KR101018245B1 - Method for Depressing Slag Foaming in Vacuum Tank Degasser - Google Patents

Abstract

The present invention relates to a method for vacuum degassing molten steel in a vacuum degassing apparatus (VTD), and when the gas degassing the molten steel in a VTD facility under appropriate conditions By suppressing slag forming phenomena more effectively, it is possible to prolong the treatment time under high vacuum, reduce the total treatment time, and provide a slag forming suppression method that can prevent the slag overflow phenomenon. There is this.

The present invention provides a method for vacuum degassing molten steel using VTD equipment, comprising: mounting a gas inlet pipe on a tank cover of a degassing facility to allow gas to flow into a slag located above the ladle; Measuring slag thickness and observing whether slag forming occurs; Through the gas inlet pipe until the slag thickness becomes less than 0.5 times the thickness T0 just before the slag forming occurs when the slag thickness becomes more than twice the thickness T0 just before the slag forming occurs. Summary of the Invention The method for suppressing slag forming in a vacuum degassing apparatus comprising introducing gas onto the slag so that the pressure applied to the upper slag is 5 to 15 kg / cm ² .

Vacuum degassing, VTD, slag, forming, containment, gas inlet pipe

Description

Method for suppressing slag foaming in vacuum degassing apparatus {Method for Depressing Slag Foaming in Vacuum Tank Degasser}             

1 is a schematic view showing a conventional degassing plant,

(a) shows RH equipment and (b) shows VTD equipment.

Figure 2 is a schematic diagram showing the process of slag foaming (slag foaming) during vacuum degassing treatment in VTD equipment (VTD)

3 is a schematic view showing a method of suppressing slag forming according to the present invention during vacuum degassing treatment in a VTD facility;

4 is a schematic view showing examples of the installation position of the inflow gas pipe according to the present invention;

The present invention relates to a method for vacuum degassing molten steel in a vacuum degassing apparatus (VTD), and more particularly, when refining molten steel in a vacuum degassing apparatus (VTD). A method of inhibiting slag foaming is disclosed.

VTD and Rheinstaal Huttenwerke und Heraus (RH) facilities are commercially available.

As shown in Fig. 1 (a), the RH equipment is evacuated inside the vessel vessel, and then the molten steel ladle is raised and gas is passed through a snorkel. It is a facility which performs degassing, decarburization, etc. of molten steel by blowing molten steel in a vacuum atmosphere.

As shown in FIG. 1 (b), the VTD facility puts the entire ladle into a tank, makes the tank vacuum, and stirs the molten steel and slag through bottom bubbling. (stirring) to induce desulfurization through molten steel and slag reaction, and also to remove harmful gases such as hydrogen, nitrogen and oxygen dissolved in the molten steel produced, and also remove carbon components in molten steel. It is configured to be.

When performing molten steel degassing with VTD equipment, as shown in FIG. 2, slag forming may occur. If slag foaming occurs, the following problems may occur. Let's go.

First, it is difficult to remove harmful gases such as hydrogen in molten steel due to the lack of molten steel, so that the hydrogen content in molten steel is high, causing internal quality defects during rolling after continuous casting.                         

Second, VTD (Vacuum Tank Degasser) processing time is increased by lowering the bottom bubbling gas flow rate (VTD) and lowering the vacuum level for slag foaming control. In addition, the drop in molten steel temperature increases during processing.

Third, when slag forming occurs excessively, slag overflow causes a facility accident.

In the past, when slag forming was difficult to control when performing degassing in VTD, the ladle free board was controlled to 500mm level for 350 ton molten steel to 800mm level. By limiting, the charging amount is reduced by about 40 tons, and the tapping is reduced, and the surface tension of the ladle slag is lowered so as not to introduce the flospar, which is known to cause slag forming. VTD pretreatment was required.

In VTD, ladles are placed in tanks to initiate bottom bubbling, and the entire tank is vacuumed to remove harmful gases such as hydrogen in molten steel, and molten steel and slag. The slag-metal reaction induced a desulfurization reaction.

However, in the conventional refining method as described above, the slag forming control is difficult.

That is, even when slag forming occurs, even if slag forming occurs excessively, even if slag freeboard is increased to 800mm from the ladle freeboard, which is generally maintained at 400mm for 350 ton molten steel, to prevent slag from overflowing the ladle, separate slag As there is no forming control method, there is a problem that an emergency stop of a process is inevitable.

In addition, in the case of the method of not adding fluorspar to the ladle can reduce the rate of slag forming during VTD processing, but there is a problem that can not be suppressed when the slag forming occurs.

In the present invention, when the vacuum degassing treatment of molten steel in VTD equipment, the gas is introduced into the upper slag under the appropriate conditions so that the slag forming phenomenon can be more effectively suppressed, thereby increasing the treatment time under high vacuum, and the total treatment time. It is to provide a method for suppressing slag forming that can shorten the shortage and also prevent the occurrence of slag overflow.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention is a method for vacuum degassing the molten steel using a VTD facility comprising a tank for receiving a ladle in which molten steel is received therein and a slag positioned thereon, and a cover provided in the tank. To

Mounting a gas inlet pipe on the cover to allow gas to flow into the slag located above the ladle; And

Measuring slag thickness and observing whether slag forming occurs;

Through the gas inlet pipe until the slag thickness becomes less than 0.5 times the thickness T0 just before the slag forming occurs when the slag thickness becomes more than twice the thickness T0 just before the slag forming occurs. It relates to a method for suppressing slag forming in a vacuum degassing apparatus comprising introducing a gas onto the slag so that the pressure applied to the upper slag is 5 to 15 kg / cm ² .

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present invention is a method for vacuum degassing the molten steel using a VTD facility comprising a tank for receiving a ladle in which molten steel is received therein and a slag positioned thereon, and a cover provided in the tank. Preferably applied.

In order to effectively control slag forming, the present inventors analyzed the cause of slag forming phenomenon and conducted various experiments, and it is most effective to break the slag foam by physical method to remove the gas trapped in the slag. Recognized and based on the present invention will be proposed.

That is, the present invention is to suppress the slag forming by installing a gas inlet pipe on the VTD tank cover to strongly enter the gas to the foamed slag portion.

In order to suppress slag forming in the vacuum degassing apparatus according to the present invention, as shown in FIG. 3, the gas inlet pipe must be mounted on the tank cover of the VTD facility.                     

The gas inlet pipe should be mounted to the cover to allow gas to flow into the slag located above the ladle as shown in FIG. 4.

4 shows examples of locations where gas inlet pipes may be installed in accordance with the present invention.

VTD equipment should be used to measure slag thickness while vacuum degassing molten steel and observe whether slag forming occurs.

When the occurrence of slag forming is observed and the thickness of the slag reaches at least two times the thickness T0 just before the slag forming occurs, gas is introduced into the slag through the gas inlet pipe, and the slag thickness is increased. Slag forming can be suppressed by terminating gas inflow when it becomes 0.5 times or less of the thickness T0 immediately before slag forming occurs.

The gas flowing through the gas inlet pipe is not particularly limited, but air, nitrogen, argon, and the like may be used. In consideration of economic aspects, a preferable gas is air.

The pressure introduced through the gas inlet pipe is preferably set so that the pressure received by the upper ladle slag is 5 to 15 kg / cm ² , because the upper slag is formed by the gas introduced through the gas inlet pipe. When the pressure received is less than 5 kg / cm ^{2, the} pressure that breaks the slag forming is not enough to suppress the slag forming sufficiently, and even if the high vacuum of less than 2 torr is reached or not reached, the high vacuum of less than 2 torr is achieved. This is because when the treatment time under the short time is not sufficient to remove gas such as hydrogen, and when the weight exceeds 15 kg / cm ² , the slag may overflow out of the ladle and contamination of molten steel may occur due to gas components.

Therefore, in the present invention, the diameter of the valve, the flow rate of the incoming gas, and the distance to the slag of the gas valve and the upper ladle can be variously changed as long as the above pressure conditions are satisfied. have.

Usually, slag forming occurs 2 to 20 minutes after the start of VTD vacuum processing.

When slag forming occurs during VT treatment, the slag thickness increases. When the slag thickness increases to less than 2.0 times the thickness T0 just before the slag forming occurs, the gas in the slag is temporarily caused by the stirring molten steel. As the slag thickness is increased and the gas is released, the foaming can be reduced without any action.

However, when slag forming occurs during VT processing, when the slag thickness increases to 2.0 times or more than the thickness T0 immediately before the slag forming occurs, the degree of slag forming is so severe that there is a risk of overflowing the ladle. It is necessary to reduce the thickness of the slag by flowing in, in the present invention, when the slag thickness is 2.0 times or more than the thickness T0 just before the slag forming occurs, gas is introduced into the slag through the gas inlet pipe.                     

The inflow of gas by the gas inlet pipe is terminated when the slag thickness is less than 0.5 times the thickness T0 just before the slag forming occurs, because the slag thickness is the thickness just before the slag forming occurs ( This is because the effect of suppressing slag forming is insufficient when the inflow of gas is terminated in a state exceeding 0.5 times T0).

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example)

Molten steel containing carbon (C): 0.06 wt%, aluminum (Al): 0.035 wt%, manganese (Mn): 1.50 wt%, silicon (Si): 0.30 wt% is transferred to a bubbling station and bubbling After the temperature and composition of the molten steel is uniformed, the ladle is transferred to VTD to perform molten steel degassing, and the molten steel is refined in VTD under the conditions shown in Table 1 below, followed by continuous casting. After the preparation, the hydrogen concentration in molten steel after refining, the oxygen concentration in the slab, and whether the slag overflowed were examined, and the results are shown in Table 1 below.

In the case of the conventional example of Table 1, but the capacity of 350 tons to reduce the slag overflow in the VDT tank according to the slag foaming to reduce the charging amount (charging amount) to 310 tons, In the case of Comparative Example (1-2) and Invention Example (1-4), the level was set to 340 to 345 tons.                     

Accordingly, in the case of the conventional example, the ladle free value is 810 mm, and in the comparative example (1-2) and the invention example (1-4), the ladle free value is 465 to 550 mm.

In VTD, slag foaming occurred at 5 to 10 minutes of vacuum treatment and 20 to 50 torr vacuum degree, and in order to prevent slag overflow, the bottom bubbling gas flow rate was decreased in the conventional case. It was reduced to 50 ~ 90% level, lowered the vacuum level to 200torr level, and repeated high vacuum treatment.

In the case of Comparative Example (1-2) and Inventive Example (1-4), the slag forming occurred and the gas inlet pipe installed in the cover of VT tank at the thickness of slag more than twice the thickness just before slag forming occurred. Air was introduced into the slag under the number of times of gas inlet pipe use and gas pressure conditions shown in Table 1 below.

The gas inlet pipe was used for about 3 to 7 seconds at a time.                     

Example No. Converter charge
(Ton / charge) Ladle Freeboard
(mm) VTD Operations Hydrogen content in molten steel
(ppm) Total slab
(ppm)
Slag overflow ≤2 torr arrival time (minutes) Total processing time
(minute) 2 thor
Under
Treatment of
Minutes Gas inflow
Pipe usage
Count Gas pressure (kg / cm ² ) Conventional example 310 810 15 22 7 unused 0 4.2 12 Overflow Comparative Example 1 340 550 18 25 7 3rd time 30 3.9 35 none Comparative Example 2 340 550 Not reached 24 0 1 time 3 8.1 13 emergency stop Inventive Example 1 340 550 7 20 13 3rd time 20 3.0 19 none Inventive Example 2 345 504 5 19 14 5 times 10 2.5 11 none Inventive Example 3 340 533 6 18 14 3rd time 23 2.3 8 none Honorable 4 345 465 6 18 14 4 times 15 2.2 9 none

As shown in Table 1, in the case of the conventional example, the overflow phenomenon of the slag occurs, it can be seen that the treatment time is less than 2 Torr, the hydrogen content in the molten steel is high and the total treatment time is long.

In addition, even when the gas is introduced through the gas inlet pipe (Comparative Example 2) in the case of low inlet pressure (Comparative Example 2) it can be seen that the high vacuum of less than 2 Torr to stop the refining, and if the gas pressure is too high ( In Comparative Example 1), the overflow phenomenon of the slag did not occur, but it can be seen that the total amount of oxygen in the slab is high.

On the other hand, in the case of Inventive Example (1-4) according to the present invention, the total treatment time is short, and the hydrogen content in molten steel and the total oxygen content in the slab are not only low, but also the slag overflow phenomenon does not occur. .

As described above, the present invention can increase the treatment time under high vacuum, shorten the total treatment time, and prevent the slag overflow phenomenon, thereby increasing productivity and preventing reduction of the cleanliness of molten steel. Not only can it do, but it is effective to prevent equipment accidents.

Claims (1)

In the method for vacuum degassing the molten steel using a VTD facility comprising a tank for receiving a ladle in which molten steel is received therein and a slag is positioned thereon, and a cover provided in the tank, Mounting a gas inlet pipe to the cover to allow gas to flow into the slag located above the ladle; And Measuring slag thickness and observing whether slag forming occurs; Through the gas inlet pipe until the slag thickness becomes less than 0.5 times the thickness T0 just before the slag forming occurs when the slag thickness becomes more than twice the thickness T0 just before the slag forming occurs. Slag forming suppression method in a vacuum degassing apparatus comprising the step of introducing a gas on the slag so that the pressure received from the slag upper slag is 5 ~ 15 kg / cm ²

Images