JP4427429B2 - Method and apparatus for controlling flow in strand pool - Google Patents

Description

  The present invention relates to a method for controlling flow in a strand pool using electromagnetic force and an apparatus therefor for producing a high-quality slab having less bubbles and inclusion system defects.

  The flow in the strand pool (here, the unsolidified molten steel pool from the mold surface to the final solidification position) in the continuous casting process greatly affects the slab quality. Therefore, how to control the flow in the strand pool is extremely important. Since electromagnetic force can control the flow in the pool without contact, various methods have been studied.

  For the purpose of preventing generation and capture of CO bubbles generated during the casting of rimmed steel and semi-killed steel, solidification corresponding to the scale thickness generated between casting and the start of rolling by an electromagnetic stirrer provided in the mold A method (Patent Document 1) in which the discharge hole of the immersion nozzle is discharged horizontally or downward under electromagnetic flow while applying electromagnetic flow to the solidified interfacial molten steel at the site until the layer is formed, or a discharge hole of the immersion nozzle The molten steel is disposed below the electromagnetic stirrer, and the molten steel near the solidification interface at a predetermined depth (while the solidification thickness is formed at least 5 mm) from the position of the molten metal surface in the mold is approximately constant to 0.1 to 1 m / s as a whole. (Patent Document 2) is disclosed.

  However, Patent Documents 1 and 2 aim to impart a film-like flow to the solidification interface because the swirling flow is applied in the horizontal section and the flow velocity is excessive, which disturbs the molten steel surface in the mold. Yes (see Fig. 1 (a)). Therefore, the frequency applied to the electromagnetic stirrer is preferably a high frequency of 5 to 20 Hz. However, when the frequency is increased, the magnetic field loss due to the mold copper plate and the magnetic field loss due to the stainless steel plate provided on the back surface of the copper plate become large. Therefore, special work is required for the thickness of the mold copper plate and the mold structure. Therefore, in normal electromagnetic stirring, about 3 to 6 Hz is selected as the frequency. At that time, electromagnetic force acts not only in the vicinity of the interface of about 20 mm from the solidification interface but also in the molten steel pool.

In addition, since the kinematic viscosity coefficient of molten steel is a low-viscosity fluid of about 1 × 10 ⁻⁶ m ² / s (equivalent to water), it is difficult to apply a film-like forced flow near the solidification interface. . For this reason, a method of forming a swirl flow in a horizontal section rather than imparting a film-like flow to the solidification interface is generally used. As a result, interference between the nozzle discharge flow and the agitation flow, and the discharge reverse flow after the nozzle discharge flow collides with the short side often interfere with the agitation flow due to electromagnetic stirring, preventing inclusion trapping at that site. There is a problem that the effect is not sufficient (see Fig. 1 (b)).

In Patent Document 3, [S] <0.015%, [C × O] <8 × 10 ⁻⁴ % ² or less molten steel having a component composition of 1 m / m or less by a magnetic stirrer provided near the mold or the mold. A method of casting while forcibly flowing at a flow rate of s or less is disclosed. Although it is ideal from the viewpoint of preventing inclusions and bubbles from being trapped in the solidified shell, it is clear that strong stirring from the solidification start position to the completion position is ideal, but it is clear that a large-scale stirring device is required. is there.

  In Patent Documents 1 and 2, it is assumed that a flow of about 0.1 to 1 m / s is given to a region of the same degree as the solidified thickness to be scaled off, but only a portion corresponding to the thickness to be scaled off is given. It is not sufficient to prevent the inclusions and bubbles from being trapped, and it is necessary to achieve the trapping prevention in the region of about 10 mm from the surface. However, even in this case, it is unclear how the flow rate required for ensuring cleanliness varies with the thickness of the solidified shell.

In addition to electromagnetic stirring in the mold, Patent Documents 4 and 5 are disclosed as relating to electromagnetic stirring located below the mold.
Patent Document 4 discloses that the left and right short sides are below the position where the discharge flow from the immersion nozzle collides with the short side solidified shell and above the position 1 m below the maximum penetration depth of the jet. In this method, an upward flow is alternately formed on the molten steel in the vicinity of the solidified shell by a magnetic stirring device. However, although the temperature in the strand pool decreases with the pool depth, molten steel with a low superheat in the pool depth is supplied to the upper part. (A mixture of iron and inclusions, bubbles, etc.) may be trapped by the solidified shell, resulting in defects in the steel sheet. Therefore, it is necessary to make the conditions of the electromagnetic stirring given to the hot-water surface vicinity into an appropriate condition.

Further, Patent Document 5 discloses a method in which an electromagnetic stirrer is installed in the vicinity of the molten metal surface in the mold and below the immersion nozzle discharge hole for supplying molten steel into the mold, and the stirring direction of both is controlled in conjunction. ing. However, when the agitation direction is switched, the time zone in which the flow rate is 0 is reliably passed. In particular, in order to prevent inclusion trapping in the solidified shell generated in the vicinity of the molten metal surface, if a flow rate higher than a certain constant flow rate is required, inclusion trapping inevitably occurs in a time zone where the flow rate is zero. Become.
JP 56-4355 A Japanese Patent Laid-Open No. 56-41054 JP 58-77755 A JP-A-11-28556 JP 2003-39141 A

As described above, the conventional technology avoids interference between the stirring flow caused by electromagnetic stirring and the discharge flow from the immersion nozzle and the discharge reverse flow, and traps bubbles and inclusions in the slab that becomes the surface defect of the steel plate. None of them is insufficient in terms of providing a method and an apparatus for producing a high-quality cast slab that does not cause powder entrainment, and it is an object of the present invention to solve these problems.
Furthermore, the requirements for inclusion-type defects are becoming stricter year by year, and expectations for high quality slabs with fewer quality defects than ever are increasing.

The configuration of the present invention is as follows.
(1) In the continuous cast strand pool, in the molten steel near the solidification interface , 0. A swirling flow having a flow rate of 6 to 1 m / s is continuously formed, and in a region from the molten metal surface to 1 m below the molten metal surface , depending on the distance L (m) from the molten metal surface and the casting speed V c (m / s). The flow control method of the flow in a strand pool characterized by providing the flow more than the flow velocity U (m / s) prescribed | regulated by following (1) Formula.
D = K√ (L / Vc)
U ≧ 0. 6 3 a K ^4/3 (D) ^2/3 ... ... (1)
However, a = 12.4 (m ^1/3 · s ^−1/3 )
K: Solidification shell growth rate coefficient (1.8 × 10 ⁻³ to 3.2 × 10 ⁻³ m · s ^−1/2 )
Solidified shell thickness D (m): 0.001 ≦ D ≦ 0.1
(2) the Te (1) odor, directed from one short side in any region from below the melt surface 1m to 7m on the other short side, and the maximum flow rate that is less and propulsion direction 0.5 m / s By applying a propulsive flow that changes in a cycle of 10 to 60 s, a strand that gives a flow that is parallel to the solidification interface and whose flow velocity periodically varies in a region from 1 m to 7 m below the molten metal surface Control method of flow in the pool.
(3) In the vicinity of the molten metal surface, a horizontal cross-section in which the stirring flow rate is 0.6 m / s or more and 1 m / s or less by applying thrust in parallel with each long side of the opposed slabs and in the opposite direction to each other. In an electromagnetic stirrer that can apply a swirl flow within one area and any area from 1 m to 7 m below the molten metal surface, the thrust is applied in the same direction in parallel to each long side of the opposed slabs. By doing so, it is possible to give a propulsion flow with a maximum flow velocity of 0.5 m / s or less from one short side to the other short side, and to periodically change the propulsion direction at a period of 10 to 60 s. A flow control device in a strand pool, comprising at least one set of electromagnetic stirring devices.

  By using the continuous casting method of the present invention and the flow control device in the strand pool, a high-quality slab having few bubbles and inclusion defects can be produced. The method of the present invention can also be applied to a vertical bending type continuous casting machine of a continuous casting machine. Moreover, when it is desired to improve the cleanliness only on the slab surface layer portion, a swirling flow having a stirring flow rate of 0.6 m / s or more and 1 m / s or less can be applied only in the vicinity of the molten metal surface.

If flow is imparted to the molten steel near the solidification interface, it is possible to prevent inclusions from being trapped in the solidification shell. However, a large-scale device is required to impart such a flow from the molten metal surface to the final solidification position. The inventors first examined the relationship between the flow rate necessary for preventing inclusion trapping in the solidified shell and the solidification rate.
The mechanism for preventing inclusion trapping in the solidified shell by flow is understood as follows. It is known that a force in proportion to the square root of the velocity gradient (Saffmann force) is generally applied to particles under flow with a velocity gradient (for example, T. Toh et al: ISIJ. Int., 41 (2001), 1245.). In the case of continuous casting of steel, a boundary layer is formed in the vicinity of the solidified shell, and the flow velocity increases with the distance from the solidified shell in the boundary layer. Therefore, force acts on the particles in the direction away from the solidified shell, Moves at a speed in a direction away from the solidified shell (hereinafter referred to as a separation speed Up) (see FIG. 2). At that time, since the solidified shell moves forward at a solidification speed V corresponding to the casting condition, if the separation speed Up is larger than the solidification speed V, it is not captured by the solidified shell.
That is, the conditions for preventing inclusion inclusion are expressed as follows.
Up ≧ V (Up: separation speed, U: flow velocity, V: solidification rate)

The inventors conducted a numerical analysis and studied the relationship between the flow velocity U and the separation velocity Up, and found the following relational expression as inclusion trapping prevention conditions. Here, U (m / s): flow velocity, V (m / s): solidification rate, a (m ^1/3 · s ^−1/3 ): coefficient.
U ≧ aV ^2/3
FIG. 3 shows the results of examining the flow rate necessary for preventing trapping when the inclusion particle size is 100 μm in relation to the solidification rate. a shows the result calculated using 12.4 (m ^1/3 · s ^−1/3 ).
In addition, the range of the coefficient a in the present invention varies depending on the inclusion diameter targeted for defect prevention. When the inclusion particle diameter is 50 μm, a = 19.7 (m ^1/3 · s ^{−1 / 3} ). When the inclusion particle size is 200 μm, a = 7.8 (m ^1/3 · s ^−1/3 ). Here, since the presence of inclusions of 100 μm or more is generally a problem, a = 12.4 (m ^1/3 · s ^−1/3 ). The coefficient a (m ^1/3 · s ^−1/3 ) here is a coefficient defined based on the physical properties (density, viscosity and inclusion diameter) of the molten steel and inclusions, and as described above, the inclusions The smaller the diameter, the larger.

Under actual continuous casting conditions, the solidification rate is defined by the solidification shell thickness D (m), that is, the distance L (m) from the molten metal surface, and the casting speed Vc (m / s). Therefore, the relationship between the distance from the molten metal surface, the casting speed and the required flow rate can be expressed as follows.
D = K√ (L / Vc)
V = K / 2√ (Vc / L )
U ≧ 0.63 aK ^4/3 / D ^2/3
Here, D: solidified shell thickness (m), 0.001 ≦ D ≦ 0.1, t: time (s), K: solidified shell growth rate coefficient (m · s ^−1/2 ), L: from the ^{molten metal} surface (M), Vc: casting speed (m / s).

FIG. 4 shows the results of investigating the relationship between the flow velocity and the solidified shell thickness using the above equation and using a K value of 3.0 × 10 ⁻³ . The K value varies depending on the thickness of the mold copper plate, the cooling water amount, the secondary cooling conditions, the powder used, etc., but is approximately in the range of 1.8 × 10 ^{−3 to} 3.2 × 10 ⁻³ (for example, the first 3rd edition "Iron & Steel Handbook II" Steelmaking and Steelmaking, edited by Japan Iron and Steel Institute, p.619).
When the solidification rate is high as in the vicinity of the molten metal surface, the required flow rate is as high as about 0.3 m / s. However, as the solidification rate decreases, the required flow rate decreases and becomes 0.05 m / s. It can be seen that the capture can be sufficiently prevented by applying a flow rate of about s.

In the following, based on this knowledge, we will examine what flow pattern is formed in the pool to prevent inclusion trapping in the solidified shell at the upper and lower parts of the pool.
As for the upper part of the pool, since the solidification speed is relatively fast, it is necessary to apply a flow rate of about 0.3 m / s or more to the molten steel near the solidification interface in the entire slab circumferential direction. Therefore, in the case of a rectangular cross section such as a slab, a method of forming a swirl flow in a horizontal cross section is preferable. Moreover, as the stirring flow rate, it is necessary to continuously apply a stirring flow rate of about 0.3 m / s or more on the molten metal surface. However, as shown in FIG. 1 (b), when the nozzle discharge reversal flow and the stirring flow interfere with each other, the stagnation of the flow at that portion occurs, and inclusions and bubbles are trapped.

The inventors conducted a detailed numerical analysis on the influence of the absolute value of the stirring flow velocity on the flow behavior near the molten metal surface and examined it in detail. As a result, as shown in FIG. 5, it was found that if the stirring flow rate of 0.6 m / s or more can be applied, interference between the nozzle discharge reversal flow and the stirring flow can be almost eliminated.
The flow velocity stagnation frequency shown on the vertical axis in FIG. 5 represents how many seconds or less of the necessary flow velocity is formed per unit time. Here, the effect becomes remarkable when the flow velocity is 0.6 m / s or more. Generally, the nozzle conditions (immersion depth and nozzle) are set so that the flow velocity at the molten metal surface is 0.2 m / s or more and 0.4 m / s or less. The angle and area of the discharge holes are set, but the flow rate formed in the molten steel near the solidification interface on the long side by the nozzle discharge flow becomes larger below the molten metal surface, and the solidification formed by the stirring flow rate and the nozzle discharge flow This is to ensure that the stirring flow becomes large when compared with the molten steel flow velocity in the vicinity of the interface .

  Next, in order to clarify the relationship between the stirring flow rate and the powder entrainment, a water model experiment was conducted and examined. In order to simulate powder, silicon oil was used, and the relationship between the silicon oil / water interface condition and the stirring flow rate was examined. Based on the result, the flow velocity at which entrainment between molten steel / powder occurred was estimated. The result is shown in FIG. It has been found that when the stirring flow rate exceeds 1 m / s, the shape of the molten metal surface becomes more turbulent and causes the powder added to the molten metal surface to be involved. From the above, the stirring flow rate is set to 0.6 m / s or more and 1 m / s or less.

Moreover, an accompanying effect arises by providing such a strong stirring flow. In general, a predetermined stirring flow is applied to a portion where the coil is installed, but the flow rate gradually decreases as the coil moves away from the coil. According to the analysis results of the inventors, when a stirring flow having a stirring flow velocity of 0.6 m / s or more is formed in the vicinity of the molten metal surface , the stirring flow can be imparted to the molten steel near the solidification interface from the molten metal surface to a depth of 1 m (Fig. 7). This effect means that inclusion trapping in the solidified shell in the region with the largest number of inclusions in the upper pool in the mold can be greatly reduced (the required flow velocity in FIG. 7 shows the same curve as FIG. 4). ing).
As described above, by providing a stirring flow velocity of 0.6 m / s or more and 1 m / s or less near the molten metal surface, inclusion capture at the slab surface layer portion where cleanliness is required can be reliably prevented.

  Furthermore, in order to reduce internal defects such as internal bubbles, a flow control method under the strand pool was studied. In general, since Ar bubbles and inclusions are smaller in density than molten steel, the number density in the pool is large near the molten metal surface, and the number density decreases with the pool depth. In addition, as shown in FIGS. 3 and 4, the solidification rate decreases with the pool depth, so the required flow rate may be small. However, even if the solidified shell thickness becomes 100 mm, the flow rate is not necessarily zero. Therefore, unlike the upper part of the pool, a method that can impart some flow over as wide a range as possible is preferable.

  In order to apply a flow over a wide range within the continuous cast strand pool, it is necessary to make the area of the circulating flow as large as possible. As a flow pattern for that purpose, the pattern shown in FIG. 8 can be considered. That is, by applying a propulsion flow from one short side to the other short side, the flow branches up and down on the short side where the propulsion flow collides, and then rises or falls along each short side. It is possible to form a flow, and to form two circulating flows having different rotational directions in the vertical direction in the vertical section. This flow system can provide flow over the widest range.

  However, when such a flow is constantly applied, the flow becomes uneven, which is not preferable because the penetration of the nozzle discharge flow is promoted on one short side. Therefore, a water model experiment was conducted to examine how to change the propulsion direction periodically. In the experiment, a hose was attached to both short sides, a hose and a pump were connected, and a water flow was pumped from one short side into the pool while being drawn from the other short side to form a flow propelled in the width direction.

As a result, it was found that by switching the propulsion direction, a flow can be imparted to the molten steel near the solidification interface in the entire width direction (see FIG. 9). This is due to the following reason.
When an upflow is added to one short side and a downflow is added to the other short side, the center of the circulating flow is the center of the width, and the flow is always stagnant at the center of the width. However, after the nozzle discharge flow collides with the short side, there is a downward flow that intrudes along the short side. In this state, when the downward flow is applied to one short side and the upward flow is applied to the other short side, The center of the flow is not in the center of the width, but moves to either short side. By switching the propulsion direction in this state, it is possible to impart flow to any part of the molten steel near the solidification interface .

  If the propulsion direction is changed, the flow velocity always passes through the time zone of 0, but if the area is lower than 1 m below the molten metal surface, the number density of Ar bubbles and inclusions is compared with the vicinity of the molten metal surface. Therefore, even if there is a time zone in which the flow velocity is 0, the probability of being trapped by the solidified shell is extremely low. In addition, since the solidification rate is slow, even if inclusions are being captured, the inclusions can be separated again into the molten steel pool by applying a flow. In other words, it is not always necessary to provide a constant flow in the region below 1 m below the surface of the molten metal, and it is necessary to improve the internal quality of the slab by applying a constantly changing flow over as wide a range as possible. It can be said that it is a simple flow pattern.

  Next, water model experiments were conducted to examine the maximum propulsion flow rate necessary to achieve the above objective. In addition, a tracer was added as an inclusion, and the situation in the pool was investigated. The results are shown in FIG. The number of tracers adhering to the wall in the circulation region decreases remarkably as the propulsion flow rate increases, especially when it exceeds 0.2 m / s, but when it exceeds 0.5 m / s, the number that reaches the lowest end of the pool is It became extremely large. If the flow velocity is small, the range covered by the circulating flow becomes small, and naturally the flow velocity value on the front surface of the solidified shell becomes small. On the other hand, if it exceeds 0.5 m / s, the effect of facilitating the downward intrusion due to the downward flow becomes remarkable, and the ascent rate is reduced. Note that the effect of reducing the number of tracers is small at less than 0.2 m / s because the range of flow is small. Further, when the region of the circulation flow is investigated by observing the tracer trace, the circulation is performed from the position where the propulsion flow is applied to about 2 to 3 m above and below from the position where the maximum flow velocity is 0.2 to 0.5 m / s. It was found that a flow was formed.

A water model experiment was similarly conducted to examine the period for switching the propulsion direction. The maximum propulsion flow velocity applied was 0.3 m / s. As a result, as shown in FIG. 11, the number of bubbles could be reduced by changing the period for switching the propulsion flow from 10 seconds to 1 minute.
From the above results, the flow conditions necessary to provide a highly clean slab from the slab surface to the inside can be summarized as follows (see FIG. 12).

First, the flow rate prescribed | regulated by the following (1) Formula is provided with the distance L (m) from a molten metal surface, and the casting speed Vc (m / s) .
U ≧ 0.63 aK ^4/3 / D ^2/3 (1)
Here, 0.001 ≦ D ≦ 0.1
As a method for that,
(I) From the inside of the mold to 1 m below the hot water surface:
1) A swirling flow having a flow rate of 0.6 to 1 m / s is continuously formed in the vicinity of the molten metal surface.
Thereby, the flow which satisfies (1) Formula can be provided to 1 m below the hot_water | molten_metal surface.
(II) From 1m to 7m below the hot water surface:
1) Form a plurality of circulating flows with a maximum flow velocity of 0.5 m / s or less, and periodically change the circulation direction.
2) The period for switching the direction of the circulating flow is preferably 10 to 60 s.

Next, an apparatus for imparting such a flow was examined.
First, an electromagnetic stirrer (coil (1)) is provided in the vicinity of the molten metal surface to apply thrusts parallel to the long sides of the opposing slabs and opposite to each other . In the vicinity of the molten metal surface, a flow (shown by a thin arrow in FIG. 13) having a flow velocity of 0.6 m / s to 1 m / s is applied. As a result, a continuous swirling flow can be formed in the horizontal cross section, and as shown in FIG. 7, the necessary flow velocity is changed depending on the thickness of the solidified shell at a depth of 1 m from the molten metal surface. Can be given inside .

Next, in the pool below the 1 m position below the hot water surface, a large circulation flow whose circulation direction periodically changes is formed. Therefore, a set of electromagnetic stirrers (coil (2)) is installed on each long side of the opposed slabs to apply a propulsive force in the same direction, and the propulsion direction is switched periodically. When the maximum value of the propulsion flow velocity is 0.2 to 0.5 m / s, the influence range of the circulating flow is about 2 to 3 m in the vertical direction of the pool. Therefore, if an electromagnetic stirrer is installed about 2 to 4 m below the hot water surface, the circulation flow (illustrated by a block arrow in FIG. 13) is formed up to an area of about 1 m to 7 m below the hot water surface.

Further, in the present invention, a stirring flow rate of 0.6 m / s to 1 m / s is applied in the vicinity of the hot water surface. As a result, a swirl flow region (illustrated by a thin arrow in FIG. 13) extends to about 1 m below the molten metal surface. In this state, when the propulsion flow is applied by the coil (2), the stirring flow in the same direction as the circulating flow (illustrated by the block arrow in FIG. 13) formed by the coil (2) is generated by the coil (1). Because it is constantly formed in the molten steel near the solidification interface of the steel , the electromagnetic stirrer (coil (2)) on the lower stage and the electromagnetic stirrer (coil (1)) provided near the hot water surface It is possible to efficiently form a circulation flow and switch the circulation direction (see FIG. 13).

In addition, in order to form a circulation flow area as widely as possible, two sets of electromagnetic stirrers (coils (2), (3)) along the long sides of the slabs facing each other at two different locations in the strand pool. Is installed. Then, in one set of electromagnetic stirring devices, propulsive force in the same direction is applied, and in different sets up and down, reverse propulsive force is applied, and the propulsion direction is periodically switched. The installation position of the coil (2) may be the same as in FIG. It is preferable that the electromagnetic stirrer (coil (3)) installed therebelow is separated by about 3 to 6 m. This is because the influence of the stirring flow formed by each stirring device affects up to about 2 to 3 m when the stirring flow rate is about 0.5 m / s. In order to stir, it is because it can stir most effectively about 3-6 m apart (refer FIG. 14).

The ultra-low carbon steel was melted by refining in the converter, processing in the reflux type vacuum degassing equipment and alloy addition. This molten steel was cast into a slab having a thickness of 250 mm and a width of 1800 mm using a 10.5 mR continuous casting machine. The casting speed was 1 m / min, and Ar gas was allowed to flow through the nozzle at 3 Nl / min. As the electromagnetic stirring coil in the mold, a coil capable of applying a maximum flow velocity of 1.2 m / s on the molten metal surface was used, and the coil center was set at a position 100 mm from the molten metal surface.
On the other hand, as for the electromagnetic stirrer of the strand, in the case of one-stage installation, one set installation along each long side of the slab facing the electromagnetic stirrer at a position of 3.5 m from the molten metal surface, in the case of two-stage installation, Two sets were installed at positions different from 3.5m and 7m positions from the hot water surface. In each case, a stirring flow rate that can give a maximum flow rate of 0.5 m / s was used, and in the case of two-stage installation, the stirring direction was up and down and reverse.

As a method for evaluating the bubble defect which becomes an internal defect, a sample having a total casting width of 10 mm in the casting length direction was cut out, an X transmission photograph was taken, and the distribution of the bubble defect was investigated. On the other hand, regarding the number of inclusions on the surface part of the slab, a sample having a total width × 200 mm in the casting direction is cut out from each of the upper and lower surfaces of the slab, and inclusions in the surface having a total width × 200 mm are removed every 1 mm from the surface. After grinding and polishing, the number of inclusions of 100 μm or more was examined.
The results are shown in Table 1. In Table 1, stirring (1) indicates the stirring condition of the electromagnetic stirring coil in the mold, and stirring (2) indicates the stirring condition of the strand by the electromagnetic stirring device. In addition, the same result was able to be obtained also when it operated using the vertical bending type continuous casting machine.

The measurement of the stirring flow rate formed by the electromagnetic stirrer was performed by measuring the dendrite inclination angle of the obtained slab and using the relational expression of the inclination angle θ and the solidification rate f and the flow rate u shown in the following formula A-1. (Okano et al .: Iron and steel, 61 (1975), 2982).
ln u = (θ + 9.73 ln f + 33.7) / (1.45 ln f + 12.5) (u ≦ 50cm / s)
ln u = (θ + 4.83 ln f + 7.2) / (0.1 ln f + 5.4) (u ≧ 50cm / s)
…………… A-1
f = k ² / 120δ …………… A-2
In the above equation, u: flow velocity (cm / s), f: solidification rate (cm / s), θ: deflection angle with respect to the normal of the slab surface (°), k: solidification shell growth rate coefficient (cm / min ^{1) / 2} ), δ: solidified shell thickness (mm).
At each casting speed, the tilt angle at the shell thickness δ corresponding to the core center height of the electromagnetic stirring coil was measured. At the same time, the solidification rate f at the solidified shell thickness was estimated by the formula A-2, and the stirring flow rate was estimated by using the formula A-1. In the above estimation, k was set to 2.2 cm / min ^1/2 .

It is a figure which shows the stirring state in the horizontal cross section of the molten steel by a conventional method (patent documents 1 and 2), (a) is the situation of a nozzle discharge reverse flow and a film-like stirring flow, (b) is a stirring flow and discharge reverse flow And the situation of swirling flow. The figure which shows the behavior of the particle | grains in the molten steel of the solidification interface vicinity . The figure which shows the relationship between the coagulation | solidification speed | rate in this invention (following and the same) and the flow velocity required for inclusion trapping prevention. The figure which shows the relationship between the solidification shell thickness and the flow velocity required for inclusion capture prevention. The figure which shows the relationship between stirring flow velocity and flow stagnation frequency. The figure which shows the relationship between a stirring flow rate and powder entrainment frequency. The figure which shows the relationship between a stirring flow rate and a stirring area | region. The schematic diagram which shows the state of the stirring flow in a lower pool. It is a schematic diagram which shows the flow condition in a pool, (a) shows the counterclockwise stirring flow + nozzle discharge flow, (b) shows the state of clockwise stirring flow + nozzle discharge flow. The figure which shows the relationship between the maximum circulation flow velocity and a tracer number index. The figure which shows the relationship between the propulsion direction switching period of a molten steel, and a tracer number index. The figure which shows the relationship between the distance from a molten metal surface, a required flow velocity, and the maximum flow velocity. It is a schematic diagram which shows the flow condition at the time of providing a set of electromagnetic stirring apparatus near the molten metal stirring and a pool lower part, (a) shows a vertical cross section, (b) shows a horizontal cross section. It is a schematic diagram which shows the flow situation at the time of providing two sets of electromagnetic stirring apparatuses near the hot water surface and the pool lower part, (a) shows a vertical section, (b) shows a horizontal section.

Claims (3)

In the continuous cast strand pool, in the molten steel near the solidification interface , 0. A swirling flow having a flow rate of 6 to 1 m / s is continuously formed, and in a region from the molten metal surface to 1 m below the molten metal surface , depending on the distance L (m) from the molten metal surface and the casting speed V c (m / s). The flow control method of the flow in a strand pool characterized by providing the flow more than the flow velocity U (m / s) prescribed | regulated by following (1) Formula.
D = K√ (L / Vc)
U ≧ 0. 6 3 a K ^4/3 (D) ^2/3 ... ... (1)
However, a = 12.4 (m ^1/3 · s ^−1/3 )
K: Solidification shell growth rate coefficient (1.8 × 10 ⁻³ to 3.2 × 10 ⁻³ m · s ^−1/2 )
Solidified shell thickness D (m): 0.001 ≦ D ≦ 0.1 Te claim 1 smell, either towards the other short side from one short side in the region, and less and propulsion direction 10~60s period maximum flow velocity thereof is 0.5 m / s from below the melt surface 1m to 7m The flow in the strand pool is characterized in that in the region from 1 m to 7 m below the molten metal surface, a flow that is parallel to the solidification interface and whose flow rate periodically varies is provided. Control method.   In the vicinity of the molten metal surface, the stirring flow velocity is 0.6 m / s or more and 1 m / s or less in a horizontal cross section by applying thrusts parallel to the long sides of the opposite slabs and opposite to each other. In an electromagnetic stirrer capable of applying a flow and in any region from 1 m to 7 m below the molten metal surface, both are applied in parallel to each long side of the opposing slabs, and both apply thrust in the same direction. Electromagnetic stirring capable of imparting a propulsion flow having a maximum flow velocity of 0.5 m / s or less from one short side to the other short side and periodically changing the propulsion direction at a period of 10 to 60 s. A flow control device in a strand pool, comprising one or more sets of devices.

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