JPH1068013A - Vacuum-decarburizing refining method of stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a vacuum-decarburizing refining method of a stainless steel excellent in the productivity which restrains the sticking of splash to the inner walls of a vacuum vessel and an immersion tube and an oxygen lance and also, can reduce the solidified sticking of slag between the immersion tube and a ladle while preventing the loss caused by the oxidation of chromium in molten stainless steel. SOLUTION: In the vacuum-decarburizing refining method of the stainless steel stirring the molten steel by inert gas while blowing oxygen gas, in the high carbon concn. range of the molten steel 11, the flow rate of the oxygen gas is kept to 3-25Nm<3> /h/t-steel and the flow rate of the inert gas is kept to 0.3-4.0NL/min/s-steel. In the successive low carbon range of the molten steel 11, the flow rate of the oxygen gas is reduced to 0.5-12.5Nm<3> /h/t-steel and also, the dipping depth (h) of the immersion tube 14 is changed in a prescribed range.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for vacuum decarburization and refining of stainless steel, which can prevent slag from sticking between a dip tube and a ladle and can perform efficient decarburization and refining.

[0002]

2. Description of the Related Art As a method for obtaining molten steel having a carbon concentration of about 0.01% by weight by further decarburizing and refining molten steel in an electric furnace or a converter, the molten steel surface in a ladle is subjected to vacuum. And an immersion pipe method in which oxygen gas is blown onto the molten steel surface in an immersion pipe immersed in the molten steel to perform vacuum refining by blowing oxygen gas onto the molten steel surface while holding the molten steel. However, in the VOD method, since the space above the molten steel surface cannot be sufficiently secured, splashes of the molten steel scattered during refining adhere to the upper blowing lance and the lid of the vacuum vessel, which hinders operation. There was a problem that becomes.

[0003] Japanese Patent Publication No. Hei 4-20967 discloses a method using an immersion tube method with less restrictions on equipment.
As shown in FIG. 12, the molten steel 51 in the ladle 50 is
Into the vacuum chamber 53, blow an inert gas from the lower part of the ladle 50 below the projection plane of the immersion pipe 52, and oxidize gas through the upper lance 54 to the molten steel surface in the vacuum chamber 53. In the vacuum refining method of molten steel spraying
The inner diameter of the dip tube 52 is determined so that the ratio (D ₁ / D ₀ ) of the inner diameter (D ₁ ) of the dip tube 52 to the inner diameter (D ₀ ) of the ladle 50 becomes 0.4 to 0.8. The ratio (H ₁ / H ₀ ) of the depth (H ₁ ) of blowing the inert gas from the surface of the molten steel to the depth (H ₀ ) of the molten steel in the ladle 50 is 0.5 to 1.0. A vacuum refining method for molten steel has been proposed for the purpose of efficiently performing decarburization by determining the injection depth of the inert gas. JP-A-2-133510 discloses a ladle for accommodating a molten metal, a vacuum chamber provided with a dip tube at the lower end immersed in the molten metal, and a vacuum source for depressurizing the inside of the vacuum chamber. A vacuum processing apparatus comprising a connected exhaust pipe and a shield disposed inside the vacuum vessel, and maintaining the shield at a height of 2 to 5 m from a molten metal surface in the immersion pipe is proposed. Have been.

[0004]

However, the method described in Japanese Patent Publication No. 4-20967 has the following problems. Since decarburization and refining conditions such as the flow rate of oxygen gas and argon gas blown into the molten steel and the degree of vacuum in the vacuum chamber 53 are not properly specified, the rocking and splash of the molten steel surface become excessive, Operation troubles due to the adhesion of gold occur. The chromium component in the molten steel is oxidized by the injected oxygen, and the oxidized chromium oxide is reduced by the carbon in the molten steel while descending in the molten steel, but due to a convection phenomenon of the inert gas blown from below, Without being reduced, it floats on the molten steel surface between the dip tube and the inner wall of the ladle, forms slag 55 and is discharged from the molten steel, and the loss of chromium increases. Due to the slag 55 containing such chromium oxide, on the molten steel surface between the immersion pipe 52 and the inner wall of the ladle, the viscosity of the molten steel surface is increased, and the slag 55 or the metal is adhered to the periphery. Due to the sticking, it is difficult to move the immersion pipe 52 from the position of the ladle 50 at the end of the refining, which hinders the refining operation. The decarbonation efficiency, which is the ratio of the amount of oxygen gas contributing to decarburization of the molten steel to the total amount of oxygen gas blown into the molten steel, is determined by the vacuum chamber 5
3 depends on the refining conditions such as the degree of vacuum, the stirring state of the molten steel, and the flow rate of the oxygen gas to be blown. However, such refining conditions are not appropriate and it is difficult to maintain the decarbonation efficiency at a high level. It is. Further, Japanese Patent Application Laid-Open No.
A shield is arranged inside the vacuum chamber of 33510,
The smelting apparatus for preventing splash has a problem of reducing productivity, for example, an operation of removing bullion adhered to the shield is required.

[0005] The present invention has been made in view of the above circumstances, while suppressing the adhesion of splash to the inner wall of the vacuum chamber or immersion tube, the oxygen lance, and while preventing the loss due to oxidation of chromium in the molten stainless steel. It is another object of the present invention to provide a method for vacuum decarburization and refining of stainless steel, which is excellent in productivity and can reduce slag adhesion between a dip tube and a ladle.

[0006]

According to the present invention, there is provided a semiconductor device comprising:
The vacuum decarburization and refining method for stainless steel described in the above is a method in which an immersion pipe is immersed in molten steel containing chromium held in a ladle, and the inside of a vacuum chamber communicating with the immersion pipe is depressurized, and the upper part of the vacuum chamber is blown upward. In a vacuum decarburization refining method for stainless steel in which the molten steel is stirred by an inert gas supplied from a bottom blowing nozzle at the bottom of the ladle while blowing oxygen gas through a lance, in the high carbon concentration region of the molten steel, The oxygen gas flow rate of the oxygen gas is ³ to 25 Nm ³ / h / t-steel.
In addition, the inert gas flow rate of the inert gas is set to 0.3 to 4.0.
N liter / min / t-steel, and in the subsequent low carbon concentration region of the molten steel, the oxygen gas flow rate is reduced at a rate of 0.5 to 12.5 Nm ³ / h / t-steel per minute. At the same time, the immersion depth of the immersion tube is increased or decreased within a predetermined range. The method for vacuum decarburization and refining of stainless steel according to claim 2 is the method for vacuum decarburization and refining for stainless steel according to claim 1, wherein the carbon concentration of the molten steel in the high carbon concentration region is 0.3 to 1 wt%. The carbon concentration of the molten steel in the low carbon concentration region is 0.01 to 0.3 wt%
It is. The method for vacuum decarburization and refining of stainless steel according to claim 3 is the method for vacuum decarburization and refining for stainless steel according to claim 1 or 2, wherein the chromium content of the molten steel is 5 to 30 watts.
t%. The vacuum decarburization refining method for stainless steel according to claim 4 is the vacuum decarburization refining method for stainless steel according to any one of claims 1 to 3, wherein the immersion depth of the immersion pipe in the high carbon concentration region is set. And the immersion ratio between the molten steel depth in the ladle and 0.1 to 0.6. The vacuum decarburizing and refining method for stainless steel according to claim 5 is the vacuum decarburizing and refining method for stainless steel according to claim 1, wherein the vacuum degree of the vacuum tank in the high carbon concentration region is set. Is maintained at 200 torr or more. The vacuum decarburization refining method for stainless steel according to claim 6 is the vacuum decarburization refining method for stainless steel according to any one of claims 1 to 5, wherein the immersion pipe is a straight body type immersion pipe.

[0007] The high carbon concentration region of the molten steel means a reaction region in which the decarburization reaction is controlled by the supply of oxygen gas blown into the molten steel from the upper blowing lance. The low carbon concentration region of the molten steel refers to a reaction region in which the decarburization reaction becomes a rate-determining movement of carbon in the molten steel. If the oxygen gas flow rate in the high carbon concentration region is less than ³ Nm ³ / h / t-steel (hereinafter referred to as Nm ³ / h / t), the decarburization rate of the molten steel decreases, the refining time becomes longer, and the productivity increases. Tend to decrease. In the following description, the oxygen gas flow rate and the inert gas flow rate refer to flow rates converted per ton of molten steel to be refined. On the other hand, the oxygen gas flow rate is 25 Nm ³ / h /
If t is exceeded, the molten steel is excessively stirred and splash is likely to occur. As a result, loss due to metal adhesion and difficulty in pulling up the dip tube due to metal adhesion make the chromium oxidation amount large. It is not preferable because adverse effects such as appear.

Further, the flow rate of the inert gas in the high oxygen concentration region is 0.3 Nl / min / t-steel (hereinafter referred to as N
Liter / minute / t), it becomes difficult to efficiently separate the molten slag floating on the surface of the molten metal in the immersion tube to cause the carbon and oxygen in the molten steel to react efficiently. descend. Conversely, the inert gas flow rate is 4.0N
When it exceeds liter / min / t, the molten slag containing chromium oxide floating on the surface of the molten metal in the dip tube easily escapes from the dip tube into the ladle, and the splash cannot be effectively suppressed. Not preferred. The rate of decrease of oxygen gas flow rate in low carbon concentration region is 0.5N / min
When the amount is less than m ³ / h / t, the amount of reduction in the amount of generated CO gas is small, so that it is impossible to suppress the splash. Further, since the supply of oxygen gas becomes excessive, the oxidation amount of chromium increases. On the other hand, the decreasing speed is 12.5 Nm ³ / h
If the ratio exceeds / t, the decarburization efficiency in the low carbon concentration region decreases, and the blowing acid speed decreases too fast, so that the time for blowing acid at a low flow rate becomes longer, resulting in a subsequent processing time. It is not preferable because it becomes longer and the productivity tends to decrease.

[0009] The carbon concentration of the molten steel in the high carbon concentration region is in the range of 0.3 to 1 wt%. This is because, in a converter or an electric furnace, if the C concentration of molten stainless steel is to be reduced to less than 0.3 wt% by coarse decarburization refining, chromium in the molten steel is oxidized and oxidization loss of chromium is easily caused. Concentration 1
If the amount exceeds wt%, the time required for finish decarburization and refining of molten stainless steel is prolonged, and the productivity tends to decrease. Also, the lower limit of the carbon concentration in the low carbon concentration region following the high carbon concentration region is set to 0.01 wt% because the decarburization refining is performed even if the carbon concentration is reduced to less than 0.01 wt%. This is because it is difficult to further improve the required properties such as the target strength, and the cost for refining increases. If the chromium content of the molten steel is less than 5 wt%, it becomes difficult to maintain the properties such as corrosion resistance required for stainless steel. The chromium content is 30 wt%
Is not preferable because the required characteristics cannot be improved so much and the cost increases.

The immersion ratio (h / H) between the immersion depth (h) of the immersion tube and the molten steel depth (H) in the ladle in the high carbon concentration region.
Is less than 0.1, the generated chromium oxide flows out of the immersion tube early without the chromium oxide being reduced by the carbon in the steel, and the decarbonation efficiency decreases. Also,
When the immersion ratio exceeds 0.6, the retention of chromium oxide in the immersion tube is promoted, but due to the deterioration of the circulation of the molten steel in the ladle, the carbon in the steel to be used in the reduction reaction tank is reduced. Since the supply into the vacuum chamber is hindered, as a result, the promotion of the reduction reaction is hindered and the decarboxylation efficiency is reduced. The degree of vacuum of the vacuum chamber in the high carbon concentration region is 200 to
If the value is smaller than rr, splash is undesirably increased. It is to be noted that maintaining the degree of vacuum at 200 torr or more has the same meaning as maintaining the degree of vacuum within a range from 200 torr or more to less than 760 torr, which is the atmospheric pressure.

The immersion pipe is capable of sucking up the stainless steel by reducing the pressure in a vacuum chamber communicated with the upper end of the immersion pipe in a state where one end is immersed in the molten steel stored in the ladle. The shape, size, and the like are not particularly limited, and are appropriately selected. For example, when the cross-sectional shape of the immersion tube is circular, it is desirable that the inner diameter of the immersion tube be substantially the same as the inner diameter of the vacuum chamber. This is because the molten steel drawn into the immersion tube can be efficiently subjected to finish decarburization and refining (the immersion tube having substantially the same inner diameter as the inner diameter of the vacuum vessel is also referred to as a straight-body immersion tube).

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the accompanying drawings to provide an understanding of the present invention. First, a vacuum decarburization refining facility 10 used in a vacuum decarburization refining method for stainless steel according to an embodiment of the present invention will be described. As shown in FIG. 1, the vacuum decarburizing and refining equipment 10 includes a ladle 13 for holding molten steel 11, a dip tube 14 whose lower end is immersed in the molten steel 11, and an upper end of the dip tube 14 extended. A vacuum chamber 15 connected to the vacuum chamber 15, an evacuation device 16 for evacuating the vacuum chamber 15, an elevating drive device 17 for moving the vacuum chamber 15 up and down, and a vacuum chamber 15
An upper blowing lance 18 provided at the upper end of the ladle for blowing oxygen gas, and a porous plug 19 which is an example of a bottom blowing nozzle for blowing an inert gas disposed at the bottom of the ladle 13 are provided. An oxygen gas flow control valve 20 for controlling the flow rate of the oxygen gas blown through the upper blowing lance 18 is disposed on the inlet side of the upper blowing lance 18,
An inert gas flow control valve 21 for controlling the flow rate of the inert gas is provided on the inlet side of the bottom blowing nozzle, and the respective flow rates can be adjusted via the control device 23 and the like. Further, a vacuum gauge 22 for measuring the degree of vacuum in the vacuum chamber 15 is attached to the vacuum chamber 15 or a predetermined portion of the exhaust system. The signal corresponding to the degree of vacuum measured by the vacuum gauge 15, the signal of the relative position between the immersion pipe 14 and the ladle 13, the signal of the carbon concentration in the molten steel 11, and the like are transmitted to the control device 23.
The control device 23 can control the exhaust device 16 and the lifting / lowering drive device 17 to perform necessary operations according to these input signals and the operation procedure described later. When obtaining the carbon concentration in the molten steel 11, the carbon concentration in the molten steel 11 may be measured directly, or based on the carbon concentration before refining and the change history of the CO gas concentration in the exhaust gas. You can also calculate. Further, it is also possible to previously determine a time change of the carbon concentration for each processing step, and to estimate the carbon concentration at a specific time according to the change. The ladle 13 is a substantially cylindrical molten steel container lined with a refractory such as alumina silica. The immersion tube 14 and the vacuum tank 15 are devices (a straight-body immersion tube 14) made of a cylindrical refractory having the same diameter.

Next, a method for vacuum decarburization and refining of stainless steel according to one embodiment of the present invention will be described. First,
A molten steel 11 for stainless steel having a chromium concentration of 8 wt% and a carbon concentration of 1.2 wt%, which has been roughly decarburized and refined in a converter, is poured into a ladle 13 and positioned below a dip tube 14. Next, the lifting drive device 17 is operated to immerse the lower end of the immersion pipe 14 into the molten steel 11 in the ladle 13,
Evacuates the vacuum chamber 15 to maintain the degree of vacuum (P) in the vacuum chamber 15 at a predetermined level. Thereby, the immersion tube 1
The molten steel 11 in 4 is pushed up by the atmospheric pressure, the molten steel surface rises, and the immersion depth (h) of the immersion pipe 14 and the molten steel depth (H) in the ladle 13 as shown in FIG. Change.
While maintaining this state, oxygen gas is supplied from the upper blowing lance 18 at a predetermined oxygen gas flow rate (Q), and argon gas is blown from the porous plug 19 at a predetermined inert gas flow rate (N) to melt the molten steel. 11 is stirred to perform decarburization refining of the molten steel 11.

FIG. 2 is a schematic diagram showing a time change of the carbon concentration in the molten steel 11 during such decarburization refining. In this case, the carbon concentration (C) defining the high carbon concentration region is set to 0.3.
To 1.0 wt%, the low carbon concentration region is set to 0.0%.
The case where it is set in the range of 1 to 0.3 wt% is shown. In FIGS. 3 to 8 described later, time (time) scales on the horizontal axis are arranged so as to be aligned, and regions corresponding to such a high carbon concentration region and a low carbon concentration region are illustrated. 3 to 8 respectively show the results during vacuum decarburization refining.
Immersion ratio (h / H), oxygen gas flow rate (Q), reduction rate of oxygen gas flow rate (R), degree of vacuum in vacuum chamber 15 (P), inert gas flow rate (N), and immersion depth of immersion tube 14 It is a schematic diagram of time change of (h).

First, the vacuum decarburization refining in the high carbon concentration region will be described with reference to FIGS. In the high carbon concentration region, while monitoring or estimating the change in the carbon concentration in the molten steel 11, the operation of the control device 23 or the operation of the operator causes the oxygen gas flow control valve 20, the inert gas flow control valve 21, the elevation drive The oxygen gas flow rate (Q) is controlled to 3 to 25 N by controlling the device 17 and the exhaust device 16.
The inert gas flow rate (N) is set to 0.3 to 4.m ³ / h / t.
0N l / min / t, immersion ratio (h / H) 0.1 ~
The decarburization refining is performed by maintaining the degree of vacuum (P) at 0.6 and a range of 200 torr or more as shown in FIGS. 4, 7, 3, and 6, respectively. In the subsequent low carbon concentration region, the oxygen gas flow rate (Q) is adjusted by adjusting the oxygen gas flow control valve 20 as shown in FIGS.
Is reduced at a decreasing rate (R) of 0.5 to 12.5 Nm ³ / h / t per minute, and the immersion depth (h) of the molten steel 11 is set as shown in FIG. The decarburization refining is continued by increasing or decreasing within a predetermined range. The decreasing rate of the oxygen gas flow rate (Q) is the magnitude of the gradient in the time change of the oxygen gas flow rate (Q), that is, the time differential amount of the oxygen gas flow rate (Q), and the unit is (Nm ³ / h / t) / min.

As described above, in the present embodiment, in the decarburization refining operation of the molten steel 11 containing chromium, the flow rate of oxygen gas (Q), the flow rate of inert gas (N), the degree of vacuum (P), and the immersion ratio (h) / H) and the immersion depth (h) of the molten steel 11 are controlled so as to satisfy predetermined conditions, thereby simultaneously satisfying the following objects (1) to (3). Maintain high levels of decarbonation efficiency and reduce splash. The object can be achieved by maintaining the oxygen gas flow rate, the inert gas flow rate, and the degree of vacuum in appropriate ranges. Prevent chrome loss. The chromium component in the molten steel 11 oxidized on the molten steel surface in the immersion pipe 14
The slag 12 is fixed between the wall of the immersion pipe 14 and the inner wall of the ladle 13 via the lower end of the slag 12. For this reason, the convection state of the molten steel 11 in the immersion pipe 14 of the chromium component (chromium oxide) is properly maintained by maintaining the immersion ratio, the flow rate of the inert gas, the flow rate of the oxygen gas and the like in a predetermined range. The chromium oxide is efficiently reduced by the carbon in the steel in the immersion tube 14 and the chromium component slag 1
2 is suppressed. The sticking phenomenon by the slag 12 between the outer wall of the immersion tube 14 and the inner wall of the ladle 13 can be avoided. Since the relative position between the immersion tube 14 and the ladle 13 is changed within a predetermined range of the low carbon concentration region, such a sticking phenomenon by the slag 12 can be prevented.

[0017]

Next, a more detailed experimental example will be described with reference to Tables 1 and 2. Examples 1 to 9 shown in Table 1 and Table 2 show the results of decarburization and refining, respectively, using the vacuum decarburization and refining equipment shown in FIG. 9 to 11 show the relationship between the immersion ratio (h / H), the inert gas flow rate (N), and the oxygen gas flow rate decreasing rate (R) with respect to the decarbonation efficiency. It is a graph. As shown in FIGS. 9 and 10, the immersion ratio (h / H) is 0.1 to
0.6, the inert gas flow rate (N) is set to 0.3 to 4.0.
Decarbonation efficiency by maintaining each in the range of 6
It can be 5% or more. Further, as is apparent from FIG. 11, the rate of decrease (R) of the oxygen gas flow rate is 0.6 to 1
It can be seen that by setting the range to 2.5 Nm ³ / h / t / min, the decarbonation efficiency can be maintained at 65% or more without deteriorating the productivity. Note that the hatched portion in FIG. 11 indicates a region where the processing time and the like in the entire refining process become longer, which leads to a decrease in productivity. For example, Embodiment 1
Indicates that the oxygen gas flow rate should be 3 to
At 25 Nm ³ / h / t, as shown in Table 1, the immersion ratio (h / H), the flow rate of the inert gas (N), and the degree of vacuum (P) were 0.3 and 1.7 N l / h, respectively. Min / t,
225 torr, and in the subsequent low carbon concentration region,
An example is shown in which the oxygen gas flow rate (Q) is reduced at a rate of 6.7 Nm ³ / h / t per minute to increase or decrease the immersion depth (h) of the immersion tube 14.

[0018]

[Table 1]

[0019]

[Table 2]

As shown in the column of results in Table 1, the occurrence state of the splash in Example 1 was small and good (○), and the decarbonation efficiency was high in the high carbon concentration region and low carbon concentration region. 74% and 72% respectively
%, Which is a predetermined level (65
%). In addition, vacuum tank 15 and ladle 1
There was no sticking between the three samples, and the chromium loss was less than the predetermined level, and a good result (○) was obtained. Therefore, in Example 1, all of the above conditions were satisfied, and the overall evaluation was determined to be good (良好). Thus, Embodiment 1
In Examples 9 to 9, it is understood that a good overall evaluation (維持) can be obtained by properly adjusting and maintaining various conditions of the decarburization refining.

On the other hand, Tables 3 and 4 show Comparative Examples 1 to 8 under conditions deviating from the scope of the present invention, and all of them have a poor evaluation (×).

[0022]

[Table 3]

[0023]

[Table 4]

Here, Comparative Example 1 is an example in which the immersion ratio (h / H) is set to 0.06 which is a value out of the range (0.1 to 0.6) of the present invention. Has a high decarbonation rate in the high carbon concentration region of 43%, which is lower than the reference value of 65%, which is a quality standard. Comparative Example 2
Indicates that the oxygen gas flow rate (Q) falls within the range of the present invention in the range of 3 to 25.
This is an example in which the value is set higher than Nm ³ / h / t, and the high rate of decarbonation in the high carbon concentration region is as low as 45%. Comparative Example 3 is an example in which the inert gas flow rate (N) is set to 0.15 N liter / min / t, which is outside the range of the present invention (0.3 to 4.0 N liter / min / t). In this case, the high rate of decarbonation in the high carbon concentration region is 38%.
And even lower rates. In Comparative Example 4, the oxygen gas flow rate in the high carbon concentration region was 3 to 25 within the range of the present invention.
This is an example in which the value is set lower than Nm ³ / h / t, and the high rate of decarbonation in the high carbon concentration region is 42%, which is determined to be defective. In Comparative Example 5, the rate of decrease (R) of the oxygen gas flow rate in the low carbon concentration region was 0.2 Nm ³ /, which is a value outside the range of the present invention (0.5 to 12.5 Nm ³ / h / t / min). An example in which h / t / min is set is shown.
In this case, the high rate of decarbonation in the low carbon concentration region is 3
The rate is as low as 1%. In Comparative Example 6, the rate of decrease (R) of the oxygen gas flow rate in the low carbon concentration region was within the range of the present invention (0.
5 which exceeds 5 to 12.5 Nm ³ / h / t / min).
This is an example in which 6.2 Nm ³ / h / t / min is set, and chromium loss or the like becomes a nonnegligible amount, and productivity is significantly impaired. Comparative Example 7 shows the degree of vacuum (P) in the high carbon concentration region.
Is set to 165 torr, which is a value outside the range (200 torr to 760 torr) of the present invention, and shows that the splash in the immersion tube 14 becomes severe. Lastly, Comparative Example 8 shows an example in which decarburization refining was performed by fixing the immersion depth (h) of the immersion pipe 14 in the low carbon concentration region. 12 shows an example in which 12 adheres to each other and sticks to each other, resulting in production failure.

The embodiment of the present invention has been described above.
The present invention is not limited to these embodiments, and all changes in conditions without departing from the gist are within the scope of the present invention. For example, in the present embodiment, the case where the operation of increasing or decreasing the immersion depth (h) of the immersion tube in the low carbon concentration region is monotonously reduced has been described. Since the sticking by the slag is prevented, not only such a monotonous decrease but also a method of vibrating the immersion depth (h) irregularly or regularly or increasing the immersion depth (h) can be adopted. .

[0026]

According to the vacuum decarburization refining method for stainless steel according to the first to sixth aspects, the oxygen gas flow rate and the inert gas flow rate in the high carbon concentration region are respectively maintained within predetermined ranges. Since the oxygen gas flow rate is reduced at a decreasing rate within a specific range, and the immersion depth of the immersion tube is increased or decreased within a predetermined range, the decarbonation efficiency can be maintained at a high level, and the Splash generation and sticking of the immersion tube can be prevented, and efficient decarburization refining with less chromium loss can be performed. In particular, claim 2
In the vacuum decarburization refining method for stainless steel described, the carbon concentration of the molten steel in the high carbon concentration region is specified in a specific range, so that it is possible to perform an appropriate operation according to the rate-determining stage of the decarburization reaction. In addition, more efficient decarburization refining can be performed. Further, in the method for vacuum decarburization and refining of stainless steel according to the third aspect, since the chromium content of the molten steel is specified, it is easy to appropriately control the chromium oxidation reaction by oxygen gas blown into the molten steel. It is.
In the vacuum decarburization refining method for stainless steel according to the fourth aspect, the immersion ratio between the immersion depth of the immersion pipe in the high carbon concentration region and the depth of the molten steel in the ladle is specified in a specific range. The amount of chromium oxide which moves outside the immersion tube due to the convection of molten steel generated in the immersion tube can be suppressed, and the chromium loss and the sticking phenomenon can be further suppressed. In the method for refining stainless steel according to claim 5, since the degree of vacuum in the vacuum chamber in the high carbon concentration region is maintained at 200 torr or more, the decarbonation efficiency can be maintained at a predetermined level, and the efficiency can be further improved. Decarburization refining is possible.
In the method for vacuum decarburization and refining of stainless steel according to the sixth aspect, since the immersion pipe is a straight-body immersion pipe, the molten steel drawn into the immersion pipe can be more efficiently decarburized and refined.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a vacuum decarburization refining facility to which a vacuum decarburization refining method for stainless steel according to an embodiment of the present invention is applied.

FIG. 2 is a schematic diagram showing a time change of a carbon concentration in molten steel during decarburization refining.

FIG. 3 is a schematic diagram showing a temporal change of an immersion ratio (h / H) during decarburization refining.

FIG. 4 is a schematic diagram showing a time change of an oxygen gas flow rate during decarburization refining.

FIG. 5 is a schematic diagram showing a time change of a decreasing speed of an oxygen gas flow rate during decarburization refining.

FIG. 6 is a schematic diagram showing a temporal change in the degree of vacuum in a vacuum chamber during decarburization refining.

FIG. 7 is a schematic diagram showing a temporal change of an inert gas flow rate during decarburization refining.

FIG. 8 is a schematic diagram showing a temporal change in immersion depth (h) of an immersion tube during decarburization refining.

FIG. 9 is a graph showing a relationship between decarbonation efficiency and immersion ratio (h / H).

FIG. 10 is a graph showing a relationship between decarbonation efficiency and an inert gas flow rate.

FIG. 11 is a graph showing the relationship between the decarbonation efficiency and the rate of decrease in the flow rate of oxygen gas.

FIG. 12 is an explanatory view of a vacuum decarburization refining method in a conventional example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 10 Vacuum decarburization refining equipment 11 Molten steel 12 Slag 13 Ladle 14 Immersion pipe 15 Vacuum tank 16 Exhaust device 17 Elevation drive device 18 Top blowing lance 19 Porous plug (bottom blowing nozzle) 20 Oxygen gas flow control valve 21 Inert gas flow control Valve 22 Vacuum gauge 23 Controller

Claims (6)

[Claims] 1. An immersion tube is immersed in molten steel containing chromium held in a ladle, and the inside of a vacuum tank communicating with the immersion tube is depressurized, and oxygen gas is supplied through an upper blowing lance on the upper portion of the vacuum tank. In the vacuum decarburization refining method for stainless steel, in which the molten steel is stirred by an inert gas supplied from a bottom blowing nozzle at the bottom of the ladle while blowing, the oxygen gas flow rate of the oxygen gas in the high carbon concentration region of the molten steel is increased. 3 to 25N
m ³ / h / t-steel, the flow rate of the inert gas is 0.3 to 4.0 Nl / min / t-steel.
In the subsequent low carbon concentration region of the molten steel, the oxygen gas flow rate is reduced at a rate of 0.5 to 12.5 Nm ³ / h / t-steel per minute, and the immersion pipe is immersed. A method for vacuum decarburization and refining of stainless steel, wherein the depth is increased or decreased within a predetermined range. 2. The carbon concentration of molten steel in the high carbon concentration region is 0.3-1 wt%, and the carbon concentration of molten steel in the low carbon concentration region is 0.01-0.3 wt%. The method for vacuum decarburization and refining of stainless steel according to claim 1. 3. The chromium content of the molten steel is 5 to 30 wt.
%. The method for vacuum decarburization and refining of stainless steel according to claim 1, wherein 4. An immersion ratio between the immersion depth of the immersion tube and the depth of molten steel in the ladle in the high carbon concentration region is set to 0.1.
The method for vacuum decarburization and refining of stainless steel according to any one of claims 1 to 3, wherein the method is set to 0.6. 5. The method for vacuum decarburization and refining of stainless steel according to claim 1, wherein the degree of vacuum of the vacuum chamber in the high carbon concentration region is maintained at 200 torr or more. 6. The method for vacuum decarburization and refining of stainless steel according to claim 1, wherein the immersion pipe is a straight-body immersion pipe.