Effect of Tempering Temperature on the Microstructure and Mechanical Properties of an Online Relaxation and Quenching High-Strength Steel

The effect of tempering temperature on microstructure and mechanical properties of high-strength medium-thickness steel plate was studied by rolling + relaxation + online quenching + tempering process. After quenching at 670 °C, the yield strength, tensile strength, elongation, and impact energy at − 20 °C of high-strength steel could reach 1002, 1240 MPa, 12%, and 148 J, respectively. After tempering between 560 and 650 °C, they could be 1051-1139 MPa, 1100-1160 MPa, 14.2-17.8%, and 150-240 J, accordingly. The precipitates and carbides were the main factors causing the increase in yield strength, while the lath width was the main factor causing the decrease in yield strength when the tempering temperature increased from 560 to 650 °C. Some dislocations were pinned by precipitates so that they were difficult to move when tempering. Thus, dislocation density decreased dramatically in the early stage of tempering (560 °C) and then decreased slowly. The increase in tempering temperature could reduce the dislocation density and increase the proportion of high-angle grain boundaries to consume more energy by changing the crack propagation direction, thus improving the impact toughness.