Making ultrahigh-strength dual-phase steels tough: experiment and simulation

Abstract

The low damage resistance and fracture toughness hinder the widespread application of ultrahigh-strength dual phase (DP) steels. In this work, we propose a novel strategy to improve the fracture toughness of ultrahigh-strength DP steels by an order of magnitude without sacrificing the tensile strength. Six ultrahigh-strength DP steels with varying microstructure but comparable tensile strength (>1400 MPa) were prepared via tailoring the heat treatment process after cold rolling. Additionally, finite element (FE) method incorporated with Gurson-Tvergaad-Needleman (GTN) model and cohesive zone model (CZM) is established to simulate the fracture behaviour of DP steel. Twelve model DP steels with different ferrite sizes and F/M strength differences are constructed. The combined experiment and simulation results demonstrate that (i) ferrite/martensite (F/M) interface decohesion prevails in all steels, (ii) the ferrite morphology has a strong influence on the fracture toughness of ultrahigh-strength DP steels, (iii) the effects of matrix type, ferrite size, and F/M hardness difference on the fracture toughness are relatively weak, (iv) the exceptional high fracture toughness of plate-like DP steel can be attributed to the crack deflection, crack divider and crack arrester mechanisms induced by F/M interface decohesion.