Control of heterogeneous microstructure for improving delayed fracture resistance of ultrahigh strength hot stamping steel sheets

Synopsis: Toward a high demand for ultrahigh strength automotive components for reducing body weight and increasing crashworthiness, hot stamped components have been employed worldwidely, and the further increase in strength of hot stamped components is being requested. A large obstacle of the strength increase is the concern about the occurrence of delayed fracture. In this study, we investigated the effects of microstructure on the resistance to delayed fracture of hot stamped steel sheets from a viewpoint of the control of heterogeneous microstructure, and the following results were obtained: (1) The grain refinement of martensite increases the resistance to delayed fracture. (2) Samples containing some amount of ferrite in the martensite matrix have a lower or higher resistance to delay fracture than samples with full martensitic microstructure, depending on the morphology of ferrite. (3) The presence of residual austenite of several % increases the resistance to delayed fracture remarkably but if it is strained and transformed to martensite, the resistance to delayed fracture drastically decreases. (4) Concerning 2000 MPa class hot stamping steel sheets, a higher carbon steel quenched and tempered at low temperature has higher resistance to delayed fracture than a quenched lower carbon steel with the same strength. In the paper, we discussed these results including their mechanisms in detail.