JP4082288B2 - Mo-containing austenitic stainless steel and method for producing the same - Google Patents
Mo-containing austenitic stainless steel and method for producing the same Download PDFInfo
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【０００１】
【発明の属する技術分野】
この発明は、例えば優れた耐食性が求められるケミカルタンカ−等の構造部材として好適な曲げ加工時のしわ模様の発生が抑制されたMo含有オ−ステナイト系ステンレス鋼材、並びにケミカルタンカ−等の構造部材として好適なMo含有オ−ステナイト系ステンレス鋼材の製造方法に関するものである。
【０００２】
【従来の技術】
国の内外を問わず、種々の薬品類の多量運搬手段としてケミカルタンカ−が重要な役割を担ってきているが、このようなケミカルタンカ−等の構造部材には強度，加工性，溶接性，耐食性等に優れた材料が求められており、従来からMoを添加したオ−ステナイト系ステンレス鋼材 (例えばＳＵＳ３１６相当鋼材）が多用されてきた。
しかしながら、オ−ステナイト系ステンレス鋼は一般に熱間加工性が良いとは言えず、熱間加工時に割れ等の欠陥が生じやすい材料であった。特に、ケミカルタンカ−の構造部材等に適用するためMo添加によって耐食性の向上を図ったオ−ステナイト系ステンレス鋼では、その熱間加工性の改善が大きな課題となっていた。
【０００３】
また、オ−ステナイト系ステンレス鋼の鋼材を製造する際には、通常、熱間圧延した鋼材を一旦冷却した後、これに再加熱して急冷する“溶体化処理”を施す工程が付加される。この溶体化処理はオ−ステナイト系ステンレス鋼に生じがちな粒界腐食を防止するために実施される熱処理であって、粒界腐食の原因となるクロム炭化物の析出を抑制するためのものである。
【０００４】
ところが、近年、オ−ステナイト系ステンレス鋼材の製造コスト低減のために溶体化処理を省略する手法が提案されるようになった。
例えば、特開昭５５−１０７７２９号公報には、８５０〜１１５０℃での累積圧下率が５０％以上で、仕上温度を８５０℃以上とした熱間圧延を行った後、引き続いて８５０〜５５０℃の温度域を急冷することからなるオ−ステナイト系ステンレス鋼材の製造法が開示されている。
【０００５】
しかし、例えば板厚が２０mmを超えるような厚板の鋼材を製造する場合には溶体化処理を省略すると粗大な結晶粒が混在した混粒組織が残留してしまい、このような鋼材に曲げ加工を施すと表面に微小な凹凸を有した“しわ模様”が発生することが分かった。
この傾向は、オ−ステナイト系ステンレス鋼の中でも特にMoを含有したステンレス鋼 (ＳＵＳ３１６相当鋼等）に多いことも明らかとなった。
【０００６】
この“しわ模様”が発生する原因は、鋼材の表層近傍に粒径が粗大なものと微細なものとが混在した言わば“混粒組織”が生じた状態で鋼材が曲げ変形を受けると、粒径の違いによって変形度合に差異が生じ、これが鋼材表面に凹凸模様となって現れることにあると考えられる。
“しわ模様”の原因となる上記混粒組織は、特に板厚が２０mmを超えるオ−ステナイト系ステンレス鋼厚板材に多く発生する傾向があることは前述した通りである。
【０００７】
ところで、前述したようにケミカルタンカ−等ではMo含有オ−ステナイト系ステンレス鋼材が多く使用されているが、運行毎に多種の薬品を入れ替えて使用されることが多いためにその都度薬品と接触する容器部材の内部を洗浄する必要がある。しかし、この際、薬品と接触する鋼材面に“しわ模様”が存在していると“しわ”の凹部に薬品が滞留して完全に除去することが難しくなり、洗浄性が悪化するという問題が注目されるようになってきた。
【０００８】
なお、オ−ステナイト系ステンレス鋼厚板材に生じがちな混粒組織の防止法として、例えば特開平９−３１０１２０号公報には、「オ−ステナイト系ステンレス鋼に８５０℃以上での圧延に続いて８５０℃未満で圧下率が２０％を超える圧延を施し、次に行う１０００〜１１５０℃での溶体化処理の均熱時間を板厚の関数で調整することからなるステンレス厚鋼板の製造方法」が示されている。
しかしながら、この方法は溶体化処理を施すことが前提となっており、近年の「製造コスト低減のために溶体化処理を省略する」という要望に沿うものではなかった。
【０００９】
【発明が解決しようとする課題】
このようなことから、本発明が目的としたのは、省力化，省エネルギ−化のために溶体化処理を省略しても曲げ加工時に“しわ模様”を表面に発生することがない高耐食性オ−ステナイト系ステンレス鋼材を提供することである。
【００１０】
【課題を解決するための手段】
本発明者らは、上記目的を達成すべく、熱間加工性が良好な上に溶体化処理を省略しても優れた耐食性を示すオ−ステナイト系ステンレス鋼の成分系について検討を行うと共に、溶体化処理を省略しても混粒組織を少なくすることができて曲げ加工時の“しわ模様”を極力抑えることができる耐食性の優れたオ−ステナイト系ステンレス鋼材の実現手段を求めて鋭意研究を重ねた結果、次のような知見を得ることができた。
【００１１】
a) 溶体化処理を施すことなくケミカルタンカ−等の構造部材に求められる耐食性を熱間加工（熱間圧延等）後のオ−ステナイト系ステンレス鋼に付与するためには、少なくとも 2.0％（以降、 化学成分割合を表す％は質量％とする）のMo含有量を確保することが欠かせない。
しかし、Moを含有したオ−ステナイト系ステンレス鋼の場合には、熱間加工中にフェライトを生成してオ−ステナイトの粒界に残存する傾向があり、このフェライトとオ−ステナイトとの強度差に起因して両者の境界部から割れが発生するなど、熱間加工性の点で満足できる材料であるとは言えない。
その上、溶体化処理を省いたオ−ステナイト系ステンレス鋼をケミカルタンカ−等に適用する場合には、Moを添加する手立てだけでは今一つ耐食性面での懸念を払拭することができない。
【００１２】
b) ところが、Mo含有オ−ステナイト系ステンレス鋼に0.20％以上のCu添加を行うと、ケミカルタンカ−等のような厳しい腐食環境でも満足できる耐食性が溶体化処理を省いた材料にも維持できるようになる上、Cu添加量を調整したMo含有オ−ステナイト系ステンレス鋼に対し強力な脱酸成分となるAlを適量含有させることにより鋼中の酸素を下げて非金属介在物を減少させると共に、鋼中のＳをCaＳとして固定するCaをも適量含有させ、更に結晶粒界に優先的に偏析して粒界の親和力を高める作用を有するＢの適量をCaと共存させた場合には、その熱間加工性も大幅に改善される。
即ち、Mo含有オ−ステナイト系ステンレス鋼にCu，Al，Ca，Ｂを必須成分として添加・共存させると共に、これら各成分の含有量を適正な範囲に調整すると、Mo含有オ−ステナイト系ステンレス鋼の熱間加工性及び耐食性が飛躍的に向上する（Cu，Al，Ca及びＢの４成分のうち１つでも欠けると圧延中に割れが発生したり、 鋼の耐食性が不十分となってケミカルタンカ−等のような厳しい腐食環境用途には適さなくなる）。
【００１３】
c) また、上記Mo含有オ−ステナイト系ステンレス鋼にTi，Nb及びＶの１種又は２種以上を適量添加すると、材料の耐食性が更に改善される上、結晶粒の微細化にも有効である。
【００１４】
d) 更に、一般にオ−ステナイト系ステンレス鋼を加熱した時に生じがちなデルタフェライトは熱間加工性の劣化をもたらすとされているが、本発明者らは適量のデルタフェライトの生成が熱間加工性においては逆に有利に働くことを見出した。従って、Mo含有オ−ステナイト系ステンレス鋼の熱間加工温度で適量のデルタフェライトが生成されるように鋼のNi-balを調整すれば、その熱間加工性はより一層改善される。
【００１５】
e) しかも、上記の如くに成分調整された各Mo含有オ−ステナイト系ステンレス鋼に対し“１０５０℃以上で累積圧下率５０％以上の圧下を施してから８００〜９５０℃の間で仕上げる条件の熱間加工”を実施すると、曲げ加工を施した後の表面の“しわ模様”が殆ど目立たないMo含有オ−ステナイト系ステンレス鋼材を得ることができる。
【００１６】
f) ただ、製品厚さが２０mmを超えるような材料では“しわ模様”の発生を十分に抑え切れない場合もあるが、このような材料の製造においても加熱して圧延する工程を少なくとも２回繰り返して最終目標寸法とするならば、累積圧下率等の条件規制を行わなくても前記“しわ模様”の発生抑制効果を十分に得ることができ、そのため製品厚さが厚いMo含有オ−ステナイト系ステンレス鋼材であっても曲げ加工後の“しわ模様”が殆ど目立たなくなる。
【００１７】
本発明は、上記知見事項等を基に完成されたもので、次の 1）項乃至 4）項に示すMo含有オ−ステナイト系ステンレス鋼材並びにMo含有オ−ステナイト系ステンレス鋼材の製造方法を提供するものである。
1）質量割合にて、Ｃ：0.03％以下，Si： 1.0％以下，Mn：0.85〜1.50％，Cu：0.20〜0.60％，Ni：12.0〜15.0％，Cr：16.0〜20.0％，Mo： 2.0〜 3.0％，Al： 0.010〜 0.100％，Ｎ： 0.050％以下，Ca：O.OO05〜0.0040％，Ｂ：0.0005〜0.0020％を含有すると共に、残部がFe及び不可避的不純物から成り、かつ下記の式（１）で表されるNi−bal が“−２〜＋１”の範囲内であって、熱間圧延による動的再結晶によって得られた再結晶組織が５０％以上である鋼材組織を有していることを特徴とする、曲げ加工時のしわ模様の発生が抑制されたMo含有オ−ステナイト系ステンレス鋼材。
Ni−bal ＝30（Ｃ＋Ｎ）＋0.5Mn ＋Ni＋8.2
− 1.1（1.5Si ＋Cr＋Mo＋0.5Nb ） …（１）
2）前記 1）項に記載の化学成分に加え、更に質量割合にてTi：0.005 〜 0.020％，Nb：0.05％以下及びＶ：0.15％以下のうちの１種又は２種以上を含み、かつ下記の式（１）で表される Ni − bal が“−２〜＋１”の範囲内であって、熱間圧延による動的再結晶によって得られた再結晶組織が５０％以上である鋼材組織を有していることを特徴とする、曲げ加工時のしわ模様の発生が抑制されたMo含有オ−ステナイト系ステンレス鋼材。
Ni−bal ＝30（Ｃ＋Ｎ）＋0.5Mn ＋Ni＋8.2
− 1.1（1.5Si ＋Cr＋Mo＋0.5Nb ） …（１）
3）前記 1）項又は 2）項に記載の化学組成（ Ni − bal を含む）の鋼に対して、１０５０℃以上での累積圧下率が５０％以上で仕上温度が８００〜９５０℃の熱間圧延を施すことを特徴とする、曲げ加工時のしわ模様の発生が抑制されたMo含有オ−ステナイト系ステンレス鋼材の製造方法。
4）前記 1）項又は 2）項に記載の化学組成（ Ni − bal を含む）の鋼に対して、加熱に次ぐ熱間圧延を施して鋼の組織を鋳造組織から圧延組織に変えると共に再結晶が進行した組織となし、更に加熱に次ぐ熱間圧延を施して再結晶が更に進行した組織となすことを特徴とする、曲げ加工時のしわ模様の発生が抑制されたMo含有オ−ステナイト系ステンレス鋼材の製造方法。
【００１８】
【発明の実施の形態】
以下、本発明において鋼の化学成分組成及び鋼材の製造条件を前記の如くに限定した理由を、本発明の実施の形態を交えながら説明する。
[A] 鋼の化学成分組成
Ｃ： Ｃにはオ−ステナイト系ステンレス鋼の強度を確保する作用があるが、その含有量が0.03％を超えるとCrと結合した炭化物が圧延後の放冷時に結晶粒界に析出して耐食性劣化を招くようになる。従って、Ｃ含有量は0.03％以下と定めたが、強度と耐食性の観点から好ましい範囲は0.01〜0.02％である。
【００１９】
Si： Siは脱酸のために必要な成分であるが、その含有量が 1.0％を超えると熱間加工性を著しく阻害することから、Si含有量の上限を 1.0％と定めた。
Mn： Mnは鋼の脱酸剤及び脱硫剤として有効な成分であり、更にオ−ステナイト相の安定化及び熱間加工性の向上に寄与する成分でもある。しかし、その含有量が0.85％未満ではオ−ステナイト相の安定化に不十分であり、一方、1.50％を超えて含有させると耐食性に悪影響を及ぼすことから、Mn含有量は0.85〜1.50％と定めた。
【００２０】
Cr： Crは鋼に耐食性を付与するために欠かせない成分であって、その含有量が16.0％未満であると所望の耐食性が確保することができない。一方、Cr含有量が20.0％を超えると鋼（鋼材）の製造性が劣化する。従って、Cr含有量は16.0〜20.0％と定めたが、好ましい範囲は16.5〜18.0％である。
Ni： Niはオ−ステナイト組織の形成及び厳しい耐酸性を確保するために12.0％以上含有させることが必要であるが、15.0％を超えて含有させると熱間加工性の劣化を招くようになる。従って、Ni含有量は12.0〜15.0％と定めた。
【００２１】
Mo： Moはオ−ステナイト系ステンレス鋼の耐食性を向上させる成分であり、ケミカルタンカ−等の部材用として欠かせないものであるが、その含有量が 2.0％未満であると溶体化処理を省いて熱間圧延後に放冷するという条件のみでは十分な耐食性を確保することが困難である。一方、Moの多量添加は鋼のコスト高を招く。従って、Mo含有量は 2.0〜 3.0％と定めた。
【００２２】
Ｎ： Ｎはオ−ステナイト系ステンレス鋼の強度と耐食性を改善するために有効な成分であるが、その含有量が 0.050％を超えると熱間加工性が劣化して圧延時に表面疵を多発させたり、再結晶温度を上昇させることにより表層に混粒を発生させるおそれが生じる。従って、Ｎ含有量は 0.050％以下としたが、好ましくは 0.025％以下とするのが良い。
【００２３】
Cu： Cuは重要な成分の一つであり、耐食性、特に耐硫酸性を向上させるために0.20％以上含有させるが、過剰な添加は熱間加工性を低下させると共に鋼材の表層部に混粒を生じさせることからその上限を0.60％とした。このように、Cu含有量は0.20〜0.60％と定めたが、好ましい範囲は0.25〜0.50％である。
【００２４】
Al： Alは強力な脱酸剤であり、鋼中の酸素を下げることによって非金属介在物を減少させ熱間加工性を改善させる作用を有しているので、本発明ではこの目的で含有させる重要な成分の一つである。この場合、Al含有量が 0.010％未満では非金属介在物が鋼中に残るために熱間加工性の改善効果が十分でない。一方、Al含有量が 0.100％を超えると溶接時にＮと結合してAlＮを析出させ溶接金属部の靱性及び耐食性を劣化させる（この意味からすればAl含有量は少ない方が良いが 0.100％を超えなければ実用上特に問題とならない）。このため、Al含有量は 0.010〜 0.100％と定めたが、好ましい範囲は 0.020〜 0.060％である。
【００２５】
Ca： Caにはその添加により鋼中のＳをCaＳとして固定し熱間加工性を改善する作用があり、本発明ではこの作用を有効活用している。しかしながら、Ca含有量が0.0005％未満では前記作用による効果が期待できず、一方、0.0040％を超えて含有させると逆に熱間加工性を害するようになるばかりか、耐食性をも劣化するようになる。従って、Ca含有量は0.0005〜0.0040％と定めた。
【００２６】
Ｂ： Ｂは、鋼の結晶粒界に優先的に偏析して粒界の親和力を高める性質を有した成分である。特に、フェライト形成元素であるMoを含有するオ−ステナイト系ステンレス鋼では熱間加工中に結晶粒界にフェライトが生成する傾向があり、フェライトとオ−ステナイトの強度の違いからその粒界が割れ起点となりがちであるが、Ｂ添加によりこのような割れの発生を防止することが可能であるため、ＢはMo含有オ−ステナイト系ステンレス鋼材の製造には欠くことのできない重要な成分の一つである。そして、Ｂは、熱間加工性改善目的で添加するCaと共存させることで広い温度域でMo含有オ−ステナイト系ステンレス鋼の熱間加工性を改善することが可能となる。しかし、その含有量が0.0005％未満では熱間加工性改善効果が十分でなく、一方、0.0020％を超えて含有させると結晶粒界にＢ炭化物が析出して耐食性が劣化するようになることから、Ｂ含有量は0.0005〜0.0020％とした。
【００２７】
Ti，Nb及びＶ： これらの成分には何れも結晶粒の微細化及び耐食性を改善する作用があるので、必要により１種又は２種以上の添加がなされるが、以下、個々の成分毎にその含有量範囲を限定した理由を説明する。
a) Tiには、鋼中のＯ及びＮと結合してTiＯx ，TiＮとして結晶粒の微細化の核となり、スラブ鋳込み時に緩冷却となる厚肉スラブの結晶粒の粗大化を防止する作用がある。また、鋼中で安定な炭化物を生成して耐食性の向上を図る作用もある。しかし、その含有量が 0.005％未満では前記作用による効果が期待できず、一方、 0.020％を超えて含有させると逆にTiＯx 及びTiＮの析出によって表面性状が劣化するようになることから、Ti含有量は 0.005〜 0.020％とした。
b) Nbは、鋼中で微細な炭化物を形成して耐食性の向上に寄与すると共に結晶粒を微細化する効果を発揮する。しかし、0.05％を超えて含有させると再結晶温度の上昇を招くことから、Nb含有量の上限を0.05％とした。
c) Ｖはフェライト生成元素であり、結晶粒の微細化と高温クリ−プ強度を向上させる効果がある。また、Ｖ成分にはCr，Mo及びCuと共に適量添加することで耐孔食性を向上させる効果もある。しかし、0.15％を超えて含有させると熱間加工性の劣化を招くことから、Ｖ含有量の上限を0.15％とした。
【００２８】
Ni-bal： 下記の式(1) で表されるNi-balはMo含有オ−ステナイト系ステンレス鋼の連続鋳造スラブを再加熱した時に生成するデルタフェライト量と相関があり、Ni-balの負の値が大きいほどデルタフェライト量が高くなる。
【００２９】
[B] 鋼材の製造条件
本発明に係るMo含有オ−ステナイト系ステンレス鋼は、熱間加工性が良好な上に溶体化処理を省略しても優れた耐食性を示す材料であるが、このMo含有オ−ステナイト系ステンレス鋼に対して“１０５０℃以上で累積圧下率が５０％以上の圧下を施してから８００〜９５０℃の間で仕上げる条件の熱間加工”を施すと、曲げ加工を施しても表面に“しわ模様”が殆ど目立たない高耐食性鋼材が得られる。
【００３０】
図１は、本発明に係る上記熱間加工工程 (図では熱間圧延工程を例示した) の概要説明図であるが、加熱した本発明に係るMo含有オ−ステナイト系ステンレス鋼（スラブ）に再結晶領域である１０５０℃以上で累積圧下率５０％以上の圧下を施すと“動的再結晶”によって微細で均一な結晶粒を有した組織が得られ、圧延後に放冷した鋼材に十分な耐食性が確保されると共に、圧延後の鋼材に曲げ加工を施しても“しわ模様”の発生は殆ど認められなくなる。
この場合、１０５０℃以上での累積圧下率が５０％を下回った場合には、動的再結晶が十分に進行しないために圧延後の鋼材に上記特性を確保することが困難である。
【００３１】
本発明では、１０５０℃以上での圧下の後にも更に加工が続けられ、８００〜９５０℃の温度で仕上げがなされるが、仕上温度を８００〜９５０℃としたのは加工硬化による鋼材の強度確保のためである。この仕上温度が９５０℃を上回っていると加工硬化による強度確保が十分になされず、また仕上加工が８００℃を下回る温度域に持ち越されると 圧延後の鋼材に曲げ加工を施した際の“しわ模様”の発生が懸念されるようになる（厚さ２０mm以上の鋼材の場合には、 強度確保の観点から仕上温度を８００〜８７０℃に調整することが望ましい）。
なお、上記仕上温度での圧下率は格別に規定する必要はなく、目標とする鋼材強度に応じて調整すれば良い。これにより、ケミカルタンカ−等の構造部材として必要な強度の確保を容易かつ安定に行うことができる。
【００３２】
また、熱間加工後の鋼材厚（例えば板厚等の製品厚）が２０mmを超える場合には、図２に示したように“１０５０℃以上で累積圧下率が５０％以上の圧下”を施してから次の加工（例えば仕上加工）までの間に６０〜１２０秒程度の“待ち時間（間隔）”を確保するのが好ましい。これにより動的再結晶の進行が遅い厚鋼材においても再結晶が進み、微細で均一な結晶粒組織が増えるので、圧延後の鋼材に曲げ加工を施した際の“しわ模様”の発生が抑えられる。
【００３３】
図３は、後述する「実施例」の「表１」に示した「鋼Ａ」を用い、前記図２で示した熱間圧延工程に従って２８０mm厚のスラブから２５mm厚の鋼板を製造した際における再結晶の状況を、“熱間圧延温度”及び“当該温度での圧下率（５０％）”と“その後の圧延までの待ち時間（保持時間）”との関係で示したグラフである。この図３からも、１０５０℃以上の温度域で５０％以上の累積圧下率を確保し、更に上記待ち時間（保持時間）として６０秒以上の間隔をとることによって、厚さが２０mmを超える鋼板の場合でも安定して全体の５０％以上を再結晶組織にすることができ、放冷後の鋼材組織が微細で均一な組織となることが分かる。
なお、板厚が２０mmを下回る場合には、１０５０℃以上の温度域で５０％以上の累積圧下率を確保すればその後に前記“待ち時間（保持時間）”をとらなくても十分に再結晶が進行することは確認済である。
【００３４】
更に、熱間加工後の鋼材厚（製品厚）が２０mmを超える場合には、本発明に係るMo含有オ−ステナイト系ステンレス鋼に対して加熱・加工の工程を少なくとも２回繰り返せば、得られる製品を曲げ加工した際に発生しがちな“しわ模様”をより一層かつ安定して抑えることができる。
即ち、例えば２ヒ−ト圧延を行えば、１ヒ−ト目の圧延時に鋼の組織が鋳造組織から圧延組織に変わり、再結晶が進行した組織となる。また、この圧延で歪の導入もなされる。この状態で次の２ヒ−ト目の圧延を実施すれば、この２ヒ−ト目の圧延時に再結晶が更に促進され、より微細で均一な結晶粒となるため、得られた鋼材を曲げ加工した際の“しわ模様”の発生はより一層減少する。
この２ヒ−ト加工では、累積圧下率等の製造条件規制を行わなくても通常の熱間圧延条件で十分な効果が得られる。
勿論、前記図１に示した工程あるいは前記図２に示した工程をそれぞれ２回繰り返しても良いし、また図４に示すように、前記図１に示した工程に続いて前記図２に示した工程を実施しても構わない。
【００３５】
次いで、本発明を実施例によって説明する。
【実施例】
まず、表１に示す化学成分組成のオ−ステナイト系ステンレス鋼を溶製し、連続鋳造で得られたスラブを熱間圧延用スラブ（サイズ：２８０mm厚×１２５０mm幅×２３００mm長）とした。なお、表１中に示されている化学成分以外の不可避不純物元素の含有量は通常のステンレス鋼と同程度であった。
【００３６】
【表１】
次に、これらのスラブを表２に示す各条件で熱間圧延してから放冷し、２５mm厚の鋼板を得た。
なお、１ヒ−ト材（試験番号１〜17）については、スラブを１２３０℃に加熱した後、スラブ厚２８０mmから最終厚２５mmまで一度で圧延した。
また、２ヒ−ト材（試験番号18〜26）については、スラブを１２３０℃に加熱した後、１次圧延においてスラブ厚２８０mmから中間厚１３０mmまで圧延を実施し（中間厚までの圧延は１０５０℃以上の温度域で終了した）、引き続いてこれを再度１２３０℃に加熱してから、２次圧延において中間厚１３０mmから最終厚２５mmまで圧延した（２次圧延での仕上温度は表２に示した通りであった）。
【００３８】
【表２】
次いで、得られた各鋼板の 1/4幅位置よりＣ方向に試験片を採取し、これを各々の試験片形状に加工して特性試験に供した。
なお、機械的特性の評価は、JIS 13Ｂ号試験片を用いて 0.2％耐力，引張強さ及び伸びを測定し、"0.2％耐力”が４００Ｎ/mm2以上、“引張強さ”が６００Ｎ/mm2以上を合格とした。“伸び”については、ケミカルタンカ−内壁としての使用を考慮し、２５％以上を合格とした。
また、“シャルピ−衝撃試験”は、Ｖノッチ試験片を使用し、JIS Ｚ２２４２に従って試験温度−１９６℃で試験を行い３個の試験の最小値で評価した。
【００４０】
「曲げ加工試験でのしわ状況」の調査は、JIS Ｚ２２０４で規定する１号試験片を使用し、半径４５mmのポンチにより曲げ角度９０°まで曲げた加工部の頂点を目視及び触診して“しわ模様”の状況を評価した。そして、目視にて“しわ模様”が視認できるものは“×”、目視では殆ど分からないものの触診にて“しわ模様”を感じるものは“△”、目視及び触診にて“しわ模様”が確認できないものは“○”と判定し、“○”及び“△”を合格とした。
そして、“耐食性”については、ケミカルタンカ−での使用状況再現テストとして５０℃の９８％濃硫酸溶液中に４８時間浸漬する試験を行い、このときの重量減量による腐食度で耐食性を評価した。
これらの結果を表２に併せて示す。
【００４１】
さて、表２の適用鋼Ａ〜Ｉについては本発明に係る鋼であるが、これらの鋼を用いて試験番号１〜７及び試験番号16〜24の条件で製造された鋼板については、圧延状況が良好であったことは勿論、機械的特性，シャルピ−衝撃試験結果及び腐食試験結果は何れも満足できるものであった。
しかし、曲げ試験の結果では、１ヒ−ト材である試験番号１〜５，試験番号６及び試験番号７では目視・触診でも殆ど分からない程度の“しわ模様”がどうにか認められただけであった。
【００４２】
一方、通常の圧延が２回繰り返された２ヒ−ト材である試験番号16〜24に係る鋼板については、曲げ試験の結果は何れも良好で、目視・触診によっても“しわ模様”は認められなかった。
【００４３】
これに対して、試験番号８〜15は化学成分組成が本発明の規定条件を外れる比較鋼Ｊ〜Ｑを用いたものであるが、このような鋼を用いた試験番号９〜12及び試験番号14〜15では何れも熱間加工性が不満足である結果となった。
また、試験番号８〜15で得られた鋼板は、機械的特性，シャルピ−衝撃試験結果及び腐食試験結果の何れかが不芳であった。
なお、試験番号８及び９については、仕上温度が低かったために伸びが低くなっている。更に、試験番号12及び13については、仕上温度が高かったために引張強さが低くなっている。そして、これら試験番号８，９及び試験番号12，13に係る鋼板は、曲げ試験の結果において目視で分かるほどの“しわ模様”が発生している。
【００４４】
【発明の効果】
以上に説明した如く、この発明によれば、溶体化処理を省略しても優れた機械的特性，熱間加工性，耐食性を示すMo含有オ−ステナイト系ステンレス鋼を提供できるだけでなく、曲げ加工を施しても表面に“しわ模様”を発生することがない高耐食性Mo含有オ−ステナイト系ステンレス鋼材を低コストで製造できるようになり、ケミカルタンカ−等の構造部材として好適な材料の安定供給が可能になるなるなど、産業上有用な効果がもたらされる。
【図面の簡単な説明】
【図１】本発明に係る熱間圧延工程を例示した概要説明図である。
【図２】本発明に係る熱間圧延工程の別例を示した概要説明図である。
【図３】「実施例」の「鋼Ａ」を用いて２５mm厚の鋼板を製造した際における再結晶の状況を、“熱間圧延温度”及び“当該温度での圧下率”と“その後の圧延までの待ち時間”との関係で示したグラフである。
【図４】本発明に係る熱間圧延工程の更なる別例を示した概要説明図である。[0001]
BACKGROUND OF THE INVENTION
The present invention is suitable as a structural member such as a chemical tanker that requires excellent corrosion resistance, for example.The generation of wrinkle patterns during bending was suppressed.Mo-containing austenitic stainless steelMaterialFurther, the present invention relates to a method for producing a Mo-containing austenitic stainless steel material suitable as a structural member such as a chemical tanker.
[0002]
[Prior art]
Regardless of domestic or overseas, chemical tankers have played an important role as a means of transporting a large amount of various chemicals. Structural members such as chemical tankers have strength, workability, weldability, A material excellent in corrosion resistance and the like has been demanded, and conventionally, austenitic stainless steel material (for example, SUS316 equivalent steel material) to which Mo is added has been widely used.
However, austenitic stainless steel generally cannot be said to have good hot workability, and is a material that is prone to defects such as cracks during hot working. In particular, in an austenitic stainless steel that has been improved in corrosion resistance by adding Mo to be applied to a structural member of a chemical tanker or the like, improvement of its hot workability has been a major issue.
[0003]
Further, when manufacturing austenitic stainless steel, usually a process of applying a “solution treatment” is performed in which the hot-rolled steel is once cooled and then reheated and rapidly cooled. . This solution treatment is a heat treatment performed to prevent intergranular corrosion that tends to occur in austenitic stainless steel, and is intended to suppress precipitation of chromium carbides that cause intergranular corrosion. .
[0004]
However, in recent years, a method has been proposed in which the solution treatment is omitted in order to reduce the manufacturing cost of the austenitic stainless steel material.
For example, in JP-A-55-107729, after performing hot rolling with a cumulative rolling reduction at 850 to 1150 ° C. of 50% or more and a finishing temperature of 850 ° C. or more, subsequently, 850 to 550 ° C. A method for producing an austenitic stainless steel material comprising rapidly cooling the temperature range is disclosed.
[0005]
However, for example, when manufacturing a steel plate having a thickness exceeding 20 mm, if the solution treatment is omitted, a mixed grain structure in which coarse crystal grains are mixed remains, and such a steel material is bent. It was found that a wrinkle pattern with minute irregularities was generated on the surface.
It has also been clarified that this tendency is more common among austenitic stainless steels, especially stainless steels containing Mo (SUS316 equivalent steel, etc.).
[0006]
The cause of this “wrinkle pattern” is that if the steel material is subjected to bending deformation in a state where a “mixed grain structure” in which coarse and fine grain sizes are mixed in the vicinity of the surface layer of the steel material, It is considered that a difference in the degree of deformation occurs due to the difference in diameter, and this appears as an uneven pattern on the steel surface.
As described above, the mixed grain structure causing the “wrinkle pattern” tends to occur particularly in the austenitic stainless steel thick plate material having a plate thickness exceeding 20 mm.
[0007]
By the way, as described above, Mo-containing austenitic stainless steel materials are often used in chemical tankers and the like, but since they are often used by replacing various chemicals for each operation, they come into contact with the chemicals each time. It is necessary to clean the inside of the container member. However, at this time, if there is a “wrinkle pattern” on the surface of the steel material that comes into contact with the chemical, the chemical will stay in the concave portion of the “wrinkle” and it will be difficult to remove it completely. It has come to be noticed.
[0008]
As a method for preventing a mixed grain structure that tends to occur in an austenitic stainless steel thick plate material, for example, in Japanese Patent Laid-Open No. 9-310120, “Following rolling to austenitic stainless steel at 850 ° C. or higher” A method for producing a stainless steel plate comprising: rolling at a temperature lower than 850 ° C. and a rolling reduction exceeding 20%, and adjusting the soaking time of the solution treatment at 1000 to 1150 ° C. performed as a function of the plate thickness ” It is shown.
However, this method is based on the premise that solution treatment is performed, and is not in line with the recent demand for “omission of solution treatment for manufacturing cost reduction”.
[0009]
[Problems to be solved by the invention]
For this reason, the present invention aims to omit the solution treatment in order to save labor and energy.Also songAn object of the present invention is to provide a highly corrosion-resistant austenitic stainless steel material that does not generate a “wrinkle pattern” on the surface during the bending process.
[0010]
[Means for Solving the Problems]
In order to achieve the above object, the inventors have studied the component system of austenitic stainless steel that exhibits excellent corrosion resistance even when the solution treatment is omitted while the hot workability is good, Intense research to find a means to realize austenitic stainless steel materials with excellent corrosion resistance that can reduce mixed grain structure even when solution treatment is omitted and can minimize the “wrinkle pattern” during bending. As a result, the following findings were obtained.
[0011]
a) In order to provide the austenitic stainless steel after hot working (hot rolling, etc.) with the corrosion resistance required for structural members such as chemical tankers without solution treatment, at least 2.0% It is indispensable to ensure the Mo content of the chemical component ratio.
However, in the case of austenitic stainless steel containing Mo, ferrite tends to form during hot working and remain at the grain boundaries of austenite, and there is a difference in strength between this ferrite and austenite. Therefore, it cannot be said that the material is satisfactory in terms of hot workability, such as cracks occurring at the boundary between the two.
In addition, when austenitic stainless steel without solution treatment is applied to a chemical tanker or the like, it is not possible to dispel the concern in terms of corrosion resistance only by means of adding Mo.
[0012]
b) However, if Cu addition of 0.20% or more is added to Mo-containing austenitic stainless steel, it is possible to maintain satisfactory corrosion resistance even in harsh corrosive environments such as chemical tankers, etc., even in materials without solution treatment. In addition, by adding an appropriate amount of Al, which is a strong deoxidizing component, to the Mo-containing austenitic stainless steel in which the Cu addition amount is adjusted, oxygen in the steel is reduced to reduce non-metallic inclusions, When an appropriate amount of Ca that fixes S in the steel as CaS is contained, and when an appropriate amount of B having the effect of preferentially segregating at the grain boundaries and increasing the affinity of the grain boundaries is coexisted with Ca, Hot workability is also greatly improved.
That is, when Cu, Al, Ca, and B are added and coexisted as essential components to the Mo-containing austenitic stainless steel, and the contents of these components are adjusted to an appropriate range, the Mo-containing austenitic stainless steel The hot workability and corrosion resistance of steel are drastically improved (if any of the four components of Cu, Al, Ca and B is missing, cracking will occur during rolling, and the corrosion resistance of the steel will be insufficient. Not suitable for severe corrosive environments such as tankers).
[0013]
c) Addition of an appropriate amount of one or more of Ti, Nb and V to the above-mentioned Mo-containing austenitic stainless steel further improves the corrosion resistance of the material and is effective in making crystal grains finer. is there.
[0014]
d) In addition, delta ferrite, which is generally prone to occur when austenitic stainless steel is heated, is said to cause deterioration of hot workability. On the contrary, it was found that it works favorably in sex. Therefore, when the Ni-bal of the steel is adjusted so that an appropriate amount of delta ferrite is generated at the hot working temperature of the Mo-containing austenitic stainless steel, the hot workability is further improved.
[0015]
e) Moreover, each Mo-containing austenitic stainless steel whose components were adjusted as described above was subjected to a reduction of “cumulative reduction ratio of 50% or more at 1050 ° C. or higher and then finished between 800 and 950 ° C.” When the “hot working” is performed, a Mo-containing austenitic stainless steel material in which the “wrinkle pattern” on the surface after the bending work is hardly noticeable can be obtained.
[0016]
f) However, in the case of a material with a product thickness exceeding 20 mm, the occurrence of “wrinkle pattern” may not be sufficiently suppressed. In the production of such a material, the process of heating and rolling is performed at least twice. If the final target dimensions are repeated, the effect of suppressing the occurrence of the “wrinkle pattern” can be sufficiently obtained without restricting the conditions such as the cumulative rolling reduction, and therefore, the Mo-containing austenite having a thick product thickness can be obtained. Even with stainless steel, the “wrinkle pattern” after bending is almost inconspicuous.
[0017]
The present invention has been completed based on the above knowledge and the like, and provides a Mo-containing austenitic stainless steel material and a method for producing a Mo-containing austenitic stainless steel material shown in the following items 1) to 4). To do.
1) By mass ratio, C: 0.03% or less, Si: 1.0% or less, Mn: 0.85-1.50%, Cu: 0.20-0.60%, Ni: 12.0-15.0%, Cr: 16.0-20.0%, Mo: 2.0 -3.0%, Al: 0.010-0.100%, N: 0.050% or less, Ca: O.OO05-0.0040%, B: 0.0005-0.0020%, the balance is composed of Fe and inevitable impurities, and Ni-bal represented by the formula (1) is within the range of “−2 to +1”,Steel structure whose recrystallization structure obtained by dynamic recrystallization by hot rolling is 50% or moreA Mo-containing austenitic stainless steel material in which generation of wrinkle patterns during bending is suppressed.
Ni-bal = 30 (C + N) + 0.5Mn + Ni + 8.2
-1.1 (1.5Si + Cr + Mo + 0.5Nb) (1)
2) In addition to the chemical components described in 1) above, in addition, one or more of Ti: 0.005 to 0.020%, Nb: 0.05% or less, and V: 0.15% or less are included by mass ratio.And is represented by the following formula (1) Ni − bal Is in the range of “−2 to +1”, and has a steel structure in which the recrystallized structure obtained by dynamic recrystallization by hot rolling is 50% or more.A Mo-containing austenitic stainless steel material in which generation of wrinkle patterns during bending is suppressed.
Ni-bal = 30 (C + N) + 0.5Mn + Ni + 8.2
-1.1 (1.5Si + Cr + Mo + 0.5Nb) (1)
3) Chemical composition as described in 1) or 2) above( Ni − bal Including)Mo with suppressed wrinkle pattern formation during bending, characterized by subjecting steel to hot rolling at a cumulative reduction rate of 50% or more at 1050 ° C. or more and a finishing temperature of 800 to 950 ° C. A method for producing a contained austenitic stainless steel material.
4) Chemical composition as described in 1) or 2) above( Ni − bal Including)Steel is hot-rolled after heating to change the steel structure from a cast structure to a rolled structure and recrystallized, and further hot-rolled after heating to further recrystallize. A method for producing a Mo-containing austenitic stainless steel material in which generation of a wrinkle pattern during bending is suppressed, wherein
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the reason why the chemical composition of steel and the production conditions of the steel material are limited as described above in the present invention will be described with reference to the embodiment of the present invention.
[A] Chemical composition of steel
C: C has the effect of ensuring the strength of austenitic stainless steel. However, if its content exceeds 0.03%, carbide combined with Cr precipitates at the grain boundaries during cooling after rolling and is corrosion resistant. Degraded. Accordingly, the C content is determined to be 0.03% or less, but a preferable range is 0.01 to 0.02% from the viewpoint of strength and corrosion resistance.
[0019]
Si: Si is a necessary component for deoxidation, but when its content exceeds 1.0%, hot workability is significantly inhibited. Therefore, the upper limit of Si content is set to 1.0%.
Mn: Mn is an effective component as a deoxidizer and desulfurizer for steel, and further contributes to stabilization of the austenite phase and improvement of hot workability. However, if its content is less than 0.85%, it is insufficient for stabilizing the austenite phase. On the other hand, if it exceeds 1.50%, the corrosion resistance is adversely affected, so the Mn content is 0.85 to 1.50%. Determined.
[0020]
Cr: Cr is an indispensable component for imparting corrosion resistance to steel, and if the content is less than 16.0%, desired corrosion resistance cannot be ensured. On the other hand, when the Cr content exceeds 20.0%, the manufacturability of steel (steel material) deteriorates. Therefore, the Cr content is determined to be 16.0 to 20.0%, but a preferable range is 16.5 to 18.0%.
Ni: Ni needs to be contained at 12.0% or more in order to secure the formation of austenite structure and severe acid resistance, but if it exceeds 15.0%, it will cause deterioration of hot workability. . Therefore, the Ni content is determined to be 12.0 to 15.0%.
[0021]
Mo: Mo is a component that improves the corrosion resistance of austenitic stainless steel, and is indispensable for components such as chemical tankers. However, if its content is less than 2.0%, solution treatment is omitted. In addition, it is difficult to ensure sufficient corrosion resistance only under the condition of cooling after hot rolling. On the other hand, the addition of a large amount of Mo leads to high cost of steel. Therefore, the Mo content is set to 2.0 to 3.0%.
[0022]
N: N is an effective component for improving the strength and corrosion resistance of austenitic stainless steel, but if its content exceeds 0.050%, hot workability deteriorates and surface flaws occur frequently during rolling. Or raising the recrystallization temperature may cause mixed grains in the surface layer. Therefore, the N content is set to 0.050% or less, preferably 0.025% or less.
[0023]
Cu: Cu is one of the important components, and it is contained in an amount of 0.20% or more in order to improve corrosion resistance, especially sulfuric acid resistance. However, excessive addition reduces hot workability and mixes in the surface layer of steel. Therefore, the upper limit was made 0.60%. Thus, although Cu content was defined as 0.20 to 0.60%, a preferable range is 0.25 to 0.50%.
[0024]
Al: Al is a strong deoxidizer and has the effect of reducing non-metallic inclusions and improving hot workability by lowering the oxygen in the steel, so it is included for this purpose in the present invention. It is one of the important ingredients. In this case, if the Al content is less than 0.010%, the non-metallic inclusions remain in the steel, so the effect of improving hot workability is not sufficient. On the other hand, if the Al content exceeds 0.100%, it combines with N during welding to precipitate AlN and deteriorate the toughness and corrosion resistance of the weld metal part (in this sense, the lower the Al content, the better 0.100%) If it does not exceed, there is no particular problem in practical use). For this reason, the Al content is determined to be 0.010 to 0.100%, but a preferable range is 0.020 to 0.060%.
[0025]
Ca: Ca has the action of fixing S in the steel as CaS and improving hot workability by the addition of Ca. In the present invention, this action is effectively utilized. However, if the Ca content is less than 0.0005%, the effect by the above action cannot be expected. On the other hand, if the Ca content exceeds 0.0040%, the hot workability is adversely affected, and the corrosion resistance is also deteriorated. Become. Therefore, the Ca content is determined to be 0.0005 to 0.0040%.
[0026]
B: B is a component having the property of preferentially segregating at the grain boundaries of steel and increasing the affinity of the grain boundaries. In particular, in the austenitic stainless steel containing Mo, which is a ferrite forming element, ferrite tends to form at the grain boundary during hot working, and the grain boundary cracks due to the difference in strength between ferrite and austenite. Although it tends to be a starting point, it is possible to prevent such cracking by adding B, so B is one of the important components indispensable for the production of Mo-containing austenitic stainless steel materials. It is. And B can improve the hot workability of Mo-containing austenitic stainless steel in a wide temperature range by coexisting with Ca added for the purpose of improving hot workability. However, if the content is less than 0.0005%, the effect of improving the hot workability is not sufficient. On the other hand, if the content exceeds 0.0020%, B carbide precipitates at the crystal grain boundary and the corrosion resistance deteriorates. The B content was 0.0005 to 0.0020%.
[0027]
Ti, Nb and V: Since these components all have the effect of improving the grain refinement and corrosion resistance, one or more types are added as necessary. The reason for limiting the content range will be described.
a) Ti combines with O and N in steel to form TiOx, TiN serves as a nucleus for refining crystal grains, and has the effect of preventing coarsening of the crystal grains of the thick-walled slab that is slowly cooled during slab casting. It also has the effect of improving the corrosion resistance by generating stable carbides in the steel. However, if the content is less than 0.005%, the effect by the above action cannot be expected. On the other hand, if the content exceeds 0.020%, TiOxAnd, since the surface properties are deteriorated by the precipitation of TiN, the Ti content is set to 0.005 to 0.020%.
b) Nb forms fine carbides in steel and contributes to the improvement of corrosion resistance, and at the same time exerts the effect of refining crystal grains. However, if the content exceeds 0.05%, the recrystallization temperature increases, so the upper limit of the Nb content is set to 0.05%.
c) V is a ferrite-forming element, and has the effect of refining crystal grains and improving high-temperature creep strength. Further, the V component has an effect of improving the pitting corrosion resistance by adding an appropriate amount together with Cr, Mo and Cu. However, if the content exceeds 0.15%, hot workability is deteriorated, so the upper limit of the V content is set to 0.15%.
[0028]
Ni-bal: Ni-bal represented by the following formula (1) is correlated with the amount of delta ferrite produced when re-heating a continuous cast slab of Mo-containing austenitic stainless steel, The larger the value of, the higher the amount of delta ferrite.
[0029]
[B] Steel production conditions
The Mo-containing austenitic stainless steel according to the present invention is a material that has excellent hot workability and exhibits excellent corrosion resistance even if the solution treatment is omitted. When "Hot working under conditions of finishing between 800 and 950 ° C after applying a reduction of 1050 ° C or higher and a cumulative reduction ratio of 50% or higher" is applied, A highly corrosion-resistant steel material in which “is hardly noticeable is obtained.
[0030]
FIG. 1 is a schematic explanatory view of the hot working process (the hot rolling process is illustrated in the figure) according to the present invention. In the heated Mo-containing austenitic stainless steel (slab) according to the present invention, FIG. When a reduction of 50% or more of the cumulative reduction ratio is applied at 1050 ° C. or higher, which is the recrystallization region, a structure having fine and uniform crystal grains is obtained by “dynamic recrystallization”, which is sufficient for steel that is allowed to cool after rolling. Corrosion resistance is ensured, and the occurrence of “wrinkle pattern” is hardly recognized even when the rolled steel material is subjected to bending.
In this case, when the cumulative rolling reduction at 1050 ° C. or more is less than 50%, the dynamic recrystallization does not proceed sufficiently, so that it is difficult to ensure the above characteristics in the rolled steel material.
[0031]
In the present invention, the processing is further continued after the reduction at 1050 ° C. or higher, and finishing is performed at a temperature of 800 to 950 ° C., but the finishing temperature is set to 800 to 950 ° C. to ensure the strength of the steel material by work hardening. For. If this finishing temperature exceeds 950 ° C, the strength cannot be secured sufficiently by work hardening, and if the finishing process is carried over to a temperature range below 800 ° C, the wrinkle when bending the steel material after rolling will be reduced. There is concern about the occurrence of “patterns” (in the case of a steel material having a thickness of 20 mm or more, it is desirable to adjust the finishing temperature to 800 to 870 ° C. from the viewpoint of securing the strength).
In addition, it is not necessary to prescribe | regulate the reduction rate in the said finishing temperature exceptionally, What is necessary is just to adjust according to the steel material strength made into the target. As a result, it is possible to easily and stably ensure the strength necessary for a structural member such as a chemical tanker.
[0032]
Also, when the steel thickness after hot working (for example, product thickness such as plate thickness) exceeds 20mm, as shown in Fig. 2, apply "rolling with a cumulative rolling reduction of 50% or more at 1050 ° C or higher". It is preferable to secure a “waiting time (interval)” of about 60 to 120 seconds between the first processing and the next processing (for example, finishing processing). As a result, recrystallization progresses even in thick steel materials where the dynamic recrystallization progresses slowly, and the fine and uniform grain structure increases, so the occurrence of “wrinkle patterns” when bending the rolled steel material is suppressed. It is done.
[0033]
FIG. 3 shows a case where a 25 mm thick steel plate was produced from a 280 mm thick slab according to the hot rolling process shown in FIG. 2 using “steel A” shown in “Table 1” of “Example” described later. It is the graph which showed the condition of recrystallization by the relationship between "hot rolling temperature" and "the rolling reduction (50%) at the said temperature", and "waiting time (holding time) until subsequent rolling". From FIG. 3 as well, a steel sheet with a thickness exceeding 20 mm is obtained by securing a cumulative reduction ratio of 50% or more in a temperature range of 1050 ° C. or more and further taking an interval of 60 seconds or more as the waiting time (holding time). Even if the whole is stableOf 5It can be seen that 0% or more can be a recrystallized structure, and the steel structure after being allowed to cool is a fine and uniform structure.
If the plate thickness is less than 20 mm, recrystallization can be performed sufficiently without taking the above “waiting time (holding time)” if a cumulative reduction ratio of 50% or more is secured in a temperature range of 1050 ° C. or more. Has been confirmed to proceed.
[0034]
Furthermore, when the steel material thickness (product thickness) after hot working exceeds 20 mm, it can be obtained by repeating the heating and working steps on the Mo-containing austenitic stainless steel according to the present invention at least twice. The “wrinkle pattern” that tends to occur when a product is bent can be further and stably suppressed.
That is, if, for example, two-height rolling is performed, the steel structure changes from a cast structure to a rolled structure during the rolling of the first heat, and a recrystallization progresses. Moreover, strain is also introduced by this rolling. If rolling of the next second heat is performed in this state, recrystallization is further promoted during the rolling of the second heat, and finer and uniform crystal grains are formed. Therefore, the obtained steel material is bent. The occurrence of “wrinkle patterns” during processing is further reduced.
In this two-heat process, normal hot rolling conditions can be used without restricting the production conditions such as the cumulative rolling reduction.InA sufficient effect can be obtained.
Of course, the process shown in FIG. 1 or the process shown in FIG. 2 may be repeated twice, or the process shown in FIG.ItAs described above, the process shown in FIG. 2 may be carried out following the process shown in FIG.
[0035]
The invention will now be illustrated by examples.
【Example】
First, austenitic stainless steel having the chemical composition shown in Table 1 was melted, and a slab obtained by continuous casting was used as a slab for hot rolling (size: 280 mm thick × 1250 mm wide × 2300 mm long). In addition, the content of inevitable impurity elements other than the chemical components shown in Table 1 was about the same as that of ordinary stainless steel.
[0036]
[Table 1]
Next, these slabs were hot-rolled under the conditions shown in Table 2 and allowed to cool to obtain a steel plate having a thickness of 25 mm.
In addition, about 1 heat material (test numbers 1-17), after heating a slab to 1230 degreeC, it rolled at once from slab thickness 280mm to final thickness 25mm.
For the two-heat material (test numbers 18 to 26), the slab was heated to 1230 ° C. and then rolled in the primary rolling from a slab thickness of 280 mm to an intermediate thickness of 130 mm (the rolling up to the intermediate thickness was 1050). Then, this was heated again to 1230 ° C. and then rolled from the intermediate thickness of 130 mm to the final thickness of 25 mm in the secondary rolling (the finishing temperature in the secondary rolling is shown in Table 2). As it was).
[0038]
[Table 2]
Then, for each steel plate obtained1/FourTest specimens were collected in the C direction from the width position, processed into respective test specimen shapes, and subjected to a characteristic test.
The mechanical properties were evaluated by measuring 0.2% proof stress, tensile strength and elongation using JIS 13B test pieces, and "0.2% proof stress" was 400 N / mm.2Above, "Tensile strength" is 600 N / mm2The above was regarded as passing. Regarding "elongation", considering use as an inner wall of a chemical tanker, 25% or more was accepted.
In the “Charpy impact test”, a V-notch test piece was used and the test was conducted at a test temperature of −196 ° C. in accordance with JIS Z2242, and the evaluation was performed with the minimum value of three tests.
[0040]
The “wrinkle condition in the bending test” was investigated by using the No. 1 test piece specified in JIS Z2204, visually and palpating the apex of the processed part bent to a bending angle of 90 ° with a punch with a radius of 45 mm. The situation of “pattern” was evaluated. And “x” indicates that the “wrinkle pattern” can be visually confirmed, “△” indicates that the “wrinkle pattern” is felt by visual inspection, but “crease pattern” is confirmed by visual inspection and palpation. Those that could not be judged as “◯”, and “○” and “△” were accepted.
As for “corrosion resistance”, a test of immersion in a 98% concentrated sulfuric acid solution at 50 ° C. for 48 hours was performed as a test to reproduce the use situation in a chemical tanker, and the corrosion resistance was evaluated by the degree of corrosion due to weight loss at this time.
These results are also shown in Table 2.
[0041]
Now, the steels A to I in Table 2 are steels according to the present invention,These steelTest numbers 1 to7And test number16~twenty fourAs for the steel sheet produced under the above conditions, the mechanical properties, Charpy impact test results, and corrosion test results were all satisfactory, as well as good rolling conditions.
However, as a result of the bending test, test numbers 1 to 5, which are one heat material, test number6And test number7Then, “wrinkle pattern” that was almost unknown even by visual inspection and palpation was somehow recognized.It was.
[0042]
On the other hand, the test number is a two-heat material in which normal rolling is repeated twice.16~twenty fourAs for the steel sheet according to No. 4, the results of the bending test were all good, and “wrinkle pattern” was not recognized by visual inspection or palpation.
[0043]
In contrast, the test number8~15Are comparative steels J to Q whose chemical composition deviates from the specified conditions of the present invention, but test numbers using such steels.9~12And test number14~15In either case, the hot workability was unsatisfactory.
Also, exam number8~15The steel sheet obtained in 1 was unsatisfactory in any of mechanical properties, Charpy impact test results and corrosion test results.
Test number8as well as9As for, the elongation was low because the finishing temperature was low. In addition, the test number12as well as13For, the finishing temperature was high, so the tensile strength was low. And these test numbers8,9And test number12,13As for the steel plate which concerns on the result of a bending test, the "wrinkle pattern" enough to understand visually has generate | occur | produced.
[0044]
【The invention's effect】
As described above, according to the present invention, not only can the Mo-containing austenitic stainless steel exhibit excellent mechanical properties, hot workability, and corrosion resistance even if the solution treatment is omitted, but also bending work can be provided. High-corrosion-resistant Mo-containing austenitic stainless steel material that does not generate “wrinkle patterns” on the surface can be manufactured at low cost, and stable supply of materials suitable for structural members such as chemical tankers This makes it possible to produce industrially useful effects.
[Brief description of the drawings]
FIG. 1 is a schematic explanatory diagram illustrating a hot rolling process according to the present invention.
FIG. 2 is a schematic explanatory diagram showing another example of the hot rolling process according to the present invention.
FIG. 3 shows the state of recrystallization when a steel plate having a thickness of 25 mm is manufactured using “steel A” in “Example”, “hot rolling temperature”, “reduction rate at the temperature”, and “after that It is the graph shown by the relationship with "waiting time until rolling".
FIG. 4 is a schematic explanatory view showing still another example of the hot rolling process according to the present invention.
Claims (4)
Ni−bal ＝30（Ｃ＋Ｎ）＋0.5Mn ＋Ni＋8.2
− 1.1（1.5Si ＋Cr＋Mo＋0.5Nb ） …（１）By mass ratio, C: 0.03% or less, Si: 1.0% or less, Mn: 0.85-1.50%, Cu: 0.20-0.60%, Ni: 12.0-15.0%, Cr: 16.0-20.0%, Mo: 2.0-3.0 %, Al: 0.010 to 0.100%, N: 0.050% or less, Ca: O.OO05 to 0.0040%, B: 0.0005 to 0.0020%, with the balance being Fe and inevitable impurities, and the following formula ( Ni-bal represented by 1) is in the range of “−2 to +1”, and has a steel structure in which the recrystallization structure obtained by dynamic recrystallization by hot rolling is 50% or more. A Mo-containing austenitic stainless steel material in which generation of wrinkle patterns during bending is suppressed.
Ni-bal = 30 (C + N) + 0.5Mn + Ni + 8.2
-1.1 (1.5Si + Cr + Mo + 0.5Nb) (1)
Ni−bal ＝30（Ｃ＋Ｎ）＋0.5Mn ＋Ni＋8.2
− 1.1（1.5Si ＋Cr＋Mo＋0.5Nb ） …（１）In addition to the chemical components according to claim 1, Ti in further weight ratio: 0.005 ~ 0.020%, Nb: 0.05% or less and V: looking contains one or two or more of 0.15% or less, and the following formula Ni - bal represented by (1) is in the range of “−2 to +1”, and has a steel structure in which the recrystallized structure obtained by dynamic recrystallization by hot rolling is 50% or more. wherein the is, bending at the wrinkle pattern Mo content Oh suppressed occurrence of - austenitic stainless steel.
Ni-bal = 30 (C + N) + 0.5Mn + Ni + 8.2
-1.1 (1.5Si + Cr + Mo + 0.5Nb) (1)
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| CN109504826B (en) * | 2018-12-22 | 2022-03-15 | 佛山培根细胞新材料有限公司 | Copper-vanadium-containing high-strength high-corrosion-resistance stainless steel and preparation method thereof |
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