JP6443200B2 - Manufacturing method of high clean steel - Google Patents

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, in Patent Document 1, metallic aluminum is added as a deoxidizer, CaO is used as a modifier for the inclusions to be generated, the inclusions are levitated by stirring the molten steel, and inclusions in the molten steel are reduced. Techniques for making them disclosed are disclosed.
Moreover, in patent document 2, in order to suppress the production | generation of the alumina inclusion of the above-mentioned patent document 1, the technique which carburizes molten steel and deoxidizes is disclosed. Specifically, by utilizing the carbon added during the vacuum degassing process, the amount of metallic aluminum used as a deoxidizing material is suppressed, and by adding carbon before the vacuum degassing process, It describes the prevention of bumping. It also describes that metallic aluminum is added during steelmaking after primary refining.
In Patent Documents 3 and 4, in order to float and remove alumina inclusions in the tundish of a continuous casting machine, the inside of the tundish is divided into a hot water receiving part (steel receiving part) and a hot water discharging part (molten steel discharging part). There has been disclosed a technique for effectively floating and removing alumina inclusions by heating molten steel in a hollow refractory provided in a weir and utilizing the upward flow of molten steel discharged from the hollow refractory.

Japanese Patent Laid-Open No. 7-300612 Japanese Patent No. 3674422 JP-A 63-93452 JP-A-8-1289

However, the conventional technique still has the following problems to be solved.
Although the technique of Patent Document 1 can be expected to reduce the corresponding alumina inclusions, it is necessary to further reduce the number of inclusions in order to improve the quality. Further, according to the knowledge of the present inventors, an effect of reducing alumina inclusions having a large particle size (for example, 70 μm or more) can be expected, but an effect of reducing alumina inclusions having a small particle size (about 10 to 50 μm) is small. .
As shown in FIG. 4, which describes the effect of suppressing inclusions, the technology of Patent Document 2 can be expected to have a corresponding effect of reducing alumina inclusions, but 50 μm or less compared to the effect of reducing alumina inclusions with a particle size of 70 μm. The effect of reducing alumina inclusions of 30 μm, particularly 20 μm or less, is small, and in order to improve quality, improvement of the effect of reducing alumina inclusions with a small particle size is desired.
In the techniques of Patent Documents 3 and 4, for example, as described in “size of 100 μm or more” (Patent Document 3, page 2, lower right column, lines 11 to 13), the corresponding reduction of alumina inclusions Although the effect can be expected, the effect of reducing the alumina inclusion having a small particle size (about 10 to 50 μm) is small.

  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 manufacturing method of the high clean steel according to the present invention that meets the above-mentioned object is to melt molten steel that has been subjected to primary refining that is blown acid decarburized under atmospheric pressure at least in the steelmaking process and the vacuum degassing process. Then, when pouring into the tundish in the continuous casting process and continuously casting, it is a method for producing highly clean steel in which metallic aluminum is added to the molten steel after the decarburization treatment instead of before the decarburization treatment by the vacuum degassing step. There,
A carbon component is added to the molten steel between the steeling step and the vacuum degassing step, the decarburization treatment is performed while stirring the molten steel in the vacuum degassing step, and the metal aluminum is added after the decarburization treatment. The molten steel was stirred for 3 minutes to 12 minutes,
A dam that divides the molten steel into a hot water receiving portion that receives molten steel and a hot water discharging portion that is poured into a mold for continuously casting the molten steel is provided inside, and one or a plurality of molten steel flow paths communicating the hot water receiving portion and the hot water discharging portion. The height position from the bottom surface of the hot water receiving portion of the opening formed on the weir and located on the hot water receiving portion side of the molten steel flow path is 0.2 of the molten steel depth on the hot water receiving portion side. The molten steel that has been subjected to the stirring treatment after the addition of the metal aluminum is poured into the tundish that has been doubled or less.

  In the method for producing highly clean steel according to the present invention, it is preferable that the molten steel flowing in the molten steel flow path is induction-heated.

The manufacturing method of the high clean steel according to the present invention is based on the precondition that metal aluminum is added to the molten steel after decarburization treatment, not before decarburization treatment by the vacuum degassing step.
Here, since metal aluminum is added after the decarburization process by a vacuum degassing process, addition of metal aluminum is performed with respect to the molten steel which reduced the dissolved oxygen concentration in molten steel, and the production | generation of an alumina inclusion 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.
And, a tundish in which a dam that divides the molten steel into a hot water receiving part and a hot water discharging part is provided inside, and a molten steel channel that communicates the hot water receiving part and the hot water discharging part is formed at a predetermined height position of the weir. In this tundish, the floating removal effect of the aggregated and combined alumina inclusions can be obtained. This is because the molten steel temperature at the surface of the hot metal (near the surface of the molten metal) decreases in the tundish, and the molten steel temperature at the surface of the hot metal becomes lower than the molten steel at the temperature of the hot water. Since a temperature difference is generated in the vertical direction, inclusions in the molten steel flowing from the molten steel flow path to the waste water portion are levitated and removed by convection (upflow) of the molten steel caused by the temperature difference.
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 graph which shows the particle size frequency distribution of the alumina inclusion in molten steel at the time of completion | finish of the stirring process 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
The alumina inclusions at the beginning of production have a small particle size (20 μm or less), and may remain in the molten steel as they are regardless of the passage of time, or the produced inclusions may aggregate gradually over time.

In the above-described carburizing and vacuum degassing treatment described in Patent Document 2, it is considered that generation of coarse inclusions can be suppressed as a result of utilizing C element for deoxidation (in FIG. 4 of Patent Document 2, the particle size is 70 μm). Inclusions are greatly reduced).
However, the effect of reducing inclusions with a small particle size (particle size of 20 μm or less class) is small.
This is because, in Patent Document 2, reducing iron such as metal aluminum and aluminum iron is added to slag accompanying molten steel at the time of steel extraction from the primary refining furnace or after steel output, FeO, MnO, etc. in the slag This is due to the reduction of the oxidizing component.

As described above, reducing the oxidizing component in the slag with metallic aluminum, aluminum soot, etc. means that alumina is produced. If the amount of production is large, a fine state (20 μm or less) The alumina inclusions produced in (1) cannot be agglomerated or levitated and remain in the molten steel and further in the cast slab.
Further, when a large number of fine alumina inclusions in the 5 to 20 μm class remain in the molten steel or in the slab, the frequency of starting defects becomes high during processing such as ultra-thinning of the steel material.

  Therefore, in order to improve quality, it is necessary to suppress the amount of alumina inclusions generated.

(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 agitated, the contribution form of agitation varies depending on the amount of alumina produced (the presence of inclusions immediately after deoxidation). The situation is as follows.

When there are relatively many alumina inclusions in the molten steel, the effect of improving the absolute value of the number of inclusions by the stirring treatment is small. In addition, the number of alumina inclusions in molten steel is the case where metallic aluminum is added to molten steel (molten steel with high dissolved oxygen concentration immediately after primary refining) before decarburization treatment (vacuum degassing treatment) performed by adding a carbon component. In addition, it increases due to the use of a large amount of metallic aluminum.
In this case, most of the energy generated by gas stirring in the ladle (same as the reflux stirring in the RH treatment) is spent on the floating movement of the existing coarse inclusions. small. In addition, since the number of fine (20 μm or less) alumina inclusions is large, the frequency of collision between the particles increases without stirring, and the alumina inclusions generated before the decarburization treatment are aggregated and coalesced over time. Ascent progresses. However, since the number of alumina inclusions 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 the alumina inclusions in the molten steel are relatively small, the collision frequency of fine inclusion particles due to the stirring treatment increases, so the particle size distribution of the inclusions tends to increase slightly (the particle size increases). It was. The number of alumina inclusions in the molten steel can be reduced when adding metallic aluminum to the molten steel after decarburization without adding metallic aluminum to the molten steel before decarburization performed by adding a carbon component. .
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 because metal aluminum is added to the molten steel after the decarburization treatment, and gas agitation is performed immediately after the addition of this metal aluminum. This is considered to be due to the effect of aggregation and coalescence accompanying the collision of inclusion particles due to (flow).

  Therefore, it is important to add metallic aluminum to the molten steel in which the dissolved oxygen concentration after the decarburization treatment is reduced, and to perform a stirring treatment immediately after that.

(3) 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.
In addition, when the molten steel temperature decreases in the tundish, the solubility product decreases, which leads to the formation of new fine alumina (2 Al +3 O → Al ₂ O ₃ ), and the increase in alumina inclusions in the cast slab is remarkable. It may become.
On the other hand, the effect of suppressing the formation of new alumina inclusions can be expected by heating the molten steel in the tundish. 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), the slag present on the surface of the hot water in the tundish is stirred. In the state where the effect is suppressed, the effect of causing inclusions in the molten steel having a particle diameter of about 30 to 50 μm to float and trapping this in the slag can be expected.

  Therefore, a dam that divides (independently arranges) the hot water receiving portion and the hot water discharge portion is erected inside the tundish, and a molten steel channel that communicates the hot water receiving portion and the hot water discharge portion with this weir. It is preferable to provide a hollow refractory to be formed and heat the molten steel in the region of the hollow refractory.

From the above, the present inventors have conceived the method for producing highly clean steel according to the present invention.
That is, as shown in FIG. 1, the method for producing a highly clean steel according to an embodiment of the present invention performs a primary refining (processed in a converter) that has been subjected to primary refining by blowing acid decarburization under atmospheric pressure. At least in the steelmaking process and the vacuum degassing process, the steel is cast and melted at the same time, and then poured into the tundish 10 in the continuous casting process to perform continuous casting, not before the decarburizing process in the vacuum degassing process. This is a method of adding metallic aluminum to the later molten steel.
This will be described in detail below.

The molten steel that has undergone primary refining is supplied to the ladle in the steelmaking process.
Immediately after primary refining, such as converter blowing, the dissolved oxygen concentration of molten steel is generally as high as about 600-900 ppm, and if a deoxidation treatment is performed by adding metallic aluminum in this state, a very large amount of fine alumina is produced. It becomes. 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.

The amount of alumina produced is governed by the concentration of dissolved oxygen to be deoxidized and the amount of metal aluminum added. That is, reducing the dissolved oxygen concentration before deoxidation treatment, reducing the amount of metallic aluminum added, and suppressing aluminum oxidation (such as slag) by oxygen other than dissolved oxygen (FeO and MnO in slag). Is extremely important.
Therefore, for the molten steel in a state where the dissolved oxygen concentration is high after the end of primary refining, the carbon component, which is a deoxidizing element that cannot generate inclusions, is introduced into the molten steel (between the steelmaking process and the vacuum degassing process). Add (carburizing treatment). Next, decarburization treatment (deoxidation treatment) is performed while stirring the molten steel in the ladle to which the carbon component has been added in a vacuum degassing (degassing treatment under vacuum) step.
Thereby, the dissolved oxygen concentration of molten steel can be reduced to about 50-200 ppm, for example.

Then, metallic aluminum is added to the molten steel after decarburization treatment in which the dissolved oxygen concentration is reduced, and the molten steel to which the metallic aluminum is added is added for 3 minutes to 12 minutes (preferably, the lower limit is 4 minutes, and the upper limit is 10 minutes).
The amount of metallic aluminum added to the molten steel is preferably reduced in order 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 is added per ton of molten steel. It is good.
Moreover, the gas stirring (bubbling) which blows inactive gas, such as Ar (argon), and the reflux stirring using RH can be used for the stirring process of molten steel. In the case of stirred refluxing with RH, the vacuum degree of ^{133~400 × 10 2 Pa (1~300Torr)} , preferably at a low vacuum of ^{133 × 10 2 ~400 × 10 2} Pa (100~300Torr) It is good to stir. In addition, the operating conditions (stirring power of gas stirring) in the ladle are the same as in the case of performing the above-described decarburizing process, or a lower flow rate (for example, 0. 3 times or more and less than 1.0 times).

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, 12 minutes, which is the upper limit of the stirring time, was determined based on gas stirring in a ladle, which is one of the methods of stirring described above.
In the reflux stirring at RH, the stirring treatment may be performed for more than 12 minutes. However, in the gas stirring in the ladle, the temperature drop of the molten steel increases by increasing the stirring time, and new alumina inclusion particles Becomes easier to generate. This is due to a decrease in the solubility product of the “2 Al +3 O → Al ₂ O ₃ ” reaction accompanying the temperature drop of the molten steel.
Therefore, the upper limit of the stirring time was determined in consideration of the gas stirring in the ladle that is affected by the temperature drop among the stirring treatment methods described above.
Thereby, the effect of the aggregation coalescence of the small alumina inclusion produced | generated in molten steel can be accelerated | stimulated.

Subsequently, the molten steel stirred after the addition of metallic aluminum is poured into the tundish 10 through the long nozzle 12 using the molten steel pan 11.
As for the tundish 10, the hot water receiving part 14 which receives molten steel from the molten steel pan 11 via the long nozzle 12 by the weir 13 inside, and the hot water discharging part 15 which inject | pours into the casting_mold | template (not shown) which casts this molten steel, It is divided into In addition, the immersion nozzle 16 is provided in the bottom part of the hot-water part 15, and it has the structure which inject | pours the molten steel in the hot-water part 15 into a casting_mold | template via the immersion nozzle 16.
The weir 13 that divides the hot water receiving portion 14 and the hot water discharge portion 15 is provided with a hollow refractory 18 that forms a molten steel channel 17 that communicates the hot water receiving portion 14 and the hot water discharge portion 15. The hollow refractory 18 receives molten steel from the opening 19 on the hot water receiving part 14 side, and discharges the molten steel from the opening 20 on the hot water discharging part 15 side to the hot water discharging part 15. The molten steel flowing in the hollow refractory 18 (molten steel flow path 17) can be heated by, for example, the induction heating device (here, the induction heating coil 21) described in Patent Documents 3 and 4 described above.

In addition, in order to prevent molten steel remaining in the hot water receiving part 14 after completion | finish of continuous casting, the opening part 19 (lower end of the opening part 19) located in the hot water receiving part 14 side of the hollow refractory 18 (molten steel flow path 17). ) 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 on the hot water receiving portion 14 side (the lower limit is For example, 0 times (0 × H), that is, the position at which the opening 19 is in contact with the bottom surface 22 of the hot water receiving portion 14).
Here, the number of the hollow refractory 18 (molten steel flow path 17) provided in the weir 13 may be one according to casting conditions, or may be two or more. In addition, when the number of hollow refractories is plural, the height positions from the bottom surface of the hot water receiving portion of the openings located on the hot water receiving portion side of all the hollow refractories are adjusted so as to satisfy the above-described conditions. . The length of the hollow refractory 18 (molten steel channel 17) (the thickness of the weir 13) is, for example, about 500 to 1500 mm.
The weir 13 and the hollow refractory 18 are both made of a refractory, but may be made of the same material or different materials depending on the intended use.
Further, the hollow refractory 18 (molten steel channel 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 hot water drainage part 15 is made deeper than the depth position of the bottom face 22 of the hot water receiving part 14, it may be the same depth.
In addition, a molten steel flow path is not limited to forming with a hollow refractory, For example, it can also form by penetrating a hole (through-hole) in a weir.

As described above, in order to effectively cause the upward flow of molten steel to act in the tundish 10, the weir 13 provided with the hollow refractory 18 is provided inside the tundish 10, and the hot water receiving portion 14 and the waste hot water are provided. It is necessary to clearly divide the space (chamber) of the portion 15 (the molten steel surface position in the tundish 10 (the hot water receiving portion 14 and the hot water discharging portion 15) is lower than the upper surface of the weir 13).
In general, since the molten steel temperature of the surface layer of the hot water discharge portion 15 is lowered in the tundish 10, the molten steel temperature of the surface layer of the hot water discharge portion 15 is lower than the molten steel temperature of the hot water receiving portion 14, and the depth of the hot water discharge portion 15 is increased. A temperature difference occurs in the molten steel in the vertical direction. For this reason, even if the molten steel discharged from the hollow refractory 18 to the hot water discharge portion 15 is not induction-heated in the hollow refractory 18, the convection (upflow) of the molten steel occurs due to the temperature difference described above. Due to the convection, inclusions in the molten steel discharged from the hollow refractory 18 to the hot water discharge section 15 are levitated and removed.

However, even if an upward flow is formed in the tundish 10, the inclusion particle size that can be lifted and removed is only a coarse diameter of about 30 to 50 μm or more, and it is difficult to float and remove a small diameter inclusion of about 5 to 20 μm. is there.
Further, when the molten steel temperature is lowered in the tundish 10 due to the longer casting time, the buoyancy of the inclusions is weakened due to the increase in the viscosity of the molten steel, and the floating efficiency of the inclusions is deteriorated. There is a concern that the solubility product of ( Al +3 O → Al ₂ O ₃ ) is lowered, and fine Al ₂ O _{3 of} less than 20 μm is newly generated (secondary generation).

Accordingly, as described above, by reducing the amount of deoxidized aluminum to be added by carburizing and decompression treatment, the amount of Al ₂ O ₃ to be generated is suppressed, and then the fine Al ₂ O ₃ is stirred by molten steel. It is important to perform continuous casting while agglomerating and agglomerating to increase the size and suppressing generation of new fine Al ₂ O _{3 in} the tundish.
Further, in order to promote the floating of the inclusions and suppress the formation of new fine Al ₂ O ₃ , a weir 13 is provided in the tundish to divide the hot water receiving part 14 and the hot water discharging part 15. It is desirable that the hot water part 14 and the hot water discharge part 15 are communicated with each other by a hollow refractory 18 provided in the weir 13 and the molten steel in the hollow refractory 18 is induction-heated.

As a result, convection is generated in the molten steel in the hot water discharge section 15 of the tundish 10, and the alumina inclusions having a particle diameter of about 30 to 50 μm that are aggregated and coalesced are efficiently levitated, and this is made into slag on the molten metal surface. The effect of capturing is obtained. Furthermore, the induction of the molten steel in the hollow refractory 18 to avoid a temperature drop of the molten steel can suppress the generation of new fine alumina in the hot water discharge section 15.
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 incompatibility (product failure) caused by inclusions even during 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.
Carburizing by adding pitch coke (carbon component) to the molten steel (carbon concentration: 0.037 mass%, dissolved oxygen concentration: 700 ppm) after steelmaking in the ladle after primary refining in a 350-ton converter Treated. Thereafter, degassing treatment (decarburization treatment) was performed by immersion of a single-legged large-diameter tube and gas stirring of bottom blowing in a ladle.
And after adding 0.1-2.4 kg of metal aluminum to the molten steel in the ladle, and further performing gas stirring (stirring treatment) for 3-14 minutes, the ladle after the stirring treatment is added. The heat treatment (heat insulation material etc.) was carried out and it conveyed rapidly to the continuous casting machine, and the molten steel after the stirring treatment was poured into the tundish to carry out continuous casting. In this tundish, the hot water receiving part and the hot water discharging part are separated by a weir, and the hot water receiving part and the hot water discharging part are communicated by a hollow refractory provided in the weir (the molten steel in the hot water receiving part is discharged only from the hollow refractory. The molten steel in the hollow refractory can be heated by induction. In addition, the height position from the bottom face of the hot water receiving portion of the lower end of the opening on the hot water receiving portion side of the hollow refractory is 0.2 times (0.2 × H) the bath depth H on the hot water receiving portion side. The position.
Table 1 shows the test conditions, the results, and the evaluation.

In Table 1, the “after carburizing” column describes the carbon concentration ([C] (%)) and dissolved oxygen concentration ([O] (ppm)) of the molten steel after adding pitch coke, The column “after deoxidation under reduced pressure C” describes the carbon concentration ([C] (%)) and the dissolved oxygen concentration ([O] (ppm)) of the molten steel after the decarburization treatment.
In the “T. [O] after ladle treatment” column, the total oxygen concentration (T. [O] (ppm)) of the molten steel after gas stirring for the time of the “ladder stirring time” column is shown. It is described.
“Presence / absence of induction heating” describes the presence / absence of induction heating described above with respect to the molten steel flowing in the hollow refractory, and “none” indicates induction of molten steel using the above-described induction-heatable tundish. This refers to the case where heating was not performed. The “temperature increase on the outlet side of the hollow refractory” means that the molten steel flowing in the hollow refractory is heated by induction heating in the molten steel in the hot water receiving part (hollow refractory outlet side) of the molten steel in the hot water receiving part. It means elevated temperature (ΔT).
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 (×).

As described above, Examples 1 to 8 in Table 1 use molten steel to which metallic aluminum is added after decarburization treatment, not before decarburization treatment, and this molten steel is used for a time within an appropriate range (3 to 12 minutes). This is a result of continuous casting after pouring into a tundish provided with a hollow refractory placed in an appropriate range (0.2 × H) after stirring in the range. That is, the amount of metallic aluminum added before decarburization is 0 kg.
In this case, the effect of suppressing the formation of alumina inclusions depending on the timing of addition of metallic aluminum, the effect of agglomeration and coalescence of small alumina inclusions by the stirring treatment of the molten steel, and the floating of alumina inclusions in the molten steel by convection in the tundish drain A removal effect was 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: ○ ).

In particular, Examples 6 to 8 are the results of studying the influence of induction heating on the molten steel flowing in the hollow refractory, but Example 6 in which induction heating was performed rather than Example 8 in which induction heating was not performed. 7 was able to reduce the number of alumina inclusions present in the slab. Moreover, by raising the molten steel temperature on the outlet side of the hollow refractory material by induction heating as compared with Example 6 as in Example 7, the number of alumina inclusions present in the slab could be further reduced.
This is because the induction effect of the molten steel increased the convection effect in the tundish hot water portion and the effect of suppressing the formation of new fine alumina in the hot water portion.

On the other hand, the comparative example 9 is a result at the time of performing a degassing process, without giving a carburizing process to the molten steel after primary refining on the conditions of Examples 1-3.
In this case, since the carburizing treatment was not performed, the amount of metallic aluminum to be added to the molten steel after the degassing treatment has to be increased, so that a large amount of alumina inclusions are formed, and agglomeration of small alumina inclusions by the stirring treatment of the molten steel The coalescence effect 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).

In Comparative Examples 10 and 11, in the conditions of Examples 1 to 5, the stirring time of the molten steel to which metallic aluminum was added was set to a time outside the appropriate range (Comparative Example 10: 2 minutes, Comparative Example 11: 14 minutes). Is the result of
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. 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 at the time of performing addition of metal aluminum to the molten steel before a decarburization process instead of after a decarburization process on the conditions of Examples 1-3 (the same method as patent document 2).
In this case, as described above, since the oxidizing component in the slag was reduced with metallic aluminum and a large amount of alumina was produced, the effect of agglomeration and coalescence of alumina inclusions by the stirring treatment of the molten steel could not be obtained sufficiently.
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 13 is a result at the time of not stirring the molten steel which added metal aluminum in the conditions of Examples 1-3.
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).

The conventional method is a result of adding metallic aluminum to the molten steel after primary refining without performing a carburizing process or a degassing process under the conditions of Examples 1 to 3 (that is, Deoxidation with metallic aluminum only).
In this case, since the carburizing treatment and degassing treatment were not performed, the amount of metallic aluminum added to the molten steel increased, a large amount of alumina inclusions were produced, and the agglomeration and coalescence effect of small alumina inclusions due to the stirring treatment of the molten steel was sufficient. It was 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 2, the results of investigating the particle size frequency distribution of alumina inclusions in the molten steel at the end of the stirring treatment in the ladle are shown in FIG. The results of investigating the particle number distribution of inclusions are shown in FIG. The vertical axis in FIG. 2 represents each of the total number of all alumina inclusions (particle size range 5 μm to 20 μm, 20 μm to 30 μm, 30 μm to 50 μm, and 50 μm) as 100%. The number ratio of alumina inclusions in the particle size range is shown.

As shown in FIG. 2, the particle size range of the alumina inclusions 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 2 than the conventional method, but the number of particles exceeding 30 μm and 50 μm or less. The ratio is higher in Example 2 than in the conventional method.
In other words, the decrease in the number ratio of 5 μm to 20 μm and more than 20 μm to 30 μm corresponds to the increase in the number ratio over 30 μm and 50 μm or less compared to the conventional method in Example 2. This is because Example 2 performs carburizing treatment and degassing treatment before the addition of metallic aluminum, so that the amount of alumina inclusions in the molten steel can be reduced. As a result, small alumina inclusions due to the stirring treatment of the molten steel can be reduced. This is considered due to the fact that the agglomeration effect was obtained.

And by pouring the above-mentioned molten steel into a tundish having a weir provided with a hollow refractory and continuously casting, for Example 2, the convection effect in the hot water discharge part of the tundish is obtained. As shown in FIG. 3, the number of detected particles with an alumina inclusion particle size range of more than 30 μm and less than 50 μm could be made lower than in the conventional method.
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.
Further, in the above embodiment, the case where the molten steel that has been subjected to primary refining is sequentially processed in the steelmaking process and the vacuum degassing process to be melted and then continuously cast in the continuous casting process has been described. Before the process, a process other than the steel output process and the vacuum degassing process may be performed as necessary.

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

Claims (2)

Molten steel that has been subjected to primary refining that is blown acid decarburized under atmospheric pressure is processed at least sequentially in the steelmaking process and vacuum degassing process, and then poured into a tundish in the continuous casting process for continuous casting. In this case, it is a manufacturing method of high clean steel in which metallic aluminum is added to the molten steel after the decarburization treatment instead of before the decarburization treatment by the vacuum degassing step,
A carbon component is added to the molten steel between the steeling step and the vacuum degassing step, the decarburization treatment is performed while stirring the molten steel in the vacuum degassing step, and the metal aluminum is added after the decarburization treatment. The molten steel was stirred for 3 minutes to 12 minutes,
A dam that divides the molten steel into a hot water receiving portion that receives molten steel and a hot water discharging portion that is poured into a mold for continuously casting the molten steel is provided inside, and one or a plurality of molten steel flow paths communicating the hot water receiving portion and the hot water discharging portion. The height position from the bottom surface of the hot water receiving portion of the opening formed on the weir and located on the hot water receiving portion side of the molten steel flow path is 0.2 of the molten steel depth on the hot water receiving portion side. A method for producing highly clean steel, characterized by pouring the agitated molten steel into the tundish after the addition of the metallic aluminum to the tundish that is less than double.   2. The method for producing high-clean steel according to claim 1, wherein the molten steel flowing through the molten steel flow path is induction-heated.

Images