Tool steel and method for producing the same

Abstract

The application provides a tool steel and a preparation method thereof. The tool steel comprises the following chemical components in percentage by mass: C: 0.50-0.59%, Si: 0.80-0.95%, Mn: 0.35-0.55%, Cr: 8.00-9.00%, W: 0.80-1.20%, Mo: 3.00-3.40%, V: 0.30-0.50%, Nb: 0.15-0.30%, Co: 0.18-0.38%, Ni: 0.15-0.35%, and the balance of Fe and impurities. The tool steel has excellent high strength, high toughness and red hardness through the design of alloy components, the increase of the content of Mo, the addition of Co and Ni elements, the design principle of multiple element combination, and the comprehensive consideration of the precipitation and growth mechanism of different element carbides in the solidification process.

Description

Technical Field

[0001] This invention relates to the field of steel alloy materials technology, and particularly to a tool steel. This invention also relates to a method for preparing the aforementioned tool steel. Background Technology

[0002] Tool steel is a type of steel used to manufacture cutting tools, measuring tools, molds, and wear-resistant tools. Tool steel has high hardness and can maintain high hardness and red hardness at high temperatures, as well as high wear resistance and appropriate toughness.

[0003] The rotary cutting disc of the forestry rotary cutting machine rotates at speeds of hundreds or even thousands of revolutions per minute. The wood fiber structure is tough and uneven, with high elastic recovery and poor thermal conductivity. In addition, the logs are not fixed during rotary cutting, causing violent vibrations, making the working conditions of the rotary cutting blades extremely harsh.

[0004] The failure modes of rotary cutting blades are wear and early failure, with wear being the most common. During operation, the cutting edge comes into high-speed contact with the wood tissue, generating intense friction and high heat, causing the blade to over-temper. Organic matter in the wood also combines with alloying elements to form organometallic compounds. Therefore, wear is a combined result of mechanical, overheating, and chemical effects. When the blade becomes dull, the cutting force and temperature surge, exacerbating machine vibration and potentially rendering the machine unusable. Early failure occurs shortly after installation, with the blade chipping, rolling, surface peeling, or even breaking, leading to serious accidents. Therefore, the steel used for rotary cutting blades needs appropriate hardness to ensure a sharp and wear-resistant cutting edge, a good combination of strength and toughness to ensure the blade's impact resistance, and a certain degree of tempering resistance and corrosion resistance. Summary of the Invention

[0005] In view of this, the present invention aims to provide a tool steel to improve its wear resistance and corrosion resistance.

[0006] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0007] A tool steel, wherein the chemical composition of the tool steel comprises, by mass percentage: C: 0.50-0.59%, Si: 0.80-0.95%, Mn: 0.35-0.55%, Cr: 8.00-9.00%, W: 0.80-1.20%, Mo: 3.00-3.40%, V: 0.30-0.50%, Nb: 0.15-0.30%, Co: 0.18-0.38%, Ni: 0.15-0.35%, with the balance being Fe and impurities.

[0008] Furthermore, the impurities include S, and S: ≤0.010%.

[0009] Furthermore, the impurities include P, and P: ≤0.030%.

[0010] Furthermore, the tool steel, after heat treatment, has an MC carbide volume fraction of 5.2-5.6%.

[0011] Furthermore, the method includes the following steps:

[0012] S1. Preparing steel ingots;

[0013] S2. Anneal the steel ingot prepared in S1, and then heat it to above the recrystallization temperature;

[0014] S3. Forge the steel ingot, then anneal it with residual heat, roll it into flat steel, and then cool it with air to 290-310℃ after rolling;

[0015] S4. Perform spheroidizing annealing treatment on the flat steel.

[0016] Furthermore, the steel ingot preparation in step S1 involves refining the molten alloy in an LF furnace, degassing it in a VD furnace under vacuum, casting it into electrode rods, and then electroslag remelting it into electroslag ingots; or refining the molten alloy in an ESH furnace and preparing steel ingots in a jet furnace.

[0017] Furthermore, the spheroidizing annealing in step S4 includes the following steps:

[0018] S401. The flat steel is heated to 770-790℃ at a rate of 90-100℃ / h and held at that temperature;

[0019] S402. Heat to 880-890℃ at a rate of 90-100℃ / h and hold at that temperature;

[0020] S403. Rapidly cool to 790-810℃ at a rate of ≥50℃ / h;

[0021] S404. Slowly cool to 670-690℃;

[0022] S405 is cooled to 550℃ in the furnace before being unloaded.

[0023] This invention, through the design of alloy composition and by increasing the content of Mo and adding Co and Ni elements to improve the hardenability of heat treatment, adopts a multi-component composite design principle, taking into account the precipitation and growth mechanism of different element carbides during solidification, thereby enabling tool steel to have excellent high strength, high toughness and red hardness.

[0024] Specifically, the carbon (C) content in the tool steel of this invention needs to be controlled between 0.50% and 0.59%, preferably between 0.52% and 0.57%. Its function is as one of the constituent elements of carbides, with some elements dissolved in the matrix to improve the matrix strength. A C content not exceeding 0.59% ensures that ledeburite structure is not generated or is minimally generated during the solidification process of the molten steel, which also helps control the amount of MC carbides and carbide segregation. A C content not less than 0.50% ensures that appropriate hardness is achieved after heat treatment.

[0025] The role of tungsten (W) is that it is a carbide-forming element, forming M6C carbides with carbon. These carbides inhibit grain growth, increasing the high-temperature hardness and wear resistance of steel. The W content should not exceed 1.20% to ensure minimal formation of M6C skeletal ledeburite during solidification, preventing difficulty in breaking down the structure during later hot working and affecting the steel's plasticity. The W content should not be less than 0.80% to ensure sufficient carbide formation, improving the steel's wear resistance and red hardness.

[0026] The role of molybdenum (Mo) is that it is a carbide-forming element. Under non-equilibrium cooling conditions, the carbides formed by Mo undergo a phase transition, producing metastable M2C carbides. These plate-like or fan-shaped M2C carbides decompose into fine M6C+MC particles during forging heating and holding after solidification and cooling, making them easier to distribute evenly, increasing the toughness and thermoplasticity of the steel. It also improves the stability of the carbides and the strength and wear resistance of the steel. In this invention, the Mo content is preferably controlled at 3.00-3.40%.

[0027] The equivalent relationship between W (tungsten), Mo (molybdenum), and C (carbon) in forming carbides is that 1.0% W is equivalent to 1.8% Mo. W and Mo can be substituted for each other. This invention controls the W equivalent [W] = W + Mo / 1.8, 2.46 ≤ [W] ≤ 3.08, and W equivalent ≥ 2.46 to ensure that the M6C content at room temperature after heat treatment is greater than 1.5%, and W equivalent ≤ 3.08 to ensure that the M6C carbides precipitated in the liquid phase are zero. This results in high wear resistance of the material while reducing the probability of complex carbide formation during smelting.

[0028] The role of chromium (Cr): Cr promotes the precipitation of carbides. Simultaneously, some Cr is partially dissolved in the matrix, primarily improving the hardenability and tempering hardness of the steel. The preferred Cr content in this invention is 8.00–9.00%.

[0029] The role of vanadium (V) is that it is a strong carbide-forming element, forming proeutectic and eutectic MC during solidification. As the V content increases, the difference between the precipitation temperature of proeutectic MC and eutectic MC becomes larger, resulting in larger proeutectic MC particles. Considering both the quenching and tempering hardness of the steel and the MC particle size, the preferred V content in this invention is 0.30-0.50%, and the MC content after heat treatment is 5.2-5.6%.

[0030] Silicon (Si) strengthens ferrite, enhances the secondary hardening ability of steel during heat treatment, reduces the critical cooling rate of steel, and improves the hardenability of steel. In this invention, the Si content is controlled at 0.80-0.95%.

[0031] The presence of manganese (Mn) makes cutting easier and helps improve the quality of the machined surface. Sulfur (S) is a metal inclusion-forming element. To mitigate the harmful effects of S forming low-melting-point non-metallic inclusions with Fe and other elements, an appropriate amount of Mn is controlled to form MnS. However, MnS extends and distributes along the rolling direction, reducing toughness in that direction. Therefore, the lower the S content, the better. In this invention, the S content is controlled at ≤0.010%, and the Mn content is controlled at 0.35-0.55%.

[0032] Co (cobalt) forms a continuous solid solution with iron. During use, Co inhibits and delays the precipitation and aggregation of carbides from other elements, significantly improving the hot strength and high-temperature hardness of steel, and enhancing the hardenability of the material. In this invention, the Co content is controlled at 0.18-0.38%.

[0033] Ni (nickel) lowers the critical transformation temperature, reduces the diffusion rate of other elements in steel, and thus improves hardenability. In this invention, the Ni content is controlled between 0.15% and 0.35%.

[0034] Niobium (Nb) can effectively refine grain size. In this invention, the Nb content is controlled at 0.15-0.30%. Attached Figure Description

[0035] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0036] Figure 1 This is a scanning electron microscope (SEM) image of the tool steel described in Embodiment 1 of the present invention;

[0037] Figure 2 This is a scanning electron microscope (SEM) image of the tool steel described in Embodiment 2 of the present invention. Detailed Implementation

[0038] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0039] A tool steel, wherein the chemical composition of the tool steel comprises, by mass percentage: C: 0.50-0.59%, Si: 0.80-0.95%, Mn: 0.35-0.55%, Cr: 8.00-9.00%, W: 0.80-1.20%, Mo: 3.00-3.40%, V: 0.30-0.50%, Nb: 0.15-0.30%, Co: 0.18-0.38%, Ni: 0.15-0.35%, with the balance being Fe and impurities.

[0040] The sulfur content in the impurities is preferably ≤0.01%, and the phosphorus content is preferably ≤0.030%.

[0041] The method for preparing high-speed steel according to the present invention specifically includes the following steps:

[0042] S1. Preparation of steel ingots. There are two methods for preparing steel ingots: One is to melt the alloy and recycled material in a neutral crucible and pour it into a ladle, then refine it in an LF furnace, degas it under vacuum in a VD furnace, cast it into R120 electrode rods, and then electroslag remelt it into φ315mm diameter electroslag ingots; the other is to melt the alloy and recycled material in a neutral crucible and pour it into a ladle, then refine it in an ESH furnace, and finally prepare φ360mm diameter steel ingots in a jet furnace.

[0043] S2. Anneal the steel ingot prepared in S1, and then heat it to above the recrystallization temperature. The heating temperature can be set to 1190-1200℃.

[0044] S3. The steel ingot is forged into 140 square billets using a precision forging machine, then subjected to residual heat annealing at 750-770℃, rolled into φ65-72 round bars using a φ600 rolling mill, and then rolled into flat bars using a φ430 rolling mill. After rolling, the flat bars are cooled to 290-310℃ by blowing air.

[0045] S4. Perform spheroidizing annealing on the flat steel. Spheroidizing annealing preferably includes the following steps:

[0046] The flat steel is heated to 770-790℃ at a rate of 90-100℃ / h and held for 6-8 hours; then heated to 880-890℃ at a rate of 90-100℃ / h and held for 5.5-7 hours; then rapidly cooled to 770-790℃ at a rate of ≥50℃ / h; and then slowly cooled to 670-690℃, with the cooling process lasting 9-11 hours; finally, all heat dissipation vents are opened, and the flat steel is cooled in the furnace to 550℃ before being removed from the furnace.

[0047] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0048] Example 1

[0049] This embodiment relates to a high-speed steel and its preparation method, comprising the following components by mass percentage: 0.55% C, 0.85% Si, 0.45% Mn, 0.025% P, 0.010% S, 8.60% Cr, 3.20% Mo, 1.10% W, 0.35% V, 0.25% Nb, 0.25% Co, 0.25% Ni, with the balance being iron.

[0050] Its preparation method includes the following steps:

[0051] S1. The alloy and scrap are melted in a neutral crucible and poured into a ladle. After ESH ladle refining, the alloy is spray-formed and directly deposited into steel ingots.

[0052] S2. After annealing, the steel ingot from the previous step is heated in a ring furnace at a temperature of 1150℃.

[0053] S3. The steel ingot is forged into 140 square billets using a precision forging machine. After residual heat annealing at 760℃, the billets are rolled into φ62-73 round bars by a rolling mill, and then rolled into flat bars by a φ430 rolling mill. After rolling, the flat bars are cooled to 300℃ by blowing air.

[0054] S4. The flat steel is heated to 780℃ at a rate of 90℃ / h and held at that temperature for 8 hours; then heated to 880℃ at a rate of 90℃ / h and held at that temperature for 7 hours; then rapidly cooled to 780℃ at a rate of 50℃ / h; finally, it is slowly cooled to 680℃, with the cooling process lasting 11 hours; all heat dissipation vents are opened, and the steel is cooled in the furnace to 550℃ before being removed from the furnace. The straightened steel sheet is then scanned by electron microscopy, and the electron micrograph is shown below. Figure 1 As shown.

[0055] The steel product prepared in this embodiment was oil quenched at 1100℃ and tempered at 525℃. After tempering, the hardness reached 62HRC, the grain size reached grade 11, and the impact toughness was 100J. The MC carbide content accounted for 5.4% by volume.

[0056] Example 2

[0057] This embodiment relates to a high-speed steel and its preparation method, comprising the following components by mass percentage: 0.55% C, 0.85% Si, 0.45% Mn, 0.025% P, 0.009% S, 8.60% Cr, 3.20% Mo, 1.10% W, 0.35% V, 0.25% Nb, 0.25% Co, 0.25% Ni, with the balance being iron.

[0058] Its preparation method includes the following steps:

[0059] S1. The alloy and scrap are melted in a neutral crucible and poured into a ladle. After being refined in an LF furnace and degassed in a VD vacuum, the alloy is cast into an electrode rod and then electroslag remelted into an electroslag ingot with a diameter of 315 mm.

[0060] S2. After annealing, the steel ingot from the previous step is heated in a ring furnace at a temperature of 1150℃.

[0061] S3. The steel ingot is forged into 140 square billets using a precision forging machine. After residual heat annealing at 760℃, the billets are rolled into φ62-73 round bars by a rolling mill, and then rolled into flat bars by a φ430 rolling mill. After rolling, the flat bars are cooled to 300℃ by blowing air.

[0062] S4. The flat steel is heated to 780℃ at a rate of 100℃ / h and held for 6 hours; then heated to 890℃ at a rate of 100℃ / h and held for 5.5 hours; then rapidly cooled to 780℃ at a rate of 55℃ / h; finally, slowly cooled to 680℃, with the cooling process lasting 9 hours; all heat dissipation vents are opened, and the steel is cooled in the furnace to 550℃ before being removed from the furnace. The straightened steel sheet is then scanned by electron microscopy, and the electron micrograph is shown below. Figure 2 As shown.

[0063] The steel product prepared in this embodiment was oil quenched at 1050℃ and tempered at 550℃. After tempering, the hardness reached 61 HRC, the grain size reached grade 11, and the impact toughness was 110 J. The MC carbide content of the high-speed steel prepared in this embodiment accounted for 5.2% by volume.

[0064] Example 3

[0065] This embodiment relates to a high-speed steel and its preparation method, comprising the following components by mass percentage: 0.50% C, 0.80% Si, 0.35% Mn, 0.029% P, 0.008% S, 8% Cr, 3% Mo, 0.9% W, 0.31% V, 0.15% Nb, 0.18% Co, 0.15% Ni, with the balance being iron.

[0066] Its preparation method includes the following steps:

[0067] S1. The alloy and scrap are melted in a neutral crucible and poured into a ladle. After ESH ladle refining, the alloy is spray-formed and directly deposited into steel ingots.

[0068] S2. After annealing, the steel ingot from the previous step is heated in a ring furnace at a temperature of 1150℃.

[0069] S3. The steel ingot is forged into 140 square billets using a precision forging machine. After residual heat annealing at 760℃, the billets are rolled into φ62-73 round bars by a rolling mill, and then rolled into flat bars by a φ430 rolling mill. After rolling, the flat bars are cooled to 300℃ by blowing air.

[0070] S4. Heat the flat steel to 780℃ at a rate of 90℃ / h and hold for 8 hours; heat to 880℃ at a rate of 90℃ / h and hold for 7 hours; rapidly cool to 780℃ at a rate of 50℃ / h and hold for 10 hours; slowly cool to 680℃, with the cooling process lasting 11 hours; open all heat dissipation vents and cool in the furnace to 550℃ before removing from the furnace. Straighten the steel and roll it out.

[0071] The steel product prepared in this embodiment was oil quenched at 1100℃ and tempered at 550℃. After tempering, the hardness reached 62HRC, the grain size reached grade 10, and the impact toughness was 95J. The MC carbide content of the high-speed steel prepared in this embodiment accounted for 5.6% by volume.

[0072] Example 4

[0073] This embodiment relates to a high-speed steel and its preparation method, comprising the following components by mass percentage: 0.59% C, 0.95% Si, 0.55% Mn, 0.015% P, 0.008% S, 9% Cr, 3.4% Mo, 1.2% W, 0.5% V, 0.3% Nb, 0.38% Co, 0.35% Ni, with the balance being iron.

[0074] Its preparation method includes the following steps:

[0075] S1. The alloy and scrap are melted in a neutral crucible and poured into a ladle. After being refined in an LF furnace and degassed in a VD vacuum, the alloy is cast into an electrode rod and then electroslag remelted into an electroslag ingot with a diameter of 315 mm.

[0076] S2. After annealing, the steel ingot from the previous step is heated in a ring furnace at a temperature of 1150℃ for homogenization.

[0077] S3. The steel ingot is forged into 140 square billets using a precision forging machine. After residual heat annealing at 760℃, the billets are rolled into φ62-73 round bars by a rolling mill, and then rolled into flat bars by a φ430 rolling mill. After rolling, the flat bars are cooled to 300℃ by blowing air.

[0078] S4. Heat the flat steel to 650℃ at a rate of 90℃ / h and hold for 8 hours; then heat it to 780℃ at a rate of 90℃ / h and hold for 7 hours; rapidly cool it to 650℃ at a rate of 30℃ / h, then slowly cool it to 550℃, with the cooling process lasting 6 hours; open all heat dissipation vents and allow it to cool in the furnace to 400℃ before removing it from the furnace. Straighten the steel and roll it out.

[0079] The steel product prepared in this embodiment was oil quenched at 1050℃ and tempered at 550℃. After tempering, the hardness reached 60HRC, the grain size reached grade 10, and the impact toughness was 92J. The MC carbide content of the high-speed steel prepared in this embodiment accounted for 5% by volume.

[0080] Comparative Example 1

[0081] This comparative example relates to a high-speed steel and its preparation method, comprising the following components by mass percentage: 0.55% C, 0.85% Si, 0.45% Mn, 0.025% P, 0.010% S, 8.00% Cr, 3.20% Mo, 1.10% W, 0.35% V, with the balance being iron.

[0082] Its preparation method includes the following steps:

[0083] S1. The alloy and scrap are melted in a neutral crucible and poured into a ladle. After ESH ladle refining, the alloy is spray-formed and directly deposited into steel ingots.

[0084] S2. After annealing, the steel ingot from the previous step is heated in a ring furnace at a temperature of 1150℃.

[0085] S3. The steel ingot is forged into 140 square billets using a precision forging machine. After residual heat annealing at 760℃, the billets are rolled into φ62-73 round bars by a rolling mill, and then rolled into flat bars by a φ430 rolling mill. After rolling, the flat bars are cooled to 300℃ by blowing air.

[0086] S4. The flat steel is heated to 780℃ at a rate of 90℃ / h and held at that temperature for 8 hours; then heated to 880℃ at a rate of 90℃ / h and held at that temperature for 7 hours; then rapidly cooled to 780℃ at a rate of 50℃ / h; finally, it is slowly cooled to 680℃, with the cooling process lasting 11 hours; all heat dissipation vents are opened, and the steel is cooled in the furnace to 550℃ before being removed from the furnace. The straightened steel sheet is then scanned using an electron microscope.

[0087] The steel product prepared in this embodiment was oil quenched at 1100℃ and tempered at 525℃. After tempering, the hardness reached 60HRC, the grain size reached grade 9.5, and the impact toughness was 90J. The MC carbide content accounted for 5.0% by volume.

[0088] Comparative Example 2

[0089] This comparative example relates to a high-speed steel and its preparation method, comprising the following components by mass percentage: 0.55% C, 0.85% Si, 0.45% Mn, 0.025% P, 0.010% S, 8.60% Cr, 3.20% Mo, 1.10% W, 0.35% V, 0.25% Nb, 0.25% Co, 0.25% Ni, with the balance being iron.

[0090] Its preparation method includes the following steps:

[0091] S1. The alloy and scrap are melted in a neutral crucible and poured into a ladle, which is then cast into F435 ingots.

[0092] S2. After annealing, the steel ingot from the previous step is heated in a ring furnace at a temperature of 1150℃.

[0093] S3. The steel ingot is forged into 140 square billets using a precision forging machine. After residual heat annealing at 760℃, the billets are rolled into φ62-73 round bars by a rolling mill, and then rolled into flat bars by a φ430 rolling mill.

[0094] S4. The flat steel is heated to 780℃ at a rate of 90℃ / h and held at that temperature for 8 hours; then heated to 880℃ at a rate of 90℃ / h and held at that temperature for 7 hours; then rapidly cooled to 780℃ at a rate of 50℃ / h; finally, it is slowly cooled to 680℃, with the cooling process lasting 11 hours; all heat dissipation vents are opened, and the steel is cooled in the furnace to 550℃ before being removed from the furnace. The straightened steel sheet is then scanned using an electron microscope.

[0095] The steel product prepared in this embodiment was oil quenched at 1100℃ and tempered at 525℃. After tempering, the hardness reached 61 HRC, the grain size reached grade 9.5, and the impact toughness was 85 J. The MC carbide content accounted for 5.1% by volume.

[0096] The steels of the above embodiments and comparative examples were subjected to performance tests, and the test data are shown in Table 1 below.

[0097] Table 1

[0098] Hardness after tempering Grain size Impact toughness MC content <![CDATA[M 23 C6 content]]> Example 1 62HRC Level 11 100J 5.4％ 13％ Example 2 61HRC Level 11 110J 5.2％ 12％ Example 3 62HRC Level 10 95J 5.6％ 14％ Example 4 60HRC Level 10 92J 5.0％ 10％ Comparative Example 1 60HRC Level 9.5 90J 5.0％ 8％ Comparative Example 2 59.8 HRC Level 9.5 85J 5.1％ 11％

[0099] The test data of each embodiment of the present invention are superior to those of the comparative examples. Comparative example 1 shows that the chemical composition ratio of the present invention is superior to that of other components, and comparative example 2 shows that the preparation method of the present invention is superior to other preparation processes, and it is easy to obtain a product with high hardness and uniform structure.

[0100] In the description of this specification, embodiments of the present invention are given. It is to be understood that the above embodiments are exemplary and should not be construed as limiting the invention. Without contradiction, those skilled in the art can combine, integrate, substitute and modify the features of the different embodiments or examples described in this specification.

Claims

1. A method for preparing tool steel, characterized in that: The chemical composition of the tool steel, by mass percentage, comprises: C: 0.50-0.59%, Si: 0.80-0.95%, Mn: 0.35-0.55%, Cr: 8.00-9.00%, W: 0.80-1.20%, Mo: 3.00-3.40%, V: 0.30-0.50%, Nb: 0.15-0.30%, Co: 0.18-0.38%, Ni: 0.15-0.35%, with the balance being Fe and impurities; The tool steel, after heat treatment, has an MC carbide volume fraction of 5.2-5.6%; The method includes the following steps: S1. Preparing steel ingots; S2. Anneal the steel ingot prepared in S1, and then heat it to above the recrystallization temperature; S3. Forge the steel ingot, then anneal it with residual heat, roll it into flat steel, and then cool it with air to 290-310℃ after rolling; S4. Perform spheroidizing annealing treatment on the flat steel.

2. The method for preparing tool steel according to claim 1, characterized in that: The impurities include S, and S: ≤0.010%.

3. The method for preparing tool steel according to claim 1, characterized in that: The impurities include P, and P: ≤0.030%.

4. The method for preparing tool steel according to claim 1, characterized in that: The steel ingot preparation in step S1 involves refining the molten alloy in an LF furnace, degassing it in a VD furnace under vacuum, casting it into electrode rods, and then electroslag remelting it into electroslag ingots; or refining the molten alloy in an ESH furnace and preparing steel ingots in a jet furnace.

5. The method for preparing tool steel according to claim 1, characterized in that, Step S4, the spheroidizing annealing, includes the following steps: S401. The flat steel is heated to 770-790℃ at a rate of 90-100℃ / h and held at that temperature; S402. Heat to 880-890℃ at a rate of 90-100℃ / h and hold at that temperature; S403. Rapidly cool to 790-810℃ at a rate of ≥50℃ / h; S404. Slowly cool to 670-690℃; S405 is cooled to 550℃ in the furnace before being unloaded.

Images