ELECTRIC RESISTANCE WELDED STEEL TUBE FOR LINE PIPE

Abstract

 An electric resistance welded steel pipe for a line pipe, in which a base metal portion includes, in terms of % by mass, 0.080 to 0.120% of C, 0.30 to 1.00% of Mn, 0.005 to 0.050% of Ti, 0.010 to 0.100% of Nb, 0.001 to 0.020% of N, 0.010 to 0.450% of Si, and 0.001 to 0.100% of Al, and the balance includes Fe and impurities, and wherein CMeq, expressed by Formula (1), is 0.170 to 0.300, a Mn/Si ratio is 2.0 or more, LR, expressed by Formula (2), is 0.210 or more, in a case in which a metallographic microstructure of the base metal portion is observed using a scanning electron microscope at a magnification of 1,000 times, an areal ratio of a first phase that is ferrite is from 60 to 98%, and a second phase, which is the balance, includes tempered bainite;

Description

Technical Field

[0001] The present disclosure relates to an electric resistance welded steel pipe for a line pipe.

Background Art

[0002] In recent years, a line pipe which is one of types of means of primarily transporting crude oil or natural gas has increased in importance.

[0003] Electric resistance welded steel pipes used as line pipes (i.e., electric resistance welded steel pipes for line pipes) may be required to have lower yield ratios in the pipe axis directions of the electric resistance welded steel pipes.

[0004] For example, Patent Document 1 discloses a technology in which the yield ratio of an obtained electric resistance welded steel pipe in a pipe axis direction is decreased by repeatedly applying a strain to an uncoiled steel sheet which is a raw material, for example, by bending-unbending processing, before pipe-making forming, thereby inducing a Bauschinger effect.

[0005] Patent Document 2 proposes a technology in which the yield ratio of an electric resistance welded steel pipe in a pipe axis direction is decreased by allowing the metallographic microstructure of a hot-rolled steel sheet for producing an electric resistance welded steel pipe to be a microstructure consisting of martensite having an areal ratio of from 1 to 20% and ferrite.

[0006] Patent Document 3 proposes a method of producing an electric resistance welded steel pipe using a slab in which the amount of Nb is from 0.003% to less than 0.02%, as a method of producing an electric resistance welded steel pipe in which a rise in yield ratio due to heating is suppressed, and a deformation property is improved, and which has excellent strain aging resistance. In the paragraph 0019 of Patent Document 3, "In a conventional electric resistance welded steel pipe with a large amount of Nb, a work strain introduced in pipe-making causes the precipitation of Nb carbide to proceed, thereby increasing a yield strength and a tensile strength. Such precipitation strengthening was found to particularly cause a yield strength to be greatly increased, thereby resulting in an increase in yield ratio." is described.

[0007] 

Patent Document 1: Japanese Patent No. 4466320

Patent Document 2: Japanese Patent Application Laid-Open (JP-A) No. H10-176239

Patent Document 3: International Publication No. WO 2012/133558

SUMMARY OF INVENTION

Technical Problem

[0008] In the technology of Patent Document 1, however, a step of applying a strain to an uncoiled steel sheet is needed, and therefore, the number of steps is increased, thereby resulting in the possibility of increasing the cost of producing an electric resistance welded steel pipe.

[0009] In the technology of Patent Document 2, further improvement in the toughness of the base metal portion of an electric resistance welded steel pipe may be required.

[0010] In the technology of Patent Document 3, a decrease in YR of an electric resistance welded steel pipe by a method other than a method of reducing the amount of Nb may be required.

[0011] An object of the present disclosure is to provide an electric resistance welded steel pipe for a line pipe, which has a certain amount of tensile strength and yield strength, which has a decreased yield ratio, and a base metal portion and an electric resistance welded portion thereof having excellent toughness.

Solution to Problem

[0012] Means for solving the problem described above includes the following aspects.

<1> An electric resistance welded steel pipe for a line pipe, 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:

0.080 to 0.120% of C,

0.30 to 1.00% of Mn,

0.005 to 0.050% of Ti,

0.010 to 0.100% of Nb,

0.001 to 0.020% of N,

0.010 to 0.450% of Si,

0.001 to 0.100% of Al,

0 to 0.030% of P,

0 to 0.0100% of S,

0 to 0.50% of Mo,

0 to 1.00% of Cu,

0 to 1.00% of Ni,

0 to 1.00% of Cr,

0 to 0.100% of V,

0 to 0.0100% of Ca,

0 to 0.0100% of Mg,

0 to 0.0100% of REM, and

the balance being Fe and impurities, wherein:

CMeq, expressed by the following Formula (1), is from 0.170 to 0.300,

a ratio of % by mass of Mn to % by mass of Si is 2.0 or more,

LR, expressed by the following Formula (2), is 0.210 or more,

in a case in which a metallographic microstructure of the base metal portion is observed using a scanning electron microscope at a magnification of 1,000 times, an areal ratio of a first phase that is ferrite is from 60 to 98%, and a second phase, which is the balance, comprises tempered bainite,

a yield strength in a pipe axis direction is from 390 to 562 MPa,

a tensile strength in the pipe axis direction is from 520 to 690 MPa,

a yield ratio in the pipe axis direction is 90% or less,

a Charpy absorbed energy in a circumferential direction of the pipe in the base metal portion is 100 J or more at 0°C, and

a Charpy absorbed energy in the circumferential direction of the pipe in the electric resistance welded portion is 80 J or more at 0°C;



wherein, in Formula (1) and Formula (2), C, Mn, Cr, Ni, Cu, Nb, Mo, and V represent % by mass of respective elements.

<2> The electric resistance welded steel pipe for a line pipe according to <1>,
wherein the chemical composition of the base metal portion comprises, in terms of % by mass, one or more of:

more than 0% but equal to or less than 0.50% of Mo,

more than 0% but equal to or less than 1.00% of Cu,

more than 0% but equal to or less than 1.00% of Ni,

more than 0% but equal to or less than 1.00% of Cr,

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

more than 0% but equal to or less than 0.0100% of Ca,

more than 0% but equal to or less than 0.0100% of Mg, or

more than 0% but equal to or less than 0.0100% of REM.

<3> The electric resistance welded steel pipe for a line pipe according to <1> or <2>, wherein an areal ratio of a precipitate having an equivalent circle diameter of 100 nm or less is from 0.10 to 1.00% in a case in which the metallographic microstructure of the base metal portion is observed using a transmission electron microscope at a magnification of 100,000 times.

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

<5> The electric resistance welded steel pipe for a line pipe according to any one of <1> to <4>, wherein the electric resistance welded steel pipe for a line pipe has a wall thickness of from 10 to 25 mm and an outer diameter of from 114.3 to 609.6 mm.

Advantageous Effects of Invention

[0013] According to the present disclosure, an electric resistance welded steel pipe for a line pipe, which has a certain amount of tensile strength and yield strength, which has a decreased yield ratio, and a base metal portion and an electric resistance welded portion thereof having excellent toughness can be provided.

BRIEF DESCRIPTION OF DRAWINGS

[0014] Fig. 1 is a scanning electron micrograph showing an example of the metallographic microstructure of a base metal portion in the present disclosure.

DESCRIPTION OF EMBODIMENTS

[0015] 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.

[0016] The content of a component (element) expressed by "%" herein means "% by mass".

[0017] The content of C (carbon) may be herein occasionally expressed as "C content". The content of another element may be expressed similarly.

[0018] 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.

[0019] An electric resistance welded steel pipe for a line pipe of the present disclosure (hereinafter also simply referred to as "electric resistance welded steel pipe") includes a base metal portion and an electric resistance welded portion, wherein the chemical composition of the base metal portion consists of, in terms of % by mass: 0.080 to 0.120% of C, 0.30 to 1.00% of Mn, 0.005 to 0.050% of Ti, 0.010 to 0.100% of Nb, 0.001 to 0.020% of N, 0.010 to 0.450% of Si, 0.001 to 0.100% of Al, 0 to 0.030% of P, 0 to 0.0100% of S, 0 to 0.50% of Mo, 0 to 1.00% of Cu, 0 to 1.00% of Ni, 0 to 1.00% of Cr, 0 to 0.100% of V, 0 to 0.0100% of Ca, 0 to 0.0100% of Mg, 0 to 0.0100% of REM, and the balance being Fe and impurities, wherein: CMeq, expressed by the following Formula (1), is from 0.170 to 0.300, a ratio of % by mass of Mn to % by mass of Si (hereinafter also referred to as "Mn/Si ratio") is 2.0 or more, LR, expressed by the following Formula (2), is 0.210 or more, in a case in which the metallographic microstructure of the base metal portion is observed using a scanning electron microscope at a magnification of 1,000 times, an areal ratio of a first phase that is ferrite (hereinafter also referred to as "ferrite fraction") is from 60 to 98%, and a second phase, which is the balance, includes tempered bainite, a yield strength in a pipe axis direction (hereinafter also referred to as "YS") is from 390 to 562 MPa, a tensile strength in the pipe axis direction (hereinafter also referred to as "TS") is from 520 to 690 MPa, a yield ratio in the pipe axis direction (hereinafter also referred to as "YR") is 90% or less, a Charpy absorbed energy in a circumferential direction of the pipe in the base metal portion is 100 J or more at 0°C, and a Charpy absorbed energy in the circumferential direction of the pipe in the electric resistance welded portion is 80 J or more at 0°C;



wherein, Formula (1) and Formula (2), C, Mn, Cr, Ni, Cu, Nb, Mo, and V represent % by mass of respective elements.

[0020] The electric resistance welded steel pipe of the present disclosure includes the base metal portion and the electric resistance welded portion.

[0021] Commonly, an electric resistance welded steel pipe is produced by forming a hot-rolled steel sheet into a pipe shape (hereinafter also referred to as "roll forming") to thereby make an open pipe, subjecting abutting portions of the obtained open pipe to electric resistance welding to form an electric resistance welded portion, and then, if necessary, subjecting the electric resistance welded portion to seam heat treatment.

[0022] In the electric resistance welded steel pipe of the present disclosure, the base metal portion refers to a portion other than the electric resistance welded portion and a heat affected zone.

[0023] The heat affected zone (hereinafter also referred to as "HAZ") refers to a portion affected by heat caused by electric resistance welding (affected by heat caused by the electric resistance welding and seam heat treatment in a case in which the seam heat treatment is performed after the electric resistance welding).

[0024] Herein, the electric resistance welded portion may be simply referred to as "welded portion".

[0025] The electric resistance welded steel pipe of the present disclosure has a certain amount of YS and TS (i.e., YS and TS in the ranges described above), has YR decreased to 90% or less, and has the excellent toughness of the base metal portion and the electric resistance welded portion.

[0026] In the present disclosure, the excellent toughness means that a Charpy absorbed energy (J) in the circumferential direction of the pipe at 0°C (hereinafter also referred to as "vE") is high.

[0027] Specifically, the electric resistance welded steel pipe of the present disclosure has a vE of 100 J or more in the base metal portion and a vE of 80 J or more in the electric resistance welded portion.

[0028] The electric resistance welded steel pipe of the present disclosure has low YR, and is therefore expected to exhibit the effect of being capable of suppressing the buckling of the electric resistance welded steel pipe.

[0029] Examples of a case in which the suppression of the buckling of a steel pipe is demanded include a case in which a steel pipe for a subsea pipeline is laid by reel-lay. In the reel-lay, the steel pipe is produced on land in advance, and the produced steel pipe is spooled on the spool of a barge. The spooled steel pipe is laid on a sea bottom while being unspooled at sea. In the reel-lay, plastic bending is applied to the steel pipe at the time of the spooling or unspooling of the steel pipe, and therefore, the steel pipe may be buckled. The occurrence of the buckling of the steel pipe unavoidably results in the stopping of a laying operation, and the damage caused by the stopping is enormous.

[0030] The buckling of the steel pipe can be suppressed by reducing the YR of the steel pipe.

[0031] Accordingly, the electric resistance welded steel pipe of the present disclosure is expected to exhibit the effect of being capable of suppressing buckling at the time of reel-lay, for example, in the case of being used as an electric resistance welded steel pipe for a subsea pipeline.

[0032] Further, the electric resistance welded steel pipe of the present disclosure has the excellent toughness of the base metal portion and the electric resistance welded portion, and is therefore expected to exhibit the effect of having the excellent property of arresting crack propagation at the time of burst in the case of being used as an electric resistance welded steel pipe for a line pipe.

[0033] YS, TS, YR, the vE of the base metal portion, and the vE of the electric resistance welded portion as described above are achieved by a combination of the chemical composition (including CMeq, a Mn/Si ratio, and LR) and the metallographic microstructure in the electric resistance welded steel pipe.

[Chemical Composition of Base Metal Portion]

[0034] With regard to the chemical composition of the base metal portion, each component in the chemical composition will be first described below, and CMeq, a Mn/Si ratio, and LR will be subsequently described.

C: 0.080 to 0.120%

[0035] C is an element necessary for forming a second phase.

[0036] A C amount of 0.080% or more facilitates achievement of a ferrite fraction of 98% or less and achievement of an LR of 0.210 or more. As a result, achievement of a YR of 90% or less is facilitated. Accordingly, the amount of C is 0.080% or more. The amount of C is preferably 0.085% or more.

[0037] In contrast, a C amount of 0.120% or less facilitates achievement of a ferrite fraction of 60% or more. As a result, achievement of a YR of 90% or less is facilitated. Accordingly, the amount of C in the present disclosure is 0.120% or less. The amount of C is preferably 0.115% or less, and more preferably 0.110% or less.

Mn: 0.30 to 1.00%

[0038] Mn is an element that enhances the hardenability of steel. In addition, Mn is an essential element for detoxification of S.

[0039] A Mn amount of less than 0.30% may result in embrittlement due to S and in the deterioration of the toughness of the base metal and the electric resistance welded portion. Accordingly, the amount of Mn is 0.30% or more. The amount of Mn is preferably 0.40% or more, and more preferably 0.50% or more.

[0040] In contrast, a Mn amount of more than 1.00% may result in generation of coarse MnS in the central portion of the sheet thickness, thereby degrading the toughness of the base metal and the electric resistance welded portion. In addition, a Mn amount of more than 1.00% may make it impossible to achieve an LR of 0.210 or more, thereby consequently making it impossible to achieve a YR of 90% or less. Accordingly, the amount of Mn is 1.00% or less. The amount of Mn is preferably 0.90% or less, and more preferably 0.85% or less.

Ti: 0.005 to 0.050%

[0041] Ti is an element forming a carbonitride and contributing to crystal grain refining.

[0042] The amount of Ti is 0.005% or more from the viewpoint of securing the toughness of the base metal and the electric resistance welded portion.

[0043] In contrast, a Ti amount of more than 0.050% may result in generation of coarse TiN, thereby deteriorating the toughness of the base metal and the electric resistance welded portion. Accordingly, the amount of Ti is 0.050% or less. The amount of Ti is preferably 0.040% or less, still more preferably 0.030 or less, and particularly preferably 0.025%.

Nb: 0.010 to 0.100%

[0044] Nb is an element contributing to improvement in toughness and a reduction in YR.

[0045] The amount of Nb is 0.010% or more for improvement in toughness due to rolling in the region of unrecrystallization temperature.

[0046] The amount of Nb is preferably 0.020% or more.

[0047] In contrast, a Nb amount of more than 0.100% results in the deterioration of toughness due to a coarse carbide. Therefore, the amount of Nb is 0.100% or less. The amount of Nb is preferably 0.090% or less.

N: 0.001 to 0.020%

[0048] N is an element that forms a nitride, thereby suppressing the coarsening of crystal grains and consequently improving the toughness of the base metal portion and the electric resistance welded portion. From the viewpoint of such an effect, the amount of N is 0.001% or more. The amount of N is preferably 0.003% or more.

[0049] In contrast, a N amount of more than 0.020% results in an increase in the amount of generated alloy carbide, thereby deteriorating the toughness of the base metal portion and the electric resistance welded portion. Accordingly, the amount of N is 0.020% or less. The amount of N is preferably 0.015% or less, more preferably 0.010% or less, and particularly preferably 0.008% or less.

Si: 0.010 to 0.450%

[0050] Si is an element that functions as a deoxidizer for steel. More specifically, a Si amount of 0.010% or more results in suppression of generation of a coarse oxide in the base metal portion and the electric resistance welded portion, thereby resulting in improvement in the toughness of the base metal portion and the electric resistance welded portion. Accordingly, the amount of Si is 0.010% or more. The amount of Si is preferably 0.015% or more, and more preferably 0.020% or more.

[0051] In contrast, a Si amount of more than 0.450% may result in generation of an inclusion in the electric resistance welded portion, thereby decreasing a Charpy absorbed energy and deteriorating toughness. Accordingly, the amount of Si is 0.450% or less. The amount of Si 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%

[0052] Al is an element that functions as a deoxidizer, similar to Si. More specifically, an Al amount of 0.001% or more results in suppression of generation of a coarse oxide in the base metal portion and the electric resistance welded portion, thereby resulting in improvement in the toughness of the base metal portion and the electric resistance welded portion. Accordingly, the amount of Al is 0.001% or more. The amount of Al is preferably 0.010% or more, and more preferably 0.015% or more.

[0053] In contrast, an Al amount of more than 0.100% may result in generation of an Al-based oxide during electric resistance welding, thereby deteriorating the toughness of a welded portion. Accordingly, the amount of Al is 0.100% or less. The amount of Al is preferably 0.090% or less.

P: 0 to 0.030%

[0054] P is an impurity element. A P amount of more than 0.030% may result in segregation in a grain boundary, thereby degrading toughness. Accordingly, the amount of P is 0.030% or less. The amount of P is preferably 0.025% or less, more preferably 0.020% or less, still more preferably 0.015% or less, and more preferably 0.010% or less.

[0055] The amount of P may be 0%. From the viewpoint of reducing a dephosphorization cost, the amount of P may be more than 0%, and may be 0.001% or more.

S: 0 to 0.0100%

[0056] S is an impurity element. A S amount of more than 0.0100% may result in degradation in toughness. Accordingly, the amount of S is 0.0100% or less. The amount of S is preferably 0.0070 or less, more preferably 0.0050% or less, and more preferably 0.0030% or less.

[0057] The amount of S may be 0%. From the viewpoint of reducing a desulfurization cost, the amount of S may be more than 0%, may be 0.0001% or more, and may be 0.0003% or more.

Mo: 0 to 0.50%

[0058] Mo is an optional element. Accordingly, the amount of Mo may be 0%.

[0059] Mo is an element improving the hardenability of a steel and contributing to the high strength of the steel. From the viewpoint of such an effect, the amount of Mo may be more than 0%, may be 0.01% or more, and may be 0.03% or more.

[0060] In contrast, a Mo amount of more than 0.50% may result in generation of a Mo carbonitride, thereby deteriorating toughness. Accordingly, the amount of Mo is 0.50% or less. The amount of Mo is preferably 0.40% or less, more preferably 0.30% or less, still more preferably 0.20% or less, and particularly preferably 0.10% or less.

Cu: 0 to 1.00%

[0061] Cu is an optional element. Accordingly, the amount of Cu may be 0%.

[0062] Cu is an element that is effective for improving the strength of a base metal. From the viewpoint of such an effect, the amount of Cu may be more than 0%, may be 0.01% or more, and may be 0.03% or more.

[0063] In contrast, a Cu amount of more than 1.00% may result in generation of fine Cu grains, thereby considerably deteriorating toughness. Accordingly, 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%

[0064] Ni is an optional element. Accordingly, the amount of Ni may be 0%.

[0065] Ni is an element that contributes to improvement in strength and toughness. From the viewpoint of such an effect, the amount of Ni may be more than 0%, may be 0.01% or more, and may be 0.05% or more.

[0066] In contrast, a Ni amount of more than 1.00% may result in excessively high strength. Accordingly, the amount of Ni 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%

[0067] Cr is an optional element. Accordingly, the amount of Cr may be 0%.

[0068] Cr is an element that improves hardenability. From the viewpoint of such an effect, the amount of Cr may be more than 0%, may be 0.01% or more, and may be 0.05% or more.

[0069] In contrast, a Cr amount of more than 1.00% may result in the deterioration of the toughness of the welded portion due to Cr-based inclusions generated in the electric resistance welded portion. Accordingly, the amount of Cr is 1.00% or less. The amount of Cr 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%

[0070] V is an optional element. Accordingly, the amount of V may be 0%.

[0071] V is an element that contributes to improvement in toughness and a reduction in YR. From the viewpoint of such an effect, the amount of V may be more than 0%, may be 0.005% or more, and may be 0.010% or more.

[0072] In contrast, a V amount of more than 0.100% may result in the deterioration of toughness due to a V carbonitride. Accordingly, the amount of V is 0.100% or less. The amount of V is preferably 0.080% or less, more preferably 0.070% or less, still more preferably 0.050% or less, and particularly preferably 0.030% or less.

Ca: 0 to 0.0100%

[0073] Ca is an optional element. Accordingly, the amount of Ca may be 0%.

[0074] Ca is an element controlling a shape of a sulfide-based inclusion and improving low-temperature toughness. From the viewpoint of such an effect, the amount of Ca may be more than 0%, may be 0.0001% or more, may be 0.0010% or more, may be 0.0030% or more, and may be 0.0050% or more.

[0075] In contrast, a Ca amount of more than 0.0100% may result in generation of a large-sized cluster or large-sized inclusion including CaO-CaS, thereby adversely affecting toughness. Accordingly, the amount of Ca is 0.0100% or less. The amount of Ca is preferably 0.0090% or less, more preferably 0.0080% or less, and particularly preferably 0.0060% or less.

Mg: 0 to 0.0100%

[0076] Mg is an optional element. Accordingly, the amount of Mg may be 0%.

[0077] Mg is an element that is effective as a deoxidizer and a desulfurization agent and that particularly forms a fine oxide, thereby contributing to improvement in the toughness of an HAZ (heat affected zone). From the viewpoint of such an effect, the amount of Mg may be more than 0%, may be 0.0001% or more, may be 0.0010% or more, may be 0.0020% or more, may be 0.0030% or more, and may be 0.0050% or more.

[0078] In contrast, a Mg amount of more than 0.0100% is prone to cause an oxide to be aggregated or coarsened, thereby resulting in the deterioration of HIC resistance (hydrogen-induced cracking resistance) or the deterioration of the toughness of the base metal or the HAZ. Accordingly, the amount of Mg is 0.0100% or less. The amount of Mg is preferably 0.0060% or less.

REM: 0 to 0.0100%

[0079] REM is an optional element. Accordingly, the amount of REM may be 0%.

[0080] "REM" refers to a rare earth element, i.e., 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.

[0081] REM is an element effective as a deoxidizer or a desulfurization agent. From the viewpoint of such an effect, the amount of REM may be more than 0%, may be 0.0001% or more, and may be 0.0010% or more.

[0082] In contrast, an REM amount of more than 0.0100% may result in generation of a coarse oxide, thereby resulting in the deterioration of HIC resistance or in the deterioration of the toughness of a base metal or HAZ. Accordingly, the amount of REM is 0.0100% or less. The amount of REM is preferably 0.0070% or less, and more preferably 0.0050% or less.

[0083] From the viewpoint of obtaining the effects offered by the optional elements described above, the chemical composition of the base metal portion may contain one or more of: more than 0% but equal to or less than 0.50% of Mo, more than 0% but equal to or less than 1.00% of Cu, more than 0% but equal to or less than 1.00% of Ni, more than 0% but equal to or less than 1.00% of Cr, more than 0% but equal to or less than 0.100% of V, more than 0% but equal to or less than 0.0100% of Ca, more than 0% but equal to or less than 0.0100% of Mg, and more than 0% but equal to or less than 0.0100% of REM.

[0084] The more preferred amount of each optional element has been described above.

Balance: Fe and Impurities

[0085] In the chemical composition of the base metal portion, the balance excluding each element described above is Fe and impurities.

[0086] The impurities refer to components which are contained in a raw material or mixed into in a production step, and which are not intentionally incorporated into a steel.

[0087] Examples of the impurities include any elements other than the elements described above. Elements as the impurities may be only one kind, or may be two or more kinds.

[0088] Examples of the impurities include O, B, Sb, Sn, W, Co, As, Pb, Bi, and H.

[0089] Among the elements described above, O is preferably controlled to have a content of 0.006% or less.

[0090] For the other elements, typically, Sb, Sn, W, Co, or As may be included in a content of 0.1% or less, Pb or Bi may be included in a content of 0.005% or less, B may be included in a content of 0.0003% or less, H may be included in a content of 0.0004% or less, and the contents of the other elements need not particularly be controlled as long as being in a usual range.

CMeq: 0.170 to 0.300

[0091] In the chemical composition of the base metal portion, CMeq expressed by the following Formula (1) is from 0.170 to 0.300.

[0092] [in Formula (1), C, Mn, Cr, Ni, Cu, Nb, Mo, and V represent % by mass of the respective elements, respectively].

CMeq has a positive correlation with a yield strength.

[0093] CMeq is 0.170 or more from the viewpoint of facilitating achievement of a yield strength of 390 MPa or more. CMeq is preferably 0.180 or more, more preferably 0.200 or more, and still more preferably 0.230 or more.

[0094] In contrast, CMeq is 0.300 or less from the viewpoint of facilitating achievement of a yield strength of 562 MPa or less. CMeq is preferably 0.290 or less, and more preferably 0.275 or less.

LR: 0.210 or more

[0095] In the chemical composition of the base metal portion, LR expressed by the following Formula (2) is 0.210 or more.

[0096] In the electric resistance welded steel pipe of the present disclosure, an LR of 0.210 or more may result in achievement of a YR of 90% or less.

[0097] An LR of less than 0.210 may result in a YR of more than 90%. The reason thereof can be considered to be because the amount of precipitate in a steel is decreased, and work hardenability is deteriorated (i.e., TS is decreased).

[0098] [in Formula (2), C, Nb, and Mn represent % by mass of the respective elements, respectively].

The technological meaning of Formula (2) is as follows.

[0099] The reason why the amounts of C and Nb are arranged in the numerator in Formula (2) can be considered to be that C and Nb form precipitates, thereby improving the work hardenability of a steel (i.e., increasing TS) and consequently decreasing the YR of the steel.

[0100] The reason why the amount of C is multiplied by "2.1" can be considered to be because, regarding the effect of improving work hardenability due to the formation of a precipitate described above, the effect of the inclusion of C is about 2.1 times the effect of the inclusion of Nb.

[0101] The reason why the amount of Mn is arranged in the denominator in Formula (2) is because, although the inclusion of Mn enables a steel to be transformed at relatively low temperature, the inclusion of Mn causes the work hardenability in itself of the steel to be deteriorated (i.e., causes TS to be decreased), thereby increasing the YR of the steel.

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

[0102] In the electric resistance welded steel pipe of the present disclosure, even in a case in which the amount of Nb is relatively large, for example, more than the amount of Nb in Patent Document 3 (International Publication No. WO 2012/133558) (from 0.003% to less than 0.02%), LR may be allowed to be 0.210 or more depending on the amounts of C and Mn by allowing LR to satisfy 0.210 or more. In this case, a YR of 90% or less can be achieved.

[0103] In the electric resistance welded steel pipe of the present disclosure, a YR of 90% or less can also be achieved by allowing LR to be 0.210 or more and allowing conditions other than LR to be satisfied, in a case in which the amount of Nb is less than 0.02%.

[0104] From the viewpoint of further facilitating achievement of a YR of 90% or less, LR is preferably 0.230 or more, and more preferably 0.270 or more.

[0105] The upper limit of LR is not particularly restricted. From the viewpoint of the production suitability of the electric resistance welded steel pipe, LR is preferably 0.500 or less, more preferably 0.450 or less, and particularly preferably 0.400 or less.

Mn/Si Ratio: 2.0 or more

[0106] In the chemical composition of the base metal portion, a Mn/Si ratio (i.e., a Mn/Si ratio which is a ratio of % by mass of Mn to % by mass of Si) is 2.0 or more.

[0107] In the electric resistance welded steel pipe of the present disclosure, a Mn/Si ratio of 2.0 or more results in improvement in the toughness of the electric resistance welded portion, thereby allowing vE in the electric resistance welded portion (i.e., a Charpy absorbed energy in the circumferential direction of the pipe at 0°C) to be 80 J or more.

[0108] In a case in which the Mn/Si ratio is less than 2.0, vE may be less than 80 J. The reason thereof can be considered to be because in a case in which the Mn/Si ratio is less than 2.0, a MnSi-based inclusion initiates brittle fracture in the electric resistance welded portion, whereby toughness is deteriorated.

[0109] The Mn/Si ratio is preferably 2.1 or more from the viewpoint of further improving the toughness of the electric resistance welded portion.

[0110] The upper limit of the Mn/Si ratio is not particularly restricted. From the viewpoint of further improving the toughness of the electric resistance welded portion and the toughness of the base metal portion, the Mn/Si ratio is preferably 50 or less, more preferably 40 or less, and particularly preferably 30 or less.

[Metallographic Microstructure of Base Metal Portion]

[0111] In the electric resistance welded steel pipe of the present disclosure, the metallographic microstructure of the base metal portion has a ferrite fraction (i.e., an areal ratio of a first phase that is ferrite) of from 60 to 98% and includes a second phase, which is a balance, including at least either tempered bainite or pearlite in a case in which the metallographic microstructure is observed using a scanning electron microscope at a magnification of 1,000 times.

[0112] In the electric resistance welded steel pipe of the present disclosure, a YR of 90% or less can be achieved by allowing a ferrite fraction to be 60% or more. The ferrite fraction is preferably 65% or more, and more preferably 70% or more.

[0113] In the electric resistance welded steel pipe of the present disclosure, a ferrite fraction of 98% or less as described above enables a TS of 520 MPa or more to be achieved. The ferrite fraction is preferably 95% or less, and more preferably 92% or less.

[0114] In the electric resistance welded steel pipe of the present disclosure, the second phase which is the balance includes tempered bainite.

[0115] The inclusion of tempered bainite in the second phase means that the electric resistance welded steel pipe of the present disclosure is an electric resistance welded steel pipe tempered after pipe-making (i.e., after electric resistance welding (after seam heat treatment in the case of performing the seam heat treatment after the electric resistance welding)).

[0116] The electric resistance welded steel pipe of the present disclosure is an electric resistance welded steel pipe tempered after pipe-making, whereby a YR of 90% or less can be achieved. The reason thereof can be considered to be because YR is decreased by the tempering after the pipe-making. The reason why YR is decreased by the tempering after the pipe-making can be considered to be because YS is decreased by decreasing a dislocation density, and cementites are precipitated on a dislocation, thereby increasing work hardening (i.e., increasing TS).

[0117] Herein, tempered bainite is distinguished from bainite which is not tempered bainite, in view of including granular cementites in the structure of the tempered bainite.

[0118] The concept of "bainite" herein includes bainitic ferrite, granular bainite, upper bainite, and lower bainite.

[0119] The second phase may include tempered bainite, may be a phase consisting of tempered bainite, or may include a structure other than tempered bainite.

[0120] Examples of the structure other than tempered bainite include pearlite.

[0121] The concept of "pearlite" herein also includes pseudo-pearlite.

[0122] The measurement of the ferrite fraction and the identification of the second phase in the metallographic microstructure of the base metal portion are performed by nital-etching a metallographic microstructure at the 1/4 position of a wall thickness in an L cross-section at a base metal 90° position, and observing micrographs of the nital-etched metallographic microstructure (hereinafter also referred to as "metallographic micrographs") with a scanning electron microscope (SEM) at a magnification of 1,000 times. Metallographic micrographs corresponding to ten 1,000-times visual fields (corresponding to an actual cross-sectional area of 0.12 mm²) are taken. The measurement of the ferrite fraction and the identification of the second phase are performed by performing image processing of the metallographic micrographs that were taken. The image processing is performed using, for example, a small-sized general-purpose image analysis apparatus LUZEX AP manufactured by NIRECO CORPORATION.

[0123] Herein, " base metal 90° position" refers to a position deviating at 90° in the circumferential direction of the pipe from an electric resistance welded portion, "L cross-section" refers to a cross section parallel to a pipe axis direction and a wall thickness direction, and "1/4 position of wall thickness" refers to a position to which a distance from the outer surface of the electric resistance welded steel pipe is 1/4 of a wall thickness.

[0124] Herein, the pipe axis direction may be referred to as "L-direction".

[0125] Fig. 1 is a scanning electron micrograph (SEM micrograph; a magnification of 1,000 times) showing an example of the metallographic microstructure of a base metal portion in the present disclosure.

[0126] The SEM micrograph in Fig. 1 is one (one visual field) of SEM micrographs used in the measurement of a ferrite fraction and the identification of a second phase in Example 17 described later.

[0127] As shown in Fig. 1, a first phase that is ferrite and a second phase including tempered bainite can be confirmed. In particular, the presence of white points (cementites) reveals that the second phase includes tempered bainite.

[0128] The metallographic microstructure of the base metal portion preferably has an areal ratio (hereinafter also referred to as "specific precipitate areal ratio") of precipitates having an equivalent circle diameter of 100 nm or less (hereinafter also referred to as "specific precipitates") of from 0.10 to 1.00% in a case in which the metallographic microstructure is observed using a transmission electron microscope at a magnification of 100,000 times.

[0129] The specific precipitate areal ratio of 0.10% or more further facilitates achievement of a YR of 90% or less. The reason thereof can be considered to be because the specific precipitates (i.e., precipitates having an equivalent circle diameter of 100 nm or less) contribute to improvement in work hardening characteristic (i.e., an increase in TS), thereby resulting in a decrease in YR.

[0130] In contrast, the specific precipitate areal ratio of 1.00% or less results in suppression of brittle fracture (i.e., excellent toughness of the base metal portion). The specific precipitate areal ratio is preferably 0.80% or less, and more preferably 0.70% or less.

[0131] The specific precipitate areal ratio of from 0.10 to 1.00% can be achieved by performing tempering at a temperature of from 400°C to an Ac1 point after pipe-making.

[0132] In the present disclosure, the precipitate areal ratio (i.e., the areal ratio of precipitates having an equivalent circle diameter of 100 nm or less) is measured by observing a metallographic microstructure at a position of 1/4 of a wall thickness in an L cross-section at a base metal 90° position with a transmission electron microscope (TEM) at a magnification of 100,000 times.

[0133] More specifically, at first, on the basis of a sample taken from the position of 1/4 of the wall thickness in the L cross-section at the base metal 90° position, a replica for TEM observation is produced by SPEED method using an electrolytic solution including 10% by volume of acetylacetone, 1% by volume of tetramethylammonium chloride, and 89% by volume of methyl alcohol. Then, by observing the obtained replica for TEM observation with TEM at a magnification of 100,000 times, TEM images with a field size of 1 µm square, corresponding to ten visual fields, are obtained. The areal ratio of precipitates having an equivalent circle diameter of 100 nm or less with respect to the total area of the obtained TEM image is calculated, and the obtained result is regarded as the specific precipitate areal ratio (%).

[0134] The condition of etching in the SPEED method is set at a condition in which a charge of 10 coulombs is applied at a voltage of -200 mV with respect to a surface area of about 80 square millimeters with the use of a saturated calomel electrode as a reference electrode.

[0135] The specific precipitates (i.e., precipitates having an equivalent circle diameter of 100 nm or less) can be specifically considered to be at least one 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.

[0136] Conceivable examples of the metals other than Fe include Ti and Nb. In a case in which the chemical composition contains at least one of V, Mo, or Cr, conceivable examples of the metals other than Fe include at least one of V, Mo, or Cr.

[Yield Strength in Pipe Axis Direction (YS)]

[0137] The electric resistance welded steel pipe of the present disclosure has a yield strength in a pipe axis direction (YS) of from 390 to 562 MPa.

[0138] YS in the pipe axis direction is preferably 430 MPa or more, more preferably 450 MPa or more, more preferably 470 MPa or more, and particularly preferably 500 MPa or more.

[0139] YS in the pipe axis direction is preferably 550 MPa or less, more preferably 540 MPa or less, and particularly preferably 530 MPa or less.

[0140] A YS in the pipe axis direction of 562 MPa or less can be achieved by performing tempering after pipe-making. The reason thereof can be considered to be because the tempering after pipe-making results in a decrease in pipe-making strain, and a dislocation density.

[Tensile Strength in Pipe Axis Direction (TS)]

[0141] The electric resistance welded steel pipe of the present disclosure has a tensile strength in a pipe axis direction (TS) of from 520 to 690 MPa.

[0142] TS in the pipe axis direction is preferably 550 MPa or more, and more preferably 580 MPa or more.

[0143] TS in the pipe axis direction is preferably 680 MPa or less, more preferably 660 MPa or less, and particularly preferably 650 MPa or less.

[Yield Ratio in Pipe Axis Direction]

[0144] The electric resistance welded steel pipe of the present disclosure has a yield ratio in a pipe axis direction (YR = (YS/TS) × 100) of 90% or less.

[0145] As a result, the buckling of the electric resistance welded steel pipe in laying or the like is suppressed.

[0146] A YR in the pipe axis direction of 90% or less can be achieved by performing tempering after pipe-making. The reason thereof can be considered to be because YS is decreased by decreasing a dislocation density, and because work hardening is increased (i.e., TS is increased) by finely precipitating cementites on a dislocation.

[Wall Thickness of Electric Resistance Welded Steel Pipe]

[0147] The wall thickness of the electric resistance welded steel pipe of the present disclosure is preferably from 10 to 25 mm.

[0148] A wall thickness of 10 mm or more is advantageous in view of facilitating a decrease in YR by using a strain caused by forming a hot-rolled steel sheet into a pipe shape. The wall thickness is more preferably 12 mm or more.

[0149] A wall thickness of 25 mm or less is advantageous in view of the production suitability of the electric resistance welded steel pipe (specifically, formability in formation of a hot-rolled steel sheet into a pipe shape). The wall thickness is more preferably 20 mm or less.

[Outer Shape of Electric Resistance Welded Steel Pipe]

[0150] The outer diameter of the electric resistance welded steel pipe of the present disclosure is preferably from 114.3 to 609.6 mm (i.e., from 4.5 to 24 inches).

[0151] An outer diameter of 114.3 mm or more is more preferred as the electric resistance welded steel pipe for a line pipe. The outer diameter is preferably 139.7 mm (i.e., 5.5 inches) or more, and more preferably 177.8 mm (i.e., 7 inches) or more.

[0152] An outer diameter of 609.6 mm or less is advantageous in view of facilitating a decrease in YR by using a strain caused by forming a hot-rolled steel sheet into a pipe shape. The outer diameter is preferably 406.4 mm (i.e., 16 inches) or less, and more preferably 304.8 mm (i.e., 12 inches) or less.

[One Example of Production Method]

[0153] One example of a method of producing an electric resistance welded steel pipe of the present disclosure is the following production method A.

[0154] The production method A includes:

a step of producing an as-rolled electric resistance welded steel pipe by using a hot-rolled steel sheet having the chemical composition described above, and

a tempering step of obtaining an electric resistance welded steel pipe by tempering the as-rolled electric resistance welded steel pipe.

According to the production method A, the inclusion of the tempering step facilitates the production of an electric resistance welded steel pipe having a YR of 90% or less by the reasons described above.

[0155] A tempering temperature (i.e., a retention temperature in the tempering) is preferably from 400°C to an Ac1 point.

[0156] A tempering temperature of 400°C or more further facilitates precipitation of cementite and a specific precipitate (precipitate having an equivalent circle diameter of 100 nm or less), and therefore further facilitates achievement of a YR of 90% or less. The tempering temperature is more preferably 420°C or more.

[0157] A tempering temperature of an Ac1 point or less enables a precipitate to be further inhibited from being coarse. Although tempering temperature depends on the Ac1 point of a steel, it is also preferably 720°C or less, also preferably 710°C or less, and also preferably 700°C or less.

[0158] The Ac1 point means a temperature at which transformation to austenite is started in the case of increasing the temperature of a steel.

[0159] The Ac1 point is calculated by the following Formula:

[0160] [where C, Si, Mn, Ni, Cu, Cr, Mo, V, Ti, Nb, and Al represent % by mass of the respective elements, respectively. Ni, Cu, Cr, Mo, and V are optional elements. Among the optional elements, an element that is not contained in a slab is set at 0% by mass, and the Ac1 point is calculated.

[0161] A tempering time (i.e., a retention time at the tempering temperature) in the tempering step is preferably 5 minutes or more in view of facilitating a more decrease in YR.

[0162] In the production method A, the as-rolled electric resistance welded steel pipe refers to an electric resistance welded steel pipe which is produced by roll-forming (i.e., forming into a pipe shape) a hot-rolled steel sheet, and which is not subjected to heat treatment other than seam heat treatment after the roll-forming.

[0163] A preferred aspect of the step of producing the as-rolled electric resistance welded steel pipe in the production method A will be described later.

[0164] The production method A preferably includes a sizer step of adjusting the shape of the as-rolled electric resistance welded steel pipe by a sizer under a condition in which the change in ovality before and after adjustment (hereinafter also referred to as "change in ovality (%) by sizer step") is 1.0% or more, between the step of producing an as-rolled electric resistance welded steel pipe and the tempering step.

[0165] In a case in which the production method A includes the sizer step, the electric resistance welded steel pipe having the specific precipitate areal ratio of from 0.10 to 1.00% described above can be more easily produced.

[0166] The reason thereof can be considered to be because a dislocation of which the amount is equal to or more than a certain amount is introduced into the as-rolled electric resistance welded steel pipe by the sizer step under the condition in which the change in ovality by sizer step is 1.0% or more, and the as-rolled electric resistance welded steel pipe is then tempered at a temperature of from 400°C to an Ac1 point, thereby facilitating precipitation of fine specific precipitates on the dislocation.

[0167] The ovality of the as-rolled electric resistance welded steel pipe is determined as described below.

[0168] First, four measurement values are obtained by measuring the outer diameter of the as-rolled electric resistance welded steel pipe in the circumferential direction of the pipe with a 45° pitch. Each of the maximum value, minimum value, and average value of the four measurement values is determined. The ovality of the as-rolled electric resistance welded steel pipe is determined by the following Formula on the basis of the maximum value, the minimum value, and the average value.

[0169] The change in ovality (%) by sizer step is determined by the following Formula on the basis of the ovality of the as-rolled electric resistance welded steel pipe before the adjustment of the shape by the sizer and the ovality of the as-rolled electric resistance welded steel pipe after the adjustment of the shape by the sizer.

[0170] The step of producing an as-rolled electric resistance welded steel pipe in the production method A preferably includes:

a hot-rolling step of heating a slab having the chemical composition described above and hot-rolling the heated slab, thereby obtaining a hot-rolled steel sheet,

a cooling step of cooling the hot-rolled steel sheet obtained in the hot-rolling step,

a coiling step of coiling the hot-rolled steel sheet cooled in the cooling step, thereby obtaining a hot coil consisting of the hot-rolled steel sheet, and

a pipe-making step of uncoiling the hot-rolled steel sheet from the hot coil, roll-forming the uncoiled hot-rolled steel sheet to thereby make an open pipe, and subjecting abutting portions of the obtained open pipe to electric resistance welding to form an electric resistance welded portion, thereby obtaining an as-rolled electric resistance welded steel pipe.

[0171] In the pipe-making step, the electric resistance welded portion may be subjected to seam heat treatment after the electric resistance welding, if necessary.

[0172] In the hot-rolling step, the slab having the chemical composition described above is preferably heated to a temperature of from 1150°C to 1350°C.

[0173] In a case in which the temperature to which the slab is heated is 1150°C or more, the toughness of the base metal portion of the electric resistance welded steel pipe can be further improved. The reason thereof can be considered to be because generation of an insoluble Nb carbide can be suppressed in a case in which the temperature to which the slab is heated is 1150°C or more.

[0174] In a case in which the temperature to which the slab is heated is 1350°C or less, the toughness of the base metal portion of the electric resistance welded steel pipe can be further improved. The reason thereof can be considered to be because coarsening of a metallographic microstructure can be suppressed in a case in which the temperature to which the slab is heated is 1350°C or less.

[0175] In the hot rolling step, the slab heated, for example, to a temperature of 1150°C to 1350°C is preferably hot-rolled at a temperature that is equal to or more than Ar3 point + 100°C. As a result, the hardenability of the hot-rolled steel sheet can be improved.

[0176] The Ar3 point is determined from the chemical composition of the base metal portion by the following Formula:

[0177] [where C, Mn, Ni, Cu, Cr, and Mo represent % by mass of the respective elements, respectively. Ni, Cu, Cr, and Mo are optional elements. Among the optional elements, an element that is not contained in the slab is set at 0% by mass, and the Ar3 point is calculated.

[0178] In the cooling step, the hot-rolled steel sheet obtained in the hot-rolling step is preferably cooled at a cooling start temperature set at the Ar3 point or more. As a result, the strength and toughness of the base metal portion can be further improved. The reason thereof can be considered to be because generation of coarse polygonal ferrite is suppressed by setting the cooling start temperature at the Ar3 point or more.

[0179] In the cooling step, the hot-rolled steel sheet obtained in the hot-rolling step is preferably cooled at a cooling rate of from 5°C/s to 80°C/s.

[0180] In a case in which the cooling rate is 5°C/s or more, the degradation of the toughness of the base metal portion is further suppressed. The reason thereof can be considered to be because generation of coarse ferrite is suppressed by setting the cooling rate in the cooling step at 5°C/s or more.

[0181] In a case in which the cooling rate is 80°C/s or less, the degradation of the toughness of the base metal portion is suppressed. The reason thereof can be considered to be because an excessive second phase fraction (i.e., a ferrite fraction of less than 60%) is suppressed by setting the cooling rate in the cooling step at 80°C/s or less.

[0182] In the coiling step, the hot-rolled steel sheet cooled in the cooling step is preferably coiled at a coiling temperature of from 450 to 650°C.

[0183] A coiling temperature of 450°C or more results in suppression of the degradation of the toughness of the base metal portion. The reason thereof can be considered to be because a coiling temperature of 450°C or more results in suppression of generation of martensite.

[0184] A coiling temperature of 650°C or less may result in suppression of an increase in YR. The reason thereof can be considered to be because a coiling temperature of 650°C or less results in suppression of excessive generation of a Nb carbonitride, thereby resulting in suppression of an increase in YS.

EXAMPLES

[0185] Examples of the present disclosure will be described below. However, the present disclosure is not limited to the following Examples.

[Examples 1 to 17, and Comparative Examples 1 to 26]

<Production of Hot Coil>

[0186] Each of slabs having chemical compositions set forth in Table 1 and Table 2 was heated to a temperature of 1250°C, the heated slab was hot-rolled to obtain a hot-rolled steel sheet, the obtained hot-rolled steel sheet was cooled at a cooling start temperature set at 820°C and a cooling rate of 50°C/s, and the cooled hot-rolled steel sheet was coiled at a coiling temperature of 550°C, whereby a hot coil consisting of the hot-rolled steel sheet was obtained.

[0187] In each Example and each Comparative Example, the balance excluding the elements set forth in Table 1 and Table 2 is Fe and impurities.

[0188] In Tables 1 and 2, REM in Example 11 is Ce, REM in Example 16 is Nd, and REM in Example 17 is La.

[0189] In Tables 1 to 3, the underlined numerical values show numerical values that fall outside the scope of the present disclosure.

<Production of As-Rolled Electric Resistance Welded Steel Pipe>

[0190] A hot-rolled steel sheet was uncoiled from the hot coil, the uncoiled hot-rolled steel sheet was roll-formed to thereby make an open pipe, abutting portions of the obtained open pipe was subjected to electric resistance welding to form an electric resistance welded portion (hereinafter also referred to as "welded portion"), and the welded portion was then subjected to seam heat treatment, thereby obtaining an as-rolled electric resistance welded steel pipe.

<Production of Electric Resistance Welded Steel Pipe (Sizer and Tempering)>

[0191] The shape of the as-rolled electric resistance welded steel pipe was adjusted by a sizer under conditions achieving each of changes in ovality (%) by sizer step set forth in Table 3.

[0192] The as-rolled electric resistance welded steel pipe of which the shape had been adjusted was tempered at each tempering temperature and for each tempering time set forth in in Table 3, thereby obtaining an electric resistance welded steel pipe.

[0193] The outer diameter of the obtained electric resistance welded steel pipe was 219 mm, and the wall thickness of this electric resistance welded steel pipe was 15.9 mm.

[0194] The above production step does not affect the chemical composition of a steel. Accordingly, the chemical composition of the base metal portion of the obtained electric resistance welded steel pipe can be considered to be the same as the chemical composition of the slab which is a raw material.

<Measurement>

[0195] The following measurement was performed for the obtained electric resistance welded steel pipe.

[0196] The results are set forth in Table 3.

(Measurement of Ferrite Fraction and Confirmation of Structure of Second Phase)

[0197] By the method described above, the ferrite fraction ("F fraction" in Table 3) was measured, and the kind of a second phase was confirmed.

[0198] In Table 3, TB means tempered bainite, P means pearlite, and MA means martensite island.

(Measurement of YS, TS, and YR)

[0199] A specimen for a tensile test was sampled in a direction where the test direction (tensile direction) in a tensile test corresponds to the pipe axis direction (hereinafter also referred to as "L-direction") of the electric resistance welded steel pipe from the base metal 90° position of the electric resistance welded steel pipe. The shape of the specimen was allowed to be a flat plate shape conforming to an American Petroleum Institute standard API 5L (hereinafter simply referred to as "API 5L").

[0200] A tensile test in which a test direction was the L-direction of the electric resistance welded steel pipe was conducted using the sampled specimen in conformity with API 5L at room temperature, and TS in the L-direction of the electric resistance welded steel pipe and YS in the L-direction of the electric resistance welded steel pipe were measured.

[0201] YR (%) in the L-direction of the electric resistance welded steel pipe was determined based on a calculation formula "(YS/TS) × 100".

(Measurement of vE (J) (Charpy Absorbed Energy at 0°C) of Base Metal Portion)

[0202] A full-size specimen with a V-notch (a specimen for a Charpy impact test) was sampled from the base metal 90°C position of the electric resistance welded steel pipe. The full-size specimen with a V-notch was sampled so that a test direction was the circumferential direction of the pipe (C-direction). The sampled full-size specimen with a V-notch was subjected to a Charpy impact test in conformity with API 5L under a temperature condition of 0°C to measure vE (J).

[0203] The above measurement was performed five times per one electric resistance welded steel pipe, and the average value of five measurement values was regarded as vE (J) of the base metal portion of the electric resistance welded steel pipe.

(Measurement of vE (J) (Charpy Absorbed Energy at 0°C) of Welded Portion)

[0204] The same operation as the measurement of vE (J) of the base metal portion was performed except that a position from which a full-size specimen with a V-notch was sampled was changed to the welded portion of the electric resistance welded steel pipe.

(Measurement of Specific Precipitate Areal Ratio)

[0205] The specific precipitate areal ratio (i.e., the areal ratio of precipitates having an equivalent circle diameter of 100 nm or less, simply referred to as "precipitate areal ratio (%)"in Table 3) was measured by the method described above.

[Table 1]

	Component (% by mass)
	C	Mn	Ti	Nb	N	Si	Al	P	S
Example 1	0.097	0.73	0.010	0.032	0.010	0.318	0.041	0.019	0.0019
Example 2	0.092	0.64	0.016	0.021	0.005	0.200	0.019	0.008	0.0044
Example 3	0.088	0.64	0.007	0.047	0.003	0.026	0.071	0.018	0.0050
Example 4	0.080	0.50	0.006	0.030	0.009	0.100	0.048	0.013	0.0026
Example 5	0.098	0.66	0.007	0.036	0.004	0.179	0.084	0.017	0.0020
Example 6	0.103	0.61	0.017	0.026	0.011	0.243	0.099	0.018	0.0029
Example 7	0.085	0.62	0.014	0.043	0.008	0.300	0.090	0.018	0.0006
Example 8	0.084	0.72	0.008	0.040	0.009	0.230	0.020	0.005	0.0030
Example 9	0.080	0.60	0.009	0.025	0.003	0.100	0.031	0.012	0.0036
Example 10	0.081	0.66	0.012	0.038	0.005	0.289	0.019	0.006	0.0024
Example 11	0.095	0.77	0.016	0.032	0.010	0.027	0.061	0.015	0.0009
Example 12	0.082	0.61	0.019	0.060	0.010	0.121	0.088	0.010	0.0008
Example 13	0.083	0.82	0.045	0.020	0.007	0.200	0.010	0.003	0.0020
Example 14	0.083	0.51	0.025	0.015	0.005	0.200	0.015	0.001	0.0019
Example 15	0.082	0.78	0.025	0.015	0.003	0.230	0.020	0.001	0.0020
Example 16	0.084	0.61	0.045	0.020	0.005	0.243	0.010	0.015	0.0008
Example 17	0.082	0.61	0.017	0.020	0.010	0.200	0.020	0.013	0.0009
Comparative Example 1	0.150	0.73	0.006	0.028	0.004	0.046	0.059	0.002	0.0032
Comparative Example 2	0.090	0.90	0.023	0.015	0.011	0.480	0.051	0.007	0.0029
Comparative Example 3	0.106	0.66	0.008	0.042	0.002	0.005	0.029	0.019	0.0030
Comparative Example 4	0.109	0.20	0.013	0.031	0.002	0.069	0.088	0.014	0.0038
Comparative Example 5	0.080	1.02	0.024	0.049	0.011	0.091	0.015	0.002	0.0017
Comparative Example 6	0.117	0.82	0.001	0.027	0.009	0.015	0.056	0.008	0.0049
Comparative Example 7	0.086	0.69	0.060	0.049	0.012	0.300	0.052	0.019	0.0047
Comparative Example 8	0.090	0.63	0.016	0.008	0.010	0.225	0.020	0.006	0.0035
Comparative Example 9	0.080	0.60	0.017	0.102	0.004	0.081	0.071	0.010	0.0011
Comparative Example 10	0.081	0.78	0.010	0.080	0.005	0.241	0.0008	0.011	0.0015
Comparative Example 11	0.093	0.65	0.014	0.051	0.011	0.093	0.110	0.018	0.0020
Comparative Example 12	0.100	0.90	0.021	0.080	0.010	0.100	0.015	0.002	0.0005
Comparative Example 13	0.080	0.30	0.021	0.010	0.008	0.100	0.015	0.002	0.0005
Comparative Example 14	0.080	0.90	0.014	0.010	0.010	0.300	0.006	0.010	0.0026
Comparative Example 15	0.113	0.64	0.017	0.056	0.008	0.184	0.054	0.000	0.0012
Comparative Example 16	0.089	0.62	0.014	0.068	0.002	0.097	0.074	0.012	0.0038
Comparative Example 17	0.087	0.73	0.021	0.050	0.012	0.046	0.016	0.006	0.0048
Comparative Example 18	0.116	0.61	0.012	0.063	0.010	0.173	0.004	0.001	0.0035
Comparative Example 19	0.087	0.65	0.008	0.044	0.0006	0.068	0.089	0.003	0.0003
Comparative Example 20	0.099	0.78	0.009	0.046	0.030	0.054	0.060	0.016	0.0015
Comparative Example 21	0.099	0.77	0.014	0.042	0.005	0.300	0.055	0.018	0.0049
Comparative Example 22	0.102	0.76	0.008	0.026	0.007	0.081	0.015	0.019	0.0004
Comparative Example 23	0.081	0.60	0.009	0.091	0.006	0.296	0.089	0.004	0.0014
Comparative Example 24	0.090	0.60	0.023	0.015	0.005	0.450	0.051	0.007	0.0029
Comparative Example 25	0.086	0.95	0.021	0.015	0.010	0.100	0.074	0.018	0.0035
Comparative Example 26	0.081	0.95	0.021	0.021	0.005	0.081	0.060	0.000	0.0003

[Table 2]

	Component (% by mass) (continued from Table 1)								CMeq	Mn/Si	LR	Ac1 (°C)
	Mo	Cu	Ni	Cr	V	Ca	Mg	REM
Example 1									0.251	2.3	0.325	746
Example 2									0.219	3.2	0.336	746
Example 3									0.242	25.1	0.363	740
Example 4									0.193	5.0	0.396	743
Example 5		0.35							0.267	3.7	0.370	730
Example 6			0.44						0.261	2.5	0.396	724
Example 7				0.20					0.272	2.1	0.354	746
Example 8				0.08		0.0032			0.260	3.1	0.301	752
Example 9	0.05								0.222	6.0	0.322	745
Example 10					0.015				0.245	2.3	0.312	751
Example 11								0.0018	0.255	28.9	0.300	737
Example 12		0.06	0.06						0.251	5.0	0.382	740
Example 13									0.240	4.1	0.237	745
Example 14	0.20								0.250	2.6	0.371	752
Example 15							0.0060		0.227	3.4	0.240	744
Example 16						0.0025		0.0015	0.205	2.5	0.323	748
Example 17						0.0028		0.0020	0.204	3.1	0.314	746
Comparative Example 1									0.299	15.7	0.469	736
Comparative Example 2									0.255	2.1	0.227	741
Comparative Example 3									0.257	131.3	0.402	745
Comparative Example 4									0.173	2.9	1.296	739
Comparative Example 5									0.299	13.2	0.213	747
Comparative Example 6									0.281	52.9	0.334	735
Comparative Example 7									0.249	2.3	0.335	748
Comparative Example 8									0.203	2.8	0.312	744
Comparative Example 9									0.282	7.5	0.450	755
Comparative Example 10									0.290	3.2	0.322	762
Comparative Example 11									0.252	7.0	0.379	736
Comparative Example 12									0.330	9.0	0.322	755
Comparative Example 13									0.140	3.0	0.593	747
Comparative Example 14									0.240	3.0	0.198	745
Comparative Example 15									0.275	3.5	0.461	747
Comparative Example 16									0.261	6.4	0.408	746
Comparative Example 17									0.259	15.8	0.320	750
Comparative Example 18									0.279	3.5	0.503	758
Comparative Example 19									0.238	9.4	0.349	737
Comparative Example 20									0.275	14.5	0.324	741
Comparative Example 21									0.270	2.6	0.323	745
Comparative Example 22									0.255	9.4	0.318	744
Comparative Example 23									0.273	2.0	0.432	753
Comparative Example 24									0.205	1.3	0.340	744
Comparative Example 25									0.259	2.0	0.206	730
Comparative Example 26									0.260	2.0	0.201	734

[Table 3]

	Change in ovality (%) by sizer step	Tempering temperature (°C)	Tempering time (min)	Electric resistance welded steel pipe
				F fraction (%)	Kind of second phase	YS (MPa)	TS (MPa)	YR (%)	vE (J)		Precipitate areal ratio (%)
									Base metal portion	Welded portion
Example 1	1.8	620	16	72	TB+P	536	657	82	181	146	0.38
Example 2	2.6	610	22	79	TB+P	490	586	84	247	212	0.35
Example 3	2.5	570	25	81	TB	527	631	83	262	228	0.31
Example 4	4.0	540	10	92	TB	438	525	83	272	245	0.22
Example 5	1.2	680	7	68	TB+P	541	633	85	200	178	0.49
Example 6	4.6	650	10	67	TB+P	546	636	86	199	179	0.43
Example 7	4.5	420	26	69	TB+P	560	663	84	260	229	0.10
Example 8	2.5	590	5	81	TB	520	635	82	251	209	0.41
Example 9	4.4	680	19	85	TB	534	623	86	308	262	0.57
Example 10	2.2	620	22	82	TB	516	633	82	272	228	0.42
Example 11	2.5	640	24	70	TB+P	544	676	81	188	150	0.42
Example 12	2.1	590	19	82	TB	532	653	82	259	218	0.39
Example 13	4.0	680	10	78	TB+P	515	600	86	300	268	0.12
Example 14	3.6	700	10	68	TB+P	550	640	86	315	285	0.42
Example 15	3.6	650	10	65	TB+P	520	590	88	325	298	0.35
Example 16	2.5	600	10	67	TB+P	480	560	86	358	318	0.42
Example 17	3.2	650	10	63	TB+P	490	560	88	380	369	0.37
Comparative Example 1	4.5	710	8	59	TB+P	590	616	96	207	166	0.52
Comparative Example 2	1.6	520	10	65	TB+P	579	679	85	262	10	0.19
Comparative Example 3	2.1	600	8	74	TB+P	539	627	86	8	18	0.36
Comparative Example 4	1.2	480	29	95	TB+P	424	530	80	5	12	0.13
Comparative Example 5	3.5	580	29	50	TB+P	673	759	89	15	18	0.35
Comparative Example 6	1.2	610	26	60	TB+P	575	705	82	10	15	0.33
Comparative Example 7	1.5	710	9	79	TB+P	575	697	83	5	8	0.63
Comparative Example 8	1.7	720	21	81	TB	474	588	81	15	171	0.53
Comparative Example 9	2.2	590	10	86	TB	598	688	87	21	24	0.47
Comparative Example 10	2.7	530	5	74	TB+P	560	660	85	15	21	0.34
Comparative Example 11	4.1	410	20	79	TB+P	533	655	81	250	8	0.15
Comparative Example 12	3.1	690	27	61	TB+P	668	829	81	218	194	0.62
Comparative Example 13	2.5	640	30	100	-	406	495	82	350	312	0.39
Comparative Example 14	2.3	480	6	68	TB+P	518	569	91	250	220	0.15
Comparative Example 15	4.6	350	27	72	TB+P	578	635	91	113	99	0.01
Comparative Example 16	4.6	750	7	82	TB+MA	450	594	76	10	15	0.02
Comparative Example 17	0.5	450	28	76	TB+P	575	632	91	251	216	0.05
Comparative Example 18	4.5	620	1	73	TB+P	581	638	91	106	85	0.05
Comparative Example 19	2.3	480	10	81	TB	511	627	81	15	15	0.14
Comparative Example 20	2.5	530	15	69	TB+P	570	666	86	5	8	0.24
Comparative Example 21	0.9	690	15	69	TB+P	568	624	91	215	190	0.05
Comparative Example 22	0.8	710	15	69	TB+P	536	585	92	143	110	0.02
Comparative Example 23	0.7	510	15	85	TB	564	618	91	287	250	0.06
Comparative Example 24	2.5	510	10	83	TB	495	592	84	279	10	0.17
Comparative Example 25	0.8	690	15	69	TB+P	520	570	91	340	310	0.02
Comparative Example 26	4.6	350	10	69	TB+P	515	568	91	330	300	0.42

[0206] As set forth in Table 1 to Table 3, the electric resistance welded steel pipe of each Example satisfied TS, YS, YR, vE (base metal portion), and vE (welded portion) in the present disclosure. In other words, it was shown that the electric resistance welded steel pipe of each Example had a certain amount of tensile strength and yield strength, which had a decreased yield ratio, and had the excellent toughness of a base metal portion and a welded portion.

[0207] Comparative Example 1 in which the amount of C was more than the upper limit resulted in a decrease in F fraction and an increase in YR.

[0208] Comparative Example 2 in which the amount of Si was more than the upper limit resulted in the deterioration of the toughness of a welded portion.

[0209] Comparative Example 3 in which the amount of Si was less than the lower limit resulted in the deterioration of the toughness of a base metal portion and a welded portion. The reason thereof can be considered to be because deoxidization became insufficient, thereby generating a coarse oxide.

[0210] Comparative Example 4 in which the amount of Mn was less than the lower limit resulted in the deterioration of the toughness of a base metal portion and a welded portion. The reason thereof can be considered to be because embrittlement due to S occurred.

[0211] Comparative Example 5 in which the amount of Mn was more than the upper limit resulted in the deterioration of the toughness of a base metal portion and a welded portion. The reason thereof can be considered to be because cracking due to MnS was facilitated.

[0212] Comparative Example 6 in which Ti was less than the lower limit resulted in the deterioration of the toughness of a base metal portion and a welded portion. The reason thereof can be considered to be because a crystal grain became coarse.

[0213] Comparative Example 7 in which Ti was more than the upper limit resulted in the deterioration of the toughness of a base metal portion and a welded portion. The reason thereof can be considered to be because coarse TiN was generated.

[0214] Comparative Example 8 in which Nb was less than the lower limit resulted in the deterioration of the toughness of a base metal portion. The reason thereof can be considered to be because rolling in the region of unrecrystallization temperature became insufficient.

[0215] Comparative Example 9 in which Nb was more than the upper limit resulted in the deterioration of the toughness of a base metal portion and a welded portion. The reason thereof can be considered to be because a coarse Nb carbonitride was generated.

[0216] Comparative Example 10 in which Al was less than the lower limit resulted in the deterioration of the toughness of a base metal portion and a welded portion. With regard to the reason thereof, the reason thereof can be considered to be because deoxidization became insufficient.

[0217] Comparative Example 11 in which Al was more than the upper limit resulted in the deterioration of the toughness of a welded portion. The reason thereof can be considered to be because a large amount of Al-based inclusion was generated.

[0218] In Comparative Example 12 in which CMeq was more than the upper limit, YS and TS were more than the upper limits.

[0219] In Comparative Example 13 in which CMeq was less than the lower limit, a F fraction was more than the upper limit, and TS was less than the lower limit.

[0220] Comparative Example 14 in which LR was less than 0.210 resulted in a yield ratio of more than 90%.

[0221] In Comparative Example 15, YS and YR were more than the upper limits. The reason thereof can be considered to be because a tempering temperature was too low, thereby resulting in the insufficient effect of reducing a pipe-making strain by tempering (i.e., the effect of reducing a dislocation density by tempering) and in insufficient precipitation on a dislocation.

[0222] In Comparative Example 16, the toughness of a base metal portion and a welded portion was deteriorated (i.e., both the base metal portion and the welded portion showed vE less than the lower limit). The reason thereof can be considered to be because a tempering temperature was too high, transformation to austenite therefore occurred in a partial region, the concentration of C was increased in the partial region, and as a result, martensite island (MA) was generated by subsequent cooling.

[0223] In Comparative Example 17, YR was more than 90%. The reason thereof can be considered to be because a change in ovality by sizer step was small, and therefore, the introduction of a dislocation and the precipitation on the dislocation were insufficient.

[0224] In Comparative Example 18, YR was more than 90%. The reason thereof can be considered to be because a tempering time was short, and therefore, precipitation on a dislocation was insufficient.

[0225] Comparative Example 19 in which the amount of N was less than the lower limit resulted in the deterioration of the toughness of a base metal portion and a welded portion. The reason thereof can be considered to be because crystal grains became coarse.

[0226] Comparative Example 20 in which the amount of N was more than the upper limit resulted in the deterioration of the toughness of a base metal portion and a welded portion. The reason thereof can be considered to be because a carbide became coarse.

[0227] In Comparative Examples 21 to 23, YR was more than 90%. The reason thereof can be considered to be because a change in ovality by sizer step was small, and therefore, the introduction of a dislocation and the precipitation on the dislocation were insufficient.

[0228] In Comparative Example 24 in which a Mn/Si ratio was 2.0 or less, the toughness of a welded portion was deteriorated.

[0229] In Comparative Examples 25 and 26 in which LR was less than 0.210, a yield ratio was more than 90%.

[0230] The entire disclosure of Japanese Patent Application No. 2016-056858 is incorporated herein by reference.

[0231] All documents, patent applications, and technical standards described in this specification are herein incorporated by reference to the same extent as if each individual document, patent application, or technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. An electric resistance welded steel pipe for a line pipe, 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:

0.080 to 0.120% of C,

0.30 to 1.00% of Mn,

0.005 to 0.050% of Ti,

0.010 to 0.100% of Nb,

0.001 to 0.020% of N,

0.010 to 0.450% of Si,

0.001 to 0.100% of Al,

0 to 0.030% of P,

0 to 0.0100% of S,

0 to 0.50% of Mo,

0 to 1.00% of Cu,

0 to 1.00% of Ni,

0 to 1.00% of Cr,

0 to 0.100% of V,

0 to 0.0100% of Ca,

0 to 0.0100% of Mg,

0 to 0.0100% of REM, and

the balance being Fe and impurities, wherein:

CMeq, expressed by the following Formula (1), is from 0.170 to 0.300,

a ratio of % by mass of Mn to % by mass of Si is 2.0 or more,

LR, expressed by the following Formula (2), is 0.210 or more,

in a case in which a metallographic microstructure of the base metal portion is observed using a scanning electron microscope at a magnification of 1,000 times, an areal ratio of a first phase that is ferrite is from 60 to 98%, and a second phase, which is the balance, comprises tempered bainite,

a yield strength in a pipe axis direction is from 390 to 562 MPa,

a tensile strength in the pipe axis direction is from 520 to 690 MPa,

a yield ratio in the pipe axis direction is 90% or less,

a Charpy absorbed energy in a circumferential direction of the pipe in the base metal portion is 100 J or more at 0°C, and

a Charpy absorbed energy in the circumferential direction of the pipe in the electric resistance welded portion is 80 J or more at 0°C;

wherein, in Formula (1) and Formula (2), C, Mn, Cr, Ni, Cu, Nb, Mo, and V represent % by mass of respective elements.

2. The electric resistance welded steel pipe for a line pipe according to claim 1,
wherein the chemical composition of the base metal portion comprises, in terms of % by mass, one or more of:

more than 0% but equal to or less than 0.50% of Mo,

more than 0% but equal to or less than 1.00% of Cu,

more than 0% but equal to or less than 1.00% of Ni,

more than 0% but equal to or less than 1.00% of Cr,

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

more than 0% but equal to or less than 0.0100% of Ca,

more than 0% but equal to or less than 0.0100% of Mg, or

more than 0% but equal to or less than 0.0100% of REM.

3. The electric resistance welded steel pipe for a line pipe according to claim 1 or 2, wherein an areal ratio of a precipitate having an equivalent circle diameter of 100 nm or less is from 0.10 to 1.00% in a case in which the metallographic microstructure of the base metal portion is observed using a transmission electron microscope at a magnification of 100,000 times.

4. The electric resistance welded steel pipe for a line pipe according to any one of claims 1 to 3, wherein a content of Nb in the chemical composition of the base metal portion is, in terms of % by mass, 0.020% or more.

5. The electric resistance welded steel pipe for a line pipe according to any one of claims 1 to 4, wherein the electric resistance welded steel pipe for a line pipe has a wall thickness of from 10 to 25 mm and an outer diameter of from 114.3 to 609.6 mm.

Drawing