JPH11189843A - Resistance welded steel tube excellent in hot dip galvanizing crack resistance, and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrence of hot dip galvanizing crack in the vicinity of a weld zone, which is the problem of a large-diameter and thick-walled resistance welded steel tube having <0.12% C content an the steel and 60 kgf/mm<2> tensile strength. SOLUTION: A weld bead 2 resultant from resistance welding is cut, and also a stepped part 3 resultant from the uncut remainder of the weld bead 2 is removed before sizer forming. Instead of the removal of the stepped part 3, a hardened part 4 resultant from the application of reduction to the stepped part 3 at the time of sizer forming is removed after sizer forming. By these procedures, the part having surface microhardness exceeding 260 Hv can be removed from the vicinity of a resistance welded zone, and the occurrence of hot dip galvanizing crack can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a large-diameter thick-walled electric resistance welded steel pipe and a method for producing the same, and more particularly, to an electric resistance welded steel pipe excellent in hot-dip galvanizing crack resistance and a method for producing the same.

[0002]

2. Description of the Related Art Among steel pipes used for power transmission towers, UOE steel pipes are used for large-diameter thick steel pipes for large towers. In addition, hot-dip galvanizing is used as the rust prevention means regardless of the steel pipe size. As is well known, hot-dip galvanizing is a method of immersing an object in hot-dip zinc at about 450 ° C. to perform plating.
This hot-dip galvanizing is performed on a welded steel structure after welding various mounting members including a flange to a steel pipe.

In hot-dip galvanizing of a welded steel structure, grain boundary cracking may occur mainly in a heat affected zone (hereinafter, referred to as a HAZ portion). In the case of a steel pipe for a steel tower, the HAZ portion accompanying the welding of a mounting member is sometimes used. Cracking at has been a problem. This hot-dip galvanizing crack is generated by superimposition of penetration of liquid zinc into the grain boundary of the coarse-grained HAZ, thermal stress generated at the time of immersion in a bath, and residual stress at a welded portion. And the hardness of the HAZ portion is 260
If it exceeds -270 Hv, the cracking sensitivity becomes extremely high.

With respect to hot-dip galvanizing cracking of welded steel structures, stress and material which are factors thereof have been studied extensively, and as methods for preventing them, reduction of residual stress in welds, bath immersion method with less thermal stress, Research and development have been conducted on the shape of structures. Further, from the viewpoint of the material, a steel material or the like in which the amount of alloying elements is specified to enhance the hot-dip galvanizing crack resistance has been proposed (Japanese Patent Publication No. 2-5814).

[0005]

By the way, UOE steel pipes have hitherto been used as large-diameter thick-walled steel pipes for steel towers. However, recently, ERW steel pipes having lower pipe manufacturing costs than UOE steel pipes have been used. Conversion to some is being promoted. The applicant started this conversion relatively early, and has been conducting various studies since then. In the course of the research, it has been found that large-diameter thick-wall ERW pipes for steel towers have the following problems.

[0006] Large-diameter thick-wall ERW steel pipes for steel towers, like ordinary ERW steel pipes, are formed from hot-rolled steel strip as pipes, ERW welding, welding bead removal processing, welding heat treatment, and shrinking with a sizer. Manufactured by each process of pipe processing, the size is larger diameter and thicker than normal ERW steel pipe, after welding of various mounting members to the manufactured ERW steel pipe, Hot-dip galvanizing for rust prevention is performed.

However, large-diameter thick-wall ERW steel pipes for steel towers are:
Despite the fact that the vicinity of the ERW weld, including the ERW HAZ, has been softened by heat treatment, there is a risk of plating cracking near the ERW weld of the steel pipe body when galvanizing is performed. There was found. UOE for the same welded steel pipe
In the case of a steel pipe, cracking may occur in the HAZ portion due to welding of the mounting member, but plating cracking does not occur on the side of the steel pipe main body including the vicinity of the seam weld. Even in the case of the same electric resistance welded steel pipe for a steel tower, in the case of a small-medium diameter pipe which is the original electric resistance welded pipe size, this plating crack is not a problem.

From these facts, it is judged that the hot-dip galvanizing cracks occurring near the ERW weld are a phenomenon peculiar to a large-diameter thick ERW steel pipe used for a large steel tower or the like.

The conventional measures against plating cracking are effective for cracking of the HAZ portion due to welding of the mounting member, but are close to the ERW weld peculiar to a large-diameter thick-wall ERW steel pipe. Has no effect on hot-dip galvanizing cracks.

An object of the present invention is to provide an electric resistance welded steel pipe having excellent resistance to hot dip galvanization cracking in the vicinity of an electric resistance welded portion peculiar to a large diameter thick wall electric resistance welded steel pipe, It is to provide a manufacturing method thereof.

[0011]

In order to achieve the above object, the present inventors have developed a phenomenon, a cause, and a countermeasure for hot-dip galvanizing cracks near an ERW weld peculiar to a large-diameter thick ERW steel pipe. We conducted research on. As a result, the following findings were obtained.

The electric resistance welded steel pipe is manufactured by using a hot rolled steel strip as a raw material in the steps of pipe forming, electric resistance welding, welding bead removal processing, welding heat treatment, and pipe contracting by a sizer. In the production of such an electric resistance welded steel pipe, since the upset is performed by electric resistance welding, as shown in FIG. 1A, the vicinity of the electric resistance welded part is expanded mainly on the outer surface side of the steel pipe 1 and the weld bead 2 is formed. Arises
The weld bead 2 is cut and removed by a tool or the like in a process following the electric resistance welding. However, the steel pipe traveling along the ERW pipe line inevitably has meandering or the like. As a result, in an actual pipe production, as shown in FIG.
The stepped portion 3 due to the uncut portion of the steel pipe is generated intermittently in the longitudinal direction of the steel pipe.

The steel pipe in which the step 3 remains is finally subjected to a tube forming process by a sizer. At this time, if the height h of the step 3 is large (even if it is large, it is usually 0.5 mm or less, 0 mm or less). .2 to 0.3 mm), FIG.
As shown in (c), the convex portion formed by the step portion 3 is strongly pressed down cold during processing, and stress concentrates on a hatched portion 4 ′ from the lower portion of the step portion 3 to the lower portion thereof. ′ Causes work hardening, and the work hardened portion 4 becomes more sensitive to hot-dip galvanizing cracking. In fact,
It has been confirmed that hot-dip galvanizing cracks near the ERW weld peculiar to large-diameter thick-wall ERW pipes are phenomena that occur in positions corresponding to both sides of the weld bead 2 in a positional manner. I have.

The reason why this crack is characteristically generated in a large-diameter thick-walled ERW steel pipe is that the steel pipe is originally U-shaped.
Large-diameter thick wall (outside diameter 457mm) as manufactured by OE pipe
As described above, since the thickness is 12 mm or more and the strength is high (the tensile strength is 50 kgf / mm ² or more), the rolling force received during the sizer molding is large, the upset amount during the electric resistance welding is large, and the welding bead 2 and The fact that the height of the stepped portion 3 is essentially large, that the meandering becomes remarkable as the outer diameter of the ERW steel pipe increases, and that the height of the stepped portion 3 tends to increase, This is considered to be due to remarkable curing. On the other hand, the reason why hot-dip galvanizing cracks do not occur in UOE steel pipes, which are conventional large-diameter thick-walled steel pipes for steel towers, is that such steel pipes are not subjected to shrink-forming by a sizer.

[0015] The tensile strength of the ERW steel pipe is usually C
When the C content is 0.12% or more, hot-dip galvanizing cracking near the ERW weld becomes remarkable, but even when the C content is less than 0.12%, high strength, In particular, when the tensile strength is 60 kgf / mm ² or more, the hot-dip galvanizing cracking property in the vicinity of the electric resistance welded portion is similarly increased.

The hardening phenomenon caused by the cold working in the sizer forming is problematic as a welded steel structure.
Z part hardening phenomena and ERW before welding heat treatment
Although it has a completely different mechanism from the hardening phenomenon of the Z part, the same result as the hardening phenomenon of the HAZ part is obtained as far as the quantitative effect of hardness on hot-dip galvanizing cracks is concerned. If it exceeds 260 to 270 Hv, the susceptibility to hot-dip galvanizing cracks is significantly increased.

As described above, in the large-diameter thick-walled electric resistance welded steel pipe, the reduction of the stepped portion by the sizer causes work hardening, thereby increasing the susceptibility to hot-dip galvanizing cracking. What has been removed has excellent hot-dip galvanizing crack resistance. In addition, even if the work hardened portion generated by the sizer molding is removed, even if the material is subjected to sizer molding without removing the stepped portion, the material has excellent hot-dip galvanizing crack resistance. Further, as a countermeasure in the pipe making process, it is effective to remove the stepped portion prior to the sizer molding and to remove the work hardened portion generated by the sizer molding after the sizer molding.

The present invention has been completed on the basis of the above findings, and has the following features of an electric resistance welded steel pipe and a method of manufacturing the same.

A first ERW steel pipe having a large diameter and a large wall thickness, wherein the C content in the base steel is less than 0.12% by weight;
Hot-dip galvanizing characterized in that the micro-hardness of portions corresponding to both side portions of the ERW bead is 260 Hv or less over substantially the entire length of the steel pipe at least in the range of 0.2 mm in the thickness direction from the outer surface. ERW steel pipe with excellent cracking properties.

A second ERW pipe having a large diameter and a large wall thickness, wherein the C content in the base steel is less than 0.12% by weight;
Undergoing a first surface treatment in which an ERW weld bead portion is removed;
An electric resistance welded steel pipe excellent in hot-dip galvanizing crack resistance, which is subjected to a second surface processing in which a step portion partially left in the vicinity of the processed part after the first surface processing is removed.

A third ERW steel pipe, which is a large-diameter thick-wall ERW pipe having a C content of less than 0.12% by weight in the base steel,
Undergoing a first surface treatment in which an ERW weld bead portion is removed;
And a second surface processing or heat treatment for removing a work hardened portion generated in the vicinity due to a step portion partially left in the vicinity of the processed portion after the first surface processing. ERW steel pipe with excellent hot-dip galvanizing resistance.

First production method: C content in steel is 0.1% by weight.
A method for producing a large-diameter and thick-walled electric resistance welded steel tube by using a hot rolled steel strip of less than 12% as a raw material in each of the steps of pipe forming, electric resistance welding, welding bead removal, heat treatment of a welded portion, and pipe contracting by a sizer. After the ERW welding bead removal processing, before the tube is contracted by the sizer, the surface step remaining near the ERW welding bead removal processing part is cut or ground to 0.1 mm or less. Manufacturing method of ERW steel pipe.

Second production method: The C content in steel is 0.
A method for producing a large-diameter and thick-walled electric resistance welded steel tube by using a hot rolled steel strip of less than 12% as a raw material in each of the steps of pipe forming, electric resistance welding, welding bead removal, heat treatment of a welded portion, and pipe contracting by a sizer. A method for manufacturing an ERW steel pipe, comprising cutting or grinding at least a surface near an ERW weld bead removal portion to a depth of 0.1 mm or more at a stage after performing a tube reduction process with a sizer.

Third production method: The C content in the steel is 0.1% by weight.
A method for producing a large-diameter and thick-walled electric resistance welded steel tube by using a hot rolled steel strip of less than 12% as a raw material in each of the steps of pipe forming, electric resistance welding, welding bead removal, heat treatment of a welded portion, and pipe contracting by a sizer. A method of manufacturing an electric resistance welded steel pipe, wherein at least a surface in the vicinity of an electric resistance welded bead removal processing part is softened and heat-treated at a stage after the pipe contracting by a sizer.

Although the present invention is directed to a large-diameter thick-walled electric resistance welded steel pipe, more specifically, a large-diameter thick-walled electric resistance welded pipe having an outer diameter of 457 mm or more and a wall thickness of 12 mm or more in which galvanizing cracking becomes a problem. Effective for steel pipe, outer diameter 500mm or more, wall thickness 1
It is particularly effective for large-diameter thick-walled electric resistance welded steel pipes of 4 mm or more. Also, in terms of the base material strength, the tensile strength is 60 kgf / m.
It is particularly effective in m ² or more high-strength electric resistance welded steel pipe.

[0026]

Embodiments of the present invention will be described below.

In the electric resistance welded pipe line, the C content in steel is set at 0.
After cutting a thick wide hot rolled steel strip having a width of less than 12% to a predetermined width, the butt edge portion is subjected to electric resistance welding to form a large-diameter thick steel pipe while being continuously formed into a tubular shape. At this time, a weld bead is generated in the vicinity of the electric resistance welded portion (see FIG. 1A).

The steel pipe after the electric resistance welding is continuously sent to a bead removing device, where the weld bead is cut and removed by a cutting tool or the like. At this time, due to the meandering of the steel pipe, etc., uncut shavings of the weld bead are generated, and the stepped portion due to this remains in the vicinity of the bead removal processing part, specifically, at the part corresponding to both sides of the weld bead. It occurs intermittently in the longitudinal direction [Fig.
(See (b)).

The steel pipe subjected to the bead removal processing is turned into a large-diameter thick-walled ERW steel pipe through a heat treatment of a welded portion and a contraction forming processing by a sizer, and the steel pipe after the bead removal processing is sent to the sizer as it is. Then, the step portion is reduced by the sizer molding, and the work-hardened portion resulting therefrom causes the hot-dip galvanizing crack susceptibility to increase.

Therefore, in the present invention, the step is removed before sizer molding. As a method of removing the stepped portion, cutting by a cutting tool, grinding by a bender (grinder), or cutting or grinding by other mechanical means, but does not include crushing. The crushing is accompanied by work hardening and causes the hot-dip galvanizing crack susceptibility to increase.

Also, even if there is a step, the height is 0.1
In the case of not more than mm, work hardening due to reduction in sizer molding is slight and its microhardness does not exceed 260 Hv, so that it does not cause an increase in hot-dip galvanizing cracking susceptibility. Therefore, the step portion having a height of 0.1 mm or less may be left as it is, while the height of the step portion needs to be 0.1 mm or less when the step portion is removed. Further, the step portion having an inclination angle θ of less than 45 ° may be left as it is because the work hardening is relatively slight.

By performing such stepped portion removal processing before sizer forming, work hardening due to reduction of the stepped part is avoided in sizer forming, and the cause of hot-dip galvanizing cracks in large-diameter thick-walled ERW steel pipes. Removed. The method for producing an electric resistance welded steel pipe including the step removal process is the first production method of the present invention.
The ERW steel pipe with improved hot-dip galvanizing crack resistance is the second ERW steel pipe of the present invention.

The step of removing the stepped portion may be performed before or after the heat treatment of the welded portion as long as it is before the sizer forming. By the way, the heat treatment of the welded part is at least HA of the steel pipe.
After heating the vicinity of the weld including the Z portion to an Ar3 point or higher, A
The cooling is performed by cooling at a temperature of c1 or lower by air cooling or higher, and then, if necessary, reheating to a temperature of 550 ° C. or higher and an Ac1 point or lower, and then cooling at a cooling rate of air cooling or higher.

Further, in the present invention, in place of the step removal before the sizer molding, or in addition to the step removal as necessary, the cold work hardened portion generated by the reduction of the step after the sizer molding is removed. It is also possible. The removal of the hardened portion can be performed mechanically or by heat treatment.

As mechanical methods, there are surface cutting with a cutting tool and surface grinding with a verder (grinder). This surface cutting or grinding need not be performed on the entire surface,
What is necessary is just to perform at least to a work hardening part. In addition, it has been confirmed that the range in which the step portion is formed by sizer and causes work hardening (260 Hv or more) which causes a problem in plating cracking is approximately 0.1 mm in the depth d from the surface in the thickness direction. Therefore, the surface shaving allowance needs to be 0.1 mm or more. However, excessive cutting results in a reduction in the wall thickness of the steel pipe and saturation of the effect. Further, as described later, the deep hardened portion does not cause plating crack. Desirable cutting allowance is 0.
15 to 0.3 mm.

On the other hand, as a method of removing the work hardened portion by heat treatment, surface heating by induction heating is the most efficient and effective. This surface heating may be performed at least on the work hardened portion, and in reality, it is reasonable to linearly execute the vicinity of the welded portion, specifically, a portion corresponding to the weld bead or both sides of the bead. . The surface heating method is not limited to the induction heating method. Although the cost is high, the re-heat treatment by quenching / tempering or normalizing the entire steel pipe is reliable.

By performing such hardened portion removal processing and treatment after sizer molding, work hardened portions that cause hot-dip galvanizing cracks in large-diameter thick-walled ERW steel pipes are removed.
Its crack sensitivity is reduced. The method for producing an electric resistance welded steel pipe including the processing and processing is the second and third production methods of the present invention. The ERW steel pipe having improved hot-dip galvanizing crack resistance by undergoing the processing and treatment is the third ERW steel pipe of the present invention.

Then, the stepped portion and the hardened portion are removed over the entire length of the steel pipe, and the micro hardness of the portions corresponding to both sides of the ERW bead is set to at least 0.2 mm in the thickness direction from the outer surface in the thickness direction. In the range above, by setting the Hp to 260 Hv or less over the substantial entire length of the steel pipe, hot-dip galvanizing cracking near the ERW weld portion becomes complete. This is the first ERW steel pipe of the present invention.

If there is a portion having a micro hardness of more than 260 Hv, this portion may be a starting point of hot-dip galvanizing cracks. Further, the elongation in the bath (defined in FIG. 2), which is an index of the hot-dip galvanizing cracking resistance, is 2% or less in this portion. Desirable micro hardness is 250 Hv or less. The hardness was measured by using a Vickers hardness tester and a load of 100
It is a value measured at about 500 g.

Although the average hardness of the entire steel pipe depends on the strength level, it is from 200 to 220 Hv at 55 kg class and at most 230 Hv at 60 kg class. The hardness of the ERW HAZ is also subject to softening heat treatment. It is managed to a degree of hardness. In conventional large-diameter thick-wall ERW steel pipes, despite the fact that softening heat treatment is applied near the weld, the portions corresponding to both sides of the weld bead are caused by the stepped portion due to the uncut portion of the bead A local work hardened portion of more than 260 Hv was formed, which was a cause of hot-dip galvanizing cracks.

Further, the occurrence of hot-dip galvanizing cracks in a large-diameter thick-walled electric resistance welded steel pipe is limited to the outer surface of the steel pipe. This is because, on the outer surface side, the residual stress of the ERW steel pipe and the thermal stress during hot-dip galvanizing become tensile stress, which accelerates the generation and propagation of cracks. This cracking is also caused by the intrusion of molten zinc from the steel pipe surface to the grain boundaries.

From these facts, except for the outer surface that comes into contact with the molten zinc, the effect is at most 0.2 mm in the thickness direction, and the depth in the thickness direction from the outer surface is less than 0.2 mm. If the micro hardness is 260 Hv or less in the range of 0.2 mm, it is sufficient from the viewpoint of hot-dip galvanizing cracking resistance. In other words, even if the hardness exceeds 260 Hv on the inner surface side more than 0.2 mm from the outer surface, the hot-dip galvanizing crack resistance is not impaired. Therefore, the micro hardness is 2
The range specified as 60 Hv or less was at least 0.2 mm in the thickness direction from the outer surface.

[0043]

EXAMPLES Next, examples of the present invention will be shown, and the effects of the present invention will be clarified by comparison with comparative examples.

Using a hot-rolled steel strip having the composition shown in Table 1 as a raw material, a large-diameter steel pipe having the specifications shown in Table 2 was obtained by each of the following steps: Thick ERW steel pipe was manufactured. Each of the material steels has a C content in the steel of less than 0.12% and a tensile strength of 60 kgf /
mm ² or more.

At this time, the bead position for welding bead cutting is changed stepwise by unit length to intentionally generate uncut portions, thereby forming step portions having various heights, and forming a step portion having various heights. The maximum height of the step portion was determined as the step height. A part of the steel pipe where the step was formed was subjected to surface grinding using a verda at a stage prior to the sizer forming. The steel pipes after sizer molding were cut into unit lengths and grouped.

One group cut out a tile-shaped specimen from the vicinity of the welded portion as it was cut and developed it so that no reduction was applied to the outer surface near the welded portion, and then left the outer surface as it was.
The tensile test piece shown in FIG. 3 was collected. Then, the highest micro hardness of the outer surface of the tensile test piece was measured, and the test piece was subjected to a tensile test in a distilled zinc bath to investigate the elongation in the bath defined in FIG. The hot-dip galvanizing crack resistance was determined to be good.

For another group, after forming the sizer, the outer surfaces on both sides of the welded portion were each about 20
By performing full length grinding in the pipe length direction with a width of mm, the work hardened portion formed by pressing down the step portion by sizer molding was mechanically removed, and the same investigation was performed.

For another group, the same investigation was conducted after the surface was softened by heat treatment. The surface softening by heat treatment was of two types: surface heating by induction heating and reheat treatment of the entire steel pipe. In the surface heating by induction heating, the steel pipe was passed twice through the heating device, so that the outer surfaces on both sides of the weld were each heated to a length of 30 to 40 mm in the pipe length direction.

Incidentally, the weld bead width w (see FIG. 1)
The average of steel A was 6 mm, and the average of steel B was 8 mm.

Table 3 shows the inspection results. In Table 2 showing the specifications of the steel pipe, YS, TS, and Hv (5 kg) are values obtained by testing the base material after forming the steel pipe. Also, H
v (5 kg) is a value measured at a load of 5 kg, and the hardening of the outer surface by cold working is not measured at the load of 5 kg.

[0051]

[Table 1]

[0052]

[Table 2]

[0053]

[Table 3]

Test Nos. 1, 2, 21, and 22 are those in which the step before the sizer exceeds 0.1 mm and this step is left as it is. As a result, the surface hardness after sizer is 260H
v, hot-dip galvanizing resistance is poor (elongation in bath is 2%
Less).

Test Nos. 3 to 5 and 23 to 25 were obtained by grinding the step before the sizer to 0.1 mm or less, even though the step exceeded 0.1 mm. Surface hardness after sizer is 260
Hv or less, and the hot-dip galvanizing resistance was good (elongation in the bath was 2% or more).

Test numbers 6 and 26 are test numbers 1, 2, 21, and 22
Similarly, the step before the sizer exceeds 0.1 mm, and
This step is left as it is. The surface hardness after the sizer exceeded 260 Hv, and the hot-dip galvanizing resistance was poor.

In test numbers 7 and 27, the step before the sizer was 0.1.
mm, the surface was subjected to verda grinding after the sizer, but the grinding amount did not reach 0.1 mm. The surface hardened layer due to the step was not sufficiently removed, and as a result, a portion where the surface hardness exceeded 260 Hv remained, and the hot-dip galvanizing resistance was poor.

In Test Nos. 8 to 10, and 28 to 30, the step before the sizer exceeded 0.1 mm, but the surface was 0.1 mm after the sizer.
It was ground by 1 mm or more. The surface hardened layer due to the step was removed, the surface hardness after the sizer did not exceed 260 Hv, and the hot-dip galvanizing resistance was good.

Test Nos. 11, 12, 31, and 32 are those in which the surface before the sizer exceeds 0.1 mm, but the surface is softened and heat-treated after the sizer. The surface hardened layer due to the step was removed, the surface hardness after the sizer became 260 Hv or less, and the hot-dip galvanizing resistance was improved.

[0060]

As is apparent from the above description, the electric resistance welded steel pipe of the present invention and the method of manufacturing the same are characteristically located in the vicinity of the welded portion of a large diameter thick electric resistance welded steel pipe used for a large steel tower or the like. The resulting hot-dip galvanizing cracks can be prevented. As a result, an inexpensive large-diameter thick-walled electric resistance welded steel pipe can be applied to various buildings such as a large steel tower, and the construction cost of various buildings can be reduced.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram showing a stepped portion which causes hot-dip galvanizing cracks and a mechanism of generation of a hardened portion caused by the stepped portion.

FIG. 2 is an explanatory diagram of elongation in a bath, which is an indicator of hot-dip galvanizing cracking resistance.

FIG. 3 is a dimensional diagram of a test piece for investigating elongation in a bath.

[Description of Signs] 1 steel pipe 2 weld bead 3 stepped part 4 hardened part

Claims (6)

[Claims] 1. A large-diameter and thick-wall ERW steel pipe having a C content of less than 0.12% by weight in a material steel, wherein a micro hardness of a portion corresponding to both side portions of the ERW weld bead is: An electric resistance welded steel pipe excellent in hot-dip galvanizing crack resistance, which is not more than 260 Hv over substantially the entire length of the steel pipe at least in a range of 0.2 mm in a thickness direction from an outer surface. 2. A large-diameter thick-wall ERW steel pipe having a C content of less than 0.12% by weight in a material steel, and subjected to a first surface treatment for removing an ERW weld bead, And an electric resistance welder for a steel tower excellent in hot-dip galvanizing crack resistance, which is subjected to a second surface processing for removing a step portion which is partially left in the vicinity of the processed part after the first surface processing. Steel pipe. 3. A large-diameter thick-walled ERW steel pipe having a C content of less than 0.12% by weight in a material steel, and subjected to a first surface treatment for removing an ERW weld bead, And a second surface processing or heat treatment for removing a work hardened portion generated in the vicinity due to a step portion partially left in the vicinity of the processed portion after the first surface processing. ERW steel pipe for steel towers with excellent hot-dip galvanizing crack resistance. 4. Using a hot rolled steel strip having a C content of less than 0.12% by weight in steel as a material, pipe forming, electric resistance welding, welding bead removing processing, heat treatment of a welded portion, contraction processing by a sizer. In the method of manufacturing a large-diameter thick ERW steel pipe in each step, the surface step remaining near the ERW weld bead removal processing portion at the stage after the ERW welding bead removal processing is completed and before the tube is contracted by the sizer. A method for producing an electric resistance welded steel pipe, characterized by cutting or grinding 0.1 mm or less. 5. A hot rolled steel strip having a C content of less than 0.12% by weight in steel as a raw material for pipe forming, electric resistance welding, welding bead removing processing, heat treatment of a welded portion, and contraction processing by a sizer. In the method for producing a large-diameter thick-walled ERW steel pipe in each step, at least the surface in the vicinity of the ERW weld bead removing portion is 0.1 m
A method for producing an electric resistance welded steel pipe, characterized by cutting or grinding to a depth of at least m. 6. Using a hot-rolled steel strip having a C content of less than 0.12% by weight in steel as a raw material, pipe forming, electric resistance welding, welding bead removing processing, heat treatment of a welded portion, contraction processing with a sizer. A method for producing a large-diameter thick-walled electric resistance welded steel pipe by each step, characterized in that at least a step near the part where the electric resistance welded bead is removed is softened and heat-treated at a stage after the contraction processing by the sizer is performed. Manufacturing method of ERW steel pipe.