Large-size maraging stainless steel bar and preparation method thereof

Abstract

The invention relates to the field of metallurgy, and discloses a large-size maraging stainless steel bar and a preparation method thereof, the diameter of the bar is larger than or equal to 300 mm, and the method comprises a vacuum induction melting stage, a vacuum consumable remelting stage, a fast forging cogging stage and a precision forging forming stage; wherein the vacuum consumable remelting stage comprises an arcing stage, a stable smelting stage and a filling smelting stage; in the arc starting stage, a molten pool is formed by adopting a current of 8-12 kA; in the stable smelting stage, the smelting speed is 4.2-4.8 kg / min, the molten drop is 3-5 s <-1 >, the molten drop short-circuit time is 0.6-0.8 s, and inert gas is started to be filled for cooling after smelting is conducted by the preset weight; and in the filling smelting stage, inert gas filling is stopped, and the current and the melting speed are controlled to descend in a stepped mode. According to the method, the uniformity of components and structures can be improved, so that the mechanical property of the large-size maraging stainless steel bar is improved.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, specifically to a large-size martensitic aging stainless steel bar and its preparation method. Background Technology

[0002] Martensitic aging stainless steel is an ultra-high-strength stainless steel with an ultra-low carbon Fe-Cr-Ni-(Co) martensite matrix. By adding trace alloying elements (such as Al, Ti, Mo, Cu, etc.), it synergistically achieves martensitic transformation strengthening, solid solution strengthening, and precipitation strengthening, thus obtaining high-strength ultra-high-strength stainless steel. The ultra-high strength of martensitic aging stainless steel is due to the precipitation of high-density intermetallic compounds during the aging process. Therefore, martensitic aging stainless steel can simultaneously maintain high toughness and corrosion resistance, and can be widely used in high-tech fields such as aerospace, marine engineering, and nuclear energy. For aerospace structural components operating in corrosive environments, martensitic aging stainless steel plays an irreplaceable role.

[0003] Martensitic aging stainless steel requires very high material purity and is typically produced using vacuum induction melting (VIM) + electroslag remelting (ESR) or vacuum induction melting (VIM) + vacuum arc remelting (VAR) processes to reduce the impact of impurities and non-metallic inclusions on the material's strength and toughness. When the ingot diameter increases from the conventional Φ410mm and Φ550mm to larger ingots such as Φ840mm, the segregation of elements such as Cr, Mo, and Ti in the martensitic aging stainless steel during solidification increases significantly, worsening the material's thermoplasticity and reducing the mechanical properties of the bar products.

[0004] Therefore, it is necessary to study a method for preparing large-size martensitic aging stainless steel bars to meet the performance requirements of large-size bars. Summary of the Invention

[0005] The main objective of this invention is to provide a large-size martensitic aging stainless steel bar and its preparation method, so as to solve the technical problem of how to improve the mechanical properties of large-size martensitic aging stainless steel bars.

[0006] According to one aspect of the present invention, a method for preparing large-diameter martensitic aging stainless steel bars is provided, wherein the bars have a diameter ≥ 300 mm. The method includes: a vacuum induction melting stage, a vacuum arc remelting stage, a rapid forging stage, and a precision forging stage; wherein the vacuum arc remelting stage includes an arc ignition stage, a stabilization melting stage, and a filling melting stage; in the arc ignition stage, a current of 8~12 kA is used to form a molten pool; in the stabilization melting stage, the melting rate is 4.2~4.8 kg / min, and the droplet length is 3~5 s. -1The short-circuit time of the molten droplets is 0.6~0.8s. After the predetermined weight is smelted, inert gas is introduced for cooling. During the filling and smelting stage, the introduction of inert gas is stopped, and the current and melting rate are controlled to decrease stepwise.

[0007] According to one embodiment of the present invention, in the arc initiation stage, a molten pool is formed after 50 to 70 minutes; in the stable melting stage, the predetermined weight is 300 to 500 kg, and the inert gas pressure is gradually increased to 200 to 300 Pa; in the filling melting stage, the filling time is 40 to 60 minutes, and the filling weight is 350 to 450 kg.

[0008] According to one embodiment of the present invention, in the vacuum self-consumable remelting stage: the vacuum degree is ≤1.0Pa, the leakage rate is ≤0.5Pa / min, the diameter of the water-cooled crystallizer is 650~950mm, and the inlet temperature of the cooling water is 22~24℃; the vacuum self-consumable remelting stage also includes a cooling stage, in which vacuum cooling is performed for 50~80min, followed by furnace cooling for at least 3 hours, and then demolding and air cooling.

[0009] According to one embodiment of the present invention, the chemical composition of the bar material, by mass percentage, is as follows: C≤0.006%, Si≤0.1%, Mn≤0.1%, Ti≤0.02%, 13.00%≤Cr≤14.00%, 11.50%≤Co≤12.50%, 5.00%≤Ni≤6.00%, 4.00%≤Mo≤5.00%, 0.55%≤Al≤0.75%, P≤0.003%, S≤0.003%, [O]≤0.002%, [N]≤0.002%, with the balance being Fe.

[0010] According to one embodiment of the present invention, during the vacuum induction melting stage, the raw material substitution amounts are controlled to be P≤0.003% and S≤0.003%; when the vacuum degree in the furnace is ≤5Pa, power is supplied for melting, and after melting, the temperature is raised to 1550±15℃, and the refining time is ≥25min; when the N≤0.002%, O≤0.002%, and Ti≤0.02% in the molten steel are sampled and analyzed, inert gas is introduced and pure aluminum is added for deoxidation; after casting, the furnace is allowed to cool for at least 90min before breaking the vacuum, and the mold is allowed to cool for at least 12h before demolding and air cooling.

[0011] According to one embodiment of the present invention, the method further includes a homogenization treatment stage between the vacuum self-consuming remelting stage and the rapid forging stage, wherein the holding temperature in the homogenization treatment stage is 1220±15℃ and the holding time is ≥40h.

[0012] According to one embodiment of the present invention, in the rapid forging stage, the steel ingot is heated to 1150±15℃ before forging and held for ≥6h; the forging temperature is ≥1050℃ and the final forging temperature is ≥850℃; the forging is carried out in at least three passes and three draws, and the holding time for each pass is ≥60min; the forging ratio for the first pass is ≥5, the forging ratio for each subsequent pass is ≥2, and the total forging ratio is ≥11.5.

[0013] According to one embodiment of the present invention, in the precision forging stage, the steel ingot is heated at 1140±15℃ for a holding time of ≥3h; the initial forging temperature is ≥1050℃ and the final forging temperature is ≥850℃; after several passes, the steel ingot is radially precision forged into a bar with a diameter of ≥300mm.

[0014] According to one embodiment of the present invention, the method further includes a heat treatment stage after the precision forging stage, the heat treatment stage including a solution treatment stage, a cryogenic stage and an aging stage; in the solution treatment stage, the temperature is held at 1040~1060℃ for 20~40 min and then cooled, and then held at 1020~1040℃ for 20~40 min and then cooled; in the cryogenic stage, the temperature is cooled at -190~-200℃ for 100~150 min; in the aging stage, the temperature is held at 500~550℃ for 220~260 min; wherein the time interval between the solution treatment stage and the cryogenic stage is ≤3h.

[0015] According to another aspect of the present invention, a large-size martensitic aging stainless steel bar is provided, which is prepared by the method described above. The bar has a tensile strength ≥1900 MPa, a yield strength ≥1600 MPa, an elongation ≥10%, a reduction of area ≥55%, and a fracture toughness ≥90 MPa•m. 1 / 2 .

[0016] In the technical solution of this invention, the process steps and parameters of the vacuum arc remelting stage are meticulously designed, enabling deep purification, improving the uniformity of composition and microstructure, reducing segregation, and facilitating the production of large steel ingots with extremely uniform chemical composition and dense, defect-free microstructure. This invention also improves the forging process to ensure sufficient fragmentation of the as-cast microstructure, homogenizing the composition and microstructure, and avoiding microstructure inhomogeneities such as mixed grains. Furthermore, other steps such as vacuum induction melting and heat treatment are also improved. Through at least one of the above improvements, the large-diameter martensitic aging stainless steel bars prepared by this invention possess excellent mechanical properties, meeting application requirements. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 A flowchart illustrating a method for preparing large-size martensitic aging stainless steel bars according to an embodiment of the present invention is shown. Figure 2 A low-magnification micrograph of the Φ300mm bar prepared in Example 1 of the present invention is shown. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be further described in detail below with reference to specific examples and the accompanying drawings.

[0020] It should be noted that all uses of "first" and "second" in the embodiments of the present invention are for the purpose of distinguishing two entities or parameters with the same name but different names. It is clear that "first" and "second" are only for the convenience of expression and should not be construed as limiting the embodiments of the present invention. Subsequent embodiments will not explain this in detail.

[0021] refer to Figure 1 This invention proposes a method for preparing large-diameter martensitic aging stainless steel bars with a diameter ≥300mm. The method includes a vacuum induction melting stage, a vacuum arc remelting stage, a rapid forging stage, and a precision forging stage. The vacuum arc remelting stage includes an arc-starting stage, a stabilizing melting stage, and a filling melting stage. In the arc-starting stage, a current of 8~12kA is used to form a molten pool. In the stabilizing melting stage, the molten pool is required to be active to the edge, with a melting rate of 4.2~4.8kg / min and a droplet duration of 3~5s. -1 The short-circuit time of the molten droplets is 0.6~0.8s. After the predetermined weight is smelted, inert gas (such as helium) is introduced for cooling. During the filling and smelting stage, the introduction of inert gas is stopped, and the current and melting rate are controlled to decrease stepwise.

[0022] In the technical solution of this invention, the process steps and parameters of the vacuum self-consuming remelting stage are refined, which can achieve deep purification, improve the uniformity of composition and structure, reduce segregation, and facilitate the production of large steel ingots with extremely uniform chemical composition and dense and defect-free structure, providing reliable billets for subsequent forging of uniform large-sized bars.

[0023] Vacuum arc remelting of steel ingots often results in annular pattern defects, reflecting compositional segregation and caused by fluctuations in the melting rate. In the initial stages of melting, the arc remelting electrode temperature is low and the melting rate is slow; however, as the melting electrode temperature increases, the melting rate accelerates at the same current and voltage. This application, by controlling the melting rate within the range of 4.2~4.8 kg / min, ensures the uniformity of the melting rate, thereby guaranteeing product quality.

[0024] In some embodiments, during the arc initiation stage, a molten pool is formed after 50 to 70 minutes; during the stable melting stage, a predetermined weight of 300 to 500 kg is used to gradually increase the inert gas pressure to 200 to 300 Pa; during the filling melting stage, the filling time is 40 to 60 minutes and the filling weight is 350 to 450 kg.

[0025] In some embodiments, during the vacuum self-consumable remelting stage: the vacuum degree is ≤1.0Pa, the leakage rate is ≤0.5Pa / min, the diameter of the water-cooled crystallizer is 650~950mm, and the inlet temperature of the cooling water is 22~24℃; the vacuum self-consumable remelting stage also includes a cooling stage after the filling and melting stage. In the cooling stage, the furnace is first vacuum-cooled for 50~80min, then cooled with the furnace for at least 3 hours, and then demolded and air-cooled.

[0026] In some embodiments, the chemical composition of the bars, by mass percentage, is: C≤0.006%, Si≤0.1%, Mn≤0.1%, Ti≤0.02%, 13.00%≤Cr≤14.00%, 11.50%≤Co≤12.50%, 5.00%≤Ni≤6.00%, 4.00%≤Mo≤5.00%, 0.55%≤Al≤0.75%, P≤0.003%, S≤0.003%, [O]≤0.002%, [N]≤0.002%, with the balance being Fe. This invention improves the composition of martensitic aging stainless steel bars, further enhancing the mechanical properties of large-diameter bars.

[0027] The carbon content is controlled at ≤0.006% to ensure that the material obtains an ultra-low carbon martensitic matrix with good strength and toughness matching, while improving the weldability and processing performance of the steel.

[0028] By controlling the Cr content between 13% and 14%, a sufficiently dense passivation film can be formed on the steel surface, ensuring excellent corrosion resistance.

[0029] Mo is an alloying element that is beneficial for improving the corrosion resistance, high-temperature mechanical properties and tempering properties of materials. During the aging process, Mo-rich intermetallic compounds (such as Ni3Mo) are precipitated. The precipitated phases are finely and uniformly distributed in the crystal, which plays an important role in maintaining the high strength of the material and improving its toughness. When the Mo content is too high, δ-ferrite may appear in the steel structure, which leads to a decrease in the toughness of the material.

[0030] Al acts similarly to Mo, promoting the formation of the nano-precipitated phase NiAl during aging, thus improving the strength and toughness of the material. However, excessive Al content will reduce the toughness and plasticity of the material.

[0031] Co is dissolved in the matrix and does not directly participate in precipitation hardening reactions. Co's greatest contribution to strengthening is its synergistic effect with Mo, which reduces the solid solubility of Mo in martensite and promotes the precipitation of Ni3Mo. At the same time, it ensures that the intermetallic compounds have a high nucleation rate on dislocation lines and a highly dispersed precipitate phase, forming an interlaced spatial lattice structure, so as to obtain extremely high strengthening effect and minimal toughness loss. Co can also increase the Ms point, ensuring that the material obtains a fully martensitic matrix structure after cooling.

[0032] Ni is an important factor in ensuring the good toughness of martensitic aging steel. Ni can reduce the interaction energy between dislocations and interstitial atoms, prevent the decomposition of screw dislocations, and ensure the occurrence of cross-slip. At the same time, during cold working, Ni can effectively reduce the ductile-brittle transition temperature. Ni causes the δ-ferrite region to move towards higher temperatures, promoting the formation of more austenite at high temperatures, which facilitates obtaining a fully martensitic structure after quenching. However, excessive Ni content will lead to insufficient material strength.

[0033] This invention strictly controls the Ti content because Ti will form square Ti(C, N) inclusions with C and N during the solidification process of molten steel, reducing the toughness and fatigue performance of the material; Ti will also form TiS with S, reducing the corrosion resistance of the material. At the same time, the content of impurity elements such as [O], [N], S, and P must also be strictly controlled during the production of bars.

[0034] In some embodiments, during the vacuum induction melting stage, the raw material substitution amounts are controlled to be P≤0.003wt% and S≤0.003wt%; when the vacuum degree in the furnace is ≤5Pa, power is supplied for melting, and after melting, the temperature is raised to 1550±15℃, and the refining time is ≥25min; when the steel liquid is sampled and analyzed to contain N≤0.002wt%, O≤0.002wt%, and Ti≤0.02%, an inert gas (e.g., argon) is introduced, and pure aluminum is added for deoxidation; after casting, the furnace is allowed to cool for at least 90min before breaking the vacuum, and the mold is allowed to cool for at least 12h before demolding and air cooling.

[0035] During the smelting process, no deoxidizers other than pure Al are allowed. During vacuum induction melting, after all the raw materials have melted, in addition to the O and N content, the Ti content must be measured: if the Ti content > 0.02%, smelting should be stopped, and the process can be changed to produce pure iron specifically for martensitic aging stainless steel; if the Ti content ≤ 0.02%, normal smelting can continue. When the O and N content is too high, degassing can be continued under vacuum conditions by extending the stirring time.

[0036] In some embodiments, the method further includes a homogenization treatment stage between the vacuum arc remelting stage and the rapid forging stage, wherein the holding temperature during the homogenization treatment stage is 1220±15℃ and the holding time is ≥40h. The homogenization treatment stage further improves the uniformity of composition and microstructure. The steel of this invention contains nearly 35% alloying elements, making it more prone to compositional segregation during solidification. To further homogenize the chemical composition of the vacuum arc remelting ingot, the large ingot is held at approximately 1220℃ for more than 40h before forging.

[0037] In some embodiments, during the rapid forging stage, the steel ingot is heated to 1150±15℃ before forging and held for ≥6h; the initial forging temperature is ≥1050℃ and the final forging temperature is ≥850℃; at least three forging passes and three drawing passes are performed, with each forging pass holding for ≥60min. Simultaneously, the forging ratio is rationally allocated: the forging ratio for the first forging pass is ≥5 (e.g., between 5 and 6) to fully break down as-cast structures such as dendrites; the forging ratio for each subsequent pass is ≥2 (e.g., between 2 and 3); and the total forging ratio is ≥11.5 (e.g., between 11.5 and 12.5).

[0038] In some embodiments, during the precision forging stage, the steel ingot is heated at 1140±15℃ for a holding time of ≥3h; the initial forging temperature is ≥1050℃ and the final forging temperature is ≥850℃; after several passes, the steel ingot is radially precision forged into a bar with a diameter of ≥300mm.

[0039] This invention improves the forging process to ensure sufficient fragmentation of the as-cast structure, thereby homogenizing the composition and structure and avoiding unevenness in the structure such as mixed crystals.

[0040] In some embodiments, the method further includes a heat treatment stage following the precision forging stage. The heat treatment stage includes a solution treatment stage, a cryogenic treatment stage, and an aging stage. In the solution treatment stage, the material is held at 1040–1060°C for 20–40 min and then cooled, followed by a holding at 1020–1040°C for 20–40 min and then cooled again. In the cryogenic treatment stage, the material is cooled at -190–-200°C for 100–150 min. In the aging stage, the material is held at 500–550°C for 220–260 min. This invention, through the heat treatment stage, ensures the complete dissolution of alloying elements, homogenizes austenite, and reduces segregation.

[0041] Strength and toughness / ductility are two contradictory properties of bar stock. The content and morphology of retained austenite in the material are one of the key phases controlling the strength and toughness of the bar stock: a high austenite content leads to a decrease in material strength; a low austenite content leads to a decrease in material toughness. In this invention, the time interval between the solution treatment stage and the cryogenic stage is ≤3 hours, ensuring that the cryogenic stage can effectively reduce the content of retained austenite in the solution state, avoiding the problem of insufficient bar strength due to excessive austenite content in the microstructure after aging. The aforementioned time interval refers to the time elapsed from the end of the solution treatment stage to the beginning of the cryogenic stage, where the end of the solution treatment stage refers to the moment after cooling to room temperature after holding at 1020~1040℃ for 20~40 minutes.

[0042] This invention also proposes a large-size martensitic aging stainless steel bar, prepared by the method described above. The bar has a tensile strength ≥1900MPa, a yield strength ≥1600MPa, an elongation ≥10%, a reduction of area ≥55%, and a fracture toughness ≥90MPa•m. 1 / 2 The large-size martensitic aging stainless steel bars prepared by this invention have excellent mechanical properties, meet the application requirements, and are suitable for applications such as aerospace structural components.

[0043] The following description is based on specific embodiments and comparative examples.

[0044] Example 1 This embodiment provides a method for preparing large-diameter maraging stainless steel bars for aerospace structural components. The detailed steps are as follows: (1) Vacuum induction melting: Selected raw materials, including high-purity iron, electrolytic nickel, electrolytic cobalt, ultra-pure metallic chromium, molybdenum bars, and pure aluminum granules, etc., control the amount of raw materials to be added (P≤0.003wt%) and (S≤0.003wt%). Before loading into the furnace, the surface of the pure iron bars and nickel plates needs to be degreased and cleaned. A 6t vacuum induction furnace is selected. Cobalt plates, nickel plates, and molybdenum bars are loaded into the crucible in sequence, and then chromium blocks and pure iron bars are loaded in 3 batches. When the vacuum degree in the furnace is 4.9Pa, the power is supplied for melting. After melting, the temperature is raised to 1560℃ to start refining. The refining time is 30min and the stirring is maintained at the power frequency. The N content in the molten steel is 0.0018wt%, the O content is 0.0017wt%, and the Ti content is 0.011wt%. High-purity argon gas is introduced at 10000Pa, and pure aluminum is added for deep deoxidation. The stirring is continued at the power frequency for 10min until the aluminum granules are melted. The temperature is measured at 1540℃ before tapping. The steel is tapped while energized. The diameter of the ingot mold is Φ600mm. Before use, the ingot mold is scalded with purified molten iron to ensure that the surface is free of rust and impurities. After pouring, the mold is furnace cooled for 90 minutes and then broken open. After the mold is cooled for 12 hours, it is demolded and air-cooled to room temperature. After obtaining the electrode rod, its surface is machined and the head and tail are cut off, with 30mm cut off at the tail and 60mm cut off at the head (riser end).

[0045] (2) Vacuum Arsenic Remelting: The vacuum arc remelting furnace is 6t, the water-cooled crystallizer diameter is Φ660mm, the vacuum degree during the smelting process is 0.8Pa, and the leakage rate is 0.4Pa / min. During the arc initiation stage, current and voltage control are used, with a large current of 10kA forming a molten pool in approximately 60 minutes. During the stable smelting stage, melting rate and droplet control are used, requiring the molten pool to be active to the edge. The melting rate is set at 4.5±0.3kg / min, and the droplet duration is 3~5s. -1 The short-circuit time of the molten droplets was 0.7 s. Helium cooling was initiated when 300 kg of ingot was smelted, with the helium pressure gradually increased to 250 Pa. Helium was shut off before smelting, and the melting rate was controlled at 2.2 ± 0.3 kg / min. The smelting time was 40 min, and the smelting weight was 370 kg. After smelting, the ingot was vacuum cooled for 50 minutes, followed by furnace cooling for 3.5 hours before demolding and air cooling. The surface of the consumable ingot was peeled down to a diameter of Φ647 mm, with 30 mm removed from the tail and 60 mm removed from the head.

[0046] (3) Homogenization treatment: The steel ingot is homogenized at 1220±15℃ for 40h. (4) Rapid forging: Before forging, the steel ingot is heated to 1150±15℃ and held for 6.5h. The initial forging temperature is 1087℃. After forging begins, the entire ingot is lightly deformed radially. The first forging is roughened to H=975mm, then drawn to an octagonal shape of 620±10mm, and held in the furnace for 86min. The second forging is roughened to H=975mm, then drawn to an octagonal shape of 620±10mm, and held in the furnace for 81min. The third forging is roughened to H=975mm, then drawn to an octagonal shape of 570±10mm, and held in the furnace for 65min. The final forging is drawn to an octagonal shape of 390±10mm, air-cooled to room temperature, and the final forging temperature is 902℃. The forging ratio for the first forging is 5.2, the forging ratio for the second forging is 2.3, the forging ratio for the third forging is 2.5, the forging ratio for the final forging is 2.0, and the total forging ratio is 12.0.

[0047] (5) Precision forging: The octagonal ingot is heated at 1150±15℃ (preheating time 6.5h), held for 3.2h, and the initial forging temperature is 1082℃. After 5 passes of radial precision forging, a black forged bar with a diameter of 310±3mm is obtained. The final forging temperature is 883℃. After the bar is air-cooled in the sheltered place, the surface is machined and the ends are flattened to finally obtain a bar with a diameter of 300mm.

[0048] The final composition of the bar stock, by mass percentage, is as follows: C 0.005%, Si 0.042%, Mn 0.0042%, Al 0.66%, Cr 13.26%, Co 11.98%, Ni 5.37%, Mo 4.51%, Ti 0.011%, P 0.0021%, S 0.0015%, [O] 0.0008%, [N] 0.0010%.

[0049] Low-magnification analysis was performed on the forged bar, and the low-magnification microstructure images are as follows: Figure 2 As shown, the results are grade A for dark spots, grade A for white spots, grade A for radial segregation, and grade A for annular patterns. According to GB / T10561, the non-metallic inclusions in steel are inspected. The coarse series items of categories A, B, C, and D are all grade 0, and the fine series items of categories A and C are grade 0, category B is grade 0.5, and category D is grade 1.0.

[0050] Longitudinal samples were taken at half the radius of the bar. The heat treatment process included: solution treatment: holding at 1050℃ for 30 min, water cooling, holding at 1030℃ for 30 min, water cooling; followed by cryogenic treatment after a 60-min interval: cryogenic treatment at -196℃ with liquid nitrogen for 120 min, then recovery to room temperature in air; aging treatment: aging at 520℃ for 240 min, air cooling. Mechanical properties were tested on the heat-treated samples, and the measured mechanical properties were: tensile strength 1921 MPa, yield strength 1765 MPa, elongation 11.0%, reduction of area 58%, and fracture toughness 95 MPa•m. 1 / 2 .

[0051] Example 2 This embodiment provides a method for preparing large-diameter maraging stainless steel bars for aerospace structural components. The detailed steps are as follows: (1) Vacuum induction melting: Selected raw materials, including high-purity iron, electrolytic nickel, electrolytic cobalt, ultra-pure metallic chromium, molybdenum bars, and pure titanium bars, etc., control the amount of raw materials to be added (P≤0.003wt%) and (S≤0.003wt%). Before loading into the furnace, the surface of pure iron bars and nickel plates must be degreased and cleaned. A 12t vacuum induction furnace is selected. Cobalt plates, nickel plates, and molybdenum bars are loaded into the crucible in sequence, and then chromium blocks and pure iron bars are loaded in 4 batches. When the vacuum degree in the furnace is 5Pa, the power is supplied for melting. After melting, the temperature is raised to 1562℃ to start refining. The refining time is 45min and the stirring is maintained at the power frequency. The N content in the molten steel is 0.002wt%, the O content is 0.0019wt%, and the Ti content is 0.009wt%. High-purity argon gas is introduced at 10000Pa, and pure aluminum is added for deep deoxidation. The stirring is continued at the power frequency for 15min until the aluminum particles are melted. The temperature is measured at 1544℃ before tapping. The steel is tapped while energized. The diameter of the ingot mold is Φ840mm. Before use, the ingot mold is scalded with purified molten iron to ensure that the surface is free of rust and impurities. After pouring, the mold is furnace cooled for 120 minutes and then broken open. After the mold is cooled for 18 hours, it is demolded and air-cooled to room temperature. After obtaining the electrode rod, its surface is machined and the head and tail are cut off, with 30mm cut off at the tail and 95mm cut off at the head (riser end).

[0052] (2) Vacuum Arsenic Remelting: The vacuum arc remelting furnace is 12t, the water-cooled crystallizer diameter is Φ920mm, the vacuum degree during the smelting process is 0.8Pa, and the leakage rate is 0.5Pa / min. During the arc initiation stage, current and voltage control are used to ensure the molten pool is active to the edge. A large current of 12kA is used to form the molten pool in about 60 minutes. During the stable smelting stage, melting rate and droplet control are used, with the melting rate set at 4.5±0.3kg / min and the droplet length at 3~5s. -1 The short-circuit time of the molten droplets was 0.6 s. Helium cooling was initiated when 450 kg of ingot was smelted, with the helium pressure gradually increased to 280 Pa. Helium was shut off before smelting, the melting rate was controlled at 2.0 ± 0.3 kg / min, the smelting time was 60 min, and the smelting weight was 450 kg. After smelting, vacuum cooling was performed for 80 minutes, followed by furnace cooling for 4 hours before demolding and air cooling. The surface of the consumable ingot was peeled down to a diameter of 904 mm, with 30 mm removed from the tail and 90 mm removed from the head.

[0053] (3) Homogenization treatment: The steel ingot is homogenized at 1220±15℃ for 54 hours; (4) Rapid forging: Before forging, the steel ingot is heated to 1150±15℃ and held for 9 hours. The forging temperature is 1083℃. After forging begins, the whole ingot is lightly deformed radially. The first forging is roughened to H=1800mm, then shaped and upset to H=1350mm. Then it is drawn to an octagonal shape of 890±10mm and held in the furnace for 107 minutes. The second forging is roughened to H=1350mm, then drawn to an octagonal shape of 890±10mm and held in the furnace for 101 minutes. The third forging is roughened to H=1350mm, then drawn to an octagonal shape of 790±10mm and held in the furnace for 98 minutes. The last forging is drawn to an octagonal shape of 540±10mm and air-cooled to room temperature. The final forging temperature is 897℃. The first forging ratio is 5.2, the second forging ratio is 2.1, the third forging ratio is 2.3, the last forging ratio is 2.0, and the total forging ratio is 11.6.

[0054] (5) Precision forging: The octagonal ingot is heated at 1150±15℃ (preheating time 9h), held for 4.5h, and the initial forging temperature is 1079℃. After 6 passes of radial precision forging, a black forging bar with a diameter of Φ360±6mm is obtained. The final forging temperature is 880℃. After the bar is air-cooled in the sheltered place, the surface is machined and the ends are flattened to finally obtain a bar with a diameter of Φ350mm.

[0055] The final composition of the bar stock, by mass percentage, is: C 0.005%, Si 0.039%, Mn 0.0049%, Al 0.59%, Cr 13.36%, Co 12.08%, Ni 5.45%, Mo 4.46%, Ti 0.009%, P 0.0023%, S 0.0015%, [O] 0.0009%, [N] 0.0013%.

[0056] Forged bars were subjected to low-magnification testing, achieving grades A for dark spots, grade A for white spots, grade B for radial segregation, and grade B for annular patterns. Non-metallic inclusions in steel were inspected according to GB / T10561, with grades 0 for coarse inclusions in categories A, B, C, and D, and grades 0 for fine inclusions in category C, A, B, and D.

[0057] Longitudinal samples were taken at half the radius of the bar. The heat treatment process included: solution treatment: holding at 1050℃ for 30 min, water cooling, holding at 1030℃ for 30 min, water cooling; followed by cryogenic treatment after an 80-min interval: cryogenic treatment at -196℃ with liquid nitrogen for 120 min, then recovery to room temperature in air; aging treatment: aging at 520℃ for 240 min, air cooling. Mechanical properties were tested on the heat-treated samples, and the measured mechanical properties were: tensile strength 1915 MPa, yield strength 1739 MPa, elongation 10.0%, reduction of area 55%, and fracture toughness 90 MPa•m. 1 / 2 .

[0058] Comparative Example 1 The consumable ingot was prepared according to the method in Example 1. After being forged and drawn to an octagonal length of 390±10mm, it was returned to the furnace and held for 60 minutes. After being taken out of the furnace, it was drawn to an octagonal length of 317±10mm. The final forging temperature was 899℃. After being air-cooled to room temperature, the ends were flattened and the surface was machined and shaped to form a forged bar with a diameter of 300±2mm.

[0059] The final composition of the bar stock, by mass percentage, is as follows: C 0.005%, Si 0.040%, Mn 0.0043%, Al 0.63%, Cr 13.29%, Co 12.02%, Ni 5.41%, Mo 4.48%, Ti 0.010%, P 0.0020%, S 0.0015%, [O] 0.0008%, [N] 0.0011%.

[0060] Forged bars were subjected to low-magnification testing to achieve Grade A for dark spots, Grade A for white spots, Grade A for radial segregation, and Grade A for annular patterns. Non-metallic inclusions in steel were inspected according to GB / T10561. The coarse series items of Class A, B, C, and D were all Grade 0, while the fine series items of Class A and C were Grade 0, Class B was Grade 0.5, and Class D was Grade 1.0.

[0061] Longitudinal samples were taken at half the radius of the bar. The heat treatment process included: solution treatment: holding at 1050℃ for 30 min, water cooling, holding at 1030℃ for 30 min, water cooling; followed by cryogenic treatment after a 60-min interval: cryogenic treatment at -196℃ with liquid nitrogen for 120 min, then recovery to room temperature in air; aging treatment: aging at 520℃ for 240 min, air cooling. Mechanical properties were tested on the heat-treated samples, and the measured mechanical properties were: tensile strength 1896 MPa, yield strength 1769 MPa, elongation 11.0%, reduction of area 52%, and fracture toughness 84 MPa•m. 1 / 2 .

[0062] Comparative Example 2 Large-diameter martensitic aging stainless steel bars were prepared using a method similar to that in Example 2, except that the melting rate fluctuation range during the stable melting stage in the vacuum consumable remelting process was 4.5 ± 0.8 kg / min.

[0063] The final composition of the bar stock, by mass percentage, is as follows: C 0.005%, Si 0.039%, Mn 0.0044%, Al 0.63%, Cr 13.31%, Co 12.01%, Ni 5.45%, Mo 4.45%, Ti 0.011%, P 0.0021%, S 0.0015%, [O] 0.0010%, [N] 0.0012%.

[0064] Forged bars were subjected to low-magnification testing to achieve grades A for dark spots, grade A for white spots, grade B for radial segregation, and grade C for annular patterns. Non-metallic inclusions in steel were inspected according to GB / T10561, with grade 0 for coarse inclusions in grades A and C, grade 0.5 for coarse inclusions in grades B and D, and grade 1.0 for fine inclusions in grades A and C, grade 1.5 for grade B, and grade 2.0 for grade D.

[0065] Longitudinal samples were taken at half the radius of the bar. The heat treatment process included: solution treatment: holding at 1050℃ for 30 min, water cooling, holding at 1030℃ for 30 min, water cooling; followed by cryogenic treatment after an 80-min interval: cryogenic treatment at -196℃ with liquid nitrogen for 120 min, then recovery to room temperature in air; aging treatment: aging at 520℃ for 240 min, air cooling. Mechanical properties were tested on the heat-treated samples, and the measured mechanical properties were: tensile strength 1910 MPa, yield strength 1742 MPa, elongation 10.5%, reduction of area 45%, and fracture toughness 69 MPa•m. 1 / 2 .

[0066] Comparative Example 3 Rods were prepared according to the method of Example 2, with the same chemical composition, low-magnification analysis, and inclusion levels as in Example 2. The heat treatment process differed in that the time interval between the end of solution treatment and the start of cryogenic treatment was 16 hours, while other heat treatment parameters remained unchanged. Mechanical properties of the heat-treated samples were tested, and the measured mechanical properties were: tensile strength 1880 MPa, yield strength 1733 MPa, elongation 10.5%, reduction of area 59%, and fracture toughness 92 MPa·m. 1 / 2 .

[0067] The chemical composition and performance data of the bars from Examples 1 and 2 and Comparative Examples 1-3 are shown in Tables 1 and 2, respectively. As can be seen from Tables 1 and 2, under similar chemical compositions, Examples 1 and 2 exhibited better mechanical properties than Comparative Example 1 through reasonable forging process design. In Comparative Example 2, the melting rate fluctuated significantly, the degree of point segregation in the ingot increased, and the level of inclusions in the bars was high, leading to a decrease in material toughness. In Comparative Example 3, the time interval between solution treatment and cryogenic treatment was 16 hours. Cryogenic treatment could not effectively reduce the residual austenite content in the solution state, resulting in a higher austenite content in the microstructure after aging and insufficient bar strength.

[0068] Table 1. Chemical composition (wt%) of the bars from Examples 1, 2 and Comparative Examples 1-3

[0069] Table 2 Performance data of Examples 1, 2 and Comparative Examples 1-3

[0070] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention (including the claims) is limited to these examples. Within the framework of the invention, technical features of the above embodiments or different embodiments can be combined, and many other variations of the different aspects of the invention as described above exist, which are not provided in the details for the sake of brevity. Therefore, any omissions, modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the invention should be included within the protection scope of the invention.

Claims

1. A method for preparing large-size martensitic aging stainless steel bars, characterized in that, The diameter of the bar is ≥300mm, and the method includes: vacuum induction melting stage, vacuum self-consumable remelting stage, rapid forging stage and precision forging stage; The vacuum consumable remelting stage includes an arc-starting stage, a stabilizing melting stage, and a filling melting stage. In the arc-starting stage, a current of 8-12 kA is used to form the molten pool. In the stabilizing melting stage, the melting rate is 4.2-4.8 kg / min, and the droplet length is 3-5 seconds. -1 The short-circuit time of the molten droplets is 0.6~0.8s. After the predetermined weight is smelted, inert gas is introduced for cooling. During the filling and smelting stage, the introduction of inert gas is stopped, and the current and melting rate are controlled to decrease stepwise.

2. The method according to claim 1, characterized in that, During the arc-starting stage, a molten pool is formed after 50 to 70 minutes; during the stable smelting stage, the predetermined weight is 300 to 500 kg, and the inert gas pressure is gradually increased to 200 to 300 Pa. During the filling and smelting stage, the filling time is 40-60 minutes and the filling weight is 350-450 kg.

3. The method according to claim 1, characterized in that, In the vacuum self-consumable remelting stage: the vacuum degree is ≤1.0Pa, the leakage rate is ≤0.5Pa / min, the diameter of the water-cooled crystallizer is 650~950mm, and the inlet temperature of the cooling water is 22~24℃; the vacuum self-consumable remelting stage also includes a cooling stage, in which vacuum cooling is performed for 50~80min, followed by furnace cooling for at least 3 hours, and then demolding and air cooling.

4. The method according to claim 1, characterized in that, The chemical composition of the rod, by mass percentage, is as follows: C≤0.006%, Si≤0.1%, Mn≤0.1%, Ti≤0.02%, 13.00%≤Cr≤14.00%, 11.50%≤Co≤12.50%, 5.00%≤Ni≤6.00%, 4.00%≤Mo≤5.00%, 0.55%≤Al≤0.75%, P≤0.003%, S≤0.003%, [O]≤0.002%, [N]≤0.002%, with the balance being Fe.

5. The method according to claim 1, characterized in that, During the vacuum induction melting stage, the raw material substitution amounts are controlled to be P≤0.003% and S≤0.003%; when the vacuum degree in the furnace is ≤5Pa, power is supplied for melting, and after melting, the temperature is raised to 1550±15℃, and the refining time is ≥25min; when the N≤0.002%, O≤0.002%, and Ti≤0.02% in the molten steel are sampled and analyzed, inert gas is introduced and pure aluminum is added for deoxidation; after casting, the furnace is allowed to cool for at least 90min before breaking the vacuum, and the mold is allowed to cool for at least 12h before demolding and air cooling.

6. The method according to claim 1, characterized in that, It also includes a homogenization treatment stage between the vacuum self-consuming remelting stage and the rapid forging stage, wherein the holding temperature in the homogenization treatment stage is 1220±15℃ and the holding time is ≥40h.

7. The method according to claim 1, characterized in that, In the rapid forging stage, the steel ingot is heated to 1150±15℃ before forging and held for ≥6h; the forging temperature is ≥1050℃ and the final forging temperature is ≥850℃; the forging is carried out in at least three passes and three draws, with a holding time of ≥60min for each pass; the forging ratio of the first pass is ≥5, the forging ratio of each subsequent pass is ≥2, and the total forging ratio is ≥11.

5.

8. The method according to claim 1, characterized in that, In the precision forging stage, the steel ingot is heated at 1140±15℃ for a holding time of ≥3h; the initial forging temperature is ≥1050℃ and the final forging temperature is ≥850℃; after several passes, the steel ingot is radially precision forged into a bar with a diameter of ≥300mm.

9. The method according to claim 1, characterized in that, It also includes a heat treatment stage after the precision forging stage, which includes a solution treatment stage, a cryogenic stage, and an aging stage; in the solution treatment stage, the temperature is held at 1040~1060℃ for 20~40 min and then cooled, and then held at 1020~1040℃ for 20~40 min and then cooled; in the cryogenic stage, the temperature is cooled at -190~-200℃ for 100~150 min; in the aging stage, the temperature is held at 500~550℃ for 220~260 min; wherein the time interval between the solution treatment stage and the cryogenic stage is ≤3h.

10. A large-size martensitic aging stainless steel bar, characterized in that, The bar is prepared by the method described in any one of claims 1-9, and the bar has a tensile strength ≥1900 MPa, a yield strength ≥1600 MPa, an elongation ≥10%, a reduction of area ≥55%, and a fracture toughness ≥90 MPa•m. 1 / 2 .

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