中文摘要：

结合膨胀法和金相观察绘制了无Nb和添加质量分数0．042％Nb的实验钢的动态CCT曲线，阐明了Nb对低碳微合金钢相变行为和相变组织的影响规律．结果表明，添加Nb可显著细化铁素体晶粒，抑制铁素体形成，促进贝氏体形成，使CCT曲线移向右下方，缩小铁素体和珠光体相变区，显著扩大贝氏体相变区．此外，添加Nb可显著增强细晶强化、析出强化和相变强化效果，使实验钢维氏硬度显著提高．

英文摘要：

The dynamic continuous-cooling-transformation （CCT） diagrams of Nb-free and 0. 042 wt% Nb tested steels were plotted by means of a dilatometry method combined with metallography. The influence of Nb on the continuous cooling transformation behavior and transformation microstructure of low carbon microalloyed steel was elucidated. The results show that the addition of Nb can significantly refine ferrite grain size and suppress the formation of ferrite, promoting the formation of bainite, and can make CCT curves move down and right, reducing the ferrite and perlite transformation field and expanding the bainite transformation field. Moreover, the Vickers-hardness can be significantly increased due to grain refinement, precipitation hardening and transformation strengthening by the addition of Nb.