High-strength electric resistance welded steel pipe with excellent resistance to hot-dip galvanized cracking, suitable for overhead line poles, and method for manufacturing the same.

Abstract

To provide an electric resistance welded steel pipe which is excellent in hot dip galvanizing wettability and has both hot dip galvanizing cracking resistance and high strength, and to provide a method of producing the same.SOLUTION: A high-strength electric resistance welded steel pipe is composed of prescribed components, satisfies CEZ≤0.44 calculated by a following formula (1), has tensile strength of 700 MPa or more in a tube axis and a direction perpendicular to the tube axis, a yield point of 520 MPa or more, and residual strain of 200 MPa or less in the direction perpendicular to the tube axis, is excellent in hot dip galvanizing crack resistance, and is suitable for a stringing column. A method of producing the high-strength electric resistance welded steel pipe is also provided. The formula (1) is: CEZ=C+Si / 17+Mn / 7.5+Cu / 13+Ni / 17+Cr / 4.5+Mo / 3+V / 1.5+Nb / 2+Ti / 4.5+420B.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an electric welded steel pipe suitable for use as an overhead line pole for railways and the like, having high strength and excellent resistance to cracking in hot-dip galvanizing, and a method for manufacturing the same.

Background Art

[0002] An overhead line pole is a pole for stretching an overhead line for supplying power to a train body. Taking the Great East Japan Earthquake in 2011 as an opportunity, the seismic design of overhead line poles has been reviewed, and there has been a demand for overhead line poles that can withstand the shaking at the level of a major earthquake. Generally, in order to improve seismic resistance, any one of increasing the diameter, increasing the wall thickness, increasing the strength, or a combination thereof can be considered. In the case of increasing the diameter of a conventional high-strength overhead line pole (590 MPa grade), it is difficult from the viewpoint of the laying space, and increasing the wall thickness increases the self-weight, which increases the horizontal shaking and makes it impossible to withstand the shaking. There is also a possibility that the transportation cost will increase. Therefore, from the viewpoint of increasing the strength, the development of a 700 MPa grade high-strength overhead line pole has been studied.

[0003] Generally, for steel towers, bridges, and buildings including overhead line poles, in order to prevent rust, a method of hot-dip galvanizing the steel materials used for them after welding them to structural members is widely used. On the other hand, when hot-dip galvanized, cracks may occur due to residual stress during cold working, residual stress in the heat-affected zone of welding, and thermal stress due to heating and cooling during hot-dip galvanizing. This phenomenon is a kind of liquid metal embrittlement phenomenon and is well known as "cracking in hot-dip galvanizing".

[0004] In the case of high-strength steel materials, elements that increase hardenability or precipitation-strengthening elements are added. However, as can be seen from the CEZ formula of the following formula (1), the elements added in large amounts to high-strength steel materials increase the value of the CEZ formula and deteriorate the resistance to cracking in hot-dip galvanizing. CEZ = C + Si / 17 + Mn / 7.5 + Cu / 13 + Ni / 17 + Cr / 4.5 + Mo / 3 + V / 1.5 + Nb / 2 + Ti / 4.5 + 420B ≦ 0.44 ··· (1) Here, the CEZ formula is described in detail on pages 1108-1114 of Non-Patent Document 1 regarding the effect of boron contamination in steel. It states that if B is 2 ppm or less and the value of CEZ in formula (1) above is 0.44 or less, hot-dip galvanizing cracking can be prevented in 590 MPa class power transmission tower steel pipes. Non-Patent Document 1 was co-authored by a fabricator and four steel companies and is currently considered the most reliable and cutting-edge technology published to date.

[0005] Furthermore, it is known that the higher the strength, the worse the resistance to hot-dip galvanizing cracking. Therefore, this project requires the development of electric resistance welded steel pipes that offer excellent wettability to hot-dip galvanizing, while simultaneously achieving both resistance to hot-dip galvanizing cracking and high strength.

[0006] Regarding methods for achieving both resistance to hot-dip galvanizing cracking and high strength, for example, Patent Document 1 states that in steel materials of grades less than 70K, the addition of ferrite-forming elements Si, Al, and Ti to form grain boundary ferrite suppresses HAZ grain boundary segregation of B, thereby improving resistance to hot-dip galvanizing cracking. Patent Document 2 describes a tempered high-tensile steel with excellent weldability and excellent resistance to hot-dip galvanizing cracking, which is tempered to a strength level of 70-80K by quenching and tempering, and a method for manufacturing the same. Patent Document 3 describes a method for manufacturing high-tensile steel with excellent resistance to hot-dip galvanizing cracking at 80K in the as-quenched state after rolling.

[0007] Patent Document 4 specifies a Si content of 0.5-1.5% for grain boundary ferrite formation, but plating burn occurs in the region of Si content above approximately 0.30%, making it unsuitable for electric resistance welded (ERW) steel pipes that are hot-dip galvanized, and the strength does not reach 70k class. Furthermore, in the case of ERW steel pipes, hot-rolled steel sheets unwound from hot coils are roll-formed into open pipes, and the butt joints of the resulting open pipes are welded using ERW to form the ERW welded joint. Consequently, ERW steel pipes are particularly susceptible to tensile residual stress in the direction perpendicular to the pipe axis, making them more prone to hot-dip galvanizing cracking than steel sheets.

[0008] Patent Document 5 describes tempering to remove residual stress, which increases manufacturing costs. Furthermore, it does not include considerations regarding residual stresses introduced during the manufacturing of electric resistance welded (ERW) steel pipes, leaving concerns about adverse effects on hot-dip galvanizing cracking. Patent Document 6 does not require heat treatment, but it also does not include considerations regarding residual stresses introduced during the manufacturing of ERW steel pipes, leaving concerns about adverse effects on hot-dip galvanizing cracking. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2005-307224 [Patent Document 2] Japanese Patent Application Publication No. 10-102195 [Patent Document 3] Japanese Patent Application Publication No. 10-110214 [Patent Document 4] Japanese Patent Publication No. 2005-307224 [Patent Document 5] Japanese Patent Application Publication No. 10-102195 [Patent Document 6] Japanese Patent Application Publication No. 10-110214 [Non-patent literature]

[0010] [Non-Patent Document 1] Iron and Steel Vol. 79 (1993) [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] The present invention provides an electric resistance welded steel pipe and a method for manufacturing the same, which have a tensile strength of 700 MPa or more in the direction perpendicular to the pipe axis, excellent wettability for hot-dip galvanizing, and achieve both resistance to hot-dip galvanizing cracking and high strength. [Means for solving the problem]

[0012] To solve the above problem, we focused on residual stress in the direction perpendicular to the pipe axis. That is, (1) The steel is characterized by having a composition of C: 0.08~0.20% by mass, Si: 0.03~0.40%, Mn: 1.00~2.00%, P: 0.000~0.030%, S: 0.000~0.010%, Al: 0.005~0.050%, N: 0.0005~0.0100%, with the remainder being Fe and impurities, satisfying CEZ ≤ 0.44 calculated by the following formula (1), having a tensile strength of 700 MPa or more in the direction perpendicular to the pipe axis, a yield point of 520 MPa or more, and a residual stress of 200 MPa or less in the direction perpendicular to the pipe axis, and is a high-strength electric resistance welded steel pipe suitable for overhead line poles with excellent resistance to hot-dip galvanized cracking. CEZ=C+Si / 17+Mn / 7.5+Cu / 13+Ni / 17+Cr / 4.5+Mo / 3+V / 1.5+Nb / 2+Ti / 4.5+420B...(1)

[0013] (2) Furthermore, the composition of steel is as follows (by mass%): C: 0.08~0.20%, Si: 0.03~0.40%, Mn: 1.00~2.00%, P: 0.000~0.030%, S: 0.000~0.010%, Al: 0.005~0.050%, N: 0.0005~0.0100%. Furthermore, the steel contains one or more of the following components by mass%, in the following proportions: B: greater than 0.00000% to 0.00020%, Ti: greater than 0.00% to 1.00%, Nb: greater than 0.00% to 1.00%, V: greater than 0.00% to 1.00%, Cu: greater than 0.00% to 1.00%, Ni: greater than 0.00% to 1.00%, Cr: greater than 0.00% to 1.00%, Mo: greater than 0.00% to 0.50%, W: greater than 0.00% to 0.50%, Ca: greater than 0.0000% to 0.0200%, Mg: greater than 0.0000% to 0.0200%, Zr: greater than 0.0000% to 0.0200%, and REM: greater than 0.0000% to 0.0200%. The remainder consists of Fe and impurities, satisfies CEZ ≤ 0.44 calculated by the following formula (1), has a tensile strength of 700 MPa or more in the direction perpendicular to the pipe axis, a yield point of 520 MPa or more, and a residual stress of 200 MPa or less in the direction perpendicular to the pipe axis, making it a high-strength electric resistance welded steel pipe suitable for overhead line poles with excellent resistance to hot-dip galvanized cracking. CEZ=C+Si / 17+Mn / 7.5+Cu / 13+Ni / 17+Cr / 4.5+Mo / 3+V / 1.5+Nb / 2+Ti / 4.5+420B...(1)

[0014] (3) Further, as the manufacturing method, using the steel having the chemical composition described in (1), the hot-rolled steel sheet is manufactured with the heating temperature during hot rolling being 1070°C or higher and 1300°C or lower, the hot finishing rolling temperature being 800°C or higher and 1050°C or lower, and the coiling temperature after cooling being 500°C or lower. By pipe-making under the conditions satisfying the following formula (3), a steel pipe is obtained that satisfies CEZ ≤ 0.44, has a tensile strength of 700 MPa or higher and a yield point of 520 MPa or higher in the pipe axis and the direction perpendicular to the pipe axis, and has a residual stress in the direction perpendicular to the pipe axis of 200 MPa or lower. A manufacturing method of a high-strength electric resistance welded steel pipe excellent in resistance to molten zinc plating cracking and suitable for overhead power transmission poles. 2.0 ≤ SZ final stage diameter reduction amount / (FP final stage diameter reduction amount + SQ diameter reduction amount) ≤ 3.5 ··· (3) Here, the SZ final stage diameter reduction amount is the final stage diameter reduction amount (mm) of the sizing process in the pipe-making process, the FP final stage diameter reduction amount is the final stage diameter reduction amount (mm) of the fin pass process, and the SQ diameter reduction amount is the diameter reduction amount (mm) of the squeeze process.

[0015] (4) In (3), it is also preferable that the steel further contains one or more of the following components in mass%: B: more than 0.00000% to 0.00020%, Ti: more than 0.00% to 1.00%, Nb: more than 0.00% to 1.00%, V: more than 0.00% to 1.00%, Cu: more than 0.00% to 1.00%, Ni: more than 0.00% to 1.00%, Cr: more than 0.00% to 1.00%, Mo: more than 0.00% to 0.50%, W: more than 0.00% to 0.50%, Ca: more than 0.0000% to 0.0200%, Mg: more than 0.0000% to 0.0200%, Zr: more than 0.0000% to 0.0200%, REM: more than 0.0000% to 0.0200%.

Advantages of the Invention

[0016] According to the present invention, in a steel grade with CEZ ≤ 0.44%, a tensile strength of 700 MPa or higher and a yield point of 520 MPa or higher in the pipe axis and the direction perpendicular to the pipe axis are achieved, and the residual stress in the direction perpendicular to the pipe axis is ≤ 200 MPa. An electric resistance welded steel pipe excellent in wettability of molten zinc plating and achieving both resistance to molten zinc plating cracking and high strength, and a manufacturing method thereof can be provided.

Brief Description of the Drawings

[0017] [Figure 1] It is a diagram showing the relationship between the tensile residual stress in the direction perpendicular to the pipe axis and the resistance to cracking of hot-dip galvanized coating in an electric resistance welded steel pipe with a tensile strength of 700 MPa or more and a yield point of 520 MPa or more. [Figure 2] It is a diagram showing the relationship between the value of SZ final stage diameter reduction amount / (FP final stage diameter reduction amount + SQ diameter reduction amount) and the tensile residual stress in the direction perpendicular to the pipe axis.

Embodiments for Carrying out the Invention

[0018] In view of the above situation, the inventors of the present invention focused on the residual stress applied during the pipe manufacturing of the electric resistance welded steel pipe, and investigated the tensile residual stress in the direction perpendicular to the pipe axis and the presence or absence of resistance to cracking of hot-dip galvanized coating in an electric resistance welded steel pipe with a tensile strength of 700 MPa or more and a yield point of 520 MPa or more. As a result, it was confirmed that cracking can be prevented when this residual stress is 200 MPa or less. The results are shown in FIG. 1.

[0019] Here, in the present invention, the tensile strength and the yield point are measured as follows. From the 90° position of the base material in the electric resistance welded steel pipe of the present invention, a JIS No. 12 tensile test piece is taken in the pipe axis direction, and from the 180° position of the base material, a JIS No. 5 tensile test piece is taken in the direction perpendicular to the pipe axis. For the taken tensile test pieces, a tensile test in the pipe axis direction is carried out in accordance with JIS Z 2241 (2011), and the tensile strengths in the pipe axis and the direction perpendicular to the pipe axis are measured. The obtained results are taken as the tensile strength and the yield point in the pipe axis direction of the electric resistance welded steel pipe of the present disclosure.

[0020] Furthermore, in this invention, the residual stress in the direction perpendicular to the pipe axis is determined by the Crampton method (e.g., The International Journal of Advanced Manufacturing Technology (2019) 103:4221-4231), which is expressed by the following equation (2). The Crampton method is a method of releasing residual stress by cutting a steel pipe in the longitudinal direction and determining the residual stress from the change in outer diameter before and after cutting. In equation (2), D0 is the average outer diameter before cutting, and D1 is the average outer diameter after cutting. Note that the length of the specimen in the Crampton method is such that L / D (ratio of specimen length L to outer diameter D) ≥ 2. Here, E is Young's modulus, ν is Poisson's ratio, and t is wall thickness. Residual stress = E (1 / D0-1 / D1) t / (1-ν2) (2)

[0021] This tensile residual stress perpendicular to the pipe axis is reduced by plastic deformation through diameter reduction processing in the pipe manufacturing process. The diameter reduction processing in the pipe manufacturing process includes the fin pass process (hereinafter referred to as FP), the squeeze process (hereinafter referred to as SQ), and the sizing process (hereinafter referred to as SZ). In the FP process, diameter reduction occurs, but since the cross section is not yet closed, only the edges undergo plastic deformation. In the SQ process, diameter reduction occurs due to bead removal, but only the area near the electric resistance weld undergoes slight plastic deformation. In the SZ process, in order to adjust the final roundness, the electric resistance welded steel pipe, which has a closed cross section after the SQ process, is drawn and plastically deformed uniformly in the circumferential direction. Therefore, in order to uniformly reduce the tensile residual stress perpendicular to the pipe axis in the circumferential direction, it is necessary to ensure a sufficient amount of diameter reduction in the SZ process.

[0022] Therefore, in order to ensure a sufficient diameter reduction in the SZ process, it is considered effective to balance the diameter reduction amounts of the FP process, SQ process, and SZ process in some way. Equation (3) was constructed, and it was found that it is important for this equation (3) to satisfy the following predetermined range. 2.0≦SZ final stage diameter reduction amount / (FP final stage diameter reduction amount + SQ diameter reduction amount)≦3.5...(3) It was found that if equation (3) above satisfies the specified range, it is possible to manufacture electric resistance welded steel pipes in which the tensile residual stress perpendicular to the pipe axis is stably 200 MPa or less, and no surface defects occur, and hot-dip galvanizing cracks can be prevented. The results are shown in Figure 2.

[0023] This invention was completed based on the above findings, and its gist is the following electric resistance welded steel pipe and method for manufacturing the same. In this specification, A numerical range represented using "~" means a range that includes the numbers written before and after "~" as the lower and upper limits, respectively. • The "%" indicating the content of a component (element) means "mass percent". The amount of a component such as C (carbon) is sometimes expressed as "C content." The amount of other elements may be expressed similarly. The term "process" includes not only independent processes, but also processes that cannot be clearly distinguished from other processes, as long as they achieve the purpose described in the specification.

[0024] In this invention, the components other than Fe are assumed to be within the following ranges.

[0025] C: 0.08~0.20% Carbon (C) is an element that improves the strength of steel. If the C content is less than 0.08%, a tensile strength of 70k class or higher may not be obtained. Therefore, the C content should be 0.08% or more. On the other hand, if the C content exceeds 0.20%, weldability and resistance to hot-dip galvanizing cracking may be impaired. Therefore, the C content should be 0.20% or less. Preferably, it should be 0.12% or less.

[0026] Si: 0.03~0.40% Si is an element used for deoxidation. If the Si content is less than 0.03%, deoxidation will be insufficient and coarse oxides may be formed. Therefore, the Si content should be 0.03% or more, preferably 0.15% or more. On the other hand, if the Si content exceeds 0.40%, the plating wettability deteriorates. Also, the Fe-Zn alloy reaction is promoted, making it easier for plating burn, a cosmetic defect where the alloy layer is exposed on the surface, to occur. Therefore, the Si content should be 0.40% or less, preferably 0.25% or less.

[0027] Mn: 1.00~2.00% Mn is an element that improves the strength of steel. If the Mn content is less than 1.00%, a tensile strength of 70k class or higher may not be obtained. Therefore, the Mn content should be 1.00% or more, preferably 1.40% or more. On the other hand, if the Mn content exceeds 2.00%, weldability and resistance to hot-dip galvanizing cracking may be impaired. Therefore, the Mn content should be 2.00% or less.

[0028] P: 0.000~0.030% P is an element that can be present as an impurity in steel. If the P content exceeds 0.030%, the alloy reaction during zinc plating becomes more active, which may cause peeling of the plating layer. Therefore, the P content should be 0.030% or less. On the other hand, in the case of the present invention, since P is substantially an impurity, a P content of 0.000% is preferable, but from the viewpoint of reducing dephosphorization costs, the P content may be 0.001% or more, or even 0.010% or more.

[0029] S: 0.000~0.010% S is an element that can be present in steel as an impurity. If the S content exceeds 0.010%, coarse MnS may be formed, which can serve as a starting point for cracking. Therefore, the S content should be 0.010% or less. Preferably, the S content is 0.005% or less. On the other hand, in the case of the present invention, since S is substantially an impurity, a S content of 0.000% is preferable, but from the viewpoint of reducing desulfurization costs, the S content may be 0.001% or more.

[0030] Al: 0.005~0.050% Al is added as a deoxidizing agent, but if the content is less than 0.005%, deoxidation will be insufficient and coarse oxides may be formed. Therefore, the Al content should be 0.005% or more. On the other hand, if the content exceeds 0.05%, the deoxidizing effect will saturate. Therefore, the Al content should be 0.05% or less. Furthermore, since it readily forms AlN and inhibits the stable precipitation of VN, it is preferably 0.02% or less.

[0031] N: 0.0005~0.0100% N generates AlN, which contributes to the refinement of austenite grains through the pinning effect during hot rolling. It also generates VN, which contributes to strength. If the N content is less than 0.0005%, no contribution to strength can be expected. Therefore, the N content is 0.0005% or more, preferably 0.001% or more. On the other hand, if the N content exceeds 0.0100%, toughness deteriorates. Therefore, the N content is 0.0100% or less, preferably 0.005% or less.

[0032] The following are elements that are preferably included one or more selectively to increase strength, toughness, etc.

[0033] B: More than 0.00000%~0.00020% B is an element that improves the hardenability of steel, but if the B content exceeds 0.00020%, it may impair the resistance to hot-dip galvanizing cracking. Therefore, it is preferable that the B content be 0.00020% or less. On the other hand, in the present invention, since B is substantially an impurity, it is preferable that the lower limit of the content is greater than 0.00000%.

[0034] Ti: More than 0.00%~1.00% Ti is an element that contributes to increasing the strength of steel, but in this invention, it is an optional element and does not need to be included. If included, the amount should be greater than 0.00%. On the other hand, if the amount of Ti is excessive, the effect may saturate, leading to an increase in cost. Also, it may degrade toughness, so the Ti content should be 1.00% or less. Preferably, it should be 0.10% or less.

[0035] Nb: More than 0.00%~1.00% Nb is an element that contributes to increasing the strength of steel, but in this invention, it is an optional element and does not need to be included. If it is included, the amount should be greater than 0.00%. On the other hand, if Nb is included in excess, the effect may saturate, leading to an increase in cost. Also, it may degrade toughness, so the Nb content should be 1.00% or less. Preferably, it should be 0.10% or less.

[0036] V: More than 0.00%~1.00% V is an element that contributes to increasing the strength of steel, but in this invention, it is an optional element and does not need to be included. If it is included, the amount should be greater than 0.00%. On the other hand, if V is included in excess, the effect may saturate, leading to an increase in cost. Also, it may degrade toughness, so the V content should be 1.00% or less. Preferably, it should be 0.10% or less.

[0037] Cu: Over 0.00% ~ 1.00% Cu is an element that contributes to increasing the strength of steel, but in this invention, it is an optional element and does not need to be included. If included, the amount should be greater than 0.00%. On the other hand, if Cu is included in excess, the effect may saturate, leading to an increase in cost. Also, it may degrade toughness, so the Cu content should be 1.00% or less. Preferably, it should be 0.10% or less.

[0038] Ni: greater than 0.00% ~ 1.00% Ni is an element that contributes to increasing the strength of steel, but in this invention, it is an optional element and does not need to be included. If included, the amount should be greater than 0.00%. On the other hand, if Ni is included in excess, the effect may saturate, leading to an increase in cost. Also, it may impair weldability and resistance to hot-dip galvanizing cracking, so the Ni content should be 1.00% or less. Preferably, it should be 0.10% or less.

[0039] Cr: More than 0.00%~1.00% Cr is an element that contributes to increasing the strength of steel, but in this invention, it is an optional element and may not be included. If included, the amount should be greater than 0.00%. On the other hand, excessive Cr content may lead to saturation of the effect and an increase in cost. It may also impair weldability and resistance to hot-dip galvanizing cracking, so the Cr content should be 1.00% or less. Preferably, it should be 0.10% or less.

[0040] Mo: Over 0.00% ~ 0.50% Mo is an element that contributes to increasing the strength of steel, but in this invention, it is an optional element and does not need to be included. If included, the amount should be greater than 0.00%. On the other hand, if Mo is included in excess, the effect may saturate, leading to an increase in cost. Therefore, the Mo content should be 0.50% or less. Preferably, it should be 0.10% or less.

[0041] W: More than 0.00%~0.500% W is an element that contributes to increasing the strength of steel, but in this invention, it is an optional element and does not need to be included. If included, the amount should be greater than 0.00%. On the other hand, if W is included in excess, the effect may saturate, leading to an increase in cost. Therefore, the W content should be 0.50% or less. Preferably, it should be 0.10% or less.

[0042] Ca: More than 0.0000%~0.0200% Ca has the effect of controlling the morphology of inclusions and improving toughness, but in this invention, it is an optional element and does not need to be included. If included, it should be greater than 0.00%. On the other hand, if Ca is included in excess, the effect may saturate, leading to an increase in cost. Therefore, the Ca content should be 0.0200% or less.

[0043] Mg: More than 0.0000%~0.0200% Mg has the effect of controlling the morphology of inclusions and improving toughness, but in this invention it is an optional element and does not need to be included. If included, it should be greater than 0.00%. On the other hand, if Mg is included in excess, the effect may saturate and lead to an increase in cost. Therefore, the Mg content should be 0.0200% or less.

[0044] Zr: Over 0.0000% ~ 0.0200% Zr has the effect of controlling the morphology of inclusions and improving toughness, but in this invention it is an optional element and does not need to be included. If it is included, it should be greater than 0.00%. On the other hand, if Zr is included in excess, the effect may saturate and lead to an increase in cost. Therefore, the Zr content should be 0.0200% or less.

[0045] REM: Greater than 0.0000% ~ 0.0200% REM refers to rare earth elements, namely at least one element selected from the group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. REM has the effect of controlling inclusions, but in this invention it is any element and does not need to be included. If included, it is greater than 0.00%. On the other hand, if REM is included in excess, the effect may saturate and lead to an increase in cost. Therefore, the REM content is 0.0200% or less.

[0046] Remainder: Fe and impurities In the chemical composition of the base material, the remainder after removing the elements mentioned above consists of Fe and impurities. Here, impurities refer to components contained in the raw materials (e.g., ore, scrap, etc.) or components mixed in during the manufacturing process, and not components intentionally included in the steel. Any elements other than those mentioned above can be considered impurities. There may be only one or more elements as impurities. Examples of impurities include Sb, Sn, Co, As, Pb, Bi, and H. Typically, the content of Sb, Sn, Co, and As may be 0.1% or less, Pb and Bi may be 0.005% or less, and H may be 0.0004% or less. The content of other elements does not need to be particularly controlled as long as it is within the normal range.

[0047] Furthermore, when the electric resistance welded steel pipe of the present invention is used as a steel material for transmission towers, it is required to be equivalent to a 590 MPa class transmission tower steel pipe, and the CEZ is 0.44 or less.

[0048] Next, an example of a method for manufacturing electric resistance welded steel pipes according to the present invention will be described below. By heating a slab having the aforementioned chemical composition at 1070°C to 1300°C, carbides, nitrogen compounds, and carbonitrylene compounds precipitated during the solidification process of molten steel can be sufficiently dissolved in the steel, thereby improving strength without degrading resistance to internal cracking. Furthermore, the coarsening of austenite grains due to heating is suppressed, preventing the precipitation of coarse AlN during hot rolling or cooling after hot rolling.

[0049] By hot-rolling the above heated slab at a finishing temperature of 800°C to 1050°C, strain is introduced in the recrystallized and non-recrystallized regions, increasing nucleation sites and refining the microstructure.

[0050] After hot rolling, the material is cooled to a winding temperature of 500°C or lower before winding, thereby suppressing the formation of soft ferrite.

[0051] By unwinding the hot coil and performing roll forming and electric resistance welding in the pipe manufacturing process within the range that satisfies formula (3), a high-strength electric resistance welded steel pipe suitable for overhead line poles can be obtained, which has a tensile strength of 700 MPa or more in the direction perpendicular to the pipe axis and the direction perpendicular to the pipe axis, a yield point of 520 MPa or more, and excellent resistance to hot-dip galvanized cracking. [Examples]

[0052] Table 1 shows the composition of the slab used in this invention.

[0053] [Table 1]

[0054] Electric resistance welded (ERW) steel pipes were manufactured and evaluated using slabs with the chemical composition listed in Table 1, under the hot rolling, cooling, winding, and pipe manufacturing conditions listed in Table 2. Table 2 also shows the tensile strength and residual stress perpendicular to the pipe axis of the obtained ERW steel pipes. Here, the method for measuring each diameter reduction during pipe manufacturing is to calculate the outer circumference by measuring it with a tape measure before and after the actual forming process at the final stage of the FP process, the SQ process, and the final stage of the SZ process.

[0055] [Table 2]

[0056] In embodiments satisfying the requirements of the present invention, the predetermined tensile strength was satisfied, and the residual stress was 200 MPa or less, and no hot-dip galvanizing cracks were observed. On the other hand, in comparative examples, the value of formula (3) was too low and outside the range of the present invention, so the effect of reducing residual stress was not obtained, and hot-dip galvanizing cracks occurred in some cases, and the value of formula (3) was too high and outside the range of the present invention, so although no hot-dip galvanizing cracks were observed, cosmetic defects occurred.

Claims

1. A high-strength electric resistance welded steel pipe suitable for overhead line poles, characterized in that the steel composition is, by mass%, C: 0.08-0.20%, Si: 0.03-0.40%, Mn: 1.00-2.00%, P: 0.000-0.030%, S: 0.000-0.010%, Al: 0.005-0.050%, N: 0.0005-0.0100%, with the remainder being Fe and impurities, satisfying CEZ ≤ 0.44 calculated by the following formula (1), having a tensile strength of 700 MPa or more in the direction perpendicular to the pipe axis, a yield point of 520 MPa or more, and a residual stress of 200 MPa or less in the direction perpendicular to the pipe axis, exhibiting excellent resistance to hot-dip galvanized cracking. CEZ=C+Si / 17+Mn / 7.5+Cu / 13+Ni / 17+Cr / 4.5+Mo / 3+V / 1.5+Nb / 2+Ti / 4.5+420B...(1)

2. The composition of the steel is, in mass%, C: 0.08-0.20%, Si: 0.03-0.40%, Mn: 1.00-2.00%, P: 0.000-0.030%, S: 0.000-0.010%, Al: 0.005-0.050%, N: 0.0005-0.0100%, and further, the composition of the steel is, in mass%, B: 0 .. More than 00000% to 0.00020%, Ti: more than 0.00% to 1.00%, Nb: more than 0.00% to 1.00%, V: more than 0.00% to 1.00%, Cu: more than 0.00% to 1.00%, Ni: more than 0.00% to 1.00%, Cr: more than 0.00% to 1.00%, Mo: more than 0.00% to 0.50%, W A high-strength electric resistance welded steel pipe for overhead line poles, characterized by having excellent resistance to hot-dip galvanized cracking and being suitable for overhead line poles, containing one or more of the following: 0.00% to 0.50%, Ca: 0.0000% to 0.0200%, Mg: 0.0000% to 0.0200%, Zr: 0.0000% to 0.0200%, and REM: 0.0000% to 0.0200%, with the remainder being Fe and impurities, satisfying CEZ ≤ 0.44 calculated by the following formula (1), having a tensile strength of 700 MPa or more in the direction perpendicular to the pipe axis, a yield point of 520 MPa or more, and a residual stress of 200 MPa or less in the direction perpendicular to the pipe axis. CEZ=C+Si / 17+Mn / 7.5+Cu / 13+Ni / 17+Cr / 4.5+Mo / 3+V / 1.5+Nb / 2+Ti / 4.5+420B...(1)

3. A method for manufacturing a high-strength electric resistance welded steel pipe suitable for overhead line poles, characterized by using steel having the chemical composition described in claim 1, manufacturing a hot-rolled steel sheet with a heating temperature of 1070°C to 1300°C during hot rolling, a hot finishing rolling temperature of 800°C to 1050°C, and a winding temperature of 500°C or less after cooling, and then using the said hot-rolled steel sheet to manufacture a pipe under conditions that satisfy the following formula (3), thereby obtaining a steel pipe that satisfies CEZ ≤ 0.44, has a tensile strength of 700 MPa or more in the direction perpendicular to the pipe axis, a yield point of 520 MPa or more, and a residual stress of 200 MPa or less in the direction perpendicular to the pipe axis, and having excellent resistance to hot-dip galvanized cracking. 2.0≦SZ final stage diameter reduction amount / (FP final stage diameter reduction amount + SQ diameter reduction amount)≦3.5...(3) Here, the final diameter reduction amount for SZ is the final diameter reduction amount for the sizer process in the pipe manufacturing process (mm), the final diameter reduction amount for FP is the final diameter reduction amount for the fin pass process in the same process (mm), and the diameter reduction amount for SQ is the diameter reduction amount for the squeeze process in the same process (mm).

4. Furthermore, in mass percent, B: greater than 0.00000% to 0.00020%, Ti: greater than 0.00% to 1.00%, Nb: greater than 0.00% to 1.00%, V: greater than 0.00% to 1.00%, Cu: greater than 0.00% to 1.00%, Ni: greater than 0.00% to 1.00%, Cr: greater than 0.00% to 1.00%, Mo: greater than 0.00% to 0.50%, W: greater than 0.00% to 0.50%, C A method for manufacturing a high-strength electric resistance welded steel pipe suitable for overhead line poles, characterized by using steel having a chemical composition containing one or more of the following: a: greater than 0.0000% to 0.0200%, Mg: greater than 0.0000% to 0.0200%, Zr: greater than 0.0000% to 0.0200%, and REM: greater than 0.0000% to 0.0200%.

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