2GPa-grade medium-carbon ultrahigh-strength steel and preparation method thereof

Abstract

The invention relates to the technical field of metal materials, in particular to 2GPa-grade medium-carbon ultrahigh-strength steel and a preparation method thereof. The ultrahigh-strength steel is prepared from the following chemical components in percentage by mass: 0.34 to 0.38 percent of C, 0.3 to 0.9 percent of Mo, 3.0 to 5.0 percent of Ni, 1.4 to 1.9 percent of Si, 0.8 to 1.5 percent of Cr, 0.2 to 0.8 percent of Mn, 0.03 to 0.05 percent of Nb and the balance of Fe and inevitable impurities. On the basis of 300M steel, by optimizing components, the cost is reduced, and meanwhile, the strength and the toughness are improved; and meanwhile, by optimizing the preparation method, the tensile strength of the obtained steel is 2 GPa or above, the room-temperature impact energy KU2 is 48 J or above, the elongation is 10% or above, and excellent plasticity and toughness are guaranteed while the ultrahigh strength of 2 GPa is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of metal materials, and particularly to a 2GPa grade medium-carbon ultra-high strength steel and a preparation method thereof. Background Art

[0002] Ultra-high strength steel, with its excellent strength, outstanding toughness, and excellent fatigue resistance, plays a crucial role in the manufacturing of key components such as aircraft engine bearings and landing gears. With the rapid progress of China's aviation industry, the demand for materials is increasing day by day. In particular, high requirements for the dual optimization of strength and toughness are put forward for ultra-high strength steel, and cost-effectiveness is emphasized. Under this background, medium-low alloy ultra-high strength steel has gradually become the preferred material for wide application due to its advantages of simple heat treatment process and low cost. For example, the main material 300M steel used in the landing gear of the current C919 large airliner demonstrates the leading technology in this field.

[0003] However, the field of medium-low alloy high-strength steel has always been committed to further improving strength. Patent CN 114774630A discloses a low-cost low-alloy ultra-high strength steel and its manufacturing method. Based on the low-alloy high-strength steel 30CrMnSiA material, Ni and V alloying elements are added to form an ultra-high strength steel. The smelting method of electric arc furnace + vacuum refining is adopted to improve the hardenability while enhancing the comprehensive performance of the material. It has high strength and good toughness at the same time, and only a small amount of Ni and V, and the cost increase is not obvious. Its mechanical properties reach Rm≥1500MPa, impact energy KU2≥45J, meeting the requirements of low-cost aerospace large structural parts under high stress. Patent CN 117626102 A discloses a high-strength and high-toughness ultra-high strength steel and its preparation method. The preparation method includes the following steps: Step 1, melting raw steel, nickel, molybdenum, and niobium, tapping slag above 1610°C, tapping crude steel above 1670°C, and adding a conditioning agent when tapping crude steel; Step 2, adding refining materials to the smelted crude steel, using calcium oxide, alumina, and lime to make slag, and then adding Cr, Cu, Mn, Si, and rare earth elements for alloying to obtain a ladle; Step 3, performing vacuum degassing on the refined ladle, tapping steel at 1500-1600°C, and casting electrodes; Step 4, performing vacuum consumable remelting on the electrodes to obtain ingots; Step 5, forging the ingots to obtain bars; Step 6, heat-treating the bars to obtain ultra-high strength steel. The obtained product meets the requirements of high strength and high toughness, and at the same time has excellent cyclic corrosion resistance and spinning performance. It can reach 1900MPa, yield strength above 1500MPa, and fracture toughness can reach above 90MPam1 / 2.

[0004] Although low-alloy high-strength steels in the prior art can meet the extreme pursuit of material properties for key components and reduce the structural weight, they are often accompanied by the challenge of decreased fracture toughness of the material, which has become a bottleneck restricting their wider application. Summary of the Invention

[0005] The object of the present invention is to provide a 2GPa-grade medium-carbon ultra-high-strength steel and a preparation method thereof. Based on 300M steel, the contents of C, Ni, and Nb are optimized. The strength of martensite is increased by enhancing the solid solution effect of Ni, the plasticity and toughness of the steel are increased by reducing the lattice distortion through decreasing the C element content, and at the same time, a small amount of Nb is used to replace V. The fine-grained ability of the steel is improved by NbC, and the strength and toughness are increased simultaneously. In addition, the present invention adopts a special heat treatment system to enable the steel to have excellent tensile properties while achieving low cost. The present invention prepares an ultra-high-strength steel with a tensile strength of 2GPa, and has low cost and excellent strength-toughness matching performance.

[0006] In order to achieve the above object, the technical solution of the present invention is as follows:

[0007] On the one hand, the present invention provides a 2GPa-grade medium-carbon ultra-high-strength steel, and its chemical composition by mass percentage is:

[0008] C: 0.34 - 0.38%, Mo: 0.3 - 0.9%, Ni: 3.0 - 5.0%, Si: 1.4 - 1.9%, Cr: 0.8 - 1.5%, Mn: 0.2 - 0.8%, Nb: 0.03 - 0.05%, and the balance is Fe and unavoidable impurities.

[0009] The reasons for the above composition design are as follows:

[0010] C: The present invention controls the C content at 0.34 - 0.38%, mainly to ensure the solid solution strengthening ability of martensite. When the C content is lower than 0.34%, the strength of the steel cannot reach the 2GPa level, and when the carbon content is higher than 0.38%, it is difficult to achieve excellent plasticity and toughness through adjusting the heat treatment process.

[0011] Ni: The present invention controls the Ni content at 3.0 - 5.0%, which can improve the stability of retained austenite by using the strong solid solution ability of Ni, improve plasticity and toughness by using the softer austenite, and Ni can also increase the stacking fault energy of the matrix and improve the plasticity of the steel by using a high density of mobile dislocations.

[0012] Nb: The present invention adds a small amount of Nb mainly to refine the grains. The precipitation temperature of NbC is higher than that of VC, and the pinning effect on the grain boundaries during the solidification process is more significant, and the ability to refine the grains is stronger. An ideal refinement effect can be achieved by adding a small amount of Nb, but Nb is prone to segregation due to its large mass, and the Nb content must be strictly controlled.

[0013] Si: In the present invention, the Si content is controlled to be 1.4 - 1.9%, mainly to increase the temperature threshold of temper brittleness, inhibit the formation of cementite during tempering, and retain fine ε-carbides to improve the yield strength and elastic limit of the steel. However, excessive Si will lead to stress concentration and a decrease in plasticity and toughness.

[0014] Cr: In the present invention, the Cr content is controlled to be 0.8 - 1.5%, to improve the oxidation resistance and provide a certain solid solution strengthening strength.

[0015] Mo: In the present invention, 0.3 - 0.9% of Mo is added to form eutectoid carbides and increase the precipitation strengthening strength.

[0016] Mn: In the present invention, 0.2 - 0.8% of Mn is added, mainly to reduce the content of harmful element S in the steel, and at the same time provide a certain solid solution strengthening. Excessive Mn content will increase the content of MnS inclusions, so the Mn content should not exceed 0.8%.

[0017] In the above technical solution, further, the tensile strength of the ultra-high strength steel ≥ 2 GPa, the yield strength > 1.6 GPa, the elongation > 10%, and the room temperature impact energy KU2 > 48 J.

[0018] On the other hand, the present invention provides a method for preparing the above medium-carbon ultra-high strength steel, and the method includes the following steps:

[0019] Step 1, after the raw materials are melted in an electric arc furnace, refined by an LF furnace and cast into a consumable electrode, and then the electrode is subjected to vacuum consumable remelting to obtain an ingot;

[0020] Step 2, subject the ingot obtained in Step 1 to staged high-temperature treatment;

[0021] Step 3, subject the ingot obtained in Step 2 to multi-pass hot deformation treatment;

[0022] Step 4, heat the ingot obtained in Step 3 to 900 - 950 °C, hold for 50 - 90 min, air cool, normalize, then heat to 650 - 700 °C, hold for 100 - 150 min, perform high-temperature tempering, then heat to 880 - 920 °C, hold for 50 - 90 min, oil quench, after quenching, perform cold treatment at -90 - -50 °C for 100 - 150 min, and finally perform low-temperature tempering treatment at 250 - 300 °C for 120 - 180 min.

[0023] The present invention adopts a special heat treatment system, which successively performs normalizing, high-temperature tempering, quenching, cryogenic treatment, and low-temperature tempering, so as to greatly improve the tensile properties and toughness of the steel. Specifically, in order to eliminate the forging inheritance structure, the steel ingot needs to be first subjected to normalizing and high-temperature tempering treatments to dissolve large-sized carbides in the steel and improve the tissue uniformity; in order to reduce the amount of retained austenite and increase the strength, a cryogenic treatment is carried out before tempering after quenching. Without a cryogenic treatment to completely eliminate the retained austenite, there is no large loss of plasticity and toughness; finally, low-temperature tempering is carried out. On the one hand, ε-carbide is fully precipitated to achieve a good precipitation strengthening effect, and on the other hand, the quenching internal stress is eliminated to adjust the plasticity and toughness of the steel.

[0024] In the above technical solution, further, in step two, the multi-stage high-temperature treatment is as follows: in the first stage, it is heated to 820 - 880°C at a rate of 50 - 80°C / h and held for 4 - 5 h, and in the second stage, it is heated to 1150 - 1250°C at a rate of 50 - 80°C / h and held for 5 - 8 h. By subjecting the steel ingot to multi-stage high-temperature treatment and carrying out multi-stage high-temperature diffusion of alloying elements, the purpose is to reduce the segregation generated during solidification and avoid cracking during the forging process. In the first stage, martensite decomposes into ferrite and carbide, providing more particles for austenite nucleation, and preventing excessive growth of austenite grains in the second stage by increasing the nucleation amount; in the second stage, alloying elements, especially difficult-to-diffuse elements such as Cr, Mo, and Nb, are fully diffused into austenite.

[0025] In the above technical solution, in step three, the multi-pass hot deformation treatment is as follows: the steel ingot is cooled to 1020 - 1080°C for forging, upset to 0.4 - 0.6 times the original height, and then drawn out into a rough square with a height-to-diameter ratio of 2.0 - 2.5. Subsequently, it is reheated in the furnace, and the upsetting and drawing operations are repeated 3 - 5 times after reheating to enhance fine grain strengthening, and the final forging temperature is 800 - 900°C. After the steel ingot is subjected to high-temperature diffusion treatment, it may still retain a certain amount of eutectoid or pro-eutectoid phase and be accompanied by coarsening of austenite grains. To solve the above problems, a multi-pass hot deformation treatment with a large deformation amount is required. By repeating the upsetting and drawing operations, fine grain strengthening is enhanced.

[0026] In the above technical solution, further, the reheating temperature in the furnace is 1150 - 1250°C.

[0027] In the above technical solution, further, the air cooling rate is 100 - 120°C / s.

[0028] In the above technical solution, further, the quenching cooling rate is 100 - 120°C / s.

[0029] The beneficial effects of the present invention are:

[0030] 1. Based on 300M steel, the present invention increases the Ni content to enhance the solid solution effect of Ni, thereby increasing the strength of martensite, reducing the C element content, and decreasing lattice distortion to increase the plasticity and toughness of the steel. Meanwhile, a small amount of Nb is used to replace V, and NbC is utilized to improve the fine grain ability of the steel, increasing strength and toughness while reducing costs.

[0031] 2. The preparation method of the present invention includes single vacuum smelting, staged high-temperature diffusion treatment of alloying elements, multi-pass hot deformation with large deformation amounts, and special heat treatment. Among them, single vacuum smelting improves the metal purity and performance and effectively controls costs; through the staged high-temperature diffusion of alloying elements, segregation generated during solidification is alleviated, and cracking during forging is avoided; multi-pass hot deformation with large deformation amounts enhances fine grain strengthening; by adopting a special heat treatment system (including normalizing, high-temperature tempering, quenching, cold treatment, and low-temperature tempering), the steel achieves excellent tensile properties at low cost. Compared with the prior art, the present invention significantly improves the tensile properties and toughness while controlling costs, and significantly broadens the application scenarios.

[0032] 3. The tensile strength of the steel of the present invention is above 2 GPa, the impact energy KU2 is above 48 J, and the elongation is above 10%. While having an ultra-high strength of 2 GPa, excellent plasticity and toughness are also ensured. Description of the Drawings

[0033] Figure 1 High-resolution scanning electron microscope (SEM) of the sample prepared in Example 1. Detailed Description of the Invention

[0034] The following examples can enable those of ordinary skill in the art to more comprehensively understand the present invention, but do not limit the present invention in any way.

[0035] Unless otherwise specified, the materials used in the examples of the present invention can be obtained through commercial channels or prepared according to conventional methods well-known to those skilled in the art.

[0036] Example 1

[0037] Step 1: After the raw materials are melted in an electric arc furnace, they are refined by an LF furnace and cast into consumable electrodes. Subsequently, the electrodes are subjected to vacuum consumable remelting to obtain ingots; samples are taken for composition detection: C: 0.36%, Mo: 0.51%, Ni: 4.03%, Si: 1.76%, Cr: 1.02%, Mn: 0.51%, Nb: 0.042%, and the balance is Fe and inevitable impurities;

[0038] Step 2: Subject the ingot obtained in Step 1 to staged high-temperature treatment to enable staged high-temperature diffusion of alloying elements. The heat preservation process is divided into two stages: In the first stage, heat it at 60°C / h to 860°C and hold for 4 h to decompose martensite into ferrite and carbide, providing more particles for austenite nucleation; in the second stage, heat it at 60°C / h to 1180°C and hold for 6 h to fully diffuse alloying elements into austenite;

[0039] Step 3: Cool the ingot obtained in Step 2 to 1050°C and start forging. Upset it to 0.5 times the original height, then draw it out to a rough square with a height-to-diameter ratio of about 2.2, and then reheat it in the furnace. The heating temperature is 1200°C. After reheating in the furnace, repeat the upsetting and drawing operations 3 times, and control the final forging temperature at 850°C;

[0040] Step 4: Special heat treatment, including normalizing, high-temperature tempering, quenching, cold treatment, and low-temperature tempering. First, heat the ingot to 920°C and hold for 60 min, then air-cool at a cooling rate of 30°C / s for normalizing. Then heat it to 680°C and hold for 120 min for high-temperature tempering to increase the defect density in preparation for subsequent austenitization. Then heat it to 890°C and hold for 60 min, and quench in oil at a cooling rate of 120°C / s. After quenching and before tempering, first perform cold treatment at -73°C for 120 min. Since deep cold treatment is not carried out to completely eliminate retained austenite, there is no significant loss of plasticity and toughness. Finally, perform low-temperature tempering at a tempering temperature of 280°C for 150 min to improve precipitation strengthening while adjusting the plasticity and toughness of the steel.

[0041] After testing, the tensile strength of the medium-carbon ultra-high-strength steel prepared in Example 1 is 2006 MPa, the yield strength is 1633 MPa, the elongation is 11.5%, and the room-temperature impact energy KU2 is 52.0 J.

[0042] Figure 1 This is the high-resolution scanning electron microscope (SEM) image of the sample prepared in Example 1. It can be seen from the figure that its matrix structure is tempered martensite, and the nano-scale ε-carbides marked by arrows are evenly and dispersedly distributed in the martensite matrix. Thus, it can be known that for the medium-low alloy ultra-high-strength steel prepared by the present invention, ε-carbides are fully precipitated, ensuring the ultra-high strength of the steel.

[0043] Example 2

[0044] Step 1: After melting the raw materials in an electric arc furnace, refine them through an LF furnace and cast them into consumable electrodes. Subsequently, subject the electrodes to vacuum consumable remelting to obtain ingots; take samples for composition detection: C: 0.38%, Mo: 0.6%, Ni: 4.1%, Si: 1.5%, Cr: 1.2%, Mn: 0.6%, Nb: 0.035%, and the balance is Fe and unavoidable impurities;

[0045] Step 2: Subject the ingot obtained in Step 1 to staged high-temperature treatment to enable staged high-temperature diffusion of alloying elements. The heat preservation process is divided into two stages: In the first stage, heat it at 70 °C / h to 840 °C and hold for 4.5 h to decompose martensite into ferrite and carbide, providing more particles for austenite nucleation; in the second stage, heat it at 70 °C / h to 1200 °C and hold for 6.5 h to fully diffuse alloying elements into austenite;

[0046] Step 3: Cool the ingot obtained in Step 2 to 1060 °C for forging. Upset it to 0.6 times the original height, then draw it out into a rough square with a height-to-diameter ratio of about 2.3. Subsequently, heat it in the furnace again, with the heating temperature being 1150 °C. After reheating in the furnace, repeat the upsetting and drawing operations 4 times, and control the final forging temperature at 880 °C;

[0047] Step 4: Special heat treatment, including normalizing, high-temperature tempering, quenching, cold treatment, and low-temperature tempering. First, heat the ingot to 900 °C and hold for 80 min, then air-cool at a cooling rate of 35 °C / s for normalizing. Then heat it to 660 °C and hold for 100 min for high-temperature tempering to increase the defect density and prepare for subsequent austenitization. Then heat it to 880 °C and hold for 80 min, followed by oil quenching at a cooling rate of 110 °C / s. After quenching and before tempering, first conduct cold treatment at -80 °C for 110 min. Without performing cryogenic treatment to completely eliminate retained austenite, it does not experience significant plastic and toughness losses. Finally, conduct low-temperature tempering at a tempering temperature of 260 °C for 160 min to improve precipitation strengthening while adjusting the plasticity and toughness of the steel.

[0048] After testing, the tensile strength of the medium-carbon ultra-high-strength steel prepared in Example 2 is 2012 MPa, the yield strength is 1658 MPa, the elongation is 10.5%, and the room-temperature impact energy KU2 is 48.4 J.

[0049] Example 3

[0050] Step 1: After melting the raw materials in an electric arc furnace, refine them through an LF furnace and pour them into a consumable electrode. Subsequently, subject the electrode to vacuum consumable remelting to obtain an ingot; take a sample for composition detection: C: 0.37%, Mo: 0.5%, Ni: 3.9%, Si: 1.6%, Cr: 0.9%, Mn: 0.7%, Nb: 0.045%, and the balance is Fe and inevitable impurities;

[0051] Step 2: Subject the ingot obtained in Step 1 to staged high-temperature treatment to enable staged high-temperature diffusion of alloying elements. The heat preservation process is divided into two stages: In the first stage, heat it at 80 °C / h to 860 °C and hold for 5 h to decompose martensite into ferrite and carbide, providing more particles for austenite nucleation; in the second stage, heat it at 80 °C / h to 1250 °C and hold for 7 h to fully diffuse alloying elements into austenite;

[0052] Step 3: Cool the ingot obtained in Step 2 to 1080°C for forging. Upset it to 0.4 times the original height, then draw it out to a rough square with a height-to-diameter ratio of about 2.4. Subsequently, heat it in the furnace again, with the heating temperature being 1200°C. After reheating in the furnace, repeat the upsetting and drawing operations 3 times, and control the final forging temperature at 900°C.

[0053] Step 4: Special heat treatment, including normalizing, high-temperature tempering, quenching, cold treatment, and low-temperature tempering. First, heat the ingot to 950°C and hold for 60 min, then air-cool with a cooling rate of 30°C / s for normalizing. Then heat it to 690°C and hold for 120 min for high-temperature tempering to increase the defect density in preparation for subsequent austenitization. Then heat it to 900°C and hold for 90 min, followed by oil quenching with a cooling rate of 120°C / s. After quenching and before tempering, first conduct cold treatment at -70°C for 120 min. Since full elimination of retained austenite by cryogenic treatment is not carried out, there is no significant loss in plasticity and toughness. Finally, conduct low-temperature tempering at 280°C for 150 min to enhance precipitation strengthening while adjusting the plasticity and toughness of the steel.

[0054] After testing, the tensile strength of the medium-carbon ultra-high-strength steel prepared in Example 3 is 2008 MPa, the yield strength is 1642 MPa, the elongation is 11.2%, and the room-temperature impact energy KU2 is 50.1 J.

[0055] Comparative Example 1

[0056] The same preparation method as in Example 1 is adopted, with the only difference being that in Step 4, the ingot is subjected to normalizing + quenching + low-temperature tempering. Among them, the normalizing temperature is 920°C. After holding for 60 min, air-cool with a cooling rate of 30°C / s, then heat it to 890°C and hold for 60 min, followed by oil quenching with a cooling rate of 120°C / s. Finally, conduct low-temperature tempering at 260°C for 120 min.

[0057] The steel prepared in Comparative Example 1 has a tensile strength of 1892 MPa, a yield strength of 1542 MPa, an elongation of 11.5%, and a room-temperature impact energy KU2 of 45 J. Its strength and toughness are both inferior to those of Example 1.

[0058] Comparative Example 2

[0059] The difference from the steel in Example 2 lies in its composition, which is as follows: C: 0.40%, Mo: 0.4%, Ni: 1.83%, Si: 1.6%, Cr: 0.9%, Mn: 0.69%, V: 0.088%, and the balance is Fe and unavoidable impurities.

[0060] The remaining steps are the same as those in Example 2, obtaining an ultra-high strength steel with a tensile strength of 1989 MPa, a yield strength of 1423 MPa, an elongation of 7.5%, and a room temperature impact energy KU2 of 40 J. Although the strength difference is not large, the plasticity and toughness indexes are much lower than those in Example 2.

[0061] Comparative Example 3

[0062] The same preparation method as in Example 3 is adopted, with the difference only that in Step 2, the staged high-temperature treatment is as follows: in the first stage, it is heated to 800 °C at a rate of 30 °C / h and held for 3 h, and in the second stage, it is heated to 1100 °C at a rate of 30 °C / h and held for 4 h;

[0063] The tensile strength of the ultra-high strength steel obtained in Comparative Example 3 is equal to 1930 MPa, the yield strength is 1542 MPa, the elongation is 8.5%, and the room temperature impact energy KU2 is 35.1 J. Its comprehensive mechanical properties are far inferior to those in Example 3.

[0064] Comparative Example 4

[0065] The same preparation method as in Example 3 is adopted, with the difference only that in Step 3, the ingot is cooled to 1000 °C for forging, upset to 0.8 times the original height, then drawn out to a rough square with a height-to-diameter ratio of about 1.5, and returned to the furnace, and the final forging temperature is controlled at 790 °C;

[0066] The tensile strength of the ultra-high strength steel obtained in Comparative Example 3 is equal to 1912 MPa, the yield strength is 1525 MPa, the elongation is 10.2%, and the room temperature impact energy KU2 is 45.2 J. Its comprehensive mechanical properties are far inferior to those in Example 3.

[0067] The above examples are only the preferred examples of the present invention and do not limit the implementation manners. The protection scope of the present invention should be subject to the scope defined by the claims. Other different forms of changes or modifications can be made on the basis of the above description. The obvious changes or modifications derived therefrom are still within the protection scope of the present invention.

Claims

1. A 2GPa grade medium carbon ultra-high strength steel, characterized in that: The chemical composition of the ultra-high strength steel is as follows in terms of mass percentage: C: 0.34-0.38%, Mo: 0.3-0.9%, Ni: 3.0-5.0%, Si: 1.4-1.9%, Cr: 0.8-1.5%, Mn: 0.2-0.8%, Nb: 0.03-0.05%, and the remainder is Fe and unavoidable impurities.

2. The 2GPa-grade medium carbon ultra-high strength steel according to claim 1, characterized in that: The ultra-high strength steel has a tensile strength of ≥2 GPa, a yield strength of >1.6 GPa, an elongation of >10%, and a room temperature impact energy KU2 of >48 J.

3. A method for preparing the 2 GPa-grade medium carbon ultra-high strength steel according to any one of claims 1 to 2, characterized in that: The method comprises the following steps: Step 1: After the raw materials are melted in an electric arc furnace, they are refined and cast into consumable electrodes in an LF furnace, and then the electrodes are vacuum remelted to obtain steel ingots; Step 2, subjecting the steel ingot obtained in step 1 to high temperature treatment in stages; Step 3, subjecting the steel ingot obtained in step 2 to multiple heat deformation treatments; Step 4, heating the steel ingot obtained in step 3 to 900-950°C for 50-90min, air cooling, normalizing, then heating to 650-700°C for 100-150min, high temperature tempering, then heating to 880-920°C for 50-90min, oil quenching, cold treatment at -90--50°C for 100-150min after quenching, and finally low temperature tempering at 250-300°C for 120-180min.

4. The preparation method according to claim 3, characterized in that: In step 2, the high temperature treatment is carried out in stages: the first stage is heated to 820-880°C at 50-80°C / h and kept warm for 4-5h, and the second stage is heated to 1150-1250°C at 50-80°C / h and kept warm for 5-8h.

5. The preparation method according to claim 3, characterized in that: The multi-pass hot deformation treatment is as follows: the steel ingot is cooled to 1020-1080℃ and forged, upset to 0.4-0.6 times of the original height, and then drawn to a rough square with a height-to-diameter ratio of 2.0-2.5, then returned to the furnace for heating, and the upsetting and drawing are repeated 3-5 times after heating. The final forging temperature is 800-900℃.

6. The preparation method according to claim 3, characterized in that: The temperature of the reheating furnace is 1150~1250℃.

7. The preparation method according to claim 3, characterized in that: The air cooling rate is 100-120℃ / s.

8. The preparation method according to claim 3, characterized in that: The quenching cooling rate is 100-120℃ / s.

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