Evolution of Ultra-Fine Microstructures During Tempering of a Niobium-Containing Mcroalloyed Steel

摘要

The evolution of microstructures during tempering of a controlled rooled plate steel was investigated. A part of samples were directily tempered while the others were rehated at 930 degC for 0.5h followed by quenching into water before tempering. Despite their lower orignial hardness, the reheated samples soften faster during temepraing. All the samples are constituted by bainite aling with little marteniste before tempering. In early stage of temepring, no obvious change is detected by means of optical microscopy while dislocation cells are formed inside bainitic laths. With further temepring, bainitic laths start to coalesce in some regions. Finally, polygonal ferrite is formed while hardness decreased dramatically. Ferrite is formed faster in reheated samples. These results indicate that the evolution of microstructure stowards equilibrium during tempering of the steel is mainly determined by whether dislocations are pinned rather than by dislocation density.