JPH0361318A - Method for refining dead-soft carbon steel - Google Patents

Abstract

PURPOSE:To accelerate the reaction between [C] and [O] and to produce a dead-soft carbon steel by detecting the amount of CO generated in the latter stage in a refining period, blowing specific amounts of gas into a molten steel according to the detected value, and increasing the agitation of the molten metal due to the resulting bubbles at the time of subjecting a molten steel to decarburizing refining in an RH-type degassing refining apparatus. CONSTITUTION:A molten steel 3 in a ladle 2 is sucked into a vacuum tank in an RH-type vacuum degassing refining apparatus 10, and CO gas as a decarburization product due to the reaction between [C] and [O] is discharged in a bubble state from the molten steel 3, and the CO gas is discharged while agitating the molten steel. This reaction exhaust gas is evacuated by means of an evacuation duct 30, and the amount of CO generated is measured by means of a flowmeter 34 and an analyzer 35 for the exhaust gas. At the point of time when reduction in the amount of CO generated takes place as a result of reduction in [C] content in the latter stage in a refining period, a gas of Ar, etc., is blown from a gas supply source 37 through a blowing nozzle 46 into the molten steel. At this time, the sum of the amount of CO generated and the amount of Ar blown is regulated so that it is higher than the amount of CO exhaust gas generated in the former stage in the refining period and agitation power due to gas bubbles in the molten steel is increased so that it is similar to that in the former stage in the refining period, by which the reaction between [C] and [O] is accelerated. By this method, a dead-soft carbon steel of <0.003% C can be produced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for producing ultra-low carbon steel in which degassing treatment is performed so that the carbon concentration [C] of molten steel is at an extremely trace level.

[Conventional technology] In recent years, with the advancement of high-grade steel materials, the carbon content has been reduced to an extremely small amount: J
There is a growing demand for ultra-low carbon steel with a high degree of stability, and there is a need for technology to quickly and stably melt it.

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 N4 molten steel is decarburized using various refining equipment to obtain a desired carbon concentration. Ultra-low carbon steel is generally required to have an extremely low carbon content of 0.003% by weight or less.
It is difficult to stably and efficiently melt a large amount of molten steel. 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 [O1] in the molten steel react to generate a large amount of CO gas. The CO gas produced 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 treatment, and it takes a long time to further reduce [C], and extremely low carbon It is not possible to melt steel stably and quickly.

In Figure 7, 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 ONI per minute.
It is a graph diagram in which various changes were made in the range of I and changes in [C] at each stage of conventional degassing treatment were investigated. In the figure, the shaded areas, white circles, and black circles indicate the argon gas blowing rate of 300 ON/min, 11.400 ONN, respectively.

The results are shown when the number is 500011. 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 an agricultural level even lower than 10 to 20 ppm.

Further, as is clear from the figure, the optimal amount of argon gas to be blown into the riser is 4000 N.Q 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 8, 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] /d t) =Kc [C] -(
1) As is clear from the figure, as the cycle time of molten steel becomes shorter, the decarburization rate constant K. will improve. 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.

In Fig. 9, the horizontal axis shows the evacuation time of the RH degassing tank, and the vertical axis shows the CO gas generation in (Nl/min) and the degree of vacuum (Torr) in the degassing tank, respectively. It is a graph diagram examining each change in (2).

As is clear from the figure, the amount of CO gas generated in region (2) is significantly lower than that in region (1).

In Figure 10, the horizontal axis shows the RH degassing treatment time, and the vertical axis shows the decarburization reaction rate constant Kc and the stirring force S in the vacuum degassing tank due to CO gas, and the relationship between the two was investigated. It is a graph diagram. Note that the decarburization reaction rate constant Kc was determined every 3 minutes based on the following equation (2).

[C] −[Cko 0 − e x p (−Kc
t) −(2) Also, gas stirring force S
was determined by the following formula (3) based on the amount of CO gas generated above.

S = (6,18XqXTs/Vs)
x (In (1+ρ, gh/P)+ (1-T,
/Ts) l...(3) where ■, is the vacuum chamber volume, h is the depth of the molten steel, T is the molten temperature, ρ is the molten steel density, P is the pressure inside the vacuum chamber, and g is the gravitational acceleration. respectively.

As is clear from the figure, the decarburization reaction rate constant KC and the gas stirring force S show the same tendency, and as the stirring force S decreases, the rate constant Kc also decreases. Therefore, late processing (region ■
), the decarburization rate is severely reduced, which is disadvantageous when producing ultra-low carbon steel.

This invention was made in view of such vehicle conditions, and aims to improve the stirring power of molten steel in the degassing tank, and
The purpose of this invention is to provide a method for producing ultra-low carbon steel that can increase the gas-liquid differential area, and an apparatus therefor.

[Means for Solving the Problems] The inventors installed various types of nozzles on the side wall of the lower RH degassing tank to bubble various gases and blow them into the molten steel, thereby increasing the gas-liquid interface area. Various studies were conducted to promote the decarburization reaction.

As a result, in the case of a porous plug, the gas blowing speed does not exceed Matsuha 1, and the bubble gas enters a bubbling state, making it impossible to increase the decarburization speed. However, when using a single or double pipe nozzle, It was found that when the gas blowing rate exceeds 1, the bubble gas becomes jetting and disperses, contributing to an increase in the decarburization rate.

Additionally, if the nozzle inner diameter is less than IIIm, molten steel will be inserted into the gas injection port, and the solidified metal will block the gas passage, causing nozzle clogging.On the other hand, if the nozzle inner diameter exceeds 5mm, the gas supply pressure (back pressure ), it was found that the reach of the blown gas was short and it was not possible to disperse the bubble gas. Furthermore, we found that when the reach of the blown gas becomes shorter, the gas rises along the side wall, bubble gas coalesces and grows, making it difficult to disperse, and the refractories on the side wall of the degassing tank are significantly eroded. Ta.

In Figure 4, the horizontal axis represents the nozzle inner diameter d, and the vertical axis represents the nozzle 1.
It is a graph diagram in which the amount of gas blown per book was measured and the relationship between the two was investigated. The shaded area in the figure is the range in which gas can be blown. As is clear from the figure, it can be seen that the optimum nozzle inner diameter d is in the range of 1 to 5 cm.

Furthermore, when the cooling capacity of the blown gas increases, porous solidified metal (hereinafter referred to as "pine mushroom") adheres to the gas inlet, and gas bubbles coalesce to form a bubbling state. Therefore, do you know that a single tube nozzle is more suitable for a gas jetting nozzle than a double tube nozzle? 11. We also found that pine mushrooms are more likely to form when pure argon gas is blown into them.

By the way, Japanese Patent Publication No. 56-49968 discloses a similar technique in which a mixed gas of argon gas and oxygen gas is side-injected into a lower RH degassing tank. According to this technology, a mixed gas consisting of oxygen gas and inert gas is blown into the molten steel in the degassing tank using a double pipe nozzle, and the molten steel is decarburized while preventing the oxidation of chromium components, etc. , K*m, and the so-called RH-OB as a technology for melting high-tensile, high-alloy steel.
The law has been disclosed. In the RH-OB method, decarburization of molten alloy steel is promoted by actively increasing the amount of dissolved oxygen in molten steel under reduced pressure and improving the rate of CO gas production.

However, in the above RH-OB method, low P
This results in an increase in dissolved oxygen [O1m] in the treated molten steel in the CO (partial pressure of CO gas in the degassing tank) region, which makes it impossible to utilize the deoxidation treatment before the start of treatment and tends to increase the amount of erosion of refractories. be. The lower degassing tank is lined with expensive magnesia chromium firebrick. In general, in the lower tank for degassing, the lifespan of bricks on the sides is shorter than that of bricks, and the lifespan of the lower tank is limited by the residual thickness of the bricks, especially those installed between the immersion pipe passages, which can cause them to bulge out. It is determined. For this reason, there is a disadvantage that the number of replacements of the lower tank increases and the cost of refractories increases.

In addition, although the above X[ER RH-OB method discloses a technology for the purpose of melting alloy steel, it also discloses a technology for melting ordinary steel with a small amount of alloy, such as ultra-low carbon steel. It's not something you do.

On the other hand, in the method for producing ultra-low carbon steel according to the present invention, bubble gas is blown into molten steel under reduced pressure to promote the decarburization reaction of [C] and [O] in the molten steel. , grasp the amount of CO gas generated by the decarburization reaction, and
The total amount of gas generation amount and the amount of bubble gas blown is,
At least exceed the amount of CO gas generated in the first stage of treatment.
The method is characterized in that the amount of the bubble gas blown is controlled. Any type of gas can be used as the blowing gas, as long as it does not actively inhibit the CO gas production reaction (for example, argon gas, helium gas, oxygen gas, nitrogen gas, Any gas among hydrogen gas, CO2 gas, CO gas, air, and mixed gases thereof can be employed.In particular, it is desirable to blow argon gas containing a small amount of oxygen gas into the molten steel as bubble gas. .In addition, when blowing CO gas into molten steel,
Considering that the CO gas production reaction is inhibited, it is preferable to use a mixed gas with other gases instead of using CO gas alone.

[Function] In the method for producing ultra-low carbon steel according to the present invention, the amount of CO gas generated during the degassing treatment is ascertained, and the amount of bubble gas blown is controlled according to the amount of CO gas generated. The total amount of the amount of CO gas generated and the amount of bubble gas blown is made to exceed the amount of CO gas generated in the first period of the treatment. That is,
[C] and [O] in molten steel in the early stage of treatment (area I)
Since it is in a high concentration region, a large amount of CO gas is generated, the molten steel is strongly stirred, a large amount of splash is generated, and the decarburization reaction rate is high. On the other hand, in the late stage of treatment (region ■), [C] and [O] in the molten steel shift to a low concentration region, so the amount of CO gas generated decreases, and the molten steel stirring force and gas-liquid interface area decrease. , the decarburization reaction rate becomes smaller. Therefore, in the latter stage of the process, the amount of bubble gas blown is increased in accordance with the decrease in the amount of CO gas generated to compensate for the deficiency in the amount of CO gas generated in region (3). This injection of compensation gas increases the molten steel stirring force and the gas-liquid interfacial area, increases the decarburization reaction rate in region (1), and promotes decarburization of the molten steel.

Incidentally, the above RH-OB method involves side injection in a low PCO region after deoxidation, whereas in this invention, injection is performed from a high PCO region in the early stage of the treatment to a low PCO region in the late stage of the treatment.
Side ejection occurs in a wide range of areas. Therefore, the inside of the tank can be moved from a high PCO region to a low PCO region (decarburization), and the molten steel can be moved from a high dissolved oxygen [O] region to a low dissolved oxygen [O] region (deoxidation). This also results in a reduction in the amount of deoxidizing agent added.

[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 unit 110 is provided on the roof 8, and a lifting table 6 for raising and lowering the ladle 2 is installed directly below this.

Degassing! 10 has an outer surface covered with an iron skin 11, and the iron skin 11 is lined with refractory bricks 12a and 12b.

An exhaust duct 30 and a shooter 32 are provided at the top of the degassing tfllO. The exhaust duct 30 communicates with a gas exhaust device (not shown). The shooter 32 is
It 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 parts are formed in the degassing lower part, one of which has an ascending pipe 24, and the other short pipe part has a down 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 4o are provided on the side wall of the lower tank, and each blowing port 47 is connected to the paving brick 12a of the lower tank.
It is open nearby. The proximal end side of the gas blowing device 40 communicates with the gas supply source 37 . The gas supply source 37 has an argon gas tank, an oxygen gas tank, and a flow control valve, and supplies oxygen gas to argon gas at a predetermined ratio.
6 of the mixed gas is supplied to the gas blowing device 40.

The output side of the process computer 36 is connected to the meteor 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 analysis=f
It is connected to 35. That is, based on the detection results of the flowmeter 34 and analyzer 35, the computer 36 detects CO.
The amount of gas generated is calculated, and based on this, a predetermined command signal is issued from the computer 36 to the gas blowing device 40, so that a predetermined amount of gas is blown into the molten steel 3 in the tank.

As shown in FIG. 3, the gas blowing device 40 is provided with two sets of four gas blowing devices radially arranged at a center angle of 25'' to avoid the passage 25 of the rising pipe and the passage 27 of the downcomer pipe, for a total of eight gas blowing devices. The larger the number of gas inlet ports 47, the better, but if the inlet ports 47 are too close to each other, the injected gas will interfere with each other and the gas bubbles will become larger in diameter, so they should be separated appropriately. In this case, the pitch interval between the gas blowing holes 47 is 1.
It is desirable to set it to 40ma+ or more, and 200 to 500
It is more preferable to set it in the range of mg.

As shown in FIG. 4, the thin tube nozzle 46 of the gas blowing device 4o is embedded in the refractory brick 12c, and its tip inlet 47 is located approximately 50 mm inward from the side wall refractory brick 12b.
It protrudes by mm. On the other hand, the base end of the thin tube nozzle 46 is
It protrudes from the iron skin 11 and is protected by a protective cover 44. The protective cover 44 is welded to the iron skin 11, and a union elbow 43 is connected to the outside thereof. Furthermore, elbow 4
3 is detachably connected to the hose 41 by a coupling 42. In this case, the capillary nozzle 46 is made of stainless steel, and has an inner diameter of 2 mm and an outer diameter of 31 Im. From the viewpoint of erosion resistance, it is preferable that the heat receiving area of the capillary nozzle 46 is small.
6 is desirably thin.

Next, using the above degassing tank, the amount of carbon was reduced to 10 ppm.
The case of producing the following ultra-low carbon steel will be explained.

Converter molten steel having a carbon concentration [C] of approximately 200 to 300 ppm is received in a ladle 2, and is transported to a degassing treatment facility.

The amount of molten fI4B is approximately 250 tons and is covered with slag 4. The ladle 2 is lifted, the dip tubes 24 and 26 are immersed in the molten M3 on the ladle pile, and the pressure inside the degassing tank 10 is reduced.

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 steel 3 is sucked up into the degassing tank 10 to a height of about 1.5 m from the ladle surface. 1 per minute to the gas blowing pipe 15
00 ONIl of argon gas is supplied, and after about 5 minutes, the argon gas supply rate is increased to 260 ONN/min. As a result, the apparent specific gravity of the melt M3 decreases, and the melt M3 rises in the passage 25 together with gas bubbles. The upper molten metal surface of the riser pipe 24 rises and splash occurs, causing the molten steel to melt [C]
reacts with [O] and gasifies, and this CO gas is emitted. The molten steel 3 flows from the rising pipe 24 toward the descending pipe 26, and circulates between w42 and the degassing tank 1o. At this time, the molten steel circulation flow rate reaches 17 Nm31i degrees 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). At this time, the gas flow rate and CO gas concentration are detected by the flowmeter 34 and analyzer 35.

These detection signals are sent to the human power section of computer 36. When the detection signal is input manually to the computer 36, the amount of co gas generated is calculated based on the detection signal. The amount of CO gas generated in the first stage of treatment is 3000 to 6000 Nf per minute.
It is about f. The calculation section of the computer 36 calculates the CO gas generation amount (
A command signal is sent to the gas supply source 37 so that a gas amount corresponding to this difference is supplied to the gas blowing device 40. As a result, the thin tube nozzle 4 of the gas blowing device 40
A mixed gas is supplied to 6, and the gas becomes fine bubbles and is vigorously injected from the blowing port 47 to the melt ff43. At this time, the travel distance of the gas bubbles is about 400 Illal.

The bubbles of the injection gas become the nucleus, and the molten steel [C]
The gas production reaction between [O] and [O] is promoted, and a large amount of CO gas is generated. As a result, decarburization of the molten steel 3 rapidly progresses. Further, since the injection gas contains oxygen, no pine mushroom is formed near the blowing hole 47. That is, the amount of heat absorbed by the adiabatic expansion of the blown gas is compensated by the amount of heat generated by oxidation of the molten steel, and solidification and adhesion of the molten steel to the blowing port 47 is prevented.

Eventually, in the late stage of processing (area ■), during the molten steel [C],
[O1 becomes smaller and the amount of CO gas generated is 1ooo per minute.
However, the shortage of stirring gas is determined by computer calculation based on the exhaust gas detection signal, and based on this, bubble gas is blown from the gas blowing device 40 to compensate for the shortage. This allows the late processing (area ■
), it is possible to obtain the same gas stirring force and gas-liquid interface area as in the first stage of treatment (region l), and the decarburization reaction is promoted.
Ultra-low carbon steel with a predetermined target composition is produced.

FIG. 5 is a graph diagram for comparatively explaining the present invention and the prior art, with the horizontal axis representing the amount of RH degassing and the vertical axis representing the carbon concentration [C] of molten steel.

In the figure, the shaded area shows the results of conventional degassing treatment without side injection, and the open circles show the results of the present invention with side injection.

FIG. 6 is a graph for comparing and explaining the present invention and the prior art, with the horizontal axis representing the carbon concentration [C] of molten steel and the vertical axis representing the decarburization reaction rate constant Kc.

In the figure, the shaded area shows the results of conventional degassing treatment without side injection, and the open circles show the results of the present invention with side injection.

As is clear from both figures, according to the embodiment of the present invention, the decarburization rate can be improved during the entire period of degassing treatment, and the decarburization rate can be particularly improved in the latter stage of the treatment (region ■). did it. Therefore, it was possible to reduce [C] in a short time to a level of 10 pp11 or less, which was a level that was difficult to achieve with conventional methods, and it was possible to stably and quickly produce ultra-low carbon steel.

In addition, in the above embodiment, the case of RHH gas method was explained, but the present invention is not limited to this only, and the DH gas method is used.
The present invention may be employed in a degassing method.

In addition, in the above embodiment, a case was explained in which a mixed gas containing 1 to 20% by volume of oxygen gas to argon gas was side-injected, but the gas type is not limited to this, and CO gas production reaction Any type of gas may be used as long as it does not actively inhibit gases, such as argon gas, helium gas,
Oxygen gas, nitrogen gas, hydrogen gas, CO2 gas, CO gas.

Either air or a mixture of these gases can be used.

In addition, in the above embodiment, a stainless steel tube was used for the thin tube nozzle for blowing gas, but the invention is not limited to this.
Other types of metal or ceramic tubes can also be used.

[Effects of the Invention] According to the present invention, since compensation gas is injected into the molten steel in an amount corresponding to the deficiency in the amount of CO gas generated, it is possible to obtain the same gas stirring force and gas-liquid interfacial area in the latter half of the treatment as in the first half of the treatment. This allows the CO gas production reaction in the latter stage of the treatment to be significantly accelerated.

Therefore, ultra-low carbon steel having a [C] of 101)I)- or less can be stably and rapidly produced.

Furthermore, the amount of splash generated in the tank also increases,
Along with the decarburization reaction, other reactions such as denitrification are promoted, and it is effective not only for producing ultra-low carbon steel but also for producing ultra-low nitrogen steel.

[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. FIGS. 4 to 6 are enlarged vertical cross-sectional views showing parts of the device, and FIGS. 7 to 10 are graphs and graphs for explaining the effects of the present invention, respectively.
Each figure is a graph diagram for explaining the prior art.

Claims (1)

[Claims] When blowing bubble gas into molten steel under reduced pressure to promote the decarburization reaction of [C] and [O] in the molten steel,
The amount of CO gas generated by the decarburization reaction is ascertained, and the amount of bubble gas blown is adjusted so that the total amount of CO gas generated and the amount of bubble gas blown exceeds at least the amount of CO gas generated in the first half of the treatment. A method for producing ultra-low carbon steel characterized by control.