JPS6043891B2 - Secondary refining method of molten steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a secondary refining method for refining molten steel discharged from a converter or the like into a ladle or the like outside the furnace.

Conventionally, molten steel that has been primarily refined in a converter or electric furnace is tapped from the furnace into a ladle, where alloys and deoxidizers are added to the molten steel to remove oxygen and impurities or to produce the desired steel. Secondary refining is carried out outside the furnace for the purpose of adjusting the ingredients to obtain the composition.

Various methods have been proposed as secondary refining methods. These methods include a vacuum degassing method that uses a vacuum degassing device to prevent molten steel from being oxidized by the atmosphere during processing, and simultaneously actively removes gas in the molten steel; There are two methods: a molten steel stirring method that simply uses gas to stir the molten steel, and a molten steel stirring method that simply uses gas to stir the molten steel. The vacuum degassing method is represented by the RH type reflux degassing method or the DH type suction degassing method, and is a very excellent method as a secondary refining method, but it requires the use of large-scale vacuum equipment. , the equipment becomes large-scale, and equipment costs and processing costs become expensive.

For this reason, vacuum degassing methods have been implemented especially for the purpose of producing high-grade steel materials. On the other hand, the molten steel stirring method is used for lower-grade steel materials compared to the vacuum degassing method, and is a so-called inert gas method in which an inert gas is blown into the molten steel and the molten steel is stirred using floating blades of gas bubbles. A typical example is the stirring method, which is a low-cost method.

This method of stirring molten steel using an inert gas does not require any special equipment, as it only uses a gas blowing nozzle or porous plug, and the processing method is simple. This method has many characteristics as a method. However, in this method, the stirring power is weak, and due to the principle of stirring, the interface between the slag and molten steel is most strongly stirred. slag tends to react with molten steel, and this is difficult to prevent. Therefore, for example, during processing, easily oxidized alloying elements such as aluminum react with the slag and its concentration changes drastically, making it difficult to adjust to the desired component concentration. There is. Another problem is that the oxygen concentration at the end of the treatment does not drop sufficiently due to the oxygen flowing in from the slag. Furthermore, when the blowing gas flow rate is increased to strengthen the stirring power, the slag layer on the surface of the molten steel bath at the gas bubble release position is removed to the surrounding area, and the molten steel comes into direct contact with the atmosphere, causing the molten steel to be heated by the oxygen in the atmosphere. It has the disadvantage of being oxidized and reducing refining efficiency. Recently, as a method to improve the above-mentioned drawbacks of the method of stirring molten steel using inert gas, the lower end of a refractory cylinder is immersed in molten steel in a container such as a ladle, and the upper end of this cylinder is connected to a pressure fluctuation generator. The gas pressure inside the cylinder is changed by a pressure fluctuation generator, and accordingly the molten steel flows into the cylinder from the ladle during the depressurization period, and flows out from the cylinder into the ladle during the pressurization period. A gas-dynamic stirring method was developed that uses repeated vibrational motion to stir molten steel, thereby deoxidizing the molten steel and adding alloys. In this method, the kinetic energy generated when the molten steel in the cylinder descends due to fluctuations in the gas pressure inside the cylinder acts as a stirring force for the molten steel in the container.

Therefore, it is important to appropriately control the flow of the melt 11A within the cylinder in terms of the refining effect. As mentioned above, in order to sufficiently stir the molten steel in the container for the purpose of improving the secondary refining effect of molten steel, as a method of controlling the flow of molten steel in the cylinder, the position of the bath surface of the molten steel in the cylinder is detected, A possible method is to control the cylinder gas pressure fluctuation based on this signal. However, this method cannot be applied to aluminum other than molten steel. However, it is difficult to detect the position of the molten steel bath surface, which is at a high temperature of 1,600°C and is rising and falling at a considerable speed. Therefore, this method is not practical for molten steel. Therefore, in j, the gas pressure inside the cylinder is continuously detected,
We are conducting research on a method to indirectly control the flow of molten steel by controlling the gas pressure inside the cylinder. The purpose of the present invention is to find gas pressure fluctuation conditions to obtain the maximum molten steel refining effect using such an indirect control method. Regarding the refining effect, various experiments are conducted to provide pressure fluctuation conditions that enhance the refining effect. According to the present invention, the lower end of the cylinder is immersed in molten steel held in a secondary refining container, and the gas pressure in the cylinder is varied to suck up molten steel from the container into the cylinder. A method for secondary refining by stirring molten steel in a container by discharging it into the container, characterized in that the fluctuation range of the gas pressure is set to 800 Hg or more.

In addition, the maximum value of the pressurization speed during the pressurization period of pressure fluctuation was set to 150 h.
Hg/Sec or more, preferably 200'Hg/Sec or more. Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 shows an example of the apparatus used to carry out the invention.

In Fig. 1, 1 indicates molten steel that has been tapped from a furnace and held in a container such as a ladle for the purpose of secondary refining, and 2 indicates the lower end of the molten steel, for example, at a depth of 10" or more. A refractory cylinder immersed in water is shown. cylinder 2 in order to vary the force within cylinder 2.
A pressurizing pipe 3 and a depressurizing exhaust pipe 4 are connected to the upper end of the pressurizing pipe 3.A solenoid on-off valve 5, a flow control valve 6, a pressure accumulating gas tank 7 and a pressure reducing valve 8 are arranged in the pressurizing pipe 3, It is connected to a high pressure gas source (not shown) through them.On the other hand, the reduced pressure exhaust pipe 4 includes an electromagnetic on-off valve 9 and a flow control valve 1.
0 and are connected to the vacuum exhaust pump 11 via these. A timer is provided in the control device 12 in order to alternately open and close the electromagnetic on-off valves 5 and 9 of the pressurizing pipe 3 and the depressurizing exhaust pipe 4 at predetermined intervals.

A pressure detector 13 is connected to continuously detect the pressure inside the cylinder 2 and send its detection signal to the control device 12.

By using the device configured as described above, the gas pressure inside the cylinder is alternately increased and decreased depending on the opening and closing cycles of the two solenoid valves, and as this pressure fluctuates, the molten steel increases during the pressurization period. is discharged from the cylinder and is sucked into the cylinder during the decompression period, repeating a periodic flow.

The pressure fluctuation within the cylinder 2 described above is continuously detected by the pressure detector 13, and the control device 12 receives the detection signal,
Based on this pressure detection signal, the control device 12 controls the flow rate of the pressurization piping 3 and the depressurization exhaust piping 4 so that the maximum pressure Pm during the pressurization period and the pressure Pmin reached during the depressurization exhaust period of pressure fluctuations become preset values. Control valves 6 and 10 are controlled.

The power to stir the molten steel in the ladle or container depends on the kinetic energy of the molten steel discharged from the cylinder into the ladle or container, and therefore, by stirring the molten steel, a sufficient refining effect can be obtained. In order to achieve this, it is necessary to appropriately control the gas pressure during the pressurization period. The present inventors conducted various experiments regarding pressure fluctuation conditions and found pressure fluctuation conditions for obtaining a sufficient molten steel refining effect.

The present invention will be described in detail below with reference to Examples.

Example 1 100 tons of molten steel held in a ladle was deoxidized using an apparatus schematically shown in FIG. Table 1 shows the dimensions of the apparatus during the experiment. The molten steel used in the experiment was a low carbon aluminum killed steel, and the temperature at the start of the treatment was 1620 to 1630°C, and its typical components are shown in Table 2. Under the above conditions, the penetration depth of the lower end of the cylinder into the molten steel is 200 to 600 TWL, and the pressure fluctuation period is 3 to 3? E
Molten steel was deoxidized under various pressure fluctuation conditions in the range c, and the oxygen concentrations reached after the treatment of 10 to 1 powder were measured and compared. As a result, the relationships shown in FIGS. 2 and 3 were obtained regarding the ultimate oxygen concentration and pressure fluctuation conditions. As is clear from Figure 2, as the difference between the maximum force during the pressurization period and the pressure reached during the depressurization period during pressure fluctuations, in other words, the width of pressure fluctuations increases, the oxygen concentration at the end of the treatment decreases, causing pressure fluctuations. Width 800T
! r! With NHg as the critical point, the oxygen concentration at the end of the treatment decreases to the lowest attainable value, and this lowest value is maintained above 800 Tr$THg.

Furthermore, as for the maximum pressurization speed during the pressurization period, as is clear from Fig. 3, 1500TIrmHg/SeCl is preferably 2000m! It has become clear that a sufficiently low oxygen concentration can be obtained when the ratio is NHg/SeC or more.

Particularly favorable conditions are that the pressure fluctuation range is 800mHg or more, and the maximum pressurization rate is 1500wrmHg/Se.
In this case, the oxygen concentration at the end of the treatment is 30 PPM or less, and the RH
A result almost equivalent to the deoxidizing effect obtained by a vacuum degassing device of type or DH type can be obtained. According to the present invention, the conventional RH
Compared to the vacuum degassing method using the vacuum degassing method or the DH method, the equipment cost and operating cost of the equipment used are about 10 to 20%, but almost the same deoxidizing effect can be obtained, and the manufacturing cost of molten steel can be reduced. can be significantly reduced.

Although the present invention relates to secondary refining of molten steel, it can also be applied to out-of-furnace smelting of hot metal by appropriately selecting the fluctuation range of gas pressure and pressurization rate in accordance with the method of the present invention described above. can.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of the equipment used to carry out the method of the present invention, Figure 2 is a curve diagram showing the relationship between the pressure fluctuation range and the oxygen concentration at the end of treatment, and Figure 3 is the maximum pressurization rate and the end of treatment. FIG. 3 is a curve diagram showing the relationship between time and oxygen concentration. 1... Molten steel to be treated, 2... Cylinder, 3.
...Pipe for pressurization, 4...Pipe for reduced pressure exhaust, 5, 9...Solenoid on-off valve, 6,10...
・Flow rate control valve, 7...Tank, 11...
・Vacuum pump, 12...Control device, 13...
...Pressure detector.

Claims (1)

[Claims] 1. The lower end of a cylinder is immersed in molten steel held in a secondary refining container, and the gas pressure in the cylinder is varied to suck up molten steel from the container into the cylinder. A method for secondary refining of molten steel by stirring the molten steel in the container by discharging the gas into the container, characterized in that the range of fluctuation of the gas pressure is set to 800 mmHg or more. 2 The maximum value of the pressurization speed during the pressurization period of the pressure fluctuation is 150
0mmHg/sec or more, preferably 200mmHg/
Claim 1 characterized in that it is not less than sec.
The method described in section.