TWI752653B - Square steel pipe, method for manufacturing the same, and building structure - Google Patents

Abstract

In order to provide a square steel pipe which has excellent mechanical properties at the flat plate portion, can sufficiently ensure the function of the scale formed on the inner and outer surfaces of the pipe, and can sufficiently ensure toughness at the corners and can suppress work hardening, and provides a manufacturing method of the square steel pipe. A method and a building structure using the square steel pipe. A plurality of flat plate portions (101) and a plurality of corner portions (102) are alternately formed in the pipe circumferential direction, the yield strength YS of the flat plate portion (101) is 295 MPa or more, and the tensile strength TS of the flat plate portion (101) is 400 MPa or more , the yield ratio YR of the flat part (101) is below 0.80, the energy absorbed by the corner part (102) in the Charpy test at 0°C is more than 70J, the thickness of the oxide scale on the inner and outer surfaces of the tube is 1μm ~ 20μm, at the apex of the corner The difference in average Vickers hardness between the predetermined position in the wall thickness direction of the inner surface and the predetermined position in the wall thickness direction of the outer surface of the flat plate portion (101) in the tube circumferential direction central portion is 5HV~60HV.

Description

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

The present invention relates to a square steel pipe which is excellent in deformability of a column material used in a building structure and reduces the influence of work hardening of a corner portion, a manufacturing method thereof, and a building structure.

Conventionally, a method for producing a square steel pipe used as a column material of a building is to press a thick steel plate into a square shape with a press machine, and then weld it (BCP method). On the other hand, in recent years, in order to reduce the cost by replacing the BCP method with low productivity, an attempt has been made to manufacture a square steel pipe by the BCR method. In the BCR method, after roll forming, welding is performed, and then square forming is performed to obtain a square shape. steel pipe method.

In the BCR method, a hot-rolled steel sheet is formed into a cylindrical open pipe shape by roll forming, and after resistance welding is performed on the butted parts, the cylindrical shape is maintained by rollers arranged on the top, bottom, left, and right sides. A drawing of several % is applied in the direction of the tube axis, and then it is formed into a square shape and manufactured. The manufacture of square steel pipes based on roll forming is performed in a cold room, and the effect of work hardening is significant. Therefore, compared with the square steel pipe produced by the BCP method, the plastic deformation ability of the flat plate portion in particular is impaired, and there is a limitation in design.

In order to alleviate this design restriction, the yield ratio YR of the flat portion of the square steel pipe produced by the BCR method must be equivalent to that of the flat portion of the square steel pipe produced by the BCP method, and the yield ratio YR should be 0.80 or less.

In addition, on the inner surface of the corner of the square steel pipe based on the BCR method, since the Zn plating treatment in the subsequent process is carried out at a temperature, the residual stress will be released, and the embrittlement caused by the work hardening will be generated as a starting point. Embrittlement cracks. Therefore, in the square forming process, it is necessary to select manufacturing conditions that can suppress excessive work hardening of the inner surfaces of the corners of the square steel pipe.

For the above reasons, when a square steel pipe is produced by the BCR method, a material that can suppress the increase in the yield ratio YR of the flat plate portion due to work hardening during cold forming is selected, and the generation of residual stress on the inner surface of the corner portion is selected. The manufacturing method and the like are effective, and among them, heat treatment for heating the entire square steel pipe is also effective.

Patent Document 1 proposes a method for producing a square steel pipe, which is a continuous process of forming a square steel pipe by the BCR method, heat treatment of relaxation annealing of the square steel pipe by an induction heating device, and plating treatment of molten zinc plating. carried out.

Patent Document 2 proposes a method of manufacturing, a square steel tube is obtained by cold forming at a temperature below the Ac ₁ transformation point for heat treatment.

Patent Document 3 proposes a manufacturing method in which square forming is performed before finishing forming of square steel pipe, heat treatment is performed in the middle, and square forming is performed in a temperature range _{exceeding the Ac 3 transformation point.} [Prior Art Literature] [Patent Literature]

Patent Document 1: Japanese Patent Application Laid-Open No. 9-155447 Patent Document 2: Japanese Patent Laid-Open No. 2005-163159 Patent Document 3: Japanese Patent No. 2852317

[Problems to be Solved by Invention]

However, in recent years, square steel pipes used in building structures with excellent seismic performance are required as the mechanical properties of the flat plate portion: the yield strength and tensile strength are more than predetermined values, and the yield ratio is 0.80 or less as described above. In addition, as the corner portion, it is required that the toughness can be sufficiently ensured, and the work hardening can be suppressed as described above. Furthermore, regarding the scale formed on the inner and outer surfaces of the pipe, it is required to ensure the function as a protective film and to suppress the peeling of the scale. However, the techniques described in the aforementioned Patent Documents 1 to 3 are insufficient as techniques for obtaining a square steel pipe that satisfies these requirements.

The present invention has been developed in view of the above-mentioned matters, and its object is to provide a square steel pipe having excellent mechanical properties at the flat plate portion, sufficiently securing the function of the scale formed on the inner and outer surfaces of the pipe, and sufficient at the corner portions. To ensure toughness and suppress work hardening, a method for manufacturing the square steel pipe and a building structure using the square steel pipe are provided. [Technical means to solve problems]

In order to solve the above-mentioned problems, the inventors of the present invention have made intensive studies. First, in the present invention, the mechanical properties required as the flat plate portion (the side portion of the vertical cross section in the pipe axis direction) are determined as long as the yielding strength YS is 295 MPa or more, the tensile strength TS is 400 MPa or more, and the yielding ratio YR is 0.80 or less. Can. In addition, it is judged that the toughness required as a corner part should just be 70 J or more of absorbed energy in the Charpy test of 0 degreeC. In addition, in order to sufficiently ensure the function of the scale generated on the inner and outer surfaces of the tube, specifically, in order to suppress peeling of the scale and ensure its function as a protective film, the present invention recognizes that the thickness should be 1 μm to 20 μm. In addition, in order to sufficiently suppress the work hardening of the corners, it is recognized that the average Vickers hardness at the position in the wall thickness direction at a distance of 1 mm ± 0.1 mm from the inner surface of the vertex of the corner, and the outer surface at the center of the tube circumferential direction of the flat plate The difference of the average Vickers hardness at the wall thickness direction position of 1mm±0.1mm may be 5HV~60HV.

In addition, the inventors of the present invention have found that, in order to obtain the above-mentioned characteristics of a square steel pipe, it is only necessary to perform annealing heat treatment on a specific square billet after finishing from a steel sheet into a square shape by cold forming, and the annealing heat treatment is lower than Ac ₁ transformation point temperature heating, the heating temperature of the pipe wall thickness direction of the deviation becomes less 50 ℃, and the heating above 500 ℃ retention time than 100sec. In detail, it was first noted that when heat treatment is performed at a temperature higher than the _{Ac 1 transformation point, the toughness may be remarkably deteriorated.} In addition, the inventors of the present invention investigated the relationship between the variation in the heating temperature in the thickness direction of the pipe and the mechanical properties of the pipe. Specifically, regarding the variation of the heating temperature in the thickness direction of the tube, attention is paid to annealing heat treatment such as relaxation annealing treatment, and the heating temperature has a great influence. It is also noted that in annealing heat treatment such as induction heating, the outer surface side is heated by resistance, and the temperature of the inner surface side becomes lower than that of the outer surface side. According to these findings, if the heating temperature deviation is large, the difference in the influence of annealing heat treatment such as relaxation annealing between the outer surface and inner surface of the tube becomes larger, and as a result, the difference in mechanical properties between the outer surface and inner surface of the tube becomes larger. A tube with non-uniform properties, which has been painstakingly researched. Furthermore, in the stress relaxation annealing treatment, it was considered that a sufficient heating holding time must be ensured in order to remove the stress.

From this discussion, it is understood that in order to make the square steel pipe have the above-mentioned characteristics, it is only necessary to perform annealing heat treatment on the square billet which is finished from the steel plate into a square shape by cold forming, and the annealing heat treatment is lower than Ac ₁ transformation. The temperature of the point is heated so that the heating temperature deviation in the thickness direction of the tube is 50°C or less, and the heating holding time at 500°C or more is 100 sec or more.

In view of the above-mentioned findings, the inventors of the present invention have examined, as an example of the annealing heat treatment, high-frequency induction heating of a square steel pipe using a working coil.

In this high-frequency induction heating, the object to be heated in the working coil connected to the AC power source is heated by Joule heat generated by resistance. Therefore, the heat loss of high-frequency induction heating is small, and the heating efficiency is excellent. In addition, by controlling the heating frequency, the penetration depth of the eddy current, which is the main cause of Joule heat generation, can be adjusted, and by reducing the frequency, the heating can be carried out to the inner side of the object to be heated. Therefore, in high-frequency induction heating, even if the thickness of the object to be heated increases, by appropriately controlling the frequency, the temperature deviation of the heating temperature between the surface and the inside of the object to be heated can be reduced.

The inventors of the present invention carried out various square steel pipes used as pillars of building structures in a work coil, and performed overall heating of the square steel pipes by high-frequency induction heating. As a result, by setting the frequency in an appropriate range, a uniform heating distribution in the wall thickness direction can be obtained. In addition, by increasing the penetration depth of the current, the heating concentration of the surface caused by the skin effect can be suppressed, and the time for the inner surface of the steel pipe to reach the target temperature can be shortened. Furthermore, it was confirmed that the above-mentioned effect can be obtained even in a small-sized heating device having an overall length of the working coil of about several meters. The above-mentioned skin effect refers to the following phenomenon. First, a current (eddy current) that cancels the magnetic field is generated on the surface of the object to be heated (steel pipe) by the magnetic field of the high-frequency current. The object to be heated is heated by the resistance by the eddy current, and the 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 has the following characteristics. [1] A square steel pipe having a plurality of flat plate portions and a plurality of corner portions alternately formed in the pipe circumferential direction, wherein the yield strength YS of the flat plate portion is 295 MPa or more, the tensile strength TS of the flat portion is 400 MPa or more, and the The yield ratio YR of the flat plate is 0.80 or less, the Charpy test absorption energy of the corner at 0°C is more than 70J, the thickness of the oxide scale on the inner and outer surfaces of the tube is 1 μm~20 μm, and the inner surface from the vertex of the corner is 1mm± The difference between the average Vickers hardness at a position in the thickness direction of 0.1 mm and the average Vickers hardness at a position in the thickness direction at a distance of 1 mm ± 0.1 mm from the outer surface of the central portion in the tube circumferential direction of the flat plate portion is 5HV to 60HV. [2] The square steel pipe according to the above [1], wherein the absolute value of the residual stress in the pipe circumferential direction at the inner surface and the outer surface of the corner vertex is 10 MPa to 200 MPa. [3] The square steel pipe according to the above [1] or [2], wherein the uniform elongation is 5% or more at positions in the wall thickness direction 6 mm±1 mm from the inner surface and the outer surface of the corner apex. [4] A method for producing a square steel pipe, which is the method for producing a square steel pipe according to any one of the aforementioned [1] to [3], which is a square billet that is finished from a steel sheet into a square shape by cold forming An annealing heat treatment is performed. The annealing heat treatment is _{performed at a temperature lower than the Ac 1} transformation point so that the heating temperature deviation in the thickness direction of the tube is 50°C or less, and the heating holding time at 500°C or higher is 100 sec or more. [5] The method for producing a square steel pipe according to the above [4], wherein in the annealing heat treatment, the heating temperature is 500°C to 700°C. [6] The method for producing 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 of the induction heating is 100 Hz to 1000 Hz. [7] A building structure using the square steel pipe according to any one of the aforementioned [1] to [3] as a column material. [Effect of invention]

According to the present invention, it is possible to provide a square steel pipe having excellent mechanical properties at the flat plate portion, which can sufficiently ensure the function of the oxide scale generated on the inner and outer surfaces of the pipe, and can sufficiently ensure toughness and suppress work hardening at the corner portions, and provide the square steel pipe. A manufacturing method of a steel pipe and a building structure using the square steel pipe.

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

＜Square steel pipe＞ FIG. 1 shows an example of the shape of the rectangular 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 (a cross section perpendicular to the pipe axis direction) perpendicular to the longitudinal direction of the pipe (a cross section perpendicular to the pipe axis direction), and a plurality of flat plate portions (a cross section perpendicular to the pipe axis direction) are alternately formed in the pipe circumferential direction. The side portion) 101 and the plurality of corner portions 102, the yield strength YS of the flat plate portion 101 is 295 MPa or more, the tensile strength TS of the flat portion 101 is 400 MPa or more, and the flat portion 101 The yield ratio YR (= yield strength /tensile strength) is 0.80 or less, the Charpy test energy absorption of the corner 102 at 0°C is more than 70J, the thickness of the oxide scale on the inner and outer surfaces of the tube is 1 μm~20 μm, and the inner surface from the apex of the corner is 1mm± The difference between the average Vickers hardness at a position of 0.1 mm in the thickness direction and the average Vickers hardness at a position in the thickness direction at a distance of 1 mm±0.1 mm from the outer surface of the central portion in the tube circumferential direction of the flat plate portion 101 is 5HV to 60HV. In addition, the square steel pipe 1 of the present invention may be a steel pipe obtained from an electric resistance welded steel pipe, and the flat plate portion 101 is provided with a welded portion (resistance welded portion) 103 .

In the present invention, although not particularly limited, the side length H of the flat plate portion 101 in the vertical 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 a vertical cross-sectional view in the pipe axis direction is preferably a square (substantially square) having the same lengths of four sides of each flat plate portion 101, or a rectangle (substantially a rectangle). In the case of a rectangle, the side length H is the average of the longitudinal side length H1 (mm) and the transverse side length H2 (mm) (H=(H1+H2)/2).

The yield strength YS of the flat plate portion 101 defined by the present invention: 295 MPa or more, the tensile strength TS: 400 MPa or more, and the yield ratio YR: 0.80 or less can be adjusted by performing an annealing heat treatment on a specific square billet. The annealing heat treatment , is lower than the Ac ₁ transformation point temperature by heating the heating tube thickness direction of the temperature deviation becomes less 50 ℃, and the heating above 500 ℃ retention time than 100sec.

In addition, the Charpy test absorption energy of the corner portion 102 defined by the present invention at 0°C: 70J or more can be adjusted by performing an annealing heat treatment. The annealing heat treatment is heated at a temperature _{lower than the transformation point of Ac 1 .} 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 more is set to 100 sec or more.

The yield strength YS, tensile strength TS, and yield ratio YR of the flat plate portion 101 can be measured as follows. That is, a JIS No. 5 tensile test is taken from the flat plate portion 101 of the square steel pipe 1 so that the tensile direction is parallel to the pipe axis direction. The test piece was used and it was implemented according to the regulation of JIS Z 2241.

In addition, the Charpy test of the corners 102 at 0°C absorbs energy by taking a V-notch test at a distance t/4 from the pipe outer surface of the corners 102 of the square steel pipe 1 so that the longitudinal direction of the test piece is parallel to the pipe longitudinal direction. The test piece was obtained by carrying out the Charpy impact test at the test temperature: 0°C according to the regulations of JIS Z 2242.

FIG. 2 is a schematic diagram for explaining the scale formed on the square steel pipe 1 of the present invention. The oxide scale located on the inner and outer surfaces of the above-mentioned steel pipe 1 has a structure as shown in FIG. 2, and is composed of layers of wirtzite (FeO) and magnetite (Fe) in this order from the base metal (base metal) side to the surface side. ₃ O ₄ ) and hematite (Fe ₂ O ₃ ).

In the present invention, the growth of scale on the surface of the steel pipe 1 is suppressed by heating at a temperature _{lower than the Ac 1} transformation point in annealing heat treatment such as induction heating. On the other hand, when heating is performed at a temperature equal to or higher than the _{Ac 1 transformation point, scale will grow.} When the thickness of the scale (hereinafter also referred to as "scale thickness") increases to exceed 20 μm, stress due to external impact or the like is likely to accumulate in the scale layer, causing peeling of the scale. On the other hand, when the scale thickness is less than 1 μm, the effect of the scale as a protective film is lost during cold forming, and a sufficient anticorrosion effect cannot be obtained. Therefore, the thickness of the scale on the inner and outer surfaces of the tube is set to 1 μm to 20 μm. 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, more preferably 8 μm or less.

The above-mentioned scale thickness can be set to 1 μm or more by adjusting the time for exposing the high-temperature blank to the atmosphere during hot rolling. 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 made 20 μm or less. In addition, the thickness of the scale formed on the inner and outer surfaces of the steel pipe 1 can be measured using a scanning electron microscope (SEM).

The square steel pipe 1 after the annealing heat treatment such as induction heating, which will be described later, and the sizing process or the straightening process, is subjected to heat treatment to release residual stress. In the unheated square steel pipe, especially on the outer and inner surfaces of the corners, a large compressive residual stress and a tensile residual stress are generated respectively. At this time, when excessive residual stress acts on the outer surface of the corner portion, the progress of work hardening of the outer surface becomes remarkable. Cracks may occur due to thermal expansion of the part. In addition, when excessive residual stress acts on the inner surface of the corner, the residual stress is released by Zn plating after the square steel pipe is formed, and Zn plating-induced cracks may occur on the inner surface of the corner. When the residual stress in the pipe circumferential direction is equal to or more than the yield stress of the base material of the steel sheet (the yield stress of the corner surface), defects are likely to occur at the corners of the square steel pipe. Therefore, in order to suppress the defects of the corner 102, the residual stress in the pipe circumferential direction on the inner surface and the outer surface of the corner apex must be reduced, and the absolute value of the residual stress should be smaller than the yield stress of the corner surface. More specifically, the absolute value of the residual stress is preferably 200 MPa or less in order to prevent abnormal notch deformation that occurs when the formed square steel pipe 1 is cut. In addition, when the absolute value of the residual stress is less than 10 Mpa, there may be cases where the elongation at the yield point of the material cannot be eliminated due to insufficient correction. Therefore, the absolute value of the residual stress in the pipe circumferential direction on the inner surface and the outer surface of the corner vertex is preferably 10 MPa to 200 MPa. More preferably, it is 20 MPa or more, and particularly preferably 50 MPa or more. Moreover, it is more preferable that it is 150 MPa or less, and it is especially preferable that it is 100 MPa or less. In addition, in the present invention, the absolute value of residual stress can be 10Mpa~200Mpa by controlling the processing amount of the straightening processing after the heat treatment, and performing the annealing heat treatment. The annealing heat treatment is performed at a temperature _{lower than the Ac 1 transformation point.} Heating is performed so that the heating temperature deviation in the thickness direction of the pipe is 50° C. or less, and the heating holding time at 500° C. or more is 100 sec or more.

In addition, as the measurement of residual stress, the steel pipe was cut, the surface layer from the measurement position to the member with a depth of 50 μm was removed by electrolytic etching, and the residual stress in the circumferential direction was measured by the cosα method of X-ray diffraction. The measurement position is the center of the length of the steel pipe, and is the vertex position of the corners of the four corners.

Here, as shown in FIG. 1 , the apex of the corner portion is such that the center position of the short side (H1) of the flat plate portion 101 in the cross section perpendicular to the pipe axis direction of the square steel pipe 1 faces the inside of the steel pipe. More specifically In other words, the straight line drawn toward the center of the opposite short side is offset by 1/2×|H2-H1| ( That is, the point (offset point) which is half of the difference between the side length H2 and the side length H1) is used as the starting point, and the straight line drawn with respect to the center position of the short side opposite to the above is formed at the offset point. The long side of the flat plate portion 101 on the opposite side is the intersection of the line at 45° and the outside of the corner portion 102 . In addition, the corner apex may be drawn from the center position of the long side (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 center position of the opposite long side. On the straight line set, the starting point is the point (offset point) offset by 1/2×|H2-H1| in the direction from the center of the square steel pipe to the short side (in the case of H1<H2, H1), relative to the above A straight line drawn toward the center of the opposite long side is drawn from the center of the short side (H1) of the flat plate portion 101 of the vertical cross-section in the pipe axis direction of the square steel pipe 1 toward the inside of the steel pipe, and is formed at the center of the steel pipe. The short side of the flat plate portion 101 on the side where the offset point is located is the intersection of the 45° line and the outside of the corner portion 102 . In addition, when the shape of the vertical cross-sectional view in the pipe axis direction is square (substantially square), the vertex of the corner is the intersection of the line at 45° from the center axis of the steel pipe 1 and the flat plate 101 and the outside of the corner 102 .

In the steel pipe just after cold forming, the influence of work hardening is remarkable, especially in the corners of the four corners, compared with the flat part, the work hardening progresses on a large scale. In the case of roll-formed square steel pipes, the influence of work hardening is greatest on the inner surface side of the corners, and the ductility is impaired. In the structure of the square steel pipe after heat treatment such as relaxation force annealing by induction heating, the stress is removed by recovery, the ductility is improved, and the influence of work hardening is almost eliminated. At this time, if the uniform elongation in the thickness direction of the inner surface and the outer surface at a distance of 6 mm ± 1 mm from the apex of the corner is less than 5%, the relaxation annealing is insufficient, and cracks may occur in the corner. Therefore, it is preferable that the uniform elongation at the position in the wall thickness direction at a distance 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.

The above-mentioned uniform elongation can be measured as follows, that is, a JIS No. 5 tensile test piece is 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 is parallel to the pipe axis direction, The use of this test piece was carried out in accordance with the provisions of JIS Z 2241.

In the present invention, a specific heat treatment by annealing the blank square tube, the uniform elongation can be 5% or more, the annealing, the temperature is lower than the Ac ₁ transformation point is heated, so that the thickness direction of the tube The heating temperature deviation was 50° C. or less, and the heating holding time of 500° C. or more was set to 100 sec or more.

In addition, by subjecting the entire steel pipe to the heat treatment described later, a square steel pipe 1 having substantially uniform mechanical properties at each portion of the flat plate portion 101 and the corner portion 102 can be obtained.

In the present invention, although the Vickers hardness of the square steel pipe 1 is not particularly limited, it is preferably set to 100 to 300 HV in order to prevent insufficient correction and excessive work hardening in the correction process after the heat treatment.

In addition, in the square steel pipe 1 of the present invention, the Vickers hardness of the apexes of the corners before heat treatment is higher than the Vickers hardness of the flat parts, because the influence still exists after the relaxation annealing, and the Vickers hardness of the apexes of the corners is higher than that of the flat parts. 101 has a higher Vickers hardness.

When the average Vickers hardness at a position in the wall thickness direction at a distance of 1 mm ± 0.1 mm from the inner surface of the apex of the corner, and at a position in the wall thickness direction at a distance of 1 mm ± 0.1 mm from the outer surface of the central part in the tube circumferential direction of the flat plate Difference in hardness ((average Vickers hardness at the wall thickness direction position 1 mm ± 0.1 mm from the inner surface of the corner apex) - (the wall at 1 mm ± 0.1 mm from the outer surface of the central part of the tube circumferential direction of the flat plate When the average Vickers hardness in the thickness direction is less than 5HV, the elongation at the yield point of the material cannot be eliminated due to insufficient correction. On the other hand, when the difference in average Vickers hardness exceeds 60HV, relaxation 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 a position in the thickness direction of 1 mm ± 0.1 mm from the inner surface of the apex of the corner portion and a position in the 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 101 The difference in average Vickers hardness was set to 5HV to 60HV. 10HV or more is preferable, and 15HV or more is more preferable. In addition, it is preferably 40HV or less, more preferably 30HV or less. In the present invention, by performing annealing heat treatment on a specific square billet, it is more preferable to control the heating temperature and annealing heat treatment time in the annealing heat treatment such as relaxation annealing, so that the difference in the above-mentioned average Vickers hardness is 5HV to 60HV. The annealing heat treatment temperature is lower than the Ac ₁ transformation point is heated, the heating temperature of the pipe wall thickness direction of the deviation becomes less 50 ℃, and the heating above 500 ℃ retention time than 100sec.

As Vickers hardness, according to the micro Vickers hardness test (JIS Z2244:2009), the position in the wall thickness direction at a distance of 1 mm ± 0.1 mm from the inner surface of the corner vertex of the four corners, and at a distance from the flat plate portion were measured. The Vickers hardness of the outer surface of the central part in the circumferential direction of the pipe of 101 at a position of 1 mm ± 0.1 mm in the wall thickness direction. The Vickers hardness was measured with a test force of 9.8 N.

The composition of the square steel pipe 1 of the present invention is not particularly limited, but preferably has the following composition, in terms of mass %, C: 0.07-0.20%, Si: less than 0.4%, Mn: 0.3-2.0% , P: 0.030% or less, S: 0.015% or less, Al: 0.01 to 0.06%, N: 0.006% or less, and the remainder is composed of Fe and inevitable impurities. The reason for the limitation of each component will be described below. Hereinafter, in the description of each component, unless otherwise specified, mass % is represented by %.

C: 0.07~0.20% C increases the strength of the steel by solid solution strengthening, and contributes to the formation of one of the second phases, that is, bleed iron. The C content is preferably 0.07% or more in order to secure the desired tensile properties and toughness, and further secure the desired steel structure. On the other hand, when the C content exceeds 0.20%, a mated iron structure is formed when square steel pipes are welded (for example, when square steel pipes are welded to each other), which may cause welding 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 an increase in the strength of the steel by solid solution strengthening, and may be contained as necessary in order to secure the desired strength of the steel. In order to obtain such an effect, the Si content is preferably more than 0.01%. However, when the Si content exceeds 0.4% or more, fayalite called red rust tends to form 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%. When Si is not intentionally added, Si is an unavoidable impurity, and its content is 0.01% or less.

Mn: 0.3~2.0% Mn is an element that increases the strength of the steel sheet by 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%, it will lead to the rise of the initial temperature of the metamorphosis of the fertilized iron, and it is easy to make the tissue excessively coarse. On the other hand, if the Mn content exceeds 2.0%, the hardness of the central segregation part increases, and there is a possibility that cracks may be caused in the joint welding of the column using the square steel pipe or the welding with the separator. Therefore, the Mn content is preferably 0.3 to 2.0%. The upper limit of the Mn content is more preferably 1.6%, and particularly preferably 1.4%.

P: 0.030% or less P is an element which segregates in the iron grain boundary of fertilizer grains and has the effect of reducing toughness, and in the present invention, it is preferable to reduce it as much as possible as an impurity. However, excessive reduction leads to an increase in refining cost, so the P content is preferably 0.002% or more. And P content can be tolerated 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 in the form of sulfide in steel, and if within the composition range of the present invention, it mainly exists in the form of MnS. MnS is thinned and stretched in the hot rolling process, and adversely affects ductility and toughness. Therefore, in the present invention, it is preferable to reduce MnS as much as possible. However, excessive reduction leads to an increase in refining cost, so the S content is preferably 0.0002% or more. And S content can be allowed to 0.015% so far. 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 which functions as a deoxidizer and has a function of fixing N in the form of AlN. In order to obtain such an effect, the Al content must be 0.01% or more. If the Al content is less than 0.01%, the deoxidation power is insufficient in the case where no Si is added, the 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 solution Al increases, and when welding a square steel pipe in the longitudinal direction (when welding a square steel pipe), especially in the case of welding in the atmosphere, let the welding The risk of oxide formation increases, which reduces the toughness of the welded part of the square steel pipe. Therefore, the Al content is preferably 0.01 to 0.06%. The Al content is more preferably, the lower limit is 0.02%, and the upper limit is 0.05%.

N: 0.006% or less N has the effect of firmly fixing the dislocation motion and reducing the toughness. In the present invention, N is preferably as small as possible as an impurity, and can be tolerated up to 0.006%. Therefore, the N content is preferably 0.006% or less. The N content is more preferably 0.005% or less.

The remainder other than the above is Fe and inevitable impurities. The above-mentioned components are the basic components of the steel material of the present invention, and in addition, may further contain: one or two kinds selected from the group consisting of Nb: 0.005-0.150%, Ti: 0.005-0.150%, and V: 0.005-0.150% or less above.

One or more selected from Nb: 0.005~0.150%, Ti: 0.005~0.150%, V: 0.005%~0.150% Nb, Ti, and V are elements that form fine carbides and nitrides in steel and contribute to the improvement of the strength of steel through precipitation strengthening, and may be contained as necessary. 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, if it contains excessively, it will cause the rise of a yield ratio and the fall of 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% are set. Preferably, Nb: 0.008% or more, Ti: 0.008% or more, and V: 0.008% or more. Further, Nb: 0.10% or less, Ti: 0.10% or less, and V: 0.10% or less are preferable.

In addition to the above components, it may further contain: Cr: 0.01-1.0%, Mo: 0.01-1.0%, Cu: 0.01-0.50%, Ni: 0.01-0.30%, Ca: 0.0005-0.010%, B: 0.0003- One or more of 0.010%.

One or two kinds selected from Cr: 0.01~1.0%, Mo: 0.01~1.0%, Cu: 0.01~0.50%, Ni: 0.01~0.30%, Ca: 0.0005~0.010%, B: 0.0003~0.010% above Cr, Mo, Cu, and Ni are elements that increase the strength of steel by solid solution strengthening, and can also improve the hardenability of steel, and are elements that contribute to the stabilization of iron in the Worcestershire. Elements that contribute to the formation of hard Matian iron and Vostian iron can be contained as necessary. 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, if it contains excessively, it will lead to the fall of toughness and the deterioration of 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%, 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. Further, Cr: 0.5% or less, Mo: 0.5% or less, Cu: 0.40% or less, and Ni: 0.20% or less are preferable.

Ca is an element which contributes to the improvement of toughness of steel by spheroidizing sulfides such as MnS thinned and stretched in the hot rolling process, and may be contained as necessary. In order to obtain such an effect, the Ca content is preferably 0.0005% or more. However, when the Ca content exceeds 0.010%, a Ca oxide cluster may be formed in the steel, thereby deteriorating the toughness. Therefore, when Ca is contained, the Ca content is set to 0.0005 to 0.010%. Preferably, the Ca content is 0.001% or more. Moreover, it is preferable that the Ca content is 0.0050% or less.

B is an element that reduces the temperature at which the metamorphosis of ferrite begins and contributes to the refinement of the tissue. In order to obtain such an effect, the B content is preferably 0.0003% or more. 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. Moreover, it is preferable that the B content is 0.0050% or less.

In addition, when it has the above-mentioned composition, in order to ensure the weldability, it is preferable that Ceq defined by the formula (1) is 0.15% to 0.50%, and Pcm defined by the formula (2) is preferably 0.30% or less. The composition of each element in the formula (1) and the formula (2) is all mass %.

在此，式(1)中，C、Mn、Si、Ni、Cr、Mo、V表示各元素的含量(質量%)。(其中，不包含的元素為0(零)%。)

在此，式(2)中，C、Si、Mn、Cu、Ni、Cr、Mo、V、B表示各元素的含量(質量%)。(其中，不包含的元素為0(零)%。) (1)式中的Ceq為碳當量，成為熔接部及熱影響部之硬度的指標。若Ceq小於0.15%，有可能無法獲得作為建築結構物之柱材所需的強度。此外，若Ceq大於0.50%，會使熔接部及熱影響部過度硬化，而使周剖面強度的偏差變大。因此，Ceq較佳為0.15%~0.50%。

Here, in Formula (1), C, Mn, Si, Ni, Cr, Mo, and V represent the content (mass %) of each element. (The elements not included are 0 (zero)%.)

Here, in Formula (2), C, Si, Mn, Cu, Ni, Cr, Mo, V, and B represent contents (mass %) of each element. (The element not included is 0 (zero)%.) (1) Ceq in the formula is a carbon equivalent, and is an index of the hardness of the welded part and the heat-affected part. If Ceq is less than 0.15%, there is a possibility that the strength required as a column material of a building structure cannot be obtained. In addition, when Ceq exceeds 0.50%, the welded portion and the heat-affected zone are excessively hardened, and the variation in circumferential cross-sectional strength increases. Therefore, Ceq is preferably 0.15% to 0.50%.

(2) Pcm in the formula is the sensitivity of welding cracks. If Pcm is greater than 0.30%, low-temperature cracks are likely to be caused in the welded part and the heat-affected part. Therefore, in the present invention, Pcm is preferably 0.30% or less, particularly preferably 0.25% or less.

<The manufacturing method of the square steel pipe> Next, the manufacturing method of the square steel pipe 1 of this invention is demonstrated. Square steel pipe of the present invention is a method for manufacturing, and is finished by cold forming from a blank sheet into a square of a square pipe annealing heat treatment of the annealing, the temperature is lower than the Ac ₁ transformation point of the heating of the tube The heating temperature deviation in the thickness direction is 50° C. or less, and the heating holding time of 500° C. or more is 100 sec or more. In order to obtain the above-mentioned steel sheet, in order to make the thickness of the scale formed on the inner and outer surfaces of the square steel pipe finally obtained to be 1 μm or more, the time for exposing the high temperature slab to the atmosphere after hot rolling and finish rolling is adjusted. Specifically, it is preferable to expose the slab having a surface temperature of 900° C. or lower to the atmosphere for 5 to 400 sec after the finish rolling of the hot rolling. Then, the obtained steel sheet is finished into a square shape by cold forming, thereby obtaining a square billet.

Here, a method for obtaining the above-mentioned square billet will be described. FIG. 3 is a schematic diagram showing an example of a manufacturing facility for electric resistance welded steel pipes for obtaining square billets. As shown in FIG. 3, a steel strip (hereinafter also referred to as a "steel plate") 4 wound into a coil is unwound and straightened by a leveler 5, and a row of rolls composed of a plurality of rolls is used. After the group 6 is intermediately formed into an open tube, the finish forming is performed by a finishing roll group 7 composed of a plurality of rollers. The above-mentioned open pipe can be formed into a cylindrical shape by cold roll forming. After finishing forming, the butted portions in the circumferential direction of the steel strip 4 are resistance-welded by a welding machine 9 while being crimped by a squeeze roll 8 to form a resistance-welded steel pipe 10 . Moreover, in this invention, the manufacturing facility of the electric resistance welding steel pipe 10 is not limited to the pipe making process like FIG. 3. FIG. In addition, in the above-mentioned resistance welding, the abutting portion is heated and melted, and is crimped and solidified, thereby completing the joining.

The subsequent steps will be described later with reference to FIG. 4 and the like. In the sizing step after resistance welding, in order to satisfy the roundness and residual stress in the pipe axis direction required by the present invention, it is preferable to make the steel pipe The diameter of the steel pipe is reduced so that the perimeter is reduced by a ratio of 0.30% or more in total. On the other hand, if the circumference of the steel pipe is reduced by a ratio exceeding 5.0% in total, the amount of bending in the pipe axis direction when passing through the rollers increases, which may cause residues in the pipe axis direction after the diameter reduction. Stress rises. Therefore, it is preferable to reduce the diameter so that the circumference of the steel pipe after the diameter reduction is reduced by a ratio of 0.30% to 5.0% with respect to the circumference of the steel pipe before the diameter reduction.

In the sizing process, in order to minimize the amount of bending in the tube axis direction when passing through the rollers and suppress the generation of residual stress in the tube axis direction, it is preferable to perform a multi-stage diameter reduction based on a plurality of stands, The diameter reduction of each stand is preferably performed so as to reduce the circumference of the steel pipe by a ratio of 1.0% or less.

Whether the square steel pipe (square billet) is made from resistance welded steel pipe can be determined by cutting the square steel pipe (square billet) perpendicular to the pipe axis direction, and cutting the cut surface including the welded part. After grinding, it was etched with a nitrate and observed with an optical microscope. As long as the width in the pipe circumferential direction of the molten and solidified part in the central part of the plate thickness of the welded part is 1 mm or less, it is produced from a resistance welded steel pipe.

Figure 4 is a schematic diagram showing the process of forming a square billet tube from an electric resistance welded steel tube. As shown in FIG. 4 , the electric resistance welded steel pipe 10 is formed by a plurality of rollers after the diameter is reduced by maintaining a cylindrical shape by a sizing roller group (sizing stand) 11 composed of a plurality of rollers. The square forming roll group (square forming stand) 12 is sequentially formed into a shape like R1, R2, and R3, and becomes a square billet. In addition, 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 diagram showing an example of an apparatus for producing a square steel pipe from the above-mentioned square billet. In the example shown in FIG. 5 , after the sizing process, the square billet tube cut into a predetermined length is conveyed on the conveying table 2 in the longitudinal direction at a predetermined speed. At this time, the working coil 3 is fixed, and the square steel pipe 1 sent out by the conveying table is heated while passing through the working coil.

As aforesaid, in the present invention, is the square of the tube blank by cold-forming the steel sheet into a square from the finished annealing heat treatment, the annealing temperature is lower than the Ac ₁ transformation point is heated, so that the wall of the tube The heating temperature deviation in the thickness direction is 50° C. or lower, and the heating holding time of 500° C. or higher is 100 sec or longer.

In the above-mentioned annealing heat treatment, in order to release the stress accumulated by the cold forming, the heat treatment is performed in the temperature range of the relaxation annealing. When heated _{above the Ac 1} transformation point, 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 oxide scale on the inner and outer surfaces of the tube may exceed 20 μm. Therefore, in the annealing heat treatment of the present invention, heating is performed at a temperature _{lower than the transformation point of Ac 1 .}

In addition, in the above-mentioned annealing heat treatment, since the heating such as induction heating is performed from the outside of the steel pipe, temperature deviation occurs between the inside and outside of the steel pipe during heating. In the case of using a heating below the Ac ₁ transformation point is the force relaxation annealing, the heating temperature is low, the stress until it takes longer to completely remove. In such a case, on the inner surface side where the heating temperature tends to be low, the progress of stress release is delayed, and the problem occurs that the mechanical properties tend to become non-uniform in the thickness direction of the tube. In order to solve the problem of temperature deviation between the heating temperature of the outer surface and the inner surface, as long as the heating temperature deviation in the thickness direction of the tube is 50°C or less, uniform mechanical properties in the thickness direction of the tube can be obtained. Therefore, in the annealing heat treatment of the present invention, the variation in the heating temperature in the thickness direction of the tube is set to 50°C or lower. Preferably it is 30 degrees C or less, More preferably, it is 10 degrees C or less.

Further, based on the heating temperature of the annealing heat treatment, it is preferably 500°C to 700°C. When heat treatment is performed below 500°C, it takes a long time until the stress is completely removed.

When the relaxation annealing is performed at 500° C. or higher, it is preferable to ensure a heating holding time of 100 sec or longer in order to remove stress. When the pipe is heated by induction heating and then naturally cooled, the cooling rate on the inner and outer surfaces of the pipe is about 0.5°C/sec. Therefore, the lower limit of the heating temperature in the annealing heat treatment is preferably 550°C (=500°C+0.5°C/sec×100sec) in order to secure a heating holding time of 100sec or more and 500°C or more after heating. The temperature of the heat treatment based on the annealing heat treatment is preferably 550°C to 700°C, and more preferably 600°C or higher. Still more preferably, it is 650°C or lower.

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

In the above-mentioned induction heating, when the frequency is lower 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 concentrated part may be lowered. As a result, since the heat conduction from the heated high temperature portion to the inner surface side of the steel pipe is reduced, the heating efficiency of the entire pipe is deteriorated, and the equipment is increased in size. On the other hand, when the frequency exceeds 1000 Hz, since the skin effect becomes large, the temperature deviation between the heating temperature of the outer surface and the inner surface of the tube may become large. Therefore, the frequency of induction heating is preferably set to 100 Hz to 1000 Hz. The frequency of induction heating is more preferably 150 Hz or more. Further, the frequency of the induction heating is more preferably 500 Hz or less, and particularly preferably 300 Hz or less. The above-mentioned skin effect refers to the following phenomenon. First, a current (eddy current) that cancels the magnetic field is generated on the surface of the object to be heated (steel pipe) by the magnetic field of the high-frequency current. The object to be heated is heated by the resistance by the eddy current, and the heating becomes more concentrated as it gets closer to the surface. This phenomenon is called the skin effect.

In addition, in the induction heating, the conveyance speed of the square billet tube is not particularly limited, but from the viewpoint of the production efficiency and the uniform heating temperature of the cross section, it is preferably 0.2 to 4 m/min. Furthermore, the electric power of the induction heating device is not particularly limited, but is preferably 3 to 12 MW in order to ensure a desired conveyance speed.

As the above-mentioned temperature management method for steel pipes, the temperature of the outer surface of the pipe is measured with a radiation thermometer, and the temperature of the inner surface of the pipe and the inside of the wall thickness can be calculated by temperature calculation based on a two-dimensional model based on thermal analysis. The temperature distribution in the wall thickness direction of the whole circumference is managed according to this method.

The square steel pipe after the annealing heat treatment by induction heating or the like described above may go through the sizing process and/or the straightening process again. These steps are for eliminating the elongation at the yield point that occurs when tensile deformation is applied to the heat-treated steel pipe base material, and it is not limited as long as a strain of 0.5 to 3% can be imparted to the entire circumference of the pipe.

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

The building structure of the present invention uses the aforementioned square steel pipe 1 of the present invention as a column material. Reference numerals 13, 14, 15, and 16 represent a partition, a girder, a trabecular, and a column in this order. The square steel pipe of the present invention, as described above, has excellent mechanical properties at the flat plate portion, can sufficiently secure the function of the scale formed on the inner and outer surfaces of the pipe, and can sufficiently secure toughness and suppress work hardening at the corner portions. Therefore, the building structure of the present invention using this square steel pipe as a column material exhibits excellent seismic performance. Example

Hereinafter, the present invention will be further described according to the embodiments.

The hot-rolled steel sheet with the composition shown in Table 1 is continuously formed into an open tube with an elliptical cross-section by a group of row rolls and a group of finishing rolls, and then the opposite end faces of the open tube are heated by high-frequency induction or High-frequency resistance heating is used to heat to a melting point or higher, and the resistance welded steel pipe is obtained by crimping with a squeeze roll. In addition, in order to make the thickness of the oxide scale formed on the inner and outer surfaces of the square steel pipe finally obtained to be 1 μm or more, the time for exposing the high-temperature slab to the atmosphere after the finish rolling of the hot rolling is adjusted. After finishing rolling, the time for exposing the blank with surface temperature below 900℃ to the atmosphere is 5~400sec.

From the obtained cylindrical steel pipe, after passing through two stages of sizing stands, and four stages of square forming stands, a square billet tube whose corner curvature is (2.5±0.5) times the plate thickness is obtained.

Next, the above-mentioned square billet tube is cut into a predetermined length, and heat-treated (annealing heat treatment) is performed using a high-frequency heating apparatus (induction heating apparatus) having a cylindrical working coil to obtain a square-shaped steel pipe. The inner diameter D of the above-mentioned working coil was 960 mm, and the length in the conveying direction (the height direction in the case of assuming a cylindrical shape) was 1 m. The square billet is heated while being inserted into the working coil by the transfer trolley. At this time, the conveyance speed, the heating frequency, and the electric power are controlled so that the predetermined heating temperature is obtained.

Regarding the temperature management of steel pipes, the temperature of the outer surface of the pipe is measured with a radiation thermometer, and the temperature of the inner surface of the pipe and the inside of the wall thickness is calculated by temperature calculation based on a two-dimensional model based on thermal analysis. In Table 2, a heating temperature (the highest temperature and the outside temperature maximum in-plane) (℃) is lower than the Ac ₁ transformation point (see Table "heating temperature <Ac ₁ transformation point (℃)" column 2). In Table 2, "○" represents the heating temperature is lower than the Ac1 transformation point, "×" represents the heating temperature is less than Ac ₁ transformation point. In addition, the heating temperature deviation is calculated by calculating the difference between the outer maximum temperature (°C) and the inner maximum temperature (°C) (refer to the column of "outside temperature - inner surface temperature (°C)" in Table 2). In addition, in Table 2, "holding time" means the heating holding time of 500 degreeC or more.

After that, straightening processing was performed using an inclined roll straightening machine, and a strain of 2% was applied to the steel pipe. A test piece was taken 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 carried out.

As a tensile test of the flat plate portion, a JIS No. 5 tensile test piece was taken from the flat plate portion of the square steel pipe so that the tensile direction was parallel to the pipe axis direction, and the test piece was used in accordance with the regulations of JIS Z2241, and the yield strength YS was measured. , and the tensile strength TS, and the yield ratio YR defined by (yield strength)/(tensile strength) was calculated.

As a Charpy impact test, a V-notch test piece is taken so that the longitudinal direction of the test piece is parallel to the longitudinal direction of the pipe at a distance of t/4 (t: wall thickness) from the outer surface of the pipe at the corner of the square steel pipe, and this test is used. The sheet was performed at a test temperature: 0°C according to the regulations of JIS Z 2242, and the absorbed energy (J) was determined. In addition, the number of test pieces was three each, and the average value thereof was used as a representative value.

The residual stress was measured by cutting a steel pipe into a length of 500 mm, and removing the member from the surface layer to a depth of 50 μm by electrolytic etching. Then, the residual stress in the circumferential direction was measured by the cosα method of X-ray diffraction. The measurement position is the length center part of the test piece steel pipe, and is the position of the outer surface and the inner surface of the corner apexes of the four corners. The corner apex, regarding steel pipe Nos. 1 to 15 and 18, is the point of intersection of the line at 45° with the flat plate portion and the outside of the corner with the central axis of the steel pipe as the starting point. In addition, regarding the steel pipe Nos. 16 and 17, the starting point is the offset point offset by 1/2×(H1-H2) from the central part of the square steel pipe toward the long side (H1) direction, with respect to the above-mentioned straight line, and the sum is formed. The intersection of the line at 45° and the outside of the corner is the flat plate portion on the opposite side to the offset point.

The thickness of the oxide scale on the surface of the steel pipe was measured using a scanning electron microscope (SEM) at positions on 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 scale surface is measured at 8 points, and the total value of the distances at these 8 points is divided by 8 (average value) as the thickness (μm) of the oxide scale. The above-mentioned 8 points are a total of 8 points in the width center of the flat plate portion of the four sides of the square steel pipe, 4 points on the inner surface and 4 points on the outer surface.

As a corner tensile test, a JIS No. 5 tensile test piece was taken from a position in the 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 the test piece was used. It implemented in accordance with the provisions of JIS Z 2241, and the uniform elongation (%) was calculated.

As the hardness measurement, according to the micro-Vickers hardness test (JIS Z2244:2009), the test force was set to 9.8 N, and the thickness direction of the inner and outer surfaces 1 mm ± 0.1 mm away from the apexes of the four corners was measured. The average Vickers hardness of the position and the average Vickers hardness (HV) of the position in the wall thickness direction of 1 mm±0.1 mm from the inner and outer surfaces of the central part in the tube circumferential direction of the four-side flat plate portion. In addition, the difference between the Vickers hardness of the vertex of the corner and the Vickers hardness of the flat plate is such that the difference between the average Vickers hardness of the vertex of the corner and the average Vickers hardness of the flat plate becomes the largest. The difference between the average Vickers hardness of the inner surface of the apex in the wall thickness direction of 1 mm ± 0.1 mm and the average Vickers hardness of the outer surface of the outer surface of the central part of the tube circumferential direction of the flat plate part of 1 mm ± 0.1 mm in the wall thickness direction. Difference ((average Vickers hardness at 1mm±0.1mm from the inner surface of the corner apex in the wall thickness direction) - (at 1mm±0.1mm from the outer surface of the flat plate portion in the tube circumferential direction central portion in the wall thickness direction The average Vickers hardness)) to calculate the hardness difference.

Side length H (mm) (longitudinal side length H1 (mm), transverse side length H2 (mm)) was measured with a caliper, and wall thickness t (mm) was measured with a micrometer.

The results are shown in Table 3.

Based on the above description, it is possible to provide a square steel pipe having excellent deformability and suppressing excessive work hardening of corners, a method for producing the same, and a building structure having excellent seismic resistance.

1: Square steel pipe (square billet pipe) 2: Carrying table 3: Working coil 4: Steel strip (steel plate) 5: Leveler 6: Roller group 7: Finishing roll group 8: Squeeze roller 9: fusion splicer 10: Resistance welding steel pipe 11: Sizing roller group 12: Square forming roll group 13: Clapboard 14: Beam 15: Trabecular 16: Pillars 101: Flat panel 102: Corner 103: Welding part (resistance welding part)

Fig. 1 is a vertical cross-sectional view in the pipe axis direction for explaining a flat plate portion and a corner portion of a square steel pipe. [FIG. 2] It is a schematic diagram for demonstrating an oxide scale. Fig. 3 is a schematic diagram showing an example of a production facility for resistance welded steel pipes. [ Fig. 4] Fig. 4 is a schematic diagram showing an example of a manufacturing facility for a square steel pipe of the present invention. [Fig. 5] is a schematic diagram showing the heat treatment process of the square billet. Fig. 6 is a schematic diagram showing an example of a building structure.

1: Square steel pipe (square billet pipe)

10: Resistance welding steel pipe

11: Sizing roller group

12: Square forming roll group

R1 to R3: the shape of the resistance welded steel pipe 10

Claims (7)

A square steel pipe is formed with a plurality of flat plate parts and a plurality of corner parts alternately in the pipe circumferential direction, the yield strength YS of the flat plate part is 295MPa or more, the tensile strength TS of the flat plate part is 400MPa or more, and the flat plate part is more than 400MPa. The yield ratio YR is less than 0.80, the energy absorbed by the Charpy test at 0°C of the corners is more than 70J, the thickness of the oxide scale on the inner and outer surfaces of the tube is 1μm~20μm, and the inner surface from the apex of the corners is 1mm±0.1 The difference between the average Vickers hardness at the thickness direction position of mm and the average Vickers hardness at the wall thickness direction position 1 mm±0.1 mm away from the outer surface of the central portion in the tube circumferential direction of the flat plate portion is 5HV to 60HV. The square steel pipe according to claim 1, wherein the absolute value of the residual stress in the pipe circumferential direction of the inner surface and the outer surface of the vertex of the corner portion is 10 MPa to 200 MPa. The square steel pipe according to claim 1 or 2, wherein the uniform elongation is 5% or more at positions in the wall thickness direction 6 mm±1 mm away from the apex of the corner portion from the inner surface and the outer surface. A method for producing a square steel pipe, which is the method for producing a square steel pipe as described in any one of Claims 1 to 3, is to perform annealing heat treatment on a square billet that is finished from a steel sheet into a square shape by cold forming. heat treatment temperature is lower than the Ac ₁ transformation point is heated, the heating temperature deviation of the pipe wall thickness direction becomes less 50 ℃, and the heating above 500 ℃ retention time than 100sec. The method for producing a square steel pipe according to claim 4, wherein, in the annealing heat treatment, the heating temperature is 500°C to 700°C. The method for manufacturing a square steel pipe according to claim 4 or 5, wherein the heating of the aforementioned annealing heat treatment is induction heating, and the frequency of the induction heating is 100 Hz to 1000 Hz. A building structure using the square steel pipe as described in any one of Claims 1 to 3 as a column material.

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