Characteristics and analysis of bonded steel breakouts in slab continuous casting

In view of the bonding breakout that often occurs in the continuous casting process, various factors causing bonding breakout are analyzed to provide reference for preventing the occurrence of bonding breakout.

Keywords: continuous casting; slab; bonding; steel breakout

Introduction

In the continuous casting production process, steel breakout is a serious accident, which not only affects the continuous casting operation rate, but also damages the equipment and destroys the balance of the production organization. According to statistics, if all harmful factors caused by steel breakouts are taken into account, a “typical” steel breakout accident in slab continuous casting can cause an economic loss of US$200,000. Therefore, in order to ensure the stability and smoothness of continuous casting production, it is necessary to reduce the occurrence of steel breakouts.

According to the literature, adhesive breakouts account for approximately 65% to 80% of the total breakouts. It can be seen that in the slab continuous casting production process, adhesive breakout is the most frequent form of steel breakout.

Bonded steel breakout

Bonded steel leakage is the main steel leakage accident during the pouring process. Adhesive breakout mainly refers to fluctuations in the mold liquid level or other reasons, resulting in poor lubrication near the meniscus and adhesion between the billet shell and the mold wall, as shown in Figure 1(a). The frictional resistance of the billet increases, the bonding point is cracked, and expands downward and on both sides, forming a “V” rupture line, as shown in Figure 1(b). As the billet moves downward, the molten steel directly contacts the copper plate, and due to the action of the cooling water, a new billet shell is formed, as shown in Figure 1(c). As the crystallizer vibrates and the slab continues to move downward, this process repeats, as shown in Figure 1(d). Until the thin billet shell is pulled apart when exiting the lower opening of the mold, and steel leakage occurs, as shown in Figure 1(e).

Figure 1 Process of bonding steel breakout

Bonded steel breakout has the following characteristics:

1) Bonded steel breakouts are mainly caused by the pulling speed being too high, changing the pulling speed too quickly or poor lubrication between the billet shell and the crystallizer;

2) When bonded steel leakage occurs, the hot spot (tear point) starts near the meniscus and then extends to the lower part of the mold. When the hot spot reaches the bottom of the mold, steel leakage occurs;

3) The moving speed of the hot spot of the bonded steel breakout is about 0.5 times the pulling speed;

4) Under normal pouring, the crystallizer will produce one vibration mark per vibration cycle; when bonding occurs, two meniscus-shaped ripple marks will be produced per vibration cycle;

(5) The distance between the surface fractures of the blast steel shell is about half of the normal vibration mark distance.

Analysis of the causes of bonded steel leakage

Pouring temperature (or superheat)

If the pouring temperature is too high, the molten steel shell will become thinner. Under the action of the static pressure of the mold slag and the molten steel, the friction between the shell and the copper plate will increase, making it easy for bonding to occur. When Tangshan Iron and Steel’s No. 3 casting machine was pouring SS400 steel, a furnace of steel with a temperature exceeding 1570°C bonded three times in a row. Therefore, appropriate superheat is beneficial to the melting and lubrication of the mold slag, and enhances the uniformity of the distribution of the mold slag film on the copper plate of the mold. According to theoretical calculations, for every 10°C increase in superheat, the shell thickness of the crystallizer is reduced by approximately 3%. It can be seen that the appropriate degree of superheat is not only beneficial to the heat transfer of the copper plate, but also conducive to the growth of the billet shell.

 Performance of mold powder

The poor lubrication performance of mold slag is the main reason for bonded steel breakouts. The main physical parameters of mold flux include melting temperature, melting speed, crystallization temperature, solidification temperature, viscosity, etc. At the appropriate pouring temperature, the mold powder plays an extremely good role, so the lubrication effect is good and the production process is smooth. If the pouring temperature is too high or too low, the lubrication effect of the mold slag will be poor, resulting in increased friction between the billet shell and the copper plate, and bonding may occur.

It is now recognized that the ideal thickness of the liquid slag layer is 10 to 15 mm. Koyama et al. proposed the following empirical formula to establish the relationship between the thickness of the liquid slag layer and the glass ratio, crystallizer size, pouring speed and mold powder consumption:

In the formula: d–liquid slag layer thickness (mm); Sg–slag formation rate;a, b–crystallizer size (m); V–pouring speed (m/min); W–mold powder consumption (kg/t).

The appropriate glass phase ratio is an important factor affecting bonding. Once there is a problem with the surface quality of the cast slab, the mold powder manufacturer will adjust the composition of the mold powder to increase the crystallization ratio of the mold powder, which can effectively reduce the occurrence of longitudinal cracks in the slab, but it can easily lead to the occurrence of steel sticking (see Figure 2 ,Figure 3). 

Figure 2 Effect of crystallization rate on surface longitudinal cracks

Figure 3 Effect of crystallization index on bonded steel breakout

Composition of molten steel

Carbon is the most basic element in steel and the element that has the greatest influence on the crystal structure. When the carbon content in the steel is around 0.12%, the heat flow is minimal (Figure 4). This is because at this time, the shell undergoes a δ→γ phase transition, causing volume shrinkage, resulting in a large air gap formed between the surface of the shell and the wall of the crystallizer (the thermal resistance of the air gap accounts for 70% of the total thermal resistance ~90%), at this time the heat flow is the smallest and the shell is the thinnest and uneven. When the thin billet shell exits the lower opening of the mold, cracks may occur due to the static pressure of the molten steel, and even steel leakage may occur.

Figure 4 Relationship between carbon content and heat flow in steel

The solubility of sulfur in steel is very large, but the solubility in solid steel is very small, and it decreases as the temperature decreases, precipitating iron sulfide. When iron sulfide crystallizes, it precipitates on the boundaries of primary grains, forming a continuous or discontinuous network structure surrounding ferrite, causing grain boundary brittleness.

Phosphorus is an element that reduces the surface tension of molten steel and tends to accumulate at the grain boundaries. As the phosphorus content increases, the surface tension of the steel decreases, thereby reducing the hot cracking performance of the steel.

As can be seen from Table 1, the higher the sulfur and phosphorus content in the steel, the higher the probability of steel leakage.

Table 1 The relationship between ([P]+[S])% in steel and the probability of steel breakage

([P]+[S])%Number of furnacesNumber of steel leakage furnacesProbability of steel breakout/%
<35150106.7
35~50145139.0
>502102210.5

Pull speed

Under normal pulling speed conditions, the flow rate of the molten steel at the tundish nozzle, the supply speed of the mold slag, and the flow rate of the cooling water for the copper plate of the mold are all maintained in a stable state. When the pulling speed suddenly changes, each link requires an adjustment process. The most significant changes during this buffering period are temperature fluctuations somewhere on the copper plate of the mold and discontinuities in the supply of mold powder. For example, when the pulling speed changes unsteadily, it often causes the liquid level of the crystallizer to fluctuate, which easily causes the thermocouple signal to jump irregularly. It is difficult for the steel breakout prediction system to identify this situation, which is often a cause of high false alarms in the detection system. The reason is shown in Figure 5. When the sudden change in drawing speed causes the supply of mold powder to be discontinuous, the shell may be in direct contact with the copper plate, causing adhesion.

Figure 5 Non-breakout temperature waveform with temperature fluctuations

Crystallizer liquid level fluctuation

The crystallizer liquid level fluctuation is the beginning of shell bonding. Through on-site observation at a domestic steel mill, when pouring SS400 steel, the normal liquid level fluctuation is ±2mm, and when bonding occurs, it exceeds ±5mm. The bends or irregularly densely spaced vibration marks observed on the bonded billet shell are caused by liquid level fluctuations. Under normal pouring conditions, one vibration mark is produced per cycle. However, when the liquid level fluctuation causes bonding, when the molten steel enters the torn gap, two meniscus-shaped ripple marks will be formed, that is, there will be two surface dents in each vibration cycle instead of one vibration mark.

Crystallizer taper

During the crystallizer transfer process, the air gap thermal resistance is the largest, accounting for 70% to 90% of the total thermal resistance. The crystallizer is designed to be large at the top and small at the bottom with appropriate inverse taper, which can reduce the thickness of the lower air gap and improve heat transfer. If the reverse taper is too large, it will increase the friction between the crystallizer wall and the green shell, which will not only destroy the stability of the protective slag layer and cause bonding, but also accelerate the wear of the lower part of the crystallizer. If the reverse taper is too small, the thermal resistance in the middle will be very large (mainly the air gap), which is not conducive to heat transfer, and the shell will be relatively thin. When exiting the lower opening of the mold, if the billet shell cannot withstand the static pressure of molten steel, it will easily cause steel breakage.

Crystallizer vibration

The crystallizer vibration device is a very important piece of equipment for the continuous casting machine. The regular reciprocating vibration of the crystallizer can prevent the billet shell from bonding with the copper plate, and at the same time obtain good billet quality. When the mold moves upward, the bonding between the new shell and the wall of the mold is reduced to prevent the shell from being subject to greater stress and reduce cracks on the surface of the cast slab. When the crystallizer moves downward, a certain amount of pressure is exerted on the shell with the help of the friction between the crystallizer wall and the shell to heal the cracks pulled out when the crystallizer rises. If the vibration of the crystallizer is unbalanced, the frictional resistance of the primary billet shell increases, which can easily crack the billet shell and lead to steel leakage.

Conclusion

The occurrence of bonded steel breakouts in slab continuous casting is closely related to the on-site operation process, pouring temperature, protective slag performance, mold vibration parameters, and fluctuations in the steel liquid level in the mold. Maintaining a good lubrication effect between the mold copper plate and the billet shell can effectively prevent steel breakouts.

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