Method for producing ultra-high purity stainless steel

Abstract

The application belongs to the field of metallurgy and provides a method for preparing ultra-high purity stainless steel. A three-in-one smelting process is provided, which comprises, after vacuum carbon deoxidation pretreatment, composite treatment deep deoxidation and deep desulfurization, and special slag material reduction of aluminum and oxygen + deep degassing and removal of inclusions. In the process of electroslag remelting, the slag material comprises 40-50% CaF2, 33-41% CaO, 3-7% MgO, 13-18% SiO2, and the balance is impurities, and the content of impurities is ≤1%. The method realizes efficient removal of aluminum, deep deoxidation, deep desulfurization, deep dehydrogenation and inclusions of ultra-high purity stainless steel, thereby meeting the urgent needs of high-end equipment for ultra-high purity stainless steel materials.

Description

Technical Field

[0001] This application relates to the field of metallurgical technology, and in particular to a method for preparing ultra-high purity stainless steel. Background Technology

[0002] With the rapid development of high-end equipment manufacturing industries such as aerospace, high-precision machinery, and nuclear industry, unprecedentedly stringent requirements have been placed on the performance of key structural materials. Ultra-high purity stainless steel, as a core basic material for high-end equipment, directly determines the equipment's service reliability, service life, and ultimate performance. For example, in key components such as aero-engine turbine disks, nuclear reactor pressure vessels, and ultra-high precision machine tool spindles, impurities such as oxygen (O), sulfur (S), and hydrogen (H), as well as non-metallic inclusions, in the steel can significantly reduce the material's ductility, toughness, corrosion resistance, and fatigue life. Excessive aluminum (Al) can form brittle alumina inclusions, leading to fatal failure risks such as cracks and fractures during service. Therefore, achieving controlled aluminum, deep removal of O, deep removal of S, deep removal of H, and efficient removal of inclusions in ultra-high purity stainless steel has become an urgent need for the development of high-end equipment manufacturing and a technical challenge that urgently needs to be overcome in the metallurgical field.

[0003] Currently, the processes for producing ultra-high purity stainless steel both domestically and internationally mainly rely on single processes or combinations thereof, such as vacuum induction melting (VIM), electroslag remelting under protective atmosphere (IESR), and vacuum arc remelting (VAR). However, existing processes still face many technical bottlenecks in actual production. In the vacuum induction melting stage, traditional processes largely rely on carbon deoxidation or aluminum deoxidation. For ultra-low carbon stainless steel, carbon deoxidation is difficult to achieve deep deoxidation; while aluminum deoxidation can reduce oxygen content, it leads to an increase in residual aluminum in the steel, forming a large number of Al2O3 inclusions, and cannot effectively solve the problem of excessively high sulfur content. Using single deoxidizers with stronger deoxidation capabilities (Mg, Ca, RE, etc.) also has limited effects and is difficult to simultaneously achieve deep deoxidation of oxygen and sulfur while effectively controlling aluminum content. In the electroslag remelting stage, existing processes generally use conventional slag systems, which can effectively remove inclusions, but have weak control over the oxygen and aluminum content in the steel. In the vacuum arc remelting stage, although traditional processes can achieve a certain degree of degassing and inclusion removal, the poor control of cleanliness in the preceding VIM and IESR processes means that the ingots cast after vacuum arc remelting cannot meet the ultimate cleanliness requirements of ultra-high purity stainless steel.

[0004] It should be noted that the above content is not necessarily prior art, nor is it intended to limit the scope of protection of this application. Summary of the Invention

[0005] This application provides a method for preparing ultra-high purity stainless steel to solve or alleviate one or more of the technical problems mentioned above.

[0006] This application provides a method for preparing ultra-high purity stainless steel. The method includes the following steps: An intermediate alloy is provided, by mass percentage, comprising Al ≤ 0.006%, O ≤ 0.008%, S ≤ 0.003%, and P ≤ 0.01%; According to the target composition of the stainless steel, the intermediate alloy is batched, and the batched product is subjected to vacuum induction melting to form the first molten steel after melting. Carbon is added to the first molten steel for vacuum carbon deoxidation pretreatment to obtain a second molten steel with an oxygen content of less than or equal to 0.005 wt%. Argon gas is introduced into the second molten steel to maintain the second molten steel at the first temperature, and magnesium alloy is added to perform deoxidation and desulfurization to obtain the third molten steel. Rare earth elements are added to the third molten steel to perform deep deoxidation and desulfurization, resulting in a fourth molten steel. The temperature of the fourth molten steel is controlled at 1520℃~1550℃, and after a 2min~10min quenching treatment, it is cast to obtain an ingot. The ingot is forged into a first consumable electrode, and the cross-sectional dimensions of the first consumable electrode are determined with a filler ratio of 0.3 to 0.6. In an argon atmosphere, the first consumable electrode is subjected to electroslag remelting. The slag material in the electroslag remelting process, by mass percentage, comprises: 40%–50% CaF2, 33%–41% CaO, 3%–7% MgO, 13%–18% SiO2, with the balance being impurities, and the impurity content being ≤1%. During the capping stage of the electroslag remelting, multi-stage current reduction and feeding are performed to obtain the electroslag remelting ingot. The electroslag remelting ingot is forged into a second consumable electrode, and the cross-sectional dimensions of the second consumable electrode are determined according to a filling ratio of 0.3 to 0.6. Vacuum degree ≤10 -3 Under Pa conditions, the second consumable electrode was subjected to vacuum arc remelting. Stainless steel is obtained by performing multi-stage current reduction and shrinkage during the vacuum arc remelting capping period.

[0007] The method described in this application provides a three-stage smelting process for preparing stainless steel, involving a vacuum induction furnace, protective atmosphere electroslag remelting, and vacuum arc remelting. The resulting stainless steel exhibits extremely high purity: Al ≤ 0.003%, O ≤ 0.0005%, S ≤ 0.0005%, N ≤ 0.002%, and H ≤ 0.0001%. Inclusions are efficiently modified and rendered harmless, with coarse inclusions at level 0 and fine inclusions (A+B+C+D types) ≤ 0.5. Simultaneously, ultra-low content control of impurity elements such as Al, O, S, N, and H in the steel is achieved.

[0008] Specifically, the vacuum induction melting stage adopts a process of "vacuum carbon deoxidation first, then magnesium deoxidation and desulfurization, and finally rare earth for deep deoxidation and desulfurization." This process is based on a scientific matching of the thermodynamic properties, metallurgical effects, and cost-effectiveness of each deoxidizing and desulfurizing agent. First, vacuum carbon deoxidation is used to achieve efficient pre-removal of initial oxygen in the molten steel at low cost. The generated CO gas does not pollute the molten steel and can improve the kinetic conditions of steel deoxidation. At the same time, it avoids the large-scale burn-off of magnesium and rare earth due to excessive initial oxygen content in the molten steel. After vacuum carbon deoxidation pretreatment, magnesium is used for deoxidation and desulfurization, which can further reduce the oxygen content, initially remove sulfur, and improve the activity of rare earth. Finally, rare earth is added to carry out deep deoxidation and desulfurization. It uses its strong affinity with oxygen and sulfur to remove residual trace oxygen and sulfur, and can also modify inclusions in the steel into spherical, high-melting-point stable forms, ultimately achieving deep purification of the molten steel.

[0009] Protective atmosphere electroslag remelting is highly effective in desulfurization, inclusion removal, and improvement of solidification structure, but it often causes problems such as increased oxygen (O) and alkanes (Al). The application of a CaF2-CaO-SiO2-MgO quaternary slag system in electroslag remelting effectively solved these problems, achieving simultaneous Al removal, O removal, and inclusion removal, ultimately resulting in further purification of the molten steel.

[0010] Vacuum arc remelting can deeply remove gaseous impurities such as O, N, and H from molten steel and remove inclusions. High-purity consumable electrode ingots obtained through vacuum induction melting and protective atmosphere electroslag remelting are subjected to vacuum arc remelting for deep purification, ultimately yielding ultra-high purity stainless steel ingots.

[0011] The triple process of VIM (vacuum carbon deoxidation pretreatment + magnesium + rare earth composite treatment for deep O and deep S removal) + IESR (special slag system for Al and O reduction) + VAR (deep degassing and inclusion removal) enables the control of aluminum in ultra-high purity stainless steel and deep O, deep S, deep H removal and efficient removal of inclusions, thereby meeting the urgent needs of high-end equipment for ultra-high purity stainless steel materials. Detailed Implementation

[0012] The embodiments of this application are described in detail below. It should be noted that the terms "first," "second," etc., used in this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0013] In this application, when numerical intervals (i.e., numerical ranges) are involved, unless otherwise specified, the distribution of selectable numerical values ​​within the numerical interval is considered continuous, and includes the two endpoints of the numerical interval (i.e., the minimum and maximum values), as well as every numerical value between these two endpoints. Unless otherwise specified, when a numerical interval refers only to integers within that numerical interval, it includes the two endpoint integers of the numerical range, as well as every integer between the two endpoints, which is equivalent to directly listing every integer. When multiple numerical ranges are provided to describe features or characteristics, these numerical ranges can be merged. In other words, unless otherwise specified, the numerical ranges disclosed in this application should be understood to include any and all subranges included therein. The "numerical value" in the numerical interval can be any quantitative value, such as a number, percentage, ratio, etc. The term "numerical interval" can be broadly included to include percentage intervals, ratio intervals, proportion intervals, etc.

[0014] This application provides a method for preparing ultra-high purity stainless steel. The method includes the following steps: (1) Provide an intermediate alloy, in mass percentage, wherein the intermediate alloy comprises Al≤0.006%, O≤0.008%, S≤0.003%, P≤0.01%; (2) According to the target composition of the stainless steel, the intermediate alloy is batched and the batched product is subjected to vacuum induction melting to form the first molten steel after melting. (3) Add carbon to the first molten steel for vacuum carbon deoxidation pretreatment to obtain a second molten steel with an oxygen content of less than or equal to 0.005 wt%; (4) Argon gas is introduced into the second molten steel to maintain the second molten steel at the first temperature, and magnesium alloy is added to carry out deoxidation and desulfurization to obtain the third molten steel; (5) Rare earth elements are added to the third molten steel to perform deep deoxidation and desulfurization, thereby obtaining the fourth molten steel; (6) The temperature of the fourth molten steel is controlled at 1520℃~1550℃, and after being calmed for 2min~10min, it is cast to obtain an ingot; (7) The ingot is forged into a first consumable electrode, and the cross-sectional size of the first consumable electrode is determined with a filling ratio of 0.3 to 0.6; (8) In an argon atmosphere, the first consumable electrode is subjected to electroslag remelting. The slag material in the electroslag remelting process, by mass percentage, includes: 40% to 50% CaF2, 33% to 41% CaO, 3% to 7% MgO, 13% to 18% SiO2, with the balance being impurities, and the impurity content being ≤1%. (9) Multi-stage current reduction feeding is performed during the capping period of the electroslag remelting to obtain the electroslag remelting ingot; (10) The electroslag remelting ingot is forged into a second consumable electrode, and the cross-sectional dimensions of the second consumable electrode are determined according to a filling ratio of 0.3 to 0.6; (11) When the vacuum degree is ≤10 -3 Under Pa conditions, the second consumable electrode was subjected to vacuum arc remelting. (12) Multi-stage current reduction and shrinkage are performed during the vacuum arc remelting capping period to obtain stainless steel.

[0015] The method described in this application provides a three-stage smelting process for preparing stainless steel, involving a vacuum induction furnace, protective atmosphere electroslag remelting, and vacuum arc remelting. The resulting stainless steel exhibits extremely high purity: Al ≤ 0.003%, O ≤ 0.0005%, S ≤ 0.0005%, N ≤ 0.002%, and H ≤ 0.0001%. Inclusions are efficiently modified and rendered harmless, with coarse inclusions at level 0 and fine inclusions (A+B+C+D types) ≤ 0.5. Simultaneously, ultra-low content control of impurity elements such as Al, O, S, N, and H in the steel is achieved.

[0016] Specifically, the vacuum induction melting stage adopts a process of "vacuum carbon deoxidation first, then magnesium deoxidation and desulfurization, and finally rare earth for deep deoxidation and desulfurization." This process is based on a scientific matching of the thermodynamic properties, metallurgical effects, and cost-effectiveness of each deoxidizing and desulfurizing agent. First, vacuum carbon deoxidation is used to achieve efficient pre-removal of initial oxygen in the molten steel at low cost. The generated CO gas does not pollute the molten steel and can improve the kinetic conditions of steel deoxidation. At the same time, it avoids the large-scale burn-off of magnesium and rare earth due to excessive initial oxygen content in the molten steel. After vacuum carbon deoxidation pretreatment, magnesium is used for deoxidation and desulfurization, which can further reduce the oxygen content, initially remove sulfur, and improve the activity of rare earth. Finally, rare earth is added to carry out deep deoxidation and desulfurization. It uses its strong affinity with oxygen and sulfur to remove residual trace oxygen and sulfur, and can also modify inclusions in the steel into spherical, high-melting-point stable forms, ultimately achieving deep purification of the molten steel.

[0017] Protective atmosphere electroslag remelting is highly effective in desulfurization, inclusion removal, and improvement of solidification structure, but it often causes problems such as increased oxygen (O) and alkanes (Al). The application of a CaF2-CaO-SiO2-MgO quaternary slag system in electroslag remelting effectively solved these problems, achieving simultaneous Al removal, O removal, and inclusion removal, ultimately resulting in further purification of the molten steel.

[0018] Vacuum arc remelting can deeply remove gaseous impurities such as O, N, and H from molten steel and remove inclusions. High-purity consumable electrode ingots obtained through vacuum induction melting and protective atmosphere electroslag remelting are subjected to vacuum arc remelting for deep purification, ultimately yielding ultra-high purity stainless steel ingots.

[0019] The triple process of VIM (vacuum carbon deoxidation pretreatment + magnesium + rare earth composite treatment for deep O and deep S removal) + IESR (special slag system for Al and O reduction) + VAR (deep degassing and inclusion removal) enables the control of aluminum in ultra-high purity stainless steel and deep O, deep S, deep H removal and efficient removal of inclusions, thereby meeting the urgent needs of high-end equipment for ultra-high purity stainless steel materials.

[0020] In step (1), an intermediate alloy is provided, which has a composition similar to that of stainless steel, and includes Al≤0.006%, O≤0.008%, S≤0.003%, and P≤0.01%. Due to its matrix composition similar to that of stainless steel and extremely low levels of Al, O, S, and P impurities, the intermediate alloy can serve as a high-quality base material or additive for the smelting of ultra-pure stainless steel, avoiding the introduction of impurities from the source, simplifying the vacuum arc remelting process, and ensuring that the final product reaches ultra-pure grade.

[0021] In step (2), the intermediate alloy is batched according to the target composition of the stainless steel, and the batched product is subjected to vacuum induction melting to form the first molten steel. The intermediate alloy has low impurities. Batching the intermediate alloy can make up for the missing elements in the intermediate alloy, thereby promoting the formation of stainless steel with a composition similar to the target composition. The first molten steel is obtained during the vacuum induction melting process, and the first molten steel can also be directly cast into the first consumable electrode in subsequent operations.

[0022] Furthermore, the crucible used in vacuum induction melting is made of Al2O3, MgO, CaO, ZrO2, or a mixture of MgO and Al2O3. In step (3), carbon is added to the first molten steel for vacuum carbon deoxidation pretreatment to obtain a second molten steel with an oxygen content of less than or equal to 0.005 wt%. Under vacuum conditions, the deoxidation capacity of carbon is significantly enhanced, thereby greatly reducing the oxygen content in the first molten steel and providing a low-oxygen environment for subsequent processing.

[0023] In some embodiments, the vacuum degree of the vacuum carbon deoxidation pretreatment is less than or equal to 20 Pa, such as 1 Pa, 3 Pa, 5 Pa, 10 Pa, 13 Pa, 16 Pa, 18 Pa, 20 Pa, etc. A low vacuum degree can facilitate the vacuum carbon deoxidation pretreatment.

[0024] In some embodiments, the amount of carbon added during the vacuum carbon deoxidation pretreatment is determined according to Formula 1. Formula 1, where w[%C] is the amount of carbon added, w[%C]0 is the target C content of the stainless steel, w[%O] is the oxygen content in the first molten steel, and w[%C]... imp The carbon content of impurities introduced into the product obtained from the ingredients is given. Formula 1 is based on the theoretical stoichiometric ratio (0.75) of the carbon deoxidation reaction (C+O→CO) under vacuum and the carbon utilization rate in actual production. It is calculated using an empirical coefficient of 2.7 and takes into account the target carbon content, the amount of oxygen to be removed (to 0.005%), and the carbon impurities introduced into the ingredients.

[0025] In some embodiments, the preprocessing time is determined according to Equation 2. Equation 2, where t is the pretreatment time, p is the vacuum degree during the vacuum carbon deoxidation pretreatment process, S is the free surface area of ​​the molten steel, and V is the volume of the second molten steel. [%O] represents the change in oxygen content in the first and second molten steel. Therefore, under the aforementioned pretreatment time control, the oxygen content can be minimized. Mass transfer and bubble kinetics in vacuum carbon deoxidation: Under vacuum conditions, carbon reacts with oxygen to generate CO bubbles. The deoxidation rate is controlled by the nucleation, growth, and escape processes of CO bubbles. Specifically, the deoxidation time t is directly proportional to the vacuum degree p (the lower the pressure, the greater the driving force for CO generation, and the less vacuum carbon deoxidation time is required), directly proportional to the change in oxygen content Δ[%O] (reflecting the influence of the initial oxygen concentration difference on the reaction rate), and directly proportional to the ratio of the free surface area to the volume of the molten steel, S / V. An empirical coefficient of 0.76 × 10⁻⁶ is obtained from experimental data. 3 This is to guide production to minimize oxygen content.

[0026] In step (4), argon gas is introduced into the second molten steel to maintain it at the first temperature, and magnesium alloy is added to perform deoxidation and desulfurization, resulting in the third molten steel. In this step, magnesium alloy is added simultaneously with argon gas to deoxidize and desulfurize the second molten steel.

[0027] Furthermore, the deoxidation and desulfurization time is 2 min to 3 min, for example, 2 min, 2.5 min, 3 min, etc.

[0028] Furthermore, the amount of magnesium alloy added is determined according to Formula 3. Equation 3, where w[%Mg] is the magnesium content in the magnesium alloy, w[%Mg]0 is the target magnesium content in the stainless steel, p is the pressure in the vacuum induction furnace when the Mg alloy is added, and T is the temperature of the second molten steel. The volatilization loss of magnesium under vacuum conditions: the vapor pressure of magnesium increases with increasing temperature and decreasing pressure, leading to a decrease in magnesium yield. The exponential term in the formula is derived from the gas-liquid equilibrium relationship of magnesium (such as the Clapeyron equation or vapor pressure formula) and experimental data fitting, where 1.38 × 10⁻⁶... 6 and 7.1×10 3 This is an empirical coefficient used to correct for the volatilization loss of magnesium under different vacuum levels p and temperatures T; w[%Mg]0+0.0025% in parentheses represents the sum of the magnesium content in the magnesium alloy and the initial residual magnesium content in the molten steel (approximately 0.0025%), thereby calculating the amount of magnesium alloy required to achieve the target magnesium content w[%Mg].

[0029] Furthermore, the magnesium alloy includes Ni-Mg alloy or Fe-Mg alloy, and the Mg alloy contains 10% to 30% Mg by mass.

[0030] In step (5), rare earth elements are added to the third molten steel to perform deep deoxidation and desulfurization, resulting in a fourth molten steel. In this step, rare earth elements can be used for deep deoxidation and desulfurization of the third molten steel.

[0031] Furthermore, the deep deoxidation and desulfurization time is 2 to 3 minutes. For example, 2 minutes, 2.5 minutes, 3 minutes, etc.

[0032] Furthermore, the amount of rare earth element added is determined according to Equation 4. Formula 4, where w[%RE] is the amount of RE added to the third molten steel, and w[%RE]0 is the target RE content of stainless steel.

[0033] Equation 4 is based on the fact that the deoxidation and desulfurization efficiency of rare earth elements (such as Ce and La) in the third molten steel varies non-linearly with the target content: the lower the target rare earth content, the lower the rare earth recovery rate, and the larger the required addition factor. The denominator in the formula reflects this empirical rule—the cube root term in the denominator comes from the regression fitting of actual production data and is used to correct for the differences in rare earth burn-off, volatilization, and reaction with refractory materials under different target contents; the part in parentheses in the numerator represents the target content plus the initial residual rare earth content in the molten steel or the empirical benchmark value. Overall, this formula is used to guide the precise addition of rare earths during the deep deoxidation and desulfurization process.

[0034] Furthermore, the rare earth element is a pure metal or a rare earth alloy, preferably La, Ce, Y, or a mixture of rare earth elements. When using it, the surface oxide scale needs to be removed to ensure that the amount of rare earth element added calculated using Formula 4 is accurate.

[0035] In step (6), the temperature of the fourth molten steel is controlled at 1520℃~1550℃. After a 2-10 minute settling treatment, it is cast to obtain an ingot. Therefore, a temperature between 1520℃ and 1550℃ provides good fluidity for the fourth molten steel while avoiding problems such as oxidation and coarse grains caused by high temperatures. Settling removes inclusions and homogenizes the mixture, resulting in an ingot with good cleanliness.

[0036] In step (7), the ingot is forged into a first consumable electrode, and the cross-sectional size of the first consumable electrode is determined with a filler ratio of 0.3 to 0.6. This allows for the production of a first consumable electrode with precise dimensions and a fine microstructure.

[0037] In step (8), the first consumable electrode is electroslag remelted in an argon atmosphere. The slag used in the electroslag remelting process comprises, by mass percentage: 40%–50% CaF2, 33%–41% CaO, 3%–7% MgO, 13%–18% SiO2, with the balance being impurities, and the impurity content being ≤1%. Using this slag for electroslag remelting can further improve the cleanliness of the ingot obtained from electroslag remelting.

[0038] Furthermore, the melting rate V1 of the electroslag remelting process is determined according to Equation 5 or Equation 6. When the stainless steel contains Fe ≥ 60wt%, T L -T S At ≤60℃, the melting rate V1 is determined by Equation 5. Equation 5.

[0039] When Fe < 60% in the stainless steel, T L -T S When the temperature is >60℃, the melting rate V1 is determined by Equation 6. Formula 6.

[0040] In equations 5 and 6, T L T is the liquidus temperature of stainless steel. SV is the solidus temperature of the stainless steel, V1 is the melting rate of the first consumable electrode, and D is the size of the protective atmosphere electroslag remelting crystallizer. The relationship between the melting rate, the solidification characteristics of the steel, and the crystallizer size during electroslag remelting is crucial to ensure a smooth surface, dense internal structure, and the absence of segregation and cracks in the ingot. Specifically: when the iron content in the stainless steel is high (≥60%) and the solidification temperature range is narrow (≤60°C), the molten steel has good fluidity and a low tendency for hot cracking, allowing for a lower melting rate range (coefficient 0.49~0.58), which is beneficial for obtaining a uniform microstructure. When the iron content is low (<60%) and the solidification temperature range is wide (>60°C), the steel has a high degree of alloying, a wide mushy region, and strong hot cracking sensitivity, requiring a higher melting rate (coefficient 0.59~0.72) to form a shallow, flat molten pool and reduce the risk of segregation and hot cracking. The coefficients (0.49, 0.58, 0.59, 0.72) and the denominator T in the two equations L -T S The data are all derived from industrial test data of electroslag remelting of different steel grades, reflecting the empirical law that the melting rate V1 is directly proportional to the crystallizer diameter D and inversely proportional to the square root of the solidification temperature range.

[0041] In some embodiments, the slag material comprises CaF2: 40-50%, CaO: 33-41%, MgO: 4%-6%, SiO2: 12%-15%, with the balance being impurities, the impurity content being ≤1%, and the sum of the mass percentages of all components being 100%.

[0042] In some embodiments, in step (8), the process parameters such as current, voltage, slag resistance and melting rate are kept stable during the protective atmosphere electroslag remelting process, and the water temperature of the crystallizer inlet is kept at 25℃~35℃ and the water temperature of the outlet does not exceed 55℃.

[0043] In step (9), multi-stage current reduction feeding is performed during the electroslag remelting capping period to obtain the electroslag remelted ingot. Multi-stage current reduction feeding is the final key process to achieve a high-density, defect-free electroslag ingot. In the multi-stage current reduction feeding process, the current is gradually reduced in stages and steps to achieve precise control of the molten pool temperature. This ensures that liquid metal is continuously added and that the added metal can solidify smoothly, avoiding defects such as looseness and slag grooves on the top of the final product.

[0044] In step (10), the electroslag remelting ingot is forged into a second consumable electrode, and the cross-sectional size of the second consumable electrode is determined according to the filling ratio of 0.3 to 0.6.

[0045] In step (11), when the vacuum degree is ≤10 -3 Under Pa conditions, the second consumable electrode is subjected to vacuum arc remelting.

[0046] Furthermore, the melting rate V2 of the vacuum arc remelting process is determined according to Equations 7 and 8. When the solidification range of the smelting alloy (Fe≥60%) is narrow, T L -T S At ≤60℃, the melting rate is determined by Equation 7: Formula 7.

[0047] When the solidification range of the smelting alloy (Fe < 60%) is relatively wide, T L -T S When the temperature is >60℃, the melting rate is determined by Equation 8: Formula 8.

[0048] In equations 7 and 8, T L T is the liquidus temperature of stainless steel. S V1 is the solidus temperature of stainless steel, V2 is the melting rate of the second consumable electrode, and D is the size of the protective atmosphere electroslag remelting crystallizer. Equations 7 and 8 are based on the matching law between the melting rate, the solidification characteristics of the steel grade, and the crystallizer size during vacuum arc remelting (VAR): the melting rate V2 is directly proportional to the crystallizer diameter D and related to the solidification temperature range T. L T S The ratios are inversely proportional. For high-speed rail (Fe≥60%) stainless steel, when the solidification range is narrow (≤60℃), a lower melting rate (coefficient 0.41~0.49) is used to promote dense microstructure and reduce segregation; when the solidification range is wide (>60℃), the melting rate needs to be appropriately increased (coefficient 0.48~0.63) to control the shape of the molten pool and prevent hot cracking. The coefficient ranges in both formulas are derived from regression analysis of industrial test data for VAR processes of different steel grades.

[0049] Furthermore, in step (11), during the vacuum arc remelting process, the process parameters such as vacuum degree, current, voltage and melting rate are kept stable, and the water temperature of the crystallizer inlet is kept at 25℃~35℃ and the water temperature of the outlet does not exceed 50℃.

[0050] In step (12), multi-stage current reduction and shrinkage are performed during the vacuum arc remelting capping period to obtain stainless steel.

[0051] In some embodiments, the impurity element content in the stainless steel prepared by the method of this application is: Al≤0.003%, O≤0.0005%, S≤0.0005%, N≤0.002%, H≤0.0001%. The inclusion rating in the prepared ultra-high purity stainless steel is: coarse inclusions are grade 0, and fine inclusions of type A+B+C+D are ≤0.5.

[0052] In some embodiments, a method for continuous production of stainless steel includes: S1: Provide an intermediate alloy, by mass percentage, wherein the intermediate alloy comprises Al≤0.006%, O≤0.008%, S≤0.003%, and P≤0.01%.

[0053] S2: According to the target composition of the stainless steel, the intermediate alloy is batched, and the batched product is subjected to vacuum induction melting to form the first molten steel.

[0054] S3: Add carbon to the first molten steel for vacuum carbon deoxidation pretreatment to obtain a second molten steel with an oxygen content of less than or equal to 0.005 wt%. The amount of carbon added is determined according to Formula 1, and the pretreatment time is determined according to Formula 2. Formula 1, In Formula 1, w[%C] represents the amount of carbon added, w[%C]0 represents the target C content of the stainless steel, w[%O] represents the oxygen content in the first molten steel, and w[%C]... imp The carbon content of impurities introduced into the product obtained from the ingredients. Equation 2, In Equation 2, t is the pretreatment time, p is the vacuum degree during the vacuum carbon deoxidation pretreatment process, S is the free surface area of ​​the molten steel, and V is the volume of the second molten steel. [%O] represents the change in oxygen content in the first and second molten steel.

[0055] S4: Argon gas is introduced into the second molten steel to maintain it at the first temperature. Magnesium alloy is added for deoxidation and desulfurization to obtain the third molten steel. The amount of magnesium alloy added is determined according to Formula 3. The deoxidation and desulfurization time is 2 min to 3 min. Formula 3, In Formula 3, w[%Mg] is the magnesium content in the magnesium alloy, w[%Mg]0 is the target magnesium content in the stainless steel, p is the pressure in the vacuum induction furnace when the Mg alloy is added, and T is the temperature of the second molten steel.

[0056] S5: Rare earth elements are added to the third molten steel to perform deep deoxidation and desulfurization, resulting in a fourth molten steel. The amount of rare earth elements added is determined according to Formula 4, and the deep deoxidation and desulfurization time is 2 min to 3 min. Equation 4, In Formula 4, w[%RE] represents the amount of RE added to the third molten steel, and w[%RE]0 represents the target RE content of the stainless steel.

[0057] S6: Control the temperature of the fourth molten steel at 1520℃~1550℃, and after a 2min~10min calming treatment, cast the steel to obtain an ingot.

[0058] S7: Forge the ingot into a first consumable electrode, and determine the cross-sectional size of the first consumable electrode with a filling ratio of 0.3 to 0.6.

[0059] S8: In an argon atmosphere, the first consumable electrode is subjected to electroslag remelting, wherein the melting rate V1 of the electroslag remelting process is determined according to Equation 5 or Equation 6. When the stainless steel contains Fe ≥ 60wt%, T L -T S At ≤60℃, the melting rate V1 is determined by Equation 5. Equation 5, When Fe < 60% in the stainless steel, T L -T S When the temperature is >60℃, the melting rate V1 is determined by Equation 6. Formula 6, In equations 5 and 6, T L T is the liquidus temperature of stainless steel. S V is the solidus temperature of stainless steel, V1 is the melting rate of the first consumable electrode, and D is the size of the protective atmosphere electroslag remelting crystallizer.

[0060] S9: The electroslag remelting air is periodically subjected to multi-stage current reduction and feeding to obtain electroslag remelting ingots.

[0061] S10: The electroslag remelting ingot is forged into a second consumable electrode, and the cross-sectional dimensions of the second consumable electrode are determined according to a filling ratio of 0.3 to 0.6.

[0062] S11: When the vacuum degree is ≤10 -3 Under Pa conditions, the second consumable electrode is subjected to vacuum arc remelting, and the melting rate V2 of the vacuum arc remelting process is determined according to Equations 7 and 8. Equation 7, When the solidification range of the smelting alloy (Fe≥60%) is relatively wide, T L -T S When the temperature is >60℃, the melting rate is determined by Equation 8: Formula 8, In equations 7 and 8, T L T is the liquidus temperature of stainless steel. S V is the solidus temperature of stainless steel, V2 is the melting rate of the second consumable electrode, and D is the size of the protective atmosphere electroslag remelting crystallizer.

[0063] S12: Stainless steel is obtained by multi-stage current reduction and shrinkage during the vacuum arc remelting capping period.

[0064] In some embodiments, the target composition of the stainless steel includes, by mass percentage, the following elements: C ≤ 0.02%; Cr 16.00%~18.00%; Ni 14.00%~16.00%; Mo 2.00%~3.00%; Si 0.30%~0.50%; Mn 0.30%~0.50%; P ≤ 0.005%; S ≤ 0.001%; ​​Al ≤ 0.003%; O ≤ 0.0008%; N ≤ 0.002%; H ≤ 0.0001%; ​​the remainder being Fe.

[0065] Exemplary embodiments according to this application will now be described in more detail. It should be understood that these exemplary embodiments may be implemented in many different forms and should not be construed as being limited to the embodiments set forth herein.

[0066] The target composition of the 316L austenitic stainless steel (hereinafter referred to as "stainless steel") in the following examples and comparative examples is as follows: Based on mass percentage, it includes the following elements: C 0.01%; Cr 17.00%; Ni 15.00%; Mo 3.00%; Si 0.40%; Mn 0.30%~0.50%; P≤0.005%; S≤0.001%; ​​Al≤0.003%; O≤0.0007%; N≤0.002%; H≤0.0001%; ​​the remainder is Fe.

[0067] Examples 1-4, Comparative Examples 1-2 A method for preparing ultra-high purity stainless steel using the VIM+IESR+VAR triple process was developed. The smelting equipment consisted of a 2t vacuum induction furnace, a 2t protective atmosphere electroslag remelting furnace, and a 2t vacuum arc remelting furnace. Four heats of experimental steel were designed, namely Examples 1 to 4. Simultaneously, ingots that underwent only vacuum induction melting were smelted as Comparative Example 1, and ingots that underwent vacuum induction melting followed by protective atmosphere electroslag remelting were smelted as Comparative Example 2.

[0068] The main impurity components of the intermediate alloy used are shown in Table 1.

[0069] Table 1. Impurity Composition of Intermediate Alloys in Smelting

[0070] The vacuum induction melting process of the intermediate alloys in Examples 1-4 and Comparative Examples 1-2 is as follows: (1) Loading: Weigh the required experimental raw materials and fill them into the vacuum induction melting furnace according to the principle of pressing down tightly and filling up loosely on top; (2) Smelting: After the furnace is closed, turn on the vacuum pump, evacuate to less than 5Pa and then start powering on, gradually increase the power until all the furnace charge is melted, and control the smelting temperature at 1520℃~1550℃. (3) Vacuum carbon deoxidation pretreatment: Add high-purity carbon blocks with a purity ≥ 99.99 wt% to the molten steel for deoxidation pretreatment, control the vacuum degree < 20 Pa, and the refining time is 12 min ~ 18 min; (4) Control the temperature at 1480℃~1500℃, add Ni-Mg alloy (Mg content is 20%) to the molten steel for deoxidation and desulfurization, and the treatment time is 2min~3min; (5) RE deep deoxidation and desulfurization treatment: After the Mg treatment is completed, rare earth RE is added to the molten steel for deep deoxidation and deep desulfurization. The treatment time is 2 min to 3 min. (6) Casting: Control the temperature of the molten steel at 1520℃~1550℃. After the molten steel is calmed, it is poured into the ingot mold. After the molten steel cools and solidifies, the ingot is removed.

[0071] The specific VIM smelting parameters for each embodiment are shown in Table 2.

[0072] Table 2 Smelting parameters of stainless steel in Examples 1-4

[0073] The C content, Ni-Mg alloy (Mg) content, and rare earth Ce content in each example are shown in Table 3.

[0074] Table 3. Amounts of each substance added during smelting

[0075] The protective atmosphere electroslag remelting (IESR) processes of Examples 1-4 and Comparative Example 2 are as follows: (1) The slag system is pre-melted, cooled, crushed and baked, and after arc ignition, the slag material is added to the crystallizer for slag formation.

[0076] (2) Forge the VIM ingot into electrodes and control the filling ratio to 0.3~0.8.

[0077] (3) Remelting is carried out in an electroslag furnace under a high-purity atmosphere, and high-purity argon gas is introduced into the IESR with a purity of ≥99.999%.

[0078] (4) Control the melting rate as follows: .

[0079] Wherein: T L T represents the liquidus temperature of the steel grade, in °C. S V1 is the solidus temperature of the steel grade, in °C; V1 is the melting rate of the vacuum arc remelting consumable electrode, in kg / min; D is the size of the vacuum arc remelting crystallizer, in mm.

[0080] The IESR slag systems used in each embodiment and comparative example are shown in Table 4.

[0081] Table 4. IESR slag composition used in Examples 1-4 and Comparative Example 2

[0082] The specific IESR smelting parameters for each embodiment are shown in Table 5.

[0083] Table 5. IESR smelting parameters for stainless steel

[0084] The vacuum arc remelting (VAR) process in Examples 1-4 is as follows: (1) Forge the IESR ingot into electrodes and control the filling ratio to 0.3~0.6.

[0085] (2) Arc remelting is performed under high vacuum conditions, where the vacuum degree is less than 10. -3 Pa (3) Control the melting rate as follows:

[0086] Among them, T L T represents the liquidus temperature of the steel grade, in °C. S V1 is the solidus temperature of the steel grade, in °C; V2 is the melting rate of the vacuum arc remelting consumable electrode, in kg / min; D is the size of the vacuum arc remelting crystallizer, in mm.

[0087] The specific IESR smelting parameters for each embodiment are shown in Table 6.

[0088] Table 6. IESR smelting parameters for stainless steel

[0089] Table 7 shows the impurity element content of the ultra-high purity stainless steel obtained by the above method. It can be seen that the VIM+IESR+VAR triple process significantly improves deoxidation and desulfurization, and effectively controls impurity elements.

[0090] Table 7 Chemical composition of stainless steel

[0091] The inclusion ratings of the 316L austenitic stainless steel prepared in the examples are shown in Table 8. It can be seen that all coarse inclusions in the examples are grade 0, and all inclusions in categories A, B, C, and D are ≤0.5. Table 8 Inclusion grades in stainless steel

[0092] In summary, the present invention employs a triple smelting process of VIM (vacuum carbon deoxidation pretreatment followed by Mg+Ce composite treatment for deep O and deep S removal) + IESR (special slag system for Al and O reduction) + VAR (deep degassing and inclusion removal) to achieve ultra-pure control and good microstructure and performance control of 316L stainless steel.

[0093] It should also be noted that the terms "some embodiments," "other embodiments," and "embodiments" used in this application refer to specific features, structures, or characteristics described in connection with those embodiments, which are included in at least one embodiment described in the general description of this application. The appearance of the same expression in multiple places in the specification does not necessarily refer to the same embodiment. Furthermore, when a specific feature, structure, or characteristic is described in connection with any embodiment, the intention is to suggest that implementing such a feature, structure, or characteristic in conjunction with other embodiments also falls within the scope of this application.

[0094] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0095] It should also be noted that the above are merely preferred embodiments of this application and do not limit the scope of protection of this application. Any equivalent structural or procedural transformations made using the content of this application specification, or direct or indirect applications in other related technical fields, are similarly included within the scope of protection of this application.

Claims

1. A method for preparing ultra-high purity stainless steel, characterized in that, Includes the following steps: An intermediate alloy is provided, by mass percentage, comprising Al ≤ 0.006%, O ≤ 0.008%, S ≤ 0.003%, and P ≤ 0.01%; According to the target composition of the stainless steel, the intermediate alloy is batched, and the batched product is subjected to vacuum induction melting to form the first molten steel after melting. Carbon is added to the first molten steel for vacuum carbon deoxidation pretreatment to obtain a second molten steel with an oxygen content of less than or equal to 0.005 wt%. Argon gas is introduced into the second molten steel to maintain the second molten steel at the first temperature, and magnesium alloy is added to perform deoxidation and desulfurization to obtain the third molten steel. Rare earth elements are added to the third molten steel to perform deep deoxidation and desulfurization, resulting in a fourth molten steel. The temperature of the fourth molten steel is controlled at 1520℃~1550℃, and after a 2min~10min quenching treatment, it is cast to obtain an ingot. The ingot is forged into a first consumable electrode, and the cross-sectional dimensions of the first consumable electrode are determined with a filler ratio of 0.3 to 0.

6. In an argon atmosphere, the first consumable electrode is subjected to electroslag remelting. The slag material in the electroslag remelting process, by mass percentage, comprises: 40%–50% CaF2, 33%–41% CaO, 3%–7% MgO, 13%–18% SiO2, with the balance being impurities, and the impurity content being ≤1%. During the capping stage of the electroslag remelting, multi-stage current reduction and feeding are performed to obtain the electroslag remelting ingot. The electroslag remelting ingot is forged into a second consumable electrode, and the cross-sectional dimensions of the second consumable electrode are determined according to a filling ratio of 0.3 to 0.

6. Vacuum degree ≤10 -3 Under Pa conditions, the second consumable electrode was subjected to vacuum arc remelting. Stainless steel is obtained by performing multi-stage current reduction and shrinkage during the vacuum arc remelting capping period.

2. The method according to claim 1, characterized in that, The vacuum degree of the vacuum carbon deoxidation pretreatment is less than or equal to 20 Pa; In the vacuum carbon deoxidation pretreatment process, the amount of carbon added is determined according to Formula 1, and the pretreatment time is determined according to Formula 2. Formula 1, In Formula 1, w[%C] represents the amount of carbon added, w[%C]0 represents the target C content of the stainless steel, w[%O] represents the oxygen content in the first molten steel, and w[%C]... imp The carbon content of impurities introduced into the product obtained from the ingredients. Equation 2, In Equation 2, t is the pretreatment time, p is the vacuum degree during the vacuum carbon deoxidation pretreatment process, S is the free surface area of ​​the molten steel, and V is the volume of the second molten steel. [%O] represents the change in oxygen content in the first and second molten steel.

3. The method according to claim 1, characterized in that, The amount of magnesium alloy added is determined according to Formula 3, and the deoxidation and desulfurization time is 2 min to 3 min. Formula 3, In Formula 3, w[%Mg] is the magnesium content in the magnesium alloy, w[%Mg]0 is the target magnesium content in the stainless steel, p is the pressure in the vacuum induction furnace when the Mg alloy is added, and T is the temperature of the second molten steel.

4. The method according to claim 1, characterized in that, The amount of rare earth element added is determined according to Formula 4, and the deep deoxidation and desulfurization time is 2 min to 3 min. Equation 4, In Formula 4, w[%RE] represents the amount of RE added to the third molten steel, and w[%RE]0 represents the target RE content of the stainless steel.

5. The method according to claim 1, characterized in that, The melting rate V1 of the electroslag remelting process is determined according to Equation 5 or Equation 6. When the stainless steel contains Fe ≥ 60wt%, T L -T S At ≤60℃, the melting rate V1 is determined by Equation 5. Equation 5, When Fe < 60% in the stainless steel, T L -T S When the temperature is >60℃, the melting rate V1 is determined by Equation 6. Formula 6, In equations 5 and 6, T L T is the liquidus temperature of stainless steel. S V is the solidus temperature of stainless steel, V1 is the melting rate of the first consumable electrode, and D is the size of the protective atmosphere electroslag remelting crystallizer.

6. The method according to claim 1, characterized in that, The melting rate V2 of the vacuum arc remelting process is determined according to Equations 7 and 8. Equation 7, When the solidification range of the smelting alloy (Fe≥60%) is relatively wide, T L -T S When the temperature is >60℃, the melting rate is determined by Equation 8: Formula 8; In equations 7 and 8, T L T is the liquidus temperature of stainless steel. S V is the solidus temperature of stainless steel, V2 is the melting rate of the second consumable electrode, and D is the size of the protective atmosphere electroslag remelting crystallizer.

7. The method according to claim 1, characterized in that, The method includes the following steps: An intermediate alloy is provided, by mass percentage, comprising Al ≤ 0.006%, O ≤ 0.008%, S ≤ 0.003%, and P ≤ 0.01%; According to the target composition of the stainless steel, the intermediate alloy is batched, and the batched product is subjected to vacuum induction melting to form the first molten steel after melting. Carbon is added to the first molten steel for vacuum carbon deoxidation pretreatment to obtain a second molten steel with an oxygen content of less than or equal to 0.005 wt%. The amount of carbon added is determined according to Formula 1, and the pretreatment time is determined according to Formula 2. Formula 1, In Formula 1, w[%C] represents the amount of carbon added, w[%C]0 represents the target C content of the stainless steel, w[%O] represents the oxygen content in the first molten steel, and w[%C]... imp The carbon content of impurities introduced into the product obtained from the ingredients. Equation 2, In Equation 2, t is the pretreatment time, p is the vacuum degree during the vacuum carbon deoxidation pretreatment process, S is the free surface area of ​​the molten steel, and V is the volume of the second molten steel. [%O] represents the change in oxygen content between the first and second molten steel. Argon gas is introduced into the second molten steel to maintain it at a first temperature. Magnesium alloy is then added for deoxidation and desulfurization to obtain a third molten steel. The amount of magnesium alloy added is determined according to Formula 3. The deoxidation and desulfurization time is 2 to 3 minutes. Formula 3, In Formula 3, w[%Mg] is the magnesium content in the magnesium alloy, w[%Mg]0 is the target magnesium content in the stainless steel, p is the pressure in the vacuum induction furnace when the Mg alloy is added, and T is the temperature of the second molten steel. Rare earth elements are added to the third molten steel to perform deep deoxidation and desulfurization, resulting in a fourth molten steel. The amount of rare earth elements added is determined according to Formula 4, and the deep deoxidation and desulfurization time is 2 to 3 minutes. Equation 4, In Formula 4, w[%RE] represents the amount of RE added to the third molten steel, and w[%RE]0 represents the target RE content of the stainless steel. The temperature of the fourth molten steel is controlled at 1520℃~1550℃, and after a 2min~10min quenching treatment, it is cast to obtain an ingot. The ingot is forged into a first consumable electrode, and the cross-sectional dimensions of the first consumable electrode are determined with a filler ratio of 0.3 to 0.

6. In an argon atmosphere, the first consumable electrode is subjected to electroslag remelting, wherein the melting rate V1 of the electroslag remelting process is determined according to Equation 5 or Equation 6. When the stainless steel contains Fe ≥ 60wt%, T L -T S At ≤60℃, the melting rate V1 is determined by Equation 5. Equation 5, When Fe < 60% in the stainless steel, T L -T S When the temperature is >60℃, the melting rate V1 is determined by Equation 6. Formula 6, In equations 5 and 6, T L T is the liquidus temperature of stainless steel. S V is the solidus temperature of stainless steel, V1 is the melting rate of the first consumable electrode, and D is the size of the protective atmosphere electroslag remelting crystallizer. The electroslag remelting air is periodically subjected to multi-stage current reduction and feeding to obtain electroslag remelting ingots; The electroslag remelting ingot is forged into a second consumable electrode, and the cross-sectional dimensions of the second consumable electrode are determined according to a filling ratio of 0.3 to 0.

6. Vacuum degree ≤10 -3 Under Pa conditions, the second consumable electrode is subjected to vacuum arc remelting, and the melting rate V2 of the vacuum arc remelting process is determined according to Equations 7 and 8. Equation 7, When the solidification range of the smelting alloy (Fe≥60%) is relatively wide, T L -T S When the temperature is >60℃, the melting rate is determined by Equation 8: Formula 8, In equations 7 and 8, T L T is the liquidus temperature of stainless steel. S V is the solidus temperature of stainless steel, V2 is the melting rate of the second consumable electrode, and D is the size of the protective atmosphere electroslag remelting crystallizer. Stainless steel is obtained by performing multi-stage current reduction and shrinkage during the vacuum arc remelting capping period.

8. The method according to claim 1, characterized in that, The slag material, by mass percentage, comprises: 40%–50% CaF2, 33%–41% CaO, 3%–7% MgO, 13%–18% SiO2, with the balance being impurities, and the impurity content ≤1%; The contents of SiO2 and CaO are calculated according to the following formula: w1 = (16.1 - 1.1 × 10) 3 w Al Equation 9: (1) × 100% w2=Rw1 (Equation 10); In Equations 9 and 10, w1 is the mass percentage of SiO2 in the slag, w2 is the mass percentage of CaO in the slag, and w Al Let w be the mass fraction of Al in stainless steel. Al ≤0.03%, where R is the binary basicity of the slag, and R takes a value of 2-3; In the stainless steel, the mass fraction of oxygen is less than or equal to 0.001%.

9. The method according to any one of claims 1-8, characterized in that, During the electroslag remelting process, the inlet water temperature of the crystallizer is 25℃~35℃, and the outlet water temperature does not exceed 55℃. During the vacuum arc remelting process, the inlet water temperature of the crystallizer is 25℃~35℃, and the outlet water temperature does not exceed 50℃.

10. The method according to any one of claims 1-8, characterized in that, The magnesium alloy is a Ni-Mg alloy or an Fe-Mg alloy, and the magnesium content in the magnesium alloy is 10% to 30% by mass. The rare earth element is selected from at least one of La, Ce, and Y.

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