High purity high strength steel and method of making same

Abstract

The application relates to a high-purity high-strength steel and a manufacturing method thereof. The manufacturing method significantly improves the purity and mechanical properties of the steel by optimizing the chemical composition and adopting a vacuum induction, electroslag remelting and vacuum consumable triple smelting process. The chemical composition comprises C, Mn, Si, Ni, Cr, Mo, V and the like, the impurity contents of P, S, Al, Ti and the like are strictly controlled, and the contents of H, O, N and the like are greatly reduced, so that the steel has excellent toughness and plasticity under the condition of ultrahigh strength. The manufacturing method combines multiple smelting and refining with quenching, tempering and heat treatment processes, so that the internal structure of the steel is uniform, the white spots and the like are eliminated, and the impact resistance and fatigue life of the steel are significantly improved. Finally, the high-purity high-strength steel has wide application prospects in high-demand fields such as aerospace, military industry and pressure-bearing components, and is especially suitable for key components requiring high strength and high toughness.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of alloy preparation, in particular to a high-purity high-strength steel and a manufacturing method thereof. BACKGROUND

[0002] High-strength steel refers to steel material with tensile strength exceeding 1500 MPa and yield strength greater than 1300 MPa at room temperature. In addition to the requirement of tensile strength of 1500 MPa or more, this type of steel material also needs to have good plasticity and toughness, reduce notch sensitivity as much as possible, have high fatigue strength, and exhibit good processability. Therefore, various medium-high alloy ultra-high strength steels have been developed and are widely used in many high-strength and high-demand fields, including but not limited to airplane landing parts, bulletproof steel plates, missile and shell casings, aerospace gear shafts, pressure-bearing components, rocket launcher casings, high-end molds, fasteners, etc. These steels exhibit good comprehensive performance under high stress and complex environments, and their application fields are still expanding, showing broad development prospects.

[0003] The development of medium-alloy ultra-high strength steel is based on the reasonable addition of elements such as C, Ni, Cr, Si, V, and Mo in steel, and through the optimization of the proportion of these elements, a large amount of martensite structure is formed after quenching and tempering. The pinning effect of carbides in the martensite dislocation structure significantly improves the strength and hardness of the material while maintaining certain plasticity and toughness. Generally, the total content of alloying elements in medium-alloy ultra-high strength steel is greater than 5%, among which C, Ni, Cr, Mo, V, and Si are the main strengthening elements. Through scientific design and organization control of these chemical compositions, the mechanical properties of medium-alloy ultra-high strength steel have been significantly improved.

[0004] For example, there is a medium-alloy ultra-high strength steel 45CrNiMoV for shell casings in the aviation field, and its chemical composition is shown in Table 1:

[0005] Table 1 Chemical composition of medium-alloy ultra-high strength steel 45CrNiMoV (wt%)

[0006]

[0007] This steel grade is produced by the following process: first, using a vacuum induction furnace to melt the raw materials into an electrode rod with a diameter of , then through electroslag remelting into an ingot with a diameter of , and then through a fast forging machine to forge into a material, and finally through annealing and bright turning to complete the processing of the steel material. However, this production process has the following significant problems:

[0008] Poor toughness and plasticity: Although the 45CrNiMoV steel has a very high tensile strength, its toughness and plasticity are relatively poor. In practical applications, especially in cases of dynamic load and impact, the material is prone to fracture or other damage, limiting its further application in high-load, high-stress environments.

[0009] Gas contamination problem in the smelting process: The smelting process using vacuum induction furnace plus electroslag remelting increases the content of gas elements such as oxygen, nitrogen and hydrogen during the electroslag remelting process, which makes it easy for the finished forging to contain a large number of oxide inclusions. Especially the increase of hydrogen content, will greatly increase the risk of white spot formation. White spot is a kind of micro crack induced by hydrogen, which is easy to cause brittle fracture of steel in subsequent processing or use, greatly reducing the strength and toughness of the material, and even may cause the product to be completely scrapped.

[0010] Uncertainty in the smelting process: Even if high-purity raw materials are used, the uncertainty in the electroslag remelting process will still cause the gas content to be high, thereby affecting the final mechanical properties of the material. Especially the content of hydrogen, oxygen, nitrogen and other gases is difficult to accurately control, which may eventually lead to fluctuations in the strength and toughness of the material. This uncertainty increases the risk in the production process and affects the reliability of the material.

[0011] Therefore, although the existing ultra-high strength steel has been widely used in many fields, the problems of insufficient toughness and plasticity, gas contamination in the smelting process, and uncertainty in the smelting process still plague the development of this field. These technical difficulties limit the popularization and use of ultra-high strength steel in more demanding application scenarios. In order to solve these problems and improve the overall performance of the steel, especially to further improve the toughness, plasticity and reduce the gas content and eliminate the tendency of white spot defects while ensuring ultra-high strength, it is necessary to improve the smelting process and optimize the material composition. SUMMARY

[0012] The technical problem to be solved by the present application is to provide a manufacturing method of high-purity high-strength steel to solve the problems of poor strength and toughness, poor reliability and gas contamination in the smelting process of the steel produced by the conventional manufacturing method in the prior art.

[0013] The present application provides a manufacturing method of high-purity high-strength steel, comprising the following steps:

[0014] S1: Vacuum induction furnace smelting

[0015] The electrode rod is prepared by vacuum induction furnace smelting with high-purity raw materials, and the raw materials are degassed after being melted, and the mass percentage of the gas content and chemical elements is: C: 0.44-0.46%, Mn: 0.60-1.00%, Si: 1.65-1.85%, S≤0.002%, P≤0.005%, Ni: 1.20-1.40%, Cr: 1.30-1.50%, Mo: 1.20-1.40%, V: 0.05-0.10%, Al: ≤0.015%, Ti: ≤0.015%, H: ≤0.00005%, O: ≤0.0008%, N: ≤0.0010%, and the balance is Fe and other unavoidable impurities;

[0016] S2: electroslag remelting

[0017] The electrode rod prepared by S1 is subjected to electroslag remelting, a slag system with a fluorite and Al2O3 slag ratio of 70:30 is used, and the melting speed is controlled to improve the segregation of chemical elements and the purity of the steel material, so as to obtain an electroslag ingot;

[0018] S3: electroslag ingot forging vacuum consumable electrode:

[0019] The electroslag ingot obtained in the step S2 is subjected to forging after heat treatment, so as to obtain a vacuum consumable electrode;

[0020] S4: vacuum consumable remelting

[0021] The vacuum consumable electrode of the step S3 is subjected to vacuum consumable remelting, and the purity of the steel is further improved by degassing, and the vacuum degree is controlled to be ≤0.5 Pa during the melting process;

[0022] S5: forging treatment

[0023] The electroslag ingot after the step S4 is subjected to forging treatment, and is gradually heated to 800℃ and kept for ≥1 hour before forging, and is further heated to 1140-1210℃ and kept for ≥2 hours, and then is forged in the temperature range of 1000-850℃, and is slowly cooled after forging;

[0024] S6: heat treatment after annealing

[0025] The steel material after forging is subjected to heat treatment after annealing, and the heat treatment after annealing includes normalizing, tempering, quenching and re-tempering, so as to optimize the mechanical properties and organizational uniformity of the steel material.

[0026] Compared with the conventional steel smelting process in the prior art, the three-in-one smelting process of vacuum induction furnace smelting + electroslag remelting + vacuum consumable remelting is adopted, so that the gas content in the steel material is greatly reduced, especially the gas impurities such as hydrogen, oxygen and nitrogen, and the purity of the steel material is significantly improved. At the same time, by optimizing the chemical composition and smelting process parameters, the steel material of the present application has good plasticity and toughness while maintaining high strength. The technical principle of the preparation method in the present application is to use a vacuum induction furnace for degassing treatment of raw materials, effectively reducing the gas impurities in the steel material, and further removing residual inclusions and segregation by electroslag remelting. Vacuum consumable remelting further improves the purity of the steel material, reduces the number and volume of inclusions, optimizes the microstructure of the steel material, thereby improving the mechanical properties of the material. The final technical effect is that through this smelting process, the produced steel material has high strength (tensile strength exceeding 1500 MPa), high plasticity and high toughness, and its fatigue performance and fracture resistance are significantly improved, and it is particularly suitable for high-end fields such as aerospace, military industry, etc.

[0027] In a possible implementation, in the step S1, the vacuum induction furnace smelting temperature is 1520-1600℃, the vacuum degree during refining is ≤5Pa, the refining period temperature is 1530-1570℃, and the refining time is ≥30 minutes.

[0028] Compared with the prior art, the above technical solution can greatly reduce the content of gas impurities (such as oxygen, nitrogen, hydrogen, etc.) in the molten steel by precisely controlling the temperature and vacuum degree of the vacuum induction furnace, and reduce the generation of inclusions. Keeping a long enough time during the refining stage is beneficial to further degassing and homogenizing the molten steel, ensuring that the steel material finally has high purity and good mechanical properties.

[0029] In a possible implementation, in the step S1, when O≤15ppm, N≤15ppm, H≤1ppm in the molten steel, Ar≥3000Pa is filled, and metal Mn or electrolytic Mn is started to be added, and the casting temperature of the electrode is 1540-1580℃.

[0030] Compared with the prior art, the above technical solution can further reduce the gas content by adding high-purity inert gas (such as argon) to the molten steel during the smelting process. At the same time, by controlling the casting temperature, it is prevented that the molten steel is not completely cast due to too low temperature, or the electrode is cracked due to too high temperature, thereby ensuring the quality of the electrode.

[0031] In a possible implementation, in the step S2, the electroslag remelting current is 3KA-19KA, and the power is 60kw-800kw; the melting speed is 3.5-10.5kg / min, and slow cooling ≥24 hours is performed after the electroslag remelting is completed.

[0032] Compared with the prior art, the above technical scheme can improve the segregation in the steel material by accurately controlling the electric current and melting speed of the electroslag remelting, and can effectively reduce the accumulation of internal stress in the steel material and prevent cracking of the material in subsequent processing by slow cooling.

[0033] In a possible implementation, the heat treatment in the step S3 includes the following steps:

[0034] Initial temperature control at less than 600℃, then gradually heated to 800℃ at a steady rate, and the entire heating process lasts more than 2 hours;

[0035] 800℃ holding stage: when the temperature reaches 800℃, the temperature is maintained at this temperature for at least 1 hour;

[0036] Further heating stage: after the 800℃ holding, continue to heat until the temperature reaches between 1140℃ and 1210℃, and the heating time is not less than 2 hours, to ensure gradual heating and avoid thermal stress or uneven organization of the material.

[0037] High-temperature holding stage: maintain the temperature in the temperature range of 1140℃ to 1210℃ for at least 2 hours.

[0038] Compared with the prior art, the above technical scheme can control the temperature rise and holding time in stages, ensure that each part of the material is uniformly heated during heating, avoid thermal stress concentration and uneven organization, and ensure that the steel material after forging has good toughness and grain size control.

[0039] In a possible implementation, the forging in the step S3 includes the following steps:

[0040] Reburning stage: heat the electroslag ingot to 1120-1160℃ to ensure the thermal plasticity of the material during forging;

[0041] This temperature range can make the material maintain good plasticity during forging and reduce the risk of cracks and other defects during processing. At the same time, the chemical composition in the material is homogenized during the reburning process, further improving the processability of the steel material.

[0042] Forging stage: forging is carried out in the temperature range of ≥1000℃ for opening forging and ≥850℃ for final forging, to ensure the best formability and grain size control of the material;

[0043] This temperature range ensures that the material is shaped within the optimal thermoplasticity interval and guarantees the best grain size control, avoiding grain growth. By gradually reducing the forging temperature, the toughness of the steel can be effectively improved while maintaining its high strength.

[0044] Slow cooling phase: After forging, slow cooling treatment is performed, and the slow cooling method is cover cooling, sand cooling, pit cooling or furnace cooling, and the slow cooling time is not less than 24 hours.

[0045] Through slow cooling, the internal stress in the steel is released, avoiding cracks or uneven organization caused by rapid cooling. This step has a significant effect on improving the material's fracture resistance and fatigue strength.

[0046] Polishing treatment: After slow cooling, the forged piece is polished to ensure that it meets the electrode size requirements matched by the vacuum consumable crystallizer. Polishing can remove the oxide layer and other impurities on the surface of the forged piece, ensuring that the electrode has good surface quality and providing an ideal electrode shape for subsequent vacuum consumable remelting.

[0047] Compared with the prior art, the above technical solution can significantly improve the plasticity and microstructure of the steel during forging. The re-burning phase controls the uniform heating and temperature of the material, reducing the generation of thermal stress in the material, and controlling the temperature gradient during forging to ensure grain refinement of the steel. Slow cooling (slow cooling) after forging further reduces the accumulation of internal stress, reduces the risk of thermal cracks during cooling, and maintains the good organization uniformity of the steel. The technical principle is to accurately control the temperature during forging, especially in the high-temperature plasticity region, to ensure that the thermal plasticity of the material is fully developed. At the same time, the slow cooling process effectively releases the residual stress in the material, making the microstructure of the steel more stable and the grain size uniform, thereby improving the mechanical properties and service life of the material. The final technical effect is that the present invention ensures the formability and toughness of the steel during forging, and the grain size of the steel is significantly refined after forging, and the material exhibits high strength, high toughness and good fatigue resistance, which is suitable for aerospace, military and other fields with high mechanical performance requirements.

[0048] In one possible implementation, in step S4, the melting speed during the vacuum consumable remelting process is 2.5-6.5 kg / min, and helium gas is used for cooling to prevent chemical composition and ingot segregation.

[0049] Compared with the prior art, the above technical solution can improve the segregation in the steel by accurately controlling the current and melting speed of electroslag remelting, and can effectively reduce the accumulation of internal stress in the steel by slow cooling (slow cooling), preventing cracking of the material during subsequent processing.

[0050] In a possible implementation, in step S5, the slow cooling after forging is performed by means of cover cooling, sand cooling, pit cooling or furnace cooling, and the slow cooling time is not less than 24 hours.

[0051] Compared with the prior art, the above technical solution can reduce the stress generated in the material during rapid cooling by means of slow cooling, ensure that the internal structure of the material is more stable, avoid cracks or other defects in subsequent processing, and improve the final mechanical properties of the material.

[0052] In a possible implementation, in step S6, the heat treatment after calcination includes the following steps:

[0053] normalizing: the temperature is 920±10℃, the holding time is 2.5mm / minute+60 minutes, and the cooling mode is air cooling;

[0054] tempering: the temperature is 680±10℃, the holding time is 120-240 minutes, and the cooling mode is air cooling;

[0055] quenching: the temperature is 890±10℃, the holding time is 60 minutes, and the cooling mode is oil cooling;

[0056] retempering: the temperature is 510-550℃, the holding time is 120 minutes, and the cooling mode is air cooling.

[0057] Compared with the prior art, the above technical solution can control the temperature rise and fall and the holding time in stages, ensure that each part of the material is uniformly heated during heating, avoid the problems of thermal stress concentration and uneven structure, and ensure that the forged steel has good toughness and grain size control.

[0058] Another technical problem to be solved by the present application is to provide a high-purity high-strength steel to solve the problem that conventional steel grades in the prior art have a contradiction between high strength and high toughness.

[0059] To solve the above technical problems, the present application also provides a high-purity high-strength steel prepared by the manufacturing method, and the composition of the high-purity high-strength steel comprises: C: 0.44-0.46%; Mn: 0.60-1.00%; Si: 1.65-1.85%; S≤0.002%; P≤0.005%; Ni: 1.20-1.40%; Cr: 1.30-1.50%; Mo: 1.20-1.40%; V: 0.05-0.10%; Al: ≤0.015%; Ti: ≤0.015%; H: ≤0.00005%; O: ≤0.0008%; N: ≤0.0010%; and the balance is Fe and other unavoidable impurities.

[0060] The technical effect of the high-purity high-strength steel lies in that, through the component proportioning of the high-purity high-strength steel, the obtained steel has excellent mechanical properties, low impurity element content and high purity, and can be widely applied to the fields of aerospace, military and high-end equipment. BRIEF DESCRIPTION OF DRAWINGS

[0061] Figure 1 Figure for the influence of Si content on the formation temperature of cementite in the tempering process;

[0062] Figure 2 Figure for the influence of Si content on the tempering hardness;

[0063] Figure 3 Figure for the heating curve of the electroslag ingot forging electrode;

[0064] Figure 4 Figure for the macrostructure of the finished material of the steel prepared in Example 1, the left and right figures correspond to the head and tail of the finished material of the steel, respectively;

[0065] Figure 5 Figure for the grain size of the finished material of the steel prepared in Example 1, the left and right figures correspond to the head and tail of the finished material of the steel, respectively. DETAILED DESCRIPTION

[0066] Firstly, those skilled in the art should understand that the embodiments are only used to explain the technical principles of the embodiments of the present application, and are not intended to limit the protection scope of the embodiments of the present application. Those skilled in the art can adjust them as needed in order to adapt to specific application occasions.

[0067] The present application provides a manufacturing method of high-purity high-strength steel, comprising the following steps:

[0068] S1: vacuum induction furnace smelting

[0069] High-purity raw materials are used, vacuum induction furnace smelting is adopted, the raw materials are degassed after being melted, and an electrode rod is prepared, the mass percentage content of the gas content and chemical elements is as follows: C: 0.44-0.46%, Mn: 0.60-1.00%, Si: 1.65-1.85%, S≤0.002%, P≤0.005%, Ni: 1.20-1.40%, Cr: 1.30-1.50%, Mo: 1.20-1.40%, V: 0.05-0.10%, Al: ≤0.015%, Ti: ≤0.015%, H: ≤0.00005%, O: ≤0.0008%, N: ≤0.0010%, and the balance is Fe and other inevitable impurities;

[0070] S2: electroslag remelting

[0071] The electrode bar prepared by S1 is subjected to electroslag remelting, a slag system with a fluorite and Al2O3 slag ratio of 70:30 is used, and the melting speed is controlled to improve the segregation of chemical elements and the purity of the steel, to obtain an electroslag ingot;

[0072] S3: forging of the electroslag ingot into a consumable vacuum electrode

[0073] The electroslag ingot obtained in S2 is subjected to heat treatment and then forging to obtain a consumable vacuum electrode;

[0074] S4: consumable vacuum remelting

[0075] The consumable vacuum electrode of S3 is subjected to consumable vacuum remelting to further degas and improve the purity of the steel, and the vacuum degree is controlled to be ≤0.5 Pa during melting;

[0076] S5: forging treatment

[0077] The electroslag ingot after S4 is subjected to forging treatment, and is gradually heated to 800℃ and kept for ≥1 hour before forging, and then further heated to 1140-1210℃ and kept for ≥2 hours, and then forged at a temperature of 1000-850℃ and then slowly cooled;

[0078] S6: heat treatment after annealing

[0079] The steel after forging is subjected to heat treatment after annealing, which includes normalizing, tempering, quenching and re-tempering to optimize the mechanical properties and microstructure uniformity of the steel.

[0080] As a preferred scheme, in S1, the vacuum induction furnace smelting temperature is 1520-1600℃, the vacuum degree during refining is ≤5 Pa, the refining temperature is 1530-1570℃, and the refining time is ≥30 minutes.

[0081] As a preferred scheme, in S1, when O≤15ppm, N≤15ppm, H≤1ppm in the steel liquid, Ar≥3000 Pa is filled, and metal Mn or electrolytic Mn is started to be added, and the casting temperature of the electrode is 1540-1580℃.

[0082] As a preferred scheme, in S2, the electroslag remelting current is 3-19 KA, and the power is 60-800 kW; the melting speed is 3.5-10.5 kg / min, and slow cooling is performed for ≥24 hours after the electroslag remelting is completed.

[0083] As a preferred scheme, in S3, the heat treatment includes the following steps:

[0084] Initial heating stage: initial temperature control below 600℃, then heating to 800℃ at a steady rate, the whole heating process lasts more than 2 hours;

[0085] 800℃ holding stage: when the temperature reaches 800℃, keep at this temperature for at least 1 hour;

[0086] Further heating stage: after holding at 800℃, continue to heat until the temperature reaches between 1140℃ and 1210℃, the heating time is not less than 2 hours, to ensure gradual heating and avoid thermal stress or uneven organization of the material.

[0087] High temperature holding stage: at a temperature range of 1140℃ to 1210℃, keep the temperature for at least 2 hours.

[0088] As a preferred solution, in the step S3, the forging includes the following steps:

[0089] Reburning stage: heat the electroslag ingot to 1120-1160℃ to ensure the thermal plasticity of the material during forging;

[0090] Forging stage: forging is carried out at a temperature range of ≥1000℃ for opening forging and ≥850℃ for final forging, to ensure the best formability and grain size control of the material;

[0091] Slow cooling stage: after forging, slow cooling treatment is carried out, the slow cooling method is cover cooling, sand cooling, pit cooling or furnace cooling, and the slow cooling time is not less than 24 hours;

[0092] Polishing treatment: after slow cooling, the forged piece is polished to ensure that it meets the electrode size requirements matched by the vacuum consumable crystallizer.

[0093] As a preferred solution, in the step S4, the melting speed during the vacuum consumable remelting process is 2.5-6.5kg / min, and helium gas is used for cooling to prevent segregation of chemical composition and ingot type.

[0094] As a preferred solution, in the step S5, after forging, slow cooling is carried out by cover cooling, sand cooling, pit cooling or furnace cooling, and the slow cooling time is not less than 24 hours.

[0095] As a preferred solution, in the step S6, the heat treatment after forging includes the following steps:

[0096] Normalizing: the temperature is 920±10℃, the holding time is 2.5mm / min+60 minutes, and the cooling method is air cooling;

[0097] Tempering: the temperature is 680±10℃, the holding time is 120-240 minutes, and the cooling method is air cooling;

[0098] Quenching: temperature 890±10℃, holding time 60 minutes, cooling mode oil cooling;

[0099] Re-tempering: temperature 510-550℃, holding time 120 minutes, cooling mode air cooling.

[0100] The application further provides a high-purity high-strength steel prepared by the manufacturing method, and the high-purity high-strength steel comprises the following components: C: 0.44-0.46%; Mn: 0.60-1.00%; Si: 1.65-1.85%; S≤0.002%; P≤0.005%; Ni: 1.20-1.40%; Cr: 1.30-1.50%; Mo: 1.20-1.40%; V: 0.05-0.10%; Al: ≤0.015%; Ti: ≤0.015%; H: ≤0.00005%; O: ≤0.0008%; N: ≤0.0010%; and the balance is Fe and other inevitable impurities.

[0101] The following is a detailed description of the elements in the high-purity high-strength steel, and a further description of the component configuration of the high-purity high-strength steel and the reasons for the configuration:

[0102] As an ultra-high-purity ultra-high-strength steel, the matching of the extremely low residual elements and the interaction and addition amount of the alloy elements is very important.

[0103] The role of carbon in the steel is as follows: ① solid solution in the martensite matrix during austenitizing, which plays a solid solution strengthening role and ensures that the steel has ultra-high strength. Research shows that when the solid solution C content in the martensite of low-carbon steel and medium-carbon steel after low-temperature tempering is in the range of 0.2%-0.5%, the linear relationship between the tensile strength and the carbon atomic mass fraction is maintained: tensile strength (MPa) = 2940*C+820. The gap solid solution strengthening of C atoms significantly improves the strength, but also endangers the toughness and plasticity of the material, and must be tempered to precipitate a part of carbon atoms in the supersaturated martensite, which significantly improves the toughness and plasticity of the material while appropriately reducing the solid solution strengthening effect. ② Fine ε-carbides are formed during tempering, which produces a precipitation strengthening effect.

[0104] The role of silicon in the steel is reflected in that Si is an element that narrows the austenite phase region, but it increases the stability of the supercooled austenite. At a higher temperature, it increases the activity of carbon atoms in the austenite, promotes the diffusion of carbon in the austenite phase region, and at a lower temperature, it increases the binding force of carbon atoms and iron atoms, increases the diffusion activation energy of carbon in the austenite, thereby inhibiting the diffusion of carbon in the austenite phase region. At the same time, Si has a strong hindering effect on the precipitation of cementite. There are currently two views: one believes that Si diffuses into ε-carbide, increases its stability, and thus it is difficult to form cementite; the other view believes that Si is insoluble in cementite, and the growing cementite sheet will push out silicon atoms, and the diffusion of silicon at low temperature is quite difficult, thereby forming a kinetic barrier to hinder the further growth of cementite. Regardless of which mechanism, the result is that Si will push the first type of temper brittleness to a high temperature direction, thereby improving its plasticity, toughness and notch sensitivity. Referring to the effects of Si content on the formation temperature of cementite in the tempering process shown in Figure 1 、 Figure 2 , it can be seen that when the Si content is below 3% (atomic percent), the formation temperature of cementite increases significantly with the increase of the content.

[0105] The role of nickel in the steel is reflected in that Ni is an element that expands the austenite phase region, strongly inhibits the transformation of austenite to pearlite and bainite during cooling after austenitizing, and improves the hardenability of martensite; Ni reduces the temperature of the transformation of supercooled austenite to martensite, increases the content of residual austenite, and is beneficial to toughness; Ni can increase the stacking fault energy of the steel matrix, so that screw dislocations are easy to produce cross slip, thereby improving the toughness.

[0106] The role of manganese in the steel is reflected in that Mn and Ni are both elements that expand the austenite phase region, and Mn reduces the temperature of the transformation of supercooled austenite to martensite, increases the content of residual austenite, and is beneficial to toughness.

[0107] The role of vanadium in the steel is reflected in that V is a strong carbide forming element, forms stable VC phase, and refines the austenite grain during quenching and heating, thereby refining the martensite structure, improving the toughness of tempered martensite, and increasing the cleavage fracture stress.

[0108] The role of chromium and molybdenum in the steel is reflected in that Cr and Mo delay the transformation of supercooled austenite to pearlite during quenching and cooling, improve the hardenability of the steel, and in the tempering process, enter the carbide to form alloy carbide, increase the stability of the carbide, and improve the tempering resistance of the steel. At the same time, Cr can also improve the corrosion resistance of the steel.

[0109] The role of aluminum and titanium in the steel is reflected in that Al and Ti in the steel are easy to combine with N in the steel to form AlN and TiN, increasing the number of E-type non-metallic inclusions, resulting in the decrease of the strength and toughness of the steel. In order to realize the theoretical base strength of the steel and improve the purity of the steel, the content of Al and Ti must be strictly controlled as low as possible.

[0110] The role of phosphorus and sulfur in the steel is reflected in that phosphorus can reduce the cold bending performance of the steel and make the weldability worse. Meanwhile, phosphorus can improve the strength, hardness, wear resistance and corrosion resistance of the steel, but these positive effects are usually offset by the cold brittleness caused by phosphorus. Phosphorus dissolves in ferrite, and due to its large atomic diameter, it can cause ferrite lattice distortion, thereby significantly reducing the plasticity and toughness of the steel, especially in low temperature environment. In addition, phosphorus is easy to segregate during the crystallization process of the steel and is difficult to eliminate by heat treatment, increasing the risk of cold brittleness of the steel. The harm of sulfur: sulfur can easily cause brittle cracking during the hot working process of the steel, which is called hot brittleness. The presence of sulfur also reduces the impact toughness, fatigue strength, weldability and corrosion resistance of the steel. Even if sulfur exists in trace amounts, it is harmful to the steel, so the content of sulfur needs to be strictly controlled. Sulfur can form brittle compounds in the weld, causing the weld joint to be weak and even cracking during welding. Sulfur also reduces the cold working forming performance of the steel, affecting its plastic deformation ability, making the steel prone to cracking and breaking during bending, stamping and other processing forming processes. The presence of phosphorus and sulfur in the steel has a negative impact on its physical properties and processing performance, so their content in the steel needs to be strictly controlled.

[0111] In summary, harmful elements P, S, Al, Ti and gases H, O, N, etc. are considered as impurity elements, and the content of these elements is as low as possible. The raw materials are selected from intermediate alloys, metal materials, etc. of the steel or similar steels, and the high-purity intermediate alloy ultra-high strength steel is obtained through vacuum induction, electroslag remelting and vacuum consumable remelting smelting.

[0112] More specifically, the above steps S1-S6 further include the following detailed steps:

[0113] In step S1: (1) Raw material requirements of vacuum induction furnace:

[0114] ① High-purity pure iron is added, and the pure iron requires S≤0.002%, P≤0.005% or refined steel material melted by electric furnace + LF + VOD + VD, and the refined steel material requires residual elements S, P, Al≤0.001%.

[0115] ② Very high purity metal molybdenum, metal nickel, metal chromium, metal vanadium, metal manganese or electrolytic manganese are added; the surface of the raw material is checked before charging, and rust, oil stains and oxidation are not allowed, and must be cleaned if any.

[0116] (2) Vacuum induction furnace smelting process:

[0117] ① Slow low temperature melting in the melting period, temperature 1520-1600℃ after full melting; the vacuum induction furnace smelting can improve the degassing effect by increasing the molten steel surface, so slow low power melting can achieve good degassing effect.

[0118] ② The vacuum degree requirement is ≤5Pa in the refining period, the refining period holding time is ≥30min, and the refining period temperature is 1530-1570℃; too low temperature cannot achieve refining effect, and too high temperature causes Al and Ti in the crucible to be reduced into the molten steel.

[0119] ③ After the refining period, when the gas meets the requirements, i.e. O≤15ppm, N≤15ppm, H≤1ppm, Ar≥3000Pa starts to add metal Mn or electrolytic Mn.

[0120] ④ Pouring temperature 1540-1580℃. Too low pouring temperature causes molten steel solidification pouring to be unable to complete; too high pouring temperature leads to electrode easy cracking.

[0121] ⑤ The electrode requires slow cooling (including sand cooling, cover cooling, pit cooling, furnace with slow cooling and other cooling methods) polishing and flat head after electroslag remelting. It effectively reduces the content of chemical element S in the steel and improves non-metallic inclusions to ensure the purity of the steel, preparing for improving the strength and toughness of the steel.

[0122] In step S2: (3) Preparation of electroslag remelting:

[0123] ① Each electrode corresponds to one ingot, requiring the shrinkage hole to be downward and the shrinkage hole end to be cut flat.

[0124] ② Slag ratio: Fluorite: Al2O3 = 70:30; slag weight 36-230kg (select appropriate slag weight according to the ingot type of electroslag ingot).

[0125] ③ Slag baking temperature: 600-1000℃; single hopper baking time: 3-10h. When there are other slag in the heating furnace, the baking time is required to be ≥5h.

[0126] ④ Arc starter: this steel bell or carbon steel conducting block.

[0127] (4) Electroslag remelting process parameters (argon protection electroslag furnace)

[0128] ① Arc starting stage: current, power control, different parameters are selected for different ingot types, the following parameter range is used for 1-6 ton steel ingot. The parameters are as follows:

[0129] Current: 3KA-19KA; power 60kw-800kw; arc starting time ≥30min.

[0130] ②Smelting stage: The smelting process is controlled by the melting rate. Different melting rates are specified for different ingot types. The melting period is based on the melting rate, and the melting rate is as low as possible to improve chemical element segregation. Melting rate: 3.5-10.5 kg / min.

[0131] ③ Hot sealing top stage: Hot sealing top is controlled by the melting rate. Different starting weights are specified for hot sealing top for different ingot types. In order to achieve the effect of shrinkage, hot sealing top needs enough weight support. The starting weight of hot sealing top is 100-550 kg. The melting rate gradually decreases from 10.5 to 2.0 kg / min.

[0132] ④ Slow cooling after ingot demoulding (including cover cooling, sand cooling, pit cooling, and furnace cooling) ≥24 h; or red delivery for forging.

[0133] Step S3: (5) Electroslag ingot forging vacuum consumable electrode process

[0134] ① Electroslag ingot forging heating curve reference Figure 3

[0135] ② Reburning temperature 1120-1160℃, open forging temperature ≥1000℃, final temperature ≥850℃. Within this temperature range, steel forming can be guaranteed in the best hot plasticity zone, and grain size can also be effectively controlled.

[0136] ③ Allow one-fire direct forging of the required electrode size.

[0137] ④ Slow cooling after forging (including cover cooling, sand cooling, pit cooling, and furnace cooling) ≥24 h.

[0138] ⑤ After slow cooling, the surface is polished according to the electrode size matched by the vacuum consumable crystallizer.

[0139] Step S4: (6) Vacuum consumable furnace smelting process:

[0140] ① Vacuum consumable furnace smelting requires a vacuum degree of ≤0.5 Pa to achieve remelting and degassing.

[0141] ② Vacuum consumable furnace smelting specifies different melting rates for different ingot types. The melting period is based on the melting rate, and the melting rate is as low as possible to improve chemical element segregation.

[0142] Parameters: Starting stage current 4.0-11.5 (KA), melting stage melting rate 2.5-6.5 kg / min, and shrinkage stage 2.5-6.5 (KA) control, which can effectively prevent segregation.

[0143] ③ Vacuum consumable furnace smelting requires opening helium cooling for accelerated cooling, melting and solidification, and utilizing melting droplet pulse technology to improve chemical composition segregation and ingot segregation and improve steel ingot purity. ​

[0144] (iv) After the vacuum consumable furnace remelting ingot, it needs to be slowly cooled (including cover cooling, sand cooling, pit cooling, furnace cooling) ≥24h for the surface of the steel ingot to be polished or ground.

[0145] In step S5: (6) Forging: the vacuum consumable steel ingot is heated to 1220℃ for more than 25h, and diffusion is carried out to make the carbide dispersedly distributed. After diffusion, the furnace is opened for forging, and the open forging temperature is 1180-1200℃, and the final forging temperature is 850-900℃. In this temperature range, the steel forming can be ensured in the best hot plasticity zone, so that the forging cracking is avoided.

[0146] In step S6: (7) Normalizing is treated at 920±10℃, and the temperature is kept for 2.5mm / min+60min, and air cooling is carried out.

[0147] (8) Tempering is treated at 680±10℃, and the temperature is kept for 2.5mm / min+120min-240min, and air cooling is carried out. This tempering is to remove the organizational stress in the steel.

[0148]

[0149] (9) Quenching is treated at 890±10℃, and the temperature is kept for 2.5mm / min+60min, and oil cooling is carried out.

[0150] (10) Re-tempering is treated at 510-550℃, and the temperature is kept for 2.5mm / min+120min, and air cooling is carried out. This tempering is to make the mechanical properties reach the best and is the heat treatment step set for this purpose.

[0151] The following provides specific examples combined with the above data range to further explain the present application:

[0152] Example 1:

[0153] Example 1 provides a high-purity high-strength steel and a manufacturing method thereof, the high-purity high-strength steel comprising the following components: C: 0.45%; Mn: 0.80%; Si: 1.75%; S≤0.002%; P≤0.005%; Ni: 1.30%; Cr: 1.40%; Mo: 1.30%; V: 0.075%; Al: ≤0.015%; Ti: ≤0.015%; H: ≤0.00005%; O: ≤0.0008%; N: ≤0.0010%; and the balance being Fe and other unavoidable impurities.

[0154] The manufacturing method comprises the following steps:

[0155] S1: Vacuum induction furnace smelting

[0156] ​The electrode rod is prepared by vacuum induction furnace smelting with high-purity raw materials, degassing after melting the raw materials, and controlling the gas content and mass percentage of chemical elements as follows: C: 0.45%; Mn: 0.80%; Si: 1.75%; S≤0.002%; P≤0.005%; Ni: 1.30%; Cr: 1.40%; Mo: 1.30%; V: 0.075%; Al: ≤0.015%; Ti: ≤0.015%; H: ≤0.00005%; O: ≤0.0008%; N: ≤0.0010%; and the balance of Fe and other unavoidable impurities. The vacuum induction furnace smelting temperature is 1560℃, when the O≤15ppm, N≤15ppm, H≤1ppm in the molten steel, Ar≥3000Pa is filled, and the metal Mn or electrolytic Mn is started to be added, and the casting temperature of the electrode is 1560℃, the vacuum degree during refining is ≤5Pa, the refining temperature is 1550℃, and the refining time is ≥30 minutes.

[0157] S2: electroslag remelting

[0158] The electrode rod prepared by S1 is subjected to electroslag remelting, a slag system with a fluorite and Al2O3 slag ratio of 70:30 is used, and the melting speed is controlled to improve the segregation of chemical elements and the purity of the steel material, to obtain an electroslag ingot;

[0159] The electroslag remelting current is 3KA-19KA, the power is 60kw-800kw, the melting speed is 7kg / min, and slow cooling is performed for ≥24 hours after the electroslag remelting is completed.

[0160] S3: electroslag ingot forging vacuum consumable electrode:

[0161] The electroslag ingot obtained in step S2 is subjected to forging after heat treatment to obtain a vacuum consumable electrode;

[0162] In step S3, the heat treatment includes the following steps:

[0163] Initial heating stage: the initial temperature is controlled below 600℃, and then heated to 800℃ at a steady rate, and the whole heating process lasts more than 2 hours;

[0164] 800℃ holding stage: when the temperature reaches 800℃, it is kept at this temperature for at least 1 hour of holding;

[0165] Further heating stage: after holding at 800℃, continue to heat until the temperature reaches 1175℃, and the heating time is not less than 2 hours, to ensure gradual heating and avoid thermal stress or uneven organization of the material.

[0166] High-temperature holding stage: at a temperature of 1175℃, the temperature is kept for at least 2 hours.

[0167] The forging includes the following steps:

[0168] Reburning stage: the electroslag ingot is heated to 1140℃ to ensure the thermal plasticity of the material during forging;

[0169] Forging stage: forging is carried out in a temperature range of open forging temperature ≥ 1000℃ and finish forging temperature ≥ 850℃ to ensure the best formability and grain size control of the material;

[0170] Slow cooling stage: after forging is completed, slow cooling treatment is carried out, and the slow cooling mode is cover cooling, sand cooling, pit cooling or furnace cooling, and the slow cooling time is not less than 24 hours;

[0171] Polishing treatment: after slow cooling, the forged piece is polished to ensure that it meets the electrode size requirements matched by the vacuum consumable crystallizer.

[0172] S4: vacuum consumable remelting

[0173] The vacuum consumable electrode of step S3 is subjected to vacuum consumable remelting to further degas and improve the purity of the steel, and the vacuum degree is controlled to be ≤0.5Pa during melting.

[0174] The melting speed during the vacuum consumable remelting process is 4.5kg / min, and helium gas is used for cooling to prevent segregation of chemical composition and ingot type.

[0175] S5: forging treatment

[0176] The electroslag ingot after step S4 treatment is subjected to forging treatment, and before forging, it is gradually heated to 800℃ and kept for ≥1 hour, and then further heated to 11750℃ and kept for ≥2 hours, and then forged at a temperature of 925℃, and then slowly cooled after forging; the slow cooling after forging adopts cover cooling, sand cooling, pit cooling or furnace cooling mode, and the slow cooling time is not less than 24 hours.

[0177] S6: heat treatment after forging

[0178] The forged steel is subjected to heat treatment after forging, and the heat treatment after forging includes normalizing, tempering, quenching and retempering to optimize the mechanical properties and organizational uniformity of the steel.

[0179] The heat treatment after forging includes the following steps:

[0180] Normalizing: the temperature is 920℃, the holding time is 2.5mm / minute+60 minutes, and the cooling mode is air cooling;

[0181] Tempering: the temperature is 680℃, the holding time is 180 minutes, and the cooling mode is air cooling;

[0182] Quenching: the temperature is 890℃, the holding time is 60 minutes, and the cooling mode is oil cooling;

[0183] Re-tempering: temperature is 530℃, holding time is 120 minutes, and cooling mode is air cooling.

[0184] Example 2:

[0185] Example 2 provides a high-purity high-strength steel and a manufacturing method thereof, the high-purity high-strength steel comprising the following components: C: 0.44%; Mn: 0.60%; Si: 1.65%; S≤0.002%; P≤0.005%; Ni: 1.20%; Cr: 1.30%; Mo: 1.20%; V: 0.05%; Al: ≤0.015%; Ti: ≤0.015%; H: ≤0.00005%; O: ≤0.0008%; N: ≤0.0010%; and the balance being Fe and other unavoidable impurities.

[0186] The manufacturing method comprises the following steps:

[0187] S1: Vacuum induction furnace smelting

[0188] High-purity raw materials are used, and the electrode rod is prepared by vacuum induction furnace smelting to degas after melting the raw materials, so that the mass percentage of the gas content and the chemical elements is: C: 0.44%; Mn: 0.60%; Si: 1.65%; S≤0.002%; P≤0.005%; Ni: 1.20%; Cr: 1.30%; Mo: 1.20%; V: 0.05%; Al: ≤0.015%; Ti: ≤0.015%; H: ≤0.00005%; O: ≤0.0008%; N: ≤0.0010%; and the balance being Fe and other unavoidable impurities; the vacuum induction furnace smelting temperature is 1520℃, when O≤15ppm, N≤15ppm, H≤1ppm in the steel liquid, Ar≥3000Pa is filled, and metal Mn or electrolytic Mn is started to be added, and the casting temperature of the electrode is 1540℃, the vacuum degree during refining is ≤5Pa, the refining period temperature is 1530℃, and the refining time is ≥30 minutes.

[0189] S2: Electroslag remelting

[0190] The electrode rod prepared by S1 is subjected to electroslag remelting, a slag system with a fluorite and Al2O3 slag ratio of 70:30 is used, and the melting speed is regulated to improve the segregation of chemical elements and the purity of the steel material, so as to obtain an electroslag ingot;

[0191] The electroslag remelting current is 3KA-19KA, the power is 60kw-800kw; the melting speed is 3.5kg / min, and slow cooling≥24 hours is carried out after the electroslag remelting is completed.

[0192] S3: Electroslag ingot forging vacuum consumable electrode:

[0193] The electroslag ingot obtained in the step S2 is subjected to heat treatment and then forged to obtain a vacuum consumable electrode;

[0194] In the step S3, the heat treatment comprises the following steps:

[0195] Initial heating stage: the initial temperature is controlled below 600℃, and then heated to 800℃ at a smooth rate, and the whole heating process lasts more than 2 hours;

[0196] 800℃ holding stage: when the temperature reaches 800℃, the temperature is kept at this temperature for at least 1 hour;

[0197] Further heating stage: after the 800℃ holding, continue to heat until the temperature reaches 1140℃, and the heating time is not less than 2 hours, to ensure gradual heating and avoid thermal stress or uneven organization of the material.

[0198] High-temperature holding stage: at 1140℃, keep the temperature for at least 2 hours.

[0199] The forging comprises the following steps:

[0200] Reburning stage: heat the electroslag ingot to 1120℃ to ensure the thermal plasticity of the material during forging;

[0201] Forging stage: forging is carried out in the temperature range of ≥1000℃ opening forging temperature and ≥850℃ final forging temperature, to ensure the best formability and grain size control of the material;

[0202] Slow cooling stage: after forging, slow cooling treatment is carried out, and the slow cooling mode is cover cooling, sand cooling, pit cooling or furnace cooling, and the slow cooling time is not less than 24 hours;

[0203] Polishing treatment: after slow cooling, the forged piece is subjected to polishing treatment to ensure that it meets the electrode size requirements matched by the vacuum consumable crystallizer.

[0204] S4: vacuum consumable remelting

[0205] The vacuum consumable electrode of the step S3 is subjected to vacuum consumable remelting to further degas and improve the purity of the steel, and the vacuum degree is controlled to be ≤0.5Pa during melting;

[0206] The melting speed during the vacuum consumable remelting process is 2.5kg / min, and helium gas is used for cooling to prevent segregation of chemical composition and ingot type.

[0207] S5: forging treatment

[0208] The electric slag ingot after the step S4 processing is subjected to a forging treatment, before the forging, it is gradually heated to 800℃ and kept for ≥1 hour, then further heated to 1140℃ and kept for ≥2 hours, then the forging is carried out at a temperature of 1000℃, and after the forging, slow cooling is carried out; the slow cooling after the forging adopts a cover cooling, sand cooling, pit cooling or furnace cooling mode, and the slow cooling time is not less than 24 hours.

[0209] S6: heat treatment after calcination

[0210] The steel material after the forging is subjected to a heat treatment after calcination, the heat treatment after calcination includes normalizing, tempering, quenching and re-tempering, so as to optimize the mechanical properties and the uniformity of the organization of the steel material.

[0211] The heat treatment after calcination includes the following steps:

[0212] Normalizing: the temperature is 910℃, the keeping time is 2.5mm / minute+60 minutes, and the cooling mode is air cooling;

[0213] Tempering: the temperature is 670℃, the keeping time is 120 minutes, and the cooling mode is air cooling;

[0214] Quenching: the temperature is 880℃, the keeping time is 60 minutes, and the cooling mode is oil cooling;

[0215] Re-tempering: the temperature is 510℃, the keeping time is 120 minutes, and the cooling mode is air cooling.

[0216] Example 3:

[0217] Example 3 provides a high-purity high-strength steel and a manufacturing method thereof, the high-purity high-strength steel includes the following components: C: 0.46%; Mn: 1.00%; Si: 1.85%; S≤0.002%; P≤0.005%; Ni: 1.40%; Cr: 1.50%; Mo: 1.40%; V: 0.10%; Al: ≤0.015%; Ti: ≤0.015%; H: ≤0.00005%; O: ≤0.0008%; N: ≤0.0010%; and the balance is Fe and other unavoidable impurities.

[0218] The manufacturing method includes the following steps:

[0219] S1: vacuum induction furnace smelting

[0220] The electrode rod is prepared by vacuum induction furnace smelting with high-purity raw materials, degassing after melting the raw materials, and controlling the gas content and mass percentage of chemical elements as follows: C: 0.46%; Mn: 1.00%; Si: 1.85%; S≤0.002%; P≤0.005%; Ni: 1.40%; Cr: 1.50%; Mo: 1.40%; V: 0.10%; Al: ≤0.015%; Ti: ≤0.015%; H: ≤0.00005%; O: ≤0.0008%; N: ≤0.0010%; and the balance of Fe and other unavoidable impurities. The vacuum induction furnace smelting temperature is 1600℃, when the O≤15ppm, N≤15ppm, H≤1ppm in the molten steel, Ar≥3000Pa is filled, and the metal Mn or electrolytic Mn is started to be added, and the casting temperature of the electrode is 1580℃, the vacuum degree during refining is ≤5Pa, the refining temperature is 1570℃, and the refining time is ≥30 minutes.

[0221] S2: electroslag remelting

[0222] The electrode rod prepared by S1 is subjected to electroslag remelting, a slag system with a fluorite and Al2O3 slag ratio of 70:30 is used, and the melting speed is controlled to improve the segregation of chemical elements and the purity of the steel material, to obtain an electroslag ingot;

[0223] The electroslag remelting current is 3KA-19KA, the power is 60kw-800kw, the melting speed is 10.5kg / min, and slow cooling is performed for ≥24 hours after the electroslag remelting is completed.

[0224] S3: electroslag ingot forging vacuum consumable electrode:

[0225] The electroslag ingot obtained in step S2 is subjected to forging after heat treatment to obtain a vacuum consumable electrode;

[0226] In step S3, the heat treatment includes the following steps:

[0227] Initial heating stage: the initial temperature is controlled below 600℃, and then heated to 800℃ at a steady rate, and the whole heating process lasts more than 2 hours;

[0228] 800℃ holding stage: when the temperature reaches 800℃, it is kept at this temperature for at least 1 hour of holding;

[0229] Further heating stage: after holding at 800℃, continue to heat until the temperature reaches 1210℃, and the heating time is not less than 2 hours, to ensure gradual heating and avoid thermal stress or uneven organization of the material.

[0230] High-temperature holding stage: at 1210℃, the temperature is kept for at least 2 hours.

[0231] The forging includes the following steps:

[0232] Reheating stage: the electroslag ingot is heated to 1160℃ to ensure the thermal plasticity of the material during forging;

[0233] Forging stage: forging is carried out in a temperature range of open forging temperature ≥ 1000℃ and finish forging temperature ≥ 850℃ to ensure the best formability and grain size control of the material;

[0234] Slow cooling stage: after forging is completed, slow cooling treatment is carried out, and the slow cooling mode is cover cooling, sand cooling, pit cooling or furnace cooling, and the slow cooling time is not less than 24 hours;

[0235] Polishing treatment: after slow cooling, the forged piece is polished to ensure that it meets the electrode size requirements matched by the vacuum consumable crystallizer.

[0236] S4: Vacuum consumable remelting

[0237] The vacuum consumable electrode of step S3 is subjected to vacuum consumable remelting to further degas and improve the purity of the steel, and the vacuum degree is controlled to be ≤0.5Pa during melting.

[0238] The melting speed during the vacuum consumable remelting process is 6.5kg / min, and helium gas is used for cooling to prevent segregation of chemical composition and ingot type.

[0239] S5: Forging treatment

[0240] The electroslag ingot after step S4 treatment is subjected to forging treatment, which is gradually heated to 800℃ and kept for ≥1 hour before forging, and then further heated to 1210℃ and kept for ≥2 hours, and then forged at 850℃, and slowly cooled after forging; Slow cooling after forging adopts cover cooling, sand cooling, pit cooling or furnace cooling mode, and the slow cooling time is not less than 24 hours.

[0241] S6: Heat treatment after forging

[0242] The forged steel is subjected to heat treatment after forging, and the heat treatment after forging includes normalizing, tempering, quenching and retempering to optimize the mechanical properties and organizational uniformity of the steel.

[0243] The heat treatment after forging includes the following steps:

[0244] Normalizing: the temperature is 930℃, the holding time is 2.5mm / minute+60 minutes, and the cooling mode is air cooling;

[0245] Tempering: the temperature is 690℃, the holding time is 240 minutes, and the cooling mode is air cooling;

[0246] Quenching: the temperature is 900℃, the holding time is 60 minutes, and the cooling mode is oil cooling;

[0247] Re-tempering: temperature is 550℃, holding time is 120 minutes, and cooling mode is air cooling.

[0248] The following compares the mechanical properties of the high-purity high-strength steel prepared in the above-mentioned embodiment 1 of the present application with the 45CrNiMoV mentioned in the background art. The two are the same in other manufacturing methods except for the heat treatment methods (i.e. normalizing, tempering, quenching corresponding to post-forging heat treatment). The tested mechanical properties comparison (GB / T 228 metal tensile test method) is shown in Table 2:

[0249] Table 2 Mechanical property comparison

[0250]

[0251] From the comparison in Table 2, it can be seen that:

[0252] (1) The steel of the present application compared with the 45CrNiMoV in the background art optimizes the chemical composition, greatly reduces the content of residual elements, so that it has very high strength while still has good plasticity;

[0253] (2) The steel of the present application through adjustment of the chemical composition has more martensite structure at room temperature, improving the strength;

[0254] (3) The present application through the special heat treatment process, combined with the above-mentioned adjustment of the chemical composition, further improves the mechanical properties of the steel.

[0255] In summary, the present application provides a method for manufacturing high-purity high-strength steel, which solves the contradiction between high strength and toughness of steel in the prior art and the gas pollution problem in the smelting process. The principle is to remove the gas impurities such as oxygen, nitrogen and hydrogen in the steel by adopting the three-in-one smelting process of vacuum induction furnace smelting, electroslag remelting and vacuum consumable remelting, and combining with precise temperature control and gas control means, further reducing the content of harmful elements, and optimizing the microstructure of the steel. The technical scheme proposed by the present application includes multiple key steps, such as selection of high-purity raw materials, precise vacuum smelting process, control of electroslag remelting, and vacuum consumable remelting and subsequent forging treatment, heat treatment process. Through these optimized manufacturing methods, the gas impurities in the steel are effectively reduced, the formation of oxides and inclusions is reduced, and the purity and uniformity of the steel are greatly improved. At the same time, through the optimization and adjustment of the composition, the strength, toughness and fatigue resistance of the steel are further enhanced; the technical effect of the present application is remarkable, the obtained steel maintains high strength (tensile strength exceeds 1500MPa), while the toughness and plasticity are significantly improved, and the fatigue strength and fracture resistance are also greatly improved. Overcome the gas pollution, poor material reliability and uncertainty in the smelting process in the prior art, and provide a high-purity, high-strength steel solution for high-end fields such as aerospace, military industry, etc.

[0256] In the description of the present application, the description of the terms "one embodiment", "some embodiments", "in this embodiment", "specific examples" or "some examples" means that the specific features, mechanisms, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, mechanisms, materials or characteristics described can be combined in any suitable manner in any one or more embodiments or examples. In addition, different embodiments or examples described in the specification and the features of different embodiments or examples can be combined and combined by those skilled in the art without contradiction.

[0257] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited thereto, and any skilled person in the art can easily think of changes or replacements within the technical scope disclosed in the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A method of manufacturing a high-purity high-strength steel, characterized by, The method comprises the following steps: S1: Vacuum induction furnace smelting uses high-purity raw materials, through vacuum induction furnace smelting, the raw materials are melted and degassed to obtain an electrode rod, the content of gas and chemical elements is controlled as follows: C: 0.44-0.46%, Mn: 0.60-1.00%, Si: 1.65-1.85%, S≤0.002%, P≤0.005%, Ni: 1.20-1.40%, Cr: 1.30-1.50%, Mo: 1.20-1.40%, V: 0.05-0.10%, Al: ≤0.015%, Ti: ≤0.015%, H: ≤0.00005%, O: ≤0.0008%, N: ≤0.0010%, and the balance is Fe and other inevitable impurities; S2: Electroslag remelting Electroslag remelting is performed on the electrode rod obtained in S1, a slag system with a fluorite and Al2O3 slag ratio of 70:30 is used, and the melting speed is controlled to improve the segregation of chemical elements and the purity of the steel to obtain an electroslag ingot; S3: Electroslag ingot forging vacuum consumable electrode: the electroslag ingot obtained in S2 is subjected to heat treatment and then forging to obtain a vacuum consumable electrode; S4: Vacuum consumable remelting Vacuum consumable remelting is performed on the vacuum consumable electrode of S3, further degassing and improving the purity of the steel, and the vacuum degree is controlled to be ≤0.5 Pa during the melting process; S5: Forging treatment Forging treatment is performed on the electroslag ingot after S4, the electroslag ingot is gradually heated to 800 ℃ and kept for ≥1 hour before forging, and then further heated to 1140-1210 ℃ and kept for ≥2 hours, and then forged at a temperature of 1000-850 ℃, and then slowly cooled after forging; S6: Annealing and heat treatment Annealing and heat treatment is performed on the forged steel, and the annealing and heat treatment comprises normalizing, tempering, quenching and re-tempering. In S5, the slow cooling after forging is performed by using cover cooling, sand cooling, pit cooling or furnace cooling, and the slow cooling time is not less than 24 hours. In S6, the annealing and heat treatment comprises the following steps: normalizing: the temperature is 920±10 ℃, the holding time is 2.5 mm / min+60 min, and the cooling mode is air cooling; tempering: the temperature is 680±10 ℃, the holding time is 120-240 min, and the cooling mode is air cooling; quenching: the temperature is 890±10 ℃, the holding time is 60 min, and the cooling mode is oil cooling; re-tempering: the temperature is 510-550 ℃, the holding time is 120 min, and the cooling mode is air cooling.

2. The production method according to claim 1, characterized by In S1, the vacuum induction furnace smelting temperature is 1520-1600 ℃, the vacuum degree during refining is ≤5 Pa, and the refining temperature is 1530-1570 ℃, and the refining time is ≥30 min.

3. The production method according to claim 1, characterized by In S1, when O≤15 ppm, N≤15 ppm and H≤1 ppm in the molten steel, Ar≥3000 Pa is filled, metal Mn or electrolytic Mn is added, and the electrode casting temperature is 1540-1580 ℃.

4. The production method according to claim 1, characterized by In the step S2, the electroslag remelting current is 3KA-19KA, the power is 60kw-800kw, the melting speed is 3.5-10.5kg / min, and slow cooling is performed for more than 24 hours after the electroslag remelting is completed.

5. The production method according to claim 1, characterized by In the step S3, the heat treatment includes the following steps: an initial temperature rising stage, in which the initial temperature is controlled to be lower than 600℃, and then the temperature is raised to 800℃ at a stable rate, and the whole temperature rising process lasts more than 2 hours; an 800℃ temperature maintaining stage, in which when the temperature reaches 800℃, the temperature is maintained at 800℃ for at least 1 hour; a further temperature rising stage, in which after the 800℃ temperature maintaining, the temperature is continuously raised until the temperature reaches between 1140℃ and 1210℃, and the temperature rising time is not less than 2 hours to ensure gradual heating; and a high temperature maintaining stage, in which the temperature is maintained at the temperature range of 1140℃-1210℃ for at least 2 hours.

6. The production method according to claim 1, characterized by In the step S3, the forging includes the following steps: a reheating stage, in which the electroslag ingot is heated to 1120-1160℃; a forging stage, in which the forging is performed at a temperature range of an open forging temperature≥1000℃ and a finish forging temperature≥850℃; a slow cooling stage, in which after the forging is completed, slow cooling treatment is performed, the slow cooling mode is cover cooling, sand cooling, pit cooling or furnace cooling, and the slow cooling time is not less than 24 hours; and a turning finishing stage, in which after the slow cooling, the forged piece is subjected to turning finishing treatment to ensure that it meets the electrode size requirements matched by the vacuum consumable crystallizer.

7. The production method according to claim 1, characterized by In the step S4, the melting speed in the vacuum self-consumption remelting process is 2.5-6.5kg / min, and helium is used for cooling.

8. A high purity high strength steel, characterized in that, The high-purity high-strength steel is prepared by the manufacturing method in any one of claims 1-7, and the composition of the high-purity high-strength steel includes: C: 0.44-0.46%; Mn: 0.60-1.00%; Si: 1.65-1.85%; S≤0.002%; P≤0.005%; Ni: 1.20-1.40%; Cr: 1.30-1.50%; Mo: 1.20-1.40%; V: 0.05-0.10%; Al: ≤0.015%; Ti: ≤0.015%; H: ≤0.00005%; O: ≤0.0008%; N: ≤0.0010%; and the balance is Fe and other inevitable impurities.

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