Continuous casting method for steel

Abstract

The present invention relates to a continuous casting method for steel, which involves casting in a continuous casting machine equipped with multiple roll segments, each having a plurality of pairs of slab support rolls that sandwich and support a slab drawn from a mold. The roll segment containing the final solidification position of the slab is designated as the first roll segment, and 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 set to 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 set to 0.1 mm or more and 5 mm or less.

Description

Technical Field

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

Background Art

[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 that 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 before solidification is completed, or reduction is carried out after solidification. However, in the soft reduction carried out before solidification is completed, a certain amount of porosity remains, and the porosity cannot be crimped 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 before solidification is completed can be rendered harmless. Therefore, a method for reduction after solidification 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 strong reduction is carried out in two steps 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 having 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.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

[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 preceding and succeeding 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. [Means for solving the problem]

[0007] In order to solve the aforementioned problems and achieve the objective, (1) The present invention relates to a continuous casting method for steel, wherein the steel is cast in a continuous casting machine provided with a plurality of roll segments 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 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 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) In 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. [Effects of the Invention]

[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. [Brief explanation of the drawing]

[0012] [Figure 1]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] Figure 2 shows the widthwise ends of a pair of cast slab support rolls using a convex roll. [Figure 3] Figure 3 shows an example of a convex roll applicable to the driving roll and driven roll of a slab support roll pair. [Figure 4] Figure 4 shows another example of a convex roll applicable to the driving roll and driven roll of a slab support roll pair. [Figure 5] Figure 5 shows the relationship between the total reduction amount in multiple roll segments after complete solidification and the porosity index. [Figure 6] Figure 6 shows the relationship between the maximum reduction amount and the porosity index. [Figure 7] Figure 7 shows an example in which three roll segments are provided in the horizontal band region. [Modes for carrying out the invention]

[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 Fig. 1 includes a tundish 14, a mold 13, a plurality of pairs of slab support rolls 16, and a plurality of spray nozzles 17. Also, as shown in Fig. 1, the slab 18 is drawn in the slab drawing direction D1. In this embodiment, the side where the tundish 14 is provided in the slab drawing direction D1 is described as the upstream side, and the side where the slab 18 is drawn is described as the downstream side.

[0017] The tundish 14 is provided 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 is stored therein. At the bottom of the tundish 14, a sliding nozzle (not shown) for adjusting the flow rate of the molten steel 12 is installed, and an immersion nozzle 15 is installed on the lower surface of this sliding nozzle.

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

[0019] The plurality of pairs of slab support rolls 16 support the slab 18 from both sides along the slab drawing direction D1. The plurality of pairs of slab support rolls 16 are composed of, for example, a plurality of support rolls including a support roll pair, a guide roll pair, and a pinch roll pair. Also, as shown in Fig. 1, a plurality of pairs of slab support rolls 16 gather to form one roll segment 20.

[0020] The plurality of spray nozzles 17 are provided between adjacent pairs of slab support rolls 16 along the slab drawing direction D1. The spray nozzle 17 is a nozzle for spraying cooling water onto the slab 18 to perform secondary cooling of the slab 18. As the spray nozzle 17, nozzles such as a water spray nozzle (a single-fluid spray nozzle) or an air mist spray nozzle (a two-fluid spray nozzle) can be used without limitation.

[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 is provided downstream of the continuous casting machine 11 in the slab withdrawal direction D1 to lightly reduce the slab 18. The light reduction zone 19 is provided as a roll segment 20 composed of multiple pairs of slab support rolls 16, with roll segment 20B, roll segment 20Aa, and roll segment 20Ab arranged sequentially 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 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 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 located within the area of ​​the light reduction zone 19 is indicated by the numeral 22.

[0023] Downstream of the continuous casting machine 11, a horizontal zone A1 is provided through which 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 multiple 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, 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 conveying 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 conveying 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, accompanying the slab 18 that is conveyed 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 collide: 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. 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 a first roll section 16aA, 16bA and a second roll section 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 shall be 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 shall be 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 castings were 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 of 30 [mm] thickness and 100 [mm] width 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 2 [g], it is considered good as the reduction is sufficient, and 2 [cm 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 total reduction amount in multiple roll segments 20 after complete solidification is set to a 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 adequately 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] Furthermore, in the continuous casting machine 11 according to the embodiment, the total reduction amount, which is the sum of the reduction amounts of the cast slab 18 in the multiple roll segments 20, is set to 6 [mm] or more in the range from the first roll segment to the second roll segment. That is, the total reduction amount in the first roll segment, roll segment 20Aa, and the second roll segment, roll segment 20Ab, is set to 6 [mm] or more. In this case, the range of the reduction amount for the cast slab 18 in each roll segment 20, in the range from the first roll segment to the second roll segment, is set to 0.1 [mm] or more and 5 [mm] or less. That is, the range of the reduction amount in the first roll segment, roll segment 20Aa, and the second roll segment, roll segment 20Ab, is set to 0.1 [mm] or more and 5 [mm] or less, in other words, the maximum reduction amount is set to 5 [mm] or less. Furthermore, in the continuous casting machine 11 according to this embodiment, the cast slab 18 is reduced by each roll segment 20 such that the thickness of the cast slab 18 after reduction by the second roll segment (roll segment 20Ab) is 200 [mm] or more and 500 [mm] or less.

[0044] Figure 7 shows an example in which three roll segments 20 are provided in the horizontal band region A1.

[0045] In the example shown in Figure 7, roll segments 20Aa, 20Ac, and 20Ab are arranged in order in the horizontal zone A1 in the slab withdrawal direction D1. In the example shown in Figure 7, roll segment 20Aa, which includes the final solidification position 18b of the slab 18, is the first roll segment. Roll segment 20Ab, which is located at the downstream end of the slab withdrawal direction D1 in the continuous casting machine 11, is the second roll segment. Furthermore, the roll segments 20 after the first roll segment correspond to the roll segments 20 after complete solidification.

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

[0047] The continuous casting method for steel applied to the continuous casting machine 11 according to the embodiment 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 casting method for steel applied to the continuous casting machine 11 according to the embodiment can be applied using existing roll segment equipment for light reduction, thereby suppressing an increase in equipment costs. Furthermore, in the continuous casting method for steel applied to the continuous casting machine 11 according to the embodiment, the reduction amount is controlled in units of roll segments, similar to many existing roll segment equipment for light reduction, so there is no need to introduce new equipment, thus suppressing an increase in equipment costs.

[0048] Based on the above, in the continuous casting method for steel applied to the continuous casting machine 11 according to the embodiment, when continuously casting slabs 18 used for thick-walled products, it is possible to reduce porosity while suppressing an increase in equipment costs. [Examples]

[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 furthest downstream in 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 1-8 and Comparative Examples 1-8, 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 (number of reduction segments) from the first roll segment to the second roll segment that reduces the cast slab 18 was set to 4. Also in Invention Example 1, 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 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 (number of reduction segments) from the first roll segment to the second roll segment that reduces the cast slab 18 was set to 3. Also in Invention Example 2, flat 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 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 (number of reduction segments) from the first roll segment to the second roll segment that reduces the cast slab 18 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 of 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 (number of reduction segments) from the first roll segment to the second roll segment that reduces the cast slab 18 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 (number of reduction segments) from the first roll segment to the second roll segment that reduces the cast slab 18 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 (number of reduction segments) from the first roll segment to the second roll segment that reduces the cast slab 18 was set to 4. Also in Invention Example 8, 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 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 (number of reduction segments) from the first roll segment to the second roll segment that reduces the cast slab 18 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 (number of reduction segments) from the first roll segment to the second roll segment that reduces the cast slab 18 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 (number of reduction segments) from the first roll segment to the second roll segment that reduces the cast slab 18 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 (number of reduction segments) from the first roll segment to the second roll segment that reduces the cast slab 18 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 (number of reduction segments) from the first roll segment to the second roll segment that reduces the cast slab 18 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 (number of reduction segments) from the first roll segment to the second roll segment that reduces the cast slab 18 was set to 4. Also in Comparative Example 6, 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 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 (number of reduction segments) from the first roll segment to the second roll segment that reduces the cast slab 18 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 (number of reduction segments) from the first roll segment to the second roll segment that reduces the cast slab 18 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 1-8 and Comparative Examples 1-8.

[0068] [Table 1]

[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. Also, 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. [Industrial applicability]

[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. [Explanation of symbols]

[0074] 11. Continuous casting machine 12 Molten steel 13. Mold 14 Tan Dish 15 Immersion nozzle 16 slab support roll pairs 16a Drive Roll 16b Driven Roll 16aA, 16bA First Roll Section 16aB, 16bB Second Roll Section 17 Spray nozzle 18 cast slabs 18a Unsolidified area 18b Final solidification position 19 Light compression zone 20, 20Aa, 20Ab, 20Ac, 20B Roll Segment 21 Conveyor Rolls 22 Lower correction position 30a, 30b Bearing members 161a, 161aA, 161aB, 161b, 161bA, 161bB Large diameter roll section 162a, 162aA, 162aB, 162b, 162bA, 162bB Small diameter roll section 163a, 163aA, 163aB, 163b, 163bA, 163bB Rotation axis 181 Top surface 182 Bottom surface 183 End face

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, The roll segment including the final solidification position of the cast slab is defined as the first roll segment. In the aforementioned continuous casting machine, the roll segment located furthest downstream in the slab withdrawal direction is designated as the second roll segment. The 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 set to 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 set to 0.1 mm or more and 5 mm or less. A continuous casting method for steel.

2. The aforementioned slab support roll pair is composed of a driving roll and a driven roll, For 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, a convex roll is used in which the diameter of the axial end is smaller than the diameter of the axial center. The method for continuous casting of steel according to claim 1.

3. The convex roll has a diameter in the roll portion corresponding to the portion of the cast slab that is wider than the triple junction, and the diameter of the roll portion corresponding to the portion of the cast slab that is wider than the triple junction. The method for continuous casting of steel according to claim 2.

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

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