JP2009256780A - 780 MPa CLASS LOW YIELD RATIO CIRCULAR STEEL PIPE FOR BUILDING STRUCTURE HAVING EXCELLENT EARTHQUAKE RESISTANCE, AND METHOD FOR PRODUCING THE SAME - Google Patents
780 MPa CLASS LOW YIELD RATIO CIRCULAR STEEL PIPE FOR BUILDING STRUCTURE HAVING EXCELLENT EARTHQUAKE RESISTANCE, AND METHOD FOR PRODUCING THE SAME Download PDFInfo
- Publication number
- JP2009256780A JP2009256780A JP2009031545A JP2009031545A JP2009256780A JP 2009256780 A JP2009256780 A JP 2009256780A JP 2009031545 A JP2009031545 A JP 2009031545A JP 2009031545 A JP2009031545 A JP 2009031545A JP 2009256780 A JP2009256780 A JP 2009256780A
- Authority
- JP
- Japan
- Prior art keywords
- steel pipe
- less
- temperature
- hardness
- circular steel
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Granted
Links
Landscapes
- Heat Treatment Of Steel (AREA)
Abstract
Description
本発明は、主に耐震性が要求される建築鉄骨用途向けの円形鋼管、およびその製造方法に関するものであり、特に引張強さが780MPa以上(780MPa級)で、降伏比が90%以下の高強度低降伏比円形鋼管、およびこうした円形鋼管を製造するための有用な方法に関するものである。 TECHNICAL FIELD The present invention relates to a circular steel pipe for architectural steel frame applications that mainly requires earthquake resistance, and a method for producing the same, and in particular, a high tensile strength of 780 MPa or more (780 MPa class) and a yield ratio of 90% or less. It relates to a high strength low yield ratio round steel pipe and a useful method for producing such a round steel pipe.
建築用鋼材は建築構造物の耐震性を確保するために、弾性変形後の塑性変形により地震エネルギーを吸収するという思想の下に、降伏応力YSと引張強さTSとの比(YS/TS)で示される降伏比YRの上限が規定されている。 The ratio of yield stress YS to tensile strength TS (YS / TS) under the idea that building steels absorb seismic energy by plastic deformation after elastic deformation in order to ensure the earthquake resistance of building structures. The upper limit of the yield ratio YR indicated by
上記のような建築構造物に適用される円形鋼管は、鋼板をプレス曲げ加工等によって成形されるため、加工硬化に起因した材質変化が生じ、降伏比YRや鋼管表裏面の硬さが上昇し靭性が低下する。特に、円形鋼管の直径Dと鋼管厚tの比(D/t)が20以下となるような強曲げ加工の成形を行った円形鋼管の外面側は、板厚中央部と比較して硬さの上昇が大きく、また引張応力場となっているため、延性の低下により脆性破壊が生じる危険性がある。 Since circular steel pipes applied to building structures as described above are formed by press bending or the like of steel sheets, material changes due to work hardening occur, yield ratio YR and steel pipe front and back hardness increase. Toughness decreases. In particular, the outer surface side of a round steel pipe formed by a strong bending process such that the ratio (D / t) of the diameter D of the round steel pipe to the thickness t of the steel pipe is 20 or less is harder than the thickness center portion. Since there is a large increase in the tensile stress field, there is a risk of brittle fracture due to a decrease in ductility.
即ち、大地震時の荷重を受けて変形した場合には、亀裂は外面側から発生しやすく、円形鋼管は四面ボックス柱では発生しない固有の問題を有している。特に、付属金型等を円形鋼管に溶接したときには、熱影響部(HAZ)の硬化部が脆性破壊の発生起点となり、更に円形鋼管外周面の延性の延性低下が脆性破壊を発生伝播させることが問題となる。 That is, when deformed by receiving a load in the event of a large earthquake, cracks are likely to occur from the outer surface side, and the circular steel pipe has an inherent problem that does not occur in a four-sided box column. In particular, when an attached mold or the like is welded to a circular steel pipe, the hardened part of the heat-affected zone (HAZ) becomes the starting point of brittle fracture, and further, the ductile decrease in the ductility of the outer peripheral surface of the circular steel pipe can cause and propagate brittle fracture. It becomes a problem.
ところで、冷間成形によって鋼管を製造する方法としては、ラインパイプ用鋼管に適用されているUOE成形法(Uing press−Oing press−expander法)の他、プレスベンド冷間成形法(以下、単に「プレスベンド法」と呼ぶことがある)が基本的に採用されている。上記成形法のうち、鋼板厚さが厚く(例えば、板厚:30mm超)、強曲げ加工が必要な場合にはプレスベンド法が採用されることになる。 By the way, as a method of manufacturing a steel pipe by cold forming, in addition to a UOE forming method (Uing press-Oing press-expander method) applied to a steel pipe for a line pipe, a press bend cold forming method (hereinafter, simply “ The “press bend method” is sometimes used. Among the above forming methods, when the steel plate thickness is large (for example, plate thickness: more than 30 mm) and strong bending work is required, the press bend method is adopted.
上記プレスベンド法では、鋼板の一部(直線部)を型押し曲げ加工し、順次型押し位置を移動させて円形に成形する方法であり、加工能力が高い方法である。こうしたプレスベンド法で、円形鋼管を成形したときには、特に円形鋼管における外表面の硬化が顕著になるのであるが、こうした硬さを低減する方法としては、590MPa級鋼管では鋼管成形後、応力除去焼鈍(Stress Relieving:以下、「SR熱処理」と呼ぶことがある)を行なうことが知られている。 The press bend method is a method in which a part (straight portion) of a steel plate is stamped and bent, and the stamping position is sequentially moved to form a circle, which has a high processing capability. When a circular steel pipe is formed by such a press bend method, the hardening of the outer surface of the circular steel pipe is particularly remarkable. As a method for reducing such hardness, a stress-annealing annealing is performed after forming the steel pipe in a 590 MPa class steel pipe. (Stress Relieving: hereinafter referred to as “SR heat treatment”) is known to be performed.
しかしながら、780MPa級鋼管の場合、SR熱処理の適用を前提として、従来からの引張強さTS:780MPa以上の鋼板を適用すると、合金元素の添加量が多いため金属組織中にマルテンサイトや下部ベイナイト等の極めて硬質な組織を含有しており、この硬質組織が主体となると低降伏比YRの特性(以下、「低YR特性」と呼ぶことがある)の確保は勿論のこと、SR熱処理後も鋼管母材靭性の確保が非常に困難であり、鋼管表面の硬さは依然として硬い。一方、鋼管表面の硬さを低減するため、SR熱処理温度を高温にすれば、鋼管厚中央部の硬さも低下し、円形鋼管としての要求強度である引張強さTS:780MPa以上を確保することは困難であった。 However, in the case of a 780 MPa class steel pipe, on the premise that SR heat treatment is applied, when a steel sheet having a conventional tensile strength TS: 780 MPa or more is applied, the amount of alloy elements added is large, so martensite, lower bainite, etc. In addition to ensuring the characteristics of the low yield ratio YR (hereinafter sometimes referred to as “low YR characteristics”) when this hard structure is the main component, the steel pipe is also subjected to SR heat treatment. It is very difficult to secure the base material toughness, and the hardness of the steel pipe surface is still hard. On the other hand, in order to reduce the hardness of the steel pipe surface, if the SR heat treatment temperature is increased, the hardness of the steel pipe thickness central portion is also reduced, and the tensile strength TS: 780 MPa or more, which is a required strength as a circular steel pipe, is ensured. Was difficult.
更に、建築材料に対する要求は、高強度化や低降伏比特性等の機械的性質は勿論のこと、建設コスト低減のための大入熱溶接特性や良好な溶接性を確保することも重要であり、むやみに合金元素を添加することはできない。 In addition to the mechanical properties such as high strength and low yield ratio characteristics, it is also important for building materials to ensure high heat input welding characteristics and good weldability to reduce construction costs. Unnecessarily, alloying elements cannot be added.
上記のような鋼管に関する技術として、これまでも様々な技術が提案されている。例えば特許文献1には、490MPa以上のプレスベンド冷間成形円形鋼管の製造方法について提案されている。この技術は、490MPa級の円形鋼管の技術としては有用なものであるが、鋼管の表・裏面の夫々から深さ1mmまでの表層部におけるビッカース硬さHvが140〜200程度であり、鋼管厚中央部の硬さは更に低くなることから、780MPa以上の引張強さTSを得ることはできない。 Various techniques have been proposed for the steel pipe as described above. For example, Patent Document 1 proposes a method for producing a press-bend cold-formed circular steel pipe of 490 MPa or more. Although this technique is useful as a technique of a 490 MPa class circular steel pipe, the Vickers hardness Hv in the surface layer portion from the front and back surfaces of the steel pipe to a depth of 1 mm is about 140 to 200, and the thickness of the steel pipe Since the hardness of the central portion is further reduced, a tensile strength TS of 780 MPa or more cannot be obtained.
また特許文献2には、主組織をフェライトとして、硬質第二相の分率を10〜70%とした厚鋼板について開示されている。この技術では、その組織からして引張強さTS:780MPa以上を安定して確保することはできない。その製造方法についても、「冷却停止温度が500℃以下」の規定だけでは、硬質相の安定確保は困難であり、また組織制御の要点となる圧延終了温度:Ar3変態点以上、且つ冷却速度:5℃/秒以下であり、板厚方向に均一な金属組織と硬さを安定して得ることはできない。 Patent Document 2 discloses a thick steel plate having a main structure of ferrite and a hard second phase fraction of 10 to 70%. With this technique, the tensile strength TS: 780 MPa or more cannot be stably secured from the structure. With respect to the production method, it is difficult to ensure the stability of the hard phase only by the provision of “cooling stop temperature of 500 ° C. or less”, and the rolling end temperature: the Ar 3 transformation point or higher, which is the main point of the structure control, and the cooling rate : 5 ° C./second or less, and a uniform metal structure and hardness cannot be stably obtained in the thickness direction.
特許文献3には、化学成分組成を適切に調整した鋼素材を用い、圧延終了温度をAr3変態点以上の温度域とする熱間圧延を施し、次いで、Ar3変態点以上の温度域から300℃以下への焼入れ後、Ac1〜Ac1+150℃の温度域へ再加熱する際、再加熱温度までの加熱速度が1℃/秒以下で且つAc1〜Ac1+150℃の温度域での滞留時間が90秒以内であるような、高強度・高靭性鋼の製造方法について提案されている。 Patent Document 3, using the steel material was appropriately adjusted chemical composition, the rolling end temperature subjected to hot rolling to a temperature range of not lower than Ar 3 transformation point, then, from the temperature range of not lower than Ar 3 transformation point after quenching into 300 ° C. or less, Ac 1 ~Ac 1 + 150 upon reheating ° C. to the temperature range, the heating rate up to the reheating temperature is below 1 ° C. / sec and Ac 1 ~Ac 1 + 150 at ° C. temperature range of Has been proposed for producing a high-strength, high-toughness steel such that the dwell time is 90 seconds or less.
しかしながらこの技術では、金属組織や板厚方向の硬さ分布については考慮されておらず、プレス曲げ成形を施した場合には、外面側の硬さの硬化を抑制できず、円形鋼管としたときの良好な耐震性は発揮されないことが予想される。また製造方法において、二相域への急速加熱と滞留時間が短いために、板厚方向に均一な組織を得ることができないという問題がある。 However, this technology does not take into account the metal structure and the hardness distribution in the thickness direction, and when press bending is performed, the hardness of the outer surface cannot be suppressed, and when a round steel pipe is formed It is expected that good earthquake resistance will not be demonstrated. In addition, in the manufacturing method, there is a problem that a uniform structure cannot be obtained in the plate thickness direction because rapid heating and residence time in the two-phase region are short.
一方、特許文献4には、超大入熱HAZ靭性に優れる建築構造物用高強度厚肉鋼板を製造するための方法が提案されている。この技術は、建築構造物として超大入熱溶接したときの良好なHAZ靭性を確保する技術である。しかしながら、この技術では対象とする鋼板は基本的に低強度のものであり(700MPa以下)、板厚方向の硬さ分布については考慮されておらず、またC含有量が比較的高く、圧延温度も高く設定されているので、プレスベンド法による成形後の円形鋼管では、外面側の硬さが高くなり、良好な耐震性を発揮することができない。 On the other hand, Patent Document 4 proposes a method for producing a high-strength thick steel plate for a building structure that is excellent in super large heat input HAZ toughness. This technique is a technique for ensuring good HAZ toughness when super-high heat input welding is performed as a building structure. However, in this technique, the target steel plate is basically of low strength (700 MPa or less), the hardness distribution in the thickness direction is not taken into consideration, the C content is relatively high, and the rolling temperature Since the hardness of the outer surface becomes high in a round steel pipe after being formed by the press bend method, good earthquake resistance cannot be exhibited.
また特許文献5には、造管後の表面硬度と降伏比が低い高強度鋼管素材について、提案されている。この技術では、鋼管素材の強度を780MPa級とするものであるが、その成分系からして780MPa以上の強度を安定して得ることは困難である。また、製造方法において、二相域焼入れ温度について何ら規定されておらず、板厚方向に均一な金属組織と硬さを得ることはできない。 Patent Document 5 proposes a high-strength steel pipe material having a low surface hardness and yield ratio after pipe making. In this technique, the strength of the steel pipe material is set to 780 MPa class, but it is difficult to stably obtain a strength of 780 MPa or more from the component system. In addition, in the production method, no two-phase region quenching temperature is defined, and a uniform metal structure and hardness cannot be obtained in the thickness direction.
特許文献6には、板厚方向の硬さ分布を均一にした高強度高靭性鋼板を製造するための方法について開示されている。この技術では、圧延途中で一旦水冷し、復熱させた後に再度圧延を行うという特殊な製造方法を適用して、表層部に微細な加工フェライトを生成させ、表面の硬さを低減することによって、板厚方向の硬さ分布の均一化を図るものである。 Patent Document 6 discloses a method for producing a high-strength, high-toughness steel plate having a uniform hardness distribution in the thickness direction. In this technology, by applying a special manufacturing method in which water is cooled once in the middle of rolling, reheated and then rolled again, fine processed ferrite is generated in the surface layer portion, and the hardness of the surface is reduced. In this case, the hardness distribution in the thickness direction is made uniform.
しかしながら、この技術では、表層部が板厚内部よりも軟化する可能性があり、安定した材質を得るための量産面での製造管理が難しいという問題がある。また、この技術では、円形鋼管に加工した後の硬さについては、考慮されていない。 However, with this technique, there is a possibility that the surface layer portion may be softer than the inside of the plate thickness, and there is a problem that it is difficult to manage production in terms of mass production in order to obtain a stable material. In this technique, the hardness after being processed into a circular steel pipe is not taken into consideration.
本発明は、こうした状況の下でなされたものであって、その目的は、建築鉄骨用途では最も高強度クラスに位置する引張強さTS:780MPa級の鋼管について、高強度と低降伏比の両立を達成すると共に、鋼管成形時の曲げ加工に起因した鋼管外面側の硬さを低減することにより、延性を確保し、併せて外表面への溶接時の耐割れ性を向上させることにより、耐震性向上に寄与できる円形鋼管、およびこうした円形鋼管を製造するための有用な方法を提供することにある。 The present invention has been made under such circumstances, and its purpose is to achieve both high strength and low yield ratio for steel pipes with a tensile strength TS: 780 MPa class, which is located in the highest strength class for building steel frame applications. By reducing the hardness of the outer surface of the steel pipe caused by bending during steel pipe forming, the ductility is ensured, and at the same time, the crack resistance when welding to the outer surface is improved. It is an object of the present invention to provide a circular steel pipe that can contribute to improvement in performance and a useful method for manufacturing such a circular steel pipe.
上記目的を達成し得た本発明の円形鋼管とは、C:0.01〜0.06%(質量%の意味、以下同じ)、Si:0.10〜0.40%、Mn:1.60〜2.50%、Al:0.025〜0.090%、Cu:0.15〜0.70%、Ni:0.90〜1.60%、Cr:0.50〜1.35%、Mo:0.10〜0.30%、Ti:0.008〜0.025%、B:0.0005〜0.0025%、N:0.0030〜0.0060%およびCa:0.0005〜0.0040%を夫々含有すると共に、下記(1)式で示されるPCM値が0.30%以下であり、残部がFeおよび不可避的不純物からなり、該不可避的不純物のうちP:0.012%以下(0%を含まない)、S:0.005%以下(0%を含まない)およびO:0.0040%以下(0%を含まない)に夫々抑制し、且つ下記(A)〜(C)の要件を満足する点に要旨を有するものである。 The circular steel pipe of the present invention that can achieve the above-mentioned object is C: 0.01 to 0.06% (meaning of mass%, the same applies hereinafter), Si: 0.10 to 0.40%, Mn: 1. 60 to 2.50%, Al: 0.025 to 0.090%, Cu: 0.15 to 0.70%, Ni: 0.90 to 1.60%, Cr: 0.50 to 1.35% , Mo: 0.10 to 0.30%, Ti: 0.008 to 0.025%, B: 0.0005 to 0.0025%, N: 0.0030 to 0.0060% and Ca: 0.0005 -0.0040%, respectively, and the PCM value represented by the following formula (1) is 0.30% or less, and the balance is composed of Fe and unavoidable impurities. 012% or less (excluding 0%), S: 0.005% or less (not including 0%), and O: 0.00 Each was suppressed to 0% or less (not including 0%), and has a gist and a point which satisfies the following requirements (A) ~ (C).
PCM値=[C]+[Si]/30+[Mn]/20+[Cu]/20+[Ni]/60+[Cr]/20+[Mo]/15+[V]/10+([B]×5) …(1)
但し、[C],[Si],[Mn],[Cu],[Ni],[Cr],[Mo],[V]および[B]は、夫々C,Si,Mn,Cu,Ni,Cr,Mo,VおよびBの含有量(質量%)を示す。
(A)鋼管の表・裏面の夫々から深さ2mmまでの表層部を除く中央部の平均ビッカース硬さHvが230〜310である、
(B)鋼管のミクロ組織において、ベイニティックフェライト相の分率が80面積%以上であり、マルテンサイト相の分率が5面積%以下である、
(C)鋼管の表・裏面の夫々から深さ2mmまでの表層部の平均ビッカース硬さHvが、前記中央部の平均ビッカース硬さHvの1.3倍以下である。
PCM value = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + ([B] × 5) (1)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are respectively C, Si, Mn, Cu, Ni, Content (mass%) of Cr, Mo, V, and B is shown.
(A) The average Vickers hardness Hv of the central part excluding the surface layer part from each of the front and back surfaces of the steel pipe to a depth of 2 mm is 230 to 310.
(B) In the microstructure of the steel pipe, the fraction of bainitic ferrite phase is 80 area% or more, and the fraction of martensite phase is 5 area% or less.
(C) The average Vickers hardness Hv of the surface layer portion from each of the front and back surfaces of the steel pipe to a depth of 2 mm is 1.3 times or less of the average Vickers hardness Hv of the central portion.
上記のような円形鋼管を製造するに当たっては、前記化学成分からなる鋳片を950〜1200℃に加熱した後、仕上げ圧延温度を800〜930℃の範囲として熱間圧延を行なって所定の板厚とし、次いでt/4(t:板厚)の位置における冷却速度が2〜25℃/秒で、表面温度が350℃以下となるまで水冷し、その後、温度:700〜900℃の範囲に再加熱して焼入れ処理を行い、450〜700℃の温度範囲で焼戻しして鋼板とし、得られた鋼板を用いてプレスベンド法によって円形鋼管に成形するようにすれば良い。 In producing the circular steel pipe as described above, the slab made of the chemical component is heated to 950 to 1200 ° C., and then hot rolled at a finish rolling temperature in the range of 800 to 930 ° C. to obtain a predetermined plate thickness. Then, it is cooled with water until the surface temperature becomes 350 ° C. or less at a cooling rate of 2 to 25 ° C./second at a position of t / 4 (t: plate thickness), and then re-entered within the range of temperature: 700 to 900 ° C. It is only necessary to perform a quenching treatment by heating, tempering in a temperature range of 450 to 700 ° C. to form a steel plate, and forming the steel plate into a circular steel pipe by a press bend method using the obtained steel plate.
本発明によれば、鋼板(鋼管を構成する鋼板)の化学成分組成を適正に調整すると共に、ミクロ組織中の各相の面積分率を適切に制御し、且つ厚さ方向の硬さ分布を適切にすることによって、780MPa以上の高強度と低降伏比の両立を達成すると共に、鋼管成形時の曲げ加工に起因した鋼管外面側の硬さを低減して延性を確保し、併せて溶接による耐割れ性をも向上させることにより、耐震性向上に寄与できる円形鋼管が実現できた。 According to the present invention, the chemical composition of the steel sheet (the steel sheet constituting the steel pipe) is appropriately adjusted, the area fraction of each phase in the microstructure is appropriately controlled, and the hardness distribution in the thickness direction is controlled. By making it appropriate, it is possible to achieve both high strength of 780 MPa or more and a low yield ratio, reduce the hardness on the outer surface side of the steel pipe due to bending during steel pipe forming, and ensure ductility, and also by welding By improving crack resistance, a round steel pipe that can contribute to improved earthquake resistance was realized.
本発明者らは、780MPa以上の高強度と低降伏比の両立を達成すること、およびプレス曲げ加工時の加工硬化に起因した円形鋼管外面側の硬化を低減するために、様々な角度から検討した。その結果、まず鋼管(即ち、鋼板)の基本的なミクロ組織として、ベイニティックフェライト相の分率(面積分率)を80%以上とし、マルテンサイト相の面積分率を5%以下とすることが重要である[前記(B)の要件]ことが判明した。ここで、ベイニティックフェライト相とは、フェライトより低温で変態する低Cのベイナイト組織の相であり、グラニュラーベイニティックフェライト組織、広義の上部ベイナイト組織や下部ベイナイト組織等を含むが、ポリゴナルフェライト組織や粒界フェライト組織は含まない(例えば、「鋼のベイナイト写真集−1」:日本鉄鋼協会ベイナイト調査研究部会、1992)。また、マルテンサイト相は、MA(Martensite−Austenite Constituent)を含む。 The present inventors have studied from various angles in order to achieve both a high strength of 780 MPa or more and a low yield ratio, and to reduce the hardening of the outer surface of the circular steel pipe caused by work hardening during press bending. did. As a result, as a basic microstructure of the steel pipe (ie, steel plate), the bainitic ferrite phase fraction (area fraction) is set to 80% or more, and the martensite phase area fraction is set to 5% or less. It was found that [Requirement (B)] is important. Here, the bainitic ferrite phase is a phase of a low C bainite structure that transforms at a lower temperature than ferrite, and includes a granular bainitic ferrite structure, broad upper bainite structure, lower bainite structure, and the like. Ferrite structure and grain boundary ferrite structure are not included (for example, “Steel Bainite Photobook-1”: Japan Iron and Steel Institute Bainite Research and Study Group, 1992). Further, the martensite phase includes MA (Martensite-Austenite Constituent).
低Cのベイニティックフェライト組織は、炭化物が少なく冷却速度依存性が小さいため鋼板の板厚方向の硬さの均一性が高く、また通常のポリゴナルフェライトと比べて転位密度が高いことから変形歪みに対する加工硬化量は小さくなる。こうしたことから、鋼管にした後の厚さ方向の硬さ分布の均一化に寄与することになる。ベイニティックフェライト相の面積分率が80%未満となり、マルテンサイト等の硬質相の面積分率が多くなると、円形鋼管外面側の硬度が上昇して変形能が劣化し、破断伸びが低下することになる。こうしたことから、ベイニティックフェライトの面積分率は少なくとも80%以上とする必要があり、好ましくは85%以上とするのが良い。 The low-C bainitic ferrite structure is deformed because it has less carbide and less cooling rate dependence, so the hardness in the thickness direction of the steel sheet is high and the dislocation density is higher than normal polygonal ferrite. The amount of work hardening with respect to strain is reduced. For this reason, it contributes to uniform hardness distribution in the thickness direction after making the steel pipe. When the area fraction of the bainitic ferrite phase is less than 80% and the area fraction of the hard phase such as martensite is increased, the hardness on the outer surface side of the circular steel pipe is increased, the deformability is deteriorated, and the elongation at break is lowered. It will be. For these reasons, the area fraction of bainitic ferrite needs to be at least 80% or more, preferably 85% or more.
一方、マルテンサイト相については、鋼管(鋼板)靭性確保という観点から、その面積分率を5%以下に抑える必要がある。即ち、マルテンサイト相の面積分率が5%を超えると、硬質のマルテンサイトが破壊の起点となって、著しく靭性が劣化するという不都合が生じる。尚、本発明の円形鋼管のミクロ組織は上記のように制御されていれば良いが、残部としてベイナイト相やフェライト相等が一部含まれていても良い。 On the other hand, for the martensite phase, it is necessary to suppress the area fraction to 5% or less from the viewpoint of securing steel pipe (steel plate) toughness. That is, when the area fraction of the martensite phase exceeds 5%, the hard martensite becomes a starting point of fracture, resulting in a disadvantage that the toughness is remarkably deteriorated. In addition, although the microstructure of the circular steel pipe of this invention should just be controlled as mentioned above, a bainite phase, a ferrite phase, etc. may be partially contained as the remainder.
上記のようなミクロ組織とするためには、製造条件も適切に制御する必要があるが、その前提として、鋼板の化学成分組成も適切に制御する必要がある。その基本的な方向としては、Cの含有量を低減することによる円形鋼管の表面硬さの低減と、それを前提として高強度と低降伏比を維持するために、Crの適正添加によるベイニティックフェライトの生成、およびそのベイニティックフェライトへのCu,Niの固溶による強化と、Bによるオーステナイト粒界からのフェライト変態制御による強化機構を活用することが有効である。 In order to obtain the microstructure as described above, it is necessary to appropriately control the manufacturing conditions, but as a premise, it is also necessary to appropriately control the chemical composition of the steel sheet. The basic direction is to reduce the surface hardness of the circular steel pipe by reducing the C content, and to maintain high strength and low yield ratio on the premise of this, the baiini by appropriate addition of Cr. It is effective to utilize the strengthening mechanism by the formation of tick ferrite, the strengthening by solid solution of Cu and Ni in the bainitic ferrite, and the ferrite transformation control from the austenite grain boundary by B.
鋼板の強度を向上させるために有効な手段は、合金元素量を増加させることである。特に、780MPa級という高強度を達成するためには、合金元素の添加量を比較的多くして、それらによる各種強化機構を利用することが必要である。しかしながら、こうした合金元素の増大は、耐割れ性といった溶接性や溶接継手の機械的特性の劣化を招くことになる。本発明者らは、適正な合金元素の添加とその含有量を適正化することによって、高強度と低YR特性の両立と、曲げ加工による加工硬化を低減できることを見出したのである。 An effective means for improving the strength of the steel sheet is to increase the amount of alloying elements. In particular, in order to achieve a high strength of 780 MPa class, it is necessary to use a relatively large amount of alloy elements and use various strengthening mechanisms. However, such an increase in alloy elements leads to deterioration of weldability such as crack resistance and mechanical properties of welded joints. The present inventors have found that by adding an appropriate alloy element and optimizing the content thereof, both high strength and low YR characteristics can be achieved and work hardening by bending can be reduced.
上記した各要件(ミクロ組織および化学成分組成)を満足させることによって、板厚方向の硬さ分布を均一化させると共に、加工硬化量を安定化させ、円形鋼管外面下2mmまでの領域(鋼板表面から深さ2mmまでの表層部)と、鋼管厚方向中央部[t/2部(t:板厚)]のビッカース硬さHvの比を抑制でき、円形鋼管としての耐震性が向上できたのである。 By satisfying the above requirements (microstructure and chemical composition), the hardness distribution in the sheet thickness direction is made uniform, the work hardening amount is stabilized, and the area up to 2 mm below the outer surface of the circular steel pipe (steel sheet surface) Since the ratio of Vickers hardness Hv of the steel layer thickness direction center part [t / 2 part (t: plate thickness)] and the steel pipe thickness direction center part can be suppressed, and the earthquake resistance as a circular steel pipe can be improved. is there.
上記した観点から、本発明の円形鋼管の化学成分組成が決定されたのであるが、上記した合金成分(C,Cr,Ni,B)を含め、各元素の範囲限定理由について説明する。本発明では、上記のように、C:0.01〜0.06%、Si:0.10〜0.40%、Mn:1.60〜2.50%、Al:0.025〜0.090%、Cu:0.15〜0.70%、Ni:0.90〜1.60%、Cr:0.50〜1.35%、Mo:0.10〜0.30%、Ti:0.008〜0.025%、B:0.0005〜0.0025%、N:0.0030〜0.0060%およびCa:0.0005〜0.0040%を夫々含有すると共に、前記(1)式で示されるPCM値を適正な範囲に制御する必要があるが、これらの元素の範囲限定理由は、次の通りである。 From the above viewpoint, the chemical composition of the circular steel pipe of the present invention has been determined. The reasons for limiting the range of each element including the above-described alloy components (C, Cr, Ni, B) will be described. In the present invention, as described above, C: 0.01 to 0.06%, Si: 0.10 to 0.40%, Mn: 1.60 to 2.50%, Al: 0.025 to 0.5. 090%, Cu: 0.15 to 0.70%, Ni: 0.90 to 1.60%, Cr: 0.50 to 1.35%, Mo: 0.10 to 0.30%, Ti: 0 0.008-0.025%, B: 0.0005-0.0025%, N: 0.0030-0.0060% and Ca: 0.0005-0.0040%, respectively, and (1) Although it is necessary to control the PCM value represented by the formula within an appropriate range, the reasons for limiting the ranges of these elements are as follows.
[C:0.01〜0.06%]
Cは、鋼板の強度を高める効果があり、硬さを制御するために重要な元素であると共に、耐割れ性等の溶接性を劣化させる元素でもある。C含有量が0.01%未満であると、必要な母材(鋼管)強度を確保することができない。しかしながら、C含有量が0.06%を超えると、表層部のマルテンサイト変態により、板厚方向の硬さ分布が大きくなる。また、溶接部に島状マルテンサイト(MA)が過剰に生成してHAZが硬くなり過ぎ、割れが発生しやすくなり、地震時の破壊の発生点となる。尚、C含有量の好ましい下限は0.02%であり、好ましい上限は0.05%である。
[C: 0.01 to 0.06%]
C has an effect of increasing the strength of the steel sheet, is an element important for controlling the hardness, and is an element that deteriorates weldability such as crack resistance. If the C content is less than 0.01%, the required base material (steel pipe) strength cannot be ensured. However, if the C content exceeds 0.06%, the hardness distribution in the thickness direction becomes large due to the martensitic transformation of the surface layer portion. In addition, island-like martensite (MA) is excessively generated in the welded portion, the HAZ becomes too hard, and cracks are likely to occur, which becomes a point of occurrence of destruction during an earthquake. In addition, the preferable minimum of C content is 0.02%, and a preferable upper limit is 0.05%.
[Si:0.10〜0.40%]
Siは、鋼管の強度向上に有効な元素である。こうした強化機構を発揮させるためには、Siは0.10%以上含有させることが必要である。しかしながら、Si含有量が過剰になると、母材靭性、HAZ靭性や溶接性が劣化するので、0.40%以下とする。尚、Si含有量の好ましい下限は0.15%であり、好ましい上限は0.35%である。
[Si: 0.10 to 0.40%]
Si is an element effective for improving the strength of a steel pipe. In order to exhibit such a strengthening mechanism, it is necessary to contain Si by 0.10% or more. However, if the Si content is excessive, the base material toughness, the HAZ toughness and the weldability deteriorate, so the content is made 0.40% or less. In addition, the minimum with preferable Si content is 0.15%, and a preferable upper limit is 0.35%.
[Mn:1.60〜2.50%]
Mnは、焼入れ性を向上させ、強度と靭性を確保する上で有効な元素である。こうした効果を発揮させるためには、Mnは1.60%以上含有させる必要がある。しかしながらMnを過剰に含有させると、靭性が劣化するので、上限を2.50%とする。尚、Mn含有量の好ましい下限は1.80%であり、好ましい上限は2.20%である。
[Mn: 1.60 to 2.50%]
Mn is an element effective in improving hardenability and ensuring strength and toughness. In order to exhibit such an effect, it is necessary to contain Mn 1.60% or more. However, if Mn is contained excessively, toughness deteriorates, so the upper limit is made 2.50%. In addition, the minimum with preferable Mn content is 1.80%, and a preferable upper limit is 2.20%.
[Al:0.025〜0.090%]
Alは、脱酸、およびフリー窒素の固定によりBの焼入れ性を確保するために必要な元素である。これらの効果を発揮させるためには、0.025%以上含有させる必要があるが、過剰に含有させると、アルミナ系の粗大な介在物を形成し母材靭性が低下するので、0.090%以下とする必要がある。尚、Al含有量の好ましい下限は0.035%であり、好ましい上限は0.080%である。
[Al: 0.025 to 0.090%]
Al is an element necessary for ensuring the hardenability of B by deoxidation and fixation of free nitrogen. In order to exert these effects, it is necessary to contain 0.025% or more. However, if excessively contained, alumina-based coarse inclusions are formed and the base material toughness is lowered. It is necessary to do the following. In addition, the minimum with preferable Al content is 0.035%, and a preferable upper limit is 0.080%.
[Cu:0.15〜0.70%]
Cuは、固溶強化によって、母材強度を向上させるのに有用な元素である。こうした効果を発揮させるためには、Cuは0.15%以上含有させる必要がある。しかしながら、Cu含有量が過剰になると、ガス切断時にCu割れが生じることがあるので、0.70%以下とする必要がある。尚、Cu含有量の好ましい下限は0.25%であり、好ましい上限は0.65%である。
[Cu: 0.15 to 0.70%]
Cu is an element useful for improving the base material strength by solid solution strengthening. In order to exhibit such an effect, it is necessary to contain 0.15% or more of Cu. However, if the Cu content is excessive, Cu cracking may occur at the time of gas cutting, so it is necessary to be 0.70% or less. In addition, the minimum with preferable Cu content is 0.25%, and a preferable upper limit is 0.65%.
[Ni:0.90〜1.60%]
Niは、母材靭性・HAZ靭性の向上および焼入れ性を高めて強度を向上させると共に、Cu割れおよび溶接割れを防止にも有効な元素である。こうした効果を発揮させるためには、Niは0.90%以上含有させる必要がある。しかしながら、Ni含有量が過剰になると、耐溶接割れ性が劣化し、圧延時にスケール疵が発生しやすくなるので、1.60%以下とする必要がある。尚、Ni含有量の好ましい下限は1.10%であり、好ましい上限は1.35%である。
[Ni: 0.90 to 1.60%]
Ni is an element that is effective in improving base metal toughness / HAZ toughness and improving hardenability to improve strength and also preventing Cu cracking and weld cracking. In order to exhibit such an effect, Ni needs to be contained by 0.90% or more. However, if the Ni content is excessive, the weld crack resistance deteriorates and scale flaws are likely to occur during rolling, so it is necessary to make it 1.60% or less. In addition, the minimum with preferable Ni content is 1.10%, and a preferable upper limit is 1.35%.
[Cr:0.50〜1.35%]
Crは、焼入れ性を高めて強度を向上させるのに有効な元素である。こうした効果を発揮させるためには、Crは0.50%以上含有させる必要がある。しかしながら、Cr含有量が過剰になると、耐溶接割れ性が劣化するので、1.35%以下とする必要がある。尚、Cr含有量の好ましい下限は0.60%であり、好ましい上限は1.25%である。
[Cr: 0.50 to 1.35%]
Cr is an element effective for improving the hardenability and improving the strength. In order to exert such an effect, it is necessary to contain 0.50% or more of Cr. However, if the Cr content is excessive, the weld crack resistance deteriorates, so it is necessary to make it 1.35% or less. In addition, the minimum with preferable Cr content is 0.60%, and a preferable upper limit is 1.25%.
[Mo:0.10〜0.30%]
Moは、焼入れ性を高めて強度を向上させる元素であり、また炭化物を生成しやすい元素である。Moによる焼入れ性向上効果を発揮させるためには、Moは0.10%以上含有させる必要がある。しかしながら、Mo含有量が過剰になると、焼入れ性が過剰となり、耐溶接割れ性が劣化するので、0.30%以下とする必要がある。尚、Mo含有量の好ましい下限は0.15%であり、好ましい上限は0.25%である。
[Mo: 0.10 to 0.30%]
Mo is an element that improves hardenability and improves strength, and is an element that easily generates carbides. In order to exert the effect of improving the hardenability by Mo, Mo needs to be contained by 0.10% or more. However, if the Mo content becomes excessive, the hardenability becomes excessive and the weld crack resistance deteriorates, so it is necessary to make it 0.30% or less. In addition, the minimum with preferable Mo content is 0.15%, and a preferable upper limit is 0.25%.
[Ti:0.008〜0.025%]
Tiは、Nと窒化物(TiN)を形成して熱間圧延前の加熱時におけるオーステナイト粒(γ粒)の粗大化を防止し、靭性向上に効果がある元素である。また、Nを固定することによりBの焼入れ性を確保するのに有効である。これらの効果を発揮させるためには、Tiは0.008%以上含有させる必要がある。しかしながら、Ti含有量が過剰になると、TiNが粗大化して母材靭性が劣化するので、0.025%以下とする必要がある。尚、Ti含有量の好ましい下限は0.010%であり、好ましい上限は0.018%である。
[Ti: 0.008 to 0.025%]
Ti is an element that forms N and nitride (TiN) to prevent coarsening of austenite grains (γ grains) during heating before hot rolling, and is effective in improving toughness. Further, fixing N is effective in securing the hardenability of B. In order to exert these effects, it is necessary to contain Ti by 0.008% or more. However, if the Ti content is excessive, TiN becomes coarse and the base material toughness deteriorates, so it is necessary to make it 0.025% or less. In addition, the minimum with preferable Ti content is 0.010%, and a preferable upper limit is 0.018%.
[B:0.0005〜0.0025%]
フリーBはγ粒界に存在し、焼入れ性を向上させて母材強度の向上をはかる上で有効な元素である。Bの含有量が0.0005%未満であると、母材強度の向上効果が少なく、引張強度:780MPa以上の強度を確保できなくなる。しかしながら、B含有量が過剰になると、介在物が生成し母材靭性が劣化するので、0.0025%以下とする必要がある。尚、B含有量の好ましい下限は0.0008%であり、好ましい上限は0.0020%である。
[B: 0.0005 to 0.0025%]
Free B exists at the γ grain boundary and is an effective element for improving the hardenability and improving the strength of the base material. If the B content is less than 0.0005%, the effect of improving the strength of the base material is small, and a tensile strength of 780 MPa or more cannot be secured. However, if the B content is excessive, inclusions are generated and the base material toughness deteriorates, so it is necessary to make the content 0.0025% or less. In addition, the minimum with preferable B content is 0.0008%, and a preferable upper limit is 0.0020%.
[N:0.0030〜0.0060%]
Nは、TiNを生成し、熱間圧延前の加熱時および溶接時におけるγ粒の粗大化を防止し、母材靭性やHAZ靭性を向上させるのに有効な元素である。Nの含有量が0.0030%未満であると、TiNが不足し、加熱γ粒が粗大になり、靭性が劣化するので、0.0030%以上含有させる必要がある。またN含有量が過剰になって0.0060%を超えると、曲げ加工による脆化により、鋼管の靭性が劣化する。尚、N含有量の好ましい下限は0.0035%であり、好ましい上限は0.0055%である。
[N: 0.0030 to 0.0060%]
N is an element effective for producing TiN, preventing coarsening of γ grains during heating and hot welding before hot rolling, and improving base metal toughness and HAZ toughness. If the N content is less than 0.0030%, TiN is insufficient, the heated γ grains become coarse, and the toughness deteriorates. Therefore, it is necessary to contain 0.0030% or more. On the other hand, if the N content becomes excessive and exceeds 0.0060%, the toughness of the steel pipe deteriorates due to embrittlement caused by bending. In addition, the minimum with preferable N content is 0.0035%, and a preferable upper limit is 0.0055%.
[Ca:0.0005〜0.0040%]
Caは、MnSの球状化による耐溶接割れ性に対する無害化に有効な元素である。こうした効果を発揮させるためには、Caは0.0005%以上含有させる必要がある。しかしながら、Ca含有量が0.0040%を超えて過剰になると、介在物を粗大化させ、母材靭性を劣化させる。尚、Ca含有量の好ましい下限は0.0015%であり、好ましい上限は0.0030%である。
[Ca: 0.0005 to 0.0040%]
Ca is an element effective for detoxifying the weld cracking resistance by spheroidizing MnS. In order to exhibit such an effect, Ca needs to be contained by 0.0005% or more. However, when the Ca content exceeds 0.0040% and becomes excessive, inclusions are coarsened and the base metal toughness is deteriorated. In addition, the minimum with preferable Ca content is 0.0015%, and a preferable upper limit is 0.0030%.
[PCM値:0.30%以下]
前記(1)式で表わされるPCM値は、溶接施工による低温割れを防止する指標として最も一般的な要件である。溶接割れを防止するためには、PCM値を0.30%以下とする必要がある。PCM値は、好ましくは0.28%以下とするのが良い。
[PCM value: 0.30% or less]
The PCM value represented by the formula (1) is the most common requirement as an index for preventing cold cracking due to welding. In order to prevent weld cracking, the PCM value needs to be 0.30% or less. The PCM value is preferably 0.28% or less.
本発明の円形鋼管において、上記成分の他は、Feおよび不可避的不純物(例えば、P,S,O等)からなるものであるが、溶製上不可避的に混入する微量成分(許容成分)も含み得るものであり(例えば、Zr,H等)、こうした円形鋼管も本発明の範囲に含まれるものである。但し、不可避的不純物としてのP,S,O等については、下記の観点から、夫々下記の範囲に抑制する必要がある。 In the circular steel pipe of the present invention, in addition to the above components, it consists of Fe and unavoidable impurities (for example, P, S, O, etc.), but there are also trace components (allowable components) that are inevitably mixed during melting. Such circular steel pipes are also included in the scope of the present invention. However, P, S, O, etc. as inevitable impurities need to be controlled within the following ranges from the following viewpoints.
[P:0.012%以下(0%を含まない)]
不可避的不純物であるPは、母材、溶接部の靭性に悪影響を及ぼすものであり、こうした不都合を招かない上でもその含有量が0.012%以下に抑制する必要があり、好ましくは0.010%以下とするのが良い。
[P: 0.012% or less (excluding 0%)]
P, which is an unavoidable impurity, adversely affects the toughness of the base metal and the welded portion. Even without causing such inconvenience, the content must be suppressed to 0.012% or less. It should be 010% or less.
[S:0.005%以下(0%を含まない)]
Sは、MnSを形成して耐溶接割れ性を劣化させるので、できるだけ少ない方が好ましい。こうした観点から、S含有量は0.005%以下に抑制する必要があり、好ましくは0.003%以下とするのが良い。
[S: 0.005% or less (excluding 0%)]
Since S forms MnS and degrades weld crack resistance, it is preferable that S be as small as possible. From such a viewpoint, the S content needs to be suppressed to 0.005% or less, and preferably 0.003% or less.
[O:0.0040%以下(0%を含まない)]
Oは種々の元素と結合して酸化物を形成する。その酸化物は、場合によっては粗大化し、母材靭性を劣化させる原因となる。こうした観点から、O含有量は0.0040%以下とする必要があり、これよりも含有量が過剰になると、酸化物が粗大化することになる。好ましくは、0.0030%以下に抑制するのが良い。
[O: 0.0040% or less (excluding 0%)]
O combines with various elements to form oxides. In some cases, the oxide becomes coarse and causes the base material toughness to deteriorate. From this point of view, the O content needs to be 0.0040% or less, and if the content is excessive, the oxide becomes coarse. Preferably, the content is suppressed to 0.0030% or less.
本発明の円形鋼管においては、鋼管の表・裏面(鋼管を構成する鋼板の表・裏面)の夫々から深さ2mmまでの表層部を除く中央部の平均ビッカース硬さHvが230〜310であることも必要である[前記(A)の要件]。このビッカース硬さHvは引張強さTSと相関があり、所望の引張強さTSと降伏比YRを得るためには、鋼管厚中央部の平均ビッカース硬さHvが230〜310であることも必要である。このときの平均ビッカース硬さHvとは、鋼管厚断面の表面から深さ4mmの位置から裏面側の方向に裏面から4mmの位置までの硬さを2mm間隔で連続的に測定し、その値を平均化したものである。鋼管厚中央部の平均ビッカース硬さHvが230未満では、低降伏比YRは確保できるが、引張強さTSが780MPa未満となり強度を満たさなくなる。また、鋼管厚中央部の平均ビッカース硬さHvが310を超えると、引張強さTSが大きくなり過ぎ、降伏比YRも高くなる。 In the circular steel pipe of the present invention, the average Vickers hardness Hv of the central part excluding the surface layer part from the front and back surfaces (front and back surfaces of the steel plate constituting the steel pipe) to a depth of 2 mm is 230 to 310. It is also necessary [Requirement (A) above]. This Vickers hardness Hv correlates with the tensile strength TS, and in order to obtain the desired tensile strength TS and yield ratio YR, it is also necessary that the average Vickers hardness Hv at the center of the steel pipe thickness is 230 to 310. It is. The average Vickers hardness Hv at this time is a continuous measurement of the hardness from the position of 4 mm depth from the surface of the steel pipe thickness section to the position of 4 mm from the back surface in the direction of the back surface at intervals of 2 mm. Averaged. If the average Vickers hardness Hv at the center of the steel pipe thickness is less than 230, the low yield ratio YR can be secured, but the tensile strength TS becomes less than 780 MPa and the strength is not satisfied. Moreover, when the average Vickers hardness Hv of the steel pipe thickness center part exceeds 310, tensile strength TS will become large too much and the yield ratio YR will also become high.
本発明の円形鋼管においては、鋼管の表・裏面の夫々から深さ2mmまでの表層部の平均ビッカース硬さHvが、前記鋼管厚中央部の平均ビッカース硬さHvの1.3倍以下であることも必要である[前記(C)の要件]。この表層部の平均ビッカース硬さHvとは、表面から深さ1mmと2mmの位置、および裏面から深さ1mmと2mmの位置の4点の平均値である。 In the circular steel pipe of the present invention, the average Vickers hardness Hv of the surface layer part from each of the front and back surfaces of the steel pipe to a depth of 2 mm is 1.3 times or less of the average Vickers hardness Hv of the steel pipe thickness center part. It is also necessary [Requirement (C) above]. The average Vickers hardness Hv of the surface layer portion is an average value of four points at positions 1 mm and 2 mm deep from the front surface and positions 1 mm and 2 mm deep from the back surface.
表層部と鋼管厚中央の硬さの比が1.3倍を超えると、表層部の塑性変形能が低下するため、大地震時の大荷重による引張応力が作用したとき表層部の延性が追随できず、表面から亀裂が発生する危険性がある。更に、付属金物溶接がある場合は、溶接によるHAZ硬化部が亀裂発生の起点となり、表層部の低延性低靭性部を脆性亀裂が発生伝播し、円形鋼管が脆性破断する可能性がある。この比の値は、好ましくは1.25倍以下である。 If the ratio of the hardness of the surface layer to the center of the steel pipe exceeds 1.3 times, the plastic deformability of the surface layer will decrease, so the ductility of the surface layer will follow when tensile stress is applied due to a heavy load during a large earthquake. There is a risk of cracking from the surface. Furthermore, when there is accessory welding, the HAZ hardened part by welding becomes a starting point of crack generation, brittle cracks are generated and propagated in the low ductility and low toughness part of the surface layer part, and there is a possibility that the circular steel pipe breaks brittlely. The value of this ratio is preferably 1.25 times or less.
本発明の円形鋼管を製造するには、上記の様な化学成分からなる鋳片を950〜1200℃に加熱した後、仕上げ圧延温度を800〜930℃の範囲として熱間圧延を行なって所定の板厚とし、次いでt/4(t:板厚)の位置における冷却速度が2〜25℃/秒で、表面温度が350℃以下となるまで水冷し、その後、温度:700〜900℃の範囲に再加熱して焼入れ処理を行い、450〜700℃の温度範囲で焼戻しして鋼板とし、得られ鋼板を用いてプレスベンド法によって円形鋼管に成形すれば良いが、各工程の条件を規定した理由は次の通りである。 In order to manufacture the circular steel pipe of the present invention, a slab made of the above chemical components is heated to 950 to 1200 ° C., and then hot rolled at a finish rolling temperature in the range of 800 to 930 ° C. The thickness is then changed to a thickness of t / 4 (t: thickness) at a cooling rate of 2 to 25 ° C./second until the surface temperature is 350 ° C. or lower, and then the temperature is in the range of 700 to 900 ° C. The steel plate is reheated to be tempered and tempered in a temperature range of 450 to 700 ° C. to form a steel plate. The resulting steel plate may be formed into a circular steel pipe by the press bend method, but the conditions for each step are defined. The reason is as follows.
[鋳片を950〜1200℃に加熱]
この加熱温度は、熱間圧延前の組織制御に大きく影響する。加熱温度が950℃未満であると、圧延最終パス(仕上げ圧延)温度が800℃未満となり、水冷前に表面からフェライトが析出し780MPa以上の母材強度を確保できなくなると共に、板厚方向の硬さ分布が均一にならない。一方、加熱温度が1200℃を超えると、γ粒径の粗大化により母材靭性が劣化する。
[Heating slab to 950-1200 ° C]
This heating temperature greatly affects the structure control before hot rolling. When the heating temperature is less than 950 ° C., the rolling final pass (finish rolling) temperature is less than 800 ° C., ferrite precipitates from the surface before water cooling, and it becomes impossible to ensure the strength of the base material of 780 MPa or more and the hardness in the plate thickness direction. The distribution is not uniform. On the other hand, if the heating temperature exceeds 1200 ° C., the base material toughness deteriorates due to the coarsening of the γ grain size.
[仕上げ圧延温度を800〜930℃の範囲として熱間圧延を行なって所定の板厚とする]
制御冷却(加速冷却)は、その前の組織制御が重要であり、そのためには制御圧延での圧延終了温度(仕上げ圧延温度)と冷却開始温度を管理する必要がある。仕上げ圧延温度が800℃未満であると、冷却開始前にフェライトが析出し、所望の強度を得ることができない。また、仕上げ圧延温度が930℃を超えると、冷却前組織が粗大となり、母材靭性が劣化し、板厚方向の硬さ分布が大きくなる。仕上げ圧延温度は、好ましくは900℃以下とするのが良い。
[Hot rolling is performed at a finish rolling temperature of 800 to 930 ° C. to a predetermined thickness]
For controlled cooling (accelerated cooling), prior structure control is important. For this purpose, it is necessary to manage the rolling end temperature (finish rolling temperature) and cooling start temperature in controlled rolling. When the finish rolling temperature is less than 800 ° C., ferrite precipitates before the start of cooling, and a desired strength cannot be obtained. On the other hand, when the finish rolling temperature exceeds 930 ° C., the structure before cooling becomes coarse, the base material toughness deteriorates, and the hardness distribution in the sheet thickness direction increases. The finish rolling temperature is preferably 900 ° C. or lower.
[t/4(t:板厚)の位置における冷却速度が2〜25℃/秒]
圧延後の冷却工程(DQ)は、組織制御のために重要な工程である。冷却速度が2℃/秒未満では、所望の組織であるベイニティックフェライト(ベイナイト)の面積分率:80%以上を確保できなくなる。冷却速度が大きい方が、ベイニティックフェライト組織を微細化し靭性が向上するが、25℃/秒を超えた場合には、表面近くの組織において、有害組織であるマルテンサイト(MAを含む)が増大し、母材靭性が劣化すると共に、強度が過大となり表面が硬化するため延性(伸び性能)が低下する。尚、冷却速度を測定する位置として、t/4(t:板厚)としたのは、鋼板の平均的な性能を発揮する位置だからである。
[The cooling rate at the position of t / 4 (t: plate thickness) is 2 to 25 ° C./second]
The cooling process (DQ) after rolling is an important process for controlling the structure. When the cooling rate is less than 2 ° C./second, it becomes impossible to ensure an area fraction of bainitic ferrite (bainite) which is a desired structure: 80% or more. When the cooling rate is larger, the bainitic ferrite structure is refined and the toughness is improved. However, when it exceeds 25 ° C./second, martensite (including MA) which is a harmful structure is present in the structure near the surface. It increases, the toughness of the base material deteriorates, and the strength becomes excessive and the surface hardens, so the ductility (elongation performance) decreases. Note that t / 4 (t: plate thickness) is used as a position for measuring the cooling rate because it is a position that exhibits the average performance of the steel sheet.
[冷却停止温度:鋼板の表面温度が350℃以下]
冷却停止温度によって、マルテンサイトや下部ベイナイトの存在形態が変化し、強度が変わる。冷却停止温度が350℃を超えると、板厚中央部で低温変態組織が少なくなり、強度が低下すると共に、板厚方向で変態組織や板厚さ方向の硬さ分布が不均一となる。板厚方向に均一に変態させるため、冷却停止温度は350℃以下とする必要がある。
[Cooling stop temperature: Steel sheet surface temperature is 350 ° C or lower]
Depending on the cooling stop temperature, the form of martensite and lower bainite changes and the strength changes. When the cooling stop temperature exceeds 350 ° C., the low temperature transformation structure is reduced at the center of the plate thickness, the strength is lowered, and the transformation structure and the hardness distribution in the plate thickness direction are not uniform in the plate thickness direction. In order to uniformly transform in the plate thickness direction, the cooling stop temperature needs to be 350 ° C. or lower.
[温度:700〜900℃の範囲に再加熱して焼入れ処理」
低YR特性を実現する軟質相と硬質相の複合組織を得るためには、Ac1とAc3の間の二相域の温度に加熱することが有効な手段である。そのための温度が700〜900℃であり、二相域の温度に加熱することにより、一部は焼戻しにより軟質組織となり、一部はオーステナイト相に逆変態してその後の冷却で硬質組織となる。この二相域温度の制御で硬質相の面積分率や硬度を変化させ、YS,TS,YRを制御することができる。再加熱温度が700℃未満の場合は、780MPa以上の強度を確保できない。再加熱温度が900℃を超えると、強度は高いが85%以下の低YRを達成できない。700〜900℃へ再加熱した後、一部がオーステナイトに逆変態しており、その後の焼入れ(水冷)により、オーステナイト相がそのまま硬質相に変態する。尚、この硬質相と軟質相の組織は極めて微細なため、光学顕微鏡では判別が困難であり、これら硬質相と軟質相を合わせた複合組織全体をベイニティックフェライト(ベイナイト)相とする。
[Temperature: quenching by reheating to 700 to 900 ° C]
In order to obtain a composite structure of a soft phase and a hard phase that realizes low YR characteristics, it is an effective means to heat to a temperature in a two-phase region between Ac 1 and Ac 3 . For this purpose, the temperature is 700 to 900 ° C., and by heating to a temperature in the two-phase region, a part becomes a soft structure by tempering, a part reversely transforms into an austenite phase, and a hard structure is obtained by subsequent cooling. By controlling the two-phase region temperature, the area fraction and hardness of the hard phase can be changed to control YS, TS, and YR. When the reheating temperature is lower than 700 ° C., a strength of 780 MPa or more cannot be ensured. When the reheating temperature exceeds 900 ° C., the strength is high, but a low YR of 85% or less cannot be achieved. After reheating to 700 to 900 ° C., a part is reversely transformed into austenite, and the austenite phase is transformed into a hard phase as it is by subsequent quenching (water cooling). Since the structure of the hard phase and the soft phase is extremely fine, it is difficult to discriminate with an optical microscope, and the entire composite structure including the hard phase and the soft phase is defined as a bainitic ferrite (bainite) phase.
[450〜700℃の温度範囲で焼戻し(T)をする]
焼戻し処理は、強度を低下させるが、降伏比YRを低下させ、靭性を向上させ、また表面部の硬さを低下させるのに有効である。その場合、焼戻し熱処理が450〜700℃の温度範囲であれば、強度の過度な低下を抑え、適正な降伏比YR、靭性を得ることができ表面硬さを低減できる。焼戻し温度が450℃未満であると、靭性向上と表面硬さの低下が十分ではない。一方、焼戻し温度が700℃を超えると、所望の強度(TS,YS)を得ることはできない。
[Tempering (T) in a temperature range of 450 to 700 ° C.]
The tempering treatment is effective in reducing the strength but reducing the yield ratio YR, improving the toughness, and reducing the hardness of the surface portion. In that case, if the tempering heat treatment is in a temperature range of 450 to 700 ° C., an excessive decrease in strength can be suppressed, and an appropriate yield ratio YR and toughness can be obtained, and the surface hardness can be reduced. When the tempering temperature is less than 450 ° C., the toughness is not improved and the surface hardness is not sufficiently reduced. On the other hand, if the tempering temperature exceeds 700 ° C., the desired strength (TS, YS) cannot be obtained.
[プレスベンド法によって円形鋼管に成形]
鋼板をプレス曲げ法によって、冷間曲げを行って円形鋼管とする。前述のように、ラインパイプに適用されるような板厚:30mm以下の鋼板であれば、UOE成形法によって円形鋼管が製造されるが、建築構造物用円形鋼管では、板厚が厚く、強度が高い場合には、プレスベンド法(即ち、プレス曲げ加工)によって円形鋼管に成形する必要がある。こうした方法の適用では、D/t:10〜20もの強加工を行うため、表面の曲げ加工歪が大きく、表面の加工硬化が大きい。そのため、上記のように製造した鋼板を用いて、プレス曲げ成形を行うことによって、表面硬さの低い、円形鋼管を製造することができる。
[Molded into a circular steel pipe by the press bend method]
A steel plate is cold bent by a press bending method to obtain a circular steel pipe. As described above, a steel plate having a thickness of 30 mm or less as applied to a line pipe can produce a circular steel pipe by the UOE forming method. However, a circular steel pipe for a building structure is thick and strong. Is high, it is necessary to form a circular steel pipe by a press bend method (that is, press bending). In application of such a method, D / t: Since strong processing of 10-20 is performed, the bending distortion of the surface is large and the work hardening of the surface is large. Therefore, a circular steel pipe with low surface hardness can be manufactured by performing press bending using the steel plate manufactured as described above.
[円形鋼管の熱処理]
円形鋼管への成形後、SR熱処理は実施してもしなくても良い。本発明方法によれば、高強度でYRが低く、鋼管厚方向の硬さ分布の均一性が優れているため、基本的にはSR熱処理は行わなくても良いが、D/t≦15程度の強曲げ加工を行なった場合は、YRが90%を超える可能性があるため、その場合はSR熱処理を行なうことができ、その熱処理温度は350〜650℃の温度範囲とする。350℃未満では、YR低減効果はない。一方、650℃を超えると、YR、TSの低下が大きく、780MPa以上の強度を確保できない。
[Heat treatment of round steel pipe]
After forming into a circular steel pipe, SR heat treatment may or may not be performed. According to the method of the present invention, since the strength is low, the YR is low, and the uniformity of the hardness distribution in the thickness direction of the steel pipe is excellent, the SR heat treatment is basically not necessary, but D / t ≦ 15 When the strong bending process is performed, the YR may exceed 90%. In this case, the SR heat treatment can be performed, and the heat treatment temperature is set to a temperature range of 350 to 650 ° C. Below 350 ° C., there is no YR reduction effect. On the other hand, when it exceeds 650 ° C., the decrease in YR and TS is large, and the strength of 780 MPa or more cannot be secured.
以下、実施例によって本発明をより具体的に説明するが、本発明はもとより下記実施例によって制限を受けるものではなく、前・後記の趣旨に適合し得る範囲で変更を加えて実施することは勿論可能であり、それらはいずれも本発明の技術的範囲に包含される。 Hereinafter, the present invention will be described in more detail by way of examples.However, the present invention is not limited by the following examples as a matter of course, and may be implemented with modifications within a range that can meet the gist of the preceding and following descriptions. Of course, they are all possible and are included in the technical scope of the present invention.
[実施例1]
下記表1、2に示す化学成分組成の鋼を通常の溶製方法によって溶製し、鋼片とした後、熱間圧延、加速冷却(圧延後の冷却)、二相域焼入れ、焼戻しを施し、鋼板を製造した。得られた鋼板を用いて、プレスベンド法によって円形鋼管に成形した。尚、表1、2には、前記(1)式で規定されるPCM値についても示した。このときの製造条件を、下記の通りである。
[Example 1]
Steels with the chemical composition shown in Tables 1 and 2 below are melted by a normal melting method to form steel pieces, which are then subjected to hot rolling, accelerated cooling (cooling after rolling), two-phase quenching, and tempering. A steel plate was manufactured. Using the obtained steel plate, it was formed into a circular steel pipe by the press bend method. Tables 1 and 2 also show the PCM values defined by the equation (1). The manufacturing conditions at this time are as follows.
[製造条件]
鋼No.1〜60のものについては、鋳片を1150±50℃に加熱した後、仕上げ圧延温度(表面温度)を900±30℃の範囲として熱間圧延を行ない、板厚:60mmとし、次いでt/4(t:板厚)の位置における冷却速度を5〜25℃/秒に制御し、冷却停止時の表面温度を250℃以下とした。更に、二相域熱処理(Q’)温度を700〜850℃として焼入れ処理(一部空冷)を行い、450〜650℃の温度範囲で焼戻して鋼板とし、得られ鋼板を用いてプレスベンド法によって円形鋼管に成形した。このときの曲げ加工度は、円形鋼管の直径をD(mm)、鋼板の厚さをt(mm)としたとき、D/tが10(t/D=0.1)である。
[Production conditions]
Steel No. For 1-60, after the slab was heated to 1150 ± 50 ° C., the final rolling temperature (surface temperature) was hot rolled in the range of 900 ± 30 ° C. to obtain a sheet thickness of 60 mm, and then t / The cooling rate at the position of 4 (t: plate thickness) was controlled to 5 to 25 ° C./second, and the surface temperature when cooling was stopped was 250 ° C. or less. Furthermore, a two-phase region heat treatment (Q ′) temperature is set to 700 to 850 ° C., quenching treatment (partial air cooling) is performed, and a steel plate is tempered in a temperature range of 450 to 650 ° C., and the obtained steel plate is used by a press bend method. Molded into a round steel pipe. The bending degree at this time is 10 (t / D = 0.1) when the diameter of the circular steel pipe is D (mm) and the thickness of the steel sheet is t (mm).
一方、鋼No.61〜64については、以下の条件を変えて鋼板を製造し、円形鋼管に成形した。鋼No.61は、上記条件のうち、焼戻し温度を720℃とした。鋼No.62は、上記条件のうち、二相域熱処理温度を930℃として焼入れた後の焼戻し温度を400℃とした。鋼No.63は、上記条件のうち、仕上げ圧延温度を750℃として圧延し冷却後の再加熱(Q’)温度を680℃とした。鋼No.64は、上記条件のうち、二相域熱処理を行なったままで、その後の焼戻しを行わなかった。これらの鋼板から、円形鋼管への成形は鋼No.1〜60と同様に、D/t=10で行なった。 On the other hand, Steel No. About 61-64, the following conditions were changed, the steel plate was manufactured, and it shape | molded in the circular steel pipe. Steel No. No. 61, among the above conditions, the tempering temperature was 720 ° C. Steel No. No. 62, among the above conditions, the tempering temperature after quenching with a two-phase region heat treatment temperature of 930 ° C. was set to 400 ° C. Steel No. No. 63, among the above conditions, the finish rolling temperature was rolled at 750 ° C., and the reheating (Q ′) temperature after cooling was 680 ° C. Steel No. No. 64 did not perform subsequent tempering while performing the two-phase region heat treatment among the above conditions. From these steel plates, forming into a circular steel pipe was performed using steel no. Similar to 1 to 60, D / t = 10.
得られた各円形鋼管について、鋼管のミクロ組織(各相の面積分率)および硬さを下記の方法で評価すると共に、材質(降伏強度YS、引張強さTS、降伏比YRおよび靭性vE-20)および溶接性を下記の方法によって評価した。 For each of the obtained steel tube was, while evaluating the microstructure (area fraction of each phase) and hardness of the steel tube in the following manner, the material (yield strength YS, tensile strength TS, yield ratio YR and toughness vE - 20 ) and weldability were evaluated by the following methods.
[ミクロ組織および硬さの測定方法]
ミクロ組織を画像解析により、ベイニティックフェライト相およびマルテンサイト相の面積分率を測定すると共に、鋼管表層部のビッカース硬度(Hv0)と中央部のビッカース硬度Hv1を測定し(荷重:98N)、その硬さ比(Hv0/Hv1)を求めた。このときの硬さHv0、硬さHv1の測定は、厚さ方向に2mm間隔で測定し、その平均値を求めたものである(例えば、表層部のビッカース硬さHv0は、表・裏面の夫々から深さ2mまでの硬さの平均値となる)。
[Measuring method of microstructure and hardness]
By analyzing the microstructure of the microstructure, the area fractions of the bainitic ferrite phase and martensite phase were measured, and the Vickers hardness (Hv 0 ) of the steel pipe surface layer portion and the Vickers hardness Hv 1 of the central portion were measured (load: 98 N ) And its hardness ratio (Hv 0 / Hv 1 ) was determined. In this case, the hardness Hv 0 and the hardness Hv 1 are measured at intervals of 2 mm in the thickness direction, and the average value thereof is obtained (for example, the Vickers hardness Hv 0 of the surface layer portion is expressed by It becomes the average value of hardness from each of the back surface to a depth of 2 m).
[降伏強度YS、引張強さTSの評価方法]
円形鋼管の外面側から鋼管のt/4部(tは鋼管厚:鋼管を構成する鋼板の厚さ)における管軸方向(鋼板の主圧延方向に相当)に、JIS Z 2201 4号試験片を採取してJIS Z 2241の要領で引張試験を行ない、鋼管の降伏応力YS(上降伏点YPまたは0.2%耐力σ0.2)、引張強さTS、降伏比YR(降伏応力YS/引張強度TS)を測定した。合格基準は、2回での平均値で、降伏応力YS:630MPa以上、引張強さTS:780〜930MPa、降伏比YR:90%以下である。
[Evaluation method of yield strength YS and tensile strength TS]
A test piece of JIS Z 2201 No. 4 in the tube axis direction (corresponding to the main rolling direction of the steel plate) from the outer surface side of the circular steel pipe to the t / 4 part of the steel pipe (t is the thickness of the steel pipe: the thickness of the steel plate constituting the steel pipe). Extracted and subjected to a tensile test in accordance with JIS Z 2241. Yield stress YS (upper yield point YP or 0.2% yield strength σ 0.2 ), tensile strength TS, yield ratio YR (yield stress YS / tensile strength TS). ) Was measured. The acceptance criteria are the average values of two times, yield stress YS: 630 MPa or more, tensile strength TS: 780-930 MPa, yield ratio YR: 90% or less.
[靭性評価方法]
円形鋼管の外面側から鋼管のt/4部(tは鋼管厚:鋼管を構成する鋼板の厚さ)における管軸方向(鋼板の主圧延方向)に、JIS Z 2204 Vノッチ衝撃試験片を採取してJIS Z 2242に準拠してシャルピー衝撃試験を行ない(3回試験の平均値)、温度:−20℃での平均吸収エネルギーvE-20を測定した。この平均吸収エネルギーvE-20が47J以上を合格とした。
[Toughness evaluation method]
JIS Z 2204 V-notch impact test specimens are collected from the outer surface side of the circular steel pipe in the tube axis direction (the main rolling direction of the steel sheet) in t / 4 part of the steel pipe (t is the thickness of the steel pipe constituting the steel pipe). Then, a Charpy impact test was conducted in accordance with JIS Z 2242 (average value of three tests), and an average absorbed energy vE- 20 at a temperature of −20 ° C. was measured. This average absorbed energy vE- 20 was determined to be 47 J or more.
[溶接性(耐溶接割れ性)]
JIS Z 3101に規定された溶接熱影響部(HAZ)の最高硬さ試験に準拠して、円形鋼管の外面側に溶接ビードを置き、最高硬さを測定した。また円形鋼管の外側に付属金物を溶接し、浸透探傷試験による表面割れの有無、超音波探傷試験による内部割れの有無について調査した。
[Weldability (weld crack resistance)]
In accordance with the maximum hardness test of the weld heat affected zone (HAZ) defined in JIS Z 3101, a weld bead was placed on the outer surface side of the circular steel pipe, and the maximum hardness was measured. In addition, attachments were welded to the outside of the circular steel pipe, and the presence or absence of surface cracks by penetration testing and the presence of internal cracks by ultrasonic testing were investigated.
鋼板のミクロ組成および硬さ分布(鋼板中央部の硬さ、硬さ比)を下記表3、4に、材質(降伏応力YS、引張強さTS、降伏比YRおよび靭性vE-20)および溶接性の評価結果を下記表5、6に示す。尚、下記表5、6には、「溶接性」として、HAZの最高硬さ(Hv)を示した。 The micro composition and hardness distribution of the steel sheet (hardness and hardness ratio at the center of the steel sheet) are shown in Tables 3 and 4 below, and the materials (yield stress YS, tensile strength TS, yield ratio YR and toughness vE -20 ) and welding. The evaluation results of the properties are shown in Tables 5 and 6 below. Tables 5 and 6 below show the maximum hardness (Hv) of HAZ as “weldability”.
これらの結果から、次のように考察できる。まず、鋼No.1〜32のもの(表1、3、5)は、本発明で規定する要件を満足するものであり、全ての特性において目標値を満足するものとなっている(総合評価:○)。 From these results, it can be considered as follows. First, steel no. 1 to 32 (Tables 1, 3, and 5) satisfy the requirements defined in the present invention, and satisfy the target values in all the characteristics (overall evaluation: ◯).
これに対して、鋼No.33〜64のもの(表2、4、6)では、本発明で規定するいずれかの要件を満足しないものであり、少なくともいずれかの要求特性が劣化している(総合評価×)。 On the other hand, Steel No. 33 to 64 (Tables 2, 4, and 6) do not satisfy any of the requirements defined in the present invention, and at least any of the required characteristics is deteriorated (overall evaluation ×).
[実施例2]
前記表1に示した鋼No.1〜11のもの(化学成分組成が本発明で規定する範囲を満足するもの)を用い、下記表7に示す各種製造条件(DQ−Q’−T)によって、鋼板を製造した(実験No.1〜21)。得られた鋼板(板厚:60mm)を用いて、プレスベンド法によって円形鋼管に成形した。得られた円形鋼管について、実施例1と同様にして材質(降伏応力YS、引張強さTS、降伏比YRおよび靭性vE-20)および溶接性を評価した。
[Example 2]
Steel No. 1 shown in Table 1 above. Steel sheets were produced under the various production conditions (DQ-Q'-T) shown in Table 7 below (Experiment No. 1) using the materials of Nos. 1 to 11 (with the chemical component composition satisfying the range specified in the present invention). 1-21). Using the obtained steel plate (plate thickness: 60 mm), it was formed into a circular steel pipe by the press bend method. About the obtained circular steel pipe, it carried out similarly to Example 1, and evaluated the material (yield stress YS, tensile strength TS, yield ratio YR, toughness vE- 20 ), and weldability.
尚、表7の実験No.12、13は鋼片加熱温度が本発明で規定する範囲を外れるもの、実験No.14、15は仕上げ圧延温度が本発明で規定する範囲を外れるもの、実験No.15、16は冷却速度が本発明で規定する範囲を外れるもの、実験No.17は冷却停止温度が本発明で規定する範囲を外れるもの、実験No.18、19は焼入れ温度(焼入れ時の加熱温度)が本発明で規定する範囲を外れるもの、実験No.20、21は焼戻し温度が本発明で規定する範囲を外れるもの、を夫々示している。 In addition, the experiment No. Nos. 12 and 13 are those in which the slab heating temperature deviates from the range specified in the present invention. Nos. 14 and 15 are those in which the finish rolling temperature is outside the range defined in the present invention. Nos. 15 and 16 are those in which the cooling rate is outside the range defined in the present invention. No. 17 has a cooling stop temperature outside the range defined in the present invention. Nos. 18 and 19 are those in which the quenching temperature (heating temperature during quenching) is outside the range defined in the present invention. Reference numerals 20 and 21 denote tempering temperatures that are outside the range defined by the present invention.
この結果から明らかなように、本発明で規定する要件を満足する円形鋼管を得るためには、製造条件も適切に制御する必要があることが分かる。 As is apparent from the results, it is understood that the manufacturing conditions must be appropriately controlled in order to obtain a circular steel pipe that satisfies the requirements defined in the present invention.
Claims (2)
PCM値=[C]+[Si]/30+[Mn]/20+[Cu]/20+[Ni]/60+[Cr]/20+[Mo]/15+[V]/10+([B]×5) …(1)
但し、[C],[Si],[Mn],[Cu],[Ni],[Cr],[Mo],[V]および[B]は、夫々C,Si,Mn,Cu,Ni,Cr,Mo,VおよびBの含有量(質量%)を示す。
(A)鋼管の表・裏面の夫々から深さ2mmまでの表層部を除く中央部の平均ビッカース硬さHvが230〜310である、
(B)鋼管のミクロ組織において、ベイニティックフェライト相の分率が80面積%以上であり、マルテンサイト相の分率が5面積%以下である、
(C)鋼管の表・裏面の夫々から深さ2mmまでの表層部の平均ビッカース硬さHvが、前記中央部の平均ビッカース硬さHvの1.3倍以下である。 C: 0.01-0.06% (meaning of mass%, the same applies hereinafter), Si: 0.10-0.40%, Mn: 1.60-2.50%, Al: 0.025-0. 090%, Cu: 0.15 to 0.70%, Ni: 0.90 to 1.60%, Cr: 0.50 to 1.35%, Mo: 0.10 to 0.30%, Ti: 0 0.008 to 0.025%, B: 0.0005 to 0.0025%, N: 0.0030 to 0.0060% and Ca: 0.0005 to 0.0040%, respectively, and (1) The PCM value represented by the formula is 0.30% or less, the balance is made of Fe and inevitable impurities, and P: 0.012% or less (not including 0%) of the inevitable impurities, S: 0.0. 005% or less (excluding 0%) and O: 0.0040% or less (not including 0%), respectively, One following (A) ~ (C) good architectural structural 780MPa class low yield ratio steel tube earthquake resistance which satisfies the requirements.
PCM value = [C] + [Si] / 30 + [Mn] / 20 + [Cu] / 20 + [Ni] / 60 + [Cr] / 20 + [Mo] / 15 + [V] / 10 + ([B] × 5) (1)
However, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V] and [B] are respectively C, Si, Mn, Cu, Ni, Content (mass%) of Cr, Mo, V, and B is shown.
(A) The average Vickers hardness Hv of the central part excluding the surface layer part from each of the front and back surfaces of the steel pipe to a depth of 2 mm is 230 to 310.
(B) In the microstructure of the steel pipe, the fraction of bainitic ferrite phase is 80 area% or more, and the fraction of martensite phase is 5 area% or less.
(C) The average Vickers hardness Hv of the surface layer portion from each of the front and back surfaces of the steel pipe to a depth of 2 mm is 1.3 times or less of the average Vickers hardness Hv of the central portion.
Priority Applications (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP2009031545A JP5368820B2 (en) | 2008-03-27 | 2009-02-13 | 780 MPa class low yield ratio circular steel pipe for building structure having excellent earthquake resistance and method for producing the same |
Applications Claiming Priority (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP2008084465 | 2008-03-27 | ||
JP2008084465 | 2008-03-27 | ||
JP2009031545A JP5368820B2 (en) | 2008-03-27 | 2009-02-13 | 780 MPa class low yield ratio circular steel pipe for building structure having excellent earthquake resistance and method for producing the same |
Publications (2)
Publication Number | Publication Date |
---|---|
JP2009256780A true JP2009256780A (en) | 2009-11-05 |
JP5368820B2 JP5368820B2 (en) | 2013-12-18 |
Family
ID=41192451
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
JP2009031545A Active JP5368820B2 (en) | 2008-03-27 | 2009-02-13 | 780 MPa class low yield ratio circular steel pipe for building structure having excellent earthquake resistance and method for producing the same |
Country Status (2)
Country | Link |
---|---|
JP (1) | JP5368820B2 (en) |
CN (1) | CN101545081B (en) |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JP2014177669A (en) * | 2013-03-14 | 2014-09-25 | Jfe Steel Corp | Non-heat treated low yield ratio high tensile thick steel plate, and manufacturing method therefor |
JP2021181589A (en) * | 2020-05-18 | 2021-11-25 | 株式会社神戸製鋼所 | Steel sheet and method for producing the same |
Families Citing this family (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
EP3680358A1 (en) * | 2017-09-08 | 2020-07-15 | JFE Steel Corporation | Steel sheet and method for producing same |
JP6816827B2 (en) * | 2018-03-16 | 2021-01-20 | Jfeスチール株式会社 | Manufacturing method of square steel pipe and square steel pipe |
JP6418361B1 (en) * | 2018-03-16 | 2018-11-07 | 新日鐵住金株式会社 | Steel plate for holding coal and ore carrier |
CN108588581B (en) * | 2018-04-28 | 2019-09-20 | 东南大学 | A kind of marine concrete structure high strength anti-corrosion ferrite/bainite two-phase reinforcing bar and preparation method thereof |
Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JPH04314824A (en) * | 1991-04-02 | 1992-11-06 | Nippon Steel Corp | Production of 70kgf/mm2 class high tensile strength steel excelent in weldability and having low yield ratio |
JP2005213534A (en) * | 2004-01-27 | 2005-08-11 | Jfe Steel Kk | Method for producing steel material excellent in toughness at welding heat affected zone |
JP2006213976A (en) * | 2005-02-04 | 2006-08-17 | Sumitomo Metal Ind Ltd | High tensile strength steel having excellent weldability and joint toughness |
-
2009
- 2009-02-13 JP JP2009031545A patent/JP5368820B2/en active Active
- 2009-03-26 CN CN2009101296952A patent/CN101545081B/en active Active
Patent Citations (3)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JPH04314824A (en) * | 1991-04-02 | 1992-11-06 | Nippon Steel Corp | Production of 70kgf/mm2 class high tensile strength steel excelent in weldability and having low yield ratio |
JP2005213534A (en) * | 2004-01-27 | 2005-08-11 | Jfe Steel Kk | Method for producing steel material excellent in toughness at welding heat affected zone |
JP2006213976A (en) * | 2005-02-04 | 2006-08-17 | Sumitomo Metal Ind Ltd | High tensile strength steel having excellent weldability and joint toughness |
Non-Patent Citations (1)
Title |
---|
JPN6013029542; 小林克壮、他2名: '建築構造用高性能鋼管KSAT440およびKSAT630' 神戸製鋼技報 Vol.58 No.1, 20080402, Page.52-56 * |
Cited By (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JP2014177669A (en) * | 2013-03-14 | 2014-09-25 | Jfe Steel Corp | Non-heat treated low yield ratio high tensile thick steel plate, and manufacturing method therefor |
JP2021181589A (en) * | 2020-05-18 | 2021-11-25 | 株式会社神戸製鋼所 | Steel sheet and method for producing the same |
Also Published As
Publication number | Publication date |
---|---|
JP5368820B2 (en) | 2013-12-18 |
CN101545081A (en) | 2009-09-30 |
CN101545081B (en) | 2010-12-08 |
Similar Documents
Publication | Publication Date | Title |
---|---|---|
RU2623551C2 (en) | High strength sheet steel having low ratio of yield strength to tensile strength, excellent as of resistance to post-deformation aging, method of its production and high strength welded steel pipe made thereof | |
JP5516784B2 (en) | Low yield ratio high strength steel sheet, method for producing the same, and high strength welded steel pipe using the same | |
JP4853575B2 (en) | High strength steel pipe for low temperature excellent in buckling resistance and weld heat affected zone toughness and method for producing the same | |
RU2502820C1 (en) | Plate steel characterised by low ratio between yield point and ultimate strength, high strength and high uniform relative elongation, and method for its manufacture | |
KR100934405B1 (en) | High strength thick steel sheet and manufacturing method thereof, and high strength steel pipe | |
JP5217556B2 (en) | High strength steel pipe for low temperature excellent in buckling resistance and weld heat affected zone toughness and method for producing the same | |
JP5146051B2 (en) | Plate thickness excellent in toughness and deformability: Steel material for high-strength steel pipes of 25 mm or more and method for producing the same | |
JP2008261046A (en) | High-tensile steel excellent in weldability and plastic deformability, and cold-formed steel pipe formed therefrom | |
JP5141073B2 (en) | X70 grade or less low yield ratio high strength high toughness steel pipe and method for producing the same | |
JP5385760B2 (en) | Cold-formed square steel pipe with excellent earthquake resistance | |
JP5480215B2 (en) | Steel plate for low yield ratio thick circular steel pipe having a tensile strength of 780 MPa or more, manufacturing method thereof, and low yield ratio thick circular steel pipe having a tensile strength of 780 MPa or more | |
CA2775043A1 (en) | Low yield ratio, high strength and high toughness steel plate and method for manufacturing the same | |
JP2006291349A (en) | Line pipe steel sheet having high deformation performance and its manufacturing method | |
JP5407478B2 (en) | High-strength thick steel plate with excellent toughness of heat-affected zone of single layer large heat input welding and method for producing the same | |
JP2011052271A (en) | High-strength cold-rolled steel sheet having excellent workability, and method for producing the same | |
JP5368820B2 (en) | 780 MPa class low yield ratio circular steel pipe for building structure having excellent earthquake resistance and method for producing the same | |
JP4547944B2 (en) | Manufacturing method of high strength and high toughness thick steel plate | |
JP2009235516A (en) | 590 MPa CLASS HIGH YIELD RATIO CIRCULAR STEEL PIPE FOR BUILDING STRUCTURE HAVING EXCELLENT EARTHQUAKE RESISTANCE, AND METHOD FOR PRODUCING THE SAME | |
JP7115200B2 (en) | Steel plate for line pipe | |
JP2008248359A (en) | Method for manufacturing on-line cooling type high tension steel sheet | |
KR101096992B1 (en) | 780MPa CLASS LOW YIELD RATIO CIRCULAR STEEL FOR CONSTRUCTION STRUCTURE EXCELLENT IN EARTHQUAKE-PROOF PERFORMANCE, AND PROCESS FOR PRODUCING THE SAME | |
JP2006265722A (en) | Production method of steel sheet for high-tension linepipe | |
JP4824142B2 (en) | Steel for line pipe with good strength and ductility and method for producing the same | |
JP7163777B2 (en) | Steel plate for line pipe | |
JP2012036499A (en) | High-tensile strength steel sheet having superior bending property and low-temperature toughness, and method for manufacturing the same |
Legal Events
Date | Code | Title | Description |
---|---|---|---|
A621 | Written request for application examination |
Free format text: JAPANESE INTERMEDIATE CODE: A621 Effective date: 20110901 |
|
A977 | Report on retrieval |
Free format text: JAPANESE INTERMEDIATE CODE: A971007 Effective date: 20130611 |
|
A131 | Notification of reasons for refusal |
Free format text: JAPANESE INTERMEDIATE CODE: A131 Effective date: 20130618 |
|
A521 | Written amendment |
Free format text: JAPANESE INTERMEDIATE CODE: A523 Effective date: 20130807 |
|
TRDD | Decision of grant or rejection written | ||
A01 | Written decision to grant a patent or to grant a registration (utility model) |
Free format text: JAPANESE INTERMEDIATE CODE: A01 Effective date: 20130910 |
|
A61 | First payment of annual fees (during grant procedure) |
Free format text: JAPANESE INTERMEDIATE CODE: A61 Effective date: 20130913 |
|
R150 | Certificate of patent or registration of utility model |
Ref document number: 5368820 Country of ref document: JP Free format text: JAPANESE INTERMEDIATE CODE: R150 Free format text: JAPANESE INTERMEDIATE CODE: R150 |