Three-dimensional Characterisation of Microstructures in Low-carbon Lath Martensite

The toughness of martensitic steels is strongly related to their fine and complex morphologies. To control the toughness of martensitic steels, the effect of carbon content on the morphology of lath martensite must be studied. However, these morphological changes are difficult to clarify using conventional two-dimensional observations because their morphologies are complex and tangled. Previously, we analysed and reported the three-dimensional microstructures of ultra-low-carbon lath martensite. Here we look further at three-dimensional microstructures of low-carbon lath martensite. Lath martensite in both specimens contains a few coarse packets in a prior austenite grain. The coarse packets in low-carbon lath martensite contain plate-like blocks stacked from end to end of the coarse packets and many fine blocks embedded in the coarse packets. The fine and included blocks are much smaller than the plate-like blocks in size. A part of the fine blocks belongs to the crystallographic packets different from the surrounding coarse packet, which should be regarded as ‘fine’ packets. The fine packets are also observed in ultra-low-carbon martensite. On the other hand, low-carbon martensite contains fine blocks that belong to the same crystallographic packet as the surrounding coarse blocks. These results suggest that the block structure in the coarse packet of low-carbon martensite is more entangled with each other than that of ultra-low-carbon martensite.