An ultra-pure 316l(n) austenitic stainless steel and a method for producing the same

Abstract

The application provides a kind of ultra-pure 316L (N) austenitic stainless steel and its preparation method, relate to metallurgy technical field.The application provides VIM+IESR double smelting process.In which, VIM adopts the ultra-pure combined control process of "first adding FeO to control aluminum, then vacuum carbon deoxidation pretreatment, finally Mg+Ca compound deoxidation and desulfurization", IESR further removes impurities and purifies, can realize 316L (N) austenitic stainless steel controlled aluminum and deep de-O, deep de-S and inclusion effective modification treatment, so as to meet the urgent needs of ultra-purity stainless steel material in the field of semiconductor equipment and nuclear power and the like.

Description

Technical Field

[0001] This invention relates to the field of metallurgical technology, and in particular to an ultrapure 316L(N) austenitic stainless steel and its preparation method. Background Technology

[0002] Ultra-pure 316L(N) austenitic stainless steel (referring to ultra-pure 316L austenitic stainless steel and ultra-pure 316LN austenitic stainless steel) possesses excellent corrosion resistance, electropolishing performance, low-temperature toughness, and neutron radiation resistance, making it a key material for components in semiconductor equipment (such as high-purity gas transmission pipelines and joints) and nuclear power equipment (such as main pipelines, magnet support plates, and bolts). With the continuous advancement of semiconductor processes and nuclear power technology, higher requirements have been placed on the purity of this steel: ultra-low O, S, and H; low Al, low P, and low C; and fine, dispersed, and harmless inclusions. All these requirements aim to ensure that the material possesses good corrosion resistance, polishing performance, low-temperature toughness, and neutron radiation resistance. However, under these stringent requirements, finding suitable deoxidation, desulfurization, and inclusion modification methods has become a challenge.

[0003] In traditional smelting processes, Al is often used as a deoxidizer. However, for ultrapure 316L(N) austenitic stainless steel, the lower the Al content, the better. Furthermore, Al deoxidation generates large, hard Al₂O₃ inclusions, making it unsuitable for ultrapure 316L(N) austenitic stainless steel. Mg and Ca treatments are commonly used to improve the purity of molten steel. They effectively remove O and S, significantly reduce the number and size of inclusions, and thus significantly improve the steel's purity. This is mainly due to the high reactivity of Mg and Ca in molten steel, their easy combination with O and S impurities, and their ability to transform harmful large Al₂O₃ inclusions into fine, dispersed MgO-Al₂O₃ and liquid calcium aluminate, promoting the flotation and removal of inclusions. Since ultrapure 316L(N) austenitic stainless steel requires very low impurity element content, it is difficult for Mg or Ca treatments alone to simultaneously control the O and S content to very low levels. Furthermore, excessive Mg and Ca can lead to the formation of large inclusions in steel, thereby deteriorating its properties. In addition, Mg and Ca have very high vapor pressures, and their reaction is very vigorous at steelmaking temperatures, making it difficult to control precisely during the smelting process.

[0004] Therefore, there is an urgent need to explore a technical solution for deep deoxidation, deep desulfurization and effective modification of inclusions in ultrapure 316L(N) austenitic stainless steel, so as to meet the urgent demand for ultrapure austenitic stainless steel materials in fields such as semiconductor equipment and nuclear power. Summary of the Invention

[0005] The purpose of this invention is to provide an ultrapure 316L(N) austenitic stainless steel and its preparation method. The method of this invention can achieve deep deoxidation, deep desulfurization and efficient modification of inclusions, and prepare 316L(N) austenitic stainless steel with ultra-high purity, good microstructure and properties, which meets the urgent demand of semiconductor equipment and other fields for ultra-high purity austenitic stainless steel materials.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] This invention provides an ultrapure 316L(N) austenitic stainless steel, comprising the following elements by mass percentage:

[0008]

[0009] The remainder is Fe;

[0010] The coarse inclusions in the ultrapure 316L(N) austenitic stainless steel are grade 0, and the fine inclusions (A+B+C+D) are ≤ grade 0.5.

[0011] When N≤0.01%, it is ultra-pure 316L austenitic stainless steel;

[0012] When N is 0.10% to 0.16%, it is an ultrapure 316LN austenitic stainless steel.

[0013] Preferably, when the stainless steel is ultrapure 316L austenitic stainless steel, the mechanical properties at room temperature are: yield strength ≥245MPa, tensile strength ≥550MPa, and elongation ≥70%.

[0014] When it is ultra-pure 316LN austenitic stainless steel, the mechanical properties at room temperature are: yield strength ≥330MPa, tensile strength ≥650MPa, and elongation ≥60%.

[0015] Preferably, the volume content of α-ferrite is 0, and the volume content of δ-ferrite is ≤0.1%.

[0016] Preferably, when the stainless steel is ultrapure 316L austenitic stainless steel, the grain size is 20-35 μm after hot rolling and annealing; when the stainless steel is ultrapure 316LN austenitic stainless steel, the grain size is 20-30 μm after hot rolling and annealing.

[0017] This invention provides a method for preparing the ultrapure 316L(N) austenitic stainless steel described above, comprising the following steps:

[0018] (1) Prepare the raw materials according to the target composition and perform vacuum induction melting to obtain molten steel; by mass percentage, the total Al introduced by the raw materials is ≤0.030%, total O is ≤0.025%, total S is ≤0.004%, and total P is ≤0.003%; the material of the crucible used for vacuum induction melting is MgO, CaO or MgO-CaO mixed material;

[0019] (2) Add FeO to the molten steel to refine and remove Al, and the refining and Al removal time is 2 to 3 minutes;

[0020] When the initial O content of the molten steel is >0.009% and ≤0.025%, the amount of FeO added is determined according to Formula 1: w[%FeO]=4×(w[%Al]-0.01%)-4.5×(w[%O]-0.009%)Formula 1; if w[%FeO]≤0 is calculated according to Formula 1, then it is not necessary to add FeO to the molten steel.

[0021] When the initial oxygen content of the molten steel is ≤0.009%, the amount of FeO added is determined according to formula 2:

[0022] w[%FeO]=4×(w[%Al]-0.01%)+4.5(0.009%-w[%O]) Formula 2;

[0023] In formulas 1 and 2: w[%FeO] is the amount of FeO added, w[%Al] is the initial Al content in the molten steel, and w[%O] is the initial O content in the molten steel;

[0024] (3) Add high-purity C to the de-Al steel liquid and perform vacuum C deoxidation pretreatment to control the O content of the steel liquid at 0.005% to 0.006%;

[0025] The amount of high-purity C added is determined according to formula 3:

[0026] w[%C]=0.7×(w[%O]-0.005%)Equation 3;

[0027] In Formula 3: w[%C] is the amount of high-purity C added, and w[%O] is the initial O content in the molten steel;

[0028] The time for the vacuum C deoxidation pretreatment is determined according to Equation 4:

[0029]

[0030] In Equation 4: t is the vacuum C deoxidation pretreatment time, s; F is the free surface area of ​​the molten steel, m². 2 V is the volume of molten steel, in meters. 3 ;w[%O] C The oxygen content (%) in steel after vacuum C deoxidation pretreatment;

[0031] (4) When preparing ultrapure 316L austenitic stainless steel, after the vacuum C deoxidation pretreatment is completed, high-purity argon gas is introduced into the vacuum induction melting furnace.

[0032] When preparing ultrapure 316LN austenitic stainless steel, after the vacuum C deoxidation pretreatment is completed, high-purity nitrogen gas is introduced into the vacuum induction melting furnace, and the temperature of the molten steel is controlled at 1480-1500℃ for vapor-phase nitriding. The vapor-phase nitriding time is 12-15 minutes. The pressure of the high-purity nitrogen gas introduced is determined according to Equation 5.

[0033]

[0034] In Formula 5: w[%x] represents the mass percentage of each target component in ultrapure 316LN austenitic stainless steel, where x represents C, Si, Cr, Ni, Mo, and Mn, and w[%N] represents the mass percentage of each component. g P represents the theoretical nitrogen content in gas-phase nitriding. N2 p is the nitrogen pressure. θ The pressure is standard atmosphere, and T is the temperature of the molten steel.

[0035] Among them, the theoretical nitrogen content w[%N] in gas-phase nitriding. g The target nitrogen content w[%N] in the steel follows Equation 6: w[%N] g = (1.19~1.25)×w[%N] Equation 6;

[0036] (5) The temperature of the molten steel is controlled at 1460-1480℃, and Mg alloy is added for deoxidation and desulfurization. The deoxidation and desulfurization time is 2-3 minutes.

[0037] The amount of Mg added to the molten steel using Mg alloy is determined according to formula 7 or formula 8:

[0038] When the initial sulfur content in the molten steel is >0.003% and ≤0.004%, it is determined according to Equation 7:

[0039] w[%Mg]=1.5×(w[%O]) C -0.002%) / 15%+0.75×(w[%S]-0.003%) / 15% Equation 7;

[0040] When the initial sulfur content in the molten steel is ≤0.003%, it is determined according to Equation 8:

[0041] w[%Mg]=1.5×(w[%O]) C -0.002% / 15% Equation 8;

[0042] In formulas 7 and 8: w[%Mg] represents the amount of Mg added to the molten steel via Mg alloy, and w[%O] represents the amount of Mg added. CThe O content in the molten steel after vacuum C deoxidation pretreatment is w[%S], and the initial S content in the molten steel is w[%S].

[0043] (6) Add Ca alloy to the deoxidized and desulfurized molten steel for deep deoxidation and desulfurization, the time of which is 2 to 3 minutes;

[0044] The amount of Ca added to molten steel via Ca alloy is determined according to Equation 9 or Equation 10:

[0045] When the initial sulfur content in the molten steel is >0.003% and ≤0.004%, it is determined according to Equation 9:

[0046] w[%Ca]=(6.25×w[%O]) C -0.625×w[%Mg]+1.25×w[%S]-0.004%) / 5% Equation 9;

[0047] When the initial sulfur content in the molten steel is ≤0.003%, it is determined according to Equation 10:

[0048] w[%Ca]=(3.75×w[%O] C -0.375×w[%Mg]+1.25×w[%S]-0.003%) / 5% Equation 10;

[0049] In formulas 9 and 10: w[%Ca] represents the amount of Ca added to the molten steel via Ca alloy, and w[%O] represents the amount of O added. C The value represents the O content in the molten steel after vacuum C deoxidation pretreatment, w[%S] represents the initial S content in the molten steel, and w[%Mg] represents the amount of Mg added to the molten steel via Mg alloy.

[0050] (7) The temperature of the molten steel after deep deoxidation and desulfurization is controlled at 1500-1550℃. After being killed, it is cast to obtain an ingot.

[0051] (8) Forging the ingot into electrodes, controlling the filling ratio to be 0.5 to 0.7;

[0052] (9) The electrode is subjected to electroslag remelting under a protective atmosphere. When preparing ultrapure 316L austenitic stainless steel, the protective atmosphere is high-purity argon; when preparing ultrapure 316LN austenitic stainless steel, the protective atmosphere is a mixture of high-purity argon and nitrogen; the melting rate is (0.008~0.011)×Dkg·min -1 D is the diameter of the crystallizer (mm);

[0053] (10) Multi-stage current reduction and feeding are carried out during the smelting capping period to obtain the ultrapure 316L(N) austenitic stainless steel.

[0054] Preferably, in step (3), the vacuum degree of the vacuum C deoxygenation pretreatment is ≤10Pa.

[0055] Preferably, in step (5), the Mg alloy is a Ni-Mg alloy or an Fe-Mg alloy, and the mass content of Mg in the Mg alloy is 10% to 30%.

[0056] Preferably, in step (6), the Ca alloy is a Si-Ca alloy, a silicon-calcium cored wire, or a Ni-Ca alloy, and the mass content of Ca in the Ca alloy is 20% to 40%.

[0057] Preferably, in step (9), the slag material used for electroslag remelting, by mass percentage, comprises CaF2: 64.0%–78.0%, CaO: 14.0%–20.0%, SiO2: 3.0%–5.0%, MgO: 2.0%–4.0%, MgF2: 1.0%–2.5%, K2O: 2.0%–4.0%, and the sum of the mass percentages of all components is 100%.

[0058] Preferably, in step (10), the multi-stage current reduction compensation time is 60-80 minutes, and the compensation cycle is 5-7 times.

[0059] The technical principle of this invention is as follows:

[0060] Ultra-pure 316L(N) austenitic stainless steel is widely used in the semiconductor industry and nuclear power equipment. Due to the requirement of very low content of impurity elements such as O, S, and Al, traditional smelting processes cannot meet these requirements. This invention proposes a dual smelting process of vacuum induction melting (VIM) + protective atmosphere electroslag remelting (IESR).

[0061] In the vacuum induction melting stage, based on the composition requirements of ultrapure 316L(N) austenitic stainless steel, the traditional Al deoxidation method cannot be used. When the Al content in the raw material is too high, a certain amount of FeO needs to be added to the furnace to control Al, and magnesium or calcium crucibles are used to adsorb the de-Al products. Mg and Ca elements are also often used to purify molten steel. Mg and Ca treatment can not only effectively remove O and S, but also significantly reduce the number and size of inclusions, thereby significantly improving the purity of steel. When the initial O content of molten steel is high, directly using Mg and Ca treatment for O and S removal will result in a large amount of Mg and Ca being burned off, which is not effective and does not conform to the principle of economy. Therefore, it is necessary to pre-de-O treatment of molten steel before adding Mg and Ca treatment. This invention uses vacuum C deoxidation to pre-treat molten steel for O removal. Vacuum C deoxidation has the following advantages: (1) the deoxidation product is CO, which does not contaminate the molten steel; (2) the CO deoxidation product agitates the molten steel during the floating process, improving the kinetic conditions. Considering the compositional requirements of ultrapure 316L(N) austenitic stainless steel (w[C]≤0.02%), the amount of C added needs to be precisely controlled. Vacuum C deoxidation pretreatment significantly increases the activity of Mg and Ca, thereby achieving stronger O and S removal effects. However, Mg and Ca have different thermodynamic affinities with O and S, making it difficult to simultaneously control the O and S content to very low levels with either Mg or Ca treatment alone. In the ultrapure 316L(N) austenitic stainless steel system, Mg is more suitable for O removal, while Ca exhibits stronger S removal ability. Therefore, this invention employs a strategy of treating Mg first, followed by Ca treatment, after vacuum C deoxidation pretreatment. Mg treatment further reduces the O content in the molten steel while also increasing the activity of Ca, thereby enhancing the deep O and S removal effects of Ca treatment.

[0062] Electroslag Remelting (IESR) under protective atmosphere has a significant effect on removing inclusions and improving solidification structure. After vacuum induction melting, ingots subjected to IESR not only have their impurity element composition (O, S) controlled, but inclusions are also significantly removed. Furthermore, IESR is a rapid solidification method, thus significantly improving the homogeneity of the steel and facilitating the acquisition of a uniform and fine solidification structure.

[0063] Based on the above technical principles, this invention provides a VIM+IESR dual-smelting process. VIM employs an ultra-pure combined control process of "first adding FeO to control aluminum, then undergoing vacuum carbon deoxidation pretreatment, and finally Mg+Ca composite deoxidation and desulfurization." IESR further removes impurities and purifies the material, enabling controlled aluminum production and deep removal of O, S, and inclusions from 316L(N) austenitic stainless steel, thus meeting the urgent demand for ultra-pure stainless steel materials in fields such as semiconductor equipment and nuclear power.

[0064] The beneficial effects of this invention include:

[0065] This invention provides a method for preparing ultrapure 316L(N) austenitic stainless steel using a dual smelting process of vacuum induction melting (VIM) + protective atmosphere electroslag remelting (IESR). The prepared 316L(N) stainless steel has ultra-high purity: O≤0.001%, S≤0.0015%, Al≤0.01%, inclusions are effectively modified and rendered harmless, coarse inclusions are grade 0, and fine inclusions of type A+B+C+D are ≤0.5.

[0066] The ultrapure 316L austenitic stainless steel prepared by the method provided in this invention has a grain size of 25-35 μm after hot rolling and annealing. The ultrapure 316LN austenitic stainless steel prepared by this invention has a grain size of 20-30 μm after hot rolling and annealing. The α-ferrite content in the ultrapure 316L(N) austenitic stainless steel is 0, and the δ-ferrite content is ≤0.1%.

[0067] The ultrapure 316L austenitic stainless steel prepared by the method provided in this invention has a yield strength ≥245MPa, tensile strength ≥550MPa, and elongation ≥70%; the ultrapure 316LN austenitic stainless steel has a yield strength ≥330MPa, tensile strength ≥650MPa, and elongation ≥60%.

[0068] The ultrapure 316L(N) austenitic stainless steel prepared by the method provided in this invention can meet the ultrapure requirements of key stainless steel materials in fields such as semiconductor equipment and nuclear power. It can then be applied to key components of semiconductor equipment (such as high-purity gas transmission pipelines, joints, etc.) and nuclear power equipment (such as main pipelines, magnet support plates, bolts, etc.). Detailed Implementation

[0069] This invention provides an ultrapure 316L(N) austenitic stainless steel, comprising the following elements by mass percentage:

[0070]

[0071] The remainder is Fe;

[0072] The coarse inclusions in the ultrapure 316L(N) austenitic stainless steel are grade 0, and the fine inclusions (A+B+C+D) are ≤ grade 0.5.

[0073] When N≤0.01%, it is ultra-pure 316L austenitic stainless steel;

[0074] When N is 0.10% to 0.16%, it is an ultrapure 316LN austenitic stainless steel.

[0075] The ultrapure 316L(N) austenitic stainless steel provided by the present invention, by weight percentage, comprises C ≤ 0.02%, preferably ≤ 0.019%. In embodiments of the present invention, specifically 0.019%, 0.015%, 0.017%, or 0.018%.

[0076] The ultrapure 316L(N) austenitic stainless steel provided by the present invention comprises 16.00% to 18.00% Cr, preferably 16.20% to 17.85%, and more preferably 16.50% to 17.30% by mass percentage. In the embodiments of the present invention, specifically 16.23%, 17.29%, 17.81%, or 16.64%.

[0077] The ultrapure 316L(N) austenitic stainless steel provided by this invention, by weight percentage, comprises 10.00% to 14.00% Ni, preferably 11.00% to 13.00%, and more preferably 11.50% to 12.50%. In embodiments of this invention, specifically 11.83%, 13.51%, 12.38%, or 10.57%.

[0078] The ultrapure 316L(N) austenitic stainless steel provided by the present invention comprises 2.00% to 3.00% Mo, preferably 2.20% to 2.80%, and more preferably 2.40% to 2.60% by mass percentage. In the embodiments of the present invention, it is specifically 2.13%, 2.52%, 2.78%, or 2.61%.

[0079] The ultrapure 316L(N) austenitic stainless steel provided by this invention, by weight percentage, comprises N ≤ 0.01% or 0.10% to 0.16%. In this invention, when N ≤ 0.01%, it is ultrapure 316L austenitic stainless steel; when N is 0.10% to 0.16%, it is ultrapure 316LN austenitic stainless steel. In embodiments of this invention, the N content of the ultrapure 316L austenitic stainless steel is specifically 0.004% or 0.005%; the N content of the ultrapure 316LN austenitic stainless steel is specifically 0.156% or 0.149%. In this invention, C and N are not impurity elements, but 316L(N) austenitic stainless steel has the highest requirements for the content of C and N. Therefore, this invention controls the content of both within the above range to meet the composition requirements of ultrapure 316L(N) austenitic stainless steel.

[0080] The ultrapure 316L(N) austenitic stainless steel provided by the present invention comprises 0.30% to 0.75% Si, preferably 0.40% to 0.70%, and more preferably 0.50% to 0.60% by mass percentage. In the embodiments of the present invention, it is specifically 0.71%, 0.36%, 0.45%, or 0.52%.

[0081] The ultrapure 316L(N) austenitic stainless steel provided by the present invention comprises 0.50% to 2.00% Mn, preferably 0.80% to 1.60%, and more preferably 1.00% to 1.40% by mass percentage. In the embodiments of the present invention, it is specifically 0.68%, 1.67%, 1.08%, or 1.86%.

[0082] The ultrapure 316L(N) austenitic stainless steel provided by this invention, by mass percentage, contains P ≤ 0.003%, preferably ≤ 0.0026%. In embodiments of this invention, specifically 0.0023%, 0.0012%, 0.0026%, or 0.0018%. In this invention, P is considered an impurity element, and the lower the content, the better.

[0083] The ultrapure 316L(N) austenitic stainless steel provided by this invention, by mass percentage, contains S ≤ 0.0015%, preferably ≤ 0.0013%. In embodiments of this invention, specifically 0.0013%, 0.0009%, 0.0012%, or 0.0010%. In this invention, S is considered an impurity element, and the lower the content, the better.

[0084] The ultrapure 316L(N) austenitic stainless steel provided by this invention, by mass percentage, contains Al ≤ 0.01%. In the embodiments of this invention, specifically 0.0096%, 0.0082%, 0.0092%, or 0.0088%. In this invention, Al is considered an impurity element, and the lower the content, the better.

[0085] The ultrapure 316L(N) austenitic stainless steel provided by this invention, by mass percentage, contains ≤0.001% O. In the embodiments of this invention, this is specifically 0.0010%, 0.0008%, or 0.0009%. In this invention, O is considered an impurity element, and the lower the content, the better.

[0086] The ultrapure 316L(N) austenitic stainless steel provided by this invention includes the Fe balance.

[0087] The ultrapure 316L(N) austenitic stainless steel provided by this invention also includes trace amounts of Mg and Ca. In this invention, Mg and Ca are not impurity elements, so there is no need to limit the content of Mg and Ca in the steel.

[0088] In this invention, the coarse inclusions of the ultrapure 316L(N) austenitic stainless steel are grade 0, and the fine inclusions of type A+B+C+D are ≤ grade 0.5; the volume content of α ferrite is 0, and the volume content of δ ferrite is ≤ 0.1%.

[0089] In this invention, when the material is ultrapure 316L austenitic stainless steel, the preferred mechanical properties at room temperature are: yield strength ≥ 245 MPa, tensile strength ≥ 550 MPa, elongation ≥ 70%; and after hot rolling and annealing (using the mature hot rolling and annealing process of 316L), the grain size is 20-35 μm.

[0090] In this invention, when the material is ultrapure 316LN austenitic stainless steel, the preferred mechanical properties at room temperature are: yield strength ≥ 330 MPa, tensile strength ≥ 650 MPa, elongation ≥ 60%; after hot rolling and annealing (using the mature hot rolling and annealing process of 316LN), the grain size is 20-30 μm.

[0091] This invention provides a method for preparing the ultrapure 316L(N) austenitic stainless steel described above, comprising the following steps:

[0092] (1) Prepare the raw materials according to the target composition and then perform vacuum induction melting to obtain molten steel;

[0093] (2) Add FeO to the molten steel for refining and Al removal;

[0094] (3) Add high-purity C to the de-Al steel liquid and perform vacuum C deoxidation pretreatment to control the O content of the steel liquid at 0.005% to 0.006%;

[0095] (4) When preparing ultrapure 316L austenitic stainless steel, after the vacuum C deoxidation pretreatment is completed, high-purity argon gas is introduced into the vacuum induction melting furnace.

[0096] When preparing ultrapure 316LN austenitic stainless steel, after the vacuum C deoxidation pretreatment is completed, high-purity nitrogen gas is introduced into the vacuum induction melting furnace, and the temperature of the molten steel is controlled at 1480-1500℃ to carry out vapor phase nitriding.

[0097] (5) Control the temperature of the molten steel at 1460-1480℃ and add Mg alloy for deoxidation and desulfurization;

[0098] (6) Add Ca alloy to the deoxidized and desulfurized molten steel for deep deoxidation and desulfurization;

[0099] (7) The temperature of the molten steel after deep deoxidation and desulfurization is controlled at 1500-1550℃. After being killed, it is cast to obtain an ingot.

[0100] (8) Forging the ingot into electrodes, controlling the filling ratio to be 0.5 to 0.7;

[0101] (9) The electrode is subjected to electroslag remelting under a protective atmosphere. When preparing ultrapure 316L austenitic stainless steel, the protective atmosphere is high-purity argon; when preparing ultrapure 316LN austenitic stainless steel, the protective atmosphere is a mixture of high-purity argon and nitrogen; the melting rate is (0.008~0.011)×Dkg·min -1 D is the diameter of the crystallizer (mm);

[0102] (10) Multi-stage current reduction and feeding are carried out during the smelting capping period to obtain the ultrapure 316L(N) austenitic stainless steel.

[0103] Unless otherwise specified, all raw materials and equipment used in this invention are commercially available.

[0104] This invention involves batching ingredients according to the target composition and then performing vacuum induction melting on the raw materials to obtain molten steel.

[0105] This invention does not have special requirements for the batching process, which is common knowledge in the art. In this invention, by mass percentage, the total Al introduced by the raw materials is ≤0.030%, total O is ≤0.025%, total S is ≤0.004%, and total P is ≤0.003%. In this invention, the vacuum induction melting temperature is preferably 1500-1550℃, more preferably 1510-1540℃, and even more preferably 1520-1530℃. In this invention, the vacuum induction melting preferably includes: weighing the required experimental raw materials and filling them into the vacuum induction melting furnace according to the principle of pressing down and loosening up; after closing the furnace, turning on the vacuum pump, evacuating to <5Pa and then starting to supply power, gradually increasing the power until all the furnace charge is melted, and obtaining molten steel.

[0106] In this invention, the crucible used for vacuum induction melting is preferably made of MgO, CaO, or a mixture of MgO and CaO; the mass ratio of MgO to CaO in the MgO-CaO mixture is preferably (1.0 to 2.5):1.

[0107] After obtaining the molten steel, the present invention adds FeO to the molten steel for refining and de-Al removal.

[0108] In this invention, the temperature of the molten steel is preferably 1500-1550°C, more preferably 1510-1540°C, and even more preferably 1520-1530°C.

[0109] In this invention, when the initial O content of the molten steel is >0.009% and ≤0.025%, the amount of FeO added is determined according to Formula 1: w[%FeO]=4×(w[%Al]-0.01%)-4.5×(w[%O]-0.009%)Formula 1; if w[%FeO]≤0 is calculated according to Formula 1, then it is not necessary to add FeO to the molten steel.

[0110] When the initial oxygen content of the molten steel is ≤0.009%, the amount of FeO added is determined according to formula 2:

[0111] w[%FeO]=4×(w[%Al]-0.01%)+4.5(0.009%-w[%O]) Formula 2;

[0112] In Formulas 1 and 2: w[%FeO] is the amount of FeO added, w[%Al] is the initial Al content in the molten steel, and w[%O] is the initial O content in the molten steel.

[0113] In this invention, the FeO is preferably added in block or disc form.

[0114] In this invention, the refining and de-Al removal time is 2-3 minutes. After adding FeO to the molten steel, the Al in the molten steel is fully oxidized. The resulting Al₂O₃ inclusions react with the magnesium or calcium crucible of this invention and are directly adsorbed onto the crucible for removal.

[0115] After completing the refining and de-Al removal process, the present invention adds high-purity C to the de-Al removed molten steel for vacuum C deoxidation pretreatment, thereby controlling the O content of the molten steel to be between 0.005% and 0.006%.

[0116] In this invention, the purity of the high-purity C is preferably ≥99.99%.

[0117] In this invention, the vacuum degree of the vacuum C deoxidation pretreatment is preferably ≤10Pa.

[0118] In this invention, the amount of high-purity C added is determined according to Formula 3:

[0119] w[%C]=0.7×(w[%O]-0.005%)Equation 3;

[0120] In Formula 3: w[%C] is the amount of high-purity C added, and w[%O] is the initial O content in the molten steel;

[0121] The time for the vacuum C deoxidation pretreatment is determined according to Equation 4:

[0122]

[0123] In Equation 4: t is the vacuum C deoxidation pretreatment time, s; F is the free surface area of ​​the molten steel, m². 2 V is the volume of molten steel, in meters. 3 ;w[%O] C The oxygen content in steel after vacuum C deoxidation pretreatment is expressed as %.

[0124] This invention reduces the O content in molten steel through vacuum C deoxidation pretreatment, which can significantly increase the activity of Mg and Ca, thereby achieving a stronger O and S removal effect.

[0125] After the vacuum C deoxidation pretreatment is completed, when preparing ultrapure 316L austenitic stainless steel, the present invention introduces high-purity argon gas into the vacuum induction melting furnace.

[0126] In this invention, the purity of the high-purity argon gas is preferably ≥99.999%; the pressure of the high-purity argon gas introduced into the vacuum induction melting furnace is preferably 0.01-0.03 MPa, more preferably 0.015-0.025 MPa.

[0127] In this invention, when preparing ultrapure 316LN austenitic stainless steel, after the vacuum C deoxidation pretreatment is completed, high-purity nitrogen gas is introduced into the vacuum induction melting furnace, and the temperature of the molten steel is controlled at 1480-1500℃ to carry out vapor phase nitriding.

[0128] In this invention, the purity of the high-purity nitrogen gas is preferably ≥99.999%.

[0129] In this invention, the pressure of the high-purity nitrogen gas is determined according to Formula 5: lgw[%N] g =1 / 2lg(P) N2 / p θ )-188 / T-1.17-{(3280 / T-0.75)(0.13w[%N] g +0.118w[%C] +0.043w[%Si]+

[0130] 0.011w[%Ni]+3.5×10 -5 w[%Ni] 2 -0.024w[%Mn]+3.2×10 -5 w[%Mn] 2 -0.01w[%Mo]+7.9×10 -5 w[%Mo] 2 -0.048w[%Cr]+3.5×10 -4 w[%Cr] 2 )}

[0131] Formula 5;

[0132] In Formula 5: w[%x] represents the mass percentage of each target component in ultrapure 316LN austenitic stainless steel, where x represents C, Si, Cr, Ni, Mo, and Mn, and w[%N] represents the mass percentage of each component. g P represents the theoretical nitrogen content in gas-phase nitriding. N2 p is the nitrogen pressure. θ The pressure is standard atmosphere, and T is the temperature of the molten steel.

[0133] Among them, the theoretical nitrogen content w[%N] in gas-phase nitriding. g The target nitrogen content w[%N] in the steel follows Equation 6: w[%N] g = (1.19~1.25)×w[%N] Equation 6.

[0134] In this invention, the gas-phase nitriding time is 12 to 15 minutes.

[0135] This invention ensures that the nitrogen content in 316LN steel reaches the target composition through vapor-phase nitriding.

[0136] After the above steps are completed, whether preparing ultrapure 316L austenitic stainless steel or ultrapure 316LN austenitic stainless steel, continue with the following steps: control the temperature of the molten steel at 1460-1480℃, and add Mg alloy for deoxidation and desulfurization.

[0137] In this invention, the Mg alloy is preferably a Ni-Mg alloy or an Fe-Mg alloy, and the mass content of Mg in the Mg alloy is preferably 10% to 30%, more preferably 15% to 25%.

[0138] In this invention, the deoxidation and desulfurization time is 2 to 3 minutes.

[0139] In this invention, the amount of Mg added to the molten steel via Mg alloy is determined according to Formula 7 or Formula 8:

[0140] When the initial sulfur content in the molten steel is >0.003% and ≤0.004%, it is determined according to Equation 7:

[0141] w[%Mg]=1.5×(w[%O]) C -0.002%) / 15%+0.75×(w[%S]-0.003%) / 15% Equation 7;

[0142] When the initial sulfur content in the molten steel is ≤0.003%, it is determined according to Equation 8:

[0143] w[%Mg]=1.5×(w[%O]) C -0.002% / 15% Formula 8;

[0144] In formulas 7 and 8: w[%Mg] represents the amount of Mg added to the molten steel via Mg alloy, and w[%O] represents the amount of Mg added. C The value represents the O content in the molten steel after vacuum C deoxidation pretreatment, and w[%S] represents the initial S content in the molten steel.

[0145] This invention uses magnesium alloy for deoxidation and desulfurization, which can reduce the content of O and S in steel, while increasing the activity of Ca in the molten steel and improving the deoxidation and desulfurization effect of Ca in the molten steel.

[0146] After completing the deoxidation and desulfurization, the present invention adds Ca alloy to the deoxidized and desulfurized molten steel for deep deoxidation and desulfurization, and the deep deoxidation and desulfurization time is 2 to 3 minutes.

[0147] In this invention, the Ca alloy is preferably a Si-Ca alloy, a silicon-calcium cored wire, or a Ni-Ca alloy, and the mass content of Ca in the Ca alloy is preferably 20% to 40%, more preferably 25% to 35%.

[0148] In this invention, the amount of Ca added to the molten steel via Ca alloy is determined according to formula 9 or formula 10:

[0149] When the initial sulfur content in the molten steel is >0.003% and ≤0.004%, it is determined according to Equation 9:

[0150] w[%Ca]=(6.25×w[%O]) C -0.625×w[%Mg]+1.25×w[%S]-0.004%) / 5% Equation 9;

[0151] When the initial sulfur content in the molten steel is ≤0.003%, it is determined according to Equation 10:

[0152] w[%Ca]=(3.75×w[%O] C -0.375×w[%Mg]+1.25×w[%S]-0.003%) / 5% Formula 10;

[0153] In formulas 9 and 10: w[%Ca] represents the amount of Ca added to the molten steel via Ca alloy, and w[%O] represents the amount of O added. C The value represents the O content in the molten steel after vacuum C deoxidation pretreatment, w[%S] represents the initial S content in the molten steel, and w[%Mg] represents the amount of Mg added to the molten steel via Mg alloy.

[0154] This invention achieves deep desulfurization and deep deoxidation of molten steel by adding Ca alloy.

[0155] After completing the deep deoxidation and desulfurization, the present invention controls the temperature of the molten steel after deep deoxidation and desulfurization at 1500-1550℃, and after being killed, it is cast to obtain an ingot.

[0156] In this invention, the sedation treatment time is preferably 2 to 3 minutes.

[0157] The process described above, which involves first adding FeO to control aluminum, then undergoing vacuum C deoxidation pretreatment, Mg+Ca composite treatment for deep O and deep S removal, and finally casting, is the vacuum induction melting (VIM) stage.

[0158] After obtaining the ingot, the present invention forges the ingot into an electrode, and controls the filling ratio to be 0.5 to 0.7, preferably 0.55 to 0.65.

[0159] After obtaining the electrode, the present invention performs electroslag remelting of the electrode under a protective atmosphere.

[0160] In this invention, the slag used for electroslag remelting preferably comprises, by mass percentage: CaF2: 64.0%–78.0%, CaO: 14.0%–20.0%, SiO2: 3.0%–5.0%, MgO: 2.0%–4.0%, MgF2: 1.0%–2.5%, K2O: 2.0%–4.0%, with the sum of the mass percentages of all components being 100%. In this invention, the content of other impurities in the slag is controlled to be ≤0.8% (wherein, Al2O3 ≤0.1%).

[0161] The present invention preferably involves pre-melting, cooling, crushing, and baking the slag material, adding the slag material to a crystallizer for slag formation after arc ignition, and then starting protective atmosphere arc melting.

[0162] When preparing ultrapure 316L austenitic stainless steel, the protective atmosphere is high-purity argon; when preparing ultrapure 316LN austenitic stainless steel, the protective atmosphere is a mixture of high-purity argon and nitrogen.

[0163] In this invention, the purity of the high-purity argon gas is preferably ≥99.999%. In this invention, the volume content of high-purity nitrogen in the mixture of high-purity argon and nitrogen gas is preferably 30%–80%, more preferably 40%–70%. By using a mixture of nitrogen and argon gas as a protective atmosphere, this invention can maintain the N content in electroslag ingots within the target composition range.

[0164] In this invention, the preferred gas flow rate of the protective atmosphere is (0.012~0.014)×D m. 3 ·h -1 D is the diameter of the crystallizer (mm).

[0165] In this invention, the melting rate of the protective atmosphere arc melting is (0.008~0.011)×Dkg·min. -1 D is the diameter of the crystallizer (mm).

[0166] During the capping stage of the smelting process, the present invention performs multi-stage current reduction and feeding to obtain the ultrapure 316L(N) austenitic stainless steel.

[0167] In this invention, the multi-stage current reduction compensation time is preferably 60-80 min, more preferably 65-75 min; the compensation cycle is preferably 5-7 times, specifically 5 times, 6 times or 7 times.

[0168] Electroslag Remelting (IESR) under protective atmosphere has a significant effect on removing inclusions and improving solidification structure. After vacuum induction melting, ingots subjected to IESR not only have their impurity element composition (O, S) controlled, but inclusions are also significantly removed. Furthermore, IESR is a rapid solidification method, thus significantly improving the homogeneity of the steel and facilitating the acquisition of a uniform and fine solidification structure.

[0169] The following detailed description of the ultrapure 316L(N) austenitic stainless steel provided by the present invention, with reference to the embodiments, should not be construed as limiting the scope of protection of the present invention.

[0170] Examples 1-4

[0171] This invention relates to a method for preparing ultrapure 316L(N) austenitic stainless steel using a VIM+IESR duplex smelting process. The smelting equipment is a 100kg vacuum induction furnace with a furnace charge of 85kg. Four heats of experimental steel were designed, namely Examples 1 to 4, and their target compositions are shown in Table 1.

[0172] Table 1 Target composition (wt.%) of ultrapure 316L(N) austenitic stainless steel

[0173]

[0174]

[0175] The main impurity components of the smelting raw materials used are shown in Table 2.

[0176] Table 2. Impurity composition of raw materials used in smelting (wt.%)

[0177] Example 1 0.024 0.019 0.0038 0.0025 Example 2 0.012 0.010 0.0021 0.0017 Example 3 0.017 0.028 0.0033 0.0029 Example 4 0.028 0.015 0.0028 0.0020

[0178] The vacuum induction melting process in Examples 1-4 is as follows:

[0179] (1) Loading: Weigh the required experimental raw materials according to the target composition shown in Table 1, and fill the raw materials into the vacuum induction melting furnace according to the principle of pressing down and loosening up;

[0180] (2) Smelting: After the furnace is closed, turn on the vacuum pump, draw the vacuum to less than 5Pa, and then start powering. Gradually increase the power until all the furnace charge is melted, and control the smelting temperature at 1500-1550℃.

[0181] (3) Al removal treatment: Add the blocky FeO into the molten steel and refine for 2 to 3 minutes;

[0182] (4) Vacuum C deoxidation pretreatment: Add high-purity C blocks with a purity ≥ 99.99% to the molten steel for deoxidation pretreatment, control the vacuum degree ≤ 10 Pa, and the refining time is 12 to 18 min;

[0183] (5) Vapor phase nitriding treatment: After deoxidation of C is completed, the vacuum valve is closed. When smelting the ultrapure 316LN austenitic stainless steel of Examples 1 and 2, a certain pressure P is introduced into the induction furnace. N2 High-purity nitrogen gas with a purity ≥99.999% was used to control the temperature of the molten steel at 1480–1500℃ and to maintain the nitriding time for 12–15 minutes to ensure that the N content in the experimental steel reached the target composition. When smelting the ultrapure 316L austenitic stainless steel of Examples 3 and 4, high-purity argon gas with a purity ≥99.999% was introduced into the induction furnace at a pressure of 0.02 MPa.

[0184] (6) Mg deoxidation and desulfurization treatment: control the temperature at 1460~1480℃, add Ni-Mg alloy (Mg content is 20%) to the molten steel for deoxidation and desulfurization, and the treatment time is 2~3min;

[0185] (7) Ca deep deoxidation and desulfurization treatment: After the Mg treatment is completed, Si-Ca alloy (Ca content is 30%) is added to the molten steel for deep deoxidation and deep desulfurization, and the treatment time is 2 to 3 minutes;

[0186] (8) Casting: The temperature of the molten steel is controlled at 1500-1550℃. After 2-3 minutes of calming treatment, the molten steel is poured into the ingot mold. After the molten steel cools and solidifies, the ingot is removed.

[0187] The specific VIM smelting parameters for each embodiment are shown in Table 3.

[0188] Table 3. Smelting parameters of austenitic stainless steel in Examples 1-4

[0189] Melting temperature (°C) 1520 1545 1538 1512 FeO-controlled Al treatment time (min) 3 2 2 3 Vacuum C deoxidation vacuum degree (Pa) 8 4 6 7 <![CDATA[Free surface area of molten steel (m 2 )]]> 0.038 0.038 0.038 0.038 <![CDATA[Volume of molten steel (m 3 )]]> 0.01 0.01 0.01 0.01 Vacuum C deoxidation time (min) 16 12 15 18 Vapor phase nitriding temperature (°C) 1482 1496 - - <![CDATA[Gas nitriding pressure P N2 (MPa)]]> 0.058 0.062 - - Vapor phase nitriding time (min) 12.5 14.6 - - Temperature of molten steel before Mg treatment (°C) 1466 1475 1472 1468 Mg treatment time (min) 2 3 2 2 Ca treatment time (min) 3 2 3 2 Temperature of molten steel before casting (°C) 1545 1537 1513 1524 Settling time of molten steel before casting (min) 2 2 3 3 Material of crucibles used in smelting MgO CaO MgO-CaO MgO

[0190] The FeO content, C content, Ni-Mg alloy (Mg) content, and Si-Ca alloy (Ca) content added in each embodiment are shown in Table 4.

[0191] Table 4. Amount of each substance added during smelting (wt.%)

[0192] Example 1 0.011 0.023 0.195 0.039 0.717 0.215 Example 2 0.004 0.023 0.150 0.030 0.515 0.155 Example 3 0 0.022 0.168 0.034 0.779 0.234 Example 4 0.045 0.024 0.200 0.040 0.573 0.172

[0193] The protective atmosphere electroslag remelting (IESR) processes in Examples 1-4 are as follows:

[0194] (1) The slag system is pre-melted, cooled, crushed and baked, and after arc ignition, the slag material is added to the crystallizer for slag formation.

[0195] (2) Forge the VIM ingot into electrodes and control the filling ratio to 0.5 to 0.7.

[0196] (3) Remelting is carried out in an electroslag furnace under a high-purity atmosphere, with the high-purity gas flow rate controlled at (0.012~0.014)×Dm. 3 ·h -1 D represents the diameter of the crystallizer (mm). When smelting the ultrapure 316LN austenitic stainless steel of Examples 1 and 2, a mixture of high-purity argon and nitrogen should be introduced into the IESR, with a purity ≥99.999%. The volume proportions of high-purity nitrogen in the mixture should be controlled at 70% and 40%, respectively, to ensure that the N content in the experimental steel reaches the target composition. When smelting the ultrapure 316L austenitic stainless steel of Examples 3 and 4, high-purity argon should be introduced into the IESR, with an argon purity ≥99.999%.

[0197] (4) Control the melting rate to (0.008~0.011)×D kg·min -1 D is the diameter of the crystallizer (mm).

[0198] (5) During the capping period of smelting, multi-stage current reduction and feeding are carried out, and the feeding time is controlled at 60-80 minutes, with a feeding cycle of 5-7.

[0199] The IESR slag systems used in each embodiment are shown in Table 5.

[0200] Table 5. Composition of IESR slag system used in Examples 1-4 (wt.%)

[0201] <![CDATA[CaF2]]> 72.4 68.0 74.1 70.1 CaO 15.0 20.0 16.0 17.5 <![CDATA[SiO2]]> 3.0 5.0 3.6 4.3 MgO 3.1 2.8 2.0 4.0 <![CDATA[MgF2]]> 2.5 1.6 1.0 2.1 <![CDATA[K2O]]> 4.0 2.6 3.3 2.0

[0202] The specific IESR smelting parameters for each embodiment are shown in Table 6.

[0203] Table 6. IESR smelting parameters of 316L(N) austenitic stainless steel in Examples 1-4.

[0204] Fill ratio 0.6 0.5 0.7 0.6 <![CDATA[High-purity gas flow rate (m 3 ·h -1 )]]> 2.7 2.8 2.4 2.5 High-purity nitrogen volume percentage (%) 70 40 0 0 Crystallizer diameter (mm) 200 200 200 200 <![CDATA[Melting rate (kg·min -1 )]]> 1.8 1.6 2.2 1.8 Compensation time (min) 69 60 76 80 Compensation cycles (times) 7 6 6 5

[0205] The chemical composition of the ultrapure 316L(N) austenitic stainless steel obtained by the above method is shown in Table 7. It can be seen that the VIM+IESR duplex process significantly improves deoxidation and desulfurization, and effectively controls impurity elements. Compared with the comparative steel grade prepared by the VIM+ESR+VAR triple smelting process in the existing patent "An Ultrapure Austenitic Stainless Steel and Its Preparation Method" (CN116445828A), this invention achieves ultrapure smelting of 316L(N) austenitic stainless steel using only a duplex process.

[0206] Table 7 Chemical composition (wt.%) of the experimental steel

[0207] Example 1 0.019 16.23 11.83 2.13 0.68 0.71 0.156 0.0096 0.0028 0.0027 0.0010 0.0013 0.0023 margin Example 2 0.015 17.29 13.51 2.52 1.67 0.36 0.149 0.0082 0.0018 0.0022 0.0008 0.0009 0.0012 margin Example 3 0.017 17.81 12.38 2.78 1.08 0.45 0.004 0.0092 0.0022 0.0024 0.0009 0.0012 0.0026 margin Example 4 0.018 16.64 10.57 2.61 1.86 0.52 0.005 0.0088 0.0031 0.0019 0.0009 0.0010 0.0018 margin Comparative Example 0.006 16.78 14.75 2.32 0.03 0.14 0.004 0.0080 - - 0.0010 0.0010 0.0060 margin

[0208] The inclusion ratings of the 316L(N) austenitic stainless steel prepared in the examples are shown in Table 8. It can be seen that the coarse inclusions in the examples are all grade 0, and the inclusions in categories A+B+C+D are all ≤ grade 0.5; the α-ferrite content of the 316L(N) austenitic stainless steel in the examples is all 0, and the δ-ferrite content is all ≤ 0.1%.

[0209] Table 8 shows the inclusion grades in the 316L(N) austenitic stainless steel in the examples.

[0210] Example 1 Level 0 ≤0.5 level Example 2 Level 0 ≤0.5 level Example 3 Level 0 ≤0.5 level Example 4 Level 0 ≤0.5 level

[0211] The prepared 316L(N) austenitic stainless steel was processed into plates and then annealed (annealing temperature was 1050℃~1080℃, and annealing time was calculated based on the plate cross-sectional thickness of 1.25~1.45min / mm). Tensile specimens of the required dimensions were prepared according to ASTM-E8 standard and tensile tests were conducted at room temperature. The experimental results are shown in Table 9. The steel in the example exhibits good microstructure and mechanical properties.

[0212] Table 9. Grain size and mechanical properties of 316L(N) stainless steel in the examples.

[0213] Example 1 24 339 662 60.4 Example 2 27 330 650 62.1 Example 3 32 245 550 71.7 Example 4 28 256 563 70.3

[0214] In summary, the present invention employs a dual process of VIM (first adding FeO to control aluminum, then undergoing vacuum carbon deoxidation pretreatment, and finally Mg-Ca composite deoxidation and desulfurization) + IESR to achieve ultra-pure control and good microstructure and property control of 316L(N) stainless steel.

[0215] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing ultrapure 316LN austenitic stainless steel, characterized in that, Includes the following steps: (1) Charging: The smelting equipment is a 100 kg vacuum induction furnace, and the charging amount is 85 kg. According to the target composition, the required raw materials are weighed and filled into the vacuum induction melting furnace according to the principle of pressing down and loosening up. The target composition of ultrapure 316LN austenitic stainless steel is C≤0.02wt.%, Cr 16.2wt.%, Ni 11.8wt.%, Mo 2.1wt.%, Mn 0.7wt.%, Si 0.72wt.%, N 0.10~0.16wt.%, Al ≤0.01wt.%, O≤0.001wt.%, S≤0.0015wt.%, P≤0.003wt.% and Fe balance. The impurity composition of the raw materials used for smelting is: Al 0.024wt.%, O 0.019wt.%, S 0.0038wt.%, P 0.0025wt.%. The material of the crucible used for melting is MgO. (2) Smelting: After the furnace is closed, turn on the vacuum pump and evacuate to less than 5 Pa. Then start powering on and gradually increase the power until all the furnace charge is melted. The melting temperature is 1520℃. (3) Al removal treatment: Add blocky FeO to the molten steel and refine for 3 min; the amount of FeO added is 0.011 wt.% of the molten steel. (4) Vacuum C deoxidation pretreatment: High-purity C blocks with a purity ≥ 99.99% are added to a free surface area of ​​0.038 m². 2 The volume is 0.01 m³. 3 The steel was subjected to deoxidation pretreatment under a vacuum of 8 Pa and a refining time of 16 min; the amount of C block added was 0.023 wt.% of the steel. (5) Gas phase nitriding treatment: After the deoxidation of C is completed, close the vacuum valve and charge the induction furnace with 0.058MPa of high-purity nitrogen gas with a purity of ≥99.999%. Control the temperature of the molten steel at 1482℃ and continue nitriding for 12.5 min to ensure that the N content in the steel reaches the target composition. (6) Mg deoxidation and desulfurization treatment: The temperature is controlled at 1466℃, and Ni-Mg alloy is added to the molten steel for deoxidation and desulfurization. The treatment time is 2 min. The Mg content in the Ni-Mg alloy is 20%. The amount of Ni-Mg alloy added is 0.195 wt.% of the molten steel, and the amount added as Mg is 0.039 wt.%. (7) Ca deep deoxidation and desulfurization treatment: After the Mg treatment, Si-Ca alloy is added to the molten steel for deep deoxidation and desulfurization, and the treatment time is 3 min; the Ca content in the Si-Ca alloy is 30%; the amount of Si-Ca alloy added is 0.717 wt.% of the molten steel, and the amount added as Ca is 0.215 wt.%; (8) Casting: The temperature of the molten steel is controlled at 1545℃. After 2 minutes of calming treatment, the molten steel is poured into the ingot mold. After the molten steel cools and solidifies, the ingot is removed. (9) Forge the ingot into an electrode, controlling the filling ratio to be 0.6; (10) The electrode is subjected to electroslag remelting under a protective atmosphere. The protective atmosphere electroslag remelting IESR process is as follows: The slag system is pre-melted, cooled, crushed, and baked. After arc ignition, the slag is added to the crystallizer for slag formation. The slag system composition is as follows: CaF2 72.4 wt.%, CaO 15.0 wt.%, SiO2 3.0 wt.%, MgO 3.1 wt.%, MgF2 2.5 wt.%, K2O 4.0 wt.%; the crystallizer diameter is 200 mm. Remelting was carried out in a high-purity atmosphere-protected electroslag furnace. A mixture of high-purity argon and nitrogen was introduced into the IESR, with a purity ≥99.999% and a flow rate of 2.7 m³ / s. 3 ·h -1 To ensure that the N content in the experimental steel reaches the target composition, the volume of high-purity nitrogen in the mixed gas is controlled to be 70%. The melting rate is controlled at 1.8 kg / min. -1 ; During the capping stage of smelting, multi-stage current reduction feeding was carried out, with the feeding time controlled at 69 min and the feeding cycle repeated 7 times to obtain the ultrapure 316LN austenitic stainless steel. The actual composition of the ultrapure 316LN austenitic stainless steel is as follows: C 0.019wt.%, Cr 16.23wt.%, Ni 11.83wt.%, Mo 2.13wt.%, Mn 0.68wt.%, Si 0.71wt.%, N 0.156wt.%, Al 0.0096wt.%, Mg 0.0028wt.%, Ca 0.0027wt.%, O 0.0010wt.%, S 0.0013wt.%, P 0.0023wt.%, and Fe balance; The ultrapure 316LN austenitic stainless steel has coarse inclusions of grade 0 and inclusions of type A+B+C+D ≤ grade 0.5; α-ferrite content is 0 and δ-ferrite content is ≤ 0.1%; after hot rolling and annealing, the ultrapure 316LN austenitic stainless steel has a grain size of 24 μm; the mechanical properties at room temperature are: yield strength 339 MPa, tensile strength 662 MPa, and elongation 60.4%.

2. A method for preparing ultrapure 316LN austenitic stainless steel, characterized in that, Includes the following steps: (1) Charging: The smelting equipment is a 100 kg vacuum induction furnace, and the charging amount is 85 kg. According to the target composition, the required experimental raw materials are weighed and filled into the vacuum induction melting furnace according to the principle of pressing down and loosening up. The target composition of the ultrapure 316LN austenitic stainless steel is C≤0.02wt.%, Cr 17.3wt.%, Ni 13.5wt.%, Mo 2.5wt.%, Mn 1.4wt.%, Si0.35wt.%, N 0.10~0.16wt.%, Al ≤0.01wt.%, O≤0.001wt.%, S≤0.0015wt.%, P≤0.003wt.% and Fe balance. The impurity composition of the raw materials used for smelting is: Al 0.012wt.%, O 0.010wt.%, S0.0021wt.%, P 0.0017wt.%. The material of the crucible used for melting is CaO. (2) Smelting: After the furnace is closed, turn on the vacuum pump and evacuate to less than 5 Pa. Then start powering on and gradually increase the power until all the furnace charge is melted. The melting temperature is 1545℃. (3) Al removal treatment: Add blocky FeO to the molten steel and refine for 2 min; the amount of FeO added is 0.004 wt.% of the molten steel. (4) Vacuum C deoxidation pretreatment: High-purity C blocks with a purity ≥ 99.99% are added to a free surface area of ​​0.038 m². 2 The volume is 0.01 m³. 3 The steel was subjected to deoxidation pretreatment under a vacuum of 4 Pa ​​for 12 min; the amount of C block added was 0.023 wt.% of the steel. (5) Gas phase nitriding treatment: After the deoxidation of C is completed, the vacuum valve is closed, and 0.062 MPa of high-purity nitrogen gas is introduced into the induction furnace. The nitrogen purity is ≥99.999%. The temperature of the molten steel is controlled at 1496℃, and the nitriding time is maintained for 14.6 min to ensure that the N content in the experimental steel reaches the target composition. (6) Mg deoxidation and desulfurization treatment: The temperature is controlled at 1475℃, and Ni-Mg alloy is added to the molten steel for deoxidation and desulfurization. The treatment time is 3 min. The Mg content in the Ni-Mg alloy is 20%; the amount of Ni-Mg alloy added is 0.150 wt.%, and the amount added as Mg is 0.030 wt.%. (7) Ca deep deoxidation and desulfurization treatment: After the Mg treatment, Si-Ca alloy is added to the molten steel for deep deoxidation and desulfurization, and the treatment time is 2 min; the Ca content in the Si-Ca alloy is 30%; the amount of Si-Ca alloy added is 0.515 wt.% of the molten steel, and the amount added as Ca is 0.155 wt.%; (8) Casting: The temperature of the molten steel is controlled at 1537℃. After 2 minutes of calming treatment, the molten steel is poured into the ingot mold. After the molten steel cools and solidifies, the ingot is removed. (9) Forge the ingot into an electrode, controlling the filling ratio to be 0.5; (10) The electrode is subjected to electroslag remelting under a protective atmosphere. The protective atmosphere electroslag remelting IESR process is as follows: The slag system is pre-melted, cooled, crushed, and baked. After arc ignition, the slag is added to the crystallizer for slag formation. The slag system composition is as follows: CaF2 68.0 wt.%, CaO 20.0 wt.%, SiO2 5.0 wt.%, MgO 2.8 wt.%, MgF2 1.6 wt.%, K2O 2.6 wt.%; the crystallizer diameter is 200 mm. Remelting was carried out in a high-purity atmosphere-protected electroslag furnace. A mixture of high-purity argon and nitrogen was introduced into the IESR, with a purity ≥99.999% and a flow rate of 2.8 m³ / s. 3 ·h -1 To ensure that the N content in the experimental steel reaches the target composition, the volume of high-purity nitrogen in the mixed gas is controlled to be 40%. The melting rate is controlled at 1.6 kg / min. -1 ; During the capping stage of smelting, multi-stage current reduction feeding was carried out, with the feeding time controlled at 60 min and the feeding cycle repeated 6 times to obtain the ultrapure 316LN austenitic stainless steel. The actual composition of the ultrapure 316LN austenitic stainless steel is as follows: C 0.015wt.%, Cr 17.29wt.%, Ni 13.51wt.%, Mo 2.52wt.%, Mn 1.67wt.%, Si 0.36wt.%, N 0.149wt.%, Al 0.0082wt.%, Mg 0.0018wt.%, Ca 0.0022wt.%, O 0.0008wt.%, S 0.0009wt.%, P 0.0012wt.%, balance Fe; The ultrapure 316LN austenitic stainless steel has coarse inclusions of grade 0 and inclusions of type A+B+C+D ≤ grade 0.5; α-ferrite content is 0 and δ-ferrite content is ≤ 0.1%; after hot rolling and annealing, the ultrapure 316LN austenitic stainless steel has a grain size of 27 μm; the mechanical properties at room temperature are: yield strength 330 MPa, tensile strength 650 MPa, and elongation 62.1%.

3. A method for preparing ultrapure 316L austenitic stainless steel, characterized in that, Includes the following steps: (1) Charging: The smelting equipment is a 100 kg vacuum induction furnace, and the charging amount is 85 kg. According to the target composition, the required experimental raw materials are weighed and filled into the vacuum induction melting furnace according to the principle of pressing down and loosening up. The target composition of the ultrapure 316L austenitic stainless steel is C ≤0.02wt.%, Cr 17.8wt.%, Ni 12.4wt.%, Mo 2.8wt.%, Mn 1.1wt.%, Si 0.46wt.%, N ≤0.01wt.%, Al ≤0.01wt.%, O ≤0.001wt.%, S ≤0.0015wt.%, P ≤0.003wt.%, with the balance being Fe. The impurity composition of the raw materials used in smelting is: Al 0.017wt.%, O 0.028wt.%, S 0.0033wt.%, P 0.0029wt.%. The material of the crucible used in melting is MgO-CaO. (2) Smelting: After the furnace is closed, turn on the vacuum pump and evacuate to less than 5 Pa. Then start powering on and gradually increase the power until all the furnace charge is melted. The melting temperature is 1538℃. (3) Al removal treatment: No FeO is added, and the refining time is 2 min; (4) Vacuum C deoxidation pretreatment: High-purity C blocks with a purity ≥ 99.99% are added to a free surface area of ​​0.038 m². 2 The volume is 0.01 m³. 3 The steel was subjected to deoxidation pretreatment under a vacuum of 6 Pa and a refining time of 15 min; the amount of C block added was 0.022 wt.% of the steel. (5) After deoxidation of C is completed, close the vacuum valve and introduce 0.02 MPa of high-purity argon gas into the induction furnace. The purity of the argon gas is ≥99.999%. (6) Mg deoxidation and desulfurization treatment: The temperature is controlled at 1472℃, and Ni-Mg alloy is added to the molten steel for deoxidation and desulfurization. The treatment time is 2 min. The Mg content in the Ni-Mg alloy is 20%; the amount of Ni-Mg alloy added is 0.168 wt.%, and the amount added as Mg is 0.034 wt.%. (7) Ca deep deoxidation and desulfurization treatment: After the Mg treatment, Si-Ca alloy is added to the molten steel for deep deoxidation and desulfurization, and the treatment time is 3 min; the Ca content in the Si-Ca alloy is 30%; the amount of Si-Ca alloy added is 0.779 wt.% of the molten steel, and the amount added as Ca is 0.234 wt.%; (8) Casting: The temperature of the molten steel is controlled at 1513℃. After 3 minutes of calming treatment, the molten steel is poured into the ingot mold. After the molten steel cools and solidifies, the ingot is removed. (9) Forge the ingot into an electrode, controlling the filling ratio to be 0.7; (10) The electrode is subjected to electroslag remelting under a protective atmosphere. The protective atmosphere electroslag remelting IESR process is as follows: The slag system is pre-melted, cooled, crushed, and baked. After arc ignition, the slag is added to the crystallizer for slag formation. The slag system composition is as follows: CaF2 74.1 wt.%, CaO 16.0 wt.%, SiO2 3.6 wt.%, MgO 2.0 wt.%, MgF2 1.0 wt.%, K2O 3.3 wt.%; the crystallizer diameter is 200 mm. Remelting was carried out in a high-purity atmosphere-protected electroslag furnace, with high-purity argon gas (≥99.999%) introduced into the IESR at a flow rate of 2.4 m³ / s. 3 ·h -1 ; The melting rate is controlled at 2.2 kg / min. -1 ; During the capping stage of smelting, multi-stage current reduction feeding was carried out, with the feeding time controlled at 76 min and the feeding cycle repeated 6 times to obtain the ultrapure 316L austenitic stainless steel. The actual composition of the ultrapure 316L austenitic stainless steel is as follows: C 0.017wt.%, Cr 17.81wt.%, Ni 12.38wt.%, Mo 2.78wt.%, Mn 1.08wt.%, Si 0.45wt.%, N 0.004wt.%, Al 0.0092wt.%, Mg 0.0022wt.%, Ca 0.0024wt.%, O 0.0009wt.%, S 0.0012wt.%, P 0.0026wt.%, balance Fe; The ultrapure 316L austenitic stainless steel has coarse inclusions of grade 0 and inclusions of type A+B+C+D ≤ grade 0.5; α-ferrite content is 0 and δ-ferrite content is ≤ 0.1%; after hot rolling and annealing, the ultrapure 316L austenitic stainless steel has a grain size of 32μm; the mechanical properties at room temperature are: yield strength 245 MPa, tensile strength 550 MPa, and elongation 71.7%.

4. A method for preparing ultrapure 316L austenitic stainless steel, characterized in that, Includes the following steps: (1) Charging: The smelting equipment is a 100 kg vacuum induction furnace, and the charging amount is 85 kg. According to the target composition, the required experimental raw materials are weighed and filled into the vacuum induction melting furnace according to the principle of pressing down and loosening up. The target composition of the ultrapure 316LN austenitic stainless steel is C ≤0.02wt.%, Cr 16.6wt.%, Ni 10.6wt.%, Mo 2.6wt.%, Mn 1.9wt.%, Si 0.52wt.%, N ≤0.01wt.%, Al ≤0.01wt.%, O ≤0.001wt.%, S ≤0.0015wt.%, P ≤0.003wt.%, balance Fe. The impurity composition of the raw materials used for smelting is: Al 0.028wt.%, O 0.015wt.%, S 0.0028wt.%, P 0.0020wt.%. The material of the crucible used for melting is MgO. (2) Smelting: After the furnace is closed, turn on the vacuum pump and evacuate to less than 5 Pa. Then start powering on and gradually increase the power until all the furnace charge is melted. The melting temperature is 1512℃. (3) Al removal treatment: Add blocky FeO to the molten steel and refine for 3 min; the amount of FeO added is 0.045 wt.% of the molten steel. (4) Vacuum C deoxidation pretreatment: High-purity C blocks with a purity ≥ 99.99% are added to a free surface area of ​​0.038 m². 2 The volume is 0.01 m³. 3 The steel was pre-treated for deoxidation under a vacuum of 7 Pa for 18 min; the amount of C block added was 0.024 wt.% of the steel. (5) After deoxidation of C is completed, close the vacuum valve and introduce 0.02 MPa of high-purity argon gas into the induction furnace. The purity of the argon gas is ≥99.999%. (6) Mg deoxidation and desulfurization treatment: The temperature is controlled at 1468℃, and Ni-Mg alloy is added to the molten steel for deoxidation and desulfurization. The treatment time is 2 min. The Mg content in the Ni-Mg alloy is 20%; the amount of Ni-Mg alloy added is 0.200 wt.%, and the amount added as Mg is 0.040 wt.%. (7) Deep deoxidation and desulfurization treatment with Ca: After the Mg treatment, Si-Ca alloy is added to the molten steel for deep deoxidation and desulfurization, and the treatment time is 2 min; the Ca content in the Si-Ca alloy is 30%; the amount of Si-Ca alloy added is 0.573 wt.% of the molten steel, and the amount added as Ca is 0.172 wt.%; (8) Casting: The temperature of the molten steel is controlled at 1524℃. After 3 minutes of calming treatment, the molten steel is poured into the ingot mold. After the molten steel cools and solidifies, the ingot is removed. (9) Forge the ingot into an electrode, controlling the filling ratio to be 0.6; (10) The electrode is subjected to electroslag remelting under a protective atmosphere. The protective atmosphere electroslag remelting IESR process is as follows: The slag system is pre-melted, cooled, crushed, and baked. After arc ignition, the slag is added to the crystallizer for slag formation. The slag system composition is as follows: CaF2 70.1wt.%, CaO 17.5wt.%, SiO2 4.3wt.%, MgO 4.0wt.%, MgF2 2.1wt.%, K2O 2.0wt.%; the crystallizer diameter is 200mm. Remelting is carried out in an electroslag remelting furnace under a high-purity atmosphere, with high-purity argon gas (≥99.999%) introduced into the IESR at a flow rate of 2.5 m³ / s. 3 ·h -1 ; The melting rate is controlled at 1.8 kg / min. -1 ; During the capping stage of smelting, multi-stage current reduction and feeding were carried out, with the feeding time controlled at 80 min and the feeding cycle repeated 5 times to obtain the ultrapure 316L austenitic stainless steel. The actual composition of the ultrapure 316L austenitic stainless steel is as follows: C 0.018wt.%, Cr 16.64wt.%, Ni 10.57wt.%, Mo 2.61wt.%, Mn 1.86wt.%, Si 0.52wt.%, N 0.005wt.%, Al 0.0088wt.%, Mg 0.0031wt.%, Ca 0.0019wt.%, O 0.0009wt.%, S 0.0010wt.%, P 0.0018wt.%, balance Fe; The ultrapure 316L austenitic stainless steel has coarse inclusions of grade 0 and inclusions of type A+B+C+D ≤ grade 0.5; α-ferrite content is 0 and δ-ferrite content is ≤ 0.1%; the ultrapure 316L austenitic stainless steel has a grain size of 28μm after hot rolling and annealing; the mechanical properties at room temperature are: yield strength 256 MPa, tensile strength 563 MPa, and elongation 70.3%.

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