JP6984785B2 - Square steel pipe and its manufacturing method and building structure - Google Patents

Description

The present invention relates to a square steel pipe which is used as a pillar material of a building structure, has excellent deformation ability, and is less affected by work hardening of corners, a method for manufacturing the same, and a building structure.

Conventionally, a square steel pipe used as a pillar material of a building has been manufactured by a method (BCP method) in which a thick steel plate is press-formed into a square shape by a press machine and then welded. On the other hand, in recent years, instead of the low-productivity BCP method, square steel pipes are manufactured by a method (BCR method) of roll forming, welding, and square forming to obtain square steel pipes from the viewpoint of cost reduction. Attempts have come to be made.

In the BCR method, a hot-rolled steel sheet is rolled into a cylindrical open tube shape, and the butt joints are welded by electric stitching. Is added, and the product is subsequently molded into a square shape. Since the production of square steel pipes by roll forming is carried out cold, the influence of work hardening is remarkable. Therefore, as compared with the square steel pipe obtained by the BCP method, the plastic deformability of the flat plate portion is particularly impaired, and design restrictions are imposed.

In order to relax this design limitation, the yield ratio YR of the flat plate portion of the square steel pipe obtained by the BCR method must be equivalent to that of the flat plate portion of the square steel pipe obtained by the BCP method, and the yield ratio YR is 0. It is .80 or less.

Furthermore, on the inner surface of the corner of the square steel pipe by the BCR method, the Zn plating process in the subsequent process is performed warmly, so that residual stress is released and embrittlement cracks occur starting from the part embrittled by work hardening. There was a problem that it occurred. Therefore, in the square forming step, it is necessary to select manufacturing conditions that suppress excessive work hardening of the inner surface of the corner of the square steel pipe.

From the above, when manufacturing a square steel pipe by the BCR method, select a material that reduces the increase in the yield ratio YR of the flat plate due to work hardening during cold forming, and select a material that reduces the residual stress on the inner surface of the corner. It is effective to select a manufacturing method that suppresses the generation, but it is also an effective means to perform heat treatment to heat the entire square steel pipe.

Patent Document 1 proposes a method for manufacturing a square steel pipe by continuously forming a square steel pipe by the BCR method, heat treatment for strain removal and annealing of the square steel pipe by an induction heating device, and plating treatment for hot dip galvanizing. ..

Patent Document 2 proposes a manufacturing method in which a square steel pipe obtained by cold forming is tempered at a temperature equal to or lower than the _{Ac 1 transformation point.}

Patent Document 3 proposes a manufacturing method in which square forming is performed in advance up to finish forming of a square steel pipe, heat treatment is performed in the middle, _{and finish square forming is performed in a temperature range exceeding the Ac 3 transformation point.}

Japanese Unexamined Patent Publication No. 9-155447 Japanese Unexamined Patent Publication No. 2005-163159 Japanese Patent No. 28522317

By the way, in recent years, as a square steel pipe for use in building structures having excellent seismic performance, the yield strength and the tensile strength are set to a predetermined value or more as the mechanical properties of the flat plate portion, and the yield ratio is set as described above. It is required to be 0.80 or less.
Further, the corners are required to sufficiently secure toughness and suppress work hardening as described above.
Further, regarding the oxide scale formed on the inner and outer surfaces of the tube, it is also required to suppress scale peeling while ensuring the function as a protective film.
However, it cannot be said that the techniques described in the above-mentioned Patent Documents 1 to 3 are sufficient as a technique for obtaining a square steel pipe satisfying these requirements.

The present invention has been made in view of the above circumstances, and has excellent mechanical properties of the flat plate portion, sufficiently secures the function of the oxide scale formed on the inner and outer surfaces of the pipe, and further, the corner portion. It is an object of the present invention to provide a square steel pipe having sufficient toughness and suppressed work hardening, a method for manufacturing the square steel pipe, and a building structure using the square steel pipe.

The present inventors have made diligent studies to solve the above problems.
First, as the mechanical characteristics required for the flat plate portion (side portion in the vertical cross section in the pipe axis direction) in the present invention, the yield strength YS is 295 MPa or more, the tensile strength TS is 400 MPa or more, and the yield ratio YR is 0.80 or less. I decided that I should do it. Further, it was determined that the charpy absorption energy at 0 ° C. should be 70 J or more as the toughness required for the corners.
Further, in order to sufficiently secure the function of the oxide scale generated on the inner and outer surfaces of the tube, specifically, in order to secure the function as a protective film while suppressing the peeling of the oxide scale, in the present invention. It was found that the thickness should be 1 μm or more and 20 μm or less.
Further, in order to sufficiently suppress work hardening of the corner portion, the average Vickers hardness at the position in the wall thickness direction of 1 mm ± 0.1 mm from the inner surface of the corner apex and the outer surface of the central portion in the tube circumferential direction of the flat plate portion are required. It was found that the difference from the average Vickers hardness at the position in the wall thickness direction of 1 mm ± 0.1 mm should be 5 HV or more and 60 HV or less.

Further, in order for the square steel pipe to have the above-mentioned characteristics, the present inventors have a temperature of less than the _{Ac 1 transformation point with respect to a specific square steel pipe finished from a steel plate to a square shape by cold forming.} It was found that the annealing heat treatment should be performed with the heating temperature deviation in the wall thickness direction of the pipe set to 50 ° C. or less and the heating holding time of 500 ° C. or higher set to 100 sec or more. In detail, first, attention was paid to the fact that the toughness may be significantly deteriorated when the heat treatment is performed at a temperature higher than the _{Ac 1 transformation point.}
In addition, the present inventors investigated the relationship between the heating temperature deviation in the wall thickness direction of the pipe and the mechanical properties of the pipe.
Specifically, we focused on the fact that the annealing heat treatment such as strain removal annealing has a large effect on the heating temperature deviation in the thickness direction of the pipe. It was also noted that in the annealing heat treatment such as induction heating, the outer surface side is heated by electric resistance, and the temperature on the inner surface side is lower than that on the outer surface side. From these, if the heating temperature deviation is large, the difference in the influence of annealing heat treatment such as strain removal annealing becomes large on the outer and inner surfaces of the pipe, and as a result, the difference in mechanical properties on the outer and inner surfaces of the pipe becomes large, which is not possible. We found that the tube had uniform characteristics, and studied this point diligently. In addition, in the strain-removing annealing treatment, we focused on the need to secure a sufficient heating and holding time to remove the strain.

Based on these studies, in order for the square steel pipe to have the above-mentioned characteristics, the square steel pipe finished from a steel plate to a square shape by cold forming is heated at a temperature below the _{Ac 1 transformation point.} It was found that the annealing heat treatment should be performed with the heating temperature deviation in the wall thickness direction of the pipe set to 50 ° C. or less and the heating holding time of 500 ° C. or higher set to 100 sec or more.

The above findings were demonstrated by the present inventors by examining performing high-frequency induction heating using a work coil on a square steel pipe as an example of annealing heat treatment.

In this high-frequency induction heating, the heated element in the work coil connected to the AC power source is heated by Joule heat due to electric resistance. Therefore, high frequency induction heating has a small heat loss and excellent heating efficiency.
In addition, by controlling the heating frequency, the penetration depth of the eddy current, which is a factor that generates Joule heat, can be adjusted, and by reducing the frequency, heating is performed to the inner side of the object to be heated. Can be done. Therefore, in high-frequency induction heating, even if the thickness of the heated body increases, the temperature deviation between the surface and the inside of the heated body can be reduced by appropriately controlling the frequency.

The present inventors heated the entire square steel pipe by high-frequency induction heating while transporting various square steel pipes used for pillar materials of building structures into a work coil. As a result, by setting the frequency in an appropriate range, it was possible to obtain a uniform heat distribution in the wall thickness direction. In addition, by increasing the penetration depth of the electric current, it was possible to suppress the heat concentration on the surface due to the skin effect and shorten the time to reach the target temperature on the inner surface of the steel pipe. Furthermore, it was confirmed that the above effect can be obtained even with a small heating facility having a total length of the work coil of about several meters.
The above-mentioned skin effect refers to the following phenomena.
First, a magnetic field of high-frequency current generates a current (eddy current) that cancels the magnetic field on the surface of the heated body (steel pipe). Due to this eddy current, the heated body is heated by the electric resistance, and the heating is concentrated as it approaches the surface. This phenomenon is called the skin effect.

The present invention is based on the above findings, and its features are as follows.
[1] A plurality of flat plate portions and corner portions are alternately formed in the circumferential direction of the pipe.
The yield strength YS of the flat plate portion is 295 MPa or more, and the yield strength is 295 MPa or more.
The tensile strength TS of the flat plate portion is 400 MPa or more, and the plate portion has a tensile strength TS of 400 MPa or more.
The yield ratio YR of the flat plate portion is 0.80 or less, and the flat plate portion has a yield ratio of 0.80 or less.
The Charpy absorption energy at 0 ° C. of the corner is 70 J or more, and the charpy absorption energy is 70 J or more.
The thickness of the oxidation scale on the inner and outer surfaces of the tube is 1 μm or more and 20 μm or less.
The average Vickers hardness at the position in the wall thickness direction of 1 mm ± 0.1 mm from the inner surface of the apex of the corner, and the average Vickers hardness at the position in the wall thickness direction of 1 mm ± 0.1 mm from the outer surface of the central portion in the circumferential direction of the flat plate portion. A square steel pipe whose difference from the above is 5 HV or more and 60 HV or less.
[2] The square steel pipe according to the above [1], wherein the absolute value of the residual stress in the circumferential direction of the inner surface and the outer surface of the apex of the corner portion is 10 MPa or more and 200 MPa or less.
[3] The square steel pipe according to the above [1] or [2], wherein the uniform elongation at the position in the wall thickness direction of 6 mm ± 1 mm from the inner surface and the outer surface of the apex of the corner portion is 5% or more.
[4] The method for manufacturing a square steel pipe according to any one of the above [1] to [3].
A square raw pipe finished from a steel plate to a square shape by cold forming _{is heated at a temperature less than the Ac 1} transformation point, the heating temperature deviation in the thickness direction of the pipe is 50 ° C. or less, and 500 ° C. or more. A method for manufacturing a square steel pipe, which is subjected to an annealing heat treatment with a heating holding time of 100 sec or more.
[5] The method for manufacturing a square steel pipe according to the above [4], wherein the heating temperature is 500 ° C. or higher and 700 ° C. or lower in the annealing heat treatment.
[6] The method for manufacturing a square steel pipe according to the above [4] or [5], wherein the heating of the annealing heat treatment is induction heating, and the frequency in the induction heating is 100 Hz or more and 1000 Hz or less.
[7] A building structure in which the square steel pipe according to any one of [1] to [3] above is used as a pillar material.

According to the present invention, the mechanical properties of the flat plate portion are excellent, the function of the oxide scale generated on the inner and outer surfaces of the pipe is sufficiently secured, and further, the toughness is sufficiently secured at the corner portion and the processing is performed. It is possible to provide a square steel pipe with suppressed hardening, a method for manufacturing the same, and a building structure using the square steel pipe.

It is a pipe axial vertical sectional view for demonstrating the flat plate part and the corner part of a square steel pipe. It is a schematic diagram for demonstrating the oxidation scale. It is a schematic diagram which shows an example of the manufacturing equipment of the electric resistance pipe. It is a schematic diagram which shows an example of the manufacturing equipment of the square steel pipe of this invention. It is a schematic diagram which shows the heat treatment process of a square raw tube. It is a schematic diagram which shows an example of a building structure.

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

<Square steel pipe>
FIG. 1 shows an example of the shape of the square steel pipe of the present invention in a vertical cross-sectional view in the pipe axis direction.
The square steel pipe 1 of the present invention has a square or rectangular cross section (vertical cross section in the pipe axis direction) perpendicular to the longitudinal direction (pipe axis direction) of the pipe, and a flat plate portion (side portion in the pipe axis vertical cross section) in the circumferential direction of the pipe. ) 101 and a plurality of square portions 102 are alternately formed, the yield strength YS of the flat plate portion 101 is 295 MPa or more, the tensile strength TS of the flat plate portion 101 is 400 MPa or more, and the flat plate portion The yield ratio YR (= yield strength / tensile strength) of 101 is 0.80 or less, the charmy absorption energy of the square portion 102 at 0 ° C. is 70 J or more, and the thickness of the oxidation scale on the inner and outer surfaces of the tube is 1 μm or more. It is 20 μm or less, and has an average Vickers hardness at a position in the wall thickness direction of 1 mm ± 0.1 mm from the inner surface of the apex of the corner, and a wall thickness direction of 1 mm ± 0.1 mm from the outer surface of the central portion in the circumferential direction of the flat plate portion 101. The difference from the average Vickers hardness at the position is 5 HV or more and 60 HV or less.
Further, the square steel pipe 1 of the present invention is a steel pipe obtained from an electrosewn steel pipe, and can have a welded portion (electrosewn welded portion) 103 on a flat plate portion 101.

In the present invention, the side length H of the flat plate portion 101 in the vertical cross section of the square steel pipe 1 in the pipe axis direction is preferably 300 to 550 mm, and the wall thickness t is preferably 16 to 30 mm.
The shape of the square steel pipe 1 in the vertical cross-sectional view in the pipe axis direction is preferably a square (substantially square) in which the side lengths H of all four sides of each flat plate portion 101 are the same, and is also a rectangle (substantially rectangular). May be good. The side length H in the case of a rectangle is the average of the vertical side length H1 (mm) and the horizontal side length H2 (mm) (H = (H1 + H2) / 2).

The yield strength YS: 295 MPa or more, the tensile strength TS: 400 MPa or more, and the yield ratio YR: 0.80 or less in the flat plate portion 101 specified in the present invention are at temperatures less than the _{Ac 1 transformation point with respect to the specific rectangular tube.} It can be adjusted by heating and performing an annealing heat treatment in which the heating temperature deviation in the wall thickness direction of the tube is 50 ° C. or less and the heating holding time of 500 ° C. or higher is 100 sec or more.

Further, the charpy absorption energy of the corner 102 specified in the present invention at 0 ° C.: 70 J or more is _{heated at a temperature lower than the Ac 1} transformation point, the heating temperature deviation in the wall thickness direction of the tube is set to 50 ° C. or less, and It can be adjusted by performing an annealing heat treatment with a heating holding time of 500 ° C. or higher for 100 sec or longer.

For the yield strength YS, tensile strength TS, and yield ratio YR of the flat plate portion 101, a JIS No. 5 tensile test piece was collected from the flat plate portion 101 of the square steel pipe 1 so that the tensile direction was parallel to the pipe axis direction, and the JIS No. 5 tensile test piece was used. It can be measured by carrying out in accordance with the regulations of JIS Z 2241.

Further, the Charpy absorption energy of the corner portion 102 at 0 ° C. is obtained from a V-notch test piece collected from the outer surface of the corner portion 102 of the square steel pipe 1 so that the longitudinal direction of the test piece is parallel to the longitudinal direction of the pipe at t / 4. It is obtained by performing a Charpy impact test at a test temperature of 0 ° C. in accordance with the regulations of JIS Z 2242.

FIG. 2 is a schematic diagram for explaining an oxidation scale formed by the square steel pipe 1 of the present invention.
The oxide scale existing on the inner and outer surfaces of the steel pipe 1 has a structure as shown in FIG. 2, and wustite (FeO), magnetite (Fe ₃ O ₄ ), and hematite (Fe ₂ O ₃ ) are ground iron (mother). It is composed of layers in order from the material) side to the surface side.

In the present invention, in the annealing heat treatment such as induction heating, the growth of the oxidation scale on the surface of the steel pipe 1 is suppressed by heating at a temperature lower than the _{Ac 1 transformation point.} On the other hand, when heated at a temperature equal to or higher than the _{Ac 1 transformation point, the oxidation scale grows.} When the thickness of the oxide scale (hereinafter, also referred to as scale thickness) increases and exceeds 20 μm, strain due to an impact force from the outside is likely to be accumulated in the scale layer, and scale peeling occurs. On the other hand, if the scale thickness is less than 1 μm, the effect of the scale as a protective film disappears during cold molding, and a sufficient anticorrosion effect cannot be obtained.
Therefore, the scale thickness of the inner and outer surfaces of the pipe is set to 1 μm or more and 20 μm or less. The scale thickness is preferably 2 μm or more, more preferably 4 μm or more. Further, the scale thickness is preferably 10 μm or less, and more preferably 8 μm or less.

The scale thickness can be increased to 1 μm or more by adjusting the time for exposing the high-temperature raw plate to the atmosphere in hot rolling. Further, by setting the heating temperature of the annealing heat treatment to be less than the Ac1 transformation point, the scale thickness can be reduced to 20 μm or less.
Further, the thickness of the oxide scale formed on the inner and outer surfaces of the steel pipe 1 can be measured by using a scanning electron microscope (SEM).

In the square steel pipe 1 after undergoing an annealing heat treatment such as induction heating as described later, a sizing step, or a straightening step, residual stress is released by the heat treatment.
In the unheat-treated square steel pipe, a large compressive residual stress and a tensile residual stress are generated, respectively, especially on the outer surface and the inner surface of the corner portion.
At this time, if excessive residual stress is applied to the outer surface of the corner, the progress of work hardening of the outer surface is remarkable, and when welding a square steel pipe as a building member with a diaphragm or the like, the vicinity of the weld is formed. Cracks may occur due to the thermal expansion that occurs in the heated part of the.
Further, when an excessive residual stress acts on the inner surface of the corner portion, the residual stress may be released by the Zn plating treatment performed after forming the square steel pipe, and plating cracks may occur on the inner surface of the corner portion.
When the residual stress in the pipe circumferential direction is equal to or greater than the yield stress of the steel plate base material (yield stress on the surface of the corners), defects are likely to occur at the corners of the square steel pipe. Therefore, in order to suppress defects in the corner 102, it is necessary to reduce the residual stress in the circumferential direction of the inner and outer surfaces at the apex of the corner, and the absolute value of the residual stress is the yield stress on the surface of the corner. It is desirable that it is less than. More specifically, the absolute value of the residual stress is preferably 200 MPa or less in order to prevent abnormal cut deformation that occurs when the square steel pipe 1 after forming is cut.
Further, when the absolute value of the residual stress is less than 10 MPa, the yield elongation of the material may not be eliminated due to insufficient straightening. Therefore, the absolute value of the residual stress in the tube circumferential direction on the inner surface and the outer surface of the corner apex is preferably 10 MPa or more and 200 MPa or less. It is more preferably 20 MPa or more, and even more preferably 50 MPa or more. Further, it is more preferably 150 MPa or less, and further preferably 100 MPa or less.
Further, in the present invention, the processing amount of the straightening process after the heat treatment is controlled _{, the heating is performed at a temperature lower than the Ac 1} transformation point, and the heating temperature deviation in the wall thickness direction of the tube is set to 50 ° C. or less. By performing an annealing heat treatment with a heating holding time of 500 ° C. or higher for 100 sec or longer, the absolute value of the residual stress can be reduced to 10 MPa or higher and 200 MPa or lower.

To measure the residual stress, the steel pipe is cut, the member up to a depth of 50 μm from the surface layer at the measurement position is removed by electrolytic etching, and then the residual stress in the circumferential direction is measured by the cosα method of X-ray diffraction. The measurement position is the central part of the longitudinal direction of the steel pipe, and is the position of the apex of the corners at the four corners.

Here, as shown in FIG. 1, the corner apex is from the center position of the short side (H1 in the case of H1 <H2) of the flat plate portion 101 in the vertical cross section in the pipe axis direction of the square steel pipe 1 toward the inside of the steel pipe. More specifically, on a straight line drawn toward the center position of the opposite short side, 1/2 × | H2-H1 | (in the case of H1 <H2, H2) direction from the center of the square steel pipe. That is, the offset point is located with respect to the straight line drawn toward the center position of the opposite short side from the point (offset point) offset by half the difference between the side length H2 and the side length H1. It can be an intersection of a line forming 45 ° with the long side of the flat plate portion 101 formed on the opposite side to the side and the outside of the corner portion 102.
Further, the apex of the corner portion is drawn from the center position of the long side (H2 in the case of H1 <H2) of the flat plate portion 101 in the vertical cross section of the square steel pipe 1 toward the center position of the opposite long side. On a straight line, starting from a point (offset point) offset by 1/2 × | H2-H1 | in the short side (H1 in the case of H1 <H2) direction from the center of the square steel pipe, the above-mentioned opposed long sides The offset point is toward the inside of the steel pipe from the center position of the short side (H1 in the case of H1 <H2) of the flat plate portion 101 in the vertical cross section in the pipe axis direction of the square steel pipe 1 with respect to the straight line drawn toward the center position of. It can be said that it is the intersection of the line forming 45 ° with the short side of the flat plate portion 101 formed on the position side and the outside of the corner portion 102.
When the shape of the vertical cross-sectional view in the pipe axis direction is a square (substantially square), the corner apex is a line forming 45 ° with the flat plate portion 101 and the outside of the corner portion 102 with the central axis of the steel pipe 1 as the starting point. Can be the intersection of.

The steel pipe immediately after cold forming is significantly affected by work hardening, and work hardening is progressing at the corners of the four corners as compared with the flat plate portion.
In the case of roll-formed square steel pipes, work hardening has the greatest effect on the inner surface side of the corners, and ductility is impaired. Strain removal by induction heating After heat treatment such as annealing, the structure of the square steel pipe is strain-removed by recovery, so that the ductility is improved and the influence of work hardening is almost eliminated. At this time, if the uniform elongation at the position in the wall thickness direction of 6 mm ± 1 mm from the inner surface and the outer surface of the corner apex is less than 5%, the strain removing annealing is insufficient and the corner may be cracked. be. Therefore, it is preferable that the uniform elongation at the position in the wall thickness direction of 6 mm ± 1 mm from the inner surface and the outer surface of the apex of the corner is 5% or more. More preferably, it is 10% or more.

For the above uniform elongation, JIS No. 5 tensile test pieces were taken from a position in the wall thickness direction of 6 mm ± 1 mm from the inner and outer surfaces of the apex of the square steel pipe so that the tensile direction was parallel to the pipe axis direction, and this was used. It can be measured by carrying out in accordance with the regulations of JIS Z 2241.

In the present invention, a specific square tube _{is heated at a temperature lower than the Ac 1} transformation point, the heating temperature deviation in the thickness direction of the tube is set to 50 ° C. or less, and the heating holding time is set to 500 ° C. or higher. By performing the annealing heat treatment for 100 sec or more, the uniform elongation can be 5% or more.

Further, by performing the heat treatment described later on the entire steel pipe, it is possible to obtain a square steel pipe 1 having substantially uniform mechanical properties at each portion of the flat plate portion 101 and the square portion 102.

In the present invention, the Vickers hardness of the square steel pipe 1 is not particularly limited, but may be 100 to 300 HV in order to prevent insufficient straightening and excessive work hardening in the straightening step after the heat treatment.

Further, in the square steel pipe 1 of the present invention, the Vickers hardness of the apex of the corner before heat treatment is higher than the Vickers hardness of the flat plate portion, and the influence remains even after the strain is removed and annealed. It may be higher than the Vickers hardness of the flat plate portion 101.

The average Vickers hardness at the position in the wall thickness direction of 1 mm ± 0.1 mm from the inner surface of the apex of the corner, and the average Vickers hardness at the position in the wall thickness direction of 1 mm ± 0.1 mm from the outer surface of the central part in the circumferential direction of the flat plate portion. ((Average Vickers hardness at the position in the wall thickness direction of 1 mm ± 0.1 mm from the inner surface of the apex of the corner)-(Thickness direction of 1 mm ± 0.1 mm from the outer surface of the central part in the circumferential direction of the flat plate portion) When the average Vickers hardness)) at the position is less than 5 HV, the yield elongation of the material cannot be eliminated due to insufficient correction. On the other hand, if the difference in average Vickers hardness exceeds 60 HV, the strain removing annealing is insufficient, and the mechanical properties of the flat plate portion and the corner portion become non-uniform. Therefore, the average Vickers hardness at the position in the wall thickness direction of 1 mm ± 0.1 mm from the inner surface of the apex of the corner and the average at the position in the wall thickness direction of 1 mm ± 0.1 mm from the outer surface of the central portion in the circumferential direction of the flat plate portion 101. The difference from the Vickers hardness is 5 HV or more and 60 HV or less. It is preferably 10 HV or more, and more preferably 15 HV or more. Further, it is preferably 40 HV or less, and more preferably 30 HV or less.
In the present invention, a specific square elemental tube _{is heated at a temperature less than the Ac 1} transformation point, the heating temperature deviation in the thickness direction of the tube is set to 50 ° C. or less, and the heating holding time of 500 ° C. or higher is 100 sec or longer. By performing the annealing heat treatment, and more preferably by controlling the heating temperature and the annealing heat treatment time in the annealing heat treatment such as strain removal annealing, the difference in the average Vickers hardness can be reduced to 5 HV or more and 60 HV or less.

As the Vickers hardness, in accordance with the regulations of the Micro Vickers hardness test (JIS Z2244: 2009), the position in the wall thickness direction of 1 mm ± 0.1 mm from the inner surface of the corner apex of the four corners and the tube circumference of the flat plate portion 101. The Vickers hardness at a position in the wall thickness direction of 1 mm ± 0.1 mm from the outer surface of the central portion in the direction is measured. The test force is 9.8 N, and the Vickers hardness is measured.

The composition of the square steel tube 1 of the present invention is not particularly limited, but in terms of mass%, C: 0.07 to 0.20%, Si: less than 0.4%, Mn: 0.3 to 2.0%, It is a component composition containing P: 0.030% or less, S: 0.015% or less, Al: 0.01 to 0.06%, N: 0.006% or less, and the balance Fe and unavoidable impurities. Is preferable. The reasons for limiting each component are described below. Hereinafter, in the description of each component, the mass% is simply expressed as% unless otherwise specified.

C: 0.07 to 0.20%
C is an element that increases the strength of steel by solid solution strengthening and contributes to the formation of pearlite, which is one of the second phases. In order to secure the desired tensile properties, toughness, and desired steel structure, it is preferable to contain C in 0.07% or more. On the other hand, if the content of C exceeds 0.20%, a martensite structure is generated at the time of welding of square steel pipes (for example, at the time of welding of square steel pipes), and there is a concern that it causes welding cracks. Therefore, the C content is preferably in the range of 0.07 to 0.20%. The C content is more preferably 0.09% at the lower limit and 0.18% at the upper limit.

Si: Less than 0.4% Si is an element that contributes to increasing the strength of steel by solid solution strengthening, and can be contained as necessary in order to secure the desired steel strength. In order to obtain such an effect, it is preferable to contain Si in an amount of more than 0.01%. However, if the content of Si is 0.4% or more, firelite called red scale is likely to be formed on the steel surface, and the appearance of the surface is often deteriorated. Therefore, when Si is contained, the Si content is preferably less than 0.4%. In particular, when Si is not added, the Si content is 0.01% or less as an unavoidable impurity.

Mn: 0.3-2.0%
Mn is an element that increases the strength of the steel sheet through solid solution strengthening, and is preferably contained in an amount of 0.3% or more in order to secure the desired strength of the steel sheet. If the content of Mn is less than 0.3%, the ferrite transformation start temperature is increased and the structure tends to be excessively coarsened. On the other hand, if Mn is contained in an amount of more than 2.0%, the hardness of the central segregated portion increases, and there is a concern that it may cause cracking during joint welding of columns using a square steel pipe or welding with a diaphragm. Therefore, the Mn content is preferably 0.3 to 2.0%. The Mn content is more preferably 1.6% at the upper limit. Even more preferably, the upper limit is 1.4%.

P: 0.030% or less P is an element having an action of segregating into ferrite grain boundaries and lowering toughness, and in the present invention, it is preferable to reduce it as an impurity as much as possible. However, since excessive reduction causes an increase in refining cost, the P content is preferably 0.002% or more. The P content can be up to 0.030%. Therefore, the P content is preferably 0.030% or less. The P content is more preferably 0.025% or less.

S: 0.015% or less S exists as a sulfide in steel and mainly exists as MnS within the composition range of the present invention. Since MnS is thinly stretched in the hot rolling step and adversely affects the ductility and toughness, it is preferable to reduce MnS as much as possible in the present invention. However, since excessive reduction causes an increase in refining cost, the S content is preferably 0.0002% or more. The S content is acceptable up to 0.015%. Therefore, the S content is preferably 0.015% or less. The S content is more preferably 0.010% or less.

Al: 0.01-0.06%
Al is an element that acts as a deoxidizing agent and also has an action of fixing N as AlN. In order to obtain such an effect, the content of Al of 0.01% or more is required. When the Al content is less than 0.01%, the deoxidizing power is insufficient when Si is not added, oxide-based inclusions increase, and the cleanliness of the steel decreases. On the other hand, if the Al content exceeds 0.06%, the amount of solid-dissolved Al increases, and welding is performed during longitudinal welding of square steel pipes (during welding during manufacturing of square steel pipes), especially in the case of welding in the atmosphere. The risk of forming oxides in the portion increases, and the toughness of the welded portion of the square steel pipe decreases. Therefore, the Al content is preferably 0.01 to 0.06%. The Al content is more preferably 0.02% at the lower limit and 0.05% at the upper limit.

N: 0.006% or less N is an element having an action of lowering toughness by firmly fixing the motion of dislocations. In the present invention, it is desirable to reduce N as an impurity as much as possible, and up to 0.006% is acceptable. Therefore, the N content is preferably 0.006% or less. The N content is more preferably 0.005% or less.

The rest other than the above is Fe and unavoidable impurities. The above components are the basic composition of the steel material in the present invention, but in addition to these, Nb: 0.005 to 0.150%, Ti: 0.005 to 0.150%, V: 0. It may contain one kind or two or more kinds selected from 005 to 0.150% or less.

Nb: 0.005 to 0.150%, Ti: 0.005 to 0.150%, V: One or more selected from 0.005% to 0.150% Nb, Ti, V , Both are elements that form fine carbides and nitrides in steel and contribute to the improvement of steel strength through precipitation strengthening, and can be contained as needed. In order to obtain such an effect, it is preferable to contain Nb: 0.005% or more, Ti: 0.005% or more, and V: 0.005% or more. On the other hand, excessive content leads to an increase in yield ratio and a decrease in toughness. Therefore, when Nb, Ti, and V are contained, Nb: 0.005 to 0.150%, Ti: 0.005 to 0.150%, and V: 0.005 to 0.150%. Preferably, Nb: 0.008% or more, Ti: 0.008% or more, V: 0.008% or more. Further, Nb: 0.10% or less, Ti: 0.10% or less, V: 0.10% or less are preferable.

In addition to the above, Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.30%, It may contain one or more selected from Ca: 0.0005 to 0.010% and B: 0.0003 to 0.010%.

Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.30%, Ca: 0.0005 to One or more selected from 0.010%, B: 0.0003 to 0.010% Cr, Mo, Cu, and Ni are elements that increase the strength of steel by solid solution strengthening. Since both are elements that enhance the hardenability of steel and contribute to the stabilization of austenite, they are elements that contribute to the formation of hard martensite and austenite, and can be contained as needed. In order to obtain such an effect, it is preferable to contain Cr: 0.01% or more, Mo: 0.01% or more, Cu: 0.01% or more, and Ni: 0.01% or more. On the other hand, excessive content causes a decrease in toughness and a deterioration in weldability. Therefore, when Cr, Mo, Cu, and Ni are contained, Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni. : 0.01 to 0.30%. Preferably, Cr: 0.1% or more, Mo: 0.1% or more, Cu: 0.1% or more, Ni: 0.1% or more. Further, Cr: 0.5% or less, Mo: 0.5% or less, Cu: 0.40% or less, Ni: 0.20% or less are preferable.

Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly stretched in the hot rolling process, and can be contained as needed. In order to obtain such an effect, it is preferable to contain 0.0005% or more of Ca. However, if the Ca content exceeds 0.010%, Ca oxide clusters may be formed in the steel and the toughness may deteriorate. Therefore, when Ca is contained, the Ca content is 0.0005 to 0.010%. Preferably, the Ca content is 0.001% or more. Further, the Ca content is preferably 0.0050% or less.

B is an element that contributes to the miniaturization of the structure by lowering the ferrite transformation start temperature. In order to obtain such an effect, it is preferable to contain 0.0003% or more of B. However, when the B content exceeds 0.010%, the yield ratio increases. Therefore, when B is contained, the B content is set to 0.0003% to 0.010%. Preferably, the B content is 0.0005% or more. Moreover, the B content is preferably 0.0050% or less.

Further, in order to ensure weldability when having the above-mentioned composition, the Ceq defined by the formula (1) is defined by the formula (1) of 0.15% or more and 0.50% or less, and the formula (2). The Pcm is preferably 0.30% or less. However, the component compositions of the various elements in the formulas (1) and (2) are all mass%.

Ceq = C + Mn / 6 + Si / 24 + Ni / 40 + Cr / 5 + Mo / 4 + V / 14 ... (1)
Here, in the formula (1), C, Mn, Si, Ni, Cr, Mo, and V are the contents (mass%) of each element. (However, the element not contained is 0 (zero)%.)
Pcm = C + Si / 30 + Mn / 20 + Cu / 20 + Ni / 60 + Cr / 20 + Mo / 15 + V / 10 + 5B ... (2)
Here, in the formula (2), C, Si, Mn, Cu, Ni, Cr, Mo, V, and B are the contents (mass%) of each element. (However, the element not contained is 0 (zero)%.)
Ceq in the formula (1) is a carbon equivalent and is an index of the hardness of the welded portion and the heat-affected zone. If Ceq is less than 0.15%, the strength required for pillars of building structures may not be obtained. Further, when Ceq exceeds 0.50%, the welded portion and the heat-affected zone are excessively hardened, and the variation in the peripheral cross-sectional strength becomes large. Therefore, the Ceq is preferably 0.15% or more and 0.50% or less.

Pcm in the formula (2) is susceptible to weld cracking, and when Pcm exceeds 0.30%, low temperature cracking is likely to occur in the welded portion and the heat-affected zone. Therefore, the Pcm is preferably 0.30% or less, more preferably 0.25% or less.

<Manufacturing method of square steel pipe>
Next, a method for manufacturing the square steel pipe 1 of the present invention will be described. In the method for manufacturing a square steel pipe 1 of the present invention, a square raw pipe finished from a steel plate to a square shape by cold forming _{is heated at a temperature less than the Ac 1} transformation point, and the heating temperature deviation in the thickness direction of the pipe is measured. The annealing heat treatment is performed with the temperature set to 50 ° C. or lower and the heating holding time of 500 ° C. or higher set to 100 sec or longer.
In order to obtain the above-mentioned steel sheet, in order to make the thickness of the oxide scale formed on the inner and outer surfaces of the finally obtained square steel pipe 1 μm or more, the high-temperature raw plate is exposed to the atmosphere after the finish rolling of hot rolling. Adjust the exposure time. Specifically, it is preferable to expose the base plate having a surface temperature of 900 ° C. or lower to the atmosphere for 5 to 400 sec after the finish rolling of hot rolling. After that, the obtained steel plate is cold-formed to form a square shape, whereby a square raw tube can be obtained.

Here, a method for obtaining the above-mentioned square tube will be described. FIG. 3 is a schematic view showing an example of a manufacturing facility for an electric resistance sewn steel pipe used for obtaining a square steel pipe.
As shown in FIG. 3, a steel strip (hereinafter, also referred to as a steel plate) 4 wound around a coil is dispensed, straightened by a leveler 5, and intermediately formed by a cage roll group 6 composed of a plurality of rolls to form an open pipe. After that, finish molding is performed with a finpass roll group 7 composed of a plurality of rolls. The open tube can be formed into a cylindrical shape obtained by cold roll molding.
After the finish forming, the circumferential butt portion of the steel strip 4 is electrically resistance welded by the welding machine 9 while being pressure-welded with the squeeze roll 8 to obtain the electrosewn steel pipe 10. In the present invention, the manufacturing equipment for the electrosewn steel pipe 10 is not limited to the pipe making process as shown in FIG. Further, in the above-mentioned electric sewing welding, the butt portion is heated, melted, pressed and solidified, and the joining is completed.

The subsequent steps will be described later with reference to FIG. 4 and the like, but in the sizing step after the electric resistance welding, in order to satisfy the roundness and the residual stress in the pipe axial direction required in the present invention, the steel pipe is used. It is preferable to reduce the diameter of the steel pipe so that the circumference is reduced at a rate of 0.30% or more in total. On the other hand, when the diameter is reduced so that the circumference of the steel pipe decreases at a rate of more than 5.0% in total, the bending amount in the pipe axis direction when passing through the roll becomes large, and the residual stress in the pipe axis direction after the diameter reduction increases. On the contrary, it may rise. Therefore, it is preferable to reduce the diameter so that the circumference of the steel pipe after the diameter is reduced by 0.30% or more and 5.0% or less with respect to the circumference of the steel pipe before the diameter is reduced.

In the sizing step, it is preferable to perform multi-step diameter reduction with a plurality of stands in order to minimize the bending amount in the pipe axis direction when passing through the roll and suppress the generation of residual stress in the pipe axis direction. The diameter reduction in each stand is preferably performed so that the tube circumference is reduced at a rate of 1.0% or less.

Whether or not a square steel pipe (square steel pipe) is obtained from an electrosewn steel pipe is determined by cutting the square steel pipe (square steel pipe) vertically in the pipe axis direction, polishing the cut surface including the welded part, and then corroding it with nital. , Can be judged by observing with an optical microscope. If the width of the melt-solidified portion in the pipe circumferential direction at the center of the plate thickness of the welded portion is 1 mm or less, the pipe is an electrosewn steel pipe.

FIG. 4 is a schematic view showing a process of forming a square steel pipe from an electrosewn steel pipe.
As shown in FIG. 4, the electrosewn steel pipe 10 is reduced in diameter by a sizing roll group (sizing stand) 11 composed of a plurality of rolls while maintaining a cylindrical shape, and then a square forming roll group (square forming stand) composed of a plurality of rolls. By 12, the pipes are sequentially formed into shapes such as R1, R2, and R3 to form a square pipe. The number of stands of the sizing roll group 11 and the square forming roll group 12 is not particularly limited. Further, the caliber curvature of the sizing roll group 11 or the square forming roll group 12 is preferably one condition.

FIG. 5 is a schematic view showing an example of equipment for manufacturing a square steel pipe from the above-mentioned square raw pipe.
In the example shown in FIG. 5, after the sizing step, the square tube cut to a predetermined length is transported in the longitudinal direction on the transport table 2 at a predetermined speed. At this time, the work coil 3 is fixed, and the square steel pipe 1 sent out by the transport table is heated while passing through the work coil.

As described above, in the present invention, a square raw tube finished from a steel plate to a square shape by cold forming _{is heated at a temperature lower than the Ac 1} transformation point, and the heating temperature deviation in the thickness direction of the tube is 50 ° C. The annealing heat treatment is performed with the following, and the heating holding time at 500 ° C. or higher is 100 sec or longer.

In the above annealing heat treatment, in order to release the strain accumulated by the cold forming, the heat treatment is performed in the temperature range of the strain removing annealing. When _{heated to the Ac 1} transformation point or higher, the structure of the steel pipe becomes a two-phase structure, and there is a problem that the toughness deteriorates. In addition, the thickness of the oxidation scale on the inner and outer surfaces of the tube exceeds 20 μm. Therefore, in the annealing heat treatment of the present invention, heating is performed at a temperature lower than the _{Ac 1 transformation point.}

Further, in the above annealing heat treatment, heating such as induction heating is performed from the outer surface of the steel pipe, so that a temperature deviation occurs on the inner and outer surfaces of the steel pipe during heating. When _{strain removal annealing is performed by heating below the Ac 1} transformation point, the lower the heating temperature, the longer it takes for the strain to be completely removed. In such a case, there is a problem that the progress of strain release is delayed on the inner surface side where the heating temperature tends to be low, and the mechanical properties tend to be non-uniform in the wall thickness direction of the pipe. Regarding the problem of the temperature deviation of the heating temperature of the outer surface and the inner surface, if the heating temperature deviation in the wall thickness direction of the pipe is 50 ° C. or less, uniform mechanical characteristics can be obtained in the wall thickness direction of the pipe. Therefore, in the annealing heat treatment of the present invention, the heating temperature deviation in the wall thickness direction of the pipe is set to 50 ° C. or less. It is preferably 30 ° C. or lower, and more preferably 10 ° C. or lower.

Further, the heating temperature by the annealing heat treatment is preferably 500 ° C. or higher and 700 ° C. or lower. When the heat treatment is performed at a temperature lower than 500 ° C., it takes a long time until the strain is completely removed.

When the strain is removed and annealed at 500 ° C. or higher, it is preferable to secure a heating holding time of 100 sec or longer in order to remove the strain. When the tube is heated by induction heating and then naturally allowed to cool, the cooling rate of the surface on the inner and outer surfaces of the tube is about 0.5 ° C./sec. Therefore, in order to secure a heating holding time of 500 ° C. or higher at 100 sec or longer after heating, the lower limit of the heating temperature in the annealing heat treatment is preferably 550 ° C. (= 500 ° C. + 0.5 ° C./sec × 100 sec).
The temperature of the heat treatment by the annealing heat treatment is preferably 550 ° C. or higher and 700 ° C. or lower, and more preferably 600 ° C. or higher. Further, it is more preferably 650 ° C. or lower.

The heating in the above annealing heat treatment is preferably induction heating, and can be performed using an induction heating device.

In the above induction heating, when the frequency is less than 100 Hz, the penetration depth of the current becomes too large and the skin effect becomes small, so that the heating temperature of the heating concentration portion may decrease. As a result, the heat conduction from the heated high temperature portion to the inner surface side of the steel pipe becomes small, so that the heating efficiency of the entire pipe deteriorates and the equipment becomes large. On the other hand, when the frequency exceeds 1000 Hz, the skin effect becomes large, so that the temperature deviation between the heating temperature of the outer surface and the inner surface of the tube may become large. Therefore, it is preferable to set the frequency of induction heating to 100 Hz or more and 1000 Hz or less. More preferably, the frequency of induction heating is 150 Hz or higher. Further, the frequency of induction heating is more preferably 500 Hz or less, and even more preferably 300 Hz or less.
The above-mentioned skin effect refers to the following phenomena.
First, a magnetic field of high-frequency current generates a current (eddy current) that cancels the magnetic field on the surface of the heated body (steel pipe). Due to this eddy current, the heated body is heated by the electric resistance, and the heating is concentrated as it approaches the surface. This phenomenon is called the skin effect.

Further, in the induction heating, the transport speed of the square tube is not particularly limited, but is preferably 0.2 to 4 m / min from the viewpoint of manufacturing efficiency and uniform heating temperature of the cross section. Further, the amount of electric power in the induction heating device is not particularly limited, but is preferably 3 to 12 MW in order to secure a desired transfer speed.

As the above-mentioned temperature control method for steel pipes, the temperature on the outer surface of the pipe is measured by a radiation thermometer, and the temperature on the inner surface of the pipe and the temperature inside the wall thickness is calculated by a two-dimensional model based on thermal analysis. It can be controlled by a method of calculating the temperature distribution in the wall thickness direction over the entire circumference of the steel pipe.

The square steel pipe after the annealing heat treatment by induction heating or the like described above can be subjected to the sizing step and / or the straightening step again. These are for eliminating the yield elongation that occurs when tensile deformation is applied to the steel pipe base material after heat treatment, and if a strain of 0.5 to 3% can be applied over the entire circumference of the pipe, this is used. Not as long.

<Building structure>
FIG. 6 is a schematic view showing an example of the building structure of the present invention.

In the building structure of the present invention, the square steel pipe 1 of the present invention described above is used as a pillar material.
Reference numerals 13, 14, 15, and 16 indicate diaphragms, girders, girders, and studs in that order.
As described above, the square steel pipe of the present invention has excellent mechanical properties of the flat plate portion, sufficiently secures the function of the oxide scale formed on the inner and outer surfaces of the pipe, and further provides sufficient toughness at the corner portion. While ensuring, work hardening is suppressed. Therefore, the building structure of the present invention using this square steel pipe as a column material exhibits excellent seismic performance.

Hereinafter, the present invention will be further described based on Examples.

A hot-rolled steel sheet having the composition shown in Table 1 is continuously formed into an open pipe having an elliptical cross section by a cage roll group and a fin pass roll group, and then the opposite end faces of the open pipe are melted by high-frequency induction heating or high-frequency resistance heating. After heating above, pressure welding was performed with a squeeze roll to obtain an electrosewn steel pipe. In order to make the thickness of the oxide scale formed on the inner and outer surfaces of the finally obtained square steel pipe 1 μm or more, the time for exposing the high temperature base plate to the atmosphere after the finish rolling of hot rolling was adjusted. Specifically, the time for exposing the base plate having a surface temperature of 900 ° C. or lower to the atmosphere after the finish rolling of hot rolling was set to 5 to 400 sec.

From the obtained cylindrical steel pipe, a square raw pipe having a curvature of the corner portion (2.5 ± 0.5) times the plate thickness was obtained through a two-stage sizing stand and then a four-stage square forming stand. ..

Next, the square raw pipe was cut to a predetermined length and heat-treated (annealed heat treatment) using a high-frequency heating device (induction heating device) having a cylindrical work coil to obtain a square steel pipe.
The inner diameter D of the work coil is 960 mm, and the length in the transport direction (height direction assuming a cylindrical shape) is 1 m.
The square raw tube was heated while being inserted into the work coil by a transport carriage. At that time, the transport speed, the heating frequency, and the amount of electric power were controlled so as to reach a predetermined heating temperature.

Regarding the temperature control of the steel pipe, the temperature on the outer surface of the pipe was measured by a radiation thermometer, and the temperature on the inner surface of the pipe and the temperature inside the wall thickness was calculated by temperature calculation using a two-dimensional model based on thermal analysis.
In Table 2, the heating temperature (the outer surface maximum temperature and the inner surface maximum temperature) (℃) indicates whether Ac less than ₁ transformation point (Table 2 "heating temperature <Ac ₁ transformation point (℃)" column references ). In Table 2, "○" indicates that the heating temperature is less than Ac1 transformation point, "×" indicates that the heating temperature is Ac ₁ transformation point or more.
The heating temperature deviation was calculated as the difference between the maximum outer surface temperature (° C.) and the maximum inner surface temperature (° C.) (see the column "Outer surface temperature-inner surface temperature (° C.)" in Table 2).
Further, in Table 2, the "holding time" refers to a heating holding time of 500 ° C. or higher.

Then, a straightening process was performed using an inclined roll straightening machine, and a strain of 2% was applied to the steel pipe.
A test piece was collected from the obtained square steel pipe, and a tensile test, a Charpy impact test, a residual stress measurement, a scale thickness measurement, and a hardness measurement were performed.

As a tensile test of the flat plate part, a JIS No. 5 tensile test piece was taken from the flat plate part of the square steel pipe so that the tensile direction was parallel to the pipe axis direction, and it was carried out in accordance with the regulations of JIS Z 2241. , Yield strength YS and tensile strength TS were measured, and the yield ratio YR defined by (yield strength) / (tensile strength) was calculated.

As a Charpy impact test, JIS Z was used as a V-notch test piece collected from the outer surface of the corner of a square steel pipe so that the longitudinal direction of the test piece was parallel to the longitudinal direction of the pipe at t / 4 (t: wall thickness). In accordance with the provisions of 2242, the test was carried out at a test temperature of 0 ° C., and the absorbed energy (J) was determined. The number of test pieces was 3 each, and the average value thereof was used as a representative value.

As a residual stress measurement, a steel pipe was cut to a length of 500 mm, a member up to a depth of 50 μm from the surface layer at the measurement position was removed by electrolytic etching, and then the residual stress in the circumferential direction was measured by the cosα method of X-ray diffraction. The measurement position was the longitudinal center of the test piece steel pipe, and the positions of the outer and inner surfaces of the corner vertices at the four corners.
The apex of the corner is the steel pipe No. For 1 to 15 and 18, starting from the central axis of the steel pipe, the intersection of the line forming 45 ° with the flat plate portion and the outside of the corner portion was set. In addition, steel pipe No. For 16 and 17, the offset point offset by 1/2 × (H1-H2) in the long side (H1) direction from the center of the square steel pipe is the starting point, which is opposite to the side where the offset point is located with respect to the straight line. The intersection of the line forming 45 ° with the flat plate formed on the side and the outside of the corner was used.

The thickness of the oxide scale on the surface of the steel pipe was measured using a scanning electron microscope (SEM) at the positions of the inner and outer surfaces of the flat plate portion of the square steel pipe.
Here, the distance between the interface between the steel pipe base material and the scale and the surface of the scale is measured at eight points, and the total value of the distances at these eight points divided by eight (average value) is the thickness of the oxide scale. It was set to (μm). The above eight points were the central portion of the width of the flat plate portion on the four sides of the square steel pipe, and were set to a total of eight points, that is, four points on the inner surface and four points on the outer surface.

As a tensile test of the corner, JIS No. 5 tensile test piece was taken from the position in the wall thickness direction of 6 mm ± 1 mm from the inner and outer surfaces of the apex of the square steel pipe so that the tensile direction was parallel to the pipe axis direction, and this was used. The uniform elongation (%) was calculated in accordance with the provisions of JIS Z 2241.

The hardness is measured in accordance with the regulations of the Micro Vickers hardness test (JIS Z2244: 2009), the test force is 9.8 N, and the position in the wall thickness direction is 1 mm ± 0.1 mm from the inner and outer surfaces of the corner apex. The average Vickers hardness in 1 mm and the average Vickers hardness (HV) at the position in the wall thickness direction of 1 mm ± 0.1 mm from the inner and outer surfaces of the central portion in the tube circumferential direction of the four flat plate portions were measured. Then, as the difference between the Vickers hardness of the corner apex and the Vickers hardness of the flat plate portion, 1 mm ± from the inner surface of the corner apex where the difference between the average Vickers hardness of the corner apex and the average Vickers hardness of the flat plate portion is maximum. The difference between the average Vickers hardness at the position in the wall thickness direction of 0.1 mm and the average Vickers hardness at the position in the wall thickness direction of 1 mm ± 0.1 mm from the outer surface of the central portion in the tube circumferential direction of the flat plate portion ((at the apex of the corner). Average Vickers hardness at 1 mm ± 0.1 mm wall thickness direction from the inner surface)-(Average Vickers hardness at 1 mm ± 0.1 mm wall thickness direction from the outer surface of the central portion of the flat plate in the tube circumference direction)) The hardness difference was calculated from.

The side length H (mm) (vertical side length H1 (mm), horizontal side length H2 (mm)) was measured with a caliper, and the wall thickness t (mm) was measured with a micrometer.

These results are shown in Table 3.

From the above, it is possible to provide a square steel pipe having excellent deformation ability and suppressing excessive work hardening of corners, a method for manufacturing the square steel pipe, and a building structure having excellent seismic performance.

1 Square steel pipe (square raw pipe)
2 Transport table 3 Work coil 4 Steel strip (steel plate)
5 Leveler 6 Cage roll group 7 Finpass roll group 8 Squeeze roll 9 Welder 10 Electric sewn steel pipe 11 Sizing roll group 12 Square forming roll group 13 Diaphragm 14 Large beam 15 Small beam 16 Stud 101 Flat plate 102 Square 103 Welded part (electric) Sewing weld)

Claims (8)

By mass%, C: 0.07 to 0.20%, Si: less than 0.4%, Mn: 0.3 to 2.0%, P: 0.030% or less, S: 0.015% or less, It contains Al: 0.01 to 0.06%, N: 0.006% or less, and has a component composition consisting of the balance Fe and unavoidable impurities.
Multiple flat plates and corners are alternately formed in the circumferential direction of the pipe.
The yield strength YS of the flat plate portion is 295 MPa or more, and the yield strength is 295 MPa or more.
The tensile strength TS of the flat plate portion is 400 MPa or more, and the plate portion has a tensile strength TS of 400 MPa or more.
The yield ratio YR of the flat plate portion is 0.80 or less, and the flat plate portion has a yield ratio of 0.80 or less.
The Charpy absorption energy at 0 ° C. of the corner is 70 J or more, and the charpy absorption energy is 70 J or more.
The thickness of the oxidation scale on the inner and outer surfaces of the tube is 1 μm or more and 20 μm or less.
The average Vickers hardness at the position in the wall thickness direction of 1 mm ± 0.1 mm from the inner surface of the apex of the corner, and the average Vickers hardness at the position in the wall thickness direction of 1 mm ± 0.1 mm from the outer surface of the central portion in the circumferential direction of the flat plate portion. A square steel pipe whose difference from the above is 5 HV or more and 60 HV or less. Further, the component composition is, in terms of mass%, Nb: 0.005 to 0.150%, Ti: 0.005 to 0.150%, V: 0.005 to 0.150%, Cr: 0.01 to 0.01. 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.30%, Ca: 0.0005 to 0.010%, B : The square steel pipe according to claim 1, which contains one or more selected from 0.0003 to 0.010%. The square steel pipe according to claim 1 or 2 , wherein the absolute value of the residual stress in the circumferential direction of the inner surface and the outer surface of the corner apex is 10 MPa or more and 200 MPa or less. The square steel pipe according to any one of claims 1 to 3, wherein the uniform elongation at the position in the wall thickness direction of 6 mm ± 1 mm from the inner surface and the outer surface of the corner apex is 5% or more. The method for manufacturing a square steel pipe according to any one of claims 1 to 4.
A square raw pipe finished from a steel plate to a square shape by cold forming _{is heated at a temperature less than the Ac 1} transformation point, the heating temperature deviation in the thickness direction of the pipe is 50 ° C. or less, and 500 ° C. or more. A method for manufacturing a square steel pipe, which is subjected to an annealing heat treatment with a heating holding time of 100 sec or more. The method for manufacturing a square steel pipe according to claim 5 , wherein the heating temperature is 500 ° C. or higher and 700 ° C. or lower in the annealing heat treatment. The method for manufacturing a square steel pipe according to claim 5 or 6 , wherein the heating of the annealing heat treatment is induction heating, and the frequency in the induction heating is 100 Hz or more and 1000 Hz or less. A building structure in which the square steel pipe according to any one of claims 1 to 4 is used as a pillar material.

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