Heterogeneous microstructure of low-carbon lath martensite with continuous yielding behavior in Fe-C-Mn alloys

Abstract

High strength steels have been rapidly developed for several kinds of application fields such as light weight automobile bodies, multistory buildings, and oil well tubulars. With an increase of strength, the use of the low-carbon lath martensite phase puts this component as the main microstructure, or a part of the dual phase one. However, the martensite is a non-equilibrium phase, and the microstructure is also a heterogeneous composite-like one including of fine carbides and retained austenite. In order to understand the hierarchical structure, multi-scale microscopic techniques have been utilized. Internal twins in a lath and some large lath domains with fine carbides are revealed by scanning electron microscope (SEM) and transmission electron microscope (TEM) observations, although a conventional low-carbon martensite is known to include mainly dislocations in the lath structure. The heterogeneous microstructure is discussed based on a model of the local difference of transformation temperature of each lath block segment during cooling. On the plasticity of low-carbon lath martensite, the continuous yielding behavior is one characteristic property, and the elastic limit is obtained by the cyclic tensile test. The heterogeneous deformation behavior is detected as a periodic strain partitioning among lath blocks in a grain by digital imaging correlation analysis using in situ SEM tensile testing.