Polygonal steel pipe, method for manufacturing same, and building structure using polygonal steel pipe

Abstract

Provided is a polygonal steel pipe having excellent buckling resistance. In a polygonal steel pipe having a plurality of flat plate sections and corner sections alternately in the circumferential direction of the pipe, the yield strength of the flat plate sections in the circumferential direction of the pipe is 0.83-1.20 times the yield strength of the flat plate sections in the axial direction of the pipe, and the yield strength of the corner sections in the circumferential direction of the pipe is 0.90-1.30 times the yield strength of the flat plate sections in the axial direction of the pipe.

Description

Technical Field

[0001] The present invention relates to a polygonal steel pipe having excellent buckling resistance and suitable for use as a column material of a building, a method for producing the same, and a building structure using the polygonal steel pipe. Background Art

[0002] Roll-formed polygonal steel pipes are widely used in polygonal steel pipes used as columns for buildings.

[0003] The roll-formed polygonal steel pipe is manufactured by forming a steel strip (hereinafter referred to as "steel plate") into a cylindrical open pipe shape by cold roll forming, and then performing electric seam welding on the butt joint of the open pipe, and then applying drawing along the pipe axis direction while maintaining the cylindrical state by rollers arranged vertically and horizontally to form a polygon. In the above-mentioned electric seam welding, the butt joint is heated and melted, and then pressed and solidified, thereby completing the joining.

[0004] For roll-formed polygonal steel pipes, high buckling resistance during bending deformation is required from the viewpoint of earthquake resistance. However, the flat plate and corners of roll-formed polygonal steel pipes are greatly work-hardened during manufacturing, so the ductility and / or toughness of the corners are reduced, and the buckling resistance is easily reduced.

[0005] In order to solve this problem, various polygonal steel pipes having improved properties of flat portions and corner portions have been proposed.

[0006] For example, Patent Documents 1 and 2 propose polygonal steel pipes in which the ductility and toughness of the corners are improved by appropriately controlling the chemical composition and microstructure.

[0007] Patent Document 3 proposes a polygonal steel pipe in which the ductility and toughness of the corners are improved by bending a steel plate and then welding it to form a semi-formed polygonal steel pipe, heating the semi-formed polygonal steel pipe to perform hot forming, and then cooling it.

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent Application Publication No. 2011-094214

[0011] Patent Document 2: Japanese Patent Application Publication No. 2018-053281

[0012] Patent Document 3: Japanese Patent Application Publication No. 2004-330222 Summary of the invention

[0013] Among them, regarding the buckling resistance of polygonal steel pipes during bending deformation, it is necessary to study the performance both when the load is applied in a direction perpendicular to the flat plate (0 degree bending) and when the load is applied in the direction of the corner vertex (45 degree bending).

[0014] It should be noted that in the case of a 0-degree bend, the flat plate portion inside the bend is sunken toward the inner surface of the tube, causing buckling. In the case of a 45-degree bend, the radius of curvature of the corner portion inside the bend increases, causing buckling.

[0015] In order to improve the buckling resistance of a polygonal steel pipe, it is believed that it is effective to suppress the deformation of the flat plate portion and the deformation of the corner portion, which are the precursor phenomena of buckling. However, no research has been conducted from such a viewpoint in the past.

[0016] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a polygonal steel pipe having excellent buckling resistance and a method for manufacturing the same, and a building structure having excellent earthquake resistance by using the polygonal steel pipe.

[0017] In the present invention, excellent buckling resistance means that the cumulative plastic deformation ratio in the 0-degree bending test and the 45-degree bending test of the polygonal steel pipe is 28 or more.

[0018] The inventors of the present invention have conducted in-depth research to solve the above-mentioned problems and found that by appropriately controlling the yield strength of the flat plate portion and corner portion of the polygonal steel pipe in the circumferential direction of the pipe, the deformation of the flat plate portion and / or the deformation of the corner portion, which are the precursor phenomena of buckling, can be effectively suppressed, thereby improving the buckling resistance performance.

[0019] In addition, the inventors have also found that by appropriately controlling the residual stress of the flat plate portion in the tube circumferential direction, the deformation of the flat plate portion, which is a precursor phenomenon of buckling, can be suppressed, thereby further improving the buckling resistance in the 0 degree direction.

[0020] The present invention has been completed by further research based on this knowledge. That is, the gist of the present invention is as follows.

[0021] 1. A polygonal steel pipe having a plurality of flat portions and corner portions alternately arranged in the pipe circumferential direction and having a welded portion extending in the pipe axial direction,

[0022] The yield strength of the flat plate portion in the tube circumferential direction is 0.83 to 1.20 times the yield strength of the flat plate portion in the tube axial direction.

[0023] The yield strength of the corner portion in the tube circumferential direction is 0.90 to 1.30 times the yield strength of the flat plate portion in the tube axial direction.

[0024] 2. The polygonal steel pipe according to item 1 above, wherein the residual stress in the pipe circumferential direction of the outer surface of the flat plate portion is -200 MPa to 150 MPa.

[0025] The residual stress indicates tensile stress when it is a positive value, and compressive stress when it is a negative value.

[0026] 3. The polygonal steel pipe according to 1 or 2 above, having the following composition:

[0027] It contains, by mass%, C: 0.020-0.350%, Si: 0.01-0.65%, Mn: 0.30-2.50%, P: 0.050% or less, S: 0.0500% or less, Al: 0.005-0.100% and N: 0.0100% or less, and the remainder is Fe and inevitable impurities.

[0028] 4. The polygonal steel pipe according to item 3 above, wherein the component composition further contains, in terms of mass%, one or more selected from the group consisting of 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, Ca: 0.0100% or less, and B: 0.0100% or less.

[0029] 5. A method for manufacturing a polygonal steel pipe, which is the method for manufacturing a polygonal steel pipe according to any one of 1 to 4 above, comprising the following steps:

[0030] In the pipe making process, the steel plate is formed into a cylindrical shape by cold rolling, and the circumferential ends of the cylindrical shape are butted together and then welded by electric welding;

[0031] The angle forming step is performed after the pipe making step, and uses an angle forming machine frame to form a polygonal steel pipe billet with a flatness of -10.0 mm to -4.0 mm at the flat plate portion and a curvature radius of the corner portion of 1.0 to 2.5 times the wall thickness of the flat plate portion.

[0032] In the internal pressure loading step, an internal pressure p (MPa) satisfying the following formula (1) is applied to the inner surface of the polygonal steel tube billet to form a polygonal steel tube having a flatness of -3.0 mm to 3.0 mm in the flat plate portion and a curvature radius of the corner portion of 2.0 to 4.0 times the wall thickness of the flat plate portion.

[0033] N(MPa)<p≤N×1.5(MPa)···(1)

[0034] Where, N = (wall thickness of polygonal steel tube billet (mm) / side length of polygonal steel tube billet (mm)) × yield strength of the flat plate portion of the polygonal steel tube billet in the circumferential direction of the tube (MPa)

[0035] 6. A building structure comprising the polygonal steel pipe according to any one of 1 to 4 above as a column material.

[0036] According to the present invention, a polygonal steel pipe having excellent buckling resistance and a method for producing the same can be provided.

[0037] In addition, the building structure using the polygonal steel pipe of the present invention as a column material can obtain more excellent earthquake resistance performance compared with the building structure using the conventional cold-formed polygonal steel pipe. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 This is a schematic diagram showing a cross section perpendicular to the tube axis direction of the polygonal steel tube (or polygonal steel tube material) of the present invention.

[0039] Figure 2 This is a schematic diagram for explaining the method of measuring flatness.

[0040] Figure 3 It is a schematic diagram showing an angle forming frame.

[0041] Figure 4 This is a schematic diagram showing an example of the building structure of the present invention.

[0042] Figure 5 This is a schematic diagram showing the positions for taking tensile test specimens in the tube axis direction of the flat plate portion of a polygonal steel tube material.

[0043] Figure 6 The diagram schematically shows the locations where specimens for tensile tests in the tube circumferential direction are taken from the flat portion and the corner portion of a polygonal steel tube.

[0044] Figure 7 This is a schematic diagram showing the locations where tensile test specimens are taken from the corners of a polygonal steel pipe.

[0045] Figure 8 This is a schematic diagram of a 0-degree bend test on a polygonal steel pipe.

[0046] Fig. 9 This is a schematic diagram of a 45-degree bend test on a polygonal steel pipe. DETAILED DESCRIPTION

[0047] The polygonal steel pipe of the present invention has a plurality of flat portions and corner portions alternately in the circumferential direction of the pipe, so that the cross section of the steel pipe is any one of a triangle and a polygon with a pentagon or more in addition to a rectangle (a quadrilateral). Moreover, the polygonal steel pipe forms a weld extending in the pipe axial direction, the yield strength of the flat portion in the circumferential direction of the pipe is 0.83 to 1.20 times the yield strength of the flat portion in the axial direction of the pipe, and the yield strength of the corner portion in the circumferential direction of the pipe is 0.90 to 1.30 times the yield strength of the flat portion in the axial direction of the pipe.

[0048] Yield strength of the flat plate in the circumferential direction: 0.83 to 1.20 times the yield strength of the flat plate in the axial direction

[0049] When the yield strength of the flat plate portion in the circumferential direction of the tube is less than 0.83 times the yield strength of the flat plate portion in the axial direction of the tube, the flat plate portion on the inner side of the bend is easily plastically deformed when bending at 0 degrees, and the flat plate portion on the inner side of the bend is easily recessed toward the inner surface of the tube, thereby reducing the buckling resistance. Thus, the yield strength of the flat plate portion in the circumferential direction of the tube is made 0.83 times or more of the yield strength of the flat plate portion in the axial direction of the tube. It should be noted that the yield strength of the flat plate portion in the circumferential direction of the tube is preferably 0.85 times or more of the yield strength of the flat plate portion in the axial direction of the tube, and more preferably 0.87 times or more.

[0050] On the other hand, when the yield strength of the flat plate portion in the circumferential direction of the tube is greater than 1.20 times the yield strength of the flat plate portion in the axial direction of the tube, the ductility of the flat plate portion is reduced, and the uneven deformation of the flat plate portion occurs early when bending at 0 degrees, so the buckling resistance is reduced. In addition, fractures on the outside of the bend are easily generated. Thus, the yield strength of the flat plate portion in the circumferential direction of the tube is made less than 1.20 times the yield strength of the flat plate portion in the axial direction of the tube. It should be noted that the yield strength of the flat plate portion in the circumferential direction of the tube is preferably less than 1.15 times the yield strength of the flat plate portion in the axial direction of the tube, and more preferably less than 1.10 times.

[0051] It should be noted that, in the conventional manufacturing method, the yield strength of the flat plate portion in the tube circumferential direction is approximately 0.80 times the yield strength of the flat plate portion in the tube axial direction.

[0052] In the present invention, the flat plate portion refers to a range excluding the corner portion described later, and both ends of the flat plate portion (connection point ( Figure 1 The points A, A', etc. shown are connected to different corners and have a flatness described later. Figure 1 This is a schematic diagram showing a cross section perpendicular to the tube axis direction of the polygonal steel tube of the present invention.

[0053] Yield strength of the corner in the circumferential direction: 0.90 to 1.30 times the yield strength of the flat plate in the axial direction

[0054] When the yield strength of the corner portion in the circumferential direction of the tube is less than 0.90 times the yield strength of the flat portion in the axial direction of the tube, the corner portion on the inner side of the bend is prone to plastic deformation when bending at 45 degrees, and the radius of curvature of the corner portion on the inner side of the bend is prone to increase, so the buckling resistance performance is reduced. Therefore, the yield strength of the corner portion in the circumferential direction of the tube is set to be 0.90 times or more of the yield strength of the flat portion in the axial direction of the tube. It should be noted that the yield strength of the corner portion in the circumferential direction of the tube is preferably 0.95 times or more of the yield strength of the flat portion in the axial direction of the tube, and more preferably 1.00 times or more.

[0055] On the other hand, when the yield strength of the corner in the circumferential direction is greater than 1.30 times the yield strength of the flat plate in the axial direction, the ductility of the flat plate is reduced, and uneven deformation of the corner occurs early when bending at 45 degrees, so the buckling resistance is reduced. In addition, fractures are likely to occur on the outside of the bend. Therefore, the yield strength of the corner in the circumferential direction is set to be less than 1.30 times the yield strength of the flat plate in the axial direction. It should be noted that the yield strength of the corner in the circumferential direction is preferably less than 1.25 times the yield strength of the flat plate in the axial direction, and more preferably less than 1.20 times.

[0056] It should be noted that, in the conventional manufacturing method, the yield strength of the corner portion in the tube circumferential direction is approximately 0.85 times the yield strength of the flat plate portion in the tube axial direction.

[0057] In addition, the corner portion in the present invention refers to a portion having a curved surface with a predetermined curvature radius described later.

[0058] In order to ensure sufficient strength and buckling resistance as a building structure, the polygonal steel pipe of the present invention preferably has a yield strength of 295 MPa or more in the pipe axis direction of the flat plate portion.

[0059] In addition, the above-mentioned yield strength can be obtained by carrying out a tensile test in accordance with the provisions of JIS Z 2241 (2011).

[0060] For the polygonal steel pipe of the present invention, in order to improve the buckling resistance in the 0 degree direction, the residual stress in the circumferential direction of the outer surface of the flat plate portion is preferably -200 MPa to 150 MPa. A positive value of the residual stress indicates tensile stress, and a negative value indicates compressive stress.

[0061] If the residual stress value of the outer surface of the flat plate portion in the circumferential direction of the tube is -200 MPa or more, the compression deformation of the outer surface of the flat plate portion is promoted when bending at 0 degrees, and the flat plate portion on the inner side of the bend is prevented from easily sinking toward the inner surface of the tube, and the buckling resistance performance can be well maintained. The above residual stress is more preferably -190 MPa or more, and further preferably -180 MPa or more.

[0062] On the other hand, if the residual stress value of the outer surface of the flat plate portion in the circumferential direction of the tube is 150 MPa or less, the tensile deformation of the outer surface of the flat plate portion is promoted during 0-degree bending, and the disadvantage that the adjacent flat plates on both sides of the flat plate portion on the inner side of the bend are easily expanded to the outer side of the tube can be suppressed. As a result, the concavity of the inner surface of the tube of the flat plate portion on the inner side of the bend can also be suppressed, and the buckling resistance performance can be well maintained. The above residual stress is more preferably 140 MPa or less, and further preferably 130 MPa or less.

[0063] It should be noted that the residual stress in the present invention can be measured by an X-ray diffraction method.

[0064] In order to ensure sufficient strength and weldability as a building structure, the polygonal steel pipe of the present invention preferably has the following component composition: containing, by mass%, C: 0.020-0.350%, Si: 0.01-0.65%, Mn: 0.30-2.50%, P: 0.050% or less, S: 0.0500% or less, Al: 0.005-0.100% and N: 0.0100% or less, and the remainder is Fe and unavoidable impurities.

[0065] In the following, "%" indicating the steel composition means "mass %" unless otherwise specified. In addition, the following component composition may also be referred to as the component composition of the flat plate portion and the corner portion excluding the welded portion of the polygonal steel pipe.

[0066] C: 0.020%~0.350%

[0067] C is an element that improves the strength of steel by solid solution strengthening. In addition, C is an element that helps to refine the structure by lowering the ferrite transformation start temperature. In order to obtain these effects, it is preferred to contain more than 0.020% C. On the other hand, C is an element that promotes the formation of pearlite, improves hardenability and helps the formation of martensite, helps stabilize austenite, and thus helps to form a hard phase. However, if the C content is greater than 0.350%, the proportion of the hard phase increases and the ductility decreases, and the buckling resistance may decrease. In addition, weldability may also deteriorate.

[0068] From the above, the C content is preferably in the range of 0.020% to 0.350%. The C content is more preferably 0.030% or more, and further preferably 0.040% or more. On the other hand, the C content is more preferably 0.330% or less, and further preferably 0.310% or less.

[0069] Si: 0.01% to 0.65% or less

[0070] Si is an element that increases the strength of steel by 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 easily generated in the electric resistance welded part, and the welded part characteristics may be reduced. In addition, the ductility of the base material part other than the electric resistance welded part may be reduced, and the buckling resistance may be reduced.

[0071] From the above, the Si content is preferably in the range of 0.01% to 0.65%. The Si content is more preferably 0.02% or more, and even more preferably 0.03% or more. On the other hand, the Si content is more preferably 0.63% or less, and even more preferably 0.61% or less.

[0072] Mn: 0.30%~2.50%

[0073] Mn is an element that improves the strength of steel by solid solution strengthening. In addition, Mn is an element that helps refine the structure by lowering the ferrite transformation start temperature. In order to obtain these effects, it is preferred to contain 0.30% or more of Mn. On the other hand, if the Mn content exceeds 2.50%, oxides are easily generated in the electric seam weld, and the weld characteristics may be reduced. In addition, due to solid solution strengthening and refinement of the structure, ductility is reduced and buckling resistance may be reduced.

[0074] From the above, the Mn content is preferably in the range of 0.30% to 2.50%. 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, and even more preferably 2.10% or less.

[0075] P: 0.050% or less

[0076] Since P segregates at grain boundaries and causes material inhomogeneity, it is preferred to reduce it as much as possible. Therefore, the P content is preferably 0.050% or less. The P content is more preferably 0.040% or less, and further preferably 0.030% or less. There is no particular lower limit for P (usually greater than 0%), but excessive reduction will lead to an increase in smelting costs, so the P content is preferably 0.002% or more.

[0077] S: 0.0500% or less

[0078] S usually exists in steel as MnS, but MnS is thinly stretched during the hot rolling process, which reduces ductility and reduces buckling resistance. Therefore, in the present invention, it is preferred 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 further preferably 0.0100% or less. There is no particular lower limit for S (usually greater than 0%), but excessive reduction increases smelting costs, so S is preferably 0.0002% or more.

[0079] Al: 0.005% to 0.100%

[0080] Al is an element that acts as a strong deoxidizer. In order to obtain such an effect, it is preferable to contain 0.005% or more of Al. On the other hand, if the Al content exceeds 0.100%, weldability deteriorates, and alumina inclusions increase, resulting in deteriorated surface properties.

[0081] Therefore, the Al content is preferably in the range of 0.005% to 0.100%. 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, and even more preferably 0.060% or less.

[0082] N: 0.0100% or less

[0083] N is an element that has the effect of reducing ductility and buckling resistance by firmly fixing the movement of dislocations. In the present invention, N is preferably reduced as much as possible. Therefore, 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 for N (usually greater than 0%), but excessive reduction leads to an increase in smelting costs, so N is preferably 0.0010% or more.

[0084] In addition, the above-mentioned component composition of the polygonal steel pipe of the present invention further contains, in terms of mass%, one or more selected from the group consisting of Ti: (greater than 0%) 0.100% or less, Nb: (greater than 0%) 0.100% or less, V: (greater than 0%) 0.100% or less, Cr: (greater than 0%) 1.00% or less, Mo: (greater than 0%) 1.00% or less, Cu: (greater than 0%) 1.00% or less, Ni: (greater than 0%) 1.00% or less, Ca: (greater than 0%) 0.0100% or less, and B: (greater than 0%) 0.0100% or less.

[0085] Ti: 0.100% or less

[0086] If the Ti content is greater than 0.100%, the ductility decreases and the buckling resistance may decrease. Therefore, the Ti content is preferably less than 0.100%. The Ti content is more preferably less than 0.090%, and further preferably less than 0.080%. On the other hand, Ti is an element that forms fine carbides and nitrides in steel and helps to increase the strength of the steel. In addition, due to its high affinity with N, it renders N in the steel harmless as nitrides, and is an element that helps to increase the ductility of the steel. In order to obtain the above-mentioned effects, the Ti content is preferably greater than 0.001%. The Ti content is more preferably greater than 0.002%, and further preferably greater than 0.005%.

[0087] Nb: 0.100% or less

[0088] If the Nb content is greater than 0.100%, ductility decreases and buckling resistance may decrease. Therefore, the Nb content is preferably less than 0.100%. The Nb content is more preferably less than 0.090%, and further preferably less than 0.080%. On the other hand, Nb is an element that helps to increase the strength of steel by forming fine carbides and nitrides in steel, and also helps to refine the structure by inhibiting the coarsening of austenite during hot rolling. In order to obtain the above-mentioned effects, the Nb content is preferably greater than 0.001%. The Nb content is more preferably greater than 0.002%, and further preferably greater than 0.005%.

[0089] V: 0.100% or less

[0090] If the V content is greater than 0.100%, ductility decreases 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, and further preferably 0.080% or less. On the other hand, V is an element that contributes to increasing the strength of steel by forming fine carbides and nitrides in steel. In order to obtain the above-mentioned effect, it is preferred to contain 0.001% or more of V. The V content is more preferably 0.002% or more, and further preferably 0.005% or more.

[0091] Cr: 1.00% or less

[0092] If the Cr content is greater than 1.00%, ductility decreases and buckling resistance may decrease. Therefore, the Cr content is preferably less than 1.00%. The Cr content is more preferably less than 0.80%, and further preferably less than 0.60%. On the other hand, Cr is an element that improves the hardenability of steel and the strength of steel. In order to obtain the above-mentioned effect, the Cr content is preferably greater than 0.01%. The Cr content is more preferably greater than 0.02%, and further preferably greater than 0.05%.

[0093] Mo: 1.00% or less

[0094] If the Mo content is greater than 1.00%, ductility decreases and buckling resistance may decrease. Therefore, the Mo content is preferably less than 1.00%. The Mo content is more preferably less than 0.80%, and further preferably less than 0.60%. On the other hand, Mo is an element that improves the hardenability of steel and the strength of steel. In order to obtain the above-mentioned effects, the Mo content is preferably greater than 0.01%. The Mo content is more preferably greater than 0.02%, and further preferably greater than 0.05%.

[0095] Cu: 1.00% or less

[0096] If the Cu content is greater than 1.00%, ductility decreases and buckling resistance may decrease. Therefore, the Cu content is preferably less than 1.00%. The Cu content is more preferably less than 0.80%, and further preferably less than 0.60%. On the other hand, Cu is an element that increases the strength of steel by solid solution strengthening. In order to obtain the above-mentioned effect, the Cu content is preferably more than 0.01%. The Cu content is more preferably more than 0.02%, and further preferably more than 0.05%.

[0097] Ni: 1.00% or less

[0098] If the Ni content is greater than 1.00%, ductility decreases and buckling resistance may decrease. Therefore, the Ni content is preferably less than 1.00%. The Ni content is more preferably less than 0.80%, and further preferably less than 0.60%. On the other hand, Ni is an element that increases the strength of steel by solid solution strengthening. In order to obtain the above-mentioned effect, the Ni content is preferably more than 0.01%. The Ni content is more preferably more than 0.02%, and further preferably more than 0.05%.

[0099] Ca: 0.0100% or less

[0100] If the Ca content is greater than 0.0100%, Ca oxide clusters are formed in the steel, ductility is reduced, and buckling resistance may be reduced. Therefore, the Ca content is preferably 0.0100% or less. The Ca content is more preferably 0.0080% or less, and further preferably 0.0060% or less. On the other hand, Ca is an element that spheroidizes sulfides such as MnS that are thinly stretched in the hot rolling process and helps to improve the ductility of the steel. In order to obtain the above-mentioned effect, it is preferred to contain 0.0002% or more of Ca. The Ca content is more preferably 0.0005% or more, and further preferably 0.0010% or more.

[0101] B: 0.0100% or less

[0102] If the B content is greater than 0.0100%, ductility decreases 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, further preferably 0.0060% or less, and further 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 contained as needed. In order to obtain the above-mentioned effect, it is preferred to contain 0.0001% or more of B. The B content is more preferably 0.0005% or more, and further preferably 0.0008% or more.

[0103] In the above composition, the remainder is Fe and inevitable impurities. Examples of the inevitable impurities in the remainder include Sn, As, Sb, Bi, Co, Pb, Zn, and O. However, within the range that does not impair the effect of the present invention, it is not rejected to contain less than 0.10% of Sn, less than 0.050% of As, Sb, and Co, respectively, and less than 0.0050% of Bi, Pb, Zn, and O, respectively.

[0104] In addition, the above-mentioned O means the total oxygen including O as an oxide.

[0105] Next, the method for producing the polygonal steel pipe of the present invention will be described.

[0106] In the method for manufacturing a polygonal steel pipe of the present invention, a steel plate is formed into a cylindrical shape by cold roll forming, and both circumferential ends of the cylindrical shape are butted and subjected to electric seam welding (pipe making process), and then, a polygonal steel pipe billet having a flatness of a flat plate portion of -10.0 mm to -4.0 mm and a curvature radius of a corner portion of 1.0 to 2.5 times the wall thickness of the flat plate portion is formed by an angle forming frame (polygon forming process), and then, an internal pressure p (MPa) satisfying the following formula (1) is applied to the inner surface of the above-mentioned polygonal steel pipe billet to form a polygonal steel pipe having a flatness of a flat plate portion of -3.0 mm to 3.0 mm and a curvature radius of a corner portion of 2.0 to 4.0 times the wall thickness of the flat plate portion (internal pressure loading process).

[0107] N(MPa)<p≤N×1.5(MPa)···(1)

[0108] Where, N = (wall thickness of polygonal steel tube billet (mm) / side length of polygonal steel tube billet (mm)) × yield strength of the flat plate portion of the polygonal steel tube billet in the circumferential direction of the tube (MPa)

[0109] It should be noted that the above-mentioned cylindrical shape means that the cross section of the tube circumference is "C"-shaped.

[0110] Figure 1 In, t 1 and t2 It is the wall thickness (mm) of the center (III in the figure) in the tube circumferential direction of each of the two flat plate parts 11 (II in the figure) adjacent to each other across the corner part 12 relative to the flat plate part 11 (I in the figure) including the welded part (electric seam welded part) 13. In addition, Figure 1 In, t 3 is the wall thickness (mm) of the center (V) in the circumferential direction of the flat plate portion 11 (IV in the figure) facing the flat plate portion 11 including the welded portion (electrical seam welded portion) 13. 1 (mm), t 2 (mm) and t 3 (mm) is calculated by adding and averaging.

[0111] The side length of the polygonal steel pipe billet is when the welded portion (electric seam welded portion) 13 is placed upward. Figure 1 H 1 : Longitudinal length (mm) and H 2 : Calculated by adding and averaging the horizontal lengths (mm).

[0112] like Figure 2 As shown in FIG. 1 , the flatness of the flat plate portion in the present invention is the amount of expansion or depression relative to a straight line passing through two points at both ends of the outer surface of the flat plate portion in the circumferential direction. The amount of expansion is a positive value, the amount of depression is a negative value, and the value is 0 when there is no expansion or depression. In addition, the above-mentioned two ends in the circumferential direction refer to the boundary points of the contact part and the non-contact part with the roller of the angle forming frame, and are the angular points where the radius of curvature of the outer circumferential surface of the tube changes.

[0113] like Figure 1 As shown, the curvature radius of the corner portion in the present invention is set to: Figure 1 In the example of the upper right corner, the radius of curvature is the intersection P of the straight lines (extension lines) L1 and L2 extending from the outside of the flat plate parts 11 on both sides adjacent to the upper right corner, and the intersection B of the straight line L forming a 45° angle with the extension line L1 or L2 and the curved line on the outside of the corner 12.

[0114] The radius of curvature is measured at the connection point between the extended lines L1, L2 and the flat plate portion 11 and the corner portion 12 ( Figure 1 The points A, A') and the outer surface of the corner 12 are arcs, and the center is on the straight line L. The center angle is 90 degrees in the sector, and the measurement is performed within the range of the center angle 65 degrees centered at the intersection point B of the straight line L and the outer surface of the corner 12. For example, the method of measuring the curvature radius can be cited, which is a method of measuring the curvature radius with a radial gauge that is very consistent with the arc within the range of the above-mentioned center angle 65 degrees.

[0115] The internal pressure load can be applied, for example, by sealing the pipe end with a rubber pad and applying water pressure to the inside of the pipe. Furthermore, in order to stabilize the shape, a metal mold of a predetermined shape can be used as an outer frame as needed.

[0116] A typical roll-formed polygonal steel pipe is manufactured by forming a flat plate portion from a cylindrical shape to a flat polygonal shape using an angle forming stand.

[0117] In contrast, the polygonal steel pipe of the present invention is temporarily recessed toward the inner surface of the pipe due to the flat portion formed by the angle forming frame. Figure 3 As shown, the electric welded steel pipe 7 obtained by electric seam welding is formed by the angle forming roller group 9 after passing through the sizing roller 8 so that the flat plate portion is temporarily recessed toward the inner surface of the pipe, thereby obtaining a polygonal steel pipe blank 10'. On this basis, by applying an internal pressure load to the inner surface of the pipe, the flat plate portion is expanded toward the outer surface of the pipe to make the flat plate portion flat, and the curvature radius of the corner portion is increased, so that the polygonal steel pipe of the present invention can be obtained.

[0118] It should be noted that it is preferred that the rolls of the final stand in the angle forming stand are four rolls, and the flat plate portion is formed at the contact portion between the rolls and the steel pipe, and the angle portion is formed at the non-contact portion.

[0119] In order to shape the flat plate portion of the polygonal steel pipe material so as to be concave toward the inner surface of the pipe, it is necessary to use a roll having a smaller curvature radius than conventional hole shapes and adjust the roll gap.

[0120] Therefore, the roll hole shape of the final stand in the angle forming stand is set to the same shape as the outer peripheral cross-sectional shape of the desired polygonal steel pipe billet (for example, a substantially regular polygonal shape), and the roll gap is adjusted as needed. The target side length of the polygonal steel pipe billet is set to H (mm), and the curvature radius (mm) of the roll hole shape is preferably H. 2 / 100~H 2 / 30.

[0121] The desired outer cross-sectional shape of the polygonal steel pipe material is, for example, that the flatness of the flat plate portion is -10.0 mm to -4.0 mm and the radius of curvature of the corner portion is 1.0 to 2.5 times the thickness of the flat plate portion.

[0122] If the gap between the rollers of the final stand in the above-mentioned angle forming stand is reduced, the radius of curvature of the corner of the polygonal steel pipe billet can be reduced, and the flatness of the flat plate portion can be reduced (the amount of concavity can be increased). On the other hand, if the gap between the rollers of the final stand in the angle forming stand is increased, the radius of curvature of the corner of the polygonal steel pipe billet can be increased, and the flatness of the flat plate portion can be increased (the amount of concavity can be reduced).

[0123] Thus, by adjusting the roll gap of the final stand in the angle forming stand, the radius of curvature of the corner of the polygonal steel pipe billet can be adjusted to a desired value. In addition, by adjusting the radius of curvature of the roll hole shape and the roll gap, the flatness of the polygonal steel pipe billet can be adjusted to a desired value.

[0124] In the present invention, since the flat plate portion of the polygonal steel pipe is repeatedly bent and deformed in the circumferential direction, the yield strength of the flat plate portion in the circumferential direction can be increased compared to a conventional polygonal steel pipe.

[0125] In addition, in the present invention, since the corners of the polygonal steel pipe are also repeatedly bent and deformed in the circumferential direction, the circumferential work hardening of the corners can be appropriately increased compared to the conventional polygonal steel pipe. That is, the circumferential yield strength of the corners can be increased.

[0126] Flatness of the flat plate of the polygonal steel pipe billet based on the angle forming machine frame: -10.0mm to -4.0mm

[0127] When the flatness of the flat plate portion of the polygonal steel tube billet using the angle forming machine frame is less than -10.0 mm, the deformation of the flat plate portion during internal pressure load increases, the work hardening of the flat plate portion increases, and the yield strength of the flat plate portion in the circumferential direction becomes too high, so the buckling resistance during 0 degree bending decreases. The above flatness is preferably -9.5 mm or more, and more preferably -9.0 mm or more.

[0128] On the other hand, when the flatness of the flat plate portion of the polygonal steel tube billet using the angle forming machine frame is greater than -4.0 mm, the deformation of the flat plate portion under internal pressure load becomes small, the work hardening amount of the flat plate portion becomes small, and the yield strength of the circumferential direction of the flat plate portion becomes too low, so the buckling resistance performance during 0-degree bending is reduced. In addition, since the absolute value of the residual stress in the circumferential direction of the outer surface of the flat plate portion becomes too large, the buckling resistance performance during 0-degree bending is reduced. The above flatness is preferably less than -4.5 mm, and more preferably less than -5.0 mm.

[0129] The curvature radius of the corner of the polygonal steel pipe billet using the angle forming machine frame: 1.0 to 2.5 times the wall thickness of the flat plate

[0130] When the radius of curvature of the corner of the polygonal steel pipe billet using the angle forming machine is less than 1.0 times the wall thickness of the flat plate, the deformation of the corner during internal pressure load increases, the work hardening of the corner increases, and the yield strength of the corner in the circumferential direction becomes too high, so the buckling resistance during 45-degree bending decreases. The above-mentioned radius of curvature is preferably 1.1 times or more of the wall thickness of the flat plate, and more preferably 1.2 times or more of the wall thickness of the flat plate.

[0131] On the other hand, when the radius of curvature of the corner of the polygonal steel pipe billet using the angle forming machine is greater than 2.5 times the wall thickness of the flat plate portion, the deformation amount of the corner during internal pressure load becomes small, the work hardening amount of the corner becomes small, and the yield strength of the corner in the circumferential direction becomes too low, so the buckling resistance during 45-degree bending is reduced. The above-mentioned radius of curvature is preferably 2.4 times or less of the wall thickness of the flat plate portion, and more preferably 2.3 times or less of the wall thickness of the flat plate portion.

[0132] Flatness of the flat plate of a polygonal steel pipe after internal pressure is applied to the inner surface of the pipe: -3.0mm to 3.0mm

[0133] When the flatness of the flat plate portion of the polygonal steel pipe after the internal pressure is applied to the inner surface of the pipe is less than -3.0 mm, the deformation of the flat plate portion under the internal pressure load becomes smaller, the work hardening amount of the flat plate portion becomes smaller, and the yield strength of the circumferential direction of the flat plate portion becomes too low, so the buckling resistance during 0-degree bending is reduced. In addition, due to the large curvature of the outer peripheral cross-section of the flat plate portion, it is difficult to weld it with the partition or beam. Furthermore, the absolute value of the residual stress in the circumferential direction of the outer surface of the flat plate portion becomes larger, and the buckling resistance during 0-degree bending is sometimes reduced. The above flatness is preferably greater than -2.9 mm, and more preferably greater than -2.8 mm.

[0134] On the other hand, when the flatness of the flat plate portion of the polygonal steel pipe after the internal pressure is applied to the inner surface of the pipe is greater than 3.0 mm, the deformation of the flat plate portion during the internal pressure load becomes large, the work hardening of the flat plate portion becomes large, and the yield strength of the circumferential direction of the flat plate portion becomes too high, so the buckling resistance performance during 0 degree bending is reduced. In addition, due to the large curvature of the outer peripheral cross section of the flat plate portion, it is difficult to weld and join with the partition or beam. The above flatness is preferably 2.7 mm or less, and more preferably 2.5 mm or less.

[0135] The radius of curvature of the corner of a polygonal steel pipe after internal pressure is applied to the inner surface of the pipe: 2.0 to 4.0 times the wall thickness of the flat plate

[0136] When the radius of curvature of the corner of the polygonal steel pipe after the internal pressure is applied to the inner surface of the pipe is less than 2.0 times the wall thickness of the flat plate portion, the deformation amount of the corner when the internal pressure is applied becomes small, the work hardening amount of the corner becomes small, and the yield strength of the corner in the circumferential direction becomes too low, so the buckling resistance during 45-degree bending is reduced. The above-mentioned radius of curvature is preferably 2.1 times or more of the wall thickness of the flat plate portion, and more preferably 2.2 times or more of the wall thickness of the flat plate portion.

[0137] On the other hand, when the radius of curvature of the corner of the polygonal steel pipe after the internal pressure is applied to the inner surface of the pipe is greater than 4.0 times the wall thickness of the flat plate portion, the deformation of the corner during the internal pressure load becomes large, the work hardening of the corner becomes large, and the yield strength of the corner in the circumferential direction becomes too high, so the buckling resistance during 45-degree bending decreases. The above-mentioned radius of curvature is preferably 3.9 times or less of the wall thickness of the flat plate portion, and more preferably 3.8 times or less of the wall thickness of the flat plate portion.

[0138] It should be noted that the wall thickness of the flat plate portion of the polygonal steel pipe after the internal pressure is applied to the inner surface of the pipe does not substantially change from the wall thickness of the flat plate portion of the polygonal steel pipe material before the internal pressure is applied to the inner surface of the pipe.

[0139] When the internal pressure p (MPa) is less than N in the above formula (1), the tensile stress generated in the circumferential direction of the tube is lower than the yield strength of the flat plate portion in the circumferential direction of the tube, and the deformation of the flat plate portion becomes smaller. As a result, the work hardening of the flat plate portion becomes smaller, and the yield strength of the flat plate portion in the circumferential direction becomes too low, so the buckling resistance during 0-degree bending is reduced. In addition, the absolute value of the residual stress in the circumferential direction of the outer surface of the flat plate portion becomes larger, and the buckling resistance during 0-degree bending is sometimes reduced. Furthermore, the deformation of the corner becomes smaller, the work hardening of the corner becomes smaller, and the yield strength of the corner in the circumferential direction becomes too low, so the buckling resistance during 45-degree bending is reduced.

[0140] On the other hand, when the internal pressure p (MPa) is greater than N×1.5 in the above formula (1), the tensile stress generated in the circumferential direction of the tube becomes too high, and the deformation of the flat plate portion becomes large. As a result, the work hardening of the flat plate portion becomes large, and the yield strength of the flat plate portion in the circumferential direction becomes too high, so the buckling resistance performance during 0-degree bending decreases. In addition, the deformation of the corner portion becomes large, the work hardening of the corner portion becomes large, and the yield strength of the corner portion in the circumferential direction becomes too high, so the buckling resistance performance during 45-degree bending decreases.

[0141] The polygonal steel pipe of the present invention preferably has a wall thickness of 6 mm or more to ensure sufficient strength and buckling resistance as a building structure. In addition, the wall thickness is preferably 40 mm or less to prevent the deformation of the outer surface of the pipe from becoming excessive during bending deformation under internal pressure load.

[0142] The building structure of the present invention includes the above-mentioned polygonal steel pipe 10 of the present invention as a column material.

[0143] Figure 4 1 is a schematic diagram showing an example of a building structure 100 according to the present invention.

[0144] In the building structure 100 of the present invention, the penetration partition 17 and the polygonal steel pipe 10 are welded, and the polygonal steel pipe 10 is used as a column material. Figure 4As shown, the building structure 100 is formed by a large beam 18, a small beam 19, and a spacer 20, and can be further formed using known components.

[0145] Here, as described above, the polygonal steel pipe 10 has excellent buckling resistance. Therefore, the building structure 100 of the present invention using the polygonal steel pipe 10 as a column material exhibits excellent earthquake resistance.

[0146] About the various conditions of the manufacturing method of the polygonal steel pipe not recorded in the above, all can be according to conventional methods.In addition, about the formation of the polygonal steel pipe not recorded in the above and the building structure, all can use known formations, etc.

[0147] Example

[0148] Hereinafter, the present invention will be further described in detail based on examples. It should be noted that the present invention is not limited to the following examples.

[0149] Molten steel having the component composition shown in Table 1 was melted to obtain slabs. The obtained slabs were hot-rolled to obtain steel plates.

[0150]

[0151] The steel plate thus obtained is formed into a cylindrical shape by cold roll forming, and the two circumferential ends of the cylindrical shape are butted and welded. Then, a polygonal steel pipe billet having four flat portions and four corner portions is formed by an angle forming stand. Then, the two pipe ends of the polygonal steel pipe billet are sealed with rubber gaskets, and the inside of the pipe is filled with water to apply an internal pressure p (MPa), thereby forming a polygonal steel pipe.

[0152] (Measurement of Flatness of Flat Plate)

[0153] The flatness of all positions of each flat plate portion, i.e., four positions, was measured at any five positions in the tube axis direction of the polygonal steel tube material and the polygonal steel tube obtained in this way, and the arithmetic average of the measured values ​​at a total of 20 positions was taken as the flatness of each polygonal steel tube material and the polygonal steel tube.

[0154] In the determination of flatness, first, Figure 2 As shown, the maximum expansion and the maximum depression with respect to the straight line passing through the two points at both ends of the outer surface of each flat plate portion are measured, and the maximum value F of the absolute value of the maximum expansion or the maximum depression at each measurement position is obtained. And, when F is the expansion, the value of flatness is set to F, when F is the depression, the value of flatness is set to -F, and when there is no expansion or depression, the value of flatness is set to 0.

[0155] (Measurement of the curvature radius of the corner)

[0156] At any five positions in the tube axis direction of the polygonal steel tube material and the polygonal steel tube, the curvature radius of the outer surface (outside of the corner) of all the positions of the corner, that is, four positions, is measured respectively, and the arithmetic average of the measured values ​​at a total of 20 positions is taken as the curvature radius of each corner of the polygonal steel tube material and the polygonal steel tube.

[0157] The radius of curvature of the corner is measured using a radial measuring instrument. Regarding the method for measuring the radius of curvature, the radius of curvature at the intersection point P of two straight lines L1 and L2 that pass through the outside of the flat plate portion on both sides adjacent to the corner and are 45 degrees to L1 or L2 and the outside of the corner is measured as the radius of curvature of the outside of the corner ( Figure 1 ).

[0158] Specifically, the measurement of the radius of curvature is performed on a sector with a central angle of 90° formed by the connection point (A, A') between the flat portion and the corner and the outer surface of the corner and centered on the above L, within a range of a central angle of 65° centered at the intersection of the above L and the outer surface of the corner, and the radius of curvature is measured by a radial measuring device that is most consistent with the outer surface of the corner within the range of the above central angle of 65° ( Figure 1 ).

[0159] (Determination of yield strength)

[0160] In addition, tensile test pieces were taken from the polygonal steel pipe billet and the polygonal steel pipe to measure the yield strength. Figure 5 As shown in W, a JIS No. 5 tensile test piece is taken from the flat plate portion of the polygonal steel tube billet 10' in a manner that the tensile direction is parallel to the tube axis direction. Figure 6 As shown in X, a JIS No. 5 tensile test piece is taken from the flat plate portion of the polygonal steel pipe 10 in a manner such that the tensile direction is parallel to the pipe circumferential direction. Figure 6 As shown in Y, a tensile test piece of 1 / 5 size of a JIS 5 tensile test piece is taken from the corner of the polygonal steel pipe 10. It should be noted that the test piece taken from the flat portion is used as a total thickness test piece, and the test piece taken from the corner is ground to a thickness of 1 / 2 of the wall thickness.

[0161] In more detail, Figure 7 As shown in Z, the tensile test piece of the corner is taken in such a way that the center of the parallel long side of the test piece is located on the line that passes through the intersection obtained by extending the outer surface of the flat plate portion on both sides adjacent to the corner and is 45 degrees to the outer surface of the flat plate portion. It should be noted that the bent clamping portion is flattened by pressure straightening.

[0162] Using these test pieces, tensile tests were carried out in accordance with the provisions of JIS Z 2241, and the yield strength LYS of the flat plate portion in the tube axis direction was determined.f , yield strength CYS of the flat plate in the circumferential direction f , Yield strength CYS of the corner in the circumferential direction of the tube c The number of test pieces was set to 2 each, and the average value thereof was calculated to calculate each measured value.

[0163] (Determination of Cumulative Plastic Deformation Ratio)

[0164] In addition, the polygonal steel pipe is subjected to a 0 degree bending test and a 45 degree bending test. Figure 8 and Fig. 9 As shown in FIG. 1 , a partition plate 2 is welded through the central position of the long side of a polygonal steel pipe 1 as a test piece. The two ends of the test piece are supported by a support material 3 for pins (rotational support), and the horizontal and vertical movements are fixed. Thus, the test piece is moved in the 0 degree direction ( Figure 8 ) or 45 degrees ( Fig. 9 ) load and calculate the cumulative plastic deformation ratio.

[0165] It should be noted that the cumulative plastic deformation ratio refers to the value obtained by dividing the sum of the plastic rotation angles until the test piece is partially buckled or fractured and the endurance is sharply reduced by the reference rotation angle corresponding to the total plastic moment. The larger the value, the better the deformation performance when used as a column material (column component) and the higher the energy absorption capacity during an earthquake.

[0166] (Measurement of residual stress in the circumferential direction of the outer surface)

[0167] In addition, the residual stress in the tube circumferential direction of the outer surface was measured by X-ray diffraction.

[0168] Specifically, the residual stress in the circumferential direction of the tube was measured on the electrolytically polished area of ​​the following test piece under the following measuring equipment and measuring conditions. It should be noted that the measurement of the residual stress in the circumferential direction of the tube was carried out on all the flat plate parts, and the average value of the residual stress in the circumferential direction of the tube obtained on each flat plate part was taken as the residual stress in the circumferential direction of the polygonal steel pipe.

[0169] ·Measurement equipment: Portable X-ray residual stress measurement device manufactured by PULSTEC INDUSTRIAL (Model μ-X360n)

[0170] ·Measurement conditions: The X-ray used was Cr-Kα ray, the tube voltage was 30 kV, and the measurement was performed using the cosα method. The measured crystal plane was bcc-Fe(211), the Poisson's ratio was 0.280, and the elastic constant was 224000 MPa.

[0171] Test piece: A polygonal steel pipe with a length of 1000 mm in the pipe axial direction was taken, and the outer surface of the pipe was electrolytically polished to 100 μm at the center of the flat plate portion in the center of the longitudinal direction in the pipe circumferential direction.

[0172] The various conditions and the obtained results are shown in Tables 2 and 3.

[0173] [Table 2]

[0174]

[0175]

[0176] In Table 2 and Table 3, No. 1 to 6 are examples of the present invention, and No. 7 to 14 are comparative examples.

[0177] For the polygonal steel pipe of the present invention example, the yield strength (CYS f ) are the yield strength of the flat plate in the tube axis direction (LYS f ) is 0.83 to 1.20 times, and the yield strength (CYS c ) are the yield strength of the flat plate in the tube axis direction (LYS f ) is 0.90 to 1.30 times.

[0178] In Comparative Example No. 7, since the internal pressure p (MPa) in the internal pressure loading process is higher than the range of formula (1), the flatness of the polygonal steel pipe is greater than 3.0 mm, and the radius of curvature of the corner is greater than 4.0 times the wall thickness of the flat plate portion. Therefore, the yield strength of the flat plate portion in the circumferential direction of the tube relative to the yield strength of the flat plate portion in the axial direction and the yield strength of the corner portion in the circumferential direction of the tube are both higher than the range of the present invention, and the cumulative plastic deformation ratio at 0 degree bending and the cumulative plastic deformation ratio at 45 degree bending do not reach their respective desired values ​​(0 degree bending and 45 degree bending, cumulative plastic deformation ratio: 28 or more).

[0179] In Comparative Example No. 8, the radius of curvature of the polygonal steel pipe material is greater than 2.5 times the wall thickness of the flat plate portion. Therefore, the yield strength of the corner portion in the pipe circumferential direction relative to the yield strength of the flat plate portion in the pipe axial direction is lower than the range of the present invention, and the cumulative plastic deformation ratio when bent at 45 degrees does not reach the desired value.

[0180] In Comparative Example No. 9, the internal pressure p (MPa) in the internal pressure loading process is lower than the range of formula (1). Therefore, the flatness of the flat plate portion of the polygonal steel pipe is less than -3.0 mm, the yield strength of the flat plate portion in the circumferential direction relative to the yield strength of the flat plate portion in the axial direction is lower than the range of the present invention, and the cumulative plastic deformation ratio at 0 degree bending does not reach the desired value. In addition, the residual stress in the circumferential direction of the outer surface is less than -200 MPa.

[0181] In Comparative Example No. 10, since the internal pressure p (MPa) in the internal pressure loading process is higher than the range of formula (1), the radius of curvature of the corner of the polygonal steel pipe is larger than 4.0 times the wall thickness of the flat plate portion. Therefore, the yield strength of the corner in the pipe circumferential direction relative to the yield strength of the flat plate portion in the pipe axial direction is higher than the range of the present invention, and the cumulative plastic deformation ratio when bent at 45 degrees does not reach the desired value.

[0182] In Comparative Example No. 11, the internal pressure p (MPa) in the internal pressure loading process is lower than the range of formula (1). Therefore, the yield strength of the flat plate portion in the tube circumferential direction relative to the yield strength of the flat plate portion in the tube axial direction and the yield strength of the corner portion in the tube circumferential direction are both lower than the range of the present invention, and the cumulative plastic deformation ratio at 0 degree bending and the cumulative plastic deformation ratio at 45 degree bending do not reach their respective desired values. In addition, the residual stress in the tube circumferential direction of the outer surface is a value greater than 150 MPa.

[0183] In Comparative Example No. 12, since the flatness of the flat plate portion of the polygonal steel pipe billet is less than -10.0 mm, the yield strength of the flat plate portion in the circumferential direction of the tube relative to the yield strength of the flat plate portion in the axial direction of the tube is higher than the range of the present invention, and the cumulative plastic deformation ratio during 0 degree bending does not reach the desired value.

[0184] In Comparative Example No. 13, the flatness of the flat plate portion of the polygonal steel pipe billet was greater than -4.0 mm. Therefore, the yield strength of the flat plate portion in the circumferential direction of the tube relative to the yield strength of the flat plate portion in the axial direction was lower than the range of the present invention, and the cumulative plastic deformation ratio at 0 degree bending did not reach the desired value. In addition, the residual stress in the circumferential direction of the outer surface was less than -200 MPa.

[0185] In Comparative Example No. 14, the internal pressure p (MPa) in the internal pressure loading step was lower than the range of formula (1), and the radius of curvature of the corner of the polygonal steel pipe was less than 2.0 times the wall thickness of the flat plate portion. Therefore, the yield strength of the corner in the pipe circumferential direction relative to the yield strength of the flat plate portion in the pipe axial direction was lower than the range of the present invention, and the cumulative plastic deformation ratio during 45-degree bending did not reach the desired value.

[0186] Explanation of symbols

[0187] 1 Polygonal steel pipe

[0188] 2 Through the bulkhead

[0189] 3 Support material

[0190] 7 Electric welded steel pipe

[0191] 8 Sizing roller

[0192] 9 angle forming roller set

[0193] 10 Polygonal steel pipe

[0194] 10' Polygonal Steel Pipe Billet

[0195] 11 Flat Plate

[0196] 12 corners

[0197] 13 Welding section (electric seam welding section)

[0198] 17 Through the bulkhead

[0199] 18 Beam

[0200] 19 Liang

[0201] 20 columns

[0202] 100 Building structures

Claims

1. A polygonal steel pipe having a plurality of flat portions and corner portions alternately arranged in the pipe circumferential direction and having a welded portion extending in the pipe axial direction, The yield strength of the flat plate portion in the tube circumferential direction is 0.83 to 1.20 times the yield strength of the flat plate portion in the tube axial direction. The yield strength of the corner portion in the tube circumferential direction is 0.90 to 1.30 times the yield strength of the flat plate portion in the tube axial direction.

2. The polygonal steel pipe according to claim 1, wherein: The residual stress of the outer surface of the flat plate portion in the circumferential direction of the tube is -200 MPa to 150 MPa. Wherein, a positive value of the residual stress indicates tensile stress, and a negative value indicates compressive stress.

3. The polygonal steel pipe according to claim 1 has the following composition: It contains, by mass%, C: 0.020-0.350%, Si: 0.01-0.65%, Mn: 0.30-2.50%, P: 0.050% or less, S: 0.0500% or less, Al: 0.005-0.100% and N: 0.0100% or less, and the remainder is Fe and inevitable impurities.

4. The polygonal steel pipe according to claim 2 has the following composition: It contains, by mass%, C: 0.020-0.350%, Si: 0.01-0.65%, Mn: 0.30-2.50%, P: 0.050% or less, S: 0.0500% or less, Al: 0.005-0.100% and N: 0.0100% or less, and the remainder is Fe and inevitable impurities.

5. The polygonal steel pipe according to claim 3, wherein: The component composition further contains, in mass %, one or more selected from the group consisting of 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, Ca: 0.0100% or less, and B: 0.0100% or less.

6. The polygonal steel pipe according to claim 4, wherein: The component composition further contains, in mass %, one or more selected from the group consisting of 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, Ca: 0.0100% or less, and B: 0.0100% or less.

7. A method for manufacturing a polygonal steel pipe, which is the method for manufacturing a polygonal steel pipe according to any one of claims 1 to 6, comprising the following steps: In the pipe making process, the steel plate is formed into a cylindrical shape by cold rolling, and the circumferential ends of the cylindrical shape are butted together and then welded by electric welding; An angle forming step is performed after the pipe making step, and uses an angle forming machine frame to form a polygonal steel pipe billet with a flatness of -10.0 mm to -4.0 mm for the flat plate portion and a curvature radius of the corner portion of 1.0 to 2.5 times the wall thickness of the flat plate portion; An internal pressure loading step, then, applying an internal pressure p (MPa) satisfying the following formula (1) to the inner surface of the polygonal steel tube billet to form a polygonal steel tube having a flatness of -3.0 mm to 3.0 mm at the flat plate portion and a curvature radius of the corner portion of 2.0 to 4.0 times the wall thickness of the flat plate portion; N(MPa)<p≤N×1.5(MPa)···(1) in, N=(wall thickness of polygonal steel pipe material (mm) / side length of polygonal steel pipe material (mm))×yield strength of the flat plate portion of the polygonal steel pipe material in the circumferential direction of the pipe (MPa).

8. A building structure comprising the polygonal steel pipe according to any one of claims 1 to 6 as a column material.

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