WO2024053169A1

WO2024053169A1 - Square steel pipe, manufacturing method for same, and building structure using square steel pipe

Info

Publication number: WO2024053169A1
Application number: PCT/JP2023/019016
Authority: WO
Inventors: 晃英松本; 直道岩田; 昌士松本; 信介井手
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-09-09
Filing date: 2023-05-22
Publication date: 2024-03-14

Abstract

Provided is a square steel pipe having excellent buckling resistance. In this square steel pipe, which has a plurality of flat portions and corner portions alternating in the pipe circumference direction, the yield strength of the flat portions in the pipe circumference direction is set to be 0.83-1.20 times the yield strength of the flat portions in the pipe axial direction, and the yield strength of the corner portions in the pipe circumference direction is set to be 0.90-1.30 times the yield strength of the flat portions in the pipe axial direction.

Description

Square steel pipes, their manufacturing methods, and building structures using square steel pipes

The present invention relates to a rectangular steel pipe with excellent buckling resistance that is suitably used as a pillar material in buildings, a method for manufacturing the same, and a building structure using such a rectangular steel pipe.

Roll-formed square steel pipes are widely used as square steel pipes for building pillars.
Roll-formed square steel pipes are made by cold roll-forming a steel strip (hereinafter referred to as "steel plate") into a cylindrical open pipe shape, and after welding the butt portions of the open pipes by electric resistance welding, they are arranged vertically and horizontally. It is manufactured by applying a drawing in the axial direction of the tube while maintaining the cylindrical shape using rolled rolls to form it into a square shape. In the electric resistance welding described above, the abutted portions are heated and melted, and are pressed and solidified to complete the joining.

From the perspective of earthquake resistance, roll-formed square steel pipes are required to have high buckling resistance during bending deformation. However, in roll-formed square steel pipes, the flat plate portions and corner portions undergo significant work hardening during manufacture, resulting in a decrease in ductility and/or toughness of the corner portions, and the buckling resistance tends to decrease.

In order to solve this problem, various square steel pipes have been proposed that have improved properties at the flat plate portion and corner portions.

For example, Patent Documents 1 and 2 propose square steel pipes with improved ductility and toughness at corners by appropriately controlling chemical components and microstructures.

In addition, in Patent Document 3, a steel plate is bent and then welded to form a semi-formed square steel tube, and the half-formed square steel tube is heated and hot-formed, and then cooled to improve the ductility and toughness of the corners. A rectangular steel pipe has been proposed.

Japanese Patent Application Publication No. 2011-094214 JP2018-053281A Japanese Patent Application Publication No. 2004-330222

Regarding the buckling resistance during bending deformation of square steel pipes, we will examine the buckling performance when the load is applied from the direction perpendicular to the flat plate part (0 degree bending) and when the load is applied from the corner apex direction (45 degree bending). Performance needs to be considered for both.
In addition, in the case of 0 degree bending, buckling occurs because the flat plate part on the inside of the bend is recessed toward the inner surface of the tube. Furthermore, in the case of 45-degree bending, buckling occurs due to the increased radius of curvature of the corner on the inside of the bend.

In order to improve the buckling resistance of a square steel pipe, it is considered effective to suppress the deformation of the flat plate portion and the deformation of the corner portion, which are the precursor phenomena of buckling described above. However, conventional studies have not been conducted from this perspective.

The present invention has been made in view of the above circumstances, and provides a square steel pipe with excellent buckling resistance, a method for manufacturing the same, and a building structure using such a square steel pipe with excellent seismic performance. The purpose is to
In the present invention, "excellent buckling resistance" means that the cumulative plastic deformation magnification of the square steel pipe in both the 0 degree bending test and the 45 degree bending test is 28 or more.

As a result of intensive studies by the present inventors to solve the above-mentioned problem, it was found that by appropriately controlling the yield strength in the circumferential direction of the flat plate portion and the corner portion of the square steel pipe, the above-mentioned precursor phenomenon of buckling can be prevented. It has been found that the deformation of the flat plate portion and/or the corner portion can be effectively suppressed and the buckling resistance can be improved.

Furthermore, by appropriately controlling the residual stress in the circumferential direction of the flat plate portion, deformation of the flat plate portion, which is a precursor phenomenon of buckling, can be suppressed, and the buckling resistance performance in the 0 degree direction can be further improved. We also discovered what can be done.

The present invention was completed based on this knowledge and further studies. That is, the gist of the present invention is as follows.

1. A square steel pipe having a plurality of flat plate parts and corner parts alternately in the pipe circumferential direction and having a welded part extending in the pipe axial direction,
The yield strength of the flat plate portion in the tube circumferential direction is 0.83 times or more and 1.20 times or less of the yield strength of the flat plate portion in the tube axis direction,
A square steel pipe, wherein the yield strength of the corner portion in the tube circumferential direction is 0.90 times or more and 1.30 times or less the yield strength of the flat plate portion in the tube axis direction.

2. 2. The square steel pipe according to 1, wherein the residual stress in the circumferential direction on the outer surface of the flat plate portion is -200 MPa or more and 150 MPa or less.
However, when the residual stress is a positive value, it represents tensile stress, and when it is a negative value, it represents compressive stress.

3. In mass%,
C: 0.020-0.350%,
Si: 0.01 to 0.65%,
Mn: 0.30-2.50%,
P: 0.050% or less,
S: 0.0500% or less,
3. The square steel pipe according to 1 or 2 above, which has a composition comprising Al: 0.005 to 0.100% and N: 0.0100% or less, with the balance being Fe and inevitable impurities.

4. The component composition further includes, in mass%,
Ti: 0.100% or less,
Nb: 0.100% or less,
V: 0.100% or less,
Cr: 1.00% or less,
Mo: 1.00% or less,
Cu: 1.00% or less,
Ni: 1.00% or less,
3. The square steel pipe according to 3 above, containing one or more selected from Ca: 0.0100% or less and B: 0.0100% or less.

5. The method for manufacturing a square steel pipe according to any one of 1 to 4 above,
A pipe-making process in which a steel plate is formed into a cylindrical shape by cold roll forming, and the circumferential ends of the cylindrical shape are abutted against each other and welded by electric resistance welding;
The flatness of the flat plate part is -10.0 mm or more and -4.0 mm or less by the square forming stand performed after the pipe making process, and the radius of curvature of the corner part is 1.0 times or more and 2.5 times the wall thickness of the flat plate part. A square forming process to make a square steel pipe material that is less than double the size of the square steel pipe;
Subsequently, an internal pressure p (MPa) satisfying the following formula (1) is applied to the inner surface of the square steel pipe material, and the flatness of the flat part of the square steel pipe material is set to -3.0 mm or more and 3.0 mm or less. , an internal pressure loading step to form a square steel pipe in which the radius of curvature of the corner portion is 2.0 times or more and 4.0 times or less the wall thickness of the flat plate portion;
A method of manufacturing a square steel pipe, including:
N(MPa)<p≦N×1.5(MPa)...(1)
However, N = (wall thickness of square steel pipe material (mm) / side length of square steel pipe material (mm)) x yield strength in the pipe circumferential direction of the flat plate part of square steel pipe material (MPa)

6. An architectural structure comprising the square steel pipe according to any one of 1 to 4 above as a column material.

According to the present invention, it is possible to provide a square steel pipe with excellent buckling resistance and a method for manufacturing the same.
Moreover, a building structure using the square steel pipe of the present invention as a column material can obtain better seismic performance than a building structure using a conventional cold-formed square steel pipe.

FIG. 1 is a schematic diagram showing a cross section perpendicular to the tube axis direction of a square steel pipe (or a square steel pipe material) of the present invention. FIG. 3 is a schematic diagram for explaining a method for measuring flatness. It is a schematic diagram showing a corner forming stand. 1 is a schematic diagram showing an example of an architectural structure of the present invention. FIG. 2 is a schematic diagram showing the sampling position of a tube axial direction tensile test piece of a flat plate portion of a square steel pipe material. FIG. 2 is a schematic diagram showing the sampling positions of circumferential direction tensile test pieces of a flat plate portion and a corner portion of a square steel pipe, respectively. FIG. 2 is a schematic diagram showing the sampling position of a tensile test piece at a corner of a square steel pipe. It is a schematic diagram of a 0 degree bending test of a square steel pipe. It is a schematic diagram of a 45 degree bending test of a square steel pipe.

The square steel pipe of the present invention has a plurality of flat plate parts and corner parts alternately in the pipe circumferential direction, so that the cross section of the steel pipe is not only rectangular (quadrilateral) but also triangular and polygonal with pentagon or more. There is. Furthermore, the square steel pipe has a welded portion extending in the tube axis direction, and the yield strength of the flat plate portion in the tube circumferential direction is 0.83 times or more the yield strength of the flat plate portion in the tube axis direction. .20 times or less, and the yield strength of the corner portion in the tube circumferential direction is 0.90 times or more and 1.30 times or less of the yield strength of the flat plate portion in the tube axis direction.

Yield strength in the tube circumferential direction of the flat plate portion: 0.83 times or more and 1.20 times or less than the yield strength in the tube axis direction of the flat plate portion. If it is less than 0.83 times, the flat plate part on the inside of the bend is likely to be plastically deformed in 0 degree bending, and the flat plate part on the inside of the bend is likely to dent toward the inner surface of the tube, resulting in a decrease in buckling resistance. Therefore, the yield strength of the flat plate portion in the tube circumferential direction is 0.83 times or more the yield strength of the flat plate portion in the tube axis direction. The yield strength of the flat plate portion in the tube circumferential direction is preferably 0.85 times or more, more preferably 0.87 times or more, the yield strength of the flat plate portion in the tube axis direction.
On the other hand, if the yield strength of the flat plate part in the circumferential direction is more than 1.20 times the yield strength of the flat plate part in the tube axis direction, the ductility of the flat plate part decreases, resulting in uneven deformation of the flat plate part in 0 degree bending. occurs early, resulting in a decrease in buckling resistance. Moreover, breakage on the outside of bending is likely to occur. Therefore, the yield strength of the flat plate portion in the tube circumferential direction is 1.20 times or less than the yield strength of the flat plate portion in the tube axis direction. The yield strength of the flat plate portion in the tube circumferential direction is preferably 1.15 times or less, more preferably 1.10 times or less, the yield strength of the flat plate portion in the tube axis direction.
In addition, in the conventional manufacturing method, the yield strength of the flat plate portion in the tube circumferential direction was about 0.80 times the yield strength of the flat plate portion in the tube axis direction.
In addition, in the present invention, the flat plate portion refers to a range excluding the corner portions described below, and each of the both ends (connection points (points A, A', etc. shown in FIG. 1)) of the flat plate portion is connected to a different corner portion. However, this is a portion having flatness, which will be described later. Note that FIG. 1 is a schematic diagram showing a cross section perpendicular to the tube axis direction of the square steel pipe of the present invention.

Yield strength in the circumferential direction of the corner: 0.90 times or more and 1.30 times or less the yield strength in the tube axis direction of the flat plate portion. If it is less than 0.90 times, the corner on the inner side of the bend tends to be plastically deformed during 45-degree bending, and the radius of curvature of the corner on the inner side of the bend tends to increase, resulting in a decrease in buckling resistance. Therefore, the yield strength of the corner portion in the tube circumferential direction is 0.90 times or more the yield strength of the flat plate portion in the tube axis direction. The yield strength of the corner portion in the tube circumferential direction is preferably 0.95 times or more, more preferably 1.00 times or more, the yield strength of the flat plate portion in the tube axis direction.
On the other hand, if the yield strength of the corner in the tube circumferential direction is more than 1.30 times the yield strength of the flat plate in the tube axis direction, the ductility of the flat plate decreases and uneven deformation of the corner occurs during 45 degree bending. Since this occurs early, buckling resistance performance deteriorates. Moreover, breakage on the outside of bending is likely to occur. Therefore, the yield strength of the corner portion in the tube circumferential direction is 1.30 times or less than the yield strength of the flat plate portion in the tube axis direction. The yield strength of the corner portion in the tube circumferential direction is preferably 1.25 times or less, more preferably 1.20 times or less, the yield strength of the flat plate portion in the tube axis direction.
In addition, in the conventional manufacturing method, the yield strength of the corner portion in the tube circumferential direction was approximately 0.85 times the yield strength of the flat plate portion in the tube axis direction.
Furthermore, in the present invention, a corner is a portion of a curved surface having a predetermined radius of curvature, which will be described later.

In order to ensure sufficient strength and buckling resistance for the square steel pipe of the present invention as a building structure, it is preferable that the flat plate portion has a yield strength of 295 MPa or more in the tube axis direction.
Note that the above yield strength is obtained by conducting a tensile test in accordance with the regulations of JIS Z 2241 2011.

In the square steel pipe of the present invention, in order to improve the buckling resistance in the 0 degree direction, it is preferable that the residual stress in the tube circumferential direction on the outer surface of the flat plate portion is −200 MPa or more and 150 MPa or less. However, when the residual stress is a positive value, it represents tensile stress, and when it is a negative value, it represents compressive stress.

If the value of residual stress in the tube circumferential direction on the outer surface of the flat plate part is -200 MPa or more, compressive deformation of the flat plate outer surface is promoted in 0 degree bending, and the flat plate part on the inside of the bend tends to dent toward the tube inner surface. It is possible to suppress such problems and maintain good buckling resistance. The residual stress is more preferably -190 MPa or more, and still more preferably -180 MPa or more.

On the other hand, if the value of the residual stress in the tube circumferential direction on the outer surface of the flat plate part is 150 MPa or less, tensile deformation of the flat plate outer surface is promoted in 0 degree bending, and the flat plate parts on both sides of the flat plate part on the inner side of the bending are It is possible to suppress problems such as easy swelling. As a result, denting of the flat plate portion on the inner side of the bending toward the inner surface of the tube is also suppressed, and buckling resistance can be maintained satisfactorily. The residual stress is more preferably 140 MPa or less, still more preferably 130 MPa or less.
Note that the residual stress in the present invention can be measured by X-ray diffraction.

In order to ensure sufficient strength and weldability as a building structure, the square steel pipe of the present invention has a mass percentage of
C: 0.020-0.350%,
Si: 0.01 to 0.65%,
Mn: 0.30-2.50%,
P: 0.050% or less,
S: 0.0500% or less,
It is preferable to have a component composition containing Al: 0.005 to 0.100% and N: 0.0100% or less, with the balance being Fe and inevitable impurities.

Hereinafter, unless otherwise specified, "%" indicating the steel composition is "% by mass". Moreover, the following composition can also be said to be the composition of the flat plate part and the corner part of the square steel pipe excluding the welded part.

C: 0.020% or more and 0.350% or less C is an element that increases the strength of steel through solid solution strengthening. Further, C is an element that contributes to the refinement of the structure by lowering the ferrite transformation start temperature. In order to obtain these effects, it is preferable to contain 0.020% or more of C. On the other hand, C is an element that promotes the formation of pearlite, improves hardenability, contributes to the formation of martensite, and contributes to the stabilization of austenite, and therefore also contributes to the formation of a hard phase. However, if the C content exceeds 0.350%, the ratio of the hard phase increases, ductility decreases, and buckling resistance may decrease. Furthermore, weldability may also deteriorate.
From the above, it is preferable that the C content is in the range of 0.020% or more and 0.350% or less. The C content is more preferably 0.030% or more, and even more preferably 0.040% or more. On the other hand, the C content is more preferably 0.330% or less, still more preferably 0.310% or less.

Si: 0.01% or more and 0.65% or less Si is an element that increases the strength of steel through solid solution strengthening. In order to obtain such an effect, it is preferable to contain 0.01% or more of Si. On the other hand, if the Si content exceeds 0.65%, oxides are likely to be generated in the electric resistance welded part, and the properties of the welded part may deteriorate. Furthermore, the ductility of the base metal portion other than the electric resistance welded portion may be reduced, and buckling resistance may be reduced.
From the above, it is preferable that the Si content is in the range of 0.01% or more and 0.65% or less. The Si content is more preferably 0.02% or more, and still more preferably 0.03% or more. On the other hand, the Si content is more preferably 0.63% or less, still more preferably 0.61% or less.

Mn: 0.30% or more and 2.50% or less Mn is an element that increases the strength of steel through solid solution strengthening. Furthermore, Mn is an element that contributes to the refinement of the structure by lowering the ferrite transformation start temperature. In order to obtain these effects, it is preferable to contain Mn in an amount of 0.30% or more. On the other hand, when the Mn content exceeds 2.50%, oxides are likely to be generated in the electric resistance welded part, and there is a possibility that the properties of the welded part may deteriorate. In addition, due to solid solution strengthening and microstructural refinement, ductility may decrease and buckling resistance may decrease.
From the above, it is preferable that the Mn content is in the range of 0.30% or more and 2.50% or less. The Mn content is more preferably 0.40% or more, and even more preferably 0.50% or more. On the other hand, the Mn content is more preferably 2.30% or less, even more preferably 2.10% or less.

P: 0.050% or less P segregates at grain boundaries and causes material inhomogeneity, so it is preferable to reduce it as much as possible. For this reason, the P content is preferably 0.050% or less. The P content is more preferably 0.040% or less, and still more preferably 0.030% or less. There is no particular lower limit for P (usually it is over 0%), but excessive reduction will lead to a rise in smelting costs, so it is preferable for the P content to be 0.002% or more.

S: 0.0500% or less S usually exists as MnS in steel, but MnS is stretched thin during the hot rolling process, reducing ductility and buckling resistance. Therefore, in the present invention, it is preferable to reduce S as much as possible. Therefore, the S content in the present invention is preferably 0.0500% or less. The S content is more preferably 0.0300% or less, and still more preferably 0.0100% or less. There is no particular lower limit to S (usually over 0%), but excessive reduction will lead to a rise in smelting costs, so S is preferably 0.0002% or more.

Al: 0.005% or more and 0.100% or less Al is an element that acts as a strong deoxidizing agent. In order to obtain such an effect, it is preferable to contain 0.005% or more of Al. On the other hand, when the Al content exceeds 0.100%, weldability deteriorates and alumina-based inclusions increase, which may deteriorate the surface quality.
Therefore, the Al content is preferably in the range of 0.005% or more and 0.100% or less. The Al content is more preferably 0.010% or more, and even more preferably 0.015% or more. On the other hand, the Al content is more preferably 0.080% or less, still more preferably 0.060% or less.

N: 0.0100% or less N is an element that firmly fixes the movement of dislocations, thereby reducing ductility and reducing buckling resistance. In the present invention, it is desirable to reduce N as much as possible. For this reason, the N content is preferably 0.0100% or less. The N content is more preferably 0.0080% or less. There is no particular lower limit to N (usually it is over 0%), but excessive reduction will lead to a rise in smelting costs, so N is preferably 0.0010% or more.

Moreover, the above-mentioned composition of the square steel pipe of the present invention further includes, in mass %,
Ti: (more than 0%) 0.100% or less,
Nb: (more than 0%) 0.100% or less,
V: (more than 0%) 0.100% or less,
Cr: (more than 0%) 1.00% or less,
Mo: (more than 0%) 1.00% or less,
Cu: (more than 0%) 1.00% or less,
Ni: (more than 0%) 1.00% or less,
It may contain one or more selected from Ca: (more than 0%) 0.0100% or less and B: (more than 0%) 0.0100% or less.

Ti: 0.100% or less If the Ti content exceeds 0.100%, ductility may decrease and buckling resistance may decrease. Therefore, the Ti content is preferably 0.100% or less. The Ti content is more preferably 0.090% or less, still more preferably 0.080% or less. On the other hand, Ti is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in steel. Furthermore, since it has a high affinity with N, it is an element that renders N in steel harmless as a nitride and contributes to improving the ductility of steel. In order to obtain the above effects, the Ti content is preferably 0.001% or more. The Ti content is more preferably 0.002% or more, and still more preferably 0.005% or more.

Nb: 0.100% or less If the Nb content exceeds 0.100%, ductility may decrease and buckling resistance may decrease. Therefore, the Nb content is preferably 0.100% or less. The Nb content is more preferably 0.090% or less, still more preferably 0.080% or less. On the other hand, Nb contributes to improving the strength of steel by forming fine carbides and nitrides in the steel, and also contributes to the refinement of the structure by suppressing the coarsening of austenite during hot rolling. It is an element that In order to obtain the above effects, the Nb content is preferably 0.001% or more. The Nb content is more preferably 0.002% or more, and still more preferably 0.005% or more.

V: 0.100% or less If the V content exceeds 0.100%, ductility may decrease and buckling resistance may decrease. Therefore, the V content is preferably 0.100% or less. The V content is more preferably 0.090% or less, still more preferably 0.080% or less. On the other hand, V is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in steel. In order to obtain the above effects, it is preferable to contain 0.001% or more of V. The V content is more preferably 0.002% or more, and still more preferably 0.005% or more.

Cr: 1.00% or less If the Cr content exceeds 1.00%, ductility may decrease and buckling resistance may decrease. Therefore, the Cr content is preferably 1.00% or less. The Cr content is more preferably 0.80% or less, still more preferably 0.60% or less. On the other hand, Cr is an element that improves the hardenability of steel and increases the strength of steel. In order to obtain the above effects, the Cr content is preferably 0.01% or more. The Cr content is more preferably 0.02% or more, and even more preferably 0.05% or more.

Mo: 1.00% or less If the Mo content exceeds 1.00%, ductility may decrease and buckling resistance may decrease. Therefore, the Mo content is preferably 1.00% or less. The Mo content is more preferably 0.80% or less, still more preferably 0.60% or less. On the other hand, Mo is an element that improves the hardenability of steel and increases the strength of steel. In order to obtain the above effects, the Mo content is preferably 0.01% or more. The Mo content is more preferably 0.02% or more, and still more preferably 0.05% or more.

Cu: 1.00% or less If the Cu content exceeds 1.00%, ductility may decrease and buckling resistance may decrease. Therefore, the Cu content is preferably 1.00% or less. The Cu content is more preferably 0.80% or less, and still more preferably 0.60% or less. On the other hand, Cu is an element that increases the strength of steel through solid solution strengthening. In order to obtain the above effects, the Cu content is preferably 0.01% or more. The Cu content is more preferably 0.02% or more, and still more preferably 0.05% or more.

Ni: 1.00% or less If the Ni content exceeds 1.00%, ductility may decrease and buckling resistance may decrease. Therefore, the Ni content is preferably 1.00% or less. The Ni content is more preferably 0.80% or less, and still more preferably 0.60% or less. On the other hand, Ni is an element that increases the strength of steel through solid solution strengthening. In order to obtain the above effects, the Ni content is preferably 0.01% or more. The Ni content is more preferably 0.02% or more, and still more preferably 0.05% or more.

Ca: 0.0100% or less When the Ca content exceeds 0.0100%, Ca oxide clusters are formed in the steel, which may reduce ductility and reduce buckling resistance. For this reason, the Ca content is preferably 0.0100% or less. The Ca content is more preferably 0.0080% or less, still more preferably 0.0060% or less. On the other hand, Ca is an element that contributes to improving the ductility of steel by spheroidizing sulfides such as MnS that are thinly stretched in the hot rolling process. In order to obtain the above effects, it is preferable to contain 0.0002% or more of Ca. The Ca content is more preferably 0.0005% or more, and still more preferably 0.0010% or more.

B: 0.0100% or less If the B content exceeds 0.0100%, ductility may decrease and buckling resistance may decrease. Therefore, the B content is preferably 0.0100% or less. The B content is more preferably 0.0080% or less, still more preferably 0.0060% or less, and even more preferably 0.0040% or less. On the other hand, B is an element that contributes to the refinement of the structure by lowering the ferrite transformation start temperature, and can be included as necessary. In order to obtain the above effects, it is preferable to contain 0.0001% or more of B. The B content is more preferably 0.0005% or more, and still more preferably 0.0008% or more.

In the above component composition, the remainder is Fe and inevitable impurities. Unavoidable impurities in the remainder include, for example, Sn, As, Sb, Bi, Co, Pb, Zn and O. However, within a range that does not impair the effects of the present invention, it contains 0.10% or less of Sn, 0.050% or less of each of As, Sb, and Co, and 0.0050% or less of each of Bi, Pb, Zn, and O. It's not something I refuse to do.
Note that the above O refers to total oxygen including O as an oxide.

Next, a method for manufacturing a square steel pipe according to the present invention will be described.
In the method for manufacturing a square steel pipe of the present invention, a steel plate is formed into a cylindrical shape by cold roll forming, both circumferential ends of the cylindrical shape are abutted and electrical resistance welded (pipe making process), and then a square forming stand is used to form the steel plate into a cylindrical shape. A square steel pipe material whose flat plate part has a flatness of -10.0 mm or more and -4.0 mm or less, and a corner radius of curvature is 1.0 times or more and 2.5 times or less than the wall thickness of the flat plate part ( After that, an internal pressure p (MPa) that satisfies the following equation (1) is applied to the inner surface of the square steel pipe material, and the flatness of the flat part of the square steel pipe material is adjusted to -3.0 mm or more3. 0 mm or less, and the radius of curvature of the corners is 2.0 times or more and 4.0 times or less than the wall thickness of the flat plate portion (internal pressure loading process).
N(MPa)<p≦N×1.5(MPa)...(1)
However, N = (wall thickness of square steel pipe material (mm) / side length of square steel pipe material (mm)) x yield strength in the pipe circumferential direction of the flat plate part of square steel pipe material (MPa)
In addition, the said cylindrical shape refers to a pipe circumferential section having a "C" shape.

In FIG. 1, t ₁ and t ₂ are two flat plate parts 11 (indicated by I in the figure) that are adjacent to each other with a corner 12 in between, including a welded part (ERW welded part) 13 (represented by I in the figure). This is the wall thickness (mm) at the center in the circumferential direction of each tube (indicated as III in the figure). In addition, in FIG. 1, _t3 is the center in the tube circumferential direction (denoted as V in the figure) of the flat plate part 11 (denoted as IV in the figure) facing the flat plate part 11 including the welded part (ERW welded part) 13. ) is the wall thickness (mm). The wall thickness of the square steel pipe material is determined by averaging t ₁ (mm), t ₂ (mm), and t ₃ (mm).

When the square steel pipe material is placed with the welded part (ERW welded part) 13 facing up, the side lengths of the square steel pipe material are as shown in Fig. 1: H ₁ : Vertical side length (mm), H ₂ : Horizontal side length (mm).

As shown in FIG. 2, the flatness of the flat plate portion in the present invention is defined as the amount of bulge or dent with respect to a straight line passing through two points at both circumferential ends of the outer surface of the flat plate portion. However, the amount of bulge is a positive value, the amount of dent is a negative value, and if there is no bulge or dent, the value is 0. Further, the circumferential ends are the boundary points between the contact portion and the non-contact portion with the roll of the square forming stand, and are angular points where the radius of curvature changes on the outer circumferential surface of the tube.

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

The radius of curvature is measured when the connection points between the extension lines L1 and L2 and the flat plate part 11 and the corner part 12 (points A and A' shown in FIG. 1) and the outer surface of the corner part 12 are circular arcs, and the center is on the straight line L. In the fan shape with a central angle of 90° that exists in the area, the test 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 12. A method for measuring the radius of curvature includes, for example, a method of measuring the radius of curvature using a radial gauge that closely matches the arc in the range of the central angle of 65 degrees.

Further, the internal pressure can be applied by, for example, sealing the end of the tube with a rubber packing and applying water pressure to the inside of the tube. Furthermore, in order to stabilize the shape, a mold with a predetermined shape can be used as the outer frame if necessary.

A typical roll-formed square steel pipe is manufactured by forming a cylindrical shape into a square shape with a flat plate part using a square-forming stand.
In contrast, the square steel pipe of the present invention is formed using a square forming stand so that the flat plate portion is once recessed toward the inner surface of the tube. Specifically, as shown in FIG. 3, for example, an ERW steel pipe 7 obtained by ERW welding is formed by a group of square forming rolls 9 after a sizing roll 8 so that the flat plate part is recessed toward the inner surface of the tube. Then, a square steel pipe material 10' is obtained. Then, the square steel pipe of the present invention is obtained by applying internal pressure to the inner surface of the tube to inflate the flat plate portion toward the outer surface of the tube, making the flat plate portion flat, and increasing the radius of curvature of the corner portion.

Note that it is preferable that the final stand in the corner forming stand has four rolls, that a flat plate part is formed in the contact area between the roll and the steel pipe, and a corner part is formed in the non-contact area.

In order to form the flat plate part of the square steel pipe material so that it is concave toward the inner surface of the tube, it is necessary to use rolls whose hole-shaped curvature radius is smaller than conventional ones and to adjust the roll gap.
Therefore, the roll hole shape of the final stand in the square forming stand is made to have the same shape as the outer circumferential cross-sectional shape (for example, approximately square shape) of the desired square steel pipe material, and the roll gap is adjusted as necessary. The radius of curvature (mm) of the roll hole shape is preferably H ² /100 or more and H ² /30 or less, where the target side length of the square steel pipe material is H (mm).
In addition, the outer peripheral cross-sectional shape of the desired square steel pipe material means that the flatness of the flat plate part is -10.0 mm or more and -4.0 mm or less, and the radius of curvature of the corner part is 1.0 times or more the wall thickness of the flat plate part. An example is 2.5 times or less.

By reducing the roll gap of the final stand in the square forming stand, it is possible to reduce the radius of curvature of the corner of the square steel pipe material, and it is also possible to reduce the flatness of the flat plate portion (increase the amount of dent). . On the other hand, if the roll gap of the final stand in such a square forming stand is increased, the radius of curvature of the corner of the square steel pipe material can be increased, and the flatness of the flat plate portion can be increased (reduced amount of dent). Can be done.
Therefore, by adjusting the roll gap of the final stand in the square forming stand, the radius of curvature of the corner of the square steel pipe material can be set to a desired value. Further, by adjusting the radius of curvature of the roll hole shape and the roll gap, the flatness of the square steel pipe material can be set to a desired value.

Since the present invention repeatedly bends and deforms the flat plate portion of a square steel pipe in the circumferential direction, the yield strength of the flat plate portion in the circumferential direction can be increased compared to a normal square steel pipe.

Furthermore, in the present invention, since the corners of the square steel pipe are also repeatedly bent and deformed in the circumferential direction, work hardening of the corners in the circumferential direction can be appropriately increased compared to ordinary square steel pipes. That is, the circumferential yield strength of the corners can be increased.

Flatness of the flat part of the square steel pipe material by the square forming stand: -10.0 mm or more and -4.0 mm or less If the flatness of the flat part of the square steel pipe material by the square forming stand is less than -10.0 mm, when internal pressure is applied The amount of deformation of the flat plate portion increases, the amount of work hardening of the flat plate portion increases, and the circumferential yield strength of the flat plate portion becomes too high, resulting in a decrease in buckling resistance in 0 degree bending. The flatness is preferably -9.5 mm or more, more preferably -9.0 mm or more.
On the other hand, if the flatness of the flat plate part of the square steel pipe material by the square forming stand is larger than -4.0 mm, the amount of deformation of the flat plate part when internal pressure is applied is small, the amount of work hardening of the flat plate part is small, and the flat plate part Since the yield strength in the circumferential direction of the part becomes too low, the buckling resistance in 0 degree bending decreases. Furthermore, since the absolute value of the residual stress in the circumferential direction on the outer surface of the flat plate portion becomes too large, the buckling resistance in 0 degree bending deteriorates. The flatness is preferably −4.5 mm or less, more preferably −5.0 mm or less.

The radius of curvature of the corner of the square steel pipe material by the square forming stand: 1.0 times or more and 2.5 times or less the wall thickness of the flat plate part The radius of curvature of the corner part of the square steel pipe material by the square forming stand If it is less than 1.0 times, the amount of deformation of the corner portion when internal pressure is applied will increase, the amount of work hardening of the corner portion will increase, and the yield strength of the corner portion in the circumferential direction will become too high. The buckling resistance is reduced. The radius of curvature is preferably 1.1 times or more the thickness of the flat plate portion, more preferably 1.2 times or more the thickness of the flat plate portion.
On the other hand, if the radius of curvature of the corner of the square steel pipe material formed by the square forming stand is more than 2.5 times the wall thickness of the flat plate part, the amount of deformation of the corner when internal pressure is applied becomes small, resulting in work hardening of the corner. Since the amount becomes small and the yield strength in the circumferential direction of the corner becomes too low, the buckling resistance in 45-degree bending decreases. The radius of curvature is preferably 2.4 times or less the thickness of the flat plate portion, more preferably 2.3 times or less the thickness of the flat plate portion.

The flatness of the flat plate part of the square steel pipe after applying internal pressure to the inner surface of the tube: -3.0 mm or more and 3.0 mm or less The flatness of the flat plate part of the square steel pipe after applying internal pressure to the inner surface of the tube is -3.0 mm, respectively If it is smaller than , the amount of deformation of the flat plate part under internal pressure load will be small, the amount of work hardening of the flat plate part will be small, and the yield strength of the flat plate part in the circumferential direction will be too low, so the buckling resistance in 0 degree bending will be reduced. Performance decreases. Further, since the outer peripheral cross section of the flat plate portion has a large curvature, it is difficult to weld the flat plate portion to the diaphragm or the beam. Furthermore, the absolute value of the residual stress in the circumferential direction on the outer surface of the flat plate portion increases, and the buckling resistance in 0 degree bending may decrease. The flatness is preferably -2.9 mm or more, more preferably -2.8 mm or more.
On the other hand, if the flatness of the flat plate part of the square steel pipe after applying internal pressure to the inner surface of the tube is larger than 3.0 mm, the amount of deformation of the flat plate part when the internal pressure is applied increases, and the amount of work hardening of the flat plate part increases. As the yield strength in the circumferential direction of the flat plate portion becomes too high, the buckling resistance in 0 degree bending deteriorates. Further, since the outer peripheral cross section of the flat plate portion has a large curvature, it is difficult to weld the flat plate portion to the diaphragm or the beam. The flatness is preferably 2.7 mm or less, more preferably 2.5 mm or less.

Radius of curvature of the corner of a square steel pipe after applying internal pressure to the inner surface of the pipe: 2.0 times or more and less than 4.0 times the wall thickness of the flat plate Curvature of the corner of a square steel pipe after applying internal pressure to the inner surface of the pipe If the radius is less than 2.0 times the wall thickness of the flat plate part, the amount of deformation of the corner part under internal pressure load will be small, the amount of work hardening of the corner part will be small, and the circumferential yield strength of the corner part will be If it becomes too low, the buckling resistance in 45 degree bending will deteriorate. The radius of curvature is preferably at least 2.1 times the thickness of the flat plate portion, more preferably at least 2.2 times the thickness of the flat plate portion.
On the other hand, if the radius of curvature of the corner of a square steel pipe after applying internal pressure to the inner surface of the pipe is more than 4.0 times the wall thickness of the flat plate part, the amount of deformation of the corner when internal pressure is applied increases, The amount of work hardening at the corners becomes large, and the yield strength in the circumferential direction of the corners becomes too high, resulting in a decrease in buckling resistance in 45-degree bending. The radius of curvature is preferably 3.9 times or less the thickness of the flat plate portion, more preferably 3.8 times or less the thickness of the flat plate portion.
Note that the wall thickness of the flat plate portion of the square steel pipe after applying internal pressure to the inner surface of the tube is substantially unchanged from the wall thickness of the flat plate portion of the square steel pipe material before applying internal pressure to the inner surface of the tube.

If the internal pressure p (MPa) is equal to or less than N in equation (1), the tensile stress generated in the circumferential direction of the pipe becomes lower than the yield strength of the flat plate portion in the circumferential direction, and the amount of deformation of the flat plate portion becomes small. As a result, the amount of work hardening of the flat plate portion becomes small, and the circumferential yield strength of the flat plate portion becomes too low, resulting in a decrease in buckling resistance in 0 degree bending. Further, the absolute value of the residual stress in the circumferential direction on the outer surface of the flat plate portion may increase, and the buckling resistance in 0 degree bending may deteriorate. Further, when the amount of deformation of the corner portion becomes smaller and the amount of work hardening of the corner portion becomes smaller, the yield strength of the corner portion in the circumferential direction becomes too low, resulting in a decrease in buckling resistance in 45-degree bending.
On the other hand, if the internal pressure p (MPa) exceeds N×1.5 in the above formula (1), the tensile stress generated in the circumferential direction of the pipe becomes too high, and the amount of deformation of the flat plate portion becomes large. As a result, the amount of work hardening of the flat plate portion increases, and the circumferential yield strength of the flat plate portion becomes too high, resulting in a decrease in buckling resistance in 0 degree bending. Further, the amount of deformation of the corner portion increases, the amount of work hardening of the corner portion increases, and the yield strength of the corner portion in the circumferential direction becomes too high, resulting in a decrease in buckling resistance in 45-degree bending.

The square steel pipe of the present invention preferably has a wall thickness of 6 mm or more in order to ensure sufficient strength and buckling resistance as a building structure. Further, from the viewpoint of suppressing the amount of deformation of the outer surface of the tube from becoming too large during bending deformation when internal pressure is applied, it is preferable that the wall thickness is 40 mm or less.

The building structure of the present invention includes the above-described square steel pipe 10 of the present invention as a column material.
FIG. 4 is a schematic diagram showing an example of the architectural structure 100 of the present invention.
In the architectural structure 100 of the present invention, a through diaphragm 17 and a square steel pipe 10 are welded together, and the square steel pipe 10 is used as a column material. In addition, as shown in FIG. 4, the building structure 100 is formed of a large beam 18, a small beam 19, and a stud 20, and may also be formed using known members.
Here, the square steel pipe 10 has excellent buckling resistance, as described above. Therefore, the building structure 100 of the present invention using this square steel pipe 10 as a column material exhibits excellent seismic performance.

Regarding the various conditions of the manufacturing method for square steel pipes that are not mentioned above, conventional methods can be used. In addition, as for the configurations of the square steel pipe and the building structure that are not described above, known configurations can be used.

Hereinafter, the present invention will be further described in detail based on Examples. Note that the present invention is not limited to the following examples.
Molten steel having the composition shown in Table 1 was melted to obtain a slab. The obtained slab was hot rolled to obtain a steel plate.

The steel plate thus obtained was formed into a cylindrical shape by cold roll forming, and both ends of the cylindrical shape in the circumferential direction were butted together and electrical resistance welded. Thereafter, a square steel pipe material having four flat plate portions and four corner portions was formed using a square forming stand. Thereafter, both ends of the square steel pipe material were sealed with rubber packing, the inside of the pipe was filled with water, and an internal pressure p (MPa) was applied to form a square steel pipe.

(Measurement of flatness of flat plate part)
At five arbitrary positions in the tube axis direction of the square steel pipe material and the square steel pipe obtained in this way, the flatness of all points of the flat plate portion, that is, four points, was measured, and the average of the measured values of the total of 20 points was calculated. The values were taken as the flatness of the square steel pipe material and the square steel pipe.

To measure the flatness, first, as shown in Figure 2, the maximum amount of bulge and the maximum amount of concavity with respect to a straight line passing through two points at both circumferential ends of the outer surface of each flat plate are measured, and the maximum amount of bulge at each measurement point is determined. Alternatively, the maximum value F of the absolute value of the maximum amount of depression was determined. If F is the amount of bulge, the value of flatness is F, if F is the amount of dent, the value of flatness is -F, and if there is no bulge or dent, the value of flatness is was set to 0.

(Measurement of corner radius of curvature)
At five arbitrary positions in the tube axis direction of the square steel pipe material and the square steel pipe described above, the radius of curvature of all corners, that is, the outer surface (outside of the corner) at four locations, was measured, and the measured values at a total of 20 locations were determined. The average value of was taken as the radius of curvature of each corner of the square steel pipe material and the square steel pipe.
A radial gauge was used to measure the radius of curvature of the corners. To measure the radius of curvature, pass through the intersection P of the two straight lines L1 and L2 that include the outer surfaces of the flat plate on both sides adjacent to the corner, and then pass through the intersection P of the straight line L making a 45° angle with L1 or L2 and the intersection of the outside of the corner. The radius of curvature at was measured as the radius of curvature on the outside of the corner (Fig. 1).
Specifically, the radius of curvature is measured in a fan shape with a center angle of 90°, which consists of the connection points (A, A') between the flat plate part and the corner part and the outer surface of the corner part, and whose center lies on the above-mentioned L. The radius of curvature was measured using a radial gauge that most coincided with the outer surface of the corner in the range of 65 degrees with the center angle centered around the intersection of the outer surfaces of the corner (FIG. 1).

(Measurement of yield strength)
In addition, tensile test pieces were taken from the rectangular steel pipe material and the rectangular steel pipe, and the yield strength was measured. As a tensile test piece, a JIS No. 5 tensile test piece was taken from the flat plate part of the square steel pipe material 10' so that the tensile direction was parallel to the tube axis direction, as shown in W in FIG. Moreover, as shown in X of FIG. 6, a JIS No. 5 tensile test piece was taken from the flat plate part of the square steel pipe 10 so that the tensile direction was parallel to the pipe circumferential direction. Further, as shown in Y in FIG. 6, a tensile test piece having a size of 1/5 of the JIS No. 5 tensile test piece was taken from the corner of the square steel pipe 10. Note that the test piece taken from the flat plate part was a full-thickness test piece, and the test piece taken from the corner part was ground to a thickness of 1/2 of the wall thickness.
More specifically, as shown in Z in FIG. 7, the tensile test piece at the corner part is tested so that the longitudinal center of the parallel length of the test piece passes through the intersection of the respective extensions of the outer surfaces of the flat plate parts on both sides adjacent to the corner part, and The samples were taken so as to be located on a line making a 45° angle with each of the outer surfaces of the flat plate parts. Note that the bent gripping portion was flattened by press straightening.
Using these test pieces, a tensile test was conducted in accordance with the provisions of JIS Z 2241, and the yield strength in the tube axis direction of the flat plate portion LYS _f , the yield strength in the tube circumferential direction of the flat plate portion CYS _f , and the tube circumference of the corner portion were determined. The yield strength CYS _c in each direction was determined. The number of test pieces was two each, and each measured value was calculated by calculating their average value.

(Measurement of cumulative plastic deformation magnification)
In addition, a 0 degree bending test and a 45 degree bending test were conducted on the square steel pipe. Specifically, as shown in FIGS. 8 and 9, a diaphragm 2 was welded through the square steel pipe 1 at the center in the longitudinal direction to prepare a test specimen. Both ends of the test specimen are pin-supported (rotationally supported) with support members 3, so that movement in the horizontal and vertical directions is fixed, and the specimen is moved in the 0 degree direction (Fig. 8) or the 45 degree direction (Fig. 9) at the position of the arrow. ) was subjected to a repeated bending test, and the cumulative plastic deformation magnification was determined.
Incidentally, the cumulative plastic deformation magnification is the value obtained by dividing the sum of plastic rotation angles until the test specimen undergoes local buckling or rupture and the yield strength suddenly decreases by the reference rotation angle corresponding to the total plastic moment. The larger this value, the better the deformation performance when used as a pillar material (column member), which means the higher the energy absorption capacity during an earthquake.

(Measurement of circumferential residual stress on the outer surface)
Further, the residual stress in the circumferential direction on the outer surface was measured by X-ray diffraction method.
Specifically, residual stress in the tube circumferential direction was measured in the electrolytically polished region of the test piece shown below using the measuring equipment and under the following measurement conditions. Note that the circumferential residual stress was measured in all the flat plate parts, and the average value of the circumferential residual stress obtained in each flat plate part was taken as the circumferential residual stress of the rectangular steel pipe.
・Measuring equipment: Portable X-ray residual stress measuring device manufactured by Pulstec Industries (model μ-X360n)
-Measurement conditions: The X-rays used were Cr-Kα rays, the tube voltage was 30 kV, and the measurements were performed using the cosα method. The measurement lattice plane was bcc-Fe (211), the Poisson's ratio was 0.280, and the elastic constant was 224000 MPa.
- Test piece: A rectangular steel pipe with a tube axial length of 1000 mm was taken, and the outer surface of the tube was electrolytically polished by 100 μm at the center of the flat plate portion in the circumferential direction of the tube.

Various conditions and obtained results are shown in Tables 2 and 3.

In Tables 2 and 3, No. 1 to 6 are examples of the present invention, No. 7 to 14 are comparative examples.
In all of the square steel pipes of the present invention, the yield strength in the tube circumferential direction (CYS _f ) of the flat plate portion is 0.83 to 1.20 times the yield strength (LYS _f ) of the flat plate portion in the tube axis direction. The yield strength of the corner portion in the tube circumferential direction ( _CYSC ) was 0.90 times or more and 1.30 times or less of the yield strength of the flat plate portion in the tube axis direction (LYS _f ).

Comparative example No. 7 is because the internal pressure p (MPa) in the internal pressure loading process exceeded the range of formula (1), so the flatness of the square steel pipe became a value larger than 3.0 mm, and the radius of curvature of the corner part was larger than the wall thickness of the flat plate part. It was more than 4.0 times that of the previous year. Therefore, with respect to the yield strength of the flat plate part in the tube axis direction, both the yield strength of the flat plate part in the tube circumferential direction and the yield strength of the corner part in the tube circumferential direction exceed the range of the present invention, and the cumulative The plastic deformation magnification and the cumulative plastic deformation magnification in 45-degree bending did not reach the respective desired values (cumulative plastic deformation magnification: 28 or more in both 0-degree bending and 45-degree bending).

Comparative example No. In No. 8, the radius of curvature of the square steel pipe material was more than 2.5 times the wall thickness of the flat plate portion. Therefore, the yield strength in the tube circumferential direction of the corner portion with respect to the yield strength in the tube axis direction of the flat plate portion fell below the range of the present invention, and the cumulative plastic deformation magnification in 45 degree bending did not reach the desired value.

Comparative example No. In No. 9, the internal pressure p (MPa) in the internal pressure loading step was below the range of formula (1). Therefore, the flatness of the flat plate part of the square steel pipe becomes a value smaller than -3.0 mm, and the yield strength in the circumferential direction of the flat plate part with respect to the yield strength in the tube axis direction of the flat plate part falls below the range of the present invention. The cumulative plastic deformation magnification in 0 degree bending did not reach the desired value. Further, the residual stress in the circumferential direction on the outer surface was smaller than -200 MPa.

Comparative example No. In No. 10, the internal pressure p (MPa) in the internal pressure loading process exceeded the range of formula (1), so the radius of curvature of the corner of the square steel pipe was more than 4.0 times the wall thickness of the flat plate part. Therefore, the yield strength in the tube circumferential direction of the corner portion with respect to the yield strength in the tube axis direction of the flat plate portion exceeded the range of the present invention, and the cumulative plastic deformation magnification in 45 degree bending did not reach the desired value.

Comparative example No. In No. 11, the internal pressure p (MPa) in the internal pressure loading step was below the range of formula (1). Therefore, with respect to the yield strength of the flat plate part in the tube axis direction, both the yield strength of the flat plate part in the circumferential direction and the yield strength of the corner part in the circumferential direction fall below the range of the present invention, and the cumulative The plastic deformation magnification and the cumulative plastic deformation magnification in 45-degree bending did not reach desired values. Further, the residual stress in the circumferential direction on the outer surface was larger than 150 MPa.

Comparative example No. In No. 12, the flatness of the flat plate part of the square steel pipe material was less than -10.0 mm, so the yield strength in the circumferential direction of the flat plate part was beyond the scope of the present invention compared to the yield strength in the tube axis direction of the flat plate part. Therefore, the cumulative plastic deformation magnification in 0 degree bending did not reach the desired value.

Comparative example No. In No. 13, the flatness of the flat plate portion of the square steel pipe material was greater than -4.0 mm. Therefore, the yield strength in the tube circumferential direction of the flat plate portion relative to the yield strength in the tube axis direction of the flat plate portion fell below the range of the present invention, and the cumulative plastic deformation magnification in 0 degree bending did not reach the desired value. Further, the residual stress in the circumferential direction on the outer surface was smaller than -200 MPa.

Comparative example No. In No. 14, the internal pressure p (MPa) in the internal pressure loading step was below the range of formula (1), and the radius of curvature of the corner of the square steel pipe was smaller than 2.0 times the wall thickness of the flat plate portion. Therefore, the yield strength in the tube circumferential direction of the corner portion with respect to the yield strength in the tube axis direction of the flat plate portion fell below the range of the present invention, and the cumulative plastic deformation magnification in 45 degree bending did not reach the desired value.

1 Square steel pipe 2 Through diaphragm 3 Support material 7 ERW steel pipe 8 Sizing roll 9 Square forming roll group 10 Square steel pipe 10' square steel pipe material 11 Flat plate part 12 Corner part 13 Welded part (ERW welded part)
17 Through diaphragm 18 Large beam 19 Small beam 20 Stud 100 Architectural structure

Claims

A square steel pipe having a plurality of flat plate parts and corner parts alternately in the pipe circumferential direction and having a welded part extending in the pipe axial direction,
The yield strength of the flat plate portion in the tube circumferential direction is 0.83 times or more and 1.20 times or less of the yield strength of the flat plate portion in the tube axis direction,
A square steel pipe, wherein the yield strength of the corner portion in the tube circumferential direction is 0.90 times or more and 1.30 times or less the yield strength of the flat plate portion in the tube axis direction.
The rectangular steel pipe according to claim 1, wherein residual stress in the circumferential direction on the outer surface of the flat plate portion is -200 MPa or more and 150 MPa or less.
However, when the residual stress is a positive value, it represents tensile stress, and when it is a negative value, it represents compressive stress.
In mass%,
C: 0.020-0.350%,
Si: 0.01 to 0.65%,
Mn: 0.30-2.50%,
P: 0.050% or less,
S: 0.0500% or less,
The square steel pipe according to claim 1, having a composition comprising Al: 0.005 to 0.100% and N: 0.0100% or less, with the balance being Fe and inevitable impurities.
In mass%,
C: 0.020-0.350%,
Si: 0.01 to 0.65%,
Mn: 0.30-2.50%,
P: 0.050% or less,
S: 0.0500% or less,
The rectangular steel pipe according to claim 2, having a composition comprising Al: 0.005 to 0.100% and N: 0.0100% or less, with the balance being Fe and inevitable impurities.
The component composition further includes, in mass%,
Ti: 0.100% or less,
Nb: 0.100% or less,
V: 0.100% or less,
Cr: 1.00% or less,
Mo: 1.00% or less,
Cu: 1.00% or less,
Ni: 1.00% or less,
The square steel pipe according to claim 3, containing one or more selected from Ca: 0.0100% or less and B: 0.0100% or less.
The component composition further includes, in mass%,
Ti: 0.100% or less,
Nb: 0.100% or less,
V: 0.100% or less,
Cr: 1.00% or less,
Mo: 1.00% or less,
Cu: 1.00% or less,
Ni: 1.00% or less,
The square steel pipe according to claim 4, containing one or more selected from Ca: 0.0100% or less and B: 0.0100% or less.
A method for manufacturing a square steel pipe according to any one of claims 1 to 6,
A pipe-making process in which a steel plate is formed into a cylindrical shape by cold roll forming, and the circumferential ends of the cylindrical shape are abutted against each other and welded by electric resistance welding;
The flatness of the flat plate part is -10.0 mm or more and -4.0 mm or less by the square forming stand performed after the pipe making process, and the radius of curvature of the corner part is 1.0 times or more and 2.5 times the wall thickness of the flat plate part. A square forming process to make a square steel pipe material that is less than double the size of the square steel pipe;
Subsequently, an internal pressure p (MPa) satisfying the following formula (1) is applied to the inner surface of the square steel pipe material, and the flatness of the flat part of the square steel pipe material is set to -3.0 mm or more and 3.0 mm or less. , an internal pressure loading step to form a square steel pipe in which the radius of curvature of the corner portion is 2.0 times or more and 4.0 times or less the wall thickness of the flat plate portion;
A method of manufacturing a square steel pipe, including:
N(MPa)<p≦N×1.5(MPa)...(1)
However, N = (wall thickness of square steel pipe material (mm) / side length of square steel pipe material (mm)) x yield strength in the pipe circumferential direction of the flat plate part of square steel pipe material (MPa)
An architectural structure comprising the square steel pipe according to any one of claims 1 to 6 as a column material.