JP2002097549A - Steel tube superior in formability and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a steel tube with a superior characteristic in forming such as hydroforming, and a manufacturing method therefor. SOLUTION: The steel tube superior in formability is characterized by satisfying 203C1/2+15.2Ni-44.7Si-104V-31.5Mo+30Mn+11Cr+20Cu-700P-200Al<-20...(1) 44.7Si+700P+200Al>80...(2) by mass%, and the following formula (3) for a relationship between tensile strength TS and n value of the steel tube, and by having a volume rate of a ferrite phase of 75% or more, an average crystal particle size of ferrite of 10 μm or more, and 90% or more of area rate of crystal particles with an aspect ratio of 0.5-3.0 out of crystal particles composing ferrite; n>=-0.126×ln(TS)+0.94...(3) It is further characterized in heating the steel tube at 850 deg.C or more, subjecting it to radius reduction working so as to make the radius reduction rate at a temperature range between a temperature of less than Ar3 and 750 deg.C or more be 20% or more, and completing radius reduction working at 750 deg.C or more.

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, suspensions, members, and the like, and a method of manufacturing the same. In particular, it is suitable for use in hydroform molding (see Japanese Patent Application Laid-Open No. 10-175027), and can improve the production efficiency of automotive parts during hydroform molding. The steel pipe of the present invention includes both a pipe not subjected to a surface treatment and a pipe subjected to a surface treatment such as hot-dip plating and electroplating for rust prevention. The types of plating include pure zinc, alloys whose main component is zinc, Al
And so on. According to the present invention, since it can be applied to high-strength steel pipes, it is possible to reduce the thickness of parts, and it is considered that the present invention can contribute to global environmental conservation.

[0002]

2. Description of the Related Art With the need to reduce the weight of automobiles, it is desired to increase the strength of steel sheets. By increasing the strength of the steel sheet, it is possible to reduce the weight by reducing the thickness of the steel sheet and to improve the safety at the time of collision. Recently, attempts have been made to form a high-strength steel pipe using a hydroforming method for a part having a complicated shape. This is aimed at reducing the number of parts and reducing the number of welding flanges in response to the need for lighter and lower cost automobiles. As described above, if a new molding method such as hydroform molding is actually adopted, great merits such as cost reduction and design flexibility are expected.

In order to make full use of the merits of such hydroform molding, materials suitable for these new molding methods are required. The present inventors have filed Japanese Patent Application No. 2000-52574.
Has filed an application for an invention relating to a steel pipe excellent in formability in which texture is controlled by diameter reduction processing.

[0004]

In order to obtain a good r value, it is effective to reduce the diameter in the α + γ range or the α range. There is a problem that the tissue remains and the n value decreases. Further, when Ti or Nb is added to increase the strength, this tendency becomes more remarkable. When manufacturing high-strength parts and difficult-to-mold parts using hydroform molding,
There is no doubt that the formability of steel pipes will be more problematic than before. The present invention provides a steel pipe having better formability and a method for stably producing the same.

[0005]

The gist of the present invention is as follows. (1) In mass%, C: 0.0001 to 0.30%, S
i: 0.001 to 2.5%, Mn: 0.01 to 2.5
%, P: 0.005 to 0.20%, S: 0.03% or less, Al: 0.01 to 2.5%, N: 0.01% or less,
And O: 0.01% or less, the formula (1) and the formula (2)
All of the relations determined from the steel components expressed in mass% shown in the equation are satisfied, the balance is composed of iron and unavoidable impurities, and the relation between the tensile strength (TS) and the n value satisfies the equation (3). , And the volume fraction of the ferrite phase is 75% or more,
The average crystal grain size of the ferrite is 10 μm or more.
A steel pipe excellent in formability, characterized in that the area ratio of crystal grains of 5 to 3.0 is 90% or more.

203 ° C + 15.2Ni-44.7Si-104V-31.5Mo + 30Mn + 11Cr + 20Cu-700P-200Al <−20 (1) 44.7Si + 700P + 200Al> 80 (2) n ≧ −0.126 × ln (TS) +0.94 (3) (2) Further, the r value in the longitudinal direction of the steel pipe is 1.0 or more and {110} <
The average value of the X-ray random intensity ratio of the group of 110> to {332} <110> orientations is 2.0 or more, and {111} <112>
Wherein the X-ray random intensity ratio is 1.5 or less. (3) Further, in mass%, Ti: 0.2% or less, Nb:
0.2% or less, B: 0.007% or less, and V: 0.
The steel pipe excellent in formability according to the above (1) or (2), containing one or more of 2% or less of the following. (4) Further, in mass%, Mo: 1% or less, Cu: 2% or less, Ni: 1% or less, Sn: 0.2% or less, Cr: 2.
0% or less, Ca: 0.01% or less, and Mg: 0.5
% Or less, containing one or more of the following (1) to
The steel pipe excellent in formability according to any one of (3). (5) A steel pipe excellent in formability obtained by plating the steel pipe according to any one of the above (1) to (4). (6) In producing the steel pipe according to any one of the above (1) to (5), when reducing the diameter of the mother pipe, 850 ° C.
Heating as described above, performing diameter reduction processing such that the diameter reduction rate in a temperature range of less than Ar ₃ points to 750 ° C. or more is 20% or more,
A method for producing a steel pipe excellent in formability, wherein diameter reduction processing is completed at 750 ° C. or more. (7) The formability according to (6), wherein, in the diameter reduction processing, the diameter reduction of the steel pipe after the diameter reduction processing with respect to the mother pipe is performed to be +5 to -30%. Excellent method of manufacturing steel pipes.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. First, the invention (1) will be described. C: Addition of 0.0001% or more is effective for increasing the strength, but if added excessively, the moldability is deteriorated, so the upper limit is set to 0.
30%. 0.001 to 0.15% is preferable,
0.001 to 0.05% is a more preferable range.

[0008] Si: an important element in the present invention.
That is, since the γ → α transformation temperature is raised and the α + γ2 phase temperature range is expanded, the optimum temperature range for diameter reduction processing shifts to a higher temperature side. Therefore, recrystallization proceeds sufficiently at the time of completion of the diameter reduction processing, and good formability can be obtained. Such an effect is recognized not only for Si but also for Al and P. S
Since i is also an element that increases the mechanical strength at low cost, its amount may be added in consideration of the required strength level and the balance with Al and P. The upper limit is set to 2.5%, because it not only deteriorates the workability and workability but also inhibits the formation of a favorable texture. The lower limit is set to 0.001% because it is difficult to make the lower limit less than this in view of steelmaking technology. 0.3 to 1.2% is a preferable range.

Mn: Since the element is effective for increasing the strength, the lower limit is set to 0.01%. For the purpose of preventing hot cracking caused by S, it is preferable to add Mn / S ≧ 15. However, excessive addition lowers the γ → α transformation temperature and lowers the ductility, so the upper limit is set to 2.
5%. 0.05 to 0.50% is a more preferable range.

P: An important element similar to Si. That is, it has the effect of increasing the γ → α transformation temperature and expanding the α + γ2 phase temperature range. It is also an element effective for increasing the strength. The amount of addition depends on the required strength level,
It may be added in consideration of the balance with Al and Al.
If more than 20% is added, defects occur during hot rolling or diameter reduction processing and formability deteriorates.
Is the upper limit. Further, if the content is less than 0.005%, the steelmaking cost increases, so the lower limit is 0.005%.
0.02 to 0.12% is a more preferable range.

S: It is an impurity and the content is preferably as low as possible. In order to prevent hot cracking, the content is set to 0.03% or less. Preferably it is 0.015% or less. Al: An important element like Si and P. That is,
This has the effect of increasing the γ → α transformation temperature and expanding the α + γ2 phase temperature range. Further, Al hardly changes the mechanical strength and is therefore an effective element for obtaining a steel pipe having relatively low strength and excellent formability. The amount may be added in consideration of the required strength level and the balance with Si and P. However, if added in excess of 2.5%, the plating wettability deteriorates and the alloying reaction proceeds. Is significantly suppressed, so the upper limit is 2.5%. Since 0.01% is necessary as a deoxidizing element, the lower limit is 0.01%. 0.1-1.5% is a more preferable range.

N: An impurity, and the lower the content, the better. Since the workability is deteriorated, the upper limit is made 0.01%. 0.005% or less is a more preferable range. O: When the content is too large, the workability is deteriorated.
01%. Equations (1) and (2) are very important equations in the present invention. That is, equation (1) is determined from the viewpoint that the γ → α transformation point of a steel pipe is made higher than that of pure iron. Equation (2) means that Si, P, and Al are actively used to increase the γ → α transformation point. By simultaneously satisfying the expressions (1) and (2), it is possible for the first time to obtain extremely excellent formability.

203√C + 15.2Ni-44.7Si-104V-31.5Mo + 30Mn + 11Cr + 20Cu-700P-200Al <−20 (1) 44.7Si + 700P + 200Al> 80 (2) The γ → α transformation point is increased to further increase In order to obtain good moldability, the following formulas (1 ′) and (2 ′) are more preferred limiting formulas.

203 ° C + 15.2Ni-44.7Si-104V-31.5Mo + 30Mn + 11Cr + 20Cu-700P-200Al <−50 (1 ′) 44.7Si + 700P + 200Al> 110 (2 ′) n value and tensile strength of the steel pipe of the present invention The strength TS (MPa) is
Equation (3) must be satisfied.

N ≧ −0.126 × ln (TS) +0.94 (3) That is, since the n value which is an index of the formability changes according to the TS, it is necessary to define the n value for each TS. is there. For example, a steel pipe having a TS of 350 MPa must have an n value of about 0.20 or more. More preferably, n ≧ −0.126 × ln (TS) +0.96.

The TS and the n value are measured by a tensile test using a JIS No. 11 tubular test piece or a JIS No. 12 arc-shaped test piece. The n value may be evaluated at 5% and 15% strain, but when the uniform elongation is less than 15%, 5% and 10%
When the uniform elongation is less than 10%, the strain is evaluated at 3% and 5%. Next, the reasons for limitation regarding the organization will be described. The structure of the steel pipe of the present invention is 7
It consists of 5% or more of ferrite phase. If this is less than 75%, good moldability cannot be ensured. 85
% Or more, more preferably 90% or more. Although the effect of the present invention can be obtained even when the volume ratio of the ferrite phase is 100%, it is preferable that the second phase is appropriately dispersed particularly when it is necessary to increase the strength. The second phase other than the ferrite phase comprises one or more of pearlite, cementite, austenite, bainite, acicular ferrite, martensite, carbonitride, and intermetallic compound.

The average crystal grain size of the ferrite is 10 μm or more. If it is less than 10 μm, it is difficult to ensure good ductility. It is more preferably at least 20 μm, further preferably at least 30 μm. The upper limit of the average grain size of the ferrite is not particularly defined, but if it is too large, the ductility is rather deteriorated or the surface becomes rough, so it is preferably 200 μm or less.

The average grain size of ferrite is determined by polishing a section (L section) of a steel sheet parallel to the rolling direction and perpendicular to the sheet surface to a mirror surface, etching with a suitable corrosive solution, and then removing the ferrite by 1/100 of the sheet thickness.
A range of 2 mm ² or more in the range of 8 to 7/8 may be randomly selected and observed, and determined by a point calculation method or the like.
In addition, the ferrite occupies 90% or more by crystal grains having an aspect ratio of 0.5 to 3.0. Since the structure of the steel pipe of the present invention is finally formed by recrystallization, the ferrite structure is sized, and the crystal grains having the above aspect ratio occupy the majority. 95
% Or more, more preferably 98% or more. 10
Even at 0%, the effect of the present invention can be naturally obtained. Further, a more preferable aspect ratio is 0.7 to 2.0.

The aspect ratio is defined as follows. That is, in a section (L section) of a steel sheet parallel to the rolling direction and perpendicular to the sheet surface, a value obtained by dividing the maximum length (X) in the rolling direction by the maximum length (Y) of the crystal grain in the thickness direction ( X
/ Y). The volume ratio of the crystal grains having the above-mentioned aspect ratio range is represented by the area ratio. The area ratio is determined by etching the L section with an appropriate etchant, and thereafter, in a range of 1/8 to 7/8 of the plate thickness. May be randomly selected and observed in a range of 2 mm ² or more, and determined by a point calculation method or the like.

Next, the invention (2) will be described. Although the r-value of the steel pipe changes variously due to the change in texture, the r-value in the longitudinal direction of the steel pipe is preferably 1.0 or more. More preferably, it is 1.5 or more.
Depending on the manufacturing conditions, the r value in the axial direction may exceed 2.5. The r-value anisotropy is not particularly limited. That is, the r value in the axial direction may be smaller than the r value in the circumferential direction or the radial direction, or vice versa.

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 necessarily becomes 1.0 or more in many cases. However, the present invention is clearly distinguished from such a steel pipe in that it has a texture described below and, at the same time, has an r value of 1.0 or more. 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. The most reliable evaluation method is to attach a strain gauge to a JIS No. 12 arc-shaped test piece and measure the r value. Because JIS11 tubular test piece and JIS1
This is because the No. 2 arc-shaped test piece has a different r-value due to the shape of the test piece because the test piece shape is different from the plate material. When a JIS No. 12 arc-shaped test piece is used, it is necessary to perform a tensile test using a chuck having the same shape as the arc of the test piece.

{110} <1 of the sheet surface at a steel sheet 1/2 sheet thickness
The orientation groups of 10> to {332} <110> and [11
1] X-ray random intensity ratio of <112>: An important characteristic value in performing hydroform molding or the like. {110} <110> to {3} when the X-ray diffraction of the plate surface at the plate thickness center position was performed and the intensity ratio of each direction to the random sample was obtained.
The average value in the orientation group of 32 ° <110> was set to 2.0 or more. The main orientations included in this orientation group are {110} <1
10>, {661} <110>, {441} <110
>, {331} <110>, {221} <110>,
{332} <110>.

The steel pipe of the present invention has {443} <110>,
{554} <110> and {111} <110> may also develop, and although these are preferred orientations for hydrated foam forming, they are also orientations generally accepted in cold-rolled steel sheets for deep drawing. I excluded it in the sense of doing it. That is, the steel pipe of the present invention has a group of crystal orientations that cannot be obtained by simply forming a steel pipe by deep seam welding or the like using a deep drawn cold-rolled steel sheet as a material.

In the present invention, there is almost no {111} <112> which is a typical crystal orientation of the high r-value cold rolled steel sheet, and these are 1.5 or less, more preferably less than 1.0. The X-ray random intensity ratio in each of these directions is ｛1
It may be obtained from a three-dimensional texture calculated by a series expansion method based on a plurality of pole figures among the pole figures of {10}, {100}, {211} and {310}. That is, to obtain the X-ray random intensity ratio of each crystal orientation, (110) [1-10] in the φ2 = 45 ° cross section of the three-dimensional texture,
(661) [1-10], (441) [1-10],
(331) [1-10], (221) [1-10],
(332) Represented by the intensity of [1-10].

Incidentally, the texture of the present invention usually has φ
2 = 45 ° cross section has the highest intensity within the range of the above orientation group, and the intensity level gradually decreases as the distance from this orientation group increases. Taking into account the problem of torsion, the problem of the accuracy of preparing an X-ray sample, and the like, the orientation showing the highest intensity may deviate from these orientation groups by about ± 5 ° to 10 °.

{110} <110> to {332} <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>. $ 1
Needless to say, as long as the average strength ratio of the 10 ° <110> to {332} <110> orientation group is 3.0 or more, it is particularly suitable as a steel pipe for hydroforming.

When molding is difficult, it is desirable that the average intensity ratio of the orientation group is 4.0 or more. Other orientations, eg, {001} <110>, {116}
<110>, {114} <110>, {113} <11
The strengths such as 0>, {112} <110>, and {223} <110> vary depending on the manufacturing conditions and are not particularly limited. However, the average strength is preferably 3.0 or less.

When performing X-ray diffraction on a steel pipe, an arc-shaped test piece is cut out from the steel pipe and pressed to form 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, it is preferable to perform the test with as low a strain as possible. 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, the measurement may be performed at a place where there is no segregation zone in the range of ／ to ／ of the sheet thickness. If X-ray measurement is more difficult, a statistically sufficient number of measurements are performed 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 plate thickness or at a plane near the center of the plate thickness.

However, the outer surface of the steel pipe to the plate thickness 1 /
Up to about 4, the texture changes due to shear deformation due to the diameter reduction processing described later, and the above texture requirements may not be satisfied. Note that {hkl} <uvw> means that the crystal orientation perpendicular to the plate surface is <hkl> and the longitudinal direction of the steel pipe is <uv
w>.

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 is 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>: 2 or less, <411>: 2 or less, <211
>: 4 or less, <111>: 8 or less, <332>: 10 or less, <221>: 15.0 or less, <110>: 20.0
Less than.

In the inverse pole figure showing the axial direction,
<110>: 8 or more, all orientations other than the above <110>: 3 or less. Next, the reasons for limiting the components of the invention (3) and the invention (4) will be described. Ti, Nb, B,
V: Since Ti, Nb, and V can each increase the mechanical strength by adding 0.005% or more, they are added according to the required strength level. However, when the addition amount of each exceeds 0.2%, the γ → α transformation point is extremely lowered, the processed structure remains after the diameter reduction processing, and the n value is reduced. Upper limit. Preferably, it is 0.1% or less. B is also added as necessary because addition of 0.0005% or more can refine the structure to increase the strength or increase the grain boundary strength. But,
If it exceeds 0.007%, the γ → α transformation point is extremely reduced, and the processed structure remains after the diameter reduction processing, and the n value decreases. Therefore, the upper limit is 0.007%. 0.005% is a more preferred upper limit.

Mo, Cu, Ni, Cr, Sn: these are strengthening elements, and if necessary, are 0.005% or more, 0.03% or more, 0.01% or more, 0.05% or more, and 0% or more. Add 0.005% or more. Mo, Cu, Ni, C
The upper limits of the amounts of addition of r and Sn are 1%, 2%, 1%, and 2%, respectively, from the viewpoint of securing workability and suppressing cost increase.
% And 0.2%.

Ca, Mg: These are elements that are effective for deoxidation as well as for controlling inclusions.
Add 01% or more, 0.0005% or more. Although the addition of an appropriate amount improves the hot workability, the excessive addition of Ca adversely promotes hot embrittlement, so the upper limit was made 0.01%.
Mg is also effective as a deoxidizing agent, and addition of an appropriate amount improves workability, but excessive addition deteriorates workability and increases costs, so the upper limit was made 0.5%.

As unavoidable impurities, Zn, P
The effects of the present invention are not lost even if b, As, Sb, W, etc. are contained in the range of 0.01% or less, respectively. In addition, in production, following ingot smelting using a blast furnace, converter, electric furnace, etc., various secondary smelting is performed, ingot casting or continuous casting is performed, and 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 the temperature of 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. However, when the heating temperature before the diameter reduction processing is in the α + γ region or the α region, it is preferable that the finishing temperature of hot rolling is in the γ single phase region. Lubrication may be performed for one or more passes of hot rolling. 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. In addition, skin pass rolling,
Cold rolling with a rolling reduction of 95% or less may be performed, or annealing may be performed subsequent to rolling.

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 a local solution heat treatment depending on the required properties, alone or in combination, and in some cases, may be applied a plurality of times. Further enhance the effect. This heat treatment is intended to be applied only to the welded portion and the heat affected zone, and can be performed online or offline during manufacturing.

Next, the invention (6) and the invention (7) will be described. The heating temperature before diameter reduction of the steel pipe is important for obtaining a good n value. This is 850
If the temperature is lower than 0 ° C., the processed structure tends to remain after the completion of the diameter reduction processing, and the n value decreases. When the heating temperature is lower than 850 ° C., if the heating is performed again by an induction heater or the like during the diameter reduction processing, the n value can be secured, but the cost increases. 900 ° C. or higher is more preferable. When a good r value is required, it is preferable that the heating temperature be in the γ single phase region. Although the upper limit of the heating temperature is not particularly set, if the heating temperature is higher than 1200 ° C., excessive scale is generated on the surface of the steel pipe, and not only the surface properties are deteriorated, but also the formability is deteriorated. 1,050 ° C. or lower is a more preferable upper limit. In addition, the heating method is not particularly limited, but it is preferable to perform heating in a short time with an induction heater in order to suppress the formation of scale and maintain good surface properties.

The descaling after the heating is performed as needed with water or the like. In the diameter reduction processing, the diameter reduction rate in a temperature range of less than the Ar ₃ transformation point to 750 ° C. or more is at least 20%.
The procedure is performed as described above. If this diameter reduction ratio is less than 20%,
Not only is it difficult to obtain a good r-value and texture, but also coarse grains are generated and the formability is deteriorated. It is preferably at least 50%, more preferably at least 65%. Although the effect of the present invention can be obtained without particularly setting the upper limit of the diameter reduction rate, it is preferably 90% or less from the viewpoint of productivity. The diameter reduction at Ar ₃ or more may be performed prior to the diameter reduction at less than Ar ₃ point. This makes it possible to obtain a better r value. The completion temperature of diameter reduction is also very important. That is, the lower limit is set to 750 ° C.
If the temperature at which the diameter reduction is completed is lower than 750 ° C., the processed structure tends to remain, and the n value becomes poor. 780 ° C. or higher is more preferable.

The diameter reduction ratio below the Ar ₃ transformation point is as follows:
｛(Diameter of steel pipe immediately before diameter reduction at less than Ar ₃ transformation point-
It is defined as (diameter of steel pipe after completion of diameter reduction) / diameter of steel pipe immediately before diameter reduction at less than Ar ₃ transformation point １００ × 100 (%).
The diameter is reduced so that the plate thickness change rate becomes + 5% to -30%.
If the rate of change of the plate thickness is not in this range, it is difficult to obtain a good texture and r value. -5 to -20% is a more preferable range.

The sheet thickness change rate is defined as {(sheet thickness of mother pipe after diameter reduction is completed-sheet thickness of steel pipe before diameter reduction) / sheet thickness of mother pipe after diameter reduction is completed} × 100 (%). Is done. In addition, the diameter of a steel pipe measures the external shape of a steel pipe. The diameter reduction completion temperature is desirably in the α + γ range. This is because it is necessary for the above-mentioned diameter reduction to be added to the α phase by a certain amount or more in order to obtain a good texture.

It is desirable to provide lubrication at the time of diameter reduction from the viewpoint of improving formability. The diameter reduction processing may be performed by combining a plurality of rolls and passing a multi-pass line, or may be performed using a die.

[0043]

EXAMPLE A hot-rolled steel sheet having the components shown in Table 1 was pickled.
Subsequently, after forming the pipe to an outer diameter of 100 to 200 mm by electric resistance welding, the pipe was heated to a predetermined temperature to reduce the diameter. 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 bulge 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 an index for the formability of the hydroform. Evaluation of the mechanical properties was performed using a JIS No. 12 arc-shaped test piece. Since the r value is affected by the shape of the test piece, a strain gauge was attached to the test piece and evaluated. The X-ray measurement was performed by cutting out an arc-shaped test piece from the steel pipe after diameter reduction and pressing it as a flat plate.
The pole figures of (110), (200), (211), and (310) were measured, and a three-dimensional texture was calculated by a series expansion method using the pole figures, and X of each crystal orientation in a φ2 = 45 ° cross section was calculated. The line random intensity ratio was determined.

Tables 2 and 3 show the heating temperature before diameter reduction processing,
Diameter reduction completion temperature, diameter reduction rate, sheet thickness change rate, tensile strength of steel pipe,
n value, total elongation, ferrite fraction, average crystal grain size, aspect ratio, r value in the axial direction, maximum expansion ratio in hydroform molding, and furthermore, {11
1 {<112>, {110} <110>, {441} <
110>, {221} <110> and {110} <11
The average value of the X-ray random intensity ratio of the orientation group of 0> to {332} <110> is shown. In the examples of the present invention, all have good moldability and the maximum expansion ratio is high, whereas in the examples other than the present invention, the maximum expansion ratio is low.

[0046]

[Table 1]

[0047]

[Table 2]

[0048]

[Table 3]

[0049]

According to the present invention, a texture of a material excellent in formability such as hydroform and a method for controlling the texture are found, and a steel pipe excellent in formability such as hydroform and a method for producing the same are provided.

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[Procedure amendment]

[Submission date] October 23, 2000 (2000.10.
23)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0045

[Correction method] Change

[Correction contents]

Tables 2 and 3 show the heating temperature before diameter reduction processing,
Diameter reduction completion temperature, diameter reduction rate, sheet thickness change rate, tensile strength of steel pipe,
n value, ferrite fraction, average grain size, aspect ratio,
The r value in the axial direction, the maximum expansion ratio in hydroforming, and further, {111} <1 at the center of the thickness of the mother pipe.
12>, {110} <110>, {441} <110
>, {221} <110> and {110} <110>
The average value of the X-ray random intensity ratio of the orientation group of {332} <110> is shown. In the examples of the present invention, all have good moldability and the maximum expansion ratio is high, whereas in the examples other than the present invention, the maximum expansion ratio is low.

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0048

[Correction method] Change

[Correction contents]

[0048]

[Table 3]

Continued on the front page (72) Inventor Nobuhiro Fujita 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division (72) Inventor Yasuhiro Shinohara 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Within the Technology Development Division (72) Inventor Tohru Yoshida 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Inside the Technology Development Division (72) Inventor Natsuko Sugiura 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation F-term in the Technology Development Division (reference)

Claims (7)

[Claims] 1. Mass%, C: 0.0001 to 0.30%, Si: 0.001 to 2.5%, Mn: 0.01 to 2.5%, P: 0.005 to 0. 20%, S: 0.03% or less, Al: 0.01 to 2.5%, N: 0.01% or less, and O: 0.01% or less. 2) Satisfies all the relations obtained from the steel components expressed by mass% shown in the equation, the balance consists of iron and unavoidable impurities, and the tensile strength (T
The relationship between S) and the n value satisfies the expression (3), the volume fraction of the ferrite phase is 75% or more, the average crystal grain size of the ferrite is 10 μm or more, and the aspect ratio of the crystal grains constituting the ferrite is A steel pipe excellent in formability, characterized in that the area ratio of crystal grains having a ratio of 0.5 to 3.0 is 90% or more. 203√C + 15.2Ni-44.7Si-104V-31.5Mo + 30Mn + 11Cr + 20Cu-700P-200Al <−20 (1) 44.7Si + 700P + 200Al> 80 (2) n ≧ −0.126 × ln (TS) +0.94 … (3) 2. The steel pipe according to claim 1, wherein the r value in the longitudinal direction is 1.0 or more, and at least ｛1 in a half plate thickness.
The average value of the X-ray random intensity ratio of the 10｝ <110> ｛{332｝ <110> orientation group is 2.0 or more, and the {111} <
112. The steel tube excellent in formability according to claim 1, wherein the X-ray random intensity ratio of 112> is 1.5 or less. 3. In addition, one or more of Ti: 0.2% or less, Nb: 0.2% or less, B: 0.007% or less, and V: 0.2% or less in mass%. The steel pipe excellent in formability according to claim 1 or 2 containing at least one kind. Further, in mass%, Mo: 1% or less, Cu: 2% or less, Ni: 1% or less, Sn: 0.2% or less, Cr: 2.0% or less, Ca: 0.01 % Or less, and Mg: 0.5% or less, containing one or more of the following:
A steel pipe excellent in formability as described in the item. 5. A steel pipe excellent in formability, wherein the steel pipe according to claim 1 is plated. 6. In producing the steel pipe according to any one of claims 1 to 5, when reducing the diameter of a mother pipe,
Heating to 50 ° C. or more, reducing the diameter in a temperature range of less than the Ar ₃ point to 750 ° C. or more, reducing the diameter to 20% or more, and completing the reducing at 750 ° C. or more. A method for producing a steel pipe having excellent formability. 7. The forming method according to claim 6, wherein, in the diameter reduction processing, the steel pipe is subjected to a diameter reduction processing such that a steel pipe thickness change rate of the steel pipe after the diameter reduction processing becomes +5 to −30%. Method for manufacturing steel pipes with excellent properties.