Production method for slab and production method for steel sheet

Abstract

Provided is a production method for a slab that makes it possible to determine the occurrence of cracking in a slab that has been cooled under prescribed cooling conditions.　A production method for a slab according to the present invention involves producing a slab by cooling under cooling conditions under which the ductile-to-brittle transition temperature of a cast, cut slab is at or below a predetermined threshold value.

Description

Slab manufacturing method and steel plate manufacturing method

[0001] The present invention relates to a method for manufacturing a slab capable of suppressing placement cracks in a slab for high strength steel (high tensile steel), and a method for manufacturing a steel plate using the slab.

[0002] In order to reduce the amount of steel used and to lighten products, not only is there a need for further strengthening of steel, but also for properties such as high ductility and high toughness to improve formability and manufacturing efficiency in the process of manufacturing final products. In order to achieve such properties, the content of elements such as Cr and Mo in addition to C, Si, and Mn in steel has increased, but as a result, the toughness of continuously cast slabs and blooms has significantly decreased compared to conventional products.

[0003] As the toughness of slabs decreases, cracks during slab cooling, so-called slab placement cracks (thermal cracks), are becoming more frequent. If placement cracks occur, the slab may break during transportation, making hot rolling impossible. Even if the slab does not break, cracks may open during hot rolling, causing the hot-rolled steel sheet to break.

[0004] When the cracks are small, they appear as surface defects such as scabs and slivers on the steel sheet after hot rolling, cold rolling, annealing, or plating. Cracks on the slab surface are usually removed with a grinder, but due to a decrease in the toughness of the slab, the cracks may grow further due to the stress of the grinder, and the cracks may not be completely removed by the grinder.

[0005] Furthermore, small cracks due to thermal cracks may be overlooked and appear as surface defects on the steel sheet after hot rolling, cold rolling, annealing, or plating. As described above, thermal cracks in slabs reduce yield and efficiency not only in the casting process but also in downstream processes such as hot rolling, so it is necessary to suppress the occurrence of thermal cracks.

[0006] The cracking of slabs is caused by the decrease in ductility and toughness of slabs due to the increase in the strength of steel. It is thought that the cracking occurs due to the thermal stress generated in the slab during cooling. As a countermeasure, various methods have been proposed to suppress the excessive stress generated during the cooling process of the slab.

[0007] Patent Document 1 discloses a slab cooling method that can suppress the occurrence of slab cracking by stacking three or more slabs and maintaining the stacked state for eight hours or more so that the total contact area ratio is 60% or more, and slowly cooling the slabs. According to Patent Document 1, the occurrence of slab cracking can be suppressed by controlling the slab cooling rate according to the length of internal cracks that occur in the slab after casting.

[0008] Patent Document 2 discloses a slab cooling method for a slab produced by continuous casting, in which the cooling rate of the slab is set to 20°C / h or less at 500 to 700°C. According to Patent Document 2, by setting the average cooling rate of a continuously cast slab of high-strength hot-rolled steel sheet at 500 to 700°C to 20°C / h or less, it is possible to prevent not only slab cracking during cooling but also the occurrence of quality defects such as scabs during hot rolling.

[0009] JP 2020-139209 A JP 2019-167560 A

[0010] The methods disclosed in Patent Documents 1 and 2 roughly specify the range of the chemical composition of the slab, but cool the slab within that range under cooling conditions that correspond to the slab that is most susceptible to cracking during heating. As a result, slabs that are less susceptible to cracking during heating are cooled under cooling conditions that correspond to slabs that are susceptible to cracking during heating, which poses the problem of needing to install excessive cooling equipment that corresponds to the cooling rate and reducing slab productivity.

[0011] The present invention has been made in consideration of the problems of the prior art, and its object is to provide a slab manufacturing method that can determine the occurrence of a crack in a slab cooled under predetermined cooling conditions and manufacture the slab. Another object of the present invention is to provide a steel plate manufacturing method that uses the slab manufactured by the slab manufacturing method.

[0012] As a result of extensive research, the present inventors discovered that the occurrence of slab cracking can be evaluated using the ductile-brittle transition temperature (hereinafter referred to as "DBTT") in a Charpy impact test as an index, and thus completed the present invention. The means for solving the above problems are as follows. The slab manufacturing method and steel plate manufacturing method according to the present invention are more preferable means for producing a slab for high-strength steel and a high-strength steel plate containing, in mass %, C: 0.10% to 0.60%, Si: 0.10% to 2.50%, and Mn: 1.00% to 5.00%.

[0013] [1] A method for manufacturing a slab, comprising: cooling the cast and cut slab under cooling conditions such that the ductile-brittle transition temperature of the slab becomes equal to or lower than a predetermined threshold value; [2] The method for manufacturing a slab according to [1], wherein the ductile-brittle transition temperature is calculated using a correspondence relationship between the ductile-brittle transition temperature and carbon equivalent that is previously determined for each microstructure; the carbon equivalent is calculated using the chemical composition of the slab; and the microstructure is determined by observing a cross section of the slab cooled under the cooling conditions. [3] The method for manufacturing a slab according to [1], wherein the ductile-brittle transition temperature is calculated using a correspondence relationship between the ductile-brittle transition temperature and carbon equivalent that is previously determined for each microstructure; the carbon equivalent is calculated using the chemical composition of the slab; and the microstructure is determined by observing a cross section of the slab cooled under cooling conditions that simulate the cooling conditions. [4] The method for producing a slab according to [2] or [3], wherein the carbon equivalent is calculated using the chemical composition of the slab and one or more of the average cooling rates in the temperature ranges where ferrite transformation, pearlite transformation, and bainite transformation occur. [5] The method for producing a slab according to [1], wherein the ductile-brittle transition temperature is calculated using the carbon equivalent of the slab, and the carbon equivalent is calculated using the chemical composition of the slab, the transformation rate of the microstructure in the slab cooled until phase transformation is complete, and the average cooling rate at the surface of the slab. [6] The method for producing a slab according to any one of [1] to [5], wherein the slab is cooled using cooling equipment with cooling conditions that result in the slab being equal to or less than the threshold, selected from among multiple cooling equipment with different cooling conditions. [7] The method for producing a slab according to [6], wherein the slab is cooled using cooling equipment with the fastest cooling rate among the cooling equipment with cooling conditions that result in the slab being equal to or less than the threshold. [8] The method for producing a slab according to [7], wherein the cooling equipment that cools the slab is changed based on the usage status of the multiple cooling equipment. [9] A method for producing a steel plate, comprising rolling a slab produced by the method for producing a slab according to any one of [1] to [5].

[0014] According to the present invention, by producing a slab by cooling it under cooling conditions that make the DBTT equal to or less than a threshold value, it is possible to suppress cracking in the slab. Since it becomes possible to determine whether cracking in the slab occurs under various cooling conditions, it becomes possible to produce a slab by cooling it under the optimal cooling conditions for the slab, thereby achieving both cracking in the slab and improved slab productivity.

[0015] Fig. 1 is a cross-sectional schematic diagram showing a continuous steel casting machine capable of carrying out the slab manufacturing method according to this embodiment. Fig. 2 is a graph showing the results of a Charpy impact test. Fig. 3 is a graph showing an example of a toughness map. Fig. 4 is a TTT curve diagram. Fig. 5 is a diagram showing the correspondence between the area fraction of the microstructure and the type of structure.

[0016] The present invention will be described below through embodiments of the present invention. However, the following embodiments are preferred examples of the present invention, and the present invention is not limited to these embodiments in any way.

[0017] 1 is a cross-sectional schematic diagram showing a continuous steel casting machine 100 capable of implementing the slab manufacturing method according to this embodiment. The continuous steel casting machine 100 mainly comprises a mold 10, a tundish 12, several pairs of cast strip support rolls 32, and slab cooling equipment 50-54. The tundish 12 is installed above the mold 10. A sliding shutter 14 is installed at the bottom of the tundish 12 to adjust the flow rate of molten steel 18 poured into the mold 10, and an immersion nozzle 16 is installed on the underside of this sliding shutter 14.

[0018] Molten steel 18 is poured into the mold 10 through the submerged nozzle 16. The molten steel 18 poured into the mold 10 is solidified as heat is removed from the inner surface of the mold 10, forming a solidified shell 20. This results in the formation of a cast 24 having the solidified shell 20 as its outer shell and an unsolidified layer 22 made of the molten steel 18 inside.

[0019] Below the mold 10, multiple pairs of strand support rolls 32, including support rolls 26, guide rolls 28, and drive rolls 30, are provided. Of these, the drive rolls 30 support the strand 24 while simultaneously withdrawing it. Spray nozzles (not shown), such as water spray nozzles or air mist spray nozzles, are provided in the gaps between adjacent strand support rolls in the casting direction, forming a secondary cooling zone. The strand 24 is cooled by the cooling water sprayed from the spray nozzles in the secondary cooling zone while being withdrawn, reducing the internal unsolidified layer 22 and growing the solidified shell 20. The strand 24 is then appropriately cooled, solidifying the unsolidified layer 22, and the strand 24 is completely solidified. Downstream of the strand support rolls 32, multiple transport rolls 34 are provided to transport the cast strand 24. Above the transport rolls 34, a strand cutter 36 is provided to cut the cast strand 24 into slabs 38 of a predetermined length. The cast 24 is cut by a cast cutter 36 into slabs 38 of a predetermined length. The slabs 38 are cooled to room temperature by cooling equipment 50 to 54 under predetermined cooling conditions.

[0020] The slabs 38 are cooled, for example, in three cooling facilities with different cooling conditions depending on the likelihood of cracking. Cooling facility 50 is a facility that cools the slabs 38 one by one. Cooling facility 52 is a facility that stacks and cools multiple slabs 38. Cooling facility 54 is a facility that stacks and cools multiple slabs 38 in a heat insulation facility 56. Of the cooling facilities 50 to 54, cooling facility 50 is the facility that cools the slabs 38 at the fastest rate, and cooling facility 54 is the facility that cools the slabs 38 at the slowest rate.

[0021] If the toughness of the slab 38 is low, the slab 38 is more likely to develop cracks during storage. Therefore, in the slab manufacturing method according to this embodiment, the DBTT of the slab 38 is focused on as an index related to the toughness of the slab 38, and the occurrence of cracks during storage in the slab is determined based on the DBTT of the slab cooled in the cooling equipment 50-54. Specifically, the DBTT of each slab cooled in the cooling equipment 50-54 is calculated, and the slab is cooled in the cooling equipment under cooling conditions that make the DBTT equal to or less than a predetermined threshold value. This makes it possible to manufacture the slab 38 while suppressing cracks during storage in the slab 38.

[0022] When there are multiple cooling facilities where the DBTT of the slab 38 is equal to or less than the threshold, it is preferable to produce the slab by cooling it using the cooling facility with the fastest cooling rate. For example, if the DBTT of the slab 38 is equal to or less than the threshold whether the slab 38 is cooled using the cooling facility 50 or the cooling facility 52, it is preferable to cool the slab 38 using the cooling facility 50 with the fastest cooling rate. This makes it possible to achieve both the prevention of cracking of the slab 38 and the improvement of the productivity of the slab 38.

[0023] Furthermore, if there are multiple cooling facilities that can reduce the DBTT of the slab 38 to a threshold or lower, the cooling facility that cools the slab 38 may be changed in consideration of the usage status of these cooling facilities. For example, if the DBTT of the slab 38 is reduced to a threshold or lower whether the slab 38 is cooled by the cooling facility 50 or the cooling facility 52, the cooling facility 50 is being fully utilized. In this case, the cooling facility that cools the slab 38 may be changed from the cooling facility 50 to the cooling facility 52. ​​This provides a wider range of options for the cooling facility that cools the slab 38, allowing the cooling facilities 50 to 54 to be used more efficiently and reducing the number of cooling facilities that need to be installed.

[0024] The DBTT threshold is determined in advance by checking the relationship between the occurrence of cracks in the slab 38 and the DBTT, and is set to a value within the range of DBTTs where no cracks in the slab 38 occur. For example, the DBTT threshold may be set to the largest value within the range of DBTTs where no cracks in the slab 38 occur.

[0025] Next, we will explain the DBTT of slabs. When the fracture morphology of the fracture surface of cracked high-strength steel slabs fractured due to thermal cracking was observed, it was confirmed that many of the thermal cracks had propagated to a depth of approximately 20 mm below the surface of the slab and had taken the form of "intergranular fracture" that had propagated to the prior austenite grain boundaries. On the other hand, the region of the slab less than 5 mm below the surface is directly cooled by the mold or by the cooling water directly below the mold, and is therefore thought to have a highly tough structure with a fine γ grain size. Therefore, since it is unlikely that thermal cracks would originate from this region, we determined that the area 5 to 20 mm below the surface of the slab should be the location where toughness should be predicted, and measured the DBTT at a position 10 mm below the surface in the center of the long side of the slab.

[0026] Charpy impact tests were conducted on slabs of various steel types using test pieces taken from a position 10 mm below the surface at the center of the long side of the slab to confirm the relationship between the occurrence of cracking in the slab and DBTT. As a result, it was confirmed that cracking in the slab occurs when the DBTT exceeds 200°C, but does not occur when the DBTT is 200°C or less.

[0027] The relationship between the slab's chemical composition and DBTT was confirmed using the above test results. As a result, a tendency was observed for the DBTT to be higher as the alloying element content of the slab increased, but the variation was large, and it was confirmed that DBTT cannot be predicted with high accuracy by chemical composition and carbon equivalent alone. Therefore, the cooling conditions of the slab were changed to confirm how DBTT changes by changing the microstructure of the slab after cooling.

[0028] FIG. 2 is a graph showing the results of Charpy impact tests. The horizontal axis of FIG. 2 represents the test temperature (°C) in the Charpy impact test, and the vertical axis represents the brittle fracture surface ratio (%). As shown in FIG. 2, when the cooling conditions for a slab of steel type 1 were changed from cooling condition 1 to cooling condition 2, thereby changing the microstructure of the slab after cooling, the DBTT, at which the brittle fracture surface ratio is 50%, also changed. Similarly, for a slab of steel type 2, which has a different chemical composition from steel type 1, the DBTT also changed when the cooling conditions were changed from cooling condition 1 to cooling condition 2, thereby changing the microstructure of the slab after cooling. These results for steel type 1 and steel type 2 confirmed that the DBTT varies not only with the steel chemical composition but also with the microstructure of the slab after cooling.

[0029] As described above, since the DBTT varies depending on the steel composition and microstructure of the slab, in the slab manufacturing method according to this embodiment, the DBTT is determined taking into consideration the microstructure after cooling in addition to the chemical composition of the slab 38. Next, a method for measuring the DBTT of the slab 38 will be described. In the slab manufacturing method according to this embodiment, the DBTT of the slab 38 is determined by the following three methods. Measurement method 1: The DBTT of the slab 38 is determined using a toughness map. Measurement method 2: The microstructure is predicted from the temperature history of the slab and the DBTT of the slab 38 is determined. Measurement method 3: The DBTT of the slab 38 is determined by performing a Charpy impact test.

[0030] First, a method for determining the DBTT of the slab 38 using a toughness map (measurement method 1) will be described. Fig. 3 is a graph showing an example of a toughness map. The horizontal axis of Fig. 3 represents Ceq (carbon equivalent mass%), and the vertical axis represents DBTT (°C).

[0031] A toughness map is a map showing the relationship between DBTT and carbon equivalent determined for each microstructure. In Measurement Method 1, the toughness map shown in Figure 3 is prepared in advance. The toughness map can be prepared by the following steps (1) to (4). (1) Test specimens are taken from the center of the long side of various slabs, 10 mm below the surface, and Charpy impact tests are performed at various test temperatures to determine DBTT. (2) The determined DBTT is plotted on a graph with carbon equivalent (Ceq) on the horizontal axis and DBTT on the vertical axis. (3) The cross-section of the sample taken from the center of the long side of the slab, 10 mm below the surface, is observed. Based on this, the slab is classified into five microstructures: "mainly ferrite + small amount of pearlite," "mainly pearlite + small amount of ferrite," "single-phase pearlite," "bainite + ferrite," and "single-phase bainite." "F to F + P" in Figure 3 refers to mainly ferrite + small amount of pearlite. "P+F" means mainly pearlite + a small amount of ferrite. "P" means single phase pearlite. "B+F" means bainite + ferrite. "B" means single phase bainite. (4) The plots on the graph are classified into five categories corresponding to the above slab classifications, and a linear regression equation for the plots classified into five categories is found. The linear regression equation is not limited to one, and it can be expressed as two linear regression equations.

[0032] To determine the DBTT of the slab 38 using Measurement Method 1, first, the microstructure of the slab 38 is measured. The microstructure is measured by taking a sample from a position 10 mm below the surface at the center of the long side of the slab that has been cooled to room temperature under predetermined cooling conditions, and observing the cross section of the sample. The microstructure may also be measured by taking a sample from a slab that has been cooled under cooling conditions that simulate the predetermined cooling conditions, for example, under laboratory-scale cooling conditions, and observing the cross section of the sample. This makes it easy to take a sample for cross-sectional observation.

[0033] In measuring the area ratio of ferrite, first, the observation cross section of the sample is mirror-polished using diamond paste. Then, the observation cross section is polished to a finish using colloidal silica, and etched with 3 vol% nital to reveal the microstructure on the observation cross section. Under the condition of an acceleration voltage of 15 kV, the observation cross section is observed at 50x magnification using a SEM (Scanning Electron Microscope) to obtain a microstructure image by observing 10 fields of view. Using Adobe's PHOTOSHOP (registered trademark), the area ratio of ferrite is calculated for 10 fields of view from this microstructure image, and the average value of these 10 fields of view is taken as the area ratio of ferrite.

[0034] Ferrite is iron containing a maximum of 0.02% carbon by mass, and is a structure close to pure iron. Ferrite is the softest and most ductile of all steel structures. Ferrite has a larger grain size than other structures (pearlite, bainite, tempered martensite, quenched martensite, and retained austenite), and its smooth surface and dark contrast make it easily distinguishable at 50x magnification.

[0035] To measure the area fractions of pearlite, bainite, and martensite (tempered martensite, quenched martensite, and retained austenite), first, the microstructure is revealed on the cross section of the sample using a method similar to that for measuring ferrite. Using an SEM with an accelerating voltage of 15 kV, the cross section is observed at a magnification of 10,000x, excluding ferrite, to obtain microstructure images. Using Adobe PHOTOSHOP (registered trademark), the area fractions of pearlite, bainite, and martensite are calculated for each of the 10 fields of view from the microstructure images. The average value of each microstructure in the 10 fields of view is calculated, and this average value is combined with the area fraction of ferrite to obtain a total of 100% to determine the area fraction of each microstructure.

[0036] Pearlite is a structure obtained when austenite is slowly cooled. Pearlite consists of ferrite and cementite layers, and is formed by alternating these layers. Bainite is a type of transformed structure formed from the austenite of carbon steel or alloy steel. It has superior ductility and impact resistance, as well as toughness and durability, compared to structures obtained by normal quenching and tempering processes. Bainite is a black needle-like crystal with mechanical properties intermediate between those of martensite and fine pearlite. Martensite is a hard and brittle structure that forms when an austenite structure is rapidly cooled.

[0037] Bainite is a structure of recesses, and pearlite is a structure of recesses containing lamellar carbides. Tempered martensite is a structure of recesses containing fine carbides. Quenched martensite is a structure with convex portions and fine irregularities within the structure. Retained austenite is a structure with convex portions and a flat structure within the structure. Since tempered martensite, quenched martensite, and retained austenite are calculated as the total area fraction of martensite, it is not necessary to distinguish between them. The area fraction of the microstructure measured in this manner is used to determine the microstructural composition ratio of the slab 38. Using this microstructural composition ratio, the microstructure of the slab 38 is determined to be "mainly ferrite + small amount of pearlite," "mainly pearlite + small amount of ferrite," "single-phase pearlite," "bainite + ferrite," or "single-phase bainite." This allows for the linear regression equation for calculating DBTT in the toughness map to be determined.

[0038] Next, a method for calculating the carbon equivalent will be described. The carbon equivalent of a slab can be calculated using the following formula (1).

[0039] Ceq = C + Σαi [Mi] (1) In the above equation (1), Ceq is the carbon equivalent (mass%), C is the carbon concentration (mass%) of the slab 38, [Mi] is the component concentration (mass%) of element i contained in the slab 38, and αi is a parameter.

[0040] Even for slabs with the same chemical composition and the same microstructure, differences in cooling rate can affect toughness. Therefore, instead of using the above formula (1), it is preferable to calculate the carbon equivalent of a slab using the following formula (2), which reflects the effect of the cooling rate in formula (1). The cooling rate CR in formula (2) is calculated using one or more average cooling rates in the temperature ranges where ferrite, pearlite, and bainite transformations occur: 850 to 700°C, 700 to 500°C, and 500 to 300°C, respectively.

[0041] Ceq=C+Σαi[Mi]+Σ(βjCRj+γj) (2) In the above formula (2), Ceq is the carbon equivalent (mass%), and C is the carbon concentration (mass%) of the slab 38. [Mi] is the content (mass%) of element i contained in the slab 38, αi is a parameter, CRj is the average cooling rate (°C / hr), and βj and γj are parameters.

[0042] The parameters in the above formulas (1) and (2) are determined by using the following formula (3) to associate DBTT, which can be measured by a Charpy impact test, with carbon equivalent. Then, each parameter of multiple regression analysis can be determined using a data set that includes a set of the actual DBTT value, the actual content of each element, and the actual cooling rate. When determining each parameter by multiple regression analysis, it is preferable to use 100 or more types of data sets. The parameters may also be determined using machine learning instead of multiple regression analysis.

[0043] DBTT=a×Ceq+b (3) In the above formula (3), DBTT is the ductile-brittle transition temperature (° C.), Ceq is the carbon equivalent (mass %), and a and b are parameters.

[0044] In this way, the carbon equivalent of the slab is calculated using the above formula (1) or (2), and the DBTT of the slab cooled under predetermined cooling conditions is calculated by using the carbon equivalent and the linear regression equation in the toughness map. This is the method for calculating the DBTT of the slab 38 using measurement method 1.

[0045] Next, a method (measurement method 2) for predicting the microstructure from the temperature history of the slab and determining the DBTT of the slab 38 will be described. In measurement method 2, the temperature history of the slab 38 is predicted from the cooling conditions for cooling the slab 38, phase transformation factors are predicted from the temperature history, and the microstructure of the slab 38 is determined from the phase transformation factors.

[0046] The temperature change of the slab 38 can be obtained by solving the three-dimensional heat conduction equation shown in the following equation (4).

[0047] In the above formula (4), ρ is the density (kg / m ３ ), Cp is the specific heat (J / (kg·°C)), λ is the thermal conductivity (W / (m×K)). qi is the transformation heat (W / m ３ ) and this transformation heat will be described later.

[0048] The amount of heat removed by the coolant (water, etc.) on the slab surface, the amount of heat removed by air and convection qh, and the amount of heat removed by radiation qr are calculated using the following equations (5) and (6).

[0049] qh=h(Ts-Tw)...(5)

[0050] qr=εσ((Ts+273) ４ - (Tw+273) ４ ) ... (6)

[0051] In the above equations (5) and (6), qh is the amount of heat removed (W / m ２ ), and h is the heat transfer coefficient (W / (m ２ × K), Ts is the surface temperature of the slab (°C), Tw is the ambient temperature (°C), and qr is the radiation (W / m ２ ) and σ is the Stefan-Boltzmann constant (W / (m ２ ×K ４ )) and ε is the emissivity (-).

[0052] The temperature history of the slab 38 can be determined by calculating the above formulas (4) to (6) and the transformation heat generation amount qi, which will be described later. Next, a method for predicting the microstructure of the slab 38 will be described. The transformation rate and transformation heat generation amount of the microstructure of the slab 38 are calculated using, for example, a TTT diagram.

[0053] Fig. 4 is a TTT curve diagram. The vertical axis of Fig. 4 is temperature (°C), and the horizontal axis is time (s). In Fig. 4, ts indicates the time when the isothermal transformation at temperature T starts, and tf indicates the time when the isothermal transformation at temperature T ends. The TTT diagram may be a diagram created by actual measurements, or one created by calculation software.

[0054] The temperature of the slab 38 changes continuously over time. This continuous temperature change is approximated by a series of stages in which a specific temperature is maintained for a short period of time (a series of isothermal transformations), resulting in a stepwise change in temperature. The transformation rates of the microstructure corresponding to this temperature change are calculated. In this embodiment, the transformation rates of ferrite, pearlite, bainite, and martensite are calculated.

[0055] First, a method for calculating the transformation ratios of ferrite, pearlite, and bainite will be described. The transformation start conditions for each phase are the equilibrium transformation start temperature T ０ Hereinafter, this is defined as the time when the consumption I of the incubation period shown in the following formula (7) reaches 1. Once the temperature drops below the equilibrium transformation start temperature and then exceeds the equilibrium transformation start temperature, the consumption of the incubation period returns to 0. This incubation period is calculated simultaneously for each of the ferrite, pearlite, and bainite phases, and if the incubation period of another phase reaches 1 during the transformation of another phase, the phase transformation of the currently transforming phase ends, and a new phase transformation of the other phase begins.

[0056]

[0057] After the start of transformation, the transformation rate is calculated assuming that isothermal transformation proceeds for a very short time dt at temperature T. That is, the start time ts and end time tf of isothermal transformation at temperature T are obtained from the TTT model, and the transformation rate f after time t + dt is calculated. Here, time t is a fictitious time that differs from actual time. The logarithmic time LT, which is dimensionless from the transformation start time and transformation end time, is given by equation (8).

[0058]

[0059] The transformation rate can be expressed by the following equation (9). Here, k and n are constants. A value of approximately 2 to 4 can be used for n. Since the transformation end time in the following equation (9) becomes infinite, it is advisable to multiply it by a constant C (1.001) close to 1, as in the following equation (10). The logarithmic time LT is calculated by using the logarithmic time LT, which is dimensionless from the transformation start time and transformation end time, so that f = 0 at T = 0 and f = feq at T = 1. Here, f = feq at T = 1, resulting in the following equation (11). feq is the equilibrium transformation rate (-) during ferrite transformation, and is the retained austenite phase fraction at the start of each phase transformation after pearlite transformation. In the ferrite transformation region, feq is the ferrite fraction calculated from the equilibrium phase diagram, and the ferrite fraction depends on the temperature. k in the following equation (9) can be calculated using the following equation (11).

[0060] f=1-exp(-k(LT) ｎ ) ... (9)

[0061] f=feq×C{1-exp(-k(LT) ｎ ) ... (10)

[0062] k=-Ln((C-1) / C)...(11)

[0063] The logarithmic time LT at a certain step is calculated by finding the time when the transformation rate becomes equivalent to that of the previous step using the above formula (8), and calculating it as the time that has progressed by a small amount from this state. By repeating the above calculations for each phase of ferrite, pearlite, and bainite, the transformation rates f Ｆ , f Ｐ , f Ｂ (-) is required.

[0064] The martensitic transformation is performed at the martensitic transformation rate f Ｍ It can be calculated based on equation (12) or (13) assuming that (-) progresses linearly or nonlinearly with respect to temperature between the phase transformation start temperature and the phase transformation finish temperature.

[0065] f Ｍ = f ｅｑ ×((T-T Ｍｆ ) / (T Ｍｓ -T Ｍｆ )) ... (12)

[0066] f Ｍ = f ｅｑ ×((1-exp{-A ０ (T-T Ｍｆ )} / (1-exp{-A ０ (T Ｍｓ -T Ｍｆ )} (13) In the above formulas (12) and (13), T is temperature (°C), and T Ｍｓ and T Ｍｆ are the start and end temperatures (°C) of martensitic transformation, and A ０ is a constant, and in this embodiment, 0.018 is used.

[0067] Total transformation rate f ＡＬＬ can be calculated using the following formula (14): In this way, the transformation rates of ferrite, pearlite, bainite, and martensite can be derived.

[0068] f ＡＬＬ = f Ｆ +f Ｐ +f Ｂ +f Ｍ ...(14)

[0069] Next, a method for calculating the transformation heat generation amount qi from the transformation rate will be described. The transformation heat generation amount qi can be calculated using the following formula (15).

[0070] qi = L × (δf ＡＬＬ / δt) (15) In the above formula (14), qi is the transformation heat value (W / m ３ ), t is time (s), and f ＡＬＬ is the total transformation rate (-). L is the latent heat of transformation (J / g), which is a physical quantity that is uniquely identified once the composition of the steel material is specified. This latent heat of transformation can be calculated by inputting the component composition of the slab into commercially available calculation software (e.g., Thermo-Calc, etc.).

[0071] The constituent ratios of the microstructure of the slab 38 are determined from the transformation rates of ferrite, pearlite, bainite, and martensite calculated in the transformation factor prediction process. Unless otherwise specified, the calculation results are determined from the calculation results when the temperature reaches room temperature and the phase transformation is complete. The transformation rates of ferrite, pearlite, bainite, and martensite are used as the constituent ratios of these microstructures, and the microstructure of the slab 38 is determined from these constituent ratios.

[0072] Fig. 5 is a diagram showing the correspondence between the area ratio of the microstructure and the type of structure. Using Fig. 5 and the composition ratio of the microstructure, it is determined whether the microstructure of the slab 38 corresponds to "mainly ferrite + small amount of pearlite", "mainly pearlite + small amount of ferrite", "single pearlite phase", "bainite + ferrite", or "single bainite phase".

[0073] Next, a method for calculating the carbon equivalent will be described. The carbon equivalent is calculated using formula (16) or formula (17) corresponding to the microstructure of the slab 38. When the microstructure of the slab 38 is "mainly ferrite + small amount of pearlite", "mainly pearlite + small amount of ferrite", or "single phase pearlite", the carbon equivalent of the slab 38 is calculated using formula (16) below. On the other hand, when the microstructure of the slab 38 is "mainly ferrite + small amount of ferrite" or "single phase bainite", the carbon equivalent of the slab 38 is calculated using formula (17) below.

[0074] Ce, fp=C+Si / 24+Mn / 24+P-Al / 50+(Cu+Ni) / 40+Cr / 6+Mo+V / 14+Nb / 2+10B+0.0035CR-0.14...(16)

[0075] Ce,b = C + Si / 24 + Mn / 6 + P - Al / 50 + (Cu + Ni) / 40 + Cr / 6 + Mo + V / 14 + 1.5Nb + 10B - 0.004CR + 0.16 (17) In the above formulas (16) and (17), Ce,fp and Ce,b are carbon equivalents (mass%), and each element symbol represents the content (mass%) of each element, or 0 if the element is not contained. CR is the average cooling rate (°C / hr) when the surface temperature of the slab 38 is 500°C or higher and 700°C or lower. The coefficients and parameters of each element and cooling rate may be changed to coefficients and parameters identified by multiple regression analysis using a data set consisting of a set of actual values ​​for DBTT, component concentration, and cooling rate.

[0076] The carbon equivalent thus determined and the above formula (3) are used to determine the DBTT of the slab 38 cooled under predetermined cooling conditions. This is the method for calculating the DBTT of a slab using measurement method 2.

[0077] Next, a method for determining the DBTT of a slab by performing a Charpy impact test (measurement method 3) will be described. In this method, the DBTT of the slab 38 is measured by performing a Charpy impact test under conditions where the measurement temperature is changed.

[0078] A sample is taken from the center of the long side of a slab cooled under specified cooling conditions, 10 mm below the surface, and the sample is subjected to a Charpy impact test at different temperatures. By performing this Charpy impact test, the DBTT, which is the temperature at which the brittle fracture surface ratio becomes 50%, can be determined. This is the method for calculating the DBTT of a slab using Measurement Method 3.

[0079] As described above, in the slab manufacturing method according to this embodiment, the DBTT of the slab is measured by carrying out a Charpy impact test, or the DBTT of the slab is measured taking into consideration the microstructure of the slab 38. This makes it possible to measure the DBTT of the slab 38 with high accuracy, and as a result, it is possible to determine with high accuracy the occurrence of thermal cracking in the slab 38 under specified cooling conditions, and it becomes possible to manufacture slabs by cooling them under optimal cooling conditions.

[0080] The slab manufacturing method according to this embodiment is suitable for manufacturing a slab for high-strength steel containing, in mass%, C: 0.10% to 0.60%, Si: 0.10% to 2.50%, and Mn: 1.00% to 5.00%. The composition of each chemical component contained in the slab for high-strength steel will be described below. In the following description, "%" indicating the content of a component element in steel means "mass%" unless otherwise specified.

[0081] C: 0.10% or more and 0.60% or less The carbon contained in the slab 38 is an element necessary for increasing the strength of high-strength steel plates made from the slab. If the carbon content is less than 0.10%, the strength required for high-strength steel plates cannot be obtained, so the lower limit of the carbon content is 0.10%. On the other hand, if the carbon content exceeds 0.60%, there may be an excess of quenched martensite in the slab, which may cause cracks to originate from these structures, which is undesirable.

[0082] Therefore, the C content of the slab 38 is preferably 0.10% or more and 0.60% or less, more preferably 0.12% or more and 0.50% or less, and even more preferably 0.15% or more and 0.40% or less.

[0083] Si: 0.10% or more and 2.50% or less The Si contained in the slab 38 is an element necessary for ensuring retained austenite in the high-strength steel sheet during the annealing process of the steel sheet made from the slab. In addition, Si is an essential additive element because it contributes to increasing the strength of the high-strength steel sheet through solid solution strengthening. If the Si content is less than 0.10%, the strength required for the high-strength steel sheet cannot be obtained. Therefore, the lower limit of the Si content is 0.10%.

[0084] On the other hand, if the Si content exceeds 2.50%, the effect of obtaining the strength required for a high-strength steel sheet saturates, and strong scales are formed on the hot-rolled steel sheet before being processed into a high-strength steel sheet. As a result, the appearance and pickling properties of the high-strength steel sheet deteriorate. Therefore, the upper limit of the Si content is 2.50%.

[0085] Therefore, the Si content of the slab 38 is preferably 0.10% or more and 2.50% or less, more preferably 0.50% or more and 2.00% or less, and even more preferably 1.00% or more and 1.80% or less.

[0086] Mn: 1.00% or more and 5.00% or less Mn contained in the slab 38 is an element necessary for further increasing the strength of the high-strength steel plate. Specifically, Mn is an element added to control the strength of the high-strength steel plate through transformation control during the hot rolling process of the continuously cast slab. If the Mn content is less than 1.00%, the high-strength steel plate cannot be sufficiently strengthened. Therefore, the lower limit of the Mn content is 1.00%. On the other hand, if the Mn content exceeds 5.00%, the quenched martensite fraction of the slab may become excessive, and the effect of improving the strength of the high-strength steel plate saturates, increasing the manufacturing cost of the high-strength steel plate, which is undesirable from an economic standpoint.

[0087] From this viewpoint, the Mn content of the slab 38 is preferably 1.00% or more and 5.00% or less, more preferably 1.50% or more and 4.50% or less, and even more preferably 1.80% or more and 4.00% or less.

[0088] The slab produced by the slab manufacturing method according to this embodiment has the above-described chemical composition, with the balance consisting of Fe and unavoidable impurities, and has an average prior austenite grain size and microstructure of appropriate composition after cooling. Insofar as this is the case, taking other properties into consideration, the slab may contain 0.001% to 0.100% P, 0.0001% to 0.0200% S, and 0.005% to 0.100% Al. Similarly, the slab may contain 0.0001% to 0.0100% N and 0.0001% to 0.0100% O. Examples of unavoidable impurities include Zn, Pb, and As. A total content of 0.100% or less of these unavoidable impurities is permitted.

[0089] P segregates at prior austenite grain boundaries and embrittles the grain boundaries, which may cause stabilization cracks in the slab. Therefore, the P content is preferably 0.100% or less. There is no particular lower limit for the P content, but since P is a solid solution strengthening element and can increase the strength of the steel sheet, it is preferably 0.001% or more. Therefore, the P content is preferably 0.001% or more and 0.100% or less, and more preferably 0.001% or more and 0.070% or less.

[0090] S exists as sulfide and is an element that embrittles the slab. Therefore, the S content is preferably 0.0200% or less. There is no particular need to specify a lower limit for the S content, but due to constraints on production technology, it is preferably 0.0001% or more. Therefore, the S content is preferably 0.0001% or more and 0.0200% or less, and more preferably 0.0001% or more and 0.0050% or less.

[0091] Al is an element that suppresses the formation of carbides during slab cooling and promotes the formation of retained austenite, thereby affecting the fraction of retained austenite in the slab. Al is preferably added in an amount of 0.005% or more for deoxidation. On the other hand, an Al content exceeding 0.100% may cause slab embrittlement. Therefore, the Al content is preferably 0.005% or more and 0.100% or less, and more preferably 0.005% or more and 0.080% or less.

[0092] N exists as a nitride and is an element that causes embrittlement of the slab. Therefore, the N content is preferably 0.0100% or less. There is no particular need to specify a lower limit for the N content, but due to constraints on production technology, the N content is preferably 0.0001% or more. Therefore, the N content is preferably 0.0001% or more and 0.0100% or less, and more preferably 0.0001% or more and 0.0050% or less.

[0093] O exists as an oxide and is an element that causes embrittlement of the slab. Therefore, the O content is preferably 0.0100% or less. There is no need to specify a lower limit for the O content, but due to constraints on production technology, the O content is preferably 0.0001% or more. Therefore, the O content is preferably 0.0001% or more and 0.0100% or less, and more preferably 0.0001% or more and 0.0050% or less.

[0094] The slab manufactured by the slab manufacturing method according to this embodiment is for use as a high-strength steel plate, and in addition to the above-mentioned chemical composition, further contains Ti: 0.001% or more and 0.200% or less, Nb: 0.001% or more and 0.200% or less, V: 0.001% or more and 0.200% or less, Ta: 0.01% or more and 0.10% or less, W: 0.01% or more and 0.10% or less, Cr: 0.01% or more and 2.00% or less, Mo: 0.01% or more and 2.00% or less, Ni: 0.01% or more and 2.00% or less, Cu: 0.01% or more and 2.00% or less, B: 0.0003% or more and 0.010% or less, At least one element selected from the group consisting of 0.001% or more and 0.200% or less, Sn: 0.001% or more and 0.200% or less, Sb: 0.001% or more and 0.200% or less, Ca: 0.0005% or more and 0.0100% or less, Mg: 0.0005% or more and 0.0100% or less, REM: 0.0005% or more and 0.0100% or less, Zr: 0.001% or more and 0.100% or less, Te: 0.001% or more and 0.100% or less, Hf: 0.01% or more and 0.10% or less, and Bi: 0.001% or more and 0.200% or less may be contained alone or in combination of two or more thereof.

[0095] When the Ti, Nb, and V contents are each 0.200% or less, large amounts of coarse precipitates and inclusions are not formed, and the toughness of the slab is not reduced. Therefore, the Ti, Nb, and V contents are preferably each 0.200% or less. The lower limits of the Ti, Nb, and V contents do not need to be particularly specified, but they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing of continuously cast slabs. Therefore, the Ti, Nb, and V contents are preferably each 0.001% or more. Therefore, when Ti, Nb, and V are contained, their contents are preferably 0.001% or more and 0.200% or less, and more preferably 0.001% or more and 0.100% or less.

[0096] If the Ta and W contents are each 0.10% or less, large amounts of coarse precipitates and inclusions are not formed, and the toughness of the slab is not reduced. Therefore, the Ta and W contents are preferably each 0.10% or less. There is no need to specify lower limits for the Ta and W contents, but they increase the strength of the steel sheet by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing of continuously cast slabs. Therefore, the Ta and W contents are preferably each 0.01% or more. Therefore, when Ta and W are contained, their contents are preferably 0.01% or more and 0.10% or less, and more preferably 0.01% or more and 0.08% or less.

[0097] Cr, Mo, Ni, and Cu have the effect of increasing the strength of steel sheets through structural control during hot rolling of slabs. This effect becomes more pronounced when one or more of Cr, Mo, Ni, and Cu are added in an amount of 0.01% or more, respectively, so it is preferable to add 0.01% or more. If the content of each element exceeds 2.00%, the weldability, hot workability, etc. of the steel sheet deteriorate. Therefore, the upper limit of the amount of each of Cr, Mo, Ni, and Cu is preferably 2.00%. Therefore, when Cr, Mo, Ni, and Cu are contained, their contents are preferably 0.01% or more and 2.00% or less, and more preferably 0.01% or more and 1.00% or less.

[0098] B may be added because it controls the structural transformation during hot rolling and annealing of slabs and affects strength through structural strengthening. If the B content is 0.0100% or less, it does not affect the toughness of the slab. Therefore, the B content is preferably 0.0100% or less. There is no particular lower limit for the B content, but since B is an element that segregates to austenite grain boundaries during hot rolling and annealing of continuously cast slabs and improves hardenability, the B content is preferably 0.0003% or more. Therefore, when B is contained, the B content is preferably 0.0003% or more and 0.0100% or less, and more preferably 0.0003% or more and 0.0080% or less.

[0099] Sn does not affect the toughness of the slab if its content is 0.200% or less. Therefore, the Sn content is preferably 0.200% or less. There is no need to specify a lower limit for the Sn content, but since Sn is an element that improves hardenability, the Sn content is preferably 0.001% or more. Therefore, when Sn is contained, its content is preferably 0.001% or more and 0.200% or less, and more preferably 0.001% or more and 0.100% or less.

[0100] If Sb is 0.200% or less, coarse precipitates and inclusions do not increase, and the toughness of the slab does not decrease. Therefore, the Sb content is preferably 0.200% or less. There is no particular lower limit for the Sb content, but since Sb is an element that suppresses decarburization and enables adjustment of the strength of the steel sheet, the Sb content is preferably 0.001% or more. Therefore, when Sb is contained, the content is preferably 0.001% or more and 0.200% or less, and more preferably 0.001% or more and 0.100% or less.

[0101] If the content of Ca, Mg, and REM is 0.0100% or less, the amount of coarse precipitates and inclusions does not increase, and the toughness of the slab does not decrease. Therefore, the content of each of Ca, Mg, and REM is preferably 0.0100% or less. There is no need to specify a lower limit for the content of each of Ca, Mg, and REM. However, since Ca, Mg, and REM are elements that spheroidize the shape of nitrides and sulfides and improve the toughness of the slab, the content of each of these elements is preferably 0.0005% or more. Therefore, when Ca, Mg, and REM are contained, the content of each of these elements is preferably 0.0005% or more and 0.0100% or less, and more preferably 0.0005% or more and 0.0050% or less.

[0102] If Zr and Te are each 0.100% or less, coarse precipitates and inclusions do not increase in the slab, and the toughness of the slab does not decrease. Therefore, the contents of Zr and Te are preferably 0.100% or less. There is no need to specify the lower limits of the contents of Zr and Te. However, since Zr and Te are elements that spheroidize the shape of nitrides and sulfides and improve the toughness of the slab, the contents of Zr and Te are preferably 0.001% or more. Therefore, when Zr and Te are contained, the contents thereof are preferably 0.001% or more and 0.100% or less, and more preferably 0.001% or more and 0.080% or less.

[0103] If Hf is 0.10% or less, the amount of coarse precipitates and inclusions does not increase, and the toughness of the slab does not decrease. Therefore, the Hf content is preferably 0.10% or less. 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, the Hf content is preferably 0.01% or more. Therefore, when Hf is contained, its content is preferably 0.01% or more and 0.10% or less, and more preferably 0.01% or more and 0.08% or less.

[0104] If Bi is 0.200% or less, the amount of coarse precipitates and inclusions does not increase, and the toughness of the slab does not decrease. Therefore, the Bi content is preferably 0.200% or less. There is no particular lower limit for the Bi content, but since Bi is an element that reduces segregation, the Bi content is more preferably 0.001% or more. Therefore, when Bi is contained, the Bi content is preferably 0.001% or more and 0.200% or less, and more preferably 0.001% or more and 0.100% or less.

[0105] When the contents of the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Cu, B, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi are less than the preferred lower limits, the effects of the present invention are not impaired. Therefore, these elements are included as unavoidable impurities. By producing a slab containing these component elements, it becomes possible to produce a high-strength steel plate having the required strength, weldability, workability, and excellent appearance using the slab.

[0106] Next, an example will be described in which slabs of steel types A to H were manufactured, the DBTTs of the slabs were measured using measurement method 1, and the occurrence of cracks in the slabs was confirmed. The chemical compositions of steel types A to H used in the example are shown in Table 1 below, and the results of the example are shown in Table 2 below.

[0107]

[0108]

[0109] In Table 2 above, the area ratio of each microstructure was determined using an observation cross-sectional sample taken 10 mm below the surface at the center of the long side of the slab, using the method described in the embodiment. In the column for slab structure type, "F + small amount of P" means a microstructure consisting mainly of ferrite + small amount of pearlite, and "P + small amount of F" means a microstructure consisting mainly of pearlite + small amount of ferrite. "P single phase" means a single-phase pearlite, "B + F" means a microstructure consisting of bainite + ferrite, and "B single phase" means a microstructure consisting of a single-phase bainite.

[0110] An "X" in the cooling condition column indicates that the slab was cooled in cooling equipment 50 (slabs were allowed to cool one by one). "Y" indicates that the slab was cooled in cooling equipment 52 (multiple slabs were stacked and allowed to cool), and "Z" indicates that the slab was cooled in cooling equipment 54 (multiple stacked slabs were allowed to cool in an insulation facility). The carbon equivalent of the slab was calculated using the chemical compositions of steel types A to H, the average cooling rates from 850 to 700°C, the average cooling rates from 700 to 500°C, and the above formula (2). The DBTT was determined by identifying a regression line for DBTT from the microstructure type and using the carbon equivalent and the regression line of the toughness map shown in Figure 3.

[0111] The presence or absence of cracks on the wide and narrow surfaces of the slab was evaluated by conducting a penetrant test in accordance with JIS Z 2343:2017. Specifically, after applying the developer, the appearance of the penetrant was visually confirmed, and any cracks or defects on the surface of the slab were visually confirmed. The criteria for determining whether a slab had cracks on its surface were as follows:

[0112] "◎": When visually inspected, there were no cracks on the surface, or only cracks of 10 mm or less were found, and no maintenance was required. "◯": When visually inspected, cracks or scratches of 50 mm or less in length were found on the surface, but the cracks could be removed by local grinding maintenance. "△": When visually inspected, cracks or scratches of 50 mm or more were found on the surface, and extensive grinding maintenance was required to remove the cracks. "×": Deep thermal cracks that could not be removed by grinding maintenance or slab fracture occurred.

[0113] As shown in Table 2, in slabs with DBTT of 200°C or less, the temperature cracking was marked as "◎" or "◯". Furthermore, in slabs with DBTT of 100°C or less, all the temperature cracking was marked as "◎". On the other hand, in slabs with DBTT exceeding 200°C, the temperature cracking was marked as "△" or "X". This confirmed that by using the DBTT of a slab as an index, it is possible to determine the occurrence of temperature cracking in a slab cooled under specified cooling conditions. From the results shown in Table 2, it was confirmed that temperature cracking in a slab can be suppressed by setting the DBTT threshold at 200°C and producing a slab by cooling under cooling conditions such that the DBTT is 200°C or less.

[0114] By calculating the DBTT of the slab for each cooling condition, it becomes possible to determine the occurrence of slab placement cracks for each cooling condition. This allows slabs to be produced by cooling them under the optimal cooling conditions, and it was confirmed that it is possible to achieve both slab placement cracks and improved slab productivity.

[0115] REFERENCE SIGNS LIST 10 Mold 12 Tundish 14 Sliding shutter 16 Submerged nozzle 18 Molten steel 20 Solidified shell 22 Unsolidified layer 24 Strand 26 Support roll 28 Guide roll 30 Drive roll 32 Strand support roll 34 Transport roll 36 Strand cutter 38 Slab 50 Cooling equipment 52 Cooling equipment 54 Cooling equipment 56 Heat retention equipment 100 Continuous steel casting machine

Claims

1. A method for manufacturing a slab, comprising: producing the slab by cooling the cast and cut slab under cooling conditions that make the ductile-brittle transition temperature of the slab equal to or lower than a predetermined threshold.

2. A method for manufacturing a slab as described in claim 1, wherein the ductile-brittle transition temperature is calculated using a correspondence relationship between the ductile-brittle transition temperature and carbon equivalent determined in advance for each microstructure, the carbon equivalent is calculated using the component composition of the slab, and the microstructure is determined by observing a cross section of the slab cooled under the cooling conditions.

3. A method for manufacturing a slab as described in claim 1, wherein the ductile-brittle transition temperature is calculated using a correspondence relationship between the ductile-brittle transition temperature and carbon equivalent determined in advance for each microstructure, the carbon equivalent is calculated using the component composition of the slab, and the microstructure is determined by observing a cross section of a slab cooled under cooling conditions that simulate the cooling conditions.

4. A method for manufacturing a slab according to claim 2 or claim 3, wherein the carbon equivalent is calculated using one or more of the average cooling rates in the temperature ranges where ferrite transformation, pearlite transformation, and bainite transformation occur, in addition to the chemical composition of the slab.

5. A method for manufacturing a slab according to claim 1, wherein the ductile-brittle transition temperature is calculated using the carbon equivalent of the slab, and the carbon equivalent is calculated using the component composition of the slab, the transformation rate of the microstructure in the slab cooled until phase transformation is complete, and the average cooling rate at the surface of the slab.

6. A method for manufacturing a slab described in any one of claims 1 to 5, wherein the slab is cooled using cooling equipment with cooling conditions below the threshold selected from among multiple cooling equipment with different cooling conditions.

7. A method for manufacturing a slab according to claim 6, wherein the slab is cooled by cooling equipment with the fastest cooling rate among cooling equipments that have cooling conditions below the threshold value.

8. The method for manufacturing a slab according to claim 7, wherein the cooling equipment for cooling the slab is changed based on the usage status of the plurality of cooling equipment.

9. A method for manufacturing a steel plate, comprising rolling a slab manufactured by the method for manufacturing a slab according to any one of claims 1 to 5 to manufacture a steel plate.

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