Effect of Austenite Conditioning on Martensitic Transformation in Commercial Grade Interstitial-Free Steel

Martensite is an important phase for the strengthening of interstitial-free (IF) steel. The as-received sample is soaked in the austenitic temperature domain at 930 °C for 5 minutes, followed by water quenching in a Gleeble 3800® thermo-mechanical simulator. The outcome shows a dramatic increase in strength by lath martensite and massive ferrite. Hot deformation is also a part of the comparative study. The as-received sample is processed in 0.5 compressive strain at the rate 0.01/s in the same austenitic temperature domain, followed by water quenching. Dynamic recrystallization has not been observed. Instead, plastic deformation in austenite leads to grain boundary nucleation of diffusional ferrite, inhibiting nucleation and growth of massive ferrite in a triplex phase mixture of lath martensite. The austenite to martensite transformation follows Kurdjumov–Sachs (K–S) orientation relationships. The calculated variants based on the K–S orientation relationships between parent and product phase designate that the transformation of differently oriented martensite can originate under constraints from deformed austenite. Thus, the strength-ductility combination is verified with martensite obtained under two different austenite conditions.