JPH06330144A - Method for removing impurity in chromium-containing molten steel - Google Patents

Abstract

PURPOSE:To provide a removing method for impurities in a chromium-containing molten steel, by which Pb, Zn, Bi and Sn adversely effecting to hot-workability of the steel are efficiently removed and the loads in the previous and the following processes are lightened, at the time of decarburize-refining the chromium-containing molten steel. CONSTITUTION:In order to promote the removal of the impurities of Pb, Zn, Bi, Sn, etc., in the molten steel in the decarburizing treatment of the chromium-containing molten steel 4, the decarburizing treatment is started at >=1.2mass% [C] concn. and executed in the range of >=0.5mass% [C] concn. with the combined blowing method for supplying gaseous oxygen or mixed gas containing the gaseous oxygen onto the bath surface and from below the bath surface. Further, in the range of <=0.2mass% [C] concn., the pressure in the vessel is reduced to <=200Torr and the decarburizing treatment is executed. By this method, in the finish refining stage of the chromium- containing molten steel, the miss of the aimed values of Pb, Zn, Bi and Sn in the product stage is eliminated and the stable removal of the impurities can be obtd. and further, as the regulation in the raw material blending stage can be relaxed, the refining cost can remarkably be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to Pb, Zn, B which adversely affects the hot workability of steel when decarburizing and refining molten steel containing chromium.
The present invention relates to a method for efficiently removing impurities such as i and Sn.

[0002]

2. Description of the Related Art Conventionally, 11 wt% such as stainless steel
Pb, Z in molten chromium-containing steel containing the above chromium
There is no quantitative knowledge regarding the removal of impurities such as n, Bi, and Sn. Generally, Pb, Zn, Bi, Sn, etc. are desired to be lowered as much as possible in order to deteriorate the hot workability of steel, and an upper limit is set depending on the steel type.

For ordinary steel, for example, "Materials and Processes", vol. 1, No. 4, page 1169-1
172 (1988), Pb, Z
n is that removal is promoted by increasing the flow rate of the stirring gas,
It has been shown that and Sn can be slightly removed. However, regarding the chromium-containing molten steel, since the effect of chromium contained in a large amount is unknown, there is no quantitative knowledge to determine the removal and removal limit value during decarburization refining.

Therefore, Pb is used as a reducing agent, a component adjusting agent, and a cooling agent added in the final refining period for controlling the impurity concentration of the molten raw material, reducing the chromium oxide in the slag, and adjusting the molten steel composition and temperature. , Zn, Bi, S
In addition to thoroughly controlling the concentration of n and the like, preferentially using a material with a low concentration, and performing operations such as excessive gas injection.

However, even if such operation management is performed,
The target Pb, Zn, Bi, and Sn concentration values may deviate from the regulation values. In addition, a material having a low concentration of Pb, Zn, Bi, and Sn has a high price, which causes an increase in cost. Furthermore, P
If excessive gas is blown in order to reduce the concentrations of b, Zn, Bi, and Sn, the cost will increase, which cannot be said to be an efficient refining method.

[0006]

DISCLOSURE OF THE INVENTION The present invention regulates the concentration and amount of impurities used in a molten raw material by promoting the removal of impurities such as Pb, Zn, Bi, and Sn during decarburization refining of molten chromium-containing steel. Relaxed and Pb, Z during final refining
The object is to achieve the target values of n, Bi, and Sn concentrations.

[0007]

The present invention advantageously solves the above-mentioned problems, and the gist thereof is as follows. (1) Pb in molten steel in decarburizing chromium-containing molten steel,
In order to accelerate the removal of impurities such as Zn, Bi and Sn,
[C] Start decarburization at a concentration of 1.2 mass% or more,
In addition, in the region where the [C] concentration is 0.5 mass% or more, the compound blowing method of supplying oxygen gas or a mixed gas containing oxygen gas from above and below the bath surface is performed, and the [C] concentration is 0.2 ma.
A method for removing impurities from molten chromium-containing steel, characterized in that decarburization is performed under a pressure of 200 Torr or less in a region of ss% or less.

(2) The method for removing impurities from chromium-containing molten steel according to the above item 1, wherein the complex blowing method is carried out in a region where the [C] concentration is 0.5 mass% or more under the condition that the following formula is satisfied. O _T / (O _T + O _B ) ≧ 0.3 …… O _T ; Top blown acid feed amount (Nm ³ / Hr) O _B ; Bottom blown acid feed amount (Nm ³ / Hr) (3) For decarburization treatment The method for removing impurities from molten chromium-containing steel according to item 1, wherein the subsequent final refining period is performed under reduced pressure of 200 Torr or less.

The present invention will be described in detail below. INDUSTRIAL APPLICABILITY The present invention is applied to a refining method for molten chromium-containing steel, which is performed under reduced pressure after atmospheric pressure treatment as illustrated in FIG.
The refining gas (5) is blown into the molten chromium-containing steel (4) through the bottom blowing tuyere (2) in the refining vessel (1). Further, the refining vessel (1) has a detachable exhaust hood (3) and can reduce the pressure to 200 Torr or less. Further, the refining vessel (1) has an upper blowing lance (6) at the upper part, and in the high [C] concentration range, gas is supplied from below the bath surface (bottom blowing) and above the bath surface (top blowing). It is possible.

In this blowing method, molten steel obtained by melting scraps, alloys and the like in an electric furnace is decarburized by blowing oxygen gas or oxygen gas and an inert gas. In the decarburization refining, the ratio of oxygen gas is high on the high [C] concentration side, and then the ratio of oxygen gas is reduced to prevent the oxidation of chromium in the molten steel and refining under reduced pressure is performed. And finally decarburized to [C] 0.1 mass% or less. Subsequently, a final refining period is provided for reducing the chromium oxide that has been oxidized during the decarburization and transferred into the slag, and at the same time, adjusting the composition and temperature of the molten steel. In the final refining period, a reducing material and an alloy material are added in order to adjust the composition and temperature of molten steel, and some pickup of impurities occurs. The allowable amount of impurities before decarburization is determined in consideration of this pickup amount, and the composition of the alloy etc. at the time of melting is determined.

The present invention relates to Pb, Zn, B in molten steel containing chromium.
When decarburizing and refining i and Sn impurities, especially [C] concentration 0.5
It is focused on that the removal proceeds remarkably by the treatment in a high coal area of mass% or more and under vacuum. The decarburization reaction of molten steel containing chromium is represented by the equation or the equation. With C + O = CO (g) ...... (Cr 2 O 3) +3 O = 2 Cr + 3CO (g) ...... decarburization time is blown a large amount of oxygen gas, a large amount of CO gas is generated. Therefore, the removal of impurities is promoted. Further, under reduced pressure, the evaporation of impurities easily progresses, so that the removal is promoted.

In FIG. 2, SUS3 was used by using a 60t scale furnace.
4 shows the relationship between the [C] concentration at the start of decarburization refining and the removal rate η of impurities of Pb, Zn, Bi, and Sn when decarburization refining of 04 stainless steel was performed. Incidentally, the oxygen-flow amount ratio blown over when the _{[O T / (O T +} O B) is in the range of 0.3 to 0.5, the [C] concentration of 0.2 mass% or less from 50 to 200
The pressure was reduced to Torr for decarburization. The impurity removal rate η is a value calculated using an equation, and is represented by a line connecting the minimum removal rates in the figure.

[0013] η = 100 × ([M] O - [M] f) / [M] O ...... here, M represents an impurity element, subscript o During the decarburization start, f is the time of decarburization end Shows the concentration of. From the figure, in particular, P
The removal rates of b, Zn, and Bi are as follows: [C] concentration 1.
Up to 2 mass%, it increases linearly as the [C] concentration increases, and when the [C] concentration is 1.2 mass% or more, the removal rate tends to be saturated at 95% or more. Moreover, although the removal rate of Sn is much smaller than that of other elements, the [C] concentration is 1.
At 2 mass%, the removal rate is about 20%, and when the [C] concentration is 1.2 mass% or more, it tends to be saturated.

From this, it is understood that the [C] concentration at the start of decarburization must be 1.2 mass% or more in order to efficiently remove impurities. In Fig. 3, when a 60t-scale furnace is used to decarburize and refine SUS304 stainless steel, the upper blowing acid ratio [O _T
/ (O _T + O _B) and shows the relationship between removal rate of Pb. In addition,
At this time, the [C] concentration at the start of decarburization is 1.2 to 1.5 ma.
The range is ss%, and the upper blowing is [C] 0.5 mass%.
Applied to the above areas. In addition, [C] concentration is 0.2mas
When the content was s% or less, the pressure was reduced to 50 to 200 Torr for decarburization. The removal rate of Pb in the figure is represented by a line connecting the minimum values. From the figure, the removal rate of Pb shows a tendency to increase with an increase in the upper blowing acid amount ratio in the range of 0 to 0.3, and is saturated when the upper blowing acid amount ratio is 0.3 or more. To do. In addition, when the amount of acid fed from above is 0.6 or more, it tends to decrease slightly. It should be noted that the upper blowing is generally applied to a [C] concentration of 0.5 mass% or more,
If the concentration is less than that, the effect of promoting decarburization is small. For this reason, the removal rate of Pb is 0.5C for the upper blowing [C] concentration.
It was confirmed that the improvement margin was small even when applied to the area of less than%.

From the above, it was confirmed that in order to efficiently remove Pb, it is necessary to set the amount of acid to be blown up to 0.3 or more in the region where the [C] concentration is 0.5 mass% or more. The same tendency is observed for the other Zn, Bi, and Sn, and the above equations can be summed up. It should be noted that, if the top-blowing acid amount ratio is 0.7 or more, the decarburization efficiency is lowered, so from the viewpoint of accelerating the decarburization reaction, it is not preferable to take the top-blowing acid ratio of more than this value.

In FIG. 4, SUS3 was used by using a 60t scale furnace.
The relationship between the degree of vacuum and the Pb removal rate in the region where the [C] concentration is 0.2 mass% or less when decarburizing and refining 04 stainless steel is shown. The [C] concentration at the start of decarburizing and refining is 1.2 to 1.5 mass%, and the [C] concentration is 0.5.
In the region of mass% or more, the upper blowing acid amount ratio is 0.3 to
Combined blowing with 0.5, then [C] concentration 0.2m
Vacuum refining was applied in the area of ass% or less. From the figure, P
The removal rate of b tends to increase as the vacuum degree decreases, and tends to saturate at a vacuum degree of 200 Torr or less. A similar tendency is observed for the other Zn, Bi, and Sn, and it can be seen that the removal of impurities is promoted by setting the vacuum degree of the pressure reduction treatment to 200 Torr or less.

In FIG. 5, SUS3 was used by using a 60t scale furnace.
The relationship between the degree of vacuum and the Pb removal rate in the final refining period when the final refining of 04 stainless steel is performed is shown. The removal rate in this case indicates the removal rate at the start and end of the final refining. From the figure, the removal rate of Pb in the final refining period tends to increase as the vacuum degree decreases, and the vacuum degree is 200 Torr.
Below, it tends to be saturated. In addition, other Zn, Bi,
A similar tendency was observed for Sn, and it was confirmed that the removal of impurities can be promoted by performing the treatment under a vacuum of 200 Torr or less even in the final refining period.

To summarize the above findings, in order to accelerate the removal of impurities such as Pb, Zn, Bi and Sn in molten steel during decarburization of molten chromium-containing steel, deoxidation is performed at a [C] concentration of 1.2 mass% or more. It is necessary to carry out decarburization refining while starting charcoal and feeding acid at a top blowing acid feeding ratio of 0.3 or more in a region where the [C] concentration is 0.5 mass% or more. Further, when the [C] concentration is 0.2 mass% or less, it is necessary to perform decarburization under a reduced pressure of 200 Torr or less, and it is preferable to reduce the pressure to 200 Torr or less even in the final refining period following the decarburizing period.

The higher the [C] concentration at the start of decarburization, the more preferable it is. However, as shown in FIG. 2, since the removal rate of the removal rate is small, the composition of the molten raw material and the load of decarburization refining It is necessary to set it in consideration. Further, the higher the [C] concentration at the start of the depressurization treatment, the more preferable, but 0.2 mass% was selected here as the [C] concentration at which the minimum effect is obtained. Further, although the removal of impurities is promoted as the degree of vacuum during the depressurization process is lower, the improvement rate of the removal rate is small as shown in FIGS. 4 and 5, and there are also operational problems such as splash on the high vacuum side. Therefore, it is preferably 10 Torr or more.

[0020]

FIG. 6 shows the relationship between the melt temperature and the vapor pressure of various metal elements in the pure metal state. Pb, Zn, Bi and Sn are F
e, Cr, Ni has a higher vapor pressure than 1450 to 175
It is considered that the removal by evaporation proceeds in the molten steel temperature state of 0 ° C. Also, the evaporation removal rate is Z with high vapor pressure.
It is considered that n, Bi, Pb, and Sn are larger in this order.

In this evaporation removal reaction, it is considered that the rate-determining process of the reaction is the movement of the evaporation element to the reaction interface in the molten steel or the separation of the evaporation element from the reaction interface to the gas phase side. Factors that accelerate this reaction include the following. 1) Increase the molten steel temperature. 2) The atmosphere is depressurized or vacuumed.

3) The gas generation rate is increased in order to increase the reaction interface area. Regarding the rise in molten steel temperature in 1), the decarburization of chromium-containing molten steel has a low molten steel temperature at the initial stage of decarburization, but thereafter, decarburization is performed at a molten steel temperature of 1650 ° C or higher in order to prevent oxidation of chromium in the molten steel. In order to carry out, the molten steel temperature pattern is rarely changed. It has been qualitatively shown in the past that the reduced pressure or the vacuum state of 2) is qualitatively, but it has not been quantified in the actual operation, because restrictions are added due to the relationship between the gas supply rate and the processing time. . In the present invention, the effect of the degree of vacuum on the removal of impurities is quantified, and the conditions of [C] concentration and degree of vacuum are derived as conditions for maximizing the effect of 2).

Conventionally, the effect of 3) has been qualitatively shown, but in the field of molten chromium-containing steel containing a large amount of chromium in particular, the effect on evaporation and removal of chromium is unknown, so it is quantitative. There was no finding. In the present invention, it was found that evaporation removal during the decarburization period is large, and Pb, Z
Zn, Bi, P with high removal rate of n, Bi, Sn with high vapor pressure
b, Sn in that order, the removal rate depends on the [C] concentration at the start of decarburization, saturation occurs at [C] concentration of 1.2 mass% or more, and in the region of [C] concentration of 0.5 mass% or more. It was found that evaporative removal is promoted when the top blown acid ratio is 0.3 or more.

In the decarburization refining of molten steel containing chromium, especially in the high carbon region where the [C] concentration is 0.5 mass% or more, the oxygen supply rate-determining region, the rapid decarburization reaction proceeds, and a large amount of CO gas is generated. . The gas-liquid interface increases due to the effect of bubble burst and the like due to the generation of CO gas, and the removal of impurities is promoted.
Therefore, the [C] concentration at the start of decarburization becomes a factor in determining the removal rate of impurities, and as shown in the present invention, if the [C] concentration at the start of decarburization is 1.2 mass% or more, the efficiency is high. Can be removed effectively.

In addition, in the top-bottom blowing composite blowing, by adding the top-blowing, as compared with bottom-blowing only, 210 by top-blowing
A high-temperature hot spot of 0 ° C. or higher is formed, the decarburization reaction is promoted, and the secondary combustion reaction promotes the movement to the gas phase side. Therefore, the removal rate of impurities is improved with an increase in the upper blowing acid amount ratio, and efficient removal can be performed at the upper blowing acid amount ratio of 0.3 or more in the present invention.

Due to the hot workability of steel and the like, the upper limit of the content of impurities is generally regulated for each steel type, and in order to satisfy the regulation value, efficient removal is performed during decarburization refining. This reduces the load on the front and rear processes.

[0027]

[Example] SUS304 stainless steel (8 mass% N
An example in which the processing of i-18 mass% Cr) 60 ton is performed in the embodiment shown in FIG. 1 will be described. The results obtained in Table 1 are shown in comparison with the method of the present invention and the conventional method. In each of the examples of the present invention, the target value after the final refining is [Pb] ≦ 5.0.
ppm, [Zn] ≦ 15.0 ppm, [Bi] ≦ 5.0
ppm, [Sn] ≤ 200 ppm, and the upper-bottom blowing composite blowing was applied to [C] concentration of 0.5 mass% or more. Further, when the [C] concentration was 0.2 mass% or less, the treatment was performed under reduced pressure, and the treatment was also performed under reduced pressure during the final refining period.

In the comparative example, the target value after the final refining is the same as that of the example of the present invention, but the [C] concentration at the start of decarburization, the upper blowing acid ratio, and the degree of vacuum during depressurization are the conditions of the present invention. It is outside. The pick-up in the final refining period when reducing chromium oxides after decarburization refining and adjusting the composition and temperature of molten steel is 1.0 to 2.0 ppm for [Pb] and 2.
0-4.0 ppm, [Bi] 1.0-2.0 ppm,
[Sn] was in the range of 5 to 10 ppm.

In the examples of the present invention, the amount of impurities removed during decarburization refining was quantified, so the concentration of impurities at the start of decarburization refining, that is, after dissolution, was kept high. That is, the dissolution was performed using a low-quality raw material. On the other hand, in the comparative example, since the amount of impurities removed is unclear, the melting raw material was selected and used so that the concentration at the start of decarburization would be low. The results of the examples are shown in Table 2. The raw material cost in the table is No. 100 in 1
It is the value converted as. In the comparative example, since the removal was not sufficient, the target value after completion of refining might not be achieved in some cases, and some sort of rescue treatment was necessary.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

EFFECTS OF THE INVENTION According to the present invention, in the final refining period of molten chromium-containing steel, it is possible to eliminate the deviation of the target values of Pb, Zn, Bi, and Sn at the product stage and to stably remove impurities.
Further, the regulation at the raw material blending stage for preventing deviation of the regulated values of Pb, Zn, Bi, and Sn can be relaxed, so that the refining cost can be significantly reduced.

[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 [C] concentration at the start of decarburization in the present invention.

FIG. 3 is a diagram showing the reason for limiting the upper blowing acid amount ratio in the present invention.

FIG. 4 is a diagram showing the reason for limiting the degree of vacuum in the region where the [C] concentration is 0.2 mass% or less in the present invention.

FIG. 5 is a diagram showing the reason for limiting the degree of vacuum in the final refining period in the present invention.

FIG. 6 is a diagram showing a relationship between a vapor pressure of an impurity element in a pure metal state and a molten metal temperature.

[Explanation of symbols]

 1 Refining container 2 Bottom blowing tuyere 3 Exhaust hood 4 Molten steel 5 Gas 6 Top blowing lance

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hiroshi Iwasaki 3434 Shimada, Hikari City, Yamaguchi Prefecture Shin Nippon Steel Co., Ltd.

Claims (3)

[Claims] 1. A decarburization treatment is started at a [C] concentration of 1.2 mass% or more in order to accelerate the removal of impurities such as Pb, Zn, Bi and Sn in the molten steel in the decarburization treatment of chromium-containing molten steel. And, in the region where the [C] concentration is 0.5 mass% or more, the compound blowing method in which oxygen gas or a mixed gas containing oxygen gas is supplied from above and below the bath surface, and the [C] concentration is 0.2 mass% or less In the region (1), the method for removing impurities from molten chromium-containing steel is characterized in that decarburization treatment is performed under a reduced pressure of 200 Torr or less. 2. The method for removing impurities from molten chromium-containing steel according to claim 1, wherein the complex blowing method is performed in a region where the [C] concentration is 0.5 mass% or more under the condition that the following formula is satisfied. O _T / (O _T + O _B ) ≧ 0.3 ... O _T ; Top blown acid supply amount (Nm ³ / Hr) O _B ; Bottom blown acid supply amount (Nm ³ / Hr) 3. The final refining period following decarburization treatment is 200To
The method for removing impurities from a chromium-containing molten steel according to claim 1, wherein the treatment is performed under a reduced pressure of rr or less.