The dominating role of austenite stability and martensite transformation mechanism on the toughness and ductile-to-brittle-transition temperature of a quenched and partitioned steel

Abstract

The effect of austenite stability and martensite transformation (MT) mechanism on the tensile and impact properties has been investigated for Fe-0.3C-2.7Mn-1.7Si (wt.%) quenched and partitioned (Q&P) steel. The stability of metastable retained austenite (γM) in the Q&P steel samples, which were subjected to different quenching temperatures (20, 120, and 240 °C), was quantitatively evaluated according to the Msσ temperature. Results suggest that the stability of γM was mainly related to the carbon content, grain size/morphology, and stress shielding effects provided by the surrounding martensite. In addition, in contrast to the consistently positive transformation-induced plasticity effect (TRIP) of γM on the tensile properties, MT had a two-side effect on the dynamic impact properties, depending on the MT mechanism and the related Msσ temperature. At high temperatures, the strain-induced MT in the QP120 and QP240 samples relieved the local stress concentration and impeded the crack initiation and propagation by the TRIP effect, whereas the stress-assisted MT at low temperatures resulted in a low crack initiation, propagation energy, and intergranular fracture mode, thereby resulting in a lower impact toughness of the QP240 sample than that of the almost austenite-free QP20 sample. Furthermore, the ductile-to-brittle transition temperature (DBTT) of the Q&P samples showed an almost linear relationship with their Msσ temperature. Finally, an excellent combination of large product of strength and elongation, low DBTT, and high toughness was obtained in the QP120 sample, which was mainly attributed to the good stability of the film γM during the tensile or impact tests.