Continuously cast slab and manufacturing method therefor

Abstract

Provided are a continuously cast slab that exhibits no slab thermal cracking while being cooled even when the continuously cast slab has a low toughness; and a manufacturing method thereof. The continuously cast slab is a continuously cast slab for high-strength steel that has a given chemical composition, in which an average prior austenite grain size ratio (d20 / d10) is 1.0 to 4.0, provided that d10 is an average prior austenite grain size at a position 10 mm below a surface layer of the continuously cast slab and d20 is an average prior austenite grain size at a position 20 mm below the surface layer of the continuously cast slab, and in which a microstructure at the position 10 mm below the surface layer of the continuously cast slab is such that a total of an area ratio of ferrite and an area ratio of pearlite is 80% or more, and a microstructure at the position 20 mm below the surface layer of the continuously cast slab is such that a total of an area ratio of ferrite and an area ratio of pearlite is 60% or more, provided that a total of the area ratio of ferrite, the area ratio of pearlite, and an area ratio of a low-temperature transformation phase is 100%.

Description

Technical Field

[0001] The present invention relates to a continuously cast slab capable of preventing cracking when being cooled; and a manufacturing method thereof. More specifically, the invention relates to a continuously cast slab that is a continuously cast slab for high-strength steel (high tensile strength steel) and exhibits no operationally disruptive product defects at the time of performing rolling as it is effective in preventing slab thermal cracking; and a manufacturing method thereof.Background Art

[0002] In recent years, in the automotive field, for the sake of balancing further lightweighting of vehicle bodies and ensuring collision safety, high-strength steel is progressively required to possess an even higher strength and be highly alloyed accordingly. When high-strength steel is highly alloyed, the toughness of a slab deteriorates significantly.

[0003] As the toughness of a slab deteriorates due to high alloying, cracking at the time of cooling the slab, i.e., slab thermal cracking has come to occur frequently. When slab thermal cracking occurs, the slab will fracture when being conveyed, whereby the slab may then not be able to be subjected to hot rolling. Further, even if the slab does not fracture, the cracks of this slab will widen while performing hot rolling thereon, whereby the hot-rolled steel sheet may fracture.

[0004] Further, as a result of performing hot rolling, cold rolling, annealing, or plating on a slab having small cracks, these small cracks of the slab will then turn into surface defects such as scabs and sliver flaws on a steel sheet produced by carrying out these treatments.

[0005] The cracks on the surface of a slab are usually removed by a grinder. However, in the case of a highly alloyed slab, the slab has a deteriorated toughness due to its high alloying, whereby the cracks of this slab will develop due to the stress applied by the grinder, thus resulting in a situation where the cracks of the slab may not be able to be removed completely.

[0006] Meanwhile, small cracks of a slab may be overlooked and then emerge as surface defects on the hot-rolled, cold-rolled, annealed, or plated steel sheet. For these reasons, slab cracking needs to be suppressed.

[0007] Fig.1 is a magnified micrograph showing a fracture surface of a cracked portion of a slab for high-strength steel that has fractured due to slab thermal cracking, the micrograph being taken via an electron scanning microscope (SEM). As is clear from Fig. 1, the fracture surface of the slab cracked portion is such that there is shown a grain boundary fracture surface along the prior austenite grain boundary. In Fig.2, the cross-sectional surface of the slab cracked portion is shown as a structure micrograph. The depth of the slab crack(s) was mostly about 20 mm from the surface layer of the slab. The slab crack was found to be propagated in the vicinity of the prior austenite grain boundary, and grain boundary ferrite was present at the tip end of the slab cracked portion. Further, pearlite, or pearlite and bainite were observed in the prior austenite grains.

[0008] Grain boundary fracture occurs when prior austenite grains are coarse and the grain boundary embrittles. When grain boundary ferrite is generated, there will occur a difference in strength to the pearlite and bainite in the grains, whereby stress will be concentrated on the grain boundary ferrite portion with a low strength such that slab cracks will develop even under a lower stress.

[0009] Meanwhile, when a slab is cooled, there will occur a stress attributed to differences in heat contraction and transformation expansion between the surface of the slab and the inner region thereof. When this stress is large, slab cracking will occur when the slab is cooled to room temperature. As for the highalloy and high-strength steel of recent years, due to a high quenching property as well as the coarse prior austenite grain size of the slab, the precipitation of a low-temperature transformation phase (e.g., bainite, martensite) cannot be suppressed with the conventional slow cooling process.

[0010] In addition, also due to the low slab toughness, it is difficult to remove deep slab cracks that have occurred in such a manner via maintenance such as one using a grinder, which has led to a problem of significantly lowering slab yield.

[0011] In this regard, there has been proposed a method of suppressing the occurrence of thermal cracking of slabs for high-tensile steel. For example, Patent Literature 1 proposes a method where slow cooling is performed in a temperature region of 700 to 500°C which is a region in which austenite transforms into ferrite, thereby suppressing bainite / martensite transformation so as to reduce the stress caused by the related transformation expansion. That is, Patent Literature 1 discloses a method that is capable of suppressing the occurrence of slab thermal cracking even in the case of a high-tensile steel type that is likely to exhibit thermal cracking. Specifically, the high-tensile steel slab cooling method disclosed in Patent Literature 1 is a method of suppressing the occurrence of thermal cracking by controlling the cooling rate of the slab in accordance with the length(s) of internal cracks that have occurred in the high-tensile steel, based on the finding that the internal stress of a high-tensile steel depends on its cooling rate.

[0012] Further, disclosed in Patent literate 2 is a method where slow cooling of the slab is started immediately after it was casted, and the slab is held at a temperature of 700°C or higher for 10 hours or longer before being further slowly cooled from 700 to 500°C, whereby the stress of the slab that occurs when there is a temperature difference and / or transformation can be reduced.

[0013] That is, disclosed in Patent Literature 2 is a method for cooling a slab for a high-strength steel sheet, that is capable of preventing quality defects such as scabs at the time of performing hot rolling other than slab cracking that occurs while cooling the slab, even when the slab is of a composition containing Si. Specifically, the high-strength steel sheet slab cooling method disclosed in Patent Literature 2 is a method in which a continuously cast slab of a high-strength hot-rolled steel sheet with specified contents of chemical components such as C, Si, and Mn is cooled at an average cooling rate of 20°C / hr or less in the range of 500 to 700°C.Citation ListPatent Literature

[0014] Patent Literature 1: JP-A-2020-139209 Patent Literature 2: JP-A-2019-167560 Summary of InventionTechnical Problem

[0015] However, the above conventional techniques have the following problems. The method disclosed in Patent Literature 1, which is a method of cooling a slab for high-tensile steel after casting, is such that when performing cooling after casting the slab, the internal stress occurring in the slab is controlled to be small by merely focusing on the slab temperature range from 700 to 500°C. For this reason, the occurrence of slab thermal cracking cannot be suppressed sufficiently by producing a slab with a heightened carbon content using the high-tensile steel slab cooling method disclosed in Patent Literature 1.

[0016] Further, the high-strength steel sheet slab cooling method disclosed in Patent Literature 2 is such that slab cracking is suppressed by focusing on a reduction in heat stress based on the finding that slab cracking is attributed to a heat stress that is caused by Si addition to the steel and the temperature unevenness in the slab. However, with regard to the high-strength steel sheet slab cooling method disclosed in Patent Literature 2, no limitation is imposed on the microstructure of the slab at all. Thus, the occurrence of slab thermal cracking cannot be suppressed sufficiently even when the slab is produced using the high-strength steel sheet slab cooling method disclosed in Patent Literature 2.

[0017] Further, as a result of diligently conducting a series of studies, the inventors of the present invention found that a slab of the conventional art that contains a large amount(s) of C, Si, and Mn has a tremendously low toughness so much so that slab thermal cracking cannot be suppressed completely, and there will occur hole defect troubles when performing rolling.

[0018] The present invention was made in view of these circumstances, and it is an object of the present invention to provide a continuously cast slab that exhibits no slab thermal cracking while being cooled and exhibits no hole defect troubles at the time of performing rolling, even when the continuously cast slab itself has a low toughness; and a manufacturing method thereof.Solution of Problem

[0019] The inventors of the present invention heartily carried out a number of studies to achieve the above object. As a result, by analyzing the fracture morphology of slab cracking, they found that at least one of a grain boundary fracture surface along the prior austenite grain boundary and an intragranular fracture surface (cleavage fracture surface) crossing the prior austenite grain boundary was present on the fracture surface thereof. Further, while the microstructure at the position where the slab cracking is extended is a structure mainly composed of ferrite and pearlite, the internal microstructure therebeyond was a structure mainly composed of bainite.

[0020] Here, the structure mainly composed of bainite refers to a structure containing at least one selected from the group consisting of bainite, tempered martensite, quenched martensite, and residual austenite. That is, the structure mainly composed of bainite means a microstructure that mainly contains bainite and may also contain at least one selected from the group consisting of quenched martensite, tempered martensite, and residual austenite that are unavoidable structures. The structure mainly composed of bainite is a low-temperature transformation phase undergoing phase transformation at a temperature lower than that of ferrite and pearlite that are contained in the microstructure.

[0021] In fact, since an object is cooled from the surface thereof, it is impossible for a low-temperature transformation phase to be precipitated thereinside. However, since the prior austenite grain size in a slab is coarse, such prior austenite grain size shall have a great impact on the transformation time of the phase composing the microstructure. That is, the inventors found that when the prior austenite grain size inside the slab is larger than the prior austenite grain size on the slab surface layer, the structure(s) precipitated in such slab as well as the timing for such structure to be precipitated were different even when cooling the slab in the same manner.

[0022] In this regard, the inventors found that at the timing when heat contraction occurs after the slab surface layer has finished undergoing transformation, a tensile stress will occur on the surface of the slab surface layer as the low-temperature transformation phase inside the slab undergoes transformation expansion, whereby such tensile stress will then lead to slab cracking. Further, the inventors conducted a number of detailed studies and arrived at the present invention based on the following findings. Specifically, they found that slab thermal cracking during the cooling process of a continuously cast slab can be suppressed, and hole defect troubles at the time of performing rolling can be prevented from happening, by controlling the microstructure of the continuously cast slab and reducing the stress that occurs when the low-temperature transformation phase inside the slab undergoes transformation.

[0023] That is, the continuously cast slab of the present invention that is capable of advantageously solving the above problems is: (a) a continuously cast slab for high-strength steel that contains, by mass%, C: 0.10 to 1.00%, Si: 0.10 to 2.50%, and Mn: 0.40 to 5.00%, and optionally contains, by mass%, 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 further optionally contains, by mass%, at least one or two or more elements selected from the group consisting of 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, with a balance being Fe and unavoidable impurities, wherein: an average prior austenite grain size ratio (d 20 / d 10 ) is between 1.0 and 4.0, provided that d 10 is an average prior austenite grain size at a position 10 mm below a surface layer of the continuously cast slab and d 20 is an average prior austenite grain size at a position 20 mm below the surface layer of the continuously cast slab; a microstructure of the continuously cast slab consists of ferrite, pearlite, and a low-temperature transformation phase, the low-temperature transformation phase containing at least one selected from the group consisting of bainite, tempered martensite, quenched martensite, and residual austenite; the microstructure at the position 10 mm below the surface layer of the continuously cast slab is such that a total of an area ratio of the ferrite and an area ratio of the pearlite is 80% or more, provided that a total of the area ratio of the ferrite, the area ratio of the pearlite, and an area ratio of the low-temperature transformation phase is 100%; and the microstructure at the position 20 mm below the surface layer of the continuously cast slab is such that a total of an area ratio of the ferrite and an area ratio of the pearlite is 60% or more, provided that a total of the area ratio of the ferrite, the area ratio of the pearlite, and an area ratio of the low-temperature transformation phase is 100%.

[0024] Further, the continuously cast slab manufacturing method of the present invention is a method for manufacturing a continuously cast slab for high-strength steel. This method is to manufacture the continuously cast slab according to (a), and includes: a first cooling step in which the continuously cast slab having the chemical composition as set forth in (a) is cooled under a condition where a total heat flux Q MD in a continuous casting mold satisfies the following relational expression (1), and a temperature T 20 of the continuously cast slab at a position that is located in a central part of the continuously cast slab along a width direction thereof and is 20 mm away from the surface layer of the continuously cast slab resides in a temperature range of 1,200 to 1,450°C for a residence time of 230 s or less; a second cooling step in which the continuously cast slab is cooled at an average cooling rate of 20°C / hr or less when a surface temperature T 0 of the continuously cast slab surface layer at a position that is located in a central part of the continuously cast slab along the width direction thereof is in a range of 700 to 850°C; and a third cooling step in which the continuously cast slab is cooled at an average cooling rate of 10°C / hr or less when the surface temperature T 0 of the continuously cast slab surface layer at the position that is located in the central part of the continuously cast slab along the width direction thereof is in a range of 500 to 700°C. [Expression 1] Q MD × L MD Vc ≦ 1.5

[0025] In the above expression (1), Q MD represents a total heat flux [MW / m 2< ] in a continuous casting mold, L MD represents a mold effective length [m], and V C represents a casting rate [m / min].Advantageous Effects of Invention

[0026] According to the present invention, there can be provided a continuously cast slab that exhibits no slab thermal cracking during the cooling process and exhibits no hole defect troubles at the time of performing rolling, even when the slab has a component system of a continuously cast slab for high-strength steel.Brief Description of Drawings

[0027] [Fig.1] is a micrograph taken by an electron scanning microscope (SEM), that shows a fracture surface of a cracked portion of a continuously cast slab for high-strength steel that has fractured due to thermal cracking. [Fig.2] is a cross-sectional structure micrograph that shows the above cracked portion. [Fig.3A] is an observational magnified micrograph taken via an optical microscope, that shows a continuously cast slab produced in an invention example (test No. D-2) of a continuously cast slab of an embodiment of the present invention; this micrograph is a magnified micrograph showing a microstructure at a position 10 mm below the surface layer of the continuously cast slab. [Fig.3B] is an observational magnified micrograph taken via an optical microscope, that shows the continuously cast slab produced in the invention example (test No. D-2) of the continuously cast slab of the embodiment of the present invention; this micrograph is a magnified micrograph showing a microstructure at a position 20 mm below the surface layer of the continuously cast slab. Description of Embodiments

[0028] Embodiments of the present invention are described in detail hereunder. Here, each drawing is schematic, and the embodiments shown therein may thus differ from those in reality. Further, the following embodiments are sets of examples of devices and / or methods embodying the technical concept of the present invention and are not to limit the configuration of the present invention to those shown below. That is, various modifications can be made to the technical concept of the present invention within the technical scope described in the claims.[First embodiment]

[0029] A continuously cast slab of a first embodiment will be described. The continuously cast slab of this embodiment is a continuously cast slab for high-strength steel that is characterized as follows. The continuously cast slab of this embodiment contains, by mass%, C: 0.10 to 1.00%, Si: 0.10 to 2.50%, and Mn: 0.40 to 5.00%, and optionally contains, by mass%, 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 further optionally contains, by mass%, at least one or two or more elements in combination selected from the group consisting of 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, with a balance being Fe and unavoidable impurities. Further, the continuously cast slab of this embodiment is characterized in that: (i) an average prior austenite grain size ratio (d 20 / d 10 ) is 1.0 to 4.0, provided that d 10 is an average prior austenite grain size at a position 10 mm below a surface layer of the continuously cast slab and d 20 is an average prior austenite grain size at a position 20 mm below the surface layer of the continuously cast slab; (ii) a microstructure of the continuously cast slab consists of ferrite, pearlite, and a low-temperature transformation phase, the low-temperature transformation phase contains at least one selected from the group consisting of bainite, tempered martensite, quenched martensite, and residual austenite, the microstructure at the position 10 mm below the surface layer of the continuously cast slab is such that a total of an area ratio of the ferrite and an area ratio of the pearlite is 80% or more, provided that a total of the area ratio of the ferrite, the area ratio of the pearlite, and an area ratio of the low-temperature transformation phase is 100%, and the microstructure at the position 20 mm below the surface layer of the continuously cast slab is such that the total of the area ratio of the ferrite and the area ratio of the pearlite is 60% or more, provided that the total 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%.

[0030] That is, by specifying the composition of the components contained in the continuously cast slab and imparting at least the aforementioned features (i) and (ii), the continuously cast slab for high-strength steel according to the present embodiment can exhibit high yield, even in recent cases where the continuously cast slab for high-strength steel demonstrates extremely low toughness, the slab does not undergo thermal cracking during cooling, nor does it generate defects such as hole formation during rolling.

[0031] First, an appropriate range of the microstructure of the continuously cast slab and the reasons for limiting such a range will be described. In the following description, the symbol "%" representing a constitutional ratio in the microstructure means "area %" unless otherwise stated. It is assumed that the microstructure of the continuously cast slab has been observed at room temperature.

[0032] As described above, as a result of observing the fracture morphology of fracture surfaces of cracked portions of continuously cast slabs for high-strength steel that had fractured due to slab thermal cracking, it was found that slab thermal cracking in many cases had developed to about 20 mm below the surface layer of the slab, that the morphology observed was "grain boundary fracture" where cracking had developed into the prior austenite crystal grain boundary, and that the microstructure of the cracked portion(s) was mainly composed of ferrite and pearlite, whereas the microstructure beyond the crack toward the inner region (center side in the thickness direction of the slab) was mainly composed of bainite.

[0033] That is, in a continuously cast slab for high-strength steel, it is considered that slab thermal cracking caused by fracture in the crystal grain boundary is attributed to the fact that the prior austenite grain size is coarse, and the fact that the microstructure of the slab surface layer is different from that of the slab inner region whereby a low-temperature transformation phase is precipitated in the slab inner region. When the prior austenite grain size is coarse, grain boundary embrittlement is likely to occur due to, for example, grain boundary segregation and precipitation of grain boundary ferrite, thus contributing to slab thermal cracking. Further, since the microstructure of the slab surface layer and the microstructure of the slab inner region are different, when a low-temperature transformation phase is precipitated in the slab inner region, the transformation expansion of the low-temperature transformation phase will contribute to slab thermal cracking.

[0034] In this regard, as conditions required for a continuously cast slab for high-strength steel that does not exhibit slab thermal cracking during the cooling process, the invention of this embodiment focuses on the following two elements on the premise that the composition of the components contained in the continuously cast slab is limited: (i) an average prior austenite grain size ratio calculated from average prior austenite grain sizes at multiple locations that are set from the surface layer of the continuously cast slab; and (ii) the microstructure of the continuously cast slab.<(i) Average prior austenite grain size ratio>

[0035] The continuously cast slab for high-strength steel of this embodiment is a continuously cast slab for high-strength steel and is characterized in that (i) an average prior austenite grain size ratio (d 20 / d 10 ) is 1.0 to 4.0, provided that d 10 is an average prior austenite grain size at a position 10 mm below the surface layer of the continuously cast slab and d 20 is an average prior austenite grain size at a position 20 mm below the surface layer of the continuously cast slab. Here, an average prior austenite grain size is a value obtained by averaging the values of a plurality of prior austenite grain sizes that are measured in a plurality of fields of view.

[0036] In a conventional continuously cast slab, the average prior austenite grain size is extremely large, which is of a size of serval mm. For this reason, the toughness of such continuously cast slab is significantly impaired. In addition, average prior austenite grain size also has a great impact on the transformation behavior of a continuously cast slab's microstructure; the larger the prior austenite grain size is, the more the transformation starting time will shift toward the longtime side. As a result, a low-temperature transformation phase is likely to be precipitated in such microstructure even when slowly cooling the continuously cast slab. Moreover, the larger the differences of the average prior austenite grain sizes are, the more likely the differences in microstructure between the slab surface layer and the slab inner region will occur.

[0037] From such technical perspective, the continuously cast slab of this embodiment is such that the average prior austenite grain size ratio (d 20 / d 10 ) is set to 1.0 to 4.0, provided that d 10 is the average prior austenite grain size at the position 10 mm below the surface layer of the continuously cast slab and d 20 is the average prior austenite grain size at the position 20 mm below the surface layer of the continuously cast slab.

[0038] As such, the upper limit of the average prior austenite grain size ratio (d 20 / d 10 ) is preferably 4.0. This is because if the average prior austenite grain size ratio (d 20 / d 10 ) is 4.0 or less, the differences in microstructure between the slab surface layer and the slab inner region can be made less significant.

[0039] Meanwhile, there is no strict limitation on the lower limit of the average prior austenite grain size ratio (d 20 / d 10 ). However, a special cooling control is required to make the average prior austenite grain size in the slab inner region smaller than the average prior austenite grain size of the slab surface layer, and equipment investments need to be made in order to perform such special cooling control. Thus, the average prior austenite grain size ratio (d 20 / d 10 ) is preferably 1.0 or more. Here, the lower limit of the average prior austenite grain size ratio (d 20 / d 10 ) is more preferably 1.2, even more preferably 1.5.

[0040] Here, the reason that the position 10 mm below the surface layer of the continuously cast slab and the position 20 mm below the surface layer of the continuously cast slab were chosen when setting the average prior austenite grain size(s) was because it is considered that since slab thermal cracking in many cases develops to about 20 mm below the slab surface layer, the position 20 mm below the surface layer of the continuously cast slab and the position 10 mm below the surface layer of the continuously cast slab, which is a position located in between the surface layer of the continuously cast slab and the position 20 mm below the surface layer of the continuously cast slab, are positions required to suppress slab thermal cracking.

[0041] Meanwhile, a region that is less than 5 mm below the surface layer of the continuously cast slab is directly and rapidly cooled by the mold or a water sprayer provided immediately below the mold.

[0042] Further, the region that is less than 5 mm below the surface layer of the continuously cast slab has a fine structure where the grains composing the continuously cast slab are those of γ grain size, which imparts a high toughness to the continuously cast slab.

[0043] Thus, it is hardly conceivable that a start point of slab thermal cracking occurs from the region that is less than 5 mm below the surface layer of the continuously cast slab.

[0044] From such technical perspective, in the case of the continuously cast slab of this embodiment, the region that is less than 5 mm below the surface layer of the continuously cast slab can be excluded from positions in the continuously cast slab that require microstructure control.

[0045] Therefore, the positions in the continuously cast slab that require structure control are a first position, which is a position that is located inside the slab and is at a depth of 20 mm into the slab thickness direction; and a second position, which is a position that is located in the slab surface layer and is at a depth of 10 mm into the slab thickness direction.

[0046] That is, as a first position below the surface layer of the continuously cast slab, with the position inside the slab that is 20 mm from the surface layer of the continuously cast slab being a reference, there may be employed, for example, average prior austenite grain sizes at 18 to 22 mm, and 15 to 25 mm in the depth direction from the surface layer of the continuously cast slab.

[0047] Further, as a second position below the surface layer of the continuously cast slab, with the position inside the slab that is 10 mm from the surface layer of the continuously cast slab being a reference, there may be employed, for example, average prior austenite grain sizes at 8 to 12 mm, and 5 to 15 mm in the depth direction from the surface layer of the continuously cast slab.

[0048] As for the continuously cast slab of this embodiment, a factor determining the average prior austenite grain size is the temperature when cooling the continuously cast slab. Austenite grains exhibit a high growth rate particularly in a temperature range of 1,200 to 1,450°C. Thus, the temperature for cooling the continuously cast slab is particularly of a range of 1,200 to 1,450°C, and a cooling rate and residence time of the continuously cast slab with regard to such temperature range shall have an impact.

[0049] In other words, in the temperature range of 1,200 to 1,450°C, the slower the cooling rate of the continuously cast slab is, or the longer the residence time of the continuously cast slab is, the coarser the average prior austenite grain size will become. The prior austenite grain size of a continuously cast slab is such that while the grain size on the slab surface layer side is fine, it becomes coarser toward the inner region of the slab. In order to minimize the difference between the average prior austenite grain size on the slab surface layer side and the average prior austenite grain size in the inner region of the slab, it is required that the average prior austenite grain size on the slab surface layer side be coarsened, and that the average prior austenite grain size in the slab inner region be made finer.

[0050] That is, in order for the continuously cast slab of this embodiment to meet the condition (i), it is critical to manufacture the continuously cast slab in the following manner. Specifically, it is critical to control cooling when temperatures T 10 and T 20 are in a range of 1,200 to 1,450°C; the temperatures T 10 and T 20 are temperatures of the continuously cast slab at the positions that are located in the central part of the continuously cast slab along the width direction thereof and are respectively 10 mm and 20 mm from the surface layer thereof. Here, the condition (i) is the condition where the average prior austenite grain size ratio (d 20 / d 10 ) is 1.0 to 4.0, provided that d 10 is the average prior austenite grain size at the position 10 mm below the surface layer of the continuously cast slab and d 20 is the average prior austenite grain size at the position 20 mm below the surface layer of the continuously cast slab. In short, T 10 and T 20 are temperatures at the positions 10 mm and 20 mm below the surface layer of the continuously cast slab, respectively.

[0051] In order to control the cooling rate when the temperature T 10 at the position 10 mm from the surface layer of the continuously cast slab is in the range of 1,200 to 1,450°C, a total heat flux in a continuous casting mold needs to be taken into consideration during a manufacturing step(s) of the continuously cast slab of this embodiment.

[0052] That is, since the temperature T 10 at the position 10 mm from the surface layer of the continuously cast slab is in the range of 1,200 to 1,450°C when the continuously cast slab passes through the continuous casting mold, it is necessary to cool the continuously cast slab so that the total heat flux in the continuous casting mold satisfies the following relational expression (1) in view of a relation among a total heat flux per unit area Q MD [MW / m 2< ] in a continuous casting mold, a mold effective length L MD [m], and a casting rate V C [m / min]. [Expression 2] Q MD × L MD Vc ≦ 1.5

[0053] In the above relational expression (1), Q MD represents a total heat flux [MW / m 2< ] in a continuous casting mold, L MD represents a mold effective length [m], and V C represents a casting rate [m / min].

[0054] The total heat flux Q MD in the continuous casting mold is calculated in such a manner that a total heat extraction is first calculated from the flow rate of a cooling water flowing into the continuous casting mold and from a difference in temperature between the inlet of the continuous casting mold and the outlet thereof, followed by dividing the calculated total heat extraction by a contact area between the mold copper plate composing the continuous casting mold and the cast slab.

[0055] The mold effective length L MD is a length of a mold (mold length) by which the molten steel poured from the tundish is able to solidify. Although depending on the type of the continuous casting mold, the mold effective length L MD is preferably 0.7 to 0.9 [m], for example.

[0056] The casting rate V C is a rate at which the molten steel is poured into the continuous casting mold. The casting rate V C is preferably 0.8 to 2.0 [m / min], for example.

[0057] The relationship among the total heat flux Q MD [MW / m 2< ] in the continuous casting mold, mold effective length L MD [m], and casting rate V C [m / min] is defined by the above relational expression (1), and the value calculated by the relational expression (1) is preferably 0.6 to 1.5. It is preferred if the value calculated by the relational expression (1) is 1.5 or less, because the average prior austenite grain size at the position 10 mm below the surface layer of the continuously cast slab can be coarsened, whereby its difference to the average prior austenite grain size in the slab inner region can be made smaller.

[0058] Meanwhile, it is preferred if the value calculated by the relational expression (1) is 0.6 or larger, because the shell thickness of the continuously cast slab on the exit side of the continuous casting mold can be secured, and the average prior austenite grain size of the continuously cast slab can be coarsened without the risk of causing breakout.

[0059] In order to control the residence time during which the temperature T 20 at the position 20 mm away from the surface layer of the continuously cast slab is in the range of 1,200 to 1,450°C, cooling by spray water cooling needs to be controlled in the manufacturing step(s) of the continuously cast slab of this embodiment.

[0060] That is, the temperature T 20 at the position 20 mm away from the surface layer of the continuously cast slab is in the range of 1,200 to 1,450°C when the corresponding continuously cast slab passes through the secondary cooling zone that is located immediately below the continuous casting mold. Therefore, it is preferred that the amount of the cooling water used in the secondary cooling zone be controlled so that the residence time during which the temperature T 20 at the position 20 mm away from the surface layer of the continuously cast slab is in the range of 1,200 to 1,450°C can be set to 230 s or shorter.

[0061] It is preferable if the total heat flux Q MD in the continuous casting mold satisfies the above relational expression (1), and the residence time during which the temperature T 20 at the position 20 mm away from the surface layer of the continuously cast slab is in the range of 1,200 to 1,450°C is 230 s or shorter. This is because if this is the case, slab thermal cracking can be suppressed as the average prior austenite grain size ratio (d 20 / d 10 ), which is the ratio between the average prior austenite grain size d 10 at the position 10 mm away from the surface layer of the continuously cast slab and the average prior austenite grain size d 20 at the position 20 mm below the surface layer of such slab, can be made 4.0 or less.

[0062] From such perspective, the residence time of the continuously cast slab is preferably 220 s or shorter, more preferably 210 s or shorter, even more preferably 200 s or shorter. Here, while there are no particular limitations on the lower limit of the residence time of the continuously cast slab, the residence time is set to 60 s or longer as an excessively short residence time will lead to a higher risk of breakout in continuous casting that is caused by inhomogeneous solidification.

[0063] That is, the residence time of the continuously cast slab is preferably 60 s or longer, because if the residence time of the continuously cast slab is shorter than 60 s, cracking will occur in the continuously cast slab due to an inhomogeneous solidification of the initially solidified shell, which will lead to the risk of breakout.

[0064] Here, from such perspective, the residence time of the continuously cast slab during which the temperature T 20 at the position 20 mm away from the surface layer of the continuously cast slab is in the range of 1,200 to 1,450°C is more preferably 80 s or longer, even more preferably 90 s or longer.

[0065] The cooling rate and residence time of the continuously cast slab with regard to the temperature range of 1,200 to 1,450°C of the temperatures T 10 and T 20 at the positions 10 mm and 20 mm away from the surface layer of the continuously cast slab can be controlled by adjusting the cooling conditions in the initial stages of slab casting. For example, in continuous casting of steel, a molten steel whose chemical composition has been adjusted is at first charged into a water-cooled copper mold to generate an initially solidified shell. Next, the extraction of a continuously cast slab from the water-cooled copper mold is started; after the continuously cast slab has exited the water-cooled copper mold, cooling of such continuously cast slab is conducted by water spray.

[0066] The temperature T 10 at the position 10 mm below the surface layer of the continuously cast slab is largely affected by cooling performed in a continuous casting mold. Thus, for example, the thermal conductivity of a mold flux used to lubricate the inner side of the continuous casting mold may be reduced, or the amount of the cooling water for the continuous casting mold may be reduced.

[0067] Meanwhile, the temperature T 20 at the position 20 mm below the surface layer of the continuously cast slab is largely affected by cooling performed immediately below the continuous casting mold. Thus, for example, this temperature can be controlled by increasing the flow rate of water spray that is immediately below the continuous casting mold. When the spraying performed immediately below the continuous casting mold is a two-fluid spraying composed of water and air, this temperature can also be controlled by increasing the amount of water flow and the amount of air flow.

[0068] By controlling these cooling conditions, the average prior austenite grain sizes at the positions 10 mm and 20 mm away from the surface layer of the continuously cast slab can be controlled.

[0069] Here, it is difficult to actually measure the temperature(s) inside a continuously cast slab. Thus, the temperatures T 10 and T 20 of the continuously cast slab can be estimated by calculating the temperature history at each of the positions 10 mm and 20 mm below the surface layer of the continuously cast slab via heat-transfer analysis. In order to achieve a longest residence time with regard to the above temperature range even in the inner region of the continuously cast slab, a heat-transfer analysis position(s) can be set to the center of the slab along its width direction.<(ii) Microstructure of continuously cast slab>

[0070] The continuously cast slab of this embodiment is characterized in that (ii) the microstructure at the position 10 mm below the slab surface layer is such that the total of the area ratio of ferrite and the area ratio of pearlite is 80% or more, and the microstructure at the position 20 mm below the slab surface layer is such that the total of the area ratio of ferrite and the area ratio of pearlite is 60% or more. That is, in addition to the characteristic that (i) the average prior austenite grain size ratio (d 20 / d 10 ), which is the ratio between the average prior austenite grain size d 10 at the position 10 mm below the surface layer of the continuously cast slab and the average prior austenite grain size d 20 at the position 20 mm below the surface layer of the continuously cast slab, is 4.0 or less, it is known that the ratio of the inner structure(s) composing the microstructure of the continuously cast slab, such as bainite and ferrite, is also a factor determining the fracture unit(s) of the slab, and that a stress applied to the slab changes depending on such ratio.

[0071] Particularly, when a low-temperature transformation phase(s) as an unavoidable structure that mainly contains bainite and contains quenched martensite, tempered martensite, and residual austenite occurs in the microstructure of the slab inner region, not only its expansion amount associated with the transformation of such low-temperature transformation phase will be large, but the ferrite and pearlite formed around the low-temperature transformation phase containing bainite and the like have already finished undergoing transformation and will undergo heat contraction thereafter, whereby a significant level of stress concertation will occur therein.

[0072] In this regard, the inventors found that the transformation stress applied to the surface layer of a slab can be reduced by controlling the cooling rate so that (ii) the microstructure at the position 10 mm below the surface layer of the continuously cast slab will be such that the total of the area ratio of ferrite and the area ratio of pearlite is 80% or more, and the microstructure at the position 20 mm below the surface layer of the continuously cast slab will be such that the total of the area ratio of ferrite and the area ratio of pearlite is 60% or more.

[0073] The low-temperature transformation phase as a microstructure other than ferrite and pearlite is mainly composed of bainite; the component(s) thereof are not only limited to bainite, but the low-temperature transformation phase may also contain quenched martensite, tempered martensite, and residual austenite in addition to bainite.

[0074] It should be noted that the area ratio of ferrite and the area ratio of pearlite can be calculated based on an observation result of the continuously cast slab's microstructure, using an observational means such as an optical microscope and an electronic microscope. Further, ferrite and pearlite contained in the microstructure of the continuously cast slab can be identified using an observational means such as an optical microscope and an electronic microscope.

[0075] Based on the identification result of the continuously cast slab's microstructure, calculated are an area S total of the microstructure of the continuously cast slab; and an area S (ferrite+pearlite) which is a total of a ferrite area S ferrite and a pearlite area S pearlite . Next, calculated is a ratio of the area S (ferrite+pearlite) which is the total of the ferrite area S ferrite and the pearlite area S pearlite to the area S total of the microstructure of the continuously cast slab, the ratio being defined as area ratio (%).

[0076] The microstructure of the continuously cast slab consists of ferrite, pearlite, and the low-temperature transformation phase; and a total of the area ratio of ferrite (%), the area ratio of pearlite (%), and the area ratio of the low-temperature transformation phase (%) is 100%. Thus, an area ratio (%) as a ratio of an area S X of the low-temperature transformation phase can be calculated by subtracting, from 100%, an area ratio obtained by adding together the area ratio of ferrite (%) and the area ratio of pearlite (%).

[0077] The continuously cast slab of this embodiment is characterized in that (ii) the microstructure at the position 10 mm below the surface layer of the continuously cast slab is such that the total of the area ratio of ferrite and the area ratio of pearlite is 80% or more, provided that the total 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%; and the microstructure at the position 20 mm below the surface layer of the continuously cast slab is such that the total of the area ratio of ferrite and the area ratio of pearlite is 60% or more, provided that the total 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%.

[0078] That is, in the case of the continuously cast slab of this embodiment, it is preferred if, (ii) with regard to the microstructure at the position 10 mm below the surface layer of the continuously cast slab, an area ratio (%) as a ratio of an area S10 (ferrite+pearlite) obtained by adding together an area S10 ferrite of ferrite and an area S10 pearlite of pearlite to an area S10 total of the microstructure at such position is 80% or more; and with regard to the microstructure at the position 20 mm below the surface layer of the continuously cast slab, an area ratio (%) as a ratio of an area S20 (ferrite+pearlite) obtained by adding together an area S20 ferrite of ferrite and an area S20 pearlite of pearlite to an area S20 total in the microstructure at such position is 60% or more. Because, in such case, slab thermal cracking of the continuously cast slab can be suppressed as heat stress and transformation stress caused by slab slow cooling that occurs on the surface layer of the slab can be alleviated.

[0079] When the area ratio of ferrite is not less than 5% but less than 10%, a small amount of grain boundary ferrite is precipitated, which will worsen the degree of thermal cracking of the slab as the grain boundary strength is now degraded. Thus, as are the cases with S10 ferrite and S20 ferrite , it is preferred that these area ratios of ferrite be set to less than 5% or not less than 10%.

[0080] Meanwhile, it is not preferable if the microstructure at the position 10 mm below the surface layer of the continuously cast slab is such that the total of the area ratio of ferrite and the area ratio of pearlite is less than 80%, because slab thermal cracking will occur as the stress at the time when the low-temperature transformation phase undergoes cooling and transformation cannot be absorbed.

[0081] It is not preferable either if the microstructure at the position 20 mm below the surface layer of the continuously cast slab is such that the total of the area ratio of ferrite and the area ratio of pearlite is less than 60%, because the transformation expansion of the low-temperature transformation phase will cause an excessively large tensile stress on the slab surface layer that has already finished undergoing transformation, which will result in the occurrence of slab thermal cracking of the continuously cast slab.

[0082] Here, the microstructure at the position 10 mm below the surface layer of the continuously cast slab consists of ferrite, pearlite, and the low-temperature transformation phase, and the total of the area ratio of the ferrite area S10 ferrite , the area ratio of the pearlite area S10 pearlite , and the area ratio of the low-temperature transformation phase area S10 x is 100%. Similarly, the microstructure at the position 20 mm below the surface layer of the continuously cast slab consists of ferrite, pearlite, and the low-temperature transformation phase, and the total of the area ratio of the ferrite area S20 ferrite , the area ratio of the pearlite area S20 pearlite , and the area ratio of the low-temperature transformation phase area S20 x is 100%.

[0083] Here, ferrite contains iron that contains up to 0.02% by mass of carbon; the structure of ferrite is close to that of pure iron. Ferrite is the softest and superior in ductility among iron and steel structures. Pearlite is a structure obtained as a result of slowly cooling austenite. Pearlie consists of ferrite layers and cementite layers and is formed in such a manner that these layers are alternately aligned.

[0084] In order to control slab thermal cracking, it is also critical to control the microstructure of a continuously cast slab in addition to reducing heat and transformation stress by cooling the slab. The precipitation of ferrite in the microstructure can be controlled by controlling a cooling rate in a region of not higher than 850°C to not lower than 700°C, which is the ferrite transformation region. Specifically, by lowering the cooling rate of a continuously cast slab in the ferrite transformation region, the amount of ferrite precipitated can be increased, whereby the ferrite precipitated from the grain boundary that is considered as problematic in terms of stress concentration can be made harmless.

[0085] Further, when suppressing slab thermal cracking, it is also critical to reduce the stress occurring at the time when the low-temperature transformation phase undergoes transformation in addition to suppressing the embrittlement of the prior austenite grain boundary. In addition to controlling the average prior austenite grain sizes at the multiple given positions that are located away from the surface layer of the continuously cast slab, by also variously controlling the cooling rate in the pearlite transformation region (not higher than 700°C to not lower than 500°C), the microstructure of the continuously cast slab can be controlled, whereby the stress occurring at the time when the low-temperature transformation phase undergoes transformation can also be reduced. Specifically, by lowering the cooling rate in the pearlite transformation region, the precipitation of the low-temperature transformation phase mainly containing bainite is suppressed, thereby preventing the excessive transformation expansion of the low-temperature transformation phase from affecting the ferrite phase and pearlite phase that have already finished undergoing transformation, thus making it possible to suppress slab thermal cracking of the continuously cast slab.

[0086] Here, cooling after the continuously cast slab has exited the continuous-casting machine can be controlled by modifying, for example, the slab temperature on the exit side of the continuous-casting machine, the time it takes for multiple slabs to be stacked together, the number of pieces of slab to be stacked together, the presence or non-presence of a heat-retaining cover, and conditions for water toughening treatment.

[0087] The measurement of the cooling rate of the continuously cast slab that has exited the continuous-casting machine can be conducted via a thermocouple. For example, after the continuously cast slab has exited the continuous-casting machine, by installing a thermocouple in a surface central portion of the wider surface (longer side) of the continuously cast slab surface layer and measuring a surface temperature T 0 of the continuously cast slab surface layer, the cooling rate can be calculated.

[0088] As described above, according to the invention of this embodiment, there can be provided a continuously cast slab for high-strength steel that exhibits a high yield as slab thermal cracking during the cooling process will not occur, and hole defect troubles or the like at the time of performing rolling can be prevented as well, even in the cases of the continuously cast slabs for high-strength steel of recent years that have an extremely low continuously cast slab toughness.

[0089] Further, the continuously cast slab of this embodiment is such that the continuously cast slab contains, by mass%, C: 0.10 to 1.00%, Si: 0.10 to 2.50%, and Mn: 0.40 to 5.00%.

[0090] Here, in the following description, the unit "%" indicating the contents of the component elements of steel means "% by mass" unless otherwise specified.<C: 0.10 to 1.00%>

[0091] With regard to the continuously cast slab of this embodiment, the reasons that each chemical component contained in the continuously cast slab is limited will be described. Here, the content of each chemical component contained in the continuously cast slab is expressed as % by mass. The reason that the content of C in the continuously cast slab is set to 0.10 to 1.00% is as follows. C contained in a continuously cast slab for high-strength steel is an element required to improve the strength of a high-strength steel sheet whose raw material is a continuously cast slab. The lower limit of the C content is 0.10%, because the strength required for a high-strength steel sheet cannot be achieved if C is contained in an amount of less than 0.10%. Meanwhile, it is not preferable if the content of C is larger than 1.00%, because the high-strength steel sheet will exhibit an insufficient weldability and workability.

[0092] Thus, from such perspective, in the case of the continuously cast slab of this embodiment, it is preferred that C be contained in the continuously cast slab by an amount of 0.10 to 1.00%, more preferably 0.12 to 0.45%, particularly preferably 0.15 to 0.40%.<Si: 0.10 to 2.50%>

[0093] Next, the reason that the content of Si in the continuously cast slab for high-strength steel is set to 0.10 to 2.50% is as follows. Si contained in a continuously cast slab is an element required to secure residual austenite in a steel sheet during an annealing step of a high-strength steel sheet whose raw material is a continuously cast slab. Further, Si contained in a continuously cast slab is an essential additive element as it also contributes to heightening the strength of a high-strength steel sheet via solid-solution strengthening. The lower limit of the Si content is 0.10%, because the strength required for a high-strength steel sheet cannot be achieved if Si is contained in an amount of less than 0.10%.

[0094] Meanwhile, if Si is contained in an amount of more than 2.50%, not only the effect of achieving the strength required for a high-strength steel sheet will saturate, but a strong scale will occur on a hot-rolled sheet that has not yet been processed into a high-strength steel sheet. As a result, the upper limit of the Si content is set to 2.50% in terms of preventing the appearance and pickling property of a high-strength steel sheet from deteriorating.

[0095] Thus, from such perspective, in the case of the continuously cast slab of this embodiment, it is preferred that Si be contained in the continuously cast slab by an amount of 0.10 to 2.50%, more preferably 0.50 to 2.00%, even more preferably 1.00 to 1.80%.<Mn: 0.40 to 5.00%>

[0096] Further, the reason that the content of Mn in the continuously cast slab is set to 0.40 to 5.00% is as follows. Mn contained in a continuously cast slab is an element required to further improve the strength of a high-strength steel sheet. Specifically, Mn is an element added to control the strength of a high-strength steel sheet via transformation control of a continuously cast slab during a hot-rolling step thereof. The lower limit of the Mn content is 0.40%, because a high-strength steel sheet cannot be strengthened in a sufficient manner if Mn is contained in an amount of less than 0.40%. Meanwhile, if Mn is contained in an amount of greater than 5.00%, not only the extent to which a high-strength steel sheet is sufficiently strengthened will saturate, but it will also be unfavorable in terms of economic efficiency due to an increase in the manufacturing cost of the high-strength steel sheet.

[0097] Thus, from such perspective, in the case of the continuously cast slab of this embodiment, it is preferred that Mn be contained in the continuously cast slab by an amount of 0.40 to 5.00%, more preferably 1.20 to 4.50%, even more preferably 1.40 to 4.00%.

[0098] The continuously cast slab of this embodiment has the abovementioned chemical composition and the balance being Fe and unavoidable impurities, and has the average prior austenite grain sizes and microstructure of the appropriate composition. On that premise and in view of other properties, the continuously cast slab of this embodiment may contain 0.100% or less of P, 0.0200% or less of S, 0.100% or less of Al, 0.0100% or less of N, and 0.0100% or less of O. Here, examples of the unavoidable impurities include Zn, Pb, and As. A permissible contained amount of these unavoidable impurities is 0.100% or less in total.

[0099] P may cause slab thermal cracking as it embrittles the prior austenite grain boundary when segregated therein. For this reason, P is preferably contained in an amount of 0.100% or less. Here, while there are no particular restrictions on the lower limit of the P content, it is preferred that P be contained in an amount of 0.001% or more because P is a solid-solution strengthening element and is capable of increasing the strength of a steel sheet. Thus, P is preferably contained in an amount of 0.100% or less. Preferably, the P content is 0.001% or more. More preferably, the P content is 0.070% or less.

[0100] S is an element that exists in the form of a sulfide(s) and causes slab embrittlement. For this reason, S is preferably contained in an amount of 0.0200% or less. Here, while there are no particular restrictions on the lower limit of the S content, it is preferred that S be contained in an amount of 0.0001% or more in terms of manufacturing technology constraints. Thus, S is preferably contained in an amount of 0.0200% or less. Preferably, the S content is 0.0001% or more. More preferably, the S content is 0.0050% or less.

[0101] Al is an element affecting the fraction of the residual austenite in a slab, since this element promotes residual austenite generation by suppressing carbide generation that occurs while cooling the slab. Further, it is preferred that Al be added in an amount of 0.005% or lager for deoxidation purpose. An Al content of greater than 0.100% may cause slab embrittlement. Thus, Al is preferably contained in an amount of 0.100% or less. More preferably, the Al content is 0.010% or more. Even more preferably, the Al content is 0.080% or less.

[0102] N is an element that exists in the form of a nitride(s) and causes slab embrittlement. Thus, N is preferably contained in an amount of 0.0100% or less. Here, while there are no particular restrictions on the lower limit of the N content, it is preferred that N be contained in an amount of 0.0001% or more in terms of manufacturing technology constraints. Thus, N is preferably contained in an amount of 0.0100% or less. Preferably, the N content is 0.0001% or more. More preferably, the N content is 0.0050% or less.

[0103] O is an element that exists in the form of an oxide(s) and causes slab embrittlement. Thus, O is preferably contained in an amount of 0.0100% or less. Here, while there are no particular restrictions on the lower limit of the O content, it is preferred that O be contained in an amount of 0.0001% or more in terms of manufacturing technology constraints. Thus, O is preferably contained in an amount of 0.0100% or less. Preferably, the O content is 0.0001% or more. More preferably, the O content is 0.0050% or less.

[0104] In addition to the above chemical composition, the continuously cast slab of this embodiment, which is a continuously cast slab for high-strength steel sheet, may further contain at least one or two or more elements selected from the group consisting of 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.

[0105] When Ti, Nb, and V are each contained in an amount of 0.200% or less, coarse precipitates and / or inclusions will not be generated in a large amount in the slab whereby the toughness of the slab will not deteriorate. Thus, it is preferred that Ti, Nb, and V be each contained in an amount of 0.200% or less.

[0106] Here, while there are no particular restrictions on the lower limits of the contained amounts of Ti, Nb, and V, it is more preferred if Ti, Nb, and V are each contained in an amount of 0.001% or more because the strength of a steel sheet can be increased as fine carbides, nitrides, or carbonitrides are formed when performing hot rolling or continuous annealing on the continuously cast slab. Therefore, if containing Ti, Nb, and V, they are each contained in an amount of 0.200% or less. More preferably, the contained amount is 0.001% or more. Even more preferably, the contained amount is 0.100% or less.

[0107] When Ta and W are each contained in an amount of 0.10% or less, coarse precipitates and / or inclusions will not be generated in a large amount whereby the toughness of the slab will not deteriorate. Thus, it is preferred that Ta and W be each contained in an amount of 0.10% or less. Here, while there are no particular restrictions on the lower limits of the contained amounts of Ta and W, it is more preferred if Ta and W are each contained in an amount of 0.01% or more because the strength of a steel sheet can be increased as fine carbides, nitrides, or carbonitrides are formed when performing hot rolling or continuous annealing on the continuously cast slab. Therefore, if containing Ta and W, they are each contained in an amount of 0.10% or less. More preferably, the contained amount is 0.01% or more. Even more preferably, the contained amount is 0.08% or less.

[0108] If necessary, within a scope that does not undermine the object of the present invention, the continuously cast slab of this embodiment may contain at least one selected from the group consisting of Cr, Mo, Ni, and Cu. Cr, Mo, Ni, and Cu bring about an effect of heightening the strength of a steel sheet via structure control when hot-rolling a continuously cast slab. This effect shall be noticeable when one or more of Cr, Mo, Ni, and Cu are each added in an amount of 0.01% or more; they are preferably each added in an amount of 0.01% or more. The upper limit of the amount of each of the elements Cr, Mo, Ni, and Cu is set to 1.00%, because the weldability, hot workability and so on of a steel sheet will deteriorate if the amount of each element exceeds the upper limit thereof. Thus, when the continuously cast slab contains Cr, Mo, Ni, and Cu, the contained amount of each element is set to 1.00% or less. The contained amount is preferably 0.01% or more. More preferably, the contained amount is 1.00% or less.

[0109] B may be added as it affects strength via structure strengthening in order to control structure transformation that takes place while hot-rolling and annealing a continuously cast slab. The toughness of a slab will not be affected if B is in an amount of 0.0100% or less. Thus, the contained amount of B is preferably set to 0.0100% or less. Here, while there are no particular restrictions on the lower limit of the B content, it is more preferred if B is contained in an amount of 0.0003% or more because this is an element that improves quenching property when segregated in the austenite grain boundary while performing hot rolling and annealing on a continuously cast slab. Thus, if contained, B is contained in an amount of 0.0100% or less. More preferably, the B content is 0.0003% or more. Even more preferably, the B content is 0.0080% or less.

[0110] Co, when added in an amount of 1.00% or less, shall not impair the toughness of a slab as there will be no increase in coarse precipitates and inclusions. Thus, the contained amount of Co is preferably set to 1.00% or less. Here, while there are no particular restrictions on the lower limit of the Co content, it is more preferred if Co is contained in an amount of 0.001% or more because this is an element that improves the quenching property of a slab. Thus, if contained, Co is contained in an amount of 1.00% or less. More preferably, the Co content is 0.001% or more. Even more preferably, the Co content is 0.80% or less.

[0111] Cu, when added in an amount of 1.00% or less, shall not impair the toughness of a slab as there will be no increase in coarse precipitates and inclusions. Thus, the contained amount of Cu is preferably set to 1.00% or less. Here, while there are no particular restrictions on the lower limit of the Cu content, it is more preferred if Cu is contained in an amount of 0.01% or more because this is an element that improves quenching property. Thus, if contained, Cu is contained in an amount of 1.00% or less. More preferably, the Cu content is 0.01% or more. Even more preferably, the Cu content is 0.80% or less.

[0112] Sn, when added in an amount of 0.200% or less, shall not impair the toughness of a slab. Thus, the contained amount of Sn is preferably set to 0.200% or less. Here, while there are no particular restrictions on the lower limit of the Sn content, it is more preferred if Sn is contained in an amount of 0.001% or more because Sn is an element that improves quenching property. Thus, if contained, Sn is contained in an amount of 0.200% or less. More preferably, the Sn content is 0.001% or more. Even more preferably, the Sn content is 0.100% or less.

[0113] Sb, when added in an amount of 0.200% or less, shall not impair the toughness of a slab as there will be no increase in coarse precipitates and inclusions. Thus, the contained amount of Sb is preferably set to 0.200% or less. Here, while there are no particular restrictions on the lower limit of the Sb content, it is more preferred if Sb is contained in an amount of 0.001% or more because this is an element capable of adjusting the strength of a steel sheet by suppressing decarburization. Thus, if contained, Sb is contained in an amount of 0.200% or less. More preferably, the Sb content is 0.001% or more. Even more preferably, the Sb content is 0.100% or less.

[0114] Ca, Mg, and REM, when added in an amount of 0.0100% or less each, shall not impair the toughness of a slab as there will be no increase in coarse precipitates and inclusions. Thus, the contained amount of each of Ca, Mg, and REM is preferably set to 0.0100% or less. Here, while there are no particular restrictions on the lower limit of the contained amount of each of Ca, Mg, and REM, it is more preferred if the contained amount of each of these elements be 0.0005% or more because these are elements that improve the toughness of a slab by spheroidizing the shapes of nitrides and sulfides.

[0115] Thus, if contained, Ca, Mg, and REM are each contained in an amount of 0.0100% or less. More preferably, the contained amount is 0.0005% or more. Even more preferably, the contained amount is 0.0050% or less.

[0116] Zr and Te, when added in an amount of 0.100% or less each, shall not impair the toughness of a slab as there will be no increase in coarse precipitates and inclusions in the slab. Thus, the contained amount of each of Zr and Te is preferably set to 0.100% or less. Here, while there are no particular restrictions on the lower limit of the contained amount of each of Zr and Te, it is more preferred if the contained amount of each of Zr and Te be 0.001% or more because these are elements that improve the toughness of a slab by spheroidizing the shapes of nitrides and sulfides. Thus, if contained, Zr and Te are each contained in an amount of 0.100% or less. More preferably, the contained amount is 0.001% or more. Even more preferably, the contained amount is 0.080% or less.

[0117] Hf, when added in an amount of 0.10% or less, shall not impair the toughness of a slab as there will be no increase in coarse precipitates and inclusions. Thus, the contained amount of Hf is preferably set to 0.10% or less. Here, while there are no particular restrictions on the lower limit of the Hf content, it is more preferred if Hf is contained in an amount of 0.01% or more because this is an element that improves the ultimate deformability of a steel sheet by spheroidizing the shapes of nitrides and sulfides. Thus, if contained, Hf is contained in an amount of 0.10% or less. More preferably, the Hf content is 0.01% or more. Even more preferably, the Hf content is 0.08% or less.

[0118] Bi, when added in an amount of 0.200% or less, shall not impair the toughness of a slab as there will be no increase in coarse precipitates and inclusions. Thus, the contained amount of Bi is preferably set to 0.200% or less. Here, while there are no particular restrictions on the lower limit of the Bi content, it is more preferred if Bi is contained in an amount of 0.001% or more because this is an element that alleviates segregation. Thus, if contained, Bi is contained in an amount of 0.200% or less. More preferably, the Bi content is 0.001% or more. Even more preferably, the Bi content is 0.100% or less.

[0119] Here, as for the above Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi, the effects of the present invention will not be impaired if the contained amount of each of these elements is lower than each corresponding preferable lower limit; in such case, they are contained as unavoidable impurities.

[0120] As described above, according to the invention of the first embodiment, there can be obtained a continuously cast slab with which a strength required for a high-strength steel can be achieved, and with which an excellent weldability, workability and appearance of a high-strength steel can be achieved as well.[Second embodiment]

[0121] A continuously cast slab manufacturing method of a second embodiment is described hereunder. The continuously cast slab manufacturing method of this embodiment is a method for manufacturing a continuously cast slab for high-strength steel, and includes: a first cooling step in which a continuously cast slab having the chemical composition described in the above embodiment is cooled under a cooling condition where the total heat flux Q MD in the continuous casting mold satisfies the following relational expression (1), and the temperature T 20 of the continuously cast slab at the position that is located in the central part of the continuously cast slab along the width direction thereof and is 20 mm away from the surface layer of the continuously cast slab resides in a temperature range of 1,200 to 1,450°C for a residence time of 230 s or less; a second cooling step in which the continuously cast slab is cooled at an average cooling rate of 20°C / hr or less when the surface temperature T 0 of the continuously cast slab surface layer at the position that is located in the central part of the continuously cast slab along the width direction thereof is in a range of 700 to 850°C; and a third cooling step in which the continuously cast slab is cooled at an average cooling rate of 10°C / hr or less when the surface temperature T 0 of the continuously cast slab surface layer at the position that is located in the central part of the continuously cast slab along the width direction thereof is in a range of 500 to 700°C. [Expression 3] Q MD × L MD Vc ≦ 1.5

[0122] In the above relational expression (1), Q MD represents a total heat flux [MW / m 2< ] in a continuous casting mold, L MD represents a mold effective length [m], and V C represents a casting rate [m / min].

[0123] Here, as for the method of this embodiment for manufacturing a slab for a high-strength steel sheet, relocation may take place depending on various conditions of the manufacturing steps thereof. If relocation takes place, the cooling rate of the slab may temporarily exceed the predetermined cooling rate. However, since the time spent on transformation is 10 hr or longer, which is extremely slow, a handling time of about the length of a relocation time (1 to 2 hr at the most) shall not lead to the occurrence of thermal cracking. For this reason, in the case of the method of this embodiment for manufacturing a slab for a high-strength steel sheet, designated as the cooling rate when cooling the continuously cast slab is not a maximum cooling rate, but an average cooling rate.

[0124] Described hereunder are the steps of the continuously cast slab manufacturing method of this embodiment.(First cooling step)

[0125] The continuously cast slab manufacturing method of this embodiment is a method for manufacturing a continuously cast slab for high-strength steel, and includes: the first cooling step in which a continuously cast slab having the chemical composition described in the above embodiment is cooled under the cooling condition where the total heat flux Q MD in the continuous casting mold satisfies the following relational expression (1), and the temperature T 20 of the continuously cast slab at the position that is located in the central part of the continuously cast slab along the width direction thereof and is 20 mm away from the surface layer of the continuously cast slab resides in the temperature range of 1,200 to 1,450°C for the residence time of 230 s or less. [Expression 4] Q MD × L MD Vc ≦ 1.5

[0126] In the above relational expression (1), Q MD represents a total heat flux [MW / m 2< ] in a continuous casting mold, L MD represents a mold effective length [m], and V C represents a casting rate [m / min].

[0127] The first cooling step is a step for controlling the average prior austenite grain size ratio (d 20 / d 10 ) to 4.0 or less, the average prior austenite grain size ratio being the ratio between the average prior austenite grain size d 10 at the position 10 mm below the surface layer of the continuously cast slab and the average prior austenite grain size d 20 at the position 20 mm below the surface layer of the continuously cast slab, as described in the above embodiment. In the continuously cast slab manufacturing method of this embodiment, a factor that determines the average prior austenite grain sizes is the temperature when cooling the continuously cast slab. In the first cooling step, the temperature for cooling the continuously cast slab is in a range of 1,200 to 1,450°C. In this way, the continuously cast slab manufacturing method of this embodiment is such that temperature control of the continuously cast slab is conducted by focusing on its temperature range of 1,200 to 1,450°C, which serves as a factor that determines the average prior austenite grain sizes.

[0128] Here, in the first cooling step, since it is difficult to actually measure the cooling temperature of the continuously cast slab in the temperature range of 1,200 to 1,450°C, the temperature history at the position 20 mm away from the surface layer of the continuously cast slab was calculated via heat-transfer analysis. The analysis position was set to a central part of the slab along its width direction, which is a position that results in the longest residence time of the above temperature range even inside the slab.

[0129] In order to reduce the average prior austenite grain size ratio (d 20 / d 10 ), which is the ratio between the average prior austenite grain size d 10 at the position 10 mm below the surface layer of the continuously cast slab and the average prior austenite grain size d 20 at the position 20 mm below the surface layer of the continuously cast slab, it is necessary to increase the average prior austenite grain size d 10 at the position 10 mm below the surface layer of the continuously cast slab, and reduce the average prior austenite grain size d 20 at the position 20 mm below the surface layer of the continuously cast slab.

[0130] In other words, cooling at the position 10 mm below the surface layer of the continuously cast slab needs to be slowed, whereas cooling at the position 20 mm below the surface layer of the continuously cast slab needs to be accelerated.

[0131] Moreover, in the first cooling step, the total heat flux Q MD in the continuous casting mold needs to satisfy the following relational expression (1) in terms of its relation with respect to the mold effective length L MD [m] and casting rate V C [m / min]. Further, the residence time of the continuously cast slab is 230 s or shorter in the abovementioned temperature range for cooling the position 20 mm away from the surface layer of the continuously cast slab.

[0132] Furthermore, it is preferable when the total heat flux Q MD in the continuous casting mold satisfies the following relational expression (1) in terms of its relation with respect to the mold effective length L MD [m] and casting rate V C [m / min], and when the residence time of the continuously cast slab in the above temperature range is 230 s or shorter, because the average prior austenite grain size ratio (d 20 / d 10 ) can be controlled to 4.0 or less, and slab thermal cracking can thus be suppressed.

[0133] Here, while there are no particular restrictions on the lower limit of the residence time of the continuously cast slab in the temperature range of 1,200 to 1,450°C, the residence time is preferably set to 60 s or longer because an excessively short residence time will lead to a higher risk of breakout in continuous casting that is caused by ununiform solidification. Particularly, a residence time of 80 s or longer is more preferred, and a residence time of 90 s or longer is even more preferred. [Expression 5] Q MD × L MD Vc ≦ 1.5

[0134] In the above relational expression (1), Q MD represents a total heat flux [MW / m 2< ] in a continuous casting mold, L MD represents a mold effective length [m], and V C represents a casting rate [m / min].(Second cooling step)

[0135] Next, the continuously cast slab manufacturing method of this embodiment includes: the second cooling step in which the continuously cast slab is cooled at the average cooling rate of 20°C / hr or less when the surface temperature T 0 of the continuously cast slab surface layer at the position that is located in the central part of the continuously cast slab along the width direction thereof is in the range of 700 to 850°C. The second cooling step is a step for increasing the area ratio of ferrite contained in the microstructure of the continuously cast slab of the above embodiment.

[0136] In the second cooling step, the temperature for further cooling the continuously cast slab is 700 to 850°C. In this way, the continuously cast slab manufacturing method of this embodiment focuses on the cooling rate of the temperature range in the ferrite transformation region capable of increasing the precipitation amount of ferrite and controls such temperature. In the second cooling step, the cooling rate was measured by a thermocouple. After the slab had exited the continuous-casting machine, the thermocouple was placed on the upper surface central portion of the wider surface (longer side) of the slab, whereby the cooling rate was able to be calculated from a temperature measured thereon. Cooling rate measurement in the later-described third cooling step may be carried out in a similar manner.

[0137] In the second cooling step, the average cooling rate of the continuously cast slab is 20°C / hr or less in the above temperature range for cooling the continuously cast slab. It is preferable when the average cooling rate of the continuously cast slab is 20°C / hr or less, because the residence time of such continuously cast slab in the ferrite transformation temperature region can be sufficiently secured, whereby the area ratio of ferrite can be increased, and the precipitation of grain boundary ferrite can be suppressed, thus resulting in an improved slab toughness.

[0138] Here, while there are no strict limitations imposed on the lower limit of the average cooling rate, the average cooling rate is preferably and desirably 2°C / hr or more as an energy source required for performing control will be needed separately. More preferably, the average cooling rate is 5 to 18°C / hr.(Third cooling step)

[0139] Further, the continuously cast slab manufacturing method of this embodiment includes: the third cooling step in which the continuously cast slab is cooled at the average cooling rate of 10°C / hr or less when the surface temperature T 0 of the continuously cast slab surface layer at the position that is located in the central part of the continuously cast slab along the width direction thereof is in the range of 500 to 700°C.

[0140] The third cooling step is a step for turning the microstructure of the continuously cast slab of the above embodiment into a structure mainly composed of pearlite, and lowering the internal stress of such microstructure.

[0141] In other words, the third cooling step is a step for suppressing the precipitation of the low-temperature transformation phase in the microstructure of the continuously cast slab so that the stress caused by such transformation expansion will not be applied to the ferrite and pearlite portions that have already finished undergoing transformation.

[0142] Specifically, the third cooling step is a step of turning the abovementioned area ratio (%) which is the ratio of the area S10 (ferrite+pearlite) obtained by adding together the area S10 ferrite of ferrite and the area S10 pearlite of pearlite to the area S10 total of the microstructure at the position 10 mm below the continuously cast slab surface layer into 80% or more, and turning the abovementioned area ratio (%) which is the ratio of the area S20 (ferrite+pearlite) obtained by adding together the area S20 ferrite of ferrite and the area S20 pearlite of pearlite to the area S20 total of the microstructure at the position 20 mm below the continuously cast slab surface layer into 60% or more.

[0143] In the third cooling step, the temperature for further cooling the continuously cast slab is 500 to 700°C. In this way, the continuously cast slab manufacturing method of this embodiment controls the temperature of the continuously cast slab by focusing on the cooling rate of the temperature range in the pearlite transformation region.

[0144] In the third cooling step, the average cooling rate of the continuously cast slab in the above temperature range for cooling such continuously cast slab is 10°C / hr or less. It is preferable when the average cooling rate of the continuously cast slab is 10°C / hr or less while controlling the average prior austenite grain size ratio (d 20 / d 10 ) to 4.0 or less, because the transformation of the low-temperature transformation phase containing bainite or the like can be suppressed, whereby the microstructure of the continuously cast slab can be turned into a structure mainly composed of pearlite, thus allowing the internal stress thereof to be reduced.

[0145] Here, while there are no strict limitations on the lower limit of the average cooling rate when cooling the continuously cast slab, the average cooling rate is desirably 2°C / hr or more as an energy source required for performing control will be needed separately. More desirably, the average cooling rate is 5°C / hr or more.

[0146] As such, the continuously cast slab manufacturing method of this embodiment is configured to finely control the average prior austenite grain size ratio and the microstructure of a continuously cast slab by employing three stages of cooling steps as steps for cooling the continuously cast slab. Thus, the manufacturing method of this embodiment is able to provide a continuously cast slab for high-strength steel that is capable of suppressing slab thermal cracking caused by cooling, and also preventing hole defect troubles or the like at the time of performing rolling.

[0147] As described above, according to the continuously cast slab manufacturing method of the second embodiment, even in the case of a component system of a continuously cast slab for high-strength steel, by dividing the cooling step into three stages and finely control each stage of the cooling step, there can be provided a continuously cast slab for high-strength steel that exhibits no thermal cracking during the cooling process, and is also capable of preventing hole defect troubles or the like at the time of performing rolling.[Other embodiments]

[0148] The invention of this application has so far been described with reference to the above embodiments; however, the invention of this application shall not be limited to the above embodiments. The configuration and particulars of the invention of this application may be subjected to various modifications that are comprehensible to those skilled in the art within the technical scope of the invention of this application. Further, systems or devices established by freely combining separate features that are included in each embodiment are also covered by the technical scope of the present invention.Examples

[0149] The effects of the present invention are described in detail hereunder based on working examples; however, the present invention shall not be limited to these working examples. Specifically, in order to verify the effects of the present invention, the inventors of the present invention manufactured continuously cast slabs using various types of steel as raw materials in comparative examples (tests No. A-1 to A-4, tests No. B-1 to B-4, and tests No. C-1 to C-4) and invention examples (tests No. D-1 to D-24). Shown in Table 1 are the components of the steels as the raw materials for the continuously cast slabs that are used in the comparative examples (tests No. A-1 to A-6, tests No. B-1 to B-4, and tests No. C-1 to C-4) and the invention examples (tests No. D-1 to D-24). [Table 1]Steel TypeC [Mass%]Si [Mass%]Mn [Mass%]P [Mass%]S [Mass%]Sol. Al [Mass%]N [Mass%]Ti [Mass%]Nb [Mass%]A0.102.503.510.0110.00110.0520.0043--B0.120.515.000.0090.00150.0480.00390.0240.010C0.160.741.490.0080.00150.0470.0032--D0.181.372.740.0070.00210.0480.00340.021-E0.251.083.230.0070.00130.0500.0036-0.034F0.501.973.020.0100.00080.0430.0030--G0.420.241.550.0190.00430.0410.0042--H0.550.190.690.0210.00450.0250.0035--I1.000.230.400.0200.00250.0020.0031--

[0150] Here, there were employed three stages of cooling steps whose cooling conditions of the continuously cast slab were composed of (I) total heat flux function [-] in a continuous casting mold, (II) residence time [s] in the range of 1,450 to 1,200°C, (III) average cooling rate [°C / hr] in the range of 850 to 700°C, and (IV) average cooling rate [°C / hr] in the range of 700 to 500°C, where cooling was performed by appropriately changing the conditions in each of these stages. The continuously cast slab cooling conditions (I) to (IV), the microstructures of the continuously cast slabs obtained, and the evaluation on slab thermal cracking are shown in Tables 2 to 4.

[0151] With regard to the continuously cast slabs manufactured in the comparative and invention examples, the measurement of the average prior austenite grain size, the calculation of the area ratios of ferrite and pearlite, and the evaluation on slab thermal cracking of the continuously cast slab, were carried out as follows.<Measurement of average prior austenite grain size>

[0152] Here, the average prior austenite grain size was as measured as follows. A sample was cut out from the cooled slab's central portion along the width direction of the slab so that a slab thickness cross-section parallel to the width direction of the slab was treated as an observation surface. Next, the observation surface was mirror-polished using a diamond paste, followed by using colloidal silica to perform finish-polishing, and then using a 3 vol.% Nital to perform etching so as to reveal a structure on the observation surface. With an optical microscope, the position 10 mm below the slab surface layer and the position 20 mm below the slab surface layer were observed at 10-fold magnification and in 5 fields of view to obtain a structure image(s). From these structure images obtained, the average value of the prior austenite grain size was calculated using an intercept method that is in accordance with JIS G 0551:2020.<Method for measuring ferrite area ratio>

[0153] A method for measuring the ferrite area ratio is such that a slab observation surface is prepared in a similar manner as the above method for measuring the average prior austenite grain size. Next, the observation surface was mirror-polished using a diamond paste, followed by using colloidal silica to perform finish-polishing, and then using a 3 vol.% Nital to perform etching so as to reveal a structure. At an accelerating voltage of 15 kV, SEM (Scanning Electron Microscope) was used to observe the structure at the position 10 mm below the slab surface layer and the structure at the position 20 mm below the slab surface layer at 50-fold magnification and in 10 fields of view, whereby structure images obtained were then subjected to PHOTOSHOP (registered trademark) by Adobe to calculate 10 fields of view worth of ferrite area ratio and then average these values so as to obtain an average value as the area ratio of ferrite.

[0154] Ferrite can be easily distinguished at 50-fold magnification since the grain size thereof is larger than that of pearlite and a low-temperature transformation phase containing bainite or the like as other structure (bainite, tempered martensite, quenched martensite, and residual austenite), and since ferrite has a dark contrast with a smooth surface.

[0155] However, it is difficult to precisely distinguish grain boundary ferrite from intragranular ferrite. In this regard, the inventors of the present invention diligently studied slabs exhibiting slab thermal cracking and found that a large quantity of detrimental ferrite was present in the grain boundary when the area ratio of ferrite was larger than 5% but smaller than 10%. That is, a structure with a ferrite area ratio of not less than 5% but less than 10% is regarded as grain boundary ferrite.<Method for measuring pearlite area ratio>

[0156] A method for measuring the area ratio of the pearlite structure is such that a structure is to be revealed on a slab observation surface in a similar manner as the above method for measuring ferrite. At an accelerating voltage of 15 kV, SEM was used to observe the structure at the position 10 mm below the slab surface layer and the structure at the position 20 mm below the slab surface layer at 10,000-fold magnification and in 10 fields of view while excluding ferrite from the fields of view, whereby structure images obtained were then subjected to PHOTOSHOP (registered trademark) by Adobe to calculate 10 fields of view worth of pearlite area ratio and then average these values so as to obtain an average value as the area ratio of pearlite.

[0157] It should be noted that pearlite is a eutectoid of ferrite and cementite; when observed via a scanning electron microscope, it is a structure in which the thin sheet-shaped layers of the two exhibit pearl-like luster.

[0158] The microstructure of the continuously cast slab of the present invention consists of ferrite, pearlite, and a low-temperature transformation phase. Thus, the area ratio of the area S X of the low-temperature transformation phase composing the microstructure can be calculated by subtracting, from 100%, the sum total of the ferrite area ratio and pearlite area ratio, in view of the fact that the total of the ferrite area ratio, pearlite area ratio, and low-temperature transformation phase area ratio is 100%.<Evaluation on slab thermal cracking>

[0159] Slab thermal cracking was evaluated by performing a test based on a penetrant test prescribed in JIS Z 2343:2017, where the presence or non-presence of cracks in the wider and narrower surface portions of the slab was evaluated. After applying a developer thereto, by visually observing the ooze of the penetrant, slab thermal cracking that had occurred on the surface was checked visually.

[0160] Here, when there is a crack(s) of a length of 50 mm or longer, there will be a high risk of slab fracture at the time of slab handling and / or in a heating furnace, or a high possibility of causing hole defect troubles when performing rolling; the evaluation criteria for slab thermal cracking is thus as follows. ·Slab thermal cracking "∘"· · · examples exhibiting no cracks of length of 50 mm or longer on slab surface ·Slab thermal cracking "△" · · ·examples exhibiting no cracks of length of 50 mm or longer on slab surface, but exhibiting scratches after rolling ·Slab thermal cracking "×"· · · examples exhibiting cracks of length of 50 mm or longer on slab surface [Table 2] Test No. Q MD × L MD Vc Steel type10 mm below slab surface layer20 mm below slab surface layerPrior austenite grain size ratio d 20 / d 10 [-][-]Residence time [s] from 1450 to 1200°CAverage cooling rate [°C / hr] from 850 to 700°CAverage cooling rate [°C / hr] from 700 to 500°CThermal crackingRemarksPrior austenite grain size d 10 [mm]Ferrite + pearlite area ratio [%]Prior austenite grain size d 20 [mm]Ferrite + pearlite area ratio[%]A-1D0.4952.1705.31.812301510△Comparative ExampleA-2D0.6922.6654.31.52280126△Comparative ExampleA-3D0.7902.9604.11.48240183△Comparative ExampleA-4D0.8854.0905.01.4228051△Comparative ExampleB-1D0.6771.8693.01.451203018×Comparative ExampleB-2D0.9682.2612.41.401503215×Comparative ExampleB-3D1.8602.4601.30.901902519×Comparative ExampleB-4D1.1771.9721.71.101202214×Comparative ExampleC-1D1.4851.7101.21.151102020×Comparative ExampleC-2D0.71002.052.91.382401225×Comparative ExampleC-3D0.9951.5201.70.921301515×Comparative ExampleC-4D1.5912.201.50.812301130×Comparative ExampleD-1D0.71002.8604.01.10210610○Invention ExampleD-2D1.5923.1802.10.802301310○Invention ExampleD-3D1.7801.9951.10.83130188○Invention ExampleD-4D1.11001.81001.60.99110165○Invention Example [Table 3] Test No. Q MD × L MD Vc Steel type10 mm below slab surface layer20 mm below slab surface layerPrior austenite grain size ratio d 20 / d 10 [-][-]Residence time [s] from 1450 to 1200°CAverage cooling rate [°C / hr] from 850 to 700°CAverage cooling rate [°C / hr] from 700 to 500°CThermal crackingRemarksPrior austenite grain size d 10 [mm]Ferrite + pearlite area ratio [%]Prior austenite grain size d 20 [mm]Ferrite + pearlite area ratio[%]D-5D1.41001.5821.10.9410029○Invention ExampleD-6D0.81001.7912.11.0590147○Invention ExampleD-7D1.2811.41001.21.0290206○Invention ExampleD-8D0.8951.9992.41.46110151○Invention ExampleD-9D0.51001.7883.41.28130202○Invention ExampleD-10A0.61002.3913.81.3021821○Invention ExampleD-11A1.1833.0732.70.98230159○Invention ExampleD-12A0.91002.2902.41.1222086○Invention ExampleD-13B1.2962.4882.00.9021853○Invention ExampleD-14B0.5881.9763.81.39200198○Invention ExampleD-15C0.91002.3882.61.0622262○Invention ExampleD-16C1.3921.5741.20.8818582○Invention ExampleD-17E0.71001.8912.61.2120994○Invention ExampleD-18E0.8991.6892.01.33195108○Invention ExampleD-19E0.5982.0824.01.46207126○Invention ExampleD-20F0.91002.5952.81.1120932○Invention Example [Table 4] Test No. Q MD × L MD Vc Steel type10 mm below slab surface layer20 mm below slab surface layerPrior austenite grain size ratio d 20 / d 10 [-][-]Residence time [s] from 1450 to 1200°CAverage cooling rate [°C / hr] from 850 to 700°CAverage cooling rate [°C / hr] from 700 to 500°CThermal crackingRemarksPrior austenite grain size d 10 [mm]Ferrite + pearlite area ratio [%]Prior austenite grain size d 20 [mm]Ferrite + pearlite area ratio[%]D-21F1.0812.4692.40.94212189○Invention ExampleD-22G1.3802.1601.60.88200207○Invention ExampleD-23H0.61001.7902.81.4918553○Invention ExampleD-24I1.4862.1791.50.85207109○Invention Example <Comparative examples (tests No. A-1 to A-4)>

[0161] A slab microstructure whose particulars were met by the continuously cast slabs manufactured in tests No. A-1 to A-4 was defined as a condition A. The condition A corresponds to an example where the ratio between the average prior austenite grain size at the position 10 mm below the slab surface layer and the average prior austenite grain size at the position 20 mm below the surface layer turned out to be larger than 4.0. In these cases, even when employing various conditions for slowly cooling the slab after it had exited the slab continuous-casting machine, differences in transformation speed occurred due to the differences in prior austenite grain sizes, which led to a significant gap in the timing of transformation between the position 10 mm below the slab surface layer and the position 20 mm below the slab surface layer, whereby a transformation stress was concentrated on the slab surface layer, and slab thermal cracking was thus unable to be suppressed.<Comparative examples (tests No. B-1 to B-4)>

[0162] A slab microstructure whose particulars were met by the continuously cast slabs manufactured in tests No. B-1 to B-4 was defined as a condition B. The condition B corresponds to an example where while the ratio between the average prior austenite grain size at the position 10 mm below the slab surface layer and the average prior austenite grain size at the position 20 mm below the surface layer was 4.0 or less, the ferrite precipitation amount at the position 10 mm below the slab surface layer was small, and grain boundary ferrite was confirmed to have precipitated. In these cases, although the cooling rate in the range of 700 to 500°C was low, and a transformation stress generated was low as well, grain boundary embrittlement occurred due to the grain boundary ferrite, thus failing to suppress slab thermal cracking.<Comparative examples (tests No. C-1 to C-4)>

[0163] A slab microstructure whose particulars were met by the continuously cast slabs manufactured in tests No. C-1 to C-4 was defined as a condition C. The condition C corresponds to an example where while the ratio between the average prior austenite grain size at the position 10 mm below the slab surface layer and the average prior austenite grain size at the position 20 mm below the surface layer was 4.0 or less, a low-temperature transformation phase containing bainite or the like was precipitated at the position 20 mm below the slab surface layer, thus failing to suppress slab cracking. Particularly, it is considered that since bainite transformation occurs at a temperature lower than that of pearlite transformation, a density difference to austenite is large, and a larger transformation stress is incurred as well, whereby such stress is then concentrated on the ferrite and pearlite portions that have already finished undergoing transformation, thus failing to suppress slab cracking.<Invention examples (tests No. D-1 to D-24)>

[0164] A slab microstructure whose particulars were met by the continuously cast slabs manufactured in tests No. D-1 to D-24 was defined as a condition D. The condition D corresponds to the condition(s) of the invention examples where the average prior austenite grain size ratio was 4.0 or less, the microstructure at the position 10 mm below the slab surface layer was such that the total of the ferrite area ratio and pearlite area ratio was 80% or more, and the microstructure at the position 20 mm below the slab surface layer was such that the total of the ferrite area ratio and pearlite area ratio was 60% or more. The continuously cast slabs manufactured in tests No. D-1 to D-24 exhibited no slab thermal cracking after cooling.

[0165] Fig.3 is an observational magnified micrograph showing a continuously cast slab of an invention example (test No. D-2) of the continuously cast slab of the present invention; this continuously cast slab was observed via an optical microscope. The observational magnified micrograph of Fig.3A shows the microstructure at the position 10 mm below the surface layer of the continuously cast slab. The observational magnified micrograph of Fig.3B shows the microstructure at the position 20 mm below the surface layer of the continuously cast slab.

[0166] As is clear from Fig.3, it was revealed that the continuously cast slab manufactured in the invention example (test No. D-2) of continuously cast slab was such that the average prior austenite grain size at the position 10 mm below the slab surface layer and the average prior austenite grain size at the position 20 mm below the slab surface layer were able to be controlled respectively, whereby there were achieved microstructures with ferrite area ratios corresponding to those at the respective positions below the slab surface layer.

[0167] According to Tables 2 to 4 and Fig.3, it became clear that slab thermal cracking when cooling a slab could be suppressed when (i) the average prior austenite grain size ratio (d 20 / d 10 ) was 1.0 to 4.0, provided that d 10 was the average prior austenite grain size at the position 10 mm below the surface layer of the continuously cast slab and d 20 was the average prior austenite grain size at the position 20 mm below the surface layer of the continuously cast slab, and (ii) the microstructure at the position 10 mm below the surface layer of the continuously cast slab was such that the total of the area ratio of ferrite and the area ratio of pearlite was 80% or more, and the microstructure at the position 20 mm below the surface layer of the continuously cast slab was such that the total of the area ratio of ferrite and the area ratio of pearlite was 60% or more. Here, the total of the area ratio of ferrite, the area ratio of pearlite, and the area ratio of the low-temperature transformation phase is 100% at each of the position 10 mm below the surface layer of the continuously cast slab and the position 20 mm below the surface layer of the continuously cast slab.

[0168] That is, the continuously cast slab of the present invention is configured in such a manner where the average prior austenite grain size ratio (d 20 / d 10 ) is 1.0 to 4.0, provided that d 10 is the average prior austenite grain size at the position 10 mm below the surface layer of the continuously cast slab and d 20 is the average prior austenite grain size at the position 20 mm below the surface layer of the continuously cast slab; and the microstructure at the position 10 mm below the surface layer of the continuously cast slab is such that the total of the area ratio of ferrite and the area ratio of pearlite is 80% or more, and the microstructure at the position 20 mm below the surface layer of the continuously cast slab is such that the total of the area ratio of ferrite and the area ratio of pearlite is 60% or more. Therefore, there can be provided a slab for high-alloy and high-strength steel that exhibits no slab cracking after performing casting, and hole defect troubles or the like at the time of rolling can be prevented as well.Industrial Applicability

[0169] The continuously cast slab of the present invention is industrially useful because it is a slab for high-strength steel that exhibits no slab thermal cracking after performing casting and is also capable of preventing hole defect troubles or the like at the time of performing rolling owing to its configuration where the average prior austenite grain size ratio (d 20 / d 10 ) is 1.0 to 4.0, provided that d 10 is the average prior austenite grain size at the position 10 mm below the surface layer of the continuously cast slab and d 20 is the average prior austenite grain size at the position 20 mm below the surface layer of the continuously cast slab; and the microstructure at the position 10 mm below the surface layer of the continuously cast slab is such that the total of the area ratio of ferrite and the area ratio of pearlite is 80% or more, and the microstructure at the position 20 mm below the surface layer of the continuously cast slab is such that the total of the area ratio of ferrite and the area ratio of pearlite is 60% or more.

Claims

1. A continuously cast slab for high-strength steel comprising, by mass%, C: 0.10 to 1.00%, Si: 0.10 to 2.50%, and Mn: 0.40 to 5.00%, and optionally comprising, by mass%, 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 further optionally contains, by mass%, at least one or two or more elements selected from the group consisting of 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, with a balance being Fe and unavoidable impurities, wherein: an average prior austenite grain size ratio (d20 / d10) is between 1.0 and 4.0, provided that d10 is an average prior austenite grain size at a position 10 mm below a surface layer of the continuously cast slab and d20 is an average prior austenite grain size at a position 20 mm below the surface layer of the continuously cast slab; a microstructure of the continuously cast slab consists of ferrite, pearlite, and a low-temperature transformation phase, the low-temperature transformation phase containing at least one selected from the group consisting of bainite, tempered martensite, quenched martensite, and residual austenite; the microstructure at the position 10 mm below the surface layer of the continuously cast slab is such that a total of an area ratio of the ferrite and an area ratio of the pearlite is 80% or more, provided that a total of the area ratio of the ferrite, the area ratio of the pearlite, and an area ratio of the low-temperature transformation phase is 100%; and the microstructure at the position 20 mm below the surface layer of the continuously cast slab is such that a total of an area ratio of the ferrite and an area ratio of the pearlite is 60% or more, provided that a total of the area ratio of the ferrite, the area ratio of the pearlite, and an area ratio of the low-temperature transformation phase is 100%.

2. A method for manufacturing a continuously cast slab for high-strength steel, which is a method for manufacturing the continuously cast slab according to claim 1, comprising: a first cooling step in which the continuously cast slab having the chemical composition as set forth in claim 1 is cooled under a condition where a total heat flux QMD in a continuous casting mold satisfies the following relational expression (1), and a temperature T20 of the continuously cast slab at a position that is located in a central part of the continuously cast slab along a width direction thereof and is 20 mm away from the surface layer of the continuously cast slab resides in a temperature range of 1,200 to 1,450°C for a residence time of 230 s or less; a second cooling step in which the continuously cast slab is cooled at an average cooling rate of 20°C / hr or less when a surface temperature T0 of the continuously cast slab surface layer at a position that is located in a central part of the continuously cast slab along the width direction thereof is in a range of 700 to 850°C; and a third cooling step in which the continuously cast slab is cooled at an average cooling rate of 10°C / hr or less when the surface temperature T0 of the continuously cast slab surface layer at the position that is located in the central part of the continuously cast slab along the width direction thereof is in a range of 500 to 700°C, the relational expression (1) being defined as follows: [Expression 1] Q MD × L MD Vc ≦ 1.5 , wherein, in the above expression (1), QMD represents a total heat flux [MW / m2] in a continuous casting mold, LMD represents a mold effective length [m], and VC represents a casting rate [m / min].

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