To improve the punchability of a steel sheet and furthermore to obtain various characteristics equal to those of semiorganic film by allowing the outermost surface layer to have a specified amt. of crystalline zinc phosphate film.
The surface layer film is composed of crystalline zinc phosphate, the form of this crystals is a thin planar one, the raggedness allows a die and the surface of the steel sheet to be point-contacted at the time of punching to improve its punchability. Since the film is formed at a high speed, it is preferable that electrolyzing treatment is executed in a soln. of phosphate contg. zinc only or zinc and nickel with the steel sheet as the cathode. The amt. of the crystalline zinc phosphate film is regulated to 0.2 to 5 g/m2. As for the base, it is possible that Zn or Ni plating is not present, but, in the case of their presence, they form the nuclei at the time of chemical conversion formation to shorten the treating time, where the contents of Zn and Ni in the final product are laminated to ≤5 g/m2. In the case the value exceeds, the plating itself adheres to the die to cause seizure and to deteriorate its punchability.
NIIGASHIRA HIDETOSHI
SATO HIROAKI
