JPH0361316A - Method for refining dead-soft carbon steel - Google Patents

Abstract

PURPOSE:To accelerate decarburizing reaction and to stably produce a dead-soft carbon steel by blowing a gas of Ar, etc., against the surface of a molten metal under reduced pressure in the latter stage in a refining period and reducing the partial pressure of CO in an exhaust gas in the vicinity of the molten steel surface at the time of subjecting a molten steel to decarburizing refining in an RH-type vacuum degassing apparatus. CONSTITUTION:In an RH-type vacuum degassing apparatus 10, [C] is oxidized and decarburized into CO by the reaction between [C] and [O] while circulating a molten steel 3 in a ladle 2 by means of a ascending tube 24 and a descending tube. The resulting exhaust gas is discharged by means of a duct 30, and the amount of CO generated in a unit time is detected by means of an exhaust gas flowmeter 34 and a CO analyzer 35. In the latter stage in a refining period, reduction in the amount of CO generated is detected by means of a process computer 36, and a gas of Ar, etc., is blown from a gas supply source 37 against the surface of the molten steel 3 to dilute CO gas and reduce the partial pressure of CO gas, by which the reaction between [C] and [O] can be accelerated. By this method, a dead-soft carbon steel of <0.003% C can be stably produced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for producing ultra-low carbon steel, in which a degassing treatment is carried out so that the carbon concentration [C] of molten steel becomes an extremely small amount. , relates to a method for increasing the amount of splash generated in the early hours of processing.

[Prior Art] In recent years, as steel materials have become more sophisticated, there has been an increasing demand for ultra-low carbon steel with an extremely low carbon content, and there is a need for technology for rapidly and stably melting this steel.

In the converter process, carbon [C] in molten steel is usually 0.0
When the content falls within the range of 2 to 0.04% by weight, steel is tapped. Furthermore, the tapped molten steel is decarburized using various refining equipment to achieve a desired carbon concentration. Ultra-low carbon steel has a carbon content of 0.003% by weight
Generally, it is difficult to stably and efficiently produce a large amount of molten steel because a very small amount of molten steel is required. Against this background, RH degassing refining is attracting attention as a technology for efficiently decarburizing molten steel.

When producing ultra-low carbon steel using the RH degassing method, a pair of immersion tubes at the bottom of the degassing tank are immersed in molten steel, and the molten steel is circulated between the pot and the degassing tank while being degassed. . That is, in RH degassing refining, inert gas is blown into one immersion pipe (rising pipe) to reduce the apparent specific gravity and raise the molten steel, and the molten steel is sucked up from the ladle into the degassing tank. Since the inside of the tank is under reduced pressure due to gas exhaust, [C] and [O] in the molten steel react to generate a large amount of Co gas. The Co gas generated by the decarburization reaction forms a splash together with the argon gas blown into the riser. This splash increases the contact area between the molten steel and the gas, further promoting the decarburization reaction. After being degassed in the tank, the molten steel is returned to the ladle via the other immersion pipe (downcomer pipe).

During such degassing (decarburization) treatment, auxiliary raw materials and alloy materials are added into the tank to make the molten steel a desired target composition.

[Problems to be Solved by the Invention] However, in the conventional melting method, the decarburization rate decreases in the latter half of the process, and it takes a long time to further reduce [C]. It is not possible to melt steel stably and quickly.

In Figure 4, the horizontal axis shows the processing time by the RH degassing method.
The carbon concentration [C] of the treated molten steel is plotted on the vertical axis, and the amount of argon gas blown into the riser pipe is set at 3000 to 500 ONN per minute.
FIG. 3 is a graph showing the changes in IC at various stages of conventional degassing treatment with various changes made within the range of . In the figure, the shaded areas, white circles, and black circles indicate the argon gas injection rate of 300ONN, 400ONN, 1°5000/min, respectively.
! The results are shown when l is used. As is clear from the figure, [C] in the molten steel rapidly decreases in the early stage of treatment (region I), but the reduction rate of [C] decreases significantly in the latter stage of treatment (region ■). When [C] reaches region (3), the decarburization rate decreases, and it takes a long time to decarburize to the region of ultra-low carbon steel with a concentration level even lower than 10 to 20 pp-. Further, as is clear from the figure, the optimal amount of argon gas to be blown into the riser is 400 ONj7 per minute, and rapid decarburization cannot be performed if the amount is at least higher than this. This is because when the amount of gas blown becomes excessive, gas blow-through occurs.

Therefore, there is a limit to improving the decarburization rate by increasing the amount of argon gas blown into the riser.

In Figure 5, the horizontal axis represents the cycle time of molten steel between the degassing tank and the ladle, and the vertical axis represents the decarburization rate constant Kc in region I, and the relationship between the two was investigated under various processing conditions. FIG. Here, the decarburization rate constant KC is an index in the primary decarburization reaction defined by the following equation (1).

(d[c]/dt)=Kc [C]-(1) As is clear from the figure, when the cycle time of molten steel becomes shorter,
The decarburization rate constant Kc is improved. Therefore, by increasing the flow rate of molten steel circulating through the degassing tank and shortening the cycle time, the decarburization reaction of molten steel can be promoted. However, it is generally difficult to increase the flow rate of molten steel, and it is not possible to dramatically improve the decarburization rate.

During the degassing process, the CO gas partial pressure PC when the air is 0 is
It is known that a decrease in O accelerates the decarburization reaction. In conventional natural decarburization (decarburization by reaction between [O1 and [C] in molten steel), there is a limit to the reduction of the CO gas partial pressure PCO. This is because the current exhaust system reaches its limit at around 1 torr of vacuum in the tank, making it impossible to reduce the PCO below a certain value. Although it is possible to increase the capacity of the exhaust system, this would significantly increase equipment costs and is not practical.

This invention was made in view of the above circumstances, and it is possible to reduce the CO gas partial pressure PCO in the vicinity of the scene during degassing treatment.
The purpose of the present invention is to provide a method for producing ultra-low carbon steel that can reduce the amount of carbon.

[Means to solve the problem]

The method for producing ultra-low carbon steel according to the present invention involves blowing gas onto molten steel under reduced pressure, and causing CO near the surface of the molten steel to be removed by the blown gas.
[C] and [ in degassed molten steel are reduced by reducing the gas partial pressure.
It is characterized by promoting the decarburization reaction of O1.

Any type of gas may be used as the blowing gas, as long as it does not actively inhibit the cO gas production reaction, such as argon gas, helium gas, nitrogen gas, hydrogen gas, air, etc. .

Also, any of these mixed gases can be employed.

[Function] In the method for producing ultra-low carbon steel according to the present invention, the melt f! 4 Blow gas toward the scene to reduce the partial pressure of CO gas near the scene, and reduce [C] and [O] in the molten steel.
promotes degassing reactions with

During the degassing process, a large amount of CO gas is generated and is exhausted to the outside of the tank.

When the gas partial pressure PCO reaches a certain level, it is no longer possible to reduce the PCO beyond that level. However, since the CO gas near the hot water surface is diluted by blowing the gas onto the hot water surface, the CO gas partial pressure PCO at the hot water surface decreases, and the decarburization reaction is further promoted.

[Embodiments] Hereinafter, embodiments of the present invention will be specifically described with reference to the accompanying drawings.

Fig. 1 is a schematic diagram showing the RH degassing device used in the method for producing ultra-low carbon steel according to the embodiment of the present invention, Fig. 2 is a cross-sectional view of the lower degassing tank, and Fig. 3 is the gas FIG. 3 is a longitudinal sectional view showing a portion of the blowing device.

Rails are laid on the first floor of the building of the RH degassing facility, and the ladle 2 is transported from the converter factory by a traveling truck. A degassing tank 10 is provided in the upper part of the building, and a lifting table 6 for raising and lowering the ladle 2 is installed directly below this.

The outer surface of the degassing tank 10 is covered with an iron skin 11.
1 is lined with refractory bricks 12a and 12b. At the top of the degassing tank 10, an exhaust duct 30 and a chute 32 are provided. The exhaust duct 30 communicates with a gas exhaust device (not shown). The shooter 32 communicates with a hopper 31 that stores auxiliary raw materials or alloy materials. The degassing tank 10 has an upper tank and a lower tank separably connected to each other by a flange joint (not shown).

A pair of short pipe sections are formed in the lower degassing tank, one of which has an ascending pipe 24, and the other short pipe section has a descending pipe 2.
6 are connected to each other by flange joints (not shown). The rising pipe 24 and the descending pipe 26 are each provided with a refractory brick on the inside of the core material (on the side of the molten steel passage 25, 27), and the outside of the core material is covered with alumina castable.

A gas blowing pipe 15 passes through the riser pipe 24 , and its gas blowing port opens in a passage 25 .

The base end side of the gas blowing pipe 15 communicates with an argon gas supply source (not shown) equipped with a flow rate control valve.

A plurality of gas blowing devices 40 are provided on the side wall of the degassing tank, and each blowing hole 47 opens above the hot water level (at a level of about 1 m or less from the level of the static hot water level). The base end side of the gas blowing device 40 is connected to a gas supply source 37.
is connected to. The gas supply source 37 includes an argon gas tank, an oxygen gas tank, and a flow control valve, and supplies a mixed gas of argon gas and oxygen gas at a predetermined ratio to the gas blowing device 40.

The output side of the process computer 36 is connected to the flow control valve of the gas supply source 37, while the input side of the computer 36 is connected to the gas flow meter 34 of the exhaust duct 30 and the analyzer 3.
5. That is, the computer 36 calculates the CO gas partial pressure PCO based on the detection results of the flowmeter 34 and the analyzer 35, and based on this, the computer 36 calculates the CO gas partial pressure PCO.
A predetermined command signal is issued to the gas blowing device 40 from
A predetermined amount of gas is blown into the molten steel 3 in the tank.

As shown in FIG. 2, the gas blowing devices 40 are arranged radially in two sets of four gas blowing devices 40 at intervals of 25'' in total, eight in total. , can be provided at a maximum of 32 locations.

As shown in FIG. 3, the thin tube nozzle 46 of the gas blowing device 40 is provided downward at an angle θ of about 20° from the horizontal. The downward angle θ of the nozzle 46 only needs to be able to obtain a predetermined gas reach distance, and is preferably within the range of 1 to 60°.

The thin tube nozzle 46 is embedded in the refractory brick 12c and formed into a unit. That is, the capillary nozzle 46 is attached to the nozzle unit by penetrating the steel skin 11 and the side wall refractory brick 12b.

The base end of the capillary nozzle 46 protrudes from the iron shell 1 and is protected by a protective cover 44. The protective cover 44 is
It is welded to the iron skin 11, and a union elbow 43 is connected to the outside thereof. Furthermore, the elbow 43 is a coupling 42
It is detachably connected to the hose 41 by. In addition,
In this case, the capillary nozzle 46 is made of stainless steel, and has an inner diameter of 2a+m and an outer diameter of 3mm.

From the viewpoint of erosion resistance, it is preferable that the heat receiving area of the capillary nozzle 46 is small, and it is desirable that the capillary nozzle 46 has a thin wall thickness. Note that the thin tube nozzle 46 has an inner diameter of 1 to
It is preferable that the diameter is in the range of 5.5 mm, and a double tube can also be used in addition to a single tube.

Next, a case will be described in which ultra-low carbon steel is melted using the above-mentioned degassing tank.

Converter molten steel ladle 2 with carbon concentration [C] of approximately 300 ppm
The steel is then transported to a degassing facility.

The amount of molten steel 3 is about 250 tons and is covered with slag 4. Lift the ladle 2, immerse the dip tubes 24 and 26 in the molten fv43 in the ladle, and degas t! The inside of 10 is depressurized. When the pressure is reduced to about 200 torr, the molten steel 3 reaches the upper surface of the paving bricks 12a in the lower tank. Furthermore, when the pressure inside the tank is reduced, the molten fI43 flows into the degassing tank 10 at a rate of about 1.5 mm from the ladle surface.
It can be sucked up to a height of 5 meters. Argon gas is supplied to the gas blowing pipe 15 at a rate of 100 ONff per minute, and after about 5 minutes, the amount of argon gas supplied is increased to 250 ONff per minute.

As a result, the apparent specific gravity of the molten steel 3 decreases, and the molten steel m3 rises in the passage 25 together with gas bubbles. The upper molten metal surface of the riser pipe 24 rises, splash occurs, and the molten steel [C
] reacts with [O] to gasify, and this CO gas is exhausted. The molten steel 3 flows from the riser pipe 24 toward the downcomer pipe 26 and circulates between the ladle 2 and the degassing tank 10. At this time, the molten steel circulation flow rate reaches about 17 Nm3 per minute.

When argon gas is started to be blown into the riser pipe 24, the molten steel is stirred and a large amount of gas containing CO gas as a main component is generated. The generated gas passes through the exhaust duct 30 and is exhausted to an exhaust device (not shown), but at this time, the flow meter 3
4 and an analyzer 35 detect the gas flow rate and CO gas concentration.

These detection signals are sent to an input of computer 36. When the detection signal is input to the computer 36, the CO gas partial pressure P is determined based on the detection signal. 0 is calculated. for example,
The PCO during processing is approximately 0.1 to 2.0 torr. Thereby, the amount of gas sprayed onto the hot water surface can be adjusted while monitoring the CO gas partial pressure PCO during treatment.

When argon gas is sprayed onto the molten metal surface, the CO gas near the molten metal surface is diluted and the decarburization reaction of the molten steel 3 is promoted. In this case, the maximum amount of argon gas sprayed is 2500 per minute! l
It is.

According to the above embodiment, since the CO gas partial pressure PCO near the hot water surface can be further reduced, the decarburization speed during the degassing process can be improved without increasing the size of the current exhaust system. Ta. As a result, the initial [C] amount was 300 p
pm of molten steel could be decarburized to 15 pp- in about 10 minutes. Incidentally, according to the conventional method, 10
Decarburization of up to 30 ppm was the limit in a one-minute treatment. Therefore, it was possible to stably and quickly produce ultra-low carbon steel.

In addition, although the above-mentioned example explained the case of RH degassing method, the present invention is not limited to this only, and the DH degassing method is described.
The present invention may be employed in a degassing method.

[Effects of the Invention] According to the present invention, it is possible to reduce the CO gas partial pressure pco during degassing treatment without increasing the size of existing exhaust equipment, and it is possible to reduce the CO gas partial pressure pco during degassing treatment, and to obtain ultra-low carbon gas with [C] below a predetermined level. Steel can be melted stably and quickly.

[Brief explanation of drawings]

Fig. 1 is a schematic diagram showing a degassing tank used in the method for producing ultra-low carbon steel according to an embodiment of the present invention, Fig. 2 is a cross-sectional view of the lower degassing tank, and Fig. 3 is a gas blowing tank. An enlarged vertical sectional view showing a portion of the device, and FIGS. 4 and 5 are graphs for explaining the prior art, respectively.

Claims (1)

[Claims] A pole characterized in that a gas is blown onto molten steel under reduced pressure, the blown gas reduces the partial pressure of CO gas near the molten metal surface, and promotes the decarburization reaction of [C] and [O] in the degassed molten steel. Low carbon steel melting method.