This study aims to investigate the effect of tundish level control on the change in element content and inclusion amount in molten steel during the low tundish-level steel grade transition. Based on multiphase flow, mass transfer, and discrete phase, a three-dimensional transient numerical simulation of the tundish was established in Ansys Fluent. The model uses moving mesh refinement technology to obtain clear steel and slag interface with a small number of meshes. The numerical simulation results were verified through industrial experiments and physical simulations. The results indicate that when the tundish is at a low level, strand 3 becomes a short-circuit flow, and the number of inclusions in strand 3 is approximately four times that in strand 1. If the old grade density is higher than that of the new grade, the unqualified length of the element content in the transition billet is 10.2 m shorter than that in the opposite order. When the filling speed of the tundish is three times the normal flow rate, the length of the transition billet with an unqualified number of inclusions is 7.1 m less than that when the filling speed is 2 times the normal flow rate. In addition, at the initial stage of the low tundish level steel grade transition, the minimum amount of inclusions in the transition billet can be reduced to 40% of the average amount of inclusions in the old grade; however, the maximum number of inclusions in the transition billet increase by a factor of 2.5 times the average number of inclusions in the new grade at the end stage of the low tundish-level steel grade transition. It can be observed that the inclusions in the initial stage of the low tundish-level steel grade transition have less effect on the quality of the old grades; however, they have a greater effect on the new grades in the final stage of the low tundish-level steel grade transition.

In continuous casting, fewer strand operations are sometimes required to match production schedules. However, the study of flow behavior and temperature distribution under fewer strand casting conditions remains insufficiently systematic, especially with regard to the grade casting process, which has not yet been explored. This study presents an innovative investigation of the grade transition process in a symmetrical 12-strand tundish under fewer strand casting conditions. Seven operational cases were analyzed: standard casting (the normal symmetric Case 0), individual closure of strands 1–6 (the asymmetric Cases 1–6), and simultaneous closure of strands 1–2 (the asymmetric Case 7). Notably, strand closures in Cases 5 and 6 significantly impair flow characteristics in their respective strands. The impact area temperature reaches approximately 1844 K (new heat) after 30 min of continuous casting. However, Case 6 exhibits persistent low-temperature regions near strands 5 and 6. The average transition billet lengths for Cases 0 to 7 are 72.41 m, 70.16 m, 70.30 m, 71.68 m, 72.95 m, 72.12 m, 76.35 m, and 65.45 m, respectively. Based on a comprehensive evaluation of flow dynamics, temperature uniformity, and transition billet length, Case 1 emerges as the most favorable single-strand closure strategy. Operational recommendations suggest avoiding strand closure patterns implemented in Cases 5 and 6 during reduced strand casting operations.

The presence of ferrite in 316 austenitic stainless steel affects its magnetic permeability, and the secondary precipitates formed by ferrite decomposition can further degrade the corrosion resistance, limiting its application in industries such as nuclear power and medical devices. In previous studies, the characteristics of ferrite and precipitates in several 316 stainless steel continuous-casting billet samples with different Ni contents were analyzed. In this work, remelting experiments were further conducted on several 316L stainless steels with different Ni contents using a tube furnace; according to the Ni content, they are designated as L-316, M-316, and H-316 stainless steels. Metallographic analysis and electron back-scattered diffraction (EBSD) characterization were employed. The effects of Ni content and cooling rate on the microstructure of 316 stainless steel were systematically investigated. The results show that for the L-316 stainless steel, the ferrite morphologies in water-cooled, oil-cooled, air-cooled, and furnace-cooled samples change successively as follows: skeletal and lath-like, clustered network, lath-like and clustered network, and short rod-like. For the M-316 stainless steel remelted samples, the ferrite morphologies are network and skeletal, network and short rod-like, semi-network and short rod-like, and semi-network, respectively. The solidification modes of the L-316, M-316, and H-316 stainless steel remelted samples are FA, FA, and AF modes, respectively. Increasing Ni content reduces the ferrite content under all cooling conditions. When the Ni content increases from 10% to 12.17%, the ferrite content decreases significantly, with a greater reduction at higher cooling rates; when the Ni content further increases to 14.25%, the decrease in ferrite content slows down, indicating that after Ni content reaches a certain level, its inhibiting effect on ferrite formation weakens. The effect of cooling rate on ferrite content depends on the solidification mode: in the FA mode, slow cooling promotes diffusional transformation of ferrite to austenite, so the ferrite content decreases with decreasing cooling rate—for L-316 stainless steel, the ferrite content drops from 22.44% in the water-cooled sample to 2.71% in the furnace-cooled sample. In the AF mode, slow cooling favors the enrichment of elements at grain boundaries and promotes ferrite nucleation and growth; the overall trend of ferrite content increases as the cooling rate decreases. For the H-316 stainless steel specimens, the ferrite content is similar between water-cooled specimens (0.36%) and oil-cooled specimens (0.26%); for air-cooled specimens, the ferrite content increases significantly to 1.49%; and finally, it reaches 1.94% for the furnace-cooled specimen. Regarding secondary precipitates, the phase constituents of the L-316 stainless steel specimens after water cooling, oil cooling, and air cooling consist of an austenite matrix and ferrite, with a secondary precipitated Chi phase forming only under furnace cooling conditions. For the M-316 stainless steel, the Sigma phase and Chi phase begin to form under oil cooling conditions, and the Sigma phase also precipitates in the oil-cooled specimens of the H-316 stainless steel. In the air-cooled and furnace-cooled specimens of both M-316 and H-316 stainless steels, the secondary precipitated phase is the Sigma phase. All three types of water-cooled stainless steel specimens exhibited no secondary precipitate phase; increasing the cooling rate suppresses atomic diffusion, thereby reducing the precipitation of Chi phase and Sigma phase; however, a higher Ni content shifts the solidification mode toward the AF mode, making secondary precipitates more prone to form.

The limited demand for specialty steel grades, which cannot meet the production quantity of a single-casting sequence in the continuous caster, is addressed through steel grade transition. This study aims to reduce the length of transition blooms generated during the steel grade transition by controlling macrosegregation in the bloom. A three-dimensional numerical simulation of the steel grade transition has been established based on models involving fluid flow, solidification, heat transfer, electromagnetic, and mass transfer. This study takes into account macrosegregation phenomena in the transition bloom that were not considered in previous research. The findings indicate that using a four-port submerged entry nozzle (SEN) and deactivating the mold electromagnetic stirring (M-EMS) can mitigate subcutaneous negative segregation, resulting in shorter transition blooms. The optimal configuration reduces the length of transition blooms by 4.3 m compared to the worst-case scenario. While lowering the casting speed can reduce center segregation, it also extends the mixing time of solute elements in the molten steel. These combined effects lead to a slight increase in the length of the transition bloom, from 10.88 m to 12.41 m. Therefore, for steel grade transition, utilizing a four-port SEN and deactivating the M-EMS can minimize the length of transition blooms, without the need to alter casting speed.