Method for cooling continuously cast steel slab

Abstract

To disclose a technique capable of reducing occurrence of season cracking of a continuously cast slab.SOLUTION: In a cooling process of a continuously cast steel slab having a predetermined chemical composition, a process for imparting a temperature distribution to the slab is started at any point of time when a representative temperature on a surface along a width direction and a longitudinal direction of the continuously cast slab reaches 600°C from 1200°C. The temperature distribution is applied to the surfaces so as to satisfy, after the start, two conditions that (A) a difference ΔT between a maximum temperature and a minimum temperature in a length of 2000 mm along a longitudinal direction of the surfaces is 150°C or more, and (B) an interval I1 between a portion having the maximum temperature and a portion having the minimum temperature in the length of 2000 mm along the longitudinal direction of the surfaces is 180 mm or more and 1000 mm or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This application discloses a method for cooling a continuously cast steel strand. [Background technology]

[0002] High-strength steel sheets, such as those used in automobiles, are required to be lighter (thinner) for CO2 reduction purposes, while also being stronger to prevent deformation of the vehicle body during a collision and ensure the safety of passengers. Cast slabs, the raw materials for components made from such steel sheets, contain high amounts of C, Si, and Mn to enhance their strength. However, these slabs are known to have low toughness and are prone to cracking during cooling to room temperature after continuous casting. This type of cracking is called "delayed cracking." High-carbon steels, used as raw materials for products such as bearings, tools, and springs, are also prone to delayed cracking due to their high C content. Generally, during the cooling and heating processes of cast slabs, thermal strain due to the temperature difference between the surface and interior of the slab and transformation strain due to transformation occur, resulting in stress. High stress in the slab can lead to cracking during the cooling process. For the above reasons, in the prior art, the cooling rate of the slab after continuous casting is controlled to cool it slowly, and the temperature difference between the surface and the interior of the slab is minimized to prevent cracking of the slab during the cooling process (e.g., Patent Documents 1 to 6). However, due to equipment constraints, such temperature control can be difficult in some cases, and there is also a concern that the long cooling time may cause process congestion. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-167560 [Patent Document 2] Japanese Patent Application Publication No. 2019-167559 [Patent Document 3] Japanese Patent Application Laid-Open No. 2007-083274 [Patent Document 4] Japanese Patent Publication No. 2023-047054 [Patent Document 5] Japanese Patent Application Publication No. 2020-139209 [Patent Document 6] Japanese Patent Application Publication No. 2020-139210 Summary of the Invention [Problem to be solved by the invention]

[0004] The present application discloses a new cooling method for continuously cast steel slabs, which is capable of reducing the occurrence of placement cracks in the slabs. [Means for solving the problem]

[0005] The present application discloses the following aspects as means for solving the above problems. <Aspect 1> A method for cooling a continuously cast steel slab, comprising the steps of: The chemical composition of the continuous cast slab is In mass%, C: 0.03 to 1.50%, Si: 0.10 to 2.50% Mn: 0.30~3.50% P: 0.200% or less, S: 0.020% or less, Al: 0.0003 to 2.5000%, N: 0.0100% or less, and O: Contains 0.0100% or less, and have a carbon equivalent (Ceq) of 0.50 or greater; or In mass%, C: 0.03 to 1.50%, Si: 0.80 to 2.50% Mn: 0.30~3.50% P: 0.200% or less, S: 0.020% or less, Al: 0.0003 to 2.5000%, N: 0.0100% or less, and O: Contains 0.0100% or less, and It has a carbon equivalent (Ceq) of 0.20 or more, In the cooling process of the continuously cast slab, a step of imparting a temperature distribution to the slab is started at any time during which a representative temperature on the surface along the width direction and the length direction of the continuously cast slab reaches 1200°C to 600°C, and after the start of the step, the following conditions (A) and (B) are met: (A) the difference between the maximum and minimum temperatures within a length of 2000 mm along the longitudinal direction of the surface is 150°C or more; (B) the distance between the part having the highest temperature and the part having the lowest temperature within a length of 2000 mm along the longitudinal direction of the surface is 180 mm or more and 1000 mm or less; A temperature distribution is applied to the surface so that A method for cooling continuous casting of steel. <Aspect 2> A method for cooling a continuously cast steel slab according to aspect 1, comprising: The step of imparting a temperature distribution includes: placing the continuously cast slab on a plurality of cold strips arranged at intervals in the longitudinal direction of the continuously cast slab at any time during which the representative temperature of the surface along the width direction and longitudinal direction of the continuously cast slab reaches 1200°C to 600°C; Including, A method for cooling continuous casting of steel. [Effects of the Invention]

[0006] According to the method of the present disclosure, it is possible to reduce the occurrence of placement cracks in continuously cast strands. [Brief explanation of the drawings]

[0007] [Figure 1] 2 is a schematic diagram illustrating an example of the temperature distribution on the surface of a continuously cast slab according to the method of the present disclosure. [Figure 2] 1 illustrates a schematic diagram of an example embodiment of the method of the present disclosure. [Figure 3] The results of a computer-based thermal stress analysis are shown below regarding the effect of the interval I1 and the temperature difference ΔT on the stress in the slab. DETAILED DESCRIPTION OF THE INVENTION

[0008] In conventional techniques, the cooling rate of a slab after continuous casting is controlled to cool it slowly, minimizing the temperature difference between the surface and interior of the slab, thereby suppressing the occurrence of slab-place cracks. However, such temperature control can be difficult due to equipment limitations. The present inventors utilized thermal stress analysis to explore cooling conditions for the slab that can reduce slab-place cracks. As a result, they found that by providing a predetermined temperature distribution on the surface along the width and length of the slab during the cooling process of the slab after continuous casting, the tensile residual stress generated at the center of the slab can be reduced, thereby reducing the occurrence of slab-place cracks. The technology disclosed herein also enables greater flexibility in slab cooling conditions compared to conventional techniques. Hereinafter, one embodiment of the cooling method for continuously casting a steel slab of the present disclosure will be described.

[0009] In the cooling method for continuously casting steel slabs disclosed herein, The chemical composition of the continuous cast slab is In mass%, C: 0.03 to 1.50%, Si: 0.10 to 2.50% Mn: 0.30~3.50% P: 0.200% or less, S: 0.020% or less, Al: 0.0003 to 2.5000%, N: 0.0100% or less, and O: Contains 0.0100% or less, and have a carbon equivalent (Ceq) of 0.50 or greater; or In mass%, C: 0.03 to 1.50%, Si: 0.80 to 2.50% Mn: 0.30~3.50% P: 0.200% or less, S: 0.020% or less, Al: 0.0003 to 2.5000%, N: 0.0100% or less, and O: Contains 0.0100% or less, and It is important to have a carbon equivalent (Ceq) of 0.20 or greater. In addition, in the cooling method for continuously cast steel slabs of the present disclosure, In the cooling process of the continuously cast slab, a step of imparting a temperature distribution to the slab is started at any time during which a representative temperature on the surface along the width direction and the length direction of the continuously cast slab reaches 1200°C to 600°C, and after the start of the step, the following conditions (A) and (B) are met: (A) the difference between the maximum and minimum temperatures within a length of 2000 mm along the longitudinal direction of the surface is 150°C or more; (B) the distance between the part having the highest temperature and the part having the lowest temperature within a length of 2000 mm along the longitudinal direction of the surface is 180 mm or more and 1000 mm or less; It is important that a temperature distribution is applied to the surface so that the following condition is satisfied.

[0010] 1. Shape of continuous cast slab In the method of the present disclosure, the continuously cast slab may be a slab, a bloom, or a billet. A slab or a bloom is particularly preferred, with a slab being more preferred. The slab may have a width and a thickness in a cross-sectional shape perpendicular to the continuous casting direction, and a length in the continuous casting direction. When the slab is a slab, its width corresponds to the long side (corresponding to the long side of the mold) in the cross-sectional shape perpendicular to the continuous casting direction, and its thickness corresponds to the short side (corresponding to the short side of the mold) in the cross-sectional shape. Furthermore, when the slab is a bloom or a billet, its aspect ratio (length of long side / length of short side) is generally smaller than that of a slab. When the slab is a slab, its width may be, for example, 800 mm or more and 1300 mm or less, its thickness may be, for example, 100 mm or more and 300 mm or less, and its length may be, for example, 6 m or more and 10 m or less. When the cast piece is a bloom, its width may be, for example, 300 mm or more and 600 mm or less, its thickness may be, for example, 200 mm or more and 400 mm or less, and its length may be, for example, 6 m or more and 10 m or less.

[0011] 2. Chemical composition of continuous cast slabs The method disclosed herein relates to a method for cooling a continuously cast steel slab containing predetermined amounts of essential elements such as C and optionally containing other elements. The continuously cast slab may be, for example, a material used for high-tensile steel plate, or a high-carbon steel used as a material for products such as bearings, tools, and springs. In this application, "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. Tensile tests of steel plates are conducted, for example, in accordance with JIS Z 2241, using JIS No. 5 test specimens taken in an orientation where the longitudinal direction of the test specimen is parallel to the direction perpendicular to the rolling direction of the steel plate.

[0012] A continuously cast slab having a chemical composition according to the first or second embodiment described below has low toughness and is prone to developing stage cracks. However, according to the method of the present disclosure, the occurrence of stage cracks can be reduced even in such continuously cast slabs with low toughness. Note that, in this specification, "%" in relation to chemical composition means mass %. Furthermore, in this specification, unless otherwise specified, the symbol "to" indicating a numerical range is used to mean that the numerical values ​​before and after it are included as the lower and upper limits.

[0013] 2.1 Basic elements of the first embodiment In the first embodiment, the chemical composition of the continuously cast slab contains, in mass%, C: 0.03 to 1.50%, Si: 0.10 to 2.50%, Mn: 0.30 to 3.50%, P: 0.200% or less, S: 0.020% or less, Al: 0.0003 to 2.5000%, N: 0.0100% or less, and O: 0.0100% or less, and has a carbon equivalent (Ceq) of 0.50 or more.

[0014] (C: 0.03 to 1.50%) 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 C content is 0.03% or more. The C content may be 0.05% or more, or 0.10% or more. On the other hand, if the C content is too high, the toughness of the steel is likely to deteriorate excessively. In this regard, the C content is 1.50% or less. The C content may be 1.30% or less, or 1.10% or less.

[0015] (Si: 0.10 to 2.50%) Si is an important element with a high strengthening ability. When obtaining high-tensile steel, it is preferable to use a high concentration of Si. In the first embodiment, the Si content is 0.10% or more. The Si content may be 0.20% or more or 0.40% or more. On the other hand, if the Si content is too high, there is a risk of deterioration in toughness and workability. In this regard, the Si content is 2.50% or less. The Si content may be 2.30% or less or 2.10% or less.

[0016] (Mn: 0.30 to 3.50%) Mn is an important element for improving hardenability and ensuring hardness deep into the steel material even when the cooling rate is insufficient. To obtain high-tensile steel, it is advisable to use a high Mn concentration. Specifically, the Mn content is 0.30% or more. The Mn content may be 0.50% or more or 1.00% or more. On the other hand, if the Mn content is too high, there is a risk of deteriorating toughness and workability. In this regard, the Mn content is 3.50% or less. The Mn content may be 3.00% or less or 2.50% or less.

[0017] (P:0.200% or less) P is an element that promotes Mn concentration in unsolidified parts during the solidification process of molten steel, reduces the Mn concentration in negative segregation parts, and promotes an increase in the area ratio of ferrite, so the lower the content, the better. Furthermore, excessive P content increases the strength of the steel, but may also lead to brittle fracture of the steel. In this regard, the P content is 0.200% or less. The P content may be 0.100% or less, 0.050% or less, or 0.020% or less. Meanwhile, the lower limit of the P content is not particularly limited. The P content may be 0% or more, or 0.001% or more.

[0018] (S:0.020% or less) S is an element that generates non-metallic inclusions such as MnS in steel, which reduces the ductility of the steel, and therefore the lower the S content, the better. In this regard, the S content is 0.020% or less. The S content may be 0.010% or less or 0.005% or less. On the other hand, the lower limit of the S content is not particularly limited. The S content may be 0% or more or 0.001% or more.

[0019] (Al: 0.0003 to 2.5000%) Al is an element that acts as a deoxidizer for steel. This effect is easily achieved when the Al content is 0.0003% or more. The Al content may be 0.0010% or more, or 0.0020% or more. On the other hand, excessive Al content may excessively promote ferrite transformation and bainite transformation, resulting in a decrease in the strength of the steel. In this regard, the Al content is 2.5000% or less. The Al content may be 2.0000% or less, 1.5000% or less, or 1.0000% or less.

[0020] (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 is 0.0100% or less, and may be 0.0070% or less, or 0.0050% or less, or may be 0% or more, 0.0001% or more, or 0.0010% or more.

[0021] (O:0.0100% or less) O is an element that can be mixed in during the manufacturing process, and the less O present, the better. However, reducing the O content to the minimum required refining time, resulting in reduced productivity. On the other hand, excessive O content can lead to the formation of coarse inclusions, which can reduce the toughness of the steel. In this regard, the O content is 0.0100% or less. The O content may be 0.0070% or less, 0.0050% or less, or 0.0030% or less. The O content may be 0% or more, 0.0005% or more, or 0.0010% or more.

[0022] (Carbon equivalent (Ceq): 0.50 or more) In the first embodiment, the carbon equivalent (Ceq) is 0.50 or more. The higher the carbon equivalent (Ceq), the lower the toughness and the more likely the problem of staging cracking occurs. However, according to the method of the present disclosure, even when the carbon equivalent (Ceq) of the continuously cast slab according to the first embodiment is 0.50 or more, the occurrence of staging cracking in the slab can be reduced. In the first embodiment, the carbon equivalent (Ceq) may be 0.55 or more or 0.60 or more. The upper limit of the carbon equivalent (Ceq) is not particularly limited and can be determined appropriately depending on the properties of the target steel. In the first embodiment, the carbon equivalent (Ceq) may be 1.50 or less, 1.30 or less, 1.10 or less, 0.90 or less, or 0.70 or less.

[0023] In this specification, the term "carbon equivalent (Ceq)" is calculated according to the following formula (1) (JIS G 3475:2014). Ceq=[C]+[Si] / 24+[Mn] / 6+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14 (1) In the above formula (1), [C], [Si], [Mn], [Ni], [Cr], [Mo], and [V] are the contents (mass%) of each element in the continuously cast slab.

[0024] 2.2 Basic elements of the second embodiment In the second embodiment, the chemical composition of the continuously cast slab contains, in mass%, C: 0.03 to 1.50%, Si: 0.80 to 2.50%, Mn: 0.30 to 3.50%, P: 0.200% or less, S: 0.020% or less, Al: 0.0003 to 2.5000%, N: 0.0100% or less, and O: 0.0100% or less, and has a carbon equivalent (Ceq) of 0.20 or more.

[0025] In the second embodiment, the C content, Mn content, P content, S content, Al content, N content, and O content are the same as those in the first embodiment, and therefore, description thereof will be omitted here.

[0026] (Si: 0.80 to 2.50%) As described above, Si is an important element with a high strengthening ability, and when obtaining high-tensile steel, it is preferable to use a high concentration of Si. In the second embodiment, the Si content is 0.80% or more. The Si content may be 0.90% or more or 1.00% or more. On the other hand, if the Si content is too high, there is a risk of deteriorating toughness and workability. In this regard, the Si content is 2.50% or less. The Si content may be 2.30% or less or 2.10% or less.

[0027] (Carbon equivalent (Ceq): 0.20 or more) In the second embodiment, the carbon equivalent (Ceq) is 0.20 or more. As described above, the lower limit of the Si content in the second embodiment is higher than that in the first embodiment. That is, even when the carbon equivalent (Ceq) is relatively small, a large amount of Si can easily cause problems with slab cracking during casting. The method of the present disclosure can reduce the occurrence of slab cracking even in continuously cast slabs having a high Si content but a low carbon equivalent (Ceq). In the second embodiment, the carbon equivalent (Ceq) may be 0.30 or more, 0.40 or more, or 0.50 or more. The upper limit of the carbon equivalent (Ceq) is not particularly limited and can be determined appropriately depending on the properties of the target steel. In the second embodiment, the carbon equivalent (Ceq) may be 1.50 or less, 1.30 or less, 1.10 or less, 0.90 or less, or 0.70 or less.

[0028] 2.3 Arbitrary elements In the first and second embodiments, the continuously cast slab may contain, in addition to the above-mentioned basic elements, optional elements other than those mentioned above. The optional elements do not substantially affect the problem-solving mechanism of the method of the present disclosure. Since the optional elements may not be contained, the lower limit is 0%. The chemical composition of the continuously cast slab may be, for example, in mass %, Ni: 0-0.50%, Cr: 0-2.00%, Mo: 0-1.50%, V: 0-0.500%, Ti: 0-0.500%, Nb: 0-0.500%, Co: 0-0.50%, B: 0-0.0100%, Cu: 0-0.50%, Te: 0-0.500%, W: 0-0.10%, The cast slab may contain one or more elements selected from the group consisting of Ta: 0-0.100%, Hf: 0-0.050%, Sn: 0-0.050%, Sb: 0-0.050%, As: 0-0.050%, Mg: 0-0.0500%, Ca: 0-0.0500%, Zr: 0-0.0500%, Bi: 0-0.050%, and REM: 0-0.050%. Note that the types and contents of the above optional elements are merely examples, and the types and amounts of optional elements that may be contained in the cast slab are not limited to those described above. Note that "REM" is a collective term for 17 elements: scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanides lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The "REM content" refers to the total content of these elements.

[0029] 3. Temperature distribution on the surface of continuously cast slab during cooling In the method of the present disclosure, during the cooling process of a continuously cast slab having the above-mentioned chemical composition, it is important that the step of imparting a temperature distribution to the slab is started at any point in time between when the representative temperature on the surface along the width direction and length direction of the continuously cast slab reaches 1200°C and 600°C, and that the temperature distribution is imparted to the surface so that the above conditions (A) and (B) are satisfied after the start of the step.

[0030] In the method of the present disclosure, the "surface along the width direction and longitudinal direction of the continuously cast slab" refers to, for example, the broad surface of the continuously cast slab. In the method of the present disclosure, the "representative temperature" refers to the temperature at the center of the surface in the length direction of the slab and the center of the slab in the width direction, immediately before the temperature distribution is applied. Furthermore, the "maximum temperature" and the "minimum temperature" are each determined based on the temperature at the center of the surface in the width direction.

[0031] 3.1 When conditions (A) and (B) are met In the method of the present disclosure, conditions (A) and (B) may be satisfied when the step of imparting a temperature distribution to the slab is started at any point during the cooling process of the continuously cast slab, between when the representative temperature on the surface along the width and length directions of the slab reaches from 1200°C to 600°C, and the cooling process continues thereafter. For example, in one embodiment, the step of imparting a temperature distribution to the slab may be started at any point during the cooling process of the continuously cast slab, between when the representative temperature on the surface along the width and length directions of the slab reaches from 1200°C to 600°C, and conditions (A) and (B) may be satisfied after the start of the step of imparting a temperature distribution to the slab, between when the representative temperature reaches 600°C. If the temperature distribution is started from a low-temperature region where the representative temperature is below 600°C, the yield stress increases, making it difficult to induce the desired plastic deformation in the slab. By starting the process of imparting a temperature distribution to the slab at any point between 1200°C and 600°C and satisfying the above conditions (A) and (B) at any point during the subsequent cooling process, compressive plastic deformation can be induced in part of the surface along the width and length directions of the continuously cast slab, and tensile plastic deformation can be induced in part. The plastic deformation induced in the continuously cast slab may remain during the subsequent cooling process. In the method disclosed herein, compressive plastic deformation and tensile plastic deformation are induced alternately at predetermined intervals along the length direction of the slab on the surface along the width and length directions of the continuously cast slab, thereby dispersing the tensile stress induced in the center of the continuously cast slab, thereby reducing the occurrence of staging cracks in the slab.

[0032] 3.2 Condition (A) Condition (A) is that the difference between the maximum and minimum temperatures on the surfaces along the width and length of the continuously cast slab is 150°C or more over a 2000 mm length along the length. FIG. 1 schematically illustrates an example of the temperature distribution on the surface of the continuously cast slab by the method of the present disclosure. As shown in FIG. 1, in the method of the present disclosure, a maximum temperature difference ΔT of 150°C or more occurs at the center of the width and length of the surfaces along the width and length of the continuously cast slab from an arbitrary position P1 to a position P2 2000 mm away in the longitudinal direction. The larger the maximum temperature difference ΔT, the more appropriately compressive plastic deformation and tensile plastic deformation can be induced on the surfaces along the width and length of the continuously cast slab. The maximum temperature difference ΔT may be 170°C or more, 190°C or more, or 210°C or more. The upper limit of the maximum temperature difference ΔT is not particularly limited. The maximum temperature difference ΔT may be, for example, 500°C or less, 490°C or less, 470°C or less, 450°C or less, 430°C or less, or 410°C or less. Note that Fig. 1 illustrates a case in which, with respect to the temperature distribution along the longitudinal direction of the continuously cast slab at the width center thereof, the maximum and minimum temperatures are repeated at regular intervals and the maximum temperature difference ΔT is the same in each interval. However, the temperature distribution in the method of the present disclosure is not limited to that illustrated. In the method of the present disclosure, the maximum and minimum temperatures may be repeated at random intervals, and the maximum temperature difference ΔT may be different for each interval.

[0033] 3.3 Condition (B) Condition (B) is that the distance between the highest temperature point and the lowest temperature point on the surface along the width and length of the continuously cast slab, within a 2000 mm length along the length, is 180 mm or more and 1000 mm or less. As shown in FIG. 1 , in the method of the present disclosure, a maximum temperature difference ΔT of 150°C or more occurs at the center of the width and length of the surface along the width and length of the continuously cast slab, from an arbitrary position P1 to a position P2 2000 mm away in the longitudinal direction. Here, there is a distance I1 between the highest temperature point Pmax and the lowest temperature point Pmin. In the method of the present disclosure, it is important that the distance I1 is 180 mm or more and 1000 mm or less. By setting the distance I1 to 180 mm or more and 1000 mm or less, compressive plastic deformation and tensile plastic deformation can be appropriately generated on the surface along the width and length of the continuously cast slab, and tensile stress can be distributed in the center of the continuously cast slab. The interval I1 may be 250 mm or more or 300 mm or more, or may be 900 mm or less or 800 mm or less. Note that, although Fig. 1 illustrates a case where the difference ΔT between the maximum and minimum temperatures from an arbitrary position P1 to a position P2 2000 m away in the longitudinal direction (2000 mm length) is the same for any 2000 mm length, the difference ΔT between the maximum and minimum temperatures may differ depending on the position in the 2000 mm length.

[0034] 3.4 Specific examples for achieving conditions (A) and (B) The above conditions (A) and (B) can be achieved, for example, by intermittently cooling the surface of the continuously cast slab along the longitudinal direction of the slab. FIG. 2 shows a specific example for achieving conditions (A) and (B). As shown in FIG. 2, the process for providing the above temperature distribution may involve placing the continuously cast slab 10 on a plurality of cold slabs 20 arranged at an interval I2 in the longitudinal direction of the continuously cast slab 10 at any point between when the representative temperature of the surface 10a along the width and longitudinal directions of the continuously cast slab 10 reaches 1200°C and 600°C. During the subsequent cooling process, a temperature distribution is provided on the surface 10a so that the above conditions (A) and (B) are satisfied. In this case, the interval I2 can be considered to be twice the interval I1. The temperature, material, shape, and size of the cold slab 20 may be appropriately determined or selected so as to satisfy the above conditions (A) and (B). For example, the temperature of the cold slab 20 may be 500°C or lower. The cold slab 20 may be a metal piece. The shape of the cold slab 20 may be a square prism. The length of the cold slab 20 may be the same as or greater than the width of the continuously cast slab 10. The volume of the cold slab 20 may be 3.0×10 7 mm 3 It may be 2.0 x 10 or more. 8 mm 3 It may be the following:

[0035] 3.5 Surface to which temperature distribution is applied In the method of the present disclosure, during the cooling process of the continuously cast slab, only one of the two surfaces along the width and length of the continuously cast slab may have a temperature distribution that satisfies the above conditions (A) and (B), or both surfaces may have temperature distributions that satisfy the above conditions (A) and (B). Furthermore, in the method of the present disclosure, the temperature distribution imparted to one of the two surfaces along the width and length of the continuously cast slab may be the same (symmetrical between the front and back) or different. Furthermore, in the method of the present disclosure, it is sufficient that the temperature distribution satisfies the above relationships (A) and (B) except for a range of 2000 mm from the longitudinal end of the continuously cast slab. Furthermore, there is no particular need to control the temperature of the surface along the thickness direction of the continuously cast slab.

[0036] In the method of the present disclosure, there is no particular limitation on the cooling rate of the continuously cast slab. After the continuously cast slab has been given a temperature distribution so as to satisfy the above conditions (A) and (B), it may, for example, be allowed to cool naturally, or it may be stacked with other slabs, i.e., stacked, and allowed to cool naturally. In this invention, "natural cooling" means cooling by leaving it in a state where it is not subjected to forced wind, water, or the like from outside.

[0037] 4.Effects As described above, according to the method of the present disclosure, by applying a predetermined temperature distribution to the continuously cast slab during the cooling process, it is possible to control the state of compressive plastic deformation and tensile plastic deformation that occur in the slab, thereby reducing the occurrence of slab-induced cracks. Furthermore, with the method of the present disclosure, there is no need to precisely control the cooling rate of the slab, as in conventional techniques. According to the method of the present disclosure, special equipment for cooling control is not required, and the degree of freedom in cooling conditions is increased compared to conventional techniques. [Example]

[0038] 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.

[0039] 1. Experiment 1 (thermal stress analysis) A thermal stress analysis was performed using a computer to examine the effect of a longitudinal temperature gradient on the stress distribution in a slab (0.25%C-1.5%Si-1.5%Mn steel slab, 240 mm thick, 1000 mm wide, and 7500 mm long) after continuous casting. The thermal stress analysis was performed using a conventional thermo-elastic-plastic analysis that took phase transformation into account. The physical properties used in the analysis were measured using conventional testing methods using test specimens. The thermal stress analysis assumed that the slab was placed on top of multiple cold slabs (as shown in Figure 2) when its representative temperature reached a predetermined temperature between 1200°C and 600°C. The calculations were performed with a temperature gradient applied to the surface along both the width and length of the slab, followed by cooling to room temperature. The difference between the maximum and minimum temperatures on the slab surface was controlled by changing the slab temperature, and the distance I2 between the slabs controlled the distance I1 between the maximum and minimum temperatures in the temperature distribution on the slab surface (see Figure 1). The maximum temperature was determined at the center of the slab width and the center between the two slabs. The minimum temperature was determined at the center of the slab width and the center between the slabs. The temperature difference between the maximum and minimum temperatures was defined as ΔT. Similar calculations were also performed on a slab of the same shape made of 0.2%C-0.3%Si-0.3%Mn steel, a steel that is less susceptible to cracking. Since slab cracking often originates in the center (in both thickness, width, and length) of the slab, the calculations focused on the stress in this area when cooled to room temperature. Figure 3 shows the results of a computer-based thermal stress analysis of the effects of I1 and the temperature difference ΔT on the stress in the slab. In Figure 3, "●" indicates that the tensile stress value at the center of a 0.25%C-1.5%Si-1.5%Mn steel slab at room temperature was less than 1.2 times the calculated value under conditions that did not include a process to impart a temperature distribution during the cooling process of a 0.2%C-0.3%Si-0.3%Mn steel slab, and "×" indicates that the stress value was 1.2 times or more.As shown in FIG. 3, it was found that in order to suppress the risk of slab placement cracks, a temperature difference ΔT of 150°C or more is applied to the surface of the slab in the longitudinal direction of the slab during the slab cooling process, and the distance I1 between the highest temperature position and the lowest temperature position is 180 mm or more and 1000 mm or less. This highly likely reduces slab placement cracks.

[0040] 2. Experiment 2 Molten steel having a predetermined chemical composition was used for continuous casting to produce a slab. Table 1 below shows the chemical composition of the slab produced by continuous casting. In the chemical composition shown in Table 1, the balance is Fe and impurities. Continuous casting was performed using a slab casting mold having a thickness of 240 to 280 mm and a width of 1050 mm, or a bloom casting mold having a thickness of 300 mm and a width of 410 mm, with a casting speed of 0.65 to 1.35 m / min. The slab was then cut into lengths of 7000 to 9700 mm, and cooling was initiated. The surface temperatures of the slab at the center of the slab length and the slab width on the surface along the width and length directions, and at the center of the slab width, immediately before intentionally creating a temperature difference in the slab, were defined as the "representative temperature" of the slab.

[0041] [Table 1]

[0042] Subsequently, in the slab cooling process, to intentionally generate a temperature difference along the length of the slab surface, multiple slabs (240 mm thick, 300 mm wide, and 1100 mm long, with temperatures listed in Table 2 below) were placed at a predetermined interval I2, as shown in Figure 2, and the slab to be cooled was placed on top of them at the representative temperature of the slab listed in Table 2. A radiation thermometer was used to measure the representative temperature of the slab. Temperature measurements were also taken starting from a position 2000 mm away from one end of the slab along its length and extending to 4000 mm from that end, i.e., a 2000 mm length of the slab surface along its length. The temperature measurements were taken six times, every 10 minutes for one hour after placement, using a thermograph (Nippon Avionics Co., Ltd., model number: InfRec R500EX), a thermometer that non-contactly measures two-dimensional temperature distribution on an object surface. Measurements were taken every 10 minutes, and the highest temperature within the measurement range was taken as the provisional maximum surface temperature (maximum temperature). However, since this method cannot be used to measure the area in contact with the slab, a thermocouple was inserted into the area corresponding to the center of the slab width for the 2000 mm length of the contact area with the slab, and temperature measurements were taken every 10 minutes for one hour after installation, and this measurement value was taken as the provisional minimum surface temperature (minimum temperature). Of the six temperature measurements, the difference between the maximum and minimum surface temperatures was calculated at the measurement time when the difference between the provisional maximum and minimum values ​​was greatest. In addition, by changing the above-mentioned interval I2, the interval I1 (see Figure 1) between the maximum and minimum temperature positions was changed.

[0043] After applying the temperature distribution to the slabs as described above, they were stacked arbitrarily and allowed to cool to room temperature. Table 2 shows the slab temperatures at the center of the broad face width and center of the length when stacked, the number of slabs stacked, and their arrangement in the stack. When stacking, the upper and lower slabs used had temperatures within ±100°C of the temperature of the slab in question. In Table 2 below, "-" in the "number of slabs stacked" column indicates that the slab was left to cool to room temperature as a single slab. In addition, the underlined values ​​in Table 2 indicate values ​​outside the range of the present invention.

[0044] After cooling to room temperature, the top and bottom surfaces of each slab's broad face were visually inspected for cracks along the central 3 m of the length to confirm the presence of cracks. If even one crack 10 mm or longer was found, the slab was judged to have cracks. Taking into account the risk of cracks occurring in the reheating furnace or hot rolling stages, hot rolling was not performed and the slab was judged to have cracks. Furthermore, 1050 mm wide slabs that did not have any cracks 10 mm or longer detected by visual inspection were hot rolled under standard conditions to produce 2.5 mm thick steel plates. If a fracture occurred during hot plate rolling, including the reheating process, or if any visual defects were found during rolling, rolling was stopped and the slab was judged to have cracks. 410 mm wide slabs that did not have any cracks 10 mm or longer were bloomed under standard conditions to produce slabs with cross sections of 150 mm square. These billets were hot-rolled under standard conditions to obtain steel bars with a diameter of 25 mm. If any fractures occurred during blooming and bar rolling, which include the heating process, or if any defects were found visually during rolling, the rolling was stopped and the product was judged to have had a "delayed crack." The results are shown in Table 2 below.

[0045] [Table 2]

[0046] As is clear from the results shown in Table 2, in Comparative Examples 1 to 9, all of the samples underwent hardening upon storage, whereas in Examples 1 to 21, no hardening upon storage occurred.

[0047] 3. Summary From the above results, it can be said that the occurrence of lay cracks in the slab can be reduced by a method for cooling a continuously cast slab that satisfies the following requirements (I) and (II).

[0048] (I) The chemical composition of the continuously cast slab is In mass%, C: 0.03 to 1.50%, Si: 0.10 to 2.50% Mn: 0.30~3.50% P: 0.200% or less, S: 0.020% or less, Al: 0.0003 to 2.5000%, N: 0.0100% or less, and O: Contains 0.0100% or less, and have a carbon equivalent (Ceq) of 0.50 or greater; or In mass%, C: 0.03 to 1.50%, Si: 0.80 to 2.50% Mn: 0.30~3.50% P: 0.200% or less, S: 0.020% or less, Al: 0.0003 to 2.5000%, N: 0.0100% or less, and O: Contains 0.0100% or less, and It has a carbon equivalent (Ceq) of 0.20 or more.

[0049] (II) In the cooling process of the continuously cast slab, a step for imparting a temperature distribution to the slab is started at any time during which the representative temperature on the surface along the width direction and the length direction of the continuously cast slab reaches 1200°C to 600°C, and after the start, the following conditions (A) and (B) are met: (A) The difference between the maximum and minimum temperatures over a length of 2000 mm along the longitudinal direction of the surface is 150°C or more; (B) The distance between the hottest part and the coldest part of the surface within a 2000 mm length along the longitudinal direction of the surface is 180 mm or more and 1000 mm or less. A temperature distribution is applied to the surface so that the following is satisfied. [Explanation of symbols]

[0050] 10 Castings 20 cold pieces

Claims

1. A method for cooling a continuously cast steel slab, comprising the steps of: The chemical composition of the continuous cast slab is In mass%, C: 0.03-1.50%, Si:0.10~2.50%, Mn: 0.30-3.50%, P: 0.200% or less, S: 0.020% or less, Al: 0.0003-2.5000%, N: 0.0100% or less, and O: 0.0100% or less, and having a carbon equivalent (Ceq) of 0.50 or greater; or In mass%, C: 0.03-1.50%, Si:0.80~2.50%, Mn: 0.30-3.50%, P: 0.200% or less, S: 0.020% or less, Al: 0.0003-2.5000%, N: 0.0100% or less, and O: 0.0100% or less, and having a carbon equivalent (Ceq) of 0.20 or greater; During the cooling process of the continuously cast slab, a step of imparting a temperature distribution to the slab is started at any time during which the representative temperature at the surface along the width direction and the longitudinal direction of the continuously cast slab reaches 1,200°C to 600°C, and after the start of the step, the following conditions (A) and (B) are met: (A) the difference between the maximum temperature and the minimum temperature within a length of 2000 mm along the longitudinal direction of the surface is 150°C or more; (B) the distance between the part having the highest temperature and the part having the lowest temperature within a length of 2000 mm along the longitudinal direction of the surface is 180 mm or more and 1000 mm or less; A temperature distribution is applied to the surface so that A method for cooling continuous cast steel slabs.

2. 2. A method for cooling a continuously cast steel slab according to claim 1, comprising: The step of imparting a temperature distribution includes: placing the continuously cast slab on a plurality of cold strips arranged at intervals in the longitudinal direction of the continuously cast slab at any time during which the representative temperature of the surface along the width direction and longitudinal direction of the continuously cast slab reaches 1200°C to 600°C; Including, A method for cooling continuous cast steel slabs.

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