Influence of κ-carbide interface structure on the formability of lightweight steels

• The reasons for the poor formability of α-matrix-based high Al steel have been found.
• Large lattice misfit and lamellar structure cause the materials cracking.
• A flash processing using electric current changes the microstructure.
• The cold formability of lightweight steel can be improved drastically by the electric current processing.

κ-Carbide (κ) in high aluminium (Al) steels is grown from austenite (γ) via γ → γ + κ or γ → α + κ (α represents ferrite), and is a lamellar structure. This work demonstrates that the formability of high Al lightweight steels is affected by the lattice misfit and interface shape between κ and matrix. The cold workability can be improved by either to change the steel chemical constitution or to implement an electro-thermo-mechanical process. For ferrite-matrix-based high Al steel, electric-current promotes the spheroidization and refinement of κ structure and reduces volume fraction of κ phase. This retards the crack nucleation and propagation, and hence improves the materials formability. The observation is caused by a direct effect of electric-current rather than side effects.

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