Ultra-pure 316L stainless steel and vacuum induction melting preparation method thereof

Abstract

The present application belongs to the field of metallurgy, and particularly relates to a kind of ultra-pure 316L stainless steel and a vacuum induction melting preparation method thereof. First, a NiCa intermediate alloy is prepared by vacuum induction melting, then a 316L alloy is prepared by vacuum induction melting, Al is added for deoxidation, the prepared NiCa intermediate alloy is added for high-temperature refining to remove Al, and finally, low-temperature refining is performed to remove Ca and further deep deoxidation and desulfurization. The present application can remove various impurity elements such as Al, O and S and inclusions of the ultra-pure 316L stainless steel by only one method of vacuum induction melting, without additional processes such as electroslag or consumable, and the equipment is simple and convenient to use, which is conducive to large-scale industrial application, and provides an effective way for the purification preparation of the ultra-pure 316L stainless steel.

Description

Technical fields:

[0001] This invention belongs to the field of metallurgical technology, specifically relating to an ultrapure 316L stainless steel and its preparation method by vacuum induction melting. Background technology:

[0002] With the development of the semiconductor industry, the requirements for material purity and performance are increasing. Ultra-pure 316L stainless steel, as a key material in semiconductor equipment, is commonly used to manufacture reaction chambers, pipes, valves, etc., and its quality and stability directly affect the performance and reliability of semiconductor devices. Specifically, the content of impurity elements and inclusions in 316L stainless steel is crucial for the stability of semiconductor equipment and the maintenance of a high-purity environment. However, the impurity element and inclusion content of ordinary commercially available 316L stainless steel is far from meeting the needs of the semiconductor industry. Therefore, developing an industrially scalable preparation process for ultra-pure 316L stainless steel is of great significance.

[0003] To obtain high-purity 316L stainless steel, it is difficult to effectively remove various impurity elements using non-vacuum induction melting methods, especially controlling the inclusion content. The patent in publication number CN 117512268 A uses an EAF+AOD+LF+VAR method to prepare 316L, but achieving a zero-level inclusion content is difficult, and impurity element control is not demonstrated. Existing vacuum melting processes for 316L typically require two or three vacuum steps to achieve suitable alloy impurity content control. For example, patents in publication numbers CN 116623105 A and CN 117987747 A use a vacuum induction melting + vacuum arc remelting process, while patent in publication number CN 118389933 A uses a vacuum induction melting + vacuum consumable melting method to prepare the alloy. This type of method has a complex process flow, requiring the electrode rod to be prepared first and then cut, turned or machined before it can enter the next process. The material utilization rate is extremely low, and the investment in equipment is large, resulting in high process costs.

[0004] It is generally believed that carbon (C) is an effective deoxidizer in vacuum induction melting. However, the deoxidation product of C is a gas, which is difficult to spontaneously form bubbles and escape from the molten steel. Furthermore, the solid solubility of gases in molten steel is limited. Therefore, C deoxidation cannot be sustained, resulting in a still high residual oxygen content in the molten steel, failing to meet the requirements of high-purity 316L. Al (Al) has a stronger deoxidizing ability than C, and its product, solid Al₂O₃, has a strong nucleation ability, allowing it to continuously combine with oxygen and reduce the oxygen content in the molten steel to extremely low levels. However, ultra-pure 316L requires a lower Al content, and the deoxidation product Al₂O₃ easily forms inclusions that are difficult to remove. Therefore, conventional methods do not use Al, but instead use Mg and Ca for deoxidation and desulfurization. However, Mg and Ca volatilize significantly under high-temperature vacuum, and their addition is difficult to control, making it difficult to achieve stable and effective deoxidation. Therefore, subsequent purification of the alloy is often required through methods such as secondary vacuum self-consumption to control the content of impurity elements and inclusions, which significantly increases equipment costs and material consumption. Summary of the Invention:

[0005] The purpose of this invention is to overcome the shortcomings of the existing technology and provide an ultrapure 316L stainless steel and its preparation method by vacuum induction melting. The method of this invention can achieve deep removal of impurity elements and precise control of inclusions by vacuum induction melting alone, and can produce ultrapure 316L stainless steel with high purity and low inclusion content. The equipment is simple and convenient to set up, which is conducive to large-scale industrial application.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] An ultrapure 316L stainless steel is composed of the following components by mass percentage: C≤0.01%, Si≤0.03%, Mn≤0.1%, P≤0.01%, S≤0.003%, Ni 12.5~15%, Cr 16~18%, Mo 2.2~3.0%, Cu≤0.2%, Al≤0.008%, Ca≤0.01%, Ti≤0.01%, Nb≤0.02%, Se≤0.001%, O≤0.0008%, N≤0.003%, H≤0.0001%, with the remainder being Fe; the coarse inclusions (A, B, C, D, and DS) of the ultrapure 316L stainless steel are all grade 0, and the fine inclusions (A+B+C+D+DS) are ≤0.5 grade.

[0008] The vacuum induction melting method for preparing ultrapure 316L stainless steel includes the following steps:

[0009] (1) NiCa master alloy was prepared by vacuum induction melting;

[0010] (2) Based on the target alloy composition, electrolytic nickel, metallic chromium, molybdenum bars and industrial pure iron are selected as raw materials and mixed in a crucible according to the proportion of each element Ni, Cr, Mo and Fe.

[0011] (3) Evacuate the air pressure to below 5Pa, then power on to start smelting and obtain molten steel;

[0012] (4) Add Al to the molten steel for refining and deoxidation. The refining temperature for refining and deoxidation is 1520-1560℃ and the refining time is 5-7 min.

[0013] The amount of Al element added is determined according to Equation 1:

[0014] w[%Al]=w[%O]-0.8*w[%C]-0.1*(w[%Si]+w[%Mn]+w[%Ti])+0.05% Formula 1;

[0015] In Formula 1, w[%Al] is the amount of Al added, w[%O] is the initial O content introduced into each raw material, w[%C] is the initial C content introduced into each raw material, w[%Si] is the initial Si content introduced into each raw material, w[%Mn] is the initial Mn content introduced into each raw material, and w[%Ti] is the initial Ti content introduced into each raw material.

[0016] (5) After the vacuum Al refining and deoxidation treatment is completed, high-purity argon gas is introduced to 0.03-0.05MPa, and the refined NiCa intermediate alloy is added for high-temperature refining and de-Al treatment. The refining temperature of the high-temperature refining and de-Al treatment is 1620-1680℃, the refining time is 15-20min, and after the refining is completed, the power is turned off and the mixture is left to stand for more than 10min.

[0017] The amount of NiCa master alloy added is determined according to formula 2:

[0018] (0.75*w[%Al]+w[%O]-0.01%) / 0.005≤w[%NiCa]≤w[%Ni] / 0.95 Equation 2

[0019] In Formula 2, w[%NiCa] is the amount of NiCa master alloy added, w[%Al] is the amount of Al added in step (4), w[%O] is the initial O content introduced into each raw material, and w[%Ni] is the amount of Ni added in step (2).

[0020] (6) After the high-temperature refining and Al removal process is completed, remove the argon gas until the pressure is 9.9 × 10⁻⁶. -2 Below Pa, low-temperature refining and decalcification and further deep deoxidation and desulfurization are carried out. The refining temperature of the low-temperature refining process is 1450℃~1550℃, and the refining time is 30~40min.

[0021] (7) After refining, the power is turned off and the temperature is allowed to drop. The temperature of the molten steel is adjusted to 1420-1470℃. A filter screen is placed in the steel guide trough and the steel is cast to obtain an ingot.

[0022] The vacuum induction melting method for preparing ultrapure 316L stainless steel, in step (1), includes the following steps in the preparation of the NiCa master alloy:

[0023] 1) Raw material preparation: Select electrolytic nickel and metallic calcium with a purity of 99wt.% or higher. Prepare raw materials according to the mass percentage of calcium as 50% to 60%, with the balance being nickel.

[0024] 2) Argon gas furnace cleaning: Evacuate to a pressure of 9.9 × 10⁻⁶. -2 Below 0.03 MPa, close the valve and purge with high-purity argon gas to 0.03–0.05 MPa; then evacuate the argon gas again until the pressure reaches 9.9 × 10⁻⁶ MPa. -2 Below 0.03 MPa, then close the valve and purge with high-purity argon gas to 0.03–0.05 MPa;

[0025] 3) Induction melting: Nickel blocks and calcium blocks are alternately stacked in a crucible, and electricity is applied to melt the raw materials to obtain molten steel. The steel is then refined at a temperature of 1400-1450℃ for 20-40 minutes. After refining, the temperature is adjusted and the steel is cast at 1300-1320℃ to obtain a NiCa master alloy.

[0026] 4) In the prepared NiCa master alloy, the mass percentage content of Ca is 5% to 15%, the mass percentage content of O is ≤0.0080%, and the remainder is Ni.

[0027] The vacuum induction melting method for preparing ultrapure 316L stainless steel uses an Al2O3 or CaO crucible.

[0028] In the vacuum induction melting preparation method of ultrapure 316L stainless steel, in step (7), the filter screen is made of zirconium oxide with a porosity of 20 ppi.

[0029] The technical principle of this invention is as follows:

[0030] Ultra-pure 316L stainless steel, a key material in semiconductor equipment, is commonly used to manufacture reaction chambers, pipes, valves, etc. Its quality and stability directly affect the performance and reliability of semiconductor devices. To obtain high-purity 316L stainless steel, it is difficult to effectively remove various impurity elements using non-vacuum induction melting methods. While vacuum induction melting traditionally considers carbon (C) as an effective deoxidizer, the deoxidation product of C is a gas, which is difficult to spontaneously form bubbles in the molten steel and escape. Furthermore, the solid solubility of gases in molten steel is limited, thus C deoxidation cannot continue, resulting in a still high residual oxygen content in the molten steel, failing to meet the requirements of high-purity 316L. Al has a stronger deoxidizing ability than C, and its product, solid Al₂O₃, has a strong nucleation ability, allowing it to continuously combine with oxygen and reduce the oxygen content in the molten steel to extremely low levels. However, ultra-pure 316L has lower requirements for Al content, and the deoxidation product Al₂O₃ easily forms inclusions that are difficult to remove. Therefore, conventional methods do not use Al, but instead use Mg and Ca for deoxidation and desulfurization. However, Mg and Ca volatilize significantly under high temperature and vacuum, and their addition is difficult to control, making it hard to achieve stable and effective deoxidation. This often requires subsequent purification of the alloy through methods such as adding secondary vacuum self-consumption furnaces, employing two or even three vacuum steps to control the content of impurity elements and inclusions, which greatly increases equipment costs and material consumption. This invention, however, differs from traditional approaches, proposing a feasible method of first adding Al for deoxidation, and then removing Al through a reaction. Ultra-pure 316L stainless steel can be prepared using only one method: vacuum induction melting.

[0031] Specifically, during vacuum induction melting, Al's deoxidation effect is far superior to that of other elements such as C. Therefore, this invention first directly adds a certain amount of Al for deoxidation during the vacuum induction melting stage. However, the amount of Al added must be strictly controlled to ensure sufficient removal of O without excessive residue. Furthermore, appropriate refining temperatures and times must be selected to ensure thorough deoxidation without the product agglomerating and growing. The Al2O3 inclusions formed after Al deoxidation are difficult to remove completely using conventional processes, which is a key factor why Al deoxidation is generally not chosen in ultra-pure 316L steel. In this invention, however, a refining process to remove Al is applied. Since Al enters the molten steel after melting and combines with free O in the molten steel, its deoxidation product, Al2O3, is fine and dispersed throughout the molten steel. Al2O3 has a high melting point and exists in solid form in the molten steel. Due to surface tension, these fine and dispersed Al2O3 inclusions are difficult to agglomerate and grow, and cannot be removed by static flotation or filtration. The refining and de-Al removal process of this invention introduces an appropriate amount of Ca to modify Al2O3, transforming it entirely into low-melting-point CaO·Al2O3 composite inclusions, while controlling suitable refining temperature and time. The refining temperature is strictly controlled to be above the melting point of the composite inclusions but below the temperature at which they react with the crucible. The refining time is controlled to ensure that the original high-melting-point solid Al2O3 inclusions are completely transformed into low-melting-point liquid composite inclusions, thereby eliminating the influence of surface tension between the solid and liquid phases and allowing them to completely dissolve in the molten steel. Because the density of the composite inclusions is lower than that of the molten steel, they can fully float to the top of the molten steel and then solidify again during a long period of static settling after refining, ultimately forming a layered structure with solid composite inclusions on top and pure molten steel at the bottom.

[0032] After high-temperature refining and Al removal, a certain amount of Ca still exists in the molten steel. Since Ca and its deoxidation and desulfurization products are highly volatile under vacuum, this invention continues refining at low temperature under high vacuum, simultaneously removing Ca and further undergoing deep deoxidation and desulfurization, thus continuously reducing the impurity content in the molten steel. Controlling the vacuum level enhances the volatilization of Ca and its deoxidation and desulfurization products, and controlling the appropriate refining temperature ensures continuous impurity removal while remaining below the melting point of the composite inclusions generated in the previous step, preventing them from re-dissolving into the molten steel. By controlling the refining time, ultra-pure 316L molten steel can be obtained, and after filtering out composite inclusions through a filter screen, high-cleanliness, high-purity 316L stainless steel can be obtained.

[0033] In this invention, the amount of Ca added needs to be precisely controlled. Commercially available calcium or nickel-calcium alloys have a high oxygen content, resulting in a significant amount of CaO. Furthermore, the content of metallic Ca is extremely unstable, leading to poor application results and making it impossible to obtain a stable and expected amount of Ca. Therefore, this invention also provides a method for preparing a NiCa master alloy. The NiCa master alloy prepared by this invention has a low oxygen content and a stable metallic calcium content. Simultaneously, by controlling the Ca mass fraction to 5%–15%, the obtained NiCa master alloy has a high melting point and minimal burn-off when added to molten steel, thus enabling precise control of the Ca addition amount.

[0034] Based on the above technical principles, this invention can remove various impurity elements and inclusions such as Al, O, and S from ultrapure 316L stainless steel using only one method of vacuum induction melting, providing an effective way for the purification preparation of ultrapure 316L stainless steel.

[0035] The advantages and beneficial effects of this invention are as follows:

[0036] 1. This invention provides a method for preparing ultrapure 316L stainless steel using vacuum induction melting, which breaks through the limitation of traditional methods that cannot be deoxidized by Al. By first adding Al for deoxidation and then removing Al, the preparation of ultrapure 316L stainless steel is achieved.

[0037] 2. The present invention also provides a method for preparing NiCa master alloy. By controlling the composition range and oxygen content, the prepared master alloy has a high melting point and stable calcium content, and the amount of Ca added can be stably controlled.

[0038] 3. The method used in this invention can be achieved simply through vacuum induction melting, without the need for additional processes such as electroslag or consumable metallurgy. The prepared ultrapure 316L stainless steel contains, by mass fraction, Al≤0.008%, Ca≤0.01%, Ti≤0.01%, Nb≤0.02%, Se≤0.001%, O≤0.0008%, N≤0.003%, H≤0.0001%, and the coarse inclusions A, B, C, D, and DS are all grade 0, while the fine inclusions A+B+C+D+DS are ≤0.5. The equipment is simple and convenient to set up, which is conducive to the large-scale industrial application of ultrapure 316L stainless steel. Detailed implementation method:

[0039] The specific embodiments of the present invention will be described in further detail below with reference to examples and comparative examples. It should be noted that these descriptions are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0040] Example 1

[0041] In this embodiment, a NiCa master alloy is prepared using a vacuum induction melting method. Electrolytic nickel (GB / T 6516-2010, Ni9950) and metallic calcium with a purity of 99 wt.% or higher are selected. The raw materials are prepared with calcium accounting for 50% of the mass, and the balance is nickel. The vacuum is evacuated to a pressure of 9.8 × 10⁻⁶. -2 Pa, close the valve and purge with high-purity argon gas (99.999% volume purity) to 0.03 MPa; then evacuate the argon gas again until the pressure reaches 9.7 × 10⁻⁶ MPa. -2 Pa, then close the valve and purge with high-purity argon to 0.03 MPa; place nickel and calcium blocks alternately in the crucible, and melt the raw materials to obtain molten steel by power supply. Refine at 1450℃ for 20 minutes. After refining, adjust the temperature and cast at 1300℃ to obtain NiCa master alloy; in the prepared NiCa master alloy, the mass percentage content of Ca is 5%, the mass percentage content of O is 0.0078%, and the remainder is Ni.

[0042] Based on the target alloy composition, electrolytic nickel (GB / T 6516-2010, Ni9950), metallic chromium (GB / T 3211-2023, JCr99-A), molybdenum bars (GB / T 3462-2017, Mo-1), and industrial pure iron were selected as raw materials, and the raw materials were proportioned according to the ratio of Ni, Cr, Mo, and Fe. The pressure was evacuated to 4.8 Pa, and smelting was initiated to obtain molten steel. Al was added to the molten steel for refining and deoxidation. The content of each element introduced into the raw materials is shown in Table 1. The amount of Al added was determined by the following formula:

[0043] w[%Al]=w[%O]-0.8*w[%C]-0.1*(w[%Si]+w[%Mn]+w[%Ti])+0.05%;

[0044] Table 1. Content (wt.%) of each element introduced into the raw materials of Example 1

[0045] content 0.0074 0.0041 0.0040 0.0030 0.0010

[0046] The amount of Al added was determined to be 0.0533 wt.% through calculation. The refining temperature for deoxidation was 1520℃, and the refining time was 7 min. After the vacuum Al refining and deoxidation treatment was completed, high-purity argon gas was introduced to 0.03 MPa, and the refined NiCa master alloy was added for high-temperature refining and de-Al treatment. The amount of NiCa master alloy added was determined according to the following formula:

[0047] (0.75*w[%Al]+w[%O]-0.01%) / 0.005≤w[%NiCa]≤w[%Ni] / 0.95

[0048] The calculated addition amount of NiCa master alloy was determined to be 13 wt.%, the high-temperature refining and Al removal temperature was 1680℃, and the refining time was 15 min. After refining, the power was turned off and the mixture was allowed to stand for 15 min. After the high-temperature refining and Al removal process was completed, the argon gas was evacuated to a pressure of 9.8 × 10⁻⁶. -2 Pa is used for low-temperature refining to remove Ca and further deep deoxidation and desulfurization. The refining temperature of the low-temperature refining process is 1450℃ and the refining time is 40min. After refining, the power is turned off and the temperature is allowed to drop. The temperature of the molten steel is adjusted to 1420℃. A zirconia filter with a porosity of 20ppi is placed in the steel guide trough and then cast to obtain an ingot.

[0049] Example 2

[0050] In this embodiment, a NiCa master alloy is prepared using a vacuum induction melting method. Electrolytic nickel (GB / T 6516-2010, Ni9990) and metallic calcium with a purity of 99 wt.% or higher are selected. The raw materials are prepared according to a calcium mass percentage of 55%, with the balance being nickel; a vacuum is drawn to a pressure of 7.9 × 10⁻⁶. -2 Pa, close the valve and purge with high-purity argon gas (99.999% volume purity) to 0.04 MPa; then evacuate the argon gas again until the pressure reaches 7.8 × 10⁻⁶ MPa. -2 Pa, then close the valve and purge with high-purity argon to 0.04 MPa; alternately stack nickel and calcium blocks in the crucible, and melt the raw materials to obtain molten steel by power supply. Refine at 1400℃ for 40 min. After refining, adjust the temperature and cast at 1320℃ to obtain NiCa master alloy; in the prepared NiCa master alloy, the mass percentage content of Ca is 12%, the mass percentage content of O is 0.0068%, and the remainder is Ni.

[0051] Based on the target alloy composition, electrolytic nickel (GB / T 6516-2010, Ni9990), metallic chromium (GB / T 3211-2023, JCr99-B), molybdenum bars (GB / T 3462-2017, Mo-2), and industrial pure iron were selected as raw materials, and the raw materials were proportioned according to the ratio of Ni, Cr, Mo, and Fe. The vacuum was evacuated to a pressure of 4.2 Pa, and smelting was initiated to obtain molten steel. Al was added to the molten steel for refining and deoxidation. The content of each element introduced into the raw materials is shown in Table 2. The amount of Al added was determined by the following formula:

[0052] w[%Al]=w[%O]-0.8*w[%C]-0.1*(w[%Si]+w[%Mn]+w[%Ti])+0.05%;

[0053] Table 2. Content (wt.%) of each element introduced into the raw materials of Example 2

[0054] content 0.0112 0.0051 0.0052 0.0060 0.0021

[0055] The amount of Al added was determined to be 0.0558 wt.%, the refining temperature for deoxidation was 1550℃, and the refining time was 5 min. After the vacuum Al refining and deoxidation treatment was completed, high-purity argon gas was introduced to 0.03 MPa, and the refined NiCa master alloy was added for high-temperature refining and de-Al treatment. The amount of NiCa master alloy added was determined according to the following formula:

[0056] (0.75*w[%Al]+w[%O]-0.01%) / 0.005≤w[%NiCa]≤w[%Ni] / 0.95

[0057] The addition amount of NiCa master alloy was determined to be 10 wt.% through calculation. The refining temperature for high-temperature Al removal was 1640℃, and the refining time was 18 min. After refining, the mixture was left to stand for 15 min with the power off. After the high-temperature Al removal process was completed, the argon gas was evacuated to a pressure of 9.0 × 10⁻⁶. -2 Pa is used for low-temperature refining to remove Ca and further deep deoxidation and desulfurization. The refining temperature of the low-temperature refining process is 1530℃ and the refining time is 32min. After refining, the power is turned off and the temperature is allowed to drop. The temperature of the molten steel is adjusted to 1470℃. A zirconia filter with a porosity of 20ppi is placed in the steel guide trough and then cast to obtain an ingot.

[0058] Example 3

[0059] In this embodiment, a vacuum induction melting method is used to prepare the NiCa master alloy. Electrolytic nickel (GB / T 6516-2010, Ni9920) and metallic calcium with a purity of 99 wt.% or higher are selected. The raw materials are prepared according to a calcium mass percentage of 58%, with the balance being nickel; a vacuum is drawn to a pressure of 6.2 × 10⁻⁶. -2 Pa, close the valve and purge with high-purity argon gas (99.999% volume purity) to 0.04 MPa; then evacuate the argon gas again until the pressure reaches 6.5 × 10⁻⁶ MPa. -2 Pa, then close the valve and purge with high-purity argon to 0.04 MPa; alternately stack nickel and calcium blocks in the crucible, and melt the raw materials to obtain molten steel by power supply. Refine at 1420℃ for 30 minutes. After refining, adjust the temperature and cast at 1310℃ to obtain NiCa master alloy; in the prepared NiCa master alloy, the mass percentage content of Ca is 8%, the mass percentage content of O is 0.0060%, and the remainder is Ni.

[0060] Based on the target alloy composition, electrolytic nickel (GB / T 6516-2010, Ni9920), metallic chromium (GB / T 3211-2023, JCr98.5), molybdenum bars (GB / T 3462-2017, Mo-1), and industrial pure iron were selected as raw materials, and the raw materials were proportioned according to the ratio of Ni, Cr, Mo, and Fe. The vacuum was evacuated to a pressure of 3.2 Pa, and smelting was initiated to obtain molten steel. Al was added to the molten steel for refining and deoxidation. The content of each element introduced into the raw materials is shown in Table 3. The amount of Al added was determined by the following formula:

[0061] w[%Al]=w[%O]-0.8*w[%C]-0.1*(w[%Si]+w[%Mn]+w[%Ti])+0.05%;

[0062] Table 3. Content (wt.%) of each element introduced into the raw materials of Example 3

[0063] content 0.0200 0.0102 0.0085 0.0040 0.0051

[0064] The amount of Al added was determined to be 0.0601 wt.%, the refining temperature for deoxidation was 1560℃, and the refining time was 6 min. After the vacuum Al refining and deoxidation treatment was completed, high-purity argon gas was introduced to 0.05 MPa, and the refined NiCa master alloy was added for high-temperature refining and de-Al treatment. The amount of NiCa master alloy added was determined according to the following formula:

[0065] (0.75*w[%Al]+w[%O]-0.01%) / 0.005≤w[%NiCa]≤w[%Ni] / 0.95

[0066] The calculated addition amount of NiCa master alloy was determined to be 8.5 wt.%, the high-temperature refining and Al removal temperature was 1620℃, the refining time was 20 min, and after refining, the power was turned off and the mixture was allowed to stand for 11 min. After the high-temperature refining and Al removal process was completed, the argon gas was evacuated to a pressure of 6.5 × 10⁻⁶. -2 Pa is used for low-temperature refining to remove Ca and further deep deoxidation and desulfurization. The refining temperature of the low-temperature refining process is 1480℃ and the refining time is 35min. After refining, the power is turned off and the temperature is allowed to drop. The temperature of the molten steel is adjusted to 1450℃. A zirconia filter with a porosity of 20ppi is placed in the steel guide trough and then cast to obtain an ingot.

[0067] Example 4

[0068] In this embodiment, a vacuum induction melting method is used to prepare the NiCa master alloy. Electrolytic nickel (GB / T 6516-2010, Ni9999) and metallic calcium with a purity of 99 wt.% or higher are selected. The raw materials are prepared according to a calcium mass percentage of 60%, with the balance being nickel; a vacuum is drawn to a pressure of 8.5 × 10⁻⁶. -2 Pa, close the valve and purge with high-purity argon gas (99.999% volume purity) to 0.05 MPa; then evacuate the argon gas again until the pressure reaches 8.6 × 10⁻⁶ MPa. -2 Pa, then close the valve and purge with high-purity argon to 0.05 MPa; alternately stack nickel and calcium blocks in the crucible, and melt the raw materials to obtain molten steel by power supply. Refine at 1440℃ for 35 min. After refining, adjust the temperature and cast at 1305℃ to obtain NiCa master alloy; in the prepared NiCa master alloy, the mass percentage content of Ca is 15%, the mass percentage content of O is 0.0044%, and the remainder is Ni.

[0069] Based on the target alloy composition, electrolytic nickel (GB / T 6516-2010, Ni9999), metallic chromium (GB / T 3211-2023, JCr98), molybdenum bars (GB / T 3462-2017, Mo-2), and industrial pure iron were selected as raw materials, and the materials were proportioned according to the ratio of Ni, Cr, Mo, and Fe; the vacuum was evacuated to a pressure of 8.2 × 10⁻⁶. -2 Pa, power is supplied to start smelting, and molten steel is obtained; Al is added to the molten steel for refining and deoxidation. The content of each element introduced into the raw materials is shown in Table 3. The amount of Al added is determined by the following formula:

[0070] w[%Al]=w[%O]-0.8*w[%C]-0.1*(w[%Si]+w[%Mn]+w[%Ti])+0.05%;

[0071] Table 4. Content (wt.%) of each element introduced into the raw materials of Example 4

[0072] content 0.0266 0.0089 0.0091 0.0061 0.0021

[0073] The calculated Al addition amount was 0.0678 wt.%, the refining temperature for deoxidation was 1525℃, and the refining time was 7 min. After the vacuum Al refining and deoxidation treatment was completed, high-purity argon gas was introduced to 0.04 MPa, and the refined NiCa master alloy was added for high-temperature refining and Al removal treatment. The amount of NiCa master alloy added was determined according to the following formula:

[0074] (0.75*w[%Al]+w[%O]-0.01%) / 0.005≤w[%NiCa]≤w[%Ni] / 0.95

[0075] The amount of NiCa master alloy added was determined to be 15 wt.%, the refining temperature for high-temperature Al removal was 1660℃, and the refining time was 20 min. After refining, the mixture was left to stand for 20 min with the power off. After the high-temperature Al removal process was completed, the argon gas was evacuated to a pressure of 7.2 × 10⁻⁶. -2 Pa is used for low-temperature refining to remove Ca and further deep deoxidation and desulfurization. The refining temperature of the low-temperature refining process is 1550℃ and the refining time is 30min. After refining, the power is turned off and the temperature is allowed to drop. The temperature of the molten steel is adjusted to 1430℃. A zirconia filter with a porosity of 20ppi is placed in the steel guide trough and then cast to obtain an ingot.

[0076] The chemical composition of the ultrapure 316L stainless steel obtained through the above embodiments is shown in Table 5. After processing by the process of the present invention, the content of impurity elements in the steel is stably and well controlled, meeting the requirements for the preparation of ultrapure 316L stainless steel.

[0077] Table 5. Element content (wt.%) of alloys prepared in Examples 1-4

[0078]

[0079]

[0080] The inclusion ratings of the ultrapure 316L prepared in the examples were performed according to ASTM E45 standard, as shown in Table 6. The coarse inclusions (A, B, C, D, and DS) in the prepared ultrapure 316L were all grade 0, and the fine inclusions (A+B+C+D+DS) were ≤ grade 0.5, all meeting the requirements.

[0081] Table 6. Inclusion ratings in alloys prepared in Examples 1-4

[0082]

[0083] In summary, the method provided by this invention first prepares a NiCa master alloy through vacuum induction melting, then prepares a 316L alloy through vacuum induction melting. Al is first added for deoxidation, followed by high-temperature refining with the refined NiCa master alloy to remove Al, and finally, low-temperature refining to remove Ca and further deep deoxidation and desulfurization are performed. Thus, the composition and inclusions of ultrapure 316L stainless steel can be controlled using only one method: vacuum induction melting.

[0084] The above description is merely an embodiment of the present invention and does not limit the scope of protection of the present invention. Those skilled in the art can make other variations or modifications based on the above description. It is neither necessary nor possible to exhaustively describe all embodiments here. All equivalent changes or modifications made according to the spirit and essence of the present invention are similarly covered within the scope of protection of the present invention.

Claims

1. A method for preparing ultrapure 316L stainless steel by vacuum induction melting, characterized in that, The composition by mass percentage of each component is as follows: C ≤0.01%, Si ≤0.03%, Mn ≤0.1%, P ≤0.01%, S ≤0.003%, Ni 12.5~15%, Cr 16~18%, Mo 2.2~3.0%, Cu ≤0.2%, Al ≤0.008%, Ca ≤0.01%, Ti ≤0.01%, Nb ≤0.02%, Se ≤0.001%, O ≤0.0008%, N ≤0.003%, H ≤0.0001%, with the remainder being Fe; the coarse inclusions (A, B, C, D, and DS) of the ultrapure 316L stainless steel are all grade 0, and the fine inclusions (A+B+C+D+DS) are ≤0.5 grade; The vacuum induction melting method for preparing ultrapure 316L stainless steel includes the following steps: (1) A NiCa master alloy was prepared by vacuum induction melting. (2) Based on the target alloy composition, electrolytic nickel, metallic chromium, molybdenum bars and industrial pure iron are selected as raw materials and mixed in the crucible according to the proportion of each element Ni, Cr, Mo and Fe. (3) Evacuate the gas pressure to below 5 Pa, then power on to start smelting and obtain molten steel; (4) Add Al to the molten steel for refining and deoxidation. The refining temperature for refining and deoxidation is 1520~1560℃ and the refining time is 5~7min. The amount of Al element added is determined according to Equation 1: w[%Al]=w[%O]-0.8*w[%C]-0.1*(w[%Si]+w[%Mn]+w[%Ti])+0.05% Formula 1; In Formula 1, w[%Al] is the amount of Al added, w[%O] is the initial O content introduced into each raw material, w[%C] is the initial C content introduced into each raw material, w[%Si] is the initial Si content introduced into each raw material, w[%Mn] is the initial Mn content introduced into each raw material, and w[%Ti] is the initial Ti content introduced into each raw material. (5) After the vacuum Al refining and deoxidation treatment is completed, high-purity argon gas is introduced to 0.03~0.05 MPa, and the refined NiCa intermediate alloy is added for high-temperature refining and de-Al treatment. The refining temperature of the high-temperature refining and de-Al treatment is 1620~1680℃, the refining time is 15~20min, and after the refining is completed, the power is turned off and the mixture is left to stand for more than 10min. The amount of NiCa master alloy added is determined according to formula 2: (0.75*w[%Al]+w[%O]-0.01%) / 0.005≤w[%NiCa]≤w[%Ni] / 0.95 Equation 2 In Formula 2, w[%NiCa] is the amount of NiCa master alloy added, w[%Al] is the amount of Al added in step (4), w[%O] is the initial O content introduced into each raw material, and w[%Ni] is the amount of Ni added in step (2). (6) After the high-temperature refining and Al removal process is completed, remove the argon gas until the pressure is 9.9 × 10⁻⁶. -2 Below Pa, low-temperature refining and decalcification and further deep deoxidation and desulfurization are carried out. The refining temperature of the low-temperature refining process is 1450℃~1550℃, and the refining time is 30~40min. (7) After refining, the power is turned off and the temperature is allowed to drop. The temperature of the molten steel is adjusted to 1420~1470℃. A filter screen is placed in the steel guide trough and the casting is carried out to obtain the ingot.

2. The method for preparing ultrapure 316L stainless steel by vacuum induction melting according to claim 1, characterized in that, In step (1), the preparation of the NiCa master alloy includes the following steps: 1) Raw material preparation: Select electrolytic nickel and metallic calcium with a purity of 99wt.% or higher. Prepare raw materials according to the mass percentage of calcium as 50%~60%, with the balance being nickel; 2) Argon gas furnace cleaning: Evacuate to a pressure of 9.9 × 10⁻⁶. -2 Below 0.03 MPa, close the valve and purge with high-purity argon gas to 0.03~0.05 MPa; then evacuate the argon gas again until the pressure is 9.9×10⁻⁶ MPa. -2 Below Pa, then close the valve and purge with high-purity argon gas to 0.03~0.05 MPa; 3) Induction melting: Nickel blocks and calcium blocks are alternately stacked in a crucible, and electricity is applied to melt the raw materials to obtain molten steel. The steel is then refined at a temperature of 1400~1450℃ for 20~40 minutes. After refining, the temperature is adjusted and the steel is cast at 1300~1320℃ to obtain NiCa master alloy. 4) In the prepared NiCa master alloy, the mass percentage content of Ca is 5%~15%, the mass percentage content of O is ≤0.0080%, and the remainder is Ni.

3. The method for preparing ultrapure 316L stainless steel by vacuum induction melting according to claim 1 or 2, characterized in that, The crucible used is an Al2O3 or CaO crucible.

4. The method for preparing ultrapure 316L stainless steel by vacuum induction melting according to claim 1, characterized in that, In step (7), the filter screen is made of zirconium oxide with a porosity of 20 ppi.

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