JP2017166026A - Manufacturing method of high cleanliness steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of a high cleanliness steel capable of reducing the number of alumina inclusions compared to conventional ones, especially reducing the number of alumina inclusions having particle diameter of 20 μm or less.SOLUTION: There is provided a method for continuously casting a molten steel by pouring the same to a tundish 10 after manufacturing the molten steel by primarily purifying, tapping and ladle treating, a slag is modification treated by inputting caustic lime during tapping and adding metal Al and/or flux containing the same to make total of Fe concentration and MnO concentration at 5 mass% or less and molten presence oxygen concentration of the molten steel at 100 to 300 ppm, then conducting a deoxidation treatment by adding the metal Al and stirring for 3 to 10 min, standing the molten steel for 10 min or more and tapping to the tundish 10 having a weir 16 separating a heat retaining part 13 retaining the molten steel and a heat discharging part 15 inputting the molten steel to a mold 14 for continuously casting inside and height at 0.3 to 0.8 times of molten steel depth H.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for producing high-clean steel, and more particularly to a method for producing high-clean steel by Al deoxidation.

The molten steel after the primary refining manufactured by blowing acid decarburization at atmospheric pressure in a converter etc. has a high dissolved oxygen concentration in the steel, so it is cast after deoxidation treatment and has the characteristics as a product. It has gained.
In deoxidation, an element that forms an oxide by combining with oxygen is generally added. In addition to Al (aluminum), Si (silicon), C (carbon), Ti (titanium), and Ca (calcium). ), Zr (zirconium), REM (rare earth metal) and the like are known to be used as deoxidizers.
Among these, Al used as a deoxidizing material is inexpensive and has a strong deoxidizing effect, and a steel material produced using this has a high versatility because it has a track record of use including beverage cans.

However, the alumina (Al ₂ O ₃ ) produced after the deoxidation reaction with Al remains as inclusions in the steel material after solidification (slab obtained by continuous casting), which may cause a deterioration in product quality. is there. For example, it causes cracking during can-making when used as a material for beverage cans, and therefore it is necessary to eliminate the adverse effects of alumina inclusions in order to improve quality.
Furthermore, when a large amount of alumina is present in the molten steel, during casting, adhesion and aggregation of alumina to the inner surface of the immersion nozzle is promoted, resulting in occurrence of drift in the mold (mold) and nozzle clogging. The amount of fluctuation of the molten metal surface becomes large, which causes quality deterioration due to mixing of mold powder (powder inclusions).
Even when a metal other than Al is used as the deoxidizer, the generated metal oxide (inclusions) may impair the product quality, and this is the same as Al.

Therefore, the following method has been proposed.
For example, Patent Document 1 discloses a slag-based intervening floating in molten steel by spraying granular flux composed of CaO (quick lime) and Al ₂ O ₃ together with an inert gas by a gas blowing lance after slag reforming. Incorporation of the material, and further blowing Ar gas (argon gas) from the bottom of the ladle and deoxidizing under inert gas while avoiding contact with the slag, promotes the rise of inclusions in the molten steel A method of reducing is disclosed.
Specifically, CaO is put into the converter, the slag is solidified and steel is discharged to the ladle, and Al is uniformly sprayed on the slag on the ladle, so that the iron oxide concentration in the slag is 3 mass% or less. Reform. Furthermore, metal Al is added as a deoxidizer, CaO is utilized as a modifier for the inclusions to be generated, and inclusions are floated by stirring the molten steel.

In addition, in Patent Document 2, in order to promote the adsorption removal of the generated alumina inclusions to the slag, the oxygen potential of the ladle slag from after the steel output to the start of casting is suppressed to a low level, A technique for preventing reoxidation and adjusting the component composition of slag to one having excellent Al ₂ O ₃ absorption capacity is disclosed.
Specifically, at the time of steel output from the refining furnace, a predetermined amount of CaO is introduced toward the steel output flow, and then the ladle slag after steel output contains single metal or metal Al as a slag modifier. Add in the form of flux. Further, degassing treatment is carried out in the RH degassing equipment, and during and / or after the degassing treatment, CaO or Al ₂ O ₃ is added to the slag in the pan, and (wt% CaO) / ( wt% Al ₂ O ₃ ) within the range of 0.4 to 0.7, SiO ₂ concentration within the range of ₂ to 15 wt%, and T.W. By maintaining the Fe concentration at 3.0 wt% or less, reoxidation due to oxygen in the slag is prevented.

Then, Patent Document 3, decarburization of the molten steel using a vacuum degassing apparatus, and, in the deacidification subsequent thereto, at the same time properly to hold the dissolved oxygen necessary for decarburization, the Al ₂ O ₃ A method of inhibiting formation is disclosed.
Specifically, the slag modifier is added at the time of steel output to adjust the concentration of the lower oxide in the slag, and after decarburization using a molten steel recirculation type degassing apparatus, before the Al deoxidation treatment and At a later time, a slag modifier is added.

Japanese Patent Laid-Open No. 7-300612 Japanese Patent Laid-Open No. 11-21614 JP-A-10-298629

  However, according to the knowledge of the present inventors, all of the conventional techniques can reduce the effect of reducing alumina inclusions having a large particle size (for example, 70 μm or more), but alumina inclusions having a small particle size (5 to 50 μm). It was clarified that there was little effect to reduce the degree).

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

The method for producing a high clean steel according to the present invention that meets the above-mentioned object is to sequentially process molten steel that has undergone primary refining that is blown acid decarburized at atmospheric pressure in a ladle treatment process that includes at least a steelmaking process and alloy addition. In the manufacturing method of high clean steel, after pouring into a tundish in the continuous casting process and continuously casting,
At the time of discharging the molten steel in the steel extraction step, lime is added to one or both of the molten steel and slag, and either one or both of the flux containing metal aluminum and metal aluminum is added to form slag. Is reformed, and the slag T.I. After making the total of Fe concentration and MnO concentration 5 mass% or less, and making dissolved oxygen concentration of molten steel into the range of 100 ppm or more and 300 ppm or less,
In the ladle treatment step, metallic aluminum is further added to the molten steel, the molten steel is stirred for 3 minutes to 10 minutes and deoxidized, and from the deoxidation treatment to the start of continuous casting in the continuous casting step. Let stand for more than 10 minutes,
In the continuous casting process, a weir for partitioning into a hot water receiving part for receiving molten steel and a hot water discharging part for pouring the molten steel into a mold for continuously casting the molten steel is provided inside, and the height of the weir is set to 0.3 of the molten steel depth. The molten steel that has been allowed to stand after the deoxidation treatment is poured into the tundish that has been doubled to 0.8 times.

Here, adding quick lime or the like (quick lime and metallic aluminum and / or a flux containing the same) at the time of steel discharge of the molten steel in the steel output process described above means that quick lime or the like is applied during or after the steel discharge of the molten steel. Is added. For example, when the molten steel is discharged, it means adding quick lime etc. during the molten steel, and after the molten steel is discharged, when the molten steel is discharged into a ladle containing quick lime, etc. It means the case where quick lime or the like is added immediately after the molten steel is poured into the ladle.
Moreover, addition of quicklime etc. is performed with respect to one or both of molten steel and slag according to the condition etc. of steel output.
And addition of quicklime, metal aluminum, and the flux (henceforth metal Al etc.) containing this may be performed simultaneously, and may be performed separately. The method of adding quick lime and metal Al can be variously changed depending on the operation status.For example, both quick lime and metal Al etc. may be added only at the time of steel output or after steel output. In addition, both quick lime and metal Al can be added at the time of steel output, and only one of quick lime and metal Al can be added after the steel output.

The method for producing high-clean steel according to the present invention is based on the slag T.S. In the state where the Fe concentration and MnO concentration and the dissolved oxygen concentration of the molten steel are high, quick lime is added at the time of steel output and metal slag is added to reform the slag. The alumina inclusions can be levitated and removed as a low melting point calcium aluminate. Further, by the slag reforming treatment, the slag T.I. In the state where the total of Fe concentration and MnO concentration is 5 mass% or less and the dissolved oxygen concentration of molten steel is reduced to 100 to 300 ppm or less, metallic aluminum is further added to the molten steel, so the formation of alumina inclusions can be suppressed. .
At this time, although small alumina inclusions are generated in the molten steel, the amount of generation is suppressed, and thus the molten steel is agitated for a predetermined time to agglomerate and coalesce the generated small alumina inclusions (aggregation coalescence). ) It is thought that the effect can be promoted. In addition, by allowing the molten steel after stirring treatment (deoxidation treatment) to stand for a predetermined time, the floating removal of alumina inclusions with a large particle size can be promoted, and the number associated with the promotion of agglomeration and coalescence of alumina inclusions with a small particle size It is thought that the decrease can be promoted.
And, since this molten steel is poured continuously into a tundish provided with a weir with a predetermined height for partitioning into a hot water receiving part and a hot water discharging part, in this tundish, the aggregated and coalesced alumina inclusions A floating removal effect is obtained.
Therefore, the number of alumina inclusions can be reduced as compared with the conventional case, 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 the highly clean steel which concerns on one embodiment of this invention. It is a front view of the weir of the same tundish. It is a graph which shows the particle size frequency distribution of the alumina inclusion in molten steel after standing 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 accompanying drawings for understanding of the present invention.
First, the background of the high clean steel manufacturing method of the present invention will be described.

(1) Knowledge about the formation of alumina inclusions The alumina inclusions (hereinafter, also simply referred to as inclusions) react with FeO, MnO in slag, dissolved oxygen in molten steel, and Al, which is a deoxidizer. Generate with
For this reason, a slag reforming process (primary removal) in which a flux (slag modifier) containing metallic aluminum or the like is added to slag (and / or molten steel) at the time of (and / or after) the steel from the converter. Reducing the concentration of FeO or MnO in the slag before the subsequent deoxidation treatment (hereinafter also referred to as final deoxidation), that is, reducing the oxidation degree of the slag, This is effective for suppressing the amount of Al ₂ O ₃ produced.

Therefore, in order to avoid re-oxidation of the molten steel after slag reforming, “(mass% T. Fe) + (mass% MnO)” is set to 5 mass% or less as the degree of slag oxidation. In addition, (mass% T.Fe) and (mass% MnO) are respectively the Fe concentration and MnO density | concentration in slag, and this (mass% T.Fe) is all the iron oxides (for example, FeO in slag). And Fe ₂ O ₃ ) converted to Fe.
However, even if the above-described slag reforming is performed, dissolved oxygen (free oxygen) remains in the molten steel, and thus it is impossible to completely suppress the production of Al ₂ O ₃ . The alumina inclusions at the beginning of production have a small particle size (20 μm or less), and remain in the molten steel as they are regardless of the passage of time, and when the produced inclusions aggregate gradually over time. is there.

Immediately after primary refining such as converter blowing, 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, and in this state, deoxidation treatment by adding metallic aluminum is performed. An extremely large amount of fine alumina is produced. As described above, some of the fine alumina thus produced is aggregated and coalesced with the passage of time to become coarse and lifted and removed, but within a limited time until casting, all inclusions, In particular, it is practically impossible to completely lift and remove inclusions of a class of 20 μm or less.
On the other hand, the amount of alumina produced during the final deoxidation is governed by the dissolved oxygen concentration in the molten steel to be deoxidized and the amount of metal aluminum added. That is, after reducing the dissolved oxygen concentration in the molten steel before final deoxidation, the amount of metal aluminum added is reduced, and aluminum oxidation (such as slag) by oxygen other than dissolved oxygen in the molten steel (FeO and MnO in the slag) It is extremely important to suppress this.

From the above, in the state where the slag oxidation degree immediately after the end of primary refining and the dissolved oxygen concentration in molten steel are high (slag oxidation degree: 15 mass% or more, dissolved oxygen concentration in molten steel: 600 to 900 ppm) Alumina inclusions produced during the treatment by adding lime to molten steel and / or slag and adding metal Al and / or flux containing metal Al during or after steelmaking Is floated and removed as a 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 final deoxidation with metallic aluminum in a state where the dissolved oxygen concentration in the molten steel after the slag reforming is reduced (100 to 300 ppm). .
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 fine inclusions in the time zone until the final deoxidation with metallic aluminum, Effective for solving problems.

The above-described levitation removal of the Al ₂ O ₃ inclusions will eventually be adsorbed (absorbed) by the slag, but if a flux containing metal Al or metal Al is added as a slag modifier, it will be caused by 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 Al ₂ O ₃ activity in the slag is high, the ability to absorb Al ₂ O ₃ into the slag decreases, so that the floating Al ₂ O ₃ particles are not adsorbed in the slag and resuspended in the molten steel. Is more likely to do.
In order to prevent this, by adding quick lime as a slag modifier during the above-described reforming treatment and reducing the activity of the Al ₂ O ₃ component in the slag, the ability to absorb Al ₂ O ₃ in the slag Therefore, the addition of quicklime is effective. In addition, since the possibility of re-mixing into the molten steel increases as the inclusions become minute (for example, 20 μm or less), the addition of quick lime has a problem of reducing minute inclusions as in the present invention. This is effective for the invention.

(2) Knowledge about stirring process of molten steel The stirring process of molten steel using a ladle is generally performed by blowing Ar gas into the molten steel from the bottom of the ladle and using the floating effect of gas bubbles. It is used to equalize the composition and temperature of molten steel, and to remove inclusions.
The present inventors have found from a number of experiments and the like that when the molten steel is stirred, the contribution form of stirring varies depending on the amount of alumina produced (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 performing slag reforming), 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 generated by gas stirring in the ladle is spent on the floating movement of the coarse inclusions that have already been generated, so the effect of significantly reducing the number of minute inclusions is small. Moreover, since the number of fine alumina inclusions (20 μm or less) is large, the collision frequency between the particles increases without stirring, and the generated alumina inclusions rise by aggregation and coalescence over time. However, since the number of alumina inclusions produced by the addition of metallic aluminum in the ladle is too large, inclusions whose particle size has not increased still remain in the molten steel.
Thus, when there are relatively many alumina inclusions, the effect of inclusion removal by stirring is unclear, and it is difficult to remove fine inclusions that cannot be aggregated and coalesced even if a predetermined stirring treatment is performed. Therefore, no significant change in the particle size distribution of inclusions due to the presence or absence of the stirring treatment is observed.

On the other hand, when there are relatively few alumina inclusions in the molten steel (when slag reforming is performed and the slag oxidation degree and the dissolved oxygen concentration of the molten steel are reduced below a predetermined amount), the addition of metallic aluminum in the ladle Even if inclusions are generated, the dissolved oxygen concentration 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 frequency of collision of fine inclusion particles due to stirring treatment increases, The particle size distribution of inclusions slightly increases (the particle size increases).
In this case, it was found that the number of fine inclusions having a particle size of 5 to 20 μm decreased and the number of inclusions of 30 to 50 μm class increased by stirring treatment.
This is by adding metal aluminum to the molten steel after slag reforming, and by performing gas stirring immediately after the addition of this metal aluminum, the effect of trapping the generated small alumina inclusions by gas bubbles, This is considered to be due to the effect of agglomeration and coalescence accompanying the collision of inclusion particles by stirring (flow).

  Therefore, it is important to further add metallic aluminum and to carry out the stirring treatment for a predetermined time immediately after the slag oxidation degree and the dissolved oxygen concentration of the molten steel are reduced by slag reforming.

(3) Findings regarding the standing of molten steel In order to further enhance the floating effect by agglomeration by the above-described stirring treatment, the standing after the stirring treatment (final deoxidation) is effective.
Increasing the buoyancy of inclusions due to coarsening due to agglomeration and coalescence, but during the stirring process, an upward flow is formed by bubbling, and a corresponding downward flow also occurs. It may be insufficient. For this reason, the floating removal of inclusions can be remarkably accelerated by taking a standing time of 10 minutes or more, preferably 30 minutes or more after the stirring treatment until the start of continuous casting.
This promotion of floating removal is particularly effective for inclusions having a particle size of 70 μm or more. In addition, with inclusions having a particle size of about 5 to 50 μm, a remarkable floating removal effect is hardly recognized, but an effect of promoting aggregation and coalescence is recognized and effective in reducing the number of inclusions having a particle size of 5 to 20 μm.
Here, standing refers to a state in which some treatment is not performed on the molten steel in the ladle, for example, without blowing gas into the molten steel or charging the alloy material. In addition, since it is not a process of molten steel to introduce | transduce a heat insulating material into a ladle, even if it inserts a heat insulating material, it does not interfere.

(4) Knowledge about tundish In continuous casting, molten steel is poured into the 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 it is difficult to expect the effect of inclusion aggregation.
However, when a weir (lower weir) is erected inside the tundish and an upward flow is generated in the molten steel in the tundish, the stirring effect of the slag present on the molten metal surface in the tundish is suppressed, It can be expected that the inclusions in the molten steel having a particle diameter of about 50 μm are levitated and captured by the slag.

  Therefore, it is necessary to erect a weir that divides (independently arranges) the hot water receiving portion and the hot water discharging portion inside the tundish.

Based on the above findings, the present inventors have developed a highly clean steel production method that complements the effect of refining by leaving the molten steel subjected to slag reforming and final deoxidation treatment by the effect of tundish. I came up with it. Specifically, the effect of refining, that is, the increase in the number of inclusions with a particle size of 30-50 μm accompanying the decrease in the number of fine inclusions with a particle size of 5-20 μm, and the rise of inclusions with a particle size of 70 μm or more By supplementing the promotion of removal with the effect of tundish, that is, the promotion of floating removal of inclusions having a particle size of about 30 to 50 μm, the number of alumina inclusions can be reduced as compared with the conventional case. The number of alumina inclusions of 20 μm or less class can be reduced.
Hereinafter, it will be described in detail with reference to FIGS.
A method for producing a high clean steel according to an embodiment of the present invention includes at least a steelmaking step and an alloy addition for a molten steel subjected to primary refining (treated in a converter) by blowing acid decarburization under atmospheric pressure. This is a method of performing continuous casting by pouring the tundish 10 in a continuous casting process after sequentially processing and melting in the ladle processing process.

First, molten steel that has undergone primary refining is supplied to the ladle in the steelmaking process.
The slag oxidation degree and the dissolved oxygen concentration in the molten steel immediately after the end of the primary refining such as converter blowing are in a high state (slag oxidation degree: 15 mass% or more, dissolved oxygen concentration in molten steel: 600 to 900 ppm). .
Therefore, a slag reforming process is performed in the steel output process.
Specifically, when putting molten steel in the converter into a ladle (at the time of or after steelmaking), quick lime is added to one or both of the molten steel and slag, and metallic aluminum (single unit) And either one or both of fluxes containing metallic aluminum.

As a result, the T.O. 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.
In addition, T. of slag. The total of the Fe concentration and the MnO concentration may be 5% by mass or less (preferably 3% by mass or less, more preferably 2% by mass or less) based on the above findings, and the lower limit value is not particularly defined. Actually, it is about 0.5 mass%, for example.

And in the ladle treatment process, 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, metallic aluminum is added.
In addition, it is preferable to reduce the amount of metallic aluminum added to the molten steel to reduce the amount of alumina produced. Depending on the amount of dissolved oxygen in the molten steel, for example, about 0.1 to 2.4 kg per ton of molten steel It is good to add.
In this ladle treatment process, the addition of metal aluminum and the addition of alloy material are performed in consideration of the component adjustment (final component) of the molten steel.

The molten steel to which the above-described metallic aluminum is added is stirred for 3 minutes to 10 minutes (preferably, the lower limit is 4 minutes, and the upper limit is 8 minutes) to perform final deoxidation.
In addition, the gas stirring (bubbling) which blows 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 (stirring time) of the stirring treatment is less than 3 minutes, the above-described action and effect of stirring cannot be remarkably obtained. On the other hand, when the stirring time exceeds 10 minutes, the temperature drop of the molten steel becomes large, and new alumina inclusion particles are easily generated. This is because the solubility product of the reaction of “2 Al +3 O → Al ₂ O ₃ ” decreases as the temperature of the molten steel decreases.
Thereby, the effect of the aggregation coalescence of the small alumina inclusion produced | generated in molten steel can be accelerated | stimulated.

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 in which the dissolved oxygen concentration and slag oxidation degree in the molten steel are reduced, and performing final deoxidation. Can do.
The final deoxidation, that is, the addition of metal aluminum and the stirring process are performed under atmospheric pressure using, for example, a simple ladle refining equipment (CAS) and under vacuum using a vacuum degassing equipment (RH). Is not something you do. For this reason, the manufacturing cost can be reduced.

Next, it is allowed to stand for 10 minutes or more (preferably 30 minutes or more) in a state where the molten steel is put in a ladle from the end of the deoxidation treatment to the start of continuous casting in the continuous casting step.
In addition, the standing time of molten steel should just be 10 minutes or more (preferably 30 minutes or more) from the above-mentioned knowledge, and although it does not prescribe | regulate especially the upper limit, as stationary time becomes long, molten steel In practice, for example, it takes about 60 minutes.
Thereby, the floating effect by the above-mentioned agglomeration coalescence of the stirring treatment can be further enhanced.

Subsequently, the molten steel that has been agitated and allowed to stand after the addition of metal aluminum is poured from the molten steel pan (the ladle described above) 11 into the tundish 10 via the long nozzle 12 (see FIG. 1).
The tundish 10 has a weir (lower weir) that divides the interior into a hot water receiving portion 13 that receives molten steel from a molten steel pan 11 through a long nozzle 12 and a hot water discharging portion 15 that is poured into a mold 14 for continuously casting molten steel. ) 16 is provided. An immersion nozzle 17 is provided at the bottom of the hot water discharge section 15, and molten steel in the hot water discharge section 15 is poured into the mold 14 through the immersion nozzle 17.
The weir 16 is erected so as to go from the bottom surface 18 of the tundish 10 to the bath surface (bath surface), and its height is 0.3 times the molten steel depth (bath depth) H (m). (0.3 × H) or more and 0.8 times (0.8 × H) or less. The molten steel depth H (m) means the distance from the bottom surface 18 of the tundish 10 where the weir 16 is disposed to the bath surface.

As described above, the height of the weir needs to be 0.3 times or more the depth of the molten steel in order for the upward flow of the molten steel to act effectively in the tundish. On the other hand, when the height of the weir exceeds 0.8 times the depth of the molten steel, the upward flow may undesirably stir the hot water surface slag in the tundish.
Therefore, the height of the weir 16 is set to 0.3 times (preferably 0.4 times) or more and 0.8 times (preferably 0.7 times) or less of the molten steel depth H (m).
A plurality of weirs can be installed at intervals in the flow direction of the molten steel in the tundish. In this case, an upper weir is installed between the weirs adjacent in the flow direction of the molten steel to form a downward flow in the molten steel, and the flow of the molten steel is zigzag in the vertical direction as viewed from the side, It is possible to increase the residence time of the molten steel.

In addition, a through hole 19 is generally provided near the bottom of the weir 16 in order to facilitate the discharge of the remaining hot water in the used tundish 10 (see FIG. 2). The shape of the through hole 19 is a quadrangle when viewed from the front. When the width of the bath surface is W, the inner width W1 in the height direction is 1/5 × W and the inner width W2 in the width direction is 1/5 ×. W. The configuration of the through hole is not particularly limited as long as the remaining hot water can be easily discharged. For example, the inner width W1 in the height direction is within a range of 1/5 × W or less. The inner width W2 in the width direction can be adjusted within a range of 1/5 × W or less.
The through-holes 19 are formed in the weir 16 in two pieces (one or a plurality of through-holes 19). However, if the through-holes 19 are of this level, the effect of generating an upward flow in the molten steel can be obtained. . Further, if the above-described through-hole has an opening area equal to or less than that, it is possible to generate an upward flow in the molten steel in the tundish, and the effect of the present invention can be obtained. It is done.

As a result, an upward flow is generated in the molten steel in the tundish 10, and the aggregated and aggregated alumina inclusions having a particle diameter of about 30 to 50 μm are levitated, and the effect of capturing this in the slag on the molten metal surface is obtained. .
Therefore, by continuously casting the obtained molten steel, the number of alumina inclusions can be reduced as compared with the prior art, and in particular, a steel material (slab) with a reduced number of alumina inclusions having a particle size of 20 μm or less can be produced. In particular, this steel material can remarkably reduce product mismatch caused by inclusions even in the production of steel plates for beverage cans, etc., which are the most demanding for inclusion content regulation.

Next, examples carried out for confirming the effects of the present invention will be described.
Here, each condition was changed based on the following method, and the cleanliness of the slab was evaluated.
After primary refining in a 350-ton converter, the molten steel (carbon concentration: 0.037 mass%, dissolved oxygen concentration in molten steel: 700 ppm) that was steeled in the ladle (after steelmaking) 0.9 kg of quicklime was introduced per ton, and simultaneously, 1.4 kg of flux (aluminum dross) containing metallic aluminum was added per ton of molten steel. Thereafter, in a simple ladle refining equipment (CAS), 0.1 to 2.4 kg of metal aluminum is added to the molten steel in the ladle, and ladle bubbling treatment (stirring treatment) for 2 to 14 minutes. ), And then allowed to stand for 6 to 49 minutes until the start of casting.
And the molten steel in this ladle was poured into a tundish having a lower weir at a height of 0.2 × H to 0.9 × H with respect to the bath depth H (m), and continuous casting was performed. .
Table 1 shows the test conditions, the results, and the evaluation.

In Table 1, in the column “Presence / absence of slag reforming”, slag reforming, that is, whether or not to add quick lime after steelmaking and whether or not to add flux is described. The case where both of these were not performed was determined as “None”.
Further, in the column “before final deoxidation”, the slag oxidation degree ((% T. Fe) + (% MnO)) before the addition of metal aluminum immediately before the stirring treatment (after the slag modification, after the modification) ) And the dissolved oxygen concentration ([O] (ppm)) of the molten steel.
And in the column of "Ladle", the time (stirring time) and the standing time of the stirring process in the ladle are described. In the column of “T. [O] after standing”, the total oxygen concentration (T. [O] (ppm)) of the molten steel after being left to stand by stirring in a ladle is described.
Further, in the “cast slab” column, the “T. [O] (ppm)” column describes the total oxygen concentration of the slab after continuous casting, and the “inclusion number” column. Shows the results (number of alumina inclusions) of a sample (25 mm square) cut out from the representative position, which was examined with an optical microscope.
Incidentally, "evaluation" is the "inclusion number" result is determined to 1.00 (pieces / cm ²⁾ or less of the case where good cleanability (○), 1.00 (pieces / cm ²⁾ greater than The case was judged to be poor (×).

In Examples 1 to 8 in Table 1, by performing slag reforming, the slag oxidation degree and the dissolved oxygen concentration of the molten steel are within an appropriate range (slag oxidation degree: 5 mass% or less, dissolved oxygen concentration in the molten steel: 100 to After adding metal aluminum to the molten steel (300 ppm) and stirring it for a time within the appropriate range (3 to 10 minutes) and allowing it to stand for a time within the appropriate range (10 minutes or more), It is the result of pouring into a tundish having a lower weir with a height within the range (range of 0.3 × H to 0.8 × H) and continuously casting.
In this case, the floating removal effect of alumina inclusions (calcium aluminate) by slag modification, the suppression effect of formation of alumina inclusions by final deoxidation after slag modification, agglomeration of small alumina inclusions by stirring treatment of molten steel The effect, the floating removal effect of large alumina inclusions by standing the molten steel, and the effect of imparting the upward flow to the molten steel by the lower weir of the 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 9 is a result at the time of performing a deoxidation process without performing slag modification | reformation after primary refining on the conditions of Example 1. FIG.
In this case, since the slag modification was not performed, the amount of metallic aluminum added to the molten steel had to be increased, so that a large amount of alumina inclusions were formed, and the effect of agglomeration and coalescence of small alumina inclusions by the stirring treatment of the molten steel, The effect of floating and removing alumina inclusions by placing was not sufficiently obtained.
As a result, as shown in Table 1, the number of alumina inclusions present in the slab increased, and the cleanability of the slab deteriorated (evaluation: x).

Comparative Examples 10 and 11 are the results when the stirring time of the molten steel to which metallic aluminum was added during the final deoxidation was set to a time outside the appropriate range (Comparative Example 10: 2 minutes, Comparative Example 11: 14 minutes). (Comparison with Examples 1, 4, and 5).
In this case, in Comparative Example 10, the agitation time is insufficient and the effect of agglomeration and coalescence of small alumina inclusions by the agitation treatment cannot be sufficiently obtained. In Comparative Example 11, the temperature of the molten steel is increased as the agitation time is prolonged. Decreased and many alumina inclusions were produced.
As a result, as shown in Table 1, the number of alumina inclusions present in the slab increased, and the cleanability of the slab deteriorated (evaluation: x).

The comparative example 12 is a result when the stationary time of the molten steel after the final deoxidation is set to a time outside the appropriate range (6 minutes).
In this case, the standing time is insufficient, and the time required for the floating and removal of the alumina inclusions by standing cannot be sufficiently secured. As shown in Table 1, the number of alumina inclusions present in the slab is large. As a result, the cleanability of the slab deteriorated (evaluation: x).

The comparative example 13 is a result at the time of not stirring the molten steel which added the metal aluminum at the time of final deoxidation.
In this case, the effect of agglomeration and coalescence of small alumina inclusions by the stirring treatment was not obtained, and as shown in Table 1, the number of alumina inclusions present in the slab increased, and the cleanability of the slab deteriorated. (Evaluation: x).

Comparative Examples 14 and 15 are the results when the lower weir provided in the tundish is set to a height outside the appropriate range (Comparative Example 14: 0.2 × H, Comparative Example 15: 0.9 × H). (Comparison with Examples 1, 7, and 8)
In this case, in Comparative Example 14, the height of the lower weir is too low to allow the upward flow of molten steel to effectively act in the tundish. In Comparative Example 15, the height of the lower weir is Too high and the upward flow stirred the hot water surface slag in the tundish.
As a result, as shown in Table 1, the number of alumina inclusions present in the slab increased, and the cleanability of the slab deteriorated (evaluation: x).

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

  Here, with respect to the above-described conventional method and Example 5, the results of investigating the particle size frequency distribution of the alumina inclusions in the molten steel after standing in the ladle are shown in FIG. 3, and the alumina inclusions in the continuously cast slab are shown in FIG. The results of investigating the particle number distribution of the products are shown in FIG. The vertical axis in FIG. 3 represents each of the total number of alumina inclusions (particle size range of 5 μm 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. 3, the particle size range of the alumina inclusion is 5 μm or more and 20 μm or less and the number ratio of 20 μm or more and 30 μm or less is lower in Example 5 than the conventional method, but the number of particles exceeding 30 μm and 50 μm or less. The ratio of Example 5 is higher than that of the conventional method.
That is, the decrease in the number ratio of 5 μm or more and 20 μm or less and more than 20 μm and 30 μm or less with respect to the conventional method of Example 5 corresponds to the increase of the number ratio of more than 30 μm and 50 μm or less with respect to the conventional method of Example 5. This is because Example 5 performs slag reforming before the final deoxidation, so that the amount of alumina inclusions in the molten steel can be reduced. As a result, agglomeration and coalescence of small alumina inclusions by stirring and standing of the molten steel This is probably due to the effect.

And by pouring the above-mentioned molten steel into a tundish having a weir of a predetermined height and continuously casting, for Example 5, the convection effect in the hot water discharge part of the tundish is obtained, which is shown in FIG. As described above, the number of detected alumina inclusions having a particle size range of more than 30 μm and 50 μm or less could be made lower than that of the conventional method.
In addition, although the case where quick lime and a flux were simultaneously added to the molten steel after steel extraction was demonstrated here, quick lime and / or a flux (metal aluminum simple substance) at the time of steel extraction (at the time of steel output and / or after steel output) However, the same tendency was obtained without being affected by the form of addition.
Therefore, it was confirmed that the number of alumina inclusions can be reduced by using the method for producing the high clean steel of the present invention, and in particular, the number of alumina inclusions having a particle size of 20 μm or less can be reduced.

As described above, the present invention has been described with reference to the embodiment. However, the present invention is not limited to the configuration described in the above embodiment, and the matters described in the scope of claims. Other embodiments and modifications conceivable within the scope are also included. For example, the case where the manufacturing method of the high clean steel of the present invention is configured by combining a part or all of the above-described embodiments and modifications is also included in the scope of the right of the present invention.
Moreover, in the said embodiment, although the molten steel which performed the primary refining was processed sequentially in the steel-making process and the ladle process process, it demonstrated the case where it casts continuously in a continuous casting process, but continuous casting. Prior to the process, a process other than the steel exit process and the ladle treatment process may be performed as necessary.
Further, in the above embodiment, the case where metallic aluminum is added during slag reforming and final deoxidation has been described. However, one or two more times between slag reforming and final deoxidation. Metal aluminum may be added a plurality of times as described above.

10: Tundish, 11: Molten steel pan, 12: Long nozzle, 13: Hot water receiving part, 14: Mold, 15: Hot water discharging part, 16: Weir, 17: Immersion nozzle, 18: Bottom surface, 19: Through hole

Claims (1)

The molten steel that has undergone primary refining, which is blown acid decarburized under atmospheric pressure, is sequentially processed in the ladle processing process that includes at least the steelmaking process and alloy addition, and then poured into the tundish in the continuous casting process. In the manufacturing method of high clean steel that continuously casts
At the time of discharging the molten steel in the steel extraction step, lime is added to one or both of the molten steel and slag, and either one or both of the flux containing metal aluminum and metal aluminum is added to form slag. Is reformed, and the slag T.I. After making the total of Fe concentration and MnO concentration 5 mass% or less, and making dissolved oxygen concentration of molten steel into the range of 100 ppm or more and 300 ppm or less,
In the ladle treatment step, metallic aluminum is further added to the molten steel, the molten steel is stirred for 3 minutes to 10 minutes and deoxidized, and from the deoxidation treatment to the start of continuous casting in the continuous casting step. Let stand for more than 10 minutes,
In the continuous casting process, a weir for partitioning into a hot water receiving part for receiving molten steel and a hot water discharging part for pouring the molten steel into a mold for continuously casting the molten steel is provided inside, and the height of the weir is set to 0.3 of the molten steel depth. A method for producing highly clean steel, characterized in that molten steel that has been allowed to stand after the deoxidation treatment is poured into the tundish that has been doubled to 0.8 times.

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