Microstructure, properties and strengthening mechanisms of low-carbon steel subjected to equal-channel angular pressing

In the work, an ultrafine-grained (UFG) state was formed in a low-carbon steel by equal-channel angular pressing (ECAP) (8 passes, 200 °С), demonstrating high mechanical properties (yield strength is 1021 MPa, tensile strength is 1072 MPa, ductility is 10.7 %) along with satisfactory corrosion resistance (0.345 mm/year). To explain the reasons for improvement of strength properties and changes in corrosion properties, UFG steel microstructure was analysed using electron microscopy and X-ray scattering methods. Specifically, electron microscopy methods revealed structural refinement of ECAP-processed steel, resulting in the formation of equiaxed grains averaged ~240 nm in size. Modified Williamson–Hall and Warren–Averbach X-ray procedures were applied to find the patterns of changes in coherent scattering domains size, density ρ and fraction fs of screw-type dislocations, effective outer cut-off radius Re of dislocations and some other parameters of low-carbon steel depending on a number of ECAP passes (degree of deformation). X-ray diffraction analysis and small-angle X-ray scattering methods were used to determine evolution trends of mass fraction, size and morphology of various precipitates depending on the number of ECAP passes. Based on the obtained data, a model of microstructure transformation during UFG state formation in steel was proposed. Furthermore, strengthening mechanisms of both coarse-grained and UFG steels were discussed. It was found that in initial state, steel strength was primarily ensured by grain-boundary strengthening and precipitation of small Ме23С6 and Ме3С2 precipitates. It was shown that during UFG structure formation, steel strength increases due to grain-boundary strengthening and dislocation density increase. The contribution of precipitates in the UFG state to the strengthening decreases and this is due to their growth during ECAP processing. It was found that an increase in corrosion rate of UFG steel results from a decrease in ferrite grain size, an increase in grain-boundary dislocations density and a cellular structure formation.