Continuous and discontinuous dynamic recrystallization in the superplastic deformation of moderately cold-deformed Cr4Mo4Ni4V martensitic steel

This study employs lath martensite as the starting structure to achieve superplasticity through limited cold deformation, revealing how the dynamic recrystallization mechanism during superplastic deformation changes with the amount of prior cold deformation. Results show that an elongation close to 700 % can be achieved with 25 % cold deformation, and the peak stress is significantly reduced. The amount of deformation required to achieve superplasticity with martensite as the initial processing structure is much lower than with ferrite as the initial processing structure. However, when the deformation increased from 25 % to 50 %, the elongation increased only slightly, from 696 % to 756 %, while the peak stress increased from 90.1 MPa to 96.4 MPa. The reason is that continuous dynamic recrystallization (CDRX) is suppressed, and softening occurs only through discontinuous recrystallization (DDRX), thus weakening the softening effect. The superplastic deformation mechanism for samples with high cold deformation mainly involves grain boundary sliding (GBS) associated with DDRX, while for samples with moderate cold deformation, it involves GBS accompanied by both CDRX and DDRX. Strain rate jump (SRJ) tests reveal that even 5 % cold deformation can accelerate the growth of the m-value during deformation. Interestingly, the m-value of the 25 % deformed sample is slightly higher than that of the 50 % deformed sample. This research offers a promising route for achieving superplasticity in high-alloy low-carbon steel, revealing continuous and discontinuous dynamic recrystallization accompanied by grain boundary sliding in superplastic cold-deformed martensitic Cr₄Mo₄Ni₄V steel.