Formation and Growth of Iron-Zinc Compound Phases in Galvannealed Coatings on Phosphorus-containing Steel

ARAI Masahiro, ADACHI Yoshitaka, NAKAMORI Toshio, USUKI Noriaki

doi:10.2355/tetsutohagane1955.83.11_713

The formation and growth of the iron-zinc compound phases, especially Γ₁-Fe₅Zn₂₁ and Γ-Fe₃Zn₁₀, at 773K in galvannealed coatings on the titanium-stabilized interstitial free (IF) steel and the interstitial-free rephosphorized (IFP) steel have been investigated by means of the cross-sectional SEM observation. The formation order of Γ₁ phase and Γ phase was different between the substrates; on the IFP steel substrate, the formation of Γ phase was retarded, which resulted in the prior formation of Γ₁ phase. The formation and growth of Γ₁ phase, on which little influence of phosphorus in steel was observed, is considered to be controlled by the diffusion of iron atoms across the Γ₁/δ₁ interface, which is controlled by the diffusion-coefficient change and the iron-concentration gradient of δ₁ phase. The reactionrate analysis reveals that Γ phase on the IF steel substrate grows by moving the Γ/substrate interface toward the substrate by the diffusion of zinc atoms from the coating into the substrate and that early formation and rapid growth in the earlier stages of alloying reaction are caused by moving Γ/Γ₁ interface toward the Γ₁ phase by the diffusion of iron atoms from the substrate to the coating. Both formation and growth of Γ phase on the IFP steel substrate were retarded, comparing to those on the IF steel substrate, which suggest that both solid-soluted and grain-boundary segregated phosphorus in the IFP substrate interferes the zinc diffusion from the coating into the substrate.

Formation and Growth of Iron-Zinc Compound Phases in Galvannealed Coatings on Phosphorus-containing Steel

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