JP2004143555A

JP2004143555A - Method for manufacturing steel product for low temperature use having excellent stress corrosion cracking resistance

Info

Publication number: JP2004143555A
Application number: JP2002311591A
Authority: JP
Inventors: Shinichi Suzuki; 鈴木　伸一; Nobuyuki Ishikawa; 石川　信行
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2002-10-25
Filing date: 2002-10-25
Publication date: 2004-05-20
Anticipated expiration: 2022-10-25
Also published as: JP3906779B2

Abstract

<P>PROBLEM TO BE SOLVED: To efficiently manufacture steel products for low temperature use used for steel structures for which stress corrosion cracking resistance is demanded, such as tanks and refining plants used for storage and transportation of various liquefied gases like liquefied ammonia, at low cost. <P>SOLUTION: The steel containing, by mass %, 0.02 to 0.12% C, 0.05 to 0.5% Si, and 0.6 to ≤1.8% Mn, is heated to a temperature range from 1,000 to 1,200°C and is then subjected to hot rolling at a draft ≥50% in a recrystallization temperature region and at a draft ≥30% in a non-recystallization temperature. The steel is cooled down to a temperature region from 400 to 600°C in average temperature in succession after the end of the rolling and is thereafter heated to a temperature range from a surface temperature of ≥500 to ≤Acl+50°C by using an induction heater of a frequency of 200 to 2,000 Hz and is then air cooled. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
この発明は、耐応力腐食割れ性に優れた低温用鋼材の製造方法、特に、液化アンモニア等の各種液化ガスの貯蔵や運搬に用いられるタンクや精製プラント等、耐応力腐食割れ性が要求される鋼構造物に用いられる低温用鋼材の製造方法に関するものである。
【０００２】
【従来の技術】
各種の液化ガスの貯蔵、運搬に用いられるタンクや精製プラント等の鋼構造物においては、共用時に低温になるため低温靭性が要求される。さらに、近年増加している混載型のタンクでは、液化アンモニアを積載する場合もあり、低温靭性に加え、液化アンモニアによる応力腐食割れ防止対策も必要となる。
【０００３】
応力腐食割れを防止するための指針としては、鋼材のＮｉ量を制限することや、降伏強度や表層部硬度を抑制すること等が知られており、特に、液化アンモニアによる応力腐食割れに対しては、鋼材の降伏強度を４４０Ｎ／ｍｍ^２以下とすることや、鋼材の表面硬度をビッカース硬さ（ＨＶ）２２０以下とすることで防止可能であることが知られている（「液体アンモニアによる鋼の応力腐食割れ」、鉄と鋼、Ｖｏｌ．６５（１９７９）、ｐ．４０２）。
【０００４】
表層部硬度の低減方法については、これまでに多くの検討が行われており、例えば、特開平１０−１３０７２１号公報および特開平１０−１６８５１６号公報には、圧延後に二相域再加熱焼入れ・焼戻しを行うことにより低降伏比を達成する方法、特開平４−１７６１３号公報には、焼入れを行った鋼板表面部に高密度エネルギー熱源による照射を行い表層部を軟化させる方法、特開２００１−２４０９３６号公報には、Ａｃ１−５０℃以下の温度域で圧延を行った後、二相域に加熱して表層部に粗粒フェライト層を生成させる方法等が開示されている。
【０００５】
【特許文献１】
特開平１０−１３０７２１号公報
【特許文献２】
特開平１０−１６８５１６号公報
【特許文献３】
特開平４−１７６１３号公報
【特許文献４】
特開２００１−２４０９３６号公報
【０００６】
【発明が解決しようとする課題】
しかしながら、特開平１０−１３０７２１号公報および特開平１０−１６８５１６号公報に開示の方法では、再加熱焼入れ、焼戻しが必要であると共に、高価な成分元素であるＮｉの添加が必須であり、生産性や製造コストの面で難点がある。また、特開平４−１７６１３号公報に開示の方法では、鋼板表面のみからの加熱であるため、表層から数ｍｍ程度の範囲で硬度が上昇している場合には、硬度低下の効果が十分でなく、また、焼入性が高い成分系では、表面温度がＡｃ１変態点を超えると、冷却時に再焼入れされ硬度が再び上昇して耐応力腐食割れ性が劣化する。また、特開２００１−２４０９３６号公報に開示の方法では、通常の鋼板圧延の後、さらに、室温もしくは温間での圧延が必要であるため圧延負荷が大きく、また、形鋼等の複雑な断面形状を有する鋼材には適用し難い。
【０００７】
従って、この発明の目的は、上述の問題点を解消し、液化アンモニアの貯蔵や運搬に用いられる混載型のタンクや精製プラント等、耐応力腐食割れ性が要求される鋼構造物に用いられる低温用鋼を安価で効率よく製造する方法を提供することにある。
【０００８】
【課題を解決するための手段】
請求項１記載の発明は、質量％で、Ｃ：０．０２〜０．１２％、Ｓｉ：０．０５〜０．５％、Ｍｎ：０．６〜１．８％以下を含有する鋼を、１０００℃以上１２００℃以下の温度範囲に加熱後、再結晶温度域において圧下率５０％以上、未再結晶温度域で圧下率３０％以上の熱間圧延を行い、圧延終了後、引き続き２℃／秒以上の冷却速度で、平均温度が４００℃以上６００℃以下の温度域まで冷却した後、周波数２００Ｈｚ以上２０００Ｈｚ以下の誘導加熱装置を用いて表面温度５００℃以上Ａｃ１＋５０℃以下の温度範囲まで加熱してから空冷することに特徴を有するものである。
【０００９】
請求項２記載の発明は、前記鋼が、さらに、質量％で、Ｃｕ：０．０５〜１．０％、Ｎｉ：０．０５〜１．０％、Ｃｒ：０．０５〜１．０％、Ｍｏ：０．０１〜１．０％のうちの少なくとも１種を含有することに特徴を有するものである。
【００１０】
請求項３記載の発明は、前記鋼が、さらに、質量％で、Ｎｂ：０．００５〜０．１％、Ｖ：０．００５〜０．１％およびＴｉ：０．００５〜０．０３％のうちの少なくとも１種を含有することに特徴を有するものである。
【００１１】
【発明の実施の形態】
本発明者等は、複雑な圧延・熱処理工程を経たり、高価な合金元素を添加したりすることなく、耐応力腐食割れ性に優れた低温用鋼材の製造方法を得るべく鋭意検討を重ねた。この結果、制御圧延に引き続く加速冷却後、高周波誘導加熱を行うことにより表層部を軟化させれば、低温靭性および耐応力腐食割れ性の両立を図ることが可能であるという知見を得た。
【００１２】
表１に示す鋼種Ａからなる鋼板を１１００℃に加熱した後、図１に示すように、熱間圧延機３により圧延終了温度８００℃の条件で圧延し、次いで、このようにして圧延した板厚２５ｍｍの鋼板２Ａを、加速冷却装置４により７８０℃から２５℃／秒の冷却速度で５００℃まで水冷し、次いで、水冷された鋼板２Ｂを誘導加熱装置５により種々の温度に加熱し、そして、ホットレベラー６により形状矯正して、鋼板２Ｃを調製した。なお、図１において、１は、圧延ラインを示す。
【００１３】
図２に、このようにして調製した鋼板２Ｃの表面加熱温度と表層部硬度および引張強さとの関係を、誘導加熱装置５に代えて雰囲気炉により鋼板２Ｂを加熱した場合の結果と併せて示す。
【００１４】
図２から明らかなように、雰囲気炉（○）を用いた場合は、表面加熱温度の上昇に伴い引張強さが低下し、高張力鋼としての４９０Ｎ／ｍｍ^２以上の引張強度が得られないのに対し、誘導加熱装置（●）を用いて表層部を５００℃からＡｃ１＋５０℃（鋼Ａの場合は７５９℃）の温度範囲に加熱した場合には、鋼板の引張強さを確保しつつ表層部の硬度低下を図ることができることが分かる。
【００１５】
以上の知見に基づき、本発明者は、鋼組成、圧延・加速冷却および表層再加熱条件を制御して、耐応力腐食割れ性に優れた低温用鋼材の製造方法を見出し、この発明を完成させた。
【００１６】
この発明によれば、液化アンモニア等の各種液化ガスの貯蔵や運搬に用いられるタンクや精製プラント等、耐応力腐食割れ性が要求される鋼構造物に用いられる低温用鋼を安価で効率よく製造することができる。
【００１７】
以下、この発明の成分添加理由、成分限定理由および製造条件の限定範囲について説明する。
【００１８】
（１）成分組成範囲
Ｃ：０．０２〜０．１２％
Ｃは、鋼の強度を確保するために０．０２％以上添加するが、０．１２％を超えて多量に含有させると低温靭性あるいは溶接性が劣化する。従って、Ｃ添加量は、０．０２〜０．１２％の範囲内とする。
【００１９】
Ｓｉ：０．０５〜０．５％
Ｓｉは、脱酸のために０．０５％以上の添加が必要であるが、０．５％を超えるとＨＡＺ靭性および溶接性が劣化する。従って、Ｓｉ添加量は、０．０５〜０．５％の範囲内とする。
【００２０】
Ｍｎ：０．６〜１．８％
Ｍｎは、鋼材の強度・靭性の向上ならびにＦｅＳの生成抑制のため０．６％以上は必要であるが、１．８％を超える多量の添加は鋼の焼き入れ性の増加を引き起こし、溶接時に硬化層が生成して割れ感受性が高くなる。従って、Ｍｎ添加量は、０．６〜１．８％の範囲内とする。
【００２１】
この発明は、以上を基本成分とし、以下の選択成分群の１種または２種以上を添加する。
【００２２】
Ｃｕ：０．０５〜１．０％
Ｃｕは、強度上昇および靭性改善に非常に有効な元素であるが、含有量が０．０５％未満では十分な効果が発揮されず、一方、１．０％を超えると析出硬化が著しくまた圧延時に鋼材表面に割れが生じやすい。従って、Ｃｕを添加する場合には、その範囲は、０．０５〜１．０％とする。
【００２３】
Ｎｉ：０．０５〜１．０％
Ｎｉは、母材の強度ならびに靭性を向上させる効果を有するが、その含有量が０．０５％未満では十分な効果が得られず、一方、１．０％を超える添加は焼入性を上昇させて表層加熱後の硬度が高くなり耐応力腐食割れ性を劣化させると共に、コストアップにつながる。従って、Ｎｉを添加する場合には、その範囲を０．０５〜１．０％とする。
【００２４】
Ｃｒ：０．０５〜１．０％
Ｃｒは、焼入性向上に有効な元素であるが、その含有量が０．０５％未満では効果が小さく、一方、１．０％を超えると溶接性やＨＡＺ靭性を劣化させる。従って、Ｃｒを添加する場合には、その範囲を０．０５〜１．０％とする。
【００２５】
Ｍｏ：０．０５〜１．０％
Ｍｏは、焼入性を高めると共に、焼き戻し軟化抵抗を高め、強度上昇に有効であるが、その含有量が０．０５％未満ではその効果が十分に発揮されず、一方、１．０％を超えると溶接性を劣化させると共に、炭化物の析出により降伏比が上昇する。従って、Ｍｏを添加する場合には、その範囲を０．０５〜１．０％とする。
【００２６】
Ｎｂ：０．００５〜０．１％
Ｎｂは、少量の添加により焼入性を向上させ、また微細炭窒化物の析出効果により強度上昇に有効に作用する元素であるが、その含有量が０．００５％未満では効果が発揮されず、０．１％以上の添加は溶接継手部の靭性を劣化させる。従って、Ｎｂを添加する場合には、その範囲を０．００５〜０．１％とする。
【００２７】
Ｖ：０．０１〜０．１％
Ｖは、少量の添加により焼入性を向上させ、また微細炭窒化物の析出効果により強度上昇に有効に作用する元素であるが、その含有量が０．０１％未満ではその効果が十分に発揮されず、一方、０．１％を超えて添加すると溶接性を劣化させる。従って、Ｖを添加する場合には、その範囲を０．０１〜０．１％とする。
【００２８】
Ｔｉ：０．００５〜０．０３％
Ｔｉは、ＴｉＮを形成して溶接ＨＡＺ部の組織粗大化を抑制して溶接継手部の靱性向上に寄与する元素である。０．００５％未満のＴｉ添加では継手靱性向上効果が発揮されない。一方、０．０３％を超えて添加すると、溶接の冷却過程でＴｉＣが析出し、ＨＡＺ靱性の劣化を招く。従って、Ｔｉを添加する場合には、その範囲を０．００５％〜０．０３％の範囲とする。
【００２９】
以上、何れも質量％である。
【００３０】
（２）製造方法
上記の成分組成範囲に調整した鋼を、１０００〜１２００℃の温度範囲に加熱後、再結晶温度域において圧下率５０％以上、未再結晶温度域で圧下率３０％以上の熱間圧延を行い、圧延終了後引き続き２℃／秒以上の冷却速度で平均温度が４００〜６００℃の温度域まで冷却した後、周波数２００〜２０００Ｈｚの誘導加熱装置を用いて表面温度５００〜Ａｃ１＋５０℃の温度範囲まで加熱してから空冷する。
【００３１】
（ａ）鋼の加熱温度：１０００〜１２００℃
１０００℃未満の加熱では、良好な熱間加工性が得られない。一方、１２００℃を超える加熱では、オーステナイト粒が過度に粗大化し、圧延後に良好な低温靭性を得がたい。従って、鋼の加熱温度は、１０００〜１２００℃の範囲内とする。
【００３２】
（ｂ）再結晶温度域での圧下率：５０％以上
再結晶温度域での圧下率が５０％未満の場合は、加熱時に粗大化したオーステナイトの再結晶が十分に行われず、良好な低温靭性を得がたい。従って、再結晶温度域での圧下率は、５０％以上とする。
【００３３】
（ｃ）未再結晶温度域での圧下率：３０％以上
未再結晶温度域での圧下率が３０％未満の場合は、フェライト変態時の核生成サイトが十分に導入されず、変態後の組織の細粒効果が小さいため、良好な低温靭性が得がたい。従って、未再結晶温度域での圧下率は、３０％以上とする。
【００３４】
（ｄ）加速冷却速度：２℃／秒以上
フェライト変態時の粒成長を抑制し低温靭性を向上させると共に、フェライト・ベイナイト混合組織として低成分系で強度を確保するため、圧延終了後に加速冷却を行う。加速冷却速度が２℃／秒未満では、これらの効果が十分に得られず、良好な強度、靭性が得がたい。従って、加速冷却速度は、２℃／秒以上とする。
【００３５】
（ｅ）加速冷却停止温度：４００〜６００℃
加速冷却停止温度が４００℃未満では、加速冷却時にマルテンサイトが生成し、短時間の表層加熱では板厚中心部の焼戻しが十分に行われないため、靭性が劣化する。一方、加速冷却の停止温度が６００℃を超えた場合、ベイナイト変態が十分に進行しないため十分な強度・靭性の確保が難しくなる。従って、加速冷却停止温度は、４００〜６００℃の範囲内とする。
【００３６】
（ｆ）誘導加熱の周波数：２００〜２０００Ｈｚ
加速冷却された鋼材は、表面から数ｍｍ程度の範囲で硬度が上昇しているため、硬度低下のためには、誘導加熱時の浸透深さ（発熱領域の深さ）が重要である。誘導加熱時の周波数が２００Ｈｚ未満であると、浸透深さが深くなり板厚中心部まで加熱されるため、鋼板全体の強度が低下して、高張力鋼としての強度が得られない。一方、誘導加熱時の周波数が２０００Ｈｚを超えると浸透深さが浅くなり、極表層部しか加熱されないため、硬度上昇した部分全体の軟化ができず、耐応力腐食割れ性が劣る。従って、誘導加熱の周波数は２００〜２０００Ｈｚの範囲内とする。
【００３７】
（ｇ）誘導加熱時の表面温度：５００〜Ａｃ１＋５０℃
誘導加熱時の表面温度が５００℃未満であると、表層硬度軟化の効果がほとんど表れない。一方、Ａｃ１＋５０℃を超えると表層部組織が冷却時にフェライト−パーライトとなる部分が増加し靭性が劣化すると共に、鋼板内部での熱伝達により板厚中心部まで温度が上昇し強度低下が大きくなる。従って、誘導加熱時の表面温度は５００〜Ａｃ１＋５０℃の範囲内とする。
【００３８】
なお、熱間圧延の終了温度は、Ａｒ１変態点以上、Ａｒ１変態点未満のいずれでもよく、低温靭性等の要求特性に応じて選択可能であるが、表層硬度が上昇しやすいＡｒ１変態点以上からの冷却時に本発明の技術が特に有効である。また、加速冷却後の誘導加熱開始温度は、加速冷却停止後の空冷によりベイナイト変態がある程度進行した後であれば特に限定しない。また、表層部の軟化が目的であるため、表層加熱時の保持は必ずしも必要ではない。また、本技術は誘導加熱コイルの形状を変更することにより、鋼板のみならず形鋼にも適用可能である。
【００３９】
以上により、耐応力腐食割れ性が要求される鋼構造物に用いられる低温用鋼材を安価で効率よく製造することができる。
【００４０】
【実施例】
以下、この発明を実施例によりさらに詳細に説明する。
【００４１】
成分系ならびに圧延、冷却、誘導加熱条件を変えて製造した鋼材の引張特性、シャルピー衝撃特性を調べた。
【００４２】
表１に供試鋼の化学成分およびＡｃ１変態点を示す。各鋼種とも本発明の範囲内であり、鋼種Ａ〜Ｃは、４９０Ｎ／ｍｍ^２級、鋼種Ｄ、Ｅは、５７０Ｎ／ｍｍ^２級の鋼種である。なお、Ａｃ１変態点は、表１欄外に示す式により求めた。
【００４３】
表２に供試鋼の製造条件および引張試験、シャルピー衝撃試験ならびに表層部硬度測定の結果を示す。引張特性は、圧延方向と直交する方向から採取した全厚引張試験片、シャルピー衝撃特性は、板厚１／４位置より採取した２ｍｍＶノッチ試験片により求めた。また、表層部最高硬さは、鋼板の板厚方向断面を１ｍｍピッチで測定し、その表層部での最高値を最高硬さとした。
【００４４】
【表１】

【００４５】
【表２】

【００４６】
表２から明らかなように、製造条件が本発明の範囲内である鋼番１〜１０は、４９０Ｎ／ｍｍ^２以上もしくは５７０Ｎ／ｍｍ^２以上の十分な引張強さ、脆性−延性破面遷移温度（ｖＴｒｓ）が−５０℃以下の良好な低温靭性、および、ＨＶ２２０以下の表層部硬度を有していた。
【００４７】
これに対して、表層部の加熱方法が本発明と異なる鋼番１１および誘導加熱の周波数が本発明の範囲よりも低い鋼番１５は、何れも、引張強度が不足していた。
【００４８】
スラブ加熱温度が本発明の範囲よりも高い鋼番１２、再結晶域での圧下率が本発明の範囲よりも低い鋼番１３、加速冷却停止温度が本発明の範囲よりも低い鋼番１８および未再結晶域での圧下率が本発明の範囲よりも低い鋼番１９は、何れも、靭性が不足していた。
【００４９】
誘導加熱の周波数が本発明の範囲よりも高い鋼番１４および表面加熱温度が本発明の範囲よりも低い鋼番２０は、何れも、表層部硬度が高く、耐効力腐食割れ特性が劣っていた。
【００５０】
圧延後の冷却速度が本発明の範囲よりも低い鋼番１６および表面加熱温度が本発明の範囲よりも高い鋼番１７は、何れも、引張強度が低くかつ靭性が劣っていた。
【００５１】
【発明の効果】
以上説明したように、この発明によれば、液化アンモニア等の各種液化ガスの貯蔵や運搬に用いられるタンクや精製プラント等、耐応力腐食割れ性が要求される鋼構造物に用いられる低温用鋼材を安価で効率よく製造することができるといった有用な効果がもたらされる。
【図面の簡単な説明】
【図１】熱処理鋼板の製造方法を示す工程図である。
【図２】引張強度および表層部硬さに及ぼす表面加熱温度の影響を示したグラフである。[0001]
BACKGROUND OF THE INVENTION
This invention requires a method for producing a low-temperature steel material having excellent stress corrosion cracking resistance, in particular, stress corrosion cracking resistance, such as tanks and purification plants used for storage and transportation of various liquefied gases such as liquefied ammonia. The present invention relates to a method for producing a low-temperature steel material used in a steel structure.
[0002]
[Prior art]
Steel structures such as tanks and refining plants used for storage and transportation of various liquefied gases require low temperature toughness because they become low temperature when used in common. Furthermore, in a mixed type tank that has been increasing in recent years, liquefied ammonia may be loaded, and in addition to low temperature toughness, measures to prevent stress corrosion cracking due to liquefied ammonia are required.
[0003]
As a guideline for preventing stress corrosion cracking, it is known to limit the amount of Ni in the steel material, to suppress the yield strength and surface layer hardness, etc., especially against stress corrosion cracking due to liquefied ammonia. Is known to be able to be prevented by setting the yield strength of the steel material to 440 N / mm ² or less, or by setting the surface hardness of the steel material to Vickers hardness (HV) 220 or less (“steel with liquid ammonia”). Stress corrosion cracking ", iron and steel, Vol. 65 (1979), p. 402).
[0004]
Many methods have been studied for reducing the surface layer hardness so far. For example, in JP-A-10-130721 and JP-A-10-168516, two-phase region reheating quenching is performed after rolling. A method for achieving a low yield ratio by tempering, Japanese Patent Application Laid-Open No. 4-17613 discloses a method for softening a surface layer part by irradiating a quenched steel sheet surface with a high-density energy heat source, Japanese Patent No. 240936 discloses a method in which rolling is performed in a temperature range of Ac1-50 ° C. or lower, and then a two-phase region is heated to form a coarse ferrite layer in the surface layer portion.
[0005]
[Patent Document 1]
JP-A-10-130721 [Patent Document 2]
Japanese Patent Laid-Open No. 10-168516 [Patent Document 3]
JP-A-4-17613 [Patent Document 4]
Japanese Patent Laid-Open No. 2001-240936
[Problems to be solved by the invention]
However, in the methods disclosed in JP-A-10-130721 and JP-A-10-168516, reheating and quenching and tempering are necessary, and addition of Ni, which is an expensive component element, is essential. There are also disadvantages in terms of manufacturing costs. Further, in the method disclosed in Japanese Patent Laid-Open No. 4-17613, since the heating is performed only from the surface of the steel sheet, when the hardness is increased in the range of several mm from the surface layer, the effect of decreasing the hardness is sufficient. In addition, in a component system having high hardenability, when the surface temperature exceeds the Ac1 transformation point, it is re-quenched during cooling, the hardness is increased again, and the stress corrosion cracking resistance is deteriorated. Further, in the method disclosed in Japanese Patent Application Laid-Open No. 2001-240936, after normal steel plate rolling, further rolling at room temperature or warm is necessary, so that the rolling load is large, and a complicated section such as a section steel is used. It is difficult to apply to steel materials having a shape.
[0007]
Accordingly, an object of the present invention is to solve the above-mentioned problems and to be used in a steel structure requiring stress corrosion cracking resistance such as a mixed tank or refinery plant used for storing and transporting liquefied ammonia. An object of the present invention is to provide a cheap and efficient method for producing steel for use.
[0008]
[Means for Solving the Problems]
The invention according to claim 1 is a steel containing, by mass%, C: 0.02 to 0.12%, Si: 0.05 to 0.5%, Mn: 0.6 to 1.8% or less. , After heating to a temperature range of 1000 ° C. or more and 1200 ° C. or less, hot rolling is performed at a reduction rate of 50% or more in the recrystallization temperature region and a reduction rate of 30% or more in the non-recrystallization temperature region. After cooling to a temperature range of 400 ° C. or more and 600 ° C. or less with an average temperature of 400 ° C. or more at a cooling rate of / sec or more, heating to a temperature range of 500 ° C. or more and Ac1 + 50 ° C. or less using an induction heating device having a frequency of 200 Hz to 2000 Hz Then, it is characterized by air cooling.
[0009]
The invention according to claim 2 is characterized in that the steel further comprises, in mass%, Cu: 0.05 to 1.0%, Ni: 0.05 to 1.0%, Cr: 0.05 to 1.0%. Mo: It is characterized by containing at least one of 0.01 to 1.0%.
[0010]
According to a third aspect of the present invention, the steel further comprises, in mass%, Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.03%. It has the characteristics in containing at least 1 sort (s) of these.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
The inventors of the present invention have made extensive studies in order to obtain a method for producing a low-temperature steel material having excellent stress corrosion cracking resistance without going through complicated rolling and heat treatment steps or adding expensive alloy elements. . As a result, it has been found that if the surface layer is softened by high-frequency induction heating after accelerated cooling subsequent to controlled rolling, both low temperature toughness and stress corrosion cracking resistance can be achieved.
[0012]
After heating the steel plate which consists of the steel type A shown in Table 1 to 1100 degreeC, as shown in FIG. 1, it rolls on the conditions of rolling completion temperature 800 degreeC with the hot rolling mill 3, and then rolled in this way The steel plate 2A having a thickness of 25 mm is water cooled from 780 ° C. to 500 ° C. at a cooling rate of 25 ° C./second by the accelerated cooling device 4, and then the water cooled steel plate 2B is heated to various temperatures by the induction heating device 5, and The shape was corrected by a hot leveler 6 to prepare a steel plate 2C. In addition, in FIG. 1, 1 shows a rolling line.
[0013]
FIG. 2 shows the relationship between the surface heating temperature, the surface layer hardness, and the tensile strength of the steel plate 2C thus prepared, together with the results when the steel plate 2B is heated by an atmospheric furnace instead of the induction heating device 5. .
[0014]
As is apparent from FIG. 2, when the atmospheric furnace (◯) is used, the tensile strength decreases as the surface heating temperature rises, and a tensile strength of 490 N / mm ² or more as high-tensile steel cannot be obtained. On the other hand, when the surface layer portion is heated to a temperature range of 500 ° C. to Ac1 + 50 ° C. (759 ° C. for steel A) using an induction heating device (●), the surface layer is secured while ensuring the tensile strength of the steel plate. It turns out that the hardness reduction of a part can be aimed at.
[0015]
Based on the above knowledge, the present inventor found a method for producing a low-temperature steel material excellent in stress corrosion cracking resistance by controlling the steel composition, rolling / accelerated cooling and surface layer reheating conditions, and completed the present invention. It was.
[0016]
According to this invention, low-temperature steel used for steel structures that require stress corrosion cracking resistance, such as tanks and refining plants used for storage and transportation of various liquefied gases such as liquefied ammonia, can be produced efficiently at low cost. can do.
[0017]
Hereinafter, the reason for adding the component, the reason for limiting the component, and the limited range of the manufacturing conditions will be described.
[0018]
(1) Component composition range C: 0.02 to 0.12%
C is added in an amount of 0.02% or more in order to ensure the strength of the steel, but if it is contained in a large amount exceeding 0.12%, the low temperature toughness or weldability deteriorates. Therefore, the amount of C added is in the range of 0.02 to 0.12%.
[0019]
Si: 0.05-0.5%
Si needs to be added in an amount of 0.05% or more for deoxidation, but if it exceeds 0.5%, the HAZ toughness and weldability deteriorate. Therefore, the Si addition amount is set in the range of 0.05 to 0.5%.
[0020]
Mn: 0.6 to 1.8%
Mn is required to be 0.6% or more in order to improve the strength and toughness of the steel material and to suppress the formation of FeS. However, a large amount of addition exceeding 1.8% causes an increase in the hardenability of the steel. A hardened layer is formed and cracking sensitivity is increased. Therefore, the amount of Mn added is in the range of 0.6 to 1.8%.
[0021]
In the present invention, the above is the basic component, and one or more of the following selected component groups are added.
[0022]
Cu: 0.05 to 1.0%
Cu is a very effective element for increasing the strength and improving the toughness. However, if the content is less than 0.05%, sufficient effects cannot be exhibited. Sometimes the surface of steel tends to crack. Therefore, when adding Cu, the range is made 0.05 to 1.0%.
[0023]
Ni: 0.05-1.0%
Ni has an effect of improving the strength and toughness of the base material, but if its content is less than 0.05%, a sufficient effect cannot be obtained, while addition over 1.0% increases the hardenability. This increases the hardness after heating the surface layer and degrades the stress corrosion cracking resistance, leading to an increase in cost. Therefore, when adding Ni, the range is made 0.05 to 1.0%.
[0024]
Cr: 0.05-1.0%
Cr is an element effective for improving the hardenability, but its effect is small when its content is less than 0.05%, while it degrades weldability and HAZ toughness when its content exceeds 1.0%. Therefore, when adding Cr, the range is made 0.05 to 1.0%.
[0025]
Mo: 0.05-1.0%
Mo enhances hardenability, increases temper softening resistance, and is effective in increasing the strength. However, if its content is less than 0.05%, the effect is not sufficiently exhibited, while 1.0% Exceeding will degrade the weldability and increase the yield ratio due to the precipitation of carbides. Therefore, when adding Mo, the range is made 0.05 to 1.0%.
[0026]
Nb: 0.005 to 0.1%
Nb is an element that improves hardenability by addition of a small amount and effectively acts to increase the strength due to the precipitation effect of fine carbonitride, but if its content is less than 0.005%, the effect is not exhibited. Addition of 0.1% or more deteriorates the toughness of the welded joint. Therefore, when adding Nb, the range is made 0.005 to 0.1%.
[0027]
V: 0.01 to 0.1%
V is an element that improves hardenability by adding a small amount and effectively acts to increase the strength due to the precipitation effect of fine carbonitrides, but if its content is less than 0.01%, the effect is sufficient. On the other hand, if it exceeds 0.1%, weldability is deteriorated. Therefore, when adding V, the range is made 0.01 to 0.1%.
[0028]
Ti: 0.005 to 0.03%
Ti is an element that forms TiN and suppresses the coarsening of the welded HAZ part and contributes to the improvement of the toughness of the welded joint part. When Ti is added to less than 0.005%, the joint toughness improving effect is not exhibited. On the other hand, if added over 0.03%, TiC precipitates during the cooling process of welding, leading to degradation of HAZ toughness. Therefore, when adding Ti, the range is made 0.005% to 0.03%.
[0029]
As mentioned above, all are mass%.
[0030]
(2) Manufacturing method After heating the steel adjusted to the above component composition range to a temperature range of 1000 to 1200 ° C., the reduction rate is 50% or more in the recrystallization temperature range, and the reduction rate is 30% or more in the non-recrystallization temperature range. Hot rolling is performed, and after the rolling is finished, the average temperature is cooled to a temperature range of 400 to 600 ° C. at a cooling rate of 2 ° C./second or more, and then a surface temperature of 500 to Ac1 + 50 using an induction heating device having a frequency of 200 to 2000 Hz. Heat to the temperature range of ° C and then air cool.
[0031]
(A) Steel heating temperature: 1000 to 1200 ° C
When the temperature is lower than 1000 ° C., good hot workability cannot be obtained. On the other hand, when heating exceeds 1200 ° C., austenite grains become excessively coarse and it is difficult to obtain good low temperature toughness after rolling. Therefore, the heating temperature of steel shall be in the range of 1000-1200 degreeC.
[0032]
(B) Reduction rate in the recrystallization temperature range: When the reduction rate in the recrystallization temperature range is less than 50%, the austenite coarsened during heating is not sufficiently recrystallized, and good low temperature toughness Hard to get. Therefore, the rolling reduction in the recrystallization temperature range is 50% or more.
[0033]
(C) Reduction ratio in the non-recrystallization temperature range: 30% or more When the reduction ratio in the non-recrystallization temperature range is less than 30%, nucleation sites during ferrite transformation are not sufficiently introduced, and Good low-temperature toughness is difficult to obtain because the fine grain effect of the structure is small. Therefore, the rolling reduction in the non-recrystallization temperature range is 30% or more.
[0034]
(D) Accelerated cooling rate: 2 ° C./sec or more In order to suppress grain growth during ferrite transformation and improve low-temperature toughness, and to ensure strength in a low component system as a ferrite-bainite mixed structure, accelerated cooling is performed after the end of rolling. Do. If the accelerated cooling rate is less than 2 ° C./second, these effects cannot be obtained sufficiently, and it is difficult to obtain good strength and toughness. Therefore, the accelerated cooling rate is 2 ° C./second or more.
[0035]
(E) Accelerated cooling stop temperature: 400-600 ° C
If the accelerated cooling stop temperature is less than 400 ° C., martensite is generated during accelerated cooling, and tempering of the central portion of the plate thickness is not sufficiently performed by heating the surface layer for a short time, so that the toughness is deteriorated. On the other hand, when the stop temperature of accelerated cooling exceeds 600 ° C., the bainite transformation does not proceed sufficiently, and it becomes difficult to ensure sufficient strength and toughness. Therefore, the accelerated cooling stop temperature is set within a range of 400 to 600 ° C.
[0036]
(F) Induction heating frequency: 200 to 2000 Hz
Since the steel material accelerated and cooled has an increased hardness within a range of several millimeters from the surface, the penetration depth at the time of induction heating (the depth of the heat generation region) is important for reducing the hardness. If the frequency during induction heating is less than 200 Hz, the penetration depth becomes deep and the plate thickness is heated to the center, so that the strength of the entire steel plate is lowered and the strength as high-tensile steel cannot be obtained. On the other hand, when the frequency during induction heating exceeds 2000 Hz, the penetration depth becomes shallow, and only the extreme surface layer portion is heated, so that the entire portion with increased hardness cannot be softened and the stress corrosion cracking resistance is poor. Therefore, the frequency of induction heating is in the range of 200 to 2000 Hz.
[0037]
(G) Surface temperature during induction heating: 500 to Ac1 + 50 ° C.
If the surface temperature during induction heating is less than 500 ° C., the effect of softening the surface hardness hardly appears. On the other hand, when the temperature exceeds Ac1 + 50 ° C., the portion of the surface layer structure that becomes ferrite-pearlite at the time of cooling is increased and the toughness is deteriorated. Therefore, the surface temperature at the time of induction heating is set within a range of 500 to Ac1 + 50 ° C.
[0038]
The end temperature of hot rolling may be any of Ar1 transformation point or higher and lower than Ar1 transformation point, and can be selected according to required characteristics such as low temperature toughness, but from the Ar1 transformation point or higher where surface layer hardness tends to increase. The technique of the present invention is particularly effective during the cooling process. Further, the induction heating start temperature after accelerated cooling is not particularly limited as long as the bainite transformation has progressed to some extent by air cooling after stopping accelerated cooling. Further, since the purpose is to soften the surface layer portion, it is not always necessary to hold the surface layer during heating. Moreover, this technique is applicable not only to a steel plate but to a shaped steel by changing the shape of the induction heating coil.
[0039]
As described above, a low-temperature steel material used for a steel structure requiring stress corrosion cracking resistance can be efficiently manufactured at low cost.
[0040]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples.
[0041]
The tensile properties and Charpy impact properties of steel materials manufactured by changing the component system, rolling, cooling, and induction heating conditions were investigated.
[0042]
Table 1 shows the chemical composition and Ac1 transformation point of the test steel. Each steel type is within the scope of the present invention, and steel types A to C are 490 N / mm grade ² , steel types D and E are 570 N / mm grade ² steel types. The Ac1 transformation point was determined by the formula shown outside Table 1.
[0043]
Table 2 shows the production conditions of the test steel and the results of the tensile test, Charpy impact test, and surface layer hardness measurement. Tensile properties were obtained with full-thickness tensile specimens taken from the direction perpendicular to the rolling direction, and Charpy impact characteristics were obtained with 2 mm V notch specimens taken from the 1/4 position of the plate thickness. Moreover, the surface layer part maximum hardness measured the board thickness direction cross section of the steel plate at 1 mm pitch, and made the maximum value in the surface layer part the maximum hardness.
[0044]
[Table 1]

[0045]
[Table 2]

[0046]
As apparent from Table 2, the steel No. 1 to 10 manufacturing conditions are within the scope of the present invention, 490 N / mm ² or more, or 570N / mm ² or more sufficient tensile strength, brittle - ductile fracture transition temperature (VTrs) had good low temperature toughness of −50 ° C. or lower and surface layer hardness of HV220 or lower.
[0047]
On the other hand, steel No. 11 whose surface layer heating method is different from that of the present invention and steel No. 15 whose induction heating frequency is lower than the range of the present invention are insufficient in tensile strength.
[0048]
Steel No. 12 having a slab heating temperature higher than the range of the present invention, Steel No. 13 having a reduction rate in the recrystallization region lower than the range of the present invention, Steel No. 18 having an accelerated cooling stop temperature lower than the range of the present invention, and Steel No. 19 having a rolling reduction in the non-recrystallized region lower than the range of the present invention was insufficient in toughness.
[0049]
Steel No. 14 having a frequency of induction heating higher than the range of the present invention and Steel No. 20 having a surface heating temperature lower than the range of the present invention both had high surface layer hardness and poor resistance to effective corrosion cracking. .
[0050]
Steel No. 16 having a cooling rate after rolling lower than the range of the present invention and Steel No. 17 having a surface heating temperature higher than the range of the present invention had low tensile strength and poor toughness.
[0051]
【The invention's effect】
As described above, according to the present invention, steel materials for low temperature used in steel structures that require stress corrosion cracking resistance, such as tanks and refining plants used for storage and transportation of various liquefied gases such as liquefied ammonia. Can be produced inexpensively and efficiently.
[Brief description of the drawings]
FIG. 1 is a process diagram showing a method for producing a heat-treated steel sheet.
FIG. 2 is a graph showing the influence of surface heating temperature on tensile strength and surface layer hardness.

Claims

Steel containing, in mass%, C: 0.02 to 0.12%, Si: 0.05 to 0.5%, Mn: 0.6 to 1.8% or less After heating to the temperature range, hot rolling is performed at a reduction rate of 50% or more in the recrystallization temperature range and a reduction rate of 30% or more in the non-recrystallization temperature range. After cooling to an average temperature of 400 ° C. or more and 600 ° C. or less, and using an induction heating device with a frequency of 200 Hz or more and 2000 Hz or less, the surface temperature is heated to a temperature range of 500 ° C. or more and Ac1 + 50 ° C. or less and then air-cooled. A method for producing a low-temperature steel material having excellent stress corrosion cracking resistance.

The steel is further in mass%, Cu: 0.05-1.0%, Ni: 0.05-1.0%, Cr: 0.05-1.0%, Mo: 0.01-1 The method for producing a low-temperature steel material excellent in stress corrosion cracking resistance according to claim 1, comprising at least one of 0.0%.

The steel further contains at least one of Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and Ti: 0.005 to 0.03% by mass%. The manufacturing method of the steel material for low temperature excellent in the stress corrosion cracking resistance of Claim 1 or 2 characterized by performing.