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

Abstract

Provided are a square steel pipe having high strength, a low yield ratio, and excellent low-temperature toughness, a method for manufacturing the same, and a building structure having excellent earthquake resistance even in a low-temperature environment. Provided is a square steel pipe having flat plate parts and corner parts, the square steel pipe having a predetermined component composition, and containing, in a location at the center of the plate thickness of the flat plate parts, 80-95% ferrite by area ratio and a total of 5-20% by area ratio of at least one selected from the group consisting of pearlite, pseudo pearlite, and upper bainite, and having a microstructure in which the average crystal grain size df of ferrite is 5.0-20.0 µm, the average crystal grain size dp of pearlite and pseudo-pearlite is 3.0-18.0 µm, the ratio dp / df of dp to df is 0.60-1.00, and the total number density of pearlite and pseudo pearlite is 2,000 / mm2 or greater.

Description

Square steel pipe, its manufacturing method and building structure

[0001] The present invention relates to a square steel pipe, a manufacturing method thereof, and a building structure, and more particularly to a square steel pipe (square column) manufactured by cold roll forming, which has a low yield ratio and excellent low-temperature toughness. In particular, the present invention relates to a square steel pipe suitable for use as a structural member for large buildings. The present invention also relates to a building structure obtained using this square steel pipe.

[0002] In recent years, structural components used in large buildings such as factories, warehouses, and commercial facilities have become stronger in order to reduce construction costs through weight reduction. In particular, square steel pipes, which have flat and square sections and are used as pillars in buildings, require high strength in the flat sections. At the same time, square steel pipes used as structural components are also required to have a low yield ratio and excellent toughness from the perspective of earthquake resistance.

[0003] Square steel pipes are generally manufactured by cold forming hot-rolled steel sheets (hot-rolled steel strips). Cold forming methods include cold press bending and cold roll forming.

[0004] In the case of square steel pipes manufactured by roll-forming hot-rolled steel sheets (hereinafter sometimes referred to as "roll-formed square steel pipes"), the hot-rolled steel sheets are cold-rolled to form cylindrical open pipes, and the butt joints are electric resistance welded to form round steel pipes. The round steel pipes are then circumferentially reduced by a few percent using rolls positioned above, below, left, and right of the round steel pipes, and subsequently formed into square shapes to produce square steel pipes. On the other hand, in the case of square steel pipes manufactured by press-bending hot-rolled steel sheets (hereinafter sometimes referred to as "press-formed square steel pipes"), the hot-rolled steel sheets are cold-press-bent to form square or U-shaped cross sections, and these pipes are then joined by submerged arc welding to produce the pipes.

[0005] Compared to press-formed square steel pipes, roll-formed square steel pipes have the advantage of being more productive and capable of being manufactured in a shorter time. However, in press-formed square steel pipes, the flat plate is not subjected to cold forming and only the corners undergo work hardening, whereas in roll-formed square steel pipes, large processing strain is introduced in the axial direction along the entire pipe circumference, especially when cold-forming into a cylindrical shape. Due to this processing strain, roll-formed square steel pipes have the problem of a high yield ratio in the axial direction even in the flat plate, resulting in low toughness.

[0006] Furthermore, the thicker the wall of a roll-formed square steel pipe, the greater the work hardening during roll-forming, which results in a higher yield ratio and a lower toughness, making it more difficult to achieve a low yield ratio and high toughness in a thick-walled roll-formed square steel pipe.

[0007] In light of the above, there is a demand for a square steel pipe having high strength, a low yield ratio, and excellent toughness. For example, Patent Document 1 proposes a method in which a steel material is hot-rolled at a predetermined reduction ratio and a rolling finish temperature, coiled at a predetermined temperature, and the reduction in peripheral length during square shaping is set to three times the plate thickness or less. The method proposed in Patent Document 1 is said to be able to provide a square steel pipe having a yield ratio of 90% or less and a Charpy absorbed energy of 27 J or more at a test temperature of 0°C.

[0008] Patent Document 2 proposes a method in which a steel material is heated, rough-rolled, and finish-rolled under predetermined conditions, and then cooled in three stages. The method proposed in Patent Document 2 is said to be capable of producing a square steel pipe having a yield ratio of 80% or less and a Charpy absorbed energy of 150 J or more at a test temperature of 0°C.

[0009] Patent Document 3 proposes a method of heating, rough rolling, and finish rolling a predetermined steel material under predetermined conditions, followed by cooling, during which the cooling conditions, such as the time required for the temperature at the center of the plate thickness to reach 650° C., are controlled. The method proposed in Patent Document 3 is said to be capable of producing a square steel pipe having a yield ratio of 80% or less and a Charpy absorbed energy of 150 J or more at a test temperature of 0° C.

[0010] Patent Document 4 proposes a method for manufacturing a square steel pipe by controlling rolling conditions, cooling conditions, etc. According to the method proposed in Patent Document 4, it is possible to manufacture a square steel pipe having a Charpy absorbed energy of 27 J or more at a test temperature of 0°C.

[0011] Patent Document 5 proposes a method for controlling the roll gap of the sizing stand and the square forming stand immediately before square forming so that the circumferential length of the steel pipe at the entry and exit sides of the square forming stand and the width of the steel plate satisfy predetermined conditions when forming a steel plate into a square shape.The method proposed in Patent Document 5 is said to be capable of producing square steel pipes with a Charpy absorbed energy of 100 J or more at -10°C at a position 1 / 4t from the outer surface of the corner in the wall thickness direction.

[0012] Japanese Patent Application Laid-Open No. 9-87743 Japanese Patent Application Laid-Open No. 2012-153963 Japanese Patent Application Laid-Open No. 2012-132088 International Publication No. 2018 / 110152 International Publication No. 2022 / 075026

[0013] However, the square steel pipes manufactured by the methods disclosed in Patent Documents 1 to 4 have insufficient low-temperature toughness when used in a sub-freezing environment.

[0014] The technology proposed in Patent Document 5 also has room for improvement in terms of toughness at temperatures far below 0° C., such as −30° C. In addition, in order to appropriately control the roll gap when forming the steel plate, it is necessary to determine optimal manufacturing conditions for each dimension of the square steel pipe, which is a problem in that it requires a lot of effort.

[0015] The present invention has been made in consideration of the above-mentioned problems, and aims to provide a square steel pipe having high strength, a low yield ratio, and excellent low-temperature toughness, a manufacturing method thereof, and an architectural structure having excellent earthquake resistance even in low-temperature environments.

[0016] The gist of the present invention for solving the above problems is as follows.

[0017] 1. A square steel pipe having a flat portion and a corner portion, comprising, in mass%, C: 0.07% or more and 0.20% or less, Si: 0.01% or more and 0.40% or less, Mn: 0.20% or more and 1.20% or less, P: 0.100% or less, S: 0.050% or less, Ti: 0.005% or more and 0.035% or less, Al: 0.005% or more and 0.100% or less, and N: 0.0100% or less, with the balance consisting of Fe and unavoidable impurities, and having a chemical composition in which the ratio of the Mn content to the Ti content, %Mn / %Ti, is 30.0 or more and 150.0 or less, and at the center position of the thickness of the flat portion: Ferrite: 80 to 95% by area ratio, and one or more selected from the group consisting of pearlite, pseudo-pearlite, and upper bainite: 5 to 20% by total area ratio, and the average grain size d of ferrite ｆ is 5.0 to 20.0 μm, the average grain size d of pearlite and pseudo-pearlite ｐ is 3.0 to 18.0 μm, d ｆ d for ｐ The ratio d ｐ / d ｆ is 0.60 or more and 1.00 or less, and the total number density of pearlite and pseudo-pearlite is 2000 pieces / mm ２ A square steel pipe having a microstructure that is equal to or greater than the above.

[0018] 2. The square steel pipe according to 1 above, wherein the chemical composition further includes, in mass%, one or more elements selected from the group consisting of Nb: 0.020% or less, V: 0.10% or less, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.40% or less, Ni: 0.40% or less, Ca: 0.0100% or less, B: 0.0100% or less, Sn: 0.100% or less, and Sb: 0.100% or less.

[0019] 3. A method for manufacturing a square steel pipe according to 1 or 2, comprising: heating a steel material having the above-mentioned chemical composition to a heating temperature of 1100°C or more and 1300°C or less; hot-rolling the heated steel material to form a hot-rolled steel plate; cooling the hot-rolled steel plate under conditions of an average cooling rate at the center of thickness of 10.0°C / s or less and a cooling stop temperature at the center of thickness of 480°C or more and 650°C or less; coiling the cooled hot-rolled steel plate at a coiling temperature at the center of thickness of 480°C or more and 650°C or less; forming the coiled hot-rolled steel plate into a cylindrical shape by roll forming and then welding the ends to form a round steel pipe; and forming the round steel pipe into the square steel pipe, wherein the hot-rolling is carried out by sequentially carrying out rough rolling and finish rolling, The rough rolling is performed under the conditions of a rough rolling end temperature of 850°C or higher and 1150°C or lower, a time from the end of heating of the steel material to the end of the rough rolling of 10.0 minutes or less, and a total reduction rate at 930°C or higher of 60% or higher, the finish rolling is performed under the conditions of a finish rolling end temperature of 750°C or higher and 850°C or lower, and the hot rolling is performed under the conditions of a total reduction rate at 930°C or lower of 40% or higher.

[0020] 4. An architectural structure in which the square steel pipe according to 1 or 2 above is used as a pillar material.

[0021] According to the present invention, a square steel pipe having high strength, a low yield ratio, and excellent low-temperature toughness can be provided.

[0022] Fig. 1 is a schematic diagram showing a cross section perpendicular to the pipe axis direction of a square steel pipe according to the present invention. Fig. 2 is a perspective view showing an example of an architectural structure according to the present invention. Fig. 3 is a schematic diagram showing the positions at which tensile test specimens are taken. Fig. 4 is a schematic diagram showing the positions at which Charpy test specimens are taken.

[0023] The present invention will be described in detail below.

[0024] First, the square steel pipe of the present invention will be described with reference to the drawings. Fig. 1 is a schematic diagram showing a cross section perpendicular to the pipe axis direction of a square steel pipe 1 according to the present invention. The square steel pipe 1 has a flat portion 2 and a corner portion 3.

[0025] Next, the chemical composition of the square steel pipe of the present invention will be described. Unless otherwise specified, "%" indicating the chemical composition is "% by mass."

[0026] C: 0.07% or more and 0.20% or less C is an element that increases the strength of steel through solid solution strengthening. C also contributes to the formation of pearlite and pseudo-pearlite contained in the second phase. In order to ensure the desired strength and yield ratio, a C content of 0.07% or more is necessary. However, if the C content exceeds 0.20%, the proportion of the hard second phase increases, resulting in a decrease in low-temperature toughness and a high yield ratio, making it impossible to obtain the desired yield ratio. Furthermore, weldability also deteriorates. Therefore, the C content is set to 0.07% or more and 0.20% or less. The C content is preferably 0.08% or more. The C content is preferably 0.19% or less, more preferably 0.18% or less.

[0027] Si: 0.01% or more and 0.40% or less Si is an element that increases the strength of steel through solid solution strengthening. To achieve this effect, a Si content of 0.01% or more is necessary. However, if the Si content exceeds 0.40%, oxides are formed in the electric resistance weld, degrading the weld characteristics and ductility. Furthermore, the toughness of the base material other than the electric resistance weld is also reduced. For this reason, the Si content is set to 0.40% or less. The Si content is preferably 0.02% or more, more preferably 0.05% or more. The Si content is also preferably 0.37% or less, more preferably 0.35% or less.

[0028] Mn: 0.20% or more and 1.20% or less Mn is an element that increases the strength of steel by increasing solid solution strengthening and / or the amount of bainite formed. Furthermore, Mn contributes to microstructural refinement by lowering the ferrite transformation start temperature. To obtain the desired strength and microstructural structure, a Mn content of 0.20% or more is necessary. However, if the Mn content exceeds 1.20%, the amount of bainite formed becomes too large, making it difficult to achieve the desired yield ratio. Therefore, the Mn content is set to 0.20% or more and 1.20% or less. The Mn content is preferably 0.25% or more, more preferably 0.30% or more. The Mn content is preferably 1.10% or less, more preferably 1.05% or less.

[0029] P: 0.100% or less P segregates at grain boundaries, causing inhomogeneity in the material, so it is preferable to reduce it as much as possible, but a content of up to 0.100% is acceptable. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.030% or less, and more preferably 0.020% or less. There is no particular lower limit for the P content, and it may be 0, but excessive reduction leads to increased refining costs, so the P content is preferably 0.002% or more.

[0030] S: 0.0500% or less S is usually present in steel as MnS, which is thinly drawn during the hot rolling process and has a negative effect on ductility. For this reason, in the present invention, it is preferable to reduce the S content as much as possible, but a content of 0.0500% or less is acceptable. Therefore, the S content is set to 0.0500% or less. The S content is preferably 0.0150% or less, more preferably 0.0100% or less, and even more preferably 0.0080% or less. There is no particular lower limit for the S content, and it may be 0, but excessive reduction leads to increased refining costs, so the S content is preferably 0.0002% or more.

[0031] Ti: 0.005% or more and 0.035% or less Ti is an element that forms fine carbides and nitrides in steel and contributes to improving the strength of steel through precipitation strengthening. To achieve this effect, a Ti content of 0.005% or more is necessary. However, if the Ti content exceeds 0.035%, coarse carbides and nitrides are formed, resulting in a decrease in low-temperature toughness and ductility. Therefore, the Ti content is set to 0.005% or more and 0.035% or less. The Ti content is preferably 0.007% or more, more preferably 0.009% or more. The Ti content is also preferably 0.032% or less, more preferably 0.030% or less.

[0032] Al: 0.005% or more and 0.100% or less Al is an element that acts as a powerful deoxidizer. To achieve this effect, it is necessary to contain 0.005% or more of Al. However, if the Al content exceeds 0.100%, the weldability deteriorates, and alumina-based inclusions increase, deteriorating the surface properties. Furthermore, the toughness of the weld is also reduced. For this reason, the Al content is set to 0.005% or more and 0.100% or less. The Al content is preferably 0.010% or more, more preferably 0.015% or more. The Al content is also preferably 0.070% or less, more preferably 0.050% or less.

[0033] N: 0.0100% or less N is an element that has the effect of firmly fixing dislocation motion and thereby reducing toughness. For this reason, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, more preferably 0.0040% or less, and even more preferably 0.0035% or less. Note that excessive reduction leads to an increase in refining costs, and the inclusion of N may further increase strength, so the N content is preferably 0.0010% or more, and more preferably 0.0015% or more.

[0034] The ratio of the Mn content to the Ti content, %Mn / %Ti: 30.0 or more and 150.0 or less. If %Mn / %Ti is less than 30.0, the average grain size of ferrite and the second phase becomes coarse, resulting in reduced low-temperature toughness. If %Mn / %Ti exceeds 150.0, the yield ratio becomes high and the desired yield ratio cannot be obtained. Therefore, %Mn / %Ti is set to 30.0 or more and 150.0 or less. %Mn / %Ti is preferably 35.0 or more, more preferably 38.0 or more. %Mn / %Ti is preferably 145.0 or less, more preferably 140.0 or less.

[0035] In the above composition, the remaining components are Fe and unavoidable impurities. Examples of unavoidable impurities include As, Bi, Co, Pb, Zn, W, and O. These components may be unavoidably mixed in due to raw materials, materials, or manufacturing equipment. Examples of raw materials include iron ore, reduced iron, ferroalloy, and scrap. For example, the unavoidable impurities may include one or more selected from the group consisting of As: 0.05% or less, Co: 0.05% or less, Bi: 0.005% or less, Pb: 0.005% or less, Zn: 0.005% or less, O: 0.005% or less, and W: 0.100% or less.

[0036] The above-mentioned components are the basic composition of the square steel pipe of the present invention. The properties targeted by the present invention can be obtained with the above-mentioned essential elements, but the following elements can also be contained as necessary.

[0037] That is, the composition of the square steel pipe of the present invention may further include, as optional elements, one or more selected from the group consisting of Nb: 0.020% or less, V: 0.10% or less, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.40% or less, Ni: 0.40% or less, Ca: 0.0100% or less, B: 0.0100% or less, Sn: 0.100% or less, and Sb: 0.100% or less. The contents of these elements are calculated as the sum of the contents of intentionally added elements and the contents of unavoidably mixed elements.

[0038] Nb: 0.020% or less Nb is an element that forms fine carbides and nitrides in steel and contributes to further improving the strength of steel through precipitation strengthening, and can be contained as needed. To achieve this effect, when Nb is contained, the Nb content is preferably 0.005% or more, and more preferably 0.007% or more. However, if the Nb content exceeds 0.020%, the amount of upper bainite formed may increase. In addition, the yield ratio increases, making it impossible to obtain the desired yield ratio. Therefore, when Nb is contained, the Nb content is set to 0.020% or less. The Nb content is preferably 0.018% or less, and more preferably 0.016% or less.

[0039] V: 0.10% or less V is an element that improves the hardenability of steel and further increases the strength of steel, and can be contained as needed. To achieve this effect, when V is contained, the V content is preferably 0.01% or more, and more preferably 0.02% or more. However, if the V content exceeds 0.10%, ductility may decrease and low-temperature toughness may also decrease. Therefore, when V is contained, the V content is set to 0.10% or less. The V content is preferably 0.08% or less.

[0040] Cr: 0.50% or less Cr is an element that improves the hardenability of steel and further increases its strength, and can be contained as needed. To achieve this effect, when Cr is contained, the Cr content is preferably 0.01% or more, and more preferably 0.10%. However, if the Cr content exceeds 0.50%, the area ratio of the second phase may become excessively high. In addition, ductility may decrease. Furthermore, there is a risk of a decrease in low-temperature toughness. Therefore, when Cr is contained, the Cr content is set to 0.50% or less. The Cr content is preferably 0.40% or less.

[0041] Mo: 0.50% or less Mo is an element that improves the hardenability of steel and further increases the strength of steel, and can be contained as needed. To achieve this effect, when Mo is contained, the Mo content is preferably 0.01% or more, and more preferably 0.10% or more. However, if the Mo content exceeds 0.50%, ductility decreases and low-temperature toughness may also decrease. Therefore, when Mo is contained, the Mo content is set to 0.50% or less. The Mo content is preferably 0.40% or less.

[0042] Cu: 0.40% or less Cu is an element that further increases the strength of steel through solid solution strengthening and can be contained as needed. To achieve this effect, when Cu is contained, the Cu content is preferably 0.01% or more, and more preferably 0.10% or more. However, if the Cu content exceeds 0.40%, low-temperature toughness decreases, making it impossible to obtain the desired low-temperature toughness. Therefore, when Cu is contained, the Cu content is set to 0.40% or less. The Cu content is preferably 0.30% or less.

[0043] Ni: 0.40% or less Ni is an element that further increases the strength of steel through solid solution strengthening and can be contained as needed. To achieve this effect, when Ni is contained, the Ni content is preferably 0.01% or more, and more preferably 0.10% or more. However, if the Ni content exceeds 0.40%, the amount of upper bainite formed may be excessively large. In addition, the yield ratio becomes too high and the desired yield ratio cannot be obtained. Therefore, when Ni is contained, the Ni content is set to 0.40% or less. The Ni content is preferably 0.30% or less.

[0044] Ca: 0.0100% or less Ca is an element that contributes to further improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly drawn in the hot rolling process, and can be contained as needed. To achieve this effect, when Ca is contained, the Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more. However, when the Ca content exceeds 0.0100%, the ductility and low-temperature toughness decrease, making it impossible to obtain the desired low-temperature toughness. This is thought to be due to the formation of Ca oxide clusters. Therefore, when Ca is contained, the Ca content is set to 0.0100% or less. The Ca content is preferably 0.0050% or less.

[0045] B: 0.0100% or less B is an element that contributes to further refining the structure by lowering the ferrite transformation start temperature. To achieve this effect, when B is contained, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more. However, if the B content exceeds 0.0100%, the amount of upper bainite formed may increase, and the yield ratio may become too high to achieve the desired yield ratio. Therefore, when B is contained, the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less.

[0046] Sn: 0.100% or less Sn is an element that further increases the strength of steel through solid solution strengthening and can be contained as needed. To achieve this effect, when Sn is contained, the Sn content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more. However, if the Sn content exceeds 0.100%, toughness decreases and the desired low-temperature toughness cannot be obtained. In addition, hot workability decreases, which may cause cracking during hot rolling. Therefore, when Sn is contained, the Sn content is set to 0.100% or less. The Sn content is preferably 0.070% or less, more preferably 0.050% or less.

[0047] Sb: 0.100% or less Sb is an element that further increases the strength of steel through solid solution strengthening and can be contained as needed. To achieve this effect, when Sb is contained, the Sb content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more. However, if the Sb content exceeds 0.100%, toughness decreases and the desired low-temperature toughness cannot be obtained. In addition, hot workability decreases, which may cause cracks during hot rolling. Therefore, when Sb is contained, the Sb content is set to 0.100% or less. The Sb content is preferably 0.070% or less, more preferably 0.050% or less.

[0048] The components that can be optionally contained in the composition of the square steel pipe of the present invention have been described above.

[0049] Next, a preferred embodiment of the chemical composition of the square steel pipe of the present invention will be described.

[0050] (Total Content of Si, Cr, Cu, Ni, Sn, and Sb) In the following description, the total content of Si, Cr, Cu, Ni, Sn, and Sb is represented by A (unit: mass%) as shown in the following formula (1). Here, the content of components that are not contained is calculated as 0. A = %Si + %Cr + %Cu + %Ni + %Sn + %Sb (1)

[0051] The upper and lower limits of A are not particularly limited. However, when A is 0.20% or more, the yield strength and tensile strength can be further increased. Furthermore, when A is 1.00% or less, the yield strength decreases, making it easier to obtain a desired yield ratio. Therefore, A is preferably 0.20% or more and 1.00% or less. A is preferably 0.20% or more, more preferably 0.30% or more, and even more preferably 0.40% or more. A is preferably 1.00% or less, more preferably 0.95% or less, and even more preferably 0.90% or less.

[0052] Next, the microstructure of the square steel pipe of the present invention will be described.

[0053] In this specification, one or more structures selected from pearlite, pseudo-pearlite, and upper bainite may be referred to as the second phase.

[0054] First, the area ratio of each structure will be described. The area ratio of each structure can be measured by etching with nital a cross section taken from the center of the width of the flat plate portion 2 of the square steel pipe 1 (the width indicated by the arrow in FIG. 1 ) and the center of the plate thickness (center of the wall thickness) and parallel to the rolling direction, and observing the cross section. More specifically, the area ratio can be measured by the method described in the Examples.

[0055] Ferrite: 80-95% by area fraction Ferrite is a soft structure and is the main phase of the microstructure in order to obtain the desired yield strength and low yield ratio. Specifically, it is contained in the microstructure at an area fraction of 80% or more. If the area fraction of ferrite is less than 80%, the yield stress becomes excessively large and the desired yield ratio cannot be obtained. The area fraction of ferrite is preferably 82% or more. On the other hand, if the area fraction of ferrite exceeds 95%, the strength decreases and the desired yield strength and tensile strength cannot be obtained. Therefore, the area fraction of ferrite is 95% or less. The area fraction of ferrite is preferably 93% or less.

[0056] One or more selected from pearlite, pseudo-pearlite, and upper bainite: total area fraction of 5 to 20%. Pearlite, pseudo-pearlite, and upper bainite are harder structures than ferrite and are important for increasing the strength of steel and achieving a low yield ratio. Therefore, the total area fraction of one or more selected from pearlite, pseudo-pearlite, and upper bainite is set to 5% or more. If the total area fraction is less than 5%, the desired yield strength, tensile strength, or yield ratio cannot be obtained. The total area fraction is preferably 6% or more, and more preferably 8% or more. On the other hand, if the total area fraction exceeds 20%, low-temperature toughness deteriorates and the yield ratio increases. Therefore, the total area fraction is set to 20% or less. The total area fraction is preferably 18% or less, and more preferably 16% or less.

[0057] The contents of each of the structures of pearlite, pseudo-pearlite, and upper bainite are not particularly limited. For example, the lower limit of the total area ratio of pearlite and pseudo-pearlite is not particularly limited and may be 0%, or neither pearlite nor pseudo-pearlite may be contained. However, the total area ratio of pearlite and pseudo-pearlite is preferably 7% or more. When the total area ratio of pearlite and pseudo-pearlite is 7% or more, the yield ratio is further reduced, thereby achieving better seismic resistance. The total area ratio of pearlite and pseudo-pearlite is more preferably 8% or more. Furthermore, the upper limit of the total area ratio of pearlite and pseudo-pearlite is not particularly limited and may be 20% or less. However, when the total area ratio is 15% or less, excellent low-temperature toughness is more easily achieved. Therefore, the total area ratio of pearlite and pseudo-pearlite is preferably 15% or less, and more preferably 14% or less.

[0058] Furthermore, the upper limit of the area fraction of upper bainite is not particularly limited and may be 20% or less, but if it is 7% or less, a low yield ratio is more likely to be obtained, so the area fraction of upper bainite is preferably 7% or less. The area fraction of upper bainite is more preferably 5% or less. The lower limit of the area fraction of upper bainite is not particularly limited and may be 0%, or no upper bainite may be present.

[0059] The microstructure may be a microstructure consisting of ferrite and a second phase. However, it may further contain other structures (remaining structures). That is, the microstructure may be a microstructure consisting of ferrite, a second phase, and other structures. Examples of other structures include martensite, austenite, and lower bainite.

[0060] When other structures are contained, the area ratio (total area ratio) of the other structures is not particularly limited. However, for example, the presence of martensite or lower bainite may reduce toughness, and the presence of austenite may reduce tensile strength or increase yield ratio. Therefore, the lower the area ratio of the other structures, the better. Specifically, it is preferably 5% or less, more preferably 1% or less, and even more preferably no other structures are contained.

[0061] Next, we will explain the grain size. As described above, the square steel pipe of the present invention is made of dual-phase steel, which is a mixture of soft and hard phases, in order to obtain the desired yield ratio, yield strength, and tensile strength. However, dual-phase steel has inferior toughness compared to single-phase steel. Therefore, in the present invention, by controlling the grain size, it is possible to simultaneously achieve the desired strength, yield ratio, and toughness.

[0062] The grain size can be measured by etching with nital a cross section parallel to the rolling direction, taken from the center of the width (the width indicated by the arrow in FIG. 1 ) of the flat portion 2 of the square steel pipe 1 and the center of the plate thickness (center of the wall thickness), and observing the cross section. More specifically, the grain size can be measured by the method described in the Examples.

[0063] Average grain size of ferrite d ｆ If the average grain size of ferrite is less than 5.0 μm, the yield ratio increases and the desired yield ratio cannot be obtained. If the average grain size of ferrite is more than 20.0 μm, the tensile strength decreases, and the desired tensile strength cannot be obtained, or the low-temperature toughness decreases. Therefore, the average grain size d of ferrite is ｆ The average grain size of ferrite d is 5.0 to 20.0 μm. ｆ is preferably 6.0 μm or more, more preferably 6.5 μm or more. ｆ is preferably 19.0 μm or less, more preferably 18.5 μm or less.

[0064] Average grain size d of pearlite and pseudo-pearlite ｐIf the average grain size of pearlite and pseudo-pearlite is less than 3.0 μm, the yield ratio increases and the desired yield ratio cannot be obtained. If the average grain size of pearlite and pseudo-pearlite is more than 18.0 μm, the desired tensile strength cannot be obtained or the low-temperature toughness decreases. Therefore, the average grain size d of pearlite and pseudo-pearlite is ｐ The average grain size d of pearlite and pseudo-pearlite is 3.0 to 18.0 μm. ｐ is preferably 4.0 μm or more, more preferably 4.5 μm or more. ｐ is preferably 17.0 μm or less, more preferably 16.5 μm or less.

[0065] Average grain size of ferrite d ｆ The average grain size d of pearlite and pseudo-pearlite ｐ The ratio d ｐ / d ｆ : 0.60 or more and 1.00 or less In addition to the above average grain size, the average grain size d of ferrite ｆ and the average grain size d of pearlite and pseudo-pearlite ｐ The ratio (= d ｐ / d ｆ ) is defined. ｐ / d ｆ If d is less than 0.60, the desired yield ratio cannot be obtained. ｐ / d ｆ If d exceeds 1.00, the desired low-temperature toughness cannot be obtained. Therefore, in order to obtain a square steel pipe having the desired low-temperature toughness and yield ratio, ｐ / d ｆ is set to 0.60 or more and 1.00 or less. ｐ / d ｆ is preferably 0.62 or more, more preferably 0.65 or more. ｐ / d ｆ is preferably 0.97 or less, more preferably 0.95 or less.

[0066] Next, the number density of each structure will be described. The number density of each structure can be measured by determining the number of crystal grains per unit area using the same method as for the crystal grain size.

[0067] Total number density of pearlite and pseudo-pearlite: 2000 pieces / mm ２ In the above microstructure, pearlite and pseudo-pearlite are present at a total of 2000 particles / mm ２ If the total number density of pearlite and pseudo-pearlite in the microstructure is less than 2000 particles / mm, the toughness decreases and the desired low-temperature toughness cannot be obtained. ２ The total is preferably 2200 pieces / mm ２ More preferably, 2400 pieces / mm ２ Although there is no particular upper limit to the total number density, if pearlite and pseudo-pearlite are contained in excess, the yield ratio may increase, so the total number density of pearlite and pseudo-pearlite is set to 6000 particles / mm ２ Preferably, 5500 pieces / mm or less ２ The following is more preferred:

[0068] The above-described chemical composition and microstructure make it possible to obtain a square steel pipe having high strength, a low yield ratio, and excellent low-temperature toughness. The square steel pipe of the present invention can be suitably used as a structural member for buildings in cold regions where the ambient temperature falls below freezing or as a structural member for refrigerated warehouses, and can ensure excellent earthquake resistance. The following describes the preferred mechanical properties of the flat plate portion of the square steel pipe of the present invention.

[0069] (Yield Strength) The square steel pipe of the present invention has a high yield strength due to the above-described composition and microstructure. The yield strength is preferably 295 MPa or more, more preferably 320 MPa or more. The upper limit of the yield strength is not particularly limited, but may be, for example, 470 MPa or less. The yield strength is the yield strength in the axial direction of the pipe, and can be measured specifically by the method described in the Examples.

[0070] (Tensile strength) The square steel pipe of the present invention has high tensile strength due to the above-mentioned composition and microstructure. The tensile strength is preferably 400 MPa or more, more preferably 420 MPa or more. The upper limit of the tensile strength is not particularly limited, but may be, for example, 620 MPa or less or 500 MPa or less. The tensile strength is the tensile strength in the pipe axial direction, and can be measured specifically by the method described in the Examples.

[0071] (Yield Ratio) The square steel pipe of the present invention has a low yield ratio due to the above-mentioned chemical composition and microstructure. The yield ratio is preferably 0.90 or less, and more preferably 0.87 or less. The lower limit of the yield ratio is not particularly limited, but may be, for example, 0.65 or more, or 0.70 or more. The yield ratio is defined as (yield strength) / (tensile strength).

[0072] (Charpy absorbed energy) As a result of having the above-described chemical composition and microstructure, the square steel pipe of the present invention has high Charpy absorbed energy at low temperatures. The Charpy absorbed energy at a test temperature of -30°C is preferably 27 J or more, more preferably 47 J or more, and even more preferably more than 100 J. The upper limit of the Charpy absorbed energy at a test temperature of -30°C is not particularly limited, but may be, for example, 250 J. Furthermore, the Charpy absorbed energy at a test temperature of -35°C is preferably 27 J or more. The upper limit of the Charpy absorbed energy at a test temperature of -35°C is also not particularly limited, but may be, for example, 250 J. The Charpy absorbed energy can be measured by the method described in the Examples.

[0073] (Ductility-to-brittle transition temperature) As a result of having the above-mentioned component composition and microstructure, the square steel pipe of the present invention has a low ductility-to-brittle transition temperature. The ductility-to-brittle transition temperature is preferably −30°C or lower, more preferably −35°C or lower, and even more preferably −50°C or lower. There is no particular restriction on the lower limit of the ductility-to-brittle transition temperature, but it may be, for example, −70°C or higher. The ductility-to-brittle transition temperature can be measured by the method described in the examples.

[0074] Next, the dimensions of the square steel pipe of the present invention will be explained.

[0075] The wall thickness (thickness of the flat portion) of the square steel pipe is not particularly limited, but is preferably 12 mm or more. Furthermore, according to the present invention, even thick square steel pipes with wall thicknesses exceeding 20 mm have a low yield ratio and excellent low-temperature toughness. Therefore, the wall thickness of the square steel pipe may be greater than 20 mm. The upper limit of the wall thickness of the square steel pipe is also not particularly limited, but may be, for example, 32 mm or less.

[0076] The length of one side of the square steel pipe is not particularly limited. The square steel pipe of the present invention is not limited to a square steel pipe whose sides are all equal (i.e., the value of (long side length / short side length) is 1.0), and the value of (long side length / short side length) may be greater than 1.0. However, if the value of (long side length / short side length) of the square steel pipe exceeds 2.5, local buckling is likely to occur on the long side, reducing the compressive strength in the axial direction of the pipe. Therefore, the value of (long side length / short side length) of the square steel pipe is preferably 1.0 or more and 2.5 or less. The value of (long side length / short side length) is more preferably 1.0 or more and 2.0 or less.

[0077] Next, an embodiment of the method for manufacturing a square steel pipe of the present invention will be described.

[0078] The method for manufacturing a square steel pipe of the present invention includes heating a steel material having the above-mentioned chemical composition, hot-rolling the heated steel material to form a hot-rolled steel plate, cooling the hot-rolled steel plate, coiling the cooled hot-rolled steel plate, forming the coiled hot-rolled steel plate into a cylindrical shape by roll forming and then welding the ends to form a round steel pipe, and forming the round steel pipe to form the square steel pipe.

[0079] The conditions for each step are explained below. In the following explanation of the manufacturing method, the temperature indicated in "°C" refers to the surface temperature of the steel material or steel plate unless otherwise specified. The surface temperature can be measured using a radiation thermometer or the like. The temperature at the center of the thickness of the steel plate can be determined by calculating the temperature distribution in the cross section of the steel plate by heat transfer analysis and correcting the result by the surface temperature of the steel plate. Furthermore, "hot-rolled steel plate" includes hot-rolled steel strip.

[0080] (Steel Raw Material) Any steel raw material can be used as long as it has the above-mentioned component composition. For example, a steel slab can be used as the steel raw material. The method for producing molten steel is not particularly limited, and any of methods such as converters, electric furnaces, and vacuum melting furnaces can be used. The molten steel may further be subjected to secondary refining such as ladle refining. Furthermore, the casting method is not particularly limited, and a steel raw material having the desired dimensions can be obtained by a casting method such as continuous casting. Instead of continuous casting, an ingot-blooming rolling method may be used.

[0081] (Heating) First, the steel material is heated. The heating method is not particularly limited, and may be a conventional method in which the obtained steel material is cooled to room temperature and then reheated, or from the viewpoint of energy saving, direct rolling may be used in which the steel material is charged into a heating furnace as a hot slab without being cooled to room temperature.

[0082] Heating temperature: 1100°C or higher and 1300°C or lower If the heating temperature is lower than 1100°C, the deformation resistance of the steel material to be rolled increases, making rolling difficult. On the other hand, if the heating temperature exceeds 1300°C, the austenite grains become coarse, making it difficult to obtain fine austenite grains in the subsequent hot rolling. As a result, it becomes difficult to obtain a fine microstructure at the center of the thickness of the hot-rolled steel sheet. Therefore, the heating temperature when heating the steel material is set to 1100°C or higher and 1300°C or lower. The heating temperature is preferably 1120°C or higher, more preferably 1140°C or higher. The heating temperature is also preferably 1280°C or lower, more preferably 1260°C or lower.

[0083] (Hot Rolling) Next, the heated steel material is hot rolled to form a hot rolled steel sheet. Hot rolling is performed by sequentially carrying out rough rolling and finish rolling.

[0084] Rough rolling end temperature: 850°C or higher and 1150°C or lower If the rough rolling end temperature is lower than 850°C, the surface temperature of the steel sheet will be lower than the ferrite transformation start temperature during the subsequent finish rolling, a large amount of ferrite will be generated, and the yield strength and tensile strength will decrease. On the other hand, if the rough rolling end temperature exceeds 1150°C, fine austenite grains will not be obtained, making it difficult to ensure the average crystal grain size of the microstructure. For this reason, the rough rolling end temperature is set to 850°C or higher and 1150°C or lower. The rough rolling end temperature is preferably 860°C or higher, more preferably 870°C or higher. Furthermore, the rough rolling end temperature is preferably 1100°C or lower, more preferably 1050°C or lower.

[0085] Time from the end of heating the steel material to the end of rough rolling: 10.0 minutes or less If the time from the end of heating the steel material to the end of rough rolling exceeds 10.0 minutes, even if large reduction rolling is performed in the subsequent finish rolling, the austenite will not be sufficiently refined. Furthermore, pearlite and pseudo-pearlite cannot be refined compared to ferrite. As a result, low-temperature toughness will be reduced. Therefore, the time from the end of heating the steel material to the end of rough rolling is set to 10.0 minutes or less. This time is preferably 8.0 minutes or less, more preferably 7.0 minutes or less, and even more preferably 5.0 minutes or less. The lower limit of this time is not limited, but may be, for example, 1.0 minute or more.

[0086] Total reduction rate at 930°C or higher in rough rolling: 60% or more If the total reduction rate at 930°C or higher in rough rolling is less than 60%, the austenite is not sufficiently refined, and even if large reduction rolling is performed in finish rolling, fine pearlite and pseudo-pearlite cannot be obtained. This results in a decrease in low-temperature toughness. Therefore, the total reduction rate at 930°C or higher is 60% or more. The total reduction rate is preferably 65% ​​or more, more preferably 68% or more. Note that there is no particular upper limit for the total reduction rate, but if the reduction rate is too large, excessive load is placed on the manufacturing equipment for hot rolling, making manufacturing difficult, so the total reduction rate is preferably 85% or less.

[0087] Here, when a part of the rough rolling is carried out at 930°C or less, the total reduction is calculated excluding the part of the rough rolling, and in other cases, the total reduction in the rough rolling is calculated as the total reduction. That is, the total reduction in the rough rolling at 930°C or more is calculated as follows: when the rough rolling finish temperature is 930°C or less, (t ｉ -t ｆ ) / t ｉ × 100, otherwise (t ｉ -t ｒ ) / t ｉ × 100, where t ｉ : Plate thickness before rough rolling, t ｆ : Plate thickness at 930 ° C, t ｒ : Plate thickness at the end of rough rolling.

[0088] Finish rolling end temperature: 750°C or higher and 850°C or lower If the finish rolling end temperature is lower than 750°C, the surface temperature of the steel sheet will fall below the ferrite transformation start temperature during finish rolling, resulting in the formation of ferrite elongated in the rolling direction, which may result in reduced workability. On the other hand, if the finish rolling end temperature exceeds 850°C, fine austenite grains will not be obtained, and the crystal grains will become coarse, making it difficult to ensure the desired strength. For this reason, the finish rolling end temperature is set to 750°C or higher and 850°C or lower. The finish rolling end temperature is preferably 770°C or higher, more preferably 780°C or higher. Furthermore, the finish rolling end temperature is preferably 830°C or lower, more preferably 820°C or lower.

[0089] Total reduction ratio of hot rolling at 930°C or less: 40% or more In the present invention, by refining subgrains in austenite during hot rolling, the ferrite, pearlite, and pseudo-pearlite formed during subsequent cooling and coiling are refined, thereby enabling the production of a hot-rolled steel sheet with desired strength and low-temperature toughness. In order to refine the subgrains in austenite during hot rolling, it is necessary to increase the reduction ratio in the austenite non-recrystallization temperature range and introduce sufficient working strain. If the total reduction ratio at 930°C or less is less than 40%, the rolling in the austenite non-recrystallization temperature range becomes insufficient, resulting in large grain sizes of ferrite, pearlite, and pseudo-pearlite, resulting in a decrease in low-temperature toughness. Therefore, the total reduction ratio of hot rolling at 930°C or less is set to 40% or more. The total reduction ratio is preferably 42% or more, and more preferably 45% or more. The upper limit of the total rolling reduction is not particularly limited, but if the rolling reduction is too large, an excessive load is placed on the manufacturing equipment for hot rolling, making manufacturing difficult, so the total rolling reduction is preferably 60% or less.

[0090] The total rolling reduction is calculated by comparing the thickness at 930°C with the thickness after hot rolling. ｆ -t ｏ ) / t ｆ × 100, where t ｆ : Plate thickness at 930 ° C, t ｏ : The thickness of the hot-rolled steel sheet after hot rolling.

[0091] (Cooling) Next, the hot-rolled steel sheet obtained by hot rolling is cooled. Examples of cooling methods include water cooling by spraying water from a nozzle, and cooling by spraying cooling gas. Here, it is preferable to perform the cooling treatment on both sides of the hot-rolled steel sheet so that both sides are cooled under the same conditions. Furthermore, from the viewpoint of preventing the crystal grain size from becoming coarse, it is preferable to start cooling immediately after the end of finish rolling.

[0092] Average cooling rate at the center of the thickness: 10.0°C / s or less. If the average cooling rate exceeds 10.0°C / s at the temperature at the center of the thickness, the amount of upper bainite produced increases, and the desired average grain size cannot be obtained. As a result, the desired yield ratio cannot be obtained. Therefore, the average cooling rate at the center of the thickness is set to 10.0°C / s or less. The average cooling rate is preferably 9.5°C / s or less, and more preferably 9.0°C / s or less. Although there is no particular limitation on the lower limit of the average cooling rate, setting the average cooling rate to 1.0°C / s or more makes it easier to keep the cooling stop temperature within the range described below. Therefore, the average cooling rate is preferably 1.0°C / s or more, more preferably 1.5°C / s or more, and even more preferably 2.0°C / s or more.

[0093] In order to control the average cooling rate, it is preferable to adjust the amount, pressure, spray time, spray angle, and conveying speed of the hot-rolled steel sheet of cooling water or cooling gas, etc. In order to control the average cooling rate, a heat transfer analysis may be performed in advance to determine the conditions for the cooling treatment of the hot-rolled steel sheet, and then the cooling may be performed based on those conditions.

[0094] The average cooling rate is calculated by ((temperature at the center of thickness of the hot-rolled steel sheet when cooling starts - temperature at the center of thickness of the hot-rolled steel sheet when cooling stops) / cooling time).

[0095] Cooling stop temperature at the center of the thickness: 480°C or higher and 650°C or lower. If the cooling stop temperature at the center of the thickness is lower than 480°C, the amount of upper bainite produced increases, resulting in an excessively high area ratio of the second phase, and the desired yield ratio cannot be obtained. Furthermore, temperature unevenness is likely to occur in the length and / or width directions of the hot-rolled steel sheet during cooling, which may result in variations in mechanical properties. On the other hand, if the cooling stop temperature at the center of the thickness exceeds 650°C, the ferrite grains become coarse, and the desired grain size cannot be obtained. Therefore, the cooling stop temperature at the center of the thickness is set to 480°C or higher and 650°C or lower. The cooling stop temperature is preferably 490°C or higher, more preferably 495°C or higher. The cooling stop temperature is preferably 640°C or lower, more preferably 635°C or lower.

[0096] (Coiling) Next, the cooled hot-rolled steel sheet is coiled.

[0097] Coiling temperature at the center of the thickness: 480°C or higher and 650°C or lower If the coiling temperature at the center of the thickness is lower than 480°C, a large amount of upper bainite is formed on the surface of the steel sheet, and the desired yield ratio cannot be obtained. If the coiling temperature at the center of the thickness exceeds 650°C, the ferrite grains become coarse, and the desired average crystal grain size cannot be obtained. Therefore, the coiling temperature at the center of the thickness is set to 480°C or higher and 650°C or lower. The coiling temperature is preferably 490°C or higher, and more preferably 495°C or higher. The coiling temperature is also preferably 640°C or lower, and more preferably 635°C or lower.

[0098] After coiling, the hot-rolled steel sheet may be allowed to cool naturally.

[0099] The square steel pipe of the present invention is made from the above-mentioned hot-rolled steel sheet. The square steel pipe of the present invention can be manufactured, for example, by forming the above-mentioned hot-rolled steel sheet into a round steel pipe and then forming the round steel pipe. The round steel pipe can be obtained by rolling the hot-rolled steel sheet (typically cold rolling) into a cylindrical open pipe and welding the ends of the hot-rolled steel sheet (the butt joints of the open pipe). The square steel pipe can then be formed into a square steel pipe using rolls arranged above, below, left, and right of the round steel pipe. That is, the square steel pipe of the present invention may be a roll-formed square steel pipe.

[0100] According to the above-mentioned manufacturing method, when a square steel pipe is obtained by forming a hot-rolled steel sheet into a square steel pipe, there is no need to specially control the roll gap of a sizing stand or a square forming stand, and the square steel pipe can be manufactured efficiently.

[0101] (Architectural structure) Figure 2 is a schematic diagram showing an example of an architectural structure of the present invention. In the architectural structure of the present invention, the square steel pipe of the present invention described above (square steel pipe 1) is used as a column material. Reference numerals 4, 5, 6, and 7 represent the main beam, sub-beam, diaphragm, and stud, respectively. As described above, the square steel pipe of the present invention has excellent mechanical properties in the flat plate portion. Therefore, the architectural structure of the present invention exhibits excellent seismic performance.

[0102] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0103] Molten steel having the chemical composition shown in Table 1 was cast to form a steel slab. In the table, %Mn / %Ti represents the ratio of the Mn content (unit: mass%) to the Ti content (unit: mass%). A represents the total content (unit: mass%) of Si, Cr, Cu, Ni, Sn, and Sb. The obtained steel slab was heated, hot rolled, cooled, and coiled under the process conditions shown in Table 2 to obtain a hot-rolled steel sheet. In Table 2, the average cooling rate, cooling stop temperature, and coiling temperature are shown as temperatures at the center of the sheet thickness.

[0104] The obtained hot-rolled steel sheet was then rolled (cold rolled) into a cylindrical open pipe, and the butt joints were electric resistance welded to form a round steel pipe. The round steel pipe was then formed using rolls arranged above, below, left, and right to form a square steel pipe that was square in cross section perpendicular to the pipe axis. The obtained square steel pipe had a corner portion and a flat portion, and had the side length and wall thickness (plate thickness of the flat portion) shown in Table 2. The wall thickness of the square steel pipe was the same as the plate thickness of the hot-rolled steel sheet used.

[0105]

[0106]

[0107] Test specimens were taken from the square steel pipes manufactured by the above method, and the following microstructure observations, tensile tests, and Charpy impact tests were carried out.

[0108] [Structural Observation] The microstructure of the flat plate portion of the square steel pipe was observed at the center of the plate thickness. The test specimen for structural observation was taken from the center of the plate width and the center of the plate thickness (position of plate thickness t / 2 (t: plate thickness of the flat plate portion)) at the 3 o'clock side when the weld of the square steel pipe is in the 12 o'clock direction. The observation surface was set to be a cross section in the rolling direction during hot rolling, polished, and then etched with nital to prepare the observation surface. The structural observation was performed using an optical microscope image taken using an optical microscope (magnification: 1000x).

[0109] First, the area ratios of ferrite, pearlite, pseudo-pearlite, and upper bainite were determined from the obtained optical microscope images.

[0110] The area fraction of each structure was calculated as the average of the values ​​obtained from five visual fields. Here, the area fraction obtained by structural observation was used as the area fraction of each structure. Here, ferrite is a product of diffusion transformation and exhibits a nearly recovered structure with low dislocation density. This includes polygonal ferrite and pseudo-polygonal ferrite. Pearlite is a structure in which cementite and ferrite are arranged in layers. Pseudo-pearlite is a structure in which cementite is observed arranged in a dotted row within ferrite. Upper bainite is a complex phase structure of lath-shaped ferrite and cementite with a high dislocation density. In addition to the above shapes, the distinction was made based on the fact that ferrite is white, pearlite is black, pseudo-pearlite is black or gray, and upper bainite is white or gray. Note that none of the examples contained structures other than ferrite, pearlite, pseudo-pearlite, and upper bainite.

[0111] In addition, the average grain sizes of ferrite, pearlite, and pseudo-pearlite were measured from the optical microscope images. Images of ferrite, pearlite, and pseudo-pearlite were separated from the optical microscope images using Fiji, an image processing package of the free software ImageJ, and the average grain sizes of ferrite, pearlite, and pseudo-pearlite were calculated in terms of circle-equivalent diameter. The grain sizes of five fields of view were calculated for each square steel pipe, and the average value was used as the average grain size of each structure in each square steel pipe. In the grain size analysis, grains with a grain size of less than 2.0 μm were excluded from the analysis as measurement noise.

[0112] Furthermore, the number density (unit: pieces / mm ２ The number of pearlite and pseudo-pearlite particles present in the five fields of view observed was totaled, and the observed area (unit: mm ２ The number of pearlite and pseudo-pearlite particles is the total number of pearlite and pseudo-pearlite particles having an equivalent circle diameter of 2.0 μm or more.

[0113] [Tensile Test] The tensile test was conducted in accordance with the provisions of JIS Z 2241 (2011). FIG. 3 is a schematic diagram showing the sampling position of the tensile test specimen (JIS No. 5 tensile test specimen) used in the tensile test. As shown in FIG. 3, the tensile test specimen was sampled from the center of the width of the flat portion at the 3 o'clock side when the weld of the square steel pipe was in the 12 o'clock direction, so that the tensile direction was parallel to the pipe axis direction. A tensile test was conducted on the sampled tensile test specimen to measure the yield strength (YS) and tensile strength (TS), and the yield ratio defined as (yield strength) / (tensile strength) was calculated. Two test specimens were used, and their average values ​​were calculated to determine the YS, TS, and yield ratio. Regarding strength, a yield strength of 295 MPa or more and a tensile strength of 400 MPa or more were considered to be acceptable. Regarding yield ratio, a yield ratio of 0.90 or less was considered to be acceptable.

[0114] [Charpy Impact Test] The Charpy impact test was conducted in accordance with the provisions of JIS Z 2242 (2018). Figure 4 is a schematic diagram showing the sampling position of the Charpy test specimen (V-notch standard test specimen) used in the Charpy impact test. As shown in Figure 4, the Charpy test specimen was sampled from the width center of the flat portion at the 3 o'clock side when the weld of the square steel pipe was positioned at 12 o'clock, so that the longitudinal direction of the test specimen was parallel to the pipe axis. Here, the Charpy test specimen was sampled from the wall thickness t / 4 position (a position 1 / 4 of the wall thickness t from the outer surface of the square steel pipe) for square steel pipes with a wall thickness of more than 19 mm, and from the wall thickness t / 2 position (a position 1 / 2 of the wall thickness t from the outer surface of the square steel pipe) for square steel pipes with a wall thickness of 19 mm or less. The Charpy impact test was conducted at test temperatures of -70°C, -50°C, -30°C, -10°C, and 10°C. The test was carried out on three specimens at each test temperature, and the average values ​​of the ductile-brittle transition temperature and Charpy absorbed energy were calculated. For low-temperature toughness, a specimen was deemed to have passed if its Charpy absorbed energy at a test temperature of -30°C was 27 J or more and its ductile-brittle transition temperature was -30°C or less.

[0115] The results obtained are shown in Table 3.

[0116]

[0117] In Table 3, square steel pipes Nos. 1 to 19, 46, and 47 are examples of the present invention, and square steel pipes Nos. 20 to 45, 48, and 49 are comparative examples.

[0118] As shown in Table 3, all of the square steel pipes of the present invention met the acceptable standards for strength, yield ratio, and low-temperature toughness. Furthermore, square steel pipes No. 2, 6, 7, 8, 12, 14, 15, 16, and 19 were manufactured under the following conditions: a total reduction of 68% or more at 930°C or higher during rough rolling, and a time from the end of heating to the end of rough rolling of 7.0 minutes or less. These square steel pipes had Charpy absorbed energy at -30°C of more than 100 J and ductile-brittle transition temperatures of -50°C or lower, demonstrating particularly excellent low-temperature toughness.

[0119] In contrast, Comparative Examples Nos. 20 to 35 did not have the above-mentioned chemical composition. Comparative Examples Nos. 36 to 45, 48, and 49 did not satisfy the above-mentioned manufacturing conditions and did not have the above-mentioned microstructure. Therefore, Comparative Examples Nos. 20 to 45, 48, and 49 were unable to produce square steel pipes with high strength, low yield ratios, and excellent low-temperature toughness.

[0120] For example, in Comparative Example No. 40 (Steel A), the time from the end of heating to the end of rough rolling exceeded the range of the present invention. As a result, the austenite became coarse, and the average crystal grain size of the structure fell outside the range of the present invention. As a result, the Charpy absorbed energy and ductile-brittle transition temperature of the flat plate at -30°C did not meet the criteria for acceptance.

[0121] 1 Square steel pipe 2 Flat plate portion 3 Corner portion 4 Large beam 5 Small beam 6 Diaphragm 7 Stud

Claims

1. A square steel pipe having a flat portion and a corner portion, containing, by mass%, C: 0.07% to 0.20%, Si: 0.01% to 0.40%, Mn: 0.20% to 1.20%, P: 0.100% or less, S: 0.050% or less, Ti: 0.005% to 0.035%, Al: 0.005% to 0.100%, and N: 0.0100% or less, with the balance consisting of Fe and unavoidable impurities, and having a chemical composition in which the ratio of the Mn content to the Ti content, %Mn / %Ti, is 30.0 to 150.0, and at the center position of the thickness of the flat portion, Ferrite: 80 to 95% by area ratio, and one or more selected from the group consisting of pearlite, pseudo-pearlite, and upper bainite: 5 to 20% by total area ratio, and the average grain size d of ferrite ｆ is 5.0 to 20.0 μm, the average grain size d of pearlite and pseudo-pearlite ｐ is 3.0 to 18.0 μm, d ｆ d for ｐ The ratio d ｐ / d ｆ is 0.60 or more and 1.00 or less, and the total number density of pearlite and pseudo-pearlite is 2000 pieces / mm ２ A square steel pipe having a microstructure that is equal to or greater than the above.

2. A square steel pipe as described in claim 1, wherein the chemical composition further includes, in mass %, one or more elements selected from the group consisting of Nb: 0.020% or less, V: 0.10% or less, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.40% or less, Ni: 0.40% or less, Ca: 0.0100% or less, B: 0.0100% or less, Sn: 0.100% or less, and Sb: 0.100% or less.

3. A method for manufacturing a square steel pipe according to claim 1 or 2, comprising: heating a steel material having the above-mentioned chemical composition to a heating temperature of 1100°C or more and 1300°C or less; hot-rolling the heated steel material to form a hot-rolled steel plate; cooling the hot-rolled steel plate under conditions of an average cooling rate at the center of thickness of 10.0°C / s or less and a cooling stop temperature at the center of thickness of 480°C or more and 650°C or less; coiling the cooled hot-rolled steel plate at a coiling temperature at the center of thickness of 480°C or more and 650°C or less; forming the coiled hot-rolled steel plate into a cylindrical shape by roll forming and then welding the ends to form a round steel pipe; and forming the round steel pipe into the square steel pipe, wherein the hot-rolling is carried out by sequentially carrying out rough rolling and finish rolling, The rough rolling is performed under the conditions of a rough rolling end temperature of 850°C or higher and 1150°C or lower, a time from the end of heating of the steel material to the end of the rough rolling of 10.0 minutes or less, and a total reduction rate at 930°C or higher of 60% or higher, the finish rolling is performed under the conditions of a finish rolling end temperature of 750°C or higher and 850°C or lower, and the hot rolling is performed under the conditions of a total reduction rate at 930°C or lower of 40% or higher.

4. An architectural structure in which the square steel pipe according to claim 1 or 2 is used as a pillar material.

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