JP4905240B2 - Manufacturing method of hot-rolled steel sheet with excellent surface quality, fracture toughness and sour resistance - Google Patents
Manufacturing method of hot-rolled steel sheet with excellent surface quality, fracture toughness and sour resistance Download PDFInfo
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Description
本発明は、腐食性のつよいサワー環境で使用されるラインパイプ向け電縫鋼管およびスパイラル鋼管等の用途に供して好適な高張力熱延鋼板の製造方法に係り、とくに表面欠陥の発生防止および破壊靭性の向上に関する。なお、鋼板には、鋼板、鋼帯を含むものとする。 The present invention relates to a method for producing a high-tensile hot-rolled steel sheet suitable for use in line-pipe ERW steel pipes and spiral steel pipes used in a corrosive sour environment, and in particular, prevention and destruction of surface defects. Related to improved toughness. In addition, a 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。また、一旦は、開発が放棄されていた腐食性の強いサワーガス田等に対する開発が盛んとなっている。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にしたがい、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. Also, once the development has been abandoned, the development of a corrosive sour gas field, etc. has become active. 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) needs to be a thick steel pipe, and a UOE steel pipe made of a thick steel plate has been used. However, recently, in line with the strong demand for further reduction in pipeline construction costs, there is a strong demand for reducing the material cost of steel pipes. Instead of UOE steel pipes made of thick steel plates as transport pipes, productivity is higher. High-strength ERW steel pipes or spiral steel pipes made from inexpensive, coil-shaped hot-rolled steel sheets (hot-rolled steel strips) have come to be used.
これら高強度鋼管には、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。このような強度と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。またさらに最近では，極寒地用の鋼管に対しては、パイプラインのバースト破壊を防止する観点から、破壊靭性、とくに優れたＣＴＯＤ特性、とくに優れたＤＷＴＴ特性を具備することが要求される場合が多い。 These high-strength steel pipes are required to maintain excellent low-temperature toughness at the same time from the viewpoint of preventing line pipe breakage. In order to manufacture steel pipes that combine such strength and toughness, steel sheets that are steel pipe materials are produced by transformation strengthening using accelerated cooling after hot rolling and precipitation of alloy elements such as Nb, V, and Ti. Strengthening by precipitation strengthening using sapphire and high toughness by refinement of structure using control rolling or the like have been attempted. More recently, steel pipes for extremely cold regions may be required to have fracture toughness, particularly excellent CTOD characteristics, particularly excellent DWTT characteristics, from the viewpoint of preventing pipeline burst fracture. Many.
このような要求に対し、例えば特許文献１には、Ｃ：0.05〜0.12％、Ca：0.0020〜0.0060％を含み、Si、Mn、Al、Ｐ、Ｓを適正量調整して含む連鋳製スラブに、950℃以下で10〜50％の圧下を行い、引続き表面の冷却速度が２℃／ｓ以上で表面温度がＡr３以下の温度になるまで冷却し、250ｓ未満の復熱後、未再結晶領域にて50％以上の圧延を行い、720〜820℃の範囲で圧延を終了し、引続いて平均冷却速度５〜30℃／ｓで冷却した後、400〜600℃の範囲で巻取る高靭性耐サワー鋼管用ホットコイルの製造方法が記載されている。特許文献１に記載された技術によれば、耐ＨＩＣ特性と、低温靭性の両特性に優れたホットコイルが製造でき、寒冷地でのラインパイプの製造が可能となるとしている。 In response to such demands, for example, Patent Document 1 includes C: 0.05 to 0.12%, Ca: 0.0020 to 0.0060%, and includes a continuous cast slab containing Si, Mn, Al, P, and S with appropriate amounts adjusted. In addition, a reduction of 10 to 50% is performed at 950 ° C. or lower, and the cooling is continued until the surface cooling rate is 2 ° C./s or higher and the surface temperature is Ar 3 or lower. Roll at 50% or more in the crystal region, finish rolling in the range of 720 to 820 ° C, subsequently cool at an average cooling rate of 5 to 30 ° C / s, and then wind in the range of 400 to 600 ° C A method of manufacturing a hot coil for high toughness sour steel pipe is described. According to the technique described in Patent Document 1, a hot coil excellent in both HIC resistance and low temperature toughness can be manufactured, and a line pipe can be manufactured in a cold region.
また、特許文献２には、Ｃ：0.01〜0.20％を含み、Si、Mn、Al、Ｎを適正量含有する鋼片を、Ａc３変態点以上1250℃以下に加熱し、900℃以上の温度での累積圧下率が10〜80％の粗圧延を行ったのち、２〜40℃／ｓの加速冷却を、該冷却速度における（Ａr３変態点＋50℃）〜（Ａr３変態点−50℃）まで行って、加速冷却後、累積圧下率30〜90％の仕上げ圧延を650℃以上で終了し、さらに仕上げ圧延終了後、５〜40℃／ｓの冷却速度で200〜450℃まで再び加速冷却する低温靭性に優れた低降伏比高張力鋼材の製造方法が記載されている。特許文献２に記載された技術によれば、複雑な熱処理工程を必要とすることなく、低降伏比と、優れた低温靭性とを両立させた熱延鋼板を製造することができるとしている。 In Patent Document 2, a steel slab containing C: 0.01 to 0.20% and containing appropriate amounts of Si, Mn, Al, and N is heated to an Ac 3 transformation point or higher and 1250 ° C. or lower, and a temperature of 900 ° C. or higher. After rough rolling at a cumulative rolling reduction of 10 to 80%, accelerated cooling at 2 to 40 ° C./s is performed at (Ar 3 transformation point + 50 ° C.) to (Ar 3 transformation point −50 ° C.) at the cooling rate. ), And after completion of accelerated cooling, finish rolling with a cumulative rolling reduction of 30 to 90% is completed at 650 ° C or higher, and after finishing rolling is further accelerated to 200 to 450 ° C at a cooling rate of 5 to 40 ° C / s. A method for producing a low-yield ratio high-tensile steel material excellent in low-temperature toughness to be cooled is described. According to the technique described in Patent Document 2, a hot-rolled steel sheet having both a low yield ratio and excellent low-temperature toughness can be produced without requiring a complicated heat treatment step.
また、特許文献３には、Ｃ：0.01〜0.10％、Nb：0.01〜0.1％を含み、Si、Mn、Ｐ、Ｓ、Ｎを適正量含み、かつMn／Si：５〜８を満足するように調整した鋼片に、1100℃以上で行う最初の圧下率：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い、５℃／ｓ以上の冷却速度で鋼板表層部をＡr３点以下まで冷却し、復熱または強制加熱により、表層部の温度が（Ａr３−40℃）〜（Ａr３＋40℃）となった時点で仕上圧延を開始し、950℃以下の合計圧下率：60％以上の条件で仕上圧延を終了し、ついで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℃の範囲で巻き取る低温靭性及び溶接性に優れた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３に記載された技術によれば、高価な合金元素を添加することなく、また熱処理する必要なく、低温靭性および溶接性に優れた高強度電縫鋼管を製造することができるとしている。
しかし、特許文献１に記載された技術で製造された熱延鋼板では、耐ＨＩＣ特性の向上は顕著であるが、ＤＷＴＴ特性やＣＴＯＤ特性の向上は顕著ではなく、さらに表面割れが発生する場合があり、問題を残していた。さらに、特許文献１に記載された技術では、オーステナイト未再結晶域における圧下率を極めて大きくする必要があり、熱間圧延プロセスにおいて仕上板厚が厚くなるに従って、シートバー厚みが厚くなり、仕上板厚が厚い鋼板を製造するには限界があった。また、特許文献２、特許文献３に記載された技術で製造された熱延鋼板では、表面割れが多発する場合があるという問題があった。 However, in the hot-rolled steel sheet manufactured by the technique described in Patent Document 1, the improvement in the HIC resistance is remarkable, but the improvement in the DWTT characteristic and the CTOD characteristic is not remarkable, and surface cracks may occur. There was a problem left. Furthermore, in the technique described in Patent Document 1, it is necessary to extremely increase the rolling reduction in the austenite non-recrystallized region. There was a limit to manufacturing thick steel plates. Moreover, in the hot-rolled steel plate manufactured by the technique described in Patent Document 2 and Patent Document 3, there is a problem that surface cracks frequently occur.
本発明は、かかる従来技術の問題を解決し、低温大圧下で熱間圧延を実施しても表面割れ等の表面欠陥の発生がなく表面品質に優れ、しかも破壊靱性に優れ、さらに耐サワー性にも優れた高張力熱延鋼板を製造することが可能となる、高張力熱延鋼板の製造方法を提供することを目的とする。なお、ここでいう「高張力鋼板」とは、降伏強さYSが448MPa以上のＡＰＩ−Ｘ65級鋼管を製造可能な鋼板をいうものとする。また、「破壊靭性に優れた」とは、ASTM規格E1290の規定に準拠したＣＴＯＤ試験を実施した際の試験温度−10℃における限界開口変位量δc（mm）が0.25mm以上である場合、およびASTM E436の規定に準拠したＤＷＴＴ試験を実施し延性破面率が85％となる温度が−10℃未満の場合をいう。また、「表面品質に優れた」とは、割れ深さが0.1mmを超える表面割れ等の表面欠陥の発生がない場合をいう。なお、ここでいう表面割れとは、圧延方向に対して90度となる方向に走るＴ方向割れを指す。 The present invention solves such problems of the prior art, and even when hot rolling is performed under low temperature and high pressure, surface defects such as surface cracks are not generated, surface quality is excellent, fracture toughness is excellent, and sour resistance is further improved. Another object of the present invention is to provide a method for producing a high-tensile hot-rolled steel sheet, which makes it possible to produce an excellent high-tensile hot-rolled steel sheet. Here, the “high-strength steel plate” refers to a steel plate capable of producing an API-X65 grade steel pipe having a yield strength YS of 448 MPa or more. In addition, “excellent fracture toughness” means that the limit opening displacement amount δc (mm) at a test temperature of −10 ° C. when a CTOD test in accordance with the standard of ASTM standard E1290 is 0.25 mm or more, and This refers to the case where the temperature at which the ductile fracture surface ratio is 85% is less than −10 ° C. when a DWTT test is performed in accordance with ASTM E436. In addition, “excellent surface quality” refers to a case where no surface defects such as surface cracks having a crack depth exceeding 0.1 mm occur. In addition, the surface crack here refers to the T direction crack which runs in the direction used as 90 degree | times with respect to a rolling direction.
本発明者らは、上記した課題を達成するために、靭性、表面品質に及ぼす各種要因について鋭意研究を重ねた。その結果、本発明者らは、表面割れ等の表面欠陥は、高靭性を確保するために低温圧延を指向したことによる、表層部の過冷却による熱間延性の低下、あるいはさらに表層部への過大な圧下による、粒界フェライトの割れにその主因があることを突き止めた。しかし、本発明者らの検討によれば、高靭性熱延鋼板を得るためには、被圧延材の温度を高靭化に有効な温度域に冷却したのち、所定範囲の圧下を施す仕上圧延を行うことが肝要であり、そのために仕上圧延前あるいは仕上圧延中に加速冷却を施し、その後の仕上圧延で所定値以上の有効圧延率を施すことが必須となることを知見した。そして、高靭性と、優れた表面品質と、さらには耐サワー性とを両立させるためには、優れた耐サワー性を維持できる組成の鋼素材を用いるとともに、さらに、表層部がＡr３変態点を下回らないように、加速冷却を調整するか、あるいはＡr３変態点を下回るような低温に冷却する場合には、その後に施す仕上圧延における1パス当たりの圧下量を、高温での一様伸び（均一伸び）値と関連する値以下とする圧延を行うことがよいことを知見した。そして、このような処理は、既存の、仕上圧延前の冷却手段、仕上圧延機内の冷却手段を積極的に活用することにより、達成できることを見い出した。 In order to achieve the above-described problems, the present inventors have conducted extensive research on various factors affecting toughness and surface quality. As a result, the present inventors have found that surface defects such as surface cracks are directed to low-temperature rolling in order to ensure high toughness, decrease in hot ductility due to supercooling of the surface layer portion, or further to the surface layer portion. We found out that the main cause of the cracking of intergranular ferrite due to excessive reduction. However, according to the study by the present inventors, in order to obtain a high toughness hot-rolled steel sheet, after the temperature of the material to be rolled is cooled to a temperature range effective for toughening, finish rolling is applied to a predetermined range of reduction. It has been found that it is essential to perform the above-mentioned, and for that purpose, it is essential to perform accelerated cooling before or during finish rolling, and to apply an effective rolling rate of a predetermined value or more in subsequent finish rolling. In order to achieve both high toughness, excellent surface quality, and sour resistance, a steel material having a composition capable of maintaining excellent sour resistance is used, and the surface layer portion has an Ar 3 transformation point. If the accelerated cooling is adjusted so that it does not fall below the temperature, or if it is cooled to a low temperature below the Ar 3 transformation point, the amount of reduction per pass in the subsequent finish rolling is uniformly stretched at a high temperature. It has been found that it is preferable to perform rolling to a value equal to or less than the value related to the (uniform elongation) value. And it discovered that such a process could be achieved by actively utilizing existing cooling means before finish rolling and cooling means in the finish rolling mill.
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通りである。
（１）質量％で、Ｃ：0.02〜0.08％、Si：0.5％以下、Mn：0.8〜1.8％、Ｐ：0.025％以下、Ｓ：0.001％以下、Al：0.005〜0.10％、Ｎ：0.005％以下、Nb：0.03〜0.10％、Ti：0.005〜0.05％、Ca：0.003％以下、Ｏ：0.005％以下を含み、さらにCa、Ｓ、Ｏが次（１）式、次（２）式
1.4≦｛［％Ca］−［％Ｏ］×（0.18＋130［％Ca］）｝／1.25［％Ｓ］＜3.3‥‥（１）
［％Ca］×［％Ｓ］0.28 ≦3.6×10−４ ‥‥（２）
（ここで、［％Ca］、［％Ｏ］、［％Ｓ］：各元素の含有量（質量％））
を満足し、残部Feおよび不可避的不純物からなる組成を有する鋼素材に、粗圧延を施しシートバーとする粗圧延工程と、該シートバーに仕上圧延を施し熱延板とする仕上圧延工程と、該熱延板を巻き取る巻取工程とを順次施す熱延鋼板の製造方法において、前記粗圧延工程後で、前記仕上圧延工程前に、前記シートバーに、表層部を50℃／ｓ以上の冷却速度でＡr３変態点超え900℃以下の温度に達するまで急冷する加速冷却を施したのち、該加速冷却を停止し、しかる後に仕上圧延工程を施すことを特徴とする表面品質、破壊靱性、および耐サワー性に優れる熱延鋼板の製造方法。
（２）質量％で、Ｃ：0.02〜0.08％、Si：0.5％以下、Mn：0.8〜1.8％、Ｐ：0.025％以下、Ｓ：0.001％以下、Al：0.005〜0.10％、Ｎ：0.005％以下、Nb：0.03〜0.10％、Ti：0.005〜0.05％、Ca：0.003％以下、Ｏ：0.005％以下を含み、さらにCa、Ｓ、Ｏが次（１）式、（２）式
1.4≦｛［％Ca］−［％Ｏ］×（0.18＋130［％Ca］）｝／1.25［％Ｓ］＜3.3‥‥（１）
［％Ca］×［％Ｓ］0.28 ≦3.6×10−４ ‥‥（２）
（ここで、［％Ca］、［％Ｏ］、［％Ｓ］：各元素の含有量（質量％））
を満足し、残部Feおよび不可避的不純物からなる組成を有する鋼素材に、粗圧延を施しシートバーとする粗圧延工程と、該シートバーに仕上圧延を施し熱延板とする仕上圧延工程と、該熱延板を巻き取る巻取工程とを順次施す熱延鋼板の製造方法において、前記仕上圧延工程で少なくとも１回、圧延パス間で、仕上圧延途中の熱延板に、表層部が50℃／ｓ以上の冷却速度でＡr３変態点超え900℃以下の温度に達するまで急冷する加速冷却を施したのち、該加速冷却を停止し、さらに仕上圧延を行い所定寸法形状の熱延板とすることを特徴とする表面品質、破壊靱性、および耐サワー性に優れる熱延鋼板の製造方法。
（３）（１）または（２）において、前記仕上圧延工程における仕上圧延が、1パス当たりの圧下率が15〜50％の圧延であることを特徴とする熱延鋼板の製造方法。
（４）（１）または（２）において、前記加速冷却に代えて、表層部が50℃／ｓ以上の冷却速度でＡr３変態点以下の温度に達するまで急冷する加速冷却とし、引き続く前記仕上圧延工程における仕上圧延、１パス当たりの圧下率が（1.1×一様伸び）％以下（ここで、一様伸び：950℃まで加熱したのちＡr３変態点以下まで冷却し、再び950℃まで加熱して高温引張を実施したときに、得られる応力−歪曲線における一様伸び（％））である圧延とすることを特徴とする熱延鋼板の製造方法。
（５）（１）〜（４）のいずれかにおいて、前記組成に加えてさらに、質量％で、Cu：0.005〜0.5％、Ni：0.005〜0.5％、Cr：0.005〜1.0％、Mo：0.005〜0.5％、Ｖ：0.005〜0.3％のうちから選ばれた１種または２種以上を含有する組成とすることを特徴とする熱延鋼板の製造方法。
（６）（１）〜（５）のいずれかにおいて、前記巻取工程における前記熱延板の巻取り温度を350〜700℃とし、巻き取ったのちの冷却速度をコイル中央部で５〜20℃／ｈとすることを特徴とする熱延鋼板の製造方法。
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) By mass%, C: 0.02 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.8%, P: 0.025% or less, S: 0.001% or less, Al: 0.005 to 0.10%, N: 0.005% In the following, Nb: 0.03 to 0.10%, Ti: 0.005 to 0.05%, Ca: 0.003% or less, O: 0.005% or less, and Ca, S, and O are the following formulas (1) and (2)
1.4 ≦ {[% Ca] − [% O] × (0.18 + 130 [% Ca])} / 1.25 [% S] <3.3 (1)
[% Ca] × [% S] 0.28 ≦ 3.6 × 10 −4 (2)
(Where [% Ca], [% O], [% S]: content of each element (mass%))
A steel material having a composition consisting of the balance Fe and inevitable impurities, rough rolling step to rough rolling and make a sheet bar, and finish rolling step to finish rolling the sheet bar to hot rolled sheet, In the method for producing a hot-rolled steel sheet, which is sequentially subjected to a winding process for winding the hot-rolled sheet, the surface layer portion of the sheet bar is set to 50 ° C./s or more after the rough rolling process and before the finish rolling process. Surface quality, fracture toughness, characterized by performing accelerated cooling that rapidly cools until reaching a temperature of 900 ° C. or more exceeding the Ar 3 transformation point at a cooling rate, then stopping the accelerated cooling and then performing a finish rolling step. And a method for producing a hot-rolled steel sheet having excellent sour resistance.
(2) By mass%, C: 0.02 to 0.08%, Si: 0.5% or less, Mn: 0.8 to 1.8%, P: 0.025% or less, S: 0.001% or less, Al: 0.005 to 0.10%, N: 0.005% In the following, Nb: 0.03 to 0.10%, Ti: 0.005 to 0.05%, Ca: 0.003% or less, O: 0.005% or less, and further, Ca, S and O are the following formulas (1) and (2)
1.4 ≦ {[% Ca] − [% O] × (0.18 + 130 [% Ca])} / 1.25 [% S] <3.3 (1)
[% Ca] × [% S] 0.28 ≦ 3.6 × 10 −4 (2)
(Where [% Ca], [% O], [% S]: content of each element (mass%))
A steel material having a composition consisting of the balance Fe and inevitable impurities, rough rolling step to rough rolling and make a sheet bar, and finish rolling step to finish rolling the sheet bar to hot rolled sheet, In the method for producing a hot-rolled steel sheet, which is sequentially subjected to a winding process for winding the hot-rolled sheet, the surface layer portion is 50 ° C. on the hot-rolled sheet during the finish rolling at least once in the finish rolling process and between the rolling passes. After accelerating cooling that rapidly cools until reaching a temperature of 900 ° C. or more exceeding the Ar 3 transformation point at a cooling rate of / s or more, the accelerated cooling is stopped, and finish rolling is performed to obtain a hot-rolled sheet having a predetermined size and shape. A method for producing a hot-rolled steel sheet having excellent surface quality, fracture toughness, and sour resistance.
( 3 ) The method for producing a hot-rolled steel sheet according to (1) or (2) , wherein the finish rolling in the finish rolling step is rolling with a rolling reduction per pass of 15 to 50%.
( 4 ) In (1) or (2) , instead of the accelerated cooling, accelerated cooling is carried out by rapidly cooling the surface layer portion at a cooling rate of 50 ° C./s or more until it reaches a temperature not higher than the Ar 3 transformation point. Finishing rolling in rolling process, reduction ratio per pass is (1.1 x uniform elongation)% or less (here, uniform elongation: after heating to 950 ° C, cooling to Ar 3 transformation point or less and heating to 950 ° C again) A method for producing a hot-rolled steel sheet, characterized in that the rolling is a uniform elongation (%) in a stress-strain curve obtained when high-temperature tensioning is performed.
( 5 ) In any one of (1) to ( 4 ), in addition to the composition, Cu: 0.005-0.5%, Ni: 0.005-0.5%, Cr: 0.005-1.0%, Mo: 0.005 A method for producing a hot-rolled steel sheet, characterized in that the composition contains one or more selected from -0.5% and V: 0.005-0.3%.
( 6 ) In any one of (1) to ( 5 ), the winding temperature of the hot-rolled sheet in the winding step is set to 350 to 700 ° C., and the cooling rate after winding is set to 5 to 20 at the coil central portion. The manufacturing method of the hot-rolled steel plate characterized by setting it as (degreeC / h).
本発明によれば、表面割れ等の表面欠陥の発生がなく表面品質に優れ、しかも限界開口変位量δcが大きく、ＤＷＴＴ温度が低く破壊靭性に優れ、さらに耐サワー性にも優れた高張力熱延鋼板を、容易にかつ生産性高く製造でき、産業上格段の効果を奏する。また、本発明によれば、鋼管の材料コストを低減でき、したがってパイプラインの施工コストの更なる低減が可能となるという効果もある。 According to the present invention, there is no generation of surface defects such as surface cracks, the surface quality is excellent, the critical opening displacement amount δc is large, the DWTT temperature is low, the fracture toughness is excellent, and the sour resistance is also excellent. A rolled steel sheet can be manufactured easily and with high productivity, and has a remarkable industrial effect. In addition, according to the present invention, the material cost of the steel pipe can be reduced, and therefore there is an effect that the construction cost of the pipeline can be further reduced.
まず、本発明で使用する鋼素材の組成限定理由について説明する。なお、とくに断らないかぎり質量％は単に％と記す。
Ｃ：0.02〜0.08％、
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、0.02％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、パーライト等の第二相の組織分率を増大させ、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0.02〜0.08％の範囲に限定した。なお、好ましくは0.02〜0.05％である。
First, the reasons for limiting the composition of the steel material used in the present invention will be described. Unless otherwise specified, mass% is simply expressed as%.
C: 0.02 to 0.08%,
C is an element having an action of increasing the strength of steel, and in the present invention, it is necessary to contain 0.02% or more in order to ensure a desired high strength. On the other hand, an excessive content exceeding 0.08% increases the structural fraction of the second phase such as pearlite, and lowers the base metal toughness and the weld heat affected zone toughness. For this reason, C was limited to the range of 0.02 to 0.08%. In addition, Preferably it is 0.02 to 0.05%.
Si：0.5％以下
Siは、固溶強化、焼入れ性の向上を介して、鋼の強度を増加させるが、同時に靭性を低下させる作用を有し、また、Siは電縫溶接時にSiを含有する酸化物を形成し、電縫溶接部の表面品質を低下させる。このため、本発明では、Siはできるだけ低減することが望ましいが、0.5％までは許容できることから、Siは0.5％以下に限定した。なお、好ましくは0.4％以下である。
Si: 0.5% or less
Si increases the strength of steel through solid solution strengthening and hardenability improvement, but at the same time has the effect of lowering toughness, and Si forms an oxide containing Si during ERW welding. , Reduce the surface quality of ERW welds. For this reason, in the present invention, it is desirable to reduce Si as much as possible, but up to 0.5% is acceptable, so Si is limited to 0.5% or less. In addition, Preferably it is 0.4% or less.
Mn：0.8〜1.8％
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、0.8％以上の含有を必要とする。一方、1.8％を超える含有は、偏析を助長し、セパレーションの発生を増加させる。この偏析を消失させるには、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0.8〜1.8％の範囲に限定した。なお、好ましくは0.9〜1.5％である。
Mn: 0.8-1.8%
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 acquire such an effect, 0.8% or more needs to be contained. On the other hand, a content exceeding 1.8% promotes segregation and increases the occurrence of separation. In order to eliminate this segregation, 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 0.8 to 1.8%. In addition, Preferably it is 0.9 to 1.5%.
Ｐ：0.025％以下
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％以下に限定した。なお、好ましくは0.015％以下である。
Ｓ：0.001％以下
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、0.001％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、熱延鋼板においては粗大なMnSを形成し、延性の低下を生じさせる。このため、Ｓは0.001％以下に限定した。なお、好ましくは0.0008％以下である。
P: 0.025% or less P is inevitably contained as an impurity in steel, but has an effect of increasing the strength of steel. However, when it exceeds 0.025% and it contains excessively, weldability will fall. For this reason, P was limited to 0.025% or less. In addition, Preferably it is 0.015% or less.
S: 0.001% or less S is inevitably contained as an impurity in steel as in P, but if it exceeds 0.001% and excessively contained, slab cracking occurs and coarse MnS is contained in the hot-rolled steel sheet. Forming and causing a reduction in ductility. For this reason, S was limited to 0.001% or less. In addition, Preferably it is 0.0008% or less.
Al：0.005〜0.10％
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、0.005％以上含有することが望ましい。一方、0.10％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0.005〜0.10％に限定した。なお、好ましくは0.08％以下である。
Al: 0.005-0.10%
Al is an element that acts as a deoxidizer, and in order to obtain such an effect, it is desirable to contain 0.005% or more. On the other hand, the content exceeding 0.10% significantly impairs the cleanliness of the welded part during ERW welding. For these reasons, Al is limited to 0.005 to 0.10%. In addition, Preferably it is 0.08% or less.
Ｎ：0.005％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0.005％以下に限定した。なお、好ましくは0.004％以下である。
Nb：0.03〜0.10％
Nbは、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とともに、炭窒化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、0.03％以上の含有を必要とする。一方、0.10％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0.03〜0.10％の範囲に限定した。なお、好ましくは0.03〜0.07％である。
N: 0.005% or less N is an element inevitably contained in steel, but excessive inclusion frequently causes cracks during slab casting. For this reason, N was limited to 0.005% or less. In addition, Preferably it is 0.004% or less.
Nb: 0.03-0.10%
Nb is an element that has the effect of suppressing the coarsening and recrystallization of austenite grains, enabling the austenite non-recrystallization temperature range rolling in hot finish rolling, and by precipitating finely as carbonitride, It has the effect | action which makes a hot-rolled steel plate high intensity | strength with little content, without impairing property. In order to obtain such an effect, the content of 0.03% or more is required. On the other hand, an excessive content exceeding 0.10% may cause 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 0.03-0.10%. In addition, Preferably it is 0.03-0.07%.
Ti：0.005〜0.05％
Tiは、窒化物を形成しＮを固定しスラブ（鋼素材）割れを防止する効果を有するとともに、炭化物として微細析出することにより、鋼板を高強度化させる。このような効果は、0.005％以上の含有で顕著となるが、0.05％を超える含有は析出強化により降伏点が著しく上昇する。このため、Tiは0.005〜0.05％に限定した。なお、好ましくは0.005〜0.035％である。
Ti: 0.005-0.05%
Ti has the effect of forming nitrides and fixing N to prevent cracking of the slab (steel material), and also makes the steel sheet high in strength by being finely precipitated as carbides. Such an effect becomes remarkable when the content is 0.005% or more, but when the content exceeds 0.05%, the yield point is remarkably increased by precipitation strengthening. For this reason, Ti was limited to 0.005 to 0.05%. In addition, Preferably it is 0.005-0.035%.
Ca：0.003％以下
Caは、ＳをCaSとして固定し、硫化物系介在物を球状化する、介在物の形態を制御する作用を有し、介在物の周囲のマトリックスの格子歪を小さくして、水素のトラップ能を下げ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、0.0005％以上含有させることが好ましいが、0.003％を超える含有は、CaOの増加を招き、耐食性、靭性を低下させる。このため、Caは0.003％以下に限定した。なお、好ましくは0.0005〜0.003％である。
Ca: 0.003% or less
Ca has the action of fixing S as CaS and spheroidizing sulfide inclusions, controlling the form of inclusions, reducing the lattice strain of the matrix around the inclusions, and trapping hydrogen. In order to obtain such an effect, it is preferably contained in an amount of 0.0005% or more. However, if it exceeds 0.003%, CaO is increased and corrosion resistance and toughness are reduced. For this reason, Ca was limited to 0.003% or less. In addition, Preferably it is 0.0005 to 0.003%.
Ｏ：0.005％以下
Ｏは、鋼中では、各種の酸化物を形成し、熱間加工性、耐食性、靭性等を低下させる。このため、本発明では、できるだけ低減することが好ましいが、0.005％までは許容できる。極端な低減は精錬コストの高騰を招くため、Ｏは0.005％以下に限定した。
本発明では、Ca、Ｓ、Ｏを、上記した範囲内で、さらに次（１）式、（２）式
1.4≦｛［％Ca］−［％Ｏ］×（0.18＋130［％Ca］）｝／1.25［％Ｓ］＜3.3‥‥（１）
［％Ca］×［％Ｓ］0.28 ≦3.6×10−４ ‥‥（２）
（ここで、［％Ca］、［％Ｏ］、［％Ｓ］：各元素の含有量（質量％））
を満足するように含有する。Ca、Ｓ、Ｏが（１）式を、Ca、Ｓが（２）式を満足することにより、腐食性のつよいサワー環境においても、耐食性、耐腐食割れ性が低下しない介在物形状に調整される。（１）式の中央値が1.4未満となる場合は、圧延方向に伸びたMnSが形成され、著しくＨＩＣ発生が助長される。一方、（１）式の中央値が、3.3以上となる場合は、CaOクラスターの生成が顕著となりＨＩＣの発生が助長される。（２）式の左辺値が3.6×10-4を超える場合は、CaSクラスターの形成が助長されＨＩＣの発生が助長される。
O: 0.005% or less O forms various oxides in steel and reduces hot workability, corrosion resistance, toughness, and the like. Therefore, in the present invention, it is preferable to reduce as much as possible, but up to 0.005% is acceptable. Since an extreme reduction leads to an increase in refining costs, O is limited to 0.005% or less.
In the present invention, Ca, S, and O are within the above-described range, and the following formulas (1) and (2)
1.4 ≦ {[% Ca] − [% O] × (0.18 + 130 [% Ca])} / 1.25 [% S] <3.3 (1)
[% Ca] × [% S] 0.28 ≦ 3.6 × 10 −4 (2)
(Where [% Ca], [% O], [% S]: content of each element (mass%))
Is contained so as to satisfy. When Ca, S, and O satisfy the formula (1) and Ca and S satisfy the formula (2), the inclusion shape is adjusted so that corrosion resistance and corrosion cracking resistance do not deteriorate even in a highly corrosive sour environment. The When the median of the formula (1) is less than 1.4, MnS extending in the rolling direction is formed, and the generation of HIC is remarkably promoted. On the other hand, when the median value of the equation (1) is 3.3 or more, the generation of CaO clusters is remarkable and the generation of HIC is promoted. When the value on the left side of equation (2) exceeds 3.6 × 10 −4 , the formation of CaS clusters is promoted and the generation of HIC is promoted.
上記した成分が基本の組成であるが、この基本の組成に加えてさらに、Cu：0.005〜0.5％、Ni：0.005〜0.5％、Cr：0.005〜1.0％、Mo：0.005〜0.5％、Ｖ：0.005〜0.3％のうちから選ばれた１種または２種以上を含有する組成としてもよい。
Cu、Ni、Cr、Mo、Ｖはいずれも、焼入れ性を向上させ、鋼板の強度を増加させる元素であり、必要に応じて１種または２種以上を選択して含有できる。
The above components are basic compositions. In addition to this basic composition, Cu: 0.005-0.5%, Ni: 0.005-0.5%, Cr: 0.005-1.0%, Mo: 0.005-0.5%, V: It is good also as a composition containing 1 type, or 2 or more types chosen from 0.005-0.3%.
Cu, Ni, Cr, Mo, and V are all elements that improve the hardenability and increase the strength of the steel sheet, and can be selected from one or more as required.
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、0.005％以上含有することが望ましいが、0.5％を超える含有は熱間加工性を低下させる。このため、Cuは0.005〜0.5％に限定することが好ましい。
Niは、焼入れ性を向上させ、鋼板の強度を増加させるとともに、靭性を向上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、0.005％以上含有することが望ましいが、0.5％を超えて含有しても効果が飽和し含有量に見合う効果が期待できなくなり、経済的に不利となる。このため、Niは0.005〜0.5％に限定することが好ましい。
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 0.005% or more, but inclusion exceeding 0.5% reduces hot workability. For this reason, it is preferable to limit Cu to 0.005-0.5%.
Ni is an element that has the effect of improving hardenability, increasing the strength of the steel sheet, and improving toughness. In order to acquire such an effect, it is desirable to contain 0.005% or more, but even if it contains more than 0.5%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous. For this reason, Ni is preferably limited to 0.005 to 0.5%.
Crは、焼入性を向上させ、鋼板強度を増加させる作用を有する元素である。このような効果は、0.005％以上の含有で顕著となる。一方、1.0％を超える過剰の含有は、電縫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0.005以上1.0％以下に限定することが好ましい。なお、より好ましくは0.01〜0.8％である。
Moは、焼入性を向上させるとともに、炭化物を形成して鋼板を高強度化する作用を有する元素であり、このような効果は0.005％以上の含有で顕著となる。一方、0.5％を超える多量の含有は、溶接性を低下させる。このため、Moは0.005〜0.5％に限定することが好ましい。なお、より好ましくは0.01〜0.3％である。
Cr is an element that has the effect of improving hardenability and increasing the strength of the steel sheet. Such an effect becomes remarkable when the content is 0.005% or more. On the other hand, an excessive content exceeding 1.0% tends to cause frequent welding defects during ERW welding. For this reason, it is preferable to limit Cr to 0.005 to 1.0%. In addition, More preferably, it is 0.01 to 0.8%.
Mo is an element that has an effect of improving hardenability and forming carbides to increase the strength of the steel sheet. Such an effect becomes remarkable when the content is 0.005% or more. On the other hand, a large content exceeding 0.5% reduces weldability. For this reason, it is preferable to limit Mo to 0.005 to 0.5%. In addition, More preferably, it is 0.01 to 0.3%.
なお、Cu、Ni、Cr、Moはともに熱間圧延後の冷却過程におけるフェライト変態、引き続くパーライト変態挙動に影響を及ぼす。Cu、Ni、Cr、Moはいずれも、ポリゴナルフェライト形成を抑制しベイニティックフェライト形成を促進するとともに、パーライト変態ノーズを長時間側へ移動させる働きがあり、圧延方向に列状に並ぶパーライトの析出を抑制する働きを有する。なお、ここでいうベイニティックフェライトとは、低温にてフェライト変態し結晶粒内の転位密度が高く、ラス間にセメンタイトの析出がほとんど見られないフェライト相を指す。 Cu, Ni, Cr, and Mo all affect the ferrite transformation and subsequent pearlite transformation behavior in the cooling process after hot rolling. Cu, Ni, Cr, and Mo all suppress the formation of polygonal ferrite, promote the formation of bainitic ferrite, and have the function of moving the pearlite transformation nose to the long time side. It has the function of suppressing the precipitation of. Here, bainitic ferrite refers to a ferrite phase that undergoes ferrite transformation at a low temperature, has a high dislocation density in crystal grains, and hardly shows cementite precipitation between laths.
Ｖは、焼入性を向上させるとともに、炭窒化物を形成して鋼板を高強度化する作用を有する元素であり、このような効果は0.005％以上の含有で顕著となる。一方、0.3％を超える過剰の含有は、溶接性を劣化させる。このため、Ｖは0.005〜0.3％とすることが好ましい。なお、より好ましくは0.01〜0.2％である。
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。なお、不可避的不純物としては、Sn：0.005％以下、Mg：0.0005％以下が許容できる。
V is an element that has an effect of improving hardenability and forming carbonitride to increase the strength of the steel sheet. Such an effect becomes remarkable when the content is 0.005% or more. On the other hand, an excessive content exceeding 0.3% deteriorates weldability. For this reason, V is preferably 0.005 to 0.3%. In addition, More preferably, it is 0.01 to 0.2%.
The balance other than the components described above consists of Fe and inevitable impurities. Inevitable impurities are Sn: 0.005% or less and Mg: 0.0005% or less.
上記した組成の鋼素材に、粗圧延を施しシートバーとする粗圧延工程と、該シートバーに仕上圧延を施し熱延板とする仕上圧延工程と、該熱延板を巻き取る巻取工程とを順次施す。なお、鋼素材の製造方法はとくに限定する必要はない。上記した組成の溶鋼を通常の転炉等の溶製法で溶製し、連続鋳造法、造塊−分塊法等の通常の鋳造方法で鋼素材とすることができる。 The steel material having the above composition is subjected to rough rolling to be a sheet bar, to a rolling process, to finish rolling the sheet bar to be a hot rolled sheet, and to wind the hot rolled sheet Are applied sequentially. In addition, the manufacturing method of a steel raw material does not need to be specifically limited. The molten steel having the above composition can be melted by a melting method such as a normal converter, and used as a steel material by a normal casting method such as a continuous casting method or an ingot-bundling method.
上記した組成の鋼素材は、加熱され、まず粗圧延工程を施される。加熱温度は、とくに限定されないが、1100〜1300℃の範囲の温度とすることが好ましい。加熱温度が1100℃未満では、変形抵抗が高く、圧延機への負荷が過大となりすぎる。1300℃を超えて高温とすると、結晶粒が粗大化しすぎて熱延板の靭性が低下する。また、スケールロスが多くなり、歩留が低下する。なお、粗圧延工程では、所定寸法のシートバーとすることができればよく、とくに粗圧延条件は限定されないが、粗圧延終了温度は1050℃以上とすることが靭性の観点から好ましい。 The steel material having the above composition is heated and first subjected to a rough rolling process. Although heating temperature is not specifically limited, It is preferable to set it as the temperature of the range of 1100-1300 degreeC. When the heating temperature is less than 1100 ° C., the deformation resistance is high and the load on the rolling mill becomes excessive. When the temperature is higher than 1300 ° C., the crystal grains become too coarse and the toughness of the hot-rolled sheet decreases. In addition, scale loss increases and yield decreases. In the rough rolling process, it is sufficient that the sheet bar has a predetermined size. The rough rolling conditions are not particularly limited, but the rough rolling finish temperature is preferably 1050 ° C. or more from the viewpoint of toughness.
粗圧延工程を経て得られたシートバーには、仕上圧延工程を施すが、仕上圧延工程前に、加速冷却を施すことが好ましい。加速冷却は、シートバー等を冷却し、高靭化に有効な温度域に冷却して、その後の仕上圧延により、靭性を有効に向上させるために施す。加速冷却を施すことにより、高靭化に有効な温度域に冷却された板厚方向の領域が拡大でき、仕上圧延による靭性向上の程度を大きくすることができる。なお、仕上圧延工程前の加速冷却は、既存のFSB、ロール冷却、ストリップクーラント等の冷却手段によって容易に行える。 The sheet bar obtained through the rough rolling process is subjected to a finish rolling process, but it is preferable to perform accelerated cooling before the finish rolling process. The accelerated cooling is performed to cool the sheet bar and the like, cool to a temperature range effective for high toughness, and then effectively improve toughness by finish rolling. By performing accelerated cooling, the region in the plate thickness direction cooled to a temperature range effective for high toughening can be expanded, and the degree of toughness improvement by finish rolling can be increased. Accelerated cooling before the finish rolling process can be easily performed by existing cooling means such as FSB, roll cooling, and strip coolant.
加速冷却は、シートバーの表層部が50℃／ｓ以上の冷却速度で、Ａr３変態点超え900℃以上の温度に達するまで急冷する冷却とすることが好ましい。これにより、シートバー中心部近傍までを、高靭化に有効な温度域である、950℃以下の温度とすることができる。
なお、「表層部」とは、表面から板厚方向に２mmの範囲をいうものとする。またここで、「表層部の温度」は、放射温度計により測定される値、あるいは伝熱計算により求められる値とする。
The accelerated cooling is preferably cooling in which the surface portion of the sheet bar is rapidly cooled at a cooling rate of 50 ° C./s or more until reaching a temperature of 900 ° C. or more exceeding the Ar 3 transformation point. Thereby, it can be set as the temperature of 950 degrees C or less which is a temperature range effective for toughening to the sheet bar center part vicinity.
The “surface layer portion” means a range of 2 mm from the surface in the plate thickness direction. Here, the “surface layer temperature” is a value measured by a radiation thermometer or a value obtained by heat transfer calculation.
加速冷却の冷却速度が50℃／ｓ未満では、冷却中にフェライト変態が生じる可能性があり、その後の仕上圧延で割れを生じやすくなる。また冷却速度が50℃／ｓ未満では、高靭化に有効な温度域に冷却される板厚方向における領域が狭く、高靭化の程度が少なくなる。
加速冷却の冷却停止温度がＡr３変態点超えの900℃を超える高温となる場合には、高靭化に有効な温度域に冷却される範囲が狭く、靭性の向上代が少ない。一方、加速冷却の冷却停止温度がＡr３変態点以下となると、その後の仕上圧延条件によっては表層部に割れが発生する危険性が高くなる。
If the cooling rate of accelerated cooling is less than 50 ° C./s, ferrite transformation may occur during cooling, and cracking is likely to occur in subsequent finish rolling. On the other hand, when the cooling rate is less than 50 ° C./s, the region in the plate thickness direction that is cooled to the temperature range effective for toughening is narrow, and the degree of toughening is reduced.
When the cooling stop temperature of accelerated cooling becomes a high temperature exceeding 900 ° C. exceeding the Ar 3 transformation point, the range of cooling to a temperature range effective for toughening is narrow, and the allowance for improving toughness is small. On the other hand, when the cooling stop temperature for accelerated cooling is below the Ar 3 transformation point, the risk of cracking in the surface layer portion increases depending on the subsequent finish rolling conditions.
また、上記した加速冷却に代えて、表層部が50℃／ｓ以上の冷却速度でＡr３変態点以下の温度に達するまで急冷する加速冷却としてもよい。これにより、シートバー中心部近傍までを、高靭化に有効な温度域である、950℃以下の温度とすることが容易となる。しかし、この場合には、加速冷却後の仕上圧延の条件を特定範囲の条件とする必要がある。なお、加速冷却の冷却停止温度がＡr３変態点以下の350℃未満では、マルテンサイト変態が進行し、その後復熱し熱間圧延された場合に不均一な展伸組織が形成され、ブリスター発生などの不具合を招くため、上記した加速冷却の冷却停止温度はAr3変態点以下350℃以上とすることが好ましい。 Further, instead of the above-described accelerated cooling, accelerated cooling in which the surface layer portion is rapidly cooled at a cooling rate of 50 ° C./s or more until it reaches a temperature not higher than the Ar 3 transformation point may be employed. Thereby, it becomes easy to set the temperature up to the vicinity of the center portion of the seat bar to a temperature of 950 ° C. or lower, which is a temperature range effective for toughening. However, in this case, the finish rolling conditions after accelerated cooling must be within a specific range. When the cooling stop temperature of accelerated cooling is less than 350 ° C. below the Ar 3 transformation point, martensitic transformation proceeds, and then when reheated and hot rolled, a non-uniform stretched structure is formed, causing blistering, etc. Therefore, it is preferable that the cooling stop temperature for the above-described accelerated cooling is 350 ° C. or higher below the Ar 3 transformation point.
加速冷却を施されたシートバーは、ついで、仕上圧延工程を施す。
なお、加速冷却は、上記したように、粗圧延工程後で仕上圧延工程前に施すことに代えて、仕上圧延工程中に行ってもよく、また粗圧延工程後で仕上圧延工程前と、仕上圧延工程中とを合わせ行ってもよい。
仕上圧延工程中に加速冷却を施す場合には、少なくとも１回、圧延のパス間で、仕上圧延途中の熱延板に施すことが好ましい。仕上圧延工程中の加速冷却は、仕上圧延ミル内のクーラントを利用することにより行うことができる。なお、仕上圧延工程中の加速冷却も、仕上圧延工程前の加速冷却と同様に、圧延途中の熱延板の表層部が50℃／ｓ以上の冷却速度でＡr３変態点超え900℃以下の温度まで急冷、あるいはＡr３変態点以下、好ましくは350℃以上の温度まで急冷、する冷却とすることが好ましい。
The sheet bar subjected to accelerated cooling is then subjected to a finish rolling process.
As described above, accelerated cooling may be performed during the finish rolling process instead of after the rough rolling process and before the finishing rolling process, and after the rough rolling process and before the finishing rolling process. The rolling process may be performed together.
When accelerated cooling is performed during the finish rolling process, it is preferably applied to the hot-rolled sheet in the middle of finish rolling at least once between rolling passes. Accelerated cooling during the finish rolling process can be performed by using a coolant in the finish rolling mill. In addition, the accelerated cooling during the finish rolling process is similar to the accelerated cooling before the finish rolling process, and the surface layer portion of the hot-rolled sheet in the middle of rolling has an Ar 3 transformation point exceeding 900 ° C. at a cooling rate of 50 ° C./s or more. It is preferable that the cooling is rapid cooling to a temperature or rapid cooling to a temperature not higher than the Ar 3 transformation point, preferably 350 ° C. or higher.
加速冷却を施された熱延板は、仕上圧延工程における所定の圧下を施されて所定寸法の熱延板とされる。なお、本発明では、仕上圧延における有効圧下率は、20％以上とすることが高靭性化の観点から好ましい。有効圧下率とは、高靭化に有効な温度域である、950℃以下の温度域での全圧下量をいう。なお、板厚全体で所望の低温靱性を達成するためには、板厚中央部で所望の有効圧下率を確保する必要がある。 The hot-rolled sheet that has been subjected to accelerated cooling is subjected to a predetermined reduction in the finish rolling process to obtain a hot-rolled sheet having a predetermined size. In the present invention, the effective rolling reduction in finish rolling is preferably 20% or more from the viewpoint of increasing toughness. The effective reduction ratio refers to the total reduction amount in a temperature range of 950 ° C. or lower, which is a temperature range effective for toughening. In order to achieve the desired low-temperature toughness over the entire plate thickness, it is necessary to ensure a desired effective rolling reduction at the center portion of the plate thickness.
加速冷却の停止温度が、Ａr３変態点超えの場合には、その後に施される仕上圧延工程における仕上圧延の圧延条件はとくに限定されないが、１パス当たりの圧下率は、15〜50％の圧延とすることが好ましい。1パス当たりの圧下率が15％未満では、所望の高靭性化が期待できなかったり、板形状を損なう恐れがある。一方、１パス当たりの圧下率が50％を超えると、表面欠陥が発生しやすくなったり、圧延負荷が増大する。なお、より好ましくは15〜35％である。 When the stop temperature of accelerated cooling exceeds the Ar 3 transformation point, the rolling conditions of finish rolling in the finish rolling process to be performed thereafter are not particularly limited, but the reduction rate per pass is 15 to 50%. Rolling is preferable. If the rolling reduction per pass is less than 15%, the desired toughness cannot be expected or the plate shape may be damaged. On the other hand, if the rolling reduction per pass exceeds 50%, surface defects are likely to occur and the rolling load increases. More preferably, it is 15 to 35%.
一方、加速冷却の冷却停止温度が、Ａr３変態点以下の温度の場合には、表層部の組織がフェライト＋オーステナイトの二相となり、表面欠陥の発生を避けるために、本発明では、加速冷却停止後の仕上圧延における１パス当たりの圧下率を、（1.1×一様伸び）％以下に限定することが好ましい。なお、ここでいう「一様伸び」は、引張試験片を950℃まで加熱したのちＡr３変態点以下まで加速冷却し、その後950℃まで加熱して、その場で高温引張を行ったときに、得られる応力−歪曲線における一様伸び値（％）をいう。１パス当たりの圧下率が、（1.1×一様伸び）％を超えて大きくなると、析出したフェライトに歪が集中し、表層に割れを誘発しやすくなる。 On the other hand, when the cooling stop temperature of accelerated cooling is a temperature below the Ar 3 transformation point, the structure of the surface layer portion becomes two phases of ferrite and austenite. It is preferable to limit the rolling reduction per pass in finish rolling after stopping to (1.1 × uniform elongation)% or less. “Uniform elongation” as used herein means that when a tensile test piece is heated to 950 ° C., acceleratedly cooled to below the Ar 3 transformation point, then heated to 950 ° C., and then subjected to high temperature tension in situ. The uniform elongation value (%) in the obtained stress-strain curve. If the rolling reduction per pass exceeds (1.1 × uniform elongation)%, the strain concentrates on the precipitated ferrite, and it is easy to induce cracks in the surface layer.
なお、表面割れは表層部と板厚中心での温度差が大きくなる厚肉材で起こりやすくなるため、本発明の効果は特に厚肉材で発揮される。ここで、厚肉材とはWT17.5mm以上、特にWT19.1mm以上を指す。
仕上圧延工程を経て得られた熱延板は、ついで巻取工程でコイル状に巻き取られる。本発明における巻取工程では、巻取り温度は350〜700℃とすることが好ましい。なお、仕上圧延終了後、熱延板に、好ましくは冷却速度：５〜50℃／ｓで、巻取り温度まで冷却する。仕上圧延後の冷却は、好ましくは仕上圧延ミルに引き続く冷却設備を有するホットランナウトの前詰（前段）にて行う。これによりホットランナウト上でフェライト変態を進行させることができ、所望のベイニティックフェライト組織を達成しやすい。巻取り温度が350℃未満では、鋼板各位置での温度ばらつきが大きくなり、材質ばらつきや形状のばらつきが生じ、さらには、コイラー能力によっては巻き取ることができない場合も生ずる。一方、巻取り温度が700℃を超えると、仕上圧延ミルに引き続く冷却設備を有するホットランナウト上で板厚中心部でフェライト変態が進行せず、巻取り後徐冷されながらフェライト変態が進行するため、結晶粒が粗大化し、靭性が低下する。このようなことから、巻取り温度は350〜700℃とすることが好ましい。また、コイル状に巻き取ったのち、コイル中央部の冷却速度で５〜20℃／ｈで室温まで冷却することが好ましい。冷却速度が、５℃／h未満では結晶粒成長が進行し、靱性が低下する場合がある。一方20℃／hを超える場合には、コイル中央部と外周部や内周部との温度差が大きくなりコイル形状の悪化を招きやすくなる。
In addition, since the surface crack is likely to occur in a thick material having a large temperature difference between the surface layer portion and the center of the plate thickness, the effect of the present invention is particularly exerted in the thick material. Here, the thick material refers to WT17.5 mm or more, particularly WT19.1 mm or more.
The hot-rolled sheet obtained through the finish rolling process is then wound into a coil in the winding process. In the winding process in the present invention, the winding temperature is preferably 350 to 700 ° C. After finishing rolling, the hot-rolled sheet is cooled to a coiling temperature, preferably at a cooling rate of 5 to 50 ° C./s. Cooling after finish rolling is preferably performed by pre-packing (previous stage) of hot runout having a cooling facility following the finish rolling mill. As a result, the ferrite transformation can proceed on the hot runout, and a desired bainitic ferrite structure is easily achieved. When the coiling temperature is less than 350 ° C., the temperature variation at each position of the steel sheet increases, resulting in material variation and shape variation, and further, depending on the coiler ability, winding may not be possible. On the other hand, if the coiling temperature exceeds 700 ° C, the ferrite transformation does not proceed at the center of the plate thickness on the hot runout having the cooling equipment following the finish rolling mill, and the ferrite transformation proceeds while being gradually cooled after winding. The crystal grains become coarse and the toughness decreases. Therefore, the winding temperature is preferably 350 to 700 ° C. Moreover, after winding up in coil shape, it is preferable to cool to room temperature at 5-20 degreeC / h by the cooling rate of the coil center part. When the cooling rate is less than 5 ° C./h, crystal grain growth proceeds and the toughness may be lowered. On the other hand, when it exceeds 20 ° C./h, the temperature difference between the coil central part and the outer peripheral part or the inner peripheral part becomes large and the coil shape is liable to be deteriorated.
表１に示す組成を有するスラブ（鋼素材）（肉厚：215mm）を、表2に示す温度に加熱し、表2に示す条件で粗圧延工程、仕上圧延工程、および巻取工程を施し、表2に示す板厚の熱延板とした。なお、加速冷却を、粗圧延工程後で仕上圧延工程前に、および／または、仕上圧延工程中のパス間に、表２に示す条件で施した。仕上圧延工程後の巻取工程では、表２に示す冷却速度で冷却し、表２に示す巻取り温度で巻き取り、コイル状としたのち、表２に示す冷却速度で室温まで冷却した。 A slab (steel material) having the composition shown in Table 1 (wall thickness: 215 mm) is heated to the temperature shown in Table 2, and subjected to a rough rolling process, a finish rolling process, and a winding process under the conditions shown in Table 2. A hot-rolled sheet having a thickness shown in Table 2 was used. In addition, accelerated cooling was performed under the conditions shown in Table 2 after the rough rolling process and before the finish rolling process and / or between passes during the finish rolling process. In the winding process after the finish rolling process, the sheet was cooled at the cooling rate shown in Table 2, wound at the winding temperature shown in Table 2, coiled, and then cooled to room temperature at the cooling rate shown in Table 2.
得られた熱延板について、表面品質試験、引張試験、衝撃試験、ＣＴＯＤ試験、ＤＷＴＴ試験、ＨＩＣ試験を実施した。試験方法は次のとおりである。
（１）表面品質試験
得られた熱延板について、鋼板の全域にわたり表面を目視またはビデオカメラで観察し、割れの有無を調査し、表面品質を評価した。割れ等の表面欠陥が発生した場合を×、発生しなかった場合を○として評価した。表面割れは深さ100μm以上の割れを指す。
（２）引張試験
得られた熱延板から引張試験片を採取し、ASTM規格E8の規定に準拠して引張試験を実施した。引張試験片は、試験方向が圧延方向に直角になるように採取した標点間距離２インチ、平行部板幅１／２インチの板状試験片を用いて、室温における降伏強さ（YS）、引張強さ（TS）を測定した。
（３）衝撃試験
得られた熱延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、試験温度：−80℃での吸収エネルギー（Ｊ）を求めた。なお、試験片は３本とし、得られた吸収エネルギー値の算術平均をもとめ、その鋼板の吸収エネルギー値vE−80（Ｊ）とした。なお、vE−80が300Ｊ以上の場合を低温靱性が優れると評価した。
（４）ＣＴＯＤ試験
得られた熱延鋼板から、試験片の長手方向が鋼板の圧延方向に直交する方向（Ｃ方向）となるように、試験片を採取し、ASTM E 1290の規定に準拠して試験温度：−10℃においてＣＴＯＤ試験を実施した。試験荷重は三点曲げ方式で負荷し、切欠に変位計を取り付けて限界開口変位量δcを測定した。このδc値が0.25mm以上である場合に、鋼板の破壊靱性が優れると評価した。
（５）ＤＷＴＴ試験
得られた熱延板から、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ＤＷＴＴ試験片を採取し、ASTM E436の規定に準拠して、ＤＷＴＴ試験を実施し、ＤＷＴＴ温度（℃）（：延性破面率が85％となる最低温度）を求め、破壊靱性を評価した。なお、ＤＷＴＴ温度が−15℃以下である場合には、破壊靱性に優れると評価した。
（６）ＨＩＣ試験
得られた熱延板から、試験片の長手方向が鋼板の圧延方向に平行となるように、試験片を採取し、NACE規格TMO284の規定に準拠してＨＩＣ試験を実施した。ＨＩＣ試験は、試験片を該規格に規定のA溶液中に浸漬したのち、ＣＬＲ値を測定した。なお、ＣＬＲ値が０％である場合には、ＨＩＣの発生が認められず、耐ＨＩＣ性(耐サワー性)に優れると評価した。
The obtained hot-rolled sheet was subjected to a surface quality test, a tensile test, an impact test, a CTOD test, a DWTT test, and an HIC test. The test method is as follows.
(1) Surface quality test About the obtained hot-rolled sheet, the surface was observed visually or with the video camera over the whole region, the presence or absence of a crack was investigated, and the surface quality was evaluated. The case where a surface defect such as a crack occurred was evaluated as x, and the case where it did not occur was evaluated as o. A surface crack refers to a crack with a depth of 100 μm or more.
(2) Tensile test Tensile test pieces were collected from the obtained hot-rolled sheet and subjected to a tensile test in accordance with the provisions of ASTM standard E8. The tensile test specimen is a yield strength (YS) at room temperature using a plate-shaped specimen with a distance between the gauge points of 2 inches and a parallel part width of 1/2 inch taken so that the test direction is perpendicular to the rolling direction. The tensile strength (TS) was measured.
(3) Impact test V-notch test specimens were taken from the center of the thickness of the obtained hot-rolled 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, and the absorbed energy (J) at a test temperature of −80 ° C. was obtained. The number of test pieces was three, and the arithmetic average of the obtained absorbed energy values was obtained to obtain the absorbed energy value vE- 80 (J) of the steel sheet. In addition, it evaluated that the case where vE- 80 was 300J or more was excellent in low temperature toughness.
(4) CTOD test From the obtained hot-rolled steel sheet, a test piece is taken so that the longitudinal direction of the test piece is in a direction (C direction) perpendicular to the rolling direction of the steel sheet, and conforms to the provisions of ASTM E 1290. Test temperature: CTOD test was conducted at -10 ° C. The test load was applied by a three-point bending method, and a displacement meter was attached to the notch to measure the limit opening displacement amount δc. When the Δc value was 0.25 mm or more, it was evaluated that the fracture toughness of the steel sheet was excellent.
(5) DWTT test From the obtained hot-rolled sheet, a DWTT test piece is collected so that the direction perpendicular to the rolling direction (C direction) is the longitudinal direction, and the DWTT test is performed in accordance with the provisions of ASTM E436. Then, the DWTT temperature (° C.) (: the lowest temperature at which the ductile fracture surface ratio becomes 85%) was determined, and the fracture toughness was evaluated. In addition, when DWTT temperature was -15 degrees C or less, it evaluated that it was excellent in fracture toughness.
(6) HIC test From the obtained hot-rolled sheet, a test piece was collected so that the longitudinal direction of the test piece was parallel to the rolling direction of the steel sheet, and the HIC test was performed in accordance with the rules of NACE standard TMO284. . In the HIC test, the CLR value was measured after immersing the test piece in the A solution defined in the standard. In addition, when CLR value was 0%, generation | occurrence | production of HIC was not recognized but it evaluated that it was excellent in HIC resistance (sour resistance).
得られた結果を表３に示す。 The obtained results are shown in Table 3.
本発明例はいずれも表面割れの発生もなく表面品質に優れ、vE−80が300J以上と低温靱性に優れ、−10℃におけるδcが0.25mm以上、ＤＷＴＴ温度が−10℃未満と破壊靱性にも優れ、ＣＬＲ値も０％と耐サワー性にも優れた熱延板となっている。
鋼No.Aの組成を有する鋼板は、いずれも耐サワー性は良好であるが、加速冷却を施さない比較例(熱延板No.１)では、表面品質は良好であるが、vE−80が300J未満と低温靱性が低下し、ＤＷＴＴ温度が−５℃と破壊靱性が低下している。また、仕上圧延が本発明の範囲を外れる比較例(熱延板No.２、No.３)では、表面割れが発生し、表面品質が低下し、さらに破壊靱性、あるいは低温靱性および破壊靱性が低下している。
All of the inventive examples have excellent surface quality with no occurrence of surface cracks, excellent low-temperature toughness with vE- 80 of 300J or higher, δc at -10 ° C of 0.25 mm or higher, and DWTT temperature of less than -10 ° C with high fracture toughness. The CLR value is also 0%, which is a hot-rolled sheet excellent in sour resistance.
All the steel plates having the composition of steel No. A have good sour resistance, but the surface quality is good in the comparative example (hot rolled plate No. 1) where accelerated cooling is not applied, but vE- 80 Is less than 300 J, the low temperature toughness is lowered, and the DWTT temperature is −5 ° C., and the fracture toughness is lowered. Further, in the comparative examples (hot rolled sheets No. 2 and No. 3) in which the finish rolling is outside the scope of the present invention, surface cracks are generated, the surface quality is deteriorated, and fracture toughness, or low temperature toughness and fracture toughness are exhibited. It is falling.
鋼No.Bの組成を有する鋼板はいずれも耐サワー性は良好であるが、加速冷却が本発明範囲を外れる比較例(熱延板No.10)では、表面品質、低温靱性は良好であるが、ＤＷＴＴ温度が−10℃と破壊靱性が低下している。
鋼No.Cの組成を有する鋼板はいずれも耐サワー性は良好であるが、仕上圧延が本発明の範囲を外れる比較例(熱延板No.13)は、有効圧下率が高い圧延のため低温靭性は極めて良好であるが、表面割れが発生し表面品質が低下し、さらにＤＷＴＴ温度が−10℃と破壊靱性が低下している。
All steel sheets having the composition of Steel No. B have good sour resistance, but in the comparative example (hot rolled sheet No. 10) in which accelerated cooling is outside the scope of the present invention, the surface quality and low temperature toughness are good. However, the DWTT temperature is -10 ° C. and the fracture toughness is lowered.
Steel plates having the composition of steel No. C have good sour resistance, but the comparative example (hot rolled plate No. 13) in which the finish rolling is outside the scope of the present invention is due to rolling with a high effective rolling reduction. Although the low temperature toughness is very good, surface cracking occurs, the surface quality is lowered, and the DWTT temperature is −10 ° C. and the fracture toughness is lowered.
鋼No.Dの組成を有する鋼板はいずれも耐サワー性は良好であるが、加速冷却、仕上圧延が本発明の範囲を外れる比較例(熱延板No.15)は、低温靱性は良好であるが、表面割れが発生し表面品質が低下し、さらにＤＷＴＴ温度が−５℃と破壊靱性が低下している。
鋼No.F、Gの組成を有する鋼板(熱延板No.17、18)は、表面品質、低温靱性、破壊靱性は良好であるが、鋼組成が(１)式を満足しないか、(１)、(２)式を両方とも満足しておらず、本発明の範囲を外れるため、耐サワー性が低下している。
Steel sheets having the composition of steel No. D all have good sour resistance, but the comparative example (hot rolled sheet No. 15) in which accelerated cooling and finish rolling are outside the scope of the present invention has good low temperature toughness. However, surface cracks are generated, the surface quality is lowered, and the DWTT temperature is -5 ° C. and the fracture toughness is lowered.
Steel sheets having the compositions of steel No. F and G (hot rolled sheets No. 17 and 18) have good surface quality, low temperature toughness and fracture toughness, but the steel composition does not satisfy the formula (1), Both the formulas 1) and (2) are not satisfied and are outside the scope of the present invention, so the sour resistance is reduced.
Claims (6)
Ｃ：0.02〜0.08％、 Si：0.5％以下、
Mn：0.8〜1.8％、 Ｐ：0.025％以下、
Ｓ：0.001％以下、 Al：0.005〜0.10％、
Ｎ：0.005％以下、 Nb：0.03〜0.10％、
Ti：0.005〜0.05％、 Ca：0.003％以下、
Ｏ：0.005％以下
を含み、さらにCa、Ｓ、Ｏが下記（１）式、（２）式を満足し、残部Feおよび不可避的不純物からなる組成を有する鋼素材に、粗圧延を施しシートバーとする粗圧延工程と、該シートバーに仕上圧延を施し熱延板とする仕上圧延工程と、該熱延板を巻き取る巻取工程とを順次施す熱延鋼板の製造方法において、
前記粗圧延工程後で、前記仕上圧延工程前に、前記シートバーに、表層部を50℃／ｓ以上の冷却速度でＡr３変態点超え900℃以下の温度に達するまで急冷する加速冷却を施したのち、該加速冷却を停止し、しかる後に仕上圧延工程を施すことを特徴とする表面品質、破壊靱性および耐サワー性に優れる熱延鋼板の製造方法。
記
1.4≦｛［％Ca］−［％Ｏ］×（0.18＋130［％Ca］）｝／1.25［％Ｓ］＜3.3‥‥（１）
［％Ca］×［％Ｓ］0.28 ≦3.6×10−４ ‥‥（２）
ここで、［％Ca］、［％Ｏ］、［％Ｓ］：各元素の含有量（質量％） % By mass
C: 0.02 to 0.08%, Si: 0.5% or less,
Mn: 0.8 to 1.8%, P: 0.025% or less,
S: 0.001% or less, Al: 0.005-0.10%,
N: 0.005% or less, Nb: 0.03-0.10%,
Ti: 0.005-0.05%, Ca: 0.003% or less,
O: A steel sheet containing 0.005% or less, further Ca, S, O satisfying the following formulas (1) and (2), and having a composition comprising the balance Fe and unavoidable impurities, and then subjected to rough rolling to a sheet bar In the method of manufacturing a hot-rolled steel sheet, sequentially performing a rough rolling process, a finish-rolling process in which the sheet bar is subjected to finish rolling and a hot-rolled sheet, and a winding process of winding the hot-rolled sheet,
After the rough rolling step and before the finish rolling step, the sheet bar is subjected to accelerated cooling that rapidly cools the surface layer portion at a cooling rate of 50 ° C./s or higher until the temperature reaches the temperature exceeding 900 ° C. above the Ar 3 transformation point. Then, the accelerated cooling is stopped, and then a finish rolling step is performed, and a method for producing a hot-rolled steel sheet having excellent surface quality, fracture toughness and sour resistance.
Record
1.4 ≦ {[% Ca] − [% O] × (0.18 + 130 [% Ca])} / 1.25 [% S] <3.3 (1)
[% Ca] × [% S] 0.28 ≦ 3.6 × 10 −4 (2)
Here, [% Ca], [% O], [% S]: Content of each element (% by mass)
Ｃ：0.02〜0.08％、 Si：0.5％以下、
Mn：0.8〜1.8％、 Ｐ：0.025％以下、
Ｓ：0.001％以下、 Al：0.005〜0.10％、
Ｎ：0.005％以下、 Nb：0.03〜0.10％、
Ti：0.005〜0.05％、 Ca：0.003％以下、
Ｏ：0.005％以下
を含み、さらにCa、Ｓ、Ｏが下記（１）式、（２）式を満足し、残部Feおよび不可避的不純物からなる組成を有する鋼素材に、粗圧延を施しシートバーとする粗圧延工程と、該シートバーに仕上圧延を施し熱延板とする仕上圧延工程と、該熱延板を巻き取る巻取工程とを順次施す熱延鋼板の製造方法において、
前記仕上圧延工程で少なくとも１回、圧延パス間で、仕上圧延途中の熱延板に、表層部が50℃／ｓ以上の冷却速度でＡr３変態点超え900℃以下の温度に達するまで急冷する加速冷却を施したのち、該加速冷却を停止し、さらに仕上圧延を行い所定寸法形状の熱延板とすることを特徴とする表面品質、破壊靭性および耐サワー性に優れる熱延鋼板の製造方法。
記
1.4≦｛［％Ca］−［％Ｏ］×（0.18＋130［％Ca］）｝／1.25［％Ｓ］＜3.3‥‥（１）
［％Ca］×［％Ｓ］0.28 ≦3.6×10−４ ‥‥（２）
ここで、［％Ca］、［％Ｏ］、［％Ｓ］：各元素の含有量（質量％） % By mass
C: 0.02 to 0.08%, Si: 0.5% or less,
Mn: 0.8 to 1.8%, P: 0.025% or less,
S: 0.001% or less, Al: 0.005-0.10%,
N: 0.005% or less, Nb: 0.03-0.10%,
Ti: 0.005-0.05%, Ca: 0.003% or less,
O: A steel sheet containing 0.005% or less, further Ca, S, O satisfying the following formulas (1) and (2), and having a composition comprising the balance Fe and unavoidable impurities, and then subjected to rough rolling to a sheet bar In the method of manufacturing a hot-rolled steel sheet, sequentially performing a rough rolling process, a finish-rolling process in which the sheet bar is subjected to finish rolling and a hot-rolled sheet, and a winding process of winding the hot-rolled sheet,
In the finish rolling step, at least once, between the rolling passes, the hot-rolled sheet in the middle of finish rolling is rapidly cooled at a cooling rate of 50 ° C./s or more until the temperature reaches 900 ° C. or below the Ar 3 transformation point. A method for producing a hot-rolled steel sheet having excellent surface quality, fracture toughness and sour resistance, characterized in that the accelerated cooling is stopped after the accelerated cooling, and then finish rolling is performed to obtain a hot-rolled sheet having a predetermined size and shape. .
Record
1.4 ≦ {[% Ca] − [% O] × (0.18 + 130 [% Ca])} / 1.25 [% S] <3.3 (1)
[% Ca] × [% S] 0.28 ≦ 3.6 × 10 −4 (2)
Here, [% Ca], [% O], [% S]: Content of each element (% by mass)
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