JPH0730386B2 - 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) Generally, it is known that when the Cr content in steel increases, the workability such as cold workability and deep drawability deteriorates. When melting carbon steel with good workability, the Cr content in the molten steel should be 0.03-
It is strictly required to adjust it to 0.05% or less.

However, in recent years, it has become increasingly difficult to completely prevent a sudden increase in the Cr content in a carbon steel melting operation using a converter.

Because, in the melting operation of carbon steel in a converter, scrap is usually compounded into the hot metal. This is because the chances of mixing steel scrap are increasing. Therefore, of course, although the management of scrap has been further strengthened, in the actual work, it was the actual situation that the problem of out-of-specification of the Cr content in the molten steel after converter blowing could not be eradicated. .

Therefore, as a countermeasure in the case where the Cr content is out of the specification as described above, a method of blowing down carbon in the converter and further extending the blowing to remove Cr is also taken. However, in this case, the Cr dissolved in the hot metal
There is a limit to the reduction of Cr because a part of it has already been oxidized during the decarburization refining and moved into the slag as Cr oxide,
In addition, there was a problem that the melting loss of the refractory material of the converter became severe during this treatment.

Therefore, under the present circumstances, "change of steel type" is inevitable in the case of molten steel with a large degree of Cr content deviation from the standard.

On the other hand, the amount of scrap used is increasing due to the recent trend of stabilizing steel demand. If more scrap can be obtained at low cost in the future, the molten pig iron ratio will be lowered during the melting of carbon steel. It is obvious that the situation will increase the scrap ratio. Therefore, in such a situation, it is considered that the probability that the Cr content of the steel material will deviate from the standard due to the inclusion of stainless steel scraps will increase, and the deviance range will become even greater. it is obvious.

(Problems to be Solved by the Invention) Therefore, it is necessary to consider the oxidation removal of Cr in molten iron such as molten pig iron and 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 [C] is blown down, it is not possible to expect very effective Cr removal. Also,
By blowing down [C], the (T.Fe) of the slag increases, and the melting loss of the converter refractory becomes severe, so this method is also economically disadvantageous.

Further, when [C] is blown down as described above, the oxidation of Mn proceeds at the same time as the decarbonization, so that after the Cr removal treatment is completed.
You have to add additional Mn, which is an expensive operation.

Therefore, an object of the present invention is to provide a simple, easy, and effective method for removing Cr from molten iron.

A more specific object of the present invention is to economically and effectively oxidize and remove Cr while reducing the oxidation loss of Mn in the converter or in the ladle after the blowing of the converter during the melting of carbon steel. Is to provide a method.

(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 remove [Cr] in molten steel by bringing the slag into contact with a ladle, it is necessary that the slag has a strong oxidizing power. is necessary. This is because in order to oxidize and remove [Cr], Cr removal is carried out with deoxidization and Mn removal due to strong oxidizing power.

In addition, in the above-mentioned strong oxidation treatment, Japanese Patent Application No. 63-83598
As described in the above issue, in order to prevent the oxidation loss of [Mn] which is a beneficial component, it is necessary that the slag contains a sufficient amount of Mn oxide.

Therefore, further investigation based on these findings revealed that it was effective to add carbonaceous material to the Cr-free slag after the Cr-free treatment in order to further improve the Mn yield. The present invention has been reached.

Here, the present invention is characterized in that molten iron is contacted with slag containing Fe oxide and Mn oxide as main components, and thereafter, carbonaceous material is added into the slag, loss of [Mn]. This is a Cr removal method for molten iron, in which Cr is removed while reducing the amount.

According to a preferred embodiment of the present invention, a flux composed of one or more of CaO, SiO ₂ , Al ₂ O ₃ , CaF ₂ , ZrO ₂ and MgO may be further added prior to the addition of the carbonaceous material.

The above "molten iron" includes, for example, molten pig iron and molten steel, and is not limited to any molten iron as long as Cr removal is performed based on the principle of the present invention, but molten steel is more preferable. In the case of molten steel, the target is carbon steel.

(Operation) The details of each processing operation of the present invention will be described.

In the following description, the molten Cr steel after the converter refining will be taken as an example of molten iron, and its Cr removal operation will be described.

As already mentioned, according to the present invention, after blowing the converter, the
The above-mentioned slag is brought into contact with molten steel for the purpose of Cr, but at that time after converter blowing, it may be before deoxidation, but preferably after removing converter slag after completion of converter blowing It is good to make contact as soon as possible. Therefore,
Generally, the treatment is carried out 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.

According to a preferred embodiment of the present invention, the flux is composed of iron oxide and Mn oxide, or in addition to them, CaO, SiO ₂ , Mg.
Form of synthetic flux obtained as a mixture with one or more of O, ZrO ₂ , Al ₂ O ₃ and CaF ₂ , or by melting and cooling the mixture before adding it to molten steel and then pulverizing it. May be used in. The latter synthetic flux is preferable from the viewpoint of stability of flux properties.

The amount of flux added and the suitable blending ratio of these components vary depending on the molten steel composition, especially the Cr content, but as a general example, the flux amount: 10 to 50 kg / T · molten steel flux component: Fe oxide ≧ 30% 40% ≧ Mn oxide ≧ 20% 50% ≧ (CaO ＋ SiO ₂ ＋ MgO ＋ ZrO ₂ ＋ Al ₂ O ₃ ＋ CaF ₂ ) ≧ 20% Also, after adding and fluxing the above flux, the carbon material to be added is as follows. is there.

Addition amount of carbonaceous material: 0.5-5kg / T Molten steel In addition, when performing Cr removal treatment with this ladle, it is desirable to remove converter slag that inevitably enters, but when reconstitution is not a problem. May control the composition of the added flux so that the slag component having the above-mentioned properties is obtained with a small amount of converter slag left.

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 Cr removal method according to the present invention can be applied to the case where an electric furnace is used to produce carbon steel using scrap as a main raw material.

Next, the composition of the slag that can effectively perform Cr removal while suppressing Mn removal 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 an oxide into the slag by an oxidation reaction represented by the following formula (1). Need to be present.

[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 summarized in the graph in FIG. The upper curve in Figure 1 is Is low, the lower curve is Is high. The amount of slag was 50 kg / T.

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

For example, the slag (T.Fe) needs to be about 20% or more in the slag for Cr removal with the Cr removal rate exceeding 50%. In other words, about 30% or more of Fe oxide is contained in the flux for Cr removal initially added. Here, Fe oxide means FeO, Fe ₂ O ₃ ,
Refers to Fe ₃ O ₄ etc.

Therefore, in the preferred embodiment, the slag for Cr removal is (T.Fe)
It contains 20% or more.

By the way, since the Cr-free slag used at this time has a strong oxidizing power, the purpose is to prevent the beneficial components in molten steel [Mn] from being simultaneously oxidized and removed at the same time as [Cr]. It is necessary to contain a large amount of (MnO).

In the shaded area in Fig. 2, (MnO) in the slag and [Mn in the molten steel
n] A graph showing the yield relationship. In the figure, the upper curve is When is high or (T.Fe) is low, the lower curve is Is low or (T.Fe) is high.

Here, the Mn yield is defined as follows.

The transfer of Mn in the slag into the molten steel is represented by the following equation (2).

(MnO) + C → [Mn] + CO ↑ (2) The results shown in FIG. 2 are obtained in the same experiment as in FIG. 1, and the oxidation loss of [Mn] is shown from the results at this time. Clearly, large amounts of (MnO) are needed to prevent. De-C
(T.Fe) in r slag was adjusted to 20 to 35%, and the amount of (MnO) was changed.

However, in order to obtain high Mn yield here (MnO)
When the value is extremely high, the melting loss to the refractory becomes severe. Therefore, if only 0.5 to 5 kg / T. Of molten steel is added to the slag as carbonaceous material at the stage when the Cr removal treatment is completed, a large amount of MnO in the slag is reduced, and the Mn yield is further improved. I understood. The timing of adding the carbonaceous material is not particularly limited. It may be added when sufficient Cr removal has proceeded. The results are shown in FIG. 2 together with plots, and five examples are shown.

The white circle plot shows the Mn yield at the end of Cr removal treatment, and the black circle plot shows the Mn yield after further addition of carbonaceous material. Thus, the Mn yield was further improved by 20-40%. here,
The reason why (MnO) does not change so much while the Mn yield is improved is that the addition of carbonaceous material significantly reduces (T.Fe) and reduces the amount of slag, so even if MnO is reduced. This is because the amount of MnO with respect to the total amount of slag did not change.
At this time, pickup of [C] and [Cr] in the molten steel was hardly seen. Cr oxide compared to Mn oxide,
The reason why it is less susceptible to reduction by carbonaceous materials is that, as a result of investigating the slag properties with an X-ray microanalyzer, Cr oxides are crystallized as solids in the Cr-free slag, so there is little contact with carbonaceous materials. it is conceivable that.

Preferably, the amount of carbonaceous material added is about 1/10 to 1/1 / the amount of flux.
Twenty. If it exceeds this, carburization may proceed mainly in some cases, while if it is less than that, the effect of increasing [Mn] is small. It should be noted that a small amount of double Cr is acceptable (for example, 0.01 ~
0.02% increase), and if the amount of [Mn] is further increased, a larger amount may be added. The particle size of the carbonaceous material is not particularly limited, but if the particle size is small, the yield of the carbonaceous material to the slag decreases and the reaction becomes too rapid. If the particles are large, the reaction is slow. Therefore, a diameter of about 1 to 5 mm is effective. Examples of such carbonaceous materials include pulverized coal, powdered coke, and graphite particles, but they may be pelletized.

By the way, as is clear from the above explanation, in order to make the final Mn yield of molten steel exceed 70%, (MnO) in the slag for Cr removal should be 15% or more, and This can be achieved by adding a material. In other words, about 20% or more of Mn oxide is contained in the initially added Cr-free flux.
Here, the Mn oxide mainly refers to MnO ₂ (or part of MnO).

The upper limit of (MnO) in the Cr-free slag is 20 to 25%. As can be seen from Fig. 2, if (MnO) is in this range, it is possible to add 100% of Mn yield by adding carbonaceous material. De-C
Approximately 40 in terms of Mn oxide added initially as flux.
%.

In the present invention, (T.Fe), (MnO) is an essential component of the slag for Cr removal, and its composition ratio is limited as described above.
According to a further preferred embodiment, CaO, SiO ₂ , MgO, Zr
Mixing at least one of O ₂ , Al ₂ O ₃ and CaF ₂ to remove C
Form r slag.

50% ≧ (CaO ＋ SiO ₂ ＋ MgO ＋ ZrO ₂ ＋ Al ₂ O ₃ ＋ CaF ₂ ) ≧ 20% where When the high and low values are compared, the Cr removal rate or
There is some difference in Mn yield. That is, even if the Cr removal rate is the same (T.Fe) in Fig. 1, When is high, it is slightly lower than when it is low. However, even if the Mn yield is the same (MnO) in FIG. When the value is high, it is slightly improved compared to when the value is low. Therefore, comparing both, there are merits and demerits, but as a guide Is optimal.

Incidentally, MgO, ZrO ₂ , Al ₂ O ₃ and CaF ₂ promote slag formation or
Add to protect refractories. Regarding MgO, the properties of Cr-free slag were investigated by an X-ray microanalyzer, and as a result, it was crystallized by selectively combining with Cr ₂ O ₃ , which is an effective component for improving the Cr-free rate. it is conceivable that.

Since ZrO ₂ is a slightly acidic oxide, it is similar to SiO ₂ .
It is considered to be an effective component for stabilizing the basic oxide Cr ₂ O ₃ in the slag.

Next, the present invention will be described more specifically by way of its examples.

Example 1 Using a Tammann furnace, 2 kg of carbon steel was melted in the atmosphere in an MgO crucible and kept at 1600 ° C., 100 g of flux having the composition shown in Table 1 was added, and bubbling stirring was performed with argon gas. The Cr removal treatment was performed for about 5 to 10 minutes.
Coke powder, particle size 1-2 mm, 10 g was then added.

The results are summarized in Table 1.

As shown in Experiment Nos. 1 to 13 in the same table, it is found that in the Cr removal treatment stage, Mn yield is further increased by performing Cr removal while suppressing Mn removal and then adding carbonaceous material.

Example 2 250 tons of molten steel (1680 ° C.), which had been subjected to normal converter blowing by a 250 tons converter, was placed in a ladle, and the flux shown in Table 2 was applied.
A 10-ton bag was put into the tapped steel in portions and added, and a deCr treatment was performed by stirring with argon gas. Next, 10 minutes later, 250 kg of coke powder (particle size 2 to 5 mm) was added.

The results are summarized in Table 2. As can be seen from the results, the Cr removal proceeds while suppressing the oxidation loss of [Mn] while accompanied by the C removal, and the carbonaceous material is added 10 minutes after the flux addition. As a result, the Mn yield was further improved. Then take off
Cr slag was removed, Al was added at RH, and the temperature was increased by utilizing the heat of oxidation reaction of Al to degas.

As the flux, Mn ore was used as the MnO ₂ source, oxide scale was used as Fe ₂ O ₃ , calcium oxide was used as CaO, and silica sand was used as SiO ₂ with a particle size of 5 mm or less.

(Advantages of the Invention) By adopting the present invention, it is possible to repair Cr out-of-spec while preventing loss of [Mn] after converter blowing, and increase scrap ratio of stainless steel and increase scrap ratio. The present invention has great significance when it comes to a situation in which operation is expected.

[Brief description of drawings]

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

Claims (2)

[Claims] 1. The loss of [Mn] is reduced by contacting molten iron with slag containing Fe oxide and Mn oxide as main components, and then adding carbonaceous material into the slag. A method for removing Cr from molten iron while removing Cr. 2. CaO, SiO ₂ , A before addition of carbonaceous material
The method for removing Cr from molten iron according to claim 1, wherein a flux comprising at least one of l ₂ O ₃ , CaF ₂ , ZrO ₂ , and MgO is blended.