TW202045745A - Rectangular steel tube and method for manufacturing same, and building structure - Google Patents

Abstract

Provided are a rectangular steel tube and a method for manufacturing same, and a building structure employing said rectangular steel tube. The present invention provides a rectangular steel tube which has flat plate portions and corner portions, and which has a specific component composition, wherein: the steel structure in a position at a depth 1/4 of a plate thickness t from an outer surface of the steel tube is such that ferrite occupies at least 55% and at most 80% by area ratio, and the mean aspect ratio of the hard phase is 0.1 to 0.8; the flat plate portions have a yield strength (YS) at least equal to 350 MPa and a tensile strength (TS) at least equal to 520 MPa; the ratio of YS in the flat plate portions relative to the corner portions is at least equal to 0.80 and at most equal to 0.90; the ratio of TS in the flat plate portions relative to the corner portions is at least equal to 0.90 and at most equal to 1.00; the Charpy absorbed energy of the flat plate portions at -40 DEG C is at least equal to 100 J; and the radius of curvature R of the corner portions is at least equal to (2.3*t) and at most equal to (2.9*t).

Description

Square steel pipe and its manufacturing method, and building structure

The present invention relates to a square steel pipe, a manufacturing method thereof, and a building structure. The square steel pipe with small strength difference between the corners and the flat plate of the present invention is suitable for use as a building structural member.

Square steel pipes (also known as "square columns") are generally manufactured by cold forming using hot-rolled steel plates (hot-rolled steel strips) or thick plates. As the cold forming method, there are press forming and roll forming. However, regardless of the method, a larger plastic strain is applied to the corners of the square steel tube than the flat part of the square steel tube, so the strength of the corner is likely to increase, which increases the strength difference between the corner and the flat part. Big fear. When the characteristics of the corners and flats are greatly different, it becomes extremely difficult to select welding materials or architectural design, so it is difficult to use square steel pipes as materials for building construction.

Regarding such a problem, although there are not many examples that directly discuss it, for example, there is the technique of Patent Document 1 which is a square steel pipe for a building structure. Patent Document 1 discloses a cold-formed square steel pipe, which is a square steel pipe obtained by cold bending a steel plate; the aforementioned steel pipes respectively contain C: 0.02 to 0.18% ("%" means "mass%" , The same for the following chemical components) Si: 0.03 to 0.5%, Mn: 0.7 to 2.5%, Al: 0.005 to 0.12%, and N: less than 0.008% (not including 0%), the remainder contains Fe and unavoidable Impurities. Among the unavoidable impurities, P: 0.02% or less (not including 0%), S: 0.01% or less (not including 0%), and O: 0.004 or less (not including 0%). The aforementioned bending process The part system is maintained in a state processed at a right angle and satisfies the following requirements (A) to (C) to thereby satisfy the shock resistance. (A) Yield strength of the flat part of the steel pipe: 355MPa or more, tensile strength: 520MPa or more, (B) The microstructure of the aforementioned flat part, the area fraction of the toughened iron structure: 40% or more, (C) The surface layer of the corner of the steel pipe has a Vickers hardness Hv: 350 or less, elongation in a tensile test: 10% or more, and Charpy absorption energy vE0 at 0°C: 70J or more. [Prior Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent No. 5385760

[The problem to be solved by the invention]

The square steel pipe manufactured by cold roll forming is formed by rolling a material (hot rolled material) that is flat in the width direction produced by hot rolling into a round steel pipe. It is a square steel pipe with corners and flat plates. In such a manufacturing method, due to the difference in work hardening, the strength difference between the corner portion and the flat portion is likely to increase. In addition, the hot rolling before roll forming is performed by cooling control from the surface of the hot rolled material to shape the material, so the strength (hardness) before processing is relatively large at the cooling rate The hot-rolled material has a larger problem near the surface layer.

However, with the technique disclosed in the aforementioned Patent Document 1, the hardness of the surface of the steel sheet does not increase excessively by the temperature control of the hot rolling, and it does not actively make the corners and the flat portions poor in strength. Zoom out. Therefore, even if the characteristics of the corners of the square steel pipe obtained by cold bending meet the predetermined criteria, the strength of the corners is obviously higher than that of the flat plate. If the plastic strain of the corner is reduced, the increase in the strength of the corner can be effectively suppressed. In order to reduce the plastic strain of the corner portion, it is considered to increase the corner portion R (roundness). However, a square steel pipe with a large R at the corners is not ideal because it may have design problems when combined with other members as a square member or the performance of the building may be reduced due to gaps.

The present invention was completed in view of such circumstances, and aims to provide a square steel pipe with a small difference in strength between the corners and the flat plate, a method for manufacturing the square steel pipe, and a building structure using the square steel pipe. [Technical means to solve the problem]

The inventors of the present invention have intensively studied to solve the aforementioned problems and obtained the following findings.

The present invention focuses on the vicinity of the surface layer (hereinafter referred to as the outer surface vicinity) of the steel pipe where the processing strain (plastic strain) introduced during the cold roll forming becomes particularly large, so that work hardening is not easily generated, thereby making The strength difference between the corner portion and the flat portion is reduced.

Therefore, the present inventors prepared a plurality of samples for changing the area ratio of the ferrous iron in the steel structure of the square steel pipe and the aspect ratio of the hard phase other than the ferrous iron (hereinafter referred to as the hard phase), and investigated the processing Ease of hardening. Here, the so-called hard phase includes toughened iron, polished iron, Asada iron, residual austenitic iron, etc., but is not limited to this.

Therefore, it has been found that in square steel pipes with a strength-based flat plate portion with YS of 350 MPa or more and TS of 520 MPa or more, the ratio of the ferrite iron is fixed or more, and the average aspect ratio of the hard phase is 0.1 to 0.8. Shape the steel structure that is not easy to work hardening. In this system, since the work hardening energy of the fat grain iron is small and the strain tends to concentrate on the fat grain iron, the work hardening energy of the entire steel structure can be reduced.

In addition, the inventors of the present invention made use of the steel structure of the material (hot rolled material) to suppress work hardening of the corners. When manufacturing square steel pipes, they were temporarily formed so that the ratio of longitudinal diameter to transverse diameter was 0.99 or more and 1.01 The following cylindrical round steel pipes are formed into an angular shape with a corner R of 2.3×t (t series plate thickness) or more and 2.9×t or less by the rollers arranged on the upper and lower and left and right. With this, it was found that a square steel pipe can be obtained without excessive work hardening of the corners. Here, the "longitudinal diameter" refers to the outer diameter in the vertical direction relative to the tube axis of the circular steel pipe, and the "transverse diameter" refers to the outer diameter in the horizontal direction relative to the tube axis of the circular steel pipe.

As described above, according to the present invention, in the steel structure near the outer surface of the square steel pipe where the processing strain is the largest when the cold roll forming is introduced, the ratio of the ferrite iron is made constant and the hard The average aspect ratio of the phase is 0.1 to 0.8. In addition, after the hot rolled material is formed into a cylindrical shape with a ratio of longitudinal diameter to transverse diameter of 0.99 or more and 1.01 or less, the rolls arranged on the upper and lower sides and on the left and right are formed into angles to produce the corners and the flat plate. Square steel pipe with small strength difference.

In addition, in the present invention, the "square steel pipe with a small difference in strength between the corner and the flat part" means that the YS ratio of the flat part to the corner is 0.80 or more and 0.90 or less, and the TS ratio of the flat part to the corner is Above 0.90 and below 1.00.

The inventors have further studied in detail to complete the present invention. The gist of the present invention is as follows. [1] A square steel pipe with flat plate and corners; its characteristics are: As an ingredient composition, it contains by mass%: C: 0.07～0.20%, Si: 1.0% or less, Mn: 0.5～2.0%, P: 0.030% or less, S: 0.015% or less, Al: 0.01～0.06%, N: less than 0.006%, The remainder is Fe and inevitable impurities, From the outer surface of the steel pipe to the 1/4 depth position of the plate thickness t, the area ratio of ferrous iron is 55% to 80%, and the average aspect ratio of the hard phase is 0.1 to 0.8. For the aforementioned flat part, YS is 350MPa or more, TS is 520MPa or more The ratio of the flat part to the YS of the corner part is 0.80 or more and 0.90 or less, and the ratio of the flat part to the corner part TS is 0.90 or more and 1.00 or less, The Charpy absorption energy at -40°C of the aforementioned flat part is above 100J, The R of the aforementioned corner is (2.3×t) or more (2.9×t) or less. [2] The square steel pipe as described in [1], in which, in addition to the aforementioned component composition, one or more groups selected from the following groups A to C are contained in mass%. Group A: Choose one or more from Nb: 0.05% or less, Ti: 0.05% or less, and V: 0.10% or less Group B: B: less than 0.008% Group C: One or two 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%, and Ca: 0.001 to 0.010%. [3] The square steel pipe according to [1] or [2], wherein the steel structure has an average equivalent circle diameter of the hard phase of 20 μm or less. [4] A method for manufacturing square steel pipes, which is the method for manufacturing square steel pipes described in any one of [1] to [3], characterized by: Carry out the pipe making step. The pipe making step is to weld the steel plate into a cylindrical shape by cold roll forming, and shape it into a cylindrical shape with a longitudinal diameter/transverse diameter ratio of 0.99 or more and 1.01 or less. Angular. [5] A method for manufacturing square steel pipes, which is the method for manufacturing square steel pipes described in any one of [1] to [3], characterized by: For the steel material, when the hot rolling step, the cooling step, the coiling step, and the pipe making step are sequentially performed to manufacture the square steel pipe, After heating the aforementioned steel material to a heating temperature: 1100 to 1300°C, a hot rolling step is performed to form a hot rolled steel sheet. The hot rolling step is: Rough rolling time when the thickness center temperature is above 1000°C: 200 seconds to 400 seconds, rough rolling end temperature: 1000～800°C, Start temperature of finishing rolling: 1000～800℃, finishing temperature of finishing rolling: 900～750℃; Next, perform a cooling step for the aforementioned hot-rolled steel sheet. This cooling step is a cooling step of 0.2s or more and less than 3.0s of the initial cooling step 10s from the beginning of cooling, at the center of the plate thickness. The average cooling rate: 4～25℃/s; Next, the aforementioned hot rolled steel sheet is subjected to a coiling step of coiling at a coiling temperature of 580°C or less to form a steel sheet, Next, a pipe making step is performed. This pipe making step involves welding the aforementioned steel plate into a cylindrical shape by cold roll forming, and forming it into a cylindrical shape with a longitudinal diameter/transverse diameter ratio of 0.99 to 1.01. , Shaped into an angular shape. [6] The method for producing a square steel pipe according to [5], wherein the cooling stop temperature in the cooling step is set to 580°C or lower. [7] A building structure using the square steel pipe described in any one of [1] to [3]. [Effects of Invention]

According to the present invention, a square steel pipe with a small difference in strength between the corners and the flat plate can be obtained. Since this square steel pipe is controlled to an appropriate size at the corner R, it can be suitably used as a square steel pipe for building structural members, for example.

Hereinafter, the present invention will be described in detail.

The square steel pipe of the present invention is as described below. As a component composition, it contains in mass %: C: 0.07 to 0.20%, Si: 1.0% or less, Mn: 0.5 to 2.0%, P: 0.030% or less, S: 0.015% or less, Al: 0.01 to 0.06%, N: Below 0.006%, the remainder is Fe and unavoidable impurities. The steel structure from the outer surface of the square steel pipe to the 1/4 depth position of the plate thickness t (hereinafter referred to as the 1/4t position), the area ratio of the ferrite iron is 55% to 80%, and the average vertical and horizontal of the hard phase The ratio is 0.1 to 0.8. In addition, the flat part of the square steel pipe has a YS of 350MPa or more and a TS of 520MPa or more. The ratio of the flat part to the corner of the YS is 0.80 or more and 0.90 or less, and the ratio of the flat part to the corner of the TS is 0.90 or more and 1.00 or less. The Charpy absorption energy at -40°C at the 1/4t position of the plate thickness of the part is 100J or more, and the R of the corner part is (2.3×t) or more (2.9×t) or less.

First, the component composition of the present invention will be explained. In addition, if not specifically limited, the mass% is only expressed in %. Moreover, in the present invention, the square steel pipe has the same component composition as the steel plate used as the material of the square steel pipe. Therefore, the reasons for limiting the composition of the square steel pipe and the steel plate used as the material will be explained below.

C: 0.07～0.20% C will increase the strength of steel plates and square steel pipes due to solid solution strengthening. On the other hand, C is an element that increases the production of hard phases and therefore reduces the production of ferrous iron. In order to ensure the required strength and the steel structure of the required steel plate and square steel pipe, C must contain 0.07% or more. On the other hand, if the content of C exceeds 0.20%, it will be difficult to ensure the required amount of fertilizer iron. Therefore, the C range is 0.07 to 0.20%. The C system is preferably 0.09% or more, more preferably 0.10% or more. In addition, C is preferably 0.18% or less, more preferably 0.17% or less.

Si: 1.0% or less The Si-based element contributes to increasing the strength of steel plates and square steel pipes due to solid solution strengthening. In order to ensure the required strength and the steel structure of the required steel plate and square steel pipe, it is better to contain more than 0.01% Si. However, if the Si content exceeds 1.0%, the toughness will decrease. Therefore, Si is 1.0% or less. Moreover, Si is preferably 0.8% or less, more preferably 0.6% or less. More preferably, it is 0.03% or more.

Mn: 0.5～2.0% Mn is an element that increases the strength of steel plates and square steel pipes through solid solution strengthening. In order to ensure the required strength of steel plates and square steel pipes, it must be contained at more than 0.5%. If the content of Mn is less than 0.5%, the start temperature of the ferrite transformation will increase, and with this, the hard phase will easily become excessively coarse. On the other hand, if the content of Mn exceeds 2.0%, the hardness of the central segregation portion will increase, which may cause cracks when the square steel pipe is welded in the field. Therefore, Mn is 0.5 to 2.0%. Mn is preferably 1.8% or less, more preferably 1.6% or less. Mn is preferably 0.6% or more, more preferably 0.7% or more.

P: 0.030% or less The P-based element has a function of segregating to the grain boundaries of the fertilizer grains and lowering the toughness of the steel plate and square steel pipe. In the present invention, it is preferable to reduce impurities as much as possible. However, excessive reduction of P will cause high refining costs, so 0.002% or more is better. In addition, the allowable content of P is up to 0.030%. Therefore, P is 0.030% or less. P is preferably 0.025% or less, more preferably 0.020% or less.

S: 0.015% or less S is present as a sulfide in steel, and if it is within the range of the composition of the present invention, it mainly exists as MnS. MnS will be stretched thinly in the hot rolling step, which will adversely affect the ductility and toughness of steel plates and square steel pipes. Therefore, in the present invention, it is preferable to reduce MnS as much as possible. However, excessive reduction of S will result in high refining costs, so S is better than 0.0002%. In addition, it is allowed to contain S up to 0.015%. Therefore, S is 0.015% or less. S is preferably 0.010% or less, more preferably 0.008% or less.

Al: 0.01～0.06% The Al-based element functions as a deoxidizer and has a role of fixing N as AlN. In order to obtain such an effect, 0.01% or more of Al must be contained. If Al is less than 0.01%, the deoxidizing power is insufficient when Si is not added, which increases the oxide-based inclusions and reduces the cleanliness of the steel sheet. On the other hand, if it contains more than 0.06% of Al, the amount of solid solution Al will increase when welding the long sides of square steel pipes (that is, when welding the length of the steel pipe in the manufacture of square steel pipes), especially in the atmosphere When welding is performed during welding, the risk of oxide formation at the welded portion increases, and the toughness of the welded portion of the square steel pipe decreases. Therefore, Al is 0.01 to 0.06%. The Al system is preferably 0.02% or more. In addition, Al is preferably 0.05% or less.

N: less than 0.006% The N-based element has a function of firmly fixing the movement of dislocations and reducing the toughness of steel plates and square steel pipes. In the present invention, it is preferable to reduce N as an impurity as much as possible, and the content can be allowed up to 0.006%. Therefore, N is 0.006% or less. N is preferably 0.005% or less. Although not particularly limited in the present invention, from the viewpoint of manufacturing cost, N is preferably 0.001% or more.

The remainder is Fe and unavoidable impurities. In addition, within a range that does not impair the effect of the present invention, as an inevitable impurity, for example, O (oxygen): 0.005% or less can be allowed to be contained.

The above is the basic composition of the present invention. The characteristics intended by the present invention can be obtained by the aforementioned essential elements, but the following elements can also be contained as necessary.

One or two or more selected from Nb: 0.05% or less, Ti: 0.05% or less, and V: 0.10% or less. Nb, Ti, and V all form fine carbides and nitrides in the steel and are precipitated An element that strengthens and helps increase the strength of steel. Therefore, it may be contained for the purpose of adjusting the strength in the present invention. In the case where Nb, Ti, and V are contained in order to obtain such an effect, they are preferably Nb: 0.05% or less, Ti: 0.05% or less, and V: 0.10% or less, and more preferably Nb: 0.04% or less, Ti: 0.04% or less, V: 0.08% or less. When Nb, Ti, and V are contained, they are preferably Nb: 0.001% or more, Ti: 0.001% or more, V: 0.001% or more, and more preferably Nb: 0.003% or more, Ti: 0.003% or more, V: 0.003% or more.

In addition, when two or more selected from Nb, Ti, and V are contained, the total is preferably 0.2% or less and 0.005% or more.

B: 0.008% or less B is an element that delays the ferrous iron phase transformation during the cooling process, promotes the formation of low-temperature phase transformation ferrite iron, and increases the strength of steel plates and square steel pipes. The inclusion of B increases the yield ratio of the steel plate, that is, the yield ratio of the square steel tube. Therefore, in the present invention, if the yield ratio of the square steel pipe is within a range of 90% or less, B can be contained as necessary for the purpose of adjusting the strength. When B is contained, B: 0.008% or less is preferable. B is preferably 0.0015% or less, more preferably 0.0008% or less. B is preferably 0.0001% or more, more preferably 0.0003% or more.

Choose one or more of Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.30%, and Ca: 0.001 to 0.010%. Cr: 0.01～1.0% Cr is an element that improves hardenability and increases the strength of steel plates and square steel pipes, and can be contained as necessary. When Cr is contained in order to obtain such an effect, it is preferable to contain 0.01% or more of Cr. On the other hand, if the content exceeds 1.0%, the toughness and weldability may decrease. Therefore, when Cr is contained, it is preferably 1.0% or less. Cr is more preferably 0.02% or more, and 0.8% or less.

Mo: 0.01～1.0% Mo is an element that increases the strength of steel plates and square steel pipes by improving hardenability, and can be contained as necessary. When Mo is contained in order to obtain such an effect, it is preferable to contain 0.01% or more of Mo. On the other hand, if it contains more than 1.0% of Mo, the toughness may be lowered, so if it contains Mo, it is preferably 1.0% or less. Mo is more preferably 0.02% or more, and 0.8% or less.

Cu: 0.01～0.50% Cu is an element that increases the strength of steel plates and square steel pipes by solid solution strengthening, and can be contained as necessary. When Cu is contained in order to obtain such an effect, it is preferable to contain 0.01% or more of Cu. On the other hand, if Cu is contained in excess of 0.50%, the toughness may be lowered. Therefore, when Cu is contained, it is preferably 0.50% or less. Cu is more preferably 0.02% or more, and 0.4% or less.

Ni: 0.01～0.30% Ni is an element that increases the strength of steel plates and square steel pipes by solid solution strengthening, and can be contained as necessary. When Ni is contained in order to obtain such an effect, it is preferable to contain 0.01% or more of Ni. On the other hand, if the content of Ni exceeds 0.30%, the area ratio of the ferrous iron may decrease. Therefore, when Ni is contained, it is preferably 0.30% or less. Ni is more preferably 0.02% or more, and 0.2% or less.

Ca: 0.001～0.010% Ca is an element that spheroidizes sulfides such as MnS thinly stretched in the hot rolling step to improve the toughness of steel, and can be contained as necessary. When Ca is contained in order to obtain such an effect, it is preferable to contain 0.001% or more of Ca. However, if the Ca content exceeds 0.010%, Ca oxide clusters are formed in the steel, which may deteriorate toughness. Therefore, when Ca is contained, the Ca content is preferably 0.001 to 0.010%. Ca is more preferably 0.0015% or more, and 0.0050% or less.

First, the steel structure of the square steel pipe of the present invention will be described.

From the outer surface of the square steel pipe of the present invention to the steel structure at the position of 1/4t, the area ratio of ferrite iron is 55% to 80%, and the average aspect ratio of the hard phase is 0.1 to 0.8. The steel structure can further make the average equivalent circle diameter of the hard phase 20 μm or less.

Fertilizer iron: area ratio is 55% above 80% In order to ensure the required strength of the square steel pipe of the present invention, the steel structure at the position of 1/4t from the outer surface of the steel pipe contains ferrite and other hard phases. Here, the so-called hard phase includes phases other than fertile iron, that is, toughened iron, polished iron, hemp iron, and residual austrian iron. In this hard phase, the total area ratio of each phase is 20 to 45%.

When the area ratio of the fat grain iron is less than 55%, the ratio of the toughened iron will be excessive in the range of the aforementioned composition of the present invention, so that the strain is easily dispersed to the hard phase, and work hardening is easy. Therefore, it is impossible to obtain a square steel pipe with a small difference in strength between the corner portion and the flat portion. On the other hand, in the case where the area ratio of ferrite iron exceeds 80%, the required strength cannot be obtained. Fertilizer iron is preferably 60% or more and 75% or less.

The average aspect ratio of the hard phase: 0.1 to 0.8 If the average aspect ratio of the hard phase is less than 0.1, the starting point of cracks is likely to occur, which reduces the toughness. On the other hand, if the average aspect ratio of the hard phase exceeds 0.8. The strain is easily dispersed to the hard phase, so work hardening is easy. Therefore, when a square steel pipe is manufactured using a steel plate, a square steel pipe with a small difference in strength between the corners and the flat plate cannot be obtained. More preferably, it is 0.2 or more and 0.7 or less. In the present invention, as described later, the aspect ratio of the hard phase is the average value of the aspect ratio of crystal grains with a nano-hardness of 3.0 GPa or more of the structure other than the fat iron.

The average equivalent circle diameter of the hard phase: 20μm or less (suitable conditions) If the average equivalent circle diameter of the hard phase exceeds 20 μm, the toughness will decrease, so it is preferably 20 μm or less. More preferably, it is 15 μm or less. In the present invention, as described later, the average equivalent circle diameter of the hard phase is the average value of the equivalent circle diameter of crystal grains with a nano-hardness of 3.0 GPa or more of the structure other than ferrous iron.

In addition, generally speaking, a square steel pipe manufactured by roll forming a steel plate (hot-rolled steel plate) as a material has the same steel structure at the 1/4t position of the corner and flat part, so the 1/4t position of the flat part is measured Or 1/4t at the corner. Here, the steel structure at the 1/4t position of the flat plate portion is defined.

In the present invention, if the aforementioned steel structure exists in the range of 3/16t to 5/16t of the steel pipe, the aforementioned effects can be obtained similarly. Therefore, the "steel structure at the 1/4t position" in the present invention means that the steel structure is present in any of the aforementioned 3/16t position to 5/16t position.

The aforementioned steel structure was observed by the following method to obtain the type and area ratio (%) of the structure. The test piece for structure observation was taken from a square steel pipe, polished so that the section in the rolling direction (L section) became the observation surface, and was produced by nitric acid corrosion. Microstructure observation is based on the surface of the test piece for self-organization observation (that is, the outer surface of the square steel tube) at the position of 1/4t of the plate thickness as the center of observation, using an optical microscope (magnification: 500 times) or a scanning electron microscope (SEM, magnification: 500 times) Observe and photograph the steel structure. The measurement area is 500 μm×500 μm. Here, "t" indicates the thickness (plate thickness) of the steel pipe. From the obtained tissue photos, an image analysis device (image analysis software: Photoshop, manufactured by Adobe) was used to identify the type of tissue, and calculate the area ratio of the fat iron. The area ratio of the tissue was obtained by observing at least 5 fields of view, and was obtained as the average value of the values obtained in each field of view.

In addition, the average aspect ratio of the hard phase is obtained as follows. First, from the tissue photos obtained above, the nanoindentation method is used to obtain the nano hardness of the tissues other than fat iron. For crystal grains with a nano hardness of 3.0 GPa or more, the value calculated from (the average of the length in the plate thickness direction/the average of the length in the rolling direction) of all the crystals is obtained and used as the average aspect ratio.

In addition, the average equivalent circle diameter of the hard phase was measured using the SEM/EBSD method. The average equivalent circle diameter is determined by determining the misorientation of adjacent grains, and measuring the boundary where the misorientation is 15˚ or more as the grain boundary. Calculate the arithmetic average of the particle size from the obtained grain boundaries, and obtain the average equivalent circle diameter. The measurement area is 500 μm×500 μm, and the measurement interval length is 0.5 μm. In addition, in the analysis of the crystal grain size, those with a crystal grain size of 2.0 μm or less are regarded as measurement noise, and those with a nano-hardness of less than 3.0 GPa are regarded as non-hard phases and excluded from the analysis target.

Next, the method of manufacturing the square steel pipe of the present invention will be described using FIGS. 1 and 2. Fig. 1 is a schematic diagram showing an example of manufacturing equipment for electric-welded steel pipe. Figure 2 is a schematic diagram showing the forming process of a square steel pipe.

The method for manufacturing a square steel pipe of the present invention is to form a square steel pipe by performing a pipe making step on a steel plate. The pipe making step of the present invention is to roll the steel plate in the cold room to form a cylindrical end face welding. Next, after the hot-rolled material is formed into a cylindrical round steel pipe with a ratio of longitudinal diameter to lateral diameter of 0.99 to 1.01, the round steel pipe is formed into corners in the cold zone by the rolls arranged on the upper and lower sides. Shaped, and manufactured into a square steel tube with corners and flat plates.

First, as shown in Fig. 1, the steel strip 1, which is the material of the electric welded steel pipe, is corrected by the leveler 2, for example, after the in-side straightening is carried out, and then an open tube is formed by the middle of the row of rollers 3 composed of a plurality of rollers. The finishing roll group 4 composed of a plurality of rolls is formed. After the completion of forming, the width ends of the steel strip 1 are resistance-welded with a welding machine 6 while being crimped by the squeeze roll 5 to form a cylindrical electric resistance welded steel pipe 7. In addition, in the present invention, the manufacturing equipment of the electric resistance welded steel pipe 7 is not limited to the pipe manufacturing process as shown in FIG. 1.

After that, as shown in Fig. 2, the electric-welded steel pipe 7 is reduced in diameter by a group of dressing rolls (dressing mill) 8 composed of a plurality of rolls, so that the ratio of longitudinal diameter/transverse diameter is 0.99 or more and 1.01 The following cylindrical shape. After that, a group of corner forming rolls (corner forming rolls) 9 composed of a plurality of rolls are sequentially formed into shapes such as R1, R2, and R3 to form a square steel pipe 10. In addition, the number of the finishing roll group 8 and the corner forming roll group 9 is not particularly limited.

Here, the reason for forming into a cylindrical shape with a longitudinal diameter/lateral diameter ratio of 0.99 or more and 1.01 or less before forming into an angular shape will be described.

In the present invention, it is important that the ratio of longitudinal diameter/lateral diameter is 0.99 or more and 1.01 or less for the following reasons. Generally speaking, when a steel pipe is manufactured by roll forming, in order to suppress springback, uneven strain is often applied in the circumferential direction during the process. However, under the premise that the final shape is a square shape, the cross-section of the cylindrical shape at the previous stage does not have to be a perfect circle. Therefore, even if it is called a cylindrical shape, it is not necessarily a perfect circle in the middle of manufacturing a square steel pipe, so the obtained square steel pipe cannot reduce the characteristic difference between the flat part and the corner part. Therefore, in the present invention, in order to reduce the difference in characteristics between the flat portion and the corner portion, the shape must be formed into a cylindrical shape with a longitudinal diameter/lateral diameter ratio of 0.99 or more and 1.01 or less in an earlier stage.

If it is not formed into a cylindrical shape with the aforementioned longitudinal diameter/transverse diameter ratio of 0.99 or more and 1.01 or less, the plastic strain of the corner portion will be too large compared to the flat portion. Therefore, the ratio of the YS of the flat part to the corner part may not reach 0.80, and the ratio of the flat part to the TS of the corner part may not reach 0.90. In addition, since the plastic strain of the corner portion is larger than that of the flat portion, of course the YS ratio of the flat portion to the corner portion is 0.90 or less, and the TS ratio of the flat portion to the corner portion is 1.00 or less. Therefore, in order to achieve the purpose of the present invention, that is, to make the YS of the flat portion 350 MPa or more, the TS of 520 MPa or more, the ratio of the YS of the flat portion to the corner portion to 0.80 or more and 0.90 or less, and the TS of the flat portion to the corner portion The ratio of is 0.90 or more and 1.00 or less, and it must be formed into a cylindrical shape with a longitudinal diameter/lateral diameter ratio of 0.99 or more and 1.01 or less before corner forming.

In addition, by forming into a cylindrical shape with the aforementioned longitudinal diameter/lateral diameter ratio of 0.99 or more and 1.01 or less, the corners are evenly formed when the corners are formed. The corner R system (2.3×t) Above (2.9×t) or less (here, t is the plate thickness). By setting the R of the corner portion to (2.3×t) or more (2.9×t) or less (here, t is the plate thickness), the difference in strength between the corner portion and the flat portion can be reduced.

As explained above, according to the present invention, the YS of the flat part is 350 MPa or more, the TS is 520 MPa or more, the ratio of the YS of the flat part to the corner part is 0.80 or more and 0.90 or less, and the ratio of the flat part to the corner TS is 0.90 or more Below 1.00, the Charpy absorption energy at -40°C of the flat part is 100J or more, and the R of the corner part is (2.3×t) or more (2.9×t) or less, so a square with a small difference in strength between the corner and the flat part can be obtained Steel Pipe. Since this square steel pipe has an appropriate size of R at the corners and a small difference in strength between the corners and the flat plate, it can be suitably used as a square steel pipe for building structural members.

In addition, as described above, as the material for the square steel pipe of the present invention, a steel sheet (hot-rolled steel sheet) obtained by sequentially performing the hot rolling step, cooling step, and coiling step described below can be used. In the present invention, the steel plate may be formed into a square steel pipe by performing the aforementioned pipe making step.

An example of a method of manufacturing a steel plate suitable for use as a material of the square steel pipe of the present invention will be described.

The method of manufacturing a steel sheet suitable for use as the material of the square steel tube of the present invention is that, for example, a steel material having the aforementioned composition can be sequentially subjected to a hot rolling step under the conditions described below (hereinafter referred to as hot The rolling step), the cooling step, and the coiling step are formed into a steel sheet (hot-rolled steel sheet).

For example, the steel material having the aforementioned composition is heated to a heating temperature of 1100 to 1300°C, and then a hot rolling step is performed to form a hot rolled steel sheet. The hot rolling step is: the thickness center temperature of the steel material Rough rolling time above 1000°C: 200 seconds to 400 seconds, rough rolling end temperature: 1000-800°C, finishing rolling start temperature: 1000-800°C, finishing rolling end temperature: 900-750°C. Next, the hot-rolled steel sheet after the hot-rolling step is subjected to a cooling step. The cooling step is: one or more times of initial cooling from the beginning of the cooling 10s between 0.2s or more and less than 3.0s of standing cooling. The average cooling rate of the thickness center temperature of the rolled steel sheet: 4～25℃/s, the cooling stop temperature: 580℃ or less; then, the hot rolled steel sheet after the cooling step is coiled at a coiling temperature: 580℃ or less. After that, a coiling step of standing cooling is performed to obtain a steel sheet (hot-rolled steel sheet).

Hereinafter, each step will be described in detail. In addition, in the following description of the manufacturing method, the temperature (°C) refers to the surface temperature of the steel material, sheet, hot-rolled steel sheet, or steel sheet, unless otherwise limited. The surface temperature of these can be measured with a radiation thermometer or the like. The average cooling rate (℃/s), if not specifically limited, is by ((Temperature before cooling-temperature after cooling)/cooling time) The value sought.

In the present invention, the method for melting the steel material (steel blank) having the aforementioned component composition is not particularly limited, and it can be melted using a known melting method such as a converter, an electric furnace, and a vacuum melting furnace. The casting method is also not particularly limited, and it can be manufactured to the required size by a known casting method such as a continuous casting method. Moreover, instead of the continuous casting method, there is no problem in using the block-dividing rolling method. For molten steel, further secondary refining such as ladle refining is also possible.

Next, a hot rolling step is performed on the obtained steel material (steel blank). In the hot rolling step, the steel material is heated to a heating temperature: 1100-1300°C. After that, the heated steel material extracted from the heating furnace is rough-rolled and then finished-rolled to form a hot-rolled steel sheet; this rough rolling is controlled to be a rough-rolling where the thickness center temperature of the steel material is 1000°C or higher Time: 200 seconds to 400 seconds, and rough rolling end temperature: 1000-800°C; the finishing rolling system: finishing rolling start temperature: 1000-800°C, finishing rolling end temperature: 900-750°C.

In addition, the temperature of the thickness center of the steel material in the hot rolling step is obtained by calculating the temperature distribution in the section of the steel material by heat transfer analysis.

Heating temperature: 1100～1300℃ If the heating temperature of the steel material does not reach 1100°C, the deformation resistance of the material to be rolled will be too large, resulting in insufficient load resistance and rolling torque in the roughing and finishing mills, making it difficult to perform rolling. On the other hand, if the heating temperature exceeds 1300° C., the austenitic iron grains will coarsen, and even if the austenitic iron grains are processed and recrystallized by repeated rough rolling and finish rolling, it is difficult to refine the austenitic iron grains. In addition, it may be difficult to ensure the required toughness of the hot-rolled steel sheet. Therefore, the heating temperature of the steel material is 1100 to 1300°C. The heating temperature is preferably 1280°C or lower. The heating temperature is preferably 1150°C or higher.

In addition, when there is a margin for the load resistance and rolling torque of each rolling mill, a temperature in the range of 1100°C or lower and Ar3 transformation point or higher may be selected as the heating temperature.

The heated steel material is then rough-rolled to form a plate or the like.

Rough rolling time when the thickness center temperature is 1000℃ or higher: After the steel material is drawn out from the heating furnace within 200 seconds or more and 400 seconds, the rough rolling time when the thickness center temperature of the steel material is 1000℃ or higher is within 400 seconds. This allows the vicinity of the surface of the rolled material to be preferentially cooled. This suppresses the coarsening of the austenitic iron grain size at a position 1/4t from the surface of the material to be rolled, and promotes the subsequent ferrite transformation. Therefore, the ratio of ferrous iron in the steel structure is 55% or more in terms of area ratio. If the rough rolling time exceeds 400 seconds, the austenitic iron will be coarse-grained, and the required amount of fat iron cannot be ensured, so a square steel pipe with a small difference in strength between the corners and the flat plate cannot be obtained. On the other hand, if the aforementioned rough rolling time is less than 200 seconds, the austenitic iron grain size will be excessively fine, and the ratio of the fat iron in the steel structure will exceed 80% in terms of area ratio, resulting in insufficient strength. The rough rolling time is preferably 220 seconds or more, more preferably 250 seconds or more. The rough rolling time is preferably 380 seconds or less, more preferably 350 seconds or less.

End temperature of rough rolling: 1000～800℃ The heated steel material will be processed, recrystallized and refined by rough rolling. If the rough rolling end temperature is less than 800°C, the rough rolling mill's load resistance and rolling torque are likely to be insufficient. On the other hand, if the rough rolling end temperature is a high temperature exceeding 1000°C, the austenitic iron grains will be coarsened, and the toughness of the square steel pipe will tend to decrease. The finishing temperature of rough rolling is preferably 820°C or higher, more preferably 840°C or higher. The finishing temperature of rough rolling is preferably 980°C or lower, more preferably 950°C or lower.

In addition, this rough rolling end temperature can be achieved by adjusting the heating temperature of the steel material, the cooling conditions in the aforementioned rough rolling, the retention during the rough rolling, the thickness of the steel material, and the like. The thickness of the material to be rolled (thickness of a sheet or the like) at the end of rough rolling is not particularly limited, as long as it can be formed into a finished plate (hot-rolled steel sheet) of the required finished thickness by finish rolling. For example, with regard to the manufacture of square steel pipes for building structural members, the thickness of the finished product is preferably about 12-28 mm.

After rough rolling, the material to be rolled is finished by, for example, an in-line rolling mill to form a hot-rolled steel sheet.

Start temperature of finishing rolling: 1000～800℃ In the finishing rolling, rolling processing and recrystallization are repeatedly performed, and the austenite (γ) grains are refined. If the finish rolling start temperature (finish rolling-in side temperature) is low, the processing strain introduced by the rolling process is likely to remain, and the γ grains are easily refined. If the finish rolling start temperature is less than 800°C, the temperature near the surface of the steel sheet in the finish rolling mill will be below the Ar3 transformation point, increasing the risk of ferrous iron formation. The resulting fat iron particles are then elongated in the rolling direction by the finish rolling process to become fat iron particles, which causes the decrease in workability. On the other hand, if the finish rolling start temperature is a high temperature exceeding 1000°C, the effect of refining the γ grains produced by the finish rolling is reduced, and the toughness of the square steel pipe is likely to be lowered. Therefore, the finishing rolling start temperature is 800 to 1000°C. The start temperature of finishing rolling is preferably 825 to 975°C.

Finish rolling end temperature: 900～750℃ If the finish rolling end temperature (finish side temperature) is a high temperature exceeding 900°C, the processing strain imparted during the finish rolling will be insufficient, the γ grains will not be refined, and the toughness of the square steel tube will tend to decrease. On the other hand, if the finishing temperature of the finishing rolling is less than 750°C, the temperature near the surface of the steel sheet in the finishing mill will be below the Ar3 transformation point, forming fertilizer grains elongated in the rolling direction, and making the fertilizer iron The granules become mixed granules. Therefore, the risk of reduced toughness increases. Therefore, the finishing temperature of finishing rolling is 900 to 750°C. The finishing temperature of finish rolling is preferably 850°C or lower. More preferably, it is 770°C or higher.

After finishing rolling, the hot-rolled steel sheet is subjected to a cooling step.

From the initial cooling of 10 seconds between the beginning of the cooling, the number of times of standing cooling of 0.2s or more and less than 3.0s: 1 time or more. In the present invention, the cooling of the hot-rolled steel sheet obtained in the hot rolling step is within 10 seconds (10s) as the initial cooling step. In the initial cooling step of the cooling step, cooling is performed by setting a standing cooling step of 0.2s or more and less than 3.0s for 1 time. This is to suppress the formation of Asada scattered iron structure on the surface and back of the steel plate. In the initial cooling step, if the placing cooling step is not set or the placing cooling step is less than 0.2s, the steel structure on the surface and back of the steel plate will become the Asada scattered iron structure, which reduces the toughness of the square steel pipe. In addition, in the initial cooling step, if the cooling step is left for 3.0 s or more, the average aspect ratio of the hard phase may not reach 0.1, resulting in insufficient toughness. Therefore, in the initial cooling step of the cooling step, the time to perform the standing cooling step once is 0.2 s or more and less than 3.0 s. The time for one cooling step is preferably 0.4 s or more and 2.0 s or less.

In order to obtain the aforementioned effect, the number of times of the standing cooling step in the initial cooling step must be one or more. In addition, the number of cooling steps may be appropriately set depending on the configuration of the cooling equipment or the cooling stop temperature. Here, the so-called standing cooling is natural cooling. Although the upper limit of the number of remaining cooling steps is not particularly limited, from the viewpoint of productivity, it is preferably 10 times or less. In the case of setting the number of stand-to-cooling to a plurality of times, for example, by stopping spraying of water from nozzles in a section of the nozzles for water cooling described later, it becomes an intermittent spray, etc., and it may be appropriately set.

Average cooling rate of plate thickness center temperature: 4-25°C/s, cooling stop temperature: 580°C or less. In the cooling step, the hot-rolled steel sheet obtained by finish rolling is performed from cooling start to cooling stop (cooling End) the average cooling rate of the plate thickness center temperature is 4-25°C/s, and the cooling stop temperature is 580°C or less. The cooling performed in the cooling step is, for example, water cooling (water cooling) such as water jet cooling, spray cooling, mist cooling, etc., or gas jet cooling where cooling gas is sprayed from a nozzle. Furthermore, it is preferable to perform a cooling operation on both sides of the hot-rolled steel sheet (steel sheet) so that both sides (front and back sides) of the hot-rolled steel sheet (steel sheet) are cooled under the same conditions.

If the average cooling rate at the center of the thickness of the hot-rolled steel sheet is less than 4°C/s, the frequency of fertilizer iron grain generation will decrease, which will coarsen the fertilizer iron grains and reduce toughness. On the other hand, if the aforementioned average cooling rate exceeds 25°C/s, the production of ferrous iron will be suppressed to less than 55%, and the average aspect ratio of the hard phase will exceed 0.8, making it impossible to obtain a steel structure that is not easy to work hardening. . Therefore, the average cooling rate of the thickness center of the hot-rolled steel sheet is 4-25°C/s. The average cooling rate of the thickness center of the hot-rolled steel sheet is preferably 5°C/s or more and 15°C/s or less.

Here, the average cooling rate of the thickness center of the hot-rolled steel sheet is determined by ((The temperature of the thickness center at the beginning of cooling (°C)-the temperature of the thickness center at the stop of cooling (°C))/cooling time (s)) Come and ask. The temperature of the thickness center of the hot-rolled steel sheet can be obtained by calculating the temperature distribution in the section of the steel sheet by heat transfer analysis.

If the cooling stop temperature exceeds 580°C, the average equivalent circle diameter of the hard phase of the steel sheet exceeds 20 μm, which may reduce the toughness. The cooling stop temperature is more preferably 560°C or less.

In addition, in order to obtain the required steel structure at the position of 1/4t, it is preferable that the average cooling rate of the surface temperature of the hot-rolled steel sheet in the temperature range of 750°C to 650°C be 20°C/s or more. If the average cooling rate in this temperature range does not reach 20°C/s, the average equivalent circle diameter of the hard phase may exceed 20 μm. It is preferable that the average cooling rate of the surface temperature of the hot-rolled steel sheet in the temperature range of 750°C to 650°C be 80°C/s or less. If the average cooling rate in this temperature range exceeds 80°C/s, the average aspect ratio of the hard phase may exceed 0.8. In order to make the average aspect ratio of the hard phase 0.8 or less, it is preferable to start the cooling step immediately (within 5 seconds) after finishing rolling.

After cooling, the hot-rolled steel sheet is subjected to a coiling step to obtain a steel sheet (hot-rolled steel sheet).

Coiling temperature: below 580℃ In the coiling step, the hot-rolled steel sheet is coiled at a coiling temperature of 580°C or less, and then left to cool. If the coiling temperature exceeds 580°C, the ferrous iron phase transformation and the corundum phase transition will progress after the coiling, resulting in an excessive ratio of cortical iron and lowering the toughness of the square steel pipe. Therefore, the coiling temperature is 580°C or less. The coiling temperature is preferably 550°C or lower. In addition, although the coiling temperature is low, there is no material problem. However, if the coiling temperature does not reach 400°C, especially for thick steel plates with a thickness of more than 25mm, the coiling deformation resistance will be extremely large, and it may not be able to properly Coiling situation. Therefore, the coiling temperature is preferably 400°C or higher.

After that, the steel sheet (hot-rolled steel sheet) after the coiling step is subjected to the aforementioned pipe making step to obtain a square steel pipe.

Next, an example of a building structure using the square steel pipe of the present invention will be described.

Fig. 3 is a perspective view schematically showing a building structure according to an embodiment of the present invention. As shown in FIG. 3, in the building structure of this embodiment, a plurality of square steel pipes 11 of the present invention are erected as columns. Between adjacent square steel pipes 11, a plurality of beams 14 made of steel materials such as H-shaped steel are erected. In addition, a plurality of small beams 15 made of steel materials such as H-shaped steel are erected between adjacent large beams 14. The square steel pipe 11 and the diaphragm 16 are welded, and the H-shaped steel as the beam 14 is welded to the square steel pipe 11, whereby a beam 14 including steel materials such as H-shaped steel is erected between adjacent square steel pipes 11. In addition, in order to install a wall or the like, a pillar 17 is installed as necessary.

The building structure of the present invention uses the square steel pipe 11 of the present invention with a small difference in strength between the corners and the flat plate. Therefore, the welding material for welding the square steel pipe 11 and the diaphragm 16 can be easily selected, and it is not easy to cause problems such as insufficient matching. The strength of the welding material is poor. Since it is not easy to produce insufficient matching, it can restrain the problems such as the fracture of the welding part. In addition, since the angle R (the R of the corner portion) of the square steel pipe 11 is controlled to an appropriate size, it can be easily combined with other structural members whose cross-section is a right angle. In addition, since the angle R of the square steel pipe 11 is controlled to an appropriate size, it can withstand a larger external force and improve the shock resistance and the like. Example

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

The molten steel was melted in a converter, and a steel blank (steel material: thickness 250mm) with the composition shown in Table 1 was formed by continuous casting. After the steel billets (steel materials) are heated to the heating temperature shown in Table 2, after the hot rolling step, the cooling step, and the coiling step under the conditions shown in Table 2, they are placed and cooled to form a plate Thickness: 16～28mm steel plate (hot rolled steel plate). In addition, immediately after finishing rolling (within 5 seconds), the cooling step is started. Cooling is performed by water cooling. In the initial cooling step, in the initial cooling step, in the initial cooling step 10s from the start of cooling, set the left cooling zone without water cooling. After that, using the obtained hot-rolled steel sheet as a material, a round steel pipe was formed by cold roll forming under the conditions shown in Table 2, and then a square steel pipe (400-550mm angle) was formed by cold roll forming. ).

In the examples of the present invention, test pieces were taken from the obtained square steel pipes, and the structure observation, the tensile test, the Charpy impact test, and the measurement of the corner R were respectively performed. In addition, tissue observation is to observe and measure by the aforementioned method. In addition, the test methods of the tensile test, the Charpy impact test, and the measurement method of the corner R are as follows.

(1) Tensile test of square steel pipe JIS No. 5 tensile test specimens were taken from the flat portion and corner portion of the obtained square steel pipe so that the tensile direction was the pipe length direction. Next, a tensile test was performed in accordance with JISZ2241 (2011), and the yield strength YS and the tensile strength TS were measured. Using the obtained measured value, calculate the yield ratio YR(%) defined by (Yield Strength)/(Tensile Strength)×100(%).

(2) Impact test of square steel pipe A V-notch test piece was taken from the position of the plate thickness 1/4t of the flat part of the obtained square steel pipe so that the length direction of the test piece was the pipe circumferential direction. Next, in accordance with the regulations of JISZ2242 (2011), a Charpy impact test was performed at a test temperature of -40°C, and the absorbed energy (J) was determined. In addition, the number of test pieces was set to three, and the average value of each of the three pieces was used as the value of the impact test results shown in Table 3-2.

(3) Measuring method of corner R (angle R) From the obtained square steel pipe, randomly cut 10 sections perpendicular to the pipe axis direction, measure the radii of curvature of the four corners of the vertical section, and use the average value as the corner R. Specifically, as shown in Figure 4, the welded part (joining part) of the steel pipe is taken as 0˚, and the 0˚ is used as a reference, and the positions of 45˚, 135˚, 225˚, and 315˚ are respectively regarded as corners In the case of the center, the so-called radius of curvature of the corner refers to the curvature at the intersection of a 45˚ line (L) with the adjacent side starting from the center of the tube and the outside of the corner (the outer surface of the corner of the tube). radius. The radius of curvature of the corner is the radius of a sector of 65˚ defined by a line drawn from the connecting point (A, A') of the flat part and the arc part of the square steel tube with the center on the aforementioned L. In addition, "t" shown in FIG. 4 is the thickness of the plate, and "H" is the length of the side of the outer shape. As a method of calculating the radius of curvature, for example, there is a method of calculating the radius of curvature using the law of sine from the measurement results of the distance relationship between three points (the intersection point on the outside of the corner and the two points that are the connection points between the flat part and the arc part), or The method of measuring the radius of curvature from a radius gauge that matches the height of the corner in the area of the aforementioned 3 points, etc., however, is not limited to this. In this embodiment, a radius gauge is used to measure the radius of curvature of the corner. In addition, the angle R is the average value of 10 cross sections perpendicular to the tube axis direction as described above.

The obtained results are shown in Table 3-1 and Table 3-2.

As far as the invention examples within the scope of the present invention are concerned, the characteristics of the present invention are obtained (the YS of the flat part is 350MPa or more, the TS is 520MPa or more, the ratio of the YS of the flat part to the corner is 0.80 or more and 0.90 or less, and the flat part has The ratio of TS is 0.90 or more and 1.00 or less, the Charpy absorption energy at -40°C of the flat part is 100J or more, and the R of the corner part is (2.3×t) or more (2.9×t) or less). Here, t is the plate thickness. ). On the other hand, with regard to comparative examples that deviate from the scope of the present invention, the characteristics of the present invention cannot be obtained.

1: Steel belt 2: Leveler 3: Row roller group 4: Finishing roll group 5: Squeeze roller 6: Welding machine 7: Electric welded steel pipe 8: Dressing roll group 9: Corner forming roll group 10: Square steel pipe 11: Square steel pipe 14: beam 15: Trabecular 16: Diaphragm 17: Column

[Fig. 1] Fig. 1 is a schematic diagram showing an example of manufacturing equipment for electric resistance welded steel pipe. [Figure 2] Figure 2 is a schematic diagram showing the forming process of a square steel pipe. [Fig. 3] Fig. 3 is a perspective view schematically showing an example of a building structure using the square steel pipe of the present invention. [Fig. 4] Fig. 4 is a schematic diagram showing a cross section of a square steel pipe.

Claims (7)

A square steel pipe with a flat part and corner parts; it is characterized by: As an ingredient composition, it contains by mass%: C: 0.07～0.20%, Si: 1.0% or less, Mn: 0.5～2.0%, P: 0.030% or less, S: 0.015% or less, Al: 0.01～0.06%, N: less than 0.006%, The remainder is Fe and inevitable impurities, From the outer surface of the steel pipe to the 1/4 depth position of the plate thickness t, the area ratio of ferrous iron is 55% to 80%, and the average aspect ratio of the hard phase is 0.1 to 0.8. For the aforementioned flat part, YS is 350MPa or more, TS is 520MPa or more The ratio of the flat part to the YS of the corner part is 0.80 or more and 0.90 or less, and the ratio of the flat part to the corner part TS is 0.90 or more and 1.00 or less, The Charpy absorption energy at -40°C of the aforementioned flat part is above 100J, The R of the aforementioned corner is (2.3×t) or more (2.9×t) or less. The square steel pipe according to claim 1, in which, in addition to the aforementioned component composition, one or more groups selected from the following groups A to C are contained in mass%; Group A: Choose one or more from Nb: 0.05% or less, Ti: 0.05% or less, and V: 0.10% or less Group B: B: less than 0.008% Group C: One or two 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%, and Ca: 0.001 to 0.010%. The square steel pipe according to claim 1 or 2, wherein the steel structure has an average equivalent circle diameter of the hard phase of 20 μm or less. A method for manufacturing square steel pipes is the method for manufacturing square steel pipes described in any one of Claims 1 to 3; and is characterized by: Carry out the pipe making step. The pipe making step is to weld the steel plate into a cylindrical shape by cold roll forming, and shape it into a cylindrical shape with a longitudinal diameter/transverse diameter ratio of 0.99 or more and 1.01 or less. Angular. A method for manufacturing square steel pipes is the method for manufacturing square steel pipes described in any one of Claims 1 to 3; and is characterized by: For the steel material, when the hot rolling step, the cooling step, the coiling step, and the pipe making step are sequentially performed to manufacture the square steel pipe, After heating the aforementioned steel material to a heating temperature: 1100 to 1300°C, a hot rolling step is performed to form a hot rolled steel sheet. The hot rolling step is: Rough rolling time when the thickness center temperature is above 1000°C: 200 seconds to 400 seconds, rough rolling end temperature: 1000～800°C, Start temperature of finishing rolling: 1000～800℃, finishing temperature of finishing rolling: 900～750℃; Next, perform a cooling step for the aforementioned hot-rolled steel sheet. This cooling step is a cooling step of 0.2s or more and less than 3.0s of the initial cooling step 10s from the beginning of cooling, at the center of the plate thickness. The average cooling rate: 4～25℃/s; Next, the aforementioned hot rolled steel sheet is subjected to a coiling step of coiling at a coiling temperature of 580°C or less to form a steel sheet, Next, a pipe making step is performed. This pipe making step involves welding the aforementioned steel plate into a cylindrical shape by cold roll forming, and forming it into a cylindrical shape with a longitudinal diameter/transverse diameter ratio of 0.99 to 1.01. , Shaped into an angular shape. The method of manufacturing a square steel pipe according to claim 5, wherein the cooling stop temperature in the cooling step is 580°C or lower. A building structure using the square steel pipe described in any one of claims 1 to 3.

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