JP3282544B2 - Demanganese method for high chromium molten iron alloy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing stainless steel having low manganese at a low cost, which relates to a method for removing manganese in refining stainless steel, particularly a stainless steel containing 5% by weight or more of chromium.

[0002]

2. Description of the Related Art Generally, stainless steel is used in an electric furnace, AOD.
When manufacturing by a process using a furnace, VOD furnace or VIM furnace, iron scrap and Fe-Cr are used as raw materials.
Use alloy. However, when the quality of these raw materials is low, the manganese contained in the raw materials increases the manganese value in the stainless steel.

[0003] The standard value of the manganese content of stainless steel products is determined variously depending on product characteristics such as weldability, but when the manganese content in the molten iron alloy is higher than the manganese standard value of the product, In addition, the manganese content of the product easily exceeds the upper limit of the standard value.

Therefore, conventionally, raw materials are selected according to the manganese standard value of each stainless steel. For this reason, when the manganese standard value of the product is, for example, 0.5 to 0.1% by weight or less, it is necessary to use high-quality and expensive raw materials, resulting in high cost.

However, the quality of raw materials has recently been declining, and the amount of manganese contained in the raw materials has increased. Regarding the manganese standard value of the product, some stainless steels having a lower manganese content than before may be required. For this reason, the tendency of high cost is more remarkable.

[0006] On the other hand, there has not yet been disclosed any method for effectively oxidizing and removing manganese which is an impurity while suppressing oxidation of chromium in a melting stage of an iron alloy containing chromium which is an element which is easily oxidized.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method for producing stainless steel, particularly stainless steel containing 5% by weight or more of chromium and having a low manganese content, at low cost. the manganese content Mn _i in the molten iron alloy pretreatment, with de manganese ratio defined by the following formula when the manganese content of the molten iron in the alloy after the treatment was Mn _f, 70% or more, preferably Is to provide a demanganese method capable of achieving 80 or more.

[0008] The de-manganese ratio _{= {(Mn i -Mn f)} / M
n _i ｝ × 100 (%)

[0009]

The gist of the present invention resides in the following method for removing manganese.

A method for removing manganese from a high chromium molten iron alloy containing 5% by weight or more of chromium, comprising a flux containing CaO as a main component and one or both of iron oxide and chromium oxide. The basicity of the slag formed on the molten iron alloy determined by the following equation was determined using the following formula:
5 or less, the Al ₂ O ₃ content in the slag is adjusted to 10 to 30% by weight, and the temperature of the molten iron alloy is raised to 1350 ° C. or more by blowing or blowing an oxidizing gas to the molten iron alloy.
A method for removing manganese from a high chromium molten iron alloy, wherein the method removes manganese while maintaining the temperature at 1550 ° C or lower.

Basicity =% CaO / (% SiO ₂ +% Al ₂ O ₃ ) where% CaO,% SiO ₂ and% Al ₂ O ₃ are
Content in slag (% by weight).

[0012] In the demanganese method of the present invention,
The flux used is not only CaO and iron oxide or / and chromium oxide as an oxidizing agent, but also SiO ₂ , Al for adjusting the basicity, melting point and fluidity of the slag formed on the molten iron alloy. ₂ O ₃ and Ca
It is desirable to use F ₂ , and a mixture of one or more of an Si alloy (for example, FeSi) and an Al alloy (for example, metal Al, FeAl, etc.) for adjusting the temperature of the molten iron alloy. .

The basicity of the slag formed on the molten iron alloy is determined by the SiO ₂ and Al _{2 in the} flux and the slag existing on the molten iron alloy before adding the flux.
O ₃ , SiO ₂ and Al ₂ O ₃ generated by the oxidation of Si and Al in the molten iron alloy, and SiO generated by the oxidation of the Si alloy and Al alloy added to the flux
_It is desirable to obtain from ₂ and Al ₂ O ₃ .

Further, the lower limit of the desirable slag basicity is about 0.3, and the upper limit of the chromium content in the molten iron alloy is 40.
%.

In the present invention, as a method of inexpensively producing stainless steel, particularly low manganese stainless steel containing 5% by weight or more of chromium, an inexpensive raw material is used irrespective of the manganese standard value of the product, and the iron alloy is melted. Attention was paid to a method of effectively oxidizing and removing only manganese without oxidizing loss of chromium at the stage.

Specifically, in the melting step of the iron alloy containing chromium, molten iron is formed by using an oxidizing flux containing CaO and one or both of iron oxide and chromium oxide as an oxidizing agent. Melting by adjusting the basicity of slag formed on the alloy and the Al ₂ O ₃ content in the slag, and adjusting the temperature of the molten iron alloy appropriately by blowing or blowing oxidizing gas to the molten iron alloy It is characterized by oxidizing and removing manganese in an iron alloy.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the reason why the subject of the method of the present invention is "a molten iron alloy containing 5% by weight or more of chromium" will be described.

In order to remove Mn in the molten iron alloy,
It is necessary to oxidize Mn to MnO by oxidative refining to make MnO a stable compound that is not easily reduced in slag. However, when the molten iron alloy containing chromium is oxidized and refined by an AOD furnace or a VOD furnace, the Cr in the molten iron alloy is oxidized simultaneously with Mn, and the Cr in the molten iron alloy is oxidized.
Also decreases. In particular, when the chromium content is 5% by weight or more, C
The oxidation of r is significant. This Cr loss causes an increase in manufacturing cost and must be avoided.

Therefore, in the method of the present invention, the object of the molten iron alloy is defined as "containing 5% by weight or more of chromium" which is required to reduce manganese. Although the upper limit of the chromium content is not limited, the method of the present invention can be applied to, for example, general stainless steel having a relatively high chromium content of about 30 to 40% by weight.

An important first feature of the method of the present invention is that CaO
Chromium is oxidized by a flux having a relatively low basicity and appropriate oxidizing power, containing one or both of iron oxide and chromium oxide as oxidizing agents. Without removing manganese. That is, manganese ([Mn]) in the molten iron alloy is oxidized by oxygen ((O)) in slag existing as iron oxide and chromium oxide according to the following formula (A).

[Mn] + (O) → (MnO) (A) Next, the manganese oxide, which is a basic oxide, is fixed by an acidic slag having a relatively low basicity, and the manganese oxide is removed from the slag. And then remove it out of the system as slag.

Specifically, a flux containing CaO and one or both of iron oxide or chromium oxide as an oxidizing agent is brought into contact with the molten iron alloy containing chromium.

At this time, it is effective to mix one or more of SiO ₂ , Al ₂ O ₃ and CaF ₂ in order to adjust the basicity, melting point and fluidity of the slag. In particular, Al ₂ O ₃ has a lower acidity than SiO ₂ , but is itself an acidic oxide, can reduce the basicity of the slag, improve the fluidity of the slag, and It is effective in lowering the melting point of the slag to physically accelerate the reaction. Therefore, it is preferable to mix an appropriate amount of Al ₂ O ₃ .

In order to further improve the fluidity of the slag, it is effective to incorporate CaF _2.
When F ₂ is blended, the refractory is easily melted.
It is preferable to use Al ₂ O ₃ as much as possible.

Here, the slag subjected to demanganese is mainly a flux for demanganese addition, and a slag brought in from a previous step before the flux is added to the molten iron alloy (hereinafter referred to as a preslag). , And oxides such as silicon, aluminum, manganese, and chromium in the molten iron alloy oxidized during the demanganese treatment.

When a flux containing CaO as a main component and either or both of iron oxide and chromium oxide as an oxidizing agent is brought into contact with a molten iron alloy containing chromium, mainly, Manganese is oxidized and chromium is hardly oxidized for the following reasons. That is, when the basicity of the slag is low (a state close to acidic), the manganese oxide is more stable in the slag than the chromium oxide.

[0027] In the method of the present invention, SiO ₂ and Al ₂ O ₃ in the flux to be added, SiO ₂ before the slag and Al ₂
O ₃ , and Si by oxidation of Si and Al in fused gold iron
O ₂ and Al ₂ O ₃ , and SiO ₂ and Al due to oxidation of Si and Al in a flux added and blended for the purpose described below
Considering ₂ O ₃ , these SiO ₂ and Al ₂ O ₃
Molten iron alloy in the basicity obtained by the following formula slag produced from a CaO in the flux of contacting 1.5 or less, it is necessary to make the content of Al ₂ O ₃ in the slag 10 to 30% by weight .

Basicity =% CaO / (% SiO ₂ +% Al ₂ O ₃ ) By the way, the above-mentioned demanganese treatment by the oxidation reaction is thermodynamically exothermic, and the oxidation reaction is low when the temperature is low. It becomes more noticeable. However, when the temperature of the molten iron alloy is low, the temperature of the slag also decreases, and the fluidity of the slag decreases. As a result, the above-mentioned oxidation reaction by the slag becomes slow, and the manganese removal efficiency decreases because the demanganese removal rate becomes slow.

An important second feature of the method of the present invention is that manganese in the molten iron alloy is more efficiently oxidized and removed by controlling the temperature of the molten iron alloy.

Specifically, an oxidizing gas is blown onto slag on the molten iron alloy, or is blown into the molten iron alloy to positively increase the temperature of the molten iron alloy and maintain the temperature at an appropriate temperature. By improving the fluidity of the slag and promoting the oxidation reaction by the slag, manganese in the molten iron alloy is oxidized and removed more efficiently.

However, there is a range of appropriate temperatures for this purpose, and the temperature range needs to be 1350 ° C. or more and 1550 ° C. or less.

At this time, in order to increase the degree of temperature rise of the molten iron alloy, an Al alloy and a S
It is preferable to include one or both of the i-alloys. The Al alloy and the Si alloy may be added to the slag on the molten iron alloy or the molten iron alloy immediately before or during or during the blowing or blowing of the oxidizing gas.

The above-mentioned Al alloy and Si alloy are used when the molten iron alloy contains sufficient amounts of Al and Si to contribute to the temperature rise of the molten iron alloy by blowing or blowing an oxidizing gas. Need not always be added, and the addition can be omitted.

Next, the results of a basic experiment which led to the above findings will be described.

18% by weight of chromium in a Tamman furnace
After melting 2 kg of the contained molten iron alloy into the atmosphere with a MgO crucible and adjusting its temperature to 1450 ° C and 1500 ° C,
While 0.2 kg of a CaO—SiO ₂ —Al ₂ O ₃ —Fe ₂ O ₃ flux was added, Al ₂ O ₃ in the slag was added.
After the content was adjusted to 10 to 30% by weight, the slag basicity was variously changed, and a demanganese treatment was performed for about 20 minutes while performing bubbling stirring with Ar gas.

The amount of Fe ₂ O ₃ in the flux was 0.02 kg. The initial components of manganese, carbon, and silicon in the molten iron alloy are 0.2% by weight, respectively.
1.0%, trace (tr.).

FIG. 1 shows the results of the experiment, and shows the effect of the slag basicity on the demanganese rate defined by the above equation.

As apparent from FIG. 1, the demanganese removal rate increases as the slag basicity decreases. Also,
The absolute value of the demanganese rate differs depending on the temperature of the molten iron alloy.

More specifically, for example, when the temperature of the molten iron alloy is 1500 ° C., if the slag basicity is set to 1.5 or less, a demanganese rate of 70% or more can be obtained. When the temperature of the molten iron alloy is 1450 ° C.,
It is understood that when the slag basicity is 1.5 or less, a demanganese rate of 80% or more can be obtained.

From the above results, it was found that the slag basicity determined by the following equation had to be 1.5 or more in order to obtain a demanganese rate of 70% or more, more preferably 80% or more. .

Basicity =% CaO / (% SiO ₂ +% Al ₂ O ₃ ) The lower limit of the slag basicity is not particularly limited, but when the slag basicity is smaller than 0.3, the fluidity of the slag is remarkable. It is not preferable because it lowers.

Next, for the same molten iron alloy as above,
While adjusting the temperature to 1450 ° C. and 1500 ° C., while keeping the slag basicity constant at 1.5, a similar experiment was conducted by changing the Al ₂ O ₃ content in the formed slag variously. .

FIG. 2 shows the results of the experiment, and shows the effect of the Al ₂ O ₃ content in the slag on the demanganese removal rate.

As apparent from FIG. 2, the demanganese removal rate increases when the content of Al ₂ O ₃ in the formed slag is within a certain range. In addition, the absolute value of the demanganese rate varies depending on the temperature of the molten iron alloy.

More specifically, when the content of Al ₂ O ₃ in the formed slag is in the range of 10 to 30% by weight, the demanganese removal rate becomes high. It is understood that when the temperature is set to 70 ° C., a demanganese rate of 70% or more can be obtained. The temperature of the molten iron alloy is set to 1
It can be seen that at 450 ° C., a demanganese rate of 80% or more can be obtained.

This is because the addition of an appropriate amount of Al ₂ O ₃ or an Al alloy or a Si alloy reduces the basicity of the slag and optimizes the temperature of the molten iron alloy, thereby improving the fluidity of the slag and improving the slag fluidity. Is lowered and the temperature of the molten iron alloy is proper, and the oxidation reaction is promoted.

From the above results, in order to obtain a demanganese ratio of 70% or more, more preferably 80% or more, the content of Al ₂ O ₃ in the formed slag should be 10 to 30% by weight.
It was found that it was necessary to adjust the amount within the range.

Further, for the same molten iron alloy as above, the slag basicity is kept constant at 1.5, and the Al ₂ O ₃ content in the formed slag is in the range of 10 to 30% by weight. A similar experiment was conducted by changing the temperature of the molten iron alloy in various amounts.

FIG. 3 shows the results of the experiment, and shows the effect of the temperature of the molten iron alloy on the demanganese rate.

As is apparent from FIG. 3, the demanganese removal rate greatly changes depending on the temperature of the molten iron alloy.

More specifically, Al ₂ O in slag
_{(3) When the} content is within the range of 10 to 30% by weight specified in the present invention, even if the slag basicity is 1.5 which is the upper limit specified in the present invention, the temperature of the molten iron alloy is 1350%. ° C
50% or more when the temperature is in the range of
70% when the temperature is in the range of 400 ° C to 1500 ° C
As described above, it can be seen that when the temperature is 1450 ° C., a demanganese rate of 80% or more can be obtained.

Although illustration of data is omitted, of the above conditions, when the slag basicity is smaller than 1.5,
It was confirmed that the demanganese rate at each of the above temperatures was improved.

From the above results, in order to obtain a demanganese rate of 70% or more, more preferably 80% or more, the temperature of the molten iron alloy during the demanganese treatment is set to 1350 to 15
It has been found that it is necessary to maintain the temperature at 50C, preferably 1400-1500C.

The reason why the demanganese rate is maximized when the temperature of the molten iron alloy is a specific temperature and the demanganese rate is deteriorated when the temperature is lower or higher than the specific temperature is as follows. . That is, the lower the temperature of the molten iron alloy is lower than the specific temperature, the lower the fluidity of the slag,
This is because the above oxidation reaction by the slag for demanganese becomes slow. On the other hand, as the temperature of the molten iron alloy becomes higher than a specific temperature, the oxidation reaction of manganese, which proceeds remarkably at a lower thermodynamic temperature, becomes more difficult to progress.

As a method for maintaining the temperature of the molten iron alloy within the above temperature range, as described above, an oxidizing gas is used.
It can be maintained by spraying on the slag on the molten iron alloy or by blowing into the molten iron alloy.

That is, by blowing or blowing the oxidizing gas, Al and Si in the molten iron alloy are more remarkably oxidized, and the temperature of the molten iron alloy rises due to the heat of oxidation. Therefore, by controlling the supply amount of the oxidizing gas, the temperature of the molten iron alloy can be adjusted and maintained within the above temperature range. In addition, since the amount of Al ₂ O ₃ and SiO _{2 in} the slag changes along with the temperature adjustment, and also the amounts of other components change, the composition (oxidizing agent) in the slag can be adjusted simultaneously with the temperature adjustment. .

It should be noted that a sufficient amount of Al and S
As described above, when i does not exist, an Al alloy or a Si alloy may be added.

Examples of the oxidizing gas include oxygen gas, carbon dioxide gas, air and the like. However, in addition to being expensive, carbon dioxide increases the amount of C (carbon) in the molten iron alloy. In addition, air has a poor thermal efficiency and most of it is nitrogen, so that the temperature of the molten iron alloy is rather lowered. Therefore, it is preferable to use oxygen gas as the oxidizing gas.

Next, other desirable refining conditions for demanganese will be described.

The range of the total amount of the oxidizing agent (iron oxide, chromium oxide) in the flux is 3 to the total amount of the slag formed.
It is desirable to mix so as to be 40% by weight.

That is, when the oxidizing agent in the total amount of slag is less than 3% by weight, demanganese does not proceed due to insufficient oxidizing power of the slag. On the other hand, if the oxidizing agent in the total amount of slag exceeds 40% by weight, the oxidation of chromium in the molten iron alloy is promoted, and the amount of Cr ₂ O ₃ in the slag increases. Of the liquid deteriorates.

The mixing ratio in the case where both iron oxide and chromium oxide are mixed is not limited as long as it is within the above range.

The iron oxide used as an oxidizing agent in the flux includes, for example, Fe ₂ O ₃ , Fe ₃ O ₄ , FeO and compounds containing these, and the chromium oxide includes, for example, Cr ₂ O ₃ And compounds and mixtures containing the same. Among them, chromium oxide is particularly effective because it has a function of preventing chromium in the molten iron alloy from being oxidized when the flux is added.

When SiO ₂ , Al ₂ O ₃ or CaF ₂ is used, the composition is as follows.

The desirable addition amount of CaF _{2 in} the flux is not more than 20% by weight of the total amount of slag formed. 2
If it exceeds 0% by weight, the refractory will be severely melted, making it difficult to use a flux.

[0066] desirable addition amount of Al ₂ O ₃ in the flux, added before Al ₂ O ₃ and fusion KimuTetsu in Al ₂ O ₃ by oxidation of Al in the slag, and further because of the temperature adjustment by the oxidizing gas Al ₂ O ₃ due to oxidation of rotating Al alloy
In consideration of the above, the basicity of the formed slag "(% Ca
O) / (% SiO ₂ +% Al ₂ O ₃ ) ”is 0.3 or more,
What is necessary is just to make it 1.5 or less.

However, the amount of Al ₂ O ₃ added must be 10 to 30% by weight of the total amount of slag to be formed, as described above. This is because if the Al ₂ O ₃ content in the slag is less than 10% by weight or more than 30% by weight of the total amount of the slag, the fluidity of the slag deteriorates, and the oxidation reaction for demanganese becomes slow.

The desirable addition amount of SiO _{2 in} the flux is as follows: SiO ₂ in the pre-slag and S _{2 in the} molten iron alloy.
SiO ₂ by oxidation of i, SiO ₂ by oxidation of the Si alloy further be added for the temperature adjustment by the oxidizing gas
In consideration of the slag basicity "% CaO / (% SiO
₂ +% Al ₂ O ₃ ) ”should be 0.3 or more and 1.5 or less.

The desirable range of the flux addition varies depending on the manganese content in the molten iron alloy, the target product manganese content, the silicon content in the molten iron alloy before treatment, and the like. 150kg / iron alloy
about t.

C (carbon) in the molten iron alloy does not directly have a large effect on the demanganese behavior, but the higher the C, the lower the melting point of the molten iron alloy. It is.

The method of adding the flux is not particularly limited as long as the flux can be brought into contact with the molten iron alloy. For example, a method of charging from the top of the molten iron alloy is sufficient, but injection into the molten iron alloy is more effective.

In the method of the present invention, the stirring of the molten iron alloy and the slag is important. That is, as a stirring method, bubbling stirring by Ar gas is generally used, but mechanical stirring by an impeller, and further, use of a stirring effect by natural fall when a molten iron alloy before processing is received in a processing vessel can be considered. .

Therefore, the container and the timing of performing the demanganese treatment are as follows: in the electric furnace after melting down in the electric furnace, when receiving from the smelting furnace to the ladle, and further in the ladle, AOD furnace, VOD In a furnace or the like.

It is desirable that the slag after the demanganese removal be removed from the molten iron alloy in order to prevent manganese reversion at the time of the deoxidation treatment in the next step. Hardening).

[0075]

EXAMPLE In melting various stainless steels, 4
Using a 0 ton AOD furnace, the content of C is 1 to 1.5% by weight, and the content of Al, Si, Mn and Cr is 10 kinds of molten iron alloys shown in Tables 1 and 2. A demanganese treatment was performed under the following conditions.

That is, with respect to the molten iron alloys other than No. 10 shown in Table 2, oxygen gas of the amount shown in Tables 1 and 2 was blown into the molten iron alloy, and the temperatures were shown in Tables 1 and 2. The fluxes having the compositions shown in Tables 1 and 2 were added while adjusting each within the range shown. At this time, N
In the molten iron alloys other than o.2, the amounts of Al alloy (metallic Al) and / or Si alloy (F
eSi) was added.

For the molten iron alloy of No. 10 shown in Table 2, oxygen gas was blown, and Al alloy or S
No i alloy was added.

However, in each case, 20 Nm from the furnace bottom
Ar gas was blown in at a rate of ³ / min to perform bottom-blowing stirring. The processing time was 10 minutes in each case.

Then, the Mn content in the molten iron alloy after the demanganese treatment was examined, and the demanganese rate was determined. The results are shown in Tables 1 and 2.

[0080]

[Table 1]

[0081]

[Table 2]

As is clear from the results shown in Tables 1 and 2, in each of the inventive examples (Nos. 1 to 5) subjected to the demanganese treatment according to the method of the present invention, the demanganese rate was 8%.
It was as good as 0% or more.

On the other hand, Comparative Examples (Nos. 5 to 10) in which any one of the temperature of the molten iron alloy, the Al ₂ O ₃ content in the slag, and the slag basicity are out of the ranges specified in the present invention.
Were unsatisfactory with a demanganese rate of 65% or less.

[0084]

According to the method for removing manganese of the present invention, stainless steel, particularly low manganese stainless steel containing 5% by weight or more of chromium, can be produced at low cost.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of slag basicity on the demanganese rate.

FIG. 2 is a diagram showing the effect of the amount of Al ₂ O _{3 in} slag on the demanganese rate.

FIG. 3 is a diagram showing the effect of the temperature of a molten iron alloy on the demanganese rate.

Claims (3)

(57) [Claims] 1. A method for removing manganese from a high chromium molten iron alloy containing at least 5% by weight of chromium, comprising CaO as a main component and one or both of iron oxide and chromium oxide. Using the flux, the basicity of the slag formed on the molten iron alloy determined by the following equation was determined as follows:
5 or less, the content of Al ₂ O ₃ in the slag is adjusted to 10 to 30% by weight, and the temperature of the molten iron alloy is raised to 1350 ° C. or more by blowing or blowing an oxidizing gas to the molten iron alloy.
A method for removing manganese from a high chromium molten iron alloy, wherein the manganese is removed at a temperature of 1550 ° C. or lower. Basicity _{=% CaO / (% SiO 2} +% Al 2 O 3) , however,% CaO,% SiO ₂ and% Al ₂ O ₃ is
Slag content (% by weight) 2. The flux according to claim 1, further comprising SiO ₂ , A
_{l 2 O 3, CaF 2,} Al de manganese method of the high chromium molten iron alloy according to claim 1, characterized in that one of alloy and Si alloy or contain two or more kinds. 3. The basicity is generated by oxidizing SiO ₂ and Al ₂ O ₃ in slag present in the flux and on the molten iron alloy before adding the flux, and Si and Al in the molten iron alloy. to SiO ₂ and Al ₂ O _3, as well as according to claim 1 or claim 2, wherein the determination by the SiO ₂ and Al ₂ O ₃ produced by oxidation of the Si alloy and Al alloy is added in the flux Demanganese method for high chromium molten iron alloy.