Abstract: SV30 bearing steel was quenched at 990-1 070℃ for 10 min and at 1 030℃ for 0-100 min by DIL805A dilatometer. The microstructure, hardness, residual austenite content of the tested steel before and after quenching were studied. The dissolution of second phase and characteristics of quenching phase transformation were analyzed. The results show that when the quenching temperature rose from 990℃ to 1 070℃, the dissolution rate of M₂₃C₆ carbides in the steel became quicker. M₂N nitrides dissolved in large quantities only when the temperature reached 1 050℃ or higher. When holding at 1 070℃ for 10 min, the carbides were completely dissolved into the austenite while about 6.3wt% nitrides still remained undissolved. After quenching at 1 010-1 030℃ for 10-30 min, the hardness of the tested steel reached over 58 HRC; after quenching at 1 030-1 070℃, the residual austenite of about 30vol%-50vol% were retained in the tested steel.