JPH0361315A - Method for refining dead-soft carbon steel - Google Patents

Abstract

PURPOSE:To stably carry out the decarburizing refining of a molten steel to a dead-soft carbon region without forming a mushroom at a blowing opening by blowing an O2-containing inert gas into a molten low carbon steel in a vacuum tank in the latter stage in a refining period at the time of subjecting a molten steel to decarburizing refining by means of an RH-type vacuum degassing refining apparatus. CONSTITUTION:A molten steel 3 in a ladle 2 in an RH-type vacuum degassing apparatus 10 is sucked into a vacuum tank and decarburized by the reaction between [C] and [O] in the molten steel, and the resulting gas composed principally of CO is discharged by means of a duct 30. At this time, the flow rate of exhaust gas and CO content are measured by means of a flowmeter 34 and an analyzer 35, and the resulting values are inputted to a process computer 36 to calculate the amount of CO generated. When C content in the molten steel 3 is reduced in the latter stage in a decarburizing refining period and the amount of CO formed by the reaction between [C] and [O], O2 gas to be allowed to react with [C] is supplied by blowing an Ar gas containing O2 gas by 2-20vol.% from a gas supply source 37 through a nozzle tip 46 in a gas-blowing pipe 40 in a state of fine bubbles into the molten steel to accelerate decarburizing reaction, by which decarburization is carried out until a dead-soft carbon grade of <0.003% C is reached and also a mushroom formed at the nozzle tip 46 is melted by the quantity of heat of oxidation reaction by O2 and removed.

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 heavy level. , relates to a method for preventing porous solidified metal (pine mushroom) from adhering to a gas inlet.

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

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 drawn W4 molten steel is decarburized using various refining equipment to obtain the desired charcoal, a
Let it be the concentration. Ultra-low carbon steel is required to have a carbon content as low as 0.003% by weight or less, so it is generally difficult to stably and efficiently produce 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 [0] 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 process, and it takes a long time to further reduce [C]. 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 ONf per minute.
FIG. 2 is a graph showing changes in [C] at various stages of conventional degassing treatment with various changes made within the range of i. In the figure, the shaded area, white circles, and black circles indicate the argon gas blowing amount of 300 ONN and 4000 rl per minute, respectively.

The results are shown when the number of ONNs is 500ONN. 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 ppm. Further, as is clear from the figure, the optimal amount of argon gas to be blown into the riser is 400 ONfi per minute, and rapid decarburization cannot be performed even if the amount is at least higher than this. This is because when the amount of gas blown becomes excessive, gas blow-through occurs.

For this reason, 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 refers to an index in the primary reaction of decarburization defined by the following equation (1).

(d [C] / d t) "Kc [C]
(1) As is clear from the figure, when the cycle time of molten steel becomes shorter, the decarburization rate constant improves. Therefore,
By increasing the flow rate of molten steel 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 significantly 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 (ffi CNI /min) and the degree of vacuum (torr) in the degassing tank, respectively. It is a graph diagram examining each change in. As is clear from the figure, the CO of region ■ is higher than that of region I.
It can be seen that the amount of gas generated is significantly reduced.

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] -CCE o-e x p (-Kc-t) = 12
) Furthermore, the gas stirring force S was determined by the following formula (3) based on the amount of CO gas generated.

S − (8,18x q x Tt /V,) x (i
n (1+ρ, ・g −h/P)+(1−T, /TJ
) l...(3) Where, V is the vacuum chamber volume, h is the depth of the molten steel, T is the molten steel temperature, ρ is the molten steel density,
P represents the pressure inside the vacuum chamber, and g represents the gravitational acceleration.

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 decreases significantly, which is disadvantageous when producing ultra-low carbon steel.

By the way, when high-pressure gas is blown into a tank under reduced pressure, the gas expands at a predetermined rate and absorbs heat from the surrounding area, causing a portion of molten steel to solidify at the gas injection port.

In particular, when the nozzle inner diameter is small and the gas cooling capacity is large, porous solidified metal (hereinafter referred to as "pine mushroom") is formed at the gas injection port.

Such pine mushrooms, for example, have a purity of 98% by volume.
When pure argon gas is blown at a speed higher than the speed of sound,
Formed in a short time.

When pine mushrooms form at the gas inlet, the gas injection speed is reduced and the distance that the injected gas travels is shortened. Further, even if the gas supply pressure (back pressure) is increased, the blown gas does not enter a jetting state but becomes a bubbling state, making it impossible to increase the desired stirring force and gas-liquid interfacial area.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing ultra-low carbon steel that can prevent the formation of pine dust at the gas injection port.

[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, porous plugs or multi-hole plugs (M
HP), the gas blowing speed is the sonic speed (Matsuha 1
), the bubble gas becomes bubbling and the decarburization rate cannot be increased.However, if a single pipe nozzle or double pipe nozzle is used, the gas blowing speed exceeds Matsuha 1 and the bubble gas becomes jetting. dispersed into a state,
We found that it contributes to increasing the decarburization rate.

In addition, when the nozzle inner diameter is less than 111 m, molten steel is inserted into the gas injection port, and the gas passage is blocked by solidified metal, causing nozzle clogging. On the other hand, when the nozzle inner diameter exceeds 5 mm, the gas supply pressure (back It was discovered that even if the pressure was increased, the distance the blown gas could reach was short and it was not possible to disperse the bubble gas. Furthermore, when the reach of the blown gas becomes shorter, the gas floats along the side walls, and the gas bubbles coalesce and grow, making it difficult to disperse.
It was discovered that the refractories on the side walls of the degassing tank were significantly eroded and damaged.

Equation (4) below shows the conditions under which nozzle clogging will not occur.

Q/N≧3.3 XIO2fρ, ρ, −ρ, )l−”X (1+H/
1.48)

Q/N≦13×(ρ, /ρ ) −12×)l 32
X d ... (5) However, Q/N is the gas flow rate per nozzle, ρ is the density of molten steel, H is the molten steel depth, d is the nozzle inner diameter, and ρ is the gas after correcting the molten steel static pressure. Each represents the density.

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 is found that the optimum nozzle inner diameter d is in the range of 1 to 5IIIl.

Incidentally, the conditions under which molten steel does not enter are when the nozzle inner diameter is 1 and the minimum velocity V of the blown gas is 321.7 m/s.
, minimum capacity Q is 15.2Nff per minute, nozzle inner diameter 2I
In the case of Ila+, the minimum velocity of the blowing gas is V or 45 per second.
4.9 m, the minimum volume QQ is 85.7 NN per minute, and the minimum velocity V of the blown gas is 7 per second when the nozzle inner diameter is 5 mm.
19.3 m, and the minimum capacity Q is 847.4 rl per minute.

The method for producing ultra-low carbon steel according to the present invention involves blowing bubble gas into molten steel under reduced pressure to promote the decarburization reaction of [C] and [0] in the molten steel. It is characterized in that oxygen gas is mixed with the gas at a ratio of 2 to 20 volumes 96, and this is blown into the molten steel through a single tube nozzle as the bubble gas.

Any type of gas may be used as the non-oxidizing gas, as long as it is not a type of gas that actively harms the skin due to the CO gas production reaction. For example, argon gas, helium gas,
Nitrogen gas, hydrogen gas.

Any of CO2 gas, CO gas, air, and a mixed gas thereof can be used.

By the way, Japanese Patent Application Laid-Open No. 64-79317 discloses that when reflux gas is blown into the riser pipe from two systems of high-pressure and low-pressure nozzles for the purpose of increasing the molten steel recirculation flow rate, A similar technique has been disclosed in which oxygen gas is mixed to prevent solidified metal from adhering to the gas inlet. However, although the technique described in the above-mentioned publication discloses blowing reflux gas into the riser, it does not disclose blowing gas into the degassing tank. Furthermore, although the above technology aims to increase the flow rate of molten steel, the technology aims to proactively increase the decarburization rate in the degassing tank to produce ultra-low carbon steel. [Function] In the method for producing ultra-low carbon steel according to the present invention, oxygen gas is mixed with non-oxidizing gas at a ratio of 2 to 20% by volume, and this is used as bubble gas through a single tube nozzle. into the molten steel in the degassing tank. The gas expands adiabatically under reduced pressure, taking heat away from the surrounding molten steel, and the solidified metal tries to adhere to the gas injection port, but since the blown gas contains oxygen gas, it solidifies due to an oxidative exothermic reaction. Metal melts immediately. This prevents the formation of pine mushrooms and
Jetting gas is stably blown into the molten steel throughout the degassing process, increasing the stirring force and gas-liquid interfacial area, and promoting the decarburization reaction.

By the way, when argon gas is used as the non-oxidizing gas,
When the argon gas flow rate is INm' per minute (normal pressure), the amount of heat absorbed due to adiabatic expansion is 61.8 kcal per minute.

This is compensated for by the oxidation calorific value of the solidified metal, but increasing the oxygen gas content does not necessarily increase the calorific value. When the oxygen gas content is 2.5% by volume, the heat transfer efficiency η(
The proportion of oxygen gas that can contribute to exothermic reactions is approximately 1
00%, but when the oxygen gas content is 4.8% by volume, the heat transfer efficiency η is about 50%, and the oxygen gas content is 19.8%.
When it is % by volume, the heat transfer efficiency η decreases to about 10%. Therefore, the appropriate oxygen gas content in the injection gas is 2 to 20% by volume.

[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 chamber 610 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). In each of the rising pipe 24 and the descending pipe 26, a refractory brick is provided inside the core material (on the side of the molten steel passage 25, 27), and the outside of the core material is covered with alumina chitostable.

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 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 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 amount of CO gas generated based on the detection results of the flowmeter 34 and the analyzer 35, and based on this, the computer 36 issues a predetermined command signal to the gas blowing device 40, and a predetermined amount of gas is injected into the tank. It is designed to be blown into the melt fl143 inside.

As shown in FIG. 2, the gas blowing device 40 has a total of eight gas blowing devices, two sets of four gas blowing devices radially arranged at a center angle of 25 degrees, so as to avoid the rising pipe passage 25 and the downcomer pipe passage 27. ing. Note that the greater the number of gas inlet ports 47, the better; however, if the inlet ports 47 are too close to each other, the injected gases will interfere with each other and the gas bubbles will become larger in diameter, so these should be appropriately spaced apart. In this case, the pitch interval between the gas blowing holes 47 is 1
．． It is desirable to set it to 40mm or more, and 200 to 500
It is more preferable to set it in the range of mm.

As shown in FIG. 3, the thin tube nozzle 46 of the gas blowing device 40 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.
11IIIl stands out. On the other hand, the base end of the thin tube nozzle 46 protrudes from the iron shell 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,
The elbow 43 is detachably connected to the hose 41 by a coupling 42. In this case, the thin tube nozzle 46 is made of stainless steel, and has an inner diameter of 2 mm and an outer diameter of 3111 mm. From the viewpoint of erosion resistance, it is preferable that the heat receiving area of the capillary nozzle 46 is small.
It is desirable that the thin tube nozzle 46 has a thin wall thickness.

Next, using the above degassing tank, the carbon content 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 the ladle 2, and is conveyed to the degassing treatment facility 63. The amount of molten steel 3 is about 250 tons and is covered with slag 4. Lift the ladle 2 and insert the dip tube 2 into the melt m3 in the ladle.
4.26 is immersed, and the inside of the degassing tank 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 of the lower part. Further, 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. 100 per minute to the gas blowing pipe 15
ONJI! of argon gas is supplied, and after about 5 minutes, the argon gas supply rate is increased to 2600 il/min. As a result, the apparent specific gravity of the molten steel 43 decreases, and the molten steel 3 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 EOE and gasifies, and this CO gas is exhausted. The molten steel 3 flows from the riser pipe 24 toward the downcomer pipe 26 and circulates between m2 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]! 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), and 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 an input 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. CO gas generation in the first stage of treatment is 3000 to 600 ONj per minute;
! That's about it. In the calculation section of the computer 36, a preset J! Semi-gas amount (gas agitation force above a predetermined level i)
The difference is obtained by subtracting the amount of CO gas generated (the amount of gas calculated based on the detected exhaust gas value) from the total amount of stirring gas required to produce the gas, and the amount of gas corresponding to this difference is supplied to the gas blowing device 40. A command signal is sent to the gas supply source 37 so that
send to As a result, the mixed gas is supplied to the capillary nozzle 46 of the gas blowing device 40, and the gas forms fine bubbles and is vigorously injected from the blowing port 47 into the melt fI 43. The injection gas contains approximately 2.5% oxygen gas by volume.
It is mixed with argon gas at a ratio of .

When gas is blown under reduced pressure, the gas expands adiabatically and causes a local temperature drop at the gas injection port 47, causing molten steel to solidify and adhere, but when a small amount of solidified metal is formed, it reacts with oxygen and generates heat due to oxidation. Dissolve again. For this reason, pine mushrooms are not formed.

Thereby, gas is stably injected during the degassing process. The bubbles of the injection gas serve as nuclei, promoting the gas production reaction between [C] and [0] in the molten steel, and the decarburization of the molten steel 3 rapidly progresses.

Eventually, in the late stage of processing (area ■), during the molten steel [C],
[0] becomes smaller, and the amount of CO gas generated is 1000~ per minute.
Although the amount of stirring gas decreases to 200ON1 or less, the amount of stirring gas shortage is determined by computer calculation based on the exhaust gas detection signal.
Based on this, bubble gas is blown from the gas blowing device 40 to make up for the shortage. This allows late processing (region ■)
Also in the first stage of the treatment (region 1), the same gas stirring force and gas-liquid interfacial area can be obtained, the decarburization reaction is promoted, and ultra-low carbon steel with a predetermined target composition is produced.

FIG. 5 is a graph for comparing and explaining the present invention and the prior art, with the horizontal axis representing the RH degassing treatment time 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 iii!i region ■). I was able to do 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, 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.

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, the amount of heat absorbed by the adiabatically expanded gas is compensated by the amount of heat generated by the oxidation reaction between the oxygen in the blown gas and the solidified metal, so that the solidified metal does not grow and the gas injection port pine mushrooms do not form. Therefore, gas can be blown into the molten steel at a speed higher than the speed of sound over the entire period of degassing treatment, and the decarburization reaction can be significantly promoted by the bubble gas. As a result, the gas agitation force and gas-liquid interface area are the same in the late stage of degassing as in the early stage of treatment. 11, and ultra-low carbon steel with a [C] level of 10 ppm or less can be stably and rapidly produced.

[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,
A method for producing ultra-low carbon steel, comprising mixing non-oxidizing gas with oxygen gas at a ratio of 2 to 20% by volume, and blowing this as the bubble gas into molten steel through a single tube nozzle.