EP4066954A1

EP4066954A1 - Line pipe-use electric resistance welded steel pipe

Info

Publication number: EP4066954A1
Application number: EP20919182.4A
Authority: EP
Inventors: Takuya Hara; Shinya Sakamoto; Shuji Iwamoto
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2020-02-10
Filing date: 2020-02-10
Publication date: 2022-10-05
Also published as: JPWO2021161366A1; KR102730861B1; WO2021161366A1; EP4066954A4; KR20220105166A; JP7226595B2

Abstract

An electric resistance welded steel pipe for a linepipe comprising a base metal portion and an electric resistance welded portion, wherein a chemical composition of the base metal portion includes, in terms of % by mass, 0.030 to 0.090% of C, 0.01 to 0.50% of Si, 0.50 to 1.50% of Mn, 0.005 to 0.060% of Nb, 0.005 to 0.030% of Ti, 0.0001 to 0.0040% of Ca, 0.0010 to 0.0080% of N, and a balance of Fe and impurities, ΔYS, which is a value obtained by subtracting a yield strength of the electric resistance welded portion from a yield strength of the base metal portion, is from 0 to 80 MPa, and AHv, which is a value obtained by subtracting a Vickers hardness of an inner surface layer of the electric resistance welded portion from a Vickers hardness of an outer surface layer of the electric resistance welded portion, is from 0 to 25 Hv.

Description

Technical Field

The present disclosure relates to an electric resistance welded steel pipe for a linepipe.

Background Art

A pipeline is a system constructed on the ground, seabed, and the like, to transfer oil or gas.
A pipeline is formed by joining a plurality of linepipes together. An electric resistance welded steel pipe may be used for such linepipes.
In recent years, areas where pipelines are being constructed have expanded to areas with severe environments, such as sour environments.
Here, a sour environment means an acid environment containing H₂S which is a corrosive gas.
Sulfide Stress Cracking (SSC) resistance is required for pipelines constructed in sour environments, linepipes to form the above-described pipelines, and steel pipes for the above-described linepipes.
Japanese Patent Application Laid-Open ( JP-A) No. H06-41684 (Patent Document 1) and JP-A No. H06-235045 (Patent Document 2) disclose a technique for improving the SCC resistance of an electric resistance welded steel pipe for a linepipe.
The electric resistance welded steel pipe disclosed in Patent Document 1 is produced using a steel containing from 0.05 to 0.35% of C, from 0.02 to 0.50% of Si, from 0.30 to 2.00% of Mn, from 0.0005 to 0.0080% of Ca, and from 0.005 to 0.100% of Al and the balance composed of Fe. In this electric resistance welded steel pipe, contents of S, O, and Ca satisfy the formula (1.0 ≤ (%Ca){1-72 (%O)}/1.25(%S) ≤ 2.5), the deoxidation product is a composite inclusion of (CaO)_m(Al₂O₃)_n, m/n < 1, and the maximum value of hardness measured within 30 mm on both sides centered on an electric resistance welded abutting surface is 250 or less in the Vickers hardness, and the difference between the maximum and minimum values is 30 or less in the Vickers hardness.
Paragraph 0063 of Patent Document 1 describes that the above-described electric resistance welded steel pipe has high strength and excellent sulfide stress corrosion cracking resistance characteristics even in low pH and severe environments.
The electric resistance welded steel pipe disclosed in Patent Document 2 is produced using a steel containing, % by weight, from 0.05 to 0.35% of C, from 0.02 to 0.50% of Si, from 0.30 to 2.00% of Mn, 0.030% or less of P, 0.005% or less of S, from 0.0005 to 0.0080% of Ca, and from 0.005 to 0.100% of Al, wherein contents of S, O, and Ca satisfy the formula (1.0 ≤ (%Ca) {1-72 (%O)}/1.25 (%S) ≤ 2.5), the relationship between the amount of O and the amount of Ca satisfies (%Ca)/(%O) ≤ 0.55, and the balance is consisting of Fe and unavoidable impurities. In this electric resistance welded steel pipe, the maximum value of hardness measured within 30 mm on both sides centered on an electric resistance welded abutting surface is 250 or less in Vickers hardness, and the difference between the maximum value and the minimum value is 30 or less in Vickers hardness.
Paragraph 0059 of Patent Document 2 describes that the above-described electric resistance welded steel pipe does not degrade the SSC properties even in low pH and severe environments.

Patent Document 1: JP-A No. H06-41684
Patent Document 2: JP-A No. H06-235045

SUMMARY OF INVENTION

Technical Problem

However, there may be a need to further improve the SCC resistance of an electric resistance welded steel pipe for a linepipe used in sour environments.
Furthermore, in recent years, not only sulfide stress cracking (SSC) but also stress-oriented hydrogen induced cracking (SOHIC), which has a different cracking mechanism from SSC, has been pointed out in electric resistance welded steel pipes for a linepipe used in sour environments.
The mechanism by which SOHIC occurs is considered to be as follows.
In an electric resistance welded steel pipe for a linepipe used in sour environments, a bulge extending in the direction of the pipe axis of the electric resistance welded steel pipe, called a "blister," may occur near the surface of the electric resistance welded steel pipe. When an electric resistance welded steel pipe is subjected to stress, a plurality of blisters and minute internal cracks in the electric resistance welded steel pipe connect in the direction of the thickness of the electric resistance welded steel pipe, resulting in SOHIC.
Patent Documents 1 and 2 do not describe SOHIC resistance. Therefore, even when the techniques disclosed in Patent Documents 1 and 2 are applied to an electric resistance welded steel pipe for a linepipe used in sour environments, excellent SOHIC resistance may not be achieved.
An object of the present disclosure is to provide an electric resistance welded steel pipe for a linepipe having excellent SSC resistance and SOHIC resistance.

Solution to Problem

Means for solving the problem described above includes the following aspects.

<1> An electric resistance welded steel pipe for a linepipe, the steel pipe comprising a base metal portion and an electric resistance welded portion, wherein:
- a chemical composition of the base metal portion consists of, in terms of % by mass: from 0.030 to 0.090% of C,
  - from 0.01 to 0.50% of Si,
  - from 0.50 to 1.50% of Mn,
  - from 0 to 0.020% of P,
  - from 0 to 0.0020% of S,
  - from 0.005 to 0.060% of Nb,
  - from 0.005 to 0.030% of Ti,
  - from 0.0001 to 0.0040% of Ca,
  - from 0 to 0.050% of Al,
  - from 0.0010 to 0.0080% of N,
  - from 0 to 0.0030% of O,
  - from 0 to 0.500% of Cu,
  - from 0 to 0.500% of Ni,
  - from 0 to 0.0020% of B,
  - from 0 to 0.100% of V,
  - from 0 to 0.500% of Cr,
  - from 0 to 0.500% of Mo,
  - from 0 to 0.500% of W,
  - from 0 to 0.0500% of Zr,
  - from 0 to 0.0500% of Ta,
  - from 0 to 0.0050% of Mg,
  - from 0 to 0.0050% of REM,
  - from 0 to 0.0050% of Hf,
  - from 0 to 0.0050% of Re, and
  - a balance of Fe and impurities;
- in a metallographic microstructure of an inner surface layer of the base metal portion, a polygonal ferrite fraction is from 80% to less than 100%, and a balance contains degenerated-pearlite and cementite;
- a yield strength of the base metal portion is 410 MPa or higher, and a tensile strength of the base metal portion is from 515 to 650 MPa;
- ΔYS, which is a value obtained by subtracting a yield strength of the electric resistance welded portion from the yield strength of the base metal portion, is from 0 to 80 MPa; and
- AHv, which is a value obtained by subtracting a Vickers hardness of an inner surface layer of the electric resistance welded portion from a Vickers hardness of an outer surface layer of the electric resistance welded portion, is from 0 to 25 Hv.
<2> The electric resistance welded steel pipe for a linepipe according to <1>, wherein
the chemical composition of the base metal portion includes one kind, or two or more kinds selected from the group consisting of, in terms of % by mass:
- more than 0% but equal to or less than 0.500% of Cu,
- more than 0% but equal to or less than 0.500% of Ni,
- more than 0% but equal to or less than 0.0020% of B,
- more than 0% but equal to or less than 0.100% of V,
- more than 0% but equal to or less than 0.500% of Cr,
- more than 0% but equal to or less than 0.500% of Mo,
- more than 0% but equal to or less than 0.500% of W,
- more than 0% but equal to or less than 0.0500% of Zr,
- more than 0% but equal to or less than 0.0500% of Ta,
- more than 0% but equal to or less than 0.0050% of Mg,
- more than 0% but equal to or less than 0.0050% of REM,
- more than 0% but equal to or less than 0.0050% of Hf, and
- more than 0% but equal to or less than 0.0050% of Re.
<3> The electric resistance welded steel pipe for a linepipe according to <1> or <2>, wherein a thickness is 13 mm or more.
<4> The electric resistance welded steel pipe for a linepipe according to any one of <1> to <3>, wherein an outer diameter is from 300 mm to 650 mm.

Advantageous Effects of Invention

According to the present disclosure, an electric resistance welded steel pipe for a linepipe having excellent SSC resistance and SOHIC resistance is provided.

DESCRIPTION OF EMBODIMENTS

A numerical range expressed by "x to y" herein includes the values of x and y in the range as the minimum and maximum values, respectively.
The content of a component (element) expressed by "%" herein means "% by mass".
The content of C (carbon) may be herein occasionally expressed as "C content". The content of another element may be expressed similarly.
The term "step" herein encompasses not only an independent step but also a step of which the desired object is achieved even in a case in which the step is incapable of being definitely distinguished from another step.
The electric resistance welded steel pipe for a linepipe of the present disclosure (hereinafter, also simply referred to as "electric resistance welded steel pipe of the present disclosure") is an electric resistance welded steel pipe for a linepipe, the steel pipe comprising a base metal portion and an electric resistance welded portion, wherein: a chemical composition of the base metal portion consists of, in terms of % by mass: from 0.030 to 0.090% of C, from 0.01 to 0.50% of Si, from 0.50 to 1.50% of Mn, from 0 to 0.020% of P, from 0 to 0.0020% of S, from 0.005 to 0.060% of Nb, from 0.005 to 0.030% of Ti, from 0.0001 to 0.0040% of Ca, from 0 to 0.050% of Al, from 0.0010 to 0.0080% of N, from 0 to 0.0030% of O, from 0 to 0.500% of Cu, from 0 to 0.500% of Ni, from 0 to 0.0020% of B, from 0 to 0.100% of V, from 0 to 0.500% of Cr, from 0 to 0.500% of Mo, from 0 to 0.500% of W, from 0 to 0.0500% of Zr, from 0 to 0.0500% of Ta, from 0 to 0.0050% of Mg, from 0 to 0.0050% of REM, from 0 to 0.0050% of Hf, from 0 to 0.0050% of Re, and a balance of Fe and impurities; in a metallographic microstructure of an inner surface layer of the base metal portion, a polygonal ferrite fraction is from 80% to less than 100%, and a balance contains degenerated-pearlite and cementite; a yield strength of the base metal portion is 410 MPa or higher, and a tensile strength of the base metal portion is from 515 to 650 MPa; ΔYS, which is a value obtained by subtracting a yield strength of the electric resistance welded portion from the yield strength of the base metal portion, is from 0 to 80 MPa; and AHv, which is a value obtained by subtracting a Vickers hardness of an inner surface layer of the electric resistance welded portion from a Vickers hardness of an outer surface layer of the electric resistance welded portion, is from 0 to 25 Hv.
In the present disclosure, the term "base metal portion" refers to a portion other than the electric resistance welded portion and a heat affected zone in the electric resistance welded steel pipe. The term "heat affected zone" (sometimes referred to as "HAZ") refers to a zone which is present in the vicinity of the electric resistance welded portion, and which has been affected by heat due to electric resistance welding and the seam heat treatment.
The electric resistance welded steel pipe of the present disclosure have excellent SSC resistance and SOHIC resistance.
This effect will now be described in detail.
The present inventors investigated the SSC resistance and the SOHIC resistance of an electric resistance welded steel pipe satisfying all the requirements in the present disclosure except the requirements for ΔYS and ΔHv (namely, ΔYS is from 0 to 80 MPa and ΔHv is from 0 to 25Hv) (hereinafter, referred to as "electric resistance welded steel pipe X").
As a result, it was found that the SSC resistance and/or the SOHIC resistance at an electric resistance welded portion may decrease in an electric resistance welded steel pipe X.
Therefore, the present inventors first considered a reason for the decrease in SSC resistance as follows.
Since an electric resistance welded portion of an electric resistance welded steel pipe is rapidly heated and rapidly cooled during electric resistance welding, it is generally considered that the strength of an electric resistance welded portion of an electric resistance welded steel pipe is higher than that of the base metal portion (namely, ΔYS is a negative value). When the strength of an electric resistance welded portion of an electric resistance welded steel pipe is too high, SSC is likely to occur in the electric resistance welded portion of the electric resistance welded steel pipe. As a method of suppressing the occurrence of SSC, a heat treatment applied to the electric resistance welded portion (hereinafter, also referred to as "seam heat treatment"), thereby decreasing the strength of the electric resistance welded portion, is considered.
However, it was found that in an electric resistance welded steel pipe X (namely, an electric resistance welded steel pipe satisfying requirements other than those for ΔYS and ΔHv in the present disclosure), SSC may occur at an electric resistance welded portion even when heat treatment is applied to the electric resistance welded portion.
Furthermore, it was also found that in an electric resistance welded steel pipe X, the yield strength of an electric resistance welded portion, which was thought to be higher than the yield strength of a base metal portion, rather tends to be lower than that of the base metal portion (namely, ΔYS tends to be a positive value).
Accordingly, the present inventors examined in more detail the relationship between SSC at an electric resistance welded portion of an electric resistance welded steel pipe X and ΔYS in the electric resistance welded steel pipe X.
As described above, although the SSC resistance at an electric resistance welded portion decreases when the yield strength of the electric resistance welded portion is too high, the SSC resistance at an electric resistance welded portion may decrease also when the yield strength of the electric resistance welded portion is too low for the following reasons.
SSC (sulfide stress cracking) in electric resistance welded steel pipes is a cracking that can occur when the internal pressure of an electric resistance welded steel pipe increases during use in a sour environment, thereby loading stress in the circumferential direction of the pipe. When the yield strength of an electric resistance welded portion in an electric resistance welded steel pipe is too low, the internal pressure of the electric resistance welded steel pipe, which increases during use, causes plastic strain (namely, strain in the plastic range) on the electric resistance welded portion. The lower the yield strength of an electric resistance weld relative to the yield strength of a base metal portion (namely, the larger the ΔYS), the more easily the applied stress is concentrated in the electric resistance weld, and as a result, the greater the plastic strain caused to the electric resistance welded portion. As a result, hydrogen is absorbed in the plastic strain of the electric resistance welded portion, and SSC are more likely to occur in the electric resistance welded portion.
In an electric resistance welded steel pipe X, in which the ΔYS tends to be a positive value, the SSC of the mechanism described above is considered to be more likely to occur.
Based on the above findings, the present inventors found that SSC in an electric resistance welded portion can be suppressed (namely, SSC resistance in an electric resistance welded portion can be improved) by bringing the yield strength of the electric resistance welded portion close to that of a base metal portion without decreasing the yield strength of the welded portion too much, specifically by adjusting the ΔYS to be from 0 to 80 MPa.
However, it was found that SOHIC may occur in an electric resistance welded portion of an electric resistance welded steel pipe X, even when the ΔYS is from 0 to 80 MPa.
Therefore, the present inventors investigated the occurrence of SOHIC in detail, focusing on an electric resistance welded portion of an electric resistance welded steel pipe.
As a result, the following is found.
The present inventors have investigated the Vickers hardness of electric resistance welded portions of electric resistance welded steel pipes X in aspects in which ΔYS is from 0 to 80 MPa. As a result, it has been found that when AHv, which is the value obtained by subtracting the Vickers hardness of the inner surface layer of an electric resistance welded portion from the Vickers hardness of the outer surface layer of an electric resistance welded portion, exceeds 25Hv, SOHIC occurs in the electric resistance welded portion.
Conventionally, the difference in hardness between the outer surface layer and the inner surface layer of an electric resistance welded portion (namely, ΔHv) has not been focused on in an electric resistance welded steel pipe.
In this regard, the present inventors have found that even in cases in which ΔYS is small, when ΔHv is too large, the SOHIC resistance at an electric resistance welded portion decreases. The reason for this is considered to be that SOHIC is a cracking caused by a plurality of blisters and minute internal cracks in an electric resistance welded portion that are connected in the thickness direction of the electric resistance welded portion.
As a result, the present inventors have found that the SSC resistance and the SOHIC resistance of an electric resistance welded portion can be improved in an electric resistance welded steel pipe X when the ΔYS is from 0 to 80 MPa and the ΔHv is from 0 to 25 Hv (namely, when the steel pipe falls under the electric resistance welded steel pipe of the present disclosure).
In other words, the present inventors have found that the electric resistance welded steel pipe of the present disclosure described above have excellent SSC resistance and SOHIC resistance.

< Chemical Composition of Base Metal Portion >

The chemical composition of the base metal portion (namely, chemical composition A) will now be described.
C: from 0.030 to 0.090%
C (carbon) is an element that increases the strength of steel.
When the C content is too low, the above-described effect may not be obtained.
On the other hand, when the C content is too high, C may form a carbide with an alloying element in steel, which may decrease the SSC resistance and the SOHIC resistance of the steel material. Furthermore, when the C content is too high, the strength of a steel material may become too high, and ΔYS and ΔHv may become too large, resulting in a decrease in the SSC resistance and the SOHIC resistance of the steel material.
Accordingly, the C content is from 0.030 to 0.090%.
The lower limit of the C content is preferably 0.035%, and more preferably 0.040%.
The upper limit of the C content is preferably 0.080%, and more preferably 0.070%.
Si: from 0.01% to 0.50%.
Si is an element that deoxidizes steel.
When the Si content is too low, the above-described effect may not be obtained. On the other hand, when the Si content is too high, the SSC resistance and the SOHIC resistance of steel may decrease.
Therefore, the Si content is from 0.01 to 0.50%.
The lower limit of the Si content is preferably 0.02%, and more preferably 0.05%.
The upper limit of the Si content is preferably 0.40%, and more preferably 0.35%.
Mn: from 0.50 to 1.50%
Mn is an element that deoxidizes steel.
Mn is also an element that increases the strength of steel. When the Mn content is too low, the above-described effects may not be obtained.
On the other hand, when the Mn content is too high, the strength of steel may become too high. In this case, the ΔYS becomes higher, and the ΔHv may further become higher. In these cases, the SSC resistance and the SOHIC resistance of a steel material decrease.
Therefore, the Mn content is from 0.50 to 1.50%.
The lower limit of the Mn content is preferably 0.60%, more preferably 0.80%, and still more preferably 1.00 %.
The upper limit of the Mn content is preferably 1.40%, and more preferably 1.35%.
P: from 0 to 0.020%
P (phosphorus) is an impurity. The P content may be 0% or may exceed 0%.
P may segregate at the grain boundary and decrease the SSC resistance and/or the SOHIC resistance of a steel material.
Therefore, the P content is from 0 to 0.020%.
The upper limit of the P content is preferably 0.015%, and more preferably 0.013%.
The P content is preferably as low as possible. However, extreme reduction of the P content may considerably increase the manufacturing cost of a steel material. Therefore, when industrial production is considered, the lower limit of the P content is preferably 0.001%, and more preferably 0.005%.
S: from 0 to 0.0020%.
S (sulfur) is an impurity. The S content may be 0%, or may exceed 0%.
S may segregate at the grain boundary, and may decrease the SSC resistance and/or the SOHIC resistance of a steel material.
Therefore, the S content is from 0 to 0.0020%.
The upper limit of the S content is preferably 0.0015%, more preferably 0.0010%, and still more preferably 0.0008%. The S content is preferably 0.0015%, more preferably 0.0010%, and still more preferably 0.0008%.
The S content is preferably as low as possible. However, extreme reduction of the S content may considerably increase the manufacturing cost of a steel material. Therefore, when industrial production is considered, the lower limit of the S content is preferably 0.0001%, and more preferably 0.0002%.
Nb: from 0.005 to 0.060%
Nb is an element that bonds with C (carbon) and/or N (nitrogen) to form a carbide, a nitride, or a carbonitride (hereinafter, referred to as "carbonitride or the like"). The carbonitride or the like refinines the sub-structure of a steel material by a pinning effect, and may increase the SSC resistance and/or the SOHIC resistance of a steel material. When the Nb content is too low, the above-described effect may not be obtained.
On the other hand, when the Nb content is too high, a carbonitride or the like may coarsen, and the SSC resistance and the SOHIC resistance of a steel material may deteriorate.
Therefore, the Nb content is from 0.005 to 0.060%.
The lower limit of the Nb content is preferably 0.008%, and more preferably 0.010%.
The upper limit of the Nb content is preferably 0.055%, and more preferably 0.050%, and still more preferably 0.045%.
Ti: from 0.005 to 0.030%
Ti is an element that bonds with N (nitrogen) to form a nitride.
The nitride refines crystal grains by a pinning effect. As a result, the SSC resistance and/or the SOHIC resistance of a steel material is increased. When the Ti content is too low, the above-described effect may not be obtained.
On the other hand, when the Ti content is too high, a nitride may coarsen, and the SSC resistance and/or the SOHIC resistance of a steel material may decrease.
Therefore, the Ti content is from 0.005 to 0.030%.
The lower limit of the Ti content is preferably 0.006%, more preferably 0.007%, and still more preferably 0.008%.
The upper limit of the Ti content is preferably 0.025%, more preferably 0.020%, and still more preferably 0.017%.
Ca: from 0.0001 to 0.0040%
Ca is an element that controls the shape of a sulfide in a steel material and increases the SSC resistance and/or the SOHIC resistance of the steel material. When the Ca content is too low, the above-described effect may not be obtained.
On the other hand, when the Ca content is too high, a coarse oxide inclusion may be formed, and the SSC resistance and/or the SOHIC resistance of the steel material may be reduced.
Therefore, the Ca content is from 0.0001 to 0.0040%.
The lower limit of the Ca content is preferably 0.0005%, and more preferably 0.0010%.
The upper limit of the Ca content is preferably 0.0035%, and more preferably 0.0030%.
Al: from 0 to 0.050%
Al: 0 to 0.050% Al
Al is an optional element. In other words, the Al content may be 0% or more than 0%.
When the Al content is too high, coarse oxide inclusions are formed and the SSC resistance and/or the SOHIC resistance of a steel decreases.
Therefore, the Al content is from 0 to 0.050%.
The upper limit of the Al content is preferably 0.045%, and more preferably 0.040%.
On the other hand, Al is an element that deoxidizes steel. From the viewpoint of obtaining such an effect, the lower limit of the Al content is preferably 0.0005%, more preferably 0.0010%, and still more preferably 0.0015%.
N: from 0.0010 to 0.0080%
N (nitrogen) is an element that bonds with Ti to form a fine nitride to refine the crystal grains of a steel material, thereby improving the SSC resistance and the SOHIC resistance of the steel material. When the N content is too low, the above-described effect may not be obtained.
On the other hand, when the N content is too high, a nitride may coarsen and the SSC resistance and/or the SOHIC resistance of the steel material may decrease.
Therefore, the N content is from 0.0010 to 0.0080%.
The lower limit of the N content is preferably 0.0015% and more preferably 0.0020%.
The upper limit of the N content is preferably 0.0070%, more preferably 0.0060%, and still more preferably 0.0050%.
O: from 0 to 0.0030%
O (oxygen) is an impurity. The O content may be 0% or more than 0%.
O is an element that forms a coarse oxide and decreases the SSC resistance and/or the SOHIC resistance of a steel material.
Therefore, the O content is 0.0030% or less.
The upper limit of the O content is preferably 0.0028%, and more preferably 0.0025%.
The O content is preferably as low as possible. However, extreme reduction of the O content considerably increases the manufacturing cost of a steel material. Therefore, considering industrial production, the lower limit of the O content is preferably 0.0001%, more preferably 0.0003%, still more preferably 0.0010%, and still more preferably 0.0015%.
Cu: from 0 to 0.500%
Cu is an optional element. In other words, the Cu content may be 0% or more than 0%.
If the Cu content is too high, the strength of the steel material becomes too high, and the SSC resistance and SOHIC resistance of the steel material may decrease.
Therefore, the Cu content is 0 to 0.500%.
The upper limit of Cu content is preferably 0.450%, and more preferably 0.400%.
On the other hand, Cu is an element that solid-dissolves in steel to increase the strength of the steel.
From the viewpoint of obtaining such effect, the lower limit of Cu content is preferably more than 0%, more preferably 0.010%, more preferably 0.020%, and even more preferably 0.030%.
Ni: from 0 to 0.500%.
Ni is an optional element. In other words, the Ni content may be 0% or more than 0%.
When the Ni content is too high, the strength of a steel material becomes too high, and the SSC resistance and the SOHIC resistance of the steel material may decrease, and the electric resistance weldability of the steel material may also decrease.
Therefore, the Ni content is from 0 to 0.500%.
The upper limit of the Ni content is preferably 0.450%, and more preferably 0.400%.
On the other hand, Ni is an element that solid-dissolves in a steel material to increase the strength of the steel material. From the viewpoint of obtaining such an effect, the lower limit of the Ni content is preferably more than 0%, more preferably 0.010%, still more preferably 0.050%, and still more preferably 0.100%.
B: from 0 to 0.0020%
B (boron) is an optional element. In other words, the B content may be 0% or more than 0%.
When the B content is too high, a coarse nitride may be formed and the SSC resistance and the SOHIC resistance of a steel material may decrease.
Therefore, the B content is from 0 to 0.0020%.
The upper limit of the B content is preferably 0.0015%, and more preferably 0.0012%.
On the other hand, B is an element that solid-dissolves in a steel material to increase the strength of the steel material. From the viewpoint of obtaining such an effect, the lower limit of the B content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0002%, and still more preferably 0.0003%.
V: from 0 to 0.100%.
V (vanadium) is an optional element. In other words, the V content may be 0% or more than 0%.
When the V content is too high, the low temperature toughness of the steel may decrease.
Therefore, the V content is from 0 to 0.100%.
The upper limit of the V content is preferably 0.090%, and more preferably 0.080%.
On the other hand, V is an element that forms a carbonitride or the like and increases the strength of a steel material. From the viewpoint of obtaining such effects, the lower limit of the V content is preferably more than 0%, more preferably 0.001%, still more preferably 0.005%, and still more preferably 0.010%.
Cr: from 0 to 0.500%.
Cr is an optional element. In other words, the Cr content may be 0% or more than 0%.
When the Cr content is too high, the SSC resistance of a steel material may decrease.
Therefore, the Cr content is from 0 to 0.500%.
The upper limit of the Cr content is preferably 0.450%, and more preferably 0.400%.
On the other hand, Cr is an element that increases the strength of a steel material by forming a carbide. From the viewpoint of obtaining such an effect, the lower limit of the Cr content is preferably more than 0%, more preferably 0.010%, still more preferably 0.050%, and still more preferably 0.100%.
Mo: from 0 to 0.500%
Mo is an optional element. In other words, the Mo content may be 0% or more than 0%.
When the Mo content is too high, an M₂C carbide is formed in excess, which may decrease the SSC resistance of a steel material.
Therefore, the Mo content is from 0 to 0.500%.
The upper limit of the Mo content is preferably 0.450%, and more preferably from 0.5 to 0.400%.
On the other hand, Mo is an element that increases the strength of steel by forming a carbide. From the viewpoint of obtaining such an effect, the lower limit of the Mo content is preferably more than 0%, and more preferably 0.010%, still more preferably 0.050%, and still more preferably 0.100%.
W: from 0 to 0.500%.
W (tungsten) is an optional element. In other words, the W content may be 0% or more than 0%.
When the W content is too high, a coarse carbide is formed in a steel material, and the SSC resistance of the steel material may decrease.
Therefore, the W content is from 0 to 0.500%.
The upper limit of the W content is preferably 0.450%, and more preferably 0.400%.
On the other hand, W is an element that increases the strength of a steel material. Furthermore, W is also an element that forms a protective corrosion coating in a hydrogen sulfide environment, inhibits hydrogen ingress, and thus increases the SSC resistance and the SOHIC resistance of the steel material. From the viewpoint of obtaining these effects, the lower limit of the W content is preferably more than 0%, more preferably 0.001%, still more preferably 0.050%, and still more preferably 0.100%.
Zr: from 0 to 0.0500%
Zr is an optional element. In other words, the Zr content may be 0%, or more than 0%.
When the Zr content is too high, an oxide in a steel material may coarsen, and the SSC resistance and the SOHIC resistance of the steel material may decrease.
Therefore, the Zr content is from 0 to 0.0500%.
The upper limit of the Zr content is preferably 0.0400%, and more preferably 0.0300%.
On the other hand, Zr is an element that refines a sulfide in a steel material and improves the SSC resistance and the SOHIC resistance of the steel material. From the viewpoint of obtaining such effects, the lower limit of the Zr content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0005%.
Ta: from 0 to 0.0500%.
Ta is an optional element. In other words, the Ta content may be 0%, or more than 0%.
When the Ta content is too high, an oxide in a steel material may coarsen, and the SSC resistance and the SOHIC resistance of the steel material may decrease.
Therefore, the Ta content is from 0 to 0.0500%.
The upper limit of the Ta content is preferably 0.0400%, and more preferably 0.0300%.
On the other hand, Ta is an element that refines a sulfide in a steel material and improves the SSC resistance and the SOHIC resistance of the steel material. From the viewpoint of obtaining such effects, the lower limit of the Ta content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0005%.
Mg: from 0 to 0.0050%.
Mg is an optional element. In other words, the Mg content may be 0%, or more than 0%.
When the Mg content is too high, an oxide in a steel material may coarsen, and the SSC resistance and the SOHIC resistance of the steel material may decrease.
Therefore, the Mg content is from 0 to 0.0050%.
The upper limit of the Mg content is preferably 0.0045%, and more preferably 0.0040%.
On the other hand, Mg is an element that detoxifies S in a steel material as a sulfide and increases the SSC resistance and the SOHIC resistance of the steel material. From the viewpoint of obtaining such effects, the lower limit of the Mg content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0005%.
REM: from 0 to 0.0050%.
An REM is an optional element. In other words, the REM content may be 0%, or more than 0%.
Here, the REM means a rare earth element, or 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. The REM content means the total content of rare earth elements.
When the REM content is too high, an oxide coarsens and the SSC resistance and the SOHIC resistance of a steel material decrease.
Therefore, the REM content is from 0 to 0.0050%.
On the other hand, the REM is an element that controls the shape of a sulfide in a steel material to improve the SSC resistance and the SOHIC resistance of the steel material. The REM bonds with P in a steel material to suppress segregation of P at the grain boundary, thereby suppressing decrease in low temperature toughness of the steel material caused by the segregation of P. From the viewpoint of obtaining these effects, the lower limit of the REM content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0005%.
Hf: from 0 to 0.0050%.
Hf is an optional element. In other words, the Hf content may be 0%, or more than 0%.
When the Hf content is too high, an oxide in a steel material may coarsen, and the SSC resistance and the SOHIC resistance of the steel material may decrease.
Therefore, the Hf content is from 0 to 0.0050%.
The upper limit of the Hf content is preferably 0.0045%, and more preferably 0.0040%.
On the other hand, Hf is an element that controls the shape of a sulfide in a steel material to enhance the SSC resistance and the SOHIC resistance of the steel material. From the viewpoint of obtaining such effects, the lower limit of the Hf content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0005%.
Re: from 0 to 0.0050%.
Re is an optional element. In other words, the Re content may be 0%, or more than 0%.
When the Re content is too high, an oxide in a steel material may coarsen, and the SSC resistance and the SOHIC resistance of the steel material may decrease.
Therefore, the Re content is from 0 to 0.0050%.
The upper limit of the Re content is preferably 0.0045%, and more preferably 0.0040%.
On the other hand, Re is an element that controls the shape of a sulfide in a steel material and enhances the SSC resistance and the SOHIC resistance of the steel material. From the viewpoint of obtaining such effects, the lower limit of the Re content is preferably more than 0%, more preferably 0.0001%, still more preferably 0.0003%, and still more preferably 0.0005%.

Balance: Fe and impurities

In the chemical composition of the base metal portion, the balance excluding the respective elements described above is Fe and impurities.
The term "impurities" as used herein refers to components which are contained in raw materials (such as ores and scraps), or components which are mixed-in during production steps, and are not intentionally incorporated into the steel.
Examples of the impurities include all elements other than the elements described above.
Only one kind, or two or more kinds of elements may be contained as the impurities. Examples of the impurities include Sb, Sn, Co, As, Pb, Bi, and H (hydrogen).
In general, any of Sb, Sn, Co and As can be contained, for example, in a content of 0.1% or less, any of Pb and Bi can be contained, for example, in a content of 0.005% or less, and H can be contained, for example, in a content of 0.0004% or less, as the impurities. The contents of other elements need not be particularly controlled, as long as the contents are within usual ranges.
From the viewpoint of obtaining effects provided by the above described respective elements, the chemical composition of the base metal portion may include one kind, or two or more kinds selected from the group consisting of: more than 0% but equal to or less than 0.500% of Cu, more than 0% but equal to or less than 0.500% of Ni, more than 0% but equal to or less than 0.0020% of B, more than 0% but equal to or less than 0.100% of V, more than 0% but equal to or less than 0.500% of Cr, more than 0% but equal to or less than 0.500% of Mo, more than 0% but equal to or less than 0.500% of W, more than 0% but equal to or less than 0.0500% of Zr, more than 0% but equal to or less than 0.0500% of Ta, more than 0% but equal to or less than 0.0050% of Mg, more than 0% but equal to or less than 0.0050% of REM, more than 0% but equal to or less than 0.0050% of Hf, and more than 0% but equal to or less than 0.0050% of Re.
Preferred ranges of the contents of these optional elements are each as described above.

< Metallographic Microstructure of Inner Surface Layer of Base Metal Portion >

The electric resistance welded steel pipe of the present disclosure have a polygonal ferrite fraction (hereinafter, also referred to as "F fraction") of from 80% to less than 100% in the metallographic structure of the inner surface layer of a base metal portion, and the balance contains degenerated-pearlite and cementite.
The above-described metallographic microstructure of the inner surface layer of a base metal portion is a metallographic microstructure that is a prerequisite for obtaining an effect of sour resistance (the SSC resistance and the SOHIC resistance in detail) due to the requirements of the ΔYS and the ΔHv under the restriction that the chemical composition of the base metal portion is the chemical composition A described above, and the tensile strength of the base metal portion is in a range of from 515 to 650 MPa.
When the F fraction in the metallographic microstructure of the inner surface layer of a base metal portion is less than 80%, the strength of the base metal portion may become too high, the ΔYS and the ΔHv may become too large, and as a result, the SSC resistance and the SOHIC resistance of a steel material may decrease. Therefore, the F fraction in the metallographic structure of the inner surface layer of a base metal portion is 80% or more, preferably 83% or more, more preferably 85% or more, and still more preferably 90% or more.
When the F fraction in the metallographic microstructure of the inner surface layer of a base metal portion is 100%, the tensile strength and the yield strength of the base metal portion may become too low. Therefore, the F fraction in the metallographic microstructure of the inner surface layer of a base metal portion is less than 100%, preferably 98% or less, and still preferably 95% or less.
In the present disclosure, in the metallographic structure in the inner surface layer of the base metal portion, the balance excluding polygonal ferrite contains degenerated-pearlite and cementite. This facilitates adjustment of the tensile strength of the base metal portion to within the range described above. The balance is preferably composed of degenerated-pearlite and cementite.
In the present disclosure, measurement of the F fraction and confirmation of the type of the balance in the metallographic microstructure of the inner surface layer of a base metal portion are performed as follows.
The L-section (namely, a section parallel to the pipe axial direction and the thickness direction of an electric resistance welded steel pipe) of a base metal 180° position (namely, a position 180° away from the electric resistance welded portion in a pipe circumferential direction) in an electric resistance welded steel pipe is subjected to nital etching.
A micrograph of the metallographic microstructure of the electric resistance welded steel pipe at a depth of 1 mm from the inner surface of the pipe in the L-section after nital etching is taken using a scanning electron microscope (SEM) at a magnification of 200 times.
By image processing the micrograph of metallographic microstructure, the F-fraction is measured and the type of the balance is confirmed.
Image processing is performed using, for example, LUZEX AP, a compact general-purpose image analyzer manufactured by NIRECO CORPORATION.

< Tensile Strength of Base Metal Portion (TS) >

In the electric resistance welded steel pipe of the present disclosure, the tensile strength (TS) of a base metal portion is from 515 to 650 MPa.
Here, the TS of the base metal portion means the tensile strength in the pipe axial direction.
When the TS of a base metal portion is 515 MPa or higher, it is easier to meet a strength required for an electric resistance welded steel pipe for a linepipe.
The TS of a base metal portion is preferably 520 MPa or higher, and more preferably 530 MPa or higher.
When the TS of a base metal portion is 650 MPa or less, the SSC resistance and the SOHIC resistance of the base metal portion are improved. The TS of the base metal portion is preferably 640 MPa or less, and more preferably 630 MPa or less.

< Yield Strength (YS) of Base Metal Portion >

In the electric resistance welded steel pipe of the present disclosure, the yield strength (YS) of a base metal portion is 410 MPa or higher.
Here, the YS of a base metal portion means the yield strength in the pipe axial direction.
When the YS of a base metal portion is 410 MPa or higher, a strength required for electric resistance welded steel pipes for a linepipe is more easily satisfied. The YS of a base metal portion is preferably 430 MPa or higher, and more preferably 450 MPa or higher.
The YS of a base metal portion is preferably 630 MPa or less. When the YS of a base metal portion is 630 MPa or less, the SSC resistance and the SOHIC resistance of the base metal portion are advantageous. When the YS of a base metal portion is 630 MPa or lower, the bending deformation or the buckling control is advantageous when laying a pipeline formed using an electric resistance welded steel pipe for a linepipe. The YS of the base metal portion is more preferably 620 MPa or less, still more preferably 610 MPa or less, and still more preferably 600 MPa or less.
The TS and the YS of a base metal portion are measured in accordance with ASTM E8 (2013) by the following method.
A round bar test piece with a diameter of 6 mm and a parallel length of 35 mm is obtained from the center of the thickness of an electric resistance welded steel pipe at the base metal 180° position, with the longitudinal direction of a parallel portion of the round bar test piece parallel to the pipe axis direction of the electric resistance welded steel pipe.
Using the round bar test piece, a tensile test (namely, a tensile test in the direction of the pipe axis) is performed at room temperature (25°C) in air, in accordance with ASTM E8 (2013).
The maximum stress during uniform elongation in the above-described tensile test is defined as the TS (MPa) of the base metal portion.
The 0.2% proof stress in the above-described tensile test is defined as the YS (MPa) of the base metal portion.
In a base metal portion of the electric resistance welded steel pipe of the present disclosure, it is preferable that substantially no yield elongation is observed when a pipe axial direction tensile test (for example, the tensile test described above) is performed.
Here, "substantially no yield elongation is observed" means that the yield elongation is less than 1%.
The fact that substantially no yield elongation is observed in the pipe axial direction tensile test of the base metal portion means that the electric resistance welded steel pipe is an as-rolled electric resistance welded steel pipe.
Here, an as-rolled electric resistance welded steel pipe means an electric resistance welded steel pipe which has not been subjected to any heat treatment other than the seam heat treatment, after pipe-making.
In contrast, in an electric resistance welded steel pipe which has been subjected to a heat treatment (such as tempering) other than the seam heat treatment after pipe-making, a substantial yield elongation (a yield elongation of 1% or more) is observed when subjected to a tensile test in the pipe axis direction.

< Yield Ratio (YR) of Base Metal Portion >

In the electric resistance welded steel pipe of the present disclosure, the yield ratio (YR) (= YS of base metal portion/TS of base metal portion) of a base metal portion is not particularly restricted.
Here, the YR of a base metal portion means the yield ratio in the pipe axial direction.
From the viewpoint of more effectively suppressing buckling when laying a pipeline formed using an electric resistance welded steel pipe for a linepipe, the YR of the base metal portion is preferably 0.97 or less, and more preferably 0.96 or less.
Examples of the lower limit of the YR of a base metal portion include 0.85 and 0.86.

< ΔYS >

In the electric resistance welded steel pipe of the present disclosure, ΔYS (namely, a value obtained by subtracting a yield strength of the electric resistance welded portion from a yield strength of the base metal portion) is from 0 to 80 MPa.
When the ΔYS is from 0 to 80 MPa, generation of SSC in an electric resistance welded portion is suppressed (namely, the SSC resistance in the electric resistance welded portion is improved).
As described above, when ΔYS exceeds 80 MPa, loaded stress is concentrated to an electric resistance welded portion and SSC may occur in the electric resistance welded portion. Therefore, the ΔYS is 80 MPa or less, and preferably 70 MPa or less.
The fact that the ΔYS is 0 MPa or higher contributes to the suitability for manufacturing the electric resistance welded steel pipe of the present disclosure.
To add to this point, as described above, the yield strength of an electric resistance welded portion has conventionally been considered to be higher than that of a base metal portion.
However, in the electric resistance welded steel pipe of the present disclosure, the yield strength of the electric resistance welded portion is equal to or lower than that of a base metal portion (specifically, the ΔYS is 0 MPa or higher).
The details of the reason for this phenomenon are unknown, but it is assumed that this phenomenon is caused by a combination of the chemical composition of a base metal portion, the metallographic structure of the inner surface layer of the base metal portion, the TS of the base metal portion, the YS of the base metal portion, and the like.
A method of measuring the yield strength (YS) of a base metal portion to calculate the ΔYS is as described above.
The yield strength (YS) of an electric resistance welded portion for calculating the ΔYS is measured in accordance with ASTM E8 (2013) by the following method.
A round bar test piece with a diameter of 6 mm and a length of parallel portion of 35 mm is obtained from an area including an electric resistance welded portion at the inner surface side of an electric resistance welded steel pipe in such a manner that the length direction of the parallel portion of the round bar test piece is parallel to the circumferential direction of the electric resistance welded steel pipe. More precisely, the above-described round bar test piece is obtained in such a manner that the center in the longitudinal direction of the parallel portion of the above-described round bar test piece and the electric resistance welding abutting line of the electric resistance welded steel pipe are approximately aligned. Here, "approximately aligned" means that the center in the longitudinal direction of the parallel portion and the electric resistance welding abutting line of the electric resistance welded steel pipe are perfectly aligned, or that the gap therebetween is 1 mm or less.
By using the obtained round bar test piece, a tensile test (namely, circumferential tensile test) is performed at room temperature (25°C) in air, in accordance with ASTM E8 (2013).
The 0.2% proof stress in the above-described tensile test is defined as the YS (MPa) of the electric resistance welded portion.

< ΔHv >

In the electric resistance welded steel pipe of the present disclosure, the ΔHv (namely, the value obtained by subtracting the Vickers hardness of the inner surface layer of an electric resistance welded portion from the Vickers hardness of the outer surface layer of the electric resistance welded portion) is from 0 to 25Hv.
When the ΔHv is from 0 to 25Hv, occurrence of SOHIC in an electric resistance welded portion is suppressed (namely, the SOHIC resistance in the electric resistance welded portion is improved).
As described above, even when the ΔYS is from 0 to 80 MPa, the SOHIC may occur at an electric resistance welded portion when the ΔHv is more than 25 Hv. Therefore, the ΔHv is 25 Hv or less, preferably 20 Hv, more preferably 18 Hv, and still more preferably 15 Hv.
The fact that the ΔHv is 0Hv or higher contributes to the suitability for manufacturing the electric resistance welded steel pipe of the present disclosure.
For example, it is easy to achieve a ΔHv of 0Hv or higher by applying seam heat treatment from the outer surface side of an electric resistance welded portion (namely, heating and cooling are applied in this order).
In the present disclosure, the Vickers hardness of the outer surface layer of an electric resistance welded portion is determined as follows.
In the C-section of an electric resistance welded steel pipe (namely, a cross-section perpendicular to the pipe axial direction), five points at a pitch of 0.5 mm are specified as measuring points on a line corresponding to a depth of 1 mm from the outer surface of the electric resistance welded steel pipe, and within a circumferential range of 2 mm centered on the electric resistance welding abutting line. At each of the five measuring points, Vickers hardness is measured under a load of 100 gf in accordance with JIS Z 2244 (2009). The arithmetic mean of the measured values at the five measuring points is defined as "Vickers hardness of the outer surface layer of an electric resistance welded portion".
In the present disclosure, the Vickers hardness of the inner surface layer of an electric resistance welded portion is determined as follows.
In the C-section of an electric resistance welded steel pipe, five points at a pitch of 0.5 mm are specified as measuring points on a line corresponding to a depth of 1 mm from the inner surface of the electric resistance welded steel pipe, and within a circumferential range of 2 mm centered on the electric resistance welding abutting line. At each of the five measuring points, Vickers hardness is measured under a load of 100 gf in accordance with JIS Z 2244 (2009). The arithmetic mean of the measured values at the five measuring points is defined as "Vickers hardness of the inner surface layer of an electric resistance welded portion".
In the present disclosure, the ΔHv is obtained by subtracting the "Vickers hardness of the inner surface layer of an electric resistance welded portion" obtained as described above from the "Vickers hardness of the outer surface layer of an electric resistance welded portion" obtained as described above.

< Thickness of Electric Resistance Welded Steel Pipe >

The above-described examination by the present inventors revealed that, in particular, when the thickness of an electric resistance welded steel pipe X (namely, an electric resistance welded steel pipe satisfying requirements other than those of the ΔYS and the ΔHv in the present disclosure) is thick (specifically, 13 mm or more), ΔYS has a strong tendency to adopt a large positive value, and SSC is likely to occur in an electric resistance welded portion.
However, in the electric resistance welded steel pipe of the present disclosure that satisfies requirements of the ΔYS and the ΔHv (namely, requirements that the ΔYS is from 0 to 80 MPa and the ΔHv is from 0 to 25Hv), the requirements of ΔYS and ΔHv ensure excellent sour resistance (in detail, the SSC resistance and the SOHIC resistance. The same applies hereafter) at the electric resistance welded portion, even when the thickness is 13 mm or more. In other words, when the thickness is 13 mm or more, an improvement in sour resistance due to the requirements of the ΔYS and the ΔHv is more effective.
Therefore, the thickness of the electric resistance welded steel pipe of the present disclosure is preferably 13 mm or more, and more preferably 14 mm or more, from the viewpoint that improvement in sour resistance due to requirements of the ΔYS and the ΔHv is highly effective.
On the other hand, the thickness of the electric resistance welded steel pipe of the present disclosure is preferably 25 mm or less, more preferably 23 mm or less, and still more preferably 20 mm or less, from the viewpoint of further suppressing occurrence of SSC in an electric resistance welded portion due to a large thickness.

< Outside Diameter of Electric Resistance Welded Steel Pipe >

The outer diameter of the electric resistance welded steel pipe of the present disclosure is not particularly limited, and is preferably from 300 to 650 mm.
When the outer diameter is 300 mm or more, performance required for a linepipe, namely, oil or gas transfer efficiency, is excellent. The outer diameter is more preferably 330 mm or more, and still more preferably 350 mm or more.
On the other hand, when the outer diameter is 650 mm or less, the electric resistance welded steel pipe is more suitable for manufacturing. The outer diameter is preferably 630 mm or less, and more preferably 610 mm or less.

< SSC Resistance and SOHIC Resistance of Electric Resistance Welded Steel Pipe >

The SSC resistance and the SOHIC resistance of an electric resistance welded steel pipe can be evaluated by a four-point bending test.
The following is a detailed description.
As a test bath, a mixed aqueous solution of 5.0% by mass sodium chloride and 0.5% by mass acetic acid (NACE solution A) is prepared.
A rectangular test piece of 120 mm long, 10 mm wide, and 2 mm thick is obtained from an area including an electric resistance welded portion at the inner surface side of an electric resistance welded steel pipe.
The above-described rectangular test piece is obtained in a direction where the length direction of the test piece is in the pipe circumferential direction of the electric resistance welded steel pipe, the width direction of the test piece is in the pipe axial direction of the electric resistance welded steel pipe, and the thickness direction of the test piece is in the thickness direction of the electric resistance welded steel pipe. In this case, the test sample is obtained in such a manner that the center of the above-described rectangular test piece in the longitudinal direction and the electric resistance welding abutting line of the electric resistance welded steel pipe are approximately aligned. Here, "approximately aligned" means that the center of the above-described rectangular test piece in the longitudinal direction and the electric resistance welding abutting line of the electric resistance welded steel pipe are perfectly aligned, or that a gap between them is within 1 mm.
A four-point bending stress is applied to the obtained rectangular test piece using a four-point bending test jig in accordance with ASTM G39-99 (2011). This 4-point bending stress is applied with the distance between support points (namely, the distance between the outer support points) as 100 mm and the distance between load points (namely, the distance between the inner support points) as 40 mm. The 4-point bending stress applied to the test piece is a stress equivalent to 90% of the YS of the base metal portion in the axial direction of the pipe.
Next, the test piece under 4-point bending stress is sealed in a test container with the 4-point bending test jig. Into this test container, the test bath described above is injected, leaving the gas phase portion, and the test piece is immersed in the test bath. Subsequently, after the test bath is degassed, the test bath is saturated with H₂S gas by continuously ventilating the test bath with 1 atm H₂S gas and agitating the test bath. Next, after sealing the test container, the test bath in which the test piece is immersed is maintained at 24°C for 720 hours, and then the test piece is removed.
The removed test piece is observed to determine whether SSC and SOHIC have occurred or not.
When neither SSC nor SOHIC occurs, the test piece can be judged to have excellent SSC resistance and SOHIC resistance.
Here, SSC and SOHIC are distinguished by the shape of a crack.
Specifically, a crack that extends in both the axial direction and the thickness direction is considered SOHIC, and a crack that extends in the thickness direction but not in the axial direction is considered SSC.

< One Example of Method of Producing Electric Resistance Welded Steel Pipe (Production Method A) >

One example of the method of producing the electric resistance welded steel pipe according to the present disclosure (hereinafter, referred to as "production method A") will now be described.
The following production method A is a method of producing the electric resistance welded steel pipes of Examples to be described below.
The production method A includes:

a slab preparation step of preparing a slab having the chemical composition A (namely, the chemical composition of the base metal portion in the present disclosure);
a hot rolling step of hot rolling the thus prepared slab under the conditions to be described later, to obtain a hot-rolled steel sheet;
a water cooling step of the hot-rolled steel sheet, in which the hot-rolled steel sheet is cooled with water until the temperature of the outer surface of the hot-rolled steel sheet reaches a coiling temperature of from 450 to 625°C;
a coiling step of coiling the cooled hot-rolled steel sheet, at the coiling temperature, to obtain a hot coil composed of the hot-rolled steel sheet;
a pipe-making step in which the hot-rolled steel sheet is uncoiled from the hot coil, the uncoiled hot-rolled steel sheet is roll-formed to prepare an open pipe, and the abutting portion in the thus prepared open pipe is subjected to electric resistance welding to form an electric resistance welded portion, thereby obtaining a raw pipe; and
a seam heat treatment step in which the electric resistance welded portion of the raw pipe is subjected to a seam heat treatment in which the raw pipe is heated to a heating temperature of from 900 to 1,000°C, soaked at the above-described heating temperature for at least 1 second, and then water-cooled at a cooling rate of 5 to 20°C/second to a cooling stop temperature of from 300 to 580°C.

In the production method A, a raw pipe means an electric resistance welded steel pipe before a seam heat treatment is applied to an electric resistance welded portion.
The production method A enables to produce the electric resistance welded steel pipe according to the present disclosure.
The respective steps in the production method A will now be described.

(Slab Preparation Step)

The slab preparation step in the production method A is a step of preparing a slab having the above described chemical composition.
The step of preparing a slab may be a step of producing a slab, or may be a step of simply preparing a slab which has been produced in advance.
In the case of producing a slab, for example, a molten steel having the above described chemical composition is produced, and the thus produced molten steel is used to produce the slab. At this time, the slab may be produced by a continuous casting method, or alternatively, the slab may be produced by forming an ingot using the molten steel, and subjecting the ingot to blooming.

(Hot Rolling Step)

The hot rolling step in the production method A is a step of heating the slab prepared above, and hot rolling the heated slab to obtain a hot-rolled steel sheet.
The slab is heated at a slab heating temperature of from 1,100°C to 1,250°C.
When the heating temperature is 1,100°C or higher, grain refinement during hot rolling and precipitation strengthening after hot rolling are more easily progressed, and as a result, the strength of a steel is more easily improved.
When the heating temperature is 1,250°C or lower, coarsening of austenite grains can be more easily suppressed, which makes it easier to refine crystal grains, and as a result, the strength of a steel can be more easily improved.
A slab is heated, for example, by a furnace.
Here, the slab heating temperature means the temperature of the outer surface of a slab.
In the hot rolling step, the slab heated as described above is hot-rolled to obtain a hot-rolled steel sheet.
Hot rolling is preferably performed under conditions where a finish rolling finishing temperature (hereinafter, also referred to as "finish rolling temperature") is from 780 to 930°C.
Hot rolling is generally performed using a roughing mill and a finishing mill. Both the roughing mill and the finishing mill are generally equipped with a number of rolling stands in a row, each of which is equipped with a roll pair. In this case, the finish rolling temperature (namely, finish rolling finishing temperature) is the surface temperature of a hot-rolled steel sheet at the exit of the final stand of the finish rolling mill.
When the finish rolling temperature is 780°C or higher, the rolling resistance of steel sheets can be reduced, resulting in higher productivity.
When the finish rolling temperature is 780°C or higher, a phenomenon of rolling in the two-phase region of ferrite and austenite is suppressed, and formation of a layered structure and decrease in mechanical properties associated with this phenomenon can be suppressed.
On the other hand, when the finish rolling temperature is 930°C or lower, a phenomenon of a hot-rolled steel sheet becoming too hard is suppressed, thereby preventing the TS of a base metal portion of a resulting electric resistance welded steel pipe from becoming too high.

(Water Cooling Step of Hot-Rolled Steel Sheet)

The water cooling step of the hot-rolled steel sheet is a step in which the hot-rolled steel sheet is cooled with water until the temperature of the outer surface of the hot-rolled steel sheet reaches a coiling temperature of from 450 to 625°C.
When the above-described coiling temperature (namely, the temperature at which the cooling of the outer surface of the hot-rolled steel sheet ends) is 450°C or higher, the productivity of the product is ensured. The coiling temperature is preferably 500°C or higher.
When the above-described coiling temperature is 625°C or lower, coarsening of crystal grains can be more suppressed, thereby increasing the strength of the hot-rolled steel sheet. The coiling temperature is preferably 600°C or lower.

(Pipe-Making Step)

The pipe-making step is a step in which the hot-rolled steel sheet is uncoiled from the hot coil, the uncoiled hot-rolled steel sheet is roll-formed to prepare an open pipe, and the abutting portion in the thus prepared open pipe is subjected to electric resistance welding to form an electric resistance welded portion, thereby obtaining a raw pipe (namely, the electric resistance welded steel pipe before seam heat treatment is applied to the electric resistance welded portion).

(Seam Heat Treatment Step)

The seam heat treatment step in the production method A is a step of performing a seam heat treatment on the electric resistance welded portion of the raw pipe (namely, the electric resistance welded steel pipe before the electric resistance welded portion is subjected to the seam heat treatment).
The seam heat treatment in the production method A is a treatment in which an electric resistance welded portion of a raw pipe is heated to a heating temperature of from 900 to 1,000°C, soaked at the above-described heating temperature for at least 1 second, and then water cooled at a cooling rate of from 5 to 20°C/second to a cooling stop temperature of from 300 to 580°C.
After water cooling, the electric resistance welded portion is air cooled until the temperature of the electric resistance welded portion reaches room temperature.
The seam heat treatment in the production method A is carried out by performing heating and cooling, in this order, on the electric resistance welded portion before being subjected to the seam heat treatment, from the outer surface side of the electric resistance welded portion.
More specifically, the seam heat treatment in the production method A is carried out as follows.
First, the electric resistance welded portion before being subjected to the seam heat treatment is heated, by induction heating, from the outer surface side until the temperature of the outer surface reaches a heating temperature within the range of from 900 to 1,000°C, and soaked for a soaking time in the range of 1 second or more (preferably from 1 to 5 seconds), in a state in which the temperature of the outer surface is kept at the heating temperature in the above-described range.
Next, the electric resistance welded portion after soaking is water cooled from the outer surface side at a cooling rate in the range of from 5 to 20°C/second to a cooling stop temperature in the range of from 300 to 580°C. In this process, examples of means to achieve a cooling rate in the range of from 5 to 20°C/second include misting water cooling shower, adjusting the flow rate of water cooling shower, and adjusting the angle of water cooling shower.
Here, the heating temperature refers to the temperature of the outer surface of an electric resistance welded portion, and the cooling rate refers to the cooling rate on the outer surface of the electric resistance welded portion.
The cooling stop temperature refers to the recuperative temperature measured on the outer surface of an electric resistance welded portion after water cooling has been stopped, and is the highest temperature measured within 1 minute after water cooling has been stopped.
When the heating temperature is less than 900°C in the seam heat treatment step, the electric resistance weld is heated insufficiently, and the YS of the electric resistance welded portion tends to be too low, resulting in a ΔYS of more than 80 MPa. In this case, furthermore, the hardness of the inner surface layer of the electric resistance welded portion is insufficient, and the ΔHv tends to be more than 25 Hv. Therefore, the heating temperature in the seam heat treatment step of the production method A is 900°C or higher.
On the other hand, if the heating temperature exceeds 1000°C in the seam heat treatment step, the Electric resistance welded portion tends to be overheated and the YS of the Electric resistance welded portion becomes too high, resulting in a ΔYS of less than 0 MPa. In this case, furthermore, the hardness of the outer surface layer of the Electric resistance welded portion tends to become too hard and ΔHv tends to be over 25Hv. Therefore, the heating temperature in the seam heat treatment step in the production method A is 1000°C or less.
In the seam heat treatment step, when the cooling stop temperature is less than 300°C, the YS of the electric resistance welded portion tends to be too high and the ΔYS is less than 0 MPa. In this case, furthermore, the hardness of the outer surface layer of the electric resistance welded portion tends to be too hard, and the ΔHv tends to be more than 25 Hv. Therefore, the cooling stop temperature in the seam heat treatment step of the production method A is 300°C or higher.
On the other hand, when the cooling stop temperature is higher than 580°C in the seam heat treatment step, the YS of the electric resistance welded portion tends to be too low and the ΔYS is higher than 80 MPa. In this case, furthermore, the hardness of the inner surface layer of the electric resistance welded portion tends to be insufficient and the ΔHv tends to be more than 25 Hv. Therefore, the cooling stop temperature in the seam heat treatment step of the production method A is 580°C or lower.
When the cooling rate is less than 5°C/sec in the seam heat treatment step, the YS of the electric resistance welded portion tends to be too low and the ΔYS is more than 80 MPa. In this case, furthermore, the hardness of the inner surface layer of the electric resistance welded portion is insufficient, and the ΔHv tends to be more than 25 Hv. Therefore, the cooling rate in the seam heat treatment step of production method A is 5°C/second or higher.
When the cooling rate is more than 20°C/sec in the seam heat treatment step, the hardness of the outer surface layer of the electric resistance welded portion tends to become too high, and as a result, the ΔHv tends to be more than 25Hv. Therefore, the cooling rate in the seam heat treatment step of the production method A is 20°C/second or less.
The production method A may include other steps other than the steps described above.
Examples of the other steps include a step of adjusting the shape of the electric resistance welded steel pipe using sizing rolls, after the seam heat treatment step.
The respective steps in the production method A do not affect the chemical composition of the steel.
Therefore, by using a molten steel having the chemical composition A or a slab having the chemical composition A, an electric resistance welded steel pipe whose base metal portion has the chemical composition A is produced

EXAMPLES

Examples of the present disclosure will now be described below. However, the present disclosure is in no way limited to the following Examples.
Underlined values in Table 1 to Table 3 indicate that the corresponding values are outside the scope of the electric resistance welded steel pipe of this disclosure or outside the scope of the production method A.
[Test Nos. 1 to 43]
An electric resistance welded steel pipe for each Test No. was produced according to the above-described production method A.
Details are described below.
Test Nos. 1 to 22 are Examples, and Test Nos. 23 to 43 are Comparative Examples.

< Production of Slab and Hot Coil >

Molten steels having the chemical compositions shown in Table 1 and Table 2 were continuously cast, and slabs having a thickness of 240 mm were produced.
In Table 1 and Table 2, numerical values shown in the column of each element are the contents of each element in % by mass.
In each of the chemical compositions for Test Nos., the balance excluding the elements shown in Table 1 and Table 2 is Fe and impurities.
REMs in Test Nos. 12, 16, and 28 are all Ce.
The above-described slab is heated in a furnace, the heated slab is hot-rolled using a plurality of hot-rolling mills to produce a hot-rolled steel sheet, the resulting hot-rolled steel sheet is water-cooled, and the water-cooled hot-rolled steel sheet is coiled to obtain a hot coil composed of the hot-rolled steel sheet.
Here, the heating temperature when heating the slab was 1,200°C, the finish rolling temperature in hot rolling was set at from 790°C to 930°C, and the coiling temperature was set at from 500°C to 600°C.
< Production of Electric Resistance Welded Steel Pipe >
Each hot-rolled steel sheet was uncoiled from the hot coil, the uncoiled hot-rolled steel sheet was roll-formed to prepare an open pipe, and the abutting portion in the thus prepared open pipe was subjected to electric resistance welding to form an electric resistance welded portion, thereby obtaining a raw pipe.
Next, a seam heat treatment was performed, on the electric resistance welded portion of the raw pipe, under the conditions shown in Table 3, and then the shape of the pipe was adjusted using sizing rolls, to obtain an electric resistance welded steel pipe having the thickness of t and the outer diameter D shown in Table 3.
Although the description is omitted in Table 3, the soaking time in the seam heat treatment was adjusted to be from 1 second to 5 seconds.
The soaking time was controlled by adjusting the timing between the end of heating and the start of showering.
The cooling rate in the seam thermal treatment was controlled by tuning a water-cooling shower into mist, and adjusting the flow rate of the water-cooling shower and/or the angle of the water-cooling shower.
The cooling stop temperature in the seam thermal treatment was controlled by adjusting the timing of stopping the shower.
It is noted here that the production steps described above do not affect the chemical composition of the resulting steel. Therefore, the chemical composition of the base metal portion of each resulting electric resistance welded steel pipe can be regarded as the same as the chemical composition of each molten steel as the raw material.

< Measurements >

For the obtained electric resistance welded steel pipes, the F fraction (namely, polygonal ferrite fraction) in the inner surface layer of the base metal portion was measured and the type of the balance was confirmed by the above-described method.
Furthermore, for the above-described electric resistance welded steel pipes, using the above-described method,

the YS (MPa) of the electric resistance welded portion,
the TS (MPa) of the electric resistance welded portion,
the YS (MPa) of the base metal portion,
the TS (MPa) of the base metal portion,
the ΔYS (namely, value obtained by subtracting the YS of the electric resistance welded portion from the YS of the base metal portion), and
ΔHv (namely, value obtained by subtracting the Vickers hardness of the inner surface layer of the electric resistance welded portion from the Vickers hardness of the outer surface layer of the electric resistance welded portion)
were measured.

The results are shown in Table 3.
In the balance of Table 3, "P + C" means degenerated-pearlite and cementite.

< Evaluation SSC Resistance and SOHIC Resistance of Electric Resistance Welded Portion; Four-point Bending Test >

The obtained electric resistance welded steel pipes were evaluated for the SSC resistance and the SOHIC resistance of the electric resistance welded portions (namely, the presence or absence of SSC and SOHIC after a four-point bending test) by the above-described method.
The results are shown in Table 3.
In Table 3,

"SSC" means that SSC had occurred,
"SOHIC" means that SOHIC had occurred,
"SSC + SOHIC" means that both SSC and SOHIC had occurred, and
"No SSC No SOHIC" means that neither SSC nor SOHIC had occurred.

[Table 1]

Test No.	Element (% by mass)
Test No.	C	Si	Mn	P	S	Nb	Ti	Ca	AI	N	O
1	0.055	0.20	1.20	0.005	0.0005	0.040	0.015	0.0035	0.025	0.0025	0.0020
2	0.040	0.30	1.30	0.010	0.0003	0.020	0.012	0.0030	0.020	0.0030	0.0015
3	0.060	0.10	1.10	0.008	0.0005	0.015	0.010	0.0025	0.030	0.0035	0.0026
4	0.045	0.20	1.00	0.006	0.0004	0.030	0.013	0.0020	0.040	0.0040	0.0025
5	0.050	0.35	1.30	0.010	0.0008	0.030	0.009	0.0015	0.015	0.0015	0.0015
6	0.055	0.25	1.30	0.012	0.0007	0.040	0.010	0.0010	0.030	0.0029	0.0017
7	0.070	0.20	1.20	0.010	0.0005	0.015	0.016	0.0015	0.035	0.0038	0.0019
8	0.080	0.15	1.10	0.009	0.0004	0.020	0.015	0.0010	0.025	0.0043	0.0023
9	0.090	0.10	1.00	0.006	0.0003	0.030	0.015	0.0023	0.025	0.0045	0.0025
10	0.040	0.05	1.30	0.008	0.0002	0.020	0.011	0.0025	0.019	0.0023	0.0019
11	0.050	0.35	1.20	0.010	0.0004	0.015	0.010	0.0030	0.015	0.0015	0.0020
12	0.060	0.20	1.30	0.009	0.0005	0.020	0.013	0.0033	0.023	0.0020	0.0023
13	0.055	0.25	1.30	0.007	0.0004	0.040	0.014	0.0024	0.033	0.0030	0.0025
14	0.070	0.30	1.40	0.006	0.0006	0.010	0.017	0.0028	0.038	0.0040	0.0019
15	0.040	0.20	1.20	0.006	0.0004	0.040	0.017	0.0018	0.030	0.0045	0.0023
16	0.055	0.15	1.30	0.010	0.0003	0.030	0.016	0.0010	0.040	0.0023	0.0025
17	0.060	0.10	1.15	0.012	0.0020	0.020	0.012	0.0029	0.025	0.0018	0.0018
18	0.070	0.05	1.20	0.013	0.0005	0.030	0.008	0.0025	0.020	0.0029	0.0019
19	0.070	0.05	1.20	0.013	0.0005	0.030	0.008	0.0025	0.020	0.0029	0.0019
20	0.070	0.05	1.20	0.013	0.0005	0.030	0.008	0.0025	0.020	0.0029	0.0019
21	0.070	0.05	1.20	0.013	0.0005	0.030	0.008	0.0025	0.020	0.0029	0.0019
22	0.070	0.05	1.20	0.013	0.0005	0.030	0.008	0.0025	0.020	0.0029	0.0019
23	0.040	0.10	1.30	0.006	0.0004	0.015	0.012	0.0030	0.020	0.0045	0.0020
24	0.050	0.05	1.30	0.010	0.0005	0.020	0.010	0.0033	0.030	0.0023	0.0015
25	0.060	0.35	1.40	0.012	0.0004	0.040	0.013	0.0024	0.040	0.0015	0.0026
26	0.060	0.20	1.20	0.010	0.0006	0.010	0.009	0.0028	0.015	0.0020	0.0025
27	0.070	0.25	1.30	0.009	0.0004	0.040	0.010	0.0018	0.030	0.0030	0.0015
28	0.040	0.30	1.15	0.006	0.0020	0.030	0.016	0.0010	0.035	0.0045	0.0017
29	0.001	0.35	1.30	0.010	0.0008	0.030	0.009	0.0015	0.015	0.0015	0.0015
30	0.100	0.25	1.30	0.012	0.0007	0.040	0.010	0.0010	0.030	0.0029	0.0017
31	0.040	0.15	0.02	0.007	0.0004	0.040	0.014	0.0024	0.033	0.0030	0.0025
32	0.060	0.10	2.00	0.006	0.0006	0.010	0.017	0.0028	0.038	0.0040	0.0019
33	0.055	0.05	1.30	0.050	0.0006	0.010	0.017	0.0028	0.038	0.0040	0.0019
34	0.050	0.35	1.30	0.006	0.0025	0.030	0.015	0.0028	0.015	0.0015	0.0015
35	0.055	1.00	1.20	0.005	0.0005	0.040	0.015	0.0035	0.025	0.0025	0.0020
36	0.040	0.20	1.20	0.008	0.0004	0.003	0.009	0.0018	0.023	0.0020	0.0017
37	0.055	0.20	1.20	0.005	0.0005	0.100	0.015	0.0035	0.025	0.0025	0.0020
38	0.085	0.20	1.20	0.005	0.0005	0.040	0.001	0.0035	0.025	0.0025	0.0020
39	0.055	0.25	1.10	0.010	0.0003	0.020	0.050	0.0010	0.033	0.0030	0.0019
40	0.060	0.30	1.00	0.009	0.0002	0.040	0.012	0.0050	0.025	0.0040	0.0023
41	0.055	0.20	1.30	0.007	0.0004	0.010	0.010	0.0028	0.080	0.0025	0.0025
42	0.060	0.15	1.20	0.007	0.0005	0.040	0.014	0.0018	0.030	0.0100	0.0030
43	0.040	0.25	1.15	0.006	0.0004	0.030	0.015	0.0010	0.020	0.0030	0.0050

[Table 2]

Test No.	Element (continued from Table 1; % by mass)
Test No.	Cu	Ni	B	V	Cr	Mo	W	Zr	Ta	Mg	REM	Hf	Re
1	-	-	-	-	-	-	-	-	-	-	-	-	-
2	0.030	-	-	-	-	-	-	-	-	-	-
3	-	0.200	-	-	-	-	-	-	-	-	-
4	-	-	0.0003	-	-	-	-	-	-	-	-	-	-
5	-	-	-	0.050	-	-	-	-	-	-	-	-	-
6	-	-	-	-	0.300	-	-	-	-	-	-	-	-
7	-	-	-	-	-	0.200	-	-	-	-	-	-	-
8	-	-	-	-	-	-	0.100	-	-	-	-	-	-
9	-	-	-	-	-	-	-	0.0030	-	-	-	-	-
10	-	-	-	-	-	-	-	-	0.0030	-	-	-	-
11	-	-	-	-	-	-	-	-	-	0.0020	-	-	-
12	-	-	-	-	-	-	-	-	-	-	0.0040	-	-
13	-	-	-	-	-	-	-	-	-	-	-	0.0020	-
14	-	-	-	-	-	-	-	-	-	-	-	-	0.0020
15	-	0.200	-	-	-	0.100	-	-	-	-	-	-	-
16	-	0.300	-	-	-	-	-	-	-	-	0.0030	-	-
17	-	-	-	-	0.300	-	-	-	-	0.0015	-	-	-
18	0.200	0.200	-	-	-	0.100	-	-	-	0.0020	-	-	-
19	0.200	0.200	-	-	-	0.100	-	-	-	0.0020	-	-	-
20	0.200	0.200	-	-	-	0.100	-	-	-	0.0020	-	-	-
21	0.200	0.200	-	-	-	0.100	-	-	-	0.0020	-	-	-
22	0.200	0.200	-	-	-	0.100	-	-	-	0.0020	-	-	-
23	-	-	-	-	-	-	-	-	-	-	-	-	-
24	-	0.300	-	-	-	-	-	-	-	-	-	-	-
25	-	-	-	-	0.300	-	-	-	-	-	-	-	-
26	-	-	-	-	-	-	-	-	-	0.0020	-	-	-
27	-	0.200	-	-	-	0.100	-	-	-	-	-	-	-
28	-	0.200	-	-	-	0.100	-	-	-	-	0.0020	-	-
29	-	-	-	-	-	-	-	-	-	-	-	-	-
30	-	0.200	-	-	-	-	-	-	-	-	-	-	-
31	-	-	-	-	0.300	-	-	-	-	-	-	-	-
32	-	0.200	-	-	-	0.100	-	-	-	-	-	-	-
33	-	-	-	-	-	-	-	-	-	-	-	-	-
34	-	-	-	-	-	-	-	-	-	-	-	-	-
35	-	-	-	-	0.300	-	-	-	-	-	-	-	-
36	-	0.200	-	-	0.200	-	-	-	-	-	-	-	-
37	-	-	-	-	0.300	-	-	-	-	-	-	-	-
38	-	-	-	-		-	-	-	-	-	-	-	-
39	-	-	-	-	-	0.100	-	-	-	-	-	-	-
40	-	-	-	-	-	0.200	-	-	-	-	-	-	-
41	-	0.200	-	-	-	-	-	-	-	-	-	-	-
42	-	-	-	0.040	-	-	-	-	-	-	-	-	-
43	0.200	0.200	-	-	-	-	-	-	-	-	-	-	-

[Table 3]

Test No.	Seam heat treatment conditions			Thickness t (mm)	Outer diameter D (mm)	Base metal portion inner surface layer		Electric resistance Welded portion YS (MPa)	Electric resistance Welded portion TS (MPa)	Base metal portion YS (MPa)	Base metal portion TS (MPa)	ΔYS (MPa)	ΔHv (MPa)	Presence or absence of SSC and SOHIC (4-point bending test results)
Test No.	Heating temperature (°C)	Cooling rate (°C/s)	Cooing stop temperature (°C)	Thickness t (mm)	Outer diameter D (mm)	F fraction %	Balance	Electric resistance Welded portion YS (MPa)	Electric resistance Welded portion TS (MPa)	Base metal portion YS (MPa)	Base metal portion TS (MPa)	ΔYS (MPa)	ΔHv (MPa)
1	910	20.0	500	19.0	559	90	P + C	468	492	518	540	50	15	No SSC No SOHIC
2	950	10.0	450	19.0	559	93	P + C	447	470	513	534	66	20	No SSC No SOHIC
3	990	10.0	550	19.0	559	85	P + C	468	492	518	540	50	15	No SSC No SOHIC
4	930	20.0	450	19.0	559	90	P + C	422	485	477	533	55	17	No SSC No SOHIC
5	920	15.0	580	19.0	559	92	P + C	509	535	548	570	39	11	No SSC No SOHIC
6	910	20.0	550	19.0	559	94	P + C	556	586	585	609	29	7	No SSC No SOHIC
7	940	20.0	500	19.0	559	87	P + C	574	604	578	607	4	1	No SSC No SOHIC
8	950	15.0	450	19.0	559	92	P + C	543	571	569	592	26	7	No SSC No SOHIC
9	930	10.0	350	19.0	559	90	P + C	556	586	585	609	29	7	No SSC No SOHIC
10	910	15.0	450	19.0	559	87	P + C	406	468	465	541	59	18	No SSC No SOHIC
11	970	20.0	500	19.0	559	90	P + C	468	492	518	540	50	15	No SSC No SOHIC
12	930	20.0	550	19.0	559	91	P + C	509	535	558	581	49	14	No SSC No SOHIC
13	910	10.0	580	19.0	559	87	P + C	495	521	551	574	56	17	No SSC No SOHIC
14	950	15.0	450	19.0	559	90	P + C	584	614	588	617	4	1	No SSC No SOHIC
15	960	10.0	450	19.0	559	89	P + C	447	471	514	535	67	20	No SSC No SOHIC
16	970	20.0	500	19.0	559	88	P + C	502	528	559	583	57	17	No SSC No SOHIC
17	920	15.0	550	19.0	559	87	P + C	526	553	568	592	42	12	No SSC No SOHIC
18	990	20.0	580	19.0	559	85	P + C	590	622	596	626	6	1	No SSC No SOHIC
19	990	20.0	580	14.0	610	88	P + C	590	622	599	630	9	8	No SSC No SOHIC
20	990	20.0	580	15.0	457	87	P + C	590	622	597	628	7	6	No SSC No SOHIC
21	990	20.0	580	20.0	406	90	P + C	590	620	590	620	0	0	No SSC No SOHIC
22	990	20.0	580	16.0	356	88	P + C	590	622	597	628	7	6	No SSC No SOHIC
23	1100	20.0	350	19.0	559	90	P + C	673	708	533	555	-140	48	SSC + SOHIC
24	800	20.0	450	19.0	559	85	P + C	438	461	538	561	100	31	SSC + SOHIC
25	930	30.0	580	19.0	559	84	P + C	554	618	591	615	37	52	SOHIC
26	940	3.0	550	19.0	559	91	P+C	300	316	563	586	263	85	SSC + SOHIC
27	950	15.0	750	19.0	559	90	P+C	400	421	574	598	174	55	SSC + SOHIC
28	920	10.0	200	19.0	559	87	P + C	650	684	518	539	-132	45	SSC + SOHIC
29	930	20.0	500	19.0	559	100	P+C	288	303	322	335	34	10	No SSC No SOHIC
30	940	20.0	450	19.0	559	70	P+C	897	944	1055	1099	158	48	SSC + SOHIC
31	910	15.0	550	19.0	559	86	P+C	220	231	240	250	20	6	No SSC No SOHIC
32	920	10.0	500	19.0	559	85	P+C	698	730	792	826	94	30	SSC + SOHIC
33	950	20.0	500	19.0	559	88	P+C	468	492	518	540	50	15	SSC + SOHIC
34	930	15.0	550	19.0	559	90	P+C	488	514	543	566	55	16	SOHIC
35	910	20.0	550	19.0	559	94	P+C	556	586	653	680	97	32	SSC + SOHIC
36	980	20.0	580	19.0	559	92	P+C	461	485	510	531	49	14	SOHIC
37	910	20.0	550	19.0	559	94	P+C	556	586	643	670	87	29	SSC + SOHIC
38	910	20.0	550	19.0	559	94	P + C	556	586	634	660	78	26	SOHIC
39	910	15.0	400	19.0	559	90	P+C	481	507	535	557	54	16	SOHIC
40	920	20.0	450	19.0	559	86	P+C	515	543	576	600	61	18	SOHIC
41	930	13.0	500	19.0	559	85	P+C	502	528	559	583	57	17	SOHIC
42	940	12.0	500	19.0	559	83	P+C	498	524	554	578	56	17	SOHIC
43	950	20.0	450	19.0	559	80	P+C	458	482	506	537	48	14	SOHIC

As shown in Tables 1 to 3,

electric resistance welded steel pipes of Examples (Test Nos. 1 to 22) in which
the chemical composition of the base metal portion is the above-described chemical composition A;
in the metallographic microstructure of the inner surface layer of the base metal portion, the F fraction is from 80% to less than 100%, the balance contains degenerated-pearlite and cementite;
the YS of the base metal portion is 410 MPa or higher, the TS of the base metal portion is from 515 to 650 MPa;
the value obtained by subtracting the YS of the electric resistance welded portion from the YS of the base metal portion, ΔYS, is from 0 to 80 MPa; and
the value obtained by subtracting the Vickers hardness of the inner surface layer of the electric resistance welded portion from the Vickers hardness of the outer surface layer of the electric resistance welded portion, AHv, is from 0 to 25 Hv

In contrast to these Examples, the results of the Comparative Examples (Test Nos. 23 to 43) were as follows.
Although the electric resistance welded steel pipe of Test No. 23 had an appropriate chemical composition (namely, the above-described chemical composition A) in the base metal portion, the heating temperature in the seam heat treatment step was too high. As a result, the YS of the electric resistance welded portion became too high, resulting in a ΔYS of less than 0 MPa. Furthermore, the Vickers hardness of the outer surface layer of the electric resistance welded portion became too hard, and ΔHv exceeded 25Hv. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC resistance and SOHIC resistance were not obtained.
Although the electric resistance welded steel pipe of Test No. 24 had an appropriate chemical composition (namely, the above-described chemical composition A) in the base metal portion, the heating temperature in the seam heat treatment step was too low. As a result, the YS of the electric resistance welded portion became too low, resulting in a ΔYS of more than 80 MPa. Furthermore, the Vickers hardness of the inner surface layer of the electric resistance welded portion was insufficient, and ΔHv exceeded 25Hv. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC resistance and SOHIC resistance were not obtained.
Although the electric resistance welded steel pipe of Test No. 25 had an appropriate chemical composition (namely, the above-described chemical composition A) in the base metal portion, the cooling rate in the seam heat treatment step was too high. As a result, the Vickers hardness of the outer surface layer became too hard, and ΔHv exceeded 25Hv. As a result, SOHIC was confirmed in the four-point bending test. In other words, excellent SOHIC resistance were not obtained.
Although the electric resistance welded steel pipe of Test No. 26 had an appropriate chemical composition (namely, the above-described chemical composition A) in the base metal portion, the cooling rate in the seam heat treatment step was too low. As a result, the YS of the electric resistance welded portion became too low, resulting in a ΔYS of more than 80 MPa. Furthermore, the Vickers hardness of the inner surface layer of the electric resistance welded portion was insufficient, and ΔHv exceeded 25Hv. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC resistance and SOHIC resistance were not obtained.
Although the electric resistance welded steel pipe of Test No. 27 had an appropriate chemical composition (namely, the above-described chemical composition A) in the base metal portion, the cooling stop temperature in the seam heat treatment step was too high. As a result, the YS of the electric resistance welded portion became too low, resulting in a ΔYS of more than 80 MPa. Furthermore, the Vickers hardness of the inner surface layer of the electric resistance welded portion was insufficient, and ΔHv exceeded 25Hv. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC resistance and SOHIC resistance were not obtained.
Although the electric resistance welded steel pipe of Test No. 28 had an appropriate chemical composition (namely, the above-described chemical composition A) in the base metal portion, the cooling stop temperature in the seam heat treatment step was too low. As a result, the YS of the electric resistance welded portion became too high, resulting in a ΔYS of less than 0 MPa. Furthermore, the Vickers hardness of the outer surface layer of the electric resistance welded portion became too hard, and ΔHv exceeded 25Hv. As a result, SSC was confirmed in the four-point bending test. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC resistance and SOHIC resistance were not obtained.
The electric resistance welded steel pipe of Test No. 29 had too low a C content in the chemical composition of the base metal portion. As a result, the YS of the base metal portion was less than 410 MPa and the TS of the base metal portion was less than 515 MPa.
The electric resistance welded steel pipe of Test No. 30 had too high a C content in the chemical composition of the base metal portion. As a result, the F fraction in the metallographic microstructure of the inner surface layer of the base metal portion was less than 80%, and the TS of the base metal portion exceeded 650 MPa. Furthermore, the YS of the base metal portion became too high, and the ΔYS exceeded 80 MPa. Furthermore, the Vickers hardness of the outer surface layer of the electric resistance welded portion became too hard, and ΔHv exceeded 25Hv. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC resistance and SOHIC resistance were not obtained.
The electric resistance welded steel pipe of Test No. 31 had too low Mn content in the chemical composition of the base metal portion. As a result, the YS of the base metal portion was less than 410 MPa, and the TS of the base metal portion was less than 515 MPa.
The electric resistance welded steel pipe of Test No. 32 had too high an Mn content in the chemical composition of the base metal portion. As a result, the TS of the base metal portion exceeded 650 MPa. Furthermore, the YS of the base metal portion became too high, and the ΔYS exceeded 80 MPa. Furthermore, the Vickers hardness of the outer surface layer of the electric resistance welded portion became too hard, and ΔHv exceeded 25Hv. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC resistance and SOHIC resistance were not obtained.
The electric resistance welded steel pipe of Test No. 33 had too high a P content in the chemical composition of the base metal portion. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC and SOHIC resistance were not obtained.
The electric resistance welded steel pipe of Test No. 34 had too high an S content in the chemical composition of the base metal portion. As a result, SOHIC were confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.
The electric resistance welded steel pipe of Test No. 35 had too high an Si content in the chemical composition of the base metal portion. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC and SOHIC resistance were not obtained.
The electric resistance welded steel pipe of Test No. 36 had too low an Nb content in the chemical composition of the base metal portion. As a result, SOHIC were confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.
The electric resistance welded steel pipe of Test No. 37 had too high an Nb content in the chemical composition of the base metal portion. As a result, SSC and SOHIC were confirmed in the four-point bending test. In other words, excellent SSC and SOHIC resistance were not obtained.
The electric resistance welded steel pipe of Test No. 38 had too low a Ti content in the chemical composition of the base metal portion. As a result, SOHIC were confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.
The electric resistance welded steel pipe of Test No. 39 had too high a Ti content in the chemical composition of the base metal portion. As a result, SOHIC were confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.
The electric resistance welded steel pipe of Test No. 40 had too high a Ca content in the chemical composition of the base metal portion. As a result, SOHIC were confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.
The electric resistance welded steel pipe of Test No. 41 had too high an Al content in the chemical composition of the base metal portion. As a result, SOHIC were confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.
The electric resistance welded steel pipe of Test No. 42 had too high an N content in the chemical composition of the base metal portion. As a result, SOHIC were confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.
The electric resistance welded steel pipe of Test No. 43 had too high an O content in the chemical composition of the base metal portion. As a result, SOHIC was confirmed in the four-point bending test. In other words, excellent SOHIC resistance was not obtained.

Claims

An electric resistance welded steel pipe for a linepipe, the steel pipe comprising a base metal portion and an electric resistance welded portion, wherein:
a chemical composition of the base metal portion consists of, in terms of % by mass:
from 0.030 to 0.090% of C,

from 0.01 to 0.50% of Si,

from 0.50 to 1.50% of Mn,

from 0 to 0.020% of P,

from 0 to 0.0020% of S,

from 0.005 to 0.060% of Nb,

from 0.005 to 0.030% of Ti,

from 0.0001 to 0.0040% of Ca,

from 0 to 0.050% of Al,

from 0.0010 to 0.0080% of N,

from 0 to 0.0030% of O,

from 0 to 0.500% of Cu,

from 0 to 0.500% of Ni,

from 0 to 0.0020% of B,

from 0 to 0.100% of V,

from 0 to 0.500% of Cr,

from 0 to 0.500% of Mo,

from 0 to 0.500% of W,

from 0 to 0.0500% of Zr,

from 0 to 0.0500% of Ta,

from 0 to 0.0050% of Mg,

from 0 to 0.0050% of REM,

from 0 to 0.0050% of Hf,

from 0 to 0.0050% of Re, and

a balance of Fe and impurities;

in a metallographic microstructure of an inner surface layer of the base metal portion, a polygonal ferrite fraction is from 80% to less than 100%, and a balance contains degenerated-pearlite and cementite;

a yield strength of the base metal portion is 410 MPa or higher, and a tensile strength of the base metal portion is from 515 to 650 MPa;

ΔYS, which is a value obtained by subtracting a yield strength of the electric resistance welded portion from the yield strength of the base metal portion, is from 0 to 80 MPa; and

AHv, which is a value obtained by subtracting a Vickers hardness of an inner surface layer of the electric resistance welded portion from a Vickers hardness of an outer surface layer of the electric resistance welded portion, is from 0 to 25 Hv.
The electric resistance welded steel pipe for a linepipe according to claim 1, wherein
the chemical composition of the base metal portion includes one kind, or two or more kinds selected from the group consisting of, in terms of % by mass:
more than 0% but equal to or less than 0.500% of Cu,

more than 0% but equal to or less than 0.500% of Ni,

more than 0% but equal to or less than 0.0020% of B,

more than 0% but equal to or less than 0.100% of V,

more than 0% but equal to or less than 0.500% of Cr,

more than 0% but equal to or less than 0.500% of Mo,

more than 0% but equal to or less than 0.500% of W,

more than 0% but equal to or less than 0.0500% of Zr,

more than 0% but equal to or less than 0.0500% of Ta,

more than 0% but equal to or less than 0.0050% of Mg,

more than 0% but equal to or less than 0.0050% of REM,

more than 0% but equal to or less than 0.0050% of Hf, and

more than 0% but equal to or less than 0.0050% of Re.
The electric resistance welded steel pipe for a linepipe according to claim 1 or 2, wherein a thickness is 13 mm or more.
The electric resistance welded steel pipe for a linepipe according to any one of claims 1 to 3, wherein an outer diameter is from 300 mm to 650 mm.