Effect of Al-based intermetallic coating layer on H uptake during austenitization in 31MnB5 steel

Currently, the Al-Si-coated steel is widely utilized in hot press forming (HPF) processes. The Al-Si coating serves as a protective barrier against oxidation and decarburization, as the HPF steel undergoes austenitization at temperatures exceeding approximately 900 °C. However, the formation of liquid Al during austenitization triggers an oxidation reaction with atmospheric moisture, generating H atoms. This leads to H uptake and subsequent embrittlement. Therefore, in the present study, to reduce the generation and absorption of H during the austenitization of Al-Si-coated HPF 31MnB5 steel, we modified the Al-Si coating layer to a fully intermetallic coating layer by performing annealing before HPF process. Consequently, there was a notable reduction in both H uptake and subsequent embrittlement. This can be attributed to three main reasons. (1) Restricted interaction between the Al-based intermetallic phase on the surface of the coating layer and the water vapor in the furnace atmosphere. (2) Low H diffusivity within the intermetallic coating layer. (3) Efficient entrapment of H within surface voids found in the intermetallic coating layer.