Influence of cross-screw rolling modes on mechanical properties and fracture toughness of pipe steel

Abstract

Influence of three modes of multi-pass cross-screw rolling (CSR) on microstructure, mechanical properties and fracture toughness of ferriteic-pearlitic 09G2S pipe steel was investigated by methods of mechanical tests, optical metallography, electron transmission and scanning microscopy. After all CSR modes there is a change in parameters of initial grain structure of the billet with formation of lamination in grain distribution by sizes. Near the surface of the billet the size of globular grains is 1 – 4 µm, the length of extended grains in the central part of the billet varies from units to tens µm, width – from 1 to 8 µm. Mechanical tests for uniaxial tension and toughness were performed on the samples cut from the central zone of the billet. It was found that CSR increases the yield and tensile strengths of all test samples with a slight decrease in overall plasticity. The greatest increase in toughness at T = –70 °C is observed after controlled CSR within the 850 – 500 °С temperature range. Electron microscopic studies have shown that features of mechanical behavior of the samples after CSR are related to structural transformations occurring in steel during rolling and cooling. The main hardening factor is the grinding of ferrite grains and the formation of a subgrain structure after CSR. The increase in fracture toughness is related to more uniform ultrafine-grained structure of rolled material, which does not contain plates of cementite and bainite. Fracture processes of initial steel samples and after CSR depending on temperature were analyzed on the basis of recorded diagrams of impact loading and fractures in breakage regions of the Charpy specimens.