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 a thickness of the chromium plated layer is 5 μm or more and 30 μm or less, arithmetic average roughness Ra of the outer peripheral surface of the chromium plated layer is 4.0 μm or more and 6.0 μm or less, and skewness Rsk is -0.4 or more and 0.1 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 temper rolling method for a metal strip, and a method for manufacturing a metal strip.

Background Art

[0002] In recent years, with the tightening of environmental regulations, the need for weight reduction of automobiles has increased, and the demand for high-strength steel sheets has been on the rise. High-strength steel sheets are manufactured by performing heat treatment on the steel sheets after cold rolling using continuous annealing equipment or continuous hot-dip galvanizing equipment, and appropriately controlling the microstructure of the steel sheets. Then, temper rolling is performed to impart an elongation rate of 2.0% or less to the steel sheets after the heat treatment process, thereby flattening the shape of the steel sheets and imparting an appropriate roughness to the steel sheet surface.

[0003] However, high-strength steel sheets with a tensile strength exceeding 980 MPa after the heat treatment process have a high yield stress, so there is a problem that in a normal temper rolling mill, the rolling load becomes too large and the elongation rate required for shape correction cannot be ensured. To address this problem, Patent Document 1 discloses a method for temper rolling a steel strip, which uses a work roll for rolling with a surface average roughness Ra of 3.0 to 10.0 μm and performs temper rolling with an elongation rate of 0.1% or more on a steel strip with a yield strength of 340 MPa or more.

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

[0005] Patent Document 3 discloses a method for temper rolling a metal strip with a 0.2% proof stress exceeding 350 MPa, which uses a dull work roll with a roll diameter of 1000 mm or more and 1400 mm or less and a Young's modulus of the roll surface layer of 450 GPa or more. [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 thickness of the chromium plating layer is 5 μm or more and 30 μm or less, the arithmetic mean roughness Ra of the outer surface of the chromium plating layer is 4.0 μm or more and 6.0 μm or less, and the skewness Rsk is -0.4 or more and 0.1 or less. [2] A method for manufacturing a work roll for a rolling mill, comprising: a dulling step of shot-dulling the outer surface of a body having a ceramic surface with a Young's modulus of 350 GPa or more; and a plating layer forming step of forming a chromium plating layer on the outer surface of the shot-dulled body, wherein in the dulling step, the dulling conditions are set according to the thickness of the chromium plating layer such that the arithmetic mean roughness Ra of the outer surface after the chromium plating layer is formed by the plating layer forming step is 4.0 μm or more and 6.0 μm or less, and the skewness Rsk is -0.4 or more and 0.1 or less. [3] The method for manufacturing a rolling work roll according to [2], wherein the average particle size of the shot particles used in the shot dulling process is 250 μm or more and 710 μm or less. [4] A method for manufacturing a rolling work roll according to [2] or [3], wherein the shot dulling process uses an air blast type dulling apparatus and the projection pressure of the shot particles is 0.3 MPa or more and 0.7 MPa or less. A method for temper rolling a metal strip, comprising temper rolling the metal strip using the rolling work rolls described in [5][1]. 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 any of [6], [2] to [4]. A method for producing a metal strip having a tensile strength of 1310 MPa or more, using the temper rolling method for metal strips described in [7][5]. A method for producing a metal strip having a tensile strength of 1310 MPa or more, using the temper rolling method for metal strips described in [8][6]. [Effects of the Invention]

[0012] By using the rolling work roll according to the present invention, even high-strength metal strips with a tensile strength of 1310 MPa or more can be given an appropriate elongation rate through temper rolling, thereby correcting the shape of the metal strip. Furthermore, by temper rolling with the rolling work roll, even high-strength metal strips with a tensile strength of 1310 MPa or more can be manufactured into metal strips with a flattened shape. [Brief explanation of the drawing]

[0013] [Figure 1] Figure 1 is a schematic diagram showing an example of a temper rolling mill equipped with rolling work rolls according to this embodiment. [Figure 2] Figure 2 is a schematic diagram showing an example of the installation of a temper rolling mill in a metal strip manufacturing line. [Figure 3] Figure 3 is a schematic diagram of the rolling work roll 12a before the chromium plating layer is formed. [Figure 4] Figure 4 is a schematic diagram of a rolling mill work roll on which a chromium plating layer has been formed. [Figure 5] Figure 5 is a schematic cross-sectional view of the outer surface of the chromium plating layer. [Figure 6] Figure 6 is a schematic cross-sectional view showing the state in which the protrusions on the outer surface of the chromium plating layer are pressed into the surface of the metal strip. [Figure 7] Figure 7 is a schematic cross-sectional view of the outer surface of the chromium plating layer. [Figure 8]FIG. 8 is a schematic cross-sectional view showing a state in which irregularities on the outer peripheral surface of the chromium plating layer are pressed into the surface of the metal strip. [Figure 9] FIG. 9 is a schematic view of a shot blast apparatus which is an example of a shot peening apparatus for performing shot peening. [Figure 10] FIG. 10 is a schematic view showing a state of irregularities formed on the surface of the workpiece by shot particles. [Figure 11] FIG. 11 is a graph showing an example of the influence of the particle size and air pressure of shot particles, which are shot peening processing conditions, on the arithmetic mean roughness Ra and skewness Rsk. [Figure 12] FIG. 12 is a graph showing the relationship between the thickness of the chromium plating layer and the surface roughness of the outer peripheral surface of the plating layer.

MODE FOR CARRYING OUT THE INVENTION

[0014] Hereinafter, the present invention will be specifically described through embodiments of the present invention. The following embodiments show a preferred example of the present invention, and the present invention is not limited by these embodiments.

[0015] The work roll for rolling according to the present embodiment is used as a work roll of a temper rolling mill. FIG. 1 is a schematic view showing an example of a temper rolling mill 10 (hereinafter, may be referred to as "rolling mill 10") including the work roll for rolling according to the present embodiment. FIG. 1(a) is a side schematic view of the rolling mill 10, and FIG. 1(b) is a front schematic view of the rolling mill 10. The rolling mill 10 is used for temper rolling a high-strength metal strip having a tensile strength of 1310 MPa or more, for example, as the metal strip.

[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] Next, shot dulling is performed on the outer surface 32a of the body, which has a Young's modulus of 350 GPa or more in the surface layer. This process is called the dulling step. This imparts microscopic irregularities to the outer surface 32a of the body. Shot dulling is a processing method that forms microscopic irregularities on the surface of a material by spraying and impacting the surface of the material to be treated with metal or ceramic particles.

[0023] In shotdal processing, metal or ceramic particles that are impacted onto the workpiece are called shot particles. Shot particles can be spherical or polygonal, and either type can be used. Spherical particles are often manufactured by gas atomization, and the surface tension of the molten metal results in a shape close to spherical. When spherical particles are used as shot particles, residual stress is formed on the surface of the workpiece. This residual stress is compressive, and is sometimes called shot peening.

[0024] On the other hand, polygonal-shaped shot particles are manufactured by water atomization or pulverization. When polygonal-shaped particles are used as shot particles, microscopic irregularities are formed on the surface of the material to be treated while removing a portion of the surface. For polygonal-shaped shot particles, cut wire, which is made by finely cutting metal wire, or sand extracted from minerals such as silica sand, may also be used (sometimes called sandblasting). In the shot dulling process in the manufacturing method of the rolling work roll according to this embodiment, any of the above shot particles may be used.

[0025] The average particle size of the shot particles used in shot dulling is preferably between 250 μm and 710 μm. If the average particle size of the shot particles is less than 250 μm, it becomes difficult to impart a sufficiently large arithmetic mean roughness Ra to the surface of the rolling work roll, which is undesirable. If the average particle size of the shot particles exceeds 710 μm, the arithmetic mean roughness Ra imparted to the surface of the rolling work roll becomes excessive, which is also undesirable. Furthermore, repeated collisions of large-particle-sized shot particles with the surface of the rolling work roll can cause the convex portions to collapse, reducing the skewness Rsk to less than -0.4, which is also undesirable.

[0026] Furthermore, it is preferable to use hard shot particles such as zirconia, alumina, black silicon carbide, and diamond for shot-dull processing, and it is preferable that these particles have a Vickers hardness of 1800 HV or higher. In order to impart relatively large irregularities to the outer surface 32a of the barrel, the shot particles are made to collide with the outer surface 32a of the barrel at a relatively high speed, so it is preferable to use hard shot particles from the viewpoint of durability.

[0027] On the other hand, means of imparting microscopic irregularities to the outer surface 32a of the body include, in addition to shotdal machining, electrical discharge machining, laser machining, and electron beam machining. However, in methods such as electrical discharge machining, laser machining, and electron beam machining, which partially melt the surface of the material to be processed to form irregularities, thermal effects occur on the outer surface 32a of the body, causing microscopic cracks to occur on the outer surface 32a of the ceramic body due to thermal effects. For this reason, shotdal machining, which has less thermal effect, is used in this embodiment.

[0028] 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 in Figure 4(a). In the manufacture of the rolling work roll 12a according to this embodiment, a chromium plating layer 34 is formed on the outer peripheral surface 32a of the body which has been shotdull processed. In this case, the shotdull processing conditions are set according to the thickness of the chromium plating layer 34 such that the arithmetic mean roughness Ra on the outer peripheral surface 34a of the chromium plating layer is 4.0 μm or more and 6.0 μm or less, and the skewness Rsk is -0.4 or more and 0.1 or less. The specific method for setting the shotdull processing conditions will be described later.

[0029] The chromium plating layer 34 formed on the outer circumferential surface 32a of the body, which has undergone shotdal processing, is a hard chromium plating layer. The hard chromium plating layer is applied primarily to ensure wear resistance. 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 between 750 HV and 1000 HV.

[0030] Forming a chromium plating layer 34 on the outer circumferential surface 32a of the body prevents the hard ceramic from contacting the backup rolls by forming a film with lower hardness than the body 32, thereby reducing damage to the backup rolls that come into contact with the rolling work rolls. Furthermore, using hard chromium plating can suppress wear on the rolling work rolls 12a that come into direct contact with the metal strip. From this viewpoint, in this embodiment, it is preferable that the thickness of the chromium plating layer 34 formed on the outer circumferential surface 32a of the body be 5 μm or more. On the other hand, if the thickness of the chromium plating layer 23 is greater than 30 μm, the microscopic irregularities on the outer circumferential surface 32a of the body, which were imparted by shotdal processing, change due to the formation of the chromium plating layer 34, making it difficult to obtain the target arithmetic mean roughness Ra and skewness Rsk. For this reason, it is preferable that the thickness of the chromium plating layer 23 be 30 μm or less. From the above viewpoint, it is more preferable that the thickness of the chromium plating layer 23 be 5 μm or more and 10 μm or less.

[0031] In this embodiment, the shotdal processing conditions are set such that the arithmetic mean roughness Ra of the chromium plating layer formed on the outer surface of the rolling work roll 12a is 4.0 μm or more and 6.0 μm or less, and the skewness Rsk is -0.4 or more and 0.1 or less. As a result, the surface roughness of the rolling work roll 12a has an arithmetic mean roughness Ra of 4.0 μm or more and 6.0 μm or less, and a skewness Rsk of -0.4 or more and 0.1 or less.

[0032] Arithmetic mean roughness Ra and skewness Rsk are line roughness values ​​specified in JIS B 0601:2013. Arithmetic mean roughness Ra is an index representing the average height of the roughness curve over a reference length. Skewness Rsk is defined by the following equation (1).

[0033]

number

[0034] 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.

[0035] 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.

[0036] Furthermore, by applying shotdal processing to the outer surface 32a of the body, the arithmetic mean roughness Ra of the outer surface 34a of the chromium plating layer is set to 4.0 μm or more and 6.0 μm or less, and the skewness Rsk is set to -0.4 or more and 0.1 or less. In this way, by appropriately controlling the shape of the microscopic irregularities on the surface of the rolling work roll 12a, it becomes possible to perform temper rolling while suppressing the increase in rolling load, even for high-strength metal strips with a tensile strength of 1310 MPa or more.

[0037] 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 a surface roughness of arithmetic mean roughness Ra of 4.0 μm or more and 6.0 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 the effect of stretching the metal strip 100 as a whole (stretching effect).

[0038] 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. Note that the schematic cross-sectional views shown in Figures 5-8 are shown with a larger vertical magnification than the horizontal magnification to make it easier to understand the shape of the irregularities. Therefore, they are shown with a greater slope than the actual slope of the irregularities.

[0039] 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.

[0040] 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.

[0041] 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 it will also become difficult to perform shot dulling on the outer surface 32a of the body to increase the arithmetic mean roughness Ra of the outer surface 34a of the chromium plating layer. 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 6.0 μm.

[0042] 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 0.1 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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. Also, when the skewness Rsk is large, there is a risk of microscopic cracks occurring on the outer circumferential surface 32a of the body during the dulling step. For this reason, the skewness Rsk applied to the outer circumferential surface 34a of the chromium plating layer must be between -0.4 and 0.1, and preferably between -0.2 and 0.1. More preferably, the skewness Rsk applied to the outer circumferential surface 34a of the chromium plating layer must be between 0.0 and 0.1. This further suppresses the increase in rolling load.

[0047] 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.

[0048] Next, the shot dulling method for the rolling work rolls in this embodiment will be described. Figure 9 is a schematic diagram of a shot blasting apparatus 40, which is an example of a shot dulling apparatus that performs shot dulling. The shot blasting apparatus 40 is an air blast type apparatus and consists of an air blast nozzle 42 and a projection chamber 44. The air blast nozzle 42 is connected to a shot particle supply unit 48 that supplies shot particles 46 to the air blast nozzle 42. The air blast nozzle 42 is also connected to a compressed air supply system 50 that supplies compressed air from a compressor (not shown).

[0049] The air blast nozzle 42 is configured with an internal shape that accelerates the compressed air supplied from the compressed air supply system 50. The compressed air accelerated inside the air blast nozzle 42 accelerates the shot particles 46 and sprays them onto the workpiece. The projection chamber 44 is configured to house the rolling work rolls inside, and is designed to prevent the shot particles 46 that are projected onto the rolling work rolls and scattered from flying out. The pressure of the compressed air supplied to the air blast nozzle 42 is called the projection pressure or air pressure.

[0050] In the air blast type shot blasting apparatus 40, the shot particles 46 projected in the projection chamber 44 are collected at the bottom of the projection chamber 44 and have a circulation system for reusing the shot particles 46. For example, the shot particle supply unit 48 is composed of a cyclone and is configured to remove the shot particles 46 that have been crushed and made smaller, and to supply only shot particles 46 of a certain size or larger to the air blast nozzle 42. On the other hand, the shot particles 46 that have been crushed and made smaller are separated by the cyclone and collected by the dust collector 52.

[0051] In shotdull machining, the microscopic irregularities formed on the surface of the workpiece vary depending on the particle size (average particle size, particle size distribution), material (hardness, Young's modulus), and shape (spherical, polygonal) of the shot particles 46. This is because the particle size, material, and shape of the shot particles 46 affect the impact force when they collide with the surface of the workpiece and the deformation state of the workpiece surface upon impact. Furthermore, the microscopic irregularities formed on the surface of the workpiece also vary depending on the impact velocity (projection velocity), projection density (weight or number of shot particles 46 projected per unit area of ​​the workpiece), and projection time (the time during which the shot particles 46 are projected onto the workpiece). The impact velocity, projection density, and projection time affect the impact force when the shot particles 46 collide with the surface of the workpiece and the amount of work done to form the irregularities, thereby affecting the deformation state of the workpiece surface.

[0052] In shot dulling, which imparts microscopic irregularities to the surface of a rolling work roll 12a, the arithmetic mean roughness Ra of the rolling work roll surface can be increased by increasing the momentum imparted to the shot particles 46. To increase the momentum imparted to the shot particles 46, a larger average particle size should be selected for the shot particles 46, and the projection speed of the shot particles 46 should be increased. This increases the impact force of the shot particles 46 colliding with the work roll surface, resulting in the formation of relatively large irregularities. On the other hand, to increase the skewness Rsk of the rolling work roll, a relatively small average particle size should be selected for the shot particles 46.

[0053] Figure 10 is a schematic diagram showing the state of irregularities formed on the surface of a workpiece by shot particles 46. Figure 10(a) shows the case when the particle size of the shot particles 46 is small, and Figure 10(b) shows the case when the particle size of the shot particles 46 is large. As shown in Figure 10(a), when the particle size of the shot particles 46 is small, many local depressions are formed, resulting in an irregular shape with pointed tops. Therefore, the skewness Rsk of the workpiece surface becomes large. On the other hand, as shown in Figure 10(b), when the particle size of the shot particles 46 is large, even if pointed protrusions are formed on the surface of the workpiece, the tops of the protrusions are crushed by subsequent impacting shot particles 46, and pointed protrusions are less likely to remain. Therefore, the skewness Rsk of the workpiece surface becomes small.

[0054] For the reasons stated above, in the method for manufacturing a rolling work roll according to this embodiment, shot particles 46 with an average particle size of 710 μm or less are used as the shot particles 46 projected onto the outer circumferential surface 32a of the body. On the other hand, if the average particle size of the shot particles 46 is too small, the arithmetic mean roughness Ra of the outer circumferential surface 34a after the chromium plating layer 34 is formed may be less than 4.0 μm. For this reason, it is preferable to use shot particles 46 with an average particle size of 250 μm or more and 710 μm or less as the shot particles 46 projected onto the outer circumferential surface 32a of the body. Furthermore, it is even more preferable to use shot particles 46 with an average particle size of 250 μm or more and 400 μm or less as the shot particles 46 projected onto the outer circumferential surface 32a of the body. The particle size of the shot particles 46 may be expressed by a grit number, in which case it is preferable to use shot particles 46 with a grit number of #24 or more and #60 or less.

[0055] Furthermore, in order to make the arithmetic mean roughness Ra of the outer peripheral surface 23a after the chromium plating layer 23 is formed 4.0 μm or more, it is preferable to control the projection speed of the shot particles 46 to 200 m / s or more. To project shot particles at a high speed such as 200 m / s or more, it is preferable to use an air blast type shot dulling apparatus. In this case, it is preferable to set the air pressure (projection pressure) for projecting the shot particles to 0.3 MPa or more. On the other hand, if the air pressure is greater than 0.7 MPa, the projection chamber 44 may be destroyed by erosion. Therefore, it is preferable that the air pressure for projecting the shot particles 46 be between 0.3 MPa and 0.7 MPa.

[0056] Figure 11 is a graph showing an example of the effects of the shot particle size 46 and air pressure, which are processing conditions for shot dulling, on the arithmetic mean roughness Ra and skewness Rsk. As shown in Figure 11, when the particle size of the shot particles 46 decreases, the arithmetic mean roughness Ra of the microscopic irregularities formed on the outer surface 32a of the body decreases, and the skewness Rsk increases. Also, when the air pressure for projecting the shot particles 46 increases, the arithmetic mean roughness Ra of the microscopic irregularities formed on the outer surface 32a of the body increases, and the skewness Rsk increases. In this way, by controlling the processing conditions for shot dulling, the arithmetic mean roughness Ra and skewness Rsk formed on the outer surface 32a of the body can be controlled to an appropriate range (the area shown in gray in Figure 11).

[0057] In this embodiment, the shotdull processing conditions are set such that the arithmetic mean roughness Ra of the outer surface 34a of the chromium plating layer 34 is 4.0 μm or more and 6.0 μm or less, and the skewness Rsk is -0.4 or more and 0.1 or less after the chromium plating layer 34 has been formed. Furthermore, it is preferable to set the processing conditions for shotdull processing according to the thickness of the chromium plating layer 34. Here, according to the thickness of the chromium plating layer 34 means that when performing shotdull processing on the outer surface 32a of the body, the processing conditions are set to correspond to the thickness of the chromium plating layer 34 that has been set in advance according to the manufacturing specifications.

[0058] The thicker the chromium plating layer 34, the less likely microscopic irregularities formed on the outer surface 32a of the body are to remain on the outer surface 34a of the chromium plating layer, thus reducing the arithmetic mean roughness Ra and skewness Rsk. The processing conditions for shotdal machining on the outer surface 32a of the body are set taking into account this reduction in arithmetic mean roughness Ra and skewness Rsk corresponding to the thickness.

[0059] Specifically, the reduction amounts ΔRa and ΔRsk in arithmetic mean roughness Ra and skewness Rsk due to the chromium plating layer 34 are predetermined according to the thickness of the chromium plating layer 23. Then, when performing shotdal machining on the outer surface 32a of the body, the machining conditions should be set so that the arithmetic mean roughness Ra and skewness Rsk obtained by adding the reduction amount ΔRa and ΔRsk corresponding to the predetermined thickness of the chromium plating layer 34 to the target arithmetic mean roughness Ra and skewness Rsk are equal.

[0060] Figure 12 is a graph showing the relationship between the thickness of the chromium plating layer 34 and the surface roughness of the outer surface of the plating layer. Figure 12(a) is a graph showing the relationship between the thickness of the chromium plating layer 34 and the arithmetic mean roughness Ra, and Figure 12(b) is a graph showing the relationship between the thickness of the chromium plating layer 34 and the skewness Rsk.

[0061] As shown in Figure 12(a), even if the arithmetic mean roughness Ra of the workpiece before chromium plating is 5.0 μm, the arithmetic mean roughness Ra after chromium plating decreases when the thickness of the chromium plating layer 34 is 30 μm or more. On the other hand, the change in skewness Rsk due to chromium plating behaves differently depending on whether the skewness Rsk at the outer surface 32a of the body is positive or negative. As shown in Figure 12(b), when the skewness Rsk at the outer surface 32a of the body is positive, the skewness Rsk after chromium plating decreases when the thickness of the chromium plating layer 34 is 30 μm or more. On the other hand, when the skewness Rsk at the outer surface 32a of the body is negative, the skewness Rsk after chromium plating increases when the thickness of the chromium plating layer 34 is 30 μm or more.

[0062] Thus, since the arithmetic mean roughness Ra and skewness Rsk change with the thickness of the chromium plating layer, if the changes with respect to the thickness are specified in advance, processing conditions can be set such that the arithmetic mean roughness Ra of the outer surface 34a of the chromium plating layer is between 4.0 μm and 6.0 μm, and the skewness Rsk is between -0.4 and 0.1.

[0063] 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 obtained. In this case, the diameter of the rolling work roll 2 is preferably 300 mm or more and 800 mm or less, similar to the work rolls of a normal temper rolling mill. If the diameter of the rolling work roll 12a is less than 300 mm, the deflection of the rolling work roll will increase when the rolling load is high, which may cause the shape of the metal strip 100 to become distorted, so this is undesirable. Also, if the diameter of the rolling work roll 12a is larger than 800 mm, the rolling load will increase, which is undesirable because the effect of using the rolling work roll 12a of this embodiment will be reduced.

[0064] In this embodiment, temper rolling is performed using one or more rolling mills 10 incorporating the rolling work rolls 12a according to this embodiment, thereby imparting an elongation of 0.1% to 0.6% to the metal strip. 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, which is necessary for correcting the shape of the metal strip 100, can be secured. However, it is preferable that the elongation imparted to high-strength metal strips be 0.6% or less. If the imparted elongation is greater than 0.6%, it is undesirable because it may cause excessive strain on the metal strip 100, leading to a decrease in the elongation (ductility) of the metal strip product due to work hardening. Furthermore, if the imparted elongation is greater than 0.6%, it is undesirable because the frictional force at the contact interface between the irregularities on the surface of the rolling work rolls 12a and the metal strip 100 increases, which may increase the rolling load during temper rolling. In addition, a rolling mill with standard equipment specifications refers to a rolling mill capable of applying a rolling load of 5 kN / mm to 15 kN / mm.

[0065] 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]

[0066] <Example 1> Next, we will describe Example 1, in which a metal strip with a tensile strength of 1310 MPa, a thickness of 2.1 mm, and a width of 1200 mm was subjected to temper rolling. The arithmetic mean roughness Ra of the metal strip before temper rolling was 0.3 μm. In Example 1, a rolling work roll was used in which a ceramic body consisting of 79 mass% tungsten carbide and the remainder being a metal binder, or a forged steel body, was shot-dulled on its outer surface, and a chromium plating layer was formed on its outer surface. The diameter of the body was 530 mm, the Young's modulus of the ceramic was 390 GPa, and the Young's modulus of the forged steel was 250 GPa. The diameter of the body was 530 mm, and the thickness of the body was 20 mm.

[0067] In Example 1, the outer surface of the body was subjected to shotdull processing using sandblasting, a type of air blasting method. Shotdull processing was performed by changing the particle size of the shot particles and the air pressure used to project the shot particles, and the arithmetic mean roughness Ra and skewness Rsk of the microscopic irregularities formed on the outer surface of the body were confirmed. Furthermore, after shotdull processing, chromium plating was applied to the outer surface of the body, and the arithmetic mean roughness Ra and skewness Rsk formed on the outer surface of the chromium plating layer were confirmed. The types of shot particles used in the example are shown in Table 1 below.

[0068] [Table 1]

[0069] 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 is 4.0 μm or more and 6.0 μm or less, and the skewness Rsk is -0.4 or more and 0.1 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 6.0 μm or less 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 0.1 or less for the skewness Rsk was designated as the comparative example.

[0070] The rolling work rolls of the inventive example and comparative example were loaded into a four-stage rolling mill, and temper rolling was performed on a metal strip with a tensile strength of 1310 MPa (yield strength of 1160 MPa). The temper rolling mill used had a backup roll diameter of 1400 mm. The temper rolling conditions were controlled to maintain a constant rolling load of 11000 kN, and the elongation rate (stretch rate) applied to the steel sheet was measured. In other words, a larger elongation rate indicates a greater reduction in rolling load. The forward and backward tensions applied during temper rolling were both controlled to remain constant at 100 kN. The surface roughness of the rolling rolls and the results of temper rolling are shown in Table 2 below.

[0071] [Table 2]

[0072] Table 2 shows that by pre-determining the difference between the surface roughness after dulling and the surface roughness after chromium plating, corresponding to the thickness of the chromium plating layer, it is possible to determine the processing conditions for shot dulling that result in appropriate arithmetic mean roughness Ra and skewness Rsk after chromium plating.

[0073] Furthermore, as shown in Table 2, for samples No. 1 to 5, where the Young's modulus of the body was 350 GPa or higher, the arithmetic mean roughness Ra after chromium plating was 4.0 μm or more and 6.0 μm or less, and the skewness Rsk was -0.4 or more and 0.1 or less, the elongation rate of the tempered rolling was 0.1% or higher. In particular, for samples No. 1 to 4, the elongation rate of the tempered rolling was 0.2%, which was a more favorable result than for No. 5. On the other hand, for sample No. 6, which used forged steel with a Young's modulus of 250 GPa for the body, the elongation rate of the tempered rolling was 0%, and the metal strip could not be given an appropriate elongation rate. Similarly, for samples No. 7 to 12, where the arithmetic mean roughness Ra after chromium plating did not satisfy 4.0 μm or more and 6.0 μm or less, or the skewness Rsk did not satisfy -0.4 or more and 0.1 or less, the elongation rate of the tempered rolling was 0%, and the metal strip could not be given an appropriate elongation rate. These results confirm that by performing temper rolling using the rolling work rolls according to this embodiment, it is possible to impart an appropriate elongation rate and correct the shape of a metal strip, even if it is a high-strength metal strip with a tensile strength of 1310 MPa.

[0074] <Example 2> As Example 2, we will describe an example of temper rolling of a metal strip with a tensile strength of 1310 MPa (yield strength of 1160 MPa). The metal strip to be rolled had a thickness of 1.2 mm and a width of 1050 mm. In Example 2, a rolling work roll was used in which the outer surface of a ceramic body, consisting of 79% by mass of tungsten carbide and the remainder being a metal binder, was shot-dal processed, and a chromium plating layer was formed on the outer surface. The Young's modulus of the ceramic was 390 GPa, and the diameter of the body was 530 mm.

[0075] In Example 2, shotdull blasting was performed on the body of the object before the chromium plating layer was formed, using the shot blasting apparatus shown in Figure 9. Zirconia alumina (a composite material of alumina and zirconia) particles were used as the shot particles. Shotdull blasting was performed by changing the particle size of the shot particles and the air pressure used to project the shot particles, and the arithmetic mean roughness Ra and skewness Rsk values ​​of the microscopic irregularities formed on the outer surface of the body were adjusted. Furthermore, a chromium plating layer with a thickness of 10 μm was formed on the outer surface of the body after shotdull blasting. The arithmetic mean roughness Ra and skewness Rsk of the outer surface of the chromium plating layer were then measured.

[0076] 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 is 4.0 μm or more and 6.0 μm or less, and the skewness Rsk is -0.4 or more and 0.1 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 6.0 μm or less 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 0.1 or less for the skewness Rsk was designated as the comparative example.

[0077] In Example 2, the metal strip was temper-rolled so that the rolling load per unit width of the metal strip was kept constant at 6.3 kN / mm. In Example 2, if the elongation rate of the metal strip after temper rolling was secured to be between 0.2% and 0.6%, it was marked as "◎", and if the elongation rate of the metal strip was between 0.1% and less than 0.2%, it was marked as "〇". On the other hand, 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 surface roughness of the outer surface of the chromium plating layer of the rolling work roll and the results of temper rolling are shown in Table 3 below.

[0078] [Table 3]

[0079] As shown in Table 3, in the inventive examples No. 20 to 26, 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. 27 and 28, an elongation rate of 0.1% to 0.6% of the metal strip could not be secured with the same rolling load.

[0080] In the invention examples No. 20-22, 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. 23-26 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 4.0 μm to 6.0 μm and the skewness Rsk is 0.00 to 0.10. 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 high-strength metal strips having a tensile strength of 1310 MPa or more. [Explanation of symbols]

[0081] 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 Shot blasting equipment 42 Air blast nozzles 44 Projection room 46 Shot Particles 48 Shot Particle Supply Unit 50 Compressed air supply system 52 Dust collector

Claims

1. A dulling step in which the outer surface of a body having a ceramic with a Young's modulus of 350 GPa or more in the surface layer is shot dulled, A plating layer forming step of forming a chromium plating layer on the outer surface of the shotdal-processed body portion, Includes, A method for manufacturing a work roll for a rolling mill, wherein in the dulling step, the dulling conditions are set according to the thickness of the chromium plating layer such that the arithmetic mean roughness Ra of the outer surface after the chromium plating layer is formed by the plating layer forming step is 4.0 μm or more and 6.0 μm or less, and the skewness Rsk is -0.4 or more and 0.1 or less.

2. The method for manufacturing a rolling work roll according to claim 1, wherein the average particle size of the shot particles used in the shotdal processing is 250 μm or more and 710 μm or less.

3. A method for manufacturing a rolling work roll according to claim 1 or claim 2, wherein the shot dulling process uses an air blast type dulling apparatus and the projection pressure of the shot particles is 0.3 MPa or more and 0.7 MPa or less.

4. 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.

5. 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 3.

6. A method for manufacturing a metal strip, comprising using the temper rolling method for a metal strip described in Claim 4 to produce a metal strip with a tensile strength of 1310 MPa or more.

7. A method for manufacturing a metal strip, comprising using the temper rolling method for a metal strip described in Claim 5 to produce a metal strip with a tensile strength of 1310 MPa or more.

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