JPH06287628A - Decarburization treatment of chromium-containing molten steel in reduced pressure - Google Patents

Abstract

PURPOSE:To provide a decarburization-treating method of decarburizing chromium-containing molten steel in reduced pressure, by which the oxidation of chromium in the molten steel is restrained and the decarburization is efficiently executed in high speed. CONSTITUTION:In the refining method of chromium-containing molten steel executing the decarburization in the vacuum after decarburizing in the atmospheric pressure in the same refining vessel, the decarburization in the vacuum is started in the condition of <=0.5mass% [C] concn. and <=30mass% (Cr2O3) concn. in slag and the pressure is reduced to <=200Torr vacuum degree. By using gaseous oxygen or mixed gas of the gaseous oxygen and inert gas as the blowing gas, until the [C[ concn. becomes 0.1mass%, the refining is executed, and after the [C[ concn. lowers to this value or further lower, the refining is executed with only the inert gas. By this method, the oxygen efficiency for decarburization is improved and the decarburizing speed is improved at the same oxygen supplying rate, and the unit consumption of Si for reduction after decarburizing can be reduced, and also as the refining time can be shortened, the productivity is improved.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention suppresses the oxidation of [Cr] in molten steel efficiently in the decarburization refining of molten steel containing chromium.
The present invention relates to a vacuum decarburization treatment method for chromium-containing molten steel that decarburizes at high speed and decarburizes to an extremely low carbon concentration.

[0002]

2. Description of the Related Art As a decarburizing method for molten steel containing chromium such as stainless steel containing 11 mass% or more of chromium, oxygen gas or a mixture of oxygen gas (hereinafter simply referred to as "oxygen") and an inert gas is used from below the bath surface. The AOD method of blowing gas is widely used. In the AOD method, when decarburization progresses and the [C] concentration in the molten steel decreases, [Cr] is easily oxidized. Therefore, as the [C] concentration decreases, Ar gas in the blown gas decreases. A method of suppressing the oxidation of [Cr] by increasing the ratio of the inert gas such as the above and decreasing the ratio of the oxygen is adopted. However, in the low [C] concentration range, it takes a long time to reach the desired [C] concentration because the decarburization rate decreases, and in order to increase the ratio of the inert gas in the blown gas. It also becomes economically disadvantageous because the consumption of inert gas increases significantly.

As a method for promoting decarburization in such a low [C] concentration range, use of a vacuum refining method can be mentioned.
For example, in Japanese Examined Patent Publication No. 60-10087, in order to decarburize high chromium stainless steel to a low [C] concentration of 0.03 mass% or less, decarburization with oxygen under atmospheric pressure [C] = 0.2 to 0.4 mass%, and then the stirring with non-oxidizing gas is continued, but the oxygen blowing is stopped, and the pressure on the steel bath is continuously reduced to about 10 Torr to cause boiling. A method for performing the desired decarburization is described.

In this method, since the supply of oxygen is stopped compared to a relatively high [C] concentration, the loss due to the oxidation of [Cr] is reduced, but a large amount of CO gas is generated by the rapid vacuum refining, which causes an explosion. Causes danger. For this reason, if the vacuum suction is loosened, there is no danger, but the elapsed time becomes longer, the molten steel temperature decreases, and the reaction becomes slower. Also, set the pressure to 1
If the vacuum is set to a high vacuum of 0 Torr or less, the splash of molten steel becomes violent, causing problems such as clogging of the hopper for feeding the alloy material.

Methods for solving these problems are described in JP-A-3-68713 and JP-A-4-254509. The refining methods for molten chromium-containing steel described in these documents are based on [C] concentration of 0.2 to 0.05 ma.
A mixed gas of a non-oxidizing gas and oxygen is used as a blowing gas up to ss%, and after the [C] concentration falls within this range, the pressure is reduced to 200 to 15 Torr and the blowing gas is a non-oxidizing gas. It uses only gas.

In this method, since refining is performed under atmospheric pressure to a relatively low [C] concentration, the oxidation loss of [Cr] is large, and decarburization under vacuum uses only an inert gas [ C
Oxidation of r] is suppressed, but the oxygen source for decarburization is [O] in the molten steel or oxygen in the slag, and especially when the slag is solidified, the oxygen supply rate slows down and the decarburization rate decreases. Therefore, it is not an efficient decarburization refining method.

[0007]

DISCLOSURE OF THE INVENTION In the decarburizing refining of molten chromium-containing steel using vacuum refining, the present invention is to start vacuum refining, [C] concentration, slag (Cr ₂ O ₃ ) concentration, vacuum refining By maintaining the degree of vacuum and the type of gas blown during vacuum refining within a suitable range, oxidation of [Cr] in molten steel is suppressed, decarburization is performed efficiently and at high speed, and at the same time, the reduction of the Si unit for reduction is reduced. It enables shortening of refining time and refining of ultra-low carbon concentration steel.

[0008]

The present invention has the above-mentioned problems.
This is a solution to the problem, and the main point is. under
It is as described. (1) Large while supplying blowing gas in the same refining vessel
Black decarburization that is performed under vacuum and then under vacuum
Start of decarburization treatment under vacuum in refining method of molten steel
Is a [C] concentration in molten steel of 0.5 mass% or less, and
(Cr in the rug ₂O₃) Articles with a concentration of 30 mass% or less
The degree of vacuum is 200 Torr or less, and the blowing gas is
As for [C] concentration up to 0.1 mass%, oxygen gas or
Or using a mixed gas of oxygen gas and an inert gas, [C]
After the concentration drops below this value, it becomes inactive as blown gas.
Of molten chromium-containing steel, which is characterized by supplying only a reactive gas
Vacuum decarburization treatment method.

(2) [C] concentration in molten steel is 0.5 ma
ss% or less, and (Cr ₂O₃) Concentration is 30
200 Torr in the refining container under the condition of less than mass%
The pressure is reduced to the following, and the flow rate of the blown gas is set to 0.1
Nm³/ Min · T or more, but do not supply blown gas
Decarburized from the chromium-containing molten steel as described in 1 above.
Decompression decarburization treatment method.

The present invention will be described in detail below. The decarburization refining of the chromium-containing molten steel of the present invention has a [C] concentration of 0.5 ma.
In the range of ss% or less, the refining method uses a smelting vessel as illustrated in FIG. The refining gas 5 is blown into the molten chromium-containing steel 4 in the refining vessel 1 through the bottom blowing tuyere 2.
Further, the refining vessel 1 has a detachable exhaust hood 3 and can reduce the pressure to 200 Torr or less.

The present invention is used for decarburizing and refining molten chromium-containing steel using vacuum refining, which has a relatively high [C] concentration of 0.5 mass.
If the degree of vacuum is s% or less and the degree of vacuum is 200 Torr or less, by using oxygen or a mixed gas of oxygen and an inert gas as a blowing gas, it is possible to suppress the oxidation of [Cr] in molten steel and maintain the decarburization rate at a high level. Possible and in slag (Cr ₂ O
₃ ) It is focused on that decarburization is further promoted when the concentration is 30 mass% or less.

FIG. 2 shows the relation between the [C] concentration and the decarboxylation efficiency in the refining under atmospheric pressure when SUS304 stainless steel is treated. The decarbonation efficiency represents the proportion of oxygen used for decarburization in the blown oxygen. Further, the [Si] concentration before blowing is 0.1 mass% or less, oxygen and Ar gas are used as the blowing gas, and the O ₂ / Ar ratio = 4 /
This is the result when blowing was performed in 1. From FIG. 2, it can be seen that the efficiency of decarboxylation decreases sharply when the [C] concentration is 0.5 mass% or less. Therefore, the [C] concentration is 0.5 mass%
If vacuum refining is applied below, it becomes possible to prevent a decrease in decarboxylation efficiency.

FIG. 3 shows a SUS304 stainless steel having a vacuum degree of 100 to 200 Torr, Ar gas as a blowing gas of 0.2 Nm ³ / min · T, and a [C] concentration of 0.1 to 0. The relationship between the (Cr ₂ O ₃ ) concentration in slag and the decarburization rate index at 4 mass% is shown. The decarburization rate index is (Cr ₂ O ₃ ) concentration of 30 mass%.
The average decarburization rate in 100 is a value converted. It can be seen from FIG. 3 that the decarburization rate stabilizes at a high level when the (Cr ₂ O ₃ ) concentration is 30 mass% or less.

In FIG. 4, SUS304 stainless steel is O ₂ /
When the Ar gas ratio = 1/1, the [C] concentration was 0.
The relationship between the degree of vacuum and the efficiency of decarboxylation at 2 to 0.5 mass% will be shown. In the slag at this time (Cr ₂ O
₃ ) The concentration was in the range of 15 to 25 mass%. Figure 4
It can be seen that the decarboxylation efficiency is stabilized at a high level at a vacuum degree of 200 Torr or less.

Therefore, the degree of vacuum applied in vacuum refining is 20.
It must be 0 Torr or less. It should be noted that a rapid increase in the degree of vacuum causes a large amount of molten steel splash, so in vacuum refining the [C] concentration is reduced to 200
It is preferable to gradually decrease from Torr. Relationship between [C] concentration and decarboxylation oxygen efficiency when the SUS304 stainless steel was treated under vacuum in 100~200Torr Figure 5, the O ₂ / Ar ratio of blowing gas 1 / 1,1
The results are shown with two levels of / 4. The total flow rate of blown gas was 0.3 Nm ³ / min · T, and the (Cr ₂ O ₃ ) concentration in the slag was in the range of 15 to 25 mass%. From FIG. 5, it can be seen that when oxygen is mixed as the blowing gas, the efficiency of decarboxylation decreases sharply at a [C] concentration of 0.1 mass% or less. Therefore, [C] concentration of 0.1
If it is less than mass%, it is possible to efficiently decarburize by using an inert gas as the blowing gas.

FIG. 6 shows a slag of SUS304 stainless steel.
Medium (Cr₂O₃) Concentration of 15 to 25 mass% and 1
Blow only Ar gas under a vacuum of 100 to 200 Torr
[C] concentration of 0.05 to 0.15mass when treated with
Relationship between blown gas flow rate and decarburization rate index in the range of s%
Indicates the person in charge. The decarburization rate index is a gas flow rate of 0.2 Nm. ³
Converted with the average value of the decarburization rate at / min · T as 100
It is the value. From Figure 6, gas flow rate 0.1 Nm³/ Min
・ It can be seen that the decarburization rate drops sharply below T.
Therefore, in order to perform decarburization at high speed
0.1 Nm³/ Min · T or more is required.

From the above, in order to suppress the oxidation of [Cr] in the molten steel and efficiently decarburize the chromium-containing molten steel at a high speed, [C] concentration of 0.5 mass% or less, in the slag (Cr ₂ O
₃ ) Apply vacuum refining at a concentration of 30 mass% or less, reduce the pressure to a vacuum degree of 200 Torr or less, and use oxygen or a mixed gas of oxygen and an inert gas as 0.1 Nm ³ /
It is understood that it is necessary to blow more than min · T. Also,
When the [C] concentration is 0.1 mass% or less, efficient decarburization is possible by using only an inert gas.

In operation, the composition of molten steel at the time of charging the crude molten steel
And the molten steel temperature, and predict when vacuum refining will start.
Measure. After that, before the end of atmospheric pressure refining, such as sampling
From [C] concentration, (Cr₂O₃) Concentration and molten steel temperature
Figure out. Inside slag (Cr ₂O₃) Concentration is 30mas
If it exceeds s%, CaO, CaF₂, Al₂O₃ etc
To secure 30 mass% or less by adding the flux of
To do so. Also, during vacuum refining, grasp the situation inside the furnace,
Gas injection conditions and vacuum conditions can be determined
Noh. Depending on the operating method, a large amount of molten steel splash
Amount generation can be prevented and stable high-speed blowing operation is possible
Is. After decarburizing and refining, do not return the atmosphere to atmospheric pressure.
, (Cr₂O₃) In order to recover the Cr in
After adding and reducing and refining, steel is tapped into the ladle.

[0019]

In the decarburization refining of molten steel containing chromium, the injected oxygen is used for the decarburization reaction represented by the following formula and simultaneously with the oxidation reaction of [Cr] in the molten steel represented by the formula. The reaction equilibrium constant K of the equation is expressed by the equation. Further, (Cr ₂ O ₃ ) generated by the formula acts on the decarburization reaction depending on the conditions, and the reaction of the formula proceeds.

[C] + [O] = CO (g) ... K = P _CO / (a _C · a _O ) ... 2 [Cr] +3 [O] = (Cr ₂ O ₃ ) ... (Cr ₂ O ₃ ) +3 [C] = 3CO (g) Here, a _C and a _O represent the activities of [C] and [Cr] in the molten steel, and P _CO represents the CO partial pressure in the atmosphere.

In the decarburization reaction, the rate-determining process changes depending on the [C] concentration. It is said that the oxygen supply rate is controlled in a high-carbon area where the [C] concentration is 0.7 mass% or more, and the [C] concentration is 0.3 in the low-carbon area where the [C] concentration is 0.3 mass% or less.
It is said to be mixed rate-determining in the range of 0.7 mass%. Therefore, the effect is small even if the vacuum refining is applied at a [C] concentration of 0.7 mass% or more. In the present invention, it has been found that applying the [C] concentration of 0.5 mass% or less is an effective condition. On the low [C] concentration side, the supply rate of oxygen becomes slow, and therefore decarburization by the formula (Cr ₂ O ₃ ) easily proceeds. The reaction of the formula is affected by the state of slag, and the decarburization reaction hardly progresses when the slag is solidified. Decarburization with slag reduces the (Cr ₂ O ₃ ) concentration to 3
It has been found that when the content is 0 mass% or less, it can be promoted by liquefying the slag or making it into a semi-molten state.

To promote decarburization at a lower [C] concentration side
Is a from the formula_OTo increase P _COTo lower
Is effective. Oxygen supply is delayed only with non-oxidizing gas
In order to_OIs considered to be smaller, and
It has been found that mixing oxygen is more effective. Also,
Oxygen is not supplied at a [C] concentration of 0.1 mass% or less.
Below this concentration, the reaction of
Supplying only active gas is effective. In addition, as the degree of vacuum
Is effective below 200 Torr as shown in FIG.
Yes, the degree of vacuum is reduced as the concentration of [C] is reduced.
It turned out that it is preferable.

[0023]

[Example] SUS304 stainless steel (8 mass% N
The i-18 mass% Cr) 60 ton treatment was carried out in the embodiment shown in FIG. FIG. 7 shows Example-1 according to the present invention.
Indicates. The [C] concentration at the start of decarburization was 1.5 mass%, and decarburization was performed under atmospheric pressure until the [C] concentration was 0.5 mass%, and then vacuum refining was applied. Vacuum refining O
₂ / Ar gas ratio is 1/1 to 1/4, 0/1, vacuum degree is 200 Torr to 100 Torr, 50 Torr
It was lowered to r and decarburized to [C] 0.04 mass%. Then, while reducing the degree of vacuum to atmospheric pressure, Fe-as a reducing material for reducing the chromium oxidized during decarburization.
Si was added, reduction treatment was performed by blowing only Ar gas, and the steel was tapped into a ladle.

FIG. 8 shows a second embodiment according to the present invention.
[C] The same treatment as in Example-1 was performed up to a concentration of 0.5 mass%. When the [C] concentration was 0.5 mass%, the (Cr ₂ O ₃ ) concentration in the slag was over 30 mass%, so a flux was added to reduce the (Cr ₂ O ₃ ) concentration to 28 mass%. After that, vacuum refining was applied at a [C] concentration of 0.5 mass% or less, and the O ₂ / Ar ratio was 1/1, 1/4, 0 /
1. The degree of vacuum was reduced to 200, 100 and 50 Torr. The reduction treatment was performed in the same manner as in Example-1.

FIG. 9 shows Comparative Example-1 in which the (Cr ₂ O ₃ ) concentration in the slag deviates from the conditions of the present invention. The [C] concentration was the same as Example-2 of the present invention up to 0.5 mass%, but the (Cr ₂ O ₃ ) concentration in the slag was 30 ma.
Even though it exceeded ss%, it proceeded to vacuum refining as it was. The refining pattern of vacuum refining and the pattern of reduction treatment were performed by the same method as in FIG.

FIG. 10 shows an example according to Japanese Patent Laid-Open No. 3-68713 (Comparative Example-2), which is shown as a conventional method.
Indicates. In this method, refining is performed at atmospheric pressure up to a [C] concentration of 0.15 mass%, and decarburization is performed up to 0.04 mass% by Ar gas injection under the condition of a [C] concentration of 0.15 mass% or less and a vacuum degree of 100 Torr. After the treatment, the reduction treatment was performed under the atmospheric pressure, and the steel was tapped in a ladle.

In each of the examples, the total gas injection flow rate was set to 1.0 Nm ³ / min · T in the atmospheric pressure treatment, and the oxygen injection flow rate was set to 0.3 Nm ³ / min · T or less in the vacuum treatment and Ar was set. The treatment was carried out at a gas injection flow rate of 0.3 Nm ³ / min · T or less. FIGS. 7, 8, 9 and 10 also show changes in the refining time and the [C] and [Cr] concentrations in each Example, but the total refining time of the present invention was shorter than that of the conventional method, and The amount of decrease in the [Cr] concentration was also small. The results of these refining are summarized in Table 1. In addition, the value of Table 1 is shown as an index when the result of Comparative Example-2 is 100.

[0028]

[Table 1]

[0029]

According to the present invention, in the decarburizing refining of molten steel containing chromium, the decarburizing efficiency is improved, and therefore the decarburizing rate can be improved with the same oxygen supply amount. Moreover, since the refining time can be shortened as well as the reduction Si basic unit is reduced, the refining cost can be greatly reduced and the productivity can be improved.

Further, since the vacuum treatment is used, the use of nitrogen gas as a substitute for Ar gas can be expanded and refining to an extremely low carbon region having a [C] concentration of 0.01 mass% or less can be facilitated.

[Brief description of drawings]

FIG. 1 is a diagram showing a refining vessel according to an embodiment of the present invention.

FIG. 2 is a diagram showing the reason for limiting the [C] concentration at the start of vacuum refining in the present invention.

[Fig. 3] In the slag at the start of vacuum refining in the present invention (C
r ₂ O ₃₎ is a diagram showing the reason for limiting concentration.

FIG. 4 is a diagram showing the reason for limiting the degree of vacuum during vacuum refining in the present invention.

FIG. 5 is a diagram showing the reason for limiting the oxygen blowing lower limit [C] concentration in vacuum refining in the present invention.

FIG. 6 is a diagram showing the reason for limiting the flow rate of blown gas during vacuum refining in the present invention.

FIG. 7 is a diagram showing a refining pattern of Example-1 of the present invention.

FIG. 8 is a diagram showing a refining pattern of Example-2 of the present invention.

FIG. 9 is a view showing a refining pattern of Comparative Example-1 which is out of the conditions of the present invention.

FIG. 10 is a diagram showing a refining pattern of an example (Comparative Example-2) according to a conventional method.

[Explanation of symbols]

 1 ... Refining container 2 ... Bottom blowing tuyere 3 ... Exhaust hood 4 ... Molten steel 5 ... Refining gas

Continuation of the front page (72) Inventor, Hiroshi Iwasaki 3434, Shimada, Hikari City, Yamaguchi Prefecture Nippon Steel Works, Ltd.

Claims (2)

[Claims] 1. A refining method for molten chromium-containing steel, which comprises decarburizing at atmospheric pressure and then performing decarburizing under vacuum while supplying a blowing gas in the same refining vessel. Is started when the [C] concentration in the molten steel is 0.5 mass% or less,
In addition, the (Cr ₂ O ₃ ) concentration in the slag is set to 30 mass% or less, the degree of vacuum is set to 200 Torr or less, and the blowing gas is oxygen gas or oxygen gas and an inert gas up to a [C] concentration of 0.1 mass%. Using a mixed gas of
A method for decarburizing decarburization of molten chromium-containing steel, which comprises supplying only an inert gas as a blowing gas after the [C] concentration has dropped below this value. 2. The [C] concentration in the molten steel is 0.5 mass%.
Below, and (Cr ₂ O ₃ ) concentration in the slag is 30mass
Under the condition of s% or less, the inside of the refining vessel is depressurized to 200 Torr or less, and the blowing gas flow rate is 0.1 Nm ³ per ton of molten steel.
The method for decarburizing a chromium-containing molten steel under reduced pressure according to claim 1, wherein decarburization is performed while supplying a blowing gas at a rate of not less than / min · T.