JP7215361B2 - 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,
A collision point at a position where an imaginary line of a runner center axis passing through the respective center positions 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 discharge chamber intersects the refractory wall surface of the water discharge chamber facing the weir. and the angle φ formed by the vertical line of the refractory wall surface at the collision point and the imaginary line of the runner center axis is 0.10≦0.5−φ/180−Z /1200≤0.30.

In the continuous casting method according to the present invention, 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, A flux having an Al ₂ O ₃ amount of 10% by mass or more with respect to the total amount of SiO ₂ amount and Al ₂ O ₃ amount is placed on the surface of the molten metal in the receiving chamber, and the thickness of the flux is 5 mm or more and 50 mm or less. It is preferable to

In the continuous casting method according to the present invention, in a plan view of the tundish, the shortest distance between the center position of a long nozzle for pouring molten metal from the ladle into the receiving chamber and the imaginary line of the center axis of the runner It is preferable that the distance is at least twice the outer diameter of the long nozzle.

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, the height Z from the bottom of the tapping chamber at the collision point where the runner center axis imaginary line intersects the refractory wall surface of the tapping chamber, and the vertical line of the refractory wall surface at the collision point and the runner center axis imaginary line. angle φ (positive when the vertical line is above the runner center axis virtual line) satisfies 0.10≦0.5−φ/180−Z/1200≦0.30 Therefore, it is possible to prevent the flow of molten metal discharged from the runner into the tapping chamber from being disturbed and the molten metal flowing downward toward the discharge hole of the tapping chamber.
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. It is a top view of the tundish to which the same continuous casting method is applied.

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 found that, when casting molten metal using a tundish having a weir with a runner, the molten metal is agitated in the vicinity of the entrance of the runner, and the accompanying flux is entrained in the molten metal. It has been found that low-cleanliness molten metal 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 inhibits the high-cleanliness 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 dispensing chamber 16, and the molten metal flows from the hot water receiving chamber 15 toward the hot water dispensing chamber 16. A weir 18 having two (2) runners 17 at its lower portion is provided, and the diameter of the runners 17 is set in the range of 100 mm or more and 300 mm or less when the cross-sectional shape is converted into a circle.
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) 24 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 24 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) collides with the refractory wall 24 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.

By optimizing the inclination of the runner 17 as described above, it is possible to suppress the flux on the surface of the molten metal (on the surface of the molten metal) of the receiving chamber 15 from being entangled in the molten metal as compared with the conventional runner. It is possible to suppress the occurrence of an upward flow in the molten metal. However, the center axis virtual line (runner channel center axis virtual line) L passing through the respective center positions C1 and C2 of the end surface of the runner 17 on the side of the hot water receiving chamber 15 and the end surface on the side of the hot water discharge chamber 16 and the water discharge chamber 16 facing the weir 18 The downward flow of the molten metal from the collision point C3, which is the intersection position with the wall surface of the refractory wall 24 (refractory wall surface), tends to be relatively strong.
Therefore, the present inventors focused on making the downward flow of the molten metal harmless from the collision point C3 in order to utilize the effect of suppressing the flux from being entrained in the molten metal in the receiving chamber 15 .
If the downward flow of the molten metal from the collision point C3 is directly supplied to the discharge hole 23 provided in the bottom surface 19 of the tapping chamber 16, improvement in cleanliness is suppressed. Therefore, it was considered preferable to set the height Z from the bottom surface 19 to the collision point C3 high to disperse the downward flow of the molten metal from the collision point C3 to the discharge hole 23 .

However, the present inventors have found that when the height Z exceeds a certain level, the effect of improving the cleanliness of the molten metal is rather suppressed.
Although it depends on the height of the surface of the molten metal in the tapping chamber 16, if the height Z is set high under the condition that the height of the surface of the molten metal is constant, the distance between the collision point C3 and the surface of the molten metal becomes shorter. It is believed that the upward flow of molten metal from C3 detracts from the cleanliness enhancing effect of the molten metal. However, even if the height of the surface of the molten metal in the tapping chamber 16 is set to a sufficiently high value, the effect of increasing the cleanness of the molten metal may be suppressed.
Based on this, the present inventors studied the influence of the downward flow of the molten metal from the collision point C3 on cleanliness when the height Z is set high.
As a result, when the height Z exceeds a certain level, the downward flow of molten metal from the impingement point C3 is as shown in FIG. A vortex (stirring flow) may be generated when viewed from the side, and a plurality of such vortices may be generated. As a result, the highly straight molten metal flow ejected from the outlet of the runner 17 into the tapping chamber 16 is disturbed and dispersed, and as a result, there is a possibility that the downward molten metal flow directly from the runner 17 to the discharge hole 23 will be strengthened. I was able to infer that

Based on the above knowledge, the α value shown in Equation (1) is defined, and by setting this α value to 0.10 or more and 0.30 or less, the collision point C3 when the inclination of the runner 17 is set appropriately. The downward flow of the molten metal from the was adjusted. The formula (1) is a formula conceived using the past operational results with the height Z and the angle φ as variables, which will be described later.
α=0.5−φ/180−Z/1200 (1)
Here, Z is the height (mm) from the bottom surface 19 of the tapping chamber 16 to the collision point C3, and 1200 is a coefficient.
Also, φ is an angle (deg.) defined as the angle formed by the runner center axis virtual line L and the perpendicular line M to the refractory wall surface at the collision point C3. When the vertical line M is above the virtual line L of the runner center axis, φ is assumed to be positive.

The reason why the angle φ term is provided in the equation (1), which is the evaluation of the height Z of the impingement point C3, is that as the angle φ decreases, the downward flow of the molten metal from the impingement point C3 increases, that is, the angle φ changes This is because the downward flow of the molten metal changes along with it, and it is thought that the range of the appropriate height Z value changes.
Here, when the α value in formula (1) is less than 0.10, the highly straight molten metal flow ejected from the runner outlet into the tapping chamber is disturbed and dispersed, and as a result, directly from the runner to the discharge hole. Since the downward flow of the molten metal is strengthened, the high cleanliness effect is suppressed.
On the other hand, when the α value in formula (1) is more than 0.30, the collision point is close to the bottom surface, and regardless of the presence or absence of agitation of the highly straight molten metal flow from the runner exit, the flow is directed downward toward the discharge hole. Since the flow of the molten metal is strengthened, the high cleanliness effect is suppressed.

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 their cleanliness may deteriorate due to mixing into the molten metal.
On the other hand, if the basicity exceeds 4.0, flux particles may be caught 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 described above, by optimizing the composition and thickness of the flux, it is possible to suppress the entrainment of the flux.
For example, boiling from the long nozzle 12 .
The long nozzle is generally attached to the bottom of the ladle, and the surrounding atmosphere is sucked into the long nozzle through the attachment. For this reason, it is common to place a cushioning material (packing) at the mounting part, but it is not possible to completely eliminate the inhalation of the atmosphere. It hinders high cleaning of the molten metal. Although it is possible to prevent oxidation of the molten metal by forming a chamber around the mounting portion to create an argon gas atmosphere, the atmosphere is sucked in, so the argon gas sucked into the long nozzle is released from the long nozzle in the receiving chamber. , boiling will occur. In particular, when the long nozzle attached to the bottom of the ladle is tilted or when the thickness of the cushioning material varies, the suction of the atmosphere becomes significant, and boiling from the long nozzle becomes significant.

Such boiling causes the flux existing on the surface of the molten metal in the receiving chamber to become involved in the molten metal, and the high cleaning effect cannot be exhibited by optimizing the inclination of the runner.
As a factor similar to the above-mentioned flux entrainment due to boiling, there is ladle replacement work in an unsteady section. Continuous casting is a common form of operation, but when the ladle is replaced during casting, the long nozzle that is removed from the ladle inevitably moves up and down.
In this case, too, the flux existing on the surface of the molten metal in the receiving chamber is involved in the molten metal, which is a factor that hinders the high cleaning effect, as in the case of boiling.
The continuous casting described above is a method of successively casting molten metal in a plurality of ladles, and the molten metal to be cast may be of the same steel grade or of different steel grades.

Therefore, the inventors of the present invention have focused on promoting the resurfacing of the flux to the surface of the molten metal during boiling and ladle replacement work.
According to the knowledge of the present inventors, the range in which the flux is involved during boiling or ladle replacement work is around the long nozzle, and the area where the flux floats to the surface of the molten metal again is also around the long nozzle.
However, as described above, the molten metal may be agitated by a vortex-like flow or the like near the entrance of the runner. It has been found that re-surfacing flux can be suppressed from being supplied to the runner inlet before surfacing.

That is, as shown in FIG. 2, in a plan view of the tundish 13, 1) the center position P of the long nozzle 12 for injecting the molten metal from the ladle 11 into the receiving chamber 15 and the center axis of one of the runners. 2) the shortest distance d1 (hereinafter also referred to as the planar view distance d1) from the virtual line L1 (corresponding to the runner center axis virtual line L); and 2) the center position P of the long nozzle 12 and the center of the other runner. The shortest distance d2 (hereinafter also referred to as the planar view distance d2) from the virtual axis line L2 (corresponding to the runner center axis virtual line L) is set to be at least twice the outer diameter D of the long nozzle 12. Here, the outer diameter of the long nozzle is the maximum outer diameter of the portion immersed in the molten metal. In addition, in FIG. 2, the distances d1 and d2 in plan view are not two times or more of the outer diameter D for simplification of the drawing.
Normally, the diameter (inner diameter) of the runner is about 300 mm or less, and the outer diameter of the long nozzle is about 150 mm to 400 mm. By doing so, it is possible to suppress the flux that resurfaces around the long nozzle from being supplied to the entrance of the runner before it floats, and it is possible to maintain and improve the high cleaning effect by optimizing the inclination of the runner.
The upper limits of the distances d1 and d2 in plan view need not be set unless there is a significant temperature drop in the molten metal. considered possible.

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 centerline of the runner extending toward the tapping chamber, is 77 degrees. Further, the position C3 of the refractory wall surface of the tapping chamber where the center line in the length direction of the runner extends toward and intersects with the tapping chamber is a position 150 to 480 mm from the bottom of the tapping chamber.
In both Examples and Comparative Examples, a flux having a basicity of 2.0 (same as the flux charged to the receiving chamber in Example 6) was previously added to 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".
The column of "collision angle with wall surface" describes the angle φ at the collision point C3.
In the column of "collision point on the hot water discharge chamber side", the height Z (mm) of the collision point C3 from the bottom surface and α defined by the formula (1) (α = 0.5 - φ / 180 - Z / 1200 ) is described.
In the "LN-sleeve distance" column, the shortest distance (that is, the distance in plan view) between the virtual line of the center axis of the runner and the center position of the long nozzle (LN) when the tundish is viewed in plan. is a multiple of the outer diameter of the long nozzle.

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 Example 2 was set to 1.00, and the numbers of detected steady portions and unsteady portions in other 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 Example 2: △ Evaluation ・Indexed value is 0.70 times or more and less than 0.95 times that of Example 2: ○ Evaluation / Index The indexed value is less than 0.70 times that of Example 2: ◎ The evaluated/indexed value is 1.60 times or more that of Example 2: × Evaluation A comprehensive evaluation is made by combining the above evaluations of the steady part and the unsteady 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 ・Comprehensive evaluation is ◎ evaluation: When one is △ evaluation and the other is ◎ evaluation・Comprehensive evaluation x evaluation: When x evaluation is included

As shown in Table 1, in any of Examples 1 to 15, the inclination of the runner is set within an appropriate range (0.5×10 ⁻² or more and 9.5×10 ⁻² or less), and the collision point C3 This is the result when the α value in is set within an appropriate range (0.10 or more and 0.30 or less). 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, and , the upward flow of the molten metal flow in the tapping chamber can be suppressed, and the downward flow of the molten steel flow in the tapping chamber can also be rendered harmless.

In addition, in Examples 6, 8, 9, and 13, the composition of the flux placed on the surface of the molten metal in the receiving chamber is within the above-mentioned preferable appropriate range (basicity: 1 or more and 4 or less, Al ₂ O ₃ concentration: 10% by mass or more ), and 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 part.
Examples 7, 11, 12, and 15 are the results when the planar view distance is set to the above-described preferable appropriate range (twice or more the outer diameter of the long nozzle). It was found that the composition and thickness of the flux placed on the surface of the molten metal in the chamber were within the appropriate range, so that the cleaning effect was the greatest.

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.
Comparative Examples 3 and 4 show the results when the inclination of the runner is set within the proper range, but the α value at the collision point C3 is set outside the proper range. For this reason, the downward flow to the discharge hole was strengthened in the tapping chamber, resulting in an x evaluation.

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 embodiments and modifications described above is also included in the scope of rights 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, 22: opening, 23: discharge hole, 24: refractory wall

Claims (3)

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,
A collision point at a position where an imaginary line of a runner center axis passing through the respective center positions 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 discharge chamber intersects the refractory wall surface of the water discharge chamber facing the weir. and the angle φ formed by the vertical line of the refractory wall surface at the collision point and the imaginary line of the runner center axis is 0.10≦0.5−φ/180−Z /1200≦0.30. In the continuous casting method according to claim 1, 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 , SiO ₂ amount, and Al ₂ O ₃ amount with respect to the total amount of Al ₂ O ₃ amount is 10% by mass or more is placed on the surface of the molten metal in the receiving chamber, and the thickness of the flux is 5 mm or more and 50 mm A continuous casting method characterized by: 3. The continuous casting method according to claim 1 or 2, wherein the tundish is viewed from above, the center position of a long nozzle for pouring molten metal from the ladle into the receiving chamber, and the virtual line of the center axis of the runner. A continuous casting method characterized in that the shortest distance between and is set to be at least twice the outer diameter of the long nozzle.

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