Fabrication of the ultrafine-grained ferrite with good resistance to grain growth and evaluation of its tensile properties

Thermomechanical treatment including cold rolling followed by annealing was conducted on a low alloy steel AISI 8620 to develop an ultrafine grain structure using bainite as the starting microstructure. The specimens were first austempered at 425 °C to form the bainitic microstructure and then cold rolled by 90% reduction in thickness. Annealing was conducted at 550–700 °C for different times. Microstructures and tensile properties of specimens were characterized during different stages of the treatment. Cold rolled bainite showed notable thermal stability during annealing. Significant recrystallization did not take place at and below 600 °C. The results show a notable potential for grain refinement using a bainitic microstructure. A ferritic microstructure with the average grain size of about 0.7 µm was developed after annealing at 700 °C and 650 °C for 10 min and 120 min. The 90% cold rolled bainite possessed the hardness of 380 HV and yield strength of 1110 MPa with the elongation of below 2%. The delamination observed in the fracture surface was considered as an effective factor for the low ductility. The ultrafine equiaxed grain structure possessed the hardness of 250 HV, yield strength of 886 MPa (about twice of the coarse-grained steel) and the total elongation of about 8% with some small delamination in the fracture surface.