A method for preparing an ultra-low temperature high strength and toughness special steel by ultra-pure smelting

Abstract

This invention relates to a method for preparing ultra-pure smelting of high-strength and high-toughness special steel at ultra-low temperatures. The chemical composition (wt%) is controlled as follows: C≤0.01%, Si≤0.05%, Mn≤0.05%, S≤0.005%, P≤0.005%, Ni: 17.5~18.5%, Co: 8.0~9.0%, Mo: 3.0~3.8%, Al: 0.05~0.15%, Ti: 0.20~0.35%, H≤1.0 ppm, O≤16 ppm, N≤15 ppm, with the balance being Fe and other unavoidable impurities. Core Technology: A five-element synthetic slag system of CaO-SiO2-Al2O3-MgO-CaF2 is introduced during the VIM stage. Utilizing the high-speed directional solidification effect of VAR steady-state remelting, and through matching the smelting environment and cooling rate, the coarse, brittle titanium sulfide phase is transformed into finely dispersed Al-Mg-Ti-S-O multiphase composite oxides. This achieves composite modification of inclusions and reduces their number density from 45 inclusions / mm². 2 Reduced to 8.5 pieces / mm 2 It effectively eliminates the hazards of large particles >4μm. The average grain diameter of the original austenite is refined from 42.37μm to 26.38μm, the elongation of the material reaches 21.06%, and the impact energy is as high as 143.63J, significantly improving the resistance to brittle fracture and possessing extremely high engineering application value.

Description

Technical Field

[0001] This invention relates to the field of special steel smelting and processing technology, specifically a method for preparing ultra-pure smelting special steel at ultra-low temperatures with high strength and toughness. Background Technology

[0002] With the rapid development of aerospace, deep space exploration, and superconducting quantum technology, modern engineering equipment places almost stringent requirements on the service performance of key structural materials. This is particularly true for core components of launch vehicle engines operating in liquid hydrogen (-253℃) and liquid nitrogen (-196℃) environments, cryogenic support structures for superconducting magnets, and deep-sea drilling and heavy equipment operating in extreme polar environments. These materials not only require extremely high yield strength (typically exceeding 930 MPa) to achieve lightweight structural design, but also must maintain excellent ductility, toughness, fatigue resistance, and resistance to brittle fracture at extremely low temperatures to ensure the service safety and long lifespan of critical equipment.

[0003] Among numerous metallic materials, ultra-low temperature high-strength and high-toughness special steels (typically represented by 18Ni200 grade martensitic aging stainless steel) stand out. These special steels consist of an ultra-low carbon, high-nickel soft martensitic matrix. Through precise control of the aging process, intermetallic compounds such as Ni3Ti and Ni3Mo are dispersed and precipitated within the matrix, producing a strong precipitation strengthening effect. Compared to traditional quenched and tempered steels or ordinary stainless steels, this special steel, while maintaining ultra-high strength, exhibits excellent fracture toughness, superior weldability, and dimensional stability after heat treatment. Therefore, the 18Ni200 series of special steels has become an irreplaceable key strategic material in major national projects.

[0004] However, ultra-low temperature high-strength and high-toughness special steels face a core challenge in practical engineering applications: the tendency for the material to undergo a "ductile-brittle transition" under ultra-low temperature environments. Although 18Ni series steels have lower ductile-brittle transition temperatures than conventional body-centered cubic (BCC) structural steels due to their high nickel content matrix, under extreme loads at liquid nitrogen (-196℃) or even lower temperatures, their fracture mode readily transitions from ductile dimple fracture to brittle quasi-cleavage fracture. This brittle fracture is often sudden and without warning; once it occurs, it can lead to catastrophic equipment failure.

[0005] Existing research indicates that the core microscopic cause of cryogenic brittle fracture in ultra-low temperature high-strength and tough special steels is not the material's chemical composition, but rather the microscopic defects within the steel that are difficult to completely eliminate, particularly gaseous impurity elements (O, N, H) and the resulting non-metallic inclusions. In cryogenic service environments, the material's deformation capacity is limited, and even tiny inclusions can cause significant stress concentration. The following are the effects of gaseous impurity elements (O, N, H) and large-size inclusions: The impact of harmful gaseous elements: Hydrogen (H) is the main cause of decreased material plasticity and delayed cracking; oxygen (O) and nitrogen (N) directly participate in the formation of non-metallic inclusions. Patent CN112301244A demonstrated that reducing the concentration of gaseous elements H to below 1.6 ppm and O to below 20 ppm is crucial for improving the matrix purity of high-strength alloys.

[0006] The destructive effect of large inclusions: Especially large inclusions larger than 4μm (such as nitrides or oxides like TiN), under ultra-low temperatures and alternating loads, will generate microcracks at the interface due to the mismatch between their thermal expansion coefficients and the matrix. Patent EP3170911A1 explicitly points out that coarse TiN or TiCN inclusions in martensitic aging steel are the origin of fatigue fracture and quasi-cleavage fracture. For ultra-low temperature high-strength and high-toughness special steels, although the Ti content is slightly lower than that of 300 grade, under conventional smelting conditions, the large particle defects formed by the aggregation and growth of nitrides still cannot be completely eliminated.

[0007] Currently, the industry primarily employs vacuum induction melting (VIM) for the conventional production of ultra-low temperature high-strength and high-toughness special steels. The VIM process, through vacuum degassing and carbon deoxidation, can effectively control the macroscopic composition of the alloy and reduce some gas content. However, the VIM process has two significant limitations: (1) Refractory material contamination: During the VIM smelting process, the molten steel is in contact with the crucible refractory material for a long time, which is very easy to cause secondary oxidation, resulting in fluctuations in the oxygen content of the steel and the generation of endogenous inclusions. (2) Uneven crystal structure: During the cooling process, the solidification rate of traditional VIM ingots is relatively slow, which easily leads to macroscopic segregation and makes it impossible to break up the already formed coarse nitrides by physical means. This results in large-sized (>4μm) inclusions still being widely present in the matrix, becoming a bottleneck in the performance of the material in ultra-low temperature environments.

[0008] To compensate for the shortcomings of single VIM (Vacuum Injection Molding), some manufacturers have attempted to introduce vacuum arc remelting (VAR) technology. Patent CN1056902A mentions the effect of VIM+VAR dual vacuum process on improving the transverse plasticity of 18Ni series steels. However, in actual engineering practice, there is still a lack of mature and stable process guidelines and parameter systems in the industry regarding how to precisely control the remelting current, voltage, and melting rate of VAR to achieve the dual goals of "ultra-high purity and elimination of inclusions larger than 4μm" for the special physical properties of ultra-low temperature high-strength and tough special steels (such as melting point, viscosity, and volatile element control).

[0009] To address the aforementioned issues, developing an ultra-pure smelting process capable of precisely controlling the extremely low gas content of ultra-low temperature high-strength and tough special steel and effectively eliminating large-sized inclusions is not only an urgent need to improve the reliability of my country's aerospace equipment, but also a key step in breaking the foreign blockade on high-performance materials in this field.

[0010] This invention is based on a profound understanding of the ultra-low temperature fracture mechanism. Through innovative optimization of the VIM+VAR dual vacuum fusion process, a set of rigorous refining, casting, and remelting control parameters is proposed. While ensuring that the H, O, and N content of the matrix reaches an extremely pure level, the high-speed directional solidification and steady-state remelting of VAR significantly refine the original austenite grain size compared to the initial VIM state, and reduce the inclusion number density to 10 inclusions / mm. 2 The process eliminates most of the hazards posed by large particle inclusions larger than 4μm, thus achieving a leapfrog synergistic improvement in the strength and impact toughness of ultra-low temperature high-strength and toughness special steel in an ultra-low temperature environment of -196℃. This provides a solid material foundation for engineering equipment used in extreme environments such as my country's deep space exploration. Summary of the Invention

[0011] To address the aforementioned issues, this invention targets ultra-low temperature high-strength and high-toughness special steel. Through parameter innovation in the VIM+VAR dual-vacuum fusion process, utilizing the high-temperature decomposition of vacuum arc and the directional rapid cooling effect of the VAR water-cooled crystallizer, it achieves deep removal of interstitial elements H, O, and N (reduced to 0.9, 16, and 12 ppm, respectively) and significant refinement of the original austenite grains. The core technology lies in precisely matching the VAR steady-state melting parameters to induce composite modification of inclusions, promoting the transformation of coarse Ti-S and composite oxides in the VIM state into finely dispersed and rounded-edge Al-Mg-Ti-SO multiphase composite inclusions. This reduces the inclusion number density from 45 inclusions / mm². 2 Significantly reduced to 8.5 per mm 2 At the same time, it effectively blocks the aggregation and damage of harmful inclusions with large particles >4μm, fundamentally reducing stress concentration and crack initiation sensitivity in ultra-low temperature environments, thereby significantly improving the comprehensive mechanical properties of ultra-low temperature high-strength and tough special steel.

[0012] The present invention provides a method for preparing ultra-pure smelting of high-strength and high-toughness special steel at ultra-low temperatures, which specifically includes the following steps: (1) Vacuum Induction Melting (VIM): The raw materials according to the target composition ratio are put into a vacuum induction furnace, vacuum is drawn and heated to melt; after the raw materials are completely melted, preheated and baked synthetic refining slag is added to the molten steel. Then, the temperature of the molten steel is raised and the refining period is entered. After the refining is completed, it is poured at a certain temperature and the initial refining ingot is obtained after demolding. (2) Vacuum self-consumable melting (VAR): The primary smelting ingot is placed in the crystallizer of the vacuum self-consumable furnace as a self-consumable electrode for remelting. After remelting, the ingot is fed back, cooled and demolded to finally obtain an ultra-pure steel ingot. The ultra-pure steel ingot is an ultra-low temperature high strength and toughness special steel.

[0013] Furthermore, the components of the synthetic refining slag described in step (1) are, by mass percentage: 50-55% CaO, 15-20% SiO2, 15-20% Al2O3, 4-6% MgO and 4-6% CaF2.

[0014] Furthermore, the amount of synthetic refining slag added in step (1) is 0.8%-1.5% of the mass of the molten steel.

[0015] Furthermore, in step (1), the step "After the raw materials are fully melted, add the preheated and baked synthetic refining slag to the molten steel, and then raise the temperature of the molten steel and enter the refining period" is replaced with "After the raw materials are fully melted, raise the temperature of the molten steel and enter the refining period, and then add the preheated and baked synthetic refining slag to the molten steel", that is, the synthetic refining slag can be added either before the start of refining or during the refining period.

[0016] Furthermore, in step (1), the refining temperature is 1540-1560℃ and the refining time is 15-30min.

[0017] Furthermore, in step (1), the vacuum level inside the furnace is controlled to be ≤1.0Pa during the refining period.

[0018] Furthermore, during the refining process in step (1), electromagnetic stirring is used to promote a full reaction at the slag-gold interface.

[0019] Furthermore, in step (1), the casting is carried out at 1520-1540℃.

[0020] Furthermore, in step (2), the initial smelting ingot is surface cleaned and then used as a consumable electrode. The surface cleaning specifically involves grinding and rust removal.

[0021] Furthermore, the compensation time in step (2) is 10-20 min.

[0022] Furthermore, during remelting in step (2), the steady-state current is controlled to be 4.8-5.2KA, the steady-state voltage is 27-29V, the steady-state melting rate is 2.2-2.6Kg / min, and the vacuum degree is ≤1.6Pa.

[0023] Furthermore, during remelting in step (2), the cooling water flow rate is controlled at 28-32 t / h.

[0024] The ultra-low temperature high strength and toughness special steel obtained by the above preparation method has the following chemical composition by mass percentage: C≤0.01%, Si≤0.05%, Mn≤0.05%, S≤0.005%, P≤0.005%, Ni: 17.5-18.5%, Co: 8.0-9.0%, Mo: 3.0-3.8%, Al: 0.05-0.15%, Ti: 0.20-0.35%, H≤1.0ppm, O≤16ppm, N≤15ppm, with the balance being Fe and other unavoidable impurities.

[0025] The scientific basis of this invention, which uses the above-mentioned specific proportion of a pentagonal synthetic refining slag in the vacuum induction melting process, lies in: ① High basicity control: By controlling the CaO / SiO2 mass ratio within the range of 2.5-3.6, an extremely high basicity slag system was constructed. This system has an extremely high sulfur capacity, which can efficiently capture sulfur elements in the original coarse Ti-S brittle phase in the molten steel, creating a key chemical potential difference for the transformation of inclusions into finely dispersed composite oxides.

[0026] ② Flowability and degassing balance: The synergistic effect of 15-20% SiO2 and 15-20% Al2O3 precisely regulates the melting point and viscosity of the slag. Experiments have shown that this ratio enables the slag layer to spread rapidly and form a uniform liquid layer at 1550℃, which can both adsorb inclusions and avoid obstruction of the vacuum degassing channel due to excessive slag layer thickness or excessive viscosity, thus ensuring the achievement of ultra-low H and O content.

[0027] ③ Protecting the crucible and inhibiting secondary oxidation: Adding 4-6% MgO to the slag makes it close to the saturation solubility of magnesia / alumina-magnesia crucible material, which significantly inhibits the erosion of refractory materials by molten slag. This not only extends the service life of the equipment, but also prevents endogenous oxide pollution caused by crucible erosion from the source.

[0028] ④ Kinetic acceleration: The addition of 4-6% CaF2 as a flux significantly reduces the interfacial tension of the liquid slag, ensuring that the slag-metal reaction can quickly reach thermodynamic equilibrium within a 20-minute refining period, and accelerating the capture and flotation absorption of large particle inclusions >4μm.

[0029] Compared with the prior art, the present invention has the following beneficial effects: (1) Significant inclusion control effect: This invention utilizes a specific ratio of CaO-SiO2-Al2O3-MgO-CaF2 pentagonal refining slag, taking advantage of its high basicity and good fluidity to form a liquid slag layer during vacuum induction melting. This slag system can effectively capture oxides and sulfides floating to the slag-metal interface and promote their chemical modification. Combined with a vacuum environment, this slag system can significantly reduce the residual oxygen and sulfur content in the molten steel. Further vacuum self-consumption further reduces the average number of inclusions per square millimeter in the steel from 45 inclusions / mm in conventional processes. 2 It dropped sharply to about 8.5 particles / mm. 2 The area ratio of inclusions decreased from 0.018% to approximately 0.006%. In particular, large particle inclusions >4μm, which are extremely harmful to materials, were effectively eliminated, thereby reducing stress concentration sources in ultra-low temperature environments from the root.

[0030] (2) Extremely low levels of gaseous and impurity elements: The VAR remelting process further removes the gas and volatile impurities remaining from the initial refining process, resulting in a reduction of C content to 0.004%, H content to 0.9ppm, and O content to 16ppm in the final product, which significantly improves the purity of the steel matrix.

[0031] (3) Significant grain refinement: Relying on the high-speed directional solidification effect of the water-cooled crystallizer in the VAR remelting process, the undercooling at the solidification front of the molten metal pool is greatly improved. Experimental data show that the average grain diameter of the original austenite in the steel prepared in this embodiment of the invention is significantly refined from 42.37 μm in the VIM state to 26.38 μm, a reduction of 37.7%. This refinement of the microstructure effectively increases the grain boundary area and significantly improves the crack propagation work, which is a key factor in ensuring that the material obtains high impact toughness at -196℃ from a microstructural perspective.

[0032] (4) Significantly improved plasticity and toughness: While maintaining extremely high room temperature strength (tensile strength > 1060 MPa, yield strength > 930 MPa), the special steel prepared by this invention has an elongation of 21.06% (nearly 70% higher than conventional processes), a reduction of area of ​​71.84% (nearly 40% higher), and an impact energy of 118.73 J to 143.63 J, which significantly improves the resistance to brittle fracture and meets the stringent requirements of ultra-low temperature engineering equipment for high strength and high toughness. Attached Figure Description

[0033] Figure 1 The ultrapure steel ingot is prepared by dual smelting of vacuum induction melting (VIM) and vacuum arc remelting (VAR) in Embodiment 1 of the present invention; Figure 2This is a comparison diagram of the original austenite grain boundaries of samples obtained by forging and sampling the initial refined ingot obtained in Comparative Example 1 and the ultra-pure steel ingot obtained in Example 1 of this invention, respectively; wherein, Figure 2 Figure (a) in the middle is Comparative Example 1. Figure 2 Figure (b) shows Example 1; Figure 3 The images show a comparison of the microstructure and energy dispersive spectroscopy (EDS) of inclusions in samples forged and sampled from the primary refined ingot obtained in Comparative Example 1 and the ultrapure steel ingot obtained in Example 1 of this invention, respectively; wherein, Figure 3 Figures (a) and (b) in the middle are comparative examples 1. Figure 3 Figures (c) and (d) in the middle are examples of Example 1. Detailed Implementation

[0034] To better understand the content of this invention, it will be further described below with reference to specific embodiments and accompanying drawings. The following embodiments are based on the technology of this invention and provide detailed implementation methods and operating steps, but the scope of protection of this invention is not limited to the following embodiments.

[0035] Example 1: A method for preparing ultra-pure smelting of high-strength and high-toughness special steel at ultra-low temperatures, the specific steps of which are as follows: (1) VIM initial refining: Alloy raw materials are taken according to the mass percentage of the chemical composition of the special steel. The alloy raw materials are placed in a vacuum induction furnace. The alloy raw materials specifically include ferrosilicon, ferromanganese, metallic cobalt, nickel plate, ferromolybdenum, ferrotitanium, industrial pure iron, and aluminum wire. The system is evacuated and heated to melt. After the raw materials are completely melted, the temperature is raised to 1550℃ to enter the refining period. Then, synthetic refining slag is added to the molten steel. The synthetic refining slag is baked and dried at 300℃, and its mass ratio is: 53% CaO, 18% SiO2, 19% Al2O3, 5% MgO, 5% CaF2, with a slag content of 1.2% of the total weight of the molten steel, is fully in contact with the molten steel during refining under the action of electromagnetic stirring, playing a role in adsorbing inclusions and deep desulfurization. The vacuum degree is maintained at 0.8 Pa during refining, and the refining time is 20 min. After refining, the temperature of the molten steel is adjusted to 1530℃ for casting. After cooling, a primary refining ingot with a diameter of ∅105 mm and a length of 1500 mm is obtained. (2) VAR remelting: The above-mentioned primary smelting ingot is cleaned on the surface and used as a consumable electrode. The surface cleaning is specifically grinding and rust removal. It is placed in a water-cooled copper crystallizer in a vacuum consumable furnace for remelting. The diameter of the crystallizer is ∅160mm. The remelting parameters are strictly controlled as follows: steady-state current 5KA, steady-state voltage 28±1V, steady-state melting rate 2.4Kg / min, vacuum degree 1.6Pa, cooling water flow rate 30t / h. After remelting, feeding is performed for 15min. After cooling and demolding, an ultra-pure steel ingot with a diameter of ∅160mm and a length of 600mm is obtained.

[0036] The ultra-low temperature high strength and toughness special steel described herein has the following chemical composition by mass percentage: C: 0.004%, Si: 0.03%, Mn: 0.04%, S: 0.005%, P: 0.005%, Ni: 18.0%, Co: 8.50%, Mo: 3.40%, Al: 0.10%, Ti: 0.28%, H: 0.9ppm, O: 16ppm, N: 12ppm, with the balance being Fe and other unavoidable impurities.

[0037] Example 2: (1) VIM initial refining: According to the chemical composition mass percentage of special steel, take alloy raw materials and place the alloy raw materials (specifically including ferrosilicon, ferromanganese, cobalt metal, nickel plate, ferromolybdenum, ferrotitanium, industrial pure iron, and aluminum wire) in a vacuum induction furnace. After the system is evacuated, heat and melt it. After the raw materials are completely melted, raise the temperature to 1545℃ to enter the refining period. Then add synthetic refining slag that has been baked and dried at 300℃ to the molten steel. The composition mass ratio of the synthetic refining slag is: 55% CaO, 15% SiO2, 20% Al2O3, 4% MgO, 6% CaF2, and the slag accounts for 1.0% of the total weight of the molten steel. Use electromagnetic stirring to promote full reaction at the slag-metal interface. During the refining period, the vacuum degree is maintained at 0.5Pa and the refining time is 30min. After the refining is completed, adjust the temperature of the molten steel to 1525℃ for casting. After demolding, a primary refining ingot with a diameter of ∅105mm and a length of 1500mm is obtained. (2) VAR remelting: The above-mentioned primary smelting ingot was surface-polished and derusted, and then used as a consumable electrode in a water-cooled copper crystallizer in a vacuum consumable furnace for remelting. The remelting process parameters were controlled as follows: steady-state current 4.8 kA, steady-state voltage 27 V, steady-state melting rate 2.2 kg / min, vacuum degree 1.2 Pa, and cooling water flow rate 28 t / h. After remelting, feeding was performed for 20 min, and after cooling and demolding, ultra-pure steel ingots were obtained.

[0038] The chemical composition (by mass percentage) of the ultra-low temperature high strength and toughness special steel obtained in this embodiment is as follows: C: 0.006%, Si: 0.02%, Mn: 0.05%, S: 0.003%, P: 0.004%, Ni: 17.8%, Co: 8.80%, Mo: 3.20%, Al: 0.08%, Ti: 0.32%, H: 0.8ppm, O: 14ppm, N: 13ppm, with the balance being Fe and other unavoidable impurities.

[0039] Example 3: (1) VIM initial refining: According to the chemical composition mass percentage of special steel, take alloy raw materials and place the alloy raw materials (specifically including ferrosilicon, ferromanganese, metallic cobalt, nickel plate, ferromolybdenum, ferrotitanium, industrial pure iron, and aluminum wire) in a vacuum induction furnace. After the system is evacuated, heat and melt it. After the raw materials are completely melted, raise the temperature of the molten steel to 1555℃ and enter the refining period. Then add the synthetic refining slag (the composition mass ratio of the synthetic refining slag is: 51% CaO, 19% SiO2, 20% Al2O3, 6% MgO, 4% CaF2) to the molten steel. The amount of slag accounts for 1.5% of the total weight of the molten steel. During the refining period, use electromagnetic stirring to promote the full reaction of slag and metal. The vacuum degree is controlled at 0.1Pa and the refining time is 15min. After the refining is completed, pour at 1535℃. After cooling and demolding, the initial refining ingot is obtained. (2) VAR remelting: After cleaning the surface of the primary smelting ingot, it is used as a consumable electrode and placed in the water-cooled copper crystallizer of the vacuum consumable furnace for remelting. The remelting parameters are controlled as follows: steady-state current 5.2 KA, steady-state voltage 29 V, steady-state melting rate 2.6 Kg / min, vacuum degree 1.0 Pa, cooling water flow rate 32 t / h. After remelting, the ingot is fed for 10 min, cooled and demolded to obtain ultra-pure steel ingot.

[0040] The chemical composition (mass percentage) of the ultra-low temperature high strength and toughness special steel obtained in this embodiment is as follows: C: 0.005%, Si: 0.04%, Mn: 0.03%, S: 0.004%, P: 0.005%, Ni: 18.2%, Co: 8.20%, Mo: 3.60%, Al: 0.12%, Ti: 0.24%, H: 0.7ppm, O: 15ppm, N: 11ppm, with the balance being Fe and other unavoidable impurities.

[0041] Comparative Example 1: The existing single vacuum induction melting process is adopted. That is, only step (1) in Example 1 is performed to obtain the initial ingot, and the vacuum consumable remelting in step (2) is not performed.

[0042] The preparation steps are the same as step (1) in Example 1, and the resulting primary smelting ingot is the sample obtained in this comparative example.

[0043] Performance comparison test and results analysis The ultrapure steel ingot obtained in Example 1 and the primary refined ingot obtained in Comparative Example 1 were forged into rectangular steel bars of 15mm×15mm×500mm at a temperature range of 1100℃. After forging, the steel bars were air-cooled to room temperature. Samples were then taken from the same location to test their chemical composition, observe and statistically analyze the original austenite grain size, investigate the evolution of inclusions, rate non-metallic inclusions, statistically analyze micro-sizes, and measure room temperature mechanical properties.

[0044] The specific test results and metallurgical analysis are as follows: Table 1. Comparison of chemical composition between the examples and the comparative examples (mass percentage, gas is ppm). As shown in Table 1, after VAR remelting in Example 1, harmful interstitial elements in the matrix were thoroughly purified. In particular, for hydrogen (H) and oxygen (O), which are highly prone to inducing low-temperature brittle fracture, the hydrogen content in Example 1 was reduced from 1.3 ppm to 0.9 ppm (eliminating the risk of hydrogen-induced delayed cracking), and the oxygen content was significantly reduced from 23 ppm to 16 ppm. In addition, the carbon (C) content was further reduced from 0.014% to 0.004%, which greatly suppressed the formation of brittle and hard inclusions such as titanium carbide (TiC) and alumina (Al2O3) from a thermodynamic source, providing a pure matrix environment for the material to maintain high fracture toughness at ultra-low temperatures.

[0045] Figure 1 The ultrapure steel ingot is prepared by dual smelting of vacuum induction melting (VIM) and vacuum arc remelting (VAR) in Embodiment 1 of the present invention; Figure 2 This is a comparison diagram of the original austenite grain boundaries of samples obtained by forging and sampling the initial refined ingot obtained in Comparative Example 1 and the ultra-pure steel ingot obtained in Example 1 of this invention, respectively. Figure 2 Figure (a) in the middle is Comparative Example 1. Figure 2 Figure (b) shows Example 1; Table 2 shows the statistical results of the comparison between the original austenite grain size of the primary refined ingot obtained in Comparative Example 1 and the ultra-pure steel ingot obtained in Example 1 of this invention, which were forged and sampled respectively.

[0046] Table 2. Statistical Comparison of Original Austenite Grain Sizes Combination Figure 2 Microstructural observation and statistical results in Table 2 show that the sample prepared in Comparative Example 1 had a relatively limited cooling rate during initial casting, allowing sufficient time for grain growth, resulting in a larger average austenite grain size of approximately 42.37 μm. In contrast, Example 1 of this invention underwent vacuum arc remelting (VAR). The precise matching of a 5 kA steady-state current with a 2.4 kg / min steady-state melting rate ensured a smooth and moderately deep molten pool. Combined with the intense heat dissipation from the water-cooled copper crystallizer, this resulted in a significantly enhanced cooling intensity at the solidification front, forcefully breaking down the original cast grains. Statistical results show that the grain diameter of Example 1 was reduced to 26.38 μm.

[0047] Figure 3This is a comparison of the microstructure and energy dispersive spectroscopy (EDS) of inclusions in samples obtained by forging and sampling the primary refined ingot obtained in Comparative Example 1 and the ultrapure steel ingot obtained in Example 1 of this invention, respectively. Figure 3 As shown in Figures (a) and (b), the inclusions in Comparative Example 1 are mainly coarse titanium sulfide (Ti content approximately 57%, S content approximately 36%), which are highly susceptible to stress concentration at ultra-low temperatures. Figure 3 Figures (c) and (d) show the inclusions in Example 1 of this invention. A comparison reveals that after vacuum arc remelting (VAR), the phase composition of the inclusions underwent a significant evolution. The proportion of sulfur decreased substantially, accompanied by the participation of elements such as Al, Mg, Mn, and O, forming multiphase composite micro-inclusions. This result demonstrates that this invention effectively alters the phase composition of the inclusions through VAR steady-state remelting, transforming them from the original brittle titanium sulfide into a more stable multiphase composite oxide. The synergistic effect of component modification and quantity reduction enhances the material's ultra-low temperature fracture resistance.

[0048] Table 3 Comparison of average grade number of non-metallic inclusions Table 4. Quantitative Statistical Comparison of Geometric Dimensions and Quantity Distribution of Non-metallic Inclusions Table 3 compares the average grade of non-metallic inclusions in samples forged and sampled from the primary refined ingot obtained in Example 1 and the ultra-pure steel ingot obtained in Example 1 of this invention. Table 4 compares the statistical size of inclusions in the above samples. The inclusion grades in 00Cr12Ni10MoTi steel were assessed according to GB / T10561—2023, "Standard Rating Chart Microscopic Examination Method for Determination of Non-metallic Inclusion Content in Steel". Under high magnification metallography, non-metallic inclusions were classified into five categories: A (sulfides), B (alumina), C (silicates), D (spherical oxides), and DS (large spherical oxide particles). Each category of inclusions was further divided into two series based on its particle width: coarse (T) and fine (H).

[0049] The data in Tables 3 and 4 clearly demonstrate the remarkable effectiveness of this invention in controlling microscopic defects. In Comparative Example 1 (single VIM process), due to slight erosion of the crucible refractory material and slow solidification cooling rate, the inclusion density reached as high as 45 inclusions / mm². 2 Statistical analysis revealed that Comparative Example 1 contained a significant number of coarse particles larger than 4 μm (approximately 1.16%). Under ultra-low temperature conditions of -196℃, these coarse inclusions, due to their mismatch with the thermal expansion coefficient of the matrix, will become fatal sources of stress concentration and cleavage fracture.

[0050] In Example 1, using a VAR process with specific parameters, the high-temperature decomposition effect of the vacuum arc and the rapid directional solidification of the water-cooled crystallizer not only reduced the overall level of Class B (alumina) and Class D (spherical oxide) inclusions, but also reduced the total area of ​​inclusions by over 60% (down to 0.006%). Extremely fine, dispersed inclusions of 1-2 μm dominate the microstructure (>60%), while highly hazardous coarse particles of >4 μm are essentially eliminated, achieving perfect control over the morphology and size of inclusions.

[0051] Table 5 Comparison of Mechanical Properties Test Results The mechanical property data in Table 5 strongly confirm the triple synergistic mechanism of "ultra-purification-microstructure refinement-inclusion modification" described in this invention. Compared with Comparative Example 1 (single VIM process), although Example 1 showed a slight decrease in tensile strength and yield strength, its yield strength remained stable at a high level of 937.00 MPa, fully meeting the high-strength load-bearing requirements of aerospace design. This slight adjustment in strength reflects the physical essence of the extremely purified matrix: on the one hand, the significant reduction in interstitial atoms and impurity elements such as H, O, and N weakens the interstitial solid solution strengthening and the pinning effect on dislocations; on the other hand, although purification leads to a slight decrease in hardness, the original austenite grain refinement achieved by the present invention through VAR directional solidification (the average diameter is significantly reduced from 42.37 μm to 26.38 μm) produces a significant grain refinement strengthening effect, effectively compensating for the strength loss caused by matrix purification.

[0052] More importantly, the material's ductility and toughness have achieved a leap forward: elongation has increased dramatically from 12.40% to 21.06%, reduction of area has increased to 71.84%, and impact energy has reached a staggering 143.63 J. This performance leap stems from the deep optimization of its microstructure: (1) Reduction in the number and size of inclusions: The number density of inclusions was reduced from 45 inclusions / mm 2 Reduced to 8.5 per mm 2 Furthermore, it effectively eliminates the brittle source of large particles >4μm, greatly reducing the probability of crack nucleation at ultra-low temperatures; (2) Composite modification of inclusions: Through VAR steady-state remelting, the original coarse Ti-S brittle inclusions are transformed into finely dispersed Al-Mn-Ti-SO multiphase composite oxides with rounded edges, which significantly alleviates the stress concentration between inclusions and matrix and enhances the continuity of matrix. (3) Refinement of matrix structure: The refined original austenite grains (refinement rate of about 38%) significantly increased the grain boundary area at -196℃, effectively blunting the crack tip and forcing the crack propagation path to deflect, consuming more fracture energy.

[0053] In summary, through a robust and perfectly synergistic microstructure design, this invention demonstrates that the ultrapure smelting preparation method has extremely high safety application value and engineering promotion potential in major national projects such as deep space probe support components and polar cryogenic pressure vessels.

[0054] The above description is merely an embodiment of the present invention and is not intended to limit the present invention in any way. The present invention can also have other embodiments based on the above structure and function, which will not be listed hereafter. Therefore, any simple modifications, equivalent changes, and alterations made by those skilled in the art to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for preparing ultra-pure smelting of ultra-low temperature high-strength and high-toughness special steel, characterized in that, Specifically, the following steps are included: (1) Vacuum induction melting: The raw materials according to the target composition ratio are put into the vacuum induction furnace, vacuum is drawn and heated to melt; after the raw materials are completely melted, the preheated and baked synthetic refining slag is added to the molten steel. Then, the temperature of the molten steel is raised and the refining period is entered. After the refining is completed, it is poured at a certain temperature and the initial refining ingot is obtained after demolding. (2) Vacuum self-consumable melting: The primary smelting ingot is placed in the crystallizer of the vacuum self-consumable furnace as a self-consumable electrode for remelting. After remelting, the ingot is fed back, cooled and demolded to finally obtain an ultra-pure steel ingot. The ultra-pure steel ingot is an ultra-low temperature high strength and toughness special steel.

2. The method for preparing ultra-pure smelting of ultra-low temperature high-strength and high-toughness special steel as described in claim 1, characterized in that, The components of the synthetic refining slag in step (1) are, by mass percentage: 50-55% CaO, 15-20% SiO2, 15-20% Al2O3, 4-6% MgO and 4-6% CaF2, and the amount of synthetic refining slag added is 0.8%-1.5% of the mass of the molten steel.

3. The method for preparing ultra-pure smelting of ultra-low temperature high-strength and high-toughness special steel as described in claim 1, characterized in that, In step (1), the step "After the raw materials are fully melted, add the preheated and baked synthetic refining slag to the molten steel, and then raise the temperature of the molten steel and enter the refining period" is replaced with "After the raw materials are fully melted, raise the temperature of the molten steel and enter the refining period, and then add the preheated and baked synthetic refining slag to the molten steel".

4. The method for preparing ultra-pure smelting of ultra-low temperature high-strength and high-toughness special steel as described in claim 1, characterized in that, In step (1), the refining temperature is 1540-1560℃; the refining time is 15-30 min; and the casting is carried out at 1520-1540℃.

5. The method for preparing ultra-pure smelting of ultra-low temperature high-strength and high-toughness special steel as described in claim 1, characterized in that, In step (1), the vacuum level inside the furnace is controlled to be ≤1.0Pa during the refining period; electromagnetic stirring is used during the refining period to promote full reaction at the slag-gold interface.

6. The method for preparing ultra-pure smelting of ultra-low temperature high-strength and high-toughness special steel as described in claim 1, characterized in that, In step (2), the surface of the primary smelting ingot is cleaned and then used as a consumable electrode.

7. The method for preparing ultra-pure smelting of ultra-low temperature high-strength and high-toughness special steel as described in claim 6, characterized in that, The surface cleaning specifically involves grinding to remove rust.

8. The method for preparing ultra-pure smelting of ultra-low temperature high-strength and high-toughness special steel as described in claim 1, characterized in that, During remelting in step (2), the steady-state current is controlled at 4.8-5.2KA, the steady-state voltage at 27-29V, the steady-state melting rate at 2.2-2.6Kg / min, and the vacuum degree at ≤1.6Pa.

9. The method for preparing ultra-pure smelting of ultra-low temperature high-strength and high-toughness special steel as described in claim 1, characterized in that, In step (2), the feeding time is 10-20 min; during remelting, the cooling water flow rate is controlled at 28-32 t / h.

10. The method for preparing ultra-pure smelting of ultra-low temperature high-strength and high-toughness special steel as described in claim 1, characterized in that, The obtained ultra-low temperature high strength and toughness special steel has the following chemical composition by mass percentage: C≤0.01%, Si≤0.05%, Mn≤0.05%, S≤0.005%, P≤0.005%, Ni: 17.5-18.5%, Co: 8.0-9.0%, Mo: 3.0-3.8%, Al: 0.05-0.15%, Ti: 0.20-0.35%, H≤1.0ppm, O≤16ppm, N≤15ppm, with the balance being Fe and other unavoidable impurities.

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