極低炭素変態強化鋼の変態組織と強靱性

抄録

The α→γ transformation behavior and its correlation with mechanical properties were systematically investigated in two types of transformation-strengthened steel under a range of very low carbon content from 0.005% to 0.05%. Those are C-high Mn-Nb-B bainite steels and C-high Mn-Mo-Nb acicular ferrite steels. The effects of C content and controlled rolling on the microstructures and mechanical properties were studied, by focusing on the difference between these two steels. The microstructure of bainite steels was recognized as type I upper bainite which was characterized by lath structure of 0.5-1μm width with a retention of austenite grain boundaries in pan-cake shape. Acicular ferrite steels showed a mixed microstructure consisting of polygonal ferrite and upper bainite.
Transformation behavior and mechanical properties in bainite steels did not change markedly with both C content and the degree of controlled rolling, while they changed remarkably in acicular ferrite steels with those variables. Metallurgical factors controlling the strength and toughness in both steels were discussed in detail.