JP2008221243A - Continuously casting method and apparatus for steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stabilize the flowing and temperature of a molten steel without causing any stagnation and interference point in a mold when the casting is performed by using an electromagnetic stirring device in a continuous casting apparatus often varied in a width, a thickness or a casting speed. <P>SOLUTION: When the steel is continuously cast by performing the casting while generating the circulating flow in the mold with the electromagnetic stirring device having a moving magnetic field coil, the casting is performed so that the axial center position L<SB>0</SB>of the coil 24 on the downstream side is set to lower position in the casting direction than the axial center position L<SB>2</SB>of the coil 22 on the upstream side in the stirring direction. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a steel continuous casting method and apparatus, and more particularly, a bubble called a hege / sleever or the like in a product suitable for use in continuous casting of steel in which a swirling flow is generated in a mold by an electromagnetic stirrer. -It is related with the continuous casting method and apparatus of steel which can reduce the defect of inclusion physical property stably.

  When the molten steel solidifies in continuous casting, if bubbles or inclusions remaining in the molten steel cannot be excluded and are brought into the product, defects called Hege-Sleeber defects occur, and stable quality can be ensured. It becomes difficult.

  On the other hand, in continuous casting, a technique for controlling the flow of molten steel in a mold (also referred to as a mold) to prevent trapping of bubbles and inclusions in the molten steel into the solidified shell and preventing entry into the lower part of the mold, Currently in common practice.

  It is known that hege and sliver defects are likely to originate in locations where stagnation of the flow velocity occurs or where the temperature decreases locally in the molten steel flow in the mold. In order to make the molten steel flow uniform in the mold long side direction (mold width direction) and the casting direction, an electromagnetic flow control technique using an alternating magnetic field or a static magnetic field has been proposed.

  For example, in Patent Document 1, the discharge port of the immersion nozzle is opened downward from the side direction of the immersion nozzle at an angle in the range of 35 degrees or more and 75 degrees or less, and the discharge port is lower than the lower surface of the core of the electromagnetic stirring device. A continuous casting method is described in which the molten steel can be agitated while minimizing the influence of the discharge flow caused by discharging the molten steel from the immersion nozzle.

  Further, in Patent Document 2, an electromagnetic stirrer is installed in the vicinity of a long side meniscus (also referred to as a molten metal surface) of a mold having a rectangular cross section in a continuous casting facility. A continuous casting method in which a magnetic flux density at the discharge port of the immersion nozzle is provided at a position where the magnetic flux density at the discharge port of the immersion nozzle is 50% or less of the maximum magnetic flux density of the electromagnetic stirring device and cast. Are listed.

  Further, in Patent Document 3, in the continuous casting method of steel using an electromagnetic stirrer, the minimum distance d between the outer wall of the immersion nozzle immersed in the molten steel and the solidified shell formed on the long side of the mold is described. A continuous casting method for casting so as to satisfy the following formula is described.

d = (t−D) / 2-18√ (L / Vc) ≧ 86√ (f) (1)
Where t: mold short side wall length (mm)
D: Outer nozzle outer diameter (mm)
L: From the meniscus to the part where the distance between the immersion nozzle and the solidified shell is minimized
Distance (m)
Vc: Casting speed (m / min)
f: Frequency of electromagnetic stirring coil (Hz)

  Patent Document 4 discloses that in the continuous casting of molten steel in which molten steel is discharged through a dipping nozzle into a mold having a rectangular cross section and a length ratio (a / b) of long and short side walls of 3 or more. The nozzle has a ratio of the short side wall length of the mold to its outer diameter (b / d) of 3 or less, and a pair of molten steel discharge ports provided symmetrically with respect to the central axis. It is described that the molten steel is continuously discharged in a direction satisfying the equation.

0 <θ <θ _M
Where θ: perpendicular to the short side wall of the mold from the discharge direction of the molten steel and the central axis of the immersion nozzle
Angle made with
θ _M : perpendicular line from the central axis of the immersion nozzle to the short side wall of the mold and the flat section of the mold
The angle between the diagonal line

  However, in continuous casting operations, the casting speed, slab width, slab thickness, etc. are generally constantly changing, and the above method may be effective for casting under certain conditions. However, the problem has not been solved completely.

  In addition to the above technique, in a continuous casting method in which a swirl flow is added in the width direction using an electromagnetic stirrer, the mold has two or more coils in the long side direction of the mold, and the difference in stirring force and applied current is In addition, a technique for reducing interference between the swirling flow and the reverse flow from the immersion nozzle has been proposed.

  For example, in Patent Document 5, a copper plate thickness of a reference surface of a casting mold having an electromagnetic stirring mechanism is measured, and an operating frequency of a power source at which the Lorentz force generated by the electromagnetic stirring mechanism is maximized corresponding to the copper plate thickness is set. A method for continuously casting a metal characterized in that a current having the operating frequency is supplied to an electromagnetic stirring coil is described.

  Further, in Patent Document 6, in a molten steel electromagnetic stirring method in which electromagnetic force is applied in a rectangular continuous casting mold in an opposing direction along opposite long sides, the flow of molten steel is extended from one short side to a long side. The ratio F2 / F1 of the Lorentz force F1 in the initial acceleration stage that is directed inward along the side and the Lorentz force F2 in the late acceleration stage that directs the flow of molten steel from the inside toward the other short side is 0.15. There is described a method for stirring molten steel, characterized in that it is controlled in the range of ~ 0.5 and the flow rate of molten steel is ensured to 20 to 60 cm / sec.

  Further, in Patent Document 7, in continuous casting of a metal slab, the molten metal is injected into the mold from an immersion nozzle provided in the center of the cross section of the mold, and provided along the two long sides of the meniscus surface. In this method, the molten metal is caused to flow in the meniscus plane with an electromagnetic stirring coil, the electromagnetic stirring thrusts along the two long sides of the mold are opposite to each other, and the electromagnetic stirring thrust in the direction from the immersion nozzle toward the mold is applied to the mold. A method for flowing a molten metal in a mold in continuous casting is described in which a uniform rotational flow is applied to the molten metal in the meniscus surface by increasing the electromagnetic stirring thrust in the direction from the short side toward the immersion nozzle. .

In Patent Document 8, an electromagnet, which is an electromagnetic force generator, is arranged along the two opposite long sides of the mold at substantially the same level as the nozzle outlet for injecting molten metal into the mold. Horizontal molten metal in the horizontal direction along the long side of the mold at the 1/4 long side width point along the long side from the casting short side of the casting mold along the long side of the molten metal that is driven by horizontal circulation in the two long sides The starting side velocity Vs, which is the velocity, is the molten metal horizontal velocity in the horizontal direction along the mold long side direction at the quarter long side width point from the short end side mold side of the flow along the long side to the long side of the mold. A molten metal flow device is described in which an electromagnetic force Vs ≧ Ve is applied to a molten metal for a certain end-side speed Ve, and the horizontal flow along the first long side and the second long piece is further described. Arranged on each end side with respect to exciting current I ₁ of the electromagnet arranged on each starting point side of A molten metal flow control device is described in which the ratio α = I ₂ / I ₁ of the excitation current I ₂ of the electromagnet is 0 ≦ α ≦ 0.5.

  The techniques of Patent Documents 5 to 8 are common in that the contents are different, but the purpose is to equalize the flow velocity in the width direction.

JP 2004-42062 A Japanese Patent Laid-Open No. 2001-47201 JP 2006-192441 A JP 2000-263199 A Japanese Patent No. 2978356 Japanese Patent No. 3129942 Japanese Patent No. 2948443 Japanese Patent No. 3577389

  However, although it is possible to improve the degree of horizontal swirling type stirring on the same plane, it is impossible to completely avoid the stagnation of the flow due to the interference between the swirling flow and the reversal flow. It was.

  The present invention has been made to solve the above-mentioned conventional problems. In a continuous casting facility in which the width / thickness and the casting speed are constantly changed, when casting using an electromagnetic stirring device in the mold, The objective is to achieve stable molten steel flow and molten steel temperature without any stagnation or interference.

  In the invention according to claim 1 of the present invention, the position of the axial center of the coil on the downstream side in the stirring direction is determined during continuous casting of steel in which a swirling flow is generated in a mold by an electromagnetic stirring device having a moving magnetic field coil. The continuous casting method of steel, characterized in that the steel is cast by being installed at a position lower than the axial center position of the coil on the upstream side in the stirring direction with respect to the casting direction.

The invention according to claim ₂ is set so that the difference L ₁ between the axial center position L ₂ of the upstream coil and the axial center position L ₀ of the downstream coil satisfies the following expression. By doing this, it is possible to produce a slab that is particularly excellent in surface quality.

L ₁ = 0.8 × L _{0 to} 1.2 × L ₀ (mm) (2)
L ₀ = 2 × 10 ⁶ × (TP / W / 2 × cos θ) ^0.8
−400 × L ₂ −2000 × (D−0.3) −1000 × W ^0.5 (mm) (3)
(However, if L ₀ <0, L ₀ = 0)
Here, L ₀ : distance (m) between the coil axis and the molten metal surface of the moving magnetic field coil on the downstream side of the swirl flow
L ₂ : Distance between the coil axis of the moving magnetic field coil on the upstream side of the swirl flow and the molten metal surface (m)
TP: Throughput (ton / min)
W: slab width (m)
θ: Downward nozzle angle (°)
D: slab thickness (m)

  The invention according to claim 3 is the steel continuous casting method according to claim 1 or 2, wherein the moving magnetic field coil is divided in the direction of the long side of the mold.

  The invention according to claim 4 is the steel continuous casting method according to claim 1 or 2, wherein the moving magnetic field coil is inclined with respect to the long side of the mold.

  According to a fifth aspect of the present invention, there is provided a continuous casting apparatus for steel in which a swirling flow is generated in a mold by an electromagnetic stirring device having a moving magnetic field coil, and the axial center position of the coil on the downstream side in the stirring direction is The continuous casting apparatus for steel is characterized in that it is installed at a position lower than the axial center position of the coil on the upstream side in the stirring direction with respect to the casting direction.

  In the present invention, as shown in FIG. 1, even when an electromagnetic stirrer having coils 22 and 24 divided into two in the long side direction (left and right direction in the figure) of the mold 10 is used, the axial center position of the coil is kept at the same height. It is a necessary condition that the stagnation region of the flow due to the interference between the swirl flow and the reversal flow is eliminated by setting the coil position corresponding to the downstream side of the swirl flow downward rather than on the vertical plane. In the figure, 12 is an immersion nozzle.

  FIG. 2 is a schematic diagram in the mold when the electromagnetic stirring coils 22 and 24 divided into two in the mold long side direction as shown in FIG. 1 are used. As shown in FIG. 2 (a), it is widely known that a flow stagnation region occurs on the molten metal surface due to interference between the reversal flow of the short side of the mold and the horizontal swirling flow, which hinders the removal of inclusions and bubbles. Yes. On the other hand, as shown in FIG. 2 (b), it is possible to improve the interference range by reducing the stirring thrust (Lorentz force) of the coil 24 on the downstream side of the horizontal swirl flow. There is a problem that interference cannot be completely avoided only by changing the area.

  Therefore, in order to solve this problem, the present inventors do not set the positions in the casting direction of the axial center of the coil divided into two in the mold long side direction to the same height in the upstream and downstream coils of the swirl flow. It is characterized by that.

  In the prior art, when the slab width, slab thickness, casting speed, etc. change during actual operation, complicated interference occurs between the swirling flow caused by electromagnetic stirring in the mold and the flow caused by the nozzle discharge flow. Although it was difficult to produce a stable product with no hege and sliver defects at the stage, by applying the method of the present invention, it is not necessary to change to the optimum nozzle shape for each of the above conditions, and high production Compatibility with high quality.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  First, a first embodiment of the present invention will be described with reference to FIG. FIG. 3 is a view of the positional relationship between the electromagnetic stirring coil and the mold, which is the object of the first embodiment, viewed from the mold side. A swirling flow is generated by electromagnetic stirring with respect to the long side direction of the mold 10. Among the coils to be applied, the axial center position of the downstream coil 24 is set so that the position from the meniscus is lower than that of the upstream coil 22.

  The reason why the position of the downstream coil 24 is lowered from the meniscus as compared with the upstream coil 22 is as follows. Normally, in continuous casting, when the throughput TP representing the casting amount of molten steel per unit time of one strand is 3 tons / min or more, it is common to use a downward two-hole type immersion nozzle. The molten steel that has passed through the nozzle collides with the short side of the mold to form a reverse flow.

  In continuous casting, if bubbles or inclusions brought in by molten steel from the immersion nozzle 12 are brought into a slab, which is a semi-finished product, it may cause product defects. Therefore, the continuous casting may be removed from the mold so as not to be captured by the solidified shell. An electromagnetic stirrer that uses a moving magnetic field as one means is generally used in continuous casting of slabs and blooms. When a swirling flow velocity of 10 to 40 cm / sec is applied to the meniscus portion by an electromagnetic stirring device, a cleaning effect is expected in which inclusions and bubbles are prevented from being trapped by the solidified shell.

  However, on the downstream side of the swirling flow, if an effective flow velocity is not given to the solidification interface due to collision and interference between the reversing flow and the swirling flow, inclusions and bubbles are trapped starting from that location. The occurrence of hege and sliver defects in products is a problem.

  The present inventors studied effective means for suppressing the interference between the swirling flow and the reversal flow by making full use of model experiments and numerical calculations using a low melting point metal. As a result, it was found that it is difficult to completely eliminate the interference part of the meniscus by simply weakening the stirring thrust (Lorentz force) of the coil divided into two in the mold long side direction on the downstream side. .

  In order to avoid interference between the swirling flow and the reversing flow, it is effective to avoid by lowering the position of the coil 24 located on the downstream side of the swirling flow in the casting direction. Further, it is better that the amount by which the coil 24 on the downstream side of the swirl flow is lowered relative to the coil 22 on the upstream side is changed according to the strength of the reverse flow.

  As a result of investigation by the present inventors on the factors that determine the flow velocity and position of the reversal flow at the solidification interface near the mold meniscus, the throughput TP, slab width W, nozzle discharge angle (downward) θ, and mold thickness D are large. It turns out that it is affecting.

When the distance L ₂ from the meniscus of the axial portion of the upstream side of the coil 22, the reversing flow is the distance from the meniscus of the axial portion of the downstream side of the coil 24 to not interfere with the swirling flow and L _0, Using the above influencing factors, it has been empirically found that the following equation can be used.

L ₁ = 0.8 × L ₀ −1.2 × L ₀ (mm) (2)
L ₀ = 2 × 10 ⁶ × (TP / W / 2 × cos θ) ^0.8
−400 × L ₂ −2000 × (D−0.3) −1000 × W ^0.5 (mm) (3)
(However, if L ₀ <0, L ₀ = 0)

  However, the throughput TP is calculated by the following equation.

TP = VR × D × W × ρFe (4)
Where VR: drawing speed (m / min)
ρFe: Molten steel density (kg / m ³ )

  The above formula is calculated based on the following concept.

When the reverse flow is large, L ₁ needs to be increased, and the term representing the magnitude of the reverse flow is (TP / W / 2 × cos θ) ^0.8 , so the coefficient of this term is a positive value.

Further, if the axis position L ₂ on the upstream side of the coil 22 is large, it is in a deep position upstream coil 22, since the downstream coil 24 is not significantly affected by reversing flow, so positioned below the upstream There is no need to make this, and the coefficient of this term is negative.

  Further, when the mold thickness D is large, the reversal flow does not reach the vicinity of the solidification interface, so the influence is small, and when the mold thickness D is 300 mm or more, the reversal flow hardly reaches the solidification interface. Therefore, as D is larger, there is no need to make a difference in the distance between the upstream and downstream coils, so the coefficient of this term becomes negative.

The value of W ^0.5 of the last slab width W corresponds to the acceleration section of the swirl flow, and the larger this value, the faster the flow velocity is accelerated by the upstream coil 22, so the swirl flow downstream is shorter. The tendency to dive down near the side becomes prominent. Accordingly, since it is not necessary to lower the coil position downward, the coefficient of this term is also negative.

  The above coefficients are values that the inventors of the present invention have empirically derived from these numerical calculations or laboratory or actual machine tests.

When the axial position L ₀ of the downstream coil 24 is lowered by the distance L ₁ relative to the upstream coil 22, the interference between the swirl flow and the reverse flow is almost eliminated, and the occurrence of surface defects due to the stagnation of the flow occurs. Can be avoided.

For steel types with extremely strict specifications for surface defects, if the downstream coil can be set lower than the upstream coil at a distance of 0.8 × L _{1 to} 1.2 × L ₁ , It has been found that the quality improvement effect is great.

  The axial center position can be set online by changing the operating conditions by providing a drive system so that the coil axial position can be automatically changed during operation.

  Further, as in the second embodiment shown in FIG. 4, the same effect can be obtained by installing the moving magnetic field coil 20 so that the coil axis has an inclination angle with respect to the mold long side direction. Also confirmed. In this case, it is possible to use the coil 20 that is not divided in the mold long side direction.

  Hereinafter, an embodiment showing the present invention will be described.

  The present inventors cast a slab having a mold thickness of 200 mm to 300 mm and a slab width of 1000 mm to 2000 mm with a slab continuous casting machine. The molten steel component is a general very low carbon steel.

  The immersion nozzle has 2 holes, the discharge angle is downward 25 degrees or 35 degrees, and the immersion depth is constant at 250 mm. The casting speed was 1.2 to 2.2 m / min in the steady speed part.

  The electromagnetic stirring coil was used at a frequency of 2 Hz and a constant applied current of 750A.

When the number of defects per coil is 1.2 × 10 ⁻³ pieces / m ² or more after carrying out a visual inspection for hege / slivers defects in the coil, the number is 4 × 10 ⁻⁴ pieces / m ² The case of 8 × 10 ⁻⁴ pieces / m ² or more was evaluated as Δ, the case of 0 pieces / m ² to 4 × 10 ⁻⁴ pieces / m ² was evaluated as ◯, and the case of 0 pieces / m ² was evaluated as ◎.

  Table 1 shows the experimental conditions and the experimental results.

  Under the conditions of the present invention, it was confirmed that hege / sliver defects in the coil could be completely suppressed.

Schematic view of the electromagnetic stirring coil viewed from the top of the mold Schematic diagram of mold longitudinal section showing interference of swirling flow and reverse flow from nozzle discharge flow by electromagnetic stirring Schematic of mold longitudinal section showing coil installation position in the first embodiment of the present invention Schematic diagram of the mold longitudinal section showing the coil installation position in the second embodiment of the present invention when using an electromagnetic stirring coil that is not divided in the mold long side direction

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Mold 12 ... Immersion nozzle 20 ... Moving magnetic field coil 22 ... Upstream coil 24 ... Downstream coil

Claims (5)

In continuous casting of steel, which performs casting by generating a swirl flow in a mold by an electromagnetic stirrer having a moving magnetic field coil,
A continuous casting method for steel, wherein the casting is performed by setting the axial center position of the coil on the downstream side in the stirring direction at a position lower than the axial position of the upstream coil in the stirring direction with respect to the casting direction. In claim 1, the difference L ₁ of the upstream side of the axis position L ₂ and axis position L ₀ of the downstream side of the coil of the coil, by setting so as to satisfy the following equation, particularly good surface quality A method for continuous casting of steel, characterized in that slabs can be manufactured.
L ₁ = 0.8 × L _{0 to} 1.2 × L ₀ (mm)
L ₀ = 2 × 10 ⁶ × (TP / W / 2 × cos θ) ^0.8
−400 × L ₂ −2000 × (D−0.3) −1000 × W ^0.5 (mm)
(However, if L ₀ <0, L ₀ = 0)
Here, L ₀ : distance (m) between the coil axis and the molten metal surface of the moving magnetic field coil on the downstream side of the swirl flow
L ₂ : Distance between the coil axis of the moving magnetic field coil on the upstream side of the swirl flow and the molten metal surface (m)
TP: Throughput (ton / min)
W: slab width (m)
θ: Downward nozzle angle (°)
D: slab thickness (m)   The continuous casting method for steel according to claim 1 or 2, wherein the moving magnetic field coil is divided in the direction of the long side of the mold.   The continuous casting method for steel according to claim 1 or 2, wherein the moving magnetic field coil is inclined with respect to the long side of the mold. In a continuous casting apparatus for steel that performs casting by generating a swirl flow in a mold by an electromagnetic stirring device having a moving magnetic field coil,
A continuous casting apparatus for steel, wherein the axial center position of the coil on the downstream side in the stirring direction is installed at a position lower than the axial center position of the coil on the upstream side in the stirring direction.

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