JPH0730387B2 - Cr removal method for molten iron - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for removing Cr from molten iron, and more particularly to a method for removing Cr from molten iron in a ladle.

(Prior art) Cr content is the target value when recovering from the Cr spec of molten metal after converter or electric furnace blowing, or when melting low Cr steel such as cold rolled steel sheet for automobiles. If it exceeds the value, it is necessary to perform Cr removal processing.

That is, if the Cr content in the carbon steel is high, the cold workability and the deep drawability are deteriorated. Therefore, for example, when melting low Cr steel such as cold-rolled steel sheet material for automobiles, the Cr content in the molten steel is ,
That is, it is necessary to reduce [Cr] to 0.03 to 0.05% by weight or less.

By the way, at the present time, when smelting carbon steel, a sudden increase in Cr occurs due to, for example, blending of scrap into hot metal in a converter. This is because stainless steel scraps may be mixed in the scrap. Currently, we are strengthening the scrap management method, but we are still using Cr after converter blowing.
The out-of-specification of is a problem.

If such a Cr out-of-specification occurs, [C] in the converter
There are cases where Cr is removed by blowing down, but there is a limit to lowering Cr by such means, and in such a method, melting damage of the converter refractory is severe. Therefore, under the present circumstances, it is inevitable to change the steel type for molten steel having a large degree of deviating from the Cr standard.

On the other hand, the amount of scrap tends to increase more and more. Therefore, in the future, if a large amount of scrap can be obtained at low cost in melting carbon steel, it is expected that the hot metal ratio in the converter will be lowered and the scrap ratio will be increased. In such a case, it is considered that the probability of deviation from the Cr standard due to mixing of stainless steel scraps as described above increases, and the range of deviation from the Cr standard also increases, and it is clear that it is not possible to deal with it only by changing the steel type. .

As described above, when melting the carbon steel, if the molten pig iron ratio is lowered and the scrap ratio is increased in the converter, Cr in the molten steel suddenly rises, and there is a high possibility that the Cr standard will be broken. However, there has been no method to easily remove Cr from molten steel in such a case.

(Problems to be Solved by the Invention) Therefore, it is necessary to consider oxidation removal of Cr in molten steel.

As described above, in order to remove Cr in the molten steel after the blowing of the converter, it is first considered to blow down [C] in the converter to promote the oxidation of [Cr]. However, since Cr contained in the hot metal or scrap has already been partially oxidized and entered as chrome oxide in the slag, even if it is blown down, effective Cr removal cannot be expected. Further, the blow-down of [C] increases the T.Fe of the slag, that is, (T.Fe), and the melting loss of the converter refractory becomes severe, and this method is economically disadvantageous.

Further, in that case, since Mn is oxidized at the same time as decarbonization, depending on the steel type, additional Mn must be added after the treatment, which is an expensive operation.

Here, an object of the present invention is to provide an economical and effective method for removing Cr from molten iron.

Further, a specific object of the present invention is, when melting carbon steel, in a converter or in a ladle after blowing the converter, Cr is economically and effectively under the condition that the melting loss of the refractory is suppressed. It is to provide a method for oxidative removal.

(Means for Solving the Problems) The present inventors have made various studies to achieve the above-mentioned object, and have obtained the following findings.

First, as described in Japanese Patent Application No. 63-52099, in order to oxidize and remove Cr in molten steel by bringing molten steel and slag into contact with each other in a ladle or the like, the slag must have a strong oxidizing power. is necessary.

Further, depending on the type of steel, as described in Japanese Patent Application No. 63-83598, the slag having a strong oxidizing power does not oxidize Mn, which is a beneficial component in molten steel, so that it contains sufficient Mn oxide. is necessary.

Subsequent studies have found that increasing (T.Fe) in the slag is preferable to improve the Cr removal rate, however, when (T.Fe) in the slag increases, refractory material Will be significantly melted and become economically disadvantageous. Therefore, when these points were examined, refractory materials were generally composed of materials containing MgO, ZnO _2, etc., so slag for Cr removal should be used in order to prevent their melting loss.
When one or more of MgO and ZrO ₂ is mixed,
The present invention has been made based on the knowledge that not only the melting loss of refractory materials can be prevented, but also the Cr removal rate is remarkably improved, and the above-mentioned problems of the prior art are effectively solved.

Here, the gist of the present invention is that, in weight%, Fe oxide is 20% or more, MgO and / or ZrO _{2 is} 10% or more in total, and mainly CaO, S as a melting point depressant.
A method for removing Cr from molten iron, which comprises adding to the molten iron a mixed or synthetic flux containing 60% or less in total of one or more of iO ₂ , Al ₂ O ₃ and CaF ₂ .

Further, according to a preferred aspect of the present invention, the flux contains
You may mix 20% or more of Mn oxides. In that case, CaO, S
One or more of iO ₂ , Al ₂ O ₃ and CaF ₂ are added in a total amount of 50% or less. This is because, depending on the type of steel, it is necessary to avoid the oxidative consumption of Mn in the slag.

As described above, according to the present invention, when MgO or ZrO ₂ is one of the main components of the refractory, it is possible to suppress the melting loss of the refractory due to the slag for Cr removal.

MgO is effective in promoting Cr removal because it selectively forms a compound with Cr oxide.

Since ZrO ₂ is an acidic oxide, it is effective in stabilizing the basic Cr oxide in the slag.

(Operation) Next, details of each processing operation of the Cr removal method of the present invention will be described.

First, according to the present invention, after the converter blowing, the above-mentioned mixed flux or synthetic flux is added for the purpose of removing Cr, but the timing may be after the converter blowing and before deoxidizing. . It is preferable to add the converter slag as soon as possible after removing the converter slag after the completion of the converter blowing. Therefore, it is generally processed in the ladle after tapping, but if necessary, the converter slag may be removed in the converter before the Cr removal treatment according to the present invention.

The reason why it is preferable to perform the Cr removal treatment before deoxidation is that the presence of oxygen is required not only in the slag but also in the molten steel in order to remove the Cr by oxidation.

It should be noted that the converter blowing itself itself prior to the Cr removal treatment is not limited in the present invention, and any ordinary one is sufficient.
Only when the Cr content finally exceeds the target value, the treatment according to the present invention may be added.

Next, in the Cr removal treatment according to the present invention, the method of adding the flux may be the injection method from the upper portion of the molten steel, but the injection into the molten steel is more effective. Also, the amount of flux added is the target amount of Cr removal (usually 0.
(About 05 to 0.1%) and the amount of decarbonization that occurs at the same time, but 10 to 50 kg / T of molten steel is usually sufficient.

Further, the flux may be used as a mixed flux containing various compounds as a mixture, or in the form of a synthetic flux obtained by melting and cooling the mixture once before adding it to molten steel and then pulverizing the mixture. . The latter synthetic flux is preferable from the viewpoint of stability of flux properties.

The amount of flux added and the mixing ratio of these components are
Although it depends on the composition of molten steel, especially Cr content, as a general example, flux amount: 10-50kg / T molten steel Flux component: (Fe oxide) ≧ 20% (MgO and / or ZrO ₂ ) ≧ 10% Depending on the steel type, (Mn oxide) ≥ 20%, and other flux components include CaO and Si.
One or more of O ₂ , CaF ₂ and Al ₂ O ₃ may be added in a total amount of 60% or less mainly for the purpose of lowering the melting point of the flux. When compounding Mn oxide, it should be 50% or less.

By the addition of such a melting point depressant, the melting point of the flux changes depending on the blending ratio, and the Cr removal treatment temperature (1700 ° C ~
1550 ° C) or lower.

Further, the more desirable mixing ratio of the flux is (Fe oxide) ≧ 30% 50% ≧ (MgO and / or ZrO ₂ ) ≧ 10% Depending on the steel type, (Mn oxide) ≧ 30%.

In addition, when performing the Cr removal process with this ladle, it is desirable to remove the converter slag that inevitably enters, but if reconstitution P is not a problem, leave the converter slag in a small amount. The composition of the added flux may be controlled so that the slag component has the above properties.

Further, in the method according to the present invention, stirring of molten steel and slag is important from a practical point of view. That is, as a stirring method, bubbling stirring with argon is generally used, but it is also conceivable to use the stirring effect of natural dropping of molten steel to which flux is added during tapping. In the case of the present invention,
This is because if the oxidizing power is sufficient, the reaction will proceed promptly.

It is desirable to remove the slag after Cr removal in order to prevent re-Cr in the degassing process such as the next process, degassing process such as RH treatment,
A method of slag chilling (hardening the slag) with lumped quicklime or the like may be used.

Needless to say, the method according to the present invention can be applied to the case where the scrap is used as the main raw material and the carbon steel is melted using the electric furnace.

Next, the composition of the slag will be described more specifically.

First, the slag for Cr removal used in the present invention has a strong oxidizing power because it is intended to remove [Cr] as Cr oxide into the slag by the oxidation reaction shown in the following formula (1). It is necessary to have

[Cr] + _x (FeO) → (CrO _x ) + _x (Fe) (1) Therefore, it is necessary that the slag for Cr removal contains a large amount of Fe oxide.

As shown in Fig. 1, consider the relationship between (T.Fe) and the Cr removal rate. The initial composition of molten steel at this time was C ≈ 0.05%, Si = tr., M
n≈0.15%, P≈0.015%, S≈0.01%, Cr≈0.10%, and the molten steel temperature was 1600 ° C. A flux with a different amount of Fe oxide was added to this, the amount of (T.Fe) in the slag was changed, and the Cr removal rate at that time was determined. The results are shown graphically in FIG.

The upper curve in FIG. 1 is the basicity of the slag, that is, If is low (for acid slag), the lower curve is the basicity of the slag, ie Is high (in the case of basic slag).

This is because the Cr oxide generated after the Cr removal treatment is slightly basic, so that the more acidic the slag is, the more stable the Cr oxide is in the slag, and therefore the Cr removal tends to proceed. The flux amount was 50 kg / T.

As is clear from the results shown in FIG. 1, the higher the (T.Fe) in the slag, the higher the Cr removal rate.

For example, when a Cr removal rate of 40% or more is required, (T.Fe) in the slag needs to be about 15% or more, and in terms of Fe oxide, about 20% or more.

Further, when a Cr removal rate of 50% or more is required, about 20% or more (T.Fe) in the slag, and about 30% or more in terms of Fe oxide are required.

Here, the Fe oxide refers to FeO, Fe ₂ O ₃ , Fe ₃ O _{4, and the} like.

However, slag for Cr removal with such a high (T.Fe) has a particularly strong erosion power with respect to the refractory material of the converter or the ladle. Therefore, in the preferred embodiment of the present invention, in order to prevent the melting of the refractory due to the slag for Cr removal, the slag for Cr removal includes:
One or more compounds that are the main components of the refractory are included.

For example, when the refractory is an MgO-based refractory (mag / carbon brick, spinel brick, etc.), it is preferable to add MgO or a MgO-containing compound such as dolomite as the “compound which is the main component of the refractory”.

Further, for example, when the refractory material is zircon brick, it is preferable to add a zirconia-containing compound, such as zircon, as the “compound which is the main component of the refractory material”.

Here, in the case of slag for Cr removal containing MgO, as a result of investigating the slag properties with an X-ray microanalyzer, MgO and Cr oxide tend to selectively form compounds. Therefore, MgO is a very effective additive not only for preventing melting damage of refractories but also for improving the Cr removal rate.

Further, in the case of slag for Cr removal containing ZrO ₂ , since ZrO ₂ is an acidic oxide, ZrO ₂ can stabilize Cr oxide, which is a basic oxide, in the slag. Therefore, not only to prevent melting damage of refractories but also to improve the Cr removal rate, Zr
O ₂ is a very effective additive.

The total content of these MgO and / or ZrO ₂ is 10%
That is all. It is preferably 10% or more and 50% or less. That is, 50% ≧ (MgO and / or ZrO ₂ ) ≧ 10%.
This lower limit is the minimum value at which the effect begins to appear, and the upper limit is a value in consideration of the fluidity of slag, but it varies depending on the blending of other compounds.

In the present invention, since the slag for Cr removal used has a strong oxidizing power, [Cr] and [Mn], which is a beneficial component in molten steel, are simultaneously removed by oxidation. Depending on the steel type, it is necessary to contain a large amount of (MnO) in order to prevent this.

Figure 2 shows the relationship between (MnO) in slag and [Mn] yield in molten steel. In the figure, the upper curve is Is higher, the lower curve is Is low.

Here, the Mn yield is defined as follows.

The results in Fig. 2 were obtained in the same manner as in Fig. 1. From the results at this time, a large amount of (MnO) is necessary to prevent the oxidation loss of [Mn]. Is clear.

For example, in order to make the Mn yield of molten steel to exceed 50%,
15% or more of (MnO) in slag, about 20 in terms of Mn oxide
% Or more is required. In addition, (MnO) in the slag must be 25% or more, and in terms of Mn oxide, about 30% or more, in order to make the Mn yield of the molten steel exceed 70%.

Here, the Mn oxide mainly refers to MnO ₂ (partly MnO).

In addition, the above Fe oxide is mixed with MgO and / or ZrO _2, and depending on the type of steel, Mn oxide is further mixed, and CaO, SiO ₂ , Al ₂ O ₃ and CaFe are mainly used for the purpose of promoting slag formation.
The Cr-free flux is adjusted using a melting point depressant consisting of And it has already been described that they are added as a mixed flux or a synthetic flux.

Next, the Cr removal treatment according to the present invention will be described more specifically by way of examples.

Example 1 2 kg of carbon steel was melted in the atmosphere in a MgO crucible using a Tammann furnace and maintained at 1600 ° C., and then 100 g of flux shown in Table 1 as test Nos. 1 to 26 was added and argon gas
After bubbling and stirring for a minute, Cr removal treatment was performed. The results are summarized in the table.

In this way, Cr removal progresses well with C removal. Further, in Test Nos. 1 to 24, it is considered that the MgO melting loss from the MgO crucible could be reduced because the increase amount of MgO in the slag was small. Further, as in Test Nos. 1 to 18 and 26, by premixing the flux with Mn oxide, Mn removal could be suppressed.

However, as shown in Test No. 27 in the table, when the (T.Fe) of the slag was small, the Cr removal did not proceed so much. In addition, as shown in test number 28 in the table, if ZrO ₂ or MgO is not mixed with the Cr-free flux in advance,
The amount of increase of MgO in the slag is large, which is considered to be caused by melting damage from the MgO crucible, and the Cr removal rate is relatively not so good.

Example 2 2 kg of carbon steel was melted in the atmosphere in a MgO crucible using a Tammann furnace and kept at 1600 ° C., and then 100 g of flux shown in test numbers 29 to 40 in Table 2 was added, and argon gas was added to give 20
After bubbling and stirring for a minute, Cr removal treatment was performed. The results are summarized in the table.

Thus, Cr removal proceeds well. Test number 29-
As for 37, the increase in ZrO _{2 in} the slag is small, so
It is considered that the ZrO ₂ melting loss from the ZrO ₂ crucible could be reduced.

However, as shown in Test No. 41 of the same table, if ZrO ₂ or MgO was not mixed in advance with the Cr-free flux, the amount of increase in ZrO _{2 in} the slag was large, and this was due to melting damage from the ZrO ₂ crucible. It is thought that the Cr removal rate is relatively poor.

Example 3 Molten steel 250 for which normal converter blowing has been completed by a 250 ton converter
Tons (1680 ℃), zircon brick (60% ZrO ₂ , 34% Si
O ₂ ) It was placed in a ladle, and the flux shown in Table 3 was packed in 10 tons of tapped steel, added in portions, and stirred by argon gas to perform a Cr removal treatment. The results are summarized in Table 3.

Thus, good Cr removal proceeded. Also, Zr in the slag
Since O ₂ is not much before and after the treatment, it is considered that the melting loss of the refractory is small. After the Cr removal process, remove the Cr removal slag,
Degassing was performed by adding Al at RH and using the heat of oxidation reaction of Al to raise the temperature.

As the flux, Fe ₂ O ₃ source was oxide scale, M
The nO ₂ source was Mn ore, CaO was quicklime, and ZrO ₂ -SiO ₂ was zircon brick with a particle size of 5 mm or less.

(Effects of the Invention) As described in detail above, by adopting the present invention, it is possible to remedy molten iron out of the Cr standard while preventing melting loss of refractory materials. Significant.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between slag (T.Fe) and the Cr removal rate. FIG. 2 is a graph showing the relationship between (MnO) in slag and [Mn] yield in molten steel. .

Claims (2)

[Claims] 1. A molten iron containing, as a main component, 20% or more by weight of Fe oxide, 10% or more of MgO and / or ZrO _{2 in} total, and further comprises CaO, SiO ₂ , Al ₂ O ₃ , and CaF _2. 60% in total of one or more of
A method for removing Cr from molten iron, which is characterized by adding a mixed or synthetic flux contained below. 2. The flux is further compounded with 20% or more of Mn oxide and 50% or less in total of at least one of CaO, SiO ₂ , Al ₂ O ₃ and CaF _2. The method for removing Cr from molten iron according to claim 1.