JP2024029929A - High-strength electric resistance welded steel pipe with excellent hot-dip galvanizing cracking resistance and suitable for overhead line poles, and its manufacturing method - Google Patents

Abstract

An object of the present invention is to provide an electric resistance welded steel pipe that has excellent hot-dip galvanizing wettability, and has both hot-dip galvanizing cracking resistance and high strength, and a method for manufacturing the same. [Solution] Consists of predetermined components, satisfies CEZ≦0.44 calculated by the following formula (1), has a tensile strength of 700 MPa or more in the tube axis and the direction perpendicular to the tube axis, has a yield point of 520 MPa or more, and has a yield point of 520 MPa or more perpendicular to the tube axis. A high-strength electric resistance welded steel pipe that has excellent resistance to hot-dip galvanizing cracking and has residual stress in the direction of 200 MPa or less and is suitable for overhead wire poles, and a method for manufacturing the same. 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) [Selection diagram] Figure 1

Description

TECHNICAL FIELD The present invention relates to an electric resistance welded steel pipe that is suitable for use as overhead line poles in railways, etc., and has high strength and excellent resistance to hot-dip galvanizing cracking, and a method for manufacturing the same.

An overhead line pole is a telephone pole on which overhead wires are attached to supply electricity to the train car body. In the wake of the Great East Japan Earthquake in 2011, the seismic design of overhead line poles was reviewed, and there was a demand for overhead line poles that could withstand earthquake-level shaking. In general, to improve earthquake resistance, one of the methods of increasing diameter, thickening, and strengthening is considered to be a combination of these. Increasing the diameter of conventional high-strength overhead line columns (590 MPa class) is difficult from the perspective of installation space, and thickening increases the weight of the overhead line, which increases horizontal shaking, making it difficult to withstand shaking. I can't do it. There is also a risk that transportation costs will increase. Therefore, from the viewpoint of increasing strength, the development of 700 MPa class high strength overhead line poles is being considered.

Generally, in order to prevent rust in steel towers, bridges, and buildings including overhead line columns, a method is widely used in which the steel used therein is welded to structural members and then hot-dip galvanized. On the other hand, in the case of hot-dip galvanizing, cracks may occur due to residual stress during cold working, residual stress in the weld heat affected zone, and thermal stress due to heating and cooling during hot-dip galvanizing. This phenomenon is a type of liquid metal embrittlement phenomenon and is well known as "hot-dip galvanizing cracking."

For high-strength steel materials, elements that increase hardenability and elements that strengthen precipitation are added, but as can be seen from the CEZ equation (1) below, the elements that are added in large quantities to high-strength steel materials are , the value of the CEZ equation becomes large, and the cracking resistance of hot-dip galvanizing deteriorates.
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 as described in Non-Patent Document 1, p. 1108-p. 1114 describes in detail the influence of boron mixed in steel, and if B is 2 ppm or less and the CEZ value of the above formula (1) is 0.44 or less, in a 590 MPa class power transmission tower steel pipe, The company says it can prevent cracking in hot-dip galvanizing. Non-Patent Document 1 was co-authored by a fabricator and four steel companies, and is positioned as the most reliable and cutting-edge technology among the technologies published so far.

It is also known that the higher the strength, the poorer the resistance to hot-dip galvanizing cracking. Therefore, in this project, there is a demand for the development of an electric resistance welded steel pipe that has excellent hot-dip galvanizing wettability, resistance to hot-dip galvanizing cracking, and high strength.

Regarding a method for achieving both hot-dip galvanizing cracking resistance and high strength, for example, in Patent Document 1, in steel materials of less than 70K class, B is produced by grain boundary ferrite generation by adding ferrite-forming elements Si, Al, and Ti. It is said that it suppresses HAZ grain boundary segregation and improves hot-dip galvanizing cracking resistance. Furthermore, Patent Document 2 discloses a tempered high-strength steel that has excellent weldability and is quenched and tempered to have a strength level of 70 to 80K, and has excellent hot-dip galvanizing cracking resistance, and a method for manufacturing the same. Are listed. Patent Document 3 describes a method for manufacturing high-strength steel of 80K class that has excellent hot-dip galvanizing cracking resistance even after being directly quenched after rolling.

In Patent Document 4, Si is set at 0.5 to 1.5% to generate grain boundary ferrite, but since the plating damage that occurs in the region of about 0.30% or more Si becomes a problem, hot-dip galvanizing is not performed. It cannot be applied to electric resistance welded steel pipes, and the strength does not reach 70k class. In the case of an electric resistance welded steel pipe, a hot rolled steel plate unwound from a hot coil is roll-formed to form an open tube, and the abutting portions of the resulting open tube are electric resistance welded to form an electric resistance welded portion. Therefore, since ERW steel pipes are particularly subjected to tensile residual stress in the direction perpendicular to the pipe axis, cracks in the hot-dip galvanizing occur more easily than steel plates.

In Patent Document 5, tempering is performed to remove residual stress, which increases manufacturing costs. Furthermore, it does not include any consideration of residual stress that is applied during the production of ERW steel pipes, and there remains concern that it may have an adverse effect on cracking in hot-dip galvanizing. Although Patent Document 6 does not require heat treatment, it does not include any consideration regarding residual stress that is applied during the production of ERW steel pipes, and there remains a concern that it may have an adverse effect on cracking in hot-dip galvanizing.

Japanese Patent Application Publication No. 2005-307224 Japanese Patent Application Publication No. 10-102195 Japanese Patent Application Publication No. 10-110214 Japanese Patent Application Publication No. 2005-307224 Japanese Patent Application Publication No. 10-102195 Japanese Patent Application Publication No. 10-110214

Iron and steel vol. 79 (1993)

The present invention provides an electric resistance welded steel pipe that has a tensile strength of 700 MPa or more in the tube axis and the direction perpendicular to the tube axis, has excellent hot dip galvanizing wettability, and has both hot dip galvanizing cracking resistance and high strength, and a method for manufacturing the same. .

In order to solve the above problem, we focused on the residual stress in the direction perpendicular to the tube axis. That is,
(1) The components of steel are C: 0.08-0.20%, Si: 0.03-0.40%, Mn: 1.00-2.00%, P: 0.000-0 .030%, S: 0.000 to 0.010%, Al: 0.005 to 0.050%, N: 0.0005 to 0.0100%, and the balance consists of Fe and impurities, and the following formula (1) It satisfies CEZ≦0.44 calculated by This is a high-strength electric resistance welded steel pipe that has excellent resistance to hot-dip galvanizing cracking and is suitable for overhead line poles.
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) Furthermore, the components of the steel are expressed 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% 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: 0.0000% It is also preferable to contain one or more of the following.

(3) Furthermore, as a manufacturing method, using steel having the chemical composition described in (1), the heating temperature during hot rolling is 1070°C or more and 1300°C or less, and the hot finishing temperature is 800°C or more and 1050°C or less. , CEZ≦0.44 can be achieved by manufacturing a hot-rolled steel sheet with a coiling temperature of 500°C or less after cooling, and using the hot-rolled steel sheet to form a pipe under conditions that satisfy the following formula (3). A steel pipe 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, is obtained. A method for manufacturing high-strength electric resistance welded steel pipes that are excellent and suitable for overhead line 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 sizer process in the pipe making process, the FP final stage diameter reduction amount is the final stage diameter reduction amount (mm) of the same fin pass process, and the SQ diameter reduction amount is the sizer process final stage diameter reduction amount (mm). This is the amount of diameter reduction (mm) in the process.

(4) In (3), the steel components are further expressed in mass %: B: more than 0.00000% to 0.00020%, Ti: more than 0.00% to 1.00%, Nb: more than 0.00% ~1.00%, V: over 0.00% ~ 1.00%, Cu: over 0.00% ~ 1.00%, Ni: over 0.00% ~ 1.00%, Cr: 0.00 % to 1.00%, Mo: 0.00% to 0.50%, W: 0.00% to 0.50%, Ca: 0.0000% to 0.0200%, Mg: 0 It is also preferable to contain one or more of the following: more than 0.0000% to 0.0200%, Zr: more than 0.0000% to 0.0200%, and REM: more than 0.0000% to 0.0200%.

According to the present invention, in a steel type with CEZ≦0.44%, a tensile strength of 700 MPa or more in the direction perpendicular to the tube axis, a yield point of 520 MPa or more in the direction perpendicular to the tube axis, and residual stress ≦200 MPa in the direction perpendicular to the tube axis, and hot-dip galvanizing. It was possible to provide an electric resistance welded steel pipe that has excellent wettability and is compatible with hot-dip galvanizing cracking resistance and high strength, and a method for manufacturing the same.

FIG. 3 is a diagram showing the relationship between tensile residual stress in the direction perpendicular to the tube axis and hot-dip galvanizing cracking resistance in an electric resistance welded steel pipe having a tensile strength of 700 MPa or more and a yield point of 520 MPa or more. It is a figure which shows 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 tube axis.

In view of the above-mentioned situation, the present inventors focused on the residual stress imparted during the manufacturing of ERW steel pipes, and found that in ERW steel pipes with a tensile strength of 700 MPa or more and a yield point of 520 MPa or more, tensile residual stress in the direction perpendicular to the pipe axis. When investigating the presence or absence of hot-dip galvanizing cracking, it was confirmed that cracking could be prevented if this residual stress was 200 MPa or less. The results are shown in Figure 1.

Here, in the present invention, tensile strength and yield point are measured as follows. In the ERW steel pipe of the present invention, a JIS No. 12 tensile test piece was taken from the 90° position of the base material in the direction of the tube axis, and a JIS No. 5 tensile test piece was taken from the 180° position of the base material in the direction perpendicular to the pipe axis. Then, a tensile test is performed on the collected tensile test piece in the tube axis direction in accordance with JIS Z 2241 (2011), and the tensile strength in the tube axis and the direction perpendicular to the tube axis is measured. The obtained results are defined as the tensile strength in the tube axis direction and the yield point of the electric resistance welded steel pipe of the present disclosure.

Further, in the present invention, the residual stress in the direction perpendicular to the tube axis is determined by the Crampton method (for example, The International Journal of Advanced Manufacturing Technology (2019) 103:4221-4231) expressed by the following equation (2). The Crampton method is a method in which the residual stress is released by cutting the steel pipe in the longitudinal direction, and the residual stress is determined from the amount of change in the outer diameter before and after cutting. In formula (2), D0 is the average external shape before cutting, and D1 is the average external shape after cutting. The length of the specimen in the Crampton method is set to satisfy 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)

This tensile residual stress in the direction perpendicular to the tube axis is reduced by plastically deforming the tube through diameter reduction processing in the tube making process. The diameter reduction process in the tube manufacturing process includes a fin pass process (hereinafter, fin pass is referred to as FP), a squeeze process (hereinafter, squeeze is referred to as SQ), and a sizing process (hereinafter, sizing is referred to as SZ). In the FP process, the diameter is reduced, but since the cross section has not yet been closed, only the edge portion in particular undergoes plastic deformation. In the SQ process, the diameter is reduced due to bead discharge, but only slight plastic deformation occurs in the vicinity of the electric resistance welding part. In the SZ process, for final roundness adjustment, 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 in the direction perpendicular to the tube axis in the circumferential direction, it is necessary to ensure a sufficient diameter reduction amount in the SZ process.

Therefore, in order to ensure a sufficient amount of diameter reduction in the SZ process, we thought that it would be effective to balance the diameter reduction amounts of the FP process, SQ process, and SZ process in some way, and constructed equation (3), It has been found that it is important that this formula (3) satisfies 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)
If the above formula (3) satisfies a predetermined range, it is possible to manufacture an ERW steel pipe in which the tensile residual stress in the direction perpendicular to the pipe axis is stably 200 MPa or less, and no appearance defects occur, and there is no cracking in the hot-dip galvanizing. It turns out that it can be prevented. The results are shown in FIG.

The present invention was completed based on the above knowledge, and its gist is the following electric resistance welded steel pipe and method for manufacturing the same. In addition, in this specification,
- A numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as lower and upper limits.
- "%" indicating the content of a component (element) means "mass%".
- The content of components such as C (carbon) is sometimes expressed as "C content". The contents of other elements may also be expressed in the same manner.
- The term "step" is used not only to refer to an independent step, but also to include the term even if the step cannot be clearly distinguished from other steps, as long as the step achieves the purpose stated in the specification.

In the present invention, the following component ranges are assumed for the components other than Fe.

C: 0.08-0.20%
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 is 0.08% or more. On the other hand, if the C content exceeds 0.20%, weldability and hot-dip galvanizing cracking resistance may be impaired. Therefore, the C content is 0.20% or less. Preferably it is 0.12% or less.

Si: 0.03-0.40%
Si is an element used for deoxidation. If the Si content is less than 0.03%, deoxidation may be insufficient and coarse oxides may be produced. Therefore, the Si content is 0.03% or more. Preferably it is 0.15% or more. On the other hand, when the Si content exceeds 0.40%, plating wettability deteriorates. In addition, the Fe--Zn alloy reaction is accelerated, and plating burn, which is a poor appearance in which the alloy layer is exposed on the surface, is likely to occur. Therefore, the Si content is 0.40% or less. The Si content is preferably 0.25% or less.

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 is 1.00% or more. Preferably it is 1.40% or more. On the other hand, if the Mn content exceeds 2.00%, weldability and hot-dip galvanizing cracking resistance may be impaired. Therefore, the Mn content is 2.00% or less.

P:0.000~0.030%
P is an element that can be contained as an impurity in steel. If the P content exceeds 0.030%, the alloy reaction during galvanizing becomes active, and the plating layer may peel off. Therefore, the P content is 0.030% or less. On the other hand, in the case of the present invention, the P content is preferably 0.000% since it is substantially an impurity, but from the viewpoint of reducing the cost of dephosphorization, the P content may be 0.001% or more. It may be 0.010% or more.

S: 0.000-0.010%
S is an element that can be contained as an impurity in steel. When the S content exceeds 0.010%, coarse MnS is generated, which may serve as a starting point to cause cracks. Therefore, the S content is 0.010% or less. The S content is preferably 0.005% or less. On the other hand, in the case of the present invention, the S content is preferably 0.000% since it is substantially an impurity, but from the viewpoint of reducing desulfurization cost, the S content may be 0.001% or more.

Al: 0.005-0.050%
Al is added as a deoxidizer, but if the content is less than 0.005%, deoxidation may be insufficient and coarse oxides may be produced. Therefore, the Al content is 0.005% or more. On the other hand, even if the content exceeds 0.05%, the deoxidizing effect is saturated. Therefore, the Al content is 0.05% or less. Note that since AlN is easily formed and stable precipitation of VN is inhibited, the content is preferably 0.02% or less.

N: 0.0005-0.0100%
N generates AlN and contributes to the refinement of austenite grains due to the pinning effect during hot rolling. It is also an element that generates VN and 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 it is 0.001% or more. On the other hand, if the N content exceeds 0.0100%, toughness will deteriorate. Therefore, the N content is 0.0100% or less. Preferably it is 0.005% or less.

The following are elements that are preferably selectively included in one or more types in order to increase strength, toughness, etc.

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

Ti: more than 0.00% to 1.00%
Ti is an element that contributes to increasing the strength of steel, but it is an optional element in the present invention and does not need to be included. If it is contained, it is more than 0.00%. On the other hand, if Ti is contained excessively, the effect may be saturated and the cost may increase. Furthermore, the Ti content is 1.00% or less since it may deteriorate the toughness. Preferably it is 0.10% or less.

Nb: More than 0.00% to 1.00%
Nb is an element that contributes to increasing the strength of steel, but in the present invention, it is an optional element and does not need to be included. If it is contained, it is more than 0.00%. On the other hand, if Nb is contained excessively, the effect may be saturated and the cost may increase. Furthermore, the Nb content is 1.00% or less since it may deteriorate the toughness. Preferably it is 0.10% or less.

V: More than 0.00% to 1.00%
Although V is an element that contributes to increasing the strength of steel, it is an optional element in the present invention and does not need to be contained. If it is contained, it is more than 0.00%. On the other hand, if too much V is contained, the effect may be saturated and the cost may increase. Further, the V content is 1.00% or less since it may deteriorate the toughness. Preferably it is 0.10% or less.

Cu: more than 0.00% to 1.00%
Cu is an element that contributes to increasing the strength of steel, but in the present invention, it is an optional element and does not need to be included. If it is contained, it is more than 0.00%. On the other hand, if Cu is contained excessively, the effect may be saturated and the cost may increase. Further, the Cu content is 1.00% or less since it may deteriorate toughness. Preferably it is 0.10% or less.

Ni: more than 0.00% to 1.00%
Ni is an element that contributes to increasing the strength of steel, but in the present invention, it is an optional element and does not need to be included. If it is contained, it is more than 0.00%. On the other hand, if Ni is contained excessively, the effect may be saturated and the cost may increase. In addition, the Ni content is 1.00% or less since it may impair weldability and hot-dip galvanizing cracking resistance. Preferably it is 0.10% or less.

Cr: more than 0.00% to 1.00%
Cr is an element that contributes to increasing the strength of steel, but in the present invention, it is an optional element and does not need to be included. If it is contained, it is more than 0.00%. On the other hand, if Cr is contained excessively, the effect may be saturated and the cost may increase. In addition, the Cr content is 1.00% or less since it may impair weldability and hot-dip galvanizing cracking resistance. Preferably it is 0.10% or less.

Mo: more than 0.00% to 0.50%
Mo is an element that contributes to increasing the strength of steel, but in the present invention, it is an optional element and does not need to be included. If it is contained, it is more than 0.00%. On the other hand, if Mo is contained excessively, the effect may be saturated and the cost may increase. Therefore, the Mo content is 0.50% or less. Preferably it is 0.10% or less.

W: More than 0.00% to 0.500%
W is an element that contributes to increasing the strength of steel, but in the present invention, it is an optional element and does not need to be included. If it is contained, it is more than 0.00%. On the other hand, if W is contained excessively, the effect may be saturated and the cost may increase. Therefore, the W content is 0.50% or less. Preferably it is 0.10% or less.

Ca: more than 0.0000% to 0.0200%
Although Ca has the effect of controlling the shape of inclusions and improving toughness, it is an optional element in the present invention and does not need to be contained. If it is contained, it is more than 0.00%. On the other hand, if Ca is contained excessively, the effect may be saturated and the cost may increase. Therefore, the Ca content is 0.0200% or less.

Mg: more than 0.0000% to 0.0200%
Although Mg has the effect of controlling the shape of inclusions and improving toughness, it is an optional element in the present invention and does not need to be contained. If it is contained, it is more than 0.00%. On the other hand, if Mg is contained excessively, the effect may be saturated and the cost may increase. Therefore, the Mg content is 0.0200% or less.

Zr: More than 0.0000% to 0.0200%
Although Zr has the effect of controlling the shape of inclusions and improving toughness, it is an optional element in the present invention and does not need to be contained. If it is contained, it is more than 0.00%. On the other hand, if Zr is contained excessively, the effect may be saturated and the cost may increase. Therefore, the Zr content is 0.0200% or less.

REM: More than 0.0000% to 0.0200%
REM includes at least one rare earth 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. Refers to one type of element. Although REM has the effect of controlling inclusions, it is an optional element in the present invention and may not be included. If it is contained, it is more than 0.00%. On the other hand, if REM is contained excessively, the effect may be saturated and the cost may increase. Therefore, the REM content is 0.0200% or less.

Remaining portion: Fe and impurities In the chemical composition of the base material, the remainder excluding each of the above-mentioned elements is Fe and impurities. Here, impurities refer to components contained in raw materials (for example, ore, scrap, etc.) or components mixed in during the manufacturing process, but not intentionally added to steel. Examples of impurities include all elements other than those mentioned above. The number of elements as impurities may be one or two or more. Examples of impurities include Sb, Sn, Co, As, Pb, Bi, and H. Usually, Sb, Sn, Co, and As are mixed in at a content of 0.1% or less, Pb and Bi are mixed in at a content of 0.005% or less, and H is mixed in at a content of 0.1% or less, for example. Contamination of up to 0.0004% is possible in each case. There is no need to particularly control the contents of other elements as long as they are within normal ranges.

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

Next, an example of a method for manufacturing the electric resistance welded steel pipe of the present invention will be described below.
By heating the slab having the above chemical composition at 1070°C to 1300°C, the carbides, nitrates, and carbonitrides that precipitate during the solidification process of molten steel are sufficiently dissolved in the steel, thereby deteriorating the internal cracking resistance. It is possible to improve the strength without causing any damage. Further, coarsening of austenite grains due to heating is suppressed, and coarse AlN can be prevented from precipitating during hot rolling or during cooling after hot rolling.

By hot rolling the heated slab at a hot rolling finish temperature of 800° C. or more and 1050° C. or less, strain is introduced in the recrystallized region and the non-recrystallized region, the number of nucleation sites is increased, and the structure is refined.

After hot rolling, the product is cooled to a coiling temperature of 500° C. or less and coiled to suppress the formation of soft ferrite.

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

Table 1 shows the components of the slab used in the present invention.

For the slabs having the chemical compositions listed in Table 1, electric resistance welded steel pipes were manufactured and evaluated under the hot rolling conditions, cooling, winding conditions, and pipe forming conditions listed in Table 2. Table 2 also shows the tensile strength and residual stress in the direction perpendicular to the pipe axis of the obtained electric resistance welded steel pipe. Here, the method for measuring each amount of diameter reduction during pipe making is calculated by actually measuring the outer circumferential length at the final stage of the FP process, SQ process, and SZ process with a tape measure before and after forming.

In the examples that satisfied 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 cracking was observed. On the other hand, in comparative examples, the value of formula (3) was too low outside the range of the present invention, so the effect of reducing residual stress could not be obtained, and hot-dip galvanizing cracking occurred. Because the temperature was too high outside the scope of the present invention, no cracks were observed in the hot-dip galvanizing, but appearance defects occurred.

Claims (4)

The components of the steel are C: 0.08-0.20%, Si: 0.03-0.40%, Mn: 1.00-2.00%, P: 0.000-0.030% in mass%. , S: 0.000 to 0.010%, Al: 0.005 to 0.050%, N: 0.0005 to 0.0100%, and the balance consists of Fe and impurities, calculated using the following formula (1) CEZ≦0.44, the tensile strength in the tube axis and the direction perpendicular to the tube axis is 700 MPa or more, the yield point is 520 MPa or more, and the residual stress in the direction perpendicular to the tube axis is 200 MPa or less, High-strength electric resistance welded steel pipe with excellent hot-dip galvanized cracking resistance and suitable for overhead line poles.
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) Further, the components of the steel are expressed 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%, and REM: more than 0.0000% to 0.0200%. High-strength electric resistance welded steel pipe with excellent hot-dip galvanized cracking resistance and suitable for overhead line poles. Using steel having the chemical composition according to claim 1, the heating temperature during hot rolling is 1070°C or more and 1300°C or less, the hot finish rolling temperature is 800°C or more and 1050°C or less, and the coiling temperature after cooling is 500°C. ℃ or less, and using the hot rolled steel plate to form a tube under conditions that satisfy the following formula (3), CEZ≦0.44 is satisfied, and the tube axis and the direction perpendicular to the tube axis are A steel pipe with a tensile strength of 700 MPa or more, 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, with excellent hot-dip galvanizing cracking resistance and high strength suitable for overhead wire columns. Manufacturing method of ERW steel pipe.
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 sizer process in the pipe making process, the FP final stage diameter reduction amount is the final stage diameter reduction amount (mm) of the same fin pass process, and the SQ diameter reduction amount is the sizer process final stage diameter reduction amount (mm). This is the amount of diameter reduction (mm) in the process. Furthermore, 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: 0.00% More than 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: 0. 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: Claim 3, characterized in that a steel having a chemical composition containing one or more of the following: more than 0.0000% to 0.0200%, and REM: more than 0.0000% to 0.0200% is used. A method for producing a high-strength electric resistance welded steel pipe that has excellent hot-dip galvanizing cracking resistance and is suitable for overhead line poles.