The surfaces of dies used for flat sliding tests of galvannealed steel sheet (GA) have been investigated using several electron microscopic techniques in order to clarify the adhesion mechanism. Two kinds of adhesive materials were identified on the die surface. One consists of an Al oxide -Fe-Zn alloy composite and tightly bonds to ridges of the die surface. This resembles the built-up edges formed on tool surfaces during metal-cutting operations. Another adhesive material is composed of Fe-Zn alloys located in the hollows of the die surface which do not bond or only loosely bond to the die surface. A new adhesion model consisting three steps is proposed; (1) the Al oxide -Fe-Zn alloy composite layers are formed on the ridges of the die surface. (2) The Fe-Zn intermetallics on the GA surfaces are cut by this layer and (3) accumulate in the hollow of the die surface.

The typical sliding conditions at the fracture-risk positions in press forming of automotive parts were identified by FE analysis and laboratory-scale sliding tests. The investigated sliding conditions fall into three categories: the inflow sections of a steel sheet (group A), the punch shoulders in drawing (group B), and the stretch sections (group C). The frictional properties of group A and group B could be evaluated by measurement of the dynamic coefficient of friction in the flat sliding tests under a high contact pressure and at low sliding speed in each group area. The frictional properties of group C could be evaluated by the static coefficient of friction, which is measured as the primary peak in a coefficient of friction -stroke curve under the specific sliding test conditions (contact pressure: 7MPa, sliding speed: 10mm/min, tool length: 10mm).

The effect of pre-strain on the stretch flange deformation limit of steel sheets was investigated by hole expansion tests under various pre-strain conditions. Pre-strain was given by stretch forming with a cylindrical punch. The strain ratio and amount of pre-strain were controlled by changing the blank shape and forming height. The hole expansion tests were carried out with a conical punch and cylindrical punch in order to clarify the effect of the strain gradient. The effect of the strain ratio of pre-strain on the stretch flange deformation limit can be expressed by equivalent strain. In the case of higher strain gradients, the stretch flange deformation limit increased with increasing pre-strain. However, at lower strain gradients, the stretch flange deformation limit was almost constant. These results suggest that the formability of the stretch flange deformation area given by pre-strain should be determined from not only the total equivalent strain but also the strain gradient and the amount of pre-strain expressed by equivalent strain.

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