This article describes the crystallization thermal resistance characteristics and other importance of continuous casting mold flux.
Key words: continuous casting; mold flux; thermal resistance characteristics
The metallurgical behavior of continuous casting mold slag mainly plays two processes, one is the process of being heated and melted by molten steel to form liquid slag, and the other is the process of liquid slag flowing into the air gap and being cooled to solidify into solid slag. Generally, the molten steel temperature is 1500-1550°C, the surface temperature of the mold wall does not exceed 400°C, and the temperature change from the molten steel to the mold wall is over 1000°C. Since the melting temperature of the mold slag is higher than the surface temperature of the mold wall, the surface temperature of the slab is also lower. It is greater than the melting temperature of mold slag, so there must be a liquid slag film and a solid slag film between the billet shell and the mold. The temperature at the interface between the liquid slag film and the solid slag film is the melting temperature of the mold slag, and the temperature at the interface between the slab solidification shell and the liquid slag film is between the melting temperature of the mold slag and the solidification temperature of molten steel. As shown in Figure 1, when the liquid slag flows into the gap between the mold wall and the new cast slab, the temperature of the liquid slag changes accordingly, and the side close to the mold wall solidifies under the strong cooling of the mold to form a solid glass slag layer, while the mold slag with a higher temperature close to the new slab shell remains liquid, and there is a temperature gradient between the mold wall and the new slab, there must be a certain point where the molten slag has a suitable temperature condition to crystallize (Crystalization is generated in a slag film with a temperature gradient, so it is called non-isothermal crystallization). Finally, the slag film in the gap presents a three-layer structure: glass layer, crystal layer and liquid slag layer. Because of the presence of crystals, the comprehensive thermal resistance of the slag film increases, and the proportion of the crystal layer in the slag film can regulate the heat transfer. Therefore, when designing mold flux, it is necessary to design different crystallization properties according to the heat transfer requirements of different steel types to meet the heat control requirements of continuous casting.
Figure 1 Schematic diagram of the state of mold flux in the crystallizer
The powder slag layer formed by the continuous casting mold slag added to the upper mouth of the mold covers the liquid steel surface, and the powder slag melts under the high temperature of the molten steel to form a liquid slag layer, and the liquid slag flows into the gap between the mold wall and the new cast slab The gap forms a slag film (as shown in Figure 2). The liquid slag on the side near the crystallizer wall forms a glass layer under the rapid cooling of the crystallizer, and the liquid slag on the side close to the new slab remains liquid due to high temperature, and there is a temperature gradient between the mold wall and the new slab (non- isothermal conditions), crystals are first formed in the area with suitable temperature conditions in the slag film (as shown in Figure 2a), and as the slag film stays in the crystallizer, the crystals will grow toward the side of the liquid slag layer; on the other hand, near the crystallizer The glass layer formed by the rapid cooling of the wall is also recrystallized under the action of reheating to form crystals, and the amount of crystal transformation is related to the properties of the slag itself (Figure 2b). Finally, the slag film between the mold wall and the new cast slab forms a stable three-layer structure, which is a glass layer, a crystallization layer and a liquid layer (due to the different recrystallization capabilities of the mold slag, the glass layer may be completely recrystallized to form Crystallization layer, so the final slag film formed by some mold flux may only have the crystallization layer and liquid slag layer 2).
Fig.2 Schematic diagram of crystal growth mechanism in slag film
The comprehensive thermal resistance of the crystalline slag layer is large, which affects the thermal control performance of the slag film, and the liquid slag layer can reduce the friction and affect the lubrication performance of the slab. Since the gap between the mold wall and the new slab is about 2mm, if the proportion of solidified and crystallized parts in the slag film is high, the liquid layer will be thinner and not conducive to the lubrication of the slab; otherwise, the liquid layer will be thinner. Although the thickness is good for lubricating the cast slab, the thinner solidification crystal layer will definitely affect the heat control performance. For different steel types, continuous casting mold fluxes with different solidification and crystallization properties are required. Therefore, it is very important to study the solidification and crystallization properties of continuous casting mold fluxes.