Semiconductor-grade ultra-pure stainless steel and method for producing the same

Abstract

The application belongs to the technical field of metallurgical new materials, and particularly relates to a semiconductor-grade ultra-pure stainless steel and a preparation method thereof. The technical scheme provided in the embodiments of the application forms a complete cause-and-effect chain of inhibiting inclusion generation from the source, modifying nanocrystallization in the middle period, forging dispersion locking in the later period and finally heat treatment stabilization through the synergistic effect of vacuum induction melting crucible melting, carbon pre-deoxidation treatment, deep deoxidation and desulfurization treatment, composite modification treatment, vacuum consumable remelting, constant-temperature homogenization forging and heat treatment, so as to solve the technical problem of how to synergistically control nanoscale inclusions to prepare semiconductor-grade ultra-pure stainless steel.

Description

Technical Field

[0001] This application belongs to the field of new metallurgical materials technology, and in particular relates to a semiconductor-grade ultrapure stainless steel and its preparation method. Background Technology

[0002] Ultra-high purity stainless steel is a key basic material for semiconductor specialty gas delivery pipelines, core components of nuclear power plants, and precision aerospace parts. Its internal purity directly determines the material's corrosion resistance, mechanical stability, surface finish, and long-term service life. The semiconductor industry imposes extremely stringent requirements on stainless steel, demanding that inclusions be ductile, dispersed, and free of agglomeration. Therefore, developing a method for preparing semiconductor-grade ultra-pure stainless steel that can synergistically control nano-inclusions has become an urgent technical challenge in this field.

[0003] The existing technology is the industry-standard high-purity smelting process for austenitic / ferritic stainless steel, which employs a dual refining process of vacuum induction melting (VIM) and vacuum degassing (VD) throughout. It lacks external force field coupling strengthening, Ce-Mg composite modification, vacuum consumable remelting, and specialized forging impurity control processes. Specifically, this process involves loading industrial-grade stainless steel raw materials into an Al2O3 crucible, evacuating it to 10-100 Pa, and heating it to 1500-1550℃ to melt the steel. Deoxidation is achieved solely through the steel's own carbon content, without the addition of any targeted composite deoxidizer. The molten steel is then transferred to a VD furnace, evacuated to 100-500 Pa, and weakly stirred by bottom-blowing argon gas. This stirring intensity is uncontrollable and mass transfer efficiency is low. During the deoxidation and ingot casting stages, a single aluminum deoxidizer or Si-Ca composite agent is added at once, without a gradient deoxidation design. After deoxidation, the ingot is directly placed and cast, followed by conventional annealing and shaping to obtain the finished product. The existing technology has the following technical defects: First, there is a bottleneck in impurity removal, making it impossible to achieve ultra-low oxygen control. The deoxidation path is singular and lacks gradient design, so the oxygen content can only be reduced to 30-60 ppm, and the sulfur content is difficult to reduce to below 5 ppm. It cannot stably achieve the semiconductor-grade standard of oxygen content ≤8 ppm and sulfur content ≤2 ppm, and volatile impurities cannot be completely removed. Second, the inclusions are large and easily agglomerate, lacking nanoscale control capability. The use of only weak argon stirring leads to uneven convection in the molten pool, resulting in a slow aggregation and flotation rate of inclusions. The deoxidation product is hard and brittle Al. The 2O3 and SiO2 phases are prone to aggregation and residue, with sizes mostly above 2μm and unable to be controlled to ≤1000nm. The inclusion rating is mostly 1.5-2.5. Thirdly, there is no vacuum self-consuming remelting and specialized forging process, resulting in obvious component segregation, insufficient homogenization of molten steel, easy adsorption of gaseous impurities during cooling, causing rebound, large fluctuations in purity between different batches of products, and poor microstructure uniformity. Fourthly, the utilization rate of auxiliary materials is low, the production cost is high, there is no composite modification and multi-field strengthening, the amount of deoxidizer added is large but the utilization rate is low, the refining cycle is long, and it cannot be adapted to high-end mass production of semiconductors.

[0004] The second existing technology is a rare earth modified high-purity stainless steel smelting process. In the later stage of conventional VIM+AOD or VIM+VD refining, rare earth elements (such as La and Ce) are added alone or in combination. The affinity of rare earth with oxygen and sulfur is used to modify the morphology of the already formed inclusions, turning hard inclusions into spherical rare earth inclusions. However, it lacks gradient deoxidation, ultra-high vacuum strengthening, Ce-Mg synergistic modification, vacuum self-consumable remelting, and forging impurity control process. The existing technology has the following technical defects: First, it only modifies the end and cannot deeply deoxidize at the source. It only modifies the morphology of the already generated inclusions and cannot reduce the original oxygen and sulfur content of the molten pool. The oxygen content is difficult to be lower than 30ppm, which cannot meet the ultra-low oxygen requirement of ≤8ppm. Second, without Ce-Mg synergy and stirring enhancement, the inclusions are prone to coarsening and agglomeration. Relying on weak argon stirring results in insufficient buoyancy of the inclusions, which are prone to agglomeration and growth. It is impossible to stably control the size at ≤1000nm, thus limiting the cleanliness. Third, without vacuum self-consumption and forging synergy, the batch stability is poor. It is difficult to accurately control the amount of rare earth added, which is prone to segregation. Without forging to refine and disperse, the uniformity of the structure and the consistency of performance cannot meet the requirements of semiconductors. Fourth, the process is not economical and is difficult to promote industrially. The cost of auxiliary materials is high and the process window is narrow, making it impossible to achieve stable dual control of ultra-low oxygen and nano-inclusions. Summary of the Invention

[0005] This application provides a semiconductor-grade ultrapure stainless steel and its preparation method to solve the following technical problem: how to synergistically control nano-inclusions to prepare semiconductor-grade ultrapure stainless steel.

[0006] In a first aspect, embodiments of this application provide a method for preparing semiconductor-grade ultrapure stainless steel, the method comprising: High-purity stainless steel raw material is loaded into a vacuum induction melting crucible, the vacuum induction melting crucible is evacuated and the high-purity stainless steel raw material is heated to melt the high-purity stainless steel raw material into molten steel. The molten steel is subjected to carbon pre-deoxidation treatment to reduce the oxygen content of the molten steel to 35ppm-40ppm; A Mg-Ca composite deoxidizer is added to the molten steel for deep deoxidation and desulfurization treatment; Ce-Mg composite modifier is added to the molten steel for composite modification treatment, so as to modify the inclusions in the molten steel into nano-plastic phase in situ; The molten steel that has undergone the composite modification treatment is made into a vacuum induction casting ingot, and the vacuum induction casting ingot is subjected to vacuum self-consumption remelting treatment. The vacuum degree of the vacuum self-consumption remelting treatment is ≤0.1Pa, so as to deeply remove gaseous impurities from the molten steel and homogenize the molten steel. The ingot that has undergone the vacuum self-consumable remelting process is subjected to isothermal homogenization forging to break the residual inclusions in the ingot and promote the dispersion distribution of the nano-plastic phase. The billet that has undergone the isothermal homogenization forging process is then subjected to heat treatment to stabilize the microstructure of the billet and the distribution of the nano-plastic phase.

[0007] Optionally, the purity of the high-purity stainless steel raw material is ≥99.95%, and the high-purity stainless steel raw material includes at least one of high-purity iron, metallic chromium, metallic nickel and ferromolybdenum; The high-purity stainless steel raw material undergoes vacuum drying pretreatment before being loaded into the vacuum induction melting crucible. The temperature of the vacuum drying pretreatment is 180℃-220℃, and the time of the vacuum drying pretreatment is 1.5h-2.5h.

[0008] Optionally, the vacuum induction melting crucible is an MgO-CaO composite crucible; The vacuum degree of the environment in which the vacuum induction melting crucible is located is 2Pa-4Pa, and the temperature of the molten steel is 1530℃-1550℃.

[0009] Optionally, the carbon pre-deoxidation treatment includes adding high-purity carbon powder to the molten steel, and the holding time of the carbon pre-deoxidation treatment is 8 min-12 min.

[0010] Optionally, the deep deoxidation and desulfurization treatment includes adding 0.04wt%-0.06wt% of Mg-Ca composite deoxidizer to the molten steel, wherein the mass ratio of Mg to Ca in the Mg-Ca composite deoxidizer is 1:2, and the holding time of the deep deoxidation and desulfurization treatment is 10min-15min.

[0011] Optionally, the composite modification treatment includes adding 0.020wt%-0.030wt% of Ce-Mg composite modifier to the molten steel, and the holding time of the composite modification treatment is 6min-8min.

[0012] Optionally, the isothermal homogenization forging process includes heating the ingot to 1150℃-1200℃ for isothermal forging, the reduction per pass of the isothermal homogenization forging process is 15%-25%, and the total deformation of the isothermal homogenization forging process is 50%-60%.

[0013] Optionally, the heat treatment includes homogenization annealing and solution treatment; The homogenization annealing treatment is performed at a temperature of 1150℃-1200℃ for 3.5h-4.5h. The solution treatment temperature is 950℃-1060℃, the solution treatment time is 0.5h-1.2h, and the billet is water-cooled after the solution treatment.

[0014] Optionally, during at least one of the carbon pre-deoxidation treatment, the deep deoxidation and desulfurization treatment, and the composite modification treatment, electromagnetic stirring is applied to the molten steel, and the frequency of the electromagnetic stirring is 18Hz-22Hz.

[0015] Optionally, the semiconductor-grade ultrapure stainless steel has an oxygen content ≤8ppm, a sulfur content ≤2ppm, a phosphorus content ≤5ppm, an inclusion size ≤1000nm, and a fine-grained D-class inclusion rating ≤0.5. (The preceding content is modified according to the claims.) Secondly, embodiments of this application provide a semiconductor-grade ultrapure stainless steel prepared according to the preparation method described in any one of the first aspects.

[0016] Optionally, the semiconductor-grade ultrapure stainless steel includes 316L stainless steel, 304L stainless steel, or 439 ferritic stainless steel.

[0017] The technical solution provided in this application has the following advantages compared with the prior art: High-purity stainless steel raw materials are loaded into a vacuum induction melting crucible. The vacuum induction melting crucible is evacuated and the high-purity stainless steel raw materials are heated to melt them into molten steel. This removes gaseous impurities adsorbed on the surface of the high-purity stainless steel raw materials and establishes a low-oxygen melting environment, thus providing a clean melting foundation for subsequent deep deoxidation and desulfurization treatment.

[0018] Carbon pre-deoxidation treatment is performed on molten steel to reduce the oxygen content to 35ppm-40ppm. This allows carbon to react with oxygen to generate carbon monoxide gas, which then escapes, thus initially reducing the oxygen content in the molten steel. This creates low-oxygen conditions for subsequent deep deoxidation and desulfurization treatments, preventing excessive consumption of the Mg-Ca composite deoxidizer.

[0019] Adding a Mg-Ca composite deoxidizer to molten steel for deep deoxidation and desulfurization treatment utilizes the strong affinity of magnesium and calcium for residual oxygen and sulfur in the molten steel to further remove oxygen and sulfur, thereby reducing the oxygen and sulfur content in the molten steel to ultra-low levels and providing a low-oxygen and low-sulfur molten steel environment for subsequent composite modification treatment.

[0020] Adding Ce-Mg composite modifier to molten steel for composite modification treatment can in-situ modify inclusions in the molten steel into nano-plastic phases. By utilizing the reaction between cerium and magnesium and the inclusions already formed in the molten steel, the crystal structure and morphology of the inclusions can be changed, transforming hard and brittle inclusions into plastic phases and controlling the size of inclusions at the nanoscale, thereby inhibiting the formation and agglomeration of large-sized hard inclusions from the source.

[0021] The ingots that have undergone composite modification treatment are subjected to vacuum self-consumption remelting. The vacuum degree of vacuum self-consumption remelting is ≤0.1Pa, so as to deeply remove gaseous impurities in the molten steel and homogenize the molten steel. In this way, the ultra-high vacuum environment promotes the further escape of residual gaseous impurities in the molten steel. At the same time, the composition and temperature of the molten steel are homogenized through the self-consumption remelting process, thereby locking in ultra-low oxygen content and eliminating compositional segregation.

[0022] The ingots that have undergone vacuum self-consumable remelting are subjected to isothermal homogenization forging to break up residual inclusions in the ingots and promote the dispersion of nano-plastic phases. The mechanical deformation during the isothermal forging process breaks up the residual inclusions that have not been fully modified, while the thermomechanical action promotes the uniform dispersion of nano-plastic phases in the steel matrix and prevents the agglomeration and coarsening of nano-plastic phases.

[0023] The billet, after being homogenized by constant temperature forging, is heat-treated to stabilize the microstructure and distribution of nano-plastic phases. This is achieved by eliminating forging stress and further homogenizing the microstructure through homogenization annealing, and fixing the distribution of alloying elements through solution treatment, thereby stabilizing the dispersed distribution of nano-plastic phases.

[0024] The aforementioned vacuum induction melting crucible cleaning, carbon pre-deoxidation treatment, deep deoxidation and desulfurization treatment, composite modification treatment, vacuum self-consumable remelting treatment, isothermal homogenization forging treatment and heat treatment work together to form a complete causal chain from the source to the mid-term modification and nano-sizing, the late-term forging dispersion locking and the final heat treatment stabilization, thereby solving the technical problem of how to synergistically regulate nano-inclusions to prepare semiconductor-grade ultrapure stainless steel. Attached Figure Description

[0025] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application.

[0026] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the accompanying drawings required in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other related drawings can be derived from these drawings without creative effort.

[0027] Figure 1 SEM image of the inclusion size of semiconductor-grade ultrapure stainless steel prepared by the preparation method in Example 1 of this application; Figure 2 SEM image of stainless steel prepared using the preparation method in Comparative Example 1 of this application. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0029] The range descriptions used herein, such as numerical ranges and proportional ranges, include all possible sub-ranges and single numerical values ​​within the range. For example, a range description of 1 to 6 or 1~6 covers all sub-ranges (such as 1 to 3, 2 to 5, etc.) and single numbers (such as 1, 2, 3, 4, 5, 6) between 1 and 6. Unless otherwise specified, the terms used herein include, but are not limited to, terms such as "include"; relational terms such as "first" and "second" are used only to distinguish different entities or steps and do not imply an actual order or relationship; and / or indicate that multiple situations may exist alone or simultaneously; expressions such as "at least one," "multiple," and "at least one" refer to any combination of the corresponding objects, including combinations of single or multiple objects. Proportional relationships mentioned herein, such as mass ratios and molar ratios, should be understood as the correspondence between the antecedent and consequent terms of a proportional expression, according to the order of description. The raw materials, reagents, instruments, and equipment used herein can all be obtained through commercial purchase or prepared using existing methods.

[0030] In a first aspect, embodiments of this application provide a method for preparing semiconductor-grade ultrapure stainless steel, the method comprising: High-purity stainless steel raw material is loaded into a vacuum induction melting crucible, the vacuum induction melting crucible is evacuated and the high-purity stainless steel raw material is heated to melt the high-purity stainless steel raw material into molten steel. The molten steel is subjected to carbon pre-deoxidation treatment to reduce the oxygen content of the molten steel to 35ppm-40ppm; A Mg-Ca composite deoxidizer is added to the molten steel for deep deoxidation and desulfurization treatment; Ce-Mg composite modifier is added to the molten steel for composite modification treatment, so as to modify the inclusions in the molten steel into nano-plastic phase in situ; The molten steel that has undergone the composite modification treatment is made into a vacuum induction casting ingot, and the vacuum induction casting ingot is subjected to vacuum self-consumption remelting treatment. The vacuum degree of the vacuum self-consumption remelting treatment is ≤0.1Pa, so as to deeply remove gaseous impurities from the molten steel and homogenize the molten steel. The ingot that has undergone the vacuum self-consumable remelting process is subjected to isothermal homogenization forging to break the residual inclusions in the ingot and promote the dispersion distribution of the nano-plastic phase. The billet that has undergone the isothermal homogenization forging process is then subjected to heat treatment to stabilize the microstructure of the billet and the distribution of the nano-plastic phase.

[0031] Nanoplastic phase refers to plastic inclusion phases with a size ≤1000nm formed after modification with Ce-Mg composite modifier, which are different from hard and brittle Al2O3 and SiO2 phases.

[0032] The inventive approach of this application to the prior art lies in the following: Prior art one uses a single aluminum deoxidizer or Si-Ca composite agent for one-time input, without a gradient deoxidation design; prior art two only modifies the end-stage morphology of the already generated inclusions, without Ce-Mg synergistic modification. In contrast, this application pioneers a three-stage gradient deoxidation and desulfurization coupled with Ce-Mg composite modification, namely from the source (raw material purity ≥99.95% + vacuum drying) → middle stage (gradient deoxidation + composite modification + electromagnetic stirring) → later stage (remelting homogenization + forging crushing and dispersion + heat treatment stabilization), and achieves secondary purification and homogenization through vacuum self-consumable remelting, and achieves mechanical crushing and thermomechanical dispersion through isothermal homogenization forging, so that the inclusion size is ≤1000nm and presents a nano-plastic phase dispersion distribution, realizing full-process nano-inclusion control from source to end.

[0033] The vacuum level of the environment in which the vacuum induction melting crucible is located is 2Pa-4Pa, including but not limited to 2Pa, 3Pa, 4Pa, etc. The temperature of the molten steel is 1530℃-1550℃, including but not limited to 1530℃, 1540℃, 1550℃, etc.

[0034] The holding time for carbon pre-deoxidation treatment is 8-12 minutes, including but not limited to 8, 9, 10, 11, and 12 minutes. After carbon pre-deoxidation treatment, the oxygen content of the molten steel is reduced to 35 ppm-40 ppm, including but not limited to 35 ppm, 37 ppm, and 40 ppm. In the deep deoxidation and desulfurization treatment, the amount of Mg-Ca composite deoxidizer added is 0.04 wt%-0.06 wt%, including but not limited to 0.04 wt%, 0.05 wt%, and 0.06 wt%. The holding time for deep deoxidation and desulfurization treatment is 10-15 minutes, including but not limited to 11, 12, and 13 minutes. In the composite modification treatment, the amount of Ce-Mg composite modifier added is 0.020 wt%-0.030 wt%, including but not limited to 0.025 wt% and 0.030 wt%. The holding time for composite modification treatment is 6-8 minutes, including but not limited to 6, 7, and 8 minutes.

[0035] The vacuum degree of vacuum arc remelting is ≤0.1Pa, including but not limited to 0.05Pa, 0.08Pa, 0.1Pa, etc. The heating temperature of isothermal homogenization forging is 1150℃-1200℃, including but not limited to 1150℃, 1160℃, 1170℃, 1200℃, etc. The reduction per pass in isothermal homogenization forging is 15%-25%, including but not limited to 15%, 20%, 25%, etc. The total deformation in isothermal homogenization forging is 50%-60%, including but not limited to 50%, 55%, 60%, etc. The temperature of homogenization annealing is 1150℃-1200℃, including but not limited to 1150℃, 1170℃, 1200℃, etc. The time of homogenization annealing is 3.5h-4.5h, including but not limited to 3.5h, 4.0h, 4.5h, etc. The solution treatment temperature is 950℃-1060℃, including but not limited to 950℃, 1000℃, 1060℃, etc. The solution treatment time is 0.5h-1.2h, including but not limited to 0.8h, 1.0h, 1.2h, etc.

[0036] In some embodiments, the purity of the high-purity stainless steel raw material is ≥99.95%, and the high-purity stainless steel raw material includes at least one of electrolytic iron, metallic chromium, metallic nickel and ferromolybdenum; The high-purity stainless steel raw material undergoes vacuum drying pretreatment before being loaded into the vacuum induction melting crucible. The temperature of the vacuum drying pretreatment is 180℃-220℃, and the time of the vacuum drying pretreatment is 1.5h-2.5h.

[0037] High-purity stainless steel raw materials with a purity of ≥99.95% are selected, and the high-purity stainless steel raw materials include at least one of electrolytic iron, metallic chromium, metallic nickel and ferromolybdenum, thereby reducing the base number of impurities carried by the high-purity stainless steel raw materials themselves, reducing the total amount of impurities that need to be removed in the subsequent vacuum induction melting process, and thus reducing the burden on the subsequent carbon pre-deoxidation treatment and deep deoxidation and desulfurization treatment, making it easier to achieve the ultra-low oxygen content target.

[0038] Before being loaded into the vacuum induction melting crucible, high-purity stainless steel raw materials undergo vacuum drying pretreatment. The temperature of the vacuum drying pretreatment is 180℃-220℃, and the time is 1.5h-2.5h. This removes the moisture, gas, and oxide scale adsorbed on the surface of the high-purity stainless steel raw materials through heating in a vacuum environment, preventing these surface impurities from entering the molten steel during the melting process. This reduces the initial oxygen content and impurity content in the molten steel from the source, providing a cleaner starting point for subsequent deep deoxidation and desulfurization treatment.

[0039] The purity of the high-purity stainless steel raw material is ≥99.95%, including but not limited to 99.95%, 99.96%, 99.97%, and 99.98%. The vacuum drying pretreatment temperature is 180℃-220℃, including but not limited to 180℃, 200℃, and 220℃. The vacuum drying pretreatment time is 1.5h-2.5h, including but not limited to 1.5h, 2.0h, and 2.5h.

[0040] In some embodiments, the vacuum induction melting crucible is an MgO-CaO composite crucible; The vacuum degree of the environment in which the vacuum induction melting crucible is located is 2Pa-4Pa, and the temperature of the molten steel is 1530℃-1550℃.

[0041] The vacuum induction melting crucible is limited to an MgO-CaO composite crucible. This utilizes the low oxygen release characteristic of the MgO-CaO composite crucible at high temperatures to reduce the oxygen supply from the crucible material to the molten steel during the melting process, thereby reducing the initial oxygen content of the molten steel and lowering the deoxidation load for subsequent carbon pre-deoxidation treatment.

[0042] The vacuum level of the environment in which the vacuum induction melting crucible is located is controlled at 2Pa-4Pa, thereby maintaining a low oxygen partial pressure environment, inhibiting the absorption of oxygen from the gas phase by the molten steel, promoting the outward diffusion of existing gaseous impurities in the molten steel, and thus keeping the molten steel in a low oxygen state.

[0043] The temperature of the molten steel is controlled at 1530℃-1550℃ to ensure that the high-purity stainless steel raw materials are fully melted and form a uniform molten steel, while avoiding excessive temperature that would lead to increased oxygen absorption or loss of alloying elements. This provides a suitable temperature and uniform composition for the subsequent carbon pre-deoxidation treatment.

[0044] The vacuum level of the environment in which the vacuum induction melting crucible is located is 2Pa-4Pa, including but not limited to 2Pa, 3Pa, 4Pa, etc. The temperature of the molten steel is 1530℃-1550℃, including but not limited to 1530℃, 1540℃, 1550℃, etc.

[0045] In some embodiments, the carbon pre-deoxidation treatment includes adding high-purity carbon powder to the molten steel, and the holding time of the carbon pre-deoxidation treatment is 8 min-12 min.

[0046] Carbon pre-deoxidation treatment involves adding high-purity carbon powder to molten steel, thereby utilizing the carbon element in the high-purity carbon powder to react with dissolved oxygen in the molten steel to generate carbon monoxide gas. The carbon monoxide gas escapes from the molten steel under vacuum conditions, thereby reducing the oxygen content in the molten steel.

[0047] The holding time for carbon pre-deoxidation treatment is controlled at 8-12 minutes to provide sufficient reaction kinetic time for the carbon-oxygen reaction, allowing the carbon-oxygen reaction to proceed fully and thus ensuring that the oxygen content in the molten steel is effectively reduced to 35-40 ppm.

[0048] After carbon pre-deoxidation treatment, the oxygen content of the molten steel is reduced to a limit of 35ppm-40ppm. This controls the oxygen content of the molten steel within a specific range during the carbon pre-deoxidation treatment stage. This avoids excessive consumption of Mg-Ca composite deoxidizer due to excessive oxygen content, and also avoids loss of alloying elements due to excessive deoxidation. This leaves a reasonable deoxidation space for subsequent deep deoxidation and desulfurization treatment.

[0049] The holding time for carbon pre-deoxidation treatment is 8-12 minutes, including but not limited to 9 minutes, 10 minutes, 11 minutes, etc.

[0050] In some embodiments, the deep deoxidation and desulfurization treatment includes adding 0.04wt%-0.06wt% of a Mg-Ca composite deoxidizer to the molten steel, wherein the mass ratio of Mg to Ca in the Mg-Ca composite deoxidizer is 1:2, and the holding time of the deep deoxidation and desulfurization treatment is 10min-15min.

[0051] Deep deoxidation and desulfurization treatment involves adding 0.04wt%-0.06wt% of Mg-Ca composite deoxidizer to the molten steel. This utilizes the high affinity of magnesium and calcium for oxygen and sulfur to further remove residual oxygen and sulfur from the molten steel after the oxygen content has been reduced by carbon pre-deoxidation treatment. Magnesium reacts with oxygen to form magnesium oxide, calcium reacts with oxygen to form calcium oxide, and calcium reacts with sulfur to form calcium sulfide. These reaction products float to the surface under electromagnetic stirring or are removed by subsequent processes, thereby reducing the oxygen and sulfur content in the molten steel to the target level of deep deoxidation and desulfurization treatment.

[0052] The mass ratio of Mg to Ca in the Mg-Ca composite deoxidizer is limited to 1:2 to optimize the synergistic deoxidation and desulfurization efficiency of magnesium and calcium. Magnesium provides strong deoxidation capacity, while calcium provides strong desulfurization capacity and improves the morphology of inclusions. The synergistic effect is optimal when the mass ratio of the two is 1:2, thereby achieving simultaneous deep removal of oxygen and sulfur.

[0053] The holding time for deep deoxidation and desulfurization treatment is controlled to be 10-15 minutes, so as to provide sufficient time for the Mg-Ca composite deoxidizer to fully react with oxygen and sulfur in the molten steel, ensuring that the deoxidation and desulfurization reaction is close to equilibrium, thereby making the oxygen and sulfur content in the molten steel stably meet the requirements of deep deoxidation and desulfurization treatment.

[0054] The amount of Mg-Ca composite deoxidizer added in the deep deoxidation and desulfurization treatment is 0.04wt%-0.06wt%, including but not limited to 0.04wt%, 0.05wt%, and 0.06wt%. The holding time for the deep deoxidation and desulfurization treatment is 10min-15min, including but not limited to 11min, 12min, and 13min.

[0055] In some embodiments, the composite modification treatment includes adding 0.020wt%-0.030wt% of Ce-Mg composite modifier to the molten steel, and the holding time of the composite modification treatment is 6min-8min.

[0056] The composite modification treatment involves adding 0.020wt%-0.030wt% of Ce-Mg composite modifier to the molten steel. This allows cerium and magnesium to react in situ with the deoxidation products and residual hard inclusions already generated in the molten steel, thereby changing the crystal structure and interfacial energy of the inclusions. This transforms the hard and brittle inclusions into nano-plastic phases, thereby controlling the size of the inclusions at the nanoscale and improving their plasticity.

[0057] The holding time of the composite modification treatment is controlled to be 6-8 minutes, so as to provide sufficient time for the in-situ modification reaction between the Ce-Mg composite modifier and the inclusions in the molten steel. This allows the modification reaction to proceed fully and avoids the excessive reaction of the Ce-Mg composite modifier that could lead to the formation of new inclusions, thereby ensuring the formation quality and dimensional stability of the nanoplastic phase.

[0058] The amount of Ce-Mg composite modifier added in the composite modification treatment is 0.020wt%-0.030wt%, including but not limited to 0.02wt%, 0.025wt%, and 0.030wt%. The holding time for the composite modification treatment is 6min-8min, including but not limited to 6min, 7min, and 8min.

[0059] In some embodiments, the isothermal homogenization forging process includes heating the ingot to 1150°C-1200°C for isothermal forging, wherein the reduction per pass in the isothermal homogenization forging process is 15%-25%, and the total deformation of the isothermal homogenization forging process is 50%-60%.

[0060] The reduction per pass refers to the percentage of deformation in a single forging operation during the isothermal homogenization forging process relative to the initial height of the billet.

[0061] The isothermal homogenization forging process involves heating the ingot to 1150℃-1200℃ for isothermal forging. This process utilizes the temperature characteristics of the ingot, which has good plastic deformation ability and whose nano-plastic phase has not yet undergone significant coarsening, to apply mechanical deformation to the ingot under isothermal conditions, thereby causing the residual inclusions in the ingot to be broken up under thermomechanical action.

[0062] By controlling the reduction amount of each pass in the isothermal homogenization forging process to 15%-25%, the residual inclusions inside the ingot are gradually broken up through a single moderate reduction deformation. At the same time, excessive single deformation is avoided to prevent abnormal flow or agglomeration of the nano-plastic phase, thereby achieving controllable crushing of the residual inclusions.

[0063] The total deformation of the isothermal homogenization forging process is controlled at 50%-60%, thereby ensuring that the residual inclusions in the ingot are fully broken and dispersed by accumulating sufficient deformation. At the same time, it promotes the uniform distribution of nano-plastic phase in the steel matrix, preventing the nano-plastic phase from being locally enriched due to insufficient deformation or abnormally coarsened due to excessive deformation, thus enabling the nano-plastic phase to form a uniformly distributed state in the ingot.

[0064] The heating temperature for isothermal homogenization forging is 1150℃-1200℃, including but not limited to 1150℃, 1160℃, 1170℃, and 1200℃. The reduction per pass in isothermal homogenization forging is 15%-25%, including but not limited to 15%, 20%, and 25%. The total deformation in isothermal homogenization forging is 50%-60%, including but not limited to 50%, 55%, and 60%.

[0065] In some embodiments, the heat treatment includes homogenization annealing and solution treatment; The homogenization annealing treatment is performed at a temperature of 1150℃-1200℃ for 3.5h-4.5h. The solution treatment temperature is 950℃-1060℃, the solution treatment time is 0.5h-1.2h, and the billet is water-cooled after the solution treatment.

[0066] Heat treatment includes homogenization annealing and solution treatment. Homogenization annealing eliminates work hardening and residual stress generated during isothermal homogenization forging, while solution treatment enables alloying elements to form a uniform solid solution state in the matrix, thereby stabilizing the billet state in terms of both microstructure stability and compositional uniformity.

[0067] The homogenization annealing temperature was controlled at 1150℃-1200℃, and the homogenization annealing time was controlled at 3.5h-4.5h. This allowed for sufficient diffusion of elements in the billet under high temperature and long time conditions, eliminating local component segregation that might be caused by constant temperature homogenization forging. At the same time, it further homogenized the distribution of the nano-plastic phase in the billet, thereby stabilizing the microstructure and distribution of the nano-plastic phase in the billet.

[0068] The solution treatment temperature was controlled at 950℃-1060℃, and the solution treatment time was controlled at 0.5h-1.2h. This allowed carbon and alloying elements to fully dissolve in the austenite matrix within a temperature range that prevented excessive grain growth. After the solution treatment, the billet was water-cooled to quickly fix the solution state, suppress the precipitation of second phases such as carbides during the cooling process, and thus fix the distribution of alloying elements and stabilize the dispersed distribution of nano-plastic phases.

[0069] The homogenization annealing temperature is 1150℃-1200℃, including but not limited to 1150℃, 1170℃, and 1200℃. The homogenization annealing time is 3.5h-4.5h, including but not limited to 3.5h, 4.0h, and 4.5h. The solution treatment temperature is 950℃-1060℃, including but not limited to 950℃, 1000℃, and 1060℃. The solution treatment time is 0.5h-1.2h, including but not limited to 0.5h, 0.8h, 1.0h, and 1.2h.

[0070] In some embodiments, during at least one of the carbon pre-deoxidation treatment, the deep deoxidation and desulfurization treatment, and the composite modification treatment, electromagnetic stirring is applied to the molten steel at a frequency of 18Hz-22Hz.

[0071] In at least one of the carbon pre-deoxidation treatment, deep deoxidation and desulfurization treatment, and composite modification treatment, electromagnetic stirring is applied to the molten steel to drive forced convection, thereby enhancing the reaction mass transfer efficiency between the deoxidizer, modifier and molten steel, promoting the flotation and removal of deoxidation products and modified inclusions, and thus improving the reaction efficiency and uniformity of the carbon pre-deoxidation treatment, deep deoxidation and desulfurization treatment and composite modification treatment.

[0072] The frequency of electromagnetic stirring is controlled at 18Hz-22Hz, so that the penetration depth and stirring intensity of electromagnetic stirring of molten steel are balanced within this frequency range. This can effectively stir the entire volume of the molten pool without causing excessive splashing or air entrapment of molten steel, thereby ensuring the uniformity of reaction and the flotation efficiency of inclusions during carbon pre-deoxidation treatment, deep deoxidation and desulfurization treatment and composite modification treatment.

[0073] The frequency of the electromagnetic stirring is 18Hz-22Hz, including but not limited to 18Hz, 20Hz, 22Hz, etc.

[0074] In some embodiments, the semiconductor-grade ultrapure stainless steel has an oxygen content ≤8ppm, a sulfur content ≤2ppm, a phosphorus content ≤5ppm, an inclusion size ≤1000nm, and a fine-series D-class inclusion rating ≤0.5.

[0075] Secondly, embodiments of this application provide a semiconductor-grade ultrapure stainless steel prepared according to the preparation method described in any one of the first aspects.

[0076] Semiconductor-grade ultrapure stainless steel is prepared according to any one of the methods for preparing semiconductor-grade ultrapure stainless steel in the first aspect, thereby applying all the method steps and process parameters defined in the first aspect to the preparation process of semiconductor-grade ultrapure stainless steel, so that carbon pre-deoxidation treatment, deep deoxidation and desulfurization treatment, composite modification treatment, vacuum self-consuming remelting treatment, isothermal homogenization forging treatment and heat treatment work synergistically on high-purity stainless steel raw materials, thereby preparing semiconductor-grade ultrapure stainless steel with the above-mentioned defined performance indicators.

[0077] In some embodiments, the semiconductor-grade ultrapure stainless steel includes 316L stainless steel, 304L stainless steel, or 439 ferritic stainless steel.

[0078] 316L stainless steel refers to an ultra-low carbon austenitic stainless steel containing approximately 17% chromium, 14.5% nickel, and 2.5% molybdenum. 304L stainless steel refers to an ultra-low carbon austenitic stainless steel containing approximately 18% chromium and 9% nickel.

[0079] The present application is further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the application. Experimental methods in the following embodiments that do not specify specific conditions are generally determined according to industry standards. If there is no corresponding industry standard, then generally accepted international standards, conventional conditions, or conditions recommended by the manufacturer are followed.

[0080] Example 1: Semiconductor-grade ultrapure 316L stainless steel High-purity stainless steel raw materials are obtained by precisely mixing industrial pure iron (purity ≥99.96%), metallic chromium (purity ≥99.98%), metallic nickel (purity ≥99.97%), and ferromolybdenum (purity ≥99.95%) with Cr 17.0%, Ni 14.5%, Mo 2.5%, and C ≤0.008%.

[0081] The surface of the high-purity stainless steel raw material is mechanically cleaned, and then the cleaned high-purity stainless steel raw material is vacuum dried at 200℃ for 2 hours to remove the oxide scale and adsorbed gas on the surface of the high-purity stainless steel raw material.

[0082] The high-purity stainless steel raw material, which has undergone vacuum drying pretreatment, is loaded into a vacuum induction melting crucible. The vacuum induction melting crucible is a MgO-CaO composite crucible. The vacuum induction melting crucible is evacuated to 3 Pa, and the high-purity stainless steel raw material is heated to 1540℃ to melt the high-purity stainless steel raw material into molten steel. An electromagnetic stirrer with a frequency of 20 Hz is turned on.

[0083] High-purity carbon powder was added to the molten steel for carbon pre-deoxidation treatment. The holding time for the carbon pre-deoxidation treatment was 10 minutes to reduce the oxygen content of the molten steel to 35 ppm.

[0084] Maintain the vacuum level of the vacuum induction melting crucible and electromagnetic stirring. Add 0.05wt% of Mg-Ca composite deoxidizer to the molten steel for deep deoxidation and desulfurization treatment. The mass ratio of Mg to Ca in the Mg-Ca composite deoxidizer is 1:2. The holding time for deep deoxidation and desulfurization treatment is 12min.

[0085] Add 0.03wt% Ce-Mg composite modifier to molten steel for composite modification treatment. The holding time of composite modification treatment is 8min, so as to modify the inclusions in the molten steel into nano-plastic phase in situ.

[0086] The ingots that have undergone composite modification treatment are subjected to vacuum arc remelting. The vacuum degree of the vacuum arc remelting treatment is 0.1 Pa, so as to deeply remove gaseous impurities from the molten steel and homogenize the molten steel.

[0087] The ingots that have undergone vacuum arc remelting are heated to 1200℃ for isothermal homogenization forging. The reduction per pass in the isothermal homogenization forging process is 20%, and the total deformation is 60%, in order to break up residual inclusions in the ingots and promote the dispersion distribution of nano-plastic phases.

[0088] The billet, after being homogenized by isothermal forging, is subjected to heat treatment, which includes homogenization annealing and solution treatment. The homogenization annealing temperature is 1200℃ and the homogenization annealing time is 4h. The solution treatment temperature is 1060℃ and the solution treatment time is 1h. After solution treatment, the billet is water-cooled to stabilize the microstructure and distribution of nano-plastic phases.

[0089] Test results: Oxygen content = 7ppm, sulfur content = 1.6ppm, phosphorus content = 4ppm, inclusion size ≤ 860nm, fine series D inclusion rating = 0, tensile strength Rm = 562MPa, elongation after fracture A = 67%.

[0090] Example 2: Semiconductor-grade ultrapure 304L stainless steel High-purity stainless steel raw materials are obtained by precisely mixing industrial pure iron with a purity ≥99.96%, metallic chromium with a purity ≥99.98%, and metallic nickel with a purity ≥99.97% at Cr 18.0%, Ni 9.0%, and C ≤0.006%.

[0091] The surface of the high-purity stainless steel raw material is mechanically cleaned, and then the cleaned high-purity stainless steel raw material is vacuum dried at 200℃ for 2 hours to remove the oxide scale and adsorbed gas on the surface of the high-purity stainless steel raw material.

[0092] The high-purity stainless steel raw material, which has undergone vacuum drying pretreatment, is loaded into a vacuum induction melting crucible. The vacuum induction melting crucible is a MgO-CaO composite crucible. The vacuum induction melting crucible is evacuated to 4 Pa, and the high-purity stainless steel raw material is heated to 1530℃ to melt the high-purity stainless steel raw material into molten steel. An electromagnetic stirrer with a frequency of 18 Hz is turned on.

[0093] High-purity carbon powder was added to the molten steel for carbon pre-deoxidation treatment. The holding time for the carbon pre-deoxidation treatment was 11 minutes to reduce the oxygen content of the molten steel to 32 ppm.

[0094] Maintain the vacuum level of the vacuum induction melting crucible and electromagnetic stirring. Add 0.04wt% of Mg-Ca composite deoxidizer to the molten steel for deep deoxidation and desulfurization treatment. The mass ratio of Mg to Ca in the Mg-Ca composite deoxidizer is 1:2. The holding time for deep deoxidation and desulfurization treatment is 13min.

[0095] Add 0.025wt% Ce-Mg composite modifier to molten steel for composite modification treatment. The holding time of composite modification treatment is 7min, so as to modify the inclusions in the molten steel into nano-plastic phase in situ.

[0096] The molten steel that has undergone composite modification treatment is subjected to vacuum arc remelting. The vacuum degree of the vacuum arc remelting treatment is 0.08 Pa, so as to deeply remove gaseous impurities from the molten steel and homogenize the molten steel.

[0097] The ingots that have undergone vacuum arc remelting are heated to 1170°C for isothermal homogenization forging. The reduction per pass in the isothermal homogenization forging process is 20%, and the total deformation is 55%, in order to break up residual inclusions in the ingots and promote the dispersion distribution of nano-plastic phases.

[0098] The billet, after being homogenized by isothermal forging, is subjected to heat treatment, which includes homogenization annealing and solution treatment. The homogenization annealing temperature is 1200℃ and the homogenization annealing time is 4h. The solution treatment temperature is 1050℃ and the solution treatment time is 1h. After solution treatment, the billet is water-cooled to stabilize the microstructure and distribution of nano-plastic phases.

[0099] Test results: Oxygen content = 7ppm, sulfur content = 1.7ppm, phosphorus content = 4ppm, inclusion size ≤ 920nm, fine series D-class inclusion rating = 0, tensile strength Rm = 609MPa, elongation after fracture A = 73%.

[0100] Example 3: Semiconductor-grade ultrapure 439 ferritic stainless steel High-purity stainless steel raw materials are obtained by precisely mixing electrolytic iron with a purity ≥99.97% and metallic chromium with a purity ≥99.98% at Cr 17.5%, C ≤0.007%, and Ti ≤0.15%.

[0101] The surface of the high-purity stainless steel raw material is mechanically cleaned, and then the high-purity stainless steel raw material that has undergone surface mechanical cleaning is subjected to vacuum drying pretreatment to remove the oxide scale and adsorbed gas on the surface of the high-purity stainless steel raw material.

[0102] The high-purity stainless steel raw material, which has undergone vacuum drying pretreatment, is loaded into a vacuum induction melting crucible. The vacuum induction melting crucible is a MgO-CaO composite crucible. The vacuum induction melting crucible is evacuated to 2 Pa, and the high-purity stainless steel raw material is heated to 1550℃ to melt the high-purity stainless steel raw material into molten steel. An electromagnetic stirrer with a frequency of 22 Hz is turned on.

[0103] High-purity carbon powder was added to the molten steel for carbon pre-deoxidation treatment. The holding time for the carbon pre-deoxidation treatment was 9 minutes to reduce the oxygen content of the molten steel to 29 ppm.

[0104] Maintain the vacuum level of the vacuum induction melting crucible and perform electromagnetic stirring. Add 0.06wt% of Mg-Ca composite deoxidizer to the molten steel for deep deoxidation and desulfurization treatment. The mass ratio of Mg to Ca in the Mg-Ca composite deoxidizer is 1:2. The holding time for deep deoxidation and desulfurization treatment is 11min.

[0105] Add 0.03wt% Ce-Mg composite modifier to molten steel for composite modification treatment. The holding time of composite modification treatment is 6min, so as to modify the inclusions in the molten steel into nano-plastic phase in situ.

[0106] The molten steel that has undergone composite modification treatment is subjected to vacuum arc remelting. The vacuum degree of the vacuum arc remelting treatment is 0.05 Pa, so as to deeply remove gaseous impurities from the molten steel and homogenize the molten steel.

[0107] The ingots that have undergone vacuum consumable remelting are heated to 1150°C for isothermal homogenization forging. The reduction per pass in the isothermal homogenization forging process is 20%, and the total deformation is 50%, in order to break up residual inclusions in the ingots and promote the dispersion distribution of nano-plastic phases.

[0108] The billet, after being homogenized by isothermal forging, is subjected to heat treatment, which includes homogenization annealing and solution treatment. The homogenization annealing temperature is 1150℃ and the homogenization annealing time is 4 hours. The solution treatment temperature is 950℃ and the solution treatment time is 1 hour. After solution treatment, the billet is water-cooled to stabilize the microstructure and distribution of nano-plastic phases.

[0109] Test results: Oxygen content = 6 ppm, sulfur content = 1.2 ppm, phosphorus content = 3 ppm, inclusion size ≤ 790 nm, fine series D inclusion rating = 0.5, tensile strength Rm = 535 MPa, elongation after fracture A = 73%.

[0110] Example 4: Semiconductor-grade ultrapure 316L stainless steel High-purity stainless steel raw materials are obtained by precisely mixing industrial pure iron (purity ≥99.96%), metallic chromium (purity ≥99.98%), metallic nickel (purity ≥99.97%), and ferromolybdenum (purity ≥99.95%) with Cr 17.0%, Ni 14.5%, Mo 2.5%, and C ≤0.008%.

[0111] The surface of the high-purity stainless steel raw material is mechanically cleaned, and then the cleaned high-purity stainless steel raw material is vacuum dried at 180℃ for 2.5h to remove the oxide scale and adsorbed gas on the surface of the high-purity stainless steel raw material.

[0112] The high-purity stainless steel raw material, which has undergone vacuum drying pretreatment, is loaded into a vacuum induction melting crucible. The vacuum induction melting crucible is a MgO-CaO composite crucible. The vacuum induction melting crucible is evacuated to 2 Pa, and the high-purity stainless steel raw material is heated to 1530℃ to melt the high-purity stainless steel raw material into molten steel. An electromagnetic stirrer with a frequency of 18 Hz is turned on.

[0113] High-purity carbon powder was added to the molten steel for carbon pre-deoxidation treatment. The holding time for the carbon pre-deoxidation treatment was 9 minutes to reduce the oxygen content of the molten steel to 36 ppm.

[0114] Maintain the vacuum level of the vacuum induction melting crucible and perform electromagnetic stirring. Add 0.04wt% of Mg-Ca composite deoxidizer to the molten steel for deep deoxidation and desulfurization treatment. The mass ratio of Mg to Ca in the Mg-Ca composite deoxidizer is 1:2. The holding time for deep deoxidation and desulfurization treatment is 11min.

[0115] Add 0.025wt% Ce-Mg composite modifier to molten steel for composite modification treatment. The holding time of composite modification treatment is 6min, so as to modify the inclusions in the molten steel into nano-plastic phase in situ.

[0116] The molten steel that has undergone composite modification treatment is subjected to vacuum arc remelting. The vacuum degree of the vacuum arc remelting treatment is 0.1 Pa, so as to deeply remove gaseous impurities from the molten steel and homogenize the molten steel.

[0117] The ingots that have undergone vacuum arc remelting are heated to 1150°C for isothermal homogenization forging. The reduction per pass in the isothermal homogenization forging process is 15%, and the total deformation is 50%, in order to break up residual inclusions in the ingots and promote the dispersion distribution of nano-plastic phases.

[0118] The billet, after being homogenized by isothermal forging, is subjected to heat treatment, which includes homogenization annealing and solution treatment. The homogenization annealing temperature is 1150℃ and the homogenization annealing time is 3.5h. The solution treatment temperature is 950℃ and the solution treatment time is 0.8h. After solution treatment, the billet is water-cooled to stabilize the microstructure and distribution of nano-plastic phases.

[0119] Test results: Oxygen content = 8.2 ppm, sulfur content = 1.8 ppm, phosphorus content = 4.5 ppm, inclusion size ≤ 950 nm, fine series D-class inclusion rating = 0, tensile strength Rm = 595 MPa, elongation after fracture A = 74%.

[0120] Example 5: Semiconductor-grade ultrapure 304L stainless steel High-purity stainless steel raw materials are obtained by precisely mixing electrolytic iron with a purity ≥99.96%, metallic chromium with a purity ≥99.98%, and metallic nickel with a purity ≥99.97% at Cr 18.0%, Ni 9.0%, and C ≤0.006%.

[0121] The surface of the high-purity stainless steel raw material is mechanically cleaned, and then the cleaned high-purity stainless steel raw material is vacuum dried at 220℃ for 1.5h to remove the oxide scale and adsorbed gas on the surface of the high-purity stainless steel raw material.

[0122] The high-purity stainless steel raw material, which has undergone vacuum drying pretreatment, is loaded into a vacuum induction melting crucible. The vacuum induction melting crucible is a MgO-CaO composite crucible. The vacuum induction melting crucible is evacuated to 4 Pa, and the high-purity stainless steel raw material is heated to 1550℃ to melt the high-purity stainless steel raw material into molten steel. An electromagnetic stirrer with a frequency of 22 Hz is turned on.

[0123] High-purity carbon powder was added to the molten steel for carbon pre-deoxidation treatment. The holding time for the carbon pre-deoxidation treatment was 11 minutes to reduce the oxygen content of the molten steel to 37 ppm.

[0124] Maintain the vacuum level of the vacuum induction melting crucible and electromagnetic stirring, add 0.06wt% of Mg-Ca composite deoxidizer to the molten steel for deep deoxidation and desulfurization treatment. The mass ratio of Mg to Ca in the Mg-Ca composite deoxidizer is 1:2, and the holding time for deep deoxidation and desulfurization treatment is 13min.

[0125] Add 0.03wt% Ce-Mg composite modifier to molten steel for composite modification treatment. The holding time of composite modification treatment is 8min, so as to modify the inclusions in the molten steel into nano-plastic phase in situ.

[0126] The molten steel that has undergone composite modification treatment is subjected to vacuum arc remelting. The vacuum degree of the vacuum arc remelting treatment is 0.05 Pa, so as to deeply remove gaseous impurities from the molten steel and homogenize the molten steel.

[0127] The ingots that have undergone vacuum arc remelting are heated to 1200℃ for isothermal homogenization forging. The reduction per pass in the isothermal homogenization forging process is 25%, and the total deformation is 60%, in order to break up residual inclusions in the ingots and promote the dispersion distribution of nano-plastic phases.

[0128] The billet, after being homogenized by isothermal forging, is subjected to heat treatment, which includes homogenization annealing and solution treatment. The homogenization annealing temperature is 1200℃ and the homogenization annealing time is 4.5h. The solution treatment temperature is 1060℃ and the solution treatment time is 1.2h. After solution treatment, the billet is water-cooled to stabilize the microstructure and the distribution of nano-plastic phases.

[0129] Test results: Oxygen content = 7.5 ppm, sulfur content = 1.5 ppm, phosphorus content = 3.5 ppm, inclusion size ≤ 820 nm, fine series D inclusion rating = 0, tensile strength Rm = 585 MPa, elongation after fracture A = 78%.

[0130] Example 6: Semiconductor-grade ultrapure 439 ferritic stainless steel High-purity stainless steel raw materials are obtained by precisely mixing electrolytic iron with a purity ≥99.97% and metallic chromium with a purity ≥99.98% at Cr 17.5%, C ≤0.007%, and Ti ≤0.15%.

[0131] The surface of the high-purity stainless steel raw material is mechanically cleaned, and then the cleaned raw material is vacuum dried at 200℃ for 2 hours to remove the oxide scale and adsorbed gas on the surface of the high-purity stainless steel raw material.

[0132] The high-purity stainless steel raw material, which has undergone vacuum drying pretreatment, is loaded into a vacuum induction melting crucible. The vacuum induction melting crucible is a MgO-CaO composite crucible. The vacuum induction melting crucible is evacuated to 3 Pa, and the high-purity stainless steel raw material is heated to 1540℃ to melt the high-purity stainless steel raw material into molten steel. An electromagnetic stirrer with a frequency of 20 Hz is turned on.

[0133] High-purity carbon powder was added to the molten steel for carbon pre-deoxidation treatment. The holding time for the carbon pre-deoxidation treatment was 10 minutes to reduce the oxygen content of the molten steel to 38 ppm.

[0134] Maintain the vacuum level of the vacuum induction melting crucible and electromagnetic stirring. Add 0.05wt% of Mg-Ca composite deoxidizer to the molten steel for deep deoxidation and desulfurization treatment. The mass ratio of Mg to Ca in the Mg-Ca composite deoxidizer is 1:2. The holding time for deep deoxidation and desulfurization treatment is 12min.

[0135] Add 0.03wt% Ce-Mg composite modifier to molten steel for composite modification treatment. The holding time of composite modification treatment is 7min, so as to modify the inclusions in the molten steel into nano-plastic phase in situ.

[0136] The molten steel that has undergone composite modification treatment is subjected to vacuum arc remelting. The vacuum degree of the vacuum arc remelting treatment is 0.08 Pa, so as to deeply remove gaseous impurities from the molten steel and homogenize the molten steel.

[0137] The ingots that have undergone vacuum arc remelting are heated to 1170°C for isothermal homogenization forging. The reduction per pass in the isothermal homogenization forging process is 20%, and the total deformation is 55%, in order to break up residual inclusions in the ingots and promote the dispersion distribution of nano-plastic phases.

[0138] The billet, after being homogenized by isothermal forging, is subjected to heat treatment, which includes homogenization annealing and solution treatment. The homogenization annealing temperature is 1170℃ and the homogenization annealing time is 4h. The solution treatment temperature is 1000℃ and the solution treatment time is 1h. After solution treatment, the billet is water-cooled to stabilize the microstructure and distribution of nano-plastic phases.

[0139] Test results: Oxygen content = 8.0 ppm, sulfur content = 1.4 ppm, phosphorus content = 3.8 ppm, inclusion size ≤ 840 nm, fine series D-class inclusion rating = 0.5, tensile strength Rm = 540 MPa, elongation after fracture A = 75%.

[0140] Comparative Example 1: Traditional VIM+VD process The process employs a conventional industrial vacuum induction melting + vacuum degassing dual refining method, using industrial-grade stainless steel raw materials. The vacuum induction melting crucible is made of Al2O3, and the vacuum level of the environment in which the vacuum induction melting crucible is located is 10Pa-500Pa. Deoxidation is performed using a single aluminum deoxidizer, with weak stirring in argon gas. There is no Ce-Mg composite modification, no vacuum self-consumable remelting treatment, no isothermal homogenization forging treatment, and conventional ingot annealing.

[0141] Test results: Oxygen content = 46 ppm, sulfur content = 7.5 ppm, phosphorus content = 9 ppm, inclusion size > 5 μm, fine series D inclusion rating = 2.0, tensile strength Rm = 480 MPa, elongation after fracture A = 52%.

[0142] Comparative Example 2: Traditional Rare Earth End-Modification Process It adopts vacuum induction melting + vacuum degassing + single rare earth end addition, industrial grade stainless steel raw materials, no gradient deoxidation design, no Ce-Mg composite modification, no vacuum self-consuming remelting treatment, no isothermal homogenization forging treatment, only the single rare earth element is added at the end to modify the morphology of inclusions.

[0143] Test results: Oxygen content = 38ppm, sulfur content = 6ppm, phosphorus content = 8ppm, inclusions are prone to agglomeration and size > 2μm, fine series D-class inclusion rating = 1.5, tensile strength Rm = 495MPa, elongation after fracture A = 55%.

[0144] Comparative Example 3: Vacuum-free consumable remelting process High-purity stainless steel raw materials are obtained by precisely mixing electrolytic iron with a purity ≥99.96%, metallic chromium with a purity ≥99.98%, metallic nickel with a purity ≥99.97%, and ferromolybdenum with a purity ≥99.95% according to the following proportions: Cr 17.0%, Ni 14.5%, Mo 2.5%, and C ≤0.008%.

[0145] The surface of the high-purity stainless steel raw material is mechanically cleaned, and then the cleaned high-purity stainless steel raw material is vacuum dried at 200℃ for 2 hours to remove the oxide scale and adsorbed gas on the surface of the high-purity stainless steel raw material.

[0146] The high-purity stainless steel raw material, which has undergone vacuum drying pretreatment, is loaded into a vacuum induction melting crucible. The vacuum induction melting crucible is a MgO-CaO composite crucible. The vacuum induction melting crucible is evacuated to 3 Pa, and the high-purity stainless steel raw material is heated to 1540℃ to melt the high-purity stainless steel raw material into molten steel. An electromagnetic stirrer with a frequency of 20 Hz is turned on.

[0147] High-purity carbon powder is added to the molten steel for carbon pre-deoxidation treatment. The holding time of the carbon pre-deoxidation treatment is 10 minutes to reduce the oxygen content of the molten steel to 35ppm-40ppm. After the carbon pre-deoxidation treatment, the oxygen content of the molten steel is reduced to 70ppm.

[0148] Maintain the vacuum level of the vacuum induction melting crucible and electromagnetic stirring. Add 0.05wt% of Mg-Ca composite deoxidizer to the molten steel for deep deoxidation and desulfurization treatment. The mass ratio of Mg to Ca in the Mg-Ca composite deoxidizer is 1:2. The holding time for deep deoxidation and desulfurization treatment is 12min.

[0149] Add 0.03wt% Ce-Mg composite modifier to molten steel for composite modification treatment. The holding time of composite modification treatment is 8min, so as to modify the inclusions in the molten steel into nano-plastic phase in situ.

[0150] The molten steel that has undergone composite modification treatment is directly cast into ingots and then subjected to vacuum-free consumable remelting.

[0151] The ingot is heated to 1200℃ for isothermal homogenization forging. The reduction per pass in the isothermal homogenization forging process is 20%, and the total deformation is 60%, in order to break up residual inclusions in the ingot and promote the dispersion distribution of nano-plastic phase.

[0152] The billet, after being homogenized by isothermal forging, is subjected to heat treatment, which includes homogenization annealing and solution treatment. The homogenization annealing temperature is 1200℃ and the homogenization annealing time is 4h. The solution treatment temperature is 1060℃ and the solution treatment time is 1h. After solution treatment, the billet is water-cooled to stabilize the microstructure and distribution of nano-plastic phases.

[0153] Test results: Oxygen content = 18ppm, sulfur content = 3.5ppm, phosphorus content = 6ppm, inclusion size ≤ 1500nm, fine series D inclusion rating = 1.0, tensile strength Rm = 520MPa, elongation after fracture A = 65%.

[0154] Comparative Example 4: Forging process without isothermal homogenization treatment High-purity stainless steel raw materials are obtained by precisely mixing electrolytic iron with a purity ≥99.96%, metallic chromium with a purity ≥99.98%, metallic nickel with a purity ≥99.97%, and ferromolybdenum with a purity ≥99.95% according to the following proportions: Cr 17.0%, Ni 14.5%, Mo 2.5%, and C ≤0.008%.

[0155] The surface of the high-purity stainless steel raw material is mechanically cleaned, and then the cleaned high-purity stainless steel raw material is vacuum dried at 200℃ for 2 hours to remove the oxide scale and adsorbed gas on the surface of the high-purity stainless steel raw material.

[0156] The high-purity stainless steel raw material, which has undergone vacuum drying pretreatment, is loaded into a vacuum induction melting crucible. The vacuum induction melting crucible is a MgO-CaO composite crucible. The vacuum induction melting crucible is evacuated to 3 Pa, and the high-purity stainless steel raw material is heated to 1540℃ to melt the high-purity stainless steel raw material into molten steel. An electromagnetic stirrer with a frequency of 20 Hz is turned on.

[0157] High-purity carbon powder is added to the molten steel for carbon pre-deoxidation treatment. The holding time of the carbon pre-deoxidation treatment is 10 minutes to reduce the oxygen content of the molten steel to 35ppm-40ppm. After the carbon pre-deoxidation treatment, the oxygen content of the molten steel is reduced to 70ppm.

[0158] Maintain the vacuum level of the vacuum induction melting crucible and electromagnetic stirring. Add 0.05wt% of Mg-Ca composite deoxidizer to the molten steel for deep deoxidation and desulfurization treatment. The mass ratio of Mg to Ca in the Mg-Ca composite deoxidizer is 1:2. The holding time for deep deoxidation and desulfurization treatment is 12min.

[0159] Add 0.03wt% Ce-Mg composite modifier to molten steel for composite modification treatment. The holding time of composite modification treatment is 8min, so as to modify the inclusions in the molten steel into nano-plastic phase in situ.

[0160] The molten steel that has undergone composite modification treatment is subjected to vacuum arc remelting. The vacuum degree of the vacuum arc remelting treatment is 0.1 Pa, so as to deeply remove gaseous impurities from the molten steel and homogenize the molten steel.

[0161] The ingots that have undergone vacuum arc remelting are directly subjected to heat treatment without isothermal homogenization forging. The heat treatment includes homogenization annealing and solution treatment. The homogenization annealing temperature is 1200℃ and the homogenization annealing time is 4h. The solution treatment temperature is 1060℃ and the solution treatment time is 1h. After solution treatment, the billet is water-cooled to stabilize the billet's microstructure and the distribution of nano-plastic phases.

[0162] Test results: Oxygen content = 10ppm, sulfur content = 1.8ppm, phosphorus content = 4.5ppm, inclusion size ≤ 1800nm, fine series D inclusion rating = 1.0, tensile strength Rm = 540MPa, elongation after fracture A = 68%.

[0163] Comparative Example 5: Ce-Mg-free composite metamorphic treatment process High-purity stainless steel raw materials are obtained by precisely mixing electrolytic iron with a purity ≥99.96%, metallic chromium with a purity ≥99.98%, metallic nickel with a purity ≥99.97%, and ferromolybdenum with a purity ≥99.95% according to the following proportions: Cr 17.0%, Ni 14.5%, Mo 2.5%, and C ≤0.008%.

[0164] The surface of the high-purity stainless steel raw material is mechanically cleaned, and then the cleaned high-purity stainless steel raw material is vacuum dried at 200℃ for 2 hours to remove the oxide scale and adsorbed gas on the surface of the high-purity stainless steel raw material.

[0165] The high-purity stainless steel raw material, which has undergone vacuum drying pretreatment, is loaded into a vacuum induction melting crucible. The vacuum induction melting crucible is a MgO-CaO composite crucible. The vacuum induction melting crucible is evacuated to 3 Pa, and the high-purity stainless steel raw material is heated to 1540℃ to melt the high-purity stainless steel raw material into molten steel. An electromagnetic stirrer with a frequency of 20 Hz is turned on.

[0166] High-purity carbon powder is added to the molten steel for carbon pre-deoxidation treatment. The holding time of the carbon pre-deoxidation treatment is 10 minutes to reduce the oxygen content of the molten steel to 35ppm-40ppm. After the carbon pre-deoxidation treatment, the oxygen content of the molten steel is reduced to 70ppm.

[0167] Maintain the vacuum level of the vacuum induction melting crucible and electromagnetic stirring. Add 0.05wt% of Mg-Ca composite deoxidizer to the molten steel for deep deoxidation and desulfurization treatment. The mass ratio of Mg to Ca in the Mg-Ca composite deoxidizer is 1:2. The holding time for deep deoxidation and desulfurization treatment is 12min.

[0168] Without Ce-Mg composite modification treatment, the inclusions in the molten steel retain their original hard and brittle phases.

[0169] The molten steel that has undergone deep deoxidation and desulfurization treatment is subjected to vacuum arc remelting. The vacuum degree of the vacuum arc remelting treatment is 0.1 Pa, so as to deeply remove gaseous impurities from the molten steel and homogenize the molten steel.

[0170] The ingot, which has undergone vacuum consumable remelting, is heated to 1200℃ for isothermal homogenization forging. The reduction per pass in the isothermal homogenization forging process is 20%, and the total deformation is 60%, in order to break up the residual inclusions in the ingot.

[0171] The billet, after being homogenized by constant temperature forging, is subjected to heat treatment, which includes homogenization annealing and solution treatment. The homogenization annealing temperature is 1200℃ and the homogenization annealing time is 4 hours. The solution treatment temperature is 1060℃ and the solution treatment time is 1 hour. After solution treatment, the billet is water-cooled to stabilize the microstructure of the billet.

[0172] Test results: Oxygen content = 10ppm, sulfur content = 1.9ppm, phosphorus content = 4.5ppm, inclusion size ≤ 3000nm, fine series D inclusion rating = 1.5, tensile strength Rm = 505MPa, elongation after fracture A = 62%.

[0173] Comparative Example 6: Process without Electromagnetic Stirring High-purity stainless steel raw materials are obtained by precisely mixing electrolytic iron with a purity ≥99.96%, metallic chromium with a purity ≥99.98%, metallic nickel with a purity ≥99.97%, and ferromolybdenum with a purity ≥99.95% according to the following proportions: Cr 17.0%, Ni 14.5%, Mo 2.5%, and C ≤0.008%.

[0174] The surface of the high-purity stainless steel raw material is mechanically cleaned, and then the cleaned raw material is vacuum dried at 200℃ for 2 hours to remove the oxide scale and adsorbed gas on the surface of the high-purity stainless steel raw material.

[0175] High-purity stainless steel raw materials that have undergone vacuum drying pretreatment are loaded into a vacuum induction melting crucible. The vacuum induction melting crucible is a MgO-CaO composite crucible. The vacuum induction melting crucible is evacuated to 3 Pa, and the high-purity stainless steel raw materials are heated to 1540℃ to melt the high-purity stainless steel raw materials into molten steel without electromagnetic stirring.

[0176] High-purity carbon powder is added to the molten steel for carbon pre-deoxidation treatment. The holding time of the carbon pre-deoxidation treatment is 10 minutes to reduce the oxygen content of the molten steel to 35ppm-40ppm. After the carbon pre-deoxidation treatment, the oxygen content of the molten steel is reduced to 70ppm.

[0177] Maintain the vacuum level of the vacuum induction melting crucible, add 0.05wt% of Mg-Ca composite deoxidizer to the molten steel for deep deoxidation and desulfurization treatment. The mass ratio of Mg to Ca in the Mg-Ca composite deoxidizer is 1:2, and the holding time for deep deoxidation and desulfurization treatment is 12min.

[0178] Add 0.03wt% Ce-Mg composite modifier to molten steel for composite modification treatment. The holding time of composite modification treatment is 8min, so as to modify the inclusions in the molten steel into nano-plastic phase in situ.

[0179] The molten steel that has undergone composite modification treatment is subjected to vacuum arc remelting. The vacuum degree of the vacuum arc remelting treatment is 0.1 Pa, so as to deeply remove gaseous impurities from the molten steel and homogenize the molten steel.

[0180] The ingots that have undergone vacuum arc remelting are heated to 1200℃ for isothermal homogenization forging. The reduction per pass in the isothermal homogenization forging process is 20%, and the total deformation is 60%, in order to break up residual inclusions in the ingots and promote the dispersion distribution of nano-plastic phases.

[0181] The billet, after being homogenized by isothermal forging, is subjected to heat treatment, which includes homogenization annealing and solution treatment. The homogenization annealing temperature is 1200℃ and the homogenization annealing time is 4h. The solution treatment temperature is 1060℃ and the solution treatment time is 1h. After solution treatment, the billet is water-cooled to stabilize the microstructure and distribution of nano-plastic phases.

[0182] Test results: Oxygen content = 15ppm, sulfur content = 3.0ppm, phosphorus content = 5.5ppm, inclusion size ≤ 1200nm, fine series D-class inclusion rating = 0.5, tensile strength Rm = 560MPa, elongation after fracture A = 70%.

[0183] Experimental methods for evaluating results: 1. Oxygen content detection method The oxygen content in semiconductor-grade ultrapure stainless steel was determined using an inert gas melting-infrared absorption method. The semiconductor-grade ultrapure stainless steel sample was placed in a graphite crucible and melted at high temperature under inert gas protection. Oxygen in the sample reacted with carbon to form carbon monoxide, which was then quantitatively determined using an infrared detector. The results are expressed in ppm.

[0184] 2. Sulfur content detection method The sulfur content in semiconductor-grade ultrapure stainless steel was determined using a high-frequency combustion-infrared absorption method. The semiconductor-grade ultrapure stainless steel sample was burned in an oxygen atmosphere in a high-frequency induction furnace, where sulfur was oxidized to sulfur dioxide. The sulfur content was then quantitatively determined using an infrared detector, and the results are expressed in ppm.

[0185] 3. Phosphorus content detection method The phosphorus content in semiconductor-grade ultrapure stainless steel was determined by spectrophotometry. After dissolving the semiconductor-grade ultrapure stainless steel sample in acid, the absorbance of the phosphomolybdenum blue complex was measured at a specific wavelength. The results were quantitatively calculated using a standard curve and expressed in ppm.

[0186] 4. Methods for detecting inclusion size The size of inclusions in semiconductor-grade ultrapure stainless steel was determined using scanning electron microscopy combined with energy dispersive spectroscopy (EDS). After metallographic preparation, the inclusion morphology was observed and the maximum cross-sectional size was measured under a scanning electron microscope. The inclusion composition was determined using EDS, and the results are expressed in nm.

[0187] 5. Detection Method for Class D Inclusions in Fine-Grained Systems According to the fine series D (spherical oxide) rating standard in GB / T 10561-2005 "Determination of Non-metallic Inclusions in Steel - Standard Rating Chart Microscopic Examination Method", the semiconductor-grade ultrapure stainless steel sample was examined by metallographic microscopy, and the level was rated by comparison with the standard chart.

[0188] 6. Method for testing tensile strength Rm According to GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Room temperature test method", a room temperature tensile test was conducted on a standard tensile specimen of semiconductor grade ultrapure stainless steel to determine the tensile strength corresponding to the maximum force. The results are expressed in MPa.

[0189] 7. Method for testing elongation at fracture (A) According to GB / T 228.1-2021 "Metallic materials - Tensile testing - Part 1: Test at room temperature", a room temperature tensile test was conducted on a standard tensile specimen of semiconductor-grade ultrapure stainless steel, and the percentage of permanent elongation of the gauge length after fracture was determined. The results are expressed as %.

[0190] As can be seen from the examples and comparative data, the technical advancements of this application include: The oxygen and sulfur contents were significantly reduced: the oxygen content in Examples 1-3 and Examples 4-6 was 7.5ppm-9ppm and the sulfur content was 1.2ppm-1.8ppm; the oxygen content in Comparative Example 1 was 46ppm and the sulfur content was 7.5ppm; the oxygen content in Comparative Example 2 was 38ppm and the sulfur content was 6ppm; the oxygen content in Comparative Example 3 was 18ppm and the sulfur content was 3.5ppm; the oxygen content in Comparative Example 5 was 10ppm and the sulfur content was 1.9ppm; and the oxygen content in Comparative Example 6 was 15ppm and the sulfur content was 3.0ppm. The inclusion size is significantly refined and is controllable at the nanometer level: the inclusion size of Examples 1-3 and Examples 4-6 is ≤790nm-≤950nm, the inclusion size of Comparative Example 1 is >5μm, the inclusion size of Comparative Example 2 is >2μm, the inclusion size of Comparative Example 3 is ≤1500nm, the inclusion size of Comparative Example 4 is ≤1800nm, the inclusion size of Comparative Example 5 is ≤3000nm, and the inclusion size of Comparative Example 6 is ≤1200nm; The fine D-type inclusion rating was significantly improved: the fine D-type inclusion ratings of Examples 1-3 and Examples 4-6 were 0-0.5, the fine D-type inclusion rating of Comparative Example 1 was 2.0, the fine D-type inclusion rating of Comparative Example 2 was 1.5, the fine D-type inclusion rating of Comparative Example 3 was 1.0, the fine D-type inclusion rating of Comparative Example 4 was 1.0, and the fine D-type inclusion rating of Comparative Example 5 was 1.5. Overall improvement in mechanical properties: The tensile strength Rm of Examples 1-3 and Examples 4-6 is 535MPa-610MPa and the elongation after fracture A is 73%-79%. The tensile strength Rm of Comparative Example 1 is 480MPa and the elongation after fracture A is 52%. The tensile strength Rm of Comparative Example 2 is 495MPa and the elongation after fracture A is 55%.

[0191] Detailed explanation of the attached diagram: Figure 1 This is a SEM image showing the inclusion size of the semiconductor-grade ultrapure stainless steel prepared by the method described in Example 1 of this application. The SEM image shows a single inclusion in the semiconductor-grade ultrapure stainless steel matrix, with a measured size of 0.851 μm, or 851 nm. The size of this inclusion is ≤1000 nm, satisfying the requirement of inclusion size ≤1000 nm in claim 10. The scale bar of this SEM image is 5 μm, the magnification is ×4000, the accelerating voltage is 23 kV, and the working distance is 11 mm. As shown in this SEM image, the inclusions in the semiconductor-grade ultrapure stainless steel prepared by the method described in Example 1 of this application are isolated and dispersed, without agglomeration, and the inclusion size is at the nanometer level. This verifies the synergistic effect of carbon pre-deoxidation treatment, deep deoxidation and desulfurization treatment, composite modification treatment, vacuum self-consumable remelting treatment, isothermal homogenization forging treatment, and heat treatment in Example 1 of this application on the regulation of nano-inclusions.

[0192] Figure 2 Scanning electron micrograph (×500) of conventional stainless steel in Comparative Example 1 of this application. It can be clearly seen from the figure that there are a large number of large-sized blocky and irregular hard and brittle inclusions in the steel matrix. The inclusions are concentrated and their size is significantly larger than 2 μm. The purity of the matrix is ​​significantly different from that of Example 1.

[0193] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing semiconductor-grade ultrapure stainless steel, characterized in that, The method includes: High-purity stainless steel raw material is loaded into a vacuum induction melting crucible, the vacuum induction melting crucible is evacuated and the high-purity stainless steel raw material is heated to melt the high-purity stainless steel raw material into molten steel. The molten steel is subjected to carbon pre-deoxidation treatment to reduce the oxygen content of the molten steel to 35ppm-40ppm; A Mg-Ca composite deoxidizer is added to the molten steel for deep deoxidation and desulfurization treatment; Ce-Mg composite modifier is added to the molten steel for composite modification treatment, so as to modify the inclusions in the molten steel into nano-plastic phase in situ; The molten steel that has undergone the composite modification treatment is made into a vacuum induction casting ingot, and the vacuum induction casting ingot is subjected to vacuum self-consumption remelting treatment. The vacuum degree of the vacuum self-consumption remelting treatment is ≤0.1Pa, so as to deeply remove gaseous impurities from the molten steel and homogenize the molten steel. The ingot that has undergone the vacuum self-consumable remelting process is subjected to isothermal homogenization forging to break the residual inclusions in the ingot and promote the dispersion distribution of the nano-plastic phase. The billet that has undergone the isothermal homogenization forging process is then subjected to heat treatment to stabilize the microstructure of the billet and the distribution of the nano-plastic phase.

2. The preparation method according to claim 1, characterized in that, The purity of the high-purity stainless steel raw material is ≥99.95%, and the high-purity stainless steel raw material includes at least one of high-purity iron, high-purity chromium, metallic nickel and metallic molybdenum. The high-purity stainless steel raw material undergoes vacuum drying pretreatment before being loaded into the vacuum induction melting crucible. The temperature of the vacuum drying pretreatment is 180℃-220℃, and the time of the vacuum drying pretreatment is 1.5h-2.5h.

3. The preparation method according to claim 1, characterized in that, The vacuum induction melting crucible is a MgO-CaO composite crucible; The vacuum degree of the environment in which the vacuum induction melting crucible is located is 2Pa-4Pa, and the temperature of the molten steel after melting is 1530℃-1550℃.

4. The preparation method according to claim 1, characterized in that, The carbon pre-deoxidation treatment includes adding high-purity carbon powder to the molten steel, and the holding time of the carbon pre-deoxidation treatment is 8 min-12 min.

5. The preparation method according to claim 1, characterized in that, The deep deoxidation and desulfurization treatment includes adding 0.04wt%-0.06wt% of Mg-Ca composite deoxidizer to the molten steel, wherein the mass ratio of Mg to Ca in the Mg-Ca composite deoxidizer is 1:2, and the holding time of the deep deoxidation and desulfurization treatment is 10min-15min.

6. The preparation method according to claim 1, characterized in that, The composite modification treatment includes adding 0.020wt%-0.030wt% of Ce-Mg composite modifier to the molten steel, and the holding time of the composite modification treatment is 6min-8min.

7. The preparation method according to claim 1, characterized in that, The isothermal homogenization forging process includes heating the ingot to 1150℃-1200℃ for isothermal forging, the reduction per pass of the isothermal homogenization forging process is 15%-25%, and the total deformation of the isothermal homogenization forging process is 50%-60%.

8. The preparation method according to claim 1, characterized in that, The heat treatment includes homogenization annealing and solution treatment; The homogenization annealing treatment is performed at a temperature of 1150℃-1200℃ for 3.5h-4.5h. The solution treatment temperature is 950℃-1060℃, the solution treatment time is 0.5h-1.2h, and the billet is water-cooled after the solution treatment.

9. The preparation method according to claim 1, characterized in that, In at least one of the carbon pre-deoxidation treatment, the deep deoxidation and desulfurization treatment, and the composite modification treatment, electromagnetic stirring is applied to the molten steel at a frequency of 18Hz-22Hz.

10. The preparation method according to claim 1, characterized in that, The semiconductor-grade ultrapure stainless steel has an oxygen content ≤8ppm, a sulfur content ≤2ppm, a phosphorus content ≤5ppm, an inclusion size ≤1000nm, and a fine D-class inclusion rating ≤0.5.

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