Continuously cast slab and its manufacturing method

Abstract

Provided are: a continuously cast slab in which slab season cracking does not occur during cooling of the continuously cast slab even when the continuously cast slab has a low toughness; and a manufacturing method therefor. This continuously cast slab for high-strength steel has a predetermined component composition, and is characterized in that: when the average prior austenite grain diameter at a position of 10 mm below a surface layer of the continuously cast slab is d10 and the average prior austenite grain diameter at a position of 20 mm below the surface layer of the continuously cast slab is d20, the ratio (d20 / d10) of the average prior austenite grain diameters is 1.0-4.0; a microstructure at a position of 10 mm below the surface layer of the continuously cast slab has a total of at least 80% of the area ratio of ferrite and the area ratio of pearlite; a microstructure at a position of 20 mm below the surface layer of the continuously cast slab has a total of at least 60% of the area ratio of ferrite and the area ratio of pearlite; and the total of the area ratio of ferrite, the area ratio of pearlite, and the area ratio of a low-temperature transformation phase is 100%.

Description

[Technical Field]

[0001] The present invention relates to a continuously cast slab that prevents cracks during cooling and a method for manufacturing the same, and more particularly to a continuously cast slab for high strength steel (hi-tensile), which is effective in preventing cracks during slab placement and does not cause the problem of holes during rolling, and a method for manufacturing the same. [Background technology]

[0002] In recent years, in the automotive industry, in order to achieve both thinner car bodies and collision safety, high-strength steels have been further strengthened and therefore highly alloyed. However, the use of highly alloyed high-strength steels significantly reduces the toughness of slabs.

[0003] As the toughness of slabs decreases due to the use of high alloys, cracks occurring during slab cooling, so-called slab placement cracks, have become more frequent. If slab placement cracks occur, the slab may break during transportation, making it impossible to use the slab for hot rolling. Even if the slab does not break, cracks in the slab may open during hot rolling, causing the hot-rolled steel sheet to break. Alternatively, when a slab having small cracks is subjected to hot rolling, cold rolling, annealing, or plating treatment, the small cracks in the slab appear as surface defects such as scabs and slivers on the steel sheet produced by these treatments. Cracks on the surface of a slab are usually removed by a grinder. However, in a highly alloyed slab, the toughness of the slab decreases due to the high alloying, and the cracks in the slab propagate due to the stress of the grinder, so that the cracks in the slab may not be completely removed. On the other hand, small cracks in the slab may be overlooked and appear as surface defects on the steel sheet after hot rolling, cold rolling, annealing, or plating. For these reasons, it is necessary to suppress cracks in the slab.

[0004] Figure 1 is an enlarged photograph taken with a scanning electron microscope (SEM) of the fracture surface of a crack in a high-strength steel slab that fractured due to slab cracking. As is clear from Figure 1, the fracture surface of the slab crack exhibited the appearance of an intergranular fracture along the prior austenite grain boundary. Figure 2 shows a microstructure photograph of the cross section of the slab crack. The depth of the slab crack was mainly about 20 mm from the slab surface. The slab crack propagated near the prior austenite grain boundary, and grain boundary ferrite was present at the tip of the slab crack. Pearlite, or pearlite and bainite, was also observed within the prior austenite grains.

[0005] Intergranular fracture occurs when the prior austenite grains are coarse and the grain boundaries are embrittled. When intergranular ferrite forms, there is a difference in strength between the intragranular pearlite and bainite, causing stress concentration in the weak intergranular ferrite, which can develop into cracks in the slab even at lower stress levels. On the other hand, when a slab is cooled, stress is generated due to differences in thermal contraction and transformation expansion between the surface and interior of the slab. If this stress is large, slab cracking occurs when the slab is cooled to room temperature. Recent high-alloy, high-strength steels have high hardenability, and the prior austenite grain size in the slab is coarse. Therefore, conventional slow cooling processes cannot prevent the precipitation of low-temperature transformation phases (bainite, martensite, etc.). In addition, since the toughness of the slab is low, deep cracks that occur in the slab are difficult to remove by manual maintenance such as using a grinder, which has been a problem that significantly reduces the yield of the slab.

[0006] From this perspective, methods for suppressing the occurrence of thermal cracking in high-tensile steel slabs have been proposed. For example, Patent Document 1 proposes a method of suppressing the bainite / martensite transformation by slow cooling from 700 to 500°C, the temperature range in which austenite transforms to ferrite, thereby reducing the stress generated by the transformation expansion. That is, Patent Document 1 discloses a method that can suppress the occurrence of thermal cracking in slabs, even in high-tensile steels of steel types that are prone to thermal cracking. Specifically, the cooling method for high-tensile steel slabs disclosed in Patent Document 1 is a method for suppressing the occurrence of thermal cracking by controlling the cooling rate of the slab in accordance with the length of internal cracks that have occurred in the high-tensile steel, based on the finding that the internal stress of high-tensile steel depends on the cooling rate.

[0007] Patent Document 2 also proposes a method of reducing the temperature difference and stress in the slab that occurs during transformation by starting slow cooling of the slab immediately after casting, and then further slow cooling the slab at a temperature of 700°C or higher for 10 hours or more, and then at a temperature of 700 to 500°C. That is, Patent Document 2 discloses a cooling method for slabs for high-strength steel plates that not only prevents slab cracking during cooling of the slab but also prevents quality defects such as scabs during hot rolling, even for slabs containing Si. Specifically, the cooling method for slabs for high-strength steel plates disclosed in Patent Document 2 sets the average cooling rate of a continuously cast slab of high-strength hot-rolled steel plate, which has limited contents of chemical components such as C, Si, and Mn, at 500 to 700°C, to 20°C / hr or less. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 2020-139209 [Patent Document 2] Japanese Patent Application Publication No. 2019-167560 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the above-mentioned conventional techniques have the following problems. The method of cooling a high-tensile steel slab after casting, described in Patent Document 1, focuses only on the temperature range from 700°C to 500°C when the slab is cooled after casting, and controls the internal stress generated in the slab to be small. Therefore, even if a slab with an increased carbon content is produced using the cooling method of a high-tensile steel slab described in Patent Document 1, it is not possible to sufficiently suppress the occurrence of slab lay cracks.

[0010] Furthermore, the cooling method for slabs for high-strength steel plates described in Patent Document 2 is based on the finding that slab cracking is caused by thermal stress generated due to the addition of Si to steel and temperature unevenness within the slab, and focuses on reducing thermal stress to suppress slab cracking. However, the cooling method for slabs for high-strength steel plates described in Patent Document 2 does not limit the microstructure of the slab. Therefore, even if a slab is produced using the cooling method for slabs for high-strength steel plates described in Patent Document 2, the occurrence of slab placement cracking cannot be sufficiently suppressed. Furthermore, as a result of intensive research by the present inventors, it was found that slabs containing large amounts of C, Si, and Mn according to conventional technology have significantly low toughness, making it impossible to completely suppress cracking during slab placement, and causing problems such as holes during rolling.

[0011] The present invention has been made in consideration of the above circumstances, and aims to provide a continuously cast slab and a manufacturing method thereof that does not cause slab placement cracks during cooling, even in the case of a continuously cast slab with low toughness, and that does not cause hole formation problems during rolling. [Means for solving the problem]

[0012] The inventors conducted extensive research to achieve the above-mentioned object. As a result, they analyzed the fracture morphology of slab cracks and found that the fracture surface contained at least one of the following fracture surfaces: an intergranular fracture along the prior austenite grain boundary, and an intragranular fracture (cleavage fracture) that crosses the prior austenite grain boundary. Furthermore, the microstructure at the position where the slab crack propagated was mainly composed of ferrite and pearlite, whereas the microstructure further inside was mainly composed of bainite. Here, the term "bainite-based structure" refers to a microstructure that contains at least one selected from bainite, tempered martensite, quenched martensite, and retained austenite. In other words, the term "bainite-based structure" refers to a microstructure that mainly contains bainite and may also contain at least one unavoidable structure selected from quenched martensite, tempered martensite, and retained austenite. The bainite-based structure is a low-temperature transformation phase that undergoes phase transformation at a lower temperature than the ferrite and pearlite contained in the microstructure. Normally, an object is cooled from the surface, so the precipitation of low-temperature transformed phases does not occur inside the object. However, because the prior austenite grain size in a slab is coarse, the prior austenite grain size significantly affects the transformation time of the phases that make up the microstructure. In other words, when the prior austenite grain size inside the slab is larger than that on the surface of the slab, the structure that precipitates in the slab and the timing of the precipitation of the structure differ, even if the slab is cooled in the same way. From this, it was discovered that, after the transformation of the surface layer of the slab is completed, the low-temperature transformation phase inside the slab undergoes transformation expansion during thermal contraction, generating tensile stress on the surface of the surface layer of the slab, leading to slab cracking. Furthermore, after extensive research, the inventors discovered that by controlling the microstructure of the continuously cast slab and reducing the stress during transformation in the low-temperature transformation phase inside the slab, it is possible to suppress slab placement cracking during the cooling process of the continuously cast slab and prevent the occurrence of hole formation problems during rolling, and thus arrived at the present invention.

[0013] That is, the continuously cast slab according to the present invention, which advantageously solves the above problems, has the following features: (a) A continuous cast slab for high strength steel, In mass%, C: 0.10% or more and 1.00% or less, Si: 0.10% or more and 2.50% or less, Mn: 0.40% or more and 5.00% or less; Optionally, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and O: 0.0100% or less; and optionally containing at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, B: 0.0100% or less, Co: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, either alone or in combination of two or more thereof, with the balance being Fe and inevitable impurities; The average prior austenite grain size at a position 10 mm below the surface of a continuously cast slab is defined as d. 10 The average prior austenite grain size at a position 20 mm below the surface of the continuously cast slab is defined as d 20 When the average prior austenite grain size ratio (d 20 / d 10 ) is between 1.0 and 4.0, The microstructure consists of ferrite, pearlite, and a low-temperature transformation phase, the low-temperature transformation phase includes at least one selected from bainite, tempered martensite, quenched martensite, and retained austenite; The microstructure at a position 10 mm below the surface of the continuously cast slab has a total area ratio of the ferrite and the pearlite of 80% or more, and a total area ratio of the ferrite, the pearlite, and the low-temperature transformation phase of 100%; and The microstructure at a position 20 mm below the surface of the continuously cast slab is characterized in that the sum of the area ratio of the ferrite and the area ratio of the pearlite is 60% or more, and the sum of the area ratio of the ferrite, the area ratio of the pearlite, and the area ratio of the low-temperature transformation phase is 100%.

[0014] Furthermore, the method for producing a continuous cast slab according to the present invention is a method for producing a continuous cast slab for high strength steel, comprising the steps of: A continuously cast slab having the composition described in (a) is Total heat dissipation in the continuous casting mold Q MD satisfies the following relational expression (1), The temperature T of the continuously cast slab at the center in the width direction of the continuously cast slab and at a position 20 mm from the surface of the continuously cast slab 20 a first cooling step in which the temperature is in the range of 1200°C or more and 1450°C or less and the residence time is 230 seconds or less; a second cooling step in which the continuous casting slab is cooled at a center in the width direction thereof and at a surface temperature T0 of the surface layer of the continuous casting slab at an average cooling rate of 20°C / hr or less at a surface temperature T0 of 700°C or more and 850°C or less; and a third cooling step in which the continuous cast slab is cooled at a center in the width direction thereof and under cooling conditions in which the average cooling rate at a surface temperature T0 of the surface layer of the continuous cast slab is 10°C / hr or less when the surface temperature T0 is 500°C or more and 700°C or less.

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[0015] According to the present invention, it is possible to provide a continuously cast slab that does not cause cracking during cooling and does not cause problems such as holes during rolling, even if the slab has a composition system similar to that of a continuously cast slab for high strength steel. [Brief explanation of the drawings]

[0016] [Figure 1] This is a photograph taken with a scanning electron microscope (SEM) of the fracture surface of a high-strength steel continuously cast slab that fractured due to a thermal crack. [Figure 2] 1 is a cross-sectional photograph of the cracked portion. [Figure 3A] This is an enlarged photograph of a continuously cast slab observed with an optical microscope, the continuously cast slab being produced in an inventive example (Test No. D-2) of a continuously cast slab according to an embodiment of the present invention, and showing the microstructure at a position 10 mm below the surface of the continuously cast slab. [Figure 3B] This is an enlarged photograph of a continuously cast slab observed with an optical microscope, the continuously cast slab being produced in an inventive example (Test No. D-2) of a continuously cast slab according to an embodiment of the present invention, and showing the microstructure at a position 20 mm below the surface of the continuously cast slab. DETAILED DESCRIPTION OF THE INVENTION

[0017] The following describes in detail embodiments of the present invention. Note that the drawings are schematic and may differ from the actual embodiments. Furthermore, the following embodiments exemplify devices and methods for embodying the technical concept of the present invention, and are not intended to limit the configuration to those described below. In other words, the technical concept of the present invention can be modified in various ways within the technical scope described in the claims.

[0018] [First embodiment] A continuous cast slab according to the first embodiment will be described. The continuous cast slab according to this embodiment is a continuous cast slab for high strength steel, In mass%, C: 0.10% or more and 1.00% or less, Si: 0.10% or more and 2.50% or less, Mn: 0.40% or more and 5.00% or less; Optionally, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and O: 0.0100% or less; and optionally containing at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, B: 0.0100% or less, Co: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, either alone or in combination of two or more thereof, with the balance being Fe and inevitable impurities; (i) The average prior austenite grain size at a position 10 mm below the surface of a continuously cast slab is d 10 The average prior austenite grain size at a position 20 mm below the surface of the continuously cast slab is defined as d 20 When the average prior austenite grain size ratio (d 20 / d 10 ) is between 1.0 and 4.0, (ii) the microstructure consists of ferrite, pearlite, and a low-temperature transformation phase; the low-temperature transformation phase includes at least one selected from bainite, tempered martensite, quenched martensite, and retained austenite; The microstructure at a position 10 mm below the surface of the continuously cast slab has a total area ratio of the ferrite and the pearlite of 80% or more, and a total area ratio of the ferrite, the pearlite, and the low-temperature transformation phase of 100%; and The microstructure at a position 20 mm below the surface of the continuously cast slab is characterized in that the sum of the area ratio of the ferrite and the area ratio of the pearlite is 60% or more, and the sum of the area ratio of the ferrite, the area ratio of the pearlite, and the area ratio of the low-temperature transformation phase is 100%. That is, according to the invention of this embodiment, by limiting the component composition contained in the continuously cast slab and providing it with at least the above characteristics (i) to (ii), even in recent continuously cast slabs for high-strength steel, which have very low toughness, cracking during slab placement does not occur during the cooling process, and problems such as hole formation during rolling of the continuously cast slab can be prevented, making it possible to provide a continuously cast slab for high-strength steel with a high yield.

[0019] First, the suitable range of the microstructure of the continuously cast slab according to this embodiment and the reasons for limiting it will be explained. In the following explanation, "%" indicating the composition ratio of the microstructure means "area %" unless otherwise specified. Furthermore, the observation of the microstructure of the continuously cast slab was performed at room temperature.

[0020] As mentioned above, when the fracture morphology of the fracture surface at the cracked portion of a high-strength steel continuous casting slab that had fractured due to slab placement cracking was observed, it was found that many of the slab placement cracks had progressed to a depth of approximately 20 mm below the surface of the slab, and that they had taken the form of "intergranular fracture" in which the crack had progressed to the prior austenite grain boundaries, and that the microstructure at the cracked portion was mainly composed of ferrite and pearlite, while the microstructure inside the crack (towards the center of the slab thickness direction) was mainly composed of bainite. In other words, in continuously cast slabs for high-strength steel, the causes of slab cracking due to fracture of grain boundaries are thought to be a coarse prior austenite grain size, differences in the microstructure between the slab surface and the interior, and the precipitation of low-temperature transformation phases within the slab. When the prior austenite grain size is coarse, embrittlement of the grain boundaries due to grain boundary segregation and precipitation of grain boundary ferrite is likely to occur, which is a cause of slab cracking. Furthermore, when the microstructure of the slab surface differs from that of the interior, and low-temperature transformation phases are precipitated within the slab, the transformation expansion of the low-temperature transformation phases is a cause of slab cracking. Therefore, the invention according to this embodiment focuses on two factors as the necessary conditions for a continuously cast slab for high-strength steel that does not generate slab placement cracks during the cooling process: (i) the average prior austenite grain size ratio, which is calculated from the average prior austenite grain size at each of several predetermined positions set from the surface layer of the continuously cast slab, after limiting the component composition contained in the continuously cast slab; and (ii) the microstructure of the continuously cast slab.

[0021] <(i) Average prior austenite grain size ratio> The continuously cast slab for high strength steel according to this embodiment is a continuously cast slab for high strength steel, and (i) the average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab is d 10 The average prior austenite grain size at a position 20 mm below the surface of the continuously cast slab is defined as d 20 When the average prior austenite grain size ratio (d 20 / d 10 ) is 1.0 or more and 4.0 or less. Here, the average prior austenite grain size refers to an average value of a plurality of prior austenite grain sizes measured in a plurality of visual fields.

[0022] In conventional continuously cast slabs, the average prior austenite grain size is very large, at several millimeters. This significantly reduces the toughness of the continuously cast slab. In addition, the average prior austenite grain size significantly affects the transformation behavior of the microstructure of the continuously cast slab. The larger the prior austenite grain size, the longer the transformation start time. As a result, even if the continuously cast slab is slowly cooled, low-temperature transformation phases are more likely to precipitate in the microstructure. Furthermore, the greater the difference in the average prior austenite grain size, the more likely it is that a difference in the microstructure will occur between the surface and the interior of the slab. From such a technical viewpoint, the continuously cast slab according to this embodiment has an average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab of d 10 The average prior austenite grain size at a position 20 mm below the surface of the continuously cast slab is defined as d 20 When the average prior austenite grain size ratio (d 20 / d 10 ) was set to be between 1.0 and 4.0. In this way, the average prior austenite grain size ratio (d 20 / d 10 The upper limit of the average prior austenite grain size ratio (d 20 / d 10 ) is 4.0 or less, the difference in microstructure between the surface layer and the interior of the slab can be reduced. On the other hand, the average prior austenite grain size ratio (d 20 / d 10 However, in order to make the average prior austenite grain size inside the slab smaller than the average prior austenite grain size in the surface layer of the slab, special cooling control is required, and this special cooling control requires capital investment. For this reason, the average prior austenite grain size ratio (d 20 / d 10 ) is preferably 1.0 or more. 20 / d 10 The lower limit of ) is more preferably 1.2 or more, and even more preferably 1.5 or more.

[0023] Here, the reason why the positions 10 mm below the surface of the continuously cast slab and 20 mm below the surface of the continuously cast slab are specified when setting the average prior austenite grain size is that, since most slab placement cracks progress to about 20 mm below the surface of the slab, the positions 20 mm below the surface of the continuously cast slab and the position 10 mm below the surface of the continuously cast slab, which is located halfway between the surface of the continuously cast slab and the position 20 mm below the surface of the continuously cast slab, are considered to be necessary positions for suppressing slab placement cracks. On the other hand, the area of ​​the continuously cast slab less than 5 mm below the surface is directly quenched by water sprays in the mold or just below the mold. The region of less than 5 mm below the surface of the continuously cast slab has a fine structure in which the grains constituting the continuously cast slab are γ grain size, and the continuously cast slab has high toughness. Therefore, it is unlikely that the origin of slab placement cracks occurs in an area less than 5 mm below the surface of the continuously cast slab. From this technical viewpoint, in the continuously cast slab according to this embodiment, the region less than 5 mm deep from the surface layer of the continuously cast slab can be excluded from the position where control of the microstructure of the continuously cast slab is required. Therefore, the positions where control of the continuously cast slab structure is required are the first position, which is located inside the slab and is 20 mm from the depth in the slab thickness direction, and the second position, which is located on the surface of the slab and is 10 mm from the depth in the slab thickness direction. That is, the average prior austenite grain size may be set, for example, at 18 to 22 mm and 15 to 25 mm in the depth direction from the continuously cast slab surface layer, based on a position 20 mm from the continuously cast slab surface layer inside the slab as a first position below the continuously cast slab surface layer. Furthermore, as a second position below the surface of the continuously cast slab, the average prior austenite grain size may be set, for example, at a depth of 8 to 12 mm and 5 to 15 mm from the surface of the continuously cast slab, based on a position 10 mm from the surface of the continuously cast slab inside the slab.

[0024] In the continuously cast slab according to this embodiment, the factor that determines the average prior austenite grain size is the temperature at which the continuously cast slab is cooled. Austenite grains grow at a high rate, particularly in the temperature range of 1200°C to 1450°C. Therefore, the temperature at which the continuously cast slab is cooled is particularly in the range of 1200°C to 1450°C, and the cooling rate and residence time of the continuously cast slab in this temperature range affect the temperature. That is, in the temperature range of 1200°C or higher and 1450°C or lower, the slower the cooling rate of the continuously cast slab or the longer the residence time of the continuously cast slab, the coarser the average prior austenite grain size becomes. The prior austenite grain size of a continuously cast slab is fine on the surface side of the slab and coarsens toward the interior of the slab. To reduce the difference between the average prior austenite grain size on the surface side of the slab and the average prior austenite grain size inside the slab, it is necessary to coarsen the average prior austenite grain size on the surface side of the slab and refine the average prior austenite grain size inside the slab.

[0025] That is, the continuously cast slab according to this embodiment has (i) an average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab of d 10 , the average prior austenite grain size at a position 20 mm below the surface of the continuously cast slab is d 20 When the average prior austenite grain size ratio (d 20 / d 10 In order to satisfy the condition that the temperature T 10 , T 20 It is important to control the cooling in the temperature range of 1200°C to 1450°C to produce a continuously cast slab. 10 , T 20 are the temperatures at 10 mm and 20 mm below the surface of the continuously cast slab, respectively. Temperature T at 10 mm from the surface of the continuously cast slab 10In order to control the cooling rate in the temperature range of 1200°C or more and 1450°C or less, it is necessary to take into consideration the total amount of heat removed in the continuous casting mold in the process for producing continuously cast slabs according to this embodiment. That is, the temperature T at a position 10 mm from the surface of the continuously cast slab 10 The temperature range of 1200°C to 1450°C is when the continuous casting slab passes through the continuous casting mold, so the total heat removal amount QMD [MW / m 2 Taking into account the relationship between the effective mold length LMD [m] and the casting speed Vc [m / min], the continuously cast slab must be cooled so that the total heat removal amount within the continuous casting mold satisfies the following relational expression (1).

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[0026] Total heat dissipation in continuous casting mold Q MD is calculated by calculating the total heat removal amount from the flow rate of cooling water to the continuous casting mold and the temperature difference between the inlet and outlet of the continuous casting mold, and dividing the calculated total heat removal amount by the contact area between the mold copper plate that constitutes the continuous casting mold and the cast piece. Effective mold length L MD is the length of the mold (mold length) in which the molten steel poured from the tundish can solidify. MD Although it depends on the shape of the continuous casting mold, it is preferable that the length is, for example, 0.7 to 0.9 [m]. The casting speed Vc is the speed at which molten steel is poured into the continuous casting mold, and is preferably, for example, 0.8 to 2.0 [m / min].

[0027] Here, the total heat dissipation amount in the continuous casting mold Q MD [MW / m 2 ] and the effective mold length L MDThe relationship between the casting speed Vc [m / min] and the casting velocity Vc [m / min] is defined by the above-mentioned relational expression (1), and the value calculated by relational expression (1) is preferably 0.6 to 1.5. If the value calculated by relational expression (1) is 1.5 or less, the average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab can be coarsened, and the difference with the average prior austenite grain size inside the slab can be reduced, which is preferable. On the other hand, if the value calculated by the relational expression (1) is 0.6 or more, it is preferable because the shell thickness of the continuously cast slab at the outlet side of the continuous casting mold can be ensured and the average prior austenite grain size of the continuously cast slab can be coarsened without the risk of breakout.

[0028] In addition, the temperature T 20 In order to control the residence time within the range of 1200°C or more and 1450°C or less, it is necessary to control the spray water cooling in the manufacturing process of the continuously cast slab according to this embodiment. That is, the temperature T at a position 20 mm from the surface of the continuously cast slab 20 The temperature T is in the range of 1200°C to 1450°C when the continuous casting slab passes through the secondary cooling zone directly below the continuous casting mold. Therefore, the amount of cooling water in the secondary cooling zone is controlled to maintain the temperature T at a position 20 mm from the surface of the continuous casting slab. 20 It is preferable that the residence time in the range of 1200°C or more and 1450°C or less is 230 seconds or less. The total heat dissipation amount QMD in the continuous casting mold satisfies the above relational expression (1), and the temperature T at a position 20 mm from the surface of the continuous casting slab 20 If the residence time corresponding to the temperature range of 1200°C to 1450°C is 230s or less, the average prior austenite grain size d 10 and the average prior austenite grain size d at a position 20 mm below the surface of the slab 20 The average prior austenite grain size ratio (d 20 / d 10 ) can be set to 4.0 or less, which is preferable because it can suppress cracks when the slab is placed.

[0029] From this viewpoint, the residence time of the continuously cast slab is preferably 220 seconds or less, more preferably 210 seconds or less, and even more preferably 200 seconds or less. The lower limit of the residence time of the continuously cast slab is not particularly limited, but is set to 60 seconds or more because too short a residence time increases the risk of breakout during continuous casting due to non-uniform solidification. In other words, if the residence time of the continuously cast slab is less than 60 seconds, cracks will occur in the continuously cast slab due to uneven solidification of the initial solidified shell, which may lead to breakout, so it is preferable to set the residence time of the continuously cast slab to 60 seconds or more. From this viewpoint, the temperature T 20 The residence time of the continuously cast slab in the temperature range of 1200°C or more and 1450°C or less is more preferably 80 seconds or more, and even more preferably 90 seconds or more.

[0030] Temperatures T at 10mm and 20mm from the surface of the continuously cast slab 10 , T 20 The cooling rate and residence time of a continuously cast slab in the temperature range of 1200°C to 1450°C can be controlled by adjusting the cooling conditions in the initial stage of slab casting. For example, in the continuous casting of steel, molten steel with adjusted composition is first poured into a water-cooled copper mold to form an initial solidified shell. Then, the continuously cast slab begins to be withdrawn from the water-cooled copper mold, and after the continuously cast slab leaves the water-cooled copper mold, it is cooled with a water spray. Temperature T at 10 mm below the surface of a continuously cast slab 10 Since the cooling inside the continuous casting mold has a large effect on the above, for example, the thermal conductivity of the mold powder used to lubricate the inside of the continuous casting mold may be reduced, or the amount of cooling water for the continuous casting mold may be reduced. On the other hand, the temperature T 20Since the cooling directly below the continuous casting mold has a large effect on the temperature, it is possible to control the temperature by, for example, increasing the flow rate of the water spray directly below the continuous casting mold. If the spray directly below the continuous casting mold is a two-fluid spray of water and air, the temperature can also be controlled by increasing the water flow rate and the air flow rate.

[0031] By controlling these cooling conditions, it is possible to control the average prior austenite grain size at positions 10 mm and 20 mm from the surface of the continuously cast slab. Here, it is difficult to actually measure the temperature inside the continuously cast slab. Therefore, the temperature history at positions 10 mm and 20 mm below the surface of the continuously cast slab was calculated by heat transfer analysis, and the temperature T 10 , T 20 In order to make the residence time in the above temperature range the longest even inside the continuously cast slab, the heat transfer analysis position can be set at the center of the slab width.

[0032] <(ii) Microstructure of Continuously Cast Slabs> The continuously cast slab according to this embodiment is characterized in that (ii) the microstructure at a position 10 mm below the surface of the slab has a total area ratio of ferrite and pearlite of 80% or more, and the microstructure at a position 20 mm below the surface of the slab has a total area ratio of ferrite and pearlite of 60% or more. That is, (i) the average prior austenite grain size d at a position 10 mm below the surface of the continuously cast slab 10 and the average prior austenite grain size d at a position 20 mm below the surface of the continuously cast slab 20 The average prior austenite grain size ratio (d 20 / d 10 ) is 4.0 or less, the ratio of the internal structures that make up the microstructure of a continuously cast slab, such as bainite and ferrite, is also a factor that determines the unit of fracture of the slab, and it is known that the stress applied to the slab changes depending on this ratio. In particular, when a low-temperature transformation phase, which is an unavoidable structure that mainly contains bainite and also includes quenched martensite, tempered martensite, and retained austenite, occurs in the microstructure inside a slab, the amount of expansion during transformation of the low-temperature transformation phase is large, and in addition, the ferrite and pearlite formed around the low-temperature transformation phase, including bainite, etc., have already completed their transformation and subsequently undergo thermal contraction, resulting in large stress concentrations there. Therefore, the inventors discovered that the transformation stress applied to the surface of the slab can be reduced by controlling the cooling rate and (ii) ensuring that the microstructure 10 mm below the surface of the continuously cast slab has a total area ratio of ferrite and pearlite of 80% or more, and that the microstructure 20 mm below the surface of the continuously cast slab has a total area ratio of ferrite and pearlite of 60% or more. Here, the low-temperature transformation phase, which is a microstructure other than ferrite and pearlite, is mainly composed of bainite, but is not limited to bainite alone, and may also include quenched martensite, tempered martensite, and retained austenite in addition to bainite. The area ratio of ferrite and the area ratio of pearlite can be calculated based on the results of observation of the microstructure of the continuously cast slab using an observation means such as an optical microscope, an electron microscope, etc. Then, the ferrite and pearlite contained in the microstructure of the continuously cast slab can be distinguished using an observation means such as an optical microscope or an electron microscope.

[0033] Based on the results of identifying the microstructure of the continuously cast slab, the area S of the microstructure of the continuously cast slab total and the area of ​​the ferrite S ferrite and the area of ​​pearlite S pearlite The total area S (ferrite+pearlite) Then, the area S of the microstructure of the continuously cast slab is calculated. total Ferrite area S ferrite and the area of ​​pearlite S pearlite The total area S (ferrite+pearlite) The ratio is defined as the area ratio (%) and calculated. The microstructure of a continuously cast slab consists of ferrite, pearlite, and low-temperature transformation phases, and the sum of the area ratio (%) of ferrite, the area ratio (%) of pearlite, and the area ratio (%) of low-temperature transformation phases is 100%. Therefore, the area ratio S of the low-temperature transformation phases X The area ratio (%), which is the ratio of the above, is calculated by subtracting the total area ratio of the ferrite area ratio (%) and the pearlite area ratio (%) from 100%.

[0034] The continuously cast slab of this embodiment is characterized in that (ii) the microstructure at a position 10 mm below the surface of the continuously cast slab has a total area ratio of ferrite and area ratio of pearlite of 80% or more, and the total area ratio of the ferrite, area ratio of pearlite, and area ratio of the low-temperature transformation phase is 100%, and the microstructure at a position 20 mm below the surface of the continuously cast slab has a total area ratio of ferrite and area ratio of pearlite of 60% or more, and the total area ratio of the ferrite, area ratio of pearlite, and area ratio of the low-temperature transformation phase is 100%. That is, in the continuously cast slab according to this embodiment, (ii) the area S10 of the microstructure at a position 10 mm below the surface of the continuously cast slab total Ferrite area S10 ferrite and the area of ​​pearlite S10 pearlite The total area of ​​S10 (ferrite+pearlite) The area ratio (%) of the microstructure at a position 20 mm below the surface of the continuously cast slab is 80% or more, and the area S20 total Ferrite area S20 ferrite and the area of ​​pearlite S20 pearlite The total area S20 (ferrite+pearlite) If the area ratio (%) is 60% or more, the thermal stress and transformation stress caused by slow cooling of the slab on the surface of the slab can be alleviated, and cracks during slab placement in continuously cast slabs can be suppressed, which is preferable. Furthermore, when the area ratio of ferrite is 5% or more but less than 10%, a small amount of grain boundary ferrite precipitates, which reduces the grain boundary strength and worsens the degree of cracking of the slab. ferrite , S20 ferriteIn both cases, the area ratio of these ferrites is preferably less than 5% or 10% or more. On the other hand, if the microstructure at a position 10 mm below the surface of a continuously cast slab has a total area ratio of ferrite and pearlite of less than 80%, the stress during cooling and transformation of the low-temperature transformation phase cannot be absorbed, which is undesirable as it causes cracks during slab placement. Furthermore, if the microstructure at a position 20 mm below the surface of a continuously cast slab is such that the sum of the area ratio of ferrite and the area ratio of pearlite is less than 60%, the transformation expansion of the low-temperature transformation phase will cause excessive tensile stress in the surface layer of the slab where transformation has already been completed, which is undesirable because it will cause slab placement cracks in the continuously cast slab. The microstructure at a position 10 mm below the surface of the continuously cast slab consists of ferrite, pearlite, and a low-temperature transformation phase. ferrite Area ratio of pearlite and area S10 pearlite Area fraction of low-temperature transformed phase and area S10 X Similarly, the microstructure at a position 20 mm below the surface of a continuously cast slab consists of ferrite, pearlite, and a low-temperature transformation phase, and the area ratio of ferrite is S20 ferrite Area ratio of pearlite and area S20 pearlite Area fraction of low-temperature transformed phase and area S20 X The total of the area ratio is 100%.

[0035] Ferrite contains iron with a maximum of 0.02% by mass of carbon, and is a structure close to pure iron. Ferrite is the softest and most ductile of all steel structures. Pearlite is a structure obtained when austenite is slowly cooled. Pearlite is formed by alternating layers of ferrite and cementite.

[0036] To control slab cracking, it is important not only to reduce thermal and transformation stresses by cooling the slab, but also to control the microstructure of the continuously cast slab. By controlling the cooling rate below 850°C and above 700°C, which is the ferrite transformation region, it is possible to control the precipitation of ferrite within the microstructure. Specifically, by reducing the cooling rate of the continuously cast slab in the ferrite transformation region, the amount of ferrite precipitation can be increased, and the ferrite precipitated from the grain boundaries, which causes stress concentration, can be rendered harmless.

[0037] In addition, to suppress slab cracking during laying, it is important not only to suppress the embrittlement of prior austenite grain boundaries but also to reduce the stresses that occur during the transformation of low-temperature transformation phases. In addition to controlling the average prior austenite grain size at multiple predetermined locations from the surface of the continuously cast slab, the microstructure of the continuously cast slab can be controlled by variously controlling the cooling rate in the pearlite transformation region (700°C or lower, 500°C or higher). Specifically, by reducing the cooling rate in the pearlite transformation region, the precipitation of low-temperature transformation phases, mainly including bainite, can be suppressed. This prevents excessive transformation expansion of the low-temperature transformation phases from affecting the ferrite and pearlite phases, which have already completed their transformation, thereby suppressing slab cracking during laying.

[0038] The cooling of the continuously cast slab after it leaves the continuous casting machine can be controlled by changing the slab temperature at the outlet of the continuous casting machine, the time it takes to stack multiple slabs, the number of slabs to be stacked, whether or not a heat-insulating cover is used, water-tight treatment, and other conditions. The cooling rate of a continuously cast slab after it leaves the continuous casting machine can be measured using a thermocouple. For example, a thermocouple is installed in the center of the wide surface (long side) of the surface layer of the continuously cast slab after it leaves the continuous casting machine, and the surface temperature T0 of the surface layer of the continuously cast slab is measured, allowing the cooling rate to be calculated.

[0039] As described above, according to the invention according to the present embodiment, even in the continuous casting slab for high-strength steel in recent years where the toughness of the continuous casting slab is very low, it is possible to prevent slab cracking during the cooling process and also prevent problems such as punching troubles during rolling, and a continuous casting slab for high-strength steel with good yield can be obtained.

[0040] Furthermore, the continuous casting slab according to the present embodiment contains C: 0.10% or more and 1.00% or less, Si: 0.10% or more and 2.50% or less, and Mn: 0.40% or more and 5.00% or less in mass%. In the following description, "%" representing the content of the component elements of steel means "mass%" unless otherwise specified.

[0041] <C: 0.10% or more and 1.00% or less> In the continuous casting slab according to the present embodiment, the reasons for limiting each chemical component contained in the continuous casting slab will be described. The content of each chemical component contained in the continuous casting slab is in mass%. The reason for setting the content of C contained in the continuous casting slab to 0.10% or more and 1.00% or less is as follows. C contained in the continuous casting slab for high-strength steel is an element necessary to increase the strength of the high-strength steel plate using the continuous casting slab as a raw material. If the content of C is less than 0.10%, the required strength of the high-strength steel plate cannot be obtained, so the lower limit of the content of C is 0.10%. On the other hand, if the content of C exceeds 1.00%, the weldability and workability of the above high-strength steel plate become insufficient, which is not preferable.

[0042] Therefore, from such a perspective, in the continuous casting slab according to the present embodiment, it is preferable that the content of C contained in the continuous casting slab is set to 0.10% or more and 1.00% or less, more preferably 0.12% or more and 0.45% or less, and particularly preferably 0.15% or more and 0.40% or less.

[0043] <Si: 0.10% or more and 2.50% or less> Next, the reason for setting the Si content in the continuously cast slab for high-strength steel to be 0.10% or more and 2.50% or less is as follows. Si contained in the continuously cast slab is an element necessary to ensure retained austenite in the high-strength steel sheet using the continuously cast slab as a raw material in the annealing process of the steel sheet. In addition, Si contained in the continuously cast slab is an essential additive element because it also contributes to the strengthening of the high-strength steel sheet by solid solution strengthening. If the Si content is less than 0.10%, the required strength of the high-strength steel sheet cannot be obtained. Therefore, the lower limit of the Si content is 0.10%.

[0044] On the other hand, when the Si content exceeds 2.50%, the effect of obtaining the required strength of the high-strength steel sheet saturates, and a strong scale forms on the hot-rolled sheet before it is processed into the high-strength steel sheet. As a result, the appearance and pickling property of the high-strength steel sheet deteriorate. Therefore, the upper limit of the Si content is 2.50%.

[0045] Therefore, from such a perspective, in the continuously cast slab according to this embodiment, it is preferable that the Si content contained in the continuously cast slab is 0.10% or more and 2.50% or less, more preferably 0.50% or more and 2.00% or less, and even more preferably 1.00% or more and 1.80% or less.

[0046] <Mn: 0.40% or more and 5.00% or less> Furthermore, the reason for setting the Mn content in the continuously cast slab to be 0.40% or more and 5.00% or less is as follows. Mn contained in the continuously cast slab is an element necessary to further increase the strength of the high-strength steel sheet. Specifically, Mn is an element added to control the strength of the high-strength steel sheet through its transformation control in the hot rolling process of the continuously cast slab. If the Mn content is less than 0.40%, sufficient strengthening of the high-strength steel sheet cannot be achieved. Therefore, the lower limit of the Mn content is 0.40%. On the other hand, when the Mn content exceeds 5.00%, the degree of sufficient strengthening of the high-strength steel sheet saturates, and the manufacturing cost of the high-strength steel sheet increases, which is not preferable from the perspective of economy.

[0047] Therefore, from this viewpoint, in the continuously cast slab according to this embodiment, the Mn content contained in the continuously cast slab is preferably 0.40% or more and 5.00% or less, more preferably 1.20% or more and 4.50% or less, and even more preferably 1.40% or more and 4.00% or less.

[0048] The continuously cast slab according to this embodiment has the above-mentioned chemical composition, with the remainder being Fe and unavoidable impurities, and has an average prior austenite grain size and microstructure of appropriate composition. Insofar as this is the case, taking other properties into consideration, the slab may contain up to 0.100% P, up to 0.0200% S, up to 0.100% Al, up to 0.0100% N, and up to 0.0100% O. Examples of unavoidable impurities include Zn, Pb, and As. A total content of up to 0.100% of these unavoidable impurities is permitted.

[0049] P segregates at prior austenite grain boundaries, embrittling the grain boundaries and potentially causing slab cracking. Therefore, the P content is preferably 0.100% or less. There is no particular lower limit for the P content, but since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferably 0.001% or more. Therefore, the P content is preferably 0.100% or less, more preferably 0.001% or more, and even more preferably 0.070% or less.

[0050] S exists as sulfide and is an element that causes slab embrittlement. Therefore, the S content is preferably 0.0200% or less. There is no particular lower limit for the S content, but due to production technology constraints, it is preferably 0.0001% or more. Therefore, the S content is preferably 0.0200% or less, preferably 0.0001% or more, and more preferably 0.0050% or less.

[0051] Al is an element that inhibits the formation of carbides during slab cooling and promotes the formation of retained austenite, thereby affecting the fraction of retained austenite in the slab. It is also preferable to add 0.005% or more for deoxidation. If the Al content exceeds 0.100%, there is a risk of slab embrittlement. Therefore, the Al content is preferably 0.100% or less, more preferably 0.010% or more, and even more preferably 0.080% or less.

[0052] N exists as a nitride and is an element that causes embrittlement of the slab. Therefore, the N content is preferably 0.0100% or less. Although there is no particular lower limit for the N content, due to constraints on production technology, the N content is preferably 0.0001% or more. Therefore, the N content is preferably 0.0100% or less, more preferably 0.0001% or more, and even more preferably 0.0050% or less.

[0053] O exists as an oxide and is an element that causes embrittlement of the slab. Therefore, the O content is preferably 0.0100% or less. Although there is no particular lower limit for the O content, due to constraints on production technology, the O content is preferably 0.0001% or more. Therefore, the O content is preferably 0.0100% or less, more preferably 0.0001% or more, and even more preferably 0.0050% or less.

[0054] In addition to the above-described chemical composition, the continuously cast slab according to this embodiment for use in a high-strength steel plate may further contain at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, B: 0.0100% or less, Co: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, either alone or in combination of two or more thereof.

[0055] If the content of Ti, Nb, and V is 0.200% or less, large amounts of coarse precipitates and inclusions are not formed in the slab, and the toughness of the slab is not reduced. Therefore, the content of Ti, Nb, and V is preferably 0.200% or less. Although there are no particular lower limits for the Ti, Nb, and V contents, the Ti, Nb, and V contents are each preferably 0.001% or more because they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing of continuously cast slabs. Therefore, when Ti, Nb, and V are contained, their respective contents are set to 0.200% or less, more preferably 0.001% or more, and even more preferably 0.100% or less.

[0056] If the Ta and W contents are each 0.10% or less, large amounts of coarse precipitates and inclusions are not formed, and the toughness of the slab is not reduced. Therefore, the Ta and W contents are preferably each 0.10% or less. Although there are no particular lower limits for the Ta and W contents, the Ta and W contents are more preferably 0.01% or more, because they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing of continuously cast slabs. Therefore, when Ta and W are contained, their contents are each 0.10% or less, more preferably 0.01% or more, and even more preferably 0.08% or less.

[0057] The continuously cast slab according to this embodiment may contain at least one element selected from Cr, Mo, Ni, and Cu, as needed, within the scope of the present invention. Cr, Mo, Ni, and Cu enhance the strength of the steel sheet through structural control during hot rolling of the continuously cast slab. This effect is enhanced by adding 0.01% or more of one or more of Cr, Mo, Ni, and Cu, respectively, and therefore adding 0.01% or more is preferable. If the amount of each element exceeds the upper limit of each element, the weldability, hot workability, and other properties of the steel sheet deteriorate. Therefore, the upper limit of the amount of each of Cr, Mo, Ni, and Cu is set to 1.00%. Therefore, when the continuously cast slab contains Cr, Mo, Ni, and Cu, the content of each element is set to 1.00% or less. Preferably, the content is set to 0.01% or more. More preferably, the content is set to 1.00% or less.

[0058] B may be added because it controls the structural transformation during hot rolling and annealing of continuously cast slabs and thus affects strength through structural strengthening. B does not affect the toughness of the slab if its content is 0.0100% or less. Therefore, the B content is preferably 0.0100% or less. While there is no particular lower limit for the B content, since B is an element that segregates to austenite grain boundaries during hot rolling and annealing of continuously cast slabs and improves hardenability, the B content is more preferably 0.0003% or more. Therefore, when B is contained, its content is 0.0100% or less, more preferably 0.0003% or more, and even more preferably 0.0080% or less.

[0059] If Co is 1.00% or less, the amount of coarse precipitates and inclusions does not increase, and the toughness of the slab does not decrease. Therefore, the Co content is preferably 1.00% or less. Although there is no particular lower limit for the Co content, since Co is an element that improves the hardenability of the slab, the Co content is more preferably 0.001% or more. Therefore, if Co is contained, its content is 1.00% or less, more preferably 0.001% or more, and even more preferably 0.80% or less.

[0060] If Cu is 1.00% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the slab will not be reduced. Therefore, the Cu content is preferably 1.00% or less. Although there is no particular lower limit for the Cu content, since Cu is an element that improves hardenability, the Cu content is more preferably 0.01% or more. Therefore, if Cu is contained, its content should be 1.00% or less. More preferably, it should be 0.01% or more. Even more preferably, it should be 0.80% or less.

[0061] Sn does not affect the toughness of the slab if it is 0.200% or less. Therefore, the Sn content is preferably 0.200% or less. Although there is no particular lower limit for the Sn content, since Sn is an element that improves hardenability, the Sn content is more preferably 0.001% or more. Therefore, if Sn is contained, its content is 0.200% or less, more preferably 0.001% or more, and even more preferably 0.100% or less.

[0062] If Sb is 0.200% or less, the amount of coarse precipitates and inclusions does not increase, and the toughness of the slab does not decrease. Therefore, the Sb content is preferably 0.200% or less. Although there is no particular lower limit for the Sb content, since Sb is an element that suppresses decarburization and enables adjustment of the strength of the steel sheet, the Sb content is more preferably 0.001% or more. Therefore, if Sb is contained, its content is 0.200% or less, more preferably 0.001% or more, and even more preferably 0.100% or less.

[0063] If the content of Ca, Mg, and REM is 0.0100% or less, the amount of coarse precipitates and inclusions will not increase and the toughness of the slab will not decrease. Therefore, the content of Ca, Mg, and REM is preferably 0.0100% or less. Although there are no particular lower limits for the contents of Ca, Mg, and REM, since Ca, Mg, and REM are elements that spheroidize the shape of nitrides and sulfides and improve the toughness of the slab, it is more preferable that the contents of these elements are each 0.0005% or more. Therefore, when Ca, Mg and REM are contained, their contents are each set to 0.0100% or less, more preferably 0.0005% or more, and even more preferably 0.0050% or less.

[0064] If Zr and Te are each 0.100% or less, the amount of coarse precipitates and inclusions in the slab will not increase, and the toughness of the slab will not be reduced. Therefore, the Zr and Te contents are preferably 0.100% or less. Although there are no particular lower limits for the Zr and Te contents, since Zr and Te are elements that spheroidize the shape of nitrides and sulfides and improve the toughness of the slab, it is more preferable that the Zr and Te contents be 0.001% or more. Therefore, when Zr and Te are contained, their contents should be 0.100% or less, more preferably 0.001% or more, and even more preferably 0.080% or less.

[0065] If Hf is 0.10% or less, the amount of coarse precipitates and inclusions does not increase, and the toughness of the slab does not decrease. Therefore, the Hf content is preferably 0.10% or less. Although there is no particular lower limit for the Hf content, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet, the Hf content is more preferably 0.01% or more. Therefore, if Hf is contained, its content is 0.10% or less, more preferably 0.01% or more, and even more preferably 0.08% or less.

[0066] If Bi is 0.200% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the slab will not be reduced. Therefore, the Bi content is preferably 0.200% or less. Although there is no particular lower limit for the Bi content, since Bi is an element that reduces segregation, the Bi content is more preferably 0.001% or more. Therefore, if Bi is contained, its content should be 0.200% or less, more preferably 0.001% or more, and even more preferably 0.100% or less.

[0067] In addition, when the content of each of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi is less than the preferable lower limit value, the effect of the present invention is not impaired, and therefore, these elements are included as inevitable impurities.

[0068] As described above, according to the invention relating to the first embodiment, it is possible to obtain the strength required for high-strength steel, and further to obtain a continuously cast slab that has excellent weldability, workability, and appearance of high-strength steel.

[0069] [Second embodiment] A method for producing a continuously cast slab according to a second embodiment will be described. The method for producing a continuously cast slab according to this embodiment is a method for producing a continuously cast slab for high strength steel, and comprises the steps of: Total heat dissipation in the continuous casting mold Q MD satisfies the following relational expression (1), and the temperature T of the continuously cast slab at the center in the width direction of the continuously cast slab and at a position 20 mm from the surface of the continuously cast slab 20 a first cooling step in which the temperature is in the range of 1200°C or more and 1450°C or less, and the residence time is 230 seconds or less; a second cooling step in which the continuous casting slab is cooled at a center in the width direction thereof and at a surface temperature T0 of the surface layer of the continuous casting slab at an average cooling rate of 20°C / hr or less at a surface temperature T0 of 700°C or more and 850°C or less; and a third cooling process in which the continuous cast slab is cooled at a center in the width direction thereof under cooling conditions in which the average cooling rate at a surface temperature T0 of the surface layer of the continuous cast slab is 10°C / hr or less when the surface temperature T0 is 500°C or more and 700°C or less.

number

[0070] In the manufacturing method for slabs for high-strength steel plates according to this embodiment, re-shipping may occur depending on the conditions of the manufacturing process. When re-shipping occurs, the cooling rate of the slab may temporarily exceed the specified cooling rate. However, since the time required for transformation is very slow, at 10 hours or more, the handling time required for re-shipping (at most 1 to 2 hours) does not lead to the occurrence of thermal cracking. Therefore, in the manufacturing method for slabs for high-strength steel plates according to this embodiment, the cooling rate when cooling the continuously cast slab is specified as an average cooling rate rather than a maximum cooling rate. Hereinafter, each step included in the method for producing a continuously cast slab according to this embodiment will be described.

[0071] (First cooling process) The method for producing a continuous cast slab according to this embodiment is a method for producing a continuous cast slab for high strength steel, which comprises: Total heat dissipation in the continuous casting mold Q MD satisfies the following relational expression (1), and the temperature T of the continuously cast slab at the center in the width direction of the continuously cast slab and at a position 20 mm from the surface of the continuously cast slab 20 The method includes a first cooling step in which the sintered body is cooled under cooling conditions in which the residence time in the temperature range of 1200°C to 1450°C is 230 seconds or less.

number

[0072] The first cooling step is carried out to determine the average prior austenite grain size d 10 and the average prior austenite grain size d at a position 20 mm below the surface of the continuously cast slab 20 The average prior austenite grain size ratio (d 20 / d 10) to 4.0 or less. In the method for producing a continuously cast slab according to this embodiment, the factor that determines the average prior austenite grain size is the temperature at which the continuously cast slab is cooled. In the first cooling step, the temperature at which the continuously cast slab is cooled is in the temperature range of 1200°C or more and 1450°C or less. In this way, the method for producing a continuously cast slab according to this embodiment focuses on the temperature range of 1200°C or more and 1450°C or less of the continuously cast slab, which is the factor that determines the average prior austenite grain size, and controls that temperature. In the first cooling process, the temperature at which the continuously cast slab is cooled in the temperature range of 1200°C to 1450°C is difficult to measure, so the temperature history at a position 20 mm from the surface of the continuously cast slab was calculated by heat transfer analysis. The analysis position was the center of the slab width, where the time spent in the above temperature range is the longest.

[0073] Average prior austenite grain size d at 10 mm below the surface of a continuously cast slab 10 and the average prior austenite grain size d at a position 20 mm below the surface of the continuously cast slab 20 The average prior austenite grain size ratio (d 20 / d 10 ), the average prior austenite grain size d 10 The average prior austenite grain size d 20 needs to be made smaller. In other words, the cooling rate must be slow at a position 10 mm below the surface of the continuously cast slab, and fast at a position 20 mm below the surface of the continuously cast slab.

[0074] Furthermore, in the first cooling process, the total heat dissipation amount Q MD is the effective length of the mold L MD The relationship between the casting speed Vc [m] and the casting velocity Vc [m / min] must satisfy the following relational expression (1): In the above temperature range for cooling the continuously cast slab at a position 20 mm from the surface, the residence time of the continuously cast slab is 230 seconds or less. Furthermore, the total heat dissipation amount Q MD is the effective length of the mold L MD In the relationship between the casting speed Vc [m / min] and the casting speed Vc [m / min], if the following relational expression (1) is satisfied and the residence time of the continuously cast slab at the above temperature is 230 s or less, the average prior austenite grain size ratio (d 20 / d 10 ) can be made 4.0 or less, which is preferable because it can suppress cracks when the slab is placed. Although there is no particular lower limit for the residence time of the continuously cast slab in the temperature range of 1200°C to 1450°C, if the residence time is too short, the risk of breakout during continuous casting due to uneven solidification increases, so it is preferably 60 seconds or more, more preferably 80 seconds or more, and even more preferably 90 seconds or more.

number

[0075] (Second cooling process) Next, the method for producing a continuously cast slab according to this embodiment includes a second cooling step in which the continuously cast slab is cooled at a center in the width direction thereof and at an average cooling rate of 20°C / hr or less when the surface temperature T0 of the surface layer of the continuously cast slab is 700°C or higher and 850°C or lower. The second cooling step is a step for increasing the area ratio of ferrite contained in the microstructure of the continuously cast slab according to the above embodiment.

[0076] In the second cooling step, the temperature at which the continuously cast slab is further cooled is 700°C or higher and 850°C or lower. In this way, the method for producing a continuously cast slab according to this embodiment controls the cooling rate in the temperature range in the ferrite transformation region, which can increase the amount of ferrite precipitation. In the second cooling step, the cooling rate was measured using a thermocouple. After the slab emerged from the continuous casting machine, a thermocouple was placed in the center of the top surface of the slab's wide surface (long side), and the cooling rate was calculated from the measured temperature. The cooling rate in the third cooling step, which will be described later, can be measured in the same manner.

[0077] In the second cooling step, the average cooling rate of the continuously cast slab within the above-mentioned temperature range is 20°C / hr or less. If the average cooling rate of the continuously cast slab is 20°C / hr or less, the residence time of the continuously cast slab in the ferrite transformation temperature range can be sufficiently ensured, the area ratio of ferrite can be increased, the precipitation of grain boundary ferrite can be suppressed, and the toughness of the slab can be improved, which is preferable. Although there is no strict lower limit to the average cooling rate, since a separate energy source is required for control, the average cooling rate is preferably 2°C / hr or more, and more preferably 5°C / hr or more and 18°C / hr or less.

[0078] (Third cooling process) Furthermore, the manufacturing method of a continuous cast slab according to this embodiment includes a third cooling step in which the continuous cast slab is cooled at the widthwise center thereof under cooling conditions in which the average cooling rate at the surface temperature T0 of the surface layer of the continuous cast slab is 10°C / hr or less when the surface temperature T0 is 500°C or more and 700°C or less. The third cooling step is a step for changing the microstructure of the continuously cast slab according to the embodiment to a structure mainly composed of pearlite, and for reducing the internal stress of the microstructure. In other words, this is a process for suppressing the precipitation of low-temperature transformation phases in the microstructure of the continuously cast slab, and preventing the stress caused by the transformation expansion from being applied to the ferrite and pearlite regions where transformation has already been completed. Specifically, the third cooling process is performed by determining the area S10 of the microstructure at a position 10 mm below the surface of the continuously cast slab. total Ferrite area S10 ferrite and the area of ​​pearlite S10 pearlite The total area of ​​S10 (ferrite+pearlite) The area ratio (%) of the microstructure at a position 20 mm below the surface of the continuously cast slab is set to 80% or more. total Ferrite area S20 ferrite and the area of ​​pearlite S20 pearlite The total area S20 (ferrite+pearlite) This is a process in which the area ratio (%) is set to 60% or more.

[0079] In the third cooling step, the temperature to which the continuously cast slab is further cooled is 500° C. or higher and 700° C. or lower. In this way, the method for producing a continuously cast slab according to this embodiment controls the temperature of the continuously cast slab by focusing on the cooling rate in the temperature range in the pearlite transformation region.

[0080] In the third cooling step, the average cooling rate of the continuously cast slab in the above-mentioned temperature range is 10°C / hr or less. 20 / d 10 ) to 4.0 or less, and the average cooling rate of the continuously cast slab to be 10°C / hr or less, is preferable because the transformation of low-temperature transformation phases including bainite and the like can be suppressed, and the microstructure of the continuously cast slab can be made to be mainly composed of pearlite, thereby reducing the internal stress. Although there is no strict lower limit to the average cooling rate when cooling a continuously cast slab, since a separate energy source is required for control, the rate is preferably 2°C / hr or more, and more preferably 5°C / hr or more.

[0081] As described above, the method for manufacturing a continuously cast slab according to this embodiment employs a three-stage cooling process as the cooling process for the continuously cast slab, and by precisely controlling the average prior austenite grain size ratio and the microstructure of the continuously cast slab, it is possible to provide a continuously cast slab for high strength steel that can suppress slab placement cracks that occur during cooling and prevent problems such as perforation during rolling.

[0082] As described above, according to the method for manufacturing a continuously cast slab of the second embodiment, even if the composition of a continuously cast slab for high strength steel is used, by dividing the cooling process into three stages and precisely controlling each cooling process, it is possible to provide a continuously cast slab for high strength steel that does not develop cracks during the cooling process and that can prevent problems such as holes during rolling.

[0083] [Other embodiments] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above embodiments. Various modifications can be made to the configuration and details of the present invention that are understandable to those skilled in the art within the technical scope of the present invention. Furthermore, systems or devices that combine the separate features included in each embodiment in any manner are also included in the technical scope of the present invention. [Example]

[0084] The effects of the present invention will be specifically explained below based on examples, but the present invention is not limited to these examples. That is, in order to confirm the effects of the present invention, the inventors produced continuously cast slabs using various steel types as raw materials in comparative examples (Test Nos. A-1 to A-4, Test Nos. B-1 to B-4, Test Nos. C-1 to C-4) and inventive examples (Test Nos. D-1 to D-24). Table 1 shows the compositions of the steels used as raw materials for the continuously cast slabs used in the comparative examples (Test Nos. A-1 to A-6, Test Nos. B-1 to B-4, Test Nos. C-1 to C-4) and inventive examples (Test Nos. D-1 to D-24).

[0085] [Table 1]

[0086] The cooling conditions for the continuously cast slabs were (I) the total heat removal function from the continuous casting mold [-], (II) a residence time of 1450-1200°C [s], (III) an average cooling rate of 850-700°C [°C / hr], and (IV) an average cooling rate of 700-500°C [°C / hr]. The cooling conditions for each of these stages were appropriately changed. Tables 2-4 show the cooling conditions for the continuously cast slabs (I)-(IV), the microstructure of the resulting continuously cast slabs, and the evaluation of slab placement cracks. The measurement of the average prior austenite grain size, the calculation of the ferrite and pearlite area ratios, and the evaluation of slab placement cracks in the continuously cast slabs produced in the comparative examples and inventive examples were carried out as follows.

[0087] <Measurement of average prior austenite grain size> The average prior austenite grain size was measured as follows. A sample was cut from the center of the cooled slab's width, and the observation surface was a cross section of the slab's thickness parallel to the slab's width direction. The observation surface was then mirror-polished using diamond paste, then finish-polished using colloidal silica, and then etched with 3 vol.% nital to reveal the structure on the observation surface. Using an optical microscope, five fields of view were observed at 10 mm and 20 mm below the slab surface at 10x magnification to obtain structural images. The average prior austenite grain size was determined from the obtained structural images using a cutting method in accordance with JIS G 0551:2020.

[0088] <Method for measuring ferrite area ratio> The ferrite area ratio was measured in the same manner as in the measurement of the average prior austenite grain size. The observation surface was then mirror-polished using diamond paste, followed by finish polishing using colloidal silica and etching with 3 vol.% nital to reveal the structure. Ten fields of view were observed at 50x magnification at positions 10 mm and 20 mm below the slab surface using a scanning electron microscope (SEM) at an acceleration voltage of 15 kV. The obtained structure images were analyzed using Adobe Photoshop (registered trademark), and the ferrite area ratio was calculated for each of the 10 fields of view. The average of these values ​​was then calculated as the ferrite area ratio. Ferrite has a larger grain size than pearlite and other low-temperature transformation phases including bainite (bainite, tempered martensite, quenched martensite, and retained austenite), and has a smooth surface with dark contrast, so it can be easily distinguished at 50x magnification. It is difficult to strictly classify grain boundary ferrite and intragranular ferrite. Therefore, the inventors have thoroughly investigated slabs in which cracking occurred during slab placement and found that when the area ratio of ferrite is greater than 5% and less than 10%, a large amount of harmful ferrite is present at the grain boundaries. In other words, a structure in which the area ratio of ferrite is 5% or more but less than 10% is determined to be grain boundary ferrite.

[0089] <Method for measuring the area ratio of pearlite> The area ratio of the pearlite structure was measured in the same manner as in the above-mentioned ferrite measurement method, by revealing the structure on the observation surface of the slab. Using an SEM at an accelerating voltage of 15 kV, 10 fields of view were observed at a magnification of 10,000x, at positions 10 mm and 20 mm below the surface of the slab, with the ferrite removed from the field of view. The obtained structure images were then used in Adobe Photoshop (registered trademark) to calculate the area ratio of pearlite for each of the 10 fields of view, and these values ​​were averaged to obtain the area ratio of pearlite. Pearlite is a eutectoid crystal of ferrite and cementite, and when observed with the above-mentioned scanning electron microscope, it is a structure in which thin flake layers of both crystals exhibit a pearl-like luster.

[0090] The microstructure of the continuously cast slab according to the present invention is composed of ferrite, pearlite, and a low-temperature transformation phase. Therefore, the area S of the low-temperature transformation phase that constitutes the microstructure is X The area ratio of can be calculated by subtracting the sum of the area ratio of ferrite and the area ratio of pearlite from 100%, taking into account that the sum of the area ratio of ferrite, the area ratio of pearlite, and the area ratio of the low-temperature transformation phase is 100%.

[0091] <Evaluation of slab cracks> The evaluation method for slab placement cracks was based on the penetrant testing specified in JIS Z 2343:2017, and the presence or absence of cracks on the wide and narrow surfaces of the slab was evaluated. After applying the developer, the appearance of the penetrant was visually checked to check for slab placement cracks that had occurred on the surface. Furthermore, if there are cracks of 50 mm or more in length, there is a high risk of the slab breaking during slab handling or in the heating furnace, and there is also a high possibility that this could lead to hole formation problems during rolling. Therefore, the evaluation criteria for slab cracks in place were set as follows: · Slab placement cracks ○··· No cracks of 50mm or more in length on the slab surface · Slab cracks △··· There are no cracks of 50 mm or more in length on the surface of the slab, but defects have appeared after rolling. · Slab cracks ×··· Cracks on the surface of the slab that are 50 mm or longer

[0092] [Table 2]

[0093] [Table 3]

[0094] [Table 4]

[0095] <Comparative Examples (Test Nos. A-1 to A-4)> The slab microstructures satisfied by the continuously cast slabs produced in Tests A-1 to A-4 are designated Condition A. Condition A represents an example in which the ratio of the average prior austenite grain size 10 mm below the slab surface to the average prior austenite grain size 20 mm below the surface was greater than 4.0. In these cases, even if the conditions for slow cooling of the slab after it left the continuous slab casting machine were varied, differences in the prior austenite grain size resulted in a difference in the transformation rate, and the transformation timing at the positions 10 mm and 20 mm below the slab surface differed significantly. This resulted in the transformation stress concentrating on the slab surface, making it impossible to suppress slab cracking.

[0096] <Comparative Examples (Test Nos. B-1 to B-4)> The microstructure of the slabs satisfied by the continuously cast slabs produced in Tests B-1 to B-4 is designated Condition B. Condition B represents an example in which the ratio of the average prior austenite grain size 10 mm below the slab surface to the average prior austenite grain size 20 mm below the surface is 4.0 or less, but the amount of ferrite precipitation 10 mm below the slab surface is small, resulting in the precipitation of grain boundary ferrite. In these cases, the cooling rate between 700 and 500°C is small and the transformation stress generated is also small, but grain boundary embrittlement due to grain boundary ferrite occurs, making it impossible to suppress slab cracking.

[0097] <Comparative Examples (Test Nos. C-1 to C-4)> The slab microstructure satisfied by the continuously cast slabs produced in Test Nos. C-1 to C-4 is designated Condition C. Condition C is an example of a condition in which the ratio of the average prior austenite grain size 10 mm below the slab surface to the average prior austenite grain size 20 mm below the surface is 4.0 or less, but low-temperature transformed phases including bainite precipitated 20 mm below the slab surface, making it impossible to suppress slab cracking. In particular, because bainite transformation occurs at a lower temperature than pearlite transformation, the density difference with austenite is large and the transformation stress is also large. This stress is concentrated in the ferrite and pearlite regions where transformation has already been completed, which is thought to be why slab cracking could not be suppressed.

[0098] <Examples of the Invention (Test Nos. D-1 to D-24)> The slab microstructures satisfied by the continuously cast slabs produced in Test Nos. D-1 to D-24 are designated Condition D. Condition D is a condition for examples of the present invention, in which the average prior austenite grain size ratio is 4.0 or less, and the microstructure at a position 10 mm below the slab surface has a ferrite area ratio and pearlite area ratio of 80% or more in total, and the microstructure at a position 20 mm below the slab surface has a ferrite area ratio and pearlite area ratio of 60% or more in total. The continuously cast slabs produced in Test Nos. D-1 to D-24 did not experience slab placement cracking after cooling.

[0099] Figure 3 shows enlarged photographs taken with an optical microscope of a continuously cast slab produced in an example of the present invention (Test No. D-2) of the continuously cast slab according to the present invention. The enlarged photograph in Figure 3A shows the microstructure at a position 10 mm below the surface of the continuously cast slab. The enlarged photograph in Figure 3B shows the microstructure at a position 20 mm below the surface of the continuously cast slab.

[0100] As is clear from Figure 3, the continuously cast slab produced in the present invention example (Test No. D-2) of the continuously cast slab had controlled average prior austenite grain sizes at positions 10 mm below the slab surface and 20 mm below the slab surface, and it was found that the slab had a microstructure corresponding to the area ratio of ferrite at each position below the slab surface.

[0101] According to Tables 2 to 4 and Figure 3, (i) the average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab is d 10 The average prior austenite grain size at a position 20 mm below the surface of the continuously cast slab is defined as d 20 When the average prior austenite grain size ratio (d 20 / d 10 It has been found that slab placement cracking during slab cooling can be suppressed by (ii) making the sum of the area fractions of ferrite and pearlite in the microstructure 10 mm below the surface of the continuously cast slab 80% or more, and the sum of the area fractions of ferrite and pearlite in the microstructure 20 mm below the surface of the continuously cast slab 60% or more. The sum of the area fractions of ferrite, pearlite, and low-temperature transformation phases at positions 10 mm below the surface and 20 mm below the surface of the continuously cast slab is 100%.

[0102] That is, in the continuously cast slab of the present invention, the average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab is d 10 , the average prior austenite grain size at a position 20 mm below the surface of the continuously cast slab is d 20 When the average prior austenite grain size ratio (d 20 / d 10) is 1.0 or more and 4.0 or less, the microstructure at a position 10 mm below the surface of the continuously cast slab has a total area ratio of ferrite and area ratio of pearlite of 80% or more, and the microstructure at a position 20 mm below the surface of the continuously cast slab has a total area ratio of ferrite and area ratio of pearlite of 60% or more. Therefore, a slab for high alloy high strength steel that is free from slab cracking after casting can be provided, and problems such as perforation during rolling can also be prevented. [Industrial Applicability]

[0103] The continuously cast slab of the present invention has an average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab, and the average prior austenite grain size is d 10 , the average prior austenite grain size at a position 20 mm below the surface of the continuously cast slab is d 20 When the average prior austenite grain size ratio (d 20 / d 10 ) is 1.0 or more and 4.0 or less, the microstructure at a position 10 mm below the surface of the continuously cast slab has a ferrite area ratio and a pearlite area ratio of 80% or more in total, and the microstructure at a position 20 mm below the surface of the continuously cast slab has a ferrite area ratio and a pearlite area ratio of 60% or more in total. This makes it possible to provide a slab for high-strength steel that is free from cracks during slab placement after casting, and also makes it possible to prevent problems such as holes during rolling, which is industrially useful.

Claims

1. A continuous casting slab for high strength steel, In mass%, C: 0.10% or more and 1.00% or less, Si: 0.10% or more and 2.50% or less, Mn: 0.40% or more and 5.00% or less; Optionally, P: 0.100% or less, S: 0.0200% or less, Al: 0.100% or less, N: 0.0100% or less, and O: 0.0100% or less; Further, optionally, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, B: 0.0100% or less, Co: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.20 at least one element selected from the group consisting of Cr: 0.0100% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, either alone or in combination of two or more elements, with the balance being Fe and inevitable impurities; The average prior austenite grain size at a position 10 mm below the surface of the continuously cast slab is defined as d １０ The average prior austenite grain size at a position 20 mm below the surface of the continuously cast slab is defined as d ２０ When the average prior austenite grain size ratio (d ２０ / d １０ ) is 1.0 or more and 4.0 or less, The microstructure consists of ferrite, pearlite, and low-temperature transformation phases (excluding pearlite), or the microstructure is composed of the ferrite, the pearlite, the low-temperature transformation phase (excluding pearlite) and retained austenite, The low-temperature transformation phase (excluding pearlite) is composed of one or more of bainite, tempered martensite, and quenched martensite, The microstructure at a position 10 mm below the surface of the continuously cast slab has an area ratio of the ferrite and an area ratio of the pearlite totaling 80% or more, and the area ratio of the ferrite, the area ratio of the pearlite, and the area ratio of the low-temperature transformation phase totaling 100%; and A continuously cast slab characterized in that the microstructure at a position 20 mm below the surface of the continuously cast slab has a total area ratio of the ferrite and the pearlite of 60% or more, and the total area ratio of the ferrite, the pearlite, and the low-temperature transformation phase is 100%.

2. A method for producing a continuous casting slab for high strength steel, comprising: A continuously cast slab having the component composition according to claim 1, Total heat dissipation in the continuous casting mold Q ＭＤ satisfies the following relational expression (1), The temperature T of the continuously cast slab at the center in the width direction of the continuously cast slab and at a position 20 mm from the surface of the continuously cast slab ２０ a first cooling step of cooling the mixture under cooling conditions in which the temperature is in the range of 1200°C or higher and 1450°C or lower and the residence time is 230 seconds or shorter; The surface temperature T ０ a second cooling step in which the temperature is 700°C or higher and 850°C or lower at an average cooling rate of 20°C / hr or lower; The surface temperature T ０ and a third cooling step of cooling the slab under cooling conditions in which the average cooling rate at 500°C or higher and 700°C or lower is 10°C / hr or lower. [Equation 1] In the above relational formula (1), Q ＭＤ : Total heat dissipation in the continuous casting mold [MW / m ２ ], L ＭＤ : effective mold length [m], Vc: casting speed [m / min].

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