JP7234837B2 - Continuous casting method - Google Patents

Description

The present invention relates to a continuous casting method for high cleanliness steel.

Steel materials for processing applications, such as tinplates, IF steels, steel bars, wire rods, etc., are required to reduce the amount of inclusions such as alumina contained in the steel materials in order to suppress the occurrence of cracks during processing. there is
For this reason, in the manufacturing process of melting steel materials, the formation of inclusions is suppressed and the inclusions are removed. In the continuous casting process, which is one of these manufacturing processes, molten metal (molten steel) is poured from a ladle (molten steel ladle) into a tundish, and then the molten metal in the tundish is poured into a mold to produce a slab. However, techniques for suppressing the formation of inclusions and removing floating inclusions in this tundish are also being studied.

For example, in Patent Document 1, in a tundish for induction heating, a sleeve-shaped molten metal passage is provided in the induction heating part to connect the molten metal receiving chamber and the molten metal dispensing chamber. It is described that by providing the above steps, the molten steel that has flowed into the tapping chamber is agitated, and the effect of floating inclusions in the molten metal can be enhanced.
Further, in Patent Document 2, a carbon component is added to molten steel between the tapping process and the vacuum degassing process, vacuum degassing is performed, final Al deoxidation is performed, and then the molten steel is poured into a tundish. A method for producing high cleanliness steel is described. This tundish is divided into a receiving part and a discharging part, and the height position of the opening of the molten steel flow path on the receiving part side from the bottom of the receiving part is set to the depth of the molten steel on the receiving part side. 0.2 times or less.
In addition, in Patent Document 3, a weir having a through hole (runner) for passing molten steel is provided in the tundish, and a heating means (for example, , induction heating means) are described. By heating the molten steel passing through the through-holes with this heating means, the molten steel after passing through the through-holes becomes hotter than the molten steel around it, so strong buoyancy acts. As a result, a flow of molten steel is formed toward the molten steel surface in the tundish, so inclusions can be floated and separated (eg, paragraph [0018]).

Japanese Utility Model Laid-Open No. 6-86849 JP 2016-204693 A JP 2008-264834 A

However, although the methods described in Patent Literatures 1 to 3 are capable of achieving correspondingly high cleanliness of molten steel, the effect is not sufficient when an extremely high degree of cleanliness is required.

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

A continuous casting method according to the present invention that meets the above object is a continuous casting method in which molten metal is poured from a ladle through a tundish into a mold to produce a slab,
The tundish divides the inside of the tundish into a hot water receiving chamber and a hot water dispensing chamber, and has at the bottom one or more and four or less runners through which the molten metal flows from the hot water receiving chamber toward the hot water dispensing chamber. and the runner has a cross-sectional shape converted to a circular shape and has a diameter in the range of 100 mm or more and 300 mm or less,
The runner is inclined downward from the hot water receiving chamber side to the hot water dispensing chamber side, and {height difference between each center position of the end surface of the runner on the side of the hot water receiving chamber and the end surface on the side of the hot water dispensing chamber (mm)}/{horizontal length of the runner (mm)} and the inclination of the runner is in the range of 0.5×10 ⁻² or more and 9.5×10 ⁻² or less,
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 Al ₂ O A flux having an Al ₂ O ₃ content of 10% by mass or more with respect to the total amount of the _three components is placed on the surface of the molten metal in the receiving chamber with a thickness of 5 mm or more and 50 mm or less .

In the continuous casting method according to the present invention, a discharge hole for discharging the molten metal in the tapping chamber to the mold is provided in the bottom of the tapping chamber of the tundish, and the opening of the runner on the tapping chamber side and the discharge are provided. It is preferable to blow gas from the bottom of the pouring chamber side toward a virtual molten metal flow path that linearly connects the holes.

In the continuous casting method according to the present invention, a tundish having a weir provided with a runner at the bottom is used, the runner is inclined downward from the hot water receiving chamber side to the hot water discharging chamber side, and {runner channel 0.5×10 Since the range is from ⁻² to 9.5×10 ⁻² , it is possible to suppress the entrainment of flux on the surface of the molten metal in the receiving chamber and to suppress the upward flow of the molten metal in the tapping chamber.
Furthermore, 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 Al Since the flux having an Al ₂ O _{3 content} of 10% by mass or more with respect to the total amount of ₂ O 3 is arranged on the surface of the molten metal in the receiving chamber, the slag state of the flux can be appropriately controlled, and the flux has a low viscosity. can be ensured.
As a result, the molten metal can be cleaned to a higher degree than the conventional method.

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

Next, specific embodiments of the present invention will be described with reference to the attached drawings for better understanding of the present invention.
First, the circumstances leading to the idea of the continuous casting method of the present invention will be described.

When a weir with a runner is arranged in the tundish, the inside of the tundish is divided into a receiving chamber and a tapping chamber, and molten metal (molten steel) is circulated through this runner for casting, inclusions are formed in the molten metal. Since it has the characteristic of floating inside, by providing the runner at the lower part of the weir, clean molten metal can be circulated in the runner. In many cases, an induction heating device (heating device) is provided inside the weir to heat (induction heating) the molten metal flowing through the runner.
Regarding the molten metal flowing through the runner, the aforementioned Patent Document 3 describes that the molten metal heated in the runner flows toward the upper part (surface of the molten metal) of the tapping chamber of the tundish after being ejected from the outlet of the runner. , that the removal of inclusions by floating is promoted.

However, the inventors of the present invention investigated the behavior of the molten metal flowing through the runner, and the results were as follows.
(1) Assuming that induction heating is applied, there are many cases where the number of runners is 4 or less due to the arrangement thereof. Further, the runner often has a diameter of 300 mm or less when the cross-sectional shape of the channel (opening) is converted into a circular shape due to the convenience of the heating device.
(2) When the molten metal to be cast is passed through four runners or less having a diameter of this order, the molten metal flow (flow of molten metal) ejected from the outlet of the runner into the tapping chamber of the tundish is straight. was found to be high. Furthermore, when the length of the runner is long (for example, 800 mm or more, or even 1000 mm or more (in recent years, the iron core of the heating device has been made compact, so considering the size of the iron core, it is about 2000 mm or less). case), this tendency was considered to be strong.

(3) The flow of molten metal ejected into the pouring chamber collides mainly with the inner wall of the tundish that exists beyond the extension of the runner (normal tundish structure, that is, the molten metal can be poured into the mold without solidifying). If the configuration is such that the molten metal flow collides with the inner wall), the flow then branches upward, downward, or the like. In addition, the flow diverges before the collision, and there are flows that diverge upward and downward from the flow of the molten metal before the collision.
(4) Furthermore, when the molten metal is heated in the runner, an upward branching flow is generated according to the degree of heating.
The inventors of the present invention have found that the molten metal flow ejected from the outlet of the runner (including the molten metal flow after colliding with the inner wall of the tundish) has the floating effect of inclusions, but the upward flow is It was found that it can also cause agitation of the surface.
Furthermore, in the molten metal in the receiving chamber, a molten metal flow toward the entrance of the runner is generated. It was also found that the upper flux may be involved.

From the above, the present inventors have found that when the molten metal is cast using a tundish in which a weir having a runner is arranged, the molten metal is agitated near the entrance of the runner, the flux is entrained in the molten metal accompanying this, and this It has been found that the molten metal with low cleanliness is supplied to the tapping chamber, and the stirring of the molten metal in the tapping chamber due to the upward flow after the outlet of the runner can be a factor that suppresses the high cleaning of the molten metal.

Based on the above knowledge, the present inventors came up with the continuous casting method of the present invention.
First, a continuous casting facility 10 to which a continuous casting method according to one embodiment of the present invention is applied will be described.
As shown in FIG. 1, a continuous casting facility 10 includes a ladle 11, a tundish 13 into which molten metal is poured from the ladle 11 through a long nozzle 12, and a molten metal from the tundish 13 through a submerged nozzle 14. and a mold (not shown) to be poured.
In the tundish 13, the inside of the tundish 13 is divided into a hot water receiving chamber 15 and a hot water discharging chamber 16, and one or more and four or less runners 17 through which molten metal flows from the hot water receiving chamber 15 toward the hot water discharging chamber 16. , and the cross-sectional shape of the runner 17 is set to a range of 100 mm or more and 300 mm or less in diameter.
The runner 17 is inclined downward from the hot water receiving chamber 15 side to the hot water discharging chamber 16 side, and the center positions C1 and C2 of the end surface of the hot water receiving chamber 15 side and the hot water discharging chamber 16 side of the runner 17 are aligned. The inclination of the runner 17 defined by the height difference (mm)}/{the horizontal length (mm) of the runner 17} is in the range of 0.5×10 ⁻² or more and 9.5×10 ⁻² or less. is set to
The continuous casting method according to the present embodiment is a method of producing (casting) a slab by injecting molten metal from a ladle 11 into a mold through a tundish 13 using the continuous casting equipment 10 described above.
A detailed description will be given below.

As described above, in the molten metal flow ejected from the outlet of the runner, the upward flow can cause agitation of the surface of the molten metal in the tapping chamber. For this reason, in order to reduce the agitation of the hot water surface of the hot water discharge chamber due to the upward flow, as shown in FIG. It is preferable to incline (the center position C1 of the end surface on the side of the hot water receiving chamber 15 is higher than the center position C2 of the end surface on the side of the hot water discharge chamber 16).
However, depending on the degree of inclination, as described above, the molten metal may be stirred by a vortex-like flow or the like near the entrance of the runner 17, and the flux on the molten metal surface of the receiving chamber 15 may be involved. In order to obtain this, the inclination of the runner must be set appropriately.

The refractory wall (the inner wall of the tundish 13) of the hot water discharge chamber 16 where the center line (axis) in the longitudinal direction of the runner 17 is extended toward the hot water discharge chamber 16 and the center line intersects is normally The angle θ (inclination angle θ with respect to the bottom surface 19 (horizontal direction) of the tapping chamber 16) is inclined at, for example, about 65 to 85 degrees (less than 90 degrees) so that the refractory wall opens outward from the right angle.
For this reason, the molten metal flow ejected from the outlet side of the runner 17 to the tapping chamber 16 (general casting speed, casting size, inner diameter of the runner, and when the number of runners is 1 to 4, 300 to 1800 kg/min of molten metal) collides with the refractory wall of the tapping chamber 16, the upward flow tends to be strong. It is presumed that this flow has a stronger effect of floating inclusions present on the surface of the molten metal in the tapping chamber 16 and involving them again in the molten metal, rather than the effect of floating and removing the inclusions in the molten metal.
Therefore, the inclination of the runner 17 can moderate the upward flow, which has become strong in this way, and suppress re-entrainment of inclusions in the molten metal.

Therefore, the configuration of the tundish 13 is defined as follows.
The reason why the runner 17 is provided below the weir 18 is that the inclusion has the property of floating in the molten metal, as described above.
Specifically, the height position of the lower end of the opening 20 of the runner 17 located on the receiving hot water receiving chamber 15 side (entrance side) from the bottom surface 21 of the receiving hot water receiving chamber 15 is the maximum molten metal depth of the receiving hot water receiving chamber 15. It is preferable that the height (bath depth) H is 0.2 times (0.2×H) or less (the lower limit is, for example, 0 times (0×H), that is, the opening 20 at the entrance of the runner 17 is the same as that of the receiving chamber 15. position in contact with the bottom surface 21).
Here, the reason why the lower end position of the opening 20 is set to 0.2 times or less of the depth H of the molten metal is that if it exceeds 0.2 times, the height position of the opening 20 becomes too high and the molten metal When the outlet of the path 17 becomes high and the floating time of the inclusions in the molten metal in the tapping chamber 16 cannot be secured sufficiently, the flow of the molten metal jetted from the outlet side of the tapping chamber 16 is not sufficient. This is because the surface may be agitated and the cleaning of the molten metal may be deteriorated. In addition, when the surface of the tundish 13 drops at the end of casting for each charge (one ladle), the time when the flux on the surface of the receiving chamber 15 is likely to be involved is 0.2 times as described above. If it exceeds, it will be early.

The number of runners 17 provided in the dam 18 is 1 or more and 4 or less, and each runner 17 has a diameter of 100 mm when the cross-sectional shape of each runner 17 is converted into a circle, in consideration of the speed of continuous casting that is usually considered. Each is set within a range of 300 mm or less.
Here, the number of runners 17 is usually an even number (2 or 4), but may be an odd number (1 or 3). A path 17 is provided (runners 17 are provided on both sides of the iron core). Therefore, the tundish 13 may or may not be equipped with an induction heating device.
The cross-sectional shape of the runner 17 is circular, and the shape is the same from the opening 20 located on the hot water receiving chamber 15 side of the runner 17 to the opening 22 located on the hot water discharge chamber 16 side (exit side). However, it is also possible to adopt a shape (trumpet shape or reverse tapered shape) that gradually increases from the hot water receiving chamber 15 side to the hot water discharging chamber 16 side (in this case, the maximum diameter of the opening on the hot water discharging chamber side is the above range). Moreover, the cross-sectional shape is not limited to a circular shape, and may be, for example, an elliptical shape, a polygonal shape, or the like.

The inclination of runner 17 is set as follows.
The thickness of the weir 18 (the length of the runner 17), which enables the application of induction heating, is 800 mm or more. The height difference between C1 and C2 (difference in the height direction: C1-C2) exceeds 0 mm and is about 200 mm or less depending on the storage amount of the tundish 13 and the like.
Here, the inclination of the runner 17 is defined by {height difference (mm) between the center positions of the end face of the runner on the receiving chamber side and the end face on the hot water discharge chamber side} / {horizontal length of the runner (mm)}. By definition, the lower limit is 0.5×10 ⁻² . On the other hand, the upper limit of the slope is 9.5×10 ⁻² , preferably 9.0×10 ⁻² .

With the inclinations illustrated in Patent Document 1 (approximately 9.7×10 ⁻² in FIG. 2) and Patent Document 2 (approximately 10.1×10 ⁻² in FIG. 1), the slag of the flux in the receiving chamber Depending on the degree of slag and the viscosity of the slag flux, the molten metal may be agitated by a vortex-like flow or the like in the vicinity of the runner entrance of the receiving chamber. For example, the basicity of the flux on the surface of the hot water receiving chamber = (mass% CaO) / {(mass% SiO ₂ ) + (mass% Al ₂ O ₃ )} is about 1.0 to 4.0, and CaO Even if the amount of Al ₂ O ₃ with respect to the total amount of the amount, the amount of SiO ₂ and the amount of Al ₂ O ₃ is about 10% by mass or more, flux may be involved. It is preferable to set it to 5×10 ⁻² or less.
If the inclination of the runner 17 is 0.5×10 ⁻² or more, the effect of suppressing the upward flow in the hot water discharge chamber 16, etc. is clearly high compared to the case where the runner 17 is horizontal. becomes.

Next, the composition of the flux placed on the surface of the molten metal in the receiving chamber 15 will be described.
By specifying the composition of the flux, re-oxidation of the molten steel in the tundish 13 and flux entrainment in the molten metal can be prevented more remarkably.
The tundish 13 generally uses a CaO-- _SiO.sub.2- based or CaO-- _SiO.sub.2 -- _{Al.sub.2O.sub.3 -} based flux.
As described above, in the receiving chamber 15, the molten metal flow toward the entrance of the runner 17 is generated, and the molten metal in the vicinity of the entrance of the runner 17 is agitated by this molten metal flow, and the flux on the molten metal surface of the receiving chamber 15 is involved. Sometimes. In particular, at the end of the casting charge, the level of the molten metal in the receiving chamber 15 may drop, and this tendency becomes stronger.

Therefore, the present inventors conducted various experiments, and found that by optimizing the flux composition on the side of the molten metal receiving chamber 15 and maintaining an appropriate composition, it is possible to suppress entrainment in the stirring vortex generated in the vicinity of the runner 17. It was found that the cleaning of the molten metal can be achieved.
That is, the present inventors came up with the idea of appropriately controlling CaO slag formation (fluidity of flux containing a large amount of CaO during melting) in order to suppress entrainment of flux in molten metal.
Specifically, the basicity is preferably 1.0 or more and 4.0 or less (more preferably 3.7 or less).
(mass % CaO)/{(mass % SiO ₂ )+(mass % Al ₂ O ₃ )} by CaO, SiO ₂ and Al ₂ O ₃ (alumina) is used to calculate the basicity. Here, when using only CaO and SiO ₂ basicity index = (% by mass CaO) / (% by mass SiO ₂ ), since Al ₂ O ₃ is not included, the alumina inclusions and CaO in the flux The generation of CaO—Al ₂ O ₃ -based low-melting-point oxides is not taken into consideration, and the influence of Al ₂ O ₃ on high cleanliness is not taken into consideration.

If the above-described basicity is less than 1.0, the scumming of the flux proceeds excessively, and the refractory constituting the tundish 13 progresses in melting, and refractory particles are present in the flux layer on the surface of the molten metal. It leads to things. These refractory particles are easily affected by the flow of molten metal in the receiving chamber 15, and there is concern that cleanliness may be deteriorated due to contamination into the molten metal.
On the other hand, if the basicity exceeds 4.0, there is a concern that flux particles may be involved in the molten metal due to insufficient slag formation.
In addition, if the viscosity is too high even if the slag state is appropriately controlled, the surface of the molten metal will be exposed and _reoxidation will be promoted when the flux _is disturbed. , _{SiO 2} amount, and _{Al 2} O ₃ amount) is 10% by mass or more. As a result, a constant low viscosity can be ensured, and the surface of the molten metal can be prevented from being exposed to the atmosphere inside the tundish 13 . Here, the upper limit of the Al ₂ O ₃ concentration is determined according to the upper and lower limits of the basicity, as described above (for example, about 50% by mass).

In this way, in order to control the slag formation 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 (100% by mass may be acceptable). (the balance is other components that can be used as components of the flux).
In other words, if _the above effect is to be obtained by setting the amount of Al ₂ O ₃ to 10% by mass or more _with respect to the total amount of CaO, SiO ₂ and Al 2 O 3 , The total concentration of CaO, SiO ₂ and Al ₂ O ₃ is preferably 70% by mass or more. That is, when the total concentration is less than 70% by mass, the effects of the three components CaO, SiO ₂ and Al ₂ O ₃ tend to become less pronounced.

The flux composition such as basicity is specified for the flux added to the hot water receiving chamber 15, but if the amount of slag mixed from the ladle 11 may increase significantly, it is difficult to obtain the above effects. Therefore, it is preferable that the slag in the ladle 11 has the same composition. In this way, when slag is mixed into the tundish 13 from the ladle 11, this slag is also caught together with the flux on the surface of the molten metal in the receiving chamber 15 as the molten metal is stirred near the entrance of the runner 17. subject to
Also, the flux composition on the hot water discharge chamber 16 side can be the same as that on the hot water receiving chamber 15 side, but is not particularly limited. can also be used.

As for the flux placed on the surface of the molten metal in the receiving chamber 15, not only the composition but also the thickness of the flux may affect entrainment of the flux.
Here, if the thickness of the flux is less than 5 mm, even if the viscosity of the flux is properly controlled, a small amount of flux will be caught in the vortex when the molten metal is stirred near the entrance of the runner 17 when the vortex is generated. Sometimes. On the other hand, if the thickness of the flux exceeds 50 mm, even if the slag-forming property of the flux is improved, a partially unmelted state may occur and a small amount of flux may be easily involved.
Therefore, the thickness of the flux placed on the surface of the molten metal in the receiving chamber 15 is preferably 5 mm or more and 50 mm or less.
In addition, the thickness of the flux is the average value of the thickness of the flux measured at a plurality of locations (for example, three locations). The thickness (measured value) of the flux at each location is obtained by immersing a thin iron rod into molten steel at the location where the flux is molten, pulling up the thin rod after the part of the thin rod immersed in molten steel melts, and measuring the thickness of the thin rod. determined based on the length of the molten flux attached to the

As shown in FIG. 1 , a molten metal flow (a part of the molten metal flow) branching toward the lower part of the tapping chamber 16 is generated from the highly straight molten metal flow ejected from the outlet of the runner 17 . This branched molten metal flow is provided at the bottom of the tapping chamber 16 and causes a flow that directly flows into the discharge hole 23 for pouring (discharging) the molten metal into the mold through the submerged nozzle 14. There is still room for improvement regarding the levitation action of objects. Especially when the runner 17 is inclined, this tendency becomes stronger.
Furthermore, at the end of the casting charge, the level of the molten metal in the tapping chamber 16 may drop. is reduced, and the flow branched to the lower part is strengthened, so that the tendency for the flow directly flowing into the discharge hole 23 to be generated is strengthened.
For this reason, as means for changing the flow direction of the flow of the molten metal directly flowing into the discharge hole 23 and for promoting the floating of inclusions, the opening 22 of the runner 17 on the side of the tapping chamber 16 and the discharge hole 23 are connected in a straight line. It is preferable to blow gas from the bottom on the tapping chamber 16 side toward the virtual molten metal flow path. In addition, it is preferable to provide a plurality of gas inlets for blowing in gas, for example, around the discharge hole 23 (at the bottom on the hot water discharge chamber 16 side).

An inert gas (for example, argon gas (Ar gas)) is used as the blown gas.
It is possible to change the direction of the flow of the molten metal directly flowing into the discharge hole 23 and to float the inclusions according to the increase in the gas blowing amount. However, when the amount of gas blowing exceeds 0.9 liters (0.9 L/(min.ton)) per unit amount of molten metal in the tapping chamber 16 and per unit time, the effect saturates. When the gas is blown in, the amount should be more than 0 and 0.9 L/(min·ton) or less.
It is preferable to continuously blow gas during casting. A corresponding effect can also be obtained by blowing in Continuous casting is a method of continuously casting molten metal in a plurality of ladles in sequence, and the molten metal to be cast may be of the same steel grade or of different steel grades.

Next, an example conducted to confirm the effects of the present invention will be described.
Here, based on the following method, the cleanliness of the cast slab was evaluated by changing each condition at the level of the actual machine. The type of steel to be evaluated was the bar and wire type.

(refining conditions)
Molten steel tapped into the ladle after primary refining in a 350-ton converter (carbon concentration: 0.05 to 0.15% by mass, dissolved oxygen concentration in molten steel: about 300 to 600 ppm by mass ) was deoxidized by adding 0.8 to 2.0 kg of metal aluminum per ton of molten steel. After the molten steel in the ladle was subjected to LF treatment, cleaning treatment was performed using a REDA-type vacuum degassing device (a device using a single large-diameter immersion pipe).

(Casting conditions)
The molten steel in the ladle processed by the above method is poured into the hot water receiving chamber of the tundish, which is divided into a hot water receiving chamber and a hot water discharge chamber by a weir equipped with two runners, and flux is added to the hot water receiving chamber. Continuous casting was performed in this state. The amount of molten steel passing through one runner (unsteady portion and steady portion) was 300 to 1800 kg/min.
The runner of the tundish had an inner diameter of 150 mm and a horizontal length of 1200 mm in the center line direction.
The position of the runner in the height direction of the weir (the height position of the lower end of the opening on the receiving chamber side) is 0.2, where H (m) is the maximum depth of molten steel in the receiving chamber during normal operation. xH or less.
The inclination angle θ of the refractory wall of the tapping chamber, which intersects with the longitudinal center line of the runner extending toward the tapping chamber, is 80 degrees. Further, the position C3 of the refractory wall of the tapping chamber at which the center line in the longitudinal direction of the runner extends toward and intersects with the tapping chamber is a position about 150 mm from the bottom of the tapping chamber.
In each of the examples, the reference examples, and the comparative examples, a flux having a basicity of 2.0 (same as the flux introduced into the receiving chamber of the reference example 1) was previously introduced into the pouring chamber side for casting. The total concentration of CaO, SiO ₂ and Al ₂ O ₃ in the flux put into the receiving chamber is 70% by mass or more.

(Experimental result)
Table 1 shows test conditions and their results and evaluations.
In Table 1, in the column of "Inclination of the runner", the height difference (mm) between the center positions of the end face of the runner on the side of the hot water receiving chamber and the end face on the side of the hot water discharge chamber, and the length of the runner in the horizontal direction (mm) ) is given as the slope calculated by dividing by
In the column of "basicity" of "flux in receiving hot water chamber", the basicity of flux added in the receiving hot water chamber "(mass% CaO) / {(mass% SiO ₂ ) + (mass% Al ₂ O ₃ ) }” is described.
In the "Al ₂ O ₃ concentration" column of "Flux in the hot water receiving chamber", the Al ₂ O ₃ concentration contained in the flux (that is, the total amount of CaO, SiO ₂ , and Al ₂ O ₃ Al ₂ O ₃ amount relative to the amount) is described.
The thickness of the flux placed on the surface of the molten metal in the receiving chamber is described in the column of "thickness" of "flux in receiving chamber".
In the "Blowing of gas" column, indicate whether or not gas is blown from the bottom of the tapping chamber side toward the virtual molten metal flow path. , and the gas flow rate per unit time is also described. The amount of molten steel stored in the tapping chamber can be calculated from the volume of the tapping chamber and the height of the melt surface obtained from drawings or the like.

The "index of the number of inclusions detected in the slab" was as follows.
For the evaluation, slabs at two locations were used: a steady portion in casting (the molten metal surface height of the tundish is constant at the maximum value) and an unsteady portion (near the joint due to ladle replacement in continuous casting).
Specifically, the slab in the stationary part was a slab containing a portion 50 tons upstream from the joint (the ladle was changed after casting 50 tons from a specific location of this slab). In addition, the unsteady part of the cast slab is a casting that includes a part that goes back 30 tons from the joint after the ladle replacement is approaching and the level of molten steel in the tundish has started to decrease (started to decrease). A slab (a ladle change after casting 30 tons from a specific location of the slab).
Incidentally, the time when the level of hot water in the tundish starts to decrease is approximately 40 tons before the seam. Therefore, after 40 tons have been traced back (in the range of 40 tons or less), the molten metal level continues to decrease.
The tonnage of the molten steel can be detected from the amount of residual hot water in the ladle, depending on the casting conditions (eg, casting speed, tundish volume, etc.).

Samples (rectangles with one side of 30 mm) cut out from representative positions of the slabs of the steady part and the slabs of the non-stationary part were each mirror-polished, and then examined with an optical microscope for the number of alumina inclusions. It was converted into the number of detected alumina inclusions per area.
Furthermore, the number of detected steady portions and unsteady portions under the conditions of Reference Example 2 was set to 1.00, and the numbers of detected steady portions and unsteady portions in other Reference Examples, Examples, and Comparative Examples were indexed. .
Here, the evaluation of the indexed value is as follows.
・Indexed value is 0.95 times or more and less than 1.60 times that of Reference Example 2: △ Evaluation ・Indexed value is 0.70 times or more and less than 0.95 times that of Reference Example 2: ○ Evaluation / Index The indexed value is less than 0.70 times that of Reference Example 2: ◎ The evaluated/indexed value is 1.60 times or more that of Reference Example 2: × Evaluation A comprehensive evaluation is made by combining the evaluations of the above-mentioned steady part and non-stationary part. A comprehensive evaluation of △ or higher was regarded as good. The evaluation criteria for comprehensive evaluation are shown below.
・Comprehensive evaluation is △ evaluation: When △ evaluation and △ evaluation ・Comprehensive evaluation is ○ evaluation: When one is △ evaluation and the other is ○ evaluation, or when ○ evaluation and ○ evaluation ・Comprehensive evaluation is ◎ evaluation: One If the evaluation is ○ and the other is ◎, or if the evaluation is ◎ and ◎ ・Comprehensive evaluation is x: If one is x and the other is △, or x and x

As shown in Table 1, Reference Examples 1 to 5, 8, and 9, and Examples 6, 7, and 10 all set the inclination of the runner within the appropriate range (0.5×10 ⁻² to 9.5×10 ^{− 2} or less), and the composition of the flux placed on the surface of the molten metal in the receiving chamber is set to an appropriate range (basicity: 1 or more and 4 or less, Al ₂ O ₃ concentration: 10% by mass or more). is. As a result, the evaluation of the inclusion detection index in the slab is △, ○, or ◎ for both the steady portion and the unsteady portion, and the entrainment of flux on the surface of the molten metal in the receiving chamber can be suppressed. It was found that the upward flow of the molten metal flow in the tapping chamber can be suppressed.

Further, Examples 6 and 7 are the results when the thickness of the flux placed on the surface of the molten metal in the receiving chamber is set within the above-mentioned preferable appropriate range (5 mm or more and 50 mm or less). An excellent cleaning effect was obtained in the non-stationary portion where the height was lowered.
Further, Reference Example 9 and Example 10 are the results when gas is blown. It was found that the cleaning effect was greatest in the non-stationary part.

On the other hand, Comparative Examples 1 and 2 are the results when the inclination of the runner is set outside the proper range. For this reason, in both the steady part and the unsteady part, inclusions in the slab cannot be detected due to the inclusion of flux on the receiving chamber side and/or the inability to suppress the upward flow of the molten metal flow in the tapping chamber. The evaluation of the index was x.
Further, in Comparative Examples 3 to 5, although the inclination of the runner is set within the above-mentioned appropriate range, the composition (basicity and Al ₂ O ₃ concentration) of the flux placed on the surface of the molten metal in the receiving chamber is set to the above-mentioned This is the result when it is set outside the appropriate range. For this reason, in the steady-state part where the influence of flux entrainment in the hot water receiving chamber is relatively small, the evaluation was △, but in the non-steady-state part where the influence of flux entrainment in the hot water receiving chamber is large, the flux entrainment is sufficiently Since it could not be suppressed, it was evaluated as ×.

In this example, the composition (basicity and concentration of Al ₂ O ₃ ) of the flux placed on the surface of the molten metal in the receiving chamber was set within the above-described appropriate range under the condition that the slope of the runner was set within the above-described appropriate range. 6 and 7, and Comparative Examples 3 to 5 in which the flux composition was set outside the appropriate range, the steady part had the same evaluation (Δ evaluation), but the unsteady part had a large difference in evaluation (that is, , Examples 6 and 7 were evaluated as ◯, and Comparative Examples 3 to 5 were evaluated as x).
From this, it can be seen that optimization of the flux composition greatly contributes to quality improvement in the vicinity of the joint portion, which is an unsteady portion.

Therefore, by using the continuous casting method of the present invention, it was confirmed that the molten metal can be cleaned to a higher degree than in the conventional method.

The present invention has been described above with reference to the embodiments, but the present invention is not limited to the configurations described in the above embodiments, and the matters described in the claims. It also includes other embodiments and variations that are possible within its scope. For example, a case where a continuous casting method of the present invention is constructed by combining some or all of the above-described embodiments and modifications is also included in the scope of the present invention.

10: continuous casting equipment, 11: ladle, 12: long nozzle, 13: tundish, 14: immersion nozzle, 15: hot water receiving chamber, 16: hot water outlet chamber, 17: runner, 18: weir, 19: bottom, 20: opening, 21: bottom surface, 22: opening, 23: discharge hole

Claims (2)

In a continuous casting method in which molten metal is poured from a ladle through a tundish into a mold to produce a slab,
The tundish divides the inside of the tundish into a hot water receiving chamber and a hot water dispensing chamber, and has at the bottom one or more and four or less runners through which the molten metal flows from the hot water receiving chamber toward the hot water dispensing chamber. and the runner has a cross-sectional shape converted to a circular shape and has a diameter in the range of 100 mm or more and 300 mm or less,
The runner is inclined downward from the hot water receiving chamber side to the hot water dispensing chamber side, and {height difference between each center position of the end surface of the runner on the side of the hot water receiving chamber and the end surface on the side of the hot water dispensing chamber (mm)}/{horizontal length of the runner (mm)} and the inclination of the runner is in the range of 0.5×10 ⁻² or more and 9.5×10 ⁻² or less,
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 Al ₂ O A continuous casting method, characterized in that a flux having an Al ₂ O ₃ content of 10% by mass or more with respect to the total amount of the _three fluxes is placed on the surface of the molten metal in the receiving chamber with a thickness of 5 mm or more and 50 mm or less . 2. The continuous casting method according to claim 1 , wherein a discharge hole for discharging the molten metal in the tapping chamber to the mold is provided in the bottom of the tapping chamber of the tundish, and an opening of the runner on the tapping chamber side and the A continuous casting method, characterized in that gas is blown from the bottom of the tapping chamber toward a virtual molten metal flow path that linearly connects a discharge hole.

Images