Temperature Effects on the Deformation and Fracture of a Quenched-and-Partitioned Steel

Temperature effects on the deformation and fracture of a commercially produced transformation-induced plasticity (TRIP) steel subject to a two-step quenching and partitioning (Q&P) heat treatment are investigated. Strain field evolution at room temperature is quantified in this 980 MPa grade Q&P steel with a stereo digital image correlation (DIC) technique from quasi-static tensile tests of specimens with 0°, 45°, and 90° orientations. Baseline tensile properties along with the variation of the instantaneous hardening index with strain were computed. Variations of the bake-hardening index were explored under simulated paint bake conditions. Tensile properties were measured at selected temperatures between -100°C and 200°C and the TRIP effect was found to be temperature-dependent due to stress-induced martensitic transformation at lower temperatures versus strain-induced transformation at higher temperatures. Electron back-scatter diffraction (EBSD) and X-ray diffraction (XRD) of regions close to the fracture area were used to explain the observed temperature variations of the ultimate tensile strength (UTS), 0.2% yield strength (0.2%YS), uniform elongation (UE) and total elongation (TE).