JPH06192719A - Method for removing impurity in chromium-containing molten steel - Google Patents

Abstract

PURPOSE:To efficiently remove impurities of Pb, Zn, Bi, Sn, etc., in molten steel and to reduce the load in the pre- and the post-processes by controlling the supplying of oxygen according to C concn. in the chromium-containing molten steel to execute decarburize-refining. CONSTITUTION:At the time of decarburize-refining the chromium-containing molten steel, the decarburization is started at >=1.2mass% C in the molten steel, and while supplying the oxygen in a range satifying the inequality OT/(OT +OB)>=0.2 (wherein, OT: top-blowing oxygen supplying velocity Nm<3>/hr, OB: bottom-blowing oxygen supplying velocity Nm<3>/hr) in the range of >=0.5mass% C, the decarburize-refining is executed. By this method, the removal of the impurities of Pb, Zn, Bi, Sn, etc., in the molten steel is promoted and the regulation of the impurity concn. and the using quantity of the melting raw material is released and the concn. aimed values of Pb, Zn, Bi, Sn, etc., at the final- refining stage are achieved.

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.

In ordinary steel, for example, “Materials and Processes”, vol. 1, No. 4, page 1169-11
72 (1988), Pb, Zn
Indicates that the removal is promoted by increasing the flow rate of the stirring gas, and that 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, regarding the reducing material, the component adjusting material, and the cooling material to be added in the final refining period in which the impurity concentration of the molten raw material is regulated, the chromium oxide in the slag is reduced, and the molten steel composition and the temperature are adjusted, , Pb, Zn, B
Thorough control of the concentrations of i, Sn, etc., preferential use of low-concentration materials, and excessive gas injection were carried out.

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 is expensive and causes high cost. Furthermore, Pb, Z
Since the concentration of n, Bi, and Sn is lowered, the cost is increased even if the gas is blown in excessively, which is not an efficient refining method.

[0006]

DISCLOSURE OF THE INVENTION The present invention facilitates the removal of impurities such as Pb, Zn, Bi and Sn during decarburization refining of molten chromium-containing steel, thereby relaxing the regulation of the impurity concentration and the amount of use of the molten raw material. And at the time of final refining Pb, Z
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 is intended to promote the removal of impurities such as Pb, Zn, Bi and Sn in molten steel during decarburization of molten steel containing chromium. In the region where [C] concentration is 1.2 mass% or more and decarburization is started, and [C] concentration is 0.5 mass% or more,
The gist is a method for removing impurities from molten chromium-containing steel, which is characterized in that decarburization refining is performed while feeding oxygen under conditions satisfying the following formula.

O _T / (O _T + O _B ) ≧ 0.2 …………………… O _T ; Top-blown acid transfer rate (Nm ³ / Hr) O _B ; Bottom-blown acid transfer rate (Nm ³ / Hr) ) The present invention will be described in detail below. The present invention is applied to a refining method for molten chromium-containing steel as illustrated in FIG. 1. FIG. 1 (a) is an AOD method, and FIG. 1 (b) is an upper-bottom blowing converter method. Is. In these blowing methods, molten steel obtained by melting scraps, alloys and the like in an electric furnace is decarburized by blowing oxygen or oxygen and an inert gas. In the decarburization refining, the proportion of oxygen gas is high on the high [C] concentration side, and then the pattern of decreasing the proportion of oxygen gas is taken in order to prevent the oxidation of chromium in the molten steel. ] It is decarburized to 0.1 mass% or less. continue,
A final refining period is provided to reduce the chromium oxide that has been oxidized during decarburization and has migrated into the slag, and at the same time adjust the composition and temperature of the molten steel.

The present invention relates to Pb, Zn, B in molten steel containing chromium.
When the impurities of i and Sn are decarburized and refined, [C] concentration is 0.5.
The focus is on the remarkable progress of the removal in the high coal area of mass% or more. The decarburization reaction of molten steel containing chromium is represented by the equation or the equation. C + O = CO (g) ……………… (Cr ₂ O ₃ ) +3 O = 2 Cr + 3CO (g) ………… At the time of decarburization, a large amount of oxygen gas is blown and a large amount of CO gas is generated. Therefore, the removal of impurities is promoted.

In FIG. 2, SUS3 was used with a 60t AOD furnace.
4 shows the relationship between the [C] concentration at the start of decarburization refining of 04 stainless steel and the removal rate η of impurities of Pb, Zn, Bi, and Sn. Incidentally, the oxygen-flow amount ratio blown over when the _{[O T / (O T + O} B ] was in the range of 0.3 to 0.5. The value is η removal rate of impurities was calculated using the formula And is represented by a line connecting the minimum removal rates.

Η = 100 × ([M] ₀ − [M] _f ) / [M] ₀ ...... From the figure, in particular, the removal rates of Pb, Zn, and Bi are [C] at the start of decarburization. Linearly up to a concentration of 1.2 mass%, [C]
Increased with increasing concentration, [C] concentration 1.2mas
If it is s% or more, the removal rate tends to be saturated at 90% or more. Further, although the removal rate of Sn is much smaller than that of other elements, the removal rate is about 10% at a [C] concentration of 1.2 mass% and tends to be saturated at a [C] concentration of 1.2 mass% or more.

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. 60tAO in Figure 3
Using D furnace, on blowing oxygen-flow amount ratio at the start of decarburization refining in the case of performing decarburization refining of SUS304 stainless steel [O _T
/ (O _T + O _B] and showing the removal rate relationship Pb. Incidentally,
At this time, the [C] concentration at the start of decarburization is 1.2 to 1.5 ma.
ss% range, top blowing is [C] 0.5 mass%
Applied to the above areas. Further, the removal rate of Pb is represented by a line connecting 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 the upper blowing acid amount ratio of 0 to 0.2. Saturate. In addition, when the upper blowing acid amount ratio is 0.6 or more, it tends to be slightly decreased. Note that the upper blowing is generally applied to [C] 0.5 mass% or more, and at a concentration lower than that, the effect of promoting decarburization is small.
Therefore, the removal rate of Pb is [C] 0.5mas
It was confirmed that the improvement margin was small even when applied to a region of less than s%.

From the above, it has been confirmed that in order to efficiently remove Pb, it is necessary to set the upper blowing acid amount ratio to 0.2 or more in the region of [C] 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. To summarize the above findings, when decarburizing molten steel containing chromium,
In order to accelerate the removal of impurities such as b, Zn, Bi, and Sn, decarburization is started at a [C] concentration of 1.2 mass% or more,
In addition, it is necessary to carry out decarburization refining while feeding oxygen in the region of [C] concentration of 0.5 mass% or more at a top blowing acid feeding ratio of 0.2 or more.

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.

[0015]

FIG. 4 shows the relationship between the molten steel 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. In addition, the removal rate by evaporation is Zn, B with high vapor pressure.
It is considered that i, 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. Since the decarburization refining of molten chromium-containing steel by the AOD method and the top-bottom blowing converter method is performed under atmospheric pressure, the effect of 2) cannot be enjoyed. Further, decarburization by these refining methods has a low molten steel temperature at the initial stage of decarburization, but thereafter, in order to prevent oxidation of chromium in the molten steel, decarburization is performed at a molten steel temperature of 1650 ° C or higher. It rarely changes the pattern. Therefore, 3) is a point to accelerate the evaporation removal reaction.

Conventionally, the effect of 3) has been qualitatively shown, but in the field of chromium-containing molten steel containing a large amount of chromium in particular, the effect on evaporation and removal of chromium is unknown, so a quantitative finding is obtained. There was no. In the present invention, it was found that evaporation removal in the decarburization period is large, and Pb, Zn,
Zn, Bi, Pb, which have a high vapor pressure for removing Bi and Sn,
Sn is larger in the order, the removal rate depends on the [C] concentration at the start of decarburization, saturation occurs when the [C] concentration is 1.2 mass% or more, and upper blowing is performed in the region where the [C] concentration is 0.5 mass% or more. It has been found that evaporative removal is promoted when the acid transfer rate is 0.2 or more.

In the decarburization refining of molten chromium-containing steel, especially in the high carbon area where the concentration of [C] is 0.5 mass% or more, the oxygen supply rate-determining area, 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 that determines 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, the top blowing 210
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 0.2 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.

[0022]

[Example] Using a 60 ton AOD furnace, SUS304
Stainless steel (8 mass% Ni-18 mass% C
An example performed for r) will be described. Table 1
The results of the above are shown in comparison with the method of the present invention and the conventional method. In all the examples, the target value after the final refining is [Pb] ≦
5.0 ppm, [Zn] ≦ 15.0 ppm, [Bi] ≦
The present invention was applied to steel types of 5.0 ppm and [Sn] ≦ 200 ppm, and the upper-bottom blowing composite blowing was applied to [C] concentration of 0.5 mass% or more. In addition, the reduction of chromium oxides after decarburization and refining, the composition of molten steel and the picking during temperature control
Up is 1.0 to 3.0 ppm for [Pb], 2.0 to 5.0 ppm for [Zn], 1.0 to 3.0 pp for [Bi]
m and [Sn] were in the range of 5 to 10 ppm.

In the example of the present invention, the amount of impurities removed during decarburization refining was semi-quantified, so the concentration of impurities at the start of decarburization refining, that is, after dissolution, was set to a high concentration state. 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 100 for the No. 1 example
It is the value converted as. In the comparative example, since the removal was not sufficient, the target value after completion of refining could not be achieved in some cases, and some kind of rescue treatment was necessary.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

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, in order to prevent the Pb, Zn, Bi, and Sn from deviating from the regulation values, the regulation at the raw material mixing stage can be relaxed, so that the refining cost can be significantly reduced.

[Brief description of drawings]

FIG. 1 is a diagram relating to an example of a refining method to which the present invention is applied,
(A) shows an AOD method, (b) shows a top-bottom blowing converter method.

FIG. 2 is a diagram showing the relationship between the [C] concentration at the start of decarburization and the impurity removal rate in the present invention.

FIG. 3 is a diagram showing the relationship between the upper blowing acid amount ratio and the Pb removal ratio in the method of the present invention.

FIG. 4 is a diagram showing a relationship between a vapor pressure of an impurity element in a pure metal state and a molten steel temperature.

[Explanation of symbols]

 1 Side blowing tuyeres 2 Top blowing lances 3 Bottom blowing tuyeres 4 Molten steel 5 Slag 6 Top blowing fire point

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

Claims (1)

[Claims] 1. Pb in molten steel during decarburization of molten steel containing chromium,
In order to accelerate the removal of impurities such as Zn, Bi and Sn,
[C] Decarburization and refining are performed while starting decarburization at a concentration of 1.2 mass% or more, and in the region of [C] concentration of 0.5 mass% or more, while feeding oxygen under conditions satisfying the following formula: Method for removing impurities from molten chromium-containing steel. _{O T / (O T + O} B) ≧ 0.2 ..................... O T; top-blown oxygen-flow-rate _{^{(Nm 3 / Hr) O B}} ; bottom-blown oxygen-flow-rate (Nm ³ / Hr)