An ultrastrong and ductile deformed and partitioned (D&P) steel developed by dislocation engineering has been reported recently. However, the microstructure evolution during the D&P processes has not yet been fully understood. The present paper aims to elucidate the process–microstructure relation in D&P process. Specifically, the evolution of phase fraction and microstructure during the corresponding D&P process are captured by means of X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). Subsequently, the effect of partitioning temperature on dislocation density and mechanical properties of D&P steel is investigated with the assistance of uniaxial tensile tests and synchrotron X-ray diffraction. It is found that a heterogeneous microstructure is firstly realized by hot rolling. The warm rolling is crucial in introducing dislocations, while deformation-induced martensite is mainly formed during cold rolling. The dislocation density of the D&P steel gradually decreases with the increase of partitioning temperature, while the high yield strength is maintained owing to the bake hardening. The ductility is firstly enhanced while then deteriorated by increasing partitioning temperature due to the strong interaction between dislocation and interstitial atoms at higher partitioning temperatures.
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