Technical field
[0001]
The present disclosure relates to electric resistance welded steel pipe for a torsion beam.
BACKGROUND
[0002]
Conventionally, study of steel used for automobile structural members (e.g., automotive underbody part) have been made.
For example, Patent Document 1, is used to machine structural steel, such as automobile underbody parts steel, hot rolled steel sheet is disclosed for superior mechanical structure steel pipe fatigue properties and bending formability.
　Further, Patent Document 2, even though fatigue moldability is good high and automobile chassis parts steel material having excellent fatigue characteristics that do not require much cost for heat treatment is disclosed.
　Further, Patent Document 3, a high strength, suitable torsion beam to be a good workability and excellent torsional fatigue property requirements, axle beam, trailing arm, for automotive structural members, such as a suspension arm, high tensile steel has been disclosed.
[0003]
　Patent Document 1: WO 2009/133965
　Patent Document 2: WO 2008/018624 Patent
　Patent Document 3: JP 2011-38155 JP
Summary of the Invention
Problems that the Invention is to Solve
[0004]
　The is one of the automotive underbody part torsion beam, high strength (in particular, tensile strength of the tube axis direction) is required.
　Meanwhile, the torsion beam is electric resistance welded steel pipe (hereinafter, also referred to as "torsion beam for ERW pipe") which may be prepared by subjecting the bending respect. In this case, the inner surface of the bending is applied portion of the electric resistance welded steel pipe, cracking (hereinafter, also referred to as "inner surface crack") may occur. From the viewpoint of bending of the electric resistance welded steel pipe, to an electric resistance welded steel pipe, it may be prompted to improve the耐内surface cracking.
　Here, the耐内surface cracking resistance means a property that can suppress the inner surface cracking when subjected to bending against the electric resistance welded steel pipe.
　In Patent Documents 1-3, studied from the viewpoint of improving the耐内surface cracking resistance of the steel pipe is not made at all, there remains room for improvement.
[0005]
　An object of the present disclosure are excellent in tensile strength in the tube axis direction, and is to provide a torsion beam for electric resistance welded steel pipe superior in 耐内 surface cracking.
Means for Solving the Problems
[0006]
　The means for solving the above problems includes the following aspects.
<1> includes a base metal and the electric-resistance welded portion,
　the chemical composition of the base metal is, by
mass%,
C: 0.04 ~ 0.12%, Si: 0.03 ~
1.20%, Mn :
0.30
~
2.50%, P: 0 ~ 0.030%, S: 0 ~
0.010%, Ti: 0.08 ~ 0.24%, Al: 0.005 ~ 0.500%, nb:
0.01 ~ 0.06%, N: 0.0005 ~ 0.0100%, Cu: 0 ~ 1.00%, Ni: 0 ~ 1.00%, Cr: 0 ~ 1.00%, Mo : 0 ~ 0.50%, V: 0 ~ 0.20%, W: 0 ~ 0.10%, Ca: 0 ~ 0.0200%, Mg: 0 ~ 0.0200%, Zr: 0 ~ 0. % 0200, REM: 0 ~ 0.0200%, and, the balance Fe and impurities, V is defined by the following formula (i) c90 is not less than 200, pair C Containing the mass ratio of Ti is the 0.85 to 5.00, 　in the thickness center portion of the metal structure in the L cross-section of the base metal, and the area ratio of ferrite is 80% or more, the average ferrite grain crystal grain size is not more 10μm or less, an average aspect ratio of ferrite grains is 2.0 or less, 　the torsion beam for ERW pipe tensile strength of the tube axis direction is 750 ~ 1000 MPa. 　log V c90 = 2.94-0.75 (.beta.a-1) ... formula (i) 　.beta.a = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + Mo ... formula (ii) in [formula (i), .beta.a the formula (ii) is a value defined by. 　Wherein (ii), each element symbol represents the mass% of each element. ]

[0007]
<2> The chemical composition of the base metal is, by
mass%, Cu: 0% than 1.00% or
less, Ni: 0% than 1.00% or
less, Cr: 0% than 1.00% or
less, Mo : 0 percent 0.50% or
less, V: over 0% 0.20% or
less, W: 0 percent
0.10%, Ca: over 0% 0.0200% or
less, Mg: over 0% 0. 0200% or
less, Zr: 0% than 0.0200% or less, and,
REM: 0% than contains one or more selected from the group consisting of 0.0200% or less ERW for torsion beam according to <1> steel pipe.
[0008]
<3> an outer diameter of 50 ~ 150 mm, the torsion beam for ERW pipe according to a wall thickness of 2.0 ~ 4.0 mm <1> or <2>.
Effect of the invention
[0009]
　According to the present disclosure, excellent tensile strength in the tube axis direction, and torsion beam for steel pipes excellent in 耐内 surface cracking resistance is provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010]
It is a metal structure photograph of an example of a metal structure of the base metal in the electric resistance welded steel pipe of FIG. 1 present disclosure (optical micrograph).
FIG. 2 is a schematic view schematically showing an outline of bending test in Examples.
In the bending test in FIG. 3 embodiment, it is a schematic cross-sectional view of the cross-section schematically showing a structure obtained by applying bending to the electric resistance welded steel pipe.
DESCRIPTION OF THE INVENTION
[0011]
　Herein, a numerical range represented by using "to" means a range including numerical values described before and after "to" as the lower and upper limits.
　As used herein, "%" indicating the content of the component (element) means "% by mass".
　Herein, the content of C (carbon), may be referred to as "C content". It may be referred Similarly, the content of the other elements.
　Herein, the term "process" not only separate steps, if even that can not be clearly distinguished from other processes is achieved intended purpose of the process, included in the term It is.
　In numerical ranges are stepwise described herein, the upper limit value or lower limit value of a graded numerical range may be replaced with the upper limit value or the lower limit of the numerical range of the other graded according or it may be replaced with the values shown in the examples.
[0012]
　Torsion beam for ERW steel pipe of the present disclosure (hereinafter, simply referred to as "ERW") includes a base metal and the electric-resistance welded portion, the chemical composition of the base metal is, by mass%, C: 0.04 ~ 0.12%, Si: 0.03 ~ 1.20%, Mn: 0.30 ~ 2.50%, P: 0 ~ 0.030%, S: 0 ~ 0.010%, Ti: 0. 08 ~ 0.24%, Al: 0.005 ~ 0.500%, Nb: 0.01 ~ 0.06%, N: 0.0005 ~ 0.0100%, Cu: 0 ~ 1.00%, Ni : 0 ~ 1.00%, Cr: 0 ~ 1.00%, Mo: 0 ~ 0.50%, V: 0 ~ 0.20%, W: 0 ~ 0.10%, Ca: 0 ~ 0. 0200%, Mg: 0 ~ 0.0200 %, Zr: 0 ~ 0.0200%, REM: 0 ~ 0.0200%, and the balance: Fe and impurities, the following formula (i) V is defined Te c90 is not less than 200, containing the mass ratio of Ti with respect to C (hereinafter referred to as "Ti / C ratio") is from 0.85 to 5.00, meat in the L cross-section of the base material portion in the thickness center portion of the metal structure, and the area ratio of ferrite is 80% or more, the average crystal grain size of the ferrite crystal grains is not more 10μm or less, an average aspect ratio of ferrite grains is 2.0 or less, the tube axis the tensile strength of the direction is 750 ~ 1000MPa.
[0013]
　log V c90 = 2.94-0.75 (.beta.a-1) ... formula (i)
　.beta.a = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + Mo ... formula (ii)
in [formula (i), .beta.a the formula (ii) is a value defined by.
　Wherein (ii), each element symbol represents the mass% of each element. ]
[0014]
　In ERW steel pipe of the present disclosure, the base metal portion and (base metal portion) is in the electric resistance welded steel pipe refers to the portion other than the electric resistance welding unit and the heat-affected zone.
　Here, the heat-affected zone (heat affected zone; hereinafter referred to as "HAZ") and the effects of heat by electric resistance welding (however, in the case of performing the seam heat treatment after electric resistance welding is electric resistance welding and seam heat treatment It refers to received portions due to the influence of heat).
[0015]
　The ERW steel pipe of the present disclosure (specifically, be a tensile strength in the tube axis direction than 750 MPa) excellent tensile strength in the tube axis direction, and also excellent in耐内surface cracking.
　Why the effect is obtained is presumed as follows. However, electric resistance welded steel pipe of the present disclosure is not to be limited by the following estimation reasons.
[0016]
　The ERW steel pipe of the present disclosure, in the metal structure, that the area ratio of ferrite is 80% or more, the average crystal grain size of the ferrite crystal grains is 10μm or less, and an average aspect ratio of ferrite grains by 2.0 or less, thereby improving the耐内surface cracking. The reason for this is presumed as follows.
　Inner surface cracking, the shear deformation due to bending, bending grain boundaries tearing the inner surface and its vicinity of the bent portion by molding, is considered to be generated by its tear progresses. Therefore, the metal structure on which a metal structure area ratio of ferrite is 80% or more, making the ferrite crystal grains and fine (i.e., be 10μm or less average grain size of the ferrite crystal grains), and, bringing the ferrite grain shape spherical (i.e., to the average aspect ratio of ferrite grains than 2.0), the inner surface cracking that may occur as described above is suppressed (i.e.,耐内surface cracking resistance is considered to be) improved.
[0017]
　Metal structure area ratio of ferrite is 80% or more, V c90 is realized such as by reducing the hardenability of steel by the more than 200.
　In general, as the area ratio of ferrite is soft phase becomes higher, the tensile strength of the tube axis direction tends to decrease. In this regard, the electric resistance welded steel pipe of the present disclosure, such as by enhancing the strength of steel, is higher than tensile strength 750MPa is realized by TiC precipitation by setting the Ti / C ratio 0.85 .
[0018]
　Hereinafter, the electric resistance welded steel pipe of the present disclosure, the chemical composition and metallurgical structure will be described.
[0019]

　Hereinafter, the chemical composition of the base metal in the electric resistance welded steel pipe of the present disclosure (hereinafter, referred to as "the chemical composition of the present disclosure".) Will be described.
[0020]
　C: 0.04 ~ 0.12%
　C is an element which improves the strength of steel. The C content is less than 0.04%, the necessary strength as a torsion beam for steel pipes can not be obtained. Therefore, C content is 0.04% or more. C content is preferably 0.05% or more, more preferably 0.06% or more.
　On the other hand, when the C content exceeds 0.12%, the耐内surface cracking resistance strength is too high to deteriorate. Therefore, C content is 0.12% or less. C content is preferably 0.10% or less.
[0021]
　Si: 0.03 ~
　1.20% Si is an element used for deoxidation. The Si content is less than 0.03%, the deoxidation to produce coarse oxides becomes insufficient. Therefore, Si content is 0.03% or more. Si content is preferably 0.10% or more, more preferably 0.20% or more.
　On the other hand, when the Si content exceeds 1.20%, SiO 2 leads to formation of inclusions, such as, in some cases very small voids during molding tend to occur. Therefore, Si content is less 1.20%.
　Si content is preferably not more than 1.00%, more preferably not more than 0.50%, more preferably 0.40% or less, more preferably not more than 0.30%.
[0022]
　Mn: 0.30 ~
　2.50% Mn is an element which improves the strength of steel. The Mn content is less than 0.30%, the necessary strength as a torsion beam for steel pipes can not be obtained. Therefore, Mn content is 0.30% or more. Mn content is preferably 0.50% or more, more preferably 0.60% or more.
　On the other hand, when the Mn content exceeds 2.50%, the耐内surface cracking resistance strength is too high to deteriorate. Therefore, Mn content is less 2.50%. Mn content is preferably not more than 2.00%, more preferably not more than 1.60%.
[0023]
　P: 0 ～ 0.030 Pasento
　P is an impurity. When the P content exceeds 0.030% tends to concentrated in the grain boundary, there is a case where耐内surface cracking resistance is deteriorated. Accordingly, P content is 0.030% or less. P content is preferably 0.020% or less.
　On the other hand, P content may be 0%. In view of dephosphorization cost, P content may be 0%, and may be 0.001% or more, may be 0.010% or more.
[0024]
　S: 0 ～ 0.010 Pasento
　S is an impurity. When S content exceeds 0.010%, coarse MnS is generated, there are cases where耐内surface cracking resistance is deteriorated. Thus, S content is 0.010% or less. S content is preferably 0.005% or less.
　On the other hand, S content may be 0%. From the standpoint of the desulfurizing cost reduction, S content can be 0%, and may be 0.001% or more.
[0025]
　Ti: 0.08 ~
　0.24% Ti, as well as improves the strength of steel by precipitation as TiC, which is an element contributing to the refinement of the crystal grains by the pinning effect of the hot rolling. The Ti content is less than 0.08%, the pinning effect of the necessary strength and grain as torsion beam for steel pipes can not be obtained. Therefore, Ti content is 0.08% or more. Ti content is preferably 0.09% or more, more preferably 0.10% or more.
　On the other hand, Ti content exceeds 0.24%, the coarse TiC and / or TiN is precipitated,耐内surface cracking resistance is deteriorated. Therefore, Ti content is less 0.24%. Ti content is preferably not more than 0.21%, more preferably not more than 0.20%, more preferably not more than 0.15%.
[0026]
　Al: 0.005 ~
　0.500% Al generates AlN, an element which contributes to grain refinement by pinning effect of hot rolling. The Al content is less than 0.005%, pinning effect of crystal grains can not be obtained in some cases ferrite becomes coarse. Therefore, Al content is 0.005% or more. Al content is preferably 0.010% or more, more preferably 0.020% or more.
　On the other hand, if the Al content exceeds 0.500%, the coarse AlN precipitates,耐内surface cracking resistance is deteriorated. Therefore, Al content is less 0.500%. Al content is preferably not more than 0.200%, more preferably 0.100% or less, further preferably 0.050% or less.
[0027]
　Nb: 0.01 ~
　0.06% Nb generates NbC, an element which contributes to grain refinement by pinning effect of hot rolling. The Nb content is less than 0.01%, pinning effect of crystal grains can not be obtained in some cases ferrite becomes coarse. Therefore, Nb content is 0.01% or more. Nb content is preferably 0.02% or more.
　On the other hand, when the Nb content exceeds 0.06%, coarse NbC precipitates, which may耐内surface cracking resistance is deteriorated. Therefore, Nb content is 0.06% or less. Nb content is preferably 0.04% or less.
[0028]
　N: 0.0005 ~ 0.0100%
　N generates AlN, an element which contributes to grain refinement by pinning effect of hot rolling. The N content is less than 0.0005%, the pinning effect of the crystal grains can not be obtained in some cases ferrite becomes coarse. Therefore, N content is 0.0005% or more. N content is preferably not less than 0.0010%, more preferably not less than 0.0030% or.
　On the other hand, N content is deposited coarse AlN exceeds 0.0100%耐内surface cracking resistance is deteriorated. Therefore, N content is 0.0100% or less. N content is preferably not more than 0.0080%, more preferably 0.0060% or less.
[0029]
　Cu: 0 ~
　1.00% Cu is an optional element and may not be contained. That, Cu content may be 0%.
　Cu is an element that contributes to increasing the strength of steel. From the viewpoint of such an effect, Cu content may be 0%, and may be 0.01% or more.
　On the other hand, if excessively contained the Cu, the effect is saturated in some cases cause an increase in cost. Therefore, Cu content is 1.00% or less. Cu content is preferably not more than 0.50%, more preferably not more than 0.30%.
[0030]
　Ni: 0 ~
　1.00% Ni is an optional element and may not be contained. That, Ni content may be 0%.
　Ni is an element that contributes to increasing the strength of steel. From the viewpoint of such an effect, Ni content may be 0%, and may be 0.01% or more.
　On the other hand, if excessively contained the Ni, the effect is saturated in some cases cause an increase in cost. Therefore, Ni content is 1.00% or less. Ni content is preferably not more than 0.50%, more preferably not more than 0.30%.
[0031]
　Cr: 0 ~
　1.00% Cr is an optional element and may not be contained. That, Cr content may be 0%.
　Cr is an element contributing to strengthening of steel. In view of such effects, Cr content may be 0%, and may be 0.01% or more.
　On the other hand, if excessively contained the Cr, the effect is saturated in some cases cause an increase in cost. Therefore, Cr content is 1.00% or less. Cr content is preferably not more than 0.50%, more preferably not more than 0.30%.
[0032]
　Mo: 0 ~
　0.50% Mo is an optional element and may not be contained. That, Mo content may be 0%.
　Mo is an element that contributes to increasing the strength of steel. From the viewpoint of such an effect, Mo content may be 0%, and may be at least 0.01% may be 0.10% or more.
　On the other hand, if excessively contained the Mo, effects are saturated which may lead to increase in cost. Therefore, Mo content is 0.50% or less. Mo content is preferably 0.40% or less.
[0033]
　V: 0 ～ 0.20 Pasento
　V is an optional element, it may not be contained. Ie, V content may be 0%.
　V is an element contributing to strengthening of steel. In view of such effects, V content may be 0%, and may be 0.005% or more.
　On the other hand, if excessively contained the V, effect is saturated which may lead to increase in cost. Therefore, V content is 0.20% or less. V content is preferably 0.10% or less.
[0034]
　W: 0 ~ 0.10%
　W is an optional element and may not be contained. Ie, W content may be 0%.
　W is an element which contributes to increasing the strength of steel. From the viewpoint of such an effect, W content may be 0%, and may be 0.005% or more.
　On the other hand, if excessively contained the W, the effect is saturated in some cases cause an increase in cost. Therefore, W content is 0.10% or less. W content is preferably 0.05% or less.
[0035]
　Ca: 0 ~
　0.0200% Ca is an arbitrary element may not be contained. That, Ca content may be 0%.
　Ca controls the inclusions, having a further effect of suppressing耐内surface cracking. From the standpoint of this effect, Ca content may be 0%, and may be 0.0001% or more, may also be 0.0010% or more, be more than 0.0030% or it may be.
　On the other hand, if excessively contained a Ca, effect is saturated which may lead to increase in cost. Therefore, Ca content is less 0.0200%. Ca content is preferably not more than 0.0100%, more preferably at most 0.0070%.
[0036]
　Mg: 0 ~
　0.0200% Mg is an optional element and may not be contained. That, Mg content may be 0%.
　Mg controls the inclusions, having a further effect of suppressing耐内surface cracking. From the viewpoint of such an effect, Mg content may be 0%, and may be 0.0001% or more.
　On the other hand, if excessively contained the Mg, the effect is saturated in some cases cause an increase in cost. Thus, Mg content is less 0.0200%. Mg content is preferably not more 0.0100% or less, more preferably 0.0050% or less, still more preferably 0.0020%.
[0037]
　Zr: 0 ~
　0.0200% Zr is an optional element and may not be contained. That, Zr content may be 0%.
　Zr controls the inclusions, having a further effect of suppressing耐内surface cracking. From the standpoint of this effect, Zr content may be 0%, and may be 0.0001% or more.
　On the other hand, if excessively contained the Zr, the effect is saturated in some cases cause an increase in cost. Accordingly, Zr content is less 0.0200%. Zr content is preferably not more than 0.0100%, more preferably 0.0050% or less, still more preferably 0.0020%.
[0038]
　REM: 0 ~
　0.0200% REM is an optional element and may not be contained. That, REM content may be 0%.
　Here, "REM" is a rare earth element, i.e., Sc, Y, La, Ce , Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and from the group consisting of Lu It refers to at least one element selected.
　REM controls the inclusions, having a further effect of suppressing耐内surface cracking. From the viewpoint of such an effect, REM content may be 0%, and may be 0.0001% or more, may be 0.0005% or more.
　On the other hand, if excessively contained the REM, effect is saturated which may lead to increase in cost. Therefore, REM content is less 0.0200%. REM content is preferably not more 0.0100% or less, more preferably 0.0050% or less, still more preferably 0.0020%.
[0039]
　Balance: Fe and impurities
　in the chemical composition of the base metal, the balance excluding the respective elements described above is Fe and impurities.
　Here, the impurities, the raw materials (e.g., ores, scraps, etc.) components contained in, or a component mixed in the manufacturing process, intentionally refer to a component not intended to be contained in the steel.
　Impurities include any element other than the above elements. Elements as impurities, may be two or more be only one.
　As impurities, e.g., B, Sb, Sn, W , Co, As, Pb, Bi, H and the like.
　Usually, Sb, Sn, W, Co, and contamination of the content of less than 0.1% for example for As are mixed content 0.005% or less for example for Pb and Bi are for B for example content 0 .0003% less contamination, contamination of the following content 0.0004% eg for H, can have, respectively. The content of the other elements, as long as the normal range, there is no particular need to control.
[0040]
　Chemical composition of the base metal, from the viewpoint of obtaining the effects of the elements, Cu: 0% than 1.00% or less, Ni: 0% than 1.00% or less, Cr: 0% than 1.00% or less , Mo: 0% than 0.50% or less, V: 0% greater than 0.20%, W: 0% greater than 0.10%, Ca: 0% than 0.0200% or less, Mg: 0% greater 0.0200% or less, Zr: 0% and 0.0200% or less, and, REM: may contain one or more kinds selected from 0 percent 0.0200% group consisting of.
[0041]
　V c90 : 200 or more
　in the chemical composition of the base metal, V being defined by the following formula (i) c90 is a value that is a hardenability index of steel.
[0042]
　log V c90 = 2.94-0.75 (.beta.a-1) ... formula (i)
　.beta.a = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + Mo ... formula (ii)
in [formula (i), .beta.a the formula (ii) is a value defined by.
　Wherein (ii), each element symbol represents the mass% of each element. ]
[0043]
　V C90 higher, the lower the hardenability of steel, ferrite is likely to generate.
　The ERW steel pipe of the present disclosure, the area ratio of ferrite is 80% or more.
　From the viewpoint of achieving the area ratio of such ferrite, V c90 is 200 or more. V c90 is preferably 220 or more, more preferably 240 or more.
　V C90 is not particularly limited to the upper limit of. From the viewpoint of production suitability of ERW pipe, V c90 is preferably 1500 or less.
[0044]
　Ti / C ratio: 0.85 to 5.00
　in the chemical composition of the base metal, Ti / C ratio (. That is, in other words containing the mass ratio of Ti with respect to C, mass% ratio of Ti to the mass% of C) is 0.85 or more. Thus, it is possible to improve the strength of the steel by TiC precipitation. Ti / C ratio is preferably 1.00 or more, more preferably 1.10 or more.
　In the chemical composition of the base metal, Ti / C ratio is 5.00 or less. Thus,耐内surface cracking resistance deterioration due to precipitation of coarse TiC and / or TiN is suppressed. Ti / C ratio is preferably 4.00 or less, more preferably 3.00 or less.
[0045]

　ERW steel pipe of the present disclosure, the thickness center part of the metal structure in the L cross section of the base metal, and the area ratio of ferrite is 80% or more, ferrite crystal grains having an average grain diameter is at 10μm or less, an average aspect ratio of ferrite grains is 2.0 or less.
　By thick central portion of the metal structure in the L cross section of the base metal satisfies these conditions, thereby improving the耐内surface cracking resistance seam welded steel pipe.
　Here, the L cross-section refers to a cross section parallel to the tube axis direction and the thickness direction.
[0046]
　Area ratio of ferrite: 80% or more
　in the thickness center portion of the metal structure in the base metal of the L cross-section, the area ratio of ferrite is 80% or more. Accordingly, since it possible to evenly distribute the forming distortion by bending in each crystal grain is improved耐内surface cracking.
　When the area ratio of ferrite is less than 80%, the hard phase (i.e., phase consisting tissues other than ferrite) and soft phase (i.e., phase made of ferrite) for distortion at the boundary between the concentrates, bending the inner surface at the time of molding cracking is likely to occur (i.e., degrades耐内surface cracking resistance).
　Area ratio of ferrite is preferably 85% or more, more preferably 90% or more.
　Area ratio of the ferrite may be a 100%, may be less than 100%.
[0047]
　Here, the ferrite area ratio of, represents the area ratio of polygonal ferrite in the total metallic structure.
　When the area ratio of ferrite is less than 100%, the balance of the metal structure is preferably at least one of pearlite and bainite.
　The concept of "bainite" herein, upper bainite forms of bainitic ferrite is lath, and the form of the bainitic ferrite is both lower bainite is a plate-like encompassed [for example, Japan Institute of Metals "Materia" Vol.46 (2007), No.5, see pp.321-326]. Also, the concept of "bainite" herein is further during air cooling after coiling the hot-rolled steel sheet (i.e., when air in the form of hot coils) is generated, tempered bainite also encompassed.
　The concept of "pearlite" herein pseudo pearlite and the like.
[0048]
　Specific measurement and the remainder of the area ratio of ferrite is performed as follows.
　Conforming to JIS G 0551 (2013 years), preform 180 ° positions in the electric resistance welded steel pipe of the present disclosure (i.e., a position shifted 180 ° from the electric-resistance welded portion to the circumferential direction of the pipe. Hereinafter the same.) Of L cross-section (observation polishing the surface), and then etched by nital corrosion solution. Metal structure photograph of the thick central portion of the etched L section (hereinafter, also referred to as "metal structure photograph") to shoot. Here, the metallic structure photograph, using an optical electron microscope, 10 fields of view for 1,000-fold magnification of the field of view (0.12 mm as the real area of the cross section 2 minutes) to shoot.
　The captured metal structure photograph image processing, based on the result of the image processing, and specific measurement and the remainder of the area ratio of ferrite. Image processing is performed using, for example, the Corporation Nireco made small general-purpose image analyzer LUZEX AP.
[0049]
　Figure 1 is a metallographic photograph showing an example of the metal structure of the base metal in the electric resistance welded steel pipe of the present disclosure (optical micrograph).
　Metal structure photograph 1, in Example 1 below, which is one of the metal structure photograph used for specific measurement and the remainder of the area ratio of ferrite (1 field).
　As shown in FIG. 1, a metal structure according to this example is a metal structure mainly composed of polygonal ferrite.
[0050]
　The average crystal grain size of ferrite crystal grains: 10 [mu] m or less
　in thickness central portion of the metal structure in the L cross section of the base metal, the average crystal grain size of the ferrite crystal grains is 10 [mu] m or less. Thus, the forming distortion by bending becomes easier to evenly allocated to each ferrite grain, as a result, it is possible to improve the耐内surface cracking.
　If the average crystal grain size of the ferrite grains exceeds 10 [mu] m, by forming distortion due bending is concentrated on the coarse ferrite grain, the inner surface cracking is induced (i.e., degrades耐内surface cracking resistance) when the is there.
　The average grain size of the ferrite grains is preferably 8μm or less.
　There is no particular restriction on the lower limit of the average crystal grain size of ferrite crystal grains. In terms of steel production suitability, the average crystal grain size of the ferrite grains is preferably 3μm or more, and more preferably is 4μm or more.
[0051]
　The average grain size of the ferrite crystal grains is measured as follows.
　Base metal (specifically, the base material 180 ° positions in the electric resistance welded steel pipe of the present disclosure) the thickness center part of the metal structure in the L section of observed using an SEM-EBSD apparatus (magnification 1000 times), inclination angle 15 ° determining the particle size of the ferrite grains surrounded by the more large angle grain boundaries. The particle size of the ferrite crystal grains is determined as a circle-equivalent diameter.
　In this way, all of the ferrite crystal grains contained in the field of view of 200 [mu] m (tube axis direction) × 500 [mu] m (thickness direction), obtains the respective grain size. The obtained measured values (particle size) and the arithmetic mean, the arithmetic mean value obtained, the average crystal grain size of the ferrite crystal grains.
[0052]
　The average aspect ratio of ferrite grains: 2.0
　in thick central portion of the metal structure in the base metal L cross-section, an average aspect ratio of ferrite grains is 2.0 or less. Thus, the tube axis direction (i.e., the rolling direction during the manufacture of hot-rolled steel sheet which is a material of the electric resistance welded steel pipe) can be suppressed inner surface cracking continuous to (i.e., is improved耐内surface cracking resistance).
　If the average aspect ratio of ferrite grains exceeds 2.0, the grain boundary of ferrite crystal grains is easily continuously in the axial direction of the tube. As a result, once a crack occurs on the inner surface of the electric resistance welded steel pipe, cracks developed in the tube axis direction, which may develop into a continuous crack (i.e., the inner surface cracking).
　The average aspect ratio of ferrite grains is preferably 1.8 or less, more preferably 1.6 or less.
[0053]
　The average aspect ratio of ferrite grains is, of course with its definition viewed, is 1.0 or more. The average aspect ratio of ferrite grains is preferably greater than 1.0, more preferably 1.1 or more, further preferably 1.2 or more.
[0054]
　The average aspect value of ferrite crystal grains is measured as follows.
　Base metal (specifically, the base material 180 ° positions in the electric resistance welded steel pipe of the present disclosure) the thickness center part of the metal structure in the L section of observed using an SEM-EBSD apparatus (magnification 1000 times), inclination angle 15 ° or more large angle ferrite grain shape surrounded by the grain boundaries of the ellipse approximation. In the obtained ellipse, the ratio of major axis length to minor axis length (i.e., major axis length / minor axis length ratio), a ferrite crystal grain aspect ratio.
　In this way, the 200 [mu] m (tube axis direction) × 500 [mu] m all ferrite crystal grains contained in the field of view of (thickness direction), obtains a respective aspect ratio. The resulting measured value (aspect ratio) is the arithmetic mean, the obtained arithmetic mean value as the average crystal grain size of the ferrite crystal grains.
[0055]
　Incidentally, generally, the long-axis direction tube axis direction of the electric resistance welded steel pipe (i.e., the rolling direction during the manufacture of hot-rolled steel sheet which is a material of the electric resistance welded steel pipe) substantially coincides with, the short axis direction of the electric resistance welded steel pipe substantially coincides with the thickness direction.
[0056]
　ERW steel pipe of the present disclosure, the tensile strength of the tube axis direction is 750 ~ 1000 MPa.
　By tensile strength of the tube axis direction is greater than or equal to 750 MPa, the strength of the electric resistance welded steel pipe for the torsion beam is secured. Tensile strength in the tube axis direction is preferably 800MPa or more.
　By tensile strength of the tube axis direction is less than 1000 MPa, improves耐内surface cracking. Tensile strength in the tube axis direction is preferably 990MPa or less.
[0057]
　Tensile strength in the tube axis direction of the electric resistance welded steel pipe of the present disclosure is measured in the following manner.
　From the base material 180 ° positions in the electric resistance welded steel pipe of the present disclosure, collecting JIS 12 No. Tensile test specimen. The collected JIS 12 No. Tensile test specimen, subjected to a tensile test in the tube axis direction in conformity with JIS Z 2241 (2011 years), to measure the tensile strength of the tube axis direction. The results obtained, the tensile strength of the tube axis direction of the electric resistance welded steel pipe of the present disclosure.
[0058]
　There is no particular limitation on the outer diameter of the electric resistance welded steel pipe of the present disclosure. From the viewpoint of production suitability of ERW pipe, the outer diameter is preferably 50 ~ 150 mm.
　To the thickness of the electric resistance welded steel pipe of the present disclosure is not particularly limited. From the viewpoint of production suitability of ERW pipe, the thickness of the electric resistance welded steel pipe of the present disclosure is preferably 2.0 ~ 4.0 mm.
[0059]
　ERW steel pipe of the present disclosure is preferably a Azuroru ERW.
　Here, the Azuroru ERW pipe (As-rolled electric resistance welded steel pipe), after pipe refers to electric resistance welded steel tube heat treatment other than the seam heat treatment is not subjected.
　The "granulation tract", the hot-rolled steel sheet and open pipe by roll forming, refers to the process up to the butt portion of the resulting open pipe by electric resistance welding to form a seam welded weld.
　The "roll forming" refers to forming an open tubular by bending the hot rolled steel sheet.
[0060]
　It is Azuroru ERW can be confirmed by the yield elongation is not observed when performing a tensile test in the tube axis direction.
　Azuroru ERW steel pipe, yield elongation is not observed when performing a tensile test in the tube axis direction.
　In contrast, after pipe, electric resistance welded steel tube heat treatment other than the seam heat treatment (e.g., tempering) has been subjected, yield elongation is observed when a tensile test was carried out in the tube axis direction.
[0061]
　ERW steel pipe of the present disclosure are used in the manufacture of the torsion beam.
　Production of torsion beam using ERW steel pipe of the present disclosure, for example, carried out by subjecting the molded bending to a part of the electric resistance welded steel pipe of the present disclosure. Bending, for example, of the electric resistance welded steel pipe of the present disclosure, a part of the parallel linear regions in the axial direction of the tube, carried out by pushing in the direction toward the inside from the outside of the electric resistance welded steel pipe (e.g., below Referring bending test shown in FIG. 2). Thus, for example, substantially V-shaped closed cross-section (for example, refer to FIG. 3 described later) is torsion beam includes a portion having a is produced.
[0062]
　In general, if the radius of curvature R of the inner surface of the bent portion by bending is small, it tends to inner surface cracking easily occurs.
　However, according to the electric resistance welded steel pipe of the present disclosure having excellent耐内surface cracking resistance, in this case, the occurrence of internal surface cracks is effectively suppressed.
　Therefore, the effect of耐内surface cracking resistance improvement by ERW steel pipe of the present disclosure, when the radius of curvature R of the inner surface of the bent portion by bending is small, is particularly effectively exhibited.
　In other words, the effect of耐内surface cracking resistance improvement by ERW steel pipe of the present disclosure, ERW steel pipe of the present disclosure, the radius of curvature R is small bend of the inner surface (e.g., the radius of curvature R of the inner surface with respect to the thickness 2 times or less (preferably from 0.7 to 2 times, more preferably 1 to 2 times) when used in the preparation of the torsion beam including a bend) a, it is particularly effectively exhibited.
[0063]

　One example of a method of manufacturing the electric resistance welded steel pipe of the present disclosure include the following method A.
　Process A is
　a slab preparation step of preparing a slab having the chemical composition of the present disclosure,
　the prepared slab heated to a slab heating temperature of 1070 ~ 1300 ° C., to the heated slab, hot rolling finishing temperature by subjecting to hot rolling to 920 ° C. or higher, and hot rolled to obtain a hot-rolled steel sheet,
　a hot-rolled steel sheet obtained by hot rolling step, 650 to at the primary cooling rate of 40 ~ 100 ° C. / s and primary cooling until the holding temperature of 700 ° C., and then the holding 3 ~ 15s (retention time) at a temperature and held, then at 60 ° C. / s or more secondary cooling rate, 550 ° C. or less of the winding temperature a cooling step of secondary cooling until,
　by the hot-rolled steel sheet after secondary cooling wind at the coiling temperature, the winding process to obtain a hot coil made of hot-rolled steel sheet,
　hot-rolled from the hot coil unwinding the steel sheet, unwound the heat By the rolled steel sheet and open pipe by roll forming to form the electric-resistance welded portion by electric resistance welding the abutting portions of the resulting open pipe, a pipe manufacturing process to obtain a seam welded steel pipe,
　resulting electrodeposition the outer diameter of the sewing steel pipe, the sizer, a reduced diameter step of diameter reduction by more than 2.0% of the diameter reduction
including.
[0064]
　According to this process A, a is 750 ~ 1000 MPa tensile strength in the tube axis direction, and, in the thickness center portion of the metal structure in the L cross section of the base metal, and the area ratio of ferrite is 80% or more, the average grain size of the ferrite crystal grains is not more 10μm or less, an average aspect ratio of ferrite grains is 2.0 or less, likely to produce ERW steel pipe of the present disclosure.
[0065]
(Slab preparation step)
　in production method A, the slab preparation step is a step of preparing a slab having the chemical composition described above.
　Slab preparation step may be a step of manufacturing the slab, or may be only steps simply preparing a slab which has been prefabricated.
　When producing a slab, for example, to produce a molten steel having the chemical composition described above, by using the molten steel prepared to produce a slab. At this time, may be produced slabs by a continuous casting process, to produce an ingot with molten steel may be an ingot slabbing to prepare a slab.
[0066]
(Hot rolling process)
　in the production method A, the hot rolling process, the slabs were prepared as described above was heated to a slab heating temperature of 1070 ~ 1300 ° C., to the heated slab, hot rolling finishing temperature 920 ° C. or higher and by performing hot rolling of a step of obtaining a hot-rolled steel sheet.
[0067]
　By heating the slab to a slab heating temperature of 1070 ~ 1300 ° C., carbides precipitated in molten steel solidification process, a nitride compound and carbonitride compounds can be dissolved in the steel. As a result, the carbide in the ferrite can be finely dispersed after hot rolling, thereby improving the strength without degrading耐内surface cracking. It is also possible to suppress the generation of microvoids in the molding.
　When the slab heating temperature is 1070 ° C. or higher, and precipitated in molten steel solidification process, carbides, nitride compounds and carbonitride compounds, can be sufficiently dissolved in the steel.
　When the slab heating temperature is 1300 ° C. or less, it is possible to suppress the coarse AlN is precipitated during cooling after hot rolling or during hot rolling.
[0068]
　It hot rolling finish temperature is 920 ° C. or more, instead of performing hot rolling in the pre-recrystallization region, it means to perform the hot rolling at a recrystallization region. Thus, in the resulting electric resistance welded steel pipe, easily it is realized that the average aspect ratio of ferrite grains is 2.0 or less.
[0069]
(Cooling step and the winding step)
　in production method A, the cooling step, the hot rolled steel sheet obtained by hot rolling step, the holding temperature of 650 ~ 700 ° C. at a primary cooling rate of 40 ~ 100 ℃ / s in step was primary cooling, and then hold the retention time of 3 ~ 15s at the holding temperature and then secondary cooling at 60 ° C. / s or more secondary cooling rate until 550 ° C. or less of the winding temperature to is there.
　In Process A, the winding process, by hot-rolled steel sheet after secondary cooling wind at the coiling temperature, a step of obtaining a hot coil made of hot-rolled steel sheet.
[0070]
　When the primary cooling rate is 40 ~ 100 ℃ / s, in the resulting electric resistance welded steel pipe, tends average crystal grain size of the ferrite grains is achieved that is 10μm or less.
[0071]
　When the holding temperature is 650 ° C. or higher, the electric resistance welded steel pipe obtained easily is achieved that the area ratio of ferrite is 80% or more.
　When the holding temperature is 700 ° C. or less, in the resulting electric resistance welded steel pipe, tends average crystal grain size of the ferrite grains is achieved that is 10μm or less.
　If the holding time is 3s above, in an electric resistance welded steel pipe obtained easily it is achieved that the area ratio of ferrite is 80% or more.
　If the holding time is less 15s, in the resulting electric resistance welded steel pipe, tends average crystal grain size of the ferrite grains is achieved that is 10μm or less.
[0072]
　When the secondary cooling rate is 60 ° C. / s or higher, the obtained electric resistance welded steel pipe, the average crystal grain size of the ferrite crystal grains is 10μm or less, and the tensile strength of the tube axis direction is not less than 750MPa it is realized easy.
[0073]
　When the winding temperature is 550 ° C. or less, in the resulting electric resistance welded steel pipe, the average crystal grain size of the ferrite crystal grains is 10μm or less, and, realizing that the tensile strength of the tube axis direction is greater than or equal to 750MPa likely to be.
[0074]
(Pipe-step)
　pipe production process, unwinding the hot rolled steel sheet from the hot coil, and an open pipe by roll forming the unwound hot rolled steel sheets, the butt portion of the resulting open pipe by electric resistance welding by forming the electric-resistance welded portion Te is a step of obtaining the electric resistance welded steel pipe.
　Pipe production step can be carried out according to known methods.
[0075]
(Diameter step)
　diameter step, the outer diameter of the electric resistance welded steel pipe obtained in pipe making process, the sizer is a step of reducing the diameter by up to 2.0% diameter reduction.
　When diameter reduction is 2.0% or less, in the resulting electric resistance welded steel pipe, easily it is realized that the average aspect ratio of ferrite grains is 2.0 or less.
[0076]
　Each step of the above method A does not affect the chemical composition of the steel.
　Therefore, the chemical composition of the base metal of the electric resistance welded steel pipe produced by production method A can be regarded as similar to the chemical composition of the raw material (molten steel or slab).
[0077]
　Process A is optionally may include other steps.
　The other steps, prior to about a by condensation径工a after pipe step, a step of the seam heat treatment of the electric resistance welding of electric resistance welded steel pipe, and the like.
Example
[0078]
　Hereinafter, Examples of the present disclosure, the present disclosure is not limited to these examples.
[0079]
[Examples 1-16, Comparative Examples 1-12]

　According aforementioned process A, to obtain a seam welded steel pipe of the examples respectively. By appropriately changing production conditions or the chemical composition of the electric resistance welded steel pipe of each example (see Table 2), to give respectively an electric resistance welded steel pipes of Comparative Examples 1-12.
　The following shows the details.
[0080]
　Molten steel having the chemical composition shown in Table 1 was smelted (steel A ~ W) in a furnace, to produce a thickness of 250mm slab by casting (slab preparation step).
[0081]
　Balance excluding the elements shown in Table 1 is Fe and impurities.
　In Table 1, REM in the steel H is La, REM in the steel P is Ce.
　Table 1 in V c90 is, V is defined by the above equations (i) and Formula (ii) c90 is.
　Ti / C in Table 1 means the content mass ratio of Ti with respect to C.
　Table 1 and underlined in Table 2 indicates that it is outside the scope of this disclosure.
[0082]
　The slab obtained above was heated to a slab heating temperature shown in Table 2, with respect to the heated slab, hot rolling finish temperature by applying hot rolling to a temperature shown in Table 2, the hot rolling It was obtained steel sheet (hot rolling process).
　To hot-rolled steel sheet obtained by hot rolling step, the conditions shown in Table 2, the primary cooling, holding, and subjected to secondary cooling, followed by winding at a winding temperature shown in Table 2, the thickness to obtain a hot coil made of hot-rolled steel sheet of 3.0 mm (the cooling step and the winding step).
　Next, unwinding hot-rolled steel sheet from the hot coil, the unwound hot rolled steel sheet and open pipe by roll forming, the electric resistance welding portion by electric resistance welding the abutting portions of the resulting open pipe formed by to obtain ERW pipe shrinkage diameter front (pipe production step).
　The outer diameter of the electric resistance welded steel pipe obtained condensation diameter front, the sizer, by diameter at diameter reduction shown in Table 2, an electric resistance welded steel pipe wall thickness outer diameter of 90mm is 3.0mm It was obtained.
[0083]

　for thick central portion of the metal structure in the L cross section of the base metal of the resulting electric resistance welded steel pipe, by the above-described method, measuring the area ratio of ferrite and, and, to confirm the type of balance.
　The results are shown in Table 2.
　In the type of balance in Table 2, "B, P" means that the balance is at least one of bainite and pearlite.
[0084]

　The thick central portion of the metal structure in the L cross section of the base metal of the resulting electric resistance welded steel pipe by the method described above, the average aspect ratio of ferrite grains (Table 2, simply It was measured to) the "average aspect ratio".
　The results are shown in Table 2.
[0085]

　The thick central portion of the metal structure in the L cross section of the base metal of the resulting electric resistance welded steel pipe by the method described above, the average crystal grain size of the ferrite crystal grains (Table 2 simply referred to as "average grain diameter") was measured.
　The results are shown in Table 2.
[0086]

　the tube axis direction of the tensile strength of the obtained electric resistance welded steel pipe (Table 2, simply referred to as "tensile strength") was measured by the method described above.
　In any of Examples and Comparative Examples, the tensile test in the tube axis direction to measure the tensile strength, yield elongation is not observed.
　The results are shown in Table 2.
[0087]
<(Evaluation of inner surface crack depth) Bending Test>
　For the obtained electric resistance welded steel pipe, make bending test simulating production of the torsion beam was evaluated inner surface crack depth. It will be described in detail below.
[0088]
　Figure 2 is a schematic diagram of the outline of the bending test shown schematically.
　As shown in FIG. 2, the electric resistance welded steel pipe 100A is a electric resistance welded steel pipe of the examples and comparative examples,
　the lower mold 10 having a cutout portion of the V-shaped, the tip section has a substantially triangular shape a punch 12 having, was prepared.
　Here, the cutout portion of the valley of the angle θ1 of the mold 10 and, the angle θ2 of the distal end of the punch 12 are both set to 60 °.
[0089]
　In this bending test, placing the electric resistance welded steel pipe 100A into the notch portion of the lower die 10, relative arranged ERW pipe 100A, by pushing the punch 12 in the direction of the arrow P, bent along the sewing steel pipe 100A It was applied. Thus, a part of the electric resistance welded steel pipe 100A, added bending direction perpendicular to the tube axis direction L of the electric resistance welded steel pipe 100A, shown in Figure 3, the structure 100B having a closed cross section of the substantially V-shaped the formed.
　Note that the tube axis direction L of the electric resistance welded steel pipe 100A corresponds to the rolling direction during the manufacture of hot-rolled steel sheet which is a material of the electric resistance welded steel pipe 100A.
[0090]
　Figure 3 is the bending test, a schematic cross-sectional view of the cross-section schematically showing a structure obtained by applying bending to the electric resistance welded steel pipe 100A. Section of the structure shown in FIG. 3 is a cross-section perpendicular to the longitudinal direction of the structure, corresponding to the C section of the electric resistance welded steel pipe before bending (i.e., a cross section perpendicular to the tube axis direction L) to.
　As shown in FIG. 3, by applying a bending to ERW pipe 100A, structure 100B having a closed cross section of the substantially V-shaped is formed. Here, the pressing amount of the punch 12, at one end 101B (bent portion) in the closed section of the structure portion 100B, the radius of curvature R of the inner surface 102B was adjusted to 4 mm. The radius of curvature R of the inner surface of the other end portion in the closed section of the structure portion 100B also adjusted to 4 mm.
　Section (specifically, cross section corresponding to FIG. 3) of the one end portion 101B of the inner surface 102B and the vicinity thereof in, by observing with a scanning electron microscope (SEM), the inner surface cracking depth (hereinafter, "inner surface cracking depth is also called ") was measured.
　Here, the inner surface cracking depth was determined as follows.
　By observing the inner surface 102B and its vicinity in the cross section of the one end portion 101B by SEM, which confirmed the presence of the inner surface cracking. If the inner surface crack is present, for each of the inner surface cracking, respectively, we obtain a linear distance from the origin of the crack to the end point to determine the depth of each inner surface cracking. The maximum depth of each inner surface cracking was evaluated as "inner surface cracking depth" in the Examples and Comparative Examples. If the inner surface cracking does not exist, "the inner surface cracking depth" in the examples and comparative examples were as "0μm".
　The resulting inner surface cracking depth shown in Table 2.
[0091]
　In the evaluation of the inner surface crack depth, the smaller the inner surface crack depth, excellent 耐内 surface cracking. It inner surface cracking depth of 0μm is the inner surface cracking does not at all occur, i.e., the better conspicuously 耐内 surface cracking.
[0092]
[Table 1]

[0093]
[Table 2]

[0094]
　As shown in Table 2, the electric resistance welded steel pipes of Examples 1 to 16 corresponding to the electric resistance welded steel pipe of the present disclosure, despite showing good tensile strength, no inner surface cracking occur at all, excellent tensile and strength and excellent耐内surface cracking resistance was has been achieved.
　In contrast, V c90 is less than 200 (i.e., hardenability of steel becomes too high) In Comparative Examples 1-4, and 6, the ferrite area ratio is less than 80%,耐内surface cracking resistance is deteriorated.
　Also, V c90 but is 200 or more, in Comparative Examples 5 and 12 Ti / C is less than 0.85, the tensile strength is lowered.
　Although having a chemical composition in the present disclosure, in Comparative Examples 7-9 The average crystal grain size of the ferrite crystal grains is 10μm greater,耐内surface cracking resistance is deteriorated.
　Although having a chemical composition in the present disclosure, in Comparative Examples 9-11 the average aspect ratio of ferrite grains is 2.0 greater,耐内surface cracking resistance is deteriorated.
[0095]
　Japanese disclosure of patent application 2016-195680 its entirety is incorporated herein by reference.
　All documents described herein, patent applications, and technical standards, each individual publication, patent application, and that the technical specification is incorporated by reference to the same extent as if marked specifically and individually, It incorporated by reference herein.

The scope of the claims
[Requested item 1]
　Includes a base metal and the electric-resistance welded portion,
　the chemical composition of the base metal is, by
mass%,
C: 0.04 ~ 0.12%, Si: 0.03 ~
1.20%, Mn: 0.
~
2.50%

0.010%,30, Ti: 0.08 ~ 0.24%, Al: 0.005 ~
0.500%, Nb: 0
~
0.06% .01, N: 0.0005
~ 0.0100%, Cu: 0 ~
1.00%, Ni: 0 ~ 1.00%, Cr: 0 ~
1.00%, Mo: 0 ~
%
0.50,
V: 0 ~
0.20%, W: 0 ~ 0.10%, Ca: 0 ~ 0.0200%,
Mg: 0 ~ 0.0200%, Zr: 0 ~ 0.0200%,
REM: 0 ~ 0.0200%, and,
the balance Fe and impurities, V
is defined by the following formula (i) c90 is not less than 200,
T for C Containing the mass ratio is 0.85 to 5.00,
　in the thickness center portion of the metal structure in the L cross-section of the base metal, and the area ratio of ferrite is 80% or more, ferrite crystal grains having an average grain diameter is at 10μm or less, an average aspect ratio of ferrite grains is 2.0 or less,
　the torsion beam for ERW pipe tensile strength of the tube axis direction is 750 ~ 1000 MPa.
　log V c90 = 2.94-0.75 (.beta.a-1) ... formula (i)
　.beta.a = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + Mo ... formula (ii)
in [formula (i), .beta.a the formula (ii) is a value defined by.
　Wherein (ii), each element symbol represents the mass% of each element. ]
[Requested item 2]
　Chemical composition of the base metal is, by
mass%, Cu: 0% than 1.00% or
less, Ni: 0% than 1.00% or
less, Cr: 0% than 1.00% or
less, Mo: 0% ultra 0.50% or
less, V: 0% greater than
0.20%, W: 0% greater than
0.10%, Ca: 0% than 0.0200% or
less, Mg: 0% than 0.0200% or less ,
Zr: 0% than 0.0200% or less, and,
REM: 0% greater torsion beam for ERW pipe according to claim 1, 0.0200% from the group consisting of containing one or more selected.
[Requested item 3]
　An outer diameter of 50 ~ 150 mm, the torsion beam for ERW pipe according to claim 1 or claim 2 wall thickness of 2.0 ~ 4.0 mm.