Effect of isothermal holding time on microstructure, precipitates and hardness of Ti–V–Mo microalloyed steel during coiling process

Abstract

The microstructural evolution, precipitation behavior, and hardness variation of the Ti-V-Mo complex microalloyed steel during isothermal holding following austenite deformation have been investigated using thermal simulation tester (Gleeble-3800), optical microscope (OM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD), transmission electron microscope (TEM), and Vickers hardness tester. The results indicate that the microstructure transforms from austenite to ferrite as the holding time increases from 0 s to 5400 s at 630 °C, with the microstructure primarily comprising martensite and ferrite. The proportion of ferrite continues to increase, while the proportion of martensite decreases. At a holding time of 600 s, the austenite has nearly completed the ferrite transformation (95%), and the average grain size is very fine (3.9 μm). At this point, the (Ti, V, Mo)C particles exhibit the highest density and a smaller size (7.7 nm), with the hardness reaching its maximum value (415 HV). Concurrently, numerous spherical precipitates particles with a size of < 10 nm formed in the ferrite region, conforming to the Baker–Nutting (BN) orientation relationship with respect to the matrix: (100)(Ti, V, Mo)C//(100)α-Fe, [011](Ti, V, Mo)C//[012]α-Fe. As the holding time increases from 30 s to 60 s, the hardness decreases rapidly due to the reduction of martensite and addition of soft-phase ferrite. However, the subsequent slow decrease in hardness is primarily attributed to weakened precipitation strengthening caused by precipitates coarsening and decreased grain refinement strengthening due to the slight increase in ferrite grain size as the holding time increases from 600 s to 5400 s.