Method for preparing powder metallurgy high-toughness high-nitrogen stainless steel

Abstract

The invention provides a method for preparing powder metallurgy high-toughness high-nitrogen stainless steel, and relates to the technical field of high-nitrogen stainless steel preparation. According to the method, stainless steel alloy powder serves as a raw material, a green body is obtained through cold isostatic pressing forming, then high-nitrogen stainless steel is obtained through nitriding treatment and nitrogen sintering, and finally the high-performance high-nitrogen stainless steel is obtained through hot working deformation. The problems that high-nitrogen stainless steel prepared through a traditional smelting method is low in nitrogen content, uneven in nitrogen element distribution, prone to precipitation of coarse nitride, low in mechanical property and the like are solved. The obtained high-nitrogen stainless steel is high in nitrogen content, fine and uniform nitride strengthening phases are formed, the structure distribution is uniform, the mechanical property is excellent, the corrosion resistance is good, the preparation method is simple and convenient, the preparation difficulty of the high-nitrogen stainless steel is reduced, the material utilization rate is high, the application range is wide, and the method is suitable for large-scale industrial production of various types of high-nitrogen stainless steel.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high-nitrogen stainless steel preparation, and particularly relates to a method for preparing powder metallurgy high-strength and high-toughness high-nitrogen stainless steel. BACKGROUND

[0002] Stainless steel is composed of iron, chromium, nickel and other alloying elements, and has excellent corrosion resistance, heat resistance, excellent mechanical properties and good cold working formability, and is widely used in aerospace, ocean engineering, manufacturing, construction, medical devices and other fields. The addition of nitrogen element in stainless steel can obtain high-nitrogen stainless steel, which has more excellent strength, toughness and corrosion resistance compared with traditional stainless steel, and the use of nitrogen element to replace the expensive nickel element in stainless steel can improve the problem of human body nickel element allergy.

[0003] Nitrogen element is a strong austenite stabilizing element, which can expand the austenite phase region, and its ability to stabilize austenite is 18 times that of nickel element. Nitrogen element in steel can play the role of solid solution strengthening, fine grain strengthening, precipitation strengthening and strain strengthening, etc., to improve the strength and hardness of the steel. In addition, nitrogen element can enrich on the surface of the steel, play a role in stabilizing the passivation film, thereby significantly improving the corrosion resistance of the steel. Compared with traditional stainless steel, the addition of nitrogen element in steel to obtain high-nitrogen stainless steel can achieve the purpose of improving the mechanical properties and corrosion resistance of the steel and reducing the cost of the material.

[0004] Developing a new high-nitrogen stainless steel preparation process is one of the effective methods to obtain high-performance high-nitrogen stainless steel. The traditional melting process of high-nitrogen stainless steel includes nitrogen gas normal pressure melting and pressurized melting. Under normal pressure, the solubility of nitrogen in the molten steel is very low, and nitrogen is easy to escape during condensation, resulting in low nitrogen content in the obtained stainless steel. The pressurized melting process significantly improves the nitrogen content in the stainless steel, but easily leads to uneven nitrogen concentration, and precipitates coarse nitrides during subsequent hot working, affecting the mechanical properties and corrosion resistance of the steel, and the pressurized melting process has problems such as high production cost, complex equipment operation, and difficult control of nitrogen content.

[0005] And as a technology for preparing high-nitrogen stainless steel, powder metallurgy can prepare high-nitrogen stainless steel products through pressing and sintering, but most of them are first nitrided with raw material powder and then pressed and sintered; These processes have technical defects such as poor strength and toughness of the prepared products, high porosity of the products, and uneven density distribution.

[0006] For example, Chinese patent CN101338385A discloses a kind of ammonia-containing / high-nitrogen stainless steel product and its preparation method, which is prepared by weighing ammonia-containing / high-nitrogen stainless steel powder, loading stainless steel powder into a package, degassing and hot isostatic pressing; Obviously, the pressure of hot isostatic pressing sintering is large, the cost is high, and the densification depends on the process; And the package is not removed, and the sintering effect is general. As for the preparation of high-nitrogen stainless steel powder, Chinese patent CN120443100A gives a preparation method of stainless steel powder by heating nitriding.

[0007] Chinese patent CN117026057A discloses a powder metallurgy preparation method of high-nitrogen stainless steel for knives and scissors, which uses martensitic stainless steel powder, first mechanical alloying in nitrogen atmosphere at room temperature, and then high-temperature solid-state nitriding activation sintering method to prepare high-performance high-nitrogen stainless steel; This method not only needs to carry out nitriding mechanical alloying on stainless steel powder first, but also needs high-temperature solid-state nitriding in nitrogen atmosphere; The final sintering environment does not need nitriding; Both nitriding processes are for stainless steel powder, and nitriding for pressing blank and sintering process is not considered.

[0008] Chinese patent CN117265365A discloses a preparation method of high-performance stainless steel powder metallurgical part, in which nitriding occurs in the high-temperature sintering process, the sintering temperature is very high, and the sintering time is also very long, and the solid solution hardening after sintering cannot synergistically improve the strength and toughness of the powder metallurgical part.

[0009] Chinese patent CN110295308A discloses a preparation method of stainless steel material, which needs to inject the feed prepared by stainless steel powder and binder into a predetermined shape blank on an injection machine, and then defat and sinter; Obviously, the porosity of the green body after defatting is high, and even after subsequent sintering treatment, the densification is difficult to improve to 95%, and the surface hardness can be improved by solid solution treatment, but the plasticity and toughness will be obviously decreased.

[0010] Therefore, it is urgent to invent a preparation method of high-performance high-nitrogen stainless steel, which can obtain high-strength high-toughness high-nitrogen stainless steel by increasing the content of nitrogen element in stainless steel. SUMMARY

[0011] In order to solve the problems of low nitrogen content in atmospheric melting high-nitrogen stainless steel, easy escape of nitrogen, uneven nitrogen concentration in pressurized melting high-nitrogen stainless steel, easy precipitation of coarse nitrides, resulting in uneven distribution of stainless steel organization, reduction of mechanical properties and corrosion resistance of stainless steel, high production cost of pressurized melting, complex equipment operation, and difficult control of nitrogen content, etc., the present application provides a method for preparing powder metallurgical high-strength high-toughness high-nitrogen stainless steel with high nitrogen content, uniform nitrogen distribution, easy control of nitrogen content, uniform organization distribution, high material utilization rate, excellent mechanical properties and good corrosion resistance.

[0012] To achieve the above object, the technical scheme of the present application is as follows:

[0013] A method for preparing a powder metallurgy high-strength and high-toughness high-nitrogen stainless steel, comprising the following steps:

[0014] S1, powder preparation: the raw material powder is a commercially available stainless steel alloy powder of various brands;

[0015] S2, cold isostatic pressing: the stainless steel alloy powder in S1 is placed into a cold isostatic pressing sleeve, vibrated and sealed, then placed into a cold isostatic pressing machine for forming, followed by demolding to obtain a green body of stainless steel alloy;

[0016] S3, nitriding treatment: the green body of stainless steel alloy prepared in S2 is placed into a sintering furnace for nitrogen sintering to obtain a high-nitrogen stainless steel ingot;

[0017] S4, pressurized nitrogen sintering: the stainless steel ingot prepared in S3 is placed into a pressurized sintering furnace for densification sintering to obtain a high-nitrogen stainless steel sintered compact;

[0018] S5, hot working deformation: the high-nitrogen stainless steel sintered compact prepared in S4 is subjected to hot deformation processing to finally obtain a high-strength and high-toughness high-nitrogen stainless steel with uniform microstructure and refined grains.

[0019] Optionally, the content of the main alloying element Cr in the stainless steel alloy powder in S1 is 12.00-30.00wt.%.

[0020] Optionally, the brand of the stainless steel alloy powder in S1 includes 201, 202, 304, 316L, and 321.

[0021] Optionally, the powder particle size of the stainless steel powder in S1 is ≤30μm, and the tap density is ≥4.60g / cm 3 .

[0022] Optionally, the material of the cold isostatic pressing sleeve in S2 is silica gel or polyurethane, the size of the green body of stainless steel alloy is Φ30mm-Φ2000mm, the pressing pressure is 150-300MPa, and the pressure holding time is 60-180s.

[0023] Optionally, the atmosphere for nitrogen sintering in S3 is nitrogen, the nitrogen pressure is 1.0-3.0MPa, the sintering temperature is 900-1100℃, the heating rate is 2-10℃ / min, the holding time is 1-3h, the purity of nitrogen is ≥99.9%, and the nitrogen flow is controlled between 0.5-5m 3 / min.

[0024] Optionally, the high-nitrogen stainless steel blank ingot obtained in S3 has a density of less than 90%, and the nitrogen content in the high-nitrogen stainless steel blank ingot is 0.4-1.0 wt.%.

[0025] Optionally, in S4, the atmosphere for sintering is nitrogen, the nitrogen pressure is 5.0-20.0 MPa, the sintering temperature is 1100-1300 DEG C, the holding time is 2-4 h, the purity of nitrogen is greater than or equal to 99.9%, and the nitrogen flow is controlled to be between 0.5-5 m 3 / min.

[0026] Optionally, the high-nitrogen stainless steel sintered blank obtained in S4 has a density of greater than or equal to 95% and a nitrogen content of greater than or equal to 0.5 wt.%.

[0027] Optionally, the hot deformation process in S5 includes rolling, extrusion, and forging, and the hot deformation process is performed at a temperature of 1200-1350 DEG C and a deformation amount of greater than or equal to 30%.

[0028] Optionally, the high-nitrogen stainless steel obtained in S5 has a density of greater than 99%, a hardness of 20-40 HRC, a nitrogen content of greater than 0.5 wt.%, an average grain size of less than 10 mu m, a tensile strength of greater than 1000 MPa, a yield strength of greater than 400 MPa, a yield ratio of greater than 0.6, an elongation of greater than 10%, and a product of strength and ductility of greater than 20 GPa.

[0029] Compared with the prior art, the above technical solution has at least the following beneficial effects:

[0030] The above scheme provides a method for preparing a powder metallurgy high-strength and high-toughness high-nitrogen stainless steel, which can solve the problems of low nitrogen content, easy nitrogen escape in normal-pressure smelting high-nitrogen stainless steel, uneven nitrogen concentration, easy precipitation of coarse nitrides in pressurized smelting high-nitrogen stainless steel, uneven distribution of stainless steel organization, and reduction of mechanical properties and corrosion resistance of stainless steel, and high production cost, complex equipment operation, and difficult nitrogen content control in pressurized smelting, and finally obtains full-dense high-nitrogen stainless steel with high nitrogen content, uniform organization distribution, excellent mechanical properties, and good corrosion resistance.

[0031] The present application is suitable for the preparation of various high-nitrogen stainless steels, such as high-nitrogen austenitic stainless steel, dual-phase steel, and high-nitrogen martensitic stainless steel, and has no limitation on the grade of stainless steel alloy powder raw material, including but not limited to 201, 202, 304, 316L, and 321, and has high optional flexibility, which reduces the difficulty of preparing high-nitrogen stainless steel and has strong industrial applicability.

[0032] The present application adopts cold isostatic pressing to form stainless steel powder, and the material utilization rate is high.

[0033] This invention involves subjecting stainless steel green billets to atmospheric pressure nitriding followed by pressurized nitrogen sintering. During the sintering stage, the stainless steel powder comes into full contact with nitrogen gas through tiny gaps, shortening the diffusion distance of nitrogen atoms, which facilitates the penetration of nitrogen and results in a more uniform distribution of nitrogen. This reduces the nitriding time and achieves the combined preparation of high-nitrogen stainless steel by integrating powder nitriding and subsequent sintering processes.

[0034] This invention proposes using nitrogen gas to nitrid stainless steel during sintering. During sintering, nitrogen atoms react in situ with elements such as Cr in the stainless steel matrix to form fine and uniform reinforcing phases such as CrN, which are stably distributed within the matrix. Furthermore, to improve the density of the nitrided stainless steel, nitrogen-pressurized sintering is employed, increasing the density of high-nitrogen stainless steel to over 95%, thus preventing defects such as cracking due to low density during subsequent hot working.

[0035] Unlike traditional smelting methods that nitrid in the δ-ferrite phase region where nitrogen solubility is low, this invention nitrids in the austenite phase region where nitrogen solubility is high, thus avoiding the δ-ferrite phase region and effectively preventing nitrogen gas from escaping.

[0036] This invention prepares high-nitrogen stainless steel using powder metallurgy, achieving a fine-grained structure that strengthens the steel. Powder metallurgy also allows for the convenient and safe production of high-nitrogen stainless steel with higher nitrogen content. Furthermore, as the nitrogen content in the stainless steel increases, nitrogen not only strengthens through solid solution but also forms fine CrN and Cr2N phases with Cr, inhibiting the precipitation of carbides in the stainless steel and thus providing precipitation strengthening, dispersion strengthening, and improved corrosion resistance.

[0037] This invention densifies high-nitrogen stainless steel sintered billets through hot working, resulting in high-nitrogen stainless steel with a density of over 99%. At the same time, hot working can further refine the grains of high-nitrogen stainless steel. The resulting high-nitrogen stainless steel has a high nitrogen content, uniform microstructure distribution, excellent mechanical properties, and good corrosion resistance.

[0038] In summary, this invention utilizes powder metallurgy combined with nitrogen sintering and hot working deformation processes to obtain high-strength, high-toughness, high-nitrogen stainless steel. Compared with traditional nitrogen atmosphere smelting, the stainless steel obtained by this invention has higher nitrogen content, more uniform nitrogen distribution, easier nitrogen content control, more uniform microstructure, higher material utilization, better mechanical properties, and better corrosion resistance. Furthermore, it is simple and convenient to operate, highly flexible, and has a wide range of applications, making it suitable for the industrial production of various types of high-nitrogen stainless steel. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 This is the stress-strain curve of the powder metallurgy high-strength and high-toughness high-nitrogen stainless steel of Embodiment 1 of the present invention;

[0041] Figure 2 This is the stress-strain curve of the powder metallurgy high-strength and high-nitrogen stainless steel of Embodiment 2 of the present invention;

[0042] Figure 3 This is the stress-strain curve of the powder metallurgy high-strength and high-toughness high-nitrogen stainless steel of Example 3 of the present invention;

[0043] Figure 4 This is the stress-strain curve of the powder metallurgy high-strength and high-toughness high-nitrogen stainless steel of Example 4 of the present invention;

[0044] Figure 5 This is the stress-strain curve of the powder metallurgy high-strength and high-nitrogen stainless steel of Example 5 of the present invention;

[0045] Figure 6 This is the stress-strain curve of the powder metallurgy high-strength and high-toughness high-nitrogen stainless steel of Embodiment 6 of the present invention. Detailed Implementation

[0046] The technical solution of the present invention will now be described with reference to the accompanying drawings.

[0047] In embodiments of the present invention, words such as "exemplarily," "for example," etc., are used to indicate that something is an example, illustration, or description. Any embodiment or design described as "exemplary" in the present invention should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of the word "exemplary" is intended to present the concept in a concrete manner. Furthermore, in embodiments of the present invention, the meaning expressed by "and / or" can be both, or either one.

[0048] In the embodiments of the present invention, the terms "image" and "picture" may sometimes be used interchangeably. It should be noted that when the distinction is not emphasized, their intended meanings are consistent.

[0049] In this embodiment of the invention, sometimes a subscript such as W1 may be written in a non-subscript form such as W1. When the difference is not emphasized, the meaning they express is the same.

[0050] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0051] A method for preparing powder metallurgy high-strength, high-toughness, high-nitrogen stainless steel, the method comprising the following steps:

[0052] S1. Powder preparation: The raw material powder is various grades of stainless steel alloy powder available on the market;

[0053] S2. Cold isostatic pressing: The stainless steel alloy powder in S1 is placed into a cold isostatic pressing sleeve, vibrated and sealed, then placed into a cold isostatic press for forming, and then demolded to obtain a stainless steel alloy green blank.

[0054] S3, Nitriding treatment: The stainless steel alloy green billet obtained in S2 is placed in a sintering furnace for nitrogen sintering to obtain a high-nitrogen stainless steel billet ingot.

[0055] S4. Pressurized nitrogen sintering: The stainless steel billet obtained in S3 is placed in a pressurized sintering furnace for densification sintering to obtain a high-nitrogen stainless steel sintered billet.

[0056] S5. Hot working deformation: The high-nitrogen stainless steel sintered billet obtained by S4 is subjected to hot deformation processing to finally obtain high-strength and high-toughness high-nitrogen stainless steel with uniform microstructure and refined grains.

[0057] Specifically, the content of Cr, the main alloying element in the stainless steel alloy powder of S1, is 12.00-30.00 wt.%.

[0058] Specifically, the grades of stainless steel alloy powder in S1 include 201, 202, 304, 316L, and 321.

[0059] Specifically, the stainless steel powder in S1 has a particle size ≤30μm and a tap density ≥4.60g / cm³. 3 .

[0060] Specifically, the S2 cold isostatic pressing jacket is made of silicone or polyurethane, the stainless steel alloy green blank has a size of Φ30mm-Φ2000mm, the pressing pressure is 150-300MPa, and the holding time is 60-180s.

[0061] Specifically, in S3, the nitrogen sintering atmosphere is nitrogen, with a nitrogen pressure of 1.0-3.0 MPa, a sintering temperature of 900-1100℃, a heating rate of 2-10℃ / min, a holding time of 1-3 h, a nitrogen purity ≥99.9%, and a nitrogen flow rate controlled at 0.5-5 m³ / min. 3 Between / min.

[0062] Specifically, the high-nitrogen stainless steel billet obtained in S3 has a density of <90% and a nitrogen content of 0.4-1.0 wt.%.

[0063] Specifically, the densification sintering atmosphere in S4 is nitrogen, with a nitrogen pressure of 5.0-20.0 MPa, a sintering temperature of 1100-1300℃, a holding time of 2-4 h, a nitrogen purity of ≥99.9%, and a nitrogen flow rate controlled at 0.5-5 m³ / h. 3 Between / min.

[0064] Specifically, the high-nitrogen stainless steel sintered billet obtained in S4 has a density ≥95% and a nitrogen content ≥0.5wt.%.

[0065] Specifically, hot deformation processing in S5 includes rolling, extrusion, and forging. The temperature of hot deformation processing is 1200-1350℃, and the deformation amount is ≥30%.

[0066] Specifically, the high-nitrogen stainless steel obtained in S5 has a density >99%, a hardness of 20-40 HRC, a nitrogen content >0.5 wt.%, an average grain size <10 μm, a tensile strength >1000 MPa, a yield strength >400 MPa, a yield-to-tensile ratio >0.6, an elongation >10%, and a strength-ductility product >20 GPa.

[0067] Example 1

[0068] A method for preparing powder metallurgy high-strength, high-toughness, high-nitrogen stainless steel, the method comprising the following steps:

[0069] S1. Powder Preparation: The raw material powder is commercially available 316L stainless steel alloy powder. The particle size of the stainless steel powder is ≤30μm, and the tap density is 4.65g / cm³. 3 The main alloying element, Cr, has a content of 16.55 wt.%.

[0070] S2. Cold Isostatic Pressing: The stainless steel alloy powder from S1 is placed into a cold isostatic pressing sleeve, vibrated and sealed. The sleeve is made of silicone. Then it is placed in a cold isostatic press for molding. The pressing pressure is 150MPa and the holding time is 90s. After that, it is demolded to obtain a stainless steel alloy green billet. The stainless steel alloy green billet is cylindrical in shape and has a size of Φ30×60mm.

[0071] S3. Nitriding Treatment: The stainless steel alloy green billet obtained in S2 is placed in a sintering furnace for nitrogen sintering. The nitrogen sintering atmosphere is nitrogen, the nitrogen pressure is 1.5 MPa, the sintering temperature is 950℃, the heating rate is 3℃ / min, the holding time is 1.5 h, the nitrogen purity is ≥99.9%, and the nitrogen flow rate is controlled at 1.0 m³ / min. 3The density of the obtained high-nitrogen stainless steel billet was 83%, and the nitrogen content in the high-nitrogen stainless steel billet was 0.43 wt.%.

[0072] S4. Pressurized Nitrogen Sintering: The stainless steel billet obtained in S3 is placed in a pressurized sintering furnace for densification sintering. The densification sintering atmosphere is nitrogen, the nitrogen pressure is 10.0 MPa, the sintering temperature is 1200℃, the holding time is 2 hours, the nitrogen purity is ≥99.9%, and the nitrogen flow rate is controlled at 3.5 m³ / s. 3 The high-nitrogen stainless steel sintered billet was obtained at a rate of [per unit weight] / min; the density of the obtained high-nitrogen stainless steel sintered billet was 95.2%, and the nitrogen content was 0.51 wt.%.

[0073] S5. Hot working deformation: The high-nitrogen stainless steel sintered billet obtained by S4 is hot forged at a forging temperature of 1200℃ and a deformation amount of 50%, finally obtaining high-strength and high-toughness high-nitrogen stainless steel with uniform microstructure and refined grains.

[0074] like Figure 1 As shown, the high-nitrogen stainless steel obtained in this embodiment has a density of 99.3%, a hardness of 29.4 HRC, a yield strength of 808 MPa, a tensile strength of 1032 MPa, a yield-to-tensile ratio of 0.783, an elongation of 27.3%, a strength-ductility product of 28.174 GPa%, an average grain size of 7.1 μm, a nitrogen content of 0.51 wt.%, a uniform microstructure, and excellent mechanical properties.

[0075] Example 2

[0076] A method for preparing powder metallurgy high-strength, high-toughness, high-nitrogen stainless steel, the method comprising the following steps:

[0077] S1. Powder Preparation: The raw material powder is commercially available 316L stainless steel alloy powder. The particle size of the stainless steel powder is ≤30μm, and the tap density is 4.80g / cm³. 3 The main alloying element, Cr, has a content of 16.93 wt.%.

[0078] S2. Cold Isostatic Pressing: The stainless steel alloy powder from S1 is placed into a cold isostatic pressing sleeve, vibrated and sealed. The sleeve is made of silicone. Then it is placed in a cold isostatic press for molding. The pressing pressure is 180MPa and the holding time is 90s. After that, it is demolded to obtain a stainless steel alloy green billet. The stainless steel alloy green billet is cylindrical in shape and has a size of Φ80×300mm.

[0079] S3. Nitriding Treatment: The stainless steel alloy green billet obtained in S2 is placed in a sintering furnace for nitrogen sintering. The nitrogen sintering atmosphere is nitrogen, the nitrogen pressure is 2.0 MPa, the sintering temperature is 1000℃, the heating rate is 5℃ / min, the holding time is 2.0 h, the nitrogen purity is ≥99.9%, and the nitrogen flow rate is controlled at 1.5 m³ / min. 3 The high-nitrogen stainless steel billet was obtained at a rate of [per unit weight] / min; the density of the obtained high-nitrogen stainless steel billet was 85%, and the nitrogen content in the high-nitrogen stainless steel billet was 0.48 wt.%.

[0080] S4. Pressurized Nitrogen Sintering: The stainless steel billet obtained in S3 is placed in a pressurized sintering furnace for densification sintering. The densification sintering atmosphere is nitrogen, the nitrogen pressure is 15.0 MPa, the sintering temperature is 1280℃, the holding time is 2.5 h, the nitrogen purity is ≥99.9%, and the nitrogen flow rate is controlled at 4.0 m³ / s. 3 The high-nitrogen stainless steel sintered billet was obtained at a rate of [per unit weight] / min; the density of the obtained high-nitrogen stainless steel sintered billet was 95.8%, and the nitrogen content was 0.66 wt.%.

[0081] S5. Hot working deformation: The high-nitrogen stainless steel sintered billet obtained by S4 is hot forged at a forging temperature of 1250℃ and a deformation amount of 75%, finally obtaining high-strength and high-toughness high-nitrogen stainless steel with uniform microstructure and refined grains.

[0082] like Figure 2 As shown, the high-nitrogen stainless steel obtained in this embodiment has a density of 99.5%, a hardness of 33.7 HRC, a yield strength of 1078 MPa, a tensile strength of 1134 MPa, a yield-to-tensile ratio of 0.951, an elongation of 21.7%, a strength-ductility product of 24.608 GPa%, an average grain size of 3.1 μm, a nitrogen content of 0.66 wt.%, a uniform microstructure, and excellent mechanical properties.

[0083] Example 3

[0084] A method for preparing powder metallurgy high-strength, high-toughness, high-nitrogen stainless steel, the method comprising the following steps:

[0085] S1. Powder Preparation: The raw material powder is commercially available 316L stainless steel alloy powder. The particle size of the stainless steel powder is ≤30μm, and the tap density is 4.85g / cm³. 3 The main alloying element, Cr, has a content of 16.88 wt.%.

[0086] S2. Cold Isostatic Pressing: The stainless steel alloy powder from S1 is placed into a cold isostatic pressing sleeve, vibrated and sealed. The sleeve is made of silicone. Then it is placed in a cold isostatic press for molding. The pressing pressure is 180MPa and the holding time is 120s. After that, it is demolded to obtain a stainless steel alloy green billet. The stainless steel alloy green billet is cylindrical in shape and has a size of Φ500×1500mm.

[0087] S3. Nitriding Treatment: The stainless steel alloy green billet obtained in S2 is placed in a sintering furnace for nitrogen sintering. The nitrogen sintering atmosphere is nitrogen, the nitrogen pressure is 2.5 MPa, the sintering temperature is 1050℃, the heating rate is 8℃ / min, the holding time is 2.5 h, the nitrogen purity is ≥99.9%, and the nitrogen flow rate is controlled at 2.0 m³ / min. 3 The density of the high-nitrogen stainless steel billet was 89%, and the nitrogen content in the high-nitrogen stainless steel billet was 0.69 wt.%.

[0088] S4. Pressurized Nitrogen Sintering: The stainless steel billet obtained in S3 is placed in a pressurized sintering furnace for densification sintering. The densification sintering atmosphere is nitrogen, the nitrogen pressure is 20.0 MPa, the sintering temperature is 1300℃, the holding time is 3 hours, the nitrogen purity is ≥99.9%, and the nitrogen flow rate is controlled at 5.0 m³ / s. 3 The high-nitrogen stainless steel sintered billet was obtained at a rate of [per unit weight] / min; the density of the obtained high-nitrogen stainless steel sintered billet was 96.2%, and the nitrogen content was 0.80 wt.%.

[0089] S5. Hot working deformation: The high-nitrogen stainless steel sintered billet obtained by S4 is hot forged at a forging temperature of 1250℃ and a deformation amount of 85%, finally obtaining high-strength and high-toughness high-nitrogen stainless steel with uniform microstructure and refined grains.

[0090] like Figure 3 As shown, the high-nitrogen stainless steel obtained in this embodiment has a density of 99.8%, a hardness of 39.0 HRC, a yield strength of 1120 MPa, a tensile strength of 1215 MPa, a yield-to-tensile ratio of 0.922, an elongation of 19.6%, a strength-ductility product of 22.599 GPa%, an average grain size of 2.7 μm, a nitrogen content of 0.80 wt.%, a uniform microstructure, and excellent mechanical properties.

[0091] Example 4

[0092] A method for preparing powder metallurgy high-strength, high-toughness, high-nitrogen stainless steel, the method comprising the following steps:

[0093] S1. Powder Preparation: The raw material powder is commercially available 316L stainless steel alloy powder. The particle size of the stainless steel powder is ≤30μm, and the tap density is 4.73g / cm³.3 The main alloying element, Cr, has a content of 16.95 wt.%.

[0094] S2. Cold Isostatic Pressing: The stainless steel alloy powder in S1 is placed into a cold isostatic pressing sleeve, vibrated and sealed. The sleeve is made of silicone. Then it is placed in a cold isostatic press for molding. The pressing pressure is 150MPa and the holding time is 120s. After that, it is demolded to obtain a stainless steel alloy green billet. The stainless steel alloy green billet is cylindrical in shape and has a size of Φ50×60mm.

[0095] S3. Nitriding Treatment: The stainless steel alloy green billet obtained in S2 is placed in a sintering furnace for nitrogen sintering. The nitrogen sintering atmosphere is nitrogen, the nitrogen pressure is 2.0 MPa, the sintering temperature is 1100℃, the heating rate is 10℃ / min, the holding time is 2h, the nitrogen purity is ≥99.9%, and the nitrogen flow rate is controlled at 1.5m³ / min. 3 The density of the high-nitrogen stainless steel billet was 87%, and the nitrogen content in the high-nitrogen stainless steel billet was 0.58 wt.%.

[0096] S4. Pressurized Nitrogen Sintering: The stainless steel billet obtained in S3 is placed in a pressurized sintering furnace for densification sintering. The densification sintering atmosphere is nitrogen, the nitrogen pressure is 15.0 MPa, the sintering temperature is 1250℃, the holding time is 2 hours, the nitrogen purity is ≥99.9%, and the nitrogen flow rate is controlled at 5.0 m³ / s. 3 The high-nitrogen stainless steel sintered billet was obtained at a rate of [per unit weight] / min; the density of the obtained high-nitrogen stainless steel sintered billet was 95.9%, and the nitrogen content was 0.69 wt.%.

[0097] S5. Hot working deformation: The high-nitrogen stainless steel sintered billet obtained by S4 is hot forged at a forging temperature of 1200℃ and a deformation amount of 80%, finally obtaining high-strength and high-toughness high-nitrogen stainless steel with uniform microstructure and refined grains.

[0098] like Figure 4 As shown, the high-nitrogen stainless steel obtained in this embodiment has a density of 99.6%, a hardness of 32.7 HRC, a yield strength of 928 MPa, a tensile strength of 1098 MPa, a yield-to-tensile ratio of 0.845, an elongation of 25.2%, a strength-ductility product of 22.670 GPa%, an average grain size of 3.5 μm, a nitrogen content of 0.69 wt.%, a uniform microstructure, and excellent mechanical properties.

[0099] Example 5

[0100] A method for preparing powder metallurgy high-strength, high-toughness, high-nitrogen stainless steel, the method comprising the following steps:

[0101] S1. Powder Preparation: The raw material powder is commercially available 316L stainless steel alloy powder. The particle size of the stainless steel powder is ≤30μm, and the tap density is 4.70g / cm³. 3 The main alloying element, Cr, has a content of 16.68 wt.%.

[0102] S2. Cold Isostatic Pressing: The stainless steel alloy powder in S1 is placed into a cold isostatic pressing sleeve, vibrated and sealed. The sleeve is made of silicone. Then it is placed in a cold isostatic press for molding. The pressing pressure is 200MPa and the holding time is 90s. After that, it is demolded to obtain a stainless steel alloy green billet. The stainless steel alloy green billet is cylindrical in shape and has a size of Φ55×65mm.

[0103] S3. Nitriding Treatment: The stainless steel alloy green billet obtained in S2 is placed in a sintering furnace for nitrogen sintering. The nitrogen sintering atmosphere is nitrogen, the nitrogen pressure is 3.0 MPa, the sintering temperature is 1080℃, the heating rate is 8℃ / min, the holding time is 2h, the nitrogen purity is ≥99.9%, and the nitrogen flow rate is controlled at 2.5m³ / min. 3 The high-nitrogen stainless steel billet was obtained at a rate of [per unit weight] / min; the density of the obtained high-nitrogen stainless steel billet was 86%, and the nitrogen content in the high-nitrogen stainless steel billet was 0.55 wt.%.

[0104] S4. Pressurized Nitrogen Sintering: The stainless steel billet obtained in S3 is placed in a pressurized sintering furnace for densification sintering. The densification sintering atmosphere is nitrogen, the nitrogen pressure is 20.0 MPa, the sintering temperature is 1250℃, the holding time is 3 hours, the nitrogen purity is ≥99.9%, and the nitrogen flow rate is controlled at 5.0 m³ / s. 3 The high-nitrogen stainless steel sintered billet was obtained at a rate of [per unit weight] / min; the density of the obtained high-nitrogen stainless steel sintered billet was 95.5%, and the nitrogen content was 0.68 wt.%.

[0105] S5. Hot working deformation: The high-nitrogen stainless steel sintered billet obtained by S4 is hot forged at a forging temperature of 1200℃ and a deformation amount of 90%, finally obtaining high-strength and high-toughness high-nitrogen stainless steel with uniform microstructure and refined grains.

[0106] like Figure 5 As shown, the high-nitrogen stainless steel obtained in this embodiment has a density of 99.8%, a hardness of 31.1 HRC, a yield strength of 753 MPa, a tensile strength of 1077 MPa, a yield-to-tensile ratio of 0.698, an elongation of 29.4%, a strength-ductility product of 31.664 GPa%, an average grain size of 3.8 μm, a nitrogen content of 0.68 wt.%, a uniform microstructure, and excellent mechanical properties.

[0107] Example 6

[0108] A method for preparing powder metallurgy high-strength, high-toughness, high-nitrogen stainless steel, the method comprising the following steps:

[0109] S1. Powder Preparation: The raw material powder is commercially available 316L stainless steel alloy powder. The particle size of the stainless steel powder is ≤30μm, and the tap density is 4.69g / cm³. 3 The main alloying element, Cr, has a content of 16.58 wt.%.

[0110] S2. Cold Isostatic Pressing: The stainless steel alloy powder in S1 is placed into a cold isostatic pressing sleeve, vibrated and sealed. The sleeve is made of silicone. Then it is placed in a cold isostatic press for molding. The pressing pressure is 200MPa and the holding time is 120s. After that, it is demolded to obtain a stainless steel alloy green billet. The stainless steel alloy green billet is cylindrical in shape and has a size of Φ55×70mm.

[0111] S3. Nitriding Treatment: The stainless steel alloy green billet obtained in S2 is placed in a sintering furnace for nitrogen sintering. The nitrogen sintering atmosphere is nitrogen, the nitrogen pressure is 3.0 MPa, the sintering temperature is 1100℃, the heating rate is 5℃ / min, the holding time is 3h, the nitrogen purity is ≥99.9%, and the nitrogen flow rate is controlled at 3.0 m³ / min. 3 The high-nitrogen stainless steel billet was obtained at a rate of [per unit weight] / min; the density of the obtained high-nitrogen stainless steel billet was 90%, and the nitrogen content in the high-nitrogen stainless steel billet was 0.73 wt.%.

[0112] S4. Pressurized Nitrogen Sintering: The stainless steel billet obtained in S3 is placed in a pressurized sintering furnace for densification sintering. The densification sintering atmosphere is nitrogen, the nitrogen pressure is 20.0 MPa, the sintering temperature is 1300℃, the holding time is 3.5 h, the nitrogen purity is ≥99.9%, and the nitrogen flow rate is controlled at 5.0 m³ / s. 3 The high-nitrogen stainless steel sintered billet was obtained at a rate of [per unit weight] / min; the density of the obtained high-nitrogen stainless steel sintered billet was 97.3%, and the nitrogen content was 0.85 wt.%.

[0113] S5. Hot working deformation: The high-nitrogen stainless steel sintered billet obtained by S4 is hot forged at a forging temperature of 1250℃ and a deformation amount of 90%, finally obtaining high-strength and high-toughness high-nitrogen stainless steel with uniform microstructure and refined grains.

[0114] like Figure 6 As shown, the high-nitrogen stainless steel obtained in this embodiment has a density of 99.9%, a hardness of 38.3 HRC, a yield strength of 1150 MPa, a tensile strength of 1252 MPa, a yield-to-tensile ratio of 0.920, an elongation of 18.2%, a strength-ductility product of 22.786 GPa%, an average grain size of 2.3 μm, a nitrogen content of 0.85 wt.%, a uniform microstructure, and excellent mechanical properties.

[0115] The above-mentioned solution presents a method for preparing high-strength, high-toughness, high-nitrogen stainless steel using powder metallurgy. This method can solve the problems of low nitrogen content in high-nitrogen stainless steel produced by atmospheric pressure melting, easy nitrogen escape, uneven nitrogen concentration and easy precipitation of coarse nitrides in high-nitrogen stainless steel produced by pressure melting, which leads to uneven distribution of stainless steel structure, reduced mechanical properties and corrosion resistance, and high production cost, complex equipment operation and difficulty in controlling nitrogen content in pressure melting. Finally, a fully dense high-nitrogen stainless steel with high nitrogen content, uniform structure distribution, excellent mechanical properties and good corrosion resistance is obtained.

[0116] This invention is applicable to the preparation of various high-nitrogen stainless steels, such as high-nitrogen austenitic stainless steel, duplex stainless steel, and high-nitrogen martensitic stainless steel. It does not limit the grade of stainless steel alloy powder raw materials, including but not limited to 201, 202, 304, 316L, and 321, which provides high flexibility and reduces the difficulty of preparing high-nitrogen stainless steel. It has strong industrial applicability.

[0117] This invention uses cold isostatic pressing to form stainless steel powder, which allows for flexible selection of molding die material and shape according to the shape and size of the stainless steel parts, resulting in near-net-shape forming and high material utilization.

[0118] This invention involves subjecting stainless steel green billets to atmospheric pressure nitriding followed by pressurized nitrogen sintering. During the sintering stage, the stainless steel powder comes into full contact with nitrogen gas through tiny gaps, shortening the diffusion distance of nitrogen atoms, which facilitates the penetration of nitrogen and results in a more uniform distribution of nitrogen. This reduces the nitriding time and achieves the combined preparation of high-nitrogen stainless steel by integrating powder nitriding and subsequent sintering processes.

[0119] This invention proposes using nitrogen gas to nitrid stainless steel during sintering. During sintering, nitrogen atoms react in situ with elements such as Cr in the stainless steel matrix to form fine and uniform reinforcing phases such as CrN, which are stably distributed within the matrix. Furthermore, to improve the density of the nitrided stainless steel, nitrogen-pressurized sintering is employed, increasing the density of high-nitrogen stainless steel to over 95%, thus preventing defects such as cracking due to low density during subsequent hot working.

[0120] Unlike traditional smelting methods that nitrid in the δ-ferrite phase region where nitrogen solubility is low, this invention nitrids in the austenite phase region where nitrogen solubility is high, thus avoiding the δ-ferrite phase region and effectively preventing nitrogen gas from escaping.

[0121] This invention prepares high-nitrogen stainless steel using powder metallurgy, achieving a fine-grained structure that strengthens the steel. Powder metallurgy also allows for the convenient and safe production of high-nitrogen stainless steel with higher nitrogen content. Furthermore, as the nitrogen content in the stainless steel increases, nitrogen not only strengthens through solid solution but also forms fine CrN and Cr2N phases with Cr, inhibiting the precipitation of carbides in the stainless steel and thus providing precipitation strengthening, dispersion strengthening, and improved corrosion resistance.

[0122] This invention densifies high-nitrogen stainless steel sintered billets through hot working, resulting in high-nitrogen stainless steel with a density of over 99%. At the same time, hot working can further refine the grains of high-nitrogen stainless steel. The resulting high-nitrogen stainless steel has a high nitrogen content, uniform microstructure distribution, excellent mechanical properties, and good corrosion resistance.

[0123] In summary, this invention utilizes powder metallurgy combined with nitrogen sintering and hot working deformation processes to obtain high-strength, high-toughness, high-nitrogen stainless steel. Compared with traditional nitrogen atmosphere smelting, the stainless steel obtained by this invention has higher nitrogen content, more uniform nitrogen distribution, easier nitrogen content control, more uniform microstructure, higher material utilization, better mechanical properties, and better corrosion resistance. Furthermore, it is simple and convenient to operate, highly flexible, and has a wide range of applications, making it suitable for the industrial production of various types of high-nitrogen stainless steel.

[0124] It should be understood that the term "and / or" in this article is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. A and B can be singular or plural. Additionally, the character " / " in this article generally indicates an "or" relationship between the preceding and following related objects, but it can also represent an "and / or" relationship. Please refer to the context for a more accurate understanding.

[0125] In this invention, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refer to any combination of these items, including any combination of a single item or a plurality of items. For example, at least one of a, b, or c can represent: a, b, c, ab, ac, bc, or abc, where a, b, and c can be a single item or multiple items.

[0126] It should be understood that, in various embodiments of the present invention, the order of the above-mentioned process numbers does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0127] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing powder metallurgy high-strength, high-toughness, high-nitrogen stainless steel, characterized in that, The method for preparing powder metallurgy high-strength, high-toughness, high-nitrogen stainless steel includes the following steps: S1. Powder preparation: The raw material powder is various grades of stainless steel alloy powder available on the market; S2. Cold isostatic pressing: The stainless steel alloy powder in S1 is placed into a cold isostatic pressing sleeve, vibrated and sealed, then placed into a cold isostatic press for forming, and then demolded to obtain a stainless steel alloy green blank. S3, Nitriding treatment: The stainless steel alloy green billet obtained in S2 is placed in a sintering furnace for nitrogen sintering to obtain a high-nitrogen stainless steel billet ingot. S4. Pressurized nitrogen sintering: The stainless steel billet obtained in S3 is placed in a pressurized sintering furnace for densification sintering to obtain a high-nitrogen stainless steel sintered billet. S5. Hot working deformation: The high-nitrogen stainless steel sintered billet obtained by S4 is subjected to hot deformation processing to finally obtain high-strength and high-toughness high-nitrogen stainless steel with uniform microstructure and refined grains.

2. The method for preparing powder metallurgy high-strength and high-toughness high-nitrogen stainless steel according to claim 1, characterized in that, The content of Cr, the main alloying element in stainless steel alloy powder in S1, is 12.00-30.00 wt.

3. The method for preparing powder metallurgy high-strength and high-toughness high-nitrogen stainless steel according to claim 1, characterized in that, The stainless steel powder in S1 has a particle size ≤30μm and a tap density ≥4.60g / cm³. 3 .

4. The method for preparing powder metallurgy high-strength and high-toughness high-nitrogen stainless steel according to claim 1, characterized in that, The S2 cold isostatic pressing sleeve is made of silicone or polyurethane. The size of the stainless steel alloy green blank is Φ30mm-Φ2000mm. The pressing pressure is 150-300MPa and the holding time is 60-180s.

5. The method for preparing powder metallurgy high-strength and high-toughness high-nitrogen stainless steel according to claim 1, characterized in that, In S3, the nitrogen atmosphere for nitrogen sintering is nitrogen, with a nitrogen pressure of 1.0-3.0 MPa, a sintering temperature of 900-1100℃, a heating rate of 2-10℃ / min, a holding time of 1-3 h, a nitrogen purity ≥99.9%, and a nitrogen flow rate controlled at 0.5-5 m³ / min. 3 Between / min.

6. The method for preparing powder metallurgy high-strength and high-toughness high-nitrogen stainless steel according to claim 1, characterized in that, The density of the high-nitrogen stainless steel billet obtained in S3 is <90%, and the nitrogen content in the high-nitrogen stainless steel billet is 0.4-1.0 wt.%.

7. The method for preparing powder metallurgy high-strength and high-toughness high-nitrogen stainless steel according to claim 1, characterized in that, The densification sintering atmosphere in S4 is nitrogen, with a nitrogen pressure of 5.0-20.0 MPa, a sintering temperature of 1100-1300℃, a holding time of 2-4 h, a nitrogen purity of ≥99.9%, and a nitrogen flow rate controlled at 0.5-5 m³ / h. 3 Between / min.

8. The method for preparing powder metallurgy high-strength and high-toughness high-nitrogen stainless steel according to claim 1, characterized in that, The high-nitrogen stainless steel sintered billet obtained in S4 has a density ≥95% and a nitrogen content ≥0.5wt.%.

9. The method for preparing powder metallurgy high-strength and high-toughness high-nitrogen stainless steel according to claim 1, characterized in that, Hot deformation processing in S5 includes rolling, extrusion, and forging. The temperature of hot deformation processing is 1200-1350℃, and the deformation amount is ≥30%.

10. The method for preparing powder metallurgy high-strength and high-toughness high-nitrogen stainless steel according to claim 1, characterized in that, The high-nitrogen stainless steel obtained in S5 has a density >99%, a hardness of 20-40 HRC, a nitrogen content >0.5 wt.%, an average grain size <10 μm, a tensile strength >1000 MPa, a yield strength >400 MPa, a yield-to-tensile ratio >0.6, an elongation >10%, and a strength-ductility product >20 GPa.

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