Macrosegregation, serving as a major defect in billets, can severely degrade material homogeneity. Better understanding of the physical characteristics of macrosegregation through numerical simulation could significantly contribute to the segregation control. The main purpose of this study was to predict macrosegregation in continuously cast billets with a newly developed three-dimensional macrosegregation model. The fluid flow, solidification, and solute transport in the entire billet region were solved and analyzed. Flow patterns, revealing a typical melt recirculation at the upper region of mold and thermosolutal convection at the secondary cooling zone, significantly affect the solidification and solute distribution. The solute redistribution occurring with thermosolutal convection at the solidification front contributes significantly to continued macrosegregation as solidification proceeds. The results of this study show that the equilibrium partition coefficient is mostly responsible for the magnitude of macrosegregation, while comparison between solute P and S indicated that diffusion coefficients also have some amount of influence. Typical macrosegregation patterns containing a positively segregated peak at the centerline and negatively segregated minima at either side were obtained via the proposed three-dimensional macrosegregation model, which validated by the measured surface temperatures and segregation degree.

This paper explores factors influencing the reduction efficiency of bloom during soft reduction. The research results show that the center solid fraction affects greatly on the reduction efficiency of bloom, while the reduction amount, the width and the thickness of casting strand and the diameter of roller have little effect. The results from the comparison of the crown rollers and normal rollers show that the reduction efficiency of normal roller is larger than that of crown roller with 1 mm of reduction amount, while when the reduction amount is 5−15 mm, the reduction efficiency of normal roller is smaller than that of crown roller. However, the load borne by crown roller is much smaller than that of normal roller.

Surface cracks are easily produced after friction between continuous casting billets and copper layers in mold cavity, but the formation mechanism is not clear. Based on a steel-based hot-dip copper plating experiment, this study simulated the action behavior of copper adhering to the surface of a continuous casting billet after mold wear and systematically analyzed the formation mechanism of cracks caused by copper infiltration on the surface of the continuous casting billet. It is shown that when the copper liquid adheres to the surface of the slab, in addition to the diffusion of Cu in the steel, Fe is also dissolved in the copper liquid, accelerating the solidification of the copper liquid on the surface of the slab and forming a stable fusion combination between copper and steel. At the same time, due to the enrichment of the Fe-C phase and a large number of vacancies at the grain boundary, the grain boundary becomes the dominant area of copper–steel fusion bonding. For a continuous casting process in which the temperature is kept higher than 900 ℃, Cu’s solubility is high and the diffusion coefficient is very low in Fe, which makes it very difficult for Cu accumulated in the grain boundary to diffuse into the steel matrix during the continuous casting process, resulting in a grain boundary with a greatly weakened strength becoming the origin of cracks in the bending and straightening deformation of the billet.

Reasonable control of the content of niobium (Nb) in a steel slab is of great significance to improve the strength of the slab and the regularity of precipitation of precipitates. In this study, the high-temperature strength and precipitates of the same steel with four different Nb contents (A: 0.006%, B: 0.031%, C: 0.050%, D: 0.065%) were tested and analyzed. The results show that when the Nb content is 0.031%, the tensile strength and yield strength of the steel reach the ideal state; the two-phase particles that precipitated in the Q355GJ steel after heat treatment are mainly square- and star-shaped (Ti, Nb) (C, N) composite precipitates, and their amount increases with the increase in Nb content. Most of the two-phase particles (84%) precipitated in the steel with the Nb content of 0.031% are smaller than 80 nm, and the continuous increase in the Nb content cannot increase the precipitation amount of the two-phase particles in the steel, but can increase the size of the two-phase particles.