Kohei Ueda

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.

Formability of polyester/melamine pre-painted steel sheets from rheological aspect

We reported that the formability of PCM in deep drawing was influenced by the maximum strain of paint films and the elastic strain energy stored in paint films after deformation. The elastic strain energy is considered to be an index of elastic deformation of paint films. It can be considered that the formability in deep drawing is associated with plastic deformation of paints. In this study, rheological property of polyester/melamine paint systems was investigated to examine the formability of paint film in deep drawing. The relationship between the rheological properties of the paint films and structures of paint were also studied. The PCM with paint films having a low rubbery modulus, high steady-state compliance, low creep viscosity and large permanent strain seems to have good formability in deep drawing. These viscoelastic properties are dominated by the molecular weight and Tg of polyester resins and inclusion of melamine resin in paint films. The formability in deep drawing increases with an increase in the molecular weight. In addition, the paint films having a low cross-linking density also have good formability in deep drawing.

Viscoelastic properties of paint films and formability in deep drawing of pre-painted steel sheets

Damage of paint films on pre-painted steel sheets (PCMs) in a deep drawing process is influenced by the mechanical properties of the paint films. We reported that paint films, which exhibit high elongation and low elastic strain energy (ESE) have good formability in a deep drawing process. However, examination of the paint films from a rheological viewpoint is needed for further clarification of the formability of PCMs. In this study, the viscoelastic properties of polyester/melamine paint films widely used for PCMs were studied concerning dynamic viscoelasticity, stress relaxation and creep. The effects of the viscoelastic properties of the films on the formability of PCMs were examined. It is confirmed that paint films, which exhibit good formability in deep drawing, have higher glass transition temperature (Tg) than the temperature at which drawing was performed, also they have both low rubbery modulus and low creep viscosity. Furthermore, the rubbery modulus and the creep viscosity of such paint film decreases with the increase of strain. These results show that such paint films have low cross-linking density and that the cross-linking networks are easily destroyed by large deformation. We conclude that the paint films, which exhibit high plasticity in a large deformation process, have good formability in deep drawing.