Method for continuously casting slab casting piece, and slab casting piece

Abstract

In this method for continuously casting a slab casting piece, the diameter of a roll section in convex rolls which corresponds to a part on the outer side in the width direction from a triple point in a slab casting piece continuously cast and drawn out from a mold is made smaller than the diameter of a roll section corresponding to the inner side in the width direction from the triple point, and the convex rolls are used as a pair of rolls in a segment of a light drawing zone where the slab casting piece is lightly drawn by the rolls and which is arranged on the downstream side in a casting piece drawing direction from a final solidification position of the slab casting piece.

Description

Continuous casting method for slab billets and slab billets

[0001] The present invention relates to a continuous casting method for slab billets and slab billets.

[0002] In conventional continuous casting machines, when performing soft reduction, it is common to use flat rolls where the roll diameters of the parts in contact with the slab billet are the same in the width direction (excluding the chock part). For example, in Patent Document 1, even in a continuous casting method aimed at reducing porosity, which is crucial in manufacturing extremely thick steel plates, flat rolls are used to perform soft reduction on the slab billet. Also, it is generally the case that the range where the slab billet is reduced in thickness using flat rolls is the non-solidified part. Note that performing soft reduction means providing a segment zone where the roll opening is narrowed with a gradient larger than the solidification shrinkage.

[0003] Also, as described in Patent Document 1 and Patent Document 2, for the purpose of preventing surface cracks in the slab billet, the amount of secondary cooling water and the spraying range of the secondary cooling water are adjusted in the width direction of the slab billet. As a result, the final solidification position of the slab billet may be at the edge part in the width direction or may be uniform in the width direction.

[0004] Japanese Unexamined Patent Application Publication No. 2023 - 141391, Japanese Unexamined Patent Application Publication No. 2006 - 315011

[0005] In recent years, as the sizes of products and buildings have increased, there has been a growing need for thicker and heavier steel plates. To manufacture extremely thick steel plates, it is necessary for the slab billets, which are semi-finished products, to also have larger cross-sections. However, when casting slab billets with large cross-sections, there was a problem that porosity remained in the center of the thickness, and these porosities were not welded even in the rolled steel plate and were detected as defects during UT inspection. When performing soft reduction using flat rolls as in Patent Document 1, the roll interfered with the corner part of the slab billet with a low temperature, and an excessive load was applied to the segment, making it difficult to apply a certain amount of soft reduction to the slab billet. Although it is conceivable to use segments with high rigidity, making all segments in the soft reduction zone highly rigid would result in extremely high introduction costs.

[0006] Furthermore, as described in Patent Documents 1 and 2, when the final solidification position is close to the edge of the slab or uniform across the width of the slab, porosity formation occurs in the final solidified area. As a result, porosity occurs at the edges of the slab, which are difficult to reduce with flat rolls, making it difficult to press them down under light pressure. As described in Patent Document 1, it is common to apply light pressure to the unsolidified area of ​​the slab, but when manufacturing large-section slabs, while it was effective in improving central segregation, it was not effective in reducing porosity.

[0007] The present invention has been made in view of the above problems, and its object is to provide a continuous casting method for slab slabs and a slab slab that can reduce the porosity of the slab slab.

[0008] In order to solve the above-mentioned problems and achieve the objective, (1) The continuous casting method for slab slabs according to the present invention is characterized in that, in a segment of a light reduction zone which is positioned downstream in the slab withdrawal direction from the final solidification position of the slab slab that has been continuously cast and withdrawn from the mold, and in which the slab slab is lightly reduced by a pair of rolls, the pair of rolls uses convex rolls in which the diameter of the roll portion corresponding to the portion of the slab slab that is outside in the width direction in the triple point is smaller than the diameter of the roll portion corresponding to the portion of the slab that is inside in the width direction in the triple point.

[0009] (2) The continuous casting method for slab slabs according to the present invention is characterized in that, in the invention of (1) above, the casting speed and the amount of cooling water used to cool the slab slab are adjusted in the width direction of the slab slab so that the final solidification position is located in the slab withdrawal direction, in the position immediately in front of the pair of rolls using the convex rolls.

[0010] (3) The continuous casting method for slab slabs according to the present invention is characterized in that, in the invention of (2) above, cooling water is supplied to a plurality of spray nozzles arranged in line in the width direction of the slab slab from a plurality of route pipes branched from a main pipe, and the cooling water is sprayed onto the slab slab from the plurality of spray nozzles to cool the slab slab, and the amount of cooling water flowing through the route pipe that supplies the cooling water to the spray nozzle that sprays the cooling water to the central part in the width direction of the slab slab is 50% or less of the total amount of cooling water flowing through the plurality of route pipes.

[0011] (4) The continuous casting method for slab slabs according to the present invention is characterized in that, in the invention of (2) above, a continuous casting machine is used which has a mold cross section with an octagonal chamfer shape in which the chamfer dimension is 10% or less of the thickness of the slab slab.

[0012] (5) The slab slab according to the present invention is characterized by being manufactured using the continuous casting method for slab slabs described in any one of (1) to (4) above.

[0013] The continuous casting method for slab slabs and the slab slabs according to the present invention have the effect of reducing the porosity of the slab slabs.

[0014] Figure 1 is a schematic diagram showing an example of a continuous casting machine to which the continuous casting method for slab slabs according to the embodiment is applied. Figure 2 shows the widthwise end of a segment using convex rolls in a pair of support rolls. Figure 3 shows an example of convex rolls applicable to the first and second support rolls. Figure 4 shows another example of convex rolls applicable to the first and second support rolls. Figure 5 shows an example of three route pipes branching off from the main pipe. Figure 6 shows an example of a widthwise region of a slab slab that is cooled by cooling water supplied from each route pipe.

[0015] The following describes the continuous casting method for slab slabs and embodiments of slab slabs according to the present invention. However, the present invention is not limited to these embodiments.

[0016] Figure 1 is a schematic diagram showing an example of a continuous casting machine 1 to which the continuous casting method for slab slabs according to the embodiment is applied.

[0017] The continuous casting machine 1 according to the embodiment shown in Figure 1 is a vertical bending type continuous casting machine. The continuous casting machine 1 according to the embodiment includes a tundish 3, a mold 5, a plurality of support roll pairs 7, 7a, 7b, and a plurality of spray nozzles 9. Also, as shown in Figure 1, the slab cast 6 is drawn out in the slab withdrawal direction D1. In this embodiment, the side where the tundish 3 is provided in the slab withdrawal direction D1 is described as the upstream side, and the side to which the slab cast 6 is drawn out is described as the downstream side.

[0018] The tundish 3 is located above the mold 5 and supplies molten steel 2 to the mold 5. Molten steel 2 is supplied to the tundish 3 from a ladle (not shown) and stored there. A sliding nozzle for adjusting the flow rate of molten steel 2 is installed at the bottom of the tundish 3, and an immersion nozzle 4 is installed on the underside of this sliding nozzle.

[0019] The mold 5 is located below the tundish 3. Molten steel 2 is poured into the mold 5 from the immersion nozzle 4 of the tundish 3. The poured molten steel 2 is cooled (primary cooling) in the mold 5, thereby forming the outer shell shape of the slab 6.

[0020] Each of the support roll pairs 7, 7a, and 7b consists of a first support roll 71, 71a, and 71b and a second support roll 72, 72a, and 72b, respectively. Of the support roll pairs 7, at least one is a drive roll pair, and the remaining support roll pairs are driven roll pairs. Also, of the support roll pairs 7a, at least one is a drive roll pair, and the remaining support roll pairs are driven roll pairs. Also, of the support roll pairs 7b, at least one is a drive roll pair, and the remaining support roll pairs are driven roll pairs. Furthermore, in each of the support roll pairs 7, 7a, and 7b, one or more driven roll pairs are positioned adjacent to the drive roll pair. Multiple support roll pairs 7, 7a, 7b are arranged along the slab withdrawal direction D1, and the first support rolls 71, 71a, 71b and the second support rolls 72, 72a, 72b support the slab 6 from both sides in the thickness direction while conveying it in the slab withdrawal direction D1.

[0021] Multiple spray nozzles 9 are provided between adjacent pairs of support rolls 7 along the slab withdrawal direction D1. The spray nozzles 9 are nozzles for spraying cooling water onto the slab 6 and for secondary cooling of the slab 6.

[0022] The slab 6 is cooled as it is drawn out along the slab withdrawal direction D1 by cooling water sprayed from multiple spray nozzles 9. In Figure 1, the unsolidified portion 2a of the molten steel 2 within the slab 6 is shown with hatched dots. Also in Figure 1, the final solidification position where the unsolidified portion 2a has disappeared and solidification is complete is indicated by the symbol Pe.

[0023] Downstream of the continuous casting machine 1, there is a light reduction zone 10 in which the slab slab 6 is transported horizontally and lightly reduced. The light reduction zone 10 includes, for example, a segment 8a in which six support roll pairs 7a are arranged along the slab withdrawal direction D1, and two segments 8b in which six support roll pairs 7b are each arranged along the slab withdrawal direction D1. The multiple support roll pairs 7a and 7b are arranged such that the roll spacing in the thickness direction of the slab slab 6 gradually narrows toward the slab withdrawal direction D1. As a result, the slab slab 6 passing through the light reduction zone 10 is lightly reduced by each support roll pair 7a and 7b.

[0024] Furthermore, in the continuous casting machine 1 according to this embodiment, as shown in Figure 1, the position of the downstream support roll pair 7a among the multiple support roll pairs 7a provided in segment 8a is located at the final solidification position Pe of the slab cast 6 in the slab withdrawal direction D1. And the position of the upstream support roll pair 7b among the multiple support roll pairs 7b provided in the upstream segment 8b of the two segments 8b is located immediately after the final solidification position Pe of the slab cast 6 in the slab withdrawal direction D1.

[0025] In the continuous casting machine 1, a plurality of conveyor rolls (not shown) are provided downstream of the light reduction zone 10 for conveying the completely solidified slab slab 6. Above the conveyor rolls, a slab cutting machine (not shown) is provided for cutting the slab slab 6 to a predetermined length. The solidified slab slab slab 6, conveyed by the conveyor rolls, is cut to a predetermined length by the slab cutting machine.

[0026] Here, when manufacturing slab slabs 6 by continuous casting, flat rolls are usually used, which are rolls with a flat surface in the axial direction, which is the same direction as the width of the slab slab 6. However, in the manufacture of extra-thick steel plates, the difference in thickness between the slab slab 6 and the finished product is small, making it difficult to achieve sufficient reduction. As a result, defects such as smears and porosity that occur inside the slab slab 6 cannot be completely eliminated by subsequent rolling, leading to the problem of defects such as smears and porosity remaining in the finished product.

[0027] In the continuous casting machine 1 according to the embodiment, at least one of a flat roll, which is flat in the axial direction, and a convex roll, which has a smaller diameter at its ends than at its center in the axial direction, is used for the support roll pairs positioned upstream of the final solidification position Pe of the slab slab 6 in the slab withdrawal direction D1. Specifically, in the continuous casting machine 1 according to the embodiment, at least one of a flat roll and a convex roll is used for the six support roll pairs 7a of segment 8a and for the plurality of support roll pairs 7 positioned upstream of segment 8a. Furthermore, in the continuous casting machine 1 according to the embodiment, a convex roll is used for the support roll pairs positioned downstream of the final solidification position Pe of the slab slab 6 in the slab withdrawal direction D1. Specifically, in the continuous casting machine 1 according to the embodiment, a convex roll, which has a smaller diameter at its ends than at its center in the axial direction, is used for the six support roll pairs 7b of at least one of the two segments 8b of segment 8b.

[0028] Figure 2 shows the widthwise end of segment 8b, which uses a convex roll for the support roll pair 7b.

[0029] As shown in Figure 2, the support roll pair 7b provided on segment 8b consists of a first support roll 71b positioned on the upper surface 61 side in the thickness direction of the slab slab 6, and a second support roll 72b positioned on the lower surface 62 side in the thickness direction of the slab slab 6. The support roll pair 7b presses down the slab slab 6 by sandwiching it in the thickness direction with the first support roll 71b and the second support roll 72b, while conveying the slab slab 6 downstream in the slab withdrawal direction D1. The first support roll 71b has a rotating shaft 713b that is rotatably supported on a bearing member 11. The second support roll 72b has a rotating shaft 723b that is rotatably supported on a bearing member 11.

[0030] As shown in Figure 2, the first support roll 71b has a small-diameter roll section 712b, which corresponds to the portion of the slab cast 6 that is outside the triple junction Pt in the width direction, and a large-diameter roll section 711b, which corresponds to the portion of the slab cast 6 that is inside the triple junction Pt in the width direction. The first support roll 71b uses a convex roll in which the diameter φ2 of the small-diameter roll section 712b is smaller than the diameter φ1 of the large-diameter roll section 711b. The triple junction Pt of the slab cast 6 is the point where dendrites solidified from three directions—the upper surface 61 and the lower surface 62 in the thickness direction of the slab cast 6 and the end surface 63 in the width direction of the slab cast 6—collide. The roll surfaces of the large-diameter roll section 711b and the small-diameter roll section 712b are continuously connected in the axial direction.

[0031] As shown in Figure 2, the second support roll 72b has a small-diameter roll section 722b, which corresponds to the portion of the slab casting 6 that is outside the triple junction Pt in the width direction, and a large-diameter roll section 721b, which corresponds to the portion of the slab casting 6 that is inside the triple junction Pt in the width direction. The second support roll 72b uses a convex roll in which the diameter φ2 of the small-diameter roll section 722b is smaller than the diameter φ1 of the large-diameter roll section 721b. In this embodiment, the first support roll 71b and the second support roll 72b use convex rolls of the same shape, but the embodiment is not limited to this.

[0032] As a result, when the slab 6 is pressed down by the first support roll 71b and the second support roll 72b, the slab 6 comes into contact with the large-diameter roll sections 711b and 721b but not with the small-diameter roll sections 712b and 722b. In other words, when the slab 6 is pressed down by the first support roll 71b and the second support roll 72b, as shown in Figure 2, a non-contact section W that does not come into contact with the first support roll 71b and the second support roll 72b exists at the end of the slab 6 in the width direction.

[0033] Furthermore, it is undesirable for the distance d between the roll surfaces of the large-diameter roll sections 711b, 721b and the small-diameter roll sections 712b, 722b in the thickness direction of the slab slab 6 to be too small, for example, 5 [mm]. This is because if the distance d between the roll surfaces is too small, the small-diameter roll sections 712b, 722b will immediately come into contact with the slab slab 6 when the first support roll 71b and the second support roll 72b press down on the slab slab 6. Also, it is undesirable for the distance d between the roll surfaces to be too large, for example, 50 [mm]. This is because if the distance d between the roll surfaces is too large, the diameter φ2 of the small-diameter roll sections 712b, 722b will become too small, which may cause strength problems. For this reason, in the continuous casting machine 1 according to this embodiment, the distance d between the roll surfaces is set to, for example, 18 [mm].

[0034] Furthermore, the position of the triple point Pt of the slab slab 6 varies depending on the thickness of the slab slab 6. At the edges in the width direction of the slab slab 6, the solidification speed from the side surface of the slab slab slab 6, the solidification speed from the front surface of the slab slab slab 6, and the solidification speed from the back surface of the slab slab slab 6 are approximately the same. Therefore, the boundary between the solidification structure extending from the side surface of the slab slab slab 6 and the solidification structure extending from the front surface of the slab slab slab 6 extends from the upper corner of the slab slab slab slab slab 6 at an angle of 45 degrees with the front surface. Similarly, the boundary between the solidification structure extending from the side surface of the slab slab slab slab slab 6 and the solidification structure extending from the back surface of the slab slab slab slab slab 6 extends from the lower corner of the slab slab slab slab slab slab slab slab at an angle of 45 degrees with the back surface. Consequently, the triple point Pt of the slab slab slab slab 6 ultimately occurs at a point in the width direction of the slab For example, if the thickness of the slab 6 is 460 mm, the position of the triple point Pt will be approximately 230 mm from the edge in the width direction of the slab 6. Therefore, for the first support roll 71b and the second support roll 72b, which are convex rolls, the length of the non-contact portion W located at the edge in the width direction of the slab 6 is preferably 230 mm. This is because by making the length of the non-contact portion W 230 mm, the triple point Pt can be pressed down by the convex portion. If the length of the non-contact portion W is too large (for example, 250 mm), the triple point Pt cannot be pressed down. Also, if the length of the non-contact portion W is too small (for example, 50 mm), when the slab 6 is pressed down by the first support roll 71b and the second support roll 72b, the fully solidified hardened portion near the edge in the width direction of the slab 6 will be pressed down. Therefore, a sufficient reduction effect cannot be obtained.

[0035] In the continuous casting machine 1 according to this embodiment, for example, the length of the non-contact portion W obtained from the relationship between the thickness t of the slab slab 6 and the length of the non-contact portion W, as shown in the following formula (1), may be applied. Regardless of the thickness t of the slab slab slab 6, the width of the fully solidified hardened portion is approximately 100 [mm] from the edge in the width direction of the slab slab slab 6, so the lower limit of the non-contact portion W is set to 100 [mm]. The upper limit of the non-contact portion W is set to t / 2 [mm] (half the thickness t of the slab slab slab 6) because if it is inside the triple point Pt in the width direction of the slab slab slab 6, the triple point Pt cannot be pressed down by the convex portion.

[0036] 100 ≤ W ≤ t / 2 (unit: mm) .... (1)

[0037] Figure 3 shows an example of a convex roll applicable to the first support roll 71b and the second support roll 72b. Note that only the second support roll 72b is shown in Figure 3.

[0038] The convex rolls used for the first support roll 71b and the second support roll 72b may be composed of divided rolls that are split in the axial direction. For example, as shown in Figure 3, taking the second support roll 72b as an example, a convex roll composed of divided rolls that are split in two in the axial direction into a first roll section 72bA and a second roll section 72bB can be used. In this case, for the first roll section 72bA, the roll section corresponding to the part outside the triple point Pt in the width direction of the slab cast 6 is made into a small-diameter roll section 722bA, and the roll section corresponding to the part inside the triple point Pt in the width direction of the slab cast 6 is made into a large-diameter roll section 721bA. Similarly, for the second roll section 72bB, the roll section corresponding to the part outside the triple point Pt in the width direction of the slab cast 6 is made into a small-diameter roll section 722bB, and the roll section corresponding to the part inside the triple point Pt in the width direction of the slab cast 6 is made into a large-diameter roll section 721bB.

[0039] Figure 4 shows another example of a convex roll applicable to the first support roll 71b and the second support roll 72b. Note that only the second support roll 72b is shown in Figure 4.

[0040] The convex rolls used for the first support roll 71b and the second support roll 72b may consist of a single roll. For example, as shown in Figure 4, the second support roll 72b is used as an example. The second support roll 72b has small-diameter roll sections 722b at both ends in the axial direction, corresponding to the portion of the slab casting 6 that is outside the triple junction Pt in the width direction. The second support roll 72b also has a large-diameter roll section 721b in the axial center, corresponding to the portion of the slab casting 6 that is inside the triple junction Pt in the width direction. The large-diameter roll section 711b in the axial center of the second support roll 72b and the small-diameter roll sections 712b at both ends in the axial direction are continuously connected at their respective roll surfaces in the axial direction.

[0041] Figure 5 shows an example of three route pipes 901, 902, and 903 branching off from the main pipe 900. Figure 6 shows an example of a region in the width direction of the slab casting 6 that is cooled by the cooling water 90 supplied from each route pipe 901, 902, and 903.

[0042] In the continuous casting machine 1, the amount of cooling water sprayed from the spray nozzle 9 toward the slab slab 6 is adjusted in the width direction of the slab slab 6 so that the final solidification position Pe is in the center of the width direction of the slab slab 6, between directly below the mold 5 and the upper straightening point in the slab withdrawal direction D1. In addition, the casting speed and the amount of cooling water in the width direction of the slab slab 6 are adjusted so that the final solidification position Pe of the slab slab 6 is located directly in front of the roll pair 7b, which consists of a first support roll 71b and a second support roll 72b using convex rolls.

[0043] In the conventional cooling of the slab casting 6 using a spray nozzle 9, cooling water is supplied from the main pipe to the spray nozzle 9, and the distance from the upper surface 61 or lower surface 62 of the slab casting 6 to the spray nozzle 9 is adjusted to control the spread of the cooling water in the width direction of the slab casting 6.

[0044] On the other hand, between directly below the mold 5 and the upper correction point in the casting slab extraction direction D1, for example, as shown in FIG. 5, it branches from the main pipe 900 into three route pipes, i.e., the first route pipe 901, the second route pipe 902, and the third route pipe 903. And cooling water is supplied to a plurality of spray nozzles 9 arranged side by side in the width direction of the slab casting 6 from each of the route pipes 901, 902, 903. Therefore, by adjusting the flow rate of the cooling water flowing through each of the route pipes 901, 902, 903, the cooling state in the width direction of the slab casting 6 can be controlled.

[0045] For example, as shown in FIG. 6, in the width direction of the slab casting 6, a first route pipe region Ro1, a second route pipe region Ro2, and a third route pipe region Ro3 are set. The first route pipe region Ro1 is set corresponding to the central portion in the width direction of the slab casting 6. In the first route pipe region Ro1, spray nozzles 9a to which cooling water 90 is supplied from the first route pipe 901 are arranged side by side in the width direction, four on each of the upper surface 61 side and the lower surface 62 side of the slab casting 6. The second route pipe region Ro2 is set on both sides adjacent to the first route pipe region Ro1 in the width direction of the slab casting 6. In the second route pipe region Ro2, spray nozzles 9b to which cooling water 90 is supplied from the second route pipe 902 are arranged, two on each of the upper surface 61 side and the lower surface 62 side of the slab casting 6, sandwiching the first route pipe region Ro1. The third route pipe region Ro3 is on both sides adjacent to the second route pipe region Ro2 in the width direction of the slab casting 6 and is set corresponding to both end portions in the width direction of the slab casting 6. In the third route pipe region Ro3, spray nozzles 9c to which cooling water 90 is supplied from the third route pipe 903 are arranged, two on each of the upper surface 61 side and the lower surface 62 side of the slab casting 6, sandwiching the first route pipe region Ro1 and the second route pipe region Ro2.

[0046] And the first route pipe region Ro1 has a narrow cooling range in the width direction of the slab slab 6. Also, the first route pipe region Ro1 + the second route pipe region Ro2 has an intermediate-width cooling range in the width direction of the slab slab 6. Further, the first route pipe region Ro1 + the second route pipe region Ro2 + the third route pipe region Ro3 has a wide-width cooling range in the width direction of the slab slab 6. Thus, in the continuous casting machine 1 according to the embodiment, it is possible to adjust the cooling range in the width direction of the slab slab 6.

[0047] Here, in the continuous casting machine 1 according to the embodiment, when the amount of water flowing into the nth (n is a natural number) route pipe branched from the main pipe 900 is represented as Qn, it is set so as to satisfy the relational expression of Q1 ≦ Q2 + Q3 + ··· + Qn. This means that the amount of water in the first route pipe 901 that supplies the cooling water 90 to the plurality of spray nozzles 9a in the first route pipe region Ro1 that preferentially cools the central portion in the width direction of the slab slab 6 is 50 [%] or less of the total cooling water amount (the amount of water in the main pipe 900). Thus, by setting the amount of cooling water 90 flowing through the first route pipe 901 so as to satisfy the above relational expression, it is possible to suppress the cooling of the central portion in the width direction of the slab slab 6.

[0048] Generally, the shape of the unfrozen portion 2a at the final solidification position Pe of the slab slab 6 is determined by the density distribution (water amount distribution) of the cooling water in the width direction of the slab slab 6. When the cooling at the end in the width direction of the slab slab 6 is weaker than that at the central portion in the width direction of the slab slab 6, the final solidification position Pe in the vicinity of the end in the width direction of the slab slab 6 is downstream in the drawing direction from the final solidification position Pe at the central portion in the width direction of the slab slab 6. Bridging is formed in the vicinity of the final solidification position Pe, and scum is likely to be generated. Also, the end in the width direction of the slab slab 6 is a portion where the temperature of the slab slab 6 is low and it is difficult to perform reduction. Therefore, the end in the width direction of the slab slab 6, where scum is likely to occur and it is also difficult to perform reduction, is disadvantageous for improving the quality inside the slab slab 6.

[0049] Therefore, in the continuous casting machine 1 according to this embodiment, the amount of water in the first route piping 901 is relatively reduced so that the final solidification position Pe is in the center of the width direction of the slab slab 6, which is easy to reduce with the convex rolls. This makes it easier to compress the porosity that has formed in the center of the width direction of the slab slab 6 by reduction using the first support roll 71b and the second support roll 72b which use convex rolls. Furthermore, in the continuous casting machine 1 according to this embodiment, the slab slab 6 is lightly reduced by the first support roll 71b and the second support roll 72b which use convex rolls, using a segment 8b provided for two segments immediately after the final solidification position Pe in the slab withdrawal direction D1.

[0050] As described above, in the continuous casting method for slab slabs according to the embodiment, the slab slab 6 is lightly compressed by a first support roll 71b and a second support roll 72b using convex rolls for two segments immediately after the final solidification position Pe of the slab slab 6. This suppresses defects such as burrs that occur inside the slab slab 6 and prevents defects such as burrs from remaining in the product. Furthermore, by adjusting the cooling conditions, which control the amount of cooling water sprayed from the spray nozzle 9 in the width direction of the slab slab 6, it is possible to further suppress the remaining defects such as burrs inside the slab slab 6.

[0051] Furthermore, in the continuous casting method for slab slabs according to the embodiment, the edge portion of the slab slab 6 may have a chamfered shape with a chamfer dimension of 10% or less (excluding 0%) of the thickness of the slab slab 6, preferably about 5%. In this case, for example, a continuous casting machine 1 is used that has a mold 5 with an octagonal chamfered shape, where the chamfer dimension is 10% or less (excluding 0%) of the thickness of the slab slab 6, preferably about 5%, as the mold cross section perpendicular to the drawing direction. This makes it possible to make the edge portion of the slab slab 6, which is difficult to reduce, into a shape that is easy to reduce in the continuous casting method for slab slabs according to the embodiment.

[0052] Table 1 shows the pass rate of the Ultrasonic Testing (UT) test (compliant with JIS G 0801) when slab slabs 6, manufactured by continuous casting under the conditions of Invention Example 1, Comparative Example 1, and Comparative Example 2, were rolled to a thickness of 120 mm and subjected to the UT test.

[0053] In Invention Example 1, a slab slab 6 is continuously cast using a continuous casting machine 1 to which the continuous casting method for slabs according to the embodiment is applied. Convex rolls are used for the first support roll 71b and the second support roll 72b that constitute the support roll pair 7b of the segment 8b of the light reduction zone 10. Furthermore, the casting speed and the amount of cooling water in the width direction of the slab slab 6 are adjusted so that the final solidification position Pe of the slab slab 6 is immediately in front of the upstream support roll pair 7b in the slab withdrawal direction D1 of segment 8b, in other words, immediately in front of the convex roll.

[0054] Comparative Example 1 is a comparative example in which the casting speed and the amount of cooling water in the width direction of the slab 6 were adjusted so that the final solidification position Pe of the slab 6 was at the position of the upstream support roll pair 7b in the slab withdrawal direction D1 of segment 8b, in other words, at the position of the convex roll.

[0055] Comparative Example 2 uses flat rolls for the first support roll 71b and the second support roll 72b that constitute the support roll pair 7b of segment 8b of the light reduction zone 10, in contrast to Invention Example 1. Furthermore, the casting speed and the amount of cooling water in the width direction of the slab slab 6 were adjusted so that the final solidification position Pe of the slab slab 6 is at the position of the upstream support roll pair 7b in the slab withdrawal direction D1 of segment 8b, in other words, at the position of the flat roll.

[0056]

[0057] As can be seen from Table 1, the UT pass rate in Comparative Example 1 was 27%, and in Comparative Example 2, the UT pass rate was 75%. On the other hand, Invention Example 1 showed a very good result with a UT pass rate of 100%.

[0058] The present invention can provide a continuous casting method for slab slabs and a slab slab that can reduce the porosity of the slab slab.

[0059] 1 Continuous casting machine 2 Molten steel 3 Tundish 4 Immersion nozzle 5 Mold 6 Slab 7, 7a, 7b Support roll pair 8a, 8b Segment 9, 9a, 9b, 9c Spray nozzle 10 Light compression zone 11 Bearing member 61 Top surface 62 Bottom surface 63 End surface 71b First support roll 72b Second support roll 900 Main piping 901 First route piping 902 Second route piping 903 Third route piping

Claims

1. A continuous casting method for slab slabs, characterized in that, in a segment of a light reduction zone positioned downstream in the slab withdrawal direction from the final solidification position of a slab slab that has been continuously cast and withdrawn from a mold, and in which a pair of rolls lightly reduces the slab slab, the pair of rolls uses convex rolls in which the diameter of the roll portion corresponding to the portion of the slab slab that is wider than the triple point is smaller than the diameter of the roll portion corresponding to the portion of the slab that is wider than the triple point.

2. The continuous casting method for a slab slab according to claim 1, characterized in that the casting speed and the amount of cooling water used to cool the slab slab are adjusted in the width direction of the slab slab so that the final solidification position is located in the slab withdrawal direction, directly in front of the pair of rolls using the convex rolls.

3. The continuous casting method for a slab slab according to claim 2, characterized in that the cooling water is supplied to a plurality of spray nozzles arranged in line in the width direction of the slab slab from a plurality of route pipes branched from a main pipe, and the cooling water is sprayed onto the slab slab from the plurality of spray nozzles to cool the slab slab, and the amount of cooling water flowing through the route pipe that supplies the cooling water to the spray nozzle that sprays the cooling water to the central part in the width direction of the slab slab is 50% or less of the total amount of cooling water flowing through the plurality of route pipes.

4. The method for continuous casting of a slab slab according to claim 2, characterized in that a continuous casting machine is used, which has a mold cross section with an octagonal chamfer shape in which the chamfer dimension is 10% or less of the thickness of the slab slab.

5. A slab slab manufactured using the continuous casting method for slab slabs described in any one of claims 1 to 4.

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