CN115896624B - A kind of nitrided steel 31CrMoV9 annealed material and its production method - Google Patents

Abstract

The invention relates to a nitriding steel 31CrMoV9 annealing material and a production method thereof, wherein the steel comprises the chemical components of 0.27-0.33% of C, 0.28-0.36% of Si, 0.63-0.70% of Mn, less than or equal to 0.025% of P, less than or equal to 0.025% of S, 2.43-2.53% of Cr, less than or equal to 0.10% of Ni, 0.17-0.23% of Mo, 0.10-0.16% of V, less than or equal to 0.10% of Cu, 0.015-0.045% of Alt, and the balance of Fe and impurities. The 31CrMoV9 steel produced by the invention can meet the requirement that the steel delivery hardness is not more than 248HBW; tensile strength after quenching and tempering is 1000-1200 MPa, and yield strength is more than or equal to 800MPa; the depth of the total decarburized layer at each side of the steel is not more than 1.0% of the nominal diameter, so that the material has good processing performance.

Description

A kind of nitrided steel 31CrMoV9 annealed material and its production method

Technical field

The invention relates to the technical field of nitrided steel production, and in particular to a nitrided steel 31CrMoV9 annealed material and a production method thereof.

Background technique

31CrMoV9 is a typical grade of nitrided steel and a representative grade in the European Union standard EN10085. After tempering and nitriding treatment, the surface hardness of 31CrMoV9 can reach over 1000 Vickers hardness. It has high hardenability and excellent toughness, wear resistance, "seizure" resistance and corrosion resistance. It is used for manufacturing precision products. Hydraulic pistons, hydraulic piston rods, connecting shafts, heavy-duty large parts, couplings, wind turbine gearbox ring gears, engine drive shafts and other important deep nitriding parts. However, due to the special composition of 31CrMoV9 steel, its hot-rolled structure is complex. Under actual production conditions, the hot-rolled state of 31CrMoV9 steel is a "B+P+M+a small amount of F" structure, with a high hardness (above 300HBW), which is not conducive to Subsequent processing and manufacturing.

At present, there is little information available at home and abroad regarding the related technologies that provide the processing performance of 31CrMoV9 nitrided steel.

The Chinese patent application with application publication number CN109402350A discloses "a heat treatment process for steel", and the steel is selected from one of 42CrMo4, 42CrMo, 34CrNiMo6 and 31CrMoV9. The heat treatment process includes the following steps: 1) Quenching treatment: The temperature is 880-940°C, the holding time is 1-7h, and then cooled to 500-600°C in 35-135s; 2) Tempering treatment: The temperature is 590-630°C, The holding time is 3-8h, and then cooled to room temperature. Heat-treated steel can effectively improve the tensile strength and yield strength of the steel, and can also effectively improve the impact energy of the steel; however, it does not consider the processability of the steel.

The Chinese patent application with application publication number CN106086671A discloses "an alloy nitrided steel for shafts", which contains C: 0.25% ~ 0.35%, Si: 0.2 ~ 0.5%, Mn: 0.5 ~ 1%, Cr in weight percentage : 1.5%～2.5%, Mo: 0.40～0.60%, V: 0.15～0.25%, S: 0.001～0.030%, Al: 0.1～0.5%, Ti: 0.6%～1.2%, Ni: 0.05～0.2%, The remainder includes Fe and inevitable impurities; and the following elements are limited to: P: 0.02% or less, O: 0.06% or less. The preparation method includes: smelting: smelting in an electric arc furnace. The ingredients are molten iron with a weight percentage greater than or equal to 40% and scrap steel with a weight percentage of 40% to 60%. The control endpoints are C≥0.25%, P≤0.020%, and the residual element content complies with The design requirements are that the tapping temperature of molten steel is 1600°C to 1650°C, in which pre-deoxidation and alloying is carried out in the ladle, and Al is added during the tapping process; then, LF refining outside the furnace is carried out, and argon is blown throughout the refining process to maintain the white slag time. 20 to 30 minutes; vacuum treatment is performed after refining, the vacuum degree is less than 67Pa, the holding time is 20 to 25 minutes, and the soft argon blowing time after vacuum treatment is more than 12 minutes; Pouring: continuous casting is used to protect the pouring throughout the casting, and electromagnetic stirring is used during continuous casting; Rolling: The continuous casting billet is slowly cooled and then rolled. The soaking temperature of the heating furnace is controlled to be 1200℃~1240℃, the heating time is 2.5~3 hours, the opening rolling temperature is 1100℃~1110℃, and the final rolling temperature is 860 ℃ ~ 960 ℃; after rolling is completed, the pile is cooled to room temperature; annealing: after being kept at 700-720 ℃ for 12 to 15 hours in the annealing furnace, it is cooled to 520-550 ℃ and released. However, the technical problems it solves are: the high aluminum content in traditional nitrided steel results in many inclusions in the steel, poor surface quality of the steel and a brittle nitrided layer during nitriding heat treatment. As a result, the production of traditional nitrided steel 38CrMoAl is difficult and the number of continuous pouring furnaces is small. Therefore, many steel plants still use the mold casting process for production, making it difficult to control the quality and cost of steel. The 31CrMoV9 described in the present invention belongs to aluminum-free nitrided steel, and the Alt in the steel is only 0.015% to 0.045%, which is far lower than the Al content (0.1 to 0.5%) in the patent application. Therefore, there is no such thing as the aluminum content in the patent application. technical issues.

The Chinese patent application with application publication number CN102443740A discloses "an alloy nitrided steel and its manufacturing method". The mass percentage of each chemical element is: carbon: 0.30~0.36wt%; silicon: 0.60~0.80wt%; manganese : 0.60～0.90wt%; Chromium: 2.90～3.40wt%; Molybdenum: 0.40～0.60wt%; Vanadium: 0.15～0.25wt%; Niobium: 0.02～0.06wt%; Sulfur: ≤0.015wt%; Phosphorus: ≤0.020 wt%; copper: ≤0.15wt%; nickel: ≤0.15wt%; the balance is Fe and other inevitable impurities. The manufacturing method includes the following steps: (1) Smelting: After vacuum induction smelting and electroslag remelting, the raw materials are melted and cast into steel ingots; (2) The steel ingots are heated in a soaking furnace at a heating rate of 80-90°C/h to 800-800°C. After holding at 820°C for 80-100 minutes, heat to 1280-1290°C at a heating rate of 180-200°C/h and then hold for 160-180 minutes; (3) The steel ingot is slowly cooled after rough rolling and then surface finishing; (3) 4) The rolling heating temperature is 1180~1220℃, the soaking temperature is 1160~1200℃, and the total heating time is 135-150 minutes; the final rolling temperature is ≥850℃. After the rolling is completed, the pile is cooled to room temperature; (5) Annealing: Annealing After being kept warm at 700-720℃ for 10 to 15 hours in the furnace, it is cooled to 500-550℃ and released. The main technical problems it solves are: 1. The strength and thermal stability of quenched and tempered 38CrMoAl steel cannot meet the matrix support strength requirements; 2. The nitriding time is too long, not only the cost is high, but the long-term nitriding treatment also reduces the strength of the matrix. the support strength. In addition, 38CrMoAl steel also has the following shortcomings: the hardenability is not high, and it can be quenched in oil only when the interface diameter is less than 30mm. After oil-cooling quenching of parts with a diameter of 60mm, the hardness difference between the surface and the center can reach about 60HB; this The critical point of steel is higher, so its normalizing and quenching temperatures are correspondingly higher; because aluminum-containing ferrite is highly stable and difficult to dissolve into austenite, the holding time is 1.5 times longer than that of general alloy structural steel. About times. In addition, 38CrMoAl steel also has the following problems during metallurgical manufacturing: due to the high aluminum content in the steel, the aluminum element is easily oxidized during smelting, making the yield very unstable, resulting in an extremely high failure rate; 38CrMoAl steel The problem of point-like segregation at low magnification is serious. The alloy nitrided steel produced by this patent application has high hardenability and should have good strength and toughness after heat treatment; after nitriding, compared with aluminum-containing nitrided steel, the nitrided layer depth and core hardness should be higher than those of aluminum-containing nitrided steel. Nitrided steel should have better heat resistance than existing nitrided steel; in addition, the alloy nitrided steel should also have good economy. It can be seen that the technical problem solved by this patent application is completely different from that of the present invention.

Contents of the invention

The invention provides a nitrided steel 31CrMoV9 annealed material and a production method thereof. The produced 31CrMoV9 steel can meet the requirement that the delivery hardness of the steel material does not exceed 248HBW; the tensile strength after quenching and tempering treatment is 1000-1200MPa, and the yield strength is ≥800MPa; The total decarburized layer depth (ferrite + transition layer) on each side of the steel does not exceed 1.0% of the nominal diameter, ensuring the material has good processing performance.

In order to achieve the above objects, the present invention adopts the following technical solutions:

A kind of nitrided steel 31CrMoV9 annealed material. The chemical composition of the steel includes, in terms of mass percentage, C: 0.27% ~ 0.33%, Si: 0.28% ~ 0.36%, Mn: 0.63% ~ 0.70%, P ≤ 0.025%, S ≤ 0.025 %, Cr: 2.43% ~ 2.53%, Ni ≤ 0.10%, Mo: 0.17% ~ 0.23%, V: 0.10% ~ 0.16%, Cu ≤ 0.10%, Alt: 0.015% ~ 0.045%, the rest is Fe and unavoidable of impurities.

A method for producing nitrided steel 31CrMoV9 annealed material. The raw materials are sequentially passed through EBT electric furnace smelting, LF furnace refining, VD furnace refining, continuous casting, heat preservation, heating, rolling, slow cooling, and annealing to obtain product steel. The specific steps are as follows :

1) EBT electric furnace smelting: smelting temperature 1640~1680℃, smelting end point C: 0.05%~0.10%, P≤0.013%, residual element content meets the design requirements; add 2.0~3.0kg/t aluminum powder for pre-production before tapping. For deoxidation, pre-deoxidizer, slagging materials, alloys and carburizing agents are added during the tapping process, and steel and slag are retained. The tapping time is controlled at 3 to 6 minutes;

2) LF furnace refining: perform bottom-blown argon stirring throughout the process, and control the argon gas pressure to 0.2~0.3MPa; perform white slag refining in the LF furnace, control the slag basicity R≥3.5, and control the A1 ₂ O ₃ content in the slag 25%~30%, FeO+MnO content <1.0%; white slag operation holding time ≥20min;

3) VD furnace refining: before starting the vacuum pump, control the argon pressure at 0.1~0.3MPa; after the vacuum reaches 100Pa, maintain it for more than 10 minutes, and adjust the argon pressure to 0.3~0.5MPa; after the vacuum is broken, deoxidize and feed the Al line for adjustment The Al content in the steel is between 0.025% and 0.035%; feed the Ca-Si line 100 to 120m to adjust the aluminum nitride inclusions; after breaking the vacuum, blow argon statically for 10~35min, adjust the argon gas pressure to 0.1~0.3MPa; end the static argon blowing Then add 30~80kg/furnace of heat preservation agent;

4) Continuous casting: A three-machine three-flow arc continuous casting machine is used. The temperature of the lifting bag of the first furnace of continuous casting is 1589~1594℃, the temperature of the lifting bag of the second furnace is 1577~1587℃, and the temperature of the lifting bag of the third furnace to the tail furnace is 1567~1577 ℃, argon gas protection casting;

5) Heating: The furnace temperature in the preheating section is 500~750℃, and the heating time is 45~50min; the heating section is divided into a heating section and a heating section according to the different heating speeds of the steel billets. The heating speed in the heating section is 140~170℃/h, and the furnace temperature 1220～1230℃, heating time 55～60min; second heating section heating speed 300～330℃/h, furnace temperature 1060～1080℃, heating time 60～65min; soaking section furnace temperature 1200～1220℃, soaking time 60 ~90min; billet exit temperature is 1180~1200℃, total heating time is 220~265min;

6) Rolling: The opening rolling temperature is 1100~1200℃, the final rolling temperature is 850~1000℃, and the steel is immediately put into the pit for heat preservation or pile cooling after being placed on the cooling bed;

7) Annealing: Adopt isothermal annealing process; first heat the steel to 760-780°C at a heating rate of 70-100°C/h, then equalize and maintain the temperature for more than 3 hours, then cool the furnace to 690-710°C, then equalize and maintain the temperature. Leave it for more than 2 hours, cool it in the furnace to below 500°C, then take it out and cool it to room temperature in the air.

Further, in the step 1), scrap steel and molten iron are used as raw materials. Scrap steel is first loaded, electricity is supplied, the well is penetrated to the bottom and a molten pool is formed, and then molten iron is added. When the iron is blended, the temperature of the molten iron is >1150°C; through oxygen supply, Slagging, power supply for decarburization, dephosphorization and temperature adjustment.

Furthermore, the molten iron components are required to be in mass percentage: C ≥ 3.5%, Mn ≤ 0.80%, Si: 0.20% to 1.00%, P ≤ 0.070%, S ≤ 0.070%.

Further, in the step 4), the crystallizer M-EMS and the solidification terminal F-EMS are used for joint stirring, and the electronic stirring current is 360-400A.

Further, in step 6), high-pressure water is used to remove scales before rolling, and the descaling pressure is 28-30MPa.

Further, in step 6), The steel is insulated when entering the furnace, the temperature when exiting the pit is ≤300℃, and the holding time is ≥24h;/> The steel is pile-cooled.

Furthermore, the hardness of the produced 31CrMoV9 steel is ≤248HBW; the tensile strength after quenching and tempering is 1000~1200MPa, and the yield strength is ≥800MPa.

Furthermore, the total decarburized layer depth on each side of the produced 31CrMoV9 steel does not exceed 1.0% of the nominal diameter.

Compared with the prior art, the beneficial effects of the present invention are:

The 31CrMoV9 steel produced can meet the requirement that the delivery hardness of the steel does not exceed 248HBW; the tensile strength after quenching and tempering is 1000~1200MPa, and the yield strength is ≥800MPa; the total decarburized layer depth (ferrite + transition layer) on each side of the steel is not Exceeding 1.0% of the nominal diameter ensures that the material has good processing properties.

Description of the drawings

Figure 1 is a heat treatment process curve of nitrided steel 31CrMoV9 according to the present invention.

Figure 2 is a photo of the metallographic structure after rolling of the nitrided steel 31CrMoV9 of the present invention (the structure is B+P+M+F).

Figure 3 is a photo of the metallographic structure of the nitrided steel 31CrMoV9 in the annealed state of the present invention (the structure is a granular P structure).

Detailed ways

The chemical composition of the nitrided steel 31CrMoV9 annealed material of the present invention includes, in terms of mass percentage, C: 0.27% ~ 0.33%, Si: 0.28% ~ 0.36%, Mn: 0.63% ~ 0.70%, P ≤ 0.025% , S ≤ 0.025%, Cr: 2.43% ~ 2.53%, Ni ≤ 0.10%, Mo: 0.17% ~ 0.23%, V: 0.10% ~ 0.16%, Cu ≤ 0.10%, Alt: 0.015% ~ 0.045%, the rest are Fe and inevitable impurities.

According to the production method of nitrided steel 31CrMoV9 annealed material of the present invention, the raw materials are sequentially passed through EBT electric furnace smelting, LF furnace refining, VD furnace refining, continuous casting, heat preservation, heating, rolling, slow cooling and annealing to obtain product steel. ,Specific steps are as follows:

1) EBT electric furnace smelting: smelting temperature 1640~1680℃, smelting end point C: 0.05%~0.10%, P≤0.013%, residual element content meets the design requirements; add 2.0~3.0kg/t aluminum powder for pre-production before tapping. For deoxidation, pre-deoxidizer, slagging materials, alloys and carburizing agents are added during the tapping process, and steel and slag are retained. The tapping time is controlled at 3 to 6 minutes;

2) LF furnace refining: perform bottom-blown argon stirring throughout the process, and control the argon gas pressure to 0.2~0.3MPa; perform white slag refining in the LF furnace, control the slag basicity R≥3.5, and control the A1 ₂ O ₃ content in the slag 25%~30%, FeO+MnO content <1.0%; white slag operation holding time ≥20min;

3) VD furnace refining: before starting the vacuum pump, control the argon pressure at 0.1~0.3MPa; after the vacuum reaches 100Pa, maintain it for more than 10 minutes, and adjust the argon pressure to 0.3~0.5MPa; after the vacuum is broken, deoxidize and feed the Al line for adjustment The Al content in the steel is between 0.025% and 0.035%; feed the Ca-Si line 100 to 120m to adjust the aluminum nitride inclusions; after breaking the vacuum, blow argon statically for 10~35min, adjust the argon gas pressure to 0.1~0.3MPa; end the static argon blowing Then add 30~80kg/furnace of heat preservation agent;

4) Continuous casting: A three-machine three-flow arc continuous casting machine is used. The temperature of the lifting bag of the first furnace of continuous casting is 1589~1594℃, the temperature of the lifting bag of the second furnace is 1577~1587℃, and the temperature of the lifting bag of the third furnace to the tail furnace is 1567~1577 ℃, argon gas protection casting;

5) Heating: The furnace temperature in the preheating section is 500~750℃, and the heating time is 45~50min; the heating section is divided into a heating section and a heating section according to the different heating speeds of the steel billets. The heating speed in the heating section is 140~170℃/h, and the furnace temperature 1220～1230℃, heating time 55～60min; second heating section heating speed 300～330℃/h, furnace temperature 1060～1080℃, heating time 60～65min; soaking section furnace temperature 1200～1220℃, soaking time 60 ~90min; billet exit temperature is 1180~1200℃, total heating time is 220~265min;

6) Rolling: The opening rolling temperature is 1100~1200℃, the final rolling temperature is 850~1000℃, and the steel is immediately put into the pit for heat preservation or pile cooling after being placed on the cooling bed;

7) Annealing: Adopt isothermal annealing process; first heat the steel to 760-780°C at a heating rate of 70-100°C/h, then equalize and maintain the temperature for more than 3 hours, then cool the furnace to 690-710°C, then equalize and maintain the temperature. Leave it for more than 2 hours, cool it in the furnace to below 500°C, then take it out and cool it to room temperature in the air.

Further, in the step 1), scrap steel and molten iron are used as raw materials. Scrap steel is first loaded, electricity is supplied, the well is penetrated to the bottom and a molten pool is formed, and then molten iron is added. When the iron is blended, the temperature of the molten iron is >1150°C; through oxygen supply, Slagging, power supply for decarburization, dephosphorization and temperature adjustment.

Furthermore, the molten iron components are required to be in mass percentage: C ≥ 3.5%, Mn ≤ 0.80%, Si: 0.20% to 1.00%, P ≤ 0.070%, S ≤ 0.070%.

Further, in the step 4), the crystallizer M-EMS and the solidification terminal F-EMS are used for joint stirring, and the electronic stirring current is 360-400A.

Further, in step 6), high-pressure water is used to remove scales before rolling, and the descaling pressure is 28-30MPa.

Further, in step 6), The steel is insulated when entering the furnace, the temperature when exiting the pit is ≤300℃, and the holding time is ≥24h;/> The steel is pile-cooled.

Furthermore, the hardness of the produced 31CrMoV9 steel is ≤248HBW; the tensile strength after quenching and tempering is 1000~1200MPa, and the yield strength is ≥800MPa.

Furthermore, the total decarburized layer depth on each side of the produced 31CrMoV9 steel does not exceed 1.0% of the nominal diameter.

The main functions and design basis of each element in the chemical composition of nitrided steel 31CrMoV9 according to the present invention are as follows:

Nitriding, also called nitriding, is one of the important processes in chemical heat treatment. The type of steel suitable for the nitriding process is called nitrided steel, also called nitrided steel. Using it to make machine parts, after nitriding treatment, can obtain high surface hardness, good wear resistance, high fatigue strength and low notch sensitivity, certain corrosion resistance, and high thermal stability.

31CrMoV9 is an aluminum-free nitrided steel. Adding nitride-forming elements to the steel can improve the steel's ability to absorb nitrogen and make the nitride surface layer have good adhesion. Such elements have a strong affinity with nitrogen and form stable and highly dispersed nitrides, thereby increasing the hardness. Such elements include chromium, vanadium, molybdenum, titanium, etc.

Chromium is an excellent nitride promoting element and the main alloying element in nitrided steel. In aluminum-free nitrided steel, chromium is the most important element, and is combined with vanadium or molybdenum to further optimize performance. Chromium is beneficial to improving the hardenability, strength and surface hardness of steel. However, when the content is too high, the volume expansion of the steel will gradually become significant after nitriding, so the deformation of the parts will increase. In severe cases, the surface layer will easily fall off.

Vanadium can form stable nitrides and is a commonly used element in aluminum-free nitrided steel. It plays a role in improving the hardness of the nitrided layer. Although the surface hardness is not as high as that of aluminum-containing nitrided steel, it has good toughness and is resistant to wear and stress. Parts with a certain impact load are very suitable.

The important role of molybdenum in nitrided steel is to avoid temper brittleness during the nitriding process.

Carbon is an element that is not conducive to nitriding. It will reduce the diffusion coefficient of nitrogen, thereby reducing the nitriding speed. Carbon also has an adverse effect on the surface hardness and distribution of the carburized layer. However, carbon is the basic element that ensures the strength of steel. Insufficient strength in the core has a negative impact on the performance of the nitrided layer, especially the fatigue strength. Therefore, in order to ensure the strength of the core of the transmission shaft, the carbon content should not be too low.

Consider the different effects of each alloy element from the perspective of mechanical properties. Silicon not only increases the hardenability of steel, but also increases its resistance to tempering after quenching. Manganese and iron form a solid solution, which increases the strength and hardness of ferrite and austenite in steel. Manganese in steel plays a role in refining pearlite by lowering the critical transformation temperature, and also indirectly improves the strength of pearlitic steel. effect. Aluminum is used to refine the grains of steel and fix nitrogen and oxygen in the steel, thereby reducing the sensitivity of the steel to notches, reducing or eliminating the aging phenomenon of the steel, and improving the impact toughness of the steel. Molybdenum has a solid solution strengthening effect on ferrite and also improves the stability of carbides, so it has a beneficial effect on the strength of steel. Adding an appropriate amount of molybdenum to quenched and tempered steel can not only improve the hardenability of the steel, thereby improving The strength and ductility of steel, and because molybdenum can eliminate or reduce the temper brittleness caused by other alloying elements, greatly benefit the impact toughness of steel. The effect of vanadium on the mechanical properties of annealed low carbon steel is: when the content is low and solid dissolved in ferrite, it will slightly increase the strength of the steel and slightly reduce its plasticity and toughness; if it exists in the form of aggregated carbides, it will Part of the carbon is fixed, but the strength of the steel is reduced. Adding chromium to steel can improve the strength and toughness of the steel within a certain content. However, adding chromium alone to carbon steel does not significantly improve the mechanical properties after normalizing or quenching and tempering. At the same time, other alloying elements such as manganese and molybdenum are added or added. Wait to show better performance.

Comprehensive consideration of the effects of various elements on the nitriding performance and mechanical properties of steel, the present invention determines that the smelting control component requirements of nitrided steel 31CrMoV9 are C: 0.27% ~ 0.33%, Si: 0.28% ~ 0.36%, Mn: 0.63% ~ 0.70 %, P: ≤ 0.025%, S: ≤ 0.025%, Cr: 2.43% ~ 2.53%, Ni: ≤ 0.10%, Mo: 0.17% ~ 0.23%, V: 0.10% ~ 0.16%, Cu: ≤ 0.10%, Alt:0.015%～0.045%. The remainder is Fe and unavoidable impurities.

The whole production process of the nitrided steel 31CrMoV9 annealed material according to the invention is: high-quality molten iron + scrap steel → EBT electric furnace → LF refining furnace → VD vacuum treatment furnace → 235×265mm medium billet continuous casting machine → end in and end out Beam-type heating furnace → 1150BD rolling mill + 850 bar continuous rolling mill unit → high-precision finishing line → physical and chemical structure inspection → finished product packaging and delivery to the warehouse. The main control processes are as follows:

Electric furnace smelting uses scrap steel plus molten iron as raw materials. The molten iron requires C content ≥ 3.5%, Mn ≤ 0.80%, Si: 0.20% ~ 1.00%, P ≤ 0.070%, S ≤ 0.070%. First load the scrap steel (the furnace bottom is padded with active lime before charging to reduce the impact on the furnace bottom, form a molten pool in advance, stabilize the arc and achieve early dephosphorization), then add electricity until the well is penetrated to the bottom and a certain molten pool is formed, and then the molten iron is added (The molten iron temperature is ≤1150℃ and cannot be mixed with iron). Decarburization, dephosphorization, and temperature adjustment are achieved through oxygen supply, slagging, and electricity. The smelting temperature is 1640~1680℃, the chemical composition C: 0.05%~0.10%, P≤0.013%, and the steel can be tapped if the residual elements meet the requirements. Before tapping, add 2.0~3.0kg/t aluminum powder for pre-deoxidation. During the tapping process, pre-deoxidizer, slagging materials, alloys and carburizing agents are added. When tapping steel, attention must be paid to retaining steel and slag to prevent oxidized slag from entering the ladle. The tapping time is controlled at 3 to 6 minutes.

During the LF refining process, bottom-blown argon stirring is performed throughout the process (it is appropriate to control the argon gas during refining so that the slag surface fluctuates and the molten steel is not exposed). When the ladle arrives at the receiving position, the argon gas is first turned on to blow open the slag surface to measure the temperature, and a primary sample is taken to analyze the full composition. After the primary sample analysis results are reported back, the composition is roughly adjusted according to the required lower limit; the ladle enters the LF station, and a large current is used to raise the temperature. The slag is heated to the target value (1580~1620℃) with one-time power supply, and the white slag is refined in the LF furnace. The basicity of the slag is controlled R≥3.5, w(A12O3) in the slag=25%~30%, w(FeO+ MnO) <1.0% (preferably <0.5%), realizes diffusion deoxidation of molten steel by slag, and simultaneously completes the task of desulfurization. After the slag is whitened, take a secondary sample to analyze all components. After the secondary sample is taken away, continue to use diffusion deoxidizer to adjust the slag, maintain appropriate slag alkalinity and good fluidity, and maintain the white slag operation throughout the process. The results of the secondary sample will be reported. Feed the Al wire and adjust the Al content in the steel to above 0.020%; fine-tune the composition according to the target composition. The white slag holding time is ≥20min, and the argon pressure is controlled at 0.2~0.3MPa throughout the process.

During VD refining, the argon gas pressure is controlled at 0.1~0.3MPa before starting the vacuum pump, so that the slag surface is not exposed to fretting molten steel. Start timing when the vacuum degree reaches 100Pa and keep the time ≥10min. At this time, adjust the argon gas pressure to 0.3~0.5MPa. After the vacuum is broken, final deoxidation is carried out, and the Al line is fed to control the Al content of the molten steel to 0.025% to 0.035%. The Ca-Si line is fed to 100 to 120m to adjust the aluminum nitride inclusions to ensure the castability of the steel; after the vacuum is broken, static blowing of argon is performed. 10~35min, argon gas pressure 0.1~0.3MPa, it is appropriate that the slag surface is fretting and the molten steel is not exposed. After the static argon blowing is completed, add 30 to 80kg/furnace of thermal insulating agent. The thermal insulating agent should be added when the molten steel level still fluctuates at the end of each furnace to ensure uniform coverage.

Continuous casting adopts a three-machine three-flow arc continuous casting machine to continuously cast steel billets. The temperature of the continuous casting head furnace bag is 1589~1594℃, the temperature of the second furnace bag is 1577~1587℃, and the temperature of the third furnace to the tail furnace bag is 1567~1577. ℃, argon gas protection pouring; electronic stirring uses the combined stirring of the crystallizer (M-EMS) and the solidification end (F-EMS), and the electronic stirring current is set to 380~420A.

In the heating stage, the furnace temperature of the preheating section is set to 500~750℃, and the heating time is 45~50min; the heating section is divided into a heating section and a heating section according to the different requirements of the billet heating speed, of which the heating speed of the heating section is 140~170℃/ h, the furnace temperature is 1220~1230℃, the heating time is 55~60min; the second heating section heating speed is 300~330℃/h, the furnace temperature is 1060~1080℃, the heating time is 60~65min; the soaking section furnace temperature is 1200~1220℃, Soaking time is 60~90min. The billet exit temperature is 1180~1200℃, and the total heating time is 220~265min.

During the rolling stage, high-pressure water is used for descaling before rolling. The descaling pressure is 28 to 30 MPa. The starting rolling temperature is 1100 to 1200°C, and the final rolling temperature is 850 to 1000°C. After the steel is placed on the cooling bed, it is immediately put into the pit for heat preservation or pile cooling. specifically is: The steel is insulated into the pit, the temperature out of the pit is ≤300℃, and the insulation time is ≥24h;/> The steel is pile cooled. The photo of the metallographic structure of nitrided steel 31CrMoV9 after rolling is shown in Figure 2. The structure is B+P+M+F.

In the annealing stage, based on the composition and performance characteristics of 31CrMoV9 steel, and based on the analysis of the principles of isothermal annealing heating temperature, heating time, isothermal temperature, and isothermal time, the isothermal annealing process is designed. The isothermal annealing process curve is shown in Figure 1. The photo of the annealed metallographic structure of nitrided steel 31CrMoV9 is shown in Figure 3. The structure is granular P structure.

The following examples are implemented on the premise of the technical solution of the present invention and provide detailed implementation modes and specific operating processes. However, the protection scope of the present invention is not limited to the following examples.

[Example 1]

In this embodiment, the method of the present invention is used to manufacture nitrided steel 31CrMoV9 annealed material. According to the mass percentage content, the chemical composition of the steel includes C: 0.29%, Si: 0.33%, Mn: 0.67%, P: 0.011%, S: 0.002 %, Cr: 2.49%, Ni: 0.02%, Mo: 0.19%, Cu: 0.03%, Alt: 0.026%, V: 0.13%, the balance is Fe and inevitable impurities.

The production process controls the following steps:

1. EBT electric furnace smelting

17t of scrap steel is loaded into the electric furnace, and after 20 minutes of power transmission, molten iron is added. The temperature of the molten iron is 1250℃~1350℃ and the weight is 35.8t. Oxygen oxidation, violent boiling, oxidation temperature 1679°C, high alkalinity, large slag volume, low temperature slag is formed in the early stage of oxidation to enhance the dephosphorization effect. When the furnace charge is melted more than 70%, the electric furnace foaming agent is sprayed into the slag, and the steel material is melted. Afterwards, samples were taken to analyze C, Mn, P, S and residual elements. During tapping, the C content is 0.05%, the P content is 0.008%, and the tapping temperature is 1678°C. When the electric furnace is tapping 1/4 of the steel, add an appropriate amount of slag, 102kg of pre-deoxidizer aluminum and iron, 201kg of silicon-manganese alloy, and 1000kg of low chromium.

2. LF furnace refining

When the ladle arrives at the receiving position, the argon gas is first turned on to blow open the slag surface to measure the temperature, and a primary sample is taken to analyze the full composition. After the primary sample analysis results are reported back, the composition is roughly adjusted according to the required lower limit; bottom blowing argon is used for stirring throughout the LF refining process, and activity is added 499kg of lime, 10kg of coke powder carburizer, and 50kg of fluorite briquettes are used to make slag. After the liquid slag is formed, 200kg of alumina balls are added for diffusion deoxidation. After the slag whiteness and temperature meet the requirements, take a secondary sample to analyze the full composition; after taking the secondary sample, continue to use diffusion deoxidizer to adjust the slag, maintain appropriate slag alkalinity and good fluidity, and maintain white slag operation throughout the process. The results are reported back and the ingredients are fine-tuned according to the target control ingredients. The retention time of white residue is ≥20min. The argon gas pressure is controlled at 0.2~0.3MPa throughout the process to prevent secondary oxidation caused by exposure of molten steel. The temperature of the lifting bag is 1690℃.

3. VD furnace refining

The temperature of molten steel entering the VD furnace is 1677°C. Before starting the vacuum pump, the argon gas pressure is controlled at 0.1~0.3MPa, so that the molten steel is not exposed on the slag surface. Start timing when the vacuum degree reaches 100Pa and keep it for 15 minutes. After breaking the vacuum, feed the Ca-Si wire for 100m. After the wire feeding is completed, argon gas is blown, and the argon gas is blown quietly for 20 to 35 minutes. The argon gas pressure is adjusted to 0.1 to 0.3MPa, so that the slag surface is fretting and the molten steel is not exposed. After the static argon blowing is completed, 80kg of carbonized rice husk is added as a heat preservation agent, covering it evenly, and the temperature of the hanging bag is 1604°C.

4. Continuous casting

A three-machine three-flow arc continuous casting machine is used to continuously cast the steel billet. The temperature of the continuous casting head furnace is 1598°C, and the casting is protected by argon gas. The middle bag is baked for 4 hours. The insertion depth of the immersed nozzle is 85mm from the crystallizer liquid level. The speed is 0.80m/min, the continuous casting head furnace superheat is 31°C, the crystallizer water flow is 160m ³ /h, the secondary cooling water specific water volume is 0.30L/kg, and the electronic stirring current is 360A.

5. Heating

The furnace temperature in the preheating section is 600℃, and the heating time is 50min; the heating section is divided into a heating section and a heating section according to the different heating speeds of the billets. The heating speed in the heating section is 150℃/h, the furnace temperature is 1230℃, and the heating time is 60min; the second heating section The heating rate is 320℃/h, the furnace temperature is 1060℃, and the heating time is 60min; the furnace temperature in the soaking section is 1220℃, the soaking time is 70min; the billet exit temperature is 1180℃, and the total heating time is 240min.

6. Rolling

High-pressure water descaling before rolling, the descaling pressure is 28~30MPa, the opening rolling temperature is 1150℃, the final rolling temperature is 910℃, and the rolling specifications are The steel is piled and cooled immediately after being placed on the cooling bed.

7. Annealing

Using the isothermal annealing process, first put the steel into a heating furnace with a furnace temperature of 300°C, raise the temperature to 770°C at a heating rate of 80°C/h, heat for a total of 6 hours, and keep it for 8 hours. After cooling the furnace to 700°C, keep it for 3 hours. After cooling in the furnace to below 500℃, it is released from the furnace and air cooled to room temperature. The furnace time is 23h.

The steel prepared in this example was tested and the results are as follows:

Yield Strength tensile strength Area shrinkage Elongation hardness decarburization layer 1022MPa 1149MPa 62% 20% 237HB 0.15mm

[Example 2]

In this embodiment, the method of the present invention is used to manufacture nitrided steel 31CrMoV9 annealed material. According to the mass percentage content, the chemical composition of the steel includes C: 0.31%, Si: 0.34%, Mn: 0.68%, P: 0.011%, S: 0.004 %, Cr: 2.52%, Ni: 0.02%, Mo: 0.19%, Cu: 0.01%, Alt: 0.024%, V: 0.14%, the balance is Fe and inevitable impurities.

The production process controls the following steps:

1. EBT electric furnace smelting

18t of scrap steel is loaded into the electric furnace, and after 20 minutes of power transmission, molten iron is added. The temperature of the molten iron is 1250℃~1350℃ and the weight is 35.5t. Oxygen oxidation, violent boiling, oxidation temperature 1683°C, high alkalinity, large slag volume, low temperature slag is formed in the early stage of oxidation to enhance the dephosphorization effect. When the furnace charge is melted more than 70%, the electric furnace foaming agent is sprayed into the slag, and the steel material is melted. Afterwards, samples were taken to analyze C, Mn, P, S and residual elements. During tapping, the C content is 0.08%, the P content is 0.008%, and the tapping temperature is 1678°C. When the electric furnace is tapping 1/4 of the steel, add an appropriate amount of slag, and add 103kg of pre-deoxidizer ferro-aluminum, 203kg of silicon-manganese alloy, and 1009kg of low-chromium steel balls.

2. LF furnace refining

When the ladle arrives at the receiving position, the argon gas is first turned on to blow open the slag surface to measure the temperature, and a primary sample is taken to analyze the full composition. After the primary sample analysis results are reported back, the composition is roughly adjusted according to the required lower limit; bottom blowing argon is used for stirring throughout the LF refining process, and activity is added 500kg of lime, 20kg of coke powder carburizer, and 80kg of fluorite briquettes are used to make slag. After the liquid slag is formed, 200kg of alumina balls are added for diffusion deoxidation. After the slag whiteness and temperature meet the requirements, take a secondary sample to analyze the full composition; after taking the secondary sample, continue to use diffusion deoxidizer to adjust the slag, maintain appropriate slag alkalinity and good fluidity, and maintain white slag operation throughout the process. The results are reported back and the ingredients are fine-tuned according to the target control ingredients. The retention time of white residue is ≥20min. The argon gas pressure is controlled at 0.2~0.3MPa throughout the process to prevent secondary oxidation caused by exposure of molten steel. The temperature of the lifting bag is 1680℃.

3. VD furnace refining

The temperature of molten steel entering the VD furnace is 1663°C. Before starting the vacuum pump, the argon gas pressure is controlled at 0.1~0.3MPa, so that the molten steel is not exposed on the slag surface. Start timing when the vacuum degree reaches 100Pa and keep it for 15 minutes. After breaking the vacuum, feed the Ca-Si wire for 100m. After the wire feeding is completed, argon gas is blown, and the argon gas is blown quietly for 20 to 35 minutes. The argon gas pressure is adjusted to 0.1 to 0.3MPa, so that the slag surface is fretting and the molten steel is not exposed. After the static argon blowing is completed, 80kg of carbonized rice husk is added as a thermal insulation agent, covering it evenly, and the temperature of the hanging bag is 1570°C.

4. Continuous casting

A three-machine three-flow arc continuous casting machine is used to continuously cast the steel billet. The temperature of the continuous casting bag is 1561°C, and the pouring is protected by argon gas. The middle bag is baked for 4 hours. The insertion depth of the immersed nozzle is: 85mm from the crystallizer liquid level; the pulling speed is 0.85 m/min, the superheat of the continuous casting head furnace is 26°C, the crystallizer water flow is 160m3/h, the secondary cooling water specific water volume is 0.30L/kg, and the electronic stirring current is 380A.

5. Heating

The furnace temperature in the preheating section is 600℃, and the heating time is 50min; the heating section is divided into a heating section and a heating section according to the different heating speeds of the billets. The heating speed in the heating section is 150℃/h, the furnace temperature is 1230℃, and the heating time is 60min; the second heating section The heating rate is 320℃/h, the furnace temperature is 1060℃, and the heating time is 60min; the furnace temperature in the soaking section is 1220℃, the soaking time is 70min; the billet exit temperature is 1190℃, and the total heating time is 240min.

6. Rolling

High-pressure water descaling before rolling, the descaling pressure is 28~30MPa, the opening rolling temperature is 1150℃, the final rolling temperature is 910℃, and the rolling specifications are The steel is piled and cooled immediately after being placed on the cooling bed.

7. Annealing

Using the isothermal annealing process, first put the steel into a heating furnace with a furnace temperature of 300°C, raise the temperature to 770°C at a heating rate of 90°C/h, heat for a total of 6 hours, and keep it for 8 hours. Then, the furnace is cooled to 690°C and kept for 3 hours. After cooling in the furnace to below 500℃, it is released from the furnace and air cooled to room temperature. The furnace time is 23h.

The steel prepared in this example was tested and the results are as follows:

Yield Strength tensile strength Area shrinkage Elongation hardness Decarburization layer depth 1018MPa 1147MPa 64% 18% 229HB 0.14mm

[Example 3]

21C5351 In this example, the method of the present invention is used to manufacture nitrided steel 31CrMoV9 annealed material. According to the mass percentage content, the chemical composition of the steel includes C: 0.29%, Si: 0.32%, Mn: 0.67%, P: 0.015%, S: 0.001%, Cr: 2.47%, Ni: 0.02%, Mo: 0.19%, Cu: 0.03%, Alt: 0.022%, V: 0.12%, the balance is Fe and inevitable impurities.

The production process controls the following steps:

1. EBT electric furnace smelting

18t of scrap steel is loaded into the electric furnace, and after 20 minutes of power transmission, molten iron is added. The temperature of the molten iron is 1250℃~1350℃ and the weight is 25.9t. Oxygen oxidation, violent boiling, oxidation temperature 1678°C, high alkalinity, large slag volume, low temperature slag is formed in the early stage of oxidation to enhance the dephosphorization effect. When the furnace charge is melted more than 70%, the electric furnace foaming agent is sprayed into the slag, and the steel material is melted. Afterwards, samples were taken to analyze C, Mn, P, S and residual elements. During tapping, the C content is 0.05%, the P content is 0.0012%, and the tapping temperature is 1676°C. When the electric furnace is tapping 1/4 of the steel, add an appropriate amount of slag, 102kg of pre-deoxidizer aluminum and iron, 201kg of silicon-manganese alloy, and 1000kg of low chromium.

2. LF furnace refining

When the ladle arrives at the receiving position, the argon gas is first turned on to blow open the slag surface to measure the temperature, and a primary sample is taken to analyze the full composition. After the primary sample analysis results are reported back, the composition is roughly adjusted according to the required lower limit; bottom blowing argon is used for stirring throughout the LF refining process, and activity is added 452kg of lime, 45kg of coke powder carburizer, and after the liquid slag is formed, add 200kg of alumina balls for diffusion deoxidation. After the slag whiteness and temperature meet the requirements, take a secondary sample to analyze the full composition; after taking the secondary sample, continue to use diffusion deoxidizer to adjust the slag, maintain appropriate slag alkalinity and good fluidity, and maintain white slag operation throughout the process. The results are reported back and the ingredients are fine-tuned according to the target control ingredients. The retention time of white residue is ≥20min. The argon gas pressure is controlled at 0.2~0.3MPa throughout the process to prevent secondary oxidation caused by exposure of molten steel. The temperature of the lifting bag is 1682℃.

3. VD furnace refining

The temperature of molten steel entering the VD furnace is 1660°C. Before starting the vacuum pump, the argon gas pressure is controlled at 0.1~0.3MPa, so that the molten steel is not exposed on the slag surface. Start timing when the vacuum degree reaches 100Pa and keep it for 15 minutes. After breaking the vacuum, feed the Ca-Si wire for 100m. After the wire feeding is completed, argon gas is blown, and the argon gas is blown quietly for 20 to 35 minutes. The argon gas pressure is adjusted to 0.1 to 0.3MPa, so that the slag surface is fretting and the molten steel is not exposed. After the static argon blowing is completed, 80kg of carbonized rice husk is added as a thermal insulation agent, covering it evenly, and the temperature of the hanging bag is 1568°C.

4. Continuous casting

A three-machine three-flow arc continuous casting machine is used to continuously cast the steel billet. The temperature of the continuous casting head furnace is 1563°C, and the casting is protected by argon gas. The middle package is baked for 4 hours. The insertion depth of the immersed nozzle is 85mm from the crystallizer liquid level. The speed is 0.85m/min, the continuous casting head furnace superheat is 27°C, the crystallizer water flow is 160m ³ /h, the secondary cold water specific water volume is 0.30L/kg, and the electronic stirring current is 380A.

5. Heating

The furnace temperature in the preheating section is 600℃, and the heating time is 50min; the heating section is divided into a heating section and a heating section according to the different heating speeds of the billets. The heating speed in the heating section is 150℃/h, the furnace temperature is 1230℃, and the heating time is 60min; the second heating section The heating rate is 320℃/h, the furnace temperature is 1060℃, and the heating time is 60min; the furnace temperature in the soaking section is 1220℃, the soaking time is 70min; the billet exit temperature is 1200℃, and the total heating time is 240min.

6. Rolling

High-pressure water descaling before rolling, the descaling pressure is 28MPa, the opening rolling temperature is 1150℃, the final rolling temperature is 910℃, and the rolling specifications are The steel is piled and cooled immediately after being placed on the cooling bed.

7. Annealing

Using the isothermal annealing process, first put the steel into a heating furnace with a furnace temperature of 300°C, raise the temperature to 770°C at a heating rate of 85°C/h, heat for a total of 6 hours, and maintain it for 8 hours. After the furnace is cooled to 690°C, it is maintained for 3 hours. After cooling in the furnace to below 500℃, it is released from the furnace and air cooled to room temperature. The furnace time is 23h.

The steel prepared in this example was tested and the results are as follows:

Yield Strength tensile strength Area shrinkage Elongation hardness Decarburization layer depth 1015MPa 1129MPa 65% 18% 239HB 0.12mm

The above are only preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person familiar with the technical field can, within the technical scope disclosed in the present invention, implement the technical solutions of the present invention. Equivalent substitutions or changes of the inventive concept thereof shall be included in the protection scope of the present invention.

Claims (7)

1. A production method of a nitriding steel 31CrMoV9 annealing material is characterized in that the steel comprises, by mass, 0.27% -0.33% of C, 0.28% -0.36% of Si, 0.63% -0.70% of Mn, less than or equal to 0.025% of P, less than or equal to 0.025% of S, 2.43% -2.53% of Cr, less than or equal to 0.10% of Ni, 0.17% -0.23% of Mo, 0.10% -0.16% of V, less than or equal to 0.10% of Cu, 0.015% -0.045% of Alt, and the balance of Fe and unavoidable impurities;

the raw materials sequentially pass through an EBT electric furnace smelting, an LF furnace refining, a VD furnace refining, continuous casting, heat preservation, heating, rolling, slow cooling and annealing to obtain the product steel, and the specific steps are as follows:

1) Smelting by an EBT electric furnace: smelting temperature 1640-1680 ℃, smelting end point C:0.05 to 0.10 percent, P is less than or equal to 0.013 percent, and the content of residual elements meets the design requirement; 2.0-3.0 kg/t aluminum powder is added for pre-deoxidation before tapping, a pre-deoxidizer, a slag-forming material, an alloy and a carburant are added in the tapping process, steel is reserved, slag is reserved, and the tapping time is controlled to be 3-6 min;

2) Refining in an LF furnace: stirring by bottom blowing argon in the whole process, and controlling the pressure of the argon to be 0.2-0.3 MPa; refining white slag in LF furnace, controlling slag alkalinity R not less than 3.5, and controlling A1 in slag ₂ O ₃ 25% -30% FeO+MnO content less than 1.0%; the operation and holding time of the white slag is more than or equal to 20min;

3) Refining by a VD furnace: the argon pressure is controlled to be 0.1-0.3 MPa before the vacuum pump is started; maintaining for more than 10min after the vacuum degree reaches 100Pa, and adjusting the argon pressure to 0.3-0.5 MPa; finally deoxidizing after breaking vacuum, and feeding Al wires to adjust the Al content in the steel to be 0.025% -0.035%; feeding Ca-Si wire 100-120 m to regulate aluminum nitride inclusion; after breaking vacuum, static argon blowing is carried out for 10-35 min, and the argon pressure is regulated to be 0.1-0.3 MPa; after the static argon blowing is finished, adding 30-80 kg of heat preservation agent into the furnace;

4) Continuous casting: adopting a three-machine three-stream arc continuous casting machine, wherein the temperature of a continuous casting head furnace ladle is 1589-1594 ℃, the temperature of a 2 nd furnace ladle is 1577-1587 ℃, and the temperature of a 3 rd furnace to a tail furnace ladle is 1567-1577 ℃, and carrying out argon protection casting;

5) Heating: the furnace temperature of the preheating section is 500-750 ℃, and the heating time is 45-50 min; the heating section is divided into a first heating section and a second heating section according to the difference of the heating speeds of the billets, the heating speed of the first heating section is 140-170 ℃/h, the furnace temperature is 1220-1230 ℃, and the heating time is 55-60 min; heating the second stage at a heating speed of 300-330 ℃/h, a furnace temperature of 1060-1080 ℃ and a heating time of 60-65 min; furnace temperature of the soaking section is 1200-1220 ℃, and soaking time is 60-90 min; the billet tapping temperature is 1180-1200 ℃, and the total heating time is 220-265 min;

6) Rolling: the initial rolling temperature is 1100-1200 ℃, the final rolling temperature is 850-1000 ℃, and the steel is immediately put into a pit for heat preservation or cold piling after being put into a cooling bed;

7) Annealing: an isothermal annealing process is adopted; firstly raising the temperature of steel to 760-780 ℃ at a heating rate of 70-100 ℃/h, then homogenizing the temperature and preserving the heat for more than 3 hours, then cooling the steel to 690-710 ℃ in a furnace, homogenizing the temperature and preserving the heat for more than 2 hours, cooling the steel to below 500 ℃ along with the furnace, discharging the steel, and cooling the steel to room temperature in an air;

the hardness of the produced 31CrMoV9 steel is less than or equal to 248HBW; the tensile strength after quenching and tempering is 1000-1200 MPa, and the yield strength is more than or equal to 800MPa.

2. The method for producing the nitriding steel 31CrMoV9 annealed material according to claim 1, wherein in the step 1), scrap steel and molten iron are adopted as raw materials, the scrap steel is firstly filled, molten iron is added after the steel is fed through a well to form a molten pool, and the temperature of the molten iron is more than 1150 ℃ when the molten iron is added; decarburization, dephosphorization and temperature adjustment are carried out through oxygen supply, slag formation and electrifying.

3. The production method of the nitrided steel 31CrMoV9 annealed material according to claim 2, wherein the molten iron composition is as follows: c is more than or equal to 3.5%, mn is less than or equal to 0.80%, si is 0.20% -1.00%, P is less than or equal to 0.070%, and S is less than or equal to 0.070%.

4. The method for producing a nitrided steel 31CrMoV9 annealed material according to claim 1, characterized in that in the step 4), a crystallizer M-EMS and a solidification end F-EMS are used for combined stirring, and an electric stirring current is 360-400A.

5. The method for producing a 31CrMoV9 annealed steel sheet according to claim 1, wherein the step 6) is performed with high-pressure water for descaling before rolling, and the descaling pressure is 28-30 MPa.

6. The method for producing a 31CrMoV9 annealed steel sheet according to claim 1, wherein in the step 6), the steel sheet with a specification of not less than 85mm is subjected to heat preservation by entering a furnace, the pit outlet temperature is not more than 300 ℃, and the heat preservation time is not less than 24 hours; for steel with the specification of < 85mm, heap cooling is adopted.

7. The method of producing a 31CrMoV9 annealed steel as set forth in claim 1, wherein the total decarburized depth of each side of the 31CrMoV9 steel to be produced is not more than 1.0% of the nominal diameter.