Method for manufacturing high-carbon steel slabs

Abstract

In the manufacturing of high-carbon steel, this method suppresses the occurrence of cracks without performing soaking treatment or reduction, and also suppresses the occurrence of holes and dents during rolling. [Solution] After casting the high-carbon steel slab, the slabs are cut and then stacked within 90 minutes. This state is maintained for at least 65 hours after the slabs are cut. While this state is maintained, for slabs with an internal crack length of 12 mm or less, the difference in cooling rate between the top and bottom surfaces of the slab is kept to 0.51°C / min. or less. For slabs with an internal crack length longer than 12 mm but 30 mm or less, the difference in cooling rate between the top and bottom surfaces of the slab is kept to 0.26°C / min. or less. Subsequently, the depth of surface defects present in the slab is measured, and if the depth of the surface defects exceeds the critical defect depth calculated based on the surface stress generated on the surface of the slab after it has cooled to room temperature, a process is performed to remove the surface defects.

Description

Technical Field

[0001] The present invention relates to a method for producing a slab of high-carbon steel.

Background Art

[0002] When producing high-carbon steel with a high carbon (C) content, in order to eliminate large carbides generated during casting and suppress center segregation during cooling, after casting, the slab is generally heat-treated ( soaking heat treatment) at a high temperature of 1000 °C or higher for several hours to several tens of hours or more (see Patent Document 1, etc.). Further, after the soaking heat treatment, the slab may be lightly rolled. By refining the structure of the surface portion of the slab, the progress of cracks from the inside to the surface of the slab is suppressed, and center segregation is suppressed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] The soaking heat treatment in which the slab is heated at a high temperature for a long time has a large energy loss. Further, since decarburization of the surface layer of the slab progresses due to the soaking heat treatment, the surface layer portion of the slab is scarfed, but the surface scar increases due to scarfing. Surface defects such as surface scars cause holes and heges during rolling.

[0005] Despite such problems, currently, when producing high-carbon steel, it is common to perform soaking heat treatment, or soaking heat treatment and rolling after casting.

[0006] An object of the present invention is to provide a method that can suppress the occurrence of center segregation and suppress the generation of holes and heges during rolling without performing soaking heat treatment or rolling after casting when producing high-carbon steel. [Means for solving the problem]

[0007] The applicant has filed Japanese Patent Application No. 2021-155953 concerning a method for suppressing the occurrence of cracks and preventing holes and dents during rolling in high-tensile steel, which is different from the high-carbon steel (hereinafter sometimes simply referred to as "high-carbon steel") that is the subject of this application. The high-tensile steel that is the subject of this application has a lower carbon content than high-carbon steel and generally does not undergo soaking treatment or reduction after casting. Therefore, the high-tensile steel that is the subject of Japanese Patent Application No. 2021-155953 is different from the high-carbon steel that is the subject of this application, but they share the common objective of suppressing the occurrence of cracks and preventing holes and dents during rolling.

[0008] Therefore, in the manufacture of high-carbon steel, the applicant implemented the method described in Japanese Patent Application No. 2021-155953 instead of performing soaking treatment and reduction. As a result, it was found that the same effects as those described in Japanese Patent Application No. 2021-155953 could not be obtained. Upon investigating the cause of this, it was found that a different phenomenon occurs in the high-carbon steel targeted by this application compared to the high-tensile steel targeted by Japanese Patent Application No. 2021-155953. The inventors of this application further researched and discovered the following method.

[0009] The method for manufacturing high-carbon steel slabs described in this specification is: A slab of high-carbon steel with a carbon content of 0.8 mass% to 0.9 mass%, a silicon content of 0.1 mass% to 0.4 mass%, and a manganese content of 0.3 mass% to 0.6 mass% is cast. After casting the slab, cut it and within 90 minutes the cut slabs are stacked. Maintain the above condition for at least 65 hours after cutting the slab, and while maintaining the above condition, ensure that for slabs with an internal crack length of 12 mm or less, the difference in cooling rate between the upper and lower surfaces of the slab is 0.51°C / min or less, and for slabs with an internal crack length longer than 12 mm but 30 mm or less, ensure that the difference in cooling rate between the upper and lower surfaces of the slab is 0.26°C / min or less. Subsequently, the depth of surface defects present in the slab is measured. If the depth of the surface defects exceeds the critical defect depth c(m) calculated using the following formula based on the surface stress σc(MPa) generated on the surface of the slab after it has cooled to room temperature, the surface defects are removed.

number

[0010] In the above method, the state described above may be a state in which, after casting and within 90 minutes of cutting the slabs, the cut slabs are stacked, and another slab cast by the same or another casting machine is placed on either or both above and below the stacked slabs. [Effects of the Invention]

[0011] In the production of high-carbon steel, the occurrence of cracks can be suppressed without performing soaking treatment or reduction after casting the slab, and the occurrence of holes and dents during rolling can also be suppressed. [Brief explanation of the drawing]

[0012] [Figure 1] This is a perspective view of the cut slab X. [Figure 2] This figure shows a cross-section of section II in Figure 1. [Figure 3] This figure shows the relationship between longitudinal stress in a slab and the temperature of the slab. [Figure 4] This figure shows the results of the fracture toughness test. [Figure 5] This diagram shows the relationship between stress (sum of internal and external stress) and internal crack length. [Figure 6] This figure shows the relationship between internal crack length and stress. [Figure 7] This figure shows the relationship between the slab surface temperature and the time elapsed since the slab was cut. [Figure 8] This is a schematic diagram showing the state of stacked slabs. [Figure 9]It is a schematic diagram showing an example of a warped slab state. [Figure 10] It is a schematic diagram for explaining how to obtain the warping amount of the slab. [Figure 11] It is a schematic diagram for explaining how to obtain the external stress. [Figure 12] It is a diagram showing the experimental results. [Figure 13] It is a diagram showing the relationship between the surface temperature of the slab and the time after cutting the slab. [Figure 14] It is a partially enlarged view of FIG. 13. [Figure 15] It is a diagram showing the relationship between the external stress and the slab length. [Figure 16] It is a diagram showing the warping amount. [Figure 17] It is a diagram showing the relationship between the internal crack length and the stress. [Figure 18] It is a diagram showing the relationship between the surface defect depth and the surface stress. [Figure 19] It is a diagram showing the experimental results.

Embodiments for Carrying out the Invention

[0013] Hereinafter, preferred embodiments of the present invention will be described.

[0014] The high-carbon steel targeted by this application has the following composition. Carbon (C) content: 0.8 mass% or more and 0.9 mass% or less Silicon (Si) content: 0.1 mass% or more and 0.4 mass% or less Manganese (Mn) content: 0.3 mass% or more and 0.6 mass% or less

[0015] The carbon content of the high-carbon steel targeted by this application is more than that of high-tensile steel. In this application, steel with higher strength than high-tensile steel is targeted. Hereinafter, the "high-carbon steel targeted by this application" may be simply referred to as "high-carbon steel".

[0016] A slab of high-carbon steel with the above composition is cast using a casting machine. The slab size is, for example, 230 mm to 270 mm in thickness and 800 mm to 1,400 mm in width. After casting, the slab is cut to a predetermined length.

[0017] [Cracked when placed on a surface] The cut slab is cooled. High-carbon steel is prone to cracking during cooling. Conventionally, when manufacturing high-carbon steel, in order to suppress cracking during cooling, the slab is subjected to soaking treatment, or soaking treatment and reduction, after cutting, and then cooled. The applicant has researched the causes of cracking that is prone to occur during cooling in the manufacture of high-carbon steel and has found the following.

[0018] Figure 1 shows an example of a cut slab (slab X). The width, thickness, and length directions shown in Figure 1 are orthogonal to each other. When slab X is cooled, internal stress is generated in slab X. When the magnitude of the internal stress was compared in the width, thickness, and length directions of slab X, it was found that the stress in the length direction was the largest. Furthermore, this stress in the length direction is the main stress that causes cracking.

[0019] Figure 2 shows an example of a cross-section along the longitudinal direction of slab X. The cross-section shown in Figure 2 is the same as cross-section II in Figure 1. As shown in Figure 2, internal cracks can occur in slab X. Internal cracks occur in a direction perpendicular to the longitudinal direction of the slab, where the internal stress is greatest. When stress acts on an internal crack, a populated crack develops, starting from the internal crack.

[0020] The internal stress generated when slab X is cooled is the result of both thermal stress due to thermal expansion and contraction, and transformation stress due to transformation expansion and contraction. Thermal stress is caused by the difference between the surface temperature and the internal temperature of slab X, and can be controlled by controlling the cooling rate through methods such as water cooling or air cooling.

[0021] Transformation stress varies depending on the difference in the timing of transformation between the surface and interior of the slab, and the strength (microstructure) of the material after transformation. When the slab's microstructure transforms from an austenite structure to a ferrite structure, pearlite structure, or a structure containing both during cooling, the transformation stress is small. However, when the slab's microstructure changes to a bainite or martensite structure, the transformation stress becomes large.

[0022] In the high-carbon steel targeted by this invention, the slab structure remains pearlite and does not undergo transformation, so transformation stress does not need to be considered.

[0023] From the above, it is thought that high-carbon steel generates internal stress, mainly due to thermal stress. Since this internal stress acts on internal cracks, causing pitted cracks, it is necessary to reduce the internal stress, mainly due to thermal stress, in high-carbon steel. As mentioned above, thermal stress can be controlled by controlling the cooling rate. Therefore, it is thought that the internal stress, mainly due to thermal stress, can be reduced by controlling the cooling rate.

[0024] Incidentally, when a slab is at a high temperature, its internal stress is relieved by creep. To investigate the stress relaxation effect due to creep, we examined the changes in the internal stress of the slab.

[0025] Since the internal stress is greatest in the longitudinal direction of the slab, the maximum internal (residual) stress acting in the longitudinal direction within the slab (slab longitudinal stress) was calculated and considered as the internal stress. The slab surface temperature was used as the slab temperature.

[0026] Figure 3 shows the relationship between the longitudinal stress of a slab and the temperature of the slab. Figure 3 shows data from three points (a, b, c) located at different positions in the thickness direction of the slab. As shown in Figure 3, the longitudinal stress of the slab remains constant until the high-temperature slab cools down to around 500°C. However, in the range below 500°C, the longitudinal stress of the slab increases as the temperature of the slab decreases.

[0027] This indicates that, after casting, until the slab reaches around 500°C, stress relaxation due to creep occurs, keeping internal stresses low. However, once the slab temperature drops below 500°C, stress relaxation due to creep decreases, and internal stresses increase.

[0028] Based on the above, when the slab temperature (surface temperature) is below 500°C, controlling the cooling rate of the slab reduces internal stress, mainly thermal stress. This is thought to reduce the internal (residual) stress at room temperature that causes cracking when the slab is cooled to room temperature.

[0029] Furthermore, the internal (residual) stress that causes cracking in high-carbon steel is primarily thermal stress caused by the temperature difference between the surface and the interior of the slab. When the slab is kept at room temperature, there is no temperature difference between the surface and the interior, so no thermal stress is generated. Therefore, the internal (residual) stress that causes cracking in high-carbon steel is generated before the slab reaches room temperature.

[0030] As described above, in the case of high-carbon steel, which is the subject of this application, it is necessary to reduce internal stress by controlling the cooling rate of the slab after casting, from the time the slab surface temperature reaches 500°C until it cools to room temperature (for example, 20°C) (when the temperature is between room temperature (for example, 20°C) and 500°C).

[0031] Furthermore, research by the inventors of this application has revealed that, in the case of high-carbon steel, which is the subject of this application, warping occurs after the slab is cast and cooled, similar to the high-tensile steel described in Japanese Patent Application No. 2021-155953. When correcting the warping, external stress is generated. It has been found that this external stress acts as the starting point for crack formation. Warping is caused by the difference in cooling rates between one wide surface of the slab and the other wide surface. Therefore, controlling the cooling rate of the slab is effective in reducing warping and, furthermore, in reducing external stress.

[0032] Thus, in the high-carbon steel targeted by this invention, cracks occur at the initiation point due to the combined action of internal stress, primarily thermal stress, and external stress generated during warping correction. Therefore, it is thought that cracks can be suppressed by reducing both the internal stress (primarily thermal stress) and the external stress, and by reducing the sum of the internal and external stresses. Furthermore, it is hypothesized that there is a limit to the stress (sum of internal and external stresses) at which cracks do not occur, and the following experiment was conducted to investigate this limit.

[0033] Slabs of the steel types shown in Table 1 were cast using a vertical bending continuous casting machine (machine length 36.6 m, bending radius 9.1 m). Steel type A shown in Table 1 is the high-carbon steel targeted by this application. The casting conditions are shown in Table 2. After casting the slabs, samples were taken from the slabs and fracture toughness tests were performed in accordance with ISO 12737, Metallic materials - Plane strain fracture toughness test method. [Table 1] [Table 2]

[0034] Figure 4 shows the results of the fracture toughness test. The vertical axis in Figure 4 represents the fracture stress at room temperature, and the horizontal axis represents the fracture initiation point (notch length). The curve shown in Figure 4 illustrates the fracture toughness characteristics. It was found that the slab fractures above the curve, but does not fracture above or below the curve.

[0035] In fracture toughness testing, "sample fracture," "fracture initiation point (notch length)," and "fracture stress" can be considered as "slab cracking," "internal crack that serves as the initiation point for the cracking (internal crack length)," and "stress acting on the slab," respectively. This "stress acting on the slab" was considered as the sum of internal and external stresses.

[0036] When "sample fracture," "initiation point of fracture (notch length)," and "fracture stress" in the fracture toughness test are replaced with "slab cracking," "internal cracking (internal crack length)," and "stress on the slab (sum of internal and external stresses)," respectively, the figure shown in Figure 5 is obtained. The vertical axis of Figure 5 is "stress (sum of internal and external stresses) δ," and the horizontal axis is "internal crack length L." The curve shown in Figure 5 can be considered as the maximum stress at which cracking does not occur. The range above the curve is the "range in which cracking occurs," and the range on and below the curve is the "range in which cracking does not occur."

[0037] The fracture toughness value Kc (MPa√m) was determined from the fracture toughness test described above. From the fracture toughness value Kc (MPa√m), the notch length a (m), and the stress intensity factor correction factor F, the fracture limit stress σc is expressed by equation (1).

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[0038] Equation (3) is shown in Figure 6. In Figure 6, the vertical axis represents "internal crack length" (a) and the horizontal axis represents "stress" (σc). The curve of equation (3) represents the limit value at which no cracking occurs. The range above the curve of equation (3) is the "range in which cracking occurs". The range on and below the curve of equation (3) is the "range in which cracking does not occur".

[0039] Based on the above, it is considered that if the cooling of the slab is controlled so that the sum of the internal and external stresses generated in the slab remains within the "range in which no cracks occur" as shown in Figure 6, after the slab has been cast and the surface temperature of the slab has risen to 500°C and then returned to room temperature (for example, 20°C) (when the temperature is between room temperature (20°C) and 500°C), the occurrence of cracks can be suppressed.

[0040] To confirm the above findings, the following experiment was conducted.

[0041] First, to determine when the slab surface temperature falls below 500°C, the following experiment was conducted.

[0042] A slab of steel type A (230 mm thick x 1230 mm wide), as shown in Table 1, was cast using a vertical bending continuous casting machine (length 36.6 m, bending radius 9.1 m). At the start of slab withdrawal, the slab was withdrawn at 0.2 m / min for approximately 100 seconds. Subsequently, the withdrawal rate was increased to 0.12 m / min. 2 The casting speed was gradually increased to the target casting speed using the specified acceleration. The casting speed was set to 0.8 m / min, and the water content was set to 1.45 g / kg-steel. After casting, the slab was cut and its surface temperature was measured.

[0043] Figure 7 shows the relationship between "slab surface temperature" and "time since slab cutting." In Figure 7, the vertical axis represents "slab surface temperature" and the horizontal axis represents "time since slab cutting." Figure 7 also shows an approximate curve X1 between "slab surface temperature" and "time since slab cutting." By shifting approximate curve X1 downwards to the point where it passes through the plot furthest below approximate curve X1, curve X2 is obtained. Curve X2 shows that for all slab surface temperatures, the "time since slab cutting" is shorter than that of approximate curve X1. By determining the "time since slab cutting" based on curve X2, a time corresponding to a rapid decrease in slab surface temperature can be obtained.

[0044] From curve X2, the slab surface temperature reached 500°C approximately 90 minutes after cutting the slab. After 90 minutes had passed since cutting the slab, the slab surface temperature remained below 500°C.

[0045] Based on the above, after casting the slab and cutting it, 90 minutes later, the cooling rate of the slab is controlled until it reaches room temperature (e.g., 20°C). This ensures that the sum of the internal and external stresses generated in the slab remains within the "range where no cracks occur" as shown in Figure 6. This is believed to suppress the occurrence of cracks.

[0046] Based on the above findings, we conducted an experiment to confirm the findings shown in Figure 6 by comparing slabs with different cooling conditions 90 minutes after cutting the slab following casting.

[0047] A vertical bending continuous casting machine (length 36.6m, bending radius 9.1m) was used to cast slabs of the steel types shown in Table 1 (thickness 230mm x width 1230mm). At the start of slab withdrawal, the slab was withdrawn at 0.2m / min for approximately 100 seconds. Subsequently, the withdrawal rate was increased to 0.12m / min. 2 The casting speed was gradually increased at the specified acceleration until the target casting speed was reached. Table 2 shows the casting speed and specific water content. After casting, the slabs were cut to the specified lengths (5.5 to 11.0 m).

[0048] After casting, the slabs were cut and then stacked within 90 minutes to reach the cooling state shown in Table 3. The slabs were left in the cooling state shown in Table 3 for 96 hours. The slabs had reached room temperature. After that, the slabs at the stacking positions shown in Table 3 were checked for cracks (presence or absence of slab fracture). The results are shown in Table 3.

[0049] [Table 3]

[0050] The items shown in Table 3 are explained below.

[0051] <Cooling state> As shown in Figure 8, eight cut slabs were stacked so that their wide surfaces overlapped. The wide surface of a slab is the surface that is nearly horizontal in the width and length directions of the slab. In this experiment, slabs with wide surfaces of approximately the same size were stacked in layers. The eight slabs were stacked so that the centers of all slabs in the width and length directions were aligned on the same vertical line (eight-slab stack). In this state, the slabs were left at room temperature for 96 hours. The slabs had reached room temperature.

[0052] The "cooling rate," "cooling rate difference between the top and bottom surfaces of the slab," "internal crack length L," "presence or absence of cracking," "internal stress," "warpage amount," "external stress," and "stress" in Table 3 are for the following slabs out of the eight stacked slabs. No. 1: Top slab No.2: Top slab No. 3: Second slab from the top

[0053] <Cooling rate> After stacking the slabs, K-type thermocouples were installed on the upper (wide) and lower (wide) surfaces of slabs No. 1-3, and the "temperature of the upper surface" and "temperature of the lower surface" of the slabs were measured for 96 hours. Cracking often occurs at the center of the longitudinal and widthwise directions of the slab. Furthermore, it is known that the internal stress is greatest at the center of the longitudinal and widthwise directions of the slab. Therefore, for slabs No. 1 and No. 2, K-type thermocouples were installed at the center of the longitudinal and widthwise directions of the upper surface of the slab (C1) and the center of the longitudinal and widthwise directions of the lower surface of the slab (C2), as shown in the enlarged view of Figure 8, and the "temperature of the upper surface" at C1 and the "temperature of the lower surface" at C2 were measured. Similarly, in No. 3, K-type thermocouples were installed at the longitudinal and widthwise center of the upper slab surface (C1) and at the longitudinal and widthwise center of the lower slab surface (C2), and the "temperature of the upper surface" at C1 and the "temperature of the lower surface" at C2 were measured. The time change of the measured temperature was defined as the cooling rate ("cooling rate of the top surface" and "cooling rate of the bottom surface").

[0054] <Cooling rate difference between the top and bottom surfaces of the slab> The "cooling rate difference between the top and bottom surfaces of the slab" shown in Table 3 is the absolute value of the difference between the "cooling rate of the top surface" and the "cooling rate of the bottom surface" of the slab.

[0055] <Internal crack length L> After maintaining the cooling conditions shown in Table 3 for 96 hours, the slabs at stacking positions No. 1 to No. 3 shown in Table 3 were further cut along the longitudinal direction at the center in the width direction (see Figures 1 and 2). Samples were taken from the cut surfaces and acid was applied to the samples. The length in the slab thickness direction of the acid-corroded portion was measured and defined as the internal crack length L. When multiple corroded portions existed, the longest corroded portion was defined as the internal crack length L. The detection limit for this method is 10 mm. All cracks shorter than 10 mm were treated as 10 mm. Furthermore, because it is difficult to distinguish between areas with large microsegregation (less than 10 mm) between dendrites and internal cracks, areas with large microsegregation were also treated as 10 mm cracks.

[0056] <Presence or absence of cracking due to placement> The cooling conditions shown in Table 3 were maintained for 96 hours, and after confirming that the slabs had reached room temperature, it was checked whether any cracks had occurred in the slabs at the stacking positions No. 1 to No. 3 shown in Table 3 (i.e., whether or not the slabs had broken).

[0057] <Internal stress> The internal stresses were determined when the slab was maintained under the cooling conditions shown in Table 3. In this experiment, the internal stresses were determined by numerical analysis using the general-purpose software ABAQUS. To determine the internal stresses, the temperature distribution of the cast slab calculated by CASTEM was provided, and the stress distribution within the cast slab was calculated by sequentially providing material properties (stress-strain relationship, coefficient of linear expansion) corresponding to the temperature and cooling rate. This analysis revealed that the internal stresses were greatest in the longitudinal direction of the slab compared to the width and thickness directions. Furthermore, it was found that the internal stresses in the width, thickness, and longitudinal directions of the slab were greatest at the center of the width direction, the center of the thickness direction, and the center of the longitudinal direction, respectively.

[0058] <Amount of curvature> It is known that slabs warp due to differences in cooling rates between the top and bottom surfaces. The slabs were left in the cooling conditions shown in Table 3, and after confirming that they had reached room temperature, the amount of warping in the longitudinal direction of the slabs was determined using the following method. As shown in the upper diagram of Figure 9, when the slab is curved so as to be convex upward with respect to its longitudinal direction, the shortest distance from the center of the underside of the slab to the straight line connecting both ends of the underside of the slab in the longitudinal direction was defined as the amount of curvature (maximum value) x1. As shown in the lower part of Figure 9, when the slab is curved so as to be convex downwards, the shortest distance from one longitudinal end of the slab's underside to the tangent to the center of the slab's underside is defined as the amount of curvature (maximum value) x2. The maximum value of curvature x (x1 or x2) is expressed by the following formula, as shown in Figure 10, using the slab's curvature radius (radius of the curved portion of the slab) R, the angle θ, and the slab length l.

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[0059] <External stress> In this experiment, after cooling the slab to room temperature, the force required to correct the slab's warping was defined as the external stress. The external stress was calculated using the following method. In the slab of thickness t shown in Figure 11, it is known that the external stress is greatest at a point t / 4 of the thickness from the inner circumference. When the center of the slab thickness is used as the reference point, the radius R1 at a point t / 4 of the thickness from the inner circumference is expressed by the following formula. R1 = R - t / 4 ... (a) Here, R is the radius of the slab thickness center. The slab length l1 at a position t / 4 of thickness from the inner circumference is expressed by the following formula: l1 = R1 × θ ... (b) The strain ε1 generated at a thickness t / 4 from the inner circumference when the warping of a slab is corrected is expressed by the following formula.

number

[0060] <stress> The stresses shown in Table 3 are the sum of internal and external stresses.

[0061] Figure 12 shows the graph from Figure 6 plotted for Nos. 1-3. As shown in Figure 12, No. 1, which experienced cracking, is located beyond the curve of equation (3) and within the "range in which cracking occurs." Nos. 2 and 3, which did not experience cracking, are located within the "range in which cracking does not occur."

[0062] From the above experiment, we were able to confirm that the range exceeding the curve of equation (3) is the "range in which cracking occurs," and the range not exceeding the curve of equation (3) is the "range in which cracking does not occur."

[0063] From the above, it was found that by ensuring that the sum of internal and external stresses falls within the "range where no cracks occur" shown in Figure 6, 90 minutes after casting and cutting the slab, and until the slab reaches room temperature, that is, from 500°C to room temperature (for example, 20°C) (for example, between 20°C and 500°C), the occurrence of cracks can be suppressed.

[0064] Furthermore, thermocouples were placed between two stacked slabs to measure the slab surface temperature. Figure 13 shows the "slab surface temperature" and the "time elapsed since the slab was cut." Figure 13 also shows the measurement results from the two thermocouples installed at different locations. Figure 14 shows a magnified view of a portion of Figure 13 (where the slab surface temperature is near room temperature). From Figures 13 and 14, it can be seen that the slab surface temperature reaches room temperature (for example, around 20°C) 65 hours after the slab is cut. Note that the above calculation assumes there is a slab above or below the slab. However, if there is no slab above or below, the time it takes for the slab surface temperature to reach room temperature (for example, around 20°C) is shorter than 65 hours. Therefore, it can be assumed that 65 hours after cutting the slabs, the surface temperature of all the stacked slabs will be at room temperature (for example, around 20°C).

[0065] From this, it was found that the slab surface temperature drops from 500°C to room temperature (for example, 20°C) between 90 minutes and 65 hours after cutting the slab. Based on the above, by ensuring that the sum of internal and external stresses remains within the "range where no cracks occur" as shown in Figure 6, 90 minutes after casting and cutting the slab, and for more than 65 hours after cutting, the occurrence of cracks can be suppressed.

[0066] Next, we considered methods to ensure that the sum of internal and external stresses falls within the "range where cracking does not occur," as shown in Figure 6.

[0067] Figure 15 shows the relationship between external stress and slab length when the amount of warpage (x) is varied, with the vertical axis representing "external stress" and the horizontal axis representing "slab length". "External stress" is the external stress at a position t / 4 of the thickness from the inner circumference of the slab, as shown in Figure 11, and is calculated from equation (d) above. "Slab length" is the longitudinal length (l) of the slab at the center in the thickness direction, as shown in Figure 11. As shown in Figure 15, the smaller the amount of warpage (x), the smaller the external stress. For the same amount of warpage (x), the longer the slab length, the smaller the external stress.

[0068] Figure 16 shows the curvature amounts for No. 1 to No. 3 in Table 3. In Figure 16, the maximum curvature amount for 10 tests is shown as "max," and the average curvature amount for 10 tests is shown as "ave." As shown in Figure 16, the maximum curvature amount for the top row of No. 1 and No. 2 was 72 mm, and the maximum curvature amount for the second row from the top of No. 3 was 20 mm. From Table 3, the maximum external stress when the curvature amount is 72 mm is approximately 90 MPa, and the maximum external stress when the curvature amount is 20 mm is approximately 50 MPa.

[0069] Both No. 1 and No. 2 are uppermost slabs, and their maximum and average curvature amounts are the same. Furthermore, the external stresses for No. 1 and No. 2 are the same. However, cracking occurred in No. 1, while cracking did not occur in No. 2. This suggests that, in the high-carbon steel targeted by this invention, even with the same amount of curvature—in other words, with the same external stress—there are cases where cracking can be suppressed and cases where it cannot. Additionally, since both No. 1 and No. 2 are uppermost slabs, they exhibit large amounts of curvature and high external stresses. This indicates that, in the high-carbon steel targeted by this invention, even with high external stresses, there are cases where cracking can be suppressed and cases where it cannot.

[0070] Although not shown here, it is known that the warpage of the third slab from the top is 0 mm, and the external stress at this time is 0 MPa. The slabs below the third slab from the top are in roughly the same cooling state as the third slab from the top, or are cooled less easily than the third slab from the top, and therefore have the same warpage (0 mm) and the same external stress (0 MPa).

[0071] Furthermore, since numerous slabs are placed on top of the second-to-last slab from the bottom, the second-to-last slab from the bottom is less prone to warping. The external stress generated in the second-to-last slab from the bottom is considered to be smaller than the external stress on the second-to-last slab from the top (50 MPa).

[0072] The bottom slab has a different cooling state than the slabs at other stacking positions. However, many slabs are placed on top of the bottom slab, and the underside of the bottom slab is often not open like the top surface of the top slab. Therefore, the amount of warping of the bottom slab is considered to be less than the amount of warping of the top slab. Also, the external stress generated in the bottom slab is considered to be less than the external stress of the top slab (90 MPa).

[0073] Regarding internal stress, Table 3 shows that the internal stress of the topmost slabs (No. 1 and No. 2) was high at 230.6 MPa, while the internal stress of the second-to-last slab (No. 3) was 215.7 MPa.

[0074] Samples No. 1 and No. 2 have the same internal stress. However, cracking occurred in No. 1, while cracking did not occur in No. 2. From this, it can be concluded that, in the high-carbon steel targeted by this invention, even if the internal stress is the same, there are cases where cracking can be suppressed and cases where it cannot.

[0075] The slabs from the third layer from the top to the second layer from the bottom are thought to be cooled at the same temperature as, or less than, the second layer from the top. Therefore, the internal stress generated in the slabs from the third layer from the top to the second layer from the bottom is estimated to be equal to or less than the internal stress of the second layer from the top (215.7 MPa or less).

[0076] Furthermore, generally, cast slabs are stacked on top of a room-temperature slab placed as a base plate. Under these conditions, the internal stress of the bottom slab (the slab on top of the base plate) was calculated to be 215.7 MPa.

[0077] Furthermore, when stacking cast slabs on a base plate, the "bottommost slab" and "second-to-last slab" mentioned above refer to the "bottommost slab" and "second-to-last slab" excluding the base plate. Similarly, in the following explanation, when stacking cast slabs on a base plate, the "bottommost slab" and "n-th slab from the bottom" (where n is a natural number greater than or equal to 2) refer to the "bottommost slab" and "n-th slab from the bottom" excluding the base plate.

[0078] Based on the above, it is considered that, in the case of high-carbon steel targeted by this application, even if the sum of internal and external stresses (stress) is the same in two or more stacked slabs, there are cases where the occurrence of cracking can be suppressed and cases where it cannot. Furthermore, it is considered that even if the sum of internal and external stresses (stress) of the slab is large, the occurrence of cracking can be suppressed in some cases. Furthermore, with the high-carbon steel targeted by this application, it is believed that the occurrence of cracks can be suppressed regardless of the stacking position of the slabs. For example, it is believed that the occurrence of cracks can be suppressed whether the slabs are stacked at the top, the bottom, or between the top and bottom layers.

[0079] As shown in Table 3, No. 1 and No. 2 have the same "internal stress," "external stress," and the sum of these "stresses," but their "internal crack length" is different. From this, it can be inferred that the "internal stress," "external stress," and the sum of these "stresses" at which cracking occurs differ depending on the "internal crack length." From the experiments described above, it has been found that there is a relationship between "internal crack length," "stress" (sum of internal and external stresses), and "presence or absence of cracking" as shown in Figure 6. Therefore, based on Table 3 and Figure 6, the following investigation was conducted.

[0080] Figure 17 shows equation (3) from Figure 6, along with the "range in which cracking occurs" and the "range in which cracking does not occur." In No. 1 and No. 2 shown in Table 3, the total stress, which is the sum of the internal stress (230.6 MPa) and external stress (90 MPa), is 320.6 MPa. When the stress is 320.6 MPa, the range in which no cracks occur, as shown in Figure 17, is reached when the internal crack length is 12.7 mm or less. From this, it can be seen that when the internal crack length is 12 mm or less, and the stress is 320.6 MPa or less, no cracks will occur. From Table 3, the total stress (internal stress + external stress) and the difference in cooling rate between the top and bottom surfaces of the slab show the same trend, and it can be seen that the smaller the stress, the smaller the difference in cooling rate between the top and bottom surfaces of the slab. When the stress is 320.6 MPa or less, it can be seen that the difference in cooling rate between the top and bottom surfaces of the slab is 0.51 °C / min. or less. From this, it can be concluded that when the "internal crack length" is 12 mm or less, setting the "cooling rate difference between the top and bottom surfaces of the slab" to 0.51 °C / min. or less will prevent cracking.

[0081] In No. 3 shown in Table 3, the total stress, which is the sum of the internal stress (215.7 MPa) and external stress (50 MPa), is 265.7 MPa. When the stress is 265.7 MPa, the range in which no cracks occur, as shown in Figure 17, is reached when the internal crack length is 20 mm or less. From this, it can be concluded that when the internal crack length is 20 mm or less and the stress is 265.7 MPa or less, no cracks will occur. From Table 3, it can be concluded that when the stress is 265.7 MPa or less, the cooling rate difference between the top and bottom surfaces of the slab is 0.26 °C / min. or less. From this, it can be concluded that when the internal crack length is 20 mm or less, keeping the cooling rate difference between the top and bottom surfaces of the slab at 0.26 °C / min. or less will prevent cracks from occurring. Furthermore, from No. 1 and No. 2, it is considered that when the "internal crack length" is 12 mm or less, setting the "cooling rate difference between the top and bottom surfaces of the slab" to 0.51 °C / min. or less will prevent sagging cracks. Therefore, when the "internal crack length" is longer than 12 mm and 20 mm or less, the "cooling rate difference between the top and bottom surfaces of the slab" should be set to 0.26 °C / min. or less. This is considered to prevent sagging cracks.

[0082] Furthermore, as mentioned above, in the slabs from the third layer from the top to the third layer from the bottom, the "internal stress" is estimated to be 215.7 MPa or less, and the "external stress" is 0 MPa. At this time, the total "stress" of "internal stress" (215.7 MPa) and "external stress" (0 MPa) is 215.7 MPa. When the "stress" is 215.7 MPa, the "range in which no cracks occur" as shown in Figure 17 occurs when the "internal crack length" is 30 mm or less. From this, it can be concluded that when the "internal crack length" is 30 mm or less, and the "stress" is 215.7 MPa or less, no cracks will occur. From Table 3, it can be concluded that when the "stress" is 215.7 MPa or less, the "cooling rate difference between the top and bottom surfaces of the slab" is lower than 0.26 °C / min. From this, it can be concluded that even when the "internal crack length" is 30 mm or less, no cracks will occur if the "cooling rate difference between the top and bottom surfaces of the slab" is kept below 0.26 °C / min. Based on the above, when the "internal crack length" is longer than 12 mm and less than or equal to 30 mm, it is considered that cracking will not occur if the "cooling rate difference between the top and bottom surfaces of the slab" is 0.26 °C / min or less.

[0083] Based on the above, when the internal crack length of the slab is 12 mm or less, the slab is cast and cut, and then cooled for 90 minutes and at least 65 hours after cutting, so that the difference in cooling rate between the top and bottom surfaces of the slab is 0.51 °C / min or less. This prevents the occurrence of slab cracks when the internal crack length of the slab is 12 mm or less. Furthermore, when the internal crack length of the slab is longer than 12 mm but 30 mm or less, the slab is cast and cut, and then cooled for 90 minutes and at least 65 hours after cutting, so that the difference in cooling rate between the top and bottom surfaces of the slab is 0.26°C / min or less. This prevents the occurrence of slab cracks when the internal crack length of the slab is longer than 12 mm but 30 mm or less. As long as the above conditions are met, the stacking position of the slabs can be any position.

[0084] The method for ensuring that the cooling rate difference between the top and bottom surfaces of the slab is 0.51°C / min. or less is not particularly limited, but for example, the following may be used.

[0085] After casting the slab, the cut slabs are stacked within 90 minutes of cutting. The stacked slabs may be cast in the same casting machine or in different casting machines, as long as they were cut within the last 90 minutes. The difference in cooling rate between the top and bottom surfaces of all slabs except the bottom slab is 0.51°C / min. or less. In addition, the difference in cooling rate between the top and bottom surfaces of the bottom slab may also be 0.51°C / min. or less.

[0086] Furthermore, the method for ensuring that the cooling rate difference between the top and bottom surfaces of the slab is 0.26°C / min. or less is not particularly limited, but may be as follows.

[0087] After casting the slab, the cut slabs are stacked within 90 minutes of cutting. At this time, slabs that have a cooling rate difference of 0.26°C / min. or less between the top and bottom surfaces are positioned in the stacking order from the second layer from the top to the second layer from the bottom (including the second layer from the top and the second layer from the bottom). The stacked slabs may be cast in the same casting machine or in different casting machines, as long as they were cut within 90 minutes.

[0088] Alternatively, a separate slab may be prepared without considering the occurrence of cracks, and within 90 minutes of cutting, the multiple cut slabs may be stacked, with the separate slab placed above, below, or both of the stacked slabs. This separate slab does not include the base slab. In this case, for all slabs, including the separate slab and the stacked slab, the difference in cooling rate between the top and bottom surfaces of the slab will be 0.26°C / min. or less at the stacking positions from the second layer from the top to the second layer from the bottom (including the second layer from the top and the second layer from the bottom). Therefore, a slab that makes the difference in cooling rate between the top and bottom surfaces of the slab 0.26°C / min. or less is positioned at the stacking positions from the second layer from the top to the second layer from the bottom (including the second layer from the top and the second layer from the bottom) for all slabs, including the separate slab and the stacked slab.

[0089] Even when the difference in cooling rate between the top and bottom surfaces of the slab is kept below 0.51°C / min, another slab may be placed above or below the stacked slab, or on either side.

[0090] By using a separate slab, it is possible to increase the number of slabs with a slower cooling rate difference between the top and bottom surfaces, thereby increasing the number of slabs that do not experience cracking.

[0091] The separate slab may be cast in the same casting machine as the stacked slab, or it may be cast in a different casting machine. For example, the separate slab may be a slab that has been cut within 90 minutes of casting. Since a slab that has been cut within 90 minutes of casting is at a temperature similar to or higher than the stacked slab, the difference in cooling rate between the stacked slabs tends to be slower. The separate slab may be one that has been left as a single piece after cutting, or it may be stored in a stack. A separate slab stored in a stack may be more than 90 minutes old since it was cut after casting, or it may be within 3 hours of cutting after casting. Since a separate slab that has been cut within 3 hours is at a temperature similar to or higher than the stacked slab (500°C or higher), the difference in cooling rate between the stacked slabs tends to be slower. The type of steel used for the separate slab is not particularly limited.

[0092] The stacking conditions are not particularly limited, but preferred conditions include, for example, stacking slabs and other slabs with approximately the same size of wide surfaces. Alternatively, all slabs may be stacked so that their widthwise and longitudinal centers lie on a vertical line. The size of the wide surfaces of all or some of the slabs may differ. The widthwise and longitudinal centers of all or some of the slabs may be slightly off-center from the vertical line. Furthermore, the ratio of the total area of ​​the wide surfaces of one slab to the total area of ​​the wide surfaces of the other slab, to the total area of ​​the wide surfaces of the two slabs, may be 60% or more. The number of slabs to be stacked is not particularly limited, as long as it is two or more. Another slab may be placed above and / or below the two or more stacked slabs.

[0093] Furthermore, as mentioned above, cast slabs may be stacked on top of slabs placed as base plates.

[0094] In the above experiment, the cooling state shown in Table 3 (with the slabs stacked) was maintained for 96 hours. However, the stacked slab state may be maintained for a shorter period than 96 hours. From Figures 13 and 14, it is known that the slab surface temperature drops to room temperature (20°C) 65 hours after cutting the slabs. Therefore, it is advisable to maintain the stacked slab state for at least 65 hours after cutting the slabs. After 65 hours have passed since cutting the slabs, the stacked slab state may or may not be maintained.

[0095] Based on the above, the high-carbon steel slab that is the subject of this application is cooled after casting by the following method. Within 90 minutes of casting and cutting the slab, the cut slabs should be stacked. Alternatively, within 90 minutes of casting and cutting the slab, the cut slabs may be stacked, and another slab may be placed on top of and / or below the stacked slab. Maintain this condition for at least 65 hours after cutting the slab. During this period, for slabs with an internal crack length of 12 mm or less, ensure that the difference in cooling rate between the top and bottom surfaces of the slab is 0.51°C / min or less. For slabs with an internal crack length longer than 12 mm but 30 mm or less, ensure that the difference in cooling rate between the top and bottom surfaces of the slab is 0.26°C / min or less.

[0096] By the above method, in the high-carbon steel targeted by this application, it is possible to prevent the occurrence of cracks during cooling without performing soaking treatment or reduction after casting the slab but before cooling.

[0097] Furthermore, "maintaining this condition for at least 65 hours after cutting the slab" means ensuring that each stacked slab satisfies this condition. To ensure that each stacked slab satisfies this condition, for example, the above condition may be maintained for at least 65 hours after cutting the last slab among the stacked slabs.

[0098] In the above experiment, the measured value of the cross-section of the slab after casting was used as the "internal crack length." However, instead of a measured value, an estimated internal crack length based on the slab's casting conditions (casting speed, specific water content, slab size, roll stand usage, etc.) may be used. Also, if an estimated length of "internal crack length L" exists, when casting a slab under the same casting conditions used to estimate that length, the "internal crack length L" of that slab may be considered the already existing estimated length. For example, if a measured value of "internal crack length L" exists, when casting a slab under the same casting conditions that yielded that measured value, the "internal crack length L" of that slab may be considered the measured value. When using a measured value for "internal crack length L," if multiple internal cracks occur, the longest internal crack length should be adopted as the "internal crack length."

[0099] [Perforated, bald] In the high-carbon steel slabs targeted by this invention, it was found that the above method could suppress the occurrence of cracks. However, it was subsequently discovered that when the slabs were rolled, the products were prone to developing holes or dents. Therefore, a method was investigated to suppress the development of holes and dents during rolling.

[0100] The perforations and cracks that occur during rolling are thought to be caused by surface stress generated on the slab surface when the slab cools after casting, which leads to the propagation of surface defects (cracks) in the depth direction. Using equation (1), which represents the fracture limit stress σc adopted in the above-mentioned study of fixed cracks, the following study was conducted.

[0101] The fracture limit stress σc is expressed by equation (1).

number

number

number

[0102] Figure 18 shows equation (5). In Figure 18, the vertical axis represents "surface defect depth" and the horizontal axis represents "surface stress". The curve of equation (5) is the "limit value of surface stress at which perforation and erosion do not occur" and is also the "maximum surface defect depth (critical defect depth) at which perforation and erosion do not occur". The range above the curve of equation (5) is the "range in which perforation and erosion occur". The range on and below the curve of equation (5) is the "range in which perforation and erosion do not occur".

[0103] Based on the above, it is considered that if the "surface defect depth" is measured before rolling and the "surface defect depth" before rolling is within the "range in which holes and bleeds occur" as shown in Figure 18, then a process to remove the surface defects of that depth before rolling (for example, grinder maintenance) can be performed to prevent holes and bleeds from occurring at that location. Specifically, the surface stress generated on the surface of the slab after it has cooled to room temperature is calculated. After casting, the surface stress is at its maximum at room temperature while the slab is cooling to room temperature. Therefore, the "surface stress generated on the surface of the slab after it has cooled to room temperature" is used as the surface stress. Using the calculated surface stress, the "maximum surface defect depth at which perforation or erosion does not occur (critical defect depth)" is determined from equation (5). Before rolling, the depth of surface defects in the slab is measured, and if the depth of the surface defects (measured value) exceeds the previously determined "maximum surface defect depth at which perforation or erosion does not occur (critical defect depth)," it is in the "range where perforation or erosion occurs" as shown in Figure 18. Therefore, a process to remove the surface defects of that depth (for example, grinder treatment) is performed before rolling. This is thought to prevent perforation or erosion from occurring.

[0104] To confirm the above findings, the following experiment was conducted.

[0105] A slab of steel type A (230 mm thick x 1230 mm wide) from Table 1 was cast using a vertical bending continuous casting machine (machine length 36.6 m, bending radius 9.1 m). At the start of slab withdrawal, the slab was withdrawn at 0.2 m / min for approximately 100 seconds. After that, the withdrawal rate was increased to 0.12 m / min. 2 The casting speed was gradually increased at the specified acceleration until the target casting speed was reached. Table 4 shows the casting speed and specific water content. After casting, the slabs were cut to the specified lengths (5.5 to 11.0 m).

[0106] After casting the slabs, the eight cut slabs were stacked within 90 minutes, as shown in Figure 8. The stacked slabs were left for 96 hours. The slabs had reached room temperature. Surface defects were checked for in slabs No. 4 to 6, and if surface defects were found, the depth of the surface defects was measured by ultrasonic testing. Table 4 shows the measured surface defect depths.

[0107] The slabs were then rolled, and the product (coil) was checked for holes or defects. The results are shown in Table 4.

[0108] [Table 4]

[0109] The "surface stress" shown in Table 4 is the surface stress that occurs on the surface of the slab after it has cooled to room temperature. Here, the "surface stress" was calculated using the same method as for the internal stress. Table 4 shows the maximum surface stress at room temperature acting longitudinally on the surface of the slab as the "surface stress". The "critical defect depth c" shown in Table 4 is the "maximum surface defect depth at which no holes or dents occur (critical defect depth)" obtained from the "surface stress" and the above formula (5).

[0110] Figure 19 shows the graph from Figure 18 with Nos. 4-6 plotted on it. As shown in Figure 19, No. 4, which had holes and bald spots, is beyond the curve of equation (5) and is in the "range where holes and bald spots occur". Nos. 5 and 6, which did not have holes or bald spots, are in the "range where holes and bald spots do not occur".

[0111] From the above experiment, we were able to confirm that the range exceeding the curve of equation (5) is the "range in which holes and bald spots occur," and the range not exceeding the curve of equation (5) is the "range in which holes and bald spots do not occur."

[0112] Based on the above, the "surface defect depth" of the slab is measured before rolling. If the "surface defect depth" exceeds the "maximum surface defect depth at which perforation or erosion does not occur (critical defect depth)" calculated from the surface stress generated on the surface of the slab after it has cooled to room temperature and equation (5), then it is in the "range at which perforation or erosion occurs" as shown in Figure 18. Therefore, a process to remove the surface defect at that depth (for example, grinder maintenance) is performed before rolling. This suppresses the occurrence of perforation or erosion during rolling. Note that even if the measured "surface defect depth" is less than or equal to the "maximum surface defect depth at which perforation or erosion does not occur (critical defect depth)," a process to remove the surface defect at that depth may also be performed. Furthermore, if the measured "surface defect depth" is less than or equal to the "maximum surface defect depth at which perforation or erosion does not occur (critical defect depth)," it is considered that perforation or erosion will not occur in that area, so a process to remove the surface defect at that depth is not necessary.

[0113] Furthermore, when comparing the high-carbon steel targeted by this application with other steels, such as high-tensile steel (Japanese Patent Application No. 2021-155953), which has a relatively high carbon content but not as high as high-carbon steel, it was found that the curve of the "maximum surface defect depth at which perforation or erosion does not occur (critical defect depth)" shown in equation (5) (see Figure 18) is completely different. For example, in high-tensile steel, the "maximum surface defect depth at which perforation or erosion does not occur (critical defect depth)" is larger at the same surface stress than in the case of high-carbon steel targeted by this application. Therefore, in high-tensile steel, the "range in which perforation or erosion does not occur" is wider than the "range in which perforation or erosion does not occur" for high-carbon steel (see Figure 18). Thus, it is considered that in high-tensile steel, perforation or erosion will not occur even if the surface defects are somewhat deeper than in high-carbon steel.

[0114] In the high-carbon steel targeted by this application, the "maximum surface defect depth at which perforation or erosion does not occur (critical defect depth)" is small, and the "range in which perforation or erosion does not occur" shown in Figure 18 is narrow. For this reason, in the high-carbon steel targeted by this application, the above-mentioned method is effective, which involves measuring the "surface defect depth" before rolling, predicting whether perforation or erosion will occur based on the measured "surface defect depth," and, if it is thought that perforation or erosion will occur, performing a surface defect removal treatment before rolling.

[0115] Although embodiments of the present invention have been described above with reference to the drawings, it should be understood that the specific configurations are not limited to these embodiments. The scope of the present invention is indicated not by the above description but by the claims, and all modifications within the meaning and scope equivalent to the claims are included. [Explanation of Symbols]

[0116] X Slab

Claims

1. A slab of high-carbon steel with a carbon content of 0.8 mass% to 0.9 mass%, a silicon content of 0.1 mass% to 0.4 mass%, and a manganese content of 0.3 mass% to 0.6 mass% is cast. After casting the slab, within 90 minutes of cutting, the cut slabs should be stacked. The above condition shall be maintained for at least 65 hours after the slab is cut, and while the above condition is maintained, for slabs with an internal crack length of 12 mm or less, the difference in cooling rate between the upper and lower surfaces of the slab shall be 0.51°C / min or less, and for slabs with an internal crack length longer than 12 mm and 30 mm or less, the difference in cooling rate between the upper and lower surfaces of the slab shall be 0.26°C / min or less. Subsequently, the depth of surface defects present in the slab is measured. If the depth of the surface defects exceeds the critical defect depth c (m), which is calculated using the following formula based on the surface stress σc (MPa) generated on the surface of the slab after it has cooled to room temperature, the surface defects are removed. A method for manufacturing high-carbon steel slabs, characterized by the following: [Math A] Here, Kc is the fracture toughness value.

2. The aforementioned state is one in which, within 90 minutes of casting and cutting the slab, the cut slabs are stacked, and another slab cast by the same or other casting machine is placed on either or both above and below the stacked slab. The method for producing a high-carbon steel slab according to feature 1.

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