JP2000038614A - Method for melting extra-low carbon steel excellent in cleanliness - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a melting method of an extra-low carbon steel excellent in the cleanliness while restraining lower oxide content to low level without blowing gaseous oxygen at the time of executing a vacuum decarburizing treatment. SOLUTION: The contents in slag in a ladle before the vacuum treatment are adjusted to 0.6-2.0 CaO/Al2O3 ratio of CaO content to Al2O3 content and 2-10 wt.% the total of FeO and MnO contents, <=12 wt.% SiO2 content and 5-10 wt.% Mg content. Successively, the decarburize-treatment is executed on the molten steel and further, in order to execute deoxidize-treatment, Al is added into the molten steel, and simultaneously or thereafter, the oxide- containing MgO is added into a vacuum vessel to produce the steel having <=0.005 wt.% carbon content.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for melting ultra-low carbon steel having excellent cleanliness.

[0002]

2. Description of the Related Art Ultra-low carbon steel is used as an exterior steel sheet of an automobile which is required to have a small number of surface defects and excellent formability. Measures for ultra-low carbon and high purification.

[0003] In the case of producing ultra-low carbon steel, a method of decarburizing undeoxidized molten steel using a vacuum processing apparatus is generally used. That is, undeoxidized molten steel having a carbon content of 0.02 to 0.1% by weight is put out from a steelmaking furnace such as a converter into a ladle, and then the oxygen and carbon in the molten steel are separated using a vacuum processing apparatus. 0.001-0.005% by weight carbon content by reaction
Decarburize until.

[0004] In the above reaction, it is known that the oxygen content necessary for obtaining a sufficient decarburization rate is 0.04% by weight or more. When such molten steel having a high oxygen content is obtained in a steelmaking furnace such as a converter, the total content of the lower oxides FeO and MnO in the slag is as high as about 15 to 20% by weight.

[0005] In molten steel of ultra-low carbon steel which has been deoxidized with Al after vacuum decarburization, the Al in the molten steel in the ladle and the lower oxide in the slag react during continuous casting after the vacuum treatment. I do. By this reaction, an oxide of Al (Al ₂ O)
₃ ) Generate. This oxide remains in the slab without being removed from the molten steel in the tundish or the mold during continuous casting and becomes a nonmetallic inclusion, thereby deteriorating the cleanliness of the steel.

The Al ₂ O ₃ non-metallic inclusions tend to accumulate in the vicinity of the surface of the cast slab, thereby causing surface defects on the exterior steel plate of automobiles and clogging of the immersion nozzle during continuous casting. It may be. If the immersion nozzle is closed, not only the casting cannot be performed successively and productivity is impaired, but also the molten steel passing through the nozzle is deflected to change the flow state in the mold and cause surface defects. Further, in order to prevent the nozzle blockage, it is necessary to increase the flow rate of an inert gas such as Ar blown from the upper portion of the nozzle. If the injected inert gas also remains near the surface of the slab and is captured, it contributes to surface defects. When the surface of a slab or a hot-rolled steel sheet material is to be treated to prevent such surface defects, there are problems in terms of economy and productivity.

[0007] Therefore, there is a measure to reduce the content of lower oxides such as FeO and MnO in slag which easily reacts with Al in steel at the time of tapping from a steelmaking furnace such as a converter or before vacuum treatment. Has been taken.

For example, in the method disclosed in Japanese Patent Application Laid-Open No. Hei 5-23937, a slag modifier is added to slag in a ladle during or after tapping from a converter to add FeO in the slag. The total content of MnO and MnO is set to 5% by weight or less, and then oxygen gas is blown from the top blowing lance to the surface of the molten steel in the vacuum tank before the vacuum decarburization treatment, and A after the decarburization treatment is performed.
After deoxidation, a method is employed in which a flux containing 50% by weight or more of CaO is blown onto the surface of molten steel from the above-mentioned upper blowing lance.

This method is based on the premise that oxygen gas is blown from a top blowing lance onto the surface of molten steel in a vacuum chamber. The reason is that the slag reforming lowers the content of lower oxides in the slag before vacuum treatment, so the amount of oxygen supplied from the lower oxides that contributes to the decarburization reaction decreases, which is necessary for the decarburization reaction. This is because of insufficient oxygen.

However, when oxygen gas is blown onto the surface of the molten steel in the vacuum chamber, Fe and Mn in the molten steel are oxidized, and lower oxides such as FeO and MnO are generated. The content increases. Further, the total content of FeO and MnO in the slag after tapping was adjusted to 5%.
%, A large amount of a modifier and a slag-making agent are used during tapping, so that the temperature drop of the molten steel is large.

In Japanese Patent Application Laid-Open No. 6-116623, a slag modifier containing Al is added to slag in a ladle before vacuum decarburization treatment, and a flux containing MgO is introduced into a vacuum chamber at the start of vacuum decarburization treatment. And blowing oxygen gas onto the surface of molten steel in a vacuum chamber has been proposed. But,
In this method, the use of oxygen gas at the time of vacuum decarburization treatment is assumed for the same reason as the method of Japanese Patent Application Laid-Open No. Hei 5-23937, so that the content of the lower oxide in the slag increases.

[0012]

DISCLOSURE OF THE INVENTION The present invention provides a method for melting ultra-low carbon steel which is excellent in cleanliness, while keeping the content of lower oxides low and not blowing oxygen gas during vacuum decarburization. It is intended to provide a manufacturing method.

[0013]

The gist of the present invention resides in a method for melting ultra-low carbon steel excellent in cleanliness as shown in the following (1).

(1) A method for melting ultra-low carbon steel having a carbon content of 0.005% by weight or less by deoxidizing molten steel in a ladle using a vacuum processing apparatus after decarburizing the steel. , before the vacuum treatment in the ladle CaO content in the slag and Al ₂ O _{3 2} wt% ratio CaO / Al content ratio O ₃ 0.6 to 2.0, the FeO and MnO content ratio The total is 2% by weight to 10% by weight, the SiO ₂ content is 12% by weight or less, and the MgO content is 5% by weight to 1%.
After adjusting to 0% by weight or less, the molten steel is decarburized, and then Al is added to the molten steel in order to deoxidize the molten steel, and at the same time or thereafter, an oxide containing MgO is added to the vacuum chamber. Smelting method for ultra-low carbon steel with excellent cleanliness.

In the method of the present invention, the sum of the contents of FeO and MnO in the slag before the vacuum treatment is adjusted to an appropriate and lower range. Thereby, reoxidation of molten steel by slag is suppressed. Further, the total content of FeO and MnO is ensured to such an extent that the decarburization rate does not stagnate during the vacuum decarburization treatment.

Further, the chemical composition of the slag before the vacuum treatment is changed in addition to the above-mentioned total contents of FeO and MnO.
By setting the values of CaO / Al ₂ O ₃ , SiO ₂ content, and MgO content in appropriate ranges, reoxidation of molten steel by slag is suppressed. The mechanism will be described below.

The slag in the ladle is divided into a solid phase and a liquid phase. Of these, the slag in the liquid phase directly contacts the molten steel and reoxidizes the molten steel. Therefore, in order to suppress the reoxidation of the molten steel by the slag, the ratio of the liquid phase of the slag after the vacuum treatment may be reduced and the ratio of the solid phase may be increased.

At this time, it is not necessary to completely convert the liquid slag into a solid phase, and it is found that the reoxidation of the molten steel can be suppressed by generating about 10% by volume of the solid phase in the liquid slag. Was.

When the chemical composition of the slag before the vacuum treatment is in the above range, M
When an oxide mainly composed of gO is added to molten steel in a vacuum chamber, a solid phase of about 10% by volume can be generated in a liquid slag.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION In order to control the slag composition before the vacuum treatment within an appropriate range, when the molten steel is discharged from a steelmaking furnace such as a converter to a ladle, the outflow of the slag is suppressed, and Quicklime or a flux mainly composed of Al ₂ O ₃ or CaO is added as a slag. Further, in order to adjust the total content of FeO and MnO in the slag, Al ash or Al-
A CaO-based slag modifier is added.

In the method of the present invention, the ratio of the CaO content of the slag before the vacuum treatment to the weight percentage of the Al ₂ O ₃ content CaO /
Al ₂ O _{3 is set} to 0.6 or more and 2.0 or less. CaO / A
In the case of slag having l ₂ O ₃ of less than 0.6, the ability to inevitably form and absorb floating Al ₂ O _3, which is inevitably generated during Al deoxidation after vacuum decarburization treatment, is insufficient. In addition, in the slag in which this ratio exceeds 2.0, the ratio of the liquid phase in the slag decreases and the fluidity decreases. Therefore, the ability to absorb Al ₂ O ₃ is undesirably reduced.

The slag before vacuum treatment has a total content of FeO and MnO of 2 to 10% by weight (hereinafter simply referred to as%). If it is less than 2%, it is impossible to secure 0.04% or more of oxygen in molten steel necessary for the vacuum decarburization reaction. Also,
Not only does the supply of oxygen from these lower oxides decrease, not only does the vacuum decarburization reaction rate slow, but it is sometimes impossible to melt ultra-low carbon steel.

FIG. 1 shows the relationship between the total content of FeO and MnO in slag before vacuum treatment and the time until the C content in molten steel reaches 0.002% during vacuum decarburization treatment. FIG. Ultra low carbon steel with 270t molten steel
This is the result of melting with a H vacuum processing apparatus. The processing conditions are as follows: C content in molten steel at the start of vacuum decarburization is 0.04 to
0.05%, CaO / Al ₂ O in slag before vacuum treatment
₃ is 0.8 to 1.2, SiO ₂ content is 12% or less, M
The gO content is between 5 and 10%.

As shown in FIG. 1, when the total content of FeO and MnO is less than 2%, the time required for vacuum decarburization is greatly increased. Above 5%, this time is minimized.
Therefore, the lower limit of the content of FeO and MnO is 2% in total, and more preferably 5%.

Further, cleanliness of molten steel can be ensured when the total content of FeO and MnO is up to 10%. However, 1
If it is contained in a large amount exceeding 0%, reoxidation of molten steel by slag occurs even after adding an oxide containing MgO into the vacuum chamber. Therefore, the upper limits of the contents of FeO and MnO are set to 10% in total.

The content of SiO _{2 in} the slag before the vacuum treatment is 12% or less. SiO ₂ is inevitably mixed in from slag or slag-making agent flowing out of a steelmaking furnace such as a converter. If this SiO ₂ exceeds 12%, reoxidation of the molten steel after deoxidation occurs. In addition, it is necessary to increase the amount of MgO-containing oxide added to generate a solid phase in the liquid phase slag after deoxidation.

Therefore, the content of SiO ₂ should be 12% or less,
Desirably, it is 10% or less. If less than 10%,
The oxide containing MgO added at the time of deoxidation can be further reduced. Further, reoxidation of molten steel by SiO ₂ can be suppressed. The lower limit is not particularly limited,
Since it is contained in the slag-making agent as gangue, it is usually about 1% or more.

The MgO content in the slag before vacuum treatment is 5
-10%. If it is less than 5%, the added amount of the oxide containing MgO for generating a solid phase in the slag of the liquid phase after deoxidation becomes too large, which causes a decrease in the temperature of the molten steel. Also,
In order to reduce the MgO content during tapping, it is necessary to add and dilute a large amount of slag-making agent, resulting in a decrease in the temperature of the molten steel. On the other hand, when the MgO content exceeds 10%, the ratio of the solid phase of the slag in the ladle increases, and the Al ₂ content of the slag increases.
This is not preferred because the ability to absorb O ₃ is reduced.

M added during Al deoxidation after vacuum decarburization treatment
The oxides containing gO include MgO alone, MgO · Ca
O, MgO.Al ₂ O ₃ and the like.

MgO alone is an oxide containing 85% or more, preferably 90% or more of MgO, which is generally called natural magnesia or seawater magnesia.

MgO.CaO has a mineral called dolomite, which contains 35% MgO and 60% CaO.
Magnesia dolomite having a MgO content of 50% or more or 70% or more is also included. M
A higher gO content is desirable because the addition amount can be suppressed. Since magnesia dolomite is used as a brick material, used bricks obtained by appropriately crushing and sizing the used bricks can be used.

MgO.Al ₂ O ₃ contains 28 MgO.
%, There is a so-called magnesia spinel spinel and MgO content containing Al ₂ O ₃ 72% was in the MgO saturated composition to enhance the 40% or 60%.
A higher MgO content is desirable because the amount of addition can be reduced. Spinel is also used as a brick material and an amorphous refractory material, and thus a used material obtained by appropriately crushing and sizing the used material can be used.

Regarding the use of natural magnesia, dolomite, and spinel, the cost and other economical factors are taken into consideration, and the slag composition CaO / Al before vacuum decarburization is used.
_{When 2} O ₃ is 0.6 or more and less than 1.0, spinel is used,
In the case of 1.0 to 2.0, it is desirable to use dolomite.
Natural magnesia has CaO / Al ₂ O ₃ of 0.6.
It can be used for all values of ~ 2.0. The reason is that when CaO / Al ₂ O ₃ is 0.6 or more and less than 1.0, the solid phase of MgO · Al ₂ O ₃ increases in the slag of the liquid phase. This is because the solid phase increases.

The particle size of these MgO-containing oxides is
0.1-30 mm is desirable. If it is less than 0.1 mm, it is likely to be scattered when added into the vacuum chamber. If it exceeds 30 mm, it is difficult to be caught in the molten steel after addition, and it is difficult to disperse uniformly in the molten steel. For this reason, in the case of the RH vacuum processing apparatus, there is a case where the RH vacuum processing apparatus floats collectively on the outer periphery of the descending dipping tube. More preferably, the particle size is 1 to 20 mm.
This is because the added oxide is more uniformly involved in the molten steel.

FIG. 2 is a diagram showing the relationship between the particle size of the oxide added into the vacuum chamber after vacuum decarburization and deoxidation using an RH vacuum processing apparatus and the dispersion ratio of the liquid phase to slag. is there. Mg
It is the result of investigating the dispersion rate of oxides using an oxide composed of 75% or more of O and 20% or more of SrO as a tracer.
Dispersion rate means that slag at any position after RH vacuum treatment is 2
The number of locations where slag containing SrO was collected over five locations was divided by the number of locations for all surveys. When the particle size of the oxide was 0.1 to 30 mm, the dispersion was 90% or more, and particularly when the particle size was 1 to 20 mm, the dispersion was 95% or more.

The amount of the MgO-containing oxide depends on the composition of the slag before the vacuum treatment, the amount of the slag, and the like.
About 0.4 to 3.2 kg / steel-ton in O pure content is desirable. As a result, 1 is contained in the liquid phase slag after deoxidation.
A solid phase of about 0% by volume or more can be generated.

[0037]

EXAMPLE Using a converter and an RH vacuum processing apparatus, 27
0 t of ultra low carbon steel was melted. In the converter, the C content was 0.1%.
02-0.06%, Mn content 0.01-0.2%, S
Refined to an i content of 0.01-0.03%, 1660-1
The molten steel at 690 ° C. was tapped into a ladle. In tapping, slag outflow from the converter was minimized. Quick lime and Al ₂ O are added to the molten slag in the ladle immediately after tapping as a slag-making agent.
Al-ash and Al-CaO-based flux were added as a slag modifier using a _3- system flux and a CaO-based flux.

Next, vacuum decarburization was performed using a RH vacuum processing apparatus until the carbon content in the molten steel became 0.005% or less. Thereafter, Al was added into the vacuum chamber to perform deoxidation, and the Al content in the molten steel was adjusted to 0.03 to 0.08%.

About 2 minutes after the addition of Al, MgO-containing oxide particles were added. As an oxide, MgO is 92
% Or more of magnesia clinker and MgO
% Magnesia dolomite clinker containing 44% CaO and magnesia spinel clinker containing 58% MgO and 33% Al ₂ O ₃ were used. The particle size of the oxide was set to a particle size containing about 95% in the range of 1 to 20 mm.
These MgO-containing oxides were added to molten steel in the RH vacuum chamber using an alloy addition chute. After the addition of the oxide, a reflux time of at least 5 minutes was ensured. In addition, in some of the tests of Comparative Examples described later, tests in which these oxides were not added were also performed.

After the vacuum treatment, the molten steel has a thickness of 250 mm.
Continuous casting was performed on a slab slab having a width of 1250 mm. A cross section sample was taken from the obtained slab slab, the total oxygen content and nonmetallic inclusions were inspected, and the cleanliness of the steel was investigated.

The total oxygen content was determined by taking samples from the １ ／ thickness and １ ／ thickness positions immediately below the surface of the cross section sample of the slab cast slab, and taking the average value of the three total oxygen contents.

The cleanliness of the slab was examined by observing a 10 cm ² sensitive area within 10 mm immediately below the surface of the slab by a microscope with a magnification of 400 times. Test No. of the present invention example described later. The inspection result of the cleanliness of 1 was set as an index of 1.00, and the inspection results of the cleanliness of other tests were indexed and displayed.

The adhesion of Al ₂ O ₃ to the inner surface of the immersion nozzle for supplying molten steel into the mold was investigated. After the end of the continuous casting, the immersion nozzle was recovered, and the cross-sectional area of the void in the cross section at the center in the height direction was measured. The value obtained by dividing the area of the void of the hole in the cross section at the same position of the unused immersion nozzle by the sectional area of the void of the hole after use was investigated. Test No. of the present invention example described later. The condition of occurrence of clogging of the immersion nozzle of No. 1, that is, the value of the above-described area ratio was set to an index of 1.00, and the status after use of the immersion nozzle of another test was indexed and displayed.

Table 1 shows the test conditions and test results.

[0045]

[Table 1]

Test No. of the present invention example 1 to No. The slag composition before the vacuum treatment of No. 11 is within the range specified in the present invention. Further, the oxide containing MgO after the deoxidation treatment was replaced with M
0.40 to 3.2 kg / steel-ton in pure gO
Was added.

The total oxygen content of the slab slab was determined by the test No. of the present invention. In Examples 1 to 11, it was confirmed that the cleanliness was good at 25 ppm or less. The cleanliness of the cast slab was determined by the test No. of the present invention. For 1 to 11, the index is 0.88
Good cleanliness was about 1.01. Further, the state of clogging of the holes on the inner surface of the immersion nozzle was determined by the test N
o. In the case of 1 to 11, the index was about 0.88 to 1.00, and almost no blockage of the immersion nozzle occurred, which was a good result. In all tests of the present invention, reoxidation of molten steel by slag after deoxidation was suppressed, and generation of nonmetallic inclusions such as Al ₂ O ₃ was able to be suppressed. Therefore, the cleanliness was improved and clogging of the immersion nozzle was also suppressed.

Test No. of Comparative Example 12, the total oxygen content of the slab slab was 33 ppm, and the cleanness of the slab slab was an index of 1.8.
9 was bad. Furthermore, the clogging condition of the immersion nozzle was clogged with an index of 1.80. The cause was that the slag composition before the vacuum treatment was within the range specified in the present invention, but the reoxidation of molten steel by the slag was remarkable because no oxide particles containing MgO were added. by.

Test No. of Comparative Example 13-No. In No. 18, the cleanliness was poor when the total oxygen content of the slab slab was 27 to 34 ppm, and the cleanness of the slab slab was an index of 1.54 to 2.03.
Was bad. Further, the clogging condition of the immersion nozzle was index 1.50 to 1.80, and the clogging occurred frequently. The reason why the cleanliness was poor in the comparative example and the nozzle was easily clogged is as follows.

Test No. 13 and test no. 16 is
CaO / Al of slag before vacuum treatment_TwoO_ThreeIs 2.09,
Or, the MgO content is set to 10.8% in the present invention.
The solid phase of the slag was out of the upper limit of the specified range.
Increased slag Al_TwoO _ThreeDecreased absorption capacity
by.

On the other hand, Test No. In No. 14, CaO / Al ₂ O ₃ of the slag before the vacuum treatment was 0.55, which was out of the lower limit of the range specified in the present invention. Therefore, the CaO content of the slag was small, and the slag had a capacity to absorb Al ₂ O _3. Due to the decline.

Test No. 15 and test no. 18 is
The total content of FeO and MnO in the slag before vacuum treatment is 10.8%, or the content of SiO ₂ is 12.7%.
Therefore, the re-oxidation of the molten steel by the slag was remarkable because the respective values were outside the upper limits of the ranges specified in the present invention.

Test No. No. 17, the MgO content of the slag before the vacuum treatment was 4.6%, which was out of the lower limit of the range specified in the present invention, the slag amount was large, and the addition amount of the pure MgO was small, so that the molten steel by the slag was small. Reoxidation was significant.

[0054]

According to the method of the present invention, it is possible to melt ultra-low carbon steel having excellent cleanliness.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the total content of FeO and MnO in slag and the time required for the C content to reach 0.002% during vacuum decarburization treatment.

FIG. 2 is a graph showing the relationship between the particle size of oxide added to a vacuum chamber and the dispersion ratio of oxide in a liquid slag.

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Claims (1)

[Claims] (1) The molten steel in a ladle is decarburized and then deoxidized by using a vacuum processing apparatus to reduce the carbon content to 0.04.
In a method of melting extremely low carbon steel of not more than 05% by weight,
CaO content and Al _{2 in} slag in ladle before vacuum treatment
The CaO / Al ₂ O ₃ ratio of the weight percentage of the O ₃ content is 0.6 or more and 2.0 or less, and the total content of FeO and MnO is 2
To 10 wt%, a SiO ₂ content of 12 wt% or less and the MgO content of 5 wt% to 10 wt%
After the following adjustments, the molten steel is decarburized, and subsequently, Al is added to the molten steel to deoxidize the molten steel, and at the same time or thereafter, the oxide containing MgO is added to the vacuum chamber. Melting method of ultra low carbon steel with excellent cleanliness.