DESCRIPTION
CONTINUOUS CASTING APPARATUS OF SLAB AND CONTINUOUS CASTING METHOD OF SAME
Technical Field
The present invention relates to a continuous casting apparatus injecting molten metal through an immersion nozzle into a continuous casting use mold to produce a slab and a continuous casting method of the same.
Background Art
In the past, in continuous casting of a slab, for the purpose of improving the product quality of the slab, the method has been employed of providing the continuous casting use mold (below, also simply called a "casting mold") with an electromagnetic stirrer and electromagnetically stirring the molten steel supplied through a discharge port of the immersion nozzle to the inside of the casting mold so as to generate a swirl flow in the casting mold.
For example, Japanese Patent Publication (A) No. 7-314104 discloses a method of setting an electromagnetic stirrer in the casting mold so that the position of the melt surface of the molten steel becomes in a range from the center of a core of the electromagnetic stirrer to a top end of the core and imparting flow to the molten steel in the casting mold for casting.
Further, Japanese Patent Publication (A) No. 2004-42062 discloses using an electromagnetic stirrer to impart flow to molten metal in a casting mold during which setting a position of a discharge port of an immersion nozzle at a position lower than a lower end position of an electromagnetic coil of the electromagnetic stirrer.
Note that, Japanese Patent Publication (A) No. 7-
256414 discloses, while for producing a casting comprised of a round billet (one type of bloom) different from a slab, a continuous casting apparatus where a moving magnetic field type electromagnetic coil is arranged around the entire outer circumference of the casting mold and an electromagnetic shield material is arranged below this electromagnetic coil.
Summary of Invention
However, the above prior art had the following problems which still remain to be solved.
In the method of Japanese Patent Publication (A) No. 7-314104, as shown in FIG. 7, at a location where the molten steel flow formed by the electromagnetic stirrer (below, also called the "stir flow") and the discharge flow from the immersion nozzle became reverse directions in heading, the flows interfered with each other, stagnation allowed air bubbles and inclusions to be taken into the cast slab, and, furthermore, powder became entrained in the cast slab due to fluctuations in the melt surface caused by disturbances in the flow of the molten steel in the interference region. For this reason, the cleanliness of the cast slab was liable to be degraded and a drop in product quality is invited.
Further, at a location where the molten steel flow and the discharge flow became the same forward direction in heading, an increase in the discharge flow of the immersion nozzle was invited, the depth of penetration of air bubbles and inclusions was increased, and obstacles were caused to their floating up, so air bubbles and inclusions were liable to stick to the solidifying shell at positions deep from the surface of the cast slab and cause product defects.
The above reverse direction and forward direction problems simultaneously occur in a casting mold and each becomes a cause of a drop in product quality. Further, with the method of Japanese Patent Publication (A) No.
2000-42062, a magnetic field is formed down to below the electromagnetic coil, so invites interference of the stir flow and discharge flow and an increase in the discharge flow. In the same way as above, this is liable to obstruct air bubbles and inclusions floating up for separation.
As the means for solving the problems of the above-mentioned Japanese Patent Publication (A) No. 7-314104 and Japanese Patent Publication (A) No. 2004-42062, the continuous casting apparatus disclosed in Japanese Patent Publication (A) No. 7-256414 may be considered, but Japanese Patent Publication (A) No. 7-256414 covers the production of a round billet. This has the differences shown below from the production of the slab covered by the present invention.
The casting mold for producing a round billet usually is a casting mold with an inside diameter of 300 mm or less or so in size. It is much smaller than a casting mold for producing a slab (for example, thickness: 120 to 300 mm or so, width: 800 to 1800 mm or so). The relationship between the flow of molten steel due to electromagnetic stirring in the casting mold and the discharge flow from the immersion nozzle is completely different.
That is, for the electromagnetic stirring flow in the casting mold for producing a round billet, the electromagnetic coil used for the electromagnetic stirring is set along the entire circumference of the casting mold wall surface and forms a swirl flow (see Japanese Patent Publication (A) No. 7-256414, FIG. 2). For this reason, in the discharge flow of molten steel from the immersion nozzle, the two problems of the interference and acceleration between the molten steel flow caused by electromagnetic stirring in the casting mold and the discharge flow from the immersion nozzle being invited do not arise. This is due to the fact that the discharge port of the immersion nozzle is arranged to
be directly below it as in Japanese Patent Publication (A) No. 7-256414.
Further, a casting mold for producing a round billet is configured with the electromagnetic coil arranged at the circumference of the casting mold. This completely differs in configuration from a facility for producing a slab such as where electromagnetic stirrers are arranged facing broad width long members (casting mold 2 surfaces).
Furthermore, a slab is used for thin plate materials. It is rolled to a large rate of reduction of thickness. Further, as represented by outer panels for automobiles, a high quality is required. Therefore, powder, air bubbles, inclusions, and other fine foreign matter in the cast slab lead to product defects. Therefore, compared to a round billet, the level of product quality demanded is extremely high.
Due to the above, in the continuous casting apparatus of Japanese Patent Publication (A) No. 7-256414, the problems of Japanese Patent Publication (A) No. 7-314104 and Japanese Patent Publication (A) No. 2004-42062 do not even arise. Not only that, their solution is not sought.
The present invention was made in consideration of this situation and has as its object the provision of a continuous casting apparatus of a slab suppressing disturbances in the flow of molten metal in a continuous casting use mold and enabling a good quality slab with few product flaws and a continuous casting method of the same.
The present invention was made to achieve the above object and is as follows:
(1) A continuous casting apparatus of a slab according to the first aspect of the invention along with the above object is a continuous casting apparatus provided with an immersion nozzle comprised of a tubular body forming a flow path for molten metal at the two side
directions of the bottom of which discharge ports are provided and having said discharge ports arranged with axial centers oriented within a range from the horizontal direction to 60° downward from the horizontal direction and with a continuous casting use mold having a rectangular cross-sectionally shaped space and provided with at least a pair of electromagnetic stirrers arranged facing broad width long members forming said space; and supplying molten metal into said continuous casting use mold through said discharge ports of said immersion nozzle and stirring said molten metal in said continuous casting use mold by said electromagnetic stirrers while allowing it to solidify to produce a slab, wherein an upper end position of a discharge port of said immersion nozzle being at a position of a lower end position of an electromagnetic stirrer or below, a magnetic shield plate for adjusting a magnetic field generated by an electromagnetic stirrer being provided at a position below said electromagnetic stirrer, and, when a thickness of a core of an electromagnetic stirrer in the height direction is h, a distance between a magnetic shield plate and said electromagnetic stirrer in the height direction being h/5 to h in range.
(2) A continuous casting apparatus of a slab according to the first aspect of the invention, wherein, preferably, an upper end position of a magnetic shield plate is made a position of an upper end position of a discharge port of said immersion nozzle or below and a lower end position of a magnetic shield plate is made a position of a lower end position of a discharge port of said immersion nozzle or below.
(3) A continuous casting apparatus of a slab according to the first aspect of the invention, wherein, preferably, a magnetic shield plate has a length in the height direction of 50 mm to 200 mm in range and has a thickness of 10 mm or more.
(4) A continuous casting apparatus of a slab
according to the first aspect of the invention, wherein, preferably, said immersion nozzle has a ratio (d/D) of an inside width d of said discharge ports and an inside width D of said immersion nozzle (d/D) set to 1.0 to 1.7 in range.
(5) A continuous casting method of a slab according to a second aspect of the invention in accordance with the above object uses a continuous casting apparatus of a slab according to the first aspect of the invention to produce said slab.
(6) A continuous casting method of a slab according to a second aspect of the invention, wherein, preferably, said slab casting velocity is 1.0 m/min or more.
Brief Description of the Drawings
FIG. 1 is a side cross-sectional view of a continuous casting use mold used for a continuous casting apparatus of a slab according to an embodiment of the present invention.
FIG. 2 is a plan view of a continuous casting use mold of a slab of FIG. 1.
FIG. 3(A) is an explanatory view of the relative positional relationship among an electromagnetic stirrer, immersion nozzle, and magnetic shield plate and of a flow of molten steel in the casting mold.
FIG. 3(B) is an explanatory view showing the effects of a distance between an electromagnetic stirrer and a magnetic shield plate on a molten steel flow in the casting mold.
FIG. 4(A) is an explanatory view of the relative positional relationship among an electromagnetic stirrer, immersion nozzle, and magnetic shield plate and of a flow of molten steel in the casting mold.
FIG. 4(B) is an explanatory view showing the effects of a relative position between a discharge port of the immersion nozzle and a magnetic shield plate on a molten steel flow in the casting mold.
FIG. 5(A) is an explanatory view of the relative positional relationship among an electromagnetic stirrer, immersion nozzle, and magnetic shield plate and of a flow of molten steel in the casting mold.
FIG. 5(B) is an explanatory view showing the effects of a length of a magnetic shield plate on electromagnetic force.
FIG. 6(A) is an explanatory view of a flow of molten steel in the casting mold.
FIG. 6(B) is an explanatory view showing the effects of various changes in a distance between an electromagnetic stirrer and a magnetic shield plate and the casting velocity on a molten steel flow in the casting mold.
FIG. 7 is an explanatory view showing a flow of molten steel in a continuous casting use mold according to the prior art.
Best Mode for Carrying Out the Invention
Below, while referring to the attached drawings, embodiments of the present invention will be explained for use in understanding the present invention.
Here, FIG. 1 is a side cross-sectional view of a continuous casting use mold used for a continuous casting apparatus of a slab according to an embodiment of the present invention, while FIG. 2 is a plan view of that continuous casting use mold of a slab.
As shown in FIG. 1 and FIG. 2, a continuous casting apparatus of a slab according to an embodiment of the present invention (below, also simply called a "continuous casting apparatus") 10 is provided with an immersion nozzle 11 and a continuous casting use mold (below, also simply called a "casting mold") 12. It supplies molten steel (one example of molten metal) through discharge ports 13 of the immersion nozzle 11 into the casting mold 12 and stirs the molten steel 14 in the casting mold 12 using electromagnetic stirrers 15
provided at the casting mold 12 while causing it to solidify to produce a slab. The casting mold 12 is provided with magnetic shield plates 16 for adjusting the magnetic field generated by the electromagnetic stirrers 15 at positions below the electromagnetic stirrers 15. Below, this will be explained in detail.
The immersion nozzle 11 has a tubular body 18 provided at a bottom of a tundish (not shown) holding molten steel and forming a flow path 17 of the molten metal. This tubular body 18 at provided with discharge ports 13 sideways at the two sides at its bottom. Here, the flow path is circular in cross-section (or elliptical in cross-section), but the discharge ports may, for example, be circular in cross-section, elliptical in cross-section, rectangular in cross-section (square or rectangular), or polygonal in cross-section as well. One or more may be formed at each side of the tubular body. At this time, when a discharge port is circular in cross-sectional shape, the "inside width" is its diameter, while when it is another shape, the value is the diameter when converting the cross-sectional area to the area of a circle. Further, when there are a plurality of discharge ports, the value is the diameter when adding up all of the cross-sectional areas and converting them to the area of a circle.
Further, the discharge ports 13 are oriented so that their axial centers are in a range from the horizontal direction, that is, 0° (preferably 15° downward from the horizontal direction) to 60° (preferably 40°) downward from the horizontal direction in range. Here, if orienting the discharge ports so that their axial centers are within a region over 60° downward from the horizontal direction, that is, orienting them downward (including straight down), inclusions and air bubbles will invade the inside of the slab and internal defects will be formed. Further, if orienting the discharge ports so that
their axial centers are upward from the horizontal direction, a strong rising flow will be formed and entrainment of powder will be abetted.
The ratio (d/D) of the inside width d of the discharge ports 13 and the inside width D of the tubular body 18 of the immersion nozzle 11 forming the flow path 17 is preferably set to 1.0 to 1.7 in range. Note that, the inside width D of the tubular body 18 is, for example, 50 mm to 90 mm (here, 70 mm) or so. Here, if the ratio (d/D) is less than 1.0, the inside width D of the tubular body 18 will be larger than the inside width d of the discharge ports 13 and the discharge flow from the discharge ports 13 will become too fast in flow rate, so the effect of the stir flow of the electromagnetic stirrers 15 will be harder to obtain. For this reason, further improvement of the product quality of the slab will not be able to be expected.
On the other hand, if the ratio (d/D) is over 1.7, the inside width D of the tubular body 18 will be smaller than the inside width d of the discharge ports 13 and the discharge flow from the discharge ports 13 will become slower in flow rate, so the striking effects of the magnetic shield plates 16 will be harder to obtain.
Due to the above, the ratio (d/D) of the inside width d of the discharge ports 13 and the inside width D of the tubular body 18 was made 1.0 to 1.7 in range, but making the upper limit 1.5, furthermore 1.3, is preferable.
The continuous casting use mold 12 has a pair of narrow width short members 19, 20 arranged facing each other across a distance in the horizontal direction and a pair of broad width long members 21, 22 arrange facing each other so as to sandwich the short members 19, 20 between them. Note that, the short members 19, 20 and the long members 21, 22 are known and are, for example, formed by cooling plates made of copper or a copper alloy for contacting the molten steel and back plates fastened
to their backs and run through by cooling water.
Due to this, inside them, a space 23 of a rectangular cross-sectional shape in the horizontal direction is formed. Note that, this space 23 has a length of the short sides in the horizontal cross-section of, for example, 120 to 300 mm or so and a length of the long sides of, for example, 800 to 1800 mm or so. The short members 19, 20 are made to slide with respect to the long members 21, 22. The distance between them may be changed or may be fixed.
At the upper sides of the long members 21, 22 (in more detail, in the back plates), known electromagnetic stirrers 15 are provided.
Each electromagnetic stirrer 15 is comprised of a core 24 made of a large number of electrical steel plates stacked together and an electromagnetic coil 25 wound around them arranged in a metal (for example, stainless steel) casing (not shown). Therefore, the "bottom end" of an electromagnetic stirrer 15 means the bottom end of the casing. Note that, in FIG. 1, the casing of the electromagnetic stirrer is not shown, so it appears like the bottom end of the electromagnetic coil is shown, but, as explained above, the bottom end of the electromagnetic stirrer is the bottom end of the casing. At least one electromagnetic stirrer 15 should be provided at each of the long members 21, 22 (that is, one pair). Here, when providing two or more electromagnetic stirrers (that is, two pairs or more), they are arranged in the width directions of the long members (that is, long side directions of long members, long side directions in horizontal cross-section of space).
Note that, the core 24 of each electromagnetic stirrer 15 has a thickness h in the height direction of, for example, 100 mm to 300 mm (here, 200 mm) or so.
Inside the space 23 of this continuous casting use mold 12, the immersion nozzle 11 is arranged so that its discharge ports 13 face the short members 19, 20 (at this
time, the upper end positions of the discharge ports 13 of the immersion nozzle 11 are arranged at positions of the lower end positions of the electromagnetic stirrers 15 or below). Through the discharge ports 13 of this immersion nozzle 11, the casting mold 12 is supplied with molten steel 14. The molten steel in the casting mold 12 is stirred by the electromagnetic stirrers 15. Due to this, inside the casting mold 12, a molten steel flow, that is, stir flow, is formed clockwise or counterclockwise around the immersion nozzle 11 and the molten steel is solidified to form a slab.
However, when producing a slab in this way, as shown in FIG. 7, problems arise due to the effects of the interference of the molten steel flow caused by the formation of the magnetic fields down to below the electromagnetic coils 25, that is, the stir flow, and the discharge flow from the discharge ports 13 of the immersion nozzle 11 and the acceleration of the discharge flow accompanying the stir flow. For this reason, slabs are produced with many product flaws and poor product quality.
Therefore, to proactively reduce these effects, magnetic shield plates 16 of the same length or more in the width direction as the electromagnetic stirrers 15 are arranged at positions below the electromagnetic stirrers 15 and their set positions are optimized to produce a good cast slab with few product flaws.
Each magnetic shield plate 16, for example, may be formed by electrical steel plate not passing a magnetic field, but may also be made of iron or general carbon steel. Note that, when made of iron or general carbon steel, it will generate heat by induction heating of an electromagnetic stirrer, so is made a water-cooled structure.
Note that the reason why the upper end positions of the discharge ports 13 of the immersion nozzle 11 are arranged at positions of the lower end positions of the
electromagnetic stirrers 15 or below is as follows:
The discharge ports are arranged so that their axial centers are oriented from the horizontal to 60° downward from the horizontal in range, so if setting the upper end positions of the discharge ports 13 above the lower end positions of the electromagnetic stirrers 15, even if setting the magnetic shield plates, difficult to suppress inference and acceleration would occur.
The magnetic shield plates 16, when the thickness of the cores 24 of the electromagnetic stirrers 15 in the height direction is h, are set at the long members 21, 22 so that the distance s between the magnetic shield plates 16 and the electromagnetic stirrers 15 in the height direction, that is, the distance between the top ends of the magnetic shield plates 16 and the bottom ends of the casings of the electromagnetic stirrers in the height direction (same below), becomes h/5 (below, also expressed as l/5h) to h in range. At this time, the magnetic shield plates 16, while depending also on the thickness of the cooling plates forming the long members 21, 22, are set in the back plates 50 mm to 100 mm in range from the surfaces 26, 27 of the long members 21, 22 contacting the molten steel.
Here, when the distance s between the magnetic shield plates 16 and electromagnetic stirrers 15 in the height direction is less than h/5, the distance between the magnetic shield plates 16 and the electromagnetic stirrers 15, that is, the distance from the cores 24, becomes too short. The required stirring force in the area where stirring is required is reduced by the magnetic shield plates 16 and the targeted product quality cannot be secured.
On the other hand, when the distance s is over h, the distance between the magnetic shield plates 16 and the electromagnetic stirrers 15 in the height direction becomes too great. While the required stirring force can be secured, interference of the discharge flow and stir
flow and acceleration of the discharge flow cannot be prevented and again the targeted product quality cannot be secured.
Due to the above, the distance s between the magnetic shield plates 16 and the electromagnetic stirrers 15 in the height direction was made h/5 to h, but the upper limit is preferably made 4h/5, more preferably 3h/5.
The magnetic shield plates 16 are preferably made ones of a length x in the height direction of 50 mm to 200 mm in range and a thickness of 10 mm or more. Further, the length of the magnetic shield plates 16 in the width direction is preferably made equal to or more than the width direction length of the electromagnetic stirrers 15.
Here, when the length x of the magnetic shield plates in the height direction is less than 50 mm, the residual effect of the magnetic field leaking to below the magnetic shield plates becomes larger. On the other hand, when the length x of a magnetic shield plates in the height direction exceeds 200 mm, the magnetic field leaking from below the magnetic shield plates becomes smaller and the effective of improvement of the stir flow by the magnetic shield plates becomes lower in level. Due to the above, the length x of the magnetic shield plates in the height direction was set to 50 mm to 200 mm in range, but the lower limit is preferably made 70 mm and the upper limit is preferably made 170 mm, more preferably 150 mm.
Further, by making the thickness of the magnetic shield plates 10 mm (preferably 20 mm) or more, the magnetic fields generated from the electromagnetic stirrers 15 can be adjusted, so the upper limit value is not defined, but, for example, considering the work efficiency when attaching these to the long members 21, 22 and economy, 100 mm or less is preferable.
Furthermore, regarding the set positions of the
magnetic shield plates 16 in the height direction, it is preferable to make the upper end positions of the magnetic shield plates 16 positions of the upper end positions of the discharge ports 13 of the immersion nozzle 11 or below and to make the lower end positions of the magnetic shield plates 16 positions of the lower end positions of the discharge ports 13 of the immersion nozzle 11 or below. Note that, the length x of the magnetic shield plates 16 in the height direction is made longer than the inside width d of the discharge ports 13.
Here, when making the upper end positions of the magnetic shield plates positions above the upper end positions of the discharge ports, that is, when arranging the magnetic shield plates at positions above the upper ends of the discharge ports, the magnetic shield plates are set at regions where the flow from the discharge ports of the immersion nozzle does not directly act and the problems of interference with and acceleration of the stir flow do not arise, but the areas stirred by the electromagnetic stirrers arranged with a predetermined distance from the top ends of the magnetic shield plates conversely end up being reduced and conversely the surface quality of the slab ends up being degraded.
Further, as the method for reducing the detrimental effects of the discharge flow and stir flow when positioning the lower end positions of the magnetic shield plates above the lower end positions of the discharge ports, increasing the depth of immersion of the immersion nozzle into the casting mold may be considered, but in this case it is necessary to excessively increase the length of the tubular body of the immersion nozzle. Not only does the preparatory work relating to the immersion nozzle for casting become impractical, but also problems such as interference with other equipment arise.
Due to the above, the upper end positions of the magnetic shield plates were made the upper end positions of the discharge ports of the immersion nozzle or below
and the lower end positions of the magnetic shield plates were made the lower end positions of the discharge ports or below, but the upper end positions of the magnetic shield plates are preferably set extremely close to the upper end positions of the discharge ports of the immersion nozzle.
Next, a continuous casting method of a slab according to an embodiment of the present invention will be explained.
When producing a slab, a tundish (not shown) is supplied with molten steel. This tundish supplies the molten steel through the immersion nozzle 11 to the continuous casting use mold 12. The molten steel 14 inside the continuous casting use mold 12 is made to solidify while stirring it by the electromagnetic stirrers 15 and the produced slab is delivered to the downstream side.
At this time, the slab casting velocity (pullout velocity) is usually 0.8 m/min or more, but to remarkably obtain the effects of the present invention, 1.0 m/min or more, preferably 1.2 m/min or more, more preferably 1.4 m/min or more, is preferable. Due to this, the slab production efficiency can be improved over the past.
Note that, the upper limit value of the slab casting velocity is not prescribed, but the currently possible upper limit value is, for example, 2.5 m/min or so.
Examples
Next, examples used for confirming the action and effect of the present invention will be explained.
First, the effect of the distance s between an electromagnetic stirrer and magnetic shield plate in the height direction on the flow of molten steel in the casting mold will be explained with reference to FIG. 3(A) and FIG. 3(B). This FIG. 3(A) shows the relative positional relationship of an electromagnetic stirrer, immersion nozzle, and magnetic shield plate and the flow
of molten steel in the casting mold at that time. Here, the upper end positions of the discharge ports of the immersion nozzle and the lower end position of the electromagnetic stirrer (bottom end of casing) are made to match. Further, FIG. 3(B) shows the evaluation of cleanliness of the slab produced at this time. This "cleanliness evaluation" evaluates the number of defects of the slab after casting (for example, inclusions, powder, air bubbles, etc.). In more detail, samples (30 mmx30 mm) were prepared for every 1 mm from the slab surface, polished, then divided longitudinally and laterally into 30 to secure 900 1 mmxl mm inspection areas. These 900 inspection areas were observed under an optical microscope to count the number of defects. A value proportional to the number of defects (/cm^) is shown. That is, if the cleanliness evaluation number is high, it means the product quality is poor, while conversely if low, it means the product quality is good (same below)
Note that, the test conditions were made arrangement of the upper end positions of the discharge ports of the immersion nozzle at the lower end position of the casing of the electromagnetic stirrer, an inside width D of the tubular body of 70 mm, a d/D of 1.0, discharge ports arranged with axial centers horizontal (0°), a length x of the magnetic shield plates in the height direction of 100 mm, a thickness fixed to 30 mm, and a slab casting velocity Vc of 1.4 m/min.
As clear from FIG. 3(A), it is learned that along with the narrowing of the distance s between an electromagnetic stirrer and a magnetic shield plate in the height direction (movement of the magnetic shield plate upward), the flow of the molten steel changes. Further, it is learned that along with the broadening of the distance s in the height direction (movement of the magnetic shield plate downward), the change in the flow
of the molten steel becomes smaller (relationship between solid lines and broken lines in FIG. 3(A)).
More particularly, as clear from FIG. 3(B), when narrowing the distance s between an electromagnetic stirrer and a magnetic shield plate in the height direction (moving the magnetic shield plate upward) and making it a distance of less than 1/5 of the core thickness h, the flow rate at the front of the electromagnetic stirrer is also affected and the necessary stirring force can no longer be given.
Next, when making the distance s between an electromagnetic stirrer and a magnetic shield plate in the height direction h/5 to h, the necessary stirring force can be obtained at the front of the electromagnetic stirrer and the stirring force below the electromagnetic stirrer can be reduced and therefore interference with and acceleration of the discharge flow from the discharge port of the immersion nozzle can be prevented. Finally, when making the distance s between an electromagnetic stirrer and a magnetic shield plate in the height direction more than the core thickness h, the stirring force below the electromagnetic stirrer cannot be reduced and interference with and acceleration of the discharge flow from the discharge port of the immersion nozzle cannot be prevented.
That is, it was confirmed that by providing magnetic shield plates at positions below the electromagnetic stirrers and setting the distance s between the electromagnetic stirrers and magnetic shield plates in the height direction the above suitable range, it is possible to perform pinpoint stirring of the necessary locations by the necessary stirring force or more required for improving the cleanliness of the slab surface having an effect on the product quality and possible to improve the cleanliness evaluation.
Next, the effect of the relative positions of a magnetic shield plate and the discharge ports of the
immersion nozzle on the molten steel flow in the casting mold will be explained with reference to FIG. 4(A) and FIG. 4(B). Here, FIG. 4(A) shows the relative positional relationship among an electromagnetic stirrer, immersion nozzle, and magnetic shield plate and the flow of molten steel in the casting mold at that time, while FIG. 4(B) shows the cleanliness evaluation of a slab produced at that time.
Note that, the test conditions were made a distance s between the electromagnetic stirrer and magnetic shield plate in the height direction of 2/5h, an inside width D of the tubular body of 70 mm, a d/D of 1.0, discharge ports arranged with axial centers horizontal (0°) , a length x of the magnetic shield plate in the height direction of 100 mm, a thickness fixed to 30 mm, and a slab casting velocity Vc of 1.4 m/min.
As clear from FIG. 4(A), in the case of Example 1 shown in FIG. 4(B) where the upper end position of the magnetic shield plate is arranged below the upper end position of the discharge ports of the immersion nozzle (top end of discharge ports at position 40 mm above top end of magnetic shield plate), interference of the discharge flow and stir flow and acceleration of the discharge flow by the stir flow partially occurs. The cleanliness evaluation can be made less than the cleanliness evaluation of 3 in the case of no magnetic shield plate, but deposits (for example inclusions or reaction products) build up at the discharge ports of the immersion nozzle, whereby the axial centers of the discharge ports change and therefore the cleanliness evaluation becomes unstable.
Further, as clear from FIG. 4(A), in the case of Example 2 shown in FIG. 4(B) where the upper end position of the magnetic shield plate is arranged at the upper end position of the discharge port of the immersion nozzle, it was confirmed that interference of the discharge flow and stir flow and acceleration of the discharge flow by
the stir flow can be prevented and the cleanliness evaluation can be improved.
Furthermore, as clear from FIG. 4(A), in the case of Example 3 shown in FIG. 4(B) where the lower end position of the magnetic shield plate is made a position 40 mm above the lower end position of the discharge port, it was confirmed that interference of the discharge flow and stir flow and acceleration of the discharge flow by the stir flow can be prevented and the cleanliness evaluation can be improved, but there are the following problems.
Usually, the immersion depth of an immersion nozzle is 200 to 300 mm or so, but in Example 3, the immersion depth of the immersion nozzle becomes 400 to 500 mm or more. As a result, the length from the top end of the casting mold to the tip of the immersion nozzle becomes 600 to 700 mm. For this reason, the immersion nozzle becomes extremely heavy in weight. For starting and ending continuous casting operation in the state attaching the immersion nozzle to a tundish, in order to prevent collision between the immersion nozzle and the casting mold and other nearby devices, it becomes necessary to transfer them by taking the ascending and descending stroke larger and excessively raising and avoiding the immersion nozzle, so this is not practical for actual operations.
Next, the effects of the length x of a magnetic shield plate in the height direction on the molten steel flow in the casting mold will be explained while referring to FIG. 5(A) and FIG. 5(B). Here, FIG. 5(A) shows the relative positional relationship among an electromagnetic stirrer, immersion nozzle, and magnetic shield plate and the flow of molten steel in the casting mold at that time, while FIG. 5(B) shows the electromagnetic force at that time. Note that, the electromagnetic force of the ordinate shown in FIG. 5(B) shows the attenuation of the electromagnetic force when making the electromagnetic force acting on an area deeper
than the upper end position of the discharge port of the immersion nozzle in the case of no magnetic shield plate shown in FIG. 5(A) (broken line in figure) "1.0" (electromagnetic force causing stir flow in area of hatched part in FIG. 5(A)) and changing the length x of the magnetic shield plate in the height direction. Further, the test conditions were made a distance s between the electromagnetic stirrer and magnetic shield plate in the height direction of 2/5h and a thickness of the magnetic shield plate fixed to 10 mm.
As clear from FIG. 5(B), it was confirmed that along with the ncreased length x of the magnetic shield plate in the height direction, the electromagnetic force becomes smaller. In particular, it was confirmed that by-making the length of the magnetic shield plate in the height direction 50 mm to 200 mm in range, it is possible to economically obtain the effect by the magnetic shield plate while preventing interference of the discharge flow and stir flow and acceleration of the discharge flow by the stir flow.
Next, the effects of the slab casting velocity on the molten steel flow in the casting mold will be explained with reference to FIG. 6(A) and FIG. 6(B). Here, FIG. 6(A) shows the flow of molten steel in the casting mold, while (B) shows the cleanliness evaluation of the slab produced when variously changing the distance between the electromagnetic stirrer and the magnetic shield plate in the height direction and the casting velocity.
Note that, the test conditions were made arrangement of the upper end position of the discharge ports of the immersion nozzle at the lower end position of the electromagnetic stirrer, an inside width D of the tubular body of 70 mm, a d/D of 1.0, discharge ports arranged with axial centers horizontal (0°), a length of the magnetic shield plate in the height direction of 100 mm, and a thickness fixed to 30 mm.
As shown in FIG. 6(A), the discharge flow from the discharge port of one side of the immersion nozzle increases along with an increase of the casting velocity. As a result, in the area where the stir flow and discharge flow interfere, the flow rate fluctuates and, for example, powder is entrained and disturbance of the melt surface is abetted and product defects due to the steelmaking increase.
Further, the discharge flow from the discharge port at the other side of the immersion nozzle also increases along with the increase in the casting velocity. As a result, in the acceleration area, the depth of penetration of inclusions becomes much deeper (flotation effect can no longer be obtained), and product flaws due to the steelmaking increase.
From the above and the results shown in FIG. 6(B), it could be confirmed that when making the distance s between an electromagnetic stirrer and a magnetic shield plate in the height direction l/5h to h in range , even if raising the slab casting velocity, it is possible to obtain the requiring stirring force by the electromagnetic stirrer while preventing interference of the discharge flow and stir flow and acceleration of the discharge flow by the stir flow and to improve the cleanliness evaluation.
In particular, by raising the slab casting velocity to 1.0 m/min, 1.4 m/min, and furthermore 1.6 m/min, results were obtained where the advantageous effects of the present invention appeared more remarkably.
Furthermore, the inventors experimented with the ratio (d/D) of the inside width d of the discharge port of the immersion nozzle and the inside width D of the flow path whereupon they confirmed that by making the ratio (d/D) 1.0 to 1.7 in range, a more superior advantageous effect is obtained by the magnetic shield plate.
Due to the above, it was confirmed by according to
the present invention, it is possible to suppress disturbances in flow of the molten steel in a continuous casting use mold and produce a good quality slab with few product defects.
Above, the present invention was explained with reference to embodiments, but the present invention is not limited in any way to the configurations described in the embodiments explained above and includes other embodiments and modifications considered within the range of matters described in the claims. For example, combinations of all or part of the above embodiments or modifications to form continuous casting apparatuses of slabs and their continuous casting methods of the present invention are also included in the scope of the present invention.
Industrial Applicability
The continuous casting apparatus of slabs described in (1) to (4) of the present invention and the continuous casting methods of slabs described in (5) and (6) provide magnetic shield plates at predetermined distances in the height direction below the electromagnetic stirrers provided at long members of the continuous casting use mold, so the effects of interference between the flow of molten metal, that is, the stir flow, and the discharge flow from the immersion nozzle and acceleration of the flow rate of the discharge flow can be lightened. Due to this, it is possible to suppress disturbances in the flow of molten metal in the continuous casting use mold and produce a good quality slab with few product flaws.
In particular, the continuous casting apparatus of a slab described in (2) defines the position of setting of the magnetic shield plates with respect to the discharge ports of the immersion nozzle, so the effects of interference between the stir flow and discharge flow and acceleration of the flow rate of the discharge flow can be further lightened.
The continuous casting apparatus of a slab described in (3) defines the length in the height direction and thickness of the magnetic shield plate, so it is possible to further improve the effect of suppressing a leakage magnetic field from the magnetic shield plate and suppressing disturbances in the flow of molten metal in the continuous casting use mold due to the magnetic shield plates.
The continuous casting apparatus of a slab described in (4) defines the ratio (d/D) between the inside width D of the immersion nozzle and the inside width d of a discharge port so can suppress excessive velocity up of the velocity of discharge flow from the discharge ports and stably supply molten metal from the discharge ports to the inside of the casting mold. Due to this, it is possible to mitigate the effects of the conventionally occurring interference between the stir flow and discharge flow and acceleration of flow rate of the discharge flow and possible to suppress disturbances in the flow of molten metal in the continuous casting use mold and produce a good quality slab with few product flaws.
The continuous casting method of a slab described in (6) makes the slab casting velocity 1.0 m/min or more and thereby enables disturbances in the flow of molten metal in the continuous casting use mold to be suppressed even at a casting velocity where the effects of interference between the stir flow and discharge flow and acceleration of the flow rate of the discharge flow remarkably occur. Due to this, it is possible to produce a good quality slab with few product flaws while improving the production efficiency over the past.

CLAIMS
1. A continuous casting apparatus of a slab provided with an immersion nozzle comprised of a tubular body forming a flow path for molten metal at the two side directions of the bottom of which discharge ports are provided and having said discharge ports arranged with axial centers oriented within a range from the horizontal direction to 60° downward from the horizontal direction and with a continuous casting use mold having a rectangular cross-sectionally shaped space and provided with at least a pair of electromagnetic stirrers arranged facing broad width long members forming said space; and supplying molten metal into said continuous casting use mold through said discharge ports of said immersion nozzle and stirring said molten metal in said continuous casting use mold by said electromagnetic stirrers while allowing it to solidify to produce a slab, wherein an upper end position of a discharge port of said immersion nozzle being at a position of a lower end position of an electromagnetic stirrer or below, a magnetic shield plate for adjusting a magnetic field generated by an electromagnetic stirrer being provided at a position below said electromagnetic stirrer, and, when a thickness of a core of an electromagnetic stirrer in the height direction is h, a distance between a magnetic shield plate and said electromagnetic stirrer in the height direction being h/5 to h in range.
2. A continuous casting apparatus of a slab as set forth in claim 1, wherein an upper end position of a magnetic shield plate is made a position of an upper end position of a discharge port of said immersion nozzle or below and a lower end position of a magnetic shield plate is made a position of a lower end position of a discharge port of said immersion nozzle or below.
3. A continuous casting apparatus of a slab as set forth in claim 1 or 2, wherein a magnetic shield plate has a length in the height direction of 50 mm to 200 mm
in range and has a thickness of 10 mm or more.
4. A continuous casting apparatus of a slab as set forth in any one of claims 1 to 3, wherein said immersion nozzle has a ratio (d/D) of an inside width d of said discharge ports and an inside width D of said immersion nozzle (d/D) set to 1.0 to 1.7 in range.
5. A continuous casting method of a slab which uses a continuous casting apparatus of a slab as set forth in any one of claims ,1 to 4 to produce said slab.
6. A continuous casting method of a slab as set forth in claim 5, wherein a casting velocity of said slab is 1.0 m/min or more.