A high-strength, high-hardness, and extremely low-magnetic stainless steel and its manufacturing method

Abstract

This invention belongs to the technical field of stainless steel materials and discloses an ultra-high strength, high hardness, and extremely low magnetic stainless steel and its preparation method. The steel, by mass percentage, comprises: carbon (C): 0.08%~0.18%; silicon (Si): 0.20%~1.00%; manganese (Mn): 19.00%~24.00%; phosphorus (P): ≤0.040%; sulfur (S): ≤0.005%; chromium (Cr): 16.00%~18.00%; nickel (Ni): 2.00%~4.00%; copper (Cu): 0.10%~0.50%; molybdenum (Mo): 0.10%~0.50%; nitrogen (N): 0.50%~0.65%; titanium (Ti): 0.05%~0.15%; niobium (Nb): 0.08%~0.25%, with the remainder being iron and unavoidable impurities. This invention, through reasonable component ratio design and production process design, obtains ultra-high strength, high hardness and extremely low magnetic stainless steel, wherein the tensile strength is ≥1000Mpa, the yield strength is ≥900Mpa, the hardness is HRC≥38, and the remanence of magnetic detection is ≤1nT. The preparation method of this invention for producing ultra-high strength, high hardness and extremely low magnetic stainless steel has a high yield, with a total forging yield of ≥85%.

Description

Technical Field

[0001] This invention belongs to the field of stainless steel material technology, specifically relating to an ultra-high strength, high hardness, and extremely low magnetic stainless steel. This stainless steel can be widely used in aerospace, non-magnetic buildings, medical devices, electronic equipment, and other fields that have strict requirements for material strength, hardness, and magnetism. Background Technology

[0002] In modern industry, with continuous technological advancements, the performance requirements for stainless steel materials are becoming increasingly stringent. In aerospace, medical, and construction fields, components need extremely high strength and hardness to withstand extreme working environments and mechanical stresses. Simultaneously, to avoid interference with precision electronic equipment, the material must possess extremely low magnetism. In the medical device field, especially in equipment such as magnetic resonance imaging (MRI), stainless steel components must also meet the requirement of low magnetism, while possessing good strength and hardness to ensure the stability and durability of the devices. In the construction field, some key research laboratories require extremely low magnetism. To avoid interference from the building itself with the magnetic field, all building materials must be of extremely low magnetic properties. Good corrosion resistance is also required to prevent rusting and magnetization, while extremely high strength and hardness are needed to ensure structural stability and prevent the impact of natural disasters on the building.

[0003] Currently, traditional stainless steel materials struggle to simultaneously meet the comprehensive performance requirements of ultra-high strength, high hardness, and extremely low magnetism. While some high-strength stainless steels possess high strength and hardness, they exhibit significant magnetism, such as martensitic stainless steels (e.g., 17-4PH, 15-5PH, 95Cr18), making them unsuitable for magnetically sensitive applications. Conversely, some low-magnetic stainless steels (e.g., 304H, 316H) fall short in terms of strength and hardness, failing to meet the demands of high-intensity working environments. Therefore, developing a stainless steel material and its manufacturing method that simultaneously possesses ultra-high strength, high hardness, and extremely low magnetism is of significant practical and social importance.

[0004] To address these issues, relevant documents were consulted. Chinese Patent No. CN 119016649 A disclosed a "forging method for high-nitrogen non-magnetic stainless steel." This invention designs a three-stage forging process: two high-temperature forging stages followed by a warm forging stage, and then water cooling. It controls the heating and forging temperatures to improve the surface quality and microstructure uniformity of the forging. However, this invention does not impose any restrictions on the compression ratio during forging. Too low a compression ratio during high-temperature forging can easily lead to core shrinkage cavities, while too high a compression ratio during warm forging can easily cause forging cracks, resulting in a sharp decrease in yield. Too low a compression ratio during warm forging cannot achieve the goal of increasing strength. Furthermore, the low temperature of this invention carries the risk of reducing the yield of the forged material.

[0005] Chinese patent with publication number CN 117660833 A discloses "a non-magnetic high-strength steel plate and its manufacturing method". This invention enables the material to have high strength, good low-temperature toughness and low-temperature non-magnetic properties through design and process optimization. However, the strength and hardness of this material are limited, and this material does not have corrosion resistance.

[0006] Chinese patent CN 118581388 A discloses "a method for manufacturing ultra-low temperature high-strength non-magnetic austenitic stainless steel bars". Through forging process design, it solves the problems of multiple forging fires, easy surface cracking and poor microstructure uniformity of ultra-low temperature high-strength non-magnetic austenitic stainless steel bars. However, the steel involved in this invention has a high nickel content and a low nitrogen content, and the material itself has low hardness, so it is not of much reference significance to this invention.

[0007] Chinese patent with publication number CN 118389952 A discloses "an economical non-magnetic high-strength and high-corrosion-resistant stainless steel and its preparation process". This invention mainly obtains non-magnetic and high-strength plates through composition design and process control. However, the strength of the material obtained by this invention is limited, and the design of Md30 cannot achieve the purpose of extremely low magnetism.

[0008] Chinese patent CN 114196893 A discloses "an ultra-wear-resistant and corrosion-resistant stainless steel for making coal washing screens and its preparation method". This invention is mainly aimed at wear-resistant screens and improving the material life. However, it cannot meet the requirements of extremely low magnetic properties, and its strength and hardness are also limited. Summary of the Invention

[0009] To address the problems in the prior art, the present invention aims to provide an ultra-high strength, high hardness, and extremely low magnetic stainless steel and its manufacturing method, which has the following advantages: 1) The stainless steel combines ultra-high strength, high hardness, and extremely low magnetism; 2) Excellent corrosion resistance, and can meet the requirement of not rusting for 120 hours in a neutral salt spray test; 3) Certain plastic deformation capacity, with an elongation rate greater than 15%; 4) High material processing yield, with a total forging yield ≥85%.

[0010] To achieve the above objectives, the present invention adopts the following technical solution.

[0011] An ultra-high strength, high hardness, and extremely low magnetic stainless steel, with the following chemical composition by mass percentage: carbon (C): 0.08%~0.18%; silicon (Si): 0.20%~1.00%; manganese (Mn): 19.00%~24.00%; phosphorus (P): ≤0.040%; sulfur (S): ≤0.005%; chromium (Cr): 16.00%~18.00%; nickel (Ni): 2.00%~4.00%; copper (Cu): 0.10%~0.50%; molybdenum (Mo): 0.10%~0.50%; nitrogen (N): 0.50%~0.65%; titanium (Ti): 0.05%~0.15%; niobium (Nb): 0.08%~0.25%, with the remainder being iron and unavoidable impurities.

[0012] Preferably, the chemical composition, by mass percentage, includes: carbon (C): 0.10%~0.15%; silicon (Si): 0.30%~0.80%; manganese (Mn): 20.00%~23.00%; phosphorus (P): ≤0.038%; sulfur (S): ≤0.003%; chromium (Cr): 16.50%~17.50%; nickel (Ni): 2.50%~3.50%; copper (Cu): 0.15%~0.30%; molybdenum (Mo): 0.15%~0.30%; nitrogen (N): 0.53%~0.60%; titanium (Ti): 0.07%~0.13%; niobium (Nb): 0.10%~0.20%, with the remainder being iron and unavoidable impurities.

[0013] Preferably, the ultra-high strength, high hardness, and extremely low magnetic stainless steel has a single austenitic microstructure. To achieve this, FN is controlled to be ≤-35; no deformed martensite is generated during cold working, and Md30 is controlled to be ≤-230℃; a large work hardening rate is achieved, and CWH is controlled to be ≥300. More preferably, FN is ≤-38, Md30 is ≤-250℃, and CWH is ≥330.

[0014]

[0015] The element symbols in formulas (1), (2), and (3) represent the content of each element in the chemical composition of steel, expressed as a percentage by mass.

[0016] Preferably, the ultra-high strength, high hardness, and extremely low magnetic stainless steel has a tensile strength ≥1000 MPa, a yield strength ≥900 MPa, a hardness HRC ≥38, a magnetic remanence ≤1 nT, a relative permeability of approximately ≤1.002, and more preferably, it can meet the requirement of not rusting for 120 hours in a neutral salt spray test, has an elongation greater than 15%, and a reduction of area of ​​not less than 30%.

[0017] The design concept of the elements in this invention: C: 0.08%~0.18%, carbon acts as an interstitial atom solid solution to strengthen the matrix, improving the strength and hardness of steel. It also assists manganese and nitrogen in stabilizing the austenitic structure. However, excessively high content (e.g., exceeding 0.2%) easily leads to the formation of Cr. 23 Carbides such as C6 increase the tendency for intergranular corrosion, so a reasonable range is needed to balance strength and corrosion resistance.

[0018] Si: 0.20%~1.00%, a deoxidizer used in steelmaking to remove oxide impurities and improve the purity of steel. A small amount of silicon dissolves in austenite, slightly increasing strength and hardness. Excessive content (>1%) may reduce toughness; therefore, it is controlled below 1.00% to avoid negative effects.

[0019] Mn: 19.00%~24.00%. Manganese, as a substitute for nickel, expands the austenite phase region, ensuring the formation of a single austenite structure at room temperature (replacing the role of high nickel in traditional stainless steel). It increases the work hardening rate of the material, significantly improving strength and hardness. In high-nitrogen stainless steel, manganese also plays a role in dissolving nitrogen. However, high manganese content may lead to an increased tendency for rolling hot cracking, so the upper limit must be reasonably controlled.

[0020] P: ≤0.040%, a harmful element. Although it can slightly improve strength, it tends to agglomerate at grain boundaries, exacerbating cold brittleness (especially in low-temperature environments). The upper limit must be strictly controlled.

[0021] S: ≤0.005%, harmful impurities: form low-melting-point sulfides (such as FeS), leading to thermal embrittlement, deteriorating processing performance and corrosion resistance. The requirement here is ≤0.005%, reflecting the high purity smelting requirements.

[0022] Cr: 16.00%~18.00%, a basic element for corrosion resistance, forming a dense Cr2O3 passivation film to resist corrosion from the atmosphere, water, and oxidizing acids (such as nitric acid). Chromium is also an important element for increasing nitrogen solubility, but chromium shrinks the austenite phase region (the opposite of manganese) while promoting the formation of δ-ferrite.

[0023] Ni: 2.00%~4.00%, a nickel austenite stabilizing element that can improve the toughness of stainless steel and reduce the tendency of brittleness caused by high manganese. However, higher nickel content will reduce the hardness of the material, so the upper limit is strictly limited.

[0024] Cu: 0.10%~0.50%. Copper is a relatively weak austenite-forming element, which plays a certain role in austenite strengthening. It also improves the ability to deform at low temperatures and increases the yield of low-temperature processing. However, higher copper content will reduce the hardness of the material, so the upper limit is strictly limited.

[0025] Mo (0.10%~0.50%) enhances the stability of the passivation film and improves corrosion resistance, especially inhibiting pitting corrosion in chloride-containing environments. However, molybdenum itself is expensive, and excessive amounts can reduce its ability to undergo hot working deformation.

[0026] Nitrogen (N): Between 0.50% and 0.65%, nitrogen is the main element for improving strength and hardness. It significantly expands the austenite phase region and is more effective than nickel (smaller atomic radius and higher solid solubility). It has a significant interstitial solid solution strengthening effect and can also improve the resistance of stainless steel to chloride ion corrosion. However, excessive nitrogen will increase the difficulty of smelting.

[0027] Titanium (Ti): 0.05%~0.15%; Niobium (Nb): 0.08%~0.25%. Titanium and niobium function similarly in materials, both forming TiC, TiN, NbC, and NbN with carbon and nitrogen in steel. These tiny precipitates act as heterogeneous nucleation sites during steel solidification and heat treatment, inhibiting austenite grain growth and thus achieving grain refinement. Fine-grained structures improve the strength and toughness of materials, especially important for high-nitrogen, high-manganese steels that rely on the strength of the austenite matrix. The synergistic effect of titanium and niobium maximizes grain refinement. Adding only one will not achieve the desired grain refinement effect, while excessive addition of one may lead to overly large precipitates, reducing the material's plasticity and corrosion resistance.

[0028] In this invention, FN is a commonly used method for calculating δ-ferrite. Ferrite is a magnetic microstructure. During the smelting and casting process, materials easily produce small amounts of ferrite. Even a very small amount of ferrite can affect the material's magnetism. When the FN value is negative, it indicates that the amount of δ-ferrite formed is very low. To achieve a finished product completely free of δ-ferrite, this invention controls FN ≤ ​​-35. Md30 (°C) can determine the tendency of austenitic stainless steel to form deformed martensite after cold working. The lower the value, the less likely deformed martensite is to form. Deformed martensite is also a magnetic microstructure. This invention controls Md30 ≤ -230°C to ensure that no deformed martensite is formed after product processing. Simultaneously, effective control of FN and Md30 ensures that the material has a single austenitic microstructure after processing, with a remanence ≤ 1 nT.

[0029] CWH is an important parameter for work hardening. The higher the value, the more obvious the work hardening. This invention controls CWH ≥ 300, which can ensure that the material has higher strength and hardness after forging, with tensile strength ≥ 1000 MPa, yield strength ≥ 900 MPa, and hardness HRC ≥ 38.

[0030] Meanwhile, the high chromium and nitrogen content and stable austenite also give the material good corrosion resistance and a certain degree of plasticity. It can withstand 120 hours of neutral salt spray test without rusting and has an elongation of more than 15%.

[0031] Through the rational design of the forging process, the forged products not only have extremely high strength and hardness, but also a high yield, with a total forging yield of ≥85%.

[0032] This invention also provides a method for preparing any of the above-mentioned ultra-high strength, high hardness, and extremely low magnetic stainless steel. The raw materials are smelted, cast, forged, forged to enhance strength, and peeled to obtain the ultra-high strength, high hardness, and extremely low magnetic stainless steel. The smelting process includes electric furnace smelting, AOD smelting, and LF refining; casting is performed by die casting; forging is performed by forging hammer; and forging to enhance strength is performed by radial forging machine.

[0033] Preferred method: electric furnace smelting: to ensure that the final P content meets ≤0.040%, scrap stainless steel with P≤0.030% is used for smelting. After the molten steel is melted, 5kg~10kg of ferrosilicon is added per ton of molten steel for reduction. Then the slag is removed and the steel is tapped at a tapping temperature of 1630℃~1680℃. AOD smelting is divided into three stages: oxygen blowing decarburization, reduction, and refining. During oxygen blowing decarburization, the temperature is controlled at 1700℃~1750℃ for 40-60 minutes, and the slag basicity is 2.8-3.5. During reduction, 30-40 kg of ferrosilicon is added per ton of molten steel, the temperature is 1600℃~1650℃, the time is 10-18 minutes, and the slag basicity is controlled at 1.2-2.5. During refining, lime is added to rebuild the slag, the refining time is 6-12 minutes, the temperature is maintained at 1580℃~1640℃, and the slag basicity is 1.5-2.2. After refining, the molten steel is removed for the next step. LF Refining: After the molten steel arrives at the station, adjust the temperature of the molten steel to 1600℃~1630℃, add the corresponding ferroalloy to fine-tune the composition of the molten steel to the target range, and then perform soft blowing for 10min~15min. After the soft blowing is completed, adjust the temperature to 1550℃~1580℃ and hoist the ladle for casting. Casting: The molten steel from the previous step is cast in a mold using a low superheat, with a superheat of 40℃~60℃, to obtain a steel billet. The preferred billet size is 18 inches to 22 inches, corresponding to a cross-sectional size of 380mm*380mm to 640mm*640mm. If the billet size is too large, the forging compression ratio will be too high, which will easily lead to cracking. If the size is too small, the compression ratio will be difficult to meet the requirements. Billets within this size range are more likely to ensure good yield while meeting performance requirements. Forging: After the steel billet is heated and held at a certain temperature in the previous step, it is forged by a forging hammer after being taken out of the furnace; Forging reinforcement: After the previous forging is reheated and held at the same temperature, it is forged using a radial forging machine to enhance its strength.

[0034] Peeling process: After forging, the surface is peeled to remove the oxide scale.

[0035] Preferably, during AOD smelting, nitrogen blowing and stirring are continuously used from the beginning of the refining period to increase the nitrogen content of the molten steel. During the LF composition adjustment period, if the nitrogen content is insufficient, the corresponding chromium nitride alloy is added.

[0036] Preferably, the dimensions of the ingot cast by die casting should meet the requirement that the compression ratio after forging is ≥7, where the compression ratio = original cross-sectional area before forging / corresponding cross-sectional area after forging.

[0037] Preferably, the forging process is divided into multiple forging passes, with a compression ratio ≤ 5 for each pass, an initial forging temperature of 1200℃~1250℃, and a final forging temperature of 1000~1050℃. When the compression ratio reaches 5 or the temperature is too low, the steel ingot is immediately reheated in the furnace, and then forged a second time. Preferably, the heating temperature is controlled at 1260℃~1300℃, the cold ingot is held in the furnace for 2.0h~3.5h, and then reheated in the furnace at 1260℃~1300℃ for 1.0h~2.5h. By adopting the temperature range of the above technical solution, cracking during forging can be further avoided; at the same time, within the aforementioned holding time range, it is more suitable for steel ingots of the dimensions described in this invention and can ensure uniform internal and external calcination and avoid overheating, further preventing cracking and thus comprehensively improving the yield.

[0038] Preferably, during the forging strengthening process, the forging compression ratio is strictly controlled between 1.30 and 1.70. A high compression ratio easily leads to cracking, while a low compression ratio results in insufficient strength. The material heating temperature is controlled at 750℃~850℃, with a holding time of 3.0h~5.0h. Then, the material is forged again, with each initial forging temperature at 700℃~800℃ and the final forging temperature at 500℃~550℃. If the temperature is too low, the material is immediately returned to the furnace for holding for 30min~40min; holding for too long will reduce hardness until the final dimensional requirements are met. By adopting the temperature range described above, the strengthening effect can be ensured while further preventing cracking during forging. Simultaneously, within the specified holding time range, it is more suitable for steel ingots of the dimensions described in this invention and ensures uniform internal and external calcination, avoiding overheating and further preventing cracking, thereby comprehensively improving the yield.

[0039] Compared with existing technologies, the ultra-high strength, high hardness, and extremely low magnetic stainless steel and its preparation method described in this invention have the following significant advantages: (1) Through the design and control of FN and Md30 (°C), the stainless steel of the present invention has extremely low magnetism; through the design and control of CWH and the forging process design, the stainless steel of the present invention has extremely high strength and hardness; thus, the stainless steel has extremely high strength, hardness and extremely low magnetism; tensile strength ≥1000Mpa, yield strength ≥900Mpa, hardness HRC≥38, magnetic remanence ≤1nT; (2) The high chromium and nitrogen content gives the stainless steel excellent corrosion resistance, and it can withstand at least 120 hours of neutral salt spray test without rusting. (3) The stable austenitic microstructure gives the stainless steel a certain plastic deformation capacity and an elongation of more than 15%; (4) Control parameters such as forging temperature, time, and forging compression ratio to achieve a high material yield and a total forging yield of ≥85%. Attached Figure Description

[0040] Figure 1 The metallographic structure of Embodiment 1 of the present invention is a uniform and single austenite.

[0041] Figure 2 The metallographic structures of Comparative Examples 1-2 of this invention are austenite and a small amount of δ-ferrite.

[0042] Figure 3 This is an ultra-high strength, high hardness, and extremely low magnetic stainless steel sample manufactured in Example 2 of the present invention. Detailed Implementation

[0043] To better clarify and understand the objectives, process solutions, and advantages of this invention, the technical solutions and implementation methods of this invention will be further described clearly, completely, and in detail below through specific embodiments and in conjunction with the accompanying drawings. It should be understood that the embodiments described in this invention are implemented under the premise of the technical solutions of this invention, providing detailed implementation methods and specific operating procedures, but are only some embodiments of this invention, not all embodiments. The specific implementation methods described are limited to illustrating and explaining this invention and do not limit this invention. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0044] Unless otherwise specified, the experimental methods and conditions used in the embodiments of this invention are conventional methods and conditions. The materials, reagents, instruments, and equipment used in the embodiments, unless otherwise specified, are all conventional substances or equipment known to those skilled in the art and can be obtained commercially or prepared by conventional methods. The reaction conditions described in the invention's content can all achieve the stated reactions and obtain the desired products. Due to space limitations, some embodiments are listed below to further illustrate the advantages of the technical solution of this invention.

[0045] Example 1

[0046] The raw materials undergo smelting, casting, forging, forging reinforcement, and peeling processes to obtain the ultra-high strength, high hardness, and extremely low magnetic stainless steel. The specific steps and parameters are as follows: Electric arc furnace smelting: Smelting is carried out in a 40t electric arc furnace, using scrap stainless steel with P: 0.025%. After the molten steel melts, 220kg of ferrosilicon is added per ton of molten steel for reduction. Then the slag is skimmed off and the steel is tapped at a temperature of 1645℃, yielding 35 tons of molten steel. AOD smelting is divided into three stages: oxygen decarburization, reduction, and refining. During oxygen decarburization, the temperature is 1700℃~1750℃ for 48 minutes, and the slag basicity is 2.9. During reduction, 1225 kg of ferrosilicon is added, the temperature is 1600℃~1650℃ for 15 minutes, and the slag basicity is controlled at 1.8. During refining, lime is added to rebuild the slag; the amount of lime added is controlled by the basicity. Refining takes 9 minutes, maintaining a temperature of 1580℃~1640℃ and a slag basicity of 2.0. After refining, the molten steel is removed for the next step. During AOD smelting, nitrogen blowing is used for stirring from the beginning of the refining stage.

[0047] LF Refining: After the molten steel arrives at the station, adjust the temperature of the molten steel to 1614℃, fine-tune the composition to meet the requirements, add 40kg of chromium nitride alloy to supplement the nitrogen content, soft blowing time is 13min, and then the steel is hoisted out of the ladle at a temperature of 1565℃.

[0048] Casting: The molten steel from the previous step is cast in a mold, with the superheat controlled at 40℃~60℃, to obtain a steel ingot billet. The steel ingot billet is 18 inches in size, with a corresponding cross-sectional size of 380mm*380mm, which can be used to forge round steel with a diameter of less than 160mm.

[0049] Forging: After the steel billet is heated and held at the previous temperature, it is forged using a forging hammer to produce 130mm diameter round bars. The total compression ratio for forging is 10.9. The forging process consists of two passes. The first forging produces 220mm diameter round bars with a compression ratio of 3.8, an initial forging temperature of 1243℃, and a final forging temperature of 1050℃. The second forging produces 130mm diameter round bars with a compression ratio of 2.9, an initial forging temperature of 1234℃, and a final forging temperature of 1045℃. The heating temperature is controlled between 1260 and 1300℃. The cold ingot is held in the furnace for 2.8 hours, and then held in the furnace for 1.5 hours after the second reheat.

[0050] Forging Enhancement: After reheating and holding the forgings from the previous step, the forgings are subjected to radial forging for enhancement. The forging diameter is 110mm round steel with a compression ratio of 1.4. The material heating temperature is controlled at 750℃~850℃, holding time is 3.8h, initial forging temperature is 780℃, final forging temperature is 550℃, followed by a second furnace loading and holding at 750℃~850℃ for 35min. The second initial forging temperature is 770℃, and the final forging temperature is 540℃, achieving the required dimensions.

[0051] Peeling process: After forging, the surface is peeled to remove the oxide scale. The peeling amount is 1mm.

[0052] Other cases

[0053] The operations of Examples 1, 1-1, 1-2, 1-3, 1-4, 2, 2, 3, 3-1, and 3-2 are similar to those of Example 1, but the process parameters are slightly different.

[0054] Table 1 shows the composition results and calculated MD30, CWH, and Fn for each case. The differences between Comparative Examples 1-1, 1-2, 1-3, and 1-4 and Example 1 lie only in their composition design; the production processes remain within the design range. Comparative Example 1-1 only adds a higher concentration of Ti; the calculated Fn in Comparative Example 1-2 does not meet the design requirement of ≤-35; the calculated MD30 in Comparative Example 1-3 does not meet the design requirement of ≤-230; and the calculated CWH in Comparative Example 1-4 does not meet the design requirement of ≥300. The remaining examples and comparative examples directly meet the design range requirements.

[0055] Table 2 shows the process control parameters for each case. The process parameters for Example 1 and Comparative Examples 1-1, 1-2, 1-3, and 1-4 are similar and all within the design range. The difference between Comparative Example 2 and Example 2 is that Comparative Example 2 uses two forging processes, with the compression ratio of each forging exceeding 5, while Example 2 uses two forging processes, with the compression ratio of each forging within 5. The difference between Comparative Examples 3-1 and 3-2 and Example 3 is that the total compression ratio of Comparative Example 3-1 is only 5.3, not exceeding 7, while the forging reinforcement compression ratio of Comparative Example 3-2 is 1.86, exceeding 1.7, while Example 3 is within the design range.

[0056]

[0057]

[0058] Test case

[0059] Table 3 shows the forging yield and low-magnification test results for each case. The total forging yields of Comparative Examples 1-1, 2, and 3-2 are significantly lower. The lower yield in Comparative Example 1-1 is due to the high Ti content in the composition; in Comparative Example 2, it's due to the excessively high compression ratio during the forging process; and in Comparative Example 3-2, it's due to the excessively high compression ratio during the forging reinforcement process. The severe central shrinkage cavity in Comparative Example 3-1 is due to the forging compression ratio not reaching 7.

[0060]

[0061] Performance testing was conducted on each case: The test methods for tensile strength, yield strength, elongation, and reduction of area are as follows: GB / T 228.1-2021 Metallic materials - Tensile testing - Part 1: Test method at room temperature.

[0062] The hardness HRC test method is: GB / T 230.1-2018 Metallic materials Rockwell hardness test - Part 1 Test method.

[0063] The salt spray test method is GB / T10125-2021 Artificial Atmosphere Corrosion Test - Salt Spray Test.

[0064] The method for detecting residual magnetism after material processing is as follows: various types of fluxgate magnetometers are used for measurement. The present invention uses a CTM-6W fluxgate magnetometer. The static magnetic field of the test environment is <10nT, the magnetic field fluctuation in the test area is less than +0.1nT, and the distance between the test position and the material test surface is less than 3cm.

[0065] Magnetic permeability test method: GB / T35690-2017 Method for measuring the relative magnetic permeability of weakly magnetic materials.

[0066] Table 4 shows the performance test results for each case. Comparative Examples 1-1 and 1-4 show significantly lower strength and hardness. The difference between Comparative Example 1-1 and Comparative Example 1-4 is due to the addition of a higher amount of Ti in its composition, while the difference between Comparative Example 1-4 and Comparative Example 1-3 is due to a lower calculated CWH. Comparative Examples 1-2 and 1-3 show higher remanence and relative permeability after cold working. Comparative Example 1-2 shows a higher calculated Fn, indicating the presence of trace amounts of δ-ferrite, such as... Figure 2 As shown, the MD30 calculated from the components of Comparative Examples 1-3 is relatively high, making it easy to generate trace amounts of deformed martensite.

[0067] In Table 4, all properties of Examples 1, 2, and 3 meet the design requirements, namely high strength, high hardness, and extremely low magnetism, and the metallographic structure exhibits a single austenitic structure, such as... Figure 1 As shown, Figure 1 The image shows the metallographic structure of Example 1. While the remanence of the austenitic non-magnetic stainless steel 316H meets the requirements, its strength and hardness are both relatively low. Although the strength and hardness of the high-hardness martensitic aging stainless steel 17-4PH meet the requirements, its remanence and relative permeability are extremely high. Therefore, neither 316H nor 17-4PH can simultaneously meet the requirements of high strength, high hardness, and extremely low magnetism.

[0068]

[0069] In summary, the ultra-high strength, high hardness, and extremely low magnetic stainless steel produced by this invention can simultaneously possess ultra-high strength, high hardness, and extremely low magnetism, while also exhibiting excellent corrosion resistance and a certain degree of plasticity, and a high yield rate, thus solving the urgent needs of aerospace, medical, construction, and other fields for this type of material.

[0070] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Other variations and modifications may be made without departing from the technical solutions described in the claims.

Claims

1. A method for manufacturing ultra-high strength, high hardness, and extremely low magnetic stainless steel, characterized in that, The raw materials are smelted, cast, forged, forged to increase strength, and peeled to obtain the ultra-high strength, high hardness, and extremely low magnetic stainless steel. The chemical composition of stainless steel, by mass percentage, includes: carbon (C): 0.08%~0.18%; silicon (Si): 0.20%~1.00%; manganese (Mn): 19.00%~24.00%; phosphorus (P): ≤0.040%; sulfur (S): ≤0.005%; chromium (Cr): 16.00%~18.00%; nickel (Ni): 2.00%~4.00%; copper (Cu): 0.10%~0.50%; molybdenum (Mo): 0.10%~0.50%; nitrogen (N): 0.50%~0.65%; titanium (Ti): 0.05%~0.15%; niobium (Nb): 0.08%~0.25%, with the remainder being iron and unavoidable impurities; among which: The smelting process includes electric furnace smelting, AOD smelting, and LF refining; steel ingots are cast using die casting; forging is performed using a forging hammer; and forging is performed using a radial forging machine to increase strength. The forging process is divided into multiple forging passes, with a compression ratio ≤ 5 for each forging. The initial forging temperature is 1200℃~1250℃, and the final forging temperature is 1000℃~1050℃. When the compression ratio reaches 5, the steel ingot is reheated in the furnace. After the reheating is completed, a second forging is performed. The heating temperature is controlled at 1260℃~1300℃. The cold steel ingot is held in the furnace for 2.0h~3.5h, and held in the furnace for 1.0h~2.5h. During the forging process to increase strength, the forging compression ratio is strictly controlled between 1.3 and 1.7; the material heating temperature is controlled between 750℃ and 850℃, and the holding time is 3.0h to 5.0h; for the forging after exiting the furnace, the initial forging temperature is 700℃ to 800℃, and the final forging temperature is 500℃ to 550℃, followed by a second furnace holding for 30min to 40min, until the final dimensional requirements are met.

2. The method for manufacturing ultra-high strength, high hardness, and extremely low magnetic stainless steel according to claim 1, characterized in that, The chemical composition, by mass percentage, includes: carbon (C): 0.10%~0.15%; silicon (Si): 0.30%~0.80%; manganese (Mn): 20.00%~23.00%; phosphorus (P): ≤0.038%; sulfur (S): ≤0.003%; chromium (Cr): 16.50%~17.50%; nickel (Ni): 2.50%~3.50%; copper (Cu): 0.15%~0.30%; molybdenum (Mo): 0.15%~0.30%; nitrogen (N): 0.53%~0.60%; titanium (Ti): 0.07%~0.13%; niobium (Nb): 0.10%~0.20%, with the remainder being iron and unavoidable impurities.

3. The method for manufacturing ultra-high strength, high hardness, and extremely low magnetic stainless steel according to claim 1 or 2, characterized in that, The stainless steel has a single austenitic microstructure; FN≤-35, Md30≤-230℃, CWH≥300; wherein, FN = 3.0 * Creq - 2.5 * Nieq - 12, where Creq = Cr + 1.5 * Si + Mo + 0.5 * Nb; Nieq = 30 * (C + N) + 0.5 * Mn + Ni; Md30 / ℃=551-462*(C+N)-9.2*Si-8.1*Mn-29*(Ni+Cu)-13.7*Cr-18.5*Mo; CWH=25*(Ni+Cu)^2-63.5*(Ni+Cu)+67.41*(C+N)-0.894*Mn+10.69*Si-0.923*Cr+492.

75.

4. The method for manufacturing ultra-high strength, high hardness, and extremely low magnetic stainless steel according to claim 1, characterized in that, The stainless steel has a tensile strength ≥1000 MPa, a yield strength ≥900 MPa, a hardness HRC ≥38, and a remanence ≤1 nT.

5. The method for manufacturing ultra-high strength, high hardness, and extremely low magnetic stainless steel according to claim 1, characterized in that, Electric furnace smelting: Smelting is carried out using scrap stainless steel with P≤0.030%. After the molten steel is melted, 5kg~10kg of ferrosilicon is added per ton of molten steel for reduction. Then the slag is skimmed off and the steel is tapped at a temperature of 1630℃~1680℃. AOD smelting is divided into three stages: oxygen blowing decarburization, reduction, and refining. During oxygen blowing decarburization, the temperature is controlled at 1700℃~1750℃ for 40-60 minutes, and the slag basicity is 2.8~3.

5. During reduction, 30-40 kg of ferrosilicon is added per ton of molten steel, the temperature is 1600℃~1650℃, the time is 10-18 minutes, and the slag basicity is controlled at 1.2~2.

5. During refining, lime is added to rebuild the slag, the refining time is 6-12 minutes, the temperature is maintained at 1580℃~1640℃, and the slag basicity is 1.5~2.

2. After refining, the molten steel is removed for the next step. LF Refining: After the molten steel arrives at the station, adjust the temperature of the molten steel to 1600℃~1630℃, add the corresponding ferroalloy to fine-tune the composition of the molten steel, and then perform soft blowing for 10min~15min. After the soft blowing is completed, adjust the temperature to 1550℃~1580℃ and hoist the ladle for casting. Casting: The molten steel from the previous step is cast in a mold using a low superheat of 40℃~60℃ to obtain a steel ingot billet; Forging: After the steel billet is heated and held at a certain temperature in the previous step, it is taken out of the furnace and forged using a forging hammer; Forging reinforcement: After the previous forging is reheated and held at the same temperature, it is taken out of the furnace and reinforced by radial forging machine.

6. The method for manufacturing ultra-high strength, high hardness, and extremely low magnetic stainless steel according to claim 5, characterized in that, During AOD smelting, nitrogen blowing and stirring are used continuously from the beginning of the refining period.

7. The method for manufacturing ultra-high strength, high hardness, and extremely low magnetic stainless steel according to claim 1, characterized in that, The dimensions of the cast steel ingot should meet the requirement that the compression ratio after forging is ≥7, where the compression ratio = original cross-sectional area before forging / corresponding cross-sectional area after forging; the steel ingot is a square billet with dimensions of 18 inches to 22 inches, corresponding to cross-sectional dimensions of 380mm*380mm to 640mm*640mm.

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