Regularities of structure modification of 09Mn2Si steel rod at helical rolling and its influence on impact toughness

Effect of structure modification induced by the helical rolling on the impact toughness of 09Mn2Si steel was studied. A metallographic structural analysis of the steel in the as-received state as well as after the helical rolling (HR) was carried out. It was revealed that the five-stage combined thermal-mechanical treatment results in ferrite grains refinement, formation of rolling texture at the depth of up to 1 mm, while ultrafine grained structure is formed in the surface layer. This is accompanied by a gradient hardening pattern over the rod’s cross-section as evidenced from microhardness measurements. Mechanical properties were assessed through impact bending tests. It is shown that helical rolling gives rise to increasing fracture toughness of the rolled specimens in the entire testing temperature range (-70 ÷ +20 °C). The maximum two-fold increase of impact toughness is registered at the testing temperature of -70 °C. Authors suggest that the main reason for this increase is formation of a gradient fiber-like structure. The latter ensures effective arresting of the main crack growth during the impact bending.