JP5831124B2 - Manufacturing method of high cleanliness steel slab by continuous casting - Google Patents

Description

  The present invention relates to a method for producing a high cleanliness steel slab by continuous casting. More specifically, the tundish promotes the floating separation of oxide-based non-metallic inclusions such as deoxidation products to improve the cleanliness of molten steel. It relates to how to increase.

  In continuous casting of steel, the molten steel in the ladle is once poured into the tundish, and with a predetermined amount of molten steel retained in the tundish, the molten steel is poured into the mold from the tundish to produce a slab. ing. The tundish not only has the function of supplying molten steel at the time of ladle replacement when continuing continuous casting of multiple heats and the function of distributing molten steel to multiple molds, but also provides a predetermined amount of molten steel in the tundish. The amount of molten steel flowing out of the tundish from the mold to the mold is accurately controlled by the retention, and further, oxide-based nonmetallic inclusions such as deoxidation products suspended in the molten steel (hereinafter simply referred to as “inclusions”). )), And the like. In particular, due to the recent demand for high-quality steel materials, a technique for efficiently levitating and separating inclusions in tundish is widely used.

  As a method for floating and separating inclusions in a tundish, a method is generally used in which a weir is installed in the tundish and the flow of molten steel is controlled by the weir. For example, in Patent Document 1, a weir that has a through hole in the lower part and extends from the bottom of the tundish to the surface of the molten steel in the tundish is placed inside the tundish with the molten steel injection site from the ladle interposed therebetween. There is disclosed a tundish that is disposed so as to be opposed to two locations, and the inside of the tundish is separated into a steel receiving region and a steel quasi-static region, and the inclusions are separated and floated in the steel quasi-static region.

  In Patent Document 2, the inside of the tundish is separated into the receiving steel side and the outgoing steel side by a weir having two through holes in contact with the bottom of the tundish, and a dam-like weir ( And the ratio L / W between the long side length L and the short side length W of the tundish is 2 to 7, and the volume ratio on the steel receiving side is 10 to 40% of the whole. A tundish is disclosed.

  Patent Document 3 discloses a tundish collision pad formed from a heat-resistant composition, the pad having a collision surface, and extending upward from the base and receiving the flow of the molten metal. An endless outer side wall that completely surrounds the internal space with the upper opening, and the outer side wall has an annular shape with at least a first portion extending inward and upward toward the opening A tundish impact pad including an inner surface is disclosed.

  A technique for improving the technique of Patent Document 3 has also been proposed. Patent Document 4 describes the flow of molten metal in a tundish that is installed in a portion where a molten metal flow injected from a ladle collides with the bottom of the tundish. A control pad having a wall portion extending upward from the bottom of the tundish surrounding the collision portion of the molten metal flow, and a hook-shaped portion extending from the upper end portion of the wall portion toward the surrounding wall center. The flow control pad which has a notch in the wall part on the side facing the long side inner wall of the tundish is disclosed.

  Further, in Patent Document 5, since the collision pad of Patent Document 3 is a refractory having an integral structure, a tank is formed relative to the molten metal flow from the ladle to the tundish so as to be a weir instead of the collision pad. A flow control weir having a wall portion extending upward from the bottom of the dish and a hook-like portion extending from the upper end portion of the wall portion toward the molten metal flow, wherein the wall portion has a height h and a hook-like shape. A weir is disclosed in which the width d of the portion satisfies the relational expression of 0.1 ≦ d / h ≦ 1.0.

Further, Patent Document 6 discloses that a portion of the molten steel flow from the ladle to the tundish collides with the bottom of the tundish, a wall portion surrounding the collision portion of the molten steel flow and extending upward from the bottom of the tundish; Using a tundish in which a flow control pad having a bowl-shaped portion extending from the upper end portion of the section toward the wall enclosing center is disposed, the molten steel injection rate q (m ³ / min) and the bowl-shaped portion are excluded. The relational expression that the area A1 (m ² ) of the top surface of the flow control pad and the area A2 (m ² ) of the bottom surface of the flow control pad is 0.5 <(q / A2) × (A1 / A2) <5.0. A method of manufacturing a highly clean steel slab that is continuously cast under satisfactory conditions is disclosed.

JP-A-53-6231 Japanese Patent Laid-Open No. 10-216909 JP-T 9-505242 JP 2004-1077 A JP 2004-98066 A JP 2004-154803 A

  By patent documents 1-6, the floating separation of the inclusion in a tundish was improved significantly, and the cleanliness of the molten steel improved significantly compared with the case where a weir is not installed. In particular, in Patent Documents 3 to 6, “an annular inner surface extending inward and upward toward the opening portion” or “a hook-shaped portion extending from the upper end portion of the wall portion toward the surrounding center of the wall portion”. The molten steel injection flow from the ladle to the tundish is agitated so as to return to the molten steel injection site side, so that the molten steel injection flow is decelerated and the floating separation of inclusions is hindered. In addition, the high-speed flow is eliminated and the inclusions are lifted.

  However, Patent Documents 3 to 6 still have room for improvement. That is, taking Patent Document 5 as an example, the molten steel injection flow from the ladle changes the direction of the flow by colliding with the wall portion extending upward from the bottom of the tundish, and further exists on the wall portion. It is stirred so as to return to the molten steel injection site side by a bowl-shaped part extending from the upper end part of the wall toward the wall enclosure center, but the upper opening area of the weir, the opening area of the notch provided in the wall part, and the bowl-shaped part The volume of the space surrounded by the weir and the planar shape of the weir are not suitable shapes according to the molten steel injection flow rate from the ladle to the tundish, and the molten steel injection flow rate and injection rate from the tundish to the mold However, the molten steel injection flow from the ladle cannot be uniformly decelerated, that is, the effect of the weir cannot be sufficiently obtained, and it is not expected to promote the floating separation of inclusions in the tundish.

  The present invention has been made in view of such circumstances, and an object thereof is to provide a wall portion extending upward from the bottom of the tundish between the molten steel injection site of the tundish and the molten steel outlet, and the wall. When continuously casting using a tundish provided with a weir having a bowl-like part that faces the molten steel injection site side and projecting in the horizontal direction at the upper end part of the part, the shape of the weir having the bowl-like part is taken. By optimizing according to the flow rate of molten steel injected from the pan to the tundish and the flow rate and speed of molten steel injected from the tundish to the mold, the flotation separation of inclusions is ensured and more effective than before. It is possible to provide a method for producing a high cleanliness steel slab by continuous casting, which can be performed and, as a result, can significantly reduce product defects due to inclusions.

The gist of the present invention for solving the above problems is as follows.
(1) When deoxidized molten steel is once poured into a tundish from a ladle and then poured into a mold from the tundish to continuously cast a steel slab,
Between the molten steel injection site where the molten steel injection flow from the ladle collides with the bottom of the tundish and the molten steel outlet from the tundish to the mold, the molten steel injection site is surrounded from four directions and upward from the bottom of the tundish. A weir having an extending wall portion and a hook-like portion protruding in the horizontal direction facing the molten steel injection portion side at an upper end portion of the wall portion, and continuous from the wall portion to the hook-like portion Use a tundish with one or more cutouts and a weir with a rectangular outer shape,
For the upper opening area of the weir, the opening area of the notch, the volume of the space surrounded by the weir, the long side length of the rectangular outer shape of the weir and the short side length of the rectangular outer shape of the weir. In order that the injection flow rate of molten steel from the pan to the tundish, the injection flow rate of molten steel from the tundish to the mold and the injection rate of molten steel from the tundish to the mold satisfy the relationship of the following formula (1):
A method for producing a high cleanliness steel slab by continuous casting, wherein the steel slab is continuously cast while controlling the molten steel injection from the ladle to the tundish and the molten steel injection from the tundish to the mold.
3.0 ≦ (m × u) × (S1 / Q) × (1 / V) × (X / Y) × (1 + S2) ≦ 50 (1)
In Equation (1), m is the flow rate of molten steel injected from the tundish to the mold (ton / second), u is the injection rate of molten steel from the tundish to the mold (m / second), and Q is the intake flow rate. The flow rate of molten steel from the pan to the tundish (m ³ / sec), S1 is the upper opening area (m ² ) of the weir with the bowl-shaped part, S2 is the opening area (m ² ) of the notch, V Is the volume (m ³ ) of the space surrounded by the weir having the hook-shaped part, X is the long side length (m) of the rectangular outer shape of the weir, Y is the short side length (m) of the rectangular outer shape of the weir It is.
(2) Molten steel injection from the ladle to the tundish is performed through a long nozzle that is installed at the bottom of the ladle and the lower end is immersed in the molten steel in the tundish, and around the long nozzle The method for producing a high cleanliness steel slab by continuous casting according to (1) above, wherein the surface flow velocity of the molten steel in tundish is controlled to be within the range of the following formula (2): .
15 ≦ u _S ≦ 50 (2)
However, in the equation (2), u _S is the surface flow velocity (cm / second) of the molten steel in the tundish around the long nozzle.
(3) The tundish is further provided with an upper weir between the weir and the molten steel outlet, wherein at least the lower end is immersed in the molten steel and the upper end protrudes from the molten steel. The height by continuous casting according to (1) or (2) above, wherein the installation position of the steel and the immersion depth in the molten steel satisfy the relationship of the following formula (3): A method for producing clean steel slabs.
0.1 <(L / M) × (1 / D) × (h / H) <1.9 (3)
However, in the formula (3), L is the long side length (m) of the tundish, M is the number of strands of the continuous casting machine (−), and D is the distance (m) from the molten steel injection site to the upper weir. , H is the immersion depth (m) in the molten steel of the upper weir during steady casting, and H is the molten steel surface height (m) in the tundish during steady casting.

  According to the present invention, the shape of the weir having the hook-like portion is optimized based on the casting conditions, and further, according to the shape of the weir, the flow rate of the molten steel from the ladle to the tundish, as well as from the tundish The amount of molten steel injected into the mold and the injection speed are controlled within a predetermined range, so that the floating separation of inclusions in the tundish is promoted, the cleanliness of the molten steel injected into the mold is increased, and the steel slab is continuously cast. This improves the cleanliness of the product and significantly reduces product defects caused by inclusions.

It is a front cross-sectional schematic diagram which shows the part of the tundish and casting_mold | template of the continuous casting equipment used by this invention. It is a top view of the tundish shown in FIG. It is a side view of the tundish shown in FIG. It is a figure which shows the result of investigating the influence of (m × u) × (S1 / Q) × (1 / V) × (X / Y) × (1 + S2) on the density of defects due to inclusions in the steel sheet. is there. Is a diagram showing the result of investigating the influence of the surface velocity u _S of the molten steel on the generation density of inclusions caused defects in the steel sheet. It is a schematic diagram showing a method of measuring the surface velocity u _S tundish molten steel. It is a figure which shows the result of having investigated the influence of the installation position of the upper dam and the immersion depth in molten steel on the generation density of the defect resulting from the inclusion in a steel plate. It is a figure which shows the investigation result of the number of inclusions in a slab in comparison with an example of the present invention, a comparative example, and a conventional example.

  Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. 1 is a diagram showing an embodiment of the present invention, and is a schematic front sectional view showing a tundish and a mold part of a continuous casting facility, FIG. 2 is a plan view of the tundish shown in FIG. FIG. 2 is a side view of the tundish shown in FIG. 1.

  1-3, reference numeral 1 is a tundish, 2 is a mold, 3 is a long nozzle attached to the bottom of a ladle (not shown), 4 is an immersion nozzle attached to the bottom of the tundish 1, A predetermined amount of molten steel 12 is introduced into the tundish while the molten steel 12 previously deoxidized with a deoxidizing material such as aluminum, silicon, titanium, etc. and is poured into the tundish 1 through the long nozzle 3. In the state of being retained, the molten steel 12 in the tundish is injected into the mold 2 through the immersion nozzle 4 to produce a steel slab 13. These drawings are diagrams in which two slab slabs are continuously cast with two molds 2.

  As shown in FIGS. 1 to 3, the tundish 1 used in the present invention has a molten steel injection flow injected into the tundish 1 from the ladle (not shown) through the long nozzle 3 at the bottom of the tundish 1. Between the molten steel injection | pouring site | part 5 which is a collision position, and the molten steel outflow port 6 from the tundish 1 to the casting_mold | template 2, the wall part 8 extended in the perpendicular direction upward from the bottom part of the tundish 1, and the upper end part of the wall part 8 A weir 7 having a rectangular projection shape on the horizontal plane of the wall 8 is disposed, which has a bowl-shaped portion 9 that faces the molten steel injection site side and protrudes in the horizontal direction. As shown in FIG. 1, the weir 7 has a shape in which the surface on the molten steel injection site side of the wall portion 8 and the surface on the lower surface side of the bowl-shaped portion 9 are smoothly connected by an arc. The surface on the molten steel injection site side of the portion 8 and the surface on the lower surface side of the bowl-shaped portion 9 may be orthogonal to each other.

  The weir 7 is also arranged on the long side surface side of the tundish 1 so as to surround the molten steel injection site 5 from four directions. That is, the molten steel injection | pouring site | part 5 is enclosed from four directions by the weir 7 whose projection external shape to a horizontal surface is a square or a rectangular rectangle. However, the weir 7 is provided with at least one notch 10 having an opening width of 1.0 mm or more continuous from the wall portion 8 to the bowl-shaped portion 9. The molten steel 12 in the enclosed space passes through the notch 10 and is discharged toward the molten steel outlet 6.

  In the tundish 1 shown in FIGS. 1 to 3, the notches 10 are arranged in two places, but there may be three or more places. In FIG. 2, the notch 10 is installed on the long side surface side of the tundish 1, but the installation position of the notch 10 is not limited to the long side surface side of the tundish 1. You may install in the surface which faced the short side surface side. However, when the notch 10 is installed on the surface facing the short side of the tundish 1, a short-circuit flow toward the molten steel outlet 6 may be formed and the floating of the inclusion may be impaired. The notch 10 is preferably installed on the long side surface side of the tundish 1. Moreover, since the effect of the weir 7 will fall if the opening width of the notch 10 is enlarged, it is preferable that the opening width of the notch 10 is 30 mm or less, respectively.

  Although the molten steel 12 injected into the molten steel injection site 5 through the long nozzle 3 collides with the molten steel injection site 5, it flows in four directions along the bottom surface of the tundish 1 by the falling energy of the molten steel injection flow. 7 and collides with the wall portion 8 in the upward direction, and further flows toward the molten steel injection site 5 by the flange 9 at the upper end of the weir 7. Flows coming from four directions facing the molten steel injection site 5 collide with each other and consume kinetic energy and decelerate. That is, the high-speed molten steel flow injected through the long nozzle 3 is greatly decelerated by the weir 7, and at the same time, the molten steel flow in the tundish is made uniform. Thereby, the short circuit flow and the high-speed flow in the tundish are eliminated, and the inclusions flowing out from the molten steel outlet 6 to the mold 2 are reduced accompanying these flows. That is, the floating separation of inclusions in the tundish 1 is promoted.

  However, in order to obtain the action / effect by the weir 7, the shape of the weir 7 is optimized according to the casting conditions, and simultaneously, the molten steel 12 is injected into the tundish 1 at a flow rate according to the shape of the weir 7, It is necessary to inject the molten steel 12 in the tundish into the mold 2 at a flow rate and a flow rate corresponding to the shape of the weir 7.

  That is, in the tundish 1 in which the weir 7 is arranged, the upper opening area of the weir 7, the opening area of the notch 10, the volume of the space surrounded by the weir 7, the long side length of the rectangular outer shape of the weir 7, From the short side length of the rectangular external shape, the injection flow rate of the molten steel 12 from the ladle to the tundish 1, the injection flow rate of the molten steel 12 from the tundish 1 to the mold 2, and the injection rate of the molten steel 12 from the tundish 1 to the mold 2 As a result of investigating the influence of the eight factors on the cleaning of the steel slab 13, the injection flow rate of the molten steel 12 from the ladle to the tundish 1, the injection flow rate of the molten steel 12 from the tundish 1 to the mold 2, and The shape of the weir 7 is determined in accordance with the injection speed (flow velocity) of the molten steel 12 from the tundish 1 to the mold 2, and after the weir 7 having the shape is installed, the casting conditions used for determining the shape of the weir 7 are used. Match the molten steel 12 with tande It poured into Mesh 1, and was found to be necessary to inject the tundish 1 into the mold 2.

The flow rate of molten steel 12 from the ladle 1 to the tundish 1 is Q (m ³ / sec), the flow rate of molten steel 12 from the tundish 1 to the mold 2 is m (tons / second), and the flow rate from the tundish 1 to the mold 2 The molten steel injection speed is u (m / sec), the upper opening area of the weir 7 is S1 (m ² ), the opening area of the notch 10 is S2 (m ² ), and the volume of the space surrounded by the weir 7 is V The investigation results will be described below, where (m ³ ), the long side length of the rectangular outer shape of the weir 7 is X (m), and the short side length of the rectangular outer shape of the weir 7 is Y (m).

  The upper opening area S1 of the weir 7 is an area in the range surrounded by the hook-shaped portion 9 on all sides, and the opening area S2 of the notch 10 is the opening width of each notch 10 and the weir 7 The total value of all the cutouts 10 for each opening area obtained by multiplication with the height of. The volume V of the space surrounded by the weir 7 is the volume of the space surrounded by the bottom surface of the tundish 1, the wall portion 8 of the weir 7, and the lower surface of the bowl-shaped portion 9 of the weir 7. The long side length X of the outer shape is the length of the longer side of the projected rectangular outer shape on the horizontal plane (in FIG. 2, the length of the wall portion 8 perpendicular to the long side surface of the tundish 1). The short side length Y of the rectangular external shape is the length of the shorter side of the rectangular external shape projected onto the horizontal plane (the length of the wall portion 8 parallel to the long side surface of the tundish 1 in FIG. 2). The molten steel injection speed u from the tundish 1 to the mold 2 is the descending speed of the molten steel 12 at the upper end position of the molten steel outlet 6.

  In FIG. 4, the horizontal axis is (m × u) × (S1 / Q) × (1 / V) × (X / Y) × (1 + S2), and the vertical axis is the number of defects due to inclusions in the steel sheet. Shows the results of investigating the influence of (m × u) × (S1 / Q) × (1 / V) × (X / Y) × (1 + S2) on the defect density due to inclusions in the steel sheet. . Incidentally, (m × u) is the momentum of the molten steel 12 when it flows out from the tundish 1 to the mold 2, and (S1 / Q) × (1 / V) is the molten steel 12 injected into the inside of the weir 7. Represents the degree to which the inclusions in the molten steel rise easily, and (X / Y) indicates the inclusion raising efficiency from the top of the weir 7, and the inclusion in the molten steel It can be thought of as a measure of the floating separation of objects.

  As shown in FIG. 4, the injection flow rate m of the molten steel 12 into the mold 2, the injection rate u of the molten steel 12 into the mold 2, the injection flow rate Q of the molten steel 12 into the tundish 1, the upper opening area S1 of the weir 7; The opening area S2 of the notch 10, the volume V of the space surrounded by the weir 7, the long side length X of the rectangular outer shape of the weir 7, and the short side length Y of the rectangular outer shape of the weir 7 are expressed by the following equation (1). It has been found that the occurrence of defects due to inclusions in the steel sheet is reduced when it is satisfied.

3.0 ≦ (m × u) × (S1 / Q) × (1 / V) × (X / Y) × (1 + S2) ≦ 50 (1)
When the value of (m × u) × (S1 / Q) × (1 / V) × (X / Y) × (1 + S2) is less than 3.0, the molten steel ascending flow rate from the weir 7 becomes too fast, There is a possibility that the molten steel surface in the dish will be disturbed and the slag in the tundish on the molten steel surface may be involved, while (m × u) × (S1 / Q) × (1 / V) × (X / When the value of (Y) × (1 + S2) exceeds 50, the flow rate of the molten steel from the weir 7 becomes slow, and the floating separation of inclusions is insufficient. Therefore, the value of (m × u) × (S1 / Q) × (1 / V) × (X / Y) × (1 + S2) needs to be within the range of the above equation (1).

  During casting, after changing the ladle, the range until the molten metal surface height in the tundish is returned to the steady state (referred to as “unsteady part”) is the molten steel supply flow rate from the ladle to the tundish 1 Although the flow rate Q differs from the flow rate m injected from the tundish 1 into the mold 2, as is apparent from FIG. 4, even in such an unsteady portion, (1 It can be seen that the cleanliness of the molten steel 12 is improved by controlling it within the range of the formula.

FIG. 5 shows the value of (m × u) × (S1 / Q) × (1 / V) × (X / Y) × (1 + S2) from the weir 7 in the range of 3.0 to 50 of the present invention. As a result, the surface flow velocity u _S of the molten steel in the tundish around the long nozzle 3 is changed, and the surface flow velocity u _S of the molten steel in the tundish around the long nozzle 3 is changed. It is a figure which shows the result of having investigated the influence which it has on the generation density of the defect resulting from inclusion. As shown in FIG. 6, the surface flow velocity u _S of the molten steel 12 in the tundish is obtained by immersing a refractory immersion rod 14 in the molten steel 12 in the tundish, and applying pressure applied to the immersion rod 14 by the molten steel flow. Measured by a method of measuring with a strain gauge 16 attached to a support fitting 15 holding the dip rod 14 and obtaining a molten steel flow velocity from the measured pressure. The surface flow velocity u _S of the molten steel 12 around the long nozzle 3 is a surface flow velocity of the molten steel 12 within a range of 100 cm from the outer surface of the long nozzle 3.

As shown in FIG. 5, when the surface flow velocity u _S (cm / sec) of the molten steel 12 around the long nozzle 3 satisfies the following formula (2), the occurrence of defects due to inclusions in the steel plate is stable. I found that it was less.

15 ≦ u _S ≦ 50 (2)
When the surface flow velocity u _S is less than 15 cm / second, the upward flow of the molten steel 12 from the weir 7 is weak, that is, the inclusion floating effect is impaired, while the surface flow velocity u _S exceeds 50 cm / second. There is a risk that the slag in the tundish may be caught due to the disturbance of the molten steel surface, and if the surface flow velocity u _S is within the range of the formula (2), it is possible to stably promote the floating separation of inclusions. It was. That is, it was found that it is preferable to control the casting conditions so as to satisfy the expression (2) after satisfying the expression (1).

  In addition, the tundish 1 used in the present invention has at least a lower end portion of the molten steel 12 between the weir 7 and the molten steel outlet 6 as shown in FIG. 1 from the viewpoint of further promoting the floating of inclusions. It is preferable that the tundish is provided with an upper weir 11 that is immersed and has its upper end protruding from the molten steel 12.

  When the molten steel 12 is continuously cast using the tundish 1 having the weir 7 and the upper weir 11 shown in FIG. 1, inclusions in the molten steel injected from the long nozzle 3 obtain an upward flow by the weir 7. Ascend to the molten steel surface in the tundish. On the other hand, the inclusions that have not floated on the molten steel surface then ride on the molten steel flow and reach the wall near the molten steel surface of the upper weir 11, and the molten steel flow is separated into upward and downward flows by the upper weir 11. . Since inclusions have a density lower than that of molten steel 12, inclusions in the molten steel flow mainly follow the upward flow formed by the upper weir 11 and float on the molten steel surface. Floating / separating is promoted. However, if the installation position of the upper weir 11 and the immersion depth in the molten steel are not appropriate, the above effect cannot be obtained sufficiently.

  Therefore, the position of the upper weir 11 and the molten steel are set under the condition that the value of (m × u) × (S1 / Q) × (1 / V) × (X / Y) × (1 + S2) is constant at 13.1. The effect of the installation position of the upper weir 11 and the immersion depth in the molten steel on the occurrence density of defects due to inclusions in the steel sheet was investigated by changing the immersion depth inside. The survey results are shown in FIG.

  As shown in FIG. 7, it has been found that the installation position of the upper weir 11 and the immersion depth in the molten steel preferably satisfy the relationship of the following expression (3). Note that (L / M) × (1 / D) × (h / H) on the horizontal axis in FIG. 7 is an index for preventing the slag in the tundish from being caught in the tundish corresponding to the upper part of the mold.

0.1 <(L / M) × (1 / D) × (h / H) <1.9 (3)
However, in Formula (3), L is the long side length (m) of the tundish 1, M is the number of strands (−) of the continuous casting machine, and D is the distance from the molten steel injection site 5 to the upper weir 11. (M), h is the immersion depth (m) in the molten steel of the upper weir 11 during steady casting, and H is the molten steel surface height (m) in the tundish during steady casting.

  When the value of (L / M) × (1 / D) × (h / H) is 0.1 or less, the installation position of the upper weir 11 is too far from the long nozzle 3 and the inclusions float and separate. It is difficult to obtain the effect. Alternatively, the immersion depth of the upper weir 11 is too small, and the inclusion floating and separation effects are difficult to obtain. On the other hand, when the value of (L / M) × (1 / D) × (h / H) is 1.9 or more, the upper weir 11 is too close to the long nozzle 3, so that the molten steel 12 is in the upper weir 11. After the collision, a descending flow having a high flow velocity is generated, and the rising of the inclusions is hindered by the descending flow. Alternatively, the immersion depth of the upper weir 11 is too large, and a downward flow having a high flow rate is easily generated at the installation position of the upper weir 11, and the rising of the inclusions is hindered by this downward flow. When the value of (L / M) × (1 / D) × (h / H) satisfies the expression (3), the effect of floating and separating inclusions by the upper weir 11 can be stably obtained.

  Here, the weir 7 is a weir on the premise that the molten steel 12 exists above. Therefore, the height of the weir 7 is at least less than the molten steel depth in the tundish at the position where the weir 7 is disposed. Is necessary. Preferably, the height of the weir 7 is not more than ½ of the molten steel depth in the tundish at the position where the weir 7 is disposed. On the other hand, if the height of the weir 7 is too low, the effect of the weir 7 cannot be obtained. Therefore, the height of the weir 7 is preferably secured to 100 mm or more.

  In addition, the actual molten steel injection site 5 is not a “point” but has a certain area, and such a molten steel injection site is surrounded from all sides, and at the same time, the absolute amount of the space surrounded by the weir 7 is ensured. For this purpose, the opening length of the upper opening of the weir 7 in the tundish long side direction is at least equal to the inner diameter of the lower end of the long nozzle 3, preferably more than that. In addition, in FIG. 1, the center position of the molten steel injection | pouring site | part which has an area is displayed as the molten steel injection | pouring site | part 5. FIG.

  Based on the injection flow rate Q of the molten steel 12 to the tundish 1 and the injection flow rate m of the molten steel 12 to the mold 2 and the injection speed u, the upper opening area S1 of the weir 7 and the opening of the notch 10 The weir so that the area S2, the volume V of the space surrounded by the weir 7, the long side length X of the rectangular outer shape of the weir 7 and the short side length Y of the rectangular outer shape of the weir 7 satisfy the above formula (1). 7 is determined, and the weir 7 having the shape is arranged in the tundish 1. Using this tundish 1, the injection flow rate Q of the molten steel 12 into the tundish 1 and the molten steel 12 into the mold 2 are determined. Continuously controlling the injection of the molten steel 12 into the tundish 1 and the injection of the molten steel 12 from the tundish 1 into the mold 2 so that the injection flow rate m and the injection speed u satisfy the relational expression (1). By casting, the melt injected from the long nozzle 3 Inclusions in, with the flow of an upward direction by the weir 7, flotation on molten steel surface in the tundish. That is, the levitation and separation of the inclusions in the molten steel are promoted by the weir 7, and the clean steel slab 13 can be manufactured.

  As described above, according to the present invention, the shape of the weir 7 having the bowl-shaped portion 9 is optimized based on the casting conditions, and the flow rate of the molten steel 12 from the ladle to the tundish 1 and the tank Since the injection flow rate and the injection speed from the dish 1 to the mold 2 are controlled within a predetermined range according to the shape of the weir 7, the floating separation of inclusions in the tundish 1 is promoted, and the molten steel 12 injected into the mold 2. This improves the cleanliness of the steel slab, improves the cleanliness of the continuously cast steel slab 13 and significantly reduces product defects caused by inclusions.

About 250 tons of aluminum-killed ultra-low carbon steel melted by decarburization and refining of hot metal in a converter and subsequent vacuum degassing and refining in an RH vacuum degassing unit, is composed of two strands with a capacity of 70 tons having the configuration shown in FIG. Using a slab continuous casting equipment with a tundish of the type, the molten steel injection flow rate from the tundish to the mold is 4.8 tons / min (m = 0.08 tons / second) per strand, and the molten steel from the ladle to the tundish A test for continuous casting on a steel slab slab was carried out at a total injection amount of two strands of 9.6 tons / min (Q≈0.02 m ³ / sec). In some tests, continuous casting was performed without an upper weir.

  At that time, the upper opening area S1 of the weir, the opening area S2 of the notch, the volume V of the space surrounded by the weir, the long side length X of the rectangular outer shape of the weir, and the short side length Y of the rectangular outer shape of the weir The test which changed and casted and satisfied the range of this invention (invention example 1-6) and the test which does not satisfy the range of this invention (comparative example 1-3) were done. In all tundishes, the protruding length of the hook-shaped portion (= the protruding length from the inner wall surface of the wall portion) was 0.12 m, and the radius of the arc connecting the wall portion and the hook-shaped portion was 0.06 m. For comparison, a casting test using the same tundish as the test casting was performed except that no weir was installed (conventional example).

  Table 1 shows the molten steel injection flow rate Q from the ladle to the tundish, the molten steel injection flow rate m and the injection speed u from the tundish to the mold, the shape of the tundish weir used, and the molten steel tundish and mold. The value of (m × u) × (S1 / Q) × (1 / V) × (X / Y) × (1 + S2) determined from the injection conditions and weir shape, the installation position of the upper weir and the molten steel The value of (L / M) × (1 / D) × (h / H) determined from the immersion depth is shown. In addition, the opening area S2 of the notch shown in Table 1 is the total value of the opening areas of the notches installed at two places.

  After casting, the number of inclusions in the slab was examined by ultrasonic flaw detection. FIG. 8 shows the result of investigation of the number of inclusions in the slab. In FIG. 8, the inclusion measurement value in the conventional example using the tundish without the weir is indexed and displayed as a reference (= 1.0).

  As shown in FIG. 8, it was confirmed that the number of inclusions in the slab slab can be significantly reduced by applying the present invention. In particular, in the inventive examples 3, 4, and 5 in which the upper weir was installed under the condition satisfying the expression (3), the number of inclusions in the slab slab could be greatly reduced. Thus, it has been confirmed that by applying the present invention, the floating effect of inclusions in the tundish can be greatly promoted.

About 250 tons of aluminum-killed ultra-low carbon steel melted by decarburization and refining of hot metal in a converter and subsequent vacuum degassing and refining in an RH vacuum degassing unit, is composed of two strands with a capacity of 70 tons having the configuration shown in FIG. Using a slab continuous casting equipment with a tundish of the type, the molten steel injection flow rate from the tundish to the mold is 4.8 tons / min (m = 0.08 tons / second) per strand, and the molten steel from the ladle to the tundish The total injection amount of the two strands is 9.6 tons / min (Q≈0.02 m ³ / sec), (m × u) × (S1 / Q) × (1 / V) × (X / Y) × ( The value of 1 + S2) was changed within the range of the present invention of 3.0 to 50, and the surface flow rate u _S of the molten steel around the long nozzle was changed, and a test for continuous casting on a steel slab slab was performed. In some tests, continuous casting was performed without an upper weir. The surface flow velocity u _{S of the} molten steel in the tundish was measured at the center position in the thickness direction of the tundish 50 cm away from the outer surface of the long nozzle toward the short side surface of the tundish. Moreover, the upper weir was installed on the conditions which satisfy | filling (3) Formula.

  In all tundishes, the protruding length of the hook-shaped portion (= the protruding length from the inner wall surface of the wall portion) was 0.12 m, and the radius of the arc connecting the wall portion and the hook-shaped portion was 0.06 m. For comparison, a casting test using the same tundish as the test casting was performed except that no weir was installed (conventional example).

Table 2 shows the flow rate Q of molten steel from the ladle to the tundish, the flow rate m and the flow rate u of molten steel from the tundish to the mold, the shape of the tundish weir used, the presence of the upper weir, the inside of the tundish (M × u) × (S1 / Q) × (1 / V) × (X / Y) × ((M × u) × (S1 / Q) × (1 / V) × () determined from the molten steel surface height H, the condition of pouring the molten steel into the tundish and mold, and the weir shape. The value of 1 + S2) and the measurement result of the surface flow velocity u _S of the molten steel are shown. The opening area S2 of the notch shown in Table 2 is the total value of the opening areas of the notches provided at two places.

  After casting, the number of inclusions in the slab was examined by ultrasonic flaw detection. Table 2 also shows the survey results of the number of inclusions in the slab. Table 2 shows the inclusion measurement value in the conventional example using a tundish without a weir as an index with the reference (= 1.0).

As is apparent from Table 2, it was confirmed that the number of inclusions in the slab slab could be further reduced in the test in which the surface flow velocity u _{S of the} molten steel was 15 to 50 cm / sec. Moreover, it was confirmed that the number of inclusions in the slab slab can be further reduced by installing the upper weir under the condition that satisfies the expression (3).

DESCRIPTION OF SYMBOLS 1 Tundish 2 Mold 3 Long nozzle 4 Immersion nozzle 5 Molten steel injection | pouring site 6 Molten steel outlet 7 Weir 8 Wall part 9 Ridge part 10 Notch 11 Upper weir 12 Molten steel 13 Steel slab 14 Immersion rod 15 Supporting metal fixture 16 Strain gauge

Claims (3)

When the deoxidized molten steel is once poured into the tundish from the pan, and then poured into the mold from the tundish to continuously cast the steel slab,
Between the molten steel injection site where the molten steel injection flow from the ladle collides with the bottom of the tundish and the molten steel outlet from the tundish to the mold, the molten steel injection site is surrounded from four directions and upward from the bottom of the tundish. A weir having an extending wall portion and a hook-like portion protruding in the horizontal direction facing the molten steel injection portion side at an upper end portion of the wall portion, and continuous from the wall portion to the hook-like portion Use a tundish with one or more cutouts and a weir with a rectangular outer shape,
When a region surrounded on all sides by the bowl-shaped portion is the upper opening of the weir, the upper opening area of the weir, the opening area of the notch, the volume of the space surrounded by the weir, The flow rate of molten steel from the ladle to the tundish, the flow rate of molten steel from the tundish to the mold, and the mold from the tundish to the mold for the long side length of the rectangular shape and the short side length of the rectangular shape of the weir In order to satisfy the relationship of the following formula (1)
A method for producing a high cleanliness steel slab by continuous casting, wherein the steel slab is continuously cast while controlling the molten steel injection from the ladle to the tundish and the molten steel injection from the tundish to the mold.
3.0 ≦ (m × u) × (S1 / Q) × (1 / V) × (X / Y) × (1 + S2) ≦ 50 (1)
In Equation (1), m is the flow rate of molten steel injected from the tundish to the mold (ton / second), u is the injection rate of molten steel from the tundish to the mold (m / second), and Q is the intake flow rate. Injection flow rate of molten steel from the pan to the tundish (m ³ / sec), S1 is the upper opening area (m ² ) of the weir having the bowl-shaped part, S2 is the opening area (m ² ) of the notch, V Is the volume (m ³ ) of the space surrounded by the weir having the hook-shaped part, X is the long side length (m) of the rectangular outer shape of the weir, and Y is the short side length (m) of the rectangular outer shape of the weir It is. Injection of molten steel from the ladle into the tundish is performed through a long nozzle that is installed at the bottom of the ladle and the lower end of the ladle is immersed in the molten steel in the tundish, and in the tundish around the long nozzle. 2. The method for producing a high cleanliness steel slab by continuous casting according to claim 1, wherein the surface flow velocity of the molten steel is controlled to be within the range of the following formula (2).
15 ≦ u _S ≦ 50 (2)
However, in (2), u _S is Ri surface velocity (cm / sec) der tundish molten steel at ambient long nozzle, the surface velocity of the tundish molten steel at ambient long nozzle, outside the long nozzle It is the surface flow velocity of the molten steel in the tundish at the center position in the thickness direction of the tundish 50 cm away from the surface toward the short side of the tundish. In the tundish, an upper weir is further provided between the weir and the molten steel outlet, and at least a lower end is immersed in the molten steel and an upper end protrudes from the molten steel. The high cleanliness steel slab by continuous casting according to claim 1 or 2, wherein the immersion depth in the molten steel satisfies the relationship of the following formula (3): Manufacturing method.
0.1 <(L / M) × (1 / D) × (h / H) <1.9 (3)
However, in the formula (3), L is the long side length (m) of the tundish, M is the number of strands of the continuous casting machine (−), and D is the distance (m) from the molten steel injection site to the upper weir. , H is the immersion depth (m) in the molten steel of the upper weir during steady casting, and H is the molten steel surface height (m) in the tundish during steady casting.

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