Microstructure and properties of 09G2S steel processed by equal-channel angular pressing

The effect of equal-channel angular pressing (ECAP) on the microstructure, mechanical, and corrosion properties of low-carbon 09G2S steel was investigated. It was established that ECAP processing leads to the formation of an ultrafine-grained (UFG) state, accompanied by ferrite grain refinement, an increase in dislocation density, fragmentation, and partial dissolution of cementite, as well as an increase in the fraction of globular carbide phases. These changes were analyzed in detail using electron microscopy and X-ray diffraction techniques. After four ECAP passes, the average ferrite grain size decreased from 11 μm to ~560 nm. Phase analysis revealed a redistribution of carbide precipitates: the mass fraction of lamellar cementite (Fe3C) decreased, while the content of globular particles Mn23C6, Mn7C3, and Cr3C2 increased. X-ray line profile analysis performed by the modified Williamson-Hall and Warren-Averbach methods demonstrated grain refinement and increased dislocation density. According to the modified Williamson-Hall approach, the dislocation density increased from 1×1013 m−2 in the initial state to 1.5×1013 m−2 after four ECAP passes, while the size of coherent scattering domains decreased more than fourfold. The modified Warren-Averbach method showed comparable trends. Corrosion tests indicated that the formation of the UFG structure was accompanied by an approximately 1.3 fold increase in corrosion rate. It is concluded that the improvement in strength and the reduction in corrosion resistance of 09G2S steel are associated with grain refinement, an increase in grain boundary area, high defect density, and the redistribution of carbide phases.