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

Abstract

The invention belongs to the technical field of metallurgy, and particularly relates to ultrapure 316L stainless steel and a vacuum induction melting preparation method thereof. The preparation method comprises the following steps: firstly preparing NiCa intermediate alloy through vacuum induction melting, then preparing 316L alloy through vacuum induction melting, firstly adding Al for deoxidation, then adding the refined NiCa intermediate alloy for high-temperature refining for Al removal, and finally performing low-temperature refining for Ca removal and further deep deoxidation and desulfurization. According to the method, various impurity elements such as Al, O and S and inclusions of the ultra-pure 316L stainless steel can be removed only through a vacuum induction melting method, extra technological processes such as electroslag or self consumption are not needed, equipment investment is simple and convenient, large-scale industrial application can be achieved easily, and an effective way is provided for purification preparation of the ultra-pure 316L stainless steel.

Description

Technical Field:

[0001] The present invention belongs to the field of metallurgical technology, and particularly relates to an ultra-pure 316L stainless steel and a method for preparing the same by vacuum induction melting. Background Art:

[0002] With the development of the semiconductor industry, the requirements for material purity and performance are increasing day by day. Ultra-pure 316L stainless steel, as a key material in semiconductor equipment, is commonly used to manufacture reaction chambers, pipelines, valves, etc. Its quality and stability are directly related to 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 content of impurity elements and inclusions in ordinary commercially available 316L stainless steel far cannot meet the requirements of the semiconductor industry. Therefore, developing a preparation process for industrially applicable ultra-pure 316L stainless steel is of great significance.

[0003] To obtain 316L stainless steel with high purity, if the method of non-vacuum induction melting is adopted, it is very difficult to effectively remove various impurity elements, especially it is very difficult to control the inclusion content. In the patent with publication number CN 117512268 A, the method of EAF+AOD+LF+VAR is used to prepare 316L, and it is very difficult to control the inclusion content to reach grade 0, while the control of impurity elements is not reflected. For the existing process routes of 316L vacuum melting, usually two-step or three-step vacuum methods are required to obtain appropriate control of alloy impurity content. For example, in the patents with publication numbers CN 116623105 A and CN 117987747 A, the process of vacuum induction melting + vacuum arc remelting is adopted, and in the patent with publication number CN 118389933 A, the method of vacuum induction melting + vacuum consumable is used to prepare the alloy. These methods have complex process flows, and it is necessary to first prepare electrode rods and go through processes such as cutting risers, turning or machining before entering the next process. The material utilization rate is extremely low, and the investment requirements for equipment are large, and the process cost is high.

[0004] Generally, it is considered that C is an effective deoxidizer in vacuum induction melting, but the product of C deoxidation is a gas, which is difficult to spontaneously form bubbles and discharge in the molten steel. And the solubility of gas in the molten steel is limited, so C deoxidation cannot continue, resulting in a still relatively high residual oxygen content in the molten steel, not meeting the requirements of high-purity 316L. The deoxidation ability of Al is stronger than that of C, and its product is solid Al 2 O 3 , with strong nucleation ability, can continuously combine with oxygen to reduce the oxygen content in the molten steel to an extremely low level. However, the requirement for the Al content in ultra-pure 316L is relatively low, and the deoxidation product Al 2 O 3Inclusions are easily formed and difficult to remove. Therefore, Al is not used in conventional methods, but Mg and Ca are used for deoxidation and desulfurization. However, Mg and Ca volatilize severely under high-temperature vacuum, and the addition amount is difficult to control. As a result, the deoxidation effect is difficult to be stable and effective. For this reason, it is often necessary to further purify the alloy by methods such as increasing secondary vacuum consumable melting, etc., in order to control the content of impurity elements and inclusions, which will increase a large amount of equipment costs and material losses. Summary of the Invention:

[0005] The object of the present invention is to overcome the deficiencies of the above-mentioned existing technologies, and provide a super-pure 316L stainless steel and its vacuum induction melting preparation method. By using the method of the present invention, deep removal of impurity elements and precise control of inclusions can be achieved only through vacuum induction melting. It is possible to prepare super-pure 316L stainless steel with high purity and low inclusion content. The equipment investment is simple and convenient, which is conducive to large-scale industrial application.

[0006] To achieve the above object, the present invention adopts the following technical solutions:

[0007] A super-pure 316L stainless steel, with the component mass percentages 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%, and the rest is Fe; the coarse inclusions of types A, B, C, D, and DS of the super-pure 316L stainless steel are all grade 0, and the fine inclusions of types A + B + C + D + DS ≤ 0.5 grade.

[0008] The vacuum induction melting preparation method of the super-pure 316L stainless steel includes the following steps:

[0009] (1) Prepare NiCa master alloy by vacuum induction melting method;

[0010] (2) According to the target composition of the alloy, select electrolytic nickel, metallic chromium, molybdenum bars, and industrial pure iron as raw materials, and carry out batching in a crucible according to the element ratios of Ni, Cr, Mo, and Fe;

[0011] (3) Evacuate to a pressure below 5 Pa, start melting by power supply, and obtain molten steel;

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

[0013] The addition amount of the Al element 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% Equation 1;

[0015] In Equation 1, w[%Al] is the addition amount of Al, w[%O] is the initial O content introduced in each raw material, w[%C] is the initial C content introduced in each raw material, w[%Si] is the initial Si content introduced in each raw material, w[%Mn] is the initial Mn content introduced in each raw material, and w[%Ti] is the initial Ti content introduced in each raw material;

[0016] (5) After the vacuum Al refining and deoxidation treatment is completed, high-purity argon gas is filled to 0.03 - 0.05 MPa, and the prepared NiCa master alloy is added for high-temperature refining to remove Al. The refining temperature for the high-temperature refining to remove Al is 1620 - 1680 °C, the refining time is 15 - 20 min. After the refining is completed, the power is cut off and the steel is left to stand for more than 10 min;

[0017] The addition amount of the NiCa master alloy is determined according to Equation 2:

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

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

[0020] (6) After the high-temperature refining to remove Al is completed, the argon gas is evacuated until the pressure is below 9.9×10 -2 Pa, and low-temperature refining is carried out to remove Ca and further deeply deoxidize and desulfurize. The refining temperature during the low-temperature refining process is 1450 °C - 1550 °C, and the refining time is 30 - 40 min;

[0021] (7) After the refining is completed, the power is cut off and the steel is left to stand and cool down. The temperature of the molten steel is adjusted to 1420 - 1470 °C, and a filter screen is placed in the ladle chute for casting to obtain an ingot.

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

[0023] 1) Raw material preparation: Electrolytic nickel and metallic calcium with a purity of more than 99 wt.% are selected. The raw materials are proportioned according to the calcium mass percentage of 50% - 60%, and the balance is nickel;

[0024] 2) Argon furnace washing: Evacuate to a pressure below 9.9×10 -2 Pa, close the valve and fill with high-purity argon to 0.03 - 0.05 MPa; Evacuate the argon again to a pressure below 9.9×10 -2 Pa, then close the valve and fill with high-purity argon to 0.03 - 0.05 MPa;

[0025] 3) Induction melting: Alternately stack nickel blocks and calcium blocks in the crucible, supply power to melt the raw materials to obtain molten steel, refine at a temperature of 1400 - 1450 °C for 20 - 40 min. After refining, adjust the temperature and cast at 1300 - 1320 °C to obtain the NiCa master alloy;

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

[0027] For the method for preparing ultra-pure 316L stainless steel by vacuum induction melting, the crucible used is an Al 2 O 3 or CaO crucible.

[0028] For the method for preparing ultra-pure 316L stainless steel by vacuum induction melting, in step (7), the filter screen is made of zirconia with a porosity of 20 ppi.

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

[0030] Ultra-pure 316L stainless steel, as a key material in semiconductor devices, is often used to make reaction chambers, pipelines, valves, etc. Its quality and stability are directly related to the performance and reliability of semiconductor devices. To obtain 316L stainless steel with high purity, if the non-vacuum induction melting method is used, it is very difficult to effectively remove various impurity elements. When using vacuum induction melting, the traditional idea is that C is an effective deoxidizer in vacuum induction melting, but the product of C deoxidation is a gas, which is difficult to spontaneously form bubbles and discharge in the molten steel, and the solubility of gas in the molten steel is limited. Therefore, C deoxidation cannot continue, resulting in a still high residual oxygen content in the molten steel, not meeting the requirements of high-purity 316L. The deoxidation ability of Al is stronger than that of C, and its product is solid Al 2 O 3 , with strong nucleation ability, can continuously combine with oxygen to reduce the oxygen content in the molten steel to an extremely low level. However, the requirement for the Al content in ultra-pure 316L is relatively low, and the deoxidation product Al 2 O3 Inclusions are easily formed and difficult to remove. Therefore, Al is not used in conventional methods, but Mg and Ca are used for deoxidation and desulfurization. However, Mg and Ca volatilize severely under high-temperature vacuum, and the addition amount is difficult to control, so the deoxidation effect is difficult to be stable and effective. For this reason, it is often necessary to purify the alloy again by subsequent methods such as increasing secondary vacuum consumable melting. By adopting a two-step or even three-step vacuum method, the control of the content of impurity elements and inclusions can be achieved, but this will greatly increase the equipment cost and material loss. Different from the traditional idea, the present invention proposes a feasible method of first adding Al for deoxidation and then removing Al through a reaction. The preparation of ultra-pure 316L stainless steel can be realized only by one method of vacuum induction melting.

[0031] Specifically, during vacuum induction melting, the deoxidation effect of Al is much better than that of other elements such as C. Therefore, in the vacuum induction melting stage of the present invention, a certain amount of Al is directly added for deoxidation first. However, the addition amount of Al element needs to be strictly controlled so that it can fully remove O while the residual amount is not too much, and appropriate refining temperature and time must be selected to make the deoxidation sufficient and its products will not aggregate and grow. The Al 2 O 3 inclusions formed after Al deoxidation are difficult to be fully removed by conventional processes, which is the key factor for not generally choosing Al deoxidation in ultra-pure 316L. In the present invention, a treatment of refining and removing Al is applied. Since the Al element enters the molten steel after melting and combines with the free O element in the molten steel, its deoxidation product Al 2 O 3 is fine and dispersed in the molten steel. The melting point of Al 2 O 3 is relatively high, and it exists in the molten steel in a solid form. Due to the action of surface tension, these fine and dispersed Al 2 O 3 are difficult to fully aggregate and grow, and cannot be removed by static floating or filtering through a filter screen. In the refining and removing Al process of the present invention, an appropriate amount of Ca is introduced to carry out a modification treatment on Al 2 O 3 so that it is all transformed into low-melting-point CaO·Al 2 O 3 composite inclusions, and appropriate refining temperature and time are controlled. The refining temperature is strictly controlled to be higher than the melting point of this composite inclusion and lower than the temperature at which it reacts with the crucible. The refining time is controlled so that the original high-melting-point solid Al 2 O 3Inclusions are all transformed into low-melting-point liquid composite inclusions, thus eliminating the influence of the surface tension between solid and liquid and enabling them to completely dissolve in the molten steel. Since the density of the composite inclusions is lower than that of the molten steel, they can fully float to the upper part of the molten steel and then solidify again during the long-term static holding after power-off at the end of refining. Eventually, a layered structure with solid composite inclusions in the upper part and pure molten steel in the lower part is formed.

[0032] After the completion of high-temperature refining and de-aluminum in the present invention, a certain amount of Ca can still exist in the molten steel. And Ca and its deoxidation and desulfurization products are extremely volatile under vacuum. Therefore, the present invention continues to carry out deep de-Ca and further deep deoxidation and desulfurization by applying high-vacuum and low-temperature refining, which can continuously reduce the impurity content in the molten steel. By controlling the vacuum degree to enhance the volatilization of Ca and its deoxidation and desulfurization products and controlling the appropriate refining temperature, the impurities can be continuously removed by reaction while being lower than the melting point of the composite inclusions generated in the previous step, so that they cannot re-dissolve in the molten steel. By controlling the refining time, ultra-pure 316L molten steel can be obtained, and the composite inclusions are filtered out through a filter screen. Finally, high-purity 316L stainless steel with high cleanliness can be obtained.

[0033] In the present invention, the addition amount of Ca needs to be precisely controlled. The oxygen content in commercially available calcium or nickel-calcium alloy is relatively high, so there will be more CaO present, and the content of Ca element in metallic form is extremely unstable, with poor application effect and unable to obtain a stable expected Ca addition amount. Therefore, the present invention also provides a preparation method of NiCa master alloy. The NiCa master alloy prepared by the present invention has a low oxygen content and a stable metallic calcium content. At the same time, by controlling the Ca mass fraction within 5% - 15%, the obtained NiCa master alloy has a high melting point and less burning loss when added to the molten steel, so that the precise control of the Ca addition amount can be realized.

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

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

[0036] 1. The present invention provides a method for preparing ultra-pure 316L stainless steel by vacuum induction melting, breaking through the limitation in the traditional method that Al deoxidation cannot be carried out. By first adding Al for deoxidation and then carrying out de-aluminum treatment, the preparation of ultra-pure 316L stainless steel is realized.

[0037] 2. The present invention also provides a preparation method of NiCa master alloy. By controlling the composition range and oxygen content, the prepared master alloy has a high melting point and a stable metallic calcium content, and can realize the stable control of the Ca addition amount.

[0038] 3. The method adopted by the present invention can be realized only through vacuum induction melting, without going through additional process steps such as electroslag or consumable electrode melting. In the ultra-pure 316L prepared, 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%. The coarse inclusions of types A, B, C, D, and DS are all grade 0, and the fine inclusions of types A + B + C + D + DS are ≤0.5 grade. The equipment investment is simple and convenient, which is conducive to the large-scale industrial application of ultra-pure 316L stainless steel. Specific Embodiments:

[0039] The following further describes the specific embodiments of the present invention in detail in conjunction with examples and comparative examples. It should be noted here that the description of these embodiments is used to help understand the present invention, but does not constitute a limitation on the present invention. In addition, 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 example, the NiCa master alloy was prepared by vacuum induction melting. Electrolytic nickel (GB / T 6516 - 2010, Ni9950) and metallic calcium with a purity of more than 99 wt.% were selected, and the raw materials were proportioned according to 50% by mass percentage of calcium, with the balance being nickel; the vacuum was pumped to a pressure of 9.8×10 -2 Pa, and the valve was closed and high-purity argon (volume purity 99.999%) was charged to 0.03 MPa; the argon was pumped away again to a pressure of 9.7×10 -2 Pa, and then the valve was closed and high-purity argon was charged to 0.03 MPa; the nickel blocks and calcium blocks were alternately stacked in the crucible, and the power was supplied to melt the raw materials to obtain molten steel, which was refined at a temperature of 1450 °C for 20 min. After the refining was completed, the temperature was adjusted, and the NiCa master alloy was cast at 1300 °C; in the prepared NiCa master alloy, the mass percentage content of Ca was 5%, and the mass percentage content of O was 0.0078%, and the rest was Ni.

[0042] According to the target composition of the alloy, 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 ingredients were proportioned according to the ratios of Ni, Cr, Mo, and Fe elements; the vacuum was pumped to a pressure of 4.8 Pa, and the power was supplied to start melting to obtain molten steel; Al was added to the molten steel for refining deoxidation. The contents of the elements introduced in the raw materials are shown in Table 1, and the addition amount of Al 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 of each element introduced in the raw materials of Example 1 (wt.%)

[0045] Element O C Si Mn Ti Content 0.0074 0.0041 0.0040 0.0030 0.0010

[0046] The addition amount of Al is determined to be 0.0533 wt.% by calculation. The refining temperature for refining deoxidation is 1520 °C, and the refining time is 7 min. After the vacuum Al refining deoxidation treatment is completed, high-purity argon gas is filled to 0.03 MPa, and the prepared NiCa master alloy is added for high-temperature refining to remove Al. The addition amount of the NiCa master alloy is 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 addition amount of the NiCa master alloy is determined to be 13 wt.% by calculation. The refining temperature for high-temperature refining to remove Al is 1680 °C, and the refining time is 15 min. After refining is completed, power is cut off and the molten metal is left standing for 15 min. After the high-temperature refining to remove Al treatment is completed, the argon gas is evacuated to a pressure of 9.8×10 -2 Pa, and low-temperature refining is carried out to remove Ca and further deeply deoxidize and desulfurize. The refining temperature during the low-temperature refining process is 1450 °C, and the refining time is 40 min. After refining is completed, power is cut off and the molten metal is left standing and cooled down. The temperature of the molten steel is adjusted to 1420 °C, and a zirconia filter with a porosity of 20 ppi is placed in the launder for casting to obtain an ingot.

[0049] Example 2

[0050] In this example, the NiCa master alloy is prepared by the vacuum induction melting method. Electrolytic nickel (GB / T 6516 - 2010, Ni9990) and metallic calcium with a purity of more than 99 wt.% are selected, and the raw materials are proportioned according to 55% of the mass percentage of calcium, and the balance is nickel; the vacuum is pumped to a pressure of 7.9×10 -2 Pa, and the valve is closed and high-purity argon gas (volume purity 99.999%) is filled to 0.04 MPa; the argon gas is evacuated again to a pressure of 7.8×10 -2Pa, then close the valve and fill with high-purity argon to 0.04 MPa; alternately stack nickel blocks and calcium blocks in a crucible, send electricity to melt the raw materials to obtain molten steel, refine at 1400 °C for 40 min, adjust the temperature after refining, and cast at 1320 °C 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 rest is Ni.

[0051] According to the target composition of the alloy, 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 are selected as raw materials, and ingredients are prepared according to the ratio of each element of Ni, Cr, Mo, and Fe; evacuate to a pressure of 4.2 Pa, send electricity to start melting to obtain molten steel; add Al to the molten steel for refining and deoxidation. The content of each element introduced in the raw materials is shown in Table 2, and the addition amount of Al is 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 of each element introduced in the raw materials of Example 2 (wt.%)

[0054] Element O C Si Mn Ti Content 0.0112 0.0051 0.0052 0.0060 0.0021

[0055] The addition amount of Al is determined to be 0.0558 wt.% by calculation. The refining temperature for refining and deoxidation is 1550 °C, and the refining time is 5 min; after the vacuum Al refining and deoxidation treatment is completed, fill with high-purity argon to 0.03 MPa, and add the prepared NiCa master alloy for high-temperature refining to remove Al. The addition amount of NiCa master alloy is determined by 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 is determined to be 10 wt.% by calculation. The refining temperature for high-temperature refining to remove Al is 1640 °C, and the refining time is 18 min. After refining, cut off the power and let it stand for 15 min; after the high-temperature refining to remove Al treatment is completed, evacuate the argon to a pressure of 9.0×10 -2Pa, perform low-temperature refining to remove Ca and further deeply deoxidize and desulfurize. The refining temperature during the low-temperature refining process is 1530 °C, and the refining time is 32 min. After the refining is completed, cut off the power supply and let it stand still to cool down. Adjust the temperature of the molten steel to 1470 °C, place a zirconia filter with a porosity of 20 ppi in the ladle chute, and carry out casting to obtain an ingot.

[0058] Example 3

[0059] In this example, a NiCa master alloy was prepared by the vacuum induction melting method. Electrolytic nickel (GB / T 6516-2010, Ni9920) and metallic calcium with a purity of more than 99 wt.% were selected, and the raw materials were proportioned according to a calcium mass percentage of 58%, with the balance being nickel; evacuate to a pressure of 6.2×10 -2 Pa, close the valve and fill with high-purity argon gas (volume purity 99.999%) to 0.04 MPa; evacuate the argon gas again to a pressure of 6.5×10 -2 Pa, then close the valve and fill with high-purity argon gas to 0.04 MPa; alternately stack nickel blocks and calcium blocks in the crucible, send electricity to melt the raw materials to obtain molten steel, refine at a temperature of 1420 °C for 30 min, adjust the temperature after the refining is completed, and cast at 1310 °C to obtain the 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 rest is Ni.

[0060] According to the target composition of the alloy, 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 ingredients were proportioned according to the ratios of Ni, Cr, Mo, and Fe elements; evacuate to a pressure of 3.2 Pa, send electricity to start melting to obtain molten steel; add Al to the molten steel for refining and deoxidation. The contents of each element introduced in the raw materials are shown in Table 3, and the addition amount of Al is 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 Contents of each element introduced in the raw materials of Example 3 (wt.%)

[0063] Element O C Si Mn Ti Content 0.0200 0.0102 0.0085 0.0040 0.0051

[0064] The addition amount of Al is determined by calculation to be 0.0601 wt.%, the refining temperature for refining and deoxidation is 1560 °C, and the refining time is 6 min; after the vacuum Al refining and deoxidation treatment is completed, high-purity argon gas is filled to 0.05 MPa, and the prepared NiCa master alloy is added for high-temperature refining to remove Al, and the addition amount of the NiCa master alloy is 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 addition amount of the NiCa master alloy is determined by calculation to be 8.5 wt.%, the refining temperature for high-temperature refining to remove Al is 1620 °C, and the refining time is 20 min. After refining is completed, power is cut off and the steel is left to stand for 11 min; after the high-temperature refining to remove Al treatment is completed, the argon gas is evacuated to a pressure of 6.5×10 -2 Pa, and low-temperature refining is carried out to remove Ca and further deeply deoxidize and desulfurize. The refining temperature during the low-temperature refining process is 1480 °C, and the refining time is 35 min; after refining is completed, power is cut off and the temperature is allowed to drop, and the temperature of the molten steel is adjusted to 1450 °C. A zirconia filter with a porosity of 20 ppi is placed in the ladle chute, and casting is carried out to obtain an ingot.

[0067] Example 4

[0068] In this example, the NiCa master alloy is prepared by the vacuum induction melting method. Electrolytic nickel (GB / T 6516-2010, Ni9999) and metallic calcium with a purity of more than 99 wt.% are selected, and the raw materials are proportioned according to 60% of the mass percentage of calcium, and the balance is nickel; the vacuum is pumped to a pressure of 8.5×10 -2 Pa, the valve is closed and high-purity argon gas (volume purity 99.999%) is filled to 0.05 MPa; the argon gas is evacuated again to a pressure of 8.6×10 -2 Pa, and then the valve is closed and high-purity argon gas is filled to 0.05 MPa; the nickel blocks and calcium blocks are alternately stacked and placed in the crucible, and power is supplied to melt the raw materials to obtain molten steel, which is refined at a temperature of 1440 °C for 35 min. After refining is completed, the temperature is adjusted, and casting is carried out at 1305 °C to obtain the 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 rest is Ni.

[0069] According to the target alloy composition, electrolytic nickel (GB / T 6516-2010, Ni9999), metallic chromium (GB / T 3211-2023, JCr98), molybdenum bar (GB / T 3462-2017, Mo-2), and industrial pure iron were selected as raw materials, and the ingredients were prepared according to the proportion of Ni, Cr, Mo, and Fe. The vacuum was evacuated to a pressure of 8.2×10 -2 Pa, power is supplied to start smelting to obtain molten steel; Al is added to the molten steel for refining and deoxidation. The contents of various elements introduced in the raw materials are shown in Table 3, and 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 of each element introduced in the raw materials of Example 4 (wt.%)

[0072] Element O C Si Mn Ti Content 0.0266 0.0089 0.0091 0.0061 0.0021

[0073] The amount of Al added was determined to be 0.0678wt.%, the refining temperature for refining and deoxidation was 1525°C, and the refining time was 7min. After the vacuum Al refining and deoxidation treatment was completed, high-purity argon was filled to 0.04MPa, 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:

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

[0075] The addition amount of NiCa master alloy was determined to be 15wt.%, the refining temperature of high-temperature refining and de-Al was 1660℃, the refining time was 20min, and after the refining was completed, the power was turned off and the temperature was left to stand for 20min. After the high-temperature refining and de-Al treatment was completed, the argon gas was pumped out until the pressure was 7.2×10 -2 Pa, carry out 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 the refining is completed, the power is turned off and the temperature is allowed to stand for cooling, the temperature of the molten steel is adjusted to 1430℃, a zirconium oxide filter with a porosity of 20ppi is placed in the steel guide trough, and casting is carried out to obtain an ingot.

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

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

[0078]

[0079]

[0080] Referring to the ASTM E45 standard, the inclusion rating of the ultra-pure 316L prepared in the examples was carried out, as shown in Table 6. The coarse series A, B, C, D, and DS inclusions in the prepared ultra-pure 316L were all grade 0, and the fine series A+B+C+D+DS inclusions were ≤ grade 0.5, all meeting the requirements.

[0081] Table 6 Inclusion rating in the alloys prepared in Examples 1-4

[0082]

[0083] In summary, by using the method provided by the present invention, first, the NiCa master alloy is prepared by vacuum induction melting, and then the 316L alloy is prepared by vacuum induction melting. First, Al is added for deoxidation, then the prepared NiCa master alloy is added for high-temperature refining to remove Al, and finally, low-temperature refining is carried out to remove Ca and further deeply deoxidize and desulfurize. Thus, the control of the composition and inclusions of ultra-pure 316L stainless steel can be achieved only by one method of vacuum induction melting.

[0084] The above are only the embodiments of the present invention, and do not limit the protection scope of the present invention accordingly. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to enumerate all the implementation manners here. Any equivalent changes or modifications made according to the spirit of the present invention are equally covered within the protection scope of the present invention.

Claims

1. An ultra-pure 316L stainless steel, characterized in that: The composition by mass percentage of each component is: 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%, and the rest is Fe; the coarse A, B, C, D, and DS inclusions of the ultra-pure 316L stainless steel are all level 0, and the fine A+B+C+D+DS inclusions are ≤level 0.

2. The vacuum induction melting method for preparing ultrapure 316L stainless steel according to claim 1, characterized in that: The steps include: (1) NiCa master alloy is prepared by vacuum induction melting method; (2) According to the target alloy composition, electrolytic nickel, metallic chromium, molybdenum bars, and industrial pure iron are selected as raw materials, and the ingredients are placed in a crucible according to the proportions of Ni, Cr, Mo, and Fe; (3) Evacuate the gas until the pressure is below 5 Pa, and then supply power to start smelting to obtain molten steel; (4) adding Al to the molten steel for refining and deoxidation, wherein the refining temperature of the refining and deoxidation is 1520-1560° C. and the refining time is 5-7 min; The amount of Al added is determined according to Formula 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 is filled to 0.03-0.05 MPa, and the refined NiCa master 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° C., and the refining time is 15-20 min. After the refining is completed, the power is turned off and the alloy is left to stand for more than 10 min; The amount of the 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 treatment is completed, the argon gas is pumped out to a pressure of 9.9×10 -2 Pa, low temperature refining to remove Ca and further deep deoxidation and desulfurization, 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 steel is left to cool down. The temperature of the molten steel is adjusted to 1420-1470°C. A filter is placed in the steel guide trough and casting is performed to obtain an ingot.

3. The vacuum induction melting method for preparing ultrapure 316L stainless steel according to claim 2, characterized in that: In step (1), the preparation of the NiCa master alloy comprises the following steps: 1) Raw material preparation: electrolytic nickel and metallic calcium with a purity of more than 99wt.% are selected, and the raw materials are prepared according to a calcium mass percentage of 50% to 60%, and the balance is nickel; 2) Argon gas cleaning furnace: evacuate to a pressure of 9.9×10 -2 Pa, close the valve and fill with high-purity argon to 0.03-0.05MPa; pump out the argon again until the pressure is 9.9×10 -2 Pa, then close the valve and fill with high-purity argon gas to 0.03-0.05MPa; 3) Induction melting: nickel blocks and calcium blocks are alternately stacked and placed in a crucible, power is supplied to melt the raw materials to obtain molten steel, and the steel is refined at a temperature of 1400-1450° C. for 20-40 min. After the refining is completed, the temperature is adjusted and poured at 1300-1320° C. to obtain a NiCa master alloy; 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 rest is Ni.

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

5. The vacuum induction melting method for preparing ultrapure 316L stainless steel according to claim 2, characterized in that: In step (7), the filter screen is made of zirconium oxide and has a porosity of 20 ppi.

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