Effect of B₂O₃ on Slag-Metal Reaction between CaO-Al₂O₃-Based Mold Flux and High Aluminum Steel

Introduction

High aluminum steel has been received increasing attention due to their excellent combination of high strength and superior formability, which can satisfy the requirements of being light, tough and available at a reasonable cost for the modern steel materials [1, 2, 3, 4, 5]. The high concentration of [Al] in steel is prone to react with SiO₂ from the mold flux during casting. There had many reports about the chemical reactions between mold flux and high aluminum steel [6, 7, 8]. This chemical reaction usually leads to the huge dynamical change of the chemical composition of mold flux, which would deteriorate both the casting operation and the surface quality of cast slabs and cause many problems such as increased crack frequency, non-uniform heat transfer across the mold flux, reduced lubrication and so on.

In order to solve the problems of high aluminum steel casting, researchers have proposed the non-reactive CaO-Al₂O₃-based mold flux to substitute for conventional CaO-SiO₂ mold flux [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17]. Blazek et al. [11] developed CaO-Al₂O₃-based mold flux and found that the interaction between flux and steel was markedly reduced and the as-cast quality was improved. However, low consumption of mold fluxes and poor lubrication problems have not been solved effectively. Cho et al. [13] found that CaO-Al₂O₃-based mold flux mold fluxes could suppress the occurrence of surface depressions and cracks, but the slag film of lime-alumina mold fluxes were prone to crystallize easily, which made the mold lubrication deteriorate rapidly. Li et al. found [18] that the viscosities and break temperatures of CaO-Al₂O₃-based mold fluxes were relatively high for casting. It can be concluded that CaO-Al₂O₃-based mold fluxes still face many existing problems.

In our previous work, F-free CaO-Al₂O₃-B₂O₃-based mold flux was developed [19], which could have similar viscosity values and melting temperatures as the traditional CaO-SiO₂-CaF₂ based mold fluxes showed in Fig.1. In addition, which could have lower liquidus temperature and weaker crystallization ability when compared with CaO-Al₂O₃-CaF₂-based mold flux [20, 21]. However, according to Figure 2, the oxygen-potential diagram showed that B₂O₃ is prone to react with [Al]. B₂O₃ is likely to be reduced by [Al] from the high aluminum steel during casting. The addition content of B₂O₃ (≤12%) is much less than SiO₂. Although a small addition of B₂O₃, the new reaction between slag and steel may still occur, which may cause the boron pollution for steel. The content of boron in steel are directly associated with slag-metal reaction. Therefore, in order to evaluate the effect of B₂O₃ on slag-metal reaction between CaO-Al₂O₃-based mold flux and high aluminum steel, the slag-metal reaction behaviors were investigated in present work. The present results would provide the theoretical basis for development of an optimized high aluminum steel mold flux based on CaO-Al₂O₃-B₂O₃ system.

Figure 1:

Comparison of the melting temperatures and viscosities of the studied CaO-Al₂O₃-B₂O₃-based mold flux with traditional CaO-SiO₂-CaF₂-based mold flux [19].

Figure 2:

The oxygen potential figure of B₂O₃, SiO₂ and Al₂O₃ calculated by Factsage 6.4.

Experimental

Results and discussion

According to the melting temperature of steel, the experimental temperature was designed at 1,773 K. The slag-metal reaction was investigated at three periods of 10 min, 30 min and 60 min. The specific reaction between B₂O₃ and [Al] as showed in eq. (1), four flux compositions with varied B₂O₃ concentration and two steel composition with varied [Al] were considered.

When reaction (1) occurred, the concentrations of [B] in molten steel are analyzed. Those experimental data measured in present study are shown in Figure 4–6.

Figure 3:

A schematic figure of the experimental equipment.

Figure 4:

The change of [B] content of high Al steel with different B₂O₃ addition in CaO-Al₂O₃-based mold flux.

Figure 5:

The change of [B] content of high Al steel with different reaction time.

Figure 6:

The change of [B] content of high Al steel with different content of [Al] in steel.

Conclusion

The effects of B₂O₃ on slag-metal reaction between CaO-Al₂O₃-based mold flux and high aluminum steel were investigated in the present work. The following results have been obtained:

1. The addition of 5 % B₂O₃, the slag-metal reaction hardly occurred. When B₂O₃ was increased, the reaction rate increased rapidly. This indicated that if only the reaction behavior was considered, it’s better for high aluminum steel casting if the addition content of B2O3 is less than 5%.

2. The chemical reaction was greatly influenced by the reaction time. With higher content of aluminum ([Al]=0.1 %), the early stage of reaction was greatly affected by the reaction time, and furtherly, the influence was decreased.

3. When aluminum content was increased, the effect of [Al] on the slag-metal reaction was comparatively small in the initial 10 min, and the content of [B] was increased slightly. But when the reaction time increased to 1 h, the slag-metal reaction acutely occurred, and the content of [B] increased rapidly.