EFFECT OF P-CONTENT IN ULTRA-LOW C-TI STE EL ON GALVANNEALING BEHAVIOR

An investigation was made on the effects of the P content on GAMMA pha se 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 t han 0.025% tends to suppress the growth of the GAMMA phase. With a low P content (0.005%), during the galvanizing process, a relatively smal l Al concentration is detectable at the interface between the Zn coati ng 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 in terface is large. This results in the formation of an Fe-Al alloy laye r having a stronger burrier effect of difusion and the outburst struct ure is significantly suppressed. During the galvannealing of low-P ste el, the GAMMA phase begins to grow after the zeta and delta1 phases ha ve developed to some extent. This is because the delta1 phase, constit uting mainly the outburst structure, directly contacts the substrate f rom, the stage of the galvanizing process. Thereafter, the iron conten t of the delta1 phase rises, accelerating the formation of the GAMMA p hase. With high-P steel, in contrast, the galvannealing process progre sses through the Fe-Al alloy layer, since no direct contact occurs bet ween the delta1 phase and the substrate. The GAMMA phase starts to gro w only after the zeta phase disappears, which means the GAMMA phase gr owth is delayed.