Papers by Author: Yoritoshi Minamino

Abstract: The effect of near surface-ultrafine grain (NSUFG) layer with grain size of about 90nm on the solid reaction between Ti-added ultra-low carbon interstitial free steel sheet and electroplated zinc layer (ZP) was basically investigated at 473K and 573K, in comparison with the reactions of the coarse grains IF steel sheet (CG-IF) and the ZP. The NSUFG structure dramatically changes the reactions between ZP and IF steel, as follows; (1) the incubation times for the formation of reaction layers, pseudo-z-phase, are quite shorter for the NSUFG/ZP reactions than the CG/ZP ones, (2) in the early stage of annealing the former has the smooth interfaces of pseudo-z-phase layers but the latter has wavier ones like a stone wall, (3) the thickness of the pseudo-z-phase layer of the former is thicker than that of the latter, (4) some cracks are observed in NSUFG layers while no crack in coarse grain IF steel, and (5) the subsequent layer appears in the reaction between pseudo-z-phase and NSUFG layers former after long annealing by the blast-like break of the interface.

Abstract: Three kinds of bulk nanocrystalline Fe-24at%Al-Xat%C (X=1,2,4) alloy were produced from Fe and Al powders with addition of methanol by MA with subsequent SPS at 1273K under 64MPa. Their microstructure and mechanical properties were investigated. The compacts have the relative densities of 99.97% (1at%C) to 99.6% (4at%C). The structure of compacts with 1at%C is composed of Fe3Al grains of 1.5µm in diameter and nano k-carbides (Fe3AlC0.5) precipitates, while those of compacts with 2 and 4at%C are composed of nanocrystalline Fe3Al of about 80nm in diameter, nano k-carbides and a-grains of about 1µm in diameter. These structures have the good thermal stability, maintaining the nanostructure even at 973K. The mechanical properties of these compacts were measured by compression tests at R.T. to 973K. The compacts with 1at%C and 2at%C of this work perform the superior mechanical properties (e.g. yield strength of 2.15GPa and rupture strain of 0.14 for compact with 2at%C at R.T.) when compared with the ordinary Fe3Al casting (e.g. 380MPa and 0.12). They also exhibit no environmental embrittlement, which is one of fatal problems for the ordinary Fe3Al mateials.

Abstract: Ultra low-carbon interstitial free (IF) steel having ferrite (b.c.c.) single phase was deformed to various equivalent strains ranging from 0.8 to 5.6 by the accumulative roll bonding (ARB) process at 500°C. The microstructure and crystallographic feature of the deformed specimens were characterized mainly by FE-SEM/EBSD analysis. Grain subdivision during the plastic deformation up to very high strain was clarified quantitatively. After heavy deformation above 4.0 of strain, the specimens showed the lamellar boundary structure uniformly, in which the mean spacing of the lamellar boundaries was about 200nm and more than 80% of the boundaries were high-angle ones. Annealing behavior of the ARB processed IF steel strongly depended on the strain. The specimens deformed to medium strains exhibited discontinuous recrystallization characterized by nucleation and growth, while the specimens deformed above strain of 4.0 showed continuous recrystallization. The recrystallization behaviors are discussed on the basis of the microstructural and crystallographic parameters quantitatively measured in the as-deformed samples.