Economical cold forging steel and manufacturing method thereof

Abstract

The invention discloses economical cold forging steel, which contains Fe and inevitable impurities, and further contains the following chemical elements in percentage by mass: 0.17 to 0.23 percent of C, 0.15 to 0.35 percent of Si, 0.8 to 1.2 percent of Mn, 1.0 to 1.45 percent of Cr, 0.002 to 0.03 percent of S, 0.04 to 0.1 percent of Ti and 0.01 to 0.04 percent of Al. The invention further discloses a manufacturing method of the economical cold forging steel. The manufacturing method comprises the following steps: smelting and casting; heating the casting blank; rolling round steel; performing controlled cooling after rolling; and annealing.

Description

Technical Field

[0001] This invention relates to a type of steel and a method for manufacturing the same, and more particularly to a cold-forged steel and a method for manufacturing the same. Background Technology

[0002] Cold forging offers several advantages over conventional processes: First, forged materials are less prone to oxide scale formation, resulting in better surface finish and dimensional precision. Second, the process induces work hardening (strain hardening), increasing strength and hardness. Furthermore, cold forging allows for specific directional orientation of the metal fibers within the forging. Additionally, cold forging eliminates the need for heat treatment, minimizing pollution. The elimination of heating costs also reduces production costs. Therefore, cold forging aligns better with future trends in clean manufacturing and environmental protection.

[0003] However, cold forging requires materials with excellent plasticity. Often, due to insufficient plasticity of the steel, parts will crack or develop micro-cracks during the extrusion process, resulting in a high scrap rate of processed parts and increased inspection costs.

[0004] Conventional cold-forged steel is usually hot-rolled, cooled to room temperature, and then spheroidized annealed. Spheroidizing annealing takes more than ten hours or even dozens of hours, which consumes a lot of time and energy. Summary of the Invention

[0005] One of the objectives of this invention is to provide an economical cold-forged steel. This economical cold-forged steel, through reasonable chemical composition design, makes full use of the influence of various alloying elements on phase transformation and microstructure, has good plasticity and reduction of area, excellent cold forging performance, and can reduce the spheroidizing annealing process during production, which greatly improves economy and saves energy consumption.

[0006] To achieve the above objectives, the present invention provides an economical cold-forged steel containing Fe and unavoidable impurities, and further containing the following chemical elements in the following mass percentages:

[0007] C: 0.17 to 0.23%, Si: 0.15 to 0.35%, Mn: 0.8 to 1.20%, Cr: 1.0 to 1.45%, S: 0.002 to 0.03%, Ti: 0.04 to 0.1%, Al: 0.01 to 0.04%.

[0008] Furthermore, in the economical cold-forged steel described in this invention, the mass percentage content of each chemical element is as follows:

[0009] C: 0.17–0.23%, Si: 0.15–0.35%, Mn: 0.8–1.2%, Cr: 1.0–1.45%, S: 0.002–0.03%, Ti: 0.04–0.1%, Al: 0.01–0.04%; balance is Fe and other unavoidable impurities.

[0010] The design principles of each chemical element in the economical cold-forged steel described in this invention are as follows:

[0011] C: In the economical cold-forged steel described in this invention, adding an appropriate amount of carbon (C) can ensure that the steel has good hardenability and appropriate strength, which is beneficial to improving the wear resistance of the final parts processed from the steel. However, it should be noted that increasing the C content in the steel will increase the hardness of the steel, leading to excessively high material strength during subsequent processing, increasing the wear of the dies during cold forging, and causing an increase in downstream processing costs. Conversely, when the C content in the steel is too low, the steel cannot obtain high tensile strength, the structural strength of the gear core is low, the gear's resistance to deformation is reduced, and the fatigue life of the gear is decreased. At the same time, C is also a key element affecting hardenability. Based on this, to achieve narrow hardenability in cold-forged steel, the mass percentage of C in the economical cold-forged steel described in this invention is controlled between 0.17% and 0.23%.

[0012] Si: In the economical cold-forged steel described in this invention, Si is a ferrite-forming element with a strong solid solution strengthening effect, which can improve the strength of the steel. Furthermore, Si acts as a deoxidizer, effectively reducing the oxygen content in molten steel. It should be noted that excessive Si content in steel will reduce the plasticity of the steel. Therefore, in the economical cold-forged steel described in this invention, the mass percentage of Si is controlled between 0.15% and 0.35%.

[0013] Mn: In the economical cold-forged steel described in this invention, when a certain amount of sulfur (S) is present in the steel, Mn readily forms ductile MnS with S, which effectively improves chip breaking and cutting performance during subsequent gear finishing. Furthermore, Mn is a core element affecting the hardenability of cold-forged steel and can reduce hardenability fluctuations. It should be noted that excessively high Mn content in the steel can lead to increased segregation, which is detrimental to the uniformity of the material's microstructure. Therefore, to improve the material's machinability while avoiding severe segregation, the mass percentage of Mn in the economical cold-forged steel described in this invention is controlled between 0.8% and 1.2%.

[0014] Cr: In the economical cold-forged steel described in this invention, the diffusion rate of Cr in austenite is relatively low, and it can hinder the diffusion of C, suppress the diffusion-type phase transformation of steel, which is beneficial to the stability of austenite and shifts the C-curve of the steel to the right, thus reducing the critical cooling rate. Furthermore, Cr can significantly affect the hardenability of cold-forged steel. However, when the Cr content in the steel is too high, coarse carbides will form, deteriorating the cold deformation performance. Therefore, in the economical cold-forged steel described in this invention, the mass percentage of Cr is controlled between 1.0% and 1.45%.

[0015] S: In the economical cold-forged steel described in this invention, sulfur (S) can combine with manganese (Mn) to form MnS, improving cutting performance and preventing tool sticking during subsequent finishing processes. Therefore, in the economical cold-forged steel described in this invention, the mass percentage of sulfur is controlled between 0.002% and 0.03%.

[0016] Ti: In the economical cold-forged steel described in this invention, Ti can form corresponding compounds with C and N in the steel. The formation temperature of TiN is above 1400℃, and it typically precipitates in the liquid phase or δ-ferrite, thereby refining the austenite grains. However, it should be noted that when the Ti content in the steel is too high, coarse TiN precipitates will form, leading to a decrease in the fatigue performance of the steel. Therefore, in the economical cold-forged steel described in this invention, the mass percentage of Ti is controlled between 0.04% and 0.1%.

[0017] Al: In the economical cold-forged steel described in this invention, Al can effectively reduce the oxygen content in the steel during the steelmaking process, forming fine AlN precipitates. These precipitates can inhibit austenite grain growth during subsequent cooling, thereby refining the austenite grains and improving the material's plasticity. However, it should be noted that excessively high Al content in the steel can lead to the formation of larger Al oxides, resulting in larger Class B inclusions. These coarse alumina hard inclusions can worsen the fatigue performance of the steel and cause tool breakage during machining. Therefore, to ensure that Al effectively exerts its beneficial effects in this invention, the mass percentage of Al in the economical cold-forged steel described in this invention is controlled between 0.01% and 0.04%.

[0018] Furthermore, in the economical cold-forged steel described in this invention, the mass percentage content of each chemical element also satisfies: 0.8 < (Si + Mn) / Cr < 1.4, where each chemical element in the formula is substituted with its corresponding mass percentage content.

[0019] Both Si and Mn are solid solution strengthening elements and do not form precipitates with carbon to change lattice distortion. They exist stably in the lattice within the matrix. A certain content of Si and Mn can ensure that the material has a certain strength, which is beneficial to the final service life of the steel. However, increasing the content of Si and Mn requires higher diffusion activation energy for C atoms to diffuse to the nucleation point and achieve spheroidization of the precipitate. Therefore, it is detrimental to the diffusion of C during the spheroidization process. Cr is a carbide forming element. When combined with C, it forms stable carbides with lower free energy, which is beneficial to the diffusion of C. However, excessive Cr content will lead to larger precipitate size, which is detrimental to the toughness of the steel and affects its final service life. Therefore, this invention controls the ratio of (Si+Mn) and Cr to achieve good plasticity of cold-forged steel and improve the final service performance of the material.

[0020] Furthermore, the economical cold-forged steel described in this invention also contains 0 < Ca ≤ 0.005 wt%.

[0021] In the economical cold-forged steel described in this invention, adding an appropriate amount of Ca can improve the castability of the molten steel. It should be noted that the Ca content in the steel should not be too high, as excessive Ca will produce large-sized DS inclusions. Therefore, in the economical cold-forged steel described in this invention, the mass percentage of Ca is controlled between 0 < Ca ≤ 0.005%.

[0022] Furthermore, among the other unavoidable impurities in the economical cold-forged steel described in this invention, the content of each impurity element satisfies at least one of the following: P≤0.02%, O≤0.003%, N≤0.015%.

[0023] It should be noted that in this invention, P, O, and N are all unavoidable impurity elements in steel. Where technical conditions permit, the content of these impurity elements in the steel should be controlled to be as low as possible. Specifically:

[0024] P (phosphorus): Phosphorus (P) in steel tends to segregate at grain boundaries, reducing the binding energy of these boundaries and worsening the steel's plasticity. Furthermore, P can combine with Fe to form a hard and brittle Fe3P phase, causing cold brittleness during cold working and resulting in decreased plasticity. This leads to intergranular fracture under impact loads, forming large cleavage planes. Therefore, to avoid increased brittleness, the mass percentage of P in the economical cold-forged steel described in this invention is controlled to P ≤ 0.02%.

[0025] O: Impurity element O can react with Al and Ti elements in steel to form impurities such as Al2O3 and TiO. Therefore, to ensure the uniformity of the steel structure, in the economical cold-forged steel described in this invention, the mass percentage of O element is controlled to O≤0.003%.

[0026] N: In the economical cold-forged steel described in this invention, although nitrogen (N) can form AlN or TiN in the steel, thus refining austenite grains, an increase in N content leads to increased enrichment at defects and the formation of coarse nitride precipitates, affecting the fatigue life of the steel. Therefore, in the economical cold-forged steel described in this invention, the mass percentage of N is controlled to N ≤ 0.015%.

[0027] Furthermore, the microstructure of the economical cold-forged steel described in this invention is ferrite, and carbide precipitates are precipitated on the ferrite matrix, wherein spherical carbide precipitates account for no less than 90% of all carbide precipitates.

[0028] This invention achieves its intended performance by obtaining a microstructure in which spherical carbide precipitates constitute at least 90% of all carbide precipitates. This is because lamellar microstructures, due to their layered structure, are prone to dislocation accumulation and subgrain refinement during deformation, leading to increased work hardening and consequently poorer plasticity. This is detrimental to cold working, increasing the likelihood of cold forging cracks and accelerated die wear. In contrast, spherical structures more effectively reduce the resistance to dislocation movement. The gaps between the spheres allow dislocations to bypass obstacles and continue moving, thus dispersing and mitigating dislocation accumulation, maintaining better plastic deformation capacity. Therefore, the higher the proportion of spherical carbides, the more significant the improvement in material plasticity.

[0029] Furthermore, the economical cold-forged steel described in this invention has the following properties: yield strength of 300-350 MPa, tensile strength of 400-500 MPa, elongation ≥38%, and reduction of area ≥66%.

[0030] Another objective of this invention is to provide an economical method for manufacturing cold-forged steel. The cold-forged steel produced by this method not only possesses excellent plasticity but also reduces the spheroidizing annealing process during production, effectively minimizing energy consumption. Furthermore, this method eliminates the need for pre-forging heating and normalizing when manufacturing parts from round steel, thereby further reducing energy consumption and process time.

[0031] To achieve the above objectives, the present invention provides an economical method for manufacturing cold-forged steel, comprising the following steps:

[0032] Smelting and casting;

[0033] Heating of the cast billet;

[0034] Rolled round steel: The finishing rolling temperature is controlled between T1+30℃ and T1+80℃, where T1 = 910-230℃. 1 / 2 -30Mn+45Si+700P-11Cr+400(Ti+Al), with unit parameters in °C;

[0035] Post-rolling controlled cooling: Cool the round steel at a temperature not lower than V min The cooling rate is reduced to below T2, where V min =10 8.81 -4.62C-1.10Mn-0.50Cr-0.00183T1 / 3600, its unit parameter is ℃ / s; T2=424-423C-30Mn-12Cr-11.0Si, the unit parameter is ℃;

[0036] Annealing: Heat to T3 -5℃~T3+5℃ and hold, where T3=733-11Mn+29Si+17Cr, and the unit parameter is ℃; then cool down to T3-35℃~T3-45℃ and hold, then remove from the furnace and air cool.

[0037] In the above formulas, the chemical elements should be substituted with the values ​​before the percentage sign for their mass percentage content.

[0038] This invention reduces the nucleation and growth time of cementite by controlling the cooling of rolled round steel, effectively preventing the growth of cementite lamellars. At the same time, solid solution of C ensures that there is a large amount of distortion energy in the matrix, which then provides phase transformation energy for subsequent microstructure transformation during annealing, which is beneficial to the precipitation and spheroidization of carbides.

[0039] Furthermore, by rationally setting the phase transformation temperature point after steel rolling, this invention maximizes the uniform solid solution of C in the matrix, ensuring the uniformity of the subsequent spheroidized structure. At the same time, it reduces the time for cementite growth-disconnection-diffusion-spheroidization, allowing carbides to precipitate and grow directly, eventually leading to spheroidization, which also saves time in the subsequent annealing process.

[0040] Meanwhile, this invention also reduces energy consumption during the heat treatment process by utilizing the residual heat of the steel after rolling, thus achieving energy conservation and consumption reduction.

[0041] Furthermore, in the method for manufacturing economical cold-forged steel according to the present invention, the steps between the billet heating step and the round steel rolling step are: intermediate billet rolling and intermediate billet heating.

[0042] Furthermore, in the method for manufacturing economical cold-forged steel according to the present invention, the billet heating temperature is controlled to be 1020-1080°C during the billet heating step.

[0043] Furthermore, in the method for manufacturing economical cold-forged steel according to the present invention, in the intermediate billet heating step, the intermediate billet is heated to 1150-1200°C and held at that temperature.

[0044] The economical cold-forged steel and its manufacturing method described in this invention have the following advantages and beneficial effects compared to the prior art:

[0045] The economical cold-forged steel described in this invention, through reasonable chemical composition design, makes full use of the influence of various alloying elements on phase transformation and microstructure, and is combined with specific controlled rolling and cooling processes, thereby forming a uniform ferrite matrix and a microstructure with a large number of spherical carbide precipitates on the matrix.

[0046] In some more specific embodiments, the present invention effectively controls the content of P, N and O, further ensuring that the obtained cold-forged gear steel has suitable strength, excellent plasticity and elongation, while effectively saving energy consumption.

[0047] The economical cold-forged steel described in this invention has good plasticity and reduction of area, and excellent cold-forging performance.

[0048] In some more specific embodiments, the cold-forged steel has a yield strength of 200-250 MPa, a tensile strength of 400-450 MPa, an elongation of ≥38%, and a reduction of area of ​​≥66%, exhibiting excellent plasticity and cold working characteristics. Attached Figure Description

[0049] Figure 1 The image shows the microstructure of the economical cold-forged steel of Embodiment 4 of the present invention under an optical microscope.

[0050] Figure 2 The image shown is a SEM image of the economical cold-forged steel of Embodiment 4 of the present invention. Detailed Implementation

[0051] The following description, in conjunction with the accompanying drawings and specific embodiments, will further explain and illustrate the economical cold-forged steel and its manufacturing method according to the present invention. However, this explanation and illustration do not constitute an undue limitation on the technical solution of the present invention.

[0052] Examples 1-6 and Comparative Examples 1-2

[0053] The economical cold-forged steels of Examples 1-6 and Comparative Example 1 were all prepared using the following steps:

[0054] (1) Smelting and casting; In the smelting process, an electric furnace or converter can be used for smelting. Table 1 lists the chemical composition ratios of each embodiment. Examples 1, 2, and 5 were cast into 320mm*425mm large square billets, while Examples 3, 4, 6, and Comparative Example 1 were directly cast into 160-220mm square billets.

[0055] (2) Heating of billet: The billet is hot-charged and sent into the heating furnace. The heating temperature of the billet is controlled at 1020-1080℃ and kept at that temperature for 4 hours.

[0056] (3) Intermediate billet rolling and intermediate billet heating: The billets (i.e. intermediate billets) of Examples 1, 2 and 5 are further rolled into square billets of 160-220 mm. The square billets are heated to 1150-1200℃ and held at that temperature for 5 hours.

[0057] (4) Rolled round steel: Large deformation rolling is adopted, and the finishing rolling temperature Tc is controlled between T1+30 and T1+80℃, where T1=910-230℃ 1 / 2 -30Mn+45Si+700P-11Cr+400(Ti+Al), with unit parameters in °C, and the final size of the round steel is controlled between 10-50mm.

[0058] (5) Post-rolling controlled cooling: After rolling, the round steel is cooled by water, with the water volume controlled according to the finishing rolling temperature. The water volume should be no less than V... min The cooling rate will cool the round steel to T2. Wherein, V min =10 8.81 -4.62C-1.10Mn-0.50Cr-0.00183Tc / 3600, its unit parameter is ℃ / s; T2=424-423C-30Mn-12Cr-11.0Si, its unit parameter is ℃.

[0059] (6) Annealing: The obtained round steel is placed in an annealing furnace for annealing. The round steel is heated to T3 -5℃~T3+5℃, where T3=733-11Mn+29Si+17Cr, and its unit parameter is ℃; then the holding time is 2 hours, and the temperature is lowered to T3-35℃~T3-45℃, and then held for 6 hours, and then taken out of the furnace and air cooled.

[0060] It should be noted that the chemical elements in the above formulas are all substituted with the values ​​before the percentage sign for their mass percentage content.

[0061] Comparative Example 2 adopted a conventional continuous furnace spheroidizing annealing process, and its specific process parameters are shown in Tables 2-1 and 2-2.

[0062] Table 1 lists the mass percentage of each chemical element in the economical cold-forged steel of Examples 1-6 and the comparative cold-forged steel of Comparative Examples 1-2.

[0063] Table 1. (wt%, balance Fe and other unavoidable impurities besides P, O and N)

[0064]

[0065] Tables 2-1 and 2-2 list the specific process parameters for the economical cold-forged steel of Examples 1-6 and the comparative cold-forged steel of Comparative Examples 1-2 in the above process steps.

[0066] Table 2-1.

[0067]

[0068] Table 2-2.

[0069]

[0070] Note: T1 = 910 - 230°C 1 / 2 -30Mn+45Si+700P-11Cr+400(Ti+Al); V min =10 8.81 -4.62C-1.10Mn-0.50Cr-0.00183Tc / 3600; T2=424-423C-30Mn-12Cr-11.0Si; T3=733-11Mn+29Si+17Cr.

[0071] Samples of the economical cold-forged steels obtained in Examples 1-6 were taken and their microstructure was observed in accordance with GB / T 15124-2009.

[0072] Through microstructural observation, the inventors discovered that the microstructure of the economical cold-forged steels of Examples 1-6, produced using the manufacturing method described in this invention, is entirely ferrite, with carbide precipitates present at the grain boundaries and within the ferrite matrix, most of which are spherical carbide precipitates. Specifically:

[0073] Figure 1 The image shows the microstructure of the economical cold-forged steel of Embodiment 4 of the present invention under an optical microscope.

[0074] like Figure 1 As shown, the microstructure of the economical cold-forged steel in Embodiment 4 of the present invention is ferrite matrix, with carbide precipitates at the grain boundaries and within the grains of the ferrite matrix.

[0075] Figure 2 The image shows an SEM photograph of the economical cold-forged steel of Embodiment 4 of the present invention.

[0076] from Figure 2 It can be seen that most of the carbide precipitates are spherical carbide precipitates.

[0077] In addition, the economical cold-forged steels of Examples 1-6 and the comparative cold-forged steels of Comparative Examples 1-2 were sampled again and subjected to relevant mechanical property tests. The results of the mechanical property tests are listed in Table 3.

[0078] The relevant mechanical property testing methods are as follows:

[0079] Tensile test: The mechanical properties of the economic cold-forged steels of Examples 1-6 and the comparative cold-forged steels of Comparative Examples 1-2 were tested in accordance with GB / T 228.1—2010 "Metallic materials - Tensile testing - Part 1: Test method at room temperature".

[0080] Table 3 lists the mechanical property test results of the economical cold-forged steels of Examples 1-6 and the comparative cold-forged steels of Comparative Examples 1-2.

[0081] Table 3.

[0082]

[0083] As can be seen from Table 3, the economical cold-forged steels of Examples 1-6 prepared by the present invention have excellent mechanical properties. The yield strength of each example is between 305-346 MPa, the tensile strength is between 471-488 MPa, the elongation is greater than or equal to 38%, and the reduction of area is greater than or equal to 67%.

[0084] In Comparative Example 1, the spherical carbide precipitates accounted for less than 90% of all carbide precipitates, while their elongation and reduction of area were significantly different from those of the embodiments of the present invention.

[0085] Table 4 lists the energy consumption of post-rolling heat treatment for the economical cold-forged steels of Examples 1-6 and the comparative cold-forged steels of Comparative Examples 1-2.

[0086] Table 4.

[0087]

[0088] As can be seen from Table 4, although the mechanical properties of Comparative Example 2 are comparable to those of the embodiments of the present invention, the energy consumption per ton of steel after heat treatment in Examples 1-6 of the present invention is at least 65 KW*h lower than that of Comparative Example 2 (converted from natural gas consumption to electricity consumption). This fully demonstrates that the present invention effectively reduces the energy consumption of heat treatment after rolling of cold forged steel and achieves the purpose of energy saving and consumption reduction.

[0089] As can be seen from Tables 3 and 4 above, the economical cold-forged steels of Examples 1-6 of the present invention can effectively reduce energy consumption. At the same time, the economical cold-forged steels of Examples 1-6 have excellent mechanical properties, good plasticity and reduction of area, and are comparable to the strength and plasticity of conventional spheroidized annealed round steels, and have excellent cold working characteristics.

[0090] It should be noted that the economical cold-forged steel described in this invention has a reasonable chemical composition and process design, and a wide process window, which can be mass-produced on bar or plate production lines. Therefore, it has good prospects for promotion and application value.

[0091] Furthermore, the combination of the technical features in this case is not limited to the combination methods described in the claims of this case or the combination methods described in the specific embodiments. All technical features described in this case can be freely combined or combined in any way, unless they contradict each other.

[0092] It should also be noted that the above examples are merely specific embodiments of the present invention, and the present invention is obviously not limited to the above embodiments, with many similar variations. All modifications that can be directly derived or conceived by those skilled in the art from the content disclosed in this invention should fall within the protection scope of this invention.

Claims

1. An economical cold-forged steel, containing Fe and unavoidable impurities, characterized in that, It also contains the following chemical elements in the following percentages by mass: C: 0.17-0.23%, Si: 0.15-0.35%, Mn: 0.8-1.2%, Cr: 1.0-1.45%, S: 0.002-0.03%, Ti: 0.04-0.1%, Al: 0.01-0.04%.

2. The economical cold-forged steel as described in claim 1, characterized in that, Its mass percentage content of each chemical element is as follows: C: 0.17–0.23%, Si: 0.15–0.35%, Mn: 0.8–1.2%, Cr: 1.0–1.45%, S: 0.002–0.03%, Ti: 0.04–0.1%, Al: 0.01–0.04%; balance is Fe and other unavoidable impurities.

3. The economical cold-forged steel as described in claim 1 or 2, characterized in that, The mass percentage of each chemical element also satisfies the following condition: 0.8 < (Si + Mn) / Cr < 1.

4.

4. The economical cold-forged steel as described in claim 1 or 2, characterized in that, It also contains 0 < Ca ≤ 0.005 wt%.

5. The economical cold-forged steel as described in claim 1 or 2, characterized in that, Among other unavoidable impurities, the content of each impurity element must satisfy at least one of the following conditions: P ≤ 0.02%, O ≤ 0.003%, N ≤ 0.015%.

6. The economical cold-forged steel as described in claim 1 or 2, characterized in that, Its microstructure is ferrite, and carbide precipitates are precipitated on the ferrite matrix, of which spherical carbide precipitates account for no less than 90% of the total number of carbide precipitates.

7. The economical cold-forged steel as described in claim 1 or 2, characterized in that, Its performance meets the following requirements: yield strength of 300-350MPa, tensile strength of 400-500MPa, elongation ≥38%, and reduction of area ≥66%.

8. The method for manufacturing economical cold-forged steel as described in any one of claims 1-7, characterized in that, It includes the following steps: Smelting and casting; Heating of the cast billet; Rolled round steel: The finishing rolling temperature Tc is controlled to be T1+30℃~T1+80℃, where T1=910-230℃ 1 / 2 -30Mn+45Si+700P-11Cr+400(Ti+Al), with unit parameters in °C; Post-rolling controlled cooling: Cool the round steel at a temperature not lower than V min The cooling rate is reduced to below T2, where T2 = 424-423C-30Mn-12Cr-11.0Si, with unit parameters in °C; Annealing: Heat to T3-5℃~T3+5℃ and hold, where T3=733-11Mn+29Si+17Cr, and the unit parameter is ℃; then cool down to T3-35℃~T3-45℃ and hold, then remove from the furnace and air cool. In the above formulas, the chemical elements should be substituted with the values ​​before the percentage sign for their mass percentage content.

9. The manufacturing method as described in claim 8, characterized in that, Between the billet heating step and the round steel rolling step, there are also steps: intermediate billet rolling and intermediate billet heating.

10. The manufacturing method as described in claim 8, characterized in that, In the billet heating step, the billet heating temperature is controlled at 1020~1080℃.

11. The manufacturing method as described in claim 9, characterized in that, In the intermediate billet heating step, the intermediate billet is heated to 1150-1200℃ and held at that temperature.

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