Microsegregation behaviors of alloy elements and their effects on the formation of banded structure in pinion steels

Banded structure is one of the main defects which influence inner quality of medium-and low-carbon pinion steels. In this paper, the banded structures of industrial pinion steel samples of 20CrMnTiH and SAE8620H were observed by optical microscope, and the results of EPMA analysis showed obvious banded segregation of alloying elements, such as chromium, manganese and silicon in these two types of samples. However, the results revealed that element segregation is, not a sufficient, but a necessary condition of forming banded structures, since in some examples even the banded segregation of elements was observed while the banded structures did not exist. In addition to micro segregation, other properties of alloying elements, such as their effect trend on the transition temperature from austenite to ferrite (A_r³), their solubility or content in steel, their effect on the carbon activity and CCT curves, also give significant influence on the formation of banded structures. In order to weaken or eliminate banded structures, several possible ways were suggested for the industrial practice.