Microstructure and Properties of Low Cost 780 MPa Hot-Rolled High-Strength Steel

The 780 MPa high-strength steels were prepared by thermo-mechanical control process(TMCP)on hot tandem mill.The microstructure and properties of the steels were investigated by means of optical microscope,transmission electron microscope and tensile test machine.The results show that the microstructure of the tested steel was mainly composed of fine-grained ferrite and carbides distributed along the ferrite grain boundaries.The yield strength and tensile strength of the steel were about 730 and 790 MPa,and the strain hardening exponent and plastic strain ratio were 0.12 and 0.85,respectively,providing well-matched strength with toughness.The fine-grained ferrite and tiny (Nb,Ti)(C,N)precipitates played an effective role in strengthening the steel.