A method for manufacturing a high-toughness alloy structural steel plate

Abstract

The application discloses a preparation method of high-toughness alloy structural steel plate. The material production process path of the method is as follows: hot metal desulfurization pretreatment, top and bottom combined blowing converter smelting, LF refining, RH vacuum treatment, slab continuous casting, slab hot charging and heating, continuous rolling, cooling, coiling and collecting, and packaging. The component mass percentage of the alloy structural steel plate is as follows: C: 0.55% to 0.57%, Si: 0.26% to 0.36%, Mn: 1.70% to 1.80%, V: 0.15% to 0.16%, Cr: 0.36% to 0.40%, impurity element P: ≤0.015%, S: ≤0.003%, Cu: ≤0.05%, Ni: ≤0.05%, N: ≤0.0040%, H: ≤0.0001%, and the balance is Fe and inevitable impurities.

Description

Technical Field

[0001] This invention relates to a method for preparing a high-toughness alloy structural steel plate, belonging to the field of iron and steel metallurgy and material forming. Background Technology

[0002] With the development of industries such as mining, construction, stone processing, and ceramics, the demand for high-strength, thermally shock resistant, fatigue-resistant, and high-toughness alloy steel plates is rapidly increasing. The quality requirements for spring sheets and mold steel of this quality are becoming increasingly stringent. The 50Mn2V alloy structural steel plate used in the market, after heat treatment, should possess good matrix strength, wear resistance, and weldability, be easy to level, and be used in the manufacture of saw blade bodies, lightweight molds, springs, etc. It should have good surface quality and plate shape, high dimensional accuracy, and good hardenability. However, the strength of 50Mn2V steel plate is relatively low. To improve the steel's oxidation resistance and creep strength, and to enhance its hardenability and quenchability, the chromium content in the steel is increased. However, a high chromium content will lead to bright bands or network defects caused by chromium segregation after steel treatment, resulting in uneven microstructure and reduced material service life.

[0003] Hardness and wear resistance: For saw blades, high hardness is required to effectively cut hard materials. Simultaneously, the steel must possess good wear resistance to extend the saw blade's lifespan. High-strength saw blade steel needs to possess the following characteristics.

[0004] Toughness: Saw blades are subjected to impact and vibration during use, so the material needs to have a certain degree of toughness to prevent fatigue fracture;

[0005] High temperature resistance: When cutting hard materials such as stone, the saw blade generates a lot of heat, so the material needs to be able to withstand high temperatures to avoid deformation or damage;

[0006] Excellent cutting performance: The saw blade material should have excellent cutting performance, enabling it to cut hard materials such as corundum and stone quickly and smoothly, thereby improving work efficiency;

[0007] Cost: Considering cost factors, saw blade materials need to have the lowest possible manufacturing cost while meeting performance requirements.

[0008] Literature search:

[0009] Patent 1, a steel for the base of a large-diameter thin saw blade (CN106319378A), discloses the following composition by mass percentage: C: 0.72%–0.95%, Si: 0.06%–0.40%, Mn: 0.5%–0.95%, Cr: 0.10%–0.39%, V: 0.03%–0.080%, Ni≤0.5%, and H≤0.0002%, with impurity elements P≤0.015%, S≤0.010%, and Al≤0.010%. The molten steel undergoes Si deoxidation, and the slab is hot-charged and hot-fed. The initial rolling temperature for finishing is 1050–1150℃, and the final rolling temperature is 900–980℃. Laminar flow cooling is used in the later stage, cooling to 680–800℃ before coiling. After coiling, the steel is slowly cooled in a slow-cooling pit within 30 minutes. After the hot-rolled strip is cold-stamped into the saw blade base, it is heated to 850–980℃, held for 10–40 minutes, and then quenched. The material produced by this technology has advantages such as high hardness, but the excessively high carbon content leads to poor plasticity and toughness, high brittleness, poor fatigue resistance of the saw blade, and defects such as easy breakage and short service life during operation.

[0010] Patent 2, a method for producing high-strength saw blade steel coils (CN103173685B), with the following composition: C: 0.49~0.52, Si: 0.2~0.35, Mn: 1.45~1.60, P: ≤0.015, S: ≤0.005, Cr: 0.2~0.25, V: 0.12~0.16, Al: 0.020~0.025, N: ≤0.006; production process. The process involves converter smelting, LF refining, RH vacuum treatment, and hot continuous rolling, including slab continuous casting and segregation control in saw blade steel production, control of the slow cooling temperature of the slab continuous casting line, control of the initial and final rolling temperatures, and control of the coiling temperature. The resulting saw blade steel coils exhibit a transverse yield strength of 600–750 MPa, a transverse tensile strength of 930–1040 MPa, an elongation at break of 19.5–26%, and a hardness of HRC 20–28. Summary of the Invention

[0011] The purpose of this invention is to solve the above-mentioned problems by providing a method for preparing high-toughness alloy structural steel plates, which can produce alloy structural steel with high strength, high hardness, high plasticity and toughness and long service life.

[0012] The technical solution adopted in this invention is a method for preparing a high-toughness alloy structural steel plate. The material production process of this method includes: hot metal desulfurization pretreatment, top and bottom blowing converter smelting, LF refining, RH vacuum treatment, slab continuous casting, slab hot charging heating, continuous rolling, cooling, coiling and collection, and packaging. The composition of the alloy structural steel plate by mass percentage is as follows: C: 0.55%~0.57%, Si: 0.26%~0.36%, Mn: 1.70%~1.80%, V: 0.15%~0.16%, Cr: 0.36%~0.40%, impurity elements P≤0.015%, S≤0.003%, Cu≤0.05%, Ni:≤0.05%, N:≤0.0040%, H:≤0.0001%, with the balance being Fe and unavoidable impurities.

[0013] In converter smelting, the sulfur content of the molten iron after desulfurization pretreatment is no more than 0.005%. The converter adopts the double-slag method for deep dephosphorization of the molten iron. The method is as follows: the high-phosphorus converter slag from the initial dephosphorization is poured out of the converter, and then a slagging agent made of lime and iron oxide scale is added. The slagging agent may be replaced by calcium ferrite sinter for a second slagging. In order to achieve deep dephosphorization, the temperature of the molten steel needs to be reduced. The molten steel is cooled to below 1430°C by adding scrap steel. Later, due to the low carbon and silicon content, ferrosilicon from the Ouye furnace is added to supplement the heat and further oxygen blowing dephosphorizes the steel. The phosphorus content of the steel tapped from the converter is no more than 0.010%, and the sulfur content is no more than 0.008%.

[0014] The alloying process for converter steelmaking involves a two-step deoxidation method. First, carbon-manganese balls are added for pre-deoxidation alloying. The composition of the carbon-manganese balls is: manganese content 12.0~12.5% ​​and carbon content 67.0~67.5%. The purpose is to deoxidize and increase carbon content. The carbon recovery rate in the steel is 40~50%. After the converter steelmaking process ends, aluminum wire is fed into the ladle molten steel at the argon blowing station for deep deoxidation.

[0015] The LF refining slag is a five-element slag system consisting of CaO—Al2O3—SiO2—CaF2—MgO. LF is smelted in two stages. The first stage involves initial smelting upon arrival at the station until deep desulfurization is completed. First, aluminum particles encased in thin-walled steel pipes are added to the ladle top slag to reduce it. Then, submerged arc welding and foaming materials are added, and electricity is supplied to raise the temperature for smelting. The refining slag composition is: CaO 60–65%, SiO2 8–10%, Al2O3 18–22%, CaF2 ≤3%, MgO 3–5%, with a processing time not exceeding 25 minutes. When the sulfur content in the molten steel... If the content is <0.0025%, proceed to the second stage. In the second stage, adjust the viscosity and alkalinity of the refining slag to facilitate the flotation, adsorption, and removal of inclusions in the molten steel. This is achieved by adding 1-1.2 kg / t of slag conditioner to the LF refining top slag. The conditioner is a mixture of high-alumina bauxite, active lime, and fluorite, sintered at high temperatures. Its composition is 70-75% Al2O3, 7-8% CaO, 10-12% CaF2, with the remainder being unavoidable impurities such as Fe2O3, TiO2, and MgO. Argon is blown into the ladle for stirring during the refining process, with weak stirring performed at the end of the refining process.

[0016] Slab continuous casting: The width of the continuously cast slab is 980-1200mm and the thickness is 220mm. The liquidus temperature of this material is low (1479℃) and the carbon content is high. The shell of the slab is brittle and the casting speed is relatively slow. Since the casting speed of the slab of this material is controlled at 0.9±0.05m / min, an aluminum carbon long-life submersible nozzle is used during casting to avoid changing the nozzle as much as possible and prevent the slab from freezing due to the reduction of casting speed.

[0017] Solutions to segregation in continuously cast slabs: Due to the high carbon, manganese, and other elements in the composition of this material, the following measures are taken to control segregation in the center of the slab:

[0018] During the continuous casting process, the superheat of the molten steel must be strictly controlled within the specified range. The superheat of the molten steel in the tundish is 20-28℃, and the casting speed is 1.10-1.2m / min. Light pressure on the slab is strictly prohibited during the continuous casting process of this material.

[0019] During continuous casting, a steel plate with the same composition as this steel grade is inserted into the crystallizer to lower the temperature of the molten pool and increase the supercooling. The steel plate is inserted 25-30 mm from the narrow face of the crystallizer and 25-30 mm from the crystallizer nozzle. The steel plate is 3-4 mm thick and inserted vertically into the molten steel on both sides of the nozzle, with the thick section of the steel plate in contact with the molten steel. The insertion position is on the centerline of the crystallizer's width direction. The steel plate material comes from the head and tail billets of this steel grade, which are rolled to a thickness of 3-4 mm and then longitudinally cut into three equal parts into steel strips. For example, a steel coil with a width of 1200 mm is longitudinally cut into steel strips with a width of 400 mm, and then transversely cut into steel plates with a length of 300-400 mm. Additionally, the cut ends from the transverse cutting process are processed into the required materials according to the above specifications.

[0020] [3] After the continuous casting slab is cut, it should be sent to the hot rolling slab warehouse for heat preservation and collection. It needs to be hot-rolled within 3 hours. Therefore, when the tail slab is produced, the drawing speed should be increased as soon as possible to avoid cracking when the slab is straightened.

[0021] Billet Heating: The slab is hot-charged and rolled, with an initial furnace temperature exceeding 630℃. To ensure uniform and thorough heating while avoiding excessively high temperatures that could lead to severe oxidation and decarburization, affecting the surface hardness of the heat-treated steel plate, the heating temperatures are: slab heating temperature 1180–1220℃, rough rolling return temperature 1075±15℃, final rolling temperature 980±10℃, and coiling temperature 720±15℃. For precise control of the coiling temperature, the fine-tuning section for cooling water is activated.

[0022] Because of the high strength and hardness of this material, increasing the rolling temperature reduces the rolling force, which helps to reduce the rolling load and improve the quality of the sheet shape.

[0023] The slab is kept warm for 1.5 to 4 hours, and the air-fuel ratio is controlled at (1.2 to 1.4):1 in the steel burning atmosphere. The total decarburization on both sides of the steel plate is not greater than 1.8% of the steel plate thickness.

[0024] Rolling: High-pressure water descaling is used for both roughing and finishing to ensure the surface quality of the finished steel plate; the initial rolling temperature of finishing is 1080℃~1110℃, and the final rolling temperature is controlled at 970℃±10℃.

[0025] Cooling: The steel sheet is cooled to 730℃±15℃ using two sets of intermittent sparse laminar flow cooling methods before coiling. After coiling, the steel sheet is placed upright on a hot steel coil for slow cooling. The surrounding steel coils are carbon steel or other types of hot steel coils. The surface temperature of the hot steel coils is not less than 200℃, and the air cooling rate of the material is not greater than 12℃ / h. The purpose is to reduce the temperature difference between the inner and outer rings of the steel coil.

[0026] The material properties after heat treatment are as follows: the quenching temperature is 838±2℃, the quenching medium is oil, the tempering temperature is 445±2℃, the tempering medium is air cooling, and the material microstructure is tempered troostite as observed under a 500x microscope.

[0027] .

[0028] Carbon (C): 0.55%–0.57%. Carbon is the main solid solution strengthening element in steel. Increasing the carbon content in steel will increase both its strength and hardness. However, too high a carbon content will increase the carbon segregation level in the slab, leading to uneven steel structure and affecting the material's service life. It is also detrimental to welding and plasticity. To ensure that the steel has excellent toughness and plasticity after quenching and tempering, comparative experiments and usage verification have determined the carbon content in the steel to be 0.55%–0.57%.

[0029] Si: 0.20%–0.36%. Silicon can improve the strength of materials: firstly, silicon can significantly increase the yield strength of medium carbon steel; secondly, silicon can increase the hardness and wear resistance of steel; thirdly, silicon can reduce the coefficient of thermal expansion of steel, making it less prone to deformation at high temperatures; and fourthly, silicon can improve the corrosion resistance of steel. However, excessively high silicon content will cause a rapid decrease in the plasticity of steel. Therefore, this invention determines the silicon content to be 0.26%–0.36% based on the service environment of the material.

[0030] Mn: 1.70%~1.80%. Based on the manganese content of 50Mn2V steel, the manganese content in the steel is significantly increased, so that the material has high strength, hardness and wear resistance after heat treatment. The determined manganese content is 1.70%~1.80%.

[0031] The material prepared by this invention is based on medium carbon and high manganese steel design, with the addition of trace amounts of V and Cr elements to improve the hardenability of the material. This solves the problem of insufficient hardenability depth in thick steel plates caused by the addition of a single element V, and avoids excessively high strength in hot-rolled steel plates, which increases rolling force, increases the difficulty of controlling the convexity and shape of the steel plates, and affects the machinability of the products.

[0032] V: Vanadium can form stable compounds with carbon in steel, refining the microstructure and grain size. Adding a small amount of vanadium to steel can significantly improve its strength and hardness while maintaining good toughness and plasticity. Vanadium can also improve the hardenability of steel, increasing the cooling rate during quenching and resulting in higher tensile strength. Furthermore, it can reduce the temper brittleness of steel. Therefore, after optimized design, the vanadium content added to the steel was determined to be 0.15–0.16%.

[0033] Cr: Cr can strengthen α solid solution and change the precipitation morphology and type of carbides. As an effective element to improve solid solution, it can improve the hardness and wear resistance of materials, and can also significantly improve the hardenability, quenchability and corrosion resistance of materials. At the same time, it is necessary to consider that high chromium content will cause segregation in the center of the billet, resulting in the appearance of bright bands in the rolled material. Scientific design of the chromium content in steel can make the heat-treated steel take into account the above characteristics. Therefore, the chromium content in the material determined in this invention is 0.36% to 0.40%.

[0034] P: ≤0.015%, S: ≤0.003%. In steel, P and S are unavoidable harmful impurities and are the main elements causing center segregation in continuously cast billets and banded structures in steel plates. Sulfur and phosphorus in steel will seriously deteriorate the toughness of steel. Therefore, the methods of dephosphorization by converter double slag retention, desulfurization by hot metal pretreatment, and deep desulfurization by LF are adopted to reduce the P and S content in steel. It is determined that the P content in steel is not greater than 0.015% and the S content is not greater than 0.003%. Implementation

[0035] A method for preparing a high-toughness alloy structural steel plate, comprising the following material production process: hot metal desulfurization pretreatment, top and bottom blowing converter smelting, LF refining, RH vacuum treatment, slab continuous casting, slab hot charging heating, continuous rolling, cooling, coiling and collection, and packaging; the alloy structural steel plate has the following composition by mass percentage: C: 0.55%–0.57%, Si: 0.26%–0.36%, Mn: 1.70%–1.80%, V: 0.15%–0.16%, Cr: 0.36%–0.40%, impurity elements P≤0.015%, S≤0.003%, Cu≤0.05%, Ni:≤0.05%, N:≤0.0040%, H:≤0.0001%, with the balance being Fe and unavoidable impurities;

[0036] In converter smelting, the sulfur content of the molten iron after desulfurization pretreatment is no more than 0.005%. The converter adopts the double-slag method for deep dephosphorization of the molten iron. The method is as follows: the high-phosphorus converter slag from the initial dephosphorization is poured out of the converter, and then a slagging agent made of lime and iron oxide scale is added. The slagging agent may be replaced by calcium ferrite sinter for a second slagging. In order to achieve deep dephosphorization, the temperature of the molten steel needs to be reduced. The molten steel is cooled to below 1430°C by adding scrap steel. Later, due to the low carbon and silicon content, ferrosilicon from the Ouye furnace is added to supplement the heat and further oxygen blowing dephosphorizes the steel. The phosphorus content of the steel tapped from the converter is no more than 0.010%, and the sulfur content is no more than 0.008%.

[0037] The alloying process for converter steelmaking involves a two-step deoxidation method. First, carbon-manganese balls are added for pre-deoxidation alloying. The composition of the carbon-manganese balls is: manganese content 12.0~12.5% ​​and carbon content 67.0~67.5%. The purpose is to deoxidize and increase carbon content. The carbon recovery rate in the steel is 40~50%. After the converter steelmaking process ends, aluminum wire is fed into the ladle molten steel at the argon blowing station for deep deoxidation.

[0038] The LF refining slag is a five-element slag system consisting of CaO—Al2O3—SiO2—CaF2—MgO. LF is smelted in two stages. The first stage involves initial smelting upon arrival at the station until deep desulfurization is completed. First, aluminum particles encased in thin-walled steel pipes are added to the ladle top slag to reduce it. Then, submerged arc welding and foaming materials are added, and electricity is supplied to raise the temperature for smelting. The refining slag composition is: CaO 60–65%, SiO2 8–10%, Al2O3 18–22%, CaF2 ≤3%, MgO 3–5%, with a processing time not exceeding 25 minutes. When the sulfur content in the molten steel... If the content is <0.0025%, proceed to the second stage. In the second stage, adjust the viscosity and alkalinity of the refining slag to facilitate the flotation, adsorption, and removal of inclusions in the molten steel. This is achieved by adding 1-1.2 kg / t of slag conditioner to the LF refining top slag. The conditioner is a mixture of high-alumina bauxite, active lime, and fluorite, sintered at high temperatures. Its composition is 70-75% Al2O3, 7-8% CaO, 10-12% CaF2, with the remainder being unavoidable impurities such as Fe2O3, TiO2, and MgO. Argon is blown into the ladle for stirring during the refining process, with weak stirring performed at the end of the refining process.

[0039] Slab continuous casting: The width of the continuously cast slab is 980-1200mm and the thickness is 220mm. The liquidus temperature of this material is low (1479℃) and the carbon content is high. The shell of the slab is brittle and the casting speed is relatively slow. Since the casting speed of the slab of this material is controlled at 0.9±0.05m / min, an aluminum carbon long-life submersible nozzle is used during casting to avoid changing the nozzle as much as possible and prevent the slab from freezing due to the reduction of casting speed.

[0040] Solutions to segregation in continuously cast slabs: Due to the high carbon, manganese, and other elements in the composition of this material, the following measures are taken to control segregation in the center of the slab:

[0041] During the continuous casting process, the superheat of the molten steel must be strictly controlled within the specified range. The superheat of the molten steel in the tundish is 20-28℃, and the casting speed is 1.10-1.2m / min. Light pressure on the slab is strictly prohibited during the continuous casting process of this material.

[0042] During continuous casting, a steel plate with the same composition as this steel grade is inserted into the crystallizer to lower the temperature of the molten pool and increase the supercooling. The steel plate is inserted 25-30 mm from the narrow face of the crystallizer and 25-30 mm from the crystallizer nozzle. The steel plate is 3-4 mm thick and inserted vertically into the molten steel on both sides of the nozzle, with the thick section of the steel plate in contact with the molten steel. The insertion position is on the centerline of the crystallizer's width direction. The steel plate material comes from the head and tail billets of this steel grade, which are rolled to a thickness of 3-4 mm and then longitudinally cut into three equal parts into steel strips. For example, a steel coil with a width of 1200 mm is longitudinally cut into steel strips with a width of 400 mm, and then transversely cut into steel plates with a length of 300-400 mm. Additionally, the cut ends from the transverse cutting process are processed into the required materials according to the above specifications.

[0043] [3] After the continuous casting slab is cut, it should be sent to the hot rolling slab warehouse for heat preservation and collection. It needs to be hot-rolled within 3 hours. Therefore, when the tail slab is produced, the drawing speed should be increased as soon as possible to avoid cracking when the slab is straightened.

[0044] Billet Heating: The slab is hot-charged and rolled, with an initial furnace temperature exceeding 630℃. To ensure uniform and thorough heating while avoiding excessively high temperatures that could lead to severe oxidation and decarburization, affecting the surface hardness of the heat-treated steel plate, the heating temperatures are: slab heating temperature 1180–1220℃, rough rolling return temperature 1075±15℃, final rolling temperature 980±10℃, and coiling temperature 720±15℃. For precise control of the coiling temperature, the fine-tuning section for cooling water is activated.

[0045] Because of the high strength and hardness of this material, increasing the rolling temperature reduces the rolling force, which helps to reduce the rolling load and improve the quality of the sheet shape.

[0046] The slab is kept warm for 1.5 to 4 hours, and the air-fuel ratio is controlled at (1.2 to 1.4):1 in the steel burning atmosphere. The total decarburization on both sides of the steel plate is not greater than 1.8% of the steel plate thickness.

[0047] Rolling: High-pressure water descaling is used for both roughing and finishing to ensure the surface quality of the finished steel plate; the initial rolling temperature of finishing is 1080℃~1110℃, and the final rolling temperature is controlled at 970℃±10℃.

[0048] Cooling: The steel sheet is cooled to 730℃±15℃ using two sets of intermittent sparse laminar flow cooling methods before coiling. After coiling, the steel sheet is placed upright on a hot steel coil for slow cooling. The surrounding steel coils are carbon steel or other types of hot steel coils. The surface temperature of the hot steel coils is not less than 200℃, and the air cooling rate of the material is not greater than 12℃ / h. The purpose is to reduce the temperature difference between the inner and outer rings of the steel coil.

[0049] The material properties after heat treatment are as follows: the quenching temperature is 838±2℃, the quenching medium is oil, the tempering temperature is 445±2℃, the tempering medium is air cooling, and the material microstructure is tempered troostite as observed under a 500x microscope.

Claims

1. A method for preparing a high-toughness alloy structural steel plate, comprising the following process: hot metal desulfurization pretreatment, top and bottom blowing converter smelting, LF refining, RH vacuum treatment, slab continuous casting, slab hot charging heating, continuous rolling, cooling, coiling and collection, and packaging; wherein the alloy structural steel plate comprises the following composition by mass percentage: C: 0.55%–0.57%, Si: 0.26%–0.36%, Mn: 1.70%–1.80%, V: 0.15%–0.16%, Cr: 0.36%–0.40%, impurity elements P≤0.015%, S≤0.003%, Cu≤0.05%, Ni:≤0.05%, N:≤0.0040%, H:≤0.0001%, with the balance being Fe and unavoidable impurities; In converter smelting, the sulfur content of the molten iron after desulfurization pretreatment is no more than 0.005%. The converter employs a double-slag method for deep dephosphorization of the molten iron. This involves removing the high-phosphorus converter slag from the converter after the initial dephosphorization, then adding a slagging agent made from lime and iron oxide scale (or calcium ferrite sinter) for a second slagging process. The resulting steel from the converter has a phosphorus content of no more than 0.010% and a sulfur content of no more than 0.008%. For alloying, a two-step deoxidation process is used. First, carbon-manganese balls are added for pre-deoxidation alloying. The carbon-manganese balls have a composition of 12.0–12.5% ​​manganese and 67.0–67% carbon. 0.5%, carbon recovery rate in steel is 40-50%. After the converter tapping, aluminum wire is fed into the ladle molten steel at the argon blowing station for deep deoxidation. LF is smelted in two stages. The first stage is from initial smelting to completion of deep desulfurization. First, aluminum particles wrapped in thin-walled steel pipes are added to the ladle top slag to reduce it. Then, submerged arc and foaming materials are added, and electricity is supplied to raise the temperature for smelting. The composition of the refined slag is: CaO content 60-65%, SiO2 content 8-10%, Al2O3 content 18-22%, CaF2 content ≤3%, MgO content 3-5%, and the processing time is no more than 25 minutes. When the sulfur content in the molten steel is <0.0025%, the second stage begins. In the second stage, the viscosity and basicity of the refining slag are adjusted by adding 1-1.2 kg / t of slag conditioner to the LF refining top slag. The slag conditioner is a mixture of high-alumina bauxite, active lime and fluorite sintered at high temperature. Its composition is 70-75% Al2O3, 7-8% CaO, 10-12% CaF2, and the remainder is Fe2O3, TiO2, MgO and unavoidable impurities. Argon gas is blown from the bottom of the ladle for stirring during the refining process, and weak stirring with argon gas is carried out at the end of the refining process. Slab continuous casting, slab width 980~1200mm, slab thickness 220mm, liquid phase temperature of molten steel of this material 1479℃, use aluminum carbon long life submerged entry nozzle during casting, do not change the nozzle to prevent the slab from freezing due to reduced casting speed. The superheat of molten steel in the tundish is controlled within the range of 20–28℃, and the casting speed is controlled within the range of 1.1–1.2 m / min. Light pressure on the slab is strictly prohibited during continuous casting. During continuous casting, a steel plate with the same composition as this steel grade is inserted into the crystallizer to lower the temperature of the molten pool and increase the supercooling. The steel plate is inserted 25-30 mm from the narrow face of the crystallizer and 25-30 mm from the crystallizer nozzle. The steel plate is 3-4 mm thick. The insertion method is that the thickness section of the steel plate contacts the molten steel and is inserted vertically into the molten steel on both sides of the nozzle. The insertion position is on the center line of the width direction of the crystallizer. The steel plate material comes from the head and tail billets of this steel grade, rolled to a thickness of 3-4 mm, and then longitudinally cut into steel strips in three equal parts. After the continuous casting slab is cut, it is immediately sent to the hot rolling slab warehouse for heat preservation and collection. It needs to be hot-charged and hot-rolled within 3 hours. When the tail slab is exited, the drawing speed is increased to avoid cracking during slab straightening. Billet heating: Slab hot charging rolling, slab entry temperature is greater than 630℃, slab heating temperature is 1180~1220℃, rough rolling return temperature is 1075±15℃, final rolling temperature is 980±10℃, coiling temperature is 720±15℃. In order to accurately control the coiling temperature, the fine adjustment section of the cooling water layer is turned on. The slab is kept warm for 1.5 to 4 hours, the air-fuel ratio is controlled at 1.2 to 1.4:1, and the total decarburization on both sides of the steel plate is not greater than 1.8% of the steel plate thickness. Rolling: Both roughing and finishing rolling use high-pressure water descaling. The initial rolling temperature of finishing rolling is 1080℃~1110℃, and the final rolling temperature is controlled at 970℃±10℃. Cooling: Laminar flow cooling adopts an intermittent sparse cooling method with two groups of cooling. The steel plate is coiled after cooling to 730℃±15℃. After coiling, the steel plate is immediately placed on a hot steel coil for slow cooling. The surrounding steel coils are carbon steel or other types of hot steel coils. The surface temperature of the hot steel coil is not less than 200℃. The air cooling rate of the material is not greater than 12℃ / h. Heat treatment requirements: Quenching temperature is 838±2℃, quenching medium is oil, tempering temperature is 445±2℃, tempering medium is air cooling. Under a 500x microscope, the material microstructure is tempered troostite.