Characterization of the formation of slag rims of mold powder during hypo-peritectic steel continuous casting based on full-sectional microstructures

Abstract

A full-sectional microstructure characterization method was developed to investigate the formation of coarse slag rims during the continuous casting of hypo-peritectic steel. The cross-sectional microstructural analysis of typical slag rims for two highly crystalline powders revealed that their formation was primarily driven by the solidification of the liquid slag. Distinct differences were observed in the microstructures of slag rims from the two powders. Powder A (characterized by a higher breaking temperature and viscosity) displayed alternating lamellar microstructures of coarse and fine phases, with the coarse phases composed of akermanite–gehlenite transition phases. In contrast, powder B (with a lower breaking temperature and viscosity) predominantly comprised regular akermanite–gehlenite crystals interspersed with a certain amount of glassy phases. Numerical simulations of a three-phase fluid flow coupled with heat transfer indicate that slag rim formation correlates with mold oscillation. Solidification of the liquid slag at the slag rim front predominantly occurs during the negative stroke of the mold oscillation. The average heating rate during the ascending stage of the mold reaches approximately 100 K·s−1, whereas the average cooling rate during the descending stage attains 400 K·s−1. This temperature variation leads to the formation of lamellar microstructures, whereas the ascending stage promotes the formation of coarse structures and thicker slag rims. Based on the powder properties, two distinct formation pathways exist for highly crystalline mold powders. For the powders with a higher breaking temperature, higher viscosity, and narrower solidification range (powder A), coarse microstructures and thicker slag rims were preferentially formed. For powders with lower breaking temperature and viscosity and wider solidification ranges (powder B), the liquid slag resisted rapid solidification, and the extended mushy zone allowed the partial liquid slag to persist at the slag rim front, promoting the formation of a thin slag rim. This study enhances the understanding of slag rim formation in highly crystalline mold powders and provides critical insights into the control of longitudinal surface cracks in hypo-peritectic steel.

1. Introduction

Longitudinal surface cracks, being a long-standing primary issue in the continuous casting process of hypo-peritectic steel [1–4], severely limits its production efficiency and energy saving capability. Even after extensive research of casting parameters for gaining deeper insights about the formation of these cracks and the subsequent measures to prevent them [5–9], these cracks remain inevitable. The occurrence of these cracks is closely linked to the irregularities in the continuous casting process [2,10], the uneven liquid slag infiltration between the shell and mold wall in particular [11]. This phenomenon is significantly responsible for the formation of coarse slag rims [12–14], especially when powders with high crystallinity are used in hypo-peritectic steel production. Investigating the formation process of slag rims is critical for gaining deeper insights on the coarse slag rim mechanisms and for suppressing longitudinal surface cracks.

The formation of coarse slag rims is frequently associated with the physical properties of the mold powder, such as viscosity [15] and breaking temperature [16]. Typically, higher viscosity corresponds to coarser slag rims, and powders with higher breaking temperatures tend to precipitate high-melting-point phases, which may eventually lead to formation of coarse slag rims. In addition to examining the physical properties that influence the slag rim size, the formation process of slag rims at different scales has been explored. Perrot et al. [17] proposed the widely accepted “painting” mechanism, where the slag rim grows vertically during mold oscillation via continuous bonding of liquid slag to slag rims formed earlier. However, this mechanism does not comprehensively explain the formation of slag rims along the thickness direction, which is critically responsible for the surface defects in the strand. In this regard, researchers have attributed slag rim growth to the sintered phase. Kromhout et al. [15] demonstrated that sintering reactions between Na2O and raw materials may promote slag rim formation, using scanning electron microscopy (SEM) analysis at the micrometer scale. Di et al. [18] found that the sintered phases constituted ~70% of the slag rims during the continuous casting of 430 stainless steel. Ren et al. [19] further identified the precipitation of the high-melting-point phase as the core of the coarse rims. In addition, variations in the properties of the mold powder significantly affect the growth of the rim. Marshall et al. [20] established the relationship between the coarse slag rims and the variations in the powder properties owing to the steel–slag reactions during the continuous casting of high-aluminum steel. Yan et al. [21] reached similar conclusions.