Continuous casting slab and method for manufacturing the same

Abstract

The present invention provides a continuously cast slab and a method for manufacturing the same, which prevents the occurrence of slab cracking during cooling, even in the case of a continuously cast slab with low toughness. [Solution] A continuous cast slab for high-strength steel having a predetermined component composition, wherein the average prior austenite grain size d is located 10 mm below the surface of the continuous cast slab. 10 and the average prior austenite grain size d at the 20mm position 20 The average prior austenite grain ratio (d 20 / d 10 The microstructure is characterized in that the ratio is 1.0 or more and 4.0 or less, the microstructure consists of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite, and the microstructure at a position 10 mm below the surface of the continuous casting slab and the microstructure at a position 20 mm below the surface of the continuous casting slab are formed from structures having a predetermined area ratio.

Description

[Technical Field]

[0001] The present invention relates to a continuously cast slab that prevents cracking during cooling and a method for manufacturing the same. More specifically, the present invention relates to a continuously cast slab for high-strength steel (high-tensile steel) that is effective in preventing slab cracking during placement and does not cause perforation problems during rolling, and a method for manufacturing the same. [Background technology]

[0002] In recent years, in the automotive sector, in order to achieve both further thinning of vehicle bodies and ensuring collision safety, there has been a trend towards further increasing the strength of high-strength steel (also known as high-tensile steel) and increasing its alloy content for that purpose. However, increasing the alloy content of high-strength steel significantly reduces the toughness of the slab.

[0003] Due to the decrease in slab toughness caused by increased alloying, cracks during slab cooling, so-called slab-standing cracks, have become more frequent. When slab-standing cracks occur, there is a risk that the slab may break during transport, making it unsuitable for hot rolling. Furthermore, even if the slab does not break, cracks in the slab may open during hot rolling, potentially causing the hot-rolled steel sheet to break. Alternatively, small cracks may appear as surface defects such as burrs or silvering on the steel sheet after hot rolling, cold rolling, annealing, or plating. Normally, cracks on the slab surface are removed by grinding. However, in the case of highly alloyed slabs, the toughness of the slab is reduced due to the alloying process, so the stress from grinding can cause the cracks in the slab to propagate, making it impossible to completely remove the cracks. On the other hand, small cracks in the slab may be overlooked and appear as surface defects in the steel sheet after hot rolling, cold rolling, annealing, or plating. For these reasons, cracking in the slab needs to be suppressed.

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

[0005] Grain boundary fracture occurs when the prior austenite grains are coarse and the grain boundaries become brittle. When grain boundary ferrite is formed, a strength difference arises between it and the pearlite and bainite within the grain. This causes stress concentration in the weaker grain boundary ferrite area, leading to crack propagation in the slab even at lower stress levels. On the other hand, when a slab is cooled, stress is generated due to differences in thermal contraction and transformation expansion between the slab surface and the slab's interior. If this stress is large, slab cracking occurs when the slab is cooled to room temperature. In recent years, high-alloy, high-strength steels have high hardenability, and coupled with the coarse grain size of the prior austenite in the slab, conventional slow cooling processes cannot suppress the precipitation of low-temperature transformation phases (bainite, martensite, etc.). In addition, because the slabs have low toughness, these deep cracks are difficult to remove by grinding or other maintenance methods, which significantly reduces the yield of the slabs.

[0006] From this perspective, methods have been proposed to suppress the occurrence of slab cracks in high-tensile steel slabs. For example, Patent Document 1 proposes a method to suppress the occurrence of slab cracks by suppressing bainite / martensitic transformation and reducing the stress caused by the transformation expansion, even in steel types of high-tensile steel that are prone to slab cracking, by slowly cooling the steel in the temperature range of 700-500°C, which is the temperature range in which austenite transforms to ferrite. Specifically, the cooling method for high-tensile steel slabs disclosed in Patent Document 1 is a method to suppress the occurrence of slab cracks by controlling the cooling rate of the slab according to the length of internal cracks that occur in the high-tensile steel, based on the finding that the internal stress of high-tensile steel depends on its cooling rate.

[0007] Furthermore, Patent Document 2 proposes a method for reducing stress on the slab caused by temperature differences and transformations by immediately starting the slow cooling of the slab after casting, maintaining it at a temperature of 700°C or higher for 10 hours or more, and then further slow cooling the slab at a temperature of 700 to 500°C. In other words, Patent Document 2 discloses a cooling method for slabs for high-strength steel plates that prevents quality defects such as slab cracking during cooling and dents during hot rolling, even in slabs containing Si. Specifically, the cooling method for slabs for high-strength steel plates disclosed in Patent Document 2 involves setting the average cooling rate of a continuously cast slab of high-strength hot-rolled steel plate with a limited content of chemical components such as C, Si, and Mn to 20°C / hr or less at 500 to 700°C. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2020-139209 [Patent Document 2] Japanese Patent Publication No. 2019-167560 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] However, the above-mentioned conventional technologies have the following problems. Higher alloy slabs have lower toughness, and the technologies in Patent Documents 1 and 2 have the problem of not being able to completely suppress slab cracking. Specifically, the method of cooling a high-tensile steel slab after casting described in Patent Document 1 focuses only on the temperature range from 700°C to 500°C when the slab is cooled after casting, and controls the internal stress generated in the slab to be small. For this reason, even if a slab with a higher carbon content is manufactured using the high-tensile steel slab cooling method described in Patent Document 1, the occurrence of slab cracking cannot be sufficiently suppressed.

[0010] Furthermore, the cooling method for high-strength steel slabs described in Patent Document 2 is based on the understanding that the cause of slab cracking lies in thermal stress generated due to Si addition to the steel and temperature unevenness within the slab, and focuses on reducing thermal stress to suppress slab cracking. However, the microstructure of the slab is not limited in the cooling method for high-strength steel slabs described in Patent Document 2. For this reason, even if a slab is manufactured using the cooling method for high-strength steel slabs described in Patent Document 2, the occurrence of slab cracking cannot be sufficiently suppressed. Furthermore, as a result of diligent research by the inventors, it was found that slabs containing large amounts of C, Si, and Mn according to conventional technology have considerably low toughness, making it impossible to completely suppress slab cracking, and that perforation problems occur during rolling.

[0011] The present invention has been made in view of the above circumstances, and aims to provide a continuously cast slab and a method for manufacturing the same that does not cause slab cracking during cooling, even if the continuously cast slab has low toughness, and does not cause perforation problems during rolling. [Means for solving the problem]

[0012] The inventors diligently conducted research to achieve the above objectives. As a result, they analyzed the fracture modes of slab cracks and found that at least one type of fracture surface exists on the fracture surface: 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. Furthermore, while the microstructure at the location where the slab crack extends is mainly composed of ferrite and pearlite, the microstructure further inside is mainly composed of bainite. Here, a bainite-dominant microstructure refers to a low-temperature transformation phase that undergoes a phase change at a lower temperature compared to the ferrite and pearlite contained in the microstructure, and includes at least one selected from bainitic ferrite, tempered martensite, quenched martensite, and retained austenite. In other words, a bainite-dominant microstructure means a microstructure that mainly contains bainitic ferrite and may also contain at least one selected from quenched martensite, tempered martensite, and retained austenite. Normally, objects are cooled from the surface inward, so it is impossible for low-temperature transformation phases to precipitate inside. However, because slabs have a coarse prior austenite grain size, this grain size significantly affects the transformation time of the phases that constitute the microstructure. In other words, we found that when the prior austenite grain size inside the slab is larger than that on the slab surface, even if the slab is cooled in the same way, the microstructure that precipitates in the slab and the timing of its precipitation will differ. From this, it was discovered that after the transformation of the slab surface is complete, at the time of thermal contraction, the low-temperature transformation phase inside the slab undergoes transformation expansion, generating tensile stress on the surface of the slab and leading to slab cracking. Furthermore, the inventors conducted detailed studies and found that by controlling the microstructure of the continuously cast slab and reducing the stress during transformation in the low-temperature transformation phase inside the slab, it is possible to suppress slab cracking during the cooling process of the continuously cast slab and prevent perforation problems during rolling, thus conceiving the present invention.

[0013] In other words, the continuous cast slab according to the present invention, which advantageously solves the above problems, (a) Continuous cast slabs for high-strength steel, In mass%, C: 0.10% or more and 0.50% or less, Si: 0.10% or more and 2.50% or less, Contains Mn: 1.00% to 5.00% It optionally contains at least one element selected from 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. Furthermore, it optionally contains at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, B: 0.0100% or less, Co: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, with the remainder being Fe and unavoidable impurities. The average prior austenite grain size at a position 10 mm below the surface of a continuous casting slab is d 10 , the average prior austenite grain size at a position 20 mm below the surface of the continuous casting slab is d 20 In this case, the average prior austenite grain ratio (d 20 / d 10 ) is between 1.0 and 4.0, The microstructure consists of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite. The sum of the area ratios of the ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite is set to 100%. The microstructure of the continuous casting slab from 0 mm to 10 mm below the surface is such that the sum of the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of retained austenite is 80% or more, and the area ratio of retained austenite is 0% or more and 20% or less, The microstructure of the continuous casting slab, from 10 mm to 20 mm below the surface, is characterized in that the sum of the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of retained austenite is 90% or more, and the area ratio of retained austenite is 0% or more and 20% or less.

[0014] Furthermore, the method for manufacturing a continuous cast slab according to the present invention is a method for manufacturing a continuous cast slab for high-strength steel, (a) After casting a continuous cast slab with the component composition described above, Total heat removal Q within a continuous casting mold MD The following relation (1) is satisfied, The temperature T of the continuous casting slab is at the center in the width direction of the continuous casting slab and 20 mm from the surface of the continuous casting slab. 20 A first cooling step in which the material is cooled under cooling conditions in which the residence time is 230 s or less at a temperature range of 1200°C to 1450°C, A second cooling step is performed at the center of the width direction of the continuously cast slab, and the average cooling rate is 25°C / hr or more when the surface temperature T0 of the surface layer of the continuously cast slab is between 700°C and 850°C. Next, a third cooling step is performed in which the average cooling rate is 15°C / hr or more at temperatures between 500°C and 700°C. Next, a fourth cooling step is performed in which the average cooling rate is 10°C / hr or more at temperatures between 400°C and 500°C. Next, the method is characterized by including a fifth cooling step in which the average cooling rate at 30°C / hr or less is maintained at 200°C to 400°C.

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Advantages of the Invention

[0015] According to the present invention, it is possible to provide a continuous casting slab that does not cause slab cracking during the cooling process and does not cause trouble with holes during rolling, even in the component system of a continuous casting slab for high-strength steel.

Brief Description of the Drawings

[0016] [Figure 1] It is a photograph taken by a scanning electron microscope (SEM) of the fracture surface of a cracked part of a continuous casting slab for high-strength steel broken due to cracking. [Figure 2] It is a cross-sectional tissue photograph of the above cracked part. [Figure 3A] It is an enlarged photograph taken by an optical microscope of a continuous casting slab manufactured in an inventive example (Test No. E-2) of a continuous casting slab according to an embodiment of the present invention, showing the microstructure at a position 10 mm below the surface of the continuous casting slab. [Figure 3B] It is an enlarged photograph taken by an optical microscope of a continuous casting slab manufactured in an inventive example (Test No. E-2) of a continuous casting slab according to an embodiment of the present invention, showing the microstructure at a position 20 mm below the surface of the continuous casting slab.

Modes for Carrying Out the Invention

[0017] Hereinafter, embodiments of the present invention will be specifically described. Note that each drawing is schematic and may differ from the actual one. Further, the following embodiments illustrate devices and methods for embodying the technical idea of the present invention, and do not specify the configuration to the following. That is, various changes can be made to the technical idea of the present invention within the technical scope described in the claims.

[0018] [First Embodiment] A continuous cast slab according to the first embodiment will be described. The continuous cast slab according to this embodiment is a continuous cast slab for high-strength steel, In mass%, C: 0.10% or more and 0.50% or less, Si: 0.10% or more and 2.50% or less, Contains Mn: 1.00% to 5.00% It optionally contains at least one element selected from 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. Furthermore, it optionally contains at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, B: 0.0100% or less, Co: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, with the remainder being Fe and unavoidable impurities. (i) The mean prior austenite grain size at a position 10 mm below the surface of the continuous casting slab is d 10 , the average prior austenite grain size at a position 20 mm below the surface of the continuous casting slab is d 20 In this case, the average prior austenite grain ratio (d 20 / d 10 ) is between 1.0 and 4.0, (ii) The microstructure consists of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite, and the sum of the area percentages of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite is 100%, and the microstructure from 0 mm to 10 mm below the surface of the continuous casting slab is such that the area percentages of ferrite and bainitic ferrite The microstructure of the continuous casting slab, from 10 mm to 20 mm below the surface, is characterized in that the sum of the area ratio of ferrite, the area ratio of tempered martensite, and the area ratio of retained austenite is 80% or more, the area ratio of retained austenite is 0% or more and 20% or less, and the sum of the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of retained austenite is 90% or more, and the area ratio of retained austenite is 0% or more and 20% or less. In other words, according to the invention of this embodiment, by limiting the component composition contained in the continuous casting slab and providing at least the above characteristics (i) to (ii), even in the case of continuous casting slabs for high-strength steel, which have very low toughness in recent years, it is possible to prevent slab cracking during the cooling process and to prevent perforation problems during the rolling of the continuous casting slab, thereby providing a continuous casting slab for high-strength steel with a good yield.

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

[0020] As mentioned above, observation of the fracture morphology of the fracture surface of cracks in continuously cast slabs for high-strength steel that fractured due to slab-standing cracks revealed that most slab-standing cracks propagated to about 20 mm below the slab surface, that they took the form of "intergranular fracture" where the cracks propagated along the prior austenite grain boundaries, and that the microstructure of the cracked area was mainly composed of ferrite and pearlite, while the microstructure inside the crack (towards the center in the thickness direction of the slab) was mainly composed of bainite. In other words, in continuous casting slabs for high-strength steel, the factors causing slab cracking due to grain boundary fracture are thought to be the coarseness of the prior austenite grain size, the difference in microstructure between the slab surface and the slab interior, and the precipitation of low-temperature transformation phases in the slab interior. When the prior austenite grain size is coarse, grain boundary embrittlement due to grain boundary segregation and precipitation of grain boundary ferrite is likely to occur, which is a factor in slab cracking. Furthermore, if the microstructure of the slab surface and the microstructure of the slab interior are different, and low-temperature transformation phases precipitate only in the slab interior, localized transformation expansion of the low-temperature transformation phases can cause slab cracking on the slab surface. Therefore, the invention according to this embodiment focuses on two factors as necessary conditions for a continuously cast slab for high-strength steel that does not experience slab cracking during the cooling process: (i) the average prior austenite grain ratio calculated from the average prior austenite grain size at multiple predetermined positions set from the surface of the continuously cast slab, and (ii) the microstructure of the continuously cast slab.

[0021] <(i) Regarding the average ratio of prior austenite grain size> The continuous cast slab for high-strength steel according to this embodiment is a continuous cast slab for high-strength steel, wherein (i) the average prior austenite grain size at a position 10 mm below the surface of the continuous cast slab is d 10 , the average prior austenite grain size at a position 20 mm below the surface of the continuous casting slab is d 20 In this case, the average prior austenite grain ratio (d 20 / d 10The characteristic is that the ratio is between 1.0 and 4.0. Here, the average prior austenite grain size refers to the average value obtained by averaging the values ​​of multiple prior austenite grain sizes measured in multiple fields of view.

[0022] In conventional continuous casting slabs, the average prior austenite grain size is very large, several millimeters in size. This significantly reduces the toughness of the slab. In addition, the average prior austenite grain size greatly affects the transformation behavior of the microstructure of the continuous casting slab; the larger the prior austenite grain size, the longer the transformation initiation time. As a result, even if the continuous casting slab is cooled slowly, low-temperature transformation phases are more likely to precipitate in its microstructure. Furthermore, the larger the difference in average prior austenite grain size, the more likely a difference in microstructure will occur between the slab surface and the slab interior. From this technical standpoint, the continuous cast slab according to this embodiment has an average prior austenite grain size of d at a position 10 mm below the surface of the continuous cast slab. 10 , the average prior austenite grain size at a position 20 mm below the surface of the continuous casting slab is d 20 In this case, the average prior austenite grain ratio (d 20 / d 10 The value was set to be between 1.0 and 4.0. Thus, the mean prior austenite grain ratio (d 20 / d 10 The upper limit of ) is preferably 4.0 or less. The reason is the average prior austenite grain ratio (d 20 / d 10 This is because if the value is 4.0 or less, the difference in microstructure between the slab surface and the slab interior can be reduced. On the other hand, the average prior austenite grain ratio (d 20 / d 10 The lower limit of ) is not strictly limited. However, in order to make the average prior austenite grain size inside the slab smaller than the average prior austenite grain size on the slab surface, special cooling control is required, and this special cooling control requires capital investment. For this reason, the average prior austenite grain size ratio (d 20 / d 10) is preferably 1.0 or higher. Note that the average prior austenite grain ratio (d 20 / d 10 The lower limit of ) is more preferably 1.2 or higher, and more preferably 1.5 or higher.

[0023] The reason why the average prior austenite grain size was specified as being 10 mm below the surface and 20 mm below the surface of the continuous casting slab is that, since most slab cracks progress to about 20 mm below the surface, the 20 mm position below the surface of the continuous casting slab, and the 10 mm position below the surface of the continuous casting slab, which is located midway between the surface and the 20 mm position below the surface, are considered to be necessary positions to suppress slab cracks. On the other hand, the region less than 5 mm below the surface of the continuous casting slab is rapidly cooled directly by water spray in or directly below the mold. Furthermore, the region less than 5 mm below the surface of the continuous casting slab has a fine structure in which the particles constituting the slab have a gamma particle size, and the toughness of the continuous casting slab in this region is high, so it is unlikely that the starting point of slab cracking originates in the region less than 5 mm below the surface of the continuous casting slab. From this technical standpoint, in the continuously cast slab according to this embodiment, the region less than 5 mm from the surface of the continuously cast slab can be excluded from the area where control of the microstructure of the continuously cast slab is required. Therefore, the locations where control of the continuous casting slab structure is required are, as a first location, 20 mm from the depth in the slab thickness direction, located inside the slab, and as a second location, 10 mm from the depth in the slab thickness direction, located on the slab surface. In other words, as the first position below the surface of the continuous casting slab, the average prior austenite grain size may be set at, for example, 18-22 mm and 15-25 mm in the depth direction from the surface of the continuous casting slab, with a reference point of 20 mm from the surface of the continuous casting slab inside the slab. Furthermore, as a second position below the surface of the continuously cast slab, the average prior austenite grain size may be set at, for example, 8-12 mm and 5-15 mm in the depth direction from the surface of the continuously cast slab, using a position 10 mm from the surface of the continuously cast slab inside the slab as a reference.

[0024] In the continuously cast slab according to this embodiment, the factor determining the average prior austenite grain size is the temperature at which the continuously cast slab is cooled. Austenite grains grow particularly rapidly in the temperature range of 1200°C to 1450°C. Therefore, the temperature at which the continuously cast slab is cooled is particularly in the temperature range of 1200°C to 1450°C, and the cooling rate and residence time in the continuously cast slab within that temperature range have an influence. In other words, in the temperature range of 1200°C to 1450°C, the slower the cooling rate of the continuously cast slab, or the longer the residence time of the continuously cast slab, the coarser the average prior austenite grain size becomes. The prior austenite grain size of a continuously cast slab is minute on the surface side of the slab and coarser towards the interior of the slab. To reduce the difference between the average prior austenite grain size on the surface side of the slab and the average prior austenite grain size inside the slab, it is necessary to coarseen the average prior austenite grain size on the surface side of the slab and finenen the average prior austenite grain size inside the slab.

[0025] In other words, the continuous cast slab according to this embodiment has (i) an average prior austenite grain size at a position 10 mm below the surface of the continuous cast slab. 10 The average prior austenite grain size at a position 20 mm below the surface of the continuous casting slab is d 20 In this case, the average prior austenite grain ratio (d 20 / d 10 In order to satisfy the condition that ) is between 1.0 and 4.0, the temperature T of the continuous casting slab at the center in the width direction of the continuous casting slab and at positions 10 mm and 20 mm from the surface of the continuous casting slab must be 10 , T 20 It is important to control the cooling in the temperature range of 1200°C to 1450°C to manufacture continuously cast slabs. Here T 10 , T 20These values ​​represent the temperatures at 10 mm and 20 mm below the surface of the continuous casting slab, respectively. Temperature T at a position 10 mm from the surface of the continuously cast slab 10 In order to control the cooling rate in the range of 1200°C to 1450°C, the total amount of heat removed within the continuous casting mold must be considered in the manufacturing process of the continuous casting slab according to this embodiment. In other words, the temperature T at a position 10 mm from the surface of the continuous casting slab. 10 The temperature range of 1200°C to 1450°C applies when the continuous casting slab passes through the continuous casting mold, and therefore the total heat removed from the continuous casting mold per unit area Q MD [MW / m 2 ] and the effective mold length L MD Considering the relationship between [m] and the casting speed Vc [m / min], it is necessary to cool the continuous casting slab such that the total amount of heat removed within the continuous casting mold satisfies the following relation (1).

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[0026] Total heat removal amount Q from the mold during continuous casting MD This is calculated by determining the total amount of heat removed from the flow rate of cooling water to the continuous casting mold and the temperature difference between the inlet and outlet of the continuous casting mold, and then dividing the calculated total amount of heat removed by the contact area between the mold copper plate and the cast slab that constitute the continuous casting mold. Effective mold length L MD L is the length of the mold through which the molten steel poured from the dundish can solidify. MD The length of the continuous casting mold is preferably 0.7 to 0.9 [m], although this depends on the type of mold used for continuous casting. The casting speed Vc is the speed at which molten steel is poured into a continuous casting mold and the mold pulls out the initial solidified shell, formed by the solidification of the surface layer of the molten steel. The casting speed Vc is preferably, for example, 0.8 to 2.0 [m / min].

[0027] Here, the total heat removal amount Q from the continuous casting mold. MD [MW / m 2 ] and the effective mold length L MD The relationship between [m] and the casting speed Vc [m / min] is defined by the above relational equation (1), and the value calculated by relational equation (1) is preferably 0.6 to 1.5. If the value calculated by relational equation (1) is 1.5 or less, it is preferable because the average prior austenite grain size at a position 10 mm below the surface of the continuous casting slab can be coarsened, and the difference with the average prior austenite grain size inside the slab can be reduced. On the other hand, if the value calculated by relation (1) is 0.6 or greater, it is preferable because it is possible to secure the shell thickness of the continuous casting slab at the exit side of the continuous casting mold, and the average prior austenite grain size of the continuous casting slab can be coarsened without the risk of breakout.

[0028] Furthermore, the temperature T at a position 20 mm from the surface of the continuous casting slab 20 In order to control the residence time to fall within the range of 1200°C to 1450°C, it is necessary to control the spray water cooling in the manufacturing process of the continuous casting slab according to this embodiment. In other words, the temperature T at a position 20 mm from the surface of the continuous casting slab. 20 The temperature range of 1200°C to 1450°C falls within this range when the continuous casting slab passes through the secondary cooling zone directly below the continuous casting mold. Therefore, the amount of cooling water in this secondary cooling zone is controlled, and the temperature T at a position 20 mm from the surface of the continuous casting slab is controlled. 20 It is preferable to set the residence time in the range of 1200°C to 1450°C to 230 seconds or less. Total heat removal amount Q from the mold during continuous casting MD The above relation (1) is satisfied, and the temperature T at a position 20 mm from the surface of the continuous casting slab is 20If the residence time for the temperature range between 1200°C and 1450°C is set to 230 s or less, then the average prior austenite grain size d at a position 10 mm from the surface of the slab will be... 10 and the average prior austenite grain size d at a position 20 mm below the surface of the slab. 20 The average prior austenite grain ratio (d) is the ratio of to 20 / d 10 This is preferable because it allows the coefficient to be set to 4.0 or less, which can suppress slab cracking.

[0029] Furthermore, from this viewpoint, the residence time of the continuously cast slab is preferably set to 220 s or less, preferably 210 s or less, and more preferably 200 s or less. While there is no particular lower limit to the residence time of the continuously cast slab, if the residence time is too short, the risk of breakout in continuous casting due to non-uniform solidification increases. Therefore, the temperature T at a position 20 mm from the surface of the continuously cast slab is important. 20 The residence time of the continuously cast slab in the temperature range of 1200°C to 1450°C shall be 60 s or more, preferably 80 s or more, and more preferably 90 s or more.

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

[0031] By controlling these cooling conditions, the average prior austenite grain size at 10 mm and 20 mm from the surface of the continuously cast slab can be controlled. Here, it is difficult to directly measure the temperature inside the continuous casting slab. Therefore, the temperature history at 10 mm and 20 mm below the surface of the continuous casting slab is calculated using heat transfer analysis, and the temperature T of the continuous casting slab is determined. 10 , T 20 This can be estimated. In order to maximize the residence time in the above temperature range within the continuously cast slab, the heat transfer analysis position can be set to the center of the slab width.

[0032] (ii) Microstructure of continuously cast slabs The continuous cast slab according to this embodiment (ii) has a microstructure consisting of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite, wherein the sum of the area ratios of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite is 100%, and the microstructure from 0 mm to 10 mm below the surface of the continuous cast slab has an area ratio of ferrite and The present invention is characterized in that the sum of the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of retained austenite is 80% or more, the area ratio of retained austenite is 0% or more and 20% or less, and the microstructure from 10 mm to 20 mm below the surface of the continuous casting slab is characterized in that the sum of the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of retained austenite is 90% or more, and the area ratio of retained austenite is 0% or more and 20% or less. That is, (i) the average prior austenite grain size d at a position 10 mm below the surface of the continuous casting slab. 10 and the average prior austenite grain size d at a position 20 mm below the surface of the continuous casting slab. 20 The average prior austenite grain ratio (d) is the ratio of to 20 / d 10 In addition to the ratio being 4.0 or less, the ratio of internal structures that make up the microstructure of a continuous cast slab, such as ferrite or bainitic ferrite, is also a factor that determines the unit of slab fracture, and it is known that the stress on the slab changes depending on this ratio. In particular, if the low-temperature transformation phase that constitutes the microstructure occurs only within the slab, the amount of expansion of that low-temperature transformation phase increases with its transformation. As a result, localized stress concentration occurs around the low-temperature transformation phase that constitutes the microstructure. Therefore, the inventors found that slab cracking could be reduced by controlling the cooling rate and ensuring that the microstructure satisfies (ii). Furthermore, the sum of the area percentages of ferrite, bainitic ferrite, tempered martensite, and retained austenite can be calculated based on the observation results of the microstructure of the continuous casting slab using observation methods such as optical microscopes and electron microscopes. Additionally, ferrite, bainitic ferrite, tempered martensite, and retained austenite contained in the microstructure of the continuous casting slab can be identified using observation methods such as optical microscopes and electron microscopes.

[0033] Based on the identification results from the observation of the microstructure of the continuous casting slab, the area S of the microstructure of the continuous casting slab was determined. total And the area S of the ferrite F , Perlite area S P The total area S of bainitic ferrite and tempered martensite. (B+AM) Area S of quenched martensite FM , area S of retained austenite rγ Calculate. And the area S of the microstructure of the continuous casting slab. total Area S of ferrite relative to F The total area S of bainitic ferrite and tempered martensite (B+AM) and the area S of retained austenite rγ The total area S (F+B+AM+rγ) The ratio is defined as the area ratio (%) and calculated accordingly.

[0034] The continuous cast slab according to this embodiment is characterized by (i) and (ii) in terms of its microstructure. A continuous cast slab that satisfies the characteristics of (i) and (ii) in this embodiment is preferable because it can suppress slab cracking. The following describes the constituent elements of the microstructure of the continuous casting slab according to this embodiment.

[0035] Bainitic ferrite and tempered martensite have higher toughness compared to pearlite and quenched martensite, which can increase the toughness of the steel and suppress slab cracking. From this technical standpoint, it is preferable that the area ratio of bainitic ferrite and tempered martensite constituting the microstructure of the continuous cast slab according to this embodiment be as high as possible. Specifically, in order to obtain the effects of the invention, the total area ratio of bainitic ferrite, tempered martensite, ferrite, and retained austenite must be 80% or more in the microstructure from 0 to 10 mm below the surface of the continuous casting slab, and 90% or more in the microstructure from 10 to 20 mm below the surface of the continuous casting slab. The area ratio of bainitic ferrite and the area ratio of tempered martensite can be measured by the method described in the examples below.

[0036] Ferrite has lower strength but better ductility compared to bainitic ferrite, tempered martensite, quenched martensite, retained austenite, and pearlite. Therefore, when stress is applied to a microstructure containing a small amount of ferrite, stress concentration can occur in the ferrite, leading to cracks originating from the ferrite. In particular, when the amount of ferrite is small, it tends to be concentrated near the prior austenite grain boundaries, promoting cracking along these boundaries. If the ferrite area ratio is 0%, it is preferable because cracking due to stress concentration in the soft ferrite will not occur. Alternatively, if the ferrite area ratio is 3% or more, it is preferable because a sufficient proportion of the ferrite portion is ensured, and cracking due to stress concentration in the ferrite portion will not propagate. The area ratio of ferrite can be measured by the method described in the examples below.

[0037] The crystal structure of retained austenite constituting the microstructure of the continuously cast slab according to this embodiment is a face-centered cubic lattice (FCC). That is, since retained austenite does not have cleavage planes, the toughness of steel manufactured from a continuously cast slab having a microstructure containing retained austenite can be dramatically improved. Furthermore, retained austenite undergoes martensitic transformation when subjected to high stress. Even if cracks occur in a continuously cast slab due to high stress, martensite is generated at the stress concentration point at the crack tip through the martensitic transformation of retained austenite. Thus, the presence of martensite generated by the martensitic transformation of retained austenite in the microstructure of the continuous casting slab relieves stress concentration and prevents crack propagation in the continuous casting slab.

[0038] From this technical standpoint, it is preferable to set the area ratio of retained austenite constituting the microstructure of the continuous cast slab according to this embodiment to 0% or more. Setting the area ratio of retained austenite constituting the microstructure to 0% or more is preferable because it prevents surface defects in the steel sheet after hot rolling, cold rolling, annealing, or plating of the continuous cast slab. On the other hand, if the area ratio of retained austenite constituting the microstructure exceeds 20%, the amount of structurally unstable retained austenite in the microstructure may increase, and even small stresses can cause martensitic transformation of the retained austenite. As a result, the toughness of steel produced from continuous cast slabs with a microstructure containing retained austenite is significantly reduced. From this technical standpoint, the area ratio of retained austenite constituting the microstructure of the continuous cast slab according to this embodiment should be 20% or less. Preferably, it should be 18% or less, and more preferably 15% or less. The area ratio of retained austenite can be measured by the method described in the examples below.

[0039] Pearlite and quenched martensite have inferior toughness compared to retained austenite, bainitic ferrite, and tempered martensite. Therefore, if pearlite and quenched martensite are present in large quantities in the microstructure, slab cracking may occur starting from these structures. To suppress such slab cracking, the sum of the area ratio of pearlite and the area ratio of quenched martensite must be kept below 20% in the microstructure from 0 to 10 mm below the surface of the continuously cast slab, and below 10% in the microstructure from 10 to 20 mm below the surface of the continuously cast slab. The area ratio of pearlite and the area ratio of quenched martensite can be measured by the method described in the examples below.

[0040] The cooling of a continuously cast slab after it leaves the continuous casting machine can be controlled by changing conditions such as the slab temperature at the exit of the continuous casting machine, the time it takes to stack multiple slabs, the number of slabs to stack, the presence or absence of an insulating cover, and water tensile treatment. The cooling rate of a continuously cast slab after it leaves the continuous casting machine can be measured using a thermocouple. For example, after the slab leaves the machine, a thermocouple can be placed in the center of a wide surface (long side) of the slab's surface, and the cooling rate can be calculated by measuring the surface temperature T0 of the slab.

[0041] As described above, according to the invention of this embodiment, even in the case of continuous casting slabs for high-strength steel, which have very low toughness in recent years, it is possible to prevent slab cracking during the cooling process and to prevent problems such as perforation during rolling, thereby obtaining continuous casting slabs for high-strength steel with a good yield.

[0042] Next, the appropriate range of the component composition of the continuously cast slab according to this embodiment and the reasons for its limitation will be explained. In the following explanation, "%" representing the content of the component elements of steel means "mass%" unless otherwise specified. The continuous casting slab according to this embodiment contains C: 0.10% or more and 0.50% or less, Si: 0.10% or more and 2.50% or less, and Mn: 1.00% or more and 5.00% or less in mass%.

[0043] <C: 0.10% or more and 0.50% or less> C is an element necessary for increasing the strength of a high-strength steel plate made of the continuous casting slab. If the content of C is less than 0.10%, the required strength of the high-strength steel plate cannot be obtained, so the lower limit of the content of C is 0.10%. On the other hand, if the content of C exceeds 0.50%, the area ratio of pearlite and the area ratio of quenched martensite constituting the microstructure of the continuous casting slab may become excessively high. Therefore, the content of C is set to 0.10% or more and 0.50% or less. Further, it is preferably 0.12% or more and 0.45% or less, and more preferably 0.15% or more and 0.40% or less.

[0044] <Si: 0.10% or more and 2.50% or less> Si is an element necessary for ensuring retained austenite in the annealing process of a high-strength steel plate made from the continuous casting slab. In addition, Si contained in the continuous casting slab is an essential additive element because it also contributes to the strengthening of the high-strength steel plate by solid solution strengthening. If the content of Si is less than 0.10%, the required strength of the high-strength steel plate cannot be obtained, so the lower limit of the content of Si is 0.10%. On the other hand, if the content of Si exceeds 2.50%, the effect of obtaining the required strength of the high-strength steel plate saturates, and a strong scale occurs on the hot-rolled sheet before it is processed into the high-strength steel plate. As a result, the appearance and pickling property of the high-strength steel plate are deteriorated, so the upper limit of the content of Si is 2.50%. Therefore, the content of Si is set to 0.10% or more and 2.50% or less. Further, it is preferably 0.50% or more and 2.00% or less, and more preferably 1.00% or more and 1.80% or less.

[0045] <Mn: 1.00% or more and 5.00% or less> Mn is an element necessary to further increase the strength of high-strength steel sheets. Specifically, Mn is added during the hot-rolling process of continuously cast slabs to control the strength of high-strength steel sheets through transformation control. If the Mn content is less than 1.00%, the high-strength steel sheet cannot be sufficiently strengthened, so the lower limit of the Mn content is 1.00%. On the other hand, if the Mn content exceeds 5.00%, there is a risk of excessive quenching martensite fraction in the slab, saturation of the strength-enhancing effect of the high-strength steel sheet, and an increase in the manufacturing cost of the high-strength steel sheet, which is undesirable from an economic standpoint.

[0046] Therefore, the Mn content should be between 1.00% and 5.00%. Furthermore, it is preferable that it be between 1.20% and 4.50%, and more preferably between 1.40% and 4.00%.

[0047] The continuously cast slab according to this embodiment has the above-described component composition, with the remainder being Fe and unavoidable impurities, and possesses an appropriate average prior austenite grain size and microstructure. To that extent, considering other properties, it may contain at least one element selected from P at 0.100% or less, S at 0.0200% or less, Al at 0.100% or less, N at 0.0100% or less, and O at 0.0100% or less. Here, unavoidable impurities include Zn, Pb, and As. The total content of these unavoidable impurities is acceptable at 0.100% or less.

[0048] P can segregate at prior austenite grain boundaries, causing them to become brittle and potentially leading to slab cracking. Therefore, it is preferable to keep the P content below 0.100%. Although there is no specific lower limit for the P content, since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferable to keep it above 0.001%. Accordingly, the P content should be below 0.100%, preferably above 0.001%, and more preferably below 0.070%.

[0049] S exists as a sulfide and is an element that causes slab embrittlement. Therefore, it is preferable to keep the S content at 0.0200% or less. Although there is no specific lower limit for the S content, it is preferable to keep it at 0.0001% or more due to production technology constraints. Therefore, the S content should be 0.0200% or less. Preferably, it should be 0.0001% or more. More preferably, it should be 0.0050% or less.

[0050] Al is an element that affects the fraction of retained austenite in a slab because it suppresses carbide formation during slab cooling and promotes the formation of retained austenite. It is also preferable to add 0.005% or more for deoxidation. If the Al content exceeds 0.100%, it may lead to slab embrittlement. Therefore, the Al content should be 0.100% or less. Preferably, it should be 0.010% or more. More preferably, it should be 0.080% or less.

[0051] N exists as a nitride and is an element that causes slab embrittlement. Therefore, it is preferable to keep the N content at 0.0100% or less. Although there is no specific lower limit for the N content, due to production technology constraints, it is preferable to keep the N content at 0.0001% or more. Therefore, the N content should be 0.0100% or less. Preferably, it should be 0.0001% or more. More preferably, it should be 0.0050% or less.

[0052] O exists as an oxide and is an element that causes slab embrittlement. Therefore, it is preferable to keep the O content at 0.0100% or less. Although there is no specific lower limit for the O content, due to production technology constraints, it is preferable to keep the O content at 0.0001% or more. Therefore, the O content should be 0.0100% or less. Preferably, it should be 0.0001% or more. More preferably, it should be 0.0050% or less.

[0053] The continuously cast slab according to this embodiment, for use in high-strength steel plates, may further contain, in addition to the above component composition, at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Cu: 1.00% or less, B: 0.0100% or less, Co: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, either alone or in combination of two or more elements.

[0054] If the content of Ti, Nb, and V is 0.200% or less, large amounts of coarse precipitates and inclusions will not form in the slab, and the toughness of the slab will not decrease. Therefore, it is preferable to keep the content of Ti, Nb, and V to 0.200% or less. While there is no specific lower limit for the Ti, Nb, and V content, it is more preferable that the Ti, Nb, and V content be 0.001% or more, since the formation of fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing of the continuously cast slab increases the strength of the steel sheet. Therefore, if Ti, Nb, and V are included, their content should be 0.200% or less, more preferably 0.001% or more, and even more preferably 0.100% or less.

[0055] If the content of Ta and W is 0.10% or less, large amounts of coarse precipitates and inclusions will not be formed, and the toughness of the slab will not be reduced. For this reason, it is preferable that the content of Ta and W be 0.10% or less. Although there is no particular lower limit for the content of Ta and W, it is more preferable that the content of Ta and W be 0.01% or more, since the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing of the continuously cast slab. Therefore, if Ta and W are included, their content should be 0.10% or less, more preferably 0.01% or more, and even more preferably 0.08% or less.

[0056] Cr, Mo, and Ni contribute to the increased strength of steel sheets through microstructure control during hot rolling of continuously cast slabs. This effect becomes significant when one or more of Cr, Mo, and Ni are added at a concentration of 0.01% or more, so it is preferable to add at least 0.01%. If the amount of each element exceeds the upper limit for each element, the weldability and hot workability of the steel sheet deteriorate, so the upper limit for the amount of each element, Cr, Mo, and Ni, is set at 1.00%. Therefore, if a continuously cast slab contains Cr, Mo, and Ni, the content of each element should be 1.00% or less. Preferably, it should be 0.01% or more. More preferably, it should be 0.80% or less.

[0057] B may be added to control the microstructure transformation during hot rolling and annealing of continuous cast slabs, as it affects strength through microstructure strengthening. B does not affect the toughness of the slab if it is 0.0100% or less. Therefore, it is preferable to keep the B content at 0.0100% or less. There is no particular lower limit for the B content, but since B is an element that segregates at austenite grain boundaries during hot rolling and annealing of continuous cast slabs and improves hardenability, it is more preferable to keep the B content at 0.0003% or more. Therefore, if B is included, its content should be 0.0100% or less. More preferably, it should be 0.0003% or more. Even more preferably, it should be 0.0080% or less.

[0058] If the Co content is 1.00% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the slab will not decrease. Therefore, it is preferable to keep the Co content at 1.00% or less. Although there is no specific lower limit for the Co content, it is more preferable to have a Co content of 0.001% or more, as Co is an element that improves the hardenability of the slab. Therefore, if Co is included, its content should be 1.00% or less. More preferably, it should be 0.001% or more. Even more preferably, it should be 0.80% or less.

[0059] If the Cu content is 1.00% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the slab will not decrease. Therefore, it is preferable to keep the Cu content at 1.00% or less. Although there is no specific lower limit for the Cu content, it is preferable to have a Cu content of 0.01% or more, as Cu is an element that improves hardenability. Therefore, if Cu is included, its content should be 1.00% or less. More preferably, it should be 0.01% or more. Even more preferably, it should be 0.80% or less.

[0060] If the Sn content is 0.200% or less, it does not affect the toughness of the slab. Therefore, it is preferable to keep the Sn content at 0.200% or less. There is no specific lower limit for the Sn content, but since Sn is an element that improves hardenability, it is more preferable to have a Sn content of 0.001% or more. Therefore, if Sn is included, its content should be 0.200% or less. More preferably, it should be 0.001% or more. Even more preferably, it should be 0.100% or less.

[0061] If the Sb content is 0.200% or less, the amount of coarse precipitates and inclusions does not increase, and the toughness of the slab does not decrease. Therefore, it is preferable to keep the Sb content at 0.200% or less. Although there is no specific lower limit for the Sb content, it is more preferable to have an Sb content of 0.001% or more, as Sb is an element that suppresses decarburization and allows for adjustment of the strength of the steel sheet. Therefore, if Sb is included, its content should be 0.200% or less. More preferably, it should be 0.001% or more. Even more preferably, it should be 0.100% or less.

[0062] If the content of Ca, Mg, and REM is 0.0100% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the slab will not decrease. Therefore, it is preferable to keep the content of Ca, Mg, and REM at 0.0100% or less. While there are no specific lower limits for the content of Ca, Mg, and REM, it is more preferable that their content be 0.0005% or higher, as these elements spheroidize the shape of nitrides and sulfides and improve the toughness of the slab. Therefore, if Ca, Mg, and REM are present, their respective contents should be 0.0100% or less. More preferably, they should be 0.0005% or more. Even more preferably, they should be 0.0050% or less.

[0063] If the content of Zr and Te is 0.100% or less, the amount of coarse precipitates and inclusions in the slab will not increase, and the toughness of the slab will not decrease. Therefore, it is preferable that the content of Zr and Te be 0.100% or less. Although there is no particular lower limit for the content of Zr and Te, since Zr and Te are elements that spheroidize the shape of nitrides and sulfides and improve the toughness of the slab, it is more preferable that the content of Zr and Te be 0.001% or more. Therefore, if Zr and Te are included, their content should be 0.100% or less each. More preferably 0.001% or more. Even more preferably 0.080% or less.

[0064] If the Hf content is 0.10% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the slab will not decrease. Therefore, it is preferable to keep the Hf content at 0.10% or less. Although there is no specific lower limit for the Hf content, since Hf is an element that spheroidizes the shape of nitrides and sulfides and improves the ultimate deformability of the steel sheet, it is more preferable to have an Hf content of 0.01% or more. Therefore, if Hf is included, its content should be 0.10% or less. More preferably, it should be 0.01% or more. Even more preferably, it should be 0.08% or less.

[0065] If the Bi content is 0.200% or less, the amount of coarse precipitates and inclusions will not increase, and the toughness of the slab will not decrease. Therefore, it is preferable to keep the Bi content at 0.200% or less. Although there is no specific lower limit for the Bi content, it is more preferable to have a Bi content of 0.001% or more, as Bi is an element that reduces segregation. Therefore, if Bi is included, its content should be 0.200% or less. More preferably, it should be 0.001% or more. Even more preferably, it should be 0.100% or less.

[0066] Furthermore, regarding the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Co, Cu, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi, if their respective contents are below the preferred lower limit, they will not impair the effects of the present invention and will therefore be included as unavoidable impurities.

[0067] As described above, according to the invention of the first embodiment, the strength required for high-strength steel can be obtained, and furthermore, a continuous cast slab with excellent weldability, workability, and appearance of high-strength steel can be obtained.

[0068] [Second Embodiment] A method for manufacturing a continuous cast slab according to the second embodiment will be described. The method for manufacturing a continuous cast slab according to this embodiment is a method for manufacturing a continuous cast slab for high-strength steel, and after casting a continuous cast slab having the component composition described in the above embodiment, Total heat removal Q within a continuous casting mold MDThe following relation (1) is satisfied, The temperature T of the continuous casting slab is at the center in the width direction of the continuous casting slab and 20 mm from the surface of the continuous casting slab. 20 A first cooling step in which the material is cooled under cooling conditions in which the residence time is 230 s or less at a temperature range of 1200°C to 1450°C, A second cooling step is performed at the center of the width direction of the continuously cast slab, and the average cooling rate is 25°C / hr or more when the surface temperature T0 of the surface layer of the continuously cast slab is between 700°C and 850°C. Next, a third cooling step is performed in which the average cooling rate is 15°C / hr or more at temperatures between 500°C and 700°C. Next, a fourth cooling step is performed in which the average cooling rate is 10°C / hr or more at temperatures between 400°C and 500°C. Next, the method is characterized by including a fifth cooling step in which the average cooling rate at 30°C / hr or less is maintained at 200°C to 400°C.

number

[0069] Steel slabs are manufactured by melting a steel material having the above-described component composition. In this embodiment, the method of melting the steel material is not particularly limited, and any known melting method such as a converter or electric furnace is suitable. Furthermore, while it is preferable to manufacture the steel slab (slab) by a continuous casting method to prevent macrosegregation, it is also possible to manufacture it by a thin slab casting method or the like.

[0070] Note that in the method for manufacturing a slab for a high-strength steel sheet according to this embodiment, repositioning may occur depending on various conditions in the manufacturing process. When repositioning occurs, the cooling rate of the slab may temporarily exceed the established cooling rate. However, since the time required for transformation is very slow at 10 hours or more, as long as the handling time due to repositioning is within a certain range (up to 1 to 2 hours at most), cracking will not occur. Therefore, in the method for manufacturing a slab for a high-strength steel sheet according to this embodiment, the cooling rate when cooling the continuous casting slab is defined as the average cooling rate rather than the maximum cooling rate. Hereinafter, each step included in the method for manufacturing a continuous casting slab according to this embodiment will be described.

[0071] (First cooling step) The method for manufacturing a continuous casting slab according to this embodiment is a method for manufacturing a continuous casting slab for high-strength steel, and includes cooling a continuous casting slab having the component composition described in the above embodiment, Total heat extraction amount Q in the continuous casting mold MD satisfies the following relational expression (1), and is at the center in the width direction of the continuous casting slab and at a position 20 mm from the surface layer of the continuous casting slab. The temperature T 20 of the continuous casting slab is cooled with a residence time of 230 seconds or less in a temperature range of 1200 °C or more and 1450 °C or less. This includes the first cooling step. [Number] In the above relational expression (1), Q MD : Total heat extraction amount in the continuous casting mold [MW / m 2 , L MD : Effective length of the mold [m], Vc: Casting speed [m / min].

[0072] The first cooling step is the ratio of the average prior austenite grain size d 10 at a position 10 mm below the surface layer of the continuous casting slab according to the above embodiment to the average prior austenite grain size d 20 at a position 20 mm below the surface layer of the continuous casting slab. The average prior austenite grain size ratio (d 20 / d 10This is a process for controlling the mean to 4.0 or less. In the method for manufacturing a continuously cast slab according to this embodiment, the factor that determines the mean prior austenite grain size is the temperature when the continuously cast slab is cooled. In the first cooling step, the temperature at which the continuously cast slab is cooled is in the temperature range of 1200°C to 1450°C. Thus, the method for manufacturing a continuously cast slab according to this embodiment focuses on the temperature range of 1200°C to 1450°C of the continuously cast slab, which is the factor that determines the mean prior austenite grain size, and controls that temperature. Furthermore, since it is difficult to measure the temperature at which the continuously cast slab is cooled in the first cooling process, within the temperature range of 1200°C to 1450°C, the temperature history at a position 20 mm from the surface of the continuously cast slab was calculated using heat transfer analysis. The analysis position was set to the center of the slab width, where the residence time in the above temperature range is longest.

[0073] Average prior austenite grain size d at a position 10 mm below the surface of a continuously cast slab 10 And, the average prior austenite grain size d at a position 20 mm below the surface of the continuous casting slab. 20 The average prior austenite grain ratio (d) is the ratio of to 20 / d 10 To reduce this, the average prior austenite grain size d at a position 10 mm below the surface of the continuous casting slab is 10 The average prior austenite grain size d at a position 20 mm below the surface of the continuous casting slab was increased. 20 It needs to be made smaller. In other words, the cooling process should be slow at a depth of 10 mm below the surface of the continuous casting slab, and fast at a depth of 20 mm below the surface.

[0074] Furthermore, in the first cooling process, the total amount of heat removed from the continuous casting mold Q MD However, the effective length of the mold L MD The relationship between [m] and the casting rate Vc [m / min] must satisfy the following relation (1). Furthermore, within the above temperature range where the continuous casting slab is cooled 20 mm from the surface, the residence time of the continuous casting slab is 230 s or less. Furthermore, the total heat removal amount Q within the continuous casting mold. MD However, the effective length of the mold L MD In the relationship between [m] and the casting rate Vc [m / min], if the following relation (1) is satisfied, and the residence time at the above temperature of the continuously cast slab is 230 s or less, then the average prior austenite grain ratio (d 20 / d 10 This is preferable because it allows the coefficient of friction to be reduced to 4.0 or less, thereby suppressing slab cracking. While there is no specific lower limit for the residence time of continuously cast slabs in the temperature range of 1200°C to 1450°C, it is preferable to set the residence time to 60 seconds or longer, as a residence time that is too short increases the risk of breakout in continuous casting due to non-uniform solidification. More preferably, it is 80 seconds or longer, and even more preferably 90 seconds or longer.

number

[0075] (Second cooling process) Next, the method for manufacturing a continuous cast slab according to this embodiment includes a second cooling step in which the slab is cooled at the center in the width direction of the continuous cast slab, and the average cooling rate is 25°C / hr or more when the surface temperature T0 of the surface layer of the continuous cast slab is 700°C or more and 850°C or less.

[0076] The temperature range between 700°C and 850°C is the temperature range in which ferrite and pearlite transformations occur, and the microstructure is controlled by focusing on the cooling rate in this temperature range. In the second cooling process, the cooling rate was measured using a thermocouple. After the slab came out of the continuous casting machine, a thermocouple was placed in the center of the upper surface of the widest side (long side) of the slab, and the cooling rate was calculated from the temperature measured. The cooling rates in the third to fifth cooling processes, described later, can be measured in the same manner.

[0077] The average cooling rate of the continuously cast slab between 700°C and 850°C is 25°C / hr or higher. An average cooling rate of 25°C / hr or higher is preferable because it suppresses the residence time of the continuously cast slab in the ferrite transformation temperature range, avoids stress concentration on the ferrite due to the precipitation of small amounts of ferrite at the grain boundaries, and improves the slab's toughness. Furthermore, if the average cooling rate of the continuously cast slab is less than 25°C / hr, carbon and other elements exceeding the solid solubility limit of ferrite will concentrate near the ferrite precipitate, making it easier for pearlite to precipitate, thus promoting pearlite precipitation, which is undesirable. While there is no specific upper limit on the average cooling rate, the maximum average cooling rate for a single continuous casting slab cooled to ambient temperature between 700°C and 850°C is 120°C / hr. Therefore, cooling at an average cooling rate faster than 120°C / hr is undesirable from an economic standpoint because it requires equipment such as water spraying or air blowing onto the continuous casting slab. From this technical standpoint, the average cooling rate between 700°C and 850°C should be 120°C / hr or less. Preferably, it is 100°C / hr or less, and more preferably 80°C / hr or less.

[0078] (Third cooling process) Furthermore, the method for manufacturing a continuous cast slab according to this embodiment includes a third cooling step in which the continuous cast slab is cooled at the center in the width direction of the continuous cast slab, and the average cooling rate is 15°C / hr or more when the surface temperature T0 of the surface layer of the continuous cast slab is between 500°C and 700°C.

[0079] The temperature range between 500°C and 700°C is primarily the temperature range where pearlite transformation occurs, and the structure is controlled by focusing on the cooling rate in this temperature range. The average cooling rate of the continuously cast slab at temperatures between 500°C and 700°C is 15°C / hr or higher. An average cooling rate of 15°C / hr or higher is preferable because it suppresses pearlite precipitation and improves slab toughness, thereby suppressing slab cracking. While there is no specific upper limit designated for the average cooling rate, the maximum average cooling rate for a single continuous casting slab cooled to atmospheric temperature between 500°C and 700°C is 70°C / hr. Therefore, cooling at an average cooling rate faster than 70°C / hr would require, for example, water spraying or air blowing onto the continuous casting slab, which necessitates additional equipment and is therefore undesirable from an economic standpoint. From this technical standpoint, the average cooling rate between 500°C and 700°C should be 70°C / hr or less. Preferably, it is 60°C / hr or less, and more preferably 50°C / hr or less.

[0080] (Fourth cooling process) Furthermore, the method for manufacturing a continuous cast slab according to this embodiment includes a fourth cooling step in which the slab is cooled at the center in the width direction of the continuous cast slab, and the average cooling rate is 10°C / hr or more when the surface temperature T0 of the surface layer of the continuous cast slab is 400°C or more and 500°C or less. The temperature range between 400°C and 500°C is the temperature range in which pearlite and bainite transformations occur, and the microstructure is controlled by focusing on the cooling rate in this temperature range. Specifically, the fourth step is a process to obtain a bainite-based microstructure mainly composed of bainite ferrite by suppressing the precipitation of pearlite.

[0081] The average cooling rate of the continuously cast slab at temperatures between 400°C and 500°C is 10°C / hr or higher. An average cooling rate of 10°C / hr or higher is preferable because it allows for improved slab toughness and suppression of slab cracking by making the microstructure of the continuously cast slab mainly composed of bainitic ferrite or the like. While there is no strict upper limit on the average cooling rate when cooling a continuous casting slab, the maximum average cooling rate for a single continuous casting slab cooled to atmospheric temperature between 400°C and 500°C is 40°C / hr. Therefore, cooling at an average cooling rate faster than 40°C / hr would require, for example, water spraying or air blowing onto the slab, which necessitates additional equipment and is therefore undesirable from an economic standpoint. From this technical standpoint, the average cooling rate between 400°C and 500°C should be 40°C / hr or less. Preferably, it is 37°C / hr or less, and more preferably 35°C / hr or less.

[0082] (Fifth cooling process) Furthermore, the method for manufacturing a continuous cast slab according to this embodiment includes a fifth cooling step in which the slab is cooled at the center in the width direction of the continuous cast slab, and the average cooling rate is 30°C / hr or less when the surface temperature T0 of the surface layer of the continuous cast slab is between 200°C and 400°C. The temperature range between 200°C and 400°C is the temperature range in which bainite and martensitic transformations occur, with bainite transformation occurring at a higher temperature than martensitic transformation. During bainite transformation, carbon is concentrated in the untransformed austenite during bainite ferrite formation, promoting the formation of retained austenite. To ensure sufficient bainite ferrite and retained austenite fractions in the slab, the cooling rate must be kept below a certain level to allow for adequate bainite transformation time. In martensitic transformation, a portion of the transformed martensite is tempered as a result of being held at high temperatures by the heat of the slab itself, becoming tempered martensite and improving its toughness. To increase the fraction of tempered martensite, the cooling rate must be kept below a certain level in the temperature range between 200°C and 400°C.

[0083] The average cooling rate of the continuously cast slab at temperatures between 200°C and 400°C is 30°C / hr or less. An average cooling rate of 30°C / hr or less is preferable because it suppresses the formation of quenched martensite, which reduces toughness, and results in a structure mainly composed of bainitic ferrite, thereby improving the toughness of the slab and suppressing slab cracking. Preferably, it is 25°C / hr or less, and more preferably 20°C / hr or less. While there is no strict lower limit to the average cooling rate when cooling continuously cast slabs, it is preferable to set it at 5°C / hr or higher from an economic standpoint.

[0084] Thus, the manufacturing method for a continuously cast slab according to this embodiment employs a five-stage cooling process as the cooling step for the continuously cast slab, and by precisely controlling the mean prior austenite grain size ratio and the microstructure of the continuously cast slab, it is possible to provide a continuously cast slab for high-strength steel that can suppress slab cracking caused by cooling and prevent problems such as perforation during rolling.

[0085] As described above, according to the manufacturing method of a continuous cast slab of the second embodiment, even with a composition system for a continuous cast slab for high-strength steel, by dividing the cooling process into five stages and precisely controlling each cooling stage, it is possible to provide a continuous cast slab for high-strength steel that does not cause cracking during the cooling process and prevents problems such as perforation during rolling.

[0086] [Other embodiments] Although the present invention has been described above with reference to embodiments, the present invention is not limited to the above embodiments. Various modifications to the structure and details of the present invention can be made, as can be understood by those skilled in the art within the technical scope of the present invention. Furthermore, any system or apparatus that combines the different features included in each embodiment is also within the technical scope of the present invention. [Examples]

[0087] The effects of the present invention will be specifically described below based on examples, but the present invention is not limited to these examples. That is, in order to confirm the effects of the present invention, the inventors manufactured continuously cast slabs using each type of steel as a raw material in Comparative Examples 1 to 18 (Test Nos. A-1 to A-6, Test Nos. B-1 to B-7, Test Nos. C-1 to C-2, Test Nos. D-1 to D-3) and Invention Examples 1 to 22 (Test Nos. D-1 to D-22). Table 1 shows the composition of the steel used as raw material for the continuously cast slabs in Comparative Examples 1 to 18 (Test Nos. A-1 to A-6, Test Nos. B-1 to B-7, Test Nos. C-1 to C-2, Test Nos. D-1 to D-3) and Invention Examples 1 to 22 (Test Nos. E-1 to E-22).

[0088] [Table 1]

[0089] Here, the cooling conditions for the continuous casting slab consisted of a five-stage cooling process comprising (I) the total heat extraction function of the continuous casting mold [-], (II) residence time at 1450~1200℃ [s], (III) average cooling rate at 850~700℃ [℃ / hr], (IV) average cooling rate at 700~500℃ [℃ / hr], (V) average cooling rate at 500~400℃ [℃ / hr], and (VI) average cooling rate at 400~200℃ [℃ / hr]. Cooling was performed by appropriately changing the conditions of each of these stages. Tables 2 to 7 show the continuous casting slab cooling conditions (I) to (VI), the microstructure of the obtained continuous casting slab, and the evaluation of slab cracking.

[0090] The microstructure was determined as follows: • Micro-tissue determination ○ • When the following conditions (i) and (ii) are met simultaneously (i) Mean prior austenite grain size d at a position 10 mm below the surface of the continuous casting slab 10 And, the average prior austenite grain size d at a position 20 mm below the surface of the continuous casting slab. 20 The average prior austenite grain ratio (d) is the ratio of to 20 / d 10 ) is 4.0 or less (ii) The sum of the area percentages of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite is 100%, and the microstructure 0-10 mm below the slab surface has a total area percentage of 80% or more for ferrite, bainitic ferrite, and retained austenite and a retained austenite area percentage of 20% or less, and the microstructure 10-20 mm below the slab surface has a total area percentage of 90% or more for ferrite, bainitic ferrite, and retained austenite and a retained austenite area percentage of 20% or less. • Microstructure determination × • If at least one of conditions (i) or (ii) is not met

[0091] Furthermore, the following procedures were followed for measuring the average prior austenite grain size, calculating the area ratios of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite in the continuously cast slabs produced in the comparative examples and inventive examples, and evaluating slab cracking in the continuously cast slabs.

[0092] <Measurement of average prior austenite grain size> The method for measuring the average prior austenite grain size is as follows: A sample was cut from the center of the slab's width after cooling, so that the slab thickness cross-section parallel to the slab width direction became the observation surface. Next, the observation surface was mirror-polished using diamond paste, followed by finish polishing with colloidal silica, and then etched with 3 vol.% nital to reveal the microstructure on the observation surface. Using an optical microscope, five fields of view were observed at 10x magnification at 10 mm and 20 mm below the slab surface to obtain microstructure images. The average value of the prior austenite grain size was determined from the obtained microstructure images using the sectioning method in accordance with JIS G 0551:2020.

[0093] <Method for measuring ferrite area ratio> The method for measuring the ferrite area ratio is the same as the method for measuring the average prior austenite grain size described above, by preparing an observation surface of the slab. Next, the observation surface is mirror-polished using diamond paste, then finished polishing is performed using colloidal silica, and finally etching is performed with 3 vol.% nital to reveal the microstructure. Under conditions of an accelerating voltage of 15 kV, 10 fields of view were observed at 50x magnification using an SEM (Scanning Electron Microscope) at positions 10 mm and 20 mm below the slab surface. The ferrite area ratio for the 10 fields of view was calculated using Adobe Photoshop®, and these values ​​were averaged to obtain the ferrite area ratio. Furthermore, ferrite has a larger grain size and a smoother surface with darker contrast compared to pearlite, bainite, tempered martensite, quenched martensite, and retained austenite, making it easily distinguishable at 50x magnification.

[0094] <Method for measuring the area ratio of pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite> The method for measuring the area fraction of the pearlite microstructure is the same as the method for measuring ferrite described above, by exposing the microstructure on the observation surface of the slab. Under conditions of an acceleration voltage of 15kV, using a SEM, 10 fields of view were observed at a magnification of 10,000x at a position 10 mm below the slab surface and 20 mm below the slab surface, with the ferrite excluded from the field of view. The obtained microstructure images were then used with Adobe Photoshop® to calculate the area fractions of pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite for the 10 fields of view, and these values ​​were averaged to obtain the area fraction of each microstructure.

[0095] Here, pearlite is a structure with concave areas and lamellar carbides, bainitic ferrite is a structure with concave areas, tempered martensite is a structure with concave areas and fine carbides, quenched martensite is a structure with convex areas and fine irregularities inside the structure, and retained austenite is a structure with convex areas and a flat inside the structure. Furthermore, since the total area ratio of bainitic ferrite and / or tempered martensite is calculated as the area ratio of bainitic ferrite and / or tempered martensite, they do not need to be distinguishable from each other.

[0096] <Evaluation of slab cracking> The evaluation method for slab cracking was based on the penetrant testing method specified in JIS Z 2343:2017, and the presence or absence of cracks in the wide and narrow surfaces of the slab was evaluated. After applying the developer, the surface cracks that occurred were visually checked by observing the appearance of the penetrant. Furthermore, if there are cracks longer than 50 mm, the risk of slab fracture during slab handling or in the heating furnace is high, and it is also highly likely to lead to perforation problems during rolling. Therefore, the evaluation criteria for slab cracks are as follows. • Slab surface without cracks: "○" - No cracks longer than 50mm on the slab surface. • Slab cracks marked with "△": No cracks longer than 50mm are present on the slab surface, but defects appeared after rolling. • Slab cracks marked with "×" - Slabs with cracks of 50mm or longer on the surface.

[0097] [Table 2]

[0098] [Table 3]

[0099] [Table 4]

[0100] [Table 5]

[0101] [Table 6]

[0102] [Table 7]

[0103] <Comparative Examples (Test No. A-1 to A-6)> Condition A is defined as the microstructural structure of the continuously cast slabs produced in Tests A-1 to A-6. Condition A is the condition in which the ratio of the average prior austenite grain size at 10 mm below the slab surface to the average prior austenite grain size at 20 mm below the surface is greater than 4.0. In these cases, even if various conditions are used for slow cooling of the slab after it leaves the continuous casting machine, differences in transformation rates due to differences in prior austenite grain size result in a large discrepancy in the transformation timing at 10 mm and 20 mm below the slab surface. This causes transformation stress to concentrate on the slab surface, making it impossible to suppress slab cracking.

[0104] <Comparative Examples (Test No. B-1 to B-7)> Condition B refers to the microstructural structure of the continuously cast slabs produced in Tests B-1 to B-7. Condition B is characterized by a ratio of the average prior austenite grain size at 10 mm below the slab surface to the average prior austenite grain size at 20 mm below the surface being 4.0 or less, but with excessive precipitation of pearlite or quenched martensite at at least one of the 10 mm and 20 mm positions below the slab surface. In these cases, the toughness of the slab decreased, and slab cracking could not be suppressed.

[0105] <Comparative Example (Test No. C-1~C-2)> Condition C is defined as the microstructural structure of the continuously cast slabs produced in Tests No. C-1 to C-2. Condition C is a condition in which the ratio of the average prior austenite grain size at 10 mm below the slab surface to the average prior austenite grain size at 20 mm below the surface is 4.0 or less, but slab cracking could not be suppressed because a large amount of retained austenite precipitated. It is thought that as a result of the increased proportion of retained austenite, the proportion of structurally unstable retained austenite increased, and due to residual stress and small stresses during slab handling, martensitic transformation of the retained austenite occurred, reducing the toughness of the slab. It is thought that slab cracking could not be suppressed for the above reasons.

[0106] <Comparative Examples (Test No. D-1 to D-3)> Condition D is defined as the microstructural structure of the continuously cast slabs produced in Tests D-1 to D-3. Condition D is when the steel composition does not satisfy the embodiment of the invention. The ratio of the average prior austenite grain size at 10 mm below the slab surface to the average prior austenite grain size at 20 mm below the surface was 4.0 or less. In particular, in Tests D-1 to D-2, slab cracking occurred despite the same slab cooling control as in the second embodiment. In this case, a large amount of pearlite precipitated, making it impossible to suppress slab cracking.

[0107] <Examples of Inventions (Test No. E-1 to E-22)> Condition E is defined as the microstructure of the slabs that satisfy the continuously cast slabs manufactured in Test Nos. E-1 to E-22. Condition E is the condition of the present invention example, wherein the average prior austenite grain ratio is 4.0 or less, and the sum of the area percentage of ferrite, the area percentage of pearlite, the area percentage of bainitic ferrite, the area percentage of tempered martensite, the area percentage of quenched martensite, and the area percentage of retained austenite is 100%, the microstructure from 0 mm to 10 mm below the surface of the continuous casting slab is such that the sum of the area percentage of ferrite, the area percentage of bainitic ferrite, the area percentage of tempered martensite, and the area percentage of retained austenite is 80% or more, and the area percentage of retained austenite is 0% or more and 20% or less, and the microstructure from 10 mm to 20 mm below the surface of the continuous casting slab is such that the sum of the area percentage of ferrite, the area percentage of bainitic ferrite, the area percentage of tempered martensite, and the area percentage of retained austenite is 90% or more, and the area percentage of retained austenite is 0% or more and 20% or less. No post-cooling cracks occurred in the continuously cast slabs produced in tests No. E-1 to E-22.

[0108] Figure 3 shows magnified optical microscope images of a continuously cast slab produced in the present invention example (Test No. E-2) of the continuously cast slab according to the present invention. The magnified image in Figure 3A shows the microstructure at a position 10 mm below the surface of the continuously cast slab. The magnified image in Figure 3B shows the microstructure at a position 20 mm below the surface of the continuously cast slab.

[0109] As is clear from Figure 3, the continuously cast slab produced in the present invention example (Test No. E-2) of the continuously cast slab was found to have a microstructure corresponding to the area ratio of ferrite at each of the following positions below the slab surface: the average prior austenite grain size at 10 mm below the slab surface and the average prior austenite grain size at 20 mm below the slab surface were controlled.

[0110] According to Tables 2-7 and Figure 3, (i) the average prior austenite grain size at a position 10 mm below the surface of the continuous casting slab is d 10 , the average prior austenite grain size at a position 20 mm below the surface of the continuous casting slab is d 20 In this case, the average prior austenite grain ratio (d 20 / d 10 It was found that slab cracking during slab cooling can be suppressed if (ii) the ratio is 1.0 or more and 4.0 or less, and (ii) the microstructure from 0 mm to 10 mm below the surface of the continuous casting slab has a total area ratio of ferrite, bainitic ferrite, tempered martensite, and retained austenite of 80% or more, with a retained austenite area ratio of 0% or more and 20% or less, and the microstructure from 10 mm to 20 mm below the surface of the continuous casting slab has a total area ratio of ferrite, bainitic ferrite, tempered martensite, and retained austenite of 90% or more, with a retained austenite area ratio of 0% or more and 20% or less. Furthermore, the sum of the area ratios of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite at a position 10 mm below the surface of the continuous casting slab and at a position 20 mm below the surface of the continuous casting slab is 100%.

[0111] In other words, the continuous cast slab of the present invention has an average prior austenite grain size of d at a position 10 mm below the surface of the continuous cast slab. 10 The average prior austenite grain size at a position 20 mm below the surface of the continuous casting slab is d 20 In this case, the average prior austenite grain ratio (d 20 / d 10 ) is between 1.0 and 4.0, the microstructure from 0 mm to 10 mm below the surface of the continuous casting slab is such that the sum of the area ratios of ferrite, bainitic ferrite, tempered martensite, and retained austenite is 80% or more, and the area ratio of retained austenite is between 0% and 20%, and the microstructure from 10 mm to 20 mm below the surface of the continuous casting slab is such that the sum of the area ratios of ferrite, bainitic ferrite, tempered martensite, and retained austenite is 90% or more, and the area ratio of retained austenite is between 0% and 20%, so it is possible to provide slabs for high-alloy, high-strength steel that do not crack after casting, and it is also possible to prevent problems such as perforation during rolling. [Industrial applicability]

[0112] The continuous cast slab of the present invention has an average prior austenite grain size of d at a position 10 mm below the surface of the continuous cast slab. 10 The average prior austenite grain size at a position 20 mm below the surface of the continuous casting slab is d 20 In this case, the average prior austenite grain ratio (d 20 / d 10) is between 1.0 and 4.0, the microstructure from 0 mm to 10 mm below the surface of the continuous casting slab is such that the sum of the area ratios of ferrite, bainitic ferrite, tempered martensite, and retained austenite is 80% or more, and the area ratio of retained austenite is between 0% and 20%, and the microstructure from 10 mm to 20 mm below the surface of the continuous casting slab is such that the sum of the area ratios of ferrite, bainitic ferrite, tempered martensite, and retained austenite is 90% or more, and the area ratio of retained austenite is between 0% and 20%, so it is possible to provide a high-strength steel slab without slab cracking after casting and to prevent problems such as perforation during rolling, thus it is industrially useful.

Claims

1. A continuous cast slab for high-strength steel, In mass percent, C: 0.10% or more and 0.50% or less, Si: 0.10% or more and 2.50% or less, Mn: Contains 1.00% to 5.00% It optionally contains at least one element selected from 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. Furthermore, it optionally contains at least one element selected from Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, B: 0.0100% or less, Co: 1.00% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less, with the remainder being Fe and unavoidable impurities. The average prior austenite grain size at a position 10 mm below the surface of a continuous casting slab is d １０ The average prior austenite grain size at a position 20 mm below the surface of the continuous casting slab is d ２０ In this case, the average prior austenite grain ratio (d ２０ / d １０ ) is between 1.0 and 4.0, The microstructure consists of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite. The sum of the area ratios of the ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite is set to 100%. The microstructure of the continuous casting slab from 0 mm to 10 mm below the surface is such that the sum of the area ratio of ferrite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, and the area ratio of retained austenite is 80% or more, and the area ratio of retained austenite is 0% or more and 20% or less, A continuously cast slab characterized in that the microstructure from 10 mm to 20 mm below the surface of the continuously cast slab has a total area ratio of ferrite, bainitic ferrite, tempered martensite, and retained austenite of 90% or more, and the area ratio of retained austenite is 0% or more and 20% or less.

2. A method for manufacturing a continuous cast slab for high-strength steel, After casting a continuous casting slab having the component composition described in claim 1, Total heat removal Q within the continuous casting mold ＭＤ The following relation (1) is satisfied, The temperature T of the continuous casting slab is at the center in the width direction of the continuous casting slab and at a position 20 mm from the surface of the continuous casting slab. ２０ A first cooling step in which the material is cooled under cooling conditions in which the residence time is 230 s or less at a temperature range of 1200°C to 1450°C, The center of the continuous casting slab in the width direction, and the surface temperature T of the continuous casting slab surface. ０ The first step is a second cooling step in which the average cooling rate is 25°C / hr or more at temperatures between 700°C and 850°C, Next, a third cooling step is performed in which the temperature is cooled at an average cooling rate of 15°C / hr or more between 500°C and 700°C, Next, a fourth cooling step is performed in which the average cooling rate is 10°C / hr or more at temperatures between 400°C and 500°C. The method for manufacturing a continuous cast slab according to claim 1, further comprising a fifth cooling step of cooling at an average cooling rate of 30°C / hr or less at a temperature of 200°C to 400°C. [Math 1] In the above relational expression (1), Q ＭＤ : Total heat extraction amount in the continuous casting mold [MW / m ２ , L ＭＤ : Effective length of the mold [m], Vc: Casting speed [m / min].

Images