JP5333074B2 - Steel pipe manufacturing method for steel tower - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing steel tubes for a steel tower with less change in the mechanical properties when manufacturing a steel tube having the diameter/thickness ratio of &le;20 and the tensile strength of &ge;780 MPa by press-bending a steel plate. <P>SOLUTION: In the method for manufacturing steel tubes for the steel tower having the diameter/thickness ratio of &le;20 and the tensile strength of &ge;780 MPa, a slab having a composition containing, by mass, 0.05-0.20% C, 0.03-0.40% Si, 0.5-2.0% Mn, &le;0.02% P, &le;0.005% S, 0.03-0.10% Al, &le;0.005% N, and the balance Fe with inevitable impurities is heated to &ge;1,100&deg;C, hot-rolled, and cooled down to &le;100&deg;C, and then, re-heated and hardened, tempered at 500-650&deg;C to manufacture a thick steel plate, which is re-heated at &ge;500&deg;C to the temperature equal to or lower than [the tempering temperature when manufacturing the thick steel plate -30&deg;C], and then, subjected to the press-bending, and welded. One or two or more kinds of Cu, Cr, Mo, V, Nb, Ni, B and Ti may be contained in place of a part of Fe. <P>COPYRIGHT: (C)2011,JPO&amp;INPIT

Description

  The present invention manufactures a press-bending steel pipe having a tensile strength of about 780 MPa to about 950 MPa after being formed into a steel pipe even if the steel sheet is subjected to severe bending with a diameter-thickness ratio of 20 or less by press bending (press-bending). More particularly, the present invention relates to a method for producing a press-bending steel pipe that can be suitably used for a steel tower structure.

  There is a round steel pipe as a steel material used for building structures such as steel towers. As a method of manufacturing a circular steel pipe, a press bending method is generally adopted in addition to a UOE (Uing press-Oing press-Expander) forming method suitable for a steel pipe for a line pipe. Circular steel pipes can be formed with high dimensional accuracy by the UOE forming method, but round steel pipes used for construction structures are often manufactured by a press bending method with higher processing capability. Here, the press bending process is a method of forming a steel plate into a circular shape by pressing the steel plate against the mold in order from the end and bending it inward. However, the smaller the diameter D of the steel pipe and the larger the thickness t of the steel pipe, the smaller the diameter / thickness ratio D / t, and the higher the workability, so the manufacturing conditions become severe. When a circular steel pipe is used for a steel tower, the design of the appearance may be required, and severe manufacturing conditions are often required.

  The mechanical properties of steel materials are generally measured by taking a test piece from the steel plate. However, even if the mechanical properties required as a steel plate are satisfied, by producing a steel pipe from the steel plate, for example, by forming the steel plate into a steel pipe by press bending, the mechanical properties change, The characteristics required for steel pipes may not be obtained. In this case, it is necessary to further heat-treat the formed steel pipe. In particular, this tendency is higher as the diameter-thickness ratio D / t is smaller.

  As an invention that solves the problem of mechanical characteristics due to such forming into a steel pipe, for example, Patent Document 1 discloses a predetermined machine without forming a steel plate into a steel pipe and performing SR (Stress Relieving) treatment. A method of manufacturing a steel pipe by press bending that can obtain the desired characteristics is disclosed.

JP 2007-270304 A

  As described above, when a steel pipe is manufactured by subjecting a thick steel plate to press bending, a change occurs in its mechanical characteristics. If the change in mechanical properties at this time can be reduced, it becomes easier to predict the mechanical properties after pipe making, and it becomes easier to manufacture a steel pipe having the intended mechanical properties.

  In particular, when producing a steel pipe by subjecting a thick steel plate to press bending, it is common that the thick steel plate is produced by a steel plate manufacturer, and the steel pipe manufacturer produces the steel pipe from the thick steel plate. If the steel pipe manufacturer can guarantee the same mechanical characteristics as the mechanical characteristics guaranteed by the steel plate manufacturer by reducing the change in mechanical properties during pipe making, the benefits for the steel pipe manufacturer are great.

  An object of the present invention is to provide a method of manufacturing a steel pipe for a steel tower with a small change in mechanical properties when a steel pipe having a diameter-thickness ratio of 20 or less and a tensile strength of 780 MPa or more is produced by press bending a steel plate. .

  The present invention has been completed based on the above findings, and the gist of the present invention resides in the following steel pipe manufacturing methods (1) to (4). Hereinafter, the present invention may be collectively referred to as the present invention.

(1) By mass%, C: 0.05-0.20%, Si: 0.03-0.40%, Mn: 0.5-2.0%, P: 0.02% or less, S: A slab containing 0.005% or less, Al: 0.03 to 0.10%, N: 0.005% or less, with the balance being Fe and impurities, heated to 1100 ° C or higher, hot-rolled, 100 ° C After cooling to below, it is again heated to 880-930 ° C. and quenched, and subsequently tempered at a temperature of 560 ° C. or higher and 650 ° C. or lower to produce a thick steel plate. A method for producing steel pipes for steel towers having a diameter-thickness ratio of 20 or less and a tensile strength of 780 MPa or more, characterized by performing press bending after reheating at a temperature equal to or lower than the tempering temperature of -30 ° C.

  (2) The method for producing a steel pipe for a steel tower according to (1), further comprising tempering the steel pipe at a temperature not lower than a reheating temperature before press bending and not higher than a tempering temperature of the thick steel plate.

  (3) Instead of a part of Fe, in mass%, Cu: 0.5% or less, Cr: 1.5% or less, Mo: 0.7% or less, V: 0.09% or less, Nb: 0 .05% or less, Ni: 1.5% or less, B: 0.0030% or less, containing one or more elements, (1) or (2) for steel towers Steel pipe manufacturing method.

  (4) The method for producing a steel pipe for a steel tower according to any one of the above (1) to (3), wherein Ti is contained in 0.05% or less by mass% instead of part of Fe.

  According to the present invention, when a steel pipe having a diameter-thickness ratio of 20 or less and a tensile strength of 780 MPa or more is manufactured by press bending a steel plate, the change in mechanical properties can be reduced.

  Therefore, the mechanical characteristics of the steel pipe as the final product can be predicted in advance at the stage of the thick steel plate. In particular, a steel pipe with a high degree of workability, that is, a steel pipe with a small diameter-thickness ratio, is likely to change in mechanical characteristics as a result of pipe making. Even with such a steel pipe, the mechanical characteristics can be predicted. Furthermore, since a steel pipe with a low diameter-thickness ratio can be manufactured, it can be suitably used as a steel pipe for steel towers that also require design.

  Further, since the mechanical characteristics can be investigated from the remaining material of the cutting allowance when manufacturing the steel pipe from the steel plate, it is possible to avoid destroying the final product and inspecting the mechanical characteristics.

  Hereinafter, the constituent requirements of the present invention will be described in detail. In addition, "%" display of the content of each component element means "mass%".

(A) About chemical composition:
First, the reason for limiting the composition of the steel used in the present invention as described above will be described.

C: 0.05-0.20%
C is an element contributing to an increase in strength. However, if the content is less than 0.05%, it is difficult to ensure the strength. On the other hand, if the content exceeds 0.20%, the weldability and toughness of the product are deteriorated. Therefore, the C content is 0.05 to 0.20%. The lower limit of the preferable C content is 0.07%, and the upper limit of the preferable C content is 0.17%.

Si: 0.03 to 0.40%
Si is an essential element for deoxidation, but if the content is less than 0.03%, the deoxidation effect is small. However, if the content exceeds 0.40%, the weldability of the product is deteriorated. Therefore, Si is 0.03 to 0.40%. The lower limit of the preferable Si content is 0.05%, and the upper limit of the preferable Si content is 0.35%. A more preferable lower limit of the Si content is 0.08%, and a more preferable upper limit of the Si content is 0.30%.

Mn: 0.5 to 2.0%
Mn is an element necessary for ensuring the strength and toughness of steel. However, when the content is less than 0.5%, such an effect is small. On the other hand, if the content exceeds 2.0%, weldability deteriorates. Therefore, the Mn content is set to 0.5 to 2.0%. The lower limit of the preferable Mn content is 0.6%, and the upper limit of the preferable Mn content is 1.8%. A more preferable lower limit of the Mn content is 0.7%, and a preferable upper limit of the Mn content is 1.6%.

P: 0.02% or less P is an element present in steel as an impurity. If its content exceeds 0.02%, it not only segregates at the grain boundaries and lowers the toughness, but also causes hot cracking during welding. Therefore, the P content is 0.02% or less. Preferably it is 0.018% or less.

S: 0.005% or less S is an element present in steel as an impurity. When the content exceeds 0.005%, the center segregation is promoted or a large amount of stretched MnS is generated, so that the mechanical properties of the base material and HAZ (Heat Affected Zone) deteriorate. Therefore, the S content is 0.005% or less. Preferably it is 0.003% or less.

Al: 0.03-0.10%
Al is an essential element for deoxidation. However, if the content is less than 0.03%, the deoxidation effect is small. Moreover, when the content exceeds 0.10%, the toughness tends to deteriorate particularly in HAZ. This is presumably because coarse cluster-like alumina inclusion particles are easily formed. Therefore, the Al content is 0.03 to 0.10%. A preferable lower limit of the Al content is 0.35%, and a preferable upper limit of the Al content is 0.09%.

N: 0.005% or less N is an element present in steel as an impurity. When N is present in a large amount in steel, both the base material and the toughness of HAZ deteriorate. Therefore, the N content is 0.005% or less. Preferably, the N content is 0.004% or less.

  The chemical composition of the steel pipe according to the present invention is such that the balance is Fe and impurities in addition to the above components. Here, the impurities are components that are mixed due to various factors in the manufacturing process including raw materials such as ore and scrap when industrially manufacturing steel pipes, and do not adversely affect the present invention. It means what is allowed in the range.

  In addition to the above alloy elements, the steel pipe according to the present invention includes one or more selected from the group consisting of Cu, Cr, Mo, V, Nb, Ni and B in place of part of Fe. You may contain. Further, Ti may be contained instead of a part of Fe. The reason why these elements may be contained and the contents at that time are as follows.

Cu: 0.5% or less Cu can be contained as necessary. If contained, it has the effect of improving strength and corrosion resistance. Further, if Cu is contained and then quenching and tempering treatment is performed, an aging effect by Cu is obtained, and the strength is further increased. However, even if the content exceeds 0.5%, the effect is only saturated, and the improvement in performance commensurate with the cost increase is not observed, so the upper limit of Cu is made 0.5% or less. A preferable upper limit is 0.4%. In order to ensure the effect of improving strength and corrosion resistance, it is preferable to contain 0.03% or more. More preferably, it is 0.10% or more.

  Further, when Cu is contained, it is preferable to contain 0.03% or more of Ni in order to prevent cracking (Cu checking) during rolling.

Cr: 1.5% or less Cr can be contained as necessary. When contained, it has the effect of enhancing hardenability and improving strength, and also has the effect of enhancing carbon dioxide corrosion resistance. However, if the content exceeds 1.5%, it becomes difficult to suppress the hardening of the HAZ. Moreover, even if it contains exceeding 1.5%, the carbon dioxide corrosion resistance improvement effect will be saturated. Therefore, the Cr content is 1.5% or less. Preferably it is 1.3% or less. In addition, in order to express these effects reliably, it is preferable to make it contain 0.03% or more. More preferably, it is 0.40% or more.

Mo: 0.7% or less Mo can be contained as necessary. If contained, it has the effect of improving the high temperature strength and the effect of significantly increasing the yield strength at high temperatures. However, if it exceeds 0.7%, the weldability is impaired, and there is a risk of causing toughness deterioration during high heat input welding. Therefore, the Mo content is 0.7% or less. Preferably it is 0.5% or less. In addition, in order to express these effects reliably, it is preferable to make it contain 0.03% or more. More preferably, it is 0.1% or more.

V: 0.09% or less V can be contained as necessary. If contained, there is an effect of improving the strength by precipitation hardening. However, if the content exceeds 0.09%, the weldability decreases. Therefore, the V content is 0.09% or less. Preferably it is 0.07% or less. In addition, in order to express this effect reliably, it is preferable to contain 0.003% or more. More preferably, it is 0.010% or more.

Nb: 0.05% or less Nb can be contained as necessary. If contained, it has an effect of improving high temperature strength (fire resistance) by precipitation hardening and transformation strengthening, and an effect of improving toughness by coarsening. However, if the content exceeds 0.05%, joint toughness may be deteriorated during high heat input welding. Therefore, the Nb content is 0.05% or less. Preferably it is 0.03% or less. In addition, in order to express these effects reliably, it is preferable to make it contain 0.003% or more. More preferably, it is 0.008% or more.

Ni: 1.5% or less Ni can be contained as necessary. If contained, it has the effect of improving the hardenability to increase the strength and the effect of increasing the toughness of the steel matrix (dough) in the solid solution state. However, even if the content exceeds 1.5%, these effects are saturated. Therefore, the Ni content is 1.5% or less. Preferably it is 1.1% or less. In addition, in order to express these effects reliably, it is preferable to make it contain 0.03% or more. More preferably, it is 0.10% or more.

  Further, the steel pipe according to the present invention can contain Cu as necessary, but in this case, in order to prevent cracking (Cu checking) during rolling, 0.03% or more of Ni is contained. It is preferable.

B: 0.0030% or less B can be contained if necessary. If contained, it has the effect of improving hardenability and increasing strength. However, when the content exceeds 0.0030%, the effect of increasing the strength is saturated, and the tendency of deterioration in toughness becomes remarkable in both the base material and HAZ. Therefore, the B content is 0.0030% or less. Preferably it is 0.0025% or less. In addition, in order to express this effect reliably, it is preferable to contain 0.0003% or more. More preferably, it is 0.0010% or more.

Ti: 0.05% or less Ti can be contained as necessary. If contained, it mainly acts as a deoxidizing element, forms an oxide phase composed of Al, Ti and Mn, refines the structure of the weld heat affected zone, and obtains an effect of improving toughness.
However, if the content exceeds 0.05%, the oxide formed is Ti oxide or Ti-Al oxide, and the dispersion density is lowered. Particularly, the structure in the heat-affected zone of the small heat input weld zone is reduced. There is a risk of losing the ability to refine. Therefore, the Ti content is 0.05% or less. Preferably it is 0.04% or less. In addition, in order to express this effect reliably, it is preferable to contain 0.003% or more. More preferably, it is 0.008% or more.

(B) About manufacturing conditions:
The steel pipe which concerns on this invention manufactures a thick steel plate from the slab which has the above-mentioned chemical composition, and manufactures it by giving a press bending process to the thick steel plate. For the production of slabs, it is not particularly necessary to specify the casting conditions. Although an ingot-splitting slab or a continuous casting slab can be used, it is preferable to use a continuous casting slab from the viewpoint of production efficiency, yield, and energy saving.

  Hereinafter, (B-1) production of a thick steel plate and (B-2) production of a steel pipe from the thick steel plate will be described separately. Note that the manufactured steel pipe may be further subjected to (B-3) post-pipe heat treatment (tempering) as necessary.

(B-1) Production of Thick Steel Plate First, the slab having the chemical component (A) is heated and austenite transformed. The heating temperature at this time shall be 1100 degreeC or more. If it is 1100 degreeC or more, it is because all the elements (for example, Nb, Ti, etc.) in steel can be dissolved, and intensity | strength and toughness can be ensured. The upper limit of the heating temperature is not particularly limited as long as the slab is in an austenite transformation range, but the heating temperature is preferably 1250 ° C. or lower. Heating at a temperature exceeding 1250 ° C. not only increases the fuel cost, but also increases scale generation, thereby reducing yield and further reducing production efficiency. The upper limit of the heating temperature is preferably 1200 ° C.

  Subsequently, hot rolling is performed on the heated slab. It is sufficient to perform the hot rolling by a commonly used method. At this time, it is preferable that the rolling end temperature finished to predetermined thickness shall be 800-900 degreeC. If it is 800 ° C. or higher, the deformation resistance of the slab is small and rolling is easy. On the other hand, if it is 900 ° C. or lower, there is no fear of lowering toughness.

  After rolling, the thick steel plate (slab after rolling) is cooled to 100 ° C. or lower. Cooling by cooling or air cooling is sufficient. For example, the thick steel plate may be removed from the production line (offline) and placed. If it is placed, it is allowed to cool, and the temperature of the thick steel plate eventually becomes room temperature. However, it is not necessary to wait for the temperature to drop to room temperature, and if the thick steel plate is cooled to 100 ° C. or less, the process may proceed to the next step.

  In the subsequent process, the thick steel plate is heated again and quenched. Reheating and quenching may be performed in an off-line heat treatment furnace. The heating temperature is preferably 880 to 930 ° C. If it is 880 degreeC, sufficient hardening will enter in hardening and it will be easy to obtain the target tensile strength of 780 Mpa or more. On the other hand, if it is 930 degrees C or less, since austenite grain coarsening is avoided, the fall of toughness can be prevented. Further, in order to obtain a sufficient quenching strength, a cooling medium such as water is uniformly applied to the entire surface of the steel sheet so that the cooling rate is preferably 1 ° C./s or more, more preferably 2 at the center of the sheet thickness. It is preferable to carry out at a cooling rate of at least ° C / s.

  Finally, this thick steel plate is tempered at a temperature of 500 ° C. or higher and 650 ° C. or lower. By tempering, the unstable structure generated by quenching is made stable, improving toughness and adjusting to the target strength. Further, the mechanical properties are made uniform, and changes in properties such as strength due to processing in the manufacturing process of the steel pipe described later can be reduced. In addition, the tempering temperature needs to be 500-650 degreeC. When the tempering temperature is less than 500 ° C., the effect of homogenizing the mechanical properties cannot be obtained, and when the temperature exceeds 650 ° C., some ferrite transformations start and eventually an unstable structure is formed.

(B-2) Manufacture of steel pipe The thick steel plate obtained in (B-1) above may be cut and adjusted in width to obtain a steel pipe having a desired diameter, if necessary. At this time, there is no influence on the properties of the steel pipe obtained by cutting.

  The thick steel plate needs to be reheated at a temperature of 500 ° C. or more and [a tempering temperature at the time of producing the thick steel plate−30 ° C.] or less. At a reheating temperature of less than 500 ° C., surface flaws are formed on the surface of the thick steel plate by press bending. On the other hand, the strength of the steel pipe decreases at a reheating temperature exceeding the tempering temperature at the time of producing the thick steel plate. In the present invention, because of the concern about the decrease in strength of the steel pipe, a further 30 ° C. margin is observed, and reheating is performed at a temperature equal to or lower than [tempering temperature at the time of thick steel plate production−30 ° C.].

  After reheating, press bending is performed. Specifically, the heated thick steel plate is pressed against a mold and processed into a cylindrical shape to form a steel pipe. It is desirable to complete the press bending process before the temperature of the thick steel plate falls below 350 ° C. This is because press bending at 350 ° C. or higher has a great effect of suppressing changes in mechanical properties such as an increase in strength.

  Thereafter, a steel pipe having a diameter-thickness ratio of 20 or less and a tensile strength of 780 MPa or more can be obtained by welding the steel pipe joint. It is sufficient to perform the welding by a conventional method.

(B-3) Heat treatment after pipe making (tempering)
If necessary, the steel pipe after welding may be subjected to post-pipe-forming heat treatment (tempering). Even if tempering is not performed, the change in mechanical properties before and after pipe making is sufficiently small, but by further tempering after pipe making, the mechanical properties of the entire steel pipe can be made uniform. The temperature at which the heat treatment (tempering) after pipe forming is performed is preferably not less than the reheating temperature before press bending and not more than the tempering temperature of the thick steel plate. If the heat treatment (tempering) temperature after pipe making is equal to or higher than the reheating temperature before press bending, the effect of making the mechanical properties of the entire steel pipe uniform is obtained, and if it is equal to or lower than the tempering temperature of the thick steel plate, This is because the tensile strength of 780 MPa or more is easy to be obtained.

  Sixteen types of steel having the chemical composition shown in Table 1 were melted by a usual method and continuously cast into slabs having a thickness of 250 to 300 mm. All the steels of components No. 1 to 16 in Table 1 have chemical compositions within the range defined by the present invention.

  Table 2 shows the manufacturing conditions of the thick steel plate and the manufacturing conditions of the steel pipe manufactured from the thick steel plate. Various steel slabs shown in Table 1 were heated to 1150 ° C. in a heating furnace and rolled. Rolling was performed in advance with a pass schedule in which the rolling end temperature was 850 ° C. up to the plate thickness shown in Table 2. After rolling, it was placed on a cooling bed, cooled by allowing to cool, and after cooling, it was again introduced into a heating furnace, reheated, and quenched to room temperature by water cooling. The cooling temperature at this time, the heating temperature at the time of quenching, and the tempering temperature are as shown in Table 2. The cooling temperature, the heating temperature at the time of quenching, and the tempering temperature refer to the temperature at the surface of the hot work material.

  A part of the steel plate was cut out from the obtained thick steel plate for a sample for investigating mechanical properties, and a steel pipe was manufactured. Prior to the manufacture of the steel pipe, the thick steel plate was cut and its width was adjusted in order to obtain a desired diameter-thickness ratio (D / t). After cutting, the steel plate was heated, pressed and bent into a tubular shape, and the seam was welded to produce a steel pipe. Some of the manufactured steel pipes were subjected to heat treatment (tempering) after pipe making. The heating temperature, press bending completion temperature, and tempering temperature of the thick steel plate at this time are as shown in Table 2. A sample of mechanical properties was taken from the obtained steel pipe.

  Subsequently, tensile properties as mechanical properties were examined for samples obtained from each of the obtained steel plates and steel pipes made from the steel plates.

  Table 3 shows the manufacturing conditions of the steel pipe and the tensile test results for the sample taken from the obtained steel pipe.

  For tensile properties, tensile test specimens according to JIS Z 2201 (1998) were collected in the “L direction”, which is parallel to the rolling direction centered on the position of the thickness ¼ (for the steel pipe, the axial direction (thick steel plate) From the direction corresponding to the L direction), and a tensile test at room temperature was conducted by the method described in JIS Z 2241 (1998) to investigate yield strength (hereinafter referred to as “YS”) and tensile strength (hereinafter referred to as “YS”). "TS").

  The above YS was obtained from the lower yield point when the tensile test speed was 10 N / (mm · s), and when a clear yield point did not appear, the 0.2% yield strength was defined as YS.

  As a result, the steel pipes produced by the production method of the present invention all have a tensile strength of 780 MPa or more, and the difference in mechanical strength between the steel plate and the steel pipe, ie, | ΔYS | and | ΔTS | It was small.

  On the other hand, the No. 4-2 steel pipe, in which the thick steel plate was heated to a temperature higher than the [tempering temperature at the time of producing the thick steel plate -30 ° C.] during pipe making, the mechanical properties after the pipe making were markedly reduced, and the tensile strength was Not only did it become below the target value, but also the difference in mechanical strength, that is, | ΔYS | and | ΔTS | both increased.

  In addition, the No. 6-1 steel pipe that heated the thick steel plate only up to 450 ° C. during the pipe making was conversely increased in mechanical properties after the pipe making, that is, the difference in mechanical strength, that is, | ΔYS | And | ΔTS | both increased.

  In addition, the No. 13-2 steel pipe with a high tempering temperature after pipe making has a significant decrease in mechanical properties after pipe making, not only the tensile strength is below the target value, but also the difference in mechanical strength, That is, both | ΔYS | and | ΔTS |

  According to the present invention, when a steel pipe having a diameter-thickness ratio of 20 or less and a tensile strength of 780 MPa or more is manufactured by press bending the steel sheet, a steel pipe having a small change in mechanical properties can be manufactured. Since the diameter-thickness ratio is low, it can be suitably used as a steel pipe for steel towers that require design.

Claims (4)

In mass%, C: 0.05 to 0.20%, Si: 0.03 to 0.40%, Mn: 0.5 to 2.0%, P: 0.02% or less, S: 0.005 % Or less, Al: 0.03 to 0.10%, N: 0.005% or less, with the remainder consisting of Fe and impurities heated to 1100 ° C or higher, hot-rolled, and cooled to 100 ° C or lower After that, it is again heated to 880 to 930 ° C. and quenched, and then tempered at a temperature of 560 ° C. or higher and 650 ° C. or lower to produce a thick steel plate. A method for producing a steel pipe for steel towers having a diameter-thickness ratio of 20 or less and a tensile strength of 780 MPa or more, characterized by performing press bending and welding after reheating at a temperature of -30 ° C or less.   Furthermore, the steel pipe is tempered at a temperature not lower than the reheating temperature before the press bending process and not higher than the tempering temperature of the thick steel sheet.   Instead of a part of Fe, in mass%, Cu: 0.5% or less, Cr: 1.5% or less, Mo: 0.7% or less, V: 0.09% or less, Nb: 0.05% The method for producing a steel pipe for a steel tower according to claim 1 or 2, wherein Ni: 1.5% or less and B: 0.0030% or less are contained. . It replaces with a part of Fe and contains Ti: 0.05% or less by mass%, The manufacturing method of the steel pipe for steel towers in any one of Claim 1 to 3 characterized by the above-mentioned.