JP3441523B2 - Refining method of chromium-containing molten steel - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the refining of chromium-containing molten steel in which [Cr] (hereinafter simply referred to as [Cr] ) in the molten steel is used.
The present invention relates to a method for refining molten chromium-containing steel, which suppresses the oxidation of aluminum, decarburizes efficiently, and reduces the cost of refining gas by supplying inexpensive air.

[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,
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. In the AOD method, decarburization progresses and the [C] concentration in molten steel (hereinafter sometimes simply referred to as [C] concentration).
That) from becoming easily it is to come when it [Cr] is oxidation decreases, Ar blow gas with a decrease in the [C] Concentration
A method of suppressing the oxidation of [Cr] by increasing the ratio of an inert gas such as N ₂ gas and decreasing the ratio of oxygen is used. 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 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%,
After that, stirring with a non-oxidizing gas is continued, but oxygen blowing is stopped, and a method for performing desired decarburization by continuously lowering the pressure on the steel bath to about 10 Torr and causing boiling is described. There is.

According to this method, since the supply of oxygen is stopped at 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 application of vacuum refining. , Risk of explosion. If the vacuum suction is made gentle, there is no danger of explosion, but the elapsed time becomes longer, the molten steel temperature drops, 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.

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 method for refining molten steel containing chromium 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%. After the [C] concentration falls within this range, the pressure is reduced to 200 to 15 Torr and the blowing gas is used. Only non-oxidizing gas is used.

However, these methods require refining [Cr] in order to carry out refining under atmospheric pressure to a relatively low [C] concentration.
Oxidation loss is increased. In addition, decarburization under vacuum can suppress the oxidation of [Cr] by using only an inert gas, but the oxygen source for decarburization is only [O] in the molten steel or oxygen in the slag, and the oxygen Since the supply rate becomes slow, the decarburization rate is lowered, and it cannot be said that the decarburization refining method is efficient.

On the other hand, as a method of reducing gas cost by decarburizing treatment under reduced pressure, a method described in Japanese Patent Laid-Open No. 4-263005 has been proposed. The method described in the publication has a [C] concentration of 0.15 mass% or less in the range of 2
This is a method in which the pressure is reduced to 0 to 150 Torr and non-oxidizing N ₂ gas is supplied as a blowing gas. Although this method can reduce the gas cost by using N ₂ gas which is relatively cheaper than Ar gas, it has a sufficient effect because the use [C] concentration range of N ₂ gas is narrow. It cannot be said that it is being done.

[0008]

DISCLOSURE OF THE INVENTION The present invention, in the refining of chromium-containing molten steel using vacuum refining, selects the [C] concentration at which vacuum refining is started, the degree of vacuum during vacuum refining, and the type of gas blown during vacuum refining. By maintaining in a suitable range,
Oxidation of [Cr] in molten steel is suppressed, decarburization is performed efficiently,
Moreover, it is an object of the present invention to provide a refining method capable of reducing refining costs by using air having a low gas cost.

[0009]

The present invention advantageously solves the above-mentioned problems, and the gist thereof is as follows. (1) In a refining method in which a gas is blown into molten chromium-containing steel in a refining vessel to perform decarburization, blowing is performed under atmospheric pressure until the [C] concentration in the molten steel decreases to 0.2 mass% or less. As a gas, a mixed gas composed of two or more kinds of gases selected from oxygen gas, an inert gas and air or an oxygen gas is supplied for decarburization treatment, and the [C] concentration in the molten steel is the above-mentioned concentration. After decreasing to below, the inside of the refining vessel was depressurized to below 200 Torr , and the [C] concentration in the molten steel was 0.05 mass%.
Gas mixture also consists of two or more kinds of gas selected from oxygen gas and an inert gas and air as the blowing gas in the above
Supplies air, and [C] concentration in molten steel is 0.05mass
A refining method for molten chromium-containing steel, characterized in that if it is less than s%, only an inert gas is supplied as a blowing gas to perform decarburization treatment.

(2) The [C] concentration in the molten steel is 0.2 mass
After decreasing to s% or less, the inside of the refining container is 200To
The pressure is reduced to rr or less, a mixed gas composed of one or more kinds of gas selected from air and oxygen gas and an inert gas is supplied as a blown gas, and the [C] concentration is decreased, and the mixed gas is mixed with the mixed gas. The method for refining molten chromium-containing steel according to the above item (1), wherein the ratio of the inert gas is gradually increased.

[0011] (3) to the target (C) concentration in the molten steel continue after decarburization performs reduction processing for recovering valuable metals such as chromium oxidized in the decarburization in the following reduced pressure 200Torr The refining method for molten chromium-containing steel according to the above item (1), characterized in that the supply amount of N ₂ gas as an inert gas is adjusted according to the target [N] concentration of the product . The present invention will be described in detail below.

The method for decarburizing and refining molten steel containing chromium of the present invention is
When the concentration of [C] is 0.2 mass% or less, the refining method is 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, in the decarburization refining of molten chromium-containing steel using vacuum refining, has a relatively high [C] concentration of 0.2 ma.
Even if ss% or less and the degree of vacuum is 200 Torr or less, it is possible to keep the decarburization rate at a high level while suppressing the oxidation of [Cr] in the molten steel even if oxygen or oxygen and an inert gas are used as the blowing gas. In addition, it was noted that the effect can be further enhanced by using air that acts as an oxidizing gas and an inert gas.

[0014] Air is generally an N ₂ 79%, containing O ₂ 21%. By utilizing this composition, it was confirmed that N ₂ acts as an inert gas and O ₂ acts as an oxidizing gas when blown into the molten steel in a pressurized state. Further, N ₂ gas leads to a large amount of increase in the [N] concentration soluble in steel if rice blast under atmospheric pressure, under reduced pressure, and by controlling the use N ₂ gas amount in the reduction treatment after decarburization It was confirmed that the [N] concentration of the product could be adjusted within the target range.

FIG. 2 shows the relationship between the concentration of [C] and the decarboxylation efficiency in refining under atmospheric pressure when SUS304 stainless steel is treated. The decarbonation efficiency represents the proportion of all oxygen blown in that was used for decarburization. In addition, before blowing soluble
The [Si] concentration in the steel is 0.1 mass% or less, oxygen and air are used as the blowing gas, and the O ₂ / air ratio = 1 /
This is the result when blowing was performed in 1. As can be seen from FIG. 2, the decarboxylation efficiency is sharply reduced when the [C] concentration is 0.2 mass% or less. Therefore, [C] concentration 0.2 mass
It can be seen that if the vacuum refining is applied at a rate of not more than%, it is possible to prevent a decrease in decarboxylation efficiency. If the O ₂ / air ratio of the gas to be blown changes, the decarboxylation efficiency decreases [C], and the concentration also changes, so vacuum refining starts [C].
The concentration is preferably about 0.2 mass% ± 0.05 mass%.

FIG. 3 shows the relationship between the degree of vacuum and the decarboxylation efficiency when SUS304 stainless steel was treated with a mixed gas of oxygen and air as a blowing gas at an O ₂ / air ratio = 1/2. [C] concentration = 0.15 to 0.20 ma
ss%, 0.10 to 0.15 mass% and 0.05
It is divided into three ranges of 0.10 mass% and shown.

[C] Concentration 0.15 to 0.20 mass%
In the range of 1, the decarboxylation efficiency is stabilized at a high level at a vacuum degree of 200 Torr or less. It should be noted that the decarboxylation efficiency tends to decrease as the [C] concentration decreases, and the stable vacuum degree tends to decrease. Therefore, the vacuum degree applied in vacuum refining is 200T.
Orr or less is required.

FIG . 4 shows SUS304 stainless steel with 10
Regarding the relationship between the [C] concentration and the decarburization rate index when treated under a vacuum of 0 to 200 Torr, the O ₂ /
Air / Ar gas ratio 1/0/0, 2/2/1, 1/1
The results are shown in 5 levels of / 2, 0/1/12 and 0/0/1. The total gas flow rate is 0.3 Nm ³ / m
The decarburization rate index as in · T is a value obtained by converting the decarburization rate at a [C] concentration of 0.2 mass% at an O ₂ / air / Ar gas ratio of 1/0/0 to 100. As shown in FIG. 4 , when the [C] concentration is 0.05 mass% or more, oxygen is more mixed when compared with the case where the O ₂ / air / Ar gas ratio is 0/0/1 in which only non-oxidizing gas is used as the blowing gas. It can be seen that the charcoal speed is improved. O, which is oxygen alone
_{It has been found that when the 2} / air / Ar gas ratio is 1/0/0, the decarburization rate is high but the [Cr] oxidation is also high, and therefore it is better to mix the dilution gas. Further, if the [C] concentration is less than 0.05 mass%, there is no difference in the decarburization rate even if the mixed gas ratio is changed, and in order to suppress the [Cr] oxidation, it is effective to blow an inert gas alone in this region. Target. Therefore, when the concentration of [C] is 0.05 mass% or more, it is better to use the air in which the diluent gas exists or the mixed gas composed of two or more kinds of gases selected from the air, oxygen and the inert gas as the blowing gas. . Also, O ₂ / air / Ar
When the gas ratio is high, that is, when the oxygen ratio is high, the decarburization rate is lower than that on the high [C] concentration side. To perform more efficient decarburization under vacuum, the [C] concentration decreases. Along with O
It can be said that it is better to lower the ₂ / air / Ar gas ratio and increase the Ar gas flow rate. Further, as compared with the case where only an inert gas is used at a [C] concentration of 0.05 mass% or more in vacuum refining, mixing of an oxidizing gas can prevent a decrease in molten steel temperature, so that more efficient refining can be performed.

FIG . 5 shows SUS304 stainless steel with 10
After decarburization under vacuum 0～200Torr, subsequently the product in the case of performing a reduction treatment under vacuum (N) concentration and the N
The relationship between the usage ratios of the _two gases is shown. Incidentally, N ₂ when the used ratio of the N ₂ gas (N) concentration 0.08 mass% of the product
It is a value obtained by converting the average value of the amount of gas used to 100. The amount of N ₂ gas used is a value including N ₂ in the air. From FIG. 5, (N) concentration of the required product, by controlling the supply amount of N ₂ gas, the products (N)
It turns out that it is possible to control the concentration.

From the above, in order to suppress the oxidation of [Cr] in molten steel, efficiently decarburize molten steel containing chromium, and to use inexpensive air for refining, vacuum refining at a [C] concentration of 0.2 mass% or less. Therefore, it is necessary to use a mixed gas or air composed of two or more kinds of gases selected from air, oxygen and an inert gas as the blowing gas. Further, when a mixed gas is used, it is efficient to gradually increase the ratio of the inert gas to air and oxygen for decarburization as the [C] concentration decreases.

In operation, the required [N] concentration of the product is determined, and from this, the range of use of air as the blowing gas is determined. Next, since the time change of [C] concentration can be predicted, the molten steel composition and molten steel temperature at the time of charging of the crude molten steel are grasped, and the time to start the vacuum refining is determined. Also, during vacuum refining, it is possible to determine the conditions of gas injection and degree of vacuum by grasping the conditions inside the furnace. By this operating method, it is possible to prevent a large amount of molten steel splash from occurring, and stable operation is possible.

[0022]

In the decarburization refining of molten chromium-containing steel, the oxidation reaction of [Cr] in the molten steel represented by the formula also proceeds simultaneously with the decarburization reaction represented by the formula below. The reaction equilibrium constant K in the equation is expressed by the equation. [C] _{+1/2 O 2 (g) = CO} (g) ......... K = P CO / a c · P O2 1/2 ......... 2 [Cr] _{+3/2 O 2 (g) = (} Cr ₂ O ₃ ) ... Here, _ac represents the activity of [C] in the molten steel, P _O2 represents the partial pressure of O ₂ gas in the atmosphere, and P _CO represents the partial pressure of CO gas in the atmosphere.

In the decarburization reaction, the rate-determining process changes depending on the [C] concentration. [C] It is said that the oxygen supply rate is controlled in a high coal area having a concentration of 0.7 mass% or more, and the transfer rate controlling of [C] is performed in a low carbon area having a [C] concentration of 0.3 mass% or less.
It is said to be mixed rate-determining in the range of 0.7 mass%. Therefore, even if vacuum refining is applied at a [C] concentration of 0.7 mass% or more, the effect is small. In the present invention, it has been found that application of a [C] concentration of 0.2 mass% or less is an effective condition in view of the cost of gas used.

In order to promote decarburization on the low [C] concentration side, it is effective to reduce P _O2 and P _CO from the above formula. However, since P _O2 = 0, that is, the supply of oxygen is delayed only with the non-oxidizing gas, it has been found that mixing oxygen with the non-oxidizing gas is more effective in the present invention. Moreover, in order further to lower the P _CO under vacuum, blowing of inert gas it is effective, it was found that use of N ₂ gas in the air is effective for this purpose. Further, as shown in FIG. 2, it was found that the effective degree of vacuum is 200 Torr or less, and it is preferable to reduce the degree of vacuum as the [C] concentration decreases.

When air is used as the blowing gas, N
Increase in [N] concentration by ₂ gas is concerned. This reaction is represented by the equation, and the reaction equilibrium constant K is represented by the equation. 1/2 N ₂ = [N] …………………… K = a _N / P _N2 ^1/2 ……………………, where a _N is the activity of molten steel [N], P _N2 Is N in the atmosphere
₂ Indicates the partial pressure of gas. Under reduced pressure, P _N2 decreases, so the denitrification rate increases and the nitrification rate decreases. Utilizing this, the relationship between the product [N] concentration and the amount of N ₂ gas used was clarified.

[0026]

[Example] SUS304 stainless steel (8 mass% N
i-18mass% Cr) in the product [N] concentration 500p
Processing of 60 tonnes of steel requiring pm was carried out in the embodiment shown in FIG. FIG. 6 shows Example 1 according to the method of the present invention. The [C] concentration at the start of decarburization was 1.5 mass%, and decarburization was performed under atmospheric pressure until the [C] concentration was 0.2 mass%, and then vacuum refining was applied. Vacuum refining O ₂
/ Air / N ₂ gas ratio is 1/2/0 to 0/1/4
Then, the degree of vacuum was reduced from 200 Torr to 100 Torr and 50 Torr, and the [C] concentration was 0.05 ma.
Decarburized to ss%. Then, only N ₂ gas was blown in to decarburize to a [C] concentration of 0.03%. After decarburization, Fe is used as a reducing material to reduce the chromium oxidized during decarburization.
-Si was added, reduction treatment was performed using Ar gas as a blowing gas, and after returning to atmospheric pressure, steel was tapped into a ladle.

FIG . 7 shows a second embodiment according to the method of the present invention.
[C] The same treatment as in Example 1 was performed up to a concentration of 0.2 mass%, vacuum refining was applied at a concentration of 0.2 C mass% or less, and [C] up to a concentration of 0.05 mass% [C].
O ₂ / air / N ₂ ratio is 1/3/0, 1 with decreasing concentration
/ 1/3, 0/1/6, vacuum degree is 200 Torr, 10
It was reduced to 0 Torr and 50 Torr. Then N ₂
Only the gas was blown in to decarburize to a [C] concentration of 0.03%. After decarburization, Fe-Si was added as a reducing agent to reduce the chromium oxidized during decarburization, and N ₂ gas was used as a blowing gas for an initial 1 min, and then Ar gas was used for a reduction treatment. After returning to atmospheric pressure, steel was tapped into the ladle.

FIG . 8 shows an embodiment according to Japanese Patent Laid-Open No. 3-68713, which is shown as a conventional method. In this method, refining is performed under atmospheric pressure up to a [C] concentration of 0.15 mass%, and a vacuum degree of 10 at a [C] concentration of 0.15 mass% or less.
0.03mass with Ar gas injection under the condition of 0 Torr
Decarburization treatment was performed up to s%, and then reduction treatment was performed under atmospheric pressure, and steel was tapped in a ladle.

In each of the examples, the total gas flow rate was set to 1.0 Nm ³ / min · T in the atmospheric pressure treatment, and the oxygen gas flow rate was set to 0.3 Nm ³ / min · T or less in the vacuum treatment. Blow flow rate is 0.3 Nm ³ / min · T or less,
N ₂ and Ar gas injection flow rate is 0.3 Nm ³ / min
・ Processing was performed with T or less. FIGS. 6 , 7 and 8 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 [Cr] ] The amount of decrease in density 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 the conventional method being 100.

[0030]

[Table 1]

[0031]

According to the method of the present invention, in the decarburization refining of molten steel containing chromium, the decarburization efficiency is improved, so that the decarburization rate can be improved with the same oxygen supply amount. In addition, the use of cheap air makes it possible to significantly reduce the gas cost and reduce the reduction Si unit consumption. Further, since the refining time can be shortened, the refining cost can be greatly reduced and the productivity can be improved.

Further, since the vacuum processing is used, the use of nitrogen gas as an alternative to Ar gas can be expanded, the control of the [N] concentration which affects the product quality can be facilitated, and, for example, the [C] concentration of 0 can be eliminated. Refining to extremely low carbon range of 0.01 mass% or less becomes easy.

[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 in the method of the present invention.

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

FIG. 4 is a diagram showing the reason for limiting the type of gas blown during vacuum refining in the method of the present invention.

FIG. 5 is a diagram showing the reason for limiting the amount of N ₂ gas used in the method of the present invention.

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

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

FIG. 8 is a diagram showing a refining pattern of an example according to a conventional method.

[Explanation of symbols]

1 refining container 2 Bottom blown tuyere 3 exhaust hood 4 Molten steel 5 gas

Front page continuation (72) Inventor Hiroaki Morishige 3434 Shimada, Hikari City, Yamaguchi Prefecture Nippon Steel Co., Ltd., Kogaku Works (56) References JP-A-6-330145 (JP, A) JP-A-7- 233408 (JP, A) JP 7-233409 (JP, A) JP 7-188727 (JP, A) JP 3-68713 (JP, A) JP 59-104421 (JP, A) JP-A-4-263005 (JP, A) JP-A-2-93016 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21C 7/068 C21C 7/10

Claims (3)

(57) [Claims] 1. A refining method in which a gas is blown into molten chromium-containing steel in a refining vessel to perform decarburization treatment , [C] in molten steel
A mixed gas or an acid consisting of two or more gases selected from oxygen gas, an inert gas, and air as a blowing gas under atmospheric pressure until the concentration decreases to 0.2 mass% or less.
After the raw gas is supplied to perform the decarburization treatment and the [C] concentration in the molten steel is reduced to the above concentration or less, the inside of the refining vessel is heated to 200 To
The pressure was reduced to rr or less, and the [C] concentration in the molten steel was 0.05 ma.
At ss% or more, a mixed gas or air composed of two or more kinds of gas selected from oxygen gas, inert gas and air is supplied as a blowing gas, and the [C] concentration in the molten steel is 0.05.
A refining method for molten chromium-containing steel, characterized in that when the content is less than mass%, only an inert gas is supplied as a blowing gas for decarburization treatment. 2. The [C] concentration in the molten steel is 0.2 mass%.
200 Torr in the smelting vessel after decreasing to below
The pressure is reduced to the following, and a mixed gas consisting of one or more kinds of gas selected from air and oxygen gas and an inert gas is supplied as a blown gas, and [C] in the mixed gas is decreased as the concentration decreases. The method for refining molten chromium-containing steel according to claim 1, wherein the ratio of the inert gas is gradually increased. After wherein decarburization treatment to the target (C) concentration in the molten steel subsequently subjected to reduction treatment for recovering valuable metals such as chromium oxidized in the decarburization in the following reduced pressure 200 Torr, The method for refining molten chromium-containing steel according to claim 1, wherein the supply amount of N ₂ gas as an inert gas is adjusted according to the target [N] concentration of the product .