1180 MPa級マルテンサイト母相Dual Phase鋼における微視的延性破壊機構の解析

Martensite-matrix dual-phase (DP) steel is increasingly used for high-strength automobile parts owing to its excellent compatibility, ductility, and tensile strength. However, its higher fracture strain, reflected by the hole expansion ratio, remains an issue hindering further adoption of this material. Therefore, this study conducted a microscale investigation of the ductile fracture behavior of 1180-MPa class martensite-matrix DP steel to obtain a guideline for microstructural design realizing improved fracture strain. In this investigation, in-situ tensile testing was conducted simultaneously with scanning electron microscope observations and crystal plasticity finite-element analysis (CP-FEA). The in-situ tensile test results indicated that microcracks initiated at particular martensite packets and did not propagate into other packets; the CP-FEA results revealed that the martensite crystal orientation caused this behavior to induce remarkable stress and strain localization at interfaces in the vicinity of ferrite islands, relaxing the stress and strain localization at distant martensite packets. Although the cracks observed around the ferrite–martensite interfaces were similar to those observed in conventional ferrite-matrix DP steel, such matrix-phase cracks have rarely been reported except immediately prior to final fracture. Thus, the optimization of ferrite island distribution to suppress the formation of stress and strain localization sites was identified as the key aspect of martensite-matrix DP steel microstructure design. This design aspect can be achieved using a combination of data science and CP-FEA.