JPH06330143A - Treatment of decarburization of chromium-containing molten steel in reduced pressure - Google Patents

Abstract

PURPOSE:To provide a decarburize-treating method for chromium-containing molten steel in the reduced pressure, by which the oxidation of [Cr] in the molten steel is restrained and the consumption of inert gas for dilution is restrained and the decarburization is efficiency executed at a high speed and the refining cost is reduced. CONSTITUTION:In the refining method for chromium-containing molten steel executing the decarburizing treatment in vacuum condition after decarburize- treatment in the same refining vessel under the atmospheric pressure, the decarburizing treatment in the vacuum is started at <=0.7mass% [C] concn. and executed in <=300Torr degree of vacuum. Further, in the range of >=0.05mass% [C] concn., gaseous carbon dioxide at >=0.1Nm<3>/min/ton of the molten steel is mixed with gaseous oxygen or the mixed gas of the gaseous oxygen and the inert gas or the inert gas and blown. By this method, the unit consumption of the gaseous oxygen and the inert gas are remarkably reduced, and by improving the decarburization oxygen efficiency, as the unit consumption of Si for reduction is reduced, the refining cost is remarkably reduced and the efficient decarburization can be executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to the efficient decarburization in decarburization and refining of molten steel containing chromium by suppressing the [Cr] oxidation in the molten steel and suppressing the amount of the inert gas for dilution. The present invention relates to a vacuum decarburization treatment method for molten chromium steel.

[0002]

2. Description of the Related Art 11 mass such as conventional stainless steel
As a decarburizing method for molten chromium-containing steel containing more than 50% chromium, the AOD method in which oxygen gas or a mixed gas of oxygen gas (hereinafter simply referred to as oxygen) and an inert gas is blown from below the bath surface is widely used. ing. 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 and N ₂ in the blown gas are reduced. A method of suppressing the oxidation of [Cr] by increasing the ratio of inert gas such as gas and decreasing the ratio of 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.

A vacuum refining method has come to be used as a method for promoting decarburization in such a low [C] concentration range. 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 continue stirring with a non-oxidizing gas, but stop blowing oxygen, and continuously lower the pressure on the steel bath to about 10 Torr to cause boiling. The method of decarburizing is described.

In this method, since the supply of oxygen is stopped at a relatively high [C] concentration, the loss of [Cr] due to oxidation is reduced, but a large amount of CO gas is generated due to the rapid application of vacuum refining. , Risk of explosion. If the vacuum suction is made gentle to avoid this, there is no danger, but the elapsed time becomes long, the molten steel temperature decreases, and the reaction becomes slow. Moreover, if the pressure is set to a high vacuum of 10 Torr or less,
The molten steel splashes violently, causing problems such as clogging of the hopper for feeding the alloy material.

As a method for solving these problems, JP-A-3-68713 and JP-A-4-25450 are available.
The method described in Japanese Patent No. 9 has been proposed. The refining method for molten chromium-containing steel described in these documents is [C] concentration 0.2-
A mixed gas of a non-oxidizing gas and oxygen is used as a blowing gas up to 0.05 mass%, and after the [C] concentration falls within this range, the pressure is reduced to 200 to 15 Torr and the blowing gas is not Only oxidizing gas is used. Since this method performs refining under atmospheric pressure to a relatively low [C] concentration, the oxidation loss of [Cr] becomes large. In addition, decarburization under vacuum uses only an inert gas [C
Although the oxidation of r] is suppressed, the oxygen source for decarburization becomes [O] in the molten steel or oxygen in the slag, and the decarburization rate is reduced due to the slow oxygen supply rate, resulting in efficient decarburization. It cannot be called a refining method.

Further, both of the above methods use a large amount of expensive inert gas for stirring and for lowering the CO partial pressure, resulting in an increase in gas cost. As a means to avoid this, carbon dioxide gas (hereinafter simply referred to as CO ₂ )
The use of is mentioned. As a method of using CO ₂ in the decarburization refining, for example, in JP-A-55-158208, and the CO ₂ from below the bath surface in the oxygen top-blown steelmaking blow, to promote agitation of the molten steel de A method of promoting charcoal is described. Further, JP-A-57-32316 describes a method in which CO ₂ is blown together with oxygen from the tuyere at the bottom of the steel bath in the converter to prevent the melt damage of the tuyere by utilizing the cooling effect of CO _2. ing. According to these methods, CO ₂ is completely decomposed into CO and O, and the effect of using CO ₂ is only the effect of stirring and cooling.
Not enough effect has been obtained.

Further, Japanese Patent Application Laid-Open No. 50-37611 describes a method of using CO ₂ as a substitute for an inert gas such as Ar gas because it does not decompose CO _{2 and} has a CO diluting effect. However, since the decomposition rate of CO ₂ into CO and O is not quantified in this method, an excessive amount of inert gas is used, and a sufficient effect cannot be obtained.

Further, in these methods, the use of CO ₂ is limited to atmospheric pressure, and no example of utilization in vacuum refining is shown.

[0009]

DISCLOSURE OF THE INVENTION The present invention provides a method for decarburizing and refining molten steel containing chromium that uses a large amount of an inert gas such as Ar gas, by applying vacuum refining and using CO ₂ [PROBLEM TO BE SOLVED] To provide a vacuum decarburization treatment method for molten chromium-containing steel that suppresses Cr] oxidation and suppresses the amount of an inert gas for dilution to efficiently perform decarburization and reduce refining costs. Is.

[0010]

SUMMARY OF THE INVENTION The present invention advantageously solves the above-mentioned problems, and the gist thereof is to perform decarburization under atmospheric pressure in the same refining vessel and then under vacuum. In the refining method of molten chromium-containing steel for performing charcoal treatment, decarburization treatment under vacuum is started at a [C] concentration of 0.7 mass% or less,
The degree of vacuum is 300 Torr or less, and the [C] concentration is 0.05 mass% or more.
Carbon dioxide gas at a rate of 0.1 Nm ³ / min or more per ton of molten steel is mixed with oxygen gas or a mixed gas of oxygen gas and an inert gas or an inert gas, and the mixture is blown. There is a charcoal 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 0.7 ma.
In the range of ss% or less, the refining method is as illustrated in FIG. A refining gas (5) such as oxygen, CO ₂ , and an inert gas is blown into the molten chromium-containing steel (4) through the bottom blowing tuyere (2) in the refining container (1). The refining vessel (1) also has a removable exhaust hood (3).
A reduced pressure of 00 Torr or less is possible.

The present invention in a vacuum decarburization of chromium-containing molten steel, CO ₂ degrades the ratio is constant in the CO and O is a constant flow rate over, CO ₂ oxidation supplies the action and oxygen as a diluent gas of CO Focusing on the action as a natural gas, we propose to utilize these actions efficiently. Further, in the vacuum refining, if the vacuum concentration is 0.7 mass% or less of a relatively high [C] concentration and the degree of vacuum is 300 Torr or less, CO ₂ is used as a blowing gas, oxygen gas or a mixed gas of oxygen gas and an inert gas or an inert gas. It was focused on that the oxidation of [Cr] in the molten steel can be suppressed and the decarburization rate can be maintained at a high level by mixing with and using.

FIG. 2 shows the relationship between the [C] concentration and the decarboxylation efficiency in atmospheric pressure refining 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, which is the result when only oxygen is used as the blowing gas. As shown in FIG. 2, the decarbonation efficiency drops sharply when the [C] concentration is 0.7 mass% or less. Therefore, it can be seen that if vacuum refining is applied at a [C] concentration of 0.7 mass% or less, it is possible to prevent a decrease in decarboxylation efficiency.

In FIG. 3, SUS304 stainless steel is O ₂ /
[C] concentration when treated with Ar gas ratio = 1/1 =
The relationship between the degree of vacuum and the efficiency of decarboxylation in the range of 0.5 to 0.7 mass% will be shown. At a vacuum degree of 300 Torr or less, the decarboxylation efficiency becomes stable at a high level. Therefore, the degree of vacuum applied in vacuum refining must be 300 Torr or less.
In addition, since a rapid increase in the degree of vacuum causes a large amount of molten steel splash, it is preferable to gradually decrease from 300 Torr along with the decrease in [C] concentration in vacuum refining.

FIG. 4 shows decarburization of SUS304 stainless steel.
CO₂CO when using only₂Flow rate of CO and CO₂
The relationship of the decomposition rate is shown. CO₂Decomposition rate of all CO blown
₂CO decomposed into CO and O₂Is the ratio of the amount of
The remaining value after subtracting this value from 100 is CO₂But that
Indicates the amount that is present as it is. The circles in the figure
Is the result under atmospheric pressure (760 Torr), △ is the degree of vacuum
The results are at 200 Torr. From Figure 4, CO₂Decomposition of
The effect of vacuum on the rate is small, and CO ₂Blowing
Flow rate is 0.1 Nm³/ Min ・ T or more CO₂Decomposition of
The rate is stable at about 30%. In other words, blowing above this flow rate
CO complicated₂30% of this acts as oxidizing gas, and the rest
70% acts as an inert gas. The present invention relates this relationship
It proposes an efficient refining method by using it.

FIG. 5 shows a vacuum degree of 10 when a mixed gas of CO ₂ and Ar gas is used for decarburizing SUS304 stainless steel.
[C] concentration from 0 to 200 Torr and [C] in molten steel
r] shows the oxidation index. The mixed gas of CO ₂ and Ar gas has a CO ₂ / Ar ratio of 1/0, 1/1, 1/4.
3 kinds of gas were used. Also, the [Cr] oxidation index is C
It is a value obtained by converting the average [Cr] oxidation amount at a [C] concentration of 0.1 mass% in the case of using a mixed gas of O ₂ / Ar ratio 1/1 to 1.0. As shown in FIG. 5, the oxidation amount of [Cr] sharply increases when the concentration of [C] is less than 0.05 mass% in any of the mixed gases. Therefore, it is necessary to limit the use of CO _{2 to} a [C] concentration of 0.05 mass% or more.

From the above, in the vacuum decarburization treatment method for molten chromium-containing steel, which uses a large amount of an inert gas such as Ar gas, the amount of the inert gas used for dilution is suppressed while suppressing [Cr] oxidation in the molten steel. In order to suppress decarburization efficiently and at high speed, vacuum refining is applied at a [C] concentration of 0.7 mass% or less, the vacuum degree is reduced to 300 Torr or less, and the [C] concentration is 0.05 mass% or more. Area is 0.1 Nm ³ / mi
It is necessary to mix and inject CO ₂ of n · T or more with oxygen gas or a mixed gas of oxygen gas and an inert gas or an inert gas.

In actual operation, about 30% of CO ₂ is CO
By considering the decomposition into O and O, CO ₂ ,
It is possible to set the mixing ratio of oxygen and the inert gas. Further, various refining operations can be performed by combining with the conditions of vacuum refining. For example, when the amount of [Cr] oxidation is large and the decarburization rate is slower than expected, the degree of vacuum is lowered, and the ratio of oxygen is decreased and the ratio of CO ₂ and inert gas is increased to increase the [Cr] oxidation. Can be suppressed.

[0019]

It has been shown that when CO ₂ is blown into a steel bath, it partially floats without being decomposed as shown by the formula. Generally, as shown by the formula below, It has been considered to be decomposed into CO and O. CO ₂ (g) = CO ₂ (g) ...... CO ₂ (g) = CO (g) + [O] ...... In the present invention, the influence of the vacuum degree on the decomposition of CO ₂ is small, and at a constant flow rate or more. When CO ₂ was blown in, it was found that the reaction in the formula proceeded about 70% and the reaction in the formula proceeded about 30%, and it was devised to apply this to the decarburization refining of molten steel containing chromium. Decarburization reaction is an equation, reaction equilibrium constant K is an equation,
Further, the oxidation reaction of [Cr] in molten steel, which proceeds at the same time as decarburization, is expressed by an equation.

[0020] [C] + [O] = CO (g) ...... K = P CO / a c · a o ...... 2 [Cr] +3 [O] = (Cr ₂ O ₃₎ ...... Here, a _c and a _o are the activities of [C] and [O] in the molten steel, P
_CO indicates the partial pressure of _CO in the atmosphere.

[O] generated by the formula reacts with [C] or [Cr] in the molten steel, or the reaction of the formula proceeds. In order to proceed decarburization efficiently, it is necessary to advance the formula preferentially. _Decreasing PCO is effective in order to advance the equation preferentially over the equation. It has been found that a vacuum atmosphere is effective as a means for reducing P _CO , and the effect is further enhanced by combining this with the use of CO ₂ . For CO ₂ to about 70 percent by blow remains remains CO _2, as it acts on the lowering of the P _CO.
Therefore, by using an appropriate amount of CO ₂ under reduced pressure of 300 Torr or less in a mixed gas of oxygen or a mixed gas of oxygen and an inert gas or an inert gas, CO 2
The action of _{2 as} an oxidizing gas and the action as an inert gas can be effectively utilized, and the expensive inert gas can be reduced.

The decarburization reaction of molten steel containing chromium is rate-controlled by oxygen supply in the high [C] concentration range, and is rate-controlled by [C] in the low [C] concentration range. In the oxygen supply rate controlling region, P is used to accelerate the decarburization reaction.
_No reduction in _CO is required, and no blowing of CO ₂ is required. In the present invention, it has been found that the [C] concentration at this boundary is 0.7 mass%. Further, about 30% of CO ₂ is decomposed, so that in the low [C] concentration range, oxygen is excessively supplied,
This causes the oxidation of [Cr] in the molten steel. In the present invention, it has been found that the [C] concentration at the CO ₂ supply limit is 0.05 mass%.

[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. 6 shows an embodiment according to the method of the present invention.
1 is shown. [C] concentration at the start of decarburization is 1.5 mass%
Then, decarburization was performed under atmospheric pressure until the [C] concentration reached 0.7 mass%, and then vacuum refining was applied. The ratio of CO ₂ / O ₂ / Ar gas during vacuum refining is from 3/7/0 to 3/2
/ 0, 2/0/1, and [C] concentration 0.05 mass%
If it is less than Ar, only Ar gas is used. Also, the degree of vacuum is reduced from 300 to 200, 100 and 50 Torr,
[C] Decarburized to 0.03 mass%. Then, while returning the degree of vacuum to atmospheric pressure, Fe-Si was added as a reducing material for reducing the chromium oxidized during decarburization, and A
Reduction treatment was performed by blowing only r gas, and steel was tapped into a ladle.

FIG. 7 shows Example-2 according to the method of the present invention. The same treatment as in Example 1 was performed up to a [C] concentration of 0.7 mass%, vacuum refining was applied at a [C] concentration of 0.7 mass% or less, and CO ₂ /
O ₂ / Ar ratios are 3/6/1, 4/5/1, 1/2/5,
0/0/1, vacuum degree is 300, 200, 100, 50T
orr. The reduction treatment was performed in the same manner as in Example-1.

FIG. 8 shows an example (comparative example-1) of a decarburizing method under atmospheric pressure which is generally performed as a conventional method. [C]
Decarburization was performed by decreasing the O ₂ / Ar ratio with the decrease in the concentration. FIG. 9 shows an example (Comparative Example-2) according to the method described in Japanese Patent Application Laid-Open No. 3-68713 as a conventional method. In this method, [C] concentration is 0.20
Refining at atmospheric pressure up to mass%, [C] concentration 0.2
Under 0 mass% or less and a vacuum degree of 100 Torr, A
Decarburization treatment was performed up to 0.03 mass% by blowing r gas, then reduction treatment was performed under atmospheric pressure, and the ladle was put out.

[0026] Incidentally, the total gas blowing flow rate at atmospheric pressure process in each embodiment and ^{1.0Nm 3 / min · T, 0.4Nm} 3 / min · the blowing flow rate of the CO ₂ in the vacuum process
The treatment was performed at a temperature of T or less, an oxygen flow rate of 0.4 Nm ³ / min · T or less, and an Ar gas flow rate of 0.3 Nm ³ / min · T. FIGS. 6, 7, 8 and 9 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 Comparative Example, and The amount of decrease in the [Cr] concentration was also small. The results of these refining are summarized in Table 1. The values in Table 1 are shown as indices with the result of Comparative Example-2 set to 100.

[0027]

[Table 1]

[0028]

EFFECTS OF THE INVENTION According to the present invention, in the decarburization treatment of molten steel containing chromium, the oxygen consumption rate and the inert gas consumption rate are significantly reduced, and the decarbonation efficiency is improved.
The i basic unit is reduced, and the refining cost is greatly reduced. If the supply rate of CO ₂ is increased, the decarburization rate is further improved, the refining time can be shortened, and the productivity can be improved.

Further, since the vacuum treatment is used, the use of nitrogen gas as an alternative to 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 starting [C] concentration of vacuum refining of the present invention.

FIG. 3 is a diagram showing the reason for limiting the degree of vacuum during vacuum refining according to the present invention.

FIG. 4 is a diagram showing the reason for limiting the flow rate of CO ₂ blown in the present invention.

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

FIG. 6 is a diagram showing a refining pattern of Example-1 of the present invention.

FIG. 7 is a diagram showing a refining pattern of Example-2 of the present invention.

FIG. 8 is a diagram showing a refining pattern of an example (Comparative Example-1) according to a conventional method.

FIG. 9 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 Gas

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

Claims (1)

[Claims] 1. In a refining method for molten chromium-containing steel, which comprises decarburizing treatment under atmospheric pressure in the same refining vessel and then decarburizing treatment under vacuum, the decarburizing treatment under vacuum has a [C] concentration of 0. .7mas
s% or less, the degree of vacuum is set to 300 Torr or less, and in a region of [C] concentration of 0.05 mass% or more, carbon dioxide gas of 0.1 Nm ³ / min or more per ton of molten steel is used as oxygen gas. Alternatively, a method of decarburizing a chromium-containing molten steel under reduced pressure, which comprises mixing oxygen gas and an inert gas or mixing and blowing the mixture with an inert gas.