Abstract
The effect of microstructural parameters, particularly, ferrite grain size, on the quality of a sheared edge in piercing was examined in 0.02C ferrite steel with grain sizes of 13μm and 0.7μm, 0.002C ferrite steel with a grain size of 0.7μm, and 0.3C ferrite+pearlite steel. The quality, component ratio of rollover, burnished surface and fracture surface was influenced by both clearance during piercing and microstructure. The ratio of rollover decreased with a decrease in clearance, on the other hand, the ratio of the burnished surface increased with a decrease in clearance, irrespective of microstructure and grain size. The ratio of rollover was markedly influenced by grain size. The ratios of rollover in ultrafine-grained ferrite steels were very small compared with those in large-grained and ferrite+pearlite steels. This is because ultrafine-grained steels have a small work hardening ability. The ratio of the burnished surface decreased with grain refinement, owing to smaller elongation. However, the ratio of the burnished surface of 0.02C ultrafine-grained steel was as high as 40%, although the elongation was as small as 5%. The ultrafine-grained ferrite steel has a large reduction in area in spite of its small elongation. This indicates that the ratio of the burnished surface is influenced by reduction in area rather than elongation.
