微量硼调控超级奥氏体不锈钢晶界偏析及对晶间腐蚀影响规律分析

Based on the resource and environment protection and the application of extreme corrosion environment in the ocean, the super austenite stainless steels are certain to face greater challenge with high strength and corrosion resistance. To solve the key and common problems as the decline of corrosion resistance of the material caused by element segregation and precipitated phases, we put forward introducing trace element boron to control interface composition and precipitated phases based on the idea of grain boundaries engineering, and establishing the methodology of alloy composition design and the regulation of the grain boundary composition to enhance the structure stability and pitting resistance. Main contents include researching the organizational stability and precipitation evolution law, exploring the influence mechanism of heat treatment system on microstructures and grain boundary distribution characteristics, clarifying the effect mechanism of the process parameters on the migration and diffusion of alloying elements, along with interfacial segregation, and the evolution law of the interface component characteristics. This project further reveals the mechanism of corrosion behavior affected by material, environment and boundary characteristics factors, as well as the relevance between grain boundary character distribution and the stability of passive film on the surface of super austenite stainless steel, and explains the effects of interface component regulation on the intergranular corrosion resistance from atomic level with the help of DFT, along with studying the relationship among B+Mo synergies, stability of the passive film, boundary character distribution and intergranular corrosion. These findings will provide new ideas to the application development and component optimizing of high strength, high corrosion resistance austenite stainless steel.

基于能源环保、海洋工程极端腐蚀环境对超级奥氏体不锈钢提出的“高强耐蚀”更高挑战，针对其元素偏析、析出相敏感导致材料耐蚀性降低等关键共性问题，本项目基于晶界工程思想，提出引入微量元素硼对界面成分和析出相调控，建立基于提升组织稳定性和耐点蚀性的合金成分优化设计和晶界成分调控方法。研究材料组织稳定性及析出相析出演变规律；探索热处理制度对微观组织和晶界特征分布的作用机制；阐明工艺参数对合金元素迁移、扩散及界面偏析的影响及界面成分特征的演化规律。揭示材料因素、环境因素、晶界特征对材料腐蚀行为的影响机制；阐明晶界特征分布与超级奥氏体不锈钢表面钝化膜稳定性间的关联性；结合密度泛函理论，从原子层次诠释界面成分调控对其抗晶间腐蚀能力的影响规律；探明B+Mo协同作用、钝化膜稳定性、晶界特征分布和晶间腐蚀的联系。研究成果将为高强高耐蚀奥氏体不锈钢的成分优化设计和应用技术开发提供新的思路。

超奥钢S31254凝固组织易偏析、析出敏感、热加工窗口极窄等问题制约了高品质产品的生产。针对这一瓶颈问题，首先研究了B含量对超奥钢凝固组织、析出相及热变形机制的影响，B加速凝固偏析的回溶，缩短了均质化时间，但伴随着晶粒长大；含硼试样使动态再结晶温度降低，改善超奥钢铸态热变形能力。然后研究了B对固溶过程中析出相回溶的影响，与无B试样相比，析出相回溶时间明显缩短，晶粒长大明显，优化出了析出相完全回溶且晶粒不过分长大的最佳固溶处理规范。接着对固溶态试样进行了时效处理，分析了B对超奥钢750-1100℃析出相析出的影响，B抑制静态、变形过程中析出相析出，含量40ppm时抑制σ析出效果最佳；B降低了σ相析出上限温度，有利于热塑性的提高；探讨了B的占位及分布，B易偏析于晶界，往往伴随着B-Mo共偏聚；B易溶于富Mo的Laves相，其相界面没有明显的贫铬区，含硼Laves较σ具有更好的抗晶间腐蚀性能。最后分析了B对超奥钢抗晶间腐蚀能力的影响，探讨了硼含量、变形、固溶处理、时效处理、腐蚀液类型、介质温度、晶界特征钝化膜与耐蚀性的关系，探索了B晶界对Mo、Cr等合金元素抑制偏析机制，原子层次分析了B+Mo+空位协同共存于晶界的可能性，解释了含硼富钼Laves细小、相界面无明显贫铬区的原因；提出了低温B晶界扩散预处理，促进B晶界富集的新思路；阐明了B促进Cr向表面的扩散、形成富铬氧化物，增加超奥钢的钝化膜致密度，提高晶界、整体耐蚀性的机制；设计出冷变形+高温短时退火获取高密度退火孪晶，利用孪晶、B组合提升材料强韧性、耐蚀性的方法；该加B超奥钢抑制析出相、改善热塑性的方法已用于太钢工业化生产。由于固溶强化效果提高强度有限、且高温晶粒易长大问题尚未根本解决，后续还需进一步借助析出强化改善力学性能和耐蚀性能。