Abstract: Owing to undergoing circulating MF induction heating-quenching process, multiple phase transitions of base metal structure of the hot-rolled seamless steel tube occur so as to refine all the martensite grains resulting from quenching of the ferrite and austenite structure.Thanks to the said circulating heat treatment process, the dualphase steel with base metal composed of ultra-fine grains of ferrite-martensite structure is obtained, and furthermore after multiple circulations, the ferrite grains of the dual-phase steel can be refined up to 1 μm approximately.