JPH11217621A - Production of clean steel in rh vacuum degassing apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably produce clean steel having little inclusions by sufficiently reducing slag being brought in contact with molten steel and preventing the reaction between the molten steel and the slag. SOLUTION: At the time of refluxing molten steel 10 containing Al between a vacuum vessel 1 and a ladle 9 by lowering the pressure in the vacuum vessel of an RH vacuum degassing apparatus and also, blowing Ar gas for refluxing into a rising side immersion tube 4 of the RH vacuum degassing apparatus, the pressure in the vacuum vessel is increased and decreased in plural times and the surface position of the molten steel in the ladle is locked to reduce the slag 11 with the Al in the molten steel. At this time, the reduction speed of the slag is raised by regulating the difference between the max. value and the min. value of the pressure in the vacuum vessel to >=100 Torr and then, the production of the clean steel is further stabilized.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method for stably producing clean steel containing few oxide-based nonmetallic inclusions by using an RH vacuum degassing apparatus.

[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
One of the main causes of inclusions is the reoxidation of molten steel by lower oxides such as iron oxide and MnO in the slag in the ladle, leading to slag as a means of producing clean steel. A number of methods have been proposed for adding a deoxidizer to reduce these lower oxides to prevent reoxidation of molten steel.

[0003] For example, Japanese Patent Application Laid-Open No. Hei 2-30711 discloses that, after tapping from a converter, a deoxidizing agent is charged onto slag in a ladle to remove T.O. A method of producing ultra-low carbon steel by reducing the Fe concentration to 5% or less and then decarburizing the molten steel by an RH vacuum degassing apparatus and then deoxidizing the molten steel is disclosed in Japanese Patent Application Laid-Open No. 2-93017. A method is disclosed in which, after decarburization of molten steel in a vacuum degassing apparatus, a slag is reduced by adding a deoxidizing agent onto slag in a ladle, and then the molten steel is deoxidized to produce ultra-low carbon steel. . In addition, T. The Fe concentration represents a value obtained by summing the iron concentrations in all iron oxides (FeO, Fe ₂ O _3, etc.) in the slag.

[0004]

However, according to the method disclosed in the above-mentioned Japanese Patent Application Laid-Open No. Hei 2-30711, the reduced slag is re-used by the dissolved oxygen in the molten steel until the molten steel is deoxidized. It is oxidized to produce lower oxides such as FeO in the slag again. For this reason, there is a problem that after the molten steel is deoxidized by adding a deoxidizing agent, the slag reacts with Al in the molten steel to generate inclusions in the molten steel, so 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,
With conventional methods, the reduction of slag is not yet sufficient,
Stable production of highly clean steel has not yet been achieved.

[0006] The present invention has been made in view of such circumstances, and an object thereof is to sufficiently reduce slag to prevent reaction between molten steel and slag, and to remove clean steel containing few inclusions by RH vacuum degassing. It is an object of the present invention to provide a method for stable production with an apparatus.

[0007]

According to a first aspect of the present invention, there is provided a method for producing clean steel using an RH vacuum degassing apparatus, wherein the pressure inside the vacuum chamber of the RH vacuum degassing apparatus is reduced and the rising side of the RH vacuum degassing apparatus is raised. When blowing Ar gas for reflux into the immersion tube and refluxing the molten steel containing Al between the vacuum chamber and the ladle,
The molten steel in the ladle is oscillated by increasing or decreasing the pressure in the vacuum chamber.

Further, the method for producing clean steel in an RH vacuum degassing apparatus according to the second invention is characterized in that, in the first invention, the difference between the maximum value and the minimum value of the pressure in the vacuum chamber is set to 100 torr or more. The internal pressure is increased or decreased.

When the pressure in the vacuum chamber is increased or decreased while circulating the molten steel between the vacuum chamber and the ladle of the RH vacuum degassing apparatus, the molten steel surface level in the vacuum chamber fluctuates, and accordingly the ladle is moved. The position of the molten steel surface also swings up and down. At this time, since the molten steel contains Al, even if the slag contains a lower oxide such as iron oxide or MnO, due to the swing of the molten steel surface in the ladle,
The reaction between Al in the molten steel and the lower oxide in the slag is promoted, and the lower oxide in the slag is rapidly reduced and reduced. As a result, after this, Al
And the lower oxide in the slag, that is, reoxidation of the molten steel by the slag can be suppressed, and the cleanliness of the molten steel is improved.

[0010] At this time, if the pressure in the vacuum chamber is increased or decreased by setting the difference between the maximum value and the minimum value of the pressure in the vacuum chamber to 100 torr or more, the molten steel in the ladle fluctuates sharply. Thus, the reaction between Al in the molten steel and the lower oxide in the slag is further promoted, the lower oxide in the slag is reduced to a lower level, and the cleanliness of the molten steel is further improved.

[0011]

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

In FIG. 1, a vacuum tank 1 of an RH vacuum degassing apparatus comprises an upper tank 2 and a lower tank 3 each having an outer shell made of an iron shell and an inner part made of a refractory material. A duct 7 that penetrates and connects to an exhaust device (not shown)
The lower tank 3 is provided with an ascending-side immersion pipe 4 and a descending-side immersion pipe 5 made of a refractory having an iron core (not shown) therein. 3 is provided so as to be separable. An circulating Ar gas blow-in pipe 6 is provided through the ascending immersion pipe 4, and the circulating Ar gas is blown into the ascending immersion pipe 4.

A ladle 9 containing molten steel 10 and slag 11 discharged from a converter (not shown) is located immediately below the vacuum chamber 1.
The ladle 9 is lifted by a lifting device (not shown).
As a result, the ascending-side immersion pipe 4 and the descending-side immersion pipe 5 are immersed in the molten steel 10 in the ladle 9.

Then, Ar gas is blown into the rising side immersion pipe 4 from the reflux Ar gas blowing pipe 6, and the inside of the vacuum chamber 1 is depressurized by evacuating the inside of the vacuum chamber 1 with an exhaust device. The pressure in the vacuum chamber 1 is reduced, and the molten steel 10 in the ladle 9
The rising side immersion pipe 4 is raised together with the Ar gas blown from the reflux Ar gas blowing pipe 6 and flows into the vacuum chamber 1.
After that, a flow returning from the vacuum tank 1 to the ladle 9 via the descending dipping tube 5, that is, a so-called reflux is formed and degassing is performed.

First, after performing a predetermined treatment such as a decarburization treatment in an RH vacuum degassing apparatus, metal Al is added to the molten steel 10 from the raw material inlet 8 to deoxidize the molten steel 10. Metal Al
Is added in such a manner that all dissolved oxygen in the molten steel 10 is removed as Al ₂ O ₃ and 0.01 wt% of Al remains in the molten steel 10. The remaining Al concentration is 0.
If it is less than 01% by weight, deoxidation is weak, and it is not possible to stably produce clean steel. The upper limit of the remaining Al concentration is
The range is determined by the type of steel. In order to accurately grasp the amount of metal Al added, the molten steel 1
It is preferable to measure the dissolved oxygen concentration of 0 with an oxygen probe or the like. In the RH vacuum degassing apparatus, when it is not necessary to perform the treatment performed on the molten steel 10 in an undeoxidized state, such as a decarburization treatment, metal Al is added immediately after the start of the converter and deoxidized in advance. Alternatively, deoxidation may be performed at the start of the processing of the RH vacuum degassing apparatus.

Next, the pressure in the vacuum chamber 1 is increased or decreased.
The pressure in the vacuum chamber 1 is controlled by increasing or decreasing the exhaust capacity of an exhaust device connected to the duct 7. For example, when the exhaust capacity is reduced, the pressure in the vacuum chamber 1 is increased by the Ar gas for recirculation blown into the rising-side immersion pipe 4, and the pressure in the vacuum chamber 1 is reduced by returning the exhaust capacity of the exhaust device to the original. descend. When increasing the pressure in the vacuum chamber 1, once 5 t
It is preferable to reduce the pressure in the vacuum chamber 1 to orr or less and then increase it. This is because if the pressure is increased or decreased while the pressure in the vacuum chamber 1 is higher than 5 torr, the ring flow rate of the molten steel 10 is reduced, which may hinder the removal of impurities such as hydrogen and nitrogen.

When the pressure in the vacuum chamber 1 is increased or decreased, the molten steel 10
Reacts with the slag 11, and the Al concentration in the molten steel 10 decreases. At this time, the Al concentration in the molten steel was 0.01 wt%
Since the rate of reduction of the slag 11 becomes slower when the temperature falls below, a sample is periodically taken from the molten steel
The Al concentration in the molten steel 10 is analyzed to be 0.01 wt% or less.
% Is additionally supplied from the raw material input port 8 so as to be at least 10%.

The pressure in the vacuum chamber 1 is preferably increased or decreased so that the difference between the maximum value and the minimum value of the pressure is 100 torr or more. That is, specifically, it is desirable that the minimum value of the pressure in the vacuum chamber 1 be 5 torr or less and the maximum value be 100 torr or more. When the difference between the maximum value and the minimum value of the pressure is 100 torr or more, the molten steel 10 in the ladle 9 swings sharply, and the molten steel 1
This is because the reaction between Al in the slag 11 and the lower oxide in the slag 11 is further promoted, and the cleanliness of the molten steel 10 is improved.

The cycle of increasing and decreasing the pressure in the vacuum chamber 1 is 5 minutes or less, preferably 3 minutes or less. If the cycle exceeds 5 minutes, the swing of the molten steel 10 in the ladle 9 becomes slow, and the molten steel 10
Reaction with slag 11 is slow, processing time is extended and RH
This is because the processing capacity of the vacuum degassing device is reduced. The shorter the cycle of increasing and decreasing the pressure, the more frequently the molten steel 10 fluctuates and the more the reaction between the molten steel 10 and the slag 11 proceeds, which is preferable. However, since the cycle of increasing and decreasing the pressure is related to the exhaust capacity of the exhaust device, The shortest time within 5 minutes and corresponding to the exhaust capacity of each exhaust device may be set as the increase / decrease cycle.

The number of times the pressure in the vacuum chamber 1 is increased or decreased while circulating the molten steel 10 is preferably at least two times, that is, two cycles or more. If the number of times of increasing and decreasing the pressure in the vacuum chamber 1 is less than two times, the reduction of the slag 11 is insufficient,
This is because the re-oxidation of the molten steel 10 by the slag 11 may occur thereafter. After the treatment for a predetermined time, the inside of the vacuum chamber 1 is returned to the atmospheric pressure, and the degassing treatment is completed.
0 is cast by a continuous casting machine or the like.

By performing the treatment in the RH vacuum degassing apparatus in this manner, the slag 11 can be sufficiently reduced, and as a result, the reoxidation of the molten steel 10 by the slag 11 is prevented, and the cleaning with few inclusions is performed. Steel can be manufactured stably.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment in which the present invention is implemented using the RH vacuum degassing apparatus shown in FIG. 1 will be described below. The target molten steel is
The hot metal spouted from the blast furnace was refined in a converter and was tapped into a ladle. The carbon concentration of the molten steel was 0.02 to 0.06 wt%.
The amount of tapping from the converter was 250 tons per heat, and was conveyed to the RH vacuum degassing apparatus in a non-deoxidized state.

The composition of the slag in the ladle is CaO--SiO _2-
Al is a ₂ O ₃ -MgO based, in some embodiments, in order to understand the impact on the cleanliness of the lower oxide content in ladle slag of CaO in slag in the ladle after tapping the main A slag modifier as a component is added, and (wt% T.Fe) in the slag is added.
And (wt% MnO) were previously adjusted to 5 wt% or less.

Then, the Ar gas flow rate for reflux is set to 3000N.
1 / min, the pressure in the vacuum chamber was reduced to 0.5 to 2 torr, the treatment was performed for 15 minutes, and then the dissolved oxygen concentration in the molten steel was measured with an oxygen probe and calculated based on the dissolved oxygen concentration. A fixed amount of metallic Al was added from the raw material inlet.
After the addition of metal Al, the pressure in the vacuum chamber was increased or decreased for 10 minutes with a cycle of increasing or decreasing the pressure being 2 minutes. At that time, the difference between the maximum value and the minimum value of the pressure in the vacuum chamber was changed in the range of 44 to 162 torr, and the influence was investigated.

Table 1 shows that (wt% T.C.) in the slag before and after the degassing treatment in the operation of 12 heats according to the present invention.
Fe) and (wt% MnO), and (wt% T.Fe) and (wt% Mn) in the slag during degassing.
O) and the difference between the maximum value and the minimum value of the pressure in the vacuum chamber. Heat Nos. 1 and 4 shown in Table 1
7 and 10 are slag modifiers for reducing the lower oxides of slag in advance to reduce (wt% T.Fe) and (wt% Mn) in the slag.
O) and 5 wt% or less.
o. 2, 3, 5, 6, 8, 9, 11, and 12 do not include a slag modifier. For comparison, the operation of the three heats in the conventional example in which the other conditions were the same as in the example without increasing or decreasing the pressure in the vacuum chamber was compared with heat Nos. 13 to 15.
Table 1 also shows the operating conditions. Heat N
o.13 added slag modifier, heat No.1
Nos. 4 and 15 do not add a modifier.

[0026]

[Table 1]

As shown in Table 1, in the embodiment of the present invention,
The lower oxides in the slag are reduced during the degassing treatment, and (wt% T. Fe) and (wt% Mn)
O) were all reduced to 4 wt% or less, and heat Nos.
In No. 2, it was reduced to 3 wt% or less. On the other hand, in the conventional examples of heat Nos. 13 to 15, the reduction of the lower oxides of the slag during the degassing treatment is small, and the reduction amount is at most 0.1.
It was 5 wt%.

In addition, (wt%) in the slag before degassing
T. In Heat Nos. 2, 5, 8, 11, and 14 in which the total value of Fe) and (wt% MnO) is around 6 wt%, samples are taken from molten steel before and after the treatment performed by increasing and decreasing the pressure in the vacuum chamber. Then, the reduction amount of Al in the molten steel by this treatment was investigated. FIG. 2 shows the results of the investigation. As shown in FIG. 2, Al in the molten steel hardly decreases in the conventional example, but in the embodiment of the present invention, the difference between the maximum value and the minimum value of the pressure in the vacuum chamber is increased and the reduction amount of Al in the molten steel is reduced. Grows up to 0.0
A reduction of 08 wt% was observed. As described above, it was found that by increasing the difference between the maximum value and the minimum value of the pressure in the vacuum chamber, the fluctuation of the molten steel was increased and the reaction between the molten steel and the slag was promoted.

After degassing, a slab cast having a thickness of 250 mm and a width of 950 mm is manufactured by casting with a continuous slab casting machine, and then hot-rolled and cold-rolled to manufacture a coiled thin steel sheet. Surface defects due to inclusions of the obtained coiled thin steel plate were inspected, and the incidence rate of surface defects per unit area was indexed as a product defect index, and was investigated for each heat. Table 1
Shows the results of the survey. In Examples of the present invention, the occurrence of surface defects in the thin steel sheet was small, and the product defect index was 1.7 or less, which was good. On the other hand, in the conventional example, the surface defect index was as good as 1.8 in the heat No. 13 to which the slag modifier was added, but the surface defect index was not good in the heat Nos. 14 and 15 to which the slag modifier was not added. Defects occurred frequently, and the product defect index was a high value of 2.8 and 5.1.

FIG. 3 shows the effect of the difference between the maximum value and the minimum value of the pressure in the vacuum chamber on the product defect index. When the pressure difference in the vacuum chamber is increased, the product defect index decreases. However, by setting the difference to 100 torr or more, it was possible to stably prevent surface defects due to inclusions.

FIG. 4 shows an embodiment of the present invention in which the difference between the maximum value and the minimum value of the pressure in the vacuum chamber is divided into (wt% T.Fe) and (wt% FIG. 3 is a diagram showing the effect of the total value of the product with MnO) on the product defect index.
In the figure, ΔP represents the difference between the maximum value and the minimum value of the pressure in the vacuum chamber, and the figure also shows the conventional example. As shown in FIG. 4, in the example of the present invention, the total value of (wt% T.Fe) and (wt% MnO) in the slag before the degassing treatment does not affect the product defect index, It can be seen that the product defect index depends on the difference between the maximum value and the minimum value of the pressure in the vacuum chamber. That is, in the present invention, even if the lower oxide in the slag is previously reduced by adding the slag modifier, the effect is small, and it is understood that the addition of the slag modifier is not necessary.
On the other hand, in the conventional example, (wt% TF
e) and (wt% MnO) increased, the product defect index increased with the increase, and the effect of the slag modifier was observed.

As described above, according to the present invention, lower oxides in slag could be reduced without adding a slag modifier, and clean steel could be stably produced.

[0033]

According to the present invention, when the molten steel is circulated between the vacuum chamber and the ladle, the pressure in the vacuum chamber is increased or decreased to fluctuate the molten steel level in the ladle. The lower oxides in the slag are reduced by Al to suppress the oxidation of the molten steel by the slag, and it is possible to stably produce clean steel with extremely few inclusions without adding a slag modifier. it can.

[Brief description of the drawings]

FIG. 1 is a schematic longitudinal sectional view of an RH vacuum degassing apparatus embodying the present invention.

FIG. 2 is a diagram showing a reduction amount of Al in molten steel after deoxidation in a comparison between an example of the present invention and a conventional example.

FIG. 3 is a diagram showing the effect of a pressure difference in a vacuum chamber on a product defect index in comparison between an example of the present invention and a conventional example.

FIG. 4 (wt% T.Fe) in slag before degassing treatment
FIG. 7 is a diagram showing the effect of the total value of (wt% MnO) and the product defect index on the product defect index in comparison between the example of the present invention and the conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Vacuum tank 2 Upper tank 3 Lower tank 4 Ascending side immersion pipe 5 Descending side immersion pipe 6 Ar gas blowing pipe for reflux 7 Duct 8 Material input port 9 Ladle 10 Molten steel 11 Slag

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Eiji Sakurai 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd.

Claims (2)

[Claims] 1. The pressure inside the vacuum chamber of the RH vacuum degassing apparatus is reduced, and Ar gas for reflux is blown into the rising side immersion pipe of the RH vacuum degassing apparatus to contain Al between the vacuum chamber and the ladle. A method for producing clean steel in an RH vacuum degassing apparatus, characterized in that when circulating molten steel, the pressure in the vacuum chamber is increased or decreased to fluctuate the molten steel surface position in the ladle. 2. The RH vacuum degassing apparatus according to claim 1, wherein the pressure in the vacuum chamber is increased or decreased by setting the difference between the maximum value and the minimum value of the pressure in the vacuum chamber to 100 torr or more. Manufacturing method of clean steel.