JP5125601B2 - High tensile welded steel pipe for automobile structural members and method for manufacturing the same - Google Patents

Description

  The present invention relates to a high-tensile welded steel pipe having a tensile strength of 590 MPa or more suitable as a steel pipe for automobile structural members such as a torsion beam, an axle beam, a trailing arm, and a suspension arm. It relates to improvement of torsional fatigue resistance after forming.

  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. The technique described in Patent Document 1 includes C: Si, Mn, P, S, Al, N adjusted to an appropriate amount, and B: 0.0003 to 0.003%, and further among Mo, Ti, Nb, and V. The steel material having a composition containing one or more of the above is subjected to finish rolling at 950 ° C or less at the Ar ₃ transformation point and hot rolling 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 technology, it is said that an ultra-high strength 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-welded steel pipe having high tensile strength: 1470 N / mm ² or more and high ductility, which is suitable for automobile door impact beams and stabilizers. . The technology described in Patent Document 2 includes C: 0.18 to 0.28%, Si: 0.10 to 0.50%, Mn: 0.60 to 1.80%, 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 normalization at 850 to 950 ° C. for an ERW steel pipe manufactured using a steel plate made of 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.
In addition, the ERW steel pipe manufactured by the technique described in Patent Document 2 has an elongation El of 18% at most, 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, is a non-quenched type that is not subjected to quenching, and is suitable for automotive structural members such as torsion beams, axle beams, trailing arms, suspension arms, An object of the present invention is to provide a high-strength welded steel pipe having a tensile strength of 590 MPa or more, excellent formability, excellent low-temperature toughness, and excellent torsional fatigue resistance after cross-section forming, and a method for producing the same.

The “excellent formability” as used in the present invention refers to a JIS No. 12 test piece compliant with JIS Z 2201 and an elongation El of 15 in a tensile test conducted in accordance with JIS Z 2241. % Or more (22% or more for JIS 11 test piece).
In addition, “excellent torsional fatigue resistance after cross-section forming” as used in the present invention means that the longitudinal center portion of the steel pipe is V-shaped as shown in FIG. 1 (FIG. 11 of JP-A-2001-331846). After the cross section was molded, both ends were fixed by chucking and a torsional fatigue test was conducted under the conditions of 1 Hz and double swing. The 5 × 10 ⁵ repeated fatigue limit σ _B was obtained, and the obtained 5 × 10 ⁵ The ratio between the repeated fatigue limit σ _B and the steel pipe tensile strength TS, (σ _B / TS) is 0.40 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. 1 (FIG. 11 of JP-A-2001-331846). After forming, expand from the flat part of the test material (steel pipe) so that the pipe circumferential direction (C direction) is the length of the test piece, and V-notch test piece (1/4 according to JIS Z 2242) Size) and when the Charpy impact test is performed, the fracture surface transition temperature vTrs is −40 ° C. or lower.

  In order to achieve the above-described object, the present inventors have various properties that affect these properties in order to achieve a high level of properties such as strength, formability, low temperature toughness, and torsional fatigue resistance after cross-section forming processing. We have made a systematic examination of the factors, especially the composition and manufacturing conditions of steel pipes. As a result, by adjusting the surface structure of the steel pipe material (hot-rolled steel strip) to an appropriate structure, and further applying an electric-welded pipe process to the steel pipe material to obtain a welded steel pipe (electric-welded steel pipe), In particular, the hardness variation of the surface layer and the hardness variation in the thickness direction are reduced, especially the torsional fatigue resistance after cross-section forming processing is significantly improved, the tensile strength is higher than 590 MPa, and excellent low temperature It has been found that a high-tensile welded steel pipe having toughness, excellent formability, and excellent torsional fatigue resistance after cross-section forming can be obtained.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) By mass%, C: 0.03 to 0.24%, Si: 0.002 to 0.95%, Mn: 1.01 to 1.99%, Al: 0.01 to 0.08%, Nb: 0.001 to 0.15%, P which is an impurity, Containing S, N, and O adjusted to P: 0.019% or less, S: 0.010% or less, N: 0.008% or less, O: 0.003% or less, and the composition comprising the balance Fe and inevitable impurities, and the outermost pipe The region from the innermost surface and the innermost surface of the pipe to the thickness direction of 50 μm has a ferrite phase whose average crystal grain size in the circumferential cross section is 2.0 to 14 μm and a structure composed of a second phase other than the ferrite phase. The ferrite phase has a volume fraction of 60% or more, and 1.5 to 60 nm of Nb carbide is precipitated in the ferrite phase, and the average hardness HV _0-50 in the region, The difference ΔHV (= HV ₅₀₋₂₀₀ −HV ₀₋₅₀ ) from the average hardness HV _{50−200 in} the range of 50 to 200 μm in the thickness direction from the surface or innermost surface of the tube is 4 in Vickers hardness. The standard deviation σ of the hardness of the area from 0 point or less to the pipe outermost surface or the innermost surface to 50 μm in the thickness direction is 20 points or less in terms of Vickers hardness, low temperature toughness, formability, and cross-section molding A high-strength welded steel pipe for automotive structural members characterized by excellent torsional fatigue resistance after processing.

  (2) In (1), in addition to the above composition, in terms of mass%, V: 0.001 to 0.15%, W: 0.001 to 0.15%, Ti: 0.001 to 0.15%, Cr: 0.001 to 0.45%, Mo: 0.001 -0.45%, Cu: 0.001-0.45%, Ni: 0.001-0.45%, B: One or more selected from 0.0001-0.0009%, and / or Ca: 0.0001-0.005% A high-tensile welded steel pipe for automobile structural members, characterized in that the composition is

(3) In (1) or (2), the inclusion ratio in the cross section in the tube axis direction in the region from the outermost surface and the innermost surface to 50 μm in the thickness direction is determined by the method described in JIS G 0555-2003. A high-tensile steel pipe for automobile structural members characterized by a measured value of 0.10% or less.
(4) In any one of (1) to (3), the following equation (2)
α _f = 1 + 2√ (d / ρ) (2)
(Where, d: depth of surface irregularities (μm), ρ: radius of curvature of the surface concave portions (μm))
A high-tensile steel pipe for automobile structural members, characterized in that the stress concentration coefficient α _f defined by

(5) When a steel pipe material is subjected to an electric forging pipe process to make a welded steel pipe, the steel pipe material is in mass%, C: 0.03-0.24%, Si: 0.002-0.95%, Mn: 1.01-1.99% , Al: 0.01 to 0.08%, Nb: 0.001 to 0.15%, impurities P, S, N, and O are P: 0.019% or less, S: 0.010% or less, N: 0.008% or less, O: After adjusting to 0.003% or less and heating to 1160-1320 ° C. to a steel material composed of the balance Fe and unavoidable impurities, finish rolling reduction: 80-97%, finish rolling finish temperature: 980-760 A hot rolling for finishing rolling at ℃ and a gradual cooling treatment for gradual cooling for 2 seconds or more in the temperature range of 750 to 650 ° C. after completion of the hot rolling, and a winding temperature of 660 to 510 ° C. It is a hot-rolled steel strip that is subjected to a hot-rolling process of winding, and after the electric sewing pipe process pickling and slitting the steel pipe material, the following (1) width drawing ratio = [(Width of steel tube) −π {(Product outer diameter) − (Product thickness)}] / π {(Product outer diameter) − (Product thickness)} × (100%) (1)
The width drawing ratio defined by the above is a process of continuously roll-forming and electro-welding to form a welded steel pipe with a width drawing ratio of 10% or less. The welded steel pipe has low-temperature toughness, formability, and resistance after cross-section forming. A method for producing a high-strength steel pipe for automobile structural members, characterized by excellent torsional fatigue characteristics.

  (6) In (5), in addition to the above composition, in terms of mass%, V: 0.001 to 0.15%, W: 0.001 to 0.15%, Ti: 0.001 to 0.15%, Cr: 0.001 to 0.45%, Mo: 0.001 -0.45%, Cu: 0.001-0.45%, Ni: 0.001-0.45%, B: One or more selected from 0.0001-0.0009%, and / or Ca: 0.0001-0.005% The manufacturing method of the high tension welded steel pipe for motor vehicle structural members characterized by the above-mentioned.

  According to the present invention, a high-strength welded steel pipe having a tensile strength of 590 MPa or more, excellent low temperature toughness, excellent formability, and excellent torsional fatigue resistance after cross-section forming processing can be easily obtained. It can be manufactured at low cost 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, mass% in the composition is simply expressed as% unless otherwise specified.
C: 0.03-0.24%
C is an element that increases the strength, and is an essential element for securing the desired steel pipe strength and improving the fatigue resistance characteristics, particularly the torsional fatigue resistance characteristics of the steel pipe. Such an effect is recognized when the content is 0.03% or more. However, when the content exceeds 0.24%, it cannot be a structure mainly composed of a ferrite phase with a volume ratio of 60% or more. Low temperature toughness cannot be secured. In addition, Preferably it is 0.08 to 0.20%.

Si: 0.002 to 0.95%
Si is a ferrite-forming element, has an action of promoting ferrite transformation in the hot rolling process, and is an essential element for ensuring a desired structure and necessary formability. Such an effect is recognized when the content is O.002% or more. On the other hand, if the content exceeds O.95%, surface properties and ERW weldability deteriorate. For this reason, Si was limited to the range of O.002 to 0.95%. In addition, Preferably it is 0.10 to 0.30%.

Mn: 1.01-1.99%
Mn is an element that increases the strength, secures the desired steel pipe strength, increases the fatigue strength of the steel pipe, and improves the fatigue resistance characteristics, particularly the torsional fatigue characteristics. Such an effect is recognized when the content is 1.01% or more, but when the content exceeds 1.99%, the ferrite transformation is suppressed and a desired ferrite phase-based structure cannot be obtained, and the desired excellent formability is ensured. become unable. For this reason, Mn was limited to the range of 1.01-1.99%. In addition, Preferably it is 1.20 to 1.80%.

Al: 0.01-0.08%
Al is an element that acts as a deoxidizer during steelmaking and suppresses the growth of austenite grains in the hot rolling process and contributes to refinement of crystal grains, particularly refinement of ferrite grains. Such an effect is recognized when the content is 0.01% or more. On the other hand, when the content exceeds O.08%, the above-described effects are saturated, and an effect commensurate with the content cannot be expected. In addition, oxide inclusions increase, and fatigue resistance characteristics deteriorate. 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: 0.001 to 0.15%
Nb combines with C in steel and precipitates as carbide, suppresses grain growth due to recovery and recrystallization in the hot rolling process, and forms a ferrite phase having a desired particle size (2.0 to 14 μm). It has the action of precipitating and securing the hardness of the surface layer, increasing the fatigue strength, and improving the fatigue resistance. Such an effect is recognized when the content is O.001% or more. On the other hand, if the content exceeds O.15%, the strength rises due to the precipitated carbides and the ductility is remarkably lowered. Therefore, Nb is limited to the range of 0.001 to 0.15%. In addition, Preferably it is 0.010 to 0.049%.

In the present invention, impurities P, S, N, and O are adjusted to P: 0.019% or less, S: 0.010% or less, N: 0.008% or less, and O: 0.003% or less.
P: O.019% or less P is an element having an adverse effect of lowering the low temperature toughness and degrading the electroweldability through solidification co-segregation with Mn, and is preferably reduced as much as possible. 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 acts as a starting point of fine cracks and fatigue cracks during forming, and has an adverse effect of reducing fatigue resistance and formability. In the present invention, it is preferable to reduce as much as possible. When the content exceeds 0.010%, the above-described adverse effects become remarkable. For this reason, S was limited to 0.010% or less. In addition, Preferably it is 0.005% or less.

N: O.008% or less N is an element having an adverse effect of lowering the formability and low-temperature toughness of a steel pipe if it remains as a solid solution N in the steel, and is preferably reduced as much as possible in the present invention. When the content exceeds 0.008%, the above-mentioned adverse effects become remarkable, so N is limited to 0.008% or less. In addition, Preferably it is O.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. If the content exceeds 0.003%, the above-described adverse effects become remarkable, so O is limited to O.003% or less. In addition, Preferably it is O.002% or less.

  The above components are basic components, but in the present invention, in addition to the above basic composition, V: 0.001 to 0.15%, W: O.001 to 0.15%, Ti: 0.001 to 0.15%, Cr : 0.001 to 0.45%, Mo: 0.001 to O.45%, Cu: 0.001 to 0.45%, Ni: O.001 to 0.45%, B: one or more selected from 0.0001 to 0.0009%, And / or Ca: O.O001-0.005%.

V, W, Ti, Cr, Mo, Cu, Ni, and B are all elements of Mn and Nb that have the function of enhancing the fatigue strength and complementing the fatigue resistance properties. It can contain 1 type (s) or 2 or more types by selecting.
In addition to the above-described effects, V further precipitates as carbides, increases the hardness in the vicinity of the surface layer, improves fatigue strength, suppresses grain growth due to recovery and recrystallization of Nb in the hot rolling process, and ferrite It also has the function of complementing the effect of making the phase a desired fine crystal grain size. 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 0.15% of range.

  In addition to the above-described effects, W further precipitates as carbides, increases the hardness in the vicinity of the surface layer, improves fatigue strength, and suppresses the grain growth due to recovery and recrystallization of Nb in the hot rolling process. It has the function of complementing the effect of making the phase a desired fine crystal grain size. 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 W to 0.001 to 0.15% of range.

  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, excess Ti precipitates as carbides, ensuring the hardness of the surface layer, improving fatigue strength, suppressing grain growth due to recovery and recrystallization of Nb in the hot rolling process, and reducing the ferrite phase to the desired fineness. It has a function of complementing the effect of making the crystal grain size. Such an effect is manifested with a content of 0.001% or more. However, when the content exceeds 0.15%, 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 0.15%. Further, it is more preferably O.0010 to O.080%.

  Cr is an element that is effective in suppressing the coarsening of precipitates and ferrite grains in the vicinity of the surface layer, in addition to the function of increasing the fatigue strength and improving the fatigue resistance properties of Mn described above. Such an effect is recognized when the content is 0.001% or more, but when the content exceeds O.45%, the moldability is lowered. 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 0.08 to O.29%.

  Mo precipitates as carbides, and has the function of supplementing the above-described effects of increasing the fatigue strength and improving the fatigue resistance of Nb. Such an effect is recognized when the content is 0.001% or more, but when the content exceeds O.45%, the moldability is lowered. For this reason, when it contains, it is preferable to limit Mo to 0.001 to 0.45% of range. When the residual stress removal annealing or the like is not performed, the content is more preferably less than 0.045%. In addition, More preferably, it is 0.045 to 0.30%.

Cu has a function of improving the corrosion resistance in addition to the above-described action. Such an effect is recognized when the content is 0.001% or more. However, when the content exceeds 0.45%, the moldability is deteriorated. For this reason, when it contains, it is preferable to limit Cu to 0.001 to 0.45% of range. More preferably, it is 0.2% or less.
Ni, like Cu, has a function of improving the corrosion resistance in addition to the above-described action. Such an effect is recognized when the content is 0.001% or more. However, when the content exceeds 0.45%, the moldability is deteriorated. 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.

B, like Cr, supplements the above-described effects of increasing the fatigue strength and improving the fatigue resistance of Mn. Such an effect is recognized when the content is 0.0001% or more, but when the content exceeds O.0009%, the moldability is deteriorated. For this reason, when it contains, it is preferable to limit B to 0.0001 to 0.0009% of range.
Ca: 0.0001 to 0.005%
Ca has a function of controlling the form of inclusions, in which the expanded inclusions (MnS) are granular inclusions (Ca (Al) S (O)). Thus, it is an element that has the effect of suppressing the occurrence of fine cracks and fatigue cracks during molding, and improving moldability, 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 components described above is Fe and inevitable impurities.
Next, the reason for limiting the structure of the steel pipe of the present invention will be described.
The steel pipe of the present invention has a structure in which at least the pipe surface layer is refined. The structure of the pipe surface layer is important for ensuring excellent formability, excellent fatigue resistance, and the like. The region of the pipe surface layer from the outermost surface of the steel pipe of the present invention and the innermost surface of the pipe up to 50 μm in the thickness direction has an average grain size of 2.0 to 2.0 in the circumferential section (cross section perpendicular to the longitudinal direction of the pipe). It has a structure composed of a fine ferrite phase of 14 μm and a second phase other than the ferrite phase.

  If the average crystal grain size of the ferrite phase is less than 2.0 μm, the desired formability cannot be ensured, and local thinning, rough surface, and fine cracks are likely to occur, and these become stress-concentrated portions, resulting in a significant reduction in fatigue resistance. To do. On the other hand, when the average crystal grain size of the ferrite phase exceeds 14 μm, the formability is lowered, the surface hardness is lowered, and the fatigue resistance is lowered. For this reason, in the region from the outermost surface and the innermost surface to 50 μm in the thickness direction, the average crystal grain size of the ferrite phase was limited to the range of 2.0 to 14 μm. The thickness is preferably 8 μm or less.

  The ferrite phase in the region of the pipe outer layer from the outermost surface of the steel pipe of the present invention and the innermost surface of the pipe up to 50 μm in the thickness direction has a structure fraction of 60% or more by volume. When the structure fraction of the ferrite phase is less than 60%, desired formability cannot be ensured, and local thinning, rough surface, and fine cracks become stress-concentrated portions and fatigue characteristics are greatly reduced. For this reason, the volume fraction of the ferrite phase was limited to 60% or more in the region from the outermost surface of the tube and the innermost surface of the tube to the thickness direction of 50 μm. In addition, Preferably they are 75% or more and 99% or less. The “ferrite phase” here includes polygonal ferrite, acicular ferrite, Widmanstatten ferrite, and bainitic ferrite. Examples of the second phase include any one of carbide, pearlite, bainite, martensite other than ferrite, or a mixed phase thereof. Note that the volume fraction of the second phase is preferably 20% or less from the viewpoint of moldability.

The size of the Nb carbide contained in the ferrite phase of the tube surface layer is an important factor for ensuring desired surface hardness, ensuring high fatigue resistance, and ensuring desired formability. In the present invention, the average particle size of Nb carbides precipitated in the ferrite phase in the tube surface region from the outermost surface of the tube and the innermost surface of the tube up to 50 μm in the thickness direction is limited to a range of 1.5 to 60 nm.
If the average particle size of Nb carbide in the ferrite phase of the tube surface layer is less than 1.5 nm, the yield strength is remarkably increased and the desired formability cannot be ensured, so local thinning, rough surface, and fine cracks are stressed. The ratio between 5 × 10 ⁵ repeated fatigue limit σ _B after cross-section forming and steel pipe tensile strength TS, σ _B / TS, is less than 0.40, and the torsional fatigue resistance after cross-section forming decreases. On the other hand, when the average particle size of Nb carbide in the ferrite phase of the tube surface layer exceeds 60 nm, the hardness of the tube surface region decreases, the hardness variation in the thickness direction increases, and 5 × 10 after cross-section forming processing ⁵ Repeat the fatigue limit and the ratio of the steel pipe tensile strength TS, σ _{B /} TS is now below 0.40, resistance torsional fatigue characteristics are lowered. For this reason, Nb carbides in the ferrite phase in the tube surface region from the outermost surface and the innermost surface to the thickness direction of 50 μm in the thickness direction were limited to an average particle size in the range of 1.5 nm to 60 nm. In addition, Preferably it is 5 nm-50 nm.

  Here, the average particle size of Nb carbide in the ferrite phase in the tube surface layer region is obtained by taking a sample for observing the structure from a steel tube and using an extraction replica method with a transmission electron microscope (TEM) (magnification: 100,000 times) Five visual fields were observed. Among the observed precipitates, EDS analysis identifies and excludes cementite and TiN that do not contain Nb, and for Nb-containing carbides (Nb carbides), the area of Nb carbides is measured by an image analyzer, The equivalent circle diameter was calculated from the area, and the arithmetic average value thereof was defined as the average particle diameter of Nb carbide in the sample. In addition, Nb single carbide, composite carbide with Mo, Ti, V, etc. including Mo, Ti, V, etc. were counted as Nb carbide.

The welded steel pipe of the present invention having the above composition and the above structure has an average hardness HV _0-50 in the region from the outermost surface and innermost surface to 50 μm in the thickness direction, and from the outermost surface and innermost surface of the tube. The difference ΔHV (= HV ₅₀₋₂₀₀ −HV ₀₋₅₀ ) from the average hardness HV ₅₀₋₂₀₀ in the region of 50 to 200 μm in the thickness direction is less than 40 points in Vickers hardness and the outermost surface of the pipe And the standard deviation σ of hardness in the region from the innermost surface to 50 μm in the thickness direction is 20 points or less in Vickers hardness, 5 × 10 ⁵ repeated fatigue limit σ _B and steel pipe tensile strength TS after section forming Is a steel pipe having a ratio σ _B / TS of 0.40 or more.

Average hardness HV _0-50 in the region from the outermost surface of the tube and the innermost surface to 50 μm in the thickness direction, and average hardness in the region in the range of ₅₀ to 200 μm in the thickness direction from the outermost surface of the tube and the innermost surface Difference from HV _50-200 ΔHV (= HV _50-200 -HV _0-50 ): 40 points or less in Vickers hardness Area from pipe outermost surface and innermost surface to 50μm in thickness direction, ie tube surface layer Compared to the inner layer portion of the tube, the strain and heat history during hot rolling differ, and the hardness is often different from the hardness of the inner layer portion of the tube. In particular, when the tube surface layer is relatively soft as compared with the tube inner layer portion, strain concentrates on the tube surface layer at the time of molding, and a large tensile residual stress remains on the tube surface layer after molding. For this reason, the fatigue strength of the pipe surface layer portion is reduced.

FIG. 2 shows the relationship between the difference between the average hardness of the tube inner layer and the average hardness of the tube surface layer and the fatigue strength after the cross-section forming process. In FIG. 2, the pipe inner layer is represented by a region in the range of 50 to 200 μm from the outermost surface of the tube and the innermost surface in the thickness direction, and a range of 50 to 200 μm in the thickness direction from the outermost surface of the tube and the innermost surface. The difference between the average hardness HV _50-200 in the Vickers hardness of the area of the tube and the average hardness HV _0-50 in the Vickers hardness of the area from the outermost surface and the innermost surface to the thickness direction of 50 μm , ΔHV = (HV ₅₀₋₂₀₀ −HV ₀₋₅₀ ), and the relationship between 5 × 10 ⁵ repeated fatigue limit σ _B after section forming and steel pipe tensile strength TS, σ _B / TS.

The average hardness of each area is divided into 3 blocks in the thickness direction (6 blocks in total), 10 points in each block (center position in the longitudinal direction of the pipe), Vickers hardness meter (load: 0.025) kgf, test force: 0.245 N), the hardness HV0.025 was measured, and the obtained 60 points of measurement results were arithmetically averaged to obtain the average hardness of each region.
FIG. 2 shows that when ΔHV exceeds 40 points, σ _B / TS is less than 0.40. Therefore, in the present invention, the difference ΔHV between HV _50-200 and HV _0-50 is limited to 40 points or less. Note that ΔHV is preferably 30 points or less.

Standard deviation of hardness in the region from the outermost surface of the tube and the innermost surface to 50 μm in the thickness direction σ _0-50 ： Vickers hardness is 20 points or less. Deviation) has a great influence on fatigue strength. This is presumably because fatigue cracks first occur at the weakest part of fatigue strength. The standard deviation σ _0-50 of the hardness in the region from the outermost surface and the innermost surface to the thickness direction of 50 μm, the 5 × 10 ⁵ repeated fatigue limit σ _B and the steel pipe tensile strength TS after cross-section forming The relationship between the ratio and σ _B / TS is shown in FIG.

The standard deviation σ _0-50 of the hardness of the region from the outermost surface and the innermost surface to 50 μm in the thickness direction is divided into 3 blocks (6 blocks in total) in the thickness direction. The hardness HV0.025 was measured with 10 points (at the center position in the longitudinal direction of the pipe) and a Vickers hardness tester (load: 0.025 kgf, test force: 0.245 N), and calculated from the measurement results obtained for a total of 60 points. .
From FIG. 3, when σ _0-50 exceeds 20 points in _terms of Vickers hardness, σ _B / TS is less than 0.40, and the torsional fatigue resistance is reduced. This is presumably because fatigue cracks are generated from the portion having a relatively low hardness in the tube surface layer, and the torsional fatigue life is reduced. For this reason, the standard deviation σ _0-50 of the hardness in the region from the outermost surface and the innermost surface to the thickness direction of 50 μm was limited to 20 points or less in _terms of Vickers hardness. In addition, Preferably it is 15 points or less.

Below, the preferable manufacturing method of this invention welded steel pipe is demonstrated.
It is preferable that the molten steel having the above-described composition is first melted by a known melting method such as a converter and made into 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. and then subjected to rough rolling, and finish rolling is performed at a rolling reduction ratio of 80 to 97% and final rolling final temperature of 980 to 760 ° C. After the hot rolling and after the hot rolling, the steel sheet is subjected to a slow cooling process in which annealing is performed at a temperature range of 750 to 650 ° C. for 2 seconds or more to obtain a hot rolled steel strip at a winding temperature of 660 to 510 ° C. It is preferable to set it as a process.

Heating temperature of steel material: 1160-1320 ℃
The heating temperature of the steel material affects the precipitate size and hardness profile of the steel sheet surface layer, especially through the re-solidification and precipitation of Nb in the steel, which is important for suppressing softening and ensuring good fatigue strength. It is. When the heating temperature is less than 1160 ° C, coarse Nb carbonitrides precipitated during continuous casting remain as undissolved carbonitrides, so the Nb carbides in the ferrite phase become coarse (average particle size exceeds 60 nm). The desired torsional fatigue resistance characteristics cannot be ensured. On the other hand, when the heating temperature exceeds 1320 ° C., the crystal grains become coarse, so that the ferrite phase in the region from the outermost surface and the innermost surface to 50 μm in the thickness direction is obtained in the subsequent hot rolling process. It becomes coarse (the average particle diameter of the circumferential cross section exceeds 14 μm), the surface hardness is lowered, the moldability is lowered, and the torsional fatigue resistance is lowered. For this reason, it is preferable to limit the heating temperature of a steel raw material to the range of 1160-1320 degreeC. In addition, it is 1200-1300 degreeC more preferably. Further, from the viewpoint of ensuring the uniformity of the solid solution state of Nb and ensuring a sufficient solid solution time, the soaking time of the steel material is preferably set to 30 min or more.

Finish rolling reduction: 80-97%
The finish rolling reduction in the hot rolling process is 50 μm from the outermost and innermost surfaces of the steel pipe material (hot-rolled steel strip) to the thickness direction through recrystallization and recovery behavior in the austenite region of steel and rolling strain-induced precipitation. In order to ensure good torsional fatigue resistance, it also affects the hardness profile and the structure of the region up to, ie, ferrite phase fraction, ferrite phase particle size, Nb carbide particle size in the ferrite phase, etc. It is an important factor.

  When the finish rolling reduction is less than 80%, the ferrite phase fraction in the region from the outermost surface and the innermost surface of the steel pipe material (hot rolled steel strip) to 50 μm in the thickness direction is less than 60% (volume ratio), In addition, the average grain size of the ferrite phase in the region exceeds 14 μm in the circumferential cross section, the surface hardness is lowered, the formability is lowered, and the torsional fatigue resistance is lowered. On the other hand, when the finish rolling reduction exceeds 97%, the average grain diameter of the ferrite phase in the region from the outermost surface and the innermost surface to the thickness direction of 50 μm is less than 2.0 μm, particularly where the rolling shear strain is concentrated, The standard deviation σ of hardness exceeds 20 points in terms of Vickers hardness, the desired formability cannot be secured, local thinning, rough surface, fine cracks become stress concentrated parts, and local strain induction Precipitation promotes the generation of fatigue cracks from relatively low hardness portions, making it impossible to ensure desired torsional fatigue resistance characteristics. For this reason, it is preferable to limit the finish rolling reduction in the range of 80 to 97%. In addition, More preferably, it is 89 to 95%.

Finishing rolling finish temperature: 980-760 ° C
The finish rolling finish temperature in the hot rolling process is important for ensuring good steel pipe formability by adjusting the ferrite phase fraction and the average grain size of the steel strip surface layer to a predetermined range. If the finish rolling finish temperature exceeds 980 ° C, the average ferrite phase particle size of the resulting steel pipe material exceeds 14 µm, and the ferrite phase fraction is less than 60% by volume, which reduces formability and surface properties. The torsional fatigue resistance is reduced. On the other hand, when the finish rolling finish temperature is below 760 ° C., the average grain size of the ferrite phase of the steel strip surface is less than 2.0 μm, the formability is lowered, and strain-induced precipitation reduces the average grain size of Nb carbide to 60 nm. The desired torsional fatigue resistance characteristics cannot be ensured. For this reason, it is preferable that the finish rolling end temperature is limited to a range of 980 to 760 ° C. In addition, it is 900-880 degreeC more preferably. Further, from the viewpoint of securing good surface properties and desired surface hardness, it is preferable to perform descaling at a water pressure of 140 kg / cm ² (1.4 MPa) 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 in the hot rolling process but before winding. Then, a slow cooling process is performed in which slow cooling is performed for 2 seconds or more. 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. As a result, a structure having a ferrite phase fraction of 60% or more in volume ratio can be secured, and a tensile property in which the elongation El in the JIS No. 12 test piece is 15% or more can be secured. If it is less than 2 s, the desired structure cannot be secured, and the desired formability and torsional fatigue resistance cannot be secured. More preferably, it is 4 seconds or longer.

Winding temperature: 660-510 ° C
The hot-rolled steel strip that has been 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, particularly the ferrite phase fraction of the steel strip surface layer and the precipitation state of the precipitates. If the coiling temperature is less than 510 ° C, the desired ferrite phase fraction cannot be obtained, the desired formability cannot be obtained, and the average grain size of Nb carbide is less than 1.5 nm, ensuring the desired torsional fatigue resistance. become unable. On the other hand, when the coiling temperature exceeds 660 ° C., the average grain size of the ferrite phase exceeds 14 μm, and the formability deteriorates, and the surface properties deteriorate due to the scale growth after winding and the torsional fatigue resistance property decreases. . Furthermore, the average dimension of Nb carbide exceeds 60 nm, and desired torsional fatigue characteristics cannot be ensured. For this reason, the winding temperature is preferably in the range of 660 to 510 ° C. In addition, it is 600-560 degreeC more preferably.

    By subjecting the steel material having the above composition to a hot rolling process under the above-described conditions, the microstructure, precipitate state, and hardness profile are optimized, and it has excellent formability and is formed into a steel pipe. Later, it is possible to obtain a steel pipe material (hot-rolled steel strip) suitable for torsion beams, which has excellent formability and low-temperature toughness and also has excellent torsional fatigue resistance after cross-section forming. 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 remove the black skin on the surface by subjecting the steel pipe material to pickling treatment, shot blasting, or the like. Furthermore, surface treatments such as galvanizing, aluminum plating, nickel plating, and organic film treatment can be applied to the steel pipe material from the viewpoint of corrosion resistance and coating film adhesion.

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%). When a steel pipe material is continuously roll-formed and electro-welded, the width drawing ratio is an important factor for ensuring a desired formability. If the width drawing ratio exceeds 10%, the formability drop due to pipe making becomes remarkable, and the desired steel pipe 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 equation (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 material having the composition and structure described above, a hot-rolled steel strip as described above, a cold-rolled annealed steel strip subjected to cold rolling-annealing, or a surface-treated steel strip subjected to various surface treatments is used. There is no problem. In addition, instead of electric sewing pipe process, pipe forming that combines roll forming, press-cut section of cut plate, heat treatment such as cold, warm, hot diameter reduction and annealing after pipe forming. There is no problem even if laser welding, arc latent welding, plasma welding, or the like is used in place of electric welding. The member formed using the steel pipe of the present invention exhibits excellent characteristics as it is formed, but additionally, after forming the member, the surface is hardened or compressed by heat treatment such as residual stress removal annealing or shot peening. There is no problem in applying residual stress.

  Next, the present invention will be further described based on examples.

Example 1
A slab (steel material) having the composition shown in Table 1 is heated to about 1250 ° C., subjected to hot rolling with a finish rolling reduction ratio of 93% and a finish rolling finishing temperature of about 860 ° C. After the hot rolling is completed, After performing a slow cooling treatment that gradually cools for 5 s in a temperature range of 750 ° C. to 650 ° C., a hot rolling process 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 steel pipe materials, pickled and slitted to a predetermined width, then continuously rolled into open pipes, and the open pipes are electro-sealed by high-frequency resistance welding. A welded steel pipe (outer diameter φ89.1 mm x wall thickness about 3 mm) was made by the electro-welded pipe process for welding. In the electric sewing tube process, the width drawing ratio defined by the equation (1) was set to 4%.
Specimens were collected from these welded steel pipes and subjected to a structure observation test, a precipitate observation test, a tensile test, a torsional fatigue test, and a low temperature toughness test. 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. The measurement was performed by dividing a region (tube surface layer) from the outermost surface of the tube and the innermost surface of the tube up to 50 μm in the thickness direction into three blocks in the thickness direction. In each block, the ferrite phase structure fraction (% by volume) and the average crystal grain size of the ferrite phase were measured, and the measured values of each block (total of 6 blocks) were arithmetically averaged. The average crystal grain size of the phase was taken.

(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 a total of 6 fields at a magnification of 100,000, identify and exclude Nb-free cementite, TiN, etc. by EDS analysis. Images of Nb-containing carbides (Nb carbides) By analysis, the area of each Nb carbide was measured, the equivalent circle diameter was calculated from the area, and the arithmetic average value thereof was taken as the average particle diameter of the Nb carbide. Note that composite carbides containing Ti, Mo and the like were also counted as Nb carbides. The measurement was performed by dividing the area from the outermost surface of the tube and the innermost surface of the tube to 50 μm in the thickness direction into three blocks in the thickness direction, and each block had two fields of view.

(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 and pulled in accordance with the provisions of JIS Z 2241 so that the pipe axis direction would be the tensile direction. Tests were conducted to determine tensile properties (tensile strength TS, yield strength YS, El), and to evaluate strength and formability.
(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. 1 (FIG. 11 of JP-A-2001-331846). As shown in the figure, after the cross section of the steel pipe was formed into a V-shaped cross section, both ends were fixed by chucking and a torsional fatigue test was performed.

The torsional fatigue test was conducted 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 determined 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 molding under the same conditions as the torsional fatigue test material. Expand the pipe circumferential direction (C direction) to be the test piece length, cut out a V-notch test piece (1/4 size) according to JIS Z 2242, conduct Charpy impact test, and transition to fracture surface The temperature vTrs was obtained and the low temperature toughness was evaluated.

  The obtained results are shown in Table 2.

Examples of the present invention (steel pipe Nos. 1 to 8 and No. 10) all have a structure fraction of ferrite phase in the region from the outermost pipe surface and the innermost pipe surface to 50 μm in the thickness direction of 60 vol% or more. And the average crystal grain size of the ferrite phase is 2 to 14 μm, the average grain size of Nb carbide in the ferrite phase is 1.5 to 60 nm, the tensile strength is 590 MPa or more, This is a welded steel pipe with excellent formability that satisfies an elongation El of 15% or more. Further, in all of the examples of the present invention, the average hardness HV _50-200 in the region of 50 to 200 μm in the thickness direction from the outermost surface of the tube and the innermost surface of the tube and the thickness direction from the outermost surface of the tube and the innermost surface of the tube The difference ΔHV (= HV _50-200 -HV _0-50 ) from the average hardness HV _0-50 in the region up to 50 μm is 40 points or less, from the outermost surface of the tube and the innermost surface of the tube to the thickness direction of 50 μm The standard deviation σ _0-50 of the hardness of the steel region is 20 points or less, and the ratio of the 5 × 10 ⁵ cyclic fatigue limit σ _B to the steel pipe strength TS (σ _B / TS in the torsional fatigue test after cross-section forming processing) ) Is O.40 or more, and the welded steel pipe has excellent torsional fatigue resistance. In addition, all of the inventive examples show excellent low-temperature toughness with fracture surface transition temperatures vTrs of −40 ° C. or lower in the Charpy impact test after cross-section forming.

On the other hand, in the comparative examples (steel pipe Nos. 11 to 33) in which the steel composition is out of the scope of the present invention, the structure is out of the scope of the present invention, and at least one of strength, formability, torsional fatigue resistance, and low temperature toughness is present. It is falling. C, Mn, Al, Nb, S, N, O, V, W, Ti, Cu, Ni, B, and Ca are comparative examples exceeding the scope of the present invention (steel pipe No. 12, No. 16, No. 18, No. .20, No.22 to 27, and No.30 to 33) are low in El with less than 15%, lack of ductility, and (σ _B / TS) is less than 0.40, resulting in reduced torsional fatigue resistance. Yes.

In addition, the comparative example (steel pipe No. 14) in which Si exceeds the scope of the present invention has (σ _B / TS) less than 0.40, low torsional fatigue resistance, vTrs exceeds −40 ° C., and low temperature toughness decreases. doing. Moreover, in the comparative examples (steel pipes No. 28 and No. 29) in which Cr and Mo exceed the scope of the present invention, El is as low as less than 15% and the ductility is insufficient. Further, all of the comparative examples (steel pipes No. 11, No. 13, No. 15, No. 17, No. 19) in which C, Si, Mn, Al, and Nb deviate from the scope of the present invention are all (σ _B / If TS) is less than 0.40, the torsional fatigue resistance is reduced.

The surface properties of the obtained welded steel pipes (steel pipes Nos. 1 to 8, Nos. 10 to 33) were also investigated. A circumferential cross section of the obtained welded steel pipe was imaged, and the depth d of the surface unevenness and the radius of curvature ρ of the tip of the surface recess were measured. Using the obtained d and ρ,
Next (2) Formula α _f = 1 + 2√ (d / ρ) (2)
(Where, d: depth of surface irregularities (μm), ρ: radius of curvature of the surface concave portions (μm))
The stress concentration coefficient α _f defined by As a result, α _{f of} each welded steel pipe was 10 or less, and the surface properties were good.

In addition, specimens were taken from the obtained welded steel pipes (steel pipes Nos. 1 to 8, Nos. 10 to 33) so that the cross section in the pipe axis direction becomes the observation surface, and after polishing, the outermost surface of the pipe and the pipe were etched without etching. The inclusion rate was measured by the method described in JIS G 0555-2003 for the region from the innermost surface to 50 μm in the thickness direction. As a result, the inclusion rate in the region from the outermost surface of the tube and the innermost surface of the tube to the thickness direction of 50 μm was O.10% or less except for the product tube with high S (steel tube No. 22).

  The results obtained are shown in Table 4.

Examples of the present invention (steel pipe No. 35, No. 38 to 40, No. 43, No. 46, No. 48 to 50, No. 52 to 58) are all made from the outermost pipe surface and the innermost pipe surface. The structure fraction of the ferrite phase in the thickness direction up to 50 μm is 60 volume% or more, the average crystal grain size of the ferrite phase is 2 to 14 μm, and the average grain size of Nb carbide in the ferrite phase is 1.5 to 60 nm. It is a welded steel pipe with excellent formability that has a structure of ## EQU1 ## and has a tensile strength of 590 MPa or more and an elongation El of JIS12 test piece of 15% or more. Further, in all of the examples of the present invention, the average hardness HV _50-200 in the region of 50 to 200 μm in the thickness direction from the outermost surface of the tube and the innermost surface of the tube and the thickness direction from the outermost surface of the tube and the innermost surface of the tube The difference ΔHV (= HV _50-200 -HV _0-50 ) from the average hardness HV _0-50 in the region up to 50 μm is 40 points or less, from the outermost surface of the tube and the innermost surface of the tube to the thickness direction of 50 μm The standard deviation σ _0-50 of the hardness of the steel region is 20 points or less, and the ratio of the 5 × 10 ⁵ cyclic fatigue limit σ _B to the steel pipe strength TS (σ _B / TS in the torsional fatigue test after cross-section forming processing) ) Is O.40 or more, and the welded steel pipe has excellent torsional fatigue resistance. In addition, all of the inventive examples show excellent low-temperature toughness with fracture surface transition temperatures vTrs of −40 ° C. or lower in the Charpy impact test after cross-section forming.

On the other hand, comparative examples (steel pipe No. 34, No. 36, No. 37, No. 41, No. 1) where the hot rolling process conditions of the steel material or the electric welding pipe process conditions of the steel pipe material are out of the scope of the present invention. No. 42, No. 44, No. 45, No. 47, No. 51, No. 59) have deteriorated any of moldability, fatigue strength, and low temperature toughness.
In comparative examples (steel pipe No. 34, No. 36, No. 37, No. 41) where either the steel material heating temperature or the finish rolling reduction ratio in the hot rolling process is out of the scope of the present invention, (σ _B / TS) Is less than 0.40, the torsional fatigue resistance is reduced. Further, a comparative example (steel pipe No. 42) in which any one of the finish rolling finish temperature in the hot rolling process of the steel material, the slow cooling treatment time, the coiling temperature, and the width drawing ratio in the electric sewing pipe process of the steel pipe is out of the scope of the present invention. , No. 44, No. 45, No. 47, No. 51, No. 59), El is less than 15%, ductility is insufficient, and (σ _B / TS) is less than 0.40. The torsional fatigue resistance is reduced.
Example 3
Steel No. D shown in Table 1 was used as the basic composition, only the S content was changed, molten steel with different inclusion amounts was melted, and cast into a slab by a continuous casting method to obtain a steel material. These steel materials are heated to about 1250 ° C, hot-rolled at a finish rolling reduction ratio of 93%, and finished at a finish rolling temperature of about 860 ° C. After the hot rolling is completed, a temperature range of 750 ° C to 650 ° C. Then, the steel sheet was subjected to a slow cooling treatment in which it was gradually cooled for 5 seconds, and 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 steel pipe materials, pickled and slitted to a predetermined width, then continuously rolled into open pipes, and the open pipes are electro-sealed by high-frequency resistance welding. A welded steel pipe (outer diameter φ89.1 mm x wall thickness about 3 mm) was made by the electro-welded pipe process for welding. In the electric sewing tube process, the width drawing ratio defined by the equation (1) was set to 4%.
From the obtained welded steel pipe, a test material (length: 1500 mm) is sampled, and about 1000 mmL at the center of the test material, as shown in FIG. 1 (FIG. 11 of JP-A-2001-331846), After the cross section was formed into a V-shaped section at the longitudinal center, both ends were fixed by chucking and a torsional fatigue test was performed. The torsional fatigue test conditions were the same as in Example 1.

In addition, the specimen was collected from the obtained welded steel pipe so that the cross section in the axial direction of the pipe became the observation surface, and after polishing, for the region up to 50 μm in the thickness direction from the outermost surface of the pipe and the innermost surface of the pipe, In the same manner as in Example 1, the inclusion ratio was measured by the method described in JIS G 0555-2003.
The obtained results are calculated from the ratio of 5 × 10 ⁵ cyclic fatigue limit σ _B and steel pipe strength TS (σ _B / TS) in the torsional fatigue test after cross-section forming, and from the outermost surface of the tube and the innermost surface of the tube FIG. 4 shows the relationship with the inclusion rate in the region up to 50 μm in the thickness direction. FIG. 4 shows that (σ _B / TS) tends to increase as the inclusion rate in the region from the outermost surface of the tube and the innermost surface of the tube to the thickness direction of 50 μm decreases. When the physical rate is reduced to 0.10% or less, (σ _B / TS) becomes O.40 or more. In addition, the observed inclusions were in the order of sulfide, oxide, and nitride from the largest. FIG. 4 shows that (σ _B / TS) becomes O.42 or more when the inclusion ratio is reduced to 0.04% or less .

It is explanatory drawing which shows typically the cross-section shaping | molding processing state of the test material used for the torsion test after a cross-section shaping | molding process. Ratio of 5 × 10 ⁵ repeated fatigue limit σ _B and steel pipe strength TS after cross-section forming (σ _B / TS), and average of 50 to 200 μm area in the wall thickness direction from the outermost pipe surface and innermost pipe surface The difference ΔHV (= HV _50-200 -HV _0-50 ) between the hardness HV _50-200 and the average hardness HV _0-50 in the region from the outermost surface of the tube and the innermost surface of the tube to the thickness direction of 50 μm It is a graph which shows the relationship. Ratio of 5 × 10 ⁵ repeated fatigue limit σ _B to steel pipe strength TS after cross-section forming processing (σ _B / TS) and hardness in the region from the outermost surface of the tube and the innermost surface of the tube to 50 μm in the thickness direction It is a graph which shows the relationship with standard deviation (sigma) _{0-50 of No} .. Ratio of 5 × 10 ⁵ cyclic fatigue limit σ _B to steel pipe strength TS after cross-section forming processing (σ _B / TS), and inclusions in the region from the outermost surface of the tube and the innermost surface of the tube up to 50 μm in the thickness direction It is a graph which shows the relationship between rate .

Claims (6)

% By mass
C: 0.03-0.24%, Si: 0.002-0.95%,
Mn: 1.01-1.99%, Al: 0.01-0.08%,
Nb: 0.001 to 0.15%
And P, S, N, and O, which are impurities, include P: 0.019% or less, S: 0.010% or less, N: 0.008% or less, O: 0.003% or less, and the balance Fe and unavoidable A composition comprising impurities;
The region from the outermost surface of the tube and the innermost surface of the tube up to 50 μm in the thickness direction is composed of a ferrite phase having an average crystal grain size of 2.0 to 14 μm in the circumferential section and a second phase other than the ferrite phase. Have
The structure fraction of the ferrite phase is 60% or more by volume, and 1.5 to 60 nm of Nb carbide is precipitated in the ferrite phase,
Difference ΔHV (= HV ₅₀₋₎ between the average hardness HV _{0-50 in} the region and the average hardness HV _50-200 in the region in the thickness direction from the outermost surface of the tube or the innermost surface of the tube to the thickness direction ₂₀₀ −HV _0-50 ) is 40 points or less in Vickers hardness, and the standard deviation σ of hardness in the region from the outermost surface of the tube or the innermost surface of the tube to 50 μm in the thickness direction is 20 points or less in Vickers hardness. A high-strength welded steel pipe for automobile structural members characterized by being excellent in low-temperature toughness, formability, and torsional fatigue resistance after cross-section forming.   In addition to the above composition, V: 0.001 to 0.15%, W: 0.001 to 0.15%, Ti: 0.001 to 0.15%, Cr: 0.001 to 0.45%, Mo: 0.001 to 0.45%, Cu: 0.001 to One or more selected from 0.45%, Ni: 0.001 to 0.45%, B: 0.0001 to 0.0009%, and / or Ca: 0.0001 to 0.005%, The high-tensile welded steel pipe for automobile structural members according to claim 1.   Inclusion ratio of the cross section in the tube axis direction in the region from the outermost surface of the tube and the innermost surface of the tube to the thickness direction of 50 μm is 0.10% or less as measured by the method described in JIS G 0555-2003 The high-tensile steel pipe for automobile structural members according to claim 1 or 2. The high-tensile steel pipe for automobile structural members according to any one of claims 1 to 3, wherein the stress concentration coefficient α _f defined by the following equation (2) on the inner and outer surface of the pipe is 10 or less.
Record
α _f = 1 + 2√ (d / ρ) (2)
Where, d: depth of surface irregularities (μm), ρ: radius of curvature of surface concave portions (μm) When the steel pipe material is subjected to the electric sewing pipe process to make a welded steel pipe,
The steel pipe material is mass%,
C: 0.03-0.24%, Si: 0.002-0.95%,
Mn: 1.01-1.99%, Al: 0.01-0.08%,
Nb: 0.001 to 0.15%
And P, S, N, and O, which are impurities, include P: 0.019% or less, S: 0.010% or less, N: 0.008% or less, O: 0.003% or less, and the balance Fe and unavoidable A steel material having a composition comprising impurities is heated to 1160 to 1320 ° C., and then subjected to finish rolling with a finish rolling reduction ratio of 80 to 97% and a finish rolling finishing temperature of 980 to 760 ° C., and the heat A hot-rolled steel strip that has been subjected to a hot-rolling process of winding at a temperature of 660 to 510 ° C., followed by annealing at a temperature range of 750 to 650 ° C., followed by annealing at a temperature of 750 to 650 ° C. Yes,
After the steel pipe material is pickled and slitted in the electric sewing pipe process, the steel pipe material is continuously roll-formed and electro-sewn with a width drawing ratio defined by the following formula (1) set to 10% or less. It is a process of welding into a welded steel pipe,
The welded steel pipe is excellent in low temperature toughness, formability, and torsional fatigue resistance after cross-section forming processing;
The manufacturing method of the high-tensile steel pipe for automobile structural members characterized by these.
Width drawing ratio = [(Material steel tube width) -π {(Product outer diameter)-(Product thickness)}] / π {(Product outer diameter)-(Product thickness)} x (100%) (1)   In addition to the above composition, V: 0.001 to 0.15%, W: 0.001 to 0.15%, Ti: 0.001 to 0.15%, Cr: 0.001 to 0.45%, Mo: 0.001 to 0.45%, Cu: 0.001 to One or more selected from 0.45%, Ni: 0.001 to 0.45%, B: 0.0001 to 0.0009%, and / or Ca: 0.0001 to 0.005%, The manufacturing method of the high tension welded steel pipe for motor vehicle structural members of Claim 5 to do.

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