Enhanced Ductility of Ferrite/Bainite Dual‐Phase Steel through the Volume Fraction Increase of Effective Ferrite

In this study, to improve the ductility of ferrite/bainite dual‐phase steels while maintaining the strength, the transformation behavior of the ferrite phase is investigated. Based on these findings, a room‐temperature microstructure comprising uniform‐grained ferrite and bainite is designed. The results illustrate that ferrite transformation always nucleates at the original austenite grain boundaries and ferrite grains grow inward, forming ferrite grains with large differences in grain size. With the help of the modified Crussard–Jaoul analysis and pre‐deformation electron backscatter diffraction, it is found that coarse‐grained ferrite is fully deformed during plastic deformation, but fine‐grain strengthening makes fine‐grained ferrite difficult to deform. It can inhibit the excessive growth of first formed ferrite grains by controlling the original austenitic grain size, improve the uniformity of ferrite grain size; under the premise of ferrite volume fraction that is unchanged, more ferrite grains (effective ferrite) involve in plastic deformation and effectively inhibit stress concentration during plastic deformation, preventing the growth and merging of microcracks. The total elongation of the experimental steels increases by about 10%, while ensuring the strength of ferrite/bainite dual‐phase steel.