Investigation Of Grain Refining By Transformation In Nb-microalloyed CFB/M Steel

The grain refining can both improve the strength and toughness of steels.In this paper,the rule of structural heredity in Nb-microalloyed CFB/M steel was investigated by experiments.The result shows:both isothermal annealing and full annealing can erase the orientation relationship of martensitic structure,eliminate the structural heredity of Nb-microalloyed CFB/M steel.In addition,It shows in the experiments that the existence of structural heredity in Nb-microalloyed CFB/M steel is in a large scope of heating rate,structural heredity will not appear when the heating rate is up to 40℃/s,and the austenite grain becomes fine,the technics of cycling phase transforming heat treatment was used to refine the prior autenitic grain size of bainite/martensite multiphase steel which was pretreated,in order to discuss the influence of heating rate, austenitizing temperature and quenching times on prior austenitic grain size.Fracture morphology analyzing and microstructure analyzing by SEM were used to estimate the size of autenitic grain.The result shows:the original structure and temperature of cycling phase transforming heat treatment are both influenceing the prior austenitic grain size. The prior austenitic grain size of sample with original structure of martensite can be 10μm by heating to 910℃and cyclic quenching of three times.