Abstract: By a special thermomechanical control process(TMCP-RPC), relaxation for a certain time after control rolling and then direct quenching/accelerated cooling, the strain induced precipitions and defect structure in deformed austenite resulted in refinement of intermediate transformation microstructure. Following this relaxation-precipitation-control phase transformation (RPC) process,800 MPa grade plate steel with an ultra-fine mixture microstmcture of bainite and martensite was obtained. The effect of processing parameters on the microstructure and the mechanical properties was revealed.