CN116323065A - Square steel pipe, method for manufacturing same, and building structure - Google Patents

Abstract

The invention provides a square steel pipe, a manufacturing method thereof, and a building structure. The present invention is a rectangular steel pipe having a flat plate portion and a corner portion, wherein when the average wall thickness of the flat plate portion is t (mm), the radius of curvature R outside the corner portion is not less than 2.0t and not more than 3.0t, and the thickness of the flat plate portion is The flatness of the outer surface is 2.5 mm or less, and the uniform elongation E2 at a position 1/4t away from the outer surface of the corner part in the direction of wall thickness is 1/4t away from the outer surface of the flat part in the direction of wall thickness. The uniform elongation E1 at the position is 0.60 times or more, and the Charpy absorbed energy at -10° C. at a position 1/4t away from the outer surface of the corner in the wall thickness direction is 100 J or more.

Description

Square steel pipe and its manufacturing method and building structure

Technical Field

The present invention relates to a square steel pipe which is particularly suitable for use as a building member in large buildings such as mid-rise buildings, factories, warehouses, etc. having a height greater than 20 m and a method for manufacturing the same. In addition, the present invention relates to a building structure using the square steel pipe of the present invention as a column material.

Background Art

From the viewpoint of earthquake resistance, high ductility and toughness are required for building columns.

When a square steel pipe having corners and flat portions used as a column is subjected to a large external force such as an earthquake force, the outer surface of the corners, in particular, is greatly deformed. Therefore, the ductility and toughness of the outer surface of the corners of the square steel pipe must be sufficiently improved.

Cold roll-formed square steel pipe (roll-formed square steel pipe) is a square steel pipe widely used as a column material for buildings. The square steel pipe is manufactured by the following method: a steel strip is formed into a cylindrical open pipe by cold roll forming, the butt joint of the open pipe is subjected to resistance welding to form a resistance welded steel pipe, and then, the resistance welded steel pipe is reduced in diameter along the pipe axis direction in a cylindrical state using rollers arranged above, below, left and right of the resistance welded steel pipe, and then formed into a square. In the above-mentioned resistance welding, the butt joint is heated to melt, pressed and solidified, thereby completing the joining.

However, although roll-formed square steel pipes have high productivity, they have a problem in that the corners are greatly work-hardened during production, and therefore the ductility and toughness of the corners are lower than those of the flat plate.

In addition, from the perspective of construction site construction and building design, the square steel pipe used for the column is also required to have a smaller curvature radius at the corner. This is because when the area of the flat plate portion of the column is large, the area that can be joined to the beam is large, which allows for a higher degree of freedom in architectural design.

However, for the roll-formed square steel tube, the larger the ratio of the average wall thickness t to the average side length H (i.e., t/H), the greater the circumferential bending strain required for forming the steel strip, and the greater the work hardening amount of the corner. In addition, the smaller the radius of curvature of the corner, the greater the circumferential bending strain required for forming the corner, and the greater the work hardening amount of the corner. Therefore, in the roll-formed square steel tube in which the ratio of the average wall thickness t to the average side length H (t/H) is large and the radius of curvature of the corner is small, the ductility and toughness of the corner are particularly low, and it is difficult to ensure sufficient seismic performance.

Here, the "average wall thickness t" refers to the average value of the wall thickness (mm) at the center position of the tube circumference of the three flat plate portions excluding the flat plate portion including the welded portion (resistance welded portion). The "average side length H" refers to the average value of the side lengths of the two adjacent flat plate portions sandwiching the corner portion.

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

Patent Document 1 proposes that a semi-formed square steel pipe is formed by bending a steel plate to which vanadium is added as a chemical component, welding the semi-formed square steel pipe, heating the semi-formed square steel pipe to the vicinity of the _A3 transformation point, hot forming the semi-formed square steel pipe, and cooling the semi-formed square steel pipe. It is disclosed that the square steel pipe has improved strength and toughness and has a sharp corner shape.

Patent Document 2 proposes a square steel pipe obtained by heat-treating a cold-formed portion. It is disclosed that the square steel pipe has improved mechanical properties and weldability of the cold-formed portion.

Patent Document 3 proposes a square steel pipe in which the toughness and plastic deformation ability of the corners are improved by appropriately controlling the chemical composition of the raw steel plate, the bainite fraction of the metal structure, and the Vickers hardness of the surface layer of the corners.

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

Therefore, for roll-formed square steel pipes, it is also required to establish a technology for improving shape characteristics, especially a technology for achieving both improvement in flatness of the flat plate portion and reduction in the radius of curvature of the corner portion. In response to this requirement, for example, Patent Documents 5 and 6 propose technologies for improving shape characteristics by adjusting manufacturing conditions during roll forming.

Specifically, Patent Document 5 proposes the following method for forming a square steel pipe: when a steel pipe is formed into a square pipe using three or four stages of square forming rolls and the reduction rate of the final stage roll is kept constant, as the wall thickness/outer diameter ratio of the steel pipe increases, the diameter of the final stage roll is reduced (from convex to concave) to perform forming.

Patent document 6 proposes a method for manufacturing a structural square tube, which includes: a first forming process, in which when a cylindrical billet tube is roll-formed into a square tube, the outer diameter of the billet tube is set to D, the wall thickness of the billet tube is set to t, and the maximum diameter height is set to H, and the set pressing rate Q defined by Q=(D-H)/(D-t)×100 is maintained in the range of 12 to 23% to form the billet tube into a rectangular cross-sectional shape; and a forming process after the second stage, in which the billet tube formed into a rectangular cross-sectional shape is formed into a target shape.

Prior art 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 Application Laid-Open No. 4-224023

Patent Document 6: Japanese Patent No. 3197661

Summary of the invention

Problem to be solved by the invention

However, the square steel pipes described in Patent Documents 1 and 2 require a heating process during or after forming, and therefore are very expensive compared to cold-formed roll-formed square steel pipes. Therefore, it is required to establish a technology for obtaining a desired square steel pipe without requiring a heating process during or after forming.

Furthermore, the square steel pipes described in Patent Documents 3 and 4 cannot sufficiently suppress a decrease in uniform elongation at the corners due to work hardening during forming, and therefore cannot be said to be able to sufficiently ensure ductility and toughness of the outer surface of the corners.

Furthermore, the techniques described in Patent Documents 5 and 6 cannot perform forming while suppressing the work hardening of the corners. Therefore, it cannot be said that they are sufficient as a technique for achieving both improved flatness of the flat plate portion of the square steel pipe and reduced curvature radius of the corners, while sufficiently ensuring ductility and toughness of the outer surface of the corners.

The present invention is completed in view of the above situation, and its purpose is to provide a square steel pipe with excellent shape characteristics, ductility and toughness of the outer surface of the corner and a manufacturing method thereof, and to provide a building structure with excellent earthquake resistance.

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

The above-mentioned “small curvature radius of the corner” means that the curvature radius R of the outer side of the corner is controlled within a specified range. Specifically, when the average wall thickness of the flat plate portion is set to t (mm), the curvature radius R of the outer side of the corner is greater than 2.0t and less than 3.0t.

The above-mentioned "flatness of the flat plate portion" means that the flatness of the outer surface of the flat plate portion in the tube axis direction is less than 2.5 mm. Specifically, in the cross-section of the surface perpendicular to the tube axis direction, the absolute value represented by the maximum bulge amount and the maximum depression amount relative to a straight line passing through two points at both ends of the circumference on the same side of the outer surface of the flat plate portion is less than 2.5 mm at most (refer to Figure 10 described later).

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

In the present invention, “the outer surface of the corner portion has excellent toughness” means that the Charpy absorbed energy of the corner portion at -10°C at a position 1/4t from the outer surface of the corner portion in the wall thickness direction is 100 J or more.

It should be noted that the above-mentioned radius of curvature, flatness, uniform elongation and toughness can be measured by the methods described in the examples described later.

Methods used to solve problems

The present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, have found that by managing the plate width of the raw steel plate and the circumference of the electric resistance welded steel pipe on the inlet side of the square forming stand within an appropriate range relative to the circumference of the square steel pipe on the outlet side of the square forming stand, it is possible to manufacture a square steel pipe having a small radius of curvature of the corner, a flat plate portion, and excellent ductility and toughness of the outer surface of the corner.

The present invention has been accomplished based on the above findings and includes the following gist.

[1] A square steel pipe having a flat portion and a corner portion, wherein:

When the average thickness of the flat plate portion is t (mm), the radius of curvature R of the outer side of the corner portion is greater than or equal to 2.0t and less than or equal to 3.0t,

The flatness of the outer surface of the flat plate portion is less than 2.5 mm.

The uniform elongation E2 at a position 1/4t from the outer surface of the corner portion in the wall thickness direction is 0.60 times or more relative to the uniform elongation E1 at a position 1/4t from the outer surface of the flat plate portion in the wall thickness direction,

The Charpy absorbed energy at -10°C at a position 1/4t away from the outer surface of the corner portion in the wall thickness direction is 100 J or more.

[2] The square steel pipe according to [1], wherein the average wall thickness t is greater than 0.030 times the average side length H (mm) of the flat plate portion.

[3] The square steel pipe according to [1] or [2], wherein the average wall thickness t is greater than or equal to 20 mm and less than or equal to 40 mm.

[4] The square steel pipe according to any one of [1] to [3], wherein the yield strength of the flat plate portion is 295 MPa or more, the tensile strength of the flat plate portion is 400 MPa or more, and the yield ratio of the corner portion is 90% or less.

[5] The square steel pipe according to any one of [1] to [4], wherein the composition of the square steel pipe comprises, in mass%, 0.020 to 0.45% C, 0.01 to 1.0% Si, 0.30 to 3.0% Mn, 0.10% or less P, 0.050% or less S, 0.005 to 0.10% Al, 0.010% or less N, 0.001 to 0.15% Ti, and the balance Fe and inevitable impurities.

In the steel structure at the center of the wall thickness of the flat plate portion, the total volume fraction of ferrite and bainite is 70% or more and 95% or less relative to the entire steel structure at the center of the wall thickness of the flat plate portion, and the remainder is composed of one or more selected from pearlite, martensite, and austenite.

When the region surrounded by the boundary where the orientation difference between adjacent crystals is 15° or more is defined as a crystal grain,

The average grain size of the above-mentioned grains is 15.0 μm or less,

The total volume ratio of the crystal grains having a grain size of 40 μm or more is 40% or less with respect to the entire steel structure at the center of the thickness of the flat plate portion.

[6] The square steel pipe according to any one of [1] to [5], which, in addition to the above component composition, further contains, in terms of mass%, one or more selected from the group consisting of Nb: 0.001 to 0.15%, V: 0.001 to 0.15%, Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 1.0%, Ni: 0.01 to 1.0%, Ca: 0.0002 to 0.010%, and B: 0.0001 to 0.010%.

[7] A method for manufacturing a square steel pipe, which is the method for manufacturing a square steel pipe according to any one of [1] to [6], wherein:

The steel plate is cold rolled and both ends of the steel plate in the width direction are resistance welded to form a resistance welded steel pipe. Then, the resistance welded steel pipe is reduced in diameter by a sizing stand and then squared by a square forming stand to produce a square steel pipe.

The gap between the rollers of the sizing stand and the gap between the rollers of the square forming stand immediately before square forming are controlled so that the ratio of the plate width W of the steel plate to the circumference C _OUT of the square steel pipe on the outlet side of the square forming stand satisfies the formula (1), and the ratio of the circumference C _IN of the electric resistance welded steel pipe on the inlet side of the square forming stand to the circumference C _OUT of the square steel pipe on the outlet side of the square forming stand satisfies the formula (2).

1.000+0.050×t/H＜W/C _OUT ＜1.000+0.50×t/H…Formula (1)

0.30×t/H+0.99≤C _IN /C _OUT ＜0.50×t/H+0.99…Equation (2)

Here, in formula (1) and formula (2),

W: the width of the steel plate as the raw material (mm),

C _IN : circumference of the electric resistance welded steel pipe at the inlet side of the first section of the square forming rack (mm),

C _OUT : The circumference of the square steel pipe at the outlet side of the final stage square forming machine frame (mm), t: The average wall thickness of the flat plate after square forming (mm),

H: average side length of the flat plate portion after square forming (mm).

Wherein, in the case of using a square forming frame of one stage to perform square forming, the square forming frame of the first stage and the square forming frame of the final stage refer to the same square forming frame.

[8] The method for manufacturing a square steel pipe according to [7], wherein the steel plate is obtained by heating a steel raw material to a heating temperature of 1100°C to 1300°C, and then performing hot rolling treatment with a rough rolling end temperature of 850°C to 1150°C, a finish rolling end temperature of 750°C to 900°C, and a total reduction ratio of 50% or more at a temperature below 950°C.

Next, cooling is performed under the conditions that the average cooling rate is 5°C/s to 30°C/s at the wall thickness center temperature and the cooling stop temperature is 400°C to 650°C.

Next, coiling is performed at 400°C to 650°C.

[9] The method for manufacturing a square steel pipe according to [7] or [8], wherein the average wall thickness t is greater than 0.030 times the average side length H of the flat plate portion.

[10] The method for producing a square steel pipe according to any one of [7] to [9], wherein the average wall thickness t is greater than or equal to 20 mm and less than or equal to 40 mm.

[11] A building structure, wherein the square steel pipe according to any one of [1] to [6] is used as a column material.

Effects of the Invention

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

Thus, a cold rolled square steel pipe having a small curvature radius at the corner, a flat flat portion, and excellent ductility and toughness of the outer surface of the corner can be manufactured. In addition, a building structure using the square steel pipe of the present invention as a column material exhibits better earthquake resistance than a building structure using a conventional cold rolled square steel pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a cross section perpendicular to the tube axis direction of a square steel tube according to the present invention.

FIG. 2 is a schematic diagram showing a pipe manufacturing process of an electric resistance welded steel pipe in the present invention.

FIG. 3 is a schematic diagram showing a forming process of a square steel pipe according to the present invention.

FIG. 4 is a schematic diagram for explaining a melt-solidified portion of a weld portion of an electric resistance welded steel pipe.

FIG. 5 is a schematic diagram showing an example of the building structure of the present invention.

FIG. 6 is a schematic diagram showing the positions at which the tensile test specimens at the flat plate portion and the corner portion are cut out in the present invention.

FIG. 7 is a schematic diagram showing the detailed cutting positions of the tensile test specimens at the corners implemented in the present invention.

FIG. 8 is a schematic diagram showing the cutting position of the Charpy test piece at the corner portion implemented in the present invention.

FIG. 9 is a schematic diagram showing detailed cutting positions of the Charpy test piece at the corner portion implemented in the present invention.

FIG. 10 is a schematic diagram for explaining a method for measuring flatness implemented in the present invention.

DETAILED DESCRIPTION

The present invention will be described with reference to the accompanying drawings. It should be noted that the present invention is not limited to the embodiments.

＜Square steel pipe＞

The present invention is a square steel pipe having a flat portion and a corner portion, wherein, when the average wall thickness of the flat portion is set to t (mm), the radius of curvature R of the outer side of the corner portion is greater than or equal to 2.0t and less than or equal to 3.0t, the flatness of the outer surface of the flat portion in the pipe axis direction is less than or equal to 2.5mm, the uniform elongation E2 at a position 1/4t from the outer surface of the corner portion in the wall thickness direction is greater than or equal to 0.60 times the uniform elongation E1 at a position 1/4t from the outer surface of the flat portion in the wall thickness direction, and the Charpy absorbed energy at -10°C at a position 1/4t from the outer surface of the corner portion in the wall thickness direction is greater than or equal to 100J.

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

In the square steel pipe 10 of the present invention, two or more flat plate portions 11 and corner portions 12 are alternately formed along the circumferential direction of the pipe. In the example shown in FIG1 , in the square steel pipe 10, four corner portions 12 and four flat plate portions 11 are sequentially formed along the circumferential direction of the pipe. The square steel pipe 10 is rectangular (approximately rectangular) or square (approximately square) when viewed from a cross section perpendicular to the pipe axis direction. In FIG1 , when the side lengths of two adjacent flat plate portions 11 sandwiching the corner portion 12 are H ₁ and H ₂ , H ₁ > H ₂ , that is, the side length H ₂ of the flat plate portion opposite to the welded portion (resistance welded portion) 13 described later is shorter than the side length H ₁ of the adjacent flat plate portion 11. In the present invention, it is not limited to this example, and the relationship may be H ₁ = H ₂ , or H ₁ < H ₂ .

The square steel pipe 10 is manufactured by forming a blank tube of a resistance welded steel pipe and forming the blank tube into a roll-formed square steel pipe. Therefore, the square steel pipe 10 has a resistance welded portion 13 extending in the tube axis direction, which forms a flat plate portion 11. Although not shown in the figure, the width of the molten solidified portion of the resistance welded portion 13 in the tube circumferential direction is 1.0 μm or more and 1000 μm or less in the total thickness of the tube.

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

When the radius of curvature R of the outer side of the corner is less than 2.0t, the circumferential bending strain of the corner increases when the steel strip is formed. As a result, the ductility and toughness targeted by the present invention cannot be obtained at the corner. On the other hand, when the radius of curvature R of the outer side of the corner is greater than 3.0t, the circumferential bending response of the flat plate portion of the square forming machine frame (and the circumferential bending strain of the corner) decreases. As a result, the flatness targeted by the present invention cannot be obtained at the flat plate portion. The above-mentioned radius of curvature R is preferably greater than 2.2t and preferably less than 2.9t.

It should be noted that in the present invention, as described in the examples described later, when the curvature radius of two or more locations is measured and the maximum and minimum values are within the above range, it is evaluated that the curvature radius R of the outer side of the corner is small. The reason for such evaluation is that the R of the corner of the square steel pipe acts independently on the earthquake resistance and construction performance, not the average value of the four locations.

The curvature radius R of the outer side of the corner refers to the curvature radius of the intersection point P of the straight lines (extension lines) L1 and L2 extended from the outer surfaces of the flat plate portion 11 on both sides adjacent to the corner 12 (the upper right corner in the example of Figure 1), the straight line L forming an angle of 45° with the extension line L1 or L2, and the curve on the outer side of the corner 12, as shown in Figure 1.

The measurement of the curvature radius R is performed within a range of a central angle of 65° centered at the intersection point B between the straight line L and the outer surface of the corner 12, in a sector having a central angle of 90° formed by the connecting points of the extended lines L1 and L2 and the flat portion 11 and the corner 12 (points A and A' shown in FIG1 ) and the outer surface of the corner 12, and having its center on the straight line L. The method for measuring the curvature radius can be, for example, a method of measuring the curvature radius by a radial measuring instrument that fully matches the outer surface of the corner 12 within the range of the central angle of 65°, but the measurement can also be performed by other methods.

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

The flatness is described using Figure 10. As shown in Figure 10, the flatness is a value obtained by measuring the maximum bulge and the maximum depression relative to a straight line passing through two points at both ends of the circumferential direction on the same side of the outer surface of the flat plate portion in a cross section of a plane perpendicular to the tube axis direction. It should be noted that the flatness is obtained by the method described in the embodiments described later in the present invention.

When the above-mentioned flatness is greater than 2.5mm, the compression resistance of the square steel pipe during bending deformation is reduced. As a result, the seismic resistance of the square steel pipe is reduced. In addition, the joint surface with the beam material is greatly bent, so it is difficult to weld and join. As a result, the construction performance is reduced. The smaller the value of flatness, the better. There is no need to specify the lower limit of flatness, and 0.6mm can be allowed as the lower limit of flatness. The lower limit of flatness is preferably 0.2mm, and more preferably 0mm. Preferably, it is less than 2.0mm, and more preferably less than 1.5mm.

In the square steel pipe 10 of the present invention, the uniform elongation E2 at a position 1/4t from the outer surface of the corner portion in the wall thickness direction is 0.60 times or more the uniform elongation E1 at a position 1/4t from the outer surface of the flat plate portion in the wall thickness direction.

When a square steel pipe is subjected to a large external force such as an earthquake force, the outer surface of the corners, in particular, deforms significantly. Therefore, the ductility and toughness of the outer surface of the corners of the square steel pipe must be sufficiently improved.

When the value of the uniform elongation E2 at a position 1/4t from the outer surface of the corner in the wall thickness direction relative to the uniform elongation E1 at a position 1/4t from the outer surface of the flat plate in the wall thickness direction (the value of E2/E1) is less than 0.60, the ductility on the outer surface side of the corner is reduced. As a result, the seismic resistance of the square steel pipe is reduced. The value of E2/E1 is preferably greater than 0.70, more preferably greater than 0.80, and further preferably greater than 0.82. The upper limit of the value of E2/E1 is not particularly specified, and is less than 1.00 from the viewpoint that the amount of work hardening during roll forming of the corner is large compared to the flat plate and the uniform elongation is small.

In addition, in the square steel pipe 10 of the present invention, the Charpy absorbed energy of the corner 12 at -10°C at a position 1/4t from the outer surface of the corner 12 in the wall thickness direction is 100 J or more. When the Charpy absorbed energy is less than 100 J, the risk of brittle fracture without plastic deformation when subjected to a large external force such as an earthquake force increases. The above-mentioned Charpy absorbed energy is preferably 150 J or more, and more preferably 200 J or more.

It should be noted that the square steel pipe 10 of the present invention preferably has the following structure in addition to the above structure.

When the average thickness of the flat plate portion of the square steel pipe 10 is t (mm) and the average side length of the flat plate portion is H (mm), the average thickness t is preferably set to be greater than 0.030 times the average side length H.

As described above, in a square steel pipe, as the ratio (t/H) of the average wall thickness t to the average side length H increases and the radius of curvature of the corner decreases, the circumferential bending strain required to form the corner increases, and the bending deformation of the corner increases. As a result, in a square steel pipe with a large ratio (t/H), the ductility and toughness of the corner tend to decrease.

When the value of the above ratio (t/H) is 0.030 or less, the endurance as a column material is reduced, so the building structures to which it can be applied are limited. Therefore, the above ratio (t/H) is preferably set to be greater than 0.030. It is more preferably 0.035 or more, and further preferably 0.040 or more. On the other hand, in order to ensure the ductility and toughness of the corner, the upper limit of the above ratio (t/H) is preferably 0.10. It is more preferably 0.080 or less.

Here, the average thickness t (mm) is calculated by the following formula (3).

t＝(t ₁ +t ₂ +t ₃ )/3…Formula (3)

In formula (3), t ₁ and t ₂ are the wall thickness (mm) of the two adjacent flat plate portions 11 sandwiching the corner portion 12 with respect to the flat plate portion 11 including the welded portion (resistance welded portion) 13 at the center position in the tube circumferential direction, and t ₃ is the wall thickness (mm) of the flat plate portion opposite to the flat plate portion including the welded portion (resistance welded portion) at the center position in the tube circumferential direction. That is, the average wall thickness t is the average value of the wall thickness of the three flat plate portions excluding the flat plate portion including the welded portion at the center position in the tube circumferential direction (see FIG. 1 ).

The average side length H (mm) is calculated by the following formula (4).

H＝(H ₁ +H ₂ )/2…Formula (4)

In formula (4), _H1 : the side length of a cross section perpendicular to the tube axis direction of any flat plate portion (the vertical side length in FIG. 1) (mm), _H2 : the side length of the flat plate portion adjacent to the flat plate portion with the side length _H1 sandwiching the corner portion (the horizontal side length in FIG. 1) (mm). That is, the average side length H is the average of the side lengths of the cross sections perpendicular to the tube axis direction of two adjacent flat plate portions 11 sandwiching the corner portion.

In addition, for the square steel pipe 10 of the present invention, in particular, from the viewpoint of being suitable for use in large buildings such as medium-rise buildings, factories, warehouses, etc. with a height greater than 20m, the average wall thickness t is preferably 20mm or more and 40mm or less. From the viewpoint of being suitable for use in medium-rise buildings and large buildings, the yield strength of the flat plate portion 11 is preferably 295MPa or more, and the tensile strength of the flat plate portion 11 is preferably 400MPa or more. From the viewpoint of better earthquake resistance, the yield ratio of the corner portion 12 is preferably 90% or less.

More preferably, the yield strength of the flat plate portion 11 is 320 MPa or more, the tensile strength of the flat plate portion 11 is 410 MPa or more, and the yield ratio of the corner portion 12 is 89.5% or less. In addition, the yield strength of the flat plate portion 11 is 500 MPa or less, the tensile strength of the flat plate portion 11 is 600 MPa or less, and the yield ratio of the corner portion 12 is 80.0% or more.

The yield strength, tensile strength and yield ratio can be obtained by performing a tensile test according to JIS Z 2241 as described in the examples below. The Charpy absorbed energy can be obtained by performing a Charpy impact test at a test temperature of -10°C using a V-notch standard test piece according to JIS Z 2242 as described in the examples below.

Next, from the viewpoint of ensuring the above-mentioned mechanical properties and weldability, preferred ranges of the chemical composition and steel structure of the square steel pipe 10 of the present invention and reasons for their limitation are described.

First, the composition will be described. The square steel pipe 10 of the present invention preferably has a composition containing, by mass%, C: 0.020-0.45%, Si: 0.01-1.0%, Mn: 0.30-3.0%, P: 0.10% or less, S: 0.050% or less, Al: 0.005-0.10%, N: 0.010% or less, Ti: 0.001-0.15%, and the balance Fe and inevitable impurities.

In this specification, "%" indicating the steel composition means "mass %" unless otherwise specified. The following chemical compositions are chemical compositions of the flat plate portion and the corner portion excluding the welded portion of the square steel pipe.

C: 0.020～0.45%

C is an element that increases the strength of steel by solid solution strengthening. In addition, C is an element that contributes to the refinement of the structure by lowering the starting temperature of ferrite transformation. In order to obtain such an effect, 0.020% or more of C is contained. In addition, C promotes the formation of pearlite, improves hardenability, contributes to the formation of martensite, and contributes to the stabilization of austenite, so it is an element that also contributes to the formation of hard phases. When the C content is greater than 0.45%, the proportion of hard phases increases, the toughness decreases, and the weldability also deteriorates. Therefore, the C content is set to 0.020-0.45%. The C content is preferably 0.040% or more, and more preferably 0.050% or more. In addition, the C content is preferably 0.40% or less, and more preferably 0.30% or less.

Si: 0.01～1.0%

Si is an element that increases the strength of steel by solid solution strengthening. In order to obtain such an effect, 0.01% or more of Si is contained. However, when the Si content is greater than 1.0%, oxides are easily generated in the resistance weld portion, and the characteristics of the weld portion are reduced. In addition, the yield ratio of the parent material portion other than the resistance weld portion increases, and the toughness decreases. Therefore, the Si content is set to 0.01-1.0%. The Si content is preferably greater than 0.02%, and more preferably greater than 0.05%. In addition, the Si content is preferably less than 0.50%, and more preferably less than 0.40%.

Mn: 0.30～3.0%

Mn is an element that increases the strength of steel by solid solution strengthening. In addition, Mn is an element that contributes to the refinement of the structure by lowering the starting temperature of ferrite transformation. In order to obtain such an effect, 0.30% or more of Mn is contained. However, when the Mn content is greater than 3.0%, oxides are easily generated in the resistance welded portion, and the characteristics of the welded portion are reduced. In addition, due to solid solution strengthening and refinement of the structure, the yield stress increases and the desired yield ratio cannot be obtained. Therefore, the Mn content is set to 0.30-3.0%. The Mn content is preferably greater than 0.40%, and more preferably greater than 0.50%. In addition, the Mn content is preferably less than 2.5%, and more preferably less than 2.0%.

P: 0.10% or less

P segregates at grain boundaries and causes inhomogeneity of the material. Therefore, as an inevitable impurity, it is preferably reduced as much as possible, but 0.10% or less is allowed. Therefore, the P content is set to 0.10% or less. The P content is preferably 0.050% or less, and more preferably 0.030% or less. It should be noted that there is no special lower limit for P, and excessive reduction will lead to a rise in smelting costs. Therefore, the P content is preferably set to 0.002% or more.

S: 0.050% or less

S usually exists in the form of MnS in steel, but MnS is thinly extended in the hot rolling process and has an adverse effect on ductility. Therefore, in the present invention, it is preferred to reduce S as much as possible, but 0.050% or less is allowed. Therefore, the S content is set to 0.050% or less. The S content is preferably 0.030% or less, and more preferably 0.010% or less. It should be noted that there is no special lower limit for S, and excessive reduction will lead to a rise in smelting costs. Therefore, S is preferably set to 0.0002% or more.

Al: 0.005～0.10%

Al is an element that acts as a strong deoxidizer. In order to obtain such an effect, it is necessary to contain 0.005% or more of Al. However, when the Al content is greater than 0.10%, weldability deteriorates, and the number of alumina inclusions increases, deteriorating the surface properties. In addition, the toughness of the weld is also reduced. Therefore, the Al content is set to 0.005-0.10%. The Al content is preferably greater than 0.010%, more preferably greater than 0.015%. The Al content is preferably less than 0.080%, more preferably less than 0.070%.

N: 0.010% or less

N is an inevitable impurity and an element that has the effect of reducing toughness by firmly fixing the movement of dislocations. In the present invention, N as an impurity is preferably reduced as much as possible, but the N content can be allowed to be less than 0.010%. Therefore, the N content is set to less than 0.010%. The N content is preferably less than 0.0080%. From the perspective of refining cost, the N content is preferably 0.0008% or more.

Ti: 0.001～0.15%

Ti is an element that contributes to improving the strength of steel by forming fine carbides and nitrides in steel. In addition, it has a high affinity with N, and therefore is an element that contributes to improving the toughness of steel by rendering N in the steel harmless in the form of nitrides. In order to obtain the above-mentioned effects, it is preferred to contain 0.001% or more of Ti. However, when the Ti content is greater than 0.15%, the yield ratio increases and the toughness decreases. Therefore, the Ti content is set to 0.15% or less. The Ti content is more preferably 0.002% or more, and further preferably 0.005% or more. The Ti content is more preferably 0.10% or less, and further preferably 0.08% or less.

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

In the present invention, the above components are preferably used as the basic component composition. The above preferred elements can obtain the properties targeted by the present invention, but in order to further improve the properties, one or more selected from Nb: 0.001-0.15%, V: 0.001-0.15%, Cr: 0.01-1.0%, Mo: 0.01-1.0%, Cu: 0.01-1.0%, Ni: 0.01-1.0%, Ca: 0.0002-0.010%, B: 0.0001-0.010% can be contained as needed.

Nb: 0.001～0.15%

Nb is an element that contributes to the improvement of the strength of steel by forming fine carbides and nitrides in steel, and also contributes to the refinement of the structure by inhibiting the coarsening of austenite during hot rolling, and can be contained as needed. In order to obtain the above-mentioned effect, when Nb is contained, it is preferred to contain 0.001% or more of Nb. However, when the Nb content is greater than 0.15%, the yield ratio increases and the toughness decreases. Therefore, when Nb is contained, the Nb content is preferably set to 0.15% or less. The Nb content is more preferably 0.002% or more, and further preferably 0.005% or more. The Nb content is more preferably 0.10% or less, and further preferably 0.08% or less.

V: 0.001～0.15%

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

Cr: 0.01～1.0%

Cr is an element that improves the hardenability of steel and increases the strength of steel, and can be contained as needed. In order to obtain the above-mentioned effect, when Cr is contained, the Cr content is preferably set to 0.01% or more. On the other hand, the inclusion of more than 1.0% Cr may lead to a decrease in toughness and deterioration of weldability. Therefore, when Cr is contained, the Cr content is preferably set to 1.0% or less. The Cr content is more preferably 0.02% or more, and further preferably 0.05% or more. In addition, the Cr content is more preferably 0.90% or less, and further preferably 0.80% or less.

Mo: 0.01～1.0%

Mo is an element that improves the hardenability of steel and increases the strength of steel, and can be contained as needed. In order to obtain the above-mentioned effect, when Mo is contained, the Mo content is preferably set to 0.01% or more. On the other hand, the inclusion of more than 1.0% Mo may lead to a decrease in toughness and deterioration of weldability. Therefore, when Mo is contained, the Mo content is preferably set to 1.0% or less. The Mo content is more preferably 0.02% or more, and further preferably 0.05% or more. In addition, the Mo content is more preferably 0.90% or less, and further preferably 0.80% or less.

Cu: 0.01～1.0%

Cu is an element that increases the strength of steel by solid solution strengthening, and can be contained as needed. In order to obtain the above-mentioned effect, when Cu is contained, the Cu content is preferably set to 0.01% or more. On the other hand, the inclusion of more than 1.0% Cu may lead to a decrease in toughness and deterioration of weldability. Therefore, when Cu is contained, the Cu content is preferably set to 1.0% or less. The Cu content is more preferably 0.02% or more, and further preferably 0.05% or more. In addition, the Cu content is more preferably 0.80% or less, and further preferably 0.60% or less.

Ni: 0.01～1.0%

Ni is an element that increases the strength of steel by solid solution strengthening, and can be contained as needed. In order to obtain the above-mentioned effect, when Ni is contained, the Ni content is preferably set to 0.01% or more. On the other hand, the inclusion of more than 1.0% Ni may lead to a decrease in toughness and deterioration of weldability. Therefore, when Ni is contained, the Ni content is preferably set to 1.0% or less. The Ni content is more preferably 0.02% or more, and further preferably 0.05% or more. In addition, the Ni content is more preferably 0.80% or less, and further preferably 0.60% or less.

Ca: 0.0002～0.010%

Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly stretched in the hot rolling process in the manufacture of raw steel plates, and can be contained as needed. In order to obtain the above-mentioned effect, when Ca is contained, it is preferred to contain 0.0002% or more of Ca. However, when the Ca content is greater than 0.010%, Ca oxide clusters are formed in the steel, and the toughness deteriorates. Therefore, when Ca is contained, the Ca content is preferably set to 0.010% or less. The Ca content is more preferably 0.0005% or more, and further preferably 0.0010% or more. In addition, the Ca content is more preferably 0.008% or less, and further preferably 0.0060% or less.

B: 0.0001～0.010%

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, when B is contained, it is preferred to contain 0.0001% or more of B. However, when the B content is greater than 0.010%, the yield ratio increases and the toughness deteriorates. Therefore, when B is contained, the B content is preferably set to 0.010% or less. The B content is more preferably 0.0005% or more, and further preferably 0.0008% or more. The B content is more preferably 0.0050% or less, further preferably 0.0030% or less, and further preferably 0.0020% or less.

Next, the steel structure is described. In the steel structure at the center of the wall thickness of the flat plate portion of the square steel pipe 10 of the present invention, the total volume ratio of ferrite and bainite is 70% or more and 95% or less relative to the entire steel structure at the center of the wall thickness of the flat plate portion, and the remainder is composed of one or more selected from pearlite, martensite, and austenite, and when the region surrounded by the boundary where the orientation difference of adjacent crystals is 15° or more is set as a grain, the average grain size of the grain is 15.0 μm or less, and the total volume ratio of the grains with a grain size of 40 μm or more relative to the entire steel structure at the center of the wall thickness of the flat plate portion is preferably 40% or less.

Total volume fraction of ferrite and bainite: 70% or more and 95% or less

Ferrite is a soft structure. In addition, bainite is harder than ferrite, softer than pearlite, martensite and austenite, and is a structure with excellent toughness. In the case where hard structures (pearlite, martensite and austenite) are mixed in ferrite and bainite, the yield ratio is reduced, but on the other hand, the interface is easily the starting point of destruction due to stress concentration caused by hardness difference, and the toughness is reduced. Therefore, in order to obtain the above-mentioned yield ratio and toughness, the total volume ratio of ferrite and bainite at the center of the wall thickness of the flat plate portion is preferably 70% or more and 95% or less relative to the overall steel structure at the center of the wall thickness of the flat plate portion. When the total volume ratio of ferrite and bainite is less than 70%, the proportion of hard structures is high, the yield stress increases, and therefore, the yield ratio increases and the toughness decreases. In addition, when the total volume ratio of ferrite and bainite is greater than 95%, the tensile strength decreases, so the yield ratio increases. More preferably, it is 73% or more and 93% or less. It is further preferably 75% or more and 92% or less.

It should be noted that the structure of the remainder (residual structure) other than ferrite and bainite is one or more selected from pearlite, martensite, and austenite. When the total volume ratio of the residual structure is less than 5%, the tensile strength decreases, so the yield ratio increases. In addition, when the total volume ratio of the residual structure is greater than 30%, the proportion of hard structures is high, the yield stress increases, and thus the yield ratio increases and the toughness decreases. Therefore, the total volume ratio of the residual structure is preferably 5% or more and 30% or less relative to the overall steel plate structure at the center of the wall thickness of the flat plate. More preferably, it is 7% or more and 27% or less. It is further preferably 8% or more and 25% or less.

The above-mentioned various structures (ferrite, bainite, pearlite, martensite) other than austenite use the austenite grain boundary or the deformation band in the austenite grain as the nucleation site. In the hot rolling of the raw steel plate in the manufacturing process of the electric resistance welded steel pipe (blank tube) used in the manufacture of square steel pipes, by increasing the reduction at a low temperature where the recrystallization of austenite is not easy to occur, a large number of dislocations can be introduced into the austenite to make the austenite finer, and a large number of deformation bands can be introduced into the grains. As a result, the area of the nucleation site increases, the nucleation frequency increases, and the steel structure can be refined.

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

As the observation of steel structure, first, the test piece for organization observation is cut in a way that the observation surface is parallel to the length direction and the wall thickness direction of the square steel pipe and becomes the center of the wall thickness of the flat plate, and after mirror polishing, it is made by nitric acid alcohol solution corrosion. In the organization observation, an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times) is used to observe and photograph the organization at the center of the wall thickness. The area ratio of ferrite, bainite and remainder (pearlite, martensite, austenite) is obtained from the obtained optical microscope image and SEM image. About the area ratio of each organization, observe in more than 5 visual fields, and calculate with the average value of the value obtained in each visual field. The area ratio obtained by organization observation is used as the volume ratio of each organization.

Here, ferrite is a product of diffusion phase transformation, and has a low dislocation density and a substantially restored structure, including polygonal ferrite and pseudo-polygonal ferrite.

Bainite is a duplex structure of lath-shaped ferrite and cementite with high dislocation density.

Pearlite is a eutectic structure of iron and iron carbide (ferrite + cementite), and presents a layered structure in which linear ferrite and cementite are alternately arranged.

Martensite is a lath-shaped low-temperature phase transformation structure with a very high dislocation density. In the SEM image, it shows a brighter contrast than ferrite and bainite. It should be noted that it is difficult to distinguish martensite from austenite using an optical microscope image and an SEM image. Therefore, the area ratio of the structure observed in the form of martensite or austenite is measured from the obtained SEM image, and the volume ratio of austenite measured by the method described later is subtracted therefrom, and the obtained value is used as the volume ratio of martensite.

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

Average grain size: 15.0 μm or less

In the present invention, average crystal grain size refers to the average equivalent circle diameter of the crystal grain when the region surrounded by the boundary with the orientation difference of adjacent crystals being 15° or more is set as the crystal grain (grain boundary). In addition, equivalent circle diameter (crystal grain size) refers to the diameter of a circle having the same area as the crystal grain as the object.

When the average crystal grain size of the crystal grains is greater than 15.0 μm, the total area of the grain boundaries that become an obstacle to crack propagation is small, so the desired toughness cannot be obtained. Therefore, in the present invention, the average crystal grain size of the crystal grains is set to be less than 15.0 μm. The average crystal grain size of the crystal grains is preferably less than 13.0 μm, more preferably less than 10.0 μm. It should be noted that the smaller the average crystal grain size, the higher the yield ratio, so the average crystal grain size is preferably more than 2.0 μm.

Total volume ratio of grains with a grain size of 40 μm or more: 40% or less

Even if the upper limit of the maximum crystal grain size is specified, if there is a certain amount of coarse grains, there will be a small area of the total area of the grain boundary that becomes an obstacle to the propagation of cracks, so the toughness is greatly reduced. Therefore, in order to obtain good toughness, it is necessary to also specify the upper limit of the proportion of coarse grains that exist. Therefore, in the present invention, the total volume rate of grains with a grain size of 40 μm or more is set to 40% or less. More preferably, it is 30% or less. Based on the above reasons, it is expected that there are fewer coarse grains, and the total volume rate of the above-mentioned grains is preferably 0%.

Here, the average crystal grain size of the grains and the total volume rate of the grains with a crystal grain size of 40 μm or more are determined as follows. First, the test piece for organization observation is cut in a manner that the observation surface is a cross section parallel to both the length direction and the wall thickness direction of the square steel pipe and becomes the center of the wall thickness of the flat plate. After mirror polishing, the histogram of the particle size distribution (a graph with the horizontal axis being the particle size and the vertical axis being the existence ratio (area ratio) of each particle size) is calculated using the SEM/EBSD method in the center of the wall thickness. The average crystal grain size is obtained from the above histogram as the arithmetic mean of the particle size. The total volume rate of grains above 40 μm is obtained from the above histogram as the total existence ratio of grains with a particle size of 40 μm or more. As measurement conditions, the acceleration voltage is set to 15 kV, the measurement area is set to 500 μm × 500 μm, and the measurement step (measurement resolution) is set to 0.5 μm. In addition, in the crystal grain size analysis, crystal grains having a grain size of less than 2.0 μm were excluded from the analysis object as measurement noise.

<Method for manufacturing square steel pipe>

Next, a method for producing the square steel pipe 10 of the present invention will be described.

The manufacturing method of the square steel pipe 10 of the present invention is as follows: a steel plate as a raw material is cold rolled, and then both ends of the cold rolled steel plate in the width direction are resistance welded to produce a resistance welded steel pipe, and then the resistance welded steel pipe is reduced in diameter by a sizing stand, and then squared by a square forming stand to produce a square steel pipe. At this time, the gap between the rollers of the sizing stand and the gap between the rollers of the square forming stand immediately before square forming are controlled so that the ratio of the plate width W of the steel plate to the circumference C _OUT of the square steel pipe on the outlet side of the square forming stand satisfies the formula (1), and the ratio of the circumference C _IN of the resistance welded steel pipe on the inlet side of the square forming stand to the circumference C _OUT of the square steel pipe on the outlet side of the square forming stand satisfies the formula (2).

1.000+0.050×t/H＜W/C _OUT ＜1.000+0.50×t/H…Formula (1)

0.30×t/H+0.99≤C _IN /C _OUT ＜0.50×t/H+0.99…Equation (2)

Here, in formula (1) and formula (2),

W: The width of the steel plate as the raw material (mm),

C _IN : circumference of the electric resistance welded steel pipe at the inlet side of the first section of the square forming rack (mm),

C _OUT : The circumference of the square steel pipe at the outlet side of the final stage square forming machine frame (mm), t: The average wall thickness of the flat plate after square forming (mm),

H: average side length of the flat plate portion after square forming (mm).

In the case of using a square forming frame of one stage to perform square forming, the square forming frame of the first stage and the square forming frame of the final stage refer to the same square forming frame.

It should be noted that the average wall thickness t is calculated by the above-mentioned formula (3), and the average side length H is calculated by the above-mentioned formula (4).

The manufacturing method of the square steel pipe 10 of the present invention is described in detail using Figures 2 and 3. Figure 2 shows a diagram for explaining the pipe making process of the blank pipe (electric resistance welded steel pipe) of the square steel pipe of the present invention. Figure 3 shows a diagram for explaining the forming process of the square steel pipe of the present invention.

First, the electric resistance welded steel pipe 7 is manufactured using a steel plate (steel strip) as a raw material (pipe manufacturing step).

As shown in FIG2 , the steel plate 1 (hot-rolled steel plate, hot-rolled steel strip) having the above-mentioned composition is unwound and straightened by a leveler 2, and intermediately formed by a row of rolls 3 consisting of two or more rolls to form a cylindrical open tube. Then, the fin-pass roll group 4 consisting of two or more rolls is used for finishing forming. The above-mentioned open tube is formed into a cylindrical shape by cold rolling.

It should be noted that the square steel pipe of the present invention preferably has the above-mentioned steel structure. As described above, the square steel pipe of the present invention is manufactured by further square-forming the electric resistance welded steel pipe (blank tube) formed by cold rolling the raw steel plate, so the raw steel plate (steel plate 1) also preferably has the above-mentioned component composition and steel structure. The preferred manufacturing conditions of the steel plate 1 will be described later, so the description here is omitted.

The open tube after fine processing is crimped by the squeeze roller 5, and at the same time, a pair of butt joints (two ends in the width direction) facing each other along the circumferential direction of the steel plate 1 are resistance welded (resistance welded) by the welding machine 6 to form a resistance welded steel tube 7. In the above-mentioned resistance welding, the butt joints are heated and melted by, for example, high-frequency induction heating or high-frequency resistance heating, and then crimped and solidified to complete the joining. As a result, the welded portion (resistance welded portion) 13 is extended in the direction of the tube axis. The manufacturing equipment used in the manufacture of the resistance welded steel tube 7 is not limited to the manufacturing equipment having the tube manufacturing process shown in FIG. 2.

It should be noted that in the present invention, in the process of manufacturing the electric resistance welded steel pipe, the upset amount by the squeeze roll 5 is preferably set to a range of 20% or more and 100% or less relative to the wall thickness of the electric resistance welded steel pipe 7. When the upset amount is less than 20% of the wall thickness, the discharge of molten steel becomes insufficient, and the toughness of the welded portion deteriorates. On the other hand, when the upset amount is greater than 100% of the wall thickness, the load on the squeeze roll increases, and the work hardening amount of the welded portion (electric resistance welded portion) 13 increases, and the hardness increases excessively.

Next, the obtained electric resistance welded steel pipe 7 is used as a base pipe to produce a square steel pipe (forming step). The forming step includes a sizing step and a square forming step.

As shown in FIG3 , the resistance welded steel pipe 7 is reduced in diameter in a cylindrical shape by a sizing roller group (sizing rack) 8 composed of two or more rollers arranged at the top, bottom, left, and right relative to the resistance welded steel pipe 7 (sizing process). Then, by a square forming roller group (square forming rack) 9 composed of two or more rollers arranged at the top, bottom, left, and right relative to the resistance welded steel pipe 7, the pipe is sequentially square-formed into the shapes shown by R1, R2, and R3 to form a square steel pipe 10 (square forming process). Each roller constituting the square forming rack 9 is a hole-shaped roller (diameter roller) having a caliber curvature, and the caliber curvature increases as it becomes a rear-stage rack. In this way, the flat plate portion and corner portion of the square steel pipe are formed.

It should be noted that the number of stands constituting the sizing roll group 8 and the square forming roll group 9 is not particularly limited. There are cases where the stands are composed of two or more stages, and there are cases where the stands are composed of one stage. In addition, when the diameter curvature of each roll in the sizing roll group 8 or the square forming roll group 9 is not constant (having two or more curvatures), it becomes a cause of irregular shape when the electric resistance welded steel pipe 7 being formed is twisted in the circumferential direction, so the diameter curvature of each roll is preferably constant.

In the present invention, as described above, it is important to _control the gap between the rollers of the sizing stand immediately before square forming and the gap between the rollers of the square forming stand so that the ratio of the plate width W of the steel plate to the circumference C _OUT of the square steel pipe on the outlet side of the square forming stand satisfies the formula (1), and the ratio of the circumference C _IN of the electric resistance welded steel pipe on the inlet side of the square forming stand to the circumference C OUT of the square steel pipe on the outlet side of the square forming stand satisfies the formula (2). Thus, even if the roll-formed square steel pipe has a large ratio (t/H) of the average wall thickness t to the average side length H and a small radius of curvature R of the corner, the ductility and toughness of the outer surface of the corner can be improved.

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

As shown in Fig. 2 and Fig. 3, when a flat steel plate 1 (raw steel plate) is cold-rolled to form a cylindrical electric resistance welded steel pipe 7 (blank pipe), and then the cylindrical electric resistance welded steel pipe is square-formed to produce a square steel pipe 10, during the manufacturing process (pipe making process, forming process), in addition to the bending deformation in the pipe circumferential direction, the steel plate 1 and the electric resistance welded steel pipe 7 are also subjected to elongation deformation in the pipe length direction due to the reduction in diameter in the pipe circumferential direction. In order to reduce the amount of reduction in diameter in the pipe circumferential direction during the manufacturing process, it is effective to appropriately control the above-mentioned two ratios "t/H" and "W/C _OUT ".

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

On the other hand, when the ratio "W/C _OUT " is equal to or greater than the value on the right side of the formula (1), the difference in the circumference of the tube (or open tube) before and after the tube making process and the forming process becomes large. As a result, the reduction in diameter in the circumferential direction of the tube becomes large, so that the corners are greatly work-hardened, and the desired ductility and toughness of the outer surface of the corners cannot be obtained.

The above ratio "W/C _OUT " is preferably not less than (1.000+0.080×t/H) and not more than (1.000+0.48×t/H), and more preferably not less than (1.000+0.10×t/H) and not more than (1.000+0.45×t/H).

Next, the reason for controlling the ratio (C IN /C OUT ) of the circumference of the electric resistance welded steel pipe 7 immediately before square forming (the circumference of the electric resistance welded steel pipe 7 on the inlet side of the first-stage square forming stand (mm), hereinafter referred to as "C _IN ") to the circumference (C _OUT ) of _the square steel pipe 10 _immediately after square forming, and the ratio (t/H) of the average wall thickness t immediately after square forming to the average side length H immediately after square forming so as to satisfy the above-mentioned formula (2) will be described.

As shown in Fig. 3, when a cylindrical electric resistance welded steel pipe 7 is square-shaped into a square steel pipe 10, the steel pipe is gradually formed from a cylindrical shape into a square shape by passing through the square forming roller group 9 as described above. In such square forming, the straight line portion (flat portion 11) of the side is bent back, the corner portion 12 is bent, and the electric resistance welded steel pipe 7 is deformed by reducing the diameter in the circumferential direction.

In particular, around the corner 12, the rollers of the square forming roller group 9 are almost not in contact with each other to complete the square forming. In the square forming, the corner 12 is formed by protruding by utilizing free deformation. At this time, the higher the rigidity of the corner 12 and the smaller the circumferential diameter reduction, the smaller the bending deformation of the corner 12 and the larger the curvature radius of the outer side of the corner. On the other hand, the lower the rigidity of the corner 12 and the larger the circumferential diameter reduction, the larger the bending deformation of the corner 12 and the smaller the curvature radius of the outer side of the corner.

The larger the ratio (t/H) of the average wall thickness t to the average side length H, the higher the rigidity of the corner portion 12 against bending deformation. The circumferential diameter reduction in square forming is obtained by the circumferential length ratio (C _IN /C _OUT ), and the larger the circumferential diameter reduction, the larger the circumferential diameter reduction.

Therefore, when t/H increases, it is difficult to form the corner 12 by bending deformation. Therefore, in order to obtain the desired corner curvature radius, it is necessary to increase the circumferential ratio (C _IN /C _OUT ) and increase the circumferential diameter reduction. For this reason, it is effective to appropriately control the above two ratios "t/H" and "C _IN /C _OUT ".

When the circumference ratio (C _IN /C _OUT ) is smaller than the value on the left side of equation (2), the difference in the circumference of the tube before and after the forming process is reduced, and the circumferential diameter reduction of the electric resistance welded steel tube 7 is reduced. As a result, the processing of the flat plate portion 11 and the corner portion 12 becomes insufficient, a flat flat plate portion cannot be obtained, and the curvature radius R of the outer side of the corner portion becomes larger than 3.0 times (3.0t) the average wall thickness t.

On the other hand, when the circumference ratio (C _IN /C _OUT ) is equal to or greater than the value on the right side of formula (2), the difference in the circumference of the tube before and after the forming process increases. As a result, the reduction in diameter in the circumferential direction of the tube is large, so that the corners are greatly work-hardened, and the desired ductility and toughness of the corners cannot be obtained. In addition, the radius of curvature R of the outer side of the corner is less than 2.0 times (2.0t) the average wall thickness t.

The circumference ratio (C _IN /C _OUT ) is preferably not less than (0.33×t/H+0.99) and not more than (0.47×t/H+0.99), and more preferably not less than (0.35×t/H+0.99) and not more than (0.45×t/H+0.99).

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

When the average wall thickness of the flat plate portion of the square steel pipe 10 is set to t (mm) and the average side length of the flat plate portion is set to H (mm), it is preferred that the above-mentioned average wall thickness t is set to be greater than 0.030 times relative to the above-mentioned average side length H. Thus, the endurance and rigidity of the column material are increased, and as a result, the earthquake resistance is improved. The ratio (t/H) of the average wall thickness t to the average side length H is more preferably 0.035 times or more. In addition, in order to ensure the ductility and toughness of the corner, it is preferably less than 0.10 times, and more preferably less than 0.080 times.

In addition, the average wall thickness t is preferably set to 20 mm or more and 40 mm or less. It should be noted that the reason is the same as the reason for controlling the average wall thickness t of the square steel pipe described above, so it is omitted.

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

In addition, the control of C _IN and C _OUT is performed by controlling the gap between the concave parts of the diameter rollers. When the difference between the maximum gap between the concave parts of the rollers of the sizing stand immediately before square forming (hereinafter also referred to as "gap of the sizing stand") and the maximum gap between the concave parts of the rollers of the square forming stand (hereinafter also referred to as "gap of the square forming stand") is Δg, it is preferred to adjust the gap of the sizing stand immediately before square forming so that the value G (=Δg/(t/H)) obtained by dividing Δg by (t/H) is 70 or more and 180 or less.

When G is less than 70, (C _IN /C _OUT ) in the above formula (2) is less than the value on the left, and as described above, the curvature radius of the flat plate portion and the outer side of the corner portion, which is the target of the present invention, cannot be obtained. On the other hand, when G is greater than 180, (C _IN /C _OUT ) in the above formula (2) is greater than the value on the right, and as described above, the ductility and toughness of the corner portion, which is the target of the present invention, cannot be obtained. Preferably, G is greater than 80 and less than 170.

It should be noted that, when there are more than two stages of sizing racks, the gap of the sizing rack just before square forming can be the same as the gap of other sizing racks. In addition, when there are more than two stages of square forming racks, the gap of the square forming rack is preferably set to the gap of the square forming rack of the first stage. The gaps of the first stage and other square forming racks can all be the same.

Here, the above-mentioned C _IN refers to the circumference (length of the outer circumference in the circumferential direction of the tube) (mm) of the electric resistance welded steel tube 7 at the inlet side of the first stage square forming stand. As shown in FIG3 , C _IN is obtained by the following method: when the tube making direction is set as the positive direction of the X axis, the X coordinate of the rotation axis of any one of the sizing roll groups 8 immediately before square forming is set as Xa (m), and the X coordinate of the rotation axis of any one of the first stage square forming roll groups 9 is set as Xb (m), the outer circumference of the circumferential section of the tube in the plane X=(Xa+Xb)/2 (m) perpendicular to the X axis is measured with a tape measure.

The above-mentioned C _OUT refers to the circumference (length of the outer circumference in the circumferential direction of the tube) (mm) of the square steel tube 10 at the outlet side of the square forming stand of the final stage. As shown in FIG3 , C _OUT is obtained by the following method: the X coordinate of the rotation axis of any one of the square forming stands of the final stage of the roll group is set as Xc (m), and the outer circumference of the circumferential section of the tube in the plane X=Xc+1 (m) perpendicular to the X axis is measured with a tape measure.

In the method for manufacturing a square steel pipe of the present invention, in order to reduce the fluctuation of the flatness of each flat plate portion and the radius of curvature of each corner portion during the process of forming the square steel pipe from the electric resistance welded steel pipe (blank pipe), in addition to the above conditions, the following conditions can be further controlled.

In the sizing process after resistance welding, in order to meet the preferred roundness, the steel pipe can be reduced in diameter in such a way that the circumference of the steel pipe is reduced by a total of 0.30% or more. As a result, in the subsequent square forming process, each flat plate portion and each corner portion are formed uniformly (symmetrically), and the fluctuation of flatness and curvature radius is reduced. The above-mentioned "preferred roundness" means that the vertical outer diameter D1 and the horizontal outer diameter D2 of the pipe are |D1-D2|/((D1+D2)/2)≤0.020.

However, when the diameter reduction is performed so that the circumference of the steel pipe is reduced by a total ratio of more than 2.0%, the bending amount in the pipe axial direction when the roll passes increases, and the yield ratio increases. Therefore, it is preferable to perform the diameter reduction so that the circumference of the steel pipe is reduced by a ratio of 0.30% or more and 2.0% or less.

In the sizing step, in order to minimize the amount of bending in the pipe axial direction when the roll passes through and to suppress the generation of residual stress in the pipe axial direction, it is preferred to perform diameter reduction in two or more stages using two or more stands. In this case, the diameter reduction in each stand is preferably performed so that the circumference of the steel pipe is reduced by a ratio of 1.0% or less compared to the diameter reduction in the stand immediately preceding the stand.

As described above, in the square steel pipe of the present invention, the electric resistance welded steel pipe is used for the blank pipe. Whether the square steel pipe 10 is obtained from the electric resistance welded steel pipe 7 can be judged by the following method: the square steel pipe 10 is cut vertically along the pipe axis direction, the cut surface including the welded portion (electric resistance welded portion) 13 is polished, corroded, and observed with an optical microscope. When the width of the molten solidified portion of the welded portion (electric resistance welded portion) 13 in the pipe circumferential direction is 1.0 μm or more and 1000 μm or less in the total pipe thickness, it is the electric resistance welded steel pipe 7. It should be noted that the etching liquid can be selected appropriately according to the steel composition and the type of steel pipe.

Here, the weld (resistance weld) is described using FIG. 4. FIG. 4 shows a schematic diagram of the molten solidified portion 16 of the weld 13. FIG. 4 is a state after the cut surface including the weld is polished and etched. The molten solidified portion 16 can be identified as a region having a different structure morphology and contrast from the base material portion 14 and the heat-affected zone 15 in FIG. 4. For example, the molten solidified portion 16 of the electric resistance welded steel pipe of carbon steel and low alloy steel can be specified as a region observed relatively white using an optical microscope in the above-mentioned cross section after etching with nital solution.

Next, a preferred method for producing a raw material steel plate of an electric resistance welded steel pipe used for producing a square steel pipe of the present invention will be described.

For example, it is preferred that the steel material having the above-mentioned composition is heated to a heating temperature of 1100°C to 1300°C, and then subjected to hot rolling treatment (hot rolling process) with a rough rolling end temperature of 850°C to 1150°C, a finish rolling end temperature of 750°C to 900°C, and a total reduction ratio of 50% or more at 950°C or below, and then cooled under the conditions of an average cooling rate of 5°C/s to 30°C/s and a cooling stop temperature of 400°C to 650°C (cooling process), and then coiled at 400°C to 650°C (coiling process) to produce a hot-rolled steel plate (steel plate 1).

It should be noted that, in the following description of the manufacturing method, unless otherwise specified, the "℃" expression related to temperature is set to the surface temperature of the steel raw material and the steel plate (hot-rolled steel plate). These surface temperatures can be measured using a radiation thermometer or the like. In addition, the temperature at the center of the steel plate wall thickness can be obtained by calculating the temperature distribution in the cross section of the steel plate using heat transfer analysis and correcting the result with the surface temperature of the steel plate. In addition, "hot-rolled steel plate" also includes hot-rolled plates and hot-rolled steel strips.

In the present invention, the smelting method of the steel raw material (steel billet) is not particularly limited, and known smelting methods such as converters, electric furnaces, and vacuum melting furnaces are suitable. The casting method is also not particularly limited, and the desired size is produced by known casting methods such as continuous casting. It should be noted that there is no problem in applying the ingot casting-opening rolling method instead of the continuous casting method. The molten steel can be further subjected to secondary refining such as ladle refining.

Hot rolling process

Heating temperature: 1100°C or higher and 1300°C or lower

When the heating temperature is lower than 1100°C, the deformation resistance of the rolled material increases, and rolling becomes difficult. On the other hand, when the heating temperature exceeds 1300°C, the austenite grains coarsen, and fine austenite grains cannot be obtained in the subsequent rolling (rough rolling, finish rolling), and it is difficult to ensure the average grain size of the steel structure of the electric resistance welded steel pipe as the target of the present invention. Therefore, the heating temperature in the hot rolling process is set to 1100°C or higher and 1300°C or lower. The heating temperature is more preferably 1120°C or higher. In addition, the heating temperature is more preferably 1280°C or lower.

It should be noted that in the present invention, in addition to the existing method of temporarily cooling the steel billet (slab) to room temperature after manufacturing and then heating it again, it is also possible to apply energy-saving direct rolling processes such as loading it into a heating furnace in a warm sheet state without cooling it to room temperature, or rolling it immediately after slightly keeping it warm.

Rough rolling end temperature: 850℃ or higher and 1150℃ or lower

When the rough rolling end temperature is lower than 850°C, the surface temperature of the steel plate becomes below the ferrite transformation start temperature in the subsequent finish rolling, a large amount of processed ferrite is generated, and the yield ratio increases. On the other hand, when the rough rolling end temperature exceeds 1150°C, the reduction amount within the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, it is difficult to ensure the average grain size of the steel structure of the above-mentioned square steel pipe, and the toughness is reduced. The rough rolling end temperature is more preferably above 860°C. In addition, the rough rolling end temperature is more preferably below 1000°C.

The start temperature of the finishing rolling is preferably above 800°C and below 980°C. When the start temperature of the finishing rolling is lower than 800°C, the surface temperature of the steel plate becomes below the start temperature of the ferrite transformation during the finishing rolling, a large amount of processed ferrite is generated, and the yield ratio increases. On the other hand, when the start temperature of the finishing rolling exceeds 980°C, the austenite coarsens and sufficient deformation bands are not introduced into the austenite. Therefore, it is difficult to ensure the average grain size of the steel structure of the above-mentioned square steel pipe, and the toughness is reduced. The start temperature of the finishing rolling is more preferably above 820°C. In addition, the start temperature of the finishing rolling is more preferably below 950°C.

Finish rolling end temperature: 750℃ or higher and 900℃ or lower

When the finishing temperature of the finishing rolling is lower than 750°C, the surface temperature of the steel plate becomes below the ferrite phase transformation start temperature during the finishing rolling, a large amount of processed ferrite is generated, and the yield ratio increases. On the other hand, when the finishing temperature of the finishing rolling exceeds 900°C, the reduction amount within the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, it is difficult to ensure the average grain size of the steel structure of the above-mentioned square steel pipe, and the toughness is reduced. The finishing temperature of the finishing rolling is more preferably above 770°C. In addition, the finishing temperature of the finishing rolling is more preferably below 880°C.

Total reduction ratio at 950℃ or below: 50% or more

In the present invention, by refining the subgrains in austenite in the hot rolling process, the ferrite, bainite and residual structure generated in the subsequent cooling process and coiling process are refined, and the steel structure of the square steel pipe having the above-mentioned strength and toughness can be obtained. In order to refine the subgrains in austenite in the hot rolling process, it is necessary to increase the reduction rate in the austenite non-recrystallization temperature range and introduce sufficient processing strain. In order to achieve this purpose, in the present invention, the total reduction rate at 950°C or below is set to 50% or more.

When the total reduction rate at 950°C or less is less than 50%, sufficient processing strain cannot be introduced in the hot rolling process, so the structure with the average grain size of the above-mentioned square steel pipe cannot be obtained. The total reduction rate at 950°C or less is more preferably 55% or more, and more preferably 57% or more. There is no particular upper limit. When it is greater than 80%, the effect of improving toughness relative to the increase in the reduction rate is reduced, and only the equipment load increases. Therefore, the total reduction rate at 950°C or less is preferably 80% or less. More preferably, it is 70% or less.

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

Cooling process

After the hot rolling step, the hot rolled sheet is cooled in a cooling step. In the cooling step, the hot rolled sheet is cooled at an average cooling rate of 5°C/s to 30°C/s and a cooling stop temperature of 400°C to 650°C.

Average cooling rate from the start of cooling to the stop of cooling (end of cooling): 5°C/s or more and 30°C/s or less

When the average cooling rate in the temperature range from the start of cooling to the stop of cooling described later is less than 5°C/s, the nucleation frequency of ferrite or bainite is reduced, and they are coarsened, so the organization with the average grain size of the above-mentioned square steel pipe cannot be obtained. On the other hand, when the average cooling rate is greater than 30°C/s, a large amount of martensite is generated, and the toughness is reduced. The average cooling rate is preferably more than 10°C/s. In addition, the average cooling rate is preferably less than 25°C/s.

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

Cooling stop temperature: 400°C or higher and 650°C or lower

When the cooling stop temperature is lower than 400°C based on the wall thickness center temperature of the hot rolled plate, a large amount of martensite is generated, and the toughness is reduced. On the other hand, when the cooling stop temperature exceeds 650°C, the nucleation frequency of ferrite or bainite is reduced, and they are coarsened, so the organization with the average grain size of the above-mentioned square steel pipe cannot be obtained. The cooling stop temperature is preferably above 430°C. In addition, the cooling stop temperature is preferably below 620°C.

It should be noted that, in the present invention, unless otherwise specified, the average cooling rate is set to a value (cooling rate) obtained by ((wall thickness center temperature of the hot-rolled plate before cooling - wall thickness center temperature of the hot-rolled plate after cooling)/cooling time). The cooling method may include water cooling such as spraying water from a nozzle, cooling by spraying a cooling gas, etc. In the present invention, in order to cool both sides of the hot-rolled plate under the same conditions, it is preferred to perform cooling operation (treatment) on both sides of the hot-rolled plate.

Coiling process

After the cooling process, in the coiling process, the hot-rolled steel sheet is coiled into a coil shape and then cooled. In the coiling process, in order to obtain the above-mentioned steel sheet structure, it is preferably coiled at a coiling temperature of 400°C or more and 650°C or less. When the coiling temperature is lower than 400°C, a large amount of martensite is generated and the toughness is reduced. When the coiling temperature exceeds 650°C, the nucleation frequency of ferrite or bainite decreases, and they coarsen, so a structure having the average grain size of the above-mentioned square steel pipe cannot be obtained. The coiling temperature is preferably above 430°C. In addition, the coiling temperature is preferably below 620°C.

＜Architectural structures＞

Next, one embodiment of a building structure using the square steel pipe 10 of the present invention will be described using Fig. 5. Fig. 5 shows an example of a building structure 100 using the square steel pipe 10 of the present invention as a member (eg, a column) of a building structure.

As shown in FIG5 , in the building structure 100 of the present invention, two or more square steel pipes 10 (column materials) are set up via a partition 17 and welded to each other. A beam 18 is set between adjacent square steel pipes 10, and a small beam 19 is set between adjacent beams 18. In addition, a spacer 20 is appropriately provided for the installation of a wall or the like. In addition, known members can be used for the building structure 100.

As described above, the corner 12 of the square steel pipe 10 of the present invention has a small radius of curvature and a flat plate 11, and has excellent shape characteristics. In addition, the outer surface of the corner 12 of the square steel pipe 10 of the present invention has excellent ductility and toughness. Therefore, the building structure 100 of the present invention using the square steel pipe 10 as a column material can ensure the plastic deformation capacity of the entire structure, and therefore, it can exert better earthquake resistance than the building structure using the conventional square steel pipe.

Example

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

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

Molten steel having the component composition shown in Table 1 was melted to produce slabs (steel raw materials). The obtained slabs were subjected to a hot rolling process, a cooling process, and a coiling process under the conditions shown in Table 2-1 to produce hot-rolled steel sheets.

The obtained hot-rolled steel sheet (raw steel sheet) is continuously cold-formed into an open tube with an elliptical cross section using a row roll group and a fin-pass roll group. Next, the opposite end faces (both ends in the width direction) of the open tube are heated to a temperature above the melting point by high-frequency induction heating or high-frequency resistance heating, and are crimped by an extrusion roll to form an electric resistance welded steel tube.

The obtained electric resistance welded steel pipe (blank pipe) was reduced in diameter by a 2-stand (2-stage) sizing roll group, and then square-formed by a 4-stand (4-stage) square forming roll group to obtain square steel pipes of the sizes shown in Table 2-2. In the square forming process, the gap between the sizing rolls and the gap between the square forming rolls immediately before square forming were controlled under the conditions shown in Table 2-2. The obtained square steel pipe was approximately rectangular when viewed from a cross section perpendicular to the pipe axis.

It should be noted that the average wall thickness t (mm) of the square steel pipe shown in Table 2-2 is calculated using the above-mentioned formula (3), and the average side length H (mm) of the square steel pipe is calculated using the above-mentioned formula (4). With respect to the side lengths _H1 and _H2 (mm) of the square steel pipe, the side lengths of the flat plate portion of the portion shown in FIG. 1 are measured. With respect to the width W (mm) of the raw steel plate, the width of the steel plate just after passing through the leveler is measured. The circumference C _IN (mm) of the electric resistance welded steel pipe on the inlet side of the first stage square forming stand, the circumference C _OUT (mm) of the square steel pipe on the outlet side of the final stage square forming stand, and the difference (△g) between the maximum gap between the caliber roller of the sizing stand immediately before square forming and the concave portion of the caliber roller of the first stage square forming stand are measured by the above-mentioned method. Then, G (=△g/(t/H)) is calculated using the above-mentioned difference (△g), the average wall thickness t, and the average side length H.

In addition, each square steel pipe obtained was cut perpendicularly to the pipe axis direction, and the cut surface including the resistance welded portion was polished, then etched with nital, and observed with an optical microscope. It was also confirmed that the width of the molten solidified portion of the resistance welded portion in the pipe circumferential direction was 1.0 μm or more and 1000 μm or less in the total thickness of the pipe. The molten solidified portion can be specified as a region observed relatively white with an optical microscope in the above cross section after etching with nital.

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

(1) Steel structure of square steel pipe

The steel structure of the square steel pipe was quantitatively measured by the above method. The obtained results are shown in Table 3.

(2) The radius of curvature of the outer surface of the corner of the square steel pipe

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

Note that a radial measuring instrument was used to measure the curvature radius of the outer side of the corner portion. The curvature radius was measured by the above method described using FIG. 1 .

(3) Flatness of the flat plate of the square steel pipe

The method for measuring flatness is described using Figure 10. In the measurement of flatness, 4 flat plate parts are respectively taken as measurement objects at any 10 positions in the tube axis direction of the square steel pipe, and the measurement is performed at a total of 40 positions. As shown in Figure 10, the maximum bulge amount and the maximum depression amount relative to the straight line at 2 points at both ends of the circumferential direction of the outer surface of each flat plate part are measured respectively. The bulge amount is set to a positive value, and the depression amount is set to a negative value. The measured values are shown in Table 4. Then, the absolute values of the maximum bulge amount and the maximum depression amount of each measured part are calculated, and the maximum value is taken as the flatness of the flat plate part, which is shown in Table 4. Among them, in the case where there is no bulge or depression, the value of the bulge amount or depression amount is set to 0.

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

(4) Tensile test of flat plate and corner of square steel tube

The obtained square steel pipe was used to perform a tensile test by the following method: FIG6 shows the positions where the tensile test pieces of the flat plate and the corner were cut, respectively, and FIG7 shows the detailed positions where the tensile test pieces of the corner were cut.

As shown in FIG6 , JIS No. 5 tensile test pieces and JIS No. 12B tensile test pieces indicated by dotted lines are cut from the flat plate and corner of the square steel pipe in such a way that the tensile direction is parallel to the pipe axis direction. The tensile test pieces are cut by grinding them in such a way that their thickness is 5 mm and the center of the thickness is at a position 1/4t of the wall thickness t from the outer surface of the pipe. It should be noted that, as shown in FIG7 , the tensile test piece of the corner is cut from a line that passes through the intersection of the outer surfaces of the flat plate on both sides adjacent to the corner and forms a 45° line with the outer surface of the flat plate.

Using these tensile test pieces, a tensile test was performed in accordance with the provisions of JIS Z 2241 to measure the yield strength YS, tensile strength TS, and uniform elongation (flat portion: E1, corner portion: E2) of the flat portion and the corner portion. The uniform elongation was set to the value of the total elongation at the maximum load. For the corner portion, the yield strength and tensile strength obtained were used to calculate the yield ratio defined by (yield strength)/(tensile strength)×100(%). In addition, the uniform elongation E2 of the corner portion was calculated relative to the uniform elongation E1 of the flat portion.

The number of tensile test pieces was set to 2 each, and the average values were calculated to obtain the yield strength YS (MPa), tensile strength TS (MPa), yield ratio (%), and uniform elongation (%). These values are shown in Table 4.

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

It should be noted that, as shown in Fig. 6, the tensile test piece of the flat plate portion is cut from the center of the width of the flat plate portion 11b adjacent to the flat plate portion 11a including the resistance welded portion 13 of the square steel pipe. The tensile test piece of the corner portion is cut from the corner portion 12a adjacent to the flat plate portion 11a including the resistance welded portion 13.

(5) Charpy impact test on the corners of square steel pipes

The obtained square steel pipe was used to perform a Charpy impact test by the following method: FIG8 shows the cutting position of the Charpy test piece at the corner, and FIG9 shows the detailed cutting position of the Charpy test piece at the corner.

As shown in Figures 8 and 9, in the Charpy impact test, a V-notch standard test piece according to the provisions of JIS Z 2242 is used, which is cut at a position of 1/4t of the wall thickness t from the outer surface of the square steel pipe so that the length direction of the test piece is parallel to the pipe axis direction. The Charpy test piece of the corner is cut from the corner 12a adjacent to the flat plate portion 11a containing the resistance welded portion 13. In more detail, as shown in Figure 9, it is cut from a line passing through the intersection extending from the outer surface of the flat plate portion on both sides adjacent to the corner 12a and forming an angle of 45° with the outer surface of the flat plate portion. According to the provisions of JIS Z 2242, the Charpy impact test is carried out at a test temperature of -10°C to obtain the Charpy absorbed energy (J). It should be noted that the number of test pieces is set to 3 each, and their average value is calculated to obtain the Charpy absorbed energy (J). Its value is shown in Table 4.

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

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

The square steel pipes of the examples of the present invention all have a radius of curvature R of the outer side of the corner that is greater than or equal to 2.0t and less than or equal to 3.0t, a flatness of the outer surface of the flat plate portion in the tube axis direction of less than or equal to 2.5mm, a uniform elongation E2 at a position 1/4t from the outer surface of the corner that is greater than or equal to 0.60 times the uniform elongation E1 at a position 1/4t from the outer surface of the flat plate portion, and a Charpy absorbed energy of the corner at -10°C of greater than or equal to 100J.

On the other hand, in Comparative Example No. 4, the value of "W/C _OUT " was below the range of the formula (1), so the radius of curvature of the outer side of the corner portion exceeded the range of the present invention, and a flat plate portion was not obtained.

In the case of Comparative Example No. 5, the value of "W/C _OUT " exceeded the range of formula (1), and therefore the ratio of uniform elongation (E2/E1) between the flat portion and the corner portion and the Charpy absorption energy at -10°C of the corner portion did not reach the expected values. In addition, the yield ratio of the corner portion also showed a value of 90% or more.

In Comparative Example No. 6, the value of "C _IN /C _OUT " was below the range of the formula (2), so the radius of curvature of the outer side of the corner portion exceeded the range of the present invention, and a flat plate portion was not obtained.

In the case of Comparative Example No. 7, the value of "C _IN /C _OUT " exceeded the range of formula (2), so the radius of curvature of the outer side of the corner was lower than the range of the present invention, and the ratio of uniform elongation of the flat plate portion to the corner (E2/E1) and the Charpy absorbed energy of the corner at -10°C did not reach the expected values. In addition, the yield ratio of the corner also showed a value of 90% or more.

Explanation of symbols

1Steel plate (steel strip)

2 Leveling Machine

3-row roller set

4-fin hole roller set

5 squeeze rollers

6Welding Machine

7. Electric resistance welded steel pipe

8 Sizing roller group

9 square forming roller set

10 Square steel pipe

11 Flat plate

12 Corner

13 Welding section (resistance welding section)

14 Parent material department

15 Heat affected zone

16 Melting and solidification section

17 Bulkhead

18 Beam

19 Xiaoliang

20 columns

100 Building structures

Claims (11)

1. A square steel pipe, which is a square steel pipe with a flat plate portion and a corner, wherein, When the average wall thickness of the flat plate portion is t (mm), the outer radius of curvature R of the corner portion is 2.0t or more and 3.0t or less, The flatness of the outer surface of the flat plate part is 2.5 mm or less, The uniform elongation E2 at a position of 1/4t from the outer surface of the corner portion in the wall thickness direction relative to the uniform elongation at a position of 1/4t from the outer surface of the flat plate portion in the wall thickness direction The ratio E1 is 0.60 times or more, The Charpy absorbed energy at −10° C. at a position 1/4t away from the outer surface of the corner portion in the wall thickness direction is 100 J or more. 2. The square steel pipe according to claim 1, wherein the average wall thickness t is greater than 0.030 times the average side length H (mm) of the flat plate portion. 3. The square steel pipe according to claim 1 or 2, wherein the average wall thickness t is not less than 20 mm and not more than 40 mm. 4. The square steel pipe according to any one of claims 1 to 3, wherein: The yield strength of the flat plate portion is 295 MPa or more, The tensile strength of the flat plate part is 400 MPa or more, The yield ratio of the corner portion is 90% or less. 5. The square steel pipe according to any one of claims 1 to 4, wherein: The composition of the square steel pipe contains C: 0.020-0.45%, Si: 0.01-1.0%, Mn: 0.30-3.0%, P: 0.10% or less, S: 0.050% or less, Al: 0.005-0.10% by mass % %, N: 0.010% or less, Ti: 0.001 to 0.15%, the balance is composed of Fe and unavoidable impurities, In the steel structure at the center of the wall thickness of the flat plate portion, the total volume fraction of ferrite and bainite is 70% to 95% of the steel structure at the center of the wall thickness of the flat plate portion, with the balance Consisting of one or two or more selected from pearlite, martensite, and austenite, When the region surrounded by the boundary between adjacent crystals with a misorientation of 15° or more is defined as a crystal grain, The average crystal grain size of the crystal grains is 15.0 μm or less, The total volume fraction of crystal grains having a crystal grain size of 40 μm or more is 40% or less of the entire steel structure at the center of the wall thickness of the flat plate portion. 6. The square steel pipe according to any one of claims 1 to 5, wherein, in addition to the composition, it further contains, in mass %, selected from the group consisting of Nb: 0.001 to 0.15%, V: 0.001 to 0.15%. , Cr: 0.01-1.0%, Mo: 0.01-1.0%, Cu: 0.01-1.0%, Ni: 0.01-1.0%, Ca: 0.0002-0.010%, B: 0.0001-0.010%, one or more . 7. A method for manufacturing a square steel pipe, which is the method for manufacturing a square steel pipe according to any one of claims 1 to 6, wherein Cold roll forming of the steel plate and resistance welding of both ends of the steel plate in the width direction to form an electric resistance welded steel pipe. The frame is square-shaped to manufacture square steel pipes, at this time, To make the ratio of the plate width W of the steel plate to the perimeter C _OUT of the square steel pipe on the outlet side of the square forming frame satisfy formula (1), and the resistance of the entrance side of the square forming frame The ratio of the circumference C _IN of the welded steel pipe to the circumference C _OUT of the square steel pipe on the outlet side of the square forming frame satisfies the mode of formula (2), for the sizing frame before the square forming The roller gap and the roller gap of the square forming frame are controlled, 1.000+0.050×t/H<W/C _OUT <1.000+0.50×t/H…Formula (1) 0.30×t/H+0.99≤C _IN /C _OUT <0.50×t/H+0.99…Formula (2) Here, in formula (1) and formula (2), W: the plate width (mm) of the steel plate used as the raw material, C _IN : the circumference (mm) of the electric resistance welded steel pipe on the inlet side of the square forming frame of the first section, C _OUT : the circumference (mm) of the square steel pipe on the outlet side of the square forming frame of the final section, t: the average wall thickness of the flat plate part after square forming (mm), H: the average side length of the flat plate part after square forming (mm), Wherein, in the case of using a section of square forming frame for square forming, the square forming frame of the first section and the square forming frame of the final section refer to the same square forming frame . 8. The method for manufacturing a square steel pipe according to claim 7, wherein the steel plate is obtained as follows: After heating the steel material to a heating temperature of 1100°C to 1300°C, the finishing temperature of rough rolling is 850°C to 1150°C, and the finish rolling temperature is 750°C to 900°C and 950°C When the total rolling reduction rate is more than 50% hot rolling treatment, Next, cooling is carried out at an average cooling rate of 5° C./s to 30° C./s and a cooling stop temperature of 400° C. to 650° C. by using a wall thickness center thermometer, Next, coiling is performed at 400°C or higher and 650°C or lower. 9. The method for manufacturing a square steel pipe according to claim 7 or 8, wherein the average wall thickness t is greater than 0.030 times the average side length H of the flat plate portion. 10 . The method for manufacturing a square steel pipe according to claim 7 , wherein the average thickness t is 20 mm to 40 mm. 11 . 11. A building structure in which the square steel pipe according to any one of claims 1 to 6 is used as a column material.

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