Phase transformation and carbon profile at the interface between Al-Si coating and steel substrate in a press-hardened steel

ABSTRACT The interface between the Al-Si coating and the steel substrate is important to the bending toughness of Al-Si coated press hardened steels (PHS), which is the critical property for automotive applications. In this work, the phase transformation and the carbon profile at the coating/steel interface are investigated by both microstructure characterisation and modelling. During hot stamping, two types of ferrite phases are found at the coating/steel interface, i.e., Al-rich ferrite and Al-free ferrite. At temperatures above 750 °C, Al-rich ferrite grows from the coating into the austenitic steel substrate due to the interdiffusion of Fe and Al. In contrast to previous findings reported in literature, the growth of Al-rich ferrite does not result in significant carbon enrichment at the interface between ferrite and austenite, due to the very high carbon diffusivity at such high temperatures. On the other hand, it was found that Al-free ferrite forms below 750 °C and causes significant carbon enrichment at the coating/steel interface. Notably, the growth of Al-free ferrite is based on the pre-existing Al-rich ferrite and bypasses the nucleation stage of ferrite transformation. Thus, Al-free ferrite can form easily at the coating/steel interface, even at a high cooling rate over 200 °C s−1. Finally, this finding suggests that lower cooling rates can lead to thicker Al-free ferrite and severer carbon enrichment at the coating/substrate interface. Other factors such as austenitisation time may only affect the Al-rich ferrite and have limited influence on carbon enrichment at the coating/steel interface of PHS.