KR102709519B1 - Square steel pipe and its manufacturing method and building structures - Google Patents

Abstract

To provide a square steel pipe having excellent mechanical properties at the flat surface, sufficiently securing the function of an oxide scale formed on the inner and outer surfaces of the pipe, and further sufficiently securing toughness at the corners while suppressing work hardening, a method for manufacturing the same, and an architectural structure using the square steel pipe.
A plurality of flat portions (101) and corner portions (102) are alternately formed in the circumferential direction of the pipe, the yield strength (YS) of the flat portion (101) is 295 MPa or more, the tensile strength (TS) of the flat portion (101) is 400 MPa or more, the yield ratio (YR) of the flat portion (101) is 0.80 or less, the Charpy absorbed energy of the corner portion (102) at 0° C. is 70 J or more, the thickness of the oxide scale on the inner and outer surfaces of the pipe is 1 ㎛ or more and 20 ㎛ or less, and the difference in average Vickers hardness between a predetermined position in the thickness direction from the inner surface of the apex of the corner portion (102) and a predetermined position in the thickness direction from the outer surface of the central portion of the circumferential direction of the flat portion (101) is 5 HV or more and 60 HV or less.

Description

Square steel pipe and its manufacturing method and building structures

The present invention relates to a square steel pipe used as a pillar material for building structures, having excellent deformation capacity and a small effect of work hardening at corners, a method for manufacturing the same, and building structures.

Conventionally, square steel pipes used as pillar materials for buildings have been manufactured by pressing thick steel plates into a square shape using a press machine and then welding them (BCP method). Recently, on the other hand, in order to reduce costs, attempts have been made to manufacture square steel pipes by a method of obtaining square steel pipes by roll forming, welding, and then forming them into square shapes (BCR method) instead of the low-productivity BCP method.

The BCR method is manufactured by forming a hot-rolled steel plate into a cylindrical open pipe shape by roll forming, and then welding the butting parts, and then applying a few percent of tightening in the axial direction of the pipe while still in a cylindrical shape by rolls arranged up, down, left, and right, and continuously forming it into a square shape. Since the manufacture of square steel pipes by roll forming is carried out in the cold, the influence of work hardening is significant. Therefore, compared to square steel pipes obtained by the BCP method, the plastic deformation ability of the flat plate portion in particular is impaired, and design restrictions are imposed.

In order to alleviate this design limitation, it is necessary to make the yield ratio (YR) of the flat plate section of the square steel pipe obtained by the BCR method equivalent to that of the flat plate section of the square steel pipe obtained by the BCP method, and the yield ratio (YR) is 0.80 or less.

In addition, since the Zn plating treatment in the post-process is performed in a warm state on the inner surface of the corners of the square steel pipes by the BCR method, there was a problem that the release of residual stress occurred and embrittlement cracks occurred starting from the part that was embrittled due to work hardening. Therefore, in the square forming process, it is necessary to select manufacturing conditions that suppress excessive work hardening on the inner surface of the corners of the square steel pipes.

From the above, in the case of manufacturing a square steel pipe by the BCR method, it is effective to select a material that reduces the increase in the yield ratio (YR) of the flat portion due to work hardening during cold forming, or to select a manufacturing method that suppresses the occurrence of residual stress on the inner surface of the corner portion, but among these, performing a heat treatment that heats the entire square steel pipe is also an effective means.

Patent Document 1 proposes a method for manufacturing a square steel pipe, which comprises forming a square steel pipe by the BCR method, performing a heat treatment for stress relief annealing of the square steel pipe using an induction heating device, and performing a plating treatment for hot-dip galvanizing.

Patent Document 2 proposes a manufacturing method for performing tempering heat treatment at a temperature below the Ac ₁ transformation point on a square steel pipe obtained by cold forming.

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

Japanese Patent Publication No. 9-155447 Japanese Patent Publication No. 2005-163159 Japanese Patent Publication No. 2852317

However, recently, for square steel pipes to be used in building structures with excellent earthquake resistance, as for the mechanical properties of the flat section, it is required that the yield strength and tensile strength be above the specified values, and the yield ratio be 0.80 or less as described above.

In addition, for the corners, it is required to sufficiently secure toughness and suppress work hardening as described above.

In addition, with regard to the oxide scale formed on the inner and outer surfaces of the pipe, it is also required to prevent scale peeling while ensuring the function of a protective film.

However, the technologies described in the above-mentioned patent documents 1 to 3 cannot be said to be sufficient as technologies for obtaining square steel pipes that satisfy these requirements.

The present invention has been made in consideration of the above circumstances, and has as its object the provision of a square steel pipe having excellent mechanical properties of a flat portion, sufficiently securing the function of an oxide scale formed on the inner and outer surfaces of the pipe, and further sufficiently securing toughness at corner portions while suppressing work hardening, a method for manufacturing the same, and an architectural structure using the square steel pipe.

The present inventors conducted a preliminary review to solve the above problem.

First, it was determined that the mechanical properties required for the flat portion (the edge portion in the cross-section perpendicular to the tube axis) in the present invention are yield strength (YS) of 295 MPa or more, tensile strength (TS) of 400 MPa or more, and yield ratio (YR) of 0.80 or less. In addition, it was determined that the toughness required for the edge portion is Charpy absorbed energy at 0°C of 70 J or more.

In addition, in order to sufficiently secure the function of the oxide scale occurring on the inner and outer surfaces of the pipe, specifically, in order to secure the function as a protective film while suppressing the peeling of the oxide scale, it was found in the present invention that the thickness should be 1 ㎛ or more and 20 ㎛ or less.

In addition, in order to sufficiently suppress work hardening at the corner portion, it was found that the difference between the average Vickers hardness at a thickness direction position of 1 mm ± 0.1 mm from the inner surface of the corner portion apex and the average Vickers hardness at a thickness direction position of 1 mm ± 0.1 mm from the outer surface of the central portion of the circumferential direction of the flat plate portion should be 5 HV or more and 60 HV or less.

In addition, the inventors of the present invention found that in order for a square steel pipe to have the above-described characteristics, a specific square tube finished into a square shape from a steel plate by cold forming 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 annealing is performed for a heating holding time of 100 seconds or more at 500°C or more. Specifically, they first noted that when heat treatment is performed at a temperature higher than the Ac ₁ transformation point, there are cases where toughness is significantly deteriorated.

In addition, the inventors of the present invention examined the relationship between the heating temperature deviation in the thickness direction of the tube and the mechanical properties of the tube.

Specifically, it was noted that the annealing heat treatment such as stress relief annealing has a large effect on the heating temperature deviation in the thickness direction of the pipe. In addition, it was noted that in annealing heat treatment such as induction heating, the outer surface is heated by electric resistance, and the temperature of the inner surface is lower than that of the outer surface. From these, it was found that if the heating temperature deviation is large, the difference in the effect of annealing heat treatment such as stress relief annealing on the outer and inner surfaces of the pipe becomes large, and as a result, the difference in mechanical properties between the outer and inner surfaces of the pipe becomes large, resulting in a pipe having non-uniform properties, and this point was carefully examined. In addition, it was noted that in stress relief annealing, it is necessary to secure a sufficient heating holding time in order to remove deformation, and this was examined.

By these examinations, it was found that in order to make a square steel pipe have the above-mentioned characteristics, an annealing heat treatment should be performed by heating a square pipe finished into a square shape from a steel plate by cold forming at a temperature lower than the Ac ₁ transformation point, setting the heating temperature deviation in the thickness direction of the pipe to 50°C or less, and further setting the heating holding time at 500°C or more to 100 seconds or more.

Regarding the above knowledge, the inventors of the present invention verified it by examining 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 object to be heated inside the work coil connected to the AC power source is heated by the Joule heat generated by the electrical resistance. Therefore, high-frequency induction heating has small heat loss and excellent heating efficiency.

In addition, by controlling the frequency of heating, the penetration depth of the eddy current, which is a factor in generating Joule heat, can be adjusted, and by reducing the frequency, the heating can be performed more internally on the heated body. Therefore, in high-frequency induction heating, even if the thickness of the heated body increases, the temperature difference between the surface and internal heating temperatures of the heated body can be reduced by appropriately controlling the frequency.

The inventors of the present invention carried out overall heating of various types of square steel pipes used as pillar materials of building structures by high-frequency induction heating while returning them into a work coil. As a result, by setting the frequency to an appropriate range, it was possible to obtain a uniform heating distribution in the thickness direction. In addition, by increasing the penetration depth of the current, it was possible to suppress surface heating concentration due to the skin effect or shorten the time it took for the inner surface of the steel pipe to reach the target temperature. In addition, it was confirmed that the above-mentioned effect was obtained even in a small heating facility with a total work coil length of several meters.

In addition, the above skin effect refers to the following phenomenon.

First, a current (eddy current) that cancels the magnetic field is generated on the surface of the heated object (steel pipe) by the magnetic field of the high-frequency current. The heated object is heated by this eddy current due to electrical resistance, and this heating becomes more concentrated as it gets closer to the surface. This phenomenon is called the skin effect.

The present invention is based on the above knowledge, and its features are as follows.

[1] Flat sections and corner sections are alternately formed in multiple directions around the pipe,

The yield strength (YS) of the above flat plate is 295 MPa or more,

The tensile strength (TS) of the above flat plate is 400 MPa or more,

The yield ratio (YR) of the above flat plate is 0.80 or less,

The Charpy absorption energy at 0°C of the above edge is 70 J or more,

The thickness of the oxide scale on the inner and outer surfaces of the tube is 1 ㎛ or more and 20 ㎛ or less,

A square steel pipe, wherein the difference between the average Vickers hardness at a thickness direction position of 1 mm ± 0.1 mm from the inner surface of the corner apex and the average Vickers hardness at a thickness direction position of 1 mm ± 0.1 mm from the outer surface of the central portion of the circumferential direction of the flat portion is 5 HV or more and 60 HV or less.

[2] A square steel pipe as described in [1] above, wherein the absolute value of the residual stress in the circumferential direction of the pipe on the inner and outer surfaces of the corner apex is 10 MPa or more and 200 MPa or less.

[3] A square steel pipe as described in [1] or [2] above, having a uniform elongation of 5% or more at a thickness direction position of 6 mm ± 1 mm from the inner and outer surfaces of the corner vertices.

[4] A method for manufacturing a square steel pipe as described in any one of [1] to [3] above,

A method for manufacturing a square steel pipe, comprising: heating a square pipe finished into a square shape from a steel plate by cold forming at a temperature lower than the Ac ₁ transformation point, setting the heating temperature deviation in the thickness direction of the pipe to 50°C or less, and performing annealing heat treatment at 500°C or more for a heating holding time of 100 seconds or more.

[5] A method for manufacturing a square steel pipe as described in [4] above, wherein in the annealing heat treatment, the heating temperature is 500°C or higher and 700°C or lower.

[6] A method for manufacturing a square steel pipe as described in [4] or [5], wherein the heating for the above annealing heat treatment is performed by induction heating, and the frequency of the induction heating is 100 Hz or more and 1000 Hz or less.

[7] A building structure in which a square steel pipe described in any one of the above [1] to [3] is used as a pillar material.

According to the present invention, it is possible to provide a square steel pipe having excellent mechanical properties of a flat portion, sufficiently securing the function of an oxide scale occurring on the inner and outer surfaces of the pipe, and further sufficiently securing toughness at corner portions while suppressing work hardening, a method for manufacturing the same, and an architectural structure using the square steel pipe.

Figure 1 is a vertical cross-sectional view along the pipe axis to explain the flat portion and corner portion of a square steel pipe.
Figure 2 is a schematic diagram explaining the oxidation scale.
Figure 3 is a schematic diagram showing an example of a manufacturing facility for a steel pipe.
Figure 4 is a schematic diagram showing an example of a manufacturing facility for a square steel pipe of the present invention.
Figure 5 is a schematic diagram showing the heat treatment process of a square tube.
Figure 6 is a schematic diagram showing an example of a building structure.

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

<Square steel pipe>

Fig. 1 shows an example of the shape of a cross-section perpendicular to the tube axis of a square steel pipe of the present invention.

The square steel pipe (1) of the present invention has a cross section perpendicular to the longitudinal direction of the pipe (the pipe axis direction) (the cross section perpendicular to the pipe axis direction) as a square or rectangular shape, and a plurality of flat portions (edge portions in the cross section perpendicular to the pipe axis direction) (101) and corner portions (102) are alternately formed in the circumferential direction of the pipe, the yield strength (YS) of the flat portion (101) is 295 MPa or more, the tensile strength (TS) of the flat portion (101) is 400 MPa or more, and further, the yield ratio (YR) (= yield strength/tensile strength) of the flat portion (101) is 0.80 or less, the Charpy absorbed energy of the corner portion (102) at 0° C. is 70 J or more, and the thickness of the oxide scale on the inner and outer surfaces of the pipe is 1 µm or more and 20 µm or less, and the thickness of the oxide scale is 1 mm ± 1 mm from the inner surface of the corner apex. The difference between the average Vickers hardness at a thickness direction position of 0.1 mm and the average Vickers hardness at a thickness direction position of 1 mm ± 0.1 mm from the outer surface of the central portion of the circumferential direction of the flat portion (101) is 5 HV or more and 60 HV or less.

In addition, the square steel pipe (1) of the present invention is a steel pipe obtained from an electric welded steel pipe and may have a welded portion (electric welded portion) (103) on a flat plate portion (101).

In the present invention, although not particularly limited, it is preferable that the side length (H) of the flat plate portion (101) in the vertical cross section in the pipe axis direction of the square steel pipe (1) is 300 to 550 mm, and the thickness (t) is 16 to 30 mm.

The shape of the square steel pipe (1) in the vertical cross-section in the pipe axis direction is preferably a square (approximately square) in which the side lengths (H) of the four sides of each flat plate portion (101) are all the same, but may also be rectangular (approximately rectangular). In the case of a rectangular shape, the side length (H) is the average of the vertical side length (H1) (mm) and the horizontal side length (H2) (mm) (H = (H1＋H2)/2).

In the present invention, 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 specified flat plate portion (101) can be adjusted by performing an annealing heat treatment in which, for a specific square tube, the temperature is 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 further, the heating holding time is set to 500° C. or more for 100 sec or more.

In addition, the Charpy absorption energy of 70 J or more at 0° C. of the edge portion (102) specified in the present invention can be adjusted by performing an annealing heat treatment in which heating is performed at a temperature lower than the Ac ₁ transformation point, the heating temperature deviation in the thickness direction of the pipe is set to 50° C. or less, and the heating holding time is set to 500° C. or more for 100 sec or more.

The yield strength (YS), tensile strength (TS), and yield ratio (YR) of the flat portion (101) can be measured by taking a JIS No. 5 tensile test piece from the flat portion (101) of the square steel pipe (1) so that the tensile direction is parallel to the pipe axis direction, and using this to conduct a test in accordance with the provisions of JIS Z 2241.

In addition, the Charpy absorbed energy at 0°C of the corner portion (102) is obtained by conducting a Charpy impact test at a test temperature of 0°C in accordance with the provisions of JIS Z 2242 using a V-notch test piece taken from the outer surface of the corner portion (102) of the square steel pipe (1) such that the length direction of the test piece is parallel to the length direction of the pipe at t/4.

Figure 2 is a schematic diagram for explaining the oxide scale formed in the square steel pipe (1) of the present invention.

The oxide scale existing on the inner and outer surfaces of the above steel pipe (1) has a structure as shown in Fig. 2, and is composed of wustite (FeO), magnetite (Fe ₃ O ₄ ), and hematite (Fe ₂ O ₃ ) in layers in order from the base iron (mother material) side to the surface side.

In the present invention, in an annealing heat treatment such as induction heating, by performing heating at a temperature lower than the Ac ₁ transformation point, the growth of oxide scale on the surface of the steel pipe (1) is suppressed. On the other hand, in the case of heating at a temperature higher than the Ac ₁ transformation point, oxide scale grows. When the thickness of the oxide scale (hereinafter also referred to as scale thickness) increases and exceeds 20 µm, deformation due to an impact force from the outside, etc., easily accumulates in the scale layer, causing scale peeling. On the other hand, when the scale thickness is less than 1 µm, the scale's effect as a protective film is lost during cold forming, and a sufficient corrosion-prevention effect is not obtained. Therefore, the scale thickness on the inner and outer surfaces of the pipe is set to 1 µm or more and 20 µm or less. Preferably, the scale thickness is 2 µm or more, more preferably 4 µm or more. Furthermore, preferably, the scale thickness is 10 µm or less, more preferably 8 µm or less.

Regarding the above scale thickness, by adjusting the time for exposing the high-temperature plate to the air during hot rolling, the scale thickness can be set to 1 ㎛ or more. In addition, by setting the heating temperature of the annealing heat treatment to be lower than the Ac ₁ transformation point, the scale thickness can be set to 20 ㎛ or less.

Additionally, the thickness of the oxide scale formed on the inner and outer surfaces of the steel pipe (1) can be measured using a scanning electron microscope (SEM).

In a square steel pipe (1) that has undergone an annealing heat treatment such as induction heating as described below, a sizing process, or a straightening process, the residual stress is released by the heat treatment.

In square steel pipes that have not been heat treated, large compressive residual stress and tensile residual stress occur, especially on the outer and inner surfaces of corners.

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 is performed on a square steel pipe as a building material such as a diaphragm, cracks may occur due to thermal expansion occurring in the heated area near the weld.

In addition, when excessive residual stress is applied to the inner surface of a corner, the residual stress may be released during the Zn plating treatment performed after forming a square steel pipe, and cracks in the plating may occur on the inner surface of the corner.

When the residual stress in the circumferential direction is higher than the yield stress of the steel plate base material (yield stress of the corner surface), a defect is likely to occur at the corner of the square steel pipe. Therefore, in order to suppress the defect at the corner (102), it is necessary to make the residual stress in the circumferential direction of the inner and outer surfaces at the corner apex small, and it is preferable that the absolute value of the residual stress is less than the yield stress of the corner surface. More specifically, in order to prevent abnormal cut surface deformation that occurs when the square steel pipe (1) after forming is cut, the absolute value of the residual stress is preferably 200 MPa or less.

In addition, when the absolute value of the residual stress is less than 10 MPa, there are cases where the yield elongation of the material cannot be lost due to insufficient correction. Therefore, the absolute value of the residual stress in the circumferential direction of the pipe at the inner surface and the outer surface of the corner vertex 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. Furthermore, it is more preferably 150 MPa or less, and even more preferably 100 MPa or less.

In addition, in the present invention, by controlling the amount of processing of the straightening processing after the heat treatment, heating at a temperature lower than the Ac ₁ transformation point, setting the heating temperature deviation in the thickness direction of the pipe to 50° C. or less, and performing annealing heat treatment of setting the heating holding time at 500° C. or more to 100 sec or more, the absolute value of the residual stress can be set to 10 MPa or more and 200 MPa or less.

In addition, for the measurement of residual stress, the steel pipe is cut, the material from the surface of the measurement position to a depth of 50 ㎛ is removed by electrolytic etching, and 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 length of the steel pipe and the vertices of the four corners.

Here, the corner portion vertex can be defined as the point (offset point) offset from the center of the short side (H1 in the case of H1 < H2) of the flat plate (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, toward the center of the opposite short side, as shown in Fig. 1, in the direction of the long side (H2 in the case of H1 < H2) from the center of the square steel pipe by 1/2 × |H2－H1| (i.e., half of the difference between the side length (H2) and the side length (H1)), and, with respect to the straight line drawn toward the center of the opposite short side, the intersection of the line forming a 45° angle with the long side of the flat plate (101) formed on the opposite side to the side where the offset point is located, and the outer side of the corner portion (102).

In addition, this corner portion vertex can also be said to be the intersection point of a line forming an angle of 45° with the short side of the flat plate portion (101) formed on the side where the offset point is located, 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), and the outer side of the corner portion (102), with respect to a straight line drawn toward the center position of the opposite long side, with a point (offset point) offset by 1/2 × |H2－H1| from the center of the square steel pipe in the direction of the short side (H1, in the case of H1 < H2) as a starting point.

In addition, when the shape of the vertical cross-section in the direction of the pipe axis is square (approximately square), the corner vertex can be the intersection of the line forming a 45° angle with the flat plate (101) based on the central axis of the steel pipe (1) and the outer side of the corner (102).

The steel pipe immediately after cold forming is significantly affected by work hardening, and work hardening is particularly advanced at the corners of the four edges compared to the flat section.

In the case of roll-formed square steel pipes, the inner surface side of the corner portion is most affected by work hardening, and ductility is impaired. Since the structure of the square steel pipe after undergoing heat treatment such as stress relief annealing by induction heating has had its deformation removed by recovery, ductility is improved, and the influence of work hardening is almost eliminated. At this time, if the uniform elongation at a thickness direction position of 6 mm ± 1 mm from the inner and outer surfaces of the corner apexes is less than 5%, stress relief annealing is insufficient, and cracks may occur at the corner portion. Therefore, it is preferable that the uniform elongation at a thickness direction position of 6 mm ± 1 mm from the inner and outer surfaces of the corner apexes is 5% or more. More preferably, it is 10% or more.

The above-mentioned uniform elongation can be measured by taking a JIS No. 5 tensile test piece from a position 6 mm ± 1 mm in the thickness direction from the inner and outer surfaces of the apex of a square steel pipe so that the tensile direction is parallel to the pipe axis, and using this to conduct a test in accordance with the provisions of JIS Z 2241.

In the present invention, by performing annealing heat treatment in which 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 more for 100 seconds or more, the uniform elongation can be made 5% or more.

In addition, by performing the heat treatment described below on the entire steel pipe, it is possible to obtain a square steel pipe (1) having almost uniform mechanical properties in each section of the flat portion (101) and the corner 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 or excessive work hardening in the straightening process after heat treatment.

In addition, since the Vickers hardness of the corner apex of the square steel pipe (1) of the present invention is higher than the Vickers hardness of the flat portion before heat treatment and the effect of this remains even after stress relief annealing, the Vickers hardness of the corner apex may be higher than the Vickers hardness of the flat portion (101).

When the difference between the average Vickers hardness at a thickness direction position of 1 mm ± 0.1 mm from the inner surface of the corner apex and the average Vickers hardness at a thickness direction position of 1 mm ± 0.1 mm from the outer surface of the central portion in the circumferential direction of the flat portion ((average Vickers hardness at a thickness direction position of 1 mm ± 0.1 mm from the inner surface of the corner apex) - (average Vickers hardness at a thickness direction position of 1 mm ± 0.1 mm from the outer surface of the central portion in the circumferential direction of the flat portion)) is less than 5 HV, the yield elongation of the material could not be lost due to insufficient correction. On the other hand, when the difference in the average Vickers hardness exceeds 60 HV, stress relief annealing is insufficient, so that the mechanical properties of the flat portion and the corner portion become uneven. Therefore, the difference between the average Vickers hardness at a thickness direction position of 1 mm ± 0.1 mm from the inner surface of the corner portion apex and the average Vickers hardness at a thickness direction position of 1 mm ± 0.1 mm from the outer surface of the central portion of the circumferential direction of the flat portion (101) is 5 HV or more and 60 HV or less. Preferably, it is 10 HV or more, and more preferably, it is 15 HV or more. Furthermore, it is preferably 40 HV or less, and more preferably, it is 30 HV or less.

In the present invention, for a specific square tube, an annealing heat treatment is performed in which heating is performed 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 further, the heating holding time is set to 500° C. or more and 100 sec or more, and more preferably, by controlling the heating temperature and the annealing heat treatment time in an annealing heat treatment such as stress relief annealing, the difference in the above-mentioned average Vickers hardness can be set to 5 HV or more and 60 HV or less.

The Vickers hardness is measured at a thickness direction position of 1 mm ± 0.1 mm from the inner surface of the corner vertices of the four corners and at a thickness direction position of 1 mm ± 0.1 mm from the outer surface of the central portion of the circumferential direction of the flat plate portion (101) in accordance with the provisions of the micro Vickers hardness test (JIS Z 2244: 2009). The Vickers hardness is measured with a test force of 9.8 N.

The component composition of the square steel pipe (1) of the present invention is not particularly limited, but is preferably a component composition containing, in 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, Al: 0.01 to 0.06%, N: 0.006% or less, with the remainder being Fe and unavoidable impurities. The reasons for the limitation of each component are described below. Hereinafter, in the description of each component, unless otherwise specified, mass% is simply described as %).

C: 0.07 ~ 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 desired tensile properties, toughness, and also desired steel structure, it is preferable to contain C of 0.07% or more. On the other hand, if the C content exceeds 0.20%, there is a concern that a martensite structure may be generated during welding of square steel pipes (for example, during welding of square steel pipes), which may cause weld cracks. Therefore, the C content is preferably in the range of 0.07 to 0.20%. The lower limit of the C content is more preferably 0.09%, and the upper limit is more preferably 0.18%.

Si: less than 0.4%

Si is an element that contributes to the increase in the strength of steel by solid solution strengthening, and can be contained as needed to secure the desired steel strength. In order to obtain this effect, it is preferable to contain Si exceeding 0.01%. However, if Si is contained at 0.4% or more, iron olivine, called red scale, is likely to form on the steel surface, and the appearance of the surface often deteriorates. Therefore, when Si is contained, it is preferable to make the Si content less than 0.4%. In addition, when Si is not specifically added, the Si content is 0.01% or less as an unavoidable impurity.

Mn: 0.3 ~ 2.0%

Manganese is an element that increases the strength of the steel plate through solid solution strengthening, and in order to secure the desired strength of the steel plate, it is preferable to contain 0.3% or more. When the Mn content is less than 0.3%, the ferrite transformation initiation temperature rises, and the structure tends to coarsen excessively. On the other hand, when the Mn content exceeds 2.0%, the hardness of the center segregation portion increases, and there is a concern that this may cause cracks when welding joints of columns using square steel pipes or welding with diaphragms. Therefore, the Mn content is preferably 0.3 to 2.0%. The upper limit of the Mn content is more preferably 1.6%. Even more preferably, the upper limit is 1.4%.

P: 0.030% or less

P is an element that segregates at ferrite grain boundaries and has the effect of 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 costs, it is preferable that the P content be 0.002% or more. Furthermore, the P content is permissible 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 sulfide in steel, and within the composition range of the present invention, exists mainly as MnS. Since MnS is thinly stretched in the hot rolling process and adversely affects ductility and toughness, it is preferable to reduce MnS as much as possible in the present invention. However, since excessive reduction leads to an increase in refining costs, it is preferable that the S content be 0.0002% or more. Furthermore, the S content is permissible 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 deoxidizer and also has the effect of fixing N as AlN. In order to obtain this effect, it is necessary to contain 0.01% or more of Al. When the Al content is less than 0.01%, the deoxidizing power is insufficient in the case of no Si addition, the oxide inclusions increase, and the cleanliness of the steel deteriorates. On the other hand, when the Al content exceeds 0.06%, the amount of solid solution Al increases, and when welding in the longitudinal direction of a square steel pipe (welding during the manufacture of a square steel pipe), especially in the case of welding in the air, the risk of forming oxides in the weld increases, and the toughness of the weld of the square steel pipe deteriorates. For this reason, the Al content is preferably 0.01 to 0.06%. The lower limit of the Al content is more preferably 0.02%, and the upper limit is 0.05%.

N: 0.006% or less

Nitrogen is an element that has the effect of lowering the toughness by firmly fixing the movement of dislocation. In the present invention, it is preferable to reduce Ni as an impurity as much as possible, and up to 0.006% is permissible. Therefore, the Ni content is preferably 0.006% or less. The Ni content is more preferably 0.005% or less.

The remainder other than the above is Fe and inevitable impurities. The above components are the basic component composition of the steel material in the present invention, but in addition to these, one or more types selected from among Nb: 0.005 to 0.150%, Ti: 0.005 to 0.150%, and V: 0.005 to 0.150% or less may be contained.

Nb: 0.005 ∼ 0.150%, Ti: 0.005 ∼ 0.150%, V: 0.005 ∼ 0.150%, 1 or 2 or more selected from among

Nb, Ti, and V are all elements that form fine carbides and nitrides in steel and contribute to improving the strength of steel through precipitation strengthening, and can be contained as needed. In order to obtain such an effect, the content is preferably Nb: 0.005% or more, Ti: 0.005% or more, and V: 0.005% or more. On the other hand, excessive content causes an increase in the 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, and V: 0.008% or more. Also, preferably, Nb: 0.10% or less, Ti: 0.10% or less, V: 0.10% or less.

In addition to the above, one or more kinds selected from 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 0.010%, and B: 0.0003 to 0.010% may be contained.

One or more selected from 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 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, and are also all elements that increase the quench hardenability of steel and contribute to the stabilization of austenite. Therefore, 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 containing Cr, Mo, Cu, and Ni, Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, and Ni: 0.01 to 0.30%. Preferably, Cr: 0.1% or more, Mo: 0.1% or more, Cu: 0.1% or more, and Ni: 0.1% or more. Also, preferably, Cr: 0.5% or less, Mo: 0.5% or less, Cu: 0.40% or less, and Ni: 0.20% or less.

Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly elongated during 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, deteriorating the toughness. Therefore, when containing Ca, the Ca content is set to 0.0005 to 0.010%. Preferably, the Ca content is 0.001% or more. Also, preferably, the Ca content is 0.0050% or less.

B is an element that contributes to the refinement of the structure by lowering the ferrite transformation initiation temperature. In order to obtain this effect, it is preferable to contain 0.0003% or more of B. However, if 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. Also, preferably, the B content is 0.0050% or less.

In addition, when having the above-mentioned composition, in order to secure weldability, it is preferable that Ceq defined by formula (1) is 0.15% or more and 0.50% or less, and Pcm defined by formula (2) is 0.30% or less. However, the compositions of various elements in formulas (1) and (2) are all in mass%.

Ceq = C + Mn/6 + Si/24 + Ni/40 + Cr/5 + Mo/4 + V/14 ···(1)

Here, in formula (1), C, Mn, Si, Ni, Cr, Mo, and V represent the content (mass%) of each element. (However, elements that are not contained are set to 0 (zero) ％.)

Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B...(2)

Here, in formula (2), C, Si, Mn, Cu, Ni, Cr, Mo, V, and B represent the content (mass%) of each element. (However, elements that are not contained are set to 0 (zero) ％.)

(1) Ceq in the formula is a carbon equivalent, which is an index of the hardness of the weld and heat-affected zone. If Ceq is less than 0.15%, the strength required as a pillar material for building structures may not be obtained. In addition, if Ceq exceeds 0.50%, the weld and heat-affected zone are excessively hardened, and the deviation in the circumferential cross-sectional strength increases. Therefore, it is preferable that Ceq be 0.15% or more and 0.50% or less.

(2) Pcm in the formula is the welding crack susceptibility. If Pcm exceeds 0.30%, low-temperature cracks are likely to occur in the weld and heat-affected zone. Therefore, Pcm is preferably 0.30% or less, and more preferably 0.25% or less.

<Method for manufacturing square steel pipes>

Next, a method for manufacturing a 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 pipe finished into a square shape from a steel plate by cold forming is heated at a temperature lower than the Ac ₁ transformation point, the heating temperature deviation in the thickness direction of the pipe is set to 50°C or less, and annealing heat treatment is performed at 500°C or more for a heating holding time of 100 seconds or more.

In addition, when obtaining the above steel plate, 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 ㎛ or more, the time for exposing the high-temperature plate to the air after the finish rolling of the hot rolling is adjusted. Specifically, it is preferable to expose the plate having a surface temperature of 900 ℃ or less to the air for 5 to 400 seconds after the finish rolling of the hot rolling. Thereafter, by finishing the obtained steel plate into a square shape by cold forming, a square pipe can be obtained.

Here, a method for obtaining the above-mentioned square tube is described. Fig. 3 is a schematic diagram showing an example of a manufacturing facility for a welded steel tube used to obtain the square tube.

As shown in Fig. 3, a steel strip (hereinafter also referred to as steel plate) (4) wound on a coil is sent out, straightened by a leveler (5), formed into an open tube by an intermediate forming process by a cage roll group (6) composed of a plurality of rolls, and then final forming is performed by a pin pass roll group (7) composed of a plurality of rolls. The open tube can be formed into a cylindrical shape obtained by cold roll forming.

After the final forming, the circumferential butting portion of the steel strip (4) is subjected to electric resistance welding with a welding machine (9) while being pressed with a squeeze roll (8), thereby forming an electric welded steel pipe (10). In addition, in the present invention, the manufacturing equipment for the electric welded steel pipe (10) is not limited to the pipe manufacturing process as shown in Fig. 3. In addition, in the electric welded welding, the butting portion is heated and melted, and the joining is completed by being pressed and solidified.

As for the process thereafter, as described later with reference to FIG. 4, etc., in the sizing process after the electric arc welding, in order to satisfy the roundness and the residual stress in the pipe-axis direction required by the present invention, it is preferable to reduce the diameter of the steel pipe so that the total circumferential length is reduced by a ratio of 0.30% or more. On the other hand, when the diameter is reduced so that the total circumferential length is reduced by a ratio exceeding 5.0%, the amount of bending in the pipe-axis direction when passing through the rolls increases, and there is a possibility that the residual stress in the pipe-axis direction after the diameter reduction may actually increase. Therefore, it is preferable to reduce the diameter so that the pipe circumferential length after the diameter reduction is reduced by a ratio of 0.30% or more and 5.0% or less with respect to the pipe circumferential length before the diameter reduction.

In addition, in the sizing process, in order to minimize the amount of bending in the pipe axis direction when passing through the roll and to suppress the occurrence of residual stress in the pipe axis direction, it is preferable to perform multi-stage axial reduction using multiple stands, and it is preferable that the axial reduction in each stand be performed so that the pipe circumference is reduced by a ratio of 1.0% or less.

Whether or not a square steel pipe (square small pipe) is obtained from an electrowelded steel pipe can be judged by cutting the square steel pipe (square small pipe) perpendicular to the pipe axis, polishing the cut surface including the weld, corroding it with nital, and observing it with an optical microscope. If the width of the molten and solidified portion in the circumferential direction of the weld in the central portion of the plate thickness of the weld is 1 mm or less, it is an electrowelded steel pipe.

Figure 4 is a schematic diagram showing the process of forming a square pipe from a steel pipe.

As shown in Fig. 4, the preformed steel pipe (10) is reduced in diameter while maintaining a cylindrical shape by a sizing roll group (sizing stand) (11) composed of a plurality of rolls, and then is sequentially formed into shapes such as R1, R2, and R3 by each forming roll group (each forming stand) (12) composed of a plurality of rolls, thereby becoming a square pipe. In addition, the number of stands of the sizing roll group (11) and each forming roll group (12) is not particularly limited. In addition, it is preferable that the caliber curvature of the sizing roll group (11) or each forming roll group (12) is 1 condition.

Figure 5 is a schematic diagram showing an example of equipment for manufacturing a square steel pipe from the above-mentioned square pipe.

In the example shown in Fig. 5, after the sizing process, a square pipe cut to a predetermined length is returned in the longitudinal direction at a predetermined speed on a return table (2). At this time, the work coil (3) is fixed, and the square steel pipe (1) that has been transferred by the return table is heated while passing through the work coil.

As described above, in the present invention, an annealing heat treatment is performed on a square pipe finished into a square shape from a steel plate by cold forming, wherein the annealing heat treatment is performed at a temperature lower than the Ac ₁ transformation point, the heating temperature deviation in the thickness direction of the pipe is set to 50°C or less, and the heating holding time is set to 500°C or more for 100 seconds or more.

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

In addition, in the above-described annealing heat treatment, since heating such as induction heating is performed from the outer surface of the steel pipe, a temperature difference occurs between the inner and outer surfaces of the steel pipe during heating. When stress relief annealing is performed by heating below the Ac ₁ transformation point, the lower the heating temperature, the longer it takes until the deformation is completely removed. In this case, on the inner side where the heating temperature tends to be low, the progress of deformation release is delayed, and there is a problem that the mechanical properties tend to become uneven in the thickness direction of the pipe. Regarding the problem of the temperature difference between the heating temperatures of the outer surface and the inner surface, if the heating temperature difference in the thickness direction of the pipe is 50°C or less, uniform mechanical properties are obtained in the thickness direction of the pipe. Therefore, in the annealing heat treatment of the present invention, the heating temperature difference in the thickness direction of the pipe is set to 50°C or less. Preferably, it is 30°C or less, and more preferably, it is 10°C or less.

In addition, it is preferable that the heating temperature for the annealing heat treatment be 500°C or higher and 700°C or lower. If the heat treatment is performed at less than 500°C, a long time is required until the deformation is completely removed.

When performing stress relief annealing at 500°C or higher, it is desirable to secure a heating holding time of 100 sec or longer in order to remove deformation. When heating a pipe by induction heating and then allowing it to cool naturally, the surface cooling rate on the inner and outer surfaces of the pipe is approximately 0.5°C/sec. Therefore, in order to secure a heating holding time of 500°C or higher for 100 sec or longer after heating, it is desirable that the lower limit of the heating temperature in the annealing heat treatment be 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, more preferably 600°C or higher. Further, it is more preferably 650°C or lower.

Heating in the above annealing heat treatment is preferably performed by 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 excessively large, so that the skin effect becomes small, and therefore the heating temperature of the heating concentration portion may decrease. As a result, since the heat conduction from the heated high-temperature portion to the inner side of the steel pipe becomes small, the heating efficiency of the entire pipe deteriorates, and the equipment becomes large. On the other hand, when the frequency exceeds 1000 Hz, since the skin effect becomes large, there is a possibility that the temperature difference between the heating temperatures of the outer surface and the inner surface of the pipe may become large. Therefore, the frequency of the induction heating is preferably set to 100 Hz or more and 1000 Hz or less. More preferably, the frequency of the induction heating is 150 Hz or more. In addition, more preferably, the frequency of the induction heating is 500 Hz or less, and even more preferably, 300 Hz or less.

In addition, the above skin effect refers to the following phenomenon.

First, a current (eddy current) that cancels the magnetic field is generated on the surface of the heated object (steel pipe) by the magnetic field of the high-frequency current. The heated object is heated by this eddy current due to electrical resistance, and this heating becomes more concentrated as it gets closer to the surface. This phenomenon is called the skin effect.

In addition, in induction heating, the return speed of the square tube is not particularly limited, but is preferably 0.2 to 4 m/min from the viewpoints of manufacturing efficiency and uniform heating temperature of the cross section. In addition, the power amount in the induction heating device is not particularly limited, but is preferably 3 to 12 MW in order to secure the desired return speed.

As for the temperature management method of the steel pipe described above, the temperature of the outer surface of the pipe can be measured by a radiation thermometer, and the temperature of the inner surface and the temperature within the thickness of the pipe can be managed by calculating the temperature distribution in the thickness direction over the entire circumference of the steel pipe by temperature calculation using a two-dimensional model based on thermal analysis.

For the square steel pipe after the annealing heat treatment by the above-mentioned induction heating, etc., a sizing process and/or a straightening process may be performed again. These are for the purpose of eliminating the yield elongation that occurs when tensile deformation is applied to the steel pipe base material after the heat treatment, and are not limited thereto as long as a deformation of 0.5 to 3% can be applied to the entire circumference of the pipe.

<Architecture Structure>

Figure 6 is a schematic diagram showing an example of an architectural 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.

Codes 13, 14, 15, and 16 represent, in order, the diaphragm, main beam, sub-beam, and intercolumn.

As described above, the square steel pipe of the present invention has excellent mechanical properties in the flat portion, sufficiently secures the function of the oxide scale formed on the inner and outer surfaces of the pipe, and further, sufficiently secures toughness at the corner portion while suppressing work hardening. Therefore, the building structure of the present invention using this square steel pipe as a pillar material exhibits excellent seismic performance.

Example

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

A hot-rolled steel plate having a composition shown in Table 1 was continuously formed into an open tube having an oval cross-section by a cage roll group and a pin pass roll group, and then the opposing end surfaces of the open tube were heated to a temperature higher than the melting point by high-frequency induction heating or high-frequency resistance heating, and pressure-welded by a squeeze roll, thereby obtaining a welded steel tube. Furthermore, in order to set the thickness of the oxide scale formed on the inner and outer surfaces of the finally obtained square steel tube to 1 µm or more, the time for exposing the high-temperature plate to the air after the finish rolling of the hot rolling was adjusted, and specifically, the time for exposing the plate having a surface temperature of 900°C or less to the air after the finish rolling of the hot rolling was set to 5 to 400 seconds.

From the obtained cylindrical steel pipe, after passing through two stages of sizing stands, a square pipe having a curvature at the corners of (2.5 ± 0.5) times the plate thickness was obtained through four stages of each forming stand.

Next, the square pipe was cut to a predetermined length, and heat treatment (annealing heat treatment) was performed using a high-frequency heating device (induction heating device) having a cylindrical work coil, thereby obtaining a square steel pipe.

The inner diameter (D) of the above work coil is 960 mm, and the length in the return direction (height direction when assumed to be cylindrical) is 1 m.

The square tube was heated while being inserted into the work coil by a return cart. At that time, the return speed, heating frequency, and power amount were controlled to achieve a predetermined heating temperature.

For temperature control of the steel pipe, the temperature on the outer surface of the pipe was measured by a radiation thermometer, and the temperature distribution on the inner surface and within the thickness of the pipe was calculated by a two-dimensional model based on thermal analysis.

Table 2 shows whether the heating temperature (outer surface maximum temperature and inner surface maximum temperature) (℃) is lower than the Ac ₁ transformation point (see “Heating temperature < Ac ₁ transformation point (℃)” in Table 2). In Table 2, “○” indicates that the heating temperature is lower than the Ac ₁ transformation point, and “×” indicates that the heating temperature is higher than the Ac ₁ transformation point.

In addition, the heating temperature deviation was calculated as the difference between the maximum temperature on the outer surface (℃) and the maximum temperature on the inner surface (℃) (see “External temperature - Internal temperature (℃)” in Table 2).

Additionally, in Table 2, “maintenance time” refers to the heating maintenance time of 500°C or higher.

After that, straightening processing was performed using a slope roll straightener, and a strain of 2% was applied to the steel pipe.

Test specimens were collected from the obtained square steel pipe, and tensile tests, Charpy impact tests, residual stress measurements, scale thickness measurements, and hardness measurements were performed.

As a tensile test of a flat plate, a JIS No. 5 tensile test piece was collected from a flat plate of a square steel pipe so that the tensile direction was parallel to the pipe axis, and this was used to conduct a test in accordance with the provisions of JIS Z 2241, to measure the yield strength (YS) and tensile strength (TS), and to calculate the yield ratio (YR), which is defined as (yield strength)/(tensile strength).

As a Charpy impact test, a V-notch test piece was taken from the outer surface of a 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: thickness), and the test was conducted in accordance with the provisions of JIS Z 2242 at a test temperature of 0℃, and the absorbed energy (J) was obtained. In addition, the number of test pieces was three each, and the average value thereof was used as the representative value.

As a residual stress measurement, the steel pipe was cut to a length of 500 mm, and the member from the surface of the measurement position to a depth of 50 ㎛ was removed by electrolytic etching, and then the circumferential residual stress was measured by the cosα method of X-ray diffraction. The measurement position was the central part of the length of the specimen steel pipe, and the positions of the outer surface and inner surface of the corner vertices of the four corners.

For steel pipes No. 1 to 15 and 18, the corner vertex was set as the intersection of the outer side of the corner and a line forming a 45° angle with the flat plate based on the central axis of the steel pipe. In addition, for steel pipes No. 16 and 17, the corner vertex was set as the intersection of the outer side of the corner and a line forming a 45° angle with the flat plate formed on the opposite side to the side where the offset point is located with respect to the straight line using an offset point offset by 1/2 × (H1－H2) in the direction of the long side (H1) from the center of the square steel pipe as the origin.

The thickness of the oxide scale on the surface of the steel pipe was measured using a scanning electron microscope (SEM) at locations on the inner and outer surfaces of the flat section of the square steel pipe.

Here, the distance between the interface of the steel pipe base material and the scale and the scale surface was measured at eight points, and the total distance of those eight points was divided by eight (the average value), and the thickness (㎛) of the oxide scale was taken as the value. In addition, the eight points are the central parts of the width of the flat section of the four sides of the square steel pipe, and the total of eight points is four points on the inner surface and four points on the outer surface.

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

For the hardness measurement, in accordance with the regulations of the micro Vickers hardness test (JIS Z 2244: 2009), the test force was set to 9.8 N, and the average Vickers hardness at a position in the thickness direction of 1 mm ± 0.1 mm from the inner and outer surfaces of the corner vertices of the four corners and the average Vickers hardness (HV) at a position in the thickness direction of 1 mm ± 0.1 mm from the inner and outer surfaces of the central portion of the flat portion in the pipe circumference direction were measured. And, as the difference between the Vickers hardness of the corner apex and the Vickers hardness of the flat portion, the difference between the average Vickers hardness at a position in the thickness direction of 1 mm ± 0.1 mm from the inner surface of the corner apex at which the difference between the average Vickers hardness of the corner apex and the average Vickers hardness of the flat portion is maximum and the average Vickers hardness at a position in the thickness direction of 1 mm ± 0.1 mm from the outer surface of the central portion in the circumferential direction of the flat portion ((average Vickers hardness at a position in the thickness direction of 1 mm ± 0.1 mm from the inner surface of the corner apex) - (average Vickers hardness at a position in the thickness direction of 1 mm ± 0.1 mm from the outer surface of the central portion in the circumferential direction of the flat portion)) was calculated.

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

Their results are shown in Table 3.

From the above, it is possible to provide a square steel pipe having excellent deformation capability and suppressing excessive work hardening at corners, a method for manufacturing the same, and a building structure having excellent earthquake-resistant performance.

1: Square steel pipe (square pipe)
2: Return Table
3: Work coil
4: Steel plate
5 : Leveler
6: Cage Roll Group
7: Pin Pass Roll Group
8: Squeeze Roll
9 : Welder
10: Electric wire
11: Sizing Roll Group
12: Each forming roll group
13 : Diaphragm
14 : Main beam
15 : Cow beam
16 : Consideration
101 : Reputation Department
102 : Corner
103: Welding (electric welding)

Claims (9)

Contains, in 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, Al: 0.01 to 0.06%, N: 0.006% or less, or additionally Nb: 0.005 to 0.150%, Ti: 0.005 to 0.150%, V: 0.005 to 0.150%, 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 0.010%, B: Contains one or more kinds selected from 0.0003 to 0.010%, and has a component composition consisting of the remainder Fe and inevitable impurities.
Flat sections and corner sections are alternately formed in multiples in the direction of the pipe circumference,
The yield strength (YS) of the above flat plate is 295 MPa or more,
The tensile strength (TS) of the above flat plate is 400 MPa or more,
The yield ratio (YR) of the above flat plate is 0.80 or less,
The Charpy absorption energy at 0°C of the above edge is 70 J or more,
The thickness of the oxide scale on the inner and outer surfaces of the tube is 1 ㎛ or more and 20 ㎛ or less,
A square steel pipe, wherein the difference between the average Vickers hardness at a thickness direction position of 1 mm ± 0.1 mm from the inner surface of the corner apex and the average Vickers hardness at a thickness direction position of 1 mm ± 0.1 mm from the outer surface of the central portion of the circumferential direction of the flat portion is 5 HV or more and 60 HV or less. In paragraph 1,
A square steel pipe having an absolute value of residual stress in the circumferential direction on the inner and outer surfaces of the corner vertices of the above-mentioned corner portion of 10 MPa or more and 200 MPa or less. In paragraph 1,
A square steel pipe having a uniform elongation of 5% or more at a thickness direction position of 6 mm ± 1 mm from the inner and outer surfaces of the corner vertices. In the second paragraph,
A square steel pipe having a uniform elongation of 5% or more at a thickness direction position of 6 mm ± 1 mm from the inner and outer surfaces of the corner vertices. A method for manufacturing a square steel pipe as described in any one of claims 1 to 4,
A method for manufacturing a square steel pipe, comprising: heating a square pipe finished into a square shape from a steel plate by cold forming at a temperature lower than the Ac ₁ transformation point, setting the heating temperature deviation in the thickness direction of the pipe to 50°C or less, and performing annealing heat treatment at 500°C or more for a heating holding time of 100 seconds or more. In paragraph 5,
A method for manufacturing a square steel pipe in the above annealing heat treatment, wherein the heating temperature is 500°C or higher and 700°C or lower. In paragraph 5,
A method for manufacturing a square steel pipe, wherein the heating for the above annealing heat treatment is performed by induction heating, and the frequency of the induction heating is 100 Hz or more and 1000 Hz or less. In paragraph 6,
A method for manufacturing a square steel pipe, wherein the heating for the above annealing heat treatment is performed by induction heating, and the frequency of the induction heating is 100 Hz or more and 1000 Hz or less. An architectural structure in which a square steel pipe as described in any one of claims 1 to 4 is used as a pillar material.