shows the effect of the sliding lengthon the flat surface area ratio of the sheet surfaces. In this study, theflat surface area ratio is defined as the percentage of the flattenedareas which are formed by sliding tests in the total of the sheetsurface areas. The flat surface area ratio increased sharply in the sliding length range from 0 mm to 40 mm, and then increased grad-ually after 40 mm. At sliding lengths over 40 mm, the real contactpressure between the tool and surface asperities of the steel sheetsdecreased and deformation of the steel sheet surface was retardedbecause the larger flat area of the sheet surface also increased thereal contact area between the tool and the sheet.