Dendrite Structure, Spot Segregation and Banded Defect of Gear Steel in Continuous Casting and Rolling Process

The homogeneity of composition and microstructure in the steel matrix takes great influence on the service performance of the final product. In order to reveal the relationship between solidification structure and solute unevenness in the continuously cast billet and hot-rolled rod in a medium carbon gear steel, the macrostructure, carbon content, dendrite evolution, spot segregation and banded defect was experimentally studied. The columnar and equiaxed structure observed in the hot-rolled rod is actually the dendrite segregation, inherited from the dendrite grain formed during solidification process in continuous cast billet. The carbon content fluctuation in the equiaxed zone of the billet is more severe than columnar zone, and a hereditary distribution is also found in the rod. Porosity type and speckle type of spot segregation in the equiaxed zone of the billet are observed, leading to the formation of single band and cluster band in the rod, respectively. The results of the solute segregation behavior, the statistical distribution of the feature size and the relative location of the spot segregation and banded defect is quantitatively corresponded. The segregation ratio of Cr and Mn in spot segregation and banded structure increases with the increasing in the size of the defect in the gear steel. Theoretically, the columnar structure composed of uniformly fine dendrite is better than coarse equiaxed dendrite regarding the control of spot segregation and banded defect for the present gear steel.