Properties of warm-rolled steel plates with ultrafine-grained ferrite and cementite structures

A novel rolling process characterized by multi-pass bi-axial reduction at warm temperatures without a fairly large reduction per pass was proposed to obtain ultrafine-grained ferrite structures for steel plates. The process was applied to 0.15%C-0.3%Si-1.5%Mn steel. Steel plates with a thickness of 18 mm a width of 75 mm and a length of I 100 turn were obtained. The ultrafine-grained structures composed of ferrite and cementite were obtained in most part of the bi-axially rolled plates, while the coarse deformed structures composed of elongated ferrite grains or subgrains remained in many part of the uni-axially rolled ones. The bi-axial reduction likely enhanced the formation of high angle boundaries. The average sizes of the newly evolved ferrite grains were approximately 0.6mum and 1.2mum at the rolling temperature of 773K and 873K respectively. The evolution of the ultrafine-grained structure through the process increased the yield strength from 330 to 780 MPa and decreased the fracture appearance transition temperature (vTrs) below 77K. Severe warm rolling at the lower temperature, however, caused the development of a deformation texture and enhanced {100} and {111} ferrite grain colony bands causing "separations" on the fracture surfaces of the Charpy impact specimens with eventual deterioration of the Carpy impact value. The bi-axial reduction was effective for controlling the microscale texture and suppressing the separation.