Roll forming machine and roll forming method

Abstract

This roll forming machine used for roll forming of a metal plate includes: a first roll group which includes at least one set of rolls forming a first pass through which a metal plate passes, and in which the inclination angle of the normal direction of a cross section of the first pass with respect to the horizontal direction is a first angle (θ1); and a second roll group which includes at least one set of rolls forming a second pass through which the metal plate passes and which is arranged upstream of the first roll group, and in which the inclination angle of the normal direction of a cross section of the second pass with respect to the horizontal direction is a second angle (θ2) larger than the first angle (θ1).

Description

Roll forming machine and roll forming method

[0001] The present disclosure relates to a roll forming machine and a roll forming method.

[0002] In manufacturing processes using roll forming machines, a strip is typically continuously fed and gradually bent with rolls to form the desired cross-sectional shape. In the case of electric resistance welded pipes made from steel or non-ferrous metals such as aluminum, the strip is formed into an open pipe with an arc-shaped cross section, and then both widthwise ends of the open pipe are heated by high-frequency induction or the like and butt-welded with squeeze rolls. In roll forming, the open pipe is typically formed by breakdown forming or cage forming in the upstream stage of the forming machine. Then, in the downstream stage of the forming machine, fin pass forming is performed to equalize the curvature distribution of the arc-shaped cross section of the open pipe, freeze the shape, and feed the open pipe to the squeeze rolls. In the target cross-sectional dimensions of an electric resistance welded pipe, the smaller the ratio of wall thickness to outer diameter, i.e., wall thickness / outer diameter, the more likely buckling of both widthwise ends of the open pipe, known as edge buckling, occurs during fin pass forming. Therefore, to prevent buckling, downhill forming is used, in which the pass line is set higher in the upstream section of the forming machine than in the downstream section, and the height is gradually lowered toward the downstream section. Downhill forming stretches the open pipe bottom, reducing the geometric trajectory difference with the open pipe width end during the forming process and reducing the longitudinal tension and compression forces acting on the open pipe width end. Therefore, many technologies related to downhill forming are concerned with suppressing edge buckling. For example, Patent Document 1 proposes a technology for reducing the impact of work hardening of the open pipe bottom, which occurs when the open pipe bottom is stretched during downhill forming, on the component performance of the product. Specifically, it proposes adjusting the downhill height of the fin pass roll to prevent the open pipe bottom from overtaking the groove bottom of the lower roll of the fin pass forming, thereby suppressing work hardening due to unnecessary longitudinal bending and unbending deformation of the open pipe bottom.

[0003] JP 2009-119479 A

[0004] As mentioned above, the purpose of utilizing downhill forming to date is mainly to suppress edge buckling by reducing the geometric trajectory difference between the open tube end and the open tube bottom during forming, and to reduce the effect of work hardening at the open tube bottom. However, no solution has been proposed to address the effect of work hardening around the open tube width end.

[0005] In conventional downhill forming, the cross section of the groove formed by each forming roll is oriented horizontally. When the difference in downhill height between the front and rear forming rolls is large, the open tube undergoes rapid longitudinal bending deformation when entering the rear forming roll. Therefore, in conventional downhill forming, the impact of work hardening around the open tube width edge on the secondary formability of the product cannot be ignored. This impact is particularly pronounced around the open tube width edge in fin-pass forming, where deformation due to winding of the rear roll around the upper roll is significant.

[0006] An object of the present disclosure is to suppress excessive work hardening around the open tube width ends in downhill forming.

[0007] (1) A roll forming machine according to one embodiment of the present disclosure is a roll forming machine used for roll forming of a metal sheet, comprising: a first roll group including at least one set of rolls forming a first groove through which the metal sheet passes, the first groove having a first inclination angle relative to the horizontal direction in the direction normal to the cross section of the first groove; and a second roll group including at least one set of rolls forming a second groove through which the metal sheet passes, the second roll group being arranged in a stage before the first roll group, the second roll group having a second inclination angle relative to the horizontal direction in the direction normal to the cross section of the second groove that is larger than the first angle.

[0008] (2) A roll forming machine according to one embodiment of the present disclosure is the roll forming machine described in (1), wherein at least one set of rolls forming the second groove includes forming rolls that adjust the shape of the metal plate.

[0009] (3) A roll forming machine according to an embodiment of the present disclosure is the roll forming machine according to (2), in which the forming roll is a breakdown roll.

[0010] (4) A roll forming machine according to one embodiment of the present disclosure is the roll forming machine described in any one of (1) to (3), wherein at least one set of rolls forming the second groove includes a guide roll that adjusts the position of the metal plate.

[0011] (5) A roll forming machine according to one embodiment of the present disclosure is the roll forming machine described in any one of (1) to (4), wherein at least one set of rolls forming the first groove includes a fin pass roll that adjusts the shape of the metal plate.

[0012] (6) A roll forming machine according to an embodiment of the present disclosure is the roll forming machine described in any one of (1) to (5), wherein the second angle is equal to or greater than 0.4° and equal to or less than 20°.

[0013] (7) A roll forming machine according to an embodiment of the present disclosure is the roll forming machine described in any one of (1) to (6), in which the first angle is 0°.

[0014] (8) A roll forming machine according to one embodiment of the present disclosure is a roll forming machine described in any one of (1) to (7), wherein, when the rear side of the axial center of the roll closest to the second roll group included in the first roll group is positive and the front side is negative, the intersection of the central axis of the first groove die and the central axis of the second groove die is located at a position away from the axial center that is at least -4 times and at most 3 times the outer diameter of the product tube manufactured by the roll forming.

[0015] (9) A roll forming machine according to one embodiment of the present disclosure is a roll forming machine described in any one of (1) to (8), and further includes a third roll group including at least one set of rolls that form a third groove through which the metal plate passes, and that is arranged in a stage before the second roll group, and in which the inclination angle of the normal direction of the cross section of the third groove relative to the horizontal direction is a third angle that is greater than the first angle.

[0016] (10) A roll forming machine according to an embodiment of the present disclosure is the roll forming machine according to (9), in which the third angle is equal to or greater than the second angle.

[0017] (11) A roll forming machine according to one embodiment of the present disclosure is the roll forming machine described in (9) or (10), wherein at least one set of rolls forming the second groove includes guide rolls that adjust the position of the metal plate, and at least one set of rolls forming the third groove includes forming rolls that adjust the shape of the metal plate.

[0018] (12) A roll forming machine according to an embodiment of the present disclosure is the roll forming machine according to any one of (1) to (11), wherein an electric resistance welded pipe is manufactured by the roll forming.

[0019] (13) A roll forming method according to one embodiment of the present disclosure is a roll forming method used for roll forming of a metal sheet, comprising: a first roll group including at least one set of rolls forming a first caliber, wherein the inclination angle of the normal to the cross section of the first caliber with respect to the horizontal is a first angle, adjusting the shape or position of the metal sheet when the metal sheet passes through the first caliber; and a second roll group including at least one set of rolls forming a second caliber, disposed upstream of the first roll group, wherein the inclination angle of the normal to the cross section of the second caliber with respect to the horizontal is a second angle larger than the first angle, adjusting the shape or position of the metal sheet when the metal sheet passes through the second caliber.

[0020] According to the present disclosure, excessive work hardening around the open pipe width end portion during downhill forming can be suppressed.

[0021] FIG. 1 is a diagram illustrating the configuration of a roll forming machine according to an embodiment of the present disclosure. FIG. 2 is a diagram illustrating the arrangement of each roll for downhill forming according to an embodiment of the present disclosure. FIG. 3 is a front view of a fin pass roll according to an embodiment of the present disclosure. FIG. 4 is a front view of a guide roll according to an embodiment of the present disclosure. FIG. 5 is a front view of a forming roll according to an embodiment of the present disclosure. FIG. 6 is a diagram illustrating a modified configuration of a roll forming machine according to an embodiment of the present disclosure. FIG. 7 is a table illustrating the chemical components of each steel type of strip used in Examples and Comparative Examples. FIG. 8 is a table illustrating the steel type of the strip, steel pipe size, downhill height, and inclination of the normal direction of the caliber cross section in Examples and Comparative Examples. FIG. 9 is a table illustrating the results of evaluation in Examples and Comparative Examples. FIG. 10 is a graph illustrating the circumferential distribution of Vickers hardness measured on the outer surface around the weld of an electric resistance welded pipe in Examples and Comparative Examples. FIG. 11 is a diagram illustrating the arrangement of each roll for downhill forming according to a Comparative Example.

[0022] Hereinafter, an embodiment of the present disclosure will be described with reference to the drawings.

[0023] In each drawing, the same or corresponding parts are denoted by the same reference numerals. In the description of this embodiment, the description of the same or corresponding parts will be omitted or simplified as appropriate.

[0024] The configuration of a roll forming machine 10 according to this embodiment will be described with reference to FIG.

[0025] The roll forming machine 10 is an apparatus used for roll forming of a metal plate. In this embodiment, the roll forming machine 10 is an apparatus for manufacturing an electric resistance welded pipe by roll forming. The metal plate is specifically a metal strip 11.

[0026] The roll forming machine 10 includes a first roll group 21 and a second roll group 22 arranged upstream of the first roll group 21. The first roll group 21 is specifically a fin pass roll group. The second roll group 22 is specifically a guide roll group.

[0027] In this embodiment, the roll forming machine 10 further includes a third roll group 23 arranged in a stage upstream of the second roll group 22. The third roll group 23 is specifically a breakdown roll group.

[0028] The roll forming machine 10 further includes a leveler 12 , an edge milling device 13 , an electrode 15 , a squeeze roll 16 , and a cutting blade 17 .

[0029] In the roll forming of electric resistance welded pipes according to this embodiment, the raw material is a coiled strip 11, which is passed through a leveler 12 and an edge milling device 13 to adjust the properties of the steel strip. The steel strip is then formed into a cylindrical open tube 20 through a third roll group 23, a second roll group 22, and a first roll group 21. The open tube 20 is then electro-resistance welded by high-frequency heating using a squeeze roll 16, resulting in the production of an electric resistance welded pipe. In this embodiment, direct current application using an electrode 15 is used as the high-frequency heating method, but other methods may also be used. After the electro-resistance welding, a bead is cut using a cutting blade 17. Downhill forming is introduced from the mill entry side. The height position of the lower roll of the third roll group 23, which corresponds to the roll group on the upstream side of the roll forming machine 10, is preset by downhill forming. In this embodiment, the height position of the groove bottom point of the lower roll is used as the reference. The height position of the lower roll of the second roll group 22, which is associated with the roll group on the upstream side of the roll forming machine 10, is also preset by downhill forming. The height position of the lower roll of the first roll group 21, which corresponds to the roll group on the rear side of the roll forming machine 10, is also set in advance by downhill forming. In order to ensure the stability of electric resistance welding, the height of the lower roll is set so that the pass line of at least the final pair of rolls among the multiple rolls in the first roll group 21 is parallel to the ground. Methods for adjusting the height of the lower roll in downhill forming include, but are not limited to, a method of adjusting the height of the roll bearing housing of the lower roll by inserting or removing a block or a method of adjusting the height in the vertical direction with a hydraulic jack.

[0030] 2 to 5, the arrangement of the rolls included in the first roll group 21, the second roll group 22, and the third roll group 23 for downhill forming will be described. Fig. 2 shows an example in which downhill forming has been completed with the first pair of rolls in the first roll group 21.

[0031] The first roll group 21 includes at least one set of rolls that form a first caliber 41 through which the metal sheet passes. In the example shown in FIG. 3 , the first caliber 41 corresponds to a caliber surrounded by a total of four rolls, one pair of rolls (upper and lower) and one pair of rolls (left and right). However, the first caliber 41 may be formed with only a total of two rolls, one pair of rolls (upper and lower). In this embodiment, the at least one set of rolls that form the first caliber 41 includes a fin pass roll 31 that adjusts the shape of the metal sheet. The inclination angle of the normal direction of the cross section of the first caliber 41 relative to the horizontal direction is defined as the first angle θ1. In other words, the inclination of the straight line L1, which is the central axis of the first caliber 41, from the horizontal is defined as the first angle θ1. In this embodiment, the first angle θ1 is 0°. Therefore, the first angle θ1 is omitted from FIG. 2 .

[0032] The second roll group 22 includes at least one set of rolls that form a second caliber 42 through which the metal sheet passes. In the example shown in FIG. 4 , the caliber sandwiched between a pair of left and right rolls corresponds to the second caliber 42. In this embodiment, at least one set of rolls that form the second caliber 42 includes a guide roll 32 that adjusts the position of the metal sheet. The inclination angle of the normal direction of the cross section of the second caliber 42 relative to the horizontal direction is defined as the second angle θ2. That is, the inclination of the straight line L2, which is the central axis of the second caliber 42, from the horizontal is defined as the second angle θ2. The second angle θ2 is greater than the first angle θ1. In this embodiment, the second angle θ2 is 0.4° or greater and 20° or less. When the second roll group 22 includes two or more sets of rolls that respectively form the second caliber 42, the second angles θ2 of the rolls of each set may be the same angle or different angles. For example, the second angles θ2 of the rolls of each set may be larger as the rolls of the previous set are positioned.

[0033] The third roll group 23 includes at least one set of rolls that form a third caliber 43 through which the metal sheet passes. In the example shown in FIG. 5 , the caliber sandwiched between a total of two rolls, an upper and lower pair, corresponds to the third caliber 43. In this embodiment, at least one set of rolls that form the third caliber 43 includes a forming roll 33 that adjusts the shape of the metal sheet. Specifically, the forming roll 33 is a breakdown roll. The inclination angle of the normal direction of the cross section of the third caliber 43 relative to the horizontal direction is defined as a third angle θ3. That is, the inclination of the straight line L3, which is the central axis of the third caliber 43, from the horizontal is defined as the third angle θ3. The third angle θ3 is greater than the first angle θ1. The third angle θ3 may be the same as the second angle θ2, but is preferably greater than the second angle θ2. In this embodiment, the third angle θ3 is also greater than 0.4° and less than 20°. When the third roll group 23 includes two or more sets of rolls each forming a third hole mold 43, the third angle θ3 of the rolls in each set may be the same angle or different angles, for example, the third angle θ3 of the rolls in each set may be larger as they are located in the earlier stages.

[0034] When the rear side of the axis of the roll closest to the second roll group 22 included in the first roll group 21 is taken as positive and the front side as negative, it is preferable that the intersection P of the line L1 which is the central axis of the first groove 41 and the line L2 which is the central axis of the second groove 42 is located at a position away from the axis that is at least -4 times and at most 3 times the outer diameter of the product tube manufactured by roll forming.

[0035] For example, as shown in Fig. 11, if the central axes M1, M2, M3 of the calibers formed by the rolls 91, 92, 93 corresponding to the rolls included in the first roll group 21, the second roll group 22, and the third roll group 23 are oriented horizontally, the open tube 90 will be subjected to abrupt bending deformation in the longitudinal direction when it enters the subsequent roll 91. As a result, in the fin pass forming using the roll 91, shear strain in the longitudinal direction and circumferential direction of the open tube 90 due to contact with the upper roll increases around the width end portion of the open tube 90, and work hardening becomes significant around the width end portion of the open tube 90. Therefore, in this embodiment, as shown in Fig. 2, the normal direction of the cross section of the calibers formed by the rolls included in the second roll group 22 and the third roll group 23 is inclined from the horizontal direction to impart an inclination in the traveling direction of the open tube 20, thereby enabling the open tube 20 to smoothly enter the subsequent rolls having different downhill heights. In this way, by tilting the normal direction of the cross section of the groove formed by each roll relative to the horizontal direction, it is possible to reduce shear strain in the longitudinal and circumferential directions of the pipe due to contact with the upper roll around the width end of the open pipe 20 during fin pass forming.

[0036] As described above, in this embodiment, the normal direction of the cross section of the groove formed by each roll pair on the front side of the roll forming machine 10, including the forming roll 33 and the accompanying guide roll 32, is inclined from the horizontal. On the other hand, the normal direction of the cross section of the groove formed by each roll pair on the rear side of the roll forming machine 10, including the fin pass roll 31, is oriented horizontally. In this arrangement of rolls, the strip 11 is formed into the open tube 20, and the trajectory of the strip 11 viewed from its longitudinal side is traced toward the groove of the next roll. Methods for adjusting the normal direction of the cross section of the groove formed by each roll pair on the front side, including the forming roll 33 and the accompanying guide roll 32, include, but are not limited to, a method of inserting a triangular prism-shaped spacer into the base of the housing of the stand corresponding to each roll pair to adjust the inclination angle of the housing as a whole, or a method of tilting the roll bearing housings of the upper and lower rolls.

[0037] The distribution of the downhill height in each pair of rolls on the upstream side, including the forming roll 33 and the accompanying guide roll 32, may be either a linear distribution such that the third angle θ3 = the second angle θ2, or a curved distribution such that the third angle θ3 ≠ the second angle θ2, when based on the height position of the groove bottom of the lower roll of the first pair of rolls on the downstream side, including the fin pass roll 31. However, considering that the geometric trajectory in the width direction of the width end face of the open tube 20 is curved, it is preferable that the distribution of the downhill height also be curved.

[0038] The downhill molding in this embodiment is adjusted to satisfy the following conditions.

[0039] In the roll group on the front side of the roll forming machine 10, the normal direction of the cross section of the groove formed by each roll pair including the forming roll 33 and the cross section of the groove formed by each roll pair including the accompanying guide roll 32 is inclined from the horizontal direction.

[0040] The inclination of the normal direction of the cross section of the groove formed by the final roll pair of the upstream roll group of the roll forming machine 10, i.e., the second angle θ2, from the horizontal is preferably 0.4° to 10°. If the inclination is less than 0.4°, the effect of work hardening caused by the winding of the open tube 20 around the upper roll of the first roll pair in fin pass forming around the width end portion may be approximately the same as when there is no inclination. On the other hand, if the inclination is more than 10°, the downhill slope is so steep that buckling may occur before the open tube 20 around the width end portion wraps around the upper roll of the first roll pair in fin pass forming. The inclination is more preferably 2° to 8°, and most preferably 4° to 7°.

[0041] In the roll group at the upstream side of the roll forming machine 10, the inclination of the normal direction of the cross section of the groove formed by each roll pair except the final roll pair, i.e., the third angle θ3, from the horizontal is preferably 0.4° to 20°. If the inclination is less than 0.4°, the distribution of the downhill height difference in the roll group at the upstream side of the roll forming machine 10 will have a locally mountain-shaped portion. Unnecessary work hardening may occur due to longitudinal bending and unbending deformation that occurs when the bottom of the open tube 20 overcomes the groove bottom point of the lower roll of the roll pair at the apex of the mountain-shaped portion. On the other hand, if the inclination is more than 20°, the downhill is too steep and the open tube 20 may buckle between the roll pairs. The inclination is more preferably 2° to 18°, and most preferably 4° to 16°.

[0042] The downhill height of the first pair of rolls in the upstream roll group of the roll forming machine 10 is preferably 0.4 to 9 times the outer diameter of the product. If the downhill height of the first pair of rolls in the upstream roll group of the roll forming machine 10 is less than 0.4 times, the effect of work hardening caused by the winding of the first pair of rolls in the fin pass forming around the upper roll of the open tube 20 around the width edge may be approximately the same as when there is no inclination. On the other hand, if the downhill height of the first pair of rolls in the upstream roll group of the roll forming machine 10 is more than 9 times, the bottom of the open tube 20 will elongate significantly during forming, and work hardening may become significant. The downhill height of the first pair of rolls in the upstream roll group of the roll forming machine 10 is more preferably 0.8 to 6 times the outer diameter of the product, and most preferably 1 to 4 times the outer diameter of the product.

[0043] The intersection point P, where the central axis of the groove formed by the final pair of rolls in the upstream roll group of the roll forming machine 10, i.e., line L2, intersects with the central axis of the groove formed by the first pair of rolls in the downstream roll group of the roll forming machine 10, i.e., line L1, is preferably within a distance range of −4 to 3 times the outer diameter of the product tube from the axial center of the first pair of rolls in the downstream roll group in the forming direction. Here, the upstream side of the roll forming machine 10 from the axial center of the first pair of rolls in the downstream roll group is considered negative, and the squeeze roll 16 side is considered positive. If the intersection point P is located at a distance less than −4 times, longitudinal bending and unbending deformation that occurs when the bottom of the open tube 20 passes over the groove bottom point of the lower roll of the first pair of rolls in the downstream roll group can become significant. This can lead to unnecessary work hardening. On the other hand, if the intersection point P is located at a distance greater than 3 times, the effect of work hardening caused by the first pair of rolls in fin pass forming wrapping around the upper roll can be significant around the width end of the open tube 20. The intersection point P is more preferably within a distance range of −2 to 1.5 times the outer diameter of the product pipe, and most preferably within a distance range of −1 to 0 times the outer diameter of the product pipe.

[0044] Although the present embodiment employs the breakdown forming method, a cage forming method may also be employed. In the breakdown forming method, the roll forming machine 10 includes a breakdown roll group as the third roll group 23, as shown in FIG. 1 , and also includes a guide roll group as the second roll group 22 to ensure the sheet threadability of the open tube 20. On the other hand, in the cage forming method, the third roll group 23 is integrated into the second roll group 22, and the roll forming machine 10 includes only the second roll group 22, which is arranged upstream of the first roll group 21, as shown in FIG. 6 . The second roll group 22 is specifically a cage roll group and includes a breakdown roll 14 in addition to a cage roll 18, which is a type of guide roll. In this modification, the second roll group 22 also includes at least one set of rolls that form a second groove 42 through which the metal sheet passes. In this modification, the at least one set of rolls that form the second groove 42 includes a forming roll that adjusts the shape of the metal sheet. Specifically, the forming roll is the breakdown roll 14. In this modification, the second angle θ2, which is the inclination angle of the normal direction of the cross section of the second cavity 42 with respect to the horizontal direction, is larger than the first angle θ1. The second angle θ2 is 0.4° or more and 20° or less, preferably 0.4° or more and 10° or less, more preferably 2° or more and 8° or less, and most preferably 4° or more and 7° or less.

[0045] As another modification, a mill configuration without a guide roll group between the fin pass roll group and the breakdown roll group may be employed. In this modification, the second roll group 22 arranged upstream of the first roll group 21 is specifically a breakdown roll group. In this modification, the second roll group 22 also includes at least one set of rolls forming the second groove 42 through which the metal sheet passes. The at least one set of rolls forming the second groove 42 includes a breakdown roll as a forming roll for adjusting the shape of the metal sheet. In this modification, the second angle θ2, which is the inclination angle of the normal direction of the cross section of the second groove 42 relative to the horizontal direction, is also greater than the first angle θ1. The second angle θ2 is 0.4° or more and 20° or less, preferably 0.4° or more and 10° or less, more preferably 2° or more and 8° or less, and most preferably 4° or more and 7° or less.

[0046] As described above, this embodiment is an improvement over the "downhill" method of creating a difference in height between rolls, specifically, making the rolls lower as they go downstream, by inclining the rolls themselves. In this embodiment, by inclining the rolls themselves upstream of the fin pass roll 31 of the electric resistance welded pipe roll forming mill, it is possible to reduce the substantial strain near the edge of the steel strip.

[0047] In this embodiment, with regard to roll forming of a strip 11, particularly in the manufacture of electric resistance welded pipes, the effect of work hardening due to shear deformation that occurs in the peripheral areas of the width ends of a blank sheet or coil during fin pass forming can be reduced, thereby improving the secondary workability of the electric resistance welded pipe after pipe formation. According to this embodiment, it is possible to provide a roll forming apparatus and an electric resistance welded pipe manufacturing method in which work hardening is reduced in the peripheral areas of welds of electric resistance welded pipes manufactured by downhill forming.

[0048] The roll forming machine 10 includes a plurality of roll groups. In the roll group on the upstream side of the forming machine, the normal direction of the cross section of the groove formed by the roll pair including the forming roll 33 and the cross section of the groove formed by the roll pair including the accompanying guide roll 32 is inclined from the horizontal. The inclination of the normal direction of the cross section of the groove formed by the final roll pair of the roll group on the upstream side of the forming machine, or the cross section of the groove formed by the accompanying roll pair, i.e., the second angle θ2, is preferably 0.4° or more and 10° or less. In the roll group on the upstream side of the forming machine, the inclination of the normal direction of the cross section of the groove formed by the roll pair excluding the final roll, or the cross section of the groove formed by the accompanying roll pair, i.e., the third angle θ3, is preferably 0.4° or more and 20° or less. The roll pair of the roll group on the downstream side of the forming machine includes a fin pass roll 31.

[0049] In this embodiment, the roll forming method used for roll forming a metal sheet includes the following two steps. Step 1: A first roll group 21 includes at least one set of rolls forming a first caliber 41, and the first caliber 41 has a first inclination angle θ1 with respect to the horizontal in the direction normal to the cross section of the first caliber 41. The first roll group 21 adjusts the shape or position of the metal sheet as it passes through the first caliber 41. Step 2: A second roll group 22 includes at least one set of rolls forming a second caliber 42, and is arranged upstream of the first roll group 21. The second roll group 22 adjusts the shape or position of the metal sheet as it passes through the second caliber 42. The second roll group 22 includes at least one set of rolls forming a second caliber 42, and the second caliber 42 has a second inclination angle θ2 with respect to the horizontal in the direction normal to the cross section of the second caliber 42. The second inclination angle θ2 is larger than the first angle θ1. These steps make it possible to suppress excessive work hardening around the width end portion of the open tube 20 during downhill forming.

[0050] The strip 11 may be made of either a steel material or a non-ferrous metal. Steel is a material for electric resistance welded pipes, which are widely used in machine structures and other applications. While there are no particular limitations on the composition, the following components are used here as an example to ensure the required product strength and weld quality. The "%" in the component composition indicates "mass %."

[0051] The base material preferably has a component composition containing, in mass%, C: 0.02% or more and 0.45% or less, Si: 0.03% or more and 0.30% or less, Mn: 0.15% or more and 2.00% or less, P: 0.030% or less, S: 0.012% or less, Al: 0.01% or more and 0.08% or less, with the balance being Fe and unavoidable impurities.

[0052] C is an element that significantly contributes to increasing the strength of steel pipes. To ensure the effects described herein, it is preferable that the steel contain 0.02% or more of C. On the other hand, if C is contained in a large amount exceeding 0.45%, it promotes the formation of hard phases such as pearlite or martensite, which may result in a decrease in toughness. Furthermore, if C is contained in a large amount exceeding 0.45%, it may excessively increase the strength of the bainite phase, i.e., hardness, and may result in a decrease in toughness. Therefore, the C content is preferably 0.02% or more and 0.45% or less. The C content is more preferably 0.03% or more, and most preferably 0.04% or more. The C content is more preferably 0.30% or less, and most preferably 0.25% or less.

[0053] Si is an element that dissolves in steel to increase the strength of steel pipes and reduce the amount of scale-off during hot rolling. To ensure the effects described above, it is preferable that the steel contain 0.03% or more Si. Si forms a highly viscous eutectic oxide together with Mn oxides. If the Si content is less than 0.03%, the Mn concentration in the eutectic oxide becomes relatively high. As a result, the melting point of the eutectic oxide exceeds the molten steel temperature, which makes the oxide more likely to remain in the weld, potentially reducing the toughness of the weld. On the other hand, if Si is contained in a large amount exceeding 0.30%, the formation of red scale becomes significant, deteriorating the appearance of the steel pipe or steel plate and causing uneven cooling during hot rolling, potentially reducing the uniformity of the steel pipe or steel plate material. Furthermore, if Si is contained in a large amount exceeding 0.30%, the Si concentration in the eutectic oxide becomes relatively high. As a result, the melting point of the eutectic oxide exceeds the molten steel temperature, the amount of oxide increases, and the oxide tends to remain in the weld, which may reduce the toughness of the weld. Therefore, the Si content is preferably 0.03% or more and 0.30% or less. The Si content is more preferably 0.10% or more, and most preferably 0.12% or more. The Si content is more preferably 0.25% or less, and most preferably 0.24% or less.

[0054] Mn is an element that dissolves in steel and contributes to increasing the strength of steel pipes through solid solution strengthening. Mn also contributes to improving the strength and toughness of steel pipes through transformation strengthening via improved hardenability. To ensure the effects described herein, it is preferable that the steel contain 0.15% or more Mn. Mn forms a highly viscous eutectic oxide together with silicon oxide. If the Mn content is less than 0.15%, the Si concentration in the eutectic oxide becomes relatively high. As a result, the melting point of the eutectic oxide exceeds the molten steel temperature, which may cause the oxide to remain in the weld and reduce the toughness of the weld. On the other hand, if Mn is contained in a large amount exceeding 2.00%, the Mn concentration in the eutectic oxide becomes relatively high, which may cause the melting point of the eutectic oxide to exceed the molten steel temperature. This increases the amount of oxide, which may cause the oxide to remain in the weld and reduce the toughness of the weld. Furthermore, if Mn is contained in a large amount exceeding 2.00%, the hardenability may be excessively improved, the martensite phase may be easily formed, and toughness may be reduced. Therefore, the Mn content is preferably 0.15% or more and 2.00% or less. The Mn content is more preferably 0.90% or more, and most preferably 0.92% or more. The Mn content is more preferably 1.80% or less, and most preferably 1.78% or less.

[0055] P is an element that has a strong tendency to segregate at grain boundaries, thereby reducing toughness. Therefore, it is preferable to reduce the P content as much as possible, but up to 0.030% is acceptable. Therefore, the P content is preferably 0.030% or less. The P content is more preferably 0.025% or less. However, excessive reduction of P may result in a longer refining time and an increase in manufacturing costs, so the P content is preferably 0.002% or more.

[0056] S is an element that forms MnS in steel and reduces toughness. Therefore, it is preferable to reduce the S content as much as possible, but up to 0.012% is acceptable. Therefore, the S content is preferably 0.012% or less. The S content is more preferably 0.004% or less. However, excessive reduction of S may result in a longer refining time and an increase in manufacturing costs, so the S content is preferably 0.002% or more.

[0057] Al is an element that acts as a deoxidizer. To ensure the effects described herein, it is preferable that the steel material contains 0.01% or more Al. On the other hand, if the Al content is greater than 0.08%, the generation of Al oxides becomes significant. In particular, Al oxides are likely to remain in the welded joint, which may reduce the toughness of the welded joint. Therefore, the Al content is preferably 0.01% or more and 0.08% or less. The Al content is more preferably 0.02% or more. The Al content is more preferably 0.07% or less.

[0058] The electric resistance welded steel pipe according to this embodiment can obtain the desired properties with the above-mentioned essential elements, but in this embodiment, optional elements can also be contained as necessary for the purpose of further improving strength or toughness. The optional elements are one or more selected from Nb: 0.01% to 0.10%, Ti: 0.01% to 0.03%, Cu: 0.50% or less, Ni: 0.50% or less, Cr: 0.500% or less, Mo: 0.500% or less, V: 0.10% or less, N: 0.006% or less, and Ca: 0.010% or less.

[0059] Nb is an element that precipitates finely as Nb carbonitrides during hot rolling in the production of steel plates, contributing to increasing the strength of steel plates used as steel pipe materials. Nb suppresses the growth of austenite grains during heat treatment of the welded portion of an electric resistance welded steel pipe, contributing to the refinement of the welded portion's structure. To ensure the effects described herein, it is preferable for the steel material to contain 0.01% or more of Nb. On the other hand, if Nb is contained in a large amount exceeding 0.10%, the amount of Nb carbonitrides precipitated increases, which may reduce the toughness of the steel plate, the base metal of the steel pipe, and the welded portion of the steel pipe. Therefore, the Nb content is preferably 0.01% or more and 0.10% or less. The Nb content is more preferably 0.02% or more, and most preferably 0.022% or more. The Nb content is more preferably 0.08% or less, and most preferably 0.078% or less.

[0060] Ti is an element that bonds with N to form TiN, thereby preventing the adverse effects of N. To ensure the effects described herein, it is preferable that the steel contain 0.01% or more of Ti. On the other hand, if Ti is contained in a large amount exceeding 0.03%, the amount of Ti carbonitrides that precipitate along the cleavage planes of iron increases, which may reduce the toughness of the steel plate, the base metal of the steel pipe, and the welded joint of the steel pipe. Therefore, the Ti content is preferably 0.01% or more and 0.03% or less. The Ti content is more preferably 0.015% or more, and most preferably 0.017% or more. The Ti content is more preferably 0.025% or less, and most preferably 0.022% or less.

[0061] Cu is an element that has the effect of increasing strength and toughness through improved hardenability. In order to ensure the effects described herein, it is preferable that the steel contains 0.05% or more Cu. The Cu content is more preferably 0.10% or more. On the other hand, if Cu is contained in a large amount exceeding 0.50%, the effects described above saturate, and it is no longer possible to expect effects commensurate with the content, which is economically disadvantageous. Therefore, when Cu is contained, the Cu content is preferably 0.50% or less. The Cu content is more preferably 0.35% or less.

[0062] Like Cu, Ni is an element that increases strength and toughness through improved hardenability. In order to ensure the effects described herein, it is preferable that the steel contains 0.05% or more Ni. The Ni content is more preferably 0.08% or more. On the other hand, if Ni is contained in a large amount exceeding 0.50%, grain boundary oxidation of Fe becomes severe during heating of the cast slab or slab, which may promote the occurrence of surface defects. Therefore, when Ni is contained, the Ni content is preferably 0.50% or less. The Ni content is more preferably 0.35% or less.

[0063] Like Cu and Ni, Cr is an element that increases strength and toughness by improving hardenability. To ensure the effects described herein, it is preferable that the steel contains 0.050% or more Cr. The Cr content is more preferably 0.100% or more. On the other hand, if Cr is contained in a large amount exceeding 0.500%, Cr oxides may form in the welded joint, which may significantly reduce the toughness of the welded joint. Therefore, when Cr is contained, the Cr content is preferably 0.500% or less. The Cr content is more preferably 0.300% or less.

[0064] Like Cu, Ni, and Cr, Mo is an element that increases strength and toughness by improving hardenability. In order to ensure the effects described herein, it is preferable that the steel contain 0.050% or more Mo. The Mo content is more preferably 0.100% or more. On the other hand, if Mo is contained in a large amount exceeding 0.500%, a hard second phase is likely to be generated in the weld during heat treatment of the weld, which may reduce the toughness of the weld. Therefore, when Mo is contained, the Mo content is preferably 0.500% or less. The Mo content is more preferably 0.250% or less.

[0065] V is an element that contributes to increasing the strength of steel sheets by dissolving in steel to strengthen the solid solution and by precipitating as carbides to strengthen the precipitation. To ensure the effects described herein, the steel preferably contains 0.005% or more of V. The V content is more preferably 0.010% or more. On the other hand, if V is contained in a large amount exceeding 0.10%, the effect saturates and it is economically disadvantageous. Therefore, when V is contained, the V content is preferably 0.10% or less. The V content is more preferably 0.085% or less.

[0066] N is an element that acts to firmly fix dislocation motion, thereby reducing toughness. In this embodiment, it is desirable to reduce N as an impurity as much as possible, and up to 0.006% is acceptable. Therefore, the N content is preferably 0.006% or less. The N content is more preferably 0.005% or less.

[0067] Ca is an element that effectively contributes to morphology control of sulfides such as MnS. In order to ensure the effects described herein, it is preferable that the steel contains 0.001% or more of Ca. On the other hand, if Ca is contained in a large amount exceeding 0.010%, the effect saturates and it is no longer possible to expect an effect commensurate with the content, which is economically disadvantageous and also increases the amount of Ca oxide, which may reduce the toughness of the weld, in particular. Therefore, when Ca is contained, the Ca content is preferably 0.010% or less. The Ca content is more preferably 0.005% or less, and most preferably 0.003% or less.

[0068] Steel material having the above-mentioned chemical composition is hot-rolled, or in some cases cold-rolled, to form a strip 11 having a thickness of 3 mm or more and 28 mm or less, and the raw material for an electric-resistance welded steel pipe having an outer diameter of φ20 mm or more and φ700 mm or less is produced.

[0069] Nine examples and three comparative examples will be described with reference to Figures 7 to 10. In Figure 8, "DH height" indicates downhill height.

[0070] In Examples No. 1, No. 2, No. 4 to No. 6, and Comparative Examples No. 3 and No. 7, various strips of steel type A or steel type B having the chemical compositions shown in Figure 7 were used as the strip 11, and electric resistance welded pipes were formed by breakdown roll forming. The roll group used consisted of four breakdown rolls including a forming roll 33, four guide rolls including a guide roll 32, three fin pass rolls including a fin pass roll 31, and a squeeze roll 16. In Figures 8 and 9, BD1 to BD4 correspond to the four breakdown rolls, CR1 to CR4 correspond to the four guide rolls, FP1 to FP3 correspond to the three fin pass rolls, and SQ corresponds to the squeeze roll 16. Spacers with inclined surfaces were placed on the bases of the housings of the stands corresponding to each breakdown roll and each guide roll to adjust the downhill height and the inclination from the horizontal of the normal direction of the cross section of the groove formed by each roll. The normal direction of the groove cross section after the fin pass roll was set to be horizontal.

[0071] In Examples No. 8 to No. 10, No. 12, and Comparative Example No. 11, various strips of steel type C, steel type D, or steel type E having the chemical compositions shown in Figure 7 were used as strip 11, and electric resistance welded pipes were formed by cage forming roll forming. The roll group used consisted of four breakdown rolls including breakdown roll 14, three fin pass rolls including fin pass roll 31, and squeeze roll 16. The four breakdown rolls were arranged in a cage zone. In Figures 8 and 9, BD1 to BD4 correspond to the four breakdown rolls, FP1 to FP3 correspond to the three fin pass rolls, and SQ corresponds to squeeze roll 16. Spacers with inclined surfaces were placed on the bases of the housings of the stands corresponding to each breakdown roll in the cage zone to adjust the downhill height and the inclination from the horizontal of the normal direction of the cross section of the groove formed by each roll. The normal direction of the groove cross section after the fin pass roll was set to be horizontal.

[0072] In each example and comparative example, the vicinity of the welded portion was cut out from the obtained electric resistance welded pipe. A micro-Vickers hardness test based on JIS Z2244-1:2023 was performed on the cylindrical cross section at a depth of 0.1 mm ± 0.05 mm from the outer surface. Using a test force of 0.98 N, the hardness distribution was measured at 0.5 mm intervals in a region extending from the welded surface to ± 15° in the circumferential direction of the pipe. From this measured hardness distribution, the maximum value of the peak hardness (HVmax) and the average value of the hardness (HVm) around the peak were calculated. As shown in Figure 9, in Examples No. 1, No. 2, No. 5, No. 6, No. 8 to No. 10, and No. 12, the difference between the maximum value (HVmax) and the average value (HVm) was the desired value of 50 HV or less. On the other hand, in Comparative Examples No. 3, No. 7, and No. 12, the difference between the maximum value (HVmax) and the average value (HVm) was the desired value of 50 HV or less. In Example No. 11, the difference between the maximum value HVmax and the average value HVm was greater than the desired value of 50 HV. In Example No. 4, the difference between the maximum value HVmax and the average value HVm was not 50 HV or less, and was actually greater than that of Comparative Example No. 3, which had the same steel type and steel pipe size as Example No. 4. However, Comparative Example No. 3 was an example in which downhill forming was not applied, and it is thought that if downhill forming were applied so that the downhill height was similar to that of Example No. 4 or Comparative Example No. 7, the difference between the maximum value HVmax and the average value HVm would be significantly increased, as in Comparative Example No. 7. Therefore, it is thought that Example No. 4 also achieved the effect of suppressing excessive work hardening around the open pipe width end during downhill forming. However, in order to make the difference between the maximum value HVmax and the average value HVm the desired value of 50 HV or less, it is possible to consider, for example, changing the inclination θ of the normal direction of the CR4 caliber cross section from 10.5° to 10° or less in Example No. 4.

[0073] 8 and 9, by inclining the normal direction of the cross section of the groove formed by each forming roll in downhill forming from the horizontal, it is possible to reduce shear strain in the longitudinal and circumferential directions of the pipe due to contact with the upper roll around the width end of the open pipe 20 in the first pair of rolls in fin pass forming, and to make the hardness around the weld less than that of the weld. Therefore, according to the present disclosure, in conventional downhill forming, the problem of work hardening around the weld deteriorates the workability of the product, but by performing downhill forming with an inclination in the direction of travel of the open pipe, it is possible to obtain an electric resistance welded pipe that is less affected by work hardening around the weld.

[0074] The present disclosure is not limited to the above-described embodiments, and modifications are possible within the scope of the present disclosure.

[0075] REFERENCE SIGNS LIST 10 Roll forming machine 11 Strip 12 Leveler 13 Edge milling device 14 Breakdown roll 15 Electrode 16 Squeeze roll 17 Cutting blade 18 Cage roll 20, 90 Open tube 21 First roll group 22 Second roll group 23 Third roll group 31 Fin pass roll 32 Guide roll 33 Forming roll 41 First groove 42 Second groove 43 Third groove 91, 92, 93 Roll

Claims

1. A roll forming machine used for roll forming of metal sheets, comprising: a first roll group including at least one set of rolls that form a first caliber through which the metal sheet passes, the first caliber having a first inclination angle relative to the horizontal direction in the direction normal to the cross section of the first caliber; and a second roll group including at least one set of rolls that form a second caliber through which the metal sheet passes, the second roll group being arranged in a stage upstream of the first roll group, the second caliber having a second inclination angle relative to the horizontal direction in the direction normal to the cross section of the second caliber that is larger than the first angle.

2. The roll forming machine according to claim 1, wherein at least one set of rolls forming the second groove includes a forming roll for adjusting the shape of the metal plate.

3. The roll forming machine of claim 2, wherein the forming roll is a breakdown roll.

4. A roll forming machine according to any one of claims 1 to 3, wherein at least one set of rolls forming the second groove includes a guide roll for adjusting the position of the metal plate.

5. A roll forming machine according to any one of claims 1 to 4, wherein at least one set of rolls forming the first groove includes a fin pass roll for adjusting the shape of the metal plate.

6. A roll forming machine according to any one of claims 1 to 5, wherein the second angle is between 0.4° and 20°.

7. A roll forming machine according to any one of claims 1 to 6, wherein the first angle is 0°.

8. A roll forming machine as described in any one of claims 1 to 7, wherein, when the rear side of the axis of the roll closest to the second roll group included in the first roll group is positive and the front side is negative, the intersection of the central axis of the first groove die and the central axis of the second groove die is located at a position away from the axis center that is at least -4 times and at most 3 times the outer diameter of the product tube manufactured by the roll forming.

9. A roll forming machine as described in any one of claims 1 to 8, further comprising a third roll group including at least one set of rolls that form a third groove through which the metal plate passes, and that is arranged in a stage before the second roll group, and in which the inclination angle of the normal direction of the cross section of the third groove relative to the horizontal direction is a third angle that is greater than the first angle.

10. The roll forming machine of claim 9, wherein the third angle is greater than or equal to the second angle.

11. A roll forming machine as described in claim 9 or claim 10, wherein at least one set of rolls forming the second groove includes guide rolls that adjust the position of the metal plate, and at least one set of rolls forming the third groove includes forming rolls that adjust the shape of the metal plate.

12. A roll forming machine according to any one of claims 1 to 11, which produces an electric resistance welded pipe by the roll forming.

13. A roll forming method used in roll forming of metal sheets, comprising: a first roll group including at least one set of rolls forming a first caliber, wherein the inclination angle of the normal to the cross section of the first caliber with respect to the horizontal is a first angle, adjusting the shape or position of the metal sheet as the metal sheet passes through the first caliber; and a second roll group including at least one set of rolls forming a second caliber, disposed in a stage upstream of the first roll group, wherein the inclination angle of the normal to the cross section of the second caliber with respect to the horizontal is a second angle larger than the first angle, adjusting the shape or position of the metal sheet as the metal sheet passes through the second caliber.

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