Zr添加超微細結晶フェライト系ステンレス鋼の機械的性質 I. メカニカルアロイング法による組織制御

Targets of this study are realization of better mechanical strength and retention of toughness for ferritic stainless steel by micro-structural modification of grain refinement in the material processing by doping of an active element and mechanical alloying (MA). MA allows the introduction of large strain energy and for materials produced by MA, the formation and presence of dispersed particles in them plays an important role in the grain refinement. The authors have been interested in zirconium which has a high affinity for gaseous impurities such as oxygen, carbon etc., and they examined the effect of zirconium on grain refinement of 12 mass% chrome ferritic stainless steel by MA. Conventional 12Cr(SUS 410) and 12Cr-1Zr ferritic stainless steels containing about 1 mass%Zr were consolidated by extrusion-forming of their powders without MA at 1073 K. They had grain diameter sizes of about 30 μm and 1 μm, respectively. On the other hand, 12Cr-1Zr steel produced through MA and the same consolidation process (designated MA-12Cr-1Zr) had a grain diameter of 0.36 μm or less. For MA-12Cr-1Zr, it seemed that Zr atoms strongly reacted with gaseous impurities such as oxygen and carbon and fixed them as oxide and carbide. These dispersed particles ranged from 5 to 30 nm in diameter. In particular, it seemed that the zirconium oxide particles were more effective in suppressing the growth of grain boundaries as TEM images showed that the particles were mainly located along them. The grain sizes obtained from the diameter and volume fraction of dispersed particles tended to support Doherty's prediction.