TW202113089A - Rectangular steel pipe and method for manufacturing same, and building structure - Google Patents

Abstract

Provided are: a rectangular steel pipe that has small variation in hardness in a circumferential cross-section and excellent ductility and toughness on corner outer surfaces, and has a flat plate section; a method for manufacturing same; and a building structure. A plurality of flat plate sections 11 and a plurality of corner sections 12 are alternately formed in the pipe peripheral direction, a weld section 13 extending in the pipe axis direction is further formed, the pipe peripheral width of a melt solidified section in the weld section 13 is 1.0-1,000 [mu]m inclusive, and the radius of curvature of the corner outer side is greater than 3.0 times and no greater than 4.0 times the average plate width. The average plate thickness t (mm) can be greater than 0.030 times the average side length H (mm).

Description

Square steel pipe, manufacturing method thereof, and building structure

The present invention relates to square steel pipes with excellent toughness, high strength, and low yield ratio used in construction components of large buildings such as middle-rise buildings, factories, warehouses, etc., with a height of more than 20m.

In the past, pillars of buildings have been widely used: four-sided box-shaped columns manufactured by welding four thick steel plates, or presses manufactured by cold bending and forming one or two thick steel plates and then welding them. In order to reduce costs and shorten the construction period for forming square steel pipes, the use of low-cost roll-forming square steel pipes that can be manufactured in a short period of time is increasing in recent years.

Roll-formed square steel pipes are formed by cold roll forming to form a steel strip into a cylindrical open pipe shape. After the butt joints are subjected to resistance welding, the cylinder is held by rollers arranged on the top, bottom, left, and right. It is manufactured by applying drawing in the direction of the tube axis, and then forming it into a square shape. In the above-mentioned resistance welding, the abutted portion is heated and melted, and is crimped and solidified, thereby completing the bonding.

The corners of a square steel pipe formed by roll forming have higher strength, lower ductility and toughness than the flat part due to work hardening during the square forming.

In particular, for roll forming square steel pipes, the greater the ratio (t/H) of the average plate thickness t to the average side length H, the greater the amount of work hardening at the corners. Therefore, in the roll-formed square steel pipe with a larger ratio (t/H), the difference in strength, ductility, and toughness in the circumferential section tends to increase.

For the above reasons, due to the poor strength, ductility and toughness in the circumferential section of the roll-formed square steel pipe, the following problems will arise, namely, the selection of the welding material and the welding method when welding with the diaphragm. , Building structure design becomes complicated. In addition, in order to further improve the seismic performance of building structures using roll-formed square steel pipes as columns, it is desirable to suppress the local decrease in ductility and toughness of the roll-formed square steel pipes, so as to increase the deformability and performance of the columns. The impact resistance is further improved. In particular, when the square steel pipe deforms due to external forces such as earthquakes, large strains are generated on the outer surface of the corner, and the ductility and toughness of the outer surface of the corner must be improved.

Patent Document 1 proposes a square steel pipe characterized in that a steel plate containing vanadium (V) as a chemical component is bent and then welded to form a semi-formed square steel pipe, and the semi-formed square steel pipe is heated to A ₃ After thermoforming near the metamorphic point, it is made by cooling it.

Patent Document 2 proposes a square steel pipe in which a cold formed part is heat treated.

However, since the square steel pipes described in Patent Documents 1 and 2 must have a heating process during or after forming, the cost is very high compared to cold-formed roll-formed square steel pipes. That is, it is desired to determine a technique that does not necessarily require a heating process during or after forming to obtain the desired square steel pipe.

In this regard, Patent Document 3 proposes a square steel pipe, by appropriately controlling the chemical composition of the blank steel plate, the bainite fraction of the metal structure, and the Vickers hardness of the surface layer of the corner, so that the corner The toughness and plastic deformation ability of the part are improved.

In addition, Patent Document 4 proposes a square steel pipe in which the chemical composition of the blank steel sheet, the hard phase of the metal structure, and the average crystal grain size of the ferrous iron are appropriately controlled to improve the toughness of the corners.

However, the square steel pipes described in Patent Documents 3 and 4 still have major problems with poor strength and poor ductility at the flat portion and the corner portion. That is, in these square steel pipes, it is impossible to sufficiently reduce the variation in hardness in the circumferential cross section including the corner portion and the flat plate portion. In addition, the ductility and toughness of the outer surface of the corner portion cannot be sufficiently ensured.

In addition, the roll forming of square steel pipes requires the establishment of a technology to improve the shape characteristics, especially the establishment of a technology to make the flat part extremely flat. Regarding this point, Patent Documents 5 and 6 disclose techniques for improving the shape characteristics by adjusting the manufacturing conditions during roll forming. Specifically, Patent Document 5 discloses a technique for forming a square steel tube. When a square forming roller of 3 or 4 stages is used and the reduction rate of the final stage roller is set to be constant, the steel tube is formed into a square shape. As the wall thickness/outer diameter ratio of the steel pipe increases, the caliber of the roller in the final stage is reduced (from a convex mold to a concave mold). In addition, Patent Document 6 discloses a technique for manufacturing a square tube for structure. When a cylindrical blank tube is roll-formed into a square tube, when the outer diameter of the blank tube is set to D, the wall of the blank tube When the thickness is set to t and the maximum hole height is set to H, the structural square tube is manufactured through the first stage of forming process and the second and subsequent forming processes. In the first stage of forming process, the Q =(DH)/(Dt)×100 defines the set indentation rate Q to be maintained in the range of 12~23% and the blank tube is formed into a rectangular cross-sectional shape; the forming process after the second stage is to be formed into a rectangular shape The blank tube after the cross-sectional shape is formed into the target shape. [Prior Technical Literature] [Patent Literature]

Patent Document 1: Japanese Patent Application Publication No. 2004-330222 Patent Document 2: Japanese Patent Application Laid-Open No. 10-60580 Patent Document 3: Japanese Patent No. 5385760 Patent Document 4: Japanese Patent Application Publication No. 2018-53281 Patent Document 5: Japanese Patent Laid-Open No. 4-224023 Patent Document 6: Japanese Patent No. 3197661

[The problem to be solved by the invention]

However, the techniques described in Patent Documents 5 and 6 are insufficient as a technique for flattening the flat portion of a square steel pipe, reducing the variation in hardness in the circumferential section, and sufficiently ensuring the ductility and toughness of the outer surface of the corner portion. .

The present invention was developed in view of the above-mentioned matters, and its purpose is to provide a square steel pipe with a small deviation in hardness in the circumferential section, excellent ductility and toughness of the outer surface of the corner portion, and a flat flat plate portion, and a manufacturing method thereof And building structures with excellent seismic performance. In the present invention, the small deviation of the hardness in the circumferential section means that the difference between the maximum value and the minimum value of the Vickers hardness in the steel pipe is 80 HV or less. In addition, in the present invention, the excellent ductility of the outer surface of the corner means that the uniform elongation at a position t/4 from the outer surface of the corner is 0.80 of the uniform elongation at a position t/4 from the outer surface of the flat plate. Times more. In addition, in the present invention, the excellent toughness of the outer surface of the corner part means that the energy absorbed by the Charpy test at the corner part at 0°C is 70 J or more. In addition, in the present invention, the flat plate part is flat means: in the circumferential section, on the same side of the outer surface of the flat plate part, the flatness is expressed by the maximum protrusion amount and the maximum depression amount for a straight line passing through the two ends of the circumferential direction. The degree is 2.5mm or less (refer to Figure 10). [Technical means to solve the problem]

The inventors of the present invention conducted painstaking studies to solve the above-mentioned problems, and found that in order to reduce the deviation of the hardness in the circumferential section of the square steel pipe (especially the deviation of the hardness of the flat part and the corner part), it is necessary to sufficiently ensure the outside of the corner. For the ductility and toughness of the surface, it is sufficient to set the radius of curvature outside the corners of the square steel pipe to be greater than 3.0 times the average plate thickness. It has also been found that in order to make the flat plate portion sufficiently flat, the radius of curvature outside the corner portion of the roll-formed square steel pipe may be set to 4.0 times or less the average plate thickness. Moreover, it was found that the perimeter of the resistance welded steel pipe on the entrance side of the square forming stand (stand) is controlled to a proper range relative to the perimeter of the square steel pipe on the exit side of the square forming stand, and the curvature of the outside corner can be manufactured. A square steel pipe with a radius greater than 3.0 to 4.0 times the thickness of the plate can reduce the deviation of the hardness in the circumferential section to the desired level, so that the ductility and toughness of the outer surface of the corner become good, and it can make the flat part It becomes sufficiently flat.

The present invention was developed based on the above-mentioned knowledge, and its gist is structured as follows. [1] A square steel pipe in which a plurality of flat plate portions and a plurality of corner portions are alternately formed in the circumferential direction of the pipe, and a welded portion extending in the direction of the pipe axis is further formed. The melting and solidification portion of the welded portion is in the pipe peripheral direction The width is 1.0μm~1000μm, and the radius of curvature on the outside of the aforementioned corners is greater than 3.0 times to 4.0 times the average plate thickness t. [2] The square steel pipe described in [1] above, wherein the average plate thickness t is greater than 0.030 times the average side length H. [3] The square steel pipe according to [1] or [2] above, wherein the difference between the maximum value and the minimum value of the Vickers hardness in the steel pipe is 80 HV or less. [4] The square steel pipe according to any one of [1] to [3], wherein the average plate thickness t is 20 mm to 40 mm, the yield strength of the flat part is 295 MPa or more, and the tensile strength of the flat part is The strength is 400 MPa or more, the yield ratio of the corner portion is 90% or less, and the energy absorption of the corner portion in the Charpy test at 0° C. is 70 J or more. [5] The square steel pipe according to any one of [1] to [4], wherein the uniform elongation at a position t/4 from the outer surface of the corner portion is t/4 from the outer surface of the flat portion The position of the uniform elongation is more than 0.80 times. [6] A method for manufacturing square steel pipes. Roll forming a steel plate, and then performing resistance welding on the steel plate after the roll forming to become a resistance welding steel pipe. Then, the aforementioned resistance welding steel pipe is passed through a sizing machine. stand) for forming, and then the square steel pipe is manufactured by square forming by a square forming machine to meet the following formula (1), and the former is controlled according to the roll gap of the aforementioned square forming machine. The roll gap of the aforementioned sizing machine base before forming, 0.30×t/H+0.99≦C _IN /C _OUT ＜0.50×t/H+0.99...Equation (1) and in Eq. (1), C _IN : in The circumference of the resistance welded steel pipe at the entrance side of the square forming machine base in the first section (mm), C _OUT : the circumference of the square steel pipe at the exit side of the square forming machine base in the final section (mm), t: square formation The average plate thickness after forming (mm), H: the average side length after square forming (mm), (but in the case of the square forming by a single stage of square forming machine, the first stage The square forming machine base of and the square forming machine base of the aforementioned final stage are the same square forming machine base). [7] The method for manufacturing a square steel pipe according to [6], wherein the average plate thickness t is 20 mm to 40 mm. [8] A building structure using the square steel pipe described in any one of [1] to [5] as a column material.

Here, the radius of curvature may be an average radius of curvature, or may be an arbitrary radius of curvature. However, from the viewpoint of ensuring a more excellent effect, the radius of curvature at any place is preferable.

In addition, the average plate thickness t can be obtained by the following formula (2). t=(t1+t2+t3)/3…Equation (2) In formula (2), t1, t2: For the flat plate part including the welded part (resistance welding part), the plate thickness (mm) of each of the two flat plate parts adjacent to each other via the corners in the circumferential direction of the tube (mm), t3: and the welded part The thickness (mm) in the center of the tube circumferential direction of the flat part facing the flat part (resistance welding part).

In addition, the average side length H can be obtained by the following formula (3). H=(H1+H2)/2…Equation (3) In formula (3), H1: the vertical cross section in the tube axis direction, the side length of a substantially rectangular shape when one side includes an arbitrary flat plate and the corners on both sides (the longitudinal side length in Figure 1 can also be said Yes, in a pair of facing flat portions, the distance from the outer surface of one flat portion to the outer surface of the other flat portion) (mm); H2: includes the distance between the flat portion with side length H1 adjacent to the corner portion The side length of the side of the flat part and the corners on both sides (the horizontal side length in Figure 1) (mm). That is, H is the sum of the side lengths H1 and H2 of the two adjacent flat plate portions in the vertical cross section of the tube axis direction and divided by two, respectively. [Effects of Invention]

According to the present invention, it is possible to provide a square steel pipe having a flat flat plate portion, a method of manufacturing the square steel pipe, and a building structure with reduced variation in hardness in the circumferential section, excellent ductility and toughness of the outer surface of the corner portion, and flat plate portion. Thereby, it is possible to manufacture a cold-rolled square steel pipe with a small difference in strength in the circumferential section and excellent ductility and toughness. In addition, a building structure using the square steel pipe of the present invention as a column material can exhibit more excellent earthquake resistance than a building structure using a conventional cold-formed square steel pipe.

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

＜Square steel pipe＞ FIG. 1 is a schematic diagram showing a cross-section perpendicular to the tube axis direction of the square steel tube 10 of the present invention. In the square steel pipe 10 of the present invention, a plurality of flat plate portions 11 and a plurality of corner portions 12 are alternately formed in the circumferential direction of the pipe. The square steel pipe 10, as shown in Figure 1, is in the order of the corner 12, the flat portion 11, the corner 12, the flat portion 11, the corner 12, the flat portion 11, the corner 12, and the flat portion 11 in the circumferential direction of the pipe Formed with 4 flat plate parts and 4 corner parts, it becomes a rectangular steel pipe with a rectangular (substantially rectangular) or square (substantially square) cross section in the tube axis direction. In addition, the square steel pipe 10 of the present invention may be a roll-formed square steel pipe made from an electric resistance welding steel pipe, and further has a welding portion (resistance welding portion) 13 formed on the flat plate portion 11 and extending in the pipe axis direction.

In the square steel pipe 10 of the present invention, the radius of curvature outside the corners is greater than 3.0 times to 4.0 times the average plate thickness t.

As shown in Figure 1, the radius of curvature on the outside of the corner part means: passing through the intersection point P of two straight lines L1 and L2 that respectively include the outer surfaces of the flat plate parts 11 on both sides adjacent to the corner part 12 and and L1 or L2 The included angle of is the radius of curvature of the intersection of the straight line L of 45° and the outside of the corner. In addition, the radius of curvature of the present invention may be an average radius of curvature, or may be an arbitrary radius of curvature. However, from the viewpoint of ensuring a more excellent effect, the radius of curvature at any place is preferable. The above-mentioned radius of curvature is measured in a sector formed by the connecting points (A, A') of the flat plate portion 11 and the corner portion 12 and the outer surface of the corner portion, and the center is located at the center angle 90° on the above L. The intersection of L and the outer surface of the corner is the center angle of 65°. In addition, the method of measuring the radius of curvature can be, for example, a method of measuring the radius of curvature using a radius gauge that exactly matches the outer surface of the corner within the range of the above-mentioned center angle of 65°, but it is not limited to these.

If the radius of curvature on the outside of the corner portion is 3.0 times or less the average plate thickness t, the variation of the hardness in the circumferential section will increase, and the desired square steel pipe will not be obtained. That is, the corners are significantly work hardened, and their strength is increased compared to the flat portions, and ductility and toughness are reduced. On the other hand, if the radius of curvature outside the corner portion is greater than 4.0 times the average plate thickness t, the flatness of the flat plate portion becomes insufficient, and the desired square steel pipe cannot be obtained. In addition, the peripheral cross-sectional area becomes small, and sufficient member strength cannot be obtained. Therefore, in the present invention, the radius of curvature outside the corner portion is set to be greater than 3.0 times to 4.0 times the average plate thickness t. Preferably, the radius of curvature outside the corner portion is 3.1 to 3.9 times the average plate thickness t, and more preferably 3.2 to 3.8 times.

In the present invention, the square steel pipe 10 is made from electric resistance welding steel pipe. Therefore, the welding part 13 is a resistance welding part. The width of the melting and solidification part of the resistance welding part in the circumferential direction of the tube is 1.0μm~1000μm in the entire tube thickness.

In addition, in the present invention, the ratio of the uniform elongation E1 at a position t/4 from the outer surface of the flat portion to the uniform elongation E2 at a position t/4 from the outer surface of the corner portion (E2/E1) is preferably 0.80 times the above. E2/E1 is more preferably 0.83 times or more, particularly preferably 0.85 times or more. In addition, E2/E1 is preferably 1.00 times or less.

In addition, in the present invention, the relationship between the average plate thickness t (mm) of the square steel pipe 10 and the average side length H can be set to t/H greater than 0.030. In a square steel pipe, the greater the ratio (t/H) of the average plate thickness t to the average side length H, the greater the amount of work hardening at the corners. Therefore, a square steel pipe with a larger ratio (t/H) has a tendency that the strength difference, ductility, and toughness difference in the circumferential section become larger. In the present invention, because the radius of curvature outside the corner portion is set to be greater than 3.0 times to 4.0 times the average plate thickness, even if t/H is greater than 0.030, the strength difference in the circumferential section can be reduced, and excellent ductility can be obtained. And toughness.

Here, the average plate thickness t can be obtained by the following formula (2). t=(t1+t2+t3)/3…Equation (2) In formula (2), t1, t2: For the flat plate portion 11 including the welded portion (resistance welded portion) 13, the plate thickness (mm) of each of the two flat plate portions 11 adjacent to each other via the corner portion 12 in the center of the pipe circumferential direction (mm), t3 : Plate thickness (mm) of the center of the tube circumferential direction of the flat plate portion 11 facing the flat plate portion 11 including the welded portion (resistance welded portion) 13.

In addition, the average side length H can be obtained by the following formula (3). H=(H1+H2)/2…Equation (3) In formula (3), H1: the length of the longitudinal side in Figure 1 (mm), H2: the length of the lateral side in Figure 1 (mm), that is, H is in the vertical section in the direction of the tube axis, which will be separated by the corner 12 The side lengths H1 and H2 of each of the two adjacent flat plate portions 11 including the corner portions 12 on both sides are added and divided by two.

In Fig. 1, H1>H2, that is, the side length H2 of the vertical cross section in the tube axis direction including the flat plate portion 11 where the welded portion 13 is formed is shorter than the side length H1 of the flat plate portion 11 adjacent to the flat plate portion 11, but The present invention is not limited to this example, and may be H1=H2, H1<H2.

In the square steel pipe 10 of the present invention, the difference between the maximum value and the minimum value of the Vickers hardness in the steel pipe is preferably 80 HV or less. Specifically, it is preferable to measure the plate portion 11, the corner portion 12, and the welding portion (resistance welding portion) 13, respectively, at a position 1 mm from the inner surface of the tube in the thickness direction, a position 1 mm from the outer surface of the tube in the thickness direction, and the plate The difference between the maximum and minimum Vickers hardness at the center of the thickness is 80HV or less. The above-mentioned Vickers hardness test can be implemented by setting the test force to 98N (10kgf) in accordance with JIS Z 2244.

In addition, the square steel pipe 10 of the present invention preferably has an average plate thickness t of 20 mm to 40 mm, a yield strength of the flat portion 11 of 295 MPa or more, a tensile strength of the flat portion 11 of 400 MPa or more, and a yield ratio of the corner portion 12 of 90. % Or less, the energy absorbed by the Charpy test at the corner 12 at 0°C is 70J or more. The above-mentioned yield strength, tensile strength, yield ratio, and uniform elongation (flat part: E1, corner part: E2) are obtained by conducting a tensile test in accordance with JIS Z 2241. The energy absorption of the Charpy test is obtained by using a V-notch standard test piece in accordance with the provisions of JIS Z 2242 and performing a Charpy impact test at a test temperature of 0°C.

In order to ensure the mechanical properties and weldability of the component composition of the square steel pipe 10 of the present invention, the Ceq defined by the formula (4) is preferably 0.15% to 0.50%. In addition, it is preferable that Pcm defined by formula (5) is 0.30% or less. However, the composition of each element in formula (4) and formula (5) is mass %. In the following, unless otherwise specified, the "%" of the component composition is the "mass%".

Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14…Equation (4) Here, in the formula (4), C, Mn, Si, Ni, Cr, Mo, and V represent the content (mass %) of each element. (Among them, the elements that are not included are 0 (zero)%.) Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+ V/10+5B…Equation (5) Here, in the formula (5), C, Si, Mn, Cu, Ni, Cr, Mo, V, and B represent the content (mass %) of each element. (Among them, the elements that are not included are 0 (zero)%.) Ceq in the formula (4) is a carbon equivalent and serves as an index of the hardness of the welded part (resistance welded part) 13 and the heat-affected zone. If the Ceq is less than 0.15%, it may not be possible to obtain the strength required for the pillars of the building structure. In addition, if Ceq is greater than 0.50%, the welded portion 13 and the heat-affected zone may be excessively hardened, which may increase the variation in the circumferential section strength. Therefore, in the present invention, Ceq is preferably 0.15% to 0.50%. In addition, Ceq is more preferably 0.20% or more, particularly preferably 0.25% or more. In addition, Ceq is more preferably 0.45% or less, particularly preferably 0.40% or less.

The Pcm in formula (5) represents the sensitivity to weld cracks. If Pcm is greater than 0.30%, low-temperature cracks are likely to occur in the welded part 13 and the heat-affected zone. Therefore, in the present invention, Pcm is preferably 0.30% or less. In addition, Pcm is more preferably 0.10% or more, particularly preferably 0.15% or more. In addition, Pcm is more preferably 0.28% or less, particularly preferably 0.25% or less.

In addition, although not particularly limited, in order to ensure mechanical properties and weldability, the square steel pipe 10 of the present invention may have the following component composition, in terms of mass %, containing C: 0.04 to 0.45%, Si: 0.02 to 0.50%, Mn: 0.40~2.5%, P: 0.10% or less, S: 0.050% or less, Al: 0.005~0.10%, N: 0.010% or less, Ti: 0.005~0.15%, and the remainder is composed of Fe and unavoidable impurities . In addition, the square steel pipe 10 of the present invention may further contain: in mass %, selected from Nb: 0.005~0.15%, V: 0.005~0.15%, Cr: 0.02~1.0%, Mo: 0.02~1.0%, Cu: 0.02~1.0%, Ni: 0.02~1.0% of one or more than two.

＜Method of manufacturing square steel pipe＞ Next, the manufacturing method of the square steel pipe 10 of this invention is demonstrated.

The method of manufacturing the square steel pipe 10 of the present invention is to roll-form a steel plate, then perform resistance welding on the rolled steel plate to become a resistance-welded steel pipe, and then shape the resistance-welded steel pipe by a sizing machine base. The square steel tube is manufactured by square forming by the square forming machine base to satisfy the following formula (1). According to the roll gap of the square forming machine base, the roll gap of the sizing machine base before the square forming is controlled . 0.30×t/H+0.99≦C _IN /C _OUT ＜0.50×t/H+0.99…Equation (1) Also in Eq. (1), C _IN : Resistance at the entrance of the square forming machine base in the first stage The circumference of the welded steel pipe (mm), C _OUT : the circumference of the square steel pipe on the exit side of the square forming machine base of the final stage (mm), t: the average plate thickness after square forming (mm), H: square forming The average side length after shape (mm).

The average plate thickness t is obtained by the following formula (2). t=(t1+t2+t3)/3…Equation (2) In formula (2), t1, t2: For the flat plate portion 11 including the welded portion (resistance welded portion) 13, the plate thickness (mm) of each of the two flat plate portions 11 adjacent to each other via the corner portion 12 in the center of the pipe circumferential direction (mm), t3 : Plate thickness (mm) of the center of the tube circumferential direction of the flat plate portion 11 facing the flat plate portion 11 including the welded portion (resistance welded portion) 13.

In addition, the average side length H can be obtained by the following formula (3). H=(H1+H2)/2…Equation (3) In formula (3), H1: the length of the longitudinal side in Figure 1 (mm), H2: the length of the lateral side in Figure 1 (mm), that is, H is in the vertical section in the direction of the tube axis, which will be separated by the corner 12 The side lengths H1 and H2 of each of the two adjacent flat plate portions 11 including the corner portions 12 on both sides are added and divided by two.

However, in the case of performing the above-mentioned square forming with a single-stage square forming machine base, the first-stage square forming machine base and the final-stage square forming machine base are the same square forming machine base.

Here, referring to FIG. 2, a method of manufacturing the electric resistance welding steel pipe used in order to obtain the square steel pipe 10 of the present invention will be described. Fig. 2 is a schematic diagram showing an example of the manufacturing equipment of the resistance welding steel pipe.

As shown in Figure 2, the steel sheet 1 (hereinafter also referred to as "steel strip 1") wound into a coil (coil) having the aforementioned composition is discharged and corrected by a leveler 2, and a plurality of After the cage roll group 3 constituted by the rollers is formed into an open tube by intermediate molding, the fin pass roll group 4 constituted by a plurality of rollers is subjected to finishing forming. The above-mentioned open tube can be formed into a cylindrical shape by cold roll forming.

After finishing forming, a pair of butting parts facing each other in the circumferential direction of the steel strip 1 are resistance-welded by a welding machine 6 while being crimped by a squeeze roll 5 to form a resistance-welded steel pipe. 7. In the present invention, the manufacturing equipment of the resistance welding steel pipe 7 is not limited to the pipe manufacturing process as shown in FIG. 2. In addition, in the above-mentioned resistance welding, the abutting portion is heated and melted, and is crimped and solidified, thereby completing the bonding. Thereby, one welding part (resistance welding part) 13 is extended in the tube axis direction (refer FIG. 1 again).

The upset amount of the squeeze roller 5 is preferably 20%-100% of the plate thickness of the resistance welding steel pipe 7. When the reduction is less than 20% of the plate thickness, the discharge of molten steel becomes insufficient, and the toughness of the welded part deteriorates. In addition, when the reduction is greater than 100% of the plate thickness, the load of the squeeze roll increases, and the work hardening amount of the welded portion (resistance welded portion) 13 becomes large, resulting in excessively high hardness.

The sizing process after resistance welding will be described later with reference to Figure 4. In order to satisfy the better roundness and residual stress in the pipe axis direction, the circumference of the steel pipe can be reduced by a total of 0.30% or more. Way to reduce the diameter of the steel pipe. However, when the diameter of the steel pipe is reduced in a proportion of more than 2.0% in total, the amount of bending in the pipe axis direction when passing through the rollers increases, and the residual stress in the pipe axis direction after the diameter reduction increases. Therefore, it is preferable to reduce the diameter of the steel pipe by a ratio of 0.30% to 2.0%.

In the sizing process, in order to minimize the amount of bending in the tube axis direction when passing through the rollers and suppress the generation of residual stress in the tube axis direction, it is preferable to perform multi-stage diameter reduction based on a plurality of bases. The diameter reduction is preferably performed so that the circumference of the steel pipe is reduced by a ratio of 1.0% or less.

Whether the square steel pipe 10 is made from the resistance welding steel pipe 7 can be judged by cutting the square steel pipe 10 perpendicular to the pipe axis direction, and grinding the cut surface including the welding part (resistance welding part) 13 After corrosion, use an optical microscope to observe. As long as the width of the melting and solidification part of the welding part (resistance welding part) 13 in the circumferential direction of the tube is 1.0 μm to 1000 μm over the entire tube thickness, it is produced from the resistance welding steel pipe 7.

Here, the corrosive liquid may be selected according to the steel composition and the type of steel pipe. FIG. 3 is a schematic diagram of the melting and solidification part of the welding part 13. As shown in FIG. 3, the above-mentioned cross-section after corrosion is visually confirmed as a region 16 having a different microstructure and contrast from the base material portion 14 and the heat-affected portion 15 in FIG. 3. For example, in the melting and solidification portion 16 of the resistance welding steel pipe of carbon steel and low-alloy steel, the above-mentioned cross section after corrosion with nital is determined as a white area observed with an optical microscope. In addition, the melting and solidification portion 16 of the UOE steel pipe of carbon steel and low-alloy steel, the above-mentioned cross section after corrosion with nitrate etching agent, is determined as a region containing a cell-like or dendritic solidification structure observed with an optical microscope .

Next, referring to FIG. 4, a method of manufacturing the square steel pipe 10 using the obtained electric resistance welding steel pipe 7 will be described. Fig. 4 is a schematic diagram showing the forming process of the square steel pipe 10 of the present invention.

As shown in Fig. 4, after the electric resistance welding steel pipe 7 is maintained in a cylindrical shape and the diameter is reduced by the sizing roller group (sizing frame) 8 composed of a plurality of rollers arranged on the top, bottom, left, and right, a plurality of A group of square forming rolls (square forming stand) 9 formed by the rollers are sequentially formed into shapes like R1, R2, and R3 to form a square steel pipe 10. The rollers of the square forming stand 9 are perforated rolls with perforated curvature. As they become the rear stage, the perforated radius of curvature increases, thereby forming the flat portion 11 and the corners 12 of the square steel pipe 10. In addition, the number of stands of the sizing roll group 8 and the square forming roll group 9 is not particularly limited. Moreover, it is preferable that the curvature of the pass of the sizing roll group 8 or the square forming roll group 9 is 1 condition.

In the present invention, the circumference of the resistance welded steel pipe 7 before square forming (the circumference (mm) of the resistance welded steel pipe 7 on the entrance side of the square forming machine base of the first stage, hereinafter referred to as "C _IN ") and The ratio (C _IN /C _OUT ) of the circumference of the square steel tube 10 immediately after square forming (the circumference of the steel pipe on the exit side of the square forming machine base in the final stage (mm), hereinafter referred to as "C _OUT "), And the ratio of the average plate thickness t after square forming to the average side length H after square forming (t/H) satisfies formula (1).

0.30×t/H+0.99≦C _IN /C _OUT ＜0.50×t/H+0.99...Equation (1) When a cylindrical blank tube, that is, a resistance welding steel pipe 7 is formed into a square steel pipe 10, as described above Generally, by passing a steel pipe through the square forming roll group 9, the forming is gradually performed from a cylindrical shape to a square shape. In such a square shape, straightening of the straight portion (flat portion 11 (refer to FIG. 1 again)) of the side, bending of the corner portion 12, and extensional deformation in the circumferential direction occur.

Especially in the periphery of the corner 12, the square forming is completed in such a way that the rollers do not substantially touch each other. That is, in the square shape, the corners 12 are formed to protrude outward through free deformation. At this time, the higher the rigidity of the corner portion 12 and the smaller the amount of extension in the circumferential direction, the smaller the bending deformation amount of the corner portion 12 and the larger the radius of curvature outside the corner portion. On the other hand, the lower the rigidity of the corner portion 12 and the greater the amount of extension in the circumferential direction, the greater the bending deformation amount of the corner portion 12 and the smaller the radius of curvature outside the corner portion.

The rigidity of the corner 12 with respect to bending deformation increases as the ratio (t/H) of the average plate thickness t to the average side length H increases.

The circumferential extension of square forming is _{calculated from the circumference ratio (C IN} /C _OUT ). The larger the circumferential ratio, the greater the circumferential extension.

Therefore, in order to obtain a square steel pipe with the same radius of curvature on the outside of the corner, because the larger the ratio of the average plate thickness t to the average side length H (t/H), the greater the circumferential extension is required, and the circumference must be compared to ( C _IN /C _OUT ) increase.

When (C _IN /C _OUT ) is smaller than the value on the left side of the formula (1), the processing becomes insufficient, and a flat flat portion cannot be obtained. In addition, because the amount of extension in the circumferential direction is small, the radius of curvature on the outside of the corner portion becomes more than 4.0 times the average plate thickness t, the circumferential cross-sectional area becomes small, and sufficient member strength cannot be obtained.

When (C _IN /C _OUT ) is greater than the value on the right side of the formula (1), the circumferential extension is large, so the radius of curvature outside the corner is 3.0 times the average plate thickness t or less, and the corner is significantly work hardened. Compared with the flat part, the strength becomes higher and the ductility and toughness are lowered.

(C _IN /C _OUT ) is preferably 0.33×t/H+0.99 or more, more preferably 0.35×t/H+0.99 or more. In addition, (C _IN /C _OUT ) is preferably 0.47×t/H+0.99 or less, more preferably 0.45×t/H+0.99 or less.

C _IN is the circumference (length of the outer circumference in the pipe circumference direction) (mm) of the resistance welding steel pipe 7 on the entrance side of the square forming machine base of the first stage. When the pipe making direction is set to the positive direction of the X axis, the X coordinate of any rotation axis of the sizing machine base before square forming is set to Xa(m), and the square forming machine base of the first stage is set to Xa(m). When the X coordinate of any one of the rotation axes is set to Xb(m), the outer circumference of the tube at the plane X=(Xa+Xb)/2(m) perpendicular to the X axis is measured with a tape measure Obtain C _IN (refer to Figure 4). In addition, C _OUT is the circumference (length of the outer circumference in the pipe circumference direction) (mm) of the square steel pipe 10 on the exit side of the square forming machine base in the final stage. When the X coordinate of any rotation axis of the square forming machine base of the final stage of the roller group is set to Xc(m), the circumference of the tube at the plane X=Xc+1(m) perpendicular to the X axis will be The outer circumference of the section is measured with a tape measure to obtain C _OUT (see Fig. 4).

The control of C _IN and C _OUT is carried out by controlling the gap between the recesses of the perforated roller. It is better to adjust the roll gap of the sizing stand before the square is formed so that the maximum roll gap between the recesses of the sizing stand before the square is formed (hereinafter also referred to as the "roll gap of the sizing stand" ``) and the maximum roll gap between the recesses of the square forming machine base (hereinafter also referred to as "the roll gap of the square forming machine base") △g divided by the value of (t/H) G (=△g/( t/H)) becomes 70~180.

If G is less than 70, (C _IN /C _OUT ) of formula (1) becomes smaller than the value on the left. If G is greater than 180, (C _IN /C _OUT ) of formula (1) becomes more than the value on the right. Also, when there are multiple sections of the sizing stand, the roll gap of the sizing stand before the square forming can be set to be the same as the roll gaps of the other sizing stand. In addition, when the square forming machine base has a plurality of stages, the roll gap of the square forming machine base is preferably the roll gap of the square forming machine base of the first stage. In addition, the roll gaps of the first stage and other square forming machines are all the same.

＜Building structure＞ The building structure of the present invention uses the aforementioned square steel pipe 10 of the present invention as a column material. FIG. 5 is a schematic diagram showing an example of the building structure 100 of the present invention. In the building structure 100 of the present invention, the partition plate 17 and the square steel pipe 10 are welded together, and the square steel pipe 10 is used as the column material. In addition, as shown in FIG. 5, the building structure 100 is formed by the beam 18, the small beam 19, and the pillar 20, and it can also be formed by other well-known members. The square steel pipe 10 has a small variation in hardness in the circumferential section as described above, and has a flat flat plate portion 11. Therefore, the building structure 100 of the present invention using the square steel pipe 10 as a column material can exhibit excellent earthquake resistance. Example

Hereinafter, the present invention will be described in further detail based on examples. In addition, the present invention is not limited to the following examples.

The hot-rolled steel sheet with the composition shown in Table 1 is continuously formed into an open tube with an elliptical cross-section through a group of rolls and a group of finishing rolls, and then the opposite end faces of the open tube are heated by high-frequency induction heating or It is heated to the melting point by high-frequency resistance heating, and is crimped by squeeze rollers to become the blank tube of the resistance welded steel pipe. For the obtained resistance welding steel pipe, after forming into a cylindrical shape with a group of sizing rollers of 2 bases (2 stages), a square forming roll group of 4 bases (4 stages) is used for square forming, as shown in Table 2. As shown, square steel pipes with a substantially rectangular cross-section in the direction of the pipe axis were obtained.

The average plate thickness t is obtained by the following equation (2). t=(t1+t2+t3)/3…Equation (2) In formula (2), t1, t2: For the flat plate part including the welding part (resistance welding part), the plate thickness (mm) of each of the two flat plate parts adjacent to each other through the corners in the circumferential direction of the tube (mm), t3: and the resistance including the resistance The plate thickness (mm) of the center of the tube circumferential direction of the opposite flat plate part of the welded part.

In addition, the average side length H is obtained by the following formula (3). H=(H1+H2)/2…Equation (3) In the formula (3), H2: in the vertical section of the tube axis, the side length including the flat plate part where the resistance welding part is formed and the corners on both sides, H1: including the flat part with the side length H2 separated by the angle The side length (mm) between the adjacent flat part and the corners on both sides.

In addition, in each square steel pipe, the square steel pipe was cut perpendicularly to the pipe axis direction, and the cut surface including the resistance welding part was polished and corroded with nitrate etching agent. Observed with an optical microscope, it was confirmed that the resistance welding part was The width of the melting and solidification part in the circumferential direction of the tube is 1.0 μm to 1000 μm throughout the thickness of the tube. The melting and solidification part was identified as a white area observed with an optical microscope in the above-mentioned cross section after etching with a nitrate etching agent.

_{Regarding the circumference C IN} (mm) of the resistance welding steel pipe at the entrance side of the square forming machine base in the first stage, when the pipe making direction is set to the positive direction of the X axis, the diameter of the sizing machine base before the square forming When the X coordinate of any rotating axis is set to Xa(m), and the X coordinate of any rotating axis of the square forming machine base of the first stage is set to Xb(m), it will be in the plane perpendicular to the X axis X= The outer circumference of the pipe at (Xa+Xb)/2(m) is measured with a tape measure, and is taken as the circumference C _IN (mm) of the electric resistance welding steel pipe (refer to Fig. 4 again).

_{Regarding the circumference C OUT} (mm) of the square steel tube on the exit side of the square forming machine base in the final stage, when the X coordinate of any one of the rotation axes of the square forming machine base in the fourth stage of the square forming roll group is set to For Xc(m), measure the outer circumference of the pipe at the plane X=Xc+1(m) perpendicular to the X axis with a tape measure, and use it as the circumference of the square steel pipe C _OUT (mm) (refer to the figure again) 4).

In addition, with respect to the above C _IN and C _OUT , the maximum roll gap between the grooved roll of the sizing stand before the square forming and the concave portion of the grooved roll of the square forming stand of the first stage were measured respectively, and they were used. Calculate G (=△g/(t/H)) with the difference △g.

Furthermore, at 10 arbitrary positions in the tube axis direction of the obtained square steel pipe, the radius of curvature of the outer surface (outside the corner) at 4 corners was measured, and the maximum value Rmax and the minimum value Rmin were obtained at a total of 40 points. . The radius of curvature on the outside of the corner is measured using a radius gauge. The method for measuring the radius of curvature is to measure the straight line L and the angle passing through the intersection P of the two straight lines L1 and L2 on the outer surfaces of the flat plate portions adjacent to the corners and the angle between L1 or L2 is 45° The radius of curvature of the intersection on the outside of the corner is taken as the radius of curvature of the outside of the corner (refer to Fig. 1 again). Specifically, the radius of curvature is measured in a sector formed by the connecting points (A, A') of the flat portion and the corner portion and the outer surface of the corner portion and centered on the above-mentioned L on the central angle of 90°. The intersection of the above-mentioned L and the outer surface of the corner is the center angle of 65°. In the range of the above-mentioned center angle of 65°, the radius of curvature is measured using a radius gauge that is exactly the same as the outer surface of the corner.

After cutting the cross section of the obtained square steel pipe perpendicular to the pipe axis direction, measure the dimensions of the flat part, the corner part, and the welded part (resistance welded part) from the inner and outer surfaces in the thickness direction at the position of 1mm and the center of the plate thickness. For the hardness, the maximum value HVmax and the minimum value HVmin are obtained respectively. The above-mentioned Vickers hardness test was carried out in accordance with JIS Z 2244 with a test force of 98 N (10 kgf). The hardness of the flat part is measured on the flat part beside the flat part including the resistance welding part; the hardness of the corner part is measured in the corner adjacent to the flat part including the resistance welding part.

Fig. 6 is a schematic diagram showing the locations of the tensile test pieces of the flat plate portion and the corner portion, respectively. Fig. 7 is a schematic diagram showing the detailed locations of the tensile test pieces at the corners. Fig. 11 is a schematic diagram showing the location of the tensile test piece taken at a position t/4 from the outer surface of the flat plate portion and a position t/4 from the outer surface of the corner portion, respectively. Fig. 12 is a schematic diagram showing the detailed location of the tensile test piece at a position t/4 from the outer surface of the corner.

As shown in Fig. 6, JIS No. 5 tensile test pieces and JIS 12B No. tensile test pieces were taken from the flat portion and corner portion of the square steel pipe so that the tensile direction was parallel to the pipe axis direction. Regarding the corner tensile test piece, as shown in FIG. 7 in more detail, it is from the intersection point of extending the outer surfaces of the flat plate portions on both sides adjacent to the corner portion and is respectively connected to the outer surface of the flat plate portion. It becomes a 45° line to take.

In addition, as shown in FIG. 11, the JIS No. 5 tensile test piece and JIS 12B No. tensile test piece shown by the dotted line are respectively taken from the flat part and the corner part of the square steel tube so that the tensile direction is parallel to the pipe axis direction. The equal thickness is 5 mm, and the thickness center is at the position t/4 of the plate thickness t from the outer surface of the tube, respectively, and ground is performed to obtain a tensile test piece. Regarding the corner tensile test piece, in more detail, as shown in FIG. 12, it is from the intersection of the outer surfaces of the flat plate portions on both sides adjacent to the corner portion and the outer surface of the flat plate portion. They are taken on the line of 45° respectively. Use them to perform a tensile test in accordance with JIS Z 2241, measure yield strength YS, tensile strength TS, uniform elongation (flat part: E1, corner part: E2), and calculate by (yield strength)/(tensile strength) ) Yield ratio defined by. The uniform elongation is the value of the total elongation at the maximum load. In addition, the tensile test piece of the flat part is taken from the position of the width center part of the flat part beside the flat part including the resistance welding part of the square steel pipe. The corner tensile test piece is taken from the corner adjacent to the flat plate including the resistance welding part. The number of test specimens is two each, and the average value of these is calculated to obtain yield strength YS, tensile strength TS, yield ratio, and uniform elongation.

Fig. 8 is a schematic diagram showing the picking position of the Charpy test piece at the corner. Fig. 9 is a schematic diagram showing the detailed location of the Charpy test piece at the corner.

In the Charpy impact test, as shown in Fig. 8 and Fig. 9, at a position t/4 from the outer surface of the square steel pipe which is the plate thickness t, the test piece length direction is parallel to the pipe axis direction. Use the V-notch standard test piece according to JIS Z 2242. According to JIS Z 2242, the Charpy impact test was carried out at the test temperature: 0°C to obtain the absorbed energy (J). The number of test pieces was 3 pieces each, and the average value thereof was calculated to obtain the absorbed energy (J).

Fig. 10 is a schematic diagram for explaining the method of measuring flatness. The flatness was measured at 10 arbitrary positions in the tube axis direction of the square steel pipe, using 4 flat plate portions as measurement objects, and performed at a total of 40 places. As shown in Fig. 10, the maximum protrusion amount and the maximum depression amount of a straight line passing through two points at both ends in the circumferential direction of the outer surface of each flat plate portion were measured respectively, and the absolute value of the maximum protrusion amount and the maximum depression amount at each measurement location was used. The maximum value is regarded as the flatness. Among them, the amount of protrusion is a positive value, and the amount of depression is a negative value. When the protrusion or depression does not exist, the value of the amount of protrusion or depression is 0.

The results obtained are shown in Table 3.

In Table 2 and Table 3, No. 1, 3, 4, 6, 7, 9, 11, 13, 14, 16, 17 are examples of the present invention, and No. 2, 5, 8, 10, 12, 15, 18 As a comparative example.

The square steel pipes of the examples of the present invention all have a circumference ratio (C _IN /C _OUT ) within the range of formula (1), and the radius of curvature (Rmin, Rmax) outside the corners is greater than 3.0 times to 4.0 times the plate thickness, The uniform elongation at the position t/4 from the outer surface of the corner is more than 0.80 times the uniform elongation at the position t/4 from the outer surface of the flat part. The hardness difference between the corner and the flat part is 1mm away from the inner and outer surfaces. The difference between the maximum and minimum Vickers hardness at the center of the plate thickness is 80HV or less. In addition, the flatness of the square steel pipe of the example of the present invention is 2.5 mm or less.

For Comparative Example Nos. 2, 10, 12, and 15, the circumference ratio (C _IN /C _OUT ) is outside the range of formula (1), and the radius of curvature outside the corner is 3.0 times the plate thickness or less, and the distance from the outside of the corner The uniform elongation at the position t/4 of the surface is less than 0.80 times the uniform elongation at the position t/4 from the outer surface of the flat portion, and the difference between the maximum and minimum Vickers hardness does not reach the expected value. In addition, in Comparative Example Nos. 2, 10, 12, and 15, the circumference ratio (C _IN /C _OUT ) is outside the range of formula (1), and the radius of curvature outside the corner is 3.0 times or less the plate thickness. Charpy The energy absorbed by the test did not reach the expected value.

For Comparative Example Nos. 5, 8, and 18, the circumference ratio (C _IN /C _OUT ) is lower than the range of formula (1) because of insufficient circumferential extension, and the radius of curvature on the outside of the corner is greater than 4.0 for the thickness of the plate. Times, a flat plate portion cannot be obtained.

Based on the above description, by setting the perimeter ratio (C _IN /C _OUT ) of the square forming within the scope of the present invention, a square steel pipe can be provided, the radius of curvature outside the corners of which is greater than 3.0 times the average plate thickness~ 4.0 times, the hardness deviation in the circumferential section is small, the outer surface of the corner has excellent ductility and toughness, and has a flat plate part. It also provides a method for manufacturing the square steel pipe and a building structure with excellent earthquake resistance.

1: Steel strip (steel plate) 2: Leveling machine 3: Row roller group 4: Finishing roll group 5: Squeeze roller 6: Welding machine 7: Resistance welding steel pipe 8: Sizing roller group 9: Square forming roll group 10: Square steel pipe 11: Flat part 12: corner 13: Welding part (resistance welding part) 14: Base metal department 15: Welding heat affected zone 16: melting and solidification part 17: partition 18: Girder 19: Trabecular 20: Pillar 100: Building structure A,A’: Connection point H1: Longitudinal side length in Figure 1 H2: Horizontal side length in Figure 1 L1, L2: straight line P: intersection t1, t2: For the flat plate part including the welding part (resistance welding part), the plate thickness of each of the two flat plate parts that are adjacent to each other through the corners in the circumferential direction of the tube t3: The thickness of the center of the tube in the circumferential direction of the flat part facing the flat part including the welding part (resistance welding part)

[Fig. 1] is a schematic diagram showing a cross section perpendicular to the pipe axis direction of the square steel pipe of the present invention. [Figure 2] is a schematic diagram showing an example of manufacturing equipment for resistance welding of steel pipes. [Fig. 3] is a schematic diagram for explaining the melting and solidification part of the welding part. [Figure 4] is a schematic diagram showing the forming process of the square steel pipe of the present invention. [Figure 5] is a schematic diagram showing an example of the building structure of the present invention. [Fig. 6] A schematic diagram showing the locations of the tensile test pieces of the flat part and the corner part, respectively. [Figure 7] is a schematic diagram showing the detailed locations of the tensile test pieces at the corners. [Figure 8] is a schematic diagram showing the location of the Charpy test piece at the corner. [Figure 9] is a schematic diagram showing the detailed location of the Charpy test piece at the corner. [Fig. 10] is a schematic diagram for explaining the method of measuring flatness. [Fig. 11] is a schematic diagram showing the location of the tensile test piece taken at a position t/4 from the outer surface of the flat plate portion and a position t/4 from the outer surface of the corner portion. [Fig. 12] is a schematic diagram showing the detailed location of the tensile test piece at a position t/4 from the outer surface of the corner.

10: Square steel pipe

11: Flat part

12: corner

13: Welding part (resistance welding part)

A,A’: Connection point

H1: Longitudinal side length in Figure 1

H2: Horizontal side length in Figure 1

L, L1, L2: straight line

P: intersection

t1, t2: For the flat plate part including the welding part (resistance welding part), the plate thickness of each of the two flat plate parts that are adjacent to each other through the corners in the circumferential direction of the tube

t3: The thickness of the center of the tube in the circumferential direction of the flat part facing the flat part including the welding part (resistance welding part)

Claims (8)

A square steel pipe in which a plurality of flat plates and a plurality of corners are alternately formed in the circumferential direction of the pipe, And further formed with a welding part extending in the direction of the tube axis, The width of the melting and solidification part of the welding part in the circumferential direction of the tube is 1.0 μm to 1000 μm, and the radius of curvature outside the corner part is greater than 3.0 times to 4.0 times the average plate thickness t. The square steel pipe as described in claim 1, in which: The aforementioned average plate thickness t is greater than 0.030 times the average side length H. The square steel pipe described in claim 1 or 2, wherein: The difference between the maximum and minimum Vickers hardness in the steel pipe is less than 80HV. The square steel pipe according to any one of claims 1 to 3, wherein: The aforementioned average plate thickness t is 20mm~40mm, The yield strength of the aforementioned flat portion is 295MPa or more, The tensile strength of the aforementioned flat portion is 400 MPa or more, The yield ratio of the aforementioned corners is 90% or less, The energy absorbed in the Charpy test of the aforementioned corner at 0°C is 70J or more. The square steel pipe according to any one of claims 1 to 4, wherein: The uniform elongation at a position t/4 from the outer surface of the corner portion is 0.80 times or more of the uniform elongation at a position t/4 from the outer surface of the flat portion. A method for manufacturing square steel pipes is to roll-form a steel plate, and then perform resistance welding on the rolled steel plate to form a resistance-welded steel pipe. Then, the aforementioned resistance-welded steel pipe is formed by a sizing machine, and then The square steel tube is manufactured by square forming by a square forming machine base to satisfy the following formula (1). According to the roll gap of the aforementioned square forming machine stand, the rolls of the aforementioned sizing stand before the square forming machine are controlled Seam, 0.30×t/H+0.99≦C _IN /C _OUT ＜0.50×t/H+0.99...Equation (1) and in Eq. (1), C _IN : at the entrance of the square forming machine base in the first section The circumference of the resistance welding steel pipe on the side (mm), C _OUT : the circumference of the square steel pipe on the exit side of the square forming machine base in the final stage (mm), t: the average plate thickness after square forming (mm), H: The average side length (mm) after square forming. (However, when the square forming is performed by the square forming machine in one stage, the square forming machine in the first stage and the final stage are The square forming machine base is the same square forming machine base). The method of manufacturing a square steel pipe as described in claim 6, wherein: The aforementioned average plate thickness t is 20 mm to 40 mm. A building structure that uses the square steel tube described in any one of claims 1 to 5 as the column material.

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