Single-vacuum high-toughness medium-alloy ultrahigh-strength steel and preparation method thereof

Abstract

The invention discloses single-vacuum high-toughness medium-alloy ultrahigh-strength steel and a preparation method thereof, and belongs to the technical field of steel smelting. In order to realize low-cost production of ultrahigh-strength steel under single vacuum, the invention provides single-vacuum high-toughness medium-alloy ultrahigh-strength steel and a preparation method thereof, and the single-vacuum high-toughness medium-alloy ultrahigh-strength steel comprises the following components: 0.26-0.31% of C, 0.6-1.2% of Mo, 3.6-6% of Ni, 1.5-2.0% of Si, 3-5% of Cr, 0.4-1.0% of Mn, 0.05-0.2% of V and the balance of Fe. The preparation method comprises the steps of vacuum induction, heating and cooling treatment, element high-temperature uniform diffusion treatment, multi-direction multi-pass deformation treatment and combined modulation heat treatment. According to the invention, alloy components are optimized, a suitable heat treatment system is provided, and the plasticity of steel is improved, so that the steel has excellent high strength, excellent plasticity and toughness are ensured, the content of alloy elements is low, and the cost is low.

Description

Technical Field

[0001] The invention belongs to the technical field of steel smelting, and specifically relates to a single vacuum high-toughness medium-alloy ultra-high-strength steel and a preparation method thereof. Background Art

[0002] Ultra-high strength steel has become an important material in the fields of aerospace, transportation, national defense and military industry due to its excellent mechanical properties and broad application prospects. In the field of aerospace, ultra-high strength steel is widely used in the manufacture of key components such as aircraft landing gear, engine shafts, and missile casings; in the field of transportation, it is used to manufacture automobile body shells, bridge structures, etc.; in the field of national defense and military industry, ultra-high strength steel is an indispensable and important material. With the continuous advancement of science and technology and the rapid development of industry, the performance of ultra-high strength steel will continue to improve, and its application field will become more extensive. If the strength and plasticity of steel can be improved through component optimization and appropriate heat treatment system. If its strength reaches more than 1.8GPa, the cost of aerospace equipment can be greatly reduced. Conventional ultra-high strength steel uses double vacuum to ensure the purity of the material, so that the steel has excellent plasticity and toughness. The production process of ultra-high strength steel is complex and has high requirements for metallurgical quality. Usually, electric arc furnace and electroslag remelting are used for smelting, and vacuum induction furnace or vacuum consumable arc furnace is used for smelting of steel with high purity. However, double vacuum is expensive and the process is complicated, while ultra-high-strength steel smelted using single vacuum is limited in its large-scale use due to its insufficient toughness.

[0003] CN117626102A discloses a high-strength and ultra-high-strength steel and a preparation method thereof, wherein the high-strength and ultra-high-strength steel comprises the following components in weight percentage: C: 0.36-0.43%, Cr: 2.6-3.6%, Ni: 0.6-1.2%, Mn: 0.5-1.8%, Ca: 0.002-0.005%, rare earth elements: 0.01-0.025%, Mo: 0.4-0.9%, Al: 0.02-0.08%, Nb: 0.05-0.2%, Si: 0.4-1%, Cu: 0.1-0.5%, V≤0.30%, and the balance is Fe and unavoidable impurities; the preparation method The method includes the following steps: Step 1, melting the raw steel, nickel, molybdenum and niobium, slag at above 1610℃, crude steel liquid at above 1670℃, and adding a conditioner to the crude steel liquid; Step 2, adding the smelted crude steel liquid to the refined material, using calcium oxide, aluminum oxide and lime to make slag, and then adding Cr, Cu, Mn, Si and rare earth elements for alloying to obtain a ladle; Step 3, vacuum degassing the refined ladle, tapping at 1500-1600℃, and casting electrodes; Step 4, vacuum consumable remelting the electrode to obtain an ingot; Step 5, forging the ingot to obtain a rod; Step 6, heat treating the rod to obtain an ultra-high strength steel. The fracture toughness of the product obtained by this method is 100MPam 1 / 2 And it needs to add rare earth elements and Nb, which leads to high cost. In particular, it adopts electric furnace smelting + refining outside the furnace + vacuum degassing + vacuum consumable smelting, and the process flow is relatively long. Summary of the invention

[0004] In order to solve the problems of the prior art, the present invention reasonably optimizes the carbon content, appropriately increases the content of Ni, Si, Mo and V elements, avoids the use of precious metals such as Nb, and optimizes the single vacuum smelting production process. Under the conditions of single vacuum smelting, ultra-high strength steel with a tensile strength of 1.8 GPa is prepared, and it has low cost and excellent plasticity.

[0005] The present invention first provides a method for preparing a single vacuum high-toughness medium-alloy ultra-high-strength steel, wherein the mass percentage content of the single vacuum high-toughness medium-alloy ultra-high-strength steel components is: C: 0.26-0.31%, Mo: 0.6-1.2%, Ni: 3.6-6%, Si: 1.5-2.0%, Cr: 3-5%, Mn: 0.4-1.0%, V: 0.05-0.2%, and the balance is Fe and unavoidable impurities;

[0006] The preparation method comprises the following steps:

[0007] A. According to the chemical composition ratio, the ingot is smelted by vacuum induction smelting method: high-purity iron is added, and after melting, refining begins. Metal nickel, metal molybdenum, electrolytic manganese, metal chromium, metal tungsten, high-purity silicon, 80 ferrovanadium and carbon powder are added for refining, and the target content of the components is adjusted. After refining, the temperature is lowered to 1450℃~1480℃, the ingot is taken out of the furnace, cast, and then the head and tail are cut and the skin is peeled to obtain a vacuum induction ingot;

[0008] B. Heating and cooling treatment: heating the vacuum induction ingot obtained in step A from room temperature to 650°C±150°C at 100-130°C / h, and heat-insulating the ingot after it is heated through. Then, cooling the ingot with the furnace to reduce the temperature to 300°C±150°C, and then air cooling the ingot at a cooling rate of 25-45°C / min to room temperature;

[0009] C. High temperature uniform diffusion treatment of elements: the steel ingot after the temperature rise and fall treatment is heated from room temperature to 1200℃~1300℃ at 100~130℃ / h, and then kept at 1200℃~1300℃;

[0010] D. Multi-directional and multi-pass deformation treatment: The steel ingot after high-temperature uniform diffusion treatment of elements is cooled to 1030±10℃~1090±10℃, and after heat preservation treatment, it is taken out of the furnace and forged. The initial forging temperature is 1030±10℃~1090±10℃, and the final forging temperature is 890℃~930℃ (if the steel ingot cools faster, it should be put into the furnace for heating in time). Multi-piercing and multi-pulling in at least three directions are carried out to strictly ensure that the steel is formed in one fire and the grain size is above 8.5. After final forging, the steel is cooled to room temperature;

[0011] E. Combined modulation heat treatment: firstly, high-temperature air cooling heat treatment is carried out: the forged steel is heated from room temperature to 900℃~1000℃, after heat preservation treatment, the cooling rate is controlled at 20~80℃ / min to air cool to room temperature; then, high-temperature quenching treatment is carried out: heating from room temperature to a quenching temperature of 870℃~1020℃, after heat preservation treatment, oil quenching, cooling rate is 100℃ / s~150℃ / s, and cooling to room temperature; finally, two low-temperature tempering treatments are carried out: both tempering treatments are heated from room temperature to 200℃~300℃, after heat preservation treatment, air cooling is carried out to room temperature, and high-toughness medium alloy ultra-high strength steel is obtained.

[0012] Among them, in the above-mentioned method for preparing single vacuum high-toughness medium-alloy ultra-high strength steel, in step A, the melting time is not less than 3 hours, and the vacuum degree is not higher than 30Pa.

[0013] Among them, in the preparation method of the above-mentioned single vacuum high-toughness medium-alloy ultra-high strength steel, in step A, the refining time is 0.5h to 1h.

[0014] Among them, in the above-mentioned method for preparing the single vacuum high-toughness medium-alloy ultra-high-strength steel, in step B, the time of the insulation treatment is 4h to 5h.

[0015] Among them, in the preparation method of the above-mentioned single vacuum high-toughness medium-alloy ultra-high strength steel, in step C, the time of the insulation treatment is 8h to 15h.

[0016] Among them, in the above-mentioned method for preparing the single vacuum high-toughness medium-alloy ultra-high-strength steel, in step D, the time of the insulation treatment is 4h to 10h.

[0017] Among them, in the method for preparing the single vacuum high-toughness medium-alloy ultra-high-strength steel, in step D, in the multiple upsetting and multiple drawing, upsetting is performed for no less than three times, and drawing is performed for no less than three times. The directions can be a combination of conventional directions in the art, including but not limited to selecting at least three directions from the X, Y, Z, XY, YZ or XZ directions.

[0018] Preferably, in the above-mentioned method for preparing the single vacuum high-toughness medium-alloy ultra-high-strength steel, in step D, the multi-upsetting and multi-drawing in at least three directions is: three-upsetting and three-drawing in three directions of X, Y and Z, or four-upsetting and four-drawing in four directions of X, Y, Z and XY, or four-upsetting and four-drawing in four directions of X, Z, YZ and ZY, or four-upsetting and four-drawing in four directions of X, Y, XY and YX, or four-upsetting and four-drawing in four directions of Z, Y, XY and YX.

[0019] Among them, in the preparation method of the above-mentioned single vacuum high-toughness medium-alloy ultra-high-strength steel, in step E, during the high-temperature air-cooling heat treatment, the insulation treatment time is 1h to 1.5h.

[0020] Among them, in the preparation method of the above-mentioned single vacuum high-toughness medium-alloy ultra-high-strength steel, in step E, during the high-temperature quenching treatment, the time of the insulation treatment is 0.5h to 1.5h.

[0021] Among them, in the preparation method of the above-mentioned single vacuum high-toughness medium-alloy ultra-high-strength steel, in step E, during the two low-temperature tempering treatments, the time of the insulation treatment is 1.5 to 3 hours.

[0022] The present invention also provides a high-toughness medium-alloy ultra-high-strength steel, which is prepared by the above-mentioned single vacuum high-toughness medium-alloy ultra-high-strength steel preparation method.

[0023] The high-toughness medium alloy ultra-high strength steel provided by the present invention has excellent high strength while also ensuring excellent plasticity and toughness. Its tensile strength is ≥1800MPa, yield strength is greater than 1500MPa, elongation is greater than 14%, and fracture toughness is greater than 110MPa·m 1 / 2 .

[0024] Beneficial effects of the present invention:

[0025] The present invention optimizes the alloy composition of ultra-high strength steel reasonably, does not need to add expensive alloy elements, adds appropriate amounts of Si, Cr, Mo and V, reduces the cost and achieves excellent tensile properties, provides a suitable heat treatment system to improve the plasticity of the steel, especially the tensile properties can stably reach 1.8GPa and KIC 110MPa·m 1 / 2 The above, at the same time, the alloy element content is small, the cost is low and has the prospect of large-scale promotion. DETAILED DESCRIPTION

[0026] Specifically, a method for preparing a single vacuum high-toughness medium-alloy ultra-high-strength steel, wherein the mass percentage content of the components of the single vacuum high-toughness medium-alloy ultra-high-strength steel is: C: 0.26-0.31%, Mo: 0.6-1.2%, Ni: 3.6-6%, Si: 1.5-2.0%, Cr: 3-5%, Mn: 0.4-1.0%, V: 0.05-0.2%, and the balance is Fe and unavoidable impurities;

[0027] The preparation method comprises the following steps: vacuum induction, temperature rise and fall treatment, element high temperature uniform diffusion treatment, multi-directional multi-pass deformation treatment and combined modulation heat treatment.

[0028] In the present invention, the C content is controlled at 0.26-0.31%, mainly to ensure that the martensite has excellent toughness while ensuring strength. A C content higher than 0.31% will result in insufficient toughness, and a C content lower than 0.26% will result in strength failing to reach the required level. In the present invention, the Si content is controlled at 1.5-2.0%, mainly to improve the yield strength and elastic limit of the steel, while inhibiting the precipitation of cementite, improving the tempering stability, preventing the ε-carbide from growing into cementite during tempering, and increasing the tempering brittleness temperature. At the same time, the C and Si components work in coordination, increasing the solid solution strengthening effect without sacrificing a large amount of toughness, and improving the yield strength and appropriate plasticity of the steel as much as possible, so that the material can ensure sufficient fracture toughness.

[0029] In the present invention, the Ni content is increased to 3.6-6% in order to ensure that the toughness is improved under the condition of single vacuum smelting, and a large amount of residual austenite is retained through subsequent quenching-tempering treatment to further improve the toughness. Ni can also increase the stacking fault energy of the matrix and improve the plasticity of the steel. However, when the Ni content exceeds 6%, the risk of unstable fracture will increase, and the residual austenite is too high, resulting in insufficient strength.

[0030] In the present invention, the Cr content is controlled at 3-5% in order to enhance the solid solution strengthening strength while ensuring the stability of the retained austenite; a small amount of V is added mainly to refine the grains, and 0.6-1.2% of Mo is combined to precipitate carbides and increase the precipitation strengthening strength; V less than 0.05% will result in insufficient precipitation strengthening effect, and V greater than 0.2% will result in coarsening of the precipitate phase.

[0031] In the present invention, 0.4-0.8% of Mn is added mainly to combine with S in the steel to form MnS, so as to prevent S from deteriorating the toughness of the steel. Too high a Mn content will lead to the precipitation of the C element, so Mn should not exceed 0.8%.

[0032] The method of the present invention includes vacuum induction, temperature rise and fall treatment, element high temperature uniform diffusion treatment, multi-directional multi-pass deformation treatment and combined modulation heat treatment:

[0033] Step 1, vacuum induction According to the chemical composition ratio, the ingot is smelted by vacuum induction smelting method, the chemical composition C: 0.26-0.31%, Mo: 0.6-1.2%, Ni: 3.6-6%, Si: 1.5-2.0%, Cr: 3-5%, Mn: 0.4-0.8%, V: 0.05-0.2%, and the balance is Fe and inevitable impurities. The melting time is set appropriately according to the ingot shape, high-purity iron is added first, the melting time is greater than 3h, the vacuum degree is below 30Pa, in order to ensure the toughness of the steel and ensure that the gas content in the molten iron is as low as possible, the refining time is controlled at 0.5h~1h, and the alloy components such as nickel, molybdenum, electrolytic manganese, chromium, tungsten, high-purity silicon, 80 vanadium iron and carbon powder are added in sequence to adjust the composition. After that, the temperature is lowered to 1450℃~1480℃, and then it can be taken out of the furnace and cast. If the temperature is too high, the smelting cycle will be increased, and the time for the ingot to completely solidify will also be increased. If the pouring temperature is too low, a filling area will appear on the surface of the ingot. The ingot is cut and the surface is peeled to remove the shrinkage holes and macro defects visible to the naked eye. Folding, slag inclusions, inclusions, and cracks are not allowed on the surface.

[0034] Step 2: The vacuum induction ingot is heated and cooled to remove the stress of the ingot and prevent the ingot from cracking during the forging process. In the heating stage, the ingot is heated to 650°C ± 150°C at 100-130°C / h. A heating rate that is too fast will result in high natural gas consumption and high costs, while a heating rate that is too slow will result in a long time and low efficiency. After the ingot is heated through, it is kept warm for 4h-5h, and then cooled with the furnace to prevent the center of the ingot from cracking due to excessive temperature drop. In the cooling stage, the temperature is reduced to 300°C ± 150°C, and air cooling is used to control the cooling rate at 25-45°C / min and cool to room temperature.

[0035] Step 3: Perform high-temperature uniform diffusion treatment on the steel ingot. In order to achieve excellent toughness, more Mo and Cr elements are added to this steel. Mo is difficult to diffuse and easily segregates, and Cr easily forms primary carbides, which affect mechanical properties. Therefore, special treatment is required. It is heated from room temperature to 1200-1300℃ at 100-130℃ / h, and kept at 1200-1300℃ for 8-15h. First, ensure that the primary carbides in the steel ingot are fully dissolved, and then ensure that the elements in the steel ingot are fully diffused, especially Mo and Cr, to avoid the formation of primary carbides of Cr. In the temperature range of 1200-1300℃, the diffusion rate of Mo is the largest, which can effectively eliminate the segregation of Mo and Cr.

[0036] Step 4: multi-directional multi-pass deformation treatment. The steel ingot is cooled to 1030±10℃~1090±10℃, and kept at 1030±10℃~1090±10℃ for 4h~10h to avoid overheating and cracking of the steel ingot caused by forging. The steel ingot is taken out of the furnace and forging begins. The initial forging temperature is 1030±10℃~1090±10℃, and the final forging temperature is 890℃~930℃. If the steel ingot cools faster, it should be put into the furnace in time for heating. At least multiple piers and multiple draws (upsetting at least three times and drawing at least three times) in multiple directions (including but not limited to X, Y, Z, XY, YZ or XZ directions) are carried out to strictly ensure that the steel is completed in one fire, and the grain size is above 8.5, and fine grain strengthening is increased to further improve the strength of the steel ingot.

[0037] Step five, combined modulation heat treatment. In order to eliminate the forging genetic organization, the steel needs to be subjected to high-temperature air-cooled heat treatment, heated from room temperature to 900-1000°C, kept warm for 1h-1.5h, and after eliminating the forging genetic organization, it is air-cooled to room temperature, and the cooling rate is controlled at 20-80°C / min. In order to obtain high-strength martensitic structure and a certain amount of residual austenite, it is subjected to high-temperature quenching + low-temperature tempering. The quenching temperature is 870-1020°C, and after keeping warm for 30min-1h30min, it is oil quenched, and the cooling rate is 100°C / s-150°C / s. After that, two low-temperature tempering treatments are carried out to adjust the toughness of the steel while allowing the carbides to fully precipitate and increase the strength. The two tempering temperatures are 200-300°C, and the tempering time is kept warm for 1.5-3h each time, after which ultra-high-strength bars can be obtained.

[0038] After heat treatment, the steel of the present invention has a tensile strength of ≥1800MPa, a yield strength of >1500MPa, an elongation of >14%, and a fracture toughness of >110MPa·m 1 / 2 , while having excellent high strength, it also ensures excellent plasticity and toughness.

[0039] The present invention is further described in detail below by way of examples, but the protection scope of the present invention is not limited to the scope of the examples.

[0040] Example 1

[0041] Step 1, vacuum induction according to the chemical composition ratio, using vacuum induction smelting method to smelt ingots: according to the ingot diameter of 200±20mm, the melting time is appropriately set, high-purity iron is added first, the melting time is 3.5h, the vacuum degree is 24Pa, the gas content in the molten iron is ensured to be as low as possible, and refining is started. The refining time is controlled at about 35 minutes, and the alloy components are added in turn to adjust the composition. Samples are taken for composition detection C: 0.31%, Mo: 0.80%, Ni: 5.1%, Si: 1.8%, Cr: 3.1%, Mn: 0.5%, V: 0.06%, and the remainder is Fe and unavoidable impurities; then the temperature is lowered to 1470℃, the furnace is removed, and casting is performed; the head and tail are cut and the surface of the ingot is peeled, the shrinkage holes and macro defects visible to the naked eye of the ingot are removed, and there is no folding, slag inclusion, inclusion, or crack on the surface.

[0042] Step 2: The vacuum induction ingot is heated from room temperature to 720°C at 100°C / h. After the ingot is heated through, it is kept warm for 4 hours, then cooled with the furnace. After the temperature drops to 450°C, it is air-cooled at a cooling rate of 30°C / min to room temperature.

[0043] Step three, subjecting the steel ingot to high-temperature uniform diffusion treatment of elements: heating from room temperature to 1280°C at 100°C / h, and keeping at 1280°C for 8 hours to ensure the dissolution of primary carbides in the steel ingot and sufficient diffusion of various elements, especially Mo and Cr, to avoid the formation of eutectic carbides of Cr.

[0044] Step 4: Multi-directional and multi-pass deformation treatment: Cool the steel ingot from 1280℃ to 1070℃, keep it at 1070±10℃ for 4h to avoid overheating and cracking of the steel ingot. Start forging after it is taken out of the furnace. The initial forging temperature is 1070℃ and the final forging temperature is above 900℃. If the steel ingot cools faster, it should be put into the furnace in time for heating. Perform three piers and three draws in the X, Y and Z directions to strictly ensure that the steel is completed in one fire and the grain size is 9.0. After final forging, air cool to room temperature.

[0045] Step 5, combined modulation heat treatment: high-temperature air cooling heat treatment, heating it from room temperature to 930℃, keeping it warm for 1h, after eliminating the forging genetic organization, air cooling it, the cooling rate is controlled at 45℃ / min, and air cooling it to room temperature; then high-temperature quenching + low-temperature tempering, directly heating it from room temperature to the quenching temperature of 900℃, keeping it warm for 55min, oil quenching, cooling rate is 120℃ / s, and cooling it to room temperature; then two low-temperature tempering treatments are performed to adjust the toughness of the steel while allowing carbides to fully precipitate to increase the strength, heating from room temperature to the tempering temperature, the two tempering temperatures are 210℃, each tempering time is kept warm for 1.5h, air cooling to room temperature, and ultra-high strength bars are obtained, the tensile strength is equal to 1838MPa, the yield strength is 1510MPa, the elongation is 15%, and the fracture toughness is 117MPa·m 1 / 2 .

[0046] Example 2

[0047] Step 1, vacuum induction according to the chemical composition ratio, using vacuum induction smelting method to smelt an ingot: according to the ingot diameter of 300±20mm, the melting time is appropriately set, high-purity iron is added first, the melting time is 4h, the vacuum degree is 20Pa, the gas content in the molten iron is ensured to be as low as possible, and refining is started. The refining time is controlled at about 35 minutes, and the alloy components are added in turn to adjust the composition. Samples are taken for composition detection C: 0.30%, Mo: 0.80%, Ni: 4.1%, Si: 1.5%, Cr: 3.2%, Mn: 0.6%, V: 0.12%, and the remainder is Fe and unavoidable impurities; then the temperature is lowered to 1460°C and the ingot is taken out of the furnace for casting; the head and tail are cut and the surface of the ingot is peeled, the shrinkage holes and macro defects visible to the naked eye of the ingot are removed, and there is no folding, slag inclusion, inclusion, or crack on the surface.

[0048] Step 2: The vacuum induction ingot is heated from room temperature to 730°C at 120°C / h. After the ingot is heated through, it is kept warm for 4.5 hours. After that, it is cooled with the furnace to 430°C. It is then air-cooled at a cooling rate of 40°C / min to room temperature.

[0049] Step three, subjecting the steel ingot to high-temperature uniform diffusion treatment of elements: heating from room temperature to 1290°C at 110°C / h, and keeping at 1290°C for 13 hours to ensure the dissolution of the primary carbide in the steel ingot and sufficient diffusion of the elements, especially Mo and Cr, to avoid the formation of eutectic carbide of Cr.

[0050] Step 4: Multi-directional and multi-pass deformation treatment: Cool the steel ingot from 1290℃ to 1060℃, keep it at 1060±10℃ for 6h to avoid overheating and cracking of the steel ingot caused by forging. Start forging after it is taken out of the furnace. The initial forging temperature is 1060℃ and the final forging temperature is above 900℃. If the steel ingot cools faster, it should be put into the furnace in time for heating. Four piers and four pulls are carried out in the four directions of X, Y, Z, and XY to strictly ensure that the steel is completed in one fire and the grain size is 9.5. After final forging, air cool to room temperature.

[0051] Step 5, combined modulation heat treatment: high-temperature air cooling heat treatment, heating it from room temperature to 980°C, keeping it warm for 1.5 hours, after eliminating the forging genetic organization, air cooling it, the cooling rate is controlled at 30°C / min, and air cooling it to room temperature; then high-temperature quenching + low-temperature tempering, directly heating it from room temperature to a quenching temperature of 915°C, keeping it warm for 60 minutes, oil quenching, cooling rate is 130°C / s, and cooling it to room temperature; then two low-temperature tempering treatments are performed to adjust the toughness of the steel while allowing carbides to fully precipitate to increase the strength, heating from room temperature to the tempering temperature, the two tempering temperatures are 225°C, each tempering time is kept warm for 2 hours, air cooling to room temperature, and ultra-high strength bars are obtained, the tensile strength is equal to 1875MPa, the yield strength is 1523MPa, the elongation is 15.5%, and the fracture toughness is 120MPa·m 1 / 2 .

[0052] Example 3

[0053] Step 1, vacuum induction: according to the chemical composition ratio, the ingot is smelted by a vacuum induction smelting method: according to the ingot diameter of 280±20mm, the melting time is appropriately set, high-purity iron is added first, the melting time is 4h, the vacuum degree is 20Pa, the gas content in the molten iron is ensured to be as low as possible, and refining is started. The refining time is controlled at about 35 minutes, and the alloy components are added in sequence to adjust the composition. Samples are taken for composition detection: C: 0.28%, Mo: 0.60%, Ni: 3.8%, Si: 1.7%, Cr: 4.2%, Mn: 0.75%, V: 0.12%, and the remainder is Fe and unavoidable impurities; then the temperature is lowered to 1460°C and the ingot is taken out of the furnace for casting; the head and tail are cut and the surface of the ingot is peeled to remove the shrinkage cavities and macro defects visible to the naked eye of the ingot, and there is no folding, slag inclusion, inclusion, or crack on the surface.

[0054] Step 2: The vacuum induction ingot is heated from room temperature to 600°C at 120°C / h. After the ingot is heated through, it is kept warm for 4 hours. After that, it is cooled with the furnace and the temperature is reduced to 390°C. It is then air-cooled at a cooling rate of 38°C / min to room temperature.

[0055] Step three, subjecting the steel ingot to a high-temperature uniform diffusion treatment of the elements: heating from room temperature to 1220°C at a rate of 100°C / h, and keeping at 1220°C for 12 hours to ensure that the primary carbides in the steel ingot are dissolved back and the elements are fully diffused, especially Mo and Cr, to avoid the formation of eutectic carbides of Cr.

[0056] Step 4: Multi-directional and multi-pass deformation treatment: Cool the steel ingot from 1220℃ to 1090℃, keep it at 1090±10℃ for 8h to avoid overheating and cracking of the steel ingot. Start forging after it is taken out of the furnace. The initial forging temperature is 1090℃ and the final forging temperature is 900℃. If the steel ingot cools faster, it should be put into the furnace in time for heating. Four piers and four pulls are carried out in the four directions of X, Z, YZ and ZY to strictly ensure that the steel is completed in one fire and the grain size is level 9. After final forging, it is air-cooled to room temperature.

[0057] Step 5, combined modulation heat treatment: high-temperature air cooling heat treatment, heating it from room temperature to 930℃, keeping it warm for 1h, after eliminating the forging genetic organization, air cooling it, the cooling rate is controlled at 78℃ / min, and air cooling it to room temperature; then high-temperature quenching + low-temperature tempering, directly heating it from room temperature to the quenching temperature of 1015℃, keeping it warm for 1h25min, oil quenching, cooling rate is 145℃ / s, and cooling it to room temperature; then two low-temperature tempering treatments are performed to adjust the toughness of the steel while allowing carbides to fully precipitate to increase the strength, heating from room temperature to the tempering temperature, the two tempering temperatures are 280℃, each tempering time is kept warm for 2h, air cooling to room temperature, and ultra-high strength bars are obtained, the tensile strength is equal to 1880MPa, the yield strength is 1516MPa, the elongation is 16%, and the fracture toughness is 114MPa·m 1 / 2 .

[0058] Comparative Example 1

[0059] The remaining steps are consistent with those in Example 1, and the components are also consistent with those in Example 1, but the difference in step 5, the combined modulation heat treatment is as follows: high temperature quenching + low temperature tempering is performed, the quenching temperature is 1040°C, after keeping warm for 60 minutes, oil quenching, cooling rate is 140°C / s, and cooling to room temperature; then two low temperature temperings are performed, the two tempering temperatures are 400°C, and the tempering time for each tempering is 2 hours, and a high-strength rod is obtained, whose tensile strength is equal to 1531MPa, yield strength is 1243MPa, elongation is 15%, and fracture toughness is 55MPa·m 1 / 2 .

[0060] Comparative Example 2

[0061] The ingredients are the same as those in Example 2, but the following production process parameters and production steps are different;

[0062] Step 1, vacuum induction: According to the chemical composition ratio, a vacuum induction smelting method is used to smelt an ingot: the melting time is appropriately set according to the ingot diameter of 300±20mm, high-purity iron is added first, the melting time is 2h, and the vacuum degree is 60Pa.

[0063] Step 2: The vacuum induction ingot is heated from room temperature to 730°C at 120°C / h. After the ingot is heated through, it is kept warm for 4.5 hours. After that, it is cooled with the furnace to 430°C. It is then air-cooled at a cooling rate of 40°C / min to room temperature.

[0064] Step 3: Multi-directional and multi-pass deformation treatment: Heat the steel ingot to 980℃, keep it at 980±10℃ for 6h, take it out of the furnace and start forging. The initial forging temperature is 980℃ and the final forging temperature is above 900℃. If the steel ingot cools down quickly, it should be put into the furnace for heating in time, and perform two piers and two pulls in the X and Y directions to strictly ensure that the steel is completed in one fire and the grain size is 6.5.

[0065] The subsequent steps were the same as those in Example 2, and a high-strength rod was obtained, whose tensile strength was equal to 1675 MPa, yield strength was 1323 MPa, elongation was 17%, and fracture toughness was 72 MPa·m 1 / 2 , its comprehensive mechanical properties are far inferior to those of Example 2.

[0066] Comparative Example 3

[0067] The difference from Example 3 lies in the composition, which is as follows: C: 0.38%, Mo: 1.60%, Ni: 3.8%, Si: 1.7%, Cr: 1.0%, Mn: 0.75%, V: 0.12%, and the remainder is Fe and unavoidable impurities.

[0068] The remaining steps are the same as those in Example 3, and a high-strength steel is obtained, whose tensile strength is 1875 MPa, yield strength is 1533 MPa, and fracture toughness is 95 MPa·m 1 / 2 , its strength is similar but its toughness index is much lower than that of Example 3.

[0069] Comparative Example 4

[0070] The difference from Example 1 lies in the composition, which is as follows: C: 0.31%, Mo: 0.90%, Ni: 1.4%, Si: 1.7%, Cr: 3.0%, Mn: 0.55%, V: 0.12%, and the remainder is Fe and unavoidable impurities.

[0071] The remaining steps are the same as those in Example 3, and a high-strength steel is obtained, whose tensile strength is 1775 MPa, yield strength is 1483 MPa, and fracture toughness is 75 MPa·m 1 / 2 , its strength is low and its toughness index is much lower than that of Example 3.

Claims

1. A method for preparing a single vacuum high-toughness medium-alloy ultra-high-strength steel, characterized in that: The mass percentage content of the single vacuum high-toughness medium-alloy ultra-high-strength steel components is: C: 0.26-0.31%, Mo: 0.6-1.2%, Ni: 3.6-6%, Si: 1.5-2.0%, Cr: 3-5%, Mn: 0.4-1.0%, V: 0.05-0.2%, and the balance is Fe and unavoidable impurities; The preparation method comprises the following steps: A. According to the chemical composition ratio, the ingot is smelted by vacuum induction smelting method: high-purity iron is added, and after melting, refining begins. Metal nickel, metal molybdenum, electrolytic manganese, metal chromium, metal tungsten, high-purity silicon, 80 ferrovanadium and carbon powder are added for refining, and the target content of the components is adjusted. After refining, the temperature is lowered to 1450℃~1480℃, the ingot is taken out of the furnace, cast, and then the head and tail are cut and the skin is peeled to obtain a vacuum induction ingot; B. Heating and cooling treatment: heating the vacuum induction ingot obtained in step A from room temperature to 650°C±150°C at 100-130°C / h, and heat-insulating the ingot after it is heated through. Then, cooling the ingot with the furnace to reduce the temperature to 300°C±150°C, and then air cooling the ingot at a cooling rate of 25-45°C / min to room temperature; C. High temperature uniform diffusion treatment of elements: the steel ingot after the temperature rise and fall treatment is heated from room temperature to 1200℃~1300℃ at 100~130℃ / h, and then kept at 1200℃~1300℃; D. Multi-directional and multi-pass deformation treatment: The steel ingot after high-temperature uniform diffusion treatment of elements is cooled to 1030±10℃~1090±10℃, and after heat preservation treatment, it is taken out of the furnace and forged. The initial forging temperature is 1030±10℃~1090±10℃, and the final forging temperature is 890℃~930℃. Multi-piercing and multi-drawing in at least three directions are carried out to strictly ensure that the material is formed in one fire and the grain size is above 8.

5. After final forging, the steel is cooled to room temperature; E. Combined modulation heat treatment: firstly, high-temperature air cooling heat treatment is carried out: the forged steel is heated from room temperature to 900℃~1000℃, after heat preservation treatment, the cooling rate is controlled at 20~80℃ / min to air cool to room temperature; then, high-temperature quenching treatment is carried out: heating from room temperature to a quenching temperature of 870℃~1020℃, after heat preservation treatment, oil quenching, cooling rate is 100℃ / s~150℃ / s, and cooling to room temperature; finally, two low-temperature tempering treatments are carried out: both tempering treatments are heated from room temperature to 200℃~300℃, after heat preservation treatment, air cooling is carried out to room temperature, and high-toughness medium alloy ultra-high strength steel is obtained.

2. The method for preparing single vacuum high-toughness medium-alloy ultra-high strength steel according to claim 1, characterized in that: In step A, the melting time is not less than 3 hours, and the vacuum degree is not higher than 30Pa.

3. The method for preparing single vacuum high-toughness medium-alloy ultra-high strength steel according to claim 1, characterized in that: In step A, the refining time is 0.5h to 1h.

4. The method for preparing single vacuum high-toughness medium-alloy ultra-high strength steel according to claim 1, characterized in that: In step B, the heat preservation treatment time is 4h to 5h.

5. The method for preparing single vacuum high-toughness medium-alloy ultra-high strength steel according to claim 1, characterized in that: In step C, the heat preservation treatment time is 8h to 15h.

6. The method for preparing single vacuum high-toughness medium-alloy ultra-high strength steel according to claim 1, characterized in that: In step D, the heat preservation treatment time is 4h to 10h.

7. The method for preparing single vacuum high-toughness medium-alloy ultra-high strength steel according to claim 1, characterized in that: In step D, in the multi-piercing and multi-drawing, upsetting is performed at least three times, and drawing is performed at least three times; preferably, in step D, the multi-piercing and multi-drawing in at least three directions is: three-piercing and three-drawing in three directions of X, Y and Z, or four-piercing and four-drawing in four directions of X, Y, Z and XY, or four-piercing and four-drawing in four directions of X, Z, YZ and ZY, or four-piercing and four-drawing in four directions of X, Y, XY and YX, or four-piercing and four-drawing in four directions of Z, Y, XY and YX.

8. The method for preparing single vacuum high-toughness medium-alloy ultra-high strength steel according to claim 1, characterized in that: At least one of the following is met: In step E, during the high temperature air cooling heat treatment, the heat preservation treatment time is 1h to 1.5h; In step E, during the high temperature quenching treatment, the heat preservation treatment time is 0.5h to 1.5h; In step E, during the two low-temperature tempering treatments, the heat preservation treatment time is 1.5 to 3 hours.

9. The high-toughness medium-alloy ultra-high-strength steel is prepared by the method for preparing the single vacuum high-toughness medium-alloy ultra-high-strength steel according to any one of claims 1 to 8.

10. The high-toughness medium alloy ultra-high strength steel according to claim 9, characterized in that: Its tensile strength is ≥1800MPa, yield strength is >1500MPa, elongation is >14%, fracture toughness is >110MPa·m 1 / 2 .