Title of invention: Electric resistance sewn steel pipe for torsion beam
Technical field
[0001]
　The present disclosure relates to electric resistance sewn steel pipes for torsion beams.
Background technology
[0002]
　Conventionally, studies have been made on steel materials used for automobile structural members (for example, automobile undercarriage parts).
　For example, Patent Document 1 discloses a hot-rolled steel sheet for machine-structured steel pipes, which is used for machine-structured steel pipes such as automobile suspension parts steel pipes and has excellent fatigue characteristics and bend formability.
[0003]
　Patent Document 1: International Publication No. 2009/133965
Outline of the invention
Problems to be solved by the invention
[0004]
　The torsion beam, which is one of the undercarriage parts of an automobile, is required to have high strength (particularly, tensile strength in the pipe axis direction).
　On the other hand, the torsion beam may be manufactured by bending a torsion beam electric resistance sewn steel pipe. In this case, cracks (hereinafter, also referred to as “inner surface cracks”) may occur on the inner surface of the bent-formed portion (hereinafter, also referred to as “bent portion”) of the electrosewn steel pipe. From the viewpoint of bending formability of the electro-sewn steel pipe, it may be required to improve the inner surface crack resistance of the electro-sewn steel pipe.
　Here, the inner surface cracking resistance means a property that can suppress inner surface cracking when bending and forming an electrosewn steel pipe.
　In the above-mentioned Patent Document 1, no study has been made from the viewpoint of improving the internal crack resistance of the steel pipe, and there is still room for further improvement.
[0005]
　An object of the present disclosure is to provide an electrosewn steel pipe for a torsion beam which is excellent in tensile strength in the pipe axial direction and also has excellent inner surface cracking resistance.
Means to solve problems
[0006]
　Means for solving the above problems include the following aspects.
<1>
　The chemical composition of the base material portion including the base material portion and the electrosewn welded portion is
C: 0.05 to 0.30%,
Si: 0.03 to 1.20%,
Mn in mass%. : 0.30 to 2.50%,
P: 0 to 0.030%,
S: 0 to 0.010%,
Ti: 0.010 to 0.200%,
Al: 0.005 to 0.500%,
Nb: 0.010 to 0.040%,
N: 0 to 0.006%,
B: 0.0005 to 0.0050%,
Cu: 0 to 1.000%,
Ni: 0 to 1.000%,
Cr : 0 to 1.00%,
Mo: 0 to 0.50%,
V: 0 to 0.200%,
W: 0 to 0.100%,
Ca: 0 to 0.0200%,
Mg: 0 to 0. It consists of 0200%,
Zr: 0 to 0.0200%,
REM: 0 to 0.0200%, and the
balance: Fe and impurities.
V c90 defined by the following formula (i) is 2 to 150, the
Ti content mass ratio to N is 3.4 or more, and
　L at a position displaced by 180 ° in the tube circumferential direction from the tempered welded portion. In the cross section, the metal structure in the central part of the wall thickness is the tempered martensite structure, the average aspect ratio of the former austenite grains in the tempered martensite structure is 2.0 or less, and in the
　central part of the wall thickness in the C cross section, the above. The metallographic structure in the region within the wall thickness distance from the electrosewn weld contains the tempered martensite phase and at least one of the tempered martensite phase and the ferrite phase,
　and yield elongation is obtained when a pipe axial tensile test is performed. Is observed, and
　the tensile strength in the pipe axis direction is 750 to 980 MPa.
　log V c90 = 2.94-0.75Betaei ... formula (i)
　.beta.a = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + 2Mo ... formula (ii)
in [formula (i), .beta.a the value defined by formula (ii) Is.
　In formula (ii), each element symbol represents the mass% of each element. ]
[0007]
<2> The chemical composition of the base material is, in mass%,
Cu: more than 0% and 1.000% or less,
Ni: more than 0% and 1.000% or less,
Cr: more than 0% and 1.00% or less,
Mo. : More than 0% and less than 0.50%,
V: More than 0% and less than 0.200%,
W: More than 0% and less than 0.100%,
Ca: More than 0% and less than 0.0200%,
Mg: More than 0% and 0. The torsion beam electric power according to <1>, which contains at least one selected from the group consisting of 0200% or less,
Zr: more than 0% and 0.0200% or less, and
REM: more than 0% and 0.0200% or less.
Sewing steel pipe.
[0008]
<3> The electrosewn steel pipe for a torsion beam according to <1> or <2>, wherein the average particle size of the packet grains in the tempered martensite structure is 10 μm or less.
<4> The electric resistance sewn steel pipe for a torsion beam according to any one of <1> to <3>, wherein the dislocation density at the central portion of the wall thickness in the L cross section is 2.0 × 10 14 m- 2 or less.
<5> The electric resistance sewn steel pipe for a torsion beam according to any one of <1> to <4>, which has an outer diameter of 50 to 150 mm and a wall thickness of 2.0 to 4.0 mm.
Effect of the invention
[0009]
　According to the present disclosure, there is provided an electrosewn steel pipe for a torsion beam which is excellent in tensile strength in the pipe axial direction and also has excellent inner surface cracking resistance.
A brief description of the drawing
[0010]
FIG. 1 is a schematic cross-sectional view conceptually showing a part of the C cross section of the electric resistance welded steel pipe according to an example of the present disclosure, and is within the distance of the wall thickness from the electric resistance welded portion in the central portion of the wall thickness of the C cross section. It is a figure for demonstrating the region (that is, the vicinity of the electric resistance welded part).
FIG. 2 is a schematic diagram schematically showing an outline of a bending test in an example.
FIG. 3 is a schematic cross-sectional view schematically showing a cross section of a structure obtained by bending a electrosewn steel pipe in a bending test in an example.
Mode for carrying out the invention
[0011]
　In the present specification, the numerical range represented by using "-" means a range including the numerical values ​​before and after "-" as the lower limit value and the upper limit value.
　In the present specification, the term "process" is included in this term not only as an independent process but also as long as the intended purpose of the process is achieved even if it cannot be clearly distinguished from other processes. Is done.
　In the numerical range described stepwise in the present specification, the upper limit value or the lower limit value of the numerical range described stepwise may be replaced with the upper limit value or the lower limit value of the numerical range described stepwise. , Or you may replace it with the value shown in the examples.
[0012]
　In the present specification, "%" indicating the content of a component (element) means "mass%".
　In the present specification, the content of C (carbon) may be referred to as "C content". The content of other elements may be described in the same manner.
　In the present specification, the "L cross section" refers to a cross section parallel to the pipe axis direction and the wall thickness direction, and the "C cross section" refers to a cross section perpendicular to the pipe axis direction.
[0013]
　The torsion beam electric sewn steel tube (hereinafter, also simply referred to as “electric sewn steel tube”) of the present disclosure includes a base metal portion and an electric sewn welded portion, and the chemical composition of the base metal portion is mass% and C: 0.05. ~ 0.30%, Si: 0.03 ~ 1.20%, Mn: 0.30 ~ 2.50%, P: 0 ~ 0.030%, S: 0 ~ 0.010%, Ti: 0. 010 to 0.200%, Al: 0.005 to 0.500%, Nb: 0.010 to 0.040%, N: 0 to 0.006%, B: 0.0005 to 0.0050%, Cu : 0 to 1.000%, Ni: 0 to 1.000%, Cr: 0 to 1.00%, Mo: 0 to 0.50%, V: 0 to 0.200%, W: 0 to 0. It consists of 100%, Ca: 0 to 0.0200%, Mg: 0 to 0.0200%, Zr: 0 to 0.0200%, REM: 0 to 0.0200%, and the balance: Fe and impurities. In the L cross section at a position where V c90 defined by the formula (i) is 2 to 150, the Ti content mass ratio to N is 3.4 or more, and the position is 180 ° deviated from the tempered welded portion in the pipe circumferential direction. The metal structure in the central part of the wall thickness is the tempered martensite structure, and the average aspect ratio of the austenite grains in the tempered martensite structure is 2.0 or less. When the metallographic structure in the region within the wall thickness distance from the portion contains the tempered martensite phase and at least one of the tempered martensite phase and the ferrite phase, yield elongation is observed when a tensile test in the pipe axis direction is performed. , The tensile strength in the tube axis direction is 750 to 980 MPa.
[0014]
　log V c90 = 2.94-0.75Betaei ... formula (i)
　.beta.a = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + 2Mo ... formula (ii)
in [formula (i), .beta.a the value defined by formula (ii) Is.
　In formula (ii), each element symbol represents the mass% of each element. ]
[0015]
　In the present specification, the above-mentioned chemical composition of the base metal portion (including the fact that V c90 is 2 to 150 and the mass ratio of Ti content to N is 3.4 or more) is described as "the present disclosure. Also called "chemical composition in".
[0016]
　The electrosewn steel pipe of the present disclosure includes a base material portion and an electrosewn welded portion.
　Generally, a hot-rolled steel pipe is formed into an open pipe by forming a hot-rolled steel plate into a tubular shape (hereinafter, also referred to as "roll forming"), and the butt portion of the obtained open pipe is electric-sewn and welded. It is manufactured by forming an electric resistance welded portion (hereinafter, the process up to this point is also referred to as "pipe making"), and then, if necessary, seam heat-treating the electric resistance welded portion.
　The electrosewn steel pipe of the present disclosure is manufactured by being tempered (hereinafter, also referred to as "post-tempering tempering") after pipe making (after seam heat treatment when seam heat treatment is performed).
　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 the heat-affected zone.
　Here, the heat affected zone (hereinafter, also referred to as “HAZ”) is the effect of heat due to electric sewing welding (when seam heat treatment is performed after electric sewing welding, heat due to electric sewing welding and seam heat treatment). Refers to the part affected by).
　The heat-affected zone referred to here and the region within the distance of the wall thickness from the electric sewing welded portion (hereinafter, also referred to as “near the electric sewing welded portion”) have an overlapping portion.
[0017]
　The hot-rolled steel sheet, which is the material of the electric resistance pipe, is manufactured by using a hot strip mill. Specifically, a hot strip mill manufactures a long hot-rolled steel sheet (hereinafter, also referred to as a hot coil) wound into a coil.
　The hot-rolled steel sheet, which is the material of the electrosewn steel pipe, differs from the steel plate manufactured by using a plate mill in that it is a long steel sheet (continuous steel sheet).
　Since a steel plate is not a continuous steel sheet, it cannot be used for roll forming, which is a continuous bending process.
　The electrosewn steel pipe is clearly distinguished from the welded steel pipe (for example, UOE steel pipe) manufactured by using the thick steel plate in that it is manufactured by using the hot-rolled steel plate described above.
[0018]
　The electrosewn steel pipe of the present disclosure is excellent in tensile strength in the pipe axial direction (specifically, the tensile strength in the pipe axial direction is 750 MPa or more), and is also excellent in internal crack resistance.
　The reason why the above effect is obtained is presumed as follows. However, the electric resistance welded steel pipe of the present disclosure is not limited by the following presumed reasons.
[0019]
　The effect of excellent tensile strength in the tube axis direction is that the base metal portion has the chemical composition in the present disclosure and that the metal structure at the center of the wall thickness in the L cross section is a tempered martensite structure. Is considered to be contributing.
[0020]
　The effect of excellent inner surface crack resistance is that the metal structure at the center of the wall thickness is in the L cross section at a position 180 ° deviated from the electrosewn welded portion in the pipe circumferential direction (hereinafter, also referred to as “base metal 180 ° position”). It is considered that the tempered martensite structure contributes to this.
　Here, the 180 ° position of the base material is a position selected as a representative position of the base material portion.
[0021]
　With respect to the electrosewn steel pipe of the present disclosure, for example, the metal structure at the center of the wall thickness in the L cross section at the position of the base metal at 180 ° is the tempered martensite phase and other phases (for example, ferrite phase, tempered bainite phase, etc.). In the case of a two-phase structure composed of, the internal crack resistance deteriorates. The reason for this is that during bending molding of an electrosewn steel pipe, molding strain due to bending molding is concentrated on the boundary between two phases having different hardness on the inner surface of the bent portion and its vicinity, and as a result, inner surface cracking is likely to occur. Conceivable.
[0022]
　Further, it is considered that the effect of excellent internal crack resistance is also contributed by the fact that the average aspect ratio of the former austenite grains in the tempered martensite structure is 2.0 or less.
　With respect to the electrosewn steel pipe of the present disclosure, when the average aspect ratio of the former austenite grains in the tempered martensite structure in the metal structure at the center of the wall thickness in the L cross section is more than 2.0, the inner surface crack resistance is improved. to degrade.
　The reason for this is that when the average aspect ratio of the old austenite grains is more than 2.0 (that is, when the old austenite grains are stretched), the aspect ratio of the packet grains in the old austenite grains is also large (that is, when the old austenite grains are stretched). , The packet grain is also stretched), and as a result, it is considered that the packet grain boundary is likely to be continuous. For this reason, it is considered that during bending molding, the tearing of the packet grain boundary is likely to propagate along the packet grain boundary on the inner surface of the bent portion and its vicinity, and as a result, the inner surface cracking which is a continuous crack is likely to occur. Be done.
　In the electrosewn steel pipe of the present disclosure, the average aspect ratio of the former austenite grains is set to 2.0 or less, and the shape of the packet grains in the former austenite grains is made closer to a spherical shape. As a result, it is considered that the continuity of the packet grain boundaries is suppressed, the progress of the tearing of the packet grain boundaries described above is suppressed, and as a result, the inner surface cracking is suppressed (that is, the inner surface cracking resistance is improved). ..
[0023]
　The tempered martensite structure at the center of the wall thickness in the L cross section at the 180 ° position of the base metal has the chemical composition in the present disclosure and the manufacturing conditions of the electric resistance pipe (the production condition of the hot-rolled steel sheet which is the material of the electric resistance steel pipe). Includes) and is built by the combination of.
　Specifically, in the hot-rolling step and the cooling step for producing the hot-rolled steel sheet having the chemical composition in the present disclosure, substantial quenching is performed, and the base metal portion (typically, the base metal at 180 ° position) is subjected to substantial quenching. As a structure, a martensite structure (that is, an unquenched martensite structure; the same applies hereinafter) is built as it is quenched, and then a tempered martensite structure is built by tempering after tube making.
　Further, the above-mentioned average aspect ratio of the austenite grains in the tempered martensite structure is 2.0 or less, which means that rolling is performed in the recrystallization region in the hot rolling process (for example, the hot rolling finish temperature is 920). Achieved by (above ℃).
　An example of the method for manufacturing the electric resistance welded steel pipe of the present disclosure will be described later.
[0024]
　In the electric resistance welded steel pipe of the present disclosure, the metal structure in the vicinity of the electric sewing welded portion (that is, the region within the distance of the wall thickness from the electric sewing welded portion) in the central portion of the wall thickness of the C cross section is tempered martensite phase and tempered. Includes at least one of the bainite phase and the ferrite phase.
　The metal structure in the vicinity of the electric resistance welded portion contains at least one of a tempered bainite phase and a ferrite phase. It shows that there is.
　In the electro-sewn steel pipe that has been hardened and tempered after the electro-sewn steel pipe of the present disclosure, the vicinity of the electro-sewn welded portion has a tempered martensite structure that substantially does not contain a tempered bainite phase or a ferrite phase.
　The manufacturing method for manufacturing the electrosewn steel pipe of the present disclosure, in which tempering is performed without quenching after pipe making, is superior in productivity as compared with the manufacturing method in which quenching and tempering are performed after pipe making.
　That is, the electrosewn steel pipe of the present disclosure can be manufactured by a manufacturing method having a higher productivity than a manufacturing method in which quenching and tempering are performed after the pipe is made, and therefore has an advantage of being excellent in economy (that is, low cost). ..
[0025]
　In addition, the yield elongation is observed when the pipe axial tensile test is performed on the electrosewn steel pipe of the present disclosure, and the electrosewn steel pipe of the present disclosure is tempered without quenching after the pipe is made. It shows that it is a hardened steel pipe.
　The electrosewn steel pipe of the present disclosure is tempered before the pipe is made (that is, the hot-rolled steel sheet which is the material), and is not tempered after the pipe is made. No yield elongation is observed when the test is performed (see, for example, Comparative Example 25 below).
　The electrosewn steel pipe of the present disclosure is superior in internal crack resistance as compared with an electrosewn steel pipe that has been tempered before pipe making and has not been tempered after pipe making. It is considered that the reason for this is that in the electrosewn steel pipe of the present disclosure, the pipe forming strain generated during the pipe making is reduced by tempering after the pipe making, and the dislocation density is reduced by this.
[0026]
Hereinafter, the chemical composition of the base material part in the electrosewn steel pipe of the present disclosure (that is, the chemical composition of the base material portion) will be described.
[0027]
　C: 0.05 to 0.30%
　C is an element that improves the strength of steel. If the C content is less than 0.05%, the intensity required for the torsion beam may not be obtained. Therefore, the C content is 0.05% or more. The C content is preferably 0.08% or more, more preferably 0.10% or more.
　On the other hand, if the C content exceeds 0.30%, the strength may be excessively increased and the inner surface crack resistance may be deteriorated. Therefore, the C content is 0.30% or less. The C content is preferably 0.25% or less, more preferably 0.20% or less.
[0028]
　Si: 0.03 to 1.20%
　Si is an element used for deoxidation. If the Si content is less than 0.03%, deoxidation may be insufficient and coarse Fe oxide may be produced. Therefore, the Si content is 0.03% or more. The Si content is preferably 0.10% or more, more preferably 0.20% or more.
　On the other hand, if the Si content exceeds 1.20% , inclusions such as SiO 2 are generated, and the inclusions are used during roll forming for producing an electric resistance welded steel pipe and / or during bending forming with respect to the electric resistance sewing steel pipe. As a starting point, minute voids may easily occur. Therefore, the Si content is 1.20% or less. The Si content is preferably 1.00% or less, more preferably 0.80% or less.
[0029]
　Mn: 0.30 to 2.50%
　Mn is an important element for improving hardenability, improving the strength of steel, and finally (that is, by tempering after pipe forming) to obtain a tempered martensite structure. Is. If the Mn content is less than 0.30%, the hardenability may be insufficient and the tempered martensite structure may not be obtained. Therefore, the Mn content is 0.30% or more. The Mn content is preferably 0.60% or more, more preferably 0.70% or more.
　On the other hand, if the Mn content exceeds 2.50%, the strength may be excessively increased and the inner surface crack resistance may be deteriorated. Therefore, the Mn content is 2.50% or less. The Mn content is preferably 2.00% or less, more preferably 1.50% or less, still more preferably 1.30% or less.
[0030]
　P: 0 to 0.030%
　P is an element that can be contained as an impurity in steel. If the P content exceeds 0.030%, it tends to thicken at the packet grain boundaries, and the internal crack resistance may deteriorate. Therefore, the P content is 0.030% or less. The P content is preferably 0.020% or less.
　On the other hand, the P content may be 0%. From the viewpoint of reducing the dephosphorization cost, the P content may be more than 0%, 0.001% or more, or 0.010% or more.
[0031]
　S: 0 to 0.010%
　S is an element that can be contained as an impurity in steel. If the S content exceeds 0.010%, coarse MnS may be generated and the internal crack resistance may be deteriorated. Therefore, the S content is 0.010% or less. The S content is preferably 0.005% or less.
　On the other hand, the S content may be 0%. From the viewpoint of reducing the desulfurization cost, the S content may be more than 0%, 0.001% or more, or 0.003% or more.
[0032]
　Ti: 0.010 to 0.200%
　Ti is an element that improves the strength of steel by precipitating as TiC. Ti is also an element that contributes to the miniaturization of austenite grains by the pinning effect during hot spreading, and as a result, contributes to the miniaturization of packet grains in the old austenite grains. If the Ti content is less than 0.010%, the strength required for the torsion beam and the pinning effect on the austenite grains may not be obtained. Further, if the Ti content is less than 0.010%, N cannot be fixed as TiN and N is precipitated together with B (that is, BN is formed), which is effective in contributing to improvement of hardenability. The amount of B cannot be secured, and as a result, the effect of improving the hardenability of B may not be obtained. Therefore, the Ti content is 0.010% or more. The Ti content is preferably 0.015% or more.
　On the other hand, if the Ti content exceeds 0.200%, coarse TiC and / or TiN may precipitate, and the internal crack resistance may deteriorate. Therefore, the Ti content is 0.200% or less. The Ti content is preferably 0.150% or less, more preferably 0.120% or less, still more preferably 0.100% or less, still more preferably 0.080% or less.
[0033]
　Al: 0.005 to 0.500%
　Al produces AlN and contributes to the miniaturization of austenite grains by the pinning effect during hot spreading, and as a result, contributes to the miniaturization of packet grains in the old austenite grains. It is an element. If the Al content is less than 0.005%, the pinning effect on the austenite grains cannot be obtained, the old austenite grains become coarse, and as a result, the packet grains may become coarse. Therefore, the Al content is 0.005% or more. The 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%, coarse AlN may precipitate and the inner surface cracking resistance may deteriorate. Therefore, the Al content is 0.500% or less. The Al content is preferably 0.100% or less, more preferably 0.050% or less.
[0034]
　Nb: 0.010 to 0.040%
　Nb produces NbC and contributes to the miniaturization of austenite grains by the pinning effect during hot spreading, and as a result, contributes to the miniaturization of packet grains in the old austenite grains. It is an element. If the Nb content is less than 0.010%, the pinning effect on the austenite grains cannot be obtained, the old austenite grains become coarse, and as a result, the packet grains may become coarse. Therefore, the Nb content is 0.010% or more. The Nb content is preferably 0.020% or more.
　On the other hand, if the Nb content exceeds 0.040%, coarse NbC may be precipitated and the inner surface crack resistance may be deteriorated. Therefore, the Nb content is 0.040% or less. The Nb content is preferably 0.036% or less.
[0035]
　N: 0 to 0.006%
　N is an element that can be contained as an impurity in steel. If the N content exceeds 0.006%, coarse AlN may be generated and the internal crack resistance may be deteriorated. Therefore, the N content is 0.006% or less.
　The N content may be 0%.
　N is also an element that produces AlN and can contribute to the miniaturization of austenite grains by the pinning effect during hot spreading, and as a result, it is also an element that can contribute to the miniaturization of packet grains in the old austenite grains. From the viewpoint of obtaining such an effect, the N content may be more than 0% or 0.001% or more.
[0036]
　B: 0.0005 to 0.0050%
　B is an element that improves the hardenability of steel, and is martensite as hardened in the hot-quenching process and cooling process for producing hot-rolled steel sheet, which is the material of electrosewn steel pipes. It is an essential element for building the site structure. If the B content is less than 0.0005%, the effect may not be obtained. Therefore, the B content is 0.0005% or more. The B content is preferably 0.0010% or more.
　On the other hand, when the B content exceeds 0.0050%, B aggregates and / or precipitates, and the solid solution B segregated at the austenite grain boundaries decreases, so that the effect of improving the hardenability of steel may decrease. is there. Therefore, the B content is 0.0050% or less. The B content is preferably 0.0040% or less, and more preferably 0.0030% or less.
[0037]
　Cu: 0 to 1.000%
　Cu is an arbitrary element and may not be contained. That is, the Cu content may be 0%.
　Cu is an element that contributes to increasing the strength of steel. From the viewpoint of such an effect, the Cu content may be more than 0%, 0.005% or more, 0.010% or more, or 0.050% or more. May be good.
　On the other hand, if Cu is contained in an excessive amount, the effect may be saturated and the cost may increase. Therefore, the Cu content is 1.000% or less. The Cu content is preferably 0.500% or less, more preferably 0.300% or less, still more preferably 0.200% or less.
[0038]
　Ni: 0 to 1.000%
　Ni is an arbitrary element and may not be contained. That is, the Ni content may be 0%.
　Ni is an element that contributes to increasing the strength of steel. From the viewpoint of such an effect, the Ni content may be more than 0%, 0.005% or more, 0.010% or more, or 0.050% or more. May be good.
　On the other hand, if Ni is excessively contained, the effect may be saturated and the cost may increase. Therefore, the Ni content is 1.000% or less. The Ni content is preferably 0.500% or less, more preferably 0.300% or less, and further preferably 0.200% or less.
[0039]
　Cr: 0 to 1.00%
　Cr is an arbitrary element and may not be contained. That is, the Cr content may be 0%.
　Cr is an element that contributes to increasing the strength of steel. From the viewpoint of such an effect, the Cr content may be more than 0%, 0.005% or more, 0.01% or more, 0.05% or more. May be good.
　On the other hand, if Cr is excessively contained, the effect may be saturated and the cost may increase. Therefore, the Cr content is 1.00% or less. The Cr content is preferably 0.50% or less, more preferably 0.30% or less, still more preferably 0.20%.
[0040]
　Mo: 0 to 0.50%
　Mo is an arbitrary element and may not be contained. That is, the Mo content may be 0%.
　Mo is an element that contributes to increasing the strength of steel. From the viewpoint of such an effect, the Mo content may be more than 0%, 0.01% or more, 0.05% or more, or 0.10% or more. May be good.
　On the other hand, if Mo is contained in an excessive amount, the effect may be saturated and the cost may increase. Therefore, the Mo content is 0.50% or less. The Mo content is preferably 0.40% or less.
[0041]
　V: 0 to 0.200%
　V is an arbitrary element and may not be contained. That is, the V content may be 0%.
　V is an element that contributes to increasing the strength of steel. From the viewpoint of such an effect, the V content may be more than 0% or 0.005% or more.
　On the other hand, if V is excessively contained, the effect may be saturated and the cost may increase. Therefore, the V content is 0.200% or less. The V content is preferably 0.100% or less, more preferably 0.050% or less.
[0042]
　W: 0 to 0.100%
　W is an arbitrary element and may not be contained. That is, the W content may be 0%.
　W is an element that contributes to increasing the strength of steel. From the viewpoint of such an effect, the W content may be more than 0% or 0.005% or more.
　On the other hand, if W is excessively contained, the effect may be saturated and the cost may increase. Therefore, the W content is 0.100% or less. The W content is preferably 0.050% or less.
[0043]
　Ca: 0 to 0.0200%
　Ca is an arbitrary element and may not be contained. That is, the Ca content may be 0%.
　Ca has the effect of controlling inclusions and further suppressing internal crack resistance. From the viewpoint of such an effect, the Ca content may be more than 0%, 0.0001% or more, or 0.0010% or more.
　On the other hand, if Ca is excessively contained, coarse Ca oxide may be generated and the inner surface cracking resistance may be deteriorated. Therefore, the Ca content is 0.0200% or less. The Ca content is preferably 0.0100% or less, more preferably 0.0070% or less.
[0044]
　Mg: 0 to 0.0200%
　Mg is an arbitrary element and may not be contained. That is, the Mg content may be 0%.
　Mg has the effect of controlling inclusions and further suppressing internal crack resistance. From the viewpoint of such an effect, the Mg content may be more than 0% or 0.0001% or more.
　On the other hand, if Mg is excessively contained, the effect may be saturated and the cost may increase. Therefore, the Mg content is 0.0200% or less. The Mg content is preferably 0.0100% or less, more preferably 0.0050% or less, still more preferably 0.0020% or less.
[0045]
　Zr: 0 to 0.0200%
　Zr is an arbitrary element and may not be contained. That is, the Zr content may be 0%.
　Zr has the effect of controlling inclusions and further suppressing internal crack resistance. From the viewpoint of such an effect, the Zr content may be more than 0% or 0.0001% or more.
　On the other hand, if Zr is excessively contained, the effect may be saturated and the cost may increase. Therefore, the Zr content is 0.0200% or less. The Zr content is preferably 0.0100% or less, more preferably 0.0050% or less, still more preferably 0.0020% or less.
[0046]
　REM: 0 to 0.0200%
　REM is an arbitrary element and may not be contained. That is, the REM content may be 0%.
　Here, "REM" is composed of rare earth elements, that is, a group consisting of Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Refers to at least one element selected.
　REM has the effect of controlling inclusions and further suppressing internal crack resistance. From the viewpoint of such an effect, the REM content may be more than 0%, 0.0001% or more, or 0.0005% or more.
　On the other hand, if REM is excessively contained, coarse oxides may be generated and the internal crack resistance may be deteriorated. Therefore, the REM content is 0.0200% or less. The REM content is preferably 0.0100% or less, more preferably 0.0050% or less, still more preferably 0.0020% or less.
[0047]
　Remaining part: Fe and impurities In
　the chemical composition of the base material part, the balance excluding each element described above is Fe and impurities.
　Here, the impurity refers to a component contained in a raw material (for example, ore, scrap, etc.) or a component mixed in a manufacturing process and not intentionally contained in steel.
　Impurities include any element other than the elements described above. The element as an impurity may be only one kind or two or more kinds.
　Examples of impurities include Sb, Sn, Co, As, Pb, Bi, and H.
　Usually, Sb, Sn, Co, and As are mixed with, for example, 0.1% or less, Pb and Bi are mixed with, for example, 0.005% or less, and H is mixed with, for example, 0.0004. There can be less than% contamination, respectively. It is not necessary to control the content of other elements as long as it is within the normal range.
[0048]
　The chemical composition of the base metal is Cu: more than 0% and 1.000% or less, Ni: more than 0% and 1.000% or less, Cr: more than 0% and 1.00% or less from the viewpoint of obtaining the above effects of each element. , Mo: more than 0% 0.50% or less, V: more than 0% 0.200% or less, W: more than 0% 0.100% or less, Ca: more than 0% 0.0200% or less, Mg: more than 0% It may contain one or more selected from the group consisting of 0.0200% or less, Zr: more than 0% and 0.0200% or less, and REM: more than 0% and 0.0200% or less.
　The preferred ranges of the contents of these elements are as described above.
[0049]
　V c90 : In
　the chemical composition of the base material portion from 2 to 150, V c90 defined by the following formula (i) is a value that is an index of hardenability of steel.
　V c90 is a value known as the critical cooling rate (unit: ° C./s) at which a 90% martensite structure is obtained [for example, the paper by Ueno et al. Prediction formula "," Iron and Steel "(The Iron and Steel Institute of Japan), 1974 (1988) No. 6, pp.1073-1080].
[0050]
　log V c90 = 2.94-0.75Betaei ... formula (i)
　.beta.a = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + 2Mo ... formula (ii)
in [formula (i), .beta.a the value defined by formula (ii) Is.
　In formula (ii), each element symbol represents the mass% of each element. ]
[0051]
　The lower the V c90, the higher the hardenability of steel.
　Therefore, when V c90 is 150 or less, the formation of ferrite phase and bainite phase is suppressed, and the formation of martensite phase is promoted as it is quenched. Therefore, tempered martensite structure can be easily obtained by tempering after tube formation.
　Further, when V c90 is 2 or more, it is advantageous from the viewpoint of cost. Since it is necessary to add a large amount of alloying elements in order to reduce V c90 to less than 2, it takes a lot of time and cost to refine in the steelmaking process.
　Therefore, V c90 is 2 to 150. The upper limit of
　V c90 is preferably 140. The lower limit of
　V c90 is preferably 10, more preferably 20.
[0052]
　Ti content mass ratio to N: 3.4 or more In
　the chemical composition of the base metal , the Ti content mass ratio to N (hereinafter, also referred to as "Ti / N ratio" or "Ti / N") is 3.4. That is all.
　When the Ti / N ratio is 3.4 or more, the effect of improving hardenability by B (boron) is effectively exhibited. This point will be described in detail below.
　As described above, B is an element that contributes to improving the hardenability of steel.
　However, even when B is contained in the steel, B existing in the form of BN (boron nitride) does not exert the function of improving hardenability. In this regard, when the Ti / N ratio in the steel is 3.4 or more, N in the steel is fixed in the form of TiN (titanium nitride). As a result, the formation of BN is suppressed, so that an effective amount of B that contributes to the improvement of hardenability is secured. As a result, the effect of improving hardenability by B (boron) is effectively exhibited.
　The Ti / N ratio is preferably 4.0 or more.
　The upper limit of the Ti / N ratio depends on the range of Ti content and the range of N content. When the N content is 0%, the Ti / N ratio is infinite. The upper limit of the Ti / N ratio is preferably 80.0, more preferably 50.0, and even more preferably 40.0.
[0053]
Next, the metal structure at the center of the wall thickness at the L cross section at the base metal 180 ° position will be described.
　Here, the central portion of the wall thickness in the L cross section at the 180 ° position of the base metal is only a position selected as a representative position of the base metal portion.
　Therefore, in the electrosewn steel pipe of the present disclosure, the metal structure at a position other than the central portion of the wall thickness in the L cross section at the position of 180 ° of the base metal in the base metal portion may have the following characteristics.
[0054]
(Tempering Martensite Structure) In
　the electrosewn steel pipe of the present disclosure, the metal structure at the center of the wall thickness is formed in the L cross section at the base metal 180 ° position (that is, the position displaced 180 ° in the pipe circumferential direction from the electrosewn welded portion). Tempering martensite structure.
　In the present disclosure, the tempered martensite structure means a single-phase structure substantially composed of the tempered martensite phase.
　Here, the single-phase structure substantially composed of the tempered martensite phase has an area ratio of the tempered martensite phase of 80% or more (preferably 90%) in the confirmation method using a scanning electron microscope (SEM) described later. It means the metal structure that is (above).
[0055]
　Whether or not the metal structure at the center of the wall thickness in the L cross section at the base metal 180 ° position is a tempered martensite structure is confirmed as follows.
　In accordance with JIS G 0551 (2013), the L cross section (observation surface) of the base metal 180 ° position in the electrosewn steel pipe is polished and then etched with a Nital corrosive liquid. A photograph of the metal structure of the central portion of the wall thickness in the etched L cross section (hereinafter, also referred to as “metal structure photograph”) is taken by a scanning electron microscope (SEM). The metallographic photograph is taken for three fields of view (one field of view is in the range of 40 μm × 40 μm) at a magnification of 3000 times.
　The area ratio of the tempered martensite phase to the entire metal structure is calculated based on the photographed metal structure photograph (SEM photograph). When the area ratio of the tempered martensite phase is 80% or more, it is determined that the tempered martensite structure is formed.
　Here, the tempered martensite phase is a phase in which the lath structure and cementite (iron-based carbide) can be confirmed on the SEM photograph, and the preferential growth direction of cementite is two or more directions (that is, random). is there.
　On the other hand, the tempered bainite phase is a phase in which the lath structure and cementite (iron-based carbide) can be confirmed on the SEM photograph, and the preferential growth direction of cementite is unidirectional.
　A phase in which it is difficult to distinguish between the tempered martensite phase and the tempered bainite phase is judged to be the tempered martensite phase.
　Further, the ferrite phase is a phase in which the lath structure cannot be confirmed on the SEM photograph.
　In addition, the as-quenched martensite phase is a phase in which the lath structure can be confirmed on the SEM photograph, but cementite cannot be confirmed.
[0056]
(Average Aspect Ratio of Old Austenite Grains) In
　the electrosewn steel pipe of the present disclosure, the average aspect ratio of the old austenite grains in the tempered martensite structure is 2.0 or less. As a result, as described above, the internal crack resistance is improved.
　When the average aspect ratio of the old austenite grains exceeds 2.0, the aspect ratio of the packet grains in the old austenite grains also becomes large, and as a result, the packet grain boundaries are easily continuous.
　Therefore, once a crack is generated on the inner surface of the bent portion when bending and forming the electrosewn steel pipe, the crack propagates along the packet grain boundary and develops into a continuous crack (that is, an inner surface crack). In some cases.
　The average aspect ratio of the former austenite grains is preferably 1.8 or less, more preferably 1.6 or less.
[0057]
　The average aspect ratio of the former austenite grains is, by definition, 1.0 or more. The average aspect ratio of the austenite grains is preferably more than 1.0, more preferably 1.1 or more.
[0058]
　Here, the average aspect ratio of the former austenite grains means the average value of the aspect ratios of the former austenite grains.
　The aspect ratio of the former austenite grain means the ratio of the major axis length to the minor axis length (that is, the major axis length / minor axis length ratio) when the former austenite grain is approximated by an ellipse.
[0059]
　The average aspect ratio of the former austenite grains is measured as follows.
　The metal structure of the central portion of the wall thickness in the L cross section of the base metal portion (specifically, the base metal 180 ° position in the electrosewn steel pipe of the present disclosure) was observed using an SEM-EBSD device (magnification: 1000 times), and the tilt angle was 15. The region surrounded by grain boundaries above ° is regarded as an old austenite grain, and the shape of this old austenite grain is approximated by an ellipse. The ratio of the major axis length to the minor axis length (that is, the major axis length / minor axis length ratio) in the obtained ellipse is defined as the aspect ratio of the old austenite grains.
　By this method, the aspect ratio of all the former austenite grains included in the visual field range of 200 μm (tube axis direction) × 500 μm (thickness direction) is determined. The obtained measured values ​​(aspect ratio) are arithmetically averaged, and the obtained arithmetic average value is used as the average aspect ratio of the former austenite grains.
[0060]
　In general, the direction of the major axis length substantially coincides with the pipe axis direction of the electric resistance pipe (that is, the rolling direction at the time of manufacturing the hot-rolled steel sheet which is the material), and the direction of the minor axis length is electric sewing. It is almost the same as the wall thickness direction of the steel pipe.
[0061]
　The average aspect ratio of the former austenite grains of 2.0 or less is achieved by rolling in the recrystallization region in the hot rolling process (for example, the hot rolling finish temperature is 920 ° C. or higher). obtain.
[0062]
(Average particle size of packet particles) In
　the electric resistance welded steel pipe of the present disclosure, the average particle size of the packet particles in the tempered martensite structure (hereinafter, also referred to as “average packet particle size”) is preferably 10 μm or less. ..
　When the average packet particle size is 10 μm or less, it is possible to prevent the molding distortion due to bending molding from concentrating on the coarse packet grains, and the molding distortion can be dispersed in each packet grain. As a result, the internal crack resistance is further improved.
　The average packet size is preferably 8 μm or less.
　There is no particular limitation on the lower limit of the average packet particle size. From the viewpoint of steel production suitability, the average packet particle size is preferably 3 μm or more, and more preferably 4 μm or more.
[0063]
　Here, the packet grain is a unit contained in one or more former austenite grains, and means a unit composed of a plurality of elongated crystals arranged substantially in parallel.
[0064]
　The average packet particle size is measured as follows.
　The metal structure at the center of the wall thickness in the L cross section at the base metal 180 ° position was observed using an SEM-EBSD device under the conditions of a magnification of 3000 times and a field of view of 40 μm × 40 μm, and an EBSD image for three fields of view was obtained. obtain.
　From the obtained EBSD image, 30 packet grains are arbitrarily selected.
　At this time, a unit composed of a plurality of elongated crystals arranged substantially in parallel (specifically, a unit surrounded by a grain boundary having an inclination angle of 10 ° or more) is regarded as a packet grain.
　Next, for the selected 30 packet grains, the circle-equivalent diameter of each packet grain is obtained, and the obtained value is used as the particle size of each packet grain.
　Next, the arithmetic average value of the particle size of the packet particles in 30 packet particles is obtained, and the obtained arithmetic average value is used as the average packet particle size (that is, the average particle size of the packet particles).
[0065]
　The average particle size of the packet grains is 10 μm or less, that
the chemical composition of the base material contains Ti, Al, and Nb in a predetermined amount or more, respectively; in the
hot rolling step, rolling is performed in the recrystallization region.
It can be achieved by doing (eg, making the hot rolling finish temperature 920 ° C. or higher); etc.
[0066]
(Dislocation Density) In
　the electrosewn steel pipe of the present disclosure, it is preferable that the dislocation density at the center of the wall thickness in the L cross section at the 180 ° position of the base metal is 2.0 × 10 14 m- 2 or less.
　When the dislocation density is 2.0 × 10 14 m- 2 or less, the internal crack resistance is further improved.
　From the viewpoint of further improving the internal crack resistance, the dislocation density is preferably 1.9 × 10 14 m- 2 or less.
　The lower limit of the dislocation density is not particularly limited. Examples of the lower limit of the dislocation density include 0.4 × 10 14 m- 2 and 0.6 × 10 14 m- 2 .
[0067]
　The dislocation density in the present disclosure is measured as follows.
　The half-value widths of the (110) plane, (211) plane, and (220) plane were measured by X-ray diffraction at the center of the wall thickness in the L cross section at the base metal 180 ° position, and Williamson was measured based on the measured values. The dislocation density is calculated according to the -Hall method (specifically, the method described in ACTA METALLURGICA Vol.1, JAN. 1953, pp.22-31).
　The above measurement and calculation are performed at three points in the central portion of the wall thickness, and the arithmetic mean value of the obtained three calculated values ​​is defined as the dislocation density in the present disclosure.
[0068]
　The conditions for X-ray diffraction are as follows. As the X-ray diffractometer used for X-ray diffraction, for example, "RINT2200" manufactured by Rigaku Corporation is used.
　Tube: Mo Tube (tube using Mo as a
　target ) Target output: 50 KV, 40 mA
　Slit: Divergence 1/2 °, Scatter 1 °, Light reception 0.15 mm
　Sampling width: 0.010 °
　Measurement range (2θ) : 34.2 ° to 36.2 °
　Maximum count number: 3000 or more
[0069]

　The electric sewing steel pipe of the present disclosure is located in the vicinity of the electric sewing welded portion (that is, within the distance of the wall thickness from the electric sewing welded portion) in the central portion of the wall thickness of the C cross section. The metallographic structure comprises a tempered martensite phase and at least one of a tempered bainite phase and a ferrite phase.
[0070]
　FIG. 1 is a schematic cross-sectional view conceptually showing a part of the C cross section of the electric resistance welded steel pipe according to an example of the present disclosure. It is a figure for demonstrating "the region within" (that is, the vicinity of the electric resistance welded portion).
　As shown in FIG. 1, in the central portion of the wall thickness of the C cross section, the region V1 within the distance of the wall thickness t from the electric stitch welded portion EW1 (that is, near the electric stitch welded portion) is the central portion of the wall thickness of the C cross section. On the curve corresponding to the above, it is a region having a length of 2 t (that is, twice the wall thickness t) centered on the electric sewing welded portion EW1. In FIG. 1, the region V1 is indicated by a chain double-dashed line.
[0071]
　The vicinity of the electric sewing welded portion in the central portion of the wall thickness of the C cross section is only a position selected as a representative position in the vicinity of the electric sewing welded portion. Therefore, the metal structure in the vicinity of the electric stitch welded portion in the portion other than the central portion of the wall thickness of the C cross section may be a metal structure containing a tempered martensite phase and at least one of a tempered bainite phase and a ferrite phase. ..
[0072]
　Whether or not the metal structure in the vicinity of the electrosewn welded portion in the central portion of the wall thickness of the C cross section contains at least one of the tempered bainite phase and the ferrite phase is confirmed as follows.
　According to JIS G 0551 (2013), the C cross section (observation surface) of the electrosewn steel pipe is polished and then etched with a Nital corrosive liquid. In the center of the wall thickness of the etched C cross section, the entire region within the distance of the wall thickness from the electric stitch welded portion (for example, the above-mentioned region V1) is observed by scanning the observation region by SEM (magnification 500 times). Then, it is confirmed whether at least one of the tempered bainite phase and the ferrite phase is present in this region.
　The method for distinguishing the tempered bainite phase, the ferrite phase, and the tempered martensite phase on the SEM photograph is as described above.
[0073]
　As described above, the fact that the metal structure in the vicinity of the electrosewn weld contains at least one of the tempered bainite phase and the ferrite phase means that the electrosewn steel pipe of the present disclosure is tempered after being made without quenching. It indicates that it is a sewn steel pipe.
　On the other hand, in the electro-sewn steel pipe that has been hardened and tempered after the pipe is formed, the metal structure in the vicinity of the electro-sewn welded portion becomes a tempered martensite structure, and substantially does not contain the tempered bainite phase or the ferrite phase.
[0074]
In the electrosewn steel pipe of the present disclosure, yield elongation is observed when a tensile test in the axial direction of the pipe is performed.
　Here, "yield elongation is observed" means that a substantial yield elongation (specifically, a yield elongation of 0.1% or more) is observed in the axial tensile test.
　The tubular axial tensile test for observing the presence or absence of yield elongation is performed under the same conditions as the tubular axial tensile test for measuring the tubular axial tensile strength, which will be described later.
[0075]
　As described above, in the electric pipe of the present disclosure, the yield elongation is observed when the tensile test in the pipe axial direction is performed. It shows that it is a steel pipe.
　For example, no yield elongation is observed in electrosewn steel pipes that have been tempered before pipe making and have not been tempered after pipe making.
[0076]

　The electric resistance welded steel pipe of the present disclosure has a tensile strength in the pipe axis direction (hereinafter, also simply referred to as "tensile strength") of 750 to 980 MPa.
　When the tensile strength is 750 MPa or more, the strength as a steel pipe for a torsion beam is secured. The tensile strength is preferably 800 MPa or more.
　When the tensile strength is 980 MPa or less, the inner surface crack resistance is improved. The tensile strength is preferably 950 MPa or less, more preferably 900 MPa or less.
[0077]
　The tensile strength of the electrosewn steel pipe of the present disclosure is measured as follows.
　A JIS No. 12 tensile test piece is collected from the 180 ° position of the base metal in the electrosewn steel pipe of the present disclosure. The collected JIS No. 12 tensile test piece is subjected to a tube axial tensile test (that is, a tensile test with the test direction as the tube axial direction) in accordance with JIS Z 2241 (2011), and the tensile strength is measured. The obtained result is taken as the tensile strength of the electrosewn steel pipe of the present disclosure (that is, the tensile strength in the pipe axial direction).
[0078]
　The outer diameter of the electrosewn steel pipe of the present disclosure is not particularly limited. From the viewpoint of manufacturing suitability of the electrosewn steel pipe, the outer diameter is preferably 50 to 150 mm.
　The wall thickness of the electrosewn steel pipe of the present disclosure is not particularly limited. From the viewpoint of manufacturing suitability of the electric resistance pipe, the wall thickness of the electric resistance pipe of the present disclosure is preferably 2.0 to 4.0 mm.
[0079]

　The electrosewn steel pipe of the present disclosure is used for manufacturing a torsion beam.
　The torsion beam using the electric resistance welded steel pipe of the present disclosure is manufactured, for example, by bending a part of the electric resistance welded steel pipe of the present disclosure. Bending molding is performed, for example, by pushing a part of a linear region parallel to the pipe axis direction from the outside to the inside of the electro-sewn steel pipe of the present disclosure (for example, described later). See the bending test shown in FIG. 2). As a result, for example, a torsion beam including a portion having a substantially V-shaped closed cross section (for example, see FIG. 3 described later) is manufactured.
[0080]
　Generally, when the radius of curvature R of the inner surface of the bent portion by bending molding is small, the inner surface cracks tend to occur easily.
　However, according to the electric resistance welded steel pipe of the present disclosure having excellent internal crack resistance, the occurrence of internal cracks is effectively suppressed even in this case as well.
　Therefore, the effect of improving the inner surface cracking resistance of the electrosewn steel pipe of the present disclosure is particularly effectively exhibited when the radius of curvature R of the inner surface of the bent portion by bending molding is small.
　In other words, the effect of improving the inner surface cracking resistance of the electro-sewn steel pipe of the present disclosure is that the electro-sewn steel pipe of the present disclosure has a bent portion having a small radius of curvature R on the inner surface (for example, the radius of curvature R of the inner surface is the wall thickness). It is particularly effective when used in the production of a torsion beam containing 2 times or less (preferably 0.7 to 2 times, more preferably 1 to 2 times).
[0081]
As an example of the method for manufacturing the electric resistance welded steel pipe of the present disclosure, the following manufacturing method A can be mentioned.
　In the production method A,
　a slab preparation step for preparing a slab having a chemical composition according to the present disclosure , and the
　prepared slab are heated to a slab heating temperature of 1070-1300 ° C., and a hot rolling finish temperature is set with respect to the heated slab. The hot-rolled steel sheet obtained by hot-rolling to 920 ° C. or higher and the hot-rolled steel sheet
　obtained in the hot-rolled step have an average cooling rate of 40 to 100 ° C. from the start of cooling to 200 ° C. A cooling step of
　cooling to a winding temperature of 200 ° C. or lower (that is, the cooling end temperature) under the condition of / s, and hot rolling by winding the cooled hot-rolled steel sheet at the above-mentioned winding temperature. A winding process for obtaining a
　hot coil made of steel plate and a hot-rolled steel plate are unwound from the hot coil, and the unwound hot-rolled steel plate is rolled to form an open pipe, and the butt portion of the obtained open pipe is welded. A pipe making process for obtaining an As-rolled electric resistance welded steel pipe by welding to form an electric resistance welded steel pipe
　, and a tempering temperature of 500 for the As-rolled electric resistance welded steel pipe without quenching. It
includes a post-forming tempering step of performing tempering under the conditions of ~ 700 ° C. and tempering time of 1 minute or more and 120 minutes or less .
　The production method A may include other steps, if necessary.
　The above hot spreading step, cooling step, and winding step are carried out using a hot strip mill.
[0082]
　Here, the azuroll electric pipe is an electric pipe that has not been subjected to a heat treatment other than the seam heat treatment after the pipe is made. That is, "azurol" in the assroll electrosewn steel pipe means "as it is rolled".
[0083]
　According to the above-mentioned production method A, in the welded steel pipe of the present disclosure, that is, in
the L cross section at the base metal 180 ° position, the metal structure at the center of the wall thickness is the tempered martensite structure, and the old austenite grains in the tempered martensite structure. The average aspect ratio of is 2.0 or less, and in the
central portion of the wall thickness of the C cross section, the metallographic structure near the electrosewn weld includes a tempered martensite phase and at least one of a tempered bainite phase and a ferrite phase.
It is
easy to manufacture the tempered steel pipe of the present disclosure, in which yield elongation is observed when a tensile test in the pipe axial direction is performed and the tensile strength in the pipe axial direction is 750 to 980 MPa .
[0084]
(Slab preparation step) In the
　production method A, the slab preparation step is a step of preparing a slab having the above-mentioned chemical composition.
　The slab preparation step may be a step of manufacturing a slab, or may be a step of simply preparing a pre-manufactured slab.
　When producing a slab, for example, a molten steel having the above-mentioned chemical composition is produced, and the produced molten steel is used to produce a slab. At this time, a slab may be produced by a continuous casting method, or an ingot may be produced using molten steel, and the ingot may be block-rolled to produce a slab.
[0085]
(Hot-rolling process) In the
　manufacturing method A, in the hot-rolling process, the slab prepared above is heated to a slab heating temperature of 1070-1300 ° C., and the hot rolling finish temperature is set to 920 ° C. or higher with respect to the heated slab. This is a process of obtaining a hot-rolled steel sheet by performing hot rolling.
[0086]
　By heating the slab to a slab heating temperature of 1070 to 1300 ° C., carbides, nitrogen compounds and carbon dioxide compounds precipitated in the molten steel solidification process can be solid-solved in the steel. As a result, the strength can be improved without deteriorating the crack resistance on the inner surface. Further, it is also possible to suppress the generation of minute voids during roll forming for manufacturing the electric resistance welded steel pipe and / or during bending forming with respect to the electric resistance sewing steel pipe.
　When the slab heating temperature is 1070 ° C. or higher, carbides, nitrogen compounds and carbon dioxide compounds precipitated in the solidification process of molten steel can be sufficiently solid-solved in the steel.
　When the slab heating temperature is 1300 ° C. or lower, coarsening of austenite grains can be suppressed, and precipitation of coarse AlN during hot rolling or cooling after hot rolling can be suppressed.
[0087]
　Further, in the hot rolling process, the hot rolling finish temperature means the end temperature of the finish rolling in the hot rolling (sometimes referred to as the finish rolling output side temperature).
　The hot rolling finish temperature of 920 ° C. or higher means that hot rolling is performed in the recrystallized region, not in the unrecrystallized region. As a result, it is easy to achieve that the average aspect ratio of the old austenite grains is 2.0 or less in the obtained electrosewn steel pipe.
　Further, the hot rolling finish temperature of 920 ° C. or higher also contributes to the miniaturization of the former austenite grains and the miniaturization of the packet grains in the former austenite grains. Therefore, when the hot rolling finish temperature is 920 ° C. or higher, it is easy to achieve that the average packet particle size is 10 μm or lower.
　The upper limit of the hot rolling finish temperature is appropriately set, but the upper limit is preferably 1000 ° C. from the viewpoint of further suppressing the coarsening of austenite grains.
[0088]
(Cooling step) In the
　manufacturing method A, the cooling step is 200 ° C. or lower under the condition that the average cooling rate of the hot-rolled steel sheet obtained in the hot-rolling step is 40 to 100 ° C. This is the process of cooling until the winding temperature is reached.
[0089]
　In the cooling step, the hot-rolled steel sheet obtained in the hot-quenching step is wound at a winding temperature of 200 ° C. or lower (that is, cooling end) under the condition that the average cooling rate from the start of cooling to 200 ° C. is 40 ° C. By cooling to the temperature), a martensite structure is formed as the metal structure of the hot-rolled steel plate as it is quenched. That is, this cooling step substantially quenches the hot-rolled steel sheet.
　In the post-tempering tempering step described later, the martensite structure is tempered while the base metal portion (for example, the base metal at 180 ° position) generated in this cooling step is tempered, and a tempered martensite structure is generated.
　On the other hand, when the average cooling rate from the start of cooling to 200 ° C. is 100 ° C./s or less, it is easy to control the cooling stop temperature. Further, when the average cooling rate is 100 ° C./s or less, the difference in cooling rate between the surface of the hot-rolled steel sheet and the inside of the wall thickness (for example, the central portion of the wall thickness) becomes small, so that the material of the hot-rolled steel sheet Better in stability.
[0090]
(Winding Step) In the
　manufacturing method A, the winding step is a step of obtaining a hot coil made of a hot-rolled steel sheet by winding the cooled hot-rolled steel sheet at the above-mentioned winding temperature.
[0091]
(Pipe-
　making process) In the manufacturing method A, in the pipe-making process, a hot-rolled steel sheet is unwound from a hot coil, and the unwound hot-rolled steel sheet is rolled to form an open pipe, and the butt portion of the obtained open pipe is formed. This is a step of obtaining an azurol electric resistance steel pipe by forming an electric sewing welded portion by electric sewing welding.
　The tube making step can be carried out according to a known method.
　In the pipe
forming process, if necessary, after the formation of the electric stitch welded portion, the electric stitch welded portion is seam heat treated;
after the formation of the electric stitch welded portion (in the case of performing the above-mentioned seam heat treatment, the seam heat treatment is performed. Later), reducing the outer diameter of the azurol electrosewn steel pipe with a sizer;
etc. may be included.
[0092]
(Tempering step after tube
　making ) In the manufacturing method A, the tempering step after tube making is performed under the conditions of a tempering temperature of 500 to 700 ° C. and a tempering time of 1 minute or more and 120 minutes or less without quenching the azuroll electrosewn steel tube. This is the process of applying.
　By this post-tempering tempering step, the martensite structure is effectively tempered while the base material portion (for example, the base material 180 ° position) is quenched, and a tempered martensite structure is generated.
　In the vicinity of the electric stitch welded portion, the martensite structure is temporarily transformed into the austenite phase and disappears while being hardened by the electric stitch welding. After electrosew welding, the austenite phase is cooled, and then tempering is performed after pipe formation to obtain a metal structure containing the tempered martensite phase and at least one of the tempered bainite phase and the ferrite phase. Will be generated.
[0093]
　In the post-tempering tempering step, if the tempering conditions are a tempering temperature of 500 ° C. or higher and a tempering time of 1 minute or longer, the tempered martensite structure can be effectively generated in the base metal portion.
　Further, by tempering under such conditions, the tube forming strain can be effectively reduced, so that the dislocation density can be effectively reduced.
[0094]
　Further, in the post-tempering tempering step, since the tempering conditions are a tempering temperature of 700 ° C. or less and a tempering time of 120 minutes or less, a decrease in strength due to excessive tempering is suppressed, so that the tensile strength is 750 MPa or more. Easy to achieve.
[0095]
　In the post-pipe tempering step, tempering under the above conditions is performed without quenching.
　Quenching here refers to an operation of heat-treating and rapidly cooling a steel pipe at a temperature of A3 or higher.
　The A3 point means the temperature at which the transformation to austenite is completed during heating, and depends on the chemical composition of the steel pipe. In the chemical composition in the present disclosure, the A3 point does not fall below 700 ° C., so that the above tempering does not correspond to quenching.
[0096]
　In the post-tempering tempering step, the cooling after the tempering is not particularly limited, and may be slow cooling (for example, air cooling) or rapid cooling (for example, water cooling).
[0097]
　Each step of the above-mentioned production method A does not affect the chemical composition of steel.
　Therefore, the chemical composition of the base material portion of the electric resistance welded steel pipe manufactured by the manufacturing method A can be regarded as the same as the chemical composition of the raw material (molten steel or slab).
Example
[0098]
　Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.
[0099]
[Examples 1 to 10 and Comparative Examples 1 to 29]
According to the
　above-mentioned production method A, electrosewn steel pipes of Examples 1 to 10 were obtained, respectively.
　Further, the chemical composition or the production conditions of the electric resistance welded steel pipes of each example were changed to obtain the electric resistance welded steel pipes of Comparative Examples 1 to 29, respectively.
　Details will be shown below.
[0100]
　Molten steels (steels AZ, AA, AB, and AC) having the chemical compositions shown in Table 1 were melted in a furnace, and then a slab having a thickness of 250 mm was produced by casting (slab preparation step).
[0101]
　In Table 1, the numerical value shown in the column of each element is the mass% of each element.
　The balance excluding the elements shown in Table 1 is Fe and impurities.
　In Table 1, the REM in steel H is La.
　V c90 in Table 1 is V c90 defined by the above formula (i) .
　Underlines in Tables 1 to 3 indicate that they are outside the scope of this disclosure.
[0102]
　The slab obtained above is heated to the slab heating temperature shown in Table 2 or Table 3, and the heated slab is subjected to hot rolling in which the hot rolling finish temperature is set to the temperature shown in Table 2 or Table 3. By applying, a hot-rolled steel plate was obtained (hot-rolling process).
　The hot-rolled steel sheet obtained in the hot-rolling step was cooled at the average cooling rate shown in Table 2 or 3 until the winding temperature (that is, the cooling end temperature) shown in Table 2 or 3 was reached. (Cooling process).
　Then, by winding at the winding temperature shown in Table 2 or Table 3, a hot coil made of a hot-rolled steel sheet having a plate thickness of 3.0 mm was obtained (winding step).
　The above hot spreading step, cooling step, and winding step were carried out using a hot strip mill.
[0103]
　In Examples 1 to 10 and Comparative Examples 1 to 24 and 26 to 29, a hot-rolled steel sheet was unwound from the hot coil, and the unwound hot-rolled steel sheet was roll-formed to form an open pipe. The butt portion of the open pipe is electrosewn to form an electrosewn welded portion, and then the diameter is reduced using a sizer to obtain an azuroll electrosewn steel pipe having an outer diameter of 90 mm and a wall thickness of 3.0 mm. Obtained (pipe making process).
[0104]
　In Comparative Example 25, the hot-rolled steel sheet was unwound from the hot coil, and the unwound hot-rolled steel sheet was pre-tempered under the conditions shown in Table 3 (tempering temperature and tempering time), and then heated again. The rolled steel plate was wound up. The wound hot-rolled steel sheet was unwound again, and the unwound hot-rolled steel sheet was used to obtain an azurol electric resistance pipe having an outer diameter of 90 mm and a wall thickness of 3.0 mm in the same manner as in Example 1. ..
[0105]
　In Examples 1 to 10 and Comparative Examples 1 to 24 and 26 to 29, the azurol electric pipe was subjected to post-tempering tempering under the conditions (tempering temperature and tempering time) shown in Tables 2 and 3. Then, by air cooling, an electrosewn steel pipe having an outer diameter of 90 mm and a wall thickness of 3.0 mm was obtained (tempering step after pipe making).
　In Comparative Example 25, the azurol electric resistance sewn steel pipe was not tempered after being formed.
[0106]
With
　respect to the electric resistance welded steel pipe obtained above (Azroll electric resistance pipe in Comparative Example 25; the same applies hereinafter), the L cross section at 180 ° position of the base material was observed, Was confirmed and measured.
[0107]
(
　Metal structure at the center of wall thickness) The metal structure at the center of the wall thickness of the L cross section at the position of 180 ° of the base metal was confirmed by the above-mentioned method.
　The results are shown in Tables 2 and 3.
[0108]
　In Tables 2 and 3,
"TM" means a tempered martensite structure,
"TM + TB" means a two-phase structure consisting of a tempered martensite phase and a tempered bainite phase, and
"F + TB" means a ferrite phase. Means a two-phase structure consisting of tempered bainite phase and
"TB" means tempered bainite structure.
[0109]
(Average aspect ratio of old austenite grains)
　By the method described above , the average aspect ratio of old austenite grains in the central portion of the wall thickness of the L cross section (in Tables 2 and 3, it is referred to as "old γ grain average aspect ratio". ) Was measured.
　The results are shown in Tables 2 and 3.
[0110]
(Average packet grain)
　The average packet grain in the central portion of the wall thickness of the L cross section was measured by the method described above.
　The results are shown in Tables 2 and 3.
[0111]
(Dislocation Density)
　The dislocation density in the central portion of the wall thickness of the L cross section was measured by the method described above. As the X-ray diffractometer used for X-ray diffraction, "RINT2200" manufactured by Rigaku Co., Ltd. was used.
　The results are shown in Tables 2 and 3.
[0112]
With
　respect to the electric resistance welded steel pipes of Examples 1 to 10, the vicinity of the electric sewing welded portion in the central portion of the wall thickness of the C cross section (that is, electric resistance) The area within the wall thickness distance from the sewn welded part) was observed.
　As a result, in any of Examples 1 to 10, it was confirmed that the metal structure in the vicinity of the electrosewn welded portion contained a tempered martensite phase and at least one of a tempered bainite phase and a ferrite phase.
[0113]
The tensile strength in the pipe axis direction of the electrosewn steel pipe (hereinafter, simply referred to as "tensile strength") was measured by the method described above.
　The results are shown in Tables 2 and 3.
[0114]
In
　a tube axial tensile test for measuring tensile strength, the presence / absence of yield elongation was confirmed.
　When a yield elongation of 0.1% or more was observed, it was determined that the yield elongation was "present", and when no yield elongation of 0.1% or more was observed, it was determined that the yield elongation was "none".
[0115]

　A bending test was performed on the electrosewn steel pipe to imitate the production of a torsion beam, and the inner surface crack depth was evaluated. The details will be described below.
[0116]
　FIG. 2 is a schematic view schematically showing an outline of a bending test.
　As shown in FIG. 2, the electric resistance pipe 100A which is the electric pipe of each Example and each comparative example, the lower die 10 having a V-shaped notch, and the tip having a substantially triangular cross section are provided. The punch 12 to have was prepared.
　Here, the angle θ1 of the valley portion of the notch portion of the mold 10 and the angle θ2 of the tip of the punch 12 are both set to 60 °.
[0117]
　In this bending test, the electric resistance pipe 100A is arranged in the notch of the lower die 10, and the punch 12 is pushed into the arranged electric resistance pipe 100A in the direction of the arrow P to bend the electric resistance pipe 100A. Molded. As a result, a part of the electric pipe 100A is bent in a direction perpendicular to the pipe axial direction L of the electric pipe 100A, and the structure 100B having a substantially V-shaped closed cross section shown in FIG. 3 is formed. Formed.
　The pipe axial direction L of the electric resistance pipe 100A corresponds to the rolling direction at the time of manufacturing the hot-rolled steel sheet which is the material of the electric resistance steel pipe 100A.
[0118]
　FIG. 3 is a schematic cross-sectional view schematically showing a cross section of a structure 100B obtained by bending a electrosewn steel pipe 100A in a bending test. The cross section of the structure 100B shown in FIG. 3 is a cross section perpendicular to the longitudinal direction of the structure 100B, and is a C cross section of the electrosewn steel pipe before bending (that is, a cross section perpendicular to the pipe axial direction L). Corresponds to.
　As shown in FIGS. 2 and 3, the structure 100B having a substantially V-shaped closed cross section was formed by bending the electrosewn steel pipe 100A. Here, the pushing amount of the punch 12 was adjusted so that the radius of curvature R of the inner surface 102B was 4 mm at one end portion 101B (bent portion) in the closed cross section of the structural portion 100B. The radius of curvature R of the inner surface of the other end of the closed cross section of the structural portion 100B was also adjusted to be 4 mm.
　By observing the inner surface 102B and its vicinity in the cross section of one end 101B (specifically, the cross section corresponding to FIG. 3) by SEM at a magnification of 1000 times, the depth of the inner surface crack (hereinafter, "inner surface crack depth"). Also called "sa") was measured.
　Here, the inner surface crack depth was obtained as follows.
　The presence or absence of cracks on the inner surface was confirmed by observing the inner surface 102B and its vicinity in the cross section of the one end portion 101B with SEM. When the inner surface cracks were present, the linear distance from the start point to the end point of each crack was obtained for each inner surface crack, and the depth of each inner surface crack was obtained. The maximum value of the depth of each inner surface crack was defined as the "inner surface crack depth" in the example or comparative example. When there was no inner surface crack, the "inner surface crack depth" in the example or comparative example was set to "0 μm".
　The obtained inner surface crack depths are shown in Tables 2 and 3.
[0119]
　In the evaluation of the inner surface crack depth, the smaller the inner surface crack depth, the better the inner surface crack resistance. The fact that the inner surface cracking depth is 0 μm means that no inner surface cracking has occurred, that is, the inner surface cracking resistance is remarkably excellent.
[0120]
[table 1]

[0121]
[Table 2]

[0122]
[Table 3]

[0123]
　As shown in Table 2, it has the chemical composition in the present disclosure, and in the L cross section at the base metal 180 ° position, the metal structure at the center of the wall thickness is the tempered martensite structure, and the former austenite grains in the tempered martensite structure. In the tempered steel pipes of Examples 1 to 10, in which the average aspect ratio of the steel pipe is 2.0 or less and the yield elongation is observed when the pipe axial tensile test is performed, the excellent tensile strength in the range of 750 MPa to 980 MPa is obtained. And was excellent in internal crack resistance.
[0124]
　The results of the comparative examples shown in Table 2 with respect to the above examples are as follows.
　In Comparative Example 1 in which the C content was too low, the tensile strength was insufficient.
　In Comparative Example 2 in which the C content was too large, the tensile strength became excessive and the internal crack resistance deteriorated.
　In Comparative Example 3 in which the Si content was too low, the internal crack resistance was deteriorated. The reason for this is considered to be that deoxidation was insufficient and coarse Fe oxide was produced.
　In Comparative Example 4 in which the Si content was too high, the internal crack resistance was deteriorated. It is considered that the reason for this is that inclusions such as SiO 2 are generated, and minute voids are generated from the inclusions as a starting point during roll forming and / or bending forming with respect to the electric resistance steel pipe. Be done.
　In Comparative Example 5 in which the Mn content is too low, the metal structure at the center of the wall thickness of the L cross section at the 180 ° position of the base metal is not a tempered martensite structure but a two-phase structure consisting of a tempered martensite phase and a tempered bainite phase. As a result, the tensile strength was insufficient and the crack resistance on the inner surface deteriorated.
　In Comparative Example 6 in which the Mn content was too large, the tensile strength became excessive and the inner surface crack resistance deteriorated.
[0125]
　In Comparative Example 7 in which the P content was too high, the internal crack resistance was deteriorated. The reason for this is considered to be that P is concentrated at the packet grain boundaries.
　In Comparative Example 8 in which the S content was too large, the internal crack resistance was deteriorated. The reason for this is considered to be that coarse MnS was generated.
　In Comparative Example 9 in which the Ti content was too low, the Ti / N was less than 3.4, and the metal structure at the center of the wall thickness of the L cross section at the 180 ° position of the base metal was not a tempered martensite structure but a ferrite phase and tempered. It has a two-phase structure consisting of a bainite phase, lacks tensile strength, and deteriorates internal crack resistance. It is considered that the reason for this is that N cannot be fixed in the form of TiN and BN is formed, and as a result, the effect of improving hardenability by B is insufficient.
　In Comparative Example 10 in which the Ti content was too high, the internal crack resistance was deteriorated. The reason for this is considered to be the precipitation of coarse TiC and / or TiN.
　In Comparative Example 11 in which the Al content was too low, the internal crack resistance was deteriorated. The reason for this is considered to be that the old austenite grains became coarse and the packet grains in the old austenite grains also became coarse.
　In Comparative Example 12 in which the Al content was too high, the internal crack resistance was deteriorated. The reason for this is considered to be that the coarseness was generated by AlN.
　In Comparative Example 13 in which the Nb content was too small, the internal crack resistance was deteriorated. The reason for this is considered to be that the old austenite grains became coarse and the packet grains in the old austenite grains also became coarse.
　In Comparative Example 14 in which the Nb content was too high, the internal crack resistance was deteriorated. The reason for this is considered to be that the coarseness was generated by NbC.
　In Comparative Example 15 in which the N content was too large, the internal crack resistance was deteriorated. The reason for this is considered to be that the coarseness was generated by AlN. Further, in Comparative Example 15, the Ti / N was less than 3.4, and the metal structure at the center of the wall thickness of the L cross section at the 180 ° position of the base metal became a two-phase structure composed of a ferrite phase and a tempered bainite phase. The tensile strength was insufficient. It is considered that the reason for this is that N cannot be fixed in the form of TiN and BN is formed, and as a result, the effect of improving hardenability by B is insufficient.
[0126]
　In Comparative Example 16 in which the B content is too low, the metal structure at the center of the wall thickness of the L cross section at the position of the base metal at 180 ° is not a tempered martensite structure but a two-phase structure consisting of a ferrite phase and a tempered bainite phase. The tensile strength was insufficient, and the crack resistance on the inner surface deteriorated. The reason for this is considered to be that the hardenability was insufficient because the B content was too low.
　In Comparative Example 17 in which the B content is too high, the metal structure at the center of the wall thickness of the L cross section at the 180 ° position of the base metal is not a tempered martensite structure but a two-phase structure consisting of a tempered martensite phase and a tempered bainite phase. As a result, the tensile strength was insufficient and the crack resistance on the inner surface deteriorated. It is considered that the reason for this is that the hardenability was lowered because the solid solution B segregated at the austenite grain boundaries was reduced due to the aggregation and / or precipitation of B.
[0127]
　The results of the comparative examples shown in Table 3 with respect to the above examples are as follows.
　In Comparative Example 18, which had the chemical composition of the present disclosure but the hot rolling finish temperature was too low, the average aspect ratio of the old austenite grains was more than 2.0, and the internal crack resistance was deteriorated.
　In Comparative Example 19 which has the chemical composition of the present disclosure but the cooling rate when cooling the hot-rolled steel plate is too slow, the metal structure at the center of the wall thickness of the L cross section at the position of the base metal at 180 ° is the tempered martensite structure. Instead, it had a two-phase structure consisting of a tempered martensite phase and a tempered bainite phase, resulting in insufficient tensile strength and deterioration of internal crack resistance.
　In Comparative Example 20, which has the chemical composition of the present disclosure but the winding temperature (that is, the cooling end temperature) when winding the hot-rolled steel plate is too high, the central portion of the wall thickness of the L cross section at the position of the base metal 180 ° The metal structure was not a tempered martensite structure but a two-phase structure consisting of a ferrite phase and a tempered bainite phase, resulting in insufficient tensile strength and deterioration of internal crack resistance.
　In Comparative Example 21, which had the chemical composition of the present disclosure but the tempering temperature in tempering after tube making was too low, no yield elongation was observed and the internal crack resistance deteriorated.
　In Comparative Example 22, which had the chemical composition of the present disclosure but the tempering temperature in the tempering after tube making was too high, the tensile strength was insufficient.
　In Comparative Example 23, which had the chemical composition of the present disclosure but the tempering time in tempering after tube making was too short, no yield elongation was observed and the internal crack resistance deteriorated.
　In Comparative Example 24, which had the chemical composition of the present disclosure but the tempering time in tempering after tube making was too long, the tensile strength was insufficient.
　In Comparative Example 25 having the chemical composition of the present disclosure but performing pre-tempering instead of post-tempering tempering, no yield elongation was observed and the internal crack resistance deteriorated.
　In Comparative Example 26, which had the chemical composition of the present disclosure but the hot rolling finish temperature was too low, the average aspect ratio of the old austenite grains was more than 2.0, and the internal crack resistance was deteriorated.
　In Comparative Example 27, which has the chemical composition of the present disclosure but the winding temperature (that is, the cooling end temperature) when winding the hot-rolled steel plate is too high, the central portion of the wall thickness of the L cross section at the position of the base metal 180 ° The metal structure was not a tempered martensite structure but a tempered bainite structure, the tensile strength was insufficient, and the internal crack resistance was deteriorated.
　Although the content of each element in the chemical composition of the present disclosure is appropriate, in Comparative Example 28 in which V c90 is more than 150, the metal structure at the center of the wall thickness of the L cross section at the position of the base material 180 ° is tempered martensite. Instead of a structure, it became a two-phase structure consisting of a tempered martensite phase and a tempered bainite phase, and the tensile strength was insufficient and the internal crack resistance deteriorated.
　Although the content of each element in the chemical composition of the present disclosure is appropriate, in Comparative Example 29 in which Ti / N is less than 3.4, the metal structure at the center of the wall thickness of the L cross section at the position of the base metal at 180 ° is formed. It had a two-phase structure consisting of a tempered martensite phase and a tempered bainite phase, and the tensile strength was insufficient. It is considered that the reason for this is that N cannot be fixed in the form of TiN and BN is formed, and as a result, the effect of improving hardenability by B is insufficient.
The scope of the claims
[Claim 1]

　The chemical composition of the base 　metal portion including the base metal portion and the electric sewing welded portion is
C: 0.05 to 0.30%,
Si: 0.03 to 1.20%,
Mn: 0. 30 to 2.50%,
P: 0 to 0.030%,
S: 0 to 0.010%,
Ti: 0.010 to 0.200%,
Al: 0.005 to 0.500%,
Nb: 0 .010 to 0.040%,
N: 0 to 0.006%,
B: 0.0005 to 0.0050%,
Cu: 0 to 1.000%,
Ni: 0 to 1.000%,
Cr: 0 to 1.00%,
Mo: 0 to 0.50%,
V: 0 to 0.200%,
W: 0 to 0.100%,
Ca: 0 to 0.0200%,
Mg: 0 to 0.0200%,
Zr: 0 to 0.0200%,
REM: 0 to 0.0200%, and the
balance: Fe and impurities, and
V c90 defined by the following formula (i) is 2 to 150.
The Ti content to mass ratio of Ti to N is 3.4 or more, and
　the metal structure at the center of the wall thickness is the tempered martensite structure in the L cross section at a position 180 ° deviated from the electrosew welded portion in the pipe circumferential direction. The average aspect ratio of the old austenite grains in the tempered martensite structure is 2.0 or less, and
　the metal structure in the region within the distance of the wall thickness from the electric stitch welded part in the central part of the wall thickness of the C cross section is A 　torsion beam containing a tempered martensite phase and at least one of a tempered bainite phase and a ferrite phase, in which
　yield elongation is observed when a tensile test in the tubular axis is performed, and
the tensile strength in the tubular axis is 750 to 980 MPa. Electric sewing steel pipe.
　log V c90 = 2.94-0.75Betaei ... formula (i)
　.beta.a = 2.7C + 0.4Si + Mn + 0.45Ni + 0.8Cr + 2Mo ... formula (ii)
in [formula (i), .beta.a the value defined by formula (ii) Is.
　In formula (ii), each element symbol represents the mass% of each element. ]
[Claim 2]
　The chemical composition of the base metal is, in terms of mass%,
Cu: more than 0% and 1.000% or less,
Ni: more than 0% and 1.000% or less,
Cr: more than 0% and 1.00% or less,
Mo: 0%. Super 0.50% or less,
V: 0% or more 0.200% or less,
W: 0% or more 0.100% or less,
Ca: 0% or more 0.0200% or less,
Mg: 0% or more 0.0200% or less The electric resistance sewn steel pipe for a torsion beam according to claim 1, which contains at least one selected from the group consisting of
Zr: more than 0% and 0.0200% or less, and
REM: more than 0% and 0.0200% or less
.
[Claim 3]
　The electrosewn steel pipe for a torsion beam according to claim 1 or 2, wherein the average particle size of the packet grains in the tempered martensite structure is 10 μm or less.
[Claim 4]
　The electrosewn steel pipe for a torsion beam according to any one of claims 1 to 3, wherein the dislocation density at the central portion of the wall thickness in the L cross section is 2.0 × 10 14 m- 2 or less.
[Claim 5]
　The electric resistance sewn steel pipe for a torsion beam according to any one of claims 1 to 4, wherein the outer diameter is 50 to 150 mm and the wall thickness is 2.0 to 4.0 mm.