The critical impact of intercritical deformation on variant pairing of bainite/martensite in dual-phase steels

We elucidate here the critical impact of intercritical deformation on variant pairing of bainite/martensite in dual-phase steels. Intercritical deformation was instrumental in significantly increasing the content of ferrite and accelerated the diffusion of carbon from ferrite to untransformed austenite, which finally transformed to martensite on quenching. In contrast, the untransformed austenite with lower enrichment of carbon transformed to bainite. Furthermore, the increase of carbon concentration In untransformed austenite, the length fraction of V1/V2 pair was increased to accommodate the strain associated with the bainite transformation. When the phase transformation product changed from bainite to martensite, all six variants with high fraction belonging to the same closed-packed (CP) group formed to accommodate the higher strain associated with martensite transformation. The steel with the lowest percentage of ferrite (25%) had the highest yield strength (583 MPa), while the yield strength (574 MPa) of steel with 58% ferrite continued to be high because of dislocation strengthening caused by intercritical deformation. Additionally, the uniform elongation was increased and yield strength to tensile strength ratio was reduced with the increase of content of ferrite in the studied steels.