Abstract: In order to investigate the phase transformation behavior of Cr8 back-up roll steel during the continuous cooling process, the phase transformation parameters of the Cr8 back-up roll steel at different cooling rates were measured by Formastor-FⅡ automatic phase transformation analyzer. The CCT curves was established by thermal expansion method and metallographic method, and the influence of different cooling rates on microstructure and hardness of the Cr8 back-up roll steel was analyzed. The results show that the microhardness of Cr8 back-up roll steel increases gradually with the increase of cooling rate in the range of 2-60 ℃/min, and the hardness value increases from 238 HV2 to 570 HV2. The supercooled austenite transforms into a mixed structure containing pearlite when the cooling rate is less than 8 ℃/min. The pearlite transformation disappears and the supercooled austenite transforms into bainite+martensite or single martensite with higher hardness when the cooling rate is between 8 ℃/min and 60 ℃/min. Compared with Cr5 steel, the supercooled austenite of the Cr8 back-up roll steel has better stability, the steel has lower critical cooling rate and good mechanical properties, and is an ideal material for manufacturing large backup roll forgings.

Key words: Cr8 back-up roll steel, thermal expansion method, CCT curves, microstructure, hardness