JP2017124444A - Manufacturing method for electroseamed steel pipe of small pipe-making strain and electroseamed steel pipe of small pipe-making strain - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method for a steel pipe with a uniform pipe circumferential distribution of processing strains added by pipe making, in particular, a steel pipe with a thickness and/or high strength the realization of which is difficult.SOLUTION: A manufacturing method for an electroseamed steel pipe enables the intermediate molding of an open pipe so that the ratio R1 (=Sc/Sm) of the maximum value SM of a distance between a screw-down direction line that is a straight line including the outer face side angular part A of a steel strip which forms an open pipe and the fin base end part B of a top roll and the outer face of the open pipe to the distance Sc between the screw-down direction line and a caliber bottom C may satisfy the following expression (1) when the outer face of the bottom part of the open pipe is made coincident with the caliber bottom C of a bottom roll to project the cross section of the open pipe orthogonal to the progressing direction of the open pipe on the inlet side of a first fin pass roll to the cross section passing through the center shafts of the first fin pass roll to be orthogonal to the progressing direction of the open pipe. 0.70≤R1....(1).SELECTED DRAWING: Figure 6

Description

  The present invention relates to a method for manufacturing an electric resistance welded steel pipe, and in particular, for oil wells, automobiles, and buildings, the load during roll forming is large and the forming itself is difficult, and the processing strain applied during pipe forming The present invention relates to a method for producing an electric resistance welded steel pipe made of a thick-walled and high-strength hot-rolled steel sheet and having a small pipe-forming strain, and an electric-resistance welded steel pipe having a small pipe-forming strain. Here, the small pipe-forming strain means that the strain applied to the steel pipe in the process of forming the hot-rolled steel sheet into a pipe shape is small. When manufacturing a steel pipe having an outer diameter D and a wall thickness t, the theoretical molding strain amount is t / D. In the conventional roll forming method, a distortion of 3 times or more of t / D is generally generated due to wrapping around the forming roll, etc. As a standard for a small pipe-forming strain, as a guideline, it is more than 3 times of t / D. Is also small.

  ERW steel pipes have excellent features such as good dimensional accuracy, beautiful surface skin, and high productivity. Steel pipes for line pipes such as oil and natural gas, steel pipes for automobiles, and construction It is used for a wide range of applications such as steel pipes. In recent years, higher strength and thicker ERW steel pipes have been required compared to conventional steel pipes, and even hot-rolled steel sheets used as ERW steel pipe materials have been developed with higher strength steel sheets and thicker steel sheets. It is being manufactured. Here, “thick” means that the hot-rolled steel sheet has a thickness of 12 to 26 mm, and “high strength” means that the hot-rolled steel sheet has a tensile strength of 460 MPa or more.

  As shown in FIG. 1, an electric resistance steel pipe is manufactured by continuously rolling a steel plate (steel strip) into a pipe shape and then welding the width end of the steel plate (steel strip). ERW steel pipe manufacturing equipment is usually capable of manufacturing steel pipes with various outer diameters and wall thicknesses by adjusting the position of forming rolls and exchanging rolls in one forming line. The production range of steel pipes in one forming line is limited by the forming force and driving force of the roll on the thick wall side, and buckling of the end of the steel strip, generally called edge wave, occurs during roll forming on the thin wall side. Is limited by

  Therefore, various techniques have been developed to expand the manufacturable range of tubes in one molding line. For example, in Patent Document 1, the inner surface of a strip-shaped metal material (semi-molded product) in a rough and intermediate molding process is moved in the direction of travel of the strip-shaped metal material by an inner surface roll that is rotatable at one or more positions in the circumferential direction. It is disclosed that edge waves are prevented by performing molding by regulating the position of a material in a vertical plane.

  Further, Patent Documents 2 and 3 disclose that a drive bottom roll, a top roll provided to be vertically adjustable and positioned on both sides of the top roll are provided in front of the fin pass stand group of the ERW steel pipe forming line. A plurality of inside rolls that can be adjusted and an outside roll that can be adjusted in the vertical and horizontal directions acting near the coil edge portion are provided in a plurality of stages in the longitudinal direction, and the drive bottom roll and the top Sufficient driving force is obtained by the roll, and the thin roll is prevented from being folded by the combination of the inside roll and the outside roll, and the edge roll is prevented by arranging a plurality of outside rolls in the longitudinal direction. The open pipe shape in front of the fin pass stand can be changed to a fin pass roll caliber. Molding apparatus ERW steel pipe can be applied are disclosed.

  Furthermore, in Patent Document 4, in the method for manufacturing a welded steel pipe using the CBR forming method, both sides are pushed by a cage roll between the final center bend exit side and the first fin pass roll exit side. By pressing both sides close to the bottom of the tube in the thickness direction from the inner surface with a presser roller, the concentration of bending stress on the bottom of the tube is reduced, and an ultra-thin, ultra-high strength strip is formed. A method and apparatus for manufacturing a welded steel pipe is disclosed that does not cause hip breakage and does not generate edge waves in the fin pass roll forming stage even when pipes are made to have a small diameter.

  Patent Document 5 discloses a cross section immediately before a fin pass roll of a vertically long oblong pipe having an opening and having a cross section consisting of bending elements of a pipe edge part, a pipe side part, a pipe bottom part, and their boundary parts. One of the features is that the ratio of the length of each bending element region to the width of the strip plate and the ratio of the bending radius of each bending element to the product radius are within an appropriate range with respect to the shape or the planned cross-sectional shape. A method of manufacturing a welded pipe suitable for hydroforming is shown.

  Furthermore, regarding the cross-sectional shape of the oval-shaped element tube (open tube) immediately before forming the fin pass roll, as a method of manufacturing a thick-walled electric resistance tube that reduces the forming load of the fin pass roll, Patent Document 6 discloses a manufacturing method in which intermediate molding is performed so that the relationship between the radius and the distance between the center of the circular tube and the distance to the outer surface in the vicinity of the tube bottom of the open tube is within an appropriate range. Patent Document 7 discloses a manufacturing method in which intermediate molding is performed so that the ratio of the diameter and the lateral diameter falls within an appropriate range.

JP 56-66323 A Japanese Patent Laid-Open No. 04-178221 Japanese Patent Laid-Open No. 04-178222 Japanese Patent Laid-Open No. 2005-66679 JP 2000-343135 A JP, 2015-147226, A Japanese Patent Laying-Open No. 2015-167773

  However, the methods disclosed in Patent Documents 1 to 4 are effective in preventing edge waves generated in thin-walled materials, but there are problems in producing thick and / or high-strength ERW steel pipes. It was insufficient to solve the shortage of the forming force and driving force of the roll. In particular, when manufacturing ERW steel pipes using thick and / or high-strength hot-rolled steel sheets, it is difficult to form a uniform strain or curvature over the entire pipe cross-section by forming with a cage roll group. Met. When forming with the cage roll group, the bending strain is often concentrated near the center of the sheet width of the hot-rolled steel sheet corresponding to the bottom of the ERW steel pipe, and in other regions, the intermediate formed shape has a small curvature. There were many. As a result, there is a problem that the steel pipe product finally has a non-uniform degree of work hardening in the pipe circumferential direction, a different yield stress in the pipe circumferential direction position, and a large wall thickness variation.

  Moreover, in the manufacturing method of the welded pipe described in Patent Document 5, in the case of a steel pipe made of a thick-walled and / or high-strength hot-rolled steel sheet, even when the edge forming machine 3 shown in FIG. As a result, one of the appropriate conditions disclosed in Patent Document 5, for example, (tube edge portion bending length) / (band width) = 0.2 to 0.4, (Pipe edge bending radius) / (Product radius) = 1.0 to 1.4 becomes difficult to fit, and may not be applicable to thick-walled and / or high-strength ERW steel pipes. all right. In an extreme case, the width region from the tube edge portion to about 3 times the plate thickness in the plate width direction was almost flat and the radius of curvature was almost infinite, far exceeding the above specified upper limit.

  Patent Document 6 and Patent Document 7 have the same problems as those of the present invention but the following problems.

  The invention described in Patent Document 6 defines the cross-sectional shape in the region of the circumferential angle 0 ≦ θ ≦ 45 ° with respect to the center position of the circular tube in contact with the bottom of the open tube. Even considering the symmetry of the shape, only the shape of the 90 ° region in the circumferential direction including the open tube bottom portion of −45 ° ≦ θ ≦ 45 ° is defined. Therefore, the influence of the shape in the vicinity of the width end portion of the steel strip forming the open pipe which is not easily bent is not considered. Therefore, when the bending deformation near the width end of the steel strip forming the open pipe is small, an increase in strain is likely to occur near the bottom of the pipe, and as a result, the fluctuation range of the thickness is ±± the predetermined thickness. There was a problem of exceeding the range of 2%.

  In addition, the invention described in Patent Document 7 defines only the ratio of the vertical diameter and the horizontal diameter of the cross-sectional shape of the open pipe immediately before the fin pass roll forming, and the curvature distribution and shape of the open pipe with respect to the fin pass forming. The impact is not considered. Therefore, either the bending strain is concentrated at the tube bottom of the open pipe and the curvature is large, or the bending deformation near the width end of the steel strip forming the open pipe is small and the curvature is small. However, in the indicators of the vertical diameter and the horizontal diameter, even when the conditions are determined to be offset with each other and determined to be no problem, as a result, after molding, strain is concentrated locally, and the fluctuation range of the thickness is ±± of the predetermined thickness. There was a problem that it might exceed the range of 2%.

  An object of the present invention is to produce an electric resistance welded steel pipe having a small processing strain applied by the pipe making and a uniform distribution in the pipe circumferential direction, and is particularly difficult to realize the thick wall and / or high strength. It is providing the manufacturing method of the ERW steel pipe.

  As a result of intensive studies to solve the above problems, the present inventors found that there is a difference between the cross-sectional shape of the semi-molded product (open pipe) on the fin-pass-rolled side and the processing strain distribution in the pipe circumferential direction after the fin pass molding. There is a correlation, there is an optimal open pipe cross-sectional shape that can reduce the amount of processing strain after fin pass molding, that is, the amount of work hardening, and uniform processing strain distribution in the pipe circumferential direction, and before fin pass molding The present invention has been completed by finding that the open tube can be formed into an optimal cross-sectional shape by a method of pressing the vicinity of the tube bottom portion on the inner surface side of the open tube with a roll during the forming process using a cage roll.

That is, this invention consists of the following summaries.
[1] An intermediate forming step of intermediate forming a steel strip with a cage roll group, a finish forming step of finish forming into a tubular shape with a fin pass roll group, and electric resistance welding of the width end of the steel strip after the finish forming step. A method of manufacturing an electric resistance welded steel pipe,
In the cross section passing through the central axis of the lower roll of the first fin pass roll composed of the upper roll, the lower roll and the side roll to be subjected to the first molding of the finish molding step, When projecting a cross section of the open tube perpendicular to the traveling direction of the open tube on the entry side of the fin pass roll with the outer surface of the bottom portion of the open tube and the caliber bottom C of the lower roll being coincident, The maximum value of the distance between the rolling direction line that is a straight line including the outer surface side corner A of the width end of the steel strip forming the open pipe and the fin base end B of the upper roll and the outer surface of the open pipe The open pipe is intermediate-molded so that a ratio R1 (= Sc / Sm) of Sm and a distance Sc between the rolling direction line and the caliber bottom C satisfies the following expression (1): ERW steel pipe with low pipe strain Manufacturing method.
0.70 ≦ R1 (1)
[2] A distance EF between a point F on the reduction direction line where the distance between the reduction direction line and the outer surface of the open pipe is the longest, and a foot E of a perpendicular line descending from the caliber bottom C to the reduction direction line; The open pipe is intermediately molded so that a ratio R2 (= EF / BE) of the distance BE from the fin base end B of the upper roll to the point E satisfies the following expression (2): The method for producing an electric resistance welded steel pipe having a small pipe-forming strain as described in [1].
R2 ≦ 0.33 (2)
[3] The open pipe is formed so that the crossing angle X on the open pipe side formed by intersecting tangents of the outer surfaces of the opposite width ends of the steel strip forming the open pipe is 120 ° or more. The method for producing an electric resistance welded steel pipe having a small pipe-forming distortion as described in [1] or [2], characterized by performing intermediate forming.
[4] The method for producing an ERW steel pipe with small pipe-forming distortion according to any one of [1] to [3], wherein the ERW steel pipe is an ERW steel pipe having a thickness of 12 to 26 mm. .
[5] The electric resistance welded steel pipe is an electric resistance welded steel pipe having a thickness to outer diameter ratio of 2 to 20%, and the pipe-forming strain according to any one of [1] to [4] is small A method for manufacturing ERW steel pipes.
[6] An electric resistance welded steel pipe having a welded portion by electric resistance welding, wherein the fluctuation width of the thickness of the electric resistance welded pipe is in a range of −90 ° to + 90 ° from the bottom of the tube when the welded portion is turned up. An electric resistance welded steel pipe having a small pipe-forming strain, characterized by being in a range of ± 2% based on a predetermined wall thickness.
[7] The electric resistance welded steel pipe having a small pipe-forming strain according to [6], wherein the wall thickness is 12 to 26 mm.
[8] The electric resistance welded steel pipe having a small pipe-forming strain according to [6] or [7], wherein the ratio of the wall thickness to the outer diameter is 2 to 20%.
[9] The steel strip forming the ERW steel pipe is a hot-rolled steel sheet having a tensile strength of 460 MPa or more. The electric pipe with low pipe-forming strain according to any one of [6] to [8], Sewn steel pipe.

According to the present invention, the peak value of equivalent plastic strain representing the degree of work hardening of the material (steel plate) by steel pipe forming is reduced by about 3% compared to the conventional one, and the thickness of the distribution of processing strain in the pipe circumferential direction is improved. And / or high strength ERW steel pipes can be produced.
This stabilizes the forming of the steel pipe and improves the workability as a steel pipe product.

It is a schematic diagram which shows one example of an electric resistance steel pipe manufacturing line. It is a mimetic diagram showing one embodiment of the present invention. It is a schematic diagram which shows the cross-sectional shape of this open pipe orthogonal to the advancing direction of the open pipe before fin pass roll shaping | molding. It is the schematic diagram which matched and projected the outer surface of the bottom part of an open pipe, and the caliber bottom C of the lower roll of a 1st fin pass roll. It is a schematic diagram which shows the contact condition of the outer surface of an open pipe and the caliber surface of a side roll in the formation process by a 1st fin pass roll. FIG. 5 is a schematic diagram in which a reduction direction line acting on the width end portion of the steel strip forming the open pipe and a distance S (Y) from the reduction direction line to the outer surface of the open pipe are shown in FIG. It is a schematic diagram which shows the contact condition of the outer surface of an open tube and the caliber surface of a lower roll in the formation process by a 1st fin pass roll. It is a figure which shows typically the cross-sectional shape of the comparative example from which R1 and R2 differ, and the open pipe of the example of this invention. It is a figure which shows the relationship between the pipe circumferential direction distribution of the equivalent plastic strain in the pipe cross section after the 1st fin pass roll shaping | molding, and the cross-sectional shape of the open pipe in the 1st fin pass roll entrance side. It is a graph which shows distribution of the pipe | tube circumference direction of the equivalent plastic strain in the cross section orthogonal to the advancing direction of the steel pipe after a welding process. It is a graph which shows the distribution of the pipe | tube thickness direction of the sheet thickness distortion in the cross section of the steel pipe orthogonal to the advancing direction of the steel pipe after a welding process.

Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is an example of equipment for manufacturing ERW steel pipes. The steel strip, which is the material of the ERW steel pipe, is formed into an open pipe by intermediate forming with the cage roll group 4 and then finish-formed with the fin pass roll group 5. After finish forming, the width end of the steel strip is subjected to electric resistance welding to form an electric resistance welded steel pipe. Here, the steel strip can be exemplified by a hot rolled steel plate of carbon steel.

  The cage roll group 4 according to the present invention is divided into three zones as shown in FIG. 2, and an inner roll is provided in the most downstream zone (# 3 zone).

  In intermediate forming by a conventional cage roll group, that is, a cage roll group that does not have an inner roll, the traveling direction of a semi-molded product (hereinafter referred to as an open pipe) immediately before finish molding (the first fin pass roll entry side) The cross section of the open tube that is orthogonal to each other has a vertically long shape as shown in FIG. FIG. 4 is a cross-sectional view of the first fin pass roll that passes through the central axis of the lower roll and is orthogonal to the traveling direction of the open pipe among the forming rolls (upper roll, lower roll, side roll) of the first fin pass roll. The figure which projected the cross section of the said open pipe orthogonal to the advancing direction of the open pipe in the entrance side of the said 1st fin pass roll by making the outer surface of the bottom part of the said open pipe, and the caliber bottom C of the said lower roll correspond. It is. Here, FIG. 4A is a view showing an example in which the open pipe is intermediately formed by a conventional method, and the cross section of the open pipe has a vertically long shape, and FIG. It is a figure which shows the example at the time of using the inner roll shown in FIG. 2, and intermediate-molding the said open pipe | tube in the cross-sectional shape which satisfy | fills the conditions of this invention method.

  In the first fin pass roll, the outer surface side corner portion A of the width end portion of the steel strip forming the open tube in contact with the upper roll and the outer surface of the bottom portion of the open tube in contact with the lower roll are the action points. 4, the cross-sectional shape of the open pipe is changed from the shape shown in FIG. 4 to the caliber surface of the upper roll, the lower roll, and the side roll in a cross section that passes through the central axis of the lower roll and is orthogonal to the traveling direction of the open pipe. It is formed into a shape as shown in FIG. 7, which is substantially similar to the shape of the hole mold to be formed.

  Therefore, in each case of FIG. 4 (a) or FIG. 4 (b), the contact state between the outer peripheral surface of the open tube and the caliber surface of the forming roll during the first fin pass roll forming, and after the finish forming The effect on the strain distribution in the pipe circumferential direction of the steel pipe was investigated. The results are shown in FIGS. 5 (a) and 5 (b). 5 (a) and 5 (b) are diagrams showing a contact state between the outer surface of the open pipe and the caliber surface of the side roll in the molding process by the first fin pass roll, and will be described later with reference to FIGS. It is the figure which expanded the contact site | part of the outer surface of the open pipe in b), and the caliber surface of a side roll.

  As shown in FIG. 4A, when the shape of the cross section of the open pipe is extremely long, that is, in the molding by the first fin pass roll, the lower roll and the upper roll are pressed against the open pipe. When the amount is large, the outer surface of the open tube and the caliber surface of the lower roll do not contact uniformly, and processing distortion due to bending deformation occurs near the boundary between the contact part and the non-contact part, The distribution of processing strain in the pipe circumferential direction becomes non-uniform. As a result, the thickness of the ERW steel pipe in the pipe circumferential direction varies. Furthermore, the local contact between the outer surface of the open tube and the caliber surface of the roll occurs between the outer surface of the open tube and the caliber surface of the side roll, as shown in FIG. In that case, the deformation of the steel strip forming the open pipe becomes more uneven in the pipe circumferential direction, and the fluctuation of the wall thickness in the pipe circumferential direction of the manufactured ERW steel pipe and the processing strain distribution become more uneven. There is.

  In order to prevent the occurrence of the above-described problem, in forming with the first fin pass roll, as shown in FIG. 5B, the outer surface of the open tube and the caliber surface of the lower roll It is preferable that the steel strip forming the open pipe is bent and formed by pressing with the upper roll and the lower roll in a form in which the contact portion is enlarged starting from the caliber bottom C.

  Therefore, the influence of the shape of the cross section of the open pipe perpendicular to the traveling direction of the open pipe on the entry side of the first fin pass roll on the forming form of the bending process was examined. FIG. 6 is a cross-sectional view of the first fin pass roll that is perpendicular to the travel direction of the open pipe including the central axis of the lower roll, and the open that is perpendicular to the travel direction of the open pipe on the entry side of the first fin pass roll. It is the figure which projected the cross section of the pipe | tube, making the outer surface of the bottom part of the said open pipe, and the caliber bottom C of the said lower roll correspond, and the width | variety edge part of the steel strip which forms an open pipe is shaping | molding by a 1st fin pass roll And a fin direction end line that is a straight line connecting the outer surface side corner portion A of the width end portion and the fin base end portion B of the first fin pass roll, and the fin base end portion B on the down direction line. The distance S (Y) from the rolling direction line to the outer surface of the open tube at the position of the distance Y to the distance Y and the maximum value Sm of the distance S (Y) are also shown.

  Overhang deformation form of the steel strip when a bending force is applied by pressing with the length of the distance S (Y) and the width end of the steel strip forming the open pipe and the caliber bottom C of the lower roll as the working point And have a correlation, the bending moment becomes maximum at the position D of the steel strip where the distance S (Y) is maximum, and the overhang deformation starts. In the case of FIG. 6A in which the position D of the outer surface of the open pipe with the maximum distance from the rolling direction line is away from the caliber bottom C of the lower roll, as shown in FIG. The position D where the bending moment becomes maximum in the forming process first comes into contact with the caliber surface of the fin pass roll, and between the caliber bottom C and position D, the outer surface of the open tube and the caliber surface of the lower roll A gap is formed between the two. On the other hand, in the case of FIG. 6B in which the position D coincides with the caliber bottom C of the lower roll, as shown in FIG. 7B, the outer surface of the open pipe and the caliber surface of the lower roll are the caliber of the lower roll. Deformation proceeds in a form in which the contact portion is enlarged starting from the bottom C, and no gap is generated between the outer surface of the open pipe and the caliber surface of the lower roll.

  Therefore, among the forming rolls of the first fin pass roll (upper roll, lower roll, side roll), the cross section of the first fin pass roll perpendicular to the traveling direction of the open pipe including the central axis of the lower roll, When projecting a cross section of the open pipe perpendicular to the direction of travel of the open pipe on the entry side of the first fin pass roll with the outer surface of the bottom of the open pipe and the caliber bottom C of the lower roll aligned. From the straight line including the outer surface side corner A of the width end of the steel strip forming the open pipe and the fin base end B of the first fin pass roll to the outer surface of the open pipe It is desirable that the position D of the outer surface of the open pipe where the distance S (Y) is maximum coincide with the caliber bottom C of the lower roll, and at least the position D and the caliber bottom C satisfy the following conditions. Intermediate molded into.

That is, when the maximum value of the distance from the down-direction line to the outer surface of the open pipe is Sm, and the distance between the down-direction line and the caliber bottom C of the lower roll is Sc, the ratio R1 of Sc to Sm (= The open pipe is intermediate-molded so that (Sc / Sm) satisfies the following formula (1).
0.70 ≦ R1 (1)
The positional relationship between the position D and the caliber bottom C is determined by R1, and when R1 = 1, the position D at which the bending moment is maximum matches the caliber bottom C, and when R1 is smaller than 1, the position D becomes caliber bottom C. Means leaving. In the forming by the fin pass roll, the outer surface of the open pipe near the position D is preferentially made to be the caliber surface of the roll by the overhanging deformation of the steel strip forming the open pipe starting at the position D where the bending moment is maximum. Approach / contact. If R1 is less than 0.70, a gap is generated between the outer surface of the open tube and the caliber surface of the roll between the caliber bottom C and the position D, and as a result, in contact with the caliber surface of the roll near the position D. Since the processing strain applied to the steel strip forming the open pipe at the position where the open pipe is formed increases, the lower limit value of R1 is limited to 0.70. From the above definition, the upper limit value of R1 is inevitably 1 when the position D matches the caliber bottom C, and the value of R1 is close to the upper limit value of 1 so that the processing strain tube circumference after molding is This is preferable in order to make the directional distribution uniform.

Further, when the point on the rolling direction line where the distance Sm is obtained is F and the point on the rolling direction line where the distance Sc is obtained is E, the ratio R2 (= EF / BE) of the distance EF to the distance BE is It is preferable that the open pipe is subjected to intermediate molding so as to satisfy the following expression (2).
R2 ≦ 0.33 (2)
Similarly to R1, R2 is a variable that determines the positional relationship between position D and caliber bottom C. When R2 = 0, position D at which the bending moment is maximum coincides with caliber bottom C, and R2 increases from 0. This means that the position D moves away from the caliber bottom C. As described above, in the forming by the fin pass roll, the position D preferentially approaches and contacts the caliber surface of the roll by the overhang forming of the steel strip forming the open pipe starting at the position D where the bending moment is maximum. To do. At this time, when R2 exceeds 0.33, a gap is generated between the outer surface of the open tube and the caliber surface of the roll between the caliber bottom C and the position D, and as a result, the roll is positioned near the position D. The processing strain applied to the steel strip forming the open pipe at the position in contact with the caliber surface of the steel increases, so the upper limit value of R2 is limited to 0.33. The lower limit value of R2 is 0 when the position D where the bending moment is maximized matches the caliber bottom C. The value of R2 is preferably closer to 0 in order to make the distribution in the pipe circumferential direction of the processed strain after molding more uniform.

  Furthermore, the crossing angle X on the open pipe side formed by intersecting the tangents of the outer surfaces at the opposite width ends of the steel strip forming the open pipe on the fin pass roll entry side is 120 ° or more. Preferably, the open pipe is formed by intermediate molding.

  If the crossing angle X is less than 120 ° and becomes more acute, in the finish forming by the fin pass roll group, the processing strain near the tube bottom becomes high due to the influence of the reduction from the upper roll, and after the finish forming, The processing strain distribution in the pipe circumferential direction and the wall thickness variation of the steel pipe tend to be non-uniform. In addition, about the upper limit of the crossing angle X, since it also depends on the shape of the forming roll of the fin pass roll, it cannot be determined uniquely. However, when the crossing angle X exceeds 150 °, the steel strip forming the open pipe When the butt-welding of both opposite width ends of each other, there is a problem that the outer surfaces of both width ends enter the inside of the tube, so a more preferable upper limit of the crossing angle X is 150 ° or less. .

  As shown in FIG. 2, the shape of the open pipe that satisfies the above conditions is a cage roll that presses the vicinity of both width ends of the steel strip from the outer surface side in the intermediate forming step by the cage roll group, This can be realized by restraining the position of the steel strip in the plane perpendicular to the material traveling direction using an inner roll that presses two or more positions in the circumferential direction from the inner wall surface side of the steel strip. That is, in order to form the shape of the open pipe so as to satisfy the above-described conditions, as shown in FIG. 2, a steel strip is formed between a pair of cage rolls in the most downstream zone (# 3 zone) of the cage roll group. In the plane perpendicular to the material traveling direction of the steel strip by the pair of cage rolls and the inner roll, using an inner roll that presses two or more positions in the circumferential direction from the inner wall surface side of the semi-molded product It is preferable to constrain the position.

  Furthermore, it is preferable that the electric resistance welded steel pipe to be manufactured is an electric resistance welded steel pipe having a thickness of 12 to 26 mm and / or a ratio of the thickness to the outer diameter of 2 to 20%. When the wall thickness is less than 12 mm and / or the ratio between the wall thickness and the outer diameter is less than 2%, the steel strip is folded by the pressing of the inner roll in addition to the low forming load of the fin pass roll even in the conventional method. There are concerns about extreme deformation such as Note that if the thickness exceeds 26 mm and / or the ratio of the thickness to the outer diameter exceeds 20%, the processing becomes extremely difficult, so that there are many restrictions on equipment and the cost is disadvantageous, so the upper limit is set.

In addition, when the strength of the steel strip to be formed is low, the inner surface of the tube may be dented by pressing the inner roll. Therefore, the tensile strength of the steel strip to be formed is preferably 460 MPa or more.
In addition, although said description demonstrated using the first shaping | molding roll (lower side roll, side roll, upper side roll) of the fin pass roll group 5, this has the effect of the first shaping roll of the fin pass roll group 5. Since it is large, the same effect can be obtained for the second and subsequent forming rolls of the fin pass roll group 5. Therefore, it is not limited to the first forming roll.

  Next, with regard to the wall thickness of the pipe, for example, in the JIS standard for carbon steel pipes for general structural use, a detailed regulation based on the size is defined, but tolerances of about ± 12.5 to 10% are generally allowed. ing. In actual production, the thickness of the coil base plate is important. In addition, depending on the reduction ratio, which is the ratio to the tube circumference of the final tube product, the average thickness is usually set by setting the coil width. Thickness increase can be controlled. However, the increase in thickness is usually processed so that the vicinity of the butt weld at the width end of the steel strip forming the open pipe is increased.

  So far, we have described the local processing strain caused by the gap and contact position between the outer surface of the open tube and the caliber surface of the forming roll. The non-uniform distribution induces a non-uniform thickness distribution in the pipe circumferential direction. When there is such a nonuniform local thickness distribution, the cost may be increased, such as readjusting the coil width and remanufacturing. In addition, when there is a locally thin portion in the pipe circumferential direction, it is assumed that stress or deformation concentration is induced during use of the pipe. Therefore, it is desirable that the wall thickness distribution in the pipe circumferential direction be more uniform, and if the method of the present invention is applied, the fluctuation range of the wall thickness is -90 ° to + 90 ° from the tube bottom when the welded portion is facing up. In this range, it is possible to be within a range of ± 2% based on a predetermined thickness.

  Here, the range of −90 ° to + 90 ° from the bottom of the tube is the other region, that is, in the half circumference of the tube close to the butt weld, as described above, the steel strip forming the open tube Since a tendency to increase the thickness appears due to the effect of increasing the thickness of the welded portion where the width end portion is butt welded, the region is limited to the half-circumference region on the tube bottom side.

  The electric resistance welded steel pipe obtained by the present invention is an electric resistance welded steel pipe having various strength levels including API standard X80 grade. Needless to say, the strength level is lower than the X80 grade, and even an ERW steel pipe having a high strength level can be applied to the present invention as long as it can be formed in terms of equipment capacity. What is necessary is just to select the steel strip used as a raw material suitably according to the target grade. In the case of an API standard X80 grade electric resistance welded steel pipe, if an example of the chemical composition of the steel strip is given, by mass%, C: 0.01 to 0.12%, Si: 0.01 to 1.0%, Mn: 0.5 to 2.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.005 to 0.10%, Nb: 0.01 to 0.10%, Ti: 0 0.001 to 0.05%, and the balance is made of iron and inevitable impurities.

Here, the reason for limiting the preferable composition will be briefly described.
C is an element having an effect of improving the strength of steel, and in order to obtain a desired strength level, the C content needs to be 0.01% or more. On the other hand, when the content of C exceeds 0.12%, there are adverse effects such as deterioration of weldability due to an increase in carbon equivalent. For this reason, the C content is limited to a range of 0.01 to 0.12%. In addition, More preferably, it is 0.02 to 0.08%.

  Si has an action of improving the strength of steel through solid solution strengthening and improvement of hardenability. In order to obtain such an effect, it is necessary to contain Si by 0.01% or more. On the other hand, Si forms an oxide containing Si during ERW welding, lowers the quality of the welded part, and lowers the toughness of the weld heat affected zone. Up to 0.0% is acceptable. For this reason, the Si content is limited to 0.01 to 1.0%. More preferably, it is 0.5% or less.

  Mn has the effect | action which improves the hardenability of steel and improves the intensity | strength of a steel plate. Further, Mn forms MnS and fixes S, thereby preventing S grain boundary segregation and suppressing slab cracking. In order to obtain such an effect, it is necessary to contain 0.5% or more of Mn. On the other hand, if the Mn content exceeds 1.8%, solidification segregation during slab casting is promoted, and a Mn-concentrated portion is generated in the steel sheet, resulting in an increase in the occurrence of separation. In order to eliminate this Mn enriched part, it is necessary to heat a slab to the temperature exceeding 1300 degreeC, and it is not realistic to implement such heat processing on an industrial scale. For this reason, content of Mn was limited to 0.5 to 2.2% of range. In addition, More preferably, it is 0.9 to 1.7%.

  P is inevitably contained as an impurity in the steel, but has an effect of increasing the strength of the steel. However, if it exceeds 0.03% and it contains excessively, weldability will fall. Therefore, the P content is limited to 0.03% or less. In addition, More preferably, it is 0.02% or less.

  S is inevitably contained as an impurity in steel as in P, but if S is contained excessively in excess of 0.005%, slab cracking occurs and coarse MnS is contained in the hot-rolled steel sheet. Forming and causing a reduction in ductility. For this reason, the S content is limited to 0.005% or less. In addition, More preferably, it is 0.004% or less.

  Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is desirable to contain 0.005% or more of Al. On the other hand, if the Al content exceeds 0.10%, the cleanliness of the welded portion during ERW welding is significantly impaired. For this reason, the content of Al is limited to 0.005 to 0.10%. In addition, More preferably, it is 0.005-0.08%.

  Nb is an element that has the effect of suppressing the coarsening and recrystallization of austenite grains, and enables rolling in the austenite non-recrystallization temperature range in hot finish rolling. Moreover, it has the effect | action which makes a hot-rolled steel plate high intensity | strength with little content, without impairing weldability by carrying out fine precipitation as a carbonitride. In order to obtain such an effect, the Nb content needs to be 0.01% or more. On the other hand, if the Nb content exceeds 0.10%, an increase in rolling load during hot finish rolling may be caused, and hot rolling may become difficult. For this reason, the Nb content is limited to a range of 0.01 to 0.10%. In addition, More preferably, it is 0.03 to 0.09%.

  Ti has a function of forming nitrides, fixing N, and preventing slab cracking. In addition, the steel sheet is strengthened by fine precipitation as carbides. Such an effect becomes remarkable when the Ti content is 0.001% or more. On the other hand, when the Ti content exceeds 0.05%, the yield point is remarkably increased by precipitation strengthening. For this reason, Ti content was limited to 0.001 to 0.05% of range. In addition, More preferably, it is 0.005 to 0.035%.

  Although the above is a basic composition, in order to improve the intensity | strength of a steel strip in addition to the said composition, V: 0.01-0.10%, Mo: 0.01-0.50%, Cr: One or more of 0.01 to 1.0%, Cu: 0.01 to 0.50%, and Ni: 0.01 to 0.50% may be included.

  V is an element that has an effect of improving hardenability and forming carbonitride to increase the strength of the steel sheet, and such an effect becomes remarkable when contained in an amount of 0.01% or more. On the other hand, if the content exceeds 0.10%, the weldability is deteriorated. For this reason, it is preferable to make content of V 0.01-0.10%. In addition, More preferably, it is 0.03 to 0.08%.

  Mo is an element that has an effect of improving hardenability and forming carbonitride to increase the strength of the steel sheet. Such an effect becomes remarkable when contained in an amount of 0.01% or more. On the other hand, a large content exceeding 0.50% reduces weldability. For this reason, it is preferable to limit the Mo content to 0.01 to 0.50%. In addition, More preferably, it is 0.05 to 0.30%.

  Cr is an element having an effect of improving hardenability and improving the strength of the steel sheet. Such an effect becomes remarkable by containing 0.01% or more. On the other hand, if the content exceeds 1.0%, welding defects tend to occur frequently during ERW welding. For this reason, it is preferable to limit the Cr content to 0.01 to 1.0%. In addition, More preferably, it is 0.01 to 0.80%.

  Cu is an element that has the effect of improving the hardenability and improving the strength of the steel sheet by solid solution strengthening or precipitation strengthening. In order to acquire such an effect, it is desirable to contain 0.01% or more, but when the content exceeds 0.50%, hot workability is lowered. For this reason, it is preferable to limit the Cu content to 0.01 to 0.50%. In addition, More preferably, it is 0.10 to 0.40%.

Ni is an element that has the effect of improving hardenability, improving the strength of the steel, and improving the toughness of the steel plate. In order to acquire such an effect, it is desirable to contain 0.01% or more. On the other hand, even if the content exceeds 0.50%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, it is preferable to limit the content of Ni to 0.01 to 0.50%. In addition, More preferably, it is 0.10 to 0.45%.
In addition to the above composition, it may further contain one or more of Ca: 0.0005 to 0.005%, B: 0.001% or less, and REM: 0.0005 to 0.005%. Good.

  Ca is an element having an action of fixing S as CaS, spheroidizing sulfide inclusions, and controlling the form of the inclusions. In addition, the lattice strain of the matrix around the inclusion is reduced, and the hydrogen trapping ability is lowered. In order to acquire such an effect, it is desirable to contain 0.0005% or more of Ca. However, if it exceeds 0.005%, CaO is increased, and corrosion resistance and toughness are lowered. For this reason, when it contains Ca, it is preferable to limit to 0.0005 to 0.005%. In addition, More preferably, it is 0.0009 to 0.003%.

  B, when contained in a small amount, significantly improves the hardenability and contributes to an increase in steel sheet strength. Such an effect becomes remarkable when the content is 0.0003% or more, but the effect is saturated even if the content exceeds 0.001%. For this reason, when B is contained, the content is preferably limited to 0.001% or less.

  REM is an element that contributes to the form control of sulfides in which expanded coarse sulfides are spherical sulfides. In order to obtain such an effect, it is desirable to contain 0.0005% or more of REM. However, if it contains more than 0.005%, the cleanliness of the steel sheet is lowered. Therefore, it is preferable to limit REM to 0.0005 to 0.005%.

  The production of the hot-rolled steel sheet is not particularly limited as long as a steel strip of API standard X80 class level can be obtained. For example, the steel material having the above composition is preferably heated to 1100 to 1300 ° C. When the heating temperature is less than 1100 ° C., the deformation resistance is high, the rolling load is increased, and the rolling efficiency is lowered. On the other hand, when the heating temperature exceeds 1300 ° C. and the temperature becomes high, the crystal grains are coarsened and the low temperature toughness is lowered, and the scale generation amount is increased, and the surface properties may be lowered. For this reason, it is preferable that the heating temperature in hot rolling shall be 1100-1300 degreeC.

Next, hot rolling is performed on the heated steel material.
The hot rolling is rolling consisting of rough rolling and finish rolling. The conditions for the rough rolling need not be particularly limited as long as the sheet bar can have a predetermined size and shape. However, in the finish rolling after the rough rolling, the hot rolling rate in the non-recrystallization temperature range is 20%. As mentioned above, it is preferable to adjust finish rolling completion temperature: 750 degreeC or more. Moreover, it is preferable that a hot reduction rate shall be 95% or less from a viewpoint of the load to a rolling mill.

For cooling after finish rolling, an appropriate means can be used as necessary.
The winding temperature should just be 400-580 degreeC. When the coiling temperature exceeds 580 ° C., precipitation of Ti and Nb carbonitrides is promoted, and the strength of the steel sheet may become too high. On the other hand, if the temperature is lower than 400 ° C., precipitation of Ti and Nb carbonitride is insufficient, and strength of X80 grade or higher may not be obtained. For this reason, winding temperature was limited to the range of 400-580 degreeC. Preferably it is 460-550 degreeC.

  In addition, although said component range and a manufacturing method are examples of API specification X80 grade, it cannot be overemphasized that a component range and a manufacturing method can be suitably selected and adjusted according to the target grade.

  By adjusting the application conditions of the inner roll for the ERW steel pipe with the strength level of API standard X56 class, outer diameter 600mm and wall thickness 25.4mm, using the steel strip of low carbon low alloy steel as the material, the first fin The cross-sectional shape of the open tube on the pass roll entrance side was manufactured as three shapes shown in FIG. Cross section 1 is R1 = 0.44, R2 = 0.36, X = 15.7 ° Comparative Example 1, Cross section 2 is R1 = 0.69, R2 = 0.34, X = 11.7 ° Comparative Example 2. Section 3 is an example of the present invention with R1 = 0.86, R2 = 0.28, and X = 12.2 °. For each of these steel pipes, the deformation behavior in the manufacturing process was calculated by forming calculation using the finite element method, equivalent plastic strain after forming by the first fin pass roll, equivalent plastic strain after forming squeeze roll, and steel pipe after electric resistance welding The thickness strain of the steel strip forming the tube was calculated, and the distribution in the pipe circumferential direction was determined. The equivalent plastic strain is a plastic strain that serves as an index for determining the yield stress (equivalent stress) after work hardening by plastic deformation using the true stress-true strain curve of the workpiece regardless of the deformation mode. is there.

  FIG. 9 shows the distribution in the pipe circumferential direction of the equivalent plastic strain after forming by the first fin pass roll. The cross-section circumferential distance from the pipe bottom portion in contact with the lower roll of the first fin path is 0 to 250 mm, and the cross-section 1 (Comparative Example 1, R1 = 0.44, R2 = 0.36, X = 15.7. In the case of °), the equivalent plastic strain had a peak value as large as about 5%, and the distribution in the pipe circumferential direction was the most nonuniform. In contrast, in the cross section 3 (example of the present invention, R1 = 0.86, R2 = 0.28, X = 12.2 °), the equivalent plastic strain peak value is as small as 2.5% (0.025), Compared with the case of the cross section 1 and the cross section 2 (Comparative Example 2, R1 = 0.69, R2 = 0.34, X = 1111.7 °), the distribution in the pipe circumferential direction of the equivalent plastic strain was also uniform.

  Next, the pipe circumferential direction distribution of the equivalent plastic strain after squeeze roll molding is shown in FIG. As shown in FIG. 10, the equivalent plastic strain in the case of the cross section 3 (example of the present invention, R1 = 0.86, R2 = 0.28, X = 12.2 °) is relatively over the entire pipe circumferential position. When compared using the average value of the equivalent plastic strain over the region, it was confirmed that it was 0.8 to 0.9% (0.008 to 0.009) smaller than those of the cross sections 1 and 2. In addition, the peak value of the equivalent plastic strain of the present invention example is 0.038, which is smaller than t (thickness) / D (outer diameter) = 0.04 of the manufactured ERW steel pipe, and the pipe-forming strain is small. Was confirmed.

  Furthermore, also in the pipe circumferential direction distribution of the thickness strain (Thickness strain) of the steel strip forming the ERW steel pipe after the finish forming process is completed and the width end of the steel strip is welded, as shown in FIG. The distribution in the pipe circumferential direction of the sheet thickness strain of the steel strip forming the ERW steel pipe manufactured as the cross section 3 (example of the present invention) becomes more uniform than in the case of other cross sections (Comparative Examples 1 and 2), The amount of change in plate thickness strain (corresponding to the rate of change in plate thickness) in the range of ± 90 ° in the circumferential direction from the tube bottom was within ± 0.6%.

  Moreover, with the manufacturing method of the present invention, an ERW steel pipe having an strength level of API standard X56 to X80, an outer diameter of 219 to 610 mm, and a wall thickness of 12 to 26 mm is manufactured, and each steel pipe at each position in the pipe circumferential direction is manufactured. Measure the yield stress at each position in the pipe circumferential direction using the thickness and tensile test specimens cut from each position in the pipe circumferential direction, and the rate of change in wall thickness due to pipe making, that is, the strip of steel strip that forms the ERW steel pipe The amount of change in thickness strain and the amount of increase in yield stress, that is, the magnitude of work hardening, were evaluated. As a result, if the thickness is 12 to 26 mm and the ratio of the thickness to the outer diameter is in the range of 2 to 20%, the change rate of the thickness is in the range of ± 90 ° in the circumferential direction from the tube bottom. In the case of API standard X65, for example, in the case of API standard X65, 48.5 MPa (processing with an equivalent plastic strain of 3.3%) is in the range of ± 1%. It was confirmed that it was 30.3 MPa (corresponding to a work hardening amount of 1.9% equivalent plastic strain) at a position of ± 90 ° from the tube bottom.

1 Uncoiler 2 Leveler 3 Edge forming machine 4 Intermediate forming machine (cage roll group)
5 Finishing machine (Finpass roll group)
6 Heating device 7 Squeeze roll 8 Bead cutting machine 9 Sizer 20 Steel strip 30 ERW steel pipe A Outer side corner of the steel strip width end of the open pipe B Fin base end C of the fin pass roll Lower roll of the fin pass roll Caliber bottom D The position of the outer surface of the open tube with the maximum distance from the rolling direction line

Claims (9)

An intermediate forming step of intermediate forming a steel strip with a cage roll group;
A finish forming step of finishing and forming into a tubular shape by a fin pass roll group;
A welding step in which a steel pipe is formed by electric resistance welding of the width end of the steel strip after the finish forming step;
In the manufacturing method of the ERW steel pipe having
In the cross section passing through the central axis of the lower roll of the first fin pass roll composed of the upper roll, the lower roll and the side roll to be subjected to the first molding of the finish molding step, When projecting a cross section of the open tube perpendicular to the traveling direction of the open tube on the entrance side of the fin pass roll with the outer surface of the bottom portion of the open tube and the caliber bottom C of the lower roll aligned,
The maximum distance between the rolling direction line that is a straight line including the outer surface side corner A of the width end of the steel strip forming the open pipe and the fin base end B of the upper roll and the outer surface of the open pipe The open pipe is subjected to intermediate molding so that the ratio R1 (= Sc / Sm) of the value Sm and the distance Sc between the rolling direction line and the caliber bottom C satisfies the following formula (1): A method for manufacturing ERW steel pipes with low pipe-forming strain.
0.70 ≦ R1 (1) A distance EF between a point F on the rolling direction line where the distance between the rolling direction line and the outer surface of the open pipe is the longest and a foot E of a perpendicular line descending from the caliber bottom C to the rolling direction line; The said open pipe is intermediate-molded so that ratio R2 (= EF / BE) of the distance BE from the fin base end part B of the roll to the said point E may satisfy | fill following (2) Formula. 2. A method for producing an electric resistance welded steel pipe having a small pipe-forming strain according to 1.
R2 ≦ 0.33 (2)   The open pipe is intermediate-molded so that the crossing angle X on the open pipe side formed by intersecting tangents of the outer surfaces of the opposite width ends of the steel strip forming the open pipe is 120 ° or more. The method for producing an electric resistance welded steel pipe according to claim 1 or 2, wherein the pipe-forming distortion is small.   4. The method for producing an electric resistance welded steel pipe having a small pipe-forming strain according to claim 1, wherein the electric resistance welded steel pipe is an electric resistance welded steel pipe having a thickness of 12 to 26 mm.   5. The electric resistance welded steel pipe according to claim 1, wherein the ratio of thickness to outer diameter is 2 to 20%. Method. An electric resistance steel pipe having a welded portion by electric resistance welding,
The variation width of the thickness of the electric resistance welded steel pipe is in a range of -90 ° to + 90 ° from the bottom of the pipe when the welded portion is up, and in a range of ± 2% based on a predetermined thickness. ERW steel pipe with small pipe-forming strain.   The wall thickness is 12 to 26 mm, and the electric resistance welded steel pipe having a small pipe-forming strain according to claim 6.   The ratio of the wall thickness to the outer diameter is 2 to 20%, and the electric resistance welded steel pipe having a small pipe-forming strain according to claim 6 or 7.   9. The electric resistance welded steel pipe having a small pipe-forming strain according to claim 6, wherein the steel strip forming the electric resistance welded pipe is a hot-rolled steel sheet having a tensile strength of 460 MPa or more.

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