低于M_(s)温度变形对等温贝氏体相变动力学和组织的影响

Effect of below M_(s) deformation on kinetics of isothermal bainitic transformation and microstructure

变形和等温热处理是高强贝氏体钢主要生产工艺,已有研究表明低于马氏体相变起始温度(M_(s))的等温热处理可以促进贝氏体相变动力学,低温奥氏体预变形也可以加速贝氏体相变。研究了低于M_(s)温度变形对后续等温贝氏体相变动力学和组织的影响,结果表明,并未出现预想的加速相变叠加效应,反而,变形温度低于M_(s)温度时,贝氏体相变动力学减弱,等温贝氏体相变孕育期延长。低于M_(s)温度等温相变时,贝氏体铁素体与母相奥氏体位向关系接近K-S关系,变形试样虽然获得了一部分先马氏体,且能提高贝氏体形核率,但并非所有的胚核都能发生长大,变形改变母相奥氏体取向,使贝氏体原本的位向关系遭受破坏,导致有效形核率降低。

Deformation and austempering are the main processes of preparing high-strength bainite steel. It was shown that both austempering below the martensite start temperature(M_(s) temperature) and ausforming can accelerate the kinetics of bainitic transformation. The effect of deformation below M_(s) temperature on the subsequent kinetics of isothermal bainitic transformation and microstructure was investigated. The results show that the expected superimposition effect of accelerating phase change does not appear. On the contrary, the deformation below M_(s) temperature retarded the kinetics of the bainitic transformation, leading to prolonged incubation period. When the isothermal transformation temperature is lower than the M_(s), the orientation relationship between bainitic ferrite and the parent phase austenite is close to the K-S type. Although the deformed sample has obtained a part of pre-martensite which can increase the bainite nucleation rate, not all nuclei can grow up. Deformation changes the orientation of the parent phase austenite and destroys the original orientation relationship of bainite, reducing the valid number of bainitic nuclei.