Change in dislocation characteristics with cold working in ultralow-carbon martensitic steel

In a previous study, the authors used X-ray analysis with the classical Williamson-Hall (CWH) method to suggest that charging a small amount of cold working markedly decreases the dislocation density of ultralow-carbon martensitic steel, although this heightens the 0.2% proof stress. However, this method does not consider the dislocation arrangement. In the present study, a modified Williamson-Hall/Warren-Averbach (MWH/WA) method was applied to ultralow-carbon martensitic steel (Fe-18%Ni alloy) in order to evaluate not only the dislocation density but also the dislocation arrangement. Their effects on the yielding behavior were examined. With the MWH/WA method, the dislocation density did not change up to 40% cold rolling. On the other hand, the dislocation arrangement parameter M was high (M > 1) in the as-quenched state and became smaller (M < 1) when a small plastic strain was charged. This means that the dislocation distribution is random in as-quenched martensite but changes the cell structure with cold working. Owing to such a dislocation arrangement, the CWH method tends to overestimate the dislocation density of as-quenched martensite compared to the MWH/WA method. Tensile testing revealed that the elastic limit was very low in as-quenched martensite and high in cold-rolled martensite. In the case of a tangled dislocation structure, a higher stress should be required because of the stable dislocation structure. On the other hand, the random dislocations introduced by martensitic transformation can easily move at a low stress level owing to their unstable distribution, which leads to the low elastic limit in asquenched martensite.