Microstructure and mechanical properties of a novel 1000Â MPa grade TMCP low carbon microalloyed steel with combination of high strength and excellent toughness

A novel low carbon Nb-V-Ti-Cr microalloyed bainitic steel with yield strength of 1000 MPa and excellent low temperature toughness was successfully processed. Two cooling procedures were adopted to optimize the mechanical properties. The microstructural evolution, precipitation behavior, and strengthening mechanisms were systematically studied, and the fracture mechanisms were analyzed via combination of fractographs and deflection-load curves. The experimental results indicated that the steel subjected to cooling rate of 65 °C/s and coiling temperature of ~380-400 °C resulted in superior mechanical properties. The yield strength, tensile strength, yield ratio, and elongation-to-fracture were 1058 MPa, 1192 MPa, 0.88, and 10.6%, respectively. The impact energy of the 1/4-size Charpy specimen and tested at −20 °C was 24 J. The ultra-high yield strength is primarily attributed to transformation strengthening from lower bainite and precipitation hardening from nano-scale (Nb,Ti)C precipitates. The excellent toughness is associated with lower bainite and acicular ferrite together with high fraction of large misorientation grain boundaries by controlling the coiling temperature.