Unveiling post-deformation transformation mechanism and ferrite microstructure evolution for tailoring mechanical properties of low-carbon martensitic steels

Tong, Z.*; Xia, C.*; Li, W.*; Ding, W.*; Guo, B.*; Min, N.*; Gong, W.   ; Harjo, S.   ; 辻 伸泰*

Tong, Z.*; Xia, C.*; Li, W.*; Ding, W.*; Guo, B.*; Min, N.*; Gong, W.; Harjo, S.; Tsuji, Nobuhiro*

This study elucidates the mechanism of post-deformation ferrite transformation during relaxation and demonstrates that controlling ferrite grain size and volume fraction enhances the strength-ductility balance of low-carbon martensitic steel. In situ neutron diffraction and microscopy show that austenite-to-ferrite transformation preferentially occurs at grain boundaries due to localized dislocation accumulation. Dislocation density remains elevated during relaxation at 755 C but fully recovers at 765 C. Thermodynamic analysis indicates that transformation behavior is governed by the combined effects of chemical driving force and stored dislocations, enabling precise control of ferrite microstructure. Exploiting grain-boundary transformation heterogeneity through controlled relaxation provides an effective strategy to overcome the strength-ductility trade-off in high-strength martensitic steels.