Method for manufacturing slabs and method for manufacturing steel plates

Abstract

A method for manufacturing a slab is provided, which can manufacture a slab by determining the occurrence of delayed cracking in a slab cooled under predetermined cooling conditions. As a method for manufacturing a slab, the slab is manufactured by cooling the cast and cut slab under cooling conditions such that the ductile-brittle transition temperature of the slab becomes below a predetermined threshold value.

Description

Technology Field

[0001] The present invention relates to a method for manufacturing a slab capable of suppressing delayed cracking in a slab for high-strength steel (high-tensile) and a method for manufacturing a steel plate using said slab. Background Technology

[0002] In order to reduce the amount of steel used or to lighten products, not only is the strength of the steel increased, but properties such as high ductility and high toughness are also required to improve formability and manufacturing efficiency during the process of manufacturing final products. In order to achieve such properties, the content of elements such as Cr and Mo is increased in addition to C, Si, and Mn in the steel, and as a result, the toughness of slabs and blooms, which are continuous casting billets, is significantly lower than before.

[0003] Along with the deterioration of slab toughness, cracking during slab cooling—so-called delayed cracking (thermal cracking)—is becoming frequent. If delayed cracking occurs, there is a risk that the slab will fracture during transport, making hot rolling impossible. Even if the slab does not fracture, there is a risk that the crack may open during hot rolling, causing the hot-rolled steel sheet to break.

[0004] Regarding small cracks in delayed cracks, the cracks appear as surface defects such as peeling scratches or sliver scratches on steel sheets after hot rolling, cold rolling, annealing, or plating. Normally, cracks on the surface of a slab are removed with a grinder, but due to a decrease in the toughness of the slab, the cracks may advance further under the stress of the grinder, and there are cases where they cannot be completely removed with the grinder.

[0005] Furthermore, small cracks in delayed cracks are often overlooked and may appear as surface defects in steel sheets after hot rolling, cold rolling, annealing, or plating. As such, since delayed cracks in slabs reduce yield and efficiency not only in the casting process but also in subsequent processes such as hot rolling, it is required to suppress the occurrence of delayed cracks.

[0006] Delayed cracking in slabs is caused by the reduction in ductility or toughness of the slab due to the increased strength of the steel. Delayed cracking in slabs is thought to be caused by thermal stress generated in the slab during cooling, and as a countermeasure, various methods have been proposed to suppress excessive stress generated during the cooling process of the slab.

[0007] Patent Document 1 discloses a method for cooling a slab that can suppress the occurrence of delayed cracking in a slab by stacking three or more slabs such that the ratio of the total contact area is 60% or more, maintaining the stack for 8 hours or more, and then slow-cooling the slabs. According to Patent Document 1, it is stated that the occurrence of delayed cracking in a slab can be suppressed by controlling the cooling speed of the slab according to the length of internal cracks occurring in the slab after casting.

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

[0009] Japanese Published Patent Application No. 2020-139209 Japanese Published Patent Application No. 2019-167560 The problem to be solved

[0010] The methods disclosed in Patent Documents 1 and 2 roughly specify the range of the composition of the slab, but cool it under cooling conditions corresponding to the slab most prone to delayed cracking among the slabs within that range. Because of this, even for slabs that are unlikely to develop delayed cracking, they are cooled under cooling conditions corresponding to the slabs prone to delayed cracking, and there was a problem that it was necessary to form an excessive number of cooling facilities corresponding to the cooling rate, or that the productivity of the slab was reduced.

[0011] The present invention is made in consideration of the problems of the prior art as described above, and its purpose is to provide a method for manufacturing a slab capable of manufacturing a slab by determining the occurrence of delayed cracking in a slab cooled under predetermined cooling conditions. Another purpose of the present invention is to provide a method for manufacturing a steel plate using a slab manufactured by the method for manufacturing the slab. means of solving the problem

[0012] The inventors, through repeated examinations, discovered that the occurrence of delayed cracking in a slab can be evaluated using the ductile-brittle transition temperature (hereinafter referred to as "DBTT") in the Charpy impact test as an indicator, and thus completed the present invention. The means for solving the above problem are as follows. The method for manufacturing a slab and the method for manufacturing a steel plate related to the present invention are more preferable solutions when manufacturing a slab for high-strength steel and a high-strength steel plate containing, in mass%, C: 0.10% or more and 0.60% or less, Si: 0.10% or more and 2.50% or less, and Mn: 1.00% or more and 5.00% or less.

[0013] [1] A method for manufacturing a slab, wherein the ductile-brittle transition temperature of the cast and cut slab is lowered to a predetermined threshold temperature by cooling the slab.

[0014] [2] The ductile-brittle transition temperature is calculated using the correspondence between the ductile-brittle transition temperature and the carbon equivalent, which is determined in advance for each microstructure, the carbon equivalent is calculated using the composition of the slab, and the microstructure is obtained by observing the cross-section of the slab cooled under the cooling conditions, [1] a method for manufacturing a slab.

[0015] [3] The ductile-brittle transition temperature is calculated using the correspondence between the ductile-brittle transition temperature and the carbon equivalent, which is determined in advance for each microstructure, and the carbon equivalent is calculated using the composition of the slab, and the microstructure is obtained by observing the cross-section of the slab cooled under cooling conditions that simulate the cooling conditions, [1] a method for manufacturing a slab.

[0016] [4] The carbon equivalent is calculated using one or more of the average cooling rates in the temperature range where ferrite transformation, pearlite transformation, and bainite transformation occur, in addition to the composition of the slab, as described in [2] or [3].

[0017] [5] The ductile-brittle transition temperature is calculated using the carbon equivalent of the slab, and the carbon equivalent is calculated using the composition of the slab, the transformation rate of the microstructure in the slab cooled until the phase transformation is completed, and the average cooling rate on the surface of the slab, as described in [1].

[0018] [6] A method for manufacturing a slab described in any one of [1] to [5], wherein the slab is cooled in a cooling facility with a cooling condition that is lower than the threshold value among a plurality of cooling facilities with different cooling conditions.

[0019] [7] A method for manufacturing a slab as described in [6], wherein the slab is cooled using the cooling equipment with the fastest cooling rate among the cooling equipment with cooling conditions below the threshold value.

[0020] [8] A method for manufacturing a slab as described in [7], which changes the cooling equipment for cooling the slab based on the usage conditions of the above multiple cooling equipment.

[0021] [9] A method for manufacturing a steel plate by rolling a slab manufactured by the method for manufacturing a slab described in any one of [1] to [5]. Effects of the invention

[0022] According to the present invention, delayed cracking of the slab can be suppressed by manufacturing the slab by cooling it under cooling conditions where DBTT is below a critical value. Since it is possible to determine whether delayed cracking of the slab occurs under various cooling conditions, the slab can be manufactured by cooling it under optimal cooling conditions, thereby enabling the simultaneous prevention of delayed cracking of the slab and improvement of slab productivity. Brief explanation of the drawing

[0023] FIG. 1 is a schematic cross-sectional view showing a continuous casting machine for steel capable of carrying out the method of manufacturing a slab related to the present embodiment. Figure 2 is a graph showing the results of the Charpy impact test. Figure 3 is a graph showing an example of a personality map. Figure 4 is a TTT curve. Figure 5 is a diagram showing the area ratio of micro-tissues and the correspondence of tissue types. Specific details for implementing the invention

[0024] The present invention will be explained below through embodiments thereof. However, the following embodiments represent preferred examples of the present invention, and the present invention is not limited in any way by these embodiments.

[0025] FIG. 1 is a schematic cross-sectional view of a steel continuous casting machine (100) capable of carrying out a method for manufacturing a slab related to the present embodiment. The steel continuous casting machine (100) mainly has a mold (10), a tundish (12), a plurality of pairs of casting support rolls (32), and a slab cooling facility (50 to 54). The tundish (12) is installed above the mold (10). A sliding shutter (14) for adjusting the injection flow rate of molten steel (18) into the mold (10) is installed at the bottom of the tundish (12), and an immersion nozzle (16) is installed on the lower surface of the sliding shutter (14).

[0026] Molten steel (18) is injected into a mold (10) through an immersion nozzle (16). The molten steel (18) injected into the mold (10) is heated from the inner surface of the mold (10) and solidifies, forming a solidified shell (20). Thus, a cast slab (24) is formed having an unsolidified layer (22) made of molten steel (18) on the inside, with the solidified shell (20) on the outside.

[0027] Below the mold (10), a plurality of pairs of casting support rolls (32) are formed, including a support roll (26), a guide roll (28), and a driving roll (30). Among these, the driving roll (30) is a roll for supporting the casting (24) and simultaneously drawing the casting (24). In the gap between adjacent casting support rolls in the casting direction, a spray nozzle (not shown), such as a water spray nozzle or an air mist spray nozzle, is formed, thereby forming a secondary cooling zone. By the cooling water sprayed from the spray nozzle of the secondary cooling zone, the casting (24) is drawn and cooled, causing the internal unsolidified layer (22) to decrease and the solidified shell (20) to grow. After that, the casting (24) is properly cooled, the solidification of the unsolidified layer (22) proceeds, and the casting (24) is completely solidified. A plurality of conveying rolls (34) for conveying the cast billet (24) are formed on the downstream side of the billet support roll (32), and a billet cutter (36) for cutting the cast billet (24) into a slab (38) of a predetermined length is formed above the conveying rolls (34). The billet (24) is cut by the billet cutter (36) to become a slab (38) of a predetermined length. The slab (38) is cooled to room temperature under predetermined cooling conditions by a cooling facility (50 to 54).

[0028] Depending on the degree to which delayed cracking of the slab (38) is likely to occur, for example, it is cooled by three cooling facilities with different cooling conditions. Cooling facility (50) is a facility that cools the slab (38) one by one. Cooling facility (52) is a facility that cools the slab (38) by stacking multiple slabs. Cooling facility (54) is a facility that cools the slab (38) by stacking multiple slabs within a thermal insulation facility (56). Among the cooling facilities (50 to 54), the facility with the fastest cooling speed of the slab (38) is cooling facility (50), and the facility with the slowest cooling speed of the slab (38) is cooling facility (54).

[0029] If the toughness of the slab (38) is low, delayed cracking is likely to occur. For this reason, in the method for manufacturing a slab related to the present embodiment, attention is paid to the DBTT of the slab (38) as an indicator related to the toughness of the slab (38), and the occurrence of delayed cracking in the slab is determined based on the DBTT of the slab cooled by cooling equipment (50 to 54). Specifically, the DBTT of the slab cooled by cooling equipment (50 to 54) is calculated, and the slab is cooled by cooling equipment with cooling conditions such that the said DBTT becomes below a predetermined threshold value. By doing so, it is possible to manufacture the slab (38) while suppressing delayed cracking in the slab (38).

[0030] When there are multiple cooling facilities where the DBTT of the slab (38) becomes below a critical value, it is preferable to manufacture the slab by cooling it with the cooling facility that has the fastest cooling rate among these cooling facilities. For example, when the DBTT of the slab (38) becomes below a critical value even when the slab (38) is cooled by cooling facility (50) or cooling facility (52), it is preferable to cool the slab (38) with the cooling facility (50) that has the fastest cooling rate. By doing so, the delay cracking of the slab (38) and the improvement of productivity of the slab (38) can be achieved.

[0031] In addition, if there are multiple cooling facilities capable of lowering the DBTT of the slab (38) below a critical value, the cooling facility used to cool the slab (38) may be changed by considering the usage conditions of these cooling facilities. For example, when the DBTT of the slab (38) is lowered below a critical value whether the slab (38) is cooled by the cooling facility (50) or by the cooling facility (52), the cooling facility (50) is in full use. In this case, the cooling facility used to cool the slab (38) may be changed from the cooling facility (50) to the cooling facility (52). By doing so, the range of choices for the cooling facility used to cool the slab (38) is widened, allowing the cooling facilities (50 to 54) to be used efficiently and reducing the number of cooling facilities installed.

[0032] The threshold value of DBTT is determined by checking the relationship between the occurrence of delayed cracks in the slab (38) and DBTT in advance, and is set to a value within the range of DBTT where delayed cracks in the slab (38) do not occur. For example, the largest value within the range of DBTT where delayed cracks do not occur may be used as the threshold value of DBTT.

[0033] Next, the DBTT of the slab will be described. Upon observing the fracture mode of the fracture surface at the cracked portion of a high-strength steel slab fractured by delayed cracking, it was confirmed that most of the delayed cracks had advanced to about 20 mm below the surface of the slab and took the form of "intergranular fracture" that advanced to the austenite grain boundaries. On the other hand, the region less than 5 mm below the surface of the slab is thought to have a high toughness structure with fine γ grains because it is directly cooled by the mold or the cooling water immediately below the mold. Therefore, since it is difficult to assume that the starting point of the delayed cracking originates from this region, the region between 5 and 20 mm below the surface of the slab was considered the location for predicting toughness, and the DBTT was measured at a position 10 mm below the surface in the center of the long side of the slab.

[0034] For slabs of various steel grades, Charpy impact tests were performed using specimens 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 delayed cracking in the slab and DBTT. As a result, it was confirmed that delayed cracking in the slab occurs when DBTT exceeds 200°C, and that delayed cracking in the slab does not occur when DBTT is 200°C or lower.

[0035] Using the above test results, the relationship between the composition of the slab and DBTT was confirmed. As a result, it was observed that DBTT tended to be higher in slabs with a higher content of alloying elements, but the variation was large, confirming that DBTT could not be predicted with high precision based solely on composition or carbon equivalent. Therefore, the cooling conditions of the slab were changed to investigate how DBTT changes by altering the microstructure of the slab after cooling.

[0036] Figure 2 is a graph showing the results of a Charpy impact test. The horizontal axis of Figure 2 is the test temperature (°C) of the Charpy impact test, and the vertical axis is the brittle fracture surface ratio (%). As shown in Figure 2, when the cooling conditions of a slab of steel grade 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 becomes 50% also changed. Likewise, for a slab of steel grade 2, which has a different composition from steel grade 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. From the results for steel grade 1 and steel grade 2, it was confirmed that the DBTT changes not only due to the steel composition but also due to the microstructure of the slab after cooling.

[0037] As such, since DBTT varies depending on the steel composition and microstructure of the slab, in the method for manufacturing a slab related to the present embodiment, DBTT is determined by considering the microstructure after cooling in addition to the composition of the slab (38). Next, a method for measuring the DBTT of the slab (38) will be described. In the method for manufacturing a slab related to the present embodiment, the DBTT of the slab (38) is determined by the following three methods.

[0038] Measurement method 1: Use a toughness map to determine the DBTT of the slab (38).

[0039] Measurement method 2: The microstructure is predicted from the temperature history of the slab to obtain the DBTT of the slab (38).

[0040] Measurement method 3: Perform a Charpy impact test to determine the DBTT of the slab (38).

[0041] First, a method (measurement method 1) for determining the DBTT of a slab (38) using a toughness map will be explained. FIG. 3 is a graph showing an example of a toughness map. The horizontal axis of FIG. 3 is Ceq (mass% carbon equivalent) and the vertical axis is DBTT (°C).

[0042] A toughness map is a map that shows the correspondence between DBTT and carbon equivalents obtained for each microstructure. In measurement method 1, the toughness map shown in FIG. 3 is prepared in advance. The toughness map can be prepared in the following order (1) to (4).

[0043] (1) A test specimen is taken from a position 10 mm below the surface of the center of the long side of various slabs, and a Charpy impact test is performed at various test temperatures to determine the DBTT.

[0044] (2) Plot the calculated DBTT on a graph with the carbon equivalent (Ceq) on the horizontal axis and DBTT on the vertical axis.

[0045] (3) A cross-sectional view is taken of a sample taken from 10 mm below the surface at the center of the long side of the slab. By doing so, the slab is classified into five microstructures: “ferrite-dominant + small amount of pearlite,” “pearite-dominant + small amount of ferrite,” “pearite-single phase,” “bainite + ferrite,” and “bainite-single phase.” “F ~ F + P” shown in Fig. 3 means ferrite-dominant + small amount of pearlite. “P + F” means pearlite-dominant + small amount of ferrite. “P” means pearlite-single phase. “B + F” means bainite + ferrite. “B” means bainite-single phase.

[0046] (4) The plots on the graph are classified into 5 categories based on the classification of the slabs, and linear regression equations are obtained for the 5 classified plots. The linear regression equations are not limited to one, and may be represented by two linear regression equations.

[0047] In determining the DBTT of the slab (38) using measurement method 1, the microstructure of the slab (38) is first measured. The microstructure is measured by taking a sample from a position 10 mm below the surface of the center of the long side of the slab, which has been cooled to room temperature under predetermined cooling conditions, and by observing the cross-section of the sample. The microstructure may also be measured by taking a sample from the slab, which has been cooled under cooling conditions that simulate predetermined cooling conditions, for example, cooling conditions at a lab scale, and by observing the cross-section of the sample. This makes it possible to easily take a cross-sectional observation sample.

[0048] In measuring the area percentage of ferrite, first, the observation cross-section of the sample is mirror-polished using diamond paste. Then, the observation cross-section is finished polished using colloidal silica, and the microstructure is revealed on the observation cross-section by etching with 3 vol% Nital. Under conditions where the acceleration voltage is 15 kV, the observation cross-section is observed in 10 fields of view using a Scanning Electron Microscope (SEM) at a magnification of 50x to acquire microstructure images. Using Adobe PHOTOSHOP (registered trademark), the area percentage of ferrite is calculated for 10 fields of view from these microstructure images, and the average value of these 10 fields of view is taken as the area percentage of ferrite.

[0049] Ferrite contains a maximum of 0.02 mass% of carbon in iron and is a structure close to pure iron. Ferrite is the softest and most ductile of the steel structures. Ferrite has a larger grain size compared to other structures (pearlite, bainite, tempered martensite, quenched martensite, and retained austenite), and also has a darker contrast on a smooth surface, so it can be easily distinguished at 50x magnification.

[0050] In measuring the area percentages of pearlite, bainite, and martensite (tempered martensite, quenched martensite, and retained austenite), first, the microstructure is visualized on the cross-section of the sample in the same manner as the measurement method for ferrite. Under conditions where the acceleration voltage is 15 kV, a microstructure image is acquired by using an SEM to exclude ferrite from the field of view and observing the cross-section at a magnification of 10,000x for 10 fields of view. Using Adobe PHOTOSHOP (registered trademark), the area percentages of pearlite, bainite, and martensite are calculated for 10 fields of view from these microstructure images. The average value of each microstructure in these 10 fields of view is calculated and added to the area percentage of the ferrite to obtain the total area percentage of each microstructure so that the sum is 100%.

[0051] Pearlite is a structure obtained when austenite is slowly cooled. Pearlite consists of ferrite and cementite layers, formed by the alternating arrangement of these layers. Bainite is a type of transformation structure generated from the austenite of carbon steel or alloy steel; it is a structure with superior ductility and impact resistance compared to structures obtained through conventional quenching and tempering, and possesses abundant toughness and durability. Bainite consists of black needle-shaped crystals and has mechanical properties intermediate between those of martensite and fine pearlite. Martensite is formed when austenite is rapidly cooled and is a hard yet soft structure.

[0052] Bainite is a concave structure, and pearlite is a concave structure containing lamellar carbides. Tempered martensite is a concave structure containing fine carbides, quenched martensite is a convex structure with fine irregularities inside, and retained austenite is a convex structure with a flat inside. Tempered martensite, quenched martensite, and retained austenite do not need to be distinguished from each other in that they are calculated as the area ratio of the total martensite. Using the area ratio of the microstructure measured in this way, the composition ratio of the microstructure of the slab (38) is determined. Using this composition ratio of the microstructure, it is determined whether the microstructure of the slab (38) corresponds to "ferrite-main + small amount of pearlite," "pealite-main + small amount of ferrite," "pearlite single phase," "bainite + ferrite," or "bainite single phase." Thus, a linear regression equation for calculating DBTT in the personality map can be specified.

[0053] Next, the method for calculating the carbon equivalent is explained. The carbon equivalent of the slab can be calculated using the following equation (1).

[0054] Ceq = C+Σαi[Mi]···(1)

[0055] 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.

[0056] Even for slabs with the same composition and microstructure, toughness may change due to differences in cooling rates. For this reason, instead of the above-mentioned equation (1), it is preferable to calculate the carbon equivalent of the slab using the following equation (2), which reflects the influence of the cooling rate on the above-mentioned equation (1). For the cooling rate CR in the following equation (2), since the temperature ranges where ferrite, pearlite, or bainite transformation occurs are 850 to 700°C, 700 to 500°C, and 500 to 300°C, respectively, one or more of the average cooling rates in these temperature ranges are used.

[0057] Ceq = C+Σαi[Mi]+Σ(βjCRj+γj)···(2)

[0058] In the above equation (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 included in the slab (38), αi is a parameter, CRj is the average cooling rate (°C / hr), and βj and γj are parameters.

[0059] Each parameter of the above equations (1) and (2) corresponds to the DBTT and carbon equivalent that can be measured by the Charpy impact test using the following equation (3). Then, each parameter of the multiple regression analysis can be determined by using a dataset in which the actual values ​​of DBTT, the actual values ​​of the content of each element, and the actual values ​​of the cooling rate are set as one set. When determining each parameter by multiple regression analysis, it is preferable to use a dataset of 100 or more types. Instead of multiple regression analysis, the above parameters may be determined using machine learning.

[0060] DBTT = a×Ceq+b···(3)

[0061] In the above equation (3), DBTT is the ductile-brittle transition temperature (°C), Ceq is the carbon equivalent (mass%), and a and b are parameters.

[0062] In this way, the carbon equivalent of the slab is calculated using the above equation (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.

[0063] Next, a method (measurement method 2) for obtaining the DBTT of a slab (38) by predicting the microstructure from the temperature history of the slab is described. In measurement method 2, the temperature history of the slab (38) is predicted from the cooling conditions that cool the slab (38), a phase transformation factor is predicted from the said temperature history, and the microstructure of the slab (38) is obtained from the said phase transformation factor.

[0064] The temperature change of the slab (38) is obtained by solving the three-dimensional heat conduction equation shown in equation (4) below.

[0065]

[0066] In the above equation (4), ρ is the density (kg / m³) 3 ) and Cp is the specific heat (J / (kg·°C)), and λ is the thermal conductivity (W / (m×K)). qi is the transformation exothermic reaction (W / m 3 ) and this transformation fever will be described later.

[0067] The amount of heat generated by the coolant (water, etc.) on the surface of the slab, the amount of heat generated by convection with air qh, and the amount of heat generated by radiation qr are calculated using the following equations (5) and (6).

[0068] qh = h(Ts-Tw)···(5)

[0069] qr = εσ((Ts+273) 4 -(Tw+273) 4 )···(6)

[0070] In the above equations (5) and (6), qh is the heat generation amount (W / m² 2 ) and h is the heat transfer rate (W / (m 2 ×K)), where Ts is the surface temperature of the cast billet (°C). Tw is the ambient temperature (°C), and qr is radiation (W / m² 2 ) and σ is a Stefan-Boltzmann integer (W / (m 2 ×K 4 )) and ε is the emissivity (-).

[0071] The temperature history of the slab (38) can be obtained by calculating the above equations (4) to (6) and the transformation heat amount qi described later. Next, a method for predicting the microstructure of the slab (38) will be explained. The transformation rate and transformation heat amount of the microstructure of the slab (38) are calculated, for example, using a TTT diagram.

[0072] Figure 4 is a TTT curve. The vertical axis of Figure 4 is temperature (°C), and the horizontal axis is time (s). In Figure 4, ts represents the time at which the isothermal transformation begins at temperature T, and tf represents the time at which the isothermal transformation ends at temperature T. The TTT curve may be a drawing created by actual measurements or one created by calculation software.

[0073] The temperature of the slab (38) changes continuously over time. The continuous temperature change is approximated as a stepwise change in temperature by stacking steps (stacking of isothermal transformation) in which the temperature is maintained at a specific temperature for a minute time, and the transformation rate of the microstructure is calculated in response to the temperature change. In this embodiment, the transformation rates of ferrite, pearlite, bainite, and martensite are calculated.

[0074] First, the method for calculating the transformation rates of ferrite, pearlite, and bainite is explained. The transformation initiation condition for each phase is defined as when the consumption of the incubation period I shown in Equation (7) below becomes 1 at an equilibrium transformation initiation temperature T0 or lower. Once the temperature is below the equilibrium transformation initiation temperature, if the temperature exceeds the equilibrium transformation initiation temperature, the consumption of the incubation period is set to return to 0. This incubation period is calculated simultaneously for each phase of ferrite, pearlite, and bainite, and if the incubation period of another phase becomes 1 during the transformation of one phase, the phase transformation of the phase currently undergoing transformation is terminated, and a new phase transformation of the other phase is initiated.

[0075]

[0076] After the transformation begins, the transformation rate is calculated by assuming that the isothermal transformation proceeds for a small time dt at temperature T. That is, the isothermal transformation start time ts and end time tf 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 different from real time. The logarithmic time LT, which is nondimensionalized from the transformation start time and transformation end time, is given by Equation (8).

[0077]

[0078] The transformation rate can be expressed by the following equation (9). Here, k and n are integers. For n, a value of approximately 2 to 4 can be used. Since the transformation end time in equation (9) becomes infinite, it is necessary to multiply by an integer C (1.001) close to 1 as in equation (10). The logarithmic time LT is defined by using the logarithmic time LT, which is nondimensionalized from the transformation start time and transformation end time, so that f = 0 at T = 0 and f = feq at T = 1. Here, from f = feq at T = 1, the following equation (11) is obtained. feq is the equilibrium transformation rate (-) in the ferrite transformation, and after the pearlite transformation, it is the residual austenite phase rate at the start of each phase transformation. In the ferrite transformation region, feq is the ferrite fraction obtained from the equilibrium phase diagram, and the ferrite fraction depends on temperature. k in the following (9) equation is obtained using the following (11) equation.

[0079] f = 1-exp(-k(LT) n )···(9)

[0080] f = feq×C{1-exp(-k(LT) n )}···(10)

[0081] k=-Ln((C-1) / C)···(11)

[0082] The logarithmic time LT at any step is calculated by determining the time equivalent to the transformation rate of the previous step from the above equation (8), and as the time elapsed from this state by a small time. By sequentially repeating the above calculation for each phase of ferrite, pearlite, and bainite, the transformation rates f of ferrite, pearlite, and bainite are obtained. F , f P , f B (-) is saved.

[0083] Martensitic transformation is, martensitic transformation rate f M(-) Between the phase transformation start temperature and the phase transformation end temperature, the process proceeds linearly or non-linearly with respect to temperature and can be calculated based on Equation (12) or (13).

[0084] f M = f eq ×((TT Mf ) / (T Ms -T Mf ))···(12)

[0085] f M = f eq ×((1-exp{-A0(TT Mf )} / (1-exp{-A0(T Ms -T Mf )}···(13)

[0086] In the above equations (12) and (13), T is the temperature (°C), and T Ms and T Mf is the initiation temperature (°C) and end temperature (°C) of the martensitic transformation, A0 is an integer, and in this embodiment, 0.018 is used.

[0087] Total transformation rate f ALL The transformation rate of ferrite, pearlite, bainite, and martensite can be calculated using the following (14) equation. In this way, the transformation rate of ferrite, pearlite, bainite, and martensite can be derived.

[0088] f ALL = f F +f P +f B +f M ···(14)

[0089] Next, a method for calculating the heat of transformation qi from the transformation rate is explained. The heat of transformation qi can be calculated using the following equation (15).

[0090] qi = L×(δf ALL / δt)···(15)

[0091] In the above equation (14), qi is the heat of transformation (W / m² 3) and t is time (s) and f ALL is the total transformation rate (-). L is the latent heat of transformation (J / g), and is a physical quantity that is uniquely determined when the composition of the steel is specified. This latent heat of transformation can be obtained by inputting the composition of the slab into commercially available calculation software (e.g., Thermo-Calc, etc.).

[0092] The composition ratio of the microstructure of the slab (38) is determined from the transformation rates of ferrite, pearlite, bainite, and martensite calculated in the transformation factor prediction process. Unless otherwise specified, the composition ratio is determined from the calculation results after the phase transformation is completed at room temperature. The transformation rates of ferrite, pearlite, bainite, and martensite are used as the composition ratio of these microstructures, and the microstructure of the slab (38) is determined from said composition ratio.

[0093] FIG. 5 is a diagram showing the area ratio of the microstructure and the correspondence of the microstructure type. Using FIG. 5 and the composition ratio of the microstructure, it is determined which of the following microstructures the microstructure of the slab (38) corresponds to: "ferrite main + small amount of pearlite," "pearite main + small amount of ferrite," "pearite single phase," "bainite + ferrite," and "bainite single phase."

[0094] Next, the method for calculating the carbon equivalent is explained. The carbon equivalent is calculated by using Equation (16) or Equation (17) in correspondence with the microstructure of the slab (38). If the microstructure of the slab (38) is “ferrite-dominant + small amount of pearlite,” “pearite-dominant + small amount of ferrite,” or “single phase of pearlite,” the carbon equivalent of the slab (38) is calculated using Equation (16) below. On the other hand, if the microstructure of the slab (38) is “bainite + ferrite” or “single phase of bainite,” the carbon equivalent of the slab (38) is calculated using Equation (17) below.

[0095] 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)

[0096] 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)

[0097] In the above equations (16) and (17), Ce, fp and Ce, b are carbon equivalents (mass%), and each element symbol is the content (mass%) of each element, and is 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 or parameters of each element and cooling rate may be changed to coefficients or parameters determined by performing multiple regression analysis using a data set in which the actual values ​​of DBTT, component concentration, and cooling rate are set as one set.

[0098] By using the carbon equivalent obtained in this way and the above equation (3), the DBTT of the slab (38) cooled under predetermined cooling conditions is obtained. This is the method for calculating the DBTT of the slab by measurement method 2.

[0099] 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.

[0100] A sample is taken from a position 10 mm below the surface of the center of the long side of a slab cooled under specified cooling conditions, and a Charpy impact test is performed on the sample by changing the measurement temperature. 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 according to measurement method 3.

[0101] In this way, in the method for manufacturing a slab related to the present embodiment, the DBTT of the slab is measured by performing a Charpy impact test or by considering the microstructure of the slab (38). As a result, the DBTT of the slab (38) can be measured with high precision, and as a result, the occurrence of delayed cracking of the slab (38) under predetermined cooling conditions can be determined with high precision, and the slab can be manufactured by cooling under optimal cooling conditions.

[0102] The method for manufacturing a slab related to the present embodiment is preferably used when manufacturing a slab for high-strength steel containing, in mass%, C: 0.10% or more and 0.60% or less, Si: 0.10% or more and 2.50% or less, and Mn: 1.00% or more and 5.00% or less. Below, the composition of each chemical component included in the slab for high-strength steel is described. In the following description, "%" indicating the content of the constituent elements of the steel means "mass%" unless otherwise specified.

[0103] C: 0.10% or more, 0.60% or less

[0104] C included in the slab (38) is an element required to increase the strength of the high-strength steel plate made from the slab. Since the strength required for the high-strength steel plate cannot be obtained if the C content is less than 0.10%, the lower limit of the C content is 0.10%. On the other hand, if the C content exceeds 0.60%, there may be an excess of quenched martensite in the slab, and cracks may occur starting from these structures, so it is not desirable.

[0105] Accordingly, 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.

[0106] Si: 0.10% or more, 2.50% or less

[0107] Si included in the slab (38) is an element necessary to secure residual austenite in the steel plate during the annealing process of the high-strength steel plate using the slab as a raw material. Additionally, Si is an essential additive element because it contributes to the high strength of the high-strength steel plate through solid solution strengthening. If the Si content is less than 0.10%, the strength required for the high-strength steel plate is not obtained. Therefore, the lower limit of the Si content is 0.10%.

[0108] Meanwhile, if the Si content exceeds 2.50%, the effect of obtaining the strength required for high-strength steel sheets becomes saturated, and a hard scale forms on the hot-rolled sheet before processing into high-strength steel sheets. As a result, the appearance and pickling properties of the high-strength steel sheets deteriorate. For this reason, the upper limit of the Si content is 2.50%.

[0109] Accordingly, 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.

[0110] Mn: 1.00% or more, 5.00% or less

[0111] Mn included in the slab (38) is an element necessary to further increase 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 continuous casting slab. If the Mn content is less than 1.00%, sufficient strengthening of the high-strength steel plate is not obtained. For this reason, 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 quenching martensite fraction of the slab will become excessive, and the effect of obtaining the strength of the high-strength steel plate will become saturated, and the manufacturing cost of the high-strength steel plate will increase, so it is not desirable from an economic perspective.

[0112] Accordingly, from this perspective, it is preferable that the Mn content of the slab (38) be 1.00% or more and 5.00% or less, more preferable that it be 1.50% or more and 4.50% or less, and even more preferable that it be 1.80% or more and 4.00% or less.

[0113] The slab produced by the method for manufacturing a slab related to the present embodiment has the above compositional composition, the remainder being Fe and unavoidable impurities, and after cooling has an average austenite grain size and microstructure of appropriate composition. Within the limits thereof, considering other properties, it may contain P in an amount of 0.001% or more and 0.100% or less, S in an amount of 0.0001% or more and 0.0200% or less, and Al in an amount of 0.005% or more and 0.100% or less. Likewise, it may contain N in an amount of 0.0001% or more and 0.0100% or less, and O in an amount of 0.0001% or more and 0.0100% or less. Here, unavoidable impurities may include Zn, Pb, and As. A total content of 0.100% or less of these unavoidable impurities is permitted.

[0114] Since P segregates at the prior austenite grain boundaries and embrittles the grain boundaries, it may cause delayed cracking in the slab. Therefore, it is desirable that the P content be 0.100% or less. Although a lower limit for the P content does not need to be specifically specified, it is desirable that it be 0.001% or more, given that P is a solid solution strengthening element and can increase the strength of the steel sheet. Accordingly, it is desirable that the P content be 0.001% or more and 0.100% or less, and more desirable that it be 0.001% or more and 0.070% or less.

[0115] S is an element that exists as a sulfide and causes embrittlement in the slab. Therefore, it is desirable to keep the S content at 0.0200% or less. Although a lower limit for the S content does not need to be specifically specified, it is desirable to have a value of 0.0001% or more due to production technical constraints. Accordingly, it is desirable for the S content to be 0.0001% or more and 0.0200% or less, and more desirable to be 0.0001% or more and 0.0050% or less.

[0116] Al is an element that affects the fraction of residual austenite in a slab in that it suppresses the formation of carbides during slab cooling and promotes the formation of residual austenite. It is desirable to add 0.005% or more of Al for deoxidation. On the other hand, if the Al content exceeds 0.100%, there is a risk of causing slab embrittlement. Therefore, it is desirable for the Al content to be 0.005% or more and 0.100% or less, and more desirable for it to be 0.005% or more and 0.080% or less.

[0117] N is an element that exists as a nitride and causes embrittlement of the slab. For this reason, it is desirable that the N content be 0.0100% or less. Although a lower limit for the N content does not need to be specifically specified, due to production technical constraints, it is desirable that the N content be 0.0001% or more. Accordingly, it is desirable that the N content be 0.0001% or more and 0.0100% or less, and more desirable that it be 0.0001% or more and 0.0050% or less.

[0118] O is an element that exists as an oxide and causes embrittlement of the slab. For this reason, it is desirable that the O content be 0.0100% or less. Although a lower limit for the O content does not need to be specifically specified, due to production technical constraints, it is desirable that the O content be 0.0001% or more. Accordingly, it is desirable that the O content be 0.0001% or more and 0.0100% or less, and more desirable that it be 0.0001% or more and 0.0050% or less.

[0119] The slab manufactured by the method for manufacturing a slab related to the present embodiment is for high-strength steel plates, and in addition to the above component composition, further comprises: 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 0.0100% or less, and Sn: 0.001% or more. It may contain at least one element selected from 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, either alone or in combination of two or more.

[0120] If the content of Ti, Nb, and V is 0.200% or less each, large amounts of coarse precipitates or inclusions are not generated, and thus the toughness of the slab is not reduced. For this reason, it is preferable that the content of Ti, Nb, and V be 0.200% or less each. Although a lower limit for the content of Ti, Nb, and V does not need to be specifically specified, the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing of the continuous casting slab. For this reason, it is preferable that the content of Ti, Nb, and V be 0.001% or more each. Therefore, when Ti, Nb, and V are present, it is preferable that their content be 0.001% or more and 0.200% or less each, and more preferable that it be 0.001% or more and 0.100% or less.

[0121] If the content of Ta and W is 0.10% or less each, large amounts of coarse precipitates or inclusions are not generated, and thus the toughness of the slab is not reduced. For this reason, it is preferable that the content of Ta and W be 0.10% or less each. Although a lower limit for the content of Ta and W does not need to be specifically specified, the strength of the steel sheet is increased by forming fine carbides, nitrides, or carbonitrides during hot rolling or continuous annealing of the continuous casting slab. For this reason, it is preferable that the content of Ta and W be 0.01% or more each. Accordingly, when Ta and W are contained, it is preferable that their content be 0.01% or more and 0.10% or less each, and more preferable that it be 0.01% or more and 0.08% or less.

[0122] Cr, Mo, Ni, and Cu bring about the effect of increasing the strength of the steel sheet through microstructure control during the hot rolling of the slab. Since this effect becomes significant when one or more of Cr, Mo, Ni, and Cu are each added in an amount of 0.01% or more, it is desirable to add them in an amount of 0.01% or more. Since the weldability and hot workability of the steel sheet deteriorate if the content of each element exceeds 2.00%, it is desirable that the upper limit of the amount of each element of Cr, Mo, Ni, and Cu be 2.00%. Therefore, when Cr, Mo, Ni, and Cu are included, it is desirable that their content be 0.01% or more and 2.00% or less, and more desirable that it be 0.01% or more and 1.00% or less.

[0123] B may be added because it controls the structural transformation during the hot rolling or annealing of the slab, thereby affecting strength through structural strengthening. If the B content is 0.0100% or less, it does not affect the toughness of the slab. For this reason, it is desirable that the B content be 0.0100% or less. Although a lower limit for the B content does not need to be specifically specified, it is desirable that the B content be 0.0003% or more, as it is an element that segregates at the austenite grain boundaries during the hot rolling or annealing of continuous casting slabs to improve quenchability. Therefore, when B is included, it is desirable that the content be 0.0003% or more and 0.0100% or less, and more desirable that it be 0.0003% or more and 0.0080% or less.

[0124] If Sn is 0.200% or less, it does not affect the toughness of the slab. For this reason, it is desirable that the Sn content be 0.200% or less. Although a lower limit for the Sn content does not need to be specifically specified, since Sn is an element that improves quenchability, it is desirable that the Sn content be 0.001% or more. Therefore, when Sn is contained, it is desirable that the content be 0.001% or more and 0.200% or less, and more desirable that it be 0.001% or more and 0.100% or less.

[0125] If the Sb content is 0.200% or less, coarse precipitates or inclusions do not increase, and thus the toughness of the slab is not reduced. For this reason, it is desirable for the Sb content to be 0.200% or less. Although a lower limit for the Sb content does not need to be specifically specified, it is desirable for the Sb content to be 0.001% or more, as it is an element that suppresses decarburization and enables the adjustment of the steel sheet's strength. Therefore, when Sb is included, it is desirable for the content to be 0.001% or more and 0.200% or less, and more desirable for it to be 0.001% or more and 0.100% or less.

[0126] If the content of Ca, Mg, and REM is 0.0100% or less each, coarse precipitates or inclusions do not increase, and thus the toughness of the slab is not reduced. Therefore, it is preferable that the content of each of Ca, Mg, and REM be 0.0100% or less. Although it is not necessary to specifically specify a lower limit for the content of each of Ca, Mg, and REM, since Ca, Mg, and REM are elements that improve the toughness of the slab by spheroidizing the shape of nitrides or sulfides, it is preferable that their content be 0.0005% or more each. Accordingly, when Ca, Mg, and REM are contained, it is preferable that their content be 0.0005% or more and 0.0100% or less each, and more preferable that it be 0.0005% or more and 0.0050% or less.

[0127] If the content of Zr and Te is 0.100% or less each, coarse precipitates or inclusions in the slab do not increase, and thus the toughness of the slab is not reduced. Therefore, it is preferable that the content of Zr and Te be 0.100% or less each. Although it is not necessary to specifically specify a lower limit for the content of Zr and Te, since Zr and Te are elements that improve the toughness of the slab by spheroidizing the shape of nitrides or sulfides, it is preferable that the content of Zr and Te be 0.001% or more each. Accordingly, when Zr and Te are contained, it is preferable that their content be 0.001% or more and 0.100% or less each, and more preferable that it be 0.001% or more and 0.080% or less.

[0128] If the Hf content is 0.10% or less, coarse precipitates or inclusions do not increase, and thus the toughness of the slab is not reduced. For this reason, it is desirable that the Hf content be 0.10% or less. Although a lower limit for the Hf content does not need to be specifically specified, since Hf is an element that improves the ultimate deformability of the steel sheet by spheroidizing the shape of nitrides or sulfides, it is desirable that the Hf content be 0.01% or more. Therefore, when Hf is contained, it is desirable that the content be 0.01% or more and 0.10% or less, and more desirable that it be 0.01% or more and 0.08% or less.

[0129] If the Bi content is 0.200% or less, coarse precipitates or inclusions do not increase, and thus the toughness of the slab is not reduced. For this reason, it is desirable for the Bi content to be 0.200% or less. Although a lower limit for the Bi content does not need to be specifically specified, it is more desirable for the Bi content to be 0.001% or more, given that it is an element that reduces segregation. Accordingly, when Bi is contained, it is desirable for the content to be 0.001% or more and 0.200% or less, and more desirable for it to be 0.001% or more and 0.100% or less.

[0130] Regarding the above-mentioned Ti, Nb, V, Ta, W, B, Cr, Mo, Ni, Cu, B, Sn, Sb, Ca, Mg, REM, Zr, Te, Hf, and Bi, if the content of each is below a desirable lower limit, it does not impair the effects of the present invention. For this reason, these are included as unavoidable impurities. By manufacturing a slab containing such component elements, it becomes possible to manufacture a high-strength steel plate with excellent strength, weldability, workability, and appearance using said slab.

[0131] Examples

[0132] Next, an example is described in which slabs of steel grades A to H were manufactured, the DBTT of the slabs was measured using measurement method 1, and the occurrence of delayed cracking in the slabs was confirmed. The composition of steel grades A to H used in the example is shown in Table 1 below, and the results of the example are shown in Table 2 below.

[0133]

[0134]

[0135] In Table 2 above, the area ratio of each microstructure was determined by the method described in the embodiment using an observation cross-sectional sample taken from 10 mm below the surface at the center of the long side of the slab. "F + small amount P" shown in the column of microstructure types of the slab refers to a microstructure of ferrite-main + small amount of pearlite, and "P + small amount F" refers to a microstructure of pearlite-main + small amount of ferrite. "P single phase" refers to a pearlite single phase, "B + F" refers to a microstructure of bainite + ferrite, and "B single phase" refers to a microstructure of bainite single phase.

[0136] "X" in the heat of the cooling conditions indicates that the slab was cooled in a cooling facility (50) (cooling the slab one by one). "Y" indicates that the slab was cooled in a cooling facility (52) (cooling multiple slabs stacked in a cooling facility), and "Z" indicates that the slab was cooled in a cooling facility (54) (cooling multiple stacked slabs in a thermal insulation facility). The carbon equivalent of the slab was calculated using the composition of steel grades A to H, an average cooling rate of 850 to 700°C, an average cooling rate of 700 to 500°C, and the above equation (2). DBTT was calculated by determining a regression line for DBTT from the microstructure grade and using the carbon equivalent and the regression line of the toughness map shown in FIG. 3.

[0137] Delayed cracking of the slab was evaluated by conducting a penetration inspection test in accordance with JIS Z 2343:2017 to assess the presence or absence of cracks on the broad and narrow surfaces of the slab. Specifically, after applying the developer, the appearance of the penetrant was visually confirmed, and delayed cracks or scratches on the surface of the slab were visually inspected. The criteria for determining delayed cracking of the slab are as follows.

[0138] "◎" : There were no cracks at all on the surface, or only cracks of 10 mm or less were visible to the naked eye, so no maintenance was necessary.

[0139] "○" : Cracks or scratches less than 50 mm in length were visible on the surface with the naked eye, but the cracks could be removed by local grinding.

[0140] "△" : Cracks or scratches of 50 mm or more were visible on the surface with the naked eye, requiring extensive grinding to remove the cracks.

[0141] "×" : Deep thermal cracks or slab fractures occurred that could not be removed by grinding.

[0142] As shown in Table 2, in slabs with a DBTT of 200°C or less, delayed cracks were marked as “◎” or “○”. In addition, in slabs with a DBTT of 100°C or less, all delayed cracks were marked as “◎”. On the other hand, in slabs with a DBTT exceeding 200°C, delayed cracks were marked as “△” or “×”. Thus, it was confirmed that by using the DBTT of the slab as an indicator, the occurrence of delayed cracks in slabs cooled under specific cooling conditions can be determined. From the results shown in Table 2, it was confirmed that delayed cracks in slabs can be suppressed by setting the critical value of DBTT to 200°C and manufacturing the slabs by cooling under conditions where the DBTT is 200°C or less.

[0143] By determining the DBTT of the slab for each cooling condition, it becomes possible to determine the occurrence of delayed cracking in the slab for each cooling condition. As a result, since the slab can be manufactured by cooling it under optimal cooling conditions, it was confirmed that the prevention of delayed cracking in the slab and the improvement of slab productivity can be achieved. Explanation of the symbols

[0144] 10 : Mold 12 : Tundish 14: Sliding shutter 16: Immersion nozzle 18 : Yonggang 20: Coagulated shell 22: Uncoagulated layer 24 : Main Section 26: Support Role 28 : Guide Roll 30: Drive Roll 32 : Main side support roll 34: Return Roll 36 : Billet cutter 38 : Slab 50: Cooling equipment 52 : Cooling equipment 54 : Cooling equipment 56 : Thermal insulation equipment 100 : Continuous casting machine for steel

Claims

Claim 1 A method for manufacturing a slab, wherein the ductile-brittle transition temperature of the cast and cut slab is cooled under cooling conditions such that the slab is below a predetermined threshold value. Claim 2 A method for manufacturing a slab according to claim 1, wherein the ductile-brittle transition temperature is calculated using the correspondence relationship between the ductile-brittle transition temperature and the carbon equivalent obtained in advance for each microstructure, the carbon equivalent is calculated using the compositional composition of the slab, and the microstructure is obtained by observing the cross-section of the slab cooled under the cooling conditions. Claim 3 A method for manufacturing a slab according to claim 1, wherein the ductile-brittle transition temperature is calculated using the correspondence relationship between the ductile-brittle transition temperature and the carbon equivalent obtained in advance for each microstructure, the carbon equivalent is calculated using the compositional composition of the slab, and the microstructure is obtained by observing a cross-section of a slab cooled under cooling conditions simulating the cooling conditions. Claim 4 A method for manufacturing a slab according to claim 2 or 3, wherein the carbon equivalent is calculated using one or more of the average cooling rates in the temperature range where ferrite transformation, pearlite transformation, and bainite transformation occur, in addition to the compositional composition of the slab. Claim 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 compositional composition of the slab, the transformation rate of the microstructure in the slab cooled until the phase transformation is completed, and the average cooling rate on the surface of the slab. Claim 6 A method for manufacturing a slab according to any one of claims 1 to 5, wherein the slab is cooled in a cooling facility having a cooling condition that is below the threshold value among a plurality of cooling facilities with different cooling conditions. Claim 7 A method for manufacturing a slab according to claim 6, wherein the slab is cooled using the cooling facility with the fastest cooling rate among cooling facilities with cooling conditions below the threshold value. Claim 8 A method for manufacturing a slab according to claim 7, wherein the cooling facility for cooling the slab is changed based on the usage conditions of the plurality of cooling facilities. Claim 9 A method for manufacturing a steel plate by rolling a slab manufactured by the method for manufacturing a slab described in any one of claims 1 to 5.

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