CN116323065A

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

Info

Publication number: CN116323065A
Application number: CN202180066462.1A
Authority: CN
Inventors: 松本晃英; 仲泽稜; 松本昌士; 井手信介
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2020-10-05
Filing date: 2021-09-15
Publication date: 2023-06-23
Also published as: JPWO2022075026A1; TWI795923B; TW202219280A; KR20230059820A; JP7306494B2; WO2022075026A1

Abstract

The invention provides a square steel pipe, a manufacturing method thereof and a building structure. The square steel pipe has 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 of the outer side of the corner portion is 2.0t or more and 3.0t or less, the flatness of the outer surface of the flat plate portion is 2.5mm or less, the uniform elongation E2 at the position of 1/4t of the outer surface of the angle portion in the wall thickness direction is 0.60 times or more relative to the uniform elongation E1 at the position of 1/4t from the outer surface of the flat plate portion in the wall thickness direction, and the Charpy absorption energy at-10 ℃ at the position of 1/4t of the outer surface of the angle portion in the wall thickness direction is 100J or more.

Description

Square steel pipe, method for manufacturing same, and building structure

Technical Field

The present invention relates to a square steel pipe which is particularly suitable for use in building members of large buildings such as middle-level buildings, factories, and warehouses having a height of more than 20m, and a method for manufacturing the same. The present invention also relates to a building structure using the square steel pipe as a column material.

Background

From the viewpoint of shock resistance, high ductility and toughness are required for the column material of a building.

When a square steel pipe having a corner and a flat plate is subjected to a large external force such as a seismic force, the outer surface of the corner is greatly deformed. Therefore, the square steel pipe needs to sufficiently improve the ductility and toughness of the outer surface of the corner.

Cold roll forming square steel pipes (roll forming square steel pipes) are square steel pipes widely used as column materials for buildings. The square steel pipe is manufactured by the following method: the steel strip is formed into a cylindrical open pipe by cold roll forming, the butt joint portions of the open pipe are resistance welded to form a resistance welded steel pipe, and then the resistance welded steel pipe is reduced in diameter in the pipe axis direction in a cylindrical state by using rolls arranged on the upper, lower, left and right sides of the resistance welded steel pipe, and then formed into a square shape. In the above resistance welding, the butt portion is heated to be melted, and then is crimped and solidified, thereby completing the joining.

However, although the roll-formed square steel pipe has high productivity, on the other hand, the corner portions are greatly work-hardened at the time of manufacture, and therefore there is a problem that the ductility and toughness of the corner portions are lower than those of the flat plate portions.

In addition, from the viewpoints of workability at construction sites and design properties of buildings, it is also required that the corner radius of the square steel pipe used for the column is small. This is because, when the area of the flat plate portion of the column is large, the area where the column and the beam can be joined is large, and thus, a building design with a higher degree of freedom can be performed.

However, in the case of roll-formed square steel pipes, the greater 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 at the corners. In addition, the smaller the radius of curvature of the corner portion, the greater the circumferential bending strain required for shaping the corner portion, and the greater the work hardening amount of the corner portion. Therefore, in the roll-formed square steel pipe having a large ratio (t/H) of the average wall thickness t to the average side length H and a small radius of curvature of the corner, the ductility and toughness of the corner are particularly low, and it is difficult to secure sufficient earthquake-resistant performance.

Here, the "average wall thickness t" refers to an average value of wall thickness (mm) at a tube circumferential center position of 3 flat plate portions other than the flat plate portion including the welded portion (resistance welded portion). The "average side length H" refers to an average value of side lengths of 2 flat plate portions adjacent to each other across the corner portion.

To meet such a demand, square steel pipes described in patent documents 1 to 4, for example, have been proposed.

Patent document 1 proposes: bending steel plate containing vanadium as chemical component, welding to obtain semi-formed square steel pipe, and heating to A ₃ And (3) performing thermoforming near the transformation point, and then cooling to obtain the square steel pipe. Disclosed is: the square steel pipe improves endurance and toughness, and makes the shape of the corner sharp.

Patent document 2 proposes a square steel pipe obtained by heat-treating a cold-formed portion. Disclosed is: the square steel pipe improves mechanical properties and weldability of the cold formed portion.

Patent document 3 proposes: a square steel pipe having improved toughness and plastic deformability of corners by properly controlling the chemical composition of a raw steel plate, the bainite fraction of a metal structure, and the Vickers hardness of the surface layer portions of the corners.

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

Therefore, there is a demand for a technique for improving shape characteristics of roll-formed square steel pipes, in particular, a technique for improving flatness of flat plate portions and reducing curvature radius of corner portions. In response to this demand, for example, patent document 5 and patent document 6 propose techniques for improving shape characteristics by adjusting manufacturing conditions at the time of roll forming.

Specifically, patent document 5 proposes a method for forming a square steel pipe as follows: when square pipes are formed by using 3-stage or 4-stage square forming rolls and keeping the rolling reduction of the final stage rolls constant, the rolling reduction of the final stage is reduced (from convex to concave) as the wall thickness/outer diameter ratio of the steel pipes increases.

Patent document 6 proposes a method for manufacturing a square tube for a structure, which includes: a first step of forming a rectangular cross-sectional shape by maintaining a set press-in ratio Q defined by q= (D-H)/(D-t) ×100 in a range of 12 to 23% when the outer diameter of a cylindrical blank tube is D, the wall thickness of the blank tube is t, and the maximum diameter height is H, in roll forming the cylindrical blank tube into a square tube; and a step of molding the blank pipe after the step 2, wherein the blank pipe molded into a rectangular cross-sectional shape is molded into a target shape.

Prior art literature

Patent literature

Patent document 1: japanese patent application laid-open No. 2004-330222

Patent document 2: japanese patent laid-open No. 10-60580

Patent document 3: japanese patent No. 5385760

Patent document 4: japanese patent laid-open publication No. 2018-53281

Patent document 5: japanese patent laid-open No. 4-224023

Patent document 6: japanese patent No. 3197661

Disclosure of Invention

Problems to be solved by the invention

However, the square steel pipes described in patent documents 1 and 2 require a heating step at the time of forming or after forming, and therefore are very costly compared to cold-formed roll-formed square steel pipes. Therefore, there is a demand for a technique for obtaining a desired square steel pipe without requiring a heating step during or after molding.

Further, the square steel pipes described in patent documents 3 and 4 cannot sufficiently suppress the decrease in the uniform elongation at the corner caused by work hardening at the time of forming, and therefore cannot be said to sufficiently ensure the ductility and toughness of the outer surface of the corner.

Further, the techniques described in patent documents 5 and 6 cannot perform forming while suppressing work hardening of the corner portions, and therefore cannot be said to be sufficient as a technique for achieving both improvement in flatness of the flat plate portion of the square steel pipe and reduction in radius of curvature of the corner portions, and sufficient securing ductility and toughness of the outer surfaces of the corner portions.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a square steel pipe excellent in shape characteristics, ductility and toughness of the outer surface of a corner portion, a method for manufacturing the same, and a building structure excellent in earthquake resistance.

The term "excellent shape characteristics" as used herein refers to a square steel pipe having a small radius of curvature at the corner and a flat plate portion.

The "the radius of curvature of the corner portion is small" means that the radius of curvature R of the outside of the corner portion is controlled within a predetermined range, specifically, when the average wall thickness of the flat plate portion is set to t (mm), the radius of curvature R of the outside of the corner portion is 2.0t or more and 3.0t or less.

The term "flat plate portion flattening" means that the flatness of the outer surface of the flat plate portion in the tube axis direction is 2.5mm or less, specifically, the absolute value represented by the maximum bulge amount and the maximum dent amount is 2.5mm or less at the maximum with respect to a straight line passing through 2 points at both ends in the circumferential direction on the same side of the outer surface of the flat plate portion in a cross section of a surface perpendicular to the tube axis direction (refer to fig. 10 described later).

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

In the present invention, "excellent toughness of the outer surface of the corner portion" means that the Charpy absorption energy of the corner portion at-10 ℃ at a position 1/4t of the outer surface of the corner portion in the wall thickness direction is 100J or more.

The radius of curvature, flatness, uniform elongation and toughness were measured by the methods described in examples described below.

Means for solving the problems

As a result of intensive studies to solve the above-described problems, the present inventors have found that a square steel pipe having a small radius of curvature at the corner, a flat plate portion, and excellent ductility and toughness at the outer surface of the corner can be produced by controlling the sheet width of a raw steel sheet and the perimeter of a resistance welded steel pipe at the inlet side of a square forming frame to be in an appropriate range relative to the perimeter of a square steel pipe at the outlet side of the square forming frame.

The present invention has been completed based on the above-described findings, and includes the following matters.

[1] A square 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 of the outer side of the corner portion is 2.0t or more and 3.0t or less,

the flatness of the outer surface of the flat plate portion is 2.5mm or less,

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 with respect 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 absorption energy at-10 ℃ at a position 1/4t from the outer surface of the corner in the wall thickness direction is 100J or more.

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

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

[4] The square steel pipe according to any one of [1] to [3], wherein the yield strength of the flat plate portion is 295MPa or more, the tensile strength of the flat plate portion is 400MPa 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 contains, in mass%, C:0.020 to 0.45 percent of Si:0.01 to 1.0 percent of Mn:0.30 to 3.0 percent, P:0.10% or less, S: less than 0.050%, al: 0.005-0.10%, N: less than 0.010%, ti:0.001 to 0.15 percent, the balance being Fe and unavoidable impurities,

the total volume ratio of ferrite and bainite in the steel structure at the center of the wall thickness of the flat plate part is 70% to 95% relative to the whole steel structure at the center of the wall thickness of the flat plate part, and the balance is one or more selected from pearlite, martensite and austenite,

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

the average crystal grain diameter of the crystal grains is 15.0 μm or less,

the total of the volume fractions of the crystal grains having a crystal grain diameter of 40 μm or more is 40% or less relative to 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 [1] to [5], wherein the square steel pipe further comprises, in mass%, a component selected from the group consisting of Nb:0.001 to 0.15 percent, V:0.001 to 0.15 percent of Cr:0.01 to 1.0 percent of Mo:0.01 to 1.0 percent of Cu:0.01 to 1.0 percent of Ni:0.01 to 1.0 percent of Ca:0.0002 to 0.010 percent, B: 0.0001-0.010% of one or more than two kinds.

[7] A method for producing a square steel pipe according to any one of [1] to [6], wherein,

cold roll forming a steel sheet, resistance welding both widthwise ends of the steel sheet to form a resistance welded steel pipe, reducing the diameter of the resistance welded steel pipe by a sizing mill, and square forming by a square forming mill to produce a square steel pipe,

so that the width W of the steel plate is relative to the perimeter C of the square steel pipe at the outlet side of the square forming frame _OUT The ratio of (2) satisfies the ratio (1) of the perimeter C of the electric resistance welded steel pipe at the inlet side of the square forming frame _IN Circumference C of square steel pipe corresponding to outlet side of the square forming frame _OUT The ratio (2) is set to control the gap between the rolls of the sizing mill immediately before square forming and the gap between the rolls of the square forming mill.

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

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

In the formulae (1) and (2),

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

C _IN : the perimeter (mm) of the resistance welded steel tube on the inlet side of the square shaped frame of the first section,

C _OUT : perimeter (mm) of square steel tube at outlet side of square forming frame of final section, t: average wall thickness (mm) of the square shaped flat plate portion,

h: average side length (mm) of the square shaped flat plate portion.

In the case of square forming by using a single-stage square forming frame, the square forming frame of the first stage and the square forming frame of the final stage are the same square forming frame.

[8] The method of producing a square steel pipe according to [7], wherein the steel sheet is obtained as follows: after heating the steel material to a heating temperature of 1100 ℃ to 1300 ℃, performing hot rolling treatment in which the rough rolling end temperature is 850 ℃ to 1150 ℃, the finish rolling end temperature is 750 ℃ to 900 ℃ and the total reduction ratio is 50% or more at 950 ℃,

Then, cooling is performed under the conditions that the average cooling rate by a wall thickness center thermometer is 5 ℃ to 30 ℃ per second, the cooling stop temperature is 400 ℃ to 650 ℃,

then, the winding is performed at 400 ℃ to 650 ℃.

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

[10] The method for producing a square steel pipe according to any one of [7] to [9], wherein the average wall thickness t is 20mm 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 excellent in shape characteristics and excellent in ductility and toughness of the outer surface of the corner, a method for producing the square steel pipe, and a building structure.

Thus, a cold roll formed square steel pipe having a small radius of curvature at the corner, a flat plate portion, and excellent ductility and toughness at the outer surface of the corner can be produced. In addition, the building structure using the square steel pipe of the present invention as a column material exhibits more excellent earthquake-resistant performance than the building structure using the conventional cold roll-formed square steel pipe.

Drawings

Fig. 1 is a schematic view showing a cross section perpendicular to the pipe axis direction of a square steel pipe according to the present invention.

Fig. 2 is a schematic diagram showing a pipe manufacturing process of the electric resistance welded steel pipe according to the present invention.

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

Fig. 4 is a schematic diagram illustrating a molten and solidified portion of a welded portion of a resistance welded steel pipe.

Fig. 5 is a schematic view showing an example of the building structure of the present invention.

Fig. 6 is a schematic diagram showing the cutting positions of the tensile test pieces of the flat plate portion and the corner portion implemented in the present invention.

Fig. 7 is a schematic diagram showing a detailed cutting position of a corner tensile test piece according to the present invention.

Fig. 8 is a schematic diagram showing a cutting position of a corner charpy test piece according to the present invention.

Fig. 9 is a schematic diagram showing a detailed cutting position of the corner charpy test piece according to the present invention.

Fig. 10 is a schematic diagram for explaining a flatness measurement method implemented in the present invention.

Detailed Description

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

< square Steel tube >)

The square steel pipe comprises a flat plate portion and a corner portion, wherein 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 portion is 2.0t or more and 3.0t or less, the flatness of the outer surface of the flat plate portion in the pipe axis direction is 2.5mm or less, the uniform elongation E2 at the position of 1/4t of the outer surface of the angle portion in the wall thickness direction is 0.60 times or more relative to the uniform elongation E1 at the position of 1/4t of the outer surface of the flat plate portion in the wall thickness direction, and the Charpy absorption energy at-10 ℃ at the position of 1/4t of the outer surface of the angle portion in the wall thickness direction is 100J or more.

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

In the square steel pipe 10 of the present invention, two or more flat plate portions 11 and corner portions 12 are alternately formed in the pipe circumferential direction. In the example shown in fig. 1, in a square steel pipe 10, 4 corner portions 12 and flat plate portions 11 are formed in this order in the pipe circumferential direction. The square steel pipe 10 has a rectangular shape (approximately rectangular shape) or a square shape (approximately square shape) in a cross section perpendicular to the pipe axis direction. In fig. 1, the side length of 2 flat plate portions 11 adjacent to each other across a corner portion 12 is H ₁ 、H ₂ When H is ₁ ＞H ₂ Namely, welding with the followingThe side length H of the flat plate portion opposite to the portion (resistance welded portion) 13 ₂ Length H of the side of the flat plate part 11 adjacent to the length H ₁ Short relations. In the present invention, the present invention is not limited to this example, and may be H ₁ ＝H ₂ May also be H ₁ ＜H ₂ Is a relationship of (3).

The square steel pipe 10 is manufactured by forming a resistance welded steel pipe into a blank pipe and forming the blank pipe to form a roll-formed square steel pipe. Therefore, the square steel pipe 10 has a resistance welded portion 13 extending in the pipe axis direction and forming a flat plate portion 11. Although not shown, the width of the fusion-solidified portion of the resistance welded portion 13 in the circumferential direction of the tube 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 wall thickness of the flat plate portion is t (mm), the radius of curvature R of the outer side of the corner portion is 2.0t or more and 3.0t or less. The average wall thickness t is a value calculated by the following formula (3).

When the radius of curvature R outside the corner is smaller 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 in the present invention are not obtained at the corners. On the other hand, when the radius of curvature R of the outer side of the corner portion is larger than 3.0t, the Zhou Xiangwan response variable of the flat plate portion of the square forming frame (and the circumferential bending strain amount of the corner portion) decreases. As a result, the flat plate portion does not have the flatness as the object of the present invention. The radius of curvature R is preferably 2.2t or more, and more preferably 2.9t or less.

In the present invention, as described in examples below, when the radii of curvature of two or more portions are measured and the maximum value and the minimum value thereof are within the above-described ranges, it is evaluated that the radius of curvature R outside the corner portion is small. The reason for this evaluation is that R at the corner of the square steel pipe does not act as an average value at 4 points, but acts independently from each other for earthquake resistance and workability.

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

The radius of curvature R is measured in a range of 65 ° centered on an intersection B of the straight line L and the outer surface of the corner 12, in a sector having a center angle of 90 ° centered on the straight line L, the sector being formed by the connection points (point a, point a' shown in fig. 1) of the extension lines L1, L2 to the flat plate 11 and the corner 12 and the outer surface of the corner 12. The method of measuring the radius of curvature includes, for example, a method of measuring the radius of curvature by a radial measuring instrument that sufficiently matches the outer surface of the corner 12 within the above-described range of the center angle of 65 °, but may be measured by other methods.

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 2.5mm or less.

The flatness is described with reference to fig. 10. As shown in fig. 10, the flatness is a value obtained by measuring the maximum bulge amount and the maximum dent amount with respect to a straight line passing through 2 points at both ends in 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. The flatness was obtained by the method described in examples described later in the present invention.

When the flatness is more than 2.5mm, the bending resistance at the time of bending deformation of the square steel pipe is lowered. As a result, the shock resistance of the square steel pipe is reduced. Further, the joint surface with the beam is greatly curved, and therefore, it is difficult to perform welding. As a result, workability is reduced. The smaller the value of flatness, the better. The lower limit of the flatness is not required to be specified, and 0.6mm may be allowed as the lower limit of the flatness. The lower limit of the flatness is preferably 0.2mm, more preferably 0mm. Preferably 2.0mm or less, more preferably 1.5mm or less.

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

When the square steel pipe receives an external force such as a seismic force, the outer surface of the corner is deformed greatly. Therefore, the square steel pipe needs to sufficiently improve the ductility and toughness of the outer surface of the corner.

When the value (value of E2/E1) of the uniform elongation E2 at the position of 1/4t of the outer surface of the pitch angle portion in the wall thickness direction with respect to the uniform elongation E1 at the position of 1/4t from the outer surface of the flat plate portion in the wall thickness direction is smaller than 0.60, the ductility on the outer surface side of the corner portion decreases. As a result, the shock resistance of the square steel pipe is reduced. The value of E2/E1 is preferably 0.70 or more, more preferably 0.80 or more, and still more preferably 0.82 or more. The upper limit of the value of E2/E1 is not particularly limited, but is 1.00 or less from the standpoint of a large work hardening amount and a small uniform elongation at the time of roll forming compared with the flat plate portion at the corner.

In the square steel pipe 10 of the present invention, the charpy absorption energy of the corner 12 at-10 ℃ is 100J or more at the position 1/4t of the outer surface of the corner 12 in the wall thickness direction. When the charpy absorption energy is less than 100J, when an external force such as a seismic force is applied, there is a high risk that plastic deformation does not occur and brittle fracture occurs. The Charpy absorption energy is preferably 150J or more, more preferably 200J or more.

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

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

As described above, in the square steel pipe, the larger the ratio (t/H) of the average wall thickness t to the average side length H and the smaller the radius of curvature of the corner portion, the larger the circumferential bending strain required for forming the corner portion, and the larger the bending deformation amount of the corner portion. As a result, the square steel pipe having a ratio (t/H) larger than the above tends to have reduced ductility and toughness at the corners.

When the value of the ratio (t/H) is 0.030 or less, the durability as a column is lowered, and therefore, the applicable building structure is limited. Therefore, the above ratio (t/H) is preferably set to be more than 0.030. More preferably 0.035 or more, still more preferably 0.040 or more. On the other hand, in order to secure ductility and toughness of the corner portion, the upper limit of the above ratio (t/H) is preferably 0.10. More preferably 0.080 or less.

The average wall thickness t (mm) is obtained by the following formula (3).

t＝(t ₁ +t ₂ +t ₃ ) 3/… type (3)

In the formula (3), t ₁ 、t ₂ : wall thickness (mm), t at a tube circumferential center position of 2 flat plate portions 11 adjacent to each other with respect to the flat plate portion 11 including a welded portion (resistance welded portion) 13 sandwiching a corner portion 12 ₃ : wall thickness (mm) at a tube circumferential center position of the flat plate portion opposite to the flat plate portion including the welded portion (resistance welded portion). That is, the average wall thickness t is an average value of wall thicknesses of 3 flat plate portions other than the flat plate portion including the welded portion, at a central position with respect to the pipe circumferential direction (refer to fig. 1).

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

H＝(H ₁ +H ₂ ) 2/… type (4)

In the formula (4), H ₁ : side length (longitudinal side length in FIG. 1) (mm), H of cross section of arbitrary flat plate portion perpendicular to tube axis direction ₂ : relative to the side length H ₁ The flat plate portions adjacent to each other across the corner portion (the lateral side length in fig. 1) (mm). That is, the average side length H is an average value of side lengths of cross sections perpendicular to the tube axis direction of the 2 flat plate portions 11 adjacent to each other across the corner.

In the square steel pipe 10 of the present invention, the average wall thickness t is preferably 20mm to 40mm from the standpoint of being suitable for use in a building member of a large building such as a middle-level building, a factory, a warehouse, etc. having a height of more than 20 m. The flat plate portion 11 preferably has a yield strength of 295MPa or more and the flat plate portion 11 preferably has a tensile strength of 400MPa or more, from the viewpoint of being suitable for use in building members for middle-level buildings and large-scale buildings, and the corner portion 12 preferably has a yield ratio of 90% or less, from the viewpoint of further excellent shock resistance.

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

The yield strength, tensile strength, and yield ratio described above can be obtained by performing a tensile test in accordance with the regulation of JIS Z2241 as described in examples below. The Charpy absorption energy can be obtained by performing a Charpy impact test at a test temperature of-10℃using a V-notched test piece in accordance with the regulation of JIS Z2242 as described in examples below.

Next, from the viewpoint of securing the mechanical properties and weldability, the preferable ranges of the composition and the steel structure of the square steel pipe 10 of the present invention and the reasons for limiting the same will be described.

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

In the present specification, "%" indicating the composition of steel is "% by mass" unless otherwise specified. The following composition was that of the flat plate portion and the corner portion except for 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. C is an element that contributes to the refinement of the structure by reducing the ferrite transformation start temperature. 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, and thus is an element that contributes to the formation of a hard phase. When the C content is more than 0.45%, the proportion of the hard phase becomes high, toughness decreases, and weldability also deteriorates. Therefore, the C content is set to 0.020 to 0.45%. The C content is preferably 0.040% or more, more preferably 0.050% or more. The C content is preferably 0.40% or less, more preferably 0.30% or less.

Si：0.01～1.0％

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

Mn：0.30～3.0％

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

P: less than 0.10%

Since P segregates in the grain boundaries to cause material inhomogeneity, it is preferable to reduce the amount of unavoidable impurities as much as possible, but 0.10% or less is allowable. Therefore, the P content is set to 0.10% or less. The P content is preferably 0.050% or less, more preferably 0.030% or less. The lower limit of P is not particularly defined, and excessive reduction may cause an increase in smelting cost, so that the P content is preferably set to 0.002% or more.

S: less than 0.050%

S is usually present in steel in the form of MnS, but MnS thinly extends in the hot rolling step and adversely affects ductility. Therefore, in the present invention, S is preferably reduced as much as possible, but 0.050% or less may be allowed. Therefore, the S content is set to 0.050% or less. The S content is preferably 0.030% or less, more preferably 0.010% or less. The lower limit of S is not particularly limited, and excessive reduction may cause an increase in smelting cost, so that S is preferably set to 0.0002% or more.

Al：0.005～0.10％

Al is an element that functions 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 more than 0.10%, weldability is deteriorated, and alumina-based inclusions are increased, and the surface properties are deteriorated. In addition, the toughness of the welded portion is also reduced. Therefore, the Al content is set to 0.005 to 0.10%. The Al content is preferably 0.010% or more, more preferably 0.015% or more. The Al content is preferably 0.080% or less, more preferably 0.070% or less.

N: less than 0.010%

N is an unavoidable impurity, and is an element having an effect of reducing toughness by firmly fixing the motion of dislocation. In the present invention, N is preferably reduced as much as possible as an impurity, but the content of N may be allowed to be 0.010% or less. Therefore, the N content is set to 0.010% or less. The N content is preferably 0.0080% or less. From the viewpoint of refining cost, the N content is preferably 0.0008% or more.

Ti：0.001～0.15％

Ti is an element that contributes to the strength improvement of steel by forming fine carbides and nitrides in steel. Further, since the alloy has high affinity with N, N in steel is also an element that contributes to improvement of toughness of steel by making N in steel harmless as a nitride. In order to obtain the above effect, it is preferable to contain 0.001% or more of Ti. However, when the Ti content is more 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 still more preferably 0.005% or more. The Ti content is more preferably 0.10% or less, and still more preferably 0.08% or less.

The balance other than the above components is Fe and unavoidable impurities. However, as an inevitable impurity, 0.0050% or less of O may be contained. O herein refers to the total oxygen comprising O in oxide form. Nb:0 to less than 0.001%, V:0 to less than 0.001%, cr:0 to less than 0.01 percent, mo:0 to less than 0.01%, cu:0 to less than 0.01 percent, ni:0 to less than 0.01 percent, ca:0 to less than 0.0002%, B: from 0 to less than 0.0001% is treated as an unavoidable impurity.

In the present invention, the above-described components are preferably used as basic components. The characteristics targeted in the present invention can be obtained by using the above preferred elements, but in order to further improve the characteristics, it may further contain, if necessary, a component selected from the group consisting of Nb:0.001 to 0.15 percent, V:0.001 to 0.15 percent of Cr:0.01 to 1.0 percent of Mo:0.01 to 1.0 percent of Cu:0.01 to 1.0 percent of Ni:0.01 to 1.0 percent of Ca:0.0002 to 0.010 percent, B: 0.0001-0.010% of one or more than two kinds.

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 the steel and also contributes to the refinement of the structure by suppressing the coarsening of austenite during hot rolling, and may be contained as needed. In order to obtain the above effect, when Nb is contained, it is preferable to contain 0.001% or more of Nb. However, when the Nb content is more than 0.15%, the yield ratio increases and the toughness decreases. Therefore, in the case of containing Nb, the Nb content is preferably set to 0.15% or less. The Nb content is more preferably 0.002% or more, and still more preferably 0.005% or more. The Nb content is more preferably 0.10% or less, and still more preferably 0.08% or less.

V：0.001～0.15％

V is an element that contributes to the strength improvement of steel by forming fine carbide and nitride in steel, and may be contained as necessary. In order to obtain the above effect, when V is contained, V is preferably contained in an amount of 0.001% or more. However, when the V content is more than 0.15%, the yield ratio increases and the toughness decreases. Therefore, in the case of containing V, the V content is preferably set to 0.15% or less. The V content is more preferably 0.002% or more, and still more preferably 0.005% or more. The V content is more preferably 0.10% or less, and still more preferably 0.08% or less.

Cr：0.01～1.0％

Cr is an element that improves hardenability of steel and increases strength of steel, and may be contained as needed. In order to obtain the above-described effects, when Cr is contained, the Cr content is preferably set to 0.01% or more. On the other hand, the inclusion of Cr of more than 1.0% may cause a decrease in toughness and a deterioration in 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 still more preferably 0.05% or more. The Cr content is more preferably 0.90% or less, and still more preferably 0.80% or less.

Mo：0.01～1.0％

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

Cu：0.01～1.0％

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

Ni：0.01～1.0％

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

Ca：0.0002～0.010％

Ca is an element contributing to the improvement of toughness of steel by spheroidizing sulfide such as MnS which thinly extends in a hot rolling step in the production of a raw steel sheet, and may be contained as necessary. In order to obtain the above effect, when Ca is contained, ca is preferably contained in an amount of 0.0002% or more. However, if the Ca content is more than 0.010%, ca oxide clusters are formed in the steel, and toughness is deteriorated. Therefore, in the case of containing Ca, the Ca content is preferably set to 0.010% or less. The Ca content is more preferably 0.0005% or more, and still more preferably 0.0010% or more. The Ca content is more preferably 0.008% or less, and still more preferably 0.0060% or less.

B：0.0001～0.010％

B is an element that contributes to the refinement of the structure by reducing the ferrite transformation start temperature, and may be contained as necessary. In order to obtain the above effect, when B is contained, it is preferable to contain 0.0001% or more of B. However, when the B content is more than 0.010%, the yield ratio increases and the toughness deteriorates. Therefore, in the case of containing B, the B content is preferably set to 0.010% or less. The content of B is more preferably 0.0005% or more, and still more preferably 0.0008% or more. The B content is more preferably 0.0050% or less, still more preferably 0.0030% or less, and still more preferably 0.0020% or less.

Next, the steel structure will be 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 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 balance is one or two or more selected from pearlite, martensite and austenite, and when a region surrounded by a boundary where the difference in orientation of adjacent crystals is 15 ° or more is defined as crystal grains, the average crystal grain diameter of the crystal grains is 15.0 μm or less, and the total of the crystal grains having a crystal grain diameter of 40 μm or more is preferably 40% or less relative to the entire steel structure at the center of the wall thickness of the flat plate portion.

Aggregate of ferrite and bainite volume ratios: 70% to 95%

Ferrite is a soft structure. The bainite is hard compared with ferrite, soft compared with pearlite, martensite, and austenite, and has a structure with excellent toughness. When a hard structure (pearlite, martensite, and austenite) is mixed in ferrite and bainite, the yield ratio is reduced, but on the other hand, the interface is likely to be a starting point of fracture due to stress concentration caused by a difference in hardness, and toughness is reduced. Therefore, in order to obtain the yield ratio and toughness described above, the total volume ratio of ferrite and bainite in the center of the wall thickness of the flat plate portion is preferably 70% to 95% with respect to the entire steel structure in 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 ratio of hard structure is high, and the yield stress increases, so that the yield ratio increases, and toughness decreases. When the total volume ratio of ferrite and bainite is more than 95%, the tensile strength is reduced, and the yield ratio is increased. More preferably 73% or more and 93% or less. More preferably 75% or more and 92% or less.

The remainder (remainder) excluding the magnetic element and the bainite is one or more selected from pearlite, martensite, and austenite. When the total volume fraction of the rest of the structure is less than 5%, the tensile strength is reduced, and thus the yield ratio is increased. When the total volume ratio of the remaining tissues is more than 30%, the ratio of the hard tissues is high, and the yield stress increases, so that the yield ratio increases, and the toughness decreases. Therefore, the total volume ratio of the surplus tissue is preferably 5% to 30% with respect to the entire steel sheet structure at the center of the wall thickness of the flat plate portion. More preferably 7% or more and 27% or less. More preferably 8% to 25%.

The above-described various structures (ferrite, bainite, pearlite, martensite) other than austenite use austenite grain boundaries or deformed bands within austenite grains as nucleation sites. In hot rolling in the process of producing a raw steel sheet of an electric resistance welded steel pipe (blank pipe) used for producing a square steel pipe, by increasing the reduction in pressure at a low temperature at which recrystallization of austenite is less likely to occur, a large amount of dislocation can be introduced into austenite to refine the austenite, and a large amount of deformation zone can be introduced into crystal grains. This increases the area of the nucleation sites and the nucleation frequency, thereby making it possible to refine the steel structure.

In the present invention, even if the above-mentioned steel structure exists within a range of ±1.0mm in the thickness direction around the center of the thickness, the above-mentioned effects can be obtained in the same manner. Accordingly, in the present invention, the term "steel structure at the center of the wall thickness" means that the steel structure exists within any range of ±1.0mm in the wall thickness direction around the center of the wall thickness.

As observation of the steel structure, first, a test piece for observation of the structure was cut so that the observation surface became a cross section parallel to both the longitudinal direction and the wall thickness direction of the square steel pipe and became the wall thickness center of the flat plate portion, and after mirror polishing, it was etched with an aqueous solution of nitric acid and ethanol. In the tissue observation, the tissue at the center of the wall thickness was observed and imaged using an optical microscope (magnification: 1000 times) or a scanning electron microscope (SEM, magnification: 1000 times). The area ratios of ferrite, bainite and the balance (pearlite, martensite, austenite) were determined from the obtained optical microscope images and SEM images. The area ratio of each tissue was observed in 5 or more fields of view, and calculated as an average value of values obtained in each field of view. The area ratio obtained by observation of the tissues was defined as the volume ratio of each tissue.

Here, ferrite is a product resulting from diffusion transformation, and exhibits a structure with low dislocation density and substantial recovery. Polygonal ferrite and pseudo polygonal ferrite are contained therein.

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

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

Martensite is a lath-like low-temperature phase transformation structure with a very high dislocation density. In SEM images, contrast is shown to be brighter than ferrite and bainite. Since it is difficult to identify martensite and austenite by using an optical microscope image and an SEM image, the area ratio of the structure observed as martensite or austenite is measured from the obtained SEM image, and the volume ratio of austenite measured by a method described later is subtracted therefrom, and the obtained value is taken as the volume ratio of martensite.

In the measurement of the volume fraction of austenite, a test piece produced by the same method as that used in the measurement of dislocation density was used, and the measurement was performed by X-ray diffraction. The volume fraction of austenite was obtained from the integrated intensities of the (200), (220), (311) planes of fcc iron and the (200), (211) planes of bcc iron obtained.

Average crystal grain diameter of crystal grains: 15.0 μm or less

In the present invention, the average crystal grain diameter refers to an average equivalent circle diameter of crystal grains (grain boundaries) when a region surrounded by boundaries where the orientation difference between adjacent crystals is 15 ° or more is defined as the crystal grains. The equivalent circle diameter (crystal grain diameter) is the diameter of a circle having an area equal to that of the target crystal grain.

When the average crystal grain diameter of the crystal grains is larger than 15.0 μm, the total area of grain boundaries which become an obstacle to crack propagation is small, and thus the desired toughness is not obtained. Therefore, in the present invention, the average crystal grain size of the crystal grains is set to 15.0 μm or less. The average crystal grain diameter of the crystal grains is preferably 13.0 μm or less, more preferably 10.0 μm or less. Since the yield ratio increases as the average crystal grain size decreases, the average crystal grain size is preferably 2.0 μm or more.

The total of the volume fractions of crystal grains having a crystal grain diameter of 40 μm or more: less than 40%

Even if the upper limit of the maximum crystal grain size is defined, if a certain amount of coarse crystal grains are present, there is a region where the total area of grain boundaries that become an obstacle to crack propagation is small, and therefore toughness is greatly reduced. Therefore, in order to obtain good toughness, it is necessary to further define the upper limit of the proportion of coarse grains present. Accordingly, in the present invention, the total of the volume fractions of crystal grains having a crystal grain diameter of 40 μm or more is set to 40% or less. More preferably 30% or less. For the above reasons, it is desirable that coarse crystal grains are small, and the total of the volume fractions of the crystal grains is preferably 0%.

The total of the average crystal grain size of the crystal grains and the volume fraction of the crystal grains having a crystal grain size of 40 μm or more was measured as follows. First, a test piece for tissue observation was cut so that the observation surface was a cross section parallel to both the longitudinal direction and the wall thickness direction of a square steel pipe and became the wall thickness center of a flat plate portion, and after mirror polishing, a histogram of particle diameter distribution (a graph in which the horizontal axis represents particle diameter and the vertical axis represents the presence ratio (area ratio) in terms of each particle diameter) was calculated using the SEM/EBSD method at the wall thickness center. The average crystal grain size was obtained from the histogram by arithmetic mean of the grain sizes. The total of the volume ratios of the crystal grains of 40 μm or more is obtained from the histogram in terms of the total of the existing ratios of the crystal grains of 40 μm or more in particle diameter. As measurement conditions, the acceleration voltage was set at 15kV, the measurement region was set at 500. Mu.m.times.500. Mu.m, and the measurement step size (measurement resolution) was set at 0.5. Mu.m. In the crystal grain size analysis, crystal grains having a crystal grain size of less than 2.0 μm were excluded from the analysis target as measurement noise.

Method for manufacturing square steel pipe

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

The method for manufacturing the square steel pipe 10 of the present invention comprises the following steps: the steel sheet as a raw material is cold roll formed, then, both widthwise ends of the cold roll formed steel sheet 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, square formed by a forming stand, whereby a square steel pipe is produced. At this time, the plate width W of the steel plate is set to be equal to the perimeter C of the square steel pipe at the outlet side of the square forming frame _OUT The ratio of (2) satisfies the ratio (1) and the perimeter C of the resistance welded steel pipe at the inlet side of the square forming frame _IN Circumference C of square steel pipe relative to outlet side of square forming frame _OUT In a manner that the ratio of (2) satisfies the formula (2), for a sizing frame immediately before square formingThe gap between the rolls and the gap between the rolls of the square forming frame are controlled.

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

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

In the formulae (1) and (2),

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

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

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

A method for manufacturing the square steel pipe 10 according to the present invention will be described in detail with reference to fig. 2 and 3. Fig. 2 is a diagram illustrating a pipe making process of a square steel pipe blank (resistance welded steel pipe) according to the present invention. Fig. 3 is a diagram illustrating a process for forming a square steel pipe according to 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 process).

As shown in fig. 2, a steel sheet 1 (hot-rolled steel sheet, hot-rolled steel strip) having the above composition, which is wound into a coil, is unwound, leveled by a leveler 2, and intermediately formed by a row roller group 3 composed of two or more rollers to form a cylindrical open pipe. Then, finish forming is performed by using the fin hole type roll set 4 composed of two or more rolls. The open pipe is formed into a cylindrical shape by cold roll forming.

The square steel pipe of the present invention preferably has the above-described steel structure. As described above, since the square steel pipe of the present invention is produced by further square-forming an electric resistance welded steel pipe (blank pipe) obtained by cold roll forming a raw steel sheet, the raw steel sheet (steel sheet 1) preferably also has the above-described composition and steel structure. The preferable production conditions of the steel sheet 1 will be described later, and therefore, the description thereof will be omitted.

The finish-formed open pipe is pressed by a squeeze roller 5, and a pair of butt portions (both widthwise end portions) opposed to each other in the circumferential direction of the steel sheet 1 are resistance welded (resistance welded) to each other by a welder 6 to produce a resistance welded steel pipe 7. In the above-described resistance welding, the butt portion is heated and melted by, for example, high-frequency induction heating or high-frequency resistance heating, and is pressure-bonded and solidified, thereby completing the bonding. Thereby, the welded portion (resistance welded portion) 13 is provided to extend in the tube axis direction. The manufacturing equipment used for manufacturing the electric resistance welded steel pipe 7 is not limited to the manufacturing equipment having the pipe manufacturing process shown in fig. 2.

In the present invention, the upsetting amount by the squeeze roller 5 is preferably set to a range of 20% to 100% with respect to the wall thickness of the electric resistance welded steel pipe 7 in the process of manufacturing the electric resistance welded steel pipe. When the upsetting amount is less than 20% of the wall thickness, the molten steel is insufficiently discharged, and the toughness of the welded portion is deteriorated. On the other hand, when the upsetting amount is more than 100% of the wall thickness, the load on the squeeze roller increases, and the work hardening amount of the welded portion (resistance welded portion) 13 increases, and the hardness excessively increases.

Next, a square steel pipe was manufactured using the obtained electric resistance welded steel pipe 7 as a blank pipe (forming step). The forming step includes a sizing step and a square forming step.

As shown in fig. 3, the electric resistance welded steel pipe 7 is reduced in diameter in a cylindrical shape by a sizing roller group (sizing frame) 8 composed of two or more rollers arranged vertically and laterally with respect to the electric resistance welded steel pipe 7 (sizing step). Then, the square steel pipe 10 is formed by square forming the steel pipe in the order indicated by R1, R2, and R3 using a square forming roller group (square forming frame) 9 composed of two or more rollers arranged vertically and horizontally with respect to the electric resistance welded steel pipe 7 (forming step). Each of the rolls constituting the forming frame 9 is a grooved roll (bore roll) having a bore curvature, and the bore curvature increases as the latter frame is formed. Thereby, a flat plate portion and corner portions of the square steel pipe are formed.

The number of frames constituting the sizing roller group 8 and the square forming roller group 9 is not particularly limited. There are cases where two or more frames are provided, and also where one frame is provided. 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 (has two or more curvatures), the diameter curvature of each roll is preferably constant because the irregular shape occurs when the electric resistance welded steel pipe 7 during forming is twisted in the circumferential direction.

In the present invention, as described above, the width W of the steel plate is set to be equal to the perimeter C of the square steel pipe on the outlet side of the square forming frame _OUT The ratio of (2) satisfies the ratio (1) and the perimeter C of the resistance welded steel pipe at the inlet side of the square forming frame _IN Circumference C of square steel pipe relative to outlet side of square forming frame _OUT It is important to control the gap between the rolls of the sizing mill immediately before square forming and the gap between the rolls of the square forming mill so that the ratio of (2) is satisfied. Thus, even if the average wall thickness t is larger than the average side length H (t/H) and the radius of curvature R of the corner is smaller, the ductility and toughness of the outer surface of the corner can be improved.

First, the sheet width W (mm) of the raw steel sheet (steel sheet) 1 is set to be equal to the circumferential length of the square steel tube 10 immediately after the formation (the circumferential length (mm) of the steel tube on the outlet side of the square forming frame in the final stage, hereinafter referred to as "C" _OUT ") ratio (W/C) _OUT ) The reason why the ratio (t/H) of the average wall thickness t after the rectangular molding to the average side length H after the rectangular molding satisfies the above-described expression (1) is explained.

As shown in fig. 2 and 3, when a flat steel plate 1 (raw steel plate) is cold roll formed 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 manufacture a square steel pipe 10, the manufacturing process (pipe maker) Sequence, forming step), the steel sheet 1 and the electric resistance welded steel pipe 7 are subjected to bending deformation in the pipe circumferential direction, and elongation deformation in the pipe longitudinal direction due to reduction in the pipe circumferential direction is also applied. To reduce the amount of tube circumferential reduction in the manufacturing process, the above-mentioned 2 ratios "t/H" and "W/C" are appropriately controlled _OUT "is effective".

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

On the other hand, the above ratio "W/C _OUT When "the value is equal to or greater than the right value of the formula (1), the difference in tube (or open tube) circumference between the tube making step and the forming step increases. As a result, the pipe is greatly reduced in diameter in the circumferential direction, and therefore, the corners are greatly work-hardened, and the desired ductility and toughness of the outer surfaces of the corners are not obtained.

The above ratio "W/C _OUT "is preferably (1.000+0.080 Xt/H) or more and (1.000+0.48 Xt/H) or less, more preferably (1.000+0.10 Xt/H) or more and (1.000+0.45 Xt/H) or less.

Next, the girth of the electric resistance welded steel pipe 7 immediately before square forming (the girth (mm) of the electric resistance welded steel pipe 7 at the inlet side of the square forming frame of the first stage, hereinafter referred to as "C" _IN ") and the circumference (C) of the square steel pipe 10 immediately after the formation _OUT ) Ratio (C) _IN /C _OUT ) The reason why the ratio (t/H) of the average wall thickness t after the rectangular molding to the average side length H after the rectangular molding satisfies the above-described expression (2) is explained.

As shown in fig. 3, when square-forming the cylindrical electric resistance welded steel pipe 7 into the square steel pipe 10, the steel pipe is gradually square-formed from a cylindrical shape by passing through the square-forming roller group 9 as described above. In such square forming, bending of the straight portions (flat plate portions 11) of the sides, bending of the corner portions 12, and reducing deformation in the circumferential direction of the resistance welded steel pipe 7 occur.

Particularly, at the periphery of the corner 12, the square forming is completed with little contact between the rolls of the square forming roll group 9. In square forming, the corner 12 is formed by protruding by free deformation. At this time, the higher the rigidity of the corner 12 and the smaller the circumferential reduction amount, the smaller the amount of bending deformation of the corner 12 and the larger the radius of curvature of the outside of the corner. On the other hand, the lower the rigidity of the corner 12 and the larger the circumferential reduction, the larger the bending deformation of the corner 12 and the smaller the radius of curvature of the outside of the corner.

Further, the greater the ratio (t/H) of the average wall thickness t to the average side length H, the higher the rigidity of the corner 12 against bending deformation. In addition, the circumferential reduction in square forming uses the circumferential length ratio (C _IN /C _OUT ) The larger it is, the larger the circumferential reduction is.

Therefore, when t/H increases, it is difficult to form the corner 12 by bending deformation. Therefore, in order to obtain a desired corner curvature radius, it is necessary to increase the circumferential length ratio (C _IN /C _OUT ) And the circumferential reduction is increased. For this reason, the above 2 ratios "t/H" and "C" are appropriately controlled _IN /C _OUT "is effective".

Circumference ratio (C) _IN /C _OUT ) When the value is smaller than the left side of the formula (2), the difference in tube circumference between before and after the forming step is reduced, and the circumferential reduction of the 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 plate portion is not obtained, and the radius of curvature R outside the corner portion becomes larger than 3.0 times (3.0 t) the average wall thickness t.

On the other hand, the circumferential length ratio (C _IN /C _OUT ) When the value is equal to or larger than the right value of the formula (2), the difference in tube circumference between before and after the molding step increases. As a result, the pipe circumferential reducing amount is large, and therefore, the corner is greatly work-hardened, and the desired ductility and toughness of the corner are not obtained. In addition, the radius of curvature R outside the corner is less than 2.0 times (2.0 t) the average wall thickness t.

Circumference ratio (C) _IN /C _OUT ) Preferably (0.33×t/H+0.99) or more and (0.47×t/H+0.99) or less, more preferably (0.35×t/H+0.99) or more and (0)45 Xt/H+0.99) or less.

In the present invention, from the viewpoint of further improving the shock resistance, it is preferable to control the conditions of the above-described formulas (1) and (2) under the following conditions.

When the average wall thickness of the flat plate portion of the square steel pipe 10 is t (mm) and the average side length of the flat plate portion is H (mm), the average wall thickness t is preferably set to be greater than 0.030 times the average side length H. As a result, the column material has improved durability and rigidity, and as a result, the shock 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 order to secure ductility and toughness of the corner portions, the ductility and toughness are preferably 0.10 times or less, more preferably 0.080 times or less.

The average wall thickness t is preferably set to 20mm to 40 mm. The reason for this is the same as the reason for controlling the average wall thickness t of the square steel pipe described above, and therefore, this is omitted.

Further, the gap between the sizing roller and the square forming roller is preferably controlled.

In addition, C _IN And C _OUT The control of the gap between the concave portions of the bore roller is performed. When the difference between the maximum gap between the recesses of the rolls of the sizing mill immediately before square forming (hereinafter also referred to as "gap of the sizing mill") and the maximum gap between the recesses of the rolls of the square forming mill (hereinafter also referred to as "gap of the square forming mill") is defined as Δg, the gap of the sizing mill immediately before square forming is preferably adjusted 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, in the above formula (2), (C) _IN /C _OUT ) The value smaller than the left side, as described above, does not give the curvature radius of the outer sides of the flat plate portion and the corner portion targeted in the present invention. On the other hand, when G is more than 180, in the above formula (2), (C) _IN /C _OUT ) The right value or more, as described above, does not give the targeted ductility and toughness of the corner in the present invention. Preferably, G is 80 or more and less than 170.

In the case where there are two or more sizing stands, the gap between the sizing stands immediately before square forming may be the same as the gap between the other sizing stands. In the case where two or more square molding frames are present, the clearance between the square molding frames is preferably set to be the clearance between the square molding frames in the first stage. The gap between the first section and the other square shaped racks may all be the same.

Here, C is as described above _IN Refers to the perimeter (length of the outer circumference in the tube circumferential direction) of the electric resistance welded steel tube 7 (mm) on the inlet side of the square-shaped forming frame of the first stage. As shown in FIG. 3, C _IN The method comprises the following steps: when the pipe making direction is the positive direction of the X axis, the X coordinate of the rotation axis of any one of the sizing roller groups 8 immediately before square forming is Xa (m), and the X coordinate of the rotation axis of any one of the square forming roller groups 9 in the first stage is Xb (m), the outer circumference of the circumferential section of the pipe in the plane x= (xa+xb)/2 (m) perpendicular to the X axis is measured by a tape measure.

C as described above _OUT Refers to the circumference (length of the outer circumference in the circumferential direction of the tube) (mm) of the square steel tube 10 on the outlet side of the square forming frame of the final stage. As shown in FIG. 3, C _OUT The method comprises the following steps: the outer circumference of the circumferential cross section of the tube in the plane x=xc+1 (m) perpendicular to the X axis is measured by a tape measure, with the X coordinate of the rotation axis of any one of the square forming frames in the final stage of the roller group being Xc (m).

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

In the sizing step after the resistance welding, the steel pipe may be reduced in diameter so that the total circumference of the steel pipe is 0.30% or more in order to satisfy the preferable roundness. In this way, each flat plate portion and each corner portion are uniformly (symmetrically) formed in the subsequent forming step, and the fluctuation in flatness and radius of curvature is reduced. The above-mentioned "preferable roundness" means that the outside diameter D1 in the vertical direction and the outside diameter D2 in the horizontal direction of the tube are |D1-D2|/(d1+d2)/2) < 0.020.

However, when the diameter of the steel pipe is reduced so that the total circumference of the steel pipe is greater than 2.0%, the amount of bending in the pipe axis direction when the roller passes increases, and the yield ratio increases. Therefore, it is preferable to reduce the diameter of the steel pipe so that the circumference of the steel pipe is reduced by 0.30% or more and 2.0% or less.

In the sizing step, it is preferable to reduce the diameter of the roll in two or more stages by using two or more stands in order to minimize the amount of bending in the tube axis direction during the passing of the roll and to suppress the occurrence of residual stress in the tube axis direction. In this case, the diameter of each frame is preferably reduced by 1.0% or less as compared with the diameter of the frame disposed immediately before the frame.

As described above, in the square steel pipe of the present invention, the electric resistance welded steel pipe is used as a blank pipe. The judgment as to whether or not the square steel pipe 10 is obtained from the resistance welded steel pipe 7 can be made by the following method: the square steel pipe 10 was cut vertically in the pipe axis direction, and the cut surface including the welded portion (resistance welded portion) 13 was polished and then corroded, and observed with an optical microscope. The width of the welded portion (resistance welded portion) 13 in the pipe circumferential direction is the resistance welded steel pipe 7 when the total thickness of the pipe is 1.0 μm or more and 1000 μm or less. The etching solution may be appropriately selected according to the type of steel component and steel pipe.

Here, a welded portion (resistance welded portion) will be described with reference to fig. 4. Fig. 4 shows a schematic view of the melt-solidified portion 16 of the welded portion 13. Fig. 4 shows a state after polishing and etching the cut surface including the welded portion. The melt-solidified portion 16 can be identified as a region having a different structure morphology and contrast from those of the base material portion 14 and the heat-affected zone 15 in fig. 4. For example, the melt-solidified portion 16 of the resistance welded steel pipe of carbon steel and low alloy steel can be designated as a region observed more clearly with an optical microscope in the above-described cross section after corrosion with an aqueous solution of nitric acid and ethanol.

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

For example, it is preferable that a steel material having the above-mentioned composition is heated to a heating temperature of 1100 ℃ or more and 1300 ℃ or less, then subjected to a hot rolling treatment (hot rolling step) in which the rough rolling end temperature is 850 ℃ or more and 1150 ℃ or less, the finish rolling end temperature is 750 ℃ or more and 900 ℃ or less, and the total reduction rate at 950 ℃ or less is 50% or more, then subjected to cooling (cooling step) in a condition in which the average cooling rate by a wall thickness center thermometer is 5 ℃/s or more and 30 ℃/s or less, and the cooling stop temperature is 400 ℃ or more and 650 ℃ or less, and then subjected to coiling (coiling step) in a temperature of 400 ℃ or more and 650 ℃ or less, thereby producing a hot rolled steel sheet (steel sheet 1).

In the following description of the production method, unless otherwise specified, the expression "°c" related to temperature is set to the surface temperature of the steel material and the steel sheet (hot-rolled steel sheet). These surface temperatures can be measured using a radiation thermometer or the like. The temperature at the center of the thickness of the steel sheet can be obtained by calculating the temperature distribution in the steel sheet cross section by heat transfer analysis and correcting the result with the surface temperature of the steel sheet. In addition, "hot-rolled steel sheet" includes a hot-rolled sheet and a hot-rolled steel strip.

In the present invention, the method for melting the steel raw material (billet) is not particularly limited, and any known melting method such as a converter, an electric furnace, and a vacuum melting furnace is suitable. The casting method is not particularly limited, and the casting method may be a known casting method such as continuous casting method, for example, to produce a desired size. The ingot-cogging rolling method was used instead of the continuous casting method, and there was no problem. The molten steel may be further subjected to secondary refining such as ladle refining.

Hot rolling process

Heating temperature: 1100 ℃ to 1300 DEG C

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

In the present invention, in addition to the conventional method of manufacturing a billet (slab) and then cooling it to room temperature once and then heating it again, the energy-saving process of directly feeding the billet into a heating furnace in the state of a hot plate without cooling to room temperature or rolling it immediately after slightly keeping heat may be applied without any problem.

Finishing temperature of rough rolling: 850 ℃ to 1150 DEG C

When the rough rolling end temperature is lower than 850 ℃, the steel sheet surface temperature becomes equal to or lower than 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 ℃, the reduction in the austenite non-recrystallization temperature range is insufficient, and fine austenite grains are not obtained. As a result, it is difficult to secure the average crystal grain size of the steel structure of the square steel pipe, and toughness is lowered. The rough rolling finishing temperature is more preferably 860 ℃ or higher. The rough rolling finishing temperature is more preferably 1000 ℃ or lower.

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

Finish finishing temperature: 750 ℃ to 900 DEG C

When the finish rolling finish temperature is less than 750 ℃, the surface temperature of the steel sheet during finish rolling becomes equal to or lower than the ferrite transformation start temperature, a large amount of processed ferrite is generated, and the yield ratio increases. On the other hand, when the finish rolling completion temperature exceeds 900 ℃, the reduction in the austenite non-recrystallization temperature range is insufficient, and fine austenite grains are not obtained. As a result, it is difficult to secure the average crystal grain size of the steel structure of the square steel pipe, and toughness is lowered. The finish rolling finishing temperature is more preferably 770℃or higher. The finish rolling finishing temperature is more preferably 880 ℃ or lower.

Total reduction at 950 ℃ or lower: more than 50 percent

In the present invention, the subgrain grains in austenite are refined in the hot rolling step, and ferrite, bainite, and the balance of the microstructure generated in the subsequent cooling step and coiling step are refined, whereby a steel structure of a square steel pipe having the above strength and toughness can be obtained. In order to refine the subgrain in the austenite in the hot rolling step, it is necessary to increase the reduction ratio in the range of the austenite unrecrystallized temperature and introduce a sufficient working strain. In order to achieve the object, in the present invention, the total reduction ratio at 950 ℃ or lower is set to 50% or more.

When the total reduction ratio is less than 50% at 950 ℃, sufficient working strain cannot be introduced in the hot rolling step, and therefore, a structure having the average crystal grain size of the square steel pipe cannot be obtained. The total reduction rate at 950 ℃ or lower is more preferably 55% or more, and still more preferably 57% or more. The upper limit is not particularly limited, and if it is more than 80%, the effect of improving the toughness with respect to the increase in the rolling reduction is reduced, and only the load on the equipment is increased. Therefore, the total reduction rate at 950 ℃ or lower is preferably 80% or lower. More preferably 70% or less.

The total reduction ratio at 950 ℃ or lower is the total reduction ratio of each rolling pass in the temperature range of 950 ℃ or lower.

Cooling process

After the hot rolling step, the hot rolled sheet is subjected to a cooling process in a cooling step. In the cooling step, cooling is performed under conditions in which the average cooling rate up to the cooling stop temperature is 5 ℃ to 30 ℃ per second, and the cooling stop temperature is 400 ℃ to 650 ℃.

Average cooling rate from start of cooling to stop of cooling (end of cooling): 5 ℃/s to 30 ℃/s

When the average cooling rate in the temperature range from the start of cooling to the stop of cooling, which will be described later, is less than 5 ℃/s, the nucleation frequency of ferrite or bainite is reduced, and these coarsens, so that the structure having the average crystal grain size of the square steel pipe is not obtained. On the other hand, when the average cooling rate is more than 30 ℃/s, a large amount of martensite is formed, and the toughness is lowered. The average cooling rate is preferably 10 ℃/s or more. The average cooling rate is preferably 25 ℃/s or less.

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

Cooling stop temperature: 400 ℃ to 650 DEG C

When the cooling stop temperature is lower than 400 ℃ by a wall thickness center thermometer 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 ℃, the nucleation frequency of ferrite or bainite decreases, and these coarsens, so that the structure having the average crystal grain size of the square steel pipe cannot be obtained. The cooling stop temperature is preferably 430 ℃ or higher. The cooling stop temperature is preferably 620 ℃ or lower.

In the present invention, unless otherwise specified, the average cooling rate is set to a value (cooling rate) obtained by ((center temperature of thickness of hot rolled sheet before cooling-center temperature of thickness of hot rolled sheet after cooling)/cooling time). The cooling method includes water cooling by spraying water or the like from a nozzle, cooling by spraying a cooling gas, and the like. In the present invention, in order to cool both surfaces of the hot rolled sheet under the same conditions, it is preferable to perform a cooling operation (treatment) on both surfaces of the hot rolled sheet.

Winding process

After the cooling step, the hot-rolled steel sheet is coiled into a coil shape in a coiling step, and then cooled. In the coiling step, in order to obtain the above-mentioned steel sheet structure, coiling is preferably performed at a coiling temperature of 400 ℃ to 650 ℃. When the coiling temperature is lower than 400 ℃, a large amount of martensite is generated, and the toughness is reduced. When the coiling temperature exceeds 650 ℃, the nucleation frequency of ferrite or bainite decreases, and these coarsens, so that a structure having the average crystal grain size of the square steel pipe is not obtained. The winding temperature is preferably 430℃or higher. The winding temperature is preferably 620 ℃ or lower.

< building Structure >)

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

As shown in fig. 5, in the building structure 100 of the present invention, two or more square steel pipes 10 (columns) are welded to each other via the partition 17. Girders 18 are installed between adjacent square steel pipes 10, and trabeculae 19 are installed between adjacent girders 18. In addition, for wall mounting or the like, the studs 20 are also provided appropriately. In addition, well-known elements may be used for the building structure 100.

As described above, the square steel pipe 10 of the present invention has a small radius of curvature at the corner 12, and the flat plate 11 is flat, and has excellent shape characteristics. Further, the external surfaces of the corners 12 of the square steel pipe 10 of the present invention are excellent in ductility and toughness. Accordingly, the building structure 100 of the present invention using the square steel pipe 10 as a column can ensure plastic deformation capability of the entire structure, and thus exhibits excellent earthquake resistance performance as compared with a building structure using a conventional square steel pipe.

Examples

The present invention will be described in more detail with reference to examples. The present invention is not limited to the following examples.

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

Molten steel having the composition shown in table 1 was melted to prepare a slab (steel stock). The obtained slab was subjected to a hot rolling step, a cooling step and a coiling step under the conditions shown in Table 2-1 to prepare a hot-rolled steel sheet.

The resulting hot rolled steel sheet (raw steel sheet) was continuously cold formed into an open pipe having an elliptical cross section using a row roll set and a fin hole roll set. Then, the opposite end surfaces (both end portions in the width direction) of the open pipe are heated to a temperature equal to or higher than the melting point by high-frequency induction heating or high-frequency resistance heating, and pressure-bonded by a squeeze roll to produce a resistance welded steel pipe.

The obtained electric resistance welded steel pipe (blank pipe) was reduced in diameter by a sizing roller group of 2 frames (2 segments), and then formed by a square forming roller group of 4 frames (4 segments), to obtain square steel pipes having the dimensions shown in table 2-2, respectively. In the square forming step, the gap between the sizing rolls immediately before square forming and the gap between the square forming rolls were controlled under the conditions shown in table 2-2. The square steel pipe obtained was approximately rectangular in cross section perpendicular to the pipe axis direction.

The average wall thickness t (mm) of the square steel pipe shown in table 2-2 was calculated using the above formula (3), and the average side length H (mm) of the square steel pipe was calculated using the above formula (4). Side length H of square steel pipe ₁ And H ₂ (mm) the side length of the flat plate portion of the portion shown in FIG. 1 was measured. The width W (mm) of the steel sheet as a stock was measured as the width of the steel sheet immediately after passing through the leveler. Perimeter C of resistance welded steel pipe on inlet side of square forming frame of first section _IN (mm), perimeter C of square Steel tube on exit side of square Forming frame of final section _OUT (mm) and the difference (Δg) in the maximum gap between the diameter roll of the sizing mill immediately before square forming and the concave portion of the diameter roll of the square forming mill of the first stage were measured by the above-described methods. Then, G (= Δg/(t/H)) is calculated using the above-described difference (Δg), average wall thickness t, and average side length H.

Further, each of the square steel pipes obtained was cut perpendicularly to the pipe axis direction, and the cut surface including the resistance welded portion was polished and then etched with an aqueous solution of nitric acid and ethanol, and observed with an optical microscope. It was also confirmed that the width of the fusion-solidified portion of the resistance welded portion in the circumferential direction of the tube was 1.0 μm or more and 1000 μm or less in the total thickness of the tube. The melt-solidified portion may be designated as a region observed more clearly with an optical microscope in the above-described cross section after etching with the nitric acid-ethanol solution.

The steel structure of the square steel pipe thus obtained was quantitatively determined, tested and evaluated by the following methods.

(1) Steel structure of square steel pipe

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

(2) Radius of curvature of outer surface of corner of square steel pipe

The radii of curvature (mm) of the outer surfaces (the outer sides of the corners) of the 4 corners were measured at arbitrary 10 positions in the tube axis direction with respect to the radii of curvature of the corners of the square steel tube obtained. The maximum value Rmax and the minimum value Rmin are obtained from the measurement values at the total 40. The values are shown in Table 4. Here, when the maximum value Rmax and the minimum value Rmin of the curvature radius are in the range of 2.0t or more and 3.0t or less, it is evaluated that the curvature radius of the outer surface of the corner portion is small.

The radius of curvature of the outer side of the corner was measured using a radial measuring instrument. The method for measuring the radius of curvature was measured by using the method described above with reference to fig. 1.

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

The method of measuring flatness will be described with reference to fig. 10. In the flatness measurement, 4 flat plate portions were measured at arbitrary 10 positions in the pipe axis direction of the square steel pipe, and a total of 40 were measured. As shown in fig. 10, the maximum bulge amount and the maximum dent amount with respect to a straight line passing through 2 points at both ends in the circumferential direction of the outer surface of each flat plate portion were measured, respectively. The bulge amount was set to a positive value, the dent amount was set to a negative value, and the measured values are shown in table 4. Then, the absolute values of the maximum bulge amount and the maximum dent amount of each measurement site were obtained, and the maximum value thereof was used as the flatness of the flat plate portion, and is shown in table 4. In the case where there is no bulge or recess, the value of the bulge or recess amount is set to 0.

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

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

Using the square steel pipe thus obtained, a tensile test was performed by the following method. The cutting positions of the tensile test pieces at the flat plate portion and the corner portion are shown in fig. 6, respectively, and the detailed cutting positions of the tensile test pieces at the corner portion are shown in fig. 7.

As shown in fig. 6, a JIS5 tensile test piece and a JIS12B tensile test piece indicated by broken lines were cut out from the flat plate portion and the corner portion of the square steel pipe, respectively, such that the tensile direction was parallel to the tube axis direction. The tensile test pieces were cut by grinding so that the thickness thereof was 5mm and the center of the thickness was located at a position 1/4t of the wall thickness t from the outer surface of the tube. As shown in fig. 7, the tensile test piece at the corner was cut from a line extending through the intersection points extending from the outer surfaces of the flat plate portions on both sides adjacent to the corner and forming 45 ° with the outer surfaces of the flat plate portions.

Using these tensile test pieces, tensile tests were carried out in accordance with the regulation of JIS Z2241 to measure the yield strength YS, tensile strength TS, and uniform elongation (flat plate portion: E1, corner portion: E2) of the flat plate portion and the corner portion. The uniform elongation is set to a value of the total elongation at maximum load. The yield ratio defined by (yield strength)/(tensile strength) ×100 (%) was calculated for the corner using the obtained yield strength and tensile strength. Further, the value of the uniform elongation E2 of the corner portion with respect to the uniform elongation E1 of the flat plate portion was calculated.

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

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

As shown in fig. 6, the tensile test piece of the flat plate portion was cut from the square steel pipe at the position of the widthwise center of the flat plate portion 11b including the adjacent position of the flat plate portion 11a of the resistance welded portion 13. The tensile test piece of the corner is cut from the corner 12a adjacent to the flat plate portion 11a including the resistance welded portion 13.

(5) Charpy impact test of corner of square steel pipe

The square steel pipe thus obtained was used for the Charpy impact test by the following method. The cutting position of the corner portion of the Charpy test piece is shown in FIG. 8, and the detailed cutting position of the corner portion of the Charpy test piece is shown in FIG. 9.

As shown in fig. 8 and 9, in the charpy impact test, a V-notch standard test piece according to JIS Z2242, which is cut so that the longitudinal direction of the test piece is parallel to the tube axis direction, was used at a position 1/4t of the wall thickness t from the tube outer surface of the square steel tube. The corner-portion-charpy test piece was cut from the corner portion 12a adjacent to the flat plate portion 11a including the resistance welded portion 13. More specifically, as shown in fig. 9, the sheet is cut from a line passing through the intersection points extending from the outer surfaces of the flat plate portions on both sides adjacent to the corner portion 12a, and forming an angle of 45 ° with the outer surfaces of the flat plate portions. The Charpy absorption energy (J) was determined by performing a Charpy impact test at a test temperature of-10℃in accordance with JIS Z2242. The number of test pieces was set to 3, and the average value was calculated to obtain the Charpy absorption energy (J). The values are shown in Table 4.

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

In tables 2-1 to 4, nos. 1 to 3 and 8 to 13 are inventive examples, and Nos. 4 to 7 are comparative examples.

The square steel pipes of the present invention each have a radius of curvature R of 2.0t or more and 3.0t or less on the outer side of the corner, a flatness of 2.5mm or less in the pipe axial direction of the outer surface of the flat plate portion, and a uniform elongation E2 at a position 1/4t of the outer surface of the angle portion of 0.60 times or more with respect to a uniform elongation E1 at a position 1/4t from the outer surface of the flat plate portion, and a Charpy absorption energy of the corner at-10 ℃ of 100J or more.

In contrast, in comparative example No.4, "W/C _OUT Since the value of "is lower than the range of the formula (1), the radius of curvature of the outer side of the corner portion exceeds the range of the present invention, and a flat plate portion is not obtained.

As for comparative example No.5, "W/C _OUT The value of "exceeds the range of the formula (1), and therefore the ratio of the uniform elongation of the flat plate portion to the corner portion (E2/E1), and the Charpy absorption energy at-10 ℃ at the corner portion do not reach the desired values. The yield ratio at the corner also shows a value of 90% or more.

As for comparative example No.6, "C _IN /C _OUT Since the value of "is lower than the range of the formula (2), the radius of curvature of the outer side of the corner portion exceeds the range of the present invention, and a flat plate portion is not obtained.

As for comparative example No.7, "C _IN /C _OUT The value of "exceeds the range of the formula (2), and therefore, the radius of curvature of the outside of the corner portion is lower than the range of the present invention, and the ratio (E2/E1) of the uniform elongation of the flat plate portion to the corner portion, and the Charpy absorption energy at-10 ℃ of the corner portion do not reach the desired values. In addition, the angleThe yield ratio of the portion also shows a value of 90% or more.

Symbol description

1 steel plate (Steel belt)

2 leveling machine

3 row roller set

4 fin hole type roller set

5 squeeze roll

6 welding machine

7 resistance welded steel pipe

8 sizing roller set

9 square forming roller set

10. Square steel pipe

11. Flat plate part

12. Corner portion

13 welding part (resistance welding part)

14. Base material part

15. Welding heat influencing part

16. Melting and solidifying part

17. Partition board

18. Girder frame

19. Trabecula (trabecula)

20. Stud

100. Building structure

Claims

1. A square steel pipe having a flat plate portion and a corner portion, wherein,

the flatness of the outer surface of the flat plate portion is 2.5mm or less,

2. A square steel tube according to claim 1, wherein the average wall thickness t is greater than 0.030 times the average edge length H (mm) of the flat plate portion.

3. A square steel pipe according to claim 1 or 2, wherein the average wall thickness t is 20mm or more and 40mm or less.

4. A square steel pipe according to claim 1 to 3,

The yield strength of the flat plate part is 295MPa or more,

the tensile strength of the flat plate part is more than 400MPa,

the yield ratio of the corner is 90% or less.

5. A square steel pipe according to claim 1 to 4,

the square steel pipe comprises the following components in percentage by mass: 0.020 to 0.45 percent of Si:0.01 to 1.0 percent of Mn:0.30 to 3.0 percent, P:0.10% or less, S: less than 0.050%, al: 0.005-0.10%, N: less than 0.010%, ti:0.001 to 0.15 percent, the balance being Fe and unavoidable impurities,

the average crystal grain diameter of the crystal grains is 15.0 μm or less,

the total of the volume fractions of the crystal grains having a crystal grain diameter of 40 [ mu ] m or more is 40% or less relative to 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, further comprising, in mass%, a component selected from the group consisting of Nb:0.001 to 0.15 percent, V:0.001 to 0.15 percent of Cr:0.01 to 1.0 percent of Mo:0.01 to 1.0 percent of Cu:0.01 to 1.0 percent of Ni:0.01 to 1.0 percent of Ca:0.0002 to 0.010 percent, B: 0.0001-0.010% of one or more than two kinds.

7. A method for producing a square steel pipe according to any one of claims 1 to 6, wherein,

cold roll forming a steel sheet, resistance welding both widthwise ends of the steel sheet to form a resistance welded steel pipe, reducing the diameter of the resistance welded steel pipe by a sizing mill, and square forming by a square forming mill to manufacture a square steel pipe,

so that the width W of the steel plate is relative to the perimeter C of the square steel pipe at the outlet side of the square forming frame _OUT The ratio of (2) satisfies the requirement of (1) and the perimeter C of the resistance welded steel pipe at the inlet side of the square forming frame _IN Circumference C of square steel pipe relative to outlet side of square forming frame _OUT In a manner that satisfies the equation (2), the gap between the rolls of the sizing mill frame and the gap between the rolls of the square mill frame immediately before square forming are controlled,

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

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

In the formulae (1) and (2),

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

C _OUT : the perimeter (mm) of the square steel tube on the outlet side of the square forming frame of the final section,

t: average wall thickness (mm) of the square shaped flat plate portion,

h: average side length (mm) of the square shaped flat plate portion,

in the case of square forming by using a square forming frame of one segment, the square forming frame of the first segment and the square forming frame of the final segment refer to the same square forming frame.

8. The method for producing a square steel pipe according to claim 7, wherein the steel sheet is obtained by:

after heating the steel material to a heating temperature of 1100 ℃ to 1300 ℃, performing hot rolling treatment in which the rough rolling end temperature is 850 ℃ to 1150 ℃, the finish rolling end temperature is 750 ℃ to 900 ℃ and the total reduction ratio is 50% or more at 950 ℃,

Then, the winding is performed at 400 ℃ to 650 ℃.

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

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

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