CN115181913A - Preparation method of low-manganese-content medium manganese steel - Google Patents
Preparation method of low-manganese-content medium manganese steel Download PDFInfo
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
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- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
- C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D2211/00—Microstructure comprising significant phases
- C21D2211/008—Martensite
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Abstract
The invention discloses a preparation method of low-manganese-content medium manganese steel, which comprises the following steps: smelting: smelting alloy ingredients of manganese steel with low manganese content and then casting the alloy ingredients into ingots; homogenizing and forging: carrying out homogenization treatment on the cast ingot and then forging the cast ingot into a blank; hot rolling: heating and insulating the forging stock, and then carrying out hot rolling to obtain a hot rolled plate; pre-quenching: carrying out complete austenitizing treatment on the hot rolled plate, and cooling to obtain a martensite initial structure; two-phase region annealing: heating to Ac 1-Ac 3 temperature to anneal in two-phase region; low-temperature carbon diffusion: heating to 280-400 ℃, preserving the heat for 20-80 min, and obtaining the low-manganese-content medium manganese steel through low-temperature carbon diffusion and secondary carbon distribution; wherein the manganese content in the low manganese content medium manganese steel is lower than 4 percent by mass. The manganese steel has the manganese element content below 3.4 wt%, reasonable component design, simple preparation process, low cost, high production efficiency and strength-plastic product up to 43GPa%.
Description
Technical Field
The invention relates to the technical field of advanced high-strength steel, in particular to a preparation method of low-manganese-content medium manganese steel.
Background
The third generation advanced high-strength steel, such as medium manganese steel, QP steel and the like, is mainly designed by chemical components with low alloy content and adopts a simple production process to obtain the advanced high-strength steel with low cost, easy preparation and 25-45 GPa percent of strength-plasticity product. As third generation advanced high strength steel, medium manganese steel is prepared by reducing the manganese content on the basis of the chemical composition of high manganese/ultrahigh manganese steel. Carbon and manganese play a key role in the thermal stability of the reverse transformation austenite of the dual-phase region of the medium manganese steel. The carbon element mainly plays a role in solid solution strengthening, and simultaneously increases the thermal stability of reverse transformation austenite, so that the reverse transformation austenite has higher content of residual austenite in a room-temperature microstructure, and can generate more remarkable transformation induced plasticity effect (namely TRIP effect) during deformation. However, too high a carbon content may degrade welding performance. Manganese is also an austenite forming element, can increase the thermal stability of super-cooled austenite, improves the hardenability of steel, and is very beneficial to increasing the content of residual austenite in a room-temperature structure. However, the microstructure is uneven due to the excessively high manganese content, a banded structure is easy to appear, the production and the processing are not facilitated, and cracking is easy to occur during rolling; in addition, because of the slow diffusion rate and long partition time of manganese atoms, in order to fully partition the manganese atoms into reverse transformation austenite so as to obtain high content of residual austenite at room temperature, the dual-phase annealing time is required to be as long as several hours or even tens of hours (Wangchang, xuhaifeng, huang Chongxiang, cao Wen Zi, dong Han, the evolution of manganese steel reverse transformation annealing structure and the partition behavior of manganese. The research and report on iron and steel, 2016, vol.28, p.38-46), which not only reduces the production efficiency, but also increases the production cost.
The Chinese invention patent CN 104651734B discloses a 1000MPa grade high-strength high-plasticity aluminum-containing medium manganese steel and a manufacturing method thereof, wherein the high-strength high-plasticity medium manganese steel is prepared by adding Al, cr, mo, cu and other elements into the steel for alloying and adopting the working procedures of hot continuous rolling, cover annealing, cold rolling, continuous annealing and the like; however, the mass percentage of manganese element in the steel is more than 7-11%, and the required annealing time is more than 10 h. The Chinese invention patent CN 110408861B discloses a cold-rolled high-strength-ductility medium manganese steel with lower manganese content and a preparation method thereof, wherein the medium manganese steel has higher strength-ductility product; however, the mass fraction of Mn element in the steel is still higher, which reaches 6%, and elements such as Al, si, cr, ni and the like with higher content are also added, and the annealing time of the two-phase region is 1-30 h. The chemical compositions of the medium manganese steels reported in the literature or published in the patent are approximately 0.1-0.6 wt% C and 4-12% Mn (Hu B, luo H, yang F, et al. Recent progress in medium-Mn steel masses with new design strategies, a review. Journal of Materials Science & Technology,2017, vol.33, p.1457-1464). If the manganese content is further reduced, the thermal stability of the reversed austenite formed in the dual-phase region may be deteriorated, resulting in difficulty in obtaining a sufficient content of residual austenite after the dual-phase region annealing, and thus failing to exert the TRIP effect of the medium manganese steel.
In addition, the literature reports Fe-Mn-Al-C series low density medium manganese steels that can greatly reduce the density of steel by adding aluminum to the steel, taking advantage of the light weight of Al (Lee S, kang S H, nam J H, et Al. Effect of testing on the microstructure and tension properties of a textured medium-Mn light steel. Metal Mater Transs A,2019, vol.50, p.2655). The density of the steel can be reduced by 17% per 12% (mass fraction) of added aluminum. In addition, attempts have been made to temper Fe-Mn-Al-C based low density medium manganese steels after dual phase zone annealing to precipitate fine cementite or coarse kappa-carbide on the martensitic matrix by tempering and to further control the properties of the steels (Lee S, kang S H, nam J H, et Al. Effect of the testing on the microstructure and tension properties of a martensitic medium-Mn light steel. Metal matrix Trans A,2019, vol.50, p.2655). However, these Fe-Mn-Al-C based low density steels have high contents of carbon, manganese and aluminum elements and often have poor mechanical properties due to the presence of coarse kappa-carbides.
Disclosure of Invention
The invention aims to provide a preparation method of high-strength-ductility low-manganese-content medium manganese steel, which has the advantages of reasonable components, simple preparation process, short two-phase region annealing time and subsequent low-temperature carbon diffusion time in the preparation process, low cost and high production efficiency.
In order to achieve the purpose, the invention adopts the technical scheme that:
the invention provides a preparation method of low-manganese-content medium manganese steel, which comprises the following steps:
1) Smelting: smelting the alloy ingredients of the manganese steel with low manganese content and then casting the alloy ingredients into ingots;
2) Homogenizing and forging: homogenizing the cast ingot obtained in the step 1), and forging into a blank;
3) Hot rolling: heating and insulating the forged forging stock, and then carrying out hot rolling to obtain a hot rolled plate;
4) Pre-quenching: carrying out complete austenitizing treatment on a hot rolled plate obtained by hot rolling, and cooling to obtain a martensite initial structure;
5) Two-phase region annealing: heating the pre-quenched sample to Ac 1-Ac 3 temperature for dual-phase zone annealing;
6) Low-temperature carbon diffusion: heating the sample annealed in the two-phase region to 280-400 ℃, preserving the heat for 20-80 min, and obtaining the low-manganese-content medium manganese steel through low-temperature carbon diffusion and secondary carbon distribution;
wherein the manganese content in the low-manganese-content medium manganese steel is lower than 4% by mass percent.
As a preferred embodiment, the chemical compositions of the low-manganese-content medium-manganese steel are as follows by mass percent: 0.26 to 0.48 percent of carbon, 2.1 to 3.4 percent of manganese, 1.0 to 2.2 percent of aluminum, 0.6 to 2.0 percent of silicon, and the balance of iron and inevitable impurities.
As a preferred embodiment, in step 1), the smelting is vacuum induction furnace smelting.
Preferably, in the step 2), the homogenization treatment is carried out at 1200-1250 ℃ for 2-3 h;
in some specific embodiments, the homogenization treatment further comprises an operation of removing a riser and a surface scale on the ingot.
In a preferred embodiment, in the step 2), the forging temperature is 1200-1250 ℃;
preferably, the forging has a finish forging temperature of not less than 900 ℃.
Preferably, in step 3), the temperature for heating and heat preservation is 1150-1200 ℃;
preferably, the heating and heat preservation time is 20-40 min;
preferably, the initial rolling temperature of the hot rolling is 1150-1200 ℃;
preferably, the finishing temperature of the hot rolling is not lower than 900 ℃.
In a preferable embodiment, in the step 4), the complete austenitizing treatment is to heat the hot-rolled plate obtained by hot rolling to a temperature higher than Ac3, and the temperature higher than Ac3 is preferably 920 to 970 ℃;
preferably, the cooling is water quenching or oil quenching cooling.
Preferably, in step 5), the heating to Ac 1-Ac 3 temperature is to heat to 730-800 ℃;
preferably, the heating time is 8-100 min;
preferably, step 5) further comprises a cooling process after the two-phase zone annealing; the cooling is water quenching or oil quenching.
As a preferred embodiment, step 6) further comprises a cooling process; the cooling is water quenching or oil quenching or natural cooling in air.
In the technical scheme of the invention, the microstructure of the manganese steel with low manganese content consists of ferrite with alternating layers, retained austenite and a small amount of tempered martensite, wherein the volume fraction of the retained austenite can reach up to 24 percent, and the content of ferrite can reach up to 65 percent; the low-manganese medium manganese steel has excellent mechanical property, and the product of strength and elongation is 32-43 GPa.
The technical scheme has the following advantages or beneficial effects:
the invention designs the components of the low-manganese-content medium manganese steel by reducing the manganese content and adding a small amount of aluminum and silicon elements, and obtains the high-strength-ductility low-manganese-content medium manganese steel by a double-phase-zone annealing and low-temperature carbon diffusion double-step carbon distribution process.
The design idea of the low-manganese-content medium manganese steel is as follows:
1) Considering the problem that the pure reduction of manganese content can cause poor thermal stability of dual-phase region reverse transformation austenite, so that the medium manganese steel cannot obtain enough content of residual austenite at room temperature or cannot obtain residual austenite at all, and thus cannot obtain excellent mechanical properties, the invention reduces the manganese content on the basis of the chemical components of the conventional medium manganese steel, utilizes the characteristic that Al element promotes C element and Mn element to be distributed into the dual-phase region austenite from ferrite in the dual-phase region annealing process, improves the thermal stability of super-cooled austenite, and further obtains enough content of residual austenite at room temperature; in the prior art, the main function of the Al element in the medium manganese steel, particularly in the Fe-Mn-Al-C series low-density medium manganese steel is to reduce weight;
2) Although the addition of a small amount of Al is beneficial to the medium manganese steel to obtain the residual austenite with sufficient content at room temperature, the mechanical stability of the residual austenite is low because the content of the manganese is low, which is unfavorable for the mechanical property of the medium manganese steel; according to the invention, by utilizing the characteristic that Si element can inhibit carbide precipitation in the low-temperature heating process, the secondary carbon distribution effect of carbon is achieved by performing low-temperature carbon diffusion on the medium manganese steel after dual-phase zone annealing, so that the residual austenite is further enriched with carbon, the mechanical stability of the medium manganese steel is improved, and the product of strength and elongation of the medium manganese steel with low manganese content is greatly improved;
the low-manganese-content medium manganese steel provided by the invention has the advantages of low manganese content, simple preparation process, short dual-phase region annealing time and low-temperature carbon diffusion time, and can improve the production efficiency and reduce the production cost.
Drawings
FIG. 1 is a stress-strain diagram of a low-manganese medium-manganese steel obtained in example 1 of the present invention.
FIG. 2 is a scanning electron microscope image of the microstructure of a low-manganese medium-manganese steel obtained in example 1 of the present invention.
FIG. 3 is a scanning electron microscope image of the microstructure of a low-manganese medium-manganese steel obtained in example 2 of the present invention.
FIG. 4 is a scanning electron micrograph of the microstructure of a low manganese medium manganese steel obtained in example 3 of the present invention.
Detailed Description
The following examples are only a part of the present invention, and not all of them. Thus, the detailed description of the embodiments of the present invention provided below is not intended to limit the scope of the invention as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the invention without inventive step, are within the scope of protection of the invention.
In the present invention, all the equipment and materials are commercially available or commonly used in the industry, if not specified. The methods in the following examples are conventional in the art unless otherwise specified.
Example 1:
in the embodiment, the manganese steel with low manganese content comprises the following chemical components in percentage by mass: 0.38% of carbon, 2.6% of manganese, 1.5% of aluminum, 1.5% of silicon and the balance of iron and inevitable impurities.
The preparation process comprises the following steps:
1) Smelting: alloy proportioning is carried out according to the chemical components, a vacuum induction furnace is adopted for smelting, molten steel is cast into ingots, and the ingots are slowly cooled to room temperature;
2) Forging and hot rolling: after dead heads and surface oxide skin of the cast ingots are removed, the cast ingots are subjected to heat preservation at 1250 ℃ for 2 hours for homogenization annealing treatment, and are taken out of the furnace and then forged, wherein the open forging temperature is 1200 ℃, and the finish forging temperature is 940 ℃; then air-cooling to room temperature, wherein the thickness of the forging stock is 35mm; then preserving the heat of the forging stock at 1200 ℃ for 0.5h, taking out the forging stock from the furnace, hot-rolling the forging stock into a hot-rolled plate with the thickness of 4mm, wherein the initial rolling temperature is 1150 ℃, the final rolling temperature is 900 ℃, and finally air-cooling the forging stock to the room temperature;
3) Pre-quenching: heating the hot rolled plate to 950 ℃, preserving heat for 20min for complete austenitizing, and then carrying out oil quenching and cooling to room temperature to obtain a martensite initial structure;
4) Annealing in a two-phase region: heating the pre-quenched sample to 745 ℃, preserving the temperature for 50min, carrying out dual-phase region annealing, and then carrying out oil quenching and cooling to room temperature;
5) Low-temperature carbon diffusion: and (3) heating the sample annealed in the two-phase region to 400 ℃, preserving the temperature for 20min, performing low-temperature carbon diffusion and secondary carbon distribution treatment, and then performing oil quenching and cooling to room temperature.
FIG. 1 is a stress-strain diagram of a manganese steel with a low manganese content obtained in the examples; FIG. 2 is a scanning electron micrograph of the microstructure of the low manganese medium manganese steel obtained in this example. The low-manganese-content medium manganese steel obtained in the embodiment contains 22% of retained austenite by volume fraction, the tensile strength is 1128MPa, the yield strength is 725MPa, the elongation is 38%, and the product of strength and elongation reaches 43GPa%, so that the level of the product of strength and elongation of the medium manganese steel with high manganese content is reached.
Example 2
In the embodiment, the low-manganese medium-manganese steel comprises the following chemical components in percentage by mass: 0.48% of carbon, 2.1% of manganese, 1.0% of aluminum, 1.0% of silicon and the balance of iron and inevitable impurities.
The preparation process comprises the following steps:
1) Smelting: alloy proportioning is carried out according to the chemical components, a vacuum induction furnace is adopted for smelting, molten steel is cast into ingots and is slowly cooled to room temperature;
2) Forging and hot rolling: removing dead heads and surface oxide skins of the cast ingots, then preserving heat at 1200 ℃ for 3 hours to carry out homogenization annealing treatment, and forging after discharging, wherein the open forging temperature is 1200 ℃, and the finish forging temperature is 900 ℃; then air-cooling to room temperature, wherein the thickness of the forging stock is 35mm; then preserving the heat of the forging stock at 1200 ℃ for 0.5h, taking out the forging stock from the furnace, hot-rolling the forging stock into a hot-rolled plate with the thickness of 4mm, wherein the initial rolling temperature is 1150 ℃, the final rolling temperature is 900 ℃, and finally air-cooling the forging stock to the room temperature;
3) Pre-quenching: heating the hot rolled plate to 920 ℃, preserving heat for 30min for complete austenitizing, and then quenching and cooling the hot rolled plate to room temperature to obtain a martensite initial structure;
4) Annealing in a two-phase region: heating the pre-quenched sample to 785 ℃, preserving heat for 8min, performing dual-phase region annealing, and then performing oil quenching and cooling to room temperature;
5) Low-temperature carbon diffusion: and (3) heating the sample after the two-phase region annealing to 320 ℃, preserving the temperature for 40min, performing low-temperature carbon diffusion and secondary carbon distribution treatment, and then cooling the sample to room temperature in air.
FIG. 3 is a scanning electron micrograph of the microstructure of the low manganese medium manganese steel obtained in this example. The low-manganese-content medium manganese steel obtained in the embodiment contains 24% of retained austenite by volume fraction, the tensile strength is 1228MPa, the yield strength is 848MPa, the elongation is 33.5%, and the product of strength and elongation reaches 41GPa%, so that the product of strength and elongation of the medium manganese steel with high manganese content is reached.
Example 3
In the embodiment, the low-manganese medium-manganese steel comprises the following chemical components in percentage by mass: 0.26% of carbon, 3.4% of manganese, 2.2% of aluminum, 1.8% of silicon and the balance of iron and inevitable impurities.
The preparation process comprises the following steps:
1) Smelting: alloy proportioning is carried out according to the chemical components, a vacuum induction furnace is adopted for smelting, molten steel is cast into ingots and is slowly cooled to room temperature;
2) Forging and hot rolling: removing dead heads and surface oxide skins of the cast ingots, then carrying out heat preservation at 1200 ℃ for 2.5h for carrying out homogenization annealing treatment, discharging and then forging, wherein the open forging temperature is 1200 ℃, and the finish forging temperature is 920 ℃; then air-cooling to room temperature, wherein the thickness of the forging stock is 30mm; then preserving the heat of the forging stock at 1200 ℃ for 0.5h, taking the forging stock out of the furnace, hot-rolling the forging stock into a hot-rolled plate with the thickness of 4mm, wherein the initial rolling temperature is 1150 ℃, the final rolling temperature is 900 ℃, and finally air-cooling the forging stock to the room temperature;
3) Pre-quenching: heating the hot rolled plate to 970 ℃, preserving heat for 10min for complete austenitizing, and then quenching and cooling the hot rolled plate to room temperature to obtain a martensite initial structure;
4) Two-phase region annealing: heating the pre-quenched sample to 765 ℃, preserving heat for 15min, carrying out dual-phase region annealing, and then carrying out oil quenching and cooling to room temperature;
5) Low-temperature carbon diffusion: and (3) heating the sample annealed in the two-phase region to 360 ℃, preserving heat for 30min, performing low-temperature carbon diffusion and secondary carbon distribution treatment, and then performing water quenching and cooling to room temperature.
FIG. 4 is a scanning electron micrograph of the microstructure of the low manganese medium manganese steel obtained in this example. The manganese steel with low manganese content obtained in the embodiment contains 20% of retained austenite by volume fraction, and has the tensile strength of 1185MPa, the yield strength of 812MPa, the elongation of 35% and the product of strength and elongation of 41.4GPa%.
Example 4
In the embodiment, the low-manganese medium-manganese steel comprises the following chemical components in percentage by mass: 0.34% of carbon, 3.0% of manganese, 1.8% of aluminum, 0.6% of silicon and the balance of iron and inevitable impurities.
The preparation process comprises the following steps:
1) Smelting: alloy proportioning is carried out according to the chemical components, a vacuum induction furnace is adopted for smelting, molten steel is cast into ingots and is slowly cooled to room temperature;
2) Forging and hot rolling: removing dead heads and surface oxide skins of the cast ingots, then preserving heat at 1200 ℃ for 3 hours to carry out homogenization annealing treatment, and forging after discharging, wherein the open forging temperature is 1200 ℃, and the finish forging temperature is 900 ℃; then air-cooling to room temperature, wherein the thickness of the forging stock is 40mm; then preserving the heat of the forging stock at 1200 ℃ for 0.5h, taking the forging stock out of the furnace, hot-rolling the forging stock into a hot-rolled plate with the thickness of 4mm, wherein the initial rolling temperature is 1150 ℃, the final rolling temperature is 900 ℃, and finally air-cooling the forging stock to the room temperature;
3) Pre-quenching: heating the hot rolled plate to 950 ℃, preserving heat for 20min for complete austenitizing, and then carrying out oil quenching and cooling to room temperature to obtain a martensite initial structure;
4) Two-phase region annealing: heating the pre-quenched sample to 730 ℃, preserving heat for 100min, carrying out dual-phase region annealing, and then quenching and cooling the sample to room temperature;
5) Low-temperature carbon diffusion: and (3) heating the sample annealed in the two-phase region to 280 ℃ and preserving heat for 80min, performing low-temperature carbon diffusion and secondary carbon distribution treatment, and then performing water quenching and cooling to room temperature.
The manganese steel with low manganese content obtained in the embodiment contains 12% of residual austenite by volume fraction, the tensile strength of the manganese steel is 1090MPa, the yield strength of the manganese steel is 650MPa, the elongation of the manganese steel is 36%, and the product of strength and elongation of the manganese steel is 39.2GPa%, so that the requirements of third-generation advanced high-strength steel on the strength, plasticity and product of strength and elongation of the manganese steel are met.
The foregoing is only a preferred embodiment of the present invention. It should be noted that: it will be apparent to those skilled in the art that various modifications and adaptations can be made without departing from the principles of the invention and these are intended to be within the scope of the invention.
Claims (10)
1. The preparation method of the low-manganese-content medium manganese steel is characterized by comprising the following steps of:
1) Smelting: smelting the alloy ingredients of the manganese steel with low manganese content and then casting the alloy ingredients into ingots;
2) Homogenization treatment and forging: homogenizing the cast ingot obtained in the step 1), and forging into a blank;
3) Hot rolling: heating and insulating the forged forging stock, and then carrying out hot rolling to obtain a hot rolled plate;
4) Pre-quenching: carrying out complete austenitizing treatment on a hot rolled plate obtained by hot rolling, and cooling to obtain a martensite initial structure;
5) Annealing in a two-phase region: heating the pre-quenched sample to Ac 1-Ac 3 temperature for dual-phase zone annealing;
6) Low-temperature carbon diffusion: heating the sample annealed in the two-phase region to 280-400 ℃, preserving the heat for 20-80 min, and obtaining the low-manganese-content medium manganese steel through low-temperature carbon diffusion and secondary carbon distribution;
wherein the manganese content in the low-manganese medium manganese steel is lower than 4% by mass percent.
2. The preparation method of claim 1, wherein the low-manganese medium-manganese steel comprises the following chemical components in percentage by mass: 0.26 to 0.48 percent of carbon, 2.1 to 3.4 percent of manganese, 1.0 to 2.2 percent of aluminum, 0.6 to 2.0 percent of silicon, and the balance of iron and inevitable impurities.
3. The method according to claim 1, wherein in step 1), the smelting is vacuum induction furnace smelting.
4. The method according to claim 1, wherein the homogenization treatment is carried out at 1200-1250 ℃ for 2-3 hours in step 2).
5. The method according to claim 1, wherein the homogenization treatment in step 2) further comprises an operation of removing a riser and a surface scale from the ingot.
6. The method according to claim 1, wherein the forging temperature in step 2) is 1200 to 1250 ℃;
preferably, the forging has a finish forging temperature of not less than 900 ℃.
7. The preparation method according to claim 1, wherein in the step 3), the temperature for heating and heat preservation is 1150-1200 ℃;
preferably, the heating and heat preservation time is 20-40 min;
preferably, the initial rolling temperature of the hot rolling is 1150-1200 ℃;
preferably, the finish rolling temperature of the hot rolling is not lower than 900 ℃.
8. The method according to claim 1, characterized in that in step 4), the complete austenitizing treatment is heating the hot-rolled plate obtained by hot rolling to a temperature higher than Ac3, preferably at a temperature of 920 to 970 ℃, for 10 to 30 min;
preferably, the cooling is water quenching or oil quenching cooling.
9. The method according to claim 1, wherein the heating to the Ac 1-Ac 3 temperature in step 5) is performed to a temperature of 730-800 ℃;
preferably, the heating time is 8-100 min;
preferably, step 5) further comprises a cooling process after the two-phase zone annealing; the cooling is water quenching or oil quenching.
10. The method according to claim 1, wherein the step 6) further comprises a cooling process; the cooling is water quenching or oil quenching or natural cooling in air.
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