JP5742123B2 - High-tensile hot-rolled steel sheet for high-strength welded steel pipe for line pipe and method for producing the same - Google Patents
High-tensile hot-rolled steel sheet for high-strength welded steel pipe for line pipe and method for producing the same Download PDFInfo
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Description
本発明は、原油、天然ガス等を輸送するラインパイプ用として、高強度、高靭性が要求される溶接鋼管、なかでも高強度電縫鋼管あるいは高強度スパイラル鋼管等の溶接鋼管向け素材として好適な、高張力熱延鋼板およびその製造方法に係り、とくに造管後の管長手方向の低降伏化に関する。
なお、ここでいう「高強度」とは、API5L−X65級以上X80級以下の高強度をいうものとする。なお、「鋼板」は、鋼板および鋼帯を含むものとする。
INDUSTRIAL APPLICABILITY The present invention is suitable as a material for welded steel pipes, such as welded steel pipes that require high strength and high toughness, especially for welded steel pipes such as high-strength ERW steel pipes or high-strength spiral steel pipes, for line pipes that transport crude oil, natural gas, etc. In particular, the present invention relates to a high-tensile hot-rolled steel sheet and a method for producing the same, and particularly to lowering the yield in the longitudinal direction of the pipe after pipe making.
Here, “high strength” means high strength of API5L-X65 grade or more and X80 grade or less. The “steel plate” includes a steel plate and a steel strip.
近年、石油危機以来の原油の高騰や、エネルギー供給源の多様化の要求などから、北海、カナダ、アラスカ等のような極寒地での石油、天然ガスの採掘およびパイプラインの敷設が活発に行われるようになっている。さらに、パイプラインにおいては、天然ガスやオイルの輸送効率向上のため、大径で高圧操業を行う傾向となっている。パイプラインの高圧操業に耐えるため、輸送管(ラインパイプ)は厚肉の鋼管とする必要があり、厚鋼板を素材とするUOE鋼管が使用され、さらにAPI5L規格のX80といった高強度グレードの鋼管が使用されるようになってきている。しかし、最近では、パイプラインの施工コストの更なる低減という強い要望や、UOE鋼管の供給能力不足などのために、鋼管の材料コスト低減の要求も強く、輸送管として、厚鋼板を素材とするUOE鋼管に代わり、生産性が高くより安価な、コイル形状の熱延鋼板(熱延鋼帯)を素材とした高強度電縫鋼管あるいは高強度スパイラル鋼管が用いられるようになってきた。 In recent years, oil and natural gas mining and pipeline construction have been actively carried out in extremely cold regions such as the North Sea, Canada and Alaska due to soaring crude oil since the oil crisis and the demand for diversified energy supply sources. It has come to be. Furthermore, in the pipeline, in order to improve the transportation efficiency of natural gas and oil, there is a tendency to perform high-pressure operation with a large diameter. In order to withstand the high-pressure operation of the pipeline, the transport pipe (line pipe) must be a thick steel pipe, UOE steel pipe made of thick steel plate is used, and steel pipe of high strength grade such as API5L standard X80 is used. It is becoming used. However, recently, due to the strong demand for further reduction of pipeline construction costs and the lack of supply capacity of UOE steel pipes, there is a strong demand for reducing the material cost of steel pipes. Instead of UOE steel pipes, high-strength ERW steel pipes or high-strength spiral steel pipes made of coil-shaped hot-rolled steel sheets (hot-rolled steel strips), which are more productive and cheaper, have come to be used.
これら高強度グレードの鋼管には、ラインパイプの破壊を防止する観点から、同時に優れた低温靭性を保持することが要求されている。このような高強度と高靭性とを兼備した鋼管を製造するために、鋼管素材である鋼板では、熱間圧延後の加速冷却を利用した変態強化や、Nb、V、Ti等の合金元素の析出物を利用した析出強化等による高強度化と、制御圧延等を利用した組織の微細化等による高靭性化が図られてきた。 These high-strength grade steel pipes are required to maintain excellent low-temperature toughness at the same time from the viewpoint of preventing breakage of the line pipe. In order to produce a steel pipe having both such high strength and high toughness, in steel sheets that are steel pipe materials, transformation strengthening using accelerated cooling after hot rolling and alloying elements such as Nb, V, Ti, etc. Strengthening by precipitation strengthening using precipitates and toughness by microstructure refinement using controlled rolling have been attempted.
例えば、特許文献1には、C:0.01〜0.07%、Si:0.5%以下、Mn:0.5〜2.0%、Nb,V,Tiの1種または2種以上を含有する鋼を、熱間圧延を完了したのち、20℃/s以上の冷却速度で冷却する加速冷却を施し、250℃以下の温度で巻き取る高強度電縫鋼管用鋼の製造方法が記載されている。しかし、特許文献1に記載された技術では、硬質な低温変態相による強化により高強度化を図るため、250℃以下という極低温での巻取りを必須の要件としている。このため、コイル状に巻取ることが困難となる場合が多く、コイル形状の悪化を招き、生産性が極めて低下するという問題があった。 For example, in Patent Document 1, steel containing C: 0.01 to 0.07%, Si: 0.5% or less, Mn: 0.5 to 2.0%, one or more of Nb, V, and Ti is hot-rolled. A method for producing high-strength ERW steel pipe steel that is subjected to accelerated cooling that is cooled at a cooling rate of 20 ° C./s or higher and wound at a temperature of 250 ° C. or lower is described. However, in the technique described in Patent Document 1, winding at an extremely low temperature of 250 ° C. or lower is an indispensable requirement in order to increase the strength by strengthening with a hard low-temperature transformation phase. For this reason, there are many cases where it is difficult to wind in a coil shape, which causes a problem that the coil shape is deteriorated and productivity is extremely lowered.
また、特許文献2には、C、Si、Mn、Nを適正量含有し、さらにSi、MnをMn/Siが5〜8を満足する範囲において含有し、さらにNb:0.01〜0.1%を含有する鋼片を、加熱後、1100℃以上で行う最初の圧延の圧下率:15〜30%、1000℃以上での合計圧下率:60%以上、最終圧延の圧下率:15〜30%の条件下で粗圧延を行ったのち、いったん5℃/s以上の冷却速度で、表層部の温度をAr1点以下まで冷却しついで、復熱または強制加熱で表層部の温度が(Ac3−40℃)〜(Ac3+40℃)となった時点で仕上圧延を開始し、950℃以下での合計圧下率:60%以上、圧延終了温度:Ar3点以上の条件で仕上圧延を終了し、仕上圧延終了後2s以内に冷却を開始し、10℃/s以上の速度で600℃以下まで冷却し、600〜350℃の温度範囲で巻き取る高強度電縫鋼管用熱延鋼板の製造方法が記載されている。特許文献2に記載された技術で製造された鋼板は、高価な合金元素を添加することなく、また鋼管全体を熱処理することなく、鋼板表層の組織が微細化され、低温靭性、とくにDWTT特性に優れた高強度電縫鋼管が製造できるとしている。しかし、特許文献2に記載された技術では、板厚が厚い鋼板では、所望の冷却速度を確保できなくなり、所望の特性を確保するためには、さらなる冷却能力の向上を必要とするという問題があった。 Patent Document 2 contains appropriate amounts of C, Si, Mn, and N, and further contains Si and Mn in a range where Mn / Si satisfies 5 to 8, and further contains Nb: 0.01 to 0.1%. After rolling the steel slab, the rolling ratio of the first rolling performed at 1100 ° C or higher: 15-30%, the total rolling ratio at 1000 ° C or higher: 60% or higher, the final rolling reduction ratio: 15-30% After performing rough rolling under, once at 5 ° C. / s or more cooling rate, cooling temperature of the surface layer portion to below 1 point Ar then the temperature of the surface layer portion at recuperation or forced heating (Ac 3 -40 C.) to (Ac 3 + 40 ° C.), and finish rolling was started, and finish rolling was completed under the conditions of a total rolling reduction at 950 ° C. or less: 60% or more and a rolling end temperature: Ar of 3 points or more, Manufacture of hot-rolled steel sheets for high-strength ERW steel pipes, starting cooling within 2 s after finishing rolling, cooling to 600 ° C or lower at a rate of 10 ° C / s or higher, and winding in a temperature range of 600 to 350 ° C The method has been described. The steel sheet manufactured by the technique described in Patent Document 2 is refined in the structure of the steel sheet surface layer without adding an expensive alloy element or heat-treating the entire steel pipe, resulting in low temperature toughness, particularly DWTT characteristics. An excellent high-strength ERW steel pipe can be manufactured. However, with the technique described in Patent Document 2, it is impossible to secure a desired cooling rate with a thick steel plate, and there is a problem that further improvement in cooling capacity is required to secure desired characteristics. there were.
また、特許文献3には、C、Si、Mn、Al、Nを適正量含有し、さらにNb:0.001〜0.1%、V:0.001〜0.1%、Ti:0.001〜0.1%を含み、Cu、Ni、Moのうちの1種または2種以上を含有し、Pcm値が0.17以下である鋼スラブを、加熱したのち、表面温度が(Ar3−50℃)以上の条件で仕上圧延を終了し、圧延後直ちに冷却し700℃以下の温度で巻き取る低温靭性および溶接性に優れた高強度電縫管用熱延鋼帯の製造方法が記載されている。 Patent Document 3 contains appropriate amounts of C, Si, Mn, Al, and N, and further includes Nb: 0.001 to 0.1%, V: 0.001 to 0.1%, Ti: 0.001 to 0.1%, Cu, Ni After heating a steel slab containing one or more of Mo and having a Pcm value of 0.17 or less, finish rolling is finished under conditions where the surface temperature is (A r3 −50 ° C.) or more, A method for producing a hot-rolled steel strip for a high-strength ERW pipe excellent in low-temperature toughness and weldability that is immediately cooled after rolling and wound at a temperature of 700 ° C. or less is described.
最近、高強度溶接鋼管用鋼板には、更なる低温靭性の向上が要求されており、特許文献3に記載された技術では、要求される低温靭性を充分に満足することができないという問題もあった。
一般的に、高強度化に伴い降伏強さも高くなり、降伏比が高くなる傾向を示す。そのため、構造部材の破壊に至るまでの変形能が低下することが懸念される。所望の変形能を確保し、パイプライン等の構造物の安全性を確保する観点から、ラインパイプ用高強度溶接鋼管においても、降伏比の低下が要望され、低降伏比高強度溶接鋼管が強く求められていた。
Recently, steel sheets for high-strength welded steel pipes have been required to further improve low-temperature toughness, and the technique described in Patent Document 3 has a problem that the required low-temperature toughness cannot be sufficiently satisfied. It was.
Generally, with increasing strength, the yield strength increases and the yield ratio tends to increase. Therefore, there is a concern that the deformability until the structural member is broken is lowered. From the viewpoint of ensuring the desired deformability and ensuring the safety of structures such as pipelines, high strength welded steel pipes for line pipes are also required to have a reduced yield ratio, and the low yield ratio and high strength welded steel pipes are strong. It was sought after.
本発明は、上記した従来技術の問題を解決し、多量の合金元素添加を必要とすることなく、X65級以上の高強度と、優れた低温靭性とを兼備し、さらに管長手方向の降伏比が85%以下の低降伏比となる高強度電縫鋼管用素材として好適な、高張力熱延鋼板およびその製造方法を提供することを目的とする高張力鋼板は、引張強さTS:520MPa以上の高強度と破面遷移温度Trs50:−80℃以下の高靭性とを兼備し、さらに造管後に85%以下の降伏比が実現できる、高張力熱延鋼板である。 The present invention solves the above-mentioned problems of the prior art, combines high strength of X65 grade or higher and excellent low temperature toughness without the need for adding a large amount of alloying elements, and further yield ratio in the longitudinal direction of the pipe Is a high strength hot-rolled steel sheet suitable for use as a material for high-strength ERW steel pipes with a low yield ratio of 85% or less, and a high-strength steel sheet intended to provide a method for producing the same, has a tensile strength of TS: 520 MPa or more. Is a high-strength hot-rolled steel sheet that has both high strength and fracture surface transition temperature Trs 50 : high toughness of −80 ° C. or less and can achieve a yield ratio of 85% or less after pipe forming.
本発明者らは、上記した目的を達成するため、降伏比に及ぼす各種要因の影響について鋭意研究した。その結果、造管後に低降伏比を実現するためには、造管時に導入される歪で、内部に可動転位が導入されることが必要であることに思い至った。そして、造管時に導入される歪で、内部に可動転位を導入させるためには、内部組織を微細なマルテンサイト相を適正量分散させた組織とする必要があることを想到した。さらに、造管後の低降伏比と、高強度と高靭性とを兼備させるためには、鋼板組織を板厚方向で異なる複層の組織とする必要があることを知見した。 In order to achieve the above-mentioned object, the present inventors diligently studied the influence of various factors on the yield ratio. As a result, in order to realize a low yield ratio after pipe making, it was thought that it was necessary to introduce movable dislocations inside with strain introduced during pipe making. In order to introduce movable dislocations into the interior with strain introduced during pipe making, the inventors have conceived that the internal structure needs to be a structure in which an appropriate amount of fine martensite phase is dispersed. Furthermore, in order to combine the low yield ratio after pipe making with high strength and high toughness, it has been found that the steel sheet structure needs to be a multi-layer structure different in the sheet thickness direction.
本発明は、上記した知見に基づき、さらに検討を加えて完成されたものである。すなわち、本発明の要旨は次のとおりである。
(1)質量%で、C:0.04〜0.08%、Si:0.4%以下、Mn:1.0〜1.8%、Al:0.1%以下、Nb:0.02〜0.08%、Cr:0.3〜0.8%を含み、残部Feおよび不可避的不純物からなる組成と、表面から1mmの位置である表層にベイナイト相が体積率で98%以上存在し、板厚中央位置である内層にベイナイト相が主相として体積率で70%以上存在し、第二相として平均粒径:3μm以下のマルテンサイト相が体積率で1〜4%分散し、第三相として体積率で0〜30%のフェライト相を含む板厚方向に複層の組織を有し、かつ引張強さTS:520MPa以上、破面遷移温度Trs 50 :−80℃以下を兼備するとともに、造管後の降伏比が85%以下であることを特徴とするラインパイプ用溶接鋼管向け高張力熱延鋼板。
The present invention has been completed based on the above findings and further studies. That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.04 to 0.08%, Si: 0.4% or less, Mn: 1.0 to 1.8%, Al: 0.1% or less, Nb: 0.02 to 0.08%, Cr: 0.3 to 0.8%, the balance a composition consisting of Fe and unavoidable impurities, bainite phase is present more than 98% by volume from the surface to the surface layer is a position of 1 mm, a volume rate bainite phase as a main phase in the inner layer is a thickness center position 70 occur more than%, an average particle diameter as the second phase: 3 [mu] m or less of martensite phase is dispersed 1-4% by volume, in the thickness direction comprising 0-30% ferrite phase by volume as the third phase have a structure of multiple layers, and tensile strength TS: 520 MPa or more, fracture appearance transition temperature Trs 50: while combines the -80 ° C. or less, the yield ratio after pipe is equal to or less than 85% High-tensile hot-rolled steel sheet for welded steel pipes for line pipes.
(2)(1)において、前記組成に加えてさらに、質量%で、Cu:0.3%以下、Ni:0.3%以下、Mo:0.3%以下、V:0.1%以下、Ti:0.03%以下、B:0.0010%以下のうちから選ばれた1種または2種以上を合計で1.0%以下含有する組成とすることを特徴とする高張力熱延鋼板。
(3)(1)または(2)において、前記組成に加えてさらに、質量%で、Ca:0.005%以下、REM:0.005%以下のうちから選ばれた1種または2種を含有する組成とすることを特徴とする高張力熱延鋼板。
(2) In (1), in addition to the above composition, in terms of mass%, Cu: 0.3% or less, Ni: 0.3% or less, Mo: 0.3% or less, V: 0.1% or less, Ti: 0.03% or less, B : A high-tensile hot-rolled steel sheet characterized by containing a total of 1.0% or less of one or more selected from 0.0010% or less.
(3) In (1) or (2), in addition to the above-mentioned composition, the composition further contains one or two kinds selected from Ca: 0.005% or less and REM: 0.005% or less by mass% A high-tensile hot-rolled steel sheet characterized by:
(4)質量%で、C:0.04〜0.08%、Si:0.4%以下、Mn:1.0〜1.8%、Al:0.1%以下、Nb:0.02〜0.08%、Cr:0.3〜0.8%を含み、残部Feおよび不可避的不純物からなる組成の鋼素材に、加熱したのち、950℃以下の温度域における累積圧下率が45%以上で、仕上圧延終了温度が(Ar3変態点−50℃)以上900℃以下となる熱間圧延を施し、該熱間圧延終了後,15s以内に、板厚中央位置の冷却速度で、5〜20℃/sの範囲の平均冷却速度で400〜600℃の温度域まで冷却する加速冷却処理を施し、該加速冷却処理終了後15s以上、放冷する放冷処理を施したのち、コイル状に巻き取って、(1)に記載の高張力熱延鋼板を製造することを特徴とするラインパイプ用溶接鋼管向け高張力熱延鋼板の製造方法。 (4) In mass%, C: 0.04 to 0.08%, Si: 0.4% or less, Mn: 1.0 to 1.8%, Al: 0.1% or less, Nb: 0.02 to 0.08%, Cr: 0.3 to 0.8%, the balance After heating a steel material composed of Fe and inevitable impurities, the cumulative rolling reduction in the temperature range of 950 ° C or lower is 45% or higher, and the finish rolling finish temperature is (Ar 3 transformation point -50 ° C) or higher and 900 ° C The following hot rolling is performed, and within 15 s after the hot rolling is completed, the cooling rate at the center position of the plate thickness is within a temperature range of 400 to 600 ° C. with an average cooling rate in the range of 5 to 20 ° C./s. A high-tensile hot-rolled steel sheet according to (1 ) is manufactured by performing an accelerated cooling process for cooling, performing a cooling process for cooling for 15 seconds or more after completion of the accelerated cooling process, and then winding it in a coil shape. A method for producing high-tensile hot-rolled steel sheets for welded steel pipes for line pipes.
(5)(4)において、前記組成に加えてさらに、質量%で、Cu:0.3%以下、Ni:0.3%以下、Mo:0.3%以下、V:0.1%以下、Ti:0.03%以下、B:0.0010%以下のうちから選ばれた1種または2種以上を合計で1.0%以下含有する組成とすることを特徴とする高張力熱延鋼板の製造方法。
(6)(4)または(5)において、前記組成に加えてさらに、質量%で、Ca:0.005%以下、REM:0.005%以下のうちから選ばれた1種または2種を含有する組成とすることを特徴とする高張力熱延鋼板の製造方法。
(5) In (4), in addition to the above composition, in terms of mass%, Cu: 0.3% or less, Ni: 0.3% or less, Mo: 0.3% or less, V: 0.1% or less, Ti: 0.03% or less, B : A method for producing a high-tensile hot-rolled steel sheet, characterized in that the composition contains 1.0% or less in total of one or more selected from 0.0010% or less.
(6) In (4) or (5), in addition to the above composition, the composition further contains one or two kinds selected from Ca: 0.005% or less and REM: 0.005% or less by mass%. A method for producing a high-tensile hot-rolled steel sheet.
本発明によれば、ラインパイプ用高強度溶接鋼管向け素材として好適な、引張強さTS:520MPa以上の高強度と破面遷移温度Trs50:−80℃以下の高靭性とを兼備し、さらに造管後に85%以下の低降伏比が実現できる、高張力熱延鋼板を、容易にしかも安価に製造でき、産業上格段の効果を奏する。本発明によれば、X65級以上の高強度と、優れた低温靭性とを兼備し、少なくとも管長手方向の降伏比が85%以下となる低降伏比を有するラインパイプ用高強度溶接鋼管(電縫鋼管)を、容易に製造できるという効果もある。 According to the present invention, it is suitable as a material for high-strength welded steel pipes for line pipes, and has both high strength of tensile strength TS: 520 MPa or higher and fracture surface transition temperature Trs 50 : high toughness of −80 ° C. or lower. A high-tensile hot-rolled steel sheet that can realize a low yield ratio of 85% or less after pipe making can be manufactured easily and inexpensively, and has a remarkable industrial effect. According to the present invention, a high-strength welded steel pipe for line pipe (electrical pipe) having both high strength of X65 grade or higher and excellent low-temperature toughness, and at least a yield ratio of 85% or less in the longitudinal direction of the pipe. There is also an effect that the sewn steel pipe) can be easily manufactured.
まず、本発明高張力熱延鋼板の組成限定理由について説明する。なお、とくに断らない限り質量%は単に%で記す。
C:0.04〜0.08%
Cは、鋼の強度を上昇させる作用を有する元素であり、本発明では所望の高強度を確保するために、0.04%以上の含有を必要とする。一方、0.08%を超える過剰な含有は、母材靭性および溶接熱影響部靭性を低下させる。このため、Cは0.04〜0.08%の範囲に限定した。なお、好ましくは0.05〜0.07%である。
First, the reasons for limiting the composition of the high-tensile hot-rolled steel sheet of the present invention will be described. Unless otherwise specified, mass% is simply expressed as%.
C: 0.04-0.08%
C is an element having an action of increasing the strength of steel. In the present invention, it is necessary to contain 0.04% or more in order to ensure a desired high strength. On the other hand, an excessive content exceeding 0.08% lowers the base metal toughness and the weld heat affected zone toughness. For this reason, C was limited to 0.04 to 0.08% of range. In addition, Preferably it is 0.05 to 0.07%.
Si:0.4%以下
Siは、脱酸剤として作用する元素であり、このような効果は0.01%以上の含有で認められる。また、Siは、電縫溶接時にSiを含有する酸化物を形成し、溶接部品質を低下させるとともに、溶接熱影響部靭性を低下させる。このような観点から、Siはできるだけ低減することが望ましいが、0.4%までは許容できる。このようなことから、Siは0.4%以下に限定した。
Si: 0.4% or less
Si is an element that acts as a deoxidizer, and such an effect is recognized with a content of 0.01% or more. Moreover, Si forms an oxide containing Si during ERW welding, lowers the weld zone quality, and lowers the weld heat affected zone toughness. From this point of view, it is desirable to reduce Si as much as possible, but it is acceptable up to 0.4%. For these reasons, Si is limited to 0.4% or less.
Mn:1.0〜1.8%
Mnは、焼入性を向上させる作用を有し、焼入性向上を介し鋼板の強度を増加させる。また、Mnは、MnSを形成しSを固定することにより、Sの粒界偏析を防止してスラブ(鋼素材)割れを抑制する。このような効果を得るためには、1.0%以上の含有を必要とする。一方、1.8%を超える含有は、スラブ鋳造時の凝固偏析を助長し、鋼板にMn濃化部を残存させ、セパレーションの発生を増加させる。このMn濃化部を消失させるには、1300℃を超える温度に加熱する必要があり、このような熱処理を工業的規模で実施することは現実的でない。このため、Mnは1.0〜1.8%の範囲に限定した。なお、好ましくは1.4〜1.8%である。
Mn: 1.0-1.8%
Mn has the effect of improving hardenability, and increases the strength of the steel sheet through the improvement of hardenability. Further, Mn forms MnS and fixes S, thereby preventing segregation of S grain boundaries and suppressing slab (steel material) cracking. In order to obtain such an effect, a content of 1.0% or more is required. On the other hand, if the content exceeds 1.8%, solidification segregation during slab casting is promoted, Mn-concentrated portions remain in the steel sheet, and the occurrence of separation increases. In order to eliminate this Mn enriched part, it is necessary to heat to a temperature exceeding 1300 ° C., and it is not practical to carry out such a heat treatment on an industrial scale. For this reason, Mn was limited to the range of 1.0 to 1.8%. In addition, Preferably it is 1.4 to 1.8%.
Al:0.1%以下
Alは、脱酸剤として作用する元素であり、このような効果を得るためには、0.01%以上含有することが望ましい。一方、0.1%を超える含有は、電縫溶接時の、溶接部の清浄性を著しく損なう。このようなことから、Alは0.1%以下に限定した。
Nb:0.02〜0.08%
Nbは、オーステナイトの粒界移動を抑制し、オーステナイト粒の粗大化、再結晶を抑制する作用を有する元素であり、熱間仕上圧延におけるオーステナイト未再結晶温度域圧延を可能にする作用を有する元素である。また、Nbは炭窒化物(析出物)として微細析出することにより、溶接性を損なうことなく、少ない含有量で熱延鋼板を高強度化する作用を有する。このような効果を得るためには、0.02%以上の含有を必要とする。一方、0.08%を超える過剰な含有は、熱間仕上圧延中の圧延荷重の増大をもたらし、熱間圧延が困難となる場合がある。このため、Nbは0.02〜0.08%の範囲に限定した。
Al: 0.1% or less
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is desirable to contain 0.01% or more. On the other hand, a content exceeding 0.1% significantly impairs the cleanliness of the welded part during ERW welding. For these reasons, Al is limited to 0.1% or less.
Nb: 0.02 to 0.08%
Nb is an element that has the effect of suppressing austenite grain boundary migration, austenite grain coarsening and recrystallization, and has the effect of enabling austenite non-recrystallization temperature range rolling in hot finish rolling. It is. Moreover, Nb has the effect | action which makes a hot-rolled steel plate high intensity | strength with little content, without impairing weldability by carrying out fine precipitation as a carbonitride (precipitate). In order to obtain such an effect, a content of 0.02% or more is required. On the other hand, an excessive content exceeding 0.08% results in an increase in rolling load during hot finish rolling, which may make hot rolling difficult. For this reason, Nb was limited to the range of 0.02 to 0.08%.
Cr:0.3〜0.8%
Crは、焼入性の向上を介して、鋼板強度を増加させる作用を有する元素である。本発明では、所望の高強度を確保するために、0.3%以上の含有を必要とする。一方、0.8%を超える含有は、抵抗溶接時に溶接欠陥を多発させる傾向となる。このため、Crは0.3〜0.8%の範囲に限定した。
Cr: 0.3-0.8%
Cr is an element having an action of increasing the strength of the steel sheet through improvement of hardenability. In the present invention, a content of 0.3% or more is required in order to secure a desired high strength. On the other hand, a content exceeding 0.8% tends to cause frequent welding defects during resistance welding. For this reason, Cr was limited to the range of 0.3 to 0.8%.
上記した成分が基本の成分であるが、必要に応じて、上記した基本の組成に加えてさらに、Cu:0.3%以下、Ni:0.3%以下、Mo:0.3%以下、V:0.1%以下、Ti:0.03%以下、B:0.0010%以下のうちから選ばれた1種または2種以上を合計で1.0%以下、および/または、Ca:0.005%以下、REM:0.005%以下のうちから選ばれた1種または2種を含有させてもよい。 The above-mentioned components are basic components. If necessary, in addition to the basic composition described above, Cu: 0.3% or less, Ni: 0.3% or less, Mo: 0.3% or less, V: 0.1% or less, One or two or more selected from Ti: 0.03% or less, B: 0.0010% or less, and 1.0% or less in total, and / or Ca: 0.005% or less, REM: 0.005% or less Alternatively, one or two of them may be contained.
Cu:0.3%以下、Ni:0.3%以下、Mo:0.3%以下、V:0.1%以下、Ti:0.03%以下、B:0.0010%以下のうちから選ばれた1種または2種以上:合計1.0%以下
Cu、Ni、Mo、V、Ti、Bはいずれも、鋼の強度を増加させる作用を有する元素であり、必要に応じて選択して含有できる。
Cuは、焼入れ性を向上させるとともに、固溶強化あるいは析出強化により鋼板の強度を増加させる作用を有する元素である。このような効果を得るためには、0.01%以上含有することが望ましいが、0.3%を超える含有は熱間加工性を低下させる。このため、Cuは0.3%以下に限定することが好ましい。
Cu: 0.3% or less, Ni: 0.3% or less, Mo: 0.3% or less, V: 0.1% or less, Ti: 0.03% or less, B: 0.0010% or less, or one or more selected from a total of 1.0 %Less than
Cu, Ni, Mo, V, Ti, and B are all elements that have an action of increasing the strength of steel, and can be selected and contained as necessary.
Cu is an element that has the effect of improving the hardenability and increasing the strength of the steel sheet by solid solution strengthening or precipitation strengthening. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.3% reduces hot workability. For this reason, it is preferable to limit Cu to 0.3% or less.
Niは、焼入性の向上を介して鋼の強度を増加させるとともに、鋼板の靭性をも向上させる作用を有する元素であり、このような効果を得るためには、0.01%以上含有することが望ましい。一方、0.3%を超える含有は、溶接性を低下させるとともに、材料コストの高騰を招く。このため、Niは0.3%以下に限定することが好ましい。
Moは、焼入性を向上させるとともに、炭窒化物(析出物)を形成して、鋼の強度を増加させる作用を有する元素であり、このような効果を得るためには0.01%以上含有することが望ましいが、0.3%を超える含有は、溶接性を低下させる。このため、Moは0.3%以下に限定することが好ましい。
Ni is an element that has the effect of increasing the strength of steel through the improvement of hardenability and also improving the toughness of the steel sheet. In order to obtain such an effect, Ni should be contained in an amount of 0.01% or more. desirable. On the other hand, if the content exceeds 0.3%, the weldability is lowered and the material cost is increased. For this reason, it is preferable to limit Ni to 0.3% or less.
Mo is an element that improves hardenability and forms carbonitrides (precipitates) to increase the strength of the steel. To obtain such effects, it is contained in an amount of 0.01% or more. Although it is desirable, the content exceeding 0.3% lowers the weldability. For this reason, it is preferable to limit Mo to 0.3% or less.
Vは、鋼中に固溶、あるいは炭窒化物(析出物)として微細析出することにより、溶接性を損なうことなく、少ない含有量で強度を増加させる作用を有する。このような効果を得るためには、0.01%以上含有することが望ましいが、0.1%を超える含有は、溶接性を低下させる。このため、含有する場合には、Vは0.1%以下に限定することが好ましい。
Tiは、炭化物を形成し、微細析出することにより、強度を増加させる作用を有する。このような効果を得るためには、0.01%以上含有することが望ましいが、0.03%を超える含有は、溶接部の靭性を劣化させる。このため、含有する場合には、Tiは0.03%以下に限定することが好ましい。
V has the effect of increasing the strength with a small content without impairing the weldability by solid solution or fine precipitation as carbonitride (precipitate) in the steel. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.1% reduces weldability. For this reason, when it contains, it is preferable to limit V to 0.1% or less.
Ti has the effect of increasing strength by forming carbides and finely precipitating. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.03% degrades the toughness of the weld. For this reason, when it contains, it is preferable to limit Ti to 0.03% or less.
Bは、焼入れ性の向上を介して、鋼の強度を増加させる作用を有する元素であり、このような効果を得るためには0.0003%以上含有することが望ましい。一方、0.0010%を超える含有は、かえって焼入れ性を低下させる。このため、含有する場合には、Bは0.0010%以下に限定することが好ましい。
なお、Cu、Ni、Mo、V、Ti、Bを複合して含有する場合には、上記した各元素の含有量の範囲内でかつ含有するそれら元素の含有量の合計が1.0%以下となるようにすることが溶接部靱性劣化の観点から好ましい。
B is an element having an action of increasing the strength of steel through improvement of hardenability. In order to obtain such an effect, B is preferably contained in an amount of 0.0003% or more. On the other hand, if the content exceeds 0.0010%, the hardenability deteriorates. For this reason, when it contains, it is preferable to limit B to 0.0010% or less.
In addition, when Cu, Ni, Mo, V, Ti, and B are contained in combination, the total content of those elements within the above-described content range of each element is 1.0% or less. It is preferable from the viewpoint of deterioration of toughness of the weld zone.
Ca:0.005%以下、REM:0.005%以下のうちから選ばれた1種または2種
Ca、REMはいずれも、展伸した粗大な硫化物を球状の硫化物とする硫化物の形態制御に寄与する元素であり、必要に応じて選択して含有できる。このような効果を得るためには、0.001%以上含有することが望ましいが、0.005%を超える多量の含有は、鋼板の清浄度を低下させる。このため、Ca、REMはいずれも0.005%以下に限定することが好ましい。
上記した成分以外の残部は、Feおよび不可避的不純物からなる。不可避的不純物としては、P:0.025%以下、S:0.005%以下、N:0.008%以下、O:0.005%以下が許容できる。
One or two selected from Ca: 0.005% or less, REM: 0.005% or less
Both Ca and REM are elements that contribute to the control of the morphology of the sulfide, in which the expanded coarse sulfide is a spherical sulfide, and can be selected and contained as necessary. In order to acquire such an effect, it is desirable to contain 0.001% or more, but if it contains more than 0.005%, the cleanliness of the steel sheet is lowered. For this reason, it is preferable to limit both Ca and REM to 0.005% or less.
The balance other than the components described above consists of Fe and inevitable impurities. Inevitable impurities include P: 0.025% or less, S: 0.005% or less, N: 0.008% or less, and O: 0.005% or less.
Pは、鋼中に不純物として不可避的に含まれるが、鋼の強度を上昇させる作用を有する。しかし、0.025%を超えて過剰に含有すると溶接性が低下する。このため、Pは0.025%以下に限定することが好ましい。
Sは、Pと同様に鋼中に不純物として不可避的に含まれるが、0.005%を超えて過剰に含有すると、スラブ割れを生起させるとともに、粒界偏析して鋼板の母材靭性を劣化させる。また、熱延鋼板においては粗大なMnSを形成し、セパレーションを発生させる。このため、Sは0.005%以下に限定することが好ましい。
P is inevitably contained as an impurity in the steel, but has an effect of increasing the strength of the steel. However, when it exceeds 0.025% and it contains excessively, weldability will fall. For this reason, it is preferable to limit P to 0.025% or less.
S is inevitably contained as an impurity in the steel as in the case of P. However, when it exceeds 0.005%, S excessively causes slab cracks and segregates at the grain boundaries to deteriorate the base metal toughness of the steel sheet. Moreover, coarse MnS is formed in a hot-rolled steel sheet, and separation is generated. For this reason, it is preferable to limit S to 0.005% or less.
N:0.008%以下
Nは、鋼中に不可避的に含まれる元素であるが、過剰な含有はスラブ鋳造時の割れを多発させる。このため、Nは0.008%以下に限定することが好ましい。
本発明の高張力熱延鋼板は、上記した組成を有し、さらに表層がベイナイト相単相で、内層がベイナイト相を主相とし、第二相として平均粒径:3μm以下のマルテンサイト相が体積率で1〜4%分散し、第三相として体積率で0〜30%のフェライト相を含む板厚方向に複層の組織を有する鋼板である。ここでいう「表層」とは、表面から1mmの位置を言い、「内層」とは、該表層に囲まれた板厚中央位置をいう。
N: 0.008% or less N is an element inevitably contained in steel, but excessive inclusion frequently causes cracks during slab casting. For this reason, it is preferable to limit N to 0.008% or less.
The high-tensile hot-rolled steel sheet of the present invention has the above-described composition, and the surface layer is a bainite phase single phase, the inner layer is a bainite phase as a main phase, and the second phase has a martensite phase with an average particle size of 3 μm or less. It is a steel sheet having a multilayer structure in the thickness direction including 1 to 4% by volume and containing a ferrite phase having a volume ratio of 0 to 30% as a third phase. As used herein, “surface layer” refers to a position 1 mm from the surface, and “inner layer” refers to the center position of the plate thickness surrounded by the surface layer.
鋼板表層の組織は、高強度と高靭性を兼備させるために、ベイナイト相単相とする。なお、ここでいう「単相」とは、該相が98%以上である場合をいうものとする。一方、鋼板内層の組織は、ベイナイト相を主相とし、第二相として平均粒径:3μm以下のマルテンサイト相が分散した組織とする。主相をベイナイト相とすることにより、高強度と高靭性を兼備させることができる。なお、ここでいう主相とは、70%以上存在する相をいうものとする。また、平均粒径:3μm以下のマルテンサイト相を分散させることにより、造管時の歪で可動転位を導入することができ、これにより造管後の降伏比を85%以下の低降伏比とすることができる。マルテンサイト相が平均粒径:3μmを超えて大きくなると、造管時の歪により可動転位の起点が減少するため、低降伏比が難しくなる。また、微細なマルテンサイト相が1%未満では、造管時の歪により所望量の可動転位の導入が困難となる。一方、4%を超える多量の含有では、鋼板の靭性が低下する。 The structure of the steel sheet surface layer is a single bainite phase in order to combine high strength and high toughness. The term “single phase” here refers to a case where the phase is 98% or more. On the other hand, the structure of the inner layer of the steel sheet is a structure in which a bainite phase is a main phase and a martensite phase having an average particle diameter of 3 μm or less is dispersed as a second phase. By making the main phase a bainite phase, it is possible to combine high strength and high toughness. In addition, the main phase here shall mean the phase which exists 70% or more. In addition, by dispersing a martensite phase with an average particle size of 3 μm or less, movable dislocations can be introduced due to strain during pipe forming, which reduces the yield ratio after pipe forming to a low yield ratio of 85% or less. can do. When the martensite phase becomes larger than the average particle size: 3 μm, the starting point of the movable dislocation decreases due to strain during pipe making, so that a low yield ratio becomes difficult. Further, if the fine martensite phase is less than 1%, it becomes difficult to introduce a desired amount of movable dislocations due to strain during pipe forming. On the other hand, if the content exceeds 4%, the toughness of the steel sheet decreases.
なお、「ベイナイト相」には、ベイニティックフェライト相をも含むものとする。
本発明では、主相、第二相以外の第三相として、0〜30%のフェライト相を含んでもよい。フェライト相が30%を超えて多量に含有されると、所望の高強度を確保することが困難となる。
なお、組織の同定、組織分率は、圧延方向断面を研磨、腐食して、光学顕微鏡等で組織観察することにより行うものとする。
The “bainite phase” includes a bainitic ferrite phase.
In the present invention, 0 to 30% of a ferrite phase may be included as a third phase other than the main phase and the second phase. If the ferrite phase is contained in a large amount exceeding 30%, it becomes difficult to ensure a desired high strength.
The structure identification and the structure fraction are performed by polishing and corroding the cross section in the rolling direction and observing the structure with an optical microscope or the like.
つぎに、本発明高張力熱延鋼板の好ましい製造方法について説明する。
上記した組成の鋼素材に、熱間圧延を施して熱延鋼板とする。
鋼素材の製造方法としては、上記した組成の溶鋼を転炉等の常用の溶製方法で溶製し、連続鋳造法等の常用の鋳造方法でスラブ等の鋼素材とすることが好ましいが、本発明では、これに限定されることはない。
Below, the preferable manufacturing method of this invention high tension hot-rolled steel plate is demonstrated.
The steel material having the above composition is hot-rolled to obtain a hot-rolled steel sheet.
As a manufacturing method of the steel material, it is preferable to melt the molten steel having the above composition by a conventional melting method such as a converter, and to make a steel material such as a slab by a conventional casting method such as a continuous casting method, The present invention is not limited to this.
上記した組成の鋼素材を、好ましくは1050〜1250℃に加熱する。
加熱温度が1050℃未満では、Nbの固溶および圧延後析出による強度増加量が低下し、所望の高強度を確保できにくくなる。一方、加熱温度が1250℃を超えて高温になると、結晶粒が粗大して低温靭性が低下するうえ、スケール生成量が増大し歩留が低下するとともに、表面性状が低下する恐れがある。このため、熱間圧延における加熱温度は1050〜1250℃とすることが好ましい。
The steel material having the above composition is preferably heated to 1050 to 1250 ° C.
When the heating temperature is less than 1050 ° C., the amount of increase in strength due to the solid solution of Nb and precipitation after rolling decreases, and it becomes difficult to secure a desired high strength. On the other hand, when the heating temperature is higher than 1250 ° C., the crystal grains are coarsened and the low-temperature toughness is reduced, the amount of scale generation is increased, the yield is lowered, and the surface properties may be lowered. For this reason, it is preferable that the heating temperature in hot rolling shall be 1050-1250 degreeC.
加熱された鋼素材に、ついで熱間圧延を施す。
熱間圧延は、粗圧延と仕上圧延からなる圧延とする。粗圧延は、所望の寸法形状のシートバーが確保できればよく、とくにその条件を限定する必要はない。仕上圧延は、950℃以下の温度域における累積圧下率が45%以上で、仕上圧延終了温度が(Ar3変態点−50℃)以上好ましくは900℃以下である圧延とする。
The heated steel material is then hot rolled.
Hot rolling is rolling consisting of rough rolling and finish rolling. In rough rolling, it is only necessary to secure a sheet bar having a desired size and shape, and it is not necessary to limit the conditions. Finish rolling is a rolling with a cumulative rolling reduction in a temperature range of 950 ° C. or lower of 45% or more and a finish rolling finish temperature of (Ar 3 transformation point −50 ° C.) or higher, preferably 900 ° C. or lower.
仕上圧延における950℃以下の温度域における累積圧下率が45%未満では、所望の高靭化を達成できない。なお、好ましくは50〜75%である。また、仕上圧延終了温度が(Ar3変態点−50℃)未満の低温となると、仕上圧延中に、表層でフェライト変態が進行し、表層組織を所望のベイナイト単相組織とすることができなくなる。また、仕上圧延終了温度が900℃を超える高温では、組織の微細化が達成できず、靭性が低下する。このようなことから、仕上圧延終了温度は(Ar3変態点−50℃)以上好ましくは900℃以下に限定することが好ましい。なお、圧延中の温度は、鋼板表面温度とする。 If the cumulative reduction in the temperature range of 950 ° C. or lower in finish rolling is less than 45%, the desired high toughness cannot be achieved. In addition, Preferably it is 50 to 75%. Further, when the finish rolling finish temperature becomes a low temperature less than (Ar 3 transformation point −50 ° C.), ferrite transformation proceeds in the surface layer during finish rolling, and the surface layer structure cannot be made into a desired bainite single phase structure. . Further, if the finish rolling finish temperature is higher than 900 ° C., the structure cannot be refined and the toughness is lowered. For this reason, the finish rolling finish temperature is preferably (Ar 3 transformation point −50 ° C.) or more, preferably 900 ° C. or less. The temperature during rolling is the steel sheet surface temperature.
熱間圧延終了後、直ちに(15s以内に)、ランアウトテーブル上で冷却(加速冷却処理)を施す。なお、加速冷却処理の冷却開始温度は、板厚中心位置での温度で、750℃以上とすることが好ましい。冷却開始温度が750℃未満では、少なくとも表層がフェライト変態し、所望の表層組織を確保することができず、強度、靭性が低下する。
また、加速冷却処理の平均冷却速度は、板厚中心位置の冷却速度で、5〜20℃/sの範囲とすることが好ましい。これにより、過剰なフェライト相の生成、パーライト相の生成が抑制され、また結晶粒の粗大化が防止できる。平均冷却速度が20℃/sを超えると、その後の処理で微細なマルテンサイト相の形成が難しくなる。一方、5℃/s未満では、フェライトが過剰に生成し、所望の高強度、高靭性が確保できにくくなる。高温で生成するフェライトが多く形成されると、微細なマルテンサイト相の形成が難しくなる。
Immediately after the hot rolling is finished (within 15 seconds), cooling (accelerated cooling treatment) is performed on the runout table. Note that the cooling start temperature of the accelerated cooling treatment is preferably 750 ° C. or higher at the center thickness position. If the cooling start temperature is less than 750 ° C., at least the surface layer undergoes ferrite transformation, a desired surface layer structure cannot be ensured, and the strength and toughness decrease.
Moreover, it is preferable to make the average cooling rate of accelerated cooling processing into the range of 5-20 degrees C / s by the cooling rate of a plate | board thickness center position. Thereby, the production | generation of an excess ferrite phase and the production | generation of a pearlite phase are suppressed, and the coarsening of a crystal grain can be prevented. When the average cooling rate exceeds 20 ° C./s, it becomes difficult to form a fine martensite phase in the subsequent treatment. On the other hand, if it is less than 5 ° C./s, ferrite is excessively generated, and it becomes difficult to ensure desired high strength and high toughness. When a large amount of ferrite formed at a high temperature is formed, it becomes difficult to form a fine martensite phase.
加速冷却処理における冷却停止温度は、600〜400℃の温度域の温度(板厚中央の位置での温度)とすることが好ましい。冷却停止温度が、400℃未満の低温になると、その後の処理で微細なマルテンサイト相の形成が少なくなる。一方、600℃を超える高温では、その後の冷却(放冷)でフェライト、パーライトが顕著に析出し、所望の組織を確保することができなくなる。なお、冷却停止温度は、より好ましくは470〜600℃である。 The cooling stop temperature in the accelerated cooling process is preferably set to a temperature in the temperature range of 600 to 400 ° C. (temperature at the center of the plate thickness). When the cooling stop temperature is a low temperature of less than 400 ° C., the formation of fine martensite phase is reduced in the subsequent treatment. On the other hand, at a high temperature exceeding 600 ° C., ferrite and pearlite are remarkably precipitated by subsequent cooling (cooling), and a desired structure cannot be secured. The cooling stop temperature is more preferably 470 to 600 ° C.
加速冷却処理後、放冷処理を施す。放冷処理の時間は、加速冷却停止後、15s以上40s以下とする。加速冷却処理後、放冷処理を施すことにより、鋼板表面が復熱し、板厚方向の温度分布が均一化するという効果が期待できる。長時間の放冷処理は生産性の低下を招く。
所定時間放冷したのち、鋼帯(鋼板)をコイル状に巻き取り、放冷する。
After the accelerated cooling process, a cooling process is performed. The cooling treatment time is set to 15 s or more and 40 s or less after the accelerated cooling is stopped. By performing the cooling treatment after the accelerated cooling treatment, the effect that the steel plate surface is reheated and the temperature distribution in the thickness direction is uniform can be expected. Prolonged cooling treatment causes a decrease in productivity.
After cooling for a predetermined time, the steel strip (steel plate) is wound into a coil shape and allowed to cool.
以下、さらに実施例に基づいて本発明を詳細に説明する。 Hereinafter, the present invention will be described in detail based on examples.
表1に示す組成の鋼素材を用いて、表2に示す熱間圧延条件で熱間圧延を施し、熱間圧延終了後、表2に示す冷却条件で加速冷却処理および放冷処理を施し、コイル状に巻取り放冷した。 Using the steel material having the composition shown in Table 1, hot rolling is performed under the hot rolling conditions shown in Table 2, and after the hot rolling, accelerated cooling treatment and cooling treatment are performed under the cooling conditions shown in Table 2, The coil was wound and allowed to cool.
得られた熱延鋼板(鋼帯)から、試験片を採取し、組織観察、引張試験、衝撃試験を実施し、組織、引張特性、靭性を調査した。なお、試験方法はつぎのとおりとした。
(1)組織観察
得られた熱延鋼板から組織観察用試験片を採取し、圧延方向断面を研磨、腐食し、光学顕微鏡(倍率:1000倍)または走査型電子顕微鏡(倍率:2000倍)で、表層(表面から1mmの位置)、および板厚中央位置で各2視野以上観察し、組織の種類、およびその組織分率を測定した。
(2)引張試験
得られた熱延鋼板から、圧延方向に直交する方向(C方向)が長手方向となるように、JIS Z 2201の規定に準拠してJIS 5号試験片を採取し、JIS Z 2241の規定に準拠して引張試験を実施し、降伏強さYS、引張強さTSを求めた。
(3)衝撃試験
得られた熱延鋼板の板厚中央部から、圧延方向に直交する方向(C方向)が長手方向となるようにVノッチ試験片を採取し、JIS Z 2242の規定に準拠してシャルピー衝撃試験を実施し、破面遷移温度Trs50(℃)を求め、靭性を評価した。
From the obtained hot-rolled steel sheet (steel strip), specimens were collected and subjected to structure observation, tensile test, and impact test, and the structure, tensile characteristics, and toughness were investigated. The test method was as follows.
(1) Microstructure observation A specimen for microstructural observation is collected from the obtained hot-rolled steel sheet, the cross section in the rolling direction is polished and corroded, and the optical microscope (magnification: 1000 times) or scanning electron microscope (magnification: 2000 times) is used. (position of 1mm from the surface) surface, and then observed by a plate Atsunaka central position each 2 or more visual fields were determined type of tissue, and the tissue fraction.
(2) Tensile test JIS No. 5 specimens were collected from the obtained hot-rolled steel sheet in accordance with the provisions of JIS Z 2201 so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction. A tensile test was performed in accordance with the provisions of Z 2241 to determine the yield strength YS and tensile strength TS.
(3) Impact test V-notch test specimens were taken from the center of the thickness of the obtained hot-rolled steel sheet so that the direction perpendicular to the rolling direction (C direction) was the longitudinal direction, and conformed to the provisions of JIS Z 2242 Then, a Charpy impact test was performed to determine the fracture surface transition temperature Trs 50 (° C.) and toughness was evaluated.
またさらに、得られた熱延鋼板を用いて、複数のロールを用いた冷間成形によりオープン管とし、該オープン管の両端部を突合せ、電縫溶接して、外径508mmφの電縫鋼管とした。得られた電縫鋼管から、管軸方向が引張方向となるように、弧状API引張試験片を採取し、引張特性(YS、TS)を測定し、降伏比を算出した。
得られた結果を表3に示す。
Furthermore, using the obtained hot-rolled steel sheet, an open pipe is formed by cold forming using a plurality of rolls, both ends of the open pipe are butt-welded and electro-welded, and an electric-welded steel pipe having an outer diameter of 508 mmφ and did. From the obtained electric resistance welded steel pipe, an arc-shaped API tensile test piece was taken so that the pipe axis direction was the tensile direction, the tensile properties (YS, TS) were measured, and the yield ratio was calculated.
The obtained results are shown in Table 3.
本発明例はいずれも、適正な組織を有し、引張強さ:520MPa以上の高強度と、Trs50が−80℃以下の高靭性と、を有し、X65級以上の高強度電縫鋼管向け熱延鋼板として充分な特性を有している。また、本発明の熱延鋼板を用いて得られた電縫鋼管(溶接鋼管)の降伏比はいずれも85%以下であり、低降伏比鋼管となっていた。
一方、本発明範囲を外れる比較例は、適正な組織が確保できておらず、強度、靭性、のいずれか、あるいは全てが低下し、高強度電縫鋼管素材用熱延鋼板として所望の特性を確保できていない。
Each of the examples of the present invention has an appropriate structure, tensile strength: high strength of 520 MPa or more, and high toughness of Trs 50 of −80 ° C. or less, and high strength ERW steel pipe of X65 class or more. It has sufficient characteristics as a hot-rolled steel sheet. Moreover, the yield ratio of the electric resistance welded steel pipe (welded steel pipe) obtained by using the hot-rolled steel sheet of the present invention was 85% or less, which was a low yield ratio steel pipe.
On the other hand, in the comparative example outside the scope of the present invention, an appropriate structure cannot be secured, and either or all of strength and toughness are lowered, and the desired characteristics as a hot rolled steel sheet for high-strength ERW steel pipe material are obtained. It is not secured.
Claims (6)
C:0.04〜0.08%、 Si:0.4%以下、
Mn:1.0〜1.8%、 Al:0.1%以下、
Nb:0.02〜0.08%、 Cr:0.3〜0.8%
を含み、残部Feおよび不可避的不純物からなる組成と、表面から1mmの位置である表層にベイナイト相が体積率で98%以上存在し、板厚中央位置である内層にベイナイト相が主相として体積率で70%以上存在し、第二相として平均粒径:3μm以下のマルテンサイト相が体積率で1〜4%分散し、第三相として体積率で0〜30%のフェライト相を含む板厚方向に複層の組織を有し、かつ引張強さTS:520MPa以上、破面遷移温度Trs50:−80℃以下を兼備するとともに、造管後の降伏比が85%以下であることを特徴とするラインパイプ用溶接鋼管向け高張力熱延鋼板。 % By mass
C: 0.04 to 0.08%, Si: 0.4% or less,
Mn: 1.0 to 1.8%, Al: 0.1% or less,
Nb: 0.02-0.08%, Cr: 0.3-0.8%
In addition, the composition comprising the balance Fe and inevitable impurities, the surface layer located 1 mm from the surface has a bainite phase of 98% or more by volume, and the inner layer at the center of the thickness has a volume of bainite phase as the main phase. A plate containing a martensite phase having an average particle diameter of 3 μm or less as a second phase and a ferrite phase having a volume ratio of 0 to 30% as a third phase. It has a multi-layered structure in the thickness direction, tensile strength TS: 520 MPa or more, fracture surface transition temperature Trs 50 : -80 ° C or less, and the yield ratio after pipe making is 85% or less High-tensile hot-rolled steel sheet for welded steel pipes for line pipes.
C:0.04〜0.08%、 Si:0.4%以下、
Mn:1.0〜1.8%、 Al:0.1%以下、
Nb:0.02〜0.08%、 Cr:0.3〜0.8%
を含み、残部Feおよび不可避的不純物からなる組成の鋼素材に、加熱したのち、950℃以下の温度域における累積圧下率が45%以上で、仕上圧延終了温度が(Ar3変態点−50℃)以上900℃以下とする熱間圧延を施し、該熱間圧延終了後,15s以内に、板厚中央位置の冷却速度で5〜20℃/sの範囲の平均冷却速度で400〜600℃の温度域まで冷却する加速冷却処理を施し、該加速冷却処理終了後15s以上、放冷する放冷処理を施したのち、コイル状に巻き取って、請求項1に記載の高張力熱延鋼板を製造することを特徴とするラインパイプ用溶接鋼管向け高張力熱延鋼板の製造方法。 % By mass
C: 0.04 to 0.08%, Si: 0.4% or less,
Mn: 1.0 to 1.8%, Al: 0.1% or less,
Nb: 0.02-0.08%, Cr: 0.3-0.8%
And a steel material having a composition comprising the balance Fe and inevitable impurities, and after heating, the cumulative rolling reduction in the temperature range of 950 ° C. or lower is 45% or more, and the finish rolling finish temperature is (Ar 3 transformation point −50 ° C. ) Hot rolling to 900 ° C. or less is performed, and after completion of the hot rolling, within 15 s, the average cooling rate in the range of 5 to 20 ° C./s at the cooling rate at the center position of the plate thickness is 400 to 600 ° C. subjected to accelerated cooling process for cooling to a temperature range, the pressurized-speed cooling process after completion 15s or more, then subjected to cooling treatment to cool, wound into a coil, a high-tensile hot-rolled steel sheet according to claim 1 A method for producing a high-strength hot-rolled steel sheet for welded steel pipes for line pipes.
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JPH05195057A (en) * | 1991-07-31 | 1993-08-03 | Kawasaki Steel Corp | Production of high cr steel type uoe steel sheet and high cr type atmosphere corrosion resisting steel sheet both excellent in ys characteristic in l direction |
JP4277405B2 (en) * | 2000-01-26 | 2009-06-10 | Jfeスチール株式会社 | Manufacturing method of hot-rolled steel sheet for high-strength ERW steel pipe excellent in low temperature toughness and weldability |
JP4273906B2 (en) * | 2003-09-30 | 2009-06-03 | Jfeスチール株式会社 | Thick hot rolled steel sheet excellent in workability and ductility and method for producing the same |
JP4466252B2 (en) * | 2004-07-28 | 2010-05-26 | Jfeスチール株式会社 | Manufacturing method of high carbon hot rolled steel sheet |
JP5011773B2 (en) * | 2005-03-24 | 2012-08-29 | Jfeスチール株式会社 | Manufacturing method of low yield ratio ERW steel pipe with excellent low temperature toughness |
JP5369639B2 (en) * | 2008-11-25 | 2013-12-18 | Jfeスチール株式会社 | High strength steel material excellent in welding heat-affected zone toughness and HIC resistance and manufacturing method thereof |
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