超级奥氏体不锈钢凝固和热加工中的多尺度组织调控

n346Super austenitic stainless steels with high Mo and N concentrations hold excellent corrosion resistance, and consequently are successfully selected as the structural materials applied in several aggressive fields, like chemical, petrochemical, the oil and gas transformation industries as well as off-shore piping. However, they cannot be produced steadily in domestic companies and mainly rely on import due to the severe issues, such as coarse solidification microstructure, serious segregation, precipitation of brittle phases, poor thermo plasticity and strong thermal deformation resistance. Thus, in this project, multiscale investigations on microstructural evolution will be carried out during solidification and hot deformation process. By composition optimization and synergistic control in the whole chain process, the manufacturing technique to achieve a fine, uniform microstructure with less segregation and brittle phase precipitation can be generated. The composition is optimized by CALPHAD method; combing Re alloying with optimized process parameters, the solidification microstructure can be refined and the ratio of equiaxed crystal also can be improve. The solidification behavior of super austenitic stainless steel and the effect of RE on the partition ratio of solutes between solid/liquid phase are studied by unidirectional solidification technique in order to relieve the interdendritic segregation. The hot deformation and dynamic recrystallization of super austenitic stainless steels are investigated by high temperature tensile and compression experiments to explore the evolution mechanism of dendrites to equiaxed grains during thermal deformation. With the help of characterization techniques including FE-SEM, EPMA, TEM, 3DAP, EBSD, CLSM, etc., the mechanism of precipitation and dissolution of brittle σ phase will be well understood and the effective precipitation control will be proposed. The results obtained in this project will solve the key scientific issues involved in solidification and hot deformation production. Meanwhile, the new technology formed would break the current technique bottleneck and realize a steady production of super austenitic stainless steel, thus to meet the requirement of critical engineering project.

高钼高氮超级奥氏体不锈钢是针对海洋、环保、化工等苛刻腐蚀环境开发的高合金不锈钢。因凝固组织粗大、偏析严重、易析出脆性相，国内尚不能稳定生产，仍然依赖进口。因此，本课题拟开展超级奥氏体不锈钢凝固和热加工的多尺度组织演变机理研究，通过成分优化和全链条多工艺协同控制，获得晶粒细小均匀、偏析程度小和无脆性相的超级奥氏体不锈钢制备技术。采用相图计算优化成分；RE微合金化与工艺参数优化相结合，细化凝固组织，提高等轴晶率；通过定向凝固实验揭示凝固规律，利用RE的吸附作用，改变溶质的再分配，减轻偏析；采用高温拉伸和压缩实验研究热变形和组织演变规律；采用FE-SEM、EPMA、TEM、3DAP、EBSD、CLSM等先进分析方法，揭示脆性σ相析出和回溶规律，掌握σ相控制技术。解决超级奥氏体不锈钢凝固和热加工的关键科学问题，突破技术瓶颈，为高端超级奥氏体不锈钢的稳定生产提供实验基础和理论指导。

高钼高氮超级奥氏体不锈钢是针对海洋、环保、化工等苛刻腐蚀环境开发的高合金不锈钢。因凝固组织粗大、偏析严重、易析出脆性相，国内尚不能稳定生产，仍然依赖进口。本课题开展超级奥氏体不锈钢凝固和热加工的多尺度组织演变机理研究，通过成分优化和全链条多工艺协同控制，获得晶粒细小均匀、偏析程度小和无脆性相的超级奥氏体不锈钢制备技术。在现有654SMo钢成分基础上，通过增Cr增Mn降Ni的方法提高钢中溶解N的能力，从而获得7Mo-0.35N系列超级奥氏体不锈钢，达到降低成本的目的。同时添加微量RE，研究其对超级奥氏体不锈钢凝固过程中相组织演变的影响。通过本项目的研究，明晰了超级奥氏体不锈钢的平衡相图及第二相析出规律。阐明了含RE高熔点化合物与奥氏体异质晶核的形成条件，揭示了σ相的形核和长大规律。从宏观、微观和纳米尺度揭示超级奥氏体不锈钢的凝固组织形成过程和演变机理，获得了超奥钢枝晶生长和固液界面溶质再分配规律。探究了合金元素对其凝固路径、偏析行为和组织演变的影响。明确了柱状晶凝固方向、枝晶状态、σ相含量对铸造超级奥氏体不锈钢动态再结晶、热加工性及热塑性的影响规律；阐明了多道次热加工过程中变形道次和弛豫时间对超级奥氏体不锈钢微观组织演变的影响，探究了纳米尺度下奥氏体晶粒发生动态再结晶时原始晶界、孪晶界及位错的演变过程。利用预先冷变形的方式在超级奥氏体钢中引入了位错等晶格缺陷，提高其热加工性能及热塑性。确立了合适的固溶工艺窗口，并明晰了热变形参数对超级钢应变诱导析出行为的影响，获得了应变诱导σ相的析出温度范围，揭示了热变形过程中以及随后保温过程中塑性变形行为、软化行为和应变诱导析出行为，以及它们之间的相互关系。本项目研究工作的开展解决了超级奥氏体不锈钢凝固和热加工的关键科学问题，突破技术瓶颈，为高端超级奥氏体不锈钢的稳定生产提供实验基础和理论指导。