A production method for 960MPa grade steel for crane boom

Abstract

A production method for 960MPa grade steel for crane boom, the process steps of which are molten iron pretreatment→converter steelmaking→external refining LF→vacuum treatment RH→casting→heating→rolling→quenching→tempering→finishing→performance inspection; the chemical composition mass percentage of the steel is C=0.15-0.17, Si≤0.20, Mn=1.0-1.20, P≤0.010, S≤0.002, Nb=0.010-0.020, V=0.04-0.08,Ti≤0.006,Als=0.06~0.10,Cr=0.15~0.40,Mo=0.40~0.60,Ni=0.2~0.4,B=0.0012~0.0020,N≤0.004,the rest are Fe and unavoidable impurity elements; carbon equivalent CEV=0.45~0.61,welding crack sensitivity coefficient Pcm=0.25~0.32. It is suitable for producing steel for crane boom with thickness of 4~30mm and maximum width of 4700mm. The yield strength of steel is ≥960MPa, tensile strength is 980~1150MPa, elongation is 15%~20%, impact energy value KV2≥170J under -40℃ condition, strain aging sensitivity coefficient is ≤20%.

Description

Technical Field

[0001] The invention belongs to the technical field of steel material preparation, and relates to a production method of high strain aging impact toughness 960MPa grade steel for crane booms. Background Art

[0002] With the rapid development of wind power generation, marine engineering, construction engineering, metallurgy and petrochemical industries, cranes are developing towards heavy and large-scale development. The number of crane boom sections increases, the deadweight increases, and the full extension length becomes longer. Among them, the boom is the most critical core load-bearing part of the crane. The material used in the equipment accounts for a high proportion, usually more than 15% of the weight, and the working environment is complex, and it is subjected to a large alternating load during operation. Improving the strength of the boom material can reduce the deadweight of the boom, reduce the structural load, and is conducive to the lightweight of the equipment and reduce emissions. Therefore, high-strength structural steel plates with a yield strength of 700 to 1100Mpa have been widely used in crane booms. The steel plates for the boom are mainly 4 to 30mm thin specifications. The steel plates are required to have high strength, high flatness, good appearance quality and weldability. Usually, they must ensure good impact toughness at -40℃ or even -60℃. In order to reduce welds, the width of the boom steel for medium and large tonnage cranes is also increasing. The current width has reached 3300 to 4600mm.

[0003] The cylinder of the crane boom is usually composed of an upper cover plate and a lower cover plate. The upper and lower cover plates are mostly made of 4-20mm thin steel plates, which are free cold bent using a V-shaped or U-shaped lower die. After multiple bending processes, the outer side of the plate is subjected to tensile stress and strain, while the inner side is subjected to compressive stress and strain. The steel plate produces obvious plastic deformation. Affected by the strain aging behavior, the strength, plasticity and toughness of the steel will change, which will have an adverse effect on the performance stability of the material during use.

[0004] Chinese patent CN114134419A discloses a method for manufacturing a high-strength steel plate with a yield strength of 960MPa, which adopts the process of online quenching (DQ water cooling) + low-temperature tempering, with a thickness of 10 to 30mm, good strength and impact toughness. Due to the use of extremely rapid cooling in the rolled state, there are large differences in the transverse and longitudinal directions of the structure and problems with cooling uniformity, large internal stress, and large deviations in the forming accuracy of the steel plate during the bending process.

[0005] Chinese patent CN112725686A discloses a steel for crane boom with a yield strength of 960 MPa and a production method thereof. The steel plate thickness is 4 to 10 mm, and the mass percentage of each element in the ingot is: C: 0.14 to 0.16%, Si≤0.07%, Mn: 1.15 to 1.25%, P≤0.012%, S≤0.005%, Als: 0.020 to 0.040%, Nb: 0.015 to 0.025%, V: 0.030 to 0.040%, Ti: 0.008-0.018%, Cr: 0.40 to 0.50%, Mo: 0.58 to 0.66%, Ni: 0.16 to 0.24%, B: 0.0010 to 0.0018%, and the others are Fe and residual elements and impurities that are inevitable in the production process. The amount of alloying elements such as Cr and Mo added to this type of steel is relatively high, and the actual CEV is controlled at 0.54-0.66%, which is not conducive to welding performance and has a high alloy cost.

[0006] Chinese patent CN116676541A discloses a low-cost 960MPa grade ultra-high strength steel plate and its preparation method, the steel contains C: 0.18-0.25%, Mn: 1.6-1.8%, Cr: 0.5-0.6%, V: 0.04-0.06%, Ti: 0.01-0.02%, B: 0.001-0.002%, and the rest is Fe and inevitable impurities. The steel mainly uses C and Mn as solid solution strengthening elements, and is matched with a relatively low tempering temperature of 450-520℃ to avoid the reduction of strength during tempering. The CEV of this component is relatively high, and the actual control range is 0.58-0.65. The welding crack sensitivity is relatively high, and the strength after welding is significantly reduced due to the low tempering softening resistance of the heat-affected zone.

[0007] Chinese patent CN112126848A discloses a method for producing a high-toughness quenched and tempered Q960 steel plate, the chemical composition and mass percentage of the steel plate are C: 0.16-0.18%, Si: 0.20-0.40%, Mn: 1.30-1.40%, P≤0.015%, S≤0.005%, Cr: 0.25-0.32%, Mo: 0.40-0.45%, Nb: 0.020-0.030%, Ti: 0.015-0.025%, B: 0.0012-0.0020%, and the balance is Fe and unavoidable impurities. The two-phase zone quenching and low-temperature tempering process adopted by the steel plate has a thickness of only 8-30 mm, and the impact energy at -40°C is 30-85 J, and the performance stability is poor.

[0008] The above patents do not consider the influence of the steel plate in the strain aging state on the performance change. Summary of the invention

[0009] In view of the above-mentioned deficiencies in the prior art, the present invention aims to provide a method for producing steel for crane booms with high strain aging impact toughness of 960MPa. The produced steel is suitable for manufacturing booms of medium and large tonnage cranes.

[0010] The technical solution of the present invention:

[0011] A production method for 960MPa grade steel for crane boom, the process flow is molten iron pretreatment → converter steelmaking → LF furnace refining → vacuum treatment → continuous casting → heating → controlled rolling → post-rolling treatment → finishing → performance inspection; the chemical composition mass percentage of the steel is C=0.15-0.17, Si≤0.20, Mn=1.0-1.20, P≤0.010, S≤0.002, Nb=0.010-0.020, V=0.04-0.08, Ti≤0.006, Als=0.06-0.10, Cr=0.15-0.40 , Mo=0.40~0.60, Ni=0.2~0.4, B=0.0012~0.0020, N≤0.004, the rest are Fe and unavoidable impurity elements, CEV=0.45~0.61, Pcm=0.25~0.32; steel plate thickness 4~30mm, maximum width 4700mm, yield strength ≥960MPa, tensile strength 980~1150MPa, elongation 15%~20%, impact energy value KV2≥170J under -40℃ condition, strain aging sensitivity coefficient ≤20%; key process steps are:

[0012] (1) LF furnace refining: temperature 1500~1650℃, composition fine-tuning, slag deoxidation, refining time ≥40min, argon blowing and stirring throughout the process;

[0013] (2) Vacuum treatment: RH furnace vacuum degassing, maintaining a vacuum degree of 0.5tor for more than 15 minutes, so that the hydrogen content of the molten steel out of the station is ≤1.5PPm, the nitrogen content is ≤40ppm, and the oxygen content is ≤20ppm; after the molten steel is degassed, Ca treatment is carried out, and the soft blowing time is not less than 15 minutes;

[0014] (3) Continuous casting: The overheat of the steel casting is controlled at 5-15°C above the liquidus line. Full protection is used for continuous casting to avoid contact between molten steel and air. The continuous casting billet is inspected and accepted according to the central segregation not exceeding C1.5 and the central porosity not exceeding C1.0;

[0015] (4) Controlled rolling: the billet thickness is 180mm, 220mm or 260mm, and the billet is heated by a walking beam heating furnace, the heating temperature is 1180-1220℃, the soaking section temperature is 1180-1220℃, the furnace time is ≥4 hours, the holding time is ≥40min, and the rough rolling adopts a large reduction system to ensure that the reduction rate of three consecutive passes after widening is more than 15%; the intermediate billet thickness is 70-100mm, the finishing rolling start temperature is 900-950℃, and the final rolling temperature is 780-820℃, so that the steel plate obtains the original structure refined before quenching and tempering treatment;

[0016] (5) Heat treatment after rolling: quenching at 860-890℃, austenitizing time is plate thickness mm×(1.6-2.5)min / mm, quenching ensures that the high pressure section pressure of the quenching machine is ≥8bar, the low pressure section pressure is ≥4bar, and the surface temperature of the steel plate after quenching is ≤50℃; tempering at 580-620℃, the time is plate thickness mm×(3.0-3.5)min / mm, air cooling.

[0017] Where: CEV = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15 (%)

[0018] Pcm=C+Si / 30+(Mn+Cr+Cu) / 20+Ni / 60+Mo / 15+V / 10+5B(%)

[0019] Principle of the invention:

[0020] 1) Chemical composition design:

[0021] The interstitial solid solution strengthening effect of C is significant. However, after the material is strain aged, a large number of dislocations accumulated in the grains can provide more diffusion channels for solid solution elements such as carbon and nitrogen in the material. During the aging process, the dislocations recover and become entangled, and the mobility becomes poor. In addition, the C atoms are segregated near the dislocation line to form Coriolis gas groups that pin the dislocations to produce hardening, resulting in a reduction in strain aging impact. Therefore, the present invention adopts a low-carbon component design and controls the range of C to 0.15% to 0.17%.

[0022] Mn is an effective element for expanding austenite phase, refining grains and improving hardenability, but Mn easily forms MnS with S and reduces plasticity, so it is not suitable to add too much. In the present invention, the Mn content is controlled in the range of 1.0% to 1.20%.

[0023] Ni can stabilize the austenite phase, improve hardenability, and reduce the ductile-brittle transition temperature. It is an effective element for improving low-temperature toughness. To ensure the stability of low-temperature impact toughness, the Ni content of the present invention is controlled in the range of 0.20% to 0.40%.

[0024] Cr can significantly improve the hardenability of steel, and at the same time Cr can significantly improve the corrosion resistance of steel, but too high a content will increase the welding crack sensitivity coefficient, which is unfavorable to the impact toughness of the welding heat affected zone. In the present invention, the Cr content is controlled at 0.15% to 0.4%.

[0025] When Mo is dissolved in ferrite and austenite, the C curve of the steel can be shifted to the right, thereby significantly improving the hardenability of the steel. Mo can also increase the tempering softening resistance of the steel plate, so that the strength of the steel plate does not decrease at a higher tempering temperature. The Mo content of the present invention is controlled in the range of 0.40% to 0.60%.

[0026] Nb can increase the recrystallization temperature of austenite, thereby achieving the purpose of refining austenite grains, improving the toughness of steel, and improving the welding performance of steel. The Nb content of the present invention is controlled in the range of 0.010% to 0.020%.

[0027] V can refine grains and improve the strength of steel. V (C / N) precipitates during tempering at about 600°C, which can improve the tempering stability of steel. However, too high a V content will damage the low-temperature toughness of the material. In the present invention, the V content is controlled at 0.04 to 0.08.

[0028] Ti element has a strong affinity with N, O and C. Ti has an obvious effect on fixing N and preserving B. However, due to the high precipitation temperature of TiN, it is easy to grow and form coarse TiN, which is harmful to the low-temperature impact toughness and strain aging impact performance. Therefore, the present invention does not add Ti element and strictly controls the residual Ti content to ≤0.006%.

[0029] On the one hand, Al element is added to molten steel as a deoxidizer. On the other hand, as the Al content increases, the starting precipitation temperature of AlN increases, the total amount of precipitation also increases, and the nitrogen fixation effect of Al is enhanced, which can replace Ti to achieve effective nitrogen fixation. The present invention controls Als=0.06%~0.10% to facilitate the formation of AlN, prevent BN precipitation, increase the solid solution amount of B, and thus ensure effective B for improving hardenability.

[0030] The N element reduces the toughness, welding performance, and thermal stress toughness of steel, making the steel more brittle; on the other hand, nitrogen can cause cracking of continuous casting billets. The harmful effects of nitrogen are mainly quenching aging and strain aging, which can reduce the toughness of steel and make the steel brittle. The N content of the present invention is controlled within the range of ≤0.004%.

[0031] P is an element that is easily segregated and easily aggregates at grain boundaries to cause grain boundary catalysis. Phosphorus also deteriorates welding performance, significantly reduces the low-temperature impact toughness of steel, and increases the brittle transition temperature of steel. The P content of the present invention is controlled within the range of ≤0.010%.

[0032] As the sulfur content increases, the crack sensitivity rate increases significantly; at the same time, sulfur also affects the impact toughness of the steel plate. As the sulfur content increases, the impact toughness value drops sharply. In addition, sulfur also causes the steel plate to be anisotropic, and the toughness in the transverse and thickness directions deteriorates. The sulfur content of the present invention is controlled within the range of ≤0.002%.

[0033] 2) Production process:

[0034] Smelting process: LF furnace refining process is refining temperature 1500 ~ 1650 ℃, composition fine-tuning, slag deoxidation, refining time ≥ 40min, argon blowing and stirring throughout the process; RH furnace vacuum degassing process is at a vacuum degree of 0.5tor, maintaining time for more than 15min, so that the hydrogen content of the outlet molten steel is ≤1.5PPm, nitrogen content ≤40ppm, oxygen content ≤20ppm, Ca treatment is carried out after the molten steel is degassed, and the soft blowing time is not less than 15 minutes to ensure that the denatured inclusions are fully floated and removed.

[0035] Continuous casting process: Molten steel that meets the composition control requirements is cast into continuous casting billets of 180, 220 or 260 mm, and the superheat of the steel is controlled at 5 to 15 °C above the liquidus. Full protection casting is used to prevent the molten steel from contacting the air, so as to control the absorption of nitrogen in the air by the molten steel during the casting process. The continuous casting billets are inspected and accepted according to the central segregation not higher than C1.5 and the central porosity not higher than C1.0. The slabs are put into the insulation cover for ≥48 hours off the line to fully remove the gas in the steel.

[0036] Rolling process: Two-stage rolling is adopted, and the grains are fully refined through high reduction in rough rolling and controlled rolling in the finishing stage.

[0037] Heat treatment process: quenching at 900-930℃, austenitizing heating time is plate thickness mm×(1.6-2.0)min / mm, quenching ensures that the high pressure section pressure of the quenching machine is ≥8bar, the low pressure section pressure is ≥4bar, and the surface temperature of the steel plate after quenching is ≤50℃.

[0038] Tempering at 580-620℃ for plate thickness mm×(3.0-4.0)min / mm, air cooling, to obtain tempered troostite structure with uniform and fine grains.

[0039] Advantages of the present invention:

[0040] 1) The appropriate content of Al is used for nitrogen fixation to ensure the acid-soluble boron ratio and reduce the adverse effects of hard and brittle second phases such as TiN and Ti (C, N) in the conventional component system on impact toughness and strain aging impact.

[0041] 2) The designed chemical composition strictly controls the N content in the molten steel and adds a trace amount of Nb, which further reduces the C and N content of the solid solution in α-Fe, making the N atoms exist completely in a combined state, reducing the influence of free N atoms on the strain failure performance of the steel plate.

[0042] 3) By adopting low superheat and full protection casting, the internal quality of the ingot is good. The H gas content of the steel can be further reduced by using the continuous casting ingot insulation cover to keep it warm and cool slowly, further improving the impact toughness.

[0043] 4) The cooling water distribution and roller speed of the quenching process are accurately controlled. The 960Mpa grade steel plates with a thickness of 4 to 30mm produced have good straightness and low residual stress after quenching and tempering. The straightness can reach 3mm / 2m and the maximum width is 4700mm. After high-temperature tempering at 580-620℃, a tempered troostite structure with uniform and fine grains is obtained. The original austenite grain size is 9-10, the low-temperature impact toughness is excellent, and it has good bending processing performance.

[0044] 5) The mechanical properties of the steel produced are excellent. The actual steel plate yield strength is ≥960MPa, the tensile strength is 980~1150MPa, the elongation is 15%~20%, the -40℃ impact energy value KV2 is ≥170J, and after 5% deformation and heat treatment at 250℃ for 1 hour, the -40℃ impact energy value KV2 is ≥150J. According to GB / T 4160-2004 "Steel Strain Aging Sensitivity Test Method", the strain aging sensitivity coefficient of the steel plate at -40℃ is ≤20%. It can meet the use requirements of steel for medium and large tonnage crane booms. DETAILED DESCRIPTION

[0045] The present invention is further described below with reference to the embodiments.

[0046] Example 1: Production of 12mm 960MPa grade steel for crane boom

[0047] The mass percentage of steel is: C = 0.15, Si = 0.16, Mn = 1.12, P = 0.008, S = 0.0013, Nb = 0.012, V = 0.05, Ti = 0.003, Als = 0.075, Cr = 0.20, Mo = 0.46, Ni = 0.20, B = 0.0014, N = 0.0032, the rest is Fe and unavoidable impurity elements, CEV = 0.49, Pcm = 0.27. Key process steps:

[0048] (1) The LF furnace refining process is as follows: refining temperature 1510~1640℃, composition fine-tuning, slag deoxidation, refining time 52min, argon blowing and stirring throughout the process;

[0049] (2) The RH furnace vacuum degassing process is to maintain a vacuum degree of 0.5tor for 18 minutes, so that the hydrogen content of the molten steel out of the station is 1.5ppm, the nitrogen content is 35ppm, and the oxygen content is 16ppm. After the molten steel is degassed, Ca treatment is carried out, and the soft blowing time is 15 minutes to ensure that the modified inclusions are fully floated and removed;

[0050] (3) Continuous casting: The superheat of the steel casting is controlled at 5 to 13°C above the liquidus line. Full protection is used for continuous casting to avoid contact between the molten steel and the air, so as to control the absorption of nitrogen in the air by the molten steel during the casting process. The center segregation of the continuous casting billet is C1.0, the center porosity is C0.5, and the billet is put into the insulation cover for cooling for 48 hours after it comes off the line;

[0051] (4) Controlled rolling: the billet thickness is 180 mm, and the billet is heated by a walking beam heating furnace, the heating temperature is 1200-1230°C, the soaking section temperature is 1210-1230°C, the furnace time is 5 hours, the holding time is 60 min, the rough rolling is widened, the reduction rate is 16% for 3 consecutive passes, the intermediate billet thickness is 75 mm, the finishing rolling start temperature is 880-920°C, and the final rolling temperature is 780-820°C;

[0052] (5) Heat treatment after rolling: quenching at 900-930℃, austenitizing time of 20min, high pressure section pressure of quenching machine of 8bar, low pressure section pressure of 4.1bar, surface temperature of steel plate after quenching of 20℃; tempering at 580-620℃, time of 25-28min, air cooling after exiting the furnace.

[0053] The performance test results of the steel are shown in Table 1.

[0054] Example 2: Production of 20mm 960MPa grade steel for crane boom

[0055] The mass percentage of steel is: C=0.16, Si=0.15, Mn=1.10, P=0.007, S=0.0008, Nb=0.014, V=0.06, Ti=0.005, Als=0.075, Cr=0.21, Mo=0.51, Ni=0.20, B=0.0016, N=0.0033, the rest is Fe and unavoidable impurity elements, CEV=0.51, Pcm=0.28. Key process steps:

[0056] (1) The LF furnace refining process is as follows: refining temperature 1520~1630℃, composition fine-tuning, slag deoxidation, refining time 52min, argon blowing and stirring throughout the process;

[0057] (2) The RH furnace vacuum degassing process is to maintain the vacuum degree of 0.5tor for 15in, so that the hydrogen content of the molten steel out of the station is 1.5ppm, the nitrogen content is 31pm, and the oxygen content is 20ppm. After the molten steel is degassed, Ca treatment is carried out, and the soft blowing time is 15 minutes;

[0058] (3) Continuous casting: The superheat of the steel casting is controlled at 5 to 14°C above the liquidus line. The continuous casting adopts full protection casting to avoid the contact between the molten steel and the air, so as to control the absorption of nitrogen in the air by the molten steel during the casting process. The center segregation of the continuous casting billet is C0.5, and the center looseness is C0.5;

[0059] (4) Controlled rolling: the billet thickness is 180 mm, and the billet is heated by a walking beam heating furnace at a heating temperature of 1200-1250°C, a soaking zone temperature of 1210-1250°C, a furnace time of 4 hours, a holding time of 40 min, and a continuous 3-pass rolling reduction rate of 16% after rough rolling and widening. The intermediate billet thickness is 90 mm, the finishing rolling start temperature is 880-920°C, and the final rolling temperature is 820-850°C;

[0060] (5) Heat treatment after rolling: quenching at 860-890°C, austenitizing time of 20-25 min, high pressure section pressure of quenching machine 8 bar, low pressure section pressure 4.1 bar, surface temperature of steel plate after quenching 42°C; tempering at 580-620°C for 50-70 min, air cooling after exiting the furnace.

[0061] The performance test results of the steel are shown in Table 1.

[0062] Table 1 Performance test results of example steel

[0063]

Claims

1. A method for producing 960MPa grade steel for crane booms, the process flow of which is hot metal pretreatment → converter steelmaking → LF furnace refining → vacuum treatment → continuous casting → heating → controlled rolling → post-rolling treatment → finishing → performance inspection, characterized in that: The chemical composition of the steel is C=0.15, Si=0.16, Mn=1.12, P=0.008, S=0.0013, Nb=0.012, V=0.05, Ti=0.003, Als=0.075, Cr=0.20, Mo=0.46, Ni=0.20, B=0.0014, N=0.0032, and the rest are Fe and unavoidable impurity elements, CEV=0.49, Pcm=0.27; the thickness of the steel plate is 12mm, the yield strength is 1009MPa, the tensile strength is 1053MPa, the elongation is 15.5%, the average impact energy value KV2=183J without strain aging at -40℃, the average impact energy value KV2=158J after 5% strain and aging at 250℃ for 1h at -40℃, and the strain aging sensitivity coefficient is 13.7%; Key process steps: (1) The LF furnace refining process is refining temperature 1510 ~ 1640 ℃, composition fine-tuning, slag deoxidation, refining time 52min, argon blowing and stirring throughout the process; (2) The RH furnace vacuum degassing process is to maintain the vacuum degree of 0.5tor for 18 minutes, so that the hydrogen content of the molten steel out of the station is 1.5ppm, the nitrogen content is 35ppm, and the oxygen content is 16ppm. After the molten steel is degassed, Ca treatment is carried out, and the soft blowing time is 15 minutes to ensure that the modified inclusions are fully floated and removed; (3) Continuous casting: The superheat of the steel is controlled at 5-13°C above the liquidus line. Full protection is used for continuous casting to avoid contact between the molten steel and the air, so as to control the absorption of nitrogen in the air by the molten steel during the casting process. The center segregation of the continuous casting billet is C1.0, and the center looseness is C0.

5. The billet is put into the insulation cover for cooling for 48 hours after it comes off the line. (4) Controlled rolling: the billet thickness is 180 mm, and the billet is heated by a walking beam heating furnace with a heating temperature of 1200-1230 °C, a soaking section temperature of 1210-1230 °C, a furnace time of 5 hours, a holding time of 60 min, a continuous 3-pass reduction rate of 16% after rough rolling and widening, an intermediate billet thickness of 75 mm, a finishing rolling start temperature of 880-920 °C, and a final rolling temperature of 780-820 °C; (5) Heat treatment after rolling: quenching at 900-930℃, austenitizing time is 20min, high pressure section pressure of quenching machine is 8bar, low pressure section pressure is 4.1bar, surface temperature of steel plate after quenching is 20℃; tempering at 580-620℃, time is 25-28min, and air cooling is performed after exiting the furnace.

2. A method for producing 960MPa grade steel for crane booms, the process flow of which is hot metal pretreatment → converter steelmaking → LF furnace refining → vacuum treatment → continuous casting → heating → controlled rolling → post-rolling treatment → finishing → performance inspection, characterized in that: The chemical composition of steel is C=0.16, Si=0.15, Mn=1.10, P=0.007, S=0.0008, Nb=0.014, V=0.06, Ti=0.005, Als=0.075, Cr=0.21, Mo=0.51, Ni=0.20, B=0.0016, N=0.0033, the rest is Fe and unavoidable impurity elements, CEV=0.51, Pcm=0.28; The thickness of the steel plate is 20mm, the yield strength is 1022MPa, the tensile strength is 1048MPa, the elongation is 16%, the average impact energy value KV2 is 176J without strain aging at -40℃, the average impact energy value KV2 is 154J after 5% strain and 250℃ for 1h at -40℃, and the strain aging sensitivity coefficient is 12.5%; Key process steps: (1) The LF furnace refining process is refining temperature of 1520-1630℃, fine-tuning of composition, slag deoxidation, refining time of 52min, and argon blowing and stirring throughout the process; (2) The RH furnace vacuum degassing process is to maintain the vacuum degree of 0.5tor for 15 minutes, so that the hydrogen content of the molten steel out of the station is 1.5ppm, the nitrogen content is 31ppm, and the oxygen content is 20ppm. After the molten steel is degassed, Ca treatment is carried out, and the soft blowing time is 15 minutes; (3) Continuous casting: The superheat of the steel casting is controlled at 5-14°C above the liquidus line. Full protection casting is adopted to avoid the contact between molten steel and air, so as to control the absorption of nitrogen in the air by molten steel during the casting process. The center segregation of the continuous casting billet is C0.5, and the center looseness is C0.

5. (4) Controlled rolling: the billet thickness is 180 mm, and the billet is heated by a walking beam heating furnace with a heating temperature of 1200-1250°C, a soaking zone temperature of 1210-1250°C, a furnace time of 4 hours, a holding time of 40 minutes, and a continuous 3-pass reduction rate of 16% after rough rolling and widening. The intermediate billet thickness is 90 mm, the finishing rolling start temperature is 880-920°C, and the final rolling temperature is 820-850°C. (5) Heat treatment after rolling: quenching at 860-890℃, austenitizing time of 20-25min, high pressure section pressure of quenching machine of 8bar, low pressure section pressure of 4.1bar, surface temperature of steel plate after quenching of 42℃; tempering at 580-620℃ for 50-70min, and air cooling after exiting the furnace.

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