論文
ばね鋼SUP12におけるベイナイト組織と機械的性質
2009 年 2009 巻 54 号 p. 27-33
詳細
2009 年 2009 巻 54 号 p. 27-33
The strength of spring steels has been increasing in order to cope with the demands of lightweightness in automobile applications. However, high strength steels, typically consisting of tempered martensite structures, accompanied by large reduction of toughness are susceptible to failure of spring properties such as fatigue resistance in corrosive conditions, because of increase in notch sensitivity. Since bainite often exhibits high strength and toughness in comparison with tempered martensite, we have studied microstructures and mechanical properties of an austempered spring steel (JIS-SUP12) by means of scanning electron microscopy and tensile tests in order to examine the potential application of bainitic spring steels. The result shows that carbide-free and lath-like bainitic ferrite was formed at austempering temperatures higher than 400°C, while plate-like bainitic ferrite with fine carbide precipitations was formed at temperatures lower than 300°C. The fraction of the bainitic ferrite increased whereas that of the martensite decreased with increased holding time in austempering. The amount of the retained austenite increased during the early stage of transformation and then decreased with further holding. At optimum austempering condition, the bainite with high strength, high elongation and reduction of area superior to tempered martensite was obtained.
