High-hardenability steel and manufacturing method therefor

Abstract

Disclosed in the present invention is a high-hardenability steel. In addition to Fe and inevitable impurities, the high-hardenability steel further comprises the following chemical elements, the mass percentage contents of which are as follows: 0.35-0.42% of C, 0.1-0.4% of Si, 0.6-1.2% of Mn, 0.05-0.3% of Cr, 0.02-0.15% of Cu, 0.015-0.045% of Al, 0.0008-0.0035% of B, 0.02-0.06% of Ti, 0.01-0.035% of S, and 0.003-0.007% of N; and the steel does not contain Mo and Ni. Also disclosed in the present invention is a method for manufacturing the high-hardenability steel. The steps thereof comprise: smelting and casting; heating and rolling; forging and forming; and surface quenching and low-temperature tempering, wherein the surface quenching is carried out at a temperature of 830-950°C, the low-temperature tempering is carried out at a temperature of 150-250°C, and the heat preservation time is 1-3 h.

Description

A high hardenability steel and its manufacturing method Technical Field

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

[0002] The constant velocity driveshaft is a crucial component in an automotive drivetrain, transmitting power from the engine or electric motor to the wheels to provide driving force. The main manufacturing process for a constant velocity driveshaft involves forging, normalizing, machining, heat treatment (normalizing, tempering, or surface induction hardening), precision machining, and assembly. Driveshafts subjected to light loads are typically made of medium carbon steel, commonly 45 or 50 grade high-quality carbon structural steel, which undergoes normalizing, tempering, and partial surface hardening to achieve the required performance specifications.

[0003] However, with the development of new power systems, the high horsepower and torque output of the drive motor need to be transmitted to the wheels through the drive shaft, which places higher demands on the performance of the constant velocity drive shaft components. During service, the working contact surfaces of the constant velocity drive shaft need to have high hardness to improve wear resistance, while other parts need to have good toughness to reduce the risk of brittle fracture upon impact.

[0004] For example, Chinese patent document CN112853211A, published on May 28, 2021, entitled "A Cold Forging Steel for Universal Joint Forks of Passenger Vehicles and Its Manufacturing Method", discloses a cold forging steel for universal joint forks of passenger vehicles and its manufacturing method. It achieves a pearlite spheroidization rate of ≥85% by spheroidizing medium carbon steel to meet the requirements of cold forging.

[0005] For example, Chinese patent document CN113604739A, published on November 5, 2021, entitled "A steel for precision forming of a car drive shaft ball cage and its manufacturing method", discloses a steel for precision forming of a car drive shaft ball cage and its manufacturing method, which refines the grains by controlling the Al / N ratio in medium carbon steel.

[0006] For example, Chinese patent document CN109852872A, published on June 7, 2019, entitled "A steel for CV joints of an automobile drive system and its production method", discloses a steel for CV joints of an automobile drive system and its production method, which adjusts the Mn and Cr content in medium carbon steel to improve hardenability.

[0007] For example, Chinese patent document CN113930606A, published on January 14, 2022, entitled "A Normalizing Process for Steel for Automobile Driveshaft Components", discloses a normalizing process for steel for automobile driveshaft components, which uses a roller hearth continuous heat treatment furnace to perform zoned temperature control on the steel and then perform controlled cooling treatment. Summary of the Invention

[0008] One of the objectives of this invention is to provide a high hardenability steel, which, through a reasonable chemical composition design and preferably combined with an appropriate heat treatment process, has high surface hardness and high hardenability.

[0009] To achieve the above objectives, the present invention provides a high hardenability steel, which, in addition to containing Fe and unavoidable impurities, contains the following chemical elements in the following mass percentages:

[0010] C: 0.35~0.42%, Si: 0.1~0.4%, Mn: 0.6~1.2%, Cr: 0.05~0.30%, Cu: 0.02~0.15%, Al: 0. 015~0.045%, B: 0.0008~0.0035%, Ti: 0.02~0.06%, S: 0.01~0.035%, N: 0.003~0.007%;

[0011] It does not contain Mo or Ni elements.

[0012] In this invention, "steel free of Mo and Ni" means that Mo and Ni are not intentionally added as beneficial elements in the steel, and their contents are below the detection limit.

[0013] In this invention, the steel performance is improved by adding element B, while the microstructure and mechanical properties are improved by utilizing alloying elements and their interactions, giving the steel high strength and good hardenability.

[0014] Preferably, in the high hardenability steel of the present invention, the mass percentage content of each chemical element is as follows:

[0015] C: 0.35–0.42%, Si: 0.1–0.4%, Mn: 0.6–1.2%, Cr: 0.05–0.30%, Cu: 0.02–0.15%, Al: 0.015–0.045%, B: 0.0008–0.0035%, Ti: 0.02–0.06%, S: 0.01–0.035%, N: 0.003–0.007%; balance Fe and unavoidable impurities.

[0016] The design principles of each chemical element in the high hardenability steel described in this invention are as follows:

[0017] C: In the high hardenability steel described in this invention, the addition of carbon (C) to the steel can improve its hardenability, enabling the steel to form a low-temperature phase transformation structure with higher hardness during surface quenching and cooling, thus significantly improving the surface hardness of the steel. When the mass percentage content of C in the steel is too high, it will increase the proportion of hard phases, such as martensite, and improve the strength of the steel, but it will lead to a decrease in toughness. When the mass percentage content of C in the steel is too low, it will lead to a decrease in martensite content, making it impossible for the steel to obtain high strength. Based on this, in the high hardenability steel described in this invention, the mass percentage content of C is controlled between 0.35% and 0.42%.

[0018] Si: In the high hardenability steel described in this invention, Si replaces Fe atoms in the steel through substitution, hindering dislocation movement and contributing to increased steel strength. Si can also reduce the diffusion capacity of C in ferrite, preventing the formation of large carbides that precipitate at defects. However, excessively high Si content in the steel reduces its impact toughness. Therefore, in the high hardenability steel described in this invention, the Si content is controlled between 0.1% and 0.4% by mass.

[0019] Mn: In the high hardenability steel described in this invention, Mn mainly exists in solid solution form. During the quenching process, Mn inhibits diffusion-type phase transformation, improves the hardenability of the steel, and forms a low-temperature phase transformation structure, which has high strength. When the mass percentage content of Mn in the steel is too high, it leads to the formation of more retained austenite, reducing the strength of the steel. Based on this, in the high hardenability steel described in this invention, the mass percentage content of Mn is controlled between 0.6% and 1.2%.

[0020] Cr: In the high hardenability steel described in this invention, chromium can form various carbides with carbon, and its affinity for carbon is greater than that for iron and manganese. When Cr is added to steel, it inhibits diffusion-type phase transformation, improves hardenability, forms a hardened martensitic structure, and yields steel with higher strength. Simultaneously, if the Cr carbides are not completely dissolved during heating, they inhibit austenite grain growth. Based on this, considering both steel cost and performance, the mass percentage of Cr in the high hardenability steel described in this invention is controlled between 0.05% and 0.30%.

[0021] Cu: In the high hardenability steel described in this invention, Cu can significantly improve the weather resistance of the steel and reduce its susceptibility to hydrogen-induced cracking. However, excessively high Cu content in the steel can negatively impact its weldability and easily lead to copper embrittlement, thus deteriorating its surface properties. Therefore, in the high hardenability steel described in this invention, the Cu content is controlled between 0.02% and 0.15% by mass.

[0022] Al: In the high hardenability steel described in this invention, Al can form fine AlN precipitates during the steelmaking process, which can inhibit austenite grain growth and refine austenite grains during subsequent cooling, thus achieving fine-grain strengthening. When the mass percentage content of Al in the steel is too high, it will lead to the formation of larger Al oxides, and coarse alumina hard inclusions will deteriorate the fatigue performance of the steel. Based on this, in the high hardenability steel described in this invention, the mass percentage content of Al is controlled between 0.015% and 0.045%.

[0023] B: In the high hardenability steel described in this invention, element B can increase the hardenability of the steel, thereby saving other rarer and more expensive precious metals such as nickel and molybdenum. Element B is also relatively reactive, exhibiting a strong affinity for both oxygen and nitrogen, easily forming boron oxide and boron nitride. This causes element B to lose its beneficial effects, resulting in increased fluctuations in the hardenability of the steel and affecting the stability of its properties. When the mass percentage content of element B in the steel is too high, it will form intermetallic compounds with element Fe, which will agglomerate at the grain boundaries, negatively impacting the toughness of the steel and increasing the tendency for temper brittleness. Therefore, in the high hardenability steel described in this invention, the mass percentage content of element B is controlled between 0.0008% and 0.0035%.

[0024] Ti: In the high hardenability steel described in this invention, Ti has a strong affinity for nitrogen, oxygen, and carbon, and its precipitation temperature is relatively high. It is an effective element for fixing nitrogen and oxygen, preventing boron (B) in the steel from combining with oxygen and nitrogen and losing its hardenability-enhancing effect. Ti also forms fine precipitates in the steel, improving its strength and toughness, especially its impact toughness at low temperatures. However, when the mass percentage of Ti in the steel is too high, it will form large, angular particles during smelting and solidification, reducing the steel's impact toughness. Therefore, in the high hardenability steel described in this invention, the mass percentage of Ti is controlled between 0.02% and 0.06%.

[0025] S: In the high hardenability steel described in this invention, sulfur (S) is a free-machining element, mainly existing in the form of MnS. During machining, it acts as an internal stress concentration source, facilitating chip breakage, and provides lubrication between the tool and the workpiece, reducing tool wear and improving the steel's machinability. However, sulfur-containing inclusions typically have low melting points. When the mass percentage of S in the steel is too high, the material will experience hot brittleness and an increased tendency for decarburization. Therefore, in the high hardenability steel described in this invention, the mass percentage of S is controlled between 0.01% and 0.035%.

[0026] Nitrogen (N): In the high hardenability steel described in this invention, nitrogen (N) is an interstitial atom that can combine with Al (Al) in the steel to form MN-type precipitates. These precipitates pin grain boundaries at high temperatures, thereby inhibiting austenite grain growth. In steel containing boron (B), nitrogen combines with B, causing B to lose its alloying effect. Therefore, to avoid nitrogen consuming B, the mass percentage of nitrogen in the high hardenability steel described in this invention is controlled between 0.003% and 0.007%.

[0027] Preferably, in the high hardenability steel of the present invention, the mass percentage content of Ti, Al, O and N elements also satisfies the range of (1.5Ti+2Al) / (3O+4N) being 3 to 6.

[0028] In this invention, besides carbon (C), the strength and toughness of the alloy are mainly adjusted through the use of manganese (Mn) and boron (B). Since boron readily combines with nitrogen (N) and oxygen (O) in steel, thus losing its effect on improving hardenability, a certain amount of titanium (Ti) and aluminum (Al) are added to the steel to fix the O and N elements. When the mass percentages of Ti, Al, O, and N in the steel meet the range of (1.5Ti + 2Al) / (3O + 4N) of 3–6, the role of Ti and Al in fixing O and N in the steel can be fully utilized, allowing boron to exist in a solid solution form and stably exert its effect of improving the hardenability of the steel. When the ratio (1.5Ti+2Al) / (3O+4N) in steel is too low, the O and N elements in the steel cannot be completely fixed by Ti and Al, and easily combine with the B element in the steel to form boron nitrides or oxides. This causes the B element in the steel to partially or completely fail, resulting in drastic fluctuations in the hardenability of the steel and affecting its performance. When the ratio (1.5Ti+2Al) / (3O+4N) in steel is too high, there are more excess Ti and Al elements in the steel. During the solidification and cooling process of the steel, these elements will combine with the C element in the steel and continue to precipitate and grow with TiN as the core, thus forming larger Ti(C,N) particles. These particles have sharp edges and reduce the fatigue performance of the steel. Therefore, in order to ensure that the B element in the steel can exist in solid solution form and play its role, the range of (1.5Ti+2Al) / (3O+4N) should be controlled between 3 and 6.

[0029] Preferably, in the unavoidable impurities of the high hardenability steel of the present invention, the content of each impurity element satisfies at least one of the following: P≤0.015%, O≤0.002%, H≤0.0002%.

[0030] In the high hardenability steel described in this invention, P, O, and H are all impurity elements in the steel. Where technical conditions permit, to obtain steel with better performance and superior quality, the content of impurity elements in the steel should be reduced as much as possible. Specifically:

[0031] P: In the high hardenability steel described in this invention, phosphorus (P) tends to segregate at grain boundaries, reducing the binding energy of the grain boundaries and worsening the impact properties of the steel. Furthermore, the coexistence of P and Mn elements exacerbates the temper brittleness of the steel. Therefore, in the high hardenability steel described in this invention, it is preferable to control the mass percentage content of P to ≤ 0.015%.

[0032] O: In the high hardenability steel described in this invention, oxygen (O) reacts with Al and Si in the steel to form compounds such as Al₂O₃ and SiO₂. Therefore, to ensure the uniformity of the steel structure and low-temperature impact resistance, the mass percentage content of oxygen (O) in the high hardenability steel described in this invention is preferably controlled to O ≤ 0.002%.

[0033] H: In the high hardenability steel described in this invention, hydrogen (H) is affected by the hydrostatic pressure field of edge dislocations in the steel, causing it to accumulate at defects and form hydrogen embrittlement, leading to delayed fracture of the steel. Therefore, in the high hardenability steel described in this invention, the mass percentage content of H is preferably controlled to H ≤ 0.0002%.

[0034] Preferably, the microstructure of the high hardenability steel of the present invention is tempered martensite.

[0035] Preferably, the high hardenability steel of the present invention includes a core and a hardened layer on the core, wherein the depth of the hardened layer is 4-8 mm.

[0036] Preferably, the surface hardness of the hardened layer of the high hardenability steel described in this invention is ≥700 HV. However, considering factors such as manufacturing cost and performance, the surface hardness is typically below 800 HV.

[0037] Preferably, the end hardenability of the high hardenability steel of the present invention meets the following requirements: J5.0mm: 48-56HRC, J9mm: 30-40HRC.

[0038] Another objective of this invention is to provide a method for manufacturing high hardenability steel. This method is simple to produce, and the steel obtained has high hardenability and low cost, thus having good application prospects and value.

[0039] To achieve the above objectives, the present invention provides a method for manufacturing high hardenability steel, comprising the following steps performed sequentially:

[0040] Smelting and casting;

[0041] Heating and rolling;

[0042] Forging and shaping;

[0043] Surface quenching + low-temperature tempering: the surface quenching temperature is 830~950℃; the low-temperature tempering temperature is 150~250℃, and the holding time is 1~3h.

[0044] In this invention, by employing a surface hardening process, the surface of the part can be rapidly heated to the austenitic region, while its core retains the balanced ferrite-pearlite structure after normalizing. During rapid water cooling, the surface layer of the part undergoes a martensitic transformation, forming a hardened layer. Simultaneously, a low-temperature tempering process reduces the internal stress of the steel, resulting in parts with excellent wear resistance and fatigue resistance, thus giving the parts superior overall performance.

[0045] Preferably, in the heating step of the manufacturing method of the present invention, the heating temperature is controlled at 1100-1220°C and the holding time is 2-5 hours.

[0046] Preferably, in the rolling step of the manufacturing method of the present invention, the initial rolling temperature is controlled to be ≥1050℃, preferably below 1150℃, and the final rolling temperature is 830~950℃.

[0047] Preferably, in the forging step of the manufacturing method of the present invention, the forging temperature is 830-1000°C.

[0048] Preferably, in the smelting step of the manufacturing method of the present invention, during vacuum degassing and refining, the mixture is stirred for 10-20 minutes under a vacuum degree of less than 67 Pa, and then calmed for 10-20 minutes.

[0049] Preferably, in the casting step of the manufacturing method of the present invention, the average superheat of the continuous casting tundish is 20-40°C.

[0050] The high hardenability steel and its manufacturing method described in this invention have the following advantages and beneficial effects compared to the prior art:

[0051] The high hardenability steel described in this invention achieves precise control of its hardenability curve through a combination of rational chemical composition design and optimized processes. This results in a surface induction hardening heat treatment that produces a hardened layer with good uniformity of hardness, while the core exhibits excellent toughness. Consequently, the high hardenability steel possesses excellent wear resistance and fatigue resistance, leading to superior overall performance of the parts.

[0052] In some embodiments, the hardened layer depth of the high hardenability steel of the present invention is 4-8 mm.

[0053] In some embodiments, the surface hardness of the hardened layer of the high hardenability steel described in this invention is ≥700HV.

[0054] In some embodiments, the end hardenability of the high hardenability steel of the present invention meets the following requirements: J5.0mm: 48-56HRC, J9mm: 30-40HRC.

[0055] The manufacturing method of the high hardenability steel described in this invention has a simple production process. The steel obtained by this manufacturing method has excellent mechanical properties and a low and narrow hardenability band, making it widely applicable and bringing huge economic benefits. Attached Figure Description

[0056] Figure 1 shows the microstructure of the high hardenability steel of Embodiment 1 of the present invention after surface quenching and low-temperature tempering heat treatment. Detailed Implementation

[0057] The high hardenability steel and its manufacturing method described in this invention will be further explained and described below with reference to the accompanying drawings and specific embodiments. However, this explanation and description do not constitute an undue limitation on the technical solution of this invention.

[0058] Examples 1-6 and Comparative Examples 1-4

[0059] The high hardenability steels of Examples 1-6 and the comparative steels of Comparative Examples 3-4 described in this invention were all prepared using the following steps:

[0060] (1) Smelting and casting: Smelting operations can be carried out using electric furnaces or converters, followed by LF and VD or RH vacuum refining treatment. During vacuum degassing refining in VD or RH furnaces, the furnace is vigorously stirred for 10-20 minutes under a vacuum degree of less than 67 Pa, and then calmed for 10-20 minutes. The steel ingots are cast using continuous casting. During continuous casting, the average superheat of the tundish is 20-40℃. The cast steel ingots are either hot-sent or cooled offline.

[0061] (2) Heating and rolling: The ingot is heated in a walking beam furnace and then rolled into finished product size, which can range from Φ20-50mm; the heating temperature is 1100~1220℃, and the holding time is 2~5h. The initial rolling temperature is ≥1050℃, and the final rolling temperature is 830~950℃. After rolling, the ingot is air-cooled or slowly cooled to room temperature.

[0062] (3) Forging and forming: After the round steel is cut into blanks, it is heated and forged into part blanks, and then machined. The forging temperature is 830~1000℃. After forging, the residual heat of the forging is used to slowly cool the part to achieve normalization and control the microstructure.

[0063] (4) Surface hardening + low temperature tempering: The machined parts are subjected to surface induction hardening and low temperature tempering. The surface induction hardening is rapidly heated to 830-950℃, and then quickly cooled to room temperature by water spraying; then low temperature tempering is performed at a temperature of 150-250℃ and a holding time (low temperature tempering time) of 1-3 hours. After tempering, the parts are air-cooled or water-cooled.

[0064] It should be noted that the composition and process of the high hardenability steels in Examples 1-6 of this invention meet the requirements of this invention, while the composition of Comparative Examples 1-2 does not meet the requirements of this invention, but they are manufactured according to a similar process to the examples, the only difference being that the specific process conditions do not meet the requirements of this invention. Comparative Examples 3-4 only have compositions that do not meet the requirements of this invention, while the processes meet the requirements of this invention.

[0065] Tables 1-1 and 1-2 list the mass percentages of each chemical element in the high hardenability steels of Examples 1-6 and the comparative steels of Comparative Examples 1-4 of the present invention.

[0066] Table 1-1. (wt%, balance Fe and other unavoidable impurities besides P, O and H)

[0067] Table 1-2. (The balance represents Fe and other unavoidable impurities besides P, O, and H.)

[0068] Tables 2-1 and 2-2 list the specific process parameters for the high hardenability steels of Examples 1-6 and the comparative steels of Comparative Examples 1-4 described in this invention.

[0069] Table 2-1.

[0070] Table 2-2.

[0071] To verify the effectiveness of this invention, samples were taken from the high hardenability steels of Examples 1-6 and the comparative steels of Examples 1-4. Samples were prepared and tested according to the Jominy test method for hardenability of steel (GB / T225-2006). The samples were held at 880℃ for 0.5 hours before end-quenching tests, and the hardness at positions J5.0 mm and J9 mm from the quenching end was measured. Simultaneously, the hardness change curves of the hardened layer at different locations were measured after surface induction hardening and low-temperature tempering. The depth of the hardened layer and the surface hardness were calculated, and the observation results are listed in Table 3. The microstructure was inspected according to GB / T13298-2015, the method for inspecting the microstructure of metals, and the depth of the hardened layer was determined according to GB / T 5617-2005, determination of the effective hardened layer depth after induction hardening or flame hardening of steel. The surface hardness of the hardened layer was determined according to GB / T4340.1-2024, Vickers hardness test for metallic materials, Part 1: Test method.

[0072] Table 3 lists the properties and microstructure types of the high hardenability steels of Examples 1-6 and the comparative steels of Comparative Examples 1-4.

[0073] Table 3.

[0074] As can be seen from Table 3 above, the hardened layer depth of Examples 1-6 of the present invention is between 4-8 mm, the surface hardness of the hardened layer is greater than 700 HV, and the end hardenability J5.0 mm is between 48-56 HRC and J9 mm is between 30-40 HRC. Therefore, it is evident that the high hardenability steel of the present invention exhibits excellent surface hardness after surface induction hardening.

[0075] Furthermore, all six embodiments of the present invention have tempered martensitic microstructure. Figure 1 shows the microstructure of the high hardenability steel of Embodiment 1 after surface induction hardening followed by low-temperature tempering heat treatment.

[0076] As shown in Figure 1, the microstructure of the high hardenability steel in Example 1 after heat treatment is tempered martensite.

[0077] 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.

[0078] 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. A type of steel, characterized in that, In addition to Fe and unavoidable impurities, the steel contains the following chemical elements in the following mass percentages: C: 0.35~0.42%, Si: 0.1~0.4%, Mn: 0.6~1.2%, Cr: 0.05~0.30%, Cu: 0.02~0.15%, Al:

0. 015~0.045%, B: 0.0008~0.0035%, Ti: 0.02~0.06%, S: 0.01~0.035%, N: 0.003~0.007%; The steel does not contain Mo or Ni.

2. The steel as described in claim 1, characterized in that, The mass percentage of each chemical element in the steel is as follows: C: 0.35–0.42%, Si: 0.1–0.4%, Mn: 0.6–1.2%, Cr: 0.05–0.3%, Cu: 0.02–0.15%, Al: 0.015–0.045%, B: 0.0008–0.0035%, Ti: 0.02–0.06%, S: 0.01–0.035%, N: 0.003–0.007%; balance Fe and unavoidable impurities.

3. The steel as described in claim 1 or 2, characterized in that, The mass percentage of Ti, Al, O and N elements in the steel also satisfies the range of (1.5Ti+2Al) / (3O+4N) being 3 to 6.

4. The steel as described in claim 1 or 2, characterized in that, The unavoidable impurities include P, O and H, and the mass percentage of impurity elements P, O and H satisfies at least one of the following: P≤0.015%, O≤0.002%, H≤0.0002%.

5. The steel as described in claim 1 or 2, characterized in that, The microstructure of the steel is tempered martensite.

6. The steel as described in claim 1 or 2, characterized in that, The steel comprises a core and a hardened layer on the core, the hardened layer having a depth of 4-8 mm.

7. The steel as described in claim 6, characterized in that, The surface hardness of the hardened layer is ≥700HV.

8. The steel as described in claim 1 or 2, characterized in that, The end hardenability of the steel meets the following requirements: J5.0mm: 48-56HRC, J9mm: 30-40HRC.

9. A method for manufacturing steel as described in any one of claims 1-8, characterized in that, The method includes the following steps performed sequentially: Smelting and casting; Heating and rolling; Forging and shaping; Surface quenching + low-temperature tempering: the surface quenching temperature is 830~950℃; the low-temperature tempering temperature is 150~250℃, and the holding time is 1~3h.

10. The method as described in claim 9, characterized in that, During the heating process, the heating temperature is controlled at 1100–1220℃, and the holding time is 2–5 hours.

11. The method as described in claim 9, characterized in that, During the rolling process, the initial rolling temperature is controlled to be ≥1050℃, and the final rolling temperature is 830~950℃.

12. The method as described in claim 9, characterized in that, In the forging process, the forging temperature is 830–1000℃.

13. The method as described in claim 9, characterized in that, During the smelting process, when performing vacuum degassing refining, the mixture is stirred for 10-20 minutes under a vacuum of less than 67 Pa, and then allowed to settle for 10-20 minutes.

14. The method as described in claim 9, characterized in that, During the casting process, the average superheat of the tundish pouring in continuous casting is 20–40°C.

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