Characterization of the microstructure and hardness of the HAZ in a 800MPa grade RPC steel

The microstructure and hardness of the HAZ in a 800MPa grade ultra-low-carbon microalloyed steel were studied.The results indicate that the heat affected zone(HAZ) of the RPC (Relaxation- Precipitation- Controlling) steel possesses a continuous gradient structure, and can be classified into three zones, i. e. CGHAZ (coarse-grain HAZ) , FGHAZ (fine-grain HAZ) and ICHAZ(intercritical temperature HAZ). The microstructures in the HAZ are all composed of bainite-like structure.The microstructure in the CGHAZ mainly consists of lath-like bainite and granular bainite . The influences of heat input and t8/5 on the hardness in the HAZ of RPC steel are notable. With the increase of heat input and t8/5, the softening tendency of HAZ becomes obvious. The hardening phenomenon that normally occurs in the CGHAZ does not take place with this steel in the range of experimental conditions. The softening in the ICHAZ is bound to occur. Hence appropriate welding technologies need to be selected.