The temperature of thermomechanical treatment affects grain size via the transformation of recrystallized austenite and its subsequent transformation to ferrite and pearlite. Lowering the forging finish temperature causes the final grain size to become smaller and improves the impact toughness.
As shown in the figure, the grain size of steels rolled at a lower temperature is much smaller, and vanadium alloying increases the effects over those of rolling at a conventional temperature without vanadium and contributes to the improvement of toughness.
However, in DHT steel processing, a decrease of hot forming temperature leads to deterioration in die life and in the accuracy of forged parts. Investigation of other methods was therefore encouraged.
For more information, click on the link below (subscription required). Then scroll to Figure 11.
H. Tahira, K. Funatani, Forge and Direct Heat Treatment Processes and Technologies, Heat Treating of Irons and Steels, Vol 4D, ASM Handbook, Edited By Jon L. Dossett, George E. Totten, ASM International, 2014, p 241–252, https://doi.org/10.31399/asm.hb.v04d.a0005994
