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Synopsis The effects of surface microstructure and chemical compositions on characteristics of Fe-Zn compounds which formed on continuously galvanized steel sheets have been investigated by using ultra low carbon and low carbon steels. From SEM observation, morphology of Fe-Zn compounds formed at interface between steel and plating layer were classified into three types; outburst structure, fine granular structure, and pillar-like structure. The amount of outburst structure increases with decreasing solute carbon content in steels. Phosphorus inhibited the formation of outburst structure when solute carbon was combined with a carbide forming element such as Nb or Ti. A comparison between distribution of Fe-Zn compounds and corresponding steel surface indicated that the outburst structure formed preferentially at grain boundaries exposed on the steel surface, whereas fine granular structure formed mainly at the interior of surface grains.

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A new type of cold rolled dual phase steel sheet with good formability and dent-resistance has been developed for automobile outer body panels. The steel, basically a low carbon and manganese aluminum killed steel, is manufactured by means of hot rolling with high temperature coiling, cold rolling, and continuous annealing with water-quenching and tempering. The mechanical properties after temper rolling are characterized by yield strength lower than 24 kgf/mm2, total elongation larger than 40% and 3-value higher than 1.5. Moreover the steel sheet has not only high bake hardenability but also high anti-aging property.