Influence of pre-deformation on the thermal stability of bainite in a microalioyed 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.