Production of super steel by multi directional forging

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Effect of large amount of dispersed particles on ultra-fine grain evolution severe deformation was investigated. For that purpose, austenitic stainless steels containing about 1 vol.% of second phase particles, which radii ranged from 30 to 70 nm, were multi-axially compressed at 637 and 873 K to a strain of ε=6.0 at maximum. Microstructure with nodes of dislocation walls or boundaries was gradually developed as strain increased. However, fine-grain evolution seemed to be stagnated by the presence of the fine particles when compared with the result of plain 304 stainless steel with no precipitates. Though ultra-fine grain evolution was not sharply took place even at ε=6.0, the grain size of the ultra-fine grains specifically evolved around deformation bands and microbands was about 0.5μm in average. Such microstructural evolution looked to be more obvious in the samples containing finer particles. This result suggests that recovery process is one of the most important factors on the ultra-fine grain evolution by severe plastic deformation.