CN112195412B

CN112195412B - Nb-V microalloyed high-strength high-toughness bainite non-quenched and tempered steel for high-power engine crankshaft and preparation method thereof

Info

Publication number: CN112195412B
Application number: CN202011085053.XA
Authority: CN
Inventors: 胡芳忠; 金国忠; 郝震宇; 汪开忠; 杨少朋; 陈世杰; 杨志强; 胡乃悦; 尹德福; 张晓瑞; 王自敏; 饶添荣
Original assignee: Maanshan Iron and Steel Co Ltd
Current assignee: Maanshan Iron and Steel Co Ltd
Priority date: 2020-10-12
Filing date: 2020-10-12
Publication date: 2021-12-24
Anticipated expiration: 2040-10-12
Also published as: CN112195412A

Abstract

The invention discloses Nb-V microalloyed high strength and toughness bainite non-quenched and tempered steel for a high-power engine crankshaft and a preparation method thereof, wherein the Nb-V microalloyed high strength and toughness bainite non-quenched and tempered steel comprises the following chemical components in percentage by weight: c: 0.30-0.40%, Si: 0.60 to 1.50%, Mn: 1.40-2.00%, Cr: 0.60-0.90%, Mo: 0.05-0.15%, V: 0.20-0.50%, Ni: 0.10-0.20%, B: 0.0010-0.0035%, Nb: 0.025 to 0.045%, Al: 0.010-0.060%, P: less than or equal to 0.010 percent, S: 0.025-0.040%, T.O: less than or equal to 10ppm, [ H ]]：≤1.0ppm，[N]: 80-110ppm, X is more than or equal to 200 and less than or equal to 260, and the balance is Fe and inevitable impurity elements; the yield strength R of the bainite non-quenched and tempered steel prepared by adopting the process production of electric arc furnace/converter smelting, LF refining, RH vacuum treatment, round billet/square billet continuous casting, rolling, induction heating, forging, finishing and temperature-controlled cooling finished product_p0.2Not less than 750MPa, tensile strength R_m: 1170-1240 MPa; elongation after fracture A is more than or equal to 12 percent, reduction of area Z is more than or equal to 40 percent, and normal temperature impact KU₂Not less than 40J, and rotary bending fatigue limit sigma_‑1≥565MPa。

Description

Nb-V microalloyed high-strength high-toughness bainite non-quenched and tempered steel for high-power engine crankshaft and preparation method thereof

Technical Field

The invention belongs to the field of non-quenched and tempered steel, and relates to Nb-V microalloyed high-strength toughness bainite non-quenched and tempered steel for a high-power engine crankshaft and a preparation method thereof.

Background

The non-quenched and tempered steel is an environment-friendly steel which meets the requirements of high performance and low cost, and the application of the non-quenched and tempered steel to automobile parts is more and more extensive along with the increase of the pressure of environmental protection, energy conservation, cost reduction and the like of automobiles and related supporting industries. The crankshaft is one of core parts of an automobile engine, is a typical die forging in the automobile industry, and the first generation of non-quenched and tempered steel 49MnVS3 for the crankshaft is developed as early as 70 years in the 20 th century by German and Satison.

The quality of the crankshaft performance determines the reliability and the service life of the engine to a great extent. The service condition of the engine crankshaft is severe, and the engine crankshaft has both torsional stress and bending stress and is simultaneously acted by torsion and impact bending force, so that the service condition of the crankshaft determines that the crankshaft material has high tensile strength and fatigue strength and also needs to have good toughness. The currently used non-quenched and tempered steel for the crankshaft is ferrite and pearlite type non-quenched and tempered steel, such as F38MnVS, and the mechanical property control range R of the steel_p0.2≥550MPa，R_mNot less than 850 MPa; elongation A after fracture is more than or equal to 12 percent, reduction of area is more than or equal to 25 percent, and KU is impacted at normal temperature₂≥25J。

Along with the improvement of the power of an automobile engine, the requirement on steel for the engine crankshaft is higher and higher, and the high-strength and high-wear-resistance performance of the crankshaft during high-speed operation is further improved, and the good toughness is also ensured. The performance of the traditional pearlite and ferrite type non-quenched and tempered steel gradually cannot meet the performance requirements of the steel for the engine crankshaft.

Disclosure of Invention

In order to solve the technical problems, the invention provides Nb-V microalloyed high-strength and high-toughness bainite non-quenched and tempered steel for a high-power engine crankshaft, the bainite non-quenched and tempered steel has higher strength and good plasticity, and can meet the requirements of the high-power engine crankshaft on strength and toughness_p0.2Not less than 750MPa, tensile strength R_m: 1170-1240 MPa; elongation after fracture A is more than or equal to 12 percent, reduction of area Z is more than or equal to 40 percent, and normal temperature impact KU₂Not less than 40J, and rotary bending fatigue limit sigma_-1≥565MPa。

The invention also provides a preparation method of the Nb-V microalloying high strength and toughness bainite non-quenched and tempered steel for the high-power engine crankshaft, which adopts the process production of electric arc furnace/converter smelting, LF refining, RH vacuum treatment, round billet/square billet continuous casting, rolling, induction heating, forging, finishing, temperature control cooling and material forming, and the Nb-V microalloying high strength and toughness bainite non-quenched and tempered steel for the high-power engine crankshaft with required performance is obtained by stably producing through controlling various process steps and process parameters.

The technical scheme adopted by the invention is as follows:

the Nb-V microalloyed high strength and toughness bainite non-quenched and tempered steel for the high-power engine crankshaft comprises the following chemical components in percentage by weight: c: 0.30-0.40%, Si: 0.60 to 1.50%, Mn: 1.40-2.00%, Cr: 0.60-0.90%, Mo: 0.05-0.15%, V: 0.20-0.50%, Ni: 0.10-0.20%, B: 0.0010-0.0035%, Nb: 0.025 to 0.045%, Al: 0.010-0.060%, P: less than or equal to 0.010 percent, S: 0.025-0.040%, T.O: less than or equal to 10ppm, [ H ]: 1.0ppm or less, [ N ]: 80-110ppm, X is more than or equal to 200 and less than or equal to 260, and the balance is Fe and inevitable impurity elements;

wherein the content of the first and second substances,

x is [25.4(0.003+0.53C) × (1+0.7Si) × (-1.12+5.1Mn) × (1+2.16Cr) × (1+1.74V) × (1+0.364Ni) × (1+3Mo) ], where each element is expressed as the content of the element × 100 in the steel.

Further, the Nb-V microalloyed bainite non-quenched and tempered steel with high strength and toughness for the high-power engine crankshaft preferably comprises the following chemical components in percentage by weight: c: 0.32-0.38%, Si: 0.65-1.25%, Mn: 1.50-1.90%, Cr: 0.65-0.85%, Mo: 0.08-0.12%, V: 0.23-0.43%, Ni: 0.12-0.18%, Nb: 0.028-0.040%, B: 0.0020 to 0.0030%, Al: 0.020-0.040%, P: less than or equal to 0.010 percent, S: 0.027-0.035%, T.O: less than or equal to 10ppm, [ H ]: 1.0ppm or less, [ N ]: 95-105ppm, 205-250% of X, and the balance Fe and inevitable impurity elements.

The invention provides a preparation method of Nb-V microalloyed high strength and toughness bainite non-quenched and tempered steel for a high-power engine crankshaft, which comprises the following steps: smelting, LF refining, RH vacuum treatment, continuous casting, rolling, slow cooling, induction heating, forging, finishing, and temperature control cooling.

Furthermore, in the rolling step, in order to promote the dissolution of sulfides, reduce the component segregation and the like, the heating temperature and the heating time of the billet are properly increased, the soaking temperature of the billet in a heating furnace is controlled to be 1210-1250 ℃, and the total time of preheating, heating and soaking is controlled to be 6.0h-12.0 h; the initial rolling temperature is 1100-1140 ℃, and the final rolling temperature is 900-.

Furthermore, the soaking temperature of the heating furnace is preferably 1235-1250 ℃, and the total time of preheating, heating and soaking is preferably 7.5-8.5 h; the initial rolling temperature is preferably 1122-1130 ℃, and the final rolling temperature is preferably 920-930 ℃.

Further, the rolling step is followed by a finishing step.

Further, in order to ensure the structure performance and proper hardness reduction of the bainite non-quenched and tempered steel, in the slow cooling step, the steel is cooled to 550 ℃ through a cooling bed after rolling, is put into a pit for slow cooling, the slow cooling time is not less than 48h, and is polished and scalped after being taken out of the pit.

Furthermore, after rolling, the steel is cooled to 495-525 ℃ by a cooling bed and is put into a pit for slow cooling for 45-55 h.

Further, on the premise of ensuring the grain size of the material, in order to promote the dissolution of sulfide, the improvement of the metal flow property and the uniformity of the overall structure of the crankshaft, in the step of induction heating, the temperature is controlled to be 1230-.

Further, in the forging step, the initial forging temperature is 1140-1180 ℃, and the final forging temperature is 850-950 ℃.

Further, the initial forging temperature is preferably 1143-1170 ℃, and the final forging temperature is preferably 920-930 DEG C

Furthermore, in the temperature-controlled cooling and forming step, in order to ensure the structure and performance of the crankshaft, the forged crankshaft is air-cooled to 400-450 ℃ and then air-cooled.

In the components of the Nb-V microalloyed high strength and toughness bainite non-quenched and tempered steel for the high-power engine crankshaft, the functions and the control of each element are as follows:

c: c is the most effective strengthening element in steel, is the most effective element for influencing strength, hardness and hardenability, in order to ensure sufficient strength, hardness and hardenability of the material, because the surface of the crankshaft needs to be subjected to induction quenching strengthening, in order to ensure the surface hardness after induction quenching, the C content cannot be lower than 0.30 percent, but the C content is too high, which can cause the toughness and plasticity of the steel to be too low, therefore, the C content cannot be higher than 0.40 percent, so the C content is determined to be 0.30-0.40 percent, and is preferably 0.32-0.38 percent.

Si: si is a deoxidizer, has a strong solid solution strengthening effect, improves the hardness and the strength of steel, can inhibit the precipitation of brittle carbides during bainite transformation in the steel during a cooling process, enables certain residual austenite to be reserved in the steel, is beneficial to improving the toughness of the steel, and can improve the elastic modulus of the steel and improve the rigidity of a crankshaft, so that the content of Si cannot be lower than 0.60 percent, but excessive silicon increases the activity of C, promotes the decarburization and graphitization tendency of the steel during rolling and heat treatment, deteriorates the toughness of the steel, and cannot be higher than 1.50 percent. The Si content is controlled between 0.60 percent and 1.50 percent, and preferably between 0.61 percent and 0.95 percent.

Mn: mn is an effective element for deoxidation and desulfurization, the hardness and the strength of steel are improved, the stability of an austenite structure can be improved by Mn, the hardenability of the steel is obviously improved, and meanwhile, bainite transformation can be promoted, and the effect is difficult to achieve when the content is less than 1.40%. However, the Mn content is too high, the plasticity of the steel is reduced, the toughness of the steel is deteriorated during hot rolling, the content of residual austenite after phase transformation is too high, the bainite phase transformation temperature is too low, the yield strength and yield ratio of the steel are too low, the internal stress is too large, and the fatigue performance of the bainite non-quenched and tempered steel is deteriorated. Thus, the Mn content is controlled to be 1.40-2.00%, preferably 1.50-1.90%.

Cr: cr can effectively improve the hardenability of steel and delay bainite transformation so as to obtain required high strength, and the hardness of bainite ferrite can be obviously improved through solid solution strengthening; meanwhile, Cr can also reduce the activity of C, can reduce the decarburization tendency of the surface of steel in the heating, rolling and forging processes, and is beneficial to obtaining high fatigue resistance. Too high Cr can reduce the toughness of steel, Cr is a strong carbide precipitation element, a large amount of carbide can be generated in the structure of an induction quenching layer, the performance of the induction quenching layer is influenced, and the content of Cr cannot be higher than 0.90%. The Cr content is controlled to be 0.60-0.90%, preferably 0.61-0.80%.

Mo: mo can obviously improve the hardenability of steel and promote bainite transformation. Mo can form strong carbide, block the diffusion of atoms, the movement of dislocation and the migration of grain boundaries, and effectively prevent the recrystallization of deformed austenite. Mo provides the strength of the steel mainly through precipitation strengthening and solid solution strengthening forms of carbides. The carbide particles of Mo are fine and can not cause stress concentration of a microstructure, and the impact toughness of the steel can be improved. But Mo has higher cost and is used as little as possible under the condition of meeting the performance requirement. Therefore, the Mo content is controlled to be 0.05 to 0.15%, preferably 0.08 to 0.12%.

V: v is a strengthening element in steel, both V and C, N have extremely strong affinity, the V mainly exists in the steel in the form of carbide, the V is mainly precipitated and strengthened due to VC and V (CN), the V mainly plays a role in refining structure grains in the steel, the solid-dissolved V can obviously inhibit C diffusion in the bainite phase transformation process due to lower bainite phase transformation temperature, the bainite ferrite can be refined, the strength and toughness of the steel can be improved, and the grain boundary proportion of the refined material is increased due to the refinement of material grains, so that the strength of the steel is increased, and the sensitivity of the material to cracks is greatly reduced. However, when the V content is high, the cost is high. Therefore, the V content is controlled to be 0.20 to 0.50%, preferably 0.23 to 0.43%.

Ni: ni can effectively improve the core toughness of steel, reduce the ductile-brittle transition temperature and improve the low-temperature impact property, and has the effect of improving the fatigue strength of steel materials, and the Ni has higher cost, and the machinability after hot working can be reduced due to the excessively high Ni content. Therefore, the Ni content is controlled to 0.10 to 0.20%, preferably 0.10 to 0.18%.

Nb: nb carbonitride can pin the grain boundary and prevent austenite grains from growing, and has the main functions of refining grains and raising the grain coarsening temperature in steel, and Nb has the function of refining grains, so that the impact toughness of the steel can be raised and the brittle transition temperature of the steel can be lowered. The strength of the steel can be improved under the condition of not influencing the plasticity or toughness of the steel by a certain amount of Nb, and the excessive Nb has no obvious effect. Therefore, the Nb content is controlled to 0.025% to 0.045%, preferably 0.028 to 0.040%.

B: b can improve the high-temperature plasticity and hardenability of the steel and promote the transformation of bainite. The effect is not significant when the B content is less than 0.0010%, and the effect is not significantly increased when the B content is more than 0.0035%, which is close to saturation. Therefore, the B content should be controlled to 0.0010 to 0.0035%, preferably 0.0020 to 0.0030%.

Al: al is an effective deoxidizer, and can form AlN refined grains, the effect is not obvious when the Al content is less than 0.010%, coarse inclusions are easily formed when the Al content is more than 0.060%, and the VN content is reduced, so that the performance of the steel is deteriorated. Therefore, the Al content should be controlled to 0.010-0.060%, preferably 0.020-0.040%.

[ N ]: can form compound with V, B, Ti, Al and the like, refine crystal grains, promote the precipitation of V by N, reduce the dosage of V and reduce the cost. While too high [ N ] causes formation of continuous casting defects such as blisters, the N content in the present invention should be controlled to 80 to 110ppm, preferably 95 to 105 ppm.

P and S: the sulfur is easy to form MnS inclusion with manganese in the steel, so that the steel generates hot brittleness, but the addition of a small amount of S can obviously improve the cutting performance of the non-quenched and tempered steel while not influencing the performance of products, and the MnS has the effect of refining grains; p is an element with strong segregation tendency, increases the cold brittleness of steel, reduces the plasticity and is harmful to the uniformity of the product structure and performance. Controlling P to be less than or equal to 0.010 percent, and S: 0.025-0.035%, S is preferably 0.026-0.030%.

T.O and [ H ]: forming oxide inclusions in the steel by the T.O, and controlling the T.O to be less than or equal to 10 ppm; [H] white spots are formed in steel, the product performance is seriously influenced, and the [ H ] is controlled to be less than or equal to 1.0 ppm.

X: the X reaction is the control of hardenability in the production process of the steel, C, Si, Mn, Cr, V, Ni and Mo elements have different influences on the properties of the steel such as hardness, plastic toughness and terminal hardenability, and in order to ensure that the steel meets the design requirements, through theoretical calculation and experimental determination, the bainite non-quenched and tempered steel needs to form a bainite structure under the control of a cooling process, so that the hardenability of the bainite non-quenched and tempered steel is ensured to reach a certain value, but the structure stress and the thermal stress generated by the overhigh hardenability during heating and cooling are larger, especially in the induction quenching stage. Therefore, X should be controlled to be 200-260, preferably 205 ≦ X ≦ 250.

The Nb-V microalloyed bainite non-quenched and tempered steel for the high-power engine crankshaft and the crankshaft blank which are produced by adopting specific components and a reasonable preparation method are tested on the mechanical properties according to GB/T228.1 and GB/T229, and the yield strength R of the Nb-V microalloyed bainite non-quenched and tempered steel is_p0.2Not less than 750MPa, tensile strength R_m: 1170-1240 MPa; elongation after fracture A is more than or equal to 12 percent, reduction of area Z is more than or equal to 40 percent, and normal temperature impact KU₂The thickness is more than or equal to 40J. The rotary bending fatigue test is carried out according to GB/T4337, and the rotary bending fatigue limit sigma of the forged crankshaft material can be ensured_-1≥565MPa。

Drawings

FIG. 1 is a metallographic structure diagram of a steel in example 1;

FIG. 2 is a metallographic structure diagram of a steel in example 2;

FIG. 3 is a metallographic structure chart of steel in example 3;

FIG. 4 is a metallographic structure chart of steel in example 4

FIG. 5 is a metallographic structure diagram of steel in comparative example 1;

FIG. 6 is a metallographic structure chart of steel in comparative example 2;

fig. 7 is a metallographic structure diagram of steel in comparative example 3.

Detailed Description

The present invention will be described in detail with reference to examples.

The invention adopts non-quenched and tempered steel with specific components as shown in table 1, produces 4 furnaces of the steel of the invention (examples 1-4) in a co-production way, and adopts the process of electric arc furnace smelting, LF refining, RH vacuum treatment, continuous casting, rolling (finishing) into a material, induction heating, forging, finishing and temperature-controlled cooling to produce, and specifically comprises the following steps: heating the continuous casting billet at 1210-1250 ℃, rolling round steel after the total heating time is more than or equal to 6 hours, wherein the initial rolling temperature is as follows: 1100-1140 ℃, the final rolling temperature is 900-.

And referring to the production of 1 furnace F38MnVS steel (low sulfur neutral line) required in GB/T15712 as the comparative steel (comparative example 1), and adopting the process of electric arc furnace smelting-LF refining-RH vacuum treatment-continuous casting-rolling (finishing) finished product-induction heating-forging-finishing-temperature control cooling for production, which specifically comprises the following steps: in the RH vacuum treatment step, the vacuum time is 22 min; carrying out round steel rolling on the continuous casting billet after heating and heat preservation at 1200-mangling temperature of 1250 ℃ for more than or equal to 4h, wherein the rolling temperature is as follows: 1100-.

The following is a specific example of the Nb-V microalloyed bainite non-quenched and tempered steel with high strength and toughness for the high-power engine crankshaft.

TABLE 1 examples and comparative examples chemical compositions (T.O, [ N ]: ppm, others:%)

TABLE 2X values in the examples and comparative examples

	X：25.4(0.003+0.53C)×(1+0.7Si)×(-1.12+5.1Mn)×(1+2.16Cr)×(1+1.74V)×(1+0.364Ni)×(1+3Mo)
		Example 1	205.9
Example 2	225.8
		Example 3	244.3
Example 4	248.9
		Comparative example 1	74.8
Comparative example 2	190.6
		Comparative example 3	275.0

TABLE 3 vacuum and Steel Rolling production Process parameters

TABLE 4 forging production Process parameters

Table 4 shows the mechanical properties of the non-heat treated steel crankshafts of examples 1 to 4 of the present invention, which are shown in Table 4, have both high strength and high toughness, and the properties of the non-heat treated steel crankshafts far exceed the performance level of comparative example 1. Comparative example 2 has a lower X value, and the structure has more ferrite, resulting in better ductility and toughness and insufficient strength; on the contrary, comparative example 3 has a high X value, a good strength and insufficient ductility and toughness.

TABLE 5 mechanical Properties of examples of the present invention

Examples	R_p0.2/MPa	R_m/MPa	A/％	Z/％	KU at Normal temperature₂/J
						F38MnVS requirement	≥550	≥850	≥12	≥25	≥25
Steel requirements of invention	≥750	1170-1240	≥12	≥40	≥40
						Example 1	806	1179	13	43	46
Example 2	823	1193	14	44	49
						Example 3	838	1213	13	46	44
Example 4	840	1224	14	42	42
						Comparative example 1	595	910	13	42	32
Comparative example 2	787	1158	18	50	53
						Comparative example 3	857	1248	11	40	37

Table 5 shows the rotary bend fatigue properties of examples and comparative examples, with a cycle characteristic R of-1, the test being carried out until the test specimen breaks or 10⁷Until now. As can be seen from Table 5, the non-quenched and tempered steel crankshafts of examples 1 to 4 of the invention all have the selected bending fatigue limits of not less than 565MPa and the fatigue properties of the crankshafts are superior to those of the crankshaftsThe comparative example performance level was exceeded.

TABLE 6 rotary bending fatigue Properties of examples of the present invention

As can be seen from tables 1 to 6, the Nb-V microalloyed high strength and toughness bainite non-quenched and tempered steel for the crankshaft of the high-power engine is provided through alloy component design and reasonable production process control, and after the Nb-V microalloyed high strength and toughness bainite non-quenched and tempered steel is forged into the crankshaft, the mechanical property of the Nb-V microalloyed high strength and toughness bainite non-quenched and tempered steel meets the yield strength R_p0.2Not less than 750MPa, tensile strength R_m: 1170-1240 MPa; elongation after fracture A is more than or equal to 12 percent, reduction of area Z is more than or equal to 40 percent, and normal temperature impact KU₂Not less than 40J, and rotary bending fatigue limit sigma_-1Not less than 565MPa, without greatly increasing the cost, and has great comprehensive advantages of cost, performance and the like in the field of non-quenched and tempered steel for crankshafts.

The above detailed description of the Nb-V microalloyed high strength and toughness bainitic non-quenched and tempered steel for high power engine crankshafts and the method of making the same with reference to the examples is illustrative and not restrictive, and several examples may be cited within the limits set forth, and it is therefore intended that all such changes and modifications as fall within the true spirit and scope of the invention be considered as within the appended claims.

Claims

1. The Nb-V microalloyed high-strength and high-toughness bainite non-quenched and tempered steel for the high-power engine crankshaft is characterized by comprising the following chemical components in percentage by weight: c: 0.32-0.38%, Si: 0.61-0.95%, Mn: 1.50-1.90%, Cr: 0.61-0.80%, Mo: 0.08-0.12%, V: 0.23-0.43%, Ni: 0.10-0.18%, Nb: 0.028-0.040%, B: 0.0020 to 0.0030%, Al: 0.020 to 0.040%, P: less than or equal to 0.010 percent, S: 0.026-0.030%, T.O: less than or equal to 10ppm, [ H ]: 1.0ppm or less, [ N ]: 95-105ppm, more than or equal to 205 and less than or equal to 250 percent of X, and the balance of Fe and inevitable impurity elements;

wherein, X =25.4(0.003+0.53C) × (1+0.7Si) × (-1.12+5.1Mn) × (1+2.16Cr) × (1+1.74V) × (1+0.364Ni) × (1+3Mo), wherein each element indicates a value = the content of the element in the steel × 100;

the yield strength R of the Nb-V microalloyed high strength and toughness bainite non-quenched and tempered steel for the high-power engine crankshaft_p0.2Not less than 750MPa, tensile strength R_m: 1170-1240 MPa; elongation after fracture A is more than or equal to 12 percent, reduction of area Z is more than or equal to 40 percent, and normal temperature impact KU₂Not less than 40J, and rotary bending fatigue limit sigma_-1≥565MPa。

2. The method for preparing the Nb-V microalloyed bainite non-quenched and tempered steel with high strength and toughness for the high-power engine crankshaft as claimed in claim 1, wherein the method comprises the following steps: smelting, LF refining, RH vacuum treatment, continuous casting, rolling, slow cooling, induction heating, forging, finishing, and temperature control cooling.

3. The preparation method according to claim 2, wherein in the rolling step, the soaking temperature of the billet in the heating furnace is controlled to be 1210-1250 ℃, and the total time of preheating, heating and soaking is controlled to be 6.0-12.0 h; the initial rolling temperature is 1100-1140 ℃, and the final rolling temperature is 900-940 ℃.

4. The preparation method as claimed in claim 2, wherein in the slow cooling step, the rolled steel sheet is cooled to 550 ℃ by a cooling bed and then is put into a pit for slow cooling, the slow cooling time is not less than 48h, and the steel sheet is polished and scalped after being taken out of the pit.

5. The method as claimed in claim 2, wherein the temperature of the induction heating step is controlled to 1230-1250 ℃ at the time of the medium frequency induction heating.

6. The method as claimed in claim 2, wherein the forging step is performed at an initial forging temperature of 1140-1180 ℃ and a final forging temperature of 850-950 ℃.

7. The method as claimed in claim 2, wherein in the step of forming the material by temperature-controlled cooling, the material is air-cooled to 400-450 ℃ after forging and then air-cooled.