Effects of Process on Microstructure and Mechanical Properties of 690MPa Grade Construction Machinery Steels

The 690 MPa grade structural steels for construction machinery were prepared using thermo-mechanical control process (TMCP) and the process combined TMCP with relaxation-precipitation-controlling phase transformation technology (RPC). The effects of different processes on the microstructure and mechanical properties of the steels were investigated by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that the microstructure of the steels prepared by TMCP and TMCP+RPC process was mainly composed of lath bainite and a small amount of granular bainite and acicular ferrite. Compared to the TMCP, the steel prepared by TMCP+RPC technology had finer microstructure, and so it had the better comprehensive mechanical properties with a yield strength of 715 MPa and a tensile strength of 860 MPa and a elongation of 20.6%.