JPH08218111A - Method for reducing slag inclusion in molten steel - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a method for reducing slag inclusion in molten steel. [Structure] When conducting gas agitation of molten steel under atmospheric pressure or reduced pressure, the gas agitation phase is divided into the first phase and the second phase, the following (1) in the first phase and the following (in the second phase: A slag entrainment reduction method that satisfies the condition 2). (1) The melting point of the molten steel upper surface slag is set to 1600 ° C or lower, and the entire slag is liquefied. (2) The melting point of the molten steel upper surface slag is set to 1700 ° C. or higher to solidify the upper layer part of the slag. [Effect] It is possible to reduce the generation of inclusions due to slag inclusion and reduce the total amount of inclusions in the product.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of reducing the amount of slag formed on the upper surface of molten steel in molten steel in order to reduce the amount of non-metallic inclusions in the molten steel.

[0002]

2. Description of the Related Art In recent years, demands for improving the quality of steel products have become strict in all fields. Particularly for steel products for line pipes, the demand for higher grades becomes even stronger as the environment in which they are used becomes more severe, and in response to this, extremely low sulfurization such as S ≦ 5 ppm, reduction of non-metallic inclusions and control of their morphology. The need is increasing.

[0003] As a general means for upgrading steel materials, it is necessary to remove slag completely when tapping the steel from the furnace, and to add synthetic slag to control the properties of the slag in the ladle to promote desulfurization. It is common to reduce inclusions. Further, as disclosed in Japanese Patent Publication No. 59-22765, C
In order to reduce the [S] (S in molten steel) before adding a alloy,
There is a method in which a CaO-based flux is added to molten steel to perform desulfurization treatment in advance, and thereafter, inclusions generated by the desulfurization treatment are spheroidized with a Ca alloy.

Japanese Unexamined Patent Publication No. 63-7318 discloses that Al ₂ O by Ca is prevented by preventing unnecessary consumption of Ca by slag.
A method of lowering (FeO + MnO) and SiO ₂ in the slag is shown in order to reproducibly control the morphology of ₃ type inclusions.

As a cleaning measure for ultra-low carbon steel, Japanese Unexamined Patent Publication No. 6-49524 discloses that steel is undeoxidized from a converter, CaO is added, and then metal Al is used as a slag deoxidizer. The ratio of CaO / Al ₂ O ₃ in the slag added is 1.3 to 1.5, and the ratio is 0.6 to 0.
8, the slag after vacuum degassing was solidified, and the T. A method of reducing Fe and suppressing outflow of slag in the ladle into the continuous casting tundish is shown.

In particular, in the case of producing steel to which metallic Ca is added, as a treatment before Ca addition, for example, low sulfurization is performed by Ar gas blowing or CaO-based flux treatment after Ar gas blowing, and CaS generation is controlled by controlling Ca addition amount. Can be reduced or suppressed. In addition, by stirring the slag existing on the upper surface of the molten steel by blowing Ar gas to reduce the concentration of, for example, FeO, MnO, SiO ₂ , Cr ₂ O ₃ (hereinafter referred to as lower oxide) in the slag, Ca T. in steel before alloy injection. It is becoming possible to reduce the O (oxygen) concentration.

[0007]

As described above, various methods have been proposed for the low sulfurization of steel, the morphology control of inclusions, the reduction of the amount of inclusions, etc., but these conventional techniques have been further enhanced in recent years. It is still difficult to deal with high grade steel.

[0008] That is, in the case of producing steel containing metal Ca, it is possible to significantly reduce non-metallic inclusions by lowering the concentration of lower oxides in the slag existing on the upper surface of molten steel, controlling desulfurization and the amount of Ca added. Became. However, due to the improvement of the cleanliness level, inclusion formation due to slag inclusion, which has not been a problem in the past, has become a big problem.

[0009] Generally, HI resistance is represented by a line pipe.
For steel materials that require C property, MnS, CaS, Al ₂ O
₃ , CaO, SiO ₂ and composite inclusions thereof (for example, CaO—Al ₂ O ₃ inclusions, CaO—Al ₂ O ₃
It is necessary to control the amount of (CaS-based inclusions) to a very low concentration. Further, in steel materials such as wheel disk materials that require deep drawability, non-metallic inclusions such as MnS and Al ₂ O ₃ pose a problem, so low sulphurization and low oxygenation are also required.

In the ordinary steelmaking process, molten steel is tapped from a converter or an electric furnace into a ladle, and CaO or Al is added to the molten steel to generate slag. The main components of the slag in this case are CaO and Al ₂ O ₃ , and FeO, Mn
Also included are lower oxides that are easily reduced by Al such as O or SiO ₂ . In some cases, oxygen may be blown into the molten steel to raise the temperature after the tapping, and a low-grade oxide is generated also here.

Since these lower oxides in the slag cause generation of non-metallic inclusions in the steel, it is necessary to reduce the concentration of the lower oxides in the slag as early as possible. further,
In order to prevent the formation of MnS and CaS, it is necessary to lower the [S] concentration in the molten steel.

In order to reduce the concentrations of the lower oxide in the slag and the [S] in the steel, it is desirable to liquefy the slag existing on the upper surface of the molten steel and stir the molten steel and the slag with an inert gas such as Ar. . That is, by blowing an inert gas such as Ar into the molten steel in a state where the slag on the molten steel upper surface is liquefied, the lowering of the concentration of the lower oxide and the desulfurization proceed rapidly.

Up to now, the low sulfurization as described above has been attempted,
It also reduces the concentration of lower oxides in the molten steel upper surface slag,
Moreover, it should have suppressed molten steel oxidation. However, as user needs become more severe, further purification of molten steel is required. In order to meet this demand, it has become necessary to suppress inclusion formation due to slag inclusion.

The amount of inclusions formed in steel depends mainly on the following.

Inclusion Formation by Reoxidation from Slag Inclusion Formation by Slag Entrainment That is, the slag is stirred by Ar blowing, etc., and the concentration of lower oxides in the molten steel upper surface slag decreases as the gas stirring time elapses. The production amount of is greatly reduced.
The above-mentioned production amount increases with the progress of liquefaction of slag. This phenomenon will be described with reference to FIG.

FIG. 5 is a diagram showing the influence of the gas stirring treatment time on the change in the slag entrainment amount (index). As shown in the figure, the slag entrainment is very small in the initial stage of gas stirring, but the slag becomes liquid as the treatment progresses, and the slag entrainment amount also increases.

Conventionally, the amount of non-metallic inclusions generated by the above-mentioned slag inclusion is negligible. But,
By reducing the amount of lower oxides in the slag and strengthening desulfurization, the total amount of inclusions decreases, and the inclusion formation due to this slag inclusion cannot be ignored.

An object of the present invention is to provide a method capable of reducing the entrainment of slag formed on the upper surface of molten steel into molten steel in order to reduce the total amount of non-metallic inclusions in view of the actual situation. Especially.

[0019]

The gist of the present invention resides in the following slag entrainment reduction method.

When performing gas stirring of molten steel under atmospheric pressure or reduced pressure, the gas stirring period is divided into a first period and a second period, the following (1) in the first period and the following in the second period: A method for reducing slag inclusion in molten steel, which satisfies the condition (2).

(1) The melting point of the slag formed on the upper surface of the molten steel is set to 1600 ° C. or lower to liquefy the entire slag.

(2) The melting point of the slag formed on the upper surface of the molten steel is set to 1700 ° C. or higher to solidify the upper part of the slag.

The above-mentioned "slag upper layer portion" means up to about 50% of the slag thickness.

The object to be treated in the above-mentioned method of the present invention is molten steel melted in a converter, an electric furnace, etc., and is a steel material such as a high-grade line pipe or a wheel disk material that requires high cleaning by low sulfurization and low oxygen. For manufacturing. Furthermore, all steel grades in which slag inclusion becomes a problem due to Ar blowing are targeted.

[0025]

A device for realizing the above-mentioned method of the present invention is a degassing device (a ladle degassing device) using a container such as a ladle having a normal gas stirring function, or an ordinary vacuum tank in which these containers are combined. A gas device, a VOD furnace, a vacuum degassing device equipped with a large-diameter cylindrical immersion pipe, etc.). The atmosphere during the gas stirring period may be either atmospheric pressure or reduced pressure.

First, in order to lower the melting point of the entire slag to 1600 ° C. or less, in consideration of the amounts of CaO for desulfurization and Al ₂ O ₃ generated by the oxidation temperature increase of Al, etc., before starting stirring. Adjust so that the slag composition has appropriate properties.

When the melting point of the slag is raised to 1700 ° C. or higher and the upper layer of the slag is solidified, ZrO ₂ , ZrO _2-
Oxides such as SiO ₂ (zircon), MgO, CaO, and Al ₂ O ₃ that increase the melting point of slag (hereinafter referred to as a high melting point adjusting agent) are used. However, oxides that are easily reduced by Al cause reoxidation of molten steel and should be avoided. The addition method may be powder blowing or addition on the slag.

Next, an example of a specific procedure for carrying out the method of the present invention under atmospheric pressure will be described.

Molten steel is tapped in a ladle, and in the first phase of gas stirring, CaO is added over the slag and Al and oxygen are added for the purpose of raising the temperature at RH. Here, Al ₂ O ₃ is further generated as slag, but the slag properties are adjusted so that the melting point of the entire slag at this time is 1600 ° C. or less. Next, Ar gas is supplied through the upper blowing lance soaked in the molten steel to stir the molten steel to reduce lower oxides in the slag and desulfurize the molten steel at the same time. If the melting point of the entire slag at this time exceeds 1600 °, the above effect is impaired.

In the subsequent second phase of gas stirring, ZrO ₂ , which is a high melting point adjusting agent, is added to the upper surface of the slag, and the slag properties are adjusted so that the slag melting point at this point becomes 1700 ° C. or higher. The addition of Ar gas is continued, but at this time, it is desirable that the Ar gas flow rate be decreased with time. As a result, the upper layer portion of the slag (up to about 50% of the slag thickness) is solidified. As a result, the amount of slag inclusion is significantly reduced. This effect becomes even greater when the Ar gas flow rate is reduced. Under the condition that the melting point of the entire slag is less than 1700 ° C. and the thickness of the solidified slag layer exceeds about 50%, the above effect cannot be sufficiently obtained.

If it is further necessary after the end of the second treatment, Ca is added to the molten steel and Ar gas stirring is continued.

The method of the present invention will be described with reference to FIG. FIG. 1 is a diagram showing the influence of the gas agitation treatment time on the change in the slag entrainment amount (index), which corresponds to FIG.

In the first stage of gas stirring, as described above,
The [S] concentration and the amount of lower oxides in the slag decrease, but as shown in the figure, the amount of slag entrainment increases contrary to these decreases. The second-stage treatment is started from the time when the amount of entrainment almost reaches the peak. In the second phase of this gas stirring, CaO, ZrO ₂ , MgO, etc. are superposedly added to the slag to raise the slag melting point to 1700 ° or higher and solidify the slag upper layer part, so that the slag inclusion in the molten steel is prevented. Reduce. This reduces the total amount of non-metallic inclusions in the molten steel.

What is important in the method of the present invention is to divide the gas stirring period into the first period and the second period as described above. In the first phase, the entire slag is liquefied to facilitate the reaction between the slag and the molten steel. However, in the second period, in order to suppress the increase in the amount of slag entrainment, the slag property is controlled to have a high melting point, and contrary to the first period, it is difficult for the slag and molten steel to react.

As described above, in the first period in which the reaction between the slag and the molten steel is required, such as the reduction of lower oxides in the slag and the desulfurization, the entire slag is liquefied to accelerate the reaction, and the slag inclusion causes the intervention. In the second stage, when the production of matter becomes a problem, the upper layer of slag is solidified and the amount of inclusions in the molten steel is reduced.

When the entire slag is liquefied and refining is performed by gas stirring as in the first period, the slag entrainment is related to the thickness of the molten portion of the slag and the kinematic viscosity. As a method of reducing the amount of slag entrained in the second phase, it is effective to reduce the thickness of the molten slag or increase the kinematic viscosity of the slag. Therefore, as described above, by solidifying the upper layer portion of the slag and reducing the amount of Ar gas blown therein, it is possible to further reduce the amount of slag entrained.

For steel materials such as line pipe materials where the presence of MnS is extremely disliked, Ca will be added after the second phase. Need not be added. When Ca is added, it is better to stir the gas with Ar or the like mildly, and the ideal stirring gas amount is slightly less than the critical amount at which slag entrainment occurs.

The desirable conditions in the above method of the present invention are as follows.

The slag composition in the first phase of gas stirring may be any composition as long as the entire slag becomes liquid by stirring the slag. However, if molten steel desulfurization is required, it is necessary to select a slag composition that can be desulfurized. In general, CaO
-Al ₂ O ₃ -SiO ₂ composition, CaO weight% /
The ratio of Al ₂ O ₃ weight% is 0.8 to 2.0, and SiO ₂ is 0.
It is desirable that the content be ˜30% by weight.

In the slag composition in the second stage of gas stirring, for example, in the case of the above CaO-Al ₂ O ₃ -SiO ₂ slag, it is preferable to increase ZrO ₂ to about 10% by weight.

The desired range of the flow rate of stirring gas in the case of atmospheric pressure, 1 to 10 nm ³ / min about the first stage, a 0.05~4Nm ³ / min approximately in the second phase. If under reduced pressure, respectively, 0.5 to 10 nm ³ / min approximately, 0.05~4Nm ³ /
It's about a minute.

The desirable range of the gas stirring period is about 5 to 15 minutes in the first period and 2 to 15 in the second period under atmospheric pressure.
It's about a minute. In the case of reduced pressure, it is about 3 to 15 minutes and about 2 to 15 minutes, respectively.

The desirable pressure range when performing under reduced pressure is
It is about 1 to 200 Torr.

[0044]

【Example】

(Test 1) In the present invention example, 1 ton of CaO was added to the upper surface of the molten steel in the ladle containing the molten steel having the chemical composition shown in Table 1 (250 tons, initial [S] was 20 ppm), and Al was added by the RH apparatus. oxygen was added and the temperature was raised to 1650 ℃, Al ₂ O ₃
About 1 ton was produced.

[0045]

[Table 1]

Next, as the first treatment, 4 Nm ^{3 of} Ar gas was passed through the top blowing lance immersed in the molten steel under atmospheric pressure.
The slag was analyzed after liquefaction of the entire slag, reduction of lower oxides in the slag, and molten steel desulfurization were carried out at the same time for 10 minutes. Table 2 shows the results of this analysis.

[0047]

[Table 2]

As shown in Table 2, the slag composition was almost uniform at this point and the melting point was 1500 ° C. or lower.

Then, as the second treatment, Z is applied to the upper surface of the slag.
2 tons of rO ₂ was added, and Ar gas was supplied for 2 minutes while reducing the flow rate thereof to 0.5 Nm ³ / minute. As a result, the upper layer of the slag was solidified. Then, C in the molten steel
a was added, and Ar gas was supplied at 0.5 Nm ³ / min or less in order to make the Ca concentration uniform.

As another example of the present invention, a test in which the first stage treatment is as described above, and the same amounts of MgO, CaO and Al ₂ O ₃ are added instead of ZrO ₂ in the second stage treatment, respectively. Was carried out. As a comparative example, the same treatment as above was carried out until the first stage treatment, and in the second stage treatment, no high melting point adjusting agent was added.

A sample was taken from a product manufactured by using the molten steel obtained as described above, and the total oxygen concentration in the product was analyzed. The result is shown in FIG.

FIG. 2 is a diagram showing the total oxygen concentration in the product compared by an index with 100 being the case of the comparative example. As shown in FIG. 2, in all the examples of the present invention, the total oxygen concentration index was low, and the case where ZrO ₂ was added was the lowest.

(Test 2) As an example of the present invention, molten steel having the chemical composition shown in Table 1 (250 tons, initial [S] is 20 ppm)
Was used in the same manner as in Test 1 to carry out the first period of gas stirring. However, Ar gas was supplied at a rate of ² Nm ³ / min for 10 minutes through a top blowing lance immersed in molten steel. The slag composition at this time was the same as in Table 2.

Subsequently, in the second treatment, 1.5 tons of zircon (ZrO ₂ --SiO ₂ ) was added to the surface of the slag, and Ar was added.
The gas was fed for 10 minutes, reducing its flow rate to 1 Nm ³ / min. As a result, the upper layer of the slag was solidified. Then, Ca was added to the molten steel, and Ar gas was supplied at 0.5 Nm ³ / min or less in order to make the Ca concentration uniform.

Molten steel was sampled throughout the gas stirring period, and the total oxygen content at the molten steel stage was analyzed. After that, the total oxygen concentration in the product was analyzed as in Test 1.
The results are shown in FIGS. 3 and 4.

FIG. 3 is a graph showing changes in the total oxygen concentration in the molten steel depending on the Ar gas blowing time.

FIG. 4 is a graph showing the total oxygen concentration in the product compared by an index with the case of the comparative example of Test 1 being 100.
As shown in FIGS. 3 and 4, the total oxygen concentration index was lower in the inventive examples.

[0058]

According to the method of the present invention, the generation of inclusions due to the inclusion of slag can be reduced, and the total amount of inclusions in the product can be reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing an influence of a gas stirring treatment time on a change in a slag entrainment amount (index) in the case of the method of the present invention.

FIG. 2 is a diagram showing the total oxygen concentration in a product as an index with 100 in the case of the comparative example.

FIG. 3 is a diagram showing changes in the total oxygen concentration in molten steel depending on the Ar gas blowing time.

FIG. 4 is a diagram showing the total oxygen concentration in a product as an index with 100 in the case of the comparative example.

FIG. 5: Slag inclusion amount (index) in the case of the conventional method
It is a figure which shows the influence of the gas stirring process time which acts on the change of.

Claims (1)

[Claims] 1. When performing gas stirring of molten steel under atmospheric pressure or reduced pressure, the gas stirring period is divided into a first period and a second period, and in the first period, the following (1) and second periods Then, a method of reducing slag entrainment in molten steel, characterized by satisfying the following condition (2). (1) The melting point of the slag formed on the upper surface of the molten steel is set to 1600 ° C or lower, and the entire slag is liquefied. (2) The melting point of the slag formed on the upper surface of the molten steel is set to 1700 ° C. or higher to solidify the upper layer part of the slag.