Electric resistance welded steel pipe, method for manufacturing the same, and automotive structural member

Abstract

The present invention relates to an electric resistance welded steel pipe having excellent workability and torsional fatigue resistance and a method for manufacturing the same. The electric resistance welded steel pipe 1 comprises a weld region 3, which is an area within ±10 degrees in the pipe circumferential direction from an electric resistance weld seam 2 formed along the pipe length, and a base material region 6 outside the weld region 3. The pipe has an r value of 1.0 or greater in the pipe length direction, and a minimum wall thickness Ts of the weld region 3. (MIN) (mm) and the average wall thickness Tb of the base material area 6 (Ave) (mm) difference (T b(Ave) ‑Ts (MIN) ) and the arc length W (mm) of the inner surface of the pipe in the weld area 3 satisfy the following formula (1), the maximum wall thickness Ts of the weld area 3 (MAX) (mm) and Tb (Ave) (mm) satisfies the following formula (2). H / W≤0.10…Formula (1) Ts (MAX) / Tb (Ave) ≤1.05…Formula (2).

Description

Technical Field

[0001] The present invention relates to a technology for electric resistance welded steel pipes having excellent torsional fatigue resistance and suitable for automobile stabilizers. Background Art

[0002] In recent years, automobile body weight reduction has been promoted to improve fuel efficiency. Among the components that make up the vehicle body, stabilizers, which were previously made of steel bars, are now becoming increasingly hollow due to the use of electric resistance welded steel pipes, which offer excellent productivity.

[0003] These automotive parts such as stabilizers require high workability because they are subjected to bending. Furthermore, since bending and torsional stresses continue to act during use as products, they are also required to have excellent fatigue properties (hereinafter also referred to as torsional fatigue resistance).

[0004] For example, Patent Document 1 proposes an electric resistance welded steel pipe that, in order to meet the required torsional fatigue resistance, specifies a region called a white layer within the electric resistance weld where the carbon content is reduced and the hardness after quenching is lower than that of the surrounding area. Furthermore, Patent Document 2 proposes an electric resistance welded steel pipe that defines and limits the area of weld defects, thereby improving torsional fatigue resistance.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent No. 5942572

[0008] Patent Document 2: Japanese Patent No. 5845623 Summary of the Invention

[0009] Problems to be solved by the invention

[0010] However, the steel pipes proposed in Patent Documents 1 and 2 cannot be said to have sufficient torsional fatigue resistance.

[0011] In order to solve the above-mentioned problems, an object of the present invention is to provide a technology for electric resistance welded steel pipe having excellent workability and torsional fatigue resistance.

[0012] Methods used to solve problems

[0013] The present inventors have conducted research on torsional fatigue of steel pipes and have found that after diameter reduction rolling, the inner diameter shape of the steel pipe near the electric resistance weld portion changes, and this shape change affects the torsional fatigue characteristics.

[0014] To achieve the above-mentioned objectives, the present inventors conducted extensive research into various measures for improving the inner diameter shape near the resistance weld seam during hot reduction rolling of steel pipes. As a result, they discovered that the inner diameter shape near the resistance weld seam varies depending on the position of the rolls used in the reduction rolling process and the position of the resistance weld seam of the steel pipe during the reduction rolling process. Furthermore, they discovered that optimizing the position at which the steel pipe is inserted into the rolls during the reduction rolling process improves the inner diameter shape, achieving both high workability and improved fatigue properties.

[0015] The present invention has been completed by further research based on this knowledge.

[0016] [1] An electric resistance welded steel pipe comprising a weld region and a base metal region outside the weld region, wherein the weld region is an area within ±10° in the circumferential direction of the pipe from a resistance weld formed along the pipe length, wherein the r value in the pipe length direction is 1.0 or greater, and the minimum wall thickness Ts of the weld region is (MIN) (mm) and the average wall thickness Tb of the above-mentioned base material area (Ave) (mm) difference (Tb (Ave) -Ts (MIN) ) and the arc length W (mm) of the inner surface of the pipe in the weld area satisfy the following formula (1), and the maximum wall thickness Ts of the weld area is (MAX) (mm) and the above Tb (Ave) (mm) satisfies the following formula (2).

[0017] H / W≤0.10…Formula (1)

[0018] Ts (MAX) / Tb (Ave) ≤1.05…Formula (2)

[0019] [2] The electric resistance welded steel pipe according to [1], wherein the Tb (Ave) (mm) and the average outer diameter Db of the base material area (Ave) (mm) satisfies the following formula (3).

[0020] (Tb (Ave) / Db (Ave) )×100≥15%…Formula (3)

[0021] [3] The electric-resistance-welded steel pipe according to [1] or [2] above, wherein the r value is the r value of the base material region in the pipe longitudinal direction.

[0022] [4] A method for manufacturing an electric resistance welded steel pipe, which is a method for manufacturing an electric resistance welded steel pipe as described in any one of [1] to [3] above, wherein a steel strip is formed into an open pipe, the open pipe is subjected to electric resistance welding to form a billet pipe, the billet pipe is heated at a heating temperature of 650°C or higher, and subjected to diameter reduction rolling with a cumulative diameter reduction ratio of 30% or higher, and during the diameter reduction rolling, at a roll stand with a diameter reduction ratio of 5.0% or higher, the electric resistance weld seam portion does not pass within a range of ±5.0° from the roll pass center of the roll and within a range of ±5.0° (n is the number of rolls per stand) at positions 360° / (n×2) to the left and right of the roll pass center.

[0023] [5] An automobile structural member formed using the electric resistance welded steel pipe according to any one of [1] to [3] above.

[0024] Effects of the Invention

[0025] According to the present invention, a technique is provided for providing an electric resistance welded steel pipe having excellent workability and torsional fatigue resistance.

[0026] Specifically, the present invention enables the production of steel pipes that exhibit high workability, such as an r-value of 1.0 or greater, and exhibit superior torsional fatigue resistance compared to conventional pipes, without causing plug seizure. This has significant industrial benefits. The electric resistance welded steel pipes of the present invention are suitable for automotive structural components such as stabilizers, which require torsional fatigue resistance after bending and cross-section forming. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a plan view showing a cross section of an electric resistance welded steel pipe.

[0028] Figure 2 is a plan view of the weld area.

[0029] Figure 3 is the maximum wall thickness Ts in the weld area (MAX) Tb is the average thickness of the base material area (Ave) Plan view of the weld area when large.

[0030] Figure 4 This is an external view of the equipment for manufacturing electric resistance welded steel pipe according to the present invention.

[0031] Figure 5 This is an external view for explaining the position of the electric resistance weld bead when the electric resistance welded steel pipe is inserted into the roll.

[0032] Figure 6 This is an external view for explaining the areas that should be avoided from contacting the resistance welding seam portion of the rolls according to the present invention.

[0033] Figure 7This is a plan view of the roll for explaining the phase angle of the roll.

[0034] Figure 8 This is a diagram showing the relationship between the position of the resistance weld bead portion inserted into the roll and the area to avoid contact in the embodiment. DETAILED DESCRIPTION

[0035] <Electric resistance welded steel pipe>

[0036] The steel pipe of the present invention is an electric resistance welded steel pipe having excellent torsional fatigue resistance. The steel pipe comprises a weld region, which is an area within ±10° in the pipe circumferential direction from an electric resistance welded weld formed along the pipe length, and a base material region outside the weld region. The steel pipe is characterized in that the r value in the pipe length direction is 1.0 or greater, and the minimum wall thickness Ts in the weld region is 1.0. (MIN) (mm) and the average wall thickness Tb of the base material area (Ave) (mm) difference (Tb (Ave) -Ts (MIN) ) and the arc length W (mm) of the inner surface of the pipe in the weld area satisfy the following formula (1), and the maximum wall thickness Ts in the weld area is (MAX) (mm) and Tb (Ave) (mm) satisfies the following formula (2).

[0037] H / W≤0.10…Formula (1)

[0038] Ts (MAX) / Tb (Ave) ≤1.05…Formula (2)

[0039] First, the measurement method of each dimension is described. Figure 1 The cross-sectional shape of the electric resistance welded steel pipe 1 is shown in FIG. Figure 2 An enlarged view of the weld seam region 3 is shown in FIG.

[0040] The weld region 3 is a region within ±10° in the pipe circumferential direction from the electric resistance weld bead portion 2 when the center of a vertical cross section in the pipe axial direction (longitudinal direction) is defined as the center of the circle Ø.

[0041] Average thickness Tb of base material region 6 (Ave) Using a single ball micrometer, with the resistance weld seam at 0°, measure the wall thickness of the base material region 6 at positions of 40°, 80°, 120°, 160°, 200°, 240°, 280°, and 320°, and take the average value of these values as Tb. (Ave) (mm).

[0042] Next, regarding the average outer diameter Db of the base material region 6, (Ave)(mm), using an outside micrometer, with the resistance weld bead portion 2 set at 0°, measure the outside diameter of the base material region 6 at positions of 40°, 80°, 120°, and 160°, and take the average value of these values as Db (Ave) (mm).

[0043] The arc length W (mm) of the pipe inner surface in the weld region 3 is defined by the following formula (4).

[0044] W(mm)=(Db after diameter reduction (Ave) -2×Tb (Ave) )×20×π / 360…Formula (4)

[0045] Ts (MIN) (mm), Ts (MAX) (mm) are respectively determined by the minimum wall thickness of the weld area 3 ( Figure 2 The wall thickness reaches its minimum value at the position indicated by symbol 5), the wall thickness of the weld area 3 reaches its maximum value ( Figure 2 , the maximum wall thickness is reached at the position shown by symbol 4).

[0046] It should be noted that Ts (MIN) (mm), Ts (MAX) (mm) are the minimum and maximum values respectively obtained by measuring the wall thickness of three sections of the weld area per 1° using a pointed micrometer.

[0047] The electric resistance welded steel pipe of the present invention is not particularly limited, and the average wall thickness Tb of the base material region 6 is (Ave) It is preferably 4.0 to 8.0 mm.

[0048] In addition, the electric resistance welded steel pipe of the present invention is not particularly limited, and the average outer diameter Db of the pipe is (Ave) It is preferably 20.0 to 45.0 mm.

[0049] R value: 1.0 or above

[0050] Next, the range of the r value (Lankford value) will be described. In the present invention, the r value can be set as the r value of the parent material region 6 in the tube length direction. By setting the r value of the steel tube 1 in the tube length direction to 1.0 or more, the workability required for manufacturing the stabilizer is met. On the other hand, when the above-mentioned r value of the steel tube 1 is less than 1.0, buckling occurs when the steel tube 1 is bent, and it cannot be bent into a specified shape. Therefore, the r value of the steel tube 1 in the tube length direction is set to 1.0 or more. Preferably, the above-mentioned r value is 1.3 or more. In addition, the upper limit of the r value is not particularly limited, but when the r value is too high, deformation is concentrated in the circumferential direction of the tube, and when the steel tube 1 is bent, the cross section of the steel tube may be reduced in diameter and necked, and the desired shape cannot be obtained. Therefore, it is preferably 2.0 or less.

[0051] The r value is determined by measuring the true strain eW in the width direction relative to the true strain eL in the length direction during a tensile test at a nominal strain of 5 to 10%, and calculating the r value = ρ / (-1-ρ) from the slope ρ. For this purpose, a JIS 12 No. A tensile test piece is cut from the base material region of the steel pipe and a strain gauge with a gauge length of 2 mm is attached for testing.

[0052] The r value can be adjusted by controlling the heating temperature and the cumulative reduction rate during the diameter reduction rolling.

[0053] H / W≤0.10…Formula (1)

[0054] Next, the reasons for limiting the ratio H / W of H (mm) to the arc length W (mm) of the pipe inner surface in the weld region 3 will be described.

[0055] H (mm) is the minimum wall thickness Ts of the weld area 3 (MIN) (mm) and the average wall thickness Tb of the base material area 6 (Ave) (mm) difference (Tb (Ave) -Ts (MIN) ), that is, H=Tb (Ave) -Ts (MIN) .

[0056] like Figure 2 As shown, the inner diameter of the electric resistance welded steel pipe 1 has a steep depression ( Figure 2 In the case of reference symbol 5), stress concentrates at the recess, and the torsional fatigue resistance is reduced. The results of the investigation show that when H / W, an indicator of the steepness of the weld region 3, is 0.10 or less, the required torsional fatigue resistance is met.

[0057] In addition, when H is negative, that is, Ts (MIN) Greater than Tb (Ave) When , there is no depression in the weld region 3 having a thickness smaller than that of the base material region 6 , and therefore, the torsional fatigue resistance is not reduced.

[0058] Therefore, in the present invention, the minimum wall thickness Ts of the weld area 3 is (min) (mm) and the average wall thickness Tb of the base material area 6 (Ave) (mm) difference (Tb (Ave) -Ts (MIN) ) defined by H (mm) to the arc length W (mm) of the weld area 3 (H / W) is set to be less than 0.10.

[0059] Furthermore, H / W is preferably 0.07 or less, and more preferably 0.05 or less.

[0060] Furthermore, H / W is preferably not less than -0.10, and more preferably not less than -0.07.

[0061] In a roll stand where reduction is performed at a reduction rate of a specific value or more per stand, the base tube 12 is loaded so that the resistance welded weld portion 2 avoids the area within a specific range from the roll pass end to the roll pass center of the roll, thereby adjusting the H / W ratio to the above-mentioned range.

[0062] Ts (MAX) / Tb (Ave) ≤1.05…Formula (2)

[0063] Next, the maximum wall thickness Ts of the weld area 3 is (MAX) (mm) and Tb (Ave) (mm) ratio (Ts (MAX) / Tb (Ave) From the perspective of torsional fatigue resistance, the thicker the weld region is compared to the base material region 6, the less likely it is to cause a fracture starting from the weld region 3.

[0064] Here, reference Figure 3 . Figure 3 is the maximum wall thickness Ts of weld zone 3 (MAX) Greater than the average wall thickness Tb of the base material area 6 (Ave) Plan view of weld area 3 at .

[0065] like Figure 3 As shown, by making Ts (MAX) ( Figure 3 , reference symbol 4) is greater than Tb (Ave) Problems may arise during the cold drawing process after the diameter reduction rolling. The cold drawing process involves inserting a plug into the steel pipe and passing the plug through a die to cold-draw the steel pipe. If a protrusion is formed on the inner diameter portion of the weld area 3, this may damage the inserted plug or create areas on the inner diameter portion where the plug does not touch.

[0066] In this regard, by using Ts as an indicator of the increase in the wall thickness of the weld area 3 (MAX) / Tb (Ave) Setting the value to 1.05 or less can suppress the occurrence of these problems.

[0067] Therefore, in the present invention, the maximum wall thickness Ts of the weld area is (MAX) (mm) and Tb (ave) (mm) ratio (Ts (MAX) / Tb (Ave) ) is set to 1.05 or less.

[0068] In addition, Ts is preferably (MAX) / Tb(Ave) It is 1.04 or less, and more preferably 1.03 or less.

[0069] In addition, Ts is preferably (MAX) / Tb (Ave) It is 0.90 or more, and more preferably 0.95 or more.

[0070] In the roller stand where the reduction rate of each stand is greater than a specific value, the resistance weld seam portion 2 is inserted into a position avoiding the area within a specific range from the roll pass end to the roll pass center, thereby reducing Ts (MAX) / Tb (Ave) Adjust to the above range.

[0071] (Tb (Ave) / Db (Ave) )×100≥15%…Formula (3)

[0072] For the steel pipe of the present invention, it is more preferable that T b(Ave) (mm) and the average outer diameter of the base material area Db (ave) (mm) ratio: (Tb (Ave) / Db (Ave) )×100 is more than 15%.

[0073] Tb (Ave) / Db (Ave) The reasons for the limitation are as follows.

[0074] By reducing Tb (Ave) / Db (Ave) And reduce the weight, but (Tb (Ave) / Db (Ave) )×100 is less than 15%, sometimes it is not possible to satisfy the rigidity and strength required as a component. Therefore, as an alternative to the bar steel, it is preferred to use (Tb (Ave) / Db (Ave) )×100≥15%.

[0075] In addition, it is preferred that (Tb (Ave) / Db (Ave) )×100 is 15.5% or more, more preferably 16.0% or more.

[0076] In addition, it is preferred that (Tb (Ave) / Db (Ave) )×100 is 45% or less, more preferably 40% or less.

[0077] Next, a preferred component composition of the steel pipe used in the present invention will be described. Hereinafter, unless otherwise specified, the component composition will be described in terms of mass %.

[0078] C: 0.55% or less

[0079] C is an element that contributes to strength and improves fatigue resistance through its addition. However, if the C content exceeds 0.55%, weldability deteriorates, and stable resistance welding quality may not be achieved. Therefore, the C content is preferably set to 0.55% or less. A C content of 0.45% or less is more preferred. Furthermore, a C content of 0.2% or more is preferred.

[0080] Si: 0.01-1.0%

[0081] Si increases the strength of steel through deoxidation and solid solution formation. To achieve these effects, the Si content is preferably set to 0.01% or higher. However, if the Si content exceeds 1.0%, the hardenability of the steel pipe may decrease. Therefore, the Si content is preferably set to 0.01-1.0%. A Si content of 0.1% or higher is more preferred. Furthermore, a Si content of 0.4% or lower is more preferred.

[0082] Mn: 0.2-3.0%

[0083] Mn improves hardenability, and this effect is achieved by containing 0.2% or more. However, if the Mn content exceeds 3.0%, resistance welding quality may deteriorate. Therefore, the Mn content is preferably set to 0.2-3.0%. A Mn content of 0.5% or more is more preferred. Furthermore, a Mn content of 2.0% or less is more preferred.

[0084] P: 0.01% or less

[0085] P segregates at grain boundaries and other locations, reducing toughness. Therefore, in the present invention, it is preferable to minimize the P content, but a P content of 0.01% or less is acceptable. Therefore, the P content is preferably set to 0.01% or less, and more preferably 0.005% or less.

[0086] S: 0.01% or less

[0087] S is an element that exists in steel as sulfide inclusions, reducing workability and fatigue resistance. Therefore, in the present invention, it is preferably reduced as much as possible, with a S content of 0.01% or less being acceptable. Therefore, the S content is preferably set to 0.01% or less. More preferably, the S content is 0.005% or less.

[0088] Cr: 2.0% or less

[0089] Cr is an element that improves hardenability, increasing steel strength and effectively improving fatigue properties. However, if the Cr content exceeds 2.0%, Cr oxides may remain in the weld zone of the resistance weld, sometimes reducing the resistance weld quality. Therefore, the Cr content is preferably set to 2.0% or less. A Cr content of 0.5% or less is more preferred. Furthermore, a Cr content of 0.001% or more is preferred.

[0090] Ti: 0.1% or less

[0091] Ti has the function of fixing nitrogen in steel as TiN. However, if the Ti content exceeds 0.1%, the workability and toughness of the steel may decrease. Therefore, the Ti content is preferably set to 0.1% or less. A Ti content of 0.04% or less is more preferred. Furthermore, a Ti content of 0.01% or more is preferred.

[0092] Al: 0.1% or less

[0093] Al is an element effective for deoxidation and essential for ensuring post-quench strength by suppressing the growth of austenite grains during quenching. However, when the Al content exceeds 0.1%, the effect becomes saturated, and fatigue strength may decrease due to an increase in Al-based inclusions. Therefore, the Al content is preferably set to 0.1% or less. A more preferred Al content is 0.08% or less. Furthermore, an Al content of 0.01% or more is preferred.

[0094] V: 0.5% or less

[0095] V is an element that forms fine carbides and contributes to increasing the strength of steel. However, if the V content exceeds 0.5%, the effect becomes saturated, and the expected effect cannot be expected, which becomes economically disadvantageous. Therefore, the V content is preferably set to 0.5% or less. A V content of 0.3% or less is more preferred. Furthermore, a V content of 0.01% or more is preferred.

[0096] Nb: 0.1% or less

[0097] Nb is an element that forms fine carbides and contributes to increasing the strength of steel. However, if the Nb content exceeds 0.1%, the effect becomes saturated, and the expected effect cannot be expected, which is economically disadvantageous. Therefore, the Nb content is preferably set to 0.1% or less. More preferably, the Nb content is 0.03% or less. Furthermore, the Nb content is preferably 0.001% or more.

[0098] Mo: 1.0% or less

[0099] Mo is an element that improves hardenability and contributes to increasing the strength of steel. However, if the Mo content exceeds 1.0%, the effect becomes saturated, and the expected effect cannot be expected, which becomes economically disadvantageous. Therefore, the Mo content is preferably set to 1.0% or less. More preferably, the Mo content is set to 0.3% or less. Furthermore, the Mo content is preferably set to 0.01% or more.

[0100] Cu: 2.0% or less

[0101] Cu is an element that improves hardenability, increases the strength of steel, and is effective in improving fatigue strength. However, when Cu exceeds 2.0%, workability may be reduced. Therefore, the Cu content is preferably set to 2.0% or less. More preferably, the Cu content is 0.5% or less. In addition, the Cu content is preferably 0.001% or more.

[0102] Ni: 2.0% or less

[0103] Nickel is an element that improves hardenability, increases steel strength, and is effective in improving fatigue strength. However, when nickel content exceeds 2.0%, workability may decrease. Therefore, the nickel content is preferably set to 2.0% or less. More preferably, the nickel content is 0.5% or less. Furthermore, the nickel content is preferably 0.001% or more.

[0104] B: 0.005% or less

[0105] Boron (B) is an element that improves the hardenability of steel in trace amounts. However, when the B content exceeds 0.005%, its effect saturates, and B segregates at grain boundaries, promoting intergranular fracture and reducing fatigue properties. Therefore, the B content is preferably set to 0.005% or less. A B content of 0.0050% or less is more preferred. Furthermore, a B content of 0.0003% or more is preferred.

[0106] N: 0.01% or less

[0107] Nitrogen is an element inevitably contained in steel. However, it combines with nitride-forming elements in steel to help suppress grain coarsening and increase strength after tempering. However, a Nitrogen content exceeding 0.01% reduces the toughness of the resistance weld and degrades workability. Therefore, the Nitrogen content is preferably set to 0.01% or less. More preferably, the Nitrogen content is set to 0.005% or less.

[0108] The balance other than the above-mentioned component composition is composed of Fe and inevitable impurities.

[0109] <Manufacturing Method of Electric Resistance Welded Steel Pipe>

[0110] Next, refer to Figure 4 The method for producing the above-mentioned steel pipe will be described.

[0111] Figure 4 A schematic diagram showing an apparatus for producing the electric-resistance-welded steel pipe of the present invention is shown.

[0112] In the present invention, first, Figure 4 As shown in (a) of FIG. 1 , a steel strip 7 is continuously formed by a continuous forming machine 8 or the like to form an open tube 9, which is then resistance welded by a welding unit 10 to form a blank tube 12. It should be noted that in the present invention, the blank tube 12 can be obtained by resistance welding the circumferential butt joints of the steel strip 7 by the welding unit 10 while simultaneously performing compression bonding by the squeeze rolls 11.

[0113] In addition, the raw tube 12 can be cut into a predetermined size by a cutter 13 .

[0114] After obtaining the blank tube 12, Figure 4 As shown in (b) of FIG. 1 , a base tube (tube body) 12 is heated at a temperature of 650° C. or higher by a heating unit 14 and subjected to diameter reduction rolling at a cumulative diameter reduction ratio of 30% or higher by rollers (hereinafter simply referred to as rollers) 15. The diameter reduction rolling by the rollers 15 can be performed sequentially using a plurality of roll stands 16-1, 16-2, ..., 16-N (N is a natural number).

[0115] It should be noted that the diameter reduction ratio is defined by the following formula (5).

[0116] Diameter reduction ratio (%) = 100 × (Db before diameter reduction) (Ave) -Db after diameter reduction (Ave) ) / Db before diameter reduction (Ave) …Formula (5)

[0117] More specifically, the cumulative diameter reduction ratio is obtained using the following formula (6).

[0118] Cumulative diameter reduction rate (%) = 100 × (Db before diameter reduction at the first rack (Ave) -Db after diameter reduction at the final rack (Ave) ) / Db before the first rack (Ave) …Equation (6)

[0119] By setting the heating temperature during steel pipe diameter reduction rolling to 650°C or higher and the cumulative diameter reduction ratio during diameter reduction rolling to 30% or higher, the r value of the steel pipe after processing can be set to 1.0 or higher, and the steel pipe can be processed into a desired shape during bending.

[0120] The heating temperature is preferably 700° C. or higher, more preferably 800° C. or higher.

[0121] The heating temperature is preferably 1050° C. or lower, and more preferably 1000° C. or lower.

[0122] The cumulative diameter reduction ratio is preferably 35% or more, and more preferably 40% or more.

[0123] Furthermore, the cumulative diameter reduction ratio is preferably 90% or less, and more preferably 85% or less.

[0124] In the present invention, during the reduction rolling, at a roll stand having a reduction ratio of 5.0% or more, the resistance weld seam portion 2 is not allowed to pass within a range of ±5.0° from the roll pass center of the roll and within a range of ±5.0° from positions 360° / (n×2) to the left and right (n is the number of rolls per stand) from the roll pass center.

[0125] The reduction ratio at the roller stand of the Nth stand is obtained by the following formula (7).

[0126] Reduction rate (%) = 100 × (Db before reduction at the Nth frame (Ave) -Db after diameter reduction at the Nth rack (Ave) ) / Db before the diameter reduction at the Nth rack (Ave) …Formula (7)

[0127] Next, the reasons for limiting the position at which the resistance-welded seam 2 is inserted into the roll stand will be explained. The insertion position of the resistance-welded seam 2 here refers to the circumferential position when the resistance-welded seam 2 is rotated counterclockwise from the apex, with the center of the steel pipe as the axis, in the direction of advancement of the steel pipe when the rolls are inserted into the reducing rolling mill. In this case, the apex direction is assumed to be 0°. Furthermore, the aforementioned positions at 360° / (n×2) on the left and right sides from the roll pass center can also be considered to be positions at ±360° / (n×2) from the roll pass center.

[0128] Figure 5 This is a diagram for explaining the loading position of electric resistance welded steel pipe. Figure 6 This is an external view illustrating the position of the roll 15 where the resistance weld 2 should avoid contact in the present invention. The present inventors have observed that when reducing the diameter of a steel pipe using the roll 15, the wall thickness increases near the roll pass ends of the roll 15 where rolling is performed, while the wall thickness decreases at the roll pass center 17 in the cross section of the steel pipe.

[0129] The rolls 15 used for reducing the steel pipe have a circumferentially defined roll pass. As the multiple rolls 15 rotate, they clamp the steel pipe (billet pipe 12) using the roll pass. When viewed from a cross-section perpendicular to the axial direction (longitudinal direction) of the steel pipe (a cross-section perpendicular to the pipe axis), the roll pass is curved to conform to the outer diameter of the steel pipe. The roll pass center 17, where the roll pass diameter 18 reaches its minimum arc centered on the roll's axis of rotation, forms the roll pass bottom.

[0130] like Figure 5 As shown in FIG. 1 , the present inventors set the resistance welding seam 2 at various positions in the circumferential direction and loaded the billet tube 12 into the diameter reduction rolling roll 15 (16-1). Thus, at the diameter reduction rolling roll stand with a diameter reduction rate of 5.0% or more per stand, as shown in FIG. Figure 6 As shown, the blank tube 12 was loaded so that the resistance weld seam portion 2 avoided the roll pass center 17 within a range of ±5.0° and was located at positions 360° / (n×2) to the left and right of the roll pass center 17 within a range of ±5.0° (n is the number of rolls per stand). As a result, it was found that deformation was small and the inner diameter shape was well maintained.

[0131] Here, the range of ±5.0° from the roll pass center 17 to the roll pass center 17 is defined as the range of ±5.0° within the sector shape, with the roll pass center 17 at 0°, when the roll pass outer edge portion of the cross section perpendicular to the pipe axis is configured as a sector shape along the cross section of the steel pipe, that is, a sector shape centered at the center of the cross section of the steel pipe. Furthermore, the range of ±5.0° from the roll pass center 17 at a position 360° / (n×2) (n being the number of rolls per stand) refers to, for example, a range of ±5.0° in the circumferential direction at positions 45° (=360° / (4×2)) to the left and right of the roll pass center 17 in rolling with four rolls, and a range of ±5.0° in the circumferential direction at positions 60° (=360° / (3×2)) to the left and right of the roll pass center 17 in rolling with three rolls.

[0132] In the present invention, from the viewpoint of workability, at the rollers 15 adjacent to the tube in the circumferential direction, an area within a range of ±5.0° at a position of 360° / (n×2) from the roll pass center 17 on the right end side of the roller 15 on the left side and an area within a range of ±5.0° at a position of 360° / (n×2) from the roll pass center 17 on the left end side of the roller 15 on the right side may overlap.

[0133] Thus, in order to obtain electric resistance welded steel pipe having excellent workability and torsional fatigue resistance, during the diameter reduction rolling, at roll stands with a diameter reduction ratio of 5.0% or more, the electric resistance weld seam portion is prevented from passing within a range of ±5.0° from the roll pass center of the rolls and within a range of ±5.0° from the positions 360° / (n×2) to the left and right (n is the number of rolls per stand) from the roll pass center.

[0134] Furthermore, during the diameter reduction rolling, as methods for restraining the circumferential displacement of the resistance weld seam portion 2 of the pipe and more reliably loading the pipe into the target loading position, there are mentioned, but not particularly limited to, a method in which roller-type guide rollers are provided between adjacent roll stands 16 to restrain the circumferential displacement of the pipe passing between the stands 16 by using these guide rollers; a method in which tensile stress is applied from behind during the diameter reduction rolling; a method in which resistance welding and diameter reduction rolling are performed continuously, etc.

[0135] The obtained steel pipe 1 can be subjected to induction hardening and tempering. Induction hardening can be performed by heating at a temperature of 850-1050°C for a holding time of 1-1800 seconds, followed by water cooling. Tempering can be performed by holding at 150-450°C for 5-60 minutes, followed by air cooling.

[0136] The electric-resistance-welded steel pipe 1 of the present invention described above can be used for automobile structural members such as stabilizers of automobiles.

[0137] Example

[0138] Hereinafter, the present invention will be further described based on examples.

[0139] Steel strips of the two steel grades A and B shown in Table 1 were continuously formed into open pipes, which were then subjected to electric resistance welding to form billet pipes. Reduction rolling was then performed by varying the cumulative reduction ratio, heating temperature, and placement position into the reduction rolling roll stand to produce steel pipes No. A-1 to A-19 and B-1 to B-19. Steel pipes Nos. A-1 to A-19 were produced from steel grade A, while steel pipes Nos. B-1 to B-19 were produced from steel grade B.

[0140] The reduction scheme is shown in Table 2. Figure 7 This is a plan view of the roll for explaining the phase angles of the rolls shown in Table 2.

[0141] For 4-roller modes A to D, Figure 7 As shown, the phase angle shown in Table 2 refers to the circumferential angle (°) from the vertex of the roll groove center (roll groove bottom) of the roll 15 located within 0 to 90° counterclockwise rotation from the vertex of each stand 16 with the center of the steel pipe as the axis.

[0142] In addition, for the 3-roll mode E, the phase angles shown in Table 2 are circumferential angles (°) of the roll groove bottoms of the rolls 15 located within 0 to 120° counterclockwise from the apex of each stand 16.

[0143] Tables 3 and 4 show the diameter reduction conditions including the diameter reduction patterns when producing steel pipes No. A-1 to A-19 and B-1 to B-19.

[0144] The positions of the steel pipes to be loaded into the rolls shown in Tables 3 and 4 are shown for each reduction mode. Figure 8 The respective angles are determined by rotating counterclockwise from the vertex toward the rolling direction (see also Figure 7 ).

[0145] Specifically, regarding the reduction patterns A and B (both with n=4), within the range of ±5.0° of the phase angles of 11.25°, 56.25°, 78.75°, 33.75°, 0°, and 45° for the 5th to 10th stands with a reduction rate of 5.0% or more, excluding the insertion position of the weld (resistance weld weld portion), and adding ±45° (=360° / (4×2)) to each phase angle, -33.75° (evaluated as 56.25° obtained by adding 90° to -33.75°), 56.25°, 11.25°, 101.25° (evaluated as 11.25° obtained by subtracting 90° from 101.25°), 33.75°, 123.75° (evaluated as 123.75° obtained by subtracting 90° from 11.25°), When the insertion position of the weld (resistance weld seam portion) is not included within the range of ±5.0° of -11.25° (evaluated at 33.75° obtained by subtracting 90° from -11.25°), 78.75°, -45° (evaluated at 45° obtained by adding 90° to -45°), 45°, 0°, and 90° (evaluated at 0° obtained by subtracting 90° from 90°), the manufacturing conditions of the present invention are satisfied, that is, the resistance weld seam portion does not pass within the range of ±5.0° from the roll pass center of the roll and within the range of ±5.0° (n is the number of rolls per stand) at positions 360° / (n×2) to the left and right of the roll pass center.

[0146] As the steel pipes adopting the diameter reduction pattern A or B, in steel pipes No. A-1 and B-1, the insertion position 18° is not included in the above range, and therefore, the condition P is satisfied.

[0147] In steel pipes No. A-2 and B-2, the angle 23° obtained by subtracting 90° from the insertion position 113° is not included in the above range, and therefore, condition P is satisfied.

[0148] In steel pipes No. A-3 and B-3, 25° obtained by subtracting 90°×2 from the insertion position 205° is not included in the above range, and therefore, condition P is satisfied.

[0149] In steel pipes No. A-4 and B-4, 67° obtained by subtracting 90°×3 from the insertion position 337° is not included in the above range, and therefore, condition P is satisfied.

[0150] In steel pipes No. A-5 and B-5, the insertion position 70° is not included in the above range, and therefore, condition P is satisfied.

[0151] In steel pipes No. A-6 and B-6, 18° obtained by subtracting 90° from the insertion position 108° is not included in the above range, and therefore, condition P is satisfied.

[0152] In steel pipes No. A-10 and B-10, the angle 23° obtained by subtracting 90° from the insertion position 113° is not included in the above range, and therefore, condition P is satisfied.

[0153] On the other hand, in steel pipes No. A-9 and B-9, the insertion position 35° is included in the above range, and therefore, condition P is not satisfied. Figure 8 (Refer to avoid areas)

[0154] In steel pipes No. A-11 and B-11, the insertion position 12° is within the above range, and therefore, condition P is not satisfied.

[0155] In steel pipes No. A-13 and B-13, 75° obtained by subtracting 90°×2 from the insertion position 255° falls within the above range, and therefore, condition P is not satisfied.

[0156] In steel pipes No. A-14 and B-14, the insertion position 35° is within the above range, and therefore, condition P is not satisfied.

[0157] In steel pipes No. A-15 and B-15, the insertion position 46° is within the above range, and therefore, condition P is not satisfied.

[0158] Regarding the reduction patterns C and D (both with n=4), within the range of ±5° of the phase angles of 11.25°, 56.25°, 78.75°, and 33.75° for the 5th to 8th stands with a reduction rate of 5.0% or more, excluding the insertion position of the weld (resistance weld weld portion), the angles obtained by adding ±45° (=360° / (4×2)) to the respective phase angles were -33.75° (evaluated as 56.25° obtained by adding 90° to -33.75°), 56.25°, 11.25°, 56.25°, 78.75°, and 33.75°, respectively, with the reduction rate being 5.0% or more. When the insertion position of the weld (resistance welding weld portion) is not included within the range of ±5.0° of 25°, 101.25° (evaluated at 11.25° obtained by subtracting 90° from 101.25°), 33.75°, 123.75° (evaluated at 33.75° obtained by subtracting 90° from 123.75°), -11.25° (evaluated at 78.75° obtained by adding 90° to -11.25°), and 78.75°, the above-mentioned condition P among the manufacturing conditions of the present invention is satisfied.

[0159] In steel pipes No. A-7 and B-7, which adopt the diameter reduction pattern C or D, the insertion position 0° obtained by subtracting 90° from 180° × 2 is not included in the above range, and therefore, the condition P is satisfied.

[0160] In steel pipes No. A-8 and B-8, 46° obtained by subtracting 90°×2 from the insertion position 226° is not included in the above range, and therefore, condition P is satisfied.

[0161] In steel pipes No. A-12 and B-12, 46° obtained by subtracting 90°×2 from the insertion position 226° is not included in the above range, and therefore, condition P is satisfied.

[0162] On the other hand, in steel pipes No. A-16 and B-16, the insertion position 79° is within the above range, and therefore, the condition P is not satisfied.

[0163] Regarding the reduction pattern E (n=3), when the insertion position of the weld (resistance weld bead portion) is not included in the range of ±5° of the phase angles of 15°, 75°, 105°, 45°, 0°, and 60° for each of the 5th to 10th stands with a reduction rate of 5.0% or more, and when the insertion position of the weld (resistance weld bead portion) is not included in the range of -45° (evaluated as 75° obtained by adding -45° to 120°), 75°, 45°, 165° (evaluated as 45° obtained by subtracting 120° from 165°), 0°, and 120° (evaluated as 0° obtained by subtracting 120° from 120°), which are angles obtained by adding ±60° (=360° / (3×2)) to each phase angle, the above-mentioned condition P among the manufacturing conditions of the present invention is satisfied.

[0164] In steel pipes No. A-17 and B-17, the insertion position 23° is not included in the above range, and therefore, condition P is satisfied.

[0165] On the other hand, in steel pipes No. A-18 and B-18, the angle 45° obtained by subtracting 120° from the insertion position 165° falls within the above range, and therefore the condition P is not satisfied.

[0166] In steel pipes No. A-19 and B-19, 102°, which is obtained by subtracting 120°×2 from the loading position 342°, falls within the above range, and therefore, condition P is not satisfied.

[0167] In addition, the target outer diameter, target plate thickness, and actual outer diameter after diameter reduction are shown in Tables 3 and 4. In addition, Ts was measured from the cross section of each steel pipe. (MAX) 、Ts (MIN) 、Tb (ave) 、Db (ave) , calculate W and H.

[0168] Next, these electric resistance welded steel pipes were subjected to induction hardening and tempering. Induction hardening involved heating at 950°C for 1 second and water cooling. Tempering involved holding at 190°C for 1 hour and air cooling.

[0169] After reducing rolling and heat treatment, tensile testing is performed to determine the tensile strength TS and r-value. For the tensile test, a JIS 12 No. A tensile test piece is cut from the base metal region of the steel pipe and a strain gauge with a gauge length of 2 mm is attached. The tensile strength TS is determined from the results of this tensile test. Furthermore, the r-value is calculated from the true strain in the width direction, eW, relative to the true strain in the longitudinal direction, eL, when stretched at a nominal strain of 5-10%. The slope ρ is used to calculate the r-value: r / (-1-ρ).

[0170] Furthermore, tubular torsional fatigue test pieces (450 mm in length) were cut from the resulting reduced-rolled steel pipe and subjected to torsional fatigue testing. The torsional fatigue test was conducted under conditions of a load stress (external surface): 600 MPa, a stress ratio: -1 (alternating), a frequency of 2 Hz, and a sinusoidal waveform. The number of repetitions until fracture was measured to evaluate fatigue resistance. In the present invention, improved torsional fatigue properties are considered when the number of repetitions until fracture is 2.0 times or greater compared to a comparative example with the same product dimensions (same combination of target wall thickness and target outer diameter) and the same steel grade, using the same reduction profile.

[0171] To evaluate plug seizure, first, a resistance-welded steel pipe is formed into a blank and cold-drawn. Specifically, a plug is inserted into the steel pipe and drawn using a die. During this process, if the inner diameter of the resistance-welded steel pipe protrudes, and scratches known as plug seizure form on the plug, this can cause defects when processing other steel pipes.

[0172] As an evaluation method, the presence or absence of defects on the plug surface during cold drawing was visually confirmed. The absence of defects was judged as excellent workability.

[0173] The obtained results are shown in Tables 3 and 4. As described above, Table 3 shows the results of Steel No. A, and Table 4 shows the results of Steel No. B.

[0174] According to Table 3 and Table 4, in the examples of the present invention, H / W is less than 0.10 and Ts (MAX) / Tb (Ave) ≤1.05, r value is 1.0 or greater. In addition, the steel pipes of the examples of the present invention have no defects on the plug surface, are excellent in workability, and have improved torsional fatigue properties in tests at higher loads than conventional electric resistance welded steel pipes.

[0175] On the other hand, in steel pipes No. A-9 and B-9 as comparative examples, the weld (resistance weld) insertion position did not satisfy the above-mentioned condition P, and H / W was outside the range of the present invention, and the desired torsional fatigue resistance characteristics were not obtained.

[0176] In addition, in Steel Pipes No. A-10 and B-10, the heating temperature during the diameter reduction rolling was lower than 650° C., and the r value was less than 1.0, and the desired workability was not obtained.

[0177] In addition, in steel pipes No. A-11 and B-11, the insertion position of the weld (resistance weld weld portion) does not satisfy the above conditions P, Ts (MAX) / Tb (Ave) Defects in the plug occur during the cold drawing process, which is outside the scope of the present invention.

[0178] In addition, in Steel Pipes No. A-12 and B-12, the cumulative diameter reduction ratio was less than 30%, and the r value was less than 1.0, and the desired workability was not obtained.

[0179] In addition, in Steel Pipes No. A-13 and B-13, the insertion position of the weld (resistance welded weld portion) did not satisfy the above-mentioned condition P, and H / W was outside the range of the present invention, and the desired torsional fatigue resistance characteristics were not obtained.

[0180] In addition, in steel pipes No. A-14 and B-14, the insertion position of the weld (resistance weld weld portion) does not satisfy the above conditions P, H / W and Ts (MAX) / Tb (Ave) Outside the scope of the present invention, the desired torsional fatigue resistance is not obtained, and defects occur in the plug during cold drawing.

[0181] In addition, in Steel Pipes No. A-15 and B-15, the insertion position of the weld (resistance welded weld portion) did not satisfy the above-mentioned condition P, and H / W was outside the range of the present invention, and the desired torsional fatigue resistance characteristics were not obtained.

[0182] In addition, in Steel Pipes No. A-16 and B-16, the insertion position of the weld (resistance welded weld portion) did not satisfy the above-mentioned condition P, and H / W was outside the range of the present invention, and the desired torsional fatigue resistance characteristics were not obtained.

[0183] In Steel Pipes No. A-18 and B-18, the insertion position of the weld (resistance weld portion) did not satisfy the above-mentioned condition P, and H / W was outside the range of the present invention, and the desired torsional fatigue resistance characteristics were not obtained.

[0184] In addition, in steel pipes No. A-19 and B-19, the insertion position of the weld (resistance weld weld portion) does not satisfy the above conditions P, Ts (MAX) / Tb(Ave) Defects in the plug occur during the cold drawing process, which is outside the scope of the present invention.

[0185]

[0186]

[0187]

[0188]

[0189] Explanation of symbols

[0190] 1 Electric resistance welded steel pipe

[0191] 2. Resistance welding weld

[0192] 3 Weld area

[0193] 4 The location where the wall thickness is the largest in the weld area

[0194] 5. The location where the wall thickness of the weld area is the smallest

[0195] 6 Base material area

[0196] 7 Steel Belt

[0197] 8 Continuous forming machine

[0198] 9 Open tube

[0199] 10 welding units

[0200] 11 Squeeze roller

[0201] 12 billet tubes

[0202] 13 cutting machine

[0203] 14 Heating unit

[0204] 15 Rollers

[0205] 16-roller stand (where the last number indicates the stand number)

[0206] 17 Roller pass center

[0207] 18 Roller pass diameter

Claims

1. An electric resistance welded steel pipe comprising a weld region and a base metal region outside the weld region, wherein the weld region is an area within ±10° in the pipe circumferential direction from the electric resistance weld portion formed along the pipe length, wherein: The r value in the tube length direction is 1.0 or more and 2.0 or less, and the r value is the r value in the tube length direction of the base material region. As the minimum wall thickness Ts of the weld area (MIN) The average wall thickness Tb of the base material area (Ave) The difference (Tb (Ave) -Ts (MIN) ) and the arc length W of the inner surface of the pipe in the weld area satisfy the following formula (1), The maximum wall thickness Ts of the weld area (MAX) and the Tb (Ave) Satisfies the following formula (2), The Tb (Ave) and the average outer diameter Db of the base material region (Ave) Satisfies the following formula (3), H / W≤0.10…Formula (1) Ts (MAX) / Tb (Ave) ≤1.05…Equation (2) (Tb (Ave) / Db (Ave) )×100≥15%…Formula (3), Among them, Ts (MIN) , Tb (Ave) ,H,W,Ts (MAX) and Db (Ave) The unit is mm.

2. A method for manufacturing an electric resistance welded steel pipe, which is a method for manufacturing the electric resistance welded steel pipe according to claim 1, wherein: The steel strip is formed into an open tube. The open tube is subjected to electric resistance welding to form a blank tube. The billet tube is heated at a temperature of 650° C. or higher and subjected to diameter reduction rolling with a cumulative diameter reduction ratio of 30% or higher. During the reduction rolling, at a roll stand having a reduction rate of 5.0% or more, the resistance weld seam portion is not allowed to pass within a range of ±5.0° from the roll pass center of the roll and within a range of ±5.0° from positions 360° / (n×2) to the left and right of the roll pass center, where n is the number of rolls per stand.

3. An automobile structural member, formed using the electric resistance welded steel pipe according to claim 1.

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