JP7259917B2 - Square steel pipes and building structures - Google Patents

Description

TECHNICAL FIELD The present invention relates to square steel pipes with excellent toughness, high strength and low yield ratio, which are used for construction members of medium-rise buildings exceeding 20 m in height and large buildings such as factories and warehouses.

For building columns, we use four-sided box columns made by welding four thick steel plates, or square press-formed square columns made by cold-press bending and then welding one or two thick steel plates. Steel pipes have long been widely used, but in recent years, the use of roll-formed square steel pipes, which are inexpensive and can be manufactured in a short period of time, is increasing in order to reduce costs and shorten the construction period.

Roll-formed square steel pipe is made by forming a steel strip into a cylindrical open pipe shape by cold roll forming, and after electric resistance welding of the butt portions, the cylindrical shape is drawn in the pipe axial direction by rolls arranged vertically and horizontally. is added, followed by square molding. In the above-described electric resistance welding, the butt portions are heated, melted, pressed and solidified to complete the joining.

The corners of a roll-formed square steel pipe are work-hardened during corner forming, and therefore have higher strength and lower ductility and toughness than flat plates.

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

For the above reasons, roll-formed square steel pipes have large differences in strength, ductility, and toughness within the circumferential cross section, which causes problems such as complicated selection of welding materials for welding with diaphragms, welding methods, and building structural design. was
In addition, in order to further improve the seismic performance of building structures that use roll-formed square steel pipes as column materials, by suppressing the partial decrease in ductility and toughness of roll-formed square steel pipes, It has been desired to further improve the deformation performance and impact resistance performance of the steel.
In particular, when square steel pipes are deformed by an external force such as an earthquake, large strain occurs on the outer surface of the corners, so it was necessary to improve the ductility and toughness of the outer surface of the corners.

In Patent Document 1, a steel plate to which vanadium is added as a chemical component is bent and then welded to form a semi-formed rectangular steel pipe, which is heated to the vicinity of the _A3 transformation point and hot-formed. , a square steel pipe characterized by being obtained by cooling has been proposed.

Patent Literature 2 proposes a square steel pipe in which the cold-formed portion is heat-treated.

However, the square steel pipes described in Patent Documents 1 and 2 require a heating process during or after forming, so the cost is very high compared to cold-formed roll-formed square steel pipes. That is, there is a need to establish a technique for obtaining a desired rectangular steel pipe without necessarily requiring a heating process during or after forming.

In this respect, in Patent Document 3, the toughness and plastic deformability of the corner are improved by appropriately controlling the chemical composition of the material steel sheet, the bainite fraction of the metal structure, and the Vickers hardness of the surface layer of the corner. A square steel pipe has been proposed.

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

However, the square steel pipes described in Patent Documents 3 and 4 have a problem that the difference in strength and ductility between the flat portion and the corner portion is still large. That is, in these square steel pipes, it cannot be said that the variations in hardness within the circumferential cross section including the corner portions and the flat plate portion can be sufficiently reduced. Moreover, it cannot be said that the ductility and toughness of the outer surface of the corner portion are sufficiently ensured.

By the way, in roll-formed square steel pipes, it is also required to establish a technique for improving shape characteristics, particularly a technique for sufficiently flattening the flat plate portion. In this regard, Patent Literatures 5 and 6 disclose techniques for improving shape characteristics by adjusting manufacturing conditions during roll forming.
Specifically, in Patent Document 5, when a steel pipe is formed into a square pipe by three- or four-stage square forming rolls with a constant rolling reduction of the final roll, the wall thickness/outer diameter ratio of the steel pipe increases. Along with this, a technique for forming a square steel pipe has been disclosed in which the final stage roll caliber is made smaller (from a convex shape to a concave shape).
Further, in Patent Document 6, when roll-forming a cylindrical tube into a square tube, Q=( DH) / (Dt) × 100 A first stage forming step of forming the blank pipe into a rectangular cross-sectional shape while maintaining the set pushing ratio Q defined by 12 to 23%, and a rectangular cross-section There is disclosed a technique for a method of manufacturing a structural rectangular tube that undergoes a second and subsequent forming process for forming a blank tube formed into a desired shape.

Japanese Patent Application Laid-Open No. 2004-330222 JP-A-10-60580 Japanese Patent No. 5385760 JP 2018-53281 A JP-A-4-224023 Japanese Patent No. 3197661

However, the techniques described in Patent Documents 5 and 6 are techniques for flattening the flat plate portion of the square steel pipe, reducing variations in hardness in the circumferential cross section, and sufficiently securing the ductility and toughness of the outer surface of the corner portion. was not sufficient.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides a square steel pipe having a flat flat plate portion with small variations in hardness in the circumferential cross section, excellent ductility and toughness on the outer surface of the corners, and a method for manufacturing the same. It also aims to provide a building structure having excellent earthquake resistance performance.
In the present invention, a small variation in hardness in the circumferential cross section means that the difference between the maximum value and the minimum value of Vickers hardness in the steel pipe is 80 HV or less.
In the present invention, the excellent ductility of the outer surface of the corner means that the uniform elongation at the position t/4 from the outer surface of the corner is 0.80 times or more the uniform elongation at the position t/4 from the outer surface of the flat plate. point to
Further, in the present invention, the phrase “excellent toughness of the outer surface of the corner” means that the Charpy absorbed energy of the corner at 0° C. is 70 J or more.
Further, in the present invention, the flatness of the flat plate portion means that, in the peripheral cross section, the degree of flatness represented by the maximum amount of swelling and the maximum amount of recession with respect to a straight line passing through two points on the same side of the outer surface of the flat plate portion in the circumferential direction. is 2.5 mm or less (see FIG. 10).

The inventors of the present invention have made intensive studies to solve the above problems, and as a result, the hardness variation in the circumferential cross section of the square steel pipe (particularly, the hardness variation between the flat plate portion and the corner portion) can be reduced. In order to ensure sufficient ductility and toughness of the outer surface, it was found that the radius of curvature of the outside corners of the square steel pipe should be more than 3.0 times the average plate thickness.
It was also found that in order to make the flat plate portion sufficiently flat, the radius of curvature of the outer corner portion of the roll-formed square steel pipe should be 4.0 times or less of the average plate thickness.
By controlling the circumference of the electric resistance welded steel pipe on the entrance side of the corner forming stand to the circumference of the square steel pipe on the exit side of the corner forming stand within an appropriate range, the radius of curvature of the outside of the corner portion is 3.5 times the plate thickness. It is possible to manufacture a square steel pipe with a hardness of more than 0 times and 4.0 times or less, reduce the variation in hardness in the circumferential cross section to a desired level, improve the ductility and toughness of the outer surface of the corner portion, and sufficiently flat plate portion. I have found that it can be flattened.

The present invention has been completed based on the above findings, and the gist and configuration thereof are as follows.
[1] A plurality of flat plate portions and corner portions are alternately formed in the pipe circumferential direction,
A welded portion extending in the pipe axis direction is further formed,
The width of the molten solidified portion in the welded portion in the pipe circumferential direction is 1.0 μm or more and 1000 μm or less,
A square steel pipe, wherein the radius of curvature of the outside corners is more than 3.0 times and less than or equal to 4.0 times the average plate thickness t.
[2] The square steel pipe according to [1], wherein the average plate thickness t is more than 0.030 times the average side length H.
[3] The square steel pipe according to [1] or [2] above, wherein the difference between the maximum value and the minimum value of Vickers hardness in the steel pipe is 80 HV or less.
[4] The average plate thickness t is 20 mm or more and 40 mm or less,
The flat plate portion has a yield strength of 295 MPa or more,
The flat plate portion has a tensile strength of 400 MPa or more,
The yield ratio of the corner is 90% or less,
The square steel pipe according to any one of [1] to [3], wherein the corner portion has a Charpy absorbed energy of 70 J or more at 0°C.
[5] Any one of the above [1] to [4], wherein the uniform elongation at a position t/4 from the corner outer surface is 0.80 times or more the uniform elongation at a position t/4 from the flat plate outer surface The square steel pipe described.
[6] A steel plate is roll-formed, and then the roll-formed steel plate is electric resistance welded to form an electric resistance welded steel pipe. A method of manufacturing a
A method for manufacturing a square steel pipe, wherein the gap of the sizing stand immediately before corner forming is controlled based on the gap of the square forming stand so as to satisfy the following formula (1).

0.30×t/H+0.99≦C _IN /C _OUT <0.50×t/H+0.99 Expression (1)
In addition, in formula (1),
C _IN : Circumferential length (mm) of the electric resistance welded steel pipe at the entry side of the first-stage corner forming stand,
C _OUT : Circumference length (mm) of the square steel pipe at the output side of the square forming stand in the final stage,
t: average plate thickness after corner forming (mm),
H: Average side length (mm) after corner forming,
is.
(However, when the corner forming is performed by a one-stage corner forming stand, the first stage corner forming stand and the final stage corner forming stand shall be the same corner forming stand.)
[7] The method for manufacturing a square steel pipe according to [6], wherein the average plate thickness t is 20 mm or more and 40 mm or less.
[8] A building structure in which the square steel pipe according to any one of [1] to [5] is used as a pillar material.

Here, the radius of curvature may be the average radius of curvature or the radius of curvature at an arbitrary location. However, from the point of view of securing a better effect, it is preferable to set the radius of curvature at an arbitrary location.

Also, the average plate thickness t is obtained from the following formula (2).
t=(t1+t2+t3)/3 Expression (2)
In formula (2), t1 and t2: plate thickness (mm) at the center of the pipe circumferential direction of each of two flat plate portions adjacent to each other across a corner with respect to the flat plate portion including the welded portion (electric resistance welded portion), t3 : Plate thickness (mm) at the center of the pipe circumferential direction of the flat plate portion facing the flat plate portion including the welded portion (electric resistance welded portion).

Also, the average side length H is obtained from the following equation (3).
H=(H1+H2)/2 Expression (3)
In the formula (3), H1: In the vertical section in the pipe axis direction, the side length of a substantially rectangular shape including an arbitrary flat plate portion and the corners on both sides (vertical side length in FIG. 1, facing 1 In a pair of flat plate portions, it can also be said that it is the distance from the outer surface of one flat plate portion to the outer surface of the other flat plate portion. and the side length (horizontal side length in FIG. 1) (mm) of the side including the corners on both sides. That is, H is obtained by adding the side lengths H1 and H2 of two flat plate portions adjacent to each other across a corner portion in the vertical cross-section in the direction of the tube axis and dividing the result by two.

ADVANTAGE OF THE INVENTION According to the present invention, it is possible to provide a square steel pipe having a small variation in hardness in the circumferential cross section, excellent ductility and toughness of the corner outer surface, and a flat plate portion, a method for manufacturing the same, and a building structure. becomes.
As a result, it is possible to manufacture a cold roll-formed square steel pipe having a small strength difference in the circumferential cross section and excellent ductility and toughness. In addition, building structures using the square steel pipes of the present invention as column materials exhibit better earthquake resistance performance than building structures using conventional cold-formed square steel pipes.

FIG. 1 is a schematic diagram showing a cross section perpendicular to the axial direction of the square steel pipe of the present invention. FIG. 2 is a schematic diagram showing an example of equipment for manufacturing an electric resistance welded steel pipe. FIG. 3 is a schematic diagram for explaining a melt-solidified portion in a welded portion. FIG. 4 is a schematic diagram showing the forming process of the square steel pipe of the present invention. FIG. 5 is a schematic diagram showing an example of the architectural structure of the present invention. FIG. 6 is a schematic diagram showing the sampling positions of the tensile test pieces of the flat plate portion and the corner portion, respectively. FIG. 7 is a schematic diagram showing detailed sampling positions of corner tensile test pieces. FIG. 8 is a schematic diagram showing the sampling positions of the corner Charpy test piece. FIG. 9 is a schematic diagram showing the detailed sampling positions of the corner Charpy test pieces. FIG. 10 is a schematic diagram for explaining a flatness measuring method. FIG. 11 is a schematic diagram showing the sampling positions of the tensile test pieces at the position t/4 from the outer surface of the flat portion and at the position t/4 from the outer surface of the corner portion. FIG. 12 is a schematic diagram showing the detailed sampling position of the tensile test piece at the position t/4 from the corner outer surface.

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

<Square steel pipe>
FIG. 1 is a schematic diagram showing a cross section perpendicular to the pipe axis direction of a square steel pipe 10 of the present invention. A square steel pipe 10 of the present invention has a plurality of flat plate portions 11 and corner portions 12 alternately formed in the pipe circumferential direction. As shown in FIG. 1, the square steel pipe 10 has four corner portions 12, flat plate portions 11, corner portions 12, flat plate portions 11, corner portions 12, flat plate portions 11, corner portions 12, and flat plate portions 11 in the circumferential direction. It is possible to form a square steel pipe that is formed one by one and has a rectangular (substantially rectangular) or square (substantially square) cross-sectional view in the vertical direction of the pipe axis. Further, the square steel pipe 10 of the present invention can be a roll-formed square steel pipe obtained from an electric resistance welded steel pipe, and also has a welded portion (electrical resistance welded portion) 13 formed in the flat plate portion 11 and extending in the pipe axial direction. .

In the square steel pipe 10 of the present invention, the radius of curvature of the corner outside is more than 3.0 times and 4.0 times or less the average plate thickness t.

As shown in FIG. 1, the radius of curvature of the outside of the corner portion passes through the intersection point P of two straight lines L1 and L2 respectively including the outer surfaces of the flat plate portions 11 on both sides adjacent to the corner portion 12, and is L1 or L2. It is the radius of curvature at the intersection of the straight line L forming 45° and the outside of the corner.
Moreover, the radius of curvature referred to in the present invention may be the average radius of curvature or the radius of curvature at an arbitrary location. However, from the point of view of securing a better effect, it is preferable to set the radius of curvature at an arbitrary location.
The radius of curvature is measured in a fan shape with a central angle of 90°, which consists of the connection point (A, A') of the flat plate portion 11 and the corner portion 12 and the outer surface of the corner portion, and whose center is on the L above. It is carried out within a central angle of 65° centering on the intersection point of the outer and outer surfaces. The method of measuring the radius of curvature is, for example, a method of measuring the radius of curvature from a radial gauge that well matches the outer surface of the corner within the range of the central angle of 65°, but is not limited to these methods.

If the radius of curvature of the outside of the corner portion is 3.0 times or less than the average plate thickness t, the variation in hardness in the circumferential cross section becomes large, and the desired square steel pipe is not obtained. That is, the corner portion is greatly work-hardened, and has higher strength and lower ductility and toughness than the flat plate portion.
On the other hand, if the radius of curvature of the outer corner portion exceeds 4.0 times the average plate thickness t, the flatness of the flat plate portion will be insufficient and the desired square steel pipe will not be obtained. In addition, the peripheral cross-sectional area becomes small, and sufficient member strength cannot be obtained.
Therefore, in the present invention, the radius of curvature of the corner outside is set to more than 3.0 times and less than or equal to 4.0 times the average plate thickness t.
Preferably, the radius of curvature of the corner outside is 3.1 to 3.9 times the average plate thickness t, more preferably 3.2 to 3.8 times.

In the present invention, the square steel pipe 10 is obtained from an electric resistance welded steel pipe. Therefore, the welded portion 13 is an electric resistance welded portion. The width of the molten solidified portion of the electric resistance welded portion in the circumferential direction of the pipe is 1.0 μm or more and 1000 μm or less over the entire thickness of the pipe.

Further, in the present invention, the ratio (E2/E1) of the uniform elongation E1 at the position t/4 from the outer surface of the flat plate portion and the uniform elongation E2 at the position t/4 from the outer surface of the corner portion is 0.80 times or more. is preferred. E2/E1 is more preferably 0.83 times or more, and still more preferably 0.85 times or more. Moreover, E2/E1 is preferably 1.00 times or less.

Further, in the present invention, the relationship between the average plate thickness t (mm) and the average side length H of the square steel pipe 10 can be t/H greater than 0.030.
In square steel pipes, the larger the ratio (t/H) between the average plate thickness t and the average side length H, the greater the amount of work hardening at the corners. Therefore, in a square steel pipe having a large ratio (t/H), the difference in strength, ductility and toughness in the circumferential cross section tends to increase.
In the present invention, since the radius of curvature of the outside of the corner portion is more than 3.0 times and less than or equal to 4.0 times the average plate thickness, even if t/H is more than 0.030, the strength difference in the circumferential cross section is minimized. small, resulting in excellent ductility and toughness.

Here, the average plate thickness t is obtained from the following formula (2).
t=(t1+t2+t3)/3 Expression (2)
In formula (2), t1 and t2: plate thickness at the center of the tube circumferential direction of two flat plate portions 11 adjacent to each other across the corner portion 12 with respect to the flat plate portion 11 including the welded portion (electric resistance welded portion) 13 ( mm), t3: Plate thickness (mm) at the center of the pipe circumferential direction of the flat plate portion 11 facing the flat plate portion 11 including the welded portion (electric resistance welded portion) 13 .

Also, the average side length H is obtained from the following equation (3).
H=(H1+H2)/2 Expression (3)
In formula (3), H1 is the length of the vertical side (mm) in FIG. 1, and H2 is the length of the horizontal side (mm) in FIG. The side lengths H1 and H2 including the corner portions 12 on both sides of each of the two flat plate portions 11 adjacent to each other with the plate portion 12 interposed therebetween are added and divided by two.

In FIG. 1, H1>H2. Although it is shorter than the length H1, the present invention is not limited to such an example, and may be H1=H2 or H1<H2.

The square steel pipe 10 of the present invention preferably has a difference of 80 HV or less between the maximum value and the minimum value of Vickers hardness in the steel pipe. Specifically, the position of 1 mm in the thickness direction from the inner surface of the pipe, the position of 1 mm in the thickness direction from the outer surface of the pipe, and the center of the plate thickness are measured at each of the flat plate portion 11, the corner portion 12, and the welded portion (ERW welded portion) 13. The difference between the maximum value and the minimum value of Vickers hardness at a position is preferably 80 HV or less.
The above Vickers hardness test can be carried out in accordance with JIS Z 2244 with a test force of 98 N (10 kgf).

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

In order to ensure mechanical properties and weldability, the component composition of the square steel pipe 10 of the present invention preferably has a Ceq defined by the formula (4) of 0.15% or more and 0.50% or less. Moreover, it is preferable that Pcm defined by the formula (5) is 0.30% or less. However, the component compositions of various elements in formulas (4) and (5) are all expressed in mass%. In addition, hereinafter, "%" indicating a component composition is "% by mass" unless otherwise specified.

Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14 Equation (4)
Here, in formula (4), C, Mn, Si, Ni, Cr, Mo, and V are contents (% by mass) of each element. (However, elements not contained shall be 0 (zero)%.)
Pcm=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B Expression (5)
Here, in formula (5), C, Si, Mn, Cu, Ni, Cr, Mo, V, and B are contents (% by mass) of each element. (However, elements not contained shall be 0 (zero)%.)
Ceq in the formula (4) is a carbon equivalent and serves as an index of the hardness of the welded portion (electric resistance welded portion) 13 and the heat affected zone. If the Ceq is less than 0.15%, it may not be possible to obtain the strength required as a pillar material for building structures. Moreover, if the Ceq exceeds 0.50%, the welded portion 13 and the heat-affected zone may be excessively hardened, resulting in large variations in circumferential cross-sectional strength. Therefore, in the present invention, Ceq is preferably 0.15% or more and 0.50% or less. Also, Ceq is more preferably 0.20% or more, and still more preferably 0.25% or more. Also, Ceq is more preferably 0.45% or less, and still more preferably 0.40% or less.

Pcm in the formula (5) is the weld crack sensitivity, and when Pcm exceeds 0.30%, cold cracking is likely to occur in the weld zone 13 and the heat affected zone. Therefore, in the present invention, Pcm is preferably 0.30% or less. Also, Pcm is more preferably 0.10% or more, and still more preferably 0.15% or more. Also, Pcm is more preferably 0.28% or less, and still more preferably 0.25% or less.

In addition, although not particularly limited, the square steel pipe 10 of the present invention also has a .50%, Mn: 0.40 to 2.5%, P: 0.10% or less, S: 0.050% or less, Al: 0.005 to 0.10%, N: 0.010% or less, It may have a component composition containing Ti: 0.005 to 0.15% and the balance being Fe and unavoidable impurities. Further, the square steel pipe 10 of the present invention further includes, in mass%, Nb: 0.005 to 0.15%, V: 0.005 to 0.15%, Cr: 0.02 to 1.0%, Mo : 0.02 to 1.0%, Cu: 0.02 to 1.0%, and Ni: 0.02 to 1.0%.

<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 includes roll forming a steel plate, then electric resistance welding the roll formed steel plate to form an electric resistance welded steel pipe, forming the electric resistance welded steel pipe with a sizing stand, and then forming the electric resistance welded steel pipe with a square forming stand. This is a method for manufacturing a square steel pipe by corner forming, in which the gap of the sizing stand immediately before corner forming is controlled based on the gap of the angle forming stand so as to satisfy the following formula (1).
0.30×t/H+0.99≦C _IN /C _OUT <0.50×t/H+0.99 Expression (1)
In addition, in formula (1),
C _IN : Circumferential length (mm) of the electric resistance welded steel pipe at the entry side of the first-stage corner forming stand,
C _OUT : Circumference length (mm) of the square steel pipe at the output side of the square forming stand in the final stage,
t: average plate thickness after corner forming (mm),
H: Average side length (mm) after corner forming.

Note that the average plate thickness t is obtained from the following formula (2).
t=(t1+t2+t3)/3 Expression (2)
In formula (2), t1 and t2: plate thickness at the center of the tube circumferential direction of two flat plate portions 11 adjacent to each other across the corner portion 12 with respect to the flat plate portion 11 including the welded portion (electric resistance welded portion) 13 ( mm), t3: Plate thickness (mm) at the center of the pipe circumferential direction of the flat plate portion 11 facing the flat plate portion 11 including the welded portion (electric resistance welded portion) 13 .

Also, the average side length H is obtained from the following equation (3).
H=(H1+H2)/2 Expression (3)
In formula (3), H1 is the length of the vertical side (mm) in FIG. 1, and H2 is the length of the horizontal side (mm) in FIG. The side lengths H1 and H2 including the corner portions 12 on both sides of each of the two flat plate portions 11 adjacent to each other with the plate portion 12 interposed therebetween are added and divided by two.

However, when the above-mentioned corner forming is performed using a one-stage corner forming stand, the first-stage corner forming stand and the final-stage corner forming stand are the same stand.

Here, a method for manufacturing an electric resistance welded steel pipe used to obtain the square steel pipe 10 of the present invention will be described with reference to FIG. FIG. 2 is a schematic diagram showing an example of equipment for manufacturing an electric resistance welded steel pipe.

As shown in FIG. 2, a steel sheet 1 (hereinafter also referred to as steel strip 1) wound into a coil and having the above-described chemical composition is paid out, straightened by a leveler 2, and rolled by a cage roll group 3 consisting of a plurality of rolls. After intermediate forming to form an open tube, finish forming is performed by a fin pass roll group 4 consisting of a plurality of rolls. The open tube may be cylindrical obtained by cold roll forming.

After finish forming, the steel strip 1 is welded by a welding machine 6 to electric resistance weld a pair of circumferentially opposed butted portions of the steel strip 1 while being pressure-welded by squeeze rolls 5 to form an electric resistance welded steel pipe 7 . In the present invention, the equipment for manufacturing the electric resistance welded steel pipe 7 is not limited to the pipe-making process shown in FIG. Also, in the electric resistance welding described above, the butt portion is heated, melted, pressed and solidified to complete the joining. As a result, a single welded portion (electric resistance welded portion) 13 (see FIG. 1 again) is extended in the pipe axial direction.

The amount of upset by the squeeze rolls 5 is preferably 20% or more and 100% or less of the plate thickness of the electric resistance welded steel pipe 7 . If the amount of upset is less than 20% of the sheet thickness, the discharge of molten steel is insufficient and the toughness of the weld deteriorates. Further, if the upset amount exceeds 100% of the plate thickness, the squeeze roll load becomes large and the amount of work hardening of the welded portion (electric resistance welded portion) 13 becomes large, resulting in an excessively high hardness.

In the sizing process after electric resistance welding, as will be described later with reference to FIG. The steel pipe may be contracted so as to decrease in proportion.
However, if the diameter of the steel pipe is reduced so that the total circumferential length of the steel pipe is reduced by more than 2.0%, the amount of bending in the axial direction of the pipe when passing through the rolls will increase, and the residual stress in the axial direction of the pipe after diameter reduction will increase. On the contrary, it rises. For this reason, it is preferable to reduce the diameter of the steel pipe so that the circumferential length of the steel pipe is reduced at a rate of 0.30% or more and 2.0% or less.

In the sizing process, in order to minimize the amount of bending in the tube axis direction when passing through the rolls and to suppress the generation of residual stress in the tube axis direction, it is preferable to perform multistage diameter reduction using multiple stands. The diameter reduction in the stand is preferably performed so that the steel pipe circumference is reduced at a rate of 1.0% or less.

Whether or not the square steel pipe 10 is obtained from the electric resistance welded steel pipe 7 is determined by cutting the square steel pipe 10 perpendicularly to the pipe axis direction, polishing and then corroding the cut surface including the welded portion (electrical resistance welded portion) 13, and examining it with an optical microscope. It can be judged by observation. If the width of the melt-solidified portion of the welded portion (electrical resistance welded portion) 13 in the pipe circumferential direction is 1.0 μm or more and 1000 μm or less over the entire thickness of the pipe, it is the electric resistance welded steel pipe 7 .

Here, an appropriate corrosive liquid may be selected according to the steel composition and the type of steel pipe. FIG. 3 is a diagram schematically showing a melt-solidified portion in the welded portion 13. As shown in FIG. As shown in FIG. 3 for the cross section after corrosion, the melt-solidified portion can be visually recognized as a region 16 having a different structural form and contrast from the base material portion 14 and the heat-affected zone 15 in FIG. For example, the molten solidified portion 16 of the electric resistance welded steel pipe of carbon steel and low alloy steel can be identified as a white region observed with an optical microscope in the cross section corroded with nital. In addition, the melt-solidified portion 16 of the UOE steel pipe of carbon steel and low alloy steel can be identified as a region containing a cellular or dendrite-like solidified structure with an optical microscope in the cross section corroded with nital.

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

As shown in FIG. 4, the electric resistance welded steel pipe 7 is diameter-reduced in a cylindrical shape by a sizing roll group (sizing stand) 8 consisting of a plurality of rolls arranged vertically and horizontally. A group (square forming stand) 9 sequentially forms shapes such as R1, R2, and R3 to form square steel pipes 10 . The rolls of the square forming stand 9 are caliber rolls having a caliber curvature. The number of stands of the sizing roll group 8 and the corner forming roll group 9 is not particularly limited. Further, it is preferable that the caliber curvature of the sizing roll group 8 or the angle forming roll group 9 is one condition.

In the present invention, the circumferential length of the electric resistance welded steel pipe 7 immediately before corner forming (the circumferential length (mm) of the electric resistance welded steel pipe 7 at the entrance side of the first stage corner forming stand, hereinafter referred to as “C _IN ”) and the angle The ratio (C _IN /C _OUT ) of the circumferential length of the square steel pipe 10 immediately after forming (the circumferential length (mm) of the steel pipe at the exit side of the square forming stand at the final stage, hereinafter referred to as “C _OUT ”), and after the square forming The ratio (t/H) between the average plate thickness t and the average side length H after corner forming satisfies the formula (1).

0.30×t/H+0.99≦C _IN /C _OUT <0.50×t/H+0.99 Expression (1)
When the electric resistance welded steel pipe 7, which is a cylindrical blank pipe, is formed into the square steel pipe 10, the steel pipe is passed through the square forming roll group 9 as described above, thereby gradually forming from a cylindrical shape into a square shape. In such corner forming, bending back of the straight portion of the side (flat plate portion 11 (see FIG. 1)), bending of the corner portion 12, and drawing deformation in the circumferential direction occur.

Especially around the corner 12, the corner forming is completed without the rolls coming into contact with each other. That is, in corner forming, the corner portion 12 is formed by projecting due to free deformation. At this time, the higher the rigidity of the corner portion 12 and the smaller the amount of circumferential drawing, the smaller the amount of bending deformation of the corner portion 12 and the larger the radius of curvature of the outside of the corner portion. On the other hand, the lower the rigidity of the corner 12 and the larger the circumferential drawing, the greater the bending deformation of the corner 12 and the smaller the radius of curvature of the outside of the corner.

The rigidity against bending deformation of the corner portion 12 increases as the ratio (t/H) between the average plate thickness t and the average side length H increases.

The amount of circumferential reduction in corner forming is determined by the circumferential length ratio (C _IN /C _OUT ), and the larger this is, the larger the amount of circumferential reduction is.

Therefore, in order to obtain a square steel pipe having the same radius of curvature on the outside of the corners, the larger the ratio (t/H) between the average plate thickness t and the average side length H, the larger the amount of drawing in the circumferential direction. It is necessary to increase the ratio (C _IN /C _OUT ).

If (C _IN /C _OUT ) is smaller than the value on the left side of the formula (1), the processing will be insufficient and a flat plate portion will not be obtained. In addition, since the amount of squeezing in the circumferential direction is small, the radius of curvature of the outside of the corner portion is more than 4.0 times the average plate thickness t, and the circumferential cross-sectional area becomes small, making it impossible to obtain sufficient member strength.

When (C _IN /C _OUT ) is equal to or greater than the value on the right side of Equation (1), the amount of circumferential reduction is large, so the radius of curvature of the outside of the corner is 3.0 times or less than the average plate thickness t, and the corner is It is greatly work-hardened and has higher strength and lower ductility and toughness than the flat plate portion.

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

C _IN is the circumferential length (perimeter in the pipe circumferential direction) (mm) of the electric resistance welded steel pipe 7 at the entry side of the first-stage corner forming stand. In C _IN , the positive direction of the X-axis is the tube-making direction, Xa (m) is the X coordinate of the rotation axis of any one of the sizing stands immediately before angle forming, and any one of the first-stage angle forming stands is When the X coordinate of one rotation axis is Xb (m), it can be obtained by measuring the outer peripheral length of the circumferential section of the pipe on the plane X = (Xa + Xb) / 2 (m) perpendicular to the X axis with a tape measure (Fig. 4).
In addition, C _OUT is the circumferential length (perimeter length in the pipe circumferential direction) (mm) of the square steel pipe 10 on the output side of the final stage square forming stand. C _OUT is the X coordinate of the rotation axis of any one of the corner forming stands at the final stage of the roll group as Xc (m), and the outer circumference of the circumferential section of the tube on the plane X = Xc + 1 (m) perpendicular to the X axis. It is obtained by measuring with a tape measure (see Figure 4).

C _IN and C _OUT are controlled by controlling the gap between recesses of the caliber roll. The difference Δg between the maximum gap between recesses of the sizing stand immediately before corner forming (hereinafter also referred to as "sizing stand gap") and the maximum gap between recesses of the corner forming stand (hereinafter also referred to as "gap of corner forming stand") is It is preferable to adjust the gap of the sizing stand immediately before corner forming so that the value G (=Δg/(t/H)) divided by (t/H) is 70 or more and 180 or less.

When G is less than 70, (C _IN /C _OUT ) in formula (1) is smaller than the value on the left side, and when it exceeds 180, (C _IN /C _OUT ) in formula (1) is greater than or equal to the value on the right side. becomes.
If there are multiple sizing stands, the gap of the sizing stand immediately before corner forming may be the same as the gap of the other sizing stands.
Further, when there are a plurality of tiers of angled stands, the gap between the angled stands is preferably the gap of the first tiered angled stand. Also, the gaps between the first stage and the other corner forming stands may all be the same.

<Building structure>
The building structure of the present invention uses the square steel pipe 10 of the present invention described above as a pillar material.
FIG. 5 is a schematic diagram showing an example of the architectural structure 100 of the present invention.
In the building structure 100 of the present invention, a diaphragm 17 and a square steel pipe 10 are welded together, and the square steel pipe 10 is used as a column material. In addition, as shown in FIG. 5, the building structure 100 may be formed of large beams 18, small beams 19, and studs 20, or may be formed of other known members.
As described above, the square steel pipe 10 has a flat plate portion 11 with a small variation in hardness within the circumferential cross section. Therefore, the building structure 100 of the present invention using this square steel pipe 10 as a column material exhibits excellent earthquake resistance performance.

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

A hot-rolled steel sheet having the chemical composition shown in Table 1 was continuously formed into an open tube with an elliptical cross section by a group of cage rolls and a group of fin pass rolls, and then the opposite end faces of the open tube were heated to the melting point by high-frequency induction heating or high-frequency resistance heating. It was heated to the above temperature and press-welded with a squeeze roll to obtain a base pipe for an electric resistance welded steel pipe. The obtained electric resistance welded steel pipe was formed into a cylindrical shape by a group of 2-stand (2-stage) sizing rolls, and then corner-formed by a group of 4-stand (4-stage) corner forming rolls. A square steel pipe having a substantially rectangular shape in a vertical cross-sectional view in the direction of the pipe axis was obtained.

The average plate thickness t was obtained from the following formula (2).
t=(t1+t2+t3)/3 Expression (2)
In formula (2), t1 and t2: plate thickness (mm) at the center of the pipe circumferential direction of each of two flat plate portions adjacent to each other across a corner with respect to the flat plate portion including the welded portion (electric resistance welded portion), t3 : Plate thickness (mm) at the center in the pipe circumferential direction of the flat plate portion facing the flat plate portion including the electric resistance welded portion.

Also, the average side length H was obtained from the following formula (3).
H=(H1+H2)/2 Expression (3)
In the formula (3), H2: side length of the side including the flat plate portion where the electric resistance welded portion is formed and the corners on both sides in the pipe axial direction vertical cross section, H1: for the flat plate portion with the side length H2 It is the side length (mm) of the side including the corners on both sides of the flat plate portion that is adjacent across the corners.

In each square steel pipe, the square steel pipe was cut perpendicular to the pipe axis direction, and the cut surface including the electric resistance welded part was polished and then nital corroded. It was also confirmed that the width of the pipe in the pipe circumferential direction is 1.0 μm or more and 1000 μm or less over the entire pipe thickness. The melt-solidified portion was identified as a white region observed with an optical microscope in the cross section corroded with nital.

Regarding the circumferential length C _IN (mm) of the electric resistance welded steel pipe at the entrance side of the first stage corner forming stand, the direction of pipe making is the positive direction of the X axis, and the rotation axis of any one of the sizing stands immediately before corner forming. When the X coordinate is Xa (m) and the X coordinate of any one of the rotation axes of the first stage corner forming stand is Xb (m), a plane perpendicular to the X axis X = (Xa + Xb) / 2 ( m) was measured with a tape measure to determine the circumference C _IN (mm) of the electric resistance welded steel pipe (see FIG. 4 again).

For the circumference C _OUT (mm) of the square steel pipe on the delivery side of the final stage square forming stand, the X coordinate of the rotation axis of any one of the fourth stage square forming stands in the group of square forming rolls is Xc (m). , the outer circumference of the circumferential cross section of the pipe on the plane X=Xc+1 (m) perpendicular to the X axis was measured with a tape measure to obtain the circumferential length C _OUT (mm) of the square steel pipe (see FIG. 4 again).

In addition, for the above C _IN and C _OUT , the maximum gap between the recesses of the caliber roll of the sizing stand immediately before corner forming and the caliber roll of the first stage corner forming stand was measured, and the difference Δg was used. to calculate G (=Δg/(t/H)).

Furthermore, the radius of curvature of the outer surface (outside of the corners) of four corners was measured at 10 arbitrary positions in the axial direction of the obtained square steel pipe, and the maximum value Rmax and the minimum value Rmin of these 40 places in total were measured. were asked for respectively.
A radial gauge was used to measure the radius of curvature of the outside corners. Regarding the method of measuring the radius of curvature, the intersection point P of two straight lines L1 and L2, which respectively include the outer surfaces of the flat plate portions on both sides adjacent to the corner portion, and the intersection point of the straight line L forming 45° with L1 or L2 and the intersection point of the outside of the corner portion The radius of curvature at was measured as the radius of curvature outside the corner (see FIG. 1 again). Specifically, the curvature radius is measured in a fan shape with a central angle of 90 °, which consists of the connection point (A, A') of the flat plate portion and the corner portion and the outer surface of the corner portion, and the center is on the above L. The radius of curvature was measured with a radial gauge that closely matches the outer surface of the corner in the range of a central angle of 65° centering on the intersection of the outer surfaces of the corners.

In the cross section of the obtained square steel pipe cut perpendicular to the pipe axis direction, the Vickers hardness was measured at the flat plate portion, the corner portion, and the welded portion (electric resistance welded portion) at positions of 1 mm in the thickness direction from the inner and outer surfaces and at the center position of the plate thickness. were measured, and their maximum value HVmax and minimum value HVmin were obtained.
The Vickers hardness test was carried out according to JIS Z 2244 with a test force of 98 N (10 kgf). The flat plate portion hardness measurement was performed at the flat plate portion adjacent to the flat plate portion including the electric resistance welded portion, and the corner hardness measurement was performed at the corner portion adjacent to the flat plate portion including the electric resistance welded portion.

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

As shown in FIG. 6, JIS No. 5 tensile test pieces and JIS No. 12B tensile test pieces were taken from flat portions and corner portions of square steel pipes so that the tensile direction was parallel to the pipe axial direction. For the tensile test piece of the corner, more specifically, as shown in FIG. It was collected from the eggplant line.

Further, as shown in FIG. 11, a JIS No. 5 tensile test piece and a JIS No. 12B tensile test piece indicated by broken lines were collected from the flat plate portion and the corner portion of the square steel pipe so that the tensile direction was parallel to the pipe axis direction, They were each ground so that their thickness was 5 mm and the thickness center was located at the position t/4 of the plate thickness t from the outer surface of the pipe, and tensile test specimens were taken. For the corner tensile test piece, more specifically, as shown in FIG. It was collected from the eggplant line.
Using these, a tensile test was performed in accordance with the provisions of JIS Z 2241, and the yield strength YS, tensile strength TS, and uniform elongation (flat plate portion: E1, corner portion: E2) were measured, and (yield strength) / ( A yield ratio defined by tensile strength) was calculated. Uniform elongation was taken as the value of total elongation at maximum load. The tensile test piece of the flat plate portion was taken from the position of the width center portion of the flat plate portion adjacent to the flat plate portion including the electric resistance welded portion of the square steel pipe. A corner tensile test piece was taken from a corner adjacent to the flat plate portion including the electric resistance welded portion.
The number of test pieces was two, and the average value of them was calculated to obtain the yield strength YS, tensile strength TS, yield ratio, and uniform elongation.

FIG. 8 is a schematic diagram showing the sampling positions of the corner Charpy test piece. FIG. 9 is a schematic diagram showing the detailed sampling positions of the corner Charpy test pieces.

For the Charpy impact test, as shown in FIGS. 8 and 9, a JIS sample was taken at a position t/4 of the plate thickness t from the outer surface of the square steel pipe so that the longitudinal direction of the test piece was parallel to the pipe axis direction. A V-notch standard specimen conforming to the Z 2242 specification was used. A Charpy impact test was performed at a test temperature of 0° C. in accordance with JIS Z 2242 to determine absorbed energy (J). Incidentally, the number of test pieces was three, and the average value was calculated to obtain the absorbed energy (J).

FIG. 10 is a schematic diagram for explaining a flatness measuring method.
The measurement of the flatness was performed at 40 points in total, with 4 points on the flat plate portion being measured at 10 arbitrary points in the axial direction of the square steel pipe. As shown in FIG. 10, the maximum amount of bulge and the maximum amount of dent are measured with respect to a straight line passing through two points on the outer surface of each flat plate portion in the circumferential direction. The value was defined as flatness. However, the amount of bulging was assumed to be a positive value, the amount of denting was assumed to be a negative value, and the value of the amount of bulging or denting was set to 0 when there was no bulging or denting.

Table 3 shows the results obtained.

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

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

Comparative example No. 2, 10, 12, and 15, the circumference ratio (C _IN /C _OUT ) exceeds the range of formula (1), and the radius of curvature of the corner outside is 3.0 times or less of the plate thickness. , The uniform elongation at the position t / 4 from the corner outer surface is less than 0.80 times the uniform elongation at the position t / 4 from the flat plate outer surface, and the difference between the maximum and minimum values of Vickers hardness is the desired value did not reach
Moreover, No. of the comparative example. 2, 10, 12, and 15, the circumference ratio (C _IN /C _OUT ) exceeds the range of formula (1), and the radius of curvature of the corner outside is 3.0 times or less of the plate thickness. , the Charpy absorbed energy did not reach the desired value.

Comparative example No. 5, 8, and 18, the circumference ratio (C _IN /C _OUT ) was below the range of formula (1), and the amount of circumferential drawing was insufficient. It was more than 4.0 times the thickness, and a flat plate portion could not be obtained.

From the above, by setting the circumference ratio (C _IN /C _OUT ) in the corner forming within the range of the present invention, the radius of curvature of the outside of the corner is set to more than 3.0 times the average plate thickness and 4.0 times or less, It is possible to provide a square steel pipe having a small variation in hardness in the circumferential cross section, excellent ductility and toughness of the corner outer surface, and a flat plate portion, a method for manufacturing the same, and a building structure having excellent earthquake resistance performance. .

1 Steel strip (steel plate)
2 leveler 3 cage roll group 4 fin pass roll group 5 squeeze roll 6 welder 7 electric resistance welded steel pipe 8 sizing roll group 9 corner forming roll group 10 square steel pipe 11 flat plate portion 12 corner portion 13 welded portion (electric resistance welded portion)
REFERENCE SIGNS LIST 14 Base metal part 15 Weld heat affected zone 16 Melt and solidify zone 17 Diaphragm 18 Large beam 19 Small beam 20 Stud 100 Building structure

Claims (3)

A plurality of flat plate portions and corner portions are alternately formed in the pipe circumferential direction,
A welded portion extending in the pipe axis direction is further formed,
The width of the molten solidified portion in the welded portion in the pipe circumferential direction is 1.0 μm or more and 1000 μm or less,
The radius of curvature of the corner outside is more than 3.0 times and less than or equal to 4.0 times the average plate thickness t,
The difference between the maximum value and the minimum value of Vickers hardness in the steel pipe is 80 HV or less,
The uniform elongation at the position t/4 from the corner outer surface is 0.80 times or more the uniform elongation at the position t/4 from the flat plate outer surface,
In the peripheral cross section, the flatness represented by the maximum swelling amount and the maximum recessing amount with respect to a straight line passing through two points at both ends in the circumferential direction on the same side of the outer surface of the flat plate portion is 2.5 mm or less,
The flat plate portion has a yield strength of 295 MPa or more,
The flat plate portion has a tensile strength of 400 MPa or more,
The yield ratio of the corner is 90% or less,
Charpy absorbed energy of the corner at 0° C. is 70 J or more,
A square steel pipe , wherein the average plate thickness t is more than 0.030 times the average side length H. 2. The square steel pipe according to claim 1, wherein the average plate thickness t is 20 mm or more and 40 mm or less. A building structure in which the square steel pipe according to claim 1 or 2 is used as a pillar material.

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