Formation and Mechanical Properties of Bimodal Microstructures in 0.2% Carbon Steel by Heavy-reduction Hot/Warm Compression

A compression test simulating heavy-reduction hot rolling was performed to investigate the compression behavior during the thermomechanical treatment and to systematically gather basic data on the formation of bimodal structures in 0.2% carbon steel at different deformation temperatures from 700 to 850 oC. The mechanical properties and the formation of such bimodal structures were revealed by field-emission scanning electron microscopy (FE-SEM), electron backscattering diffraction (EBSD) and tensile tests. Microstructures were considerably refined and equiaxed with higher fraction of high-angle grain boundaries when increasing deformation temperature from 700 to 850 oC. Especially, specimen compressed at 850 oC had average ferrite grain sizes of 1.4 μm and indicated high a likelihood of the formation of bimodal structure. Ultimate tensile strength and uniform elongation of this specimen were 677 MPa and 8%, respectively, which showed higher strength and twice improved uniform elongation than those of other specimens. Furthermore, their fractography showed dimples with less than 1 and 2-4 μm diameter on the fracture surface in 850 oC-compressed specimen, which indicated the bimodal-type dimple.