Thermal stability of the ЭИ-961Ш steel structure after combined processing

A crucial aspect in the development of materials with improved functional properties is ensuring their ability to withstand the operating temperatures of a finished product. To increase the service life and efficiency of products made of ferrite-martensite steels, various types of deformation and thermal treatments are used. The authors studied the influence of different temperature regimes on the structure and thermal stability of ЭИ-961Ш ferrite-martensite steel subjected to rolling and additional hardening. As a method of deformation and heat treatment, the authors used cold rolling followed by re-quenching from a temperature above the ferrite/austenite phase transition. The samples were rolled during several passes on a laboratory rolling mill with the deformation of 6 % per pass for a final thickness of 4.3 mm to a reduction degree of 70 %. The authors carried out structural studies by transmission electron microscopy and scanning electron microscopy. The study showed that as a rolling result, a bimodal band structure forms with the distribution of Cr₂₃C₆ carbide particles along the grain boundaries. When using additional hardening, an increase in the globular carbides proportion is observed, and during the study by transmission electron microscopy, nano-twins were found in the structure. The bands’ width after the reduction by 50 % was 0.5 microns and after cold rolling and additional heat treatment – 0.4 microns. The authors carried out short annealing in the operating temperature range to study the thermal stability of ferrite/martensite steel structure after cold rolling and additional heat treatment. The thermal stability study showed that many structural features formed during previous deformation and heat treatment are preserved, however, after annealing at 600 °C, there are no visually observable nano-twins in the structure.