JP2020110840A - Electroseamed steel pipe and method for manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electric resistance welded steel pipe in which fluctuation of the pipe thickness in the pipe circumferential direction due to pipe making is small around the pipe bottom and buckling can be suppressed, and a manufacturing method thereof.
An electric resistance welded steel pipe having a seam portion, the center of the electric resistance welded steel pipe being a center coordinate, and a pipe circumferential direction from the pipe bottom portion starting from the pipe bottom portion with the seam portion facing upward. The pipe thickness of the electric resistance welded steel pipe in the range of the angle θ1 of 0° to ±45° is in the range of 95.0% or more and 105.0% or less based on the average of the pipe thickness in the pipe circumferential direction. ERW steel pipe to be.
[Selection diagram] Fig. 3

Description

The present invention relates to an electric resistance welded steel pipe and a method for manufacturing the same. In particular, for oil wells, automobiles, or construction, the load during roll forming is large and the forming itself is difficult, and it is difficult to make the distribution of strain and pipe thickness variation in the pipe circumferential direction uniform. It is suitable for a thick-walled electric resistance welded steel pipe and a method for manufacturing the same.

ERW steel pipes have the excellent features of good dimensional accuracy, beautiful surface, and high productivity. They are used for line pipes such as oil and natural gas, automobiles, and construction. It is used in a wide range of applications such as steel pipes. In recent years, higher strength and thicker electric resistance welded steel pipes than ever have been required, and in the field of manufacturing hot-rolled steel sheets that are the raw material of electric resistance welded steel pipes, higher strength steel plates and thicker steel plates than before. Is being developed and manufactured. Here, the thick wall means a steel plate having a thickness of 26 mm or more and 32 mm or less.

FIG. 1 is a schematic diagram showing an example of an electric resistance welded steel pipe manufacturing facility. A steel strip 1 which is a material of an electric resistance welded steel pipe is straightened on the entrance side by, for example, a leveler 2 and then intermediately formed by a cage roll group 3 composed of a plurality of rolls into an open pipe, and then a fin pass roll group composed of a plurality of rolls. At 4, the tube is shaped into a tube. After that, the width end of the steel strip 1 is resistance-welded by a welding machine 6 while being pressed against the squeeze roll 5 to form an electric resistance welded steel pipe 7. The facility for producing electric resistance welded steel pipes is usually capable of producing steel pipes having various outer diameters and pipe thicknesses by adjusting the position of the forming rolls and exchanging the rolls in one forming line.

In the fin pass rolls having a plurality of stages arranged upstream of the welding machine, bending deformation and drawing deformation in the width direction of the steel plate are applied while the width ends of the steel plate are constrained by fins. As a result, the effect of improving the electric resistance weldability in the subsequent process by freezing the blank tube shape and optimizing the end surface of the steel sheet can be obtained. The amount of drawing with this fin pass roll is properly managed at each stand, and if the amount of drawing is too small, the electric resistance weldability will deteriorate significantly due to freezing of the raw pipe shape and defective end surface of the steel plate. .. On the other hand, if the drawing amount is excessively large, the mechanical characteristics are significantly deteriorated due to an increase in the circumferential thickness deviation of the steel pipe, especially around the pipe bottom, and work hardening. Particularly in the case of thick-walled steel pipes, such a problem becomes remarkable due to an increase in the circumferential length difference between the inner and outer surfaces of the steel pipes, so that fin-pass forming is difficult to control.

Therefore, various techniques have been developed in order to expand the manufacturable range of tubes in one molding line. For example, in Japanese Patent Laid-Open No. 2004-242242, the outer peripheral length drawing amount when a necessary drawing amount is given to the center of the plate thickness is calculated for each stand from the radius of curvature and bending angle of the mold and the plate thickness, and based on this, A method of forming using a designed fin pass roll is disclosed.

Japanese Patent No. 6007777

However, the method disclosed in Patent Document 1 is in a state where the semi-molded product (open pipe) is in contact with the entire surface of the fin pass roll in the caliber circumferential direction. In the one-stand fin-pass roll, there is a region where the roll and the open pipe are not in circumferential contact. In such a case, the radius of curvature and the bending angle at each circumferential position on the outer circumference of the raw pipe do not conform to the design, and the problem of an excessively small or excessive drawing cannot be solved.

When the open pipe is processed and formed, the open pipe is subjected to strain (work hardening). Particularly in roll forming, the work hardening tends to be different in the circumferential direction of the open pipe. Therefore, the yield ratio (YR) and the tube thickness are nonuniform in the tube circumferential direction. If the yield ratio and the pipe thickness in the pipe circumferential direction are not uniform, buckling is likely to occur when an impact is applied.

The present invention has been made in view of the above circumstances, and provides an electric resistance welded steel pipe capable of suppressing buckling and reducing the fluctuation of the pipe thickness in the pipe circumferential direction due to pipe forming around the pipe bottom, and a method for manufacturing the same. The purpose is to

As a result of diligent studies, the present inventors have found that the cross-sectional shape of the semi-formed product (open pipe) on the fin pass roll entrance side and the deviation of the pipe thickness in the pipe circumferential direction after the finish forming (fin pass forming) and the strain distribution in the circumferential direction. It has been found that there is a correlation between them, and that there exists an optimum condition that can suppress excessive strain after fin-pass forming, that is, work hardening, and can equalize the residual strain distribution in the pipe circumferential direction.

The present invention is based on the above findings, and its features are as follows.
[1] An electric resistance welded steel pipe having a seam portion, wherein the center of the electric resistance welded steel pipe is the center coordinate, and an angle in the pipe circumferential direction from the pipe bottom portion starting from the pipe bottom portion with the seam portion facing upward The pipe thickness of the electric resistance welded steel pipe in the range of θ1 of 0° to ±45° is in the range of 95.0% or more and 105.0% or less based on the average of the pipe thickness in the pipe circumferential direction. ERW steel pipe.
[2] The electric resistance welded steel pipe according to [1], wherein the yield strength of the pipe bottom is 295 MPa or more and 450 MPa or less, and the tensile strength of the pipe bottom is 430 MPa or more and 550 MPa or less.
[3]% by mass, C: 0.07 to 0.20%, Mn: 0.3 to 2.0%, P: 0.03% or less, S: 0.015% or less, Al: 0.01 ~ 0.06%, N: 0.006% or less, having a composition of the balance Fe and unavoidable impurities,
The steel structure of the central portion of the pipe thickness is composed of a main phase made of ferrite and one or more kinds selected from pearlite, pseudo-pearlite and upper bainite, and the area fraction is 8% or more and 20% or less. Two phases, and the average crystal grain size of the steel structure of the pipe thickness central portion is 7 μm or more and 20 μm or less,
The steel structures of the inner surface and the outer surface of the steel pipe are ferrite single phase or bainitic ferrite single phase, and the average crystal grain size of the steel structure of the inner surface and outer surface of the steel pipe is 2 μm or more and 20 μm or less. The electric resistance welded steel pipe according to [1] or [2].
[4] The electric resistance welded steel pipe according to [3], wherein the component composition further contains, by mass%, Si: less than 0.4%.
[5] The component composition further contains, in mass %, one or more selected from Nb: 0.05% or less, Ti: 0.05% or less and V: 0.10% or less. The electric resistance welded steel pipe according to [3] or [4], which is characterized in that
[6] The electric resistance welded steel pipe according to any one of [3] to [5], wherein the component composition further contains B: 0.008% or less by mass %.
[7] The electric resistance welded steel pipe according to any one of [1] to [6], which has a pipe thickness of 26 mm or more and 32 mm or less.
[8] An intermediate forming step of forming an open tube by intermediate forming a steel strip with a cage roll group, a finish forming step of finish forming into a tubular shape with a fin pass roll group, and a width end portion of the steel strip after the finish forming step. In a method for producing an electric resistance welded steel pipe having a welding step of forming a pipe by electric resistance welding
In the finish forming step, the center O of the outer peripheral circle formed by the first fin pass rolls of the fin pass roll group in the cross section orthogonal to the traveling direction of the open pipe is located at position O, and the bottom position in the open pipe cross section is When the position P is set and the angle θ2 is set in the circumferential direction from a straight line connecting the position O and the position P with the position O as the center, in the range where the angle θ2 is 0° to ±45°, the first fin-pass roll entrance side is described. A method for manufacturing an electric resistance welded steel pipe, characterized in that finish forming is performed so that the outer diameter curvature ρ _P of the open pipe and the curvature ρ _R of the first fin pass roll satisfy the following formula (1).
0.20≦ρ _P /ρ _R <1.49 (1)

According to the present invention, it is possible to manufacture an electric resistance welded steel pipe in which fluctuations in the pipe thickness in the pipe circumferential direction due to pipe forming are small around the pipe bottom, excessive work hardening is suppressed, and buckling can be suppressed.

FIG. 1 is a schematic diagram showing an example of an electric resistance welded steel pipe production line. FIG. 2 is a schematic diagram showing a cross-sectional shape of the open pipe immediately before finish forming by the fin pass roll. FIG. 3 is a diagram in which the roll gap space formed by the forming rolls (upper roll, lower roll, side roll) of the first fin-pass roll and the vertically long pre-forming shape of the open pipe are overlapped. FIG. 3A is a diagram showing an example in which the open pipe 8 is intermediately formed by a conventional method and has a vertically long cross section, and FIG. 3B shows a case where the open pipe 8 has a cross section satisfying the conditions of the present invention. It is a figure which shows the example of. FIG. 4 is a graph showing the influence of the shape of the open pipe on the fluctuation of the pipe thickness of the product. FIG. 5 is a graph showing the influence of the shape of the open pipe on the distribution of circumferential strain in the central portion of the pipe thickness of the product.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic diagram showing an example of an electric resistance welded steel pipe manufacturing facility. As described above, the steel strip 1 which is the material of the electric resistance welded steel pipe is straightened on the entrance side by, for example, the leveler 2 and then intermediately formed by the cage roll group 3 including a plurality of rolls to form an open pipe, and then from the plurality of rolls. The fin pass roll group 4 is used to finish and form into a tubular shape. After that, the width end of the steel strip 1 is resistance-welded by a welding machine 6 while being pressed against the squeeze roll 5 to form an electric resistance welded steel pipe 7. Here, the steel strip 1 can be exemplified by a hot rolled steel sheet of carbon steel.

In the intermediate forming by the cage roll group 3, the cross section of the semi-formed product (hereinafter referred to as the open tube 8) immediately before the finish forming (the entry side of the first fin pass roll) has a vertically long shape as shown in FIG. There is. In FIG. 3, the roll gap space formed by the forming rolls (upper roll, lower roll, side roll) of the first fin pass roll of the fin pass roll group 4 and the vertically long pre-forming shape of the open pipe 8 are superimposed. It is a figure, and in the cross section orthogonal to the traveling direction of the open pipe including the central axis of the lower roll, the first fin-pass roll is inserted into the cross section of the space generally surrounded by the outer diameter line of the roll cross section called roll caliber. It is the figure which made the outer surface of the bottom of the open tube 8 and the caliber bottom of the lower roll correspond, and projected the section of the open tube which intersects perpendicularly with the advancing direction of the open tube 8 in the side. Here, FIG. 3(a) is a diagram showing an example in which the open pipe 8 is intermediately formed by a conventional method and has a vertically long cross section, and FIG. 3(b) shows the open pipe 8 having a cross section of the present invention. It is a figure which shows the example in the case of the shape which satisfy|fills conditions.

Here, in the cross section orthogonal to the traveling direction of the open pipe, the center of the outer circumferential circle formed by the first fin pass roll is position O, the position of the bottom portion in the cross section of the open pipe 8 is position P, and the position O is the center. If the distance from the outer peripheral position X of the open pipe 8 to the position O at an angle θ2 (degrees) in the circumferential direction from the straight line connecting the O and the position P is r (mm), the cross-sectional shape of the open pipe 8 is the position O. Can be represented by a locus of polar coordinates (r, θ2) with the origin as. Similarly, the distance from the outer peripheral position X′ of the first fin-pass roll to the position O at an angle θ2 (degrees) in the circumferential direction from the straight line OP is r′ (mm). The shapes of the open pipe 8 and the first fin pass roll can be evaluated by converting the curvatures ρ _P and ρ _R , which are the reciprocal numbers thereof, using the distance r and the distance r′, and the function of the shape can be evaluated. Can be expressed as P(ρ _P , θ2) and R(ρ _R , θ2), respectively.

Therefore, the cross-sectional shape of the open pipe 8 is assumed to be bilaterally symmetric with respect to the straight line OP connecting the position O and the position P, and the cross-section of the open pipe 8 is defined by using the function P(ρ _P , θ2) of the shape of the open pipe 8 as a variable. The shape of the contact between the outer peripheral surface of the open pipe 8 and the caliber surface of the fin-pass roll during the finish forming (in the latter stage), the wall thickness increase distribution and the circumferential strain distribution of the steel pipe after the finish forming, etc. Numerical analysis was carried out to investigate the effect on. Here, the cross-sectional shape of the open pipe 8 is a shape 1 m away from directly below the first fin pass roll in the inlet side direction, that is, the upstream direction.

As a result, when the angle θ2 is in the range of 0° to ±45° in the circumferential direction, the outer diameter curvature ρ _P of the open pipe 8 on the inlet side of the first fin pass roll and the curvature ρ _R of the first fin pass roll are as follows. When the relational expression (1) is satisfied, the effect of strain (work hardening) is small at the pipe bottom portion in the circumferential direction of 0° to ±45°, and the yield ratio (YR) and the pipe thickness in the pipe circumferential direction are not uniform. It turned out that it can suppress.
0.20≦ρ _P /ρ _R <1.49 (1)
In the above formula (1),
ρ _P : outer diameter curvature of the open pipe on the inlet side of the first fin pass roll ρ _R : curvature of the first fin pass roll

When ρ _P /ρ _R is less than 0.20, when the open pipe 8 passes through the first fin pass roll, the bending work within the range of the angle θ2 becomes large, so that the load of the first fin pass roll becomes high. As a result, finish molding cannot be performed normally. On the other hand, when ρ _P /ρ _R is 1.49 or more, the difference between the function P (ρ _P , θ2) of the shape of the open pipe 8 and the function R (ρ _R , θ2) of the shape of the first fin pass roll is separated. Becomes large, sufficient bending deformation is not completed in the open tube 8 during finish forming with the first fin pass roll, and an open tube having a predetermined curvature cannot be obtained. When the drawing process by the fin pass forming is performed in such a state, the open pipe shape becomes vertically long, and the curvature ρ _P of the open pipe 8 becomes larger than the curvature ρ _R of the first fin pass roll, so that the first fin Since the open pipe 8 does not fit in the circumferential region of the first fin pass roll only by forming the pass roll, the first fin pass roll in a vertical section in the pipe axis direction immediately below the first fin pass roll. The contact portion in the circumferential direction of the open pipe that comes into contact with is extremely reduced. As a result, the strain (work hardening) differs depending on the circumferential position of the open pipe, and the yield ratio (YR) and the pipe thickness become uneven in the pipe circumferential direction, which causes buckling.

Further, when there is a large difference between the function P (ρ _P , θ) of the shape of the open pipe 8 and the function R (ρ _R , θ) of the shape of the first fin pass roll, the open pipe 8 and the first fin pass roll In a part of the non-contact portion, bending that causes excessive bending deformation occurs, and the wall thickness inside the bending tends to excessively increase.

The reason why the above relational expression (1) is limited to 0° to ±45° in the circumferential direction from the bottom of the tube is as follows. When a thick base plate is formed, bending due to excessive deviation between the function P (ρ _P , θ) of the shape of the open tube 8 and the function R (ρ _R , θ) of the shape of the first fin pass roll , Caused by forming on the lower roll of the first fin pass roll. Therefore, the circumferential direction is limited to 0° to ±45° from the bottom of the tube that comes into contact with the lower roll of the first fin pass roll.

The technical significance of formula (1) will be described below in detail.

FIG. 4 is a graph showing the influence of the shape of the open pipe 8 on the variation of the pipe thickness of the product, and shows the relationship between the angle in the circumferential direction of the pipe and the variation of the pipe thickness. The horizontal axis represents the angle θ1 from the pipe bottom of the steel pipe when the seam portion (welded portion) is up, and the vertical axis represents the pipe thickness variation rate when the pipe thickness at the pipe bottom is used as a reference.

FIG. 5 is a graph showing the effect of the shape of the open pipe on the distribution of the circumferential strain in the central portion of the pipe thickness of the product, showing the relationship between the angle in the circumferential direction of the pipe and the circumferential strain in the central portion of the pipe thickness. Showing. The horizontal axis is the angle θ1 from the bottom of the steel pipe when the seam is up, and the vertical axis is the strain in the circumferential direction of the central part of the pipe thickness when the central part of the pipe thickness is the reference (measurement method is (Calculated from finite element simulation on roll forming).

Regarding the horizontal axes of FIGS. 4 and 5, the tube was considered to be bilaterally symmetric, and the angle θ1 was a positive value (0° or more).

In the conventional manufacturing method, for example, when a thick-walled steel pipe having a thickness of 26 mm or more is formed, it is difficult to form it into a desired shape, the cross-sectional shape of the open pipe 8 becomes vertically long, and the outer diameter of the open pipe 8 is increased. The curvature ρ _P becomes larger than the curvature ρ _R of the first fin-pass roll, and ρ _P /ρ _R becomes 1.49 or more in the range where the angle θ2 of the open pipe 8 is 0° to ±45° in the circumferential direction. .. In such a case, in the finish forming with the first fin-pass roll, the angle θ2 of the open pipe 8 is in the range of 0° to ±45° in the circumferential direction, and in particular, the compressive strain in the circumferential direction is 0° to ±35°. The pipe thickness increased, and the pipe thickness increased drastically. Further, in particular, since the compressive strain increases from 0° to ±35° for the angle θ2 of the open pipe 8, work hardening becomes remarkable, and the pipe receives a large strain, so that the yield ratio in the pipe circumferential direction is increased. The distribution is considered to be non-uniform. Therefore, the plastic deformability of the steel pipe differs depending on the circumferential position of the pipe, and when a load is applied to the steel pipe, the deformation energy cannot be absorbed near the bottom of the pipe where the yield ratio is large, and local deformation proceeds at this position. , It is easy to go to destruction.

Regarding the method of setting ρ _P /ρ _{R to} be 0.20 or more and less than 1.49 when the angle θ2 of the open pipe 8 is in the range of 0° to ±45° in the circumferential direction, the finish forming with the first fin pass roll is performed. There is a method of controlling the shape of the open pipe by immediately applying an inner roll that projects the raw pipe from the inner surface side to the outer surface side of the pipe, but is not limited to this.

Next, the electric resistance welded steel pipe obtained by the manufacturing method of the present invention will be described.

The electric resistance welded steel pipe of the present invention is an electric resistance welded steel pipe having a seam portion, with the center of the electric resistance welded steel pipe being the center coordinates, the pipe circumferential direction from the pipe bottom portion with the pipe bottom portion as the starting point when the seam portion is on the top. Is characterized in that the tube thickness of the electric resistance welded steel pipe in the range of the angle θ1 of 0° to ±45° is in the range of 95.0% or more and 105.0% or less based on the average of the pipe thickness in the pipe circumferential direction. To do.

As described above, when the angle θ2 of the open pipe 8 is in the range of 0° to ±45° in the circumferential direction, ρ _P /ρ _R is 0.20 or more and less than 1.49, so that the first fin pass roll In the finish forming, the angle θ2 of the open pipe 8, that is, the angle θ1 in the pipe circumferential direction from the pipe bottom starting from the pipe bottom when the seam is up with the center of the center of the electric resistance welded steel pipe as the center coordinate is 0°. The increase of compressive strain in the range of ±45° is small, and the pipe thickness of the electric resistance welded steel pipe should be within the range of 95.0% or more and 105.0% or less based on the average of the pipe thickness in the pipe circumferential direction. You can Therefore, the variation in the amount of strain applied to the open pipe due to the circumferential position is smaller than in the conventional case, so that the yield ratio (YR) and the pipe thickness can be made uniform in the pipe circumferential direction, and buckling can be suppressed. I found out that

The average of the reference pipe thickness in the pipe circumferential direction is a value obtained by measuring the pipe thickness from the seam position to the entire circumference of the pipe using a micrometer and averaging the measured values.

In the electric resistance welded steel pipe of the present invention, it is preferable that the yield strength of the pipe bottom is 295 MPa or more and 450 MPa or less, and the tensile strength of the pipe bottom is 430 MPa or more and 550 MPa or less. When the yield strength of the bottom portion of the steel pipe exceeds 450 MPa, it is difficult to form a thick-walled steel pipe having a thickness of 26 mm or more because the forming load exceeds the withstand load of the mill. Further, when the yield strength of the bottom portion of the steel pipe is less than 295 MPa, even if the thick steel pipe having a thickness of 26 mm or more is formed, the pre-deformation proceeds to the open pipe on the entry side of the formation of the first fin pass. Since the open pipe fits into the first fin pass roll without causing breakage in the open pipe, problems such as work hardening and thickening around the pipe bottom are unlikely to occur. Further, if the tensile strength of the bottom portion of the steel pipe is less than 430 MPa, the problem of warpage in the longitudinal direction of the pipe after forming tends to occur. Further, if the tensile strength of the bottom portion of the steel pipe exceeds 550 MPa, there is a problem that the dimensional accuracy of the cylindrical shape of the formed pipe in the circumferential direction deteriorates.

Next, the electric resistance welded steel pipe of the present invention is, in mass%, C: 0.07 to 0.20%, Mn: 0.3 to 2.0%, P: 0.03% or less, S: 0.015. % Or less, Al: 0.01 to 0.06%, N: 0.006% or less, and has a composition of the balance Fe and unavoidable impurities. And a second phase consisting of one or more selected from pearlite, pseudo-pearlite and upper bainite, and having an area fraction of 8% or more and 20% or less. The average crystal grain size of the steel structure is 7 μm or more and 20 μm or less, the steel structures of the inner and outer surfaces of the steel pipe are ferrite single phase or bainitic ferrite single phase, and the average crystal grain size is 2 μm or more and 20 μm or less. Is desirable.

The preferred component composition of the electric resistance welded steel pipe of the present invention will be described below. In addition, all% of the unit which shows a component composition mean the mass %.

C: 0.07 to 0.20%
C is an element that increases the strength of the steel sheet by solid solution strengthening and contributes to the formation of pearlite, which is one of the second phases. In order to secure desired tensile properties, toughness, and desired steel sheet structure, it is preferable to contain 0.07% or more. On the other hand, if the content exceeds 0.20%, a martensitic structure is generated during welding of the electric resistance welded steel pipe, which may cause weld cracking. Therefore, C is preferably in the range of 0.07 to 0.20%. More preferably, C: 0.09 to 0.18%.

Mn: 0.3-2.0%
Mn is an element that increases the strength of the steel sheet through solid solution strengthening, and is preferably contained in an amount of 0.3% or more in order to secure the desired steel pipe strength, and in the content of less than 0.3%, ferrite is contained. The transformation start temperature rises and the structure tends to become excessively coarse. On the other hand, if the content exceeds 2.0%, the hardness of the center segregation portion increases, which may cause cracking during electric resistance welding. Therefore, Mn is preferably in the range of 0.3 to 2.0%. More preferably, it is 0.3 to 1.6%, and further preferably 0.3 to 1.4%.

P: 0.03% or less P is an element having a function of segregating at ferrite grain boundaries and lowering toughness. In the present invention, it is desirable to reduce it as an impurity as much as possible, but excessive reduction is a refining cost. Therefore, 0.002% or more is preferable, and 0.03% is allowable. Therefore, P is preferably 0.03% or less. It is more preferably 0.025% or less.

S: 0.015% or less S exists as a sulfide in steel, and exists mainly as MnS in the composition range of the present invention. Since MnS is thinly stretched in the hot rolling process and adversely affects ductility and toughness, it is desirable to reduce it as much as possible in the present invention. However, excessive reduction leads to a high refining cost, so 0.002% or more is preferable. In addition, 0.015% is acceptable. Therefore, S is preferably 0.015% or less. More preferably, it is 0.010% or less.

Al: 0.01 to 0.06%
Al is an element that acts as a deoxidizing agent and has the action of fixing N as AlN. In order to obtain such an effect, it is preferable to contain 0.01% or more. If it is less than 0.01%, the deoxidizing power becomes insufficient when Si is not added, and oxide-based inclusions increase, which causes the weld zone to be welded during longitudinal welding of electric resistance welded steel pipe, especially when welding in air. The risk of forming oxides increases, and the toughness of the welded portion of the electric resistance welded steel pipe decreases. On the other hand, if it exceeds 0.06%, the weldability deteriorates and the amount of alumina-based inclusions increases, so that the surface quality deteriorates. Therefore, Al is preferably 0.01 to 0.06%. More preferably, it is 0.02-0.05%.

N: 0.006% or less N is an element having the effect of lowering the toughness by firmly fixing the movement of dislocations. In the present invention, it is desirable to reduce it as an impurity, but 0.006% Up to acceptable. Therefore, N is preferably 0.006% or less. More preferably, it is 0.005% or less.

The preferred main components of the electric resistance welded steel pipe of the present invention are as described above. If necessary, the following elements may be included as appropriate.

Si: less than 0.4% Si is an element that contributes to the strength increase of the steel sheet by solid solution strengthening, and can be contained as necessary in order to secure a desired steel sheet strength. In order to obtain such an effect, it is desirable that the content is more than 0.01%, but if the content is 0.4% or more, a fire light called red scale is easily formed on the surface of the steel sheet, and In many cases, the appearance properties deteriorate. Therefore, when it is contained, it is preferably less than 0.4%. It is more preferably 0.2% or less. When Si is not added, Si is an unavoidable impurity and its level is 0.01% or less.

One or more selected from Nb: 0.05% or less, Ti: 0.05% or less, V: 0.10% or less Nb, Ti, and V are all fine carbides and nitrides in steel. It is an element that forms a substance and contributes to the strength improvement of steel through precipitation strengthening. The inclusion of these elements tends to increase the yield ratio after forming the steel pipe. Therefore, in the present invention, it is desirable not to contain it, but it may be contained for the purpose of adjusting the strength as long as the yield ratio of the electric resistance welded steel pipe is 90% or less. When they are contained, Nb: 0.05% or less, Ti: 0.05% or less, and V: 0.10% or less, respectively.

B: 0.008% or less B is an element that delays the ferrite transformation during the cooling process, promotes the formation of low-temperature transformed ferrite, that is, the ashular ferrite phase, and increases the strength of the steel sheet. The content of B increases the yield ratio of the steel plate and the yield ratio of the electric resistance welded steel pipe. Therefore, in the present invention, if the yield ratio of the electric resistance welded steel pipe is 90% or less, it can be contained as needed for the purpose of adjusting the strength. When it is contained, B: 0.008% or less is preferable. It is more preferably 0.0001 to 0.0015%, further preferably 0.0003 to 0.0008%.

The balance other than the above components is Fe and inevitable impurities. The unavoidable impurities may be, for example, O: 0.005% or less.

Next, the steel structure of the electric resistance welded steel pipe of the present invention will be described. In the steel structure of the present invention, the steel structure in the central portion of the pipe thickness has a main phase and a second phase. The main phase is made of ferrite, and the area fraction of the main phase is 80% or more and 92% or less. The second phase is composed of one or more selected from pearlite, pseudo pearlite and upper bainite, and the area fraction of the second phase is 8% or more and 20% or less. If the area fraction of the second phase is less than 8%, the desired tensile strength cannot be satisfied. If the area fraction of the second phase exceeds 20%, the desired low temperature toughness cannot be secured. Therefore, the area fraction of the second phase is limited to the range of 8% or more and 20% or less. The average crystal grain size of the steel structure at the center of the pipe thickness is 7 μm or more and 20 μm or less. The "average grain size of the steel structure in the central portion of the pipe thickness" as used herein refers to all the grains of the ferrite phase that constitutes the main phase, and the pearlite phase, the pseudo pearlite phase, and the upper bainite phase that constitute the second phase. It means the measured average crystal grain size. If the average crystal grain size is less than 7 μm, it is too fine to secure a yield ratio of 90% or less for the electric resistance welded steel pipe. On the other hand, if the average crystal grain size exceeds 20 μm and is coarsened, the toughness of the electric resistance welded steel pipe decreases, and it becomes impossible to secure the desired toughness. From the viewpoint of ensuring higher toughness, the average crystal grain size is preferably 15 μm or less.

The steel structure in the central part of the pipe thickness is obtained by observing the structure by the following method to determine the types of the main phase and the second phase, the area fraction, and the average crystal grain size of the steel structure in the central part of the pipe thickness. First, a structure observation test piece collected from an electric resistance welded steel pipe was polished so that a cross section (C cross section) perpendicular to the pipe axis direction became an observation surface, subjected to nital corrosion, and the tube thickness was measured from the surface of the structure observation test piece. A steel structure is observed and imaged using an optical microscope (magnification: 500 times) or a scanning electron microscope (magnification: 500 times) with the 1/2t position as the observation center. Note that t is the thickness of the steel pipe. Then, for the obtained structure photograph, the type of the main phase and the second phase was specified using an image analysis device (image analysis software: Photoshop, manufactured by Adobe), the area fraction was calculated, and JIS G 0551 is described. The average crystal grain size of the steel structure in the central part of the pipe thickness (main phase and second phase) is calculated by the method of.

The steel structures of the inner surface and the outer surface of the steel pipe are ferrite single phase or bainitic ferrite single phase, and the average crystal grain size is 2 μm or more and 20 μm or less. The term "single phase" as used herein means a case where the area fraction is 95% or more (the balance may contain less than 5% of pearlite, martensite, and austenite). The inner surface and the outer surface of the electric resistance welded steel pipe specifically refer to regions up to 1 mm from both surfaces of the electric resistance welded steel pipe. When the average crystal grain size is less than 2 μm, the yield strength of the inner surface and outer surface of the electric resistance welded steel pipe excessively increases, the load during roll forming increases, and it becomes difficult to form a round steel pipe. Further, if it exceeds 20 μm and becomes coarse, the toughness of the electric resistance welded steel pipe is lowered, and it becomes impossible to secure desired toughness. Therefore, the average crystal grain size is limited to 2 μm or more and 20 μm or less. The upper limit of the average crystal grain size is preferably 15 μm.

The steel structures of the inner surface and the outer surface of the steel pipe are located from the surface of the test piece for microstructure observation (the inner surface of the electric resistance welded steel pipe is the inner surface of the steel pipe, and the outer surface of the electric resistance welded steel pipe is the outer surface of the steel pipe) at a pipe thickness of 1/2 t Type of steel structure in the same manner as the observation method and measurement method of the steel structure at the center of the pipe thickness, except that the observation field of view is within 1 mm from the surface of the electric resistance welded steel pipe instead of the observation center. , Find the average grain size.

Thus, the composition, the type of steel structure at the center of the pipe thickness, the area fraction and the average crystal grain size, and the type and average crystal grain size of the steel structure on the inner and outer surfaces of the steel pipe are all within the desired range. By this, in addition to making the yield ratio (YR) and the pipe thickness uniform in the pipe circumferential direction, an electric resistance welded steel pipe having excellent strength and toughness can be obtained.

The thickness of the electric resistance welded steel pipe of the present invention is preferably 26 mm or more and 32 mm or less. More preferably, it is 28 mm or more.

Next, a method for manufacturing a steel strip, which is a material for the electric resistance welded steel pipe of the present invention, will be described. Specifically, a steel material having the above composition is heated, hot-rolled, and cooled after hot-rolling to form a hot-rolled steel strip, and the hot-rolled steel strip is wound into a coil material. A manufacturing process can be performed to obtain a steel strip that is a material of an electric resistance welded steel pipe.

Although the manufacturing conditions of the steel strip are not particularly limited, for example, the steel material having the above composition is preferably heated to 1100 to 1300°C. When the heating temperature is less than 1100°C, the deformation resistance is high, the rolling load increases, and the rolling efficiency decreases. On the other hand, when the heating temperature is higher than 1300° C. and becomes high temperature, the crystal grains become coarse and the low temperature toughness is deteriorated, and moreover, the scale generation amount is increased and the surface quality may be deteriorated. Therefore, the heating temperature in hot rolling is preferably 1100 to 1300°C.

Then, the heated steel material is subjected to hot rolling. The hot rolling is rolling including rough rolling and finish rolling. As for the conditions of rough rolling, it is desirable to use a sheet bar having a predetermined size and shape at a rough rolling finish temperature in the range of 950 to 1150°C. If the finish temperature of the rough rolling is less than 950°C, the load resistance and rolling torque of the rough rolling mill are likely to be insufficient. On the other hand, when the temperature exceeds 1150° C. and the temperature becomes high, the austenite grains become coarse, and even if finish rolling is performed thereafter, it becomes difficult to secure a desired average grain size of 20 μm or less.

In the finish rolling after the rough rolling, the finish rolling start temperature is in the range of 1100 to 850°C, the finish rolling end temperature (finish rolling exit temperature) is in the range of 900 to 750°C, and the product having the desired product thickness is obtained. It is preferable to use a plate (hot rolled steel plate). When the finish rolling start temperature (finish rolling entry temperature) is less than 850° C., the temperature near the surface of the steel sheet in the finish rolling mill becomes below the Ar ₃ transformation point, and the risk of ferrite formation increases. The generated ferrite becomes ferrite grains elongated in the rolling direction due to the subsequent finish rolling, which causes deterioration in workability. On the other hand, when the finish rolling start temperature (finish rolling entry temperature) exceeds 1100° C. and becomes high, the effect of refining the γ grains due to the above-described finishing rolling is reduced, and the desired average grain size of 20 μm or less is desired. It becomes difficult to secure the particle size. Therefore, the finish rolling start temperature is preferably limited to the range of 1100 to 850°C. The finish rolling start temperature is more preferably 1050 to 850°C.

When the finishing rolling end temperature (finishing rolling outlet side temperature) exceeds 900° C. and reaches a high temperature, the processing strain added during the finishing rolling becomes insufficient and the γ grains cannot be refined. Therefore, the average grain size is 20 μm. It becomes difficult to secure the following desired average crystal grain size. On the other hand, when the finish rolling end temperature (finish rolling exit side temperature) is less than 750° C., the temperature in the vicinity of the steel sheet surface in the finish rolling mill becomes the Ar ₃ transformation point or lower, ferrite grains elongated in the rolling direction are formed, and ferrite grains are formed. There is an increase in the risk that the particles become mixed and the workability decreases. For this reason, it is preferable that the finish rolling finish temperature (finish rolling exit temperature) be in the range of 900 to 750°C. The upper limit of the finish rolling end temperature is more preferably 850°C.

It is preferable to perform a cooling step after finishing rolling. In the cooling step, the hot-rolled sheet obtained by finish rolling is cooled at the plate thickness center temperature at a cooling rate such that the average cooling rate from the start of cooling to the stop of cooling (end of cooling) is 4°C/s or more and 25°C/s or less. It is preferable to cool to a stop temperature of 580° C. or lower. Cooling performed in the cooling step is performed by water injection such as water jet cooling, water column cooling, spray cooling, mist cooling, or the like, or gas jet cooling that injects cooling gas. In addition, it is preferable to perform a cooling operation on both sides of the steel sheet (hot rolled sheet) so that both sides are cooled under the same conditions.

When the average cooling rate of the plate thickness center of the steel plate is less than 4° C./s, the generation frequency of ferrite grains decreases, the ferrite crystal grains become coarse, and the desired average crystal grain size of 20 μm or less in the plate thickness center portion is desired. The diameter cannot be secured. On the other hand, if it exceeds 25° C./s, the production of pearlite is suppressed and the upper bainite structure is formed, so that it becomes impossible to secure a desired average crystal grain size in the central portion of the plate thickness. For this reason, the average cooling rate at the plate thickness center is preferably 4° C./s or more and 25° C./s or less, more preferably the lower limit is 5° C./s and the upper limit is 15° C./s. The average cooling rate at the plate thickness center is determined by ((temperature at the plate thickness center at the start of cooling-temperature at the plate thickness center at the time when cooling is stopped)/cooling time). The temperature at the center of the steel plate thickness is obtained by calculating the temperature distribution in the steel plate cross section by heat transfer analysis and correcting the result according to the actual temperatures of the outer surface and the inner surface. If the cooling stop temperature exceeds 580° C., the desired average crystal grain size in the central portion of the plate thickness of 7 μm or more and 20 μm or less cannot be satisfied. In order to obtain the desired front and back surface steel structure, it is preferable that the average cooling rate in the temperature range of 750°C or more and 650°C or less at the steel plate surface temperature is 20°C/s or more. Further, it is preferable to start the cooling step immediately (within 5 seconds) after finishing rolling.

Then, in the cooling step, an initial cooling step that is for 10 seconds from the start of cooling, that is, a cooling step of 0.2 seconds or more and less than 3.0 seconds for 10 seconds (10 seconds) after starting cooling the hot-rolled sheet is performed. It is preferable to provide and cool it once or more. This is performed in order to suppress the formation of martensite structure or upper bainite structure on the front and back surfaces of the plate. If the cooling step is not provided in the initial cooling step or if the cooling step is less than 0.2 s, the steel structure on the front and back of the plate thickness becomes a martensite structure, bainite structure or upper bainite structure, and ferrite single phase or bainite structure is obtained. Tick ferrite single phase structure cannot be obtained. Further, if a cooling step of 3.0 s or more is provided in the initial cooling step, the structure will be composed of ferrite and pearlite, and the desired steel structure cannot be obtained. Therefore, in the cooling step, the time of the cooling step performed during the initial cooling step, which is 10 seconds from the start of cooling, is preferably 0.2 s or more and less than 3.0 s. The time of the cooling step is more preferably 0.4 s or more and 2.0 s or less. The number of cooling steps performed during the initial cooling step may be appropriately determined according to the cooling equipment arrangement, the cooling stop temperature, etc., and the upper limit is not particularly limited.

After the cooling is completed, a winding process is performed. In the winding step, it is preferable to wind at a winding temperature of 580° C. or less and then to cool. When the coiling temperature exceeds 580° C., ferrite transformation and pearlite transformation proceed after coiling, so that it becomes impossible to satisfy the desired average crystal grain size of 7 μm or more and 20 μm or less in the central portion of the plate thickness. Even if the coiling temperature is lowered, there is no problem with the material, but if it is less than 400°C, the coiling deformation resistance becomes large and it can be coiled cleanly, especially in the case of thick steel plates with a plate thickness of over 26 mm. May not be. Therefore, the winding temperature is preferably 400° C. or higher.

The electric resistance welded steel pipe of the present invention is a steel strip obtained by the above-mentioned process, which is intermediately formed by the cage roll group described above, and an intermediate forming process for forming an open pipe, and a finish forming process in which the fin-pass roll group is formed into a tubular shape. It can be manufactured through a step and a welding step in which the width end of the steel strip is subjected to electric resistance welding to form a pipe after the finish forming step. In the finish forming step, the above-mentioned formula (1) may be satisfied. In addition, in the finish forming process, in order to obtain a good weld in the subsequent welding process, it is necessary to freeze the shape of the open pipe and optimize the shape by pressing the open pipe end face against the fin while drawing. There is. It is preferable that the condition of the draw forming is that the draw ratio in the circumferential direction is 0.05% or more and 1.4% or less with reference to the central portion of the pipe thickness. If the drawing ratio exceeds 1.4%, the influence of work hardening due to draw forming becomes large, and the yield ratio of the entire formed pipe becomes excessive. If the drawing ratio is less than 0.05%, a good weld cannot be obtained for the above reason. More preferably, it should be 0.2% or more and 1.0% or less. Regarding the caliber conditions of each fin-pass roll, it is possible to design one or more types of curvature for one roll, but there is no designation for the combination as long as it can be formed into a predetermined shape.

As described above, according to the present invention, it is possible to obtain the electric resistance welded steel pipe in which the fluctuation of the pipe thickness in the pipe circumferential direction due to the pipe making is small around the pipe bottom and the buckling can be suppressed. That is, according to the present invention, the yield ratio (YR) and the pipe thickness in the pipe circumferential direction, which are considered to be caused by buckling, can be made uniform. In addition, in the present invention, the yield strength: 295 MPa or more, the tensile strength: 430 MPa or more, a low yield ratio of 90% or less is obtained by controlling the component composition and structure, and the Charpy at a test temperature: 0° C. It is possible to obtain an electric resistance welded steel pipe excellent in strength and toughness such that the absorbed energy in the impact test is 27 J or more. For example, the electric resistance welded steel pipe of the present invention can be used as a building structure member by forming a square steel pipe by square forming.

Hereinafter, for further understanding of the present invention, examples will be described. The examples do not limit the present invention.

Hot-rolled steel sheets having the compositions shown in Table 1 were produced under the production conditions shown in Table 2. Next, by using the obtained hot-rolled steel sheet as a raw material, the shape of the open pipe is controlled at the pushing position of the inner roll in a cold state, and then finish forming (fin pass forming) and electric resistance welding are performed to obtain a predetermined dimension (table An electric resistance welded steel pipe having a product pipe thickness and outer diameter shown in 3 was obtained. Regarding the outer diameter curvature ρ _P of the open pipe on the inlet side of the first fin pass and the curvature ρ _R of the first fin pass roll, the curvature in the range of 0° to ±45° from the pipe bottom of the open pipe with reference to the bottom of the open pipe. And The curvature ρ _R of the first fin pass roll in the range of 0° to ±45° is a constant value. The curvature ρ _R of the first fin-pass roll in the range of 0° to ±45° was measured using an outer diameter measuring gauge at a pitch of 15 degrees in the circumferential direction and calculated from the average value thereof. The outer diameter curvature ρ _P of the open pipe was measured at a position 1 m away from directly below the first fin pass roll in the direction opposite to the forming direction. Table 3 shows the curvature ratio (ρ _P /ρ _R ) and the drawing ratio (%) of the first fin-pass roll.

The obtained electric resistance welded steel pipe was sliced into vertical sections in the direction of the pipe axis, and the pipe thickness of the obtained sliced sample was measured with a micrometer. The measurement position of the pipe thickness was measured at a pitch of 15° in the circumferential direction of the pipe with reference to the welded portion. The average value of the measured pipe thickness was taken as the average of the pipe thickness in the pipe circumferential direction. On the other hand, the ratio of the maximum value of the pipe thickness measured in the range of 0° to ±45° from the pipe bottom of the pipe with the pipe bottom located opposite to the welded portion as a reference, divided by the average of the pipe thickness in the pipe circumferential direction. Was calculated as the pipe bottom pipe thickness increment rate based on the pipe thickness average in the pipe circumferential direction.

Further, a test piece was cut out from the bottom of the electric resistance welded steel pipe and a tensile test was performed. The tensile test method was as follows.
(1) Tensile test A JIS No. 5 tensile test piece was sampled from the bottom of the tube so that the tensile direction was the axial direction of the tube, and a tensile test was carried out in accordance with the provisions of JIS Z 2241. Yield strength YS, tensile strength The TS was measured, and the yield ratio YR (%) defined by (yield strength)/(tensile strength)×100 (%) was calculated.

Also, tensile test pieces were cut out from θ=±30°, ±60°, ±90°, ±120°, ±150° with reference to the bottom of the pipe, the yield ratio YR was calculated, and the yield ratio YR was calculated. The minimum value was calculated with. The difference between this minimum value and the yield ratio YR obtained from the tensile test piece cut out from the tube bottom was calculated. A difference in yield ratio YR of 15% or less was regarded as acceptable.

Further, the steel structure of the electric resistance welded steel pipe was observed and the average crystal grain size was obtained. The measuring method is as described above.

Regarding the toughness of the electric resistance welded steel pipe, a V-notch Charpy test piece was sampled in the circumferential direction from the center portion of the thickness of the obtained electric resistance welded steel pipe at the center of the pipe thickness, in accordance with JIS Z 2242, and tested. A Charpy impact test was carried out at a temperature of 0° C., and an absorbed energy vE ₀ (J) was obtained. The number of test pieces was measured as an average value of 3 pieces.

The measurement results are shown in Table 3.

From the results of Table 3, in the example of the present invention, in the range of 0° to ±45° from the bottom of the pipe, the average thickness in the circumferential direction of the pipe is within the range of 95.0% or more and 105.0% or less. .. In addition, the steel pipe No. The invention examples 1 to 4, 13 to 15 and 18 to 23 have yield strengths of 295 MPa or more, tensile strengths of 430 MPa or more and low yield ratios of 90% or less by controlling the component composition and structure. It is possible to obtain an electric resistance welded steel pipe having excellent strength and toughness, such as the absorption energy of the Charpy impact test at a test temperature of 0° C. of 27 J or more.

1 Steel strip 2 Leveler 3 Cage roll group 4 Fin pass roll group 5 Squeeze roll 6 Welding machine 7 ERW steel pipe 8 Open pipe θ1 Position P is the position of the bottom of the open pipe cross section, and position O is centered on position O and position P Angle from the straight line that connects θ2 with the center of the center of the ERW steel pipe as the center coordinate, and the angle from the pipe bottom to the pipe bottom when the seam is up

Claims (8)

An electric resistance welded steel pipe having a seam portion, wherein an angle θ1 in the pipe circumferential direction from the pipe bottom portion starting from the pipe bottom portion when the seam portion is on the center coordinates of the center of the electric resistance welded steel pipe is 0. The electric resistance welded steel pipe is characterized in that the pipe thickness of the electric resistance welded steel pipe in the range of ° to ±45° is in the range of 95.0% or more and 105.0% or less based on the average of the pipe thickness in the pipe circumferential direction. .. The electric resistance welded steel pipe according to claim 1, wherein the yield strength of the pipe bottom is 295 MPa or more and 450 MPa or less, and the tensile strength of the pipe bottom is 430 MPa or more and 550 MPa or less. % By mass, C: 0.07 to 0.20%, Mn: 0.3 to 2.0%, P: 0.03% or less, S: 0.015% or less, Al: 0.01 to 0. 06%, N: 0.006% or less, having a composition of the balance Fe and unavoidable impurities,
The steel structure of the central portion of the pipe thickness is composed of a main phase made of ferrite and one or more kinds selected from pearlite, pseudo-pearlite and upper bainite, and the area fraction is 8% or more and 20% or less. Two phases, and the average crystal grain size of the steel structure of the pipe thickness central portion is 7 μm or more and 20 μm or less,
The steel structures of the inner surface and the outer surface of the steel pipe are ferrite single phase or bainitic ferrite single phase, and the average crystal grain size of the steel structure of the inner surface and outer surface of the steel pipe is 2 μm or more and 20 μm or less. The electric resistance welded steel pipe according to claim 1 or 2. The electric resistance welded steel pipe according to claim 3, wherein the component composition further contains, by mass%, Si: less than 0.4%. The component composition further contains, in mass %, one or more selected from Nb: 0.05% or less, Ti: 0.05% or less and V: 0.10% or less. The electric resistance welded steel pipe according to claim 3 or 4. The ERW steel pipe according to any one of claims 3 to 5, wherein the component composition further contains B: 0.008% or less by mass %. The electric resistance welded steel pipe according to any one of claims 1 to 6, wherein the pipe thickness is 26 mm or more and 32 mm or less. The steel strip is intermediately formed by a cage roll group to form an open tube, a finish forming step of finish-forming a tube by a fin pass roll group, and the width end portion of the steel strip after the finish forming step has an electric resistance. In a method of manufacturing an electric resistance welded steel pipe having a welding step of welding to form a pipe,
In the finish forming step, the center of the outer circumferential circle formed by the first fin pass rolls of the fin pass roll group in the cross section orthogonal to the traveling direction of the open pipe is located at position O, and the position of the bottom portion in the open pipe cross section is defined as When the position P is set and the angle θ2 is a circumferential direction from a straight line connecting the position O and the position P with the position O as the center, in the range where the angle θ2 is 0° to ±45°, the first fin pass roll entrance side is described. A method for manufacturing an electric resistance welded steel pipe, characterized in that the outer diameter curvature ρ _P of the open pipe and the curvature ρ _R of the first fin pass roll are finish-formed so as to satisfy the following formula (1).
0.20≦ρ _P /ρ _R <1.49 (1)

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