Growth Kinetics of Al-Si-Fe Intermetallics during Hot Dipping of Steel

Electrical steels are used in flux carrying machines, divided in grain oriented and non oriented electrical steels mainly used in transformers and electrical motors, respectively. Their industrial production is not always easy due to the alloying elements which produce brittle order structures in the steel. Therefore hot dipping was found to be an alternative way of producing electrical steel with a high concentration of Al and/or Si: in a first series of experiments different steel substrates were coated by immersion in an Al + 23 m.-% Si hypereutectic alloy, followed by a high temperature diffusion annealing. The present contribution reports on the growth kinetics of Al-Si-Fe intermetallics formed during the dipping process in a hypoeutectic Al – 5 m.-% Si bath of Fe-substrates with 3 m.-% Si, previously cold rolled to different thickness. This bath composition allows a liquid phase at temperatures lower than the hypereutectic one with 23 m.-% Si and also less amount of eutectic formation. No Na-addition was made to the bath (the occurrence of a needle-like morphology of the Al-Si eutectic was not relevant for these experiments), furthermore this element might lower the magnetic properties of the steel. The preheating of samples and bath temperatures were not varied and set to 670°C. Short dipping times of 1 to 60 sec. were applied. The different layers formed were characterised by Scanning Electron Microscopy (SEM), using the Back Scattered Electron (BSE) detector and Energy Dispersive Spectroscopy (EDS).