JP4080729B2 - Stainless steel for food plant - Google Patents

Description

【００１０】
実験２
このような有機酸による腐食性増大のメカニズムを探るために、本発明者らは有機酸を含有する高濃度食塩水に長期間浸漬したＳＵＳ３１６Ｌの表面分析と、その溶液中での電気化学的測定を実施した。具体的には、
２−▲１▼：１７％食塩水
２−▲２▼：１７％食塩水＋１％グルタミン酸
２−▲３▼：１７％食塩水＋１％乳酸
の３種類の試験溶液を調製し、これらを３５℃に保持して、エメリー紙４００番で湿式研磨したＳＵＳ３１６Ｌ平板試験片を１週間浸漬し、その表面の不動態皮膜構造をオージェ電子分光分析装置（以下ＡＥＳと記す）にて解析した。また同じ浸漬後の試験片表面を走査電子顕微鏡（以下ＳＥＭと記す）にて観察した。更には１週間の浸漬期間中における各試験片の自然浸漬電位を、飽和カロメルを参照電極として測定した。なお、この自然浸漬電位の測定前は、予め各試験溶液に空気を２４時間吹き込み、溶存酸素が飽和状態になるようにした。
初めに各溶液に１週間浸漬した後の試験片表面のＡＥＳ分析結果を図１に示す。なお図１はＡｒ加速電圧を１ｋＶとして深さ方向に表面不動態皮膜構成元素を分析し、不動態皮膜の強さの指標となる［Ｃｒ］／［Ｃｒ］＋［Ｆｅ］として整理した数値を示す。ここで［Ｃｒ］、［Ｆｅ］はそれぞれの原子％を表し、この指標が高いほど不動態皮膜が強い、即ち耐食性が良好なことを示唆する。図１から明らかなように、１７％食塩水（２−▲１▼）、あるいは１７％食塩水に１％乳酸を添加した溶液（２−▲３▼）では、表面不動態皮膜構造に違いは認められないが、１７％食塩水に１％グルタミン酸を添加した溶液（２−▲２▼）では、２−▲１▼や２−▲３▼に比べ最表層部における［Ｃｒ］／［Ｃｒ］＋［Ｆｅ］の値が低下していることが認められた。これはグルタミン酸が不動態皮膜を劣化させる働きをしていることを示唆している。また各溶液中での自然浸漬電位測定結果を図２に示すが、溶液２−▲１▼や２−▲３▼では測定開始からの自然浸漬電位の変化は僅かであるが、グルタミン酸を含有する溶液２−▲２▼では開始直後から急激に自然浸漬電位が低下することが認められた。以上の結果より、有機酸の中でもアミノ酸であるグルタミン酸は還元剤として作用し、その結果、表面不動態皮膜を不安定にすることが知見された。
【００１１】
一方、各溶液への１週間浸漬後の試験片表面をＳＥＭで観察したところ、溶液２−▲１▼や２−▲２▼では浸漬前と変化がないが、乳酸を含有する溶液２−▲３▼のみ微小孔が表面に形成されていることが認められた。この部分は元々介在物が存在していた所であるが、詳細な観察の結果、同じ介在物でもＡｌ₂Ｏ₃あるいはＳｉＯ₂を主体とした介在物は浸漬後も存在しているが、ＣａＯやＭｇＯ含有比率が高い介在物は選択的に溶け落ちていることが判明した。このメカニズムとして、乳酸はキレート構造を有しているため、これと親和力の強いＣａやＭｇと優先的に反応し、結果的にＣａＯやＭｇＯ系介在物を選択的に溶解させ、すきま腐食や応力腐食割れの起点となるものと考えている。従って以上の理由により、有機酸の中でもキレート構造を有する乳酸やクエン酸等が腐食性を増大させていること、またＣａＯやＭｇＯが主体の介在物が存在すると耐食性が劣化することが知見された。
【００１２】
実験３
以上、有機酸が存在する高濃度食塩含有醤油製造プラント環境の特異性、及び有機酸が不動態皮膜の劣化、あるいはＣａＯ、ＭｇＯを主体とする介在物を選択的に溶解させることで腐食性を増大させるメカニズムについて述べたが、次に本発明者らは、このような環境において良好な耐食性を示し、適用可能なステンレス鋼の成分組成を見出すため、以下の実験を実施した。
Ｃ：０．００８〜０．０３５％、Ｓｉ：０．０２〜０．２４％、Ｍｎ：０．１３〜０．３４％、Ｐ：０．０１７〜０．０３４％、Ｓ：０．００１〜０．００３％、Ｎｉ：６．４４〜３４．８３％、Ｃｒ：１６．５１〜２５．１２％、Ｍｏ：２．０６〜７．４７％、Ｃｕ：０．０１〜０．８６％、Ｗ：０．０１〜０．７３％、Ｃｏ：０．０１〜０．７５％、Ａｌ：０．００６〜０．０９２％、Ｎ：０．０２〜０．３０％、Ｃａ：０．０００１〜０．００５２％、Ｍｇ：０．０００１〜０．００１８％、Ｂ：０．０００１〜０．００３６％の組成範囲で、しかも鋼中酸化物系介在物中のＣａＯ＋ＭｇＯの質量比が様々な比率となるステンレス鋼を大気溶解炉によって溶製し、インゴットを得た。これに１２５０℃、８時間の鋼塊熱処理、鍛造、冷間圧延及び１１５０℃、３０分加熱後水冷する溶体化処理を施して、厚さ２ｍｍの冷延板を作製した。次いで、２ｍｍ冷延板から上述の実験１と同様に試験片を採取し、スポット抵抗溶接により溶接すきま付き試験片を作製した。腐食試験は、約１７％の食塩を含有する発酵調味料である醤油を試験溶液とし、これを３５℃に保持し、上述の試験片を５ヶ月間浸漬した。浸漬後、溶接ナゲット部中心を通るように切断し、光学顕微鏡にて断面観察を行い、すきま腐食、あるいは応力腐食割れの発生状況を評価した。なお、何れの腐食が生じても評価は×とし、まったく腐食が生じなかった材料を〇とした。
【００１３】
図３に鋼中酸化物系介在物中のＣａＯ＋ＭｇＯの質量比率が２０％以下の材料と、それ以上になる材料とに分け、それぞれに対する腐食試験結果を示す。なお図３の横軸には、合金成分の内、耐食性への寄与が大きいＣｒ、Ｍｏ、Ｎを取り上げ、その寄与の程度から各元素がほぼ等価となるように重み付けした総量 Ｃｒ＋３．３Ｍｏ＋２０Ｎ（但しＣｒ、Ｍｏ、Ｎは各成分元素の含有量（質量％））を示してある。この図３より、酸化物系介在物中のＣａＯ＋ＭｇＯの質量比率が２０％以上の場合、Ｃｒ＋３．３Ｍｏ＋２０Ｎの値が４４を超えて初めて腐食が発生しなくなるのに対し、ＣａＯ＋ＭｇＯの質量比率が２０％以下になるとＣｒ＋３．３Ｍｏ＋２０Ｎの値が３８以上で腐食が発生しなくなることが認められた。Ｃｒ＋３．３Ｍｏ＋２０Ｎの値が大きいほど耐食性が良好になるのは自明であるが、その分高価な元素を合金中へ添加しなければならず、コストの上昇に繋がる。しかしながら酸化物系介在物組成をＣａＯ＋ＭｇＯの質量比率で２０％以下になるように制御することで、耐食性に必要なＣｒ＋３．３Ｍｏ＋２０Ｎの下限値を下げられることが判明した。但し、このような制御を行ってもＣｒ＋３．３Ｍｏ＋２０Ｎの指標は少なくとも３８以上なければ高濃度食塩と有機酸を含有する醤油製造プラントで材料に腐食が発生する可能性があることが示された。続いて本発明者らは、鋼中酸化物系介在物中のＣａＯ＋ＭｇＯの質量比率が２０％以下になるように安定して制御するための研究を重ねた結果、溶解炉のレンガ等から混入するＣａ、Ｍｇを考慮し、脱酸材成分であるＳｉ、Ａｌの含有量をある範囲にすれば上述の比率が達成できることが判明した。即ち、図４に示す如く、Ｓｉ、Ａｌの含有量をそれぞれ０．０１〜０．２５％、０．００５〜０．１００％の範囲内で、且つＣａとＭｇ含有量との関係がＳｉ＋Ａｌ−１００（Ｃａ＋Ｍｇ）≧０を満足すれば、介在物中のＣａＯ＋ＭｇＯの質量比率を安定的に２０％以下にすることが可能であることを見出した。以上のように、Ｃｒ、Ｍｏ、Ｎ、及びＳｉ、Ａｌの成分範囲と介在物の組成を制御することで、高濃度食塩と有機酸を含有する醤油製造プラントで耐食性の良好なオーステナイトステンレス鋼を提供できるとの知見を得た。
【００１４】
次に各成分の限定理由を以下に説明する。
Ｃ：０．０５％以下
Ｃは特に溶接時に鋭敏化を誘発し耐食性を低下させる元素であるので少ない方が望ましいが、極端に低減させることは強度の低下を招くと共に製造コストが増加する。Ｃの含有量は０．０５％までは許容できるのでこの値を上限値とした。
Ｓｉ： ０．０１〜０．２５％
Ｓｉは脱酸のために有効な元素であり、特に鋼中酸化物系介在物中のＣａＯ＋ＭｇＯ比率を下げてＡｌと共に酸化物系介在物の主体を構成するため必須な元素であるので０．０１％以上の添加が必要である。しかしながら過剰の添加はその効果が飽和すると共に、延性の低下や強度の上昇を招き、更にはσ相やχ相などの金属間化合物の析出を助長して耐食性を劣化させるため、０．２５％以下にする必要がある。望ましくは０．２０％以下、より望ましくは０．１０％以下がよい。
Ｍｎ：０．４０％以下
Ｍｎはσ相やχ相などの金属間化合物の析出を抑制する上で、また耐食性劣化を抑えるため極力低減させる必要のある元素であり、そのためには０．４０％以下にする必要がある。望ましくは０．３０％以下、より望ましくは０．２０％以下が良い。
Ｐ：０．０４０％以下
Ｐは不純物として不可避的に混入する元素であり、結晶粒界に偏析し易く耐食性及び熱間加工性の観点からは少ない方が望ましい。しかしながら、Ｐの含有量を極端に低減させることは製造コストの増加を招く。Ｐの含有量は０．０４０％までは許容できるのでこの値を上限値とした。ただし、望ましくは０．０３０％以下が良い。
【００１５】
Ｓ：０．００３％以下
ＳはＰと同様に不純物として不可避的に混入する元素であり、結晶粒界に偏析し易く耐食性及び熱間加工性の観点からは少ない方が望ましい。特に、０．００３％を超えて含有するとその有害性が顕著に現れるので、含有量を０．００３％以下とした。ただし、望ましくは０．００２％以下が良い。
Ｎｉ：４０．０％以下
Ｎｉはσ相やχ相などの金属間化合物の析出を抑制する上で有効な元素であり、また組織をオーステナイトにする場合には必須な元素である。更には耐応力腐食割れ向上にも効果のある元素であるが、その含有量が４０．０ｗｔ％を上回ると熱間加工性の劣化や熱間変形抵抗の増大を招く。よって、Ｎｉの含有量は４０．０％以下とした。なお、Ｎｉの含有量は１８．０〜３０％であることが好ましく、２４．０〜２６％であればさらに好ましい。
Ｃｒ：１６．０％≦Ｃｒ≦２６．０％
Ｃｒは耐すきま腐食性を向上させるのに有効な元素であり、その効果を得るためには１６．０％以上含有する必要がある。しかしながら、２６．０％を超えて含有するとσ相やχ相などの金属間化合物の形成を助長し、かえって耐すきま腐食性を劣化させるので、１６．０％〜２６．０％とした。なお、Ｃｒの含有量は２０．０％以上であることが好ましく、２２．０％以上であればさらに好ましい。
【００１６】
Ｍｏ：２．０％≦Ｍｏ≦８．０％
Ｍｏも耐すきま腐食性を向上させるのに有効な元素であり、その効果を得るためには２．０％以上含有する必要がある。しかしながら、８．０％を超えて含有すると、金属間化合物の析出を助長し、耐食性を逆に劣化させてしまうので、２．０％〜８．０％とした。なお、Ｍｏの含有量は３．０％以上であることが好ましく、５．０％以上であればさらに好ましい。
Ａｌ：０．００５％≦Ａｌ≦０．１００％
Ａｌは強力な脱酸剤であり、実験３に示した通り、特に鋼中酸化物系介在物中のＣａＯ＋ＭｇＯ比率を下げ、Ｓｉと共に酸化物系介在物の主体を構成させるためには積極的に添加する必要があるが、０．１０％を超えて含有させるとその効果が飽和すると共に、金属間化合物の析出を助長させるので、その含有量を０．１０％以下とした。
Ｎ：０．１０％≦Ｎ≦０．３０％
ＮはＣｒ、Ｍｏと同様に耐すきま腐食性を向上させるとともに、金属間化合物の析出を抑制する有効な元素であり、その効果を得るためには、０．１０％以上含有させる必要がある。しかしながら、０．３０％を超えて含有すると、熱間変形抵抗が極めて上昇して熱間加工性を阻害するので、Ｎの含有量は０．１０％〜０．３０％とした。なお、Ｎの含有量は０．１５％以上であることが好ましい。
Ｍｇ：０．０００５％以下
Ｍｇは通常鋼中酸化物系介在物中に不可避的に含まれるものであるが、実験３の結果から明らかなように、耐食性の観点から０．０００５％以下にする必要がある。即ち０．０００５％を超えるとキレート構造を有する有機酸に可溶な介在物を形成し易くなり、耐食性劣化を招く。
【００１７】
Ｃａ：０．００１０％以下
ＣａもＭｇと同様、鋼中酸化物系介在物中に不可避的に含まれるものであるが、実験３の結果から明らかなように、耐食性の観点から０．００１０％以下にする必要がある。即ち０．００１０％を超えるとキレート構造を有する有機酸に可溶な介在物を形成し易くなり、耐食性劣化を招く。
Ｃｕ：０．０１〜１．０％
Ｗ：０．０１〜〜１．０％
Ｃｏ：０．０１〜１．０％
本発明では、上記成分に加えて、０．０１％≦Ｃｕ≦１．０％、０．０１％≦Ｗ≦１．０％、０．０１％≦Ｃｏ≦１．０％の１種または２種以上を含有することができる。これら元素は一般的な耐食性の向上に有効であるが、その効果を得るためには０．０１％以上含有させる必要がある。一方、１．０％を超えて含有すると熱間加工性を阻害するので、それぞれの含有量を０．０１％〜１．０％とした。
Ｂ：０．００１≦Ｂ≦０．０１０％
本発明では、上記成分に加えて、０．００１≦Ｂ≦０．０１０％を含有することができる。Ｂは熱間加工性の向上に極めて有効であるが、０．００１％以下ではその効果が少なく、０．０１０％を上回ると逆に熱間加工性が劣化する。よって、Ｂの含有量は０．００１％〜０．０１０％とした。
Ｃｒ＋３．３Ｍｏ＋２０Ｎ≧３８
【００１８】
本発明においてＣｒ、Ｍｏ、Ｎを次の関係式
Ｃｒ＋３．３Ｍｏ＋２０Ｎ≧３８
（但しＣｒ、Ｍｏ、Ｎは各成分元素の含有量（質量％））に限定した理由は、実験３の結果から明らかなように、Ｃｒ＋３．３Ｍｏ＋２０Ｎが３８を下回ると、本発明の主要な構成要素である鋼中酸化物系介在物中のＣａＯ＋ＭｇＯの質量比率をＳｉ、Ａｌ、Ｃａ、Ｍｇ含有量の最適化により制御しても、高濃度食塩と有機酸を含有する醤油製造プラントで十分な耐食性を有さないためである。なお、Ｃｒ＋３．３Ｍｏ＋２０Ｎは４０以上であることが好ましく、４４以上であればさらに好ましい。鋼中酸化物系介在物中のＣａＯ＋ＭｇＯの質量比率を２０％以下Ｓｉ＋Ａｌ−１００（Ｃａ＋Ｍｇ）≧０
本発明において、鋼中酸化物系介在物中のＣａＯ＋ＭｇＯの質量比率を２０％以下とし、且つ、Ｓｉ、Ａｌ、Ｃａ、Ｍｇを次の関係式
Ｓｉ＋Ａｌ−１００（Ｃａ＋Ｍｇ）≧０（但しＳｉ、Ａｌ、Ｃａ、Ｍｇは各成分元素含有量（質量％））に限定した理由は、実験３の結果から明らかなように、これらを満たさないと高濃度食塩と有機酸を含有する醤油製造プラントで十分な耐食性を有さないためである。なお、本発明では、鋼中の全ての酸化物系介在物がＳｉＯ₂、Ａｌ₂Ｏ₃、ＣａＯ、ＭｇＯの単独、あるいは複合酸化物である必要はなく、どのような介在物でも単にＣａＯ＋ＭｇＯ比率が２０％以下であることを満たせばよい。当然の如くその他の酸化物が単独、あるいは上述の酸化物と共に複合酸化物を形成する場合もある。その他の酸化物としてはＭｎＯ、ＦｅＯ、ＴｉＯ₂等が考えられる。
【００１９】
【実施例及び比較例】
次に本発明を以下に示す実施例に基づいて説明する。なおここでは上述の実験３で示した各種成分鋼も併せて記す。まず、表２及び表３に示す成分組成を有する本発明鋼、及び比較鋼を、大気溶解炉によって溶製しインゴットを得た。これに１２５０℃、８時間の鋼塊熱処理、鍛造、冷間圧延、及び１１５０℃、３０分加熱後水冷する溶体化処理を施して、厚さ２ｍｍの冷延板を作製した。次いで２ｍｍ冷延板から８０ｍｍ×２５ｍｍ×２ｍｍ、６０ｍｍ×２０ｍｍ×２ｍｍの２枚の試片を採取して、エメリー紙４００番にて湿式研磨、脱脂後、スポット抵抗溶接により溶接すきま付き試験片を作製した。腐食試験は、約１７％の食塩を含有する発酵調味料である醤油を試験溶液とし、これを３５℃に保持して試験片を５ヶ月間浸漬した。浸漬終了後、スポット溶接により形成されたナゲット部の中心を切断し、そのすきま部の断面を光学顕微鏡により観察して、すきま腐食あるいは応力腐食割れの発生状況を評価した。ここで何れの腐食も発生せず良好な耐食性を示した材料を○印、また何れか、あるいは両方の腐食が発生した材料を×印として耐食性を評価した。その結果を表２に示す。表２にはＣｒ＋３．３Ｍｏ＋２０Ｎ、及びＳｉ＋Ａｌ−１００（Ｃａ＋Ｍｇ）の指標、更には鋼中酸化物系介在物中の平均ＣａＯ＋ＭｇＯ質量比率（％）も併せて示すが、Ｃｒ＋３．３Ｍｏ＋２０Ｎ≧３８で、且つＳｉ＋Ａｌ−１００（Ｃａ＋Ｍｇ）≧０であり、更には介在物中のＣａＯ＋ＭｇＯの質量比率が２０％以下である本発明鋼は、このような高濃度食塩と有機酸を含有する醤油環境において腐食の発生がなく、比較鋼に比べて優れた耐食性を有する材料であることがわかる。
【００２０】
【表２】

【００２１】
【表３】

【００２２】
【発明の効果】
以上説明したように、本発明のステンレス鋼では、Ｃｒ、Ｍｏ、Ｎの総量に独自の重み付けをして所定以上とし、しかもＳｉ、Ａｌ、Ｃａ、Ｍｇを所定範囲内にして鋼中酸化物系介在物の組成を制御しているので、食品プラント、特に高濃度食塩と発酵過程で生成する有機酸を含有する醤油に対し優れた耐食性を有するステンレス鋼を開発することができた。
【図面の簡単な説明】
【図１】 試験溶液中に１週間浸漬した試験片表面のＡＥＳ分析結果を示したグラフである。
【図２】 試験溶液中に試験片を浸漬したときの自然浸漬電位の経時変化を示したグラフである。
【図３】 関係式Ｃｒ＋３．３Ｍｏ＋２０Ｎ及び介在物中のＣａＯ＋ＭｇＯ比率と腐食試験における腐食の有無を示したグラフである。
【図４】 関係式Ｓｉ＋Ａｌ−１００（Ｃａ＋Ｍｇ）と介在物中のＣａＯ＋ＭｇＯ比率の関係を示したグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention is excellent in crevice corrosion resistance and stress corrosion cracking resistance, food production plants, particularly food plants in which organic acids such as amino acids, citric acid and acetic acid are produced in the production process, and high salt content, particularly soy sauce The present invention relates to a stainless steel suitable for a production plant.
[0002]
[Prior art]
Traditionally, food production plants use stainless steel, inorganic or organic coated steel, or FRP depending on the operating conditions such as the ingredients and temperature of the food to be handled. The use of stainless steel is increasing from the viewpoint of reduction and further cleaning. Generally, in general food production plants such as soft drinks, beer, and milk, general-purpose stainless steels such as SUS304 and SUS316 are used, and no serious problems such as leakage due to corrosion have occurred. In addition, foods containing salt are sufficiently resistant to local corrosion such as pitting corrosion, crevice corrosion, and local corrosion such as stress corrosion cracking if used at around room temperature. However, when producing seasonings such as soy sauce containing a large amount of salt, for example, SUS304 and SUS316 cause significant local corrosion even at room temperature, and the corrosion resistance is often insufficient. SUS329 stainless steel, which has higher corrosion resistance than the above stainless steel, is less likely to cause local corrosion. However, if the temperature rises above room temperature, crevice corrosion or stress corrosion cracking of the weld may occur. , Its use is limited. Therefore, in such a soy sauce production plant that becomes a special environment, it is necessary to use inorganic or organic coated steel, FRP, nickel base alloy or titanium more expensive than stainless steel without using stainless steel. Is the actual situation.
[0003]
[Problems to be solved by the invention]
The present invention has been made in view of the above circumstances, and its purpose is to provide a stainless steel suitable for a food production plant, particularly a soy sauce production plant or a vinegar production plant in which an organic acid is produced in a fermentation process and contains high-concentration sodium chloride. It is to be.
[0004]
[Means for Solving the Problems]
As a result of various investigations on stainless steel suitable for food production plants, particularly food production plants including fermentation processes such as seasonings such as soy sauce containing a large amount of salt, amino acids, citric acid, It has been found that when organic acids such as lactic acid are produced, this accelerates corrosion of stainless steel, especially crevice corrosion and stress corrosion cracking. As a mechanism that the corrosion of stainless steel is accelerated by organic acids, amino acids generated in the fermentation process act as a reducing agent and degrade the surface passive film that imparts corrosion resistance to stainless steel, while citric acid, lactic acid, etc. chelate. Acts as a starting point for crevice corrosion and stress corrosion cracking by promoting the dissolution of water-soluble CaO and MgO oxide inclusions in the steel, which acts on the surface of stainless steel and is not covered with a surface passive film. I got the knowledge to let you. Therefore, in order to improve the corrosion resistance of the passive film on the stainless steel surface and the base metal in a high-concentration salt-containing environment where an organic acid is present, it is first necessary to satisfy the following equation (1). found.
Cr + 3.3Mo + 20N >> 38 (1)
(In the formula, Cr, Mo and N are the contents of each component (mass%)
Furthermore, CaO and MgO contained in stainless steel inclusions are reduced, and the composition is reduced to SiO.₂And Al₂O_ThreeIt has been found that the corrosion resistance is improved in an environment containing an organic acid-containing high-concentration salt. That is, the following formula (2) is satisfied by the experimental result,
Si + Al-100 (Ca + Mg) ≧ 0 (2)
(In the formula, Si, Al, Ca and Mg indicate the content (%) of each component)
And CaO + MgO in inclusionsmassIt has been clarified that the occurrence of crevice corrosion and stress corrosion cracking, which are main corrosions in stainless steel, can be suppressed when the ratio is 20% or less, and the present invention has been completed.
In addition, the ratio (%) shown in this-application specification is the mass%.
[0005]
  The gist of the first invention of the present application is mass%, C: 0.05% or less, 0.01% ≦ Si ≦ 0.25%, Mn: 0.40% or less, P: 0.040% or less. , S: 0.003% or less,18.0% ≦ Ni ≦ 40.0%, 16.0% ≦ Cr ≦ 26.0%, 2.0% ≦ Mo ≦ 8.0%, 0.005% ≦ Al ≦ 0.100%, 0.10% ≦ N ≦ 0.30%, Mg: 0.0005% or less, Ca: 0.0010% or less, the balance is made of Fe and inevitable impurities, and the mass ratio of CaO + MgO in the oxide-based inclusions in steel is 20% or less and the following (1), (2)
  Cr + 3.3Mo + 20N ≧ 38 (1)
  (In the formula, Cr, Mo and N indicate the content (% by mass) of each component)
Si + Al-100 (Ca + Mg) ≧ 0 (2)
(In the formula, Si, Al, Ca and Mg indicate the content (% by mass) of each component)
And stainless steel characterized in that it is used in an environment containing an organic acid and a salt content. The gist of the second invention of the present application is C: 0.05% or less ,: 0.01% ≦ Si ≦ 0.25%, Mn: 0.40% or less, P: 0.040% or less, S: 0.003% or less, 15.0% ≦ Ni ≦ 40.0%, 16.0% ≦ Cr ≦ 26.0%, 2.0% ≦ Mo ≦ 8.0%, 0.005% ≦ Al ≦ 0.100%, 0.10% ≦ N ≦ 0.30%, the balance being Fe and inevitable impurities And the following (1), (2) formula
  Cr + 3.3Mo + 20N ≧ 38 (1)
  (In the formula, Cr, Mo and N indicate the content (% by mass) of each component)
Si + Al-100 (Ca + Mg) ≧ 0 (2)
(In the formula, Si, Al, Ca and Mg indicate the content (% by mass) of each component)
Is an austenitic stainless steel characterized by being used in an environment containing an organic acid and a salt content. The gist of the third invention of the present application is that the organic acid is an amino acid, citric acid, acetic acid, The stainless steel according to the first invention or the second invention, characterized in that it contains one or more of lactic acid, and the gist of the fourth invention of the present application is C: 0.05% or less , 0.01% ≦ Si ≦ 0.25%, Mn: 0.40% or less, P: 0.040% or less, S: 0.003% or less,18.0% ≦ Ni ≦: 40.0%, 16.0% ≦ Cr ≦ 26.0%, 2.0% ≦ Mo ≦ 8.0%, 0.005% ≦ Al ≦ 0.100%, 0.10% ≦ N ≦ 0.30%, Mg: 0.0005% or less, Ca: 0.0010% or less, the balance being Fe and inevitable impurities, and the mass ratio of CaO + MgO in the oxide-based inclusions in steel Is 20% or less, and the following formula (1)And (2)
  Cr + 3.3Mo + 20N ≧ 38 (1)
  (In the formula, Cr, Mo and N indicate the content (% by mass) of each component)
Si + Al-100 (Ca + Mg) ≧ 0 (2)
(In the formula, Si, Al, Ca and Mg indicate the content (% by mass) of each component)
In the fifth invention of the present application, the gist of the invention is C: 0.05% or less, 0.01% ≦ Si ≦ 0.25%, Mn = 0. .40% or less, P: 0.040% or less, S: 0.003% or less, 15.0% ≦ Ni ≦ 40.0%, 16.0% ≦ Cr ≦ 26.0%, 2.0% ≦ Mo ≦ 8.0%, 0.005% ≦ Al ≦ 0.100%, 0.10% ≦ N ≦ 0.30%, the balance is made of Fe and inevitable impurities, and the following formula (1) and (2) Formula
  Cr + 3.3Mo + 20N ≧ 38 (1)
  (In the formula, Cr, Mo and N indicate the content (% by mass) of each component)
  Si + Al-100 (Ca + Mg) ≧ 0 (2)
(In the formula, Si, Al, Ca and Mg indicate the content (% by mass) of each component)
Austenitic stainless steel for food plants, characterized by satisfying,BookNo. of wish6The gist of the invention is that the stainless steel is used in a soy sauce production plant or a vinegar production plant.5The stainless steel according to the invention of the present invention,7The gist of the invention is that 0.01% ≦ Cu ≦ 1.0%, 0.01 ≦ W ≦ 1.0%, and 0.01 ≦ Co ≦ 1.0%, further containing one or more of them The first invention to the first invention characterized in that6The stainless steel according to the invention of the present invention,8The gist of the present invention is that 0.001% ≦ B ≦ 0.010% is contained.7The stainless steel described in the invention.
[0006]
  In mass%C: 0.05%0.01%≦ Si ≦ 0.25%, Mn: 0.40%Hereinafter, P: 0.040%Hereinafter, S: 0.003%Hereinafter, 15.0%≦ Ni ≦ 40.0%, 16.0%<= Cr <= 26.0%2.0%≦ Mo ≦ 8.0%, 0.005%≦ Al ≦ 0.100%, 0.10%≦ N ≦ 0.30%The balance is made of stainless steel for soy sauce production plant containing a corrosive organic acid and high-concentration salinity characterized by comprising Fe and inevitable impurities and satisfying the following formula (1).
      Cr + 3.3Mo + 20N ≧ 38 (1)
    (In the formula, Cr, Mo and N are the contents of each component.(mass%)
BookWishThe gist of the second invention is that in the stainless steel of the first invention, Mg: 0.0005%Hereinafter, Ca: 0.0010%The following (2) formula is satisfied, and CaO + MgO in oxide inclusions in steelmassIt is an austenitic stainless steel for soy sauce production plant containing a corrosive organic acid and a high concentration salinity characterized in that the ratio is 20% or less
        Si + Al-100 (Ca + Mg) ≧ 0 (2)
      (In the formula, Si, Al, Ca, Mg are the contents of each component (mass%))
In the stainless steel of the first or second invention, 0.01% ≦ Cu ≦ 1.0%, 0.01%≦ W ≦ 1.0%, 0.01%≦ Co ≦ 1.0%It is preferable to further contain one or more of these,1%≦ B ≦ 0.010%It is preferable to contain.
[0007]
DETAILED DESCRIPTION OF THE INVENTION
As described above, the stainless steel of the present invention includes (i) predetermined chemical components and their proper content ranges, (ii) the relationship among Cr, Mo, and N that particularly contribute to improving corrosion resistance, and (iii) inclusions in the steel. The composition and the proper content ranges of Al, Si, Ca, and Mg constituting them will be described below, and the experimental results on which the present invention is based will be described below.
[0008]
Experiment 1
First, the inventors have a fermentation process, and how the environment of a soy sauce manufacturing plant in which organic acids such as amino acids and lactic acid are produced in the process differs from the case where such organic acids are not present. We examined whether. In the experiment, commercially available SUS316L with a thickness of 2 mm was used as a test material. Two specimens cut into 80 mm × 25 mm × 2 mm and 60 mm × 20 mm × 2 mm were overlapped, spot resistance welding was performed at four points, and the welding clearance was An attached corrosion test piece was prepared. Ordinary soy sauce contains many organic acids, but in order to simplify the system, glutamic acid and aspartic acid, which are typical organic acids generated during the fermentation process, and lactic acid and citric acid are not amino acids. The following four types of test solutions to which acetic acid was added were prepared.
1- (1): 17% saline
1- (2): 17% saline + 1% glutamic acid
1- (3): 17% saline + 1% glutamic acid + 1% lactic acid
1- (4): 17% saline + 1% glutamic acid + 1% asparagine
+ 1% lactic acid + 0.2% citric acid + 0.15% acetic acid
These four kinds of test solutions were kept at 35 ° C., and the above-mentioned test pieces were immersed in each for one month. After the immersion, the sample was cut with a cutting machine so as to pass through the center of the weld nugget formed by spot resistance welding, and the cross section was observed with an optical microscope to evaluate the crevice corrosion depth and the stress corrosion cracking length. The results are shown in Table 1. The corrosion occurring in the solution containing sodium chloride alone (1- ▲ 1 ▼) is crevice corrosion, whereas the solution containing the amino acid glutamic acid (1- ▲ 2). In ▼), it was confirmed that stress corrosion cracking occurred in addition to crevice corrosion. On the other hand, it was observed that the depth and length of crevice corrosion and stress corrosion cracking increase in a solution (1- (3)) containing lactic acid having a chelate structure but not amino acid together with glutamic acid. Furthermore, it was also confirmed that the corrosion significantly increases in the solution (1- (4)) in which various organic acids are contained in salt in a complex manner. Based on the above results, the environment in soy sauce production plants containing high-concentration salt that produce organic acids such as amino acids and lactic acid during the fermentation process is more corrosive than the environment where these organic acids do not exist even if they contain the same high-concentration salt. The properties were found to increase significantly.
[0009]
[Table 1]

[0010]
Experiment 2
In order to investigate the mechanism of the increase in corrosivity by such an organic acid, the present inventors conducted surface analysis of SUS316L immersed in a high concentration saline containing an organic acid for a long time and electrochemical measurement in the solution. Carried out. In particular,
2- (1): 17% saline
2- (2): 17% saline + 1% glutamic acid
2- (3): 17% saline + 1% lactic acid
SUS316L flat plate test pieces wet-polished with emery paper No. 400 were immersed for one week while maintaining these at 35 ° C., and the passive film structure on the surface was subjected to Auger electron spectroscopy analyzer (Hereinafter referred to as AES). Moreover, the surface of the test piece after the same immersion was observed with a scanning electron microscope (hereinafter referred to as SEM). Furthermore, the natural immersion potential of each test piece during the immersion period of 1 week was measured using saturated calomel as a reference electrode. Before measuring the natural immersion potential, air was previously blown into each test solution for 24 hours so that the dissolved oxygen was saturated.
FIG. 1 shows the result of AES analysis on the surface of the test piece after first dipping in each solution for 1 week. In FIG. 1, the constituent elements of the surface passive film are analyzed in the depth direction with an Ar acceleration voltage of 1 kV, and the numerical values arranged as [Cr] / [Cr] + [Fe] that serve as an index of the strength of the passive film Show. Here, [Cr] and [Fe] represent respective atomic%, and the higher the index, the stronger the passive film, that is, the better the corrosion resistance. As is clear from FIG. 1, the surface passive film structure is different in 17% saline solution (2- (1)) or a solution in which 1% lactic acid is added to 17% saline solution (2- (3)). Although not recognized, in the solution (2- (2)) in which 1% glutamic acid was added to 17% saline, [Cr] / [Cr] in the outermost layer portion was compared to 2- (1) and 2- (3). It was recognized that the value of + [Fe] was lowered. This suggests that glutamic acid functions to degrade the passive film. Moreover, although the natural immersion potential measurement result in each solution is shown in FIG. 2, in the solutions 2- (1) and 2- (3), the change of the natural immersion potential from the start of measurement is slight, but it contains glutamic acid. In Solution 2- (2), it was observed that the natural immersion potential suddenly decreased immediately after the start. From the above results, it was found that glutamic acid, which is an amino acid among organic acids, acts as a reducing agent, resulting in destabilizing the surface passive film.
[0011]
On the other hand, when the surface of the test piece after one week immersion in each solution was observed by SEM, the solutions 2- (1) and 2- (2) were not changed from those before immersion, but the solution 2- ▲ containing lactic acid was not changed. It was recognized that micropores were formed on the surface only in 3 ▼. This part is where the inclusions originally existed, but as a result of detailed observation, even with the same inclusions, Al₂O_ThreeOr SiO₂It was found that inclusions mainly composed of were present after immersion, but inclusions with a high CaO or MgO content ratio were selectively melted away. As this mechanism, since lactic acid has a chelate structure, it reacts preferentially with Ca and Mg, which have a strong affinity with this, resulting in selective dissolution of CaO and MgO inclusions, crevice corrosion and stress. This is considered to be the starting point of corrosion cracking. For these reasons, it has been found that among organic acids, lactic acid or citric acid having a chelate structure has increased corrosivity, and the presence of inclusions mainly composed of CaO or MgO deteriorates the corrosion resistance. .
[0012]
Experiment 3
As described above, peculiarity of the high-concentration salt-containing soy sauce manufacturing plant environment in which organic acids are present, and deterioration of the passive film or organic acid is selectively dissolved by inclusions mainly composed of CaO and MgO. Having described the increasing mechanism, the present inventors then conducted the following experiment in order to find good corrosion resistance in such an environment and find applicable stainless steel component composition.
C: 0.008 to 0.035%, Si: 0.02-0.24%, Mn: 0.13-0.34%, P: 0.017-0.034%, S: 0.001 to 0.003%, Ni: 6.44-34.83%, Cr: 16.51 to 25.12%, Mo: 2.06 to 7.47%Cu: 0.01 to 0.86%, W: 0.01 to 0.73%, Co: 0.01 to 0.75%, Al: 0.006 to 0.092%, N: 0.02-0.30%, Ca: 0.0001 to 0.0052%Mg: 0.0001 to 0.0018%, B: 0.0001 to 0.0036%In addition, the composition of CaO + MgO in steel oxide inclusionsmassStainless steels having various ratios were melted in an atmospheric melting furnace to obtain ingots. This was subjected to steel ingot heat treatment at 1250 ° C. for 8 hours, forging, cold rolling, and solution treatment in which water cooling was performed after heating at 1150 ° C. for 30 minutes to produce a cold-rolled sheet having a thickness of 2 mm. Next, a test piece was collected from a 2 mm cold-rolled plate in the same manner as in Experiment 1 described above, and a test piece with a weld gap was produced by spot resistance welding. In the corrosion test, soy sauce, which is a fermented seasoning containing about 17% salt, was used as a test solution, which was maintained at 35 ° C., and the above-mentioned test piece was immersed for 5 months. After immersion, the sample was cut so as to pass through the center of the weld nugget, and the cross-section was observed with an optical microscope to evaluate the occurrence of crevice corrosion or stress corrosion cracking. In addition, evaluation was made x even if any corrosion occurred, and the material which did not produce corrosion at all was marked as ◯.
[0013]
  Figure 3 shows the CaO + MgO content in the oxide inclusions in steel.massThe material is divided into materials with a ratio of 20% or less and materials with more than 20%. The horizontal axis of FIG. 3 shows Cr, Mo, and N, which have a large contribution to corrosion resistance, among the alloy components. Cr, Mo, N is the content of each component element (mass%)). From FIG. 3, it can be seen that CaO + MgO in the oxide inclusions.massWhen the ratio is 20% or more, corrosion does not occur until the value of Cr + 3.3Mo + 20N exceeds 44, whereas CaO + MgOmassWhen the ratio was 20% or less, it was confirmed that corrosion did not occur when the value of Cr + 3.3Mo + 20N was 38 or more. Obviously, the larger the value of Cr + 3.3Mo + 20N, the better the corrosion resistance. However, an expensive element has to be added to the alloy, which leads to an increase in cost. However, the oxide inclusion composition is CaO + MgOmassIt was found that the lower limit of Cr + 3.3Mo + 20N required for corrosion resistance can be lowered by controlling the ratio to 20% or less. However, it was shown that even if such control is performed, if the index of Cr + 3.3Mo + 20N is not more than 38, the material may corrode in the soy sauce production plant containing high-concentration sodium chloride and organic acid. Subsequently, the present inventors have found that the CaO + MgO in the oxide inclusions in the steel.massAs a result of repeated research to stably control the ratio to 20% or less, considering the Ca and Mg mixed in from the bricks of the melting furnace, the content of Si and Al as deoxidizer components It was found that the above ratio can be achieved within a certain range. That is, as shown in FIG. 4, the contents of Si and Al are 0.01 to 0.2 respectively.5%0.005-0.100%And the relationship between Ca and Mg content satisfies Si + Al-100 (Ca + Mg) ≧ 0, the content of CaO + MgO in the inclusionsmassIt has been found that the ratio can be stably reduced to 20% or less. As described above, by controlling the component ranges of Cr, Mo, N, Si, and Al and the composition of inclusions, austenitic stainless steel with high corrosion resistance in a soy sauce manufacturing plant containing high-concentration sodium chloride and organic acid can be obtained. The knowledge that it can provide was obtained.
[0014]
  Next, the reason for limitation of each component is demonstrated below.
C: 0.05%Less than
Since C is an element that induces sensitization during welding and lowers corrosion resistance, it is desirable to reduce the amount. However, extremely reducing causes a decrease in strength and an increase in manufacturing cost. C content is 0.05%Since this is permissible, this value was taken as the upper limit.
Si: 0.01-0.25%
Since Si is an effective element for deoxidation, it is an essential element for constituting the main component of oxide inclusions together with Al by lowering the CaO + MgO ratio in the oxide inclusions in steel.1%The above addition is necessary. However, excessive addition causes saturation of the effect, leading to a decrease in ductility and an increase in strength, and further promotes precipitation of intermetallic compounds such as σ phase and χ phase, thereby deteriorating corrosion resistance. Must be: Desirably 0.20% or less, more desirably 0.10% or less.
Mn: 0.40%Less than
Mn is an element that needs to be reduced as much as possible in order to suppress the precipitation of intermetallic compounds such as the σ phase and the χ phase, and to suppress the deterioration of corrosion resistance. For that purpose, it is necessary to make it 0.40% or less. Preferably it is 0.30% or less, more preferably 0.20% or less.
P: 0.040%Less than
P is an element that is inevitably mixed as an impurity, and is preferably segregated at the grain boundary from the viewpoint of corrosion resistance and hot workability. However, extremely reducing the P content causes an increase in manufacturing cost. P content is 0.040%Since this is permissible, this value was taken as the upper limit. However, preferably 0.030%The following is good.
[0015]
S: 0.003%Less than
S is an element that is inevitably mixed as an impurity as in the case of P, and is preferably segregated at a grain boundary from the viewpoint of corrosion resistance and hot workability. In particular, 0.003%If it is contained in excess of the content, its harmfulness will appear remarkably.3%It was as follows. However, preferably 0.002%The following is good.
Ni: 40.0%Less than
Ni is an effective element for suppressing the precipitation of intermetallic compounds such as σ phase and χ phase, and is an essential element when the structure is austenite. Furthermore, although it is an element that is also effective in improving stress corrosion cracking resistance, when its content exceeds 40.0 wt%, hot workability is deteriorated and hot deformation resistance is increased. Therefore, the Ni content is 40.0%It was as follows. The Ni content is 18.0-3.0%Preferably, 24.0-26%More preferably.
Cr: 16.0%<= Cr <= 26.0%
Cr is an effective element for improving crevice corrosion resistance.0%It is necessary to contain the above. However, 26.0%If it exceeds V, it promotes the formation of intermetallic compounds such as σ phase and χ phase, and rather deteriorates crevice corrosion resistance.0%-26.0%It was. The Cr content is 20.0%Or more, preferably 22.0%The above is more preferable.
[0016]
Mo: 2.0%≦ Mo ≦ 8.0%
Mo is also an effective element for improving crevice corrosion resistance.0%It is necessary to contain the above. However, 8.0%If it exceeds V, it promotes the precipitation of intermetallic compounds and adversely deteriorates the corrosion resistance.0%~ 8.0%It was. The Mo content is 3.0%It is preferable that it is above.0%The above is more preferable.
Al: 0.005%≦ Al ≦ 0.100%
Al is a strong deoxidizer, and as shown in Experiment 3, in particular, to reduce the CaO + MgO ratio in the oxide inclusions in steel and to make the main constituent of oxide inclusions together with Si It is necessary to add, but 0.10%When the content exceeds 0.1%, the effect is saturated and the precipitation of intermetallic compounds is promoted.0%It was as follows.
N: 0.10%≦ N ≦ 0.30%
N is an effective element that improves crevice corrosion resistance and suppresses the precipitation of intermetallic compounds in the same manner as Cr and Mo.0%It is necessary to contain above. However, 0.30%When the content exceeds N, the hot deformation resistance is extremely increased and the hot workability is hindered.0%~ 0.30%It was. The N content is 0.15%The above is preferable.
Mg: 0.0005%Less than
Mg is inevitably contained in the oxide inclusions in steel, but as is clear from the results of Experiment 3, from the viewpoint of corrosion resistance, 0.0005%Must be: That is, when it exceeds 0.0005%, it becomes easy to form inclusions soluble in an organic acid having a chelate structure, resulting in deterioration of corrosion resistance.
[0017]
Ca: 0.0010%Less than
Ca is inevitably contained in the oxide-based inclusions in the steel as in the case of Mg. As is apparent from the results of Experiment 3, 0.001 is added from the viewpoint of corrosion resistance.0%Must be: Ie 0.0010%If it exceeds 1, it becomes easy to form inclusions that are soluble in an organic acid having a chelate structure, leading to deterioration in corrosion resistance.
Cu: 0.01-1.0%
W: 0.01 ~ 1.0%
Co: 0.01-1.0%
In the present invention, in addition to the above components, 0.01%≦ Cu ≦ 1.0%, 0.01%≦ W ≦ 1.0%, 0.01%≦ Co ≦ 1.0%1 type (s) or 2 or more types can be contained. These elements are effective for improving the general corrosion resistance.1%It is necessary to contain above. On the other hand,0%Since the hot workability is hindered if it exceeds V, each content is set to 0.0.1%~ 1.0%It was.
B: 0.001 ≦ B ≦ 0.010%
In the present invention, in addition to the above components, 0.001 ≦ B ≦ 0.010%Can be contained. B is extremely effective in improving hot workability, but 0.001%Below, the effect is small, 0.010%On the contrary, hot workability deteriorates. Therefore, the content of B is 0.001%~ 0.010%It was.
Cr + 3.3Mo + 20N ≧ 38
[0018]
In the present invention, Cr, Mo and N are expressed by the following relational expressions.
Cr + 3.3Mo + 20N ≧ 38
(However, Cr, Mo, N is the content of each component element (massThe reason for limiting to%)) is, as is clear from the results of Experiment 3, when Cr + 3.3Mo + 20N is less than 38, the main component of the present invention is that of CaO + MgO in the oxide-based inclusions in steel.massThis is because even if the ratio is controlled by optimizing the contents of Si, Al, Ca, and Mg, the soy sauce manufacturing plant containing high-concentration sodium chloride and organic acid does not have sufficient corrosion resistance. Note that Cr + 3.3Mo + 20N is preferably 40 or more, and more preferably 44 or more. CaO + MgO in oxide inclusions in steelmassRatio is 20% or less Si + Al-100 (Ca + Mg) ≧ 0
In the present invention, CaO + MgO in oxide inclusions in steel.massThe ratio is set to 20% or less, and Si, Al, Ca, and Mg are represented by the following relational expressions.
Si + Al-100 (Ca + Mg) ≧ 0 (However, Si, Al, Ca, Mg are the content of each component element (massThe reason for limiting to%)) is that the soy sauce production plant containing high-concentration sodium chloride and organic acid does not have sufficient corrosion resistance unless these conditions are satisfied, as is apparent from the results of Experiment 3. In the present invention, all oxide inclusions in the steel are SiO.₂, Al₂O_ThreeIt is not necessary to use CaO, MgO alone, or a composite oxide, and any inclusion may simply satisfy that the CaO + MgO ratio is 20% or less. As a matter of course, other oxides may form composite oxides alone or together with the above-mentioned oxides. Other oxides include MnO, FeO, and TiO₂Etc. are considered.
[0019]
[Examples and Comparative Examples]
Next, this invention is demonstrated based on the Example shown below. Here, various component steels shown in Experiment 3 are also described. First, the invented steel having the composition shown in Tables 2 and 3 and the comparative steel were melted in an atmospheric melting furnace to obtain an ingot. This was subjected to steel ingot heat treatment at 1250 ° C. for 8 hours, forging, cold rolling, and solution treatment in which water cooling was performed after heating at 1150 ° C. for 30 minutes to produce a cold-rolled sheet having a thickness of 2 mm. Next, 80 mm x 25 mm x 2 mm, 60 mm x 20 mm x 2 mm specimens were taken from a 2 mm cold-rolled plate, wet-polished with emery paper No. 400, degreased, and spot-welded specimens with welding clearance were obtained. Produced. In the corrosion test, soy sauce, which is a fermented seasoning containing about 17% sodium chloride, was used as a test solution, which was maintained at 35 ° C., and the test piece was immersed for 5 months. After the immersion, the center of the nugget portion formed by spot welding was cut, and the cross section of the gap portion was observed with an optical microscope to evaluate the occurrence of crevice corrosion or stress corrosion cracking. Here, the corrosion resistance was evaluated with a circle indicating a material that did not generate any corrosion and exhibited good corrosion resistance, and a symbol that indicated either or both of the corrosion occurred. The results are shown in Table 2. Table 2 shows the indices of Cr + 3.3Mo + 20N and Si + Al-100 (Ca + Mg), as well as the average CaO + MgO in oxide inclusions in steel.massAlthough the ratio (%) is also shown together, Cr + 3.3Mo + 20N ≧ 38 and Si + Al-100 (Ca + Mg) ≧ 0, and further, the CaO + MgO in the inclusions.massIt can be seen that the steel of the present invention having a ratio of 20% or less is a material having no corrosion in such a soy sauce environment containing high-concentration sodium chloride and organic acid, and having excellent corrosion resistance as compared with the comparative steel.
[0020]
[Table 2]

[0021]
[Table 3]

[0022]
【The invention's effect】
As explained above, in the stainless steel of the present invention, the total amount of Cr, Mo, N is uniquely weighted to a predetermined value or more, and Si, Al, Ca, Mg are within a predetermined range and the oxide in steel Since the composition of inclusions is controlled, stainless steel having excellent corrosion resistance against food plants, particularly soy sauce containing high-concentration sodium chloride and organic acids produced in the fermentation process, could be developed.
[Brief description of the drawings]
FIG. 1 is a graph showing the results of AES analysis of the surface of a test piece immersed in a test solution for 1 week.
FIG. 2 is a graph showing a time-dependent change in natural immersion potential when a test piece is immersed in a test solution.
FIG. 3 is a graph showing the relation Cr + 3.3Mo + 20N, the CaO + MgO ratio in inclusions, and the presence or absence of corrosion in a corrosion test.
FIG. 4 is a graph showing the relationship between the relational expression Si + Al-100 (Ca + Mg) and the ratio of CaO + MgO in inclusions.

Claims (8)

18 % by mass, C: 0.05% or less, 0.01% ≦ Si ≦ 0.25%, Mn: 0.40% or less, P: 0.040% or less, S: 0.003% or less, 18 0.0 % ≦ Ni ≦ 40.0%, 16.0% ≦ Cr ≦ 26.0%, 2.0% ≦ Mo ≦ 8.0%, 0.005% ≦ Al ≦ 0.100%, 0.10 % ≦ N ≦ 0.30%, Mg: 0.0005% or less, Ca: 0.0010% or less, the balance being Fe and inevitable impurities, and the mass of CaO + MgO in the oxide-based inclusions in steel Stainless steel having a ratio of 20% or less and satisfying the following (1) and (2) and containing an organic acid and a salt content.
Cr + 3.3Mo + 20N ≧ 38 (1)
(In the formula, Cr, Mo and N indicate the content (% by mass) of each component)
Si + Al-100 (Ca + Mg) ≧ 0 (2)
(In the formula, Si, Al, Ca and Mg indicate the content (% by mass) of each component) C: 0.05% or less, 0.01% ≦ Si ≦ 0.25%, Mn: 0.40% or less, P: 0.040% or less, S: 0.003% or less, 15% by mass 0.0% ≦ Ni ≦ 40.0%, 16.0% ≦ Cr ≦ 26.0%, 2.0% ≦ Mo ≦ 8.0%, 0.005% ≦ Al ≦ 0.100%, 0.10 % ≦ N ≦ 0.30%, Mg: 0.0005% or less, Ca: 0.0010% or less, the balance is made of Fe and inevitable impurities, and satisfies the following formulas (1) and (2) : Austenitic stainless steel characterized by being used in an environment containing organic acid and salt.
Cr + 3.3Mo + 20N ≧ 38 (1)
(In the formula, Cr, Mo and N indicate the content (% by mass) of each component)
Si + Al-100 (Ca + Mg) ≧ 0 (2)
(In the formula, Si, Al, Ca and Mg indicate the content (% by mass) of each component)   The stainless steel according to claim 1 or 2, wherein the organic acid contains an amino acid and one or more of citric acid, acetic acid and lactic acid. 18 % by mass, C: 0.05% or less, 0.01% ≦ Si ≦ 0.25%, Mn: 0.40% or less, P: 0.040% or less, S: 0.003% or less, 18 . 0 % ≦ Ni ≦: 40.0%, 16.0% ≦ Cr ≦ 26.0%, 2.0% ≦ Mo ≦ 8.0%, 0.005% ≦ Al ≦ 0.100%, 0.10 % ≦ N ≦ 0.30%, Mg: 0.0005% or less, Ca: 0.0010% or less, the balance being Fe and inevitable impurities, and the mass of CaO + MgO in the oxide-based inclusions in steel A stainless steel for food plants, wherein the ratio is 20% or less and satisfies the following formulas (1) and (2) .
Cr + 3.3Mo + 20N ≧ 38 (1)
(In the formula, Cr, Mo and N indicate the content (% by mass) of each component)
Si + Al-100 (Ca + Mg) ≧ 0 (2)
(In the formula, Si, Al, Ca and Mg indicate the content (% by mass) of each component) C: 0.05% or less, 0.01% ≦ Si ≦ 0.25%, Mn: 0.40% or less, P: 0.040% or less, S: 0.003% or less, 15% by mass 0.0% ≦ Ni ≦ 40.0%, 16.0% ≦ Cr ≦ 26.0%, 2.0% ≦ Mo ≦ 8.0%, 0.005% ≦ Al ≦ 0.100%, 0.10 An austenitic stainless steel for food plants, wherein% ≦ N ≦ 0.30%, the balance being Fe and inevitable impurities and satisfying the following formulas (1) and (2):
Cr + 3.3Mo + 20N ≧ 38 (1)
(In the formula, Cr, Mo and N indicate the content (% by mass) of each component)
Si + Al-100 (Ca + Mg) ≧ 0 (2)
(In the formula, Si, Al, Ca and Mg indicate the content (% by mass) of each component) The stainless steel according to any one of claims 1 to 5 , wherein the stainless steel is used in a soy sauce production plant or a vinegar production plant. Further, by mass%, 0.01% ≦ Cu ≦ 1.0%, 0.01 ≦ W ≦ 1.0%, 0.01 ≦ Co ≦ 1.0% stainless steel according to claim 1 to 6, characterized in. By mass%, stainless steel according to claim 1-7, characterized in that it contains 0.001% ≦ B ≦ 0.010%.

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