Manufacturing method of hot rolled coil

Abstract

To provide a hot-rolled coil manufacturing method capable of preventing deformation of a hot-rolled coil drawn out from a mandril of a coiler in order to prevent rewinding or chipping of the hot-rolled coil.SOLUTION: This hot-rolled coil manufacturing method takes up a range equivalent to an inner part at a temperature of 450°C-650°C and a range equivalent to an outer peripheral part at a temperature of 50°C-800°C higher than the temperature of the inner part when a range having a thickness 5%-30% of a total coil thickness from the outermost periphery of a hot-rolled coil is the outer peripheral part and the remaining range of the inside is the inner part.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a method for manufacturing a hot-rolled coil in which a hot-rolled steel sheet after hot finish rolling is wound by a coiler and then the hot-rolled coil is pulled out from the mandrel of the coiler without being significantly deformed by its own weight. [Background technology]

[0002] After the hot-rolled steel sheet is finish-rolled, it is wound by a coiler and transported to the next process. When the hot-rolled coil is unwound from the mandrel of the coiler, it may deform under its own weight. If this deformation is large, it may not be possible to smoothly insert the hot-rolled coil into the mandrel of the unwinder in the subsequent process. In such cases, it is necessary to carry out a coil re-coiling process after passing through a coil refinement process, but if the deformation is significant, it is not possible to re-coil the hot-rolled coil, and there is a problem that the hot-rolled coil must be scrapped.

[0003] Conventionally, it has been believed that deformation of hot-rolled coils due to their own weight is caused by volume expansion due to the γ (fcc) → α (bcc) transformation. For example, in a high-strength hot-rolled steel sheet containing a required amount of alloying elements, the gamma → alpha transformation may not be completed on the runout table after hot rolling, and may continue during and after winding on a coiler, and even after the coil is pulled out from the mandrel. In such cases, the volume expansion of the steel sheet structure accompanying the subsequent gamma → alpha transformation reduces the radial surface pressure of the hot-rolled coil, reducing the rigidity of the entire hot-rolled coil, and the coil, which is a multiple-winding cylinder under tension, behaves like a single-winding cylinder, causing the hot-rolled coil to deform under its own weight.

[0004] Therefore, techniques have been proposed to prevent deformation of hot-rolled coils, mainly from the viewpoints of (a) mechanically maintaining the shape or correcting deformation using a V-shaped skid with a small angle (see, for example, Patent Documents 1 to 4), (b) retaining the steel in a coiler for a certain period of time and cooling the steel under cooling conditions such as natural cooling, air cooling, mist cooling, and water cooling (see, for example, Patent Documents 5 to 8), and (c) controlling the transformation rate (see, for example, Patent Documents 9 to 12).

[0005] However, these techniques have the following problems. In the mechanical method of (a), since it takes time to place the coil on the V-shaped skid, there is a problem that it is not possible to take measures in time to prevent deformation that occurs immediately after the hot-rolled coil is pulled out from the mandrel.

[0006] Method (b) is effective depending on the steel type, but it is a symptomatic deformation prevention measure based on experience and cannot be said to be a reliable and efficient deformation prevention measure. In addition, a very long residence time is required, which can significantly reduce work efficiency. Furthermore, when the cooling rate is high, such as water cooling, the edge part and several turns of the outermost circumference of the hot-rolled steel sheet become excessively hardened depending on the steel type, resulting in strength differences within the hot-rolled coil. This can be dealt with by discarding the outermost circumference, but this reduces the yield and the hardening of the edge part can cause breakage during cold rolling, so there is a problem that the applicable steel types are limited.

[0007] Method (c) has a narrow control range and may be less robust, and generally requires rolling at a constant speed with low production efficiency in hot rolling equipment that performs accelerated rolling, which leads to a decrease in efficiency, and requires major modifications since existing equipment and control logic cannot be used as is, making it uneconomical. Method (c) also includes a method in which a certain length range from the tip of the coil is coiled at a higher temperature than the rest to control the volume expansion rate due to transformation within the coil, but the coiling temperature is not selected based on the mechanism by which coil deformation occurs, so coil deformation is not sufficiently suppressed, and there are problems in that the effect is hardly observed depending on the steel composition. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Publication No. 61-037647 [Patent Document 2] Japanese Utility Model Application Publication No. 01-143609 [Patent Document 3] Japanese Utility Model Application Publication No. 06-034807 [Patent Document 4] Japanese Patent Application Publication No. 07-246422 [Patent Document 5] JP 2001-179335 A [Patent Document 6] JP 2010-089107 A [Patent Document 7] JP 2010-094710 A [Patent Document 8] JP 2012-024793 A [Patent Document 9] JP 2012-055950 A [Patent Document 10] Patent No. 5447744 [Patent Document 11] JP 2014-065077 A [Patent Document 12] Patent No. 5594578 Summary of the Invention [Problem to be solved by the invention]

[0009] In view of the prior art, an object of the present invention is to provide a method for manufacturing a hot-rolled coil, which can prevent deformation of the hot-rolled coil drawn out from the mandrel of a coiler in a manner simpler than conventional means, and can avoid rewinding the hot-rolled coil and scrapping. [Means for solving the problem]

[0010] In order to solve the above problems, the present inventors have extensively studied the deformation mechanism of hot rolled coils. As a result, it was found that the deformation of the hot-rolled coil immediately after it is pulled out from the mandrel or thereafter is not due to loosening of the hot-rolled coil caused by volume expansion accompanying the γ → α transformation, as has been conventionally believed, but is due to a decrease in the rigidity of the entire coil caused by the transformation plasticity phenomenon that occurs during the γ → α transformation.

[0011] From this perspective, a method for preventing deformation of the coil was investigated. Since the strain caused by transformation plasticity increases in proportion to the rate of the γ → α transformation, the idea was reached that by taking advantage of the difference in cooling rate between the outer and inner parts of the coil, a difference in transformation rate could be created between the outer and inner parts of the hot-rolled coil, and the coil could be supported at points where the transformation rate is slow and where strain due to transformation plasticity is less likely to occur, thereby preventing deformation of the coil. It was found that high-temperature coiling is advantageous for generating a difference in transformation rate between the outer and inner parts of a hot-rolled coil. However, high-temperature coiling has the problem that internal oxidation may occur in the inner part.

[0012] As a result of further investigation into means for preventing coil deformation and avoiding the problems of internal oxidation, it was found that by changing the coiling temperature to a higher temperature midway when coiling the hot-rolled steel sheet, it is possible to delay the transformation of the part that will become the outer part of the coil (the outer periphery) during the initial stage of drawing, when deformation is likely to occur, and to cause transformation of the outer periphery to begin after the transformation rate of the part that will become the inner part (the inside) has slowed down, thereby preventing coil deformation and avoiding internal oxidation of the inner part.

[0013] The present invention has been made based on the above findings, and the gist of the present invention is as follows. (1) In the hot-rolled steel sheet coiling process, the hot-rolled steel sheet after hot rolling is cooled on a run-out table and wound up by a coiler to form a hot-rolled coil, A method for manufacturing a hot-rolled coil, characterized in that the area from the outermost periphery of a hot-rolled coil that is 5% to 30% of the total coil thickness is defined as the outer periphery, and the remaining area inside is defined as the inside, wherein the area corresponding to the inside is coiled at a temperature of 450°C to 650°C, and the area corresponding to the outer periphery is coiled at a temperature that is more than 50°C higher than the temperature of the inside and is 800°C or less.

[0014] (2) A method for manufacturing a hot rolled coil according to (1) above, characterized in that the coiling temperature of the hot rolled steel sheet is set to an optimal internal coiling temperature for the hot rolled steel sheet in a range from the front end to the inside of the hot rolled steel sheet of its entire length, and set to an outer periphery coiling temperature that is more than 50°C higher than the internal coiling temperature for the remaining range corresponding to the outer periphery.

[0015] (3) A method for manufacturing a hot-rolled coil according to the above (2), characterized in that a Formaster test is performed in advance, in which a temperature history equivalent to that during cooling of a hot-rolled steel sheet is imparted, to investigate the time progression of the transformation rate during coil cooling, and the results are used to set the coiling temperatures at the inside and outer periphery. Effect of the Invention

[0016] According to the present invention, a method for manufacturing a hot-rolled coil can be provided that can avoid scrapping and recoiling of the hot-rolled coil by a simpler means. Furthermore, it is no longer necessary to increase the coiling temperature inside the coil more than necessary, and internal oxidation that occurs when the coiling temperature is high can be avoided. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a diagram showing the time progression of the transformation rate up to 300 seconds after the start of cooling, calculated by a Formaster test assuming coiling at 600° C. [Diagram 2] FIG. 1 is a diagram showing the time progression of the transformation rate up to 300 seconds after the start of cooling, calculated by a Formaster test assuming coiling at 690° C. [Diagram 3]This graph shows the time progression of the transformation rate when the internal coiling temperature is 600°C and the external coiling temperature is 690°C, and was created from the time progression of the transformation rate in Figures 1 and 2. [Figure 4] FIG. 1 is a diagram showing an example of a temperature history assuming winding at 600° C. used in a Formaster test. [Diagram 5] FIG. 1 is a diagram illustrating a mechanism by which a hot-rolled coil is not deformed when the coiling temperature is increased only at the outer periphery of the coil. [Figure 6] FIG. 1 is a diagram showing a schematic diagram of the progress of phase transformation inside the coil and on the outer periphery of the coil during cooling of the hot-rolled coil in the case where the coiling temperature is increased only on the outer periphery of the coil. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] From the viewpoint of operation, when a hot-rolled coil in which a hot-rolled steel sheet in which the γ → α transformation is not completely completed is immediately pulled out from the mandrel of a coiler after winding is completed, the coil is required to be able to be quickly transported to the next process without being deformed after the pulling out, and further, that the coil is required to be able to be quickly transported to the next process without being deformed until the coil is cooled thereafter.

[0019] The deformation accompanying phase transformation is thought to include deformation due to transformation plasticity in addition to deformation due to volume change. Here, transformation plasticity is a phenomenon in which, when stress acts during phase transformation, deformation occurs in the direction in which the stress acts. In the case of hot-rolled coils, this phenomenon is expected to manifest itself as deformation progressing in the direction that relieves the stress due to its own weight, i.e., vertically downward, during γ→α transformation, resulting in deformation that causes the coil to collapse.

[0020] In order to clarify the deformation mechanism of a hot-rolled coil, the present inventors performed an FEM analysis of deformation caused by the weight of a steel sheet formed into a cylindrical body. As a result, it was found that when the transformation plasticity phenomenon was not taken into account, even in a simulation in which a 4 mm thick steel plate was used as a single-wound cylinder, there was almost no deformation due to its own weight, and even less deformation occurred in a hot-rolled coil, which is a cylinder with multiple turns. On the other hand, a simulation that took into account the phenomenon of transformation plasticity revealed that a single-wound cylinder made of 4 mm thick steel plate deformed significantly, and that a coil, which is a cylinder with multiple turns, also deformed.

[0021] Since deformation actually occurred in a coil, which is a multiple-turn cylinder that is more resistant to deformation due to its own weight than a single-turn cylinder, it was predicted that the deformation of the coil was largely related to a decrease in rigidity due to the transformation plasticity phenomenon, rather than loosening of the coil due to volume expansion accompanying the γ → α transformation.

[0022] It is known that the magnitude of strain caused by transformation plasticity increases in proportion to the magnitude of the transformation rate of the phase transformation. Therefore, the inventors performed a Formaster test simulating the temperature history from the start of cooling for hot-rolled coils cooled on a runout table and coiled at various coiling temperatures, calculated the magnitude of the transformation rate at the center and outer periphery of the coil for each coiling temperature, and investigated the relationship with the deformation of the coil.

[0023] In the Formastor test, a steel billet containing, by mass%, 0.15% C, 0.50% Si, and 2.6% Mn was first produced, hot-rolled at a finishing exit temperature of 900°C, cooled, and then coiled at 600°C and 690°C. After coiling was completed (76 seconds after cooling of the coil tip started), the hot-rolled coil was pulled out from the mandrel and the deformation state of the coil after pulling was confirmed. In addition, a sample was taken from the obtained hot-rolled coil and cooled on a runout table, and a Formastor test was performed to simulate the temperature history of the coil after coiling on a coiler, and the time transition of the transformation rate of the coil after cooling started was calculated.

[0024] Here, the transformation rate in the Formastor test was calculated as follows. The sample steel plate was heated from room temperature to a temperature equivalent to the exit temperature of the finishing rolling mill at a heating rate of 10°C per second, and held for 30 seconds. After that, a temperature history equivalent to that experienced by an actual hot-rolled steel plate when it is cooled was given, and the transformation rate and transformation speed from austenite (γ: fcc) to ferrite (α: bcc) (which indicates the percentage of transformation that progresses per second, expressed in "% per second") were calculated from the expansion behavior of the steel plate during this period.

[0025] The temperature history used in the Formaster test was equivalent to that of an actual steel sheet, with cooling starting from a temperature equivalent to the finish rolling exit temperature, and the temperature history was obtained for the center of the coil thickness direction (50% of the thickness, with the outermost circumference being 100%) and the outer circumference of the coil thickness direction (95% of the thickness) after the hot-rolled steel sheet after finish rolling was cooled on a runout table and wound up by a coiler. Figure 3 shows an example of the temperature history when the finish rolling exit temperature is 900°C and the winding temperature is 600°C.

[0026] Figures 1 and 2 show the time progression of the transformation rate obtained by the Formastor test. FIG. 1 shows the time progression of the transformation rate when coiling at 600°C, and FIG. 2 shows the time progression of the transformation rate when coiling at 690°C, up to 300 seconds after the start of cooling.

[0027] The steel used has traditionally been problematic due to its tendency to deform, and deformation was observed in coils wound at a winding temperature of 600°C, but not in coils wound at 690°C. Therefore, we investigated the time course of the transformation rate shown in Figures 1 and 2.

[0028] In the case of Figure 1, where the coil was wound at 600°C, the difference in transformation rate between the outer periphery and the center of the coil was small, and the transformation rate immediately after the coil was pulled out (76 seconds after cooling began) was high. As a result, it was thought that the entire coil was in a state where it was susceptible to deformation due to transformation plasticity immediately after being pulled out, and the rigidity of the entire coil was reduced, leading to deformation. In contrast, in the case of Figure 2, where the coil was wound at 690°C, the transformation of the coil was delayed and the difference in the transformation rate between the outer periphery and the center of the coil increased when the coil was pulled out from the mandrel. As a result, the entire coil did not simultaneously become susceptible to deformation due to transformation plasticity. Furthermore, even if the transformation rate of the outer periphery of the coil increased immediately after pulling out, causing a decrease in rigidity, it was thought that the shape of the coil was maintained due to the rigidity of the center of the coil.

[0029] From the above results, it was confirmed that in order to delay the transformation in the outer periphery of the coil, thereby slowing the transformation rate and maintaining the rigidity of the coil during cooling and suppressing deformation, it is effective to increase the coiling temperature from the current level. However, on the other hand, if the coiling temperature is increased, the inside of the coil, which has a slow cooling rate, will be exposed to high temperatures for a long period of time, which will cause internal oxidation and deterioration of the surface properties, which may result in problems such as a decrease in pickling ability, deterioration of the surface appearance, and platability. Therefore, it is necessary to simultaneously suppress deformation of the coil and avoid deterioration of the surface properties.

[0030] As a result of further investigation into means for preventing deformation of the coil and avoiding the problem of internal oxidation, the inventors came up with the idea of ​​increasing the winding temperature only on the outer periphery of the coil, noting that internal oxidation becomes an issue inside the coil, where the cooling rate is slow, and that internal oxidation is not particularly a problem on the outer periphery of the coil, where the cooling rate is faster than the inside, even at a relatively high winding temperature. Figure 3 shows the time progression of the transformation rate, created based on the time progression of the transformation rate in Figures 1 and 2, when the internal coiling temperature is 600°C and the outer coiling temperature is 690°C. After cooling begins, the internal transformation rate has dropped sufficiently at the time the coil is pulled out, and it is expected that sufficient coil rigidity can be ensured even when only the outer coil is coiled at a high coiling temperature.

[0031] As a result of the above, it was found that it is possible to suppress deformation of the coil while avoiding deterioration of the surface properties by winding the inside of the coil at a winding temperature at which internal oxidation is not a problem, and winding only the outer periphery of the coil at a higher winding temperature, thereby reducing the transformation rate of that portion and ensuring sufficient rigidity to suppress deformation of the entire coil.

[0032] The reason why deformation can be suppressed by increasing the winding temperature only at the outer periphery of the coil is believed to be as follows. In the hot-rolled coil 1 in which the hot-rolled steel sheet is wound around the mandrel 2, the coil is prevented from being deformed by the physical support provided by the mandrel 2 (see FIG. 5(a)).

[0033] After coiling begins, cooling progresses inside the coil and at the outer periphery of the coil, as shown in Figure 6(a). Accordingly, in the chemical composition of high-tensile steel sheet, for example, the γ → α transformation progresses as shown in Figures 6(b) and 6(c). That is, since the outer circumferential portion is coiled at a higher temperature than the inner circumferential portion, the γ→α transformation proceeds first in the inner portion 3 of the hot rolled coil 1, reducing the rigidity and making the coil more susceptible to transformation plasticity. On the other hand, in the outer circumferential portion 4 of the hot rolled coil 1, the progression of the γ→α transformation is delayed due to high temperature coiling, so the rigidity is maintained. Therefore, even if the rigidity of the internal zone where the transformation rate is high decreases after drawing, the hot-rolled coil is held in place by the rigidity of the zone where the transformation is delayed on the periphery of the hot-rolled coil 1 and does not deform (see Figure 5(b)).

[0034] As time passes, the γ → α transformation progresses further. However, because the cooling histories of the outer periphery and the center of the coil are different as shown in Figure 6(a), by the time the transformation rate increases and the rigidity decreases in the outer periphery 4 of the hot-rolled coil 1, the transformation rate in the inner region 3 has already decreased sufficiently and the rigidity has increased, preventing deformation of the coil (see Figure 5(c)). As a result, it is possible to prevent the hot-rolled coil from being deformed after it is cooled (see FIG. 5(d)).

[0035] Based on the above basic concept, specific manufacturing conditions were examined, and as a result, the inventors arrived at the present invention, which is characterized in that, when coiling the steel sheet after hot rolling, as specified in (1) above, the range corresponding to the inside of the coil is coiled at a temperature of 450°C or more and 650°C or less, and the range corresponding to the outer periphery is coiled at a temperature more than 50°C higher than the internal temperature and 800°C or less. Such requirements and preferred requirements of the present invention will be further explained below.

[0036] (chemical composition) This method is effective for manufacturing hot-rolled coils using steel types that have a component composition that causes problems with coil deformation and that cannot be coiled at high temperatures due to internal oxidation and other reasons. Such steel types include, for example, high-tensile steels with compositions as shown in Table 1 of the Examples. In particular, C, Si, and Mn are listed as influential components, and steels containing 0.05% or more C, 0.2% or more Si, and 1.3% or more Mn are targeted. The higher the content of these elements, the more likely it is that the gamma → alpha transformation will be delayed on the runout table, causing problems with coil deformation. Mo and B also have the effect of delaying the gamma → alpha transformation. Furthermore, when the Si content is 0.2% or more and the Si / Mn value, which is the component ratio with Mn, is 0.13 or more, internal oxidation will occur when the steel is wound at a high temperature, so the entire length of the coil cannot be wound at a high temperature.

[0037] (Hot rolling conditions) The hot rolling conditions may be in accordance with conventional methods and are not particularly limited. For example, the conditions include heating a steel slab to 1150°C or more and less than 1280°C, and using a tandem rolling mill, hot rolling is performed so that the finish rolling exit temperature is in the γ range, i.e., for example, 850°C or more and less than 980°C, and then cooling on a runout table and coiling at a temperature of 450°C or more.

[0038] (Cooling and coiling conditions after hot rolling) In the hot-rolled steel sheet winding process in which the hot-rolled steel sheet after hot rolling is cooled on a runout table and wound up by a coiler to form a hot-rolled coil, the cooling is weakened or stopped midway and the winding temperature of the outer circumferential part is made higher than the previous temperature. In other words, if the range from the outermost periphery of the hot-rolled coil that is 5% to 30% of the total coil thickness is defined as the outer periphery, and the remaining range inside is defined as the inside, the range corresponding to the inside is coiled at a temperature of 450°C to 650°C, and the range corresponding to the outer periphery is coiled at a temperature that is more than 50°C higher than the temperature of the inside and not higher than 800°C.

[0039] The reason why the outer periphery is set to 5% or more of the total coil thickness is to ensure the necessary thickness to ensure the rigidity to resist deformation of the entire coil immediately after the coil is pulled out. In order to ensure even greater rigidity, 10% or more is preferable. The wider the outer periphery area where the temperature is high, the better, but if it is too wide, the problem of internal oxidation will occur. To avoid internal oxidation, it must be less than 30% of the total coil thickness. To further avoid internal oxidation, it is preferable to keep it less than 25%.

[0040] The reason why the internal coiling temperature is set to 450°C or higher and 650°C or lower is that below 450°C, the temperature passes through the transition boiling region of water, causing a mixture of film boiling and nucleate boiling, resulting in extremely large temperature variations in the steel sheet being cooled, which has a detrimental effect on the mechanical properties, and above 650°C, internal oxidation becomes a problem. The internal coiling temperature is desirably 500°C or higher, which can more effectively avoid the transition boiling region.

[0041] The coiling temperature of the outer periphery is set to be more than 50°C higher than the inner coiling temperature because it is necessary to shift the period when the transformation rate of the outer periphery is high from the period when the transformation rate of the inner portion is high, and the upper limit is set to 800°C or less because the cooling rate of the outer periphery is higher than that of the inner portion, and although the time during which the outer periphery is held at high temperature is short, internal oxidation is very likely to occur at temperatures above 800°C. In addition, the higher the coiling temperature, the more likely it is that a thicker outer oxide layer (black scale) will be formed between finish rolling and coiling by a coiler, which will lead to a reduction in the effective plate thickness after removal by pickling, thereby worsening the yield. For this reason, the upper limit of the coiling temperature of the outer periphery is set to 800°C or less, preferably 750°C or less.

[0042] (How to select the winding temperature) The method for selecting the coiling temperatures of the outer periphery and the inside is as follows: within the entire length of the hot-rolled steel sheet to be coiled, the range from the front end to the inside is set as the inside coiling temperature optimum for the steel type of the hot-rolled steel sheet, and the remaining range corresponding to the outer periphery is set as the outer periphery coiling temperature which is more than 50° C. higher than the inside coiling temperature.

[0043] The coiling temperature of the outer periphery can be set within the above range by trial and error, but it can also be set by investigating the time transition of the transformation rate during coil cooling in advance by a Formaster test as shown in Figures 1 and 2. Specifically, for example, a temperature history when winding is performed at various winding temperatures is set, and the temperature history is used to determine the time transition of the transformation rate when winding is performed at various winding temperatures. Then, within the winding temperature range of the inner and outer periphery, a combination of winding temperatures is selected such that the transformation rate does not simultaneously take a large value in the outer periphery and inner part. -3 One method is to select a winding temperature that remains low and does not exceed 100% per second. (Other uses) In the manufacturing method of the present invention, since the coiling temperature at the end of the coil is different, some steel types cannot be used for applications in which the product is a hot-rolled steel sheet that is not cold-rolled. However, cold-rolled steel sheets can be used without restrictions because they are annealed after cold rolling. EXAMPLES

[0044] Next, an embodiment of the present invention will be described. However, the conditions in the embodiment are an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions.

[0045] A slab having the chemical composition shown in Table 1 was smelted, and a hot-rolled coil was produced under the conditions shown in Table 2. The hot-rolled coil was then pulled out from the mandrel in order to be transported to the next process. The hot rolled coils thus manufactured were investigated for the rate of defective coils due to deformation (NG rate) and the degree of internal oxidation. These results are also shown in Table 2.

[0046] [Table 1]

[0047] [Table 2]

[0048] As shown in Table 2, in the invention examples (production Nos. 3, 5, 7, 9, 10, 13, and 16) whose production conditions were within the range of the present invention, the NG rate due to coil deformation was 0%, and no internal oxidation was observed. On the other hand, in the comparative examples (Production Nos. 1, 2, 4, 6, 8, 11, 12, 14, and 15) in which the production conditions were outside the range of the present invention, the NG rate due to coil deformation exceeded 0%, or internal oxidation was observed. [Industrial Applicability]

[0049] As described above, according to the present invention, it is possible to provide a hot rolled coil that can prevent scrapping and recoiling of the hot rolled coil. Therefore, the present invention has a high applicability in the steel industry. [Explanation of symbols]

[0050] 1 Hot rolled coil 2 Mandrels 3 inside 4 Outer periphery

Claims

[Claim 1] In the hot-rolled steel sheet winding process, the hot-rolled steel sheet after hot rolling is cooled on a runout table and wound up into a hot-rolled coil by a coiler. When the range from the outermost periphery of a hot rolled coil to 5% to 30% of the total thickness of the coil is defined as the outer periphery, and the remaining range inside is defined as the inside, In advance, a Formaster test was conducted in which the same temperature history as that of a hot-rolled steel sheet was applied to investigate the time transition of the transformation rate during coil cooling, and using the results, A range from the front end to the inside of the entire length of the hot-rolled steel sheet is set as an optimal internal coiling temperature for the hot-rolled steel sheet, The range corresponding to the outer periphery of the remaining part is set to an outer periphery winding temperature that is more than 50° C. higher than the internal winding temperature. The area corresponding to the inside is heated to a temperature of 450°C or higher and 650°C or lower. The area corresponding to the outer periphery is wound at a temperature 50°C higher than the internal temperature and up to 800°C. A method for manufacturing a hot rolled coil comprising the steps of:

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