JP2019214057A - Continuous casting method - Google Patents

Abstract

To provide a continuous casting method capable of the higher cleanliness of a molten metal.SOLUTION: A continuous casting method has a process where a molten metal is poured from a ladle 10 into a mold via a tundish 11 to produce a slab. The tundish 11 has a weir 15 dividing the inside of the tundish 11 into a molten metal receiving chamber 12 and a molten metal tapping chamber 13 and also provided with 1 to 4 pieces of runners 14 through which the molten metal flows from the molten metal receiving chamber 12 toward the molten metal tapping chamber 13 at the lower part, in each runner 14, regarding to its cross-sectional shape, a diameter is set to a range of 100 to 300 mm expressed in terms of a circular chape, and flux in which a basicity=(mass%CaO)/{(mass%SiO)+(mass%AlO)} is 1.0 or more to 4.0 or less, and also, an AlOcontent to the total content of the CaO content, the SiOcontent and the AlOcontent is 10 mass% or more is arranged at the surface of the molten metal in the molten metal receiving chamber 12.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for continuously casting high-purity steel.

In steel materials for processing applications, for example, tin, IF steel, steel bars, steel materials used for wires, etc., it is required to reduce the amount of inclusions such as alumina contained in the steel materials in order to suppress cracking during processing. I have.
For this reason, in the manufacturing process at the time of smelting steel materials, generation of inclusions is suppressed and floating removal is performed. In the continuous casting process, which is one of the manufacturing processes, a slab is manufactured by pouring molten metal (molten steel) from a ladle (molten steel pan) into a tundish, and further pouring the molten metal in the tundish into a mold. However, also in this tundish, a technique for suppressing the generation of inclusions and removing the levitation is being studied.

For example, Patent Literature 1 discloses that the basicity ((CaO) / (SiO ₂ )) of slag in a tundish is set to 0.5 or more so that the T.T. It is described that the [O] concentration is reduced to reduce the alumina concentration to produce a high-purity steel.
In Patent Document 2, a weir having a through hole (runner) for passing molten steel is provided in a tundish, and a heating means (for example, a heating means for heating molten steel passing through the through hole) is provided in the through hole. , Induction heating means). By heating the molten steel passing through the through-hole by this heating means, the molten steel after passing through the through-hole is at a higher temperature than the surrounding molten steel, so that strong buoyancy acts. Thereby, the flow of the molten steel toward the molten steel surface in the tundish is formed, so that the inclusions can be floated and separated (for example, paragraph [0018]).

JP 2000-212628 A JP 2008-264834 A

  However, although the methods described in Patent Documents 1 and 2 described above can achieve a correspondingly high degree of cleaning of the molten metal, their effects are not sufficient.

  The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a continuous casting method capable of further purifying molten metal.

A continuous casting method according to the present invention that meets the above-mentioned object is a continuous casting method in which a molten metal is poured into a mold from a ladle via a tundish to produce a cast piece,
The tundish divides the inside of the tundish into a hot-water receiving room and a hot-water outlet room, and has at the bottom one or more and four or less runners through which the molten metal flows from the hot-water receiving room toward the hot-water outlet room. Having a weir, the runner has a diameter in the range of 100 mm or more and 300 mm or less when the cross-sectional shape is converted into a circle,
Basicity = (% by mass CaO) / {(% by mass SiO ₂ ) + (% by mass Al ₂ O ₃ )} is 1.0 or more and 4.0 or less, and the amount of CaO, the amount of SiO ₂ , and the amount of Al ₂ O _A flux in which the amount of Al ₂ O ₃ is 10% by mass or more with respect to the total amount of the _three is placed on the surface of the molten metal in the hot water receiving chamber.

  In the continuous casting method according to the present invention, it is preferable that the runner is inclined downward from the hot-water receiving room side to the tapping-room side.

  In the continuous casting method according to the present invention, at the bottom of the tapping chamber of the tundish, a discharge hole for discharging the molten metal in the tapping chamber to the mold is provided, and the opening of the tap on the tapping chamber side and the discharge are provided. It is preferable that gas is blown from the bottom on the side of the tapping chamber toward a virtual molten metal flow path connecting the holes in a straight line.

The continuous casting method according to the present invention uses a tundish having a weir provided with a runner at the bottom, and has a basicity of (% by mass CaO) / {(% by mass SiO ₂ ) + (% by mass Al ₂ O ₃ )}. Is from 1.0 to 4.0, and a flux in which the amount of Al ₂ O ₃ is 10% by mass or more based on the total amount of CaO, SiO ₂ and Al ₂ O ₃ Since it is arranged on the surface of the molten metal in the chamber, it is possible to appropriately control the slagging state of the flux and to ensure low viscosity of the flux.
Thereby, the entrainment of the flux on the surface of the molten metal in the receiving chamber can be suppressed, so that the molten metal can be further purified as compared with the related art.

BRIEF DESCRIPTION OF THE DRAWINGS It is explanatory drawing of the continuous casting method which concerns on one Embodiment of this invention.

Next, embodiments of the present invention will be described with reference to the accompanying drawings to provide an understanding of the present invention.
First, the background that led to the continuous casting method of the present invention will be described.

When a weir with a runner is placed in a tundish, the inside of the tundish is divided into a receiving room and a tapping room, and when casting by flowing molten metal (molten steel) through this runner, the inclusion is molten metal. Since it has the property of floating inside, by providing a runner at the lower part of the weir, clean molten metal can be circulated through the runner. In addition, by providing an induction heating device (heating device) inside the weir, heating (induction heating) the molten metal flowing through the runner is often performed at the same time.
Regarding the molten metal flowing through the runner, Patent Literature 2 described above states that the molten metal heated by the runner flows toward the upper part (surface of the molten metal) in the tapping chamber of the tundish after being spouted from the outlet of the runner. It is described that floating removal of inclusions is promoted.

However, as a result of studying the behavior of the molten metal flowing through the runner, the present inventors have found that the behavior is as follows.
(1) On the premise of applying induction heating, the number of runners is often four or less due to the arrangement. In addition, in many cases, the runner has a diameter of 300 mm or less when the cross-sectional shape of the flow path (opening) is converted into a circle due to the convenience of the heating device.
(2) When the molten metal to be cast flows through four or less runners having such a diameter, the flow of the molten metal (flow of the molten metal) jetted from the outlet of the runner to the tapping room of the tundish is straight. Turned out to be high. Further, when the length of the runner is long (for example, 800 mm or more, furthermore, 1000 mm or more (in recent years, since the size of the iron core of the heating device has been reduced, the size of the iron core is about 2000 mm or less). Case), this tendency was considered to be strong.

(3) The flow of the molten metal jetted into the tapping chamber mainly collides with the inner wall of the tundish existing at the end of the runner (the structure of a normal tundish, that is, it can be poured into the mold without solidifying the molten metal. With such a configuration, a phenomenon occurs in which the flow of the molten metal collides with the inner wall), and then the flow branches upward or downward. In addition, the dispersion of the flow is observed before the collision, and there is a flow that branches upward or downward from the flow of the molten metal before the collision.
(4) Further, when the molten metal is heated in the runner, a flow that branches upward depending on the degree of heating is also generated.
The present inventors have found that the upward flow of the molten metal jet (including the molten metal flow after colliding against the inner wall of the tundish) spouted from the outlet of the hot runner has an effect of floating the inclusions, although there is an effect of floating the inclusions. It has been found that it can be a cause of surface agitation.
Further, the molten metal in the receiving room has a molten metal flow toward the entrance of the runner. However, the molten metal may be agitated by a vortex-like flow in the vicinity of the entrance of the molten bath. It was also found that the above flux could be involved.

  From the above, the present inventors, when casting a melt using a tundish in which a weir having a runner is arranged, stirring of the melt near the entrance of the runner and entrainment of the flux accompanying the melt into the melt, and this The present inventor has found that the molten metal having low cleanliness is supplied to the tapping chamber, and that the agitation of the molten metal in the tapping chamber by the upward flow after the outlet of the runner can be a factor for suppressing the high purification of the molten metal.

Based on the above findings, the present inventors have conceived of the continuous casting method of the present invention.
That is, as shown in FIG. 1, the continuous casting method according to one embodiment of the present invention is a method of manufacturing a cast piece by pouring a molten metal from a ladle 10 into a mold (not shown) via a tundish 11. Yes,
The tundish 11 divides the inside of the tundish 11 into a hot-water receiving room 12 and a hot-water outlet room 13, and forms one or more and four or less runners 14 through which the molten metal flows from the hot-water receiving room 12 to the hot-water outlet room 13. It has a weir 15 provided in the lower part, and the runner 14 sets the diameter in the range of 100 mm or more and 300 mm or less by converting the cross-sectional shape into a circle,
Basicity = (% by mass CaO) / {(% by mass SiO ₂ ) + (% by mass Al ₂ O ₃ )} is 1.0 or more and 4.0 or less, and the amount of CaO, the amount of SiO ₂ , and the amount of Al ₂ O _A flux in which the amount of Al ₂ O _{3 with} respect to the total amount of the _three amounts is 10% by mass or more is arranged on the surface of the molten metal in the hot water receiving chamber 12.
In addition, the code | symbol 16 in FIG. 1 is a long nozzle, and the code | symbol 17 is an immersion nozzle.
The details will be described below.

1) Configuration of Tundish As described above, the runner 14 is provided below the weir 15 because inclusions have the property of floating inside the molten metal.
Specifically, the height position of the lower end of the opening 18 located on the hot-water receiving chamber 12 side (entrance side) of the hot-water path 14 from the bottom surface 19 of the hot-water receiving chamber 12 is determined by the maximum melt depth of the hot-water receiving chamber 12. The depth is preferably 0.2 times (0.2 × H) or less (0.2 × H) or less, for example, 0 times (0 × H), that is, the opening 18 at the entrance of the runner 14 is (Position in contact with bottom surface 19).
Here, the reason why the lower end position of the opening 18 is set to 0.2 times or less of the depth H of the molten metal is that if the depth exceeds 0.2 times, the height of the opening 18 becomes too high. In the case where the exit of the road 14 becomes high and the floating time of the inclusions in the molten metal in the molten metal chamber 13 cannot be sufficiently secured, leading to insufficient floating, or the flow of the molten metal spouted from the outlet of the molten metal path 14 is This is because the surface may be agitated to deteriorate the cleaning of the molten metal. Also, when the level of the tundish 11 drops at the end of casting for each charge (one ladle), the time when the flux on the level of the hot water in the hot water receiving chamber 12 becomes easy to entrain is 0.2 times as described above. It becomes early if it exceeds.

In consideration of the speed of continuous casting that can be considered normally, the number of the runners 14 provided in the weir 15 is 1 or more and 4 or less, and the cross-sectional shape of each runner 14 is converted into a circle and the diameter is 100 mm. Each is set in the range of not less than 300 mm and not more than 300 mm.
Here, the number of the runners 14 is usually an even number (2 or 4), but may be an odd number (1 or 3). A path 14 is provided (a hot run 14 is provided on both sides of the iron core). For this reason, the tundish 11 may be either one provided with an induction heating device or one not provided.
The cross section of the runner 14 is circular, and has the same shape from the opening 18 located on the hot-water receiving chamber 12 side of the runner 14 to the opening 20 located on the hot-water chamber 13 side (outlet side). However, a shape (a trumpet shape or an inversely tapered shape) gradually increased from the hot-water receiving chamber 12 side to the hot-water chamber 13 side or the like (in this case, the maximum diameter of the opening on the hot-water chamber side is the above-described value) Range). The cross-sectional shape is not limited to a circle, but may be, for example, an ellipse or a polygon.

2) Flux (Seal Flux) By defining the composition of the flux disposed on the surface of the molten metal in the hot water receiving chamber 12, reoxidation of the molten steel in the tundish 11 and prevention of the flux from being entrained in the molten metal are performed.
In the tundish 11, typically, a flux of CaO-SiO ₂ system and _CaO-SiO 2 _-Al 2 _{O 3} system is used.
As described above, the flow of the molten metal toward the entrance of the runner 14 is generated in the receiving room 12, and the molten metal near the entrance of the runner 14 is agitated by the flow of the molten metal, and the flux on the surface of the receiving room 12 is entrained. There are cases. In particular, at the end of the charge being cast, the height of the surface of the hot water receiving chamber 12 may decrease, and this tendency becomes stronger.

Thus, the present inventors have conducted various experiments, and as a result, by suppressing the entrainment into the stirring vortex generated near the runner 14 by optimizing the flux composition on the side of the hot water receiving chamber 12 and maintaining an appropriate mixture. And found that the molten metal can be cleaned.
That is, the present inventors have conceived of appropriately controlling the slagging of CaO (fluidity of a flux containing a large amount of CaO at the time of melting) in order to suppress the flux from being involved in the molten metal.
Specifically, the basicity may be 1.0 or more and 4.0 or less (further 3.7 or less).
For calculating the basicity, (mass% CaO) / {(mass% SiO ₂ ) + (mass% Al ₂ O ₃ )} by CaO, SiO ₂ , and Al ₂ O ₃ (alumina) is used. Here, when the basicity index using only CaO and SiO ₂ = (% by mass CaO) / (% by mass SiO ₂ ), since Al ₂ O ₃ is not included, alumina inclusions and CaO in the flux are used. The generation of the generated CaO-Al ₂ O ₃ -based low-melting oxide is not considered, and the effect of Al ₂ O ₃ on high purification is not considered.

When the above basicity is less than 1.0, slagging of the flux proceeds excessively, erosion of the refractory constituting the tundish 11 proceeds, and refractory particles are present in the flux layer on the surface of the molten metal. Leads to things. These refractory particles are easily affected by the flow of the molten metal in the receiving chamber 12, and there is a concern that the cleanliness may be deteriorated due to mixing in the molten metal.
On the other hand, when the basicity exceeds 4.0, there is a concern that the flux particles are caught in the molten metal due to insufficient slagging.
If the viscosity is too high even if the slagging state is appropriately controlled, the surface of the molten metal is exposed and reoxidation is promoted when the disturbance of the flux occurs, so that the Al ₂ O ₃ concentration (CaO amount) in the flux is increased. , SiO ₂ amount, and Al ₂ O ₃ amount based on the total amount of Al ₂ O ₃ amount) is 10% by mass or more. Thereby, a certain low viscosity can be secured, and the exposure of the molten metal surface to the atmosphere in the tundish 11 can be suppressed. Here, the upper limit value of the Al ₂ O ₃ concentration is determined according to the upper limit value and the lower limit value of the basicity as described above (for example, about 50% by mass).

As described above, in order to control the slagging and viscosity of the flux, the total concentration of CaO, SiO ₂ , and Al ₂ O _{3 in} the flux is, for example, 70% by mass or more (may be 100% by mass). What is necessary is just (the remainder is other components that can be used as components of the flux).
In other words, in order to obtain the above-mentioned effect by setting the amount of Al ₂ O ₃ to 10% by mass or more with respect to the total amount of the amounts of CaO, SiO ₂ and Al ₂ O ₃ , if the above-mentioned effects are to be obtained, It is preferable that the total concentration of CaO, SiO ₂ , and Al ₂ O ₃ is 70% by mass or more. That is, when the total concentration is less than 70% by mass, the remarkable effect of the three components CaO, SiO ₂ , and Al ₂ O ₃ tends to decrease.

The flux composition such as the basicity is specified for the flux added to the hot water receiving chamber 12, but when the amount of slag mixed from the ladle 10 may be significantly increased, the above-described effects are difficult to obtain. Therefore, it is preferable that the slag of the ladle 10 has the same composition. When the slag is mixed into the tundish 11 from the ladle 10 as described above, the slag is also entrained along with the flux on the surface of the molten metal in the receiving chamber 12 with the stirring of the molten metal near the entrance of the runner 14. Be targeted.
Further, the flux composition on the tapping room 13 side may be the same as the flux composition on the receiving room 12 side, but it is not particularly limited, and a flux having a different composition (a flux conventionally used) may be used. Can also be used.

With the configuration described above, flux entrainment due to the stirring of the molten metal in the vicinity of the entrance of the runner 14 of the hot water receiving chamber 12 is suppressed.
As shown in FIG. 1, when the straight runner 14 is inclined downward from the hot-water receiving room 12 side to the hot-water outlet room 13 side (the center position C1 of the end surface of the hot runner 14 on the hot-water receiving room 12 side). Is higher than the center position C2 of the end face on the side of the hot-water chamber 13), the hot-water-receiving-chamber 12 side of the hot-runner 14 faces the hot-water level, so that the flux on the hot-water-receiving-chamber 12 hot face can be more easily involved. However, the runner 14 can be inclined by setting the flux to the above-described composition (suppressing the flux entrainment).
By inclining the runner 14 in this manner, the upward flow of the branch flow generated after the molten metal flow ejected from the outlet of the runner 14 into the tapping chamber 13 collides with the inner wall of the tundish 11 is reduced. You. As a result, it is possible to suppress inclusions floating on the bath surface of the tapping chamber 13 of the tundish 11 from being caught in the molten metal again.

In addition, the refractory wall (the inner wall of the tundish 11) of the tapping room 13 at which the centerline intersects with the center line (axial center) in the length direction of the tapping tube 14 extending toward the tapping room 13 is usually extended. The angle θ (inclination angle θ with respect to the bottom surface 21 (horizontal direction) of the tapping chamber 13) is inclined at, for example, about 65 to 85 degrees (less than 90 degrees) so that the refractory wall opens more outward than a right angle.
For this reason, when the flow of the molten metal (general casting speed, casting size, inner diameter of the runner, and number of runners is 1 to 4), the flow per one runner is When the flow rate of the molten metal is assumed to be 300 to 1800 kg / min), the upward flow tends to be strong when the molten metal collides with the refractory wall of the tapping chamber 13. It is presumed that this flow has a stronger effect of floating the inclusions existing on the surface of the molten metal into the melt again than the effect of floating and removing the inclusions in the molten metal.
Therefore, the upward flow that has become stronger due to the inclination of the runner 14 can be reduced, and re-entanglement of inclusions into the molten metal can be suppressed.

The inclination of the runner 14 may be set as follows.
The thickness (length of the runner 14) of the weir 15 that can also apply induction heating is 800 mm or more, and each center (axial center) position of the end surface of the runner 14 on the side of the hot water receiving chamber 12 and the end surface of the runner 13 The height difference between C1 and C2 (difference in height direction: C1-C2) exceeds 0 mm, and is about 200 mm or less due to the storage amount of the tundish 11 and the like.
Here, the inclination of the runner 14 is defined as {height difference (mm) of each center position between the end surface of the hot runner on the side of the receiving chamber and the end surface of the runner on the side of the hot water} / {the horizontal length of the runner (mm)}. By definition, the lower limit is preferably more than 0.0 (horizontal), and more preferably 0.5 × 10 ⁻² or more (to the extent that a difference can be clearly confirmed compared to horizontal). On the other hand, the upper limit of the inclination is preferably less than 10.0 × 10 ⁻² , preferably 9.5 × 10 ⁻² or less, and more preferably 9.0 × 10 ⁻² or less.
As described above, the upward flow of the molten metal can be alleviated with the increase in the inclination of the runner. However, when the increase in the inclination becomes significant, In this case, the upper limit value of the inclination when the runner is inclined may be determined according to the suppression of the inclusion of the flux and the cleanliness required of the steel to be manufactured.

As shown in FIG. 1, the molten metal flow jetted from the outlet of the hot water runner 14, and has a high straightness, generates a molten metal flow that is partially branched toward the lower part of the tapping chamber 13. Since this branched molten metal flow is provided at the bottom of the tapping chamber 13 and causes a flow directly flowing into the discharge hole 22 for pouring (discharging) the molten metal into the mold via the immersion nozzle 17, the molten metal flow is interposed in the tapping chamber 13. There is room for improvement in the floating action of objects. In particular, when the runner 14 is inclined, this tendency becomes stronger.
Further, at the end of the charge being cast, the level of the molten metal in the tapping chamber 13 may decrease. However, the flow diverging upward from the molten metal jet ejected from the runner 14 due to the decrease in the level of the molten metal. Is decreased, and the flow branched to the lower part is increased, so that the flow directly flowing into the above-described discharge hole 22 is more likely to occur.
Therefore, as a means for changing the flow direction of the molten metal flow directly flowing into the discharge hole 22 and promoting the floating of inclusions, the opening 20 on the tapping chamber 13 side of the runner 14 and the discharge hole 22 are linearly connected. It is preferable to blow gas from the bottom on the side of the tapping chamber 13 toward the virtual molten metal flow path (for example, to blow gas into the tapping chamber 13 from around the discharge hole 22).

An inert gas (for example, argon gas (Ar gas)) is used as the gas to be blown.
In accordance with the increase in the gas blowing amount, it is possible to promote changing the direction of the molten metal flowing directly into the discharge hole 22 and floating the inclusions. However, when the gas injection amount exceeds 0.9 liter (0.9 L / (min · ton)) per unit amount and unit time of the molten metal in the tapping chamber 13, the effect is saturated. When the gas is blown, the amount is preferably set to more than 0 and 0.9 L / (min · ton) or less, since the gas may be reduced.
It is preferable that the gas is blown continuously during the casting, but since the effect of blowing the gas at the end of charging is more remarkable, at least the gas is blown when the level of the molten metal drops at the time of changing the charge of the continuous casting (continuous casting). Injecting also produces a corresponding effect. The continuous casting is a method in which the molten metal in a plurality of ladles is continuously and sequentially cast, and the molten metal to be cast may be the same steel type or different steel types.

Next, examples performed to confirm the operation and effect of the present invention will be described.
Here, based on the following method, each condition was changed at the actual machine level, and the cleanliness of the slab was evaluated. The steel type to be evaluated was a thin container material (tin material).

(Refining conditions)
After performing primary refining in a 350 ton converter, molten steel (carbon concentration: 0.02 to 0.04 mass%, dissolved oxygen concentration in molten steel: about 600 to 800 ppm by mass ratio) Was moved to a simple ladle refining facility (CAS), and 1.2 to 2.4 kg of metallic aluminum was added to molten steel in the ladle per ton of molten steel to perform deoxidation treatment.

(Casting conditions)
The molten steel in the ladle treated by the above method is poured into a tundish receiving room divided into a receiving room and a tapping room by a weir equipped with two runners, and flux is added to the receiving room. In this state, continuous casting was performed. The flow rate of molten steel per runner (unsteady part and steady part) was 300 to 1800 kg / min.
The tundish runner had an inner diameter of 150 mm and a horizontal length in the center line direction of 1200 mm.
The position of the runner in the height direction of the weir (the height position of the lower end of the opening on the hot-water receiving chamber side) is defined as 0.2 mm, where H (m) is the maximum depth of molten steel in the hot-water receiving chamber during normal operation. × H or less.
The angle θ of the inclination of the refractory wall of the tapping room which extends while extending the center line in the length direction of the tapping water toward the tapping room is 80 degrees.
In each of the embodiment, the comparative example, and the conventional example, a flux having a basicity of 1.0 (same as the flux charged into the hot water receiving chamber of Example 1) is cast in advance on the tapping room side and cast. Note that the total concentration of CaO, SiO ₂ , and Al ₂ O ₃ in the flux charged into the hot water receiving chamber is 70% by mass or more.
The conventional method is an operation in which no flux is added to the hot water receiving chamber and the hot water receiving chamber is purged with Ar gas.

(Experimental result)
Table 1 shows the test conditions, results and evaluation.
In Table 1, in the column of “basicity” of “hot-water receiving chamber flux”, the basicity of the flux added to the hot-water receiving chamber is “(% by mass CaO) / ｛(% by mass SiO ₂ ) + (% by mass Al _2). O ₃ )} ”.
The column of "concentration of _Al 2 _{O 3"} of "受湯chamber flux", the concentration of _Al 2 _{O 3} contained in the flux (i.e., CaO amount, SiO ₂ amount, and the total amount of the amount of _Al 2 _{O 3} Al ₂ O ₃ amount).
In the column of "Slope of runner", the presence or absence of inclination of the runner is described. When the runner is inclined, the height difference (mm ) Is divided by the horizontal length (mm) of the runner.
In the column of "gas blowing", the presence or absence of gas blowing is described, and in the case of gas blowing, the gas flow rate per unit amount of molten steel stored in the tapping room and per unit time is also described. In addition, the amount of molten steel stored in the tapping room can be calculated from the volume of the tapping room and the level of the molten metal obtained from drawings and the like.

The inclusion number index in the slab was as follows.
For the evaluation, two slabs were used: a steady portion in casting (the height of the molten metal surface of the tundish was constant at the maximum value) and an unsteady portion (near the seam due to ladle replacement in continuous casting).
In detail, the slab of the stationary part was a slab including a portion that was 50 tons from the seam (the ladle was replaced after casting 50 tons from a specific location of the slab). In addition, the slab of the unsteady part includes a part including a part which is 30 tons from the seam after the ladle replacement is approached and the surface level of the tundish is started to be lowered (beginning to be lowered). A piece (replacement of ladle after casting 30 tons from a specific place of the slab).
The start time of the drop in the surface level of the tundish is approximately 40 tons from the seam. Therefore, after the time point of going back 40 tons (in the range of 40 tons or less), the level of the molten metal continues to decrease.
The tonnage of the molten steel described above can be detected by the casting conditions (for example, casting speed, tundish volume, etc.) and the amount of remaining hot metal in the ladle.

Samples (rectangular rectangles each having a side of 30 mm) cut out from the representative positions of these slabs in the stationary part and the slabs in the unsteady part were each mirror-polished, and the number of alumina inclusions was examined with an optical microscope. It was converted to the number of detected alumina inclusions per area.
Further, the detected number of each of the stationary part and the non-stationary part under the condition of the first embodiment is set to 1.00, and the detected numbers of the stationary part and the non-stationary part of the other examples, the comparative example, and the conventional example are respectively indexed. It has become.
Here, the evaluation of the indexed value is as follows.
The indexed value is 0.95 times or more and less than 1.60 times the value of Example 1: ΔEvaluation The indexed value is 0.70 times or more and less than 0.95 times of Example 1: ○ Evaluation / index The converted value is less than 0.70 times that of Example 1. ◎ The value evaluated and indexed is 1.60 times or more than that of Example 1. X Evaluation. 1.70 to 2.99 times that of Example 1, and all evaluations were x.

As shown in Table 1, in all of Examples 1 to 9, the composition of the flux disposed on the surface of the molten steel in the hot water receiving chamber was set to an appropriate range (basicity: 1.0 to 4.0, Al ₂ O ₃ concentration: 10). (% By mass). Thereby, for both the steady portion and the unsteady portion, the evaluation of the inclusion detection index in the slab was Δ, ○, or ◎, indicating that the effect of suppressing the entrapment of the flux in the hot water receiving room was obtained. I understand.
Examples 1 to 9 describe only 10% by mass of the Al ₂ O ₃ concentration, but when the basicity is in the range of 1.0 to 4.0, the Al ₂ O ₃ concentration is increased to more than 10% by mass. In this case, substantially the same results were obtained.
The concentration of Al ₂ O _3, if the basicity of 1.0 up to 50 wt% of the concentration of _Al 2 _{O 3,} In addition, if the basicity is 4.0 to 20 wt% of the concentration of _Al 2 _{O 3,} It is possible to increase each.
In particular, the number of detected inclusions (values not indexed) in the slab in the non-stationary part of Example 1 was equivalent to that of the stationary part of the conventional example. This indicates that in the unsteady part where the surface level of the tundish in which the flux entrainment on the hot water receiving chamber side is apt to occur is reduced, the effect of suppressing the entrapment of the flux on the hot water receiving chamber side is obtained. .

Examples 4 and 6 to 9 are the results when the slope of the runner (the slope is within the preferable range described above) and / or when gas is blown. As a result, the upward flow of the branch flow in the tapping room is reduced by the inclination of the runner, and / or the flow direction of the molten steel flow directly flowing into the discharge hole due to the gas injection and the floating of inclusions are promoted. The effect was obtained. For this reason, the evaluation of the inclusion detection index in the slab was improved compared to Examples 1 to 3 in which both the slope of the runner and the gas injection were not performed for both the steady portion and the unsteady portion ( Evaluation: 又 は or ◎).
In particular, in Examples 7 and 8 in which gas was blown in the unsteady portion, the index of the number of inclusions detected in the slab was improved as compared with Example 1. For this reason, by injecting gas at the time of casting molten steel around the seam portion (for example, in the range of 40 tons before and after the seam), the vicinity of the seam portion which is an unsteady portion is equivalent to the steady portion of the conventional example. The above-described effect of suppressing inclusion defects can be expected.

On the other hand, Comparative Examples 1 to 3 and the conventional example are results when the basicity and / or Al ₂ O ₃ concentration of the flux disposed on the surface of the molten steel in the hot water receiving chamber is set outside the above-mentioned appropriate range. For this reason, in any of the steady part and the unsteady part, the entrainment of the flux on the hot water receiving chamber side could not be suppressed, and the evaluation of the inclusion detection index in the slab was evaluated as x.

  Therefore, it was confirmed that the use of the continuous casting method of the present invention can further improve the purity of the molten metal as compared with the conventional method.

  As described above, the present invention has been described with reference to the embodiments. However, the present invention is not limited to the configurations described in the above-described embodiments, and the matters described in the claims are not limited. Other embodiments and modifications that can be considered within the scope are also included. For example, a case where the continuous casting method of the present invention is configured by combining some or all of the above-described embodiments and modifications is also included in the scope of the present invention.

10: Ladle, 11: Tundish, 12: Hot-water chamber, 13: Hot-water chamber, 14: Runner, 15: Weir, 16: Long nozzle, 17: Immersion nozzle, 18: Opening, 19: Bottom, 20 : Opening, 21: bottom, 22: discharge hole

Claims (3)

In a continuous casting method of manufacturing a slab by pouring the molten metal into the mold from a ladle through a tundish,
The tundish divides the inside of the tundish into a hot-water receiving room and a hot-water outlet room, and has at the bottom one or more and four or less runners through which the molten metal flows from the hot-water receiving room toward the hot-water outlet room. Having a weir, the runner has a diameter in the range of 100 mm or more and 300 mm or less when the cross-sectional shape is converted into a circle,
Basicity = (% by mass CaO) / {(% by mass SiO ₂ ) + (% by mass Al ₂ O ₃ )} is 1.0 or more and 4.0 or less, and the amount of CaO, the amount of SiO ₂ , and the amount of Al ₂ O how continuous casting, characterized in that the amount of Al ₂ O ₃ to the total amount of ₃ weight flux is 10 mass% or more, placing the molten metal surface of the受湯chamber.   2. The continuous casting method according to claim 1, wherein the runner is inclined downward from the hot-water receiving chamber side to the hot-water chamber side.   3. The continuous casting method according to claim 1, wherein a discharge hole is provided at a bottom of the tapping chamber of the tundish to discharge molten metal in the tapping chamber to the mold, and an opening of the runner on the tapping chamber side. A continuous casting method, wherein gas is blown from a bottom portion on the side of the tapping chamber toward a virtual molten metal flow path that linearly connects the discharge hole and the discharge hole.

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