Surface-treated steel sheet having improved corrosion resistance after forming

摘要

A material which can be improved in its resistance to corrosion caused by alcohol-containing fuels after formation without detriment to weldability and without any substantial cost increase is developed. Constitution A chromate film is applied to a Zn-X alloy electroplating layer, in which X is one or more of Ni: 3-18 wt %, Co: 0.02-3 wt %, Mn: 25-45 wt %, or Cr: 8-20 wt %. The Zn-Ni alloy plating layer underlying the chromate film has cracks with a density of 1000-150000 in terms of the number of plated regions surrounded by cracks in a 1 mmx1 mm visual field, with cracks having a maximum width of 0.5 mu m or less comprising 90% or more of the total number of the cracks, and with cracks having a depth of 80% or more of the thickness of the plating layer comprising 80% or more of the total number of the cracks.