Effect of columnar crystal ratio on microstructure, composition segregation, precipitate distribution and microtexture of Hi-B steel hot rolled plate

The columnar crystal ratio is a key factor influencing the subsequent complex long processes of Hi-B steel, such as rolling and heat treatment, and its regulatory role in microstructure, compositional homogeneity, inhibitor distribution, and crystal orientation is crucial. In this study, Hi-B steel slabs with an initial thickness of 230 mm and different columnar crystal ratios (53.05%, 67.40%, 80.87%, and 86.96%) were hot rolled to a final thickness of 2 mm via a unified process. The results show that the differences in the surface microstructure among the specimens were diminished after hot rolling. However, an increase in the columnar crystal ratio expanded the width of the subsurface layer and compressed the width of the central layer. The lower the columnar crystal ratio, the more significant the fluctuation of compositional segregation in the rolled material. Meanwhile, with the increase in the columnar crystal ratio, the number density of precipitates in the subsurface and central layers increases, and their sizes tend to be uniform. In addition, increasing the columnar crystal ratio can also increase the volume fractions of Goss texture and brass texture, as well as the proportion of high-angle grain boundaries, while elevating the dislocation density in the central layer. This study confirms that controlling the solidification structure to be dominated by a high proportion of columnar crystal (> 80%) during the continuous casting process is an effective approach to improving the hot-rolled microstructure and texture, as well as optimizing the compositional segregation and precipitate distribution of Hi-B steel.