Formation of fragmented structure during warm deformation of low-carbon martensite steel

The structure of low-carbon martensite steels VKS-7 and VKS-10 subjected to warm rolling or compression in alpha state at 600 and 700 °C is studied by metallography, scanning and transmission electron microscopy. The role of dissolution and precipitation of carbide phases, sinking of dislocations to high-angle lath boundaries, polygonization and recrystallization in the process of formation of fragmented structure during warm deformation is considered. It is shown that the rolling to 40 % reduction at 600 °C does not produce fragmented structure, and laths divided into cells are retained in martensite. When deformation increases up to 60 %, separate fragments with high-angle boundaries appear. It is established that the fraction of fragmented structure increases significantly as deformation increases up to 80 %, and at the same time coalescence of subgrains having similar orientation occurs, resulting in the formation of large fragments of alpha phase. The increase of rolling temperature up to 700 °C and deformation up to 80 % produces effective fragmentation of structure in VKS-7 steel, though in the VKS-10 with higher alloy content some traces of lath structure remain. It is shown that the change of deformation mode of VKS-7 steel from rolling to overall compression by 50 % at 600 °C initiates in situ recrystallization of alpha phase and formation of large fraction of ultrafine-grained structure (grain size 0.2-0.5 m). In the VKS-10 steel these phenomena are observed at higher temperature of warm compression, in particular, at 700 °C.