Diffusion driven columnar grain growth induced in an Al–Si-coated steel substrate

• Hot dipping and diffusion annealing increases Al/Si content in electrical steel.
• Hot dipping forms different intermetallics and a 5 μm solid solution of Al in Fe.
• Diffusion annealing sometimes causes the development of a columnar grain structure.
• Substrate grains that form a solid solution are the nuclei for the columnar grains.
• Columnar grains grow via Al diffusion from intermetallics and γ→α transformation.

An alternative method to increase the Al or Si content in steels for electrical applications consists of dipping the steel substrate in a molten Al–Si bath, followed by an annealing treatment to diffuse Al/Si into the substrate. Under specific conditions, a columnar grain structure develops during diffusion annealing. It is demonstrated that a diffusion driven mechanism is responsible for the development of this particular structure. During hot dipping, a 5 μm thick layer of the substrate forms a solid solution of Al in Fe, with Al-concentrations up to the solubility limit. The nuclei for the columnar structure are grains of this 5 μm thick layer, as this small zone does not transform into austenite during diffusion annealing. The intermetallics formed during hot dipping merely serve as an Al source during diffusion annealing.