节镍型奥氏体不锈钢组织性能及控制机理研究

节镍型奥氏体不锈钢生产中合理控制其C、N含量和Cr、Ni当量,使其冷加工硬化小,拉深成形性能优异,形变诱导马氏体量少,时效开裂风险小,室温下奥氏体组织稳定是其生产应用的关键技术难点。为此,研究了不同化学成分节镍型奥氏体不锈钢在热轧、退火、冷轧退火后的金相组织及力学性能,分析了奥氏体稳定性和冷轧形变诱导马氏体相变的控制规律。结果表明:试验钢在热轧后奥氏体组织呈未完全再结晶状态,退火后奥氏体组织再结晶充分,晶粒尺寸为12～14 μm,且低的碳含量有利于改善碳化物的析出情况;试验钢冷轧变形过程中马氏体转变受奥氏体稳定性的影响,即受M_d30/50温度控制及化学成分的影响,M_d30/50温度值越高,镍当量越小,奥氏体稳定性越差,形变诱导马氏体含量越高,冷轧变形抗力越大,在退火过程越容易发生马氏体向奥氏体的逆转变,形成晶粒尺寸呈“双峰”状分布的混晶组织。因此,化学成分设计是实现节镍型奥氏体不锈钢性能的基础;同时,将本试验钢冷轧退火温度从1 080 ℃提高到1 100 ℃,且降低退火工艺速度,以延长带钢在退火炉内的时间,使奥氏体晶粒充分长大,控制晶粒尺寸为8.0～9.0级,才能保证钢卷获得良好的使用性能。

Study on microstructure,mechanical properties and control mechanism of low-nickel austenitic stainless steel

In the production of low-nickel austenitic stainless steel, the content of C and N, the equivalent of Cr and Ni were reasonably controlled, so that the steel featured with small cold hardening, excellent deep drawing performances, little amount of deformation induced martensite, and small risk of aging cracking, as well as stable austenite structure at room temperature which were the key technical difficulties in its production and application.For this purpose, the metallographic structure and mechanical properties of low-nickel austenitic stainless steel with different chemical compositions after hot rolling, annealing and cold-rolled annealing were studied, and the control laws of austenite stability and cold rolling deformation induced martensitic transformation were analyzed. The results showed that the austenite structure of the tested steel was not fully recrystallized after hot rolling, but it was fully recrystallized after annealing. The grain size was 12-14 μm, and the low carbon content was beneficial to improve the carbide precipitation. The martensitic transformation of the tested steel during the process of cold rolling deformation was affected by the austenite stability, that is, by the influence of M_d30/50 temperature control and chemical compositions. The higher the M_d30/50 temperature value was, the smaller the nickel equivalent was. Then the worse the austenite stability was, the higher the deformation induced martensite content was, the greater the cold rolling deformation resistance was, and it was more likely to reverse the transformation of martensite to austenite in the annealing process, and form a mixed grain structure with " bimodal " grain size distribution. Therefore, chemical composition design is the basis to realize the properties of low-nickel austenitic stainless steel. At the same time, the cold-rolled annealing temperature of this tested steel was increased from 1 080 ℃ to 1 100 ℃, and the annealing speed was decreased to extend the time of strip in the annealing furnace and make the austenite grains grow up fully, and the grain size was controlled to level 8.0-9.0, so as to ensure the good performance of coil.