Correlation Between Structure and Properties of Low-Carbon Cu-Ni-Mo-Ti-Nb Ultrahigh-Strength Steel

Low-carbon Cu-Ni-Mo-Ti-Nb steel having more than 1700 MPa tensile strength and good low-temperature impact toughness was successfully designed and processed for automobile, defence and structural applications. The steel was thermomechanical controlled processed at a different finish rolling temperatures (750-850 °C) and cooled at two different conditions: air cooling and water quenching. Evolutions of microstructure and precipitates were thoroughly characterised to understand the effect of different factors, such as steel composition, finish rolling temperature and cooling rate on the mechanical properties. The results indicate that the steel processed at 750 °C finish rolling temperature (FRT) followed by water quenching has exhibited superior tensile properties with a yield strength of 1034-1449 MPa, the tensile strength of 1598-1726 MPa and total elongation of 10-13%. Such an ultrahigh-strength level is primarily attributed to the combined effect of lower bainite dominated microstructure along with precipitation hardening. The satisfactory low-temperature toughness is promoted by low finish rolling temperature and higher amount of misorientation offered by the boundaries within the lower bainite.