利用Gleeble热模拟、SEM、EBSD和EPMA等方法,研究了3Mn-0.2C中锰钢热变形中发生的形变诱导铁素体相变的组织转变行为,分析了中锰钢形变诱导超细晶组织的形成机理及其在热变形后亚动态过程中的组织稳定性。结果表明,3Mn-0.2C中锰钢在α + γ两相区变形时会诱发形变诱导铁素体相变,通过相变形成由超细晶铁素体、细小残余奥氏体和马氏体组成的多相组织。形变诱导铁素体以不饱和形核和有限生长的模式进行相变,这是导致铁素体晶粒超细化的重要机理。同时,在超细晶铁素体晶界及三叉晶界处形成的细小富Mn残余奥氏体使形变诱导相变组织具有优异的组织稳定性。

Medium Mn steels (MMSs) have Mn contents of 3%-12% (mass fraction), and have been energetically investigated as the most promising candidates of the third-generation advanced high-strength steel. Their phase transformations and microstructures during various heat treatments and thermomechanical processes have received wide attention with the purpose to achieve an optimal balance of cost-efficient alloying compositions and mechanical properties. The aim of this work is to investigate the microstructural behavior of deformation-induced ferrite transformation (DIFT), starting from austenite, which occurs in MMS. Then, improved understandings of the formation of ultrafine ferrite via the DIFT and conservation of this microstructure during the post-deformation period can be obtained. For this purpose, a 3Mn-0.2C MMS with lower contents of alloying elements was selected. Microstructures and alloying element distributions of the thermomechanically processed samples were analyzed via EBSD and EPMA. The results showed that the DIFT occurred in the thermomechanically processed 3Mn-0.2C MMS in the α + γ region. Characteristic multiphase microstructures consisting isolated martensite and fine-grained equiaxed ferrite concomitant with fine islands of retained austenite dispersed between ferrite grains can be obtained. During the DIFT, the enhanced nucleation of ferrite at α/γ interfaces can not only increase the ferrite nucleation density but also facilitate extensive impingement among the neighboring grains. Formation of ultrafine ferrite via the DIFT in MMS can be interpreted in terms of unsaturated nucleation and limited growth. In addition, partitioning of Mn between the ultrafine ferrite and austenite is accelerated during the DIFT such that a large number of Mn-enriched fine islands of austenite are left untransformed at the α/α grain boundaries or at triple junctions. These islands of austenite are considered to play critical roles not only for obtaining retained austenite at room temperature but also for conserving the ultrafine microstructure of the DIFT during the post-deformation processing.