Steel continuous casting method and continuous casting machine

Abstract

The present invention addresses the problem of providing a continuous casting method and a continuous casting machine for steel, with which it is possible to reduce the overall segregation level of center segregation in the width direction of a cast slab, and to reduce variation of the degree of segregation in the width direction of the cast slab. This continuous casting method for steel is for continuously casting steel by means of a continuous casting machine (1). When the center solid-phase ratio, which is the solid-phase ratio at the thickness-direction center, of a slab to be subjected to continuous casting is in a range of not less than 0.2 but less than 1.0, and if rolling is performed on the slab at a rolling speed of 0.3-2.0 mm / min, a deflection τ, of a segment (18), in the thickness direction of a segment frame of the segment (18) that imparts rolling to the slab satisfies formula (1). (1): 2×τ-A / (n-1)≤3.25×2×δ≤1.3 Wherein, δ represents an actually measured bulging amount (mm), τ represents the segment deflection (mm), A represents a rolling set amount (mm) of light rolling, and n represents the number of pairs of slab supporting rolls present in a light rolling zone.

Description

[Technical field]

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

[0002] During the solidification process of steel, solute elements such as carbon (C), phosphorus (P), sulfur (S), and manganese (Mn) are concentrated in the unsolidified liquid phase due to redistribution during solidification. These concentrated solute elements become microsegregation formed between dendrites. In addition, during the continuous casting process of steel in a continuous casting machine, if a gap or negative pressure is formed in the center of the thickness of the slab due to solidification shrinkage, thermal shrinkage, or bulging of the solidified shell between the rolls of the continuous casting machine, molten steel will be absorbed in this area. However, since there is not enough molten steel in the unsolidified layer at the end of solidification, the molten steel concentrated by the above microsegregation flows and accumulates in the center of the slab and solidifies. The segregation spot formed in this way has a much higher concentration of solute elements than the initial concentration of the molten steel. This is generally called macrosegregation, and is also called central segregation due to its location.

[0003] The quality of line pipe materials used to transport crude oil, natural gas, etc. deteriorates due to center segregation. When manganese sulfides or niobium carbides are formed in the center segregation area, hydrogen that penetrates into the steel due to corrosion reactions diffuses and accumulates around the manganese sulfides and niobium carbides in the steel, and cracks occur due to the internal pressure. Furthermore, since the center segregation area is hard, the cracks propagate. This type of cracking is called hydrogen induced cracking (also known as "HIC"), and is the main cause of deterioration in the quality of line pipe materials used in sour gas environments.

[0004] In order to address this issue, many measures have been proposed to reduce or neutralize center segregation in the slab, from the continuous casting process to the rolling process. For example, Patent Documents 1 and 2 propose a method of casting a slab having an unsolidified layer in the final stage of solidification in a continuous casting machine while gradually reducing the slab by a slab support roll by an amount of reduction equivalent to the sum of the amount of solidification shrinkage and the amount of thermal shrinkage.

[0005] The technique of gradually reducing the slab during casting in a continuous casting machine by an amount of reduction equivalent to the sum of the solidification shrinkage and thermal shrinkage, as in Patent Documents 1 and 2, is called "soft reduction" or "soft reduction method." This soft reduction technique uses multiple pairs of rolls aligned in the casting direction to gradually reduce the slab by an amount of reduction corresponding to the sum of the solidification shrinkage and thermal shrinkage, thereby reducing the volume of the unsolidified layer. This prevents the formation of voids or negative pressure areas in the center of the slab, and at the same time prevents the flow of concentrated molten steel formed between dendrites, thereby reducing the center segregation of the slab.

[0006] In recent years, segment type continuous casting machines composed of segments with multiple pairs of rolls have become mainstream, and in the case of segment type continuous casting machines, the group of reduction rolls that perform soft reduction (called the "soft reduction zone") is also composed of multiple segments. In the soft reduction zone composed of segments, the opening between the rolls facing each other in the thickness direction of the slab is adjusted so that the entrance side is larger than the exit side, thereby imparting a predetermined amount of reduction to the slab.

[0007] Incidentally, it is known that there is a close relationship between the shape of the solidification completion position of the slab in the width direction of the slab and the center segregation. For example, Patent Document 3 proposes a method of detecting the solidification completion position in the width direction of the slab, and adjusting the flow of molten steel in the mold or adjusting the width cut amount of secondary cooling so that the difference between the shortest part and the longest part of the detected solidification completion position is within a standard range. When the solidification completion position differs in the width direction of the slab, the reduction amount in the light reduction zone differs at each position in the width direction of the slab, so that the reduction amount is small at the position where the solidification completion position extends downstream in the casting direction, and it becomes impossible to obtain a sufficient improvement effect on center segregation. However, according to the method of Patent Document 3, even if the solidification completion position differs in the width direction of the slab, the improvement effect on center segregation can be obtained.

[0008] It is also known that bulging between rolls of a slab also affects center segregation. For example, Patent Document 4 proposes a continuous casting method in which inter-roll bulging of a slab in a soft reduction zone is calculated by unsteady heat transfer and solidification calculation, and the reduction rate applied to the slab is changed according to the calculated inter-roll bulging. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Publication No. 8-132203 [Patent Document 2] Japanese Patent Application Publication No. 8-192256 [Patent Document 3] JP 2006-198644 A [Patent Document 4] JP 2012-45552 A Summary of the Invention [Problem to be solved by the invention]

[0010] As described above, in order to improve the center segregation of the slab, various measures have been taken regarding the reduction rate during soft reduction, the shape of the solidification completion position in the width direction of the slab, and inter-roll bulging. However, the quality requirements for recent continuous cast slabs are becoming higher and higher, and the variation in the degree of segregation in the width direction of the slab is also becoming a problem. In particular, in the case of steel materials with strict segregation requirements such as line pipe materials, if there is even one part with large segregation in the width direction at the slab stage, it becomes difficult to use it as line pipe material.

[0011] Even when the above-mentioned conventional techniques are applied, problems with center segregation such as a deterioration in the overall segregation level in the width direction of the slab and an increase in the variation in the degree of segregation in the width direction of the slab may occur, and therefore a technique for more stably reducing center segregation is desired. The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method and machine for continuous casting of steel that can reduce the overall level of center segregation in the width direction of the cast slab and reduce variation in the degree of segregation in the width direction of the cast slab. [Means for solving the problem]

[0012] (1) According to one aspect of the present invention, there is provided a method for continuously casting steel, which comprises continuously casting steel with a curved-type continuous casting machine or a vertical bending-type continuous casting machine, wherein the method applies a reduction to the slab at a reduction rate of 0.3 mm / min or more and 2.0 mm / min or less when the solid phase ratio at the thickness center of the continuously cast slab is at least in the range of 0.2 or more and less than 1.0, and wherein the segment deflection τ, which is the deflection in the thickness direction of the segment frame of the segment that applies the reduction to the slab, satisfies formula (1). 2×τ-A / (n-1)≦3.25×2×δ≦1.3 (1) Where: δ: Measured bulging amount (mm) τ: Deflection of the segment (mm) A: The reduction setting amount (mm) of soft reduction in the soft reduction zone where the solid fraction of the slab is 0.2 or more and less than 1.0 n: The number of pairs of supporting rolls in the soft reduction zone where the solid fraction of the slab is 0.2 or more and less than 1.0

[0013] (2) In the method for continuous casting of steel described in (1) above, the deflection deviation σ of the segment frame corresponding to the final solidification position is kept at 0.2 mm or less, and the displacement difference t between the left and right sides of the segment frame is always kept at 0.2 mm or less.

[0014] (3) In the method for continuous casting of steel described in (1) or (2) above, in an intentional bulging zone of the continuous casting machine, the roll gap of multiple pairs of slab support rolls is gradually increased toward the downstream side in the casting direction to bulge the long side surface of the slab with a total bulging amount of 3 mm or more and 20 mm or less, and in a light reduction zone downstream of the intentional bulging zone in the casting direction, the roll gap of multiple pairs of slab support rolls is gradually decreased toward the downstream side in the casting direction to impart the above-mentioned reduction to the slab.

[0015] (4) According to one aspect of the present invention, there is provided a continuous casting machine for steel, which is a curved type continuous casting machine or a vertical bending type continuous casting machine, and which includes a light reduction zone that reduces the thickness direction of a slab being continuously cast, and in the light reduction zone, the slab is reduced at a reduction rate of 0.3 mm / min or more and 2.0 mm / min or less, with a central solid fraction, which is the solid fraction at the center of the thickness of the slab, being at least in the range of 0.2 to less than 1.0, and a segment deflection τ, which is the deflection in the thickness direction of a segment frame of a segment that reduces the slab in the light reduction zone, satisfies formula (1). Effect of the Invention

[0016] According to one aspect of the present invention, there is provided a method and a machine for continuous casting of steel, which are capable of reducing the overall level of center segregation in the width direction of the cast slab and reducing the variation in the degree of segregation in the width direction of the cast slab. [Brief description of the drawings]

[0017] [Figure 1]FIG. 1 is a side schematic view showing a continuous casting machine according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic front view showing the structure of a segment. [Diagram 3] FIG. 2 is a schematic side view showing the structure of a segment. [Figure 4] 1 is a graph showing an example of a profile of the roll gap of a slab support roll. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] In the following detailed description, embodiments of the present invention will be described with reference to the drawings. Each drawing is a schematic diagram, and may differ from the actual one. Furthermore, the embodiments shown below are merely examples of devices and methods for embodying the technical idea of ​​the present invention, and the technical idea of ​​the present invention does not specify the materials, structures, arrangements, etc. of the components to be described below. The technical idea of ​​the present invention can be modified in various ways within the technical scope defined by the claims.

[0019] In one embodiment of the present invention, the continuous casting machine 1 is an apparatus for continuously casting steel to produce a slab, which is a cast piece having a rectangular cross-sectional shape, which is the shape of a cross section perpendicular to the longitudinal direction, and is a curved type continuous casting machine or a vertical bending type continuous casting machine. The continuous casting machine 1 according to this embodiment is common in principle, and can be applied to either a curved type continuous casting machine or a vertical bending type continuous casting machine. In the following description of this embodiment, the continuous casting machine 1 is assumed to be a vertical bending type continuous casting machine as an example.

[0020] FIG. 1 shows a schematic side view of a continuous casting machine 1, which is a vertical bending type slab continuous casting machine used in carrying out the present invention. In this embodiment, the longitudinal direction of the slab 3 and the direction of movement of the slab 3 and the solidified shell 4 in the continuous casting machine 1 are referred to as the casting direction. The short side direction of the rectangle in the cross section (cross section perpendicular to the longitudinal direction) of the slab 3 (direction perpendicular to the casting direction in the cross section of the slab 3 in FIG. 1) is referred to as the thickness direction, and the longitudinal direction of the rectangle in the cross section of the slab 3 (front-rear direction in FIG. 1) is referred to as the width direction. The length of the slab 3 in the thickness direction is referred to as the thickness, and the length of the slab 3 in the width direction is referred to as the width. In addition, in the cross section of the slab 3, the side facing the thickness direction is longer than the side facing the width direction, and the side facing the thickness direction is referred to as the long side, and the side facing the width direction is referred to as the short side.

[0021] As shown in Fig. 1, a continuous slab casting machine 1 is provided with a mold 13 into which molten steel 2 is poured, and which is cooled and solidified to form the outer shell shape of a cast slab 3 having a rectangular cross section. A tundish 10, which is an intermediate vessel for relaying the molten steel 2 supplied from a ladle (not shown) to the mold 13, is provided at a predetermined position above the mold 13. A sliding nozzle 11 for adjusting the flow rate of the molten steel 2 is provided at the bottom of the tundish 10, and a submerged nozzle 12 is provided below the sliding nozzle 11.

[0022] Below the mold 13, multiple pairs of slab support rolls 14, each consisting of a support roll, a guide roll, and a pinch roll, are arranged. Spray nozzles (not shown), such as water spray nozzles or air mist spray nozzles, are arranged in the gaps between adjacent slab support rolls 14 in the casting direction, and a secondary cooling zone is formed in the area from just below the mold to the slab support rolls 14 at the end of the machine. As the slab 3 is withdrawn, it is cooled by secondary cooling water sprayed from the spray nozzles in the secondary cooling zone.

[0023] A plurality of pairs of slab support rolls 14 are arranged downstream in the casting direction from a position about 1 m to 4 m away from the outlet of the mold 13 in the casting direction, and constitute an upper straightening zone 173 where the support and guide direction of the slab 3 changes from a vertical direction to a curved direction. That is, the flat slab 3 pulled out of the mold 13 in the vertical direction is gradually bent into an arc shape in the upper straightening zone 173 and straightened into a curved part with a constant radius. In the upper straightening zone 173, tensile stress acts on the lower surface side of the slab 3 and compressive stress acts on the upper surface side. Therefore, in the upper straightening zone 173, surface cracks are likely to occur on the lower surface side of the slab 3, but generally do not occur on the upper surface side. In this case, the inner and outer radial sides of the curved part are defined as the upper surface side and the lower surface side, respectively, with the center position in the thickness direction of the slab 3 as the boundary. Regarding the surface on the long side of the cast slab 3 (long side surface), the surface on the radially inner side of the curved portion is referred to as the upper surface, and the surface on the radially outer side of the curved portion is referred to as the lower surface.

[0024] Similarly, pairs of slab support rolls 14 arranged near the position where the casting direction of the curved portion is parallel to the horizontal direction constitute a lower straightening zone 174 where the support and guide direction of the slab 3 changes from the curved direction to the horizontal direction. That is, the arc-shaped slab 3 is gradually bent back into a flat plate shape in the lower straightening zone 174 and straightened to the horizontal portion. In the lower straightening zone 174, tensile stress acts on the upper surface side of the slab 3 and compressive stress acts on the lower surface side. Therefore, in the lower straightening zone 174, surface cracks are likely to occur on the upper surface side of the slab 3, but generally do not occur on the lower surface side.

[0025] A plurality of transport rolls 15 for transporting the cast strand 3 are installed downstream in the casting direction of the strand support roll 14a, which is the strand support roll 14 on the most downstream side in the casting direction. In addition, a strand cutter 16 for cutting a strand 3a of a predetermined length from the cast strand 3 is disposed above the transport rolls 15.

[0026] Soft reduction zones 171 are installed on the upstream and downstream sides of the solidification completion position 6 of the slab 3 in the casting direction. The soft reduction zone 171 is composed of a plurality of pairs of slab support rolls in which the distance between two slab support rolls 14 that face each other in the thickness direction and sandwich the slab 3 (referred to as the "roll gap") is set so that it gradually narrows toward the downstream side in the casting direction, that is, a reduction gradient is applied. In the light reduction zone 171, it is possible to perform soft reduction on the slab 3 in the entire area or in a partially selected area. As described above, the soft reduction is a technique for gradually reducing the slab being cast in the continuous casting machine 1 by an amount of reduction equivalent to the sum of the solidification shrinkage amount and the thermal shrinkage amount. A spray nozzle for cooling the slab 3 is arranged between the slab support rolls 14 adjacent to each other in the casting direction in the light reduction zone 171. The slab support rolls 14 arranged in the light reduction zone 171 are also called reduction rolls 14b.

[0027] In the continuous casting machine 1 shown in FIG. 1, the soft reduction zone 171 is configured by three segments 18, each of which has three pairs of reduction rolls 14b, connected in the casting direction. In FIG. 1, the soft reduction zone 171 is configured by three segments 18, but the number of segments 18 is not particularly limited and may be one, two, or four or more. In FIG. 1, one segment 18 has three pairs of reduction rolls 14b, but the number is not limited to three. One segment may have any number of pairs of reduction rolls 14b as long as there are multiple pairs, and may have any number of pairs as long as there are two or more pairs. In addition, in the continuous casting machine 1, although not shown, the slab support rolls 14 other than the light reduction zone 171 also have a segment structure, and the segment 18 of the light reduction zone 171 is also referred to as a light reduction segment 18a.

[0028] As shown in FIG. 2 and FIG. 3, the soft reduction segment 18a has an upper surface side frame 180, a lower surface side frame 181, and a support pillar 182. The upper surface side frame 180 supports a plurality of (three in this embodiment) slab support rolls 14 on the upper surface side. The lower surface side frame 181 supports a plurality of (three in this embodiment) slab support rolls 14 on the lower surface side. The upper surface side frame 180, the lower surface side frame 181, and the support pillar 182 are collectively referred to as a segment frame. The support pillar 182 is a pillar that connects and supports the upper surface side frame 180 and the lower surface side frame 181. The support pillar 182 is provided at both ends in the width direction, and when a pair of support pillars 182 arranged in the width direction is considered as one set, a plurality of sets are provided in the casting direction. In this embodiment, as shown in FIG. 3, a pair of support pillars 182 is provided at two locations, an inlet end and an outlet end in the casting direction. In addition, the support pillars 182 have a mechanism for adjusting the gap between the upper surface frame 180 and the lower surface frame 181 in the thickness direction by using hydraulic pressure or the like to adjust the roll gap.

[0029] Usually, the reduction gradient in the soft reduction zone 171 is expressed as the amount of roll gap reduction per 1 m in the casting direction, that is, in units of "mm / m." Therefore, the reduction speed (mm / min) of the slab 3 in the light reduction zone 171 is obtained by multiplying the reduction gradient (mm / m) by the slab withdrawal speed (m / min).

[0030] The slab support rolls 14 arranged between the lower end of the mold 13 and the liquidus crater end position of the slab 3 preferably constitute an intentional bulging zone 172. The liquidus crater end position is a position in the casting direction inside the continuous casting machine 1 where the central temperature of the slab 3 becomes the liquidus temperature of the steel being cast, and is determined by heat transfer solidification calculations (for example, the calculation method described in Patent Document 4) according to the molten steel temperature and the cooling capacity of the continuous casting machine 1. In the intentional bulging zone 172, each slab support roll 14 is set so that the roll gap is gradually increased toward the downstream side in the casting direction for each roll pair or for each plurality of roll pairs until the roll gap expansion amount reaches a predetermined value. The slab support rolls 14 arranged downstream in the casting direction from the intentional bulging zone 172 and upstream in the casting direction from the soft reduction zone 171 are narrowed to a constant value or to a degree corresponding to the amount of contraction accompanying the temperature drop of the slab 3.

[0031] Here, the bulging in this embodiment refers to intentional bulging, and hereinafter may be simply referred to as "bulging". Note that intentional bulging is preferably started when the solid fraction in the center is 0, and is terminated when the total amount of bulging on the long side surface of the slab (total amount of bulging on the upper and lower surfaces) is 3 mm or more and 20 mm or less. If the total amount of bulging in the intentional bulging is less than 3 mm, there is a risk that the short side of the slab 3 will come into contact with the slab support roll 14, making it impossible to impart sufficient soft reduction. Note that when intentionally causing bulging, from the viewpoint of suppressing internal cracks, the total amount of bulging is preferably 15 mm or less, and more preferably 10 mm or less.

[0032] FIG. 4 shows an example of a roll gap profile of the slab support rolls 14 in this embodiment. In the roll gap profile shown in FIG. 4, the slab long side surface is intentionally bulged by the molten steel static pressure in the intentional bulging zone 172 to increase the thickness of the central part of the slab long side surface (region b). In the roll gap profile shown in FIG. 4, the roll gap is narrowed to a constant value or to a degree corresponding to the amount of contraction caused by the temperature drop of the slab 3 downstream of the intentional bulging zone 172 (region c). Thereafter, in the roll gap profile shown in FIG. 4, the slab long side surface is reduced in the soft reduction zone 171 (region d). In FIG. 4, a and e are regions where the roll gap is narrowed to a degree corresponding to the amount of contraction caused by the temperature drop of the slab 3. In FIG. 4, a' is an example of the roll gap in the conventional method in which the roll gap is narrowed to a degree corresponding to the amount of contraction caused by the temperature drop of the slab 3.

[0033] In the intentional bulging zone 172, the roll gap of the slab support rolls 14 is gradually widened toward the downstream side in the casting direction, so that the long side surface of the slab 3, except for the vicinity of the short side, is intentionally bulged following the slab support rolls 14 by the static pressure of molten steel due to the unsolidified layer. The long side surface near the short side of the slab 3 is fixed and restrained by the short side surface of the slab 3 that has completed solidification, and therefore maintains the thickness it had at the time when the intentional bulging was started. Therefore, only the bulged portion of the long side surface of the slab 3 comes into contact with the slab support rolls 14 due to the intentional bulging. In the soft reduction zone 171, by setting the total reduction amount to be equal to or less than the total bulging amount, only the bulged portion of the long side surface of the slab 3 is reduced, making it possible to perform efficient soft reduction. The total reduction amount is the amount of reduction of the slab 3 from the start of reduction in the light reduction zone 171 to the end of reduction, and the total bulging amount is the amount of bulging from the start of intentional bulging to the end of intentional bulging in the intentional bulging zone 172. The amount of bulging is the maximum amount of bulging in the width direction of the slab 3, and is the length (mm) calculated as the difference between the maximum thickness in the cross section of the slab 3 (basically the thickness at the center in the width direction) and the thickness on the short side.

[0034] In addition, when intentionally bulging the slab 3, it is preferable to arrange the intentional bulging zone 172 between the lower end of the mold 13 and the liquidus crater end position of the slab 3. The reason for this is that the center of the thickness of the slab is entirely unsolidified layer 5 (liquid phase) on the upstream side of the liquidus crater end position of the slab 3 in the casting direction, and the solidified shell 4 of the slab 3 has a high temperature and a small deformation resistance, so that bulging can be easily performed. In addition, when intentionally bulging the slab 3, if bulging is performed at a time when the amount of unsolidified layer 5 present inside the slab 3 is small, center segregation will worsen. However, when bulging is performed upstream of the liquidus crater end position of the slab 3 in the casting direction, molten steel with an initial concentration that is not enriched in solute elements is abundantly present inside the slab at this time, and this molten steel flows easily. Even if this molten steel flows, segregation does not occur, and therefore bulging at this time does not cause center segregation.

[0035] The liquidus of the slab 3 is the solidification start temperature determined by the chemical components of the slab 3, and can be calculated, for example, from the following formula (2). T L =1536-(78×[%C]+7.6×[%Si]+4.9×[%Mn] +34.4×[%P]+38×[%S]+4.7×[%Cu] +3.1×[%Ni]+1.3×[%Cr]+3.6×[%Al]) ···(2)

[0036] However, in equation (2), T Lis the liquidus temperature (℃), [%C] is the carbon concentration (mass%) of the molten steel, [%Si] is the silicon concentration (mass%) of the molten steel, [%Mn] is the manganese concentration (mass%) of the molten steel, [%P] is the phosphorus concentration (mass%) of the molten steel, [%S] is the sulfur concentration (mass%) of the molten steel, [%Cu] is the copper concentration (mass%) of the molten steel, [%Ni] is the nickel concentration (mass%) of the molten steel, [%Cr] is the chromium concentration (mass%) of the molten steel, and [%Al] is the aluminum concentration (mass%) of the molten steel. In the study of this embodiment, low-carbon aluminum-killed steel with C: 0.03 mass% or more and 0.2 mass% or less, Si: 0.05 mass% or more and 0.5 mass% or less, Mn: 0.8 mass% or more and 1.8 mass% or less, P: less than 0.02 mass%, and S: less than 0.005 mass% was used. However, the scope of application of the present invention is not limited to the above-mentioned component ranges.

[0037] The intentional bulging zone 172 does not require any special mechanism and can be configured simply by adjusting the roll gap, so it can be installed at any position within the range from the lower end of the mold 13 to the liquidus crater end position of the cast slab 3.

[0038] In the continuous slab casting machine 1 having such a configuration, molten steel 2 poured from a tundish 10 into a mold 13 through a submerged nozzle 12 is cooled in the mold 13 to form a solidified shell 4. A slab 3 having the solidified shell 4 as an outer shell and an unsolidified layer 5 inside is continuously drawn below the mold 13 while being supported by a slab support roll 14 provided below the mold 13. The slab 3 is cooled by secondary cooling water in a secondary cooling zone while passing through the slab support roll 14, and the thickness of the solidified shell 4 increases. Then, the thickness of the slab 3 increases in the intentional bulging zone 172 except for the narrow side end of the long side surface, and the slab 3 is lightly reduced in the light reduction zone 171 while completing solidification to the inside at the solidification completion position 6. After solidification is completed, the slab 3 is cut by a slab cutter 16 to become a slab 3a. Mold powder (not shown) is added to the mold 13, which functions as a heat insulator, lubricant, antioxidant, etc.

[0039] In this embodiment, continuous casting is performed in the soft reduction zone 171 under the following casting conditions. In the soft reduction zone 171, the slab 3 is reduced (softly reduced) at a reduction speed of 0.3 mm / min to 2.0 mm / min, with the central solid fraction being at least 0.2 to less than 1.0, which is the solid fraction at the thickness center of the slab 3. In this embodiment, the thickness center of the slab 3 means the center in the thickness direction at the width direction position where the solid fraction at the thickness center is the lowest in the cross section of the slab 3. If the reduction speed of the soft reduction within the above range of the central solid fraction is less than 0.3 mm / min, there is a high possibility that V-segregation will occur. On the other hand, if the reduction speed of the soft reduction within the above range of the central solid fraction is more than 2.0 mm / min, there is a high possibility that inverse V-segregation will occur.

[0040] Furthermore, in this embodiment, in the light reduction zone having a central solid fraction in the range of at least 0.2 to less than 1.0, the deflection τ of the light reduction segment 18a which is the segment 18 constituting the light reduction zone satisfies the following formula (1). 2×τ-A / (n-1)≦3.25×2×δ≦1.3 (1) Where: δ: Measured bulging amount (mm) τ: Deflection of the segment (mm) A: The reduction setting amount (mm) of the soft reduction in the soft reduction zone where the central solid fraction of the slab is 0.2 or more and less than 1.0 n: The number of slab support roll pairs in the soft reduction zone where the central solid fraction of the slab is 0.2 or more and less than 1.0

[0041] In the formula (1), the deflection τ of the segment is the deflection in the thickness direction at the widthwise center of the segment frame, and is calculated from the height in the thickness direction between both ends in the widthwise direction and the widthwise center of the segment frame. Specifically, as shown by the dashed line in FIG. 2, the deflection τ is the deflection τ in both the upper frame 180 and the lower frame 181. 1 , τ 2 Sum of (τ = τ 1 +τ 2) The deflection τ is preferably the maximum value of the deflection at a plurality of positions in the casting direction of the segment frame. For example, the deflection τ may be the maximum value of the deflection at at least two positions on the exit side and the entry side in the casting direction of the segment frame. In this case, as shown in FIG. 3, the deflection τ of the segment may be obtained by measuring the deflection at each of two positions in the casting direction where the support columns 182 are provided. The deflection τ of the soft reduction segment 18a may be a value measured when the slab 3 is actually continuously cast, or may be a value measured when the same type of steel is continuously cast by the continuous casting machine 1.

[0042] The measured amount of bulging δ is the amount of bulging after the central solid fraction of the slab 3 has changed to exceed 0.0. The measured amount of bulging δ is the amount of bulging on either the top or bottom surface, and for ease of measurement, it is preferable to use the measured amount of bulging on the top surface. Since the amount of bulging is usually the same on the top and bottom surfaces, the total amount of bulging can be obtained by measuring the amount of bulging on one of the long side surfaces and doubling the measured amount of bulging. In formula (1), the measured value of the total amount of bulging is shown by multiplying the measured amount of bulging δ by a coefficient of 2.

[0043] Furthermore, the set amount A of soft reduction is the amount (set amount) by which the slab 3 is reduced from a center solid fraction of 0.2 or more until it is completely solidified. The set amount of reduction for soft reduction is a set amount of reduction that is determined only by the roll profile, i.e., the set roll gap.

[0044] Here, the inventors confirmed that in the soft reduction in an actual continuous casting machine, the soft reduction may not be performed at the desired reduction amount (set reduction amount) due to the influence of bending of the segments, etc. Then, they discovered that the center segregation can be reduced more stably by using the segments 18 that satisfy the relationship of formula (1) in the soft reduction zone 171 during continuous casting, which led to the present invention.

[0045] The load applied to the light reduction segment 18a, which is a segment 18 constituting the soft reduction zone 171, is determined by the size of the slab 3, the reduction gradient in the light reduction zone 171, and the proportion of the unsolidified layer 5 of the slab 3 to be lightly reduced. In order to prevent the flow of molten steel at the end of solidification, which causes center segregation, it is necessary to apply a light reduction of an amount corresponding to the amount of solidification shrinkage and thermal shrinkage. However, if the set reduction gradient is large or the slab size is large, the load applied to the light reduction segment 18a becomes large. When the load applied to the light reduction segment 18a becomes large, the roll gap in the light reduction segment 18a operates in a direction to widen. Therefore, even if the slab size and the reduction gradient are set to the same, the load applied to the light reduction segment 18a varies depending on the shape of the slab in the width direction at the solidification completion position 6, and the roll gap also varies depending on the load. For this reason, the reduction speed actually applied to the slab 3 also varies from the set value.

[0046] However, by making the soft reduction segment 18a of the soft reduction zone 171 satisfy formula (1) within the above-mentioned range of the central solid fraction of the slab 3, the rigidity of the soft reduction segment 18a can be sufficiently obtained, and soft reduction can be performed with a sufficient reduction amount, so that the central segregation can be reduced more stably. In addition, since the set soft reduction conditions can be uniformly applied to the width direction of the slab, it is possible to reduce the variation in the segregation degree due to the central segregation in the width direction, and the segregation degree at all points in the width direction can be reduced. On the other hand, if the soft reduction segment 18a of the soft reduction zone 171 does not satisfy formula (1), a sufficient reduction amount cannot be reliably obtained. As a result, the overall segregation level of the central segregation in the slab width direction of the slab 3 may deteriorate, or the variation in the segregation degree in the slab width direction may increase. Furthermore, when examining conventional techniques such as those in Patent Documents 1 to 4, all of them are based on the premise that the set soft reduction conditions are directly applied to the slab, and do not take into account deviations from the soft reduction opening caused by actual mechanical bending of the segments or mechanical elongation of the support columns.

[0047] Furthermore, as a casting condition of this embodiment, it is preferable that the deflection deviation σ of the segment frame for the segment 18 corresponding to the solidification completion position 6 is 0.2 mm or less, and the displacement difference t between the left and right of the segment frame is always 0.2 mm or less. The deflection deviation σ is the deflection deviation in each of the upper surface side frame 180 and the lower surface side frame 181 of the segment 18, and is the difference between the maximum value and the minimum value of the deflection at multiple positions in the casting direction of each of the upper surface side frame 180 and the lower surface side frame 181. In other words, the deflection deviation σ being 0.2 mm or less means that the deflection deviation in the casting direction of each of the upper surface side frame 180 and the lower surface side frame 181 is 0.2 mm or less. For example, the deflection deviation may be the difference between the deflection at the inlet end and the outlet end in the casting direction of each of the upper surface side frame 180 and the lower surface side frame 181. In this case, the position at which the deflection is measured may be the position where the support pillar 182 is provided. The difference between the maximum and minimum values ​​of the deflection at three or more positions in the casting direction may be the deflection deviation σ. The left-right displacement difference t is the displacement difference between both ends of the segment frame in the width direction. Since the structure of the lower part of the segment 18 is usually fixed and does not move, the left-right displacement difference t is the displacement difference between both ends of the upper frame 180 in the width direction of the segment 18. The displacement difference t may also be the difference in height between the vertical upper ends of a pair of support columns 182 connected to both ends of the upper frame 180 in the width direction. Furthermore, it is preferable that the left-right displacement difference t is always 0.2 mm or less at different positions in the casting direction. By setting such casting conditions, the center segregation can be more stably reduced.

[0048] <Modification> Although the present invention has been described above with reference to specific embodiments, it is not intended that the invention be limited by these descriptions. By referring to the description of the present invention, other embodiments of the present invention including various modifications in addition to the disclosed embodiments will be apparent to those skilled in the art. Therefore, it should be understood that the embodiments of the invention described in the claims also include embodiments including these modifications described herein, either alone or in combination.

[0049] For example, in the above embodiment, the support pillars 182 adjust the roll gap, but the present invention is not limited to such an example. For example, the roll gap may be adjusted by expanding and contracting the support members connecting the slab support rolls 14 and the segment frames using hydraulic pressure or the like. EXAMPLES

[0050] Examples carried out by the present inventors will be described. In the examples, in order to investigate the influence of the casting conditions on thickness center segregation of the slab 3, two types of segments 18 (frame 1 and frame 2) with different rigidity were experimentally mounted, and continuous casting was carried out under various casting conditions. The casting conditions were those in the case of continuous casting with soft reduction using a vertical bending type continuous casting machine 1 as shown in FIG. 1. Then, the cast slab 3 was evaluated by measuring the Mn segregation degree and conducting a hydrogen induced cracking resistance test (HIC test). The detailed casting conditions are as follows.

[0051] In the embodiment, the slab withdrawal speed was constant at 1.1 m / min in order to efficiently soft reduce the slab 3. The total amount of bulging in the intentional bulging zone 172 was changed for each level in the range of 0.0 mm to 15.0 mm. The set reduction speed in the soft reduction zone 171 was changed for each level in the range of 0 mm / min to 4.0 mm / min. The size of the slab 3 to be cast was a slab slab with a width of 2100 mm and a thickness of 250 mm. Regarding the segment frames of the segment 18, the frame 1 was made to have a higher rigidity than the frame 2. The amount of secondary cooling water was adjusted to be the same for all levels. The final solidification position was determined by a heat transfer solidification calculation similar to that in Patent Document 4.

[0052] During casting, in the light reduction segment where the solidification completion position 6, which was the most downstream in the casting direction as determined in advance by heat transfer and solidification calculations, was located, the displacement of the roll gap, the center of the segment, and the displacement of the support pillar of the segment frame were measured using non-contact sensors.

[0053] After casting, the relationship between the reduction rate actually applied to slab 3 and the segregation of slab 3 was examined by etching the cross section (corresponding to the longitudinal cross section) of a test piece taken from the slab with picric acid to check for the presence or absence of V-segregation, inverted V-segregation, and internal cracks. In addition, the Mn segregation at the center of the thickness of the slab was analyzed by EPMA in the test piece taken from slab 3, and the degree of Mn segregation at each position in the slab width direction was investigated. In addition, a hydrogen induced cracking test (HIC test) was conducted on the test piece taken from each position in the slab width direction.

[0054] Table 1 shows the conditions and test results for each level in the examples. Levels 1 to 13 are cases where segments with increased frame rigidity are used, and levels 14 to 28 are cases where segments with insufficient rigidity are used. In the examples, frames 1 and 2 with different rigidity were used in the soft reduction zone 171, and the other casting conditions were the same. The number of soft reduction segments 18a provided in the light reduction zone 171 was one. In Table 1, the measured bulging amount is the measured value of the bulging amount of the cast slab, the value α is a value calculated by the following formula (2), and the value β is a value calculated by the following formula (3). Furthermore, the set reduction amount A in formula (1) is calculated by the following formula (4), and the length L of the soft reduction zone 171 in the casting direction is set to 1.89 m from the length of the light reduction segment 18a, and the casting speed V cThe speed was set to 1.1 m / min. Furthermore, the number n of slab support roll pairs in the soft reduction zone 171 was set to 8 pairs based on the number of roll pairs provided in the soft reduction segment 18a. In addition, in Table 1, the judgment indicates whether the value β is 1.3 or less, and is indicated as "○" when the value β is 1.3 or less, and "×" when the value β is more than 1.3. In other words, the condition for the value β to be equal to or greater than the value α and for the judgment to be "○" is to satisfy the formula (1). Furthermore, in Table 1, the deflection deviation of the segment displacement is the deflection deviation σ of the segment at the final solidification position, which is the sum of the deflection deviation σ of the upper surface frame 180 and the deflection deviation σ of the lower surface frame 181. Furthermore, "in" and "out" of the segment displacement indicate the displacement of the end portions (positions where the support pillars 182 are installed) at the inlet and outlet sides in the casting direction of the segment at the final solidification position. Additionally, the "south" and "north" of the segment displacement indicate the displacement of the ends (positions where the support pillar 182 is installed) at one end (the south end in the actual machine) and the other end (the north end in the actual machine) in the width direction of the segment. Furthermore, the "difference" of the segment displacement is the difference between the displacements at the south and north, and is the displacement difference t between the left and right. In this embodiment, the segment displacement indicates how much the upper end position has displaced from the reference, using the upper end position of the support pillar 182 when the segment is not deflected as the reference.

[0055] α = 2 × τ-A / (n-1) (2) Where: τ: Deflection of the segment (mm) A: The reduction setting amount (mm) of soft reduction in the soft reduction zone where the solid fraction of the slab is 0.2 or more and less than 1.0 n: The number of pairs of supporting rolls in the soft reduction zone where the solid fraction of the slab is 0.2 or more and less than 1.0 β = 3.25 × 2 × δ (3) Where: δ: Measured bulging amount (mm) A=V p ×L / V c (4) Where: V p : Set reduction speed (mm / min) L: Length in the casting direction under soft rolling (m) Vc : Casting speed (m / min)

[0056] [Table 1]

[0057] As shown in Table 1, comparing Frame 1 and Frame 2, center segregation was improved by increasing the frame rigidity, keeping the central deflection to 0.4 mm or less, and keeping the left-right displacement difference to 0.2 mm or less. In addition, although the conditions of levels 1 to 6 are within the range of the present invention, the results of both the degree of segregation and the variation σ were good. In the conditions of levels 9 to 11, 16 to 17, and 22 to 24 where the reduction rate in the soft reduction zone 171 was less than 0.3 mm / min, V segregation occurred, and in the conditions of levels 12, 13, 25, and 26 where the reduction rate exceeded 2.0 mm / min, inverted V segregation occurred.

[0058] Regarding center segregation, when V segregation and inverse V segregation occurred, the Mn segregation degree deteriorated, and the HIC result, the crack segregation area ratio (%) = CAR (CRACK AREA RATIO), also deteriorated. The criteria for judging center segregation were that the Mn segregation degree was 1.06 or less and the HIC result was 2.0 or less. Therefore, it was found that the reduction rate in the soft reduction zone 171 needs to be controlled to 0.3 mm / min or more and 2.0 mm / min or less. The reduction rate actually applied to the slab 3 was calculated as the product of the reduction gradient calculated from the displacement of the roll gap in the soft reduction segment measured by a non-contact sensor and the slab withdrawal speed. [Explanation of symbols]

[0059] 1. Continuous slab casting machine 10 Tundish 11 Sliding nozzle 12 Submerged nozzle 13 Mold 14, 14a Cast piece support roll 14b Reduction roll 15 Transport roll 16 Slab cutting machine 171 Lightly Pressed Zone 172 Intentional bulging zone 173 Upper Correction Belt 174 Lower Correction Belt 18 Segments 18a Light reduction segment 180 Upper frame 181 Lower frame 182 Support column 2. Molten Steel 3,3a slab 4 Solidified shell 5 Unsolidified layer 6 Solidification completion position

Claims

1. A method for continuously casting steel, comprising the steps of: continuously casting steel using a curved type continuous casting machine or a vertical bending type continuous casting machine; The slab is subjected to a reduction at a reduction rate of 0.3 mm / min or more and 2.0 mm / min or less in a range where the central solid fraction, which is the solid fraction at the thickness center of the slab to be continuously cast, is at least 0.2 or more and less than 1.0, The method for continuous casting steel, wherein a deflection τ of a segment, which is a deflection in a thickness direction of a segment frame of a segment that applies a rolling reduction to the slab, satisfies formula (1). 2×τ-A / (n-1)≦3.25×2×δ≦1.3 (1) Where: δ: Measured bulging amount (mm) τ: Deflection of the segment (mm) A: reduction setting amount (mm) of soft reduction in the soft reduction zone where the central solid fraction of the slab is 0.2 or more and less than 1.0 n: the number of slab support roll pairs present in the soft reduction zone in which the central solid fraction of the slab is 0.2 or more and less than 1.0

2. 2. The method for continuous casting of steel according to claim 1, wherein a deflection deviation σ of a segment frame corresponding to a final solidification position is set to 0.2 mm or less, and a displacement difference t between the left and right sides of said segment frame is always set to 0.2 mm or less.

3. In the intentional bulging zone of the continuous casting machine, the roll gap between the pairs of cast piece support rolls is gradually increased toward the downstream side in the casting direction to bulge the long side surface of the cast piece with a total bulging amount of 3 mm or more and 20 mm or less, 3. The method for continuous casting steel according to claim 1, wherein the reduction is applied to the slab by gradually decreasing a roll gap between a plurality of pairs of slab support rolls toward the downstream side in the casting direction in a light reduction zone downstream of the intentional bulging zone in the casting direction.

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