Austenitic stainless steel is the most widely used stainless steel in industry. In austenitic stainless steel, nickel is the main austenitizing element. Its main role is to form and stabilize austenite, so that the stainless steel obtains good strength, plasticity and toughness. However, nickel has a potential sensitizing effect and has teratogenic and carcinogenic hazards to organisms; and nickel is a precious rare element, and the production of austenitic stainless steel consumes a large amount of nickel, resulting in high prices for its products Not bad, not conducive to resource-saving materials.
Therefore, the development of low-nickel and nickel-free austenitic stainless steels that replace nickel with other elements or reduce nickel content is not only conducive to reducing costs and saving expensive strategic elements, but also can improve the safety of stainless steel use, especially in the field of biomedicine. The task of medical nickel-free austenitic stainless steel is more urgent.
In recent years, nickel-free austenitic stainless steel has developed rapidly. Nickel-free Cr-Mn-N austenitic stainless steel has a high nitrogen content. While replacing nickel, it significantly improves the mechanical properties of stainless steel. Cold-rolling, cold-drawing and aging treatment after deformation and thinning. The yield strength of nickel-free Cr-Mn-N austenitic stainless steel can reach 3400MPa. It can manufacture springs, spring coils and high-pressure hoses. Among them, 0Cr19Mn10N0.5 steel has been widely used in the mining industry: 0Cr18Mn18N0.6 and 0Cr18Mn18N0.9 steel have achieved good results in the application of non-magnetic, high strength, toughness and corrosion resistance in the power plant generator positioning ring.
The 00Cr17Mn15Mo2N new medical high-nitrogen nickel-free austenitic stainless steel developed by the Chinese Academy of Sciences has passed biological performance tests such as cytotoxicity, hemolysis, sensitization, acute toxicity and genotoxicity.
The key to the development of nickel-free Cr-Mn-N austenitic stainless steel is the addition of nitrogen. The steel alloyed with nitrogen must dissolve all the nitrogen in the austenite in order to stabilize the austenite. For chromium-manganese-nitrogen stainless steel with a carbon content of less than 0.1%, when the chromium content is greater than 16%, in the absence of nickel, the nitrogen content in the steel should not be less than 0.3% to obtain a complete austenite structure. However, the solubility of nitrogen in steel is very small. At 1535℃, nitrogen can dissolve 0.0394% in steel, and only 0.015% at room temperature. Therefore, the solubility of nitrogen in steel has become the key to the development of nickel-free Cr-Mn-N stainless steel.
The high-pressure metallurgy method can effectively increase the nitrogen content in the steel. The high-nitrogen nickel-free austenitic stainless steel manufactured by the high-pressure metallurgy method invented in Bulgaria can control the nitrogen content at a relatively high level, even up to 2.1%. However, the high-pressure metallurgical method has problems of high cost and complicated operation, and it has certain difficulties in large-scale application in production. At present, the smelting of nitrogen-containing steel usually adopts methods such as pressurized electroslag remelting, back pressure casting, hot isostatic pressing melting, pressure induction melting and powder metallurgy. my country Huzhou Jiuli Special Steel Company successfully smelted 1Cr22Mn15N high-nitrogen stainless steel with argon-oxygen decarburization method, with a nitrogen content of 0.56%, and produced hot-rolled round steel for the concrete structure of marine buildings.
The high-temperature nitriding process can increase the nitrogen content of the steel surface. Under the technological conditions where the heating temperature of the Chinese Academy of Sciences is 1200℃, the holding time is 24h, and the ammonia pressure is 0.2~0.5MPa, a nitriding layer with a thickness of 2.0mm or more is obtained on the surface of nickel-free duplex stainless steel, and the surface ammonia content reaches 1.0 More than 100%, duplex stainless steel undergoes surface austenite transformation through nitriding, and a multi-phase composite stainless steel with a change in tissue gradient is obtained. The high-nitrogen austenitic stainless steel obtained by the high-temperature nitriding process has excellent corrosion resistance in 0.9% NaCl physiological saline and simulated plasma solution, while increasing strength, the elongation and shrinkage of the section remain high s level.
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