純チタン冷延薄板の延性に及ぼす結晶粒径，板厚および引張方向の影響

抄録

The effects of thickness (t), grain size (d), ratio t/d, and tensile direction on tensile properties were carefully examined using ASTM grade 1 pure titanium having strong B texture with controlled thickness (0.2-0.4 mm), grain size (20-175 mm), and t/d (2.3-19.4). 0.2％-proof stress followed a Hall-Petch relationship in each tensile direction regardless of thickness, and this indicates that 0.2％-proof stress does not depend on t/d. On the other hand, tensile strength and uniform elongation were confirmed to depend on t/d and significantly decreased in some thin sheets, in comparison with thick sheets with the same grain size, except for the case tested in the transverse direction. Namely, both tensile strength and uniform elongation exhibited a drastic decrease when t/d becomes smaller than the critical values. The critical value of t/d depended on tensile direction and decreased with an increasing tensile angle in the rolling direction. Local elongation increased with decreasing grain size except for the case tested in the transverse direction. In addition, local elongation was not related to t/d. Tensile properties that depend on t/d are inferred to be affected by work-hardening behavior, enhanced by twinning deformation in hcp titanium. However, the formation of deformation twinning in the surface regions was found to be suppressed, presumably due to the relaxed, constrained conditions. Consequently, the work-hardening rate decreased with decreasing t/d because of the attribution from the surface region where twinning deformation is retarded.

（a）,（b）,（c）True stress/true stain curves together with work hardening rate of dσ_t/dε and（d）,（e）,（f）number of twins per grain of pure titanium sheets. The tensile angles were（a）,（d）0°,（b）,（e）45° and（c）,（f）90° in the rolling direction. The grain size of the specimen was around 80 μm.