The transformation kinetics of a low carbon bainitic steel was investigated using in-situ confocal microscopy and dilatometry for isothermal treatments above and below the martensite start (M_S) temperature. Both martensite and bainite are formed during isothermal holding below the M_S, but bainite was the dominant transformation product. The prior martensite formed during cooling to the holding temperature strongly influenced the bainite precipitation kinetics. A critical volume fraction of prior martensite, $f_c^{PM}$, was identified. In order to accelerate the bainite transformation kinetics by austempering below the M_S, the prior martensite fraction should be less than $f_c^{PM}$. The best combinations of strength and ductility were obtained in steels by austempering below the M_S temperature with an optimum martensite fraction (∼5%). Finally, the isothermal bainite transformation kinetics is analyzed by Johnson-Mehl-Avrami-Kolgomorov (JMAK) kinetics model. The analysis suggests that the nucleation of bainite approaches site saturation when the isothermal holding temperature is below the M_S.

使用原位低碳贝氏体钢的转变动力学研究了共聚焦显微镜和膨胀计为上方和下方的马氏体开始（M等温处理_小号）温度。马氏体和贝氏体都在等温保持在M _S以下时形成，但贝氏体是主要的相变产物。在冷却至保持温度期间形成的先质马氏体强烈影响贝氏体沉淀动力学。临界体积分数的马氏体，$F_C^{P\mathrm{中号}}$，已被识别。为了通过在M _S以下回火来加速贝氏体转变动力学，先前的马氏体分数应小于$F_C^{P\mathrm{中号}}$。通过在低于M _S的温度下以最佳马氏体含量（约5％）进行奥氏体化，可获得强度和延展性的最佳组合。最后，通过Johnson-Mehl-Avrami-Kolgomorov（JMAK）动力学模型分析了等温贝氏体转变动力学。分析表明，当等温保温温度低于M _S时，贝氏体形核接近饱和。