预变形方式对微合金钢中贝氏体热稳定性的影响

采用光学金相、硬度测量结合透射电镜观察,研究了经过弯曲或扭转变形的含铌微合金钢在等温受热时的组织稳定性问题. 研究发现:弯曲与扭转变形都可以大幅度提高微合金钢的硬度;在随后的550℃等温受热过程中,弯曲变形区的硬度迅速下降,同时伴随贝氏体向平衡组织的演变;而扭转区在等温过程中的硬度始终高于未变形区,同时钢中贝氏体组织基本得以保持. 扭转应变量越大,硬化效果越强,并且在随后的等温受热过程中能保持这种硬度上的优势. 弯曲与扭转变形均导致贝氏体板条内位错密度显著增加. 弯曲变形区的位错分布不均匀,其中的低位错密度区易于在随后的等温受热过程中演变为平衡组织多边形铁素体的形核核心;而扭转变形区内位错分布均匀,并且在随后的等温受热过程中位错分布不发生显著改变. 这些结果表明,不同的冷变形方式对微合金钢中贝氏体热稳定性的影响存在显著差异.

Influence of pre-deformation on the thermal stability of bainite in a microalloyed steel

Optical microscopy, hardness measurement and transmission electron microscopy (TEM) were employed to investigate the thermal stability of microstructures in a Nb-bearing microalloyed steel during isothermal holding, which had been curved or twisted before being heated. It is found that both curving and torsion increase the hardness of the microalloyed steel in a large scale. In subsequent isothermal holding at 550℃, the hardness of the zone subjected to curving drops rapidly in company with the evolution of bainite towards equilibrium microstructure while the hardness of the zone undergoing torsion is higher than that of the undeformed zone all the time and bainite remains in the zone. The larger the torsion degree is, the stronger the hardening effect is. The higher hardness can be preserved during isothermal holding. Both curving and torsion result in an obvious increase of dislocation density in bainite laths. Dislocations distribute inhomogeneously in the curved zone and polygonal ferrite tends to nucleating at sites with low dislocation density during subsequent heating, while dislocations distribute homogeneously in the twisted zone and do not markedly change the distribution during subsequent isothermal holding. These results indicate that different deformation methods produce different influences on the thermal stability of bainite in a microalloyed steel.