Evolution of inclusion and microstructure in Ti–Zr deoxidized steel during hot compression

Abstract

The effects of different hot deformation amounts on the evolution of inclusion and microstructure in Ti–Zr deoxidized steel were studied by utilizing the Thermecmaster-Z hot simulation test machine, automatic scanning electron microscope equipped with energy-dispersive spectrometer, and electron backscattered diffraction. The results indicated that hot deformation amount has no significant effect on the number density of oxide, but the MnS that precipitated on the Ti–Zr oxide surface undergoes extension and breakage, resulting in the changes in oxide aspect ratio. Moreover, the fracture of nitride mainly occurs in the sample with the second pass deformation amount of 42.9% and 71.4%, and the degree of fragmentation of nitride is more serious with the deformation amount increasing. During the hot compression, sulfide undergoes breakage and extension, and with the second pass deformation amount increasing, the breakage and extension of sulfide present a periodic change. Finally, with the increase in hot compression amount, the ferrite types in microstructure change from acicular ferrite and bainitic ferrite to polygonal ferrite, and the ferrite grain size is refined. When the total deformation amount increases from 30% to 80%, the ferrite grain sizes of grain boundary with the misorientation of 4° and 15° decrease from 4.14 and 5.67 μm to 3.47 and 4.40 μm, respectively. However, when the total deformation amount increases to 80%, the harmful ferrite/pearlite banded structure appears in the microstructure. Refining ferrite grain size and avoiding harmful microstructure are the key for the optimization of hot compression process.