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AssigneeSumitomo Metal Industries, Ltd.
InventorsSachio Matsuo, Toshiaki Shiota, Maki Itoh, Hideo Kawaguchi, Hiroki Hanabata, Yukihiro Yoshikawa, Takao Taka, Kiyoyuki Fukui
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A precoated steel sheet having improved corrosion resistance and formability is disclosed, which comprises a Zn- or Zn alloy-plated steel sheet having on the plated surface either a colloidal silica-containing chromate undercoat layer and a polyhydroxypolyether resin-based topcoat of 0.3 to 10 .mu.m in thickness in which resin is derived by polycondensation of a dihydric phenol component selected from a mononuclear dihydric phenol, dinuclear dihydric phenol, and a mixture of both, with an epihalohydrin or a non-colloidal material-containing chromate underlayer and an epoxy resin based topcoat containing colloidal silica of a thickness of from 0.3-1.6 .mu.m in thickness. In spite of the absence of zinc powder, the precoated steel sheet can be satisfactorily welded by resistance welding when the thickness of the topcoat layer is not greater than 2.5 .mu.m, and even with such a thin topcoat, the precoated steel sheet retains its improved corrosion resistance and formability. The precoated steel sheet can be satisfactorily finish-coated by electrodeposition. The undercoat layer is produced by a two stage reduction of Cr.sup.+6 to Cr.sup.+3 in an aqueous suspension containing chromic acid.
A precoated steel sheet having improved corrosion resistance and formability produced by
A precoated steel sheet having improved corrosion resistance and weldability produced by (i) applying or firing after application an undercoat of a chromate film with a weight of 20-100 mg/m.sup.2 as Cr to the plated surface of a Zn or Zn alloy-plated steel sheet, said undercoat is an aqueous suspension containing partially reduced chromic acid which is produced by (a) introducing an effective amount of a reducing agent under effective temperatures to provide a chromic acid having a ratio of Cr.sup.3+ /(Cr.sup.3+ +Cr.sup.6+) of 0.4-0.6 and (b) introducing an additional amount of a reducing agent so as to provide an amount of reducing agent which is 1-4 times larger than that required to reduce the remaining Cr.sup.6+ to Cr.sup.3+, further wherein the aqueous suspension is substantially free from colloidal materials, and (ii) applying or firing after application a topcoating of 0.3-1.6 .mu.m in thickness and which comprises a resin-containing solution which contains as a base resin an epoxy resin together with colloidal silica in amounts of 10-25% by weight based on the total amount of resin solids and colloidal silica in the resin-containing solution.
Dependencies are inferred from the claim text.