Different morphologies of carbide-free bainite were obtained through a series of isothermal heat treatments of a new medium-carbon bainitic steel, and the evolution of microstructures during tensile deformation was then observed. The results showed that the strength-ductility balance could reach its highest value near 350 °C. The retained austenite sustaining martensitic transformation, long bainite ferrite sheaf, and phase transformation dynamics were the main factors that caused high plasticity of the steel at 350 °C isothermal transformation. It is noteworthy that 350 °C is also a phase change sensitive point for most bainitic steels. Maintaining high work hardening rate at high strain is beneficial to increase elongation, which is attributed to the continuous martensitic transformation of the retained austenite with high volume fraction. The (200) austenite peak was separated using the Gaussian multi-peaks fitting method. It was found that the (200) austenite peak moves to the left with the increase of strain. The proportion of low angle peaks also increased with strain. This indicates that the transformation of the retained austenite always occurs in the low carbon region.

通过一系列新型中碳贝氏体钢的等温热处理，获得了不同形态的无碳化物贝氏体，然后观察了拉伸变形过程中微观组织的演变。结果表明，强度-延性平衡可以在350°C附近达到最高值。残余奥氏体维持马氏体相变，贝氏体长铁素体束长和相变动力学是导致钢在350°C等温转变时具有高塑性的主要因素。值得注意的是，对于大多数贝氏体钢，350°C也是相变敏感点。在高应变下保持较高的加工硬化速率有利于增加伸长率，这归因于高体积分数的残余奥氏体的连续马氏体相变。使用高斯多峰拟合方法分离（200）奥氏体峰。发现（200）奥氏体峰随着应变的增加而向左移动。低角度峰值的比例也随应变而增加。这表明残留奥氏体的相变总是发生在低碳区。