JP2003147427A - Blowing method in converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control ferro-oxide quantity in slag when blowing is completed for reduction of a cost and improvement of a quality degradation caused by the ferro-oxide securing dephosphorizing performance of the ferro-oxide in the slag in a converter. SOLUTION: At the blowing time, when a temperature in the completion of blowing reaches to 1650 deg.C, the slag quantity in the converter and MgO concentration in the slag at the completion of blowing satisfies following formula (1) 3.0<=0.0045 (WSLAG-200) [(MgO)-21]<=8.0. In this furmula, WSLAG shows the slag quantity (kg/t) in the converter at the completion of blowing and (MgO) shows the MgO concentration (mass%) in the slag in the converter at the completion of blowing.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a converter blowing method for controlling the amount of iron-based oxide in the slag in the converter during blowing in the converter.

[0002]

2. Description of the Related Art It is necessary to supply a large amount of oxygen at a high speed to hot metal produced in a blast furnace in a converter through a top blowing lance or tuyeres installed at the bottom of the furnace. 4.8% by mass
The amount of C included in the range of 0.03 to 0.
The maximum purpose is to reduce the content to about 8% by mass, and the impurities (C, Si, P) in the hot metal are oxidized and removed, and the heat of oxidation of these impurities is used to bring the molten steel to a predetermined temperature. The purpose of this is to raise the temperature of and to secure the temperature required for casting.

Since a large amount of oxygen is supplied to oxidize and remove the impurities, the slag in the converter (hereinafter sometimes simply referred to as "slag") at the end of blowing has a high degree of oxidation. . As an index showing the oxidation degree of slag, FeO, Fe ₂ O ₃ and Fe ₃ O ₄ in the slag, which generally represent the amount of Fe (T.Fe) [unit: mass%], are generally used, and
(T.Fe) at the end of blowing is about 15 to 25% by mass.

The iron-based oxide in the converter slag is generally FeO out of FeO, Fe ₂ O ₃ and Fe ₃ O ₄ , so that (T.Fe) will be described below. The discussion will focus on FeO inside.

By the way, in the smelting process, an auxiliary material such as quick lime or light burned dolomite is added to the hot metal, and the phosphorus content in the hot metal is transferred to slag for dephosphorization. In this dephosphorization treatment,
FeO is required for the reaction as shown in the following reaction formula (3).

2P + 5FeO + 3CaO = 3 (CaO.P ₂ O ₅ ) + 5Fe (l) (3) On the other hand, when (T.Fe) becomes excessive, the converter operating cost and product quality are greatly adversely affected. In terms of cost, converter life may be shortened and replacement may be frequent, but this is because FeO has a low melting point and other oxides are easily dissolved in the FeO, so the FeO concentration in the slag [below (Sometimes referred to as FeO)] is high, the oxides forming the refractory lining of the converter are dissolved, that is, the refractory is melted and damaged.
In particular, when expensive MgO-C or the like is used for the refractory so as to endure refining reaction under high temperature and oxidizing atmosphere,
It will spur the cost increase.

Further, as an adverse effect on the quality, there is an increase in product defects due to a large amount of precipitation of alumina-based inclusions. This is because the converter slag also flows out with the molten steel when the molten steel after blowing is transferred to the ladle.
This is because when the FeO concentration is high, the Al alloy added as a deoxidizing material after the completion of blowing is oxidized by the FeO, and a large amount of alumina-based inclusions are generated.

Although it is necessary to secure a certain amount of (T.Fe) at the end of blowing as described above, a high concentration has a serious adverse effect on manufacturing cost and quality.
A blowing method in which e) is controlled to a low level is being proposed, and the following two types of methods are typical of them.

The first method is to blow the gas such as carbon monoxide, argon, nitrogen, etc. singly or in combination with each other from the bottom of the converter to enhance the stirring of the steel bath to suppress the generation of FeO. The second method is to reduce the upper blowing oxygen flow rate in the final stage of blowing. These are methods that have become popular in recent years because they are effective in reducing (T.Fe), and all of them are techniques derived based on the estimation of the reaction mechanism of FeO production and decarburization as shown below. Is.

It is considered that the decarburizing reaction in the converter represented by the following reaction formula (4) has two rate controlling steps depending on the blowing timing. In the early and middle stages of blowing,
Since the amount of C in the molten steel is large relative to the amount of oxygen blown in, and almost all of the supplied oxygen is consumed in the reaction with C in the molten steel, the rate-determining step of the decarburization reaction is the supply of oxygen. I can say.

On the other hand, since C in the molten steel is reduced to about 0.2 to 0.4 mass% at the final stage of blowing,
The decarburization reaction represented by the following reaction formula (4) is suppressed, and the supplied oxygen is consumed by the oxidation reaction with Fe represented by the following reaction formula (5). Therefore, in the final stage of blowing, a competing reaction between the decarburization reaction and the Fe oxidation reaction occurs. Therefore, the following reaction formula (4)
It can be said that the rate-determining step of the decarburization reaction shown in (1) is the supply of C to the decarburization reaction site such as the fire point where oxygen collides with the steel bath.

C + 1/2 O ₂ (g) = CO (g) (4) Fe (l) + 1/2 O ₂ (g) = FeO (5) And the first method (bottom) According to the strong stirring with a blowing gas), C is efficiently supplied to the decarburization reaction site to improve the efficiency of the decarburization reaction, that is, the reaction of the above formula (4) is promoted to promote the above reaction formula (5). Fe oxidation reaction can be suppressed, and (T.Fe) can be lowered.
For example, the 100th and 101st Nishiyama Memorial Technical Lectures (1984, 176th)
P.) And "Iron and Steel" (No. 76 (1990) No. 11, p. 1793), it is shown that (T.Fe) is lowered by using such a technique.

The second method (reduction of the upper blowing oxygen flow rate at the final stage of blowing) reduces the supply amount of oxygen used in the reaction equations (4) and (5), resulting in (T.Fe) is lowered by decreasing the amount of FeO generated as
(1984) S248) shows that (T.Fe) could be reduced by lowering the oxygen flow rate at the end of blowing in a 300-ton converter.

[0014]

Although the first method (strong stirring by bottom blowing gas) is effective from the viewpoint of (T.Fe) reduction in slag, only the bottom blowing stirring (T. In order to reduce Fe) to the desired concentration, it is necessary to supply a large amount of bottom blowing gas of about 0.3 Nm ³ / T · min or more, but in the region where the bottom blowing gas flow rate is high, (T. Since the Fe) reduction effect becomes smaller, it is necessary to further increase the gas flow rate. Therefore, such a technique has a problem that the cost of bottom blowing gas and bottom blowing equipment increases.

Further, in the case of performing the blowing of the dephosphorized pig iron (dephosphorized pig iron) in the hot metal stage, the amount of slag generated is smaller than that of the blowing of the pig iron not subjected to the dephosphorizing treatment. If the top blowing oxygen flow rate is the same, instead (TF
e) tends to increase. Therefore, it is difficult to reduce (T.Fe) to a desired concentration only by improving the bottom blowing technique.

The second method (reduction of the upper blowing oxygen flow rate in the final stage of blowing) has the effect of reducing the amount of FeO generated as described above, but on the other hand, it prolongs the blowing time. The problem of impeding productivity in the converter is inevitable.

The present invention has been made in view of the above circumstances, and an object thereof is (T) at the end of blowing without increasing the bottom blowing gas flow rate and reducing the top blowing oxygen flow rate. It is to provide a useful blowing method capable of lowering .Fe).

[0018]

According to the converter blowing method of the present invention, the amount of slag in the converter at the end of blowing and the MgO concentration in the slag satisfy the following formula (1) during blowing. The point is to do so.

3.0 ≦ 0.0045 (W _SLAG −200) [(MgO) −21] ≦ 8.0 (1) [In the formula, W _SLAG is the amount of slag in the converter at the end of blowing (kg /
t), and (MgO) indicates the MgO concentration (mass%) in the converter slag at the end of blowing.] The present invention also relates to the amount of slag in the converter at the end of blowing and the slag during blowing. It also defines a converter blowing method having a gist in that the MgO concentration in the inside and the blowing end temperature satisfy the following formula (2).

−64.0 ≦ 0.0045 (W _SLAG −200) [(MgO) −21] −0.040T _TD ≦ −59.0 (2) [where W _SLAG is the amount of slag in the converter at the end of blowing (kg /
t), (MgO) indicates the MgO concentration (mass%) in the converter slag at the end of blowing, and T _TD (Temperatureat Tur)
n Down) indicates the blowing end temperature (° C.)] The blowing end temperature means the temperature of the molten steel at a depth of about 500 mm from the stationary steel bath surface at the end of blowing.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION Under the circumstances as described above, the inventors of the present invention firstly investigated the appropriate range of (T.Fe) at the end of blowing and ensured the dephosphorization ability and the influence on quality and cost. From the viewpoint of
It has been found that it is preferable to set it within the range of 5% by mass. (T.
The specific reason for defining the lower limit of Fe) is as follows. That is, in the recent hot metal technology, the hot metal dephosphorization treatment for removing phosphorus in the hot metal is widely used, and the dephosphorization operation load in the converter is considerably reduced. In this way, when the phosphorus content in the hot metal is low and the (T.Fe) after the end of blowing is too low, the P ₂ O ₅ in the slag for the entire charge adhering to the refractory in the converter is Reduced and phosphorus concentration in hot metal (hot metal phosphorus concentration)
A so-called re-phosphorus phenomenon occurs in which the phosphorus concentration after the end of the blowing (the blowing phosphorus concentration) becomes higher than that. Figure 1
Is the relationship between (T.Fe) at the end of blowing and the dephosphorization rate. In the experiment, oxygen blowing is performed on the blast furnace hot metal after desiliconization and dephosphorization in a 240 ton converter. Was done,
(T.Fe) is the slag floating on the molten steel in the furnace at the time of tapping, collected after cooling, measured by fluorescent X-ray analysis, and measured by phosphorus X-ray analysis before the start of blowing and after the completion of blowing. The phosphorus concentration in the molten steel was measured by a vacuum photoelectric photometric emission spectrophotometer (Cantovac).

As shown in FIG. 1, (T.Fe) is 10% by mass.
When the amount is less than the above value, a re-phosphorization phenomenon occurs, and therefore blowing again for dephosphorization is required. On the other hand, the upper limit of (T.Fe) is 1
5% by mass means that the dephosphorization rate is almost constant even if (T.Fe) exceeds 15% by mass as shown in Fig. 1, and as described above, the quality increases as (T.Fe) increases. This is because the adverse effect on the surface and cost will be great.

The inventors of the present invention have thus described (T.
Various experiments were repeated for a specific control method so that Fe) was within the range of 10 to 15 mass%. As a result, we found that it is very effective to control the amount of slag at the end of blowing and the MgO concentration in the slag at the end of blowing to satisfy a certain relationship. The details will be described below.

The (T.Fe) at the end of the blowing has a correlation with the MgO concentration in the slag at the end of the blowing [hereinafter referred to as (MgO)], and this (T.Fe) and (MgO) It was found that, when the correlation of the above was expressed as a linear expression, the gradient was expressed as a function of the slag amount at the end of blowing. In FIG. 2, various kinds of hot metal having different amounts of slag and the like are blown, and at the end of blowing (blowing end temperature:
The relationship between (T.Fe) and (MgO) at 1640 to 1660 ° C) is shown for each slag amount. In the experiment, blast furnace hot metal after desiliconization and dephosphorization treatment was oxygen-blown in a 240-ton converter, and (T.Fe) and (MgO) were collected from the molten steel in the furnace after the completion of blowing. The slag was subjected to fluorescent X-ray analysis and measured. The amount of slag was calculated by the same method as (T.Fe) and (MgO) above (CaO), and quick lime was inserted into the furnace before the start of blowing, during blowing and after the completion of blowing before tapping. Calculated from CaO content of light burned dolomite, etc.

When the relationship in FIG. 2 is formulated as a linear expression, (T.Fe) at the end of blowing is about 1650 ° C. which is the end temperature of a general blowing operation, and the following expression (6) is given.
It is expressed as a function of (MgO) and slag amount (W _SLAG ).

(T.Fe) = 0.0045 (W _SLAG −200) [(MgO) −21] +7.1 (6) [where W _SLAG is the amount of slag in the converter at the end of blowing (kg /
t), and (MgO) indicates the MgO concentration (mass%) in the converter slag at the end of blowing.] Therefore, at the end of blowing (blowing end temperature: about 1650 ° C) (T.Fe ) Within the range of 10 to 15% by mass,
It is necessary to control so that (MgO) at the end of blowing and the amount of slag satisfy the following formula (1) derived from the above formula (6).

3.0 ≦ 0.0045 (W _SLAG −200) [(MgO) −21] ≦ 8.0 (1) [where W _SLAG is the amount of slag in the converter at the end of blowing (kg /
t), and (MgO) indicates the MgO concentration (mass%) in the converter slag at the end of blowing.] FIG. 3 shows the slag amount (W _SLAG ) at the end of _blowing on the horizontal axis, (Mg
The range of the above formula (1) is shown on the vertical axis of (O).
At a blowing end temperature of about 1650 ° C., if the relationship between the amount of slag and (MgO) is within the range shown in FIG. 3, (T.Fe) can be within the proper range.

Considering the equation (6), the amount of slag is generally about 20 to 100 kg / t. Therefore, if the equation (6) is considered as a linear equation with (MgO) as a variable, its gradient is negative. Indicates a value. That is, from the above formula (6), (MgO)
It can be read that (T.Fe) can be reduced by increasing. The mechanism that causes such a tendency can be explained as follows.

As described above, since a large amount of C is present in the hot metal from the early stage to the middle stage of blowing, the decarburization reaction represented by the above reaction formula (4) becomes the main reaction. However, since C in the hot metal decreases at the end of blowing, the decarburization reaction of the reaction formula (4) is suppressed, and the supplied oxygen is consumed in the Fe oxidation reaction of the reaction formula (5). (T.Fe) is determined by the balance between the oxide formation reaction shown in the reaction formula (5) and the reduction reaction shown in the reaction formula (7) below. (T.Fe) increases because it becomes more dominant than the reduction reaction shown in equation (7).

FeO + C → Fe + CO (g) (7) Under such circumstances, a typical blowing end temperature is 1650.
At around ℃, when MgO is present in the slag with a saturated solubility of more than about 7%, the slag is in a solid-liquid coexisting state because MgO that is not completely dissolved and remains solid is present in the slag.
The viscosity of slag increases with the increase of (MgO). When the viscosity of the slag increases, the stirring and mixing of FeO and slag generated in the reaction formula (5) become insufficient, and a region of high (FeO) locally occurs near the slag-molten steel interface. Then, since the reduction reaction of FeO is promoted near the slag-molten steel interface, FeO in the slag is reduced and (T.Fe) becomes low.

Next, the equation (1) will be considered. When the above equation (1) is considered as a linear equation of (MgO), the term indicating the gradient is a function of the slag amount. When (MgO) is constant and the slag amount (W _SLAG ) changes in the range of 20 to 100 kg / t, (T.Fe) decreases as the slag amount increases.
Moreover, the (T.Fe) reduction effect of MgO is affected by the amount of slag, and if the amount of slag increases (MgO) increases (TF
e) It was found that the reduction effect was small. It is considered that these phenomena are due to the following mechanism.

In the FeO production reaction shown in the above reaction formula (5), the oxygen supply is rate-determining, so if the oxygen supply amount is the same,
Even if the slag amount is different, the FeO production amount is the same.
(T.Fe) represents the ratio of the amount of Fe in FeO to the amount of slag. Therefore, when the amount of FeO produced is the same (the amount of Fe is almost the same), the amount of slag increases (T.Fe). The value of .Fe) decreases.

When the amount of slag increases, the effect of reducing (T.Fe) is small even if the amount of (MgO) increases. The reason is that even if FeO is generated when the amount of slag increases, the FeO is diluted by the slag. It is considered that this is because the amount of FeO in the vicinity of the molten steel interface is small and therefore the reduction rate of FeO is small.

In the above, control of (T.Fe) at about 1650 ° C., which is the blowing end temperature in a general blowing operation, has been examined, but the blowing end temperature is the target steel grade. It varies somewhat depending on the composition of No. 1, and changes in the range of approximately 1610 ° C to 1700 ° C depending on the solidification temperature of each steel type.

When the blowing end temperature changes in this way, FIG.
As shown in, the MgO saturation solubility in the slag changes. Therefore, even if the amount of MgO in the slag is constant, the solid fraction of the slag changes when the blowing end temperature changes, and the diffusion rate of FeO generated in the reaction formula (5) into the slag changes. Of. Blowing end temperature is high, therefore MgO in slag
When the saturated solubility is high, the solid phase ratio of slag is low, so that FeO easily diffuses into the slag. On the other hand, when the blowing end temperature is low, since the MgO saturated solubility in the slag is low and the solid fraction of the slag is high, the FeO hardly diffuses into the slag and remains near the slag-molten steel interface, and the reaction formula The reduction reaction shown in (7) occurs and (T.Fe) is reduced.

As described above, since the (T.Fe) at the end of blowing varies depending on the end temperature of blowing, even if the slag amount and (MgO) at the end of blowing are constant, the present inventors Is (T.
In order to control Fe) with higher accuracy, it was found that it is effective to introduce the term of the blowing end temperature into the formula (6) as shown in the following formula (9).

(T.Fe) = 0.0045 (W _SLAG −200) [(MgO) −21] −0.040T _TD +74 (8) [wherein W _SLAG is the amount of slag in the converter at the end of blowing ( kg /
t), (MgO) indicates the MgO concentration (mass%) in the slag in the converter at the end of blowing, and T _TD indicates the blowing end temperature (° C.)] 10 to 15 (T.Fe) at the end of smelting
The following formula (2) can be derived as a condition for controlling the mass%.

−64.0 ≦ 0.0045 (W _SLAG −200) [(MgO) −21] −0.040T _TD ≦ −59.0 (2) [where W _SLAG is the amount of slag in the converter at the end of blowing (kg /
t), and (MgO) is M in the slag in the converter at the end of blowing.
gO concentration (mass%) is shown, and T _TD shows the blowing end temperature (° C)] FIG. 5 and FIG. 6 are, for example, when the blowing end temperature is 1620 ° C. and 1680 ° C. Slag amount (W _SLAG ) on the horizontal axis and (MgO) on the vertical axis in the above formula (2)
At the blowing end temperatures of 1620 ° C. and 1680 ° C., if the relationship between the amount of slag and (MgO) satisfies the regions shown in FIG. 5 or 6, respectively (T.Fe) It can be set within the range of 10 to 15% by mass.

The present invention does not specify a specific operating method for satisfying the expression (1) or the expression (2), and various operating methods can be adopted, but the target value (MgO) is used. Set the blowing end temperature (T _TD ) and set the slag amount (W
_SLAG ) is within the range of formula (1) or formula (2),
Amount of quicklime added, amount of lightly burnt dolomite and Fe for heating
A method of calculating the amount of Si and inserting it is an operation method.

[0040]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited to the following examples, and may be appropriately applied within a range compatible with the gist of the preceding and the following. Modifications can be made and implemented, and all of them are included in the technical scope of the present invention.

Example 1 A blowing experiment was conducted in a 240 ton converter using blast furnace hot metal after desiliconization and dephosphorization at a blowing end temperature of about 1650 ° C. (T.Fe) and (MgO) were measured by subjecting the slag collected from the molten steel in the furnace after the completion of blowing to fluorescent X-ray analysis. The amount of slag (W _SLAG ) at the end of _blowing is the above (T.Fe)
And (MgO) were calculated by the same method as (CaO) and CaO contents of quicklime and light-burnt dolomite inserted into the furnace before the start of blowing, during blowing and after the end of blowing before tapping.
Further, the blowing end temperature (T _TD ), that is, the temperature of the molten steel at a depth of about 500 mm from the stationary steel bath surface at the end of blowing is the sublance equipped with a probe incorporating a thermocouple for temperature measurement at the tip. Was immersed in molten steel for measurement.

The amount of slag (W _SLAG ) at the end of the _blowing , (M
The results of measuring gO) and (T.Fe) and the blowing end temperature (T _TD ) are shown in Tables 1 and 2.

[0043]

[Table 1]

[0044]

[Table 2]

From Tables 1 and 2, Table 1 in which the slag amounts (W _SLAG ) and (MgO) were adjusted so as to satisfy the above formula (1)
No. In No. 1 to No. 15 of Table 2, (T.Fe) could be controlled within the range of 10 to 15% by mass, while the slag amount (W _SLAG ) and (MgO) were not controlled.
It can be seen that in all of 1 to 13, (T.Fe) is out of the range of 10 to 15% by mass.

[0046]Example 2 Uses blast furnace hot metal after desiliconization and dephosphorization in a 240 ton converter
The blowing end temperature (T _TD) As shown in Tables 3 and 4
The blowing experiment was performed by changing the temperature between 1610 and 1700 ℃.
It was.

The amount of slag at the end of _blowing (W _SLAG ), (M
gO) and (T.Fe) and the blowing end temperature (T _TD ) were measured in the same manner as in Example 1 above.

[0048]

[Table 3]

[0049]

[Table 4]

From Tables 3 and 4, the slag amounts (W _SLAG ), (MgO) and the blowing end temperature were adjusted so as to satisfy the above expression (2). In 1 to 15, (T.Fe) is 10
The amount of slag (W _SLAG ) and (MgO) were not controlled, while the amount could be within the range of 15 mass% to 15% by mass. It can be seen that in all of 1 to 15, (T.Fe) is out of the range of 10 to 15 mass%.

[0051]

EFFECTS OF THE INVENTION According to the present invention, (T.Fe) can be accurately lowered to a low level without performing strong agitation by bottom blowing gas or reducing the amount of top blowing oxygen at the end of blowing in blowing as in the prior art. It has become possible to control the amount of iron-based oxides in the slag to the extent that it does not adversely affect the quality and cost of the steel material while maintaining the dephosphorization ability.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between (T.Fe) at the end of blowing and the dephosphorization rate.

FIG. 2 is a graph showing the relationship between (MgO) and (T.Fe) at the end of blowing by slag amount.

FIG. 3 is a graph exemplifying the specified range of the present invention in the formula (1).

FIG. 4 is a graph showing the relationship between the blowing end temperature (T _TD ) and the MgO saturated solubility in slag.

FIG. 5 is a graph exemplifying the specified range of the present invention in the formula (2) when the blowing end temperature is 1620 ° C.

FIG. 6 is a graph exemplifying the specified range of the present invention in the formula (2) when the blowing end temperature is 1680 ° C.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Takashi Fujita             1 Kanazawa Town, Kakogawa City, Hyogo Prefecture             To Steel Works, Kakogawa Works (72) Inventor Takeshi Inaba             1 Kanazawa Town, Kakogawa City, Hyogo Prefecture             To Steel Works, Kakogawa Works F-term (reference) 4K070 AB06 AB13 AB18 AC03 BC01                       BE04 BE05 EA02 EA06 EA07

Claims (2)

[Claims] 1. A blowing method for a converter, wherein the amount of slag in the converter at the end of the blowing and the MgO concentration in the slag satisfy the following formula (1) during the blowing. 3.0 ≦ 0.0045 (W _SLAG −200) [(MgO) −21] ≦ 8.0 (1) [In the formula, W _SLAG is the amount of slag in the converter at the end of blowing (kg /
t), and (MgO) indicates the MgO concentration (mass%) in the converter slag at the end of blowing.] 2. A converter blower characterized in that upon blowing, the amount of slag in the converter at the end of blowing, the MgO concentration in the slag, and the blowing end temperature satisfy the following formula (2). Ren method. −64.0 ≦ 0.0045 (W _SLAG −200) [(MgO) −21] −0.040T _TD ≦ −59.0 (2) [W _SLAG is the amount of slag in the converter at the end of blowing (kg /
t), (MgO) indicates the MgO concentration (mass%) in the converter slag at the end of blowing, and T _TD indicates the blowing end temperature (° C)]