JP2018130765A - Rolling method for billet, and rolling facility - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress occurrence of a product defect such as a surface defect or a mechanical property defect by suppressing occurrence of billet cracking or plate break during rolling, in a subsequent process, caused by porosity or segregation of a thickness center portion of a billet and an internal defect or a nonuniform tissue caused by columnar crystal formation.SOLUTION: The present invention relates to a rolling method for a billet that rolls a billet of which the cross-sectional shape is substantially rectangular, for two paths or more. The rolling for at least one or more paths excluding a final path is performed by a roller integrated as one of a pair of work rolls in which a pressure roll having an outer peripheral surface shape that is periodically changed in a circumferential direction is abutted with the billet. The pressure roll has the outer peripheral surface shape of which the cycle is 1/50 times or more and double or less as long as a contact arc length and the amplitude is 1/30 times or more and 1/3 times or less as much as rolling reduction, and which is periodically changed in the circumferential direction.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a rolling method and rolling equipment for a steel slab for rolling a steel slab having a substantially rectangular cross-sectional shape as a material for a plate product.

  In a continuous casting machine that continuously solidifies molten steel into a rectangular cross-section slab, it has traditionally been a subsequent process due to porosity or segregation in the center part of the slab thickness, internal defects due to columnar crystal formation, or a heterogeneous structure. Occurrence of product defects (surface defects and mechanical property defects) such as cracks in steel slabs and sheet breakage during rolling has been a problem.

  For example, Patent Document 1 discloses a method of improving the central structure by rolling the steel slab center after the solidification of the steel slab by prescribing the rolling reduction (amount) according to the temperature difference between the surface layer part and the central part. It is disclosed. According to this technology, compared with the case where the temperature difference between the surface layer portion and the center portion is small, the hydrostatic pressure stress in the center portion of the slab increases and the amount of plastic strain also increases, and porosity defects and center segregation are improved. can get.

  Patent Document 2 discloses a technique for reducing center segregation by rolling down a deformed cross-section steel piece having a thick part at the center of the width after the center part of the steel piece is completely solidified. In the technique, by making the cross-section of the steel slab, compared to the technique disclosed in the above-mentioned Patent Document 1, the plastic strain due to rolling is increased at the center of the steel slab, and the hydrostatic pressure stress is also increased. Central organization improves.

Japanese Patent Laid-Open No. 2004-237291 Japanese Patent Laid-Open No. 2006-016450

  However, with respect to the above-mentioned Patent Document 1, the temperature difference between the surface layer portion and the central portion is generally determined by the surface heat transfer coefficient and the steel piece thickness, and the reduction amount is substantially set from the steel piece thickness and the product thickness. The upper limit of the rolling reduction is restricted by the difference with the thickness of the intermediate product (usually called a coarse bar). For this reason, there are many cases where a sufficient amount of reduction cannot be realized, and due to the insufficient amount of reduction, particularly when manufacturing a product with a large plate thickness, a sufficient structure and quality improvement effect may not be obtained. Many.

  Moreover, in the said patent document 2, the plastic strain which arises in the surface layer vicinity from the position of 1/4 of a steel piece thickness from a surface layer reduces sharply, and a columnar crystal structure remains. For this reason, the segregation in the said part gets worse extremely. Further, it is difficult to continuously cast the irregular cross-section steel slab, and the equipment cost is enormous.

  Therefore, the present invention has been made in view of the above problems, and the object of the present invention is to provide porosity and segregation in the central part of the thickness of the steel slab, and internal defects and heterogeneity due to columnar crystal formation. A steel slab rolling method and rolling facility capable of suppressing the occurrence of steel slab cracking in subsequent processes caused by the structure and the occurrence of sheet breakage during rolling, and suppressing the occurrence of product defects such as surface defects and poor mechanical properties. It is to provide.

  In order to solve the above-described problem, according to one aspect of the present invention, there is provided a method for rolling a steel slab in which rolling is performed for two or more passes on a steel piece having a substantially rectangular cross-sectional shape, excluding the final pass, Rolling of one or more passes is performed by a rolling mill in which a rolling roll having an outer peripheral surface shape that periodically changes in the circumferential direction is incorporated as at least one of a pair of work rolls that come into contact with a steel piece. The rolling roll has a period of 1/50 to 2 times the contact arc length and an amplitude of 1/30 to 1/3 times the rolling reduction, and the outer peripheral surface periodically changes in the circumferential direction. A method of rolling a billet having a shape is provided.

  Here, the outer peripheral surface shape of the rolling roll is the circumferential direction of the difference between the outer peripheral surface radius which is the distance from the rotation center of the rolling roll to the outer peripheral surface and the average value of the outer peripheral surface radius in the entire circumference. The distribution may be formed in a sine wave shape.

  Alternatively, the outer peripheral surface shape of the rolling roll is the circumferential distribution of the difference between the outer peripheral surface radius that is the distance from the rotation center of the rolling roll to the outer peripheral surface and the average value of the outer peripheral surface radius in the entire circumference. May be formed to have a continuous and periodic shape including a straight line and a plurality of line segments including at least one of an arc or a high-order function curve.

  Further, the outer peripheral surface shape of the rolling roll is the maximum value of the uphill angle representing the inclination angle of the portion where the outer peripheral surface radius, which is the distance from the rotation center of the rolling roll to the outer peripheral surface, increases in the rotation direction of the rolling roll. It may be formed to have a friction angle or less and a continuous and periodic shape.

  Alternatively, the outer peripheral surface shape of the rolling roll is the minimum value of the downward gradient angle indicating the inclination angle of the portion where the outer peripheral surface radius, which is the distance from the rotation center of the rolling roll to the outer peripheral surface, decreases in the rotation direction of the rolling roll. The absolute value may be equal to or smaller than the bite angle of the roll bite, and may be formed to have a continuous and periodic shape.

  Further, the outer peripheral surface shape of the rolling roll is the circumferential distribution of the difference between the outer peripheral surface radius which is the distance from the rotation center of the rolling roll to the outer peripheral surface and the average value of the outer peripheral surface radius in the entire circumference. However, you may form so that it may become a continuous and periodic shape, changing the phase in the body length direction of a rolling roll.

  Alternatively, the outer peripheral surface shape of the rolling roll is the circumferential distribution of the difference between the outer peripheral surface radius that is the distance from the rotation center of the rolling roll to the outer peripheral surface and the average value of the outer peripheral surface radius in the entire circumference. However, you may form so that it may become a continuous and periodic shape, changing the amplitude and the perimeter average radius value in the body length direction of a rolling roll.

  A rolling pass for rolling a steel slab by a rolling mill incorporating a rolling roll is performed in a rolling pass in which the temperature difference between the surface temperature of the steel slab and the plate thickness center temperature at the plate thickness center portion is 20 ° C. or more.

  Moreover, in order to solve the said subject, according to another viewpoint of this invention, it is rolling equipment provided with two or more rolling mills which roll the steel slab whose cross-sectional shape is a substantially rectangular shape, Comprising: The steel material conveyance direction most downstream In at least one or more rolling mills excluding the rolling mill located in the at least one of the pair of work rolls in contact with the steel slab, the period is 1/50 to 2 times the contact arc length, and A rolling facility having an outer peripheral surface shape whose amplitude is 1/30 times or more and 1/3 times or less of the reduction amount and periodically changes in the circumferential direction is provided.

  Here, the outer peripheral surface shape of the work roll is the circumferential direction of the difference between the outer peripheral surface radius which is the distance from the rotation center of the work roll to the outer peripheral surface and the average value of the outer peripheral surface radius in the entire circumference. The distribution may be formed in a sine wave shape.

  In addition, the outer peripheral surface shape of the work roll is the circumferential distribution of the difference between the outer peripheral surface radius, which is the distance from the rotation center of the work roll to the outer peripheral surface, and the average value of the outer peripheral surface radius in the entire circumference. May be formed to have a continuous and periodic shape including a straight line and a plurality of line segments including at least one of an arc or a high-order function curve.

  Alternatively, the outer peripheral surface shape of the work roll has a maximum value of an upward gradient angle representing an inclination angle of a portion where an outer peripheral surface radius, which is a distance from the rotation center of the work roll to the outer peripheral surface, increases in the rotation direction of the work roll. It may be formed to have a friction angle or less and a continuous and periodic shape.

  Further, the outer peripheral surface shape of the work roll is the minimum value of the downward gradient angle indicating the inclination angle of the portion where the outer peripheral surface radius, which is the distance from the rotation center of the work roll to the outer peripheral surface, decreases in the rotation direction of the work roll. The absolute value may be equal to or smaller than the bite angle of the roll bite, and may be formed to have a continuous and periodic shape.

  Alternatively, the outer peripheral surface shape of the work roll is the circumferential distribution of the difference between the outer peripheral surface radius that is the distance from the rotation center of the work roll to the outer peripheral surface and the average value of the outer peripheral surface radius in the entire circumference. However, you may form so that it may become a continuous and periodic shape, changing the phase in the trunk | drum length direction of a work roll.

  In addition, the outer peripheral surface shape of the work roll is the circumferential distribution of the difference between the outer peripheral surface radius, which is the distance from the rotation center of the work roll to the outer peripheral surface, and the average value of the outer peripheral surface radius in the entire circumference. However, you may form so that it may become a continuous and periodic shape, changing the amplitude and the perimeter average radius value in the body length direction of a work roll.

  The rolling mill provided with the work roll having an outer peripheral surface shape that periodically changes may be a two-high rolling mill or a four-high rolling mill.

  The rolling equipment may be arranged on the downstream side in the casting direction with respect to the continuous casting machine for casting the steel slab.

  Or the said rolling equipment may be arrange | positioned with respect to the scale removal apparatus which removes the scale of the steel slab heated by the heating furnace downstream in the conveyance direction of a steel slab.

  As described above, according to the present invention, steel slab cracking or rolling during subsequent processes due to porosity or segregation in the central part of the steel slab, and internal defects or heterogeneous structure due to columnar crystal formation. It is possible to suppress the occurrence of breakage and suppress the occurrence of product defects such as surface defects and mechanical property defects.

It is a schematic explanatory drawing which shows the rolling equipment which concerns on one Embodiment of this invention, and shows the case where three rolling mills are installed. It is a schematic explanatory drawing which shows the rolling equipment which concerns on the same embodiment, and shows the case where two rolling mills are installed. It is a schematic perspective view which shows an example of a corrugated roll. It is explanatory drawing which shows the state which applied the corrugated roll of FIG. 3 to the upper work roll and lower work roll of a 4-high rolling mill. It is explanatory drawing which shows the other example of a corrugated roll. It is a schematic explanatory drawing which shows the example which installed the rolling equipment shown in FIG. 1 in the casting direction downstream of a continuous casting machine. It is a schematic explanatory drawing which shows the example which installed the rolling equipment shown in FIG. 2 in the casting direction downstream of the continuous casting machine. It is a schematic explanatory drawing which shows the example which installed the rolling equipment shown in FIG. 1 in the conveyance direction downstream side with respect to a scale removal apparatus. It is a conceptual diagram which shows the plastic deformation state (stress, plastic distortion, and metal flow) inside a steel slab when rolling a steel slab using a corrugated roll. For corrugated rolls and flat rolls, use flat rolls for the hydrostatic pressure stress at the center of the steel slab thickness, the distribution of roll bite and its vicinity, and the maximum hydrostatic pressure stress (positive compression side) It is explanatory drawing which shows the increment value at the time of using a corrugated roll with respect to the case where it had. Each pass for the first pass rolled by the corrugated roll or the upper and lower flat rolls shown in FIG. 9 and the second pass rolled by the upper and lower flat rolls provided on the most downstream side in the conveying direction after rolling the first pass. It is explanatory drawing which shows the length direction distribution of the plastic strain (strain amount which generate | occur | produced in each pass) in the thickness center position of the steel slab after rolling, and a 1/4 thickness position from the surface layer. A flat roll was used in the first pass with respect to the total strain value obtained by summing the plastic strains in the first pass and the second pass shown in FIG. 11, the average value in the length direction, and the average value in the length direction of the total strain value. It is explanatory drawing which shows the increment value at the time of using the corrugated roll with respect to a case. It is explanatory drawing which shows the relationship between the period of the waveform formed in the outer peripheral surface of a corrugated roll, and a total distortion increment value. It is explanatory drawing which shows the relationship between the period of the waveform formed in the outer peripheral surface of a corrugated roll, and a hydrostatic pressure increment value. It is explanatory drawing which shows the relationship between the amplitude of the waveform formed in the outer peripheral surface of a corrugated roll, and a total distortion increment value. It is explanatory drawing which shows the relationship between the amplitude of the waveform formed in the outer peripheral surface of a corrugated roll, and a hydrostatic pressure increment value. It is explanatory drawing explaining the definition of the outer peripheral surface shape of a corrugated roll, and its gradient angle. It is explanatory drawing explaining the limiting conditions of the up-slope angle of the outer peripheral surface shape of a corrugated roll. It is explanatory drawing explaining the limiting conditions of the downward distribution angle of the outer peripheral surface shape of a corrugated roll. It is explanatory drawing which shows the relationship between the plate thickness direction temperature difference of a steel piece, and plate | board thickness center strain increment. It is explanatory drawing which shows the relationship between the plate thickness direction temperature difference of a steel slab, and an isostatic pressure incremental amount.

  Exemplary embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In addition, in this specification and drawing, about the component which has the substantially same function structure, duplication description is abbreviate | omitted by attaching | subjecting the same code | symbol.

<1. Rolling equipment>
[1-1. Configuration of rolling equipment]
First, the structure of the rolling equipment 20 which concerns on one Embodiment of this invention is demonstrated based on FIGS. FIG. 1 is a schematic explanatory view showing a rolling facility 20 according to the present embodiment, and shows a case where three rolling mills are installed. FIG. 2 is a schematic explanatory view showing the rolling equipment 20 according to this embodiment, and shows a case where two rolling mills are installed. FIG. 3 includes a work roll (hereinafter also referred to as a “corrugated roll”) having an outer peripheral surface shape that periodically changes in the circumferential direction among the rolling mills disposed in the rolling equipment 20 according to the present embodiment. It is a schematic perspective view which shows an example of a corrugated roll about a rolling mill. FIG. 4 is an explanatory view showing a state in which the corrugated roll of FIG. 3 is applied to the upper work roll and the lower work roll of the four-high rolling mill. FIG. 5 is an explanatory view showing another example of a corrugated roll.

  The rolling equipment 20 according to the present embodiment is equipment provided with a plurality of rolling mills for rolling the steel slab 5 having a substantially rectangular cross-sectional shape. Here, the steel slab 5 having a substantially rectangular cross section refers to a steel slab having a substantially rectangular cross section (cross section) perpendicular to the length direction of the steel slab 5, and generally has a steel slab width W. And a steel slab thickness H ratio (W / H) of approximately 1 or more. For example, as shown in FIG. 1, the rolling equipment 20 according to the present embodiment may include three rolling mills 21, 22, and 23, and includes two rolling mills 24 and 25 as illustrated in FIG. 2. Also good. Further, the rolling mill may be a two-stage rolling mill composed of a pair of work rolls, such as the rolling mills 21, 22, and 23 of FIG. 1, and is composed of a pair of work rolls and a pair of reinforcing rolls. A step rolling mill may be used. Furthermore, the rolling facility 20 may include four or more rolling mills.

  The rolling equipment 20 according to the present embodiment includes at least one of a pair of work rolls in contact with the steel slab 5 in at least one rolling mill excluding the rolling mill arranged at the most downstream side in the conveyance direction of the steel slab 5. On the other hand, a roll having a periodically changing outer peripheral shape is used. Hereinafter, the roll having the periodically changing outer peripheral shape is also referred to as a corrugated roll.

  The outer peripheral surface shape of the corrugated roll is not particularly limited as long as the periodic change of the outer peripheral surface satisfies the requirements of the period and amplitude described later. However, for example, since the two-high rolling mill shown in FIGS. 1 and 2 does not have a reinforcing roll that supports the work roll, the maximum load that the work roll can tolerate is limited. Therefore, when rolling a hard material, the 4-high rolling mill shown in FIG. 7 described later is suitable. However, even in the case of a four-high rolling mill, if the phase, amplitude, and average radius of the periodic shape of the outer peripheral surface of the work roll are constant in the body length direction of the roll, the contact point between the work roll and the reinforcing roll ( Since the distance between the straight line in the lengthwise direction) and the work roll axis varies significantly during rotation, vibrations of the rolling mill become intense and durability of the rolling mill may be reduced. Therefore, when using a corrugated roll as a work roll of a four-high rolling mill, the outer peripheral surface shape of the corrugated roll is set so that the distance between the contact point with the reinforcing roll and the axis of the work roll is substantially constant during rotation. It is desirable to be

  One structural example of a corrugated roll is shown in FIG. The corrugated roll 100 shown in FIG. 3 is used as any one of the upper work roll 21a and the lower work roll 22b of the rolling mill 22 shown in FIG. 1, and the upper work roll 24a and the lower work roll 24b shown in FIG. Is also applicable. For example, as shown in FIG. 3, the corrugated roll 100 may be configured to have a flat portion 111 and a corrugated portion 113 on the outer peripheral surface 110. For example, the flat portion 111 may be provided at both end portions (flat portions 111a and 111c) and the center portion (flat portion 111b) in the body length direction, and the corrugated portions 113a and 113b may be provided between the end portions and the center portion in the body length direction. Good.

  The corrugated portion 113 has an outer peripheral surface radius as a distance from the rotation center of the corrugated roll 100 to the outer peripheral surface, and an average value of the outer peripheral surface radius and the outer peripheral surface radius in the entire periphery (also referred to as “overall average radius”). The circumferential distribution of the difference between them may be formed in a sine wave shape. Alternatively, the circumferential distribution of the difference between the outer peripheral surface radius and the overall average radius is a continuous and periodic shape composed of a straight line and a plurality of line segments including at least an arc or a higher-order function curve. It may be formed as is.

  The radius of the flat portion 111 is the maximum radius of the corrugated portion 113, that is, the distance between the peak of the corrugated mountain formed periodically in the circumferential direction of the outer peripheral surface and the center point of the work roll (hereinafter referred to as “corrugated” This is substantially equivalent to “the maximum radius of the part”. The outer peripheral surface shape of the corrugated roll shown in FIG. 3 is the amplitude of the periodically changing shape formed on the outer peripheral surface in the body length direction of the work roll while fixing the phase of the periodic change of the outer peripheral surface shape. And by changing the average radius, it can be determined geometrically. That is, the wavy portion 113 has a periodically changing outer peripheral surface shape that satisfies the amplitude requirements described later and the average radius requirement that is normally determined from restrictions on the rolling mill design, and the flat portion 111 has an amplitude of zero. Yes, the average radius is set to be the maximum radius of the corrugated portion 113. At the boundary between the flat portion 111 and the corrugated portion 113, it is more preferable to smoothly change the amplitude and average radius of the outer peripheral surface shape that changes periodically in the body length direction of the work roll.

  Thus, the flat part 111 is provided at both ends (111a, 111c) in the trunk length direction and the middle part (111b) in the trunk length direction, and the wavy part 113a is provided between the flat parts 111a, 11b, and the flat parts 111a, 11b. A corrugated portion 113b is provided between the two. Thereby, even if the phase of the periodic shape of the outer peripheral surface of the corrugated roll 100 is constant in the body length direction of the roll, the corrugated roll 100 and the roll contacting the roll are always in contact at a plurality of points. For example, when the corrugated roll 100 shown in FIG. 3 is applied to the upper work roll 24a and the lower work roll 24b of the rolling mill 24 shown in FIG. 2, the result is as shown in FIG. Thereby, for example, the distance between the contact point between the upper work roll 24a, which is a corrugated roll, and the upper reinforcing roll 24c, and the axis of the upper work roll 24a is the radius of the flat portion 111 of the upper work roll 24a (that is, The maximum radius of the wave-like portion 113), and the distance can be made substantially constant during rotation. Further, the distance between the contact point between the lower work roll 24b and the lower reinforcement roll 24d, which is a corrugated roll, and the axis of the lower work roll 24b is the radius of the flat portion 111 of the lower work roll 24b (that is, the wavy portion). 113), and the distance can be made substantially constant during rotation.

  Or the reinforcement which contacts the said corrugated roll 200 by changing the phase of the corrugated part 212 formed in the outer peripheral surface 210 to a trunk | drum length direction like the corrugated roll 200 shown, for example in FIG. During rotation, the roll can always be brought into contact with the wave peaks of the outer peripheral surface 210 at a plurality of locations in the trunk length direction. Thereby, the distance between these contact points and the work roll axis can be made substantially constant at the maximum radius of the waved portion during rotation, and the durability of the rolling mill can be maintained.

  It is sufficient that the corrugated portion 113 of the corrugated roll 100 shown in FIG. 3 is usually formed in accordance with the maximum width of the steel slab 5 to be rolled, and the flat portions 111a and 111c are provided on the end side of the corrugated portion 113. It may be formed. The flat portion 11b at the center of the trunk length may be omitted. Further, the phase of the corrugated portion 212 formed on the corrugated roll 200 shown in FIG. 5 may be a linear or broken line change in the body length direction, or a discontinuous change. It is desirable that the change in the outer peripheral surface 210 of the corrugated roll 200 is symmetrical with respect to the center of the trunk length. Moreover, after changing the phase of the wave shape in the body length direction like the waved roll 200, the amplitude and average radius of the wave shape may be changed to form a flat portion like the waved roll 100. . When a corrugated roll is used in a two-high rolling mill as shown in FIG. 1, the corrugated shape in which the phase, amplitude, and average radius of the corrugated shape formed on the outer peripheral surface are constant in the body length direction It may be formed on the surface.

  In addition, since the outer peripheral surface shape of a corrugated roll is reflected on the surface of the steel slab 5 when rolling with a corrugated roll, it is necessary to finally prepare the surface of the steel slab 5 to be flat. For this reason, the rolling mill disposed at the most downstream side in the conveying direction of the steel slab 5 is provided with a work roll having a flat outer peripheral surface in the vertical direction, like the rolling mill 23 in FIG. 1 and the rolling mill 25 in FIG. .

[1-2. Example of rolling equipment installation]
6 to 8 show installation examples of the rolling equipment 20 according to the present embodiment. FIG. 6 is a schematic explanatory view showing an example in which the rolling equipment 20 shown in FIG. 1 is installed on the downstream side in the casting direction of the continuous casting machine 10. FIG. 7 is a schematic explanatory view showing an example in which the rolling equipment 20 shown in FIG. 2 is installed on the downstream side in the casting direction of the continuous casting machine 10. FIG. 8 is a schematic explanatory view showing an example in which the rolling equipment 20 shown in FIG. 1 is installed on the downstream side in the conveying direction of the steel slab 5 with respect to the scale removing device 40.

(Rolling steel slabs produced by a continuous casting machine)
For example, as shown in FIG. 6 or FIG. 7, the rolling equipment 20 according to the present embodiment is installed on the downstream side in the casting direction of a continuous casting machine 10 that casts a steel piece 5 having a substantially rectangular cross-sectional shape. 10 can be used as equipment for rolling the steel slab 5 manufactured according to No. 10. In addition, the kind and size of the steel piece 5 manufactured with the continuous casting machine 10 are not specifically limited. The slab 5 may be any of slab, billet and bloom, for example. Below, a slab is assumed and demonstrated as an example of the steel slab 5.

  As shown in FIG. 6, the continuous casting machine 10 is an apparatus for continuously casting a molten metal using a casting mold 13 to produce a steel slab 5 such as a slab. The continuous casting machine 10 includes a tundish 11, an immersion nozzle 12, a mold 13, and a secondary cooling device 14.

  The tundish 11 is arrange | positioned above the casting_mold | template 13 and stores the molten metal conveyed by the ladle (not shown). In the tundish 11, the inclusions in the molten metal are removed while the molten metal is stored. An immersion nozzle 12 that supplies molten metal to the mold 13 is provided at the bottom of the tundish 11. The immersion nozzle 12 continuously supplies the molten metal from which inclusions have been removed by the tundish 11 to the mold 13.

  The mold 13 is a mold having a rectangular hollow formed according to the width and thickness of the steel piece 5 to be manufactured. The mold 13 is configured by combining, for example, a mold plate made of four water-cooled copper plates. The molten metal supplied into the mold 13 through the immersion nozzle 12 is cooled by coming into contact with the mold plate, and a solidified shell 5a in which the molten metal is solidified is formed in the outer shell. With the outer shell solidified, the steel slab 5 is pulled out from the mold 13.

  The secondary cooling device 14 is provided on the downstream side in the casting direction with respect to the mold 13, supports the steel piece 5 pulled out from the lower end of the mold 1, and cools it while being conveyed. The secondary cooling device 14 includes a plurality of pairs of support rolls 14 a disposed on both sides in the thickness direction of the steel slab 5, and a plurality of spray nozzles (not shown) that inject cooling water onto the steel slab 5. Have. Although there is an unsolidified portion 5b in the solidified shell 5a of the steel piece 5 immediately after being drawn from the mold 13, solidification of the unsolidified portion 5b inside proceeds while moving the secondary cooling device 14, The thickness of the outer solidified shell 5a gradually increases. When the steel slab 5 is almost completely solidified, it is continuously conveyed from the continuous casting machine 10 to the rolling equipment 20.

  The continuous casting machine 10 according to the present invention is not limited to the vertical bending type continuous casting machine 10 as shown in FIG. 1, and may be various other continuous casting machines such as a curved type or a vertical type.

  The rolling equipment 20 according to the present embodiment is provided downstream of the continuous casting machine 10 in the casting direction. For example, as shown in FIG. 1, a rolling equipment 20 composed of three rolling mills 21, 22, and 23 may be provided on the downstream side in the casting direction of the continuous casting machine 10, and as shown in FIG. You may provide the rolling equipment 20 comprised from these rolling mills 24 and 25. FIG.

(Rolling steel slab heated by heating furnace)
Moreover, the rolling equipment 20 which concerns on this embodiment can be utilized as equipment which rolls the steel piece 5 whose cross-sectional shape heated by the heating furnace 30, for example, as shown in FIG. For example, when the steel slab 5 manufactured by the continuous casting machine 10 or the like is cooled and then rolled, the steel slab 5 placed on the roll 18 and transported is regenerated by the heating furnace 30 as shown in FIG. After the heating, the scale on the surface of the steel slab 5 generated by the heating is removed by the scale removing device 40. The rolling equipment 20 according to the present embodiment is installed on the downstream side in the transport direction of the steel slab 5 with respect to the scale removing device 40 so as to roll the steel slab 5 after the scale removal. About the steel slab which does not require scale removal, you may arrange | position the rolling equipment 20 which concerns on this embodiment in the conveyance direction downstream side of the steel slab 5 with respect to the heating furnace 30. FIG. Alternatively, a cooling device including forced air cooling, mist cooling, and forced water cooling is interposed between the heating furnace 30 and the steel slab 5 having a desired surface temperature is rolled to the steel slab 5 with respect to the cooling device. You may arrange | position the rolling equipment 20 which concerns on this embodiment in the conveyance direction downstream.

  In addition, although the rolling equipment 20 shown in FIG. 2 was applied to FIG. 8, you may install the rolling equipment 20 shown in FIG. The rolling mill constituting the rolling equipment 20 may be a two-high rolling mill as shown in FIG. 8 or a four-high rolling mill.

<2. Improvement of mechanical properties by corrugated roll>
In the rolling equipment 20 according to the present embodiment, as shown in FIG. 1 and FIG. 2, at least one rolling mill excluding the rolling mill arranged at the most downstream in the conveying direction of the steel slab 5, the steel slab 5 A rolling roll having a periodically changing outer peripheral surface shape (hereinafter also referred to as a “corrugated roll”) is used as at least one of the pair of work rolls that come into contact with each other. By using corrugated rolls, deformation conditions such as stress and strain inside the material during rolling can be significantly and intentionally changed under the same steel slab reduction conditions without changing the steel slab thickness. It is possible to improve the billet quality. That is, an increase in hydrostatic pressure stress at the center of thickness is effective for porosity compression, and an increase in plastic strain leads to improved segregation and refinement of crystal grains. Hereafter, the effect | action which arises by using a corrugated roll as shown in FIG.3 and FIG.5 for a work roll, and the specific conditions of the outer peripheral surface shape of a corrugated roll are demonstrated in detail.

[2-1. Action by corrugated roll]
First, based on FIGS. 9-12, the reason why it rolls using a corrugated roll in the rolling equipment 20 which concerns on this embodiment is demonstrated. FIG. 9 shows the state (stress) of the plastic deformation inside the steel slab under and near the contact area with the work roll (hereinafter referred to as “roll bite”) when the steel slab 5 is rolled using a corrugated roll. It is a conceptual diagram which shows a plastic strain and a metal flow. FIG. 10 shows the magnitude of hydrostatic pressure stress generated at the center of the thickness of the steel slab, the distribution of the roll bite and the vicinity thereof, and the maximum value of hydrostatic pressure stress (positive on the compression side) for corrugated rolls and flat rolls. It is explanatory drawing which shows the increment value (henceforth a "hydrostatic pressure increment value") at the time of using a corrugated roll with respect to the case where a flat roll is used. FIG. 11 shows the first pass rolled by the corrugated roll or the upper and lower flat rolls shown in FIG. 9, and the second pass rolled the steel slab after the first pass rolling by the upper and lower flat rolls provided on the most downstream side in the conveying direction. It is explanatory drawing which shows the length direction distribution of the plastic strain (strain amount which arose in each pass) in the thickness center position of the steel slab after each pass rolling, and a 1/4 thickness position from the surface layer. FIG. 12 shows the total strain value obtained by summing the plastic strains in the first pass and the second pass shown in FIG. 11, the average value in the length direction, and the length direction average value of the total strain values in the first pass. It is explanatory drawing which shows the increment value at the time of using the roll with a wave with respect to the case where a roll is used (henceforth "total strain increment value"). In addition, in FIG. 9, the case where both of a pair of work rolls which roll the steel slab 5 are made into a corrugated roll (that is, the case where a steel slab deform | transforms up and down symmetrically) is considered.

  When the steel slab 5 is rolled by the work roll 26 which is a corrugated roll, the deformation state in the vicinity of the surface of the steel slab varies significantly in the length direction of the steel slab due to the wave shape of the outer peripheral surface of the work roll 26, Propagates to the center of the thickness of the piece. That is, as shown in FIG. 6, wave-shaped convex portions 26 a in which the radius of the work roll 26 is increased are periodically formed on the outer peripheral surface of the work roll 26. When this convex part 26a presses the steel slab 5, the pressing force applied to the steel slab 5 from the work roll 26 increases, and the plastic strain of the steel slab 5 increases. In FIG. 9, a portion where a large plastic strain is generated by the convex portion 26a is shown as a deformation increasing portion 5c. The deformation increasing portion 5c is generally similar to the portion where the hydrostatic pressure stress increases.

As shown by the solid line in FIG. 10, the rolling direction distribution of the hydrostatic stress at the thickness center of the steel slab when the steel slab 5 is pressed using a flat roll is shown in the contact area between the work roll 26 and the steel slab 5. The maximum compressive stress value σ ^peak _½ is shown in the vicinity of the exit from the substantially central portion. Moreover, the broken line of FIG. 10 has shown the rolling direction distribution of the hydrostatic pressure stress at the time of using the work roll 26, and here, the deformation | transformation increase part 5c in FIG. 9 is the said position (the rolling direction where hydrostatic pressure stress becomes the maximum). The distribution when passing through (position) is displayed. At this time, since the outer peripheral surface of the work roll 26 has a wave shape, the influence of the wave-shaped convex portion 26a propagates through the region of the deformation increasing portion 5c to reach the center of the thickness, and the peak value of the hydrostatic pressure stress Is larger by dσ ^peak _{½ than} when a flat roll is used. From this, it is understood that the hydrostatic stress can be increased by using the corrugated roll as compared with the case of using the flat roll.

  The deformation increasing portion 5 c is periodically generated in the rotation direction of the work roll 26, that is, in the length direction of the steel slab 5. When the deformation increasing portion 5c and other portions coexist in the same roll bite, a hydrostatic stress gradient is generated, and a secondary metal flow from the deformation increasing portion 5c toward another portion is generated. Compared with the case where the outer peripheral surface of the work roll is flat and the plastic deformation occurs uniformly as a result of repeated appearance of a large plastic deformation portion and a small plastic deformation portion in a roll bit having a length substantially equal to the contact length. Metal flow occurs and plastic strain increases. Therefore, the plastic strain can be increased more effectively by using the corrugated roll.

  The distribution of the plastic strain in the length direction of the steel slab 5 after rolling when rolling by the work roll 26 which is a corrugated roll shown in FIG. 9 is shown on the upper side of FIG. 11 (first pass). As shown in the upper side of FIG. 11, the plastic strain periodically changes into a substantially sinusoidal shape at both the center position in the thickness direction of the steel slab 5 and the 1/4 thickness position by rolling with a corrugated roll. The fluctuation range of the plastic strain at the central position in the thickness direction is equivalent to the amount of periodic fluctuation of the roll gap caused by the upper and lower corrugated rolls (that is, approximately twice the amplitude of the periodically changing outer peripheral surface shape of the corrugated rolls). ) Is roughly equivalent to the fluctuation range of the rolling strain calculated by dividing the steel piece thickness. At this time, it is closer to the steel slab surface than the thickness center, and is a position from the steel slab surface toward the thickness center by ¼ of the steel slab thickness (hereinafter also referred to as “¼ thickness position”). ) Has a larger amplitude than the center position. Further, the phase at the center position is delayed from the phase at the 1/4 thickness position. As shown in FIG. 6, this phase lag is such that the vicinity of the surface of the deformation increasing portion 5 c is positioned upstream of the thickness central portion in the rolling direction, and the deformation increasing portion 5 c is inclined with respect to the thickness direction. It agrees with

In addition, the upper side of FIG. 11 shows the plastic strain of the steel slab 5 when a flat roll having a flat outer peripheral surface is used as the upper and lower work rolls for rolling the steel slab 5 as a comparison. The amount of reduction when a flat roll was used was substantially the same as the average amount of reduction averaged in the length direction when rolled by a corrugated roll. At this time, the plastic strain increases in the most part in the length direction by using a corrugated roll for both the central position in the thickness direction of the steel slab 5 and the 1/4 thickness position. It can be seen that the strain value also increases. In FIG. 11, the increment amount of the plastic strain when using the corrugated roll from the plastic strain when using the flat roll at the center position and the 1/4 thickness position in the thickness direction of the steel slab 5 (hereinafter, The peak values of “plastic strain increments”) are represented as peak strain increments dε ^peak _1/2 and dε ^peak _1/4 , respectively.

  In the first pass, when the steel slab 5 rolled with the corrugated roll shown in FIG. 9 is rolled with the upper and lower flat roll rolling mill in the next pass (second pass), The lengthwise distribution of the plastic strain of the steel slab 5 is as shown in the lower side of FIG. The second pass shown in the lower part of FIG. 11 has a larger amplitude than the first pass shown in the upper part of FIG. 11, and the cyclic fluctuation in the length direction of the plastic strain generated in the second pass is increased. To do. This is because the thickness of the steel slab 5 is thinner than that in the first pass, and the periodic fluctuation amount of the thickness formed (transferred) on the steel slab by rolling with a corrugated roll in the first pass (first pass). This is because the divisor becomes smaller when converting the amount of fluctuation of the roll gap in the second pass) into the amount of fluctuation in the second pass rolling strain.

  As a comparison, let us consider a case where the average reduction amount averaged in the length direction is substantially the same as the case where the corrugated roll is used, and a flat roll having a flat outer peripheral surface is used for the upper and lower work rolls in the first pass. At this time, in the second pass, the steel slab 5 is rolled using flat rolls as upper and lower work rolls. Note that the roll gap condition in the second pass is the same as the rolling condition in the second pass in which the steel slab 5 using the corrugated roll is rolled in the first pass. By using a corrugated roll in the first pass, the plastic strain of the steel slab 5 when rolled with a flat roll is as shown in the lower side of FIG. As with the plastic strain distribution in the first pass, the plastic strain generated in the second pass rolling increases in the majority of the length direction and the average strain value also increases at any thickness position. You can see that

  In addition, when a corrugated roll is used in the first pass, the lengthwise distribution of plastic strain in the first pass and the second pass for both the thickness direction central position and the quarter thickness position of the steel slab 5 You can see the relationship that the phase is almost reversed. This is because the thickness of the steel slab 5 after the first pass rolling at the portion where the strain increased in the first pass (the plastic deformation increasing portion 5c in FIG. 9) is thinner than the other portions. This is because the substantial reduction strain of the part in the rolling with the flat roll of the eyes is reduced.

The total strain amount obtained by adding the plastic strain generated in the steel slab 5 in the first pass after the corrugated roll rolling shown in FIG. 11 and the plastic strain generated in the second pass after the flat roll rolling is shown in FIG. As shown in FIG. The one-dot chain line in FIG. 12 is the average value in the length direction of the total strain amount at both the central position in the thickness direction of the steel piece 5 and the 1/4 thickness position (hereinafter referred to as “total strain average value”). Indicates. In FIG. 12, the total strain average value when using the corrugated roll from the plastic strain when using the flat roll in the first pass for the center position and the quarter thickness position of the steel piece 5 in the thickness direction. Are expressed as total strain increments Σdε _1/2 and Σdε _1/4 , respectively. It can be seen that the total strain increments Σdε _1/2 and Σdε _1/4 are increased by about 5% compared to the case where the rolls are rolled only by the flat roll. From the above, by rolling using a corrugated roll, the plastic strain can be effectively increased even in the central portion of the thickness of the steel slab 5, and as a result, center segregation can be improved. When rolling with a corrugated roll is performed in the first pass, a slight change in plastic strain remains along the length direction of the steel slab 5 even when rolling with a flat roll is performed in the second pass. However, this variation in the length direction of the plastic strain is sufficiently detoxified in the subsequent process and does not affect the quality.

[2-2. Wave outer peripheral shape (basic shape)]
In order to realize an increase in the plastic strain of the steel slab 5 by using a corrugated roll as described above, it is necessary to appropriately provide periodic changes in the outer peripheral surface. As a result of studying this condition, the inventor of the present application has found that the outer peripheral surface of the corrugated roll has a period of 1/50 to 2 times the contact arc length and an amplitude of 1/30 to 1/3 times the rolling reduction. By adopting a shape that changes in the following cycle, the plastic strain inside the steel slab can be significantly increased over the entire thickness direction under the same steel slab reduction condition. It was found that not only the effect of compressing porosity and center segregation but also microsegregation due to the formation of columnar crystals near the surface layer of the steel slab were improved.

  Hereinafter, the wave period and amplitude formed on the outer peripheral surface of the corrugated roll will be described with reference to FIGS. FIG. 13 is an explanatory diagram showing the relationship between the waveform period formed on the outer peripheral surface of the corrugated roll, the peak strain increment, and the total strain increment. FIG. 14 is an explanatory diagram showing the relationship between the period of the wave shape formed on the outer peripheral surface of the corrugated roll and the amount of incremental hydrostatic stress. FIG. 15 is an explanatory diagram showing the relationship between the amplitude of the wave shape formed on the outer peripheral surface of the corrugated roll and the total strain increment. FIG. 16 is an explanatory diagram showing the relationship between the amplitude of the wave shape formed on the outer peripheral surface of the corrugated roll and the amount of incremental hydrostatic stress. In FIGS. 13 and 14, as an index corresponding to the wave shape period of the corrugated roll, the horizontal axis represents the ratio of the contact arc length between the roll and the steel piece to the wave shape period of the corrugated roll (contact Arc length / waveform period).

(Cycle of wave roll)
First, the period of the wave shape formed on the outer peripheral surface of the corrugated roll is at least 1/50 times the contact arc length (corresponding to 50 or less in the horizontal axis range in FIG. 13) or less (in FIG. This corresponds to an axial range of 0.5 or more.) As shown in FIG. 13, the relationship between the period of the wave shape and the plastic strain increment is the peak strain increment dε ^{peak at} the center position and the 1/4 thickness position in the thickness direction of the steel slab 5 shown in FIG. Each of _1/2 , dε ^peak _1/4 and the combined strain increments Σdε _1/2 , Σdε _1/4 shown in FIG. 12 has a convex curve. Here, on the basis of the plastic strain when rolled using a flat roll, considering that the rolling strain in normal rolling is on the order of 0.1, an increment of plastic strain of 0.01 (ie, 1%) or more If so, the effect is considered to be substantially significant. From FIG. 13, the peak strain increments dε ^peak _1/2 and dε ^peak _1/4 and the total strain increments Σdε _1/2 and Σdε _{1 for} the center position and the 1/4 thickness position in the thickness direction of the steel slab 5. _{/ 4} plastic strain increment exceeds 0.01, the ratio between the contact arc length of the roll and the steel slab and the period of the wave shape of the corrugated roll is 0.5 to 50, that is, the wave The period of the wave shape of the attached roll is in the range of 1/50 to 2 times the contact arc length.

  If the ratio of the contact arc length between the roll and the steel slab and the period of the wave shape is smaller than 0.5, the wave period of the corrugated roll is too long, and the change in the roll gap within the contact arc length becomes small. That is, the change in the rolling strain in the roll bite is reduced, and the gradient of the hydrostatic pressure stress between the above-described plastic deformation increasing portion 5c and the other portions becomes excessively small. For this reason, the amount of increase in plastic strain generated by rolling with a corrugated roll is reduced through a reduction in secondary metal flow, and a sufficient effect cannot be obtained.

  On the other hand, if the ratio of the contact arc length between the roll and the steel slab to the waveform period is greater than 50, the waveform period of the corrugated roll becomes very short. Strictly speaking, the propagation from the surface of the steel slab of the increased plastic deformation portion 5c shown in FIG. 9 to the center of the thickness is not uniformly transmitted in the thickness direction, but the work hardening characteristics and strain rate in the plastic deformation of the steel slab. Due to the influence of the dependency, the width of the plastic deformation increasing portion 5c gradually increases toward the center of the thickness, and the gradient of the hydrostatic stress (rolling direction) decreases. Therefore, if the wave-shaped period, that is, the gap between the plastic deformation increasing portions 5c in the roll bite becomes too short, the difference between the plastic deformation increasing portion 5c and the other portions, which is clear near the surface of the steel slab, is the thickness center. In this case, it becomes unclear, the secondary metal flow at the center of the thickness disappears, and it is considered that a sufficient plastic strain increment at the center of the thickness of the steel slab 5 does not occur.

  Therefore, in order to significantly increase the plastic strain, it is preferable that the wave-shaped period of the corrugated roll is in the range of 1/50 to 2 times the contact arc length. In addition, as shown in FIG. 14, the effect of increasing the hydrostatic stress is such that the ratio of the contact arc length between the roll and the steel piece to the period of the wave shape is in the range of 0.5 to 50, that is, the wave shape of the corrugated roll. Is within the range of 1/50 times or more and 2 times or less of the contact arc length, a hydrostatic stress increment of 30 MPa or more is always obtained. A practically sufficient effect can be enjoyed.

(Amplitude of wave roll)
Next, the amplitude of the wave shape formed on the outer peripheral surface of the corrugated roll is 1/30 to 1/3 times the amount of reduction. As shown in FIG. 15, the relationship between the amplitude of the wave shape and the plastic strain increment is the sum of the strain increments Σdε _{1/2 at} the center position and the quarter thickness position of the steel piece 5 shown in FIG. ₂ and Σdε1 _{/ 4} , the plastic strain increment increases as the amplitude increases. Here, if the increment of the plastic strain is 0.01 (that is, 1%) or more based on the plastic strain when rolled using a flat roll, the effect is considered to be substantially significant as described above. . From FIG. 15, the range in which the total strain increments Σdε _1/2 and Σdε _{1/4 at} the central position and the 1/4 thickness position in the thickness direction of the steel slab 5 exceed 0.01 is the wave of the corrugated roll. The amplitude of the shape is in the range of 1/30 times or more.

  When the wave shape amplitude of the corrugated roll exceeds 1/3 times the reduction amount, lap-like defects frequently occur on the surface of the rolled steel slab, so the wave shape amplitude is 1/3 of the reduction amount. Must be less than double. On the other hand, when the amplitude of the wave shape of the corrugated roll is less than 1/30 times the amount of reduction, the fluctuation of the wave shape, that is, the roll gap becomes too small, and the fluctuation of the reduction strain hardly occurs. This is synonymous with a decrease in the difference in strain and hydrostatic pressure stress between the plastic deformation increasing portion 5c shown in FIG. 9 and other portions, and a secondary increase in metal flow and an increase in hydrostatic pressure stress. The effect of the periodic change of the corrugated roll can hardly be obtained. Therefore, in order to significantly develop the effect of the corrugated roll and not cause a quality defect of a new steel slab, the amplitude of the corrugated roll wave shape is not less than 1/30 times and not more than 1/3 times the reduction amount. It is better to be in the range. As shown in FIG. 16, the effect of increasing the hydrostatic pressure stress is that if the amplitude of the wave shape of the corrugated roll is within the range of 1/30 times to 1/3 times the reduction amount, the amount of increase in the hydrostatic pressure stress is Since the pressure is always 10 MPa or more, a practically sufficient effect can be obtained with respect to porosity pressure bonding.

[2-3. Outer surface shape of corrugated roll (shape setting according to the material to be rolled)]
In the rolling equipment 20 which concerns on this embodiment, the shape of the corrugated roll provided in the at least 1 or more rolling mill except the rolling mill arrange | positioned at least in the conveyance direction downstream of the steel slab 5 is used as the corrugated roll. The shape of the outer peripheral surface of the electrode may change in a period of 1/50 to 2 times the contact arc length and an amplitude of 1/30 to 1/3 times the rolling reduction. However, it is desirable to consider a more appropriate outer peripheral shape according to the characteristics of the steel piece 5 to be manufactured. Below, the setting of the outer peripheral surface shape of a corrugated roll according to the characteristic of the steel slab 5 which is a to-be-rolled material by the rolling equipment 20 is demonstrated.

(A) In the case of a rolled material with poor surface ductility For example, when a rolled material with poor surface ductility is rolled by a corrugated roll, excessive tensile stress is generated on the surface of the rolled material, and surface cracking occurs. There is concern about the occurrence. The mechanism will be described with reference to FIGS. In addition, FIG. 17 is explanatory drawing explaining the definition of the outer peripheral surface shape of a corrugated roll, and its gradient angle. FIG. 18 is an explanatory diagram for explaining the limiting condition of the upward gradient angle of the outer peripheral surface shape of the corrugated roll.

First, in the description, as shown in FIG. 17, the roll rotation direction distribution of the outer peripheral surface radius of the outer peripheral surface shape of the corrugated roll is defined as R (x), and the average of the entire periphery is defined as the average average radius R _ave . x represents the rotation direction coordinate of the corrugated roll. From this, the outer peripheral surface shape of the corrugated roll can be represented by the difference dR (= R (x) −R _ave ) between the entire average radius R _ave and the outer peripheral surface radius R (x). Further, in the outer peripheral surface shape of the corrugated roll, the inclination angle of the portion where the outer peripheral surface radius increases in the rotational direction is the upward gradient (angle) θ _U, and the inclination angle of the portion where the outer peripheral surface radius decreases in the rotational direction is the downward inclination. and slope (angle) theta _L. The upward gradient θ _U has a positive value, and the downward gradient θ _L has a negative value.

In FIG. 18, the deformation | transformation state in the roll bite exit vicinity of the steel slab 5 rolled with the corrugated roll 26 is shown. As shown in FIG. 9, the steel slab 5 is greatly deformed from the vicinity of its surface due to the periodically changing outer peripheral surface shape of the corrugated roll 26, and the entire steel slab extends in the length direction. Here, as shown in FIG. 18, when the ascending slope of the outer peripheral surface shape of the corrugated roll 26 reaches the vicinity of the roll bite outlet, the surface of the steel slab 5 is changed from the ascending slope to the slope. A vertical rolling force P and a frictional force (= μP, where μ is a friction coefficient) acting in the tangential direction of the inclined surface with roll rotation act. As apparent from FIG. 18, the rolling direction component force (= −P · sin θ _U ) of the rolling force P acts on the surface of the steel slab 5 in the backward direction and the frictional force (μP). The component force in the rolling direction (= μP · cos θ _U ) acts in the forward direction. At this time, when the absolute value of the rolling direction component force of the rolling force P becomes larger than the absolute value of the rolling direction component force of the friction force (μP), the steel slab 5 is about to exit the roll bite while being advanced in the rolling direction. A force (F) for pushing back in the rolling direction from the uphill slope is applied. As a result, since the material in the vicinity of the surface in contact with the inclined surface starts to recede, there is a concern that a tensile stress is generated in the material in the vicinity of the rear portion of the part and a surface crack defect is generated. Surface cracking is particularly a concern when rolling a material to be rolled that has poor surface ductility.

  Therefore, the corrugated roll 26 is corrugated so that the material in the vicinity of the surface in contact with the slope of the corrugated roll 26 does not recede, that is, a significant pushing force is not generated between the slope of the corrugated roll 26 and the surface of the steel slab 5. The outer peripheral surface shape of the roll 26 is determined. The pushing-back force F generated by the upward slope is expressed by the following formula (1). Note that P is a rolling force by the corrugated roll 26, and μ is a friction coefficient.

F = P (tan θ _U −μ) × cos θ _U (1)

If the push-back F is smaller than 0, the push-back force does not act between the inclined surface of the corrugated roll 26 and the surface of the steel slab 5, and a force to be sent forward can be generated. Therefore, the upward gradient θ _U may be set so that the right side of the expression (1) has a negative value, and tan θ _U may be set to be equal to or less than the friction coefficient μ. At this time, for example, the maximum ascending slope angle at the portion that becomes the ascending slope may be set to be equal to or less than the friction angle θ _f . Here, the friction angle θ _f is an angle satisfying μ = tan θ _f .

(B) In the case of a material to be rolled whose surface unevenness at the end of the continuous casting machine is remarkable. For example, when the rolling equipment 20 is arranged downstream in the casting direction of the continuous casting machine 10 as shown in FIG. In the end of the continuous casting machine, the surface unevenness of the material to be rolled becomes large. When rolling such a material to be rolled, there is a concern about the generation of surface wrinkles. The mechanism will be described with reference to FIG. FIG. 19 is an explanatory diagram for explaining a limiting condition for the downward gradient angle of the outer peripheral surface shape of the corrugated roll.

In FIG. 19, the state of the steel piece 5 bitten by the corrugated roll 26 at the rolling mill entry side is shown. As shown in FIG. 19, when the descending slope of the outer peripheral shape of the corrugated roll 26 reaches the vicinity of the roll bite entrance, the outer peripheral surface of the corrugated roll 26 is in the roll rotation direction (ie, the vertical direction of the roll radial direction). On the other hand, it inclines in the direction in contact with the steel piece 5 by the amount corresponding to the downward gradient. Beyond this downward slope theta angle theta _B absolute value biting of _L, the top near the slopes 26 _L of the downward slope is in contact with the steel strip surface prior to the slope 26 _L a descending slope, in contact with the steel strip surface region A non-contact region (free surface) V sandwiched between the two is generated. Incidentally, biting angle theta _B may be defined as the angle which satisfies example the following equation (2). Δh is a nominal (average) reduction amount.

R _ave (1-cos θ _B ) = Δh / 2 (2)

  In the contact area where the pressure is high, the material of the steel slab 5 tends to extend (flow) toward the non-contact area V. For this reason, as shown in FIG. 19, the free surface of the material in the non-contact region V sandwiched between the contact regions between the two corrugated rolls 26 and the steel slab 5 is compressed in the rolling direction, that is, the tangential direction of the free surface. The As a result, the risk of wrinkle wrinkles or wrap wrinkles increases. In general, the surface of the continuous cast slab has irregularities such as oscillation marks, and when this is subjected to significant compressive deformation in the tangential direction of the free surface in the non-contact region V, there is a concern that a lap flaw defect may occur.

Therefore, the outer peripheral surface shape of the corrugated rolls 26, by the smaller than the angle theta _B narrowing the absolute value of the downward slope angle theta _L chewed, no non-contact region V, the outer peripheral surface of the corrugated rolls 26 It is always possible to make contact in one continuous area on the surface of the billet 5. At this time, for example, it is sufficient that the maximum downward gradient angle at the portion where the downward gradient is provided is smaller than the biting angle θ _B calculated by the above equation (2).

[2-4. About the temperature of the billet rolled by the corrugated roll]
For example, as shown in FIGS. 3 to 5, the rolling equipment 20 according to the present embodiment is disposed on the downstream side in the casting direction of the continuous casting machine 10 and on the downstream side in the conveyance direction of the steel slab 5 with respect to the scale removing device 40. The In this case, the steel slab 5 rolled by the rolling mill is immediately after complete solidification or after heating, and there is a large temperature difference between the surface and the center in the thickness direction of the steel slab 5. The larger the temperature difference between the slab surface and the thickness center (relatively low surface temperature), the easier the deformation state near the slab surface harder than the thickness center is transmitted in the thickness direction. Variations in plastic strain and hydrostatic stress occur remarkably. That is, by rolling the steel slab 5 having a temperature difference of a predetermined value or more in the thickness direction with a corrugated roll, the effect of increasing the above-described plastic strain and hydrostatic pressure stress inside the steel slab can be further enhanced. This is because not only the effect of pressing the porosity near the center of the steel piece 5 in the thickness direction and the improvement of the center segregation but also the microsegregation due to the formation of columnar crystals in the vicinity of the surface layer of the steel piece 5 are further improved. Specifically, the temperature difference between the surface temperature of the steel slab 5 in the thickness direction and the temperature at the center of the thickness (hereinafter referred to as “temperature difference in the thickness direction” (= (temperature at the thickness center) − (surface temperature)). It is also effective to carry out rolling by a rolling mill equipped with a corrugated roll in a rolling pass in which the temperature is 20 ° C. or higher.

  Based on FIG.20 and FIG.21, about the reason why it is effective to use a corrugated roll when the plate thickness direction temperature difference of the steel slab 5 is 20 ° C. or more, the strain increment amount and hydrostatic pressure stress at the plate thickness center. This will be described based on the increment amount. FIG. 20 is an explanatory diagram showing the relationship between the plate thickness direction temperature difference of the steel slab 5 and the plate thickness central strain increment. FIG. 21 is an explanatory diagram showing the relationship between the plate thickness direction temperature difference of the steel slab 5 and the hydrostatic stress increment.

First, the strain when the temperature difference in the thickness direction is changed with reference to the strain increment Σdε _1/2 at the center position in the thickness direction of the steel slab 5 when the temperature difference in the thickness direction of the steel slab 5 is zero. The relative ratio of the incremental amount Σdε _1/2 was examined. As a result, as shown in FIG. 20, when the temperature difference in the plate thickness direction increases in the positive direction (that is, when the temperature at the center of the plate thickness becomes higher than the surface temperature of the steel slab 5), the strain increment Σdε _{1 /} The relative ratio of ₂ also increased. Here, if the relative ratio of the strain increment Σdε1 _{/ 2} is 1.1 or more, the plastic strain inside the steel slab can be significantly increased over the entire thickness direction under the same steel slab reduction condition. it is conceivable that. From FIG. 20, if the plate thickness direction temperature difference is 20 ° C. or more, the relative ratio of the strain increments Σdε _1/2 can be 1.1 or more. It is good to arrange | position the rolling mill which has a corrugated roll in the position used as ° C or more.

Further, from the viewpoint of hydrostatic pressure stress, based on the hydrostatic stress increment dσ ^peak _1/2 at the center position in the thickness direction of the steel slab 5 when the plate thickness direction temperature difference of the steel slab 5 is 0, As a result of investigating the relative ratio of the hydrostatic stress increment dσ ^peak _1/2 when the plate thickness direction temperature difference is changed, as shown in FIG. 21, when the plate thickness direction temperature difference increases in the positive direction (ie, steel When the temperature at the center of the plate thickness is higher than the surface temperature of the piece 5), the relative ratio of the hydrostatic stress increment dσ ^peak _1/2 also increased. From FIG. 21, when the plate thickness direction temperature difference of the steel slab 5 is 20 ° C. or more, the relative ratio of the hydrostatic stress increment dσ ^peak _1/2 is 1.1 or more. From this, it is good to arrange | position the rolling mill which has a corrugated roll in the position where the plate thickness direction temperature difference of the steel slab 5 becomes 20 degreeC or more.

  In Example 1, two rolling mills were installed at the end of a continuous casting machine capable of casting a slab having a thickness of 100 mm and a width of 600 mm, and a continuous casting and rolling test was performed. The components of the molten steel were C: 0.11%, Si: 0.31%, Mn: 1.88%, P: 0.016%, S: 0.0008%, Al: 0.007%. 0%, N: 0.0044%, and O: 0.0011%. The molten steel temperature was 1570 ° C., and casting was performed at a speed of 1.2 m / min. The slab surface temperature on the inlet side of the first rolling mill when the present technology was applied was 1050 ° C.

  All of the installed rolling mills were two-stage rolling mills, and the work roll diameter was φ600 mm. The reduction amount in both rolling mills was 20 mm. However, when the technique of the present invention was applied, the amount of reduction was defined by the difference between the sheet thickness averaged in the length direction before and after rolling. In the application example of the technology of the present invention, a sinusoidal profile having a period of about 10 mm and an amplitude of 6 mm was processed in the circumferential direction of the work roll. The period corresponds to about 1/8 times the contact arc length, and the amplitude corresponds to 3/10 times the reduction amount. In the roll body length direction, the same phase, that is, the sinusoidal groove processing parallel to the roll body length direction was performed.

  In Example A to which the present invention was applied, the upper work roll of the first rolling mill provided upstream in the casting direction was a corrugated roll, and the lower work roll was a flat roll. In Example B to which the present invention is applied, both the upper work roll and the lower work roll of the first rolling mill provided upstream in the casting direction are wavy rolls. In the second rolling mill, flat rolls were used for both the upper work roll and the lower work roll in both Examples A and B.

  On the other hand, in Comparative Example a to which the prior art is applied, flat rolls were used for the upper work roll and the lower work roll in both of the rolling mills. Further, as Comparative Example b, a case where no reduction is performed is shown. Furthermore, in Comparative Examples c, d, and e, both the upper work roll and the lower work roll of the first rolling mill use corrugated rolls having outer peripheral surface shapes outside the scope of the present invention, and the second rolling mill The upper work roll and the lower work roll were both rolled using a flat roll. In comparative example c, the period is about 1 mm (about 1/80 times the contact arc length), the amplitude is about 0.5 mm (about 1/40 times the reduction amount), and in comparative example d, the period is about 10 mm (about 1 / the contact arc length). 8 times), amplitude about 8 mm (about 1 / 2.5 times the amount of reduction), comparative example e, period about 250 mm (about 3 times the contact arc length), amplitude about 6 mm (about 3/10 times the amount of reduction) Was processed in the circumferential direction of the work roll. Similar to Examples A and B to which the present invention was applied, sinusoidal grooves were connected in the same direction in the roll body length direction, that is, parallel to the roll body length direction.

  Table 1 shows the rolling results of Example 1. In Table 1, λ0, d0, and A0 are the primary arm spacing, the average crystal grain size (all in the thickness direction) of the dendrite structure in Comparative Example b, and the porosity ratio remaining in the center of the thickness. That is, λ / λ0, d / d0, and A / A0 represent the reduction rates of the primary arm interval, crystal grain size, and porosity space factor, respectively, and the smaller the value, the better the improvement in the internal quality of the slab. Represents high. In the remarks column, the surface properties of the rolled material after rolling and cooling are described.

  As shown in Table 1, in Examples A and B, the primary arm spacing, average crystal grain size, and porosity space factor of the dendrite structure were significantly reduced as compared with Comparative Example a. Moreover, in the comparative example c and the comparative example e, the improvement effect of the internal quality of a slab is not remarkable, and it is clear that the problem of a surface defect arises in the comparative example d. From this, it is clear that the application of the present invention significantly improves the segregation, crystal structure and internal defects of the steel slab delivered to the subsequent process without causing the problem of surface defects.

  In Example 2, two rolling mills were installed at the end of a continuous casting machine capable of casting a slab having a thickness of 100 mm and a width of 600 mm, and a continuous casting and rolling test was performed. The components of the molten steel are: C: 0.06%, Si: 0.11%, Mn: 1.27%, P: 0.016%, S: 0.0030%, Al: 0.880%, N: 0 .0024%, O: 0.0021%. The molten steel temperature was 1570 ° C., and casting was performed at a speed of 1.2 m / min. The slab surface temperature on the entrance side of the first rolling mill when the present technology was applied was 1020 ° C.

  All of the installed rolling mills were two-stage rolling mills, and the work roll diameter was φ800 mm. The reduction amount in both rolling mills was 40 mm. However, when the technique of the present invention was applied, the amount of reduction was defined by the difference between the sheet thickness averaged in the length direction before and after rolling.

  In Examples C and D to which the technology of the present invention is applied, the upper work roll and the lower work roll are both wavy rolls in the first rolling mill provided on the upstream side in the casting direction. The second rolling mill provided on the downstream side in the casting direction was provided with flat rolls for both the upper work roll and the lower work roll. The corrugated roll used in Examples C and D has a periodically changing outer peripheral shape with a period of about 30 mm and an amplitude of about 6 mm, and has the same phase in the roll cylinder length direction, that is, in the roll cylinder length direction. Grooves connected in parallel were performed. The period corresponds to 1/4 times the contact arc length, and the amplitude substantially corresponds to 1/7 times the amount of rolling reduction.

In Example C, the outer peripheral surface shape was determined using a sinusoidal periodic function. The maximum value of the upward gradient angle at this time was about 32 °. In Example D, a periodic function is used by using a piecewise function in which straight lines having upward and downward inclination angles of about 22 ° and about −22 ° are connected by circular arcs having a curvature radius of 5 mm. The outer peripheral surface shape was determined and processed in the circumferential direction of the work roll. The upward slope angle (about 22 °) in Example D is estimated by back calculation from the results of continuous casting and rolling tests using flat rolls as upper and lower work rolls, the friction coefficient μ is 0.45, and the friction angle θ _f (θ _f = The friction angle θ _f obtained by conversion to tan ⁻¹ μ), that is, an angle not exceeding 24.2 ° was selected.

  On the other hand, in Comparative Example f to which the prior art is applied, flat rolls were used for the upper work roll and the lower work roll in both of the rolling mills. Moreover, the case where no reduction was performed is shown as Comparative Example g.

  Table 2 shows the rolling results of Example 2. In order to compare the internal quality, λ0, d0 and A0 in Table 2 are the same as in Example 1 above, the primary arm interval of dendritic structure in Comparative Example g, the average crystal grain size (all average values in the thickness direction) ) And the space factor of the porosity remaining in the center of the thickness, the primary arm spacing improvement rate (λ / λ0), the crystal grain size improvement rate (d / d0), and the porosity space rate reduction rate (A / A0) was used to evaluate and compare the improvement in the internal quality of the steel slab. Moreover, in order to confirm the influence on the surface quality of the steel slab over a downstream process, the observation result of the steel slab surface by visual observation on the exit side of two rolling mills was also compared.

  As shown in Table 2, with respect to the internal quality of the steel slab, in the same manner as in Example 1, in Examples C and D, compared with Comparative Example f, the primary arm spacing, average crystal grain size, and porosity of the dendrite structure Both of the occupancy rates decreased significantly. From this, it can be seen that the segregation, crystal structure, and internal defects of the steel slab passed to the subsequent process are remarkably improved.

  In addition, as for the surface quality of the steel slab, shallow surface cracks were observed in Example C in which the ascending angle of the outer peripheral surface shape was large, but this was improved in Example D and was as smooth as that in Comparative Example f. It became. However, the shallow surface cracks observed in Example C are only removed by the scale growth and descaling processes in the subsequent processes, and the surface quality of the final product is similar to that of the as-cast surface in Comparative Example g. Has been confirmed not to cause any problems. Although this shallow surface crack was not observed in Example 1, it is interpreted that this is due to an increase in (non-ductile) inclusions in the vicinity of the surface as compared with Example 1 due to the difference in the molten steel composition. That is, the circumferential distribution of the difference between the distance from the rotation center of the work roll to the outer peripheral surface and the average radius of the entire circumference is the maximum gradient angle (positive value) when the outer peripheral surface shape is displayed in the rotation direction of the work roll. By setting the friction angle to be equal to or less than the friction angle, it is possible to ensure sufficient surface quality even in a steel slab of a component system having poor surface ductility.

  In Example 3, a rectangular cross-section slab having a thickness of 100 mm and a width of 600 mm was heated to 1150 ° C. in a heating furnace, and then subjected to a rolling test with two rolling mills installed downstream of the heating furnace in the slab conveyance direction. went. The components of the slab were C: 0.11%, Si: 0.31%, Mn: 1.88%, P: 0.016%, S: 0.0008%, Al: 0.007%. 0%, N: 0.0044%, and O: 0.0011%. The surface temperature of the slab on the entrance side of the first rolling mill was 1050 ° C.

  All of the installed rolling mills were two-stage rolling mills, and the work roll diameter was φ600 mm. The reduction amount in both rolling mills was 20 mm. However, when the technique of the present invention was applied, the amount of reduction was defined by the difference between the sheet thickness averaged in the length direction before and after rolling.

  In Examples E and F to which the technology of the present invention was applied, a work roll having a periodically changing outer peripheral surface shape in which a sinusoidal profile having a period of about 10 mm and an amplitude of 6 mm was processed in the circumferential direction was used. The period corresponds to about 1/8 times the contact arc length, and the amplitude corresponds to 3/10 times the reduction amount. In the roll body length direction, the same phase, that is, the sinusoidal groove processing parallel to the roll body length direction was performed.

  In Example E, the upper work roll of the first rolling mill was a corrugated roll, and the lower work roll was a flat roll. In Example F, the upper work roll and the lower work roll of the first rolling mill were both wavy rolls. Further, in both Examples E and F, a flat roll was provided on the upper work roll and the lower work roll on the second rolling mill provided on the downstream side in the slab conveying direction with respect to the first rolling mill. . On the other hand, in Comparative Example h to which the conventional technology was applied, flat rolls were used for the upper work roll and the lower work roll in both of the rolling mills. Further, as Comparative Example i, a case where no reduction is performed is shown.

  Table 3 shows the rolling results of Example 3. In order to compare the internal quality, d0 and A0 in Table 3 are set as the average crystal grain size (average value in the thickness direction) in Comparative Example i and the space factor of the porosity remaining in the center of the thickness. The improvement rate of the internal quality of the steel slab was evaluated and compared using the improvement rate (d / d0) and the decrease rate of the porosity space factor (A / A0). The smaller these values are, the higher the improvement in slab quality is.

  As shown in Table 3, in Examples E and F, both the average crystal grain size and the porosity space factor significantly decreased even when compared with Comparative Example h in which normal rolling was performed. From this, it can be seen that the segregation, crystal structure, and internal defects of the steel slab passed to the subsequent process are remarkably improved.

  For Examples E and F and Comparative Example h in Table 3, the particle size of the final plate after hot rolling of 4 passes performed after the rolling by the two rolling mills is also shown. In the hot rolling, a flat roll was used for both the upper work roll and the lower work roll. Moreover, the plate | board thickness of the slab after completion | finish of rolling by the said 2 rolling mills was 60 mm, and the target plate | board thickness by each pass after that was 30 mm, 15 mm, 9 mm, and 6 mm. From the average crystal grain size (thickness direction average value) of the final plate material that has been reduced to a plate thickness of 6 mm, the effect of refining the plate material by applying the technique of the present invention is clear.

  The preferred embodiments of the present invention have been described in detail above with reference to the accompanying drawings, but the present invention is not limited to such examples. It is obvious that a person having ordinary knowledge in the technical field to which the present invention pertains can come up with various changes or modifications within the scope of the technical idea described in the claims. Of course, it is understood that these also belong to the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 1 Mold 5 Steel piece 5a Solidified shell 5b Unsolidified part 5c Deformation increase part 10 Continuous casting machine 11 Tundish 12 Immersion nozzle 13 Mold 14 Secondary cooling device 14a Support roll 21, 22, 23, 24, 25 Rolling machines 21a, 22a , 23a, 24a, 25a Upper work roll 21b, 22b, 23b, 24b, 25b Lower work roll 26 Work roll (rolled roll)
26a Convex part 100, 200 Corrugated roll 110, 210 Outer peripheral surface 111, 111a, 111b, 111c Flat part 113, 113a, 113b, 212 Corrugated part

Claims (19)

It is a rolling method of a steel slab that performs rolling of two or more passes on a steel slab having a substantially rectangular cross section,
The rolling of at least one pass excluding the final pass is at least one of the pair of work rolls in which a rolling roll having an outer peripheral surface shape periodically changing in the circumferential direction contacts the steel piece. Made by a rolling mill incorporated as
The rolling roll has an outer peripheral surface whose period is 1/50 times to 2 times the contact arc length and whose amplitude is 1/30 times to 1/3 times the rolling reduction, and periodically changes in the circumferential direction. A method for rolling a steel slab having a shape.   The outer peripheral surface shape of the rolling roll is the circumferential distribution of the difference between the outer peripheral surface radius which is the distance from the rotation center of the rolling roll to the outer peripheral surface and the average radius of the outer peripheral surface in the entire circumference. The steel slab rolling method according to claim 1, wherein the slab is formed to have a sinusoidal shape.   The outer peripheral surface shape of the rolling roll is the circumferential distribution of the difference between the outer peripheral surface radius which is the distance from the rotation center of the rolling roll to the outer peripheral surface and the average radius of the outer peripheral surface in the entire circumference. Is formed so as to have a continuous and periodic shape composed of a straight line and a plurality of line segments including at least one of an arc or a high-order function curve. Rolling method.   The outer peripheral surface shape of the rolling roll is the maximum value of the ascending gradient angle representing the inclination angle of the portion where the outer peripheral surface radius is the distance from the rotation center of the rolling roll to the outer peripheral surface in the rotation direction of the rolling roll. The steel slab rolling method according to any one of claims 1 to 3, wherein is a friction angle or less and is formed so as to have a continuous and periodic shape.   The outer peripheral surface shape of the rolling roll is the minimum value of the downward gradient angle representing the inclination angle of the portion where the outer peripheral surface radius, which is the distance from the rotation center of the rolling roll to the outer peripheral surface, decreases in the rotation direction of the rolling roll. The steel slab rolling method according to any one of claims 1 to 4, wherein the absolute value of is not larger than the biting angle of the roll bite and is formed to have a continuous and periodic shape.   The outer peripheral surface shape of the rolling roll is the circumferential distribution of the difference between the outer peripheral surface radius which is the distance from the rotation center of the rolling roll to the outer peripheral surface and the average radius of the outer peripheral surface in the entire circumference. The rolling of the steel slab according to any one of claims 1 to 5, wherein the steel slab is formed so as to have a continuous and periodic shape while changing its phase in the body length direction of the rolling roll. Method.   The outer peripheral surface shape of the rolling roll is the circumferential distribution of the difference between the outer peripheral surface radius which is the distance from the rotation center of the rolling roll to the outer peripheral surface and the average radius of the outer peripheral surface in the entire circumference. Is formed so as to have a continuous and periodic shape while changing the amplitude and the entire circumference average radius value in the body length direction of the rolling roll. The method of rolling a steel slab as described.   In the rolling pass in which the steel slab is rolled by the rolling mill in which the rolling roll is incorporated, the temperature difference between the surface temperature of the steel slab and the plate thickness center temperature in the plate thickness center portion is 20 ° C. or more. The method for rolling a steel slab according to any one of claims 1 to 7, which is carried out. A rolling facility comprising a plurality of rolling mills for rolling steel pieces having a substantially rectangular cross section,
In at least one of the rolling mills excluding the rolling mill located at the most downstream side in the conveying direction of the steel slab, at least one of the pair of work rolls in contact with the steel slab has a period of contact arc length 1 to 50 times and 2 times or less, and the amplitude is 1/30 times to 1/3 times the reduction amount, and the rolling equipment has an outer peripheral surface shape that periodically changes in the circumferential direction.   The outer peripheral surface shape of the work roll is a circumferential distribution of a difference between an outer peripheral surface radius which is a distance from the rotation center of the work roll to the outer peripheral surface and an average radius of the outer peripheral surface in the entire circumference. The rolling equipment according to claim 9, which is formed to have a sine wave shape.   The outer peripheral surface shape of the work roll is a circumferential distribution of a difference between an outer peripheral surface radius which is a distance from the rotation center of the work roll to the outer peripheral surface and an average radius of the outer peripheral surface in the entire circumference. The rolling equipment according to claim 9, wherein the rolling facility is formed in a continuous and periodic shape including a straight line and a plurality of line segments including at least one of an arc or a high-order function curve.   The outer peripheral surface shape of the work roll is a maximum value of an upward gradient angle representing an inclination angle of a portion where an outer peripheral surface radius, which is a distance from the rotation center of the work roll to the outer peripheral surface, increases in the rotation direction of the work roll. The rolling equipment according to any one of claims 9 to 11, wherein is a friction angle or less and is formed to have a continuous and periodic shape.   The outer peripheral surface shape of the work roll is the minimum value of the downward gradient angle that represents the inclination angle of the portion where the outer peripheral surface radius, which is the distance from the rotation center of the work roll to the outer peripheral surface, decreases in the rotation direction of the work roll. The rolling equipment according to any one of claims 9 to 12, wherein the absolute value of is not more than the biting angle of the roll bite and is formed in a continuous and periodic shape.   The outer peripheral surface shape of the work roll is a circumferential distribution of a difference between an outer peripheral surface radius which is a distance from the rotation center of the work roll to the outer peripheral surface and an average radius of the outer peripheral surface in the entire circumference. The rolling equipment according to any one of claims 9 to 13, wherein the rolling equipment is formed so as to have a continuous and periodic shape while changing its phase in the body length direction of the work roll.   The outer peripheral surface shape of the work roll is a circumferential distribution of a difference between an outer peripheral surface radius which is a distance from the rotation center of the work roll to the outer peripheral surface and an average radius of the outer peripheral surface in the entire circumference. Is formed so as to have a continuous and periodic shape that is continuously changed while changing the amplitude and the entire circumference average radius value in the body length direction of the work roll. The rolling equipment described.   The rolling mill according to any one of claims 9 to 15, wherein the rolling mill provided with the work roll having the outer peripheral surface shape that changes periodically is a two-stage rolling mill.   The rolling mill according to claim 14 or 15, wherein the rolling mill provided with the work roll having the outer peripheral surface shape that changes periodically is a four-stage rolling mill.   The said rolling equipment is rolling equipment of any one of Claims 9-17 arrange | positioned with respect to the continuous casting machine which casts the said steel slab in the casting direction downstream. The said rolling equipment is arrange | positioned in the conveyance direction downstream side of the said steel slab with respect to the scale removal apparatus which removes the scale of the said steel slab heated by the heating furnace in any one of Claims 9-17. The rolling equipment described.

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