Slab Cooling Method

Abstract

To suppress season cracking of a slab in a cooling process after continuous casting.SOLUTION: When cooling a plurality of stacked slabs, the first and second slabs arranged at both ends in the stacking direction satisfy the following temperature conditions 1 at the time of stacking and a third slab sandwiched between the first and second slabs satisfies the following temperature condition 2 at the time of stacking. The temperature condition 1: surface temperature at the widthwise center of the first and second slabs is +(higher than 0 to 200)°C at the widthwise center of the third slab, and center temperature of the first and second slabs is equal to or higher than the surface temperature at the widthwise center of the first and second slabs. The temperature condition 2: surface temperature at the widthwise center of the third slab is 500°C or higher and 900°C or lower, and center temperature of the third slab is at least 150°C higher than the surface temperature at the widthwise center of the third slab.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present application discloses a method for cooling a slab obtained by continuous casting. [Background technology]

[0002] In recent years, high-tensile steel has been used in various technical fields. For example, in the field of automobiles, the application of high-tensile steel has been promoted to reduce the weight of automobile bodies in order to improve fuel efficiency. In addition, high-tensile steel has been widely used in automobile bodies to ensure the safety of passengers.

[0003] High-tensile steel is made by adding large amounts of C, Si, and Mn in order to improve strength. Here, it is known that adding large amounts of C, Si, and Mn embrittles steel. Therefore, when a slab is obtained by continuously casting steel to which large amounts of C, Si, and Mn have been added, and then the slab is cooled, cracks are likely to occur in the inside or surface of the slab due to thermal stress caused by the temperature difference between the surface and the inside of the slab. For example, so-called "rest cracking" may occur as disclosed in Patent Document 1. To prevent rest cracking of slabs, measures such as hot charge rolling (HCR) are taken for some steel types, but it is difficult to perform HCR on all slabs.

[0004] In the prior art, when cooling a slab after continuous casting, the cooling rate in the embrittlement region of the slab is controlled to be gentle, suppressing stress and distortion occurring in the slab and preventing cracking of the slab. For example, Patent Document 1 discloses a technique for preventing cracking of the slab by precisely controlling the average cooling rate at 500°C or less when cooling a slab after continuous casting to room temperature. Patent Documents 2 and 3 disclose techniques for preventing cracking of the slab during the cooling process by setting the average cooling rate at 500 to 700°C to 20°C / hr or less when cooling a slab after continuous casting. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2007-083274 A [Patent Document 2] JP 2019-167559 A [Patent Document 3] JP 2019-167560 A Summary of the Invention [Problem to be solved by the invention]

[0006] As disclosed in Patent Documents 1 to 3, in order to prevent the slab from cracking during the cooling process, it is effective to perform slow cooling by precisely controlling the cooling rate in the embrittlement region. However, it may be difficult to precisely control the cooling rate of the slab due to equipment restrictions. In this regard, a new technology capable of suppressing the slab from cracking during the cooling process is needed. [Means for solving the problem]

[0007] As one of the means for solving the above problems, the present application provides: Obtaining a plurality of slabs by continuous casting; Stacking the plurality of slabs so that the widthwise surfaces of the plurality of slabs overlap each other to obtain a slab stack; and cooling the slab stack; Including, The slab stack has a first slab arranged on one end side in the stacking direction, a second slab arranged on the other end side in the stacking direction, and at least one third slab sandwiched between the first slab and the second slab, The third slab contains, by mass%, C: 0.02 to 0.60%, Si: 0.5 to 3.0%, Mn: 1.0 to 3.0%, P: 0.100% or less, S: 0.010% or less, Al: 0.005 to 1.000%, and N: 0.0100% or less; The first slab and the second slab satisfy the following temperature condition 1 when stacked, The third slab satisfies the following temperature condition 2 when laminated: Slab Cooling Method Disclose.

[0008] Temperature condition 1: The surface temperature of each of the first slab and the second slab at the width center is higher than the surface temperature of the third slab at the width center, the temperature difference between the surface temperature of each of the first slab and the second slab at the width center and the surface temperature of the third slab at the width center is within 200°C, and the central temperature of each of the first slab and the second slab is higher than or equal to the surface temperature of each of the first slab and the second slab at the width center.

[0009] Temperature condition 2: The surface temperature of the third slab at the width center is 500° C. or higher and 900° C. or lower, and the central temperature of the third slab is 150° C. or higher than the surface temperature at the width center of the third slab.

[0010] In the slab cooling method of the present disclosure, regarding the temperature condition 2, the surface temperature at the widthwise center of the third slab may be 550°C or higher and 750°C or lower. Effect of the Invention

[0011] According to the slab cooling method of the present disclosure, cracks in the slab during the cooling process are easily prevented. [Brief description of the drawings]

[0012] [Figure 1] 1 shows a schematic diagram of an example of a slab stack configuration; [Diagram 2] 1 shows a schematic diagram of an example of the temperature distribution change through the thickness of a slab. [Diagram 3] This is an example of the results of a thermal stress analysis immediately after the slabs are stacked, showing (a) the temperature of the slab, (b) the maximum principal stress generated in the slab, and (c) the state of plastic strain generated in the slab. [Figure 4] 1 shows a schematic of an example of a region where compressive strain occurs in a slab. [Diagram 5] The results of the examples and comparative examples are shown. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] In the conventional technology, the cooling rate of the slab after continuous casting is controlled to cool it slowly, and the temperature difference between the surface and the inside of the slab is minimized, thereby suppressing cracking of the slab during the cooling process. However, due to equipment restrictions, such temperature control may be difficult. The present inventor has used thermal stress analysis to search for conditions that can suppress cracking of the slab during the cooling process of the slab after continuous casting. As a result, it has been found that the tensile residual stress of the slab in the temperature range (for example, 100 to 500 ° C) where cracking of the slab is a concern can be greatly reduced by intentionally providing a temperature difference between the surface and the inside of the slab after continuous casting, stacking the slabs, and reheating the surface of the slab. According to the technology of the present disclosure, unlike the conventional viewpoint, by imparting a thermal history to the slab utilizing the temperature distribution at the end of the continuous casting machine, it is possible to suppress cracking of the slab and increase the degree of freedom of the cooling conditions of the slab. The slab cooling method of the present disclosure will be described in detail below.

[0014] The slab cooling method of the present disclosure includes obtaining a plurality of slabs by continuous casting (hereinafter sometimes referred to as a "first step"), stacking the plurality of slabs so that the surfaces in the width direction of the plurality of slabs overlap each other to obtain a slab stack (hereinafter sometimes referred to as a "second step"), and cooling the slab stack (hereinafter sometimes referred to as a "third step"). The slab stack has a first slab arranged at one end side in the stacking direction, a second slab arranged at the other end side in the stacking direction, and at least one third slab sandwiched between the first slab and the second slab. The third slab contains, by mass%, C: 0.02 to 0.60%, Si: 0.5 to 3.0%, Mn: 1.0 to 3.0%, P: 0.100% or less, S: 0.010% or less, Al: 0.005 to 1.000%, and N: 0.0100% or less. In the slab cooling method of the present disclosure, it is important that the first slab and the second slab satisfy the following temperature condition 1 when stacked, and that the third slab satisfies the following temperature condition 2 when stacked.

[0015] Temperature condition 1: The surface temperature of each of the first slab and the second slab at the width center is higher than the surface temperature of the third slab at the width center, the temperature difference between the surface temperature of each of the first slab and the second slab at the width center and the surface temperature of the third slab at the width center is within 200°C, and the central temperature of each of the first slab and the second slab is higher than or equal to the surface temperature of each of the first slab and the second slab at the width center.

[0016] Temperature condition 2: The surface temperature of the third slab at the width center is 500° C. or higher and 900° C. or lower, and the central temperature of the third slab is 150° C. or higher than the surface temperature at the width center of the third slab.

[0017] 1.First step In the first step, a plurality of slabs are obtained by continuous casting. The continuous casting conditions of the slabs are not particularly limited. The shape of the slab at the end of the continuous casting machine may be a general one. The slab may have a width (long side in the cross-sectional shape) and a thickness (short side in the cross-sectional shape) in a cross-sectional shape perpendicular to the continuous casting direction, and may have a length in the continuous casting direction. The width of the slab may be, for example, 800 mm or more and 1300 mm or less, the thickness of the slab may be, for example, 200 mm or more and 300 mm or less, and the length of the slab may be, for example, 5 m or more and 9 m or less. The temperature of the slab at the end of the continuous casting machine is not particularly limited as long as the temperature conditions described later can be satisfied when stacking the slabs. As described later, in the first step, the surface of the slab may be cooled to provide a predetermined temperature difference between the surface and the center of the slab. The cooling method is not particularly limited, and may be natural cooling or forced cooling (cooling by forced convection, water cooling, etc.). In particular, when the surface of the slab is forcibly cooled, the temperature difference between the surface and the center of the slab is likely to become larger.

[0018] 2.Second process In the second step, a plurality of slabs obtained by continuous casting are stacked so that the surfaces (surfaces on the long side) in the width direction of the slabs overlap each other to obtain a slab stack. FIG. 1 shows an example of the form of the slab stack. The left-right direction of the paper in FIG. 1 is the width direction of the slab, the up-down direction of the paper is the thickness direction of the slab, and the back-to-front direction of the paper is the length direction of the slab. As shown in FIG. 1, the slab stack 100 is formed by stacking a plurality of slabs 10 in the thickness direction so that the surfaces in the width direction of the plurality of slabs 10 overlap each other. The slab stack 100 has a first slab 10 arranged at one end side in the stacking direction, a second slab 20 arranged at the other end side in the stacking direction, and at least one third slab 30 sandwiched between the first slab 10 and the second slab 20. As shown in FIG. 1, in the slab stack 100, the widthwise surface of the third slab 30 is overlapped with the widthwise surface of another slab, while the thicknesswise surface of the slab may be exposed to the outside of the slab stack 100 without being overlapped with another slab.

[0019] 2.1 First and second slabs 1, the first slab 10 and the second slab 20 may be outermost slabs arranged at both ends in the stacking direction of the slab stack 100. Each of the first slab 10 and the second slab 20 may be composed of one slab or multiple slabs. In other words, the multiple stacked slabs may be regarded as one first slab 10 or one second slab 20.

[0020] 2.1.1 Steel composition The first slab 10 and the second slab 20 may or may not have a steel composition that can be a high-tensile steel. In addition, the first slab 10 and the second slab 20 may have the same steel composition or may have different steel compositions.

[0021] 2.1.2 Size The first slab 10 and the second slab 20 sandwich a third slab 30, which will be described later. The size of the width direction surface of the first slab 10 and the second slab 20 is not particularly limited, but may be substantially the same size as or larger than the width direction surface of the third slab 30 in order to sandwich the entire width direction surface of the third slab 30.

[0022] 2.1.3 Temperature conditions The first slab 10 and the second slab 20 sandwich the third slab 30 described later, thereby promoting the reheating of the surface of the third slab 30. In order to properly reheat the surface of the third slab 30, the first slab 10 and the second slab 20 satisfy the above-mentioned temperature condition 1 when stacked. "When stacked" refers to the time when stacking of the slabs begins to form the slab stack 100 (the time when one slab overlaps another slab). The slab that satisfies the above-mentioned temperature condition 1 can be obtained, for example, by controlling the cooling conditions in the continuous casting machine and the cooling conditions after continuous casting. When obtaining the first slab 10 and the second slab 20 that satisfy the temperature condition 1, the slabs after continuous casting may or may not be cooled, and when cooling, they may be left to cool or may be forced to cool (for example, cooling by forced convection or water cooling). Whether or not the first slab 10 or the second slab 20 satisfies the above temperature condition 1 can be determined based on the cooling conditions of the slabs, the measured surface temperatures of the slabs, the results of three-dimensional heat transfer analysis, and the like.

[0023] In temperature condition 1, the surface temperature of each of the first slab and the second slab at the widthwise center is higher than the surface temperature of the third slab at the widthwise center. If the surface temperature of each of the first slab and the second slab at the widthwise center is lower than the surface temperature of the third slab at the widthwise center, the temperature of the surface of the third slab will decrease when the first to third slabs are laminated, which will hinder the reheating of the surface of the third slab, and it may be difficult to properly reheat the surface of the third slab so as to generate a compressive stress described later. In addition, in temperature condition 1, the temperature difference between the surface temperature of each of the first slab and the second slab at the widthwise center and the surface temperature of the third slab at the widthwise center is within 200°C. If the temperature difference is too large, it may be difficult to properly reheat the surface of the third slab so as to generate a compressive stress described later. The temperature difference may be within 180°C, 160°C, 140°C, 120°C, or 100°C. The surface temperature of each of the first slab and the second slab at the width center is more than 500°C and not more than 1100°C in relation to the temperature condition 2 of the third slab. In addition, in the above temperature condition 1, the center temperature of each of the first slab and the second slab may be equal to or higher than the above surface temperature at the width center of the first slab and the second slab. That is, the center temperature of the first slab is equal to or higher than the surface temperature at the width center of the first slab, and the center temperature of the second slab is equal to or higher than the surface temperature at the width center of the second slab. There is no particular limit to the upper limit of the slab center temperature, and it may be any temperature that does not cause problems in laminating the slabs. For example, in the temperature condition 1, the slab center temperature may be 50°C or higher, 100°C or higher, or 150°C or higher, or 250°C or lower, or 200°C or lower, higher than the surface temperature at the width center of the slab. It is sufficient that the first slab and the second slab each satisfy the temperature condition 1, and the temperature of the first slab and the temperature of the second slab may be the same or different from each other.

[0024] 2.1.4 Supplementary Information In the slab cooling method of the present disclosure, the first slab 10 and the second slab 20 are intended only to promote appropriate reheating of the surface of the third slab 30. In other words, the first slab 10 and the second slab 20 may be excluded from temperature control during the cooling process, and it is inevitable that cracks will occur in the first slab 10 and the second slab 20 during the cooling process. Alternatively, slabs made of steel that is less likely to crack (steel other than high tensile steel) may be used as the first slab 10 and the second slab 20 to suppress the occurrence of cracks.

[0025] 2.2 Third Slab In the slab cooling method of the present disclosure, the third slab 30 is a target for suppressing the occurrence of cracks during the cooling process. The third slab 30 contains a predetermined amount or more of C, Si, and Mn, and may be used as a material for a high-tensile steel plate. In the present application, the term "high-tensile steel plate" refers to a steel plate having a tensile strength of 500 MPa or more. The tensile strength may be 780 MPa or more, 980 MPa or more, 1180 MPa or more, or 1470 MPa or more, and may be 2100 MPa or less, 2000 MPa or less, or 1900 MPa or less. The tensile test of the steel plate is performed, for example, in accordance with JIS Z 2241, by taking a JIS No. 5 test piece from a direction in which the longitudinal direction of the test piece is parallel to the rolling perpendicular direction of the steel plate.

[0026] 2.2.1 Steel composition The third slab 30 contains, in mass%, C: 0.02-0.60%, Si: 0.5-3.0%, Mn: 1.0-3.0%, P: 0.100% or less, S: 0.010% or less, Al: 0.005-1.000%, and N: 0.0100% or less. When the contents of C, Si, and Mn are within such ranges, the brittleness of the slab is likely to decrease and the problem of cracking is likely to occur, but the cooling method of the slab disclosed herein can suppress the cracking. In addition, in the present application, the numerical range "to" is used to mean that the numerical values ​​before and after it are included as the lower limit and upper limit unless otherwise specified.

[0027] (C: 0.02~0.60%) C is the most fundamental element that affects not only the static strength of steel but also its fatigue strength, toughness, and ductility. If the C content is too low, the static strength and fatigue strength of the steel may be insufficient. In this regard, the lower limit of the C content may be 0.02 mass%, 0.05 mass%, 0.10 mass%, or 0.15 mass%. Also, if the C content is too high, the toughness of the steel is likely to deteriorate excessively. In this regard, the upper limit of the C content may be 0.60 mass%, 0.50 mass%, 0.40 mass%, or 0.30 mass%.

[0028] (Si: 0.5 to 3.0%) Silicon is an important element with the second highest solid solution strengthening ability after carbon. To obtain high tensile steel, the concentration of silicon is increased. Specifically, the lower limit of the silicon content may be 0.5 mass%, 0.8 mass%, or 1.0 mass%. On the other hand, if the silicon content is too high, there is a risk of the toughness and workability being deteriorated. In this regard, the upper limit of the silicon content may be 3.0 mass%, 2.5 mass%, or 2.0 mass%.

[0029] (Mn: 1.0-3.0%) Mn is an important element for improving hardenability and ensuring hardness to the inside of the steel material even when the cooling rate is insufficient. To obtain high tensile steel, the Mn concentration is set to a high concentration. Specifically, the lower limit of the Mn content may be 1.0 mass% or 1.5 mass%. On the other hand, if there is too much Mn, there is a risk of deteriorating toughness and workability. In this regard, the upper limit of the Mn content may be 3.0 mass% or 2.8 mass%.

[0030] (P:0.100% or less) P is an element that promotes Mn concentration in unsolidified parts during the solidification process of molten steel, lowers the Mn concentration in negative segregation parts, and promotes an increase in the area ratio of ferrite, and the less the better. In addition, excessive P content increases the strength of the steel, but may lead to brittle fracture of the steel. In this regard, the upper limit of the P content may be 0.100 mass%, 0.050 mass%, or 0.010 mass%. On the other hand, the lower limit of the P content is not particularly limited and may be 0 mass%, but controlling the P content to less than 0.001 mass% may increase the refining time and increase the manufacturing cost. In order to prevent an increase in manufacturing costs, the P content may be 0.001 mass% or more.

[0031] (S:0.010% or less) S is an element that generates nonmetallic inclusions such as MnS in steel and reduces the ductility of the steel, and therefore the less the better. In this regard, the upper limit of the S content may be 0.010 mass%, 0.008 mass%, or 0.005 mass%. On the other hand, the lower limit of the S content is not particularly limited and may be 0 mass% or 0.001 mass%.

[0032] (Al: 0.005 to 1.000%) Al is an element that acts as a deoxidizer for steel and stabilizes ferrite, and is added as necessary. When the Al content is 0.005 mass% or more, such an effect is easily obtained. The Al content may be 0.010 mass% or more. On the other hand, if an excessive amount of Al is contained, the ferrite transformation and bainite transformation during the cooling process in the annealing in the downstream process may be excessively promoted, resulting in a decrease in the strength of the steel. When the Al content is 1.000 mass% or less, such a problem is easily avoided. The Al content may be 0.800 mass% or less.

[0033] (N:0.0100% or less) N is an element that forms coarse nitrides and reduces the workability of steel, so the lower the content, the better. The N content may be 0 mass%, 0.0001 mass% or more, 0.0010 mass% or more, 0.0100 mass% or less, or 0.0050 mass% or less.

[0034] The third slab 30 may contain, in addition to the above basic elements, any other element. The optional elements may not be contained, so the lower limit is 0%. The third slab 30 may contain, for example, in mass%, Ti: 0-0.500%, Co: 0-0.500%, Ni: 0-0.500%, Mo: 0-0.500%, Cr: 0-2.000%, O: 0-0.0100%, B: 0-0.0100%, Nb: 0-0.500%, V: 0-0.500%, Cu: 0-0.500%, W: 0-0.1000%, T ... and Ni: 0-0.500%. The third slab 30 may contain one or more elements selected from the group consisting of a: 0-0.1000%, Sn: 0-0.0500%, Sb: 0-0.0500%, As: 0-0.0500%, Mg: 0-0.0500%, Ca: 0-0.0500%, Y: 0-0.0500%, Zr: 0-0.0500%, La: 0-0.0500%, and Ce: 0-0.0500%. The types and contents of the optional elements described above are merely examples, and the types and amounts of the optional elements that may be contained in the third slab 30 are not limited to those described above.

[0035] 2.2.2 Temperature conditions The third slab 30 satisfies the above-mentioned temperature condition 2 when stacked. A slab that satisfies the above-mentioned temperature condition 2 can be obtained, for example, by controlling the cooling conditions in a continuous casting machine or the cooling conditions after continuous casting. In particular, the third slab 30 that satisfies the temperature condition 2 is easily obtained by cooling the surface of the slab during or after continuous casting. When cooling the surface of the slab, the slab may be left to cool or may be forced to cool (for example, cooling by forced convection or water cooling). In particular, when forced cooling is performed, the third slab 30 that satisfies the temperature condition 2 is easily obtained. Whether or not the third slab 30 satisfies the above-mentioned temperature condition 2 can be determined based on the cooling conditions of the slab, the actual measured value of the temperature of the slab, the results of a three-dimensional heat transfer analysis, etc.

[0036] In the technology of the present disclosure, the third slab 30, which satisfies the above-mentioned temperature condition 2, is sandwiched between the first slab 10 and the second slab 20, which satisfies the above-mentioned temperature condition 1. This causes the surface of the third slab 30 to reheat. Hereinafter, the effects of appropriately reheating the surface of the third slab 30 will be described.

[0037] The slab cooling method of the present disclosure is characterized in that it utilizes the temperature gradient of the slab generated by continuous casting and subsequent cooling. That is, instead of making the temperature of the slab as uniform as possible as in the conventional technology, a temperature difference is intentionally created between the surface and center of the slab. When a plurality of slabs are stacked under similar temperature conditions under the condition that the slab center temperature (temperature at the center in the width direction and thickness direction) is higher than the surface temperature at the center in the width direction of the slab, as shown in FIG. 2, in the temperature distribution in the thickness direction of the slab, the slab surface reheats and the inside of the slab is balanced in the direction of decreasing temperature. At this time, compressive stress occurs on the slab surface due to reheating, and plastic strain occurs. That is, as shown in FIG. 2 and FIG. 3(a), when the surface of the slab reheats, the surface wants to expand but is restrained inside, and as shown in FIG. 3(b) and (c), compressive stress occurs on the surface and compressive plastic strain occurs. According to the knowledge of the present inventor, the residual stress at 500°C or less, where the slab is concerned about the placement crack, is mainly determined by the amount of plastic strain generated during the cooling process. If tensile plastic deformation occurs on the surface of the slab, as the temperature inside the slab drops due to the restraint of the stretched outer frame, tensile stress occurs inside the slab, which leads to cracking. Conversely, if compressive plastic deformation occurs on the surface of the slab, tensile stress is less likely to occur inside the slab as described above, and cracking is more likely to be prevented. From various calculation results, the inventor has found that in order to cause compressive plastic deformation on the surface of the third slab, the first slab and the second slab must satisfy the above temperature condition 1, and the third slab must satisfy the above temperature condition 2, that is, the surface temperature of the third slab at the center in the width direction must be 500°C or higher and 900°C or lower, and a positive temperature gradient of 150°C or higher must be generated from the surface of the center in the width direction of the third slab toward the center of the slab (the temperature of the center of the third slab must be 150°C or higher than the surface temperature of the center in the width direction of the slab).

[0038] Furthermore, in the above temperature condition 2, when the surface temperature at the width direction center of the third slab is 550°C or more and 750°C or less, the tensile stress inside the slab can be further suppressed, and the occurrence of cracks during cooling can be further prevented. This is considered to be because, as shown in FIG. 4, in addition to the recuperation in the recuperation layer 30a near the slab surface, which is at a low temperature, ferrite transformation occurs in the transformation layer 30b, and the transformation strain associated with the transformation causes compressive plastic deformation in the same manner as the thermal strain described above. More specifically, in this temperature range, the transformation proceeds from the slab surface (the inside of the slab is a temperature range where transformation does not occur, or it takes time for transformation), so that a difference in the ferrite fraction occurs between the slab surface and the inside of the slab, which causes transformation strain and causes compressive plastic deformation on the slab surface. In this way, by considering the effects of transformation strain in addition to thermal strain, a higher effect can be achieved in preventing cracks during cooling of the slab.

[0039] As described above, in the above temperature condition 2, the surface temperature at the widthwise center of the third slab is 500°C or higher and 900°C or lower, and particularly when it is 550°C or higher and 750°C or lower, a higher effect can be expected. The upper limit of the temperature may be 650°C. In addition, in the above temperature condition 2, the temperature difference between the temperature at the center of the third slab and the surface temperature at the widthwise center of the third slab is 150°C or higher, and may be 160°C or higher or 170°C or higher. The upper limit of the temperature difference is not particularly limited. For example, it may be 300°C or lower, 250°C or lower, or 230°C or lower.

[0040] 2.3 Slab stack 1, a slab stack 100 is constructed by stacking a first slab 10 and a second slab 20 that satisfy the above-mentioned temperature condition 1 and a third slab 30 that satisfies the above-mentioned temperature condition 2. In the slab stack 100, when the first slab 10, the second slab 20, and the third slab 30 are stacked, reheating of the third slab 30 may occur.

[0041] In the slab stack 100, the third slab 30 is sandwiched between the first slab 10 and the second slab 20. In the slab cooling method of the present disclosure, the number of the third slabs 30 may be at least one, and may be more than one, and the upper limit is not particularly limited. When considering workability and the like, the number of the third slabs 30 may be 10 or less.

[0042] 3. 3rd process In the third step, the slab stack 100 is cooled. The cooling of the slab stack 100 may be performed, for example, by natural cooling. The cooling stop temperature of the slab stack 100 is not particularly limited, and may be cooled, for example, to room temperature. When the third slab 30 is taken into consideration, from the start point of cooling of the slab stack 100 (for example, the completion point of stacking), the surface temperature of the third slab 30 increases due to recuperation, while the internal temperature of the third slab 30 decreases. The temperature of the entire third slab 30 gradually decreases while the temperature is balanced between the surface and the internal. In the technology disclosed herein, it is not necessary to control the cooling rate during the cooling process of the third slab 30. According to the knowledge of the present inventor, even if the cooling rate of the third slab 30 after the completion of recuperation is faster than the cooling rate in the conventional technology, the third slab 30 is less likely to have a crack. The average cooling rate between 700° C. and 500° C. on the surface at the center in the width direction of the third slab 30 after the completion of reheating may be, for example, more than 20° C. / h or 40° C. / h or less.

[0043] After the cooling of the slab stack 100 is completed, for example, the third slab 30 is removed from the slab stack 100, and the third slab 30 is reheated and hot rolled to produce a hot-rolled steel sheet. Then, coiling, pickling, cold rolling, annealing, surface treatment, etc. may be performed. By adjusting the hot rolling conditions, coiling conditions, pickling conditions, cold rolling conditions, annealing conditions, surface treatment, etc., a steel sheet having the desired performance (for example, the above-mentioned high tensile steel sheet) can be produced.

[0044] As described above, according to the slab cooling method of the present disclosure, the third slab 30 is less likely to develop cracks during the cooling process of the third slab 30. Moreover, in the slab cooling method of the present disclosure, there is no need to precisely control the cooling rate of the slab as in the conventional technology. Moreover, even if the cooling rate is made faster than in the conventional technology, the third slab 30 is less likely to develop cracks. EXAMPLES

[0045] The following examples of the present invention are given, but the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the invention and the object is achieved.

[0046] Molten steel consisting of a predetermined composition was continuously cast to produce a slab. Table 1 below shows the steel composition of the slab produced by continuous casting. Continuous casting was performed using a mold for slab casting, with a casting speed of 0.7-1.5 m / min to produce a slab with a thickness of 240-280 mm and a width of 1100 mm. The slab was then cut to a predetermined length, and the surface of the slab was cooled by leaving it to cool or by blowing air onto it to generate forced convection. The surface temperature of the slab was measured with a contact thermocouple, and the difference between the surface temperature at the center of the slab in the width direction and the temperature at the center of the slab was calculated from the central temperature of the slab (the central temperature in the width direction and thickness direction) estimated by three-dimensional heat transfer analysis.

[0047] In Examples 1 to 10 and Comparative Examples 2 to 7, the above-mentioned slab (third slab) was sandwiched between two slabs (first slab and second slab) to obtain a slab laminate, which was then allowed to cool to room temperature.

[0048] In Comparative Example 1, a slab stack was obtained by stacking and stacking only two slabs (a first slab and a second slab), and the slab stack was allowed to cool to room temperature.

[0049] For each of the examples and comparative examples, the following Table 2 shows (1) the surface temperature at the widthwise center and (2) the temperature difference between the surface temperature at the widthwise center and the central part temperature for each of the first slab, the second slab, and the third slab at the start of stacking. In Table 1, the "surface temperature difference between the first and second slabs and the third slab" refers to the surface temperature difference between the first slab and the third slab and the surface temperature difference between the second slab and the third slab, whichever has the larger absolute value. In addition, the following Table 2 shows the number of stacks in the slab stack (the total number of the first slab, the second slab, and the third slab). In addition, a thermocouple was installed on the widthwise center surface (upper surface) of the third slab, and the average cooling rate (°C / h) of the widthwise center surface of the third slab at 700 to 500°C was measured during the cooling process of the slab stack. The average cooling rate shown in Table 2 is the average cooling rate from 700 to 500°C when the temperature at the start of stacking is 700°C or higher, and the average cooling rate from the temperature at the start of stacking to 500°C when the temperature at the start of stacking is less than 700°C. In addition, when there are multiple third slabs, Table 2 shows the average cooling rate of one third slab located at the center of the stacking direction of the slab stack. Specifically, when the number of slabs in the slab stack is an even number, it is the average cooling rate on the upper surface of the slab at a position that is halfway from the bottom of the slab stack (for example, when the number of slabs in the slab stack is 6, it is the third layer from the bottom of the slab stack), and when the number of slabs in the slab stack is an odd number, it is the average cooling rate on the upper surface of the slab at the center of the stacking direction of the slab stack (for example, when the number of slabs in the slab stack is 5, it is the third layer from both the top and bottom of the slab stack).

[0050] After cooling the slab stack to room temperature, the third slab was observed for the presence or absence of cracks (only for Comparative Example 1, the first and second slabs for the presence or absence of cracks) and evaluated. The results are shown in Table 2 and FIG. 5. In the graph shown in FIG. 5, the vertical axis is "stacking temperature" (surface temperature at the center of the width of the third slab when the slabs are stacked), and the horizontal axis is "temperature difference" (difference between the surface temperature at the center of the width of the third slab when the slabs are stacked and the temperature at the center of the slab), and "x" indicates a case where a crack occurs in the third slab, "●" and "◯" indicate a case where no crack occurs in the third slab, and among them, "●" indicates a case where the cooling rate can be significantly increased and no crack occurs. Note that Comparative Example 1 is not plotted in FIG. 5 because there is no third slab. Comparative Examples 6 and 7 are also excluded from FIG. 5 because they are examples that cannot be organized by the "stacking temperature" and "temperature difference" in FIG. 5.

[0051] [Table 1]

[0052] [Table 2]

[0053] The results shown in Table 2 reveal the following:

[0054] In Comparative Example 1, the number of third slabs in the slab stack was zero, and there was no target for preventing cracking during placement. In Comparative Example 1, cracking occurred in the first and second slabs. If the number of tiers in the slab stack is too small, sufficient heat recovery cannot be obtained on the surface of the slab, and compressive stress cannot be generated on the surface of the slab. As a result, tensile stress is easily generated inside the slab, which is thought to have led to cracking during placement.

[0055] In addition, in Comparative Example 2, the temperature difference between the surface and center of the third slab was too small, causing the third slab to crack. If the temperature difference between the surface and center of the third slab is too small, sufficient heat recovery cannot be obtained on the surface of the third slab after the slabs are stacked, and compressive stress cannot be generated on the surface of the slab. As a result, tensile stress is easily generated inside the slab, which is thought to have led to the crack.

[0056] In addition, in Comparative Examples 3 and 4, the surface temperature of the third slab was too low, causing the third slab to undergo cracking. If the surface temperature of the third slab is too low, sufficient heat recovery cannot be obtained on the surface of the third slab after the slabs are stacked, and compressive stress cannot be generated on the surface of the slab. As a result, tensile stress is likely to be generated inside the slab. In addition, the temperature range is already such that cracking is likely to occur, which is thought to have led to the cracking.

[0057] Furthermore, as shown in Comparative Example 5, when the surface temperature of the third slab is too low, it is difficult to prevent cracking even if the first and second slabs are heated to a high temperature and the surface of the third slab is reheated. In other words, when the surface temperature of the third slab is too low, it is in a temperature range where cracking is likely to occur, and tensile stress is already generated inside the third slab at the stage of stacking the slabs, and even if the surface of the third slab is sufficiently reheated after stacking the slabs, it is not enough to relieve the tensile stress inside the slab, which is thought to have led to cracking.

[0058] In addition, in Comparative Example 6, the surface temperatures of the first and second slabs were too high compared to the surface temperature of the third slab, and not only did the surface of the third slab reheat, but the surface temperature of the third slab became higher than the central temperature, making it easier for tensile stress to occur inside the third slab, which is thought to have led to placement cracks.

[0059] In addition, in Comparative Example 7, the surface temperatures of the first and second slabs were lower than the surface temperature of the third slab, so that a placement crack occurred in the third slab. When the surface temperatures of the first and second slabs are lower than that of the third slab, the surface of the third slab is cooled, so that sufficient heat recovery cannot be obtained on the surface of the third slab and compressive stress cannot be generated on the surface of the slab. As a result, tensile stress is easily generated inside the slab, which is thought to have led to the placement crack.

[0060] In contrast, no cracks were generated in the third slab in any of Examples 1 to 10. After the slabs were stacked, sufficient heat recovery was obtained on the surface of the third slab, compressive stress was generated on the surface of the slab, and tensile stress inside the slab was also suppressed, which is thought to have led to the prevention of cracks. In particular, in Examples 3 to 9, in which the surface temperature of the third slab was in the range of 550 to 750°C, cracks were prevented even when the average cooling rate at 700 to 500°C was increased to 25°C / h or more. [Explanation of symbols]

[0061] 10 First Slab 20 Second Slab 30 Third slab (target for preventing cracks) 100 Slab laminate

Claims

1. Obtaining a plurality of slabs by continuous casting; Stacking the plurality of slabs so that the widthwise surfaces of the plurality of slabs overlap each other to obtain a slab stack; and cooling the slab stack; Including, The slab stack has a first slab arranged on one end side in the stacking direction, a second slab arranged on the other end side in the stacking direction, and at least one third slab sandwiched between the first slab and the second slab, the first slab and the second slab are steel; The third slab contains, in mass %, C: 0.02-0.60%, Si: 0.5-3.0%, Mn: 1.0-3.0%, P: 0.100% or less, S: 0.010% or less, Al: 0.005-1.000%, N: 0.0100% or less, Ti: 0-0.500%, Co: 0-0.500%, Ni: 0-0.500%, Mo: 0-0.500%, Cr: 0-2.000%, O: 0-0.0100%, B: 0-0.0100%, Nb: 0-0. .500%, V: 0-0.500%, Cu: 0-0.500%, W: 0-0.1000%, Ta: 0-0.1000%, Sn: 0-0.0500%, Sb: 0-0.0500%, As: 0-0.0500%, Mg: 0-0.0500%, Ca: 0-0.0500%, Y: 0-0.0500%, Zr: 0-0.0500%, La: 0-0.0500%, and Ce: 0-0.0500%, with the balance being Fe and impurities; The first slab and the second slab satisfy the following temperature condition 1 when stacked, The third slab satisfies the following temperature condition 2 when laminated: A method for cooling a continuous cast slab. Temperature condition 1: The surface temperature of each of the first slab and the second slab at the width center is higher than the surface temperature of the third slab at the width center, the temperature difference between the surface temperature of each of the first slab and the second slab at the width center and the surface temperature of the third slab at the width center is within 200°C, and the central temperature of each of the first slab and the second slab is higher than or equal to the surface temperature of each of the first slab and the second slab at the width center. Temperature condition 2: The surface temperature of the third slab at the width center is 500° C. or higher and 900° C. or lower, and the central temperature of the third slab is 150° C. or higher than the surface temperature at the width center of the third slab.

2. Regarding the temperature condition 2, the surface temperature at the width direction center of the third slab is 550 ° C or more and 750 ° C or less; The method for cooling a continuously cast slab according to claim 1.

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