Rectangular steel pipes, methods for manufacturing the same, and building structures

Abstract

The present invention provides a rectangular steel pipe having high strength, a low yield ratio, and excellent low-temperature toughness, as well as a method for manufacturing the same, and a building structure having excellent seismic performance even in low-temperature environments. The rectangular steel pipe has a flat section and a rectangular section, and has a predetermined component composition, wherein at the center of the thickness of the flat section, it contains ferrite: 80-95% by area ratio, and one or more types selected from the group consisting of pearlite, pseudo-perlite, and upper bainite: 5-20% by total area ratio, and the average crystal grain size of the ferrite is d f The average crystal grain size of perlite and pseudo-perlite is 5.0 to 20.0 μm. p 3.0~18.0 μm, d f d for p ratio d p / d f The number density is between 0.60 and 1.00, and the total number density of perlite and pseudo-perlite is 2000 particles / mm³. 2 A rectangular steel pipe having the above-described microstructure.

Description

[Technical Field]

[0001] This invention relates to square steel pipes, methods for manufacturing the same, and building structures, and more particularly to square steel pipes (square columns) manufactured by cold roll forming, having a low yield ratio and excellent low-temperature toughness. In particular, this invention relates to square steel pipes suitably used as building structural members in large buildings. This invention further relates to building structures obtained using these square steel pipes. [Background technology]

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

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

[0004] In the case of square steel pipes manufactured by roll forming of hot-rolled steel sheets (hereinafter sometimes referred to as roll-formed square steel pipes), the hot-rolled steel sheets are roll-formed at a cold temperature to form cylindrical open pipes, and the butt joints are welded with electric resistance welding to form round steel pipes. Then, rolls positioned above, below, and to the left and right of the round steel pipe are used to reduce the diameter of the round steel pipe by a few percent in the circumferential direction, and the pipe is then formed into a square shape to manufacture the square steel pipe. 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 press-bent at a cold temperature to form a cross-sectional shape of a square (square shape) or a U-shape (U shape). These are then joined together by submerged arc welding to manufacture the pipes.

[0005] Roll-formed square steel pipes have the advantage of higher productivity and shorter manufacturing times compared to press-formed square steel pipes. However, in press-formed square steel pipes, cold forming is not applied to the flat sections, and only the corners are work-hardened, whereas in roll-formed square steel pipes, particularly during cold forming into a cylindrical shape, a large processing strain is introduced in the axial direction of the pipe along the entire circumference of the steel pipe. Due to this processing strain, roll-formed square steel pipes have a problem of high yield ratio in the axial direction and low toughness, even in the flat sections.

[0006] Furthermore, in roll-formed square steel pipes, the greater the wall thickness, the greater the work hardening during roll forming, resulting in a higher yield ratio and lower toughness. Therefore, achieving a low yield ratio and high toughness in thick-walled roll-formed square steel pipes becomes even more difficult.

[0007] Based on the above, there is a need for square steel pipes with 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 rolling completion temperature, wound at a predetermined temperature, and the outer circumference reduction in the square forming process is set to three times or less the plate thickness. According to the method proposed in Patent Document 1, it is possible to provide square steel pipes with a yield ratio of 90% or less and a Charpy absorption 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, roughly rolled, and finish-rolled under predetermined conditions, followed by a three-stage cooling process. According to the method proposed in Patent Document 2, it is possible to manufacture square steel pipes with a yield ratio of 80% or less and a Charpy absorption energy of 150 J or more at a test temperature of 0°C.

[0009] In Patent Document 3, a method is proposed in which a predetermined steel material is heated, roughly rolled, finish-rolled, and then cooled under predetermined conditions, and during this process, control is exerted over cooling conditions such as the time until the temperature at the center of the plate thickness reaches 650°C. According to the method proposed in Patent Document 3, it is said that square steel pipes can be manufactured with a yield ratio of 80% or less and a Charpy impact energy of 150 J or more at a test temperature of 0°C.

[0010] In Patent Document 4, a method is proposed for manufacturing square steel pipes by controlling rolling conditions, cooling conditions, and the like. According to the method proposed in Patent Document 4, it is said that square steel pipes can be manufactured with a Charpy impact energy of 27 J or more at a test temperature of 0°C.

[0011] In Patent Document 5, when forming a steel plate into a square shape, a method is proposed for controlling the roll gaps of a sizing stand and a square forming stand immediately before square forming so that the circumferences of the steel pipe on the inlet and outlet sides of the square forming stand, as well as the plate width of the steel plate, satisfy predetermined conditions. According to the method proposed in Patent Document 5, it is said that square steel pipes can be manufactured with a Charpy impact energy of 100 J or more at -10°C at a position 1 / 4t in the thickness direction from the outer surface of the corner.

Prior Art Documents

Patent Documents

[0012]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

Problems to be Solved by the Invention

[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 environments where the temperature drops below freezing.

[0014] The technology proposed in Patent Document 5 also has room for improvement in toughness at temperatures well below 0°C, such as -30°C. Furthermore, properly controlling the roll gap when forming the steel plate requires determining the optimal manufacturing conditions for each dimension of the square steel pipe, which presents a problem as it requires a great deal of effort.

[0015] This invention has been made in view of the above problems, and aims to provide a square steel pipe having high strength, a low yield ratio, and excellent low-temperature toughness, as well as a method for manufacturing the same, and a building structure having excellent seismic resistance even in low-temperature environments. [Means for solving the problem]

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

[0017] 1. A rectangular steel pipe having a flat section and a corner section, 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% to 0.100%, and N: 0.0100% or less It contains, with the remainder consisting of Fe and unavoidable impurities, The composition has a component ratio of %Mn / %Ti, where %Mn / %Ti is between 30.0 and 150.0. At the center position of the plate thickness of the flat plate portion, Ferrite: 80-95% by area ratio, One or more materials selected from the group consisting of perlite, pseudoperlite, and upper bainite: comprising 5-20% of the total area, d average crystal grain size of ferrite f The size is 5.0 to 20.0 μm. Average grain size d of perlite and pseudo-perlite p The size is 3.0 to 18.0 μm. d f d for p ratio d p / d f is 0.60 or more and 1.00 or less, The total number density of perlite and pseudo-perlite is 2000 particles / mm². 2 A rectangular steel pipe having the above-described microstructure.

[0018] 2. The above component composition is further expressed in mass %, 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 rectangular steel pipe according to item 1, comprising one or more types selected from the group consisting of the above.

[0019] 3. A method for manufacturing a rectangular steel pipe as described in 1 or 2 above, The steel material having the above-mentioned component composition is heated to a heating temperature of 1100°C to 1300°C, The heated steel material is hot-rolled to form a hot-rolled steel sheet, The hot-rolled steel sheet is cooled under the conditions that the average cooling rate at the center of the sheet thickness is 10.0°C / s or less and the cooling stop temperature at the center of the sheet thickness is 480°C or higher and 650°C or lower. The cooled hot-rolled steel sheet is wound at a winding temperature of 480°C to 650°C at the center of the sheet thickness, The hot-rolled steel sheet, after being wound, is formed into a cylindrical shape by roll forming, and then the ends are welded to form a round steel pipe. This includes forming the aforementioned round steel pipe into the aforementioned square steel pipe, The hot rolling described above is carried out by sequentially performing rough rolling and finish rolling. The rough rolling is carried out under the following conditions: rough rolling completion temperature: 850°C or higher and 1150°C or lower; time from the end of heating of the steel material to the end of the rough rolling: 10.0 minutes or less; and total reduction ratio at 930°C or higher: 60% or more. The aforementioned finish rolling is carried out under the condition that the finish rolling completion temperature is between 750°C and 850°C. A method for manufacturing square steel pipes, wherein the hot rolling is performed at a temperature of 930°C or lower with a total reduction ratio of 40% or more.

[0020] 4. A building structure in which the square steel pipes described in 1 or 2 above are used as column members. [Effects of the Invention]

[0021] According to the present invention, it is possible to provide a square steel pipe that has high strength, a low yield ratio, and excellent low-temperature toughness. [Brief explanation of the drawing]

[0022] [Figure 1] Figure 1 is a schematic diagram showing a cross-section of a rectangular steel pipe according to the present invention, perpendicular to the pipe axis. [Figure 2] Figure 2 is a schematic perspective view showing an example of the building structure of the present invention. [Figure 3] Figure 3 is a schematic diagram showing the sampling locations for tensile test specimens. [Figure 4] Figure 4 is a schematic diagram showing the sampling locations for Charpy test specimens. [Modes for carrying out the invention]

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

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

[0025] Next, the component composition of the rectangular steel pipe of the present invention will be described. Unless otherwise specified, the "%" indicating the component composition refers to "mass%".

[0026] C: 0.07% or more and 0.20% or less Carbon (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 in the second phase. To ensure the desired strength and yield ratio, it is necessary to contain 0.07% or more C. However, if the C content exceeds 0.20%, the proportion of the hard second phase increases, reducing low-temperature toughness and increasing the yield ratio, making it impossible to obtain the desired yield ratio. Weldability also deteriorates. Therefore, the C content should be between 0.07% and 0.20%. Preferably, the C content is 0.08% or more. More preferably, the C content is 0.19% or less, and 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 obtain this effect, it is necessary to contain 0.01% or more Si. However, if the Si content exceeds 0.40%, oxides are formed in the electric resistance welded area, reducing the weld properties and decreasing ductility. Furthermore, the toughness of the base material outside the electric resistance welded area also decreases. For this reason, the Si content should be 0.40% or less. The Si content is preferably 0.02% or more, and more preferably 0.05% or more. In addition, the Si content is preferably 0.37% or less, and 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 solid solution strengthening and / or by increasing the amount of bainite formed. Mn also contributes to microstructure refinement by lowering the ferrite transformation initiation temperature. To obtain the desired strength and microstructure, it is necessary to contain 0.20% or more Mn. 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 should be between 0.20% and 1.20%. The Mn content is preferably 0.25% or more, more preferably 0.30% or more. Furthermore, the Mn content is preferably 1.10% or less, more preferably 1.05% or less.

[0029] P:0.100% or less Since phosphorus (P) segregates at grain boundaries and causes material heterogeneity, it is preferable to reduce its content as much as possible, but a content of up to 0.100% is acceptable. Therefore, the P content should be 0.100% or less. Preferably, the P content is 0.030% or less, and more preferably 0.020% or less. There is no specific lower limit for the P content, and it may be 0, but excessive reduction will lead to a surge in refining costs, so it is preferable that the P content be 0.002% or more.

[0030] S: 0.0500% or less S usually exists as MnS in steel, but MnS is thinned during the hot rolling process, which negatively affects ductility. For this reason, it is preferable to reduce S as much as possible in this invention, but a content of 0.0500% or less is acceptable. Therefore, the S content is set to 0.0500% or less. Preferably, the S content is 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 will lead to a surge in refining costs, so it is preferable that the S content be 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, contributing to improved steel strength through precipitation strengthening. To obtain this effect, it is necessary to contain 0.005% or more Ti. However, if the Ti content exceeds 0.035%, coarse carbides and nitrides are formed, leading to a decrease in low-temperature toughness and ductility. Therefore, the Ti content should be between 0.005% and 0.035%. Preferably, the Ti content is 0.007% or more, and more preferably 0.009% or more. Furthermore, preferably, the Ti content is 0.032% or less, and 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 strong deoxidizing agent. To obtain this effect, it is necessary to contain 0.005% or more Al. However, if the Al content exceeds 0.100%, the weldability deteriorates, the amount of alumina-based inclusions increases, and the surface properties deteriorate. The toughness of the weld also decreases. For this reason, the Al content should be between 0.005% and 0.100%. The Al content is preferably 0.010% or more, more preferably 0.015% or more. Furthermore, the Al content is preferably 0.070% or less, more preferably 0.050% or less.

[0033] N: 0.0100% or less N is an element that reduces toughness by firmly fixing the movement of dislocations. For this reason, the N content should be 0.0100% or less. Preferably, the N content is 0.0080% or less, more preferably 0.0040% or less, and even more preferably 0.0035% or less. However, excessive reduction can lead to increased refining costs, and the inclusion of N may further increase strength, so it is preferable that the N content be 0.0010% or more, and more preferably 0.0015% or more.

[0034] The ratio of Mn content to Ti content (%Mn / %Ti): 30.0 to 150.0 If the %Mn / %Ti ratio is less than 30.0, the average grain size of the ferrite and the second phase becomes coarser, reducing the low-temperature toughness. Furthermore, if the %Mn / %Ti ratio exceeds 150.0, the yield ratio becomes higher, making it impossible to obtain the desired yield ratio. Therefore, the %Mn / %Ti ratio should be between 30.0 and 150.0. Preferably, the %Mn / %Ti ratio is 35.0 or higher, more preferably 38.0 or higher. Also, preferably, the %Mn / %Ti ratio is 145.0 or lower, more preferably 140.0 or lower.

[0035] In the above component composition, the remaining components (the remainder) consist of Fe and unavoidable impurities. Examples of unavoidable impurities include As, Bi, Co, Pb, Zn, W, and O, which may be inevitably mixed in from raw materials, supplies, or manufacturing equipment. Examples of raw materials include iron ore, reduced iron, ferroalloys, or 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 components constitute the basic composition of the rectangular steel pipe of the present invention. While the properties desired in this invention can be obtained with the above-mentioned essential elements, the following elements may be included as needed.

[0037] In other words, the component composition of the rectangular steel pipe of the present invention may further include, as an optional element, 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. The content of these components is calculated as the sum of the content of intentionally added components and the content of components that are inevitably mixed in.

[0038] Nb: 0.020% or less Nb is an element that forms fine carbides and nitrides in steel, contributing to further strength improvement of steel through precipitation strengthening, and can be included as needed. To obtain such effects, when Nb is included, 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. Also, the yield ratio will increase, and the desired yield ratio cannot be obtained. Therefore, when Nb is included, the Nb content should be 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 enhances the hardenability of steel and further increases its strength, and can be included as needed. To obtain these effects, when V is included, 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 will decrease, and there is a risk of a decrease in low-temperature toughness. Therefore, when V is included, the V content should be 0.10% or less. The V content is preferably 0.08% or less.

[0040] Cr:0.50% or less Cr is an element that enhances the hardenability of steel and further increases its strength, and can be included as needed. To obtain these effects, when Cr is included, it is preferable that the Cr content be 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. Furthermore, ductility decreases. In addition, it may lead to a decrease in low-temperature toughness. Therefore, when Cr is included, the Cr content should be 0.50% or less. Preferably, the Cr content is 0.40% or less.

[0041] Mo: 0.50% or less Mo is an element that enhances the hardenability of steel and further increases its strength, and can be included as needed. To obtain these effects, when Mo is included, 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 will decrease, and there is a risk of a decrease in low-temperature toughness. Therefore, when Mo is included, the Mo content should be 0.50% or less. Preferably, the Mo content is 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 included as needed. To obtain this effect, when Cu is included, the Cu content is preferably 0.01% or more, and more preferably 0.10% or more. However, if the Cu content exceeds 0.40%, it leads to a decrease in low-temperature toughness, and the desired low-temperature toughness cannot be obtained. Therefore, when Cu is included, the Cu content should be 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 included as needed. To obtain this effect, when Ni is included, 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 become excessively large. Also, the yield ratio will increase, and the desired yield ratio cannot be obtained. Therefore, when Ni is included, the Ni content should be 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, which are thinned during the hot rolling process, and can be included as needed. To obtain this effect, when Ca is included, 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, and the low-temperature toughness also decreases, 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 included, the Ca content should be 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 refinement of the microstructure by lowering the ferrite transformation initiation temperature. To obtain this effect, when B is included, 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 produced may increase, and the yield ratio may become high, making it impossible to obtain the desired yield ratio. Therefore, when B is included, the B content should be 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 included as needed. To obtain such effects, when Sn is included, 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%, the toughness decreases and the desired low-temperature toughness cannot be obtained. Also, the hot workability decreases and cracking may occur during hot rolling. For this reason, when Sn is included, the Sn content should be 0.100% or less. The Sn content is preferably 0.070% or less, and 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 included as needed. To obtain such effects, when Sb is included, 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%, the toughness decreases and the desired low-temperature toughness cannot be obtained. Also, the hot workability decreases and cracking may occur during hot rolling. For this reason, when Sb is included, the Sb content should be 0.100% or less. The Sb content is preferably 0.070% or less, and more preferably 0.050% or less.

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

[0049] Next, preferred embodiments of the component composition of the rectangular steel pipe of the present invention will be described.

[0050] (Total content of Si, Cr, Cu, Ni, Sn, and Sb) In the following explanation, the total content of Si, Cr, Cu, Ni, Sn, and Sb will be expressed as A (in mass%), as shown in formula (1) below. Here, components that are not present are calculated as having a content of 0. A=%Si+%Cr+%Cu+%Ni+%Sn+%Sb...(1)

[0051] The upper and lower limits of A are not particularly limited. However, if A is 0.20% or more, the yield strength and tensile strength can be further increased. Also, if A is 1.00% or less, the yield strength decreases, making it easier to obtain the desired yield ratio. For this reason, it is preferable that A is between 0.20% and 1.00%. 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 rectangular steel pipe of the present invention will be described.

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

[0054] First, the area ratio of each tissue will be explained. The area ratio of each tissue can be measured by taking a cross-section parallel to the rolling direction from the center of the width of the flat plate section 2 of the rectangular steel pipe 1 (the width indicated by the arrow in Figure 1) and at the center of the plate thickness (center of the wall thickness), and then etching it with Nital and observing it. More specifically, it can be measured by the method described in the examples.

[0055] Ferrite: 80-95% by area Ferrite is a soft material and is the main phase of the microstructure to obtain the desired yield strength and low yield ratio. Specifically, it should make up 80% or more of the microstructure by area. If the area ratio of ferrite is less than 80%, the yield stress will be excessively high, and the desired yield ratio cannot be obtained. The area ratio of ferrite is preferably 82% or more. On the other hand, if the area ratio of ferrite exceeds 95%, the strength will decrease, and the desired yield strength and tensile strength cannot be obtained. Therefore, the area ratio of ferrite should be 95% or less. The area ratio of ferrite is preferably 93% or less.

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

[0057] The content of each of the perlite, pseudo-perlite, and upper bainite structures is not particularly limited. For example, the lower limit of the total area percentage of perlite and pseudo-perlite is not particularly limited. do not have However, it is preferable that the total area ratio of perlite and pseudo-perlite be 7% or more. When the total area ratio of perlite and pseudo-perlite is 7% or more, the yield ratio can be kept lower, resulting in better seismic resistance. It is more preferable that the total area ratio of perlite and pseudo-perlite be 8% or more. Furthermore, there is no particular upper limit to the total area ratio of perlite and pseudo-perlite, and it may be 20% or less, but it is preferable that the total area ratio of perlite and pseudo-perlite be 15% or less, and more preferably 14% or less, because it is easier to obtain excellent low-temperature toughness when it is 15% or less.

[0058] Furthermore, the upper limit of the area percentage of upper bainite is not particularly limited and may be 20% or less, but it is preferable that the area percentage of upper bainite be 7% or less, as this makes it easier to obtain a low yield ratio. More preferably, the area percentage of upper bainite is 5% or less. The lower limit of the area percentage of upper bainite is not particularly limited and may be 0%, and upper bainite may not be present at all.

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

[0060] When other microstructures are included, the area ratio (total area ratio) of the other microstructures is not particularly limited. However, for example, if martensite or lower bainite is present, the toughness may decrease, and if austenite is present, the tensile strength may decrease or the yield ratio may increase. Therefore, the lower the area ratio of the other microstructures, the more preferable, specifically, 5% or less is preferable, 1% or less is more preferable, and it is even more preferable that no other microstructures are included.

[0061] Next, the crystal grain size will be described. As described above, in order to obtain a desired yield ratio, yield strength, and tensile strength, the square steel pipe of the present invention is a composite structure steel in which a soft structure and a hard structure are mixed. However, the composite structure steel is inferior in toughness compared to a single structure steel. Therefore, in the present invention, by controlling the crystal grain size, it is possible to simultaneously achieve a desired strength, yield ratio, and toughness.

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

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

[0064] Average grain size d of perlite and pseudo-perlite p : 3.0~18.0μm If 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. Furthermore, if the average grain size of pearlite and pseudo-pearlite exceeds 18.0 μm, the desired tensile strength cannot be obtained or the low-temperature toughness decreases. Therefore, the average grain size of pearlite and pseudo-pearlite d p The grain size should be 3.0 to 18.0 μm. The average grain size d of perlite and pseudo-perlite. p The grain size is preferably 4.0 μm or larger, and more preferably 4.5 μm or larger. Also, the average grain size d of the perlite and pseudo-perlite. p The particle size is preferably 17.0 μm or less, and more preferably 16.5 μm or less.

[0065] d average crystal grain size of ferrite f The average crystal grain size d of perlite and pseudo-perlite relative to p ratio d p / d f :0.60 or more and 1.00 or less In addition to the average crystal grain size mentioned above, the average crystal grain size d of ferrite f and the average crystal grain size d of perlite and pseudo-perlite p The ratio (=d p / d f ) is stipulated. d p / d f If d is less than 0.60, the desired yield ratio cannot be obtained. On the other hand, d p / d f If it is greater than 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, d p / d f Let it be between 0.60 and 1.00. p / d f It is preferably 0.62 or higher, and more preferably 0.65 or higher. Also, d p / d f It is preferably 0.97 or less, and more preferably 0.95 or less.

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

[0067] Total number density of perlite and pseudo-perlite: 2000 pieces / mm² 2 That's all. In the above microstructure, there are a total of 2000 perlite and pseudo-perlite particles / mm². 2 If the value is less than 2000, the toughness will decrease, and the desired low-temperature toughness will not be obtained. Therefore, in the above microstructure, the total number density of pearlite and pseudo-pearlite should be 2000 particles / mm³. 2 The above is complete. The total is preferably 2200 pieces / mm 2 The above is more preferable at 2400 pieces / mm 2 That concludes the explanation. There is no particular upper limit to the total number density, but since an excess of perlite and pseudo-perlite may increase the yield ratio, the total number density of perlite and pseudo-perlite should be 6000 particles / mm³. 2 The following is preferable: 5500 pieces / mm 2 The following are preferable.

[0068] By having the above-described component composition and microstructure, it is possible to obtain a square steel pipe with 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 of a building or a cold storage warehouse in cold regions where the ambient temperature is below freezing, and excellent seismic resistance can be ensured. The following describes the preferred mechanical properties of the flat portion of the square steel pipe of the present invention.

[0069] (yield strength) The rectangular steel pipe of the present invention has a high yield strength as a result of having the above-described component composition and microstructure. The yield strength is preferably 295 MPa or higher, and more preferably 320 MPa or higher. There is no particular upper limit to the yield strength, but it may be, for example, 470 MPa or lower. 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 rectangular steel pipe of the present invention has high tensile strength as a result of having the above-mentioned component composition and microstructure. The tensile strength is preferably 400 MPa or higher, and more preferably 420 MPa or higher. There is no particular upper limit to the tensile strength, but it may be, for example, 620 MPa or lower, or 500 MPa or lower. The tensile strength is the tensile strength in the axial direction of the pipe, and can be measured specifically by the method described in the examples.

[0071] (yield ratio) The rectangular steel pipe of the present invention has a low yield ratio as a result of having the above-mentioned component composition and microstructure. The yield ratio is preferably 0.90 or less, and more preferably 0.87 or less. Furthermore, 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 absorption energy) The rectangular steel pipe of the present invention, having the above-described component composition and microstructure, exhibits a high Charpy absorption energy at low temperatures. At a test temperature of -30°C, the Charpy absorption energy is preferably 27 J or higher, more preferably 47 J or higher, and even more preferably over 100 J. There is no particular upper limit to the Charpy absorption energy at a test temperature of -30°C, but it may be, for example, 250 J. Furthermore, at a test temperature of -35°C, the Charpy absorption energy is preferably 27 J or higher. There is no particular upper limit to the Charpy absorption energy at a test temperature of -35°C, but it may be, for example, 250 J. The Charpy absorption energy can be measured by the method described in the examples.

[0073] (ductile-brittle transition temperature) The rectangular steel pipe of the present invention, having the above-described component composition and microstructure, exhibits a low ductile-brittle transition temperature. The ductile-brittle transition temperature is preferably -30°C or lower, more preferably -35°C or lower, and even more preferably -50°C or lower. The lower limit of the ductile-brittle transition temperature is not particularly limited, but may be, for example, -70°C or higher. The ductile-brittle transition temperature can be measured by the method described in the examples.

[0074] Next, the dimensions of the rectangular steel pipe of the present invention will be described.

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

[0076] The length of one side of the rectangular steel pipe is not particularly limited. The rectangular steel pipe of the present invention is not limited to rectangular steel pipes in which all sides are equal in length ((long side length / short side length) value is 1.0), and the (long side length / short side length) value may be greater than 1.0. However, if the (long side length / short side length) value of the rectangular steel pipe exceeds 2.5, local buckling is more likely to occur on the long side, and the compressive strength in the direction of the pipe axis decreases. For this reason, it is preferable that the (long side length / short side length) value of the rectangular steel pipe be between 1.0 and 2.5. More preferably, the (long side length / short side length) value is between 1.0 and 2.0.

[0077] Next, an embodiment of the method for manufacturing rectangular steel pipes according to the present invention will be described.

[0078] The present invention provides a method for manufacturing a rectangular steel pipe, comprising: heating a steel material having the above-mentioned component composition; hot-rolling the heated steel material to form a hot-rolled steel sheet; cooling the hot-rolled steel sheet; winding the cooled hot-rolled steel sheet; forming the wound hot-rolled steel sheet 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 rectangular steel pipe.

[0079] The conditions for each process are described below. In the following description of the manufacturing method, unless otherwise specified, the "°C" temperature indication refers to the surface temperature of the steel material or steel plate. The surface temperature can be measured using a radiation thermometer or similar device. The temperature at the center of the thickness of the steel plate can be determined by calculating the temperature distribution within 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 strip.

[0080] (Steel material) Any steel material having the above-described component composition can be used as the steel material. For example, a steel slab can be used as the steel material. The method of melting the molten steel is not particularly limited, and any melting method such as a converter, electric furnace, or vacuum melting furnace can be used. The molten steel may be further subjected to secondary refining such as ladle refining. The casting method is also not particularly limited, but a steel material having the desired dimensions can be obtained by a casting method such as continuous casting. Alternatively, the ingot-parting rolling method may be applied instead of the continuous casting method.

[0081] (heating) First, the steel material is heated. The heating method is not particularly limited; in addition to the conventional method of cooling the obtained steel material to room temperature and then heating it again, a direct rolling method may be used, in which the hot slab is loaded into the heating furnace without cooling to room temperature, from the viewpoint of energy saving.

[0082] Heating temperature: 1100℃ or higher and 1300℃ or lower If the heating temperature is below 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 coarser, and fine austenite grains cannot be obtained in 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 should be between 1100°C and 1300°C. The heating temperature is preferably 1120°C or higher, and more preferably 1140°C or higher. Furthermore, the heating temperature is preferably 1280°C or lower, and 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 carried out by sequentially performing rough rolling and finish rolling.

[0084] Rough rolling completion temperature: 850°C to 1150°C If the rough rolling completion temperature is below 850°C, the steel sheet surface temperature will fall below the ferrite transformation initiation temperature during 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 completion temperature exceeds 1150°C, fine austenite grains cannot be obtained, making it difficult to secure the average grain size of the microstructure. For this reason, the rough rolling completion temperature should be between 850°C and 1150°C. Preferably, the rough rolling completion temperature is 860°C or higher, and more preferably 870°C or higher. Furthermore, preferably, the rough rolling completion temperature is 1100°C or lower, and more preferably 1050°C or lower.

[0085] Time from the end of heating of 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 high-reduction rolling is performed in the subsequent finish rolling, the austenite will not be sufficiently refined. Furthermore, pearlite and pseudo-pearlite cannot be refined as much as ferrite. As a result, the low-temperature toughness will decrease. For this reason, the time from the end of heating the steel material to the end of rough rolling should be 10.0 minutes or less. The aforementioned 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 the aforementioned time is not limited, but may be, for example, 1.0 minute or more.

[0086] Total reduction ratio at temperatures above 930°C during rough rolling: 60% or more If the total reduction ratio at 930°C or above during rough rolling is less than 60%, the austenite refinement will be insufficient, and even if high-reduction rolling is performed during finish rolling, fine pearlite and pseudo-pearlite cannot be obtained. This leads to a decrease in low-temperature toughness. Therefore, the total reduction ratio at 930°C or above should be 60% or more. The total reduction ratio is preferably 65% ​​or more, and more preferably 68% or more. There is no particular upper limit to the total reduction ratio, but if the reduction ratio is too high, it will place an excessive load on the manufacturing equipment for hot rolling, making manufacturing difficult, so the total reduction ratio is preferably 85% or less.

[0087] Here, if a portion of the rough rolling is carried out at a temperature of 930°C or lower, the total reduction ratio is calculated excluding that portion of the rough rolling; otherwise, the total reduction ratio in the rough rolling is used as the total reduction ratio. That is, the total reduction ratio at 930°C or higher in the rough rolling is calculated as follows: (t i -t f ) / t i It is calculated by multiplying by 100, otherwise (t i -t r ) / t i This is the value calculated by multiplying by 100. Here, t i : Plate thickness before rough rolling, t f : Plate thickness at 930℃, t r : This is the plate thickness at the end of rough rolling.

[0088] Finish rolling completion temperature: 750°C to 850°C If the finish rolling completion temperature is below 750°C, the steel sheet surface temperature may fall below the ferrite transformation initiation temperature during finish rolling, potentially leading to the formation of elongated ferrite in the rolling direction and a decrease in workability. On the other hand, if the finish rolling completion temperature exceeds 850°C, fine austenite grains may not be obtained, resulting in coarse grains and making it difficult to secure the desired strength. For this reason, the finish rolling completion temperature should be between 750°C and 850°C. Preferably, the finish rolling completion temperature is 770°C or higher, and more preferably 780°C or higher. Furthermore, preferably, the finish rolling completion temperature is 830°C or lower, and more preferably 820°C or lower.

[0089] Total reduction ratio at 930°C or below during hot rolling: 40% or more In this invention, by refining the subgrains in austenite during hot rolling, the ferrite, pearlite, and pseudo-pearlite generated during subsequent cooling and winding are refined, thereby obtaining 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 pre-recrystallization temperature range and introduce sufficient processing strain. If the total reduction ratio below 930°C is less than 40%, the rolling in the austenite pre-recrystallization temperature range becomes insufficient, and the grain size of ferrite, pearlite, and pseudo-pearlite increases, leading to a decrease in low-temperature toughness. Therefore, the total reduction ratio below 930°C during hot rolling should be 40% or more. The total reduction ratio is preferably 42% or more, and more preferably 45% or more. While there is no particular upper limit to the total reduction ratio, if the reduction ratio is too high, it will place an excessive load on the manufacturing equipment for hot rolling, making manufacturing difficult. Therefore, the total reduction ratio is preferably 60% or less.

[0090] The total reduction ratio mentioned above is calculated by comparing the plate thickness at 930°C with the plate thickness after hot rolling. That is, the total reduction ratio is (t f -t o ) / t f This is the value calculated by multiplying by 100. Here, tf : Plate thickness at 930℃, t o This is 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. Cooling methods include, for example, water cooling by spraying water from a nozzle, or cooling by spraying a cooling gas. Here, it is preferable to apply the cooling treatment to both sides of the hot-rolled steel sheet so that both sides are cooled under the same conditions. Furthermore, from the viewpoint of suppressing the coarsening of the grain size, it is preferable to start cooling immediately after the completion of finish rolling.

[0092] Average cooling rate at the center of the plate thickness: 10.0°C / s or less If the average cooling rate exceeds 10.0°C / s at the center of the plate thickness, the amount of upper bainite produced increases, and the desired average grain size cannot be obtained. Therefore, the desired yield ratio cannot be obtained. For this reason, the average cooling rate at the center of the plate thickness should be 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. The lower limit of the average cooling rate is not particularly limited, but by setting the average cooling rate to 1.0°C / s or more, it becomes easier to keep the cooling stop temperature within the range described later. For this reason, 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] To control the average cooling rate, it is preferable to adjust the amount of cooling water or cooling gas, pressure, injection time, injection angle, and the transport speed of the hot-rolled steel sheet. To control the average cooling rate, it is also possible to determine the conditions for the cooling process of the hot-rolled steel sheet by performing a heat transfer analysis in advance, and then perform the cooling based on those conditions.

[0094] The average cooling rate is calculated as ((temperature at the center of the thickness of the hot-rolled steel sheet at the start of cooling - temperature at the center of the thickness of the hot-rolled steel sheet at the end of cooling) / cooling time).

[0095] Cooling stop temperature at the center of the plate thickness: 480°C to 650°C If the cooling stop temperature at the center of the plate thickness is less 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. In addition, temperature unevenness is likely to occur in the length and / or width directions of the hot-rolled steel sheet during cooling, which may lead to variations in mechanical properties. On the other hand, if the cooling stop temperature at the center of the plate thickness exceeds 650°C, the ferrite grains become coarser, and the desired grain size cannot be obtained. Therefore, the cooling stop temperature at the center of the plate thickness should be between 480°C and 650°C. 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] (Rewinding) Next, the cooled hot-rolled steel sheet is rolled up.

[0097] Winding temperature at the center of the plate thickness: 480°C to 650°C If the winding temperature at the center of the sheet thickness is less than 480°C, a large amount of upper bainite will form on the surface of the steel sheet, and the desired yield ratio cannot be obtained. If the winding temperature at the center of the sheet thickness exceeds 650°C, the ferrite grains will coarse, and the desired average grain size cannot be obtained. Therefore, the winding temperature at the center of the sheet thickness should be between 480°C and 650°C. The winding temperature is preferably 490°C or higher, and more preferably 495°C or higher. Furthermore, the winding temperature is preferably 640°C or lower, and more preferably 635°C or lower.

[0098] Furthermore, the hot-rolled steel sheet may be allowed to cool after being rolled up.

[0099] The square steel pipe of the present invention is made from the hot-rolled steel sheet described above. The square steel pipe of the present invention can be manufactured, for example, by forming the hot-rolled steel sheet into a round steel pipe and then shaping the round steel pipe. A round steel pipe can be obtained by forming a hot-rolled steel sheet into a cylindrical shape by roll forming (typically cold roll forming) to make an open pipe, and then welding the ends of the hot-rolled steel sheet (the butt joint portion of the open pipe). Then, a square steel pipe can be obtained by shaping the round steel pipe into a square steel pipe using rolls positioned above, below, left, and right of the round steel pipe. In other words, the square steel pipe of the present invention may be a roll-formed square steel pipe.

[0100] The square steel pipe of the present invention is manufactured by the method described above. According to the above manufacturing method, when obtaining a square steel pipe from a hot-rolled steel sheet by forming, there is no need to specially control the roll gap of the sizing stand or the 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 a building structure according to the present invention. In the building structure of the present invention, the square steel pipe (square steel pipe 1) described above is used as a column material. Reference numerals 4, 5, 6, and 7 indicate the main beam, secondary beam, diaphragm, and intermediate column, respectively. As described above, the square steel pipe of the present invention has excellent mechanical properties in its flat section. Therefore, the building structure of the present invention exhibits excellent seismic resistance. [Examples]

[0102] The present invention will be described in more detail below based on the following examples. However, the present invention is not limited to the following examples.

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

[0104] Subsequently, the obtained hot-rolled steel sheet was formed into a cylindrical open pipe by roll forming (cold roll forming), and the butt joint was welded with electric resistance welding to form a round steel pipe. Then, the round steel pipe was formed by rolls positioned above, below, left, and right of it to form a square steel pipe in a cross-sectional view perpendicular to the pipe axis. The resulting square steel pipe had corners and flat sections, and had the side length and wall thickness (thickness of the flat section) shown in Table 2. The wall thickness of the square steel pipe was the same as the thickness of the hot-rolled steel sheet used.

[0105] [Table 1]

[0106] [Table 2] TIFF0007848946000003.tif233140

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

[0108] [Histological observation] The microstructure of the flat section of the above-mentioned rectangular steel pipe was observed at the center of its thickness. The specimen for microstructure observation was taken from the 3 o'clock position of the rectangular steel pipe, with the weld facing 12 o'clock, at the center of the width and thickness of the flat section (at a position of thickness t / 2 (t: thickness of the flat section)). The observation surface was prepared by polishing and then etching with Nital, ensuring the observation surface was in the rolling direction during hot rolling. Microstructure observation was performed using optical microscope images (magnification: 1000x).

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

[0110] The area percentage of each tissue was calculated by observing in five fields of view and averaging the values ​​obtained in each field. Here, the area percentage obtained from tissue observation was used as the area percentage of each tissue. Here, ferrite is a product of diffusion transformation and exhibits a nearly recovered tissue with a low dislocation density. Polygonal ferrite and pseudopolygonal ferrite are included in this category. Perlite is a tissue in which cementite and ferrite are arranged in layers. Pseudo-perlite is a tissue in which cementite arranged in a dotted row is observed within the ferrite. Upper bainite is a multiphase tissue of lath-like ferrite and cementite with a high dislocation density. In addition to the above shapes, differentiation was made by utilizing the fact that ferrite is white, perlite is black, pseudo-perlite is black or gray, and upper bainite is white or gray. Note that in all examples, no tissues other than ferrite, perlite, pseudo-perlite, and upper bainite were included.

[0111] Furthermore, the average grain size of ferrite, pearlite, and pseudo-pearlite was measured from the optical microscope images described above. Separated images of ferrite, pearlite, and pseudo-pearlite were obtained from the optical microscope images using Fiji, an image processing package of the free software ImageJ, and the average grain size of ferrite, pearlite, and pseudo-pearlite was calculated in terms of equivalent circle diameter. The grain size of five fields was calculated for each rectangular steel pipe, and the average value was taken as the average grain size of each microstructure in each rectangular steel pipe. In the grain size analysis, grain sizes smaller than 2.0 μm were excluded from the analysis as measurement noise.

[0112] Furthermore, the number density of perlite and pseudo-perlite (unit: pieces / mm²) was obtained from the above optical microscope images. 2 The number of perlite and pseudo-perlite particles present in the five fields of view observed was totaled, and the observed area (unit: mm²) was calculated. 2 The number density was calculated by dividing by ). Note that the number of perlite and pseudo-perlite particles is the total number of perlite and pseudo-perlite particles with an equivalent circle diameter of 2.0 μm or more.

[0113] [Tensile test] Tensile tests were conducted in accordance with the provisions of JIS Z 2241 (2011). Figure 3 is a schematic diagram showing the sampling location of the tensile test specimens (JIS No. 5 tensile test specimens) used in the tensile tests. As shown in Figure 3, the tensile test specimens were taken from the center of the width of the flat plate section at the 3 o'clock position when the welded part of the square steel pipe is at the 12 o'clock position, so that the tensile direction was parallel to the pipe axis. Tensile tests were performed on the sampled tensile test specimens 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 YS, TS, and the yield ratio. For strength, a yield strength of 295 MPa or higher and a tensile strength of 400 MPa or higher were considered acceptable. For yield ratio, a yield ratio of 0.90 or lower was considered 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 location of the Charpy impact specimen (V-notch standard specimen) used in the Charpy impact test. As shown in Figure 4, the Charpy impact specimen was taken from the center of the width of the flat plate section at the 3 o'clock position when the welded part of the square steel pipe is at the 12 o'clock position, so that the longitudinal direction of the specimen is parallel to the pipe axis. Here, for square steel pipes with a wall thickness of more than 19 mm, the Charpy impact specimen was taken from the position of wall thickness t / 4 (1 / 4 of the wall thickness t from the outer surface of the square steel pipe), and for square steel pipes with a wall thickness of 19 mm or less, the specimen was taken from the position of wall thickness t / 2 (1 / 2 of the wall thickness t from the outer surface of the square steel pipe). The Charpy impact test was conducted at test temperatures of -70°C, -50°C, -30°C, -10°C, and 10°C. Three tests were conducted at each test temperature, and the average values ​​of the ductile-brittle transition temperature and Charpy absorption energy were determined. For low-temperature toughness, a Charpy absorption energy of 27 J or higher and a ductile-brittle transition temperature of -30°C or lower were considered acceptable.

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

[0116] [Table 3] TIFF0007848946000005.tif233140

[0117] In Table 3, rectangular steel pipes No. 1-19, 46, and 47 are examples of the present invention, while rectangular steel pipes No. 20-45, 48, and 49 are comparative examples.

[0118] As shown in Table 3, all of the square steel pipes of the present invention met the acceptance criteria 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 conditions where the total reduction ratio at 930°C or higher during rough rolling was 68% or more, and the time from the end of heating to the end of rough rolling was 7.0 minutes or less. These square steel pipes had a Charpy absorption energy at -30°C exceeding 100 J and a ductile-brittle transition temperature of -50°C or lower, exhibiting particularly excellent low-temperature toughness.

[0119] In contrast, comparative examples No. 20 to 35 did not have the above component composition. Furthermore, comparative examples No. 36 to 45, 48, and 49 did not meet the above manufacturing conditions and did not have the above microstructure. Therefore, comparative examples No. 20 to 45, 48, and 49 could not be used to obtain square steel pipes with high strength, low yield ratio, 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 grain size of the microstructure fell outside the range of the present invention. Consequently, the Charpy absorption energy and ductile-brittle transition temperature of the plate portion at -30°C did not meet the acceptance criteria. [Explanation of symbols]

[0121] 1. Rectangular steel pipe 2 Flat plate part 3 corners 4 girder 5 Small beam 6 Diaphragm 7 bay pillars

Claims

1. A rectangular steel pipe having a flat section and a corner section, In mass percent, 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, N: 0.0100% or less It contains, with the remainder consisting of Fe and unavoidable impurities, The composition has a component structure in which the ratio of Mn content to Ti content (%Mn / %Ti) is 30.0 or more and 150.0 or less. The aforementioned rectangular steel pipe has a welded joint, At the center of the plate thickness of the flat plate portion, when the welded portion is viewed in a cross-sectional view perpendicular to the pipe axis and the direction is 12 o'clock, at the center of the plate thickness on the 3 o'clock side, Ferrite: 80-95% by area ratio, One or more materials selected from the group consisting of perlite, pseudoperlite, and upper bainite: comprising 5-20% of the total area, The total area ratio of perlite and pseudo-perlite is 6% or more. The average crystal grain size d of ferrite with an equivalent circle diameter of 2.0 μm or more. ｆ The size is 5.0 to 20.0 μm. The average crystal grain size d of pearlite with an equivalent circle diameter of 2.0 μm or more and pseudo-pearlite with an equivalent circle diameter of 2.0 μm or more. ｐ The size is 3.0 to 18.0 μm. d ｆ d for ｐ ratio d ｐ / d ｆ is 0.60 or more and 1.00 or less, A rectangular steel pipe having a microstructure in which the total number density of perlite with an equivalent circle diameter of 2.0 μm or more and pseudo-perlite with an equivalent circle diameter of 2.0 μm or more is 2,000 to 6,000 particles / mm².

2. The aforementioned component composition is further expressed in mass%, 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, Sb: 0.100% or less The rectangular steel pipe according to claim 1, comprising one or more types selected from the group consisting of the following.

3. A method for manufacturing a rectangular steel pipe according to claim 1 or 2, The steel material having the above-mentioned component composition is heated to a heating temperature of 1100°C or higher and 1300°C or lower. The heated steel material is hot-rolled to form a hot-rolled steel sheet, The hot-rolled steel sheet is cooled under the conditions that the average cooling rate at the center of the sheet thickness is 10.0°C / s or less and the cooling stop temperature at the center of the sheet thickness is 480°C or higher and 650°C or lower. The cooled hot-rolled steel sheet is wound at a winding temperature of 480°C to 650°C at the center of the sheet thickness, The hot-rolled steel sheet, after being wound, is formed into a cylindrical shape by roll forming, and then the ends are welded to form a round steel pipe. This includes forming the aforementioned round steel pipe into the aforementioned square steel pipe, The hot rolling described above is carried out by sequentially performing rough rolling and finish rolling. The rough rolling is carried out under the following conditions: rough rolling completion temperature: 850°C or higher and 1150°C or lower; time from completion of heating of the steel material to completion of the rough rolling: 10.0 minutes or less; and total reduction ratio at 930°C or higher: 60% or more. The aforementioned finish rolling is carried out under the condition that the finish rolling completion temperature is between 750°C and 850°C. A method for manufacturing square steel pipes, wherein the hot rolling is performed at a temperature of 930°C or lower with a total reduction ratio of 40% or more.

4. A building structure in which the rectangular steel pipe described in claim 1 or 2 is used as a column material.

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