Effects of mechanical properties of paint film on the forming of pre-painted steel sheets

Formability is one of the most important factors for pre-painted steel sheets (PCMs). Both high formability of the steel substrate and high integrity of the paint films on it after processing are required. In this study, elongation, tensile strength and elastic strain energy of various paint films were examined by tensile test, and the relationship between the mechanical properties of paint films and the integrity of paint films of PCM after bending and drawing were studied. We found that the required properties of paint films of PCM depended on the forming mode. In the bending mode, in which steel sheets were mainly subjected to tensile strain, the damage of paint films was dominated by the maximum strain and the tensile strength of the paint films. The paint films having higher maximum strain and higher tensile strength had better formability in bending. On the other hand, the damage of paint films was dominated by the maximum strain of paint films and elastic strain energy stored in paint films after deformation in the drawing mode in which steel sheets were subjected to tensile and compressive strain. The paint films having higher maximum strain and lower elastic strain energy had better formability in drawing. The relationship between the lubrication attributes of paint films and the limit of drawing ratio (LDR) of PCM was also studied. It was observed that the PCM with paint films having low friction coefficient had a higher LDR and it was the same ratio as the unpainted substrates.