Hot Workability of a Typical Ultrahigh Strength Steel During the Isothermal Forging Process

Hot compression experiments are conducted on the Gleeble-3500 device to study the flow behaviors and hot workability of typical ultrahigh strength steel (UHSS) during the isothermal forging process. An improved Johnson–Cook (J–C) constitutive model is proposed to describe the flow behaviors, which considers the interaction effect of different deformation parameters. The proposed constitutive model shows a high prediction accuracy with an average absolute error of 5.07%. Furthermore, according to the critical strain and the logarithm of the Zener–Hollomon parameter, the mathematical models for the distribution and size of austenite grains are established, which can be used to quantitatively evaluate the effects of dynamic recrystallization (DRX) on the microstructure evolution. Considering the importance of microstructures, a new evaluation approach for hot workability is proposed, which integrates the grain size and DRX degree in the traditional processing maps. In comparison to the traditional processing maps, the newly proposed optimization method can assure the full occurrence of DRX, uniformity and fineness of grains, and absence of flow instability within the optimized processing parameter range. The processing parameters can be optimized to 0.01–0.075 s −1 and 1263–1363 K by the newly proposed evaluation approach. The average grain size of the UHSS is 20.85–40.66 μm within the optimized processing parameters, which well meets the grain size requirements for forgings. Graphical Abstract