Influence of Initial Structures on the Mechanical Properties in Cold‐Rolled Transformation‐Induced Plasticity Steel

A novel transformation‐induced plasticity steel is analyzed to study the effects of initial structures and the intercritical annealing temperatures on austenization behavior, strain distribution, and the characteristics of retained austenite (RA). The resulting microstructure and its relationship with mechanical properties are measured using dilatometry, scanning electron microscopy, electron backscatter diffraction, X‐ray diffraction, and tensile testing. The results show that the initial structure is composed of martensite (M) or tempered martensite (TM) and can significantly refine the final microstructure and its strain distribution is more homogeneous compared to the ferrite + perlite (F + P) initial structure samples. However, it is found that the larger size of the RA mainly distributes at the ferrite grain boundaries while the smaller size mainly distributes in the bainite matrix and outside martensite. Meanwhile, most of the RA in the bainite matrix are films and the carbon concentration is high. In addition, the initial M samples have many homogeneous lath interfaces and the ferrite has a strong carbon discharge ability during high‐temperature annealing, so the fraction of RA is the highest at 23.1% when annealed at 820 °C, and the corresponding carbon concentration is 1.16 wt%, so the final combination of strength and ductility is up to 32.1 GPa%. Pretreatment is designed to obtain martensite initial structure and then conducts short‐time overaging heat treatment. Initial martensite structure appears in lath. Significant differences are found among the intercritial temperatures and the fractions of retained austenite (RA) are more. Results indicate that RA in the initial martensite samples is high in volume fraction and also high in stability.