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

Abstract

Provided is a square steel pipe having a low yield ratio and excellent low-temperature toughness and ductility.　The square steel pipe according to the present invention has a prescribed steel component composition. The flat plate section of the pipe has, at the central area thereof in the plate thickness, a steel structure in which: ferrite is the main phase; the area ratio of a second phase, which is composed of one or more selected from perlite, pseudo-pearlite, and upper bainite, is 5-30%; and the average crystalline particle diameter is 10.0-30.0 μm. The steel structure includes at least 500 pieces, per mm2, of pearlite and pseudo-pearlite having an equivalent circle diameter of 2 μm to 20 μm. The yield strength of the flat plate section is 295 MPa or more, the tensile strength of the flat plate section is 400 MPa or more, the yield ratio of the flat plate section is 0.85 or less, the uniform elongation of the flat plate section is 8.0% or more, and the elongation at break of the flat plate section is 27.0 or more. The Charpy absorption energy at -10°C of a corner section is 80 J or more, and the ductile-to-brittle transition temperature of the corner section is -10°C or less.

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. In particular, the present invention relates to a square steel pipe (square column) manufactured by cold roll forming, which has a low yield ratio, excellent low-temperature toughness, and high ductility. 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 (hereinafter referred to as "buildings") have been increasing in strength 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 for 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, excellent low-temperature toughness, and high ductility from the perspective of earthquake resistance.

[0003] Square steel pipes are generally manufactured from hot-rolled steel sheets (hot-rolled steel strips) or thick steel plates by cold forming the material, which can be done by cold press bending or cold roll forming.

[0004] Square steel pipes (hereinafter sometimes referred to as "roll-formed square steel pipes") are manufactured by roll-forming a raw material. Hot-rolled steel sheets are cold-rolled to form cylindrical open pipes, and the butt joints are then electric resistance welded to form round steel pipes. Then, rolls positioned above, below, left, and right of the round steel pipe are used to reduce the round steel pipe by a few percent in the axial direction, and the round steel pipe is subsequently formed into a square shape to produce a square steel pipe. On the other hand, square steel pipes (hereinafter sometimes referred to as "press-formed square steel pipes") are manufactured by press-bending a raw material. Thick steel plates are cold-press-bent to form a square or U-shaped cross section. These pipes are then joined by submerged arc welding.

[0005] The manufacturing method of roll-formed square steel pipes has the advantage of being more productive and capable of being manufactured in a shorter period of time than the manufacturing method of press-formed square steel pipes. However, in press-formed square steel pipes, the flat portion 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 around the entire steel pipe, 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 and low toughness, even in the flat portion. Because the processing strain introduced in the corners of roll-formed square steel pipes is greater than in the flat portion, the corners exhibit a higher yield ratio and lower toughness than the flat portion.

[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. Therefore, when manufacturing thick-walled roll-formed square steel pipes, it is necessary to select a material that can withstand the increase in yield ratio and the decrease in toughness caused by roll forming.

[0007] In response to such demands, for example, Patent Document 1 proposes a technology for manufacturing 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, by hot rolling a steel material containing, by weight, 0.20% or less C and one or two of 0.40 to 0.90% Mn, 0.005 to 0.040% Nb, and 0.005 to 0.050% Ti into a coil at a reduction rate of 55% or more in a non-recrystallization temperature range, a rolling finish temperature of 730 to 830°C, and a coiling temperature of 550°C or less, and by setting the reduction in circumference in the steel pipe forming process to three times the plate thickness or less.

[0008] Patent Document 2 describes a method for heating a steel containing, by mass%, 0.07 to 0.18% C and 0.3 to 1.5% Mn to a heating temperature of 1100 to 1300°C, followed by rough rolling at a rough rolling end temperature of 1150 to 950°C, and finish rolling at a finish rolling start temperature of 1100 to 850°C and a finish rolling end temperature of 900 to 750°C. The steel is then subjected to primary cooling, in which the steel is cooled to a surface temperature with a cooling end temperature of 550°C or higher, and air-cooled for 3 to 15 seconds. A technology has been proposed for manufacturing square steel pipes that exhibit a low yield ratio of 80% or less and have mechanical properties such as an absorbed energy of 150 J or more in a Charpy impact test at a test temperature of 0°C, by performing secondary cooling, in which the temperature is cooled to 650°C or less at an average cooling rate of 4 to 15°C / s in the temperature range of 750 to 650°C at the center of the plate thickness, and tertiary cooling, in which the temperature is cooled to 650°C or less at an average cooling rate of 4 to 15°C / s in the temperature range of 750 to 650°C at the center of the plate thickness, and by setting the value of the frequency of second phases contained in the steel structure to 0.20 to 0.42.

[0009] Patent Document 3 proposes a technique for producing a square steel pipe having mechanical properties such as a low yield ratio of 80% or less and an absorbed energy of 150 J or more in a Charpy impact test at a test temperature of 0 ° C. The square steel pipe is then heated to a heating temperature of 1100 to 1300 ° C., followed by rough rolling with a rough rolling end temperature of 1150 to 950 ° C. and finish rolling with a finish rolling start temperature of 1100 to 850 ° C. and a finish rolling end temperature of 900 to 750 ° C., and then cooled to a coiling temperature of 500 to 650 ° C. so that the average cooling rate in the temperature range of 750 to 650 ° C. at the surface is 20 ° C. / s or less, the time until the temperature at the center of the plate thickness reaches 650 ° C. is 35 seconds or less, and the average cooling rate in the temperature range of 750 to 650 ° C. at the center of the plate thickness is 4 to 15 ° C. / s.

[0010] Patent Document 4 discloses a steel having a composition containing, by mass%, C: 0.07 to 0.20%, Mn: 0.3 to 2.0%, P: 0.03% or less, S: 0.015% or less, Al: 0.01 to 0.06%, N: 0.006% or less, with the balance being Fe and unavoidable impurities. The steel is heated to a heating temperature of 1100 to 1300°C, and then subjected to rough rolling with a rough rolling end temperature of 1150 to 950°C, and finish rolling with a finish rolling start temperature of 1100 to 850°C and a finish rolling end temperature of 900 to 750°C. The steel is then cooled to a cooling end temperature of 580°C or less at a cooling rate of 4 to 25°C / s from the start of cooling to the end of cooling at the center of the plate thickness, and the cooling end temperature is 0.2 s / s in an initial cooling step that is 10 s from the start of cooling. The proposed technology involves one or more natural cooling steps of 3.0 seconds or more for 3.0 seconds or less, followed by coiling at a coiling temperature of 580°C or less and natural cooling, whereby the steel structure in the center of the plate thickness has a main phase made of ferrite and a second phase made of one or more types selected from pearlite, pseudo-pearlite, and upper bainite and having an area ratio of 8 to 20%, the steel structure including the main phase and the second phase having an average crystal grain size of 7 to 20 μm, the steel structure on the front and back surfaces of the plate thickness being a single phase of ferrite or bainitic ferrite with an average crystal grain size of 2 to 20 μm, the square steel pipe exhibiting a low yield ratio of 90% or less, and having mechanical properties such as an absorbed energy of 27 J or more in a Charpy impact test at a test temperature of 0°C.

[0011] Patent Document 5 describes a method for forming a square steel pipe by cold rolling a steel plate having a predetermined composition into a square shape, in which the circumferential length C of the square steel pipe at the outlet side of the square forming stand is ＯＵＴ The ratio of the width W of the steel plate to the square steel pipe satisfies a predetermined formula, and the perimeter C of the square steel pipe at the outlet side of the square forming stand ＯＵＴ Circumference C of the electric resistance welded steel pipe at the entry side of the square forming stand ＩＮA technology has been proposed for manufacturing square steel pipes in which the uniform elongation E2 at a position 1 / 4t from the outer surface of the corner in the thickness direction is 0.60 times or more the uniform elongation E1 at a position 1 / 4t from the outer surface of the flat portion in the thickness direction, and the Charpy absorbed energy at -10°C at a position 1 / 4t from the outer surface of the corner is 100 J or more, by manufacturing square steel pipes by controlling the roll gap of the sizing stand immediately before corner forming and the roll gap of the corner forming stand so that the ratio satisfies a predetermined formula.

[0012] Japanese Patent Publication No. 9-87743 Japanese Patent No. 5594165 Japanese Patent No. 5589885 Japanese Patent No. 6388091 International Publication No. 2022 / 075026

[0013] Here, the square steel pipes manufactured by cold roll forming have a problem that the larger the wall thickness or the smaller the side length, the more processing strain is introduced into the square steel pipe, resulting in an increase in the yield ratio and a greater decrease in ductility and toughness. Therefore, the square steel pipes manufactured by the methods disclosed in Patent Documents 1 to 3 have a problem that the yield ratio becomes high and the yield ratio of 90% or less cannot be satisfied, especially when the plate thickness exceeds 25 mm.

[0014] The square steel pipes manufactured using the technology described in Patent Document 4 have an absorbed energy of 27 J or more in a Charpy impact test at a test temperature of 0°C, and depending on the environment in which the square steel pipes are used, their low-temperature toughness may be insufficient.

[0015] The technology described in Patent Document 5 has the problem that it requires appropriate control of the roll gap of the sizing stand and the square forming stand when forming the steel plate into a square shape, and it is necessary to determine the optimal manufacturing conditions for each dimension of the square steel pipe, which requires a lot of effort.

[0016] The present invention has been made in view of the above problems, and has an object to provide a square steel pipe having a low yield ratio, excellent low-temperature toughness, and high ductility.

[0017] Another object of the present invention is to provide a method for manufacturing the above-mentioned square steel pipe and a building structure using the above-mentioned square steel pipe.

[0018] The present inventors have conducted extensive research to solve the above problems and to develop a square steel pipe that satisfies the strength (yield strength, tensile strength), yield ratio, ductility (uniform elongation, fracture elongation), and low-temperature toughness (absorbed energy and ductile-brittle transition temperature) that are the objectives of the present invention. As a result, the following findings (1) to (4) were obtained.

[0019] (1) The C content must be 0.07 mass% or more. Furthermore, the main phase of the steel structure in the thickness center portion of the square steel pipe must be ferrite. (2) The steel structure in the thickness center portion of the square steel pipe must have a second phase consisting of one or more types selected from pearlite, pseudo-pearlite, and upper bainite, and the area ratio of the second phase must be 5 to 30%. Furthermore, in the steel structure, the average grain size must be 10.0 to 30.0 μm, and the pearlite and pseudo-pearlite with an equivalent circle diameter of 2 μm to 20 μm must be present at a density of 500 grains / mm in total. ２ It is necessary that the above-mentioned steel structure is present. (3) In order to obtain the above-mentioned steel structure, it is extremely important to set the reduction ratio in the hot rolling when manufacturing the hot-rolled steel sheet that will be used as the raw material for the square steel pipe within an appropriate range. Specifically, the total reduction ratio at 930°C or less must be 40% or more, and the total reduction ratio of the final two passes of finish rolling must be 15% or more. (4) Furthermore, when manufacturing the square steel pipe, it is necessary to impart a compressive strain of 0.20 to 3.00% in the axial direction of the pipe after forming the hot-rolled steel sheet into a round steel pipe and / or after forming it into a square steel pipe.

[0020] The present invention has been completed based on these findings and comprises the following: [1] A square steel pipe having a flat portion and a square portion, wherein the steel composition contains, 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: 0.10% or less, S: 0.050% or less, Al: 0.005% or more and 0.100% or less, and N: 0.010% or less, with the balance being Fe and unavoidable impurities, and the steel structure in the center of the plate thickness of the flat portion has ferrite as the main phase, and an area ratio of a second phase consisting of one or more selected from pearlite, pseudo-pearlite, and upper bainite is 5 to 30%, and the average grain size is 10.0 to 30.0 μm, and the steel structure contains pearlite and pseudo-pearlite having an equivalent circle diameter of 2 μm to 20 μm in a total density of 500 grains / mm ２or more, wherein the flat plate portion has a yield strength of 295 MPa or more, a tensile strength of 400 MPa or more, a yield ratio of 0.85 or less, a uniform elongation of 8.0% or more, and a fracture elongation of 27.0% or more, and the corner portion has a Charpy absorbed energy of 80 J or more at -10°C and a ductile-brittle transition temperature of -10°C or less. [2] The square steel pipe according to [1], wherein the steel composition further contains, in mass%, one or more elements selected from Nb: 0.020% or less, Ti: 0.045% or less, V: 0.10% or less, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.30% or less, Ni: 0.30% 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 [1] or [2], wherein a steel material having the steel composition is heated at a heating temperature of 1100°C or more and 1300°C or less for a heating time of 40 minutes or more and 200 minutes or less, and then subjected to hot rolling with a rough rolling end temperature of 850°C or more and 1150°C or less, a finish rolling end temperature of 750°C or more and 850°C or less, and 930°C or less, with a total reduction of 40% or more, and a total reduction of the final two passes of finish rolling of 15% or more, and then cooled at an average cooling rate of 10.0°C / s or less at the center of the plate thickness and a cooling end temperature of 480°C or more and 650°C or less at the center of the plate thickness, and then coiled to form a hot-rolled steel sheet at a temperature of 480°C or more and 650°C or less at the center of the plate thickness, and then A method for manufacturing a square steel pipe, comprising: forming the hot-rolled steel sheet into a cylindrical shape by roll forming, welding the ends of the hot-rolled steel sheet to form a round steel pipe, and then forming the hot-rolled steel sheet into a square steel pipe, and then applying a compressive strain of 0.20 to 3.00% in the pipe axial direction after forming into the round steel pipe and / or after forming into the square steel pipe. [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 a low yield ratio, excellent low-temperature toughness, and high ductility can be provided.

[0022] Fig. 1 is a perspective view showing an example of a building structure using the square steel pipe of the present invention. Fig. 2 is a schematic diagram showing the positions where tensile test specimens were taken in the tensile test performed in the present invention. Fig. 3 is a schematic diagram showing the positions where Charpy test specimens were taken in the Charpy impact test performed in the present invention.

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

[0024] The square steel pipe of the present invention has a steel composition containing, by mass%, C: 0.07% to 0.20%, Si: 0.01% to 0.40%, Mn: 0.20% to 1.20%, P: 0.10% to 0.050%, Al: 0.005% to 0.100%, and N: 0.010% or less, with the balance being Fe and unavoidable impurities. The square steel pipe of the present invention also has a flat portion and a corner portion, and the steel structure in the center of the thickness of the flat portion is composed of ferrite as the main phase, with an area ratio of 5 to 30% of a second phase consisting of one or more types selected from pearlite, pseudo-pearlite, and upper bainite, an average grain size of 10.0 to 30.0 μm, and the steel structure contains pearlite and pseudo-pearlite having an equivalent circle diameter of 2 μm to 20 μm at a density of 500 grains / mm ２ The square steel pipe of the present invention has a flat plate yield strength of 295 MPa or more, a flat plate tensile strength of 400 MPa or more, and a low yield ratio of 0.85 or less. The square steel pipe of the present invention also has high ductility in that the flat plate uniform elongation is 8.0% or more and the flat plate fracture elongation is 27.0% or more, and excellent low-temperature toughness in that the corners have a Charpy absorbed energy at -10°C of 80 J or more and a ductile-brittle transition temperature of -10°C or less.

[0025] The steel composition of the square steel pipe of the present invention will be described below. Unless otherwise specified, "%" indicating the content of each component means "% 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. Furthermore, C contributes to the formation of pearlite and pseudo-pearlite, which are second phases. In order to ensure the desired strength and yield ratio in the present invention, a C content of 0.07% or more is necessary. However, if the C content exceeds 0.20%, the proportion of hard phase increases, low-temperature toughness decreases, and the yield ratio exceeds 0.85, making it impossible to achieve the desired yield ratio. Furthermore, ductility decreases, making it impossible to achieve the desired uniform elongation and elongation at break. 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 likely to form in the electric resistance weld, degrading the weld characteristics and ductility, making it impossible to achieve the desired uniform elongation and elongation to fracture. 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. In order to ensure the desired strength and microstructural quality in the present invention, a Mn content of 0.20% or more is necessary. However, if the Mn content exceeds 1.20%, the amount of bainite formed increases, resulting in a yield ratio exceeding 0.85, making it impossible to achieve the desired yield ratio. Furthermore, if the Mn content exceeds 1.20%, the hardness of the center segregation increases, which may cause cracking during welding. 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: 0.10% or less P is an inevitable impurity that 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 0.10% is acceptable. Therefore, the P content is set to a range of 0.10% 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, but since excessive reduction leads to an increase in smelting costs, the P content is preferably 0.002% or more.

[0030] S: 0.050% 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 0.050% or less is acceptable. Therefore, the S content is set to 0.050% or less. The S content is preferably 0.015% or less, more preferably 0.010% or less, and even more preferably 0.008% or less. Note that there is no particular lower limit for the S content, but since excessive reduction leads to an increase in smelting costs, the S content is preferably 0.0002% or more.

[0031] 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.

[0032] N: 0.010% or less N is an inevitable impurity and an element that has the effect of reducing toughness by firmly fixing dislocation motion. In the present invention, it is desirable to reduce N as an impurity as much as possible, but an N content of up to 0.010% is acceptable. Therefore, the N content is set to 0.010% 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 smelting costs, so the N content is preferably 0.0010% or more, and more preferably 0.0015% or more.

[0033] The basic components (essential components) of the square steel pipe of the present invention have been described above. The remaining components (balance) of the steel composition of the square steel pipe of the present invention can be Fe and unavoidable impurities. Examples of the unavoidable impurities in the remaining components include As, Bi, Co, Pb, Zn, W, and O, which are inevitably mixed in due to raw materials, materials, or manufacturing equipment. Examples of the raw materials include iron ore, reduced iron, ferroalloys, and scrap. However, within the range that does not impair the effects of the present invention, it is acceptable to include up to 0.05% As and Co, up to 0.005% Bi, Pb, Zn, and O, and up to 0.100% W. The above steel composition allows the desired properties of the present invention to be obtained.

[0034] The steel composition of the square steel pipe of the present invention may further contain, as optional elements, one or more selected from Nb: 0.020% or less, Ti: 0.045% or less, V: 0.10% or less, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.30% or less, Ni: 0.30% or less, Ca: 0.0100% or less, B: 0.0100% or less, Sn: 0.100% or less, and Sb: 0.100% or less.

[0035] Nb: 0.020% or less, Ti: 0.045% or less Nb and Ti are elements that form fine carbides and nitrides in steel, respectively, and contribute to improving the strength of steel through precipitation strengthening. To achieve this effect, Nb and Ti are preferably contained in an amount of 0.005% or more, and more preferably 0.007% or more. However, if the Nb content exceeds 0.020%, the yield ratio exceeds 0.85, making it impossible to obtain the desired yield ratio. Furthermore, if the Ti content exceeds 0.045%, coarse carbides and nitrides are formed, which may result in a decrease in low-temperature toughness and ductility. Therefore, when Nb and Ti are contained, the Nb and Ti contents are set to 0.020% or less and 0.045% or less, respectively. The Nb content is preferably 0.018% or less, more preferably 0.016% or less. The Ti content is preferably 0.040% or less, and more preferably 0.035% or less.

[0036] V: 0.10% or less, Cr: 0.50% or less, Mo: 0.50% or less V, Cr, and Mo are elements that improve the hardenability and strength of steel, and can be contained as needed. To achieve these effects, when V, Cr, and Mo are contained, they are preferably contained in an amount of 0.01% or more of V, 0.01% or more of Cr, and 0.01% or more of Mo, respectively, and more preferably contained in an amount of 0.02% or more of V, 0.10% or more of Cr, and 0.10% or more of Mo. However, if the V, Cr, and Mo contents exceed 0.10%, 0.50%, and 0.50%, respectively, the ductility decreases, making it impossible to obtain the desired uniform elongation and elongation at break, and there is a risk of a decrease in low-temperature toughness. Therefore, when V, Cr, and Mo are contained, the V content is set to 0.10% or less, the Cr content is set to 0.50% or less, and the Mo content is set to 0.50% or less, respectively. The V, Cr, and Mo contents are preferably set to 0.08% or less, the Cr content is set to 0.40% or less, and the Mo content is set to 0.40% or less, respectively.

[0037] Cu: 0.30% or less, Ni: 0.30% or less Cu and Ni are elements that increase the strength of steel through solid solution strengthening and can be contained as needed. To achieve these effects, when Cu and Ni are contained, the Cu and Ni contents are preferably 0.01% or more and 0.01% or more, respectively, and more preferably 0.10% or more and 0.10% or more. However, a Cu content exceeding 0.30% leads to a decrease in low-temperature toughness, making it impossible to achieve the desired low-temperature toughness. Furthermore, a Ni content exceeding 0.30% results in a yield ratio exceeding 0.85, making it impossible to achieve the desired yield ratio. Therefore, when Cu and Ni are contained, the Cu and Ni contents are set to 0.30% or less and 0.30% or less, respectively. The Cu and Ni contents are preferably 0.20% or less and 0.20% or less, respectively.

[0038] Ca: 0.0100% or less Ca is an element that contributes to improving the toughness of steel by spheroidizing sulfides such as MnS that are thinly drawn during 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, if the Ca content exceeds 0.0100%, the ductility decreases, making it impossible to obtain the desired uniform elongation and elongation at break, and the low-temperature toughness also decreases, making it impossible to obtain the desired low-temperature toughness. Therefore, when Ca is contained, the Ca content is set to 0.0100% or less. The Ca content is preferably 0.0050% or less.

[0039] B: 0.0100% or less B is an element that contributes to 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 increases, the yield ratio exceeds 0.85, and the desired yield ratio cannot be obtained. Therefore, when B is contained, the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less.

[0040] Sn: 0.100% or less Sn is an element that increases the strength of steel through solid solution strengthening and can be contained as needed. In order to obtain the above-mentioned effects, 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%, hot workability may be reduced and cracks may occur 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.

[0041] Sb: 0.100% or less Sb is an element that increases the strength of steel through solid solution strengthening and can be contained as needed. In order to obtain the above-mentioned effects, 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%, hot workability may be reduced and cracks may occur 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.

[0042] Next, the reasons for limiting the steel structure of the square steel pipe of the present invention will be explained.

[0043] The square steel pipe of the present invention has a steel structure in the center of the plate thickness in which the area ratio of a main phase consisting of ferrite and a second phase consisting of one or more types selected from pearlite, pseudo-pearlite, and upper bainite is 5 to 30%, the average crystal grain size is 10.0 to 30.0 μm, and the steel structure contains pearlite and pseudo-pearlite with a circle equivalent diameter of 2 μm to 20 μm at a total density of 500 grains / mm ２ There are more than one.

[0044] In the present invention, the crystal grain size is defined as the diameter of a circle having the same area as the target crystal grain (circle equivalent diameter).

[0045] Main Phase: Ferrite Ferrite is a soft structure and is defined as the main phase in the present invention because it provides the desired yield strength and low yield ratio. In the present invention, the main phase refers to a phase that occupies 70% or more in area fraction. If the area fraction of ferrite is less than 70%, the yield stress becomes excessively large and the desired yield ratio may not be obtained. Therefore, the area fraction of ferrite is 70% or more, preferably 72% 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 may not be obtained. Therefore, the area fraction of ferrite is preferably 95% or less, and more preferably 93% or less.

[0046] Area fraction of the second phase consisting of one or more types selected from pearlite, pseudo-pearlite, and upper bainite: 5 to 30%. 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. In order to achieve the desired yield strength, tensile strength, and yield ratio in the present invention, the area fraction of the second phase consisting of one or more types selected from pearlite, pseudo-pearlite, and upper bainite must be 5% or more. This area fraction is preferably 6% or more, and more preferably 8% or more. On the other hand, if the area fraction of the second phase consisting of one or more types selected from pearlite, pseudo-pearlite, and upper bainite exceeds 30%, low-temperature toughness may deteriorate and the yield ratio may increase. Therefore, the area fraction of the second phase consisting of one or more types selected from pearlite, pseudo-pearlite, and upper bainite must be 30% or less. This area fraction is preferably 28% or less, and more preferably 26% or less.

[0047] Furthermore, the total area ratio of pearlite and pseudo-pearlite is preferably 5% or more. When the total area ratio of pearlite and pseudo-pearlite is 5% or more, the yield ratio is kept low when a square steel pipe is manufactured, thereby obtaining better earthquake resistance. When the total area ratio of pearlite and pseudo-pearlite is 20% or less, good low-temperature toughness is more easily obtained stably, so the total area ratio of pearlite and pseudo-pearlite is preferably 20% or less.

[0048] Furthermore, when the area fraction of upper bainite is 15% or less, the low yield ratio targeted in the present invention is more likely to be obtained stably, so the area fraction of upper bainite is preferably 15% or less, and more preferably 13% or less.

[0049] The area ratios of ferrite, pearlite, pseudo-pearlite, and upper bainite can be measured by the method described in the examples.

[0050] Average grain size of the steel structure at the center of the plate thickness: 10.0 to 30.0 μm. As described above, the steel structure of the present invention is a steel (hereinafter referred to as "dual-phase steel") that combines soft and hard structures to achieve the low yield ratio, yield strength, and tensile strength desired in the present invention. However, dual-phase steel has inferior toughness compared to single-phase steel. Therefore, in the present invention, to achieve both the above-mentioned mechanical properties and excellent toughness, the average grain size of the steel structure at the center of the plate thickness is specified, assuming that the region surrounded by the boundary where the misorientation (crystallographic misorientation) of adjacent crystals is 15° or more. If the average grain size (circle equivalent diameter) of the steel structure at the center of the plate thickness is less than 10.0 μm, the yield ratio increases and the desired yield ratio cannot be achieved. On the other hand, if the average grain size of the steel structure exceeds 30.0 μm, low-temperature toughness deteriorates. Therefore, the average grain size of the steel structure at the center of the plate thickness must be 10.0 to 30.0 μm. The average crystal grain size is preferably 11.0 μm or more, more preferably 11.5 μm or more, and is preferably 28.0 μm or less, more preferably 26.0 μm or less.

[0051] The crystal orientation difference and the average crystal grain size can be measured by a scanning electron microscope (SEM) / electron backscatter diffraction (EBSD) method, as described in the examples.

[0052] In the steel structure at the center of the plate thickness of the flat plate, pearlite and pseudo-pearlite having a circle equivalent diameter of 2 μm or more and 20 μm or less are present at a total of 500 particles / mm ２ Existence of more than

[0053] In order to obtain a square steel pipe having both low-temperature toughness and high ductility, the number density of pearlite and pseudo-pearlite is specified. The number density of pearlite and pseudo-pearlite particles with a circle equivalent diameter of 2 μm to 20 μm, which are present in the steel structure at the center of the plate thickness, is 500 particles / mm in total. ２If the grain size is less than 500 / mm, the toughness and ductility decrease, and the desired yield ratio, low-temperature toughness, and ductility cannot be obtained. Therefore, the steel structure should contain pearlite having an equivalent circle diameter of 2 μm or more and 20 μm or less and pseudo-pearlite having an equivalent circle diameter of 2 μm or more and 20 μm or less at a density of 500 / mm in total. ２ The total number of particles is preferably 550 / mm ２ More preferably, 580 pieces / mm ２ The upper limit of the total is not particularly set, but it is 1200 pieces / mm ２ If the yield ratio exceeds 1200 / mm, the yield ratio may increase. ２ Preferably, 1150 pieces / mm or less ２ It is more preferable that the total number of pearlite particles having an equivalent circle diameter of more than 20 μm and pseudo-pearlite particles having an equivalent circle diameter of more than 20 μm present in the steel structure at the center of the plate thickness of the flat plate portion is 50 / mm ２ When the total number of pearlite particles and pseudo-pearlite particles having an equivalent circle diameter of more than 20 μm in the steel structure is 50 / mm ２ It is preferable that the number of particles is 40 or less per mm. ２ The total number of pearlite and pseudo-pearlite particles having a circle equivalent diameter of more than 20 μm is 0 particles / mm ２ may be.

[0054] By satisfying the above-mentioned steel composition and steel structure, it is possible to obtain a square steel pipe having the strength (yield strength, tensile strength), yield ratio, ductility (uniform elongation, fracture elongation) and low-temperature toughness (Charpy absorbed energy, ductile-brittle transition temperature) that are the objectives of the present invention.

[0055] The square steel pipe of the present invention has, in the axial direction of the pipe, a yield strength of 295 MPa or more, a tensile strength of 400 MPa or more, and a low yield ratio of 0.85 or less in the flat portion. It also has high ductility: uniform elongation of 8.0% or more and elongation at break of 27.0% or more. Furthermore, the corners have excellent low-temperature toughness, with an absorbed energy of 80 J or more in a Charpy impact test at a test temperature of -10°C and a ductile-brittle transition temperature of -10°C or less. The square steel pipe of the present invention is a square steel pipe that combines a low yield ratio with excellent low-temperature toughness and high ductility. These characteristic values ​​can be measured using the methods described in the examples.

[0056] The yield strength is preferably 320 MPa or more. The upper limit of the yield strength is not particularly limited, but an example is 470 MPa or less. The tensile strength is preferably 420 MPa or more. The upper limit of the tensile strength is not particularly limited, but an example is 620 MPa or less. The yield ratio is preferably 0.80 or less. The lower limit of the yield ratio is not particularly limited, but an example is 0.60 or more. The uniform elongation is preferably 9.0% or more. The elongation at break is preferably 29.0% or more. The absorbed energy in a Charpy impact test at -10°C is preferably 90 J or more. The ductile-brittle transition temperature is preferably -15°C or less.

[0057] The square steel pipe of the present invention preferably has a plate thickness (wall thickness) of 12 mm or more, and may be more than 22 mm. According to the present invention, even a square steel pipe with a large plate thickness of more than 22 mm has a low yield ratio, excellent low-temperature toughness, and high ductility. The square steel pipe of the present invention preferably has a plate thickness of 12 to 32 mm.

[0058] Next, a method for manufacturing a square steel pipe of the present invention will be described. First, one embodiment of a method for manufacturing a hot-rolled steel sheet (hereinafter also referred to as a hot-rolled steel sheet for a square steel pipe) that is the raw material for the square steel pipe of the present invention will be described.

[0059] The method for manufacturing a hot-rolled steel sheet for rectangular steel pipes of the present invention includes, for example, a hot rolling step, a cooling step, and a coiling step. In the hot rolling step, a steel material having the above-mentioned steel composition is heated at a heating temperature of 1100°C to 1300°C for a heating time of 40 to 200 min. Subsequently, hot rolling is performed with a rough rolling end temperature of 850°C to 1150°C, a finish rolling end temperature of 750°C to 850°C and 930°C, a total reduction of 40% or more, and a total reduction of 15% or more for the final two passes of finish rolling. In the cooling step after the hot rolling step, cooling is performed at an average cooling rate of 10.0°C / s or less at the center of the plate thickness and a cooling stop temperature of 480°C to 650°C at the center of the plate thickness. In the coiling process after the cooling process, the steel sheet is coiled at a temperature of 480°C or higher and 650°C or lower at the center of the sheet thickness to obtain a hot-rolled steel sheet.

[0060] In the following description of the manufacturing method, the temperature indicated in "°C" refers to the surface temperature of the steel material or steel plate (hot-rolled steel plate, base steel plate described below, etc.) unless otherwise specified. These surface temperatures can be measured using a radiation thermometer or the like. The temperature at the center of the steel plate thickness can be determined by calculating the temperature distribution in the cross section of the steel plate using heat transfer analysis and correcting the result by the surface temperature of the steel plate. Furthermore, "hot-rolled steel plate" includes hot-rolled steel plate and hot-rolled steel strip.

[0061] In the present invention, the method for producing the steel material (steel slab) is not particularly limited, and any of the known methods such as converter, electric furnace, and vacuum melting furnace are suitable. The casting method is also not particularly limited, and the steel material of the desired dimensions can be produced by a known casting method such as continuous casting. However, there is no problem if an ingot-making and blooming rolling method is used instead of the continuous casting method. The molten steel may further be subjected to secondary refining such as ladle refining.

[0062] Next, the obtained steel material (steel slab) is heated at a heating temperature of 1100°C or higher and 1300°C or lower for a heating time of 40 minutes or higher and 200 minutes or lower, and then subjected to rough rolling with a rough rolling end temperature of 850°C or higher and 1150°C or lower, finish rolling with a finish rolling end temperature of 750°C or higher and 850°C or lower, and a hot rolling process with a total reduction rate of 40% or higher at 930°C or lower and a total reduction rate of 15% or higher for the final two passes of finish rolling, to produce a hot-rolled sheet (material steel sheet for hot-rolled steel sheet, hereinafter also referred to as material steel sheet).

[0063] Heating temperature of steel material: 1100°C or higher and 1300°C or lower If the heating temperature of the steel material 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 of the steel material exceeds 1300°C, the austenite grains become coarse, and fine austenite grains cannot be obtained in subsequent rolling (rough rolling, finish rolling). As a result, it becomes difficult to ensure the average grain size of the steel structure at the center of the thickness of the hot-rolled steel sheet (hereinafter, the steel structure at the center of the thickness is also simply referred to as the steel structure) that is the objective of the present invention. For this reason, the heating temperature of the steel material in the hot rolling process is set to 1100°C or higher and 1300°C or lower. The heating temperature is preferably 1120°C or higher. Furthermore, the heating temperature is preferably 1280°C or lower. In addition to the conventional method of producing a steel material (steel slab), once cooling it to room temperature and then reheating it, the present invention can also easily apply an energy-saving direct rolling process in which the slab is charged into a heating furnace as a hot slab without being cooled to room temperature.

[0064] Heating time of steel material: 40 min to 200 min In order to obtain the desired steel structure (average crystal grain size, number density of pearlite and pseudo-pearlite with equivalent circle diameters of 2 μm to 20 μm), the present invention strictly specifies the heating temperature and heating time of the steel material. If the heating time of the steel material is less than 40 min, the austenite grains become too fine, and excessive ferrite is formed in the cooling process, resulting in reduced yield strength and tensile strength. On the other hand, if the heating time of the steel material exceeds 200 min, the austenite becomes coarse, making it difficult to obtain a steel structure with an average crystal grain size of 10.0 to 30.0 μm. In addition, pearlite and pseudo-pearlite with equivalent circle diameters of 2 μm to 20 μm are formed at a density of 500 grains / mm ２ It becomes difficult to obtain a steel structure having a microstructure of 40 minutes or more and 200 minutes or less. Therefore, the heating time of the steel material is set to 40 minutes or more and 200 minutes or less. The heating time is preferably 45 minutes or more, and more preferably 50 minutes or more. The heating time is preferably 190 minutes or less, and more preferably 185 minutes or less. The heating time is the time from when the steel material is charged into the heating furnace to when it is removed.

[0065] 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 fall below the ferrite transformation start temperature during the subsequent finish rolling, resulting in the formation of a large amount of ferrite and a decrease in yield strength and tensile strength. On the other hand, if the rough rolling end temperature exceeds 1150°C, the reduction amount in the austenite non-recrystallization temperature range will be insufficient, and fine austenite grains will not be obtained. As a result, it will be difficult to obtain a steel structure with an average grain size of 10.0 to 30.0 μm. 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. The rough rolling end temperature is also preferably 1100°C or lower, more preferably 1050°C or lower.

[0066] 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, the reduction in the austenite non-recrystallization temperature range will be insufficient, and fine austenite grains will not be obtained. As a result, the crystal grains will become coarse, making it difficult to ensure the strength (yield strength, tensile strength) targeted in the present invention. 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. The finish rolling end temperature is also preferably 830°C or lower, more preferably 820°C or lower.

[0067] Total reduction at 930°C or less: 40% or more In the present invention, by refining subgrains in austenite in the hot rolling process, the ferrite and bainite formed in the subsequent cooling and coiling processes are refined, thereby obtaining a steel structure of a hot-rolled steel sheet having the strength and low-temperature toughness targeted in the present invention. In order to refine subgrains in austenite in the hot rolling process, it is necessary to increase the reduction in the austenite non-recrystallization temperature range and introduce sufficient working strain. If the total reduction at 930°C or less is less than 40%, the grain size of ferrite and bainite will increase, resulting in a decrease in low-temperature toughness. Therefore, the total reduction at 930°C or less is set to 40% or more. The total reduction is preferably 42% or more, and more preferably 45% or more. Note that if the total reduction is too large, excessive load will be placed on the manufacturing equipment in the hot rolling process, making manufacturing difficult, so the total reduction is preferably 60% or less.

[0068] The reason why the total reduction ratio at 930°C or less is specified is that austenite recrystallizes in the hot rolling process in a temperature range above 930°C, and dislocations introduced by rolling disappear, making it impossible to obtain refined austenite. The above-mentioned total reduction ratio refers to the sum of the reduction ratios of each rolling pass in a temperature range of 930°C or less.

[0069] When hot rolling a steel material (steel slab), both the rough rolling and the finish rolling described above may be hot rolling with a total reduction of 40% or more at 930°C or less. Alternatively, hot rolling with a total reduction of 40% or more at 930°C or less may be performed only by finish rolling. In the latter case, if the total reduction of 40% or more at 930°C or less cannot be achieved by finish rolling alone, the slab can be cooled to a temperature of 930°C or less during rough rolling, and then the total reduction of 40% or more at 930°C or less in both rough rolling and finish rolling can be achieved.

[0070] Total reduction ratio of the final two passes of finish rolling: 15% or more In order to obtain a desired steel structure, particularly a desired number density of pearlite and pseudo-pearlite having an equivalent circle diameter of 2 μm or more and 20 μm or less, the present invention specifies the total reduction ratio of the final two passes of finish rolling. If the total reduction ratio of the final two passes of finish rolling is less than 15%, coarse pearlite will be generated, and therefore, the number of pearlite and pseudo-pearlite having an equivalent circle diameter of 2 μm or more and 20 μm or less will be 500 / mm ２ It becomes difficult to obtain a steel structure having a thickness of 15% or more. Therefore, the total reduction rate in the final two passes of finish rolling is set to 15% or more. It is preferably 17% or more, and more preferably 19% or more. There is no particular upper limit to the total reduction rate in the final two passes of finish rolling, but if the total reduction rate exceeds 50%, the load on the rolling equipment increases, so the total reduction rate in the final two passes of finish rolling is preferably 50% or less, and more preferably 48% or less.

[0071] The total reduction ratio of the final two passes of finish rolling can be calculated as follows: ｎ When the thickness of the plate after each rolling pass is t １ , t ２ , ..., t ｎ－２ , t ｎ－１ , t ｎ Then, the total reduction rate of the final two passes of finish rolling is ((t ｎ－２ -t ｎ ) / t ｎ－２ ) × 100 (%).

[0072] The hot rolling process produces a hot-rolled sheet (base steel sheet). After the hot rolling process, the hot-rolled sheet is then subjected to a cooling process. In the cooling process, the sheet is cooled at an average cooling rate of 10.0°C / s or less to a cooling stop temperature at the center of the sheet thickness, and at a cooling stop temperature of 480°C or more and 650°C or less at the center of the sheet thickness.

[0073] Average cooling rate at the center of the thickness: 10.0°C / s or less If the average cooling rate at the center of the thickness of the hot-rolled sheet in the temperature range from the start of cooling to the stop of cooling, which will be described later, exceeds 10.0°C / s, the amount of upper bainite produced increases, the desired average grain size cannot be obtained, and the target yield ratio of the present invention cannot be achieved. Therefore, the average cooling rate 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. While there is no particular limitation on the lower limit of the average cooling rate, if the average cooling rate is less than 1.0°C / s, it becomes difficult to achieve the cooling stop temperature, which will be described later. 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. In the present invention, in order to prevent the grain size from becoming coarse, it is preferable to start cooling immediately after the end of finish rolling.

[0074] Cooling stop temperature: 480°C or higher and 650°C or lower. If the cooling stop temperature at the center of the thickness of the hot-rolled sheet is lower than 480°C, the amount of upper bainite produced increases, resulting in a yield ratio exceeding 0.85, and the desired yield ratio may not be obtained. Furthermore, temperature unevenness is likely to occur in the length and / or width directions of the hot-rolled sheet during cooling, potentially resulting in variations in mechanical properties. On the other hand, if the cooling stop temperature at the center of the thickness of the hot-rolled sheet exceeds 650°C, ferrite grains become coarse, and the desired grain size cannot be obtained. Therefore, the cooling stop temperature at the center of the thickness of the hot-rolled sheet 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.

[0075] In the present invention, the average cooling rate is a value calculated by ((temperature at the center of the thickness of the hot-rolled sheet before cooling - temperature at the center of the thickness of the hot-rolled sheet after cooling) / cooling time). The average cooling rate can be calculated from the temperature distribution in the cross section of the hot-rolled sheet obtained by heat transfer analysis. Examples of cooling methods include water cooling, such as spraying water from a nozzle, and cooling by spraying cooling gas. In the present invention, it is preferable to perform a cooling operation (treatment) on both sides of the hot-rolled sheet so that both sides of the hot-rolled sheet are cooled under the same conditions. In addition, to obtain the above-mentioned average cooling rate, the amount and pressure of cooling water or cooling gas, the spray time and angle, and the conveying speed of the hot-rolled sheet, etc. are adjusted. In order to obtain the average cooling rate specified in the present invention, it is necessary to perform heat transfer analysis in advance to determine the conditions for the cooling treatment of the hot-rolled sheet before manufacturing.

[0076] After the cooling step, the hot-rolled sheet is subjected to a coiling step. In the coiling step, from the viewpoint of the steel structure, the hot-rolled sheet is coiled at a coiling temperature of 480°C or higher and 650°C or lower, which is the temperature at the center of the sheet thickness. If the coiling temperature 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 exceeds 650°C, the ferrite grains become coarse and the desired average crystal grain size cannot be obtained. Therefore, the coiling temperature 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. Furthermore, the coiling temperature is preferably 640°C or lower, and more preferably 635°C or lower.

[0077] After the coiling step, the hot-rolled steel sheet is subjected to a cooling step in which the hot-rolled steel sheet is naturally cooled. In this manner, the hot-rolled steel sheet (hot-rolled steel sheet for square steel pipes) of the present invention can be produced.

[0078] 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 is obtained by rolling (cold rolling) the hot-rolled steel sheet to form a cylindrical open pipe, and then welding the butt joints (the ends of the hot-rolled steel sheet) to form a round steel pipe. The round steel pipe is then formed into a square steel pipe using rolls arranged above, below, left, and right of the round steel pipe to obtain the square steel pipe. After the hot-rolled steel sheet has been formed into the round steel pipe and / or the square steel pipe, a compressive strain of 0.20 to 3.00% is applied in the axial direction of the pipe.

[0079] Compressive strain in the axial direction of a steel pipe: 0.20 to 3.00%. Applying compressive strain in the axial direction of the steel pipe reduces the yield ratio in the axial direction and increases ductility (increases in uniform elongation and fracture elongation). Therefore, the process of applying compressive strain is crucial for achieving the low yield ratio and high ductility (high uniform elongation and fracture elongation) desired in the present invention. A compressive strain of less than 0.20% does not reduce the yield ratio or increase ductility (increases in uniform elongation and fracture elongation). On the other hand, a compressive strain exceeding 3.00% may cause buckling of the steel pipe. Therefore, the compressive strain in the axial direction of the steel pipe is set to 0.20 to 3.00%. The compressive strain is preferably 0.25% or more, more preferably 0.30% or more. The compressive strain is preferably 2.80% or less, more preferably 2.60% or less. The method of applying compressive strain is not important as long as buckling does not occur. The compressive strain may be imparted to either the formed round or square steel pipe, or to both the round and square steel pipes. When the compressive strain is imparted to both the round and square steel pipes, the total compressive strain imparted to the round and square steel pipes is set to fall within the above-mentioned range.

[0080] Note that the square steel pipes in this invention are not limited to those whose side lengths are all equal (long side length / short side length) of 1.0, but also include square steel pipes whose (long side length / short side length) value exceeds 1.0. However, if the (long side length / short side length) value of a square steel pipe exceeds 2.5, local buckling is more likely to occur on the long side, reducing the compressive strength in the axial direction of the pipe. Therefore, the (long side length / short side length) value of a square steel pipe is preferably 1.0 or more and 2.5 or less. The (long side length / short side length) value is more preferably 1.0 or more and 2.0 or less.

[0081] The square steel pipe of the present invention is manufactured in this manner. According to the present invention, a square steel pipe can be obtained having a flat portion with a yield strength of 295 MPa or more, a tensile strength of 400 MPa or more, a yield ratio of 0.85 or less, a uniform elongation of 8.0% or more, a fracture elongation of 27.0% or more, a Charpy absorbed energy of 80 J or more at -10°C in the corners, and a ductile-brittle transition temperature of -10°C or less at the corners. The square steel pipe of the present invention has a ductile-brittle transition temperature of -10°C or less, making it suitable for use as a structural member for buildings in cold regions where the air temperature or room temperature falls below freezing, and can ensure excellent earthquake resistance. Furthermore, according to the present invention, when forming a square steel pipe from a hot-rolled steel sheet, 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 efficiently manufactured.

[0082] <Architectural Structure> FIG. 1 is a schematic diagram showing an example of an architectural structure of the present invention.

[0083] The architectural structure of the present invention uses the square steel pipe (square steel pipe 1) of the present invention described above as a column material. Reference numerals 4, 5, 6, and 7 respectively represent the main beam, sub-beam, diaphragm, and stud. 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 using this square steel pipe as a column material exhibits excellent seismic performance.

[0084] 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.

[0085] Molten steel having the steel composition shown in Table 1 was cast to prepare a steel material (steel slab). The obtained steel slab was subjected to a hot rolling process, a cooling process, and a coiling process under the conditions shown in Tables 2-1 and 2-2 to prepare a hot-rolled steel sheet. Thereafter, the following pipe-making process was carried out.

[0086]

[0087]

[0088]

[0089] The obtained hot-rolled steel sheet was formed into a cylindrical open pipe by roll forming (cold roll forming), and the butt joints were electric resistance welded to form a round steel pipe. Then, the round steel pipe was formed into a rectangular steel pipe (square in cross section perpendicular to the pipe axis direction) by rolls arranged above, below, left and right of the round steel pipe. The obtained rectangular steel pipe had a corner portion and a flat portion, and had the side length (mm) and wall thickness (mm) shown in Table 3-1 and Table 3-2.

[0090] The obtained rectangular steel pipe was cut into a length of 3.0 m, and then compressive strain was applied in the longitudinal direction (pipe axis direction) to form a rectangular steel pipe. To prevent buckling of the rectangular steel pipe, the cut surface was ground and parallelized before applying compressive strain. The compressive strain value was evaluated by the value of a strain gauge attached to the center of the width of the flat part on the outer surface of the steel pipe (the 3 o'clock side when the weld is at the 12 o'clock position) at the center of the length of the rectangular steel pipe (1.5 m from the end).

[0091] Test specimens were taken from the hot-rolled steel sheets or square steel pipes produced as described above, and the following microstructure observations, tensile tests, and Charpy impact tests were carried out.

[0092] [Structural Observation] Test specimens for structural observation were taken from the widthwise center and thicknesswise center of the hot-rolled steel sheet (position at t / 2 of the sheet thickness (t: sheet thickness)). The specimens were polished and then etched with nital so that the observation surface corresponded to a cross section in the rolling direction during hot rolling.

[0093] The microstructure was observed at the thickness t / 2 position (the center of the thickness) of the hot-rolled steel sheet using an optical microscope (magnification: 1000x), and optical microscope images were taken. From the obtained optical microscope images, the area ratios of ferrite, pearlite, pseudo-pearlite, and upper bainite were determined.

[0094] The area fraction of each structure was calculated as the average of the values ​​obtained from five visual fields. Here, the area fraction obtained through 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, and pseudo-pearlite is a structure in which cementite is observed arranged in dotted rows 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, ferrite is white, pearlite is black, pseudo-pearlite is black or gray, and upper bainite is white or gray.

[0095] In addition, the number density of pearlite and pseudo-pearlite (pieces / mm ２ ) was observed in five visual fields, and the number of pearlite and pseudo-pearlite observed in the five visual fields was totaled and the observed area (mm ２ ) was calculated. The number of pearlite and pseudo-pearlite was calculated by measuring the total number of pearlite and pseudo-pearlite having an equivalent circle diameter of 2 μm or more and 20 μm or less, and the total number of pearlite and pseudo-pearlite having an equivalent circle diameter of more than 20 μm. Then, the number density of pearlite and pseudo-pearlite having an equivalent circle diameter of 2 μm or more and 20 μm or less, and the number densities of pearlite and pseudo-pearlite having an equivalent circle diameter of more than 20 μm were calculated.

[0096] The average grain size (average circle equivalent diameter) was measured at the sheet thickness t / 2 position (sheet thickness center) using the SEM / EBSD method. The measurement area was 500 μm × 1000 μm (= 0.5 mm ２ ), the measurement step size was 0.5 μm. The crystal grain size was measured by determining the misorientation between adjacent crystal grains, and the boundary with a misorientation of 15° or more was defined as the crystal grain boundary. The grain size (circle equivalent diameter) of each crystal grain was calculated from the obtained crystal grain boundary, and the arithmetic mean was calculated to be the average crystal grain size (average circle equivalent diameter). In the crystal grain size analysis, crystal grains with a grain size of less than 2.0 μm were excluded from the analysis as measurement noise.

[0097] [Tensile Test] Figure 2 is a schematic diagram showing the location of tensile test specimens taken from the flat plate portion. As shown in Figure 2, tensile tests were conducted using JIS No. 5 tensile test specimens taken from the flat plate portion of a square steel pipe so that the tensile direction was parallel to the pipe axis. Tensile tests were conducted on the taken tensile test specimens in accordance with JIS Z 2241 (2011). The yield strength (YS), tensile strength (TS), uniform elongation (Eu), and fracture elongation (Et) were measured, and the yield ratio, defined as (yield strength) / (tensile strength), was calculated. The uniform elongation was taken as the total elongation at the maximum load. The tensile test specimens from the flat plate portion were taken from the width center of the flat plate portion and the longitudinal center of the square steel pipe at the 3 o'clock side when the weld of the square steel pipe is at the 12 o'clock direction (see Figure 2). Two test specimens were taken, and their average values ​​were calculated to determine YS, TS, yield ratio, uniform elongation, and fracture elongation. Regarding strength, a plate with a yield strength of 295 MPa or more and a tensile strength of 400 MPa or more was deemed acceptable. Regarding yield ratio, a plate with a yield ratio of 0.85 or less was deemed acceptable as having a low yield ratio. Regarding ductility, a plate with a uniform elongation of 8.0% or more and a fracture elongation of 27.0% or more was deemed acceptable as having high ductility.

[0098] [Charpy Impact Test] Charpy impact tests were conducted using V-notch standard test specimens in accordance with JIS Z 2242 (2018). Figure 3 is a schematic diagram showing the sampling locations of Charpy test specimens used in the Charpy impact test. As shown in Figure 3, Charpy test specimens were sampled from the corners of the square steel pipes at a position t / 4 of the wall thickness t from the outer surface of the square steel pipe, with the longitudinal direction of the test specimen parallel to the pipe axis. However, for square steel pipes with a wall thickness of 19 mm or less, V-notch standard test specimens were sampled at a position t / 2 of the wall thickness t from the outer surface of the square steel pipe. In accordance with JIS Z 2242 (2018), Charpy impact tests were conducted at test temperatures of -70°C, -50°C, -30°C, -10°C, and 10°C. Three test specimens were used at each test temperature, and the ductile-brittle transition temperature and average absorbed energy (J) were determined. A test temperature of -10°C and an absorbed energy of 80 J or more in a Charpy impact test and a ductile-brittle transition temperature of -10°C or less were deemed to be excellent in low-temperature toughness and to pass the test.

[0099] The results obtained are shown in Tables 3-1 and 3-2.

[0100]

[0101]

[0102] In Tables 3-1 and 3-2, Steel Nos. 1 to 21, 36, and 37 are examples of the present invention, and Steel Nos. 22 to 35 and 38 to 50 are comparative examples.

[0103] As shown in Tables 3-1 and 3-2, the rectangular steel pipes of the present invention all had a yield strength of 295 MPa or more in the flat portion and a tensile strength of 400 MPa or more in the flat portion. They also had a low yield ratio of 0.85 or less in the flat portion. They also had high ductility, with a uniform elongation of 8.0% or more in the flat portion and a fracture elongation of 27.0% or more in the flat portion. They also had excellent low-temperature toughness, with a Charpy absorbed energy of 80 J or more at the corners at -10°C and a ductile-brittle transition temperature of -10°C or less at the corners.

[0104] In contrast, Comparative Example No. 22 (Steel V) had a C content below the range of the present invention, resulting in an area ratio of the second phase outside the range of the present invention, and the yield strength, tensile strength, and yield ratio did not reach the desired values. Comparative Example No. 23 (Steel W) had a C content above the range of the present invention, resulting in an area ratio of the second phase outside the range of the present invention, and the Charpy absorbed energy at the corner at -10°C, ductile-brittle transition temperature, uniform elongation, and fracture elongation did not reach the desired values. Comparative Example No. 24 (Steel X) had a Si content above the range of the present invention, resulting in a decrease in toughness, and the Charpy absorbed energy at the corner at -10°C, ductile-brittle transition temperature, uniform elongation, and fracture elongation did not reach the desired values. Comparative Example No. 25 (Steel Y) had a Mn content below the range of the present invention, resulting in an area ratio of the second phase outside the range of the present invention, and the tensile strength and yield ratio did not reach the desired values. Comparative Example No. In No. 26 (Steel Z), the Mn content exceeded the range of the present invention, so the amount of upper bainite formed was large and the yield ratio did not reach the desired value.

[0105] Comparative Example No. 27 (Steel AA) had an Nb content exceeding the range of the present invention, resulting in an excessive increase in the area ratio of the second phase. As a result, the yield ratio did not reach the desired value. Comparative Example No. 28 (Steel AB) had a Ti content exceeding the range of the present invention, which is believed to have resulted in the formation of coarse carbides and nitrides. As a result, the Charpy absorbed energy at the corner, ductile-brittle transition temperature, and uniform elongation at -10°C did not reach the desired values. Comparative Example No. 29 (Steel AC) had a V content exceeding the range of the present invention, resulting in decreased toughness and ductility. As a result, the Charpy absorbed energy at the corner, ductile-brittle transition temperature, uniform elongation, and fracture elongation at -10°C did not reach the desired values. Comparative Example No. 30 (Steel AD) had a Cr content exceeding the range of the present invention, resulting in decreased toughness and ductility. As a result, the Charpy absorbed energy, ductile-brittle transition temperature, uniform elongation, and fracture elongation at the corners at -10°C did not reach the desired values. Comparative example No. 31 (Steel AE) had a Mo content exceeding the range of the present invention, resulting in decreased toughness and ductility. As a result, the Charpy absorbed energy, ductile-brittle transition temperature, uniform elongation, and fracture elongation at the corners at -10°C did not reach the desired values.

[0106] In Comparative Example No. 32 (Steel AF), the Cu content exceeded the range specified in the present invention, which is believed to have resulted in coarse Cu precipitation. As a result, the Charpy absorbed energy and ductile-brittle transition temperature at the corners at -10°C did not reach the desired values. In Comparative Example No. 33 (Steel AG), the Ni content exceeded the range specified in the present invention, resulting in a large amount of upper bainite formation and a yield ratio that did not reach the desired value. In Comparative Example No. 34 (Steel AH), the Ca content exceeded the range specified in the present invention, which is believed to have resulted in the formation of Ca oxide clusters. As a result, the Charpy absorbed energy, ductile-brittle transition temperature, uniform elongation, and elongation to fracture at the corners at -10°C did not reach the desired values. In Comparative Example No. 35 (Steel AI), the B content exceeded the range specified in the present invention, resulting in a yield ratio at the flat plate that was outside the range specified in the present invention. In No. 38 (Steel U), the heating temperature of the steel material (steel slab) exceeded the range of the present invention, causing the crystal grains to become coarse and resulting in an average crystal grain size outside the range of the present invention. As a result, the yield strength, tensile strength, Charpy absorbed energy at the corners at -10°C, and ductile-brittle transition temperature did not reach the desired values.

[0107] In Comparative Example No. 39 (Steel U), the rough rolling end temperature exceeded the range of the present invention, resulting in an average crystal grain size outside the range of the present invention. As a result, the yield strength, tensile strength, Charpy absorbed energy at corners at -10°C, and ductile-brittle transition temperature did not reach the desired values. In Comparative Example No. 40 (Steel U), the finish rolling end temperature exceeded the range of the present invention, resulting in coarsening of the austenite, and the average crystal grain size fell outside the range of the present invention. As a result, the yield strength, tensile strength, Charpy absorbed energy at corners at -10°C, and ductile-brittle transition temperature did not reach the desired values. In Comparative Example No. 41 (Steel U), the total reduction at 930°C or less was below the range of the present invention, resulting in coarsening of the austenite, and the average crystal grain size fell outside the range of the present invention. As a result, the Charpy absorbed energy at corners at -10°C and ductile-brittle transition temperature did not reach the desired values. In Comparative Example No. 42 (Steel U), the average cooling rate at the center of the sheet thickness exceeded the range of the present invention, resulting in a large amount of upper bainite being formed and an average grain size outside the range of the present invention. As a result, the yield ratio did not reach the desired value. In Comparative Example No. 43 (Steel U), the cooling stop temperature and coiling temperature were below the range of the present invention, resulting in a large amount of upper bainite being formed and the yield ratio did not reach the desired value.

[0108] In Comparative Example No. 44 (Steel U), the cooling stop temperature and coiling temperature exceeded the ranges of the present invention, resulting in an average crystal grain size outside the range of the present invention. As a result, the yield strength, tensile strength, Charpy absorbed energy at the corner at -10°C, ductile-brittle transition temperature, uniform elongation, and elongation at break did not reach the desired values. In Comparative Example No. 45 (Steel U), the coiling temperature was below the range of the present invention, resulting in a large amount of upper bainite formation and a yield ratio that did not reach the desired value. In Comparative Example No. 46 (Steel U), the compressive strain was below the range of the present invention, resulting in a yield ratio, uniform elongation, and elongation at break that did not reach the desired values. In Comparative Example No. 47 (Steel U), the compressive strain exceeded the range of the present invention, resulting in buckling of the square steel pipe. Therefore, tensile tests and Charpy tests were not performed.

[0109] In Comparative Example No. 48 (Steel U), the heating time of the steel material was below the range of the present invention, resulting in excessive ferrite formation and an area ratio of the second phase outside the range of the present invention. As a result, the yield strength, tensile strength, and yield ratio did not reach the desired values. In Comparative Example No. 49 (Steel U), the heating time of the steel material exceeded the range of the present invention, resulting in an average grain size and a number density of pearlite and pseudo-pearlite with an equivalent circle diameter of 2 μm to 20 μm outside the range of the present invention. As a result, the uniform elongation, fracture elongation, Charpy absorbed energy at corners at -10°C, and ductile-brittle transition temperature did not reach the desired values. In Comparative Example No. 50 (Steel U), the total reduction rate of the final two passes of finish rolling was below the range of the present invention, resulting in a number density of pearlite and pseudo-pearlite with an equivalent circle diameter of 2 μm to 20 μm outside the range of the present invention. As a result, the uniform elongation, the elongation at break, the Charpy absorbed energy at the corner at -10°C, and the ductile-brittle transition temperature did not reach the desired values.

[0110] 1 Square steel pipe 4 Large beam 5 Small beam 6 Diaphragm 7 Stud

Claims

1. A square steel pipe having a flat portion and a corner portion, the steel composition comprising, in mass%, C: 0.07% to 0.20%, Si: 0.01% to 0.40%, Mn: 0.20% to 1.20%, P: 0.10% or less, S: 0.050% or less, Al: 0.005% to 0.100% or less, and N: 0.010% or less, with the balance being Fe and unavoidable impurities, the steel structure in the center of the plate thickness of the flat portion has ferrite as the main phase, with an area ratio of a second phase consisting of one or more types selected from pearlite, pseudo-pearlite, and upper bainite being 5 to 30%, the average crystal grain size being 10.0 to 30.0 μm, and the steel structure contains pearlite and pseudo-pearlite having a circle equivalent diameter of 2 μm to 20 μm in a total of 500 particles / mm ２ or more, the flat portion has a yield strength of 295 MPa or more, a tensile strength of 400 MPa or more, a yield ratio of 0.85 or less, a uniform elongation of 8.0% or more, and a fracture elongation of 27.0% or more, the corner portion has a Charpy absorbed energy of 80 J or more at -10°C and a ductile-brittle transition temperature of -10°C or less.

2. The square steel pipe as described in claim 1, wherein the steel composition further contains, by mass%, one or more elements selected from the following: Nb: 0.020% or less, Ti: 0.045% or less, V: 0.10% or less, Cr: 0.50% or less, Mo: 0.50% or less, Cu: 0.30% or less, Ni: 0.30% 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 the steps of: heating a steel material having the above-mentioned steel composition at a heating temperature of 1100°C to 1300°C for a heating time of 40 min to 200 min, followed by hot rolling with a rough rolling end temperature of 850°C to 1150°C, a finish rolling end temperature of 750°C to 850°C, and a finish rolling end temperature of 930°C or less with a total reduction of 40% or more, and a total reduction of the final two passes of finish rolling of 15% or more, followed by cooling at an average cooling rate of 10.0°C / s or less at the center of the plate thickness and a cooling end temperature of 480°C to 650°C, followed by coiling at a temperature of 480°C to 650°C at the center of the plate thickness to form a hot-rolled steel sheet, followed by A method for manufacturing a square steel pipe, comprising the steps of: forming the hot-rolled steel sheet into a cylindrical shape by roll forming; welding the ends of the hot-rolled steel sheet into a round steel pipe; and forming the hot-rolled steel sheet into a square steel pipe, wherein a compressive strain of 0.20 to 3.00% is imparted in the axial direction of the pipe after forming into the round steel pipe and / or after forming into the square steel pipe.

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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