TWI724782B - Square steel pipe and its manufacturing method, and building structure - Google Patents

Square steel pipe and its manufacturing method, and building structure Download PDF

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TWI724782B
TWI724782B TW109103845A TW109103845A TWI724782B TW I724782 B TWI724782 B TW I724782B TW 109103845 A TW109103845 A TW 109103845A TW 109103845 A TW109103845 A TW 109103845A TW I724782 B TWI724782 B TW I724782B
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steel pipe
square steel
cooling
temperature
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TW202045745A (en
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井手信介
松本晃英
松本昌士
岡部能知
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日商Ｊｆｅ鋼鐵股份有限公司
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21C—MANUFACTURE OF METAL SHEETS, WIRE, RODS, TUBES OR PROFILES, OTHERWISE THAN BY ROLLING; AUXILIARY OPERATIONS USED IN CONNECTION WITH METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL
    • B21C37/00—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape
    • B21C37/06—Manufacture of metal sheets, bars, wire, tubes or like semi-manufactured products, not otherwise provided for; Manufacture of tubes of special shape of tubes or metal hoses; Combined procedures for making tubes, e.g. for making multi-wall tubes
    • B21C37/08—Making tubes with welded or soldered seams
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D5/00—Bending sheet metal along straight lines, e.g. to form simple curves
    • B21D5/06—Bending sheet metal along straight lines, e.g. to form simple curves by drawing procedure making use of dies or forming-rollers, e.g. making profiles
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/10—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of tubular bodies
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • E—FIXED CONSTRUCTIONS
    • E04—BUILDING
    • E04B—GENERAL BUILDING CONSTRUCTIONS; WALLS, e.g. PARTITIONS; ROOFS; FLOORS; CEILINGS; INSULATION OR OTHER PROTECTION OF BUILDINGS
    • E04B1/00—Constructions in general; Structures which are not restricted either to walls, e.g. partitions, or floors or ceilings or roofs
    • E04B1/18—Structures comprising elongated load-supporting parts, e.g. columns, girders, skeletons
    • E04B1/24—Structures comprising elongated load-supporting parts, e.g. columns, girders, skeletons the supporting parts consisting of metal
    • E—FIXED CONSTRUCTIONS
    • E04—BUILDING
    • E04C—STRUCTURAL ELEMENTS; BUILDING MATERIALS
    • E04C3/00—Structural elongated elements designed for load-supporting
    • E04C3/30—Columns; Pillars; Struts
    • E04C3/32—Columns; Pillars; Struts of metal

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  • Chemical & Material Sciences (AREA)
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Abstract

提供一種方形鋼管及其製造方法，以及使用該方形鋼管的建築構造物。本發明係一種方形鋼管，具有平板部及角部；其特徵為：具有特定的成分組成，自鋼管的外表面至板厚t之1/4深度位置之鋼組織，係肥粒鐵之面積率為55%以上80%以下，硬質相的平均縱橫比為0.1～0.8，平板部的YS為350MPa以上、TS為520MPa以上，平板部對於角部之YS的比為0.80以上0.90以下、平板部對於角部之TS的比為0.90以上1.00以下，平板部之-40℃的夏比吸收能為100J以上、角部的R為(2.3×t)以上(2.9×t)以下。Provided is a square steel pipe and a manufacturing method thereof, and a building structure using the square steel pipe. The present invention is a square steel pipe with flat plate parts and corner parts; it is characterized in that it has a specific composition, the steel structure from the outer surface of the steel pipe to the 1/4 depth position of the plate thickness t, and the area ratio of the ferrite iron 55% or more and 80% or less, the average aspect ratio of the hard phase is 0.1-0.8, the YS of the flat part is 350MPa or more, the TS is 520MPa or more, the ratio of the flat part to the corner part is 0.80 or more and 0.90 or less. The TS ratio of the corners is 0.90 or more and 1.00 or less, the Charpy absorption energy at -40°C of the flat plate is 100J or more, and the R of the corners is (2.3×t) or more (2.9×t) or less.

Description

方形鋼管及其製造方法，以及建築構造物Square steel pipe and its manufacturing method, and building structure

本發明，係關於方形鋼管及其製造方法以及建築構造物。本發明之角部及平板部的強度差小的方形鋼管，係適合用作為建築構造構件。The present invention relates to a square steel pipe, a manufacturing method thereof, and a building structure. The square steel pipe with small strength difference between the corners and the flat plate of the present invention is suitable for use as a structural member of a building.

方形鋼管(亦稱為「方形柱」)，係一般而言以熱軋鋼板(熱軋鋼帶)或是厚板為素材，藉由冷成形製造。作為冷成形的方法，係有沖壓成形及輥壓成形。然而，無論是何種方法，皆會對於方形鋼管的角部施加有比方形鋼管的平板部更大的塑性應變，故會有角部的強度容易上升而使角部與平板部的強度差增大之虞。在角部與平板部的特性有大幅差異的情形，選擇焊接材料或建築設計會變得極為困難，故難以將方形鋼管使用作為建築構造用材料。Square steel pipes (also called "square columns") are generally manufactured by cold forming using hot-rolled steel plates (hot-rolled steel strips) or thick plates. As cold forming methods, there are press forming and roll forming. However, regardless of the method, a larger plastic strain is applied to the corners of the square steel tube than the flat part of the square steel tube, so the strength of the corner is likely to rise, which increases the strength difference between the corner and the flat part. Big danger. When the characteristics of the corners and flats are greatly different, it becomes extremely difficult to select welding materials or architectural design, so it is difficult to use square steel pipes as materials for building construction.

對於如此之問題，雖直接進行探討的例子並不多，然而例如有作為建築構造物用的方形鋼管之專利文獻1的技術。於專利文獻1揭示有一種冷成形方形鋼管，其係將鋼板進行冷間彎曲加工而獲得的方形鋼管；前述鋼管，係分別含有C：0.02～0.18%(「%」係意指「質量%」，針對以下的化學成分亦相同)Si：0.03～0.5%、Mn：0.7～2.5%、Al：0.005～0.12%以及N：0.008%以下(不包含0%)，剩餘部分包含Fe及不可避免的雜質，該不可避免的雜質當中係分別抑制為P：0.02%以下(不包含0%)、S：0.01%以下(不包含0%)以及O：0.004以下(不包含0%)，前述彎曲加工部係保持在加工為直角的狀態，且滿足下述(A)～(C)之要件以藉此滿足耐震性。 (A)鋼管的平坦部之降伏強度：355MPa以上、拉伸強度：520MPa以上， (B)前述平坦部之顯微組織，變韌鐵組織的面積分率：40%以上， (C)鋼管的角部之表層部係維氏硬度Hv：350以下、拉伸試驗的伸度：10%以上、0℃之夏比吸收能vE0：70J以上。 [先前技術文獻] [專利文獻] Regarding such a problem, although there are not many examples that have been directly discussed, for example, there is the technique of Patent Document 1 which is a square steel pipe used as a building structure. Patent Document 1 discloses a cold-formed square steel pipe, which is a square steel pipe obtained by cold bending a steel plate; the aforementioned steel pipes respectively contain C: 0.02 to 0.18% ("%" means "mass%" , The following chemical components are also the same) Si: 0.03 to 0.5%, Mn: 0.7 to 2.5%, Al: 0.005 to 0.12%, and N: less than 0.008% (excluding 0%), the remainder contains Fe and unavoidable Impurities, the unavoidable impurities are respectively suppressed to P: 0.02% or less (not including 0%), S: 0.01% or less (not including 0%), and O: 0.004 or less (not including 0%), the aforementioned bending process The part system is maintained in a state processed at a right angle, and meets the following requirements (A) to (C) to thereby satisfy the shock resistance. (A) Yield strength of the flat part of the steel pipe: 355MPa or more, tensile strength: 520MPa or more, (B) The microstructure of the aforementioned flat part, the area fraction of the toughened iron structure: 40% or more, (C) The surface layer of the corner of the steel pipe has Vickers hardness Hv: 350 or less, elongation in tensile test: 10% or more, and Charpy absorption energy vE0 at 0°C: 70J or more. [Prior Technical Literature] [Patent Literature]

[專利文獻1]日本特許第5385760號公報[Patent Document 1] Japanese Patent No. 5385760

[發明所欲解決之問題][The problem to be solved by the invention]

藉由冷間之輥壓成形所製造的方形鋼管，係將藉由熱間輥軋產生之寬度方向為平坦的材料(熱間輥軋材料)進行輥壓成形而形成為圓形鋼管之後，成形為具有角部及平板部的方形鋼管。在如此之製法上，會因加工硬化的差異導致角部與平板部的強度差容易變大。並且，於輥壓成形之前所進行的熱間輥軋，係藉由從熱間輥軋材料的表面進行之冷卻控制進行材料的塑造，故會有加工前的強度(硬度)在冷卻速度相對大的熱間輥軋材料的表層附近較大的問題。The square steel pipe manufactured by cold roll forming is formed by rolling a material (hot rolled material) that is flat in the width direction produced by hot roll into a round steel pipe. It is a square steel pipe with corners and flat plates. In such a manufacturing method, the difference in strength between the corner portion and the flat portion is likely to increase due to the difference in work hardening. In addition, the hot rolling before roll forming is performed by cooling control from the surface of the hot rolled material to shape the material, so the strength (hardness) before processing is relatively large at the cooling rate. The hot-rolled material has a larger problem near the surface layer.

然而，以前述之專利文獻1所揭示的技術而言，僅是藉由於熱間輥軋之溫度控制來使鋼板的表面的硬度不致過度上升，而並非積極地使角部與平板部的強度差縮小。因此，冷間彎曲加工所獲得的方形鋼管，即便角部的特性滿足預定的基準，角部的強度顯然會相對比平板部的強度更高。若使角部的塑性應變減小，則能夠有效抑制角部的強度上升。為了使角部的塑性應變減小，考慮到使角部R(圓度)增大。然而，角部的R較大的方形鋼管，在作為方形的構件與其他構件組合之際，會有設計上的問題或是因產生間隙等導致建築物的性能降低的問題，故不甚理想。However, the technique disclosed in the aforementioned Patent Document 1 only prevents the surface hardness of the steel sheet from excessively increasing due to the temperature control of hot rolling, and does not actively make the corner portion and the flat portion strength worse. Zoom out. Therefore, even if the characteristics of the corner portion of the square steel tube obtained by cold bending meets the predetermined standard, the strength of the corner portion is obviously higher than that of the flat portion. If the plastic strain of the corner portion is reduced, the increase in the strength of the corner portion can be effectively suppressed. In order to reduce the plastic strain of the corner portion, it is considered that the corner portion R (roundness) is increased. However, a square steel pipe with a large R at the corner is not ideal because it has design problems when combined with other members as a square member or the performance of the building is reduced due to gaps.

本發明係有鑑於如此情事而完成者，以提供一種角部與平板部的強度差小的方形鋼管及其製造方法以及使用該方形鋼管的建築構造物為目的。 [解決問題之技術手段] The present invention was completed in view of such circumstances, and aims to provide a square steel pipe with a small difference in strength between the corners and the flat plate, a method for manufacturing the square steel pipe, and a building structure using the square steel pipe. [Technical means to solve the problem]

本發明者們，為解決前述課題而精心鑽研，係獲得以下見解。The inventors of the present invention have intensively studied to solve the aforementioned problems and obtained the following findings.

本發明，係著眼於在冷間之輥壓成形被導入之加工應變(塑性應變)會變得特別大的鋼管的表層附近(以下稱為外表面附近)，使加工硬化不易產生，藉此使角部與平板部的強度差減小。The present invention focuses on the vicinity of the surface layer (hereinafter referred to as the outer surface vicinity) of the steel pipe where the processing strain (plastic strain) introduced during the cold roll forming becomes particularly large, so that work hardening is difficult to occur, thereby making The strength difference between the corner portion and the flat portion is reduced.

因此，本發明者們，係備有複數個作為使方形鋼管的鋼組織之肥粒鐵的面積率及肥粒鐵以外的硬質相(以下稱為硬質相)的縱橫比變化的試料，調查加工硬化的容易度。在此，所謂硬質相，雖包含變韌鐵、波來鐵、麻田散鐵及殘留沃斯田鐵等，然而不限於此。Therefore, the present inventors prepared a plurality of samples for changing the area ratio of the ferrous iron in the steel structure of the square steel pipe and the aspect ratio of the hard phase other than the ferrous iron (hereinafter referred to as the hard phase), and investigated the processing Ease of hardening. Here, the so-called hard phase includes toughened iron, bleached iron, Asada iron, residual austenitic iron, etc., but is not limited to this.

因此，發現於具有強度係平板部之YS為350MPa以上、TS為520MPa以上的方形鋼管中，使肥粒鐵的比例為一定以上，並且使硬質相的平均縱橫比為0.1～0.8，藉此能夠塑造不易加工硬化的鋼組織。此係因肥粒鐵的加工硬化能較小且應變容易集中於該肥粒鐵，故能夠使鋼組織整體的加工硬化能小。Therefore, it has been found that in square steel pipes with a strength-based flat plate portion with YS of 350 MPa or more and TS of 520 MPa or more, the ratio of the ferrite iron is fixed or more, and the average aspect ratio of the hard phase is 0.1 to 0.8. Shape the steel structure that is not easy to work hardening. In this system, since the work hardening energy of the fat grain iron is small and the strain tends to concentrate on the fat grain iron, the work hardening energy of the entire steel structure can be reduced.

並且，本發明者們，為了活用素材(熱間輥軋材料)的鋼組織以抑制角部的加工硬化，在製造方形鋼管之際，係暫時成形為縱徑/橫徑的比為0.99以上1.01以下之圓筒狀的圓形鋼管之後，藉由配置於上下及左右的輥，成形為角部之R為2.3×t(t係板厚)以上2.9×t以下之角狀。藉此，發現能夠不致使角部過度加工硬化便獲得方形鋼管。在此，所謂「縱徑」係指對於前述圓形鋼管的管軸之垂直方向的外徑，所謂「橫徑」係指對於前述圓形鋼管的管軸之水平方向的外徑。In addition, the inventors of the present invention made use of the steel structure of the material (hot rolled material) to suppress work hardening of the corners. When manufacturing square steel pipes, they were temporarily formed so that the ratio of longitudinal diameter/transverse diameter was 0.99 or more and 1.01 The following cylindrical round steel pipes are formed into an angular shape with a corner R of 2.3×t (t series plate thickness) or more and 2.9×t or less by the rollers arranged on the upper and lower sides and on the left and right. With this, it was found that a square steel pipe can be obtained without excessive work hardening of the corners. Here, the "longitudinal diameter" refers to the outer diameter in the vertical direction relative to the tube axis of the circular steel pipe, and the "transverse diameter" refers to the outer diameter in the horizontal direction relative to the tube axis of the circular steel pipe.

如以上所述，就本發明而言，在冷間之輥壓成形被導入之加工應變為最大的方形鋼管的外表面附近的鋼組織中，使肥粒鐵的比例為一定以上，並且使硬質相的平均縱橫比為0.1～0.8。並且，在將熱間輥軋材料成形為縱徑/橫徑的比為0.99以上1.01以下之圓筒狀之後，藉由配置於上下及左右的輥成形為角狀，製造角部與平板部的強度差小的方形鋼管。As described above, according to the present invention, in the steel structure near the outer surface of the square steel pipe where the processing strain is the largest when the cold roll forming is introduced, the ratio of ferrite iron is set to a certain level or more, and the hard The average aspect ratio of the phase is 0.1 to 0.8. In addition, after the hot rolled material is formed into a cylindrical shape with a longitudinal diameter/lateral diameter ratio of 0.99 or more and 1.01 or less, the rolls arranged on the upper and lower sides and on the left and right are formed into a corner shape to produce a corner portion and a flat plate portion. Square steel pipe with small strength difference.

又，於本發明中，所謂「角部與平板部的強度差小的方形鋼管」，係表示平板部對於角部之YS的比為0.80以上0.90以下，平板部對於角部之TS的比為0.90以上1.00以下。In addition, in the present invention, the "square steel pipe with a small difference in strength between the corner and the flat part" means that the ratio of the YS of the flat part to the corner is 0.80 or more and 0.90 or less, and the ratio of the flat part to the TS of the corner is 0.90 or more and 1.00 or less.

本發明者們進一步詳加鑽研，以至完成本發明。本發明之要旨，係如以下所述。 [1] 一種方形鋼管，係具有平板部及角部；其特徵為： 作為成分組成，以質量%含有： C：0.07～0.20%、 Si：1.0%以下、 Mn：0.5～2.0%、 P：0.030%以下、 S：0.015%以下、 Al：0.01～0.06%、 N：0.006%以下， 剩餘部分為Fe及不可避免的雜質， 自鋼管的外表面至板厚t之1/4深度位置之鋼組織，係肥粒鐵之面積率為55%以上80%以下，硬質相平均縱橫比為0.1～0.8， 前述平板部，係YS為350MPa以上，TS為520MPa以上 前述平板部對於前述角部之YS的比為0.80以上0.90以下，前述平板部對於前述角部之TS的比為0.90以上1.00以下， 前述平板部的-40℃的夏比吸收能為100J以上， 前述角部的R為(2.3×t)以上(2.9×t)以下。 [2] 如[1]所述之方形鋼管，其中，除了前述成分組成，以質量%含有自下述A群～C群當中選擇之1群或2群以上。 A群：自Nb：0.05%以下、Ti：0.05%以下、V：0.10%以下當中選擇之1種或2種以上 B群：B：0.008%以下 C群：自Cr：0.01～1.0%、Mo：0.01～1.0%、Cu：0.01～0.50%、Ni：0.01～0.30%、Ca：0.001～0.010%當中選擇之1種或2種以上。 [3] 如[1]或[2]所述之方形鋼管，其中，前述鋼組織，係前述硬質相的平均當量圓直徑係20μm以下。 [4] 一種方形鋼管之製造方法，係[1]至[3]中任一項所述之方形鋼管之製造方法，其特徵為： 進行造管步驟，該造管步驟，係將鋼板於冷間輥壓成形而形成為圓筒狀的端面焊接，成形為縱徑/橫徑之比0.99以上1.01以下的圓筒狀之後，成形為角狀。 [5] 一種方形鋼管的製造方法，係[1]至[3]中任一項所述之方形鋼管的製造方法，其特徵為： 對於鋼素材，依序進行熱間輥軋步驟、冷卻步驟、捲取步驟及造管步驟而製造方形鋼管之際， 將前述鋼素材加熱至加熱溫度：1100～1300℃之後，進行熱間輥軋步驟而形成為熱軋鋼板，該熱間輥軋步驟，係： 板厚中心溫度為1000℃以上之粗軋時間：200秒以上400秒以內、粗軋結束溫度：1000～800℃， 精軋開始溫度：1000～800℃、精軋結束溫度：900～750℃； 接著，對於前述熱軋鋼板進行冷卻步驟，該冷卻步驟，係：具有1次以上之從冷卻開始10s之間的初期冷卻步驟之0.2s以上且未達3.0s之放置冷卻，於板厚中心溫度之平均冷卻速度：4～25℃/s； 接著，對於前述熱軋鋼板進行以捲取溫度：580℃以下進行捲取的捲取步驟而形成鋼板， 接著，進行造管步驟，該造管步驟，係將前述鋼板於冷間輥壓成形而形成為圓筒狀的端面焊接，成形為縱徑/橫徑之比0.99以上1.01以下的圓筒狀之後，成形為角狀。 [6] 如[5]所述之方形鋼管的製造方法，其中，使前述冷卻步驟的冷卻停止溫度為580℃以下。 [7] 一種建築構造物，係使用[1]至[3]中任一項所述之方形鋼管。 [發明之效果] The inventors have further studied in detail to complete the present invention. The gist of the present invention is as follows. [1] A square steel pipe with flat plate and corners; its characteristics are: As an ingredient composition, it contains by mass%: C: 0.07～0.20%, Si: 1.0% or less, Mn: 0.5～2.0%, P: 0.030% or less, S: 0.015% or less, Al: 0.01～0.06%, N: less than 0.006%, The remainder is Fe and unavoidable impurities, From the outer surface of the steel pipe to the 1/4 depth position of the plate thickness t, the area ratio of ferrous iron is 55% to 80%, and the average aspect ratio of the hard phase is 0.1 to 0.8. For the aforementioned flat part, YS is 350MPa or more, TS is 520MPa or more The ratio of the flat part to the YS of the corner part is 0.80 or more and 0.90 or less, and the ratio of the flat part to the corner part TS is 0.90 or more and 1.00 or less, The Charpy absorption energy at -40°C of the aforementioned flat part is more than 100J, The R of the aforementioned corner is (2.3×t) or more (2.9×t) or less. [2] The square steel pipe as described in [1], in which, in addition to the aforementioned component composition, one or more groups selected from the following groups A to C are contained in mass%. Group A: Choose one or more from Nb: 0.05% or less, Ti: 0.05% or less, and V: 0.10% or less Group B: B: less than 0.008% Group C: One or more selected from Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.30%, and Ca: 0.001 to 0.010%. [3] The square steel pipe according to [1] or [2], wherein the steel structure has an average equivalent circle diameter of the hard phase of 20 μm or less. [4] A method for manufacturing square steel pipes, which is the method for manufacturing square steel pipes described in any one of [1] to [3], characterized by: Carry out the pipe making step. The pipe making step is to weld the steel plate into a cylindrical shape by cold roll forming, and shape it into a cylindrical shape with a longitudinal diameter/transverse diameter ratio of 0.99 or more and 1.01 or less. Horny. [5] A method for manufacturing square steel pipes, which is the method for manufacturing square steel pipes described in any one of [1] to [3], characterized by: For the steel material, when the hot rolling step, the cooling step, the coiling step, and the pipe making step are carried out in order to manufacture the square steel pipe, After heating the aforementioned steel material to a heating temperature: 1100-1300°C, a hot-rolling step is performed to form a hot-rolled steel sheet. The hot-rolling step is: Rough rolling time when the thickness center temperature is above 1000℃: 200 seconds or more and within 400 seconds, rough rolling end temperature: 1000～800℃, Start temperature of finishing rolling: 1000～800℃, finishing temperature of finishing rolling: 900～750℃; Next, perform a cooling step for the aforementioned hot-rolled steel sheet. This cooling step is a cooling step of 0.2 s or more and less than 3.0 s of the initial cooling step between 10 s from the beginning of cooling, and the temperature is at the center of the plate thickness. The average cooling rate: 4～25℃/s; Next, the aforementioned hot-rolled steel sheet is subjected to a coiling step of coiling at a coiling temperature of 580°C or less to form a steel sheet, Next, a pipe making step is performed. This pipe making step is to weld the aforementioned steel plate into a cylindrical shape by cold roll forming, and shape it into a cylindrical shape with a longitudinal diameter/transverse diameter ratio of 0.99 or more and 1.01 or less. , Shaped into an angular shape. [6] The method for producing a square steel pipe according to [5], wherein the cooling stop temperature in the cooling step is set to 580°C or lower. [7] A building structure using the square steel pipe described in any one of [1] to [3]. [Effects of Invention]

依據本發明，能夠獲得角部及平板部的強度差小的方形鋼管。該方形鋼管，因角部的R被控制在恰當的大小，故例如能夠適合用作為建築構造構件用的方形鋼管。According to the present invention, a square steel pipe with a small difference in strength between the corners and the flat plate can be obtained. Since this square steel pipe has a corner R controlled to an appropriate size, it can be suitably used as a square steel pipe for building structural members, for example.

以下，針對本發明之進行詳細說明。Hereinafter, the present invention will be described in detail.

本發明之方形鋼管，係如以下所述。作為成分組成，以質量%含有：C：0.07～0.20%、Si：1.0%以下、Mn：0.5～2.0%、P：0.030%以下、S：0.015%以下、Al：0.01～0.06%、N：0.006%以下，剩餘部分為Fe及不可避免的雜質。自該方形鋼管的外表面至板厚t之1/4深度位置(以下稱為1/4t位置)之鋼組織，係肥粒鐵之面積率為55%以上80%以下，硬質相之平均縱橫比為0.1～0.8。並且，方形鋼管的平板部，係YS為350MPa以上，TS為520MPa以上，平板部對於角部之YS的比為0.80以上0.90以下，平板部對於角部之TS的比為0.90以上1.00以下，平板部的板厚1/4t位置之-40℃的夏比吸收能為100J以上，角部的R為(2.3×t)以上(2.9×t)以下。The square steel pipe of the present invention is as described below. The component composition contains in mass%: C: 0.07 to 0.20%, Si: 1.0% or less, Mn: 0.5 to 2.0%, P: 0.030% or less, S: 0.015% or less, Al: 0.01 to 0.06%, N: Below 0.006%, the remainder is Fe and unavoidable impurities. The steel structure from the outer surface of the square steel pipe to the 1/4 depth position of the plate thickness t (hereinafter referred to as the 1/4t position), the area ratio of the ferrite iron is 55% to 80%, and the average vertical and horizontal of the hard phase The ratio is 0.1 to 0.8. In addition, for the flat part of the square steel pipe, YS is 350MPa or more, TS is 520MPa or more, the ratio of the YS of the flat part to the corner part is 0.80 or more and 0.90 or less, and the ratio of the flat part to the corner TS is 0.90 or more and 1.00 or less. The Charpy absorption energy at -40°C at the position where the plate thickness of the part is 1/4t is more than 100J, and the R at the corner part is (2.3×t) or more (2.9×t) or less.

首先，針對本發明之成分組成進行說明。又，若未特別限定，質量%係僅以%表記。又，於本發明中，方形鋼管與使用為方形鋼管的素材之鋼板的成分組成相同。因此，以下，說明方形鋼管及使用作為素材之鋼板的成分組成的限定理由。First, the component composition of the present invention will be explained. In addition, if it is not particularly limited, the mass% is only expressed in %. In addition, in the present invention, the square steel pipe has the same component composition as the steel plate used as the material of the square steel pipe. Therefore, the reasons for limiting the composition of the square steel pipe and the steel plate used as the material will be explained below.

C：0.07～0.20% C會因固熔強化而使鋼板及方形鋼管的強度增加。另一方面，C係使硬質相的生成增加而因此使肥粒鐵的生成量減少的元素。為了確保所要求的強度並確保所要求的鋼板及方形鋼管的鋼組織，C必須含有0.07%以上。另一方面，若含有C超過0.20%，則難以確保所要求的肥粒鐵量。因此，C係0.07～0.20%。C係較佳為0.09%以上，更佳為0.10%以上。並且，C較佳為0.18%以下，更佳為0.17%以下。 C: 0.07～0.20% C will increase the strength of steel plates and square steel pipes due to solid solution strengthening. On the other hand, the C-based element increases the production of hard phases and therefore reduces the production of ferrous iron. In order to ensure the required strength and the steel structure of the required steel plate and square steel pipe, C must contain 0.07% or more. On the other hand, if the content of C exceeds 0.20%, it will be difficult to ensure the required amount of fertilizer iron. Therefore, the C range is 0.07 to 0.20%. The C system is preferably 0.09% or more, more preferably 0.10% or more. In addition, C is preferably 0.18% or less, more preferably 0.17% or less.

Si：1.0%以下 Si係會因固熔強化而有助於使鋼板及方形鋼管的強度增加的元素。為了確保所要求的強度並確保所要求的鋼板及方形鋼管的鋼組織，含有超過0.01%之Si為佳。然而，若含有超過1.0%之Si，則韌性會降低。因此，Si係1.0%以下。又，Si較佳為0.8%以下，更佳為0.6%以下。再更佳為0.03%以上。 Si: 1.0% or less The Si-based element contributes to increasing the strength of steel plates and square steel pipes due to solid solution strengthening. In order to ensure the required strength and the steel structure of the required steel plate and square steel pipe, it is better to contain more than 0.01% Si. However, if more than 1.0% of Si is contained, the toughness will decrease. Therefore, Si is 1.0% or less. Moreover, Si is preferably 0.8% or less, more preferably 0.6% or less. More preferably, it is 0.03% or more.

Mn：0.5～2.0% Mn係藉由固熔強化使鋼板及方形鋼管的強度增加的元素，為了確保所要求之鋼板及方形鋼管的強度，必須含有0.5%以上。若含有之Mn未達0.5%，則會導致肥粒鐵相變開始溫度上升，伴隨於此，會使硬質相容易過度地粗化。另一方面，若含有之Mn超過2.0%，則中心偏析部的硬度會上升，而有成為方形鋼管在現場進行焊接時發生龜裂的原因之虞。因此，Mn係0.5～2.0%。Mn較佳為1.8%以下，更佳為1.6%以下。Mn較佳為0.6%以上，更佳為0.7%以上。 Mn: 0.5～2.0% Mn is an element that increases the strength of steel plates and square steel pipes through solid solution strengthening. In order to ensure the required strength of steel plates and square steel pipes, it must be contained at 0.5% or more. If the content of Mn is less than 0.5%, the start temperature of the ferrite phase transformation will increase, and with this, the hard phase will easily become excessively coarse. On the other hand, if the content of Mn exceeds 2.0%, the hardness of the central segregation area will increase, which may cause cracks in the square steel pipe when welded in the field. Therefore, Mn is 0.5 to 2.0%. Mn is preferably 1.8% or less, more preferably 1.6% or less. Mn is preferably 0.6% or more, more preferably 0.7% or more.

P：0.030%以下 P係具有偏析至肥粒鐵晶界而使鋼板及方形鋼管的韌性降低的作用的元素。於本發明中，較佳為盡可能減少雜質。然而，過度減少P，會導致精煉成本高漲，故為0.002%以上為佳。又，容許含有P至0.030%為止。因此，P係0.030%以下。P較佳為0.025%以下，更佳為0.020%以下。 P: Below 0.030% The P-based element has a function of segregating to the grain boundaries of the ferrous iron and lowering the toughness of the steel plate and the square steel pipe. In the present invention, it is preferable to reduce impurities as much as possible. However, excessive reduction of P will lead to high refining costs, so 0.002% or more is better. In addition, the allowable content of P is up to 0.030%. Therefore, P is 0.030% or less. P is preferably 0.025% or less, more preferably 0.020% or less.

S：0.015%以下 S係於鋼中作為硫化物存在，若係本發明之成分組成的範圍，則主要作為MnS存在。MnS會在熱間輥軋步驟被薄薄地延伸，而對於鋼板及方形鋼管的延性及韌性造成不良影響。因此，於本發明中，較佳為盡可能減少MnS。然而，過度減少S，會導致精煉成本高漲，故S為0.0002%以上為佳。又，容許含有S至0.015%為止。因此，S係0.015%以下。S較佳為0.010%以下，更佳為0.008%以下。 S: 0.015% or less S is present in steel as a sulfide, and if it is in the range of the composition of the present invention, it mainly exists as MnS. MnS will be stretched thinly in the hot rolling step, which will adversely affect the ductility and toughness of steel plates and square steel pipes. Therefore, in the present invention, it is preferable to reduce MnS as much as possible. However, excessive reduction of S will lead to high refining costs, so S is better than 0.0002%. Also, it is permissible to contain S up to 0.015%. Therefore, S is 0.015% or less. S is preferably 0.010% or less, more preferably 0.008% or less.

Al：0.01～0.06% Al係作為脫氧劑發揮作用，且具有作為AlN固定N的作用的元素。為了獲得如此之效果，必須含有0.01%以上的Al。若Al未達0.01%，在未添加Si之情形脫氧力不足，使氧化物系夾雜物增加而使鋼板的潔淨度降低。另一方面，若含有超過0.06%之Al，則固熔Al量會增加，在焊接方形鋼管的長邊時(亦即，於方形鋼管之製造中電焊焊接鋼管長度方向時)，特別是在大氣中進行焊接時，於焊接部形成氧化物危險性會提高，而使方形鋼管焊接部的韌性降低。因此，Al係0.01～0.06%。Al係較佳為0.02%以上。並且，Al較佳為0.05%以下。 Al: 0.01～0.06% The Al-based element functions as a deoxidizer and has a role of fixing N as AlN. In order to obtain such an effect, it is necessary to contain 0.01% or more of Al. If Al is less than 0.01%, the deoxidizing power is insufficient when Si is not added, so that oxide-based inclusions increase and the cleanliness of the steel sheet decreases. On the other hand, if it contains more than 0.06% of Al, the amount of solid solution Al will increase when welding the long sides of square steel pipes (that is, when welding the length of the steel pipe in the manufacture of square steel pipes), especially in the atmosphere When welding is performed during welding, the risk of oxide formation in the welded portion will increase, and the toughness of the welded portion of the square steel pipe will decrease. Therefore, Al is 0.01 to 0.06%. The Al system is preferably 0.02% or more. In addition, Al is preferably 0.05% or less.

N：0.006%以下 N係具有將位錯的運動牢固地固著而使鋼板及方形鋼管的韌性降低的作用之元素。於本發明中，較佳為盡可能減少作為雜質之N，可容許含有至0.006%為止。因此，N係0.006%以下。N較佳為0.005%以下。於本發明中雖未特別限定，然而從製造成本的觀點而言，N較佳為0.001%以上。 N: less than 0.006% The N-based element has a function of firmly fixing the movement of dislocations and lowering the toughness of steel plates and square steel pipes. In the present invention, it is preferable to reduce N as an impurity as much as possible, and the content can be allowed up to 0.006%. Therefore, N is 0.006% or less. N is preferably 0.005% or less. Although not particularly limited in the present invention, from the viewpoint of manufacturing cost, N is preferably 0.001% or more.

剩餘部分為Fe及不可避免的雜質。又，在不損及本發明之效果的範圍內，作為不可避免的雜質，例如能夠容許含有O(氧)：0.005%以下。The remainder is Fe and unavoidable impurities. In addition, within a range that does not impair the effect of the present invention, as an inevitable impurity, for example, O (oxygen): 0.005% or less can be allowed to be contained.

以上為本發明之基本的成分組成。能夠藉由前述之必須元素獲得本發明作為目的之特性，然而亦能夠視必要含有下述之元素。The above is the basic composition of the present invention. The characteristics intended by the present invention can be obtained by the aforementioned essential elements, but the following elements can also be contained as necessary.

自Nb：0.05%以下、Ti：0.05%以下、V：0.10%以下當中選擇之1種或2種以上Nb、Ti、V，皆係於鋼中形成細微的碳化物、氮化物，並透過析出強化而有助於使鋼的強度提升的元素。因此，於本發明中亦可以調整強度為目的來含有。在為了獲得如此之效果而含有Nb、Ti、V之情形，較佳為分別係Nb：0.05%以下、Ti：0.05%以下、V：0.10%以下，更佳為分別係Nb：0.04%以下、Ti：0.04%以下、V：0.08%以下。在含有Nb、Ti、V之情形，較佳為分別係Nb：0.001%以上、Ti：0.001%以上、V：0.001%以上，更佳為分別係Nb：0.003%以上、Ti：0.003%以上、V：0.003%以上。Nb: 0.05% or less, Ti: 0.05% or less, and V: 0.10% or less. Nb, Ti, and V form fine carbides and nitrides in the steel, and they are precipitated An element that strengthens and helps increase the strength of steel. Therefore, it may be contained for the purpose of adjusting the strength in the present invention. In the case where Nb, Ti, and V are contained in order to obtain such an effect, Nb: 0.05% or less, Ti: 0.05% or less, V: 0.10% or less, and more preferably Nb: 0.04% or less, Ti: 0.04% or less, V: 0.08% or less. When Nb, Ti, and V are contained, they are preferably Nb: 0.001% or more, Ti: 0.001% or more, and V: 0.001% or more, and more preferably Nb: 0.003% or more, Ti: 0.003% or more, respectively. V: 0.003% or more.

又，在含有自Nb、Ti、V當中選擇之2種以上的情形，合計係較佳為0.2%以下、0.005%以上。In addition, when two or more selected from Nb, Ti, and V are contained, the total is preferably 0.2% or less and 0.005% or more.

B：0.008%以下 B係具有使冷卻過程之肥粒鐵相變延遲而促進低溫相變肥粒鐵形成，使鋼板及方形鋼管的強度增加的作用之元素。B之含有，會使鋼板的降伏比亦即方形鋼管的降伏比增加。因此，於本發明中，若係使方形鋼管的降伏比成為90%以下的範圍，能夠以調整強度為目的視必要含有B。在含有B的情形，較佳為B：0.008%以下。B較佳為0.0015%以下，更佳為0.0008%以下。B較佳為0.0001%以上，更佳為0.0003%以上。 B: less than 0.008% B is an element that delays the phase transformation of ferrous iron in the cooling process, promotes the formation of low-temperature phase-transformed ferrite iron, and increases the strength of steel plates and square steel pipes. The inclusion of B increases the yield ratio of the steel plate, that is, the yield ratio of the square steel pipe. Therefore, in the present invention, if the yield ratio of the square steel pipe is within a range of 90% or less, B can be contained as necessary for the purpose of adjusting the strength. When B is contained, B: 0.008% or less is preferable. B is preferably 0.0015% or less, more preferably 0.0008% or less. B is preferably 0.0001% or more, more preferably 0.0003% or more.

自Cr：0.01～1.0%、Mo：0.01～1.0%、Cu：0.01～0.50%、Ni：0.01～0.30%、Ca：0.001～0.010%當中選擇之1種或2種以上。 Cr：0.01～1.0% Cr係提高淬透性而使鋼板及方形鋼管的強度上升的元素，能夠視必要含有。在為了獲得如此效果而含有Cr的情形，較佳為含有0.01%以上之Cr。另一方面，若含有超過1.0%則有使韌性及焊接性降低之虞，故在含有Cr的情形較佳為1.0%以下。Cr更佳為0.02%以上，且為0.8%以下。 Choose one or more from Cr: 0.01 to 1.0%, Mo: 0.01 to 1.0%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.30%, and Ca: 0.001 to 0.010%. Cr: 0.01～1.0% Cr is an element that improves hardenability and increases the strength of steel plates and square steel pipes, and can be contained as necessary. In the case where Cr is contained in order to obtain such an effect, it is preferable to contain 0.01% or more of Cr. On the other hand, if the content exceeds 1.0%, the toughness and weldability may decrease. Therefore, when Cr is contained, it is preferably 1.0% or less. Cr is more preferably 0.02% or more, and 0.8% or less.

Mo：0.01～1.0% Mo係藉由提高淬透性而使鋼板及方形鋼管的強度上升的元素，能夠視必要含有。在為了獲得如此效果而含有Mo的情形，較佳為含有0.01%以上之Mo。另一方面，若含有超過1.0%之Mo則有使韌性降低之虞，故在含有Mo的情形較佳為1.0%以下。Mo更佳為0.02%以上，且為0.8%以下。 Mo: 0.01～1.0% Mo is an element that increases the strength of steel plates and square steel pipes by improving hardenability, and can be contained as necessary. When Mo is contained in order to obtain such an effect, it is preferable to contain Mo in an amount of 0.01% or more. On the other hand, if it contains more than 1.0% of Mo, the toughness may be lowered, so if it contains Mo, it is preferably 1.0% or less. Mo is more preferably 0.02% or more, and 0.8% or less.

Cu：0.01～0.50% Cu係藉由固熔強化使鋼板及方形鋼管的強度上升的元素，能夠視必要含有。在為了獲得如此效果而含有Cu的情形，較佳為含有0.01%以上之Cu。另一方面，若含有超過0.50%之Cu則有使韌性降低之虞，故在含有Cu的情形較佳為0.50%以下。Cu更佳為0.02%以上，且為0.4%以下。 Cu: 0.01～0.50% Cu is an element that increases the strength of steel plates and square steel pipes by solid solution strengthening, and can be contained as necessary. In the case where Cu is contained in order to obtain such an effect, it is preferable to contain 0.01% or more of Cu. On the other hand, if Cu is contained in excess of 0.50%, the toughness may decrease. Therefore, when Cu is contained, it is preferably 0.50% or less. Cu is more preferably 0.02% or more, and 0.4% or less.

Ni：0.01～0.30% Ni係藉由固熔強化使鋼板及方形鋼管的強度上升的元素，能夠視必要含有。在為了獲得如此效果而含有Ni的情形，較佳為含有0.01%以上之Ni。另一方面，若含有超過0.30%之Ni則有使肥粒鐵的面積率降低之虞，故在含有Ni的情形較佳為0.30%以下。Ni更佳為0.02%以上，且為0.2%以下。 Ni: 0.01～0.30% Ni is an element that increases the strength of steel plates and square steel pipes by solid solution strengthening, and can be contained as necessary. In the case where Ni is contained in order to obtain such an effect, it is preferable to contain 0.01% or more of Ni. On the other hand, if the content of Ni exceeds 0.30%, the area ratio of the ferrite grains may decrease. Therefore, when Ni is contained, it is preferably 0.30% or less. Ni is more preferably 0.02% or more, and 0.2% or less.

Ca：0.001～0.010% Ca係使在熱間輥軋步驟被薄薄地延伸的MnS等之硫化物球狀化而使鋼的韌性提升的元素，能夠視必要含有。在為了獲得如此效果而含有Ca的情形，較佳為含有0.001%以上之Ca。然而，若Ca含量超過0.010%，則會有於鋼中形成Ca氧化物團簇而使韌性惡化之虞。因此，在含有Ca的情形，Ca含量較佳為0.001～0.010%。Ca更佳為0.0015%以上，且為0.0050%以下。 Ca: 0.001～0.010% Ca is an element that spheroidizes sulfides such as MnS thinly stretched in the hot rolling step to improve the toughness of steel, and can be contained as necessary. When Ca is contained in order to obtain such an effect, it is preferable to contain 0.001% or more of Ca. However, if the Ca content exceeds 0.010%, Ca oxide clusters may be formed in the steel to deteriorate toughness. Therefore, when Ca is contained, the Ca content is preferably 0.001 to 0.010%. Ca is more preferably 0.0015% or more, and 0.0050% or less.

首先，針對本發明之方形鋼管的鋼組織進行說明。First, the steel structure of the square steel pipe of the present invention will be described.

自本發明之方形鋼管的外表面至1/4t位置之鋼組織，係肥粒鐵之面積率為55%以上80%以下，硬質相之平均縱橫比為0.1～0.8。鋼組織，係能夠進一步使硬質相的平均當量圓直徑為20μm以下。From the outer surface of the square steel pipe of the present invention to the steel structure at the position of 1/4t, the area ratio of the ferrite iron is 55% to 80%, and the average aspect ratio of the hard phase is 0.1 to 0.8. The steel structure can further make the average equivalent circle diameter of the hard phase 20 μm or less.

肥粒鐵：面積率為55%以上80%以下 本發明之方形鋼管，為了確保所要求的強度，自鋼管的外表面之1/4t位置的鋼組織，係包含肥粒鐵及除此之外的硬質相。在此，所謂硬質相，係包含肥粒鐵以外之相，亦即變韌鐵、波來鐵、麻田散鐵及殘留沃斯田鐵等。該硬質相，係各相的面積率之合計為20～45%。 Fertilizer iron: area rate is 55% above 80% In order to ensure the required strength of the square steel pipe of the present invention, the steel structure at a position of 1/4t from the outer surface of the steel pipe contains ferrite and other hard phases. Here, the so-called hard phase includes phases other than fertile iron, that is, toughened iron, pleite iron, aspartite iron, residual austenitic iron, and the like. The total area ratio of each phase of the hard phase is 20 to 45%.

在肥粒鐵的面積率未達55%的情形，於本發明之前述成分組成的範圍中變韌鐵的比例會過剩，使應變容易分散至硬質相，而容易加工硬化。因此，無法獲得角部與平板部的強度差小的方形鋼管。另一方面，在肥粒鐵的面積率超過80%的情形，無法獲得所要求的強度。肥粒鐵較佳為60%以上，且為75%以下。When the area ratio of the fat grain iron is less than 55%, the ratio of the toughened iron will be excessive in the range of the aforementioned composition of the present invention, so that the strain is easily dispersed to the hard phase, and work hardening is easy. Therefore, it is impossible to obtain a square steel pipe with a small difference in strength between the corner portion and the flat portion. On the other hand, in the case where the area ratio of the ferrous iron exceeds 80%, the required strength cannot be obtained. Fertilizer iron is preferably 60% or more and 75% or less.

硬質相的平均縱橫比：0.1～0.8若硬質相的平均縱橫比未達0.1，則容易產生龜裂的起點，故會使韌性降低。另一方面，若硬質相的平均縱橫比超過0.8。則應變容易分散至硬質相，故容易加工硬化。因此，在使用鋼板製造方形鋼管之際，無法獲得角部及平板部的強度差小的方形鋼管。更佳為0.2以上，且為0.7以下。於本發明中，如後述般，硬質相的縱橫比係肥粒鐵以外的組織之奈米硬度3.0GPa以上的晶粒的縱橫比之平均值。The average aspect ratio of the hard phase: 0.1 to 0.8 If the average aspect ratio of the hard phase is less than 0.1, the starting point of cracks is likely to occur, which reduces the toughness. On the other hand, if the average aspect ratio of the hard phase exceeds 0.8. The strain is easily dispersed to the hard phase, so work hardening is easy. Therefore, when a square steel pipe is manufactured using a steel plate, a square steel pipe with a small difference in strength between the corners and the flat plate cannot be obtained. More preferably, it is 0.2 or more, and 0.7 or less. In the present invention, as described later, the aspect ratio of the hard phase is the average value of the aspect ratio of crystal grains with a nano-hardness of 3.0 GPa or more in structures other than ferrous iron.

硬質相的平均當量圓直徑：20μm以下(合適條件) 若硬質相的平均當量圓直徑超過20μm，則韌性會降低，故較佳為20μm以下。更佳為15μm以下。於本發明中，如後述般，硬質相的平均當量圓直徑係肥粒鐵以外的組織之奈米硬度3.0GPa以上的晶粒的當量圓直徑之平均值。 The average equivalent circle diameter of the hard phase: 20μm or less (suitable conditions) If the average equivalent circle diameter of the hard phase exceeds 20 μm, the toughness will decrease, so it is preferably 20 μm or less. More preferably, it is 15 μm or less. In the present invention, as described later, the average equivalent circle diameter of the hard phase is the average value of the equivalent circle diameter of crystal grains with a nano-hardness of 3.0 GPa or more in the structure other than the ferrous iron.

又，一般而言，將鋼板(熱軋鋼板)作為素材進行輥壓成形而製造的方形鋼管，係角部及平板部在1/4t位置的鋼組織皆相同，故測定平板部1/4t位置或是角部的1/4t位置皆可。在此，界定平板部的1/4t位置的鋼組織。Also, generally speaking, a square steel pipe manufactured by roll forming a steel plate (hot-rolled steel plate) as a material has the same steel structure at the 1/4t position of the corner part and the flat part, so the 1/4t position of the flat part is measured. Or the 1/4t position of the corner can be used. Here, the steel structure at the position of 1/4t of the flat plate portion is defined.

於本發明中，若在鋼管的3/16t位置～5/16t位置的範圍內存在有前述之鋼組織，亦能夠同樣地獲得前述之效果。因此，本發明中所謂「1/4t位置之鋼組織」，係意指於前述之3/16t位置～5/16t位置之範圍中之任一者存在有前述鋼組織。In the present invention, if the aforementioned steel structure exists in the range of 3/16t to 5/16t of the steel pipe, the aforementioned effects can be obtained in the same way. Therefore, the "steel structure at the 1/4t position" in the present invention means that the steel structure is present in any of the aforementioned 3/16t position to 5/16t position.

前述鋼組織，藉由以下方法進行觀察，求取組織的種類及面積率(%)。組織觀察用試驗片，係從方形鋼管採取，以使輥軋方向剖面(L剖面)成為觀察面的方式進行研磨，並進行硝酸腐蝕而製作。組織觀察，係以自組織觀察用試驗片的表面(亦即方形鋼管的外表面)之板厚1/4t位置的組織作為觀察的中心，使用光學顯微鏡(倍率：500倍)或掃描式電子顯微鏡(SEM，倍率：500倍)觀察鋼組織並攝影。測定區域為500μm×500μm。在此，「t」係表示鋼管的厚度(板厚)。自所獲得的組織相片，使用圖像分析裝置(圖像分析軟體：Photoshop，Adobe公司製)，辨明組織的種類，算出肥粒鐵的面積率。組織的面積率，係進行5視野以上觀察，作為各視野所獲得的值之平均值而求得。The aforementioned steel structure was observed by the following method to obtain the type and area ratio (%) of the structure. The test piece for microstructure observation was taken from a square steel pipe, polished so that the section in the rolling direction (L section) became the observation surface, and was produced by nitric acid corrosion. Microstructure observation is based on the structure of the surface of the test piece for self-organization observation (that is, the outer surface of the square steel tube) at the thickness of 1/4t as the center of observation, using an optical microscope (magnification: 500 times) or a scanning electron microscope (SEM, magnification: 500 times) Observe and photograph the steel structure. The measurement area is 500 μm×500 μm. Here, "t" represents the thickness (plate thickness) of the steel pipe. From the obtained tissue photos, an image analysis device (image analysis software: Photoshop, manufactured by Adobe) was used to identify the type of tissue, and calculate the area ratio of fat iron. The area ratio of the tissue was obtained by observing at least 5 fields of view, and was obtained as the average value of the values obtained in each field of view.

並且，硬質相的平均縱橫比，係如以下般求取。首先，自前述所獲得的組織相片，對於肥粒鐵以外的組織藉由奈米壓痕法求取奈米硬度。奈米硬度為3.0GPa以上之晶粒，係求取以該所有之晶粒的(板厚方向的長度的平均/輥軋方向的長度的平均)計算的值，而作為平均縱橫比。In addition, the average aspect ratio of the hard phase is obtained as follows. First, from the tissue photos obtained above, the nanoindentation method is used to obtain the nano hardness of the tissues other than fat iron. For the crystal grains with a nano hardness of 3.0 GPa or more, the value calculated from (the average of the length in the thickness direction/the average of the length in the rolling direction) of all the crystals is obtained and used as the average aspect ratio.

並且，硬質相的平均當量圓直徑，係使用SEM/EBSD法進行測定。平均當量圓直徑，係求取相鄰之晶粒的取向差，將取向差為15˚以上的邊界作為晶界而測定。自所獲得的晶界求取粒徑的算術平均，而為平均當量圓直徑。測定區域為500μm×500μm，測定區間長度為0.5μm。又，於結晶粒徑分析中，將結晶粒徑為2.0μm以下者作為測定雜訊，並且，將奈米硬度未達3.0GPa者作為非硬質相而自分析對象剔除。In addition, the average equivalent circle diameter of the hard phase was measured using the SEM/EBSD method. The average equivalent circle diameter is determined by determining the misorientation of adjacent grains, and measuring the boundary where the misorientation is 15˚ or more as the grain boundary. Calculate the arithmetic average of the particle size from the obtained grain boundaries, and obtain the average equivalent circle diameter. The measurement area is 500 μm×500 μm, and the measurement interval length is 0.5 μm. In addition, in the analysis of the crystal grain size, those with a crystal grain size of 2.0 μm or less are regarded as measurement noise, and those with a nano hardness of less than 3.0 GPa are regarded as non-hard phases and excluded from the analysis target.

接著，使用圖1、圖2，針對本發明之方形鋼管的製造方法進行說明。圖1，係表示電焊鋼管的製造設備之一例的示意圖。圖2，係表示方鋼管的成形過程的示意圖。Next, the method of manufacturing the square steel pipe of the present invention will be described using FIGS. 1 and 2. Fig. 1 is a schematic diagram showing an example of manufacturing equipment for electric-welded steel pipes. Figure 2 is a schematic diagram showing the forming process of a square steel pipe.

本發明之方形鋼管的製造方法，係對於鋼板進行造管步驟而形成方形鋼管。本發明之造管步驟，係將鋼板於冷間輥壓成形而形成為圓筒狀的端面焊接。接著，在將熱軋材料成形為縱徑/橫徑的比為0.99以上1.01以下之圓筒狀的圓形鋼管之後，藉由配置於上下及左右的輥在冷間將圓形鋼管成形為角狀，而製造成形為具有角部及平板部的方形鋼管。The method for manufacturing a square steel pipe of the present invention is to form a square steel pipe by performing a pipe making step on a steel plate. The pipe making step of the present invention is to roll the steel plate in the cold room to form a cylindrical end face welding. Next, after the hot-rolled material is formed into a cylindrical round steel pipe with a ratio of longitudinal diameter to lateral diameter of 0.99 or more and 1.01 or less, the round steel pipe is formed into corners in the cold zone by the rolls arranged on the upper and lower sides and on the left and right. Shaped, and manufactured and shaped into a square steel pipe with corners and flat plates.

首先，如圖1所示，作為電焊鋼管之素材的鋼帶1，例如藉由校平器2進行入側矯正之後，藉由複數個輥構成的排輥群3中間成形而形成開口管之後，藉由複數個輥構成之精整軋輥群4完工成形。完工成形之後，一邊藉由擠壓輥5壓接一邊將鋼帶1的寬度端部以焊接機6電阻焊接，而形成為圓筒狀的電焊鋼管7。另外，於本發明中，電焊鋼管7的製造設備不限於如圖1般之造管步驟。First, as shown in Fig. 1, the steel strip 1, which is the material of the electric welded steel pipe, is corrected by the leveler 2, for example, after the in-side straightening is performed, and then an open tube is formed by the middle of the row of rollers 3 composed of a plurality of rollers. The finishing roll group 4 composed of a plurality of rolls is finished and formed. After the completion of the forming, the width ends of the steel strip 1 are resistance-welded by the welding machine 6 while being crimped by the squeeze roll 5 to form a cylindrical electric-welded steel pipe 7. In addition, in the present invention, the manufacturing equipment of the electric resistance welded steel pipe 7 is not limited to the pipe manufacturing process as shown in FIG. 1.

之後，如圖2所示，電焊鋼管7係藉由以複數個輥構成之修整輥群(修整軋機)8保持為圓筒狀進行縮徑，而形成縱徑/橫徑之比為0.99以上1.01以下之圓筒狀。之後，藉由以複數個輥構成之角成形輥群(角成形軋機)9，依序成形為R1、R2、R3般之形狀，而成為方形鋼管10。又，修整輥群8及角成形輥群9的數量並未特別限定。After that, as shown in Fig. 2, the electric-welded steel pipe 7 is reduced in diameter by a group of dressing rolls (dressing mill) 8 composed of a plurality of rolls, so that the ratio of longitudinal diameter/transverse diameter is 0.99 or more and 1.01. The following cylindrical shape. After that, a group of corner forming rolls (corner forming rolls) 9 composed of a plurality of rolls are sequentially formed into shapes like R1, R2, and R3 to form a square steel pipe 10. In addition, the number of the finishing roll group 8 and the corner forming roll group 9 is not particularly limited.

在此，針對成形為角狀之前，成形為縱徑/橫徑之比為0.99以上1.01以下的圓筒狀之理由進行說明。Here, the reason for forming into a cylindrical shape with a longitudinal diameter/lateral diameter ratio of 0.99 or more and 1.01 or less before forming into an angular shape will be described.

於本發明中，因以下理由，形成為縱徑/橫徑之比為0.99以上1.01以下係至關重要。一般而言，在藉由輥壓成形製造鋼管的情形，於其過程中，為了抑制彈回，多會在圓周方向施加不均勻的應變。然而，在以最後成形為方形為前提的情形下，於其前階段之圓筒形的剖面並非必須為正圓。因此，即便謂之圓筒狀，在製造方形鋼管的中途階段並非必定為正圓，故所獲得的方形鋼管無法縮小平板部與角部的特性差。因此，於本發明中，為了縮小平板部與角部的特性差，必須在前階段將形狀成形為縱徑/橫徑之比為0.99以上1.01以下的圓筒狀。In the present invention, it is important that the ratio of longitudinal diameter/lateral diameter is 0.99 or more and 1.01 or less for the following reasons. Generally speaking, in the case of manufacturing steel pipes by roll forming, in order to suppress springback, uneven strains are often applied in the circumferential direction during the process. However, under the premise that the final shape is a square shape, the cross-section of the cylindrical shape at the previous stage does not have to be a perfect circle. Therefore, even in the so-called cylindrical shape, it is not necessarily a perfect circle in the middle of manufacturing the square steel pipe, so the obtained square steel pipe cannot reduce the difference in characteristics between the flat portion and the corner portion. Therefore, in the present invention, in order to reduce the difference in characteristics between the flat portion and the corner portion, it is necessary to shape the shape into a cylindrical shape having a longitudinal diameter/lateral diameter ratio of 0.99 or more and 1.01 or less in a previous stage.

若不成形為前述之縱徑/橫徑之比為0.99以上1.01以下的圓筒狀，則與平板部相比，角部的塑性應變會過大。因此，平板部對於角部之YS的比會未達0.80，且平板部對於角部之TS的比會未達0.90。又，與平板部相比，角部的塑性應變較大，故當然會使平板部對於角部之YS的比為0.90以下，且平板部對於角部之TS的比為1.00以下。因此，為了達成本發明之目的，亦即為了使平板部的YS為350MPa以上、使TS為520MPa以上、使平板部對於角部之YS的比為0.80以上0.90以下、平板部對於角部之TS的比為0.90以上1.00以下，須在進行角成形之前，成形為縱徑/橫徑之比為0.99以上1.01以下的圓筒狀。If it is not formed into a cylindrical shape with the aforementioned longitudinal diameter/transverse diameter ratio of 0.99 or more and 1.01 or less, the plastic strain of the corner portion will be too large compared to the flat portion. Therefore, the ratio of the flat portion to the corner portion YS may not reach 0.80, and the ratio of the flat portion to the corner portion TS may not reach 0.90. In addition, since the plastic strain of the corner portion is larger than that of the flat portion, of course the YS ratio of the flat portion to the corner portion is 0.90 or less, and the TS ratio of the flat portion to the corner portion is 1.00 or less. Therefore, in order to achieve the purpose of the invention, that is, to make the YS of the flat plate part 350 MPa or more, the TS to be 520 MPa or more, the ratio of the flat part to the corner part YS to be 0.80 or more and 0.90 or less, and the flat part to the corner part TS The ratio of is 0.90 or more and 1.00 or less, and it must be formed into a cylindrical shape with a longitudinal diameter/transverse diameter ratio of 0.99 or more and 1.01 or less before corner forming is performed.

並且，藉由成形為前述之縱徑/橫徑之比為0.99以上1.01以下的圓筒狀，因此在成形為角成形之際均等地成形為角部，角部的R係(2.3×t)以上(2.9×t)以下(在此，t係板厚)。藉由使角部的R為(2.3×t)以上(2.9×t)以下(在此，t係板厚)，能夠使角部與平板部的強度差縮小。In addition, by forming into a cylindrical shape with the aforementioned longitudinal diameter/lateral diameter ratio of 0.99 or more and 1.01 or less, the corners are evenly formed when the corners are formed. The corners are R-system (2.3×t) Above (2.9×t) or less (here, t is the plate thickness). By setting the R of the corner part to be (2.3×t) or more (2.9×t) or less (here, t is the plate thickness), the strength difference between the corner part and the flat part can be reduced.

如以上所說明般，依據本發明，平板部的YS為350MPa以上、TS為520MPa以上，平板部對於角部之YS的比為0.80以上0.90以下、平板部對於角部之TS的比為0.90以上1.00以下，平板部之-40℃的夏比吸收能為100J以上、角部的R為(2.3×t)以上(2.9×t)以下，因此能夠獲得角部與平板部的強度差小的方形鋼管。該方形鋼管，因角部的R被控制在恰當的大小，且角部與平板部的強度差小，故能夠適合用作為建築構造構件用的方形鋼管。As explained above, according to the present invention, the YS of the flat part is 350 MPa or more, the TS is 520 MPa or more, the ratio of the YS of the flat part to the corner is 0.80 or more and 0.90 or less, and the ratio of the flat part to the TS of the corner is 0.90 or more Below 1.00, the Charpy absorption energy at -40°C of the flat part is 100J or more, and the R of the corner part is (2.3×t) or more (2.9×t) or less, so a square with a small difference in strength between the corner and the flat part can be obtained. Steel Pipe. Since this square steel pipe has an appropriate size of R at the corners and a small difference in strength between the corners and the flat plate, it can be suitably used as a square steel pipe for building structural members.

又，如前述般，作為本發明之方形鋼管的素材，能夠使用依序進行以下所說明之熱軋步驟、冷卻步驟及捲取步驟而獲得之鋼板(熱軋鋼板)。於本發明中，對於該鋼板進行前述之造管步驟而形成為方形鋼管亦可。In addition, as described above, as the material of the square steel pipe of the present invention, a steel sheet (hot-rolled steel sheet) obtained by sequentially performing the hot rolling step, the cooling step, and the coiling step described below can be used. In the present invention, the steel plate may be formed into a square steel pipe by performing the aforementioned pipe making step.

針對適合使用作為本發明之方形鋼管的素材之鋼板的製造方法之一例進行說明。An example of a method of manufacturing a steel plate suitable for use as the material of the square steel pipe of the present invention will be described.

適合使用作為本發明之方形鋼管的素材之鋼板的製造方法，係例如能夠對於具有前述之成分組成的鋼素材，藉由以下所說明之條件依序進行熱間輥軋步驟(以下，稱為熱軋步驟)、冷卻步驟及捲取步驟而形成為鋼板(熱軋鋼板)。The method of manufacturing a steel sheet suitable for use as the material of the square steel tube of the present invention is that, for example, a steel material having the aforementioned composition can be sequentially subjected to a hot rolling step (hereinafter, referred to as hot rolling) under the conditions described below. The rolling step), the cooling step, and the coiling step are formed into a steel sheet (hot-rolled steel sheet).

例如，將具有前述成分組成之鋼素材加熱至加熱溫度：1100～1300℃之後，進行熱間輥軋步驟而形成為熱軋鋼板，該熱間輥軋步驟，係：鋼素材之板厚中心溫度為1000℃以上之粗軋時間：200秒以上400秒以內、粗軋結束溫度：1000～800℃，精軋開始溫度：1000～800℃、精軋結束溫度：900～750℃。接著，對於熱軋步驟之後的熱軋鋼板進行冷卻步驟，該冷卻步驟，係：具有1次以上之從冷卻開始10s之間的初期冷卻之0.2s以上且未達3.0s之放置冷卻，於熱軋鋼板的板厚中心溫度之平均冷卻速度：4～25℃/s，冷卻停止溫度：580℃以下；接著，對於冷卻步驟之後的熱軋鋼板進行以捲取溫度：580℃以下進行捲取且之後進行放置冷卻的捲取步驟而獲得鋼板(熱軋鋼板)。For example, the steel material having the aforementioned composition is heated to a heating temperature of 1100 to 1300°C, and then a hot rolling step is performed to form a hot rolled steel sheet. The hot rolling step is: the thickness center temperature of the steel material Rough rolling time above 1000°C: 200 seconds to 400 seconds, rough rolling end temperature: 1000 to 800°C, finishing rolling start temperature: 1000 to 800°C, finishing rolling end temperature: 900 to 750°C. Next, the hot-rolled steel sheet after the hot-rolling step is subjected to a cooling step. This cooling step is: having one or more times of initial cooling from the beginning of cooling 10s between 0.2s or more and less than 3.0s of standing cooling. The average cooling rate of the thickness center temperature of the rolled steel sheet: 4-25°C/s, the cooling stop temperature: 580°C or less; then, the hot-rolled steel sheet after the cooling step is coiled at a coiling temperature: 580°C or less. After that, a coiling step of standing cooling is performed to obtain a steel sheet (hot-rolled steel sheet).

以下，針對各步驟進行詳細說明。又，於以下之製造方法的說明中，若未特別限定，溫度(℃)係指鋼素材、板片、熱軋鋼板或是鋼板等之表面溫度。該等之表面溫度，能夠藉由輻射溫度計等測定。平均冷卻速度(℃/s)，若未特別限定，係藉由 ((冷卻前的溫度-冷卻後的溫度)/冷卻時間) 所求取的值。 Hereinafter, each step will be described in detail. In addition, in the following description of the manufacturing method, if not particularly limited, the temperature (°C) refers to the surface temperature of the steel material, plate, hot-rolled steel sheet, or steel sheet. The surface temperature of these can be measured with a radiation thermometer or the like. The average cooling rate (℃/s), if not specifically limited, is by ((Temperature before cooling-temperature after cooling) / cooling time) The value sought.

於本發明中，具有前述之成分組成之鋼素材(鋼胚)的熔製方法，並未特別限定，能夠使用轉爐、電爐、真空熔解爐等之公知之熔製方法進行熔製。鑄造方法亦未特別限定，能夠藉由連續鑄造法等之公知的鑄造方法製造為所要求的尺寸。又，取代連續鑄造法，運用造塊-分塊壓延法亦無任何問題。對於熔鋼進一步進行盛桶精煉等之二次精煉亦可。In the present invention, the method for melting the steel material (steel blank) having the aforementioned component composition is not particularly limited, and it can be melted using a known melting method such as a converter, an electric furnace, and a vacuum melting furnace. The casting method is also not particularly limited, and it can be manufactured to the required size by a known casting method such as a continuous casting method. In addition, instead of the continuous casting method, there is no problem in using the block-dividing rolling method. For molten steel, further secondary refining such as ladle refining is also possible.

接著，對於所獲得的鋼素材(鋼胚)進行熱軋步驟。於熱軋步驟中，將鋼素材加熱至加熱溫度：1100～1300℃。之後，對於從加熱爐抽出之被加熱的鋼素材，進行粗軋之後進行精軋而形成為熱軋鋼板；該粗軋，係：控制為鋼素材之板厚中心溫度為1000℃以上之粗軋時間：200秒以上400秒以內，且粗軋結束溫度：1000～800℃；該精軋，係：精軋開始溫度：1000～800℃、精軋結束溫度：900～750℃。Next, a hot rolling step is performed on the obtained steel material (steel blank). In the hot rolling step, the steel material is heated to a heating temperature: 1100 to 1300°C. After that, the heated steel material extracted from the heating furnace is rough-rolled and then finished-rolled to form a hot-rolled steel sheet; this rough rolling is controlled to be a rough rolling where the thickness center temperature of the steel material is 1000°C or higher Time: 200 seconds to 400 seconds, and rough rolling end temperature: 1000-800°C; the finishing rolling system: finishing rolling start temperature: 1000-800°C, finishing rolling end temperature: 900-750°C.

又，熱軋步驟之鋼素材的板厚中心的溫度，係藉由傳熱分析計算鋼素材剖面內的溫度分佈而求取。In addition, the temperature of the thickness center of the steel material in the hot rolling step is obtained by calculating the temperature distribution in the section of the steel material by heat transfer analysis.

加熱溫度：1100～1300℃ 若鋼素材的加熱溫度未達1100℃，則被輥軋材料的變形阻力會過大，使粗軋機及精軋機之產生耐荷重、輥軋轉矩之不足，而難以進行輥軋。另一方面，若加熱溫度超過1300℃，則沃斯田鐵晶粒會粗化，即便藉由粗軋及精軋反覆進行沃斯田鐵粒之加工及再結晶，亦難以細粒化。並且，亦有難以確保熱軋鋼板具有所要求的韌性之情形。因此，鋼素材之加熱溫度係1100～1300℃。加熱溫度，較佳為1280℃以下。加熱溫度，較佳為1150℃以上。 Heating temperature: 1100～1300℃ If the heating temperature of the steel material does not reach 1100°C, the deformation resistance of the material to be rolled will be too large, resulting in insufficient load resistance and rolling torque in the roughing and finishing mills, making it difficult to perform rolling. On the other hand, if the heating temperature exceeds 1300°C, the austenitic iron grains will be coarsened, and even if the austenitic iron grains are processed and recrystallized by repeated rough rolling and finish rolling, it is difficult to refine the austenitic iron grains. In addition, it may be difficult to ensure the required toughness of the hot-rolled steel sheet. Therefore, the heating temperature of the steel material is 1100 to 1300°C. The heating temperature is preferably 1280°C or lower. The heating temperature is preferably 1150°C or higher.

又，在各輥軋機之耐荷重、輥軋轉矩有餘裕的情形，將1100℃以下Ar3相變點以上之範圍的溫度選擇作為加熱溫度亦可。In addition, when there is a margin for the endurance load and rolling torque of each rolling mill, a temperature in the range of 1100°C or less and Ar3 transformation point or more may be selected as the heating temperature.

被加熱之鋼素材，接著被施以粗軋，而形成為板片等。The heated steel material is then rough-rolled to form a plate or the like.

板厚中心溫度為1000℃以上之粗軋時間：200秒以上400秒以內將鋼素材從加熱爐抽出之後，使鋼素材的板厚中心溫度為1000℃以上之粗軋時間為400秒以內，藉此使被輥軋材料的表面附近優先被冷卻。藉此，抑制自被輥軋材料的表面1/4t位置的沃斯田鐵粒徑粗化，且促進之後的肥粒鐵相變。因此，使鋼組織之肥粒鐵的比例以面積率計為55%以上。若前述粗軋時間超過400秒，則沃斯田鐵會粗粒化，而無法確保所要求的肥粒鐵量，故無法獲得角部及平板部的強度差小的方形鋼管。另一方面，若前述粗軋時間未達200秒，則沃斯田鐵粒徑會過度細微，使鋼組織之肥粒鐵的比例以面積率計超過80%，而使強度不足。粗軋時間，較佳為220秒以上，更佳為250秒以上。粗軋時間，較佳為380秒以下，更佳為350秒以下。Rough rolling time with a thickness center temperature of 1000°C or higher: After the steel material is drawn out of the heating furnace within 200 seconds or more and 400 seconds, the rough rolling time when the thickness center temperature of the steel material is 1000°C or higher is within 400 seconds. This allows the vicinity of the surface of the rolled material to be preferentially cooled. This suppresses the coarsening of the austenitic iron grain size at a position 1/4t from the surface of the material to be rolled, and promotes the subsequent fat iron phase transformation. Therefore, the ratio of ferrous iron in the steel structure is 55% or more in terms of area ratio. If the rough rolling time exceeds 400 seconds, the austenitic iron will be coarse-grained, and the required amount of fat iron cannot be ensured. Therefore, a square steel pipe with a small difference in strength between the corners and the flat plate cannot be obtained. On the other hand, if the aforementioned rough rolling time is less than 200 seconds, the austenitic iron grain size will be excessively fine, and the ratio of the fat iron in the steel structure will exceed 80% in terms of area ratio, resulting in insufficient strength. The rough rolling time is preferably 220 seconds or more, more preferably 250 seconds or more. The rough rolling time is preferably 380 seconds or less, more preferably 350 seconds or less.

粗軋結束溫度：1000～800℃ 被加熱的鋼素材，會藉由粗軋使沃斯田鐵粒被加工、再結晶而細微化。若粗軋結束溫度未達800℃，則粗軋機之耐荷重、輥軋轉矩容易產生不足。另一方面，若粗軋結束溫度為超過1000℃之高溫，則沃斯田鐵粒會粗化，而容易使方形鋼管的韌性降低。粗軋結束溫度，較佳為820℃以上，更佳為840℃以上。粗軋結束溫度，較佳為980℃以下，更佳為950℃以下。 Finishing temperature of rough rolling: 1000～800℃ The heated steel material will be processed, recrystallized and refined by rough rolling. If the rough rolling end temperature is less than 800°C, the load resistance and rolling torque of the rough rolling mill are likely to be insufficient. On the other hand, if the rough rolling end temperature is a high temperature exceeding 1000°C, the austenitic iron grains will be coarsened, and the toughness of the square steel pipe will tend to decrease. The finishing temperature of rough rolling is preferably 820°C or higher, more preferably 840°C or higher. The finishing temperature of rough rolling is preferably 980°C or lower, more preferably 950°C or lower.

又，該粗軋結束溫度，能夠藉由調整鋼素材的加熱溫度、前述粗軋中之冷卻條件、於粗軋之銜接的滯留、鋼素材厚度等來達成。在粗軋結束的階段之被輥軋材料的厚度(板片等之厚度)不須特別限定，只要能夠藉由精軋形成為所要求之成品厚度的成品板(熱軋鋼板)即可。例如，就用於建築構造構件之方形鋼管的製造而言，成品厚度較佳為12～28mm左右。In addition, this rough rolling end temperature can be achieved by adjusting the heating temperature of the steel material, the cooling conditions in the rough rolling described above, the retention during the rough rolling, the thickness of the steel material, and the like. The thickness of the material to be rolled (thickness of the slab, etc.) at the end of rough rolling is not particularly limited, as long as it can be formed into a finished plate (hot-rolled steel sheet) of the required finished thickness by finish rolling. For example, for the manufacture of square steel pipes used for building structural members, the thickness of the finished product is preferably about 12-28 mm.

粗軋之後，被輥軋材料會例如藉由直列式軋機被精軋，而形成為熱軋鋼板。After rough rolling, the material to be rolled is finished by, for example, an in-line rolling mill to form a hot-rolled steel sheet.

精軋開始溫度：1000～800℃ 於精軋中，係反覆進行輥軋加工及再結晶，而使沃斯田鐵(γ)粒之細微化發生進展。若精軋開始溫度(精軋入側溫度)低，則輥軋加工所導入的加工應變容易殘留，而容易達成γ粒的細微化。若精軋開始溫度未達800℃，則於精軋機內之鋼板表面附近的溫度會成為Ar3相變點以下，而使生成肥粒鐵的危險性增大。所生成的肥粒鐵，之後會藉由精軋加工往輥軋方向伸長而成為肥粒鐵粒，而成為加工性降低的原因。另一方面，若精軋開始溫度為超過1000℃之高溫，則前述精軋所產生之γ粒的細微化效果會降低，而容易使方形鋼管的韌性降低。因此，使精軋開始溫度為800～1000℃。精軋開始溫度，較佳為825～975℃。 Start temperature of finishing rolling: 1000～800℃ In the finishing rolling, rolling processing and recrystallization are performed repeatedly, and the miniaturization of austenitic iron (γ) grains progresses. If the finish rolling start temperature (finish rolling-in side temperature) is low, the processing strain introduced by the rolling process is likely to remain, and it is easy to achieve the refinement of γ grains. If the finish rolling start temperature is less than 800°C, the temperature near the surface of the steel sheet in the finish rolling mill will be below the Ar3 transformation point, which increases the risk of ferrous iron formation. The resulting fat iron particles are then elongated in the rolling direction by the finish rolling process to become fat iron particles, which causes the decrease in workability. On the other hand, if the finish rolling start temperature is a high temperature exceeding 1000°C, the effect of refining the γ grains produced by the finish rolling will be reduced, and the toughness of the square steel pipe will tend to be lowered. Therefore, the start temperature of finish rolling is 800 to 1000°C. The start temperature of finishing rolling is preferably 825 to 975°C.

精軋結束溫度：900～750℃ 若精軋結束溫度(精軋出側溫度)為超過900℃之高溫，則精軋時所賦予的加工應變會不足，無法達成γ粒的細微化，而容易使方形鋼管的韌性降低。另一方面，若精軋結束溫度未達750℃，則於精軋機內之鋼板表面附近的溫度會成為Ar3相變點以下，形成往輥軋方向伸長的肥粒鐵粒，而使肥粒鐵粒成為混粒。因此，韌性降低的危險性會增大。因此，使精軋結束溫度為900～750℃。精軋結束溫度，較佳為850℃以下。更佳為770℃以上。 Finish rolling end temperature: 900～750℃ If the finish rolling end temperature (finish rolling-out side temperature) is a high temperature exceeding 900°C, the processing strain imparted during the finish rolling will be insufficient, the γ grains will not be refined, and the toughness of the square steel tube will tend to decrease. On the other hand, if the finishing temperature of the finishing rolling is less than 750°C, the temperature near the surface of the steel sheet in the finishing mill will be below the Ar3 transformation point, forming fertilizer iron grains elongated in the rolling direction, causing the fertilizer iron The granules become mixed granules. Therefore, the risk of a decrease in toughness increases. Therefore, the finishing temperature of finish rolling is set to 900 to 750°C. The finishing temperature of finish rolling is preferably 850°C or lower. More preferably, it is 770°C or higher.

精軋結束之後，對於熱軋鋼板進行冷卻步驟。After finishing rolling, the hot-rolled steel sheet is subjected to a cooling step.

從冷卻開始10s之間的初期冷卻之0.2s以上未達3.0s之放置冷卻的次數：1次以上於本發明中，將自以熱軋步驟所獲得之熱軋鋼板的冷卻開始10秒之間(10s間)作為初期冷卻步驟。於冷卻步驟之初期冷卻步驟中，設定1次以上之0.2s以上未達3.0s的放置冷卻步驟而進行冷卻。此係為了抑制在鋼板的表裏面生成麻田散鐵組織。於初期冷卻步驟中，在未設定放置冷卻步驟或是放置冷卻步驟未達0.2s的情形，鋼板的表裏面的鋼組織會成為麻田散鐵組織，使方形鋼管的韌性降低。並且，於初期冷卻步驟中，在放置冷卻步驟為3.0s以上的情形，硬質相的平均縱橫比會未達0.1，使韌性不足。因此，使於冷卻步驟之初期冷卻步驟中進行1次放置冷卻步驟的時間為0.2s以上未達3.0s。1次放置冷卻步驟的時間，較佳為0.4s以上，且為2.0s以下。From the initial cooling of 10 seconds between the beginning of the cooling, the number of times of standing cooling of 0.2s or more and less than 3.0s: 1 time or more. In the present invention, the cooling of the hot-rolled steel sheet obtained in the hot rolling step is within 10 seconds (10s) as the initial cooling step. In the initial cooling step of the cooling step, cooling is performed by setting a standing cooling step of 0.2 s or more and less than 3.0 s at least once. This is to suppress the formation of Asada scattered iron structure on the surface and back of the steel plate. In the initial cooling step, if the placing cooling step is not set or the placing cooling step has not reached 0.2s, the steel structure on the front and back of the steel plate will become the Asada scattered iron structure, which reduces the toughness of the square steel pipe. In addition, in the initial cooling step, if the cooling step is left for 3.0 s or more, the average aspect ratio of the hard phase may not reach 0.1, resulting in insufficient toughness. Therefore, in the initial cooling step of the cooling step, the time to perform the standing cooling step once is 0.2 s or more and less than 3.0 s. The time for one cooling step is preferably 0.4 s or more and 2.0 s or less.

為了獲得前述效果，初期冷卻步驟中進行放置冷卻步驟的次數，必須為1次以上。又，放置冷卻步驟的次數，視冷卻設備的配置或冷卻停止溫度等適當設定即可。在此，所謂放置冷卻為自然冷卻。放置冷卻步驟的次數之上限雖未特別限定，然而以生產性的觀點而言，較佳為10次以下。在將放置冷卻的次數設定為複數次的情形，例如藉由使來自後述之水冷用的噴嘴的一部分區間之噴嘴的水停止噴射，藉此成為間歇噴射等，適當設定即可。In order to obtain the aforementioned effects, the number of times of the standing cooling step in the initial cooling step must be one or more. In addition, the number of cooling steps may be appropriately set depending on the configuration of the cooling equipment, the cooling stop temperature, and the like. Here, the so-called standing cooling is natural cooling. Although the upper limit of the number of leaving the cooling step is not particularly limited, from the viewpoint of productivity, it is preferably 10 times or less. In the case of setting the number of stand-to-cooling to a plurality of times, for example, by stopping spraying water from nozzles in a part of the section of the nozzle for water cooling described later, it becomes intermittent spraying, etc., which may be appropriately set.

板厚中心溫度之平均冷卻速度：4～25℃/s、冷卻停止溫度：580℃以下於冷卻步驟中，係對於藉由精軋所獲得的熱軋鋼板，進行從冷卻開始至冷卻停止(冷卻結束)的板厚中心溫度之平均冷卻速度為4～25℃/s、冷卻停止溫度為580℃以下之冷卻。於冷卻步驟所進行之冷卻，係例如進行從噴嘴噴射水之水柱冷卻、噴霧冷卻、霧氣冷卻等之水冷(水冷卻)，或是噴射冷卻氣體之氣體噴射冷卻等。又，較佳為以使熱軋鋼板(鋼板)的兩面(表裏面)在相同條件下冷卻的方式，對於熱軋鋼板的兩面進行冷卻操作。Average cooling rate of plate thickness center temperature: 4-25°C/s, cooling stop temperature: 580°C or less. In the cooling step, the hot-rolled steel sheet obtained by finish rolling is performed from cooling start to cooling stop (cooling End) The average cooling rate of the plate thickness center temperature is 4-25°C/s, and the cooling stop temperature is 580°C or less. The cooling performed in the cooling step is, for example, water cooling (water cooling) such as water jet cooling, spray cooling, mist cooling, etc. by spraying water from nozzles, or gas spray cooling by spraying cooling gas. Furthermore, it is preferable to perform a cooling operation on both sides of the hot-rolled steel sheet (steel sheet) so that both sides (front and back sides) of the hot-rolled steel sheet (steel sheet) are cooled under the same conditions.

若熱軋鋼板的板厚中心之平均冷卻速度未達4℃/s，則肥粒鐵粒的生成頻率會減少，使肥粒鐵晶粒粗化而使韌性降低。另一方面，若前述平均冷卻速度超過25℃/s，則肥粒鐵的生成會受到抑制而未達55%，且硬質相的平均縱橫比會超過0.8，而無法獲得不易加工硬化的鋼組織。因此，使熱軋鋼板的板厚中心之平均冷卻速度為4～25℃/s。熱軋鋼板的板厚中心之平均冷卻速度，較佳為5℃/s以上，且為15℃/s以下。If the average cooling rate at the center of the thickness of the hot-rolled steel sheet is less than 4°C/s, the frequency of fertilizer iron grain generation will decrease, which will coarsen the fertilizer iron grains and reduce toughness. On the other hand, if the aforementioned average cooling rate exceeds 25°C/s, the formation of ferrous iron will be suppressed to less than 55%, and the average aspect ratio of the hard phase will exceed 0.8, making it impossible to obtain a steel structure that is not easy to work hardening. . Therefore, the average cooling rate of the thickness center of the hot-rolled steel sheet is 4-25°C/s. The average cooling rate of the thickness center of the hot-rolled steel sheet is preferably 5°C/s or more and 15°C/s or less.

在此，使熱軋鋼板的板厚中心之平均冷卻速度，係藉由 ((冷卻開始時之板厚中心的溫度(℃)-冷卻停止時之板厚中心的溫度(℃))/冷卻時間(s)) 來求取。熱軋鋼板之板厚中心的溫度，能夠藉由傳熱分析計算鋼板剖面內的溫度分佈而求取。 Here, the average cooling rate of the thickness center of the hot-rolled steel sheet is determined by ((The temperature of the thickness center at the beginning of cooling (°C)-the temperature of the thickness center at the stop of cooling (°C))/cooling time (s)) Come ask for it. The temperature of the thickness center of the hot-rolled steel sheet can be obtained by calculating the temperature distribution in the section of the steel sheet by heat transfer analysis.

若冷卻停止溫度超過580℃，則鋼板的硬質相的平均當量圓直徑超過20μm，則會有韌性降低之虞。冷卻停止溫度，更佳為560℃以下。If the cooling stop temperature exceeds 580°C, the average equivalent circle diameter of the hard phase of the steel sheet exceeds 20 μm, and the toughness may decrease. The cooling stop temperature is more preferably 560°C or less.

又，為了獲得所要求之1/4t位置的鋼組織，較佳為使熱軋鋼板的表面溫度於750℃～650℃的溫度範圍之平均冷卻速度為20℃/s以上。若該溫度範圍之平均冷卻速度未達20℃/s，則會有硬質相的平均當量圓直徑超過20μm的情形。較佳為使熱軋鋼板的表面溫度於750℃～650℃的溫度範圍之平均冷卻速度為80℃/s以下。若該溫度範圍之平均冷卻速度超過80℃/s，則會有硬質相的平均縱橫比超過0.8的情形。並且，為了使硬質相的平均縱橫比為0.8以下，較佳為在精軋結束之後立即(5秒以內)開始進行冷卻步驟。In addition, in order to obtain the required steel structure at the position of 1/4t, it is preferable that the average cooling rate of the surface temperature of the hot-rolled steel sheet in the temperature range of 750°C to 650°C be 20°C/s or more. If the average cooling rate in this temperature range does not reach 20°C/s, the average equivalent circle diameter of the hard phase may exceed 20 μm. It is preferable that the average cooling rate of the surface temperature of the hot-rolled steel sheet in the temperature range of 750°C to 650°C be 80°C/s or less. If the average cooling rate in this temperature range exceeds 80°C/s, the average aspect ratio of the hard phase may exceed 0.8. In addition, in order to make the average aspect ratio of the hard phase 0.8 or less, it is preferable to start the cooling step immediately (within 5 seconds) after finishing rolling.

冷卻結束之後，對於熱軋鋼板進行捲取步驟而獲得鋼板(熱軋鋼板)。After cooling, the hot-rolled steel sheet is subjected to a coiling step to obtain a steel sheet (hot-rolled steel sheet).

捲取溫度：580℃以下 於捲取步驟中，將熱軋鋼板以捲取溫度：580℃以下進行捲取，之後放置冷卻。若捲取溫度超過580℃，則在捲取之後肥粒鐵相變及波來鐵相變會進展，使波來鐵的比例過剩，而使方形鋼管的韌性降低。因此，捲取溫度為580℃以下。捲取溫度，較佳為550℃以下。又，雖捲取溫度低亦不致產生材質的問題，然而若捲取溫度未達400℃，則特別是就板厚超過25mm的厚鋼板而言，捲取變形阻力會極大，而有無法妥善地捲取情形。因此，捲取溫度較佳為400℃以上。 Coiling temperature: below 580℃ In the coiling step, the hot-rolled steel sheet is coiled at a coiling temperature of 580°C or less, and then left to cool. If the coiling temperature exceeds 580°C, the ferrous iron phase transformation and the corundum phase transition will progress after the coiling, resulting in an excessive ratio of cortical iron and lowering the toughness of the square steel pipe. Therefore, the coiling temperature is 580°C or less. The coiling temperature is preferably 550°C or lower. In addition, even though the coiling temperature is low, it does not cause material problems. However, if the coiling temperature does not reach 400°C, especially for thick steel plates with a thickness of more than 25mm, the coiling deformation resistance will be extremely large, and it may not be able to properly Coiling situation. Therefore, the coiling temperature is preferably 400°C or higher.

之後，對於捲取步驟之後的鋼板(熱軋鋼板)進行前述之造管步驟而獲得方形鋼管。After that, the steel plate (hot-rolled steel plate) after the coiling step is subjected to the aforementioned pipe making step to obtain a square steel pipe.

接著，針對使用本發明之方形鋼管的建築構造物之一例進行說明。Next, an example of a building structure using the square steel pipe of the present invention will be described.

圖3，係示意性表示本發明之實施形態之建築構造物的立體圖。如圖3所示，本實施形態之建築構造物，係豎設有複數個本發明之方形鋼管11作為柱材。在相鄰的方形鋼管11之間，架設有複數個包含H形鋼等之鋼材的大樑14。並且，在相鄰的大樑14之間，架設有複數個包含H形鋼等之鋼材的小樑15。將方形鋼管11與隔膜16焊接，並對其焊接作為大樑14的H形鋼，藉此在相鄰的方形鋼管11之間架設有包含H形鋼等之鋼材的大樑14。並且，為了安裝牆壁等，視必要設置間柱17。Fig. 3 is a perspective view schematically showing a building structure according to an embodiment of the present invention. As shown in FIG. 3, in the building structure of this embodiment, a plurality of square steel pipes 11 of the present invention are erected as columns. Between adjacent square steel pipes 11, a plurality of girders 14 made of steel materials such as H-shaped steel are erected. In addition, a plurality of small beams 15 made of steel materials such as H-shaped steel are erected between adjacent large beams 14. The square steel pipe 11 and the diaphragm 16 are welded, and the H-shaped steel as the beam 14 is welded to the square steel pipe 11, whereby a beam 14 including steel materials such as H-shaped steel is erected between adjacent square steel pipes 11. In addition, in order to install a wall or the like, a pillar 17 is installed as necessary.

本發明之建築構造物，因使用角部與平板部之強度差小的本發明之方形鋼管11，故能夠輕易選擇焊接方形鋼管11與隔膜16的焊接材料，而不易產生如匹配不足等之與焊接材料的強度差。因不易產生匹配不足之情事，故能夠抑止於焊接部之斷裂等問題。並且，因方形鋼管11的角R(角部的R)被控制在恰當的大小，故能夠輕易地與剖面為直角的其他構造構件組合。並且，因方形鋼管11的角R被控制在恰當的大小，故能夠承受更大的外力，而使耐震性等提升。 實施例 The building structure of the present invention uses the square steel pipe 11 of the present invention with a small difference in strength between the corners and the flat plate. Therefore, the welding material for welding the square steel pipe 11 and the diaphragm 16 can be easily selected, and it is not easy to cause problems such as insufficient matching. The strength of the welding material is poor. Since it is not easy to produce insufficient matching, it can restrain the problems such as the fracture of the welded part. In addition, since the angle R (the R of the corner portion) of the square steel pipe 11 is controlled to an appropriate size, it can be easily combined with other structural members whose cross-section is at right angles. In addition, since the angle R of the square steel pipe 11 is controlled to an appropriate size, it can withstand a larger external force and improve the shock resistance and the like. Example

以下，根據實施例，進一步詳細說明本發明。又，本發明不限於以下之實施例。Hereinafter, the present invention will be described in further detail based on examples. In addition, the present invention is not limited to the following examples.

將熔鋼以轉爐熔製，並藉由連續鑄造法形成表1所示之成分組成之鋼胚(鋼素材：厚度250mm)。將該等鋼胚(鋼素材)加熱至表2所示之加熱溫度之後，進行表2所示之條件之熱間輥軋步驟、冷卻步驟、捲取步驟之後，進行放置冷卻，藉此形成板厚：16～28mm之鋼板(熱軋鋼板)。又，精軋結束之後，立即(5秒以內)開始進行冷卻步驟。冷卻係藉由水冷進行。初期冷卻步驟中之放置冷卻步驟，在從冷卻開始10s之間之初期冷卻步驟中，設定不進行水冷的放置冷卻區間。之後，將所獲得的熱軋鋼板作為素材，以表2所示之條件藉由於冷間之輥壓成形形成圓形鋼管，接著，藉由於冷間之輥壓成形形成方形鋼管(400～550mm角)。The molten steel was melted in a converter, and a steel blank (steel material: thickness 250mm) with the composition shown in Table 1 was formed by continuous casting. After heating the steel billets (steel materials) to the heating temperature shown in Table 2, the hot rolling step, the cooling step, and the coiling step under the conditions shown in Table 2 are carried out and then placed and cooled to form a plate Thickness: 16～28mm steel plate (hot rolled steel plate). In addition, immediately after finishing rolling (within 5 seconds), the cooling step is started. The cooling is carried out by water cooling. In the initial cooling step, in the initial cooling step, in the initial cooling step 10s from the start of cooling, set the uncooled zone where water cooling is not performed. After that, the obtained hot-rolled steel sheet was used as a material, and a round steel pipe was formed by cold roll forming under the conditions shown in Table 2. Then, a square steel pipe (400-550mm angle) was formed by cold roll forming. ).

於本發明之實施例中，從所獲得的方形鋼管採取試驗片，分別實施組織觀察、拉伸試驗、夏比衝擊試驗、角部的R之測定。又，組織觀察係藉由前述之方法進行觀察、測定。並且，拉伸試驗、夏比衝擊試驗的試驗方法，以及角部的R之測定方法，係如以下所述。In the examples of the present invention, test pieces were taken from the obtained square steel pipes, and the structure observation, the tensile test, the Charpy impact test, and the measurement of the R of the corners were respectively carried out. In addition, tissue observation is observed and measured by the aforementioned method. In addition, the test methods of the tensile test, the Charpy impact test, and the measuring method of the corner R are as follows.

(1)方形鋼管拉伸試驗 從所獲得的方形鋼管之平板部及角部以使拉伸方向為管長度方向的方式採取JIS5號拉伸試驗片。接著，依據JISZ2241(2011)之規定實施拉伸試驗，測定降伏強度YS、拉伸強度TS。使用所獲得之測定值，算出以(降伏強度)/(拉伸強度)×100(%)所定義之降伏比YR(%)。 (1) Tensile test of square steel pipe A JIS No. 5 tensile test piece was taken from the flat portion and corner portion of the obtained square steel pipe so that the tensile direction was the pipe length direction. Next, a tensile test was performed in accordance with the regulations of JISZ2241 (2011), and the yield strength YS and the tensile strength TS were measured. Using the obtained measured value, calculate the yield ratio YR(%) defined by (Yield Strength)/(Tensile Strength)×100(%).

(2)方形鋼管衝擊試驗 從所獲得的方形鋼管之平板部之板厚1/4t位置以使試驗片長度方向為管周方向的方式採取V缺口試驗片。接著，依據JISZ2242(2011)的規定，以試驗溫度：-40℃實施夏比衝擊試驗，求取吸收能(J)。又，使試驗片的數量各為3個，並將各3個的平均值作為表3-2所示之衝擊試驗結果的值。 (2) Impact test of square steel pipe A V-notch test piece was taken from the position of the plate thickness 1/4t of the flat plate portion of the obtained square steel pipe so that the length direction of the test piece was the pipe circumferential direction. Next, in accordance with the regulations of JISZ2242 (2011), a Charpy impact test was performed at a test temperature: -40°C, and the absorbed energy (J) was obtained. In addition, the number of test pieces was set to three each, and the average value of each three was used as the value of the impact test result shown in Table 3-2.

(3)角部的R(角R)的測定方法 從所獲得的方形鋼管，任意切出10處對於管軸方向垂直之剖面，測定垂直剖面之4角落的角部之曲率半徑，並將其平均值作為角部的R。具體而言，如圖4所示，在以鋼管的焊接部(接合部)作為0˚，並以該0˚作為基準，將45˚、135˚、225˚、315˚的位置分別作為角部中央的情形，所謂角部的曲率半徑，係指在以管的中心作為起點與相鄰的邊呈45˚的線(L)和角部外側(角部的管外表面側)的交點之曲率半徑。角部的曲率半徑，係使中心位於前述L上並往方形鋼管的平坦部與圓弧部的連接點(A、A’)拉出的線所訂定之中心角為65˚之扇形的半徑。又，圖4所示之「t」係板厚，「H」係指外形的邊的長度。作為算出曲率半徑的方法，有例如從3點(角部外側的交點，以及作為平坦部與圓弧部之連接點的2點)的距離關係之測定結果使用正弦定理算出曲率半徑的方法，或從與前述3點之區域內的角落部高度一致之半徑規測量曲率半徑的方法等，然而不限於此。於本實施例中，使用半徑規進行角部之曲率半徑的測定。又，角R係如前述般為10處對於管軸方向垂直之剖面的平均值。 (3) Measuring method of corner R (angle R) From the obtained square steel pipe, randomly cut 10 sections perpendicular to the pipe axis direction, measure the radii of curvature of the corners of the four corners of the vertical section, and use the average value as the R of the corners. Specifically, as shown in Figure 4, the welded part (joining part) of the steel pipe is taken as 0˚, and the 0˚ is used as a reference, and the positions of 45˚, 135˚, 225˚, and 315˚ are respectively regarded as corners. In the case of the center, the so-called radius of curvature of the corner refers to the curvature at the intersection of a 45˚ line (L) with the adjacent side starting from the center of the tube and the outside of the corner (the outer surface of the corner of the tube). radius. The radius of curvature of the corner is the radius of a sector of 65˚ defined by the line drawn from the center on the aforementioned L and drawn to the connection point (A, A') of the flat part and the arc part of the square steel tube. In addition, "t" shown in FIG. 4 is the thickness of the plate, and "H" is the length of the side of the outer shape. As a method of calculating the radius of curvature, for example, there is a method of calculating the radius of curvature from the measurement result of the distance relationship between three points (the intersection point on the outside of the corner and the two points that are the connection points of the flat part and the arc part) using the law of sine, or The method of measuring the radius of curvature from a radius gauge that matches the height of the corner in the area of the aforementioned 3 points, etc., however, it is not limited to this. In this embodiment, a radius gauge is used to measure the radius of curvature of the corner. In addition, the angle R is the average value of 10 cross sections perpendicular to the tube axis direction as described above.

將所獲得的結果示於表3-1及表3-2。The obtained results are shown in Table 3-1 and Table 3-2.

Figure 02_image001
Figure 02_image001

Figure 02_image003
Figure 02_image003

Figure 02_image005
Figure 02_image005

Figure 02_image007
Figure 02_image007

就本發明範圍之發明例而言，皆獲得本發明之特性(平板部的YS為350MPa以上、TS為520MPa以上，平板部對於角部之YS的比為0.80以上0.90以下、平板部對於角部之TS的比為0.90以上1.00以下，平板部之-40℃的夏比吸收能為100J以上、角部的R為(2.3×t)以上(2.9×t)以下)。在此，t係板厚。)。另一方面，就偏離本發明範圍之比較例而言，係無法獲得本發明之特性。As far as the invention examples within the scope of the present invention are concerned, the characteristics of the present invention are obtained (the YS of the flat part is 350 MPa or more, the TS is 520 MPa or more, the ratio of the YS of the flat part to the corner part is 0.80 or more and 0.90 or less, and the flat part to the corner is 0.80 or more and 0.90 or less. The ratio of TS is 0.90 or more and 1.00 or less, the Charpy absorption energy at -40°C of the flat part is 100J or more, and the R of the corner part is (2.3×t) or more (2.9×t) or less). Here, t is the plate thickness. ). On the other hand, for comparative examples that deviate from the scope of the present invention, the characteristics of the present invention cannot be obtained.

1:鋼帶 2:校平器 3:排輥群 4:精整軋輥群 5:擠壓輥 6:焊接機 7:電焊鋼管 8:修整輥群 9:角成形輥群 10:方形鋼管 11:方形鋼管 14:大樑 15:小樑 16:隔膜 17:間柱 1: Steel belt 2: Leveler 3: Row roller group 4: Finishing roll group 5: Squeeze roller 6: Welding machine 7: Electric welded steel pipe 8: Dressing roll group 9: Corner forming roll group 10: Square steel pipe 11: Square steel pipe 14: Girder 15: Trabecular 16: Diaphragm 17: Column

[圖1]圖1，係表示電焊鋼管的製造設備之一例的示意圖。 [圖2]圖2，係表示方鋼管的成形過程的示意圖。 [圖3]圖3，係示意性表示使用了本發明之方形鋼管的建築構造物之一例的立體圖。 [圖4]圖4，係表示方鋼管的剖面的示意圖。 [Fig. 1] Fig. 1 is a schematic diagram showing an example of a manufacturing facility for electric resistance welded steel pipe. [Fig. 2] Fig. 2 is a schematic diagram showing the forming process of a square steel pipe. [Fig. 3] Fig. 3 is a perspective view schematically showing an example of a building structure using the square steel pipe of the present invention. [Fig. 4] Fig. 4 is a schematic diagram showing a cross section of a square steel pipe.

Claims (7)

一種方形鋼管，係具有平板部及角部；其特徵為：作為成分組成，以質量%含有：C：0.07~0.20%、Si：1.0%以下、Mn：0.5~2.0%、P：0.030%以下、S：0.015%以下、Al：0.01~0.06%、N：0.006%以下，剩餘部分為Fe及不可避免的雜質，自鋼管的外表面至板厚t之1/4深度位置之鋼組織，係肥粒鐵之面積率為55%以上80%以下，硬質相的平均縱橫比為0.1~0.8，前述平板部，係降伏強度YS為350MPa以上，拉伸強度TS為520MPa以上前述平板部對於前述角部之降伏強度YS的比為0.80以上0.90以下，前述平板部對於前述角部之拉伸強度TS的比為0.90以上1.00以下，前述平板部的-40℃的夏比吸收能為100J以上，前述角部的曲率半徑為(2.3×t)以上(2.9×t)以下。 A square steel pipe with flat plate parts and corner parts. It is characterized in that it contains as a component composition: C: 0.07 to 0.20%, Si: 1.0% or less, Mn: 0.5 to 2.0%, P: 0.030% or less , S: 0.015% or less, Al: 0.01~0.06%, N: 0.006% or less, the remainder is Fe and inevitable impurities, the steel structure from the outer surface of the steel pipe to the depth of 1/4 of the plate thickness t, is The area ratio of the fertilizer grain iron is 55% or more and 80% or less, the average aspect ratio of the hard phase is 0.1 to 0.8, the aforementioned flat part has a yield strength YS of 350 MPa or more, and a tensile strength TS of 520 MPa or more. The ratio of the yield strength YS of the flat portion is 0.80 or more and 0.90 or less, the ratio of the tensile strength TS of the flat portion to the corner portion is 0.90 or more and 1.00 or less, and the Charpy absorption energy at -40°C of the flat portion is 100J or more. The radius of curvature of the corner is (2.3×t) or more (2.9×t) or less. 如請求項1所述之方形鋼管，其中，除了前述成分組成，以質量%含有自下述A群至C群當中選擇之 1群或2群以上；A群：自Nb：0.05%以下、Ti：0.05%以下、V：0.10%以下當中選擇之1種或2種以上；B群：B；0.008%以下；C群：自Cr：0.01~1.0%、Mo：0.01~1.0%、Cu：0.01~0.50%、Ni：0.01~0.30%、Ca：0.001~0.010%當中選擇之1種或2種以上。 The square steel pipe according to claim 1, in which, in addition to the aforementioned component composition, it contains in mass% selected from the following groups A to C Group 1 or 2 or more; Group A: Nb: 0.05% or less, Ti: 0.05% or less, V: 0.10% or less, select one or more than 2 types; Group B: B; 0.008% or less; Group C: Choose one or more of Cr: 0.01~1.0%, Mo: 0.01~1.0%, Cu: 0.01~0.50%, Ni: 0.01~0.30%, Ca: 0.001~0.010%. 如請求項1或2所述之方形鋼管，其中，前述鋼組織，係前述硬質相的平均當量圓直徑係20μm以下。 The square steel pipe according to claim 1 or 2, wherein the steel structure has an average equivalent circle diameter of the hard phase being 20 μm or less. 一種方形鋼管的製造方法，係請求項1至3中任一項所述之方形鋼管的製造方法；其特徵為：進行造管步驟，該造管步驟，係將鋼板於冷間輥壓成形而形成為圓筒狀的端面焊接，成形為縱徑/橫徑之比0.99以上1.01以下的圓筒狀之後，成形為角狀。 A method for manufacturing a square steel pipe is the method for manufacturing a square steel pipe according to any one of claims 1 to 3; it is characterized in that: a pipe making step is performed, and the pipe making step is formed by rolling a steel plate in a cold room. The end face formed into a cylindrical shape is welded and formed into a cylindrical shape with a longitudinal diameter/lateral diameter ratio of 0.99 or more and 1.01 or less, and then formed into an angular shape. 一種方形鋼管的製造方法，係請求項1至3中任一項所述之方形鋼管的製造方法；其特徵為：對於鋼素材，依序進行熱間輥軋步驟、冷卻步驟、捲取步驟及造管步驟而製造方形鋼管之際，將前述鋼素材加熱至加熱溫度：1100~1300℃之後，進行熱間輥軋步驟而形成為熱軋鋼板，該熱間輥軋步驟，係：板厚中心溫度為1000℃以上之粗軋時間：200秒以上400秒以內、粗軋結束溫度：1000~800℃， 精軋開始溫度：1000~800℃、精軋結束溫度：900~750℃；接著，對於前述熱軋鋼板進行冷卻步驟，該冷卻步驟，係：具有1次以上之從冷卻開始10秒之間的初期冷卻步驟之0.2秒以上且未達3.0秒之放置冷卻，於板厚中心溫度之平均冷卻速度：4~25℃/秒；接著，對於前述熱軋鋼板進行以捲取溫度：580℃以下進行捲取的捲取步驟而形成鋼板，接著，進行造管步驟，該造管步驟，係將前述鋼板於冷間輥壓成形而形成為圓筒狀的端面焊接，成形為縱徑/橫徑之比0.99以上1.01以下的圓筒狀之後，成形為角狀。 A method for manufacturing a square steel pipe is the method for manufacturing a square steel pipe according to any one of claims 1 to 3; it is characterized in that: for the steel material, a hot rolling step, a cooling step, a coiling step, and When the square steel pipe is manufactured by the pipe making step, the aforementioned steel material is heated to a heating temperature of 1100 to 1300°C, and then a hot rolling step is performed to form a hot rolled steel sheet. The hot rolling step is based on the center of the plate thickness. Rough rolling time with temperature above 1000℃: 200 seconds or more within 400 seconds, rough rolling end temperature: 1000~800℃, Finish rolling start temperature: 1000~800°C, finish rolling end temperature: 900~750°C; Next, perform a cooling step for the aforementioned hot-rolled steel sheet. This cooling step is: once or more than 10 seconds from the start of cooling The average cooling rate at the center temperature of the plate thickness during the initial cooling step of 0.2 seconds or more and less than 3.0 seconds for cooling in the initial cooling step: 4~25°C/sec; then, the coiling temperature of the aforementioned hot-rolled steel sheet: 580°C or less The coiling step is used to form a steel plate, and then a pipe making step is performed. The pipe making step is to roll the aforementioned steel plate in a cold room to form a cylindrical end surface and weld it to a longitudinal diameter/transverse diameter. After a cylindrical shape with a ratio of 0.99 or more and 1.01 or less, it is formed into an angular shape. 如請求項5所述之方形鋼管的製造方法，其中，使前述冷卻步驟的冷卻停止溫度為580℃以下。 The method for manufacturing a square steel pipe according to claim 5, wherein the cooling stop temperature in the cooling step is set to 580°C or lower. 一種建築構造物，係使用請求項1至3中任一項所述之方形鋼管。A building structure using the square steel pipe described in any one of claims 1 to 3.
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