Abstract
An investigation was made on the effects of the P content on Γ phase growth during the process of galvannealing ultra-low-carbon-0.05%-Ti-steel sheet. The results showed that a P content of equal or more than 0.025% tends to suppress the growth of the Γphase. With a low P content (0.005%), during the galvanizing process, a relatively small Al concentration is detectable at the interface between the Zn coating layer and the steel substrate and this accelerates the formation of the outburst structure. With a high P content (0.052%) in which more P is concentrated at the steel surface, the Al concentration in the interface is large. This results in the formation of an Fe-Al alloy layer having a stronger burrier effect of difusion and the outburst structure is significantly suppressed. During the galvannealing of low-P steel, the Γphase begins to grow after the ζ and δ1 phases have developed to some extent. This is because the δ1 phase, constituting mainly the outburst structure, directly contacts the substrate from the stage of the galvanizing process. Thereafter, the iron content of the δ1 phase rises, accelerating the formation of the Γphase. With high-P steel, in contrast, the galvannealing process progresses through the Fe-Al alloy layer, since no direct contact occurs between the δ1 phase and the substrate. The Γphase starts to grow only after the ζ phase disappears, which means the Γ phase growth is delayed.
