JP3996824B2 - Steel for liquid phase diffusion bonding with excellent low temperature transformation cracking resistance - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は液相拡散接合を用いた部品または装置などの接合構造体を可能ならしむる鋼材に関し、さらに詳しくは液相拡散接合を一部または全部に適用して製造した強度が600MPa以上、特に1000MPa以上の高強度構造体あるいは圧力容器を構成することのできる液相拡散接合用鋼材に関する。
【０００２】
【従来の技術】
【特許文献１】
特開２００１−１６４３３２号公報
【特許文献２】
特開２００１−１３１６８２号公報
【０００３】
金属材料どうしの工業的な新しい接合技術として液相拡散接合が普及しつつある。液相拡散接合は、接合しようとする材料の接合面すなわち開先間に、拡散律速の等温凝固過程を経て継手を形成する能力を有する元素、例えばＢあるいはＰとこれらを開先間に介在させるために基材となるNiないしはFeからなる多元合金を介在させ、継手を挿入した低融点合金の融点以上の温度に加熱保持し、継手を形成する技術であって、通常の溶接技術と異なり、溶接残留応力が殆どないこと、あるいは溶接のような余盛りを発生しない平滑かつ精密な継手を形成できるなどの特徴を有する。特に、面接合であるため、接合面の面積によらず接合時間が一定でかつ比較的短時間で接合が完了する点は、従来溶接と全く異なっている。従って、開先さえ挿入した低融点金属以上の温度に所定の時間保持できれば、開先形状を選ばず、面どうしの接合を実現することができるという特徴を有する。
【０００４】
しかし、開先間に介在させる低融点の合金（以降インサートメタルと称する）の融点は、その拡散元素がＢまたはＰである以上、900℃〜1300℃の温度であり、特にフェライト構造を有する鋼の変態点、Ac１あるいはAc３を超える温度を開先で達成しなければならない。この時、工業的には拡散律速の等温凝固を早く終了させるためには実質的に接合温度が1000℃を超えることとなり、当然鋼材は変態点以上に加熱されることから、接合終了後は冷却時に再変態が生じ、材料の特性はこの時の変態組織で決定される。従って、後に熱処理を附加して調質処理を実施する場合がある。
【０００５】
しかし、液相拡散接合で継手の等温凝固を達成する場合、ＢまたはＰの拡散律速凝固が生起し、どうしても継手組織の粒界にはこれら元素が濃縮しやすく、場合によっては化合物として析出する場合もある。さらに、等温凝固部は母材の溶融と融合を伴っており、母在中に存在する不純物もまた一度再溶融し接合部組織の形成時に粒界に偏析しやすくなる傾向があることが本発明者らの詳細な研究で明らかとなった。
【０００６】
特に強度を高めるべく焼入れ、焼準しなどの調質処理で低温変態組織、すなわちマルテンサイト組織やベイナイト組織を得る場合、それらの旧γ粒界への不純物元素とＢあるいはＰの濃化は避けられず、粒界は脆化し易くなり、場合によっては冷却時の変態膨張によって焼割れが発生したりする。あるいは冷媒を使用して加速冷却する際に生じる構造物の内部と外表面の温度差に起因した変態の時間差で、構造物の表面に引張り応力が残留する場合には続く焼き戻し過程において、炭化物や窒化物を粒界に析出して脆化する条件では粒界に沿った割れ、すなわち焼き戻し割れ（再熱割れ、ＳＲ割れとも称する）を生じたり、割れないまでも著しく衝撃値の低い脆い継手となってしまう場合がある。このような脆い性質を有する継手は構造部材に衝撃的な応力が室温以下の低温で付加されたとき、あるいは鋼中に水素が侵入した場合などにある程度経時もしくは経年してから割れを生起する場合があり、問題となっていた。
【０００７】
これらは全て粒界破壊の様相を呈し、特に６００ＭＰａ以上、場合によっては１０００ＭＰａ以上の強度を必要とする、低温変態組織およびその焼き戻し組織からなる高強度鋼で顕著であり、粒界の脆化が材料の脆化を誘引していることから、粒界の性質改善が被接合材料に求められていた。
この現象はＢまたはＰを大量に拡散させる液相拡散接合に特徴的であって、通常の低合金鋼で生じる焼き戻し脆化とは異なり、ＢとＰの粒界偏析が前提となる脆化現象であるため、その対策に関する類似技術は殆ど見あたらない。
【０００８】
合金鋼の焼き戻し脆化対策技術は特開２００１−１６４３３２号公報、あるいは特開２００１−１３１６８２号公報等に最新の合金鋼における耐再熱割れ性向上技術が開示されているものの、これらの技術に液相拡散接合の継手に関する知見は全く認められない。したがって、液相拡散接合継手で生じる特徴的な焼き戻し脆化を有効に防止しうる技術の開示は、現時点では全くない。
【０００９】
【発明が解決しようとする課題】
本発明は上記のような従来技術が抱える問題点を解決して、Ｂを拡散原子として用いて液相拡散接合した後に、600MPa以上、場合によっては1000MPa以上の高強度の継手を、調質熱処理で得ようとする場合に生じる焼割れ、あるいは焼き戻し割れ等の粒界脆化に対する耐性のある鋼材を、主に鋼材不純物成分の限定によって得ることを目的とする。
【００１０】
【課題を解決するための手段】
本発明は上記のような課題に対して、鋼材の粒界脆化に寄与する化学成分を限定し、特に脆化因子として影響の大きいＡｓ，Ｓｎ，Ｓｂ，Ｐｂ，Ｚｎの個別および総合含有量を規定して粒界の脆化が液相拡散接合継手に生じないように設計した鋼材であり、具体的には以下に示すとおりである。
(1) 請求項１に記載の発明は、質量％で、Ｃ：０．０５〜１．０％，Ｓｉ：０．０１〜１．０％，Ｍｎ：０．０５〜３．０％，Ｔｉ：０．００５〜０．１％，Ａｌ：０．０１〜０．２％，Ｂ：０．０００３〜０．００５％，Ｎ：０．０１％以下を含有し、かつＰ：０．０１％以下，Ｓ：０．００３％以下，Ｏ：０．０１％以下に制限し、加えてＡｓ，Ｓｎ，Ｓｂ，Ｐｂ，Ｚｎの何れも０．００５％以下に制限し、かつ（Ａｓ％＋Ｓｎ％＋Ｓｂ％＋Ｐｂ％＋Ｚｎ％）の値が０．０１５％以下、残部が不可避的不純物およびＦｅからなる液相拡散接合用鋼材であって、この鋼材を液相拡散接合した継手をAc３変態点以上に再加熱してから冷却速度0.1℃/s以上で加速冷却するか、前記加速冷却後Ac１変態点以下に焼き戻したときの０℃におけるシャルピー吸収エネルギーが４７Ｊ以上となることを特徴とする耐低温変態割れ性に優れた液相拡散接合用鋼材である。
【００１１】
(2) 請求項２の発明は、請求項１に記載の鋼であって、更にＣａ：０．０００５〜０．０１％，Ｍｇ：０．０００５〜０．００５％，Ｙ：０．０００５〜０．０２％，Ｃｅ：０．０００５〜０．０２％，Ｌａ：０．０００５〜０．０２％，Ｚｒ：０．００１〜０．０２％の一種または２種以上を含有することを特徴とする耐低温変態割れ性に優れた液相拡散接合用鋼材である。
【００１２】
(3) 請求項３の発明は、請求項１〜２の何れかに記載の鋼であって、更にＮｉ：０．０１〜５．０％，Ｃｏ：０．０１〜５．０％，Ｃｕ：０．０１〜５．０％，Ｃｒ：０．０１〜１３．０％，Ｍｏ：０．０１〜５．０％，Ｗ ：０．０１〜５．０％の一種または二種以上を含有し、この鋼材を液相拡散接合した継手を Ac ３ 変態点以上に再加熱してから冷却速度 0.1 ℃ /s 以上で加速冷却するか、前記加速冷却後 Ac １ 変態点以下に焼き戻したときの継手の強度が１０００ＭＰａ以上であることを特徴とする耐低温変態割れ性に優れた液相拡散接合用鋼材である。
【００１３】
(4) 請求項４の発明は、請求項１〜３の何れかに記載の鋼であって，更にＮｂ：０．００５〜０．５％，Ｖ：０．００５〜１．０％ , Ｔａ：０．００５〜０．５％，Ｈｆ：０．００５〜０．５％，Ｒｅ：０．００５〜０．５％の一種または二種以上を含有し、この鋼材を液相拡散接合した継手を Ac ３ 変態点以上に再加熱してから冷却速度 0.1 ℃ /s 以上で加速冷却するか、前記加速冷却後 Ac １ 変態点以下に焼き戻したときの継手の強度が１０００ＭＰａ以上であることを特徴とする耐低温変態割れ性に優れた液相拡散接合用鋼材である。
【００１４】
【００１５】
【発明の実施の形態】
最初に、本発明に記載の液相拡散接合用鋼材の化学成分を限定した理由について以下に述べる。
Ｃは鋼の焼き入れ性と強度を制御する最も基本的な元素である。０．０１％未満では強度が確保できず、１．０％を超えて添加すると強度は向上するものの、焼戻し後においても継手の靱性を確保することができないことから０．０１〜１．０％に限定した。
【００１６】
Ｓｉは鋼材の脱酸元素であり、通常Ｍｎとともに鋼の酸素濃度を低減する目的で添加される。同時に粒内強化に必要な元素であって、その不足は強度低下をきたす。本発明でも同様に、脱酸と粒内強化を主目的として添加し、０．０１％以上で効果を発揮し、１．０％を超えて添加した場合には鋼材の脆化を招く場合があることから、その添加範囲を０．０１〜１．０％に限定した。
【００１７】
ＭｎはＳｉとともに脱酸にも効用があるが、鋼中にあって材料の焼き入れ性を高め、強度向上に寄与する。その効果は０．０５％より発現し、３．０％を超えると粗大なＭｎＯ系酸化物を晶出し、かえって靱性を低下させる場合があることからその添加範囲を０．０５〜３．０％に限った。
【００１８】
Ｔｉは微細な炭化物を析出して結晶粒を微細化し鋼の靭性を高める。この目的のためには０．００５％以上の添加が必要であるが、０．１％を超えると炭化物が粗大化して靭性の低下を招く。したがって、Ｔｉの範囲を０．００５〜０．１％に限定した。
【００１９】
ＡｌはＮと結合してＡｌＮとして析出し、液相拡散接合の温度範囲においても溶解せず、γ粒の移動を抑制する効果を有する。多く添加しても炭化物を形成しないことから、根本的にＴｉやＺｒ等の窒化物形成元素とは挙動が異なり、鋼の靱性低下をきたさない。従って、効果を発揮する最低量として０．０１％の添加が必要であり、０．２％を超えて添加した場合にはＡｌＮそのものが粗大化して靱性に影響があることから、接合温度に応じて０．０１〜０．２％の範囲で適宜添加することとした。
【００２０】
なお、本鋼のような高強度鋼において靱性を高めるには、粒界への不純物濃化は極力これを回避する必要があり、ＰおよびＳは、この目的のためにそれぞれ０．０１％および０．００３％以下に制限した。また、Ａｌを効率よくＮと結合させるためにＯは０．０１％以下に制限されなければならない。
【００２１】
また、Ａｓ，Ｓｎ，Ｓｂ，Ｐｂ，Ｚｎはいずれも本発明においては不純物に分類する。これらは全て液相拡散接合継手の粒界に偏析しやすく、焼き戻し割れの原因となるため、これらを低減する必要があるが、その範囲は各個に０．００５％が上限であり、たとえ一元素であってもこれを超えて添加すると焼き戻し割れを誘引する。同時に、これら元素の総和が０．０１５％を超えることもまた同様に焼き戻し割れを助長する事が、本発明者らの研究で明らかとなった。すなわち質量％で、
（Ａｓ％＋Ｓｎ％＋Ｓｂ％＋Ｐｂ％＋Ｚｎ％）≦０．０１５％
が接合継手で達成されている必要がある。しかも、これは同様に焼き戻し脆性に有害なＳを０．００３％以下に制限した鋼材で同時に達成されなければならない。
【００２２】
これらの不純物元素の制限範囲は以下のような実験によって求めた。
実験室規模真空溶解、あるいは実機鋼板製造設備において、100kg, 300kg, 2ton, 10ton, 100ton, 300tonの真空溶解、あるいは通常の高炉−転炉−炉外精錬−脱ガス／微量元素添加−連続鋳造−熱間圧延によって製造した、請求項１〜５に記載の化学成分範囲鋼材を含む種々の炭素鋼、低合金鋼、合金鋼を、圧延方向と平行な方向から１０mmΦあるいは２０ｍｍ角で長さ５０ｍｍの簡易小型試験片に加工した。試験片の端面をRmax<100μmに研削加工して脱脂洗浄し、その端面を２つ突き合わせて接合試験片対となし、１５０ｋＷの出力を有する高周波誘導加熱装置を備えた引っ張り／圧縮試験機を用い、接合面間には液相拡散接合を１０００〜１３００℃において実現可能なＮｉ基−Ｂ系、Ｆｅ基−Ｂ系、Ｎｉ基−Ｐ系、Ｆｅ基−Ｐ系の、実質的に体積分率で５０％以上が非晶質である厚み２０〜５０μmのアモルファス箔を介在させ、必要な接合温度まで試験片全体を加熱し、30秒から６０分の間、１〜２０ＭＰａの応力下で液相拡散接合し、接合後放冷した。
【００２３】
続いて得られた継手全体を、９００〜１０００℃（実質的に母材のAc3変態点以上の温度）へ再加熱して１０〜２００分保持の後、０．１℃／ｓ以上の冷却速度で加速冷却してベイナイト〜マルテンサイトの低温変態組織となし、これを光学顕微鏡にて観察して確認した後に、必要な場合に適宜２００〜７００℃の各温度で０．１〜５００時間の範囲で焼き戻して調質組織とした。得られた丸棒接合試験片対からは直径６mmΦの引張り試験片を採取し、強度評価に供するとともに、角棒試験片対からはＪＩＳ４号２mmＶノッチつきのシャルピー衝撃試験片を採取して、ノッチ位置を接合部とすることで継手の靱性を評価し、これをもって継手の焼き戻し割れ感受性指標とした。
【００２４】
不純物成分は、接合前の母材で通常の湿式化学分析にて分析し、析出の有無に拘わらず鋼中含有量で評価した。図１には質量％での（Ａｓ％＋Ｓｎ％＋Ｓｂ％＋Ｐｂ％＋Ｚｎ％）の値と焼き戻し割れの一指標としての０℃における継手のシャルピー吸収エネルギーの関係を示した。（Ａｓ％＋Ｓｎ％＋Ｓｂ％＋Ｐｂ％＋Ｚｎ％）の値が０．０１５％以下の場合には継手の０℃における継手のシャルピー吸収エネルギーが常に４７Ｊを超えるが、逆に（Ａｓ％＋Ｓｎ％＋Ｓｂ％＋Ｐｂ％＋Ｚｎ％）の値が０．０１５％超の場合には継手の０℃における継手のシャルピー吸収エネルギーが４７Ｊに達しない。
【００２５】
また、実施例でも触れるが、各元素が０．００５％を超える場合も同様に継手の靱性は０℃において４７Ｊに達しない。さらにＳが０．００３％を超える場合でも同様に継手靱性が確保できないことも実験的に求めた。
これらの値は、ＢおよびＰを拡散元素として使用する液相拡散接合に特徴的であって、通状の溶接継手や合金鋼の熱処理において見られる焼き戻し脆化のパラメータあるいは評価指標とは異なる制限であり、かつ指標である。この指標と制限が満足できないと、完全に焼き戻し脆化を抑制することは困難である。
【００２６】
鋼材は上記した化学成分を有し、残部不可避的不純物およびＦｅからなる。このような鋼材を液相拡散接合した場合には、接合部の近傍に熱影響部が形成されるが、この部分は結晶粒が粗大化して靭性が低い。この靭性の低い熱影響部に熱処理を施せば靭性が回復する。このためには継手をAc３変態点以上に加熱してから冷却速度0.1℃/s以上で加速冷却する。Ac３変態点未満の加熱ではオーステナイトへの変態が不充分で靭性が十分回復しない。また、冷却速度が0.1℃/s未満ではマルテンサイトやベイナイトなどの強度の高い低温変態組織を得ることが困難である。なお、加速冷却は焼入れ、あるいは焼準しによって行う。
【００２７】
加速冷却後は靭性を更に回復させるために必要に応じてAc１変態点以下に焼き戻すことができる。焼戻し温度がAc１変態点を超えると鋼が部分的にオーステナイトに変態することになるので焼戻し温度はAc１変態点以下とする。以上のような熱処理を施した接合部の靭性は、０℃におけるシャルピー吸収エネルギーで４７Ｊ以上であることが必要である。すなわち、２ｍｍＶノッチ付き試験片を用いて０℃にてシャルピー衝撃試験を行った時の吸収エネルギーが４７Ｊ未満では鋼材は十分な靭性を有していないからである。鋼を以上のような化学成分と靭性を有するものとすることによって、焼割れまたは再熱割れを生じない耐低温変態割れ性に優れた液相拡散接合用鋼材を得ることができる。
【００２８】
なお、本発明では、Ｂ：０．０００３〜０．００５％，Ｎ：０．０１％以下、またはＣａ：０．０００５〜０．０１％，Ｍｇ：０．０００５〜０．００５％，Ｙ：０．０００５〜０．０２％，Ｃｅ：０．０００５〜０．０２％，Ｌａ：０．０００５〜０．０２％，Ｚｒ：０．００１〜０．０２％の一種または２種以上、またはＮｉ：０．０１〜５．０％，Ｃｏ：０．０１〜５．０％，Ｃｕ：０．０１〜５．０％，Ｃｒ：０．０１〜１３．０％，Ｍｏ：０．０１〜５．０％，Ｗ ：０．０１〜５．０％の一種または二種以上、またはＮｂ：０．００５〜０．５％，Ｖ：０．００５〜１．０％,Ｔａ：０．００５〜０．５％，Ｈｆ：０．００５〜０．５％，Ｒｅ：０．００５〜０．５％の一種または二種以上を含有することができる。
【００２９】
上記した合金成分は以下の理由から添加範囲を制限してある。
Ｂは微量で鋼の焼き入れ性を大きく高めるが、０．０００３％未満の添加量では焼き入れ性向上効果が小さい。一方、０．００５％を超えて添加すると炭硼化物を形成して、かえって焼き入れ性を低下させることになる。したがって、Ｂの添加範囲を０．０００３〜０．００５％とするのが望ましい。
【００３０】
ＮはＡｌと結合して微細なＡｌＮを析出して結晶粒を微細化する。しかしながら、ＮはＢと結合して焼入れ性に有効な固溶Ｂの量を低減させる。Ｎが０．０１％を超えると、固溶Ｂの確保が困難となりＮの固定のために多量のＴｉを必要となってコスト高を招くので、Ｎは０．０１％以下とするのが望ましい。
【００３１】
Ｃａ，Ｍｇ，Ｙ，Ｃｅ，Ｌａ，Ｚｒは何れも硫化物形態制御能を有する元素であって、この効果を発揮させるためには、Ｃａ：０．０００５％以上、Ｍｇ：０．０００５％以上、Ｙ：０．０００５％以上、Ｃｅ：０．０００５％以上、Ｌａ：０．０００５％以上、Ｚｒ：０．００１％以上添加する必要がある。しかしながら、Ｃａ：０．０１％、Ｍｇ：０．００５％、Ｙ：０．０２％、Ｃｅ：０．０２％、Ｌａ：０．０２％、Ｚｒ：０．０２％を超えると粗大酸化物が生成されて鋼の靱性が低下する。したがって、Ｃａ：０．０００５〜０．０１％，Ｍｇ：０．０００５〜０．００５％，Ｙ：０．０００５〜０．０２％，Ｃｅ：０．０００５〜０．０２％，Ｌａ：０．０００５〜０．０２％，Ｚｒ：０．００１〜０．０２％の範囲とするのが望ましい。
【００３２】
Ｎｉ、Ｃｏ、Ｃｕはいずれもγ安定化元素であって、鋼材の変態点を下げて低温変態を促すことで焼き入れ性を向上させる元素であり、それぞれ０．０１％以上の添加で効果が得られる。一方、５．０％を超えて添加すると残留γが増加して鋼材の靱性に影響を及ぼすことから、その添加範囲をＮｉ：０．０１〜５．０％，Ｃｏ：０．０１〜５．０％，Ｃｕ：０．０１〜５．０とするのが望ましい。
【００３３】
Cr、Mo、Wは何れもα安定化元素であるが、Ｃｒは同時に耐食性の向上に有用である。何れも０．０１％添加で効果が認められるが、Ｃｒは１３．０％を超えるとδフェライトを生成して低温変態組織を生成し難くなり、かえって強度靱性を低下させる場合があるため、その上限を１３．０％に制限した。MoとＷは著しい固溶強化を発揮するが、何れも５．０％を超えて添加すると、液相拡散接合の拡散原子であるＢおよびＰと硼化物あるいは燐化物を生成し、継手の靱性を劣化させる場合がある。したがって、それぞれの添加量をＣｒ：０．０１〜１３．０％，Ｍｏ：０．０１〜５．０％，Ｗ ：０．０１〜５．０％とするのが望ましい。
【００３４】
また、Ｎｂ、Ｖ、Ｔａ、Ｈｆ、Ｒｅは微細な炭化物を析出して鋼の強度を高める。何れも０．００５％以上の添加で効果がある。しかし、Ｎｂ、Ｔａ、Ｈｆ、Ｒｅは０．５%で、またＶは１．０％を超える添加で炭化物が粗大化して靱性の低下を来すので、Ｎｂ：０．００５〜０．５％，Ｖ：０．００５〜１．０％，Ｔａ：０．００５〜０．５％，Ｈｆ：０．００５〜０．５％，Ｒｅ：０．００５〜０．５％とするのが望ましい。
【００３５】
上記した各群の元素の１種または２種以上を適宜組み合わせて複合添加しても、また各元素を単独で添加しても良く、本発明の効果を妨げることなく、鋼材に各種特性を付与することができる。
【００３６】
なお、本発明鋼材の製造工程は、通常の高炉−転炉による銑鋼一貫プロセスを適用するだけでなく、冷鉄源を使用した電炉製法、転炉製法も適用でき、さらに連続鋳造工程を経ない場合でも通常の鋳造、鍛造工程を経て製造する事も可能である。また、製造した鋼材の形状は全く自由であって、適用する部材の形状に必要な成型技術を適用できる。すなわち、鋼板、鋼管、棒鋼、線材、形鋼などの広範囲に適用することが可能である。また、本鋼は溶接性にも優れており、液相拡散接合に適していることから液相拡散接合継手を含む構造体であれば、一部に溶接を適用して、あるいは併用して構造体を製造することが可能であり、本発明の効果を何ら妨げるものではない。
【００３７】
【実施例】
実験室規模真空溶解、あるいは実機鋼板製造設備において、100kgから300ton重量の真空溶解、あるいは通常の高炉−転炉−炉外精錬−脱ガス／微量元素添加−連続鋳造−熱間圧延によって製造した、請求項１〜４に記載の化学成分範囲鋼材を、圧延方向と平行な方向から１０mmΦあるいは２０ｍｍ角で長さ５０ｍｍの簡易小型試験片に加工した。試験片の端面をRmax<100μmに研削加工して脱脂洗浄し、その端面を２つ突き合わせて接合試験片対となし、１５０ｋＷの出力を有する高周波誘導加熱装置を備えた引っ張り／圧縮試験機を用い、接合面間には液相拡散接合を１０００〜１３００℃において実現可能なＮｉ基−Ｂ系、Ｆｅ基−Ｂ系、Ｎｉ基−Ｐ系、Ｆｅ基−Ｐ系の、実質的に体積分率で５０％以上が非晶質である厚み２０〜５０μmのアモルファス箔を介在させ、必要な接合温度まで試験片全体を加熱し、30秒から６０分の間、１〜２０ＭＰａの応力下で液相拡散接合し、接合後放冷した。
【００３８】
続いて得られた継手全体を、９００〜１０００℃（実質的に母材のAc３変態点以上の温度）へ再加熱して１０〜２００分保持の後、０．１℃／ｓ以上の冷却速度で加速冷却してベイナイト〜マルテンサイトの低温変態組織となし、これを光学顕微鏡にて観察して確認した後に、必要な場合に適宜２００〜７００℃の各温度で０．１〜５００時間の範囲で焼き戻して調質組織とした。得られた丸棒接合試験片対からは直径６mmΦの引張り試験片を採取し、強度評価に供するとともに、角棒試験片対からはＪＩＳ４号２mmＶノッチつきのシャルピー衝撃試験片を採取して、ノッチ位置を接合部とすることで継手の靱性を評価し、これをもって継手の焼き戻し割れ感受性指標とした。
不純物成分は、接合前の母材で通常の湿式化学分析にて分析し、析出の有無に拘わらず鋼中含有量で評価した。
【００３９】
表１〜４には本発明鋼の化学成分実施例と継手の衝撃吸収エネルギー（Ｊ）、および継手の引張り強さ(MPa)、（Ａｓ％＋Ｓｎ％＋Ｓｂ％＋Ｐｂ％＋Ｚｎ％）の質量％総和、接合後の熱処理条件をそれぞれ示した。本発明鋼では熱処理後の継手の靱性が４７Ｊ以上であって、焼き戻し割れ感受性が低いと考えられる。また、実際に焼き戻し割れ、焼割れは継手に発生しなかった。なお、表２の左端は表１の右端に接続されるものであって、以下同様である。
【００４０】
【表１】
【表２】
【表３】
【表４】
これに対して表５、６には従来技術のみを用いて製造した鋼材による液相拡散接合継手の化学成分と評価結果の例を示してある。表５、６のうち、第８１番鋼はＰが０．０１％を超えたため、継手の靱性が接合部のみならず全体的に低下し、確保できなかった例、第８２番鋼は母材Ｓが高かったため、接合部継手の粒界に原子状Ｓが偏析して継手靱性が低下した例、第８３番から第８７番鋼は、それぞれＡｓ，Ｓｎ，Ｓｂ，Ｐｂ，Ｚｎの含有量が各個に０．００５％を超えたため、（Ａｓ％＋Ｓｎ％＋Ｓｂ％＋Ｐｂ％＋Ｚｎ％）の質量％総和は０．０１５％以下であったが、継手靱性が低下した例、第８８番鋼はＡｓ，Ｓｎ，Ｓｂ，Ｐｂ，Ｚｎの含有量は各個には０．００５％以下であったものの、総和が０．０１５％を超えたため、継手の靱性が低下した例、第８９番鋼はＴｉが過剰となり、Ｔｉの炭化物が多量に析出し、継手靱性が低下した例、第９０番鋼はＡｌ含有量が不足し、Ｔｉとともに鋼中に存在して接合中の結晶粒径制御に作用しなければならない窒化物の量が低下し、継手の靱性が低下した例、第９１番鋼はＣの量が不足し、継手の接合後の熱処理にもかかわらず、継手の強度が不足した例である。
【００４５】
【表５】
【表６】
【発明の効果】
本発明は液相拡散接合を用いて継手を形成し、構造体を製造する際に、構造体に６００MPaを超える高い強度、場合によっては１０００ＭＰａを超える超高強度と継手における熱処理時の焼き戻し割れの発生がないことを求められる場合に好適な液相拡散接合用鋼を提供するものであり、液相拡散接合の技術適用および難接合材の組立、さらには省工程による安価な部品の製造など、液相拡散接合の適用によって達成されうる構造体の機能向上に大きく寄与する。
【図面の簡単な説明】
【図１】 焼き戻し割れ感受性を高める不純物元素の総和(Ａｓ％＋Ｓｎ％＋Ｓｂ％＋Ｐｂ％＋Ｚｎ％)と液相拡散接合継手の０℃における靱性、すなわち焼き戻し割れ感受性指標との関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel material that enables a joining structure such as a component or an apparatus using liquid phase diffusion bonding, and more particularly, a strength produced by applying liquid phase diffusion bonding to a part or all of 600 MPa or more, particularly The present invention relates to a steel for liquid phase diffusion bonding that can constitute a high-strength structure or pressure vessel of 1000 MPa or more.
[0002]
[Prior art]
[Patent Document 1]
JP 2001-164332 A [Patent Document 2]
Japanese Patent Laid-Open No. 2001-131682
Liquid phase diffusion bonding is spreading as a new industrial joining technique between metal materials. In liquid phase diffusion bonding, elements having the ability to form a joint through a diffusion-controlled isothermal solidification process, for example, B or P, are interposed between the grooves between the bonding surfaces of the materials to be bonded, that is, the grooves. In order to form a joint, a multi-element alloy composed of Ni or Fe as a base material is interposed, heated to a temperature equal to or higher than the melting point of the low-melting-point alloy with the joint inserted, and unlike a normal welding technique, It has characteristics such that there is almost no residual welding stress, and a smooth and precise joint can be formed that does not generate surplus as in welding. In particular, since surface bonding is used, the welding time is constant regardless of the area of the bonding surface and the bonding is completed in a relatively short time, which is completely different from conventional welding. Therefore, as long as the groove can be maintained at a temperature equal to or higher than the inserted low melting point metal for a predetermined time, it is possible to realize bonding between faces regardless of the groove shape.
[0004]
However, the melting point of the low melting point alloy (hereinafter referred to as insert metal) interposed between the grooves is 900 ° C. to 1300 ° C. as long as the diffusing element is B or P, and particularly a steel having a ferrite structure. A temperature exceeding the transformation point of Ac 1 or Ac 3 must be achieved in the groove. At this time, in order to end diffusion-controlled isothermal solidification quickly industrially, the bonding temperature will substantially exceed 1000 ° C, and naturally the steel is heated above the transformation point. Sometimes retransformation occurs and the properties of the material are determined by the transformation structure at this time. Therefore, there is a case where a tempering process is performed by adding a heat treatment later.
[0005]
However, when isothermal solidification of a joint is achieved by liquid phase diffusion bonding, diffusion-controlled solidification of B or P occurs, and these elements tend to concentrate at the grain boundaries of the joint structure, and in some cases, precipitate as a compound. There is also. Furthermore, the isothermal solidification part is accompanied by melting and fusion of the base material, and impurities existing in the base material also tend to be remelted once and tend to segregate at the grain boundary when forming the joint structure. Their detailed study revealed this.
[0006]
In particular, when obtaining a low temperature transformation structure, that is, a martensite structure or a bainite structure by tempering such as quenching or normalizing to increase the strength, avoid enrichment of impurity elements and B or P in the former γ grain boundaries. In other words, the grain boundaries are easily embrittled, and in some cases, cracking occurs due to transformation expansion during cooling. Alternatively, if tensile stress remains on the surface of the structure due to the time difference of transformation caused by the temperature difference between the internal and external surfaces of the structure that occurs when accelerated cooling is performed using a refrigerant, Or under the condition of precipitation of nitrides at the grain boundaries and embrittlement, cracks along the grain boundaries, that is, tempering cracks (also referred to as reheat cracking, SR cracking), or brittleness with a remarkably low impact value even if not cracked It may become a joint. When such a brittle joint is subjected to impact stress on a structural member at a low temperature of room temperature or below, or when hydrogen enters the steel, cracks will occur after some time or aging. There was a problem.
[0007]
All of these exhibit an aspect of grain boundary fracture, particularly in high-strength steel composed of a low-temperature transformation structure and its tempered structure, which requires a strength of 600 MPa or more, and in some cases, 1000 MPa or more. Therefore, the material to be joined has been required to be improved in the properties of grain boundaries.
This phenomenon is characteristic of liquid phase diffusion bonding in which a large amount of B or P is diffused. Unlike temper embrittlement that occurs in ordinary low alloy steels, embrittlement is premised on grain boundary segregation of B and P. Since it is a phenomenon, there are few similar techniques for countermeasures.
[0008]
Although temper embrittlement countermeasure technology of alloy steel is disclosed in Japanese Patent Application Laid-Open No. 2001-164332 or Japanese Patent Application Laid-Open No. 2001-131682, the latest technology for improving reheat cracking resistance in alloy steel is disclosed. However, no knowledge about the joints of liquid phase diffusion bonding is found. Accordingly, there is no disclosure of a technology that can effectively prevent characteristic temper embrittlement that occurs in a liquid phase diffusion bonding joint.
[0009]
[Problems to be solved by the invention]
The present invention solves the problems of the prior art as described above, and after performing liquid phase diffusion bonding using B as a diffusion atom, a high-strength joint of 600 MPa or more, and in some cases 1000 MPa or more, is subjected to a tempering heat treatment. It is an object to obtain a steel material that is resistant to grain boundary embrittlement such as tempering cracking or tempering cracking that occurs when trying to obtain by mainly limiting the steel material impurity components.
[0010]
[Means for Solving the Problems]
The present invention limits the chemical components that contribute to the grain boundary embrittlement of the steel material for the above-mentioned problems, and particularly the individual and total contents of As, Sn, Sb, Pb, and Zn, which have a great influence as an embrittlement factor. The steel material is designed so that grain boundary embrittlement does not occur in the liquid phase diffusion bonded joint, and is specifically as follows.
(1) The invention according to
[0011]
(2) The invention of claim 2 is the steel of
[0012]
(3) The invention of claim 3 is the steel according to any one of
[0013]
(4) The invention of claim 4 is the steel according to any one of
[0014]
[0015]
DETAILED DESCRIPTION OF THE INVENTION
First, the reason why the chemical components of the steel for liquid phase diffusion bonding described in the present invention are limited will be described below.
C is the most basic element that controls the hardenability and strength of steel. If it is less than 0.01%, the strength cannot be secured, and if added over 1.0%, the strength is improved, but the toughness of the joint cannot be secured even after tempering. Limited to.
[0016]
Si is a deoxidizing element for steel and is usually added together with Mn for the purpose of reducing the oxygen concentration of the steel. At the same time, it is an element necessary for intragranular strengthening, and its deficiency causes a decrease in strength. Similarly, in the present invention, deoxidation and intragranular strengthening are added as main purposes, and the effect is exhibited at 0.01% or more, and when added over 1.0%, the steel material may be embrittled. Therefore, the addition range was limited to 0.01 to 1.0%.
[0017]
Mn has an effect on deoxidation as well as Si, but it is in steel and improves the hardenability of the material and contributes to the improvement of strength. The effect is manifested from 0.05%, and if it exceeds 3.0%, a coarse MnO-based oxide is crystallized, and on the contrary, the toughness may be lowered, so the addition range is 0.05-3.0%. Limited to.
[0018]
Ti precipitates fine carbides, refines the crystal grains, and increases the toughness of the steel. For this purpose, 0.005% or more must be added, but if it exceeds 0.1%, the carbides become coarse and the toughness is reduced. Therefore, the range of Ti is limited to 0.005 to 0.1%.
[0019]
Al binds to N and precipitates as AlN, does not dissolve even in the temperature range of liquid phase diffusion bonding, and has an effect of suppressing the movement of γ grains. Even if a large amount is added, no carbide is formed, so the behavior is fundamentally different from nitride forming elements such as Ti and Zr, and the toughness of the steel is not lowered. Therefore, it is necessary to add 0.01% as the minimum amount to exert the effect, and when adding over 0.2%, AlN itself becomes coarse and affects toughness. Therefore, it was decided to add appropriately in the range of 0.01 to 0.2%.
[0020]
In order to increase toughness in a high-strength steel such as this steel, it is necessary to avoid impurity concentration at the grain boundaries as much as possible. P and S are 0.01% and Limited to 0.003% or less. Further, O must be limited to 0.01% or less in order to bind Al to N efficiently.
[0021]
Further, As, Sn, Sb, Pb, and Zn are all classified as impurities in the present invention. All of these are easily segregated at the grain boundaries of the liquid phase diffusion joint and cause tempering cracks. Therefore, it is necessary to reduce these, but the upper limit is 0.005% for each piece. Even if it is an element, adding more than this will induce tempering cracks. At the same time, it has been clarified by the present inventors that the sum of these elements exceeding 0.015% also promotes tempering cracks. That is, by mass%
(As% + Sn% + Sb% + Pb% + Zn%) ≦ 0.015%
Must be achieved with the joint joint. Moreover, this must be achieved simultaneously with a steel material in which S which is also harmful to temper brittleness is limited to 0.003% or less.
[0022]
The limiting range of these impurity elements was determined by the following experiment.
100kg, 300kg, 2ton, 10ton, 100ton, 300ton vacuum melting or normal blast furnace-converter-off-furnace refining-degassing / trace element addition-continuous casting- Various carbon steels, low alloy steels, and alloy steels including the chemical composition range steel materials according to
[0023]
Subsequently, the entire joint obtained was reheated to 900 to 1000 ° C. (substantially the temperature above the Ac 3 transformation point of the base material), held for 10 to 200 minutes, and then cooled to 0.1 ° C./s or more. After accelerating cooling at a speed to form a low temperature transformation structure of bainite to martensite, and confirming this by observing with an optical microscope, it is appropriately performed at 200 to 700 ° C. for 0.1 to 500 hours. The tempered structure was tempered in the range. A tensile test piece with a diameter of 6 mmΦ was taken from the obtained round bar joint test piece pair for strength evaluation, and a Charpy impact test piece with a JIS No. 2 mm V notch was taken from the square bar test piece pair to obtain a notch position. Was used as a joint, and the toughness of the joint was evaluated, and this was used as a tempering crack sensitivity index of the joint.
[0024]
Impurity components were analyzed by ordinary wet chemical analysis on the base material before joining, and evaluated by the content in steel regardless of the presence or absence of precipitation. FIG. 1 shows the relationship between the value in mass% (As% + Sn% + Sb% + Pb% + Zn%) and the Charpy absorbed energy of the joint at 0 ° C. as an index of temper cracking. When the value of (As% + Sn% + Sb% + Pb% + Zn%) is 0.015% or less, the Charpy absorbed energy of the joint at 0 ° C. always exceeds 47 J, but conversely (As% + Sn% + Sb%) When the value of (+ Pb% + Zn%) exceeds 0.015%, the Charpy absorbed energy of the joint at 0 ° C. does not reach 47 J.
[0025]
In addition, as described in Examples, the toughness of the joint does not reach 47 J at 0 ° C. when each element exceeds 0.005%. Furthermore, it was experimentally determined that joint toughness could not be secured in the same manner even when S exceeds 0.003%.
These values are characteristic for liquid phase diffusion bonding using B and P as diffusing elements, and are different from temper embrittlement parameters or evaluation indices found in heat treatments of general welded joints and alloy steels. It is a limit and an indicator. If this index and restriction cannot be satisfied, it is difficult to completely suppress temper embrittlement.
[0026]
The steel material has the above-described chemical components, and the balance is inevitable impurities and Fe. When such a steel material is subjected to liquid phase diffusion bonding, a heat-affected zone is formed in the vicinity of the bonded portion, but in this portion, crystal grains become coarse and the toughness is low. If heat treatment is applied to the heat-affected zone having low toughness, the toughness is recovered. For this purpose, the joint is heated to the Ac 3 transformation point or higher and then cooled at a cooling rate of 0.1 ° C./s or higher. Heating below the Ac 3 transformation point results in insufficient transformation to austenite and does not sufficiently recover toughness. Further, when the cooling rate is less than 0.1 ° C./s, it is difficult to obtain a low-temperature transformation structure having high strength such as martensite and bainite. The accelerated cooling is performed by quenching or normalizing.
[0027]
After accelerated cooling, it can be tempered below the Ac 1 transformation point as needed to further restore toughness. When the tempering temperature exceeds the Ac 1 transformation point, the steel partially transforms to austenite, so the tempering temperature is set to the Ac 1 transformation point or less. The toughness of the joint subjected to the heat treatment as described above needs to be 47 J or more in Charpy absorbed energy at 0 ° C. In other words, the steel material does not have sufficient toughness when the absorbed energy is less than 47 J when a Charpy impact test is performed at 0 ° C. using a test piece with a 2 mmV notch. By making the steel have the above chemical components and toughness, a liquid phase diffusion bonding steel material excellent in low temperature transformation cracking resistance that does not cause firing cracking or reheat cracking can be obtained.
[0028]
In the present invention, B: 0.0003 to 0.005%, N: 0.01% or less, or Ca: 0.0005 to 0.01%, Mg: 0.0005 to 0.005%, Y: 0.0005-0.02%, Ce: 0.0005-0.02%, La: 0.0005-0.02%, Zr: one or more of 0.001-0.02%, or Ni : 0.01 to 5.0%, Co: 0.01 to 5.0%, Cu: 0.01 to 5.0%, Cr: 0.01 to 13.0%, Mo: 0.01 to 5 0.0%, W: 0.01 to 5.0% or two or more, or Nb: 0.005 to 0.5%, V: 0.005 to 1.0%, Ta: 0.005 One or more of 0.5%, Hf: 0.005 to 0.5%, and Re: 0.005 to 0.5% can be contained.
[0029]
The range of addition of the above alloy components is limited for the following reason.
A small amount of B greatly increases the hardenability of the steel, but the effect of improving the hardenability is small with an addition amount of less than 0.0003%. On the other hand, if added over 0.005%, a carbonized boride is formed and the hardenability is lowered. Therefore, it is desirable that the addition range of B is 0.0003 to 0.005%.
[0030]
N combines with Al to precipitate fine AlN to refine crystal grains. However, N combines with B to reduce the amount of solid solution B effective for hardenability. If N exceeds 0.01%, it is difficult to secure solute B, and a large amount of Ti is required to fix N, resulting in high costs. Therefore, N is preferably 0.01% or less. .
[0031]
Ca, Mg, Y, Ce, La, and Zr are all elements having sulfide form control ability, and in order to exert this effect, Ca: 0.0005% or more, Mg: 0.0005% or more Y: 0.0005% or more, Ce: 0.0005% or more, La: 0.0005% or more, Zr: 0.001% or more must be added. However, when Ca: 0.01%, Mg: 0.005%, Y: 0.02%, Ce: 0.02%, La: 0.02%, and Zr: 0.02%, coarse oxides are formed. As a result, the toughness of the steel decreases. Therefore, Ca: 0.0005-0.01%, Mg: 0.0005-0.005%, Y: 0.0005-0.02%, Ce: 0.0005-0.02%, La: 0.00. It is desirable to set the range of 0005 to 0.02% and Zr: 0.001 to 0.02%.
[0032]
Ni, Co, and Cu are all γ-stabilizing elements, and are elements that improve the hardenability by lowering the transformation point of the steel material and promoting low-temperature transformation. can get. On the other hand, if adding over 5.0%, the residual γ increases and affects the toughness of the steel material, so the addition ranges are Ni: 0.01 to 5.0%, Co: 0.01 to 5. It is desirable that 0%, Cu: 0.01 to 5.0.
[0033]
Cr, Mo, and W are all α-stabilizing elements, but Cr is also useful for improving corrosion resistance. In any case, the effect is observed when 0.01% is added. However, when Cr exceeds 13.0%, it is difficult to generate a low temperature transformation structure by forming δ ferrite, and the strength toughness may be lowered. The upper limit was limited to 13.0%. Mo and W exhibit remarkable solid solution strengthening, but if both are added in excess of 5.0%, B and P, which are diffusion atoms of liquid phase diffusion bonding, and boride or phosphide are formed, and the toughness of the joint May deteriorate. Therefore, it is desirable that the respective addition amounts be Cr: 0.01 to 13.0%, Mo: 0.01 to 5.0%, and W: 0.01 to 5.0%.
[0034]
Nb, V, Ta, Hf, and Re precipitate fine carbides to increase the strength of the steel. In any case, the addition of 0.005% or more is effective. However, Nb, Ta, Hf, and Re are 0.5%, and V exceeds 1.0%, and carbides coarsen and lower toughness. Therefore, Nb: 0.005 to 0.5% , V: 0.005 to 1.0%, Ta: 0.005 to 0.5%, Hf: 0.005 to 0.5%, Re: 0.005 to 0.5%.
[0035]
One or two or more of the elements of each group described above may be combined and added in combination, or each element may be added alone, giving various properties to the steel without impairing the effects of the present invention. can do.
[0036]
In addition, the manufacturing process of the steel of the present invention can be applied not only to an integrated steelmaking process using a normal blast furnace and converter, but also to an electric furnace manufacturing method and a converter manufacturing method using a cold iron source, and further through a continuous casting process. Even if not, it can be manufactured through normal casting and forging processes. Moreover, the shape of the manufactured steel material is completely free, and a molding technique necessary for the shape of the member to be applied can be applied. That is, it can be applied to a wide range of steel plates, steel pipes, steel bars, wire rods, shaped steels and the like. In addition, this steel has excellent weldability and is suitable for liquid phase diffusion bonding, so if it is a structure that includes a liquid phase diffusion bonding joint, it is a structure that uses welding in part or is used in combination. It is possible to manufacture a body and does not interfere with the effects of the present invention.
[0037]
【Example】
In laboratory-scale vacuum melting, or in actual steel plate manufacturing equipment, manufactured by vacuum melting of 100 kg to 300 ton weight, or ordinary blast furnace-converter-external refining-degassing / trace element addition-continuous casting-hot rolling, The chemical composition range steel according to
[0038]
Subsequently, the entire joint obtained was reheated to 900 to 1000 ° C. (substantially the temperature above the Ac 3 transformation point of the base material), held for 10 to 200 minutes, and then cooled to 0.1 ° C./s or more. After accelerating cooling at a speed to form a low temperature transformation structure of bainite to martensite, and confirming this by observing with an optical microscope, it is appropriately performed at 200 to 700 ° C. for 0.1 to 500 hours. The tempered structure was tempered in the range. A tensile test piece with a diameter of 6 mmΦ was taken from the obtained round bar joint test piece pair for strength evaluation, and a Charpy impact test piece with a JIS No. 2 mm V notch was taken from the square bar test piece pair to obtain a notch position. Was used as a joint, and the toughness of the joint was evaluated, and this was used as a tempering crack sensitivity index of the joint.
Impurity components were analyzed by ordinary wet chemical analysis on the base material before joining, and evaluated by the content in steel regardless of the presence or absence of precipitation.
[0039]
Tables 1 to 4 show examples of chemical components of the steel of the present invention, the impact absorption energy (J) of the joint, and the tensile strength (MPa) of the joint, and the mass% sum of (As% + Sn% + Sb% + Pb% + Zn%). The heat treatment conditions after bonding are shown respectively. In the steel of the present invention, the toughness of the joint after heat treatment is 47 J or more, and it is considered that the temper cracking sensitivity is low. Further, tempering cracks and tempering cracks did not actually occur in the joint. The left end of Table 2 is connected to the right end of Table 1, and so on.
[0040]
[Table 1]
[Table 2]
[Table 3]
[Table 4]
On the other hand, Tables 5 and 6 show examples of chemical components and evaluation results of liquid phase diffusion bonding joints made of steel manufactured using only the prior art. Of Tables 5 and 6, since steel No. 81 exceeded P of 0.01%, an example in which the toughness of the joint was lowered not only at the joint portion but could not be secured. Since S was high, atomic S was segregated at the grain boundary of the joint joint, and the joint toughness decreased. For example, No. 83 to No. 87 steel contained As, Sn, Sb, Pb, and Zn, respectively. Since it exceeded 0.005% in each piece, the total mass% of (As% + Sn% + Sb% + Pb% + Zn%) was 0.015% or less, but the joint toughness was lowered. , Sn, Sb, Pb, Zn content was 0.005% or less for each piece, but the total exceeded 0.015%, so the toughness of the joint was reduced. An example of excessive Ti carbide precipitates and a decrease in joint toughness. An example in which the amount of nitride, which is present in steel together with Ti and has to act to control the grain size during bonding, is reduced due to a shortage of the amount of titanium and the toughness of the joint is reduced. This is an example in which the strength of the joint is insufficient despite the heat treatment after joining the joint.
[0045]
[Table 5]
[Table 6]
【The invention's effect】
In the present invention, when a joint is formed using liquid phase diffusion bonding and a structure is manufactured, the structure has a high strength exceeding 600 MPa, and in some cases an ultrahigh strength exceeding 1000 MPa, and tempering cracks during heat treatment in the joint. Liquid phase diffusion bonding steel suitable for cases where it is required that there is no generation of defects, including application of liquid phase diffusion bonding technology, assembly of difficult-to-bond materials, and production of inexpensive parts through reduced processes, etc. This greatly contributes to the improvement of the function of the structure that can be achieved by application of liquid phase diffusion bonding.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the sum of impurity elements (As% + Sn% + Sb% + Pb% + Zn%) and the toughness at 0 ° C. of a liquid phase diffusion joint, that is, the temper cracking susceptibility index. It is.
Claims (4)
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