COMPOSITE-LAYER PLATED STEEL SHEET HAVING EXCELLENT PLATING ADHESIVENESS AND PERFORMANCE AFTER PAINTING

摘要

PURPOSE:To improve plating adhesiveness and performance after painting by subjecting a steel sheet successively to Fe, Ni and Co plating, composite Zn plating and further Fe alloy plating via a chemical conversion film. CONSTITUTION:This composite-layer plated steel sheet has, successively from the front, a 1st plating layer consisting of =1 kinds among Fe, Ni and Co ad the 2nd plating layer consisting of Zn or Zn alloy contg. fine particles having =5mu average grain size on at least one face thereof. The 3rd plating layer consisting of an Fe alloy is further provided on said layer via the chemical conversion film; for example, phosphate film. The 1st plating layer is required in order to enhance the adhesiveness of the 2nd plating layer with such composite-layer plated steel sheet but there is a need for relieving the impact force of chipping at a low temp. The formation of the 3rd plating layer is industrially best for the above-mentioned purpose; further, the chemical conversion film has the ability to slightly relieve the impact force. The impact force is greatly relieved if a phosphate film is formed as a subsurface treatment for painting in the 3rd plating layer.