Continuous casting method and continuous casting apparatus for cast slabs

Abstract

To provide a slab continuous casting method which can improve the qualities of the surface and surface layer of a slab.SOLUTION: A slab continuous casting method in this disclosure has a step for discharging molten steel from an immersion nozzle into a mold having a rectangular cross-sectional shape. The immersion nozzle comprises: an inner bottom; and a pair of discharge holes which are present above the inner bottom. The immersion nozzle is arranged in such a manner that either discharge hole faces either short side of the mold and the other discharge hole faces the other short side of the mold. The immersion nozzle passes through the centroid of the openings of the discharge holes, and also, a gradient is made at at least a part from the inner bottom to the lower end faces of the discharge holes in a cross section along the axis of the immersion nozzle. The gradient has a gradient angle α of above 0° to below 90°against the horizontal.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] This application discloses a method for continuous casting of a slab and a continuous casting apparatus.

Background Art

[0002] During continuous casting of a slab, in the tundish and the mold, it is necessary to float and separate bubbles and non-metallic inclusions contained in the molten steel, reduce defects in the slab, and improve quality. For example, when producing a hot-rolled steel sheet by hot-rolling a slab, the quality of the surface and the surface layer of the slab is emphasized. If the quality is insufficient, it is necessary to grind and repair the surface of the slab, which causes a reduction in yield. The quality of the slab produced by continuous casting is strongly affected by the flow of the molten steel in the mold. Therefore, in order to improve the quality of the slab, it is important to appropriately control the flow of the molten steel in the mold. One of the factors that affects the flow of the molten steel in the mold is the discharge flow from the immersion nozzle into the mold.

[0003] The immersion nozzle is a nozzle made of refractory with a hollow structure. The upper part is directly or indirectly connected to the bottom of the tundish, and the lower part is immersed in the molten steel in the mold and used. The immersion nozzle generally used in continuous casting is a two-hole nozzle provided with a pair of discharge holes on the side wall. When the two-hole nozzle is used for continuous casting, the molten steel supplied from the tundish into the two-hole nozzle is discharged obliquely downward from each discharge hole and toward the short side of the mold. The direction of the discharge flow from the discharge hole is adjusted by the angle of the discharge hole, and an immersion nozzle having an appropriate discharge hole angle is used according to the casting conditions (for example, see Patent Documents 1 and 2).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

[0005] As described above, the discharge flow is discharged from the discharge hole of the immersion nozzle into the mold at an oblique downward direction. That is, the flow velocity of the discharge flow discharged from the discharge hole may include a horizontal velocity component toward the short side of the mold and a vertical downward velocity component. Here, according to the inventor's knowledge, if the vertical downward velocity component of the discharge flow is large, air bubbles and nonmetallic inclusions are carried deep into the cast slab, which tends to degrade the quality of the cast slab. In this respect, a new technology is needed to control the direction of the discharge flow from the immersion nozzle. [Means for solving the problem]

[0006] This application discloses several embodiments as means for solving the above-mentioned problems. <Aspect 1> A continuous casting method for cast slabs, comprising discharging molten steel from an immersion nozzle into a mold having a rectangular cross-sectional shape, The immersion nozzle has an inner bottom and a pair of discharge holes located above the inner bottom. One of the discharge holes of the immersion nozzle is directed toward one short side of the mold, and the other discharge hole is directed toward the other short side of the mold. The immersion nozzle has an inclination in a cross-section that passes through the centroid of the opening of the discharge hole and along the axis of the immersion nozzle, from the inner bottom to the lower end surface of the discharge hole, The aforementioned inclination has an inclination angle α greater than 0° and less than 90° with respect to the horizontal. A method for continuous casting of cast slabs. <Aspect 2> This includes applying a dynamic magnetic field to the molten steel discharged from the immersion nozzle into the mold using an electromagnetic stirring device. A method for continuous casting of a cast slab according to Embodiment 1. <Aspect 3> This includes applying a static magnetic field to the molten steel discharged from the immersion nozzle into the mold using an electromagnetic brake device. A method for continuous casting of a cast slab according to embodiment 1 or 2. <Aspect 4> The longest side of the aforementioned mold is 600 mm or more and 2400 mm or less. A method for continuous casting of a cast slab according to any of embodiments 1 to 3. <Aspect 5> The immersion depth D from the surface of the molten steel to the upper end of the discharge hole in the mold is 60 mm or more and 300 mm or less. A method for continuous casting of a cast slab according to any of embodiments 1 to 4. <Aspect 6> The opening shape of each of the aforementioned discharge holes is rectangular. The height H of the aforementioned opening shape is 30 mm or more and 150 mm or less. The width W of the opening shape is 50 mm or more and 90 mm or less. A method for continuous casting of a cast slab according to any of embodiments 1 to 5. <Aspect 7> The inner diameter I of the immersion nozzle is 10 mm or more and 70 mm or less. A method for continuous casting of a cast slab according to any of embodiments 1 to 6. <Aspect 8> The casting speed is 1.2 m / min or more. A method for continuous casting of a cast slab according to any of embodiments 1 to 7. <Pattern 9> A continuous casting apparatus for cast slabs, comprising a mold having a rectangular cross-sectional shape and an immersion nozzle for supplying molten steel to the mold, The immersion nozzle has an inner bottom and a pair of discharge holes located above the inner bottom. One of the discharge holes of the immersion nozzle is directed toward one short side of the mold, and the other discharge hole is directed toward the other short side of the mold. The immersion nozzle has an inclination in a cross-section that passes through the centroid of the opening of the discharge hole and along the axis of the immersion nozzle, from the inner bottom to the lower end surface of the discharge hole, The aforementioned inclination has an inclination angle α greater than 0° and less than 90° with respect to the horizontal. A continuous casting apparatus for cast slabs. [Effects of the Invention]

[0007] According to the technology of the present disclosure, since at least a part from the inner bottom of the immersion nozzle to the lower end surface of the discharge hole is provided with an inclination at an angle α, a part of the molten steel flow is discharged while being corrected upward from the inner bottom of the immersion nozzle toward the discharge hole. Therefore, compared with the case where no inclination is provided, the flow from the discharge hole toward the lower part of the mold is weakened, and it becomes difficult for bubbles, non-metallic inclusions, etc. to be brought deep into the slab, and the quality of the slab is likely to be improved.

Brief Description of the Drawings

[0008] [Figure 1] Schematically shows the flow of the continuous casting method of the slab. [Figure 2] It is a cross-sectional shape of the immersion nozzle and the mold, and schematically shows the cross-sectional shape passing through the centroid of the opening of the discharge hole of the immersion nozzle and along the axis of the immersion nozzle. [Figure 3] It is a cross-sectional shape of the immersion nozzle and the mold, and schematically shows the cross-sectional shape along the horizontal direction. [Figure 4] Schematically shows an example of the shape of the inclination from the inner bottom of the immersion nozzle to the lower end surface of the discharge hole. (A) is the case where the inclination angle α is constant, and (B) and (C) are the cases where the inclination angle α is not constant. [Figure 5] Schematically shows the effect of the continuous casting method of the slab according to the present disclosure. [Figure 6] Schematically shows an example of the shape of the part of the immersion nozzle below the molten steel surface. [Figure 7] Shows an example of the evaluation of the slab surface quality by numerical simulation. [Figure 8] Shows an example of the evaluation of the slab surface quality by numerical simulation. [Figure 9] Shows an example of the evaluation of the slab surface quality by numerical simulation. [Figure 10] Shows an example of the evaluation of the slab surface quality by numerical simulation. [Figure 11] Shows an example of the evaluation of the slab surface quality by numerical simulation. [Modes for carrying out the invention]

[0009] 1. Continuous casting method for cast slabs Hereinafter, an embodiment of the continuous casting method for slabs of the present disclosure will be described with reference to the drawings. As shown in Figures 1 to 6, the continuous casting method for slabs of the present disclosure includes discharging molten steel 1 from an immersion nozzle 20 into a mold 10 having a rectangular cross-sectional shape. The immersion nozzle 20 has an inner bottom 21 and a pair of discharge holes 22 located above the inner bottom 21. One of the discharge holes 22 of the immersion nozzle 20 is directed toward one short side of the mold 10, and the other discharge hole 22 is directed toward the other short side of the mold 10. The immersion nozzle 20 has an inclination 23 in a cross-section that passes through the centroid of the opening of the discharge hole 22 and along the axis of the immersion nozzle 20, extending at least a portion from the inner bottom 21 to the lower end surface 22y of the discharge hole 22. The inclination 23 has an inclination angle α greater than 0° and less than 90° with respect to the horizontal.

[0010] 1.1 Mold The mold 10 may be a general mold for continuous casting. As shown in Figure 3, the mold 10 has a rectangular cross-sectional shape. That is, the opening shape of the mold 10 is rectangular. In this application, "rectangle" means that it is substantially rectangular. That is, it does not have to be a perfect rectangle, and for example, the corners may be rounded. The cross-sectional shape of the mold 10 corresponds to the cross-sectional shape of the cast slab 5 after casting. As shown in Figure 3, the rectangular cross-sectional shape of the mold 10 may have a long side and a short side. When the method of this disclosure is applied as a continuous casting method for slabs, the ratio of the long side of the mold 10 to the short side of the mold 10 (long side / short side) may be, for example, 3 or more. The long side of the mold 10 may be, for example, 600 mm or more, or 700 mm or more, and may be 2400 mm or less, or 1900 mm or less. The shorter side of the mold 10 may be, for example, 100 mm or more, or 200 mm or more, or 300 mm or less, or 250 mm or less.

[0011] 1.2 Immersion Nozzle As shown in Figures 1-3, the immersion nozzle 20 discharges molten steel 1 supplied from the tundish 50 into the mold 10. The immersion nozzle 20 is made of a known refractory material. The immersion nozzle 20 can have the same configuration as conventional nozzles, except that it has an inner bottom 21, discharge hole 22, and inclination 23, which will be described later. That is, the configuration of the immersion nozzle 20 upstream of the discharge hole 22 may be the same as conventional nozzles. For example, the immersion nozzle 20 may be substantially cylindrical (e.g., cylindrical) upstream of the discharge hole 22, and the orientation of the axis of the immersion nozzle 20 (the longitudinal direction of the immersion nozzle 20) may substantially coincide with the vertical direction. That is, the immersion nozzle 20 may be a cylindrical two-hole nozzle extending downward from the upstream side to the downstream side. The upper end of the immersion nozzle 20 may be connected directly or indirectly to the tundish 50. The connection method between the immersion nozzle 20 and the tundish 50 is not particularly limited.

[0012] As shown in Figure 2, the immersion nozzle 20 has an inner bottom 21 and a pair of discharge holes 22, 22 located above the inner bottom 21. As shown in Figures 2 and 3, one discharge hole 22 of the immersion nozzle 20 is directed toward one short side of the mold 10, and the other discharge hole 22 is directed toward the other short side of the mold 10. That is, as shown in Figures 2 and 3, the molten steel 1 supplied into the immersion nozzle 20 is discharged from each of the discharge holes 22, 22 of the immersion nozzle 20 toward the short side of the mold 10. Also, as shown in Figure 2, the immersion nozzle 20 has an inclination 23 in a cross section that passes through the centroid of the opening of the discharge hole 22 and along the axis of the immersion nozzle 20, in at least a portion from the inner bottom 21 to the lower end surface 22y of the discharge hole 22. The inclination 23 has an inclination angle α greater than 0° and less than 90° with respect to the horizontal. The inclined section 23 is located directly below each of the discharge holes 22, 22.

[0013] 1.2.1 Inner bottom As shown in Figure 2, the inner bottom 21 of the immersion nozzle 20 can be the lower end inside the immersion nozzle 20. In a cross-section passing through the centroid of the opening of the discharge hole 22 and along the axis of the immersion nozzle 20, the inner bottom 21 may be a point or may be substantially flat, as shown in Figure 2.

[0014] 1.2.2 Discharge hole As described above, the immersion nozzle 20 is arranged such that a pair of discharge holes 22, 22 are each directed toward the short side of the mold 10. The discharge holes 22 may be formed by drilling a portion of the side wall of the immersion nozzle 20. As shown in Figure 2, the discharge hole 22 may have its upper end defined by an end face 22x and its lower end defined by an end face 22y. As shown in Figure 2, the discharge angle β of the discharge hole 22 is determined by the end faces 22x, 22y. The discharge angle β is not particularly limited and may be, for example, 10° or more, 20° or more, 30° or more, or 40° or more, or 80° or less, 70° or less, 60° or less, or 50° or less. The angle β at the upper end face 22x and the angle β at the lower end face 22y may be the same or different. Furthermore, at the upper end surface 22x and the lower end surface 22y, the angle β may be constant, or it may change continuously or intermittently as you move from the inner wall side towards the outer wall side.

[0015] 1.2.3 Slope As shown in Figure 2, the immersion nozzle 20 has a slope 23 in at least a portion from the inner bottom 21 to the lower end surface 22y of the discharge hole 22. The slope 23 has a slope angle α greater than 0° and less than 90° with respect to the horizontal. As shown in Figure 4(A), the slope angle α of the slope 23 may be constant from the inner bottom 21 to the lower end surface 22y of the discharge hole 22, or it may change from the inner bottom 21 to the lower end surface 22y of the discharge hole 22, as shown in Figures 4(B) and (C). In other words, the slope 23 may be straight, curved, or folded, or a combination of these shapes, in a cross-section that passes through the centroid of the opening of the discharge hole 22 and along the axis of the immersion nozzle 20. In particular, it is thought that cracking of the slope 23 is suppressed when it is curved. Furthermore, the inclination angle α of the inclination 23 may increase (Figure 4(B)) or decrease (Figure 4(C)) as it moves from the inner bottom 21 toward the lower end surface 22y of the discharge hole 22. In particular, when the inclination angle α of the inclination 23 increases as it moves from the inner bottom 21 toward the lower end surface 22y of the discharge hole 22 (Figure 4(B)), cracking of the inclination 23 is more easily suppressed, and it is thought that the molten steel flow that collides with the inner bottom 21 is more easily corrected into an upward flow. Alternatively, when the inclination angle α of the inclination 23 decreases as it moves from the inner bottom 21 toward the lower end surface 22y of the discharge hole 22 (Figure 4(C)), the connection between the inclination 23 and the lower end surface 22y becomes relatively smooth, and it is expected that turbulence in the discharge flow will be reduced, resulting in a smoother flow. Furthermore, assuming that the inclination 23 has a starting point on the inner bottom 21 side and an ending point on the lower end surface 22y side of the discharge hole 22 in a cross-section passing through the centroid of the discharge hole 22 and along the axis of the immersion nozzle 20, the position of the starting point may be on an extension line (dotted line in Figures 4(A) to (C)) that extends vertically downward from the inner wall of the nozzle upstream of the discharge hole 22, or it may be inside the extension line (between the extension line and the nozzle axis), or it may be outside the extension line (towards the outer wall of the nozzle). Figures 4(A) to (C) illustrate an example where the position of the starting point is inside the extension line.Furthermore, the position of the endpoint may be on an extension line drawn vertically downward from the inner wall of the nozzle upstream of the discharge hole 22, or it may be inside the extension line (between the extension line and the nozzle axis), or it may be outside the extension line (towards the outer wall of the nozzle). Figures 4(A) and (B) illustrate an example where the position of the endpoint is outside the extension line, and Figure 4(C) illustrates an example where the position of the endpoint is on the extension line.

[0016] The inclination angle α of the inclination 23 is greater than 0° and less than 90°. As shown in Figures 4(A) to (C), the inclination 23 is present in at least a portion of the area from the inner bottom 21 to the lower end surface 22y of the discharge hole 22. It is particularly preferable that the inclination 23 is provided from the inner bottom 21 to the lower end surface 22 of the discharge hole 22, and that there are substantially no portions where the inclination angle α is 0° or 90° from the inner bottom 21 to the lower end surface 22 of the discharge hole 22. During continuous casting of a slab, the molten steel 1 flows at high speed, for example, from the tundish 50 into the interior of the immersion nozzle 20. According to the inventors' findings, if the inclination angle α is 0° or 90° (i.e., if the inclination 23 is absent), as shown in Figure 5(A), the direction of the discharge flow tends to be downward compared to the direction along the angle β of the discharge hole 22 (the direction along the end surfaces 22x and 22y having angle β). In contrast, when the inclination angle α is greater than 0° and less than 90°, as shown in Figure 5(B), the molten steel flow that collides with the inner bottom 21 of the immersion nozzle 20 is corrected upward by the inclination 23, and this upward molten steel flow pushes up the discharge flow, so that the direction of the discharge flow is relatively upward compared to when the inclination 23 is not present. Thus, according to the method of this disclosure, by providing an inclination 23 at an angle α from the inner bottom 21 of the immersion nozzle 20 to the lower end surface 22y of the discharge hole 22, the molten steel flow is discharged from the inner bottom 21 of the immersion nozzle 20 toward the discharge hole 22 while being corrected upward. Compared to when the inclination 23 is not provided, the flow toward the bottom of the mold from the discharge hole 22 is weakened, making it difficult for bubbles and nonmetallic inclusions to be carried deep into the cast slab, and thus the quality of the cast slab is more easily improved. In particular, a better effect is more likely to be observed when the inclination angle α is 10° or more, 20° or more, or 30° or more, and also 80° or less, 75° or less, or 70° or less.

[0017] 1.2.4 Dimensions The dimensions of the immersion nozzle 20 are not particularly limited, and the optimal dimensions should be selected according to the continuous casting conditions, etc. An example of the dimensions of the immersion nozzle 20 is shown below.

[0018] (Nozzle inner diameter I) As shown in Figure 6, the immersion nozzle 20 may have an inner diameter I. The inner diameter I of the nozzle refers to the inner diameter upstream of the discharge hole 22. The inner diameter I of the immersion nozzle 20 is not particularly limited, but may be, for example, 10 mm or more, or 30 mm or more, or 70 mm or less, or 50 mm or less.

[0019] (Nozzle outer diameter 0) As shown in Figure 6, the immersion nozzle 20 may have an outer diameter O. The outer diameter O of the nozzle refers to the outer diameter upstream of the discharge hole 22. The outer diameter O of the immersion nozzle 20 is not particularly limited, but may be, for example, 50 mm or more, or 70 mm or more, or 100 mm or less, or 90 mm or less.

[0020] (Height H and width W of the discharge port) As shown in Figure 6, the opening shape of each discharge hole 22 of the immersion nozzle 20 may be, for example, rectangular. In this case, the height H of the opening shape may be, for example, 30 mm or more, or 50 mm or more, or 150 mm or less, or 90 mm or less. The width W of the opening shape may be, for example, 50 mm or more, or 60 mm or more, or 140 mm or less, or 90 mm or less.

[0021] (Slope height h) As shown in Figure 6, the inclination 23 of the immersion nozzle 20 may have a height h between the inner bottom 21 and the lower end of the discharge hole 22. The height h of the inclination 23 is not particularly limited as long as the inclination angle α described above is ensured. The height h of the inclination 23 may be, for example, 10 mm or more, or 20 mm or more, or 30 mm or less, or 40 mm or less.

[0022] (Immersion depth D) As shown in Figure 6, the lower end of the immersion nozzle 20 is immersed below the surface of the molten steel 1 in the mold 10, and an immersion depth D may exist from the surface of the molten steel 1 to the upper end of the discharge hole 22. The immersion depth D is not particularly limited. However, if the position of the discharge hole 22 of the immersion nozzle 20 is too close to the surface of the molten steel 1, the discharge flow may disturb the surface, potentially causing powder to be entrained on the surface. In the method of this disclosure, from the viewpoint of avoiding such problems, the immersion depth D from the surface of the molten steel 1 in the mold 10 to the upper end of the discharge hole 22 of the immersion nozzle 20 may be 60 mm or more, 70 mm or more, 80 mm or more, or 85 mm or more. On the other hand, if the position of the discharge hole 22 of the immersion nozzle 20 is too far from the surface of the molten steel 1 (the immersion depth D is too large), there is a risk that bubbles and metal inclusions will be easily carried deep into the cast slab by the discharge flow directed downward from the discharge hole 22. In the method of this disclosure, from the viewpoint of avoiding such problems, the immersion depth D from the surface of the molten steel 1 in the mold 10 to the upper end of the discharge hole 22 of the immersion nozzle 20 may be 300 mm or less, 200 mm or less, 150 mm or less, or 100 mm or less.

[0023] (Nozzle length) The immersion nozzle 20 may have a length from its upper end to its lower end. The lower end is immersed in the molten steel 1 in the mold 10. The upper end may be connected directly or indirectly to the tundish 50. The total length of the immersion nozzle 20 (not shown) should be such that it can adequately supply the molten steel 1 to the mold 10, and may be, for example, 500 mm or more, or 700 mm or more, or 1000 mm or less, or 800 mm or less.

[0024] 1.3 Other configurations and conditions In the continuous casting method for cast slabs of this disclosure, various components and devices may be used in combination with the mold 10 and immersion nozzle 20 described above. For example, an electromagnetic stirring device 30, an electromagnetic brake device 40, a tundish 50, etc. In addition, vibration (oscillation) may be applied to the mold 10 during continuous casting. Rolls or spray nozzles for cooling the cast slab 5 may also be arranged downstream of the mold 10.

[0025] 1.3.1 Electromagnetic stirring device As shown in Figure 1, the continuous casting method for slabs of the present disclosure may include applying a dynamic magnetic field to the molten steel 1 discharged from the immersion nozzle 20 into the mold 10 using an electromagnetic stirring device 30. The electromagnetic stirring device 30 applies a dynamic magnetic field to the molten steel 1 in the mold 10, generating a Lorentz force, i.e., an electromagnetic force, in the molten steel 1, causing it to flow. For example, the dynamic magnetic field from the electromagnetic stirring device 30 can impart a swirling flow to the molten steel 1 in the mold 10 in a horizontal cross-section. By generating flow at the solidification shell interface of the molten steel 1 through electromagnetic stirring, it is possible to suppress the trapping of air bubbles in the molten steel 1 at the solidification shell interface, resulting in bubbly defects. In other words, it is easier to obtain a higher quality slab 5. The electromagnetic stirring device 30 may be positioned, for example, above the electromagnetic brake device 40 described later. Alternatively, the electromagnetic stirring device 30 may be positioned, for example, below the surface of the molten steel 1. The distance from the center position of the electromagnetic stirring device 30 (the position where the stirring thrust is greatest in the height direction) to the surface position of the molten steel 1 may be, for example, 50 mm or more and 200 mm or less. Such an electromagnetic stirring device 30 can be any of the electromagnetic stirring devices commonly used for continuous casting. For example, the electromagnetic stirring device 30 may have a magnetic core. Specifically, the electromagnetic stirring device 30 may be configured such that a copper coil is wound around a magnetic core, and a magnetic field is generated by applying an electric current to the coil.

[0026] 1.3.2 Electromagnetic Brake System As shown in Figure 1, the continuous casting method for slabs of the present disclosure may include applying a static magnetic field to the molten steel 1 discharged from the immersion nozzle 20 into the mold 10 using an electromagnetic brake device 40. The electromagnetic brake device 40 applies a static magnetic field to the discharge flow from the immersion nozzle 20 in the thickness direction of the mold 10, thereby providing a braking force to the discharge flow. For example, the static magnetic field from the electromagnetic brake device 40 can cause at least a portion of the discharge flow from the immersion nozzle 20 to be decelerated by the electromagnetic brake. Alternatively, at least a portion of the discharge flow may be repelled by the electromagnetic brake, creating an upward flow toward the molten surface. The electromagnetic brake device 40 may be positioned, for example, below the electromagnetic stirring device 30. The distance from the center position of the electromagnetic brake device 40 (the position where the magnetic flux density is maximum in the height direction) to the center position of the electromagnetic stirring device 30 (the position where the stirring thrust is greatest in the height direction) may be, for example, 400 mm or more and 600 mm or less. Such an electromagnetic brake device 40 can employ any of the electromagnetic brake devices commonly used for continuous casting. For example, the static magnetic field may be applied by a permanent magnet or by an electromagnet with a coil wound around an iron core and energized. In particular, if the electromagnetic brake device 40 is equipped with an electromagnet with a coil wound around an iron core and energized, it becomes easy to change the magnetic field strength.

[0027] 1.3.3 Mold Powder As shown in Figure 1, in the continuous casting method for cast slabs of this disclosure, mold powder 2 may be supplied to the surface of the molten steel 1 in the mold 10. By supplying mold powder 2 to the surface of the molten steel 1, oxidation of the molten steel 1 is suppressed, and lubrication between the inner wall of the mold 10 and the solidified shell can be ensured. The mold powder should be selected from known mold powders according to the type of steel, etc.

[0028] 1.3.4 Tan Dish As shown in Figure 1, in the continuous casting method for cast slabs of this disclosure, molten steel 1 can be supplied from the tundish 50 to the mold 10 via the immersion nozzle 20. By using the tundish 50 as an intermediate container, nonmetallic inclusions and the like in the molten steel 1 are more easily floated up and removed. Any known type of tundish 50 can be used.

[0029] 1.3.5 Casting speed The continuous casting method for cast slabs described herein is compatible with any casting speed, from low-speed casting to high-speed casting. The casting speed is not particularly limited, but may be, for example, 0.7 m / min or more, 0.9 m / min or more, or 1.2 m / min or more, or 2.0 m / min or less, 1.8 m / min or less, or 1.6 m / min or less.

[0030] 1.3.6 Steel type There are no particular restrictions on the type of steel used in the continuous casting method for cast slabs described herein. An appropriate type of steel should be selected depending on the intended use of the cast slab.

[0031] 2. Continuous casting apparatus for cast slabs The technology of the present disclosure also has aspects as a continuous casting apparatus for cast slabs. As shown in Figure 1, the continuous casting apparatus for cast slabs 100 of the present disclosure includes a mold 10 having a rectangular cross-sectional shape and an immersion nozzle 20 for supplying molten steel 1 to the mold 10. The immersion nozzle 20 has an inner bottom 21 and a pair of discharge holes 22, 22 located above the inner bottom 21. One of the discharge holes 22 of the immersion nozzle 20 is directed toward one short side of the mold 10, and the other discharge hole 22 is directed toward the other short side of the mold 10. The immersion nozzle 20 has an inclination 23 in a cross-section that passes through the centroid of the opening of the discharge hole 22 and along the axis of the immersion nozzle 20, at least in part from the inner bottom 21 to the lower end surface 22y of the discharge hole 22. The inclination 23 has an inclination angle α greater than 0° and less than 90° with respect to the horizontal.

[0032] 2.1 Basic configuration The basic components of the continuous casting apparatus 100, namely the mold 10 and the immersion nozzle 20, are as described above. For example, the dimensions and conditions described in the continuous casting method can be adopted as the dimensions and conditions for the continuous casting apparatus 100. That is, the lengths of the long and short sides of the mold 10, the immersion depth D, the height H and width W of the discharge hole 22, and the inner diameter I and outer diameter O of the immersion nozzle 20 can have the predetermined dimensions described above.

[0033] 2.2 Optional configuration In the continuous casting apparatus 100, various components and devices may be used in combination with the mold 10 and the immersion nozzle 20. For example, an electromagnetic stirring device 30, an electromagnetic brake device 40, a tundish 50, etc. A vibration device may also be used to vibrate the mold 10. Furthermore, there may be rolls or spray nozzles for cooling the cast slab 5 downstream of the mold 10. The same optional configurations as those described above may be adopted. The continuous casting apparatus 100 may be a curved type continuous casting apparatus or a vertically curved type continuous casting apparatus.

[0034] 3. Immersion nozzle for continuous casting The technology of this disclosure also has an aspect as an immersion nozzle for continuous casting of cast slabs. That is, the immersion nozzle 20 of this disclosure has an inner bottom 21 and a pair of discharge holes 22, 22 located above the inner bottom 21, and in a cross section passing through the centroid of the opening of the discharge hole 22 and along the axis of the immersion nozzle 20, an inclination 23 is provided in at least a portion from the inner bottom 21 to the lower end surface 22y of the discharge hole 22, and the inclination 23 has an inclination angle α of less than 90° with respect to the horizontal. The configuration of the immersion nozzle 20 is as described above, and a detailed explanation is omitted here.

[0035] 4. Effects As described above, according to the technology of this disclosure, by providing an inclination 23 at an angle α to at least a portion of the area from the inner bottom 21 of the immersion nozzle 20 to the lower end surface 22y of the discharge hole 22, a portion of the molten steel flow is discharged while being corrected upward from the inner bottom 21 of the immersion nozzle 20 toward the discharge hole 22. Compared to the case where no inclination is provided, the flow toward the bottom of the mold 10 from the discharge hole 22 is weakened, making it difficult for air bubbles, nonmetallic inclusions, etc. to be carried deep into the cast slab 5 (relatively downstream), thereby reducing defects in the cast slab 5 and improving the quality of the cast slab 5. [Examples]

[0036] The present invention will be further described below with reference to examples, but the present invention is not limited to the following examples. The present invention allows for the adoption of various conditions without departing from its gist and insofar as it achieves its objective.

[0037] 1. Evaluation using a conventional immersion nozzle To understand the relationship between the immersion nozzle and slab quality, a numerical simulation was performed to evaluate the relationship between the discharge hole height H and slab quality for a conventional immersion nozzle (outer diameter O: 150 mm, inner diameter I: 85 mm, discharge hole width W: 85 mm, discharge hole angle β: 45°, inclination angle α: 90°). The simulation was performed using the method disclosed in Japanese Patent Publication No. 2015-157309 (Japanese Patent No. 6379515), namely, a thermal fluid simulation using the electromagnetic force distribution obtained by electromagnetic field simulation. The evaluation method for cellular defects was also the same as the method described in the same publication. Note that "(bubble volume fraction contained in a depth of 2 mm to 15 mm from the slab surface) × 10 6 The value was calculated as the "blowhole index". For all parts except the immersion nozzle, the mold dimensions were set as follows: mold long side: 1250 mm, mold short side: 248 mm, slab withdrawal speed (casting speed): 1.4 m / min, and electromagnetic stirring was applied at a set current of 620 A. The other dimensions and shape of the immersion nozzle are shown in Table 1 below. Parameters O, I, W, β, α, H, and D correspond to those shown in Figures 2 and 6, respectively.

[0038] [Table 1]

[0039] Figure 7 shows an example of slab surface quality evaluation obtained by numerical simulation. The blowhole index shown in Figure 7 is an index representing the degree of defects beneath the surface of the slab, and a smaller value indicates better surface quality. From Figure 7, it can be seen that the blowhole index changes significantly with the discharge hole height H, and is best when the discharge hole height H is 80 mm. However, when the immersion depth D is 107-117 mm, it is very shallow compared to general casting conditions, and the slab surface quality tends to deteriorate easily. In this regard, further investigation was conducted into immersion nozzle shapes that can further improve the surface quality of the slab.

[0040] 2. When the tilt angle α is changed For each case shown in Figure 7, numerical simulations were conducted under the condition that a slope was provided directly below the inside of the discharge hole, with the aim of further improving quality. Figure 8 shows the dependence of the blowhole index on the slope angle α at each discharge hole height H. Here, the case where the slope angle α is 90° is the conventional nozzle shape, and the blowhole index matches the value shown in Figure 7. From Figure 8, it can be seen that the blowhole index changes with the slope angle α, and that by providing a slope angle α greater than 0° and less than 90° at any discharge hole height H, the quality of the cast slab is improved compared to the conventional method.

[0041] 3. When the immersion depth D is changed The effect of the inclination angle α directly below the discharge hole was evaluated when the immersion depth D was changed. The above-mentioned immersion nozzle c (discharge hole height H: 70 mm) was used for the evaluation. Figure 9 shows the blowhole index for immersion depths D of 85 mm, 117 mm, and 200 mm. From Figure 9, it can be seen that even when the immersion depth D is changed, the blowhole index is lower when using a nozzle with an inclination angle α of 60° than when using a conventional nozzle with an inclination angle α of 90°. In other words, even when the immersion depth D is changed, it can be seen that the quality of the cast slab can be improved by providing an inclination directly below the discharge hole of the nozzle.

[0042] 4. When the mold width is changed The effect of the inclination angle α directly below the discharge hole was investigated when the mold width (long side) was changed. For the evaluation, the above-mentioned immersion nozzle d (discharge hole height H: 80 mm) was used, and the immersion depth D was set to 117 mm. Figure 10 shows the blowhole index for mold widths (long side) of 700 mm, 1250 mm, and 1900 mm. From Figure 10, it can be seen that even when the mold width (long side) is changed, the blowhole index is lower when using a nozzle with an inclination angle α of 60° than when using a conventional nozzle with an inclination angle α of 90°. In other words, even when the mold width (long side) is changed, it can be seen that the quality of the cast slab can be improved by providing an inclination directly below the discharge hole of the nozzle.

[0043] 5. If the casting speed is changed The effect of the inclination angle α directly below the discharge hole was investigated when the casting speed was changed. For the evaluation, the above-mentioned immersion nozzle d (discharge hole height H: 80 mm) was used, and the immersion depth D was set to 117 mm. Figure 11 shows the blowhole index for casting speeds of 1.2 m / min, 1.4 m / min, and 1.6 m / min. From Figure 11, it can be seen that even when the casting speed is changed, the blowhole index is lower when using a nozzle with an inclination angle α of 60° than when using a conventional nozzle with an inclination angle α of 90°. In other words, even when the casting speed is changed, it can be seen that the quality of the cast slab can be improved by providing an inclination directly below the discharge hole of the nozzle.

[0044] 6. Summary From the above results, it was found that when using an immersion nozzle that satisfies the following requirements (1) to (4) during continuous casting of slabs, the quality of the slabs is improved compared to when using a conventional immersion nozzle with an inclination angle α of 90° (or 0°). This is thought to be due to the following mechanism: By providing an inclination angle α (0° < α < 90°) in at least a portion from the inner bottom of the immersion nozzle to the lower end surface of the discharge hole, a portion of the molten steel flow is corrected upward from the inner bottom of the immersion nozzle towards the discharge hole as it is discharged. Compared to cases where no inclination is provided (when α = 90° or α = 0°), the flow from the discharge hole towards the bottom of the mold is weakened, making it more difficult for air bubbles and nonmetallic inclusions to be carried deep into the slab, thus reducing defects in the slab and improving the quality of the slab.

[0045] (1) The immersion nozzle has an inner bottom and a pair of discharge holes located above the inner bottom. (2) One discharge port of the immersion nozzle is directed toward one short side of the mold, and the other discharge port is directed toward the other short side of the mold (i.e., the molten steel supplied into the immersion nozzle is discharged from the discharge port of the immersion nozzle toward the short side of the mold). (3) The immersion nozzle has an inclination in a cross-section that passes through the centroid of the opening of the discharge hole and along the axis of the immersion nozzle, from the inner bottom to the lower end surface of the discharge hole. (4) The inclination has an inclination angle α greater than 0° and less than 90° with respect to the horizontal. [Explanation of Symbols]

[0046] 1 Molten steel 2. Mold Powder 5 cast slabs 10 molds 20 Immersion nozzles 21 Inner bottom 22 Discharge hole 22x top surface 22y lower end surface 23 Slope 30 Electromagnetic stirring device 40 Electromagnetic brake system 50 Tan Dish 100 Continuous Casting Machines

Claims

1. A continuous casting method for cast slabs, comprising discharging molten steel from an immersion nozzle into a mold having a rectangular cross-sectional shape, The immersion nozzle has an inner bottom and a pair of discharge holes located above the inner bottom. One of the discharge holes of the immersion nozzle is directed toward one short side of the mold, and the other discharge hole is directed toward the other short side of the mold. The immersion nozzle has an inclination in a cross-section that passes through the centroid of the opening of the discharge hole and along the axis of the immersion nozzle, from the inner bottom to the lower end surface of the discharge hole, The aforementioned inclination has an inclination angle α greater than 0° and less than 90° with respect to the horizontal. The inclination angle α of the inclination decreases as it moves from the inner bottom toward the lower end surface of the discharge hole. A method for continuous casting of cast slabs.

2. This includes applying a dynamic magnetic field to the molten steel discharged from the immersion nozzle into the mold using an electromagnetic stirring device. A method for continuous casting of a cast slab according to claim 1.

3. This includes applying a static magnetic field to the molten steel discharged from the immersion nozzle into the mold using an electromagnetic brake device. A method for continuous casting of a cast slab according to claim 1.

4. The longest side of the mold is 600 mm or more and 2400 mm or less. A method for continuous casting of a cast slab according to any one of claims 1 to 3.

5. The immersion depth D from the surface of the molten steel to the upper end of the discharge hole in the mold is 60 mm or more and 300 mm or less. A method for continuous casting of a cast slab according to any one of claims 1 to 3.

6. The opening shape of each of the aforementioned discharge holes is rectangular. The height H of the opening shape is 30 mm or more and 150 mm or less. The width W of the opening shape is 50 mm or more and 90 mm or less. A method for continuous casting of a cast slab according to any one of claims 1 to 3.

7. The inner diameter I of the immersion nozzle is 10 mm or more and 70 mm or less. A method for continuous casting of a cast slab according to any one of claims 1 to 3.

8. The casting speed is 1.2 m / min or more. A method for continuous casting of a cast slab according to any one of claims 1 to 3.

9. A continuous casting apparatus for cast slabs, comprising a mold having a rectangular cross-sectional shape and an immersion nozzle for supplying molten steel to the mold, The immersion nozzle has an inner bottom and a pair of discharge holes located above the inner bottom. One of the discharge holes of the immersion nozzle is directed toward one short side of the mold, and the other discharge hole is directed toward the other short side of the mold. The immersion nozzle has an inclination in a cross-section that passes through the centroid of the opening of the discharge hole and along the axis of the immersion nozzle, from the inner bottom to the lower end surface of the discharge hole, The aforementioned inclination has an inclination angle α greater than 0° and less than 90° with respect to the horizontal. The inclination angle α of the inclination decreases as it moves from the inner bottom toward the lower end surface of the discharge hole. A continuous casting apparatus for cast slabs.

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