工艺参数对低碳钢形变强化相变的影响

Influence of Process Parameters on the Deformation Enhanced Ferrite Transformation in a Low Carbon Steel

研究了低碳钢过冷奥氏体变形时,工艺参数即变形温度、变形速率和原始奥氏体晶粒大小对形变强化相变组织演变、转变动力学及相变完成时临界应变量εc的影响。结果表明,εc随变形温度降低而减小,随形变速率和原始奥氏体晶粒大小增大而增加。其中,变形温度对εc的影响最大。在相同应变速率的条件下,降低变形温度、减小原始奥氏体晶粒尺寸,都起到了促进相变的作用,使转变动力学提前。在所研究的不同工艺中,组织演变和转变动力学均可分为两个阶段。第一阶段与晶界、孪晶界或形变带作为相变优先形核位置的“位置饱和”机制有关;第二阶段为晶内铁素体奥氏体相界前沿高畸变区的反复快速形核,是以形核为主导的过程,表现为“形核位置不饱和”机制。晶粒的长大在时间与空间上受到限制,形变强化相变完成时,可以使铁素体晶粒细化到2～3μm。

Based on the theory of deformation enhanced ferritic transformation (DEFT), the effects of process parameters, i.e. deformation temperature, strain rate and prior austenite grain size, on the microstructural evolution, transformation kinetics and the critical strain(Εc) for the completion of DEFT were investigated during deformation of undercooled austenite in a low carbon steel. The experimental results indicate that Εc. decreases with decreasing the deformation temperature and increases with increasing the strain rate and prior austenite grain size, and the deformation temperature is of the most significant effect. In addition, lower deformation temperature and small prior austenite grain size can both accelerate ferrite transformation at a same strain rate. Microstructural evolution and transformation kinetics can be both divided into two different stages for the different processes in the test. The first stage is in accordance with site saturation mechanism corresponding to the nucleation at grain boundaries, annealing twins and deformation bands. The second stage corresponds to the nucleation at the regions ahead of the formed ferrite grains where new nucleation are kept creating and obviously is not saturated. Nucleation is the dominant process for the transformation, which restricts the grain growth in time and space. When DEFT is completed, the ferrite grain size of 2-3 μm can be achieved.