Suppression of Coating Layer Crack by Diffusion Heat Treatment in Al-Si Coated HPF Steels

The effect of diffusion heat treatment (DHT) on the cracking of the coating layer in Al-Si coated hot press forming (HPF) steel was investigated. Cracking-induced micro-delamination in the coating layer has limited applications of the Al-Si coated HPF steels. Introduction of DHT process can modify the microstructure and constituent phase in the coating layer which can reduce the cracking. DHT was performed at 650 °C for 360 min to drive Fe diffusion into the coating layer. Formation of band-shaped τ1-Fe₃Al₂Si₃ in the coating layer during the DHT was observed by EBSD and SEM. After HPF process of the diffusion heat treated coating specimen, the phase fraction of FeAl increased and that of η-Fe₂Al₅ decreased compared to the conventional Al-Si coating specimen, due to the increase of τ1-Fe₃Al₂Si₃. Owing to the higher fracture toughness of FeAl compared to η-Fe₂Al₅, application of DHT improved the toughness of the coating layer and reduced the crack density in it. DHT did not significantly affect the mechanical properties of the base steel.