Steel continuous casting method

Abstract

A steel continuous casting method according to the present invention is a method in which casting is performed using a continuous casting machine provided with a plurality of roll segments each having a plurality of pairs of cast slab supporting roll pairs that sandwich and support a cast slab withdrawn from a mold, wherein, if the roll segment that includes the final solidification position of the cast slab is defined as a first roll segment and the roll segment positioned farthest downstream in the direction in which the cast slab is withdrawn in the continuous casting machine is defined as a second roll segment: a total reduction amount, which is the sum of the reduction amounts of the cast slab from the first roll segment to the second roll segment, is 6 mm or greater; and the range of the reduction amounts in each roll segment from the first roll segment to the second roll segment is 0.1 mm to 5 mm inclusive.

Description

Continuous casting method for steel

[0001] The present invention relates to a continuous casting method for steel.

[0002] Conventionally, it has been known that porosity and center segregation occurring in the thickness center of a slab produced by continuous casting lead to defects and deterioration of properties in the product. In recent years, an increase in the thickness of the product has been demanded. When the product thickness is increased, the reduction ratio decreases, so porosity in the slab is likely to remain in the product. Therefore, further reduction of porosity in the slab is demanded. In order to reduce the porosity of the slab in the continuous casting process, for example, soft reduction is carried out until solidification is completed, or reduction is carried out after solidification. However, in the soft reduction carried out until solidification is completed, a certain amount of porosity remains, and the porosity cannot be pressed and remains in the product produced at a low reduction ratio. On the other hand, when reduction is carried out after solidification, the porosity generated until solidification is completed can be rendered harmless, and a post-solidification reduction method for rendering porosity harmless is disclosed in, for example, Patent Documents 1 and 2.

[0003] Patent Document 1 discloses a technique in which a stepped roll rolling mill and a flat roll rolling mill are installed on the downstream side of a slab cutting device of a continuous casting machine, and heavy reduction is carried out in two stages while there is a temperature difference between the center part and the surface part of the slab. Patent Document 2 discloses a technique in which a slab is reduced using two pairs of reduction rolls discretely arranged in a range where the roll interval is 3 [m] or more and 7 [m] or less. In the technique disclosed in Patent Document 2, the slab with a solid fraction of 0.8 or more and less than 1 in the thickness center part of the slab is reduced by 3 [mm] to 15 [mm] with the first-stage reduction roll, and the slab after complete solidification is reduced with the second-stage reduction roll.

[0004] Japanese Patent Laid-Open No. 55-106601 International Publication No. 2014 / 203902

[0005] However, the technology disclosed in Patent Document 1 requires the installation of two types of rolling mills, a stepped roll rolling mill and a flat roll rolling mill, downstream of the continuous casting machine, which leads to a significant increase in equipment costs. Furthermore, the technology disclosed in Patent Document 2 requires the installation of large-diameter rolls to reduce the slab by 3 mm or more before complete solidification, or to reduce the slab after complete solidification using a pair of reduction rolls. As a result, the gap between the front and rear reduction rolls becomes wider, causing unsteady bulging and affecting the quality of the slab. This also leads to a significant increase in equipment costs, similar to Patent Document 1.

[0006] The present invention has been made in view of the above problems, and its objective is to provide a continuous casting method for steel that can reduce porosity while suppressing an increase in equipment costs when continuously casting slabs used for thick-walled products.

[0007] In order to solve the above-mentioned problems and achieve the objective, (1) The present invention provides a continuous casting method for steel, which is a continuous casting method for steel in which a plurality of roll segments are provided, each having a plurality of pairs of slab support rolls that sandwich and support a slab drawn from a mold, wherein the roll segment including the final solidification position of the slab is designated as the first roll segment, the roll segment located at the downstream end in the slab drawing direction in the continuous casting machine is designated as the second roll segment, the total reduction amount which is the sum of the reduction amounts of the slab from the first roll segment to the second roll segment is 6 [mm] or more, and the range of reduction amounts in each roll segment from the first roll segment to the second roll segment is 0.1 [mm] or more and 5 [mm] or less.

[0008] (2) The continuous casting method for steel according to the present invention, in the invention of (1) above, the slab support roll pair is composed of a drive roll and a driven roll, and the drive roll and the driven roll of the slab support roll pair in each roll segment from the first roll segment to the second roll segment are convex rolls in which the diameter of the axial end is smaller than the diameter of the axial center.

[0009] (3) In the continuous casting method for steel according to the present invention, in the invention of (2) above, the diameter of the roll portion of the convex roll that corresponds to the portion of the cast slab that is outside in the width direction of the triple point is smaller than the diameter of the roll portion that corresponds to the portion of the cast slab that is inside in the width direction of the triple point.

[0010] (4) In the continuous casting method for steel according to the present invention, in any one of the inventions (1) to (3) above, the thickness of the cast slab after reduction by the second roll segment is 200 [mm] or more and 500 [mm] or less.

[0011] The continuous steel casting method according to the present invention can reduce porosity with a lower reduction amount compared to conventional techniques that reduce porosity by reducing the cast slab after complete solidification. Therefore, the continuous steel casting method according to the present invention can be applied to existing roll segment equipment for light reduction, thereby suppressing increases in equipment costs. Furthermore, since the continuous steel casting method according to the present invention controls the reduction amount in units of roll segments, similar to many existing roll segment equipment for light reduction, there is no need to introduce new equipment, thus suppressing increases in equipment costs. Thus, the continuous steel casting method according to the present invention has the effect of reducing porosity while suppressing increases in equipment costs when continuously casting cast slabs used for thick-walled products.

[0012] Figure 1 is a schematic diagram showing an example of a continuous casting machine to which the steel continuous casting method according to the present invention is applied. Figure 2 shows the widthwise ends of a slab support roll pair using convex rolls. Figure 3 shows an example of a convex roll applicable to the drive roll and driven roll of a slab support roll pair. Figure 4 shows another example of a convex roll applicable to the drive roll and driven roll of a slab support roll pair. Figure 5 shows the relationship between the total reduction amount and the porosity index in multiple roll segments after complete solidification. Figure 6 shows the relationship between the maximum reduction amount and the porosity index. Figure 7 shows an example in which three roll segments are provided in the horizontal zone.

[0013] Embodiments of the present invention will be described below. However, the present invention is not limited to these embodiments.

[0014] The following describes an embodiment of the continuous casting method for steel according to the present invention. However, the present invention is not limited to this embodiment.

[0015] Figure 1 is a schematic diagram showing an example of a continuous casting machine 11 to which the steel continuous casting method according to the present invention is applied. The continuous casting machine 11 shown in Figure 1 is a vertical bending type continuous casting machine. However, it is not limited to a vertical bending type; a curved type or a completely vertical type continuous casting machine can also be used.

[0016] The continuous casting machine 11 shown in Figure 1 includes a tundish 14, a mold 13, a plurality of slab support roll pairs 16, and a plurality of spray nozzles 17. Also, as shown in Figure 1, the slab 18 is withdrawn in the slab withdrawal direction D1. In this embodiment, the side where the tundish 14 is located in the slab withdrawal direction D1 is described as the upstream side, and the side to which the slab 18 is withdrawn is described as the downstream side.

[0017] The tundish 14 is located above the mold 13 and supplies molten steel 12 to the mold 13. Molten steel 12 is supplied to the tundish 14 from a ladle (not shown) and stored there. A sliding nozzle (not shown) for adjusting the flow rate of molten steel 12 is installed at the bottom of the tundish 14, and an immersion nozzle 15 is installed on the underside of this sliding nozzle.

[0018] The mold 13 is located below the tundish 14. Molten steel 12 is poured into the mold 13 from the immersion nozzle 15 of the tundish 14. The poured molten steel 12 is cooled (primary cooling) in the mold 13, thereby forming the outer shell shape of the cast slab 18.

[0019] Multiple slab support roll pairs 16 support the slab 18 from both sides along the slab withdrawal direction D1. The multiple slab support roll pairs 16 are composed of multiple support rolls, for example, a support roll pair, a guide roll pair, and a pinch roll pair. Also, as shown in Figure 1, multiple slab support roll pairs 16 come together to form a single roll segment 20.

[0020] Multiple spray nozzles 17 are provided between adjacent pairs of slab support rolls 16 along the slab withdrawal direction D1. The spray nozzles 17 are nozzles for secondary cooling of the slab 18 by spraying cooling water onto it. Any nozzle can be used as the spray nozzles 17, such as water spray nozzles (single-fluid spray nozzles) or air mist spray nozzles (two-fluid spray nozzles).

[0021] The cast slab 18 is cooled as it is drawn out along the slab drawing direction D1 by cooling water (secondary cooling water) sprayed from multiple spray nozzles 17. In Figure 1, reference numeral 18a indicates the unsolidified portion of the molten steel 12 within the cast slab 18. Also in Figure 1, reference numeral 18b indicates the solidification completion position where the unsolidified portion 18a has disappeared and solidification is complete.

[0022] A light reduction zone 19 for lightly reducing the cast slab 18 is provided downstream of the continuous casting machine 11 in the slab withdrawal direction D1. The light reduction zone 19 is provided with roll segments 20, which are composed of multiple pairs of slab support rolls 16, in the order of roll segment 20B, roll segment 20Aa, and roll segment 20Ab in the slab withdrawal direction D1. The multiple pairs of slab support rolls 16 in the light reduction zone 19 are arranged such that the distance between each pair of slab support rolls 16 in the thickness direction of the cast slab 18 gradually narrows toward the slab withdrawal direction D1. As a result, the multiple pairs of slab support rolls 16 in the light reduction zone 19 lightly reduce the cast slab 18 as it passes through the light reduction zone 19. Also, in Figure 1, the lower straightening position of the continuous casting machine 11, which is located within the area of ​​the light reduction zone 19, is indicated by reference numeral 22.

[0023] Downstream of the continuous casting machine 11, a horizontal zone A1 is provided where the cast slab 18 is transported horizontally. In Figure 1, the horizontal zone A1 contains roll segments 20Aa and 20Ab.

[0024] In the continuous casting machine 11, a plurality of transport rolls 21 for transporting the completely solidified cast slab 18 are provided downstream of the horizontal zone A1. Above the transport rolls 21, a cast slab cutting machine (not shown) is provided for cutting the cast slab 18 to a predetermined length.

[0025] In the continuous casting machine 1 according to this embodiment, flat rolls that are flat in the axial direction are used as the plurality of slab support roll pairs 16. On the other hand, in the continuous casting machine 1 according to this embodiment, convex rolls with a smaller diameter at the ends than at the center in the axial direction may be used for the slab support roll pairs 16 of the roll segments 20Aa and 20Ab that are located in the horizontal band region A1 of the light reduction band 19.

[0026] Figure 2 shows the widthwise ends of a pair of cast slab support rolls 16 using convex rolls. Note that while Figure 2 shows one end of the pair of cast slab support rolls 16 in the widthwise direction, the other end has a similar configuration.

[0027] As shown in Figure 2, the slab support roll pair 16, which is part of the roll segments 20Aa, 20Ab, etc., is composed of a drive roll 16a positioned on the upper surface 181 side in the thickness direction of the slab 18, and a driven roll 16b positioned on the lower surface 182 side in the thickness direction of the slab 18. The slab support roll pair 16 presses down on the slab 18 by clamping it in the thickness direction with the drive roll 16a and the driven roll 16b, while transporting the slab 18 downstream in the slab withdrawal direction D1. The drive roll 16a has a rotating shaft 163a rotatably supported on a bearing member 30a, and rotates by a rotational driving force from a drive source (not shown), applying a transport force to the slab 18. The driven roll 16b has a rotating shaft 163b rotatably supported on a bearing member 30b, and rotates in a driven manner, following the slab 18 that is transported as the drive roll 16a rotates.

[0028] The drive roll 16a has a small-diameter roll section 162a, which corresponds to the portion of the cast slab 18 that is outside the width direction of the triple junction, and a large-diameter roll section 161a, which corresponds to the portion of the cast slab 18 that is inside the width direction of the triple junction. The drive roll 16a uses a convex roll in which the diameter φ12 of the small-diameter roll section 162a is smaller than the diameter φ11 of the large-diameter roll section 161a. The triple junction of the cast slab 18 is the point where dendrites solidified from three directions—the upper surface 181 and the lower surface 182 in the thickness direction of the cast slab 18 and the end surface 183 in the width direction of the cast slab 18—collide. The roll surfaces of the large-diameter roll section 161a and the small-diameter roll section 162a are continuously connected in the axial direction.

[0029] The driven roll 16b has a small-diameter roll section 162b, which corresponds to the portion of the cast slab 18 that is outside the width direction of the triple junction, and a large-diameter roll section 161b, which corresponds to the portion of the cast slab 18 that is inside the width direction of the triple junction. The driven roll 16b uses a convex roll in which the diameter φ22 of the small-diameter roll section 162b is smaller than the diameter φ21 of the large-diameter roll section 161b. In this embodiment, the diameters φ11 = φ21 and φ12 = φ22, and the same shape of convex roll is used for both the driven roll 16a and the driven roll 16b, but it is not limited to this.

[0030] When the cast slab 18 is pressed down by the drive roll 16a and driven roll 16b, which use convex rolls as shown in Figure 2, the cast slab 18 comes into contact with the large-diameter roll sections 161a and 161b but not with the small-diameter roll sections 162a and 162b. In other words, when the cast slab 18 is pressed down by the drive roll 16a and driven roll 16b, there are non-contact areas at the ends of the cast slab 18 in the width direction that do not come into contact with the drive roll 16a and driven roll 16b.

[0031] Figure 3 shows an example of a convex roll applicable to the drive roll 16a and driven roll 16b of a cast slab support roll pair 16.

[0032] As shown in Figure 3, the convex rolls used as the slab support roll pair 16, consisting of a drive roll 16a and a driven roll 16b, are composed of, for example, a single roller. The drive roll 16a and the driven roll 16b have small-diameter roll sections 162a and 162b at both ends in the axial direction, corresponding to the portion of the slab 18 outside the triple junction in the width direction. The drive roll 16a and the driven roll 16b also have large-diameter roll sections 161a and 161b in the axial center, corresponding to the portion of the slab 18 inside the triple junction in the width direction. As shown in Figure 3, the large-diameter roll sections 161a and 161b and the small-diameter roll sections 162a and 162b of the drive roll 16a and the driven roll 16b are continuously connected in the axial direction at their respective roll surfaces.

[0033] Figure 4 shows another example of a convex roll applicable to the drive roll 16a and driven roll 16b of a cast slab support roll pair 16.

[0034] As shown in Figure 4, the convex rolls used for the drive roll 16a and the driven roll 16b may be, for example, composed of divided rollers that are divided in the axial direction. The drive roll 16a and the driven roll 16b can use convex rollers composed of divided rollers that are divided in the axial direction into first roll sections 16aA, 16bA and second roll sections 16aB, 16bB.

[0035] The first roll sections 16aA and 16bA have their rotating shafts 163aA and 163bA rotatably supported by bearing members 30a and 30b. For the first roll sections 16aA and 16bA, the roll sections corresponding to the parts of the cast slab 18 outside the triple point in the width direction are small-diameter roll sections 162aA and 162bA. Also, for the first roll sections 16aA and 16bA, the roll sections corresponding to the parts of the cast slab 18 inside the triple point in the width direction are large-diameter roll sections 161aA and 161bA. The second roll sections 16aB and 16bB have their rotating shafts 163aB and 163bB rotatably supported by bearing members 30a and 30b. For the second roll sections 16aB and 16bB, the roll sections corresponding to the parts of the cast slab 18 outside the triple junction in the width direction are small-diameter roll sections 162aB and 162bB. Also, for the second roll sections 16aB and 16bB, the roll sections corresponding to the parts of the cast slab 18 inside the triple junction in the width direction are large-diameter roll sections 161aB and 161bB.

[0036] Furthermore, if non-divided rolls are used as the drive roll 16a and driven roll 16b, there is a risk of breakage or damage due to the load generated during reduction. On the other hand, if divided rolls are used as the drive roll 16a and driven roll 16b, the load capacity of the rolls can be increased, preventing breakage or damage and allowing for reduction of a larger amount.

[0037] Here, the inventors of the present invention, using a general light reduction roll segment 20 for reducing a cast slab 18 including an unsolidified portion 18a, found through test castings manufacturing conditions that ensure a reduction amount after complete solidification without requiring equipment upgrades, and that prevent the retention of harmful porosity.

[0038] Test casting was performed using the continuous casting machine 11 shown in Figure 1, with a slab thickness of 250 mm and a slab width of 2000 mm. The casting speed was varied to change the final solidification position 18b in the slab withdrawal direction D1. The roll spacing (set roll opening) of each slab support roll pair 16 in the thickness direction of the slab 18 was also varied to change the reduction amount of each roll segment 20 and the total reduction amount of multiple roll segments 20 after complete solidification. The final solidification position 18b was calculated using solidification heat transfer calculations. Using the actual values ​​of this final solidification position 18b and the roll spacing of the slab support roller pair 16, the actual reduction amount of each roll segment 20 and the total reduction amount of multiple roll segments 20 after complete solidification were calculated.

[0039] Figure 5 shows the relationship between the total reduction amount in multiple roll segments 20 after complete solidification and the porosity index. Here, the porosity index was calculated by density measurement. This density measurement was performed using the water displacement method by taking a sample with a thickness of 30 [mm] and a width of 100 [mm] from the center of the slab 18 in a cross-section perpendicular to the slab drawing direction D1. In this embodiment, the porosity index was 2 [cm]. ３ If the porosity is less than or equal to 2 [cm], it is considered good as the porosity has been sufficiently reduced. ３ If the value is greater than [ / g], it is considered a defect because the porosity is not properly compressed and a large amount of porosity remains.

[0040] Furthermore, density measurements were also performed on samples near the surface in the central part of the width direction of the cast slab 18. Of these samples, the porosity index was calculated by subtracting the reciprocal of the density of the sample at the outermost width position in the width direction of the cast slab 18 from the reciprocal of the density of the sample with the lowest density. In Figure 5, "I" represents the region where porosity has been sufficiently reduced. In Figure 5, "II" represents the region where porosity has not been sufficiently compressed and remains. From Figure 5, it can be seen that porosity can be rendered harmless by making the total reduction amount of the cast slab 18 in each roll segment 20 after complete solidification 6 [mm] or more. The upper limit of the total reduction amount is preferably 0.1 × D [mm], where D [mm] is the thickness of the cast slab. This is because, with a total reduction amount greater than 0.1 × D [mm], the opening of internal cracks at the end face 183 and the amount of surface step caused by the convex roll become too large in the cross-section of the cast slab 18 in the direction perpendicular to the cast slab drawing direction D1, which may lead to an increase in surface defects in downstream processes.

[0041] Figure 6 shows the relationship between the maximum reduction amount and the porosity index. In Figure 6, the condition is that the total reduction amount in multiple roll segments 20 after complete solidification is in the range of 6 [mm] to 10 [mm]. From Figure 6, it can be seen that when the maximum reduction amount in any of the multiple roll segments 20 after complete solidification exceeds 5 [mm], the reduction amount in the other roll segments 20 decreases, making it impossible to sufficiently compress the porosity, resulting in a large amount of porosity remaining.

[0042] Therefore, in the continuous casting machine 11 according to the embodiment, for example, as shown in Figure 1, the roll segment 20Aa that includes the final solidification position 18b of the cast slab 18 is designated as the first roll segment. Also, in the continuous casting machine 1 according to the embodiment, as shown in Figure 1, the roll segment 20Ab located at the downstream end of the cast slab withdrawal direction D1 in the continuous casting machine 11 is designated as the second roll segment. Furthermore, the multiple roll segments 20 after the first roll segment correspond to the multiple roll segments 20 after complete solidification.

[0043] In the continuous casting machine 11 according to the embodiment, in the range from after the first roll segment to the second roll segment, the total reduction amount, which is the sum of the reduction amounts of the slab 18 in the plurality of roll segments 20, is set to 6 [mm] or more. That is, the total reduction amount in the roll segment 20Aa which is the first roll segment and the roll segment 20Ab which is the second roll segment is set to 6 [mm] or more. And at that time, in the range from after the first roll segment to the second roll segment, the range of the reduction amount for the slab 18 in each roll segment 20 is set to 0.1 [mm] or more and 5 [mm] or less. That is, the range of the reduction amount in each of the roll segment 20Aa which is the first roll segment and the roll segment 20Ab which is the second roll segment is 0.1 [mm] or more and 5 [mm] or less. In other words, the maximum value of the reduction amount is set to 5 [mm] or less. Further, in the continuous casting machine 11 according to the embodiment, the slab 18 is reduced in each roll segment 20 so that the thickness of the slab 18 after reduction in the second roll segment (roll segment 20Ab) is 200 [mm] or more and 500 [mm] or less.

[0044] FIG. 7 is a diagram showing an example in which three roll segments 20 are provided in the horizontal band region A1.

[0045] In the example shown in FIG. 7, in the horizontal band region A1 in the slab withdrawal direction D1, the roll segment 20Aa, the roll segment 20Ac, and the roll segment 20Ab are arranged in order. And in the example shown in FIG. 7, the roll segment 20Aa including the final solidification position 18b of the slab 18 is the first roll segment. Also, the roll segment 20Ab located at the most downstream in the slab withdrawal direction D1 in the continuous casting machine 11 is the second roll segment. Also, the plurality of roll segments 20 after the first roll segment correspond to the plurality of roll segments 20 after complete solidification.

[0046] And in the example shown in FIG. 7, in the range from after the first roll segment to the second roll segment, the total reduction amount, which is the sum of the reduction amounts of the slab 18 in each roll segment 20, is set to 6 [mm] or more. That is, in the example shown in FIG. 7, the total reduction amount in the roll segment 20Aa (the first roll segment), the roll segment 20Ac, and the roll segment 20Ab (the second roll segment) is set to 6 [mm] or more. And at that time, the range of the reduction amount in each of the roll segment 20Aa, the roll segment 20Ac, and the roll segment 20Ab is set to 0.1 [mm] or more and 5 [mm] or less. In other words, the maximum value of the reduction amount is set to 5 [mm] or less. Further, in the example shown in FIG. 7, the slab 18 is reduced in each roll segment 20 so that the thickness of the slab 18 after reduction in the second roll segment (roll segment 20Ab) is 200 [mm] or more and 500 [mm] or less.

[0047] In the continuous casting method of steel applied to the continuous casting machine 11 according to the embodiment, compared with the conventional technique of reducing porosity by reducing the slab after complete solidification, porosity can be reduced with a small reduction amount. Therefore, in the continuous casting method of steel applied to the continuous casting machine 11 according to the embodiment, it can be applied with the existing roll segment equipment for soft reduction as it is, and an increase in equipment cost can be suppressed. Also, in the continuous casting method of steel applied to the continuous casting machine 11 according to the embodiment, like many existing roll segment equipment for soft reduction, since the reduction amount is controlled in units of roll segments, there is no need to introduce new equipment, and an increase in equipment cost can be suppressed.

[0048] As described above, in the continuous casting method of steel applied to the continuous casting machine 11 according to the embodiment, when continuously casting the slab 18 used for thick products, it is possible to reduce porosity while suppressing an increase in equipment cost.

[0049] In this embodiment, the following tests were conducted to evaluate the effectiveness of the steel continuous casting method of the present invention. In this embodiment, a cast slab 18 drawn from a mold 13 using the continuous casting machine 11 shown in Figure 1 was pressed down by a plurality of slab support roll pairs 16, each having a plurality of roll segments 20. In this embodiment, the roll segment 20 containing the unsolidified portion 18a (for example, roll segment 20Aa) among the plurality of roll segments 20 is designated as the first roll segment. Note that the slower the casting speed, the further upstream the final solidification position 18b in the cast slab 18 moves in the cast slab withdrawal direction D1. Therefore, the roll segment 20 containing the unsolidified portion 18a that is designated as the first roll segment may also be a roll segment 20 located upstream of roll segment 20Aa in the cast slab withdrawal direction D1 (for example, roll segment 20B of the light reduction zone 19). In this embodiment, the roll segment 20Ab located at the downstream end of the cast slab withdrawal direction D1 in the continuous casting machine 11 is designated as the second roll segment. Furthermore, the thickness of the cast slab 18 after reduction at the roll segment 20Ab located at the downstream end of the cast slab withdrawal direction D1 in the second roll segment of the continuous casting machine 11 was set to 200 [mm] or more and 500 [mm] or less, and the width was set to 2000 [mm].

[0050] The test conditions included specifying the number of segments (number of reduction segments) from the first roll segment to the second roll segment that reduces the cast slab 18. The test conditions also included specifying the type of rolls used for the cast slab support roll pair 16 in each roll segment 20 from the first to the second roll segment. Furthermore, the test conditions included specifying the maximum reduction amount in any of the roll segments 20 from the first to the second roll segment. Finally, the test conditions included specifying the total reduction amount from the first to the second roll segment. These conditions were then varied in various ways in Invention Examples 1 to 8 and Comparative Examples 1 to 8, and the roll spacing (roll opening) of the cast slab support roll pair 16 in each roll segment 20 from the first to the second roll segment was set accordingly.

[0051] In Invention Example 1, the thickness of the cast slab 18 after reduction in the second roll segment was set to 300 [mm]. Also in Invention Example 1, the number of segments from the first roll segment to the second roll segment that reduces the cast slab 18 (number of reduction segments) was set to 4. Also in Invention Example 1, convex rolls were used for the slab support roll pairs 16 on each roll segment 20 from the first roll segment to the second roll segment. Also in Invention Example 1, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 4.8 [mm]. Also in Invention Example 1, the total reduction amount from the first roll segment to the second roll segment was set to 9.8 [mm].

[0052] In Invention Example 2, the thickness of the cast slab 18 after reduction in the second roll segment was set to 300 [mm]. Also in Invention Example 2, the number of segments from the first roll segment to the second roll segment that reduces the cast slab 18 (number of reduction segments) was set to 3. Also in Invention Example 2, flat rolls were used for the cast slab support roll pairs 16 on each roll segment 20 from the first roll segment to the second roll segment. Also in Invention Example 2, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 5.0 [mm]. Also in Invention Example 2, the total reduction amount from the first roll segment to the second roll segment was set to 6.6 [mm].

[0053] In Invention Example 3, the thickness of the cast slab 18 after reduction in the second roll segment was set to 250 [mm]. Also in Invention Example 3, the number of segments from the first roll segment to the second roll segment that reduces the cast slab 18 (number of reduction segments) was set to 4. Also in Invention Example 3, convex rolls were used for the cast slab support roll pairs 16 of each roll segment 20 from the first roll segment to the second roll segment. Also in Invention Example 3, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 4.5 [mm]. Also in Invention Example 3, the total reduction amount from the first roll segment to the second roll segment was set to 9.2 [mm].

[0054] In Invention Example 4, the thickness of the cast slab 18 after reduction in the second roll segment was set to 250 [mm]. Also in Invention Example 4, the number of segments from the first roll segment to the second roll segment that reduces the cast slab 18 (number of reduction segments) was set to 3. Also in Invention Example 4, flat rolls were used for the cast slab support roll pairs 16 on each roll segment 20 from the first roll segment to the second roll segment. Also in Invention Example 4, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 4.2 [mm]. Also in Invention Example 4, the total reduction amount from the first roll segment to the second roll segment was set to 6.3 [mm].

[0055] In Invention Example 5, the thickness of the cast slab 18 after reduction in the second roll segment was set to 400 [mm]. Also in Invention Example 5, the number of segments from the first roll segment to the second roll segment that reduces the cast slab 18 (number of reduction segments) was set to 4. Also in Invention Example 5, convex rolls were used for the cast slab support roll pairs 16 of each roll segment 20 from the first roll segment to the second roll segment. Also in Invention Example 5, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 4.7 [mm]. Also in Invention Example 5, the total reduction amount from the first roll segment to the second roll segment was set to 12.8 [mm].

[0056] In Invention Example 6, the thickness of the cast slab 18 after reduction in the second roll segment was set to 400 [mm]. Also in Invention Example 6, the number of segments from the first roll segment to the second roll segment that reduces the cast slab 18 (number of reduction segments) was set to 4. Also in Invention Example 6, convex rolls were used for the cast slab support roll pairs 16 of each roll segment 20 from the first roll segment to the second roll segment. Also in Invention Example 6, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 4.8 [mm]. Also in Invention Example 6, the total reduction amount from the first roll segment to the second roll segment was set to 12.0 [mm].

[0057] In Invention Example 7, the thickness of the cast slab 18 after reduction in the second roll segment was set to 500 [mm]. Also in Invention Example 7, the number of segments from the first roll segment to the second roll segment that reduce the cast slab 18 (number of reduction segments) was set to 4. Also in Invention Example 7, convex rolls were used for the cast slab support roll pairs 16 of each roll segment 20 from the first roll segment to the second roll segment. Also in Invention Example 7, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 5.0 [mm]. Also in Invention Example 7, the total reduction amount from the first roll segment to the second roll segment was set to 11.2 [mm].

[0058] In Invention Example 8, the thickness of the cast slab 18 after reduction in the second roll segment was set to 500 [mm]. Also in Invention Example 8, the number of segments from the first roll segment to the second roll segment that reduces the cast slab 18 (number of reduction segments) was set to 4. Also in Invention Example 8, convex rolls were used for the slab support roll pairs 16 on each roll segment 20 from the first roll segment to the second roll segment. Also in Invention Example 8, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 4.8 [mm]. Also in Invention Example 8, the total reduction amount from the first roll segment to the second roll segment was set to 10.4 [mm].

[0059] In Comparative Example 1, the thickness of the cast slab 18 after reduction in the second roll segment was set to 300 [mm]. Also in Comparative Example 1, the number of segments from the first roll segment to the second roll segment that reduces the cast slab 18 (number of reduction segments) was set to 2. Also in Comparative Example 1, flat rolls were used for the cast slab support roll pairs 16 on each roll segment 20 from the first roll segment to the second roll segment. Also in Comparative Example 1, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 2.0 [mm]. Also in Comparative Example 1, the total reduction amount from the first roll segment to the second roll segment was set to 2.4 [mm].

[0060] In Comparative Example 2, the thickness of the cast slab 18 after reduction in the second roll segment was set to 300 [mm]. Also in Comparative Example 2, the number of segments from the first roll segment to the second roll segment that reduces the cast slab 18 (number of reduction segments) was set to 4. Also in Comparative Example 2, flat rolls were used for the cast slab support roll pairs 16 on each roll segment 20 from the first roll segment to the second roll segment. Also in Comparative Example 2, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 5.7 [mm]. Also in Comparative Example 2, the total reduction amount from the first roll segment to the second roll segment was set to 9.6 [mm].

[0061] In Comparative Example 3, the thickness of the cast slab 18 after reduction in the second roll segment was set to 250 [mm]. Also in Comparative Example 3, the number of segments from the first roll segment to the second roll segment that reduces the cast slab 18 (number of reduction segments) was set to 2. Also in Comparative Example 3, flat rolls were used for the cast slab support roll pairs 16 on each roll segment 20 from the first roll segment to the second roll segment. Also in Comparative Example 3, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 1.8 [mm]. Also in Comparative Example 3, the total reduction amount from the first roll segment to the second roll segment was set to 2.5 [mm].

[0062] In Comparative Example 4, the thickness of the cast slab 18 after reduction in the second roll segment was set to 250 [mm]. Also in Comparative Example 4, the number of segments from the first roll segment to the second roll segment that reduces the cast slab 18 (number of reduction segments) was set to 4. Also in Comparative Example 4, flat rolls were used for the cast slab support roll pairs 16 on each roll segment 20 from the first roll segment to the second roll segment. Also in Comparative Example 4, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 5.5 [mm]. Also in Comparative Example 4, the total reduction amount from the first roll segment to the second roll segment was set to 8.8 [mm].

[0063] In Comparative Example 5, the thickness of the cast slab 18 after reduction in the second roll segment was set to 400 [mm]. Also in Comparative Example 5, the number of segments from the first roll segment to the second roll segment that reduces the cast slab 18 (number of reduction segments) was set to 4. Also in Comparative Example 5, flat rolls were used for the cast slab support roll pairs 16 on each roll segment 20 from the first roll segment to the second roll segment. Also in Comparative Example 5, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 2.2 [mm]. Also in Comparative Example 5, the total reduction amount from the first roll segment to the second roll segment was set to 4.8 [mm].

[0064] In Comparative Example 6, the thickness of the cast slab 18 after reduction in the second roll segment was set to 400 [mm]. Also in Comparative Example 6, the number of segments from the first roll segment to the second roll segment that reduces the cast slab 18 (number of reduction segments) was set to 4. Also in Comparative Example 6, flat rolls were used for the slab support roll pairs 16 on each roll segment 20 from the first roll segment to the second roll segment. Also in Comparative Example 6, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 6.6 [mm]. Also in Comparative Example 6, the total reduction amount from the first roll segment to the second roll segment was set to 10.8 [mm].

[0065] In Comparative Example 7, the thickness of the cast slab 18 after reduction in the second roll segment was set to 500 [mm]. Also in Comparative Example 7, the number of segments from the first roll segment to the second roll segment that reduces the cast slab 18 (number of reduction segments) was set to 4. Also in Comparative Example 7, flat rolls were used for the cast slab support roll pairs 16 on each roll segment 20 from the first roll segment to the second roll segment. Also in Comparative Example 7, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 2.4 [mm]. Also in Comparative Example 7, the total reduction amount from the first roll segment to the second roll segment was set to 5.6 [mm].

[0066] In Comparative Example 8, the thickness of the cast slab 18 after reduction in the second roll segment was set to 500 [mm]. Also in Comparative Example 8, the number of segments from the first roll segment to the second roll segment that reduces the cast slab 18 (number of reduction segments) was set to 4. Also in Comparative Example 8, flat rolls were used for the cast slab support roll pairs 16 on each roll segment 20 from the first roll segment to the second roll segment. Also in Comparative Example 8, the maximum reduction amount in any of the roll segments 20 from the first roll segment to the second roll segment was set to 6.4 [mm]. Also in Comparative Example 8, the total reduction amount from the first roll segment to the second roll segment was set to 10.0 [mm].

[0067] Table 1 shows the test conditions and test results for Invention Examples 1 to 8 and Comparative Examples 1 to 8.

[0068]

[0069] In the evaluation in Table 1, "◎" and "〇" indicate that the internal quality of the cast slab 18 is good in each item, while "×" indicates that the internal quality of the cast slab 18 is poor in each item. Specifically, "◎" means there are no porosity defects at the slab stage and no porosity-induced defects in the product UT inspection. "〇" means there are no porosity-induced defects in the product UT inspection. The difference between "◎" and "○" is that "◎" means there are no porosity defects at all, and "○" means there are some porosity defects remaining, but it is usable if the grade is downgraded. "×" means there are porosity-induced defects in the product UT inspection. Note that "×" includes all of the following: porosity defects, internal cracks, and central segregation defects. Furthermore, items with a UT defect index below the threshold are marked "×".

[0070] As shown in Table 1, Invention Examples 1, 3, 5, 6, 7, and 8 received an evaluation of "◎", while Invention Examples 2 and 4 received an evaluation of "○". In Invention Examples 1 to 8, the maximum reduction amount was 5.0 [mm] or less, the total reduction amount was 6 [mm] or more, and a cast slab 18 with good internal quality was obtained. In particular, Invention Examples 1, 3, 5, 6, 7, and 8 had better internal quality compared to Invention Examples 2 and 4, as indicated by "◎" in Table 1. Therefore, in Invention Examples 1, 3, 5, 6, 7, and 8, when continuously casting cast slabs 18 for thick-walled products, it is possible to reduce porosity while keeping equipment costs down.

[0071] Furthermore, as shown in Table 1, all evaluations for Comparative Examples 1 to 8 were "×". Comparative Examples 1, 3, 5, and 7 did not reach the required total reduction amount of less than 6 [mm], resulting in insufficient reduction of the cast slab 18. Comparative Examples 2, 4, 6, and 8 had a total reduction amount of 6 [mm] or more, but the maximum reduction amount was greater than 5 [mm], and the reduction amount by the roll segments 20 other than the roll segment 20 with the maximum value was insufficient, so the porosity could not be sufficiently reduced.

[0072] Embodiments of the present invention have been described above. However, the embodiments described above are merely illustrative examples for carrying out the present invention. Therefore, the present invention is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of the invention.

[0073] The present invention provides a continuous casting method for steel that can reduce porosity while suppressing increases in equipment costs when continuously casting slabs used for thick-walled products.

[0074] 11 Continuous casting machine 12 Molten steel 13 Mold 14 Tundish 15 Immersion nozzle 16 Slab support roll pair 16a Driven roll 16b Driven roll 16aA, 16bA First roll section 16aB, 16bB Second roll section 17 Spray nozzle 18 Slab 18a Unsolidified section 18b Final solidification position 19 Lightly compressed lower zone 20, 20Aa, 20Ab, 20Ac, 20B Roll segment 21 Conveyor roll 22 Lower straightening position 30a, 30b Bearing member 161a, 161aA, 161aB, 161b, 161bA, 161bB Large diameter roll section 162a, 162aA, 162aB, 162b, 162bA, 162bB Small diameter roll portion 163a, 163aA, 163aB, 163b, 163bA, 163bB Rotating shaft 181 Upper surface 182 Lower surface 183 End surface

Claims

1. A continuous casting method for steel, comprising casting in a continuous casting machine equipped with multiple roll segments, each having multiple pairs of slab support rolls that sandwich and support a slab drawn from a mold, wherein the roll segment containing the final solidification position of the slab is designated as the first roll segment, the roll segment located furthest downstream in the slab drawing direction in the continuous casting machine is designated as the second roll segment, the total reduction amount, which is the sum of the reduction amounts of the slab from the first roll segment to the second roll segment, is 6 mm or more, and the range of reduction amounts for each roll segment from the first roll segment to the second roll segment is 0.1 mm or more and 5 mm or less.

2. The continuous casting method for steel according to claim 1, wherein the slab support roll pair is composed of a drive roll and a driven roll, and the drive roll and driven roll of the slab support roll pair in each roll segment from the first roll segment to the second roll segment are convex rolls in which the diameter of the axial end is smaller than the diameter of the axial center.

3. The continuous casting method for steel according to claim 2, wherein the diameter of the roll portion of the convex roll that corresponds to the portion of the cast slab that is outside in the width direction of the triple junction is smaller than the diameter of the roll portion that corresponds to the portion of the cast slab that is inside in the width direction of the triple junction.

4. The continuous casting method for steel according to any one of claims 1 to 3, wherein the thickness of the cast slab after reduction by the second roll segment is 200 mm or more and 500 mm or less.

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