Newly Developed Organic Composite-Coated Steel Sheet with Bake-Hardenability

A new organic composite-coated steel sheet with bake-hardenability has been developed. The steel sheet consists of Zn-Ni alloy electroplated layer, chromate and zinc powder free organic film. To obtain better formability (anti-powdering) and weldability, the thickness of the organic film is reduced to 0.8∼1.5µm (1.0∼2.0g/m²). The addition of bake-hardenability to this organic coated steel sheet was realized by controlling the amount of residual solute carbon in continuous annealed extra low carbon steels.

Corrosion resistance, formability and weldability of the obtained steel sheet were investigated.

1. 1)
   In two kinds of the cyclic corrosion test, the thickness reduction of unpainted steel sheet is the same as that of our “Zincrometal-KII” (1), which has zinc rich paint layer of 7µm on Zn-Ni alloy electroplated layer.
2. 2)
   In the formability test, the exfoliated film weight after cylindrical cup drawing is less than one fifth of “Zincrometal-KII”.
3. 3)
   In the weldability test, the number of continuous spot welding is over 5000 welds.