JP2005205441A - Method for continuously casting slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To especially prevent the entrapment of foreign matter, such as inclusion, into the inner part of a slab, regarding a method for continuously casting the slab. <P>SOLUTION: This method for continuously casting the slab is performed as the following. In a continuous casting facility composed of a casting velocity Vc>1.5 m/min and a through-put quantity TP>4 t/min and further suitably, the casting velocity Vc>2.0 m/min and the through-put quantity TP>5 t/min, a magnetic field applicator is provided at both short wall sides in a mold and at the lower position from the mold and the ascending flow is generated with the electromagnetic force toward the upper part in the molten steel by this magnetic field applicator. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a slab continuous casting method, and more particularly to a material slab manufacturing technique suitable for use in can steel plates, automobile outer plates, and the like that require strict workability.

  Based on FIG. 2, the example of the slab continuous casting line which manufactures a raw material slab is demonstrated.

  First, molten steel 2 is poured into the tundish 1 from an upstream process (not shown). Next, the molten steel 2 is cast into the mold 4 from the nozzle 3 at the bottom of the tundish 1 and is drawn from the mold 4 by the casting machine 5 to obtain a continuous slab 10. Then, the slab 10 is held by the roll 6, changed in the horizontal direction by the bending roll 7, corrected by the straightening machine 8, and then cut by the torch 11 to form one slab 12 for each process. Take it out.

  FIG. 3 schematically shows the inside of the mold into which the slab 10 is cast in the continuous casting line as described above, by describing only the short sides 4b and the nozzles 3 of the molds on both sides. . In FIG. 3, the nozzle 3 is immersed in the mold 4 and its discharge holes 3a are opened toward the short sides 4b of the molds on both sides. This is for supplying the molten steel uniformly in the width direction of the slab in the mold.

  In the following, as shown in FIG. 4, the width of the short side 4b of the mold is D, and the width of the discharge hole 3a of the nozzle 3 is d.

  The molten steel discharged from the discharge hole 3a of the nozzle 3 toward the short sides 4b of the molds on both sides flows separately as shown by arrows in FIG. 3, but this flow varies greatly depending on the time. It has been known. In particular, the downward flow flowing downward along the short side 4b of the molds on both sides always fluctuates at a pitch of about 10 to 20 seconds. For example, when one of the downward flows is about 6 m deep, The downward flow may flow only about 1 m, and the subduction may be reversed at the next pitch.

  And by the flow of such molten steel, the nonmetallic inclusions etc. in molten steel are involved, and it becomes the cause of a defect.

  It is known that when non-metallic inclusions (hereinafter simply referred to as inclusions) in molten steel remain in the slab, defects such as cracks start from the inclusions during processing.

  Moreover, when it remains in the surface layer part of a slab, when it hot-rolls and cold-rolls and it is set as a thin steel plate, it will cause a surface defect.

  The inclusions are alumina, magnesia, titania, etc., which are deoxidation products formed by combining oxygen with deoxidizers such as aluminum, magnesium, titanium, etc., which are introduced in the step of deoxidizing oxygen in the molten steel, These cannot be removed and remain as inclusions in the molten steel. And when a slab is hot-rolled and cold-rolled to make a thin steel plate, such inclusions cause cracks and cause surface defects such as scabs and blisters on the surface of the steel plate.

  In addition to inclusions, there is mold gas added to the molten steel surface in the mold during continuous casting, and argon gas supplied to prevent clogging of the immersion nozzle for supplying molten steel from the tundish into the mold. It is known that even if a bubble entrained in molten steel remains in the molten steel as a bubble alone or as a bubble combined with a deoxidized product, it causes defects similar to the above inclusions.

  For this reason, conventionally, for the defects existing in the surface portion of the slab, surface care is performed in the slab refining process as necessary. However, performing slab surface care in the refining process has a problem of reducing the yield of the slab and causing the process to stay.

  In addition, it was necessary to perform a flaw detection inspection for defects entrained inside the slab and then reject them according to the degree of the defects after hot rolling and cold rolling to form a thin steel plate.

  In order to solve this problem, it is necessary to separate these inclusions or the like in the stage of casting the slab or to suppress the capture to the slab.

The basic concept of such an attempt at the stage of manufacturing a slab in continuous casting will be described below.
(1) Increase the cross section of the slab (slab width is limited during rolling, so increase the slab thickness) and reduce the casting speed V _c (m / min) without affecting productivity. In addition, the residence time of the molten steel in the mold is lengthened, thereby obtaining a time for the deoxidation product, mold powder, bubbles and other foreign matters from the molten steel to rise in the mold.
(2) Casting with a continuous casting machine having a vertical part in order to promote floating and separation of deoxidation products, mold powder or bubbles from molten steel in the mold.
(3) Applying a horizontal flow in the vicinity of the meniscus by electromagnetic force to prevent foreign matter floating in the molten steel from being trapped by the solidified shell. This action is particularly called a cleaning effect.
(4) Reduce mold powder entrainment in molten steel by making mold powder viscosity appropriate.
(5) Optimization of the oscillation (vertical vibration) conditions of the casting mold for continuous casting and generation of claw of the solidified shell formed in the mold (a phenomenon in which a part of the solidified shell falls to the molten steel side due to the oscillation) This reduces the amount of trapped deoxidation products, mold powder, bubbles, etc. in this part.
(6) Electromagnetic stirring and electromagnetic brake are added to the molten steel discharge flow supplied from the immersion nozzle into the mold to optimize the flow of the molten steel, and the molten steel discharge flow with deoxidation products is deep in the mold. Prevent entry into position.

  However, in any of the above-described techniques, when producing a slab for use such as a steel plate for cans and an automobile outer plate, the casting speed is, for example, higher than 1.5 m / min. It was not possible to sufficiently separate inclusions or the like, or to suppress the capture to the slab, and it was impossible to stably provide a high-quality slab.

  By the way, the following has already been proposed as a technique for optimizing the flow of molten steel by adding the above-described electromagnetic stirring and electromagnetic brake.

  Patent Document 1 discloses that an electromagnetic stirrer is disposed facing the long side direction of a slab to perform electromagnetic stirring of molten steel. In particular, in Patent Document 1, the position of the maximum depth position ± 500 mm of the nozzle jet is appropriate as a stirring position.

  In Patent Document 2, an electromagnetic stirrer is installed directly under the short side support part of the slab in a curved continuous casting machine, and an upward electromagnetic flow is formed at the molten steel solidification interface of the short side part. However, in Patent Document 2, the maximum throughput amount is set to 4 t / min, and if the throughput amount exceeds that, the amount of inclusions in the 1/4 integration band increases, which is harmful.

  Patent Document 3 discloses that an upward flow is imparted to the short side portion of the slab at a position above 1 m below the maximum penetration depth (penetration depth) of the discharged molten steel from the nozzle. However, the casting speed assumed in Patent Document 3 is about 1 m / min, and high-speed casting exceeding 1.5 m / min is not assumed.

In Patent Document 4, casting is performed by applying a static magnetic field by a DC coil to a molten steel in a mold and applying a moving magnetic field that is superimposed on the molten steel and moves in a casting direction by an AC coil. The surface and internal defects are reduced. However, in Patent Document 4, these DC coils and AC coils are arranged on the long side of the mold so as to be in the slab width direction, and a static magnetic field and a moving magnetic field are applied. It does not apply a magnetic field to.
JP 57-97849 A JP 61-269960 A Japanese Patent Laid-Open No. 11-28556 JP 2000-351048

  The present invention provides a slab continuous casting method in which an electromagnetic force is applied to molten steel in a mold to control the flow of molten steel, and the casting speed of a slab exceeds 1.5 m / min, and more preferably exceeds 2 m / min. It can be applied to high-speed casting, and can stably supply a high-quality slab free from inclusions and bubbles.

As a result of intensive studies to achieve the above object, the present inventor has determined a throughput amount TP in high-speed casting in which the casting speed V _c exceeds 1.5 m / min, and more preferably exceeds 2.0 m / min. Even if the casting conditions exceed 4 t / min, and more preferably exceed 5 t / min, an upward flow is induced in the vicinity of the short side of the mold by electromagnetic force while ensuring the molten steel flow velocity above the mold. It has been found that the molten steel flow discharged from the nozzle discharge hole can be suppressed from entering along the short side, and the inclusion of foreign substances such as inclusions can be prevented most effectively.

  Further, in this case, it has been found that the molten steel flow rate can be secured more appropriately by appropriately controlling the ratio D / d of the short side length D of the casting space in the mold and the discharge hole width d of the nozzle.

That is, the present invention promotes the floating separation of inclusions by reducing the submerged flow velocity at the lower part of the mold while ensuring the flow speed at the solidification interface at the upper part of the mold. The above problems have been solved by a casting method.
1. A slab continuous casting method in which the flow of molten steel is controlled by electromagnetic force in a casting space in a continuous casting facility with a casting speed V _c of V _c > 1.5 m / min and a throughput amount TP of TP> 4 t / min. The magnetic field application device is provided on the short side of the mold and below the mold, and the magnetic field application device generates an upward flow due to electromagnetic force upward in the molten steel. A slab continuous casting method characterized by that.
2. The slab continuous casting method according to 1. above, wherein in the continuous casting equipment, the casting speed V _c is V _c > 2.0 m / min and the throughput amount TP is TP> 5 t / min.
3. The slab continuation according to 1 or 2 above, wherein a relation of a ratio D / d between the short side width D of the mold and the discharge hole width d of the nozzle is 3 ≧ D / d ≧ 1.5. Casting method
4. A magnetic field is applied over the entire width of the long side of the mold to the position including the level of the molten metal surface in the mold on the upper side and / or the position on the lower side with respect to the position of the discharge hole of the nozzle. 3. A slab continuous casting method as described in 3 above, comprising a magnetic field applying device for controlling the flow of molten steel.

  According to the present invention, it is possible to stably provide a high-quality slab in which generation of both surface defects and internal inclusions due to inclusions, bubbles, mold flux, etc. is minimized.

  The present invention makes it possible to provide a slab continuous casting method suitable for producing a material slab suitable for use in steel plates for cans, automobile outer plates, and the like that require particularly strict workability.

  The best mode for carrying out the present invention will be described with reference to FIG.

In the present invention, in high-speed casting in which the casting speed V _c is over 1.5 m / min, more preferably over 2.0 m / min, the throughput amount TP is over 4 t / min, more preferably over 5 t / min. In such a casting condition, it is possible to effectively suppress the molten steel flow flowing along the short side 4b of the mold. And in order to acquire the suppression effect, the magnetic field application apparatus 9 is arrange | positioned under the short side 4b of a casting_mold | template, and the upward flow 9a is given to molten steel. By doing so, it is possible to suppress the downward flow that causes the inclusion of foreign substances such as inclusions, or to reduce the downward flow by applying a braking force.

  The magnetic field application device 9 is installed at a position below the mold 4, but is preferably installed at a position from the position directly below the mold 4 to 6 m from the position of the molten steel surface in the mold 4. By doing so, it is possible to effectively prevent the downward flow of the molten steel that digs up to about 6 m at the maximum, and to effectively prevent the inclusion of foreign substances such as inclusions accompanying the downward flow.

  Here, the magnitude (magnetic flux density) of the DC magnetic field applied by the magnetic field application device 9 is preferably about 0.03T to 0.35T.

  In addition, when the continuous casting operation is not high speed casting and the throughput amount is not so large, foreign substances such as inclusions are not deeply involved in the downward flow of the molten steel. And easily separated.

  Furthermore, the present inventor can minimize the deviation of the molten steel flow velocity by setting the ratio D / d of the short side width D of the mold to the discharge hole width d of the nozzle within the range of 1.5 to 3.0. It has been found that the fluctuation of the downward flow flowing downward along the short side 4b can be more effectively suppressed.

  The molten steel ejected from the nozzle discharge hole widens and decelerates until it collides with the short side of the mold, but the degree and velocity distribution of the molten steel jet are the short side width D of the mold, the casting speed, And depends on the ratio of D / d.

  In particular, if the nozzle discharge hole width d is too small (that is, the ratio of D / d is too large) with respect to the short side width D of the mold, the slab thickness of the region width where the jet velocity colliding with the short side of the mold is large. Since the proportion of the (short side width) decreases, the growth of the solidified shell is uneven and easily inhibited. When the solidified shell becomes extremely thin, breakout occurs.

  On the other hand, if the nozzle discharge hole width d is too large (ie, the D / d ratio is too small) with respect to the mold short side width D, the jet collides with the long side shell before colliding with the short side. As a result, the growth of the solidified shell on the long side is hindered, transverse cracks and oblique cracks occur, and if the solidified shell becomes extremely thin, it will also lead to breakout.

  Moreover, even if the ratio of D / d is too large or too small, due to the deviation of the molten steel flow velocity in the slab thickness direction, it also contributes to fluctuations in the meniscus flow velocity, and the amount of foreign matter involved increases.

  Here, when the influence of the ratio of D / d on the product quality after rolling was investigated in detail, when the magnetic field applying device 9 was disposed below the short side 4b of the mold as in the present invention, D / d It was found that the ratio of d is preferably in the range of 1.5 to 3.

  Further, in the present invention, as shown in FIG. 5, in order to enhance the cleaning effect of the foreign matter to be trapped in the solidified shell on the long side 4a side of the mold, the upper side of the nozzle is set to the position of the discharge hole 3a. Controlling the flow of the molten steel with a magnetic field applying device 19 that applies a magnetic field over the entire width of the long side of the mold at a position including the molten metal level in the mold and / or at a position on the lower side. It is preferable. Here, the magnetic field application device 19 controls the molten steel flow by appropriately applying a static magnetic field and / or an alternating magnetic field.

  With the continuous casting machine, the short side length (slab thickness) of the casting space in the mold is 110mm (test continuous casting machine), 200, 215, 220, 235, 260 mm (above the vertical bending mold production series) Slabs with a slab width of 400 mm (test continuous casting machine) and 900-2200 mm (above are vertical bending type production continuous casting machines) were continuously cast under the conditions shown in Table 1. .

  At this time, the mold height is 900 mm (production continuous casting machine), 700 mm (test continuous casting machine), the nozzle used is alumina graphite, the wall thickness is 25 mm, and the shape of the discharge hole is square (slab thickness: (When 220 mm or less) or round shape (when the slab thickness exceeds 220 mm), the discharge angle was constant at 20 ° downward.

The mold flux used had physical properties of solidification temperature: 1000 ° C., viscosity: 0.05 to 0.2 Pa · s (0.5 to 2.0 poise) (1300 ° C.), and basicity (CaO / SiO ₂ ) = 1.0. In addition, the molten steel superheat degree in a tundish was 10-30 degreeC.

  Molten steel components are C: 0.0005 to 0.0090 mass%, Si <0.05 mass%, Mn <0.50 mass%, P <0.035 mass%, S <0.020 mass%, Al: 0.005 to 0.060 mass%, Ti <0.080 mass%, It was set as the ultra-low carbon steel composition of Nb <0.050mass% and B <0.0030mass%.

  And each slab obtained by continuous casting is slab heated at 1100 to 1200 ° C. for 2 to 2.5 hours, and then subjected to hot rolling, cold rolling and finish annealing according to a conventional method ( The thickness of the surface defects was evaluated (surface thickness: 0.8 mm), and the surface quality was evaluated. In addition, the number of internal inclusions in the plate detected by slab surface layer defects and ultrasonic inclusion sensors was indexed to evaluate internal inclusions. In these surface quality evaluation and internal inclusion evaluation, the index is evaluated as 1 to 10, the best is 1 and the worst is 10, and the interval is linearly divided.

  The above evaluation results are also shown in Table 1.

  In Table 1, it can be seen that by applying the slab continuous casting method of the present invention, both the surface quality evaluation and the internal inclusion evaluation are evaluated to be close to the best evaluation. In particular, even when the throughput amount was large and high-speed casting was performed, it was confirmed that there were no surface defects and no internal defects in the obtained slab, and there were very few.

  That is, according to the present invention, it was confirmed that a high quality slab can be stably supplied.

It is a schematic diagram of the mold vicinity of the continuous casting line to which the present invention is applied, and shows a front view (a) and a side view (b). It is a schematic diagram of the continuous casting line which continuously casts a slab. It is a schematic diagram which shows a mode that molten steel flows in a casting_mold | template. It is a schematic diagram for demonstrating the short side width D of a casting_mold | template, and the discharge hole width d of a nozzle. It is a schematic diagram near the continuous casting line mold formed by applying another aspect of the present invention.

Explanation of symbols

1 Tundish 2 Molten steel 3 Nozzle
3a Discharge hole 4 Mold
4a Long side of mold
4b Short side of mold 5 Casting machine 6 Pinch roll 7 Bending roll 8 Straightening machine 9 Magnetic field application device
9a Ascending force
10 slab
11 Torch
12 Slab
19 Magnetic field application device D Short side width of mold d Nozzle discharge hole width V _c Casting speed
TP throughput

Claims (4)

This is a slab continuous casting method in which the flow of molten steel in the casting space in the mold of a continuous casting facility with a casting speed V _c of V _c > 1.5 m / min and a throughput amount TP of TP> 4 t / min is controlled by electromagnetic force. And
A magnetic field application device is provided on both short sides of the mold and at a position below the mold,
A slab continuous casting method, characterized in that an upward flow by electromagnetic force is generated upward in the molten steel by the magnetic field application device. 2. The slab continuous casting method according to claim 1, wherein in the continuous casting equipment, the casting speed V _c is V _c > 2.0 m / min, and the throughput amount TP is TP> 5 t / min. The relationship of the ratio D / d between the short side width D of the mold and the discharge hole width d of the nozzle,
3 ≧ D / d ≧ 1.5
The slab continuous casting method according to claim 1 or 2, wherein:   A magnetic field that applies a magnetic field over the entire width on the long side of the mold to the position including the level of the molten metal surface in the mold on the upper side and / or the position on the lower side with respect to the position of the discharge hole of the nozzle. The slab continuous casting method according to claim 3, further comprising an application device to control the flow of the molten steel.

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