超微細複合組織鋼の組織制御による引張変形特性の改善  [in Japanese] Improvement of Tensile Properties by Microstructural Control for Ultrafine-Grained Multi-Phase Steels  [in Japanese]

Effect of microstructural control on tensile behavior for a 17Ni-0.2C steel was studied. Ultrafine microstructures with less than 1μm grain size were prepared by cold rolling of martensite structure followed by appropriate tempering. A drawback of little uniform elongation in ultrafine grained alloys has been overcome by introducing austenite grains in the ultrafine grained ferrite matrix. In this ferrite-austenite steel, work hardening after Luders deformation increased due to stress-induced martensitic transformation, so that the onset of necking was delayed resulting in large uniform elongation. The Luders deformation was suppressed by skin-pass rolling for the ferrite-austenite steel. Another method to avoid the appearance of Luders deformation was realized by changing the tempering condition, i.e., by introducing fresh martensite upon cooling after tempering. In this case, microstructure was composed of ferrite, austenite and martensite and an excellent combination of tensile strength and elongation was obtained.