Influence of Cross Rolling Patterns on the Mechanical Properties and Fracture Toughness of Pipe Steel

The influence of three multipass cross rolling patterns on the microstructure, mechanical properties and fracture toughness of ferrite-pearlitic pipe steel 09G2S is investigated using mechanical testing, optical metallography, transmission and scanning electron microscopy. Upon all cross-rolling patterns, a change in the parameters of the initial workpiece grain structure is observed with the formation of layered grain size distribution. Near the workpiece surface, the size of globular grains is 1–4 μm, the length of elongated grains in the central part of the workpiece varies from units to tens of μm, and the width varies from 1 to 8 μm. Mechanical tests for uniaxial tension and impact strength are carried out on specimens machined from the central part of the bar. It is established that cross rolling leads to an increase in the yield point and ultimate strength of steel after all investigated patterns with a slight decrease in the overall plasticity. The greatest increase in impact toughness at T = –70°C is observed after controlled cross rolling in the temperature range 850–500°C. Using electron microscopic studies, it is shown that the mechanical behavior of the cross-rolled specimens is associated with structural transformations that occur in steel during rolling and cooling. The main strengthening factor is the refinement of ferrite grains and the formation of a subgrain structure as cross rolled. The increase in impact toughness is associated with a more uniform finely dispersed structure of rolled products, which does not contain cementite and bainite plates. Temperature dependent fracture processes of the initial cross-rolled steel specimens are analyzed based on recorded shock loading diagrams and structures in the rupture areas of Charpy specimens.