Fracture toughness characteristics of thermo-mechanically rolled direct quenched and partitioned steels

Abstract

The propensity of the retained austenite to transform to martensite under external strain is known to induce plasticity (TRIP) in steels. If the retained austenite has a high volume fraction, it generally has blocky morphology and increases the tensile ductility via TRIP effect but can potentially deteriorate the yield strength. The present work demonstrates improved balance of strength and fracture toughness in a Si-containing 0.2%C steel having a refined martensitic matrix and evenly distributed and finely divided retained austenite (RA). Such a microstructure is obtained by thermo-mechanical rolling followed by direct quenching and partitioning (TMR-DQP) of the steel that results in establishing a constrained carbon equilibrium stabilising the austenite to room temperature. Transmission electron microscopy was used to characterise the nano-twinned martensite and the interlath nanometer thick austenite. An improved combination of yield strength, YS (1171 MPa), ultimate tensile strength, UTS (1419 MPa) and elastic-plastic fracture toughness, KJC (154 MPa √m) was achieved as compared to YS 1240 MPa, UTS 1654 MPa and KJC 111 MPa √m, respectively, for the fully martensitic counterpart. X-ray diffraction combined with Rietveld analysis revealed a reduction in the retained austenite from 9% away from the crack, which undergoes little strain, to 4 vol% near the crack, under higher strain, demonstrating strain induced martensitic transformation. The constrained nature of the austenite-to-martensite transformation within the rigid surrounding martensite is believed to increase the energy required to drive the crack forward, which raises the fracture toughness. Importantly, the results show that the uniformly distributed and nanoscale retained austenite is effective in imparting transformation-induced- plasticity at relatively small strength penalty.