Development of mold powder for low carbon and ultra-low carbon steel

Molding powder in the molding machine is very important for stabilizing continuous casting and improving casting quality. Through the application of newly developed high-viscosity mold powder, inclusion defects and bubble defects in low-carbon and ultra-low carbon steel thin coils for automotive steel sheets have been greatly reduced, and the quality of the coils has been improved.

Keywords: mold powder; inclusions; low carbon; bubbles; viscosity

1 Introduction

In recent years, the requirements for steel quality have become more stringent. At the same time, there is an urgent need to use mold powder to improve the quality of cast slabs, and great expectations are placed on it. How to eliminate physical inclusion defects and bubble defects in low-carbon and ultra-low carbon thin coils used in automobiles is a very important research topic. The current status quo is how to reduce the billet cleaning work, so the mold powder technology is more important.

This article introduces the development and use results of mold mold powder that first improves the quality of low-carbon and ultra-low carbon sheets.

2 Quality of low carbon and ultra-low carbon steel

The representative defects of low carbon and ultra-low carbon steel are inclusion defects such as alumina and protective slag and bubble defects. Figure 1 is the speculated generation mechanism of the above two defects.

Figure 1 Defect generation mechanism

Mold slag is brought into the molten steel in two situations. One is that the molten steel is scraped against the mold slag by countercurrent flow, and the other is that the turbine near the immersed nozzle is involved in the mold slag. When the entangled mold slag, aluminum oxide and other non-metallic inclusions and bubbles float up in the molten steel, the front end of the solidification shell in the meniscus area forms a claw-shaped solidification hook, and the inclusions and bubbles are sucked in, forming defects under the surface of the cast slab.

Carbon formation is also a problem in ultra-low carbon steels. It is generally believed that carbon formation is mainly caused by direct contact between molten steel and unmelted mold slag layer.

3. Development concept of mold powder for low carbon and ultra-low carbon steel

3.1 Countermeasures against mold slag inclusion defects

High viscosity mold flux can effectively reduce mold flux inclusion defects in low carbon and ultra-low carbon steel. By increasing the viscosity of the mold slag and reducing the droplet breakage of the slag, it is possible to reduce the meniscus steel flow scraping into the slag and the eddy current that is involved in the mold slag.

However, as shown in Figure 2, if the viscosity of the mold powder is increased, the consumption of mold powder will be reduced, resulting in insufficient lubrication and increasing the risk of constrained leakage. Therefore, in order to ensure the consumption of mold powder in high viscosity areas, it was found to be very effective to utilize Li₂O.

Figure 2 Relationship between mold powder consumption and viscosity

Figure 3 shows the effect of adding Li₂O on the slag viscosity and temperature dependence. If TiO₂ is added to the mold flux, its activation energy will be reduced. In the high temperature area, the viscosity will be relatively high and the mold flux will not be easily involved. However, in the low-temperature area, the viscosity is very low compared to the mold powder without adding Li₂O, so the consumption of the mold powder can be guaranteed.

3.2 Countermeasures against alumina and bubble inclusion defects

In order to reduce the bubbles and inclusions absorbed by the insufficiently grown claw-shaped solidification hooks in the meniscus area, it is necessary to control the growth and premature solidification of the claw-shaped solidification hooks. The key is to improve the meniscus’ thermal insulation performance through mold mold powder.

Figure 3 Effect of adding Li₂O on the relationship between slag viscosity and temperature

Figure 4 Effect of slag layer thickness on thermal insulation

Figure 4 shows the results of the investigation of the effect of slag layer thickness on heat retention. Melt the pig iron in the graphite crucible with an electric furnace that is heated only from below, put in mold slag with different surface thicknesses and the same composition, and then use a thermocouple to continuously measure the temperature of the molten iron at a position 5 mm away from the interface between the slag and the molten iron. According to the test results, the thicker the molten layer, the higher the temperature of the molten steel, so it can be said that the thicker the slag layer can improve the heat retention. In addition, in order to ensure the thickness of the molten layer, when local fluctuations in the molten steel level occur, it can prevent the carbon-containing slag layer from being in direct contact with the molten steel, and can also prevent ultra-low carbon steel from adhering to carbon.

To further improve thermal insulation, the thermal insulation performance of the raw slag layer is also important. Figure 5 shows the effect of mold flux shape on insulation performance. Alumina and hollow alumina balls with different particle sizes were selected as protective slag, heated in one direction from the bottom of the crucible in an electric furnace, and the surface temperature of the sample was measured. The results obtained from the test are: the powdery surface has the lowest temperature and the best thermal insulation performance, while the hollow shape has the worst thermal insulation performance.

3.3 Countermeasures against carbon adhesion in ultra-low carbon steel

BH steel plates (bake-hardened steel plates) and enameled steel plates need to prevent carbon adhesion as much as possible from the perspective of steel type characteristics. In order to prevent carbon adhesion, it is most effective to add no carbon to the mold powder, but this makes it difficult to ensure heat preservation and control the slag rate.

In order to ensure heat preservation and control the slag rate without adding carbon, a heating system with added metal was developed. Figure 6 shows the results of differential thermal analysis of the newly developed mold powder with heat-generating properties. The newly developed metal-added mold powder has higher thermal insulation properties than traditional carbon-containing mold powder.

Figure 5 Effect of mold powder shape on thermal insulation properties

Figure 6 Results of differential thermal analysis

4 Industrial applications of newly developed mold powder

4.1 Quality of newly developed mold powder

Table 1 shows the characteristics of the mold flux for low carbon and ultra-low carbon steel developed this time. Among them, the newly developed mold flux C is a carbon-free heating type for ultra-low carbon.

Table 1 Characteristics of newly developed mold powder

Newly developed protective powderTraditional mold powder
ABC
CaO/Si020.860.840.860.96
Li₂0/%1.01.22.3
F/%4.55.46.37.4
T.C/%2.92.43.2
Viscosity (1300℃)/Pa·s0.480.600.400.15

4.2 Consumption and molten layer thickness of newly developed mold powder

The consumption of the newly developed mold slag for industrial continuous casting and the changes in the thickness of the molten layer of the newly developed mold slag are shown in Figures 7 and 8 respectively. The newly developed mold powder has high viscosity, but the consumption reduction is very small, which can ensure that the suitable lubricity target value is above 0.20kg·m-². Moreover, the slag formation speed and consumption speed are stable, the molten layer is also guaranteed to be at an appropriate thickness, and the fluctuations along with the continuous casting time are also reduced.

In addition, the newly developed mold flux C is a carbon-free mold flux, and the thickness of the molten layer during continuous casting is stably maintained at 10~15mm.

Figure 7 Changes in mold powder consumption

Figure 8 Changes in thickness of slag layer

4.3 Cold rolled coil quality

Figure 9 shows the defect inclusion index of cold-rolled coils when casting automotive steel sheets using newly developed mold fluxes A and B. Figure 10 shows the defect inclusion index of cold-rolled coils using newly developed protective powder C on low-carbon tinned thin steel sheets. In any steel grade, the application of the newly developed mold flux has resulted in improved cold-rolled coil quality. This reduces the slab cleaning rate and increases productivity.

Figure 9 Defect mixing index of cold-rolled automotive thin steel sheets

4.4 Results of reducing carbon formation

Figure 11 shows the surface quality of the slab when the newly developed mold flux C is used for continuous casting of enamel steel plates. Compared with traditional mold powder, the newly developed mold powder reduces carbon formation and significantly reduces bubbles.

Figure 10 Defect mixing index of cold-rolled low-carbon tin-plated steel sheets

Figure 11 Surface defect index of enamel steel plate

5 Conclusion

This time, mold powder for low-carbon and ultra-low carbon steel was developed based on new ideas, and a new test method was established to evaluate the characteristics of the mold powder. Concluded as follow:

(1) Developed high-viscosity mold powder with Li₂O added;

(2) Due to the high viscosity characteristics, defects caused by slab mold powder can be greatly reduced;

(3) The newly developed mold slag has a high viscosity, but due to the addition of Li₂O, it can ensure the proper flow of the slag and the thickness of the molten layer, achieving stable operation;

(4) By improving thermal insulation, bubble defects and alumina inclusion defects are greatly reduced;

(5) The application of carbon-free heating flux has greatly improved the quality of ultra-low carbon steel;

(6) Through the application of newly developed mold powder, the quality of coils has been greatly improved.

The newly developed mold powder has been used by major steel companies in China, Japan, South Korea and the United States, and has achieved good results in improving steel quality and increasing productivity.

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