Hot-rolled steel for automotive structural use having tensile strength of 700mpa and method for manufacturing the same

Abstract

The present application relates to the technical field of hot-rolled high-strength steel for automobiles, and discloses a hot-rolled automobile structural steel with a tensile strength of 700 MPa and a preparation method thereof.The hot-rolled automobile structural steel with a tensile strength of 700 MPa contains the following chemical elements in percentage by weight: 0.04%≤C≤0.07%, 1.30%≤Mn≤1.7%, 0.15%≤Als≤0.25%, 0.015%≤Nb≤0.045%, 0.07%≤Ti≤0.11%, 0.2%≤Cr≤0.4%, N≤0.005%, P≤0.02%, and S≤0.005%.The hot-rolled automobile structural steel with a tensile strength of 700 MPa has good surface quality which is easier to control in the production process, higher elongation after fracture, and better flanging and hole expanding performance while ensuring the yield strength and tensile strength, and can meet the requirements of high coating, high load bearing, and high complex forming parts of automobile chassis.

Description

Technical Field

[0001] This invention relates to the field of hot-rolled high-strength steel for automobiles, specifically to hot-rolled automotive structural steel with a tensile strength of 700 MPa and its preparation method. Background Technology

[0002] Driven by the trends of safety, energy conservation, environmental protection, and emission reduction, high-strength automotive structural steel has become the mainstream steel for automotive chassis. Hot-rolled dual-phase automotive structural steel, due to its excellent strength-ductility balance, holds a very important position among high-strength steels, accounting for up to 40% of their production and sales volume. With the continuous iteration and upgrading of automotive lightweighting, component structures are becoming increasingly complex, forming methods are becoming increasingly diversified, and customer personalized requirements are also increasing. In recent years, there has been a high market demand for hot-rolled automotive structural steel with high surface quality, high strength and toughness, and a tensile strength of 700MPa.

[0003] In the prior art, Chinese patent application CN 106609335 A (hereinafter referred to as Document 1) discloses a high-expansion hot-rolled steel plate with a tensile strength of 700MPa and its manufacturing method. The chemical composition of the hot-rolled steel plate is as follows: C: 0.05~0.09%, Si: 0.15~0.35%, Mn: 1.20~1.80%, P ≤ 0.015%, S ≤ 0.008%, Al: 0.015~0.050%, Ti: 0.08~0.13%, V: 0.035~0.065%, with the balance being Fe and other unavoidable impurities; Chinese patent application CN 105803334 Reference A (hereinafter referred to as Reference 2) discloses a hot-rolled multiphase steel with a tensile strength of 700 MPa and its production method. The chemical composition of the multiphase steel is as follows (mass fraction): C: 0.06~0.10%, Si: 0~0.3%, Mn: 1.0~1.4%, P ≤ 0.025%, S ≤ 0.008%, Als: 0.02~0.07%, Nb: 0.015~0.035%, with the balance being Fe and other unavoidable impurities. References 1-2 above can obtain hot-rolled high-strength steel with a tensile strength ≥700 MPa through three-stage and five-stage laminar cooling, respectively. However, the laminar cooling process is difficult to control, requiring long cooling channels and strong cooling capacity, resulting in high production difficulty. Furthermore, the presence of Si in the chemical composition makes it difficult to guarantee surface quality, easily leading to surface red rust defects.

[0004] For example, Chinese patent application CN 116288042 A discloses a hot-rolled automotive structural steel with a tensile strength greater than 700MPa and a thickness of 2-4mm, and its production method. The chemical composition of the high-strength steel is as follows: C: 0.06~0.08%, Si: 0.15~0.25%, Mn: 1.50~1.65%, P ≤ 0.020%, S ≤ 0.005%, Alt: 0.02~0.06%, Cr: 0.35~0.45%, Mo: 0.22~0.32%, V: 0.10~0.20%, Ca: 0.001~0.003%, with the balance being Fe and unavoidable impurities. Although high-strength steel with a yield strength of 687-720 MPa, a tensile strength of 780-795 MPa, and an elongation of 16-21% can be obtained through its chemical composition and production method, the high cost of the Mo alloy in the chemical composition will significantly increase the cost per ton of steel. In addition, the elongation value in this invention is relatively small, which cannot meet the current demand for some high-elongation forming automotive chassis parts. Summary of the Invention

[0005] The purpose of this invention is to overcome the problem that existing hot-rolled automotive structural steel with a tensile strength of 700MPa cannot simultaneously achieve low alloy cost, good mechanical properties, high surface quality, and stable production process control. This invention provides hot-rolled automotive structural steel with a tensile strength of 700MPa and its preparation method. Compared with traditional automotive structural steel, the hot-rolled automotive structural steel with a tensile strength of 700MPa obtained by this method has better surface quality control during production. While ensuring a yield strength of 650~750MPa and a tensile strength of 700~800MPa, it also has higher elongation after fracture (20~26%) and better flanging and hole expansion performance (65% and above), which can meet the needs of automotive chassis for high-painting, high-load-bearing, and highly complex formed parts.

[0006] To achieve the above objectives, the present invention provides a hot-rolled automotive structural steel with a tensile strength of 700 MPa, wherein the hot-rolled automotive structural steel with a tensile strength of 700 MPa contains the following chemical elements in weight percentage: 0.04%≤C≤0.07%, 1.3%≤Mn≤1.7%, 0.15%≤Als≤0.25%, 0.015%≤Nb≤0.045%, 0.07%≤Ti≤0.11%, 0.2%≤Cr≤0.4%, N≤0.005%, P≤0.02%, and S≤0.005%.

[0007] Furthermore, the hot-rolled automotive structural steel of the present invention with a tensile strength of 700MPa may contain V and / or Mo in addition to the above-mentioned chemical element composition, or it may not contain them.

[0008] When the hot-rolled automotive structural steel with a tensile strength of 700MPa contains V, the weight percentage of V is controlled to be ≤0.1%. When the hot-rolled automotive structural steel with a tensile strength of 700MPa contains Mo, the weight percentage of Mo is controlled to be ≤0.2%.

[0009] When the hot-rolled automotive structural steel with a tensile strength of 700MPa contains both V and Mo, in addition to the above requirements for the content of these two elements, the content of Nb+V+Ti must be controlled to be ≤0.22% and (C+Mn / 6+Cr / 5+Mo / 4+V / 14) ≤0.42%.

[0010] The main chemical elements mentioned above play the following roles in this invention: Carbon (C) is the most basic strengthening element in steel, effectively balancing its strength and ductility. Too low a C content reduces the hardenability of austenite, leading to lower strength. Since Ti, Nb, and C form carbide precipitation, increased C content results in coarser carbide particles, which are detrimental to the plasticity of the steel plate and reduce its weldability. Therefore, a C weight fraction of 0.04% to 0.07% is considered appropriate.

[0011] Mn plays a role in solid solution strengthening and stabilizing austenite in steel. Too low a Mn content will not meet the strength requirements of the steel plate, while too high a Mn content will easily lead to segregation at the center of the billet and strip, which is detrimental to the toughness of the steel plate and makes it prone to delamination and cracking during processing. Therefore, a Mn weight fraction of 1.3% to 1.7% is more suitable.

[0012] Al: Al plays a role in deoxidation, nitrogen fixation, and solid solution strengthening in steel, and is a key element in this invention. The addition of Nb and Ti elements to the composition of this invention easily forms large inclusions with O and N, which will affect the material's plasticity and crack resistance. The addition of Al can deoxidize and fix nitrogen, refine the grains, and improve the purity of the steel. Al replaces the commonly used element Si, which is more conducive to obtaining steel plates with high surface quality. However, increased Al content is detrimental to the weldability of the steel plate. Therefore, the weight fraction of Als (Als is acid-soluble aluminum, referring to the portion of steel that exists in the form of elemental aluminum or aluminum nitride and is soluble in acid) is best controlled at 0.15~0.25%.

[0013] Nitrogen (Nb) plays a significant role in grain refinement and precipitation strengthening. It forms Nb(C,N) with C and N, which contributes to precipitation strengthening and inhibits austenite growth, thus refining the grain. However, Nb prices have been rising steadily in recent years, and excessively high Nb content is detrimental to cost control per ton of steel. Conversely, excessively low Nb content has little effect on grain refinement and precipitation strengthening of the steel plate. Therefore, a Nb weight fraction of 0.015–0.045% is considered appropriate.

[0014] Ti (Ti) plays a significant role in grain refinement and precipitation strengthening. It forms TiC and TiN particles with C and N, contributing to precipitation strengthening, and also inhibits austenite growth, thus refining the grain. However, excessive Ti content can lead to the formation of large precipitates at high temperatures during hot rolling, deteriorating the steel's toughness. Conversely, insufficient Ti content has little effect on grain refinement and precipitation strengthening. Therefore, a Ti weight fraction of 0.07–0.11% is considered optimal.

[0015] Cr: In steel, it plays a role in solid solution strengthening and can improve the hardenability of austenite. Excessive Cr content can easily lead to the formation of carbides at grain boundaries, reducing the toughness of the steel plate; conversely, insufficient Cr content will not improve the strength of the steel plate. Therefore, a Cr weight fraction of 0.2% to 0.4% is considered appropriate.

[0016] V (V): It plays a significant role in grain refinement and precipitation strengthening. It forms VC and VN particles with C and N, thus promoting precipitation strengthening, and also inhibits austenite growth, thereby contributing to grain refinement. Excessive V content increases the manufacturing cost per ton of steel, while insufficient V content has little effect on grain refinement and precipitation strengthening of the steel plate. Therefore, when V is present, its weight fraction should not exceed 0.1%.

[0017] Mo (Mo) can effectively refine grain size and strengthen the material. It forms MoC and MoN particles with C and N, inhibiting austenite growth and thus strengthening the grain. The addition of Mo can also delay the ferrite-pearlite phase transformation, which is beneficial for obtaining bainitic microstructure. However, its high price is detrimental to material cost control. Therefore, when Mo is present, its weight fraction should not exceed 0.2%.

[0018] Nitrogen (N): An impurity element in steel, it forms coarse compounds with Al and Ti, which are detrimental to the toughness of the steel plate. Therefore, the weight fraction of N should not exceed 0.005%.

[0019] S and P are impurity elements in steel. S easily combines with Mn to form MnS inclusions, which can create voids during part forming. P tends to segregate at grain boundaries, reducing the bonding force between grain boundaries. Both are detrimental to the ductility of steel sheets. Therefore, the weight fractions of P and S should not exceed 0.02% and 0.005%, respectively.

[0020] The microstructure of the hot-rolled automotive structural steel with a tensile strength of 700MPa in this invention is mainly composed of ferrite + bainite or ferrite + Mao islands.

[0021] The hot-rolled automotive structural steel of the 700MPa tensile strength grade described in this invention has a yield strength of 650~750MPa, a tensile strength of 700~800MPa, and an elongation after fracture A. 80 The value is 20-26%, and the porosity is ≥65%.

[0022] In a preferred embodiment, the thickness of the hot-rolled automotive structural steel with a tensile strength of 700MPa described in this invention is 1.8~6mm, and the width is 900~1500mm.

[0023] A second aspect of the present invention provides a method for preparing the above-mentioned hot-rolled automotive structural steel with a tensile strength of 700 MPa, comprising: Molten steel is continuously cast to obtain a billet. The billet is then heated, descaled, and rough rolled to obtain an intermediate billet. The intermediate billet is then fine rolled, laminar cooled, and coiled to obtain hot-rolled automotive structural steel with a tensile strength of 700 MPa. The winding temperature is 450~560℃.

[0024] In a specific embodiment of the present invention, the hot-rolled automotive structural steel with a tensile strength of 700MPa is smelted according to its chemical composition to obtain molten steel, which is then continuously cast.

[0025] In actual production, before continuous casting, chemical composition testing and analysis will be conducted on samples taken from the tundish. Based on the results, billets that meet the composition requirements will proceed to subsequent production processes.

[0026] In a specific implementation, during continuous casting, dynamic light reduction and electromagnetic stirring technology are employed to improve the uniformity of the billet microstructure and avoid or reduce segregation at the center of the billet. In one specific implementation, during electromagnetic stirring, the upper electromagnetic roller is controlled to have an alternating current of 300A and a frequency of 3Hz, while the lower electromagnetic roller is controlled to have an alternating current of 400A and a frequency of 5Hz (the upper and lower electromagnetic rollers refer to the upper and lower stirrers, which are two independent electromagnetic stirrers installed at different heights and in different process sections of the casting flow).

[0027] Preferably, the liquidus temperature of the continuous casting is 1519~1523℃, and the temperature of the molten steel in the tundish of the continuous casting is 1536~1551℃.

[0028] Furthermore, the thickness of the billet is 210~250mm. After the billet is produced, it is placed in a heat preservation pit for slow cooling. Once the hot rolling process is ready for production, it is then transported into a heating furnace for heating.

[0029] The billet is heated in the heating furnace. The billet can be hot-charged or cold-charged. When using the hot-charged process, the billet temperature should be controlled to be ≥450℃. When using the cold-charged process, the billet quality should be checked before entering the heating furnace to ensure that there are no cracks.

[0030] In this invention, the cast billet is heated in a heating furnace. The heating process includes a preheating section, a first heating section, a second heating section, and a soaking section. The temperature of the first heating section is 1160~1200℃, and the heating time is 25~35min. The temperature of the second heating section is 1230~1270℃, and the heating time is 30~40min. The temperature of the soaking section is 1230~1270℃, and the soaking time is 20~30min.

[0031] The heating time described in this invention refers to the total time in the (heating) furnace, and the heating time is preferably 130~160 min.

[0032] Preferably, after heating, the billet exiting the furnace is at a temperature of 1230~1270℃ (the billet exiting the furnace is the same as the temperature of the soaking zone during heating).

[0033] In a preferred embodiment of the present invention, the temperature of rough rolling is 1100~1200℃.

[0034] In this invention, the temperature of the intermediate billet is 1050~1090℃, and the thickness of the intermediate billet is 35~42mm.

[0035] Preferably, the finishing rolling temperature is 860~920℃.

[0036] In one specific embodiment of the present invention, in order to avoid slippage, rough rolling is generally carried out in 8 passes (3+5). The reduction rate of each pass increases sequentially, and the reduction rate of each pass is ≤30%. The total reduction rate of rough rolling is controlled at 80~87%. The specific reduction rate of each pass only needs to be ≤30%, and it is necessary to ensure that the reduction rate increases sequentially while the total reduction rate is controlled at 80~87%.

[0037] In a preferred embodiment of the present invention, laminar flow cooling employs front-end centralized rapid cooling, and the cooling rate of laminar flow cooling is controlled to be ≥40℃ / s.

[0038] In actual production, the laminar cooling time is related to the number of manifolds and the rolling speed, which are automatically calculated by the model; the front-end concentrated rapid cooling refers to water cooling, and the rear-end cooling is air cooling after water cooling, with the final target temperature being the coiling temperature.

[0039] The microstructure of the 700MPa grade hot-rolled automotive structural steel of this invention is affected by the hot rolling process, mainly related to the coiling temperature. When the coiling temperature is 450~530℃, the microstructure of the 700MPa grade hot-rolled automotive structural steel is mainly ferrite + bainite; when the coiling temperature is 530~560℃, the microstructure of the 700MPa grade hot-rolled automotive structural steel is mainly ferrite + Mao islands.

[0040] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) The present invention uses C, Mn and Nb, Ti and Cr trace alloying elements as strengthening elements, and the cost is controllable. The key innovation of the present invention is that it uses 0.15~0.25% Al element as the main deoxidation and nitrogen fixation element. It replaces Si, a commonly used deoxidation element in high-strength steel, to obtain steel plates with high surface quality. It can also be combined with the matching design of hot rolling process to obtain automotive structural steel with good strength and toughness matching performance of longitudinal yield strength of 650~750MPa, tensile strength of 700~800MPa, elongation of 20~26% and hole expansion rate of not less than 65%. (2) The layer cooling process of the present invention adopts conventional front-end centralized rapid cooling, which is easier to produce and control stably compared with segmented or multi-segment cooling. The winding temperature is 450~560℃. Compared with higher temperature winding, the yield strength ratio is lower (0.86~0.90), which can reduce the springback of parts during the stamping process. Attached Figure Description

[0041] Figure 1 These are microstructure images of hot-rolled automotive structural steel with a tensile strength of 700 MPa, as described in Example 1. Figure 2 These are microstructure images of hot-rolled automotive structural steel with a tensile strength of 700 MPa, as described in Example 4. Figure 3 This is a surface quality diagram of hot-rolled automotive structural steel with a tensile strength of 700 MPa, as shown in Example 1. Figure 4 This is a surface quality diagram of hot-rolled automotive structural steel with a tensile strength of 700 MPa, as shown in Comparative Example 1. Figure 5 This is a surface quality diagram of hot-rolled automotive structural steel with a tensile strength of 700 MPa, as shown in Comparative Example 2. Detailed Implementation

[0042] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0043] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein.

[0044] Furthermore, the technical solutions provided in the various embodiments of the present invention can be combined with each other, but only if they are feasible to those skilled in the art. If the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such combination of technical solutions does not exist and is not within the scope of protection claimed by the present invention.

[0045] The present invention will be described in detail below through embodiments. Unless otherwise specified, the experimental methods in the following embodiments are conventional methods in the art.

[0046] Example 1 The steel was smelted according to the chemical composition of hot-rolled automotive structural steel with a tensile strength of 700MPa. The molten steel was then continuously cast using dynamic light pressure and electromagnetic stirring. During electromagnetic stirring, the upper surface of the electromagnetic roller was controlled with an alternating current of 300A and a frequency of 3Hz; the lower surface was controlled with an alternating current of 400A and a frequency of 5Hz. The liquidus temperature was 1520℃, and the molten steel temperature in the tundish was 1540℃, resulting in a 230mm thick billet. After production, the billet was placed in a heat-insulating pit for slow cooling, and then heated in a furnace using a hot-charging process. The furnace entry temperature was controlled at 524℃, and the heating time was 152 minutes. The heating process included a preheating section, a first heating section, a second heating section, and a soaking section. The temperature of the first heating section was 1185℃, and the heating time was 30 minutes. The temperature of the second heating section was 1248℃, and the heating time was 38 minutes. The temperature of the soaking section was 1245℃. The soaking time was 27 minutes. After heating, the billet exited the furnace at a temperature of 1245℃. Then, the heated billet underwent descaling and rough rolling (the rough rolling was carried out in 8 passes (3+5), with the reduction rate of each pass increasing sequentially, and the reduction rate of each pass ≤30%, and the total reduction rate of the rough rolling was 84.22%). The rough rolling temperature was 1100~1190℃, resulting in an intermediate billet with a temperature of 1072℃ and a thickness of 36 mm. The intermediate billet is 3mm thick and then finished rolled at a final rolling temperature of 872℃. It is then subjected to laminar flow cooling (using concentrated rapid cooling at the front end, with a controlled cooling rate of 62℃ / s) and coiling at a temperature of 480℃ to obtain hot-rolled automotive structural steel with a tensile strength of 700MPa. The main chemical element composition of this 700MPa-grade hot-rolled automotive structural steel is shown in Table 1, with the balance being Fe and unavoidable impurities. Examples 2-7 and Comparative Examples 1-4 were all carried out according to the method of Example 1. The main chemical element composition of the hot-rolled automotive structural steel with a tensile strength of 700MPa is shown in Table 1, with the balance being Fe and unavoidable impurities. The specific process parameters of each example and comparative example are shown in Table 2. Except for Table 2, the operations and process parameters in Examples 2-7 and Comparative Examples 1-4 are the same as those in Example 1.

[0047] Table 1 Table 2 Test Example 1 The microstructure, mechanical properties, porosity, and surface quality of the 700 MPa grade hot-rolled automotive structural steels from Examples 1-7 and Comparative Examples 1-4 were tested respectively. The mechanical properties were determined according to the national standard GB / T228.1-2021 "Metallic materials - Tensile testing - Part 1: Test at room temperature", with the specimen orientation being longitudinal. The porosity was determined according to the national standard GB / T 24524-2021 "Metallic materials - Test method for porosity expansion in thin plates and strips". The microstructure was analyzed using a thermal field emission scanning electron microscope. The surface quality of the steel plates was tested using a production line surface quality control instrument.

[0048] The above test results, as well as the thickness and width of the 700MPa grade hot-rolled automotive structural steel of Examples 1-7 and Comparative Examples 1-4, are shown in Table 3. The microstructure image of the 700MPa grade hot-rolled automotive structural steel of Example 1 is shown below. Figure 1 As shown in the image, the microstructure of the 700MPa grade hot-rolled automotive structural steel of Example 4 is as follows: Figure 2 As shown, the surface quality of the 700MPa grade hot-rolled automotive structural steel of Example 1 is as follows: Figure 3 As shown, the surface quality of the hot-rolled automotive structural steel with a tensile strength of 700 MPa in Comparative Example 1 is as follows: Figure 4 As shown, the surface quality of the hot-rolled automotive structural steel with a tensile strength of 700 MPa in Comparative Example 2 is as follows: Figure 5 As shown.

[0049] Table 3 Note: In Table 3, F represents ferrite, GB represents granular bainite, and M / A represents Mao islands; According to Table 3 and Figure 1-5It can be seen that the mechanical properties and expansion rates of Examples 1-7 all meet the performance requirements of yield strength 650~750MPa, tensile strength 700~800MPa, elongation after fracture 20~26%, and expansion rate ≥65%, and the performance is good. The yield strength of Comparative Examples 1 and 3 is lower than 650MPa, and the tensile strength of Comparative Example 4 is greater than 800MPa, the elongation after fracture is lower than 20%, and the expansion rate is lower than 65%, and the performance is poor. The surface quality of the steel plates of Examples 1-7 is high, while the surface of the steel plates of Comparative Examples 1-2 shows different degrees of red rust defects.

[0050] As can be seen from the comparison between Comparative Examples 1-2 and Example 1, the amount of Si added has different effects on the surface quality and performance of the steel plate. If the amount added is too small, the strength is difficult to meet the requirements. If the amount added is too large, the surface red rust defects are serious. The present invention can avoid such problems by replacing Si with Al. As can be seen from Comparative Examples 3-4, the coiling temperature exceeds the range required by the present invention, which will weaken the high strength and toughness of the steel plate, resulting in the material not meeting the performance requirements of the present invention.

[0051] It should be understood that any parts not described in detail in this specification belong to the prior art.

[0052] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A hot-rolled automotive structural steel with a tensile strength of 700 MPa, characterized in that, This 700MPa grade hot-rolled automotive structural steel contains the following chemical elements by weight percentage: 0.04%≤C≤0.07%, 1.3%≤Mn≤1.7%, 0.15%≤Als≤0.25%, 0.015%≤Nb≤0.045%, 0.07%≤Ti≤0.11%, 0.2%≤Cr≤0.4%, N≤0.005%, P≤0.02%, and S≤0.005%.

2. The hot-rolled automotive structural steel with a tensile strength of 700 MPa according to claim 1, characterized in that, The hot-rolled automotive structural steel of the 700MPa tensile strength grade has a yield strength of 650~750MPa, a tensile strength of 700~800MPa, and an elongation after fracture A. 80 The value is 20-26%, and the porosity is ≥65%.

3. The method for preparing hot-rolled automotive structural steel with a tensile strength of 700 MPa as described in claim 1 or 2, characterized in that, include: Molten steel is continuously cast to obtain a billet. The billet is then heated, descaled, and rough rolled to obtain an intermediate billet. The intermediate billet is then fine rolled, laminar cooled, and coiled to obtain hot-rolled automotive structural steel with a tensile strength of 700 MPa. The winding temperature is 450~560℃.

4. The preparation method according to claim 3, characterized in that, The liquidus temperature in continuous casting is 1519~1523℃, and the temperature of molten steel in the tundish during continuous casting is 1536~1551℃.

5. The preparation method according to claim 3, characterized in that, The thickness of the cast billet is 210~250mm.

6. The preparation method according to claim 3, characterized in that, The heating time is 130~160 minutes.

7. The preparation method according to claim 3, characterized in that, The billet exiting the furnace is at a temperature of 1230~1270℃.

8. The preparation method according to claim 3, characterized in that, The intermediate billet has a temperature of 1050~1090℃ and a thickness of 35~42mm.

9. The preparation method according to claim 3, characterized in that, The finishing rolling temperature is 860~920℃.

10. The preparation method according to claim 3, characterized in that, The cooling rate of the laminar flow cooling is ≥40℃ / s.

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