| In this paper, the long-life bainite/martensite multiphase steel was studied. Isothermal heat treatment used to determine the transformation start temperature of BU and BL in different austenitizing temperatures. Super rapid cooling was achieved by the regulatory implementation of the phase transformation instrument, and the CCT curves of steel was measured. The effect of different cooling rate on microstructure was analyzed combined with the metallographic examination method, and the heat treatment process parameters was designed. By stepped cooling after austenitizing to avoid ferrite transformation, combined with rapid heating cycle of phase-transformation (three times) to obtain a suitable grain refined microstructure, the resulet of very high cycle fatigue testing show that Nb micro-alloying B/M multiphase steel has great strength and toughness properties. The mixed grain phenomenon after normal quenching of the high temperature quenching and high temperature tempering Nb micro-alloying carbide-free bainite/martensite (CFB/M) multiphase steel specimen was studied, and it proved that the steel has typical grain boundary inheritance.
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