JP2003191052A - Method of manufacturing extra-low carbon thin steel sheet and billet for the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing molten extra-low carbon steel of a blank for a thin steel sheet which surely prevents surface flaws by finely depositing oxides on the surface during solidification substantially without forming inclusions in the molten steel. <P>SOLUTION: The method of manufacturing the molten extra-low carbon thin steel sheet comprises decarburizing the molten steel down to a carbon concentration of 0.002 mass%, then casting the molten steel regulated in the dissolved oxygen concentration in the molten steel to ≥0.1 to ≤0.06 mass% by adding Nb and V to the molten steel. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an ultra-low carbon thin steel sheet which is excellent in workability and formability and is resistant to surface defects.

[0002]

2. Description of the Related Art A large amount of dissolved oxygen is contained in molten steel refined in a converter or a vacuum processing vessel, and this excess oxygen is deoxidized by Al, which is a strong deoxidizing element having a strong affinity with oxygen. It is generally done. However, Al produces alumina-based inclusions by deoxidation, and these are aggregated to form coarse alumina clusters. This alumina cluster causes surface defects during the production of the steel sheet and significantly deteriorates the quality of the thin steel sheet. Especially, in the case of ultra-low carbon molten steel, which is a material for thin steel plates with low carbon concentration and high dissolved oxygen concentration after refining,
Since the amount of alumina clusters is very large and the occurrence rate of surface flaws is extremely high, reduction measures for alumina-based inclusions have become a major issue.

On the other hand, in the past, the conventional technique is Japanese Patent Laid-Open No. 5-1042.
The method of removing the alumina-based inclusions by adding the flux for adsorbing the inclusions of JP-A No. 19 to the surface of the molten steel, or JP-A-Sho 6
A method has been proposed and implemented in which a CaO flux is added to molten steel by utilizing the injection flow of Japanese Patent No. 3-149057, and thereby alumina inclusions are adsorbed and removed. On the other hand, as a method of not generating alumina inclusions but removing them, a method for producing molten steel for thin steel sheets in which molten steel is deoxidized with Mg and hardly deoxidized with Al as disclosed in JP-A-5-302112. Is also disclosed.

[0004]

However, in the method of removing the alumina-based inclusions described above, it is extremely difficult to reduce the amount of the alumina-based inclusions formed in the ultra-low carbon molten steel to the extent that surface flaws do not occur. It's difficult. In addition, in Mg deoxidation that does not form alumina inclusions at all, since the vapor pressure of Mg is high and the yield of molten steel is very low, molten steel with a high dissolved oxygen concentration such as ultra-low carbon steel is deoxidized with Mg alone. Acid requires a large amount of Mg, which is not a practical process considering the manufacturing cost.

In view of these problems, the present invention is an extremely low-priced raw material for a thin steel plate capable of reliably preventing surface defects by precipitating oxides during solidification with almost no inclusions in molten steel. Provided is a method for producing carbon steel.

[0006]

In order to solve the above problems, the present invention has the following structures. That is, (1) in the method for producing an ultra-low carbon thin steel sheet, after decarburizing the molten steel to a carbon concentration of 0.002 mass% or less, Nb and V are added to the molten steel to reduce the dissolved oxygen concentration in the molten steel to 0. A method for producing an ultra-low carbon thin steel sheet is characterized by casting molten steel adjusted to 0.01% by mass or more and 0.06% by mass or less. Also,
(2) In the method for producing an ultra-low carbon thin steel sheet, after decarburizing the molten steel to a carbon concentration of 0.002 mass% or less, Nb and V are added to the molten steel so that the Nb concentration is 0.005 mass% or more. , 0.05 mass% or less, the V concentration was 0.005 mass% or more and 0.05 mass% or less, and the dissolved oxygen concentration in the molten steel was adjusted to 0.01 mass% or more and 0.06 mass% or less. It is a method for producing an ultra-low carbon thin steel sheet, which is characterized by casting molten steel. In addition, (3) the carbon concentration of the molten steel is 0.00
The method for producing an ultra-low carbon thin steel sheet according to (1) or (2) above, characterized in that vacuum degassing treatment is performed when decarburizing to 2% by mass or less. Further, (4) in the method for producing an ultra-low carbon thin steel sheet, casting is performed with a mold having a function of applying electromagnetic stirring or an electromagnetic field, wherein the pole is any of (1) to (3) above. It is a method of manufacturing a low carbon thin steel sheet. Further, (5) in the method for producing an ultra-low carbon thin steel sheet, the molten steel at the meniscus position is 40 cm / s or more and 100 c by using a mold having an electromagnetic stirring function.
The method for producing an ultra-low carbon thin steel sheet according to any one of the above (1) to (3), characterized in that casting is performed while swirling at an average flow rate of m / s or less. Further, (6) in the method for producing an ultra-low carbon thin steel sheet, the molten steel at the meniscus position is 0.1 Hz or more, using a mold having an electromagnetic coil,
The method for producing an ultra-low carbon thin steel sheet according to any one of (1) to (3) above, characterized in that casting is performed while vibrating in a horizontal direction at 100 Hz or less. (7) A method for producing an ultra-low carbon thin steel sheet, which comprises casting a molten steel produced by the production method according to any one of (1) to (6) above into a thin slab during continuous casting into a slab. Is. Further, (8) a slab produced by the production method according to any one of the above (1) to (7) and continuously cast has a diameter of 0.
Fine oxide of 5 μm to 30 μm is 20 m from the surface of the slab
The continuous cast slab is characterized in that it is dispersed in the range of m in the range of 1000 pieces / cm ² or more and less than 100,000 pieces / cm ² . Further, (9) in a slab obtained by the continuous casting method produced by the production method according to any one of the above (1) to (7), a fine oxide having a diameter of 0.5 μm to 30 μm is 20 mm from the surface layer of the slab. 1000 pieces / cm ² or more within the range of 1
It is a continuous cast slab characterized in that less than 00000 pieces / cm ² are dispersed, and 60% or more thereof is a spherical oxide.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is described in detail below.
In the manufacturing method of the present invention, Nb and V are added to molten steel having a carbon concentration of 0.002% by mass or less by refining in a steelmaking furnace such as a converter or an electric furnace, or by further performing vacuum degassing treatment. ,
Moreover, the dissolved oxygen concentration is adjusted to be 0.01 to 0.06 mass%. The basic idea of this manufacturing method is to reduce the carbon concentration to the extent that CO gas is not generated by reacting with oxygen during casting, and to add a large amount of dissolved oxygen by adding Nb and V without adding Al at all. Thereby, almost no inclusions are generated in the molten steel and the material for the steel plate for thin plate is secured.

A large amount of dissolved oxygen is contained in the molten steel that has been decarburized in a converter or a vacuum treatment vessel, and this dissolved oxygen is usually almost deoxidized by the addition of Al (equation (1)). Therefore, a large amount of alumina-based inclusions are generated.

2Al + 3O = Al ₂ O ₃ (1) Immediately after deoxidation, these alumina-based inclusions agglomerate and coalesce with each other to form coarse alumina-based inclusions, which causes surface defects during steel sheet production. However, if Al is not added at all in the molten steel after decarburization treatment and almost no deoxidation is performed, a large amount of dissolved oxygen is contained in the molten steel, but almost no inclusions are formed, and the cleanliness is very high. High molten steel can be obtained. Usually, when such molten steel with high dissolved oxygen is cast, CO gas is generated during solidification, a violent bumping phenomenon occurs, and a large amount of bubbles are trapped in the slab, which not only deteriorates castability. However, the quality of the slab is also greatly reduced.

However, in the present invention, CO gas generation is suppressed by reducing the C concentration as much as possible, instead of leaving the dissolved oxygen without adding Al at all. In an experimental study, C
It was found that the CO gas generation rate was extremely reduced when the concentration was set to 0.002 mass% or less. In addition, recently
The continuous casting machine is equipped with an in-mold electromagnetic stirrer, and the inventors of the present invention ensure a dissolved oxygen concentration of 0.06 by ensuring a molten steel flow rate of about 40 to 100 cm / s in the in-mold meniscus during solidification. It has been found that casting can be performed without trapping CO bubbles in the slab even if it is about mass%.

It should be noted that the swirling flow velocity of molten steel by electromagnetic stirring is 4
If it is less than 0 cm / s, a sufficient cleaning effect of CO bubbles cannot be obtained, and if the swirling flow rate exceeds 100 cm / s, the CO bubbles are cleaned, but the mold powder on the molten steel surface is involved,
Surface defects occur.

In order to prevent CO bubbles from being trapped in the slab, it is also effective to vibrate the molten steel in the mold at a frequency of 0.1 to 100 Hz by an electromagnetic coil provided in the mold. I have found. In this case, frequency 1
If it exceeds 00 Hz, the molten steel flow cannot follow the change in the vibration direction, and if it is less than 0.1 Hz, on the contrary, the change speed in the vibration direction is slow.

When the C concentration in the molten steel is made extremely low, dissolved oxygen is precipitated as FeO inclusions during casting. This F
The eO-based inclusions do not form in the molten steel but precipitate during solidification, so they are finely dispersed in the slab without agglomeration and coalescence. The FeO-based inclusions include not only pure FeO but also oxides complexed with SiO ₂ , MnO and the like. When the FeO-based inclusion dispersed state within a range of 20 mm from the surface layer of the cast piece obtained by the present invention was evaluated, fine oxides having a diameter of 0.5 μm to 30 μm were 1000 pieces / cm ² or more and 100000 pieces in the cast piece. / cm and less than ² distributed to a 60% or more spherical oxide, such oxide dispersion state, surface defects after rolling the cast slab having a composition and shape did not occur.

The distribution of inclusions within a range of 20 mm from the surface layer is focused on because inclusions in this range are likely to be exposed on the surface layer after rolling and become surface defects. The dispersed state of the inclusions was 100 times that of the polished surface of the slab.
The particle size distribution of the inclusions within the unit area was evaluated by observation with an optical microscope at a magnification of 000.

From the above results, according to the present invention, it is possible to precipitate and finely disperse FeO-based oxides during solidification in the molten steel with almost no inclusions generated. It does not cause the generation, and the quality of the thin steel sheet is greatly improved.

Usually, the C concentration is adjusted to 0.
When decarburizing to 002 mass% or less, the dissolved oxygen concentration in the molten steel becomes about 0.03 to 0.06 mass%. Even if these dissolved oxygens are deoxidized with elements other than Al, such as Ti,
Inclusions are generated in the molten steel. According to an experimental study, if the dissolved oxygen concentration is reduced to less than 0.01% by mass, the amount of inclusions other than alumina will increase,
It causes surface defects. On the contrary, the dissolved oxygen concentration is 0.0
If it exceeds 6% by mass, the C concentration is reduced to 0.002% by mass or less, and CO bubbles are trapped in the slab even if electromagnetic stirring is used, so that defects in the bubble system occur after rolling. Therefore, the dissolved oxygen concentration in the molten steel needs to be 0.01 mass% or more and 0.06 mass% or less.

The steel sheet for thin plates is used for severe applications such as outer panels for automobiles, so it is necessary to add workability. In order to improve the workability of the thin steel sheet, it is important to reduce the C concentration as much as possible and then fix C and N dissolved in the steel by adding other elements. The C concentration is preferably 0.01% by mass or less, and more preferably 0.005% by mass or less, from the viewpoint of workability. However, the condition for preventing the generation of CO bubbles during solidification is a C concentration of 0.002 mass% or less, so that the C concentration determined by the processability condition is sufficiently satisfied in the present invention. Further, Al, Ti and the like are usually used as elements for fixing C and N in steel, but these elements strongly deoxidize molten steel.

Therefore, in the present invention, V and Nb are added as elements capable of fixing N and C and having extremely weak deoxidizing power.
Nb is mainly used for fixing C, and V is mainly used for fixing N.
When the Nb concentration is less than 0.005 mass%, C cannot be sufficiently fixed, and when the Nb concentration exceeds 0.05 mass%, the workability is deteriorated. Therefore, the amount of Nb added is 0.005 mass% or more,
It is preferable that the amount is not more than 05% by mass. Further, if the amount of Nb added is in this range, the oxygen concentration equilibrating with Nb is 0.
It is 01 mass% or more, and 0.01 mass% or more of dissolved oxygen can be secured even if Nb is added. Also, the V concentration is 0.0
If it is less than 05% by mass, N cannot be sufficiently fixed, and the V concentration is 0.
If it exceeds 05 mass%, the workability is deteriorated. Therefore, the addition amount of V is preferably 0.005 mass% or more and 0.05 mass% or less. With the amount of V added in this range, the oxygen concentration in equilibrium with V is 0.01% by mass or more, and even if V is added, dissolved oxygen can be secured by 0.01% by mass or more.

The Si concentration in the steel sheet is preferably 0.005 mass% or more and 0.03 mass% or less. This is because if the Si concentration is less than 0.005 mass%, the strength of the plate will be insufficient, and if the Si concentration exceeds 0.03 mass%, the workability of the plate will decrease. Moreover, the Si concentration is 0.03 mass%.
If it is below, the equilibrium oxygen concentration will be more than 0.02 mass%,
It is possible to secure a dissolved oxygen concentration of 0.01% by mass or more.

If the Mn concentration in the steel sheet is less than 0.08% by mass, dent defects are likely to occur during hot rolling, and Mn
If the concentration exceeds 0.3% by mass, the workability of the plate will deteriorate.
Therefore, the Mn concentration in the steel sheet is 0.08 mass% or more,
It is preferably 0.3% by mass or less. Further, since Mn has a very weak deoxidizing power compared to Si, even if the Mn concentration is 0.3% by mass, the equilibrium oxygen concentration is more than 0.1% by mass, and 0.01% by mass in the molten steel. Therefore, 0.06 mass% of dissolved oxygen can be secured.

In the present invention, it is necessary to add no Al to the molten steel so as to prevent the formation of alumina inclusions that tend to agglomerate. However, there is a problem with alumina inclusions that inevitably enter from refractory materials. It does not become. This is because if there is a small amount of alumina-based inclusions, the dissolved oxygen in the molten steel is high, so the interfacial energy between the molten steel and the alumina-based inclusions is low, and agglomeration and coalescence hardly occurs. Further, since Ti in steel fixes C and N as TiN and TiC, it is effective in improving workability, but when the amount of addition of Ti increases, for example, the Ti concentration becomes 0.009 mass% or more. If so, the equilibrium oxygen concentration will be less than 0.01% by mass, so that a sufficient dissolved oxygen concentration cannot be secured. Therefore,
When Ti is added for the purpose of further improving the workability, it may be added in the range of 0.009 mass% or less.

The present invention is not only applied to ordinary slab casting having a thickness of about 250 mm, but also exerts a sufficient effect for thin slab casting having a mold thickness of a continuous casting machine smaller than that, for example, 150 mm or less. It is possible to obtain a slab with extremely few surface defects.

[0023]

EXAMPLES The present invention will be described below with reference to Examples and Comparative Examples. <Example 1> Molten steel 3 having a C concentration of 0.0018 mass% by refining in a converter and treatment in a reflux type vacuum degasser
00t was melted. Add alloy to this molten steel,
Mass% Si, 0.15 mass% Mn, 0.015 mass% N
b, 0.01 mass% V, and 0.045 mass% dissolved oxygen. This molten steel is continuously cast to a thickness of 250 mm and a width of 180.
It was cast into a 0 mm slab. The cast slab is 8500 m
It was cut into m lengths to make one coil unit. The slab thus obtained is hot-rolled and cold-rolled by a conventional method,
Finally, a cold rolled steel sheet having a coil thickness of 0.7 mm and a width of 1800 mm was obtained. Regarding the slab quality, the number of surface defects generated per coil was evaluated by visual observation on the inspection line after cold rolling. As a result, no surface defect was generated. <Example 2> Molten steel 30 having a C concentration of 0.0015 mass% by refining in a converter and treatment in a reflux type vacuum degassing device
0t was melted. An alloy is added to this molten steel, 0.01 mass% Si, 0.15 mass% Mn, 0.015 mass% N
b, 0.01 mass% V, 0.001 mass% Ti, 0.0
It was 4 mass% dissolved oxygen. Using a continuous casting machine having an electromagnetic stirring device in the mold, the molten steel in the meniscus was electromagnetically stirred at an average flow rate of 45 cm / s, and a thickness of 2 was obtained.
It was cast into a slab having a width of 50 mm and a width of 1800 mm. The cast slab was cut into a length of 8500 mm to make one coil unit. The slab thus obtained is hot-rolled and cold-rolled by a conventional method, and finally has a thickness of 0.7 mm and a width of 18 mm.
It was a cold rolled steel plate with a 00 mm coil. Regarding the slab quality, the number of surface defects generated per coil was evaluated by visual observation on the inspection line after cold rolling. As a result, no surface defect was generated. <Comparative Example 1> Molten steel in a ladle having a carbon concentration of 0.0015 mass% by refining in a converter and treatment with a reflux type vacuum degassing device was deoxidized with Al to obtain an Al concentration of 0.04 mass%. The dissolved oxygen concentration was 0.0002% by mass. This molten steel was cast into a slab having a thickness of 250 mm and a width of 1800 mm by the continuous casting method. The cast slab was cut into a length of 8500 mm to make one coil unit. The thus-obtained slab was hot-rolled and cold-rolled by a conventional method to finally obtain a cold-rolled steel sheet having a coil thickness of 0.7 mm and a width of 1800 mm. Regarding the slab quality, the number of surface defects generated per coil was evaluated by visual observation on the inspection line after cold rolling. As a result, 5 slabs / coil surface defects were generated.

[0024]

As described above, according to the present invention, since oxides can be finely precipitated during solidification with almost no inclusions in molten steel, surface defects can be reliably prevented. It becomes possible to produce ultra-low carbon molten steel for thin steel sheets, which has excellent workability and formability.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) B22D 11/115 B22D 11/115 C C21C 7/04 C21C 7/04 E 7/10 7/10 Z C22C 38 / 00 301 C22C 38/00 301A (72) Inventor Toru Matsumiya 20-1 Shintomi, Futtsu City, Chiba Prefecture Nippon Steel Co., Ltd. Technology Development Headquarters (72) Manabu Takahashi 20-1 Shintomi, Futtsu City, Chiba Prefecture New Japan Steelmaking Co., Ltd. F-Term in Technology Development Division (Reference) 4E004 AA09 AD00 MB12 NC04 4K013 AA07 BA02 BA14 CE06 DA03 DA08 EA18 FA02

Claims (9)

[Claims] 1. A method for producing an ultra-low carbon thin steel sheet, which comprises decarburizing molten steel to a carbon concentration of 0.002% by mass or less,
Manufacture of an ultra-low carbon thin steel sheet, characterized in that Nb and V are added to the molten steel and the dissolved oxygen concentration in the molten steel is adjusted to 0.01% by mass or more and 0.06% by mass or less. Method. 2. A method for producing an ultra-low carbon thin steel sheet, which comprises decarburizing the molten steel to a carbon concentration of 0.002% by mass or less,
Nb and V are added to the molten steel so that the Nb concentration is 0.005 mass% or more and 0.05 mass% or less and the V concentration is 0.005 mass% or more and 0.05 mass% or less. The method for producing an ultra-low carbon thin steel sheet is characterized by casting molten steel in which the dissolved oxygen concentration is adjusted to 0.01 mass% or more and 0.06 mass% or less. 3. The method for producing an ultra-low carbon thin steel sheet according to claim 1, wherein a vacuum degassing treatment is carried out when decarburizing the molten steel to a carbon concentration of 0.002% by mass or less. 4. The ultra low carbon thin steel sheet manufacturing method according to claim 1, wherein the ultra low carbon thin steel sheet is cast with a mold having a function of applying electromagnetic stirring or an electromagnetic field. Manufacturing method of carbon thin steel sheet. 5. A method for producing an ultra-low carbon thin steel sheet, which comprises casting a molten steel at a meniscus position at an average flow rate of 40 cm / s or more and 100 cm / s or less using a mold having an electromagnetic stirring function. The method for producing an ultra-low carbon thin steel sheet according to any one of claims 1 to 3, which is characterized. 6. A method for manufacturing an ultra-low carbon thin steel sheet, characterized in that the molten steel at the meniscus position is cast while horizontally vibrating at 0.1 Hz or more and 100 Hz or less using a mold having an electromagnetic coil. Claims 1-3
The method for producing an ultra-low carbon thin steel sheet according to any one of 1. 7. A very low carbon thin steel sheet produced by continuously casting the molten steel produced by the production method according to any one of claims 1 to 6 into a thin slab. Method. 8. A slab produced by the production method according to any one of claims 1 to 7 and obtained by continuous casting,
A fine oxide having a diameter of 0.5 μm to 30 μm is 1000 pieces / cm ² or more within a range of 20 mm from the surface of the cast slab,
A continuously cast slab characterized in that less than 000 pieces / cm ² are dispersed. 9. A slab obtained by continuous casting, which is produced by the production method according to claim 1.
A fine oxide having a diameter of 0.5 μm to 30 μm is 1000 pieces / cm ² or more within a range of 20 mm from the surface of the cast slab,
A continuously cast slab, wherein less than 000 pieces / cm ² are dispersed, and 60% or more of which are spherical oxides.