KR102129296B1 - Electrode Steel Pipe for Line Pipe - Google Patents

Abstract

The chemical composition of the base material part is, in mass%, C: 0.030% or more and less than 0.080%, Mn: 0.30 to 1.00%, Ti: 0.005 to 0.050%, Nb: 0.010 to 0.100%, N: 0.001 to 0.020%, Si: 0.010 To 0.450%, and Al: 0.001 to 0.100%, the balance contains Fe and impurities, CNeq represented by formula (1) is 0.190 to 0.320, Mn/Si ratio is 2.0 or more, formula (2) When the LR is represented by 0.210 or more, and when the metal structure of the base material is observed at a magnification of 1000 times using SEM, the ferrite area ratio is 40 to 80%, and the remainder is an electric wire for line pipes containing tempering bainite. Steel pipe.
CNeq=C+Mn/6+Cr/5+(Ni+Cu)/15+Nb+Mo+V… Equation (1)
LR=(2.1×C+Nb)/Mn… Equation (2)

Description

Electrode Steel Pipe for Line Pipe

The present invention relates to an electric resistance welded steel pipe for line pipes.

In recent years, the importance of line pipes, which are mainly one of the means of transporting crude oil or natural gas, has become more important.

Various studies have been made on the electric resistance steel pipe used as a line pipe (that is, the electric resistance steel pipe for line pipe).

For example, in Patent Literature 1, a hot rolled steel sheet for high-strength electric resistance welded steel pipe having a bainitic ferrite in a steel structure of 95 vol% or more is proposed.

In Patent Document 2, before forming the tubular pipe, the yield ratio in the tube axis direction of the electric resistance welded steel pipe obtained is lowered by giving the large steel material, for example, a repetitive strain by bending-spinning treatment to induce the Bauschinger effect. Technology is disclosed.

In addition, in Patent Document 3, as a method of manufacturing an electric resistance welded steel pipe excellent in strain aging resistance that suppresses an increase in yield ratio due to coating heating and improves deformation characteristics, an electric resistance welded using steel pieces having an Nb amount of 0.003% or more and less than 0.02%. A method of manufacturing a steel pipe has been proposed. In paragraph 0019 of Patent Document 3, "In a conventional electric resistance welded steel pipe having a large amount of Nb, precipitation of Nb carbides proceeds due to processing deformation introduced at the time of pipe construction, and yield strength and tensile strength increase. It was explained that in such precipitation strengthening, the yield strength was particularly increased, and as a result, the yield ratio was rather increased.”

Japanese Patent No. 4305216 Japanese Patent No. 4466320 International Publication No. 2012/133558

In recent years, the demand for line pipes for transporting crude oil containing sour gas or natural gas containing sour gas is increasing.

Under this background, there are cases in which it is required to further improve the sour resistance (ie, resistance to sour gas) of the steel pipe for line pipes.

On the other hand, from the viewpoint of suppressing the buckling of the line pipe when laying the line pipe, it is required to reduce the yield ratio of the steel pipe for line pipe.

However, in the technique described in Patent Document 1, the yield ratio may not be reduced in some cases. The reason for this is thought to be that the steel structure is mainly made of bainitic ferrite.

Moreover, in the technique of patent document 2, since the process of giving a deformation|transformation to a rough steel is required, the number of processes increases, and as a result, the manufacturing cost of a steel pipe may increase.

In addition, it is sometimes required to reduce the yield ratio of the electric resistance welded pipe by a method other than the method of reducing the amount of Nb for the technique of Patent Document 3.

An object of the present disclosure is to provide an electric resistance welded steel pipe for line pipes having excellent sour resistance, a certain tensile strength and yield strength, a reduced yield ratio, and excellent toughness in a base material and an electric welded portion.

The following aspects are included in the means for solving the above problems.

<1> includes a base material part and an electric welding part,

The chemical composition of the base material portion, in mass%,

C: 0.030% or more and less than 0.080%,

Mn: 0.30 to 1.00%,

Ti: 0.005 to 0.050%,

Nb: 0.010 to 0.100%,

N: 0.001 to 0.020%,

Si: 0.010 to 0.450%,

Al: 0.0010 to 0.1000%,

P: 0 to 0.030%,

S: 0 to 0.0010%,

Mo: 0 to 0.50%,

Cu: 0 to 1.00%,

Ni: 0 to 1.00%,

Cr: 0 to 1.00%,

V: 0 to 0.100%,

Ca: 0 to 0.0100%,

Mg: 0 to 0.0100%,

REM: 0 to 0.0100%, and

Residue: made of Fe and impurities,

CNeq represented by the following formula (1) is 0.190 to 0.320,

The ratio of the mass% of Mn to the mass% of Si is 2.0 or more,

LR represented by the following formula (2) is 0.210 or more,

When the metal structure of the base material is observed at a magnification of 1000 times using a scanning electron microscope, the area ratio of the first phase made of ferrite is 40 to 80%, and the second phase, which is the remainder, includes tempering bainite,

Yield strength in the axial direction is 390 to 562 MPa,

The tensile strength in the tube axis direction is 520 to 690 MPa,

The yield ratio in the tube axis direction is 93% or less,

The Charpy absorbed energy in the circumferential direction of the base material portion is 100 J or more at 0°C,

The electric resistance steel pipe for line pipes in which the Charpy absorbed energy in the pipe circumferential direction in the electric resistance welding portion is 80 J or higher at 0°C.

CNeq=C+Mn/6+Cr/5+(Ni+Cu)/15+Nb+Mo+V… Equation (1)

LR=(2.1×C+Nb)/Mn… Equation (2)

[In Formulas (1) and (2), C, Mn, Cr, Ni, Cu, Nb, Mo, and V each represent the mass% of each element]

<2> The chemical composition of the base material portion is in mass%,

Mo: more than 0% and 0.50% or less,

Cu: more than 0% and less than 1.00%,

Ni: more than 0% and 1.00% or less,

Cr: more than 0% and less than 1.00%,

V: more than 0% and 0.100% or less,

Ca: more than 0% and 0.0100% or less,

Mg: more than 0% and 0.0100% or less, and

REM: more than 0% and less than 0.0100%

The electric resistance steel pipe for line pipes as described in <1> containing 1 or 2 types or more.

<3> When the metal structure of the base material is observed at a magnification of 100000 times using a transmission electron microscope, the area ratio of precipitates having a circle equivalent diameter of 100 nm or less is 0.11 to 1.000% or <2>. Electrode steel pipe for line pipes listed.

<4> The electric resistance steel pipe for line pipe according to any one of <1> to <3>, wherein the content of Nb in the chemical composition of the base material portion is 0.020% or more in mass%.

<5> The electric resistance steel pipe for line pipes according to any one of <1> to <4>, having a thickness of 10 to 25 mm and an outer diameter of 114.3 to 609.6 mm.

<6> In the case where a hydrogen-organic cracking test is performed on a test piece taken from the base material part, any one of <1> to <5> in which CLR, which is a percentage of the total length of cracks relative to the test piece length, is 8% or less. Electrode steel pipe for line pipe.

According to the present disclosure, there is provided an electric resistance welded steel pipe for line pipes having excellent sour resistance, a certain tensile strength and yield strength, a reduced yield ratio, and excellent toughness in a base material and an electric welded portion.

1 is a scanning electron microscope photograph showing an example of a metal structure of a base material portion in the present disclosure.

In the present specification, a numerical range displayed using “to” means a range including numerical values described before and after “to” as lower and upper limits.

In this specification, "%" indicating the content of a component (element) means "mass%".

In this specification, the content of C (carbon) may be referred to as "C amount". The content of other elements may also be indicated in the same way.

In this specification, the term "process" is included in this term when the desired purpose of the process is achieved even if it is not clearly distinguishable from other processes as well as independent processes.

The electric resistance steel pipe for line pipes of the present disclosure (hereinafter simply referred to as "electric resistance steel pipe") includes a base material portion and an electric resistance welded portion, and the chemical composition of the base material portion is in mass %, C: 0.030% or more and less than 0.080%, Mn : 0.30 to 1.00%, Ti: 0.005 to 0.050%, Nb: 0.010 to 0.100%, N: 0.001 to 0.020%, Si: 0.010 to 0.450%, Al: 0.0010 to 0.1000%, P: 0 to 0.030%, S: 0 to 0.0010%, Mo: 0 to 0.50%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Cr: 0 to 1.00%, V: 0 to 0.100%, Ca: 0 to 0.0100%, Mg: 0 To 0.0100%, REM: 0 to 0.0100%, and balance: Fe and impurities, CNeq represented by the following formula (1) is 0.190 to 0.320, and the ratio of the mass% of Mn to the mass% of Si (hereinafter , Also referred to as "Mn/Si ratio") is 2.0 or more, LR represented by the following formula (2) is 0.210 or more, and when the metal structure of the base material is observed at a magnification of 1000 times using a scanning electron microscope , The area ratio of the first phase made of ferrite (hereinafter also referred to as "ferrite fraction") is 40 to 80%, the remaining second phase contains tempering bainite, and yield strength in the tube axis direction (hereinafter also referred to as "YS") 390 to 562 MPa, the tensile strength in the axial direction (hereinafter also referred to as "TS") is 520 to 690 MPa, the yield ratio in the axial direction (hereinafter also referred to as "YR") is 93% or less, and the base material portion The Charpy absorbed energy in the circumferential direction of the pipe is 100 J or more at 0°C, and the Charpy absorbed energy in the circumferential direction of the electric resistance welded portion is 80 J or more at 0°C.

CNeq=C+Mn/6+Cr/5+(Ni+Cu)/15+Nb+Mo+V… Equation (1)

LR=(2.1×C+Nb)/Mn… Equation (2)

[In Formulas (1) and (2), C, Mn, Cr, Ni, Cu, Nb, Mo, and V each represent the mass% of each element]

The electric resistance welded steel pipe of the present disclosure includes a base material portion and an electric resistance welded portion.

In general, the hot-rolled steel pipe is formed into an open pipe by forming a hot-rolled steel sheet into a tubular shape (hereinafter, also referred to as "roll forming"), and forms an electric resistance welded portion by welding the butt portion of the obtained open pipe, Subsequently, it is manufactured by seam heat treatment of the electric welding part as necessary.

In the electric resistance welded steel pipe of the present disclosure, the base metal portion refers to portions other than the electric resistance welded portion and the heat-affected portion.

Here, the heat-affected zone (hereinafter also referred to as "HAZ") is a part that is affected by heat by electro-welding (when seam heat treatment is performed after electro-welding, heat by seam welding and seam heat treatment) Points to.

In this specification, the electric resistance welded portion may be simply referred to as a “welded portion”.

The electric resistance welded steel pipe of the present disclosure is excellent in sour resistance, is provided with a certain degree of YS and TS (that is, YS and TS in the above-described range), and the YR is reduced to 93% or less, so that the base material portion and the electric resistance welded portion Good toughness.

In the present disclosure, having excellent toughness means that the Charpy absorbed energy J in the circumferential direction of the tube at 0°C (hereinafter also referred to as "vE") is large.

Specifically, in the electric resistance welded steel pipe of the present disclosure, vE in the base material portion is 100 J or more, and vE in the electric resistance welded portion is 80 J or more.

In the present specification, "excellent sour resistance" means excellent resistance to hydrogen organic cracking (HIC; Hydrogen-Induced Cracking) (hereinafter also referred to as "HIC resistance").

The HIC resistance is evaluated by CLR (ie, Crack to Length Ratio) when a hydrogen organic crack test (hereinafter, also referred to as a "HIC test") is performed on a specimen taken from a base material part.

CLR means the percentage of the total length of the crack with respect to the test piece length, that is, the value obtained by the following equation.

CLR (%) = (total length of crack/length of test piece) x 100 (%)

The HIC test is performed according to NACE-TM0284.

Specifically, in a test solution saturated with 100% H ₂ S gas in Solution A solution (5 mass% NaCl + 0.5 mass% glacial acetic acid aqueous solution), the test piece collected from the base material part is immersed for 96 hours.

After immersion, the above-mentioned CLR (%) is determined by an ultrasonic flaw test.

CLR means that the lower the value, the better the HIC resistance (that is, sour resistance).

It is preferable that CLR is 8% or less.

Since the YR of the electric resistance steel pipe of the present disclosure is low, an effect capable of suppressing the buckling of the electric resistance steel pipe is expected.

An example of a case where buckling suppression of a steel pipe is required is a case where a steel pipe for subsea line pipe is laid by reeling. In the reeling, steel pipes are prepared in advance on land, and the prepared steel pipes are wound onto a spool of a barge. The unwound steel pipe is unloaded from the sea and laid on the seabed. In this reeling, since plastic bending is imparted to the steel pipe during winding or unwinding of the steel pipe, the steel pipe may sometimes buckle. When buckling of the steel pipe occurs, the laying work is forced to stop, and the damage is enormous.

Buckling of the steel pipe can be suppressed by reducing the YR of the steel pipe.

Therefore, according to the electric resistance welded steel pipe of this disclosure, the effect that the buckling at the time of reeling can be suppressed when it is used as an electrical resistance welded pipe for subsea line pipes is expected, for example.

In addition, since the toughness of the welded steel pipe of the present disclosure is excellent in the toughness of the base material portion and the welded portion, it is expected that the effect that the stopping properties of crack propagation during bursting is excellent is expected.

The above-mentioned, sour resistance (i.e., CLR), YS, TS, YR, vE of the base material and vE of the electrode welded portion, and the chemical composition (including CNeq, Mn/Si ratio and LR) in the electrodeposited steel pipe It is achieved by combination with the metal structure.

〔Chemical composition of base material part〕

Hereinafter, with respect to the chemical composition of the base material portion, first, each component in the chemical composition will be described, and then CNeq, Mn/Si ratio and LR will be described.

C: 0.030% or more and less than 0.080%

C is an element necessary to improve the work hardenability of the steel and to achieve low YR of the welded steel pipe. From the viewpoint of this effect, the C content is 0.030% or more. C amount is preferably 0.033% or more, and more preferably 0.035% or more.

On the other hand, when the C content is less than 0.080%, the sour resistance of the base material portion is improved. Therefore, the C content is less than 0.080%. C amount is preferably 0.077% or less, and more preferably 0.070% or less.

Mn: 0.30 to 1.00%

Mn is an element that enhances the steel's stiffness. In addition, Mn is also an essential element for the detoxification of S.

When the Mn content is less than 0.30%, embrittlement by S occurs, and the toughness of the base material part and the electric resistance welded part may deteriorate. Therefore, the Mn content is 0.30% or more. The Mn content is preferably 0.40% or more, and more preferably 0.50% or more.

On the other hand, when the Mn content exceeds 1.00%, coarse MnS is generated in the central portion of the plate thickness, and the hardness of the central portion of the plate increases, so that the sour resistance may be impaired. Moreover, when Mn content exceeds 1.00 %, LR 0.210 or more may not be achieved, and as a result, YR 90% or less may not be achieved. Therefore, the Mn content is 1.00% or less. The Mn content is preferably 0.90% or less, and more preferably 0.85% or less.

Ti: 0.005 to 0.050%

Ti is an element that forms carbonitrides and contributes to the refinement of the grain size.

The amount of Ti is 0.005% or more from the viewpoint of securing the toughness of the base material portion and the electric resistance welded portion.

On the other hand, when the amount of Ti exceeds 0.050%, coarse TiN is generated, and the toughness of the base material portion and the electric welding portion may deteriorate. Therefore, the Ti content is 0.050% or less. Ti amount is preferably 0.040% or less, more preferably 0.030% or less, and particularly preferably 0.025%.

Nb: 0.010 to 0.100%

Nb is an element contributing to the improvement of toughness of the base material portion.

In order to improve toughness by unrecrystallized rolling, the Nb content is 0.010% or more. The Nb content is preferably 0.015% or more, and more preferably 0.020% or more.

On the other hand, when the Nb content exceeds 0.100%, toughness is deteriorated by coarse carbide. For this reason, the amount of Nb is 0.100% or less. The Nb content is preferably 0.095% or less, and more preferably 0.090% or less.

N: 0.001 to 0.020%

N is an element that suppresses coarsening of crystal grains by forming a nitride, and as a result, improves the toughness of the base material portion and the electric resistance welded portion. From the viewpoint of this effect, the N content is 0.001% or more. N amount is preferably 0.003% or more.

On the other hand, when the amount of N exceeds 0.020%, the amount of nitride generated increases, and the toughness of the base material portion and the electric welding portion deteriorates. Therefore, the N content is 0.020% or less. N amount is preferably 0.008% or less.

Si: 0.010 to 0.450%

Si is an element that functions as a deoxidizing agent for steel. More specifically, when the Si content is 0.010% or more, generation of coarse oxides in the base material and the weld is suppressed, and as a result, the toughness of the base material and the weld is improved. Therefore, the Si content is 0.010% or more. The Si content is preferably 0.015% or more, and more preferably 0.020% or more.

On the other hand, when the Si content exceeds 0.450%, inclusions are formed in the electric welding part, and the Charpy absorbed energy is lowered, and the toughness may deteriorate. Therefore, the Si content is 0.450% or less. The Si content is preferably 0.400% or less, more preferably 0.350% or less, and particularly preferably 0.300% or less.

Al: 0.001 to 0.100%

Al, like Si, is an element that functions as a deoxidizer. More specifically, when the amount of Al is 0.001% or more, generation of coarse oxides in the base material and the weld is suppressed, and as a result, the toughness of the base material and the weld is improved. Therefore, the Al content is 0.001% or more. The Al content is preferably 0.010% or more, and more preferably 0.015% or more.

On the other hand, when the amount of Al exceeds 0.100%, the toughness of the welded portion may deteriorate with the production of Al-based oxide during electric welding. Therefore, the Al content is 0.100% or less. The amount of Al is preferably 0.090% or less.

P: 0 to 0.030%

P is an impurity element. When the amount of P exceeds 0.030%, the toughness may be impaired by segregation at the grain boundary. Therefore, the P content is 0.030% or less. The amount of P is preferably 0.025% or less, more preferably 0.020% or less, still more preferably 0.010% or less.

The amount of P may be 0%. From the viewpoint of reducing the dephosphorization cost, the P content may be more than 0%, or may be 0.001% or more.

S: 0 to 0.0010%

S is an impurity element. When the amount of S exceeds 0.0010%, sour resistance may be impaired. Therefore, the amount of S is 0.0010% or less. S amount is preferably 0.0008% or less.

The amount of S may be 0%. From the viewpoint of reducing the desulfurization cost, the S content may be more than 0%, or 0.0001% or more, or 0.0003% or more.

Mo: 0 to 0.50%

Mo is an arbitrary element. Therefore, the Mo content may be 0%.

Mo is an element that contributes to the high strength of the steel by improving the ??hardening property of the steel. From the viewpoint of these effects, the Mo content may be more than 0%, or may be 0.01% or more, or 0.03% or more.

On the other hand, when the Mo content exceeds 0.50%, there is a possibility that the toughness is lowered by the production of Mo carbonitride. Therefore, the Mo content is 0.50% or less. The Mo amount is preferably 0.40% or less, more preferably 0.30% or less, further preferably 0.20% or less, and particularly preferably 0.10% or less.

Cu: 0 to 1.00%

Cu is an arbitrary element. Therefore, the amount of Cu may be 0%.

Cu is an element effective for improving the strength of the base material. From the viewpoint of such an effect, the amount of Cu may be more than 0%, or may be 0.01% or more, or may be 0.03% or more.

On the other hand, when the amount of Cu exceeds 1.00%, fine Cu particles are generated, and there is a fear that toughness is significantly deteriorated. Therefore, the amount of Cu is 1.00% or less. The amount of Cu is preferably 0.80% or less, more preferably 0.70% or less, still more preferably 0.60% or less, and particularly preferably 0.50% or less.

Ni: 0 to 1.00%

Ni is an arbitrary element. Therefore, the Ni content may be 0%.

Ni is an element contributing to the improvement of strength and toughness. From the viewpoint of these effects, the Ni content may be more than 0%, or may be 0.01% or more, or may be 0.05% or more.

On the other hand, when the Ni content exceeds 1.00%, there is a fear that the strength is too high. Therefore, the Ni content is 1.00% or less. The amount of Ni is preferably 0.80% or less, more preferably 0.70% or less, and still more preferably 0.60% or less.

Cr: 0 to 1.00%

Cr is an arbitrary element. Therefore, the Cr content may be 0%.

Cr is an element that improves ??hardness. From the viewpoint of such an effect, the Cr content may be more than 0%, or may be 0.01% or more, or may be 0.05% or more.

On the other hand, when the amount of Cr exceeds 1.00%, there is a fear that the toughness of the welding portion is deteriorated by the Cr-based inclusion generated in the electric welding portion. Therefore, the Cr content is 1.00% or less. The Cr content is preferably 0.80% or less, more preferably 0.70% or less, still more preferably 0.50% or less, and particularly preferably 0.30% or less.

V: 0 to 0.100%

V is any element. Therefore, the amount of V may be 0%.

V is an element that contributes to the improvement of toughness. From the viewpoint of such an effect, the V content may be more than 0%, 0.005% or more, or 0.010% or more.

On the other hand, when the V content exceeds 0.100%, there is a fear that the toughness is deteriorated by the V carbonitride. Therefore, the V content is 0.100% or less. The V amount is preferably 0.080% or less, more preferably 0.070% or less, further preferably 0.050% or less, and particularly preferably 0.030% or less.

Ca: 0 to 0.0100%

Ca is an arbitrary element. Therefore, the amount of Ca may be 0%.

Ca is an element that improves low-temperature toughness by controlling the form of sulfide-based inclusions. From the viewpoint of these effects, the amount of Ca may be more than 0%, may be 0.0001% or more, or may be 0.0010% or more, or may be 0.0030% or more, or may be 0.0050% or more.

On the other hand, when the amount of Ca exceeds 0.0100%, a large cluster or large inclusion formed of CaO-CaS is generated, which may adversely affect toughness. Therefore, the Ca content is 0.0100% or less. The Ca content is preferably 0.0090% or less, more preferably 0.0080% or less, and particularly preferably 0.0060% or less.

Mg: 0 to 0.0100%

Mg is an arbitrary element. Therefore, the amount of Mg may be 0%.

Mg is an effective element as a deoxidizing agent and a desulfurizing agent. In particular, it is an element that generates fine oxides and contributes to the improvement of toughness of the HAZ (Heat affected zone). From the viewpoint of these effects, the amount of Mg may be more than 0%, may be 0.0001% or more, or may be 0.0010% or more, or may be 0.0020% or more.

On the other hand, when the amount of Mg exceeds 0.0100%, oxides tend to aggregate or coarse, and as a result, there is a possibility of lowering the HIC resistance (Hydrogen-Induced Cracking Resistance) or lowering the toughness of the base material or HAZ. . Therefore, the amount of Mg is 0.0100% or less. The amount of Mg is preferably 0.0080% or less.

REM: 0 to 0.0100%

REM is an arbitrary element. Therefore, the REM amount may be 0%.

Here, "REM" is selected from the group consisting of rare earth elements, namely Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. It refers to at least one element.

REM is an effective element as a deoxidizing agent and a desulfurizing agent. From the viewpoint of these effects, the REM amount may be greater than 0%, or may be 0.0001% or more, or may be 0.0010% or more.

On the other hand, if the amount of REM exceeds 0.0100%, coarse oxides are generated, and as a result, there is a fear that the HIC resistance is lowered or the toughness of the base material or HAZ is lowered. Therefore, the REM amount is 0.0100% or less. The amount of REM is preferably 0.0070% or less, and more preferably 0.0050% or less.

The chemical composition of the base material portion is greater than 0% and 0.50% or less, Cu: greater than 0% and 1.00% or less, Ni: greater than 0% and 1.00% or less, and Cr: 0% from the viewpoint of obtaining an effect by any of the elements described above. Contains more than 1.00%, V: more than 0% and 0.100% or less, Ca: more than 0% and 0.0100% or less, Mg: more than 0% and 0.0100% or less, and REM: more than 0% and contains one or two or more types You may do it.

The more preferable amounts of each arbitrary element are as described above, respectively.

Balance: Fe and impurities

In the chemical composition of the base material portion, the balance excluding each element described above is Fe and impurities.

Here, the impurity refers to a component contained in a raw material or a component incorporated in a manufacturing process, and not intended to be contained in steel.

As an impurity, all elements other than the above-mentioned elements are mentioned. The element as an impurity may be one type or two or more types.

Examples of impurities include O, B, Sb, Sn, W, Co, As, Pb, Bi, and H.

Among the above-mentioned elements, it is preferable to control so that the content of O is 0.006% or less.

In addition, for other elements, the mixing is usually 0.1% or less for Sb, Sn, W, Co and As, the mixing is 0.005% or less for Pb and Bi, and the content is 0.0003% or less for B. Incorporation may be incorporated in an amount of 0.0004% or less, respectively, for H, but the content of other elements is not particularly controlled as long as it is within a normal range.

CNeq: 0.190 to 0.320

In the chemical composition of the base material portion, CNeq represented by the following formula (1) is 0.190 to 0.320.

CNeq=C+Mn/6+Cr/5+(Ni+Cu)/15+Nb+Mo+V… Equation (1)

[In formula (1), C, Mn, Cr, Ni, Cu, Nb, Mo, and V each represent the mass% of each element]

CNeq has a positive correlation with yield strength.

CNeq is 0.190 or more from the viewpoint of easy to achieve a yield strength of 390 MPa or more. CNeq is preferably 0.200 or more, and more preferably 0.210 or more.

On the other hand, CNeq is 0.320 or less from the viewpoint of easy to achieve yield strength of 562 MPa or less. CNeq is preferably 0.310 or less, and more preferably 0.300 or less.

LR: 0.210 or more

In the chemical composition of the base material portion, LR represented by the following formula (2) is 0.210 or more.

In the electric resistance welded steel pipe of the present disclosure, when the LR is 0.210 or more, YR 93% or less can be achieved.

When LR is less than 0.210, YR may exceed 93%. It is considered that this is because the amount of precipitates in the steel decreases and the work hardenability decreases (that is, TS decreases).

LR=(2.1×C+Nb)/Mn… Equation (2)

[In formula (2), C, Nb, and Mn each represent the mass% of each element]

The technical meaning of equation (2) is as follows.

In the formula (2), the reason for arranging the C and Nb amounts in the molecule is that C and Nb form precipitates, thereby improving the work hardenability of the steel (i.e., TS increases) and, as a result, the YR of the steel. It is because it is thought to be reduced.

The reason why the amount of C is multiplied by "2.1" is that with respect to the effect of improving the work hardenability due to the above-mentioned precipitate formation, the effect by the inclusion of C is considered to be about 2.1 times the effect by the inclusion of Nb.

In the formula (2), the reason for placing the amount of Mn in the denominator is that although it is possible to transform the steel at a relatively low temperature by the inclusion of Mn, the work hardenability of the steel itself is impaired by the inclusion of Mn (i.e. , TS decreases), as a result, the YR of the steel rises.

As described above, LR has a positive correlation with the amount of Nb and C, and negative correlation with the amount of Mn.

In the electric resistance welded pipe of the present disclosure, when the amount of Nb is relatively large by satisfying that LR is 0.210 or more, for example, the amount of Nb in Patent Document 3 (International Publication No. 2012/133558) (0.003% or more and less than 0.02%) Even in more cases, the LR may be 0.210 or more depending on the amount of C and amount of Mn. In this case, YR 93% or less can be achieved.

In addition, in the electric resistance welded steel pipe of the present disclosure, even when the amount of Nb is less than 0.02%, YR 93% or less can be achieved by satisfying the conditions other than LR and the point where LR is 0.210 or more.

From the viewpoint of more easily achieving YR 93% or less, LR is preferably 0.220 or more, and more preferably 0.230 or more.

There is no particular limitation on the upper limit of LR. LR is preferably 0.600 or less from the viewpoint of manufacturing suitability of the electric resistance welded steel pipe.

Mn/Si ratio: 2.0 or more

In the chemical composition of the base material portion, the Mn/Si ratio (that is, the ratio Mn/Si ratio of Mn to mass% of Si to mass%) is 2.0 or more.

In the electric resistance welded steel pipe of the present disclosure, when the Mn/Si ratio is 2.0 or more, the toughness of the weld is improved, and the vE (ie, Charpy absorption energy in the circumferential direction of the tube at 0°C) of the weld is 80 J or more.

When the Mn/Si ratio is less than 2.0, vE may be less than 80J. The reason for this is considered to be that, when the Mn/Si ratio is less than 2.0, the toughness deteriorates when the MnSi-based inclusion becomes a starting point for brittle fracture in the weld.

The Mn/Si ratio is preferably 2.1 or more from the viewpoint of further improving the toughness of the weld.

The upper limit of the Mn/Si ratio is not particularly limited. The Mn/Si ratio is preferably 50 or less from the viewpoint of further improving the toughness of the weld and the toughness of the base material.

〔Metal structure of base material part〕

In the electric resistance welded steel pipe of the present disclosure, the metal structure of the base material portion has a ferrite fraction (that is, the area ratio of the first phase made of ferrite) when the metal structure is observed at a magnification of 1000 times using a scanning electron microscope. 40 to 80%, the remaining second phase contains tempering bainite.

In the electric resistance welded steel pipe of the present disclosure, when the ferrite fraction is 40% or more, YR 93% or less can be achieved. From the viewpoint of further reducing YR, the ferrite fraction is preferably 45% or more, and more preferably 50% or more.

In the electric resistance welded steel pipe of the present disclosure, when the ferrite fraction is 80% or less, sour resistance is improved. From the viewpoint of improving sour resistance, the ferrite fraction is preferably 75% or less.

In the electric resistance steel pipe of the present disclosure, the second phase that is the balance includes tempering bainite.

The second phase containing the tempering bainite is that the seam steel pipe of the present disclosure is a seam steel pipe after tempering (ie, after seam welding (after seam heat treatment if seam heat treatment is performed after seam welding)). it means.

Since the electric resistance welded steel pipe of the present disclosure is an electric resistance welded steel pipe subjected to tempering after forming, YR 93% or less can be achieved. This reason is considered to be because the YR goes down by tempering after the pipe. It is considered that the reason why YR decreases by tempering after the pipe formation is that YS decreases due to a decrease in dislocation density, and work hardening increases (ie, TS increases) by depositing cementite on the dislocation.

In this specification, tempering bainite is distinguished from bainite which is not tempering bainite in that the structure contains granular cementite.

The concept of "bainite" in this specification includes bainitic ferrite, granular bainite, upper bainite and lower bainite.

The second phase may include tempering bainite, a phase composed of only tempering bainite, or a structure other than tempering bainite.

As a structure other than tempering bainite, pearlite is mentioned.

The pseudo pearlite is also included in the concept of "perlite" in this specification.

In the metal structure of the base material portion, the measurement of the ferrite fraction and the identification of the second phase are performed by nittal etching the metal structure at a position of 1/4 of the thickness in the L cross section at the 90° position of the base material, and photographing the metal structure after the nittal etching (Hereinafter, also referred to as "metal structure photograph") is performed by observing at a magnification of 1000 times using a scanning electron microscope (SEM). Here, a metallographic photograph is taken at a field of view of 1000 times for 10 o'clock (0.12 mm2 as the actual area of the cross section). By processing the photographed metal structure photograph, the ferrite fraction is measured and the second phase is specified. The image processing is performed using, for example, a small general-purpose image analysis device LUZEX AP manufactured by Nireco Corporation.

In the present specification, the “base material 90° position” refers to a position shifted 90° from the weld to the tube circumferential direction, and the “L cross section” refers to a cross section parallel to the tube axis direction and the thickness direction, and “thickness 1/4”. "Position" refers to a position where the distance from the outer circumferential surface of the electric wire is 1/4 of the thickness.

In addition, in this specification, a tube axis direction may be called "L direction."

1 is a scanning electron microscope photograph (SEM photograph; magnification of 1000 times) showing an example of a metal structure of a base material portion in the present disclosure.

The SEM photograph of FIG. 1 is one (1 field of view) of the SEM photograph used for the measurement of the ferrite fraction and the identification of the second phase in Example 1 to be described later.

As shown in Fig. 1, a first phase made of ferrite and a second phase comprising tempering bainite can be identified. Particularly, it can be seen that the second phase contains tempering bainite because a white dot (cementite) is present.

When the metal structure of the base material portion is observed at a magnification of 100000 times using a transmission electron microscope, the area ratio of the precipitate having a diameter of 100 nm or less (hereinafter referred to as "specific precipitate") equivalent to the circle (hereinafter, It is preferable that the "specific precipitate area ratio") is 0.100 to 1.000%.

When the specific precipitate area ratio is 0.100% or more, it is easier to achieve that the YR is 93% or less. The reason for this is considered to be that a specific precipitate (that is, a precipitate having a circle equivalent diameter of 100 nm or less) contributes to the improvement of work hardening properties (that is, an increase in TS), and as a result, YR decreases.

On the other hand, if the specific precipitate area ratio is 1.000% or less, brittle fracture is suppressed (that is, the toughness of the base material portion is excellent). The specific precipitate area ratio is preferably 0.900% or less, and more preferably 0.800% or less.

The specific precipitate area ratio of 0.100 to 1.000% can be achieved by performing tempering at a temperature of 400° C. or higher and Ac 1 point or lower after forming.

In the present disclosure, the area ratio of precipitates (that is, the area ratio of precipitates having a circle equivalent diameter of 100 nm or less) indicates a metal structure having a thickness of 1/4 in the L cross-section at the base material 90° position by a transmission electron microscope (TEM). It is measured by observing at a magnification of 100000 times using ).

More specifically, first, based on the sample taken at the thickness 1/4 position in the L cross section at the base material 90° position, 10% by volume of acetylacetone, 1% by volume of tetramethylammonium chloride, and 89% by volume of methyl alcohol A replica for TEM observation is produced by a SPEED method using an electrolytic solution consisting of. By observing the obtained replica for TEM observation at a magnification of 100,000 times using TEM, a TEM image of a square field of view with a size of 1 µm on one side is obtained at 10 o'clock. The area ratio of precipitates having a circle equivalent diameter of 100 nm or less with respect to the total area of the obtained TEM image is calculated, and the obtained result is taken as the specific precipitate area ratio (%).

In addition, the etching condition in the SPEED method is a condition in which a saturated caramel electrode is used as a reference electrode, and a charge of 10 coulombs is applied at a voltage of -200 mm2 to a surface area of about 80 square millimeters.

In addition, specific precipitates (i.e., precipitates having a circle equivalent diameter of 100 nm or less) are specifically selected from the group consisting of carbides of metals other than Fe, nitrides of metals other than Fe, and carbonitrides of metals other than Fe. I think it is at least one.

Ti and Nb are considered as metals other than Fe mentioned here. In addition, when the chemical composition contains at least one of V, Mo and Cr, at least one of V, Mo and Cr is also considered as a metal other than the Fe.

〔Yield strength in the axial direction (YS)〕

The electric resistance steel pipe of the present disclosure has a yield strength (YS) in the tube axis direction of 390 to 562 MPa.

The YS in the tube axis direction is preferably 410 MPa or more, more preferably 450 MPa or more, further preferably 470 MPa or more, and particularly preferably 500 MPa or more.

The YS in the tube axis direction is preferably 550 MPa or less, more preferably 540 MPa or less, and particularly preferably 530 MPa or less.

The YS in the tube axis direction of 562 MPa or less can be achieved by performing tempering after the pipe. The reason for this is considered to be that the deformation of the tubular pipe is alleviated by the tempering after the tubular pipe, and the dislocation density is lowered.

〔Tensile strength in the axial direction (TS)〕

The electric resistance steel pipe of the present disclosure has a tensile strength (TS) in the tube axis direction of 520 to 690 MPa.

The TS in the tube axis direction is preferably 550 MPa or more, and more preferably 580 MPa or more.

The TS in the tube axis direction is preferably 680 MPa or less, more preferably 660 MPa or less, and particularly preferably 650 MPa or less.

〔Yield ratio in the axis direction〕

The electric resistance steel pipe of the present disclosure has a yield ratio in the tube axis direction (YR=(YS/TS)×100) of 93% or less.

Thereby, buckling of the electric resistance steel pipe at the time of laying is suppressed.

The YR in the tube axis direction of 93% or less can be achieved by performing tempering after pipe forming. This reason is considered to be because YS decreases as the dislocation density decreases, and work hardening increases (i.e., TS increases) by depositing cementite on the dislocation.

〔Thickness of electrode steel pipe〕

The thickness of the electric resistance welded pipe of the present disclosure is preferably 10 to 25 mm.

When the thickness is 10 mm or more, it is advantageous in that it is easy to lower the YR by using the deformation in forming the hot rolled steel sheet into a tubular shape. The thickness is more preferably 12 mm or more.

When the thickness is 25 mm or less, it is advantageous in terms of manufacturing suitability of the electric resistance welded steel pipe (specifically, moldability when forming a hot-rolled steel sheet into a tubular shape). The thickness is more preferably 20 mm or less.

〔Outer diameter of the electric wire

The outer diameter of the electric resistance steel pipe of the present disclosure is preferably 114.3 to 609.6 mm (ie, 4.5 to 24 inches).

If the outer diameter is 114.3 mm or more, it is more suitable as an electric resistance steel pipe for line pipes. The outer diameter is preferably 139.7 mm (ie, 5.5 inches) or more, more preferably 177.8 mm (ie, 7 inches) or more.

When the outer diameter is 609.6 mm or less, it is advantageous in that it is easy to lower the YR by using the deformation in forming the hot-rolled steel sheet into a tubular shape. The outer diameter is preferably 406.4 mm (ie, 16 inches) or less, more preferably 304.8 mm (ie, 12 inches) or less.

[Example of manufacturing method]

The following manufacturing method A is mentioned as an example of the electric resistance steel pipe manufacturing method of this indication.

Method A is

A step of manufacturing an as-roll electrode steel pipe using a hot rolled steel sheet having the above-described chemical composition,

Tempering process to obtain a rolled steel pipe by performing tempering on the as rolled rolled steel pipe

Have

According to the said manufacturing method A, by having a tempering process, for the above-mentioned reason, it is easy to manufacture an electric resistance steel pipe whose YR is 93% or less.

It is preferable that the tempering temperature (that is, the holding temperature in tempering) is 400°C or higher and Ac1 point or lower.

When the tempering temperature is 400°C or higher, cementite and specific precipitates (precipitates having a circle equivalent diameter of 100 nm or less) are more easily precipitated, so YR 93% or less is more easily achieved. As the tempering temperature, it is more preferably 420°C or higher.

When the tempering temperature is equal to or less than Ac1, coarsening of the metal structure is suppressed, and as a result, toughness is improved. The tempering temperature varies depending on the Ac1 point of the steel, but is preferably 720°C or less, preferably 710°C or less, and also preferably 700°C or less.

Here, the Ac1 point means the temperature at which transformation into austenite starts when the temperature of the steel is increased.

The Ac1 point is calculated by the following equation.

Ac1 point(℃)=750.8-26.6C+17.6Si-11.6Mn-22.9Cu-23Ni+24.1Cr+22.5Mo-39.7V-5.7Ti+232.4Nb-169.4Al

[Here, C, Si, Mn, Ni, Cu, Cr, Mo, V, Ti, Nb, and Al are the mass% of each element, respectively. Ni, Cu, Cr, Mo, and V are arbitrary elements, and among those arbitrary elements, the Ac1 point is calculated as 0 mass% for elements not contained in the steel piece]

The tempering time in the tempering step (that is, the holding time at the tempering temperature) is preferably 5 minutes or more from the viewpoint of easily lowering the YR by precipitation of cementite and specific precipitates.

In the manufacturing method A, an as rolled electric resistance steel pipe is an electric resistance steel pipe manufactured by roll-forming (i.e., tubular molding) a hot rolled steel sheet, and refers to an electric resistance steel pipe that is not subjected to heat treatment other than seam heat treatment after roll molding.

The preferable form of the process of manufacturing the as roll electrode steel pipe in the manufacturing method A will be described later.

The manufacturing method A is the amount of change in roundness before and after the adjustment of the shape of the as rolled rolled steel pipe by a sizer between the step of manufacturing the as rolled rolled steel pipe and the tempering step (hereinafter, referred to as "the amount of change in size of the rounded rounder (%)". It is also desirable to have a sizer process adjusted under the condition that is also 1.0% or more).

When the manufacturing method A has a sizer process, it is easier to manufacture an electric resistance welded steel pipe having a specific precipitate area ratio of 0.100 to 1.000% as described above.

The reason for this is that, by the above-described sizer process in which the amount of change in the size of the sizer is 1.0% or more, a potential of a certain amount or more is introduced into the inside of the roll rolled steel pipe, and thereafter, about the roll rolled steel pipe It is considered that this is because fine specific precipitates are easily precipitated on the dislocation by tempering at a temperature of 400°C or higher and Ac1 point or lower.

Here, the roundness of the as roll electric pipe is obtained as follows.

First, four measurement values are obtained by measuring the outer diameter of the roll rolled steel pipe at a 45° pitch with respect to the pipe circumferential direction. The maximum, minimum, and average values of the four measured values obtained are respectively determined. Based on the maximum value, the minimum value, and the average value, the roundness of the as roll electrode steel pipe is obtained by the following equation.

Roundness of Asroll Jeonbong Steel Pipe=(maximum-minimum)/average

The amount of change in the roundness of the sizer (%) is determined by the following formula based on the roundness of the asroll rolled steel pipe before shape adjustment by the sizer and the roundness of the asroll rolled steel pipe after shape adjustment by the sizer.

Amount of change in roundness before and after the sizer (%) = (||roundness of the as rolled rolled steel pipe after shape adjustment by sizer-as before shape adjustment by sizer roundness of rounded steel pipe by sizer|/ as before shape adjustment by sizer Roundness of the roll electrode steel pipe)×100

The process of manufacturing the as roll electrode steel pipe in the manufacturing method A,

A hot rolling step of obtaining a hot rolled steel sheet by heating a steel piece (slab) having the above-described chemical composition and hot rolling the heated steel piece;

A cooling process for cooling the hot rolled steel sheet obtained in the hot rolling process;

A winding step of obtaining a hot coil made of a hot rolled steel sheet by winding the hot rolled steel sheet cooled in the cooling step;

Winding hot rolled steel sheet from a hot coil, roll forming the unrolled hot rolled steel sheet into an open tube, and forming an electric resistance weld by welding the butt of the obtained open tube to form an electric resistance weld, thereby forming an as roll electric resistance steel pipe

It is preferred to have.

The seam heat treatment may be performed after seam welding, if necessary, by seam heat treatment on the seam weld.

In the hot rolling step, it is preferable to heat a steel piece (slab) having the above-described chemical composition to a temperature of 1150°C to 1350°C.

If the temperature for heating the steel piece is 1150°C or higher, the toughness of the base material portion of the electric resistance welded steel pipe can be further improved. This reason is considered to be because the production of unused Nb carbide can be suppressed if the temperature for heating the steel piece is 1150°C or higher.

When the temperature for heating the steel piece is 1350°C or less, the toughness of the base material portion of the electric resistance welded pipe can be further improved. This reason is considered to be because the coarsening of a metal structure can be suppressed if the heating temperature of a steel piece is 1350 degreeC or less.

In the hot rolling step, for example, it is preferable to hot-roll a steel piece heated to a temperature of 1150°C to 1350°C to a temperature of Ar3 point + 100°C or higher. Thereby, the stiffness of the hot rolled steel sheet can be improved. As a result, it is possible to improve the sour resistance of the finally obtained electric resistance welded steel pipe (that is, the electric resistance welded steel pipe).

Here, Ar3 point is calculated|required by the following formula from the chemical composition of a base material part.

Ar3(℃)=910-310C-80Mn-55Ni-20Cu-15Cr-80Mo

[Here, C, Mn, Ni, Cu, Cr, and Mo are the mass% of each element, respectively. Ni, Cu, Cr, and Mo are arbitrary elements, and among those arbitrary elements, the Ar3 point is calculated as 0 mass% for elements not contained in the steel piece]

The cooling step is a step of cooling the hot rolled steel sheet obtained in the hot rolling step.

In the cooling step, it is preferable to cool the hot-rolled steel sheet obtained in the hot rolling step with a cooling start temperature of Ar 3 or higher. Thereby, the strength and toughness of a base material part can be improved more. This reason is considered to be because the generation of coarse ferrite is suppressed by setting the cooling start temperature to Ar3 or higher.

It is preferable to start cooling in a cooling process within 10 second after completion|finish of rolling in a hot rolling process (namely, after completion of the last rolling in a hot rolling process). Thereby, it is easy to adjust the ferrite fraction of the finally obtained electric resistance welded pipe to 80% or less.

In the cooling step, it is preferable to cool the hot rolled steel sheet obtained in the hot rolling step at a cooling rate of 5°C/s to 80°C/s.

When the cooling rate is 5°C/s or more, toughness deterioration of the base material portion is further suppressed. This reason is considered to be because the generation of coarse ferrite is suppressed when the cooling rate in the cooling step is 5°C/s or more.

When the cooling rate is 80°C/s or less, deterioration in toughness of the base material portion is suppressed. The reason for this is considered to be that the second phase fraction being excessive (that is, the ferrite fraction being less than 40%) is suppressed when the cooling rate in the cooling step is 80° C./s or less.

In the coiling step, it is preferable to wind the hot rolled steel sheet cooled in the cooling step at a coiling temperature of 450 to 650°C.

When the coiling temperature is 450°C or higher, toughness deterioration of the base material portion is suppressed. This reason is considered to be because the formation of martensite is suppressed when the coiling temperature is 450°C or higher.

When the coiling temperature is 650°C or lower, the rise of YR can be suppressed. It is considered that this reason is that when the coiling temperature is 650°C or lower, excessive production of Nb carbonitride is suppressed, and as a result, an increase in YS is suppressed.

Example

Hereinafter, examples of the present disclosure are shown, but the present disclosure is not limited to the following examples.

[Examples 1 to 26, Comparative Examples 1 to 31]

<Production of hot coil>

Steel pieces having chemical compositions shown in Tables 1 and 2 were prepared.

The steel piece of Comparative Example 28 (S: 0.0015%) was manufactured under normal conditions.

In the process of manufacturing the steel slabs of Examples 1 to 26 and Comparative Examples 1 to 27 and 29 to 31, the steel slag was used by using a technique of optimizing the composition of slag used during refining and a technique of exchanging slag during refining. The amount of S in the control was controlled to be 0.0010% or less.

The hot-rolled steel sheet is heated to 1250° C., the hot-rolled steel sheet is hot rolled to obtain a hot rolled steel sheet, and the obtained hot rolled steel sheet is cooled to a cooling rate of 50° C./s to wind the cooled hot rolled steel sheet at a winding temperature of 550° C. A hot coil made of a steel sheet was obtained.

Here, the time from the end of the final rolling in the hot rolling to the start of cooling was set as the time shown in Table 3.

In each of Examples and Comparative Examples, the remainder excluding elements shown in Tables 1 and 2 are Fe and impurities.

In Table 2, REM in Examples 18 and 19 is Ce, REM in Examples 23 and 24 is Nd, and REM in Example 25 is La.

In Tables 1 to 3, underlined values are values outside the scope of the present disclosure.

<Production of as roll electrode steel pipe>

The hot rolled steel sheet was unwound from the hot coil, and the rolled hot rolled steel sheet was roll-formed to form an open tube, and the abutted portion of the obtained open tube was electro-welded to form a weld, and then the seam was subjected to seam heat treatment to obtain an as roll electro-rolled steel pipe. .

<Production of electric wires (sizer and tempering)>

The shape of the as rolled electric resistance steel pipe was adjusted with a sizer under conditions such that the amount of change in the size of the sizer roundness shown in Table 3 (%).

The as-rolled steel pipe after shape adjustment was obtained by performing tempering with the tempering temperature and the tempering time shown in Table 3 to obtain a welded steel pipe.

The outer diameter of the obtained electric resistance steel pipe was 219 mm, and the thickness of this electric resistance steel pipe was 15.9 mm.

In addition, the above manufacturing process does not affect the chemical composition of steel. Therefore, it can be considered that the chemical composition of the base material part of the obtained electric resistance steel pipe is the same as the chemical composition of the steel piece as a raw material.

<Measurement>

The following measurement was performed about the obtained electric resistance steel pipe.

Table 3 shows the results.

(Measurement of ferrite fraction and organization of second phase)

The ferrite fraction was measured by the method described above, and the type of the second phase was confirmed.

In Table 3, TB means tempering bainite and P means pearlite.

(Measurement of YS, TS and YR)

From the 90-degree position of the base steel pipe, the test piece for tensile testing was taken in a direction in which the test direction (tensile direction) of the tensile test is the tube axis direction (hereinafter also referred to as the "L direction") of the front-barged steel pipe. Here, the shape of the test piece was a flat plate conforming to the American Petroleum Institute standard API 5L (hereinafter simply referred to as "API 5L").

Using the collected test piece, at room temperature, in accordance with API 5L, a tensile test is performed in which the test direction is in the L direction of the welded steel pipe, and TS in the L direction of the welded steel pipe and YS in the L direction of the welded steel pipe are respectively measured. Did.

In addition, YR (%) in the L direction of the electric resistance welded pipe was determined by the calculation formula “(YS/TS)×100”.

(Measurement of vE(J) (Charpy absorption energy at 0°C) of the base material part)

A full-size test piece (test piece for Charpy impact test) with a V notch was taken from the base material 90° position of the electrode steel pipe. A full-size test piece having a V notch was taken so that the test direction was in the pipe circumferential direction (C direction). The full-size test piece having the V-notch collected was subjected to a Charpy impact test according to API 5L under a temperature condition of 0°C, and vE(J) was measured.

The above measurement was performed 5 times per 1 electric resistance steel pipe, and the average value of the 5 measurement values was made into the vE(J) of the base material part of the electric resistance steel pipe.

(Measurement of vE(J) (Charpy absorption energy at 0°C) of the weld)

The operation similar to the measurement of vE(J) of a base material part was performed except having changed the position which collected the full-size test piece which has a V notch into the weld part of the electric resistance welded pipe.

(Measurement of specific precipitate area ratio)

By the above-described method, a specific precipitate area ratio (i.e., the area ratio of precipitates having a circle equivalent diameter of 100 nm or less; in Table 3, simply referred to as "precipit area ratio (%)") was measured.

(CLR(%) of HIC test; Sour resistance)

The HIC test was conducted in accordance with NACE-TM0284.

A test solution obtained by saturating 100% of H ₂ S gas in Solution A solution (5 mass% NaCl + 0.5 mass% glacial acetic acid aqueous solution) by collecting the entire thickness test piece for the HIC test from the base material 90° position of the electrode steel pipe and collecting the collected full thickness test piece. In the meantime, it was immersed for 96 hours. The presence or absence of HIC was measured by an ultrasonic flaw detector on the test piece after immersion for 96 hours. Based on the measurement results, CLR (%) was determined by the following equation.

The smaller the CLR, the better the sour resistance.

CLR (%) = (total length of crack/length of test piece) x 100 (%)

As shown in Table 1 to Table 3, the fact that the electric resistance welded steel pipe of each example has excellent sour resistance, has a certain degree of tensile strength and yield strength, reduces the yield ratio, and is excellent in the toughness of the base material and the weld. Can be seen.

For each example, the results of each comparative example were as follows.

In Comparative Example 1 in which the amount of C exceeded the upper limit, sour resistance was lowered.

In Comparative Example 2 in which the amount of C was lower than the lower limit, YR was increased. The reason for this is considered to be that the work hardenability of the steel has deteriorated.

In Comparative Example 3 in which the amount of Si exceeded the upper limit, the toughness of the welded portion decreased.

In Comparative Example 4 in which the amount of Si was lower than the lower limit, the toughness of the base material portion and the weld portion decreased. This reason is considered to be because deoxidation was insufficient and coarse oxide was generated.

In Comparative Example 5 in which the amount of Mn was lower than the lower limit, the toughness of the base material portion and the weld portion decreased. The reason for this is considered to be that embrittlement of the S group has occurred.

In Comparative Example 6 in which the amount of Mn exceeded the upper limit, the toughness of the base material portion and the weld portion decreased, and sour resistance decreased. The reason for this is considered to be that cracks due to MnS have occurred.

In Comparative Example 7 in which the amount of Ti was lower than the lower limit, the toughness of the base material portion decreased. This reason is considered to be because the crystal grains became coarse.

In Comparative Example 8 in which the amount of Ti exceeded the upper limit, the toughness of the base material portion and the weld portion decreased. It is thought that this reason is because coarse TiN was produced.

In Comparative Example 9 in which Nb was lower than the lower limit, the toughness of the base material portion was lowered. This reason is considered to be because the unrecrystallized rolling is insufficient.

In Comparative Example 10 in which Nb exceeded the upper limit, the toughness of the base material portion and the weld portion decreased. This reason is considered to be because coarse Nb carbonitride was produced.

In Comparative Example 11 in which Al was lower than the lower limit, the toughness of the base material portion and the weld portion decreased. This reason is considered to be because deoxidation became insufficient.

In Comparative Example 12 in which Al exceeded the upper limit, the toughness of the base material portion and the weld portion decreased. It is thought that this reason is because Al-type inclusions were produced in large quantities.

In Comparative Example 13 in which CNeq exceeded the upper limit, YS exceeded the upper limit.

In Comparative Example 14, in which CNeq was lower than the lower limit, TS was lower than the lower limit.

In Comparative Example 15 in which LR was less than 0.210, YR exceeded the upper limit.

In Comparative Example 16, TS was lower than the lower limit, and YR exceeded the upper limit. The reason for this is considered to be that the tempering temperature is too low, so that the effect of alleviating the deformation of the tubular tube by tempering (that is, the effect of reducing the dislocation density) is insufficient, and the precipitation over the dislocation is insufficient.

In Comparative Example 17, the toughness of the base material portion was lowered (that is, vE of the base material portion was lower than the lower limit). This reason is considered to be because the tempering temperature was too high, transformation to austenite occurred, the metal structure became coarse, and the toughness of the base material portion was lowered.

In Comparative Examples 18 to 21, YR exceeded the upper limit. The reason for this is considered to be that, since the amount of change in the roundness by the sizer is small, sufficient dislocation is not introduced and dislocation phase precipitation does not occur.

In Comparative Example 22 in which the amount of N was lower than the lower limit, the toughness of the base material portion and the weld portion was lowered. This reason is considered to be because the crystal grains became coarse.

In Comparative Example 23 in which the amount of N exceeded the upper limit, the toughness of the base material portion and the weld portion decreased. This reason is considered to be because the amount of nitride produced increased.

In Comparative Example 24, in which the Mn/Si ratio was lower than the lower limit, the toughness of the weld portion was lowered.

In Comparative Example 25 in which the ferrite fraction exceeded the upper limit, sour resistance was lowered.

In Comparative Example 26, YR exceeded the upper limit. The reason for this is thought to be that the effect of alleviating the deformation of the tubular tube by tempering (that is, the effect of reducing the dislocation density) is insufficient because the tempering time is short, and the precipitation over the dislocation is insufficient.

In Comparative Example 27 in which CNeq exceeded the upper limit, both YS and TS exceeded the upper limit.

In Comparative Example 28 in which the amount of S exceeded the upper limit, sour resistance decreased.

In Comparative Examples 29 to 31 in which LR was less than 0.210, YR exceeded the upper limit.

As for the indication of the Japan patent application 2016-134289, the whole is taken in into this specification by reference.

All documents, patent applications, and technical specifications described in this specification are incorporated by reference in the present specification to the same extent that individual documents, patent applications, and technical specifications are incorporated by reference, and to the same extent as described individually. .

Claims (7)

It includes a base material part and an electric welding part,
The chemical composition of the base material portion, in mass%,
C: 0.030% or more and less than 0.080%,
Mn: 0.30 to 1.00%,
Ti: 0.005 to 0.050%,
Nb: 0.010 to 0.100%,
N: 0.001 to 0.020%,
Si: 0.010 to 0.450%,
Al: 0.0010 to 0.1000%,
P: 0 to 0.030%,
S: 0 to 0.0010%,
Mo: 0 to 0.50%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Cr: 0 to 1.00%,
V: 0 to 0.100%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
REM: 0 to 0.0100%, and
Residue: made of Fe and impurities,
CNeq represented by the following formula (1) is 0.190 to 0.320,
The ratio of the mass% of Mn to the mass% of Si is 2.0 or more,
LR represented by the following formula (2) is 0.210 or more,
When the metal structure of the base material is observed at a magnification of 1000 times using a scanning electron microscope, the area ratio of the first phase made of ferrite is 40 to 80%, and the second phase, which is the remainder, includes tempering bainite.
Yield strength in the axial direction is 390 to 562 MPa,
The tensile strength in the tube axis direction is 520 to 690 MPa,
The yield ratio in the tube axis direction is 93% or less,
The Charpy absorbed energy in the circumferential direction of the base material portion is 100 J or more at 0°C,
The electric resistance steel pipe for line pipes in which the Charpy absorbed energy in the pipe circumferential direction in the electric resistance welding portion is 80 J or higher at 0°C.
CNeq=C+Mn/6+Cr/5+(Ni+Cu)/15+Nb+Mo+V… Equation (1)
LR=(2.1×C+Nb)/Mn… Equation (2)
[In Formulas (1) and (2), C, Mn, Cr, Ni, Cu, Nb, Mo, and V each represent the mass% of each element] According to claim 1,
The chemical composition of the base material portion, in mass%,
Mo: more than 0% and 0.50% or less,
Cu: more than 0% and less than 1.00%,
Ni: more than 0% and 1.00% or less,
Cr: more than 0% and less than 1.00%,
V: more than 0% and 0.100% or less,
Ca: more than 0% and 0.0100% or less,
Mg: more than 0% and 0.0100% or less, and
REM: more than 0% and less than 0.0100%
Steel pipe for line pipes, containing one or two or more of the above. The method according to claim 1 or 2,
When the metal structure of the base part is observed at a magnification of 100,000 times using a transmission electron microscope, the area ratio of precipitates having a circle equivalent diameter of 100 nm or less is 0.100 to 1.000%. The method according to claim 1 or 2,
The electric resistance welded steel pipe for line pipes in which the content of Nb in the chemical composition of the base material portion is 0.020% or more in mass%. According to claim 3,
The electric resistance welded steel pipe for line pipes in which the content of Nb in the chemical composition of the base material portion is 0.020% or more in mass%. The method according to claim 1 or 2,
An electric resistance steel pipe for line pipes having a thickness of 10 to 25 mm and an outer diameter of 114.3 to 609.6 mm. The method according to claim 1 or 2,
When a hydrogen-organic cracking test is performed on a test piece taken from the base material section, the CLR, which is a percentage of the total length of the cracks relative to the test piece length, is 8% or less.

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