentration gradient of the supercooled austenite between slow cooling and rapid cooling, thus determining the transformation of bainite and martensite in subsequent processes. Subsequently, it is cooled at a relatively high rate to 250-400℃ to obtain 25%-30% martensite and the remaining untransformed supercooled austenite. Too low a martensite content leads to reduced steel plate strength, while too high a martensite content leads to a decrease in retained austenite content.

[0056] The continuous annealing galvanizing process for stamped steel sheets is as follows:

[0057] The cold-rolled steel sheet is heated to 820–860℃ and held at a constant temperature for 60–120 seconds. It is then slowly cooled to 700–740℃ at a cooling rate of 1.2–3.6℃ / s, followed by rapid cooling to 250–350℃ at a rate of 18–25℃ / s. Subsequently, it is heated to 380–420℃ at a rate of 20℃ / s or higher for over-aging treatment, held at a constant temperature for 20–40 seconds, and then heated to 455–470℃ before being placed in a zinc bath. Finally, it is heated to 480–520℃ for alloy galvanizing.

[0058] During the galvanizing soaking stage (heating to 820–860℃, isothermal for 60–120s), the stamping process obtains a critical zone ferrite structure of 35%–45%, balancing the strength of the steel plate and ensuring the carbon concentration in the austenite under austenitization conditions. A slow cooling temperature of 700–740℃ yields 5%–10% oriented epiphytic ferrite, preventing excessive ferrite content from reducing strength, while also preventing excessively low ferrite content from resulting in excessively high strength. More importantly, it ensures the carbon concentration gradient of the supercooled austenite between slow cooling and rapid cooling, thus determining the transformation of bainite and martensite in subsequent processes. Subsequently, a relatively high cooling rate is applied to 250–350℃ to obtain a martensite structure of 25%–30% and the remaining untransformed supercooled austenite structure. Too low a martensite content leads to reduced steel plate strength, while too high a martensite content leads to a decrease in the retained austenite content.

[0059] Table 2 lists the continuous casting and hot rolling process parameters of the steel plates in the examples, Table 3 lists the cold rolling and continuous annealing process parameters of the steel plates in the examples, and Table 4 lists the mechanical properties of the steel plates in the examples.

[0060] Table 2 Hot rolling process of steel in the examples

[0061]

[0062] Table 3. Cold rolling, annealing, and galvanizing processes of steel in the examples.

[0063]

[0064] Table 4 Mechanical properties of the steel in the examples

[0065]

[0066] 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 galvanized steel sheet for automobiles with 980MPa grade resistance to embrittlement of spot welded liquid metal, characterized in that, The steel plate is a roll-formed steel plate; The chemical composition of the steel plate, by mass percentage, includes: C: 0.13%~0.16%, Mn: 1.60%~2.30%, Si: 0.80%~1.00%, Al: 0.035~0.06%, P: 0.008%~0.015%, S≤0.003%, with the balance being Fe and unavoidable impurities; The chemical composition of the steel plate, by mass percentage, further includes: Ti: 0.005%~0.025%, Nb: 0.005%~0.025%, and Ti+Nb≤0.03%, B: 0.002%~0.005%; The chemical composition of the steel plate also includes at least one of Ni, Cr, and Mo, wherein, by mass percentage: Ni: 0.10%~0.30%, Cr: 0.10%~0.30%, Mo: 0.10%~0.30%, and Mn+Ni+Cr+Mo≤2.60%; The method for preparing the steel plate includes the following steps: (1) Continuous casting: Continuous casting is carried out according to the chemical composition of steel, and the casting temperature is 1580~1620℃; (2) Hot rolling: The heating temperature is 1230~1280℃, the furnace time is 180~240min, the rough rolling temperature is 1150~1200℃, the finishing rolling temperature is 1070~1130℃, the final rolling temperature is ≥920℃, and the coiling temperature is 550~600℃. (3) Cold rolling: The rolling reduction rate is 46.7%~48.6%; (4) Continuous annealing galvanizing: The continuous annealing galvanizing process of the steel plate is as follows: After cold rolling, the steel sheet is heated to 900~950℃, isothermaled for 80~180s, and then slowly cooled to 750~820℃ at a cooling rate of 1.2~3.6℃ / s. It is then rapidly cooled to 250~400℃ at a rate of 15~25℃ / s, followed by aging treatment at a rate of 20℃ / s or higher. It is then galvanized in a zinc bath and finally alloyed galvanized at a temperature of 480~520℃.

2. The galvanized steel sheet for automobiles with a 980MPa grade resistance to embrittlement of spot welded liquid metal according to claim 1, characterized in that, The steel plate has a yield strength ≥780MPa, a tensile strength ≥980MPa, an elongation of 12%~15%, and a hole expansion rate of 40%~70%.

3. A method for preparing a 980MPa grade galvanized steel sheet for automobiles with resistance to embrittlement of spot welded liquid metal as described in any one of claims 1-2, characterized in that, The method includes the following steps: (1) Continuous casting: Continuous casting is carried out according to the chemical composition of steel, and the casting temperature is 1580~1620℃; (2) Hot rolling: The heating temperature is 1230~1280℃, the furnace time is 180~240min, the rough rolling temperature is 1150~1200℃, the finishing rolling temperature is 1070~1130℃, the final rolling temperature is ≥920℃, and the coiling temperature is 550~600℃. (3) Cold rolling: The rolling reduction rate is 46.7%~48.6%; (4) Continuous annealing galvanizing: The continuous annealing galvanizing process of the steel plate is as follows: After cold rolling, the steel sheet is heated to 900~950℃, isothermaled for 80~180s, and then slowly cooled to 750~820℃ at a cooling rate of 1.2~3.6℃ / s. It is then rapidly cooled to 250~400℃ at a rate of 15~25℃ / s, followed by aging treatment at a rate of 20℃ / s or higher. It is then galvanized in a zinc bath and finally alloyed galvanized at a temperature of 480~520℃.

4. The preparation method according to claim 3, characterized in that, In step (1), the thickness of the billet is 220~280mm.

5. The preparation method according to claim 3, characterized in that, In step (2), the thickness of the intermediate billet is 50~80mm.

6. The preparation method according to claim 3, characterized in that, In step (3), the thickness of the cold-rolled plate is 1.4 / 1.6 / 1.8mm. The 1.4mm plate thickness corresponds to the 2.8mm hot-rolled steel plate, and the 1.6mm and 1.8mm plate thicknesses correspond to the 3.0~3.5mm hot-rolled steel plates.

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