JP3887155B2 - Steel pipe excellent in formability and manufacturing method thereof - Google Patents

Description

【００４７】
【表２】

【００４８】
【表３】

【００４９】
【発明の効果】
本発明では、ハイドロフォーム等の成形性に優れた材料の集合組織およびその制御方法を見出し、ハイドロフォーム等の成形性に優れた鋼管とその製造方法を提供するものである。[0001]
BACKGROUND 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 for manufacturing the steel pipe. In particular, it is suitable for use in hydroforming (see Japanese Patent Application Laid-Open No. 10-175027) and can improve the production efficiency of automobile parts during hydroforming. The steel pipe of the present invention includes both those not subjected to surface treatment and those subjected to surface treatment such as hot dipping and electroplating for rust prevention. The type of plating includes pure zinc, an alloy whose main component is zinc, and Al. According to the present invention, since it can be applied to a high-strength steel pipe, it is possible to reduce the plate thickness of the component, and the present invention is considered to contribute to global environmental conservation.
[0002]
[Prior art]
Along with the need for lighter automobiles, higher strength of steel sheets is desired. By increasing the strength of the steel plate, it is possible to reduce the weight by reducing the plate thickness and improve the safety at the time of collision. Recently, an attempt has 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 accordance with the need for lighter and lower cost vehicles. In this way, if a new molding method such as hydroform molding is actually adopted, significant advantages such as cost reduction and increased design freedom are expected.
[0003]
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 applied for an invention related to a steel pipe excellent in formability in which a texture is controlled by diameter reduction processing in Japanese Patent Application No. 2000-52574.
[0004]
[Problems to be solved by the invention]
In order to obtain a good r value, diameter reduction processing in the α + γ region or α region is effective. However, in a normal steel, when the diameter reduction processing temperature is slightly lowered, the processed structure remains and the n value is reduced. The problem of deteriorating arises. Further, when Ti or Nb is added to increase the strength, this tendency becomes more prominent. When manufacturing high-strength parts and difficult-to-form parts using hydroform molding, there is no doubt that the formability of steel pipes becomes more problematic than before. The present invention provides a steel pipe with better moldability and a method for stably producing it.
[0005]
[Means for Solving the Problems]
The gist of the present invention is as follows.
(1) By 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, O: 0.01% or less, Ti: 0.2% or less, Nb : 0.2% or less, B: 0.007% or less, V: 0.2% or less, 1 type or 2 types or more, expressed by mass% shown in formulas (1) and (2) The relationship obtained from the components of the steel is satisfied, the balance is made of iron and inevitable impurities, the relationship between the tensile strength (TS [MPa]) and the n value satisfies the formula (3), and the ferrite phase The volume ratio of ferrite is 75% or more, the average crystal grain size of ferrite is 10 μm or more, and among the crystal grains constituting the ferrite, the aspect ratio is 0.5 to 3 Steel pipe excellent in formability, characterized in that 0 of the crystal grains is in the area of 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) By 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, O: 0.01% or less, 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, Mg: 0.5% or less , (1) and (2) satisfy the relationship obtained from the steel components expressed in mass%, and the balance consists of iron and inevitable impurities, and the tensile strength (TS [MPa]) And the value of n satisfy the formula (3), the volume fraction of the ferrite phase is 75% or more, the average crystal grain size of ferrite is 10 μm or more, and the ferrite Of the crystal grains constituting the aspect ratio is excellent in formability, characterized in that 90% or more in the crystal grains area ratio of 0.5 to 3.0 steel.
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)
(3) By 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, O: 0.01% or less, Ti: 0.2% or less, Nb : 0.2% or less, B: 0.007% or less, V: 0.2% or less, one or two or more, 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, Mg: 0.5% or less, containing one or more, (1) and (2) All the relationships obtained from the steel components expressed in mass% shown in the formula are satisfied, the balance is made of iron and inevitable impurities, and the relationship between the tensile strength (TS [MPa]) and the n value is (3) Fill the expression Further, the volume fraction of the ferrite phase is 75% or more, the average crystal grain size of ferrite is 10 μm or more, and among the crystal grains constituting the ferrite, crystal grains having an aspect ratio of 0.5 to 3.0 A steel pipe excellent in formability characterized by having an area ratio of 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)
[0006]
(4) Further, the average of the X-ray random intensity ratios of {110} <110> to {332} <110> orientation groups in which the r value in the longitudinal direction of the steel pipe is 1.0 or more and at least 1/2 plate thickness The value is 2.0 or more, and the X-ray random intensity ratio of {111} <112> is 1.5 or less, and the formability according to any one of (1) to (3) is excellent. Steel pipe.
(5) A steel pipe excellent in formability, wherein the steel pipe according to any one of (1) to (4) is plated.
(6) In manufacturing the steel pipe according to any one of (1) to (5), when reducing the diameter of the mother pipe, it is heated to 850 ° C. or higher, and Ar _Three A steel pipe excellent in formability characterized by performing a diameter reduction process so that the diameter reduction rate in a temperature range of less than the point to 750 ° C. or more is 20% or more, and completing the diameter reduction process at 750 ° C. or more. Production method.
(7) Formability as described in (6) above, wherein in the diameter reduction processing, the diameter reduction processing is performed so that the plate thickness change rate of the steel pipe after the diameter reduction processing with respect to the mother pipe becomes +5 to -30%. Excellent steel pipe manufacturing method.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below. First (1) Reasons for limiting the components of steel used in the invention of (3) and formulas (1) to (3) Will be described.
C: Effective for increasing strength and added in an amount of 0.0001% or more, but if added excessively, the moldability deteriorates, so the upper limit is made 0.30%. 0.001 to 0.15% is preferable, and 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 shifts to the high temperature side. Therefore, recrystallization proceeds sufficiently when the diameter reduction processing is completed, and good moldability can be obtained. Such an effect is recognized not only for Si but also for Al and P. Since Si is an element that increases mechanical strength at a low cost, its addition amount may be added in consideration of the required strength level and balance with Al and P, but excessive addition will wet the plating. The upper limit was set to 2.5% because not only deterioration of the workability and workability but also hindering good texture formation. The reason why the lower limit is set to 0.001% is that it is difficult to make it lower than this in terms of steelmaking technology. 0.3 to 1.2% is a preferable range.
[0009]
Mn: Since the element is effective for increasing the strength, the lower limit was made 0.01%. For the purpose of preventing hot cracking due to S, it is preferable to add so that Mn / S ≧ 15. However, excessive addition lowers the γ → α transformation temperature or lowers the ductility, so the upper limit is made 2.5%. 0.05 to 0.50% is a more preferable range.
[0010]
P: It is an important element like 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 addition amount may be added in consideration of the required strength level and the balance with Si and Al, but if over 0.20% is added, defects may occur during hot rolling or diameter reduction processing. Since formability deteriorates, the upper limit is 0.20%. Further, to make it less than 0.005%, the steelmaking cost becomes high, so 0.005% is made the lower limit. 0.02 to 0.12% is a more preferable range.
[0011]
S: It is an impurity and its content is preferably as low as possible. In order to prevent hot cracking, the content is made 0.03% or less. Preferably it is 0.015% or less.
Al: Si and P are important elements. That is, it has the effect of increasing the γ → α transformation temperature and expanding the α + γ2 phase temperature range. In addition, Al hardly changes the mechanical strength, so it is an element effective for obtaining a steel pipe having a relatively low strength and excellent formability. The addition amount may be added in consideration of the required strength level and the balance with Si and P. However, if over 2.5% is added, the plating wettability deteriorates or the alloying reaction proceeds. Is significantly suppressed, so 2.5% is made the upper limit. Moreover, since 0.01% is necessary as a deoxidizing element, 0.01% is made the lower limit. 0.1 to 1.5% is a more preferable range.
[0012]
N: It is 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: If the amount is too large, the workability deteriorates, so the upper limit is made 0.01%.
Ti, Nb, B, V: Since Ti, Nb, and V can each increase mechanical strength by adding 0.005% or more, they are added according to the required strength level. However, if each addition amount exceeds 0.2%, the γ → α transformation point is extremely lowered and the processed structure remains after the diameter reduction processing, and the n value is lowered. The upper limit. Preferably, the content is 0.1% or less. B is also added as necessary because addition of 0.0005% or more refines the structure to increase the strength or increase the grain boundary strength. However, if it exceeds 0.007%, the γ → α transformation point is extremely lowered, the processed structure remains after the diameter reduction processing, and the n value is lowered, so 0.007% is made the upper limit. 0.005% is a more preferable upper limit.
Mo, Cu, Ni, Cr, Sn: These are strengthening elements, and 0.005% or more, 0.03% or more, 0.01% or more, 0.05% or more, 0.005%, respectively, as necessary. Add more. The upper limit of each addition amount of Mo, Cu, Ni, Cr, and Sn is set to 1%, 2%, 1%, 2%, and 0.2%, respectively, from the viewpoint of securing workability and suppressing cost increase.
Ca, Mg: These are elements effective for deoxidation in addition to inclusion control, and 0.001% or more and 0.0005% or more are added as necessary. Although an appropriate amount improves hot workability, excessive addition of Ca promotes hot embrittlement, so the upper limit was made 0.01%. Mg is also effective as a deoxidizing material, and the addition of an appropriate amount improves the workability, but excessive addition deteriorates the workability and increases the cost, so the upper limit was made 0.5%.
Moreover, even if Zn, Pb, As, Sb, W, etc. are contained in the range of 0.01% or less as inevitable impurities, the effect of the present invention is not lost.
Equations (1) and (2) are very important in the present invention. That is, equation (1) is determined from the viewpoint of making the γ → α transformation point of the steel pipe higher than that of pure iron. The expression (2) means that Si, P and Al are actively used in order to raise the γ → α transformation point. By satisfying the equations (1) and (2) simultaneously, it becomes possible for the first time to obtain extremely excellent moldability.
[0013]
203√C + 15.2Ni-44.7Si-104V-31.5Mo
+ 30Mn + 11Cr + 20Cu-700P-200Al <-20 (1)
44.7Si + 700P + 200Al> 80 (2)
In order to increase the γ → α transformation point and obtain better moldability, the following formulas (1 ′) and (2 ′) are more preferable limiting formulas.
[0014]
203√C + 15.2Ni-44.7Si-104V-31.5Mo
+ 30Mn + 11Cr + 20Cu-700P-200Al <-50 (1 ')
44.7Si + 700P + 200Al> 110 (2 ′)
The n value and the tensile strength TS (MPa) of the steel pipe of the present invention must satisfy the formula (3).
[0015]
n ≧ −0.126 × ln (TS) +0.94 (3)
That is, since the n value, which is an index of formability, changes according to the TS, it is necessary to define the n value for each TS. 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
It is.
[0016]
TS and n value are measured by a tensile test using a JIS No. 11 tubular specimen or a JIS No. 12 arc specimen. The n value may be evaluated at 5% and 15% strain, but when the uniform elongation is less than 15%, the strain is 5% and 10%. When the uniform elongation is less than 10%, 3% and Evaluate with 5% strain.
Next, the reason for limitation regarding the organization will be described. The structure of the steel pipe of the present invention is composed of 75% or more of a ferrite phase. This is because if it is less than 75%, good moldability cannot be secured. It is preferably 85% or more, and more preferably 90% or more. The effect of the present invention can be obtained even when the volume fraction of the ferrite phase is 100%, but it is preferable to disperse the second phase appropriately, particularly when the strength needs to be increased. The second phase other than the ferrite phase is composed of one or more of pearlite, cementite, austenite, bainite, acicular ferrite, martensite, carbonitride, and intermetallic compounds.
[0017]
The average crystal grain size of ferrite is 10 μm or more. If it is less than 10 μm, it is difficult to ensure good ductility. More preferably, it is 20 micrometers or more, More preferably, it is 30 micrometers or more. The upper limit of the average particle diameter of ferrite is not particularly defined, but if it is too large, ductility is rather deteriorated or rough skin is caused.
[0018]
The average grain size of the ferrite is 1/8 to 7/8 of the plate thickness after polishing the cross section (L cross section) of the steel plate parallel to the rolling direction and perpendicular to the plate surface to a mirror surface and etching with a suitable corrosive liquid. 2mm in the range of ² The above range may be selected and observed at random and determined by a point algorithm or the like.
Further, ferrite is 90% or more occupied 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 is preferable, and 98% or more is more desirable. Even at 100%, the effect of the present invention can be obtained. A more preferable aspect ratio is 0.7 to 2.0.
[0019]
The aspect ratio is defined as follows. That is, the value obtained by dividing the maximum length (X) in the rolling direction by the maximum length (Y) in the plate thickness direction of crystal grains in the cross section (L cross section) of the steel plate parallel to the rolling direction and perpendicular to the plate surface ( X / Y). The volume ratio of the crystal grains having the above aspect ratio range is represented by the area ratio. The area ratio is determined by etching the L cross section with an appropriate corrosive solution and then the range of 1/8 to 7/8 of the plate thickness. 2mm in ² The above range may be selected and observed at random and determined by a point algorithm or the like.
[0020]
Next, the invention (4) will be described.
Although the r value of a steel pipe changes variously according to the change in texture, the r value in the longitudinal direction of the steel pipe is preferably 1.0 or more. If it is 1.5 or more, it is more desirable. Depending on manufacturing conditions, the r value in the axial direction may exceed 2.5. The anisotropy of the r value 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, and vice versa.
[0021]
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 is inevitably often 1.0 or more. However, the present invention is clearly distinguished from such a steel pipe in that it has the texture described below and at the same time the r value is 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 test piece. The amount of strain at that time is evaluated with an elongation rate of 15%. When the uniform elongation is less than 15%, the strain amount is evaluated within the range of uniform elongation. In addition, as for a test piece, it is desirable to collect a sample from other than a seam part. The most reliable evaluation method is to measure the r value by attaching a strain gauge to a JIS No. 12 arc specimen. This is because the JIS No. 11 tubular test piece and the JIS No. 12 arc-shaped test piece are different from the plate material in the shape of the test piece, and the r value may be lowered due to the shape. When using a JIS No. 12 arc specimen, it is necessary to perform a tensile test using a chuck having the same shape as the arc of the specimen.
[0022]
{110} <110> to {332} <110> orientation group on the plate surface at 1/2 steel plate thickness, and [111] <112> X-ray random strength ratio: This is an important characteristic value. The average value in the {110} <110> to {332} <110> orientation groups when the X-ray diffraction of the plate surface at the center position of the plate thickness was performed and the intensity ratio of each orientation to the random sample was determined. 2.0 or more. The main orientations included in this orientation group are {110} <110>, {661} <110>, {441} <110>, {331} <110>, {221} <110>, {332} <110>.
[0023]
{443} <110>, {554} <110>, and {111} <110> may also develop in the steel pipe of the present invention, and these are preferred orientations for hide forming, but for deep drawing Since it is also an orientation generally accepted for cold-rolled steel sheets, it was specifically excluded for the purpose of distinction.
That is, the steel pipe of the present invention has a crystal orientation group that cannot be obtained simply by using a deep-drawn cold-rolled steel sheet as a raw material by electric-welding welding or the like.
[0024]
In the present invention, {111} <112>, which is a typical crystal orientation of the high r value cold-rolled steel sheet, is hardly present, and these are 1.5 or less, more preferably less than 1.0. The X-ray random intensity ratio in each direction is a three-dimensional set calculated by a series expansion method based on a plurality of pole figures among {110}, {100}, {211} and {310} pole figures. Find it from your organization. That is, in order to obtain the X-ray random intensity ratio of each crystal orientation, (110) [1-10], (661) [1-10], (441) [1] in the φ2 = 45 ° cross section of the three-dimensional texture −10], (331) [1-10], (221) [1-10], and (332) [1-10].
[0025]
The texture of the present invention usually has the highest intensity within the range of the above azimuth group in the cross section of φ2 = 45 °, and the intensity level gradually decreases as the distance from the azimuth group increases. Taking into account the measurement accuracy problems, the torsion around the axis when manufacturing steel pipes, the accuracy of X-ray sample preparation, etc., the orientation showing the maximum strength deviates from these orientation groups by about ± 5 ° to 10 °. There may be cases.
[0026]
The average X-ray random intensity ratio of the {110} <110> to {332} <110> azimuth group is an arithmetic average of the X-ray random intensity ratios of the above-mentioned respective directions. If all the intensities in the above azimuth cannot be obtained, an arithmetic average of intensity ratios in the {110} <110>, {441} <110>, and {221} <110> orientations may be used instead. Needless to say, the average strength ratio of the {110} <110> to {332} <110> orientation groups is 3.0 or more, particularly as a steel pipe for hydroforming.
[0027]
In addition, when molding is difficult, it is desirable that the average intensity ratio of the orientation group is 4.0 or more. Intensities in other directions, for example, {001} <110>, {116} <110>, {114} <110>, {113} <110>, {112} <110>, {223} <110> However, the average strength is preferably 3.0 or less.
[0028]
When performing X-ray diffraction of 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. In addition, when the arc-shaped test piece is used as a flat plate, it is preferably performed with as low strain as possible in order to avoid the influence of crystal rotation due to processing of the test piece.
The plate-like sample thus obtained 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 buffing, and at the same time the distortion is removed by electrolytic polishing or chemical polishing. Adjust so that the center layer is the measurement surface.
[0029]
In addition, when a segregation band is recognized in the sheet thickness center layer of the steel sheet, it may be measured in a place where there is no segregation band in the range of 3/8 to 5/8 of the sheet thickness. In addition, when X-ray measurement is difficult, a statistically sufficient number of measurements are performed by the EBSP method or ECP method.
As described above, the texture of the present invention is defined by the X-ray measurement result on the surface of the plate thickness center or in the vicinity of the plate thickness center, but it is preferable to have the same texture at plate thicknesses other than the vicinity of the center.
[0030]
However, from the outer surface of the steel pipe to about ¼ of the plate thickness, the texture changes due to shear deformation by the diameter reduction process described later, and the above-mentioned texture requirements may not be satisfied. Note that {hkl} <uvw> means that when an X-ray sample is collected by the above-described method, the crystal orientation perpendicular to the plate surface is <hkl> and the longitudinal direction of the steel pipe is <uvw>. To do.
[0031]
Although the characteristics related to the texture of the present invention cannot be expressed only by a normal reverse pole figure or a positive pole figure, for example, when a reverse pole figure representing a radial orientation of a steel pipe is measured in the vicinity of the center of the plate thickness, 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 or less.
[0032]
Moreover, in the reverse pole figure showing an axial direction, <110>: 8 or more, all azimuth | directions other than said <110>: 3 or less.
[0035]
Furthermore, in the production of steel, ingot casting and continuous casting are performed by smelting in a blast furnace, converter, electric furnace, etc., followed by various secondary smelting. It may be produced by combining production methods such as rolling CC-DR.
[0036]
Needless to say, the cast ingot or cast slab may be reheated for hot rolling. The heating temperature of the hot rolling is not particularly limited as long as it is an appropriate temperature for realizing the target finishing temperature. The finishing temperature of hot rolling may be 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 the α + γ region or the α region, it is preferable that the hot rolling finishing temperature is a 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 finish hot rolled continuously. The rough rolled bar may be wound once or unwound and then subjected to finish hot rolling. The cooling rate and coiling temperature after hot rolling are not particularly limited. It is desirable to pickle after hot rolling. Further, skin pass rolling or cold rolling with a reduction rate of 95% or less may be performed, and annealing may be performed following the rolling.
[0037]
In manufacturing the steel pipe, electric seam welding is usually used, but welding and pipe making techniques such as TIG, MIG, laser welding, UO and forge welding can also be used. In the production of these welded steel pipes, local solution heat treatment may be applied to the heat affected zone according to the required properties, either alone or in combination, and in some cases, it may be repeated several times. Further enhance the effect. This heat treatment is intended to be applied only to the weld zone and the weld heat affected zone, and can be performed online or offline at the time of manufacture.
[0038]
Next, the invention (6) and the invention (7) will be described.
The heating temperature before reducing the diameter of the steel pipe is important for obtaining a good n value. When the temperature is less than 850 ° C., the processed structure tends to remain after the diameter reduction processing is completed, and the n value decreases. When the heating temperature is less than 850 ° C., if the heating is performed again by an induction heater or the like in the middle of the diameter reducing process, the n value can be secured, but the cost increases. 900 degreeC or more is more preferable. When a good r value is required, it is preferable that the heating temperature is a γ single phase region. Although the upper limit of the heating temperature is not particularly provided, when the heating temperature is higher than 1200 ° C., scale is excessively generated on the surface of the steel pipe, and the surface properties are deteriorated and the formability is also deteriorated. 1050 ° C. or lower is a more preferable upper limit. Further, the heating method is not particularly limited, but in order to suppress the generation of scale and to keep the surface property good, it is preferable to heat with an induction heater in a short time.
[0039]
Descaling after heating is appropriately performed as necessary with water or the like.
The diameter reduction process is Ar _Three The reduction in diameter in the temperature range below the transformation point to 750 ° C. or higher is performed to be at least 20% or more. When the diameter reduction ratio is less than 20%, it is difficult to obtain a good r value and texture, and coarse grains are generated and formability is deteriorated. 50% or more is preferable, and 65% or more is more preferable. 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. Ar _Three Prior to diameter reduction below the point, Ar _Three The above diameter reduction may be performed. This makes it possible to obtain a better r value. The completion temperature of the diameter reduction processing is also extremely important. That is, the lower limit is set to 750 ° C. When the temperature at which the diameter reduction is completed is less than 750 ° C., the processed structure tends to remain, and the n value becomes poor. 780 ° C. or higher is more preferable.
[0040]
Ar _Three The diameter reduction ratio below the transformation point is {(Ar _Three The diameter of the steel pipe just before the diameter reduction processing below the transformation point-the diameter of the steel pipe after the completion of the diameter reduction) / Ar _Three The diameter of the steel pipe immediately before the diameter reduction processing below the transformation point} × 100 (%).
The diameter is reduced so that the plate thickness change rate is + 5% to −30%. If the change rate of the plate thickness is not within this range, it is difficult to obtain a good texture and r value. -5 to -20% is a more preferable range.
[0041]
The plate thickness change rate is defined by {(plate thickness of the main pipe after completion of diameter reduction-plate thickness of the steel pipe before diameter reduction processing) / plate thickness of the main pipe after completion of diameter reduction} × 100 (%).
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 region. This is because it is necessary to obtain a good texture that the above-mentioned diameter reduction processing is added to the α phase by a certain amount or more.
[0042]
In addition, it is desirable to lubricate when the diameter is reduced from the viewpoint of improving formability.
The diameter reduction processing may be performed by combining a plurality of rolls and passing through a multi-stage pass line, or may be performed by drawing with a die.
[0043]
【Example】
The hot-rolled steel sheet having the components shown in Table 1 was pickled and subsequently piped to an outer diameter of 100 to 200 mm by electric-welding welding, and then heated to a predetermined temperature to perform diameter reduction processing.
The workability of the obtained steel pipe was evaluated by the following method. In advance, a scribed circle of 10 mmφ was transferred to the steel pipe, and the inner pressure and the axial push amount were controlled to perform the overhang forming in the circumferential direction. Strain εΦ in the axial direction and strain εθ in the circumferential direction of the portion showing the maximum tube expansion rate immediately before the burst (tube expansion rate = maximum circumferential length after molding / circumferential length of the mother tube) were measured.
[0044]
The ratio of these two strains ρ = εΦ / εθ and the maximum tube expansion ratio were plotted, and the tube expansion ratio Re at which ρ = −0.5 was used as the formability index of the hydroform. The mechanical properties were evaluated using JIS No. 12 arc specimens. Since the r value is affected by the shape of the test piece, a strain gauge was attached to the test piece for evaluation. The X-ray measurement was performed by cutting out an arc-shaped test piece from the steel pipe after the diameter reduction and pressing it into a flat plate. Each pole figure of (110), (200), (211), (310) is measured, and using these, a three-dimensional texture is calculated by the series expansion method, and X of each crystal orientation in the φ2 = 45 ° cross section is calculated. The line random intensity ratio was determined.
[0045]
Tables 2 and 3 show heating temperature before diameter reduction processing, diameter reduction completion temperature, diameter reduction rate, plate thickness change rate, steel pipe tensile strength, n value, ferrite fraction, average crystal grain size, aspect ratio, axis Direction r value, maximum tube expansion ratio in hydroforming, and {111} <112>, {110} <110>, {441} <110>, {221} <110> at the center of the thickness of the mother pipe And {110} <110> to {332} <110>, the average value of the X-ray random intensity ratios of the orientation group. The examples of the present invention all have good moldability and the maximum tube expansion rate is high, whereas the examples outside the present invention have a low maximum tube expansion rate.
[0046]
[Table 1]

[0047]
[Table 2]

[0048]
[Table 3]

[0049]
【The invention's effect】
In the present invention, a texture of a material excellent in formability such as hydroform and a control method thereof are found, and a steel pipe excellent in formability such as hydroform and a manufacturing method thereof are provided.

Claims (7)

% By 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 ,
O: 0.01% or less,
Further,
Ti: 0.2% or less,
Nb: 0.2% or less,
B: 0.007% or less,
V: 0.2% or less,
1 or 2 or more, satisfying both of the relationships obtained from the steel components expressed by mass% shown in the formulas (1) and (2), the balance consisting of iron and inevitable impurities, In addition, the relationship between the tensile strength (TS [MPa] ) and the n value satisfies the formula (3), the volume fraction of the ferrite phase is 75% or more, the average grain size of the ferrite is 10 μm or more, A steel pipe excellent in formability, wherein crystal grains having an aspect ratio of 0.5 to 3.0 are 90% or more in area ratio among the constituting crystal grains.
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) % By 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,
O: 0.01% or less,
Further,
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,
Mg: 0.5% or less,
1 or two or more of the above, satisfying both of the relationships obtained from the steel components expressed by mass% shown in the formulas (1) and (2), the balance consisting of iron and inevitable impurities, In addition, the relationship between the tensile strength (TS [MPa]) and the n value satisfies the formula (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, A steel pipe excellent in formability, wherein crystal grains having an aspect ratio of 0.5 to 3.0 are 90% or more in area ratio among the constituting crystal grains.
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) % By 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,
O: 0.01% or less,
Further,
Ti: 0.2% or less,
Nb: 0.2% or less,
B: 0.007% or less,
V: 0.2% or less,
One or more of and
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,
Mg: 0.5% or less,
1 or two or more of the above, satisfying both of the relationships obtained from the steel components expressed by mass% shown in the formulas (1) and (2), the balance consisting of iron and inevitable impurities, In addition, the relationship between the tensile strength (TS [MPa]) and the n value satisfies the formula (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, A steel pipe excellent in formability, wherein crystal grains having an aspect ratio of 0.5 to 3.0 are 90% or more in area ratio among the constituting crystal grains.
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)   Furthermore, the r value in the longitudinal direction of the steel pipe is 1.0 or more, and the average value of the X-ray random intensity ratios of the {110} <110> to {332} <110> orientation groups at least 1/2 sheet thickness is 2 The steel pipe excellent in formability according to any one of claims 1 to 3, wherein the X-ray random intensity ratio of {111} <112> is 1.5 or more. A steel pipe excellent in formability, wherein the steel pipe according to any one of claims 1 to 4 is plated. In producing the steel pipe according to any one of claims 1 to 5, when reducing the diameter of the mother pipe, it is heated to 850 ° C or higher, and the diameter is reduced in a temperature range of less than Ar ₃ point to 750 ° C or higher. A method for producing a steel pipe excellent in formability, characterized in that the diameter reduction is performed so that the rate is 20% or more and the diameter reduction is completed at 750 ° C. or more.   The steel pipe excellent in formability according to claim 6, wherein in the diameter reduction processing, a diameter reduction processing is performed so that a plate thickness change rate of the steel pipe after the diameter reduction processing is +5 to -30% with respect to the mother pipe. Manufacturing method.