Method for manufacturing work rolls for rolling, method for temper rolling a metal strip, and method for manufacturing a metal strip.

Abstract

To provide a work roll for rolling that can correct a shape of a metallic band by applying proper expansion, in temper rolling, although the metallic band is a high-strength metallic band whose tensile strength is above 1310 MPa.SOLUTION: The work roll for rolling comprises a trunk part having a ceramic whose young's modulus is above 350 GPa on a surface part thereof and a chromium plated layer formed on an outer peripheral surface of the trunk part, where arithmetic average roughness Ra of the outer peripheral surface of the chromium plated layer is 4.0 μm or more and 10.0 μm or less, and skewness Rsk is -0.4 or more and 0.5 or less.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a work roll for rolling, a method for manufacturing a work roll for rolling, a method for temper rolling a metal strip, and a method for manufacturing a metal strip. [Background technology]

[0002] In recent years, the need for lighter automobiles due to environmental regulations has increased, leading to a rise in demand for high-strength steel sheets. High-strength steel sheets are manufactured by applying heat treatment to cold-rolled steel sheets using continuous annealing equipment or continuous hot-dip galvanizing equipment, thereby appropriately controlling the microstructure of the steel sheet. Furthermore, temper rolling is performed on the heat-treated steel sheet to impart an elongation rate of 2.0% or less, which flattens the shape of the steel sheet and gives it an appropriate roughness to the surface.

[0003] However, high-strength steel sheets with a tensile strength exceeding 980 MPa after heat treatment have a high yield stress, which can lead to problems where the rolling load becomes too large in a normal temper rolling mill, making it impossible to secure the elongation necessary for shape correction. To address this problem, Patent Document 1 discloses a temper rolling method for steel strips with a yield strength of 340 MPa or higher, using a rolling work roll with an average surface roughness Ra of 3.0 to 10.0 μm, and achieving an elongation of 0.1% or more.

[0004] Patent Document 2 discloses a temper rolling mill that uses rolling work rolls having a Young's modulus of 500 GPa or more in the surface layer and a Vickers hardness of 1050 HV or more. According to Patent Document 2, by using these rolling work rolls, it is possible to impart an elongation rate that allows for shape correction even to high-strength steel strips with a tensile strength of 980 MPa or more.

[0005] Patent Document 3 discloses a temper rolling method for temper rolling a metal strip with a 0.2% yield strength exceeding 350 MPa using a dull work roll having a roll diameter of 1000 mm or more and a Young's modulus of 450 GPa or more on the roll surface. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2008-173684 [Patent Document 2] Japanese Patent Publication No. 2017-119303 [Patent Document 3] Japanese Patent Publication No. 2011-189404 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] In the method described in Patent Document 1, steel strips are temper-rolled with an arithmetic mean roughness Ra of the rolling work rolls in the range of 3.0 to 10.0 μm. However, when temper-rolling steel strips with a tensile strength of 1180 MPa or higher, the rolling load becomes large, and there is a problem that it may not be possible to secure the elongation rate necessary for shape correction for steel plates with a tensile strength of 1310 MPa or higher.

[0008] Patent Document 2 describes a process in which a high-strength steel sheet with a tensile strength of 980 MPa or more is temper-rolled using a Young's modulus of 500 GPa or more on the surface of a rolling work roll. However, even in this case, there is a problem in that it may not be possible to secure the elongation rate necessary for shape correction for steel sheets with a tensile strength of 1310 MPa or more.

[0009] Patent Document 3 describes using a dull work roll with a Young's modulus of 450 GPa or higher on the roll surface as a rolling work roll for temper rolling. However, even in this case, there is a problem in that it may not be possible to secure the elongation rate necessary for shape correction for steel sheets with a tensile strength of 1310 MPa or higher.

[0010] The present invention was made to solve the above problems, and its objective is to provide a rolling work roll that can impart an appropriate elongation rate and correct the shape of a metal strip, even if the metal strip has a tensile strength of 1310 MPa or more, during temper rolling. Another objective of the present invention is to provide a method for manufacturing a rolling work roll, a method for temper rolling a metal strip using the rolling work roll, and a method for manufacturing a metal strip whose shape has been flattened by temper rolling. [Means for solving the problem]

[0011] The means to solve the above problems are as follows: [1] A rolling work roll having a body with a ceramic surface having a Young's modulus of 350 GPa or more, and a chromium plating layer formed on the outer surface of the body, wherein the arithmetic mean roughness Ra of the outer surface of the chromium plating layer is 4.0 μm or more and 10.0 μm or less, and the skewness Rsk is -0.4 or more and 0.5 or less. [2] The rolling work roll according to [1], wherein the roll diameter is 150 mm or more and less than 1000 mm, and the thickness of the chromium plating layer is 50 μm or more and 0.15% or less of the roll diameter. [3] A method for manufacturing a work roll for a rolling mill, comprising: a plating layer forming step of forming a chromium plating layer on the outer circumferential surface of a body having a ceramic surface layer with a Young's modulus of 350 GPa or more; and a dulling step of dulling the outer circumferential surface of the chromium plating layer, wherein the dulling step imparts a surface roughness to the outer circumferential surface of the chromium plating layer such that the arithmetic mean roughness Ra is 4.0 μm or more and 10.0 μm or less, and the skewness Rsk is -0.4 or more and 0.5 or less. [4] The method for manufacturing a rolling work roll according to [3], wherein the outer surface of the chromium plating layer is dulled by electrical discharge machining in the dulling step. [5] In the dulling step, the product of the pulse current and pulse time is 500 A·μs or more and 12000 A·μs or less, and the number of discharges per unit area of ​​the outer surface of the chromium plating layer is 30 times / mm 2 Over 800 times / mm 2The method for manufacturing a work roll for rolling according to [4], which performs electrical discharge machining under the following electrical discharge machining conditions. A method for temper rolling a metal strip, which uses the work roll for rolling according to [6][1] or [2] to perform temper rolling on the metal strip. A method for temper rolling a metal strip, which uses the work roll for rolling manufactured by the method for manufacturing a work roll for rolling according to any one of [7][3] to [5] to perform temper rolling on the metal strip. A method for manufacturing a metal strip, which uses the method for temper rolling a metal strip according to [8][6] or [7] to manufacture a metal strip having a tensile strength of 1310 MPa or more.

Effects of the Invention

[0012] By using the work roll for rolling according to the present invention, even a high-strength metal strip having a tensile strength of 1310 MPa or more can be corrected in shape by imparting an appropriate elongation rate through temper rolling. Further, by performing temper rolling with the work roll for rolling, a metal strip having a flattened shape can be manufactured even if it is a high-strength metal strip having a tensile strength of 1310 MPa or more.

Brief Description of the Drawings

[0013] [Figure 1] FIG. 1 is a schematic view showing an example of a temper rolling machine including a work roll for rolling according to the present embodiment. [Figure 2] FIG. 2 is a schematic view showing an installation example of a temper rolling machine provided in a production line of a metal strip. [Figure 3] FIG. 3 is a schematic view of a work roll for rolling 12a before a chromium plating layer is formed. [Figure 4] FIG. 4 is a schematic view of a work roll for rolling on which a chromium plating layer is formed. [Figure 5] FIG. 5 is a schematic cross-sectional view of the outer peripheral surface of the chromium plating layer. [Figure 6] FIG. 6 is a schematic cross-sectional view showing a state where the convex portion on the outer peripheral surface of the chromium plating layer is pressed into the surface of the metal strip. [Figure 7] FIG. 7 is a schematic cross-sectional view of the outer peripheral surface of the chromium plating layer. [Figure 8] Figure 8 is a schematic cross-sectional view showing the state in which the irregularities on the outer surface of the chromium plating layer are pressed into the surface of the metal strip. [Figure 9] Figure 9 is a schematic diagram of an electrical discharge machining (EDM) equipment 40 that dulls the outer surface of a chromium plating layer. [Figure 10] Figure 10 is a schematic diagram showing pulse current in electrical discharge machining. [Figure 11] Figure 11 is a graph showing an example of the effect of the product of pulse current P and pulse time t1, and the number of discharges N, which are the electrical discharge machining conditions, on the arithmetic mean roughness Ra and skewness Rsk. [Modes for carrying out the invention]

[0014] The present invention will be described in detail below through embodiments of the present invention. The following embodiments are preferred examples of the present invention, and the present invention is not limited in any way by these embodiments.

[0015] The rolling work rolls according to this embodiment are used as work rolls in a temper rolling mill. Figure 1 is a schematic diagram showing an example of a temper rolling mill 10 (hereinafter sometimes referred to as "rolling mill 10") equipped with the rolling work rolls according to this embodiment. Figure 1(a) is a schematic side view of the rolling mill 10, and Figure 1(b) is a schematic front view of the rolling mill 10. The rolling mill 10 is used to temper roll a metal strip, for example, a high-strength metal strip with a tensile strength of 1310 MPa or more.

[0016] The rolling mill 10 has as its main components a pair of rolling work rolls (hereinafter sometimes referred to as "work rolls") 12a and 12b, backup rolls 14a and 14b, and a housing 16. The work rolls 12a and 12b are rolls that directly contact the metal strip 100, which is the material to be rolled, and apply reduction. The backup rolls 14a and 14b are provided to support the pair of work rolls 12a and 12b from above and below, and to suppress deflection of the work rolls. One end of the pair of work rolls 12a and 12b is connected to a drive motor via a coupling or reduction gear, and the work rolls 12a and 12b are rotated by the drive motor. However, in some rolling mills, the backup rolls 14a and 14b are rotated by the drive motor, causing the work rolls 12a and 12b to rotate in a driven manner. A load sensor (load cell) 20 is provided between the housing 16 of the rolling mill 10 and the bearing housings 18 of the backup rolls 14a and 14b. The load sensor 20 detects the rolling load of the rolling mill 10. A reduction device 21 is also provided on the upper side of the housing 16. The reduction device 21 adjusts the roll gap, which is the gap between the work rolls 12a and 12b, by displacing the bearing housings 18 vertically. In the rolling mill 10, the pair of work rolls 12a and 12b that directly contact and reduce the metal strip 100 are the rolling work rolls according to this embodiment.

[0017] The rolling mill 10 shown in Figure 1 is installed in the production line for the metal strip 100. Figure 2 is a schematic diagram showing an example of the installation of a temper rolling mill installed in the production line for the metal strip 100. The rolling mill 10 is installed, for example, downstream of the continuous annealing line for the metal strip 100, and performs temper rolling on the metal strip 100 that has undergone heat treatment through a heating zone, a soaking zone and a cooling zone. An entry bridle roll 22, an entry tension meter 24, an exit tension meter 26, and an exit bridle roll 28 are installed before and after the rolling mill 10.

[0018] When performing temper rolling of a metal strip 100 using a rolling mill 10, the heat-treated metal strip 100 is rolled at a predetermined elongation rate. The elongation rate (also called the stretching rate) refers to the elongation rate of the metal strip 100 before and after rolling, and is defined as the rate of increase in the length of the metal strip 100 before and after temper rolling. The elongation rate is measured by the difference in peripheral speed between the entry bridle roll 22 and the exit bridle roll 28. Under conditions where the elongation rate is small, it may be the same as the reduction rate, which is the rate of change in plate thickness. In this case, temper rolling may be performed using the reduction rate as an indicator instead of the elongation rate.

[0019] When conventional rolling work rolls are incorporated into the rolling mill 10 shown in Figures 1 and 2 to temper-roll a metal strip with a tensile strength of 1310 MPa or higher, the rolling load required to impart an appropriate elongation rate to the metal strip exceeds the equipment specifications of the rolling mill 10. Therefore, when using conventional rolling work rolls, it may not be possible to secure the elongation rate necessary for correcting the shape of the metal strip. In contrast, by incorporating the rolling work rolls 12a and 12b according to this embodiment, the increase in the rolling load in the rolling mill 10 is suppressed, making it possible to impart an appropriate elongation rate even to a metal strip with a tensile strength of 1310 MPa or higher.

[0020] Next, a rolling work roll and a method for manufacturing the same according to this embodiment will be described. Figure 3 is a schematic diagram of the rolling work roll 12a before the chromium plating layer 34 is formed. Figure 3(a) is a side view, and Figure 3(b) is a cross-sectional view AA of Figure 3(a). Since the rolling work roll 12a and the rolling work roll 12b are the same work roll, the following description will use the rolling work roll 12a.

[0021] Before forming the chromium plating layer 34, the rolling work roll 12a has a structure in which the body portion 32 is fixed to the shaft portion 30. The body portion 32 is made of ceramic with a Young's modulus of 350 GPa or more. Ceramic is a sintered body mainly composed of inorganic compound materials, for example, a cemented carbide consisting of 70-86% by mass of tungsten carbide (WC) and the remainder being cobalt (Co). This forms a body portion 32 with a surface Young's modulus of 350 GPa or more. The shaft portion 30 is made of forged steel or cast steel containing chromium and some vanadium in its composition.

[0022] Figure 4 is a schematic diagram of a rolling work roll 12a on which a chromium plating layer 34 is formed. Figure 4(a) is a side view, and Figure 4(b) is a cross-sectional view of BB of Figure 4(a). The rolling work roll 12a according to this embodiment has a chromium plating layer 34 formed on the outer circumferential surface 32a of the body portion, the Young's modulus of the surface layer being 350 GPa or more. The chromium plating layer 34 is a hard chromium plating layer. The hard chromium plating layer is applied primarily to ensure wear resistance.

[0023] The hard chromium plating layer can be formed by immersing the body 32 in a plating bath whose composition mainly consists of chromic anhydride (chromium trioxide (CrO3)) with a small amount of sulfuric acid added as a catalyst. This process is the plating layer formation step. The hardness of the hard chromium plating layer is 750 HV to 1000 HV. The surface roughness of the outer circumferential surface 34a of the chromium plating layer on which the hard chromium plating is formed is an arithmetic mean roughness Ra of 0.2 μm to 0.5 μm. Before forming the chromium plating layer 34, it is preferable to pre-treat the outer circumferential surface 32a of the body by etching so that the arithmetic mean roughness Ra is 0.5 μm to 3.0 μm. This improves the adhesion between the body 32 and the chromium plating layer 34.

[0024] Furthermore, after forming a chromium plating layer 34 on the outer circumferential surface 32a of the body, dulling is performed on the outer circumferential surface 34a of the chromium plating layer. Dulling may be performed using known methods applicable to imparting roughness to the rolling work roll 12a. For example, shot dulling, electrical discharge machining, electron beam machining, laser machining, etc., may be used as dulling. Shot dulling involves projecting shot particles onto the work surface to create surface irregularities. Electrical discharge machining involves generating an electrical discharge between an electrode and the work surface to create surface irregularities. Electron beam machining involves irradiating the work surface with an electron beam in a reduced-pressure atmosphere within a chamber to create surface irregularities. Laser machining involves irradiating the work surface with laser light to melt a portion of the surface and create irregularities. This process constitutes the dulling step.

[0025] The rolling work roll 12a of this embodiment is manufactured by dulling the outer surface 34a of the chromium plating layer to which an arithmetic mean roughness Ra of 4.0 μm or more and 10.0 μm or less and a skewness Rsk of -0.4 or more and 0.5 or less are given. The arithmetic mean roughness Ra and skewness Rsk are line roughness values ​​specified in JIS B 0601:2013. The arithmetic mean roughness Ra is an index that represents the average height of the roughness curve over a reference length. The skewness Rsk is defined by the following formula (1).

[0026]

number

[0027] Skewness Rsk is known as a parameter closely related to the friction and wear phenomena of contacting sliding surfaces. Skewness Rsk is an index that represents the symmetry between the convex and concave parts of an uneven surface. If the statistical distribution of convex and concave parts follows a normal distribution, Rsk will be zero. Surfaces with predominantly convex parts will have a large Rsk, while surfaces with predominantly concave parts will have a small Rsk. It is preferable to calculate the arithmetic mean roughness Ra and skewness Rsk from the linear roughness measured in the axial direction of the roll barrel.

[0028] As described above, the rolling work roll 12a according to this embodiment uses ceramic with a Young's modulus of 350 GPa or more for the body portion 32. As a result, even if the rolling work roll 12a receives a large contact pressure from the metal strip 100 during temper rolling, the flattening deformation of the rolling work roll 12a is suppressed, and the rolling load does not become excessively large even when temper rolling is performed on a high-strength metal strip 100.

[0029] Furthermore, if the outer surface of a ceramic with a Young's modulus of 350 GPa or higher is dulled, cracks and fissures may occur on the surface of the ceramic, reducing its durability as a rolling work roll 12a. In contrast, in the rolling work roll 12a according to this embodiment, a chromium plating layer 34 is formed on the outer surface 32a of the body, and the outer surface 34a of the chromium plating layer is dulled to suppress the occurrence of cracks on the surface of the ceramic. It is preferable to form a hard chromium plating with a hardness of 700 HV or higher as the chromium plating layer 34. This improves the wear resistance of the rolling work roll 12a and also improves the durability of the dull marks (irregularities on the surface of the rolling roll) applied to the outer surface 34a of the chromium plating layer.

[0030] Furthermore, the outer surface 34a of the chromium plating layer of the rolling work roll 12a according to this embodiment is dulled to impart a surface roughness with an arithmetic mean roughness Ra of 4.0 μm or more and 10.0 μm or less, and a skewness Rsk of -0.4 or more and 0.5 or less. By appropriately controlling the microscopic irregularities on the surface of the rolling work roll 12a in this way, it becomes possible to perform temper rolling while suppressing an increase in rolling load, even for high-strength metal strips with a tensile strength of 1310 MPa or more.

[0031] Next, regarding the surface roughness of the rolling work roll 12a, first, the arithmetic mean roughness Ra will be explained. The outer circumferential surface 34a of the chromium plating layer of the rolling work roll 12a according to this embodiment has an arithmetic mean roughness Ra of 4.0 μm or more and 10.0 μm or less. This promotes the pressing of the protrusions formed on the outer circumferential surface 34a of the chromium plating layer into the surface of the metal strip 100 during temper rolling, and the pressing of these protrusions results in an effect of stretching the metal strip 100 as a whole (stretching effect).

[0032] Figure 5 is a schematic cross-sectional view of the outer surface 34a of the chromium plating layer. Figure 5(a) shows a rolling work roll with Ra ≥ 4.0 μm, and Figure 5(b) shows a rolling work roll with Ra < 4.0 μm. As shown in Figures 5(a) and (b), the larger the arithmetic mean roughness Ra, the greater the height of the protrusions formed on the outer surface. In order to make the shape of the irregularities easier to understand, the schematic cross-sectional views shown in Figures 5 to 8 are shown with a larger magnification in the vertical direction than in the horizontal direction. Therefore, these drawings depict the irregularities with a greater slope than the actual slope of the irregularities.

[0033] Figure 6 is a schematic cross-sectional view showing the state in which the protrusions on the outer peripheral surface 34a of the chromium plating layer are pressed into the surface of the metal strip 100. Figure 6(a) shows the state of the metal strip 100 when a rolling work roll with Ra ≥ 4.0 μm is used, and Figure 6(b) shows the state of the metal strip 100 when a rolling work roll with Ra < 4.0 μm is used.

[0034] As shown in Figure 6(a), when the arithmetic mean roughness Ra is 4.0 μm or greater, even if the protrusions on the outer surface are pressed into the surface of the metal strip 100, there is no interference between adjacent plastic deformation regions (plastic strain regions). This suppresses the increase in resistance to the indentation of the protrusions, thus suppressing the increase in rolling load. In contrast, when the arithmetic mean roughness Ra is less than 4.0 μm, the spacing between the protrusions formed on the outer surface becomes smaller, and the area of ​​the plastic deformation region (plastic strain region) formed near the surface of the metal strip 100 becomes larger. This significantly increases the resistance to the indentation of the protrusions, resulting in a noticeable increase in rolling load.

[0035] Thus, in the rolling work roll 12a according to this embodiment, the calculated mean roughness Ra is set to 4.0 μm or more in order to suppress the increase in rolling load. However, if the arithmetic mean roughness Ra becomes too large, wear of the convex parts on the outer surface will progress more easily, and dulling to increase the arithmetic mean roughness Ra will also become difficult. For this reason, the arithmetic mean roughness Ra applied to the outer surface 34a of the chromium plating layer must be between 4.0 μm and 10.0 μm, and preferably between 6.0 μm and 10.0 μm.

[0036] Next, the skewness Rsk will be described. The outer circumferential surface 34a of the chromium plating layer of the rolling work roll 12a according to this embodiment has a surface roughness of skewness Rsk of -0.4 or more and 0.5 or less. This promotes the indentation of the protrusions formed on the outer circumferential surface into the surface of the metal strip 100 during temper rolling, and promotes the stretching effect of the metal strip 100 due to the indentation of the protrusions.

[0037] Figure 7 is a schematic cross-sectional view of the outer surface 34a of the chromium plating layer. Figure 7(a) shows a rolling work roll with a skewness Rsk of -0.4 or higher, and Figure 7(b) shows a rolling work roll with a skewness Rsk of less than -0.4. When the skewness Rsk is -0.4 or higher, the tips of the convex parts on the surface are sharp, and the length of the bottom of the valleys is long, resulting in a cirque-like shape. On the other hand, when the skewness Rsk is less than -0.4, the tips of the convex parts on the outer surface are not sharp, and the length of the top of the convex parts is long, resulting in a plateau-like shape.

[0038] Figure 8 is a schematic cross-sectional view showing the state in which the irregularities of the outer surface 34a of the chromium plating layer are pressed into the surface of the metal strip 100. Figure 8(a) shows the state of the metal strip 100 when a rolling work roll with a skewness Rsk of -0.4 or higher is used, and Figure 8(b) shows the state of the metal strip 100 when a rolling work roll with a skewness Rsk of less than -0.4 is used.

[0039] When the skewness Rsk is -0.4 or higher, the protrusions on the outer surface are more easily pressed into the surface of the metal strip 100, and there is no interference between adjacent plastic deformation regions. As a result, the resistance to the indentation of the protrusions is suppressed, and the increase in rolling load is suppressed. In contrast, when the skewness Rsk is less than -0.4, the contact area between the protrusions on the outer surface and the surface of the metal strip 100 becomes larger, and interference between adjacent plastic deformation regions is more likely to occur. Therefore, the resistance to the indentation of the protrusions increases significantly, and the increase in rolling load becomes pronounced.

[0040] As described above, in the rolling work roll 12a according to this embodiment, the skewness Rsk is set to -0.4 or higher in order to suppress the increase in rolling load. However, if the skewness Rsk becomes too large, wear of the surface protrusions will progress more easily. For this reason, the skewness Rsk applied to the outer surface 34a of the chromium plating layer must be between -0.4 and 0.5, and preferably between -0.2 and 0.4. More preferably, the skewness Rsk applied to the outer surface 34a of the chromium plating layer must be between 0.0 and 0.4. This further suppresses the increase in rolling load.

[0041] Furthermore, if a granular deposition chromium plating method is applied as the dulling method, in which a chromium plating layer 34 is deposited in a granular manner on the outer circumferential surface 32a of the body, the protrusions on the surface are formed in a spherical shape, and the skewness Rsk applied to the outer circumferential surface 34a of the chromium plating layer becomes less than -0.4. As described above, when the skewness Rsk becomes less than -0.4, the resistance to indentation of the protrusions increases, and the increase in rolling load becomes significant. For this reason, the granular deposition chromium plating method is excluded as a dulling method for the rolling work roll 12a according to this embodiment.

[0042] Next, a processing method for dulling the outer surface 34a of the chromium plating layer using electrical discharge machining will be described. Electrical discharge machining is preferred as the dulling method. By using electrical discharge machining and controlling processing conditions such as pulse current, pulse time, and the number of discharges per unit area of ​​the surface, the arithmetic mean roughness Ra and skewness Rsk of the outer surface 34a of the chromium plating layer can be easily controlled to a target range.

[0043] Figure 9 is a schematic diagram of an electrical discharge machining (EDM) machine 40 that dulls the outer surface 34a of a chromium plating layer. The EDM machine 40 comprises a roll support section 42 and an EDM section 44. The roll support section 42 includes a roll chuck 46, a roll rotation motor 48, and a tailstock 50. The roll chuck 46 supports the rolling work roll 12a from one end in the axial direction. The roll rotation motor 48 rotates the rolling work roll 12a at a predetermined rotational speed. The tailstock 50 supports the rolling work roll 12a from the other end in the axial direction.

[0044] The electrical discharge machining unit 44 includes an electrode 52, an electrode position control unit 54, a power supply unit 56 that supplies current to the electrode 52, and a control device 58 that controls the current waveform from the power supply unit 56. The electrical discharge machining unit 44 further includes an oil tank 62 that immerses the discharge area generated between the rolling work roll 12a and the electrode 52 with electrical discharge machining oil 60. The electrical discharge machining oil 60 is, for example, Metalwork EDF-K2 manufactured by ENEOS Corporation or Daphne Marg Plus ED3 manufactured by Idemitsu Kosan Co., Ltd.

[0045] The electrode 52 may be made of a copper alloy or graphite formed into a plate or rod shape. The electrode 52 may be provided with a hole for passing electrical discharge machining oil 60 through, and by supplying the electrical discharge machining oil 60 from the tip of the electrode 52, electrical discharge chips may not accumulate between the rolling work roll 12a and the electrode 52. The electrode 52 is positioned facing the surface of the rolling work roll 12a, and one or more electrodes may be arranged in the axial or circumferential direction of the rolling work roll 12a. The electrode position control unit 54 has a servo feed mechanism that supports the electrode 52 and maintains the distance (gap) between the rolling work roll 12a and the tip of the electrode 52 at a set condition. The electrical discharge machining equipment 40 may use either a method in which the electrode 52 is moved to perform electrical discharge machining when machining the body of the rolling work roll 12a, or a method in which the electrode 52 is fixed and the rolling work roll 12a is moved in the axial direction to perform electrical discharge machining.

[0046] Figure 10 is a schematic diagram showing pulse current in electrical discharge machining. Figure 10(a) is a schematic diagram of pulse current at machining time T, and Figure 10(b) is an enlarged schematic diagram showing a portion of the pulse current. The control device 58 controls the current supplied from the power supply device 56 to the electrode 52 to generate an arc discharge between the rolling work roll 12a and the electrode 52 (gap), thereby creating irregularities on the surface of the rolling work roll 12a. The control device 58 controls the pulse current value P supplied to the electrode 52, and the time during which the pulse current P is supplied (pulse time) t1 and the time during which the pulse current is not supplied (pulse stop time) t2 within one cycle time t, so that they meet preset conditions.

[0047] In the electrical discharge machining conditions for dulling the rolling work roll 12a according to this embodiment, it is preferable to control the product of the pulse current P and pulse time t1 so that it is between 500 A·μs and 12000 A·μs. If the product of the pulse current P and pulse time t1 is less than 500 A·μs, the height of the irregularities on the outer surface 34a of the chromium plating layer generated by the arc discharge becomes smaller. This is undesirable because it may not be possible to ensure that the arithmetic mean roughness Ra on the outer surface 34a of the chromium plating layer is 4.0 μm or more. On the other hand, if the product of the pulse current P and pulse time t1 exceeds 12000 A·μs, the arithmetic mean roughness Ra on the outer surface 23a of the chromium plating layer 23 may exceed 10.0 μm. Furthermore, if the product of the pulse current P and pulse time t1 exceeds 12000 A·μs, the amount of heat generated by the rolling work roll 12a increases, which may cause abnormalities in the arc discharge, which is undesirable. In the electrical discharge machining conditions for dulling the rolling work roll 12a, it is more preferable to control the product of the pulse current P and the pulse time t1 so that it is between 2000 A·μs and 10000 A·μs.

[0048] In the electrical discharge machining conditions for dull machining of the rolling work roll 12a, it is further preferable to control the pulse current P to be 15 A or more and 150 A or less, and the pulse time t1 to be 20 μs or more and 300 μs or less. If the pulse current P exceeds 150 A or the pulse time t1 exceeds 300 μs, the amount of heat generated in the rolling work roll 12a becomes large, and abnormal arc discharge may occur, which is not preferable. On the other hand, if the pulse current P is less than 15 A or the pulse time t1 is less than 20 μs, the height of the unevenness on the outer peripheral surface 34a of the chromium plating layer generated by arc discharge becomes small, and a predetermined unevenness may not be obtained, which is not preferable.

[0049] In the electrical discharge machining conditions for the rolling work roll 12a, further, the number of discharges of arc discharge per unit area of the outer peripheral surface 34a of the chromium plating layer is 30 times / mm 2 or more and 800 times / mm 2 or less. It is preferable to control the number of discharges to the outer peripheral surface 34a of the chromium plating layer to be 30 times / mm 2 or less. If the number of discharges to the outer peripheral surface 34a of the chromium plating layer is less than 30 times / mm, the number of arc discharges decreases, and a dent is formed in a part of the outer peripheral surface. As a result, the outer peripheral surface 34a of the chromium plating layer has a mesa-like shape with a long top of the convex part, and the skewness Rsk of the outer peripheral surface is likely to be less than -0.4. Further, the unevenness formed on the outer peripheral surface 34a of the chromium plating layer is likely to be non-uniform, and the surface appearance of the metal strip 100 subjected to temper rolling may be inferior, which is not preferable. On the other hand, if the number of discharges to the outer peripheral surface 34a of the chromium plating layer exceeds 800 times / mm 2 , the number of arc discharges becomes excessive, the formed chromium plating layer becomes thin, and the durability as the rolling work roll 12a may decrease, which is not preferable. The number of discharges to the outer peripheral surface 34a of the chromium plating layer is preferably controlled to be 40 times / mm 2 or more and 700 times / mm 2 or less, more preferably controlled to be 100 times / mm 2 or more and 400 times / mm 2 or less.

[0050] The number of discharges on the outer surface 34a of the chromium plating layer can be controlled by the control device 58 setting the processing time T, which is the total time for performing electrical discharge machining on the rolling work roll 12a. Specifically, the area Q (mm²) of the outer surface 34a of the chromium plating layer to be subjected to electrical discharge machining. 2 Using the number of electrodes 52 (M) and the time of one cycle (t(s)), the number of discharges N (times / mm) is calculated. 2 ) can be calculated by the following equation (2).

[0051] N = M × T / (t × Q) ... (2)

[0052] Figure 11 is a graph showing an example of the effect of the product of pulse current P and pulse time t1, and the number of discharges N, which are the electrical discharge machining conditions, on the arithmetic mean roughness Ra and skewness Rsk. As shown in Figure 11, by controlling the product of pulse current P and pulse time t1 and the number of discharges N in electrical discharge machining, the arithmetic mean roughness Ra and skewness Rsk formed on the outer surface 34a of the chromium plating layer can be controlled to appropriate values ​​(the area shown in gray in Figure 11).

[0053] The roll diameter of the rolling work roll 12a is preferably 150 mm or more and less than 1000 mm, and the thickness of the chromium plating layer 34 formed on the outer peripheral surface 32a of the body is preferably 50 μm or more and 0.15% or less of the roll diameter. The roll diameter of the rolling work roll 12a is the diameter of the outer peripheral surface of the body 32 plus the thickness of the chromium plating layer 34.

[0054] When the roll diameter of the rolling work roll 12a is less than 150 mm, the thickness of the chromium plating layer 34 relative to the roll diameter becomes relatively large. As a result, the effect of suppressing flattening deformation obtained by using ceramic with a Young's modulus of 350 GPa or more as the body portion 32 is relatively reduced, and the advantages of using ceramic with a Young's modulus of 350 GPa or more for the body portion 32 may be diminished, which is undesirable. When the roll diameter of the rolling work roll 12a is 1000 mm or more, the contact arc length when performing temper rolling on the high-strength metal strip 100 becomes large. As a result, even if roll flattening is reduced by using ceramic with a Young's modulus of 350 GPa or more for the body portion 32, the rolling load becomes large, and it may not be possible to impart the necessary elongation to the metal strip 100, which is undesirable.

[0055] Furthermore, if the thickness of the chromium plating layer 34 is less than 50 μm, it is undesirable because the chromium plating layer in the recesses of the surface becomes thin when the outer surface 34a of the chromium plating layer is dulled. If the chromium plating layer becomes thin, the durability (lifespan) of the rolling work roll may be shortened when it is used continuously as a rolling work roll. Also, if the thickness of the chromium plating layer 34 exceeds 0.15% of the roll diameter, the adhesion between the body 32 and the chromium plating layer 34 may decrease, which is undesirable. In particular, in the rolling work roll 12a according to this embodiment, since ceramic is used for the body 32, it is preferable to ensure sufficient adhesion between the body 32 and the chromium plating layer 34.

[0056] For example, when a rolling work roll with a roll diameter of 530 mm is used for temper rolling of a high-strength metal strip with a tensile strength of 1180 MPa or more, a rolling load of approximately 6.5 kN / mm is applied per unit width. If the thickness of the chromium plating layer 34 exceeds 0.15% of the roll diameter, the position where the internal shear stress is maximum is not inside the body 32, but at the interface between the body 32 and the chromium plating layer 34 or inside the chromium plating layer 34. Therefore, if the shear stress acting between the body 32 and the chromium plating layer 34 becomes greater than the adhesion strength between the body 32 and the chromium plating layer 34, delamination of the chromium plating layer 34 may occur.

[0057] By incorporating the rolling work roll 12a according to this embodiment into the rolling mill 10, a temper rolling mill suitable for temper rolling of high-strength metal strip 100 is created. Then, using one or more such rolling mills 10, temper rolling is performed to impart an elongation of 0.1% to 0.6% to the metal strip 100. This suppresses the increase in rolling load during temper rolling, so that even when using a rolling mill with normal equipment specifications, an elongation of 0.1% or more necessary for shape correction of the metal strip 100 can be secured. However, it is preferable that the elongation imparted to high-strength metal strip be 0.6% or less. If the imparted elongation is greater than 0.6%, too much strain will be applied to the metal strip 100, which may reduce the elongation (ductility) of the metal strip product due to work hardening, so this is undesirable. Furthermore, if the applied elongation rate is greater than 0.6%, the frictional force at the contact interface between the irregularities on the surface of the rolling work roll 12a and the metal strip 100 increases, which may increase the rolling load in temper rolling, and is therefore undesirable. Note that a rolling mill with normal equipment specifications refers to a rolling mill capable of applying a rolling load of 5 kN / mm to 15 kN / mm.

[0058] By performing temper rolling in a rolling mill 10 incorporating the rolling work roll 12a according to this embodiment, even a metal strip with a tensile strength of 1310 MPa or more that has undergone rapid cooling such as water quenching in the continuous annealing process and has developed irregularities in shape (flatness) can be flattened, and a metal strip with good flatness can be manufactured. [Examples]

[0059] <Example 1> Next, we will describe Example 1, in which a metal strip with a thickness of 2.0 mm and a tensile strength of 1470 MPa was subjected to temper rolling. In Example 1, a ceramic with a Young's modulus of 350 GPa was used for the body, and a chromium plating layer was formed on the outer surface of the body. After that, a rolling work roll was used in which the outer surface of the chromium plating layer was dulled by electrical discharge machining.

[0060] The body of the piece was made of ceramic composed of 70% tungsten carbide (WC) by mass and the remainder being cobalt (Co). The chromium plating layer was formed by immersing the body in a plating bath whose composition mainly consisted of chromic anhydride (chromium trioxide CrO3) with a small amount of sulfuric acid added as a catalyst, and the thickness was adjusted to 200 μm under all conditions. Before forming the chromium plating layer, the arithmetic mean roughness Ra of the body was adjusted to be between 0.2 μm and 0.5 μm.

[0061] The electrodes used in the electrical discharge machining were made of copper alloy and were formed into a plate shape. The electrical discharge machining conditions were controlled by changing the pulse current P within the range of 25A to 150A, and by changing the pulse time t1, so that the product of the pulse current P and pulse time t1 was between 400A·μs and 20000A·μs. In addition, as an electrical discharge machining condition for the rolling work roll, the number of times an arc discharge occurs per unit area on the outer surface of the chromium plating layer (number of discharges) was set to 30 times / mm 2 More than 800 / mm 2 The following conditions were met. As a result, the outer surface of the chromium plating layer was given a surface roughness of arithmetic mean roughness Ra of 4.0 μm to 9.6 μm and skewness Rsk of -0.65 to 0.40.

[0062] Among the rolling work rolls manufactured in this manner, a rolling work roll having a surface roughness in which the arithmetic mean roughness Ra on the outer surface of the chromium plating layer is 4.0 μm or more and 10.0 μm or less, and the skewness Rsk is -0.4 or more and 0.5 or less was designated as the inventive example. On the other hand, a rolling work roll having a surface roughness outside the range of 4.0 μm or more and 10.0 μm for the arithmetic mean roughness Ra on the outer surface of the chromium plating layer, and / or outside the range of -0.4 or more and 0.5 for the skewness Rsk was designated as the comparative example.

[0063] Using the rolling work rolls of the inventive example and comparative example, temper rolling was performed on a metal strip with a tensile strength of 1470 MPa (yield strength of 1300 MPa). In Example 1, temper rolling was performed so that the rolling load per unit width of the metal strip remained constant at 6.57 kN / mm. A result of 0.1% to 0.6% elongation of the metal strip due to temper rolling was marked as "○", and a result of 0.1% to 0.6% elongation of the metal strip due to temper rolling was marked as "×". The surface roughness of the rolling work rolls and the results of temper rolling are shown in Table 1 below.

[0064] [Table 1]

[0065] As shown in Table 1, in the inventive examples No. 1, 2, 6, and 7, it was confirmed that an elongation rate of 0.1% to 0.6% could be secured for the metal strip, and that the shape of the metal strip could be corrected. On the other hand, in the comparative examples No. 3 to 5, an elongation rate of 0.1% to 0.6% could not be secured for the metal strip at the same rolling load.

[0066] The results from Example 1 confirmed that by using the rolling work roll according to this embodiment, it is possible to impart an appropriate elongation rate and correct the shape of a high-strength metal strip with a tensile strength of 1310 MPa or more. Furthermore, it was confirmed that by performing temper rolling with the rolling work roll, it is possible to produce a metal strip with a flattened shape even if the metal strip has a tensile strength of 1310 MPa or more.

[0067] <Example 2> As Example 2, we will describe an example of temper rolling performed on a metal strip with a tensile strength of 1470 MPa. In Example 2, as in Example 1, the body of the rolling work roll was made of ceramic consisting of 70% tungsten carbide (WC) by mass and the remainder being cobalt (Co). The Young's modulus of the surface layer of the body was 350 GPa. In Example 2, the thickness was changed to form a chromium plating layer on the outer surface of the body, and the processing conditions of the electrical discharge machining (EDM), which is a dulling process, were changed to impart a predetermined arithmetic mean roughness Ra and skewness Rsk to the outer surface of the chromium plating layer.

[0068] The chromium plating layer formed on the outer surface of the body is a hard chromium plating process in which the body is immersed in a plating bath whose composition mainly consists of chromic anhydride (chromium trioxide: CrO3) with a small amount of sulfuric acid added as a catalyst. The chromium plating conditions were adjusted by changing the current and plating time so that the thickness of the chromium plating layer was between 100 μm and 200 μm.

[0069] The electrodes used for electrical discharge machining were made of copper alloy or graphite and were formed into a plate shape. The EDM conditions were adjusted by changing the pulse current P within the range of 20A to 120A and the pulse time t1 within the range of 10μs to 170μs. This controlled the product of the pulse current P and pulse time t1 to be within the range of 300A·μs to 17000A·μs. The pulse stop time t2 was also adjusted within the range of 20μs to 1500μs. The number of arc discharges per unit area on the outer surface of the chromium plating layer (number of discharges) was adjusted by changing the processing time T within the range of 10min to 150min, resulting in 20 discharges / mm². 2 Over 150 times / mm 2 The following ranges were controlled. The arithmetic mean roughness Ra and skewness Rsk of the rolling work rolls formed by electrical discharge machining are shown in Table 2 below, and the electrical discharge machining conditions are shown in Table 3 below.

[0070] [Table 2]

[0071] [Table 3]

[0072] As shown in Tables 2 and 3, it can be seen that the arithmetic mean roughness Ra and skewness Rsk of the rolling work roll can be controlled by changing the electrical discharge machining conditions such as the electrode, pulse current, pulse time, and number of discharges. Among the rolling work rolls manufactured in this way, a rolling work roll having a surface roughness in which the arithmetic mean roughness Ra on the outer surface of the chromium plating layer is 4.0 μm or more and 10.0 μm or less, and the skewness Rsk is -0.4 or more and 0.5 or less was designated as the inventive example. On the other hand, a rolling work roll having a surface roughness outside the range of 4.0 μm or more and 10.0 μm for the arithmetic mean roughness Ra on the outer surface of the chromium plating layer, and / or outside the range of -0.4 or more and 0.5 for the skewness Rsk was designated as a comparative example.

[0073] Using the rolling work rolls of the inventive example and comparative example, temper rolling of a metal strip with a tensile strength of 1470 MPa class was performed. The temper rolling was carried out so that the rolling load per unit width of the metal strip was kept constant at 6.57 kN / mm. In Example 2 as well, if the elongation rate of the metal strip after temper rolling was secured to be between 0.1% and 0.6%, it was marked as "○", and if the elongation rate of the metal strip after temper rolling was not secured to be between 0.1% and 0.6%, it was marked as "×". The evaluation results are also shown in Table 2 above.

[0074] As shown in Table 2, the invention examples No. 10-14 and 18 were able to impart the necessary elongation to the metal strip. On the other hand, the comparative examples No. 15-17 could not sufficiently secure the elongation of the metal strip for the same rolling load. From these results, it was confirmed that by using the rolling work roll according to this embodiment, even high-strength metal strips with a tensile strength of 1310 MPa or more can be given an appropriate elongation and their shape corrected. Furthermore, it was confirmed that by adjusting the electrical discharge machining conditions, a rolling work roll having a surface roughness with an arithmetic mean roughness Ra of 4.0 μm or more and a skewness Rsk of -0.4 or more and 0.5 or less on the outer surface of the chromium plating layer can be manufactured.

[0075] <Example 3> As Example 3, we will describe an example of temper rolling of a metal strip with a thickness of 1.2 mm and a width of 1140 mm, with a tensile strength of 1470 MPa (yield strength of 1300 MPa). In Example 3, as in Examples 1 and 2, the body of the rolling work roll was made of ceramic consisting of 70% tungsten carbide (WC) by mass and the remainder being cobalt (Co). The Young's modulus of the surface layer of the body was 350 GPa. In Example 3, a 200 μm thick chromium plating layer was formed on the outer surface of the body, and then the outer surface of the chromium plating layer was dulled by electrical discharge machining to produce the rolling work roll. The diameter of the body of the rolling work roll used in Example 3 was 450 mm.

[0076] The electrode used for electrical discharge machining was a plate-shaped graphite electrode. The electrical discharge machining conditions were controlled by varying the pulse current P within the range of 20A to 120A and the pulse time t1 within the range of 10μs to 170μs, so that the product of the pulse current P and pulse time t1 was within the range of 300A·μs to 17000A·μs. The pulse stop time t2 was also varied within the range of 20μs to 1500μs. The number of arc discharges per unit area on the outer surface of the chromium-plated layer (number of discharges) was controlled by varying the machining time T within the range of 10min to 150min, resulting in 20 discharges / mm². 2Over 150 times / mm 2 The following parameters were controlled to be within the specified range. As a result, the outer surface of the chromium plating layer was given a surface roughness of 3.0 μm to 9.5 μm with an arithmetic mean roughness Ra of 3.0 μm or more and a skewness Rsk of -0.50 to 0.40.

[0077] Among the rolling work rolls manufactured in this manner, a work roll having a surface roughness in which the arithmetic mean roughness Ra on the outer surface of the chromium plating layer was 4.0 μm or more and 10.0 μm or less, and the skewness Rsk was -0.4 or more and 0.5 or less, was designated as the inventive example. On the other hand, a rolling work roll having a surface roughness outside the range of 4.0 μm or more and 10.0 μm for the arithmetic mean roughness Ra on the outer surface of the chromium plating layer, and / or outside the range of -0.4 or more and 0.5 for the skewness Rsk, was designated as the comparative example. Using the rolling work rolls of the inventive example and the comparative example, temper rolling of a metal strip with a tensile strength of 1470 MPa class was performed.

[0078] In Example 3, temper rolling was performed so that the rolling load per unit width of the metal strip remained constant at 6.57 kN / mm. In Example 3, a result of 0.2% to 0.6% elongation of the metal strip due to temper rolling was marked as "◎", and a result of 0.1% to less than 0.2% elongation of the metal strip was marked as "〇". On the other hand, a result of 0.1% to 0.6% elongation of the metal strip due to temper rolling was not marked as "×". The surface roughness of the rolling work rolls and the results of temper rolling are shown in Table 4 below.

[0079] [Table 4]

[0080] As shown in Table 4, in the inventive examples No. 20 to 31, an elongation rate of 0.1% to 0.6% of the metal strip could be secured, confirming that the shape of the metal strip could be corrected by temper rolling. On the other hand, in the comparative examples No. 32 and 33, an elongation rate of 0.1% to 0.6% of the metal strip could not be secured with the same rolling load.

[0081] In the invention examples No. 20 to 26, an elongation rate of 0.2% to 0.6% of the metal strip was secured, and a greater elongation rate was achieved than in No. 27 to 31 when the same rolling load was applied. From these results, it was confirmed that it is preferable to impart a surface roughness to the outer surface of the chromium plating layer of the rolling work roll such that the arithmetic mean roughness Ra is 6.0 μm to 10.0 μm and the skewness Rsk is 0.00 to 0.40. Furthermore, it was confirmed that by using a rolling work roll with this surface roughness, a more appropriate elongation rate can be imparted and the shape of the metal strip can be corrected even with a high-strength metal strip having a tensile strength of 1310 MPa or more. [Explanation of Symbols]

[0082] 10 Rolling mill 12a Work Roll 12b Work Roll 14a Backup Roll 14b Backup role 16 Housing 18 Bearing housing 20 Load Detectors 21. Pressure reduction device 22 Bridle Rolls 24 Inlet tension meter 26 Outlet tension meter 28 Outer bridle roll 30 Shaft section 32 Torso 32a Outer surface of the fuselage 34 Chromium plating layer 34a Outer surface of the chromium plating layer 40 Electrical discharge machining equipment 42 Roll support section 44 Electrical discharge machining department 46 Roll Chuck 48 Roll Rotating Motor 50 tailstock 52 electrodes 54 Electrode position control unit 56 Power supply 58 Control device 60 Electric discharge machining oil 62 Oil tank

Claims

1. A plating layer forming step in which a chromium plating layer is formed on the outer surface of a body having a ceramic surface with a Young's modulus of 350 GPa or more, A dulling step in which the outer surface of the chromium plating layer is dulled, Includes, A method for manufacturing a work roll for a rolling mill, wherein in the dulling step, the outer surface of the chromium plating layer is dulled by electrical discharge machining to impart a surface roughness to the outer surface of the chromium plating layer such that the arithmetic mean roughness Ra is 4.0 μm or more and 10.0 μm or less, and the skewness Rsk is -0.4 or more and 0.5 or less.

2. In the dulling step, the product of the pulse current and pulse time is 500 A·μs or more and 12000 A·μs or less, and the number of discharges per unit area of ​​the outer surface of the chromium plating layer is 30 times / mm². ２ Over 800 times / mm ２ A method for manufacturing a rolling work roll according to claim 1, wherein electrical discharge machining is performed under the following electrical discharge machining conditions.

3. A method for temper rolling a metal strip, comprising temper rolling the metal strip using a work roll manufactured by the method for manufacturing a work roll for rolling described in Claim 1 or Claim 2.

4. A method for producing a metal strip having a tensile strength of 1310 MPa or more, using the temper rolling method for a metal strip described in Claim 3.

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