ばね鋼SUP7におけるオースフォームドベイナイト

抄録

In order to clarify the applicability of ausformed bainite in spring wire, effect of ausforming on the microstructure and the hardness of isothermally held JIS-SUP7 spring steel was investigated. The acceleration of isothermal transformation by ausforming was qualitatively confirmed. Ausforming made the pearlite colony size small, but the hardness of proeutectoid ferrite+pearlite structure only slightly increased by deformation. The bainitic ferrite size, especially its thickness, was decreased by ausforming. The bainitic packet which is an unit of bainitic ferrites with the same direction was refined by 20% deformation, while after 30% to 35% deformation the bainitic ferrite within an identical prior-austenite grain tended to align to a same direction so that the packet rather coarsened. Independent of deformation, large amount of austenite up to 35% retained after quenching to room temperature in the specimens tempered at upper bainite region. In the specimens tempered at lower bainite region, the amount of retained austenite largely increased by ausforming. In spite of the increase in retained austenite, ausforming by 30% to 35% strengthened the material tempered at bainite regions. Especially, the ausformed lower bainite showed large hardness of HV650 which is superior to a conventional quenched and tempered material with tempered martensite structure. These results indicate that the ausformed lower bainite could be applied to practical use.