Ultrafine-Grained Structure and its Thermal Stability in Low-Carbon Steel

The influence of annealing temperature on ultrafine-grained structure (UFG) of low-carbon steel 05G2MFB, produced by multiple isothermal forging (MIF) and warm rolling is investigated. Following 1-hour annealing (from 20 to 550A degrees D), the fibrous UFG-structure formed as a result of rolling remains virtually the same. The equiaxial UFG-structure is found to be stable as the annealing temperature is increased up to 600A degrees D. An examination of the resulting UFG-states by the method of electron backscatter diffraction (EBSD) provided a way to identify the differences in softening processes for a number of structure types. When the temperature of annealing treatment of as-rolled steel specimens is increased to 600A degrees D, the fraction of low-angle boundaries (LAB) is found to remain at about 56%, while the average grain/subgrain size in the rod cross-section increases from 0.4 to 0.9 mu m. In the MIF-processed specimens, an increase in the annealing temperature up to 625A degrees D gives rise to a gradual decrease in the fraction of LAGBs from 53 to 30%, with the average grain/subgrain size increasing from 0.4 to 0.6 mu m.