Manufacturing method of hot rolled coil

Abstract

To improve the flatness of a hot-rolled steel sheet in a coil when the coil of the hot-rolled steel sheet is manufactured in a hot rolling process.SOLUTION: A medium wave of prescribed steepness is formed in at least a width-direction center with respect to a hot rolled steel sheet by a finish rolling machine, and the hot rolled steel sheet having the medium wave formed therein is taken up by a mandril of a coiler to manufacture a coil. The prescribed steepness of the medium wave is allowed to be based on a body crown generated in the finish rolled hot rolled steel sheet. The medium wave is allowed to be formed within the range of within 200 m from a tip in the rolling direction of the hot rolled steel sheet.SELECTED DRAWING: Figure 17

Description

[Technical field]

[0001] The present invention relates to a method for producing a coil of hot rolled steel sheet 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 mill to the coiler by a run-out table, and then wound around a coiler (mandrel) 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 a 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 may meander, or processing or rolling may cause problems such as squeezing, so the shape needs to be corrected. 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, developing technology that can reduce the need for shape correction threading in the finishing process is an important technology, particularly for conventional mills, mini mills, and thin slab processes targeting 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 improving the flatness of the hot-rolled steel sheet.

[0007] The present invention has been made in consideration of the above circumstances, and has an object to improve the flatness of a hot-rolled steel sheet in a coil when the hot-rolled steel sheet is produced as 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 hot-rolled steel sheets after the hot rolling process. Specifically, it was revealed that flatness deterioration occurs due to a combination of two factors: temperature and tightening. The first temperature factor is that thermal strain occurs due to uneven temperature distribution in the width direction of the hot-rolled steel sheet just before it is wound around the coiler (mandrel), and this thermal strain becomes an elongation strain difference (residual strain). The second tightening factor is that the crown generated in the hot-rolled steel sheet after finish rolling causes the tension acting on the hot-rolled steel sheet to be unevenly distributed in the width direction when it is wound around the coiler (mandrel), and further, the inner circumference of the coil is plastically deformed due to the tightening with uneven tension distribution, 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] The deterioration of flatness due to the cause of winding tightness will be explained in more detail. A hot-rolled steel sheet after finish rolling has a crown in which the widthwise center is convex. When another hot-rolled steel sheet having a crown is wound around a hot-rolled steel sheet having such a crown, the center of the inner hot-rolled steel sheet comes into contact with the center of the outer hot-rolled steel sheet. Therefore, when the hot-rolled steel sheet is wound around a mandrel, the tension acting on the hot-rolled steel sheet is larger in the widthwise center than at the ends. This uneven tension causes the flatness of the hot-rolled steel sheet to deteriorate.

[0011] The inventors have found that, in order to make this tension uniform, it is sufficient to impart a circumferential difference corresponding to the crown of the hot-rolled steel sheet to the hot-rolled steel sheet being wound, i.e., to form a medium wave on the hot-rolled steel sheet. The present invention has been made based on this finding, and is a method for producing a coil of a hot-rolled steel sheet in a hot rolling process, which comprises forming a medium wave of a predetermined steepness on at least the central part in the width direction of the hot-rolled steel sheet by a finishing rolling mill, and winding the hot-rolled steel sheet on which the medium wave has been formed by the mandrel of a coiler to produce a coil. The predetermined steepness of the medium wave is determined based on the body crown generated in the hot-rolled steel sheet after finish rolling. In the present invention, the medium corrugation refers to a corrugation shape formed such that the corrugation height periodically varies in the rolling direction at least in the central portion in the width direction of the hot-rolled steel sheet.

[0012] According to the present invention, since a medium wave is formed in the hot-rolled steel sheet in the finish rolling, a circumferential difference is imparted to the hot-rolled steel sheet, and the tension acting on the hot-rolled steel sheet wound around the mandrel can be made uniform in the width direction. As a result, the flatness of the hot-rolled steel sheet can be improved.

[0014] In such a case, the relationship between the steepness of the medium wave at a predetermined position in the rolling direction of the hot-rolled steel sheet and the flatness of the hot-rolled steel sheet at the predetermined position in the coil may be determined in advance for each body crown, the body crown may be measured after finish rolling, and the predetermined steepness of the medium wave may be determined from the measurement results and the relationship.

[0015] Also, the body crown after finish rolling may be measured, and a predetermined steepness of the medium wave may be determined according to the number of turns of the hot-rolled steel sheet in the coil using the measurement result and the following formula (1), and when the medium wave is formed in the hot-rolled steel sheet by a finishing rolling mill, the steepness of the medium wave may be changed according to the position in the rolling direction of the hot-rolled steel sheet based on the predetermined steepness of the medium wave determined. Note that the adjustment coefficients α and β in the following formula (1) may be set based on the measurement result of the steepness of the medium wave in actual operation.

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[0016] In the method for producing a hot-rolled coil, the predetermined steepness of the medium wave may be 0.5% to 5%.

[0017] In the method for producing a hot-rolled coil, the medium wave may be formed within a range of 200 m from a leading end in the rolling direction of the hot-rolled steel sheet.

[0018] In the method for producing a hot-rolled coil, the hot-rolled steel sheet wound around the mandrel 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, by forming a medium wave in the hot-rolled steel sheet in the finish rolling, a circumferential difference is imparted to the hot-rolled steel sheet, and the tension acting on the hot-rolled steel sheet during coiling can be made uniform in the width direction. As a result, the flatness of the hot-rolled steel sheet can be improved. [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 explaining the mechanism of deterioration of flatness due to winding tightening. (a) shows the compressive stress (zero) acting on the ends of the mandrel in the width direction, (b) shows the compressive stress acting between the ends and the center in the width direction (arrows in the figure), and (c) shows the compressive stress acting in the center in the width direction (arrows in the figure). [Figure 8] FIG. 4 is an explanatory diagram showing how a mandrel segment is deformed by compressive stress. [Figure 9] FIG. 4 is an explanatory diagram showing a state in which a mandrel is deformed due to a compressive stress. [Figure 10] The figure shows the tension distribution (arrows in the figure) acting on a hot-rolled steel sheet with a convex crown when the hot-rolled steel sheet is coiled, with (a) showing the state after one coil and (b) showing the state after two coils. [Figure 11] FIG. 2 is an explanatory diagram showing the relationship between the internal pressure in the mandrel and the tension (circumferential stress) applied to the hot-rolled steel sheet. [Figure 12] FIG. 2 is an explanatory diagram showing a method for forming a medium wave in a hot-rolled steel sheet in a finishing rolling mill. [Figure 13] FIG. 2 is an explanatory diagram showing the definition of a body crown. [Figure 14] FIG. 2 is an explanatory diagram showing the geometric relationship between the first and second turns of a hot-rolled steel sheet wound around a mandrel. [Figure 15] 1 is a graph showing the relationship between medium wave steepness after finish rolling and flatness of the hot-rolled steel sheet after hot rolling in Example 1. [Figure 16] 4 is a graph showing the relationship between the crown of a hot-rolled steel sheet body and the target steepness of a medium wave in Example 1. [Figure 17] 1 is a graph comparing the flatness of the hot-rolled steel sheet after hot rolling in the present invention with the flatness of the hot-rolled steel sheet after hot rolling in Example 2 in a comparative example. 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 improves the flatness of a hot-rolled steel sheet in a coil manufactured by the hot rolling equipment having the above-mentioned configuration. The coiling temperature of the hot-rolled steel sheet varies depending on the material, but is in the range of approximately 100 to 800°C. The coil manufactured by the hot rolling equipment is transported to a coil yard, cooled to room temperature, and then unwound. The flatness improved by 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 a "strip wave" occurs at the width direction end of the hot-rolled steel sheet. Here, the flatness deterioration that occurs in many hot-rolled steel sheets is a strip wave, and the present invention is intended to improve this strip wave.

[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 widthwise 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. This definition of steepness also applies to medium waves that occur in the widthwise center of the hot-rolled steel sheet.

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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 flatness deterioration due to tight winding will be described in detail with reference to Figures 6 and 7. Specifically, flatness deterioration occurs due to the following phenomena (A) to (D). In Figures 6 and 7, the symbol T indicates tensile stress and S indicates compressive stress. In Figure 7, (a) indicates compressive stress (zero) acting on the width direction ends of the mandrel, (b) indicates compressive stress (arrows in the figure) acting between the width direction ends and the center, and (c) indicates compressive stress (arrows in the figure) acting on the width direction center.

[0036] (A) First, when the hot-rolled steel sheet H is wound at a constant tension by the coiler 4, a tensile stress T acts on the hot-rolled steel sheet H on the surface of the coil C, as shown in Figures 6 and 7, but a compressive stress S acts on the hot-rolled steel sheet H on the inner circumference of the coil C near the mandrel 12.

[0037] (B) In addition, a typical hot-rolled steel sheet H has a crown that is convex in the widthwise center. The crown is empirically formed in the hot-rolled steel sheet H due to the stability of the sheet threading. When a hot-rolled steel sheet H having such a crown is wound around another hot-rolled steel sheet H having a crown, the center of the inner hot-rolled steel sheet H comes into contact with the center of the outer hot-rolled steel sheet H. For this reason, a larger compressive stress S acts on the widthwise center of the inner circumference of the coil C than on the ends.

[0038] (C) In actual operation, the mandrel 12 waits for the hot-rolled steel sheet H to be transported at a standby diameter, and when the hot-rolled steel sheet H is wound a predetermined number of times, it is expanded further (overexpanded). Then, when the pushing force of the cylinder part 24 that tries to expand the mandrel 12 and the surface pressure from the coil C are balanced, the expansion stops and the mandrel 12 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 H, the plate thickness of the hot-rolled steel sheet H, the frictional force between the hot-rolled steel sheets H, etc., and the pushing force of the cylinder part 24 is defeated, and the diameter of the mandrel 12 gradually decreases from the point when the winding tightening is completed. Due to such a reduction in the diameter of the mandrel 12, the hot-rolled steel sheet H on the inner circumference of the coil C is forced to bear the compressive winding tightening force that should be received by the mandrel 12.

[0039] (D) When the above phenomena occur at the same time, the compressive stress S in the sheet-threading direction (circumferential direction) becomes large at the inner periphery of the coil C, especially at the widthwise center, and as a result, the radial compressive stress S acting on the surface of the mandrel 12 becomes large. As shown in FIG. 4, the mandrel 12 has four mandrel segments 20 (segment-wedge portion 27), and as shown in FIG. 7, the radial compressive stress S acts evenly on these mandrel segments 20. Then, since the mandrel segments 20 are in a cantilever state as shown in FIG. 8, the mandrel segments 20 are deformed (dotted lines in the figure) and cannot receive the radial compressive stress S evenly. Furthermore, as shown in FIG. 7, the compressive stress S generated in the radial direction is smaller at the ends than at the widthwise center, so the amount of deflection of the mandrel segments 20 is also smaller at the ends than at the widthwise center. As a result, as shown in Fig. 9, a geometric difference in circumferential length appears in the mandrel 12 in the width direction, and the diameter of the mandrel 12 is smaller in the width direction central portion than at the ends. The hot-rolled steel sheet H is then wound into a coil around the mandrel 12 in this deformed state. Since this winding operation is performed in a hot state, compressive plastic deformation, transformation plastic deformation, and creep deformation occur in the width direction central portion, freezing the shape. This causes deterioration in flatness (edge ​​waves) in the hot-rolled steel sheet H.

[0040] <Flatness Improvement Method in the Present Embodiment> The above is the mechanism of flatness deterioration of a hot-rolled steel sheet, and the 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 address this. 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 improved.

[0041] (medium wave formation) The present inventors have discovered that in order to improve the deterioration of flatness due to the above-mentioned convex crown, a medium wave is formed in the hot-rolled steel sheet after finish rolling so that the tension acting on the hot-rolled steel sheet becomes uniform when the hot-rolled steel sheet is wound around a mandrel.

[0042] FIG. 10 shows the tension distribution (arrows in the figure) acting on the hot-rolled steel sheet H when the hot-rolled steel sheet H with a convex crown is wound, where (a) shows the state of one winding and (b) shows the state of two windings. As shown in FIG. 10(a), when the hot-rolled steel sheet H is wound once around the mandrel 12, the tension σ is distributed almost uniformly in the width direction. On the other hand, as shown in FIG. 10(b), when the hot-rolled steel sheet H is wound twice around the mandrel 12, the center of the hot-rolled steel sheet H in the first winding comes into contact with the center of the hot-rolled steel sheet H in the second winding, and the tension σ is larger in the center in the width direction than at the ends, resulting in a non-uniform distribution. When the hot-rolled steel sheet H is further wound, the non-uniform distribution of the tension σ becomes even greater.

[0043] 11 is an explanatory diagram showing the relationship between the internal pressure P in the mandrel 12 and the tension σ (circumferential stress) applied to the hot-rolled steel sheet H. Here, the relationship between the internal pressure P in the mandrel 12 and the tension σ applied to the hot-rolled steel sheet H is expressed by the following formula (3). Therefore, in order for the tension σ to be uniform from the center to the ends in the width direction, the internal pressures P need to be equal. In order to equalize the internal pressures P, the geometric shape of the hot-rolled steel sheet H wound around the mandrel 12 should be given a drum-shaped elongation strain difference in which the circumferential length of the center in the width direction is longer than the circumferential length of the ends. σ = P r / h (3) where r is the radius of the mandrel, and h is the thickness of the hot-rolled steel sheet at the center in the width direction.

[0044] Therefore, in order to make the tension uniform, a medium wave is formed on the hot-rolled steel sheet. The medium wave is a corrugated out-of-plane deformation formed such that the wave height periodically varies in the rolling direction at least in the widthwise center of the hot-rolled steel sheet. When the medium wave is formed on the hot-rolled steel sheet after the finish rolling, the hot-rolled steel sheet wound around the mandrel has a circumferential difference in which the circumferential length of the widthwise center is longer than that of the end portions, that is, a circumferential difference that is commensurate with the convex crown of the hot-rolled steel sheet described above is given. This makes it possible to make the tension acting on the wound hot-rolled steel sheet uniform in the width direction, and as a result, the flatness of the hot-rolled steel sheet can be improved.

[0045] Specifically, it is possible to improve the flatness to a level where there is no need to transport the coil to a finishing process to correct the shape. As a result, it is possible to reduce the manufacturing cost and stabilize and shorten the manufacturing period. It is also possible to suppress the defects that occur on the surface of the hot-rolled steel sheet in the finishing process, thereby improving the product yield.

[0046] The range in which the medium wave is formed on the hot-rolled steel sheet after finish rolling is until the tip of the hot-rolled steel sheet is wound around the mandrel, for example, within 200 m from the tip. Hereinafter, this range of the hot-rolled steel sheet may be referred to as the tip portion. From experience in actual operation, the shape of the hot-rolled steel sheet wound on the coil becomes flat in the range after the tip portion. 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, a medium wave is formed at the tip portion of the hot-rolled steel sheet.

[0047] In addition, since no medium wave is formed after the tip of the hot-rolled steel sheet, the hot-rolled steel sheet with a convex crown is wound around the mandrel. In such a case, as described above, a larger compressive stress is generated in the center in the width direction than in the end. Therefore, even if the hot-rolled steel sheet is wound around the mandrel with the medium wave formed at the tip, the compressive stress acts on the medium wave at the tip, and as a result, the medium wave disappears. That is, when the tip is wound, the shape (edge ​​wave) can be improved by making the tension distribution uniform in the width direction by the medium wave, and further, when the tip and subsequent parts are wound, the compressive stress can eliminate the medium wave and improve the shape. Therefore, the shape of the hot-rolled steel sheet can be made flat.

[0048] The method of forming a medium wave in a hot-rolled steel sheet in a finishing mill is arbitrary. For example, as shown in Fig. 12, among the multiple stands of a finishing mill 2, in the final stand on the most upstream side in the rolling direction, a moment M is applied to the work rolls 2a, 2a by a bender (not shown), and the central part of the work rolls 2a, 2a in the width direction is protruded (dotted line in the figure). Then, the central part of the hot-rolled steel sheet in the width direction is pressed by the work rolls 2a, 2a, so that a medium wave is formed in the hot-rolled steel sheet. It is also possible to form a medium wave in the hot-rolled steel sheet by varying the rolling load in the finishing mill.

[0049] (Medium wave target steepness) Next, a specific steepness of the medium wave formed at the tip of the hot-rolled steel sheet as described above will be described. The predetermined steepness of the target medium wave (hereinafter referred to as the target steepness) is determined based on the crown generated in the hot-rolled steel sheet immediately after finish rolling. That is, the target steepness λ is calculated as a function of the crown Cr, λ=f(Cr). There are, for example, two methods for determining the target steepness based on the crown.

[0050] In the present invention, the crown is defined as the body crown. As shown in Fig. 13, 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 H is defined as the body crown excluding the edge drop.

[0051] (First embodiment) A method for determining a target steepness according to the first embodiment will be described. In this method, a relationship between the steepness of the medium wave at a predetermined position in the rolling direction of the hot-rolled steel sheet after finish rolling and the flatness (e.g., steepness) of the hot-rolled steel sheet at the predetermined position in the coil after coiling is obtained in advance for each body crown of the hot-rolled steel sheet. In other words, this relationship is an index showing the degree to which the flatness of the hot-rolled steel sheet is improved by the medium wave formed after finish rolling.

[0052] In this method, first, the body crown of the hot-rolled steel sheet immediately after finish rolling is measured. The body crown can be measured, for example, by a thickness gauge provided in the finishing rolling mill and the cooling device shown in FIG. 1. Next, based on the measurement results, the above-mentioned relationship between the steepness of the medium wave and the flatness of the hot-rolled steel sheet is extracted. From this relationship, the steepness of the medium wave corresponding to the target flatness of the hot-rolled steel sheet (for example, 2%) is derived, and this steepness is determined as the target steepness. Then, in the finishing rolling mill, a medium wave of the target steepness is formed in the hot-rolled steel sheet.

[0053] Second embodiment A method for determining a target steepness according to the second embodiment will be described. In this method, the body crown of a hot-rolled steel sheet immediately after finish rolling is measured, and the target steepness λ of the medium wave is determined using the measurement result and the following formula (1). The target steepness λ of the medium wave is determined according to the number of turns of the hot-rolled steel sheet on the coil. Then, a medium wave with the target steepness is formed on the hot-rolled steel sheet in the finishing rolling mill.

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[0054] The adjustment coefficients α and β are set based on the measurement results of the steepness of medium waves in actual operation, for example. Specifically, the theoretical steepness value calculated when the adjustment coefficients α and β are set to 1 and 0 (zero) in the above formula (1), is compared with the measured steepness value in actual operation, and the deviation is adjusted by the adjustment coefficients α and β.

[0055] Fig. 14 is an explanatory diagram showing the geometric relationship between the first and second turns of the hot-rolled steel sheet H wound around the mandrel 12. In Fig. 14, the lower half of the mandrel 12 shows one turn of the hot-rolled steel sheet H, and the upper half shows two turns of the hot-rolled steel sheet H.

[0056] When the first turn of the hot-rolled steel sheet is wound around the mandrel, the tension is distributed almost uniformly as shown in Fig. 10(a) above, so that the first turn of the hot-rolled steel sheet does not form a medium wave. In other words, the target steepness of the medium wave λ1 is 0%.

[0057] When the second roll of hot-rolled steel sheet is wound around the mandrel, the circumferential difference ΔL between the circumferential length Lc of the width direction center part and the circumferential length Le of the end part of the first roll of hot-rolled steel sheet that has already been wound is calculated by the following formula (4). Then, the elongation strain difference Δε2 for the second roll of hot-rolled steel sheet to absorb the circumferential length difference ΔL and be wound with a uniform tension is calculated by the following formula (5). In addition, the target steepness λ2 of the medium wave formed on the second roll of hot-rolled steel sheet is calculated by the following formula (6) from the above formula (2) and the following formula (5).

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[0058] Similarly, when the nth turn of the hot-rolled steel sheet is wound around the mandrel, the target steepness λ of the medium wave is set so that the nth turn of the hot-rolled steel sheet absorbs the difference in circumference and is wound with uniform tension. n is calculated by the following formula (7): When adjustment coefficients α and β based on the measurement results of medium wave steepness in actual operation are added to the right-hand side of formula (7), the above formula (1) is derived.

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[0059] In this method, as shown in the following formula, the target steepness of the medium wave changes depending on the number of turns of the hot-rolled steel sheet, i.e., the position in the rolling direction. Then, in the finishing rolling mill, the rolling conditions are controlled according to this target steepness until the leading end of the hot-rolled steel sheet (within 200 m from the leading end) is wound around the mandrel, and a medium wave is formed in the hot-rolled steel sheet. Here, for example, if the steepness of the medium wave formed at the leading end of the hot-rolled steel sheet is large, the sheet passing property becomes poor. Therefore, gradually increasing the steepness of the medium wave in this way is useful in practical operation.

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[0060] Two methods (first and second embodiments) for determining the target steepness of medium waves based on the body crown have been described above. The present inventors actually derived the target steepness and found that 0.5% to 5% is preferable. When the target steepness is 0.5 to 5%, the effect of improving the flatness of the hot-rolled steel sheet in the present invention can be sufficiently obtained.

[0061] <Target hot-rolled steel sheets> The above-mentioned method for improving flatness due to the winding tightening factor is 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 is 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 a medium wave is formed in the hot-rolled steel sheet after finish rolling as in the present invention, the tension acting on the hot-rolled steel sheet wound around the mandrel can be made uniform in the width direction even if the hot-rolled steel sheet is not transformed or is being transformed, and the flatness of the hot-rolled steel sheet can be improved.

[0062] 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 method for improving flatness of the present invention is also useful in cases where creep occurs during such high-temperature coiling.

[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. EXAMPLES

[0064] An embodiment of the present invention will be described below. In this embodiment, a hot-rolled steel sheet having a thickness of 4 mm, a width of 1200 mm, a body crown of 40 μm, and a coiling temperature of 600° C. was coiled at a constant tension of 20 MPa. At this time, the steepness of the medium wave of the hot-rolled steel sheet was measured immediately after the finish rolling at the exit side of the finishing rolling mill, and further, the flatness (steepness) was measured at a position 100 m from the tip of the hot-rolled steel sheet (a position where the number of turns of the hot-rolled steel sheet is 35) in the coil after the hot rolling process. The results are shown in FIG. 15.

[0065] The horizontal axis of Fig. 15 shows the steepness of the medium wave after finish rolling, and the vertical axis shows the flatness of the hot-rolled steel sheet after hot rolling. Fig. 15 shows that there is a correlation between the medium wave shape created by the finish rolling mill and the flatness of the hot-rolled steel sheet after the hot rolling process. Furthermore, it was confirmed that by increasing the steepness of the medium wave, the edge wave shape after the hot rolling process gradually approaches the medium wave, and a flat shape (flatness) can be created with a medium wave steepness of about 3.5%.

[0066] In addition, the point where the flatness of the hot-rolled steel sheet after the hot rolling process becomes almost 0% at this medium wave steepness of 3.5% was considered to be the target shape of the medium wave after finish rolling in the case of a body crown of 40 μmm, and the target steepness of the medium wave for each body crown was obtained. The results are shown in Figure 16.

[0067] The horizontal axis of FIG. 16 indicates the body crown of the hot-rolled steel sheet, and the vertical axis indicates the target steepness of the medium wave. The value of this target steepness changes slightly depending on the tension during winding, the winding temperature, and the thickness of the hot-rolled steel sheet, so it is corrected by an adjustment coefficient according to the actual situation. This adjustment coefficient is the adjustment coefficient α of the above formula (1) in the above second embodiment. In this embodiment, when the body crown is 40 μm, the actual measured value of the target steepness is 3.5%, and the theoretical value calculated from the above formula (1) is 3.3%, so the adjustment coefficient α is set to 1.05 and β is set to 0 (zero). If the target steepness of the medium wave can be determined for each body crown as shown in FIG. 16, the hot-rolled steel sheet can be formed to the desired flatness.

[0068] Even when a medium wave having a target steepness is not formed as in this embodiment, the effect of improving the flatness of the hot-rolled steel sheet can be achieved as long as a medium wave is formed in the hot-rolled steel sheet after at least finish rolling. EXAMPLES

[0069] Next, another embodiment of the present invention will be described. In this embodiment, a hot-rolled steel sheet having a thickness of 4 mm, a width of 1200 mm, a body crown of 40 μm, and a winding temperature of 600° C. was wound at a constant tension of 20 MPa. At this time, the average steepness of the medium wave of the hot-rolled steel sheet was measured immediately after the finish rolling at the exit side of the finishing rolling mill, and further, the flatness (steepness) at a position 200 m from the tip of the hot-rolled steel sheet was measured in the coil after the hot rolling process. The average steepness is the average steepness of the medium wave formed in the range from the tip of the hot-rolled steel sheet to 200 m.

[0070] Example 1 is a case where a medium wave is formed in a hot rolled steel sheet in finish rolling, and the steepness of the medium wave is not limited to a target steepness.

[0071] In Example 2, the method for determining the target steepness in the second embodiment is used to determine the target steepness of a medium wave by the above formula (1), and a medium wave having the target steepness is formed in the hot-rolled steel sheet during finish rolling. The value of this target steepness varies slightly depending on the tension during coiling, the coiling temperature, and the thickness of the hot-rolled steel sheet, and is therefore corrected by adjustment coefficients α and β according to the actual situation. In this Example 2, the adjustment coefficients α and β are set to 0.95 and 0 (zero), respectively.

[0072] Comparative Example 1 is a case where the medium corrugations are not formed on the hot-rolled steel sheet in the finish rolling process, as in the present invention.

[0073] The measurement results are shown in Fig. 17. Fig. 17 is a bar graph of Example 1, Example 2, and Comparative Example 3, with the vertical axis showing the flatness of the hot-rolled steel sheet after hot rolling. According to Fig. 17, the flatness of the hot-rolled steel sheet in the coil was 5.5% in Comparative Example 1, but was greatly improved to 2% in Example 1, and further improved dramatically to 0.5% in Example 2. [Industrial Applicability]

[0074] The present invention is useful in producing coils of hot-rolled steel sheet in a hot rolling process. [Explanation of symbols]

[0075] 1. Hot rolling equipment 2 Finishing rolling mill 2a Work roll 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 of hot-rolled steel sheet in a hot rolling process, comprising the steps of: A finishing rolling mill forms a medium wave having a predetermined steepness at least in a central portion in the width direction of the hot rolled steel sheet; The hot-rolled steel sheet having the medium corrugation is wound around a mandrel of a coiler to produce a coil; A method for manufacturing a hot-rolled coil, characterized in that the predetermined steepness of the medium wave is determined based on a body crown generated in a hot-rolled steel sheet after finish rolling.

2. A relationship between the steepness of the medium wave at a predetermined position in the rolling direction of the hot-rolled steel sheet and the flatness of the hot-rolled steel sheet at the predetermined position in the coil is obtained in advance for each body crown; 2. The method for manufacturing a hot rolled coil according to claim 1, further comprising the steps of: measuring the body crown after finish rolling; and determining the predetermined steepness of the medium wave from the measurement result and the relationship.

3. The body crown after finish rolling is measured, and the predetermined steepness of the medium wave is determined according to the number of turns of the hot-rolled steel sheet in the coil using the measurement result and the following formula (1); 2. The method for manufacturing a hot-rolled coil according to claim 1, characterized in that, when forming the medium wave in the hot-rolled steel sheet by a finishing rolling mill, the steepness of the medium wave is changed according to the rolling direction position of the hot-rolled steel sheet based on the determined predetermined steepness of the medium wave. [0010] where λ is a predetermined steepness of the medium wave, n is the number of turns of the hot-rolled steel sheet in the coil, Cr is the body crown generated in the hot-rolled steel sheet after finish rolling, r is the radius of the mandrel, h is the sheet thickness at the center in the width direction of the hot-rolled steel sheet, and α and β are adjustment coefficients.

4. 4. The method for producing a hot-rolled coil according to claim 3, wherein the adjustment coefficients α and β in the formula (1) are set based on a measurement result of a steepness of a medium wave in an actual operation.

5. The method for manufacturing a hot rolled coil according to any one of claims 1 to 4, characterized in that the predetermined steepness of the medium wave is 0.5% to 5%.

6. The method for manufacturing a hot-rolled coil according to any one of claims 1 to 5, characterized in that the medium wave is formed within a range of 200 m from the tip in the rolling direction of the hot-rolled steel sheet.

7. The method for producing a hot-rolled coil according to any one of claims 1 to 6, characterized in that the hot-rolled steel sheet wound around the mandrel is at 700°C or higher before or during transformation, or after completion of transformation.

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