WO2022075026A1

WO2022075026A1 - Rectangular steel pipe and production method therefor, and building structure

Info

Publication number: WO2022075026A1
Application number: PCT/JP2021/034008
Authority: WO
Inventors: 晃英松本; 稜仲澤; 昌士松本; 信介井手
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2020-10-05
Filing date: 2021-09-15
Publication date: 2022-04-14
Also published as: JPWO2022075026A1; TW202219280A; KR20230059820A; CN116323065A; JP7306494B2; TWI795923B

Abstract

Provided are a rectangular steel pipe, a production method therefor, and a building structure. The present invention is a rectangular steel pipe having flat plate parts and corner parts, wherein, when the average thickness of the plate parts is t (mm), the outer radius of curvature R of the corner parts is 2.0-3.0t, the flatness of the outer surface of the flat plate parts is 2.5 mm or less, the uniform elongation E2 at a position 1/4t in the thickness direction from the outer surface of the corner parts is 0.60 times or greater than the uniform elongation E1 at a position 1/4t in the thickness direction from the outer surface of the flat plate parts, and the Charpy absorbed energy at -10°C at a position 1/4t in the thickness direction from the outer surface of the corner parts is 100J or greater.

Description

Square steel pipe and its manufacturing method and building structure

The present invention relates to a square steel pipe and a method for manufacturing the same, which are particularly preferably used for a medium-rise building having a height of more than 20 m and a building member of a large building such as a factory or a warehouse. Further, the present invention relates to a building structure using the square steel pipe of the present invention as a column material.

High ductility and toughness are required for pillar materials of buildings from the viewpoint of earthquake resistance.

A square steel pipe having corners and flat plates used for columns is greatly deformed especially on the outer surface of the corners when it receives a large external force such as seismic force. Therefore, the square steel pipe needs to sufficiently enhance the ductility and toughness of the outer surface of the corner portion.

Cold roll-formed square steel pipe (roll-formed square steel pipe) is a square steel pipe widely used as a pillar material for buildings. This is done by using rolls arranged on the top, bottom, left, and right of the electric pipe after the steel strip is made into a cylindrical open pipe by cold roll forming and the butt portion of the open pipe is electric-sewn to make an electric-sewn steel pipe. , It is manufactured by drawing a pipe in the axial direction of the electric pipe while keeping it cylindrical, and then forming it into a square shape. In the above-mentioned electric sewing welding, the butt portion is heated and melted, and is pressure-welded and solidified to complete the joining.

However, while the roll-formed square steel pipe has high productivity, there is a problem that the ductility and toughness of the corners are lower than those of the flat plate because the corners are work-hardened greatly during manufacturing.

There is also a demand that the square steel pipe used for the pillar material should have a small radius of curvature at the corner of the square steel pipe from the viewpoint of workability at the construction site and the design of the building. This is because the larger the area of the flat plate portion of the column material, the larger the area where the column material and the beam material can be joined, and the more flexible the architectural design becomes possible.

However, in the roll-formed square steel pipe, the larger the ratio of the average wall thickness t to the average side length H (that is, t / H), the larger the circumferential bending strain required for forming the steel strip, and the greater the circumferential bending strain of the square portion. The amount of work hardening increases. Further, the smaller the radius of curvature of the corner portion, the larger the circumferential bending strain required for forming the corner portion, and the larger the amount of work hardening of the corner portion. Therefore, in the roll-formed square steel pipe having a large ratio (t / H) of the average wall thickness t to the average side length H and a small radius of curvature of the corners, the ductility and toughness of the corners are particularly low, which is sufficient. It was difficult to ensure seismic performance.

Here, the above-mentioned "average wall thickness t" is an average value of the wall thickness (mm) at the center position in the pipe circumferential direction of the three flat plate portions excluding the flat plate portion including the welded portion (electrically sewn welded portion). .. The above-mentioned "average side length H" is an average value of the side lengths of two flat plate portions adjacent to each other with a corner portion sandwiched between them.

In response to such a request, for example, the square steel pipes described in Patent Documents 1 to 4 have been proposed.

In Patent Document 1, a steel plate to which vanadium is added as a chemical component is bent and then welded to form a semi-formed square steel pipe, and this semi-formed square steel pipe is heated to the vicinity of the A3 transformation point and hot _- formed. Later, a square steel pipe obtained by cooling has been proposed. It is disclosed that this square steel pipe improves the yield strength and toughness and forms the shape of the corner portion sharply.

Patent Document 2 proposes a square steel pipe in which a cold-formed portion is heat-treated. It is disclosed that this square steel pipe has improved the mechanical properties and weldability of the cold formed portion.

Patent Document 3 describes a square steel tube with improved toughness and plastic deformability of the corners by appropriately controlling the chemical composition of the material steel sheet, the bainite fraction of the metal structure, and the Vickers hardness of the surface layer of the corners. Has been proposed.

Patent Document 4 proposes a square steel pipe having improved toughness at the corners by appropriately controlling the chemical composition of the raw steel sheet and the average crystal grain size of the hard phase of the metal structure and ferrite.

By the way, for roll-formed square steel pipes, it is also required to establish a technology for improving the shape characteristics, particularly a technology for improving the flatness of the flat plate portion and reducing the radius of curvature of the corner portion. In response to this requirement, for example, Patent Document 5 and Patent Document 6 propose techniques for improving shape characteristics by adjusting manufacturing conditions during roll molding.

Specifically, in Patent Document 5, when a steel pipe is formed into a square pipe with a three-stage or four-stage square forming roll and the reduction rate of the final stage roll is constant, the wall thickness / outer diameter ratio of the steel pipe is large. As a result, a method for forming a square steel pipe in which the roll caliber in the final stage is made smaller (from a convex shape to a concave shape) has been proposed.

In Patent Document 6, when a cylindrical raw tube is roll-formed into a square tube, when the outer diameter of the raw tube is D, the wall thickness of the raw tube is t, and the maximum caliber height is H, Q = (D). The first-stage molding step of forming the raw tube into a rectangular cross-sectional shape while maintaining the set push-in ratio Q defined by −H) / (Dt) × 100 in the range of 12 to 23%, and the rectangular cross-sectional shape. A method for manufacturing a structural square tube has been proposed, which undergoes a molding step of the second and subsequent stages for forming the raw tube formed in the above into a target shape.

Japanese Unexamined Patent Publication No. 2004-330222 Japanese Unexamined Patent Publication No. 10-60580 Japanese Patent No. 5385760 Japanese Unexamined Patent Publication No. 2018-53281 Japanese Unexamined Patent Publication No. 4-224023 Japanese Patent No. 3197661

However, since the square steel pipes described in Patent Document 1 and Patent Document 2 require a heating step at the time of forming or after forming, the cost is very high as compared with the roll-formed square steel pipe formed cold. Therefore, it is required to establish a technique for obtaining a desired square steel pipe without requiring a heating step at the time of forming or after forming.

Further, since the square steel pipes described in

Patent Documents

3 and 4 cannot sufficiently suppress the decrease in uniform elongation of the corners due to work hardening during forming, the ductility and toughness of the outer surface of the corners are sufficiently ensured. I couldn't say it was done.

Further, since the techniques described in

Patent Documents

5 and 6 cannot be formed while suppressing work hardening of the corners, the flatness of the flat plate portion of the square steel pipe is improved and the radius of curvature of the corners is reduced at the same time. At the same time, it could not be said that it was sufficient as a technique for sufficiently ensuring the ductility and toughness of the outer surface of the corner.

The present invention has been made in view of the above circumstances, and provides a square steel pipe having excellent shape characteristics, ductility and toughness of the outer surface of a corner portion, and a method for manufacturing the square steel pipe, and a building structure having excellent seismic performance. The purpose is to provide things.

Here, "excellent in shape characteristics" in the present invention refers to a square steel pipe having a small radius of curvature at the corner and a flat flat plate.

The above-mentioned "small radius of curvature of the corner" means that the radius of curvature R outside the corner is controlled within a predetermined range, specifically, the average wall thickness of the flat plate is t (mm). When this is done, it means that the radius of curvature R on the outside of the corner is 2.0t or more and 3.0t or less.

The above-mentioned "flat plate portion" means that the flatness of the outer surface of the flat plate portion in the pipe axis direction is 2.5 mm or less, specifically, in the cross section of the surface perpendicular to the pipe axis direction, the flat plate is used. It means that the maximum absolute value represented by the maximum bulge amount and the maximum dent amount with respect to a straight line passing through two points at both ends in the circumferential direction on the same side of the outer surface of the portion is 2.5 mm or less (FIG. 10 described later). reference).

Further, "excellent in ductility of the outer surface of the corner portion" in the present invention means that when the average wall thickness of the flat plate portion and the corner portion is t, uniform elongation at the 1/4 t position from the outer surface of the corner portion in the wall thickness direction. It means that E2 is 0.60 times or more the uniform elongation E1 at the 1 / 4t position in the wall thickness direction from the outer surface of the flat plate portion.

Further, "excellent in toughness of the outer surface of the corner portion" in the present invention means that the Charpy absorption energy of the corner portion at −10 ° C. at a position 1 / 4t in the wall thickness direction from the outer surface of the corner portion is 100J or more. Point to.

The radius of curvature, flatness, uniform elongation and toughness described above can be measured by the methods described in Examples described later.

The present inventors have made diligent studies to solve the above problems. As a result, the radius of curvature of the corner is controlled by controlling the circumference of the square steel pipe on the exit side of the square forming stand and the circumference of the electrosewn steel pipe on the entrance side of the square forming stand within an appropriate range. It has been found that a square steel pipe having a small size, a flat flat plate portion, and excellent extensibility and toughness on the outer surface of the corner portion can be manufactured.

The present invention has been completed based on the above findings, and has the following gist.
[1] A square steel pipe having a flat plate portion and a corner portion.
The radius of curvature R on the outside of the corner portion is 2.0 t or more and 3.0 t or less when the average wall thickness of the flat plate portion is t (mm).
The flatness of the outer surface of the flat plate portion is 2.5 mm or less.
The uniform elongation E2 at the position of 1 / 4t in the wall thickness direction from the outer surface of the corner portion is 0.60 times or more the uniform elongation E1 at the position of 1 / 4t in the wall thickness direction from the outer surface of the flat plate portion. ,
A square steel pipe having a Charpy absorption energy of 100 J or more at −10 ° C. at a position 1/4 t in the wall thickness direction from the outer surface of the corner portion.
[2] The square steel pipe according to [1], wherein the average wall thickness t is more than 0.030 times the average side length H (mm) of the flat plate portion.
[3] The square steel pipe according to [1] or [2], wherein the average wall thickness t is 20 mm or more and 40 mm or less.
[4] The yield strength of the flat plate portion is 295 MPa or more, and the yield strength is 295 MPa or more.
The tensile strength of the flat plate portion is 400 MPa or more, and the plate portion has a tensile strength of 400 MPa or more.
The square steel pipe according to any one of [1] to [3], wherein the yield ratio of the corner portion is 90% or less.
[5] The composition of the square steel pipe is mass%.
C: 0.020 to 0.45%,
Si: 0.01-1.0%,
Mn: 0.30 to 3.0%,
P: 0.10% or less,
S: 0.050% or less,
Al: 0.005 to 0.10%,
N: 0.010% or less,
Ti: contains 0.001 to 0.15%, the balance consists of Fe and unavoidable impurities.
The steel structure at the center of the wall thickness of the flat plate portion is
The total volume fraction of ferrite and bainite is 70% or more and 95% or less with respect to the entire steel structure at the center of the thickness of the flat plate, and the balance is one or more selected from pearlite, martensite, and austenite. Consists of
When a region surrounded by a boundary with an orientation difference of 15 ° or more between adjacent crystals is used as a crystal grain.
The average crystal grain size of the crystal grains is 15.0 μm or less, and the average crystal grain size is 15.0 μm or less.
The square according to any one of [1] to [4], wherein the total volume fraction of the crystal grains having a crystal grain size of 40 μm or more is 40% or less with respect to the entire steel structure at the center of the wall thickness of the flat plate portion. Steel pipe.
[6] In addition to the above-mentioned composition, by mass%,
Nb: 0.001 to 0.15%,
V: 0.001 to 0.15%,
Cr: 0.01-1.0%,
Mo: 0.01-1.0%,
Cu: 0.01-1.0%,
Ni: 0.01-1.0%,
Ca: 0.0002 to 0.010%,
B: 0.0001 to 0.010%
The square steel pipe according to any one of [1] to [5], which comprises one kind or two or more kinds selected from.
[7] The method for manufacturing a square steel pipe according to any one of [1] to [6].
A steel plate is cold-rolled, and both ends of the steel plate in the width direction are welded and welded to form an electric resistance pipe. When manufacturing steel pipes
The ratio of the plate width W of the steel plate to the peripheral length C _OUT of the square steel pipe on the outlet side of the square forming stand satisfies the formula (1), and the peripheral length C _OUT of the square steel pipe on the exit side of the square forming stand. The gap between the rolls of the sizing stand and the gap between the rolls of the square forming stand immediately before square forming are controlled so that the ratio of the peripheral length _CIN of the electric resistance pipe on the entrance side of the square forming stand satisfies the formula (2). How to manufacture a square steel pipe.
1.000 + 0.050 × t / H <W / C _OUT <1.000 + 0.50 × t / H ... Equation (1)
0.30 × t / H + 0.99 ≦ C _IN / C _OUT <0.50 × t / H + 0.99 ... Equation (2)
Here, in equations (1) and (2),
W: Plate width (mm) of the steel plate that is the material,
C _IN : Perimeter (mm) of the electric resistance sewn steel pipe on the entrance side of the first stage square forming stand,
C _OUT : Perimeter (mm) of the square steel pipe on the exit side of the square forming stand in the final stage,
t: Average wall thickness (mm) of the flat plate portion after square forming,
H: Average side length (mm) of the flat plate portion after square forming,
Is.
However, when square forming is performed by a one-stage square forming stand, the first-stage square forming stand and the final stage square forming stand refer to the same square forming stand.
[8] In the steel sheet, after the steel material is heated to a heating temperature of 1100 ° C. or higher and 1300 ° C. or lower, the rough rolling end temperature: 850 ° C. or higher and 1150 ° C. or lower, the finish rolling end temperature: 750 ° C. or higher and 900 ° C. or lower, and , Total reduction rate at 950 ° C or lower: Hot-rolled at 50% or higher,
Next, cooling was performed at the center temperature of the wall thickness at an average cooling rate of 5 ° C./s or more and 30 ° C./s or less, and a cooling shutdown temperature: 400 ° C. or more and 650 ° C. or less.
Next, the method for manufacturing a square steel pipe according to [7], which is wound at 400 ° C. or higher and 650 ° C. or lower.
[9] The method for manufacturing a square steel pipe according to [7] or [8], wherein the average wall thickness t is more than 0.030 times the average side length H of the flat plate portion.
[10] The method for manufacturing a square steel pipe according to any one of [7] to [9], wherein the average wall thickness t is 20 mm or more and 40 mm or less.
[11] A building structure using the square steel pipe according to any one of [1] to [6] as a column material.

According to the present invention, it is possible to provide a square steel pipe having excellent shape characteristics and excellent ductility and toughness on the outer surface of the corner portion, a method for manufacturing the square steel pipe, and a building structure.

This makes it possible to manufacture a cold roll-formed square steel pipe having a small radius of curvature at the corners, a flat flat plate, and excellent ductility and toughness on the outer surface of the corners. Further, the building structure using the square steel pipe of the present invention as a pillar material exhibits more excellent seismic performance as compared with the building structure using the conventional cold roll formed square steel pipe.

FIG. 1 is a schematic view showing a cross section perpendicular to the pipe axis direction of the square steel pipe of the present invention. FIG. 2 is a schematic view showing a pipe making process of an electrosewn steel pipe in the present invention. FIG. 3 is a schematic view showing a forming process of the square steel pipe of the present invention. FIG. 4 is a schematic view illustrating a melt-solidified portion in a welded portion of an electrosewn steel pipe. FIG. 5 is a schematic view showing an example of the building structure of the present invention. FIG. 6 is a schematic view showing the sampling positions of the tensile test pieces of the flat plate portion and the corner portion carried out in the present invention. FIG. 7 is a schematic view showing a detailed sampling position of the tensile test piece at the corner portion carried out in the present invention. FIG. 8 is a schematic view showing the collection position of the Charpy test piece at the corner portion carried out in the present invention. FIG. 9 is a schematic view showing a detailed collection position of the Charpy impact test piece at the corner portion carried out in the present invention. FIG. 10 is a schematic view illustrating the method for measuring flatness carried out in the present invention.

The present invention will be described with reference to the drawings. The present invention is not limited to this embodiment.

<Square steel pipe>
The present invention is a square steel pipe having a flat plate portion and a corner portion, and the radius of curvature R outside the corner portion is 2.0 t or more and 3.0 t or less when the average wall thickness of the flat plate portion is t (mm). The flatness of the outer surface of the flat plate portion in the pipe axis direction is 2.5 mm or less, and the uniform elongation E2 at the 1/4 t position in the wall thickness direction from the outer surface of the corner portion is from the outer surface of the flat plate portion in the wall thickness direction. It is 0.60 times or more with respect to the uniform elongation E1 at the 1 / 4t position, and the charmy absorption energy at −10 ° C. at the 1 / 4t position in the wall thickness direction from the outer surface of the corner portion is 100J or more.

FIG. 1 shows a cross section perpendicular to the pipe axis direction of the square steel pipe 10 of the present invention.

In the square steel pipe 10 of the present invention, a plurality of flat plate portions 11 and square portions 12 are alternately formed in the circumferential direction of the pipe. In the example shown in FIG. 1, in the square steel pipe 10, four square portions 12 and four flat plate portions 11 are formed in order in the circumferential direction of the pipe. The square steel pipe 10 is a rectangle (substantially rectangular) or a square (substantially square) in a cross-sectional view perpendicular to the pipe axis direction. In FIG. 1, when the side lengths of two flat plate portions 11 adjacent to each other across the corner portion 12 are H ₁ and H ₂ , H ₁ > H ₂ , that is, facing the welded portion (electrically sewn welded portion) 13 described later. The side length H ₂ of the flat plate portion to be welded is shorter than the side length H ₁ of the flat plate portion 11 adjacent thereto. The present invention is not limited to this example, and the relationship may be H ₁ = H ₂ or H ₁ <H ₂ .

The square steel pipe 10 is manufactured by using an electrosewn steel pipe as a raw pipe and forming the raw pipe into a roll-formed square steel pipe. Therefore, the square steel pipe 10 is formed on the flat plate portion 11 and has an electric resistance welded portion 13 extending in the pipe axis direction. Although not shown, the width of the melt-solidified portion of the electric stitch welded portion 13 in the tube circumferential direction is 1.0 μm or more and 1000 μm or less over the entire thickness of the pipe.

Further, in the square steel pipe 10 of the present invention, the radius of curvature R on the outside of the corner portion is 2.0 t or more and 3.0 t or less when the average wall thickness of the flat plate portion is t (mm). The average wall thickness t is a value calculated by the formula (3) described later.

When the radius of curvature R on the outside of the corner is less than 2.0t, the circumferential bending strain of the corner when forming the steel strip becomes large. As a result, the ductility and toughness desired in the present invention cannot be obtained at the corners. On the other hand, when the radius of curvature R on the outside of the corner portion exceeds 3.0 t, the amount of circumferential bending back strain (and the amount of circumferential bending strain of the corner portion) of the flat plate portion in the square forming stand becomes small. As a result, the flatness desired in the present invention cannot be obtained in the flat plate portion. The radius of curvature R described above is preferably 2.2 t or more, and preferably 2.9 t or less.

In the present invention, as described in Examples described later, when the radius of curvature at a plurality of locations is measured and the maximum and minimum values thereof are within the above range, the radius of curvature R outside the corner portion is small. Evaluate as. The reason for this evaluation is that the R of the corners of the square steel pipe acts independently on the seismic resistance and workability, not as the average value of the four points, but as the individual values.

As shown in FIG. 1, the radius of curvature R on the outside of the corner portion is a straight line (extension line) extended from the outer surface of the flat plate portions 11 on both sides adjacent to the corner portion 12 (the corner portion on the upper right side in the example of FIG. 1). ) The radius of curvature at the intersection B of the straight line L passing through the intersection P of L1 and L2 and forming an angle of 45 ° with the extension line L1 or L2 and the curve outside the corner 12.

The measurement of the radius of curvature R consists of the connection points (points A and A'in FIG. 1) between the extension lines L1 and L2 and the flat plate portion 11, the corner portion 12, and the outer surface of the corner portion 12, and the center is a straight line L. In the fan shape having a central angle of 90 ° existing above, this is performed within a range of a central angle of 65 ° centered on the intersection B of the straight line L and the outer surface of the corner portion 12. As a method for measuring the radius of curvature, for example, a method of measuring the radius of curvature from a radial gauge that matches well with the outer surface of the corner portion 12 in the above-mentioned range of a central angle of 65 ° can be mentioned, but measurement other than this method is also possible. Is.

Further, in the square steel pipe 10 of the present invention, the flatness of the outer surface of the flat plate portion 11 in the pipe axis direction is 2.5 mm or less.

The flatness will be described with reference to FIG. As shown in FIG. 10, the flatness is the maximum bulge amount and maximum dent amount with respect to a straight line passing through two points in the circumferential direction on the same side of the outer surface of the flat plate portion in the cross section of the surface perpendicular to the pipe axis direction. It is a value obtained by measuring. In the present invention, the flatness was determined by the method described in Examples described later.

If the flatness exceeds 2.5 mm, the buckling resistance of the square steel pipe during bending deformation will decrease. As a result, the seismic resistance of the square steel pipe is reduced. In addition, since the joint surface with the beam material is greatly curved, welding joint becomes difficult. As a result, workability is reduced. The smaller the value, the better the flatness. It is not necessary to specify the lower limit of flatness, but 0.6 mm is acceptable as the lower limit of flatness. The lower limit of flatness is preferably 0.2 mm, more preferably 0 mm. It is preferably 2.0 mm or less, and more preferably 1.5 mm or less.

Further, in the square steel pipe 10 of the present invention, the uniform elongation E2 at the 1 / 4t position in the wall thickness direction from the outer surface of the corner portion is 0 with respect to the uniform elongation E1 at the 1 / 4t position in the wall thickness direction from the outer surface of the flat plate portion. .60 times or more.

The outer surface of the square steel pipe is greatly deformed especially when it receives a large external force such as seismic force. Therefore, the square steel pipe needs to sufficiently enhance the ductility and toughness of the outer surface of the corner portion.

The value of uniform elongation E2 (value of E2 / E1) at the 1 / 4t position from the outer surface of the corner portion in the wall thickness direction is 0.60 with respect to the uniform elongation E1 at the 1 / 4t position in the wall thickness direction from the outer surface of the flat plate portion. If it is less than, the ductility on the outer surface side of the corner becomes small. As a result, the seismic resistance of the square steel pipe is reduced. The value of E2 / E1 is preferably 0.70 or more, more preferably 0.80 or more, and further preferably 0.82 or more. The upper limit of the value of E2 / E1 is not particularly specified, but the corner portion is 1.00 or less because the work hardening amount at the time of roll forming is larger and the uniform elongation is smaller than that of the flat plate portion.

Further, the square steel pipe 10 of the present invention has a Charpy absorption energy of 100 J or more at −10 ° C. at a position 1 / 4t in the wall thickness direction from the outer surface of the corner portion 12. If the Charpy absorption energy is less than 100 J, there is a high risk of brittle fracture without plastic deformation when a large external force such as an earthquake force is applied. The Charpy absorption energy is preferably 150 J or more, and more preferably 200 J or more.

The square steel pipe 10 of the present invention preferably has the following configuration in addition to the above configuration.

When the average wall thickness of the flat plate portion of the square steel pipe 10 is t (mm) and the average side length of the flat plate portion is H (mm), the average wall thickness t is more than 0.030 times the average side length H. Is preferable.

As described above, in a square steel pipe, the larger the ratio (t / H) of the average wall thickness t to the average side length H and the smaller the radius of curvature of the corner, the more the circumference required to form the corner. The directional bending strain increases, and the amount of bending deformation at the corners increases. As a result, in a square steel pipe having a large ratio (t / H), the ductility and toughness of the corners tend to be low.

When the value of the above ratio (t / H) is 0.030 or less, the proof stress as a pillar material becomes low, so that the applicable building structure is limited. Therefore, the ratio (t / H) is preferably more than 0.030. It is more preferably 0.035 or more, still more preferably 0.040 or more. On the other hand, in order to secure the ductility and toughness of the corners, the upper limit of the above ratio (t / H) is preferably 0.10. More preferably, it is 0.080 or less.

Here, the average wall thickness t (mm) is obtained by the following formula (3).
t = (t ₁ + t ₂ + t ₃ ) / 3 ... Equation (3)
In formula (3), t ₁ , t ₂ : meat at the center position in the pipe circumferential direction of two flat plate portions 11 adjacent to the flat plate portion 11 including the welded portion (electrically sewn welded portion) 13 with the corner portion 12 sandwiched between them. Thickness (mm), t ₃ : The wall thickness (mm) at the center position in the pipe circumferential direction of the flat plate portion facing the flat plate portion including the welded portion (electrically sewn welded portion). That is, the average wall thickness t is the average value of the wall thickness at the center position with respect to the pipe circumferential direction in the three flat plate portions excluding the flat plate portion including the welded portion (see FIG. 1).

The average side length H (mm) is calculated by the following equation (4).
H = (H ₁ + H ₂ ) / 2 ... Equation (4)
In equation (4), H ₁ : the side length of the cross section perpendicular to the pipe axis direction of any flat plate portion (vertical side length in FIG. 1) (mm), H ₂ : the side length is H ₁ . It is the side length (horizontal side length in FIG. 1) (mm) of the flat plate portion adjacent to the flat plate portion with the corner portion interposed therebetween. That is, the average side length H is the average value of the side lengths of the cross sections perpendicular to the pipe axis direction in the two flat plate portions 11 adjacent to each other with the corner portion interposed therebetween.

Further, the square steel pipe 10 of the present invention has an average wall thickness t of 20 mm or more, particularly from the viewpoint that it can be suitably used for a medium-rise building having a height of more than 20 m and a building member of a large building such as a factory or a warehouse. It is preferably 40 mm or less. From the viewpoint that it can be suitably used for building members of medium-rise buildings and large buildings, the yield strength of the flat plate portion 11 is preferably 295 MPa or more, and the tensile strength of the flat plate portion 11 is preferably 400 MPa or more, depending on the seismic resistance. It is preferable that the yield ratio of the corner portion 12 is 90% or less because it is more excellent.
More preferably, the yield strength of the flat plate portion 11 is 320 MPa or more, the tensile strength of the flat plate portion 11 is 410 MPa or more, and the yield ratio of the corner portion 12 is 89.5% or less. Further, preferably, the yield strength of the flat plate portion 11 is 500 MPa or less, the tensile strength of the flat plate portion 11 is 600 MPa or less, and the yield ratio of the corner portion 12 is 80.0% or more.

The above-mentioned yield strength, tensile strength, and yield ratio can be obtained by carrying out a tensile test in accordance with the provisions of JIS Z 2241 as described in Examples described later. Charpy absorption energy can be obtained by conducting a Charpy impact test at a test temperature of -10 ° C using a V-notch standard test piece in accordance with JIS Z 2242, as described in Examples described later. ..

Next, from the viewpoint of ensuring the above-mentioned mechanical properties and weldability, the component composition and the preferable range of the steel structure in the square steel pipe 10 of the present invention and the reasons for their limitation will be described.

First, the component composition will be described. The square steel pipe 10 of the present invention has a mass% of C: 0.020 to 0.45%, Si: 0.01 to 1.0%, Mn: 0.30 to 3.0%, P: 0.10. % Or less, S: 0.050% or less, Al: 0.005 to 0.10%, N: 0.010% or less, Ti: 0.001 to 0.15%, and the balance is Fe and unavoidable. It is preferable to have a component composition composed of impurities.
In the present specification, unless otherwise specified, "%" indicating the steel composition is "mass%". The following composition is the composition of the flat plate portion and the corner portion excluding the welded portion of the square steel pipe.

C: 0.020 to 0.45%
C is an element that increases the strength of steel by solid solution strengthening. Further, C is an element that contributes to the miniaturization of the structure by lowering the ferrite transformation start temperature. In order to obtain such an effect, it contains 0.020% or more of C. In addition, C is an element that promotes the formation of pearlite, enhances hardenability, contributes to the formation of martensite, and contributes to the stabilization of austenite, and thus contributes to the formation of a hard phase. When the C content exceeds 0.45%, the proportion of the hard phase becomes high, the toughness decreases, and the weldability also deteriorates. Therefore, the C content is set to 0.020 to 0.45%. The C content is preferably 0.040% or more, more preferably 0.050% or more. The C content is preferably 0.40% or less, more preferably 0.30% or less.

Si: 0.01-1.0%
Si is an element that increases the strength of steel by solid solution strengthening. In order to obtain such an effect, it contains 0.01% or more of Si. However, when the Si content exceeds 1.0%, oxides are likely to be generated in the electrosewn welded portion, and the characteristics of the welded portion deteriorate. In addition, the yield ratio of the base metal portion other than the electric stitch welded portion becomes high, and the toughness decreases. Therefore, the Si content is set to 0.01 to 1.0%. The Si content is preferably 0.02% or more, more preferably 0.05% or more. The Si content is preferably 0.50% or less, more preferably 0.40% or less.

Mn: 0.30 to 3.0%
Mn is an element that increases the strength of steel by solid solution strengthening. Mn is an element that contributes to the miniaturization of the structure by lowering the ferrite transformation start temperature. In order to obtain such an effect, Mn of 0.30% or more is contained. However, when the Mn content exceeds 3.0%, oxides are likely to be generated in the electrosewn welded portion, and the characteristics of the welded portion deteriorate. In addition, the yield stress becomes high due to the solid solution strengthening and the miniaturization of the structure, and the desired yield ratio cannot be obtained. Therefore, the Mn content is set to 0.30 to 3.0%. The Mn content is preferably 0.40% or more, more preferably 0.50% or more. The Mn content is preferably 2.5% or less, more preferably 2.0% or less.

P: 0.10% or less Since P segregates at the grain boundaries and causes inhomogeneity of the material, it is preferable to reduce it as an unavoidable impurity as much as possible, but up to 0.10% is acceptable. Therefore, the P content is set to 0.10% or less. The P content is preferably 0.050% or less, more preferably 0.030% or less. Although the lower limit of P is not specified, the P content is preferably 0.002% or more because excessive reduction causes an increase in smelting cost.

S: 0.050% or less S usually exists as MnS in steel, but MnS is thinly stretched in the hot rolling process and adversely affects ductility. Therefore, in the present invention, it is preferable to reduce S as much as possible, but up to 0.050% is acceptable. Therefore, the S content is set to 0.050% or less. The S content is preferably 0.030% or less, more preferably 0.010% or less. Although the lower limit of S is not specified, it is preferable that S is 0.0002% or more because excessive reduction causes an increase in smelting cost.

Al: 0.005 to 0.10%
Al is an element that acts as a powerful deoxidizer. In order to obtain such an effect, it is necessary to contain 0.005% or more of Al. However, when the Al content exceeds 0.10%, the weldability deteriorates, the amount of alumina-based inclusions increases, and the surface texture deteriorates. In addition, the toughness of the weld is reduced. Therefore, the Al content is set to 0.005 to 0.10%. The Al content is preferably 0.010% or more, more preferably 0.015% or more. The Al content is preferably 0.080% or less, more preferably 0.070% or less.

N: 0.010% or less N is an unavoidable impurity and is an element having an action of lowering toughness by firmly fixing the motion of dislocations. In the present invention, it is desirable to reduce N as an impurity as much as possible, but the content of N is acceptable up to 0.010%. Therefore, the N content is 0.010% or less. The N content is preferably 0.0080% or less. From the viewpoint of refining cost, the N content is preferably 0.0008% or more.

Ti: 0.001 to 0.15%
Ti is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel. Further, since it has a high affinity with N, it is an element that detoxifies N in steel as a nitride and contributes to improvement of toughness of steel. In order to obtain the above-mentioned effects, it is preferable to contain 0.001% or more of Ti. However, when the Ti content exceeds 0.15%, the yield ratio becomes high and the toughness decreases. Therefore, the Ti content is set to 0.15% or less. The Ti content is more preferably 0.002% or more, still more preferably 0.005% or more. The Ti content is more preferably 0.10% or less, still more preferably 0.08% or less.

The rest other than the above components are Fe and unavoidable impurities. However, as an unavoidable impurity, O may be contained in an amount of 0.0050% or less. Here, O refers to total oxygen including O as an oxide. Nb: 0 to less than 0.001%, V: 0 to less than 0.001%, Cr: 0 to less than 0.01%, Mo: 0 to less than 0.01%, Cu: 0 to less than 0.01%, Ni: 0 to less than 0.01%, Ca: 0 to less than 0.0002%, B: less than 0 to 0.0001% are treated as unavoidable impurities.

In the present invention, it is preferable to use the above-mentioned components as the basic component composition. Although the above-mentioned suitable elements can obtain the characteristics desired in the present invention, further, for the purpose of further improving the characteristics, Nb: 0.001 to 0.15%, V: 0.001 to, if necessary. 0.15%, Cr: 0.01-1.0%, Mo: 0.01-1.0%, Cu: 0.01-1.0%, Ni: 0.01-1.0%, Ca It can contain one or more selected from: 0.0002 to 0.010% and B: 0.0001 to 0.010%.

Nb: 0.001 to 0.15%
Nb is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in steel, and also contributes to the miniaturization of structure by suppressing the coarsening of austenite during hot rolling. And can be contained as needed. In order to obtain the above-mentioned effects, when Nb is contained, it is preferable to contain 0.001% or more of Nb. However, when the Nb content exceeds 0.15%, the yield ratio becomes high and the toughness decreases. Therefore, when Nb is contained, the Nb content is preferably 0.15% or less. The Nb content is more preferably 0.002% or more, still more preferably 0.005% or more. The Nb content is more preferably 0.10% or less, still more preferably 0.08% or less.

V: 0.001 to 0.15%
V is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in the steel, and can be contained as needed. In order to obtain the above-mentioned effect, when V is contained, it is preferable to contain 0.001% or more of V. However, when the V content exceeds 0.15%, the yield ratio becomes high and the toughness decreases. Therefore, when V is contained, the V content is preferably 0.15% or less. The V content is more preferably 0.002% or more, still more preferably 0.005% or more. The V content is more preferably 0.10% or less, still more preferably 0.08% or less.

Cr: 0.01-1.0%
Cr is an element that enhances the hardenability of steel and increases the strength of steel, and can be contained as needed. In order to obtain the above-mentioned effect, when Cr is contained, the Cr content is preferably 0.01% or more. On the other hand, if the content of Cr exceeds 1.0%, the toughness may be lowered and the weldability may be deteriorated. Therefore, when Cr is contained, the Cr content is preferably 1.0% or less. The Cr content is more preferably 0.02% or more, still more preferably 0.05% or more. The Cr content is more preferably 0.90% or less, still more preferably 0.80% or less.

Mo: 0.01-1.0%
Mo is an element that enhances the hardenability of steel and increases the strength of steel, and can be contained as needed. In order to obtain the above-mentioned effects, when Mo is contained, the Mo content is preferably 0.01% or more. On the other hand, if the content of Mo exceeds 1.0%, the toughness may be lowered and the weldability may be deteriorated. Therefore, when Mo is contained, the Mo content is preferably 1.0% or less. The Mo content is more preferably 0.02% or more, still more preferably 0.05% or more. The Mo content is more preferably 0.90% or less, still more preferably 0.80% or less.

Cu: 0.01-1.0%
Cu is an element that increases the strength of steel by solid solution strengthening, and can be contained as needed. In order to obtain the above-mentioned effects, when Cu is contained, the Cu content is preferably 0.01% or more. On the other hand, if the content of Cu exceeds 1.0%, the toughness may be lowered and the weldability may be deteriorated. Therefore, when Cu is contained, the Cu content is preferably 1.0% or less. The Cu content is more preferably 0.02% or more, still more preferably 0.05% or more. The Cu content is more preferably 0.80% or less, still more preferably 0.60% or less.

Ni: 0.01-1.0%
Ni is an element that increases the strength of steel by solid solution strengthening, and can be contained as needed. In order to obtain the above-mentioned effects, when Ni is contained, the Ni content is preferably 0.01% or more. On the other hand, if the content of Ni exceeds 1.0%, the toughness may be lowered and the weldability may be deteriorated. Therefore, when Ni is contained, the Ni content is preferably 1.0% or less. The Ni content is more preferably 0.02% or more, still more preferably 0.05% or more. The Ni content is more preferably 0.80% or less, still more preferably 0.60% or less.

Ca: 0.0002 to 0.010%
Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly stretched in the hot rolling process in the production of raw steel sheets, and can be contained as needed. In order to obtain the above-mentioned effects, when Ca is contained, it is preferable to contain 0.0002% or more of Ca. However, when the Ca content exceeds 0.010%, Ca oxide clusters are formed in the steel and the toughness deteriorates. Therefore, when Ca is contained, the Ca content is preferably 0.010% or less. The Ca content is more preferably 0.0005% or more, still more preferably 0.0010% or more. The Ca content is more preferably 0.008% or less, still more preferably 0.0060% or less.

B: 0.0001 to 0.010%
B is an element that contributes to the miniaturization of the structure by lowering the ferrite transformation start temperature, and can be contained as needed. In order to obtain the above-mentioned effect, when B is contained, it is preferable to contain 0.0001% or more of B. However, when the B content exceeds 0.010%, the yield ratio increases and the toughness deteriorates. Therefore, when B is contained, the B content is preferably 0.010% or less. The B content is more preferably 0.0005% or more, still more preferably 0.0008% or more. The B content is more preferably 0.0050% or less, further preferably 0.0030% or less, and even more preferably 0.0020% or less.

Next, the steel structure will be explained. In the steel structure at the center of the flat plate portion of the square steel pipe 10 of the present invention, the total volume ratio of ferrite and bainite is 70% or more and 95% or less of the entire steel structure at the center of the flat plate portion. When the balance consists of one or more types selected from pearlite, martensite, and austenite, and the region surrounded by the boundary where the orientation difference between adjacent crystals is 15 ° or more is defined as the crystal grains, the average of the crystal grains is used. It is preferable that the crystal grain size is 15.0 μm or less, and the total of the crystal grains having a crystal grain size of 40 μm or more is 40% or less in terms of volume ratio with respect to the entire steel structure at the center of the wall thickness of the flat plate portion.

Total volume fraction of ferrite and bainite: 70% or more and 95% or less Ferrite has a soft structure. In addition, bainite is harder than ferrite, softer than pearlite, martensite and austenite, and has an excellent toughness structure. When a hard structure (pearlite, martensite and austenite) is mixed with ferrite and bainite, the yield ratio decreases, but on the other hand, the stress concentration caused by the difference in hardness tends to cause the interface to become the starting point of fracture and the toughness decreases. do. Therefore, in order to obtain the above-mentioned yield ratio and toughness, the total volume fraction of ferrite and bainite at the center of the plate thickness is 70% or more and 95% or less of the entire steel structure at the center of the plate portion. Is preferable. When the total volume fraction of ferrite and bainite is less than 70%, the proportion of hard structure is high and the yield stress increases, so that the yield ratio increases and the toughness decreases. Further, when the total volume fraction of ferrite and bainite exceeds 95%, the tensile strength decreases and the yield ratio increases. More preferably, it is 73% or more and 93% or less. More preferably, it is 75% or more and 92% or less.

The remaining structure (residual structure) excluding ferrite and bainite is one or more selected from pearlite, martensite, and austenite. When the total volume fraction of the residual structure is less than 5%, the tensile strength decreases and the yield ratio increases. Further, when the total volume fraction of the residual structure exceeds 30%, the proportion of the hard structure is high and the yield stress increases, so that the yield ratio increases and the toughness decreases. Therefore, the total volume fraction of the remaining structure is preferably 5% or more and 30% or less with respect to the entire steel plate structure at the center of the wall thickness of the flat plate portion. More preferably, it is 7% or more and 27% or less. More preferably, it is 8% or more and 25% or less.

For the above-mentioned various structures (ferrite, bainite, pearlite, martensite) except austenite, the austenite grain boundary or the deformation zone in the austenite grain is the nucleation site. A large amount of dislocations are introduced into austenite by increasing the amount of reduction at low temperatures where recrystallization of austenite is unlikely to occur in hot rolling in the process of manufacturing steel sheets made of electric resistance steel pipes (bare pipes) used in the manufacture of square steel pipes. As a result, austenite can be refined and a large amount of deformation zone can be introduced into the grain. As a result, the area of the nucleation site increases, the frequency of nucleation increases, and the steel structure can be miniaturized.

In the present invention, the above-mentioned effect can be similarly obtained even if the above-mentioned steel structure exists within a range of ± 1.0 mm in the wall-thickness direction centering on the center of the wall-thickness. Therefore, in the present invention, the "steel structure at the center of the wall thickness" means that the above-mentioned steel structure exists in any of the range of ± 1.0 mm in the wall thickness direction centering on the center of the wall thickness. ..

To observe the steel structure, first, a test piece for structure observation is collected so that the observation surface has a cross section parallel to both the longitudinal direction and the wall thickness direction of the square steel pipe and the wall thickness center of the flat plate portion, and a mirror surface is used. After polishing, it is produced by nital corrosion. For tissue observation, an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) is used to observe and image the tissue at the center of the wall thickness. From the obtained optical microscope image and SEM image, the area ratio of ferrite, bainite and the balance (pearlite, martensite, austenite) is determined. The area ratio of each tissue is calculated as the average value of the values obtained in each visual field by observing in 5 or more visual fields. The area ratio obtained by observing the tissue is defined as the volume fraction of each tissue.

Here, ferrite is a product of diffusion transformation, and exhibits a structure with low dislocation density and almost recovery. This includes polygonal ferrite and pseudopolygonal ferrite.

Bainite is a double-phase structure of lath-like ferrite and cementite with high dislocation density.

Pearlite is an eutectoid structure of iron and iron carbide (ferrite + cementite), and exhibits a lamellar structure in which linear ferrite and cementite are alternately arranged.

Martensite is a lath-like low-temperature transformation structure with a very high dislocation density. The SEM image shows a bright contrast as compared with ferrite and bainite. Since it is difficult to distinguish between martensite and austenite in the optical microscope image and the SEM image, the area ratio of the tissue observed as martensite or austenite is measured from the obtained SEM image, and then the volume of austenite measured by the method described later. The value obtained by subtracting the rate is taken as the volume ratio of martensite.

The volume fraction of austenite is measured by X-ray diffraction using a test piece prepared by the same method as the test piece used for measuring the dislocation density. The volume fraction of austenite is obtained from the integrated intensities of the (200), (220) and (311) planes of the obtained fcc iron and the (200) and (211) planes of the bcc iron.

Average crystal grain size of crystal grains: 15.0 μm or less In the present invention, the average crystal grain size is defined as the crystal grain (grain boundary) in the region surrounded by the boundary where the orientation difference between adjacent crystals is 15 ° or more. The diameter corresponding to the average circle of the crystal grains. The circle equivalent diameter (crystal grain size) is the diameter of a circle having the same area as the target crystal grain.

When the average crystal grain size of the crystal grains exceeds 15.0 μm, the desired toughness cannot be obtained because the total area of the crystal grain boundaries that hinder crack propagation is small. Therefore, in the present invention, the average crystal grain size of the crystal grains is 15.0 μm or less. The average crystal grain size of the crystal grains is preferably 13.0 μm or less, more preferably 10.0 μm or less. The smaller the average crystal grain size, the higher the yield ratio. Therefore, the average crystal grain size is preferably 2.0 μm or more.

Total volume ratio of crystal grains with a crystal grain size of 40 μm or more: 40% or less Even if the upper limit of the maximum crystal grain size is specified, if a certain amount of coarse crystal grains are present, grain boundaries that hinder crack propagation Since there is a region where the total area of the crystal is small, the toughness is greatly reduced. Therefore, in order to obtain good toughness, it is necessary to specify the upper limit of the proportion of coarse crystal grains present. Therefore, in the present invention, the total volume fraction of crystal grains having a crystal grain size of 40 μm or more is set to 40% or less. More preferably, it is 30% or less. For the above reasons, it is desirable that the number of coarse crystal grains is small, and the total volume fraction of the crystal grains is preferably 0%.

Here, the measurement of the average crystal grain size of the crystal grains and the total volume ratio of the crystal grains having a crystal grain size of 40 μm or more is as follows. First, a test piece for microstructure observation was collected so that the observation surface had a cross section parallel to both the longitudinal direction and the wall thickness direction of the square steel tube and the wall thickness center of the flat plate portion, and after mirror polishing, the wall thickness center. In, a histogram of the particle size distribution (horizontal axis: particle size, vertical axis: graph with abundance ratio (area ratio) at each particle size) is calculated using the SEM / EBSD method. The average crystal grain size is obtained from the above histogram as an arithmetic mean of the grain size. The total volume fraction of crystal grains having a particle size of 40 μm or more is obtained as the total abundance ratio of crystal grains having a particle size of 40 μm or more from the above histogram. The measurement conditions are an acceleration voltage of 15 kV, a measurement area of 500 μm × 500 μm, and a measurement step size (measurement resolution) of 0.5 μm. In the crystal grain size analysis, those having a crystal grain size of less than 2.0 μm are excluded from the analysis target as measurement noise.

<Manufacturing method of square steel pipe>
Next, a method for manufacturing the square steel pipe 10 of the present invention will be described.

In the method for manufacturing a square steel pipe 10 of the present invention, a steel plate as a material is cold-roll-formed, and then both ends of the cold-roll-formed steel plate in the width direction are welded to form an electric-sewn steel pipe, and then the electric-sewn steel pipe is obtained. Is reduced in diameter by a sizing stand, and then squarely formed by a square forming stand to manufacture a square steel pipe. At this time, the ratio of the plate width W of the steel plate to the peripheral length C _OUT of the square steel pipe on the exit side of the square forming stand satisfies the formula (1), and the circumference C _OUT of the square steel pipe on the exit side of the square forming stand is satisfied. The gap between the rolls of the sizing stand immediately before square forming and the gap between the rolls of the square forming stand are controlled so that the ratio of the peripheral length _CIN of the electric resistance pipe on the entrance side of the square forming stand satisfies the equation (2).
1.000 + 0.050 × t / H <W / C _OUT <1.000 + 0.50 × t / H ... Equation (1)
0.30 × t / H + 0.99 ≦ C _IN / C _OUT <0.50 × t / H + 0.99 ... Equation (2)
Here, in equations (1) and (2),
W: Plate width (mm) of the steel plate that is the material,
C _IN : Perimeter (mm) of the electric resistance sewn steel pipe on the entrance side of the first stage square forming stand,
C _OUT : Perimeter (mm) of the square steel pipe on the exit side of the square forming stand in the final stage,
t: Average wall thickness (mm) of the flat plate portion after square forming,
H: Average side length (mm) of the flat plate portion after square forming,
Is.
However, when square forming is performed by the one-stage square forming stand, the first-stage square forming stand and the final stage square forming stand refer to the same square forming stand.

The average wall thickness t is calculated by the above formula (3), and the average side length H is calculated by the above formula (4).

The method for manufacturing the square steel pipe 10 of the present invention will be described in detail with reference to FIGS. 2 and 3. FIG. 2 shows a diagram illustrating a pipe making process of a raw pipe (electric pipe) of a square steel pipe of the present invention. FIG. 3 shows a diagram illustrating a forming process of the square steel pipe of the present invention.

First, the electric pipe 7 is manufactured using a steel plate (steel strip) as a material (pipe making process).

As shown in FIG. 2, the steel sheet 1 (hot-rolled steel sheet, hot-rolled steel strip) having the above-mentioned composition, which is wound around the coil, is dispensed, straightened by the leveler 2, and is a cage roll group composed of a plurality of rolls. It is intermediately molded in step 3 to form a cylindrical open tube. After that, it is finish-molded by the finpass roll group 4 composed of a plurality of rolls. The above open tube is formed into a cylindrical shape by cold roll forming.

The square steel pipe of the present invention preferably has the above-mentioned steel structure. As described above, since the square steel pipe of the present invention is manufactured by further square-forming an electrosewn steel pipe (bare pipe) obtained by cold-rolling a material steel plate, the material steel plate (steel plate 1) also has the above-mentioned composition and composition. It is preferable to have a steel structure. Since the preferable manufacturing conditions for the steel sheet 1 will be described later, the description thereof is omitted here.

The finish-formed open pipe is pressure-welded with a squeeze roll 5 and a pair of butt portions (both ends in the width direction) facing each other in the circumferential direction of the steel plate 1 are electrically resistance welded (electrically welded) with a welding machine 6. It is a welded steel pipe 7. In the above-mentioned electric sewing welding, for example, by high frequency induction heating or high frequency resistance heating, the butt portion is heated and melted, and the butt portion is pressed and solidified to complete the joining. As a result, the welded portion (electrically sewn welded portion) 13 is extended in the pipe axis direction. The manufacturing equipment used for manufacturing the electric resistance pipe 7 is not limited to the manufacturing equipment having the pipe making process shown in FIG.

In the present invention, in the process of manufacturing the electric resistance pipe, the amount of upset by the squeeze roll 5 is preferably in the range of 20% or more and 100% or less with respect to the wall thickness of the electric resistance pipe 7. When the amount of upset is less than 20% of the wall thickness, the discharge of molten steel becomes insufficient and the toughness of the welded portion deteriorates. On the other hand, when the amount of upset is more than 100% of the wall thickness, the load on the squeeze roll becomes large, the work hardening amount of the welded portion (electrically sewn welded portion) 13 becomes large, and the hardness becomes excessively high.

Next, the obtained electric resistance pipe 7 is used as a bare pipe to manufacture a square steel pipe (molding process). The molding step includes a sizing step and a square molding step.

As shown in FIG. 3, the electric resistance pipe 7 is reduced in diameter (sizing) in a cylindrical shape by a sizing roll group (sizing stand) 8 composed of a plurality of rolls arranged vertically and horizontally with respect to the electric resistance pipe 7. Process). After that, it is squarely formed into the shapes shown in R1, R2, and R3 by a square forming roll group (square forming stand) 9 composed of a plurality of rolls arranged vertically and horizontally with respect to the electrosewn steel pipe 7. (Square forming process). Each roll constituting the square forming stand 9 is a hole-shaped roll (caliber roll) having a caliber curvature, and the caliber curvature increases as the stand becomes a subsequent stand. As a result, a flat plate portion and a square portion of the square steel pipe are formed.

The number of stands constituting the sizing roll group 8 and the square forming roll group 9 is not particularly limited. It may be composed of a multi-stage stand, or it may be composed of a single-stage stand. Further, when the caliber curvature of each roll in the sizing roll group 8 or the square forming roll group 9 is not constant (has a plurality of curvatures), the shape irregularity occurs when the electric resistance pipe 7 being formed is twisted in the circumferential direction. Therefore, it is preferable that the caliber curvature of each roll is constant.

In the present invention, as described above, the ratio of the plate width W of the steel plate to the peripheral length C _OUT of the square steel pipe on the outlet side of the square forming stand satisfies the formula (1), and the square steel pipe on the exit side of the square forming stand. The gap between the rolls of the sizing stand immediately before square forming and the roll of the square forming stand so that the ratio of the circumferential length C _IN of the electric resistance pipe on the entrance side of the square forming stand to the circumferential length C _OUT satisfies the formula (2). It is important to control the gap. As a result, even in a roll-formed square steel pipe in which the ratio (t / H) of the average wall thickness t to the average side length H is large and the radius of curvature R of the corner portion is small, the ductility and toughness of the outer surface of the corner portion can be improved. Can be improved.

First, the plate width W (mm) of the material steel plate (steel plate) 1 and the peripheral length of the square steel pipe 10 immediately after square forming (the peripheral length of the steel pipe (mm) on the outlet side of the square forming stand in the final stage, hereinafter "C _OUT ". The ratio (W / C _OUT ) of (referred to as) and the ratio (t / H) of the average wall thickness t immediately after square forming and the average side length H immediately after square forming satisfy the above formula (1). The reason for controlling is explained.

As shown in FIGS. 2 and 3, a flat plate-shaped steel plate 1 (material steel plate) is cold-rolled to form a cylindrical electric resistance pipe 7 (bare pipe), and then a cylindrical electric pipe is squarely formed. In the case of manufacturing a square steel pipe 10, during the manufacturing process (pipe making process, forming process), the steel plate 1 and the electrosewn steel pipe 7 are caused by bending deformation in the pipe circumferential direction and drawing in the pipe circumferential direction. Extension deformation in the longitudinal direction of the pipe is added. In order to reduce the amount of drawing in the tube circumferential direction in the manufacturing process, it is effective to appropriately control the above-mentioned two ratios "t / H" and "W / C _OUT ".

When the "W / C _OUT " of the above ratio is equal to or less than the value on the left side of the formula (1), the circumferential bending strain amount of the steel plate 1 in the pipe making process, the circumferential bending strain amount of the wrought steel pipe 7 in the forming process, And the amount of bending back strain becomes small. As a result, the processing of the steel plate 1 and the electric resistance pipe 7 becomes insufficient, a flat flat plate portion cannot be obtained, and the radius of curvature R on the outside of the corner portion exceeds 3.0 times (3.0 t) the average wall thickness t. become.

On the other hand, when the “W / C _OUT ” of the above ratio is equal to or greater than the value on the right side of the equation (1), the difference in the circumference of the pipe (or open pipe) before and after the pipe making step and the molding step becomes large. As a result, since the amount of drawing in the circumferential direction of the pipe is large, the corners are largely work-hardened, and the desired ductility and toughness of the outer surface of the corners cannot be obtained.

The “W / C _OUT ” of the above ratio is preferably (1.000 + 0.080 × t / H) or more and (1.000 + 0.48 × t / H) or less, and more preferably (1.000 + 0.10). × t / H) or more (1.000 + 0.45 × t / H) or less.

Subsequently, the circumference of the electric resistance pipe 7 immediately before square forming (the circumference (mm) of the electric resistance pipe 7 on the entrance side of the first stage square forming stand, hereinafter referred to as "C _IN ") and the corner. The ratio of the circumference (C _OUT ) of the square steel pipe 10 immediately after forming (C _IN / C _OUT ) and the ratio of the average wall thickness t immediately after square forming to the average side length H immediately after square forming (t / H) are as follows. The reason for controlling the above equation (2) to be satisfied will be described.

As shown in FIG. 3, when the cylindrical electric resistance pipe 7 is squarely formed into a square steel pipe 10, as described above, the steel pipe is passed through the square forming roll group 9 to gradually form from a cylindrical shape to a square shape. .. In such square forming, bending back of the straight side portion (flat plate portion 11), bending of the corner portion 12, and drawing deformation in the circumferential direction occur in the electrosewn steel pipe 7.

In particular, around the corner portion 12, the corner forming is completed with almost no contact between the rolls of the square forming roll group 9. In square forming, the corner portion 12 is formed by projecting by free deformation. At this time, the higher the rigidity of the corner portion 12 and the smaller the amount of throttle in the circumferential direction, the smaller the bending deformation amount of the corner portion 12, and the larger the radius of curvature on the outside of the corner portion. On the other hand, the lower the rigidity of the corner portion 12 and the larger the circumferential diaphragm, the larger the bending deformation of the corner portion 12 and the smaller the radius of curvature on the outside of the corner portion.

The rigidity of the corner portion 12 against bending deformation increases as the ratio (t / H) of the average wall thickness t and the average side length H increases. Further, the circumferential drawing amount in square forming is obtained by the peripheral length ratio (C _IN / C _OUT ), and the larger this is, the larger the circumferential drawing amount is.

Therefore, when t / H becomes large, it becomes difficult to form the corner portion 12 due to bending deformation. Therefore, in order to obtain a desired angular radius of curvature, it is necessary to increase the peripheral length ratio (C _IN / C _OUT ) to increase the amount of peripheral aperture. For this reason, it is effective to appropriately control the above-mentioned two ratios "t / H" and "C _IN / C _OUT ".

When the peripheral length ratio (C _IN / C _OUT ) is smaller than the value on the left side of the equation (2), the difference in the peripheral length of the pipe before and after the forming process becomes small, and the amount of drawing in the circumferential direction of the wrought steel pipe 7 becomes small. As a result, the processing of the flat plate portion 11 and the corner portion 12 becomes insufficient, a flat flat plate portion cannot be obtained, and the radius of curvature R on the outside of the corner portion becomes more than 3.0 times (3.0 t) the average wall thickness t. Become.

On the other hand, when the circumference ratio (C _IN / C _OUT ) is equal to or greater than the value on the right side of the equation (2), the difference in the circumference of the pipe before and after the molding process becomes large. As a result, since the amount of drawing in the circumferential direction of the tube is large, the corners are largely work-hardened, and the desired ductility and toughness of the corners cannot be obtained. Further, the radius of curvature R on the outside of the corner is less than 2.0 times (2.0 t) the average wall thickness t.

The circumference ratio (C _IN / C _OUT ) is preferably (0.33 × t / H + 0.99) or more and (0.47 × t / H + 0.99) or less, and more preferably (0.35 × t). / H + 0.99) or more (0.45 × t / H + 0.99) or less.

In the present invention, from the viewpoint of further improving the seismic resistance, it is preferable to control under the following conditions in addition to the conditions of the above equations (1) and (2).

When the average wall thickness of the flat plate portion of the square steel pipe 10 is t (mm) and the average side length of the flat plate portion is H (mm), the average wall thickness t is more than 0.030 times the average side length H. Is preferable. As a result, the strength and rigidity of the pillar material are increased, and as a result, the seismic resistance is improved. The ratio (t / H) of the average wall thickness t and the average side length H is more preferably 0.035 times or more. Further, in order to secure the ductility and toughness of the corner portion, it is preferably 0.10 times or less, and more preferably 0.080 times or less.

Further, it is preferable that the average wall thickness t is 20 mm or more and 40 mm or less. The reason is the same as the reason for controlling the average wall thickness t of the square steel pipe, and is therefore omitted.

Further, it is preferable to control the gap between the sizing roll and the square forming roll.

Further, C _IN and C _OUT are controlled by controlling the gap between the recesses of the caliber roll. The maximum gap between the recesses of the roll of the sizing stand immediately before square forming (hereinafter, also referred to as "sizing stand gap") and the maximum gap between the recesses of the roll of the square forming stand (hereinafter, also referred to as "gap of the square forming stand"). When the difference is Δg, the sizing stand immediately before square forming so that G (= Δg / (t / H)), which is the value obtained by dividing Δg by (t / H), is 70 or more and 180 or less. It is preferable to adjust the gap of.

When G is less than 70, in the above equation (2), (C _IN / C _OUT ) becomes smaller than the value on the left side, and as described above, the outside of the flat flat plate portion and the corner portion intended in the present invention. The radius of curvature of is not obtained. On the other hand, when G is more than 180, (C _IN / C _OUT ) is equal to or greater than the value on the right side in the above equation (2), and as described above, the ductility and toughness of the corner portion targeted by the present invention are obtained. I can't. Preferably, G is 80 or more and less than 170.

If there are multiple stages of sizing stands, the gap of the sizing stand immediately before the above-mentioned square forming and the gap of other sizing stands may be the same. When there are a plurality of stages of the square forming stand, it is preferable that the gap of the above-mentioned square forming stand is the gap of the first stage of the square forming stand. The gaps between the first stage and the other square forming stands may all be the same.

Here, the above-mentioned C _IN is the peripheral length (length of the outer circumference in the peripheral direction of the pipe) (mm) of the electric resistance sewn steel pipe 7 on the entrance side of the square forming stand of the first stage. As shown in FIG. 3, in C _IN , the pipe forming direction is the positive direction of the X axis, the X coordinate of any one of the rotation axes of the sizing roll group 8 immediately before the square forming is Xa (m), and the first stage. When the X coordinate of any one of the rotation axes of the square forming roll group 9 of the eyes is Xb (m), the outer peripheral length of the peripheral cross section of the tube in the plane X = (Xa + Xb) / 2 (m) perpendicular to the X axis. Is obtained by measuring with a scale.

The above C _OUT is the peripheral length (the length of the outer circumference in the peripheral direction of the pipe) (mm) of the square steel pipe 10 on the exit side of the square forming stand in the final stage. As shown in FIG. 3, C _OUT is in a plane X = Xc + 1 (m) perpendicular to the X axis, where the X coordinate of the rotation axis of any one of the square forming stands in the final stage of the roll group is Xc (m). It is obtained by measuring the outer peripheral length of the peripheral cross section of the tube with a tape measure.

The above-mentioned method for manufacturing a square steel pipe of the present invention aims to reduce variations in the flatness of each flat plate portion and the radius of curvature of each corner portion in the process of forming a square steel pipe from an electrosewn steel pipe (bare pipe). In addition to the conditions of, it can be further controlled by the following conditions.

In the sizing step after electric stitch welding, the diameter of the steel pipe may be reduced so that the circumference of the steel pipe is reduced at a rate of 0.30% or more in total in order to satisfy the preferable roundness. As a result, each flat plate portion and each corner portion are uniformly (symmetrically) formed in the subsequent corner forming step, and the variation in flatness and radius of curvature is reduced. The above-mentioned "preferable roundness" means that the vertical outer diameter D1 and the horizontal outer diameter D2 of the pipe are | D1-D2 | / ((D1 + D2) / 2) ≦ 0.020.

However, if the diameter is reduced so that the circumference of the steel pipe decreases at a rate of more than 2.0% in total, the amount of bending in the pipe axis direction when passing through the roll becomes large, and the yield ratio increases. Therefore, it is preferable to reduce the diameter so that the circumference of the steel pipe decreases at a rate of 0.30% or more and 2.0% or less.

In the sizing step, it is preferable to perform multi-step diameter reduction with a plurality of stands in order to minimize the bending amount in the pipe axis direction when passing through the roll and to suppress the generation of residual stress in the pipe axis direction. In this case, the diameter reduction of each stand is preferably performed so that the circumference of the steel pipe is reduced by 1.0% or less as compared with the diameter reduction of the stand installed immediately before the stand.

As described above, the square steel pipe of the present invention uses an electric resistance sewn steel pipe as a raw pipe. To determine whether the square steel pipe 10 was obtained from the electric resistance pipe 7, the square steel pipe 10 was cut perpendicularly in the pipe axis direction, the cut surface including the welded portion (electrosewn welded portion) 13 was polished, and then corroded. It can be judged by observing with an optical microscope. If the width of the melt-solidified portion of the welded portion (electrically sewn welded portion) 13 in the pipe circumferential direction is 1.0 μm or more and 1000 μm or less over the entire thickness of the pipe, the electric resistance pipe 7 is used. The corrosive liquid may be appropriately selected according to the steel composition and the type of steel pipe.

Here, the welded portion (electrically sewn welded portion) will be described with reference to FIG. FIG. 4 shows a schematic view of the melt-solidified portion 16 in the welded portion 13. FIG. 4 shows a state after the cut surface including the welded portion is polished and corroded. The melt-solidified portion 16 can be visually recognized as a region having a structure shape and contrast different from those of the base metal portion 14 and the heat-affected zone 15 in FIG. For example, the melt-solidified portion 16 of the electrosewn steel pipe of carbon steel and low alloy steel can be identified as a region observed white by an optical microscope in the above cross section corroded by nital.

Next, a preferable manufacturing method of the material steel plate of the electrosewn steel pipe used for manufacturing the square steel pipe of the present invention will be described.

For example, after heating a steel material having the above-mentioned composition to a heating temperature of 1100 ° C. or higher and 1300 ° C. or lower, rough rolling end temperature: 850 ° C. or higher and 1150 ° C. or lower, finish rolling end temperature: 750 ° C. or higher and 900 ° C. or lower, In addition, a hot rolling treatment with a total rolling reduction at 950 ° C or lower: 50% or more is performed (hot rolling step), and then the average cooling rate at the wall thickness center temperature: 5 ° C / s or more and 30 ° C / s or less, cooling. Stopping temperature: It is preferable to cool the product at 400 ° C. or higher and 650 ° C. or lower (cooling step), and then wind it at 400 ° C. or higher and 650 ° C. or lower (winding step) to obtain a hot-rolled steel plate (steel plate 1).

In the following description of the manufacturing method, the "℃" indication regarding the temperature shall be the surface temperature of the steel material and the steel plate (hot-rolled steel plate) unless otherwise specified. These surface temperatures can be measured with a radiation thermometer or the like. Further, the temperature at the center of the thickness of the steel sheet can be obtained by calculating the temperature distribution in the cross section of the steel sheet by heat transfer analysis and correcting the result by the surface temperature of the steel sheet. In addition, the "hot-rolled steel plate" includes hot-rolled plates and hot-rolled steel strips.

In the present invention, the melting method of the steel material (steel slab) is not particularly limited, and any known melting method such as a converter, an electric furnace, or a vacuum melting furnace is suitable. The casting method is also not particularly limited, but it is manufactured to a desired size by a known casting method such as a continuous casting method. It should be noted that there is no problem even if the ingot-breaking rolling method is applied instead of the continuous casting method. The molten steel may be further subjected to secondary refining such as ladle refining.

Hot rolling process Heating temperature: 1100 ° C or more and 1300 ° C or less When the heating temperature is less than 1100 ° C, the deformation resistance of the material to be rolled increases and rolling becomes difficult. On the other hand, when the heating temperature exceeds 1300 ° C., the austenite grains become coarse and fine austenite grains cannot be obtained in the subsequent rolling (rough rolling, finish rolling). It becomes difficult to secure the average crystal grain size. Therefore, the heating temperature in the hot rolling step is set to 1100 ° C. or higher and 1300 ° C. or lower. This heating temperature is more preferably 1120 ° C. or higher. Further, this heating temperature is more preferably 1280 ° C. or lower.

In the present invention, in addition to the conventional method in which a steel slab (slab) is manufactured, cooled to room temperature, and then heated again, the steel slab is not cooled to room temperature and is charged into a heating furnace as a hot piece. These direct rolling energy-saving processes, which roll immediately after a small amount of heat retention, can also be applied without problems.

Rough rolling end temperature: 850 ° C or higher and 1150 ° C or less When the rough rolling end temperature is less than 850 ° C, the surface temperature of the steel sheet becomes lower than the ferrite transformation start temperature during the subsequent finish rolling, a large amount of processed ferrite is generated, and yielding occurs. The ratio goes up. On the other hand, when the rough rolling end temperature exceeds 1150 ° C., the amount of rolling in the austenite unrecrystallized temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, it becomes difficult to secure the average crystal grain size of the steel structure of the above-mentioned square steel pipe, and the toughness is lowered. The rough rolling end temperature is more preferably 860 ° C. or higher. The rough rolling end temperature is more preferably 1000 ° C. or lower.

The finish rolling start temperature is preferably 800 ° C. or higher and 980 ° C. or lower. When the finish rolling start temperature is less than 800 ° C., the steel sheet surface temperature becomes lower than the ferrite transformation start temperature during finish rolling, a large amount of processed ferrite is generated, and the yield ratio increases. On the other hand, when the finish rolling start temperature exceeds 980 ° C., the austenite becomes coarse and a sufficient deformation zone is not introduced into the austenite, so that it becomes difficult to secure the average crystal grain size of the steel structure of the above-mentioned square steel pipe. , The toughness decreases. The finish rolling start temperature is more preferably 820 ° C. or higher. The finish rolling start temperature is more preferably 950 ° C. or lower.

Finish rolling end temperature: 750 ° C or more and 900 ° C or less When the finish rolling end temperature is less than 750 ° C, the steel sheet surface temperature becomes below the ferrite transformation start temperature during finish rolling, a large amount of processed ferrite is generated, and the yield ratio is high. Rise. On the other hand, when the finish rolling end temperature exceeds 900 ° C., the amount of rolling in the austenite unrecrystallized temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, it becomes difficult to secure the average crystal grain size of the steel structure of the above-mentioned square steel pipe, and the toughness is lowered. The finish rolling end temperature is more preferably 770 ° C. or higher. The finish rolling end temperature is more preferably 880 ° C. or lower.

Total reduction rate at 950 ° C or lower: 50% or more In the present invention, by refining the subgrain in austenite in the hot rolling process, the ferrite, bainite and the residual structure produced in the subsequent cooling process and winding process are made fine. The steel structure of the square steel pipe having the above-mentioned strength and toughness can be obtained. In order to miniaturize the subgrains in austenite in the hot rolling process, it is necessary to increase the rolling reduction in the austenite unrecrystallized temperature range and introduce sufficient machining strain. In order to achieve this, in the present invention, the total reduction rate of 950 ° C. or lower is set to 50% or more.

When the total reduction rate at 950 ° C. or lower is less than 50%, sufficient machining strain cannot be introduced in the hot rolling process, so that a structure having the average crystal grain size of the above-mentioned square steel pipe cannot be obtained. The total reduction rate at 950 ° C. or lower is more preferably 55% or more, still more preferably 57% or more. Although the upper limit is not specified, if it exceeds 80%, the effect of improving the toughness on the increase in the reduction rate becomes small, and the equipment load only increases. Therefore, the total reduction rate at 950 ° C. or lower is preferably 80% or less. More preferably, it is 70% or less.

The above-mentioned total rolling reduction rate at 950 ° C. or lower refers to the total rolling reduction rate of each rolling path in the temperature range of 950 ° C. or lower.

Cooling process After the hot rolling process, the hot rolled plate is cooled in the cooling process. In the cooling step, cooling is performed at an average cooling rate up to the cooling stop temperature: 5 ° C./s or more and 30 ° C./s or less, and a cooling stop temperature: 400 ° C. or more and 650 ° C. or less.

Average cooling rate from the start of cooling to the stop of cooling (end of cooling): 5 ° C / s or more and 30 ° C / s or less At the center temperature of the wall thickness of the hot-rolled plate, the average cooling rate in the temperature range from the start of cooling to the stop of cooling described later. If the temperature is less than 5 ° C./s, the frequency of nucleation of ferrite or bainite decreases and these become coarse, so that a structure having the average crystal grain size of the above-mentioned square steel tube cannot be obtained. On the other hand, when the average cooling rate exceeds 30 ° C./s, a large amount of martensite is generated and the toughness is lowered. The average cooling rate is preferably 10 ° C./s or higher. The average cooling rate is preferably 25 ° C./s or less.

In the present invention, it is preferable to start cooling immediately after the finish rolling from the viewpoint of suppressing the formation of ferrite on the surface of the steel sheet before cooling.

Cooling stop temperature: 400 ° C. or higher and 650 ° C. or lower When the cooling stop temperature is less than 400 ° C. at the center temperature of the wall thickness of the hot-rolled sheet, a large amount of martensite is generated and the toughness is lowered. On the other hand, when the cooling stop temperature exceeds 650 ° C., the frequency of nucleation of ferrite or bainite decreases and these become coarse, so that a structure having the average crystal grain size of the above-mentioned square steel pipe cannot be obtained. The cooling shutdown temperature is preferably 430 ° C. or higher. The cooling shutdown temperature is preferably 620 ° C. or lower.

In the present invention, the average cooling rate is a value obtained by ((the center temperature of the wall thickness of the hot-rolled plate before cooling-the center temperature of the wall thickness of the hot-rolled plate after cooling) / cooling time) unless otherwise specified. (Cooling rate). Examples of the cooling method include water cooling such as injection of water from a nozzle, cooling by injection of cooling gas, and the like. In the present invention, it is preferable to perform a cooling operation (treatment) on both sides of the hot-rolled plate so that both sides of the hot-rolled plate are cooled under the same conditions.

Winding process After the cooling process, the hot-rolled steel sheet is wound into a coil in the winding process and then allowed to cool. In the winding step, in order to obtain the above-mentioned steel sheet structure, it is preferable to wind at a winding temperature of 400 ° C. or higher and 650 ° C. or lower. If the take-up temperature is less than 400 ° C., a large amount of martensite is generated and the toughness is lowered. When the winding temperature exceeds 650 ° C., the frequency of nucleation of ferrite or bainite decreases, and these become coarse, so that a structure having the average crystal grain size of the above-mentioned square steel pipe cannot be obtained. The take-up temperature is preferably 430 ° C. or higher. The winding temperature is preferably 620 ° C. or lower.

<Building structure>
Next, an embodiment of a building structure using the square steel pipe 10 of the present invention will be described with reference to FIG. FIG. 5 shows an example of a building structure 100 in which the square steel pipe 10 of the present invention is used as a member (for example, a pillar material) of a building structure.

As shown in FIG. 5, in the building structure 100 of the present invention, a plurality of square steel pipes 10 (pillars) established via a diaphragm 17 are welded together. A girder 18 is erected between the adjacent square steel pipes 10, and a girder 19 is erected between the adjacent girders 18. In addition, studs 20 are appropriately provided for mounting the wall or the like. In addition, known members can be used for the building structure 100.

As described above, in the square steel pipe 10 of the present invention, the radius of curvature of the square portion 12 is small, the flat plate portion 11 is flat, and the shape characteristics are excellent. Further, the square steel pipe 10 of the present invention is excellent in ductility and toughness on the outer surface of the square portion 12. Therefore, the building structure 100 of the present invention using the square steel pipe 10 as a pillar material can secure the plastic deformability of the entire structure, and therefore has excellent earthquake resistance as compared with the conventional building structure using the square steel pipe. Demonstrate performance.

Hereinafter, the present invention will be described in more detail based on Examples. The present invention is not limited to the following examples.

The square steel pipe of the present invention was manufactured under the following conditions.

The molten steel having the composition shown in Table 1 was melted and used as a slab (steel material). The obtained slab was subjected to a hot rolling step, a cooling step, and a winding step under the conditions shown in Table 2-1 to obtain a hot-rolled steel sheet.

The obtained hot-rolled steel sheet (material steel sheet) was continuously cold-formed into an open pipe having an elliptical cross section using a cage roll group and a fin pass roll group. Next, the opposite end faces (both ends in the width direction) of the open pipe were heated above the melting point by high frequency induction heating or high frequency resistance heating, and pressed with a squeeze roll to obtain an electrosewn steel pipe.

The obtained electric resistance pipe (bare pipe) was reduced in diameter with a 2-stand (2-stage) sizing roll group, and then square-formed with a 4-stand (4 stage) square forming roll group. Square steel pipes with the dimensions shown in 2 were obtained respectively. In the square forming step, the gap of the sizing roll and the gap of the square forming roll immediately before the square forming were controlled under the conditions shown in Table 2-2. The obtained square steel pipe was substantially rectangular in the vertical cross-sectional view in the pipe axis direction.

The average wall thickness t (mm) of the square steel pipe shown in Table 2-2 is calculated by using the above formula (3), and the average side length H (mm) of the square steel pipe is calculated by using the above formula (4). Calculated using. For the side lengths H ₁ and H ₂ (mm) of the square steel pipe, the side lengths of the flat plate portions at the locations shown in FIG. 1 were measured. For the width W (mm) of the material steel sheet, the width of the steel sheet immediately after passing through the leveler was measured. Circumferential length C _IN (mm) of the electric resistance pipe on the entrance side of the first stage square forming stand, circumference C _OUT (mm) of the square steel pipe on the exit side of the square forming stand of the final stage, and immediately before square forming. The difference (Δg) between the recesses of the caliber roll of the sizing stand and the caliber roll of the first-stage square forming stand was measured by the above method. Then, G (= Δg / (t / H)) was calculated using the above-mentioned difference (Δg), average wall thickness t, and average side length H.

In the obtained square steel pipe, each square steel pipe was cut perpendicular to the pipe axis direction, the cut surface including the electrosewn weld was polished, and then nital corroded and observed with an optical microscope. It was also confirmed that the width of the melt-solidified portion of the electric stitch welded portion in the pipe circumferential direction was 1.0 μm or more and 1000 μm or less over the entire thickness of the pipe. The melt-solidified portion was identified as a region observed white with an optical microscope in the above cross section corroded by nital.

The steel structure of the obtained square steel pipe was quantified, tested and evaluated by the method shown below.

(1) Steel structure of square steel pipe The steel structure of square steel pipe was quantified by the method described above. The results obtained are shown in Table 3.

(2) Radius of curvature of the outer surface of the corner of the square steel pipe The radius of curvature of the corner of the obtained square steel pipe is the radius of curvature of the outer surface of the four corners (outside of the corner) at 10 arbitrary positions in the pipe axis direction. The radius of curvature (mm) was measured respectively. The maximum value Rmax and the minimum value Rmin were obtained from the measured values at a total of 40 points. The values are shown in Table 4. Here, when the maximum value Rmax and the minimum value Rmin of the radius of curvature are in the range of 2.0t or more and 3.0t or less, it is evaluated that the radius of curvature of the outer surface of the corner portion is small.

A radial gauge was used to measure the radius of curvature on the outside of the corner. The method of measuring the radius of curvature was measured by the above-mentioned method described with reference to FIG.

(3) Flatness of a flat plate portion of a square steel pipe A method of measuring flatness will be described with reference to FIG. The flatness was measured at 10 points at arbitrary positions in the pipe axis direction of the square steel pipe, with 4 flat plate portions as measurement targets, respectively, at a total of 40 points. As shown in FIG. 10, the maximum bulge amount and the maximum dent amount with respect to the straight line passing through the two points at both ends in the circumferential direction of the outer surface of each flat plate portion were measured. The amount of swelling was a positive value, the amount of dent was a negative value, and the measured values are shown in Table 4. Then, the absolute values of the maximum bulge amount and the maximum dent amount at each measurement point were obtained, and the maximum values were defined as the flatness of the flat plate portion and are shown in Table 4. However, when there was no bulge or dent, the value of the bulge amount or dent amount was set to 0.

Here, when the flatness (mm) of the flat plate portion is 2.5 mm or less, it is evaluated that the flat plate portion is flat.

(4) Tensile test of flat plate and corner of square steel pipe Using the obtained square steel pipe, a tensile test was conducted by the following method. FIG. 6 shows the sampling positions of the flat plate portion and the corner tensile test piece, respectively, and FIG. 7 shows the detailed sampling positions of the corner tensile test pieces.

As shown in FIG. 6, JIS No. 5 tensile test pieces and JIS No. 12B tensile test pieces shown by broken lines were taken from the flat plate portion and the corner portion of the square steel pipe so that the tensile direction was parallel to the pipe axis direction, respectively. The tensile test pieces were collected by grinding them so that the thickness was 5 mm and the center of the thickness was 1/4 t of the wall thickness t from the outer surface of the pipe. As shown in FIG. 7, the tensile test piece at the corner is collected from an intersection that extends the outer surfaces of the flat plates on both sides adjacent to the corner, and from a line forming 45 ° with the outer surface of the flat plate. bottom.

Tensile tests were carried out using these tensile test pieces in accordance with JIS Z 2241 regulations, and the yield strength YS, tensile strength TS, and uniform elongation of the flat plate and corners (flat plate: E1, corners: E2). Was measured. The uniform elongation is the value of the total elongation at the maximum load. For the corners, the yield strength and tensile strength obtained were used to calculate the yield ratio defined by (yield strength) / (tensile strength) × 100 (%). Further, the value of the uniform elongation E2 of the corner portion with respect to the uniform elongation E1 of the flat plate portion was calculated.

The number of tensile test pieces was two each, and the average value thereof was calculated to obtain the yield strength YS (MPa), the tensile strength TS (MPa), the yield ratio (%), and the uniform elongation (%). These values are shown in Table 4.

Here, when the value of the uniform elongation E2 of the corner portion with respect to the uniform elongation E1 of the flat plate portion is 0.60 or more, it is evaluated that the ductility of the outer surface of the corner portion is excellent. It was evaluated that the yield ratio of the corner portion was good when it was 90% or less, the yield strength YS of the flat plate portion was good when it was 295 MPa or more, and the tensile strength TS of the flat plate portion was good when it was 400 MPa or more.

As shown in FIG. 6, the tensile test piece of the flat plate portion was taken from the position at the center of the width of the flat plate portion 11b located next to the flat plate portion 11a including the electric resistance welded portion 13 of the square steel pipe. The tensile test piece at the corner was taken from the corner 12a adjacent to the flat plate portion 11a including the electric stitch welded portion 13.

(5) Charpy Impact Test of Corners of Square Steel Pipe Using the obtained square steel pipe, a Charpy impact test was conducted by the following method. FIG. 8 shows the collection position of the Charpy test piece at the corner, and FIG. 9 shows the detailed collection position of the Charpy test piece at the corner.

As shown in FIGS. 8 and 9, in the Charpy impact test, JIS was taken so that the longitudinal direction of the test piece was parallel to the axial direction of the pipe at a position 1/4 t of the wall thickness t from the outer surface of the pipe of the square steel pipe. A V-notch standard test piece conforming to the provisions of Z 2242 was used. The Charpy test piece at the corner was taken from the corner 12a adjacent to the flat plate portion 11a including the electric stitch welded portion 13. More specifically, as shown in FIG. 9, the samples were taken from the intersections extending the outer surfaces of the flat plate portions on both sides adjacent to the corner portion 12a, and from the line forming an angle of 45 ° with the outer surface of the flat plate portion. A Charpy impact test was carried out at a test temperature of -10 ° C in accordance with JIS Z 2242 to determine the Charpy absorption energy (J). The number of test pieces was 3 each, and the average value of them was calculated to obtain the Charpy absorption energy (J). The values are shown in Table 4.

Here, it was evaluated that the toughness of the outer surface of the corner is excellent when the Charpy absorption energy of the corner at -10 ° C is 100 J or more.

No. in Tables 2-1 to 4 1 to 3 and 8 to 13 are examples of the present invention, No. 4 to 7 are comparative examples.

In each of the square steel pipes of the present invention, the radius of curvature R on the outside of the corner portion is 2.0 t or more and 3.0 t or less, the flatness of the outer surface of the flat plate portion in the pipe axis direction is 2.5 mm or less, and the angle is square. The uniform elongation E2 at the 1 / 4t position from the outer surface of the flat plate portion is 0.60 times or more the uniform elongation E1 at the 1 / 4t position from the outer surface of the flat plate portion, and the charmy absorption energy of the corner portion at −10 ° C. is 100J. That was all.

On the other hand, No. of Comparative Example. In No. 4, since the value of "W / C _OUT " was below the range of the equation (1), the radius of curvature outside the corner portion was above the range of the present invention, and a flat flat plate portion could not be obtained.

Comparative example No. In No. 5, since the value of "W / C _OUT " exceeded the range of the formula (1), the ratio of uniform elongation between the flat plate portion and the corner portion (E2 / E1) and the Charpy absorption energy of the corner portion at −10 ° C. The desired value was not reached. In addition, the yield ratio of the corners also showed a value of 90% or more.

Comparative example No. In No. 6, since the value of "C _IN / C _OUT " was below the range of the equation (2), the radius of curvature on the outside of the corner portion exceeded the range of the present invention, and a flat flat plate portion could not be obtained.

Comparative example No. In No. 7, since the value of "C _IN / C _OUT " exceeded the range of the equation (2), the radius of curvature outside the corner portion fell below the range of the present invention, and the ratio of the uniform elongation of the flat plate portion to the corner portion. (E2 / E1), and the Charpy absorption energy at −10 ° C. at the corners did not reach the desired value. In addition, the yield ratio of the corners also showed a value of 90% or more.

1 Steel plate (steel strip)
2 Leveler 3 Cage roll group 4 Finpass roll group 5 Squeeze roll 6 Welder 7 Electric sewn steel pipe 8 Sizing roll group 9 Square forming roll group 10 Square steel pipe 11 Flat plate part 12 Square part 13 Welded part (electric sewn welded part)
14 Base metal part 15 Welding heat-affected zone 16 Melt solidification part 17 Diaphragm 18 Large beam 19 Small beam 20 Stud 100 Building structure

Claims

A square steel pipe with a flat plate and corners,
The radius of curvature R on the outside of the corner portion is 2.0 t or more and 3.0 t or less when the average wall thickness of the flat plate portion is t (mm).
The flatness of the outer surface of the flat plate portion is 2.5 mm or less.
The uniform elongation E2 at the position of 1 / 4t in the wall thickness direction from the outer surface of the corner portion is 0.60 times or more the uniform elongation E1 at the position of 1 / 4t in the wall thickness direction from the outer surface of the flat plate portion. ,
A square steel pipe having a Charpy absorption energy of 100 J or more at −10 ° C. at a position 1/4 t in the wall thickness direction from the outer surface of the corner portion.
The square steel pipe according to claim 1, wherein the average wall thickness t is more than 0.030 times the average side length H (mm) of the flat plate portion.
The square steel pipe according to claim 1 or 2, wherein the average wall thickness t is 20 mm or more and 40 mm or less.
The yield strength of the flat plate portion is 295 MPa or more, and the yield strength is 295 MPa or more.
The tensile strength of the flat plate portion is 400 MPa or more, and the plate portion has a tensile strength of 400 MPa or more.
The square steel pipe according to any one of claims 1 to 3, wherein the yield ratio of the corner portion is 90% or less.
The composition of the square steel pipe is mass%.
C: 0.020 to 0.45%,
Si: 0.01-1.0%,
Mn: 0.30 to 3.0%,
P: 0.10% or less,
S: 0.050% or less,
Al: 0.005 to 0.10%,
N: 0.010% or less,
Ti: contains 0.001 to 0.15%, the balance consists of Fe and unavoidable impurities.
The steel structure at the center of the wall thickness of the flat plate portion is
The total volume fraction of ferrite and bainite is 70% or more and 95% or less of the total steel structure at the center of the thickness of the flat plate, and the balance is one or more selected from pearlite, martensite, and austenite. Consists of
When a region surrounded by a boundary with an orientation difference of 15 ° or more between adjacent crystals is used as a crystal grain.
The average crystal grain size of the crystal grains is 15.0 μm or less, and the average crystal grain size is 15.0 μm or less.
The square steel pipe according to any one of claims 1 to 4, wherein the total volume fraction of the crystal grains having a crystal grain size of 40 μm or more is 40% or less with respect to the entire steel structure at the center of the wall thickness of the flat plate portion.
In addition to the composition of the above components, by mass%,
Nb: 0.001 to 0.15%,
V: 0.001 to 0.15%,
Cr: 0.01-1.0%,
Mo: 0.01-1.0%,
Cu: 0.01-1.0%,
Ni: 0.01-1.0%,
Ca: 0.0002 to 0.010%,
B: 0.0001 to 0.010%
The square steel pipe according to any one of claims 1 to 5, which comprises one kind or two or more kinds selected from.
The method for manufacturing a square steel pipe according to any one of claims 1 to 6.
A steel plate is cold-rolled, and both ends of the steel plate in the width direction are welded and welded to form an electric resistance pipe. When manufacturing steel pipes
The ratio of the plate width W of the steel plate to the peripheral length C OUT of the square steel pipe on the outlet side of the square forming stand satisfies the formula (1), and the peripheral length C OUT of the square steel pipe on the exit side of the square forming stand. The gap between the rolls of the sizing stand and the gap between the rolls of the square forming stand immediately before square forming are controlled so that the ratio of the peripheral length CIN of the electric resistance pipe on the entrance side of the square forming stand satisfies the formula (2). How to manufacture a square steel pipe.
1.000 + 0.050 × t / H <W / C OUT <1.000 + 0.50 × t / H ... Equation (1)
0.30 × t / H + 0.99 ≦ C IN / C OUT <0.50 × t / H + 0.99 ... Equation (2)
Here, in equations (1) and (2),
W: Plate width (mm) of the steel plate that is the material,
C IN : Perimeter (mm) of the electric resistance sewn steel pipe on the entrance side of the first stage square forming stand,
C OUT : Perimeter (mm) of the square steel pipe on the exit side of the square forming stand in the final stage,
t: Average wall thickness (mm) of the flat plate portion after square forming,
H: Average side length (mm) of the flat plate portion after square forming,
Is.
However, when square forming is performed by a one-stage square forming stand, the first-stage square forming stand and the final stage square forming stand refer to the same square forming stand.
In the steel sheet, after the steel material is heated to a heating temperature of 1100 ° C. or higher and 1300 ° C. or lower, the rough rolling end temperature: 850 ° C. or higher and 1150 ° C. or lower, the finish rolling end temperature: 750 ° C. or higher and 900 ° C. or lower, and 950 ° C. Total reduction rate in the following: After hot rolling treatment of 50% or more,
Next, cooling was performed at the center temperature of the wall thickness at an average cooling rate of 5 ° C./s or more and 30 ° C./s or less, and a cooling shutdown temperature: 400 ° C. or more and 650 ° C. or less.
The method for manufacturing a square steel pipe according to claim 7, wherein the steel pipe is then wound at 400 ° C. or higher and 650 ° C. or lower.
The method for manufacturing a square steel pipe according to claim 7 or 8, wherein the average wall thickness t is more than 0.030 times the average side length H of the flat plate portion.
The method for manufacturing a square steel pipe according to any one of claims 7 to 9, wherein the average wall thickness t is 20 mm or more and 40 mm or less.
A building structure using the square steel pipe according to any one of claims 1 to 6 as a column material.