Ultra-high Strength Steel Treated by Using Quenching–Partitioning–Tempering Process

Quenching–Partitioning–Tempering (Q–P–T) process is developed to yield ultra-high strength and high toughness steel based on Q–P process suggested by Speer et al. The chemical composition of Q–P–T steel is designed as <0.5C, 1.5Si (or Al), 1.5Mn, 0.02Nb, 0.2Mo (mass%) in which complex carbide precipitation during tempering may contribute further hardening; or <0.5C, 1.5Si, 1.5Mn (mass%) in which η (θ) carbide precipitation may offer strengthening beside the martensite formation. Ultimate tensile strength and total elongation of >2,000 MPa and >10%, ~1,500 MPa and ~15%, and ~900 MPa and ~20% are obtained with Q–T–P processed steels containing 0.4, 0.2 and 0.1C, respectively. Steels processed by Q–P–T generally contain ~5% retained austenite. Several examples are given in this article. The thermodynamics and kinetics as well as strengthening and toughing mechanisms for Q–P–T process are briefly discussed.