Austenitic stainless steel and preparation method thereof

Abstract

The present invention discloses an austenitic stainless steel and a preparation method thereof, belonging to the technical field of special metallurgy. In terms of weight percentage, the chemical components in the stainless steel include: C 0.001 - 0.005%, Si 0.05 - 0.15%, Cr 18.0 - 19.5%, Ni 14.5 - 16.5%, Mo 3.5 - 4.5%, Cu 0.1 - 0.5%, Re 0.005 - 0.01%, B 0.001 - 0.01%, Mg 0.0005 - 0.009%, N 0.01 - 0.025%, and the rest are Fe and inevitable impurities, where Re is one or two of La, Ce or Y; among them, the weight percentage of the inevitable impurities is w, w ≤ 179 ppm, the diameter of the inclusions in the stainless steel ≤ 5 μm, and the number of inclusions ≤ 15 pieces / mm<supgt;2< / supgt;. This stainless steel is smelted by three processes. The first process is vacuum induction melting, the second process is electroslag remelting under atmosphere, and the third process is gas shielded electroslag remelting. After the above three processes, the contents of impurities, gases and inclusions in the stainless steel are effectively reduced, and the requirements for the purity of stainless steel in special industries can be met.

Description

Technical Field

[0001] The invention belongs to the technical field of special metallurgy, and more specifically, relates to an austenitic stainless steel and a preparation method thereof. Background Art

[0002] Ultra-pure austenitic stainless steel has a wide range of uses, including food, medical, aerospace, semiconductors and other fields, which have extremely high requirements on the impurity content in stainless steel, especially the inclusion content and size. Although a variety of high-purity austenitic stainless steels have been developed, the purity of current stainless steel cannot meet the corrosion resistance requirements of the use environment for critical use occasions, and still needs to be further improved.

[0003] After searching, Chinese invention patent 201911287153.8 discloses a remelting method for high-purity 316LN stainless steel. The patent is produced by vacuum induction melting-electroslag remelting-vacuum consumable process, and the remelting slag is CaF2: 40-50%, CaO: 20-25%, SiO2: 20-25%, MgO: 5-10%. After remelting, the D-type inclusions in the steel are 0.5 level, and the gas content is low. However, the smelting method adopted in this patent is the traditional triple process. Although the inclusion rating is low, the specific size and quality of the inclusions are not given, which is one of the key indicators of ultra-pure stainless steel. In addition, the steel contains 0.007% aluminum, which cannot effectively inhibit the formation of large-sized alumina inclusions; the patent also does not specify the content of S impurities in the steel, and S is also an important indicator for measuring the purity of ultra-pure stainless steel. Therefore, although the use of this patent can improve the purity of stainless steel to a certain extent, there is still a certain gap from ultra-pure stainless steel. Chinese patent 202111120319.4 discloses an ultra-pure stainless steel for semiconductor manufacturing, containing the following components (weight percentage): carbon 0.03, manganese 0.5, silicon 0.75, phosphorus 0.45, sulfur 0.005-0.010, chromium 16-18, nickel 10-14, molybdenum 2-3, copper 0.30, nitrogen 0.51, and the balance is iron. The triple process of vacuum induction melting-electroslag remelting-vacuum consumable melting is adopted. However, the impurity content in the stainless steel component of this patent is significantly higher, and the oxygen content and inclusion content reflecting the purity index are not specified; the process flow is a traditional triple process, which is difficult to meet the preparation requirements of ultra-pure stainless steel. Chinese patent 202110346617.9 discloses a method for preparing ultra-pure G102Cr18Mo stainless bearing steel. The patent adopts a process flow of parent material smelting-two vacuum consumable processes for smelting, which can effectively improve the purity of G102Cr18Mo stainless bearing steel. However, the patent is aimed at high-carbon stainless steel with a carbon content of more than 1.0%. Under vacuum consumable conditions, oxygen in the steel can be removed by carbon-oxygen reaction, thereby reducing the content of inclusions. However, for ultra-low carbon stainless steel (carbon less than 0.01%), the trend of carbon-oxygen reaction is greatly reduced. Therefore, for ultra-low carbon stainless steel, this patent still has certain limitations.

[0004] Chinese patent 202311687894.1 discloses an ultra-pure 316LVV stainless steel ingot and its triple smelting process and application. The ultra-pure 316LVV stainless steel ingot of the patent includes, by mass percentage, C≤0.01%, Si≤0.05%, Mn≤0.05%, Ni: 14.50-15.00%, Cr: 16.50-17.00%, Mo: 2.2-2.5%, Nb≤0.5%, and Fe balance. The ultra-pure 316LVV stainless steel ingot is made by a triple smelting process, (1) an induction electrode is made by a vacuum induction melting process; (2) after the surface of the induction electrode is treated, electroslag melting is performed; (3) after the electroslag melting treatment, surface treatment is performed, and then vacuum consumable melting is performed. However, the patent does not make further restrictions on the impurity content. For example, during the stainless steel welding process, if the S and Ca content in the material is too high, secondary pollution will occur. For example, the nitrogen content in the patent is controlled at a low level, but too low a nitrogen content is not conducive to improving the strength of the material. For example, the aluminum content in stainless steel is very easy to produce aluminum oxide inclusions. Even if the inclusions are observed as "0" under a microscope, it cannot be completely guaranteed that such inclusions do not exist in the steel. Although the patent describes O, N, H and inclusions, it lacks effective control over other impurities such as P, S, Al, Ca, and Ti, resulting in certain defects in the purity of the 316LVV stainless steel. In terms of preparation process, the 316LVV stainless steel adopts the traditional three-way process, which is a common process for producing special alloy products. The electroslag remelting slag system also adopts the traditional CaF2, Al2O3, CaO ternary slag system. In particular, the high Al2O3 content in the slag system is very likely to increase the aluminum content of the stainless steel, which in turn increases the inclusion content in the stainless steel. Protective remelting is used in the electroslag remelting stage. Although protective remelting can prevent the entry of air, it is not conducive to reducing the impurity S content in the steel, and the ultra-pure stainless steel used in the semiconductor field has high requirements for sulfur content. Although vacuum consumable remelting can reduce the impurity content in the steel, it will inevitably further remove the nitrogen content in the stainless steel, making it difficult to control the nitrogen content at an ideal level, thereby reducing the mechanical properties of the stainless steel. Therefore, the ultra-pure 316LVV stainless steel disclosed in the patent still has certain defects in terms of purity and preparation process.

[0005] In summary, although there are related technologies for ultra-pure stainless steel and its preparation method, the purity of the stainless steel mentioned in the prior art needs to be further improved under extremely harsh conditions of use, such as when used as a semiconductor ultra-pure gas delivery pipeline; the current preparation method cannot meet the preparation of higher purity stainless steel. Therefore, fully improving the purity of stainless steel and reducing the impurity content are key means to ensure the performance of stainless steel. At present, it is urgently necessary to design an austenitic stainless steel and its preparation method. Summary of the invention

[0006] 1. Problem to be solved

[0007] In view of the problems of low purity, large number and large size of inclusions in the prior art, the present application provides an austenitic stainless steel, and ultra-pure austenitic stainless steel is obtained by strictly controlling the raw material formula. A preparation method of austenitic stainless steel is further provided, and a vacuum induction melting-atmospheric electroslag remelting-argon gas protection electroslag remelting process route is designed, and the process parameters are strictly controlled, so that the impurity content in austenitic stainless steel can be controlled, especially ultra-pure austenitic stainless steel with extremely low inclusion content, and the corrosion resistance of stainless steel can be improved, thereby effectively solving the problem of high purity requirements for stainless steel in complex environments.

[0008] 2. Technical solution

[0009] In order to solve the above problems, the technical solution adopted by the present invention is as follows:

[0010] Austenitic stainless steel, the chemical composition of which, by weight percentage, comprises: C 0.001-0.005%, Si 0.05-0.15%, Cr 18.0-19.5%, Ni 14.5-16.5%, Mo 3.5-4.5%, Cu 0.1-0.5%, Re 0.005-0.01%, B 0.001-0.01%, Mg 0.0005-0.009%, N 0.010-0.025%, the rest is Fe and inevitable impurities, wherein Re is one or two of La, Ce or Y; wherein the weight percentage of the inevitable impurities is w, w≤179ppm, the diameter of the inclusions in the stainless steel is ≤5μm, and the number of the inclusions is ≤15 / mm 2 .

[0011] Furthermore, unavoidable impurities include but are not limited to S, O, and Al, wherein the relationship between the content of S, O, and Al and the size of inclusions is:

[0012] (1) 28ppm<S+O+Al≤33ppm, the diameter of inclusions in stainless steel ≤5.0μm;

[0013] (2) 23ppm<S+O+Al≤28ppm, the diameter of inclusions in stainless steel ≤4.2μm;

[0014] (3) 20ppm<S+O+Al≤23ppm, the diameter of inclusions in stainless steel ≤3.5μm;

[0015] (4) 15ppm<S+O+Al≤20ppm, the diameter of inclusions in stainless steel ≤3.0μm;

[0016] (5) 0<S+O+Al≤15ppm, the diameter of inclusions in stainless steel ≤2.0μm.

[0017] Furthermore, the inevitable impurity contents are as follows in percentage: Al≤0.002%, S≤0.0005%, P≤0.005%, O≤0.0008%, H≤0.00010%, Mn≤0.008%, Ca≤0.0005%, Ti≤0.001%.

[0018] Furthermore, the inclusions are mainly non-metallic inclusions, mainly including aluminum oxide, manganese sulfide, and magnesium aluminum spinel, and the diameters and numbers are shown in the following table:

[0019]

[0020] The stainless steel of the present invention strictly controls the impurity content, and in particular clearly defines the number and size of inclusions, thereby effectively improving the corrosion resistance of the stainless steel.

[0021] The preparation method of the above-mentioned austenitic stainless steel, the specific preparation steps are as follows:

[0022] S1. Vacuum induction melting process:

[0023] S101, slow melting treatment: before starting the vacuum induction, all the solid materials of the above chemical composition (except rare earth, magnesium and nitrogen) are added into the furnace at one time, and slowly melted under vacuum to obtain molten steel. The melting time is controlled within 4 to 8 hours, preferably within 6 to 8 hours. Controlling the melting time is used to control the vacuum induction power.

[0024] It should be noted that during the induction melting process, oxygen and carbon in the steel will react to produce CO, which will be extracted by the vacuum system, thereby reducing the oxygen in the steel. If rare earths are added before induction, after the raw materials are melted, the rare earths will preferentially combine with oxygen, and C and O will not be able to form CO, which is not conducive to the removal of oxygen. Therefore, after the refining is completed, the oxygen has been removed in large quantities, and adding rare earths at this time will not affect the removal of oxygen. If Mg-containing materials are added to the furnace before starting, Mg will enter the molten steel after the molten steel is melted. However, the vapor pressure of magnesium is very large, and under high vacuum conditions, magnesium will volatilize, so magnesium cannot exist in steel. Therefore, after the refining is completed, a certain pressure of gas is introduced into the furnace, and then magnesium-containing solid materials are added to ensure that Mg enters the steel.

[0025] S102, rapid refining treatment: refining molten steel under vacuum:

[0026] The refining time is controlled to be 2 to 4 hours. During the refining process, the power remains unchanged. The present invention shortens the refining time to prevent the decomposition of the furnace lining to generate oxygen that enters the molten steel, thereby contaminating the molten steel. Conventional molten steel refining requires a longer time to fully reduce the nitrogen content in the molten steel. However, the refining process of the present invention does not require strict reduction of the nitrogen content too much. Instead, the nitrogen is further increased by remelting the argon / nitrogen mixed gas at the end of the refining.

[0027] The above-mentioned vacuum induction melting process, slow melting, and vacuum refining process can reduce nitrogen to below 50ppm: in the slow melting process, nitrogen on the surface of the solid raw material is extracted under high vacuum; in the rapid refining process, nitrogen in the molten steel is extracted and removed under high vacuum. In order to reduce the target nitrogen content in the finished stainless steel product, the nitrogen content in the molten steel is controlled not to exceed 50ppm after refining, so as to reduce the increase of nitrogen caused by the remelting of argon and nitrogen mixed gas after refining. The specific control method of nitrogen content is as follows:

[0028]

[0029] Wherein, N is the nitrogen content in the molten steel after refining; N0 is the nitrogen content in the raw material, ≤350ppm; t1 is the melting time of the slow melting treatment, h; p1 is the vacuum pressure of the slow melting treatment, Pa; t2 is the refining time of the rapid refining treatment, h; p2 is the vacuum pressure of the rapid refining treatment, Pa.

[0030] S103, casting treatment: after refining, a mixed gas of argon and nitrogen is introduced, on the one hand to facilitate the entry of rare earth into the steel, and on the other hand to achieve the purpose of nitrogen addition, and then rare earth metal ingots and magnesium alloys are added to cast the molten steel into steel ingots;

[0031] S2, first electroslag remelting process: using the steel ingot obtained in step S103 as a consumable electrode, electroslag remelting is performed in an atmospheric environment to prepare a first electroslag remelting ingot;

[0032] S3, a second electroslag remelting process: using the first electroslag remelting ingot obtained in step S2 as a consumable electrode to perform electroslag remelting, controlling the volume proportion of oxygen in the electroslag remelting environment to be less than 0.5%, to prepare a second electroslag remelting ingot;

[0033] S4. Forging and heat treating the second electroslag remelting ingot obtained in step S3 to obtain austenitic stainless steel.

[0034] It should be noted that the order of the steps in the above preparation method cannot be adjusted:

[0035] (1) Vacuum induction melting is mainly used to prepare high-purity steel ingots as consumable electrodes. Its main function is to ensure the basic composition of the consumable electrode and to purify it appropriately, such as reducing the content of gases such as oxygen, nitrogen, and hydrogen. However, it cannot solve the problem of S, so further desulfurization is required in the electroslag process.

[0036] (2) The electroslag process is divided into two types: gas protection and atmospheric remelting. Remelting in the atmosphere can effectively remove sulfur from the steel, but remelting in the atmosphere will also increase the oxygen content in the steel, which is unfavorable. Therefore, remelting in the atmosphere is completed and then remelted under gas protection. Electroslag remelting under gas protection can completely shield air pollution and effectively remove oxygen and inclusions through the action of slag; the present application can achieve effective control of S, N, O and inclusions through the above process; on the contrary, if gas protection electroslag is used first and then remelted in the atmosphere, the oxygen content in the product will increase, which will reduce the purity of the product.

[0037] Furthermore, in step S101, during the slow melting process, the vacuum pressure is controlled to be 10-20 Pa. In step S102, during the rapid refining process, the refining temperature is 1530-1550° C., and the vacuum pressure is ≤2 Pa.

[0038] Furthermore, in step S102, since the refining time is short and the oxygen content in the molten steel is high, the present application adds 0.005% to 0.008% aluminum by weight of the molten steel to the molten steel one hour before the end of refining. The purpose is to combine aluminum with oxygen to form Al2O3 and remove oxygen. The aluminum added here can be removed by two subsequent electroslag remeltings.

[0039] Furthermore, in step S103, the total pressure of the introduced argon and nitrogen is 4 kPa to 10 kPa, and the volume ratio of argon to nitrogen is (6 to 8):(4 to 2).

[0040] Furthermore, in step S103, the magnesium alloy is a mixture of Ni2Mg alloy and SiMg2 alloy, and the mass ratio of the two is (5-53):(3-48). Since magnesium has a low boiling point and a high vapor pressure, if pure magnesium ingots are added, it will volatilize quickly under high temperature and high vacuum (even if the argon / nitrogen mixed gas is remelted in the furnace). Adding it in the form of a magnesium alloy will greatly reduce the vapor pressure of magnesium. Under a pressure of 4kPa to 10kPa, magnesium can smoothly enter the steel. After Si2Mg is added to the steel, a certain amount of magnesium vapor will be generated. At the same time, due to the introduction of nitrogen / argon into the furnace, there is already a certain pressure. At this time, adding Ni2Mg can inhibit the volatilization of magnesium in Ni2Mg and increase the recovery rate of magnesium in steel. Therefore, the mixed addition of the two can ensure that magnesium smoothly enters the steel.

[0041] Furthermore, in step S2, the first electroslag remelting uses the first remelting slag system, which has a composition of 50-60 parts of CaF2, 20-30 parts of CaO, and 15-20 parts of Al2O3. In order to reduce the oxygen content in the first remelting slag system and prevent it from contaminating the molten steel, silicon-calcium alloy is added during the remelting process, wherein the weight ratio of the addition amount of silicon-calcium alloy to the molten steel is (1.0-2.0kg):1t.

[0042] Furthermore, in step S3, the second electroslag remelting is to further argon-protected electroslag remelting of the first electroslag remelting ingot obtained by remelting in the atmosphere. Before remelting, the first electroslag remelting ingot is peeled, and the oxygen volume fraction in the atmosphere is adjusted by controlling the flow ratio of the air and argon gas introduced. Under the protection of argon, remelting is carried out in the electroslag furnace, and no material is added during the remelting process. The second electroslag remelting uses the second remelting slag system, and the composition is CaF2 40-50 parts, MgF2 15-20 parts, Na3AlF6 5-10 parts, MgO 10-15 parts, CaO 10-15 parts, Al2O3 5-10 parts, SiO2 5-10 parts.

[0043] The second remelting slag system contains 40-50 parts of CaF2, 15-20 parts of MgF2, and 5-10 parts of Na3AlF6, which can reduce the melting point of the entire remelting slag system, reduce the interfacial tension between the slag system and the inclusions in the electrode, promote the adsorption of inclusions, and then purify the molten steel; and the slag contains 5-10 parts of Al2O3, which can not only make the slag system have a certain conductivity and stable process, but also avoid the decomposition of alumina caused by excessive alumina, resulting in an increase in the aluminum content in the steel; the slag system contains 10-15 parts of MgO, which can form a slag film with high viscosity on the surface of the slag pool to prevent trace oxygen in the remelting atmosphere from entering the slag pool; the slag system contains a certain amount of CaO and a small amount of SiO2, which can increase the basicity of the slag system and promote the desulfurization of steel. Through the reasonable combination of the above slag system components, the remelting process can be stabilized, and the inclusions in the steel can be removed to improve the purity of the material. In other words, the second heavy melting slag system has a lower melting point and better thermal stability, and is easier to adsorb alumina and magnesia-aluminum spinel inclusions in the electrode, resulting in a better purification effect; under the protection of argon, it can shield the air from oxidizing the consumable electrode, thereby preventing the slag shell on the surface of the consumable electrode from entering the slag and affecting the physical and chemical properties of the slag; therefore, under the protection of argon, the second heavy melting slag system has stable performance and can give full play to the slag system's ability to adsorb inclusions.

[0044] 3. Beneficial effects

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] (1) The ultra-pure austenitic stainless steel of the present invention comprises, by weight percentage, C 0.001-0.005%, Si 0.05-0.15%, Cr 18.0-19.5%, Ni 14.5-16.5%, Mo 3.5-4.5%, Cu 0.1-0.5%, Re 0.005-0.01%, B 0.001-0.01%, Mg 0.0005-0.009%, N 0.010~0.025%, the rest are Fe and inevitable impurities, among which Re is one or two of La, Ce or Y, and the contents of Cr, Ni, Mo and other components are appropriately increased, which increases the corrosion resistance of stainless steel; the C carbon content is controlled to be extremely low, which reduces its adverse effect on the corrosion resistance of stainless steel; the Mn and Si contents are reduced, which reduces the volatilization of elements in the stainless steel welding process and reduces the pollution to the stainless steel surface; at the same time, trace amounts of Re, B and Mg are added to further improve the corrosion resistance and mechanical properties of stainless steel.

[0047] The impurity content is as follows in percentage: Al≤0.002%, S≤0.0005%, P≤0.005%, O≤0.0008%, H≤0.0001%, Mn≤0.008%, Ca≤0.0005%, Ti≤0.001%; ​​inclusion diameter ≤5μm, number of inclusions ≤15 / mm 2 The extremely low control of impurity content effectively reduces the impact on various properties of stainless steel, giving full play to the performance of alloy elements in stainless steel, so that stainless steel can be used in more demanding environments. In particular, the nitrogen content is controlled at 0.01-0.025%, which prevents the adverse effects of nitrogen and fully utilizes the strengthening and austenitizing effects of nitrogen to improve the corrosion resistance and mechanical properties of stainless steel.

[0048] (2) A method for preparing ultra-pure austenitic stainless steel according to the present invention, wherein the vacuum induction process adopts a smelting method of slow melting and rapid refining. By reducing the melting rate, the solid surface is melted layer by layer, and the gas impurities in the molten layer have sufficient time to overflow and be removed under high vacuum; due to the slow melting rate and low melting temperature, the furnace lining is not easy to decompose, thereby reducing the contamination of the molten steel by the furnace lining. The slow melting can fully remove the oxygen, nitrogen, hydrogen and other gas contents in the steel, thereby reducing the refining time. The refining temperature must be higher than the melting temperature. A high refining temperature will promote the decomposition of the furnace lining and pollute the molten steel. A reduction in the refining time will slow down the decomposition of the furnace lining and reduce the contamination of the molten steel. Therefore, the vacuum induction process adopts a smelting method of slow melting and rapid refining to fully reduce the gas impurity content in the molten steel. It should be noted that, on the one hand, by adding 0.005-0.008% aluminum for deoxidation before the end of vacuum induction, there is a certain amount of aluminum in the steel, and in the subsequent electroslag remelting process, the slag absorbs a certain amount of alumina inclusions, which can reduce the aluminum content to below 0.002%. On the other hand, the nitrogen at the end of vacuum induction melting is controlled within 50ppm. After the refining is completed, the nitrogen content can be controlled to the target value by filling the vacuum chamber with a mixed gas of nitrogen and argon and adjusting the ratio of the two.

[0049] (3) A method for preparing ultra-pure austenitic stainless steel of the present invention adopts atmospheric electroslag remelting-gas shielded electroslag remelting. The first remelting adopts 50-60 parts of CaF2, 20-30 parts of CaO, and 15-20 parts of Al2O3 high-basicity slag system remelting. High basicity + atmospheric remelting can effectively reduce the S content in the steel. At present, the remelting of high-purity stainless steel often adopts argon shielded remelting, but argon shielding will inhibit desulfurization. The second remelting is carried out under the protection of high-purity argon, which can completely shield the influence of the atmosphere, fully purify the stainless steel, and reduce the content of oxygen and inclusions; at the same time, the self-developed low-oxidizing and low-melting-point slag system CaF2 40-50 parts, MgF2 15-20 parts, Na3AlF6 5-10 parts, MgO 10-15 parts, CaO 10-15 parts, Al2O3 5-10 parts, SiO2 5-10 parts are used for remelting, which is beneficial to the adsorption of fine inclusions in the steel and can improve the surface quality and purity of the stainless steel ingot. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 This is an optical microscope photo of inclusions in Example 1;

[0051] Figure 2 This is the SEM spectrum of inclusions in Example 1;

[0052] Figure 3 This is an optical microscope photograph of inclusions in Example 2;

[0053] Figure 4 It is the SEM spectrum of Example 2;

[0054] Figure 5 This is the SEM photo of Example 3;

[0055] Figure 6 This is the SEM photo of Example 4;

[0056] Figure 7 This is the SEM photograph of Example 5. DETAILED DESCRIPTION

[0057] The following more detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but is merely for illustration and not to limit the description of the features and characteristics of the present invention, in order to propose the best way to perform the present invention, and is sufficient to enable those skilled in the art to implement the present invention. However, it should be understood that various modifications and variations may be made without departing from the scope of the present invention as defined by the appended claims. The detailed description should be considered only as illustrative and not restrictive, and if there are any such modifications and variations, they will all fall within the scope of the present invention described herein. In addition, the background technology is intended to illustrate the current status and significance of the research and development of the present technology, and is not intended to limit the present invention or the application field of the present application and the present invention. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art of the present invention; the terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments, and are not intended to limit the present invention; the term "and / or" used herein includes any and all combinations of one or more related listed items.

[0058] A method for preparing austenitic stainless steel is provided in a specific embodiment, and the specific preparation steps are as follows:

[0059] S101, slow melting treatment: all solid raw materials except rare earth, magnesium and nitrogen (nitrogen content in stainless steel is a trace element in solid raw materials) are added into the furnace at one time before vacuum induction is started according to Table 1, and slowly melted under vacuum to obtain molten steel, the melting time is controlled within 4 to 8 hours, and the vacuum pressure is controlled within 10 to 20 Pa;

[0060] S102, rapid refining treatment: refining molten steel under vacuum:

[0061] The refining time is controlled to be 2 to 4 hours, the refining temperature is 1530 to 1550°C, the vacuum pressure is ≤2Pa, and 0.005% to 0.008% aluminum is added to the molten steel one hour before the end of refining. The nitrogen content in the molten steel is controlled not to exceed 50ppm after the refining. The specific control method of the nitrogen content is as follows:

[0062]

[0063] Wherein, N is the nitrogen content in the molten steel after refining; N0 is the nitrogen content in the raw material, ≤350ppm; t1 is the melting time of the slow melting treatment, h; p1 is the vacuum pressure of the slow melting treatment, Pa; t2 is the refining time of the rapid refining treatment, h; p2 is the vacuum pressure of the rapid refining treatment, Pa.

[0064] S103. Casting treatment: After refining, a mixed gas of argon and nitrogen is introduced with a total pressure of 4 kPa to 10 kPa, and the ratio of argon to nitrogen is (6 to 8): (4 to 2). Then, rare earth metal ingots and magnesium alloy (a mixture of Ni2Mg alloy and SiMg2 alloy, with a ratio of 9:1) are added according to Table 1, and the molten steel is cast into steel ingots.

[0065] S2. The first electroslag remelting process:

[0066] The steel ingot is used as the consumable electrode, and the first remelting slag system is selected, with the composition of 50-60 parts of CaF2, 20-30 parts of CaO, and 15-20 parts of Al2O3. Electroslag remelting is carried out in an atmospheric environment. During the remelting process, silicon-calcium alloy is added, wherein the weight ratio of the addition amount of silicon-calcium alloy to the molten steel is (1.0-2.0kg):1t, and the first electroslag remelting ingot is prepared.

[0067] S3, Second electroslag remelting process:

[0068] The electroslag remelting ingot is used as the consumable electrode for electroslag remelting, and the volume proportion of oxygen is controlled to be less than 0.5%, and the rest is argon to prepare the second electroslag remelting ingot; the second electroslag remelting uses the second heavy melting slag system, and the composition is 40-50 parts of CaF2, 15-20 parts of MgF2, 5-10 parts of Na3AlF6, 10-15 parts of MgO, 10-15 parts of CaO, 5-10 parts of Al2O3, and 5-10 parts of SiO2.

[0069] S4, forging and heat treating the second electroslag remelting ingot to obtain austenitic stainless steel. Tables 1 to 4 respectively show the parameters of each embodiment and comparative example in each process.

[0070] Table 1 Solid raw materials

[0071]

[0072] Table 2 Vacuum induction melting process

[0073]

[0074] Table 3 The first electroslag remelting process

[0075]

[0076] Table 4 Second electroslag remelting process

[0077]

[0078]

[0079] It is worth noting that in order to obtain ultra-pure stainless steel, a sequence of two electroslag remelting processes is necessary.

[0080] Comparative Example 6

[0081] Comparative Example 6 is basically the same as Example 1, except that the order of the first electroslag remelting process and the second electroslag remelting process is interchanged, that is:

[0082] A method for preparing austenitic stainless steel, the specific preparation steps are as follows:

[0083] S101, slow melting treatment: solid raw materials (0.003% C, 0.10% Si, 18.0% Cr, 15.0% Ni, 4.0% Mo, 0.3% Cu, 0.005% La, 0.0025% Y, 0.005% B, about 62.5% Fe) except rare earth and magnesium are added into the furnace at one time before vacuum induction is started, and slowly melted under vacuum to obtain molten steel. The melting time is controlled within 8 hours, and the vacuum pressure is controlled to be 20Pa.

[0084] S102, rapid refining treatment: refining molten steel under vacuum:

[0085] The refining time is controlled to be 2 hours, the refining temperature is 1530°C, the vacuum pressure is 2Pa, and 0.005% aluminum is added to the molten steel one hour before the end of refining. The nitrogen content in the molten steel is controlled not to exceed 50ppm after the end of refining.

[0086] S103. Casting treatment: After refining, a mixed gas of argon and nitrogen is introduced with a total pressure of 4 kPa and a ratio of argon to nitrogen of 8:2. Then, rare earth metal ingots and magnesium alloy (a mixture of Ni2Mg alloy and SiMg2 alloy, wherein, based on the total solid raw materials as 1, the added mass of Ni2Mg alloy accounts for 0.053% and the added mass of SiMg2 alloy accounts for 0.026%) are added according to the formula of Example 1 in Table 1, and the molten steel is cast into a steel ingot.

[0087] S2. The first electroslag remelting process:

[0088] The steel ingot was used as the consumable electrode for electroslag remelting, and the volume proportion of oxygen was controlled to be 0.4%, and the rest was argon to prepare the first electroslag remelting ingot; the first electroslag remelting selected the first remelting slag system, and the composition was 40 parts of CaF2, 20 parts of MgF2, 5 parts of Na3AlF6, 15 parts of MgO, 10 parts of CaO, 5 parts of Al2O3, and 5 parts of SiO2.

[0089] S3, Second electroslag remelting process:

[0090] The first electroslag remelting ingot is used as the consumable electrode, and the second remelting slag system is selected, with the composition of 50 parts of CaF2, 20 parts of CaO, and 15 parts of Al2O3. Electroslag remelting is carried out in an atmospheric environment. During the remelting process, silicon-calcium alloy is added, wherein the weight ratio of the added silicon-calcium alloy to the molten steel is 1.0:1t, to prepare a second electroslag remelting ingot.

[0091] S4. Forging and heat treating the second electroslag remelting ingot to obtain austenitic stainless steel.

[0092] Performance Testing and Product Characterization

[0093] 1. Stainless steel mechanical properties and corrosion resistance test

[0094] The stainless steel obtained in the above examples and comparative examples was subjected to mechanical and corrosion resistance tests under extremely harsh use conditions. The test results are shown in Tables 5 and 6.

[0095] Table 5 Mechanical properties of stainless steel obtained in each embodiment and comparative example

[0096]

[0097] Note: In the mechanical properties data of Table 5, the lower the hardness of stainless steel, the better, and the higher the other indicators, the better.

[0098] Table 6 Corrosion resistance (corrosion rate) of stainless steel obtained in various examples and comparative examples in different acids, unit: mm.a -1

[0099] Example / Comparative Example 10% Oxalic Acid 20% Phosphoric acid 10% sulfuric acid Example 1 1.10 0.010 13.15 Example 2 1.06 0.008 12.89 Example 3 1.05 0.009 11.05 Example 4 1.04 0.01 12.60 Example 5 1.08 0.008 11.77 Comparative Example 1 1.25 0.026 15.59 Comparative Example 2 1.28 0.031 15.78 Comparative Example 3 1.10 0.028 15.26 Comparative Example 4 1.30 0.037 14.93 Comparative Example 5 1.41 0.040 15.50 Comparative Example 6 1.43 0.045 16.25

[0100] 2. Detection of impurities and inclusions in stainless steel

[0101] According to the preparation method of this embodiment, smelting was carried out, and the impurity elements, gas content, inclusion size and content of the ultra-pure austenitic stainless steel are as shown in Table 7:

[0102] Table 7 Impurity elements, Mg, N content, number and size of inclusions in the stainless steel obtained in each embodiment and comparative example

[0103]

[0104] Figure 1 This is a photo of the stainless steel prepared in Example 1 magnified 200 times under an optical microscope, and no inclusions are found in the photo; Figure 2 This is the SEM spectrum of inclusions in the stainless steel prepared in Example 1. The inclusions in the figure are aluminum oxide inclusions, and their size is only 2 μm; Figure 3 This is a photo of the stainless steel prepared in Example 2 magnified 200 times by an optical microscope. No inclusions are found in the photo. Figure 4 This is the SEM spectrum of inclusions in the stainless steel prepared in Example 2. The inclusions in the figure are magnesia-alumina spinel inclusions, and their size does not exceed 4.0 μm; Figure 5 This is a SEM photo of four inclusions in the stainless steel prepared in Example 3. The size of the largest inclusion does not exceed 3.5 μm; Figure 6 This is a SEM photo of four inclusions in the stainless steel prepared in Example 4. The size of the largest inclusion does not exceed 3.0 μm. Figure 7 This is a SEM photo of four inclusions in the stainless steel prepared in Example 5. The size of the largest inclusion does not exceed 2.0 μm.

[0105] The above-mentioned embodiments have elaborated on the purpose and implementation effects of the present invention in detail. It should be understood that the above-mentioned embodiments are only descriptions of the preferred embodiments of the present invention, and do not limit the concept and scope of the present invention. All modifications, equivalent substitutions, improvements, etc. made by engineers and technicians in this field or using the technical concept and technical solutions of the present invention within the spirit and principles of the present invention and without departing from the design concept of the present invention are within the protection scope of the present invention.

Claims

1. An austenitic stainless steel, characterized in that: Its chemical composition, by weight percentage, includes: C 0.001-0.005%, Si 0.05-0.15%, Cr 18.0-19.5%, Ni 14.5-16.5%, Mo 3.5-4.5%, Cu 0.1-0.5%, Re 0.005-0.01%, B 0.001-0.01%, Mg 0.0005-0.009%, N 0.01~0.025%, the rest are Fe and inevitable impurities, wherein Re is one or two of La, Ce or Y; wherein the weight percentage of inevitable impurities is w, w≤179ppm, and the inevitable impurity content includes the following percentages: Al≤0.0020%, S≤0.0005%, P≤0.005%, O≤0.0008%, H≤0.00010%, Mn≤0.008%, Ca≤0.0005%, Ti≤0.001%; ​​the diameter of inclusions in stainless steel is ≤5μm, and the number of inclusions is ≤15 / mm 2 ; The preparation steps are as follows: S1. Vacuum induction melting process: S101, slow melting treatment: adding solid materials into the furnace before vacuum induction starting, slowly melting under vacuum to obtain molten steel, and the melting time is controlled within 4 to 8 hours; the solid materials include: C 0.001 to 0.005%, Si 0.05 to 0.15%, Cr18.0 to 19.5%, Ni 14.5 to 16.5%, Mo 3.5 to 4.5%, Cu 0.1 to 0.5%, B 0.001 to 0.01%, 63.8334 to 58.791% Fe; S102, rapid refining treatment: refining the molten steel under vacuum, controlling the refining time to be 2 to 4 hours, and controlling the nitrogen content in the molten steel to be no more than 50 ppm after the refining; S103, casting treatment: after refining, a mixed gas of argon and nitrogen is introduced, and then 0.005-0.01% of rare earth metal ingots and the rest of magnesium alloy are added to cast the molten steel into steel ingots; the rare earth metal is one or two of La, Ce or Y; S2. The first electroslag remelting process: The steel ingot obtained in step S103 is used as a consumable electrode, and electroslag remelting is performed in an atmospheric environment to prepare a first electroslag remelting ingot; the first electroslag remelting uses a first remelting slag system, and the composition is 50-60 parts of CaF2, 20-30 parts of CaO, and 15-20 parts of Al2O3; during the first electroslag remelting, silicon-calcium alloy is added, wherein the weight ratio of the addition amount of silicon-calcium alloy to the molten steel is (1.0-2.0kg):1t; S3, Second electroslag remelting process: The first electroslag remelting ingot obtained in step S2 is used as a consumable electrode for electroslag remelting, and the volume proportion of oxygen in the electroslag remelting environment is controlled to be less than 0.5%, so as to prepare a second electroslag remelting ingot; the second electroslag remelting uses a second remelting slag system, and the composition is 40-50 parts of CaF2, 15-20 parts of MgF2, 5-10 parts of Na3AlF6, 10-15 parts of MgO, 10-15 parts of CaO, 5-10 parts of Al2O3, and 5-10 parts of SiO2; S4. Forging and heat treating the second electroslag remelting ingot obtained in step S3 to obtain austenitic stainless steel.

2. The austenitic stainless steel according to claim 1, characterized in that: The unavoidable impurities include S, O, and Al, wherein the relationship between the content of S, O, and Al and the size of inclusions is: (1) 28ppm<S+O+Al≤33ppm, the diameter of inclusions in stainless steel ≤5.0μm; (2) 23ppm<S+O+Al≤28ppm, the diameter of inclusions in stainless steel ≤4.2μm; (3) 20ppm<S+O+Al≤23ppm, the diameter of inclusions in stainless steel ≤3.5μm; (4) 15ppm<S+O+Al≤20ppm, the diameter of inclusions in stainless steel ≤3.0μm; (5) 0<S+O+Al≤15ppm, the diameter of inclusions in stainless steel ≤2.0μm.

3. The austenitic stainless steel according to claim 1, characterized in that: The inclusions are non-metallic inclusions, including alumina, manganese sulfide, and magnesium aluminum spinel. The diameter of the alumina inclusions is ≤5μm, and the number of alumina inclusions is ≤4 / mm. 2 , the diameter of manganese sulfide inclusions ≤ 2μm, the number of manganese sulfide inclusions ≤ 2 / mm 2 , the diameter of magnesia-alumina spinel inclusions ≤ 5μm, the number of magnesia-alumina spinel inclusions ≤ 6 / mm 2 .

4. A method for preparing austenitic stainless steel according to any one of claims 1 to 3, characterized in that: The preparation steps are as follows: S1. Vacuum induction melting process: S101, slow melting treatment: adding solid materials into the furnace before vacuum induction starting, slowly melting under vacuum to obtain molten steel, and the melting time is controlled within 4 to 8 hours; the solid materials include: C 0.001 to 0.005%, Si 0.05 to 0.15%, Cr18.0 to 19.5%, Ni 14.5 to 16.5%, Mo 3.5 to 4.5%, Cu 0.1 to 0.5%, B 0.001 to 0.01%, 63.8334 to 58.791% Fe; S102, rapid refining treatment: refining the molten steel under vacuum, controlling the refining time to be 2 to 4 hours, and controlling the nitrogen content in the molten steel to be no more than 50 ppm after the refining; S103, casting treatment: after refining, a mixed gas of argon and nitrogen is introduced, and then 0.005-0.01% of rare earth metal ingots and the rest of magnesium alloy are added to cast the molten steel into steel ingots; the rare earth metal is one or two of La, Ce or Y; S2. The first electroslag remelting process: Using the steel ingot obtained in step S103 as a consumable electrode, electroslag remelting is performed in an atmospheric environment to prepare a first electroslag remelted ingot; S3, Second electroslag remelting process: Using the first electroslag remelting ingot obtained in step S2 as a consumable electrode for electroslag remelting, controlling the volume percentage of oxygen in the electroslag remelting environment to be less than 0.5%, to prepare a second electroslag remelting ingot; S4. Forging and heat treating the second electroslag remelting ingot obtained in step S3 to obtain austenitic stainless steel.

5. The method for preparing austenitic stainless steel according to claim 4, characterized in that: After refining, the nitrogen content in molten steel satisfies the following formula Wherein, N is the nitrogen content in the molten steel after refining; N0 is the nitrogen content in the raw material, ≤350ppm; t1 is the melting time of the slow melting treatment, h; p1 is the vacuum pressure of the slow melting treatment, Pa; t2 is the refining time of the rapid refining treatment, h; p2 is the vacuum pressure of the rapid refining treatment, Pa.

6. The method for preparing austenitic stainless steel according to claim 4, characterized in that: In step S101, during the slow melting process, the vacuum pressure is controlled to be 10-20 Pa; in step S102, during the rapid refining process, the refining temperature is 1530-1550° C., and the vacuum pressure is ≤2 Pa.

7. The method for preparing austenitic stainless steel according to claim 4, characterized in that: In step S102, one hour before the end of refining, 0.005% to 0.008% aluminum by weight of the molten steel is added to the molten steel.

8. The method for preparing austenitic stainless steel according to claim 4, characterized in that: In step S103, the total pressure of the introduced argon and nitrogen is 4 kPa to 10 kPa, and the volume ratio of argon to nitrogen is (6 to 8):(4 to 2); the magnesium alloy is a mixture of Ni2Mg alloy and SiMg2 alloy, and the mass ratio of the two is (5 to 53):(3 to 48).

9. A method for preparing austenitic stainless steel according to any one of claims 4 to 8, characterized in that: In step S2, the first electroslag remelting uses the first remelting slag system, the composition of which is 50-60 parts of CaF2, 20-30 parts of CaO, and 15-20 parts of Al2O3; during the first electroslag remelting process, silicon-calcium alloy is added, wherein the weight ratio of the addition amount of silicon-calcium alloy to the molten steel is (1.0-2.0kg):1t; In step S3, the second electroslag remelting uses the second remelting slag system, the composition of which is 40-50 parts of CaF2, 15-20 parts of MgF2, 5-10 parts of Na3AlF6, 10-15 parts of MgO, 10-15 parts of CaO, 5-10 parts of Al2O3, and 5-10 parts of SiO2.

10. The method for preparing austenitic stainless steel according to claim 9, characterized in that: In step S101, the melting time is controlled to be 6 to 8 hours.

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