JP2002294405A - Steel tube superior in formability and the production method for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel tube with excellent formability in hydro-forming or the like, and a production method for the same. SOLUTION: This steel tube has an excellent formability characterized by the r value of 1.5 or more in the axial direction of tube, the average value of X-ray random intensity ratio of 2.0 or more in the group of directions of 110}<110>- 111}<110> on the plane at one half of the thickness of the steel sheet face, the X-ray random intensity ration of 3.0 or more in the direction of 110}<110> on the plane at one half of the thickness of the steel sheet face and the average value of X-ray random intensity ratio of 3.0 or less in the group of directions of 100}<110>- 223}<110> on the plane at one half of the thickness of the steel sheet face.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel pipe used for, for example, automobile panels, undercarriages, members, and the like, and a method of manufacturing the same. In particular, a steel pipe formed by a hydroforming method and suitable for these uses and a method of manufacturing the same. It is about the method.

[0002]

2. Description of the Related Art Along with the need to reduce the weight of an automobile, it is desired to increase the strength of a steel sheet. This makes it possible to reduce the thickness of the steel sheet and improve the safety in a collision. Recently, for members having a complicated shape, attempts have been made to form a high-strength steel pipe by a hydroforming method (see Japanese Patent Application Laid-Open No. 10-175027). This aims to reduce the number of parts and reduce the number of weld flanges due to the need for weight reduction and low cost of automobiles. As described above, by using a new forming method such as hydroforming, a great advantage such as a reduction in cost and an increase in design freedom can be expected. In order to make full use of the advantages of this hydroform molding, materials suitable for this new molding method are required. The present inventors have already proposed in Japanese Patent Application No. 2000-52574 a steel pipe having a controlled texture and excellent formability.

[0003]

In such a situation, there is an increasing demand for hydroform molding for a steel pipe having higher strength than ever, and it is inevitable that a higher formability is required. Conceivable. An object of the present invention is to provide a steel pipe which is more excellent in formability and which is inexpensive, and a method for producing the same.

[0004]

DISCLOSURE OF THE INVENTION The present invention finds a texture of a material having excellent formability such as a hydroform and a control method thereof, and specifies a condition of the control method to provide a new form of hydroform. It proposes an excellent steel pipe and its manufacturing method, and the gist is as follows. (1) In mass%, C: 0.0005 to 0.5%, S
i: 0.001 to 2.0%, Mn: 0.01 to 3.0
%, With the balance being Fe and inevitable impurities, having a tensile strength TS (MPa) and uniform elongation U.P. EL
(%) Is TS + 60U. EL> 1300 and {110} <
The average of the X-ray random intensity ratio of the orientation group of 110> to {111} <110> is 2.0 or more, and the X-ray random intensity ratio of {110} <110> on the sheet surface at a steel pipe sheet thickness of 1/2. Characterized by having one or both textures of 3.0 or more. (2) The texture of the steel pipe has an average of the X-ray random intensity ratios of the {100} <110> to {223} <110> orientation groups of the sheet surface at a steel pipe thickness of １ ／, and the steel pipe thickness. 1/2
Characterized in that one or both of the X-ray random intensity ratios of {100} <110> on the sheet surface in the above have a texture of 3.0 or less, the steel pipe having excellent formability according to the above (1), . (3) The texture of the steel pipe has a texture in which the X-ray random intensity ratio of {100} <100> of the plate surface at a steel pipe plate thickness of 1/2 is 1 or more and 8 or less. 1)
Or a steel pipe excellent in formability according to (2). (4) The steel pipe excellent in formability according to any one of the above (1) to (3), wherein the r value in the pipe axis direction of the steel pipe satisfies 1.5 or more. (5) The steel pipe according to any one of (1) to (4), wherein the steel pipe further contains Al: 0.3% or less and P: 0.1% or less by mass%. Steel tube with excellent formability. (6) The steel pipe has a mass percentage of Ti, Zr, V, N
b, one or more of Mg in total of 0.001 to
(1) to (5), characterized by containing 0.5%.
The steel pipe excellent in formability according to any one of the above items. (7) The steel pipe contains Cr, Cu, and N in mass%.
One, two or more of i, Co, W, and Mo are used in a total of 0.
(1) characterized by containing 001 to 3.0%.
A steel pipe excellent in formability according to any one of (6) to (6). (8) The steel pipe is in mass%, and B is 0.001 to
(1) to (1), which are contained at 0.01%.
The steel pipe excellent in formability according to any one of the above items (7). (9) In the steel pipe, at least 50% by volume of the metal structure is made of ferrite, and the crystal grain size of the ferrite grains is 0.5 to 0.5%.
(1) to (1) to (4), wherein the ferrite particles have a range of 500 μm and have an average particle size of 10 μm or more.
The steel pipe excellent in formability according to any one of the above items (7). (10) In the production of a steel pipe excellent in formability according to any one of the above (1) to (8), after producing a mother pipe, Ac ₃
After heating to the transformation point + 80 ° C or higher and 1350 ° C or lower,
A method for producing a steel pipe excellent in formability, comprising performing diameter reduction processing at a diameter reduction ratio of 25% or more in a temperature range of an Ar ₃ transformation point or more and an Ar ₃ transformation point + 200 ° C. (11) In the production of a steel pipe excellent in formability according to any one of the above (1) to (8), after producing a mother pipe, Ac ₃
After heating to the transformation point + 80 ° C or higher and 1350 ° C or lower,
Ar ₃ transformation point or higher, subjected to diameter reduction of radial contraction rate of 25% or more at a temperature range of Ar ₃ transformation point + 200 ° C., subsequently Ar ₃
A steel pipe excellent in formability, characterized in that after diameter reduction at a transformation point or lower and a diameter reduction ratio of 2 to 20%, diameter reduction processing is completed at an Ar ₃ transformation point of −100 ° C. or more and an Ar ₃ transformation point or less. Production method. (12) In the manufacture of a steel pipe having excellent formability as described above, the change in the thickness of the steel pipe after the diameter reduction is from -30% to +20.
%. A method for producing a steel pipe having excellent formability, wherein

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One of the necessary materials for a steel pipe capable of withstanding unprecedented severe processing such as hydroforming is uniform elongation. Generally, uniform elongation decreases with increasing tensile strength. As a steel pipe excellent in formability that can withstand severe processing expected in the future, tensile strength TS (M
Pa) and uniform elongation. The relationship with EL (%) is TS + 6
0U. It is necessary to satisfy EL> 1300.

Under these restrictions, a high r value is important for good processing in which drawing is dominant, such as branching, for example. The improvement of the r value is achieved by devising the texture formation. The inventors have
A new finding was found that can increase the r value in the pipe axis direction of a steel pipe by reducing the diameter of the steel pipe, and clarified a steel pipe having a high uniform elongation and a high r value in the pipe axis direction and a manufacturing method.

Hereinafter, the present invention will be described in detail. First,
The requirement (1) will be described. The component content is% by mass. C: Addition of 0.0005% or more is effective for increasing the strength, but excessive addition is not preferable in controlling the texture, and the upper limit is set to 0.50%. 0.001
0.3% is more preferable, and 0.002 to 0.2% is a still more preferable range.

Si: It is possible to increase mechanical strength at low cost, and it is sufficient to add Si according to the required strength level. However, excessive addition not only causes deterioration of wettability and workability of plating, but also is good. Therefore, the upper limit was set to 2.0%, since the formation of an unusual texture was inhibited. The lower limit is set to 0.001% because the lower limit is difficult due to steelmaking technology. Mn: Since the element is effective for increasing the strength, the lower limit is 0.01.
%. Further, since Mn has a favorable effect on variant selection during transformation and has an effect of improving texture, it is preferable to add 0.5% or more. On the other hand, since excessive addition causes a decrease in ductility, the upper limit is set to 3.0%.

The {110} <of the plate surface at a half plate thickness of the steel plate
Orientation group of {110> to {111} <110> and {11}
X-ray random intensity ratio of 0 ラ ン ダ ム <110>: the most important characteristic value in performing hydroform molding or the like. {110} <110> when X-ray diffraction of the plate surface at the plate thickness center position was performed and the intensity ratio of each direction with respect to the random sample was obtained.
The average in the orientation group of {111} <110> was 2.0 or more.

The main azimuth included in this azimuth group is $ 11.
0} <110>, {661} <110>, {441} <
110>, {331} <110>, {221} <110
>. These are crystal orientation groups that cannot be obtained simply by forming a steel pipe from a deep drawn cold-rolled steel sheet as a material by electric resistance welding or the like. In the present invention, {111} <112> and {554} <, which are typical crystal orientations of a high r-value cold-rolled steel sheet, are used.
225> is scarcely present, and these are all 2.0 or less, more preferably less than 1.0. The X-ray random intensity ratio in each of these directions can be calculated from the three-dimensional texture calculated by the vector method from the {110} pole figure, {110},
What is necessary is just to obtain | require from the three-dimensional texture calculated by the series expansion method based on several pole figures among {100}, {211}, and {310} pole figures.

For example, to determine the X-ray random intensity ratio of each crystal orientation by the latter method, the (110) [1-10],
(661) [1-10], (441) [1-10],
(331) Intensity of [1-10] and (221) [1-10] are represented. Note that the texture of the present invention usually has the highest intensity within the range of the above orientation group in a section of φ2 = 45 °, and the intensity level gradually decreases as the distance from the orientation group increases. Considering the problem of measurement accuracy, twisting around the axis at the time of steel pipe production, and the problem of accuracy of X-ray sample preparation, when the orientation showing the highest intensity deviates from these orientation groups by about ± 5 ° to 10 ° Is also possible.

{110} <110> to {111} <11
The average X-ray random intensity ratio of the group of 0> azimuths is an arithmetic average of the X-ray random intensity ratios of the above-described directions. If all the intensities in the above orientations cannot be obtained, {110} <110
>, {441} <110>, and {221} <110> may be substituted by the arithmetic mean. Among them, {110} <
110> is important, and in the present invention, the X-ray random intensity ratio in this direction is 3.0 or more. When molding is difficult, the average intensity ratio of the above orientation group is 4.0 or more and # 11
It is desirable that the intensity ratio of 0｝ <110> be 8.0 or more.

Further, {001} <110> and {11}
Each of 2｝ <110> must have a strength of 2.0 or less. This is because these are the directions that decrease the r value in the axial direction. Preferably it is 1.0 or less. Other orientations, for example {116} <110>, {114} <
110>, {113} <110>, {223} <110
> And the like are not particularly limited.
In order to reduce the value, it is preferable that each is 2.0 or less.

Further, in order to develop {110} <110> more sharply, γ is heated at a high temperature of 1000 ° C. or more to perform γ.
Texture after transformation is $ 10
It was newly found that the orientation of 0 ° <100> developed simultaneously. The orientation of {100} <100> is good for shape preservation after molding, but if it is excessively large, the r value will be reduced.
Therefore, the X-ray random intensity ratio is set to 1 or more and 8 or less.

{001} <110>, {116} <11
0>, {114} <110>, {113} <110>,
The X-ray random intensity ratios of {112} <110> and {223} <110> are (001) [1-10] and (116) [1-
10], (114) [1-10], (113) [1-1
0], (112) [1-10], (223) [1-1
0].

When X-ray diffraction of a steel pipe is performed, an arc-shaped test piece is cut out from the steel pipe and pressed to make a flat plate for X-ray analysis. Also, when making a flat plate from an arc-shaped test piece,
In order to avoid the influence of crystal rotation due to the processing of the test piece, the processing was performed with as low a strain as possible, and the upper limit of the amount of strain introduced by the processing was set to 10% or less. The plate-like sample obtained in this manner is polished to the vicinity of the center of the plate thickness by mechanical polishing or chemical polishing, and finished to a mirror surface by buff polishing, and then the distortion is removed by electrolytic polishing or chemical polishing. Adjust so that the center layer is the measurement surface. When a segregation zone is observed in the thickness center layer of the steel sheet,
What is necessary is just to measure about the place where there is no segregation zone in the range of ３ to ５ of the plate thickness. If X-ray measurement is more difficult,
It may be measured by the EBSP method or the ECP method.

As described above, the texture of the present invention is defined by the X-ray measurement results at the center of the sheet thickness or at the surface near the center of the sheet thickness. preferable. However, from the outer surface of the steel pipe to a plate thickness of about 1/4, the texture changes due to shear deformation due to the diameter reduction described below, and the above-mentioned requirements for the texture may not be satisfied.

Although the features relating to the texture of the present invention cannot be represented only by a normal inverse pole figure or a positive pole figure, for example, an inverse pole figure representing the radial direction of a steel pipe was measured in the vicinity of the center of the sheet thickness. In this case, the X-ray random intensity ratio in each direction is preferably as follows. <100>: 1.5 or less, <411>: 1.5 or less, <
211>: 1.5 or less, <111>: 5 or less, <332
>: 10 or less, <221>: 30.0 or less, <110
>: 50.0 or less.

In the reverse pole figure showing the axial direction,
<110>: 15 or more, all orientations other than <110>:
3 or less. The r-value of the steel pipe changes variously due to the change in texture, but at least the r-value in the axial direction is 1.5 or more. The r value in the axial direction may exceed 3.5 depending on the manufacturing conditions. Although the anisotropy of the r value is not particularly limited, in the present invention, the r value in the axial direction is always larger than the r value in the circumferential direction or the radial direction. For example, when a high r-value cold rolled steel sheet is simply made into a steel pipe by electric resistance welding, the r value in the axial direction may be 1.7 or more depending on the stripping. However, the present invention has the texture described above and at the same time r
It is clearly distinguished from such steel pipes in that the value is at least 1.5.

The evaluation of the r value may be performed using a JIS No. 11 tubular test piece or a JIS No. 12 arc-shaped test piece. The amount of strain at that time is evaluated at an elongation percentage of 15%.
When it is less than 5%, it is evaluated by the amount of strain within the range of uniform elongation. In addition, it is desirable that a test piece be collected from a portion other than the seam portion. It should be noted that the steel pipe is rewound into a plate shape and JIS1
In the case of the No. 3 plate-shaped tensile test piece, the r value tends to be larger than that of the arc-shaped test piece.

In the present invention, in addition to the components described above,
The following components can be added: Al, Zr, Mg: effective as a deoxidizing element. on the other hand,
Excessive addition causes a large amount of crystallization or precipitation of oxides, sulfides, or nitrides, deteriorating cleanliness, lowering ductility, and impairing plating properties. Therefore, if necessary, one or more of these may be used in a total amount of 0.0001 to 0.5.
0%.

Ti, V, Nb, P: added as needed. By forming carbide, nitride or carbonitride, Ti, V, and Nb not only can increase the strength of the steel material and improve the workability, but are also preferable for forming a texture. % Or more. If the total amount exceeds 0.5%, a large amount of carbides, nitrides or carbonitrides precipitates in ferrite grains or grain boundaries as a parent phase and lowers ductility. 0.001 to 0.5% by mass. More preferably, it is 0.01 to 0.08%. Since P is also an element effective for increasing the strength, it is added in an amount of 0.4% or less. If added in excess of 0.4%, defects may occur during hot rolling and diameter reduction,
Since the moldability deteriorates, the upper limit is 0.4%.

Cr, Cu, Ni, Co, W, Mo: these are strengthening elements, and if necessary, one or two of these
0.001% or more is added in total of the species or more. In addition, excessive addition causes cost increase and decrease in ductility,
3.0% or less. B: B is effective for strengthening grain boundaries and increasing the strength of steel materials. However, if the added amount exceeds 0.01%, not only the effect is saturated, but also the steel sheet strength is unnecessarily increased, 0.0001 to 0.01%, since workability is also reduced
And

Further, in the production, ingot casting or continuous casting is performed by performing various secondary smelting following smelting using a blast furnace, an electric furnace, or the like. In the case of continuous casting, hot rolling is performed without cooling to around room temperature. It may be manufactured by combining manufacturing methods such as CC-DR. It goes without saying that hot rolling may be performed by reheating the cast ingot or cast slab. The heating temperature of the hot rolling is not particularly limited, and may be any temperature that is appropriate for realizing the desired finishing temperature.

The finishing temperature of hot rolling may be performed in any temperature range of α + γ2 phase region, α single phase region, α + pearlite, and α + cementite in addition to the normal γ single phase region. Hot rolling 1
Lubrication may be applied to more than the pass. Further, the rough rolling bars may be joined to each other and the finish hot rolling may be continuously performed. The rough rolling bar may be wound once or rewound again and then subjected to finish hot rolling. The cooling rate and winding temperature after hot rolling are not particularly limited. After hot rolling, it is desirable to perform pickling. Furthermore, skin pass rolling and 5
Cold rolling with a rolling reduction of 0% or less may be performed.

In the production of steel pipes, generally, electric resistance welding is used, but welding and pipe forming techniques such as TIG, MIG, laser welding, UO, forging and the like can also be used. In the production of these welded steel pipes, the heat-affected zone of the weld may be subjected to local solution heat treatment alone or in combination depending on the required properties, and in some cases, may be performed a plurality of times. Further enhance. This heat treatment is intended to be applied only to the welded portion and the heat affected zone, and can be applied online or offline during manufacturing.

The heating temperature before diameter reduction of the steel pipe and the conditions of the subsequent diameter reduction are important in the present invention. The present invention is based on the following new findings. In other words, after the steel pipe is heated to the γ region, the steel tube is continuously reduced in diameter in the γ region. It was found that the texture near <110> remarkably developed.

Regarding the heating temperature, increasing the γ particle size affects the texture formation, so that the Ac 3 point +80
It is necessary to be higher than ° C. However, considering the surface properties of the steel pipe, the maximum temperature is 1350 ° C. The diameter reduction in the γ region is performed so that the diameter reduction rate is 25% or more. If it is less than 20%, since the texture does not develop in the γ region, it is difficult to finally obtain a desirable r value and texture. Diameter reduction rate 30 in γ range
% Or more, more preferably 50% or more. In this case, the diameter reduction ratio is {(diameter of mother pipe before diameter reduction-diameter of steel pipe after completion of diameter reduction in γ region) / diameter of mother pipe before diameter reduction)} × 100 ( %).

Further, it is desirable to perform a diameter reduction process with a diameter reduction ratio of 25% or more in the temperature range of Ar 3 points or more and Ar 3 points + 200 ° C. That is, γ under the stress or strain state of diameter reduction
And recrystallized + processed texture is greatly developed, and the desired texture formation in the subsequent transformation, ie, {11}
This is because the {0} <110> to {111} <110> orientation groups are essential for the sharp development of {110} <110>.

After the above diameter reduction processing, if the diameter reduction processing is performed at three points or less of Ar, the target texture further develops. By slightly producing a ferrite structure in a temperature range of three points or less of Ar, a larger strain is concentrated on γ, and a γ texture develops. As a result, the target texture after transformation further develops. In order to exert the effect, Ar3
The diameter reduction ratio below the point needs to be 2% or more. However, if the diameter is excessively reduced, the uniform elongation is greatly reduced. Therefore, the diameter reduction at three or less Ar points is set to 20% or less.

The total diameter reduction ratio of the steel pipe manufactured in this way must be 29% or more. If the total diameter reduction ratio is less than 29%, the texture is not sufficiently developed. Preferably 50
% Or more. The total diameter reduction ratio is defined by the following equation. {(Diameter of mother pipe before diameter reduction-diameter of steel pipe after completion of diameter reduction) / diameter of mother pipe before diameter reduction)} × 100 (%).

It is preferable that the rate of change in the thickness of the steel pipe after diameter reduction processing on the mother pipe is + 15% to -30%. The sheet thickness reduction rate is {(sheet thickness of steel pipe after diameter reduction is completed-thickness of mother pipe before diameter reduction) / sheet thickness of mother pipe before diameter reduction)} × 100
(%). In addition, the diameter of a steel pipe measures the external shape of a steel pipe. Target {110} <110> to $ 11
In order to develop a texture of the 1｝ <110> orientation group, particularly the {110｝ <110> orientation, it is more effective to reduce the thickness during the diameter reduction processing. As the plate thickness increases, the texture formation becomes weaker and must be suppressed to 15% or less. Further, considering productivity, the reduction rate is up to about 30%, and from these, the change in plate thickness is preferably -30% to + 15%.

The diameter reducing process may be performed by combining a plurality of rolls and passing a multi-pass line, or may be performed by using a die. Further, it is desirable to provide lubrication at the time of diameter reduction from the viewpoint of improving formability. The steel pipe according to the present invention preferably contains ferrite in an area ratio of 50% or more in order to secure ductility. However, as a metal structure other than ferrite, a structure such as pearlite, bainite, martensite, austenite, and carbonitride is used. May be included.

The ferrite grain size becomes mixed when processed near the Ar3 transformation point, and the grain size ranges from 0.5 μm to 500 μm.
μm. If the average particle size is 10 μm or less, the uniform elongation or the n value is greatly reduced. Therefore, the average particle size is preferably 10 μm or more.

[0035]

EXAMPLES Each steel having the components shown in Table 1 was melted at 1250 ° C.
, And then hot-rolled at the finishing temperature shown in Table 1 and wound up. 100 to 20 in diameter by ERW after pickling
After forming the tube to 0 mm, the tube was heated to a predetermined temperature to perform a diameter reduction process. The workability of the obtained steel pipe was evaluated by the following method.

A scribed circle having a diameter of 10 mm was transferred to a steel pipe in advance, and the inner pressure and the amount of axial pressing were controlled to form a stretch in the circumferential direction. The strain εΦ in the axial direction and the strain εθ in the circumferential direction of the portion showing the maximum expansion rate immediately before the burst (expansion rate = maximum perimeter after molding / perimeter of the mother pipe) were measured.
The ratio of these two strains, ρ = εΦ / εθ, and the maximum expansion ratio were plotted, and the expansion ratio Re, at which ρ = −0.5, was used as the formability index of the hydroform. The X-ray measurement was performed by cutting out an arc-shaped test piece from the mother pipe before the diameter reduction and the steel pipe after the diameter reduction and pressing it as a flat plate. (110), (200),
(211), (310) The pole figures are measured, and a three-dimensional texture is calculated by using the pole figures by a series expansion method, and φ2 = 4
The X-ray random intensity ratio of each crystal orientation in the 5 ° cross section was determined.

Tables 2 and 3 show various conditions for diameter reduction and characteristics of the steel pipe after diameter reduction. In the examples of the present invention, all have a good texture and r value, and the maximum expansion ratio during hydroform molding is high, whereas in the examples other than the present invention, the texture and r value are not preferable, and the maximum expansion ratio is large. Is also low.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3]

[0041]

As described above, the steel pipe according to the present invention is particularly excellent in hydroform moldability in which axial pushing force is exerted, and can improve the production efficiency of automotive parts during hydroform molding. . Further, since the present invention can be applied to a high-strength steel pipe, it is possible to reduce the thickness of a part.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/58 C22C 38/58 (72) Inventor Naoki Yoshinaga 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation (72) Inventor Hitoshi Asahi 20-1 Shintomi, Futtsu City, Chiba Prefecture Nippon Steel Corporation (72) Inventor Manabu Takahashi 20-1 Shintomi, Futtsu City, Chiba Prefecture Nippon Steel Corporation F-term in the R & D Headquarters of the Company (reference)

Claims (12)

[Claims] 1. The composition contains, by mass%, C: 0.0005 to 0.5%, Si: 0.001 to 2.0%, and Mn: 0.01 to 3.0%, with the balance being Fe and inevitable. Steel tube made of natural impurities, with tensile strength TS (MPa) and uniform elongation U. EL (%)
Is TS + 60U. EL> 1300 and {110} <
The average of the X-ray random intensity ratio of the orientation group of 110> to {111} <110> is 2.0 or more, and the X-ray random intensity ratio of {110} <110> on the sheet surface at a steel pipe sheet thickness of 1/2. Characterized by having one or both textures of 3.0 or more. 2. A steel pipe having a texture of steel plate thickness 1/2.
{100} <110> to {223} <110
> The average of the X-ray random intensity ratios of the orientation groups and the one or both of the {100} <110> X-ray random intensity ratios of the sheet surface at a steel pipe thickness of 1/2 are set to 3.0 or less. The steel pipe excellent in formability according to claim 1, having a structure. 3. The texture of the steel pipe has a steel pipe plate thickness of １ ／.
The steel pipe having excellent formability according to claim 1 or 2, wherein the steel pipe has a texture in which the X-ray random intensity ratio of {100} <100> of the plate surface is 1 or more and 8 or less. 4. The steel pipe excellent in formability according to claim 1, wherein the r value of the steel pipe in the pipe axis direction satisfies 1.5 or more. 5. The steel pipe according to claim 1, further comprising:
The steel pipe excellent in formability according to any one of claims 1 to 4, which contains 3% or less and P: 0.1% or less. 6. The steel pipe according to claim 1, further comprising Ti, Z
One, two or more of r, V, Nb, and Mg may be used in a total of 0.
001-0.5% is contained.
Item 5. The steel pipe excellent in formability according to any one of Items 5. 7. The steel pipe according to claim 1, further comprising Cr, C
The moldability according to any one of claims 1 to 6, wherein one or more of u, Ni, Co, W, and Mo are contained in a total amount of 0.001 to 3.0%. Excellent steel pipe. 8. The steel pipe according to claim 1, wherein B is 0.0% by mass.
The content is from 0.01 to 0.01%.
7. The steel pipe excellent in formability according to any one of items 7 to 7. 9. The steel pipe according to claim 1, wherein the volume ratio of the metal structure is 50%.
The above is composed of ferrite, the crystal grain size of the ferrite grains is in the range of 0.5 to 500 μm, and the average grain size is 10
The steel pipe excellent in formability according to any one of claims 1 to 7, wherein the steel pipe has ferrite grains of μm or more. 10. The method for producing a steel pipe excellent in formability according to claim 1, wherein after the mother pipe is formed, Ac is produced.
Molding characterized in that after heating to a temperature of ₃ transformation points + 80 ° C. or more and 1350 ° C. or less, a diameter reduction process of 25% or more is performed in a temperature range of Ar ₃ transformation points or more and Ar ₃ transformation point + 200 ° C. Method for manufacturing steel pipes with excellent properties. 11. The method for producing a steel pipe excellent in formability according to claim 1, wherein after the mother pipe is formed, Ac is produced.
After heating to a temperature of ₃ transformation points + 80 ° C. or more and 1350 ° C. or less, a diameter reduction process of 25% or more in the temperature range of Ar ₃ transformation points or more and Ar ₃ transformation point + 200 ° C. is performed.
After reducing the diameter to 2 to 20% at the r ₃ transformation point or lower, Ar
_A method for producing a steel pipe excellent in formability, characterized in that diameter reduction processing is completed at a temperature of not less than ₃ transformation point -100 ° C. and not more than Ar ₃ transformation point. 12. In the production of a steel pipe excellent in formability as described above, a change in the thickness of the steel pipe after diameter reduction is -30% or more.
+ 20%, a method for producing a steel pipe excellent in formability.