Although reduction of the thickness of materials used in the automobile body is important for weight reduction, reducing the thickness of outer panels deteriorates dent resistance and surface distortion. To investigate the potential for weight reduction, the factors which influence the surface distortion and dent resistance properties were evaluated quantitatively with the aim of securing these properties. The materials used in these experiments were a tensile strength (TS) 340MPa grade bake hardenable (BH) steel sheet, which is often used in door outers, and a TS 440MPa grade BH steel sheet for outer panels.
Surface distortion increases as a result of higher yield point (YP). It is possible to suppress the increase in surface distortion by increasing the blank holding force (BHF) in press forming. However, because this reduces the BHF range to the forming limit, application of low YP material is considered to be more advantageous for suppressing surface distortion.
It was clear that increasing the value of YP after paint baking (YP’) was effective for high dent resistance. Compared with TS 340MPa grade BH, the TS 440MPa grade BH displayed a dent resistance effect equivalent to a thickness of 0.05mm. This suggested the possibility that panel thickness can be reduced by 0.05mm by replacing TS 340MPa grade BH with TS 440MPa grade BH.
A further increase of YP’ is expected by increasing the thickness reduction rate in press forming, as this will cause an increase in work hardening. As a result, it is considered possible to secure dent resistance with an even thinner thickness.