JPH02163310A - Method for removing cr in molten iron - Google Patents

Abstract

PURPOSE:To produce a steel containing little Cr by bringing molten metal having much Cr content into contact with molten slag having much iron oxide and manganese oxide contents, oxidizing Cr in the molten steel and discharging Cr into the slag. CONSTITUTION:At the time of producing a steel slab for carbon steel plate having excellent workability, in raw material scrap charged into a converter, the scrap of stainless steel is mixed and this becomes the carbon steel having much Cr content and as cold workability and deep drawability are deteriorated at the time of working into a strip, the Cr content is reduced to 0.03-0.05%. In order to attain this purpose, by bringing the molten slag having strong oxidization containing of >=30% oxide of Fe (FeO, Fe2O3, Fe3O4) and 20-40% Mn oxide of MnO, etc., into contact with the molten steel having much Cr content, Cr in the molten steel is oxidized into Cr2O3 and removed in the molten slag. Successively, after adding flux composed of one or more kinds among CaO, SiO2, Al2O3, CaF2, ZrO2 and MgO to the molten slag, a carbonaceous material is added at 0.5-5kg/ton of molten steel and MnO oxidized and removed in the molten slag is reduced into Mn and lowering of the Mn yield is prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for decarbonizing molten iron, particularly a method for decarbonizing molten iron in a ladle.
Regarding the method.

(Prior art) It is generally known that an increase in Cr content in steel leads to deterioration of processing properties such as cold workability and female drawability. When producing carbon steel with good workability, it is strictly required to adjust the Cr content in f8tM to 0.03 to 0.05% or less.

However, recently, it has become increasingly difficult to completely prevent a sudden increase in Cr content in carbon steel melting operations using converters.

This is because when melting carbon steel in a converter, scrap is normally mixed into hot metal, but in recent years, reflecting the growth in demand for stainless steel, stainless steel has been added to the scrap that is the raw material for converter mixing. This is because the chances of steel scrap getting mixed in are increasing.

Therefore, although scrap management has of course been further strengthened, in actual operations, it is still not possible to eradicate the problem of non-standard Cr content in molten steel after converter blowing. .

Therefore, as a countermeasure against the above-mentioned deviation of the Cr content from the standard, a method of blowing down carbon in a converter and further extending the blowing period to remove Cr has been taken. However, in this case, there is a limit to the reduction in Cr because some of the Cr that had entered the hot metal has already been oxidized during decarburization and moved into the slag as Cr oxides. In addition, there was also the problem that during this treatment, the converter refractories were severely eroded.

Therefore, in the case of molten steel whose Cr content is significantly out of specification, it is currently necessary to change the steel type.

On the other hand, there are signs that the amount of scrap used will further increase due to the recent stabilization of demand for steel materials.In the future, if it becomes possible to produce even more scrap at a lower cost by hand, it will be possible to lower the hot metal ratio when melting carbon steel. It is clear that the situation will lead to an increase in the scrap ratio. Therefore, in such a situation, the probability that the Cr content of the steel material will be out of specification due to the contamination of stainless steel scraps will become higher and higher, and the range of deviations from the specification will also increase, and it will not be possible to deal with it simply by changing the steel type. it is obvious.

(Problems to be Solved by the Invention) Therefore, it is necessary to consider oxidation removal of Cr in molten iron such as hot metal and molten steel.

As mentioned above, in order to remove C'' from molten steel after converter blowing, the first idea is to blow down the (C) in the converter and accelerate the oxidation of [C'']. Or, since the Cr contained in the scrap has already been partially oxidized and is contained in the slag as chromium oxide, very effective removal of Cr cannot be expected even if (C) is blown down. As the slag is blown down, the (T, Fe) content of the slag increases, and the melting of the converter refractories becomes more intense, making this method economically disadvantageous.

Furthermore, when (C) is blown down as described above, the oxidation of Mn proceeds at the same time as the removal of C, which necessitates the additional injection of Mn after the Cr removal process, making the operation rather expensive.

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

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

(Means for Solving the Problems) The present inventors have conducted various studies to achieve the above-mentioned objectives, and have obtained the following knowledge.

First, as described in Japanese Patent Application No. 63-52099, slag is brought into contact with the molten steel in a ladle etc.
In order to remove Cr), the slag needs to have strong oxidizing power. This is because, in order to oxidize and remove CCr1, Cr removal is performed while C1 removal and Mn removal is performed using strong oxidizing power.

In addition, in the above-mentioned strong oxidation treatment,
As stated in No. 3598, it is a beneficial ingredient [1n
] In order to prevent oxidation loss, the slag needs to contain a sufficient amount of Mn oxide.

Therefore, after further investigation based on these findings, we found that in order to further improve the Mn yield, we decided to remove C.
It was found that it is effective to add carbonaceous material to the Cr-free slag after the r treatment, leading to the present invention.

Here, the present invention is characterized by contacting molten iron with a slag whose main components are Fe oxide and 60 oxide, and then adding a carbonaceous material into the slag. This is a method for removing Cr from molten iron while reducing carbon content.

According to preferred embodiment B of the present invention, CaOlSing, 8Q203, CaFz, Z'02
, and MIO may be blended.

The above-mentioned "?'a iron" includes, for example, molten pig iron and molten steel, and is not limited to any molten iron that can be chromium-removed based on the principles of the present invention, but is more preferably molten steel. It is. In the case of molten steel, the target is carbon steel.

(effect) Details of each processing operation of the present invention will be explained.

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

As already mentioned, according to the present invention, the above-mentioned slag is brought into contact with molten steel for the purpose of removing Cr after converter blowing, but the timing is after converter blowing and before deoxidation. However, it is preferable that the contact be made as soon as possible after the converter slag is removed after the converter blowing is completed. therefore,
Generally, the treatment is carried out in the ladle after tapping, but if necessary, the Cr removal treatment according to the present invention may be carried out after removing the converter slag in the converter. The reason why it is preferable to perform a carbon removal treatment before deoxidation is that oxygen needs to exist not only in the slag but also in the molten steel in order to oxidize and remove Cr.

Note that the converter blowing itself prior to the Cr removal treatment is not limited in any way in the present invention, and a normal one is sufficient. The treatment according to the present invention only needs to be applied when the Cr content finally exceeds the target value.

Next, in the carbon removal treatment according to the present invention, it is sufficient to add flux from the top of the molten steel, but injection into the molten steel is more effective.

In addition, the amount of flux added should be determined to achieve the desired removal of Crff1.
(usually about 0.05 to 0.1%) and is determined by simultaneous declination, etc., but usually 10 to 50 kg/
T-molten steel is sufficient.

According to a preferred embodiment of the present invention, the flux comprises iron oxide and Mn oxide, or further combinations thereof with CaO, Si.
O2, MgO, Zr0z, AQzos, and CaFg
It may be used as a mixture with one or more of the above, or in the form of a synthetic flux obtained by melting, cooling, and pulverizing a mixture of these before adding it to molten steel. The latter synthetic flux is preferred from the viewpoint of stability of flux properties.

The amount of flux added and the preferred blending ratio of these components vary depending on the molten steel composition, especially the Cr content, but -
As a typical example, flux t: 10 to 50 kg/T -i steel flux composition: Fell oxide ≧ 30% 40 After the carbonization, the carbon materials to be added are as follows.

Carbon material addition ffi: 0.5~5kg/T? ? ! t
! Note that when carrying out the Cr removal process using this ladle, it is desirable to remove the converter slag that inevitably enters, but
If this is not much of a problem, the composition of the added flux may be controlled so that the slag component has the above properties while leaving a small amount of converter slag.

Further, in the method according to the present invention, stirring of molten steel and slag is important from a practical viewpoint.

That is, as a stirring method, bubbling stirring using argon is generally used, but it is also possible to use the stirring effect of natural falling of molten steel to which flux is added during tapping. This is because, in the case of the present invention, if the oxidizing power is sufficient, the reaction proceeds quickly.

It is desirable to remove the slag after removing Cr in order to prevent re-Cr in the next deoxidation and degassing steps such as R11 treatment, but slag chilling (hardening the slag) with lumps of quicklime etc. is also possible. .

It goes without saying that the Cr removal method according to the present invention can also be applied to the case where carbon steel is produced using scrap as a main raw material using an electric furnace.

Next, the composition of the slag that can effectively remove Cr while suppressing Mn removal will be described in more detail.

First, the slag for removing Cr used in the present invention has strong oxidizing power because the purpose is to remove [C''] as an oxide into the slag by the oxidation reaction shown in the following formula (1). It is necessary that the

(Cr) +x(Pea) = (Cry,) +x
(Fe) (1) Therefore, the slag for removing Cr needs to contain 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 the molten steel at this time was C#0.05%, S
i2ke9, Mn #0.15%, P #0.015%, S
-0.01%, Cr#0.10%, and the molten steel temperature is 1
The temperature was 600°C.

To this, a flux that changed the star of Fe oxide was added,
The T and Fefl in the slag were varied and the Cr removal rate was determined. The results are summarized in a graph in Figure 1.
In the figure, the upper curve is (SiOz) + (AQtOs) (St (h) ÷ (AQzo,)) The amount of gas was 50 kg/T.

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

For example, in a slag for removing Cr having a Cr removal rate exceeding 50%, the content of (T, Fe) in the slag is about 20% or more. In other words, Fe oxide is about 30% or more in the initially added flux for removing Cr. Here, Fe oxide refers to FeO% FezO, Pe5Oa, etc.

Therefore, in a preferred embodiment, the slag for removing Cr is (T,
Fe) 20% or more.

By the way, since the decarbonizing slag used at this time has strong oxidizing power, the purpose of the decarbonization is to prevent (Cr) and (Mn), which is a beneficial component in molten steel, from being oxidized and removed at the same time. Therefore, it is necessary to contain a large amount of (MnO).

The hatched area in FIG. 2 graphically shows the relationship between (MnO) in slag and (Mn) yield in /8 steel. In the figure, top (Si
02M(AQzOs) Or if (T, Fe) is low, lower curve) yo <5i(
h)+(AQzOs) is high.

Here, the Mn yield is defined as follows.

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

(MnO) + C-= (Mn) +COf...
+2) The results shown in Figure 2 were obtained in the same experiment as in Figure 1, and from this result it is clear that a large amount of (MnO) is required to prevent the oxidation loss of [Mn]. it is obvious. (T, Fe) in the slag for removing Cr is 20 to 3
It was adjusted to 5% and the amount of (MnO) was varied.

However, in order to obtain a high Mn yield here (Mn
If O) is made extremely high, corrosion damage to the refractory becomes severe. Therefore, when the Cr removal process is completed, 0.5 to 5 kg/T of coke is added to the slag as a carbon material.

It was found that by adding only No. 18 steel, the large amount of MnO present in the slag was reduced and the Mn yield was further improved.

The timing of adding the carbonaceous material is not particularly limited. enough C removal
It is sufficient to add it when the process has progressed.The results are shown in five plots in Figure 2.

The white circle plot shows the Mn yield at the end of the Cr removal process, and the black circle plot shows the Mn yield after further addition of carbonaceous material. In this way, the Mn yield was further significantly improved by 20-40%. Here, the reason why (MnO) does not change much while the Mn yield improves is due to the addition of carbonaceous material (T, Fe
) is significantly reduced and the slag phase is reduced, so MnO
This is because even though MnOil was reduced, the MnOil for the entire slurry device did not change. At this time) (C) in the trunk,
(Cr) pifuquan was hardly seen.

Cr acid oxides are less susceptible to reduction by carbon materials than n-oxides when the slab properties are X! As a result of investigation using a microanalyzer, it is thought that this is because Cr oxide crystallizes as a solid in the decarbonized slag, so there is little contact with the carbonaceous material.

Preferably, the amount of carbon material added is about 1/10 of the amount of flux.
~1/20. This is because if it exceeds this range, carburization mainly proceeds depending on the case, while if it is less than this range, the effect of increasing (Mn) is small. Note that if a small amount of Cr (for example, 0.01 to 0.02% increase) is desired, and a further increase in (Mn) is desired, 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 in the slag will decrease and the reaction will be too rapid. Also, if the particles are large, the reaction will be slow. Therefore, diameter 1-5
A compound of about IIIm is effective. Examples of such carbon materials include powdered coal, coke, graphite grains, etc., but pellet-like materials may also be used.

By the way, as is clear from the above explanation, in order to make the final Mn yield of molten steel exceed 70%, decarbonization is necessary.
(MnO) in the slag for r is set to 15% or more, and this can be achieved by adding carbonaceous material. In other words, the Cr-removal flux initially added contains about 20% or more of Mn oxide. Here, Mn oxide is mainly Mn0t
(or partially refers to Mn0).

The upper limit of (MnO) in the slag for removing Cr is 20 to 2.
5% is sufficient. As can be seen from FIG. 2, if (MnO) is within this range, it is possible to achieve a Mn yield of 100% by adding carbonaceous material. Mn added initially as Cr-free flanks
In terms of oxides, it is approximately 40%.

In the present invention, (T, Fe) and (MnO) are essential components of the slag for removing Cr, and the composition ratio is y1 as described above.
However, according to a more preferred embodiment, further CaO1S
iO2, MgO%Zr0tSAQzOi, and Ca
A slag for removing Cr is formed by blending at least one type of Pz.

50% ≧ (CaO+SiO2+MgO+ZrO2 18 QzOt+CaPz) ≧20% (Si(h)+(
AQxOz) When comparing the cases, there are some differences in the Cr removal rate or Mn yield. That is, in FIG. 1, even if the Cr removal rate is the same (T, Fe), it is slightly lower than that of (Si02)+(AQzOi). However, in Fig. 2 (St
When ow) + (AQz(h) is high, there is a slight improvement compared to when it is low.Therefore, there are advantages and disadvantages when comparing the two, but as a guide and (Sing)
÷(8Qzo3) Yes.

Note that MgO, Zrot-, AQzOs, and CaFz are added to promote slag formation or protect refractories. Furthermore, regarding MgO, we will remove C and improve the properties of the slag.
As a result of investigation using a line microanalyzer, it was found that it was selectively combined with CrtO= and crystallized, so it is considered to be an effective component for improving the Cr removal rate.

Also, since ZrO is a slightly acidic oxide, SiO□
Similarly, it is considered to be an effective component for stabilizing the basic oxide Cr101 in the slag.

Next, the present invention will be explained in more detail with reference to examples thereof.

Example 1 After atmospherically melting 2 kg of carbon in a MgO crucible using a Tammann furnace and holding it at 1600°C, a long flux having the composition shown in Table 1 was added, and while stirring by bubbling with argon gas, The membrane Cr treatment was performed for about 5 to 10 minutes. Next, coke powder, particle size 1-2
nu+, Log were added.

The results are summarized in Table 1.

As shown in Experiments 1 to 13 in the same table, it can be seen that in the Cr removal treatment stage, Cr removal is performed while suppressing Mn removal, and then by adding carbon material, the Mn yield is further increased.

Example 2 Molten steel 2 after normal converter blowing in a 250-ton converter
50 tons (1680'c) were placed in a ladle, 10 tons of the flux shown in Table 2 was packed in bags, and υI minutes were added to the steel tapping, followed by stirring with argon gas.
processed. Then, after 10 minutes, coke powder (particle size 2~
5mm) 250 kg was input.

The results are summarized in Table 2, and as can be seen, decarbonization proceeded while suppressing the oxidation loss of [Mn], and carbonaceous material was added 10 minutes after adding flux. By doing this, the Mn yield was further improved.Then, the Cr-free slag was removed, and AQ was added in R11.
The heat of the oxidation reaction of M was used to raise the temperature and degas the mixture.

In addition, as for the flux, Mnn Mangoku F is used as the MnO□ source.
e, 03 has oxide scale, CaO has quicklime, Sin
For g, silica sand with a particle size of 5 mm or less was used.

(Left below) (Effects of the invention) By adopting the present invention, it is possible to prevent the loss of [Mn] after converter blowing, and it is possible to remedy the failure of Cr standard, which increases the amount of stainless steel scrap. At the same time, a situation has arisen in which operations with a high scrap ratio are expected, and the present invention has great significance.

[Brief explanation of the drawing]

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

Claims (2)

[Claims] (1) This method is characterized by contacting molten iron with slag mainly composed of Fe oxide and Mn oxide, and then adding carbonaceous material to the slag, while reducing the loss of [Mn]. A method for removing Cr from molten iron. (2) Prior to addition of carbonaceous material, further CaO, SiO_2
, Al_2O_3, CaF_2, ZrO_2, and M
2. The method for removing Cr from molten iron according to claim 1, further comprising blending a flux consisting of one or more types of gO.