Ultrahigh-nitrogen austenitic stainless steel and preparation method and application thereof

Abstract

To provide an ultrahigh-nitrogen austenitic stainless steel and a preparation method and application thereof.SOLUTION: A method of preparing an ultrahigh-nitrogen austenitic stainless steel comprises the steps of: subjecting martensitic precipitation-hardening stainless steel powder to gas nitriding to obtain nitrided powder; and sequentially subjecting the nitrided powder to pre-compression molding and spark plasma sintering to obtain the ultrahigh-nitrogen austenitic stainless steel. The martensitic precipitation-hardening stainless steel powder is treated by adopting a gas nitriding method to obtain stainless steel powder with ultrahigh nitrogen content, and then the stainless steel powder with ultrahigh nitrogen content is sintered and molded by adopting a rapid sintering method of spark plasma sintering. This approach not only allows a large amount of nitrogen to be solid-solved into a stainless steel matrix, resulting in an austenite structure with a large amount of nitrogen solid solution, but also avoids the precipitation of large amounts of Cr- and N-containing phases.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to the technical field of high-nitrogen stainless steel, and in particular to an ultra-high-nitrogen austenitic stainless steel and its manufacturing method and application. [Background technology]

[0002] Adding nitrogen to austenitic stainless steel can replace nickel to improve the stability of austenite, reduce the amount of nickel used, and reduce the manufacturing cost of austenitic stainless steel. At the same time, dissolving nitrogen in austenite can significantly improve hardness, strength, and corrosion resistance. It is widely used in fields such as aerospace, marine equipment, medical equipment, and chemical engineering. Among nitrogen-containing stainless steels, stainless steels with a nitrogen content of 0.4 wt.% or more are generally called high-nitrogen stainless steels, and stainless steels with a nitrogen content of 1.0 wt.% or more are called ultra-high-nitrogen stainless steels. To improve the solubility of nitrogen in stainless steel, higher amounts of alloying elements such as Cr, Mn, and Mo are usually added to the stainless steel, and then the stainless steel is refined using metallurgical techniques, resulting in high-nitrogen stainless steels with a nitrogen content of up to 0.9 wt.%. However, the addition of many alloying elements such as Cr, Mn, and Mo and the complex manufacturing techniques significantly increase the manufacturing costs of high-nitrogen stainless steels.

[0003] Compared with traditional smelting metallurgy, powder metallurgy can more easily produce stainless steel with ultra-high nitrogen content, even stainless steel materials with a nitrogen content of over 3.0 wt.%, but powder metallurgy is prone to nitride precipitation, especially Cr-containing nitrides, and the precipitation of a large amount of Cr-containing nitrides makes the stainless steel matrix Cr-poor, resulting in poor corrosion resistance of the stainless steel matrix.

[0004] Therefore, how to produce an austenitic stainless steel with an ultra-high nitrogen content under conditions that avoid the precipitation of a large amount of nitrides is a technical problem that needs to be solved promptly in the field. Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention aims to provide an ultra-high nitrogen austenitic stainless steel, a manufacturing method thereof, and applications thereof. The ultra-high nitrogen austenitic stainless steel obtained by the manufacturing method provided by the present invention has a high nitrogen content and only a small amount of nitride precipitation, and the ultra-high nitrogen austenitic stainless steel has excellent mechanical properties. [Means for solving the problem]

[0006] To achieve the above objectives, the present invention provides the following technical solutions: The present invention provides a method for producing an ultra-high nitrogen austenitic stainless steel, Step (1) of gas nitriding a martensitic precipitation hardening stainless steel powder to obtain a nitrided powder; and step (2) of sequentially subjecting the nitrided powder obtained in step (1) to pre-pressing and spark plasma sintering to obtain an ultra-high nitrogen austenitic stainless steel.

[0007] Preferably, the components of the martensitic precipitation hardening stainless steel powder in step (1) include Cr: 14.0-16.0 wt.%, Ni: 3-5 wt.%, Cu: 3-5 wt.%, Mn: 1.0-1.5 wt.%, Si: 0.8-1.0 wt.%, Nb: 0.3-0.5 wt.%, C: 0.03-0.05 wt.%, P: ≦0.015 wt.%, S: ≦0.015 wt.%, and a balance of Fe.

[0008] Preferably, the martensitic precipitation hardening stainless steel powder in step (1) is a spherical powder having an average particle size of 35 to 50 μm.

[0009] Preferably, the gas nitriding method in step (1) is to spread the martensitic precipitation hardening stainless steel powder evenly on a stainless steel wire mesh, then place the stainless steel wire mesh on which the martensitic precipitation hardening stainless steel powder has been spread in layers on a tool, and finally place the tool on which the stainless steel wire mesh has been placed in a pit-type gas nitriding furnace to perform gas nitriding.

[0010] Preferably, the gas used in the gas nitriding in step (1) is ammonia, the temperature for gas nitriding is 400 to 600° C., and the time for gas nitriding is 5 to 60 minutes.

[0011] Preferably, the pre-pressure in step (2) is 15 to 30 MPa.

[0012] Preferably, the temperature for spark plasma sintering in step (2) is 1050 to 1150° C., the temperature for spark plasma sintering is 10 to 30 minutes, and the pressure for spark plasma sintering is 40 to 60 MPa.

[0013] Preferably, the cooling method of the spark plasma sintering in step (2) is to cool to 750-900°C under the protection of flowing argon gas, and then cool to room temperature in a vacuum environment. [Effects of the Invention]

[0014] The present invention provides an ultra-high nitrogen austenitic stainless steel manufactured by the manufacturing method described in the above technical solution.

[0015] The present invention provides applications of the ultra-high nitrogen austenitic stainless steel described in the above technical solution in the fields of aerospace, marine equipment, medical equipment and chemical engineering.

[0016] The present invention provides a method for producing ultra-high nitrogen austenitic stainless steel, comprising step (1) gas nitriding martensitic precipitation-hardening stainless steel powder to obtain nitrided powder, and step (2) sequentially pre-pressing and spark plasma sintering the nitrided powder obtained in step (1) to obtain ultra-high nitrogen austenitic stainless steel. The present invention uses martensitic precipitation-hardening stainless steel powder as raw material, employs gas nitriding to process the raw material powder to obtain ultra-high nitrogen stainless steel powder, and then employs a rapid sintering method known as spark plasma sintering to sinter the ultra-high nitrogen stainless steel powder. This not only dissolves a large amount of nitrogen into the stainless steel matrix, resulting in a high-nitrogen dissolved austenitic structure, but also avoids the large precipitation of Cr- and N-containing element phases, enabling the production of ultra-high nitrogen dissolved austenitic stainless steel, and providing important technical support for the development and application of ultra-high nitrogen stainless steel. The results of the examples show that the ultra-high nitrogen austenitic stainless steel produced by the production method provided by the present invention has a nitrogen content of 1.5 to 2.0 wt.%, the matrix structure of the ultra-high nitrogen austenitic stainless steel is austenite plus a small amount of nano-nitrided niobium precipitate phase, and a large amount of nitrogen is dissolved in the austenite matrix, and the ultra-high nitrogen austenitic stainless steel has a microhardness of 300 to 350 HV0.1 and a pressure yield strength of 800 to 1000 MPa, and therefore has excellent mechanical properties. [Brief explanation of the drawings]

[0017] [Figure 1] 1 is a flow chart for producing an ultra-high nitrogen austenitic stainless steel according to the present invention. [Figure 2] FIG. 1 is a micromorphology diagram of the martensitic precipitation hardening stainless steel powder used in Example 1 of the present invention. [Figure 3] FIG. 1 is a micromorphology diagram of the nitride powder obtained in Example 1 of the present invention. [Figure 4]1 is an XRD diffraction spectrogram of the martensitic precipitation hardened stainless steel powder used in Example 1 of the present invention and the resulting nitrided powder. [Figure 5] FIG. 1 is a micromorphology diagram of the ultra-high nitrogen austenitic stainless steel obtained in Example 1 of the present invention. [Figure 6] 1 is an XRD diffraction spectrogram of the ultra-high nitrogen austenitic stainless steel obtained in Example 1 of the present invention. [Figure 7] FIG. 2 is a compressive stress-strain curve of the ultra-high nitrogen austenitic stainless steel obtained in Example 1 of the present invention. [Figure 8] FIG. 1 is a bright-field TEM image of the ultra-high nitrogen austenitic stainless steel obtained in Example 1 of the present invention. [Figure 9] FIG. 1 is a dark-field TEM image of the ultra-high nitrogen austenitic stainless steel obtained in Example 1 of the present invention. [Figure 10] FIG. 2 is a scanning distribution diagram of the N element plane of the ultra-high nitrogen austenitic stainless steel obtained in Example 1 of the present invention. [Figure 11] FIG. 2 is a scanning distribution diagram of the Nb element plane of the ultra-high nitrogen austenitic stainless steel obtained in Example 1 of the present invention. [Figure 12] FIG. 2 is a TEM diffraction dot diagram of the ultra-high nitrogen austenitic stainless steel obtained in Example 2 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention provides a method for producing an ultra-high nitrogen austenitic stainless steel, Step (1) of gas nitriding a martensitic precipitation hardening stainless steel powder to obtain a nitrided powder; and step (2) of sequentially subjecting the nitrided powder obtained in step (1) to pre-pressing and spark plasma sintering to obtain an ultra-high nitrogen austenitic stainless steel.

[0019] In the present invention, a nitrided powder is obtained by gas nitriding a martensitic precipitation hardened stainless steel powder.

[0020] In the present invention, the components of the martensitic precipitation hardening stainless steel powder preferably include Cr: 14.0-16.0 wt.%, Ni: 3-5 wt.%, Cu: 3-5 wt.%, Mn: 1.0-1.5 wt.%, Si: 0.8-1.0 wt.%, Nb: 0.3-0.5 wt.%, C: 0.03-0.05 wt.%, P: ≦0.015 wt.%, S: ≦0.015 wt.%, and a balance of Fe. The present invention is not particularly limited to a specific source of the martensitic precipitation hardening stainless steel powder, and it is possible to use commercially available products well known to those skilled in the art, or to produce it yourself. The present invention uses the above martensitic precipitation hardening stainless steel powder, in which the added Ni and Cu elements can expand the austenite phase region and improve the stability of austenite, and at the same time, are less likely to form compounds with N element, promoting the solid solution of nitrogen in austenite. The added Nb element is more likely to form compounds with N element, helping to reduce the formation of precipitates between Cr and N, preventing the matrix from becoming poor in Cr and ensuring the corrosion resistance of the matrix. This improves the nitrogen content of the stainless steel after nitriding treatment, laying the foundation for subsequently obtaining ultra-high nitrogen austenitic stainless steel.

[0021] In the present invention, the martensitic precipitation hardening stainless steel powder is preferably a spherical powder with an average particle size of 35 to 50 μm, more preferably a spherical powder with an average particle size of 40 to 45 μm. The use of small particle spherical powder in the present invention can increase the specific surface area of ​​the stainless steel powder, thereby improving contact with the nitriding gas and thereby further increasing the nitrogen content of the stainless steel after nitriding treatment.

[0022] In the present invention, the gas nitriding method is preferably to spread martensitic precipitation hardening stainless steel powder evenly on a stainless steel wire mesh, then place the stainless steel wire mesh on which the martensitic precipitation hardening stainless steel powder has been spread in layers on a tool, and finally place the tool on which the stainless steel wire mesh has been placed in a pit-type gas nitriding furnace to perform gas nitriding.

[0023] In the present invention, the stainless steel wire mesh is preferably a circular stainless steel wire mesh with a diameter of 400 mm, and the mesh pore size of the stainless steel wire mesh is preferably less than 35 μm, more preferably 30 μm. By controlling the mesh pore size of the stainless steel wire mesh, the present invention can prevent the mesh pore size from exceeding the particle size of the stainless steel powder.

[0024] In the present invention, the thickness of the martensitic precipitation hardening stainless steel powder laid flat on the stainless steel wire mesh is preferably 1 to 2 mm. By controlling the thickness of the laid stainless steel powder, the present invention can ensure that all of the stainless steel powder can be in sufficient contact with the nitriding gas, thereby avoiding the problem of some of the stainless steel powder not being nitrided due to the laid thickness being too thick.

[0025] In the present invention, the spacing between the layers of the stainless steel wire mesh in the tool is preferably 15 to 30 mm, more preferably 20 mm.

[0026] In the present invention, the pit-type gas nitriding furnace is preferably a pit-type gas nitriding furnace with an inner cavity diameter of 500 mm and a depth of 1000 mm. The present invention does not particularly limit the specific supply source of the pit-type gas nitriding furnace, and commercially available products well known to those skilled in the art may be used.

[0027] In the present invention, the gas used in the gas nitriding is preferably ammonia, more preferably dried high-purity ammonia. The present invention is not particularly limited to the drying method, and any drying method known to those skilled in the art may be used. By using high-purity ammonia in the gas nitriding, the present invention can reduce the content of impurity gases such as oxygen and avoid problems such as oxidation of the stainless steel powder during the nitriding process.

[0028] In the present invention, the temperature for gas nitriding is preferably 450 to 550°C, more preferably 500°C, and the temperature for gas nitriding is preferably 5 to 30 minutes, more preferably 10 to 25 minutes, and even more preferably 15 to 20 minutes. In the present invention, there are no particular limitations on the heating rate and cooling rate for gas nitriding, and these may be determined according to the common technical knowledge of those skilled in the art. In the present invention, the nitrogen content of the stainless steel powder can be further increased by controlling the parameters of gas nitriding.

[0029] In the present invention, the gas nitriding is preferably carried out in a flowing ammonia atmosphere. In the present invention, the gas nitriding is carried out in a flowing ammonia atmosphere. During the temperature rise, the ammonia flow rate required per 1000 g of martensitic precipitation hardening stainless steel powder is preferably 1.5 to 2 m 3 / h, and the gas nitriding requires a flow rate of ammonia of 3 to 5 m per 1000 g of martensitic precipitation hardening stainless steel powder during the heat retention period. 3 / h, more preferably 4m 3 / h, and the gas nitriding requires an ammonia flow rate of 1.5 to 2 m per 1000 g of martensitic precipitation hardening stainless steel powder during cooling. 3 / h. The present invention can prevent oxidation of the powder by performing gas nitriding in a flowing ammonia atmosphere.

[0030] In the present invention, the gas nitriding is preferably carried out under conditions of agitation by a blower. In the present invention, the rotation speed of the agitation by the blower is preferably 250 to 350 r / min, more preferably 300 r / min. In the present invention, the powder can be uniformly and sufficiently nitrided by agitation by a blower.

[0031] In the present invention, the nitrogen content in the nitride powder is preferably 1.6 to 2.2 wt. %.

[0032] After obtaining the nitrided powder, the present invention sequentially performs pre-pressing and spark plasma sintering on the nitrided powder to obtain an ultra-high nitrogen austenitic stainless steel.

[0033] In the present invention, the pre-pressing operation preferably involves placing the nitride powder in a cylindrical graphite mold, separating the nitride powder from the inner wall of the cylindrical graphite mold with graphite paper, and then pre-pressing with a graphite indenter, separating the graphite indenter from the powder with graphite paper. In the present invention, the pre-pressing pressure is preferably 15 to 30 MPa, more preferably 20 MPa. The pre-pressing time is not particularly limited in the present invention and can be determined according to the technical common sense of those skilled in the art. In the present invention, the pre-pressing method described above does not require demolding after molding, and spark plasma sintering can be performed directly, reducing the process. Separating the stainless steel powder from the inner wall of the cylindrical graphite mold with graphite paper avoids adhesion to the mold during sintering.

[0034] In the present invention, the temperature for the spark plasma sintering is preferably 1050 to 1150°C, more preferably 1100°C. The temperature-holding time for the spark plasma sintering is preferably 10 to 30 minutes, more preferably 15 to 25 minutes, and even more preferably 20 minutes. The pressure for the spark plasma sintering is preferably 40 to 60 MPa, more preferably 50 MPa. In the present invention, the heating rate for the spark plasma sintering is preferably 100 to 300°C / min, more preferably 150 to 250°C / min, and even more preferably 200°C / min. In the present invention, the pressure for the spark plasma sintering is preferably axial pressure applied by a graphite indenter. The present invention employs a high heating rate to achieve sintering under the assistance of an electric field, accelerating the diffusion of N element in the stainless steel matrix and forming a uniform and stable solid solution structure. The large amount of nitrogen dissolved in the stainless steel matrix significantly improves the stability of supercooled austenite, accelerating the transformation of martensitic precipitation-hardened stainless steel into austenitic stainless steel.

[0035] In the present invention, the cooling method of the spark plasma sintering is preferably to cool to 750-900°C under the protection of flowing argon gas, and then to room temperature in a vacuum environment. In the present invention, the flow rate of the flowing argon gas is preferably 10 mL / min. By adopting this cooling method, the present invention can prevent the stainless steel from being oxidized in air.

[0036] The flow chart for producing ultra-high nitrogen austenitic stainless steel according to the present invention is shown in Figure 1. As can be seen from Figure 1, the raw material powder (i.e., the martensitic precipitation hardening stainless steel powder of the present invention) is spread out and then placed in a pit-type gas nitriding furnace to undergo gas nitriding to obtain nitrided powder, which is then placed in a graphite mold and pre-pressurized, followed by spark plasma sintering to obtain ultra-high nitrogen austenitic stainless steel.

[0037] The present invention uses martensitic precipitation-hardening stainless steel powder as raw material, employs a gas nitriding method to process the raw material powder to obtain stainless steel powder with an ultra-high nitrogen content, and then employs a rapid sintering method known as spark plasma sintering to sinter and form the ultra-high nitrogen content stainless steel powder. In this way, a large amount of nitrogen is dissolved into the stainless steel matrix, not only obtaining an austenitic structure with a large amount of dissolved nitrogen, but also avoiding the large amount of precipitation of Cr- and N-containing element phases, thereby realizing the production of austenitic stainless steel with an ultra-high amount of dissolved nitrogen, and providing important technical support for the development and application of ultra-high nitrogen stainless steel.

[0038] The present invention also provides an ultra-high nitrogen austenitic stainless steel manufactured by the manufacturing method described in the above technical solution. In the present invention, the nitrogen content in the ultra-high nitrogen austenitic stainless steel is preferably 1.5-2.0 wt.%, and the matrix structure of the ultra-high nitrogen austenitic stainless steel is preferably austenite plus a small amount of nano-nitrided niobium precipitate phase, with a large amount of nitrogen dissolved in the austenitic matrix.

[0039] The present invention also provides applications of the ultra-high nitrogen austenitic stainless steel described in the above technical solution in the fields of aerospace, marine equipment, medical equipment and chemical engineering. The present invention does not have any particular limitations on the specific application manner, and application manners well known to those skilled in the art can be used.

[0040] The technical solutions of the present invention will be described clearly and completely below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without any creative efforts fall within the scope of protection of the present invention.

[0041] Example 1 The method for producing ultra-high nitrogen austenitic stainless steel comprises the following steps: (1) Martensitic precipitation hardening stainless steel powder is spread evenly on a stainless steel wire mesh, then the stainless steel wire mesh on which the martensitic precipitation hardening stainless steel powder is spread is placed in layers on a tool, and finally the tool on which the stainless steel wire mesh is placed is placed in a pit-type gas nitriding furnace with an inner cavity diameter of 500 mm and a depth of 1000 mm to carry out gas nitriding to obtain nitrided powder, the components of the martensitic precipitation hardening stainless steel powder are Cr: 16.0 wt.%, Ni: 3 wt.%, Cu: 5 wt.%, Mn: 1.5 wt.%, Si: 0.8 wt.%, Nb: 0.3 wt.%, C: 0.05 wt.%, P: ≦ 0.015 wt.%, S: ≦ 0.015 wt.% and the remaining amount of Fe, and the martensitic precipitation hardening stainless steel powder is The martensitic precipitation hardening stainless steel powder was spherical with an average particle size of 50 μm, the stainless steel wire mesh was a circular stainless steel wire mesh with a diameter of 400 mm, the mesh pore size of the stainless steel wire mesh was 30 μm, the martensitic precipitation hardening stainless steel powder was laid flat on the stainless steel wire mesh to a thickness of 2 mm, and the spacing between layers of the stainless steel wire mesh on the tool was 20 mm, the gas used in the gas nitriding was dried high-purity ammonia, the gas nitriding was maintained at a temperature of 550°C, the gas nitriding was maintained for 5 minutes, the gas nitriding was carried out in a flowing ammonia atmosphere, and the ammonia flow rate required per 1000 g of martensitic precipitation hardening stainless steel powder during the temperature rise was 2 m 3 / h, and the gas nitriding requires an ammonia flow rate of 3 m per 1000 g of martensitic precipitation hardening stainless steel powder during the heat retention period. 3 / h, and the gas nitriding requires an ammonia flow rate of 2 m per 1000 g of martensitic precipitation hardening stainless steel powder during cooling. 3 / h, the gas nitriding is carried out under the condition of agitation by a blower, and the rotation speed of the agitation by the blower is 300 r / min, (2) The nitride powder obtained in step (1) is placed into a cylindrical graphite mold, and the nitride powder is separated from the inner wall of the cylindrical graphite mold with graphite paper. Then, a graphite indenter is used to pre-press the nitride powder. The graphite indenter and the powder are separated with graphite paper. Then, spark plasma sintering is performed to obtain an ultra-high nitrogen austenitic stainless steel. The pre-pressing pressure is 20 MPa, the spark plasma sintering temperature is 1150°C, the spark plasma sintering time is 10 minutes, the spark plasma sintering pressure is 50 MPa, the spark plasma sintering pressure is the axial pressure applied by the graphite indenter, and the spark plasma sintering heating rate is 100°C / min. The spark plasma sintering cooling method is to cool to 800°C under the protection of flowing argon gas at a flow rate of 10 mL / min, and then the axial pressure is removed and the spark plasma sintering is cooled to 40°C in a vacuum environment.

[0042] The nitrogen content of the ultra-high nitrogen austenitic stainless steel produced in Example 1 was tested using an oxygen / nitrogen analyzer, and the test standard was ASTM E1019. The nitrogen content of the final ultra-high nitrogen austenitic stainless steel was 1.8 wt.%.

[0043] Example 2 The method for producing ultra-high nitrogen austenitic stainless steel comprises the following steps: (1) Martensitic precipitation hardening stainless steel powder is spread evenly on a stainless steel wire mesh, then the stainless steel wire mesh on which the martensitic precipitation hardening stainless steel powder is spread is placed in layers on a tool, and finally the tool on which the stainless steel wire mesh is placed is placed in a pit-type gas nitriding furnace with an inner cavity diameter of 500 mm and a depth of 1000 mm to carry out gas nitriding to obtain nitrided powder, the components of the martensitic precipitation hardening stainless steel powder are Cr: 14.0 wt.%, Ni: 5 wt.%, Cu: 3 wt.%, Mn: 1.0 wt.%, Si: 1.0 wt.%, Nb: 0.5 wt.%, C: 0.03 wt.%, P: ≦ 0.015 wt.%, S: ≦ 0.015 wt.% and the remaining amount of Fe, and the martensitic precipitation hardening stainless steel powder is The martensitic precipitation hardening stainless steel powder was spherical with an average particle size of 35 μm, the stainless steel wire mesh was a circular stainless steel wire mesh with a diameter of 400 mm, the mesh pore size of the stainless steel wire mesh was 30 μm, the martensitic precipitation hardening stainless steel powder was laid flat on the stainless steel wire mesh to a thickness of 1 mm, and the spacing between layers of the stainless steel wire mesh on the tool was 20 mm, the gas used in the gas nitriding was dried high-purity ammonia, the gas nitriding was maintained at a temperature of 450°C, the gas nitriding was maintained for 30 minutes, the gas nitriding was carried out in a flowing ammonia atmosphere, and the ammonia flow rate required per 1000 g of martensitic precipitation hardening stainless steel powder during the temperature rise of the gas nitriding was 2 m 3 / h, and the required ammonia flow rate per 1000g of martensitic precipitation hardening stainless steel powder during the gas nitriding is 5m 3 / h, and during the cooling of the gas nitriding, the required ammonia flow rate per 1000 g of martensitic precipitation hardening stainless steel powder is 2 m 3 / h, the gas nitriding is carried out under the condition of agitation by a blower, and the rotation speed of the agitation by the blower is 300 r / min, (2) The nitride powder obtained in step (1) is placed into a cylindrical graphite mold, and the nitride powder is separated from the inner wall of the cylindrical graphite mold with graphite paper. Then, a graphite indenter is used to pre-press the nitride powder. The graphite indenter and the powder are separated with graphite paper. Then, spark plasma sintering is performed to obtain an ultra-high nitrogen austenitic stainless steel. The pre-pressing pressure is 20 MPa, the spark plasma sintering temperature is 1050°C, the spark plasma sintering time is 30 minutes, the spark plasma sintering pressure is 50 MPa, the spark plasma sintering pressure is an axial pressure applied by the graphite indenter, and the spark plasma sintering heating rate is 300°C / min. The spark plasma sintering cooling method is to cool to 800°C under the protection of argon gas flowing at a flow rate of 10 mL / min, and then the axial pressure is removed and the spark plasma sintering is cooled to 40°C in a vacuum environment.

[0044] The nitrogen content of the ultra-high nitrogen austenitic stainless steel produced in Example 2 was tested using an oxygen / nitrogen analyzer, and the test standard was ASTM E1019. The nitrogen content of the final ultra-high nitrogen austenitic stainless steel was 1.5 wt.%.

[0045] Example 3 The method for producing ultra-high nitrogen austenitic stainless steel comprises the following steps: (1) Martensitic precipitation hardening stainless steel powder is spread evenly on a stainless steel wire mesh, then the stainless steel wire mesh on which the martensitic precipitation hardening stainless steel powder is spread is placed in layers on a tool, and finally the tool on which the stainless steel wire mesh is placed is placed in a pit-type gas nitriding furnace with an inner cavity diameter of 500 mm and a depth of 1000 mm to perform gas nitriding to obtain nitrided powder, the components of the martensitic precipitation hardening stainless steel powder being Cr: 15.0 wt.%, Ni: 4 wt.%, Cu: 4 wt.%, Mn: 1.3 wt.%, Si: 0.9 wt.%, Nb: 0.4 wt.%, C: 0.04 wt.%, P: ≦ 0.015 wt.%, S: ≦ 0.015 wt.% and the remaining amount of Fe, and the martensitic precipitation hardening stainless steel powder is spread evenly on a stainless steel wire mesh. The stainless steel wire mesh was a circular stainless steel wire mesh with a diameter of 400 mm, and the mesh pore size of the stainless steel wire mesh was 30 μm. The martensitic precipitation hardening stainless steel powder was laid flat on the stainless steel wire mesh to a thickness of 1.5 mm. The spacing between layers of the stainless steel wire mesh on the tool was 20 mm. The gas used in the gas nitriding was dried high-purity ammonia. The gas nitriding temperature was 500°C, the gas nitriding time was 20 minutes, and the gas nitriding was carried out in a flowing ammonia atmosphere. During the temperature rise in the gas nitriding, the ammonia flow rate required per 1000 g of martensitic precipitation hardening stainless steel powder was 2 m 3 / h, and the required ammonia flow rate per 1000g of martensitic precipitation hardening stainless steel powder during the gas nitriding is 4m 3 / h, and during the cooling of the gas nitriding, the required ammonia flow rate per 1000 g of martensitic precipitation hardening stainless steel powder is 2 m 3 / h, the gas nitriding is carried out under the condition of agitation by a blower, and the rotation speed of the agitation by the blower is 300 r / min, (2) The nitride powder obtained in step (1) is placed into a cylindrical graphite mold, and the nitride powder is separated from the inner wall of the cylindrical graphite mold with graphite paper. Then, a graphite indenter is used to pre-press the nitride powder. The graphite indenter and the powder are separated with graphite paper. Then, spark plasma sintering is performed to obtain an ultra-high nitrogen austenitic stainless steel. The pre-pressing pressure is 20 MPa, the spark plasma sintering temperature is 1100°C, the spark plasma sintering time is 20 minutes, the spark plasma sintering pressure is 50 MPa, the spark plasma sintering pressure is an axial pressure applied by the graphite indenter, and the spark plasma sintering heating rate is 200°C / min. The spark plasma sintering cooling method is to cool to 800°C under the protection of argon gas flowing at a flow rate of 10 mL / min, and then the axial pressure is removed and the spark plasma sintering is cooled to 40°C in a vacuum environment.

[0046] The nitrogen content of the ultra-high nitrogen austenitic stainless steel produced in Example 3 was tested using an oxygen / nitrogen analyzer, and the test standard was ASTM E1019. The nitrogen content of the final ultra-high nitrogen austenitic stainless steel was 2.0 wt.%.

[0047] The performance of the ultra-high nitrogen austenitic stainless steels produced in Examples 1 to 3 was tested. The test method was to sequentially process and polish the cross-sections of the ultra-high nitrogen austenitic stainless steels produced in Examples 1 to 3 to obtain cylindrical samples 1 to 3 with a diameter of 7.5 mm and a height of 15 mm. The microhardness of Samples 1 to 3 was tested using a general-purpose microhardness tester at room temperature, and the compression performance of Samples 1 to 3 was tested using a general-purpose microcomputer-controlled electrohydraulic servo universal testing machine at room temperature. The results are shown in Table 1.

[0048] Table 1. Performance of ultra-high nitrogen austenitic stainless steels produced in Examples 1 to 3 JPEG2025137323000002.jpg47162

[0049] As can be seen from Table 1, the microhardness of the ultra-high nitrogen austenitic stainless steel obtained by the manufacturing method provided by the present invention is 300 to 350 HV 0.1 The pressure-resistant yield strength is 800 to 1000 MPa, and it has excellent mechanical properties.

[0050] Figure 2 is a micro-morphology diagram of the martensitic precipitation hardened stainless steel powder used in Example 1 of the present invention. Figure 3 is a micro-morphology diagram of the nitrided powder obtained in Example 1 of the present invention. As can be seen from a comparison of Figures 2 and 3, the powder after nitriding still retains a better spherical shape, but a small amount of cracks are generated on the surface due to stress caused by nitriding.

[0051] Figure 4 shows XRD diffraction spectrograms of the martensitic precipitation-hardened stainless steel powder used in Example 1 of the present invention and the resulting nitrided powder. As can be seen from Figure 4, the powders before and after nitriding are all mainly composed of the α phase.

[0052] Figure 5 is a micro-morphology diagram of the ultra-high nitrogen austenitic stainless steel obtained in Example 1 of the present invention. As can be seen from Figure 5, the ultra-high nitrogen austenitic stainless steel obtained in the present invention has a uniform and consistent microstructure, and at the same time, a small amount of small white granular precipitates can be seen.

[0053] Figure 6 is an XRD diffraction spectrogram of the ultra-high nitrogen austenitic stainless steel obtained in Example 1 of the present invention. As can be seen from Figure 6, all of the ultra-high nitrogen austenitic stainless steel obtained in the present invention is composed of γ phase, and as can be seen from this, the matrix of all of the ultra-high nitrogen austenitic stainless steel is transformed into an austenite structure, and as can be seen with reference to Figure 5, the matrix structure is austenite plus a small amount of small white granular precipitate phase.

[0054] 7 is a compressive stress-strain curve of the ultra-high nitrogen austenitic stainless steel obtained in Example 1 of the present invention. As can be seen from FIG. 7, the ultra-high nitrogen austenitic stainless steel has higher compressive yield strength and better plasticity.

[0055] Figure 8 is a bright-field TEM image of the ultra-high nitrogen austenitic stainless steel obtained in Example 1 of the present invention. As can be seen from Figure 8, the precipitates in the austenite matrix of the ultra-high nitrogen austenitic stainless steel are rod-shaped, with a grain size of <100 nanometers, and are accompanied by a small number of dislocation lines.

[0056] Figure 9 is a dark-field TEM image of the ultra-high nitrogen austenitic stainless steel obtained in Example 1 of the present invention. As can be seen from Figure 9, the precipitates in the austenite matrix of the ultra-high nitrogen austenitic stainless steel are rod-shaped, with a grain size of <100 nanometers, and are accompanied by a small number of dislocation lines.

[0057] Fig. 10 is a scanning distribution diagram of the N element plane of the ultra-high nitrogen austenitic stainless steel obtained in Example 1 of the present invention. As can be seen from Fig. 10, the distribution of nitrogen element in the matrix is ​​uniform, and the precipitate phase is enriched in nitrogen element.

[0058] Fig. 11 is a scanning distribution diagram of the Nb element plane of the ultra-high nitrogen austenitic stainless steel obtained in Example 1 of the present invention. As can be seen from Fig. 11, the distribution of the matrix Nb element is uniform, and the precipitate phase is enriched in Nb element. From the results in Fig. 10, it can be determined that the precipitate phase is an Nb-N compound phase.

[0059] Figure 12 is a TEM diffraction spot diagram of the ultra-high nitrogen austenitic stainless steel obtained in Example 2 of the present invention. As can be seen from Figure 12, by locating the diffraction spots, it is again confirmed that the matrix structure of the ultra-high nitrogen austenitic stainless steel is a γ phase, i.e., an austenitic structure, and it can be determined that the manufacturing method provided by the present invention can produce and obtain ultra-high nitrogen austenitic stainless steel material.

[0060] It should be understood that the above are only preferred embodiments of the present invention, and that those skilled in the art can make further improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also fall within the protection scope of the present invention.

Claims

1. A method for producing ultra-high nitrogen austenitic stainless steel, comprising: Step (1) of gas nitriding a martensitic precipitation hardened stainless steel powder to obtain a nitrided powder; and step (2) of sequentially subjecting the nitrided powder obtained in step (1) to pre-pressing and spark plasma sintering to obtain an ultra-high nitrogen austenitic stainless steel.

2. 2. The method of claim 1, wherein the martensitic precipitation hardening stainless steel powder in step (1) has a composition comprising Cr: 14.0-16.0 wt.%, Ni: 3-5 wt.%, Cu: 3-5 wt.%, Mn: 1.0-1.5 wt.%, Si: 0.8-1.0 wt.%, Nb: 0.3-0.5 wt.%, C: 0.03-0.05 wt.%, P: ≦0.015 wt.%, S: ≦0.015 wt.%, and a balance of Fe.

3. 3. The method according to claim 1, wherein the martensitic precipitation hardening stainless steel powder in step (1) is a spherical powder having an average particle size of 35 to 50 μm.

4. 2. The manufacturing method according to claim 1, wherein the gas nitriding method in step (1) is to spread the martensitic precipitation hardening stainless steel powder evenly on a stainless steel wire mesh, then place the stainless steel wire mesh on which the martensitic precipitation hardening stainless steel powder has been spread in layers on the tool, and finally place the tool on which the stainless steel wire mesh has been spread in a pit-type gas nitriding furnace to perform gas nitriding.

5. The manufacturing method according to claim 1, characterized in that the gas used for gas nitriding in step (1) is ammonia, the temperature for gas nitriding is 400 to 600°C, and the time for gas nitriding is 5 to 60 minutes.

6. 2. The method according to claim 1, wherein the pre-pressing pressure in step (2) is 15 to 30 MPa.

7. The method according to claim 1, wherein the spark plasma sintering temperature in step (2) is 1050 to 1150 ° C., the spark plasma sintering temperature retention time is 10 to 30 min, and the spark plasma sintering pressure is 40 to 60 MPa.

8. The manufacturing method according to claim 1 or 7, characterized in that the cooling method of the spark plasma sintering in step (2) is to cool to 750-900 ° C under the protection of flowing argon gas, and then cool to room temperature in a vacuum environment.

9. 2. An ultra-high nitrogen austenitic stainless steel produced by the method of claim 1.

10. 10. Applications of the ultra-high nitrogen austenitic stainless steel according to claim 9 in the fields of aerospace, marine equipment, medical equipment and chemical engineering.

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