JP2001279330A - High strength cold rolled steel sheet and steel tube excellent in formability and producing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique for improving the r-value in the L direction which has been the point at issue in a high strength cold rolled steel sheet which can not only be applied for a high strength cold rolled steel sheet, but is also suitable for a steel tube for hydroform. SOLUTION: Si and Mn are added to extra-low carbon steel in which a fixed amount or more of Ti is added so as to satisfy Si/Mn=0.5 to 2.0, and also, the content of P is controlled to <=0.02%, the high strength cold rolled steel sheet high in the r-value in the L direction can be obtained. Moreover, by making this into a steel tube, the high strength steel tube high in the r-value in the axial direction can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet and a steel pipe used for, for example, panels, undercarriages, members and the like of an automobile, and a method of manufacturing the same. In the case of steel pipes, in particular, hydroform molding (JP-A-10-175027)
No. 1). The cold-rolled steel sheet and the steel pipe of the present invention include both those not subjected to surface treatment and those subjected to surface treatment such as hot-dip galvanizing and electroplating for rust prevention. Zinc plating includes plating of an alloy whose main component is zinc, in addition to pure zinc. The cold-rolled steel sheet and the steel pipe according to the present invention have both strength and workability, and can be used in a smaller thickness than conventional steel sheets, that is, the weight of the vehicle body can be reduced. Therefore, it is considered that it 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, it is possible to reduce the weight by reducing the plate thickness and to improve the safety in the event of a collision. Also,
Recently, attempts have been made to form a complex-shaped portion from a high-strength steel plate or a steel pipe by a hydroforming method. 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 forming method such as hydroforming is actually adopted, great merits such as reduction of costs and expansion of design freedom are expected. In order to fully utilize the merits of such hydroform molding, materials suitable for these new molding methods are required. For example,
The effect of r value on hydroform forming is shown in the 50th Joint Lecture on Plastic Working (1999, p. 447). According to this, it is shown that it is important that rL (r value in the rolling direction, hereinafter also referred to as rL) is high when an axial force acts during hydroforming. The L direction of the r value is rL and rC for a soft drawn steel sheet for deep drawing.
The difference in the r-value of the so-called V-shape is lower in rD (r-value in the 45 ° direction in the plane with respect to the rolling direction RD) than in r-value in the 90 ° direction in the plane with respect to the rolling direction RD. On the other hand, it has been pointed out that rL of the high-strength cold-rolled steel sheet reinforced with Mn and P exhibits a special anisotropy lower than that of rC or rD (the fourth International Con
ference on Recrystallization and Related Phenomen
a, Tsukuba, JIM, (1999), p. 393.). According to the document, it is stated that, even in a high-strength cold-rolled steel sheet, it is possible to slightly increase rL than rD by strictly restricting hot rolling conditions and the like, but the effect is small, and
Manufacturing conditions are also special and are not realistic.
Further, the present inventors have disclosed in Japanese Patent Application No. 12-52574 that rL
Has filed an application for a technology to provide high-quality steel pipes. However, since this is manufactured by diameter reduction processing in a warm state, there is a problem that the manufacturing cost is high.

[0003]

As described above, the existing high-strength cold-rolled steel sheet has a low rL, and therefore it is urgently necessary to improve it. Further, even when a high-strength cold-rolled steel sheet is welded by electric resistance welding to form a steel pipe and is used for hydroforming, it is essential to improve rL. The present invention has been achieved as a result of intensive studies in order to solve a new problem of improving rL, which occurred while a high-strength steel plate was in the spotlight.

An object of the present invention is to provide a high-strength cold-rolled steel sheet and a steel pipe having excellent rL, which cannot be obtained by a conventional high-strength cold-rolled steel sheet, and a method of manufacturing them without increasing costs. I do.

[0005]

The present invention relates to a high-strength cold-rolled steel sheet and a steel pipe having a high rL and a method therefor. In particular, the steel pipe according to the present invention is excellent in hydroform formability. The gist of the present invention is as follows: (1) By weight%, C = 0.010% or less, Si = 0.2 to 2.5%, Mn = 0.4
2.5%, Si / Mn = 0.5-2.0, P = 0.02% or less, S = 0.015% or less, Al = 0.005-0.2%, N = 0.0070% or less, Ti = (3.4N + 4C) ~
Contains 0.20%, has a chemical composition consisting of the balance Fe and unavoidable impurities, and has a tensile strength of 320 MPa or more and 600 MPa or less,
A high-strength cold-rolled steel sheet, wherein the r value (rL) in the rolling direction (RD) of the steel sheet is 1.3 or more. (2) The high-strength cold-rolled steel sheet according to (1), wherein Nb = 0.001 to 0.019% by weight. (3) The high-strength cold-rolled steel sheet according to the above (1) or (2), wherein the content of B is 0.0001 to 0.0009% by weight. (4) V = 0.002 to 0.04%, W = 0.002 to 0.05% by weight
, Mo = 0.003-0.25%, Sn: 0.002-0.5%, Cu: 0.003-0.5
%, Cr: 0.005 to 2.0%, Ni: 0.005 to 0.3%, Ca: 0.0002
The composition according to any one of the above (1) to (3), wherein one or more of Zr: 0.002 to 0.02%, Mg: 0.0005 to 0.02% are contained. Strength cold rolled steel sheet. (5) A high-strength cold-rolled steel sheet, wherein the zinc plating according to any one of the above (1) to (4) is applied. (6) It has the chemical composition and tensile strength described in any one of the above (1) to (4), and has an r value (r
φ) is 1.3 or more. (7) A steel pipe obtained by subjecting the steel pipe according to (6) to galvanization. (8) The slab having the chemical composition according to any one of the above (1) to (4) is hot-rolled, and then the reduction ratio is 80.
%, And cold-rolled to 650 ° C to 1100 ° C
A method for producing a high-strength cold-rolled steel sheet, comprising annealing at the following temperature. (9) The slab having the chemical composition according to any one of the above (1) to (4) is hot-rolled, and then the reduction is 75%.
A method for producing a high-strength hot-dip galvanized cold-rolled steel sheet, comprising performing cold rolling so as to be as follows, annealing at a temperature of 650 ° C. to 1100 ° C., and further performing electrogalvanizing or hot-dip galvanizing. (10) Using the cold-rolled steel sheet manufactured by the method according to the above (8) or (9) as a raw material, electric resistance welding or forging is performed such that the RD direction of the cold-rolled steel sheet matches the φ direction of the steel pipe. Method of manufacturing forged steel pipe. It is in. (11) A method for producing a steel pipe, wherein the steel pipe produced by the method according to the above (10) is electrogalvanized or hot-dip galvanized.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. Since C degrades the r value, it is better that C is small. C is 0.
If it exceeds 01%, a large amount of Ti is required to detoxify it, which not only increases the cost but also deteriorates the elongation. Therefore, more preferably, the upper limit is 0.0040%. The lower limit is not particularly limited, but from the viewpoint of steelmaking technology, 0.0005% is a practical lower limit.

[0007] Si is important in the present invention. A new fact has been found that rL, which is inferior to ordinary high-strength cold-rolled steel sheets, is dramatically improved by utilizing Si. In order to exert such an effect of Si, it is necessary to add at least 0.2%. Further, Si is an element that increases strength at a low cost. The amount of addition varies depending on the intended strength level, but in the present invention, since P is not basically used as described later, in order to obtain a tensile strength of 320 MPa or more, the addition of 0.2% or more is required. Required. 0.5%
Is a more preferred lower limit. If the addition amount exceeds 2.5%, ductility decreases, so this is made the upper limit. Further, the chemical conversion property may be reduced. Therefore, 2.0% is a more desirable upper limit. When hot-dip galvanizing is performed, problems such as a reduction in plating adhesion and a reduction in productivity due to a delay in alloying reaction occur. Therefore, the content is preferably 1.0% or less.

Mn is a solid solution strengthening element effective for increasing the strength. Mn is an element that does not essentially deteriorate rL. That is, as a result of a detailed study, Mn was found to coexist with a large amount of P, and
It was clarified that it deteriorated. Mn is MnS
To suppress the edge cracking due to S during hot rolling. Therefore, 0.4% or more is added. On the other hand, if it exceeds 2.5%, ductility is deteriorated and rL is also reduced. 0.7%
A range of at least 2.0% is a more preferable range.

It has been found that adding P in excess of 0.02% significantly reduces rL, so the upper limit is made 0.02%. Although the reason for this is not necessarily clear, it is considered that the solid solution P segregates and affects the orientation selection during recrystallization. On the other hand, if P exceeds 0.02%, the strength of the welded portion when welding such as electric resistance welding when manufacturing steel pipes, spot welding when used as automobile parts, and arc welding may be insufficient. In addition, the fatigue strength of the weld is significantly reduced. Further, it may cause a burst during hydroform molding. Therefore, 0.01% is a more preferred upper limit.

[0010] Si / Mn is important in the present invention.
That is, in order to realize good rL, both Si and Mn must be added simultaneously and in an appropriate balance, and the range is Si / Mn = 0.5 to 2.0. Although the reason why the optimum value exists in Si / Mn is not necessarily clear, if the balance between the Si amount and the Mn amount is inappropriate, the crystal grain size of the hot-rolled sheet becomes excessively fine or conversely coarse. It is presumed that it is caused by the loss. Furthermore, Si
/ Mn is also important for the mechanical properties of the weld when welding such as ERW when manufacturing a steel pipe or spot welding or arc welding when used as an automobile member. That is, if the Si / Mn ratio is inappropriate, the welded portion becomes too hard or too soft, resulting in deterioration of the strength and fatigue strength of the welded portion and also causes a burst during hydroforming. .

If S exceeds 0.015%, it causes hot cracking and deteriorates workability, so the upper limit is 0.015%. Preferably, it is 0.01% or less. Al is used for the preparation of deoxidation, but if it is less than 0.005%, its effect is insufficient. On the other hand, 0.
If it exceeds 2%, the cost is increased and the surface properties are deteriorated. Therefore, the upper limit is set to 0.2%.

If the amount of N is too large, a large amount of Ti and Al is required for fixing N, and the workability is deteriorated. Therefore, the upper limit is set to 0.0070%. Ti is an element that is extremely effective in increasing the r value, and is therefore actively added. It is necessary to add at least (3.4N + 4C)% corresponding to the amount necessary for fixing C and N. on the other hand,
Even if it is added in excess of 0.20%, no particular effect can be expected, the cost is increased, and the surface properties and the powdering properties after hot-dip galvanizing are degraded and the ductility is degraded. To solve the problem of increasing rL, Ti is added to {(3.4N + 4C) +0.01}% to 0.1%.
10% is a more preferred range.

Nb also has the effect of increasing the r value like Ti, but in the case of using Mn and Si to increase the strength as in the present invention, the amount of Nb is not added or only a small amount is added. Was found to be essential. That is, when Nb is added, 0.001 to 0.019%
Must be within the range. When Nb is less than 0.001%, no particular effect is exhibited, and when Nb is 0.01% or less.
When more than 19% is added, the average value of r values (rL + rC
(+ 2rD) / 4 hardly changed, but rL was found to be significantly reduced. Therefore, this is the upper limit. 0.002-0.009% is a more appropriate range for maintaining or improving rL. Also,
Nb also has the effect of improving the joint strength and fatigue strength of the weld.

B may be added because it has an effect of suppressing the brittleness of secondary working of a steel plate or a steel pipe and improving the joining strength and fatigue strength of a welded portion. In order to exert such an effect, it is necessary to add at least 0.0001%. on the other hand,
If the amount of B is too large, not only does the ductility of the steel sheet or steel pipe deteriorate, but also the rL, which is particularly important in the present invention, decreases. Therefore, the upper limit is made 0.0009%. 0.0002-
0.0006% is a more appropriate range for rL.

V, W, Mo, Sn, Cu, C
V = 0.002 to 0.04%, W = 0.002 to 0.05% for r, Ni, Ca, Zr, Mg
, Mo = 0.003-0.25%, Sn: 0.002-0.5%, Cu: 0.003-0.5
%, Cr: 0.005 to 2.0%, Ni: 0.005 to 0.3%, Ca: 0.0002
-0.02%, Zr: 0.002-0.04%, Mg: 0.0005-0.02%. V, W, Zr, and Mo have the effect of increasing the r value, but if added too much, conversely impair the r value, especially rL, so that the above range is set. Sn, C
Since u, Cr, and Ni are effective for increasing the strength and do not significantly impair the r value within the above range, they may be added in the above range. Since Ca forms Ca sulfide,
MnS is reduced, ductility is improved, and it is also effective as a deoxidizing element. On the other hand, if it is too much, the surface properties are degraded and the cost is increased, so that 0.0002 to 0.02%
Range. Mg is effective as a deoxidizing element and improves the workability by improving the solidification structure. On the other hand, if it is too much, the cost increases, so the content is made 0.0005 to 0.02%.

[0016] The tensile strength of the cold rolled steel sheet or the steel pipe obtained by the present invention is from 320 MPa to 600 MPa. When the tensile strength is less than 320 MPa, a good rL can be obtained without taking any special measures, and thus is out of the scope of the present invention. If the tensile strength exceeds 600 MPa, the ductility is extremely reduced and the rL is also reduced. Especially 380-550MP tensile strength
In the range of a, it becomes very difficult to improve rL, so that the usefulness of the present invention becomes extremely remarkable. The evaluation of the tensile strength may be performed using a JIS No. 5 test piece in the case of a cold-rolled steel sheet, or in the case of a steel pipe, using a JI from a steel pipe.
An S12 arc-shaped test piece may be cut out and evaluated, or a JIS No. 11 test piece may be evaluated as a tube. further,
RL of cold rolled steel sheet and r of steel pipe obtained by the present invention
φ is 1.3 or more, and 1.5 or more is more suitable for hydroform molding.

Although the effects of the present invention can be obtained without specifying the upper limits of rL and rφ, if these are too high, the r value in other directions will be extremely low, so that rL is 5.0.
Hereinafter, it is preferable that rφ be 5.0 or less. next,
The reason for limiting the manufacturing conditions will be described. The slab to be subjected to hot rolling is not particularly limited. That is, it may be any one manufactured with a continuous cast slab or a thin slab caster. It is also suitable for processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting. After the rough rolling in the hot rolling, the sheet bars may be joined and the hot finish rolling may be continuously performed.

The heating temperature of the hot rolling is not particularly limited, but is set to 900 ° C. or more to reduce the deformation resistance at the time of hot rolling, and 1350 ° C. or less to suppress excessive generation of surface scale. Is preferred. The finishing temperature of hot rolling is not particularly limited. That is, ordinary Ar3
It may be carried out in a γ-phase single-phase region at or above the transformation temperature, or in an α + γ2-phase region or an α-single-phase region below the Ar 3 point. In any case, lubrication may be performed. Especially α + γ2
When hot rolling is performed in the phase region or α single phase region, lubrication increases the rL of the product.

Cooling after hot rolling is not limited, but when hot rolling is performed at an Ar3 point or higher, the average cooling rate from the hot rolling finishing temperature to 550 ° C. is less than 50 ° C./s. Is desirable to increase the rL of the product. The winding temperature is not particularly limited, but is desirably 650 to 800 ° C. This promotes the formation and growth of TiC, and secures a good rL.

[0020] Cold rolling is important in the present invention.
When the cold rolling reduction exceeds 80%, rL is remarkably reduced. Although the lower limit is not particularly limited, it is preferable to perform cold rolling at a rolling reduction of about 30% in order to make rL 1.3 or more. 40-75% is a more desirable range. As the annealing temperature of the continuous galvanizing equipment of the continuous annealing or the in-line annealing method, a temperature at which the recrystallization fraction becomes 90% or more in the range of 650 to 1100 ° C is selected. If the annealing temperature is lower than 650 ° C., recrystallization does not proceed sufficiently, so that not only the ductility is deteriorated, but also the rL is far below 1.3. If the annealing temperature exceeds 1100 ° C., the productivity is impaired and the cost increases, so this is made the upper limit.
The annealing temperature may be in the α + γ2 phase region in addition to the normal α single phase region. Further, when Mn is added in an amount of 1% or more, even if annealing is performed in the γ single phase region, a good r value is secured, so annealing in the γ single phase region may be performed. Annealing may be box annealing in addition to the continuous annealing described above.

The cooling conditions after annealing are not particularly limited. However, when the annealing temperature is in the α + γ2 phase region or the γ single phase region, it is easier to ensure ductility if the cooling rate is less than 50 ° C./s. The overaging treatment after cooling does not affect the formation of the texture, and may be performed as needed.

When electrogalvanizing is performed, a Zn, Zn—Ni, Zn—Fe layer or the like is deposited in an electroplating cell. In the case of continuous hot-dip galvanizing, it is immersed in a plating bath after cooling, and if it is necessary to alloy the zinc plating with Fe, heat treatment is performed at a temperature of 460 to 600 ° C. for 1 second or more.
If the temperature is lower than 460 ° C, the alloying does not proceed sufficiently.If the temperature is higher than 600 ° C, the alloying proceeds excessively, which causes problems such as powdering when forming a member for automobiles by press working. Range. The alloying time is not particularly limited, but is preferably within 60 seconds from the viewpoint of production efficiency.

After annealing, skin pass rolling may be performed to correct the shape and secure aging resistance. The cold rolled steel plate manufactured in this way may be welded to form a steel pipe. In the production of steel pipes, electric resistance welding or forging welding is suitable, but other welding and pipe forming methods such as TIG, MIG, laser welding, UO and forging welding can 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 may be repeated a plurality of times in some cases, further enhancing the effects of the present invention. Enhance. 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. In addition, an electric furnace or induction heating can be used for heating below the Ac ₃ point in the diameter reduction, and the diameter can be reduced using a stretch reducer or the like. Further, it is desirable to provide lubrication at the time of diameter reduction from the viewpoint of improving formability. If the elongation or n-value is deteriorated as compared with the cold-rolled steel sheet before the steel sheet is manufactured by work hardening due to pipe forming or diameter reduction, the strain relief annealing in the α region and the heat treatment at _one or more points of Ac are appropriately performed. You may go.

The steel pipe may be subjected to electrogalvanizing or galvanizing as required. The cold rolled steel sheet obtained by the present invention has high rL of 1.3 or more and excellent press formability while having high strength. Further, since the steel pipe also has a high rφ, it is suitable for, for example, hydroforming. Next, the present invention will be described with reference to examples.

[0025]

<Example 1> A steel having a composition shown in Table 1 was melted.
Slab heating temperature 1250 ° C, finishing temperature Ar _ThreePoint
Top, hot rolled at a winding temperature of 700 ° C and a 6.9mm thick steel strip
did. After pickling, cold-rolled at a rolling reduction of about 68% to 2.2 mm
Thick cold-rolled sheet, then heated at 10 ° C / s with continuous annealing equipment
After heating and annealing at various temperatures for 60 seconds, the annealing temperature is reduced to 550
The average cooling rate to 15 ° C was cooled at 15 ° C / s. Got
A JIS No. 5 tensile test piece was sampled from a cold-rolled steel sheet and the r value (10% or
15% tensile). Also, tensile strength, yield strength, total elongation
Evaluated using JIS No. 5 tensile test pieces.

The results are shown in Table 2. As is clear from Table 2, the steel of the present invention shows a good rL, but the comparative steel outside the range of the present invention had an rL of less than 1.3.

[0027]

[Table 1]

[0028]

[Table 2]

<Example 2> Using steels A, I and K shown in Table 1,
As shown in Table 3, the cold rolling reduction was changed variously. The annealing conditions were all maintained at 830 ° C. for 50 seconds, and the heating and cooling rates at that time were in accordance with Example 1. Note that A was hot-dip galvanized, I was electroplated Zn-Ni, and K was electroplated Zn-Fe. JIS No. 5 tensile test pieces were collected from these galvanized cold-rolled steel sheets, and the r value was measured. As is clear from Table 3, when the cold rolling reduction is within the range of the present invention, a high rL is obtained, whereas when cold rolling is performed under conditions outside the range of the present invention, the rL deteriorates. I understand.

[0030]

[Table 3]

Example 3 2.2 m produced in Example 1
A cold-rolled steel sheet having a thickness of m was formed using ERW to have an outer diameter of 50 to 100 mm. Thereafter, annealing was performed at 800 ° C. to remove distortion. The hydroform molding was performed under the conditions of a shaft pushing amount of 1 mm and 100 bar / mm until a burst was reached. Table 4 shows the mechanical properties of the ERW welded steel pipes of each steel and the pipe expansion rate up to the burst in hydroforming (= maximum diameter at the time of the burst / diameter of the original pipe).

[0032]

[Table 4]

[0033]

The present invention provides a technique for improving the r value in the L direction, which has been a problem of a high-strength cold-rolled steel sheet. The present invention can be applied not only to high-strength cold-rolled steel sheets but also to steel pipes, and is also suitable as a steel pipe for hydroforming.

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Nobuhiro Fujita 20-1 Shintomi, Futtsu-shi, Chiba F-term in the Technology Development Division, Nippon Steel Corporation 4K037 EA01 EA02 EA04 EA09 EA11 EA13 EA14 EA15 EA16 EA17 EA18 EA19 EA20 EA23 EA25 EA27 EA28 EA31 EA32 EA33 EA35 EB02 EB03 FA01 FA02 FA03 FC07 FD01 FD02 FD03 FE03 FE05 FJ04 FJ05 FJ06 FJ07 GA05

Claims (11)

[Claims] (1) In mass%, C = 0.010% or less, Si = 0.2-2.5%
%, Mn = 0.4-2.5%, Si / Mn = 0.5-2.0, P = 0.02% or less,
S = 0.015% or less, Al = 0.005-0.2%, N = 0.0070% or less, Ti =
(3.4N + 4C) ~ 0.20%, with chemical composition consisting of balance Fe and unavoidable impurities, tensile strength of 320MPa or more
A high-strength cold-rolled steel sheet, wherein the r value (rL) in the rolling direction (RD) of the steel sheet is not more than 1.3 MPa. 2. The high-strength cold-rolled steel sheet according to claim 1, comprising Nb = 0.001 to 0.019% by mass%. 3. The high-strength cold-rolled steel sheet according to claim 1, wherein B = 0.0001 to 0.0009% by mass%. 4. In mass%, V = 0.002 to 0.04%, W = 0.002
~ 0.05%, Mo = 0.003-0.25%, Sn: 0.002-0.5%, Cu: 0.0
03-less than 0.5%, Cr: 0.005-2.0%, Ni: 0.005-0.3%, Ca:
0.0002 to 0.02%, Zr: 0.002 to 0.04%, Mg: 0.0005 to 0.0
The high-strength cold-rolled steel sheet according to any one of claims 1 to 3, wherein the steel sheet contains one or more of 2%. 5. A high-strength cold-rolled steel sheet, wherein the high-strength cold-rolled steel sheet according to claim 1 is galvanized. 6. It has the chemical composition and tensile strength according to claim 1, and has an r value (r) in an axial direction (φ direction).
φ) is 1.3 or more. 7. A steel pipe according to claim 6, wherein the steel pipe is galvanized. 8. A slab having the chemical composition according to claim 1, which is hot-rolled, and then the slab has a reduction ratio of 8%.
0% or less, cold-rolled to 650 ° C or more
A method for producing a high-strength cold-rolled steel sheet, comprising annealing at a temperature of not more than ℃. 9. A slab having the chemical composition according to any one of claims 1 to 4, which is hot-rolled, and then a reduction ratio of 75 is obtained.
%, And cold-rolled to 650 ° C to 1100 ° C
A method for producing a high-strength cold-rolled steel sheet, comprising annealing at the following temperature and further performing electrogalvanizing or hot-dip galvanizing. 10. A cold-rolled steel sheet produced by the method according to claim 8 or 9, wherein the cold-rolled steel sheet is subjected to ERW or forging so that the rolling direction of the cold-rolled steel sheet coincides with the pipe axis direction of the steel pipe. Steel pipe manufacturing method. 11. A method for producing a steel pipe, wherein the steel pipe produced by the method according to claim 10 is electrogalvanized or hot-dip galvanized.