JP2000144232A - Production of highly cleaned steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a producing method of a highly cleaned steel, in which the reaction between molten steel and slag is prevented by sufficiently reducing the slag and the clean steel having little inclusion is stably produced. SOLUTION: The molten steel 5 obtd. with converter refining, is tapped from the converter 2 to a ladle 3 and slag modifier 7 is added into the slag 6 floated up on the tapped molten steel in undeoxidizing state to reduce the iron oxide in the slag. Successively, the molten steel in the undeoxidizing state is decarburized in an RH vacuum degassing apparatus 4 and after decarburizing, Al is added into the molten steel to execute the deoxidation, and after deoxidizing, the surface position of the molten steel in the ladle is vertically shifted to reduce the iron oxide in the slag. At this time, after deoxidizing, the slag modifier is desirable to supplement into the slag on the molten steel in the ladle.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for stably producing a clean steel having a small amount of oxide-based nonmetallic inclusions.

[0002]

2. Description of the Related Art In recent years, demands for higher functionality and higher quality of steel materials have increased, and oxidation elements originating from impurity elements such as phosphorus and sulfur, deoxidation products, converter slag, mold powder, and the like have been developed. Nonmetallic inclusions (hereinafter referred to as "inclusions")
It is desired to reduce as much as possible. Of these, inclusions are the main cause of surface defects in thin steel products, so from the refining to casting, the causes of their occurrence were examined, and
As one of the main causes of inclusions, reoxidation of molten steel by lower oxides such as iron oxide and MnO in slag in a ladle has led to the production of clean steel with few inclusions. There have been proposed many methods of adding a deoxidizing agent to slag to reduce these lower oxides and prevent reoxidation of molten steel.

[0003] For example, Japanese Patent Application Laid-Open No. 4-72009 discloses that Al slag and quick lime are added to slag floating on undeoxidized molten steel discharged from a converter to reduce lower oxides in the slag. While reducing, the basicity of the slag (CaO / SiO
₂ ) is adjusted to a range convenient for absorption of inclusions and then deoxidized in a vacuum degassing facility to produce clean steel. JP-A-2-93017 discloses the method. ,
After decarburizing the molten steel in the RH vacuum degassing device, Al or Al and quick lime are added to the slag in the ladle to reduce FeO in the slag, and then deoxidize the molten steel to obtain a highly clean ultra-low. A method for producing carbon steel is disclosed.

[0004]

However, in the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 4-72009, the reduced slag is reused by the dissolved oxygen in the molten steel until the molten steel is deoxidized. It is oxidized and produces lower oxides such as FeO in the slag again. Therefore, after deoxidizing the molten steel, the slag reacts with Al in the molten steel to generate inclusions in the molten steel, and there is a problem that sufficient cleanliness cannot be obtained.

In the method disclosed in JP-A-2-93017, slag and molten steel in a ladle cannot be stirred in an RH vacuum degassing apparatus. The deoxidizer added above has a problem that slag cannot be sufficiently reduced. in this way,
In the prior art, slag reduction cannot be said to be sufficient yet, and it has not yet been possible to stably produce highly clean steel.

The present invention has been made in view of such circumstances, and has as its object to reduce the slag sufficiently to prevent the reaction between molten steel and slag, and to stably produce clean steel having few inclusions. Is to provide a way to

[0007]

According to a first aspect of the present invention, there is provided a method for producing high-purity steel, comprising: producing molten steel obtained by converter refining from a converter into a ladle; A slag modifier is added to the slag suspended in the slag to reduce iron oxides in the slag, and then the undeoxidized molten steel is decarburized by an RH vacuum degassing device, and after decarburization, the molten steel Deoxidation by adding Al,
After deoxidation, the molten steel in the ladle is moved up and down to reduce the iron oxide in the slag.

[0008] The method for producing high-purity steel according to the second invention is as follows.
The molten steel obtained by the converter refining is tapped from the converter into a ladle, and a slag modifier is added to the slag floating on the undeoxidized molten steel to reduce iron oxides in the slag. , Then R
In a H vacuum degassing device, undeoxidized molten steel is decarburized. After decarburization, deoxidation is performed by adding Al to the molten steel, and a slag modifier is added to the slag on the molten steel in the ladle. The slag reforming is carried out simultaneously or before and after, and thereafter, the molten steel in the ladle is moved up and down to reduce the iron oxide in the slag.

[0009] The method for producing high-purity steel according to the third invention is as follows.
In the first invention or the second invention, the vertical movement of the molten metal surface position is continued for 5 minutes or more.

[0010] According to a fourth aspect of the present invention, there is provided a method for producing high-purity steel.
In any one of the first to third inventions, the reflux Ar gas flow rate after deoxidation with Al is set to 16 Nl / min or less per ton of treated molten steel.

In the present invention, the molten steel refined by the converter is tapped in a ladle in a non-deoxidized state, and a slag modifier is added to the slag in the ladle to remove FeO or Fe ₂ O ₃ in the slag. Reduces iron oxides. By adding a slag modifier, T.C.
[Fe] concentration is reduced to about 2%.

[0012] For steel types that require decarburization treatment under a high vacuum in an RH vacuum degassing apparatus, such as extremely low carbon steel, the molten steel is not deoxidized immediately after tapping, but is not deoxidized. The molten steel in the state is conveyed to an RH vacuum degassing apparatus. In the RH vacuum degassing apparatus, first, decarbonization is performed using undeoxidized molten steel, and after decarburization, Al is added to deoxidize the molten steel. This is because the decarburization reaction is an oxidation reaction and requires dissolved oxygen in the molten steel.

However, until the molten steel is deoxidized by Al, the oxygen potential of the molten steel is higher than the oxygen potential of the slag. Therefore, the slag is oxidized by the dissolved oxygen in the molten steel, and the T.O. [Fe] concentration increases. In addition, since the molten steel has been deoxidized for a long time since tapping, the surface of the slag is solidified, and the reactivity between the molten steel and the slag is extremely poor. Accordingly, when the molten steel is deoxidized while being covered with the slag in this state and the degassing process is completed, T.T. The slag with a high [Fe] concentration is carried over to the casting process, and during casting, Al in the molten steel and iron oxide in the slag gradually react to form Al ₂ O ₃ in the molten steel, thereby improving the cleanliness of the molten steel. However, in the present invention, after deoxidation with Al, the molten steel in the ladle is moved up and down so that the slag is melted by the heat of the molten steel, so that the reaction between the molten steel and the slag is promoted. The iron oxide in it can be reduced. As a result, T.C.
[Fe] concentration is reduced, oxidation of Al in molten steel during casting by slag is suppressed, and a highly clean steel can be stably manufactured.

Simultaneously with or before or after Al deoxidation, a new slag modifier is added to the slag in the ladle, and then the molten steel is moved up and down to improve the newly added slag. Since the agent and the iron oxide in the slag react directly,
The reduction of iron oxide in the slag is further promoted, and it is possible to produce steel with extremely high cleanliness. Further, by continuing the vertical movement of the molten steel surface in the ladle for 5 minutes or more, the iron oxide in the slag can always be stably reduced.

The present inventors have also confirmed that the amount of reflux Ar gas blown from the rising side immersion pipe after Al deoxidation affects the cleanliness of the molten steel. FIG. 4 shows that the amount of Ar gas for reflux after Al deoxidation is 8 to 27 Nl / m per ton of treated molten steel.
It is a figure which shows the result of having investigated the generation | occurrence | production state of the inclusion defect in the rolled thin steel plate, changing to the range of in. As shown in FIG. 4, by setting the amount of reflux Ar gas after Al deoxidization to 16 Nl / min or less per ton of treated molten steel, inclusion defect is further reduced. This means that the amount of reflux Ar gas after Al deoxidation is reduced to 16 per ton of treated molten steel.
By controlling the flow rate to Nl / min or less, the stirring of molten steel in the vacuum chamber is suppressed, and as a result, the metal with a high oxygen concentration attached to the inner wall of the vacuum chamber before Al deoxidation is prevented from being dissolved. This is because re-oxidation of the molten steel due to re-melting of the ingot having a high level is prevented. In addition, by blowing Ar for reflux after Al deoxidation, flotation separation of Al ₂ O ₃ generated by Al deoxidation is promoted.

The slag modifier used in the present invention may be metal Al, Al alloy, Al slag, a mixture thereof, or
It is a mixture of these and fluxes such as quicklime,
T. [Fe] refers to all iron oxides (FeO
, Fe ₂ O _3, etc.)
Further, the amount of molten steel treated is the amount of molten steel for one heat in the ladle.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described with reference to the drawings.
FIG. 1 is a schematic view showing the steps of the present invention, and FIG. 2 is a schematic longitudinal sectional view of an RH vacuum degassing apparatus embodying the present invention.

As shown in FIG. 1, hot metal poured from a blast furnace 1 is charged into a converter 2 and oxygen gas is blown onto the hot metal through an upper-blowing oxygen lance 8 to perform decarburization refining. A molten steel 5 and a slag 6 are obtained in 2. Then, Al, C
The steel is poured onto the ladle 3 in the undeoxidized state without adding a strong deoxidizing agent such as a, Ti, and Zr. After tapping from the blast furnace 1, hot metal pretreatment such as desulfurization or dephosphorization may be performed.

After tapping, the slag modifier 7 is added to the slag 6 in the ladle 3 from the hopper 9 located above the ladle 3 to reduce the iron oxide in the slag 6. At this time, it is preferable that the amount of the slag modifier 7 added is such that the amount of Al in the slag modifier 7 is in the range of 0.1 to 0.5 kg per kg of the slag 6. 0.1 per kg of slag 6
If the amount is less than 0.5 kg, the iron oxide in the slag 6 may not be sufficiently reduced. If the amount exceeds 0.5 kg, the amount of Al reacting with air increases, and the yield deteriorates. Ladle 3
The amount of the slag 6 in the ladle 3 can be grasped, for example, by measuring the thickness of the slag 6 in the ladle 3. When the slag modifying agent 7 is added to the ladle 3, since the reaction between the slag 6 and the slag modifying agent 7 is promoted, it is preferable immediately after tapping when most of the slag 6 is in a molten state. If the addition is not possible immediately after tapping on the equipment, it may be immediately before the start of treatment in the RH vacuum degassing device 4. After the addition of the slag modifier 7, the ladle 3 is transported to the RH vacuum degassing device 4.

As shown in FIG. 2, the RH vacuum degassing device 4 comprises a vacuum tank 10 comprising an upper tank 11 and a lower tank 12;
It comprises an ascending immersion pipe 13 and a descending immersion pipe 14 provided below the lower tank 12, an elevator 20 for raising and lowering the ladle 3, and a hopper 19 for storing the slag modifier 7. The upper tank 11 is provided with an upper blowing lance 16, a raw material inlet 17, and a duct 18 connected to an exhaust device (not shown). The rising side immersion pipe 13 has an Ar gas blowing pipe 15. Ar gas for reflux is provided in the rising side immersion pipe 13.
Gas is blown.

The ladle 3 is carried in just below the vacuum chamber 10, and then the ladle 3 is raised by the elevating device 20, and the ascending-side immersion pipe 13 and the descending-side immersion pipe 14 are transferred to the molten steel 5 in the ladle 3. Let it soak. Then, Ar gas is blown into the ascending side immersion pipe 13 from the Ar gas blowing pipe 15 and the inside of the vacuum tank 10 is depressurized by evacuating the inside of the vacuum tank 10 with an exhaust device. When the pressure in the vacuum chamber 10 is reduced, the molten steel 5 in the ladle 3 rises along the rising side immersion pipe 13 together with the Ar gas blown from the Ar gas blowing pipe 15 and flows into the vacuum chamber 10.
The flow returning to the ladle 3 via the descending side immersion pipe 14, forming a so-called reflux, is subjected to degassing.

First, a decarburization treatment is performed. Although the decarburization reaction occurs only by circulating the undeoxidized molten steel 5 into the vacuum chamber 10, the decarburization reaction is accelerated by increasing the oxygen potential of the molten steel 5. For this purpose, it is preferable to blow oxygen gas from the upper blowing lance 16 toward the molten steel 5 in the vacuum chamber 10. When the carbon concentration of the molten steel 5 reaches a predetermined value by the decarburization reaction, metal Al is added from the raw material inlet 17 to deoxidize the molten steel 5. As described above, the amount of the Ar gas for recirculation after Al deoxidation is preferably 16 Nl / min or less per ton of treated molten steel.

The amount of metal Al to be added is determined in advance by measuring the dissolved oxygen concentration in the molten steel 5 with an oxygen meter (not shown) or the like, and removing the dissolved oxygen as Al ₂ O ₃ in total.
An amount corresponding to the amount of 0.01 wt% or more of Al remaining therein. If the residual Al concentration is less than 0.01% by weight, deoxidation is weak, and it is not possible to stably produce clean steel. The upper limit of the residual Al concentration is in a range determined by the type of steel.

Thereafter, the elevator 20 is moved up or down, or the pressure in the vacuum tank 10 is increased or decreased to move the molten steel 5 in the ladle 3 vertically. The lower limit of the movement of the metal surface position is set so as not to exceed the lower ends of the ascending immersion pipe 13 and the descending immersion pipe 14, and the upper limit is a flange (which fixes the ascending immersion pipe 13 and the descending immersion pipe 14 to the lower tank 12). (Not shown) is set so as not to be damaged by the heat of the molten steel 5. That is, the position of the molten metal surface is moved while performing the degassing process. This is because the degassing promotes the floating and separation of inclusions in the molten steel 5 and improves the cleanliness of the molten steel 5. Molten steel 5
The slag 6 is melted by the heat of the molten steel 5 by moving the molten metal surface position, and the reaction between Al in the molten steel 5 and iron oxide in the slag is promoted.

At this time, it is preferable that the slag modifier 7 is previously added to the slag 6 via the hopper 19. The slag modifier 7 is melted by the heat of the molten steel 5 and the slag 6, and the aluminum in the slag modifier 7 is moved up and down by the molten metal surface.
And the iron oxide in the slag 6 react directly to promote the reduction of the slag 6. The amount of the slag modifier 7 added is such that the amount of Al in the slag modifier 7 is 0.0
The range is 2 to 0.2 kg. In this case, since the slag modifier 7 has already been added,
It has been confirmed that 0.2 kg can be sufficiently reduced.

The vertical movement of the molten metal surface is preferably continued for 5 minutes or more. It is preferable that the cycle of the molten metal surface movement be 3 minutes or less. If the period exceeds 3 minutes, the relative movement between the molten steel 5 and the slag 6 becomes slow, the reaction between the molten steel 5 and the slag 6 is slow, and the processing time is prolonged, and the processing capacity of the RH vacuum degassing device 4 is reduced. It is because it falls. The shorter the period of the vertical movement of the molten metal surface, the more preferable the reaction between the molten steel 5 and the slag 6 is. However, taking into account the capacity of the elevating device 20 or the exhaust device, the shortest time within 3 minutes and the equipment capacity May be set as the movement cycle. Also, the number of movements
It is preferable that the period be at least two times, that is, two periods or more. If the number of times is less than two, the reduction of the slag 6 is insufficient, and the slag 6 may cause the reoxidation of the molten steel 5 even thereafter.

When the position of the molten metal in the ladle 3 is moved up and down, the molten steel 5 reacts with the slag 6, and the Al concentration in the molten steel 5 decreases. When the Al concentration in the molten steel 5 is 0.01 wt% or less, the reduction rate of the slag 6 is reduced.
Is periodically collected and the Al concentration in the molten steel 5 is analyzed. When the Al concentration is 0.01 wt% or less, the metal is adjusted so that the Al concentration of the molten steel 5 becomes 0.01 wt% or more. It is preferable that Al is additionally introduced from the material inlet 17.

After adjusting the composition of the molten steel 5, etc.,
The inside of the vacuum chamber 10 is returned to the atmospheric pressure to end the degassing process, the elevator 20 is lowered, the ladle 3 is carried out to a casting facility such as a continuous casting facility in the next step, and the molten steel 5 is cast.

By modifying the slag 6 on the molten steel 5 in this way, the slag 6 can be sufficiently reduced, and as a result, the reoxidation of the molten steel 5 by the slag 6 is prevented, and the amount of inclusions is reduced. Clean steel can be manufactured stably.

In the above description, the converter 2 of the top-blowing type has been described. However, the converter 2 is not limited to the top-blowing type. The present invention can be applied according to the above. In addition, the method has been described in which the slag modifier 7 is added once after the tapping from the converter 2 and before the processing in the RH vacuum degassing device 4 is started. It may be added.

[0031]

EXAMPLE An example of an ultra-low carbon steel of 18 heat to which the present invention is applied will be described. The hot metal that has been desulfurized and dephosphorized in the pretreatment is charged into a top-bottom blow-type converter, and calcined and refined by adding quicklime as a flux, and 250 tons with a carbon concentration of 0.04% to 0.05%. Was poured into a ladle. Using metal Al as a slag modifier, after tapping, 350 kg of metal Al was added to the ladle. The amount of slag in the ladle is 1.5 ~
2.0 tons. After the addition of the slag modifier, a sample was taken from the slag and T.C. [Fe] concentration was analyzed. Then, the ladle was transported to the RH vacuum degassing apparatus without being deoxidized.

In the RH vacuum degassing apparatus, oxygen is blown from the upper blowing lance to reduce the carbon concentration of the molten steel to 50 pp.
m or less. After the decarburization, the slag in the ladle is used for T. A sample for analysis having a [Fe] concentration was collected, and T.C. [Fe] concentration was analyzed. Next, the dissolved oxygen concentration was measured with an oxygen meter, and a predetermined amount of metal Al calculated based on the dissolved oxygen concentration was added to the molten steel to be deoxidized. In Examples 1 to 9, 100 was used as the slag modifier after deoxidation.
kg of metal Al was added on the slag. Example 10
Example 18 does not add a slag modifier after deoxidation.

After the deoxidation of the molten steel, the slag was reduced by moving the ladle up and down in one-minute cycles using an elevator. The time of this vertical movement was changed from 2 minutes to 8 minutes, and the effect was also investigated. Then, after adjusting the molten steel component, the slag in the ladle is subjected to T.P. A sample for analysis of [Fe] concentration was collected to complete the degassing process, and then the molten steel was transported to a continuous slab casting machine and cast into a slab having a thickness of 220 mm and a width of 950 mm.
The cast slab was hot-rolled and cold-rolled to produce a thin steel sheet, surface defects due to inclusions in the thin steel sheet were investigated, and cleanliness of the slab was investigated. Investigation results of surface defects are expressed as an index, and those without surface flaws are set to index = 0,
The index was increased according to the amount of flaws, and the acceptable level was set to index = 2 from experience. Table 1 shows the operation conditions and operation results of the example of 18 heats.

[0034]

[Table 1]

For comparison, the operation of ultra-low carbon steel which did not move up and down the molten metal surface after deoxidation in the RH vacuum degassing apparatus was performed for 10 heats. Five of them (Comparative Examples 1 to 5)
In 5), the slag modifier was added twice after tapping and after deoxidation, and in the other five heats (conventional examples 1 to 5), the slag modifier was added only after deoxidation. Also in the comparative example and the conventional example, the slag T.D. [Fe] concentration and surface defects in a thin steel plate were investigated. Table 1 shows operation conditions and operation results in the comparative example and the conventional example.

As shown in Table 1, the decarburization in the RH vacuum degassing apparatus caused T.C. Although the [Fe] concentration increases once, in the embodiment of the present invention, the iron oxide in the slag is reduced and reduced by the subsequent vertical movement of the molten metal surface. On the other hand, in the comparative example and the conventional example, the reduction of the slag is not sufficient, which suggests that the reduction reaction of the slag continues to occur in the subsequent casting process. FIG.
Based on the results in Table 1, T.O.
FIG. 4 shows the relationship between the [Fe] concentration and the product defect index. In the example, the T.F. [F
e] The concentration could be 1.5% or less, and product defects could be stably suppressed. Also, in Examples 1 to 9 in which the slag modifier was added twice, in the heat in which the vertical movement of the molten metal surface was 5 minutes or more, there was no surface defect in the thin steel sheet, and the cleanliness was particularly excellent. I was

[0037]

According to the present invention, the slag existing on the molten steel can be sufficiently reduced in the steel type subjected to the vacuum decarburization treatment by the RH vacuum degassing apparatus in the undeoxidized state. Reoxidation is prevented, and clean steel with extremely few inclusions can be stably manufactured.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing the steps of the present invention.

FIG. 2 is a schematic longitudinal sectional view of an RH vacuum degassing apparatus embodying the present invention.

FIG. 3 shows T.G. in slag after degassing. It is a figure which shows the relationship between [Fe] density | concentration and a product defect index.

FIG. 4 is a graph showing the results of an investigation on the effect of the amount of reflux Ar gas after Al deoxidation on the occurrence of inclusion defect.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Blast furnace 2 Converter 3 Ladle 4 RH vacuum degassing device 5 Molten steel 6 Slag 7 Slag modifier 8 Top blown oxygen lance 9 Hopper 10 Vacuum tank 11 Upper tank 12 Lower tank 13 Upside dipping pipe 14 Downside dipping pipe 15 Ar gas injection pipe 16 Top blowing lance 17 Raw material input port 18 Duct 19 Hopper 20 Elevator

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Keiji Yoshioka 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Shinichi Sugiyama 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Sun (72) Inventor Hidetoshi Matsuno 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Nihon Kokan Co., Ltd. (72) Inventor Tsuyoshi Murai 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Co., Ltd. (72) Inventor Koichi Ozawa 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Takuji Teraoka 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. (72) Inventor Satoshi Kodaira 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd. (72) Masao Nori Chiyoda-ku, Tokyo Chome No. 1 No. 2 Date. This steel pipe Co., Ltd. in the F-term of the (reference) 4K013 AA07 BA14 CA02 CA04 CB02 CE01 CF13 DA01 DA08 EA19

Claims (4)

[Claims] 1. The molten steel obtained by the converter refining is tapped from a converter into a ladle, and a slag modifier is added to the slag floating on the undeoxidized molten steel that has been tapped to add iron in the slag. The oxide is reduced, and then the undeoxidized molten steel is decarburized by an RH vacuum degassing device. After decarburization, Al is added to the molten steel to deoxidize it. A method for producing high-purity steel, characterized by reducing the iron oxide in slag by moving the molten metal surface up and down. 2. The molten steel obtained by the converter refining is tapped from a converter into a ladle, and a slag modifier is added to the slag floating on the tapped undeoxidized molten steel to add iron in the slag. The oxide is reduced, and then the undeoxidized molten steel is decarburized by an RH vacuum degassing apparatus. After decarburization, Al is added to the molten steel to perform deoxidation and slag on the molten steel in the ladle. Slag reforming by adding a slag modifying agent to, simultaneously or before and after, then,
A method for producing high-purity steel, characterized by reducing the iron oxide in slag by moving the molten steel surface in a ladle up and down. 3. The method for producing high-cleanliness steel according to claim 1, wherein the vertical movement of the surface level is continued for 5 minutes or more. 4. The method according to claim 1, wherein the flow rate of the Ar gas for reflux after the deoxidation is set to 16 Nl / min or less per 1 ton of treated molten steel. Manufacturing method of clean steel.