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

Abstract

Provided is an angular steel pipe having a low yield ratio and excellent low-temperature toughness and ductility. This angular steel pipe has a prescribed steel component composition, and the steel structure in the center of the plate thickness of the flat plate section has ferrite as the main phase, the area ratio of a second phase comprising one or more elements selected from pearlite, pseudo-pearlite, and upper bainite is 5-30%, and the average crystal grain size is 10.0-30.0 [mu] m. The number of pearlites and pseudo pearlites having an equivalent circle diameter of 2-20 [mu] m inclusive is 500 / mm2 or more, 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, the elongation at break of the flat plate section is 27.0% or more, and the Charpy absorption energy of the corner section at-10 DEG C is 80 J or more. And the ductile-brittle transition temperature of the corner section is-10 DEG C or less.

Description

Technical Field

[0001] This invention relates to angular steel tubes and their manufacturing methods, as well as building structures, particularly to angular steel tubes (corner columns) manufactured by cold rolling, exhibiting low yield ratio, excellent low-temperature toughness, and high ductility. Specifically, this invention relates to angular steel tubes suitable for use as structural components in large buildings. The invention also relates to building structures obtained using these angular steel tubes. Background Technology

[0002] In recent years, for structural building components used in large buildings such as factories, warehouses, and commercial facilities (hereinafter referred to as buildings), there has been a push towards higher strength in order to reduce construction costs through lightweighting. In particular, angular steel tubes with flat sections and corner sections used as columns in buildings require high strength in the flat sections. Meanwhile, from a seismic perspective, angular steel tubes used in structural building components are also required to have a low yield ratio, excellent low-temperature toughness, and high ductility.

[0003] Angle-shaped steel pipes are typically manufactured from hot-rolled steel plates (hot-rolled steel strips) or thick steel plates by cold forming. Methods for cold forming include cold stamping and bending or cold rolling.

[0004] Angle-shaped steel pipes (sometimes referred to as roll-formed angle-shaped steel pipes) are manufactured by cold-rolling hot-rolled steel plates to form cylindrical open tubes, and then resistance welding the butt joints to create circular steel pipes. Then, using rolls positioned above, below, to the left, and right of the circular steel pipe, a few percent of the circular steel pipe is drawn along its axial direction, and then shaped into an angle to manufacture angle-shaped steel pipes. On the other hand, angle-shaped steel pipes (sometimes referred to as stamped angle-shaped steel pipes) are manufactured by cold-stamping and bending thick steel plates to achieve a cross-sectional shape of either a ロ (quadrilateral) or コ (U) shape. They are then joined together by submerged arc welding.

[0005] Compared to the stamping method, the roll forming method for angled steel tubes offers advantages such as higher productivity and faster production time. However, in stamping, the flat portion of the angled steel tube is not cold-formed, and only the corners undergo work hardening. In contrast, in roll forming, especially when cold-formed into a cylindrical shape, a large processing strain is introduced along the tube axis throughout the entire circumference of the tube. Due to this processing strain, roll forming also exhibits a high yield ratio and low toughness in the tube axis direction in the flat portion. For roll forming, the corners experience greater processing strain compared to the flat portion, thus exhibiting a higher yield ratio and lower toughness compared to the flat portion.

[0006] Furthermore, for roll-formed angled steel pipes, the greater the wall thickness, the greater the work hardening during roll forming, resulting in a higher yield ratio and a further decrease in toughness. Therefore, especially in the manufacture of thick-walled roll-formed angled steel pipes, it is necessary to select raw materials that can also withstand the increase in yield ratio and decrease in toughness caused by roll forming.

[0007] For such requirements, for example, Patent Document 1 proposes the following technology: by hot rolling a steel raw material containing C: less than 0.20% by weight, and also containing Mn: 0.40 to 0.90%, Nb: 0.005 to 0.040%, and Ti: 0.005 to 0.050% by weight, to produce coils through a hot rolling process with a reduction rate of more than 55% in the non-recrystallization temperature range, a rolling end temperature of 730 to 830°C, and a coiling temperature of less than 550°C, the outer perimeter drawing rate in the steel pipe forming process is less than 3 times the plate thickness, thereby manufacturing an angled steel pipe with a yield ratio of less than 90% and a Charpy absorption energy of more than 27J at a test temperature of 0°C.

[0008] Patent document 2 proposes the following technique: Steel containing 0.07–0.18% C and 0.3–1.5% Mn (by mass%) is heated to a temperature of 1100–1300°C; then, rough rolling is performed with a roughing end temperature of 1150–950°C, and finish rolling is performed with a finish rolling start temperature of 1100–850°C and a finish rolling end temperature of 900–750°C; finally, the process is carried out in a manner where the cooling stop temperature, measured by a surface thermometer, is 550°C or higher. The process involves a primary cooling, a secondary cooling with air cooling for 3–15 seconds, and a tertiary cooling to below 650°C at an average cooling rate of 4–15°C / s within a temperature range of 750–650°C measured at the center of the plate thickness. This process results in a second phase frequency of 0.20–0.42 within the steel microstructure, thereby producing an angled steel tube exhibiting a low yield ratio of less than 80% and a Charpy impact test absorption energy of 150 J or more at a test temperature of 0°C.

[0009] Patent document 3 proposes the following technique: steel containing 0.07-0.18% C and 0.3-1.5% Mn by mass is heated to a heating temperature of 1100-1300°C. Then, roughing is performed with a roughing end temperature of 1150-950°C and a finishing start temperature of 1100-850°C and a finishing end temperature of 900-750°C. Then, the steel is cooled to a coiling temperature of 500-650°C in a manner that the average cooling rate in the temperature range of 750-650°C (surface temperature) is 20°C / s or less, the time until the temperature at the center of the plate reaches 650°C is 35s or less, and the average cooling rate in the temperature range of 750-650°C at the center of the plate is 4-15°C / s. This produces an angled steel tube with a low yield ratio of 80% or less and a Charpy impact energy absorption of 150J or more at a test temperature of 0°C.

[0010] Patent document 4 proposes the following technique: Steel having a composition containing, by mass percent, C: 0.07–0.20%, Mn: 0.3–2.0%, P: less than 0.03%, S: less than 0.015%, Al: 0.01–0.06%, N: less than 0.006%, with the balance being Fe and unavoidable impurities, is heated to a heating temperature of 1100–1300°C. Then, roughing is performed with a roughing end temperature of 1150–950°C, and finishing is performed with a finishing start temperature of 1100–850°C and a finishing end temperature of 900–750°C. Finally, cooling is performed at an average cooling rate of 4–25°C / s (based on the center-thickness temperature) from the start of cooling to the end of cooling until a cooling stop temperature below 580°C is reached. The steel tube, manufactured by means of an initial cooling process of 0.2s or more but less than 3.0s during the initial cooling process of 10s from the start of cooling, is then wound at a coiling temperature of 580°C or below and then cooled again. The steel structure of the central part of the plate thickness has a main phase composed of ferrite and a second phase composed of one or more of pearlite, pseudo-pearlite and upper bainite with an area fraction of 8 to 20%, and the average grain size of the steel structure containing the main phase and the second phase is 7 to 20 μm. The steel structure of the surface and back of the plate thickness is a single phase of ferrite or bainitic ferrite with an average grain size of 2 to 20 μm. It exhibits a low yield ratio of 90% or less and mechanical properties of absorbing energy of 27 J or more in a Charpy impact test at a test temperature of 0°C.

[0011] Patent document 5 proposes the following technology: when a steel plate with a specified composition is cold-rolled into an angle shape, the width W of the steel plate is relative to the perimeter C of the angled steel tube on the exit side of the angle forming machine stand. OUTThe ratio satisfies the specified formula, and the perimeter C of the resistance welded steel pipe on the entrance side of the angle forming machine frame. IN The perimeter C of the angled steel tube on the exit side of the angle forming machine frame OUT Angle steel pipes are manufactured by controlling the gap between the rollers of the sizing stand and the rollers of the angle forming stand in a manner that satisfies the prescribed formula. This produces angle steel pipes with mechanical properties such that the uniform elongation E2 at a position 1 / 4t away from the outer surface of the corner in the wall thickness direction is 0.60 times or more than the uniform elongation E1 at a position 1 / 4t away from the outer surface of the flat plate in the wall thickness direction, and the Charpy absorption energy at -10°C at a position 1 / 4t away from the outer surface of the corner is 100J or more.

[0012] Existing technical documents

[0013] Patent documents

[0014] Patent Document 1: Japanese Patent Application Publication No. 9-87743

[0015] Patent Document 2: Japanese Patent No. 5594165

[0016] Patent Document 3: Japanese Patent No. 5589885

[0017] Patent Document 4: Japanese Patent No. 6388091

[0018] Patent Document 5: International Publication No. 2022 / 075026 Summary of the Invention

[0019] The problem that the invention aims to solve

[0020] In this context, for angled steel pipes manufactured by cold rolling, the greater the wall thickness or the smaller the side length, the greater the processing strain introduced into the angled steel pipe, resulting in a greater increase in yield ratio, a greater decrease in ductility, and a greater decrease in toughness. Therefore, for angled steel pipes manufactured by the methods disclosed in Patent Documents 1 to 3, especially when the plate thickness exceeds 25 mm, there is a problem that the yield ratio becomes too high and cannot meet the requirement of a yield ratio of 90% or less.

[0021] The angled steel pipe manufactured using the technology described in Patent Document 4 has an absorbed energy of over 27 J in a Charpy impact test at a test temperature of 0°C. However, depending on the environment in which the angled steel pipe is used, its low-temperature toughness is sometimes insufficient.

[0022] The technology described in Patent Document 5 has the following problems: the roller gap between the sizing frame and the corner forming frame when forming the steel plate into an angle needs to be properly controlled, and the optimal manufacturing conditions need to be determined for each size of the angled steel pipe, thus requiring a large amount of labor.

[0023] The present invention was made in view of the above-mentioned problems, and its purpose is to provide an angled steel pipe with a low yield ratio, excellent low-temperature toughness, and high ductility.

[0024] Furthermore, the object of the present invention is to provide a method for manufacturing the aforementioned angled steel pipe and a building structure using the aforementioned angled steel pipe.

[0025] Methods for solving problems

[0026] In order to solve the above problems, the inventors conducted in-depth research on angular steel pipes that meet the strength (yield strength, tensile strength), yield ratio, ductility (uniform elongation, elongation at break), and low-temperature toughness (absorbed energy and ductile-brittle transition temperature) as the objectives of this invention. As a result, the following insights were obtained (1) to (4).

[0027] (1) The C content needs to be 0.07% by mass or more. In addition, the main phase of the steel structure in the center of the angled steel pipe needs to be ferrite.

[0028] (2) The steel microstructure of the center portion of the angular steel pipe should contain a second phase composed of one or more of pearlite, pseudo-pearlite, and upper bainite, with an area fraction of 5-30%. Furthermore, the steel microstructure should contain a total of 500 pearlite and pseudo-pearlite particles per mm, with an average grain size of 10.0-30.0 μm and an equivalent circle diameter of 2 μm or more but less than 20 μm. 2 above.

[0029] (3) In order to obtain the above-mentioned steel structure, it is extremely important to make the hot rolling reduction rate in the hot rolling process when manufacturing the hot-rolled steel sheet as the raw material for the angled steel tubes within an appropriate range. Specifically, it is necessary to make the total reduction rate at 930°C or below 40% or more, and the total reduction rate of the last two passes of finishing rolling 15% or more.

[0030] (4) In addition, when manufacturing angular steel pipes, a compressive strain of 0.20 to 3.00% needs to be applied along the pipe axis after hot-rolled steel plates are formed into circular steel pipes and / or angular steel pipes.

[0031] This invention is based on these insights and is constituted by the following principles.

[0032] [1] An angular steel pipe having a flat plate portion and an angular portion, wherein the steel composition comprises, by 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 consisting of Fe and unavoidable impurities. The steel microstructure of the central portion of the flat plate portion is mainly composed of ferrite, with a second phase consisting of one or more of pearlite, pseudo-pearlite, and upper bainite having an area fraction of 5 to 30%, an average grain size of 10.0 to 30.0 μm, and in the steel microstructure, a total of 500 pearlite and pseudo-pearlite with an equivalent circle diameter of 2 μm or more and 20 μm or less are present per mm. 2 The above-mentioned 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, an elongation at break of 27.0% or more, a Charpy absorption energy of -10°C of the corner portion of 80 J or more, and a ductile-brittle transition temperature of -10°C or less.

[0033] [2] According to the angled steel pipe described in [1], the steel composition further comprises one or more of the following by mass%: Nb: less than 0.020%, Ti: less than 0.045%, V: less than 0.10%, Cr: less than 0.50%, Mo: less than 0.50%, Cu: less than 0.30%, Ni: less than 0.30%, Ca: less than 0.0100%, B: less than 0.0100%, Sn: less than 0.100%, and Sb: less than 0.100%.

[0034] [3] A method for manufacturing an angled steel pipe, which is the method for manufacturing an angled steel pipe described in [1] or [2] above, wherein a steel raw material having the above-mentioned steel composition is heated at a heating temperature of 1100°C or higher and 1300°C or lower, and a heating time of 40 minutes or higher and 200 minutes or lower, and then hot rolling is performed with a roughing end temperature of 850°C or higher and 1150°C or lower, a finishing end temperature of 750°C or higher and 850°C or lower, and a finishing end temperature of 930°C or lower, resulting in a total reduction rate of 40% or higher, and a total reduction rate of 15% or higher for the last two passes of finishing rolling, and then flattening the plate at the center of the thickness. Cooling is performed at a cooling rate of 10.0°C / s or less and a cooling stop temperature of 480°C or more and 650°C or less at the center of the plate thickness. Then, the plate is rolled at a temperature of 480°C or more and 650°C or less at the center of the plate thickness to produce a hot-rolled steel sheet. Next, the hot-rolled steel sheet is formed into a cylindrical shape by roll forming. Then, the ends of the hot-rolled steel sheet are welded to form a circular steel pipe. Then, it is formed into an angular steel pipe to produce an angular steel pipe. At this time, after forming the circular steel pipe and / or after forming the angular steel pipe, a compressive strain of 0.20% to 3.00% is applied along the pipe axis.

[0035] [4] A building structure that uses the angular steel tubes described in [1] or [2] above as column material.

[0036] Invention Effects

[0037] According to the present invention, it is possible to provide an angled steel pipe with a low yield ratio, excellent low-temperature toughness, and high ductility. Attached Figure Description

[0038] Figure 1 This is a perspective view schematically showing an example of a building structure using the angular steel tube of the present invention.

[0039] Figure 2 This is a schematic diagram showing the cutting position of the tensile test piece in the tensile test implemented in this invention.

[0040] Figure 3 This is a schematic diagram showing the cutting position of the Charpy test piece in the Charpy impact test implemented in this invention. Detailed Implementation

[0041] The present invention will now be described in detail.

[0042] The angular steel pipe of the present invention comprises, by 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. Furthermore, the angular steel pipe of the present invention has a flat plate portion and a corner portion. The steel microstructure of the center portion of the flat plate portion is dominated by ferrite, with a second phase consisting of one or more of pearlite, pseudo-pearlite, and upper bainite having an area fraction of 5 to 30%, an average grain size of 10.0 to 30.0 μm, and in the above-mentioned steel microstructure, there are 500 pearlite and pseudo-pearlite particles with an equivalent circle diameter of 2 μm or more and 20 μm or less per mm. 2 The angled steel pipe of the present invention has a yield strength of 295 MPa or more and a tensile strength of 400 MPa or more for the flat plate portion, and a low yield ratio of 0.85 or less for the flat plate portion. Furthermore, the angled steel pipe of the present invention exhibits high ductility with a uniform elongation of 8.0% or more for the flat plate portion and an elongation at break of 27.0% or more for the flat plate portion, and excellent low-temperature toughness with a Charpy absorption energy of 80 J or more at -10°C and a ductile-brittle transition temperature of -10°C or less for the corner portion.

[0043] The steel composition of the angular steel pipe of the present invention will be described below. It should be noted that, unless otherwise specified, the "%" indicating the content of each component refers to "mass %".

[0044] C: Above 0.07% and below 0.20%

[0045] 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, which are secondary phases. To ensure the target strength and yield ratio in this invention, a C content of 0.07% or more is required. However, when the C content exceeds 0.20%, the proportion of the hard phase increases, low-temperature toughness decreases, and the yield ratio exceeds 0.85, failing to achieve the desired yield ratio. Additionally, ductility decreases, resulting in undesirable uniform elongation and elongation at break. Weldability also deteriorates. Therefore, the C content is set to 0.07% or more and 0.20% or less. A C content of 0.08% or more is preferred. Furthermore, a C content of 0.19% or less, more preferably 0.18% or less, is preferred.

[0046] Si: ≥0.01% and ≤0.40%

[0047] 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 required. However, when the Si content exceeds 0.40%, oxides are easily formed in the resistance weld, reducing the weld characteristics and decreasing ductility, thus failing to achieve the desired uniform elongation and elongation at break. Furthermore, the toughness of the base material outside the resistance weld also decreases. Therefore, 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. Additionally, the Si content is preferably 0.37% or less, more preferably 0.35% or less.

[0048] Mn: ≥0.20% and ≤1.20%

[0049] Mn is an element that increases the strength of steel through solid solution strengthening and / or by increasing the amount of bainite formation. Additionally, Mn contributes to the refinement of the microstructure by lowering the ferrite transformation initiation temperature. To ensure the target strength and microstructure in this invention, a Mn content of 0.20% or more is required. However, when the Mn content exceeds 1.20%, the amount of bainite formation increases, and the yield ratio exceeds 0.85, failing to achieve the desired yield ratio. Furthermore, when the Mn content exceeds 1.20%, the hardness of the central segregation region increases, potentially causing cracks during welding. Therefore, the Mn content is set to be 0.20% or more and 1.20% or less. 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.

[0050] P: below 0.10%

[0051] Phosphorus (P) segregates at grain boundaries, causing inhomogeneity in the material. Therefore, as an unavoidable impurity, it is preferable to minimize its concentration, although a content as low as 0.10% is permissible. Thus, the P content is set to be 0.10% or less. Preferably, the P content is 0.030% or less, more preferably 0.020% or less. It should be noted that while there is no specific lower limit for the P content, excessive reduction would lead to increased smelting costs; therefore, the P content is preferably set to 0.002% or more.

[0052] S: below 0.050%

[0053] Sulfur (S) typically exists in steel as MnS, but MnS is stretched very thin during hot rolling, negatively impacting ductility. Therefore, in this invention, it is preferable to minimize S content, although a content of less than 0.050% is permissible. Thus, the S content is set to 0.050% or less. Preferably, the S content is 0.015% or less, more preferably 0.010% or less, and even more preferably 0.008% or less. It should be noted that there is no specific lower limit for the S content, but excessive reduction leads to increased smelting costs; therefore, the S content is preferably set to 0.0002% or more.

[0054] Al: Above 0.005% and below 0.100%

[0055] Al is an element that functions as a strong deoxidizer. To achieve this effect, an Al content of 0.005% or more is required. However, when the Al content exceeds 0.100%, weldability deteriorates, alumina inclusions increase, and surface properties worsen. Furthermore, the toughness of the weld also decreases. Therefore, the Al content is set to be 0.005% or more and 0.100% or less. The Al content is preferably 0.010% or more, more preferably 0.015% or more. Additionally, the Al content is preferably 0.070% or less, more preferably 0.050% or less.

[0056] N: below 0.010%

[0057] Nitrogen (N) is an unavoidable impurity, an element that reduces toughness by firmly fixing the movement of dislocations. In this invention, N is preferably minimized as much as possible as possible as an impurity, but the N content can be as low as 0.010%. 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. It should be noted that excessive reduction will lead to a high smelting cost, therefore the N content is preferably set to 0.0010% or more, more preferably 0.0015% or more.

[0058] The basic components (essential components) of the angled steel pipe of the present invention have been described above. In the steel composition of the angled steel pipe of the present invention, the components other than those described above (the balance) may be Fe and unavoidable impurities. It should be noted that unavoidable impurities in the balance include, for example, As, Bi, Co, Pb, Zn, W, and O, which are inevitably mixed in due to their origin from raw materials, materials, or manufacturing equipment. Examples of such raw materials include iron ore, reduced iron, alloy iron, or scrap. However, without impairing the effects of the present invention, the presence of less than 0.05% As and Co, less than 0.005% Bi, Pb, Zn, and O, and less than 0.100% W is not excluded.

[0059] By having the above-described steel composition, the properties that are the target of this invention can be obtained.

[0060] The steel composition of the angled steel pipe of the present invention may also contain one or more of the following optional elements: Nb: less than 0.020%, Ti: less than 0.045%, V: less than 0.10%, Cr: less than 0.50%, Mo: less than 0.50%, Cu: less than 0.30%, Ni: less than 0.30%, Ca: less than 0.0100%, B: less than 0.0100%, Sn: less than 0.100%, and Sb: less than 0.100%.

[0061] Nb: below 0.020%, Ti: below 0.045%

[0062] Nb and Ti are elements that contribute to the strength of steel by forming fine carbides and nitrides and strengthening them through precipitation. To achieve this effect, Nb and Ti are preferably contained at least 0.005%, and more preferably at least 0.007%. However, when the Nb content exceeds 0.020%, the yield ratio exceeds 0.85, which is not the desired yield ratio. Furthermore, when the Ti content exceeds 0.045%, coarse carbides and nitrides may form, leading to a decrease in low-temperature toughness and ductility. Therefore, when Nb and Ti are present, the Nb and Ti contents are set to Nb: 0.020% or less and Ti: 0.045% or less, respectively. The Nb content is preferably Nb: 0.018% or less, and more preferably 0.016% or less. The Ti content is preferably Ti: 0.040% or less, and more preferably Ti: 0.035% or less.

[0063] V: less than 0.10%, Cr: less than 0.50%, Mo: less than 0.50%

[0064] V, Cr, and Mo are elements that improve the hardenability and strength of steel, respectively, and can be included as needed. To achieve this effect, when V, Cr, and Mo are present, it is preferable to have V: 0.01% or more, Cr: 0.01% or more, and Mo: 0.01% or more, more preferably V: 0.02% or more, Cr: 0.10% or more, and Mo: 0.10% or more. However, when the contents of V, Cr, and Mo are respectively greater than 0.10%, Cr: greater than 0.50%, and Mo: greater than 0.50%, ductility may decrease, resulting in unsatisfactory uniform elongation and elongation at break, and low-temperature toughness may also decrease. Therefore, when V, Cr, and Mo are present, the contents are set to V: 0.10% or less, Cr: 0.50% or less, and Mo: 0.50% or less, respectively. The preferred contents of V, Cr, and Mo are V: 0.08% or less, Cr: 0.40% or less, and Mo: 0.40% or less, respectively.

[0065] Cu: less than 0.30%, Ni: less than 0.30%

[0066] Cu and Ni are elements that enhance the strength of steel through solid solution strengthening, and can be included as needed. To achieve this effect, when Cu and Ni are present, the Cu and Ni contents are preferably set to Cu: 0.01% or more and Ni: 0.01% or more, more preferably Cu: 0.10% or more and Ni: 0.10% or more. However, when the Cu content exceeds 0.30%, the low-temperature toughness decreases, and the desired low-temperature toughness is not obtained. Furthermore, when the Ni content exceeds 0.30%, the yield ratio exceeds 0.85, and the desired yield ratio is not obtained. Therefore, when Cu and Ni are present, the Cu and Ni contents are set to Cu: 0.30% or less and Ni: 0.30% or less, respectively. The Cu and Ni contents are preferably Cu: 0.20% or less and Ni: 0.20% or less, respectively.

[0067] Ca: below 0.0100%

[0068] Ca is an element that helps improve the toughness of steel by spheroidizing sulfides such as MnS, which are stretched very thinly during the hot rolling process. It can be included as needed. To achieve this effect, when Ca is present, the Ca content is preferably set to 0.0005% or more, more preferably 0.0010% or more. However, when the Ca content exceeds 0.0100%, ductility decreases, and the desired uniform elongation and elongation at break are not obtained; furthermore, low-temperature toughness decreases, and the desired low-temperature toughness is not achieved. Therefore, when Ca is present, the Ca content is set to 0.0100% or less. The Ca content is preferably 0.0050% or less.

[0069] B: Below 0.0100%

[0070] Boron (B) is an element that lowers the ferrite phase transformation initiation temperature, thus contributing to microstructure refinement. To achieve this effect, when B is present, the B content is preferably 0.0003% or more, more preferably 0.0005% or more. However, when the B content exceeds 0.0100%, the formation of upper bainite increases, and the yield ratio exceeds 0.85, failing to obtain the desired yield ratio. Therefore, when B is present, the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less.

[0071] Sn: below 0.100%

[0072] Sn is an element that increases the strength of steel through solid solution strengthening and can be included as needed. To achieve the above-mentioned effect, when Sn is present, the Sn content is preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.005% or more. However, when the Sn content exceeds 0.100%, hot workability may decrease, leading to cracking during hot rolling. Therefore, when Sn is present, 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.

[0073] Sb: below 0.100%

[0074] Sb is an element that increases the strength of steel through solid solution strengthening and can be included as needed. To achieve the aforementioned effect, when Sb is present, 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 decrease, leading to cracking during hot rolling. Therefore, when Sb is present, 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.

[0075] Next, the reasons for defining the steel structure of the angular steel tube of the present invention will be explained.

[0076] The steel microstructure of the central portion of the angular steel pipe of the present invention is composed of ferrite as the main phase, and the area fraction of the second phase composed of one or more selected from pearlite, pseudo-pearlite and upper bainite is 5 to 30%, with an average grain size of 10.0 to 30.0 μm. Furthermore, in the above-mentioned steel microstructure, a total of 500 pearlite and pseudo-pearlite particles with an equivalent circle diameter of 2 μm or more and 20 μm or less are present per mm. 2 above.

[0077] It should be noted that, in this invention, the crystal grain size is set as the diameter of a circle with an area equal to that of the grain being targeted (equivalent circle diameter).

[0078] Principal phase: Ferrite

[0079] Ferrite is a soft microstructure, and in order to obtain the desired yield strength and low yield ratio, it is the dominant phase in this invention. In this invention, the dominant phase refers to a phase that accounts for 70% or more in terms of area fraction. When the area fraction of ferrite is less than 70%, the yield stress sometimes becomes too high, and the desired yield ratio is not obtained. Therefore, the ferrite content is 70% or more, preferably 72% or more, in terms of area fraction. On the other hand, when the area fraction of ferrite exceeds 95%, the strength sometimes decreases, and the desired yield strength and tensile strength are not obtained. Therefore, it is preferable that the ferrite content is 95% or less in terms of area fraction, and more preferably 93% or less in terms of area fraction.

[0080] The area fraction of the second phase, composed of one or more selected from pearlite, pseudo-pearlite, and upper bainite, is 5%–30%.

[0081] Pearlite, pseudo-pearlite, and upper bainite are harder microstructures than ferrite, and are important steel microstructures for improving steel strength and achieving a low yield ratio. To obtain the yield strength, tensile strength, and yield ratio targeted in this invention, the area fraction of the second phase composed of one or more of pearlite, pseudo-pearlite, and upper bainite needs to be 5% or more. Preferably, this area fraction is 6% or more, more preferably 8% or more. On the other hand, when the area fraction of the second phase composed of one or more of pearlite, pseudo-pearlite, and upper bainite exceeds 30%, low-temperature toughness sometimes deteriorates and the yield ratio increases. Therefore, the area fraction of the second phase composed of one or more of pearlite, pseudo-pearlite, and upper bainite needs to be 30% or less. Preferably, this area fraction is 28% or less, more preferably 26% or less.

[0082] Furthermore, the combined area ratio of pearlite and pseudo-pearlite is preferably 5% or more. When the combined area ratio of pearlite and pseudo-pearlite is 5% or more, the yield ratio can be suppressed to a lower level when manufacturing angled steel pipes, thus achieving better seismic resistance. Additionally, when the combined area ratio of pearlite and pseudo-pearlite is 20% or less, it is easier to obtain good low-temperature toughness more stably; therefore, the combined area ratio of pearlite and pseudo-pearlite is preferably 20% or less.

[0083] Furthermore, when the area ratio of upper bainite is 15% or less, it is easier to obtain the low yield ratio that is the target of this invention more stably; therefore, the area ratio of upper bainite is preferably 15% or less. More preferably, the area ratio of upper bainite is 13% or less.

[0084] It should be noted that the area ratios of ferrite, pearlite, pseudo-pearlite, and upper bainite can be determined using the methods described in the examples.

[0085] Average grain size of the steel microstructure in the center of the plate thickness: 10.0–30.0 μm

[0086] As described above, in order to obtain the low yield ratio, yield strength, and tensile strength targeted in this invention, the steel microstructure of this invention is set as a steel containing a mixture of soft and hard microstructures (hereinafter referred to as "composite microstructure steel"). However, compared with single-microstructure steel, composite microstructure steel has poorer toughness. Therefore, in this invention, in order to balance the above-mentioned mechanical properties and excellent toughness, when the region surrounded by the boundary where the orientation difference (crystal orientation difference) between adjacent crystals is 15° or more is defined as the grain, the average grain size of the steel microstructure in the center of the plate thickness of the flat plate portion is specified. When the average grain size (equivalent circle diameter) of the steel microstructure in the center of the plate thickness of the flat plate portion is less than 10.0 μm, the yield ratio increases and the yield ratio targeted in this invention is not obtained. On the other hand, when the average grain size of the steel microstructure exceeds 30.0 μm, the low-temperature toughness deteriorates. Therefore, the average grain size of the steel microstructure in the center of the plate thickness of the flat plate portion needs to be 10.0 to 30.0 μm. The above-mentioned average grain size is preferably 11.0 μm or more, and more preferably 11.5 μm or more. Furthermore, the average crystal grain size is preferably 28.0 μm or less, more preferably 26.0 μm or less.

[0087] It should be noted that the crystal orientation difference and average crystal grain size can be determined by SEM (Scanning Electron Microscope) / EBSD (Electron Backscatter Diffraction). Here, the determination can be performed using the method described in the examples.

[0088] In the steel microstructure at the center of the plate thickness of the flat section, there are a total of 500 pearlite and pseudo-pearlite particles with an equivalent circular diameter of 2μm or more but less than 20μm per mm. 2 above

[0089] To obtain a angular steel tube that combines the low-temperature toughness and high ductility desired in this invention, the number density of pearlite and pseudo-pearlite is specified. The total number of pearlite and pseudo-pearlite particles with an equivalent circular diameter of 2 μm or more and 20 μm or less present in the steel microstructure at the center of the plate thickness is less than 500 particles / mm. 2 At this point, toughness and ductility decrease, and the desired yield ratio, low-temperature toughness, and ductility cannot be obtained. Therefore, in the above-mentioned steel microstructure, a total of 500 pearlites with an equivalent circle diameter of 2μm or more but less than 20μm and pseudo-pearlites with an equivalent circle diameter of 2μm or more but less than 20μm exist per mm. 2 The above is the preferred total. 2 The above, and more preferably 580 pieces / mm2 That's all. There's no specific upper limit for the total above, but it exceeds 1200 pieces / mm. 2 At that time, the yield ratio may increase, therefore the above total is preferably 1200 pieces / mm. 2 The following, and more preferably, is 1150 pieces / mm 2 The following should be noted: In the steel microstructure at the center of the plate thickness of the flat section, the total number of pearlites with an equivalent circle diameter greater than 20 μm and pseudo-pearlites with an equivalent circle diameter greater than 20 μm is 50 per mm. 2 When the following conditions are met, the desired low-temperature toughness can be obtained more stably. Therefore, the total number of pearlite and pseudo-pearlite particles with an equivalent circular diameter greater than 20 μm present in the above-mentioned steel microstructure is preferably 50 per mm. 2 The following, and more preferably, is 40 pieces / mm 2 The following should be noted: the total number of pearlites and pseudo-pearlites with an equivalent circle diameter greater than 20 μm can also be 0 per mm. 2 .

[0090] By satisfying the above-mentioned steel composition and steel structure, it is possible to obtain angular steel pipes with the strength (yield strength, tensile strength), yield ratio, ductility (uniform elongation, elongation at break) and low-temperature toughness (Charpy absorption energy, ductile-brittle transition temperature) as targeted by this invention.

[0091] The angular steel pipe of the present invention has a yield strength of 295 MPa or more and a tensile strength of 400 MPa or more in the flat portion along the tube axis, and a low yield ratio of 0.85 or less. Furthermore, it exhibits high ductility with a uniform elongation of 8.0% or more and an elongation at break of 27.0% or more. Additionally, the corner portion possesses excellent low-temperature toughness, exhibiting 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 angular steel pipe of the present invention combines a low yield ratio with excellent low-temperature toughness and high ductility. It should be noted that these characteristic values ​​can be measured using the methods described in the examples.

[0092] The yield strength is preferably 320 MPa or higher. Furthermore, the upper limit of the yield strength is not particularly limited; for example, the yield strength is 470 MPa or lower. The tensile strength is preferably 420 MPa or higher. Furthermore, the upper limit of the tensile strength is not particularly limited; for example, the tensile strength is 620 MPa or lower. The yield ratio is preferably 0.80 or lower. Furthermore, the lower limit of the yield ratio is not particularly limited; for example, the yield ratio is 0.60 or higher. The uniform elongation is preferably 9.0% or higher. The elongation at break is preferably 29.0% or higher. The absorbed energy of the Charpy impact test at -10°C is preferably 90 J or higher. The ductile-brittle transition temperature is preferably -15°C or lower.

[0093] The thickness (wall thickness) of the angled steel pipe of the present invention is preferably 12 mm or more, but can also be greater than 22 mm. According to the present invention, even angled steel pipes with a large plate thickness exceeding 22 mm have a low yield ratio and excellent low-temperature toughness and high ductility. The plate thickness of the angled steel pipe of the present invention is preferably 12 to 32 mm.

[0094] Next, the manufacturing method of the angled steel pipe of the present invention will be described. First, one embodiment of the manufacturing method of the hot-rolled steel plate (hereinafter also referred to as hot-rolled steel plate for angled steel pipe) which is the raw material of the angled steel pipe of the present invention will be described.

[0095] The method for manufacturing hot-rolled steel sheet for angled steel pipes of the present invention includes, for example, a hot rolling process, a cooling process, and a coiling process. In the hot rolling process, steel raw material having the above-described steel composition is heated at a temperature of 1100°C or higher and 1300°C or lower, and for a heating time of 40 minutes or higher and 200 minutes or lower. Then, hot rolling is performed with a roughing end temperature of 850°C or higher and 1150°C or lower, a finishing end temperature of 750°C or higher and 850°C or lower, and 930°C or lower, resulting in a total reduction of 40% or higher, and a total reduction of 15% or higher for the final two passes of finishing rolling. In the cooling process following the hot rolling process, cooling is performed with an average cooling rate of 10.0°C / s or lower at the center of the plate thickness and a cooling stop temperature of 480°C or higher and 650°C or lower at the center of the plate thickness. In the coiling process following the cooling process, coiling is performed with a temperature of 480°C or higher and 650°C or lower at the center of the plate thickness to produce a hot-rolled steel sheet.

[0096] It should be noted that, unless otherwise specified, in the following description of the manufacturing method, the temperature-related "°C" designation refers to the surface temperature of the steel raw material and steel plate (hot-rolled steel plate, raw material steel plate, etc., described later). These surface temperatures can be measured using a radiation thermometer or similar device. Furthermore, the temperature at the center of the steel plate's thickness can be obtained by calculating the temperature distribution within the steel plate's cross-section using heat transfer analysis and correcting the result using the steel plate's surface temperature. Additionally, "hot-rolled steel plate" includes both hot-rolled steel sheet and hot-rolled steel strip.

[0097] In this invention, the smelting method for the steel raw material (steel billet) is not particularly limited; known smelting methods such as converters, electric furnaces, and vacuum melting furnaces are all suitable. The casting method is also not particularly limited; known casting methods such as continuous casting can be used to manufacture steel raw materials of the desired dimensions. It should be noted that there is no problem in using the ingot-rolling method instead of the continuous casting method. Secondary refining, such as ladle refining, can be further performed on the molten steel.

[0098] Next, the obtained steel billet is heated at a 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. Then, a rough rolling process is carried out with a rough rolling end temperature of 850°C or higher and 1150°C or lower, a finish rolling process is carried out with a finish rolling end temperature of 750°C or higher and 850°C or lower, and a hot rolling process is carried out 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 last two passes of the finish rolling process, to produce a hot-rolled plate (hot-rolled steel plate raw material steel plate, hereinafter also referred to as raw material steel plate).

[0099] Heating temperature of steel raw materials: above 1100℃ and below 1300℃

[0100] When the heating temperature of the steel raw material is below 1100°C, the deformation resistance of the steel raw material, as the material to be rolled, increases, making rolling difficult. On the other hand, when the heating temperature of the steel raw material exceeds 1300°C, the austenite grains coarsen, and fine austenite grains cannot be obtained in subsequent rolling processes (roughing and finishing). As a result, it is difficult to ensure the average grain size of the steel microstructure (hereinafter, the steel microstructure in the center of the plate thickness, which is the target of this invention) in the hot-rolled steel sheet. Therefore, the heating temperature of the steel raw material in the hot rolling process is set to 1100°C or higher and 1300°C or lower. The above heating temperature is preferably 1120°C or higher. In addition, the above heating temperature is preferably 1280°C or lower. It should be noted that in this invention, after manufacturing the steel raw material (steel billet), in addition to the existing method of temporarily cooling to room temperature and then reheating, an energy-saving process of direct feeding rolling in which the billet is loaded into the heating furnace in a warm state without cooling to room temperature can also be used without problems.

[0101] Heating time for steel raw materials: 40 minutes or more but less than 200 minutes

[0102] To obtain the desired steel microstructure (a number density of pearlite and pseudo-pearlite with an average grain size and an equivalent circle diameter of 2 μm or more but less than 20 μm), this invention strictly specifies the heating time in addition to the heating temperature of the steel raw material. When the heating time of the steel raw material is less than 40 minutes, the austenite grains are too fine, resulting in excessive ferrite formation during the cooling process, leading to a decrease in yield strength and tensile strength. On the other hand, when the heating time of the steel raw material exceeds 200 minutes, the austenite becomes coarse, making it difficult to obtain a steel microstructure with an average grain size of 10.0 to 30.0 μm. Furthermore, it is difficult to obtain a pearlite and pseudo-pearlite number density of 500 particles / mm with an equivalent circle diameter of 2 μm or more but less than 20 μm. 2 The above describes the steel structure. Therefore, the heating time for the steel raw material is set to be 40 minutes or more and 200 minutes or less. The heating time is preferably 45 minutes or more, more preferably 50 minutes or more. Furthermore, the heating time is preferably 190 minutes or less, more preferably 185 minutes or less. It should be noted that the heating time refers to the time from when the steel raw material is loaded into the heating furnace until it is removed.

[0103] Roughing finishing temperature: above 850℃ and below 1150℃

[0104] When the roughing rolling end temperature is below 850°C, the surface temperature of the steel plate falls below the ferrite transformation initiation temperature during subsequent finishing rolling, resulting in the formation of a large amount of ferrite and a decrease in yield strength and tensile strength. On the other hand, when the roughing rolling end temperature exceeds 1150°C, the reduction within the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, it is difficult to obtain a steel microstructure with an average grain size of 10.0 to 30.0 μm. Therefore, the roughing rolling end temperature is set to 850°C or higher and 1150°C or lower. The roughing rolling end temperature is preferably 860°C or higher, more preferably 870°C or higher. Furthermore, the roughing rolling end temperature is preferably 1100°C or lower, more preferably 1050°C or lower.

[0105] Finishing rolling temperature: above 750℃ and below 850℃

[0106] When the finishing rolling temperature is below 750°C, the surface temperature of the steel plate during finishing rolling falls below the ferrite phase transformation initiation temperature, resulting in elongated ferrite along the rolling direction, which may reduce machinability. On the other hand, when the finishing rolling temperature exceeds 850°C, the reduction within the austenite non-recrystallization temperature range is insufficient, and fine austenite grains cannot be obtained. As a result, the grains become coarse, making it difficult to ensure the strengths (yield strength, tensile strength) targeted in this invention. Therefore, the finishing rolling temperature is set to be 750°C or higher and 850°C or lower. The finishing rolling temperature is preferably 770°C or higher, more preferably 780°C or higher. Furthermore, the finishing rolling temperature is preferably 830°C or lower, more preferably 820°C or lower.

[0107] Total reduction rate below 930℃: 40% or more

[0108] In this invention, by refining the subgrains in austenite during the hot rolling process, the ferrite and bainite generated in subsequent cooling and coiling processes can be refined, thereby obtaining a hot-rolled steel sheet with the strength and low-temperature toughness targeted in this invention. To refine the subgrains in austenite during the hot rolling process, it is necessary to increase the reduction rate within the austenite non-recrystallization temperature range and introduce sufficient processing strain. When the total reduction rate below 930°C is less than 40%, the grain size of ferrite and bainite increases, leading to a decrease in low-temperature toughness. Therefore, the total reduction rate below 930°C is set to 40% or more. The aforementioned total reduction rate is preferably 42% or more, more preferably 45% or more. It should be noted that if the aforementioned total reduction rate is too high, it will place an excessive load on the manufacturing equipment in the hot rolling process, making manufacturing difficult; therefore, the aforementioned total reduction rate is preferably 60% or less.

[0109] It should be noted that the total reduction rate specified for temperatures below 930°C is because, above 930°C, austenite recrystallizes during the hot rolling process, and the dislocations introduced by rolling disappear, resulting in the inability to obtain refined austenite. Furthermore, the aforementioned total reduction rate refers to the sum of the reduction rates of all rolling passes within the temperature range below 930°C.

[0110] It should be noted that when hot rolling steel raw materials (steel billets), the total reduction rate below 930°C can be set to 40% or more in both the roughing and finishing rolling processes mentioned above. Alternatively, the total reduction rate below 930°C can be set to 40% or more only in the finishing rolling process. In the latter case, if it is not possible to achieve a total reduction rate of 40% or more below 930°C in the finishing rolling process alone, the steel billet can be cooled to below 930°C midway through the roughing rolling process, and then the total reduction rate below 930°C in both the roughing and finishing rolling processes can be set to 40% or more.

[0111] The combined reduction rate of the final two passes in the finishing rolling process is 15% or more.

[0112] To obtain the desired steel microstructure, particularly the desired number density of pearlite and pseudo-pearlite with an equivalent circle diameter of 2 μm or more but less than 20 μm, the present invention specifies the total reduction rate of the final two passes of finishing rolling. When the total reduction rate of the final two passes of finishing rolling is less than 15%, coarse pearlite is generated, making it difficult to obtain a pearlite and pseudo-pearlite number density of 500 particles / mm with an equivalent circle diameter of 2 μm or more but less than 20 μm. 2 The above describes the steel structure. Therefore, the total reduction rate of the final two passes in finishing rolling is set to 15% or more. Preferably, it is 17% or more, and more preferably 19% or more. There is no specific upper limit set for the total reduction rate of the final two passes in finishing rolling, but when the total reduction rate exceeds 50%, the load applied to the rolling equipment increases. Therefore, the total reduction rate of the final two passes in finishing rolling is preferably 50% or less, and more preferably 48% or less.

[0113] The total reduction rate of the final two passes in finishing rolling can be calculated as follows. Assuming the finishing rolling has n passes and the final plate thickness is t... n If the thickness of the plate after each rolling pass is set as t1, t2, ..., t... n-2 t n-1 t n Then the total reduction rate of the final two passes of finishing rolling can be obtained by ((t) n-2 -t n ) / t n-2 Calculate by multiplying 100 (%).

[0114] Hot-rolled sheet (raw steel sheet) is produced through the above-described hot rolling process. Next, after the hot rolling process, a cooling process is performed on the hot-rolled sheet. During the cooling process, cooling is carried out under the following conditions: the average cooling rate at the center of the sheet thickness up to the cooling stop temperature is 10.0°C / s or less, and the cooling stop temperature at the center of the sheet thickness is 480°C or more but 650°C or less.

[0115] Average cooling rate at the center of the plate thickness: below 10.0℃ / s

[0116] When the average cooling rate, measured using a thermometer at the center of the hot-rolled plate thickness, exceeds 10.0 °C / s from the start of cooling to the end of cooling (described later), the amount of upper bainite formed increases, and the desired average grain size cannot be obtained, thus failing to achieve the yield ratio targeted in this invention. Therefore, the average cooling rate is set to 10.0 °C / s or less. Preferably, the average cooling rate is 9.5 °C / s or less, more preferably 9.0 °C / s or less. It should be noted that while the lower limit of the average cooling rate is not particularly limited, it is difficult to obtain the cooling stop temperature (described later) when the average cooling rate is less than 1.0 °C / s. 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. It should be noted that, in this invention, from the viewpoint of suppressing the coarsening of the grain size, it is preferable to begin cooling immediately after finishing rolling.

[0117] Cooling stop temperature: above 480℃ and below 650℃

[0118] When the cooling stop temperature at the center of the hot-rolled plate thickness is below 480°C, the amount of upper bainite formed increases, and sometimes the yield ratio exceeds 0.85, failing to achieve the desired yield ratio. Furthermore, temperature unevenness easily occurs in the length and / or width directions of the hot-rolled plate during cooling, potentially causing fluctuations in mechanical properties. On the other hand, when the cooling stop temperature at the center of the hot-rolled plate thickness exceeds 650°C, the ferrite grains coarsen, failing to achieve the desired grain size. Therefore, the cooling stop temperature at the center of the hot-rolled plate thickness is set to 480°C or higher and 650°C or lower. Preferably, the cooling stop temperature is 490°C or higher, more preferably 495°C or higher. Preferably, the cooling stop temperature is 640°C or lower, more preferably 635°C or lower.

[0119] It should be noted that in this invention, the average cooling rate is a value calculated as ((temperature at the center of the hot-rolled plate before cooling - temperature at the center of the hot-rolled plate after cooling) / cooling time). This average cooling rate can be calculated based on the temperature distribution within the cross-section of the hot-rolled plate obtained through heat transfer analysis. Cooling methods include water cooling (e.g., spraying water from a nozzle) and cooling using a cooling gas jet. In this invention, it is preferable to perform the cooling operation (processing) on ​​both sides of the hot-rolled plate under the same conditions. Furthermore, to obtain the aforementioned average cooling rate, the amount, pressure, spraying time and angle of the cooling water or cooling gas, as well as the conveying speed of the hot-rolled plate, are adjusted. To obtain the average cooling rate specified in this invention, it is necessary to perform heat transfer analysis beforehand to determine the cooling conditions of the hot-rolled plate before manufacturing.

[0120] Following the aforementioned cooling process, a coiling process is performed to coil the hot-rolled sheet. In this coiling process, from the viewpoint of steel microstructure, coiling is carried out at a coiling temperature of 480°C or higher and 650°C or lower, using a thermometer at the center of the hot-rolled sheet thickness. When the coiling temperature is below 480°C, a large amount of upper bainite is generated on the steel sheet surface, resulting in an undesirable yield ratio. When the coiling temperature exceeds 650°C, the ferrite grains coarsen, and the desired average grain size is not obtained. Therefore, the coiling temperature is set to 480°C or higher and 650°C or lower. Preferably, the coiling temperature is 490°C or higher, more preferably 495°C or higher. Furthermore, preferably, the coiling temperature is 640°C or lower, more preferably 635°C or lower.

[0121] A cooling process is performed after the coiling process to allow the hot-rolled steel sheet to cool. Through the above process, the hot-rolled steel sheet (hot-rolled steel sheet for angled steel pipes) of the present invention can be manufactured.

[0122] The angular steel pipe of the present invention uses the aforementioned hot-rolled steel sheet as raw material. For the angular steel pipe of the present invention, the hot-rolled steel sheet is rolled (cold-rolled) to form a cylindrical open pipe, and its butt joint (the end of the aforementioned hot-rolled steel sheet) is welded to form a circular steel pipe. Then, it is formed into an angular steel pipe by rollers arranged above, below, to the left, and to the right of the circular steel pipe, thus obtaining the angular steel pipe. After the hot-rolled steel sheet is formed into the aforementioned circular steel pipe and / or formed into an angular steel pipe, a compressive strain of 0.20 to 3.00% is applied along the pipe axis direction.

[0123] Compressive strain in the tube axis direction: 0.20–3.00%

[0124] When compressive strain is applied along the tube axis of the steel pipe, the yield ratio in the tube axis direction decreases, and the ductility increases (uniform elongation and elongation at break increase). Therefore, the process of applying compressive strain is crucial in order to obtain the desired low yield ratio and high ductility (high uniform elongation and high elongation at break) in this invention. When the compressive strain is less than 0.20%, the effect of reducing the yield ratio and increasing ductility (increasing uniform elongation and elongation at break) is not achieved. On the other hand, when the compressive strain exceeds 3.00%, the steel pipe may buckle. Therefore, the compressive strain in the tube axis direction 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 limited as long as buckling does not occur. In addition, compressive strain can be applied to either a formed circular steel pipe or a angular steel pipe, or both a circular steel pipe and a angular steel pipe. When applying compressive strain to both circular and angular steel pipes, the total compressive strain applied to the circular and angular steel pipes respectively shall be within the range described above.

[0125] It should be noted that the angular steel pipe in this invention is not limited to angular steel pipes with all sides having equal lengths (the ratio of the length of the longer side to the length of the shorter side is 1.0), but also includes angular steel pipes with a ratio of the length of the longer side to the length of the shorter side greater than 1.0. However, when the ratio of the length of the longer side to the length of the shorter side of the angular steel pipe exceeds 2.5, local buckling is easily generated on the longer side, and the compressive strength in the axial direction of the pipe decreases. Therefore, the ratio of the length of the longer side to the length of the shorter side of the angular steel pipe is preferably set to 1.0 or more and 2.5 or less. The ratio of the length of the longer side to the length of the shorter side is more preferably 1.0 or more and 2.0 or less.

[0126] Through the above process, the angular steel pipe of the present invention can be manufactured. According to the present invention, an angular steel pipe can be obtained with a yield strength of 295 MPa or more for the flat portion, a tensile strength of 400 MPa or more for the flat portion, a yield ratio of 0.85 or less for the flat portion, a uniform elongation of 8.0% or more for the flat portion, an elongation at break of 27.0% or more for the flat portion, a Charpy absorption energy of 80 J or more at -10°C for the corner portion, and a ductile-brittle transition temperature of -10°C or less for the corner portion. The ductile-brittle transition temperature of the angular steel pipe of the present invention is below -10°C, making it suitable for use as a structural component in buildings in cold regions where the air temperature or room temperature is below freezing, ensuring excellent seismic resistance. Furthermore, according to the present invention, when forming the angular steel pipe from hot-rolled steel sheet, there is no need to specifically control the roll gap of the sizing mill or the corner forming mill, enabling efficient manufacturing of the aforementioned angular steel pipe.

[0127] <Building Structures>

[0128] Figure 1 This is a schematic diagram illustrating an example of a building structure according to the present invention.

[0129] The building structure of the present invention uses the aforementioned angled steel tube (angled steel tube 1) as the column material. Symbols 4, 5, 6, and 7 respectively represent main beams, secondary beams, diaphragms, and intermediate columns. As described above, the flat plate portion of the angled steel tube of the present invention has excellent mechanical properties. Therefore, the building structure of the present invention using this angled steel tube as the column material exhibits excellent seismic performance.

[0130] Example

[0131] The present invention will now be described in further detail based on the embodiments. It should be noted that the present invention is not limited to the following embodiments.

[0132] Molten steel with the steel composition shown in Table 1 is cast to produce steel billets. The resulting billets are then subjected to hot rolling, cooling, and coiling processes under the conditions shown in Tables 2-1 and 2-2 to produce hot-rolled steel sheets. Then, the following tube-making process is performed.

[0133]

[0134]

[0135]

[0136] The obtained hot-rolled steel sheet is formed into a cylindrical open tube by roll forming (cold roll forming), and the butt joints are resistance welded to produce a circular steel tube. Then, it is formed into an angular steel tube (square in the vertical cross-section view along the tube axis) by rolls arranged above, below, left, and right of the above circular steel tube. The resulting angular steel tube has corner portions and flat portions, and has the side lengths (mm) and wall thicknesses (mm) shown in Tables 3-1 and 3-2.

[0137] The resulting angular steel pipe is cut to a length of 3.0 m and then subjected to compressive strain along its length (pipe axis) to form an angular steel pipe. It should be noted that to prevent buckling of the angular steel pipe, the cut surface is ground to achieve parallelism before applying the compressive strain. The value of the compressive strain is evaluated by measuring the strain gauge reading at the center of the width of the flat plate (the 3 o'clock edge when the weld is positioned at the 12 o'clock position) attached to the outer surface of the steel pipe at a location 1.5 m from the end along the length of the angular steel pipe.

[0138] Test pieces were cut from the hot-rolled steel plates or angled steel pipes manufactured above, and the following microstructure observation, tensile test, and Charpy impact test were performed.

[0139] [Organizational Observation]

[0140] The test specimens used for tissue observation were cut from the central portion of the width direction and the center of the thickness of the hot-rolled steel plate (position t / 2 of the plate thickness (t: plate thickness)). After grinding so that the observation surface is the section in the rolling direction during hot rolling, the observation surface was prepared by etching with nitric acid ethanol solution.

[0141] For microstructure observation, an optical microscope (magnification: 1000x) was used to observe the microstructure at the t / 2 position (center of the plate thickness) of the hot-rolled steel plate, and optical microscope images were taken for microstructure observation. Based on the obtained optical microscope images, the area ratios of ferrite, pearlite, pseudopearlite, and upper bainite were calculated.

[0142] The area ratio of each microstructure was observed in five fields of view and calculated as the average of the values ​​obtained in each field of view. Here, the area ratio obtained through microstructure observation is taken as the area ratio of each microstructure. Ferrite is a product of diffusion phase transformation, exhibiting a low dislocation density and largely restored microstructure. Ferrite includes polygonal ferrite and quasi-polygonal ferrite. Pearlite is a microstructure in which cementite and ferrite are arranged in layers, while pseudo-pearlite is a microstructure in which cementite is found to be arranged in a dotted pattern within ferrite. Upper bainite is a multiphase microstructure of lath-shaped ferrite and cementite with high dislocation density. In addition to the above shapes, it is distinguished by the color of ferrite (white), pearlite (black), pseudo-pearlite (black or gray), and upper bainite (white or gray).

[0143] Additionally, regarding the number density (numbers / mm) of pearlite and pseudo-pearlite. 2 The observations were performed in five fields of view. The total number of pearlites and pseudo-pearlites observed in each of the five fields of view was calculated and divided by the observed area (mm²). 2 The number densities of pearlite and pseudo-pearlite were calculated accordingly. It should be noted that the total number of pearlite and pseudo-pearlite with an equivalent circle diameter of 2 μm or more but less than 20 μm, and the total number of pearlite and pseudo-pearlite with an equivalent circle diameter greater than 20 μm, were measured separately. Then, the number densities of pearlite and pseudo-pearlite with an equivalent circle diameter of 2 μm or more but less than 20 μm, and the number densities of pearlite and pseudo-pearlite with an equivalent circle diameter greater than 20 μm, were calculated separately.

[0144] The average grain size (average equivalent circle diameter) was measured using SEM / EBSD at the location of t / 2 of the plate thickness (the center of the plate). The measurement area was set to 500 μm × 1000 μm (= 0.5 mm). 2 The measurement step size was set to 0.5 μm. Regarding grain size, the orientation difference between adjacent grains was calculated, and boundaries with an orientation difference greater than 15° were considered grain boundaries for measurement. The grain size (equivalent circle diameter) of each grain was calculated based on the obtained grain boundaries, and their arithmetic mean was obtained as the average grain size (average equivalent circle diameter). In grain size analysis, grains with a grain size less than 2.0 μm were excluded from the analysis as measurement noise.

[0145] [Tension Test]

[0146] Figure 2 This is a schematic diagram showing the cutting position of the tensile test specimen for the flat plate. (Example) Figure 2As shown, the tensile test is performed by cutting JIS No. 5 tensile test pieces from the flat section of the angled steel tube with the tensile direction parallel to the tube axis. For the cut tensile test pieces, tensile tests are performed according to JIS Z 2241 (2011), determining the yield strength (YS), tensile strength (TS), uniform elongation (Eu), and elongation at break (Et), and calculating the yield ratio defined as (yield strength) / (tensile strength). Uniform elongation is set as the total elongation at maximum load. It should be noted that the tensile test pieces for the flat section are cut from the 3 o'clock position (when the welded part of the angled steel tube is positioned at the 12 o'clock direction), at the center of the width of the flat section, and at the center of the length of the angled steel tube (refer to...). Figure 2 Two test pieces were used, and their average values ​​were calculated to determine YS, TS, yield ratio, uniform elongation, and elongation at break. Regarding strength, a yield strength of 295 MPa or higher and a tensile strength of 400 MPa or higher for the plate portion were considered acceptable. Regarding yield ratio, a yield ratio of 0.85 or lower was considered acceptable due to low yield ratio. Regarding ductility, a uniform elongation of 8.0% or higher and an elongation at break of 27.0% or higher were considered acceptable due to high ductility.

[0147] Charpy Impact Test

[0148] Charpy impact tests were conducted using standard V-notch test pieces as specified in JIS Z 2242 (2018). Figure 3 This is a schematic diagram showing the cutting position of the Charpy test piece used in the Charpy impact test. (Example) Figure 3 As shown, Charpy test pieces are cut from the corner of the angled steel pipe at a position t / 4 of the wall thickness t from the outer surface of the pipe, with the length direction of the test piece parallel to the pipe axis. However, for angled steel pipes with a wall thickness of 19 mm or less, V-notch standard test pieces are cut from a position t / 2 of the wall thickness t from the outer surface of the pipe. Charpy impact tests are conducted at test temperatures of -70℃, -50℃, -30℃, -10℃, and 10℃, according to JIS Z 2242 (2018). Three test pieces are used at each test temperature, and the average value of the ductile-brittle transition temperature and absorbed energy (J) is calculated. A Charpy impact test at a test temperature of -10℃ with an absorbed energy of 80 J or more and a ductile-brittle transition temperature below -10℃ is considered to have excellent low-temperature toughness and is thus deemed acceptable.

[0149] The results are shown in Tables 3-1 and 3-2.

[0150]

[0151]

[0152] In Tables 3-1 and 3-2, steels No. 1 to 21, 36, and 37 are examples of the present invention, and steels No. 22 to 35 and 38 to 50 are comparative examples.

[0153] As shown in Tables 3-1 and 3-2, the angled steel pipes of the present invention all have a yield strength of 295 MPa or more for the flat plate portion and a tensile strength of 400 MPa or more for the flat plate portion. Furthermore, they have a low yield ratio, with a yield ratio of 0.85 or less for the flat plate portion. Additionally, they exhibit high ductility, with a uniform elongation of 8.0% or more for the flat plate portion and an elongation at break of 27.0% or more for the flat plate portion. Furthermore, they possess excellent low-temperature toughness, with a Charpy absorption energy of 80 J or more at -10°C for the corner portion and a ductile-brittle transition temperature of -10°C or less for the corner portion.

[0154] In contrast, in Comparative Example No. 22 (steel V), the C content was lower than that of the present invention. Therefore, the area ratio of the second phase was outside the scope of the present invention, and the yield strength, tensile strength, and yield ratio did not reach the desired values.

[0155] In Comparative Example No. 23 (Steel W), the C content exceeded the scope of the present invention. Therefore, the area ratio of the second phase was outside the scope of the present invention, and the Charpy absorption energy, ductile-brittle transition temperature, uniform elongation, and elongation at break at the corners at -10°C did not reach the expected values.

[0156] In Comparative Example No. 24 (Steel X), the Si content exceeded the scope of the present invention. As a result, the toughness was reduced, and the Charpy absorption energy, ductile-brittle transition temperature, uniform elongation, and elongation at break at the corners at -10°C did not reach the expected values.

[0157] In Comparative Example No. 25 (steel Y), the Mn content was lower than the range of the present invention. Therefore, the area ratio of the second phase was outside the range of the present invention, and the tensile strength and yield ratio did not reach the desired values.

[0158] In Comparative Example No. 26 (Steel Z), the Mn content exceeded the scope of this invention, therefore, the amount of upper bainite formed increased, and the yield ratio did not reach the desired value.

[0159] In Comparative Example No. 27 (steel AA), the Nb content exceeded the scope of this invention, resulting in an excessive increase in the area fraction of the second phase. As a result, the yield ratio did not reach the desired value.

[0160] It is believed that in Comparative Example No. 28 (steel AB), the Ti content exceeded the scope of this invention, resulting in the formation of coarse carbides and nitrides. As a result, the Charpy absorption energy, ductile-brittle transition temperature, and uniform elongation at the corners at -10°C did not reach the desired values.

[0161] In Comparative Example No. 29 (steel AC), the V content exceeded the scope of this invention, thus reducing toughness and ductility. As a result, the Charpy absorption energy, ductile-brittle transition temperature, uniform elongation, and elongation at break at the corners at -10°C did not reach the desired values.

[0162] In Comparative Example No. 30 (steel AD), the Cr content exceeded the scope of this invention, thus reducing toughness and ductility. As a result, the Charpy absorption energy, ductile-brittle transition temperature, uniform elongation, and elongation at break at the corners at -10°C did not reach the desired values.

[0163] In Comparative Example No. 31 (steel AE), the Mo content exceeded the scope of this invention, thus reducing toughness and ductility. As a result, the Charpy absorption energy, ductile-brittle transition temperature, uniform elongation, and elongation at break at the corners at -10°C did not reach the desired values.

[0164] It is believed that in Comparative Example No. 32 (steel AF), because the Cu content exceeded the scope of this invention, Cu precipitated coarsely. As a result, the Charpy absorption energy and ductile-brittle transition temperature at the corners at -10°C did not reach the desired values.

[0165] In Comparative Example No. 33 (steel AG), the Ni content exceeded the scope of this invention, therefore, the amount of upper bainite formed increased, and the yield ratio did not reach the desired value.

[0166] It is believed that in Comparative Example No. 34 (steel AH), because the Ca content exceeded the scope of this invention, Ca oxide clusters were formed. As a result, the Charpy absorption energy, ductile-brittle transition temperature, uniform elongation, and elongation at break at the corners at -10°C did not reach the expected values.

[0167] In Comparative Example No. 35 (steel AI), the content of B exceeds the scope of the present invention, therefore the yield ratio of the plate portion is outside the scope of the present invention.

[0168] In Comparative Example No. 38 (Steel U), the heating temperature of the steel raw material (steel billet) exceeded the range of the present invention, resulting in coarsened grains and an average grain size outside the range of the present invention. As a result, the yield strength, tensile strength, Charpy absorption energy at the corners at -10°C, and ductile-brittle transition temperature did not reach the desired values.

[0169] In Comparative Example No. 39 (steel U), the rough rolling finish temperature exceeded the range of the present invention, and therefore the average grain size was outside the range of the present invention. As a result, the yield strength, tensile strength, Charpy absorption energy at the corners at -10°C, and ductile-brittle transition temperature did not reach the desired values.

[0170] In Comparative Example No. 40 (steel U), the finishing rolling temperature exceeded the range of this invention, resulting in coarsened austenite and an average grain size outside the range of this invention. As a result, the yield strength, tensile strength, Charpy absorption energy at the corners at -10°C, and ductile-brittle transition temperature did not reach the desired values.

[0171] In Comparative Example No. 41 (Steel U), the total reduction rate below 930°C was lower than the range of the present invention, the austenite became coarse, and the average grain size was outside the range of the present invention. As a result, the Charpy absorption energy at the corners and the ductile-brittle transition temperature at -10°C did not reach the desired values.

[0172] In Comparative Example No. 42 (steel U), the average cooling rate at the center of the plate thickness exceeded the range of the present invention. Therefore, the amount of upper bainite formed increased, and the average grain size was outside the range of the present invention. As a result, the yield ratio did not reach the desired value.

[0173] In Comparative Example No. 43 (steel U), the cooling stop temperature and coiling temperature were lower than the range of the present invention. As a result, the amount of upper bainite generated increased, and the yield ratio did not reach the desired value.

[0174] In Comparative Example No. 44 (steel U), the cooling stop temperature and coiling temperature exceeded the range of the present invention, and therefore the average grain size was outside the range of the present invention. As a result, the yield strength, tensile strength, Charpy absorption energy at the corners at -10°C, ductile-brittle transition temperature, uniform elongation, and elongation at break did not reach the desired values.

[0175] In Comparative Example No. 45 (steel U), the coiling temperature was lower than the range of the present invention. As a result, the amount of upper bainite formed increased, and the yield ratio did not reach the desired value.

[0176] In Comparative Example No. 46 (steel U), the compressive strain was below the range of the present invention; therefore, the yield ratio, uniform elongation, and elongation at break did not reach the desired values.

[0177] In Comparative Example No. 47 (steel U), the compressive strain exceeded the range of the present invention, resulting in buckling of the angular steel tube. Therefore, tensile and Charpy tests were not performed.

[0178] In Comparative Example No. 48 (Steel U), the heating time of the steel raw material was less than that of the present invention. Therefore, excessive ferrite was formed, and the area fraction of the second phase was outside the scope of the present invention. Consequently, the yield strength, tensile strength, and yield ratio did not reach the desired values.

[0179] In Comparative Example No. 49 (Steel U), the heating time of the steel raw material exceeded the scope of the present invention. Therefore, the number density of pearlite and pseudo-pearlite with an average grain size and equivalent circular diameter of 2 μm or more and 20 μm or less was outside the scope of the present invention. Consequently, the uniform elongation, elongation at break, Charpy absorption energy at the corners at -10°C, and ductile-brittle transition temperature did not reach the desired values.

[0180] In Comparative Example No. 50 (steel U), the total reduction of the final two passes in the finishing rolling was lower than the range of the present invention. Therefore, the number density of pearlite and pseudo-pearlite with an equivalent circle diameter of 2 μm or more and 20 μm or less was outside the range of the present invention. Consequently, the uniform elongation, elongation at break, Charpy absorption energy at the corners at -10°C, and ductile-brittle transition temperature did not reach the desired values.

[0181] Symbol Explanation

[0182] 1. Angle steel pipe

[0183] 4 main beams

[0184] 5. Small beam

[0185] 6 partitions

[0186] 7-column

Claims

1. An angular steel pipe, comprising a flat section and an angle section, wherein, The steel composition contains, by mass percent, C: ≥0.07% and ≤0.20%, Si: ≥0.01% and ≤0.40%, Mn: ≥0.20% and ≤1.20%, P: ≤0.10%, S: ≤0.050%, Al: ≥0.005% and ≤0.100%, and N: ≤0.010%, with the balance consisting of Fe and unavoidable impurities. The steel microstructure at the center of the plate thickness of the plate portion is dominated by ferrite, with a second phase consisting of one or more of pearlite, pseudo-pearlite, and upper bainite accounting for 5-30% of the area. The average grain size is 10.0-30.0 μm, and the steel microstructure contains a total of 500 pearlite and pseudo-pearlite particles with an equivalent circular diameter of 2 μm or more and 20 μm or less per mm. 2 above, The plate portion has a yield strength of 295 MPa or higher, a tensile strength of 400 MPa or higher, a yield ratio of 0.85 or lower, a uniform elongation of 8.0% or higher, and an elongation at break of 27.0% or higher. The Charpy absorption energy of the corner portion at -10°C is above 80J, and the ductile-brittle transition temperature of the corner portion is below -10°C.

2. The angled steel pipe according to claim 1, wherein, The steel composition further includes one or more of the following components selected by mass percentage: Nb: less than 0.020%, Ti: less than 0.045%, V: less than 0.10%, Cr: less than 0.50%, Mo: less than 0.50%, Cu: less than 0.30%, Ni: less than 0.30%, Ca: less than 0.0100%, B: less than 0.0100%, Sn: less than 0.100%, and Sb: less than 0.100%.

3. A method for manufacturing an angled steel pipe, which is the method for manufacturing an angled steel pipe according to claim 1 or claim 2, wherein, The steel raw material having the aforementioned steel composition is heated at a 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. Then, hot rolling is performed with a roughing end temperature of 850°C or higher and 1150°C or lower, a finishing end temperature of 750°C or higher and 850°C or lower, and a finishing end temperature of 930°C or lower, resulting in a total reduction of 40% or higher, and a total reduction of 15% or higher in the final two finishing passes. Cooling is performed under the conditions that the average cooling rate at the center of the plate thickness is less than 10.0℃ / s and the cooling stop temperature at the center of the plate thickness is more than 480℃ and less than 650℃. Then, Hot-rolled steel sheets are produced by coiling the steel at a temperature between 480°C and 650°C in the center of the sheet thickness. Then... The hot-rolled steel sheet is formed into a cylindrical shape by roll forming, and then the ends of the hot-rolled steel sheet are welded to form a circular steel pipe. Then, it is formed into an angular steel pipe to produce an angular steel pipe. At this time, after forming the circular steel pipe and / or forming the angular steel pipe, a compressive strain of 0.20 to 3.00% is applied along the pipe axis direction.

4. A building structure that uses the angular steel tubes as described in claim 1 or claim 2 as column material.

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