JP6547638B2 - Method of manufacturing high purity steel - Google Patents

Description

  The present invention relates to a method of producing high purity steel, and more particularly, to a method of producing high purity steel by Al deoxidation.

A molten steel after primary refining that is produced by blowing and decarburizing at atmospheric pressure in a converter or the like is high after the concentration of dissolved oxygen in the steel, so it is cast after being subjected to deoxidation treatment, and the product characteristics are It has gained.
In deoxidation, the addition of an element that combines with oxygen to form an oxide is generally performed, and in addition to Al (aluminum), Si (silicon), C (carbon), Ti (titanium), Ca (calcium) ), Zr (zirconium), REM (rare earth metal), etc. are known to be used as the deoxidizer.
Among them, Al used as a deoxidizing material is inexpensive and has a strong deoxidizing effect, and a steel manufactured using this has a track record of use including the use of a beverage can, and therefore has high versatility.

However, alumina (Al ₂ O ₃ ) formed after the deoxidation reaction by Al may remain as inclusions in the steel after solidification (slabs obtained by continuous casting), which may cause deterioration of product quality. is there. For example, since it causes a crack at the time of can-making processing at the time of using as a raw material of a beverage can, when aiming at a quality improvement, it is necessary to exclude the bad influence of an alumina inclusion.
Furthermore, when a large amount of alumina is present in the molten steel, adhesion and aggregation of the alumina to the inner surface of the immersion nozzle are promoted during casting, and this is due to the occurrence of drifting and nozzle clogging in the mold. The amount of fluctuation of the surface of the molten metal becomes large, which causes the deterioration of quality due to the mixing of the mold powder (powder type inclusions).
In addition, even when metal other than Al is used as a deoxidizer, the produced metal oxide (inclusion) may impair product quality, and it is the same as Al in this point.

Therefore, the following methods have been proposed.
For example, Patent Document 1 discloses that slag-based intermediation suspended in molten steel is sprayed with a granular flux consisting of CaO (fresh lime) and Al ₂ O ₃ together with an inert gas by a lance for gas injection after slag reforming. By incorporating Ar gas (argon gas) from the bottom of the ladle and deoxidizing under inert gas while avoiding contact with the slag, thereby promoting floating of inclusions in the molten steel A method of reducing is disclosed.
Specifically, CaO is introduced into the converter, the slag is solidified, and steel is discharged to a ladle, and Al is uniformly dispersed on the ladle upper slag so that the iron oxide concentration in the slag is 3% by mass or less Reform. Furthermore, metal Al is added as a deoxidizing material, CaO is utilized as a modifier of the generated inclusions, and the inclusions are surfaced by stirring the molten steel.

Further, in Patent Document 2, in order to promote the adsorption removal of the alumina inclusions generated to the slag, the oxygen potential of the ladle slag from after the steel removal to the start of the casting is suppressed to a low level. There is disclosed a technique for preventing reoxidation and adjusting the composition of the slag to one excellent in Al ₂ O ₃ absorption capacity.
Specifically, when tapping steel from the smelting furnace, a predetermined amount of CaO is introduced toward the tapping steel flow, and then the ladle slag after tapping steel contains elemental Al alone or metallic Al as a slag modifier. Add in the form of flux. Furthermore, the degassing treatment is carried out in the RH degassing equipment, and CaO or Al ₂ O ₃ is added to the slag in the ladle during and / or after the degassing treatment, and (wt% CaO) / ( The value of wt% Al ₂ O ₃ ) is adjusted in the range of 0.4 to 0.7, the SiO ₂ concentration is adjusted in the range of ₂ to 15 wt%, and T. By maintaining the Fe concentration at 3.0 wt% or less, re-oxidation by oxygen in the slag is prevented.

Further, in Patent Document 3, in the decarburization treatment of molten steel using a vacuum degassing apparatus and the subsequent deoxidation treatment, the dissolved oxygen necessary for the decarburization is properly maintained while at the same time Al ₂ O ₃ Methods for inhibiting formation are disclosed.
Specifically, a slag modifier is added at the time of tapping to adjust the concentration of lower oxides in the slag, and after decarburizing treatment using a molten steel reflux type degassing apparatus, before the Al deoxidation treatment and And / or later add a slag modifier.

JP 7-300612 A Japanese Patent Application Publication No. 11-21614 Unexamined-Japanese-Patent No. 10-298629

  However, according to the findings of the present inventors, although any of the above-mentioned conventional techniques can expect an effect of reducing alumina inclusions having a large particle diameter (eg, 70 μm or more), alumina inclusions having a small particle diameter (5 to 50 μm It was revealed that the effect of reducing the degree was small.

  The present invention has been made in view of the above circumstances, and provides a method for producing a highly clean steel capable of reducing the number of alumina inclusions compared to the prior art, and in particular, reducing the number of alumina inclusions having a particle size of 20 μm or less. The purpose is to

The method for producing highly clean steel according to the present invention, which meets the above object, sequentially processes molten steel subjected to primary refining to blow acid decarburizing under atmospheric pressure in a ladle processing step including at least steel extraction step and alloy addition. In a method of producing high purity steel, the molten steel is melted and poured into a tundish in a continuous casting process and continuously cast,
During tapping of molten steel in the tapping step, quicklime is introduced into one or both of molten steel and slag, and one or both of metallic aluminum and a flux containing metallic aluminum are added, and slag is added. Reformed, and the slag T. After setting the total of Fe concentration and MnO concentration to 5 mass% or less and the dissolved oxygen concentration of molten steel to be in the range of 100 ppm to 300 ppm,
Metal aluminum is further added to the molten steel in the ladle processing step, the molten steel is deoxidized by stirring for 3 minutes or more and 10 minutes or less, and continuous casting is started in the continuous casting step from the deoxidation treatment Let stand for more than 10 minutes,
In the continuous casting step, a weir that divides the steel receiving section for receiving the molten steel and the discharge section for injecting the molten steel into the mold for continuous casting is provided inside, and the receiving section and the discharge section are communicated 1 or A plurality of molten steel flow channels are formed in the crucible, and the height position of the opening from the bottom surface of the receiving section of the molten steel flow channel on the receiving section side is the molten steel depth of the receiving section The molten steel that has been allowed to stand after the deoxidation treatment is poured into the tundish that is 0.2 times or less of the above, and induction heating is performed on the molten steel flowing through the molten steel flow path.

Here, adding quicklime etc. (quick lime and flux containing metallic aluminum and / or this) when tapping the molten steel in the above-mentioned tapping process means quicklime etc when tapping or after tapping the molten steel. Means adding. For example, tapping of molten steel means adding quicklime etc. to tapping of molten steel, and tapping after tapping molten steel means tapping of molten steel in a ladle containing quicklime etc. This means that quick lime (immediately after tapping) is added after pouring of molten steel into a ladle.
Moreover, the addition of quicklime etc. is performed to one or both of molten steel and slag according to the condition etc. of steel tapping.
Then, the addition of the quicklime and the metal aluminum or the flux containing the same (hereinafter also referred to as metal Al etc.) may be performed simultaneously or separately. The method of adding this quicklime and metallic Al etc. can be changed variously depending on the operating conditions, but for example, both quicklime and metallic Al etc. may be added only at one or the other after tapping. It is also possible to add both quicklime and metal Al at the time of tapping, and further add only either quicklime or metal Al after tapping.

The method of producing high purity steel according to the present invention is characterized in that the slag T.O. In the condition of high iron content and MnO concentration and dissolved oxygen concentration of molten steel, while quick lime is added at the time of tapping steel and metal Al etc. is added to reform the slag, it is generated during this treatment The alumina inclusions can be floated and removed as a low melting point calcium aluminate. Furthermore, by the reforming process of the slag, the T.V. Since metallic aluminum is further added to the molten steel in a state where the total of the Fe concentration and the MnO concentration is 5% by mass or less and the dissolved oxygen concentration of the molten steel is reduced to 100 to 300 ppm, the formation of alumina inclusions can be suppressed.
At this time, small alumina inclusions are formed in the molten steel, but since the amount of formation is suppressed, the generated small alumina inclusions are aggregated and coalesced by stirring the molten steel for a predetermined time (aggregation ) It is considered that the effect can be promoted. Further, by leaving the molten steel after stirring (deacidification) for a predetermined time, floating removal of alumina inclusions having a large particle diameter can be promoted, and the number of particles accompanying promotion of aggregation and integration of alumina inclusions having a small particle diameter It is believed that the reduction can be promoted.
A tundish is internally provided with a weir that divides the molten steel into a hot-water receiving part and a hot-water discharging part, and a molten steel flow path communicating the hot-water receiving part and the hot-water discharging part at a predetermined height position of this weir. Since the molten steel is poured and continuously cast while induction heating is performed in the molten steel flow path, in this tundish, the floating removal effect of the agglomerated and integrated alumina inclusions can be obtained. This is because the temperature of the molten steel in the surface layer (in the vicinity of the surface of the molten metal) of the drainage portion decreases in the tundish, the molten steel temperature of the surface layer of the drainage portion becomes lower than the molten steel temperature of the receiving portion, and the depth of the drainage portion Since a temperature difference occurs in the longitudinal direction, the convection (upflow) of the molten steel caused by the temperature difference causes the inclusions in the molten steel flowing from the molten steel flow path to the discharge portion to float and be removed.
Therefore, the number of alumina inclusions can be reduced compared to the prior art, and in particular, the number of alumina inclusions having a particle size of 20 μm or less can be reduced.

It is explanatory drawing of the tundish which applies the manufacturing method of highly clean steel which concerns on one embodiment of this invention. It is a graph which shows the particle size frequency distribution of the alumina inclusion in molten steel after leaving-to-stand in a ladle. It is a graph which shows the particle size number distribution of the alumina inclusion in the slab cast continuously.

Next, embodiments of the present invention will be described with reference to the attached drawings for understanding of the present invention.
First, the process of achieving the method for producing high purity steel of the present invention will be described.

(1) Findings on Formation of Alumina Inclusions In alumina inclusions (hereinafter, also simply referred to as inclusions), FeO and MnO in slag, dissolved oxygen of molten steel, etc. react with Al which is a deoxidizer. Generate with
For this reason, a slag reforming process (primary removal) in which a flux (slag modifier) containing metal aluminum etc. is added to the slag (and / or molten steel) at the time of tapping (and / or after tapping) from the converter. It is possible to reduce the concentration of FeO or MnO in the slag, that is, to lower the degree of oxidation of the slag, before performing acid treatment or primary deoxidation) and subsequent deacidification treatment (hereinafter also referred to as final deoxidation) It is effective to suppress the generation amount of Al ₂ O ₃ .

Therefore, in order to avoid re-oxidation of the molten steel after the slag reforming, “(mass% T. Fe) + (mass% MnO)” is set to 5 mass% or less as the slag oxidation degree. Note that (mass% T. Fe) and (mass% MnO) are respectively the Fe concentration and MnO concentration in the slag, and this (mass% T. Fe) represents all iron oxides (eg, FeO) in the slag And Fe ₂ O ₃ ) is converted to Fe.
However, even if the above-described slag reforming is performed, it is impossible to completely suppress the formation of Al ₂ O ₃ because dissolved oxygen (free oxygen) remains in the molten steel. The alumina inclusions at the time of formation are small in particle size (20 μm or less) and remain as they are in the molten steel regardless of the passage of time, and the inclusions formed are gradually aggregated with the passage of time. is there.

Immediately after primary refining such as converter blasting, generally, the dissolved oxygen concentration of molten steel (hereinafter, also referred to as dissolved oxygen concentration in molten steel) is as high as about 600 to 900 ppm. An extremely large amount of fine alumina will be produced. As described above, some of the fine alumina thus formed is agglomerated and coarsened over time, and some are floated away, but all inclusions are within a limited time until casting. In particular, it is virtually impossible to completely lift and remove inclusions of the 20 μm or less class.
On the other hand, the amount of alumina produced at the time of final deoxidation described above is governed by the concentration of oxygen dissolved in molten steel to be deoxidized and the amount of addition of metallic aluminum. That is, after the concentration of dissolved oxygen in the molten steel before final deoxidation is reduced, the amount of addition of metallic aluminum is reduced, and aluminum oxidation by oxygen other than dissolved oxygen in molten steel (FeO or MnO in slag) (slag, etc.) Control is extremely important.

From the above, when the slag oxidation degree immediately after the end of primary refining is high and the dissolved oxygen concentration in the molten steel is high (slag oxidation degree: 15 mass% or more, the dissolved oxygen concentration in molten steel: 600 to 900 ppm), Alumina-based inclusions formed by adding slag to molten steel and / or slag and adding flux containing metal Al and / or metal Al to the molten steel and / or slag during steel processing or after tapping) Is floated and removed as low melting point calcium aluminate (CaO-Al ₂ O ₃ ). Furthermore, the amount of alumina remaining in the molten steel can be reduced by performing the final deoxidation with metallic aluminum in a state (100 to 300 ppm) in which the dissolved oxygen concentration in the molten steel after slag reforming is reduced. .
As described above, by setting the dissolved oxygen concentration in the molten steel after the slag reforming to 300 ppm or less, it is possible to suppress the formation of micro inclusions in the time period until the final deoxidation by metal aluminum, and the present invention It is effective in problem solution.

Although the floating removal of the Al ₂ O ₃ based inclusions mentioned above will eventually be adsorbed (absorbed) in the slag, when adding a flux containing metal Al or metal Al as a slag modifier, it is due to aluminum A reduction reaction of lower oxides (FeO, MnO) in the slag occurs, and the activity of the Al ₂ O ₃ component in the slag increases. In addition, when the activity of Al ₂ O _{3 in} the slag is high, the ability to absorb Al ₂ O _{3 in} the slag is lowered, and thus floated Al ₂ O ₃ particles are not adsorbed in the slag and are re-suspended in the molten steel You are more likely to
In order to prevent this, quick lime as a slag modifier is added at the time of the above-mentioned modification processing, and the activity of the Al ₂ O ₃ component in the slag is lowered to absorb the capacity of Al ₂ O ₃ to the slag. The addition of quicklime is effective because it As the inclusions become smaller (for example, 20 μm or less), the possibility of re-inclusion in the molten steel increases, so addition of quicklime is a problem to reduce the inclusions as in the present invention. It is effective for the invention to be

(2) Findings on Stirring Treatment of Molten Steel Stirring treatment of molten steel using a ladle is generally carried out by blowing Ar gas into the molten steel from the bottom of the ladle, and using the floating effect of gas bubbles, in the ladle It is used for homogenization of the composition and temperature of molten steel, and for floating removal of inclusions.
The present inventors have found from the numerous experiments and the like that when carrying out the stirring treatment of molten steel, the contribution form of the stirring differs depending on the amount of alumina formation (the presence of inclusions immediately after the final deoxidation). The situation is as follows.

When the amount of alumina inclusions in the molten steel is relatively large (when deoxidation treatment is performed without slag modification), the effect of improving the absolute value of the number of inclusions by the stirring treatment is small.
In this case, most of the energy from the gas agitation in the ladle is spent on the levitation movement of the already generated coarse inclusions, so the effect of reducing the number of minute inclusions is small. In addition, since the number of fine (20 μm or less) alumina inclusions is large, the collision frequency of the particles becomes high without stirring, and the generated alumina inclusions float with aggregation and aggregation over time. However, since the number of alumina inclusions generated by the addition of metal aluminum in the ladle is too large, the inclusions of which the particle size is not increased still remain in the molten steel.
As described above, when the amount of alumina inclusions is relatively large, the effect of removing the inclusions by stirring is unclear, and it is difficult to remove fine inclusions that can not be aggregated and aggregated even if predetermined agitation processing is performed. Therefore, no significant change in particle size distribution of inclusions due to the presence or absence of agitation treatment is observed.

On the other hand, when the amount of alumina inclusions in the molten steel is relatively small (when the slag reforming is performed to reduce the degree of slag oxidation and the dissolved oxygen concentration of the molten steel to a predetermined amount or less), alumina is added by adding metal aluminum in the ladle. Even if inclusions are formed, the concentration of oxygen dissolved in the molten steel is reduced, so there is a limit to the increase in the amount of alumina inclusions in the molten steel, and the collision frequency of fine inclusion particles due to stirring increases. The particle size distribution of inclusions increases slightly (particle size increases).
In this case, it was found that the number of minute inclusions having a particle diameter of 5 to 20 μm class decreased and the number of inclusions of 30 to 50 μm class increased by stirring treatment.
This is because metallic aluminum is added to the molten steel after the slag reforming, and gas agitation is carried out immediately after the addition of the metallic aluminum, so that the trapping effect of the small number of fine alumina inclusions generated by the number of gas bubbles, It is considered that this is attributed to the fact that the effect of aggregation and coalescence accompanying collision of inclusion particles by stirring (flow) is obtained.

  Therefore, it is important to add metal aluminum and to perform the stirring treatment for a predetermined time immediately after that while reducing the degree of slag oxidation and the dissolved oxygen concentration of the molten steel by slag reforming.

(3) Findings on the Settling of Molten Steel In order to further enhance the floating effect of the aggregation and aggregation by the above-described stirring treatment, the setting after the stirring treatment (final deoxidation) is effective.
Although the buoyancy of the inclusions is increased by the coarsening due to aggregation and coalescence, the upflow due to bubbling is also generated during the stirring process, and the corresponding downflow is also generated. It may be insufficient. For this reason, the floating removal of inclusions can be remarkably promoted by taking the standing time of 10 minutes or more, preferably 30 minutes or more after the stirring process to the start of continuous casting.
The acceleration of the ascent removal is particularly effective for inclusions having a particle size of 70 μm or more. In the case of inclusions having a particle size of about 5 to 50 μm, a remarkable floating removal effect is hard to be recognized, but an accelerating effect on aggregation and aggregation is recognized, which is effective in reducing the number of inclusions of 5 to 20 μm.
Here, stationary means, for example, a state in which the molten steel in the ladle is not subjected to any treatment without performing gas injection or alloy material injection into the molten steel. In addition, since putting a heat insulating material into a ladle is not a process of molten steel, it does not matter even if it inputs a heat insulating material during stilling.

(4) Findings on tundish In continuous casting, molten steel is poured into tundish in an amount corresponding to the continuous casting speed (for example, an amount of about 8 tons / min or less). The flow rate is smaller than the stirring flow rate of the molten steel in the gas stirring of the ladle, and the effect of aggregation and aggregation of inclusions is less desirable.
In addition, when the molten steel temperature decreases in the tundish, the decrease of the solubility product causes the formation of new fine alumina (2 Al + 3 O → Al ₂ O ₃ ), and the increase of alumina inclusions in the cast slab is remarkable. May be
On the other hand, by heating the molten steel in the tundish, the effect of suppressing the formation of new alumina inclusions can be expected. In addition, when a weir (partition wall) is erected inside the tundish and an upward flow is generated in the molten steel in the tundish (generated in the molten steel after heating), stirring of the slag present on the hot water surface in the tundish In the state where the effect is suppressed, it is expected that the inclusion in the molten steel having a particle diameter of about 30 to 50 μm is floated and the effect of causing the inclusion in the slag is expected.

  Therefore, in the inside of the tundish, a weir for dividing (separately arranging) the hot-water receiving part and the hot-water discharging part is erected, and further, one or more communicating the hot-water receiving part and the hot-water discharging part to this weir A hollow refractory forming a molten steel flow path is provided, and the molten steel is heated in the region of the hollow refractory.

Based on the above findings, the inventors of the present invention have developed a method for producing highly clean steel, which complements the effect of refining by leaving molten steel treated with each of slag modification and final deoxidation with the effect of tundish. I thought. Specifically, the effect of refining, that is, the increase in the number of inclusions with a particle size of 30 to 50 μm and the surfacing of inclusions with a particle diameter of 70 μm or more, due to the decrease in the number of minute inclusions with a particle size of 5 to 20 μm. The number of alumina inclusions can be reduced more than before by complementing the promotion of removal by the effect of tundish, that is, the promotion of floating removal of inclusions with a particle diameter of about 30 to 50 μm, and the number of alumina inclusions can be reduced more than before, in particular The number of alumina inclusions of 20 μm or less can be reduced.
Hereinafter, this will be described in detail with reference to FIG.
The method for producing highly clean steel according to one embodiment of the present invention comprises, at least, a tapping process and alloy addition for a molten steel (treated by a converter) subjected to primary refining (blowing with degassing of oxygen) at atmospheric pressure. After sequentially processing and melting in the ladle processing step, the tundish 10 is poured and continuously cast in the continuous casting step.

First, molten steel subjected to primary refining is supplied to a ladle in a steel tapping process.
The slag oxidation degree and the dissolved oxygen concentration in the converter immediately after the end of the primary refining such as converter blasting are high (slag oxidation degree: 15 mass% or more, dissolved oxygen concentration in molten steel: 600 to 900 ppm) .
Therefore, the slag reforming process is performed in the steel extraction process.
Specifically, when tapping molten steel in a converter into a ladle (during or after tapping), quicklime is added to either or both of molten steel and slag, and metallic aluminum (single body) And / or one or both of fluxes containing metallic aluminum.

Thereby, the T.S. The total of the Fe concentration and the MnO concentration is 5% by mass or less, and the dissolved oxygen concentration in the molten steel is in the range of 100 ppm to 300 ppm.
The slag T. From the above findings, the total of Fe concentration and MnO concentration may be 5% by mass or less (preferably 3% by mass or less, more preferably 2% by mass or less), and the lower limit is not particularly defined. In practice, for example, it is about 0.5% by mass.

Then, in the ladle processing step, the molten oxygen in the ladle is reduced in a state where the dissolved oxygen concentration and the slag oxidation degree of the molten steel are reduced (the dissolved oxygen concentration in the molten steel: 100 to 300 ppm, the slag oxidation degree: 5 mass% or less) , Further add metallic aluminum.
The amount of metal aluminum added to the molten steel is preferably small in order to reduce the amount of alumina formation, and for example, about 0.1 to 2.4 kg per ton of molten steel depending on the amount of oxygen dissolved in the molten steel. It is good to add.
In this ladle processing step, addition of metal aluminum and addition of alloy material are performed in consideration of adjustment of the component (final component) of molten steel.

The final deacidification is carried out by stirring the molten steel to which the above-described metal aluminum is added in the range of 3 minutes to 10 minutes (preferably, the lower limit is 4 minutes and the upper limit is 8 minutes).
In addition, gas stirring (bubbling) which blows in inert gas, such as Ar (argon), from the bottom part of a ladle can be used for the stirring process of molten steel.
Here, when the time of the stirring process (stirring time) is less than 3 minutes, the above-mentioned effect of the stirring can not be obtained remarkably. On the other hand, if the stirring time is more than 10 minutes, the temperature drop of the molten steel becomes large, and new alumina inclusion particles are easily generated. This is attributed to the decrease in the solubility product of the “2 Al +3 O → Al ₂ O ₃ ” reaction accompanying the temperature decrease of the molten steel described above.
This can promote the effect of aggregation of small alumina inclusions formed in the molten steel.

As described above, the amount of alumina remaining in the molten steel is reduced by further adding metal aluminum to the molten steel in a state where the concentration of oxygen dissolved in the molten steel and the degree of oxidation of slag are reduced, and performing final deoxidation. Can.
In addition, the above-mentioned final deacidification, that is, addition of metallic aluminum and stirring treatment are performed under atmospheric pressure using, for example, a simple ladle refining facility (CAS), and under vacuum using a vacuum degassing facility (RH). It is not something to do. Therefore, the manufacturing cost can be reduced.

Next, from the end of the deoxidation treatment to the start of continuous casting in the continuous casting process, the molten steel is left in the ladle for 10 minutes or more (preferably 30 minutes or more).
The standing time of the molten steel may be 10 minutes or more (preferably 30 minutes or more) based on the above-mentioned findings, and the upper limit thereof is not particularly specified, but the molten steel becomes molten In fact, the temperature drop is about 60 minutes, for example, because new alumina inclusion particles are likely to be generated.
Thereby, the floating effect by the aggregation of the above-described stirring process can be further enhanced.

Subsequently, the molten steel stirred and allowed to stand after the addition of metal aluminum is poured from the molten steel pan (the above-described ladle) 11 into the tundish 10 through the long nozzle 12 (see FIG. 1).
The tundish 10 has a hot water receiving portion 14 for receiving the molten steel from the molten steel ladle 11 through the long nozzle 12 by a crucible 13 and a waste water portion 15 for injecting the molten steel into a mold (not shown) for continuously casting the molten steel It is divided into An immersion nozzle 16 is provided at the bottom of the drainage portion 15, and the molten steel in the drainage portion 15 is injected into the mold through the immersion nozzle 16.
A hollow refractory 18 forming a molten steel flow path 17 communicating the hot water receiving portion 14 and the hot water discharging portion 15 is provided in the crucible 13 which divides the hot water receiving portion 14 and the hot water discharging portion 15. The hollow refractory material 18 receives molten steel from the opening 19 on the hot water receiving portion 14 side, and discharges the molten steel from the opening 20 on the hot water discharging portion 15 side to the hot water discharging portion 15. The molten steel flowing in the hollow refractory 18 (the molten steel flow path 17) is heated by the induction heating device (here, the induction heating coil 21).

In addition, in order to prevent molten steel from remaining in the hot-water receiving portion 14 after completion of continuous casting, the lower end of the opening 19 (opening 19) located on the hot-water receiving portion 14 side of the hollow refractory 18 (molten steel flow path 17) The height position from the bottom surface 22 of the hot-water receiving portion 14) is 0.2 times (0.2 × H) or less of the molten steel depth (bath depth) H of the hot-water portion 14 (the lower limit is, for example, 0 times (0 × H), that is, the position where the opening 19 is in contact with the bottom surface 22 of the receiving portion 14).
Here, the number of hollow refractories 18 (molten steel channels 17) provided in the crucible 13 may be one, or two or more, depending on the casting conditions, for example. In addition, when the number of hollow refractories is plural, the height position from the bottom of the receiving section of the opening located on the receiving section side of all the hollow refractories is adjusted to satisfy the above-mentioned condition. . The length (the thickness of the crucible 13) of the hollow refractory 18 (the molten steel flow path 17) is, for example, about 500 to 1,500 mm.
And although both the crucible 13 and the hollow refractory 18 are comprised with a refractory, according to a use application, you may comprise with the same material and may comprise with a different material.
Furthermore, the hollow refractory 18 (the molten steel flow path 17) is inclined downward from the hot water receiving portion 14 to the hot water discharging portion 15, but may be horizontal. Moreover, although the depth position of the bottom face 23 of the drainage part 15 is made deeper than the depth position of the bottom face 22 of the hot water receiving part 14, the same depth may be sufficient.
In addition, a molten steel flow path is not limited to forming by a hollow refractory, For example, it can also be formed by making a hole penetrate (penetration hole) to a crucible.

As described above, in order to make the upward flow of molten steel work effectively in the tundish 10, the crucible 13 provided with the hollow refractory 18 is erected inside the tundish 10, and the receiving portion 14 and the waste water It is necessary to clearly divide the space (chamber) of the part 15 (the surface position of the molten steel in the tundish 10 (the hot water receiving part 14 and the drainage part 15) is lower than the upper surface of the crucible 13).
Generally, since the molten steel temperature of the surface layer of the drainage portion 15 decreases in the tundish 10, the molten steel temperature of the surface layer of the drainage portion 15 becomes lower than the molten steel temperature of the receiving portion 14. There is a temperature difference in the molten steel in the longitudinal direction. For this reason, even if the molten steel discharged from the hollow refractory 18 to the discharge part 15 is not inductively heated in the hollow refractory 18, convection (upflow) of the molten steel occurs due to the above-mentioned temperature difference. By convection, inclusions in the molten steel discharged from the hollow refractory 18 to the drainage portion 15 are floated away.

However, even if an upward flow is formed in the tundish 10, the inclusion particle size that can be removed by floating is only a coarse particle size of about 30 to 50 μm or more, and the floating removal of small inclusions of about 5 to 20 μm is difficult. is there.
In addition, when the casting time becomes long and the molten steel temperature decreases in the tundish 10, the buoyancy of inclusions weakens due to the increase in molten steel viscosity, and the floating efficiency of the inclusions is deteriorated, and the alumina formation reaction (2 There is a concern that the solubility product of Al + 3 O → Al ₂ O ₃ ) is reduced and fine Al ₂ O ₃ smaller than 20 μm is newly formed (secondary formation).

Therefore, as described above, by leaving the molten steel subjected to each of the slag reforming and final deoxidation treatments, aggregation of fine Al ₂ O ₃ is promoted and coarsened, and in the tundish, It is important to perform continuous casting while suppressing the formation of new fine Al ₂ O ₃ .
Furthermore, in order to promote the floating of the above-mentioned inclusions and to suppress the formation of new fine Al ₂ O ₃ , a weir 13 is provided in the tundish to divide the hot water receiving portion 14 and the hot water discharging portion 15. The hollow refractory 18 provided in the crucible 13 is communicated with the hot water 14 and the drainage portion 15 to inductively heat the molten steel in the hollow refractory 18.

As a result, convection is generated in the molten steel in the discharge water portion 15 of the tundish 10, and the agglomerated alumina inclusions having a particle diameter of about 30 to 50 μm are efficiently floated up, and this is used as slag on the hot water surface. The effect of capturing is obtained. Furthermore, the induction heating of the molten steel in the hollow refractory 18 to avoid the temperature drop of the molten steel can suppress the formation of new fine alumina in the drainage portion 15.
Therefore, by continuously casting the obtained molten steel, it is possible to reduce the number of alumina inclusions more than before, and in particular, it is possible to manufacture a steel material (slab) in which the number of alumina inclusions having a particle diameter of 20 μm or less is reduced. In particular, this steel material can significantly reduce product inconsistencies (product defects) due to inclusions even when manufacturing a steel plate for a beverage can that is most required for the content regulation of inclusions.

Next, an example carried out to confirm the operation and effect of the present invention will be described.
Here, each condition was changed on the basis of the following method, and the cleanliness of the slab was evaluated.
After performing primary refining in a converter of 350 tons, steel was extracted (after steel removal) in a ladle (carbon concentration: 0.037 mass%, dissolved oxygen concentration in molten steel: 700 ppm), molten steel 1 0.9 kg of quick lime was added per ton, and at the same time, 1.4 kg per ton of molten steel was added with a flux containing aluminum (aluminum dross). After that, add 0.1 to 2.4 kg of metal aluminum per ton of molten steel to the molten steel in the ladle in the simplified ladle refining facility (CAS), and boil the ladle for 2 to 14 minutes (stirring treatment) ) And then allowed to stand for 7 to 45 minutes until the start of casting.
And the molten steel in this ladle was poured into a tundish, and continuous casting was implemented. In this tundish, the hot-water receiving part and the hot-water discharging part are separated by the weir and the hollow refractories having the hot-water receiving part and the hot-water discharging part provided in the weir communicate (the molten steel in the hot-water receiving part The molten steel in the hollow refractory is supplied to a hot water portion and has a structure capable of induction heating. The height position of the lower end of the opening on the hot-water reservoir side of the hollow refractory from the bottom of the hot-water reservoir is 0.2 times (0.2 × H) the bath depth H of the hot-water reservoir. And
The test conditions and the results and evaluations are shown in Table 1.

In Table 1, in the column of "presence or absence of slag modification", slag modification, that is, the presence or absence of the addition of quick lime and the addition of flux after steel removal is described, and the case where both of them are performed is "present". And, it was "absent" when not doing both.
Also, in the column "before final deoxidation", the degree of slag oxidation ((% T. Fe) + (% MnO) before addition of metal aluminum immediately after the stirring treatment (after reforming if slag reforming is performed) And dissolved oxygen concentration ([O] (ppm)) of molten steel are described.
And in the column of "ladle", the time (stirring time) and stationary time of the stirring process in a ladle are described. In the column “After standing T. [O]”, the total oxygen concentration (T. [O] (ppm)) of the molten steel after stirring and standing in a ladle is described.

And "presence or absence of induction heating" describes the presence or absence of the above-mentioned induction heating to the molten steel which flows in the inside of a hollow refractory, and "presence" induces molten steel using the above-mentioned induction heatable tundish. Indicates the case of heating.
Furthermore, in the "slab" column, the total oxygen concentration of the slab after continuous casting is described in the "T. [O] (ppm)" column, and the "inclusion number" column The results (the number of alumina inclusions) of a sample (25 mm square) cut out from the representative position are examined by an optical microscope.
“Evaluation” is judged as good (○) when the result of “number of inclusions” is 1.00 (pieces / cm ² ) or less, and it is more than 1.00 (pieces / cm ² ) The case was judged to be bad in cleanliness (x).

In Examples 1 to 6 in Table 1, the slag oxidation degree and the dissolved oxygen concentration of the molten steel are within the appropriate range by performing the slag reforming (the slag oxidation degree: 5% by mass or less, the dissolved oxygen concentration in the molten steel: 100 to Metal aluminum is further added to the molten steel of 300 ppm), and the mixture is stirred for a time (within 3 to 10 minutes) within the appropriate range, and allowed to stand for a time (more than 10 minutes) within the appropriate range. It is the result of pouring into a tundish equipped with a hollow refractory placed in the range (0.2 × H) and continuously casting.
In this case, the flotation removal effect of alumina type inclusions (calcium aluminate) by slag modification, the formation suppression effect of alumina inclusions by final deoxidation after slag modification, aggregation of small alumina inclusions by stirring treatment of molten steel The effect, floating removal effect of large alumina inclusions due to the standing of molten steel, and floating removal effect of alumina inclusions aggregated by tundish were obtained.
As a result, as shown in Table 1, the total oxygen concentration of the slab could be reduced, the number of alumina inclusions present in the slab could be reduced, and the cleanliness of the slab could be improved (evaluation: :) ).

On the other hand, the comparative example 7 is a result at the time of performing a deoxidation process, without giving slag modification, after primary refining on the conditions of Examples 1-3.
In this case, since the slag modification was not performed, the amount of metal aluminum added to the molten steel must be increased, a large amount of alumina inclusions are generated, and the aggregation and consolidation effect of small alumina inclusions due to the stirring treatment of the molten steel The floating removal effect of alumina inclusions by the placement was not sufficiently obtained.
As a result, as shown in Table 1, the number of alumina inclusions present in the slab increased, and the cleanliness of the slab deteriorated (Evaluation: x).

In Comparative Examples 8 and 9, the stirring time of the molten steel to which metallic aluminum was added at the final deoxidation under the conditions of Examples 1 to 5 was a time outside the appropriate range (Comparative Example 8: 2 minutes, Comparative Example 9: 14 Minutes)).
In this case, in Comparative Example 8, the agglutination effect of small alumina inclusions due to the stirring process is not sufficiently obtained due to insufficient stirring time, and in Comparative Example 9, the molten steel temperature is increased with the prolonged stirring time. Decreased to form many alumina inclusions.
As a result, as shown in Table 1, the number of alumina inclusions present in the slab increased, and the cleanliness of the slab deteriorated (Evaluation: x).

Comparative Example 10 is a result in the case where the standing time of the molten steel after final deoxidation is set to a time (7 minutes) outside the appropriate range under the conditions of Examples 1 to 3.
In this case, the settling time is insufficient, and the time required for floating removal of alumina inclusions due to settling can not be sufficiently secured. As shown in Table 1, the number of alumina inclusions present in the slab is large. And the cleanability of the slab deteriorated (evaluation: x).

The comparative example 11 is a result in the case of not carrying out the stirring process of the molten steel which added the metal aluminum at the time of final deoxidation under the conditions of Examples 1-3.
In this case, the aggregation and aggregation effect of small alumina inclusions was not obtained by the stirring treatment, and as shown in Table 1, the number of alumina inclusions present in the slab increased, and the cleanability of the slab became worse. (Evaluation: x).

The conventional method is a result of the case where molten steel to which metallic aluminum is added without performing slag modification after primary refining is poured into tundish and continuously cast without performing stirring and standing.
In this case, since the slag reforming was not performed, the amount of metal aluminum added to the molten steel was large, a large amount of alumina inclusions were generated, and the effect of the stirring treatment and the standing of the molten steel was not obtained.
As a result, as shown in Table 1, the number of alumina inclusions present in the slab increased, and the cleanliness of the slab deteriorated (Evaluation: x).

  Here, the results of investigation of the particle size frequency distribution of alumina inclusions in molten steel after standing in a ladle in the conventional method and Example 2 described above are shown in FIG. The results of investigation of the particle size number distribution of objects are shown in FIG. 3 respectively. Note that the vertical axis in FIG. 2 indicates the total number of all alumina inclusions (particle size range is 5 to 20 μm, more than 20 μm to 30 μm, more than 30 μm to 50 μm, and more than 50 μm) as 100%. The number ratio of alumina inclusions in the particle size range is shown.

As shown in FIG. 2, although the particle size range of alumina inclusions is lower than that of the conventional method in Example 2 while the particle ratio of both 5 μm or more and 20 μm or less and 20 μm or more and 30 μm or less is lower than the conventional method The ratio is higher in Example 2 than in the conventional method.
That is, the reduction of the number ratio of 5 to 20 μm and the number ratio of 20 to 30 μm with respect to the conventional method corresponds to the increase of the number ratio of 30 to 50 μm and the number ratio of 30 to 50 μm with respect to the conventional method. This is because Example 2 performs the slag reforming before final deoxidation, so the amount of alumina inclusions in the molten steel can be reduced, and as a result, aggregation and aggregation of small alumina inclusions due to stirring and standing of the molten steel It is considered to be attributable to the effect obtained.

Then, the molten steel described above is poured into a tundish having a crucible provided with a hollow refractory and continuously cast, in the second embodiment, a convection effect in the drainage portion of the tundish is obtained, As shown in 3, the number of detected alumina inclusions in the particle size range of more than 30 μm and 50 μm or less was also lower than in the conventional method.
In addition, although the case where quick lime and a flux were simultaneously added to the molten steel after tapping steel was described here, quick tapping and / or flux (metallic aluminum alone) at the time of tapping steel (when tapping and / or after tapping steel) The same tendency was obtained by the addition of the compound (1), without being influenced by the form of the addition.
Therefore, it has been confirmed that the number of alumina inclusions can be reduced compared to the prior art, and in particular, the number of alumina inclusions having a particle size of 20 μm or less can be reduced by using the method for producing high purity steel of the present invention.

Although the present invention has been described above with reference to the embodiment, the present invention is not limited to the configuration described in the above-described embodiment, and the items described in the appended claims It also includes other embodiments and modifications that are considered within the scope. For example, it is also included in the scope of the present invention when combining a part or all of each above-mentioned embodiment or modification example, and constituting a manufacturing method of highly clean steel of the present invention.
Moreover, in the said embodiment, after processing and melting the molten steel which performed primary refining in a steel-making process and a ladle process process one by one in a steel-making process and a ladle process process, continuous casting process demonstrated the case where continuous casting was carried out Before the process, if necessary, processes other than the steel extraction process and the ladle treatment process may be performed.
Furthermore, in the above-mentioned embodiment, although the case where metal aluminum was added at the time of slag reforming and final deoxidation was described, one or two more times are performed between slag reforming and final deoxidation. The metal aluminum may be added a plurality of times as described above.

10: Tundish, 11: Molten steel pan, 12: Long nozzle, 13: Hot water, 14: Hot water part, 15: Waste water part, 16: Immersion nozzle, 17: Molten steel flow path, 18: Hollow refractory, 19, 20: opening, 21: induction heating coil, 22, 23: bottom surface

Claims (1)

The molten steel that has been subjected to primary refining, in which the blasting acid is decarburized under atmospheric pressure, is sequentially processed and melted in at least a steelmaking process and a ladle treatment process including alloy addition, and then poured into a tundish in a continuous casting process. In the method of manufacturing high purity steels for continuous casting,
During tapping of molten steel in the tapping step, quicklime is introduced into one or both of molten steel and slag, and one or both of metallic aluminum and a flux containing metallic aluminum are added, and slag is added. Reformed, and the slag T. After setting the total of Fe concentration and MnO concentration to 5 mass% or less and the dissolved oxygen concentration of molten steel to be in the range of 100 ppm to 300 ppm,
Metal aluminum is further added to the molten steel in the ladle processing step, the molten steel is deoxidized by stirring for 3 minutes or more and 10 minutes or less, and continuous casting is started in the continuous casting step from the deoxidation treatment Let stand for more than 10 minutes,
In the continuous casting step, a weir that divides the steel receiving section for receiving the molten steel and the discharge section for injecting the molten steel into the mold for continuous casting is provided inside, and the receiving section and the discharge section are communicated 1 or A plurality of molten steel flow channels are formed in the crucible, and the height position of the opening from the bottom surface of the receiving section of the molten steel flow channel on the receiving section side is the molten steel depth of the receiving section A method for producing a high-purity steel, comprising pouring molten steel left still after the deoxidation treatment into the tundish which is 0.2 times or less of the above and induction heating the molten steel flowing in the molten steel flow path.

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