Modification of structure and mechanical properties of 09Mn2Si pipe steel by helical rolling

Studies on the impact of helical rolling on the structure and mechanical properties of 09Mn2Si pipe steel have been carried out. With the use of transmission electron microscopy it was revealed that the helical rolling gives rise to refinement of ferrite grains as well as fracture of cementite plates within the pearlite phase. This is particularly evident in the surface layer where the degree of strain resulting from the rolling achieved the maximum value. Microhardness measurements have confirmed gradient hardening pattern over the cross section of a rod being induced during the rolling. It is concluded that the most manifested increase of the microhardness takes place at the depth of up to 3 mm. Mechanical properties of the treated steel was estimated under static tension tests. It is shown that helical rolling of 09Mn2Si steel gives rise to increasing of the flow stress (at the yield plateau) and the proportionality limit, while the value of relative elongation has decreased. At the same time, despite of the strengthening induced by the helical rolling the strain hardening takes place in the steel. The latter is manifested through the parabolic hardening at the loading diagram followed by a gradual decrease in the flow stress after the neck formation.