CN111471934B - Carbide bainite-free steel for self-reinforcing gear and preparation method thereof - Google Patents

Abstract

The invention discloses a carbide bainite-free steel for a self-reinforcing gear and a preparation method thereof, wherein the steel comprises the following chemical components in percentage by weight: c: 0.10 to 0.15 wt%, Si: 0.85-0.95 wt%, Mn: 1.35-1.55 wt%, Al: 3.1-3.9 wt%, P is less than or equal to 0.01 wt%, S is less than or equal to 0.01 wt%, W: 0.1 to 0.3 wt%, V: 0.05 to 0.07 wt%, and the balance of Fe and inevitable impurities. The invention introduces carbide-free bainitic steel, adopts a brand-new high-silicon and aluminum-containing component system and a production process, comprehensively reforms the component system and the metallographic structure of the gear steel, provides a brand-new solution for the development of the new generation of gear steel, and realizes the self-strengthening, quality-adjustment-free treatment and almost deformation-free pioneering results of the gear steel.

Description

Carbide bainite-free steel for self-reinforcing gear and preparation method thereof

Technical Field

The invention relates to gear steel and a production method thereof, belongs to steel (long material) for automobile parts and a production method thereof, and particularly relates to carbide bainite-free self-strengthening gear steel and a preparation method thereof.

Background

The production and sales of new energy vehicles in China are the first in the world for 5 continuous years, and the sales of new energy vehicles in 2018 and 2019 reach 125.6 thousands and 120.6 thousands respectively, which account for more than 50% of the global sales of new energy vehicles. Based on the industrial policy of China, China mainly uses a pure electric vehicle (BEV), and compared with the traditional automobile, the electric vehicle cancels systems such as an engine, a clutch, a transmission, exhaust and the like, adopts a brand-new electric driving mode, and has a new service environment of parts: high speed and large torque, the data shows that the rotating speed and the torque of the motor exceed 3 times of those of the traditional automobile engine, and the service environment of parts is severer.

The gear is a precise mechanical part, generally needs to be forged and machined into teeth and then carburized and quenched, and the manufacturing and assembling precision of the gear has great influence on vibration, noise and the service life of gear teeth. When the gear works, the gear is affected by various stresses such as impact force, contact stress, pulsating bending stress, friction force and the like of variable load for a long time, and is also affected by various factors such as machining precision, assembly precision, grinding of external hard particles and the like, and is a part which is easy to damage, so that the gear steel for manufacturing the gear is required to have higher obdurability, fatigue strength and wear resistance.

At present, gear steel is mainly medium-low carbon alloy steel, in order to improve the strength, hardness and wear resistance of teeth, formed gear parts need to be subjected to quenching and tempering treatment (high-temperature carburization and quenching), the process flow is long, the energy consumption is high (data shows that the gear quenching and tempering process cost accounts for more than 30% of the part processing cost), heat treatment process parameters (heating temperature, time, carburizing process and other factors) directly influence the performance of the gears, and the controlled deformation (the martensite is formed by quenching and the volume expansion causes macroscopic deformation) is difficult to control in the heat treatment, so that the quality of the gears is further restricted. Therefore, there is an increasing demand for more excellent, less expensive, and environmentally friendly gear steels.

In recent years, new bright spots appear in the research and development of high-performance novel steel types, carbide-free bainite steel (also called super bainite steel, nano bainite steel and the like) discovered by Bhadeshia et al in 2004 has a structure formed by alternately forming elongated bainite ferrite and carbon-rich film-shaped residual austenite, wherein the ferrite in the bainite structure is in a semi-coherent relationship with a parent phase, and the fine grain structure brought by low-temperature transformation, the fine structure in the ferrite and high-density dislocation jointly determine the high strength of the steel; the residual austenite belongs to a face-centered cubic structure, has a plurality of sliding systems, can relieve stress concentration, belongs to a soft phase, and obviously improves the toughness of the carbide-free bainite steel.

The prior art searches and analyzes: the gear steel is mainly fixed in GB/T3077, GB/T5216 or AISI, EN and other standards or subjected to component fine adjustment, and the hot-rolled structure of the gear steel is as follows: ferrite + pearlite; tissue after tempering: the tempered martensite contains usually 0.15 to 0.50% of C and contains a certain amount of Cr, Ni, Mo, V, W, etc. The Chinese patent publication No. CN106967931 discloses 20Cr2Ni4 gear steel and a production process thereof, wherein the mass percentage of Mo is adjusted to 0.15-0.30% on the basis of traditional components so as to meet the requirements of users on hardenability, grain size, heat strength and the like. The Chinese patent with publication number CN100569983 discloses a preparation method of Cr-Mn-Ti gear steel, and in consideration of the angle of ensuring the strength and the fatigue limit of the gear steel, the patent regulates the Ti content in the traditional 20 CrMnTiH% to be down-regulated to 0.01-0.038%. Chinese patent publication No. CN109972024 discloses a steel for a gear steel bar, a preparation method thereof and a preparation method of the steel bar, and the steel bar comprises the following chemical components: 0.17 to 0.24 percent of C, 0.85 to 1.07 percent of Mn, 0.19 to 0.30 percent of Si, 0.98 to 1.10 percent of Cr, 0.05 to 0.10 percent of Mo, 0.15 to 0.25 percent of Ni, 0.01 to 0.05 percent of Al, less than or equal to 0.02 percent of P and less than or equal to 0.02 percent of S. Publication number CN108531804 relates to an aluminum killed gear steel and a sulfide morphology control method thereof, which comprises the following chemical components: 0.15 to 0.25 percent of C, 0.40 to 1.60 percent of Mn, 0.05 to 0.40 percent of Si, 0.8 to 2.0 percent of Cr, 0 to 0.60 percent of Mo, 0 to 2.00 percent of Ni, 0.015 to 0.06 percent of Al, less than or equal to 0.015 percent of P, 0.015 to 0.040 percent of S, 0.005 to 0.040 percent of Te,0.008 to 0.015 percent of [ N ], lessthan or equal to 0.0015 percent of T.0 and less than or equal to 0.25 percent of Cu. At present, all the patent documents of the gear steel aim at the improvement of the existing products or processes, and the problems of obdurability and deformability of the gear steel are not fundamentally solved through quenching and tempering in the production.

Disclosure of Invention

The invention aims to overcome the defects of high energy consumption of quenching and tempering procedures, difficult control of quenching heat treatment deformation and the like of the gear steel in the prior art; the invention introduces non-carbide bainite steel, adopts a brand new high-silicon and aluminum-containing component system and a production process, comprehensively renovates the component system and the metallographic structure of the gear steel, provides the self-strengthening gear steel without carbide bainite and a preparation method thereof, and realizes the pioneering results of self-strengthening, hardening and tempering-free and almost no deformation of the gear steel. The gear made of the self-reinforcing gear steel has silence performance, service life and the like exceeding 25Cr2Ni4WA, and the energy consumption and cost of users are reduced. Meanwhile, the steel for the self-strengthening gear is a novel metallographic structure-carbide-free bainite, wherein under the action of stress, the residual austenite serving as a soft phase generates a TRIP effect to form martensite and reduce the generation of cracks (the microstructure can realize self-repairing of microcracks through phase transformation), so that the strength of a stressed area is increased, the crack expansion is restrained, the self-strengthening is realized, and the service life of the gear is obviously prolonged. Meanwhile, as the problem of quenching deformation does not exist (no quenching and tempering process), the overall dimension of the gear can be accurately controlled, the silence of the gear is improved, the stress condition is improved, and a series of advantages are brought.

In order to achieve the purpose, the invention designs carbide bainite-free steel for a self-strengthening gear and a preparation method thereof, wherein the steel for the gear comprises the following chemical components in percentage by weight: c: 0.10 to 0.15 wt%, Si: 0.85-0.95 wt%, Mn: 1.35-1.55 wt%, Al: 3.1-3.9 wt%, P is less than or equal to 0.01 wt%, S is less than or equal to 0.01 wt%, W: 0.1 to 0.3 wt%, V: 0.05 to 0.07 wt%, and the balance of Fe and inevitable impurities.

Further, the steel for the gear comprises the following chemical components in percentage by weight: c: 0.12 to 0.15 wt%, Si: 0.90-0.95 wt%, Mn: 1.45-1.55 wt%, Al: 3.5-3.9 wt%, P is less than or equal to 0.01 wt%, S is less than or equal to 0.01 wt%, W: 0.2 to 0.3 wt%, V: 0.06 to 0.07 wt%, and the balance Fe and inevitable impurities.

Still further, the steel for gears comprises the following chemical components in percentage by weight: c: 0.13 wt%, Si: 0.93 wt%, Mn: 1.50 wt%, Al: 3.7 wt%, P: 0.005 wt%, S: 0.005 wt%, W: 0.25 wt%, V: 0.065 wt%, and the balance of Fe and inevitable impurities.

Still further, the rolling performance of the steel for the gear reaches: the tensile strength is 1500 MPa-1650 MPa, the yield strength is 1250 MPa-1450 MPa, the elongation after fracture is 15% -30%, the reduction of area is 50% -65%, and the impact energy is 80J-100J.

The invention also provides a preparation method of the steel for the carbide bainite-free self-strengthening gear, the method is to smelt according to the chemical components according to the traditional smelting method to obtain the steel for the carbide bainite-free self-strengthening gear with the microstructure, wherein,

1) in the casting blank process, after being heated by a heating furnace, controlled rolling and controlled cooling are carried out: the temperature of the finish rolling stage is controlled between 850 ℃ and 950 ℃, and 3-6S is adopted^-1The final rolling temperature is less than or equal to 850 ℃ when the final rolling of 10% of deformation is carried out;

2) and (4) placing the rolled finished product in a slow cooling pit, wherein slow cooling is more than or equal to 24 hours.

The relevant working mechanism of the invention is as follows:

c: carbon is an important constituent element in steel, and has the most significant influence on the strength and plasticity of the gear steel. The method is different from the prior art, adopts an ultra-low carbon component system innovatively, and changes the current situation that the traditional gear steel mainly depends on the carbon content to improve the strength. The carbon content of the invention is 0.10-0.15 wt%.

Si: silicon is a strengthening element in steel and can inhibit the formation of carbides, promote the formation of carbide-free bainite, and generate high temperature on the meshing surface of a gear in operation, so that softening resistance is needed, and the content of Si is also needed to be increased. Therefore, comprehensive analysis shows that the Si content of the invention is controlled to be 0.85-0.95 wt%.

Mn: proper amount of manganese can improve the strength and hardenability, and lath bainite with small size can be obtained; in addition, manganese and sulfur are combined to generate MnS, so that the harm of sulfur is reduced, but the overhigh manganese can increase the overheating sensitivity of steel, so that crystal grains are easy to grow during heat treatment. The Mn content of the invention is controlled to be 1.35-1.55 wt%.

Al: the aluminum plays a main role in inhibiting the formation of carbides and promoting the formation of carbide-free bainite, is a key element in a component system, can obviously shorten the bainite phase transition time, and shortens the conventional phase transition process which needs several hours or even several days to meet the requirements of industrial production, but the overhigh aluminum pair causes difficulty in smelting and continuous casting. The invention Al: 3.1 to 3.9 wt%.

P, S: phosphorus and sulfur are harmful elements in the steel, phosphorus is easy to produce cold brittleness, sulfur is easy to produce hot brittleness, and further the processing conditions of steel wire drawing and heat treatment are deteriorated, so the content of the phosphorus and the sulfur needs to be reduced as much as possible. The invention has P less than or equal to 0.01 wt% and S less than or equal to 0.01 wt%.

W: tungsten has a high melting point of about 3410 c, and is one of the strong carbide formers. Tungsten and carbon often form its own special carbides. Tungsten in steel is partially dropped into iron to form solid solution except carbide formed by combination with carbon, so that the tempering stability, red hardness and heat strength of the steel are mainly improved, the crystal grain growth of the steel can be prevented, the crystal grain can be refined, and the tungsten belongs to precious rare metals, so that the tungsten alloy is comprehensively considered to be 0.10-0.30 wt%.

V: vanadium has a promoting effect on carbide-free bainite transformation, and can significantly refine grains in a hot rolling dynamic recrystallization process, but excessive vanadium can cause coarse VCN grains, lose the effect of refining grains and be harmful to the performance of steel. The invention V: 0.05 to 0.07 wt%.

Rolling process (at 850-950 deg.C for 3-6S^-1The final rolling temperature is less than or equal to 850 ℃ when the final rolling of 10% of deformation is carried out; after rolling, placing the finished product in a slow cooling pit, wherein slow cooling is more than or equal to 24 h): according to low temperature and small deformation, the transformation amount of the carbide-free bainite can be accelerated, the transformation time of the carbide-free bainite can be obviously shortened, and the carbide-free bainite structure with the lath width of about 200nm can be obtained to improve the toughness.

The invention has the beneficial effects that:

the invention creatively introduces the carbide-free bainitic steel, adopts a brand-new high-silicon and aluminum-containing component system and a production process, comprehensively renovates the component system and the metallographic structure of the gear steel, provides a brand-new solution for the development of the new generation of gear steel, and realizes the creative results of self-strengthening, quality-adjustment-free treatment and almost no deformation of the gear steel. The invention develops the self-strengthening gear steel and the preparation method thereof by taking the performance of the current gear steel 25Cr2Ni4WA for large load as the target, the silence, the service life and the like of the gear prepared by the self-strengthening gear steel exceed 25Cr2Ni4WA, and the energy consumption and the cost of the use of a user are reduced. In addition, a brand new novel metallographic structure, namely carbide-free bainite, takes place the TRIP effect as a residual austenite of a soft phase under the action of stress to form martensite and reduce the generation of cracks (the microstructure can realize the self-repair of microcracks through phase transformation), so that the strength of a stressed area is increased, the crack expansion is restrained, the self-strengthening is realized, and the service life of the gear is obviously prolonged. Meanwhile, as the problem of quenching deformation does not exist (no quenching and tempering process), the overall dimension of the gear can be accurately controlled, the silence of the gear is improved, the stress condition is improved, and a series of advantages are brought.

Drawings

Fig. 1 is a carbide-free bainite microstructure.

Detailed Description

The present invention is described in further detail below with reference to specific examples so as to be understood by those skilled in the art.

Example 1

The carbide bainite-free steel for the self-reinforcing gear 1 comprises the following chemical components in percentage by weight: c: 0.13 wt%, Si: 0.93 wt%, Mn: 1.50 wt%, Al: 3.7 wt%, P: 0.005 wt%, S: 0.005 wt%, W: 0.25 wt%, V: 0.065 wt%, and the balance of Fe and inevitable impurities.

The preparation method of the steel for the carbide bainite-free self-reinforcing gear 1 comprises the following steps of smelting according to the chemical components according to the traditional smelting method to obtain the steel for the carbide bainite-free self-reinforcing gear with the microstructure, wherein,

1) in the casting blank process, after being heated by a heating furnace, controlled rolling and controlled cooling are carried out: the temperature of the finish rolling stage is controlled within 890 ℃, and 3S is adopted^-1The final rolling with 10% deformation is carried out at the final rolling temperature of 850 ℃;

2) and (4) placing the rolled finished product in a slow cooling pit, wherein slow cooling is more than or equal to 24 hours.

The steel for self-reinforcing gears 1 prepared in the embodiment has the rolling performance as follows: the tensile strength is 1600MPa, the yield strength is 1400MPa, the elongation after fracture is 20%, the reduction of area is 55%, the impact energy is 90J, various mechanical properties are comprehensively superior to those of a standard steel grade 25Cr2Ni4WA after quenching and tempering, the steel grade can be finished into a final finished gear without quenching and tempering after conventional forging, and the gear has the characteristics of nearly zero deformation, energy conservation and environmental protection. In addition, a brand new novel metallographic structure, namely carbide-free bainite, takes place the TRIP effect as a residual austenite of a soft phase under the action of stress to form martensite and reduce the generation of cracks (the microstructure can realize the self-repair of microcracks through phase transformation), so that the strength of a stressed area is increased, the crack expansion is restrained and self-reinforcement is realized, and the service life and the safety of the gear are obviously improved.

Example 2

The carbide bainite-free steel for the self-reinforcing gear 2 comprises the following chemical components in percentage by weight: c: 0.12 wt%, Si: 0.85 wt%, Mn: 1.43 wt%, Al: 3.5 wt%, P: 0.005 wt%, S: 0.005 wt%, W: 0.25 wt%, V: 0.068 wt%, and the balance of Fe and inevitable impurities.

The preparation method of the steel 2 for the carbide bainite-free self-reinforcing gear comprises the following steps of smelting according to the chemical components according to the traditional smelting method to obtain the steel 2 for the carbide bainite-free self-reinforcing gear with the microstructure, wherein,

1) in the casting blank process, after being heated by a heating furnace, controlled rolling and controlled cooling are carried out: the temperature of the finish rolling stage is controlled within 930 ℃ and is controlled in 3S^-1The final rolling with 10% deformation is carried out at the final rolling temperature of 830 ℃;

2) and (4) placing the rolled finished product in a slow cooling pit, wherein slow cooling is more than or equal to 24 hours.

The rolling state performance of the self-strengthening gear steel 2 prepared by the embodiment can reach: the tensile strength is 1550MPa, the yield strength is 1350MPa, the elongation after fracture is 20%, the reduction of area is 50%, the impact energy is 85J, various mechanical properties are comprehensively superior to those of a standard steel grade 25Cr2Ni4WA after quenching and tempering, the steel grade can be finished into a final finished gear without quenching and tempering after conventional forging, and the gear has the characteristics of nearly zero deformation, energy conservation and environmental protection. In addition, a brand new novel metallographic structure, namely carbide-free bainite, takes place the TRIP effect as a residual austenite of a soft phase under the action of stress to form martensite and reduce the generation of cracks (the microstructure can realize the self-repair of microcracks through phase transformation), so that the strength of a stressed area is increased, the crack expansion is restrained and self-reinforcement is realized, and the service life and the safety of the gear are obviously improved.

Example 3

The carbide bainite-free steel for the self-reinforcing gear 3 comprises the following chemical components in percentage by weight: c: 0.13 wt%, Si: 0.88 wt%, Mn: 1.42 wt%, Al: 3.3 wt%, P: 0.005 wt%, S: 0.006 wt%, W: 0.26 wt%, V: 0.058 wt%, and the balance Fe and inevitable impurities.

The preparation method of the steel 3 for the carbide bainite-free self-reinforcing gear comprises the following steps of smelting according to the chemical components according to the traditional smelting method to obtain the steel 3 for the carbide bainite-free self-reinforcing gear with the microstructure, wherein,

1) in the casting blank process, after being heated by a heating furnace, controlled rolling and controlled cooling are carried out: the temperature of the finish rolling stage is controlled within the range of 920 ℃ and 5S^-1The final rolling with 10% deformation is carried out, and the final rolling temperature is 820 ℃;

2) and (4) placing the rolled finished product in a slow cooling pit, wherein slow cooling is more than or equal to 24 hours.

The rolling state performance of the self-strengthening gear steel 3 prepared by the embodiment can reach: the tensile strength is 1650MPa, the yield strength is 1450MPa, the elongation after fracture is 25%, the reduction of area is 55%, the impact energy is 100J, and various mechanical properties are comprehensively superior to the performance of a standard steel grade 25Cr2Ni4WA after quenching and tempering. In addition, a brand new novel metallographic structure, namely carbide-free bainite, takes place the TRIP effect as a residual austenite of a soft phase under the action of stress to form martensite and reduce the generation of cracks (the microstructure can realize the self-repair of microcracks through phase transformation), so that the strength of a stressed area is increased, the crack expansion is restrained and self-reinforcement is realized, and the service life and the safety of the gear are obviously improved.

Example 4

The carbide bainite-free steel for the self-reinforcing gear 4 comprises the following chemical components in percentage by weight: c: 0.12 wt%, Si: 0.88 wt%, Mn: 1.45 wt%, Al: 3.4 wt%, P: 0.005 wt%, S: 0.004 wt%, W: 0.18 wt%, V: 0.063 wt%, the balance being Fe and unavoidable impurities.

The preparation method of the steel 4 for the carbide bainite-free self-reinforcing gear comprises the following steps of smelting according to the chemical components according to the traditional smelting method to obtain the steel 4 for the carbide bainite-free self-reinforcing gear with the microstructure, wherein,

1) in the casting blank process, after being heated by a heating furnace, controlled rolling and controlled cooling are carried out: the temperature of the finish rolling stage is controlled within 930 ℃ and 4S is adopted^-1The final rolling with 10% deformation is carried out at the final rolling temperature of 830 ℃;

2) and (4) placing the rolled finished product in a slow cooling pit, wherein slow cooling is more than or equal to 24 hours.

The rolling state performance of the steel 4 for the self-reinforcing gear prepared by the embodiment can reach: the tensile strength is 1620MPa, the yield strength is 1400MPa, the elongation after fracture is 20%, the reduction of area is 58%, the impact energy is 92J, and various mechanical properties are comprehensively superior to those of a standard steel grade 25Cr2Ni4WA after quenching and tempering. In addition, a brand new novel metallographic structure, namely carbide-free bainite, takes place the TRIP effect as a residual austenite of a soft phase under the action of stress to form martensite and reduce the generation of cracks (the microstructure can realize the self-repair of microcracks through phase transformation), so that the strength of a stressed area is increased, the crack expansion is restrained and self-reinforcement is realized, and the service life and the safety of the gear are obviously improved.

Example 5

The carbide bainite-free steel for the self-reinforcing gear 5 comprises the following chemical components in percentage by weight: c: 0.14 wt%, Si: 0.92 wt%, Mn: 1.53 wt%, Al: 3.7 wt%, P: 0.004 wt%, S: 0.005 wt%, W: 0.28 wt%, V: 0.068 wt%, and the balance of Fe and inevitable impurities.

The preparation method of the steel 5 for the carbide bainite-free self-reinforcing gear comprises the following steps of smelting according to the chemical components according to the traditional smelting method to obtain the steel 5 for the carbide bainite-free self-reinforcing gear with the microstructure, wherein,

1) in the casting blank process, after being heated by a heating furnace, controlled rolling and controlled cooling are carried out: the temperature of the finishing rolling stage is controlled within 910 ℃ and 5S^-1The final rolling with 10% deformation is carried out at the final rolling temperature of 840 ℃;

2) and (4) placing the rolled finished product in a slow cooling pit, wherein slow cooling is more than or equal to 24 hours.

The steel 5 for the self-reinforcing gear, prepared in the embodiment, has rolling properties of: the tensile strength is 1590MPa, the yield strength is 1350MPa, the elongation after fracture is 25%, the reduction of area is 62%, the impact energy is 95J, various mechanical properties are comprehensively superior to the performance of a standard steel grade 25Cr2Ni4WA after quenching and tempering, the steel grade can be finished into a final finished gear without quenching and tempering after conventional forging, and the gear has the characteristics of nearly zero deformation, energy conservation and environmental protection. In addition, a brand new novel metallographic structure, namely carbide-free bainite, takes place the TRIP effect as a residual austenite of a soft phase under the action of stress to form martensite and reduce the generation of cracks (the microstructure can realize the self-repair of microcracks through phase transformation), so that the strength of a stressed area is increased, the crack expansion is restrained and self-reinforcement is realized, and the service life and the safety of the gear are obviously improved.

Other parts not described in detail are prior art. Although the present invention has been described in detail with reference to the above embodiments, it is only a part of the embodiments of the present invention, not all of the embodiments, and other embodiments can be obtained without inventive step according to the embodiments, and the embodiments are within the scope of the present invention.

Claims (3)

1. A carbide bainite-free steel for self-reinforcing gears is characterized in that: the steel for the gear comprises the following chemical components in percentage by weight: c: 0.12 to 0.15 wt%, Si: 0.90-0.95 wt%, Mn: 1.45-1.55 wt%, Al: 3.5-3.9 wt%, P is less than or equal to 0.01 wt%, S is less than or equal to 0.01 wt%, W: 0.2 to 0.3 wt%, V: 0.06-0.07 wt%, and the balance of Fe and inevitable impurities; wherein the rolling performance of the steel for the gear reaches: the tensile strength is 1500 MPa-1650 MPa, the yield strength is 1250 MPa-1450 MPa, the elongation after fracture is 15% -30%, the reduction of area is 50% -65%, and the impact energy is 80J-100J.

2. The steel for carbide bainite-free self-reinforcing gears according to claim 1, wherein: the steel for the gear comprises the following chemical components in percentage by weight: c: 0.13 wt%, Si: 0.93 wt%, Mn: 1.50 wt%, Al: 3.7 wt%, P: 0.005 wt%, S: 0.005 wt%, W: 0.25 wt%, V: 0.065 wt%, and the balance of Fe and inevitable impurities.

3. A method for producing the steel for a self-reinforcing gear free from carbide bainite according to claim 1, which is a method for obtaining the steel for a self-reinforcing gear free from carbide bainite in microstructure by smelting according to the above chemical components in a conventional manner, characterized in that:

1) in the casting blank process, after being heated by a heating furnace, controlled rolling and controlled cooling are carried out: the temperature of the finish rolling stage is controlled between 850 ℃ and 950 ℃ for 3-6 s^-1The final rolling temperature is less than or equal to 850 ℃ when the final rolling of 10% of deformation is carried out;

2) and (4) placing the rolled finished product in a slow cooling pit, wherein slow cooling is more than or equal to 24 hours.

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