JP7620196B2 - Manufacturing method of hot rolled coil - Google Patents

Description

The present invention relates to a manufacturing method for producing coils of hot-rolled steel sheet in a hot rolling process.

After finish rolling in the hot rolling process, the hot-rolled steel sheet is cooled to a specified 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 produce a coil (hot-rolled coil).

The coils manufactured as described above are wound at a specified 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 coils to be shipped or transported to the next process are unwound for processing, the flatness of the hot-rolled steel sheet may be poor. In such cases, the hot-rolled steel sheet has poor sheet passing properties and may meander, or problems such as squeezing may occur during processing or rolling, so the shape must be corrected. However, since the shape (flatness) of the hot-rolled steel sheet cannot be determined in the coil state, coils with a flatness specification that cannot be shipped with poor flatness at present are transported to a refinement 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 refinement 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 to 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.

For example, Patent Document 1 discloses a method for predicting the shape of a hot-rolled steel sheet, which separates the residual stress of the 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 predicts 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, the tension acting on the hot-rolled steel sheet when it is wound on a coiler, and ultimately the width-direction temperature distribution of the hot-rolled steel sheet at the time of winding is generated as residual stress. Furthermore, based on the results of the predicted shape, the width-direction temperature distribution is controlled, for example, by an edge heater or edge mask, thereby improving the flatness of the hot-rolled steel sheet.

Patent No. 4262142

However, when the inventors of the present invention investigated the shape of the steel sheet after the hot rolling process in detail, they found that there was a deterioration in flatness that could not 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. They also found that the flatness of the hot-rolled steel sheet could not be sufficiently improved by simply controlling the temperature distribution in the width direction based on the shape prediction results. Therefore, there is room for improvement in improving the flatness of the hot-rolled steel sheet.

The present invention was made in consideration of the above circumstances, and aims to improve the flatness of hot-rolled steel sheet in a coil when producing a coil of hot-rolled steel sheet in a hot rolling process.

In order to solve the above problem, the inventors conducted intensive research and elucidated the mechanism of the deterioration of the flatness of hot-rolled steel sheets after the hot rolling process. Specifically, it was revealed that the deterioration of flatness 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 on 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 on the coiler (mandrel), and furthermore, the uneven tension distribution causes the inner circumference of the coil to plastically deform, causing plastic strain, and this plastic strain becomes an elongation strain difference (residual strain).

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 traditionally been considered a factor in deterioration of flatness, and measures have been taken to address this. On the other hand, the second, the tightening factor, is a new finding by the inventors that deformation due to tightening that occurs in the cold rolling process also occurs in the hot rolling process.

The deterioration of flatness due to winding tightening will be explained in more detail. After finish rolling, the hot-rolled steel sheet has a crown that is convex in the widthwise center. When another hot-rolled steel sheet with a crown is wound around a hot-rolled steel sheet with 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. For this reason, when the hot-rolled steel sheet is wound around the mandrel, the tension acting on the hot-rolled steel sheet is greater in the widthwise center than at the ends. This uneven tension causes the flatness of the hot-rolled steel sheet to deteriorate.

The inventors have found that, in order to make the tension uniform, it is sufficient to give the hot-rolled steel sheet to be wound a difference in circumferential length corresponding to the crown of the hot-rolled steel sheet, 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 is characterized in that a medium wave having a predetermined steepness is formed on the hot-rolled steel sheet at least in the widthwise center portion by a finishing rolling mill, the hot-rolled steel sheet on which the medium wave is formed is wound on a mandrel of a coiler to produce a coil , and the predetermined steepness of the medium wave is determined based on the body crown generated on the hot-rolled steel sheet after the finish rolling . Note that the medium wave in the present invention refers to a wave shape formed such that the wave height periodically varies in the rolling direction at least in the widthwise center portion of the hot-rolled steel sheet.

According to the present invention, a medium wave is formed in the hot-rolled steel sheet during finish rolling, so that a difference in circumferential length 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.

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 result and the relationship.

但し、λ：中波の所定急峻度、ｎ：コイルにおける熱延鋼板の巻き数、Ｃｒ：仕上げ圧延後の熱延鋼板に生じるボディークラウン、ｒ：マンドレルの半径、ｈ：熱延鋼板の幅方向中央部における板厚、α及びβ：調整係数 In addition, the body crown after finish rolling may be measured, and the 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 rolling direction position 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.

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.

In the hot-rolled coil manufacturing method, the predetermined steepness of the medium wave may be 0.5% to 5%.

In the manufacturing method of the hot-rolled coil, the medium wave may be formed within a range of 200 m from the tip in the rolling direction of the hot-rolled steel sheet.

In the method for manufacturing hot-rolled coils, the hot-rolled steel sheet wound around the mandrel may be at 700°C or higher while untransformed, during transformation, or after transformation is complete.

According to the present invention, by forming a medium wave in the hot-rolled steel sheet during finish rolling, a difference in circumferential length is imparted to the hot-rolled steel sheet, and the tension acting on the hot-rolled steel sheet during winding can be made uniform in the width direction. As a result, the flatness of the hot-rolled steel sheet can be improved.

FIG. 2 is an explanatory diagram showing an outline of the configuration of a hot rolling facility after a finishing mill. FIG. 2 is an explanatory diagram showing an outline of the configuration of a coiler. FIG. 2 is an explanatory diagram showing an outline of the configuration of a mandrel, in an axial cross-sectional view. 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. FIG. 13 is an explanatory diagram showing the definition of steepness, which indicates the level of ear waves. FIG. 1 is a conceptual diagram for explaining the mechanism of deterioration of flatness due to tight winding. FIG. 1 is a conceptual diagram for explaining the mechanism of deterioration of flatness due to winding tightening, in which (a) shows the compressive stress (zero) acting on the width direction ends of the mandrel, (b) shows the compressive stress acting between the width direction ends and the center (arrows in the figure), and (c) shows the compressive stress acting on the width direction center (arrows in the figure). FIG. 4 is an explanatory diagram showing how a mandrel segment is deformed by compressive stress. FIG. 4 is an explanatory diagram showing a state in which a mandrel is deformed due to a compressive stress. The figure shows the tension distribution (arrows in the figure) acting on a hot-rolled steel sheet having a convex crown formed thereon when the hot-rolled steel sheet is wound, with (a) showing the state after one winding and (b) showing the state after two windings. 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. 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. FIG. 2 is an explanatory diagram showing the definition of a body crown. 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. 1 is a graph showing the relationship between medium wave steepness after finish rolling and flatness of hot-rolled steel sheet after hot rolling in Example 1. 1 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. 1 is a graph comparing the flatness of a hot-rolled steel sheet after hot rolling in the present invention with the flatness of a hot-rolled steel sheet after hot rolling in a comparative example in Example 2.

Embodiments of the present invention will be described below with reference to the drawings. Note that in this specification and the drawings, elements having substantially the same functional configuration are designated by the same reference numerals to avoid redundant description.

<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 onward.

In the hot rolling equipment 1, a finishing rolling mill 2 that continuously rolls the steel sheet H discharged from a heating furnace (not shown) and rolled by a rough rolling 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 transport direction of the hot-rolled steel sheet H. Between the finishing rolling mill 2 and the coiler 4, a run-out table 5 that transports the hot-rolled steel sheet H is provided. The hot-rolled steel sheet H rolled by the finishing rolling mill 2 is cooled by the cooling device 3 while being transported on the run-out table 5, and then wound up by the coiler 4 to be manufactured as a coil C.

Between the finishing mill 2 and the cooling device 3 of the hot rolling equipment 1, a thickness gauge 6 is provided to measure the thickness of the hot-rolled steel sheet H rolled by the finishing mill 2. The thickness gauge 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.

Figure 2 is an explanatory diagram showing the outline of the configuration of the coiler 4. The example in Figure 2 shows the state when the coiler 4 starts winding operation. The coiler 4 has a pinch roll 10, a chute 11, a mandrel 12, and a wrapper roll 13.

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 (in contact with 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 when the hot-rolled steel sheet H is wound up by a predetermined number of turns on the coil C, it begins to expand, and when the expanding force and the force of the coil C tightening are balanced, the expansion of the diameter stops, and the wrapper roll 13 opens and moves away from the coil C.

Figures 3 and 4 are explanatory diagrams showing the outline of the configuration of the mandrel 12. As shown in Figure 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.

As shown in FIG. 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.

<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 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.

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 is also applied to medium waves occurring in the widthwise center of the hot-rolled steel sheet.

The inventors conducted extensive research and elucidated the mechanism behind the deterioration of flatness of hot-rolled steel sheets after the hot rolling process. In other words, it became clear that the deterioration of flatness is caused by a combination of two factors: a temperature factor in which thermal strain occurs due to uneven temperature distribution in the width direction of the hot-rolled steel sheet, and a tightening factor in which plastic deformation occurs at the inner circumference of the coil due to tightening caused by uneven tension distribution in the width direction that occurs during coiling. These two factors are explained below.

(Temperature factors)
The first cause of flatness deterioration due to temperature will be explained. A hot-rolled steel sheet immediately before being wound by a coiler is subjected to thermal strain due to non-uniform 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.

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 other publicly known documents (Akashi et al., "Study on flatness prediction method after cooling of hot-rolled steel sheet," 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 uneven 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 that is tensioned 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 as it passes through, it is wrapped around the lower pinch roll (comes in surface contact) while being tensioned, 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, and is corrected. Furthermore, when the temperature distribution in the width direction of the hot-rolled steel sheet during winding is lowered to room temperature when the coil is unwound, a difference in elongation strain occurs in the hot-rolled steel sheet, causing the flatness to deteriorate.

(Cause of tightening)
The second cause of flatness deterioration caused by tightening is explained below. 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 a coiler is distributed unevenly in the width direction. Tightening under this uneven tension distribution plastically deforms the inner circumference of the coil, causing plastic strain. This plastic strain becomes an elongation strain difference (residual strain), causing flatness deterioration (shape deterioration) of the hot-rolled steel sheet.

Next, the mechanism of flatness deterioration caused by winding tightening will be explained 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 widthwise ends of the mandrel, (b) indicates compressive stress acting between the widthwise ends and the center (arrows in the figure), and (c) indicates compressive stress acting on the widthwise center (arrows in the figure).

(A) First, when the hot-rolled steel sheet H is wound with a constant tension by the coiler 4, as shown in Figures 6 and 7, a tensile stress T acts on the hot-rolled steel sheet H on the surface of the coil C, 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.

(B) In addition, a typical hot-rolled steel sheet H has a crown that is convex in the widthwise center. The crown is formed in the hot-rolled steel sheet H empirically based on the stability of the sheet during threading. When a hot-rolled steel sheet H having such a crown is wound over 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 greater compressive stress S acts on the widthwise center of the inner circumference of the coil C than on the ends.

(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 when the hot-rolled steel sheet H is wound, 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 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.

(D) When the above phenomena occur at the same time, the compressive stress S in the sheet passing direction (circumferential direction) becomes large at the inner circumference 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. As a result, since the mandrel segment 20 is in a cantilever state as shown in FIG. 8, the mandrel segment 20 is deformed (dotted line 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 deflection of the mandrel segment 20 is also smaller at the ends than at the widthwise center. As a result, as shown in FIG. 9, the mandrel 12 appears to have a geometric difference in circumference in the width direction, and the diameter of the mandrel 12 is smaller in the width direction center 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 center, freezing the shape. This causes deterioration in flatness (ear waves) in the hot-rolled steel sheet H.

<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.

(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.

Figure 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, with (a) showing the state of one winding and (b) showing the state of two windings. As shown in Figure 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, when the hot-rolled steel sheet H is wound twice around the mandrel 12 as shown in Figure 10(b), 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 width direction center than at the ends, resulting in an uneven distribution. Furthermore, when the hot-rolled steel sheet H is wound further, the uneven distribution of the tension σ becomes even greater.

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, it is necessary to give the hot-rolled steel sheet H taken up around the mandrel 12 a geometric shape with 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.

Therefore, to make the tension uniform, a medium wave is formed on the hot-rolled steel sheet. A 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 a medium wave is formed on the hot-rolled steel sheet after finish rolling in this way, a circumferential difference occurs in the hot-rolled steel sheet wound around the mandrel, where the circumferential length of the widthwise center is longer than that of the ends, i.e., a circumferential difference that is commensurate with the convex crown of the hot-rolled steel sheet described above is imparted. This makes it possible to make the tension acting on the wound hot-rolled steel sheet uniform in the width direction, thereby improving the flatness of the hot-rolled steel sheet.

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 manufacturing costs and stabilize and shorten the manufacturing period. It is also possible to improve product yield by suppressing the defects that occur on the surface of the hot-rolled steel sheet during the finishing process.

The range in which medium waves are formed on 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 a range of 200 m from the tip. Hereinafter, this range of 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 beyond 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, correcting the shape. Therefore, in the present invention, medium waves are formed at the tip portion of the hot-rolled steel sheet.

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 a medium wave formed at the tip, compressive stress acts on the medium wave at the tip, and as a result, the medium wave disappears. In other words, when the tip is wound, the shape (ear wave) can be improved by making the tension distribution uniform in the width direction using the medium wave, and further, when the tip and subsequent parts are wound, the shape can be improved by eliminating the medium wave using compressive stress. Therefore, the shape of the hot-rolled steel sheet can be made flat.

The method of forming medium waves in the hot-rolled steel sheet in the finishing mill is arbitrary. For example, as shown in FIG. 12, in the final stand on the most upstream side in the rolling direction among the multiple stands of the finishing mill 2, a moment M is applied to the work rolls 2a, 2a by a bender (not shown), and the center of the width of the work rolls 2a, 2a is protruded (dotted line in the figure). In this way, the center of the width of the hot-rolled steel sheet 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.

(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.

In the present invention, this 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 widthwise end (edge) and the sheet thickness at the widthwise center (center), 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 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.

(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 winding 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.

In this method, first, the body crown of the hot-rolled steel sheet is measured immediately after finish rolling. The body crown can be measured, for example, by a thickness gauge installed in the finishing rolling mill and 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 (e.g., 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.

但し、λ：中波の目標急峻度、ｎ：コイルにおける熱延鋼板の巻き数、Ｃｒ：仕上げ圧延後の熱延鋼板に生じるボディークラウン、ｒ：マンドレルの半径、ｈ：熱延鋼板の幅方向中央部における板厚、α及びβ：線形多変量解析で言うところの相関係数であり調整係数である 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 having the target steepness is formed on the hot-rolled steel sheet in the finishing rolling mill.

where λ is the target 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 correlation coefficients and adjustment coefficients in linear multivariate analysis.

The adjustment coefficients α and β are set, for example, based on the results of measurements of the steepness of medium waves in actual operation. 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 measured in actual operation, and the deviation is adjusted using the adjustment coefficients α and β.

Figure 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 Figure 14, the lower half of the mandrel 12 shows the state where the hot-rolled steel sheet H is wound once, and the upper half shows the state where the hot-rolled steel sheet H is wound twice.

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 no medium wave is formed in the first turn of the hot-rolled steel sheet, i.e., the target steepness λ ₁ of the medium wave is 0%.

When the second winding of the hot-rolled steel sheet is wound around the mandrel, the circumferential length 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 winding of the hot-rolled steel sheet that has already been wound is calculated by the following formula (4). Then, the elongation strain difference Δε ₂ for the second winding of the 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 λ ₂ of the medium wave formed in the second winding of the hot-rolled steel sheet is calculated by the following formula (6) from the above formula (2) and the following formula (5).

Similarly, when the nth turn of the hot-rolled steel sheet is wound around the mandrel, the target steepness _λn of the medium wave for absorbing the circumferential difference and winding the nth turn of the hot-rolled steel sheet with a uniform tension is calculated by the following formula (7). When adjustment coefficients α and β are added to the right side of formula (7) based on the measurement results of the medium wave steepness in actual operation, for example, the above formula (1) is derived.

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 tip of the hot-rolled steel sheet (within 200 m from the tip) 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 tip 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.

Above, two methods (first and second embodiments) for determining the target steepness of medium waves based on the body crown have been explained. When the inventors actually derived the target steepness, they found that 0.5% to 5% is preferable. If the target steepness is 0.5 to 5%, the effect of improving the flatness of the hot-rolled steel sheet in this invention can be sufficiently obtained.

<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.

In addition, for example, if a hot-rolled steel sheet is wound at a high temperature of 700°C or higher after the transformation is complete, the hot-rolled steel sheet may be deformed due to creep. Therefore, the flatness improvement method of the present invention is also useful in cases where creep occurs during such high-temperature winding.

Although the embodiments of the present invention have been described above, the present invention is not limited to these examples. It is clear that a person skilled in the art can come up with various modified or revised examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

Below, an example of the present invention will be described. In this example, a hot-rolled steel sheet with a thickness of 4 mm, width of 1200 mm, body crown of 40 μm, and winding temperature of 600°C was wound 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 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 Figure 15.

The horizontal axis of Figure 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. Figure 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 ear 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%.

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

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 second embodiment. In this embodiment, at a body crown of 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%, and the adjustment coefficient α is set to 1.05 and β is set to 0 (zero). And, 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.

Even if a medium wave with the 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.

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 waves of the hot-rolled steel sheet was measured immediately after finish rolling at the exit side of the finishing rolling mill, and the flatness (steepness) was measured at a position 200 m from the tip of the hot-rolled steel sheet in the coil after the hot rolling process. The average steepness is the average steepness of the medium waves formed in the range from the tip of the hot-rolled steel sheet to 200 m.

In Example 1, a medium wave is formed in a hot-rolled steel sheet during finish rolling, and the steepness of the medium wave is not limited to the target steepness.

Example 2 is a case where the target steepness of a medium wave is determined by the above formula (1) using the method for determining the target steepness in the second embodiment, and a medium wave of the target steepness is formed in the hot-rolled steel sheet during finish rolling. Since the value of this target steepness varies slightly depending on the tension during winding, the winding temperature, and the thickness of the hot-rolled steel sheet, it is corrected by adjustment coefficients α and β according to the actual situation. In this Example 2, adjustment coefficients α and β are set to 0.95 and 0 (zero), respectively.

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

The measurement results are shown in Figure 17. Figure 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 Figure 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.

The present invention is useful in producing coils of hot-rolled steel sheets during the hot rolling process.

REFERENCE SIGNS LIST 1 Hot rolling equipment 2 Finishing rolling mill 2a Work roll 3 Cooling device 4 Coiler 5 Run-out table 6 Plate thickness gauge 10 Pinch roll 11 Chute 12 Mandrel 13 Wrapper roll 20 Mandrel segment 21 Wedge 22 Slide rod 23 Wedge shaft 24 Cylinder portion 25 Segment flange portion 26 Wedge jaw portion 27 Segment-wedge portion C Coil H Hot-rolled steel sheet

Claims (7)

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 . 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. 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.

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. 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. 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%. 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. 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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