Formation process and mechanical properties of 0.2% carbon steel with bimodal microstructures subjected to heavy-reduction single-pass hot/warm compression

A compression test simulating heavy-reduction single-pass rolling was carried out to investigate deformation behavior and to collect basic data on the formation of bimodal structures in 0.2% plain carbon steel at deformation temperatures between 700 and 850 °C. The mechanical properties of such bimodal structures and the likelihood of their formation were revealed by field-emission scanning electron microscopy (FE-SEM), electron backscattering diffraction (EBSD) and tensile tests. These microstructures were significantly refined and had a higher fraction of high-angle grain boundaries when the deformation temperature was increased from 700 to 850 °C. The average ferrite grain sizes at deformation temperatures of 800 and 850 °C were 1.2 and 1.4 μm, respectively. In particular, there was a high probability of bimodal structures being formed in the specimen deformed at 850 °C, for which the proportions of ferrite grains with sizes of less than 1.2 μm and from 1.2 to 4 μm were 37% and 63%, respectively. This specimen had a high strength (677 MPa) and its average uniform elongation (7-8%) was about twice that of the other specimens. Moreover, bimodal dimples consisting of submicron-size (<1 μm) and fine (2-4 μm) dimples were observed at the fracture surface in the specimen deformed at 850 °C.