JP4910694B2 - High tensile welded steel pipe for automobile structural members and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high tensile strength welded steel pipe for an automobile structural member having excellent low temperature toughness, excellent formability and excellent twisting fatigue resistance after cross-sectional forming-stress relief annealing, and to provide a method for producing the same. <P>SOLUTION: A steel stock having a composition comprising C, Si and Al in proper ranges, comprising 1.01 to 1.70% Mn, 0.017 to 0.15% Nb and 0.045 to 0.45% Mo, and comprising P, S, N and O regulated to prescribed values is subjected to hot rolling in which heating temperature and finish rolling completing temperature are controlled to proper ranges, is annealed in the temperature range of 750 to 650&deg;C for &ge;2s after the completion of the hot rolling, and is coiled at a coiling temperature of 660 to 510&deg;C, so as to be a hot rolled steel strip having a structure where the ratio of a ferrite phase with the average grain size of 2 to 14 &mu;m is &ge;60 vol.%, and Nb carbides with the average grain size of 1.5 to 30 nm are precipitated into the ferrite phase. The hot rolled steel strip is subjected to an electric resistance welding process where a width contraction rate is &le;10%. In this way, the high tensile strength welded steel pipe having high strength satisfying the tensile strength of &ge;660 MPa, and having excellent low temperature toughness, formability and twisting fatigue resistance after cross-sectional forming-stress relief annealing treatment is obtained. <P>COPYRIGHT: (C)2008,JPO&amp;INPIT

Description

  The present invention relates to a high-tensile welded steel pipe having a tensile strength of 660 MPa or more suitable for automobile structural members such as a torsion beam, an axle beam, a trailing arm, and a suspension arm. It relates to the improvement of torsional fatigue resistance after processing-stress relief annealing.

  In recent years, there has been a strong demand for improving the fuel efficiency of automobiles from the viewpoint of protecting the global environment. Therefore, a thorough weight reduction of the body of an automobile or the like is aimed at. Structural members such as automobiles are no exception, and in order to achieve both weight reduction and safety, some structural members are adopting highly-strengthened ERW steel pipes. Conventionally, after forming a material (electrically welded steel pipe) into a predetermined shape, a tempering process such as a quenching process is performed to increase the strength of the member. However, adopting the tempering treatment has a problem in that the process becomes complicated, the manufacturing period of the member becomes longer, and the manufacturing cost of the member increases.

For such a problem, for example, Patent Document 1 describes a method of manufacturing an ultra-high-strength ERW steel pipe for a structural member such as an automobile. In the technique described in Patent Document 1, C, Si, Mn, P, S, Al, and N are adjusted to appropriate amounts, and B: 0.0003 to 0.003% is included, and among Mo, Ti, Nb, and V Finishing and rolling the steel material with a composition containing at least one of the above at 950 ° C or less at the Ar ₃ transformation point and hot rolling it at 250 ° C or less to form a steel strip for pipes. This is a method for producing an ERW steel pipe, which is made into an ERW steel pipe and then subjected to an aging treatment at 500 to 650 ° C. According to this technique, an ultrahigh-tensile steel pipe exceeding 1000 MPa can be obtained without tempering treatment by strengthening the transformation structure of B and precipitation hardening of Mo, Ti, Nb and the like.

Patent Document 2 describes a method for producing an electric resistance welded steel pipe having a high tensile strength: 1470 N / mm ² or more and a high ductility, suitable for use in automobile door impact beams and stabilizers. . In the technique described in Patent Document 2, C: 0.18 to 0.28%, Si: 0.10 to 0.50%, Mn: 0.60 to 1.80% are included, and P and S are adjusted to an appropriate range, and Ti: 0.020 to 0.050% , B: 0.0005 to 0.0050% and further subjected to a normal treatment at 850 to 950 ° C. for an ERW steel pipe manufactured using a steel plate made of a material steel having a composition containing at least one of Cr, Mo and Nb This is a method for producing an electric resistance welded steel pipe, which is further subjected to quenching treatment. According to this technique, an electric resistance welded steel pipe having a high strength of 1470 N / mm ² or more and a ductility of about 10 to 18% is obtained, which is said to be suitable for use in automobile door impact beams and stabilizers.
Japanese Patent No. 2588648 Japanese Patent No. 2814882

However, the ERW steel pipe manufactured by the technique described in Patent Document 1 is inferior in formability because of its low ductility, with an elongation El of 14% or less, and a torsion beam, axle beam, tray with press molding or hydroform molding. There is a problem that it is not suitable for automobile structural members such as ring arms and suspension arms.
On the other hand, the ERW steel pipe manufactured by the technique described in Patent Document 2 has an elongation El of at most 18% and is suitable for a stabilizer formed by bending. For the accompanying member, there is a problem that the ductility is insufficient and it is not suitable for an automobile structural member such as a torsion beam and an axle beam accompanied by press molding or hydroforming. Further, the technique described in Patent Document 2 requires a normalization process and a quenching process, has a complicated process, and has left a problem in terms of dimensional accuracy and economy.

  The present invention advantageously solves the above-mentioned problems of the prior art, and is particularly suitable for automobile structural members such as torsion beams, axle beams, trailing arms, suspension arms, etc., and is a quenching omission type that is not subjected to quenching treatment. An object of the present invention is to provide a high-strength welded steel pipe having a tensile strength of 660 MPa or more and excellent in torsional fatigue resistance after low temperature toughness, formability, and cross-section forming-stress relief annealing, and a method for producing the same. .

In the present invention, “excellent formability” means that the elongation El in a tensile test conducted in accordance with the JIS Z 2241 standard using a JIS No. 12 test piece in accordance with the JIS Z 2201 standard. It shall indicate the case of 16% or more (24% or more for JIS 11 test piece).
In addition, the term “cross-section forming process—excellent torsional fatigue resistance after stress relief annealing” as used in the present invention refers to a longitudinal central portion of a steel pipe as shown in FIG. 3 (FIG. 11 of JP-A-2001-331846) After forming a V-shaped cross-section, and applying stress relief annealing at 550 ° C x 10 min, both ends are fixed by chucking and a torsional fatigue test is performed at 1 Hz for both swings. 5 x 10 ^{The 5} cycle fatigue limit σ _B is obtained, and the ratio of the obtained 5 × 10 ⁵ cycle fatigue limit σ _B to the steel pipe tensile strength TS (σ _B / TS) is 0.50 or more. In addition, the above-mentioned “cross-section forming process—excellent torsional fatigue resistance after stress-relieving annealing” is the change in cross-sectional hardness after the above-described cross-section forming process and further subjected to a stress-relieving annealing process at 550 ° C. × 10 min. It can be ensured when the rate satisfies -15% or more and the residual stress reduction rate satisfies 60% or more.

  In addition, “excellent low temperature toughness” as used in the present invention means that the longitudinal center portion of the test material (steel pipe) has a V-shaped cross section as shown in FIG. 3 (FIG. 11 of JP-A-2001-331846). After forming and after forming, or after applying a stress-relieving annealing treatment at 550 ° C x 10 min, expand from the flat part of the test material so that the pipe circumferential direction (C direction) becomes the test piece length. When the V-notch specimen (1/4 size) is cut out in accordance with the provisions of 2242 and the Charpy impact test is performed, the fracture surface transition temperature vTrs is -40 ° C. or less.

In order to achieve the above-described problems, the present inventors have made compatible these properties at a high level, such as strength, low temperature toughness, formability, and cross-section forming processing-torsional fatigue resistance after stress relief annealing. Systematic investigations on various factors affecting the quality of steel, especially the composition and production conditions of steel pipes.
As a result, after adjusting C, Si, Mn, and Al within the appropriate range, the steel material having a composition that essentially contains Mo and Nb is subjected to hot rolling under appropriate conditions to obtain an average crystal of the circumferential cross section. After forming a steel pipe material (hot rolled steel strip) having a structure in which a ferrite phase having a particle size of 2 to 14 μm occupies 60% by volume or more and Nb carbide having an average particle size of 1.5 to 30 nm is precipitated in the ferrite phase. The steel pipe material is welded steel pipe (electrically welded steel pipe) by subjecting the steel pipe material to appropriate conditions, and has a tensile strength of 660 MPa or more, excellent low temperature toughness, excellent formability, and cross-section molding. It has been found that a high-tensile welded steel pipe having excellent torsional fatigue resistance after work-stress relief annealing can be obtained.

The present invention has been completed by further studies based on such knowledge, and the gist thereof is as follows.
(1) By mass%, C: 0.08 to 0.24%, Si: 0.002 to 0.35%, Mn: 1.01 to 1.70%, Al: 0.01 to 0.08%, Nb: 0.017 to 0.15%, Mo: 0.045 to 0.45% Contains P, S, N, and O as impurities, P: 0.019% or less, S: 0.010% or less, N: 0.008% or less, O: 0.003% or less, the balance Fe and unavoidable impurities And a ferrite phase having an average crystal grain diameter in the circumferential cross section of 2 to 14 μm and a second phase other than the ferrite phase, and the structure fraction of the ferrite phase is 60% by volume. It has the above structure and has a structure in which 1.5 to 30 nm of Nb carbide is precipitated in the ferrite phase, and has formability, low temperature toughness, and torsional fatigue resistance after cross-section forming-stress relief annealing A high-tensile welded steel pipe for automobile structural members characterized by being excellent in

  (2) In (1), in addition to the above composition, in terms of mass%, V: 0.001 to 0.150%, W: 0.001 to 0.150%, Ti: 0.001 to 0.040%, Cr: 0.001 to 0.45%, B: 0.0001 An automobile characterized by containing one or more selected from -0.0009%, Cu: 0.001-0.45%, Ni: 0.001-0.45%, and / or Ca: 0.0001-0.005% High-tensile welded steel pipe for structural members.

(3) In (1) or (2), the arithmetic average roughness Ra of the inner and outer surfaces of the steel pipe is 1.5 μm or less, the maximum height roughness Rz is 20 μm or less, and the ten-point average roughness Rz _JIS is 13 μm or less. A high-tensile welded steel pipe for automobile structural members, characterized by being a hollow tubular body.
(4) When a steel pipe material is subjected to an electric forging pipe process to obtain a welded steel pipe, the steel pipe material is in mass%, C: 0.08 to 0.24%, Si: 0.002 to 0.35%, Mn: 1.01 to 1.70. %, Al: 0.01 to 0.08%, Nb: 0.017 to 0.15%, Mo: 0.045 to 0.45%, and impurities P, S, N, and O, P: 0.019% or less, S: 0.010% or less , N: 0.008% or less, O: Adjusted to 0.003% or less, steel material having the composition consisting of the balance Fe and inevitable impurities, after heating to 1160-1320 ° C, temperature in the range of 980-760 ° C The hot rolling to finish the finish rolling in step 1 and the slow cooling treatment in which annealing is performed for 2 seconds or more in the temperature range of 750 to 650 ° C. after the hot rolling is completed, and the winding is performed at the winding temperature of 660 to 510 ° C. The hot-rolled steel strip is subjected to a hot-rolling process, and the electric forging pipe process is performed by the following formula (1): width drawing = [(steel pipe material width) −π {(product outer diameter) − (product meat Thickness)}] / π {(outside product ) - (product thickness)} × (100%) ...... (1)
The steel pipe material is continuously roll-formed and electro-welded to form a welded steel pipe with a width drawing ratio defined by 10% or less, and the welded steel pipe has an average crystal in a circumferential section. It consists of a ferrite phase having a particle size of 2 to 14 μm and a second phase other than the ferrite phase, and the ferrite fraction has a volume fraction of 60% or more by volume, and Nb of 1.5 to 30 nm in the ferrite phase A method for producing a high-strength welded steel pipe for automotive structural members, characterized by having a structure in which carbides are precipitated and having excellent formability, low-temperature toughness, and torsional fatigue resistance after cross-section forming-stress relief annealing.

  (5) In (4), in addition to the above composition, in terms of mass%, V: 0.001 to 0.150%, W: 0.001 to 0.150%, Ti: 0.001 to 0.040%, Cr: 0.001 to 0.45%, B: 0.0001 An automobile characterized by containing one or more selected from -0.0009%, Cu: 0.001-0.45%, Ni: 0.001-0.45%, and / or Ca: 0.0001-0.005% Manufacturing method of high-tensile welded steel pipe for structural members.

  According to the present invention, a high-tensile welded steel pipe having a tensile strength of 660 MPa or more and having excellent low-temperature toughness, excellent formability, and excellent torsional fatigue resistance after cross-section forming-stress relief annealing treatment. It can be manufactured easily and inexpensively without any tempering treatment, and has a remarkable industrial effect. Moreover, according to the present invention, there is an effect that it contributes remarkably to the improvement of the characteristics of the automobile structural member.

First, the reasons for limiting the composition of the high-tensile welded steel pipe of the present invention (hereinafter also referred to as the present invention steel pipe) will be described. Hereinafter, the mass% in the composition is simply indicated by%.
C: 0.08 ~ 0.24%
C is an element that increases the strength of the steel, and is an essential element for securing the strength of the steel pipe. Also, C diffuses during stress relief annealing, prevents dislocation movement by interaction with dislocations introduced during the electro-sewing tube process and cross-section forming processing, suppresses the occurrence of initial fatigue cracks, and prevents torsion resistance. It is an element that improves fatigue characteristics. Such an effect becomes remarkable when the content is 0.08% or more. If it is less than 0.08%, the above-described interaction effect with dislocations does not appear, and the desired torsional fatigue resistance characteristics cannot be ensured. On the other hand, if the content exceeds 0.24%, the steel pipe structure cannot be a ferrite phase-based structure whose ferrite phase is 60% by volume or more, and a desired elongation value cannot be secured, and the formability of the steel pipe is reduced. As it decreases, the low temperature toughness also decreases. For this reason, C was limited to 0.08 to 0.24% of range. In addition, Preferably it is 0.10 to 0.20%.

Si: 0.002 to 0.35%
Si is an element that promotes ferrite transformation in the hot rolling process, and in the present invention, it is necessary to contain 0.002% or more in order to ensure a desired structure and excellent formability. On the other hand, if the content exceeds 0.35%, the residual stress reduction rate at the time of stress relief annealing after cross-section forming decreases, and it becomes impossible to secure the desired torsional fatigue resistance properties, and further the surface properties and electro-weldability deteriorate. To do. For this reason, Si was limited to the range of 0.002 to 0.35%. In addition, Preferably it is 0.10 to 0.25%.

Mn: 1.01-1.70%
Mn contributes to increasing the strength of steel, affects the interaction between C and dislocations, prevents dislocation movement, suppresses strength reduction due to stress relief annealing after cross-section forming, and generates initial fatigue cracks It is an element having a function of increasing the effect of suppressing torsional fatigue resistance and improving torsional fatigue resistance. In order to obtain such an effect, a content of 1.01% or more is required. On the other hand, if the content exceeds 1.70%, ferrite transformation is suppressed, and a desired structure and excellent formability cannot be secured. For this reason, Mn was limited to the range of 1.01-1.70%. In addition, Preferably it is 1.20 to 1.60%.

Al: 0.01-0.08%
Al is an element that acts as a deoxidizing agent during steel making, and has the effect of combining with N to suppress the growth of austenite grains in the hot rolling process and to refine crystal grains. In order to ensure a desired ferrite particle size (2 to 14 μm), it is necessary to contain 0.01% or more of Mn. If it is less than 0.01%, these effects cannot be obtained, and ferrite grains become coarse. On the other hand, even if the content exceeds 0.08%, the effect is saturated and an effect commensurate with the content cannot be expected, and the fatigue resistance is lowered due to an increase in oxide inclusions. For this reason, Al was limited to the range of 0.01 to 0.08%. In addition, Preferably it is 0.02 to 0.06%.

Nb: 0.017 to 0.15%
Nb combines with C in steel, precipitates as carbide, suppresses recovery / recrystallization grain growth in the hot rolling process, and acts as a ferrite phase having a desired grain size (2 to 14 μm). . Furthermore, Nb suppresses strength reduction due to stress relief annealing after cross-section forming and improves torsional fatigue resistance. In order to obtain such an effect, a content of 0.017% or more is required. On the other hand, when the content exceeds 0.15%, the strength increase and the ductility decrease due to the precipitated carbide become remarkable. For this reason, Nb was limited to the range of 0.017 to 0.15%. In addition, Preferably it is 0.022 to 0.049%.

Mo: 0.045-0.45%
Mo, like Nb, precipitates as carbides in steel, suppresses strength reduction due to stress relief annealing after cross-section forming, suppresses the occurrence of initial fatigue cracks, and improves torsional fatigue resistance. Have. Such an effect is manifested at a content of 0.045% or more, but a content exceeding 0.45% reduces moldability. For this reason, Mo was limited to 0.045 to 0.45%. In addition, Preferably it is 0.12-0.35%.

In the present invention, impurities P, S, N, and O are adjusted so that P: 0.019% or less, S: 0.010% or less, N: 0.008% or less, and O: 0.003% or less.
P: 0.019% or less P is an element having an adverse effect of reducing low temperature toughness after cross-section forming-stress relief annealing and reducing electroweldability through solidification co-segregation with Mn, and reduces as much as possible. It is preferable. If the content exceeds 0.019%, the above-described adverse effects become remarkable, so P is limited to 0.019% or less.

S: 0.010% or less S is an element that exists as an inclusion such as MnS in steel, and has an adverse effect on reducing the electric resistance weldability, torsional fatigue resistance, formability, and low-temperature toughness of steel, and is reduced as much as possible. It is preferable. If the content exceeds 0.010%, the above-described adverse effects become remarkable, so S is made 0.010% as the upper limit. In addition, Preferably it is 0.005% or less.

N: 0.008% or less When N remains in the steel as solid solution N, N is an element having an adverse effect of lowering the formability and low temperature toughness of the steel pipe. In the present invention, N is preferably reduced as much as possible. If the content exceeds 0.008%, the above-described adverse effects become remarkable, so N is made 0.008% as the upper limit. In addition, Preferably it is 0.0049% or less.

O: 0.003% or less O is an element that exists as an oxide inclusion in steel and has an adverse effect of lowering the fatigue resistance and low temperature toughness of the steel, and is preferably reduced as much as possible in the present invention. When the content exceeds 0.003%, the above-mentioned adverse effects become remarkable, so O was made 0.003% as the upper limit. In addition, Preferably it is 0.002% or less.

The above-described components are basic components. In the present invention, in addition to the basic components described above, V: 0.001 to 0.150%, W: 0.001 to 0.150%, Ti: 0.001 to 0.040%, Cr: 0.001 to 0.45%, B: 0.0001 to 0.0009%, Cu: 0.001 to 0.45%, Ni: one or more selected from 0.001 to 0.45%, and / or Ca: 0.0001 to 0.005% .
V, W, Ti, Cr, B, Cu, and Ni all suppress the decrease in strength after cross-section forming-stress relief annealing of Mn, Nb, Mo, suppress the occurrence of initial fatigue cracks, and torsion resistance It is an element that has a function of complementing the action of improving fatigue characteristics, and can be selected as necessary and contained in one or more kinds.

V: 0.001 to 0.150%
In addition to the above-described action, V further precipitates as carbides, suppresses recovery / recrystallization grain growth in the hot rolling process of Nb, and supplements the action of making the ferrite phase a desired fine crystal grain. Have. Such an effect is manifested at a content of 0.001% or more, but a content exceeding 0.15% degrades moldability and low-temperature toughness. For this reason, when it contains, it is preferable to limit V to 0.001 to 0.150% of range.

W: 0.001 to 0.150%
In addition to the above-described functions, W further precipitates as carbides, suppresses recovery / recrystallization grain growth in the hot rolling process of Nb, and supplements the function of making the ferrite phase a desired fine crystal grain. Have. Such an effect is manifested with a content of 0.001% or more. However, when the content exceeds 0.150%, the moldability and the low temperature toughness are lowered. For this reason, when it contains, it is preferable to limit W to 0.001 to 0.150% of range.

Ti: 0.001 to 0.040%
In addition to the above-described action, Ti further contributes to improving the formability of the steel pipe by combining with N and reducing the amount of solute N. Furthermore, surplus Ti precipitates as carbides, suppresses recovery / recrystallization grain growth in the hot rolling process of Nb, and has a function of complementing the action of making the ferrite phase a desired fine crystal grain. Such an effect is manifested with a content of 0.001% or more. However, when the content exceeds 0.040%, an increase in strength, a decrease in ductility, and a decrease in low-temperature toughness due to precipitated carbides become significant. For this reason, when it contains, it is preferable to limit Ti to the range of 0.001 to 0.040%.

Cr: 0.001 to 0.45%
As described above, Cr has the function of suppressing the decrease in strength after cross-section forming-stress relief annealing of Mn, suppressing the occurrence of initial fatigue cracks, and complementing the effect of improving torsional fatigue resistance. In order to acquire such an effect, it is desirable to contain 0.001% or more, but inclusion exceeding 0.45% reduces moldability. For this reason, when it contains, it is preferable to limit Cr to 0.001 to 0.45% of range. Further, it is more preferably 0.29% or less.

B: 0.0001-0.0009%
B, like Cr, has the function of suppressing the decrease in strength due to stress relief annealing after cross-section forming processing, suppressing the occurrence of initial fatigue cracks, and complementing the effect of improving torsional fatigue resistance. In order to acquire such an effect, it is desirable to contain 0.0001% or more, but inclusion exceeding 0.0009% reduces moldability. For this reason, when it contains, it is preferable to limit B to 0.0001 to 0.0009% of range.

Cu: 0.001 to 0.45%
Cu has the function of suppressing the decrease in strength after cross-sectional forming of Mn-stress relief annealing, suppressing the occurrence of initial fatigue cracks, complementing the effect of improving torsional fatigue resistance, and further improving the corrosion resistance Has a function. In order to acquire such an effect, it is desirable to contain 0.001% or more, but inclusion exceeding 0.45% reduces moldability. For this reason, when it contains, it is preferable to limit Cu to 0.001 to 0.45% of range. In addition, More preferably, it is 0.2% or less.

Ni: 0.001 to 0.45%
Ni, like Cu, has the function of suppressing the decrease in strength after cross-sectional forming of Mn-stress relief annealing, suppressing the occurrence of initial fatigue cracks, and complementing the effect of improving torsional fatigue resistance, Furthermore, it has a function of improving corrosion resistance. In order to acquire such an effect, it is desirable to contain 0.001% or more, but inclusion exceeding 0.45% reduces moldability. For this reason, when it contains, it is preferable to limit Ni to 0.001 to 0.45% of range. In addition, More preferably, it is 0.2% or less.

Ca: 0.0001 to 0.005%
Ca has an action of controlling the form of inclusions, in which the expanded inclusions (MnS) are granular inclusions (Ca (Al) S (O)). Thus, it is an element having the effect of improving formability, torsional fatigue resistance, and low temperature toughness, and can be contained as required. Such an effect becomes remarkable when the content is 0.0001% or more. However, when the content exceeds 0.005%, the non-metallic inclusions are increased, and the torsional fatigue resistance is lowered. For this reason, when it contains, it is preferable to limit Ca to 0.0001 to 0.005% of range.

The balance other than the above components is Fe and inevitable impurities.
Next, the reason for limiting the structure of the high-strength welded steel pipe of the present invention will be described.
In the steel pipe of the present invention, the microstructure is an important material factor for ensuring excellent formability and excellent torsional fatigue resistance after cross-section forming-stress relief annealing.
The steel pipe of the present invention has a structure composed of a ferrite phase and a second phase other than the ferrite phase.
Here, the “ferrite phase” includes polygonal ferrite, acicular ferrite, Widmanstatten ferrite, and bainitic ferrite. The second phase is preferably any one of carbide, pearlite, bainite, martensite, or a mixed phase thereof other than the ferrite phase.

In the steel pipe of the present invention, the ferrite phase has an average crystal grain size (average grain size) of 2 to 14 μm in the circumferential cross section (cross section orthogonal to the longitudinal direction of the pipe) and a structure fraction of 60% by volume or more, The ferrite phase is a ferrite phase formed by precipitation of Nb carbide having an average particle size of 1.5 to 30 nm.
Ferrite phase structure fraction: 60 volume% or more If the ferrite phase structure fraction is less than 60 volume%, the desired formability cannot be ensured, and local thinning and rough surface of the surface occur during stress concentration. Thus, the torsional fatigue resistance after cross-section forming and stress relief annealing is greatly reduced. For this reason, in the steel pipe of the present invention, the structure fraction of the ferrite phase is limited to 60% by volume or more. In addition, Preferably it is 75 volume% or more.

Average particle size of ferrite phase: 2-14μm
If the average grain size of the ferrite phase is less than 2 μm, the desired formability cannot be ensured, and local thinning and surface roughness that occur during molding become stress-concentrated parts, and the torsion resistance after cross-section forming processing-stress relief annealing Fatigue properties are greatly reduced. On the other hand, if the average grain size of the ferrite phase exceeds 14 μm, the low temperature toughness and torsional fatigue resistance after cross-section forming-stress relief annealing deteriorate. For this reason, in this invention, the average particle diameter of the ferrite phase was limited to 2 micrometers or more and 14 micrometers or less. In addition, Preferably it is 8 micrometers or less.

Average particle size of Nb carbide in ferrite phase: 1.5nm-30nm
Nb carbide in the ferrite phase is important for balancing the cross-section hardness change rate and residual stress reduction rate after cross-section forming-stress relief annealing, ensuring high torsional fatigue strength, and ensuring the desired formability. Organization factors. Fig. 1 shows the relationship between the average grain size of Nb carbide in the ferrite phase and the cross-section hardness-change rate and residual stress reduction rate after cross-section forming-stress relief annealing (SR annealing). The average grain size of Nb carbide in the ferrite phase FIG. 2 shows the relationship between the diameter, the ratio between the 5 × 10 ⁵ repeated fatigue limit and the steel pipe strength TS (σ _B / TS), and the elongation El (JIS No. 12 specimen) of the steel pipe before cross-section forming.

Note that the rate of change in cross-sectional hardness (%) due to stress relief annealing (SR) after cross-section forming processing is the following formula: cross-sectional hardness change rate = {(cross-sectional hardness after SR) − (cross-sectional hardness before SR)} / (SR Previous cross section hardness) x (100%)
The value defined in is used. The residual stress reduction rate (%) due to stress relief annealing (SR) after cross-section forming processing is the following formula: residual stress reduction rate = {(residual stress before SR)-(residual stress after SR)} / (before SR Residual stress) x (100%)
The value defined in is used.

In addition, as shown in FIG. 3 (FIG. 11 of Japanese Patent Laid-Open No. 2001-331846), the cross-section forming process is performed by forming the cross section into a V-shape as shown in FIG. Furthermore, after stress-relieving annealing at 550 ° C. × 10 min, both ends are fixed by chucking and a torsional fatigue test is performed under the conditions of 1 Hz and double swing to obtain a 5 × 10 ⁵ repeated fatigue limit σ _B , The ratio between the obtained 5 × 10 ⁵ repeated fatigue limit σ _B and the steel pipe tensile strength TS, (σ _B / TS), was evaluated.

When the average particle size of Nb carbide is less than 1.5 nm, as shown in FIG. 1, the rate of change in cross-sectional hardness due to stress relief annealing after cross-section forming processing is below a predetermined value (−15%), and the rate of decrease in residual stress is a predetermined value. (60%) below. In addition, when the average particle size of Nb carbide is less than 1.5 nm, as shown in FIG. 2, the elongation El of the steel pipe is less than 16% and the formability deteriorates, and the 5 × 10 ⁵ repeated fatigue limit σ _B and the steel pipe tensile strength The ratio with TS, (σ _B / TS) is less than 0.50.

On the other hand, when the average particle size of Nb carbide exceeds 30 nm and becomes coarse, as shown in FIG. 1, the rate of change in cross-sectional hardness due to stress relief annealing after cross-section forming processing falls below a predetermined value (−15%). As shown in FIG. 5, the ratio of 5 × 10 ⁵ repeated fatigue limit σ _B to steel pipe tensile strength TS, (σ _B / TS) is less than 0.50.
Thus, when the average particle size of the Nb carbide in the ferrite phase is out of the range of 1.5 nm to 30 nm, it has excellent formability and excellent torsional fatigue resistance after cross-section processing-stress relief annealing. Can not be. For this reason, the average particle diameter of the Nb carbide in the ferrite phase is limited to a range of 1.5 nm to 30 nm. In addition, Preferably, they are 2 nm-25 nm.

  In the present invention, the average particle size of Nb carbide in the ferrite phase is determined as follows. Samples for tissue observation are collected from steel pipes using the extraction replica method, and observed using a transmission electron microscope (TEM) at five fields of view at 100,000 times, and EDS analysis identifies cementite and TiN that do not contain Nb. For carbides containing Nb (Nb carbides), the area of Nb carbides is measured by an image analyzer, the equivalent circle diameter is calculated from the area, and the arithmetic average value of these is the average particle diameter of Nb carbides. It was. Note that Nb composite carbides including Mo, Ti, etc. were also counted as Nb carbides.

In the steel pipe of the present invention, the surface roughness of the inner and outer surfaces of the steel pipe is in accordance with the provisions of JIS B 0601-2001, arithmetic average roughness Ra: 1.5 μm or less, maximum height roughness Rz is 20 μm or less, ten points It is preferable to have a surface texture with an average roughness Rz _JIS of 13 μm or less. With the surface properties of the steel pipe that deviate from the surface roughness described above, the formability deteriorates, and a stress concentration portion is generated during processing such as cross-section forming processing, and the subsequent torsional fatigue resistance properties are reduced.

Next, a preferred method for producing the above-described steel pipe of the present invention will be described.
First, it is preferable to melt the molten steel having the above-described composition by a known melting method such as a converter and to obtain a steel material by a known casting method such as a continuous casting method. Subsequently, it is preferable to subject these steel materials to a hot-rolling step to obtain a steel pipe material such as a hot-rolled steel strip.
In the hot rolling process, the steel material is heated to 1160 to 1320 ° C., then subjected to rough rolling, hot rolling for finishing finish rolling at a temperature in the range of 980 to 760 ° C., and after completion of the hot rolling, 750 It is preferable to perform a slow cooling process in which annealing is performed for 2 seconds or more in a temperature range of ˜650 ° C., and winding at a winding temperature of 660 ° C. to 510 ° C. to form a hot rolled steel strip.

Heating temperature of steel material: 1160-1320 ℃
The heating temperature of the steel material affects the rate of change in cross-sectional hardness after stress relief annealing through the re-solution and precipitation of Nb in the steel, and is an important factor for suppressing softening. When the heating temperature is less than 1160 ° C, the coarse Nb carbonitrides precipitated during continuous casting remain as undissolved carbonitrides, resulting in coarsening of the Nb carbides in the ferrite phase (average particle size exceeding 30 nm) However, the rate of change in cross-sectional hardness due to stress-relieving annealing after cross-section forming is less than −15%, and the desired torsional fatigue resistance characteristics cannot be ensured. On the other hand, when the heating temperature exceeds 1320 ° C, the crystal grains become coarser, so the ferrite phase obtained in the subsequent hot rolling process becomes coarser (the average grain diameter in the circumferential cross section exceeds 14 µm). However, the formability and the cross-section forming process-low temperature toughness and torsional fatigue resistance after stress-relieving annealing are deteriorated. For this reason, it is preferable to limit the heating temperature of a steel raw material to the range of 1160-1320 degreeC. The temperature is more preferably 1200 to 1300 ° C. Further, from the viewpoint of ensuring the uniformity of the solid solution state of Nb and securing a sufficient solid solution time, the soaking time during heating of the steel material is preferably 10 min or more.

Finishing rolling finish temperature: 980-760 ° C
The finish rolling finishing temperature in the hot rolling process is an important factor for ensuring good steel pipe formability by adjusting the structure fraction of the ferrite phase and the average grain diameter of the ferrite phase in a predetermined range. When the finish rolling finish temperature exceeds 980 ° C, the average particle diameter of the ferrite phase of the obtained steel pipe material exceeds 14 μm, and the structure fraction of the ferrite phase becomes less than 60% by volume, and the formability of the steel pipe decreases. Arithmetic average roughness Ra on the inner and outer surfaces of steel pipe exceeds 1.5μm, maximum height roughness Rz exceeds 20μm, 10-point average roughness Rz _JIS exceeds 13μm, surface properties decrease, fatigue resistance decreases To do. On the other hand, when the finish rolling finish temperature is lower than 760 ° C, the average particle diameter of the ferrite phase of the obtained steel pipe material is less than 2 µm, the formability is lowered, and the average particle diameter of Nb carbide is reduced to 30 nm by strain-induced precipitation. Exceedingly, the rate of change in cross-sectional hardness due to stress-relieving annealing after cross-section forming processing is less than −15%, and the desired torsional fatigue resistance characteristics cannot be ensured. For this reason, it is preferable that finishing rolling completion temperature shall be the range of 980-760 degreeC. In addition, More preferably, it is 880-820 degreeC. Further, from the viewpoint of ensuring good surface properties of the steel pipe, it is preferable to perform descaling with high-pressure water of 11.8 MPa (120 kg / cm ² ) or more before finish rolling.

Slow cooling treatment: Slow cooling for 2 s or more in the temperature range of 750 to 650 ° C. In the present invention, the temperature range of 750 to 650 ° C. is not taken up immediately after the finish rolling of hot rolling but before winding. A slow cooling process is performed in which slow cooling is performed. Here, slow cooling refers to cooling at a cooling rate of 20 ° C./s or less. The slow cooling time in the above temperature range is preferably 2 s or more. If it is less than 2 s, a desired tissue cannot be secured. In addition, More preferably, it is 4 s or more. By this slow cooling treatment, the structure fraction of the ferrite phase can be made 60% by volume or more, and the elongation El of the steel pipe becomes 16% or more by the JIS No. 12 test piece, and the desired formability can be secured.

Winding temperature: 660-510 ° C
The hot-rolled steel strip subjected to the slow cooling treatment is then wound into a coil shape. The winding temperature is preferably in the temperature range of 660 to 510 ° C. The coiling temperature is an important factor that determines the structure fraction of the hot-rolled steel strip and the precipitation state of the precipitates. If the coiling temperature is less than 510 ° C, the desired fraction of the microstructure of the ferrite phase cannot be ensured, the desired formability cannot be ensured, and the average particle size of Nb carbide is less than 1.5 nm, resulting in reduced strength during stress relief annealing. And the desired torsional fatigue resistance characteristics cannot be ensured. On the other hand, when the coiling temperature exceeds 660 ° C, the average particle size of the ferrite phase exceeds 14 µm, the formability deteriorates, the scale formation after winding becomes remarkable, the surface property of the steel strip decreases, and the steel pipe The arithmetic average roughness Ra of the inner and outer surfaces exceeds 1.5μm, the maximum height roughness Rz exceeds 20μm, the ten-point average roughness Rz _JIS exceeds 13μm, the surface properties of the steel pipe deteriorate, and the resistance of the steel pipe Torsional fatigue properties are reduced. Furthermore, the Nb carbide coarsens due to the Ostwald growth of the Nb carbide, the average particle diameter exceeds 30 nm, the rate of change in cross-sectional hardness by stress relief annealing after cross-section forming processing is less than -15%, and the desired torsional fatigue resistance characteristics It cannot be secured. For this reason, the winding temperature is preferably in the range of 660 to 510 ° C. In addition, More preferably, it is 620-560 degreeC.

  By subjecting the steel material having the above composition to the hot rolling process under the above-described conditions, the microstructure and the precipitate state are optimized, the surface property is excellent, the moldability is excellent, and the steel pipe is manufactured. Even after it is piped, it has excellent formability and low temperature toughness, and further has a low rate of change in cross-sectional hardness after cross-section forming-stress relief annealing (550 ° C x 10 min), ensuring the desired excellent torsional fatigue resistance. It can be a steel pipe material (hot rolled steel strip). In addition, the excellent surface properties of the steel pipe material (hot rolled steel strip) greatly contributes to ensuring excellent torsional fatigue resistance.

In the present invention, the above-described steel pipe material (hot-rolled steel strip) is further subjected to an electric sewing pipe process to obtain a welded steel pipe. Next, a preferred electric sewing tube process will be described.
The steel pipe material may be hot-rolled, but it is preferable to subject the steel pipe material to pickling treatment, shot blasting, or the like in order to remove the black skin on the surface. Furthermore, from the viewpoint of corrosion resistance and coating film adhesion, the steel pipe material can be subjected to surface treatment such as galvanization, aluminum plating, nickel plating, and organic film treatment.

An electric sewing pipe process is performed on the steel pipe material that has been pickled or surface-treated.
The electric sewing pipe process is a process in which a steel pipe material is continuously roll-formed and electro-welded to form a welded steel pipe. In the electric sewing tube process, it is preferable to apply an electric sewing tube having a width drawing ratio of 10% or less (including 0%). The width drawing ratio is an important factor for ensuring the desired formability. When the width drawing ratio exceeds 10%, the formability deteriorates due to pipe making, and the desired formability cannot be ensured. For this reason, it is preferable that the width drawing ratio is 10% or less (including 0%). More preferably, it is 1% or more. The width drawing ratio (%) is expressed by the following formula (1): width drawing ratio (%) = [(width of steel pipe material) −π {(outer diameter of product steel pipe) − (product steel pipe wall thickness)}] / π {(product Steel pipe outer diameter)-(product steel pipe wall thickness)} x (100%) ......... (1)
The value defined in.

  In the present invention, the steel pipe material is not limited to the hot-rolled steel strip. If it is a raw material having the composition and structure described above, instead of the hot-rolled steel strip as described above, a cold-rolled annealed steel strip subjected to cold rolling-annealing, or a surface-treated steel strip further subjected to various surface treatments There is no problem with using. Also, instead of the electric sewing tube process, it can also be a tube forming process that combines roll forming, press-cut section of the cut plate, cold / warm / hot diameter reduction rolling and heat treatment after pipe forming. In addition, there is no problem even if laser welding, arc latent welding, plasma welding, or the like is used in place of ERW welding.

  In addition, the high-tensile welded steel pipe of the present invention is subjected to various forming processes, and further subjected to stress relief annealing as necessary to form an automobile structural member such as a torsion beam. In the high-tensile welded steel pipe of the present invention, the conditions for stress relief annealing after the forming process need not be particularly limited. In addition, the fatigue life improvement effect by stress removal annealing is remarkable in the range of about 100 ° C. or higher where the effect of preventing dislocation movement due to C diffusion starts to be less than about 650 ° C., where the hardness reduction by stress removal annealing is less than 650 ° C. Become. For this reason, it is also possible to substitute the coating baking process of about 150-200 degreeC as a stress removal annealing process. In particular, the fatigue life improving effect is increased in the temperature range of 460 ° C. or more and 630 ° C. or less. Further, the soaking time in the stress relief annealing is preferably in the range of 1 s to 5 h. More preferably, it is 2 min to 1 h.

Example 1
Molten steel having the composition shown in Table 1 was melted and made into a steel material (slab) by a continuous casting method. These steel materials are heated to about 1250 ° C and subjected to hot rolling at a finish rolling finish temperature of about 860 ° C. After the hot rolling is completed, the steel is gradually cooled for 5 seconds at a temperature range of 750 to 650 ° C. Then, a hot rolling step of winding at a winding temperature of 590 ° C. was performed to obtain a hot rolled steel strip (plate thickness: about 3 mm).

Next, these hot-rolled steel strips are used as a steel pipe material, pickled, slitted to a predetermined width, continuously rolled into an open pipe, and the open pipe is electro-welded by high-frequency resistance welding. A welded steel pipe (outer diameter φ89.1 mm × wall thickness of about 3 mm) was made by the electric sewing pipe process. In the electric sewing tube process, the width drawing ratio defined by the equation (1) was set to 4%.
Test specimens are collected from these welded steel pipes, microstructure observation test, precipitate observation test, tensile test, surface roughness test, torsional fatigue test, low temperature toughness test, cross section hardness measurement test after stress relief annealing, stress relief annealing. A later residual stress measurement test was conducted. The test method was as follows.

(1) Microstructure observation test From the obtained welded steel pipe, a specimen for microstructural observation was collected so that the circumferential cross-section became the observation surface, polished, and subjected to nital corrosion, and a scanning electron microscope (3000 times) ) Were observed and imaged, and using an image analyzer, the structure fraction (volume%) of the ferrite phase and the average crystal grain size (equivalent circle diameter) of the ferrite phase were measured.

(2) Precipitate observation test From the obtained welded steel pipe, a precipitate observation test piece is collected so that the circumferential cross section becomes the observation plane, and a sample for structure observation is prepared using the extraction replica method. Using a transmission electron microscope (TEM), observe five fields at a magnification of 100,000, identify and exclude Nb-free cementite, TiN, etc. by EDS analysis, and analyze Nb-containing carbides (Nb carbides) Thus, the area of each Nb carbide was measured, the equivalent circle diameter was calculated from the area, and the arithmetic average value thereof was defined as the average particle diameter of the Nb carbide. In addition to Nb, composite carbides containing Ti, Mo and the like were also counted as Nb carbides.

(3) Tensile test JIS No. 12 test piece was cut out from the obtained welded steel pipe in accordance with the provisions of JIS Z 2201 so that the L direction is the tensile direction, and the tensile test was conducted in accordance with the provisions of JIS Z 2241. The tensile properties (tensile strength TS, yield strength YS, El) were determined, and the strength and formability were evaluated.
(4) Surface roughness test The surface roughness of the inner and outer surfaces of the obtained welded steel pipe was measured using a stylus type roughness meter in accordance with the provisions of JIS B 0601-2001, As the roughness parameters, arithmetic average roughness Ra, maximum height roughness Rz, and ten-point average roughness Rz _JIS were determined. The measurement direction of the roughness curve was the circumferential direction (C direction) of the tube, the low-frequency cut-off value was 0.8 mm, and the evaluation length was 4 mm. As the representative value, the larger one of the inner surface and the outer surface was adopted.

(5) Torsional fatigue test From the obtained welded steel pipe, a test material (length: 1500 mm) was sampled, and the center part of the test material was about 1000 mmL, as shown in FIG. 3 (FIG. 11 of JP-A-2001-331846). As shown in the figure, the cross-section of the steel pipe was formed into a V-shaped cross section, and after stress-relieving annealing at 550 ° C. × 10 min, both ends were fixed by chucking, and a torsional fatigue test was performed. .

The torsional fatigue test was performed under the conditions of 1 Hz and double swing, various stress levels were changed, and the number of repetitions N until the fracture at the load stress S was obtained. The 5 × 10 ⁵ repeated fatigue limit σ _B (MPa) was obtained from the obtained SN diagram, and torsional fatigue resistance was evaluated by σ _B / TS (where TS is the tensile strength of the steel pipe MPa). The load stress was measured by first conducting a torsion test with a dummy piece, confirming the fatigue crack position, and attaching a triaxial strain gauge at that position.

(6) Low temperature toughness test From the obtained welded steel pipe, a test material (length: 1500mm) is sampled and subjected to cross-section forming and stress relief annealing under the same conditions as the torsional fatigue test material. From the flat part of the test material after processing-stress relief annealing, expand the pipe circumferential direction (C direction) to be the test piece length, and V-notch test piece (1/4 size) in accordance with the provisions of JIS Z 2242 The Charpy impact test was conducted, the fracture surface transition temperature vTrs was determined, and the low temperature toughness was evaluated.

(7) Cross-sectional hardness measurement test after stress-relieving annealing Perform cross-section molding under the same conditions as the torsional fatigue test material. From the position corresponding to the fatigue crack of the test material, before and after stress-relief annealing SR (550 ° C x 10 min), A test piece for measuring the cross-sectional hardness was taken, and the Vickers hardness HV was measured with a Vickers hardness tester (test force: 98 N (10 kgf)). Hardness was measured at three points of 1/4, 1/2, and 3/4 of the wall thickness, and the average value was the cross-sectional hardness before and after stress removal annealing (SR) of the test piece. From this hardness measurement result, the following formula cross-sectional hardness change rate = {(cross-sectional hardness after SR) − (cross-sectional hardness before SR)} / (cross-sectional hardness before SR) × (100%)
Thus, the rate of change in cross-sectional hardness (%) after the cross-section forming process-stress removal annealing (SR) was obtained and used as a parameter of the softening resistance during the cross-section forming process-stress removal annealing.

(8) Residual stress measurement test after stress-relieving annealing Cross-section molding is performed under the same conditions as the torsional fatigue test material, and residual stress is measured at the stress crack equivalent position of the test material, stress-relieving annealing SR (550 ° C x 10 min) The measurement was performed by a strain gauge cutting method using a triaxial gauge before and after. From this measurement result, the following formula residual stress reduction rate = {(residual stress before SR) − (residual stress after SR)} / (residual stress before SR) × (100%)
The residual stress reduction rate (%) at the time of cross-section forming processing-stress removal annealing was determined.

  The obtained results are shown in Table 2.

In each of the inventive examples (steel pipe Nos. 1 to 10), the ferrite phase has a structure fraction of 60% by volume or more, the ferrite crystal has an average crystal grain size of 2 to 14 μm, and the Nb carbide has an average particle diameter of 30 nm. It is a welded steel pipe with a structure of ~ 1.5nm, tensile strength of 660MPa or more, high strength satisfying elongation El of JIS No. 12 test piece of 16% or more, and excellent formability. Moreover, both the inventive examples, cross molding - after stress relief annealing, is -15% or more cross-sectional hardness change ratio, residual stress reduction rate is 60% or more, 5 × 10 ⁵ repeatedly in torsional fatigue test The ratio between fatigue limit σ _B and steel pipe tensile strength TS, σ _B / TS is 0.50 or more, and it is a high-tensile welded steel pipe with excellent torsional fatigue resistance. In addition, all of the examples of the present invention are high-tensile welded steel pipes having excellent low-temperature toughness with a fracture surface transition temperature vTrs of −40 ° C. or lower as it is after cross-section forming and after cross-section forming-stress relief annealing.

On the other hand, the comparative examples (steel pipe Nos. 11 to 31) in which the steel composition is out of the scope of the present invention are out of the scope of the present invention in terms of the structure, etc., and the strength, formability, cross-section forming process-torsional fatigue resistance after stress relief annealing The low temperature toughness as it is in the cross-section forming process or the low-temperature toughness after the cross-section forming process-stress relief annealing is lowered.
Comparative examples (steel pipes No. 12, No. 14, No. 16, No. 18, No. 18, C, Si, Mn, Al, Nb, Mo, Ti, P, S, N, O, V exceeding the scope of the present invention) .20, No.22 to 28) are low in elongation El and less than 16%, lack ductility, and (σ _B / TS) is less than 0.50, torsional fatigue resistance decreases, and fracture surface transition The temperature vTrs exceeds −40 ° C., and the low temperature toughness is reduced.

In addition, comparative examples (steel pipe No. 11, No. 13, No. 15, No. 17, No. 19, No. 21) in which C, Si, Mn, Al, Nb, and Mo are out of the scope of the present invention are as follows. In both cases, the torsional fatigue resistance is reduced when the rate of change in cross-sectional hardness after cross-section forming-stress relief annealing is less than -15% and (σ _B / TS) is less than 0.50.
Moreover, the comparative examples (steel pipe No. 29, No. 30, No. 31) in which Cr, B, and Cu exceed the scope of the present invention all have low elongation El of less than 16% and lack ductility. Also, the torsional fatigue resistance is reduced when the residual stress reduction rate after cross-section forming-stress relief annealing is less than 60% and (σ _B / TS) is less than 0.50.

Steel pipes (product pipes) Nos. 1 to 31 have a surface roughness of arithmetic average roughness Ra: 0.6 to 1.4 μm, maximum height roughness Rz: 18 to 17 μm, ten-point average roughness Rz _JIS : 6 The surface properties of the inner and outer surfaces of the steel pipe were good.
(Example 2)
The steel material (slab) having the composition of steels A and D shown in Table 1 was subjected to a hot rolling process under the conditions shown in Table 3 to obtain a hot rolled steel strip. Then, these hot-rolled steel strips are used as steel pipe materials, pickled, slitted to a predetermined width, then continuously rolled into open pipes, and the open pipes are electro-sealed by high-frequency low resistance welding. A welded steel pipe (outer diameter: 70 to 114.3 mmφ × wall thickness of 2.0 to 6.0 mm) was formed by the electric sewing pipe process for welding. In the electric sewing tube process, the width drawing ratio defined by the equation (1) was set to the values shown in Table 3.

A specimen was collected from the obtained welded steel pipe in the same manner as in Example 1, and in the same manner as in Example 1, the structure observation test, the precipitate observation test, the tensile test, the surface roughness test, the torsional fatigue test, the low temperature toughness test, Cross section hardness measurement test after cross section forming process-stress relief annealing and residual stress measurement test after cross section molding process-stress relief annealing were performed.
Table 4 shows the obtained results.

Examples of the present invention (steel pipes No. 33, No. 36, No. 39, Nos. 41 to 43, Nos. 45 to 51) all have a ferrite phase structure fraction of 60% by volume or more, and the average of the ferrite phase It has a structure in which the crystal grain size is 2 to 14 μm, the average grain size of Nb carbide is 30 nm to 1.5 nm, the tensile strength is 660 MPa or more, and the elongation El in the JIS 12 test piece is 16% or more. The welded steel pipe has high strength and excellent formability. Moreover, both the inventive examples, cross molding - after stress relief annealing, is -15% or more cross-sectional hardness change ratio, residual stress reduction rate is 60% or more, 5 × 10 ⁵ repeatedly in torsional fatigue test The ratio between fatigue limit σ _B and steel pipe tensile strength TS, σ _B / TS is 0.50 or more, and it is a high-tensile welded steel pipe with excellent torsional fatigue resistance. In addition, all of the examples of the present invention are high-tensile welded steel pipes having excellent low-temperature toughness with a fracture surface transition temperature vTrs of −40 ° C. or lower as it is after cross-section forming and after cross-section forming-stress relief annealing.

On the other hand, comparative examples (steel pipes No. 32, No. 34, No. 35, No. 37, No. 37) where the conditions of the hot rolling process of the steel material or the conditions of the electric seam pipe process of the steel pipe are outside the scope of the present invention. .38, No.40, No.44, and No.52) decrease in strength, formability, torsional fatigue resistance, low-temperature toughness of cross-section forming, and low-temperature toughness after cross-section forming-stress relief annealing is doing.
In the comparative example (steel pipe No. 38) in which the slow cooling time in the hot rolling process falls outside the scope of the present invention, the comparative example (steel pipe No. 44) in which the coiling temperature in the hot rolling process falls outside the scope of the present invention, However, when El is less than 16%, ductility is lowered, and (σ _B / TS) is less than 0.50, torsional fatigue resistance is lowered.

Moreover, the comparative example (steel pipe No. 35) in which the finishing rolling finish temperature in the hot rolling process deviates from the range of the present invention is high, and the comparative example (steel pipe No. 40) in which the coiling temperature in the hot rolling process deviates from the range of the present invention. In El, less than 16%, ductility is reduced, arithmetic average roughness Ra is 1.5 μm or more, maximum height roughness Rz is 20 μm or more, ten-point average roughness Rz _JIS is 13 μm or more, and surface properties are reduced. Furthermore, when (σ _B / TS) is less than 0.50, the torsional fatigue resistance is reduced.

In addition, a comparative example (steel pipe No. 32) in which the heating temperature of the steel material in the hot rolling process deviates from the scope of the present invention is high, and a comparative example in which the width drawing ratio in the electro-sewing pipe process of the steel pipe deviates from the scope of the present invention ( Steel tube No.52) has a torsional fatigue resistance of (σ _B / TS) after cross-section forming-stress relief annealing of less than 0.50, and the torsional fatigue resistance is reduced. The surface transition temperature vTrs exceeds −40 ° C., and the low temperature toughness is reduced.

Moreover, the comparative example (steel pipe No. 34) in which the heating temperature of the steel material in the hot rolling process deviates from the scope of the present invention is low, and the comparative example (steel pipe No. 34) in which the finish rolling finish temperature in the hot rolling process deviates from the scope of the present invention. In 37), (σ _B / TS) is less than 0.50, and the torsional fatigue resistance is reduced.

It is a graph which shows the relationship between the average particle diameter of Nb carbide | carbonized_material in a ferrite phase, the cross-sectional hardness change rate after cross-section shaping | molding-stress relief annealing SR, and a residual stress fall rate. The average grain size of Nb carbide in the ferrite phase and the ratio of the 5 × 10 ⁵ cyclic fatigue limit to the steel pipe tensile strength TS after cross-section forming-stress relief annealing SR (σ _B / TS), JIS No. 12 of steel pipe It is a graph which shows the relationship between the elongation El in a test piece. It is explanatory drawing which shows typically the cross-section shaping | molding processing state of the test material used for the torsional fatigue test after cross-section shaping | molding-stress removal annealing.

Claims (5)

% By mass
C: 0.08 to 0.24%, Si: 0.002 to 0.35%,
Mn: 1.01-1.70%, Al: 0.01-0.08%,
Nb: 0.017 to 0.15%, Mo: 0.045 to 0.45%,
P, S, N, O which are impurities,
P: 0.019% or less, S: 0.010% or less,
N: 0.008% or less, O: Adjusted to 0.003% or less, a composition comprising the balance Fe and inevitable impurities, and a ferrite phase having an average crystal grain size in the circumferential cross section of 2 to 14 μm and the ferrite Comprising a second phase other than the phase, the structure fraction of the ferrite phase is 60% or more by volume, and a structure in which 1.5 to 30 nm of Nb carbide is precipitated in the ferrite phase, A high-tensile welded steel pipe for automobile structural members, which is excellent in formability, low-temperature toughness, and torsional fatigue resistance after cross-section forming-stress relief annealing.   In addition to the above-described composition, V: 0.001 to 0.150%, W: 0.001 to 0.150%, Ti: 0.001 to 0.040%, Cr: 0.001 to 0.45%, B: 0.0001 to 0.0009%, Cu: 0.001 to The automotive structural member according to claim 1, comprising 0.45%, Ni: one or more selected from 0.001 to 0.45%, and / or Ca: 0.0001 to 0.005%. High tensile welded steel pipe. Further, the present invention is a hollow tubular body having an arithmetic average roughness Ra of the inner and outer surfaces of the steel pipe of 1.5 μm or less, a maximum height roughness Rz of 20 μm or less, and a ten-point average roughness Rz _JIS of 13 μm or less. A high-tensile welded steel pipe for automobile structural members according to 1 or 2. The steel pipe material is subjected to the electric forging pipe process to make a welded steel pipe.
The steel pipe material is mass%,
C: 0.08 to 0.24%, Si: 0.002 to 0.35%,
Mn: 1.01-1.70%, Al: 0.01-0.08%,
Nb: 0.017 to 0.15%, Mo: 0.045 to 0.45%,
P, S, N, O which are impurities,
P: 0.019% or less, S: 0.010% or less,
N: 0.008% or less, O: Adjusted to 0.003% or less, a steel material having a composition comprising the balance Fe and inevitable impurities, heated to 1160-1320 ° C, and then at a temperature in the range of 980-760 ° C Hot rolling to finish the finish rolling, and after the hot rolling, annealing is performed at a temperature range of 750 to 650 ° C. for 2 seconds or more, and winding is performed at a winding temperature of 660 to 510 ° C. It is a hot-rolled steel strip that has been subjected to a hot-rolling process,
The electric forging pipe process is a pipe making process in which a width drawing ratio defined by the following formula (1) is set to 10% or less, and the steel pipe material is continuously roll-formed and electro-welded to form a welded steel pipe. ,
The welded steel pipe is composed of a ferrite phase having an average crystal grain size of 2 to 14 μm in the circumferential cross section and a second phase other than the ferrite phase, and the structure fraction of the ferrite phase is 60% or more by volume. It has a structure in which 1.5 to 30 nm of Nb carbide precipitates in the ferrite phase, and is characterized by excellent formability, low temperature toughness, and torsional fatigue resistance after cross-section forming-stress relief annealing Manufacturing method of high-tensile welded steel pipe for automobile structural members.
Serial width aperture = [(width of steel tube material) - [pi] {(product OD) - (product thickness)}] / [pi {(product OD) - (product thickness)} × (100%) ...... (1) In addition to the above-described composition, V: 0.001 to 0.150%, W: 0.001 to 0.150%, Ti: 0.001 to 0.040%, Cr: 0.001 to 0.45%, B: 0.0001 to 0.0009%, Cu: 0.001 to The automobile structural member according to claim 4, comprising 0.45%, Ni: one or more selected from 0.001 to 0.45%, and / or Ca: 0.0001 to 0.005%. Of manufacturing high-strength welded steel pipes.

Images