Formation of Micro- and Macro-segregation and Precipitation of Iron-Phosphide during the Solidification of High Phosphorous and Hyper-peritectic Steel

Abstract

In the production of high strength steel, it is necessary to improve the inner quality of steel slab. Among of the internal defects, the center segregation deteriorates the ductility and fatigue strength of high strength steel. Therefore, it is necessary to understand the generation mechanism of center segregation and develop the countermeasure technologies. In the center-segregated region of high phosphorous and hyper-peritectic steel slabs, iron-phosphide may be observed. However, the precipitation mechanism of iron-phosphide has not been fully clarified. Therefore, in this study, in order to clarify the formation mechanism of micro- and macro-segregation in the final solidification of CC slab, the melting behavior of the center segregation region of high phosphorous and hyper peritectic steel slab has been investigated by using high temperature observation equipment installed with the image furnace. Additionally, the enrichment of solute in the inter-dendritic region has been analyzed by using micro- segregation model with consideration of delta-gamma transition of steel. Under the assumption that the enriched solute in the inter dendritic region was transferred to the final solidification region because of fluid flow and the center-segregation was formed, the re-solidification of micro-segregated liquid was analyzed by using the modified micro-segregation model. At the same time, the possibility of precipitation of iron-phosphide was examined by using solubility limit of iron-phosphide. Moreover, under the basis of Fe-C-P ternary eutectic solidification system, the precipitation mechanism of iron-phosphide during the formation of center segregation was discussed.