Revealing the evolution mechanism from spot segregation to banded defect above the austenitization temperature

Abstract

Spot segregation and banded defect are unsolved technical problems of high-quality steel. In this paper, spot segregation of different sizes and irregular morphology were found in equiaxed crystal zones of slab. The characteristics of speckle type and porosity type spot segregation were compared, where the segregation degree of large-size spot segregation was worse. After heat treatment and hot rolling processes, the concentration changes of carbon, manganese, and iron were obtained by EPMA and SEM-EDS. The results showed that the carbon distribution became more even but the change in manganese concentration was not obvious, and the maximum concentration of carbon was almost unchanged, which indicated that severe segregation cannot be alleviated by the existing processes. Circular and irregular atoms were found in the banded defect. These atoms cooperate with thermodynamic coupling caused by hot rolling and contribute to the composition homogenization and void healing of the banded defect. In addition, the connection between holding time and diffusion efficiency was studied by simulating the uphill diffusion process of carbon and manganese at a holding temperature of 1250 °C. When the diameter of the spot segregation was less than 3000 μm, the carbon could uniformly diffuse before 12,000 s, and then if the spot segregation was kept warm for 36,000 s, the mean concentration of manganese was still less than 1.0 wt%, which is too high to be considered homogenized. Finally, an evolution model was established to explain the formation process and evolution mechanism from spot segregation to band defect.

Keywords

• Spot segregation
• Banded defect
• Heredity
• Uphill diffusion
• Evolution mechanism