双相不锈钢热变形过程中动态软化行为的研究

Hot deformation within the austenite/δ-ferrite dual-phase temperature region is one of the most important methods for processing of duplex stainless steels (DSS).Due to coexistence of ferrite and austenite in the microstructures, the softening behaviors in DSSs are more complicated than that in single-phase austenitic or ferritic stainless steels. This thus makes it essential to investigate the softening mechanism during hot deformation with a purpose to improve hot workability of the DSS. In this study, the microstructural development correlated with the dynamic softening process in DSSs are to be investigated by both experimental characterization and numerical modeling. Different from previous studies mostly related with specific processing techniques or materials of DSSs, this study places particular interests on associated transformation mechanisms in DSS. For this purpose, fine characterizations of the micro-substructures developed during the hot deformation inside the two consituent phases is going to be carefully made using SEM, TEM, EBSD ect. in order to provide deep insights into the complicated microstructural mechanisms both of dynamic recrystallization and phase transformations as well as their correlations with the softening behaviors of the DSS. In addition, the mechanism of strain partitioning inside the duplex microstructure due to hot deformation is going to be analyzed at a grain scale using a crystal plasticity finite element method (CPFEM). Kinetics of the phase interface migration during hot deformation is also to be studied by the famous DICTRA sofeware using a diffusional transformation concept. Moreover, a modified Cellular Automaton (CA) model is going to be developed to depict the unique microstructural behavior of the dynamic recrystallization with the coexistence of ferrite and austenite in DSS in incorporating with the fundamental transformation theories. The proposed study can not only enhance the theoretical understandings about the complex microstructural mechanism of the dynamic softening in DSS, but also promote optimising the DSS thermomechanical processing technology in practice and improve the simulation techniques at the mesoscale as well.

在奥氏体/铁素体高温两相区热变形是双相不锈钢的重要加工方式之一。与单相不锈钢相比，双相不锈钢的软化行为更为复杂，研究其动态软化规律对热加工性能控制至关重要。本项目拟通过实验研究与模拟计算结合对双相不锈钢热变形中动态软化过程的微观组织行为进行深入研究。与以往以材料或工艺为主导的双相不锈钢动态软化研究不同，本研究以阐述物理机理为首要研究目标。拟结合EBSD、SEM、TEM等对微观亚结构的精细表征，研究双相并存情况下组成相内发生的动态再结晶及相变的发生规律，从微观亚结构行为的角度阐述双相不锈钢动态软化的微观机理；结合介观尺度材料计算研究双相结构热变形微观应力应变分配规律及热变形条件下相界面迁移动力学，并基于转变机理对双相不锈钢动态再结晶的微观组织行为建模。本研究对理论上加深和正确认识两相结构动态软化微观机理、实际应用中优化双相不锈钢热加工工艺以及促进介观尺度材料计算技术的发展等方面均具重要意义。

现代钢铁工业中，准确认知和理解钢铁材料热变形中的微观组织行为及其微观机理对钢铁热加工中的组织调控、工艺优化及合金设计等均具有重要意义。本项目分别以单相奥氏体合金和双相不锈钢为原型材料，从微观亚结构行为的角度系统分析和阐述了奥氏体/铁素体双相共存及奥氏体单相热变形中的动态软化行为及其微观机理。1）对双相不锈钢热变形中的微观亚结构的演化特征进行了分析，阐明了两相共存时热变形中的动态软化机制及其应变协调机理，阐述了双相组织并存情况下发生的反常动态再结晶现象及其微观机理。2）证实双相不锈钢热变形中发生的形变诱导铁素体-奥氏体相变也是一种潜在软化机制，分析了铁素体亚结构演化与奥氏体析出相变之间的关系。3）通过应变速率瞬变变形工艺研究了奥氏体中的变形亚结构对后续再结晶行为的影响，发现前期形成的亚结构在后续变形过程中继续演变为大角度晶界，促使动态再结晶机制由非连续再结晶向连续再结晶转变。奥氏体动态再结晶变形条件有关，而且会受到变形历史的影响。4）研究了奥氏体相中变形亚结构对变形后的亚动态行为的作用，发现变形亚结构的恢复不仅会引起软化，而且会影响亚动态再结晶的发生，而静态再结晶并非亚动态软化的必要过程。5）分析了热变形不同阶段奥氏体中的孪晶界和亚晶界的微观特征，阐明奥氏体内的孪晶及亚晶界演化对动态再结晶形核的作用。进一步分析了粗大尺寸（>1mm）初始奥氏体晶粒内部的微观亚结构的行为，发现晶内亚结构的演化使再结晶晶粒以连续动态再结晶的形式呈项链状向晶内延伸，发生了由非连续动态再结晶向连续动态再结晶的模式转化。6）发展了双相结构变形与再结晶的晶体塑性有限元-元胞自动机耦合模型，在晶粒尺度上描述双相结构的塑性变形的应变分配，与CA耦合对双相结构中的再结晶组织演变及其动力学开展了模拟计算。