저탄소강 베이나이트와 마르텐사이트의 응집 조직

Abstract

This work aims to illuminate the formation principle of coalesced structure in bainite and martensite of low–c arbon steels. It was also tried to suggest a solution to avoid the phenomenon efficiently. There were four kinds of specific work included in the thesis.Firstly, coalescence in martensitic structure for investigated four alloys was identified by its bimodal distribution of grain size, which is never explained by simple geometrical sectioning effect. Considering martensite transformation proceeds as temperature decreases, the related free energy change between austenite and ferrite increases, which led to the coalescence in martensitic structure due to the resultant large driving force. Coalescence in bainite occurred at low–transformation temperatures, where carbon partitions laggingly and consequently the formation of austenitefilm becomes retarded. These enhance the adjacent martensitic ferrites to meet. The critical driving force for coalescence was estimated for a specific alloy by tracing the transformation temperature where no coalescenceoccurred.Secondly, misorientation within coalesced martensite was examined using transmission electron microscopy. Considering that austenite accommodates the associated shape deformation plastically, the bending of corresponding slip planes relates to the rotation of austenite crystal. This affected the orientation of martensitic ferrites which formed successively.Two active slip systems were specified using well established Taylor theory combined with literature data on the growth of bainitic ferrite.Thirdly, effect of external force on coalescence was studied using confocal laser scanning microscopy and thermo–mechanical simulation. Martensite transformation was promoted near to free surface, which is consistentwith theoretical calculation to predict large strain energy relaxation there. Interestingly, coalesced martensite tended to form near to free surface as a same manner. Direct tensile stress during isothermal bainite transformation induced the coalescence in bainitic ferrites, however, it occurred selectively in terms of crystal orientation of bainitic ferrites.Finally, it was attempted to block the coalescence by fragmenting the crystallographically homogeneous domains. Intercritical annealing after martensite transformation was designed to construct substitutional elements–enriched area using a program DICTRA version 25. Energy dispersive spectrometry showed that nickel and manganese–enriched areas were survived from full austenitisation and it was confirmed that the intended fragmentation was successfully conducted.