1. Introduction
Mold fluxes containing various fluxes and carbon are functional materials based on silicate. They play an important role in ensuring the smoothness of the continuous casting process and the quality of steel in the continuous casting process [
1,
2]. Mold fluxes, which sit on top of the mold, cover the molten steel and provide thermal insulation to prevent the steel from freezing, prevent oxidation of the steel surface, and absorb non-metallic inclusions from the molten steel. Mold fluxes in the lower half of the mold infiltrate the mold/shell channel, regulate the heat transfer between the shell and the mold, and lubricate the newly formed shell, which are two of the most important effects among all of their functions.
Surface defects such as longitudinal cracks in the continuous casting of peritectic steel slabs, with the carbon content being 0.09–0.16%, can be formed due to the violent release of stress caused by the peritectic reaction and peritectic phase transformation. Controlling the heat transfer from the shell to the mold can solve this problem [
3]. Presently, the main method involves regulating the crystallization behavior of mold fluxes, such as increasing the basicity of mold fluxes to produce many crystals that control the horizontal heat transfer over the meniscus region. However, lubrication of the shell can be deteriorated by the high-alkalinity mold fluxes, and it can even enhance the possibility of breakout by sticking [
4,
5,
6]. Therefore, the low-basicity and high-crystallization mold fluxes become the ideal slags for the casting of peritectic steels.
During the continuous casting process of steel, mold fluxes infiltrating into the shell-mold gap are subjected to shear stress from the mold oscillation and the slab movement [
7,
8,
9,
10,
11,
12,
13], as shown in
Figure 1. The crystallization of mold fluxes is affected by the shear stress. Therefore, the authors of the current study hope to develop low-basicity and high-crystallizing mold fluxes through shear stresses to overcome the contradiction between the lubrication and heat transfer in the peritectic steel casting process, so that the quality of casting slabs can be improved, and sequence casting can be carried out. Saito et al. [
14] studied the effect of shear stress fields exerted by agitating liquid slag using a Pt-Rh rod on the crystallization of CaO-SiO
2-R
2O (R = Li, Na, or K) melts. The crystallization temperature was determined using a technique that was based on the difference in the electric permittivity of the ionic liquid and solid. The results showed that the crystallization temperatures of the molten slags were increased by agitation; Harada et al. [
15,
16] investigated the effect of agitation on the crystallization of CaO-SiO
2-CaF
2 and CaO-SiO
2-CaF
2-RO (R = Mg, or Sr) melts with the same method. The results showed that the influence of agitation on the crystallization was related to crystal growth and nucleation mechanisms. The crystallization of dendritic CaO·SiO
2 crystals was strongly affected by agitation, while the crystallization of faceted Ca
4Si
2O
7F
2 crystals showed little dependence on agitation. Meanwhile, Li et al. [
17] studied the effect of shear stress fields on the crystallization performance of CaO-Al
2O
3-SiO
2-Na
2O-CaF
2 multi-component slag systems. The results pointed out that the effect of shear stress fields on the crystallization of mold fluxes was achieved by affecting the crystallization kinetics, and the quantitative effect of shear stress exerted by a rotating molybdenum rod at a speed of 100 rpm on the crystallization ability of the mold flux was obtained by image analysis.
The above studies indicate that the shear stress fields can promote the crystallization of slags. However, the experiments of these researchers were carried out during cooling. The temperature change has a great influence on the crystallization of slags. Second, the selected shear rates of those experiments were low (0–21.6 s
−1) (during the experiments, the shear stress was generated by the rotating rotor, and it was used to simulate the shear stress experienced in the actual continuous casting process. The relationship between the shear rate and the rotational speed of the rotor is shown in Equation (1) [
16,
18]).
where
γ,
N,
ra, and
rb are the shear rate, rotating speed, radius of the rotor, and inner radius of the crucible, respectively.
The maximum speed selected in the above studies was 100 rpm (21.6 s
−1). Watanabe et al. [
18] pointed out that during the continuous casting process of steel, the shear rate to which mold fluxes were subjected between the mold and the strand ranged from 20 s
−1 to 125 s
−1, corresponding to rotating speeds between 87–513 rpm. According to Equation (1), it differed greatly with the selected value in experiments by the above-mentioned researchers. Therefore, it is greatly significant that the influence of shear stress according to the actual casting process on crystallization should be studied. Finally, the previous studies were limited to qualitative study on the crystallization behavior by the shear stress. Although Li et al. [
17] found a quantitative relationship between the shear stress and the crystallization performance, the selected rotational speed was constant, and the actual shear stress during the continuous casting process was a changing value. More importantly, the scanning electron microscope (SEM), which was the image analysis method used by Li et al. to analyze the crystalline fraction on a small point of the slag film, does not reflect the crystalline fraction of the entire slag film [
17]. Thus, the quantitative relationship between the different shear rates and the content of crystal produced by the agitation in the slag film cannot be established. Therefore, it is worthwhile to develop a better method to quantify the crystalline fraction of the mold fluxes under a constant temperature.
The quantitative influence of the shear stress on the crystallization of mold powders was carried out under a constant temperature of 9 °C above the liquidus temperature and with rotational speeds of 100 rpm, 200 rpm, 300 rpm, and 400 rpm, respectively. The aim of this study is to provide a theoretical basis for preparing the low-basicity and high-crystallization mold fluxes that were applied in the casting process of peritectic steel.