Manufacturing method of hot rolled coil

Abstract

To highly accurately predict a flatness of a hot rolled steel sheet in a coil to thereby improve the flatness thereof when winding the hot rolled steel sheet by a coiler in a hot rolling process and producing the coil.SOLUTION: A steepness of a hot rolled steel sheet in a coil is predicted on the basis of a tensile force when winding the hot rolled steel sheet by a coiler, a body crown generated on the hot rolled steel sheet after finish rolling, and the number of winding of the hot rolled steel sheet in the coil, the tensile force and the body crown are adjusted so that the predicted steepness becomes 2.1% or less.SELECTED DRAWING: Figure 13

Description

[Technical field]

[0001] The present invention relates to a method for producing a coil by winding a hot-rolled steel sheet with a coiler in a hot rolling process. [Background technology]

[0002] In the hot rolling process, the hot-rolled steel sheet after finish rolling is cooled to a predetermined temperature by a cooling device while being transported from the finishing rolling mill to the coiler by a run-out table, and then wound onto the coiler to be produced as a coil (hot-rolled coil).

[0003] The coil manufactured as described above is once wound at a predetermined winding temperature, transported to a coil yard, cooled to room temperature, and then shipped to the user or transported to the next process. At this time, when the coil to be shipped or transported to the next process is unwound for processing, the flatness of the hot-rolled steel sheet may be poor. In such a case, the hot-rolled steel sheet has poor sheet passing properties and meanders, or processing or rolling causes problems such as squeezing, so it is necessary to correct the shape. However, since the shape (flatness) of the hot-rolled steel sheet cannot be known in the coil state, currently, coils with a flatness specification that cannot be shipped with poor flatness are transported to a finishing process and corrected regardless of whether the shape is good or bad, but this is costly. Therefore, in order to transport only hot-rolled steel sheets with poor shapes to the finishing process, it is required to previously make the flatness of the hot-rolled steel sheet in the coiled state within a standard value. In addition, building a technology to reduce the number of shape corrections required for sheet passing in the finishing process is an important technology, especially in conventional mills, mini mills, and thin slab processes that target hot finalization.

[0004] As a method for predicting the shape of a hot-rolled steel sheet, for example, Patent Document 1 discloses a method for separating the residual stress of a hot-rolled steel sheet (metal sheet) into a stress component that is converted into a wave shape when buckling and a stress component that remains in the hot-rolled steel sheet even after buckling, and predicting the shape of the hot-rolled steel sheet using the stress component that is converted into a wave shape. In addition, in this shape prediction method, the wave shape of the hot-rolled steel sheet generated after finish rolling is corrected by, for example, tension acting on the hot-rolled steel sheet when it is wound up by a coiler, so that the width-direction temperature distribution of the hot-rolled steel sheet at the time of winding is ultimately generated as residual stress. Furthermore, based on the prediction result of the shape predicted in this way, for example, the width-direction temperature distribution is controlled by an edge heater or an edge mask, thereby improving the flatness of the hot-rolled steel sheet. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 4262142 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the inventors of the present invention have investigated the shape of the steel sheet after the hot rolling process in detail, and have found that there is a deterioration in flatness that cannot be explained by simply predicting the shape using the residual stress (elongation strain difference) caused by the temperature distribution of the hot-rolled steel sheet, as disclosed in Patent Document 1. It has also been found that the flatness of the hot-rolled steel sheet cannot be sufficiently improved by simply controlling the temperature distribution in the width direction based on the shape prediction result. Therefore, there is room for improvement in accurately predicting the flatness of the hot-rolled steel sheet and further improving the flatness based on the prediction result.

[0007] The present invention has been made in consideration of the above circumstances, and has an object to accurately predict the flatness of a hot-rolled steel sheet in a coil and improve the flatness when the hot-rolled steel sheet is wound by a coiler to produce a coil in a hot rolling process. [Means for solving the problem]

[0008] In order to solve the above problems, the present inventors conducted intensive research and elucidated the mechanism of flatness deterioration of a hot-rolled steel sheet after a hot rolling process. Specifically, it was revealed that flatness deterioration occurs due to a combination of two factors, a temperature factor and a winding tightening factor. The first temperature factor is a factor in which thermal strain occurs due to uneven temperature distribution in the width direction of the hot-rolled steel sheet just before it is wound by a coiler, and this thermal strain becomes an elongation strain difference (residual strain). The second winding tightening factor is a factor in which, for example, due to a crown (defined as a body crown in the present invention as described later) generated in the hot-rolled steel sheet after finish rolling, the tension acting on the hot-rolled steel sheet when it is wound by a coiler is unevenly distributed in the width direction, and further, due to winding tightening with uneven tension distribution, the inner circumference of the coil is plastically deformed, causing plastic strain, and this plastic strain becomes an elongation strain difference (residual strain).

[0009] Of the two factors, the first, the temperature factor, is an example of the shape prediction method disclosed in the above-mentioned Patent Document 1, and has been considered as a factor of deterioration in flatness and measures have been taken. On the other hand, the second, the tightening factor, is a new finding by the present inventors that deformation due to tightening that occurs in the cold rolling process also occurs in the hot rolling process.

[0010] As a result of further intensive research, the inventors have found that the deterioration of flatness due to the tightening of the winding can be predicted based on the tension when the hot-rolled steel sheet is wound by a coiler, the body crown generated in the hot-rolled steel sheet after finish rolling, and the number of turns of the hot-rolled steel sheet in the coil. Details of this prediction method will be described in the embodiment described later.

[0011] The present invention has been made based on such findings, and is a method for manufacturing a coil by winding a hot-rolled steel sheet with a coiler in a hot rolling process, characterized in that the steepness of the hot-rolled steel sheet in the coil is predicted based on the tension when winding the hot-rolled steel sheet with the coiler, the body crown generated in the hot-rolled steel sheet after finish rolling, and the number of turns of the hot-rolled steel sheet in the coil, and the tension and the body crown are adjusted so that the predicted steepness is 2.1% or less.

[0012] According to the present invention, the flatness of a hot-rolled steel sheet, which is deteriorated due to the tightening factor, can be predicted by predicting the steepness of the hot-rolled steel sheet using the above-mentioned three parameters, tension, body crown, and number of turns. Furthermore, the flatness of the hot-rolled steel sheet can be improved by adjusting the tension and body crown so that the predicted steepness is 2.1% or less.

[0013] Here, for example, JP 2014-65935 A discloses controlling the tension when winding a hot-rolled steel sheet, for example, a low tension of 10 MPa. However, JP 2014-65935 A does not disclose controlling the crown. In addition, the tension control is originally intended to suppress the occurrence of surface defects in the hot-rolled steel sheet, and is not intended to improve the flatness as in the present invention.

[0014] For example, Japanese Patent Application Laid-Open No. 8-66701 discloses a method for controlling the crown generated in a hot-rolled steel sheet after finish rolling, and discloses a low crown of, for example, 18 μm. However, Japanese Patent Application Laid-Open No. 8-66701 does not disclose controlling the tension.

[0015] Thus, while controlling either tension or crown has been disclosed in the prior art, adjusting both tension and crown (body crown) as in the present invention has not been disclosed, making the present invention an unprecedented and novel invention.

[0016] In the method for producing a hot-rolled coil, the steepness of the hot-rolled steel sheet in the coil may be predicted by the following formula (1).

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[0017] In the method for producing a hot-rolled coil, the number of turns may be estimated from the sheet thickness, sheet width, and unit weight of the coil.

[0018] In the method for producing a hot-rolled coil, the hot-rolled steel sheet wound around the coiler may be at 700° C. or higher before or during transformation, or after completion of transformation. Effect of the Invention

[0019] According to the present invention, the flatness of a hot-rolled steel sheet, which is deteriorated due to the tightening factor, can be predicted by predicting the steepness of the hot-rolled steel sheet based on the tension when the hot-rolled steel sheet is wound by a coiler, the body crown generated in the hot-rolled steel sheet after finish rolling, and the number of turns of the hot-rolled steel sheet in a coil. Furthermore, the flatness of the hot-rolled steel sheet, which is deteriorated due to the tightening factor, can be improved by adjusting the tension and the body crown so that the predicted steepness is 2.1% or less. [Brief description of the drawings]

[0020] [Figure 1] FIG. 2 is an explanatory diagram showing an outline of the configuration of a hot rolling facility after a finishing mill. [Diagram 2] FIG. 2 is an explanatory diagram showing an outline of the configuration of a coiler. [Diagram 3] FIG. 2 is an explanatory diagram showing an outline of the configuration of a mandrel, in an axial cross-sectional view. [Figure 4] FIG. 2 is an explanatory diagram showing an outline of the configuration of a mandrel, in a cross-sectional view taken in a direction perpendicular to the axis. [Diagram 5] FIG. 13 is an explanatory diagram showing the definition of steepness, which indicates the level of ear waves. [Figure 6] FIG. 1 is a conceptual diagram for explaining the mechanism of deterioration of flatness due to tight winding. [Figure 7] FIG. 1 is a conceptual diagram for explaining the mechanism of deterioration of flatness due to tight winding. [Figure 8] 1 is a graph showing a situation in which the phenomenon of mandrel diameter reduction is captured. [Figure 9] This is a graph verifying the phenomenon in which ear waves are generated due to tightness of winding. [Figure 10] FIG. 2 is an explanatory diagram showing the definition of a body crown. [Figure 11] FIG. 1 is an explanatory diagram showing the steepness of the hot-rolled steel sheet at the inner periphery of the coil when the number of turns is 50, the tension is varied from 0 to 20 MPa, and the body crown is varied from 0 to 80 μm. [Figure 12] FIG. 1 is an explanatory diagram showing the steepness of the hot-rolled steel sheet at the inner periphery of the coil when the number of turns is 70, the tension is varied from 0 to 20 MPa, and the body crown is varied from 0 to 80 μm. [Figure 13] FIG. 1 is an explanatory diagram showing the steepness of the hot-rolled steel sheet at the inner periphery of the coil when the number of turns is 75, the tension is varied from 0 to 20 MPa, and the body crown is varied from 0 to 80 μm. [Figure 14] FIG. 1 is an explanatory diagram showing the steepness of the hot-rolled steel sheet at the inner periphery of the coil when the number of turns is 100, the tension is varied from 0 to 20 MPa, and the body crown is varied from 0 to 80 μm. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In this specification and the drawings, elements having substantially the same functional configurations are denoted by the same reference numerals, and duplicated description will be omitted.

[0022] <Hot rolling equipment> First, the configuration of the hot rolling facility according to the present invention will be described. Fig. 1 is an explanatory diagram showing an outline of the configuration of the hot rolling facility 1 from a finishing mill 2 onwards.

[0023] In the hot rolling equipment 1, a finishing rolling mill 2 that continuously rolls a steel sheet H discharged from a heating furnace (not shown) and rolled by a roughing mill (not shown) to a predetermined thickness, a cooling device 3 that cools the steel sheet H after finish rolling (hereinafter, hot-rolled steel sheet H) to a predetermined temperature, and a coiler 4 that winds up the cooled hot-rolled steel sheet H are provided in this order in the conveying direction of the hot-rolled steel sheet H. A run-out table 5 that conveys the hot-rolled steel sheet H is provided between the finishing rolling mill 2 and the coiler 4. Then, the hot-rolled steel sheet H rolled by the finishing rolling mill 2 is cooled by the cooling device 3 while being conveyed on the run-out table 5, and then wound up by the coiler 4 to be manufactured as a coil C.

[0024] In addition, a thickness meter 6 for measuring the thickness of the hot-rolled steel sheet H rolled by the finishing mill 2 is provided between the finishing mill 2 and the cooling device 3 of the hot rolling equipment 1. The thickness meter 6 measures the thickness distribution in the width direction of the hot-rolled steel sheet H and can measure the body crown of the hot-rolled steel sheet H.

[0025] 2 is an explanatory diagram showing an outline of the configuration of the coiler 4. The example in FIG. 2 shows a state where a winding operation is started in the coiler 4. The coiler 4 has a pinch roll 10, a chute 11, a mandrel 12, and a wrapper roll 13.

[0026] In the coiler 4, the hot-rolled steel sheet H is bent toward the mandrel 12 by the pinch rolls 10 and passed through the chute 11. Here, before the tip of the hot-rolled steel sheet H reaches the mandrel 12, the wrapper roll 13 is closed (contacting the mandrel 12) and waits while rotating at a speed several percent faster than the steel sheet speed. Then, when the hot-rolled steel sheet H reaches the mandrel 12 and the wrapper roll 13, the hot-rolled steel sheet H is sandwiched between the mandrel 12 and the wrapper roll 13 and wound up. The mandrel 12 can expand and contract its diameter by the cylinder section 24 as described later, and starts to expand when the hot-rolled steel sheet H is wound up by a predetermined number of turns on the coil C, and stops expanding the diameter when the expanding force and the tightening force of the coil C are balanced, and the wrapper roll 13 opens and moves away from the coil C.

[0027] 3 and 4 are explanatory diagrams showing an outline of the configuration of the mandrel 12. As shown in Fig. 3, the mandrel 12 is of a segment type and has a mandrel segment 20, a wedge 21, a slide rod 22, and a wedge shaft 23. Of these components, the slide rod 22 and the wedge shaft 23 form a cylinder section 24. By sliding the wedge 21 on this cylinder section 24, the mandrel segment 20 slides in the radial direction in the expanding or contracting direction along the gradient applied to the wedge shaft 23.

[0028] 4, the mandrel 12 has a gap A between the segment flange portion 25 and the wedge jaw portion 26, and when it rotates, the centrifugal force eliminates the gap A and causes it to expand. The segment-wedge portion 27 comprises a set of a mandrel segment 20 and a wedge 21, and the mandrel 12 is made up of four sets of segment-wedge portions 27.

[0029] <Mechanism of deterioration of flatness> The present invention predicts the flatness of a hot-rolled steel sheet in a coil manufactured by the hot rolling equipment having the above configuration, and further improves the flatness based on the prediction result. The winding temperature of the hot-rolled steel sheet varies depending on the material, but is in the range of approximately 100 to 800 ° C., and the coil manufactured by the hot rolling equipment is transported to a coil yard, cooled to room temperature, and then unwound. The flatness predicted in the present invention is the flatness of the hot-rolled steel sheet after unwound from the coil (more specifically, the flatness of the hot-rolled steel sheet due to the tightening factor as described later), and in such a case, a wavy out-of-plane deformation called an ear wave occurs at the width direction end of the hot-rolled steel sheet. Here, the deterioration of flatness that occurs in many hot-rolled steel sheets is an ear wave, and the present invention intends to predict this ear wave and further improve it. In other cases, when the crown profile of the coiler shaft or pinch roll is convex and the winding tension is abnormally large, a medium wave that appears in the center rather than the width direction end may occur, but this is outside the scope of the present invention.

[0030] Fig. 5 is an explanatory diagram showing the definition of steepness, which indicates the degree of edge waves. The steepness λ is expressed as a percentage by dividing the wave height H at the width direction end of the hot-rolled steel sheet by the wave pitch L, and then multiplying the result by 100. The steepness λ is expressed by the following formula (2) using the elongation strain difference Δε.

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[0031] The inventors of the present invention have conducted extensive research and have clarified the mechanism of the deterioration of the flatness of a hot-rolled steel sheet after the hot rolling process. In other words, it has become clear that the deterioration of the flatness is caused by a combination of two factors: a temperature factor in which thermal strain occurs due to non-uniform temperature distribution in the width direction of the hot-rolled steel sheet, and a winding tightening factor in which the inner peripheral part of the coil is plastically deformed due to winding tightening caused by non-uniform tension distribution in the width direction during coiling. These two factors will be described below.

[0032] (Temperature factors) The first cause of flatness deterioration due to temperature will be explained. Just before being wound into the coiler, a hot-rolled steel sheet undergoes thermal strain due to uneven temperature distribution in the width direction. This thermal strain becomes an elongation strain difference (residual strain), which causes flatness deterioration (shape deterioration) of the hot-rolled steel sheet.

[0033] The deterioration of flatness due to temperature factors has been known for a long time, and is disclosed in, for example, the above-mentioned Patent Document 1 and publicly known documents (Akashi et al., "Study on flatness prediction method after cooling of hot-rolled steel sheet" at the 148th Autumn Lecture Meeting of the Iron and Steel Institute of Japan, September 2004). That is, when a hot-rolled steel sheet that has been finish-rolled by a finishing mill is cooled by a cooling device while being transported on a run-out table, an elongation strain difference occurs due to non-uniform temperature distribution in the width direction. However, this elongation strain difference becomes almost zero before and after the hot-rolled steel sheet passes through the lower pinch roll due to the following correction action. For example, a hot-rolled steel sheet in a state where tension is applied by a coiler just before winding passes through the lower pinch roll with an infinite radius of curvature in the sheet passing direction until just before the lower pinch roll, but when passing through, it is wrapped around the lower pinch roll (comes in surface contact) while being applied with tension, so that bending deformation is forcibly applied at the radius of the lower pinch roll, and after passing through, the radius of curvature becomes infinite again, so that the correction is performed. Furthermore, when the temperature distribution in the width direction of the hot-rolled steel sheet during coiling is lowered to room temperature when the coil is uncoiled, a difference in elongation strain occurs in the hot-rolled steel sheet, and the flatness deteriorates.

[0034] (Factor of tightening) The second cause of flatness deterioration caused by tightening of the coil will now be explained. For example, due to the crown that occurs in the hot-rolled steel sheet after finish rolling, the tension acting on the hot-rolled steel sheet when it is wound on the coiler is distributed unevenly in the width direction, and tightening of the coil with this uneven tension distribution causes plastic deformation of the inner circumference of the coil, resulting in plastic strain. This plastic strain becomes an elongation strain difference (residual strain), which causes the flatness deterioration (shape deterioration) of the hot-rolled steel sheet.

[0035] Next, the mechanism of deterioration of flatness due to tightening will be described in detail with reference to Figures 6 and 7. In Figures 6 and 7, the symbol T indicates tensile stress and the symbol S indicates compressive stress. (A) First, when a hot-rolled steel sheet is wound up by a coiler at a constant tension, tensile stress acts on the hot-rolled steel sheet on the surface of the coil, but compressive stress acts on the hot-rolled steel sheet on the inner circumference of the coil near the mandrel. (B) In addition, a typical hot-rolled steel sheet has a crown that is convex at the center in the width direction. When a hot-rolled steel sheet having such a crown is wound around another hot-rolled steel sheet having a crown, the center of the inner hot-rolled steel sheet comes into contact with the center of the outer hot-rolled steel sheet. As a result, a larger compressive stress acts on the center in the width direction at the inner circumference of the coil than on the ends. (C) In actual operation, the mandrel waits for the hot-rolled steel sheet to be transported at a standby diameter, and when the hot-rolled steel sheet is wound a predetermined number of times, it is expanded further (overexpanded). Then, when the pushing force of the cylinder part which tries to expand the mandrel and the surface pressure from the coil are balanced, the expansion stops and the mandrel maintains a constant diameter. However, in reality, the winding tightening force becomes excessive due to the influence of the tension during winding of the hot-rolled steel sheet, the plate thickness of the hot-rolled steel sheet, the frictional force between the hot-rolled steel sheets, etc., the pushing force of the cylinder part is defeated, and the diameter of the mandrel gradually decreases from the point when winding tightening is completed. Due to such a reduction in the mandrel diameter, the hot-rolled steel sheet on the inner circumference of the coil is forced to bear the compressive winding tightening force that should be borne by the mandrel. (D) When the above phenomena occur at the same time, the compressive stress increases in the inner circumference of the coil, especially in the widthwise center, and because the operation is performed in a hot state, compressive plastic deformation, transformation plastic deformation, and creep deformation occur in the widthwise center. As a result, the widthwise center appears to shrink and the ends stretch, generating selvedge waves.

[0036] FIG. 8 is a graph showing the situation where the mandrel diameter reduction phenomenon was captured in (C) above. The horizontal axis of FIG. 8 is the time from the start of coil winding. The vertical axis is the mandrel diameter (radial distance of the outer surface of the segment) calculated from the stroke amount of the cylinder part. Point a is the point where the mandrel starts winding the hot-rolled steel sheet, over-expands after several turns, and stops expanding once the tightening force and the mandrel expansion force are balanced. It can be seen that thereafter, as time passes, the number of turns of the hot-rolled steel sheet in the coil increases and the tightening force on the inner circumference of the coil increases, and the mandrel diameter becomes smaller than point a. It is known that the amount of tightening during winding (amount of mandrel diameter reduction) is not constant but varies for each coil.

[0037] FIG. 9 is a graph verifying the phenomenon in (D) above where selvedge waves are generated due to winding tightening. The horizontal axis of FIG. 9 indicates the mandrel diameter reduction amount (MD reduction amount). The vertical axis indicates the selvedge wave steepness. According to FIG. 9, it can be seen that there is a certain correlation between the mandrel diameter reduction amount and the selvedge wave steepness.

[0038] <Flatness prediction method and flattening method based on winding tightening factors> The above is the mechanism of flatness deterioration of a hot-rolled steel sheet, and the present inventors have found that flatness deterioration occurs due to a combination of temperature and tightening factors. As described above, flatness deterioration due to temperature factors has been known for some time, and measures have been taken to prevent it. Specifically, for example, an edge heater installed in front of a finishing rolling mill or an edge mask installed in a cooling device is used to control the temperature distribution in the width direction to be uniform, thereby making it possible to improve the flatness of the hot-rolled steel sheet. Therefore, in the present invention, the flatness of the hot-rolled steel sheet that is deteriorated due to the tightening factor is predicted, and the flatness is improved based on the prediction result.

[0039] (Flatness prediction) Based on the mechanism of flatness deterioration due to the above-mentioned tightening factor, the inventors have considered the following measures to improve flatness (shape improvement): first, to reduce the tension distribution difference occurring in the width direction after winding the hot-rolled steel sheet, and second, to reduce the tightening force to suppress diameter reduction due to the tightening of the mandrel. To reduce the first tension distribution difference, it is necessary to reduce the crown occurring in the hot-rolled steel sheet. To reduce the second tightening force, it is necessary to reduce the tension when winding the hot-rolled steel sheet, and it is also necessary to reduce the number of turns of the hot-rolled steel sheet.

[0040] Here, in the present invention, the crown occurring in the hot-rolled steel sheet after finish rolling is defined as the body crown. As shown in Fig. 10, the crown of the hot-rolled steel sheet H is the difference between the sheet thickness at the end (edge) in the width direction and the sheet thickness at the center in the width direction, and is the sum of the body crown and the edge drop. The body crown is the difference between the sheet thickness at the edge and the sheet thickness at a point 75 mm away from the edge (hereinafter referred to as the 75 mm point). The edge drop is the difference between the sheet thickness at the 75 mm point and the sheet thickness at the center. The crown of the hot-rolled steel sheet is defined as the body crown excluding the edge drop.

[0041] The following formula (1) was derived based on such knowledge, and predicts the steepness of the hot-rolled steel sheet at the inner circumference of the coil as the flatness of the hot-rolled steel sheet due to the winding tightening factor. The inner circumference of the coil is the range of 200 m from the tip of the hot-rolled steel sheet. From experience in actual operation, the shape of the hot-rolled steel sheet wound on the coil becomes flat in the range of 200 m from the tip. This is presumably because tension is generated in the hot-rolled steel sheet when the tip of the hot-rolled steel sheet reaches the mandrel, and the shape is corrected. Therefore, in the present invention, the steepness of the hot-rolled steel sheet at the inner circumference of the coil is predicted. The three parameters in formula (1), tension Ut, body crown Cr, and number of turns N, can be measured and grasped in real time.

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[0042] The tension Ut is the force (average tension over the entire length of the coil) that pulls the hot-rolled steel sheet when it is wound up by a coiler, and the magnitude of the tension can be grasped during winding. If the tension Ut is small, the steepness λ becomes small and the hot-rolled steel sheet can be flattened. In formula (1), the tension Ut is expressed as a square root. This is because the tension Ut is considered to have a linear relationship with the elongation strain difference Δε, and the steepness λ in formula (2) is calculated from the square root of the elongation strain difference Δε.

[0043] The body crown Cr can be measured, for example, by a thickness gauge installed in the finishing rolling mill and cooling device shown in Figure 1. If the body crown Cr is small, the steepness λ is small and the hot-rolled steel sheet can be flattened. In formula (1), the body crown Cr is also expressed by a square root, and the reason is the same as that of the tension Ut. The body crown Cr is the average value of the body crown over the entire coil length.

[0044] The number of turns N can be grasped when the hot-rolled steel sheet is wound by a coiler. Specifically, it can be estimated from the coil thickness, width, and unit weight, for example, by the number of turns N = (unit weight) / (thickness x width). Alternatively, the number of turns may be actually counted. If the number of turns N is small, the steepness λ becomes small and the hot-rolled steel sheet can be flattened. In addition, the number of turns N is also expressed by the square root in formula (1), and the reason is the same as that for the tension Ut.

[0045] The adjustment coefficient α is a reference steepness (hereinafter sometimes referred to as base steepness) and is the steepness when the tension Ut is 20 MPa, the body crown Cr is 80 μm, and the number of turns N is 100. Specifically, the adjustment coefficient α is determined by measuring actual machine data, and is, for example, a value between 0 (zero) and 10. The adjustment coefficient β is also determined by measuring actual machine data.

[0046] In this embodiment, the steepness λ of the hot-rolled steel sheet is predicted by the formula (1), but is not limited thereto. The steepness λ may be predicted based on three parameters, namely, tension Ut, body crown Cr, and number of turns N, that is, calculated by the function λ=f(Ut, Cr, N). Specifically, the steepness λ may be calculated by the following formula (3), or may be calculated by the difference using the following formula (1') which is a modification of formula (1).

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[0047] Similarly to the steepness λ, the elongation strain difference Δε can also be predicted based on three parameters, namely, tension Ut, body crown Cr, and number of turns N, that is, it can be calculated by the function Δε=g(Ut, Cr, N). Specifically, the elongation strain difference Δε is calculated by the following formula (4) or formula (5), etc. Then, the steepness λ can be calculated by inserting the elongation strain difference Δε calculated from these formulas (4) and (5) into the above formula (2). Δε=γ×Ut×Cr×N+b (4) Δε=a1×Ut+a2×Cr+a3×N+b (5) where γ is the correlation coefficient and adjustment coefficient in linear multivariate analysis.

[0048] (Flattening method) As described above, the flatness of a hot-rolled steel sheet deteriorated due to factors of winding tightening can be predicted by functions f(Ut, Cr, N) and g(Ut, Cr, N) (hereinafter referred to as shape prediction functions) of three parameters, namely tension Ut, body crown Cr, and number of turns N, such as formula (1), formula (1'), formula (3), formula (4), and formula (5). Next, a method for improving the flatness of a hot-rolled steel sheet based on the prediction results will be described.

[0049] The inventors decided to set the steepness predicted by the shape prediction function to 2.1% or less in order to improve the flatness of the hot-rolled steel sheet. This steepness threshold of 2.1% is an allowable value for a product, and if the steepness is 2.1% or less, it can be said that the hot-rolled steel sheet is sufficiently flattened. As described above, the cause of flatness deterioration is temperature factor in addition to the tightening factor, but in the present invention, it is assumed that the flatness deterioration due to this temperature factor is improved by using, for example, an edge heater or an edge mask.

[0050] In order to make the steepness predicted by the shape prediction function 2.1% or less, the three parameters, tension Ut, body crown Cr, and number of turns N, should be reduced. In other words, to improve flatness, it is good to have a low tension, low crown, and low number of turns. However, the number of turns N is predetermined depending on the product, and is difficult to change in actual operation. Therefore, in the present invention, tension Ut and body crown Cr are adjusted so that the steepness predicted by the shape prediction function is 2.1% or less.

[0051] As described above, by calculating the steepness of the hot-rolled steel sheet at the inner periphery of the coil using the shape prediction function, the flatness of the hot-rolled steel sheet, which is deteriorated due to the winding tightening factor, can be appropriately predicted. Furthermore, by adjusting the tension Ut and the body crown Cr so that the steepness calculated by this shape prediction function is 2.1% or less, the selvage of the hot-rolled steel sheet at the inner periphery of the coil can be reduced, and the flatness can be improved. After extensive research, the inventors have confirmed that the steepness prediction result by the shape function of the present invention and the steepness in actual operation are highly consistent, and further confirmed that the steepness of the hot-rolled steel sheet can actually be improved to 2.1% or less by adjusting the tension Ut and the body crown Cr as in the present invention. Specifically, it is also possible to improve the flatness to a level where it is not necessary to convey the coil to a finishing process to correct the shape. As a result, the manufacturing cost can be reduced, and the manufacturing period can be stabilized and shortened. It is also possible to suppress defects that occur on the surface of the hot-rolled steel sheet during the finishing process, thereby improving product yield.

[0052] <Target hot-rolled steel sheets> The above-mentioned flatness prediction method and flattening method based on the winding tightening factor are particularly useful when the hot-rolled steel sheet wound by the coiler is not transformed or is being transformed. For example, when the hot-rolled steel sheet is wound after the transformation is completed, the shape of the hot-rolled steel sheet does not deteriorate more than when it was wound. On the other hand, if the hot-rolled steel sheet wound by the coiler is not transformed or is being transformed, the hot-rolled steel sheet may be further deformed. In this regard, if the tension Ut and the body crown Cr are adjusted so that the steepness predicted by the shape prediction function is 2.1% or less, as in the present invention, the flatness of the hot-rolled steel sheet can be improved even if the hot-rolled steel sheet is not transformed or is being transformed.

[0053] In addition, for example, when a hot-rolled steel sheet is coiled at a high temperature of 700° C. or higher after the completion of transformation, the hot-rolled steel sheet may be deformed due to creep. Therefore, the flatness prediction and flattening method of the present invention is also useful in cases where creep occurs during such high-temperature coiling.

[0054] <Specific flatness prediction method and flattening method> The method for predicting flatness and the method for flattening due to the tightening of winding have been described above. Next, a specific example of the method will be described.

[0055] When a hot-rolled steel sheet with a thickness of 2.5 mm and a width of 1200 mm is wound by a coiler, the tension Ut is varied from 0 to 20 MPa, the body crown Cr is varied from 0 to 80 μm, and the number of turns N is varied from 50 to 100 turns, and the steepness of the hot-rolled steel sheet at the inner circumference of the coil is calculated from formula (1). The ranges of the tension Ut, body crown Cr, and number of turns N are each assumed to be within the ranges expected in actual operation. The adjustment coefficient α (base steepness) in formula (1) is set to 6, and the adjustment coefficient β is set to 0 (zero).

[0056] The calculation results of the steepness are shown in Fig. 11 to Fig. 14. Fig. 11 shows the steepness when the number of turns N is 50 turns, Fig. 12 shows the steepness when the number of turns N is 70 turns, Fig. 13 shows the steepness when the number of turns N is 75 turns, and Fig. 14 shows the steepness when the number of turns N is 100 turns. In Fig. 11 to Fig. 14, the shaded areas are areas where the steepness is 2.1% or less, and the darker shaded areas are areas where the steepness is 1% or less. In other words, by adjusting the tension Ut and the body crown Cr in the shaded areas, respectively, the hot-rolled steel sheet can be sufficiently flattened.

[0057] Referring to Fig. 11, when the number of turns N is 50, the hot-rolled steel sheet can be flattened by coiling the hot-rolled steel sheet with a tension Ut of 5 to 10 MPa. In this case, the body crown Cr is not particularly limited. In addition, the hot-rolled steel sheet can be flattened by rolling the hot-rolled steel sheet with a body crown Cr of 0 to 40 µm, more preferably 0 to 25 µm. In this case, the tension Ut is not particularly limited.

[0058] Also, referring to Figures 11 to 13, regardless of whether the number of turns N is 50, 70, or 75, the hot-rolled steel sheet can be flattened by winding the hot-rolled steel sheet with a tension Ut of 5 to 10 MPa and rolling the hot-rolled steel sheet with a body crown Cr of 0 to 25 µm.

[0059] 14, when the number of turns N is 100, the tension Ut is 10 MPa, and the body crown Cr is 25 μm, the steepness of the hot-rolled steel sheet is 2.4%, which is slightly outside the allowable range. However, if the tension Ut is 5 to 10 MPa and the body crown Cr is 0 to 25 μm, the steepness of the hot-rolled steel sheet can be brought into the allowable range, and the hot-rolled steel sheet can be flattened.

[0060] From the above, it can be said that in practical operation, the low tension Ut that can flatten a hot-rolled steel sheet is 5 to 10 MPa, and the low body crown Cr is 0 to 25 μm. In fact, under conventional operating conditions, for example, when the number of turns N is 200 turns, the tension Ut is 20 MPa, and the body crown Cr is 70 μm, the steepness is 7%. In contrast, under the operating conditions of the present invention (tension Ut is 5 to 10 MPa, and body crown Cr is 0 to 25 μm), the steepness can be reduced to 2.1% or less, and the flatness of the hot-rolled steel sheet can be improved.

[0061] The smaller the tension Ut, the smaller the steepness of the hot-rolled steel sheet, which is preferable for improving the shape. However, if the tension Ut is less than 5 MPa, the telescope phenomenon occurs, the coil winding shape deteriorates, and stable winding may not be possible. Therefore, the tension Ut is preferably 5 MPa or more.

[0062] In addition, the smaller the body crown Cr, the smaller the steepness of the hot-rolled steel sheet, which is preferable for improving the shape. However, if the negative crown is less than 0 (zero) μm, the hot-rolled steel sheet may meander during threading, causing unstable threading. Therefore, the body crown Cr is preferably 0 (zero) μm or more.

[0063] Although the embodiment of the present invention has been described above, the present invention is not limited to such examples. It is clear that a person skilled in the art can come up with various modified or altered examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally belong to the technical scope of the present invention. [Industrial Applicability]

[0064] INDUSTRIAL APPLICABILITY The present invention is useful when a hot-rolled steel sheet is wound by a coiler to produce a coil in a hot rolling process. [Explanation of symbols]

[0065] 1. Hot rolling equipment 2 Finishing rolling mill 3 Cooling device 4. Coiler 5 Runout Table 6 Thickness gauge 10 Pinch Roll 11 SHOTS 12 Mandrel 13. Wrapper Roll 20 mandrel segments 21 Wedge 22 Slide rod 23 Wedge shaft 24 Cylinder section 25 Segment flange 26 Wedge jaw 27 Segment-Wedge Section C coil H Hot rolled steel plate

Claims

1. A method for producing a coil by winding a hot-rolled steel sheet with a coiler in a hot rolling process, comprising the steps of: The steepness of the hot-rolled steel sheet in the coil is predicted based on the tension when the hot-rolled steel sheet is wound by the coiler, the body crown generated in the hot-rolled steel sheet after finish rolling, and the number of turns of the hot-rolled steel sheet in the coil; A method for manufacturing a hot rolled coil, comprising adjusting the tension and the body crown so that the predicted steepness is 2.1% or less.

2. The method for manufacturing a hot-rolled coil according to claim 1 , characterized in that the steepness of the hot-rolled steel sheet in the coil is predicted by the following formula (1): [0010] where λ is the steepness (%) of the hot-rolled steel sheet in the coil, α and β are adjustment coefficients, Ut is the tension (MPa) when the hot-rolled steel sheet is wound by the coiler, Cr is the body crown (μm) generated in the hot-rolled steel sheet after finish rolling, and N is the number of turns of the hot-rolled steel sheet in the coil.

3. The method for manufacturing a hot rolled coil according to claim 2, characterized in that the number of turns is estimated from the sheet thickness, sheet width, and unit weight of the coil.

4. The method for producing a hot-rolled coil according to any one of claims 1 to 3, characterized in that the hot-rolled steel sheet wound around the coiler is at 700 ° C. or higher before or during transformation, or after completion of transformation.

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