Slab manufacturing method
By refining to adjust hydrogen concentration and controlling cooling water ratios during casting, and stacking high-temperature slabs post-casting, the method addresses UT defects in steel slabs, achieving defect reduction without large-scale equipment or fuel.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Patents
- Current Assignee / Owner
- KOBE STEEL LTD
- Filing Date
- 2023-03-03
- Publication Date
- 2026-06-30
Smart Images
Figure 0007882794000002 
Figure 0007882794000003 
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Abstract
Description
Technical Field
[0001] The present invention relates to a method for manufacturing a slab.
Background Art
[0002] Products obtained by rolling a cast slab may have defects caused by hydrogen (hereinafter referred to as "UT defects"). UT defects occur when hydrogen dissolved in steel accumulates due to segregation or the like during cooling after rolling, the solubility decreases as the temperature drops, and it becomes a gas, and pores are formed by the pressure.
[0003] As a method for reducing UT defects, a method for reducing segregation or the like that is the starting point of UT defects is known (see Patent Document 1). Also, a method is known in which after casting and before rolling, the cast slab is covered with a heat-insulating cover or placed in a heating furnace to keep the cast slab at a high temperature, and during that time, hydrogen in the cast slab is diffused to remove hydrogen from the cast slab.
Prior Art Documents
Patent Documents
[0004]
Patent Document 1
Summary of the Invention
Problems to be Solved by the Invention
[0005] It has been found that UT defects cannot be sufficiently reduced by conventional methods for reducing segregation or the like. Also, in the method of covering the cast slab with a heat-insulating cover or placing it in a heating furnace after casting, large-scale equipment such as a heat-insulating cover and a heating furnace and a large amount of fuel are required.
[0006] An object of the present invention is to provide a method capable of reducing UT defects by a method different from conventional methods.
Means for Solving the Problems
[0007] Our research has shown that by manufacturing slabs under the following conditions during the series of processes of refining, casting, and cooling before rolling, UT defects can be reduced without using large-scale equipment or large amounts of fuel.
[0008] The method for manufacturing a slab disclosed in this specification comprises: a refining step of adjusting the hydrogen concentration in molten steel to 1.5 ppm or less by degassing treatment; a casting step of casting a slab using a vertical bending type continuous casting machine or a bending type continuous casting machine, wherein the molten steel adjusted in the refining step is supplied to a tundish, and the ratio of "IN surface water content / OUT surface water content" at both ends of the slab in the width direction from directly below the mold to the end of the straightening band is set to ≤ 3.0 and the ratio of IN surface water content at both ends of the slab in the width direction is 0.60 L / kg-steel or less; and a cooling step of cutting the slab cast in the casting step, and within 50 minutes after casting, placing slabs cast within 50 minutes using the same casting machine or other casting machines on and below the cut slab, and maintaining the state in which slabs cast within 50 minutes are placed on and below the cut slab for 40 hours or more.
[0009] Previously, even if a slab with reduced UT defects could be manufactured, UT defects could still occur due to subsequent rolling conditions. For example, if the temperature at the end of rolling is low, or if accelerated cooling is performed after rolling, hydrogen in the steel may not diffuse to the surface of the product, resulting in UT defects. Also, if the product is thick, hydrogen in the steel may not diffuse sufficiently from the surface of the product, resulting in UT defects. The slab manufacturing method described above was found to reduce UT defects even under conditions where UT defects are likely to occur after slab manufacturing. [Effects of the Invention]
[0010] The above method can reduce UT defects. [Brief explanation of the drawing]
[0011] [Figure 1] This is a schematic diagram of a vertical bending type continuous casting machine. [Figure 2A] It is a schematic cross-sectional view of the widthwise end of the slab passing through the leveling zone. [Figure 2B] It is a schematic cross-sectional view of the widthwise end of the slab passing through the leveling zone when the IN surface is strongly cooled with respect to the OUT surface from directly below the mold to the end position of the leveling zone. [Figure 3A] It is a schematic cross-sectional view of the widthwise end of the slab passing through the leveling zone when the IN surface is strongly cooled from directly below the mold to the end position of the leveling zone. [Figure 3B] It is a schematic cross-sectional view of the widthwise end of the slab passing through the leveling zone. [Figure 4] It is a cross-sectional view of the slab for explaining the widthwise end of the slab. [Figure 5] It is a schematic view showing an example of the cut surface of the slab. [Figure 6] It is a figure showing the result of internal cracks. [Figure 7] It is a figure showing the relationship between the hydrogen concentration in the slab and the UT defect rate. [Figure 8] It is a figure showing the change over time of the slab temperature after stacking. [Figure 9] It is a figure showing the hydrogen concentration inside the slab. [Figure 10] It is a figure showing the change over time of the hydrogen concentration in the slab after stacking. [Figure 11] It is a figure showing the result of UT defects.
Mode for Carrying Out the Invention
[0012] Hereinafter, preferred embodiments of the present invention will be described.
[0013] The inventors of the present application cast a slab using a vertical bending type continuous casting machine or a bending type continuous casting machine.
[0014] Fig. 1 schematically shows the configuration of a vertical bending type continuous casting machine 1. The vertical bending type continuous casting machine 1 includes a tundish 2, a submerged nozzle 3 attached to the bottom of the tundish 2, a mold 4 disposed below the submerged nozzle 3, and a plurality of rolls 5. The plurality of rolls 5 are arranged along the casting path Q on both sides of the casting path Q. A cooling nozzle 6 is disposed between two adjacent rolls 5 in the casting direction. In the present embodiment, the side closer to the mold 4 along the casting path Q is referred to as the upstream side, and the side farther from the mold 4 is referred to as the downstream side.
[0015] In the casting path Q, a vertical zone 11 extending in the vertical direction, a bending zone 12 gently curved from the vertical zone 11, an arc zone 13 connected to the bending zone 12 and having a constant radius of curvature, a correction zone 14 provided downstream of the arc zone 13 and increasing the radius of curvature, and a horizontal zone 15 extending horizontally from the correction zone 14 are provided. In the present embodiment, the most downstream position of the correction zone 14 is referred to as the "correction zone end position".
[0016] Molten steel is supplied to the tundish 2. The molten steel 10 in the tundish 2 is injected into the mold 4 through the submerged nozzle 3, drawn downward while forming a solidified shell, and solidifies to the inside. Thereby, a slab is cast. During casting, water is sprayed from the cooling nozzle 6 onto the slab.
[0017] In the present embodiment, the side corresponding to the lower side of the slab is referred to as the "OUT side", and the side corresponding to the upper side of the slab is referred to as the "IN side". Also, the slab surface on the OUT side is referred to as the "OUT surface", and the slab surface on the IN side is referred to as the "IN surface". Note that the "OUT side" and the "OUT surface" may be referred to as the "reference side" and the "reference surface", respectively. The "IN side" and the "IN surface" may be referred to as the "opposite reference side" and the "opposite reference surface", respectively.
[0018] The vertical bending type continuous casting machine 1 described above is provided with a vertical band 11, but the bending type continuous casting machine (not shown) does not have a vertical band extending vertically downstream of the mold. In the casting path of the bending type continuous casting machine, a gently curved bending band, an arc band, a straightening band, and a horizontal band are provided sequentially downstream from directly below the mold.
[0019] Conventionally, methods for reducing UT defects by reducing segregation and other factors are known. While this method can reduce UT defects to some extent, UT defects sometimes remained in product areas corresponding to the slab widthwise ends. The inventors of this invention investigated the cause of this and obtained the following findings.
[0020] As the slab passes through the straightening band 14 (see Figure 1), the slab is straightened to become horizontal. Figures 2A, 2B, 3A, and 3B show examples of cross-sections of the slab passing through the straightening band 14 (cross-sections along line II-II in Figure 1). Figures 2A, 2B, 3A, and 3B show the widthwise ends of the slab. The widthwise direction of the slab is the direction perpendicular to the casting direction (or longitudinal direction) and the thickness direction of the slab.
[0021] When a slab is straightened, as shown in Figure 2A, the IN side of the slab shrinks in the width direction, while the OUT side stretches in the width direction. This deforms the solidified shell at the widthwise end of the slab (narrow-side solidified shell), generating forces (a1) directed towards the center of the width direction on the IN side and forces (b1) directed towards the OUT side. This causes strain at the solidification interface, resulting in cracks (internal cracks) in the segregated areas near the solidification interface. Hydrogen accumulates in these internal cracks and gasifies, causing UT defects.
[0022] Furthermore, during casting, cooling water is sprayed onto the slab from the cooling nozzle 6 (see Figure 1). This cools the slab, causing the IN side and OUT side of the slab to contract. The specific water content of the IN side and the OUT side may differ. When the specific water content of the IN side and the OUT side differ, a difference in the amount of contraction on the IN side and the OUT side occurs. For example, if the specific water content of the IN side is increased compared to the OUT side in order to strongly cool the IN side, the deformation shown in Figure 2A is amplified, and the force toward the center in the width direction of the IN side (a1) and the force toward the OUT side (b1) shown in Figure 2A become larger, as shown in Figure 2B, and these are thought to promote internal cracking.
[0023] Furthermore, internal cracking can be exacerbated not only when the water content at the intake surface is higher than that at the outlet surface, but also simply when the water content at the intake surface is excessively high. For example, if the water content on the IN surface from directly below the mold to the end of the straightening band becomes too high, the area near the IN side corner of the slab hardens, and deformation occurs as shown in Figures 2A and 2B. Then, as shown in Figure 3A, a large localized strain occurs near the IN side corner. This promotes internal cracking.
[0024] Internal cracking is exacerbated, making UT defects more likely to occur. For these reasons, conventional methods of reducing UT defects by reducing segregation, etc., likely resulted in UT defects being present in the product area corresponding to the slab widthwise end.
[0025] On the other hand, if the above is improved, for example, if the amount of water content on the IN surface is reduced so that it does not become too high from directly below the mold to the end of the straightening band, the corners will become softer. Therefore, even if the deformation shown in Figures 2A and 2B occurs, the areas where strain occurs will be dispersed as shown in Figure 3B, and the strain at each location will be smaller. As a result, internal cracking is less likely to occur.
[0026] The above phenomenon is thought to occur in the straightening belt of both vertical bending type continuous casting machines and bending type continuous casting machines.
[0027] Based on the above findings, we focused on the ratio of the specific water content of the IN surface to the specific water content of the OUT surface, specifically "specific water content of the IN surface / specific water content of the OUT surface" and "specific water content of the IN surface," which are factors that promote internal cracking. Furthermore, segregation and internal cracking, which cause UT defects, are caused by deformation during straightening and strain determined by the cooling method up to that point; therefore, cooling in the horizontal band 15 has little effect on segregation and internal cracking. For this reason, we focused on the "specific water content of the IN surface / specific water content of the OUT surface" and the "specific water content of the IN surface" from directly below the mold to the end of the straightening band. Furthermore, since the reason why conventional methods cannot reduce UT defects is thought to be that internal cracking is easily exacerbated at the widthwise ends of the slab, we focused on the "specific water content of the IN surface / specific water content of the OUT surface" and the "specific water content of the IN surface" at both ends in the widthwise direction of the slab. Based on the above, we decided to focus on the "specific water content of the IN surface / specific water content of the OUT surface" at both ends in the slab width direction, from directly below the mold to the end of the straightening band, and the "specific water content of the IN surface" at both ends in the slab width direction.
[0028] Here, the ends of the slab in the width direction refer to the region (X1) extending from one end of the slab in the width direction to half the slab thickness (D) (D / 2), and the region (X2) extending from the other end of the slab in the width direction to half the slab thickness (D) (D / 2). These regions are prone to internal cracking because they are the areas where strain is applied, as explained in Figures 2A, 2B, and 3A.
[0029] Furthermore, "IN surface specific water content" refers to the specific water content sprayed from the cooling nozzle 6 (see Figure 1) onto the IN surface of the slab. "OUT surface specific water content" refers to the specific water content sprayed from the cooling nozzle 6 (see Figure 1) onto the OUT surface of the slab. Specific water content is the amount of water sprayed onto 1 kg of slab (unit: L / kg-steel). Furthermore, the "specific water content of the IN surface at both ends in the slab width direction from directly below the mold to the end of the straightening zone" refers to the amount of water sprayed from the cooling nozzle 6 onto the IN surface at both ends in the slab width direction per 1 kg of slab at both ends in the slab width direction, from directly below the mold to the end of the straightening zone. The "specific water content of the OUT surface at both ends in the slab width direction from directly below the mold to the end of the straightening zone" refers to the amount of water sprayed from the cooling nozzle 6 onto the OUT surface at both ends in the slab width direction per 1 kg of slab at both ends in the slab width direction, from directly below the mold to the end of the straightening zone.
[0030] To investigate the effect of internal cracking caused by the "specific water content of the IN surface / specific water content of the OUT surface" at both ends in the slab width direction, and the "specific water content of the IN surface" at both ends in the slab width direction, from directly below the mold to the end of the straightening band, the following experiment (Experiment 1) was conducted.
[0031] (Experiment 1) The slab was manufactured using the following method.
[0032] After smelting in a converter, the molten steel was tapped into a 250-ton ladle, and degassing and compositional adjustment were performed using RH vacuum degassing to adjust the composition of the molten steel to the following specifications. <Steel composition> C:0.03~0.20mass% Mn: 0.4~1.7 mass%, Si: 0.03~0.49 mass% Nb: 0.00~0.05mass% Ti: 0.000~0.029 mass% N: 60ppm or less P:0.015mass% or less S: 0.005mass% or less H:1.5ppm or less
[0033] Molten steel adjusted to the above composition was supplied to a tundish, and a slab (width: 2100 mm, thickness: 280 mm) was cast using a vertical bending type continuous casting machine under the following conditions. • Vertical bending continuous casting machine: Machine length 40.4m Casting speed: 1.0~1.2m / min • Specific water content: Total specific water content (specific water content from directly below the mold to the end of the horizontal zone) 0.1~1.5 L / kg-steel
[0034] Multiple slabs were cast by varying the "water content on the IN surface / water content on the OUT surface" at both ends in the slab width direction and the "water content on the IN surface" at both ends in the slab width direction, from directly below the mold to the end of the straightening band. The internal crack length of each slab was measured using the following method.
[0035] <Internal crack length> After casting, the slab was cut to a predetermined length, the cut surfaces were polished along the width and thickness directions of the slab, and then ammonium persulfate was applied. The length of the corroded portion was measured. If there was only one corroded portion, the length of that portion was defined as the "total internal crack length." If there were multiple corroded portions, the lengths of all the corroded portions were measured, and their sum was defined as the "total internal crack length." Figure 5 schematically shows the cross-section of a slab where internal cracks occurred.
[0036] Figure 6 shows the results for "Specific Water Content on IN Surface / Specific Water Content on OUT Surface", "Specific Water Content on IN Surface", and "Total Internal Crack Length". The "Specific Water Content on IN Surface / Specific Water Content on OUT Surface" and "Specific Water Content on IN Surface" shown in Figure 6 are the "Specific Water Content on IN Surface / Specific Water Content on OUT Surface" at both ends in the slab width direction and the "Specific Water Content on IN Surface" at both ends in the slab width direction, respectively, from directly below the mold to the end of the straightening band.
[0037] Based on past experience, when the total internal crack length is 150 mm or less, products with UT defects may be obtained, but the number of UT defects in the product is small. When the total internal crack length is 70 mm or less, the number of UT defects is even smaller. On the other hand, when the total internal crack length exceeds 150 mm, products with UT defects are often obtained, and the number of UT defects is large. In Figure 6, a total internal crack length of 70 mm or less is marked with "○", a total internal crack length exceeding 70 mm but 150 mm or less is marked with "△", and a total internal crack length exceeding 150 mm is marked with "×". As shown in Figure 5, internal cracks were present at both ends in the width direction of the slab.
[0038] From Figure 6, it can be seen that when the ratio of "specific water content on the IN surface to the specific water content on the OUT surface" exceeds 3.0 and the "specific water content on the IN surface" exceeds 0.6 L / kg-steel, it is thought that a product will be obtained with many internal cracks at both ends in the slab width direction and UT defects in the areas corresponding to both ends in the slab width direction. On the other hand, when the ratio of "specific water content on the IN surface to the specific water content on the OUT surface" is 3.0 or less and the "specific water content on the IN surface" is 0.6 L / kg-steel or less, it is thought that a product will be obtained with few internal cracks at both ends in the slab width direction and, even if there are UT defects in the areas corresponding to both ends in the slab width direction, the number of UT defects will be small. Therefore, by cooling using a method that reduces the total length of internal cracks to 150 mm or less, that is, by making the "specific water content of the IN surface / specific water content of the OUT surface" at both ends in the slab width direction from directly below the mold to the end of the straightening band to 3.0 or less, and the "specific water content of the IN surface" at both ends in the slab width direction to 0.6 L / kg-steel or less, internal cracks at both ends in the slab width direction can be reduced. Furthermore, by removing hydrogen, which causes UT defects, during cooling after casting, it is thought that UT defects in the product parts corresponding to both ends in the slab width direction, which could not be solved by conventional methods, can be reduced.
[0039] Based on the above findings, we investigated cooling methods after casting.
[0040] After casting, the slab is cut and placed in a heating furnace for pre-rolling heating before being rolled to become the final product. However, before entering the heating furnace, the slab is left in the atmosphere, causing it to cool and its temperature to drop. If the slab is left unattended, it cools rapidly, the slab temperature drops, and the diffusion rate of hydrogen in the slab decreases, making it difficult to adequately remove hydrogen from the slab. Therefore, the slab is cooled slowly, and the hydrogen in the slab is removed during this slow cooling process. When slow cooling is performed, the slab temperature is maintained at a high temperature, increasing the diffusion rate of hydrogen and promoting the removal of hydrogen from the slab. Existing slow cooling methods involve covering the slab with an insulating cover or placing it in a heating furnace, which requires large-scale equipment such as insulating covers and heating furnaces, as well as large amounts of fuel. We investigated a slow cooling method that can adequately remove hydrogen from the slab without using large-scale equipment or large amounts of fuel.
[0041] First, we investigated the relationship between the hydrogen concentration in the slab after cooling and the UT defect rate of the product. The "hydrogen concentration in the slab" was determined by simulation based on the hydrogen concentration in the molten steel after degassing in the refining process and the conditions under which the slab was air-cooled after casting. The "UT defect rate" of the product was determined by the following method. Ultrasonic testing was used to check for UT defects at multiple locations on the product. The ultrasonic testing was performed according to the method specified in JIS Z 2344. A vertical probe was used for the ultrasonic testing, and the detection sensitivity was set to JIS STB N1 = 100% or higher. The ratio of products judged to have UT defects (number of products with UT defects / number of products investigated) was defined as the UT defect rate of the product.
[0042] Figure 7 shows the relationship between hydrogen concentration in the slab and the UT defect rate, indicated by diamonds. Figure 7 also shows the correlation line between hydrogen concentration in the slab and the UT defect rate. From the correlation line in Figure 7, it can be inferred that the UT defect rate is zero when the hydrogen concentration in the slab is 0.8 ppm or less. Therefore, in order to manufacture products free of UT defects, the hydrogen concentration in the slab must be 0.8 ppm or less, and hydrogen must be removed during the slow cooling of the slab so that the hydrogen concentration in the slab is 0.8 ppm or less.
[0043] One possible method for slowly cooling a slab without using large-scale equipment or large amounts of fuel is to utilize the high-temperature slab after casting. For example, it is thought that the slab can be slowly cooled by placing the high-temperature slab after casting above and below the slab to which UT defects are to be reduced. The inventors of this application have investigated a method to slowly cool the slab by stacking high-temperature slabs after casting and placing the high-temperature slabs above and below the slab to which the slab is to be slowly cooled, and to reduce the hydrogen concentration in the slab to 0.8 ppm or less while the slab is being slowly cooled.
[0044] Furthermore, while the target slab is thought to be cooled slowly when high-temperature slabs are placed above and below it, the target slab cools rapidly once the high-temperature slabs are removed above and / or below it. Therefore, it is necessary to reduce the hydrogen concentration in the target slab to 0.8 ppm or less while high-temperature slabs are placed above and below it, that is, while the slabs are stacked.
[0045] Therefore, we investigated how long the stacked slabs needed to be maintained in order to reduce the hydrogen concentration in the target slabs to 0.8 ppm or less.
[0046] First, the time-dependent temperature change of the target slab when the slabs were stacked was determined using the following method.
[0047] In order to place high-temperature slabs above and below the target slab, it was decided that three or more cast slabs would be stacked within 50 minutes after casting, so that the high-temperature slabs would be placed above and below the target slab. Hereafter, the process of placing high-temperature slabs above and below the target slab will be referred to as "stacking" ("stacking complete"), and the state in which the high-temperature slabs are placed above and below the target slab will be referred to as the "stacking state."
[0048] The temperature change over time of the target slab for 50 minutes after casting until stacking was determined under the following conditions using a two-dimensional heat transfer and solidification calculation. CASTEM (a heat transfer and solidification program) was used for the heat transfer and solidification calculation. • Slab size: 280mm thick, 2100mm wide Casting speed: 1.2 m / min • Vertical bending continuous casting machine: Length 40.3m, number of rolls 136 ·Specific water volume: 0.82L / kg-steel The specific water content here is the total specific water content of the continuous casting machine.
[0049] Using two-dimensional heat transfer calculations, the time-dependent temperature change of the target slab after stacking (50 minutes after casting) was determined under the following conditions. Here, the target slab temperature was determined so that the calculated target slab temperature fits to the previously measured target slab temperature (temperature at the center of the narrow face (measured value)) (see Figure 8). CASTEM (heat transfer solidification program) was used for the calculations. The initial temperature (stacking completion temperature) was set to the last temperature of the "target slab temperature during the 50 minutes after casting until stacking" (temperature 50 minutes after casting). • Slab size: 280mm thick, 2100mm wide • Temperature of the central surface of the slab in contact with the target slab: 800℃
[0050] The hydrogen concentration in the slab after stacking was calculated using the target slab temperature after stacking (slab temperature in the stacked state), the hydrogen concentration in the molten steel (hydrogen concentration in the molten steel after degassing treatment in the refining process), and the hydrogen diffusion coefficient, as shown in Figure 8. The hydrogen concentration in the molten steel (hydrogen concentration in the molten steel after degassing treatment in the refining process) was assumed to be 1.5 ppm. The hydrogen diffusion coefficient was obtained by the following method.
[0051] Using the hydrogen diffusion coefficient (literature value) described in the following literature, the hydrogen concentration inside the slab after slow cooling was calculated, and a correction factor was determined to make it as close as possible to the measured hydrogen concentration inside the slab (measured value). The hydrogen diffusion coefficient for calculating the "hydrogen concentration in the slab" above was obtained by multiplying the literature value by the aforementioned correction factor. Figure 9 shows the measured hydrogen concentration inside the slab and the calculated hydrogen concentration inside the slab after slow cooling that best fits the measured value. The results shown in Figure 9 were obtained with a slab size of 280 mm thickness x 2100 mm width, with the initial conditions being the hydrogen concentration in the slab at the completion of stacking being the hydrogen concentration after molten steel treatment (1.7 ppm) (the measured hydrogen concentration after molten steel treatment for the slab whose hydrogen concentration inside the slab was measured), assuming a hydrogen concentration outside the slab of 0 ppm as a boundary condition, and a slab slow cooling time of 70 hours. <Literature> 1) W. Eichenauer, H. Kiinzig and A. Pebler: Z.Metalik., 49(1958), p.220.2)TM Stross and FC Tompkins: J. Chem. Soc.London,(1956), Part 1, p.230. 3) JY Choi: Met. Trans., 1(1970), p.911.
[0052] Figure 10 shows the change in hydrogen concentration in the slab over time after stacking is complete (change in hydrogen concentration in the stacked slab over time). To manufacture products without UT defects, the hydrogen concentration in the slab needs to be 0.8 ppm or less (see Figure 7). From Figure 10, the hydrogen concentration in the slab becomes 0.80 ppm or less when the stacked state is maintained for 40 hours or more. From this, it can be concluded that products without UT defects can be manufactured by maintaining the stacked state for 40 hours or more.
[0053] Note that the hydrogen concentration in the slab shown in Figure 10 was calculated assuming a hydrogen concentration of 1.5 ppm in the molten steel (hydrogen concentration in the molten steel after gas treatment in the refining process). The lower the hydrogen concentration in the molten steel, the lower the hydrogen concentration in the slab after stacking is complete. Therefore, if the hydrogen concentration in the molten steel is lower than 1.5 ppm, the hydrogen concentration in the slab is expected to be lower than 0.80 ppm when the stacked state is maintained for 40 hours. Accordingly, by adjusting the hydrogen concentration in the molten steel to 1.5 ppm or less in the refining process, it is considered possible to maintain a hydrogen concentration of 0.80 ppm or less in the slab when the stacked state is maintained for 40 hours or more.
[0054] Furthermore, in the above example, stacking is completed within 50 minutes after casting. The shorter the time to complete stacking, the higher the temperature of the target slab, as well as the temperatures of the slabs placed above and below it, resulting in a higher temperature for the target slab in the stacked state. The higher the temperature of the target slab, the easier it is for hydrogen in the target slab to diffuse, thus making it easier to remove hydrogen from the slab. From this, it is thought that if stacking is completed in less than 50 minutes after casting and the stacked state is maintained for 40 hours, the hydrogen concentration in the slab will be lower than 0.80 ppm. Therefore, it is thought that if stacking is completed within 50 minutes after casting and the stacked state is maintained for 40 hours or more, the hydrogen concentration in the target slab can be reduced to 0.80 ppm or less.
[0055] Furthermore, in order to slowly cool the target slab, it is preferable to place a slab with a temperature close to or higher than the target slab above or below it. Since the target slab is stacked within 50 minutes after casting, it is considered that by placing a slab that has been cast within 50 minutes above or below the target slab, it is possible to place a slab with a temperature close to or higher than the target slab above and below it. Therefore, slabs that have been cast within 50 minutes are placed above and below the target slab. The slabs placed above and below the target slab only need to be slabs that have been cast within 50 minutes, and may be slabs cast in the same casting machine as the target slab, or slabs cast in a different casting machine. Even if the casting machine is different, the temperature of slabs that have been cast within 50 minutes does not change significantly. Furthermore, slabs cast using the same casting machine as the target slab may be cast under the same conditions as the target slab (casting conditions, steel type, hydrogen concentration in molten steel, etc.), or under different conditions. Even if the conditions until casting is complete differ, the temperature of the slab within 50 minutes after casting will not change significantly. Also, slabs cast using the same casting machine as the target slab may be made of the same cast as the target slab, or of a different cast. Even if the cast is different, the temperature of the slab will be close to that of the target slab as long as it is within 50 minutes of casting.
[0056] Furthermore, the slab is cut after casting, and this cutting often occurs immediately after it passes the final roll of the casting machine. If the slab is cut immediately after it passes the final roll of the casting machine, "within 50 minutes after casting" can also mean "within 50 minutes from the time the slab is cut." If the slab is not cut immediately after it passes the final roll of the casting machine, "within 50 minutes after casting" means "within 50 minutes from the time the cutting point of the slab passes the final roll."
[0057] The target slab for which UT defects are to be reduced may be one slab or multiple slabs in a stack of slabs. Slabs placed above and / or below the target slab may also be the target slab for which UT defects are to be reduced. For example, if five cut slabs are stacked in five layers within 50 minutes after casting, the three layers excluding the top and bottom layers may be the target slabs, or one or two of the three layers excluding the top and bottom layers may be the target slabs.
[0058] Based on the above, the following cooling procedure is followed after casting. Within 50 minutes of casting, place slabs cast within 50 minutes of casting (using the same or other casting machines) on top of and below the slab to be targeted for reduction of UT defects (a stacked arrangement). Maintain this stacked arrangement (with slabs cast within 50 minutes of casting) for 40 hours or more to slowly cool the target slab. Here, the target slab is a slab obtained by casting molten steel in which the hydrogen concentration in the molten steel has been adjusted to 1.5 ppm or less during the refining process. This removes the hydrogen that causes UT defects.
[0059] Furthermore, the above cooling method is based on the cooling method that was found in Experiment 1 to reduce internal cracking at both ends in the slab width direction. Specifically, it involves setting the "specific water content of the IN surface / specific water content of the OUT surface" at both ends in the slab width direction from directly below the mold to the end of the straightening band to ≤ 3.0, and setting the specific water content of the IN surface at both ends in the slab width direction to 0.60 L / kg-steel or less. By performing this cooling after reducing internal cracking at both ends in the slab width direction, it is thought that UT defects in the product parts corresponding to both ends in the slab width direction, which cannot be solved by conventional methods, can be reduced. To confirm this, the following experiment (Experiment 2) was conducted.
[0060] (Experiment 2) In Experiment 1, the target slabs were selected from those cast with a total internal crack length exceeding 150 mm, and from those with a total internal crack length of 150 mm or less, where the "specific water content on the IN surface / specific water content on the OUT surface" was close to 3.0 and the "specific water content on the IN surface" was close to 0.6 L / kg-steel (see Figure 6). Within 50 minutes of casting the target slabs, slabs cast within 50 minutes using the same or other casting machines were placed above and below the target slab (stacking completed), and this state (stacked state) was maintained for more than 40 hours for slow cooling. After slow cooling, the slabs were rolled to produce products with a thickness of 30 mm. Ultrasonic testing was used to check for the presence of UT defects in the products. Ultrasonic testing was performed according to the method specified in JIS Z 2344. A vertical probe was used for ultrasonic testing, and the detection sensitivity was set to JIS STB N1 = 100% or higher.
[0061] Figure 11 shows the results for "IN surface water content / OUT surface water content," "IN surface water content," and "UT defects." The "IN surface water content / OUT surface water content" and "IN surface water content" shown in Figure 11 are the "IN surface water content / OUT surface water content" at both ends in the slab width direction and the "IN surface water content" at both ends in the slab width direction, respectively, from directly below the mold to the end of the straightening band. "×" indicates the presence of UT defects, and "○" indicates the absence of UT defects. Products marked with "○" had no UT defects throughout the entire product.
[0062] Products with UT defects (marked "×") had UT defects in the product areas corresponding to both ends in the slab width direction. Products with UT defects ("×") had a total internal crack length exceeding 150 mm, and were cast under conditions where the "specific water content of the IN surface / specific water content of the OUT surface" exceeded 3.0, and the "specific water content of the IN surface" exceeded 0.6 L / kg-steel. It was found that when slabs were cast under these conditions, even if the slabs were slowly cooled after casting using the method described above, the UT defects in the product areas corresponding to both ends in the slab width direction could not be reduced. Products without UT defects ("○") were those with a total internal crack length of 150 mm or less, a "water content at IN surface / water content at OUT surface" ratio of 3.0 or less, and cast under the conditions that the "water content at IN surface" was 0.6 L / kg-steel or less.
[0063] From the above, it was found that by using a cooling method that reduces internal cracking at both ends in the width direction of the slab, specifically by setting the "specific water content of the IN surface / specific water content of the OUT surface" at both ends in the width direction of the slab from directly below the mold to the end of the straightening band to 3.0 or less, and the "specific water content of the IN surface" at both ends in the width direction of the slab to 0.6 L / kg-steel or less, internal cracking at both ends in the width direction of the slab is reduced during casting. Furthermore, by slowly cooling the slab after casting using the method described above, it is possible to reduce UT defects in the product parts corresponding to both ends in the width direction of the slab, which could not be solved by conventional methods, and ultimately reduce UT defects in the entire product.
[0064] Based on the above, the slab will be manufactured using the following method. In the refining process, the hydrogen concentration in the molten steel is adjusted to 1.5 ppm or less through degassing treatment. In the subsequent casting process, the adjusted molten steel is supplied to the tundish, and the slab is cast using a vertical bending type continuous casting machine or a bending type continuous casting machine. At this time, from directly below the mold to the end of the straightening band, the ratio of "water content on the IN surface / water content on the OUT surface" at both ends in the width direction of the slab is set to ≤ 3.0, and the water content on the IN surface at both ends in the width direction of the slab is set to 0.60 L / kg-steel or less. The target slab (the slab from which UT defects are to be reduced) cast by the above casting process is cut. Within 50 minutes after casting, slabs cast within 50 minutes using the same casting machine or other casting machines are placed on top of and below the target slab, and the target slab is slowly cooled. The state in which slabs cast within 50 minutes are placed on top of and below the target slab is maintained for 40 hours or more (cooling process).
[0065] By manufacturing slabs under the above conditions during the series of processes of refining, casting, and cooling before rolling, internal cracks that cause UT defects are reduced at both ends in the slab width direction during casting. Furthermore, after casting, the slab is slowly cooled, and hydrogen in the slab that causes UT defects is sufficiently removed during slow cooling. This reduces UT defects in the product parts corresponding to both ends in the slab width direction, which could not be sufficiently reduced by conventional methods, and consequently reduces UT defects in the entire product. Moreover, UT defects can be reduced in a different way from conventional methods without using large-scale equipment or large amounts of fuel.
[0066] For the degassing treatment in the refining process, a general degassing device (RH, DH, etc.) may be used. The degassing treatment time, vacuum level, etc., may be adjusted to adjust the hydrogen concentration to 1.5 ppm or less. It is not necessary to drastically reduce the hydrogen that causes UT defects in the refining process; adjusting it to 1.5 ppm or less is sufficient. There is no particular lower limit to the hydrogen concentration. In addition to adjusting the hydrogen concentration in the refining process, decarburization, desulfurization, component adjustment, etc., may also be performed.
[0067] Furthermore, in the casting process, there is no particular limit to the lower limit of the "water content of the IN surface / water content of the OUT surface" at both ends in the width direction of the slab, from directly below the mold to the end of the straightening band. Even if the "water content of the IN surface / water content of the OUT surface" is less than 1.0, UT defects can be reduced by the above method. From the standpoint of suppressing breakout, it is preferable to make the "water content of the IN surface / water content of the OUT surface" greater than 0. For example, the "water content of the IN surface / water content of the OUT surface" may be set to 0.3 or higher.
[0068] Furthermore, the lower limit of the "IN surface specific water content" at both ends in the width direction of the slab, from directly below the mold to the end of the straightening zone, is not particularly limited. From the standpoint of suppressing breakout, it may be set to, for example, 0.05 L / kg-steel or higher, or 0.1 L / kg-steel or higher.
[0069] Furthermore, Experiment 2 showed that UT defects can be reduced in the region of the slab excluding both ends in the width direction by slow cooling after casting using the method described above. Therefore, the region of the slab excluding both ends in the width direction may be cooled under general cooling conditions, for example. The region of the slab excluding both ends in the width direction may be cooled to satisfy a total specific water content of 0.1 to 1.5 L / kg-steel, for example. Alternatively, the region of the slab excluding both ends in the width direction may be cooled under the same conditions as the cooling conditions for both ends in the width direction, or under different conditions.
[0070] Furthermore, as mentioned above, cooling in the horizontal zone 15 has little effect on segregation or internal cracking, so the horizontal zone 15 may be cooled under general cooling conditions, for example. For example, the specific water content of the horizontal zone may be 0 to 0.5 L / kg-steel. Also, the cooling conditions of the horizontal zone may be the same as or different from the cooling conditions from directly below the mold to the end of the straightening zone.
[0071] After manufacturing the slab using the method described above, finishing such as scarf treatment and rolling are performed. The conditions after slab manufacturing may be normal or different from normal. For example, the temperature at the end of rolling may be lowered, or accelerated cooling may be performed after rolling. The thickness of the product obtained by rolling may be a typical thickness or thicker than a typical thickness. By manufacturing the slab using the method according to this embodiment, it was found that UT defects did not occur even when the conditions after slab manufacturing were conditions that are prone to UT defects, as shown in the experiments described later.
[0072] The slab manufacturing method described above can be used, for example, to manufacture slabs with the following compositions. C:0.03~0.20mass% Mn: 0.4~1.7%, Si: 0.030~0.49 mass% Nb: 0.00~0.05mass% Ti: 0.000~0.029 mass% N: 60ppm or less P:0.015mass% or less S: 0.005mass% or less H:1.5ppm or less Furthermore, the slab manufacturing method described above can be used, for example, when manufacturing slabs with a thickness of 250 mm or more and 300 mm or less.
[0073] To confirm the above findings, the following experiment (Experiment 3) was conducted.
[0074] (Experiment 3) The slab was manufactured using the following method.
[0075] After smelting in a converter, the molten steel was tapped into a 250-ton ladle. Subsequently, the molten steel underwent compositional adjustments using an RH vacuum degassing unit, followed by degassing. Table 1 shows the composition of the molten steel and the hydrogen concentration after degassing. The hydrogen concentration in the molten steel was measured using a Hydris sensor manufactured by Heraeus.
[0076] Molten steel as shown in Table 1 was supplied to the tundish, and a slab (width: 2100 mm, thickness: 280 mm) was cast using a vertical bending type continuous casting machine (machine length: 40.4 m, vertical length of mold: 0.9 m) under the casting conditions shown in Table 1. The "total specific water content" shown in Table 1 is the specific water content from directly below the mold to the end of the horizontal zone.
[0077] After casting, the slab (target slab) was cut and cooled under the conditions shown in Table 1. In No. 1, the target slab was left undisturbed without stacking (air cooling). In Nos. 2 to 7, within 50 minutes of casting the target slab, slabs cast within 50 minutes using the same or different casting machines were placed on and below the target slab (stacking completed). In Table 1, "Time from casting to stacking" refers to the time from the casting of the target slab until slabs cast within 50 minutes are placed on and below the target slab (until stacking is complete). In Table 1, "Time in stacked state" refers to the time the slabs cast within 50 minutes are placed on and below the target slab.
[0078] The target slab was cooled under the conditions shown in Table 1, and then rolled to obtain the product. The rolling conditions are shown in Table 1.
[0079] Ultrasonic testing was used to check for UT defects in products No. 1 to No. 7. The ultrasonic testing was performed according to the method specified in JIS Z 2344. A vertical probe was used for the ultrasonic testing, and the detection sensitivity was set to JIS STB N1 = 100% or higher. The results are shown in Table 1. In Table 1, products without UT defects are marked as pass ("○"), and products with UT defects are marked as fail ("×").
[0080] [Table 1]
[0081] From Table 1, the following was found: No. 1, which was air-cooled after casting, failed to pass inspection. No. 1 was rolled under conditions where the rolling stop temperature was low and UT defects were likely to occur in the product. When air-cooled after casting and then rolled under conditions that were likely to cause UT defects, it failed to pass inspection.
[0082] No. 2 underwent strong cooling of the IN surface during casting, failing to meet the casting cooling conditions according to this embodiment. Although it was slowly cooled after casting using the method according to this embodiment, it was unsatisfactory. Furthermore, No. 2 was rolled under conditions where the rolling stop temperature was low, making it prone to UT defects in the product. If the casting cooling conditions do not meet the cooling method according to this embodiment, the product will be unsatisfactory even if it is slowly cooled after casting.
[0083] On the other hand, products No. 3 to No. 7 were cooled during casting under the conditions of this embodiment, and then slowly cooled after casting using the method of this embodiment. Products No. 3 to No. 7 passed the test. In particular, product No. 3 had a thickness of 40mm, which is a condition prone to UT defects, but it passed the test. In No. 4, the rolling process was carried out under conditions where the rolling stop temperature was low and UT defects were likely to occur in the product, but it passed the test. In samples No. 5 and No. 6, accelerated cooling was performed after rolling, and the product thickness was over 50 mm, creating several conditions that make UT defects likely to occur, but they passed the test. In case No. 7, the product thickness was 55mm, which is a condition prone to UT defects, but it passed the test.
[0084] From the above, it was found that products free of UT defects can be obtained from slabs manufactured by the method according to this embodiment. Furthermore, in the above experiment, even though products were manufactured under conditions that are prone to UT defects after the slabs were manufactured by the method according to this embodiment, the number of UT defects was reduced. From this, it can be said that by manufacturing slabs by the method according to this embodiment, UT defects can be reduced regardless of whether the subsequent manufacturing conditions are prone to UT defects or not.
[0085] In the above experiment, slabs were cast using a vertical bending type continuous casting machine. However, even when casting slabs using a bending type continuous casting machine, UT defects can be reduced by manufacturing the slab using the method according to this embodiment. Furthermore, even when casting slabs using a bending type continuous casting machine, UT defects can be reduced by manufacturing the slab using the method according to this embodiment, even under subsequent manufacturing conditions that are prone to causing UT defects.
[0086] Although embodiments of the present invention have been described above based on examples, it should be understood that the specific configurations are not limited to these embodiments. The scope of the present invention is indicated by the claims rather than the above description, and all modifications within the meaning and scope equivalent to the claims are included. [Explanation of symbols]
[0087] 1. Continuous casting machine 2 Tan Dish 3 Immersion nozzle 4. Mold 5 rolls 6 Cooling nozzles 10 Molten steel 11 Vertical band 12 Bent band 13. Arc-shaped band 14 Corrective belt 15 Horizontal band Widthwise ends of X1 and X2 slabs
Claims
[Claim 1] A refining process in which the hydrogen concentration in molten steel is adjusted to 1.5 ppm or less by degassing treatment, In a casting process in which a slab is cast using a vertical bending type continuous casting machine or a bending type continuous casting machine, The molten steel prepared in the aforementioned refining process is supplied to the tundish. A casting process in which, from directly below the mold to the end of the straightening band, the "specific water content of the IN surface / specific water content of the OUT surface" at both ends in the width direction of the slab is ≤ 3.0 and the specific water content of the IN surface at both ends in the width direction of the slab is 0.60 L / kg-steel or less, A cooling process comprising: cutting the slab cast by the above casting process; placing slabs cast within 50 minutes of casting using the same casting machine or another casting machine on and below the cut slab; and maintaining the state in which the slabs cast within 50 minutes of casting are placed on and below the cut slab for 40 hours or more; A method for manufacturing slabs, characterized by having the following features.