Continuously cast slab and method for manufacturing same

Abstract

The present invention provides: a continuously cast slab in which slab season cracking does not occur during cooling of the continuously cast slab even if the continuously cast slab has a low toughness; and a method for manufacturing the same. Disclosed is a continuously cast slab for high-strength steel, which has a specific component composition, the continuously cast slab being characterized in that: the average prior austenite grain size ratio (d20 / d10) of the average prior austenite grain size d20 at the position of 20 mm below a surface layer of the continuously cast slab to the average prior austenite grain size d10 at the position of 10 mm below the surface layer of the continuously cast slab is 1.0 to 4.0 inclusive; the microstructure is composed of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite; and the microstructure at the position of 10 mm below the surface layer of the continuously cast slab and the microstructure at the position of 20 mm below the surface layer of the continuously cast slab are formed of a structure that has a specific area ratio.

Description

Continuous casting slab and method for manufacturing the same

[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 laying and does not cause perforation problems during rolling, and a method for manufacturing the same.

[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] With the decrease in slab toughness due to high alloying, cracks during slab cooling, so-called slab-standing cracks, have become more frequent. When slab-standing cracks occur, the slab may break during transport, making it unsuitable for hot rolling. 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 scratches on the steel sheet after hot rolling, cold rolling, annealing, or plating. Normally, cracks on the slab surface are removed by grinding. However, with high-alloy slabs, the reduced toughness due to the high alloying can cause the cracks to propagate under the stress of grinding, making it impossible to completely remove them. 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 grains. This causes stress concentration in the weaker grain boundary ferrite areas, leading to crack propagation in the slab even at lower stress levels. On the other hand, when the slab is cooled, stress is generated due to differences in thermal contraction and transformation expansion between the slab surface and the slab 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 toughness of the slab is low, deep cracks in the slab that occur in this way are difficult to remove by grinding or other methods, which has been a problem that significantly reduces the yield of the slab.

[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 to 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 between 700°C and 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 500-700°C.

[0008] Japanese Patent Publication No. 2020-139209 Japanese Patent Publication No. 2019-167560

[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 suppresses slab cracking by focusing on reducing thermal stress, based on the finding that the cause of slab cracking lies in thermal stress generated due to the addition of Si to the steel and temperature unevenness within the slab. 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. In addition, as a result of diligent research by the present inventors, it has been found that slabs containing large amounts of C, Si, and Mn according to the 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.

[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 is a low-temperature transformation phase that undergoes a phase transformation at a lower temperature than 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 coarse prior austenite grain sizes, these grain sizes significantly affect the transformation time of the phases constituting the microstructure. Specifically, we found that when the prior austenite grain size inside the slab is larger than that on the slab surface, even with the same cooling process, the resulting microstructure and the timing of its precipitation differ. From this, we discovered that after the transformation of the slab surface is complete, the low-temperature transformation phase inside the slab undergoes transformation expansion during thermal contraction, generating tensile stress on the slab surface and leading to slab cracking. Furthermore, through detailed investigations, the inventors discovered that by controlling the microstructure of continuously cast slabs 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 continuously cast slabs 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, is (a) a continuous cast slab for high-strength steel, which contains, by mass%, C: 0.10% to 0.50%, Si: 0.10% to 2.50%, Mn: 1.00% to 5.00%, and 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 1.0 or more and 4.0 or less, the microstructure consists of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite and retained austenite, 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 has a 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 of 80% or more, and the area percentage 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, wherein after casting a continuous cast slab having the component composition described in (a), the total heat removed from the continuous casting mold Q ＭＤ The following relation (1) is satisfied, and 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 slab 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, and a second step in which the slab is cooled at the center of the width direction of the continuous casting slab and the surface temperature of the surface layer of the continuous casting slab. ０A second cooling step of cooling at an average cooling rate of 25 ° C. / hr or more at 700 ° C. or more and 850 ° C. or less, then a third cooling step of cooling at an average cooling rate of 15 ° C. / hr or more at 500 ° C. or more and 700 ° C. or less, then a fourth cooling step of cooling at an average cooling rate of 10 ° C. / hr or more at 400 ° C. or more and 500 ° C. or less, and then a fifth cooling step of cooling at an average cooling rate of 30 ° C. / hr or less at 200 ° C. or more and 400 ° C. or less, characterized in that it includes these steps. In the above relational expression (1), Q ＭＤ represents the total heat extraction amount in the continuous casting mold [MW / m ２ , L ＭＤ represents the effective length of the mold [m], and Vc represents the casting speed [m / min].

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

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

[0017] Hereinafter, embodiments of the present invention will be specifically described. Note that each drawing is schematic and may be different 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, and contains, by mass%, C: 0.10% to 0.50%, Si: 0.10% to 2.50%, Mn: 1.00% to 5.00%, and 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 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(ii) The ratio is 1.0 or more and 4.0 or less, and the microstructure consists of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite and retained austenite, and the sum of the area ratio of ferrite, the area ratio of pearlite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, the area ratio of quenched martensite and the area ratio of 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 ratio of ferrite and the bainitic ferrite The invention 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 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. In other words, according to the invention of this embodiment, by limiting the component composition contained in the continuous casting slab and having 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 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, and that they took the form of "grain boundary fracture" where the cracks propagated along the prior austenite grain boundaries. It was also found 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 continuously cast slabs for high-strength steel, the factors causing slab-standing cracks due to grain boundary fracture are thought to be the coarse grain size of the prior austenite, 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-standing cracks. Furthermore, if the microstructure of the slab surface differs from that of the slab interior, and the low-temperature transformation phase precipitates only in the interior of the slab, the localized transformation expansion of the low-temperature transformation phase can cause surface cracking of the slab. 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 surface cracking during the cooling process. These factors are defined as (i) the average prior austenite grain ratio calculated from the average prior austenite grain size at multiple predetermined positions on the surface of the continuously cast slab, and (ii) the microstructure of the continuously cast slab.

[0021] <(i) Regarding the average prior austenite grain size ratio> 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 １０ The 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 continuous casting 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 casting 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 casting 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 mean prior austenite grain ratio (d) was set to be between 1.0 and 4.0. ２０ / d １０ The upper limit of ) is preferably 4.0 or less. The reason is the mean prior austenite grain size ratio (d ２０ / d １０ If the mean prior austenite grain ratio (d) is 4.0 or less, the difference in microstructure between the slab surface and the slab interior can be reduced. ２０ / d １０ The lower limit of the average prior austenite grain size ratio (d) 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 The mean prior austenite grain ratio (d) is preferably 1.0 or greater. ２０ / d １０ The lower limit of ) is more preferably 1.2 or higher, and more preferably 1.5 or higher.

[0023] Here, 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 and the 10 mm position below the surface, 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 continuous casting slab have a γ grain size, and the toughness of the continuous casting slab in this region is high, so it is unlikely that the starting point of slab cracks 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 location where control of the microstructure of the continuously cast slab is required. Therefore, the locations where control of the continuously cast 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. That is, as the first location below the surface of the continuously cast slab, the average prior austenite grain size may be set at, for example, 18 to 22 mm and 15 to 25 mm in the depth direction from the surface of the continuously cast slab, based on the 20 mm position from the surface of the continuously cast slab inside the slab. Alternatively, as the second location below the surface of the continuously cast slab, the average prior austenite grain size may be set at, for example, 8 to 12 mm and 5 to 15 mm in the depth direction from the surface of the continuously cast slab, based on the 10 mm position from the surface of the continuously cast slab inside the slab.

[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 of the continuously cast slab in that temperature range have an effect. That is, 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. In order 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 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 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 within a temperature range of 1200°C to 1450°C to manufacture continuously cast slabs. Here T １０ , T ２０ These are the temperatures at 10 mm and 20 mm below the surface of the continuous casting slab, respectively. Temperature T at 10 mm below the surface of the continuous casting 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. That is, 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 relational equation (1). In the above relational equation (1), Q ＭＤ Total heat removal from the mold during continuous casting [MW / m ２ ], L ＭＤ : represents the effective length of the mold [m], and Vc represents the casting speed [m / min].

[0026] Total heat removal amount Q from the mold during continuous casting ＭＤ This is calculated by determining the total 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 heat removed by the contact area between the mold copper plate and the cast slab that constitute the continuous casting mold. The 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 casting speed Vc is preferably 0.7 to 0.9 [m], although this depends on the form of the continuous casting mold. The casting speed Vc is the speed at which the molten steel poured into the continuous casting mold and the mold pull out the initial solidified shell, which is formed when the surface layer of the molten steel solidifies. The casting speed Vc is preferably 0.8 to 2.0 [m / min], for example.

[0027] Here, the total heat removal amount Q from the continuous casting mold. ＭＤ [MW / m ２ ] and the effective mold length L ＭＤThe relationship between [m] and the casting speed Vc [m / min] is defined by the above relational formula (1), and the value calculated by relational formula (1) is preferably between 0.6 and 1.5. If the value calculated by relational formula (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 relational formula (1) is 0.6 or more, it is preferable because the shell thickness of the continuous casting slab can be secured 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. That is, 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 that the residence time in the range of 1200°C to 1450°C be 230 s or less. Total heat removal amount Q from continuous casting mold 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 ２０ If 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 the average prior austenite grain size (d) 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. The temperature T at a position 10 mm below the surface of the continuously cast slab. １０ Since the cooling inside 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 ２０ Since 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 flow rate of water and air.

[0031] By controlling these cooling conditions, the average prior austenite grain size at 10 mm and 20 mm below the surface of the continuous casting slab can be controlled. However, it is difficult to measure the temperature inside the continuous casting slab directly. 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 the continuous cast slab> In this embodiment, the continuous cast slab has a microstructure consisting of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite, with the sum of the area ratios of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite being 100%, and the microstructure from 0 mm to 10 mm below the surface of the continuous cast slab being such that the area ratio of ferrite is equal to the area ratio of retained austenite. The above 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 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 the average prior austenite grain size (d) ２０ / d １０In addition to the ratio being 4.0 or less, the ratio of internal structures constituting the microstructure of a continuously cast slab, such as ferrite and 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 constituting the microstructure occurs only inside the slab, the amount of expansion of the low-temperature transformation phase increases with its transformation. As a result, localized stress concentration occurs around the low-temperature transformation phase constituting the microstructure. Therefore, the inventors have found that slab cracking can be reduced by controlling the cooling rate and making the microstructure satisfy (ii). The sum of the area ratios of ferrite, bainitic ferrite, tempered martensite, and retained austenite can be calculated based on the observation results of the microstructure of the continuously cast slab using observation means such as an optical microscope or an electron microscope. Furthermore, ferrite, bainitic ferrite, tempered martensite, and retained austenite contained in the microstructure of a continuously cast slab can be identified using observation means such as an optical microscope or an electron microscope.

[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 The area S of the microstructure of the continuous casting slab is calculated. total Area S of ferrite 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 satisfying the characteristics of (i) and (ii) in this embodiment is preferable because it can suppress slab cracking. The constituent elements of the microstructure of the continuous cast slab according to this embodiment will be described below.

[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 casting 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 should 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 tempered martensite can be measured by the method described in the examples below.

[0036] Ferrite has lower strength but superior 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 occurs in the ferrite, which can lead to cracking 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. A ferrite area ratio of 0% is preferable because it prevents the cracking caused by stress concentration in the soft ferrite, or a ferrite area ratio of 3% or more is preferable because it ensures a sufficient proportion of ferrite, preventing cracking due to stress concentration in the ferrite. The ferrite area ratio can be measured by the method described in the examples below.

[0037] The crystal structure of retained austenite constituting the microstructure of the continuous cast slab according to this embodiment is a face-centered cubic lattice (FCC). That is, retained austenite does not have cleavage planes, so the toughness of steel manufactured from a continuous 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 a crack occurs in the continuous cast slab due to slab cracking caused by high stress, martensite is generated at the stress concentration point at the crack tip through martensitic transformation of retained austenite. In this way, the presence of martensite generated by the martensitic transformation of retained austenite in the microstructure of the continuous cast slab alleviates stress concentration and can stop the propagation of cracks in the continuous cast 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 martensitic transformation of retained austenite may occur due to small stresses. As a result, the toughness of the steel produced from a continuous cast slab having 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 ratios of pearlite and quenched martensite should be kept below 20% in the microstructure from 0 to 10 mm below the surface of the continuous casting slab, and below 10% in the microstructure from 10 to 20 mm below the surface of the continuous casting slab. The area ratios of pearlite and 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 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 the continuously cast slab after it leaves the machine can be measured using a thermocouple. For example, after the continuously cast slab leaves the machine, a thermocouple can be placed in the center of the wide surface (long side) of the slab's surface to measure the surface temperature T of the continuously cast slab. ０ By measuring this, the cooling rate can be calculated.

[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 percent" unless otherwise specified. The continuously cast slab according to this embodiment contains, by mass percent, C: 0.10% to 0.50%, Si: 0.10% to 2.50%, and Mn: 1.00% to 5.00%.

[0043] <C: 0.10% or more and 0.50% or less> Carbon (C) is an element necessary to increase the strength of high-strength steel sheets made from continuously cast slabs. If the C content is less than 0.10%, the necessary strength for high-strength steel sheets cannot be obtained, so the lower limit of the C content is 0.10%. On the other hand, if the C content exceeds 0.50%, the area ratio of pearlite and the area ratio of quenched martensite that constitute the microstructure of the continuously cast slab may become excessively high. Therefore, the C content should be 0.10% or more and 0.50% or less. Furthermore, it is preferable to have a C content of 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% to 2.50%> Si is an essential element for securing retained austenite in high-strength steel sheets made from continuously cast slabs during the annealing process. In addition, Si contained in continuously cast slabs is an essential additive element because it contributes to increasing the strength of high-strength steel sheets through solid solution strengthening. If the Si content is less than 0.10%, the required strength for high-strength steel sheets cannot be obtained, so the lower limit of the Si content is 0.10%. On the other hand, if the Si content exceeds 2.50%, the effect of obtaining the required strength for high-strength steel sheets saturates, and a strong scale is generated on the hot-rolled sheet before it is processed into high-strength steel sheets. As a result, the appearance and pickling properties of the high-strength steel sheets deteriorate, so the upper limit of the Si content is 2.50%. Therefore, the Si content should be between 0.10% and 2.50%. Furthermore, it is preferable that the amount be between 0.50% and 2.00%, and more preferably between 1.00% and 1.80%.

[0045] <Mn: 1.00% to 5.00%> Mn is an element necessary to further increase the strength of high-strength steel sheets. Specifically, Mn is an element added in the hot-rolling process of continuous casting 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 that the quenched martensite fraction of the slab will become excessive, the strength-enhancing effect of the high-strength steel sheet will saturate, and the manufacturing cost of the high-strength steel sheet will increase, 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, Zn, Pb, and As are examples of unavoidable impurities. The total content of these unavoidable impurities is acceptable at 0.100% or less.

[0048] P segregates at prior austenite grain boundaries, embrittles them, and can cause slab cracking. Therefore, it is preferable to keep the P content at 0.100% or less. 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 at 0.001% or more. Therefore, the P content should be 0.100% or less. Preferably, it should be 0.001% or more. More preferably, it should be 0.070% or less.

[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 continuous cast slab according to this embodiment is for high-strength steel plates and, in addition to the above component composition, further comprises: 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% The product may contain at least one element selected from the following percentages: 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 be formed in the slab, and the toughness of the slab will not be reduced. For this reason, it is preferable that the content of Ti, Nb, and V be 0.200% or less. There is no particular lower limit for the content of Ti, Nb, and V, but 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, it is more preferable that the content of Ti, Nb, and V be 0.001% or more. Therefore, when Ti, Nb, and V are included, their content should be 0.200% or less each. More preferably, it should be 0.001% or more. Even more preferably, it should be 0.100% or less.

[0055] If the content of Ta and W is 0.10% or less each, 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 each. There is no particular lower limit for the content of Ta and W, but 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, it is more preferable that the content of Ta and W be 0.01% or more each. Therefore, if Ta and W are included, their content should be 0.10% or less each. More preferably 0.01% or more. 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 0.01% or more. 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. If B is present in a quantity of 0.0100% or less, it does not affect the toughness of the slab. 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 have a B content of 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. There is no specific lower limit for the Cu content, but since Cu is an element that improves hardenability, it is preferable to have a Cu content of 0.01% or more. 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] Sn content of 0.200% or less does not affect the toughness of the slab. Therefore, it is preferable to keep the Sn content at 0.200% or less. Although there is no specific lower limit for the Sn content, 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 will not increase, and the toughness of the slab will 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 Ca, Mg, and REM are present in amounts of 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 that the content of Ca, Mg, and REM be 0.0100% or less. Although there is no specific lower limit for the content of Ca, Mg, and REM, since Ca, Mg, and REM are elements that spheroidize the shape of nitrides and sulfides and improve the toughness of the slab, it is more preferable that their content be 0.0005% or more. Therefore, if Ca, Mg, and REM are present, their content should be 0.0100% or less each. More preferably, 0.0005% or more. Even more preferably, 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. For this reason, it is preferable that the content of Zr and Te be 0.100% or less. There is no particular lower limit for the content of Zr and Te, but 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. For this reason, it is preferable to keep the Hf content at 0.10% or less. There is no particular lower limit for the Hf content, but 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. For this reason, it is preferable to keep the Bi content at 0.200% or less. There is no specific lower limit for the Bi content, but since it is an element that reduces segregation, it is more preferable to have a Bi content of 0.001% or more. 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, it is possible to obtain the strength required for high-strength steel, and furthermore, to obtain a continuous cast slab of high-strength steel with excellent weldability, workability, and appearance.

[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, the total heat removed from the continuous casting mold Q ＭＤ The following relation (1) is satisfied, and 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 slab 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, and a second step in which the slab is cooled at the center of the width direction of the continuous casting slab and the surface temperature of the surface layer of the continuous casting slab. ０A second cooling step of cooling at an average cooling rate of 25 °C / hr or more at 700 °C or more and 850 °C or less, then a third cooling step of cooling at an average cooling rate of 15 °C / hr or more at 500 °C or more and 700 °C or less, then a fourth cooling step of cooling at an average cooling rate of 10 °C / hr or more at 400 °C or more and 500 °C or less, and then a fifth cooling step of cooling at an average cooling rate of 30 °C / hr or less at 200 °C or more and 400 °C or less, characterized in that it includes these steps. In the above relational expression (1), Q ＭＤ : Total heat extraction amount in the continuous casting mold [MW / m ２ , L ＭＤ : Represents the effective length of the mold [m], Vc: casting speed [m / min].

[0069] A steel slab is produced by melting a steel material having the above-described component composition. In the present embodiment, the melting method of the steel material is not particularly limited, and any known melting method such as a converter or an electric furnace is suitable. Further, the steel slab (slab) is preferably produced by a continuous casting method in order to prevent macrosegregation, but it can also be produced by a thin slab casting method or the like.

[0070] Note that in the manufacturing method of the slab for high-strength steel sheets according to the present embodiment, repositioning may occur depending on various conditions of the manufacturing process. When repositioning occurs, the cooling rate of the slab may temporarily exceed the predetermined cooling rate. However, since the time required for transformation is very slow at 10 hr or more, as long as the handling time for the degree of repositioning (at most 1 to 2 hr), cracking will not occur. Therefore, in the manufacturing method of the slab for high-strength steel sheets according to the present 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 manufacturing method of the continuous casting slab according to the present embodiment will be described.

[0071] (First cooling step) The manufacturing method of the continuous casting slab according to the present embodiment is a manufacturing method of a continuous casting slab for high-strength steel, and the continuous casting slab having the component composition described in the above embodiment is cooled by the total heat extraction amount Q in the continuous casting mold. ＭＤsatisfies the following relational expression (1), is at the center in the width direction of the continuously cast slab, and is the temperature T of the continuously cast slab at a position 20 mm from the surface layer of the continuously cast slab ２０ includes a first cooling step of cooling such that the residence time in the temperature range of 1200 °C or higher and 1450 °C or lower is 230 s or less. In the above relational expression (1), Q ＭＤ : total heat extraction amount in the continuously cast mold [MW / m ２ , L ＭＤ : effective length of the mold [m], Vc: casting speed [m / min].

[0072] The first cooling step is a step for controlling the average prior austenite grain size ratio (d １０ / d ２０ ), which is the ratio between the average prior austenite grain size d ２０ at a position 10 mm below the surface layer of the continuously cast slab according to the above embodiment and the average prior austenite grain size d １０ at a position 20 mm below the surface layer of the continuously cast slab, to 4.0 or less. In the method for manufacturing a continuously cast slab according to the present embodiment, the factor for determining the average prior austenite grain size is the temperature when cooling the continuously cast slab. In the first cooling step, the temperature for cooling the continuously cast slab is in the temperature range of 1200 °C or higher and 1450 °C or lower. Thus, the method for manufacturing a continuously cast slab according to the present embodiment focuses on the temperature range of 1200 °C or higher and 1450 °C or lower of the continuously cast slab, which is a factor for determining the average prior austenite grain size, and controls that temperature. Note that in the first cooling step, since it is difficult to measure the temperature for cooling the continuously cast slab in the temperature range of 1200 °C or higher and 1450 °C or lower, the temperature history at a position 20 mm from the surface layer of the continuously cast slab was calculated by 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 the longest even inside the slab.

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

[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, in 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. In addition, the total heat extraction amount Q in 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 slab to have a value of 4.0 or less, thereby suppressing slab cracking. There is no specific lower limit for the residence time of a continuously cast slab in the temperature range of 1200°C to 1450°C, but if the residence time is too short, the risk of breakout in continuous casting due to non-uniform solidification increases, so it is preferable to have a residence time of 60 s or more. More preferably 80 s or more, and even more preferably 90 s or more. In the above relational equation (1), Q ＭＤ Total heat removal from the mold during continuous casting [MW / m ２ ], L ＭＤ : represents the effective length of the mold [m], and Vc represents the casting speed [m / min].

[0075] (Second Cooling Step) Next, the method for manufacturing a continuous cast slab according to this embodiment is to cool the surface of the continuous cast slab surface at the center in the width direction of the continuous cast slab T ０This includes a second cooling step in which the temperature is 700°C or higher and 850°C or lower, with an average cooling rate of 25°C / hr or higher.

[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 continuous 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 continuous 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. Conversely, an average cooling rate of less than 25°C / hr is undesirable because it promotes pearlite precipitation by causing a concentration of carbon exceeding the solid solubility limit of ferrite near the precipitated ferrite, making pearlite precipitation more likely. There is no specific upper limit for the average cooling rate, but the maximum average cooling rate of a single continuous cast slab between 700°C and 850°C when cooled in the atmosphere 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 for temperatures 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 step) Furthermore, the method for manufacturing a continuous cast slab according to this embodiment is such that the center of the continuous cast slab in the width direction and the surface temperature T of the surface layer of the continuous cast slab are ０ This includes a third cooling step in which the average cooling rate is 15°C / hr or more at temperatures between 500°C and 700°C.

[0079] The temperature range between 500°C and 700°C is the temperature range where pearlite transformation mainly occurs, and the microstructure is controlled by focusing on the cooling rate in this temperature range. The average cooling rate of the continuous casting slab 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. There is no particular upper limit for the average cooling rate, but the maximum average cooling rate of a single continuous casting slab when cooled in the atmosphere between 500°C and 700°C is 70°C / hr. For this reason, cooling at an average cooling rate faster than 70°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 such a technical standpoint, it is preferable to set the average cooling rate between 500°C and 700°C to 70°C to 70°C or lower. Preferably, the temperature is 60°C / hr or less, and more preferably 50°C / hr or less.

[0080] (Fourth Cooling Step) Furthermore, the method for manufacturing a continuous cast slab according to this embodiment is such that the center of the continuous cast slab width direction and the surface temperature T of the continuous cast slab surface layer are ０ The process includes a fourth cooling step in which the average cooling rate is 10°C / hr or higher in the temperature range of 400°C to 500°C. The temperature range of 400°C to 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 step to obtain a bainite-based microstructure mainly composed of bainite ferrite by suppressing the precipitation of pearlite.

[0081] The average cooling rate of the continuous 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 improves the toughness of the slab by making the microstructure of the continuous cast slab mainly composed of bainitic ferrite, and suppresses slab cracking. Although there is no strict upper limit to the average cooling rate when cooling a continuous cast slab, the maximum average cooling rate of a single continuous cast slab cooled by air at temperatures between 400°C and 500°C is 40°C / hr. Therefore, cooling at an average cooling rate faster than 40°C / hr is undesirable from an economic standpoint because it requires equipment such as water spraying or air blowing onto the slab. From such a technical standpoint, the average cooling rate at temperatures between 400°C and 500°C should be 40°C / hr or lower. Preferably, it is 37°C / hr or lower, and more preferably 35°C / hr or lower.

[0082] (Fifth Cooling Step) Furthermore, the manufacturing method of the continuous cast slab according to this embodiment is centered in the width direction of the continuous cast slab, and the surface temperature T of the surface layer of the continuous cast slab ０ The process includes a fifth cooling step in which the average cooling rate is 30°C / hr or less in the temperature range of 200°C to 400°C. The temperature range of 200°C to 400°C is the temperature range in which bainite and martensitic transformations occur, with bainite transformation occurring at a higher temperature than martensitic transformation. In bainite transformation, carbon is concentrated in the untransformed austenite during bainite ferrite formation, promoting the formation of retained austenite. In order to ensure a sufficient fraction of bainite ferrite and retained austenite in the slab, it is necessary to maintain a cooling rate below a certain level to ensure sufficient time for bainite transformation. In martensitic transformation, a portion of the transformed martensite is tempered as a result of being held at a high temperature by the heat of the slab itself, becoming tempered martensite and improving its toughness. In order to increase the fraction of tempered martensite, it is necessary to maintain a cooling rate below a certain level in the temperature range of 200°C to 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. Although there is no strict lower limit to the average cooling rate when cooling the continuously cast slab, it is preferable to set it to 5°C / hr or more 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 the embodiments, the present invention is not limited to the embodiments described above. Various modifications to the structure and details of the present invention can be made, which 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.

[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]

[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 to 1200°C [s], (III) average cooling rate at 850 to 700°C [°C / hr], (IV) average cooling rate at 700 to 500°C [°C / hr], (V) average cooling rate at 500 to 400°C [°C / hr], and (VI) average cooling rate at 400 to 200°C [°C / 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: • Microstructure determination ○ ...When the following conditions (i) and (ii) are met simultaneously (i) 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 the average prior austenite grain size (d) ２０ / d １０) is 4.0 or less (ii) The sum of the area ratio of ferrite, the area ratio of pearlite, the area ratio of bainitic ferrite, the area ratio of tempered martensite, the area ratio of quenched martensite, and the area ratio of retained austenite is 100%, and the microstructure 0 to 10 mm below the slab surface has a total area ratio of ferrite, bainitic ferrite, and retained austenite of 80% or more and a retained austenite area ratio of 20% or less, and the microstructure 10 to 20 mm below the slab surface has a total area ratio of ferrite, bainitic ferrite, and retained austenite of 90% or more and a retained austenite area ratio of 20% or less. Microstructure determination × ... If at least one of conditions (i) and (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, then finished polishing was performed using colloidal silica, and finally etching 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 positions 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 surface of the slab. The ferrite area ratio for the 10 fields of view was calculated using Adobe's 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 ratio 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 the condition of an acceleration voltage of 15 kV, an SEM was used to observe 10 fields of view 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's PHOTOSHOP® to calculate the area ratios of pearlite, bainitic ferrite, tempered martensite, quenched martensite, and retained austenite for 10 fields of view, and these values ​​were averaged to obtain the area ratio of each microstructure.

[0095] Here, pearlite is a structure with concave areas containing lamellar carbides, bainitic ferrite is a structure with concave areas, tempered martensite is a structure with concave areas containing 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 interior. Note that bainitic ferrite and tempered martensite do not need to be distinguishable from each other, as their combined area ratio is calculated as the area ratio of bainitic ferrite and / or tempered martensite.

[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 cracking was visually checked by observing the appearance of the penetrant. In addition, if there is a crack of 50 mm or more in length, the risk of slab fracture during slab handling or in the heating furnace is high, and it is also likely to lead to perforation problems during rolling. Therefore, the evaluation criteria for slab cracking were set as follows: - Slab cracking "○": No cracks of 50 mm or more in length on the slab surface - Slab cracking "△": No cracks of 50 mm or more in length on the slab surface, but defects appeared after rolling - Slab cracking "×": Cracks of 50 mm or more in length are present on the slab surface

[0097]

[0098]

[0099]

[0100]

[0101]

[0102]

[0103] <Comparative Examples (Test Nos. A-1 to A-6)> Condition A is defined as the microstructural structure of the slabs that satisfy the conditions of the continuously cast slabs manufactured in Test Nos. A-1 to A-6. Condition A is the condition in which the ratio of the average prior austenite grain size at a position 10 mm below the slab surface to the average prior austenite grain size at a position 20 mm below the surface becomes greater than 4.0. In these cases, even if the conditions for slow cooling of the slab after it leaves the continuous casting machine are varied, differences in transformation rates occur due to the difference in prior austenite grain size. As a result, the transformation timing at the positions 10 mm and 20 mm below the slab surface is greatly shifted, and transformation stress concentrates on the slab surface, making it impossible to suppress slab cracking.

[0104] <Comparative Examples (Test Nos. B-1 to B-7)> Condition B is defined as the microstructural structure of the slabs that satisfy the continuously cast slabs manufactured in Test Nos. B-1 to B-7. Condition B 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 slab surface is 4.0 or less, but pearlite or quenched martensite precipitates excessively at at least one of the positions 10 mm below the slab surface and 20 mm below the slab surface. In these cases, the toughness of the slab decreases, and slab cracking cannot be suppressed.

[0105] <Comparative Example (Test No. C-1 to C-2)> Condition C is defined as the microstructural structure of the slabs that satisfy the continuously cast slabs manufactured in Test 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. As a result of the increased proportion of retained austenite, the proportion of structurally unstable retained austenite increased, and it is thought that residual stress and small stresses during slab handling caused martensitic transformation of the retained austenite, reducing the toughness of the slab. It is thought that slab cracking could not be suppressed for the above reasons.

[0106] <Comparative Examples (Test Nos. D-1 to D-3)> Condition D is defined as the microstructural structure of the slabs that satisfy the continuously cast slabs manufactured in Test Nos. D-1 to D-3. Condition D is the case where the steel composition does not satisfy the embodiment of the invention. The ratio of the average prior austenite grain size at a position 10 mm below the slab surface to the average prior austenite grain size at a position 20 mm below the surface was 4.0 or less. In particular, in Test Nos. 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, so slab cracking could not be suppressed.

[0107] <Examples of Inventions (Test Nos. 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, where the average prior austenite grain size 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. Test No. No post-cooling cracks occurred in the continuously cast slabs manufactured in 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 a position 10 mm below the slab surface and the average prior austenite grain size at a position 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 １０It 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 casting 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 casting 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 1.0 to 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 0% to 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 0% to 20%, so it is possible to provide a slab for high-alloy, high-strength steel that does not crack after casting, and it is also possible to prevent problems such as perforation during rolling.

[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 1.0 to 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 0% to 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 0% to 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, containing, by mass%, C: 0.10% to 0.50%, Si: 0.10% to 2.50%, Mn: 1.00% to 5.00%, and optionally containing 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 1.0 or more and 4.0 or less, the microstructure consists of ferrite, pearlite, bainitic ferrite, tempered martensite, quenched martensite and retained austenite, 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 has a 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 of 80% or more, and the area percentage 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 continuously cast slab for high-strength steel, wherein after casting the continuously cast slab having the component composition described in claim 1, the total heat extraction amount Q in the continuously casting mold ＭＤ satisfies the following relational expression (1), and the temperature T of the continuously cast slab, which is at the center in the width direction of the continuously cast slab and at a position 20 mm from the surface layer of the continuously cast slab ２０ is cooled in a first cooling step under a cooling condition where the residence time is 230 s or less in a temperature range of 1200°C or higher and 1450°C or lower; the surface temperature T of the surface layer of the continuously cast slab, which is at the center in the width direction of the continuously cast slab ０ is cooled in a second cooling step with an average cooling rate of 25°C / hr or higher at 700°C or higher and 850°C or lower; then, in a third cooling step with an average cooling rate of 15°C / hr or higher at 500°C or higher and 700°C or lower; then, in a fourth cooling step with an average cooling rate of 10°C / hr or higher at 400°C or higher and 500°C or lower; and then, in a fifth cooling step with an average cooling rate of 30°C / hr or lower at 200°C or higher and 400°C or lower. The method for manufacturing a continuously cast slab according to claim 1, characterized by including these steps. In the above relational expression (1), Q ＭＤ : total heat extraction amount in the continuously casting mold [MW / m ２ , L ＭＤ : effective length of the mold [m], Vc: casting speed [m / min].

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