Ultra-pure 316l austenitic stainless steel for semiconductor manufacturing device and preparation method and application thereof

Abstract

This invention discloses an ultra-pure 316L austenitic stainless steel for semiconductor manufacturing equipment, its preparation method, and its applications, belonging to the field of special alloy smelting technology. The chemical composition of this stainless steel includes: Cr: 17.5%~18.0%; Mo: 2.5%~3.0%; Ni: 13.5%~14.0%; S ≤ 0.003%; Al ≤ 0.01%; the balance being Fe and unavoidable impurities. The preparation method employs a triple process of vacuum induction melting, electroslag remelting, and vacuum arc remelting. Vacuum induction melting uses low-sulfur pure iron; electroslag remelting uses a completely aluminum-free slag system; vacuum arc remelting achieves deep deoxidation without residue in a high vacuum; subsequently, it undergoes high-temperature diffusion treatment, forging, and hot rolling. This invention completely eliminates Class B alumina inclusions and δ-ferrite, has an oxygen content of less than 5 ppm, a finer austenitic grain size than ASTM Grade 8, and achieves a mirror finish through electrolytic polishing, meeting the requirements for semiconductor applications.

Description

Technical Field

[0001] This invention belongs to the field of special alloy smelting and metal material processing technology, specifically relating to an ultra-pure 316L austenitic stainless steel for semiconductor manufacturing equipment, its preparation method and application. Background Technology

[0002] In semiconductor manufacturing processes, including integrated circuits, panel displays, and photovoltaics, high-purity specialty gas delivery systems (such as pipes, valves, connectors, and cavities) place extremely stringent requirements on the base materials. These materials not only need to possess extremely high purity and excellent corrosion resistance, but also require their internal surfaces to achieve an exceptionally smooth finish through electropolishing (EP) to prevent the release of impurity gases and the shedding of particles. 316L austenitic stainless steel is the most widely used core base material in this field. The SEMI F20-0706E standard, "Specifications for General Purity, High Purity, and Ultra-High Purity 316L Stainless Steel for Semiconductor Manufacturing Applications," specifies the following chemical composition requirements: C≤0.03%, Si≤1.00%, Mn≤1.50%, P≤0.045%, S≤0.01%, Ni:10.0-14.0%, Cr:16.0-18.0%, Mo:2.0-3.0%, Al≤0.01%.

[0003] However, the production of conventional 316L stainless steel faces the following technical bottlenecks. Regarding non-metallic inclusions, conventional steelmaking processes often employ aluminum deoxidation, which inevitably produces trace amounts of hard non-metallic inclusions such as alumina (Al2O3) (Class B inclusions). Because 316L stainless steel contains high levels of chromium, nickel, and molybdenum, the molten steel has a high viscosity, hindering the flotation of inclusions and increasing the difficulty of controlling the purity of the molten steel. The purity of the molten steel directly affects the surface quality of the subsequent electropolished products. During electropolishing, due to the different dissolution rates of inclusions and the metal matrix, hard inclusions are easily detached, leading to "micro-pit" defects on the polished surface and becoming potential sources of contamination. Furthermore, as an austenitic stainless steel, 316L is prone to producing trace amounts of δ-ferrite during solidification due to solidification segregation. δ-ferrite not only reduces the material's corrosion resistance but also causes surface inhomogeneity during electropolishing due to the difference in electrochemical potential between it and the austenitic matrix.

[0004] In existing technologies, patent documents have proposed a dual process of "vacuum induction melting (VIM) + vacuum arc remelting (VAR)" to prepare high-purity 316L stainless steel, such as Chinese patents with application numbers CN202410375876.8 and CN202310904405.7. Although the above-mentioned dual process can reduce the oxygen content to a low level, it has been found in actual industrial production and semiconductor component manufacturing that this process still has a fatal flaw: in the vacuum induction melting (VIM) stage, the high-temperature molten steel can cause a small amount of refractory material to fall off the crucible, or occasional small amounts of slag to be added, which can easily introduce large particles of foreign non-metallic inclusions into the steel ingot. Pure vacuum arc remelting (VAR) processes are primarily effective at degassing and removing tiny inclusions, but they are extremely ineffective at removing large foreign inclusions like these. More seriously, these sporadic large inclusions are often difficult to detect during routine metallographic sampling inspections, exhibiting a high degree of randomness. Only when the stainless steel is finally processed into semiconductor components or valves and undergoes extremely rigorous internal surface electropolishing (EP) will these large inclusions be exposed and detached, directly causing huge pits on the polished surface. This "hidden damage" leads to an extremely high scrap rate for end products, a technical problem that semiconductor material manufacturers urgently need to solve.

[0005] To address the aforementioned challenge of removing large particle inclusions, those skilled in the art might readily conceive of a feasible solution: adding an electroslag remelting (ESR) process to the existing dual-process of "vacuum induction (VIM) + vacuum self-consumption (VAR)". This ESR process would leverage its powerful ability to absorb and remove large particle inclusions, ensuring that the levels remain within acceptable limits. However, this seemingly straightforward solution has proven problematic in practice, creating a vicious cycle of contradictions and unforeseen consequences. Traditional electroslag remelting (ESR) processes often employ aluminum-containing slag systems (such as ternary slag CaF2-Al2O3-CaO). These slag systems offer excellent adsorption of inclusions and good process stability. However, the following reaction inevitably occurs between the molten steel and the remelted slag during ESR, increasing the aluminum content: [Si] + (Al2O3) = (SiO2) + [Al] (where [Si] and [Al] represent silicon and aluminum in the steel, and (Al2O3) and (SiO2) represent aluminum oxide and silicon dioxide in the slag), resulting in an Al content exceeding the 0.01% standard. Simply using non-aluminum slag systems, on the other hand, leads to the remelted slag losing its desulfurization capacity, resulting in an increase in sulfur content in the material, ultimately reducing its thermoplasticity and causing excessive levels of Class A inclusions.

[0006] In addition, in order to improve the pitting corrosion resistance of materials, simply controlling elements such as Cr and Mo at high levels will promote the production of a large amount of δ ferrite during the solidification process, as Cr and Mo are strong ferrite-forming elements.

[0007] In the subsequent hot working process, there is also a technical contradiction: in order to fully homogenize the alloying elements and completely dissolve the δ-ferrite generated during solidification, the steel ingot needs to be subjected to high-temperature and long-term diffusion treatment. However, those skilled in the art know that excessively high heating temperature and excessively long holding time will lead to abnormal growth of austenite grains, thereby deteriorating the mechanical properties and corrosion resistance of the material.

[0008] Therefore, there is an urgent need to develop an ultra-pure 316L stainless steel for semiconductor manufacturing equipment with extremely low inclusions, no δ-ferrite, fine grains, and excellent corrosion resistance, as well as its supporting preparation process. Summary of the Invention

[0009] To overcome the aforementioned shortcomings of existing technologies, this invention provides an ultra-pure 316L austenitic stainless steel for semiconductor manufacturing equipment, its preparation method, and its applications. This invention achieves ultra-pure stainless steel material with extremely low inclusion content, complete elimination of δ-ferrite, and meeting the requirements for electrolytic polishing mirror surfaces in the semiconductor industry through extreme compositional control and the synergistic matching of a triple melting process of "vacuum induction + electroslag remelting + vacuum self-consumption" and specific heat treatment processes.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: An ultrapure 316L austenitic stainless steel for semiconductor manufacturing equipment, the chemical composition of which, by weight percentage, comprises the following: C ≤ 0.03%; Si ≤ 1.0%; Mn ≤ 1.5%; Cr: 17.5% ~ 18.0%; Mo: 2.5% ~ 3.0%; Ni: 13.5% ~ 14.0%; P ≤ 0.045%; S ≤ 0.003%; Al ≤ 0.01%; balance is Fe and unavoidable impurities.

[0011] Preferably, in the stainless steel, O ≤ 0.0005% and N: 0.02%~0.05%.

[0012] Preferably, the δ-ferrite content in the microstructure of the stainless steel is 0%. The austenite grain size of the stainless steel is finer than or equal to ASTM grade 8, and the non-metallic inclusions of type A, type C, and type D do not exceed grade 0.5, and it does not contain type B non-metallic inclusions (the grade of type B inclusions is 0).

[0013] Preferably, the stainless steel has a room temperature tensile strength ≥500MPa, a yield strength ≥200MPa, an elongation after fracture ≥50%, and a reduction of area ≥60%, so as to fully meet the processing and forming requirements of semiconductor applications.

[0014] The present invention also provides a method for preparing the above-mentioned ultrapure 316L austenitic stainless steel material for semiconductor manufacturing apparatus, comprising the following steps: Step 1: Vacuum Induction Melting (VIM). Low-sulfur pure iron with a sulfur content of ≤0.002% is used as raw material for vacuum induction melting. Target components are precisely controlled, and no aluminum is added throughout the process, ensuring that the molten steel has S ≤0.003% and Al ≤0.01%, which is then cast into vacuum induction electrode rods. By forcibly using extremely high-purity low-sulfur pure iron in the vacuum induction melting stage, the desulfurization process is brought forward, effectively compensating for any potential lack of desulfurization capacity in subsequent processes.

[0015] Step 2: Electroslag Remelting (ESR). The vacuum induction electrode rod obtained in Step 1 is subjected to electroslag remelting. An aluminum-free slag system is used during the remelting process to remove large non-metallic inclusions from the steel ingot, resulting in an electroslag remelted ingot. This slag system fully utilizes the efficient washing and interception of inclusions by the electroslag remelting process while avoiding the aluminum increase problem in the molten steel caused by conventional aluminum-containing slag systems, thus preventing excessive Al content.

[0016] Preferably, the aluminum-free slag system is a CaF2-CaO-MgO-SiO2 system completely free of Al2O3. More preferably, the composition of this aluminum-free slag system, by weight percentage, is 55%–65% CaF2, 15%–25% CaO, 5%–10% MgO, and 10%–20% SiO2, with the Al2O3 content strictly controlled to not exceed 0.5%. Even more preferably, it is a 60% CaF2-15% CaO-5% MgO-20% SiO2 aluminum-free slag system. In conventional ESR slag systems, Al2O3 is an important component, typically containing 15%–25%, playing a role in regulating the conductivity and fluidity of the slag system. Removing Al2O3 will lead to significant changes in the conductivity and viscosity of the slag system, making it difficult to stabilize the remelting process. This invention overcomes the industry's technical prejudice that aluminum-free slag systems have poor stability in electroslag remelting applications by using SiO2 to replace Al2O3 to adjust the high-temperature physicochemical properties of the slag system and optimizing the ratio of the four components CaF2, CaO, MgO and SiO2.

[0017] Step 3: Vacuum Arsenic Remelting (VAR). The electroslag remelted ingot obtained in Step 2 is used as an electrode for vacuum arc remelting. The vacuum level is controlled below 0.5 Pa. Carbon is used for deep deoxidation, and fine particulate inclusions are removed, resulting in an ultra-pure vacuum arc remelted ingot. In this step, the high vacuum environment promotes the reaction of trace carbon in the molten steel with oxygen (C + O → CO↑). The generated CO gas is extracted by a vacuum pump, thus replacing the traditional aluminum deoxidation process. This achieves deep deoxidation without solid residues, ensuring thorough deoxidation of the molten steel while removing aluminum as a deoxidizing agent.

[0018] It is important to note that the carbon content in the molten steel is only ≤0.03%, which falls within the low-carbon range. There is a common misconception that the carbon-oxygen reaction is insufficient for effective deep deoxidation under low carbon content conditions, often necessitating the use of strong deoxidizers such as aluminum and titanium. This invention, by strictly controlling the vacuum level to below 0.5 Pa in an extremely high vacuum environment, strongly pushes the carbon-oxygen reaction equilibrium towards CO gas generation. This allows even trace amounts of carbon (0.01%–0.03%) to remove oxygen to below 5 ppm. By eliminating aluminum as a deoxidizer, this method ensures thorough deoxidation of the molten steel while avoiding the formation of new non-metallic inclusions due to the addition of deoxidizers.

[0019] Step 4: High-Temperature Diffusion Treatment. The vacuum consumable ingot obtained in Step 3 is placed in a heating furnace and subjected to a heat treatment diffusion process at a temperature of 1200℃~1250℃ for no less than 48 hours. This temperature is much higher than the conventional annealing temperature of stainless steel (usually 1050℃~1100℃), which can provide sufficient diffusion activation energy to fully homogenize the alloying elements in the austenitic matrix and completely dissolve the remaining δ-ferrite.

[0020] Preferably, after the high-temperature diffusion treatment is completed, the furnace can be removed and air-cooled or water-cooled.

[0021] Step 5: Forging and Billet Preparation. The vacuum consumable ingot, after high-temperature diffusion treatment, is hot-forged into a billet with a cross-sectional dimension of not less than 260mm × 260mm. This ensures sufficient deformation for subsequent rolling. Although the austenite grains tend to coarsen after extreme high-temperature diffusion treatment, the large-section billet can provide sufficient recrystallization driving force during large-deformation rolling, promoting sufficient grain refinement and homogenization.

[0022] Preferably, the initial forging temperature of the billet forging is 1150℃~1180℃, and the final forging temperature is not lower than 900℃.

[0023] Step Six: Hot Rolling. The forged billet is heated and then rolled, with the rolling heating temperature controlled between 1100℃ and 1150℃, ultimately forming the finished product. The large cross-section of the forged billet provides sufficient rolling deformation, and combined with strict control of the rolling heating temperature, it effectively breaks up the as-cast structure and inhibits abnormal grain growth, ensuring the fineness of the grain size in the final product.

[0024] Preferably, the total rolling deformation is not less than 70% (preferably 70% to 90%), and the deformation per pass is controlled at 10% to 20%.

[0025] The present invention also provides an application of the above-mentioned ultrapure 316L austenitic stainless steel for semiconductors in the field of semiconductors and their preparation.

[0026] Compared with existing technologies, the beneficial effects of this invention are as follows: This invention provides ultra-pure 316L austenitic stainless steel for semiconductor manufacturing equipment and its preparation method. By designing Cr and Mo to the upper limit of the standard, the corrosion resistance of the material is improved. Ni is designed to the upper limit to suppress the formation of δ-ferrite. At the same time, Al≤0.01% and S≤0.003% eliminates Class B alumina and Class A sulfide inclusions from the source. The inclusions of Class A, B, and C in the product are all Class 0. Furthermore, addressing the defect that the VIM+VAR dual-process cannot remove occasional large-particle inclusions in the VIM stage, an innovative "VIM+ESR+VAR" triple-process is constructed. Combined with a specially formulated aluminum-free slag system, aluminum content is avoided from exceeding the standard. Low-sulfur raw materials are used, and high-vacuum carbon-oxygen reaction deoxidation in VAR effectively controls inclusions. Further, high-temperature limiting diffusion completely dissolves δ-ferrite. Combined with large-section forging billets and low-temperature rolling, the contradiction between homogenization and grain refinement is resolved, resulting in a product grain size finer than ASTM Class 8. The product of this invention has a mirror-like surface after semiconductor-grade electrolytic polishing, without micro-pits or corrosion grooves, meeting the ultra-pure requirements of high-purity special gas delivery systems and having broad market application prospects. Attached Figure Description

[0027] Figure 1 This is a schematic flowchart of the method for preparing stainless steel according to the present invention. Detailed Implementation

[0028] To better clarify and understand the objectives, process solutions, and advantages of this invention, the technical solutions and implementation methods of this invention will be further described clearly, completely, and in detail below through specific embodiments and in conjunction with the accompanying drawings. It should be understood that the embodiments described in this invention are implemented under the premise of the technical solutions of this invention, providing detailed implementation methods and specific operating procedures, but are only some embodiments of this invention, not all embodiments. The specific implementation methods described are limited to illustrating and explaining this invention and do not limit this invention. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0029] Unless otherwise specified, the experimental methods and conditions used in the embodiments of this invention are conventional methods and conditions. The materials, reagents, instruments, and equipment used in the embodiments, unless otherwise specified, are all conventional substances or equipment known to those skilled in the art and can be obtained commercially or prepared by conventional methods. The reaction conditions described in the invention's content are all capable of achieving the reactions and obtaining the desired products.

[0030] This invention provides an ultra-pure 316L austenitic stainless steel for semiconductor manufacturing equipment, the chemical composition of which, by weight percentage, includes the following: C ≤ 0.03%; Si ≤ 1.0%; Mn ≤ 1.50%; Cr: 17.5% ~ 18.0%; Mo: 2.5% ~ 3.0%; Ni: 13.5% ~ 14.0%; P ≤ 0.045%; S ≤ 0.003%; Al ≤ 0.01%; the balance being Fe and unavoidable impurities.

[0031] In terms of composition design, this invention designs Cr and Mo at the upper limit of the standard range, significantly improving the corrosion resistance of the material; at the same time, Ni is designed at the upper limit of the standard range to expand the austenite phase region and suppress the formation of δ-ferrite from the composition source. This extreme fine-tuning of composition lays the thermodynamic foundation for obtaining a final microstructure free of δ-ferrite in subsequent processes.

[0032] In one specific embodiment of the present invention, in the stainless steel, O ≤ 0.0005% and N: 0.02~0.05%.

[0033] In one specific embodiment of the present invention, the δ-ferrite content in the microstructure of the stainless steel is 0%. More preferably, the austenite grain size of the stainless steel is finer than or equal to ASTM grade 8, and the non-metallic inclusions of type A, type C, and type D do not exceed grade 0.5, and it does not contain type B non-metallic inclusions (the grade of type B inclusions is 0).

[0034] In one specific embodiment of the present invention, the stainless steel has a room temperature tensile strength ≥500MPa, a yield strength ≥200MPa, an elongation after fracture ≥50%, and a reduction of area ≥60%, so as to fully meet the processing and forming requirements of semiconductor applications.

[0035] The present invention also provides a method for preparing the above-mentioned ultrapure 316L austenitic stainless steel material for semiconductors, comprising the following steps: Step 1: Vacuum Induction Melting (VIM). Low-sulfur pure iron with a sulfur content of ≤0.002% is used as raw material for vacuum induction melting. Target components are precisely controlled, and no aluminum is added throughout the process, ensuring that the molten steel has S ≤0.003% and Al ≤0.01%, which is then cast into vacuum induction electrode rods. By forcibly using extremely high-purity low-sulfur pure iron in the vacuum induction melting stage, the desulfurization process is brought forward, effectively compensating for any potential lack of desulfurization capacity in subsequent processes.

[0036] In addition to the specific requirements mentioned above, the other steps of the VIM process are known to those skilled in the art, and the operating process can be selected according to the actual situation. Preferably, but not limited to, the specific implementation of "precise control of each target component" can be referred to as follows: the raw materials are accurately weighed and batched before entering the furnace, and the charging amount is calculated according to the target content of each alloying element and the burn-off rate during the melting process; during the melting process, a batch feeding method is adopted, and low-sulfur pure iron, metallic chromium, pure nickel plate and molybdenum bar are added in sequence. After the furnace charge is completely melted, a sample of molten steel is taken for spectral analysis. According to the analysis results, an aluminum-free intermediate alloy or pure metal is used for micro-adjustment to ensure that the content of each element is within the target range; during the refining period, the melting temperature is controlled at 1550℃~1600℃, and the refining time is maintained for no less than 30 minutes under the condition of vacuum degree higher than 3Pa to fully remove the gas and volatile impurities in the molten steel; Preferred, but not limited to, casting temperature should be controlled between 1520℃ and 1560℃.

[0037] Step 2: Electroslag Remelting (ESR). The vacuum induction electrode rod obtained in Step 1 is subjected to electroslag remelting. An aluminum-free slag system is used during the remelting process to remove large non-metallic inclusions from the steel ingot, resulting in an electroslag remelted ingot. This slag system fully utilizes the efficient washing and interception of inclusions by the electroslag remelting process while avoiding the aluminum increase problem in the molten steel caused by conventional aluminum-containing slag systems, thus preventing the aluminum content from exceeding 0.01%.

[0038] In one specific embodiment of the present invention, the aluminum-free slag system is a CaF2-CaO-MgO-SiO2 system completely free of Al2O3. More preferably, the composition of the aluminum-free slag system, by weight percentage, is 55%–65% CaF2, 15%–25% CaO, 5%–10% MgO, and 10%–20% SiO2, with the Al2O3 content strictly controlled to not exceed 0.5%. More preferably, it is a 60% CaF2-15% CaO-5% MgO-20% SiO2 aluminum-free slag system. In conventional ESR slag systems, Al2O3 is an important component, typically containing 15%–25%, playing a role in regulating the conductivity and fluidity of the slag system. Removing Al2O3 will cause significant changes in the conductivity and viscosity of the slag system, making it difficult to stabilize the remelting process. This invention overcomes the industry's technical prejudice that aluminum-free slag systems have poor stability in electroslag remelting applications by using SiO2 to replace Al2O3 to adjust the high-temperature physicochemical properties of the slag system and optimizing the ratio of the four components CaF2, CaO, MgO and SiO2.

[0039] In addition to the specific requirements mentioned above, the other steps of the ESR process are known to those skilled in the art, and the operating process can be selected according to the actual situation. Preferably, but not limited to, during the electroslag remelting process, the remelting current is controlled at 8000A to 12000A, the melting rate is 4kg / min to 6kg / min, the slag layer thickness is 60mm to 100mm, the slag-to-metal ratio (slag amount / molten steel amount) is 0.03 to 0.05, and forced water cooling is used.

[0040] Step 3: Vacuum Arsenic Remelting (VAR). The electroslag remelted ingot obtained in Step 2 is used as an electrode for vacuum arc remelting. The vacuum level is controlled below 0.5 Pa. Carbon is used for deep deoxidation, and fine particulate inclusions are removed, resulting in an ultra-pure vacuum arc remelted ingot. In this step, the high vacuum environment promotes the reaction of trace carbon in the molten steel with oxygen (C + O → CO↑). The generated CO gas is extracted by a vacuum pump, thus replacing the traditional aluminum deoxidation process. This achieves deep deoxidation without solid residues, ensuring thorough deoxidation of the molten steel while removing aluminum as a deoxidizing agent.

[0041] In addition to the specific requirements mentioned above, the other steps of the VAR process are known to those skilled in the art, and the operating process can be selected according to the actual situation. Preferably, but not limited to, during the vacuum arc remelting process, the remelting current is controlled at 6000A to 9000A, the melting rate is 3kg / min to 6kg / min, and forced water cooling is used.

[0042] Step 4: High-Temperature Diffusion Treatment. The vacuum consumable ingot obtained in Step 3 is placed in a heating furnace and subjected to a heat treatment diffusion process at a temperature of 1200℃~1250℃ for no less than 48 hours. This temperature is much higher than the conventional annealing temperature of stainless steel (usually 1050℃~1100℃), which can provide sufficient diffusion activation energy to fully homogenize the alloying elements in the austenitic matrix and completely dissolve the remaining δ-ferrite.

[0043] In one specific embodiment of the present invention, after the high-temperature diffusion treatment is completed, the furnace can be removed and air-cooled or water-cooled.

[0044] Step 5: Forging and Billet Preparation. The vacuum consumable ingot, after high-temperature diffusion treatment, is hot-forged into a billet with a cross-sectional dimension of not less than 260mm × 260mm. This ensures sufficient deformation for subsequent rolling. Although the austenite grains tend to coarsen after extreme high-temperature diffusion treatment, the large-section billet can provide sufficient recrystallization driving force during large-deformation rolling, promoting sufficient grain refinement and homogenization.

[0045] In one specific embodiment of the present invention, the initial forging temperature of the billet forging is 1150℃~1180℃, and the final forging temperature is not lower than 900℃.

[0046] Step Six: Hot Rolling. The forged billet is heated and then rolled, with the rolling heating temperature controlled between 1100℃ and 1150℃, ultimately forming the finished product. The large cross-section of the forged billet provides sufficient rolling deformation, and combined with strict control of the rolling heating temperature, it effectively breaks up the as-cast structure and inhibits abnormal grain growth, ensuring the fineness of the grain size in the final product.

[0047] In one specific embodiment of the present invention, the total rolling deformation is not less than 70% (preferably 70% to 90%), and the deformation per pass is controlled at 10% to 20%.

[0048] In the preparation method described above, the three-stage melting process of vacuum induction melting, electroslag remelting, and vacuum arc remelting constitutes a tightly integrated whole. The core technical logic of this invention lies in addressing the problem of large particle inclusions introduced during the vacuum induction melting stage in existing dual-stage vacuum induction and vacuum arc remelting processes, leading to dark damage during final electrolytic polishing. This is achieved by innovatively introducing an electroslag remelting (ESR) process between VIM and VAR. Relying on the refining and washing effect of the ESR slag, large particle inclusions are thoroughly intercepted and removed.

[0049] However, conventional ternary slag systems (such as those containing Al2O3) used in the conventional electroslag remelting (ESR) process lead to a significant increase in aluminum content in the molten steel, resulting in excessive aluminum content. To resolve this contradiction, this invention solves the aluminum increase problem by using a specially formulated completely aluminum-free slag system. However, this aluminum-free slag system loses its desulfurization capability in terms of metallurgical physicochemistry. To compensate for this fatal flaw, this invention mandates the use of extremely high-purity, low-sulfur pure iron (S≤0.002%) in the first VIM stage, placing the desulfurization process upstream to ensure that even if desulfurization is not performed in the intermediate ESR process, the final product's S content remains ≤0.003%. The extremely low sulfur content avoids the problem of decreased high-temperature thermoplasticity. This "pre-desulfurization" design overturns the conventional "refining desulfurization" process logic and is one of the key non-obvious technical means of this invention.

[0050] After successfully eliminating aluminum as a deoxidizer, this invention further faces the challenge of achieving deep deoxidation under aluminum-free conditions. To address this, this invention overcomes technical biases in the field by utilizing the high-vacuum environment of vacuum arc remelting to induce a reaction between trace amounts of carbon and oxygen in the molten steel (C + O → CO↑). The generated CO gas is then removed, achieving deep deoxidation with no solid residue. This invention significantly reduces the CO partial pressure by implementing VAR in an ultra-high vacuum environment below 0.5 Pa. Based on the principle of chemical reaction equilibrium shift (Le Chatelier's principle), the carbon-oxygen reaction equilibrium shifts strongly to the right—achieving an oxygen content below 5 ppm even with only 0.01%–0.03% carbon. This effectively avoids dependence on conventional strong deoxidizers such as aluminum and titanium. This effect is unpredictable and difficult to conceive of by those skilled in the art based on conventional carbon deoxidation knowledge, and is a significant manifestation of the invention's high level of inventiveness. These four interconnected and indispensable processes solve the industry problem of existing dual-process methods being unable to simultaneously achieve extremely low aluminum levels and the absence of large particle inclusions.

[0051] Furthermore, in terms of hot working, the technical means of this invention also embody the non-obviousness of overcoming technical bias. Those skilled in the art know that excessively high heating temperatures and excessively long holding times can lead to abnormal austenite grain growth. In conventional processes, the heating temperature of 316L stainless steel is usually controlled below 1150℃, and the holding time is usually no more than 12 hours to avoid grain coarsening. This invention breaks through this conventional understanding by holding the vacuum consumable ingot at a high temperature of 1200℃~1250℃ (higher than conventional temperatures) for more than 48 hours. Extreme high-temperature diffusion thoroughly homogenizes the alloying elements in the austenite matrix, dissolving the remaining δ-ferrite to 0%. Simultaneously, combined with a forging billet cross-section ≥260×260mm, it provides a large amount of rolling deformation. Coupled with a strict rolling heating temperature of 1100~1150℃, it effectively breaks down the as-cast structure and inhibits abnormal grain growth, ensuring that the final product has a grain size finer than ASTM grade 8. This invention has uniquely formed a chain of processes consisting of "extreme high-temperature diffusion + large-size forging billet + low-temperature rolling". The three processes are interdependent and indispensable. This systematic process combination is something that those skilled in the art would not easily conceive of using the conventional "avoiding grain coarsening" technical approach.

[0052] In summary, this invention solves the comprehensive technical challenge of existing dual-processes being unable to simultaneously achieve extremely low aluminum content, no large particle inclusions, no δ-ferrite, and excellent electropolished surface quality by synergistically matching the extreme fine-tuning of composition with the triple smelting process and specific hot working parameters.

[0053] Due to space limitations, some embodiments are listed below to illustrate the main technical solutions of the present invention, thereby further demonstrating the advantages of the technical solutions of the present invention. The present invention will also be described in further detail with reference to comparative examples to verify the irreplaceability of the process and parameters of the present invention.

[0054] Example 1 The chemical composition of the ultrapure 316L austenitic stainless steel for semiconductors in this example, by weight percentage (wt%), includes: C 0.015%, Si 0.45%, Mn 1.2%, Cr 17.8%, Mo 2.8%, Ni 13.8%, P 0.020%, with the balance being Fe.

[0055] The preparation methods include the following: (1) Low-sulfur pure iron with a sulfur content of 0.0015% is smelted in a vacuum induction furnace (VIM) without adding aluminum for deoxidation and is then cast into electrode rods.

[0056] (2) The vacuum induction electrode rod obtained was subjected to electroslag remelting (ESR) using a 60%CaF2-20%CaO-5%MgO-15%SiO2 aluminum-free slag system to remove large non-metallic inclusions in the steel ingot and obtain an electroslag remelted ingot.

[0057] (3) The electroslag remelted ingot is used as an electrode to perform vacuum self-consumable remelting (VAR) under a vacuum degree of 0.5 Pa, and vacuum carbon deoxidation is used to obtain vacuum self-consumable ingot.

[0058] (4) Heat the VAR steel ingot to 1205℃ and keep it at that temperature for 50 hours for high-temperature homogenization diffusion treatment. After high-temperature diffusion, the steel ingot is air-cooled.

[0059] (5) After high temperature diffusion, the billet is opened by a fast forging machine and forged into a square billet with a cross-sectional size of 280mm×280mm. The initial forging temperature is controlled at 1150℃~1180℃ and the final forging temperature is not lower than 900℃.

[0060] (6) The billet is heated to 1120℃ and taken out of the furnace, and rolled into Φ50mm bars by a continuous rolling mill.

[0061] The mechanical properties of the steel obtained in this example, after testing, meet the following requirements: room temperature tensile strength ≥ 500 MPa, yield strength ≥ 200 MPa, elongation after fracture ≥ 50%, and reduction of area ≥ 60%, fully satisfying the processing and forming requirements for semiconductor applications. Specifically: tensile strength approximately 550 MPa, yield strength approximately 280 MPa, elongation after fracture approximately 55%, and reduction of area approximately 65%. That is, the method of this invention does not affect the basic mechanical performance requirements for subsequent semiconductor applications.

[0062] Example 2 The chemical composition of the ultrapure 316L austenitic stainless steel for semiconductors in this example, by weight percentage (wt%), includes: C 0.018%, Si 0.5%, Mn 1.3%, Cr 17.9%, Mo 2.9%, Ni 13.9%, P 0.025%, with the balance being Fe.

[0063] The preparation method is the same as in Example 1, except that: the VAR vacuum degree is 0.8 Pa; the diffusion temperature is 1220℃ and the holding time is 48 hours; the billet cross section is 300mm×300mm; and the rolling heating temperature is 1140℃.

[0064] The mechanical properties of the steel obtained in this example were tested and found to meet the following requirements: room temperature tensile strength ≥500MPa, yield strength ≥200MPa, elongation after fracture ≥50%, and reduction of area ≥60%, which fully meets the processing and forming requirements for semiconductor applications.

[0065] Example 3 The chemical composition of the ultrapure 316L austenitic stainless steel for semiconductors in this example, by weight percentage (wt%), includes: C 0.010%, Si 0.3%, Mn 1.1%, Cr 17.6%, Mo 2.6%, Ni 13.6%, P 0.015%, with the balance being Fe.

[0066] The preparation method is the same as in Example 1, except that the diffusion temperature is 1240℃ and the holding time is 55 hours; the rolling heating temperature is 1110℃.

[0067] The mechanical properties of the steel obtained in this example were tested and found to meet the following requirements: room temperature tensile strength ≥500MPa, yield strength ≥200MPa, elongation after fracture ≥50%, and reduction of area ≥60%, which fully meets the processing and forming requirements for semiconductor applications.

[0068] Comparative Example 1 Using the same raw materials and VIM process as in Example 1, but without the electroslag remelting (ESR) process, the VIM ingot was directly remelted using VAR, and the subsequent hot working process was the same as in Example 1. This comparative example only uses the VIM+VAR duplex process to simulate the conventional vacuum induction plus vacuum consumable duplex process in the prior art.

[0069] Comparative Example 2 The same raw materials and triple smelting process as in Example 1 were used, but the electroslag remelting in step (2) used a conventional ternary slag system CaF2-Al2O3-CaO, with an Al2O3 content of approximately 20%, and the remainder being similar to that in Example 1, i.e., the ternary slag system ratio was 60% CaF2-20% Al2O3-20% CaO. This comparative example used a conventional aluminum-containing slag system for ESR to verify the impact of the conventional aluminum-containing slag system on the product.

[0070] Comparative Example 3 Instead of using low-sulfur (S≤0.002%) pure iron, ordinary pure iron (S=0.012%) was used, employing the same other components and triple smelting process as in Example 1. This comparative example did not use low-sulfur pure iron to verify the impact of initial sulfur content on the product when using aluminum slag-free materials.

[0071] Comparative Example 4 The same composition and triple melting process as in Example 1 were used, but in step (4), the consumable ingot was only held at 1150°C for 6 hours as usual, and then directly forged and rolled. This comparative example lacks an ultra-long-term high-temperature diffusion treatment, which was used to verify the influence of high-temperature diffusion treatment process parameters on δ-ferrite elimination and grain size control.

[0072] Comparative Example 5 The same composition and triple melting process as in Example 1 were used, but in step (3), vacuum arc remelting (VAR) was performed under a vacuum of 1.5 Pa to obtain vacuum arc remelting ingots by vacuum carbon deoxidation. This comparative example verifies the effect of vacuum on oxygen content and purity.

[0073] Performance Testing and Comparison The bars obtained in the above embodiments and comparative examples were tested. Non-metallic inclusions were tested according to GB / T 10561 standard, grain size was tested according to GB / T 6394, and δ-ferrite was tested using metallographic method and ferrite measuring instrument. The results are shown in Table 1.

[0074] Table 1: Comparison of performance test results between each embodiment and the comparative example

[0075] The test results above show that all performance indicators of Examples 1 to 3 reached excellent levels: Class B inclusions were all at grade 0, δ-ferrite content was all 0%, austenite grain size was all finer than or equal to ASTM grade 8, and semiconductor electropolishing performance reached excellent mirror finish. Furthermore: 1. Comparing the examples and Comparative Example 1, it is evident that the conventional VIM+VAR dual-process cannot effectively remove large particle inclusions formed by occasional slag inflow or refractory material detachment within the VIM furnace. These large particle inclusions exhibit strong randomness and concealment in conventional metallography, making them easy to miss. However, they detach during the final EP electrolytic polishing, causing fatal large micro-pit defects. This invention, by introducing a specific ESR process between vacuum induction and vacuum self-consumption, completely eliminates large particle inclusions, achieving a mirror-like polish.

[0076] 2. Comparing the examples and Comparative Example 2, it is evident that while introducing a conventional aluminum-containing slag-based ESR between VIM and VAR can intercept large particle inclusions, it leads to severe aluminum enrichment in the molten steel, resulting in substandard aluminum content and failing to meet the semiconductor industry's Al≤0.01% standard. This demonstrates that the synergistic effect of the invention's unique "aluminum-free slag system + VIM low-sulfur pure iron pretreatment" possesses irreplaceable inventiveness and is not obvious.

[0077] 3. Comparing the examples and Comparative Example 3, it can be seen that without the extreme high-temperature diffusion treatment of 1200℃ / 48h or higher of the present invention, conventional heating cannot completely dissolve the δ-ferrite in the as-cast microstructure (residual 1.5%), which will seriously affect the uniformity of electrolytic polishing of the inner wall of the pipe. At the same time, under the conventional heating regime, the grains grow unevenly, and the grain size only reaches level 5, which cannot meet the requirements of semiconductor applications for microstructure uniformity.

[0078] 4. Comparing the examples and Comparative Example 4, it can be seen that if the low-sulfur pure iron combined with aluminum-free slag system of the present invention is not used, the conventional pure iron combined with aluminum-free slag system will seriously affect the sulfur content and Class A inclusions of the finished product, thereby affecting the surface quality after electrolytic polishing.

[0079] 5. Comparing the examples and Comparative Example 5, it can be seen that if the ultra-high vacuum of ≤0.5Pa of the present invention is not used, the oxygen content and non-metallic inclusions will increase, which will further seriously affect the surface quality of electropolishing.

[0080] In summary, this invention, through precise composition design and synergistic matching of a triple melting process (vacuum induction melting, electroslag remelting, and vacuum arc remelting) and specific thermal processing parameters, successfully prepares ultra-pure 316L austenitic stainless steel for semiconductor manufacturing equipment, which features extremely low non-metallic inclusions, no δ-ferrite, fine grains, and excellent electropolishing performance, effectively solving the technical problems existing in the prior art.

[0081] The above description is merely a preferred embodiment of the present invention and does not limit the scope of the patent. Any equivalent structural or procedural modifications made based on the content of this specification are similarly included within the scope of patent protection of this invention.

Claims

1. A method for preparing ultrapure 316L austenitic stainless steel for semiconductor manufacturing equipment, characterized in that, Includes the following steps: Step 1, Vacuum Induction Melting: Low-sulfur pure iron with a sulfur content of S≤0.002% is used as raw material for vacuum induction melting. The target components are precisely controlled, and no aluminum is added throughout the process to ensure that the S ≤0.003% and Al ≤0.01% in the molten steel. The molten steel is then cast into vacuum induction electrode rods. Step 2, Electroslag Remelting: The vacuum induction electrode rod is electroslag remelted. During the remelting process, an aluminum-free slag system is used to remove large non-metallic inclusions in the steel ingot, and an electroslag remelted ingot is obtained. Step 3, Vacuum self-consumption remelting: Using the electroslag remelted ingot as an electrode, vacuum self-consumption remelting is performed, controlling the vacuum degree below 0.5 Pa, and deep deoxidation is carried out using carbon to obtain vacuum self-consumption ingot; Step 4, High-temperature diffusion treatment: The vacuum consumable ingot is subjected to heat preservation diffusion treatment at a temperature of 1200℃~1250℃ for a holding time of ≥ 48 hours; Step 5, forging blank: The vacuum consumable ingot after high temperature diffusion is hot forged to obtain a forging blank; in step 5, the blank is forged into a forging blank with a cross-sectional dimension of not less than 260mm×260mm; Step Six: Hot Rolling Forming: The forged billet is heated and then rolled into finished product. The rolling heating temperature in Step Six is ​​controlled at 1100℃ ~ 1150℃. The chemical composition of the obtained stainless steel, by weight percentage, includes the following: C ≤ 0.03%; Si ≤ 1.0%; Mn ≤ 1.50%; Cr: 17.5% ~ 18.0%. Mo: 2.5% ~ 3.0%; Ni: 13.5% ~ 14.0%; P ≤0.045%; S ≤ 0.003%; Al ≤ 0.01%; balance is Fe and unavoidable impurities.

2. The preparation method according to claim 1, characterized in that, In the stainless steel, O ≤ 0.0005%, N: 0.02%~0.05%.

3. The preparation method according to claim 1, characterized in that, The stainless steel has a δ-ferrite content of 0%.

4. The preparation method according to claim 3, characterized in that, The stainless steel has an austenitic grain size finer than or equal to ASTM grade 8, and the non-metallic inclusions of type A, C, and D do not exceed grade 0.5, and it does not contain type B non-metallic inclusions.

5. The preparation method according to claim 1, characterized in that: The aluminum-free slag system mentioned in step two is CaF2-CaO-MgO-SiO2, which is completely free of Al2O3.

6. The preparation method according to claim 5, characterized in that: The aluminum slag-free system is composed of the following components by weight percentage: CaF2 55%~65%, CaO 15%~25%, MgO 5%~10%, SiO2 10%~20%, with Al2O3 content controlled to be ≤0.5%.

7. The preparation method according to claim 6, characterized in that: The composition of the aluminum slag-free system by weight percentage is: 60% CaF2-15% CaO-5% MgO-20% SiO2.

8. The preparation method according to claim 1, characterized in that: In step five, the initial forging temperature of the billet forging is 1150℃~1180℃, and the final forging temperature is not lower than 900℃.

9. An ultrapure 316L austenitic stainless steel for semiconductor manufacturing apparatus prepared by the preparation method according to any one of claims 1-8.

10. The application of ultrapure 316L austenitic stainless steel for semiconductor manufacturing apparatus as described in claim 9 in the field of semiconductors and their preparation.

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