JP2003201543A - Steel pipe excellent in workability and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength steel pipe for an automobile structural member having a high tensile strength of more than 580 MPa and excellent workability, and a method for producing the same. SOLUTION: C: 0.05 to 0.30%, Si: 0.01 to 1.0%,
Mn: 1.0 to 4.0%, Al: 0.005 to 0.10%, S: 0.003%
After heating and soaking the raw steel pipe having the following composition, the rolling end temperature: 400 ° C or higher and lower than 800 ° C,
Cumulative diameter reduction: Reduced by 20% or more to produce product pipes. As a result, the as-rolled tensile strength exceeds 580 MPa, the yield ratio is 70% or less, and after a heat treatment at 150 to 300 ° C. for 10 to 20 min, the yield stress increases significantly, and the yield ratio increases.
80% or more. In addition, Cu, Ni, Cr, Mo, Nb,
One or more selected from Ti and B and / or
Alternatively, it may contain one or two selected from REM and Ca.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel pipe for automobile structural members, and more particularly to a steel pipe for automobile structural members, which has high strength and excellent workability, and further has low temperature short time heat treatment hardenability (paint bake hardenability). The present invention relates to a steel pipe.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of conservation of the global environment,
There is a strong demand for weight reduction of automobile bodies. Along with such demand for weight reduction of automobile bodies, steel pipes used as automobile structural members are also required to be thin and have high strength. Steel pipes are often used for suspension mechanism-related applications among automobile structural members. A steel pipe used as a member for a suspension mechanism may be used as a simple straight pipe, but is often used after being bent, reduced in diameter, or expanded. Therefore, the characteristics of the steel pipe are required to be excellent in workability in addition to high strength.

As a suspension mechanism part of an automobile, for example,
There is a stabilizer. The stabilizer is a component that improves riding comfort and traveling stability when the vehicle turns, and is designated as an important safety component. Therefore,
Stabilizers are required to have sufficient strength and durability. DESCRIPTION OF RELATED ART Conventionally, a stabilizer as a suspension mechanism part of an automobile uses a hot-rolled steel bar or a hot-rolled wire material as a raw material, and after hot-forging and hot-bending the raw material into a predetermined shaped article, the molded article is then formed. It was manufactured by a method of quenching and tempering. However,
This method has a problem in that a complicated process and large-scale equipment are required, the manufacturing cost rises, and deformation may occur during the heating and quenching processes, and the correction process is essential. There is also a problem that the fatigue resistance is deteriorated due to roughening of the skin due to oxidation during heating and generation of decarburization.

[0004] Recently, hollowing of component materials such as stabilizers has been studied in response to demands for weight reduction of automobile bodies and demands for improvement in production efficiency of body components. Traditionally,
In order to manufacture hollow members having various cross-sectional shapes, a method has been adopted in which parts formed by press forming steel plates are spot-welded at a flange portion which is a welding allowance. However, recently, even such a hollow member has been required to have a higher impact absorbing capacity at the time of collision, and a steel plate having higher strength has been used as a material. Therefore, in the conventional method using press molding, there is no molding defect, and the shape and dimensional accuracy of the molded product are excellent.
It is becoming difficult to manufacture hollow members.

Further, it is conceivable to use a steel pipe for hollowing the material, but if the conventional method is used as it is as a method for manufacturing the member, the above-mentioned straightening step is required, Problems such as deterioration of fatigue resistance remain unsolved. Hydroforming has recently attracted attention as a new molding method for solving such problems. Hydroforming is a method of injecting a high-pressure liquid into the inside of a steel pipe and forming it into a member with the required shape.By changing the cross-sectional dimension of the steel pipe by expanding, etc., it is possible to integrally form a member with a complicated shape. This is an excellent molding method that has the function of increasing rigidity.

[0006]

Since a steel pipe used for hydroforming needs to have workability capable of withstanding hydroforming, C: 0.02 to
Tensile strength made of 0.1% by mass low carbon steel plate: 580MPa
The following ERW pipes were frequently used. However, since such a low-strength steel pipe cannot cope with the recent trend toward thinning and high strength of automobile members, there has been earnest demand for development of a high-strength steel pipe having workability capable of withstanding hydroforming.

The present invention solves the above-mentioned problems of the prior art and proposes a high-strength steel pipe for automobile structural members having a high tensile strength: more than 580 MPa and excellent workability, and a method for producing the same. With the goal.

[0008]

[Means for Solving the Problems] In order to achieve the above-mentioned objects, the present inventors have achieved a steel pipe having high strength and excellent workability without performing quenching or off-line heat treatment that causes strain. The method of manufacturing the was researched earnestly. As a result, a material steel pipe having a composition in a specific range is subjected to reduction rolling at a rolling end temperature of 400 ° C or higher and lower than 800 ° C, and air-cooled after the rolling, whereby the structure is martensite and ferrite or further retained austenite and / or It was also found that a product tube having a bainite mixed structure and having high strength and excellent workability can be obtained without performing off-line heat treatment.

The present invention has been completed by further studies based on the above findings. That is, the first
In the present invention, the mass% is C: 0.05 to 0.30%, Si: 0.01 to
2.0%, Mn: 1.0 to 4.0%, Al: 0.005 to 0.10%, S:
Contains 0.003% or less, or even Cu: 1% or less,
Ni: 1% or less, Cr: 0.05 to 1.0%, Mo: 1% or less, Nb:
0.01 to 0.1%, Ti: 0.01 to 0.1%, B: 0.005% or less and one or more selected from RE and / or RE
M: 0.02% or less, Ca: 0.01% or less selected 1
Type 1 or 2 type, with the composition consisting of balance Fe and unavoidable impurities, tensile strength above 580Mpa and yield ratio: 70
% Or less, the yield ratio after low-temperature short-time heat treatment is 80% or more, is a high-strength steel pipe for automobile structural members excellent in workability, and in the present invention, the structure , Martensite, ferrite, or a mixed structure containing retained austenite and / or bainite.

In the second aspect of the present invention, C: 0.
05 to 0.30%, Si: 0.01 to 1.0%, Mn: 1.0 to 4.0%, A
l: 0.005 to 0.10%, S: 0.003% or less, or further Cu: 1% or less, Ni: 1% or less, Cr: 0.05 to
1.0%, Mo: 1% or less, Nb: 0.01 to 0.1%, Ti: 0.01 to
0.1%, B: 0.005% or less, 1 or 2 or more selected, and / or REM: 0.02% or less, Ca: 0.01
% Or less, and preferably a raw steel tube having a composition consisting of the balance Fe and unavoidable impurities is heated and soaked, and then the rolling end temperature is 400 ℃ or more. Less than 800 ℃, cumulative reduction rate: 20%
A method for producing a high-strength steel pipe for automobile structural members, which is excellent in workability, characterized by performing the above-mentioned drawing rolling to obtain a product pipe. The product pipe manufactured in this way has an as-rolled tensile strength of more than 580 MPa and a yield ratio of 70% or less, and after being subjected to a low-temperature short-time heat treatment at 150 to 300 ° C for 10 to 20 min, It has an excellent low-temperature short-time heat treatment hardening property (paint bake hardening property) such that the yield stress is greatly increased and the yield ratio is 80% or more.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The high-strength steel pipe for an automobile structural member according to the present invention has a tensile strength TS of more than 580 MPa and a yield ratio Y.
R (yield strength YS / tensile strength TS) is 70% or less,
A high-strength steel pipe having a limit expansion ratio of 15% or more and excellent in workability is preferable. The steel pipe of the present invention may be a welded steel pipe such as an electric resistance welded pipe, a forged steel pipe or the like, or a seamless steel pipe, and is not limited to the method for producing the raw material steel pipe.

The limit expansion rate LBR (%) is the maximum outer diameter d _max of a test body when bursting in a free bulge test in which a free bulge is deformed in a circular cross section and bursted, and the following formula LBR (%) = {(D _max −d ₀ ) / d ₀ } × 100 (where d ₀ is the outer diameter of the product pipe (test body)).

Further, the high-strength steel pipe for automobile structural member of the present invention has a characteristic that the yield stress is greatly increased and the yield ratio YR is 80% or more when subjected to the heat treatment at a low temperature for a short time. The high-strength steel pipe for automobile structural members of the present invention,
The same high yield ratio can be obtained by performing a similar low temperature and short time heat treatment after performing a pre-deformation with a strain amount of 5% or less. The term "low-temperature short-time heat treatment" as used in the present invention means at a temperature of 150 to 300 ° C.
This is the process of holding for 10 to 20 minutes. If the temperature of the low-temperature short-time heat treatment is less than 150 ° C, a large effect of increasing the yield stress cannot be obtained. On the other hand, if the temperature exceeds 300 ° C, the tensile strength decreases and the cost increases. Also, it is preferable that the time of low temperature short time heat treatment is as short as possible from the viewpoint of productivity and cost, but if it is less than 10 min, a sufficient effect cannot be obtained, while if it exceeds 20 min, the tensile strength decreases. To do.

Further, in the case of automobile parts, after the parts are shaped, they are often painted and baked. Generally, the baking conditions are 170-180 ° C.
It takes about 20 minutes. In the case of the steel pipe of the present invention, it is not necessary to perform QT (Quench-Temper) treatment to secure the strength and workability as in the conventional case, but it is possible to secure the desired strength by utilizing the heat during baking of the coating. it can. Further, within the above-mentioned low-temperature short-time heat treatment condition, there is little deterioration in workability, and it becomes possible to secure desired strength while maintaining workability.

First, the reasons for limiting the composition of the high-strength steel pipe for automobile structural members of the present invention will be described. Hereinafter, mass% is simply referred to as%. C: 0.05 to 0.30% C is an element which forms a solid solution in the matrix or combines with other elements to precipitate as a carbide, which contributes to an increase in the strength of the steel. In order to obtain such effects, the content of 0.05% or more is required. On the other hand, if the content exceeds 0.30%, the weldability deteriorates. Therefore, in the present invention, C is limited to the range of 0.05 to 0.30%.

Si: 0.01 to 2.0% Si is an element that acts as a deoxidizing agent and also forms a solid solution in the matrix to increase the strength of steel. These effects are 0.
It is recognized that the content is 01% or more, preferably 0.10% or more, but the content exceeding 2.0% deteriorates ductility. Therefore, Si is limited to the range of 0.01 to 2.0%. In addition, it is preferably 0.01 to 1.0%.

Mn: 1.0 to 4.0% Mn is an element having the functions of increasing the strength of steel, improving the hardenability, and promoting the formation of martensite and retained austenite during cooling after rolling. Such effects are recognized when the content is 1.0% or more, but the content exceeding 4.0% deteriorates ductility. Therefore, Mn is 1.0 to 4.0
It was limited to the range of%. The content is preferably 2.0 to 4.0%. Further, in order to secure high strength of 980 MPa or more without performing off-line heat treatment, it is preferable that Mn is in the range of 2.5 to 4.0%. More preferably, 2.5-3.0%
Is.

Al: 0.005 to 0.10% Al is an element which acts as a deoxidizing agent and, in addition, has an action of refining crystal grains. Due to the effect of refining the crystal grains, the structure of the material steel pipe is refined, and the effect of the present invention is further enhanced. Such an effect is recognized when the content is 0.005% or more, but the content exceeding 0.10% increases the amount of oxide inclusions and reduces the cleanliness. Therefore, Al is limited to the range of 0.001 to 0.10%.

S: 0.003% or less S is present as non-metallic inclusions in steel, and when a non-metallic inclusion is used as a starting point for processing a hydroform or the like, the steel pipe may break. Therefore, it is preferable to reduce S as much as possible from the viewpoint of improving the fracture resistance (hydroforming property). But 0.00
If it is reduced to 3% or less, the effect of improving the breakage resistance (hydroforming property) appears. Therefore, S is limited to 0.003% or less. The content is preferably 0.0010% or less.

In addition to the above-mentioned basic composition, it is preferable to contain the following alloying elements as needed. Cu: 1% or less, Ni: 1% or less, Cr: 0.05 to 1.0%, Mo:
1% or less, Nb: 0.01 to 0.1%, Ti: 0.01 to 0.1%, B:
One or more selected from 0.005% or less, Cu, Ni, Cr, Mo, Nb, Ti, and B are all elements that increase the strength, and one or more selected if necessary. Can be included.

Cu, Ni and Mo have the effect of lowering the transformation point and refining the structure in addition to increasing the strength. Such effects are as follows: Cu: 0.1% or more, N
It becomes remarkable by containing i: 0.1% or more and Mo: 0.1% or more, respectively. On the other hand, if Cu is contained in a large amount exceeding 1%, hot workability deteriorates. Further, Ni improves not only the strength but also the toughness, but even if it is contained in a large amount over 1%, the effect is saturated and the effect commensurate with the content cannot be expected. Further, if Mo is contained in a large amount exceeding 1%, weldability and ductility are deteriorated and the cost becomes high. Therefore, Cu: 1% or less, Ni: 1% or less, Mo: 1% or less is preferable.

Cr is an element that improves hardenability, facilitates the formation of martensite, increases strength, and improves corrosion resistance. Such an effect
It is recognized when the content is 0.05% or more. On the other hand, if the content exceeds 1.0%, the weldability and ductility deteriorate. Therefore, Cr is
It is preferable to limit the range to 0.05 to 1.0%. Nb is
It is an element that precipitates as carbides, nitrides, or carbonitrides and contributes to the strengthening of steel. In particular, in a welded steel pipe having a joint that is heated to a high temperature, it has the effect of refining the grains in the heating process during joining and acting as ferrite precipitation nuclei in the cooling process, and preventing hardening of the joint. . Such an effect is recognized when the content is 0.01% or more. on the other hand,
If added in excess of 0.1%, both weldability and toughness will deteriorate. Therefore, Nb is preferably limited to 0.01 to 0.1%.

Ti, like Nb, is an element that precipitates as carbides, nitrides, or carbonitrides and contributes to strengthening. In the present invention, it is necessary to contain 0.01% or more,
If added in excess of 0.1%, both weldability and toughness will deteriorate. Therefore, Ti is preferably limited to 0.01 to 0.1%. B is an element that improves strength by improving hardenability. Such an effect becomes remarkable when the content is 0.0005% or more. On the other hand, if the content exceeds 0.005% in a large amount, both weldability and toughness deteriorate. Therefore, B is 0.005
It is preferably limited to not more than%.

REM: 0.02% or less, Ca: 0.01% or less One or two kinds of REM and Ca are present in the steel as sulfides, oxides, or oxysulfides. It has the effect of making the shape of inclusions spherical and improving the workability of steel, and can be selected and contained as necessary. Further, both REM and Ca have an action of preventing hardening of the joint of the welded steel pipe having the joint. Such an effect becomes remarkable when REM: 0.003% or more and Ca: 0.003% or more.

On the other hand, REM is 0.02% or Ca is 0.01%
If it exceeds, the inclusions will be excessive and the cleanliness will be lowered and the ductility will be deteriorated. Therefore, REM: 0.02% or less and Ca: 0.01% or less are preferable. The balance other than the above components is Fe
And unavoidable impurities. As unavoidable impurities, N: 0.01% or less, O: 0.01% or less, and P: 0.10% or less are acceptable.

Next, a preferable structure of the steel pipe of the present invention will be described. The steel pipe of the present invention is a mixed structure of martensite and ferrite, or martensite, a mixed structure of ferrite and retained austenite, or a mixed structure of martensite, ferrite and bainite,
Alternatively, it preferably has a mixed structure of martensite, ferrite, retained austenite and bainite. The area ratio of the ferrite in the mixed structure is preferably 70% or less. The amount of ferrite is the area ratio
If it exceeds 70%, the desired high strength cannot be secured. Incidentally, it is more preferably 50% or less.

The balance other than ferrite is mainly martensite. The area ratio of retained austenite is preferably 15% or less, and that of bainite is preferably 25% or less. If the retained austenite exceeds 15% or the bainite exceeds 25%, the desired high strength cannot be secured. Next, a method for manufacturing the high strength steel pipe of the present invention will be described.

In the manufacturing method of the present invention, a steel pipe having the above-mentioned composition is used as a raw steel pipe (base pipe), and the raw steel pipe is heated and soaked, and then drawn and rolled to obtain a product pipe. In the present invention, the manufacturing method of the material steel pipe,
Any of the commonly known pipe making methods is suitable and is not particularly limited. As a method for producing a pipe, for example, a steel strip is cold or hot roll-formed or bent to form an open pipe, and both edge portions of the open pipe are heated to a melting point or higher by using a high frequency current and squeeze roll is used. Butt-joining electric resistance welding method (name of raw tube: ERW pipe, hot-welding pipe in case of hot pipe), solid-state pressure welding method of heating both edges of open pipe to solid-state pressure welding temperature range and pressure welding It is possible to suitably use any of the name of the raw pipe: solid-phase pressure welded pipe), the forge welding method (name of the raw pipe: forged pipe) and the Mannesmann-type piercing and rolling method (name of the raw pipe: seamless steel pipe).

The material steel pipe having the above-mentioned composition and preferably produced by any of the above-mentioned pipe-making methods is first subjected to heating or soaking. The conditions of the heat treatment or soaking treatment applied to the raw material steel pipe are not particularly limited as long as they can satisfy the reduction rolling conditions described below, but the Ac ₃ transformation point to
It is preferable to adjust so as to satisfy the squeeze rolling conditions by heating between the Ac ₁ transformation points or by heating to the Ac ₃ transformation point or higher and cooling. The production of the raw material steel pipe is carried out warm or hot, and at the time of drawing rolling, if it has a sufficient temperature, without cooling to room temperature,
It is sufficient to carry out soaking for the uniform temperature distribution of the tube temperature. When the temperature of the raw steel pipe is low, it is preferable to perform heat treatment. Note that the heat treatment does not necessarily need to be cooled to room temperature. In the present invention, it is preferable to appropriately adjust the cooling after heating or soaking to satisfy the desired draw rolling condition.

The material steel pipe that has been subjected to the heat treatment or soaking is then subjected to squeeze rolling. The squeeze rolling is preferably performed with a cumulative diameter reduction ratio of 20% or more, more preferably 30% or more. If the cumulative diameter reduction ratio is less than 20%, the rolling texture is not sufficiently developed and a high r value cannot be obtained, so that excellent workability cannot be secured. From the viewpoint of increasing the r value, the cumulative diameter reduction ratio is preferably high, but from the viewpoint of productivity, the upper limit is set.
95% is preferable. In addition, preferably 40-80%
Is.

The rolling end temperature of the draw rolling is 400 ° C. or higher 800
The temperature is preferably lower than ℃, more preferably from 400 ℃ to less than 700 ℃. In addition, it is more preferable to process in the α + γ two-phase region or directly above the two-phase region. α + γ
By processing in the two-phase region or directly above the two-phase region,
Since the austenite becomes unstable, the transformation to ferrite is promoted, and the austenite (γ) fraction decreases,
Concentration of C and Mn in γ is facilitated, and hardenability is improved.

If the rolling finish temperature of the draw rolling is less than 400 ° C., the load during rolling becomes excessive and the amount of work hardening during rolling increases, so that the workability decreases and the desired excellent workability is secured. Can not do. On the other hand, when the rolling end temperature of the drawing rolling is 800 ° C. or higher, the low temperature short time heat treatment hardenability deteriorates, and desired properties after the low temperature short time heat treatment cannot be secured. The rolling end temperature is preferably equal to or higher than the martensite or bainite transformation end temperature from the viewpoint of increasing strength.

After the squeeze rolling is finished, it may be cooled according to a conventional method. Cooling may be either air cooling or water cooling.
Further, it is preferable to use a rolling mill train in which a plurality of hole rolling mills called reducers are arranged in tandem for the reduction rolling. In the present invention, the reduction rolling is preferably rolling under lubrication (lubrication rolling). When the reduction rolling is the lubrication rolling, the strain distribution in the thickness direction becomes uniform, so that the fine structure and the formation of texture can be made uniform in the thickness direction. In unlubricated rolling, rolling strain tends to concentrate only on the surface layer of the material due to the shearing effect, and a nonuniform structure tends to be formed in the thickness direction.

[0034]

Example A hot rolled steel sheet (2.3 mm thick) having the composition shown in Table 1 was
After roll-forming into a tubular shape, both ends were induction-heated and abutted to form a welded steel pipe (electric resistance welded pipe: outer diameter 146 mmφ). These welded steel pipes were used as raw material pipes, the raw steel pipes were subjected to the heat treatment shown in Table 2, and further subjected to drawing rolling under the conditions shown in Table 2 to obtain product pipes. The squeeze rolling was performed using a reducer arranged in tandem.

The resulting product pipe was examined for structure, tensile properties, low temperature short time heat treatment hardening properties, tensile bending properties, and tube expansion ratio. In addition, some steel pipes were hydroformed by a free bulge test, and the pipe expansion ratio when burst was investigated. The test method is as follows. (1) Tissue A test piece was taken from each product tube, and the tissue of the cross section orthogonal to the tube longitudinal direction was imaged using an optical microscope and a scanning electron microscope. The type of tissue and the tissue fraction of the obtained tissue photograph were determined using an image analyzer. (2) Tensile property JIS 12 A test piece (gauge distance between each product pipe in the longitudinal direction)
50 mm) is sampled and subjected to a tensile test in accordance with JIS Z 2241, yield stress YS, tensile strength TS, elongation E
1, the work hardening index n value, and the Rankford value r value were determined.

The n value is the value of the change in true strain with respect to the change in true stress between 5 and 10%, that is, n = (lnσ _10% -ln
σ _5% ) / (lne _10% −lne _5% ). Note that σ
Is the true stress, and e is the true strain. The r value is defined by the ratio of the true strain of the plate width to the true strain of the plate thickness in the tensile test.

R = ln (W _i / W _f ) / ln (T _i / T _f ), where W _i : initial plate width, W _f : final plate width, T _i :
First plate thickness, T _f : Final plate thickness. However, since the measurement of the plate thickness involves a considerable error, it is assumed that the volume of the test piece is usually constant, and the r value is calculated by the following equation.

R = ln (W _i / W _f ) / ln (L _f W _f
/ L _i W _i ) where W _i : the initial plate width, W _f : the final plate width, L _i :
First length, L _f : Final length. In the present invention, the r value is the true strain in the longitudinal direction and the true strain in the width direction when a strain gauge having a gauge length of 2 mm is attached to a tensile test piece and a tensile strain of 6 to 7% is performed. It was measured and calculated by the above formula. (3) Low-temperature short-time heat treatment Hardening property JIS 12 A test pieces (gauge length 50 mm) are sampled in the longitudinal direction from each product pipe obtained, and first pre-deformed to give 5% tensile plastic strain. did. Then, on the test piece, 180 ℃ × 20
Heat treatment was performed at low temperature for a short time of min. Then, the tensile test piece is re-pulled, and the maximum stress at the time of pre-deformation and the
The difference from the 0.2% proof stress is calculated, and the low temperature short time heat treatment hardening amount (B
As the H content), the low temperature short time heat treatment hardening characteristics were evaluated. The yield ratio after the low temperature short time heat treatment was also measured. (4) Expansion ratio A product pipe with an outer diameter of 38.1 mm or more was cut into a length of 500 mm to obtain a test piece for hydroform. As shown in FIG. 1, this hydroform test body was set in a hydroform processing apparatus, and water was supplied from both ends of the test body to cause free bulge deformation in a circular cross section and burst (break). The maximum outer diameter d _max of the test piece when bursted was measured, and the following equation LBR (%) = {(d _max −d ₀ ) / d ₀ } × 100 (where d ₀ : product pipe (test piece) Outer diameter) was used to calculate the limit expansion rate LBR. A cross-sectional view of the mold used is shown in FIG.
Shown in.

The upper die 2a and the lower die 2b each have a steel pipe holding portion 3 formed on each of both ends in the length direction, which is a semi-cylindrical surface having a diameter substantially equal to the outer diameter d _O of the steel pipe. At the center in the length direction, there is a deforming portion 6 including a semi-cylindrical deforming portion 4 having a diameter d _c and a tapered deforming portion 5 having an inclination angle θ = 45 °. In addition,
Length 1c of the deformation portion 6 is two times the d _O. Further, the diameter d _c of the semi-cylindrical deformable portion 4 may be about twice the outer diameter d _O of the steel pipe.

The details of the test method will be further described.
As shown in FIG. 1, the steel pipe 1 is sandwiched between the upper mold 2a and the lower mold 2b so that the steel pipe 1 is fitted in the steel pipe holding portion 3 of each mold. In this state, when a liquid such as water is supplied from both ends of the steel pipe 1 to the inner surface side of the steel pipe 1 through the shaft pressing cylinder 7a to apply the hydraulic pressure P, the circular cross-section is bulged and the burst occurs. measuring the maximum outer diameter d _{ma x} of.
It should be noted that reference numerals 8 and 9 in FIG. 1 denote a mold holder and an outer ring for holding the molds 2a and 2b, respectively, with the steel pipe sandwiching the steel pipe. In hydroform, there are cases where both ends of the tube are fixed and cases where a compressive force is applied from both ends of the tube (called axial compression), but in general,
It is possible to obtain a higher expansion ratio by tube end compression. Here, a compressive force was appropriately applied from both ends of the tube so that a high expansion rate was obtained. The load of the compressive force is the compressive force F in the axial direction with respect to the shaft pressing cylinders 7a and 7b in FIG.
Can be carried out by loading. The mold dimensions used here (Fig. 2) are l _C = 127.0 mm and d _c = 127.0.
mm, r _d is 5 mm, l _O is 550 mm, theta was 45 °.

The results obtained are shown in Table 3.

[0042]

[Table 1]

[0043]

[Table 2]

[0044]

[Table 3]

Each of the examples of the present invention has a tensile strength of more than 580 MPa, a low yield ratio of 70% or less and a high ductility, and further has a high pipe expansion ratio LBR and an excellent hydroforming property.
Further, a large amount of BH is obtained by the low temperature short time heat treatment, and a high YR with a yield ratio of 80% or more is obtained. Therefore, in all the examples of the present invention, high strength can be ensured after processing. On the other hand, in Comparative Examples out of the range of the present invention, the workability is lowered depending on whether the tensile strength is low, the yield ratio is high, or the LBR is low.

[0046]

As described above, according to the present invention, it is possible to inexpensively produce a high-strength steel pipe having high strength and excellent workability without performing off-line heat treatment, which is a significant industrial advantage. Play. Further, the high-strength steel pipe of the present invention has not only high strength but also high ductility and high limit pipe expansion ratio, is excellent in hydroformability, and has an effect of contributing to weight reduction of an automobile body as an automobile structural member.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing an example of the configuration of a hydroforming apparatus used for a free bulge test.

FIG. 2 is a sectional view showing an example of a mold used for a free bulge test.

[Explanation of symbols]

1 test body (test steel pipe) 2 mold 2a Upper mold 2b Lower mold 3 Steel pipe holder 4 Semi-cylindrical deformation part 5 Tapered deformation part 6 Deformation part 7a, 7b Axial push cylinder 8 Mold holder 9 Outer ring

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Yoshikazu Kawabata             1-1 Kawasaki-cho, Handa-shi, Aichi Made in Kawasaki             Chita Works of Iron Co., Ltd. (72) Inventor Masanori Nishimori             1-1 Kawasaki-cho, Handa-shi, Aichi Made in Kawasaki             Chita Works of Iron Co., Ltd. (72) Inventor Mitsuo Kimura             1-1 Kawasaki-cho, Handa-shi, Aichi Made in Kawasaki             Chita Works of Iron Co., Ltd. (72) Inventor Yuji Hashimoto             1-1 Kawasaki-cho, Handa-shi, Aichi Made in Kawasaki             Chita Works of Iron Co., Ltd.

Claims (5)

[Claims] 1. In mass%, C: 0.05 to 0.30%, Si: 0.01 to 2.0
%, Mn: 1.0 to 4.0%, Al: 0.005 to 0.10
%, S: 0.003% or less, having a composition consisting of the balance Fe and unavoidable impurities, having a tensile strength of more than 580 Mpa and a yield ratio of 70% or less. High-strength steel pipe for automobile structural members with excellent workability characterized by a yield ratio of 80% or more. 2. The high-strength steel pipe for an automobile structural member according to claim 1, wherein the structure is martensite, ferrite, or a mixed structure containing retained austenite and / or bainite. 3. In addition to the above composition, further, in% by mass, C
u: 1% or less, Ni: 1% or less, Cr: 0.05 to 1.0%, Mo:
1% or less, Nb: 0.01 to 0.1%, Ti: 0.01 to 0.1%, B:
The high-strength steel pipe for an automobile structural member according to claim 1 or 2, containing one or more selected from 0.005% or less. 4. In addition to the composition, further, in% by mass, RE
M: 0.02% or less, Ca: 0.01% or less selected 1
A high-strength steel pipe for an automobile structural member according to any one of claims 1 to 3, wherein the high-strength steel pipe contains one or two kinds. 5. In mass%, C: 0.05 to 0.30%, Si: 0.01 to 2.0
%, Mn: 1.0 to 4.0%, Al: 0.005 to 0.10
%, S: 0.003% or less, the raw material steel pipe is subjected to heating and soaking, and then subjected to reduction rolling at a rolling end temperature of 400 ° C to less than 800 ° C and a cumulative reduction ratio of 20% or more. , Tensile strength is 58
High strength for automobile structural members with excellent workability characterized by a product pipe having a yield ratio of 70% or less at 0 Mpa or more and a yield ratio of 80% or more after heat treatment at low temperature for a short time. Steel pipe manufacturing method.