| 摘要 |
PROBLEM TO BE SOLVED: To obtain a steel sheet capable of assuring metal tin sufficient for high-speed seam weldability without the progression of alloying of tin even after coating and baking at a high temp. for a short time by having granular tin plating on the steel sheet, laminar tin plating thereof and further a metal chromium layer and chromium hydroxide layer on the upper layer thereof. SOLUTION: The grain size of the granular tin plating is specified to 0.2 to 10μm and the number thereof is regulated to 1×10<3> to 1×10<6> for each 1 mm<2> . Since the entire surface of the steel sheet is not coated by this granular tin plating alone, the granular tin plating is provided thereon with the laminar tin plating in order to eliminate the exposed parts of the steel sheet and to improve the corrosion resistance thereof. In such a case, the total tin plating quantity which is the sum of the granular and laminar tin plating quantities is regulated to 200 to 1500 mg/m<3> . Further, the upper layer of the tin plating layer is provided with the metal chromium layer and chromium hydroxide layer in order to assure a good coating material adhesion property. The coating weight of the metal chromium layer is regulated to 2 to 15 mg/m<2> and the coating weight of the chromium hydroxide layer to 2 to 20 mg/m<2> .
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