JP2006247744A - Continuous casting method for steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a continuous casting method for steel where, even in the case a casting rate is high, the surface crack of a slab can be suppressed, and also, the thickness of a solidified shell is secured, so as to suppress the generation of breakout. <P>SOLUTION: A slab of medium carbon steel having a C content of 0.08 to 0.25 mass% is subjected to continuous casting at a casting rate of ≥1.6 m/min using mold powder in which the mass ratio of CaO/SiO<SB>2</SB>is controlled to 1.5 to 2.5, the content of Na<SB>2</SB>O is controlled to <2 mass%, the content of Li<SB>2</SB>O is controlled to ≥1 mass%, and recrystallization temperature is controlled to <1,100°C. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a continuous casting method of steel in which steel is continuously cast while supplying mold powder onto the surface of molten steel in a mold.

  In continuous casting of steel, mold powder is added on the surface of the molten steel in the mold. The mold powder receives heat from the molten steel, and then melts and melts to form a molten slag layer. The molten slag thus formed sequentially flows into the gap between the mold and the solidified shell and is consumed. The main roles of the mold powder during this period are (1) lubrication between the mold and the solidified shell, (2) absorption of inclusions floating from the molten steel, (3) reoxidation prevention and heat retention of the molten steel, and (4) solidified shell. For example, control of the heat removal rate from the mold to the mold.

  By the way, in recent years, demands on the quality of steel have become more severe. For example, in the case of medium carbon steel having a C concentration of 0.08 to 0.25% by mass in steel, the surface of a slab can be obtained by using a conventional mold powder. In order to prevent cracking and to prevent slab surface cracking, various casting techniques in which mold powder is improved as shown in Patent Documents 1 to 10 have been proposed.

In these patent documents and the like, when casting a middle carbon steel or the like where slab surface cracks are likely to occur, the basicity (CaO / SiO ₂ ) of the mold powder is increased, and the amount of F (CaF ₂ ) is increased to increase caspidine ( 3CaO.2SiO ₂ .CaF ₂ ) is promoted to increase the crystallization temperature (solidification temperature).

  In the mold powder having a high crystallization temperature, crystals develop in the slag film flowing between the mold and the solidified shell. The crystallized slag film has a higher heat transfer resistance than that which forms a slag film as it is without being crystallized, and has a function of reducing heat removal from the solidified shell to the mold.

The technique according to the above-mentioned patent document aims to cool the solidified shell uniformly by reducing the heat removal in this way, and the thickness of the initial solidified shell formed in the meniscus portion is uniform in the width direction. It is intended to prevent the occurrence of single-surface cracks.
Japanese Patent Laid-Open No. 5-269560 JP-A-8-141713 Japanese Patent No. 3119999 JP-A-11-320058 Japanese Patent No. 3179358 JP 2001-179408 A Japanese Patent No. 2671644 JP 2000-218348 A JP 2000-158105 A JP 2000-239893 A

  However, even if continuous casting is performed using the mold powder having a high crystallization temperature (solidification temperature) according to the above-mentioned prior art, slab surface crack defects still occur, and it is difficult to eliminate such defects. It is. In recent years, increasing the casting speed has been aimed at improving productivity. However, increasing the casting speed may cause an increase in cracking defects in the slab. It is a cause to inhibit.

  In order to further suppress slab surface cracks, it is possible to increase the crystallization temperature (solidification temperature) and increase the crystallinity of the mold powder. The solidified shell may be bulged or breakout may occur after the molding. In particular, such inconvenience is likely to occur in the case of ultra-high speed casting that is currently oriented.

  The present invention has been made in view of such circumstances. Even when the casting speed is high, the slab surface crack can be suppressed, and the thickness of the solidified shell is ensured to suppress the occurrence of breakout. It is an object of the present invention to provide a continuous casting method for steel.

  In order to suppress slab surface cracks, it is important to uniformly remove heat from the slab to the mold to form a uniform initial solidified shell. Increase the crystallization temperature or solidification temperature (point) to increase the thickness of the crystal layer in the slag film, or increase the interfacial thermal resistance between the slag film and the mold. It has been effective to lower the value and has relied on this technique.

  However, even with this method, the thickness of the solidified shell necessary for maintaining stable operation cannot be ensured, and the risk of breakout increases in high-speed casting.

  Thus, as a result of repeated investigations to ensure a solidified shell thickness that can achieve stable operation while suppressing slab surface cracking, the present inventors have rapidly formed dense and uniform crystals at the meniscus portion in the mold. It was discovered that the meniscus portion was allowed to cool slowly to suppress cracking of the slab, and in the lower portion of the meniscus portion, it was necessary to suppress the occurrence of breakout as strong cooling to the extent that the solidified shell thickness necessary for operation could be secured. .

  Specifically, by making the mold powder easy to crystallize and increasing the crystallization speed, it is possible to quickly form a dense and uniform crystal at the meniscus portion to achieve ultra-slow cooling. It was found that it is effective to make strong cooling in the lower part of the meniscus portion by making the composition of the above composition low in the crystallization temperature and capable of suppressing crystal growth.

  This invention is made | formed based on the above knowledge, and provides the following (1)-(7).

  (1) A steel continuous casting method, characterized in that continuous casting of steel is performed using mold powder that is slowly cooled in a meniscus portion in a mold and strongly cooled in a lower portion of the meniscus portion.

  (2) The steel is continuously cast by using a mold powder in which the crystallization speed is increased at the meniscus portion in the mold for slow cooling and the crystal growth is suppressed at the lower portion of the meniscus portion for strong cooling. Steel continuous casting method.

(3) In the above (1) or (2), the cooling property in the meniscus part in the mold is controlled by adjusting the crystallization speed by the content of Na ₂ O in the mold powder, and the cooling property in the lower part of the meniscus part is controlled. A method for continuous casting of steel, characterized by controlling the crystallization temperature by adjusting the Li ₂ O content of the mold powder.

  (4) The continuous casting method for steel according to any one of the above (1) to (3), wherein the slab has a thickness of 220 mm or more and a casting speed of 1.6 m / min or more.

(5) the CaO / SiO ₂ mass ratio is 1.5 to 2.5, less than 2 wt% of Na ₂ O, using a mold powder having an adjusted Li ₂ O to 1 mass% or more, more than the thickness of 220mm A continuous casting method for steel, characterized by continuously casting a slab at a casting speed of 1.6 m / min or more.

  (6) The steel continuous casting method according to (5), wherein the mold powder contains 1 to 18% by mass of C.

  (7) The continuous casting method for steel according to any one of the above (1) to (6), wherein the crystallization temperature of the mold powder is 1100 ° C. or lower.

  (8) The continuous casting method for steel according to any one of the above (1) to (7), wherein the thickness of the solidified shell at a position directly below the mold is 12 mm or more.

  According to the present invention, even under ultra-high speed casting, slab cracking is difficult to occur, and the thickness of the solidified shell can be secured to prevent the occurrence of breakout, resulting in high quality steel and improved productivity. There is provided a continuous casting method of steel capable of achieving both of the above.

Hereinafter, the present invention will be specifically described.
First, the background to the present invention will be described.
As described above, in the continuous casting of steel, by slow cooling in the mold, there is an effect of suppressing surface cracks (longitudinal cracks) on the slab, and conventionally, the mold powder CaO is effective for such slow cooling. The crystallization temperature was increased by adjusting the / SiO ₂ mass ratio (basicity). The relationship between the casting speed and mold heat removal at this time is shown in FIG. Thus, by using a mold powder (slow cooling powder) that has a higher basicity than conventional powders to increase the crystallization temperature and is slowly cooled, a thick plate material of 220 mm or more (slab thickness) is 1.6 m. Even in the case of casting at a high speed of at least / min, it is possible to make the slab surface crack (longitudinal crack) difficult to occur. However, when such a slow cooling powder having a high basicity is used, breakout is likely to occur after molding.

  FIG. 2 shows the thickness of the solidified shell immediately under the mold estimated from the investigation result of the breakout slab. As shown in FIG. 2, in the case of using a slowly cooled powder having a high basicity and a high crystallization temperature, the casting speed was increased to 1.6 m / If it exceeds the minute vicinity, the thickness of the solidified shell becomes less than 12 mm, which is a breakout occurrence risk area. That is, in the casting of medium carbon steel, in order to realize high speed casting while suppressing slab surface cracking, it is necessary to secure a certain thickness of solidified shell directly under the mold.

  In view of the above, in the present invention, as shown in FIG. 3, the lower part of the meniscus portion that influences the breakout, more slowly cooling the meniscus portion in the mold that affects the slab surface crack. Then, continuous casting of steel is performed using mold powder having a function of increasing cooling and increasing the thickness of the solidified shell.

  Hereinafter, it will be described in detail by dividing into slow cooling at the meniscus portion and strong cooling at the lower portion of the meniscus portion.

(1) Slow cooling at the meniscus part Conventionally, in order to cool slowly in the mold, it was common to increase the basicity of the powder to promote crystallization, but in the present invention only the meniscus part is used. As a result, the slab surface cracking is effectively prevented and the slab surface cracks are effectively prevented. For this purpose, the basicity of the powder, that is, the CaO / SiO ₂ mass ratio is increased to a range of 1.5 to 2.5 so that caspidine is easily crystallized. The crystallization rate was measured on the assumption that it was necessary to shorten the time from when the liquid flowed between the mold and the solidified shell until the crystal layer was formed.

The crystallization speed is obtained by calculating the time from the start of crystallization (shrinkage) to reaching a certain amount of shrinkage when the molten powder is cooled at a constant speed, which is defined as the crystallization speed index, and is obtained from this value. . Here, the base composition of the mold powder is CaO: 42.9 mass%, SiO ₂ : 23.2 mass%, Al ₂ O ₃ : 4.0 mass%, MgO: 0.8 mass%, Li ₂ O: 7 .3% by mass, F: 9.8% by mass, T.I. C: 10.5 mass% (CaO / SiO ₂ mass ratio = 1.9), and the amount of Na ₂ O added was changed to measure the crystallization rate index. The result is shown in FIG. As shown in FIG. 4, the crystallization rate increased as the added amount of Na ₂ O decreased. FIG. 5 shows the crystal structure of the powder film. From this figure, it can be seen that the smaller the amount of Na ₂ O added, the more dense the crystal structure can be obtained. This is thought to be because nucleation of caspidyne (3CaO-2SiO ₂ -CaF ₂ ) is promoted by reducing Na ₂ O, the number of crystal nucleation increases, and the crystal structure can be densified. It is done. For this reason, it is preferable to control the crystallization rate by adjusting the Na ₂ O content in the mold powder.

(2) Strong cooling at the lower part of the meniscus part In the part below the meniscus part, it is necessary to increase the heat removal rate to ensure the solidified shell thickness. For this purpose, it was considered effective to suppress the growth of the dense crystal layer generated at the meniscus portion, and therefore the crystallization temperature was reduced. Here, the base composition of the mold powder CaO: 42.9 _wt%, SiO 2: 23.2 _{wt%, Al} 2 O 3: 4.0 wt%, MgO: 0.8 wt%, _Na 2 O: 0 .2% by mass, F: 9.8% by mass, T.I. C: The crystallization temperature was measured by changing the addition amount of Li ₂ O to 10.5% by mass. The result is shown in FIG. As shown in FIG. 6, it was found that the crystallization temperature can be reduced by adding Li ₂ O. Then, the amount of Li 2 _O crystallization temperature decreases with increasing. Therefore, it is preferable to control the crystallization temperature by adjusting the Li ₂ O content in the mold powder. The amount of heat removed from the mold at this time is preferably 1.3 Gcal / m ² · hr or more at a casting speed of 1.6 m / min.

As described above, the composition of the mold powder is basically defined by setting the CaO / SiO ₂ mass ratio (basicity) to 1.5 to 2.5 so as to be slowly cooled at the meniscus portion in the mold. Further, the crystallization rate is controlled by reducing Na ₂ O to achieve ultra-slow cooling in the meniscus part, and the crystallization temperature is lowered by adjusting the content of Li ₂ O, and strong in the lower part of the meniscus part. For cooling, the thickness of the solidified shell immediately below the mold is set to 12 mm or more. Thereby, even if it is medium carbon steel (C amount is 0.08-0.25 mass%) etc. which are easy to generate | occur | produce a slab surface crack, a thick plate with a plate thickness of 220 mm or more is a high speed of 1.6 m / min or more. In continuous casting, it is possible to suppress the occurrence of cracks on the slab surface and breakout. In addition, when the casting speed is expressed by the passing amount of molten steel, surface cracks and breakout of the slab may occur in high-speed casting of 6 ton / min / str or more, and further 6.5 ton / min / str or more. Can be continuously cast.

Conventionally, the crystallization temperature of mold powder has been increased for gradual cooling of the mold to promote crystallization of caspidine, and surface cracking of the slab has been suppressed. In the present invention, 2% by mass of Na ₂ O is used. By limiting to the following, caspidine is rapidly crystallized in the meniscus part, only the meniscus part is slowly cooled, while the crystallization temperature of the mold powder is lowered to increase the total heat removal in the mold, It achieves both slow cooling at the meniscus and strong cooling at the lower part of the meniscus.

  In the present invention, steel is continuously cast using the mold powder designed as described above. Hereinafter, the composition of the mold powder will be described in more detail.

The chemical composition of the mold powder used in the steel continuous casting method of the present invention is mainly composed of CaO and SiO ₂ , the CaO / SiO ₂ mass ratio is in the range of 1.5 to 2.5, and Na ₂ O is contained. It is preferable to use a material having less than 2% by mass and Li ₂ O of 1% by mass or more, particularly 4% by mass or more.

In addition to the above, it is preferable that MgO is less than 1% by mass and the Na ₂ O / Li ₂ O mass ratio is in the range of 0 to 0.4.

The CaO / SiO ₂ mass ratio is important for producing uniform caspidine crystals. When this value is 1.5 or less, a glass phase is formed, so that the uniformity of the crystal is impaired. On the other hand, if this value exceeds 2.5, a plurality of slag film crystal mineral phases formed on the mold wall are formed, and a uniform crystal layer cannot be obtained. In addition, it is described in the said patent document 5 (patent 3179358) that a heterogeneous crystal layer is formed when such a crystal mineral phase becomes plurality. The CaO / SiO ₂ mass ratio is more preferably 1.6 to 2.5, and even more preferably 1.7 to 2.5.

Na ₂ O is preferably not contained, but is an impurity inevitably contained in the raw material constituting the mold powder. However, when Na ₂ O is contained in an amount of 2% by mass or more, the crystallization temperature of the slag film is delayed because the precipitation of caspidyne is hindered, and the crystallization temperature of the slag film is diversified. In this case, it becomes difficult to sufficiently slowly cool the meniscus portion in the mold. Therefore, the content of Na ₂ O is less than 2% by mass. Specifically, as shown in FIG. 4, Na ₂ O is less than 2% by mass, and the crystallization rate index is approximately 15 or less.

Li ₂ O has a function of lowering the crystallization temperature as described above, and in order to exert such a function, it is preferable to add 1% by mass or more. When Li ₂ O is less than 1% by mass, such a function is not exhibited effectively, but also a softening point (° C.) / (CaO / SiO ₂ mass ratio + Na ₂ O / Li ₂ O mass ratio) described later. If the value range is satisfied, the softening point of the mold powder becomes high, a stable molten layer thickness cannot be obtained, the amount of scale generated on the slab is not sufficient, and the peelability also deteriorates. Therefore, it is not preferable. The content of Li ₂ O is preferably 2% by mass or more, more preferably 3% by mass or more and 10% by mass or less. In particular, in order to obtain the preferred crystallization temperature 1100 below ℃ in order to ensure a sufficient solidified shell thickness at the lower portion of the meniscus is also balance with other components, the Li 2 _O Referring to FIG. 6 The addition amount is preferably 4% by mass or more.

From the viewpoint of obtaining the desired crystal uniformity, the CaO / SiO ₂ mass ratio is preferably in the above range, and the MgO content is preferably less than 1 mass%. When the amount of MgO is 1% by mass or more, the crystal uniformity is impaired regardless of the CaO / SiO ₂ mass ratio.

The Na ₂ O / Li ₂ O mass ratio is preferably 0 to 0.4, but when the Na ₂ O / Li ₂ O mass ratio exceeds 0.4, a uniform crystal layer cannot be obtained, This is because the peelability of the scale from the slab deteriorates and cracks and flaws on the slab surface tend to remain.

  The crystallization temperature is preferably less than 1100 ° C. If the crystallization temperature is 1100 ° C. or higher, the heat removal rate in the mold tends to be low, the solidified shell thickness is not sufficient, and there is a possibility that breakout in the ultra-high speed casting region cannot be sufficiently suppressed. is there.

The softening point of the mold powder is preferably 1110 ° C. or lower. When the softening point exceeds 1110 ° C., the mold powder is difficult to soften and melt, and thus it is difficult to secure a stable melt layer thickness in the mold. A more preferable range of the softening point is 1100 ° C. or less. Furthermore, after satisfying the crystallization temperature of 1100 ° C. or lower, as shown below, the A value which is the ratio between the softening point (° C.) and (CaO / SiO ₂ mass ratio + Na ₂ O / Li ₂ O mass ratio) Preferably satisfies the range of 345 to 750.
A value = softening point (° C.) / (CaO / SiO ₂ mass ratio + Na ₂ O / Li ₂ O mass ratio)
If the A value does not satisfy the above range, the amount of scale produced is reduced, and the peelability of the scale by secondary cooling in a continuous casting facility is deteriorated, which is not preferable. In addition, also when a softening point exceeds 1110 degreeC, the production amount of a scale falls and the peelability also deteriorates.

The mold powder may contain Al ₂ O ₃ but is preferably 10% by mass or less. If it exceeds 10% by mass, a phenomenon that the molten slag separates appears, so that it is difficult to uniformly flow between the mold and the solidified shell. More preferably, it is 1-8 mass%, More preferably, it is 2-8 mass%.

The F amount in the mold powder is preferably 15% by mass or less and F × (Na ₂ O / Li ₂ O mass ratio) is preferably 0 to 7. Even if it exceeds 15% by mass, the effect of promoting crystallization of caspidyne is saturated, and conversely, crystals of CaF ₂ tend to crystallize and are inappropriate. On the other hand, if F × (Na ₂ O / Li ₂ O mass ratio) exceeds 7, the amount of scale formation is reduced, and at the same time, a uniform and dense crystal layer cannot be obtained, which is not preferable. A more preferable range of F × (Na ₂ O / Li ₂ O mass ratio) is in the range of 0-6. Further, when the F amount is less than 5% by mass, caspidine is not preferred because it is difficult to crystallize in the slag film, and therefore 5% by mass or more is preferable.

If necessary, the mold powder may contain one or more of BaO, SrO, MnO, B ₂ O ₃ , ZrO ₂ , and TiO _{2 in} a total amount of less than 6% by mass. However, if the total amount of these is 6% by mass or more, the amount of scale produced on the slab decreases, and the peelability of the scale deteriorates, which is not preferable.

  In addition, a carbon raw material can be added to the mold powder as an adjusting agent for hatching or melting rate. The amount added is preferably 1 to 18% by mass, and particularly when high speed casting is performed, the amount of heat supplied from the molten steel is increased. .

  The shape of the mold powder is not limited, and can be used for all shapes such as powder, extruded granule, hollow spray granule, and stirred granule.

  The properties of the raw material of the mold powder are not particularly limited, but the bulk specific gravity is preferably less than 1, more preferably 0.95 or less for both powder and granule. If the bulk specific gravity exceeds 1, the meltability in the mold deteriorates, which is not preferable.

Examples of the present invention will be described below.
Casting experiments were performed using mold powders having the compositions shown in Table 1. No. in Table 1 No. 1 is designed to be super slow cooling at the meniscus part in the mold and strong cooling at the lower part of the meniscus part. No. 2 is a conventional one that does not achieve slow cooling at the meniscus portion, No. 2; 3 is intended only for slow cooling at the meniscus portion. In addition, No. 1 mold powder is used for adjusting the melting rate of the powder. C was increased.

  Using the above powder, a slab having a thickness of 220 mm was continuously cast at different casting speeds. As molten steel, JIS SS400 (C = 0.16%, Si = 0.15%, Mn = 0.70%, P = 0.020%, S = 0.010%, sol.Al = 0.035%) Was used. FIG. 7 shows the relationship between the casting speed and mold heat removal at that time. As shown in FIG. In the case of the present invention example in which continuous casting is performed using the mold powder of No. 1, No. 1 is a conventional powder that is not slowly cooled. The heat removal behavior is similar to that in the case of the conventional example in which continuous casting is performed using the mold powder of No. 2, and it is considered that the same solidified shell thickness (12 mm or more directly under the mold) can be secured. On the other hand, no. In the case of the comparative example in which continuous casting was performed using the mold powder of No. 3, the total heat removal amount in the mold is small, and the thickness of the solidified shell directly under the mold is considered to be small. For this reason, no. When the mold powder No. 3 is continuously cast at a high speed, there is a concern about the occurrence of breakout.

  FIG. 8 shows a structure photograph of the powder film collected from the mold. No. In the case of the present invention example using the mold powder No. 1, The thickness of the crystal layer was reduced by 30% compared to the case of the conventional example using the conventional mold powder of No. 2, and it was also confirmed that the crystal layer growth was suppressed in the case of the present invention example. .

  FIG. 9 shows the relationship between the casting speed and the incidence of slab surface cracks (longitudinal cracks). In the case of the present invention example using No. 1 mold powder, the vertical crack occurrence rate is low and stable even at a casting speed of 1.6 m / min or more and 2.4 m / min, and good surface quality is ensured. You can see that Moreover, the maintenance rate of the slab (slab) was also comparable with the past. In contrast, no. In the case of the conventional example using the mold powder of No. 2, the vertical crack occurrence index suddenly increased when the casting speed exceeded 1.6 m / min, and it was confirmed that it could not cope with high speed casting.

The figure which shows the relationship between the casting speed in the case where the conventional mold powder is used, and the case where the mold powder which aimed at the slow cooling in the meniscus part in a mold is used. The figure which shows the relationship between the casting speed in the case where the conventional mold powder is used, and the case where the mold powder which aimed at the slow cooling in the meniscus part in a mold is used, and the solidified shell thickness just under a casting_mold | template. The figure for demonstrating the view of the continuous casting method of steel of this invention. Shows the effect of Na 2 _O amount on the crystallization rate of the mold powder. Photo Na ₂ O content in the mold powder exhibits a tissue of the crystal layer in the case of 4.0 wt% and 0.2 wt%. Shows the effect of Li 2 _O amount on the crystallization temperature of the mold powder. In the case of continuous casting using a mold powder designed based on the idea of the present invention, in the case of continuous casting using a conventional mold powder, and in the case of continuous casting using a mold powder intended only for slow cooling, The figure which shows the relationship between casting speed and mold heat removal. The photograph which compares the structure of a powder film with the case where it casts continuously using the mold powder designed based on the idea of this invention, and the case where it casts continuously using the conventional mold powder. The figure which shows the reduction effect of the slab vertical crack occurrence rate index | exponent of this invention.

Claims (8)

  A continuous casting method of steel, characterized in that continuous casting of steel is performed using mold powder that is slowly cooled in a meniscus portion in a mold and strongly cooled in a lower portion of the meniscus portion.   A steel characterized in that continuous casting of steel is performed using a mold powder in which a crystallization speed is increased at a meniscus portion in a mold to be slowly cooled, and crystal growth is suppressed at a lower portion of the meniscus portion to be strongly cooled. Continuous casting method. The cooling property at the meniscus part in the mold is controlled by adjusting the crystallization speed according to the Na ₂ O content of the mold powder, and the cooling property at the lower part of the meniscus part is the crystallization temperature due to the Li ₂ O content of the mold powder. The method for continuous casting of steel according to claim 1 or 2, wherein the control is performed by adjusting the amount of the steel.   The steel continuous casting method according to any one of claims 1 to 3, wherein the slab has a thickness of 220 mm or more and a casting speed of 1.6 m / min or more. A slab having a thickness of 220 mm or more is formed using a mold powder in which the CaO / SiO ₂ mass ratio is 1.5 to 2.5, Na ₂ O is less than 2 mass%, and Li ₂ O is adjusted to 1 mass% or more. A continuous casting method for steel, characterized by performing continuous casting at a casting speed of 1.6 m / min or more.   The said mold powder contains 1-18 mass% C, The continuous casting method of steel of Claim 5 characterized by the above-mentioned.   The method for continuous casting of steel according to any one of claims 1 to 6, wherein the crystallization temperature of the mold powder is 1100 ° C or lower.   The method for continuous casting of steel according to any one of claims 1 to 7, wherein the thickness of the solidified shell at a position directly below the mold is 12 mm or more.

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