Lath martensite is an important structure in steels; however, its quantitative volume fractions have not been measured to date. In this study, we seek to clarify the morphologies and volume fractions of the structural units of lath martensite-packets and blocks - by using three-dimensional observations with local crystallographical analyses. Lath martensite structures in ultra-low carbon and high-carbon steels contain both coarse and small packets, whose threshold of volume fraction in prior austenite grain is 10%. The volume fractions of the coarse packets in prior austenite grain in the specimens are greater than 70%, although the number of packets is fewer than five. In ultra-low carbon steel, the coarse packets have flat packet boundaries and contain three types of blocks with a plate-like morphology. On the other hand, in high carbon steel the morphology of the coarse packets is sponge-like and contains six types of blocks. There are more small packets than coarse packets in the observed specimens, and the volume fraction of the small packets is lower. The number of small packets in high carbon martensite is more than that in the ultra-low carbon martensite. Volume measurements show a distribution of volume fractions of Kurdjumov-Sachs (K-S) variants. In high carbon martensite, all the K-S variants appear continuously. The volume fractions of some K-S variants are relatively low in ultra-low carbon martensite.
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