Fracture mechanisms of quenching and partitioning steels and press-hardened steels

Abstract

The present thesis aimed to study the fracture mechanisms of quenching and partitioning (Q&P) steels and press-hardened steels (PHS) for automotive lightweighting applications. Two major aspects were covered, i.e., fracture resistance and hydrogen embrittlement. For solutions, the thesis focused mainly on the matrix design of the Q&P steels and the Al-Si coating design of the PHS. Firstly, two ferrite-containing Q&P steels with ultimate tensile strength (UTS) of 980 MPa and 1180 MPa were studied. The ferrite phase can interrupt the continuity of prior austenite grain boundaries and martensite packet boundaries, blunt the hydrogen-assisted cracks initiated from deformation-induced martensite, and suppress the hydrogen-assisted intergranular fracture. The work suggested that both the ferrite fraction and distribution in the martensite matrix were important for improving the delayed fracture resistance of Q&P steels. Next, Q&P steels with a UTS of 1500 MPa and a full martensite matrix were investigated. In addition to the design of metastable retained austenite, the work proposed that the “martensite matrix design” was crucial to improve the fracture resistance and hydrogen embrittlement resistance of Q&P steels with a full martensite matrix. In the optimum microstructure, the martensite matrix was sufficiently tempered, with higher dislocation mobility due to dislocation recovery, solute carbon depletion, and transformation of transition carbides into fine cementite. The enhanced dislocation mobility can promote plastic deformation in front of the crack tip, thus improving the fracture resistance and the hydrogen embrittlement resistance. The second half of the thesis turned to the Al-Si coated PHS. In literature, the toughness degradation caused by the Al-Si coating was attributed to the carbon enrichment related to the Al-rich ferrite transformation in the coating. However, the thesis found that the Al-rich ferrite formed mainly at austenitisation temperatures and could not result in carbon enrichment due to the high carbon diffusivity. Instead, the Al-free ferrite transformation, controlled by carbon diffusion below 750 ℃, was the primary cause of the carbon enrichment. The Al-free ferrite grew based on the pre-existing Al-rich ferrite, bypassing the nucleation obstacle, and could not be suppressed even at a cooling rate faster than 100 ℃ s-1. By studying the surface cracks during bending tests of PHS coated with different thickness of Al-Si coating, it was found that the surface cracks can propagate continuously from the intermetallics layer to the steel substrate, resulting in a high stress intensity factor (SIF) at the crack tip and therefore degrading the bending toughness. The work proposed that the thinner Al-Si coating could produce shorter surface coating cracks and smaller SIF at the crack tip, suppressing the bendability degradation. Finally, hydrogen embrittlement of Al-Si coated PHS was investigated using pre-charged bending tests. It was found that hydrogen-induced cracks could initiate from both the Al-Si coating on the sample surface and the non-metallic inclusions inside the sample. With pre-charged hydrogen, the Al-Si coating cracks can induce quasi-cleavage and intergranular fracture in the steel substrate, propagate into the steel with a depth over tens of micrometres, promote localised shear deformation, and degrade the hydrogen embrittlement resistance.