JP4325141B2 - Stainless steel for use in environments containing organic acids and salt - Google Patents
Stainless steel for use in environments containing organic acids and salt Download PDFInfo
- Publication number
- JP4325141B2 JP4325141B2 JP2002020854A JP2002020854A JP4325141B2 JP 4325141 B2 JP4325141 B2 JP 4325141B2 JP 2002020854 A JP2002020854 A JP 2002020854A JP 2002020854 A JP2002020854 A JP 2002020854A JP 4325141 B2 JP4325141 B2 JP 4325141B2
- Authority
- JP
- Japan
- Prior art keywords
- less
- mass
- stainless steel
- content
- corrosion
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
Links
Images
Landscapes
- Heat Treatment Of Steel (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Description
【0001】
【発明の属する技術分野】
この発明は、耐すきま腐食性や耐応力腐食割れ性に優れ、食品製造プラント、特に製造過程においてアミノ酸やクエン酸、酢酸等の有機酸が生成し、且つ含有食塩濃度が高い食品プラント、特に醤油製造プラントに好適なステンレス鋼に関するものである。
【0002】
【従来技術】
従来から食品の製造プラントには、取り扱う食品の含有成分や温度などの操業条件によって、ステンレス鋼や無機または有機被覆鋼、あるいはFRP等が使い分けられているが、近年メンテナンスのし易さや維持コストの低減、更には洗浄性の観点からステンレス鋼の使用が増えつつある。通常、清涼飲料水やビール、あるいは牛乳等の食品製造プラントにおいてはSUS304やSUS316等の汎用ステンレス鋼が多く使用されており、特に腐食による漏れなどの重大な問題は生じていない。また塩分を含む食品においても、常温付近の使用ならば孔食やすきま腐食、あるいは応力腐食割れ等の局部腐食の懸念は少なく、十分に使用に耐えている。しかしながら、例えば塩分を多量に含む醤油等の調味料を製造する場合、常温においてもSUS304やSUS316では著しい局部腐食が発生し、耐食性が不充分であることが多い。また上記ステンレス鋼より耐食性の高いSUS329系ステンレス鋼では、局部腐食の発生する可能性は少なくなるが、それでも常温より温度が上昇した場合、すきま腐食や溶接部の応力腐食割れが生じる懸念があるので、その使用は制限される。従ってこのような特殊な環境になる醤油製造プラントでは、ステンレス鋼を用いずに無機または有機被覆鋼、あるいはFRP、更にはステンレス鋼より高価なニッケル基合金やチタン等を使用せざるを得ないのが実情である。
【0003】
【発明が解決しようとする課題】
本発明は上記事情に鑑みてなされたもので、その目的は食品プラント、特に発酵過程において有機酸が生成し、且つ高濃度食塩を含有する醤油製造プラント又は食酎プラントに適するステンレス鋼を提供することである。
【0004】
【課題を解決するための手段】
本発明者らは、食品製造プラント、特に塩分を多量に含有する醤油等の調味料など発酵過程を含む食品の製造プラントに適するステンレス鋼について種々検討した結果、発酵過程においてアミノ酸や、クエン酸、乳酸等の有機酸が生成する場合、これがステンレス鋼の腐食、特にすきま腐食や応力腐食割れを加速させることが判明した。ステンレス鋼の腐食が有機酸によって加速するメカニズムとして、発酵過程で生じるアミノ酸は還元剤として作用し、ステンレス鋼に耐食性を付与している表面不働態皮膜を劣化させる一方、クエン酸、乳酸等はキレートとしてステンレス鋼表面に作用し、表面不働態皮膜に覆われていない水溶性のCaO、MgOといった鋼中酸化物系介在物の溶解を促進してすきま腐食や応力腐食割れの起点となり、耐食性を劣化させるとの知見を得た。そこで有機酸が存在する高濃度食塩含有環境においてステンレス鋼表面の不働態皮膜及びその下地金属の耐食性を向上させるために下記(1)に示す式を満足することが第一に必要であることが判明した。
Cr+3.3Mo+20N≧38 (1)
(式中Cr、Mo、Nは各成分の含有量(質量%)を示す)更にステンレス鋼の介在物中に含まれるCaO、MgOを低減させ、その組成をSiO2やAl2O3主体にすると有機酸含有高濃度食塩含有環境で耐食性が向上することが判明した。即ち、実験結果により下記(2)式を満足し、Si+Al−100(Ca+Mg)≧0 (2)
(式中Si、Al、Ca、Mgは各成分の含有量(質量%)を示す)且つ、介在物中のCaO+MgO質量比率が20%以下であることによってステンレス鋼に主要な腐食であるすきま腐食や応力腐食割れの発生が抑えられることが明らかになり、本発明を完成したものである。
なお、本願明細書中に示されている比率(%)は質量%である。
【0005】
本発明の第1の発明における要旨は、質量%で、C:0.05%以下、:Si≦1.0%、0.4%<Mn≦1.0%、P:0.040%以下、S:0.003%以下、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%以下、Ca:0.0010%以下で、残部はFeおよび不可避的不純物からなり、さらに、鋼中酸化物系介在物中のCaO+MgOの質量比率が20%以下であり、且つ、下記(1)式及び(2)式
Cr+3.3Mo+20N≧38 (1)
(式中Cr、Mo、Nは各成分の含有量(質量%)を示す)
Si+Al−100(Ca+Mg)≧0 (2)
(式中Si、Al、Ca、Mgは各成分の含有量(質量%)を示す)
を満たし、有機酸と塩分を含有する環境下で使用されることを特徴とするステンレス鋼であり、本発明の第2の発明における要旨は質量%で、C:0.05%以下、:Si≦1.0%、0.4%<Mn≦1.0%、P:0.040%以下、S:0.003%以下、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%で、残部はFeおよび不可避的不純物からなり、且つ、下記(1)式及び(2)式
Cr+3.3Mo+20N≧38 (1)
(式中Cr、Mo、Nは各成分の含有量(質量%)を示す)
Si+Al−100(Ca+Mg)≧0 (2)
(式中Si、Al、Ca、Mgは各成分の含有量(質量%)を示す)
を満たし、有機酸と塩分を含有する環境下で使用されることを特徴とするオーステナイトステンレス鋼であり、本発明の第3の発明における要旨は、上記有機酸が、アミノ酸及び、クエン酸、酢酸、乳酸の1種または2種以上を含むものであることを特徴とする第1の発明又は第2の発明に記載のステンレス鋼であり、本発明の第4の発明における要旨は質量%で、C:0.05%以下、:Si≦1.0%、0.4%<Mn≦1.0%、P:0.040%以下、S:0.003%以下、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%以下、Ca:0.0010%以下で、残部はFeおよび不可避的不純物からなり、さらに、鋼中酸化物系介在物中のCaO+MgOの質量比率が20%以下であり、且つ、下記(1)式及び(2)式
Cr+3.3Mo+20N≧38 (1)
(式中Cr、Mo、Nは各成分の含有量(質量%)を示す)
Si+Al−100(Ca+Mg)≧0 (2)
(式中Si、Al、Ca、Mgは各成分の含有量(質量%)を示す)
を満足することを特徴とする食品プラント用ステンレス鋼であり、本発明の第5の発明における要旨は質量%で、C:0.05%以下、:Si≦1.0%、0.4%<Mn≦1.0%、P:0.040%以下、S:0.003%以下、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%で、残部はFeおよび不可避的不純物からなり、且つ、下記(1)式及び(2)式
Cr+3.3Mo+20N≧38 (1)
(式中Cr、Mo、Nは各成分の含有量(質量%)を示す)
Si+Al−100(Ca+Mg)≧0 (2)
(式中Si、Al、Ca、Mgは各成分の含有量(質量%)を示す)
を満足することを特徴とする食品プラント用オーステナイトステンレス鋼であり、本発明の第6の発明における要旨は、上記ステンレス鋼が醤油製造プラント又は食酢製造プラントに用いられることを特徴とする上記第1の発明〜第5の発明に記載のステンレス鋼であり、本発明の第7の発明における要旨は、質量%で0.01%≦Cu≦1.0%、0.01≦W≦1.0%、0.01≦Co≦1.0%のうち1種または2種以上をさらに含有することを特徴とする上記第1の発明〜第6の発明に記載のステンレス鋼であり、本発明の第8の発明における要旨は質量%で、0.001%≦B≦0.010%を含有することを特徴とする上記第1の発明〜第7の発明に記載のステンレス鋼である。
【0006】
第1または第2の発明のステンレス鋼において質量%で、0.01%≦Cu≦1.0%、0.01%≦W≦1.0%、0.01%≦Co≦1.0%のうち1種または2種以上をさらに含有することが好ましく、更に、0.001%≦B≦0.010%を含有することが好ましい。
【0007】
【発明の実施の形態】
本発明のステンレス鋼は、上述の通り、(i)所定の化学成分とそれらの適正含有量範囲、(ii)耐食性向上に特に寄与するCr、Mo、Nの関係、(iii)鋼中介在物組成とこれらを構成するAl、Si、Ca、Mgの適正含有量範囲から構成されるが、以下、この発明の基礎となった実験結果について説明する。
【0008】
実験1
本発明者らはまず、発酵過程を有し、アミノ酸や乳酸等の有機酸がその過程にて生成する醤油製造プラントの環境が、そのような有機酸が存在しない場合に対してどのように異なるかを検討した。実験には市販の2mm厚さのSUS316Lを供試材として用い、80mm×25mm×2mmと60mm×20mm×2mmに切断した2枚の試片を重ね合わせ、スポット抵抗溶接を4点施し、溶接すきま付き腐食試験片を作製した。通常の醤油は数多くの有機酸を含有するが、系の単純化を図るため、発酵過程で生成する代表的有機酸であるアミノ酸の一種のグルタミン酸とアスパラギン酸、及びアミノ酸ではないが乳酸とクエン酸、酢酸を添加した以下に示す4種類の試験溶液を用意した。
この4種類の試験溶液を35℃に保持し、それぞれに上述の試験片を1ヶ月間浸漬した。浸漬終了後、スポット抵抗溶接により形成された溶接ナゲット部の中心を通るように切断機で切断し、その断面を光学顕微鏡により観察し、すきま腐食深さ及び応力腐食割れ長さを評価した。その結果を表1に示すが、食塩を単独で含有する溶液(1−▲1▼)で発生する腐食はすきま腐食のみであるのに対し、アミノ酸であるグルタミン酸が含まれる溶液(1−▲2▼)では、すきま腐食の他に応力腐食割れが発生することが認められた。一方、アミノ酸ではないがキレート構造を有する乳酸がグルタミン酸と共に含まれる溶液(1−▲3▼)では、すきま腐食や応力腐食割れの深さや長さが増大することが認められた。更に、食塩に様々な有機酸が複合的に含まれる溶液(1−▲4▼)では、腐食が顕著に増大することも確認された。以上の結果より、発酵過程によりアミノ酸や乳酸等の有機酸が生成する高濃度食塩含有醤油製造プラント内の環境は、同じ高濃度食塩を含有してもこれら有機酸が存在しない環境に比べ、腐食性は著しく増大することがわかった。
【0009】
【表1】
【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:0.45〜0.92%、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○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: 1.0%以下
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に好ましい元素であるが、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、1.0%以下にする必要がある。望ましくは0.70%以下、0.50%以下、0.20%以下、より望ましくは0.10%以下がよい。
Mn:0.40%<Mn≦1.0%
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1.0%以下にする必要がある。好ましくは0.7%以下、より好ましくは0.50%以下がよい。なお、Nの固溶限を考慮する場合にはMnを0.4%を超えて含有させることがより好ましい場合がある。そこで、0.40%<Mn≦1.0%とした。好ましくは0.42%<Mn≦1.0%、より好ましくは0.45%<Mn≦1.0%である。
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】
Si:1.0%以下
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に好ましい元素であるのが、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、1.0%以下にする必要がある。望ましくは0.70%以下、0.50%以下、0.25%以下、0.20%以下、より望ましくは0.10%以下が良い。
Mn:0.40%<Mn≦1.0%
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低減させる必要のある元素であり、そのためには1.0%以下にする必要がある。より好ましくは、0.70%以下、0.50%以下がよい。なお、Nの固溶限を考慮する場合にはMnを0.4%を越えて含有させることがより好ましい場合がある。そこで、0.40%<Mn≦1.0%とした。
【0016】
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40.0%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。
【0017】
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。
【0018】
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
【0019】
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。
【0020】
【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。
【0021】
【表2】
【0022】
【表3】
【0023】
【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. Also, SUS329 stainless steel, which has higher corrosion resistance than the above stainless steel, is less likely to cause local corrosion. , 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 an object of the present invention is to provide a stainless steel suitable for a food plant, in particular, a soy sauce manufacturing plant or a food plant where organic acids are generated in the fermentation process and contain high-concentration sodium chloride. That is.
[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%)) And further reducing CaO and MgO contained in stainless steel inclusions, and the composition of SiOO is reduced to SiO.2And Al2OThreeIt 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, and Si + Al-100 (Ca + Mg) ≧ 0 (2)
(In the formula, Si, Al, Ca, Mg are the contents of each component (massAnd the mass ratio of CaO + MgO in the inclusions is 20% or less, it becomes clear that the occurrence of crevice corrosion and stress corrosion cracking, which are the main corrosions in stainless steel, can be suppressed. It has been completed.
In addition, the ratio (%) shown in this-application specification is the mass%.
[0005]
The subject matter in the first invention of the present invention is mass%, C: 0.05% or less, Si ≦ 1.0%, 0.4% <Mn ≦ 1.0%, 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 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)
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 invention is mass%, C: 0.05% or less,: Si ≦ 1.0%, 0.4% <Mn ≦ 1.0%, 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 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)
The austenitic stainless steel is characterized in that it is used in an environment containing an organic acid and a salt. The gist of the third invention of the present invention 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 invention is mass%, and C: 0.05% or less ,: Si ≦ 1.0%, 0.4% <Mn ≦ 1.0%, 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 aspect of the present invention, the gist of the present invention is mass%, C: 0.05% or less ,: Si ≦ 1.0%, 0.4% <Mn ≦ 1.0%, P: 0.040% or less, S: 0.003% or less, 15.0% ≦ Ni ≦ 40.0%, 16.0% ≦ Cr ≦ 26.0%, 0% ≦ Mo ≦ 8.0%, 0.005% ≦ Al ≦ 0.100%, 0.10% ≦ N ≦ 0.30%, the balance is Fe and inevitable impurities, and the following (1 )formulaAnd (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)
Austenitic stainless steel for food plants, characterized by satisfying,BookInvention No.6The 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 present invention is one or more of 0.01% ≦ Cu ≦ 1.0%, 0.01 ≦ W ≦ 1.0%, 0.01 ≦ Co ≦ 1.0% by mass%. The first invention to the first invention, further comprising6The stainless steel according to the invention of the present invention.8The gist of the present invention is mass%, and contains 0.001% ≦ B ≦ 0.010%.7The stainless steel described in the invention.
[0006]
In the stainless steel of the first or second inventionIn mass%, 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 welding clearance was obtained. 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.
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]
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], which is an index of the strength of the passive film, are shown. 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, Al2OThreeOr 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]
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: 0.45 to 0.92%, 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%, and the corrosion test results for each are shown. The horizontal axis in FIG. 3 shows Cr, Mo, and N that have a large contribution to the corrosion resistance among the alloy components, and the total amount Cr + 3.3Mo + 20N (provided that each element is almost equivalent from the degree of contribution) Cr, Mo, N is the content of each component element (mass%)). From FIG. 3, when the mass ratio of CaO + MgO in the oxide inclusion is 20% or more, corrosion does not occur until the value of Cr + 3.3Mo + 20N exceeds 44, whereas the mass ratio of CaO + MgO is 20%. It was confirmed that corrosion does not occur when the value of Cr + 3.3Mo + 20N is 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%Less than
Si is an effective element for deoxidation, and is a preferable element for lowering the CaO + MgO ratio in the oxide inclusions in the steel and constituting the main component of the oxide inclusions together with Al. Addition of saturates the effect, lowers the ductility and increases the strength, and further promotes the precipitation of intermetallic compounds such as σ phase and χ phase to deteriorate the corrosion resistance. There is a need to. Desirably, it is 0.70% or less, 0.50% or less, 0.20% or less, and more desirably 0.10% or less.
Mn: 0.40%<Mn ≦ 1.0%
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. Preferably it is 0.7% or less, more preferably 0.50% or less. In addition, when considering the solid solubility limit of N, it may be more preferable to contain Mn exceeding 0.4%. Therefore, 0.40%<Mn ≦ 1.0%It was. Preferably 0.42%<Mn ≦ 1.0%, More preferably 0.45%<Mn ≦ 1.0%It is.
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]
Si: 1.0%Less than
Si is an effective element for deoxidation. However, it is a preferable element for reducing the CaO + MgO ratio in the oxide inclusions in steel and constituting the main body of oxide inclusions together with Al. Excessive addition saturates the effect, lowers the ductility and increases the strength, and further promotes the precipitation of intermetallic compounds such as σ phase and χ phase, thereby deteriorating the corrosion resistance. It is necessary to. Desirably, it is 0.70% or less, 0.50% or less, 0.25% or less, 0.20% or less, and more desirably 0.10% or less.
Mn: 0.40%<Mn ≦ 1.0%
Mn is an element that needs to be reduced 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. More preferably, it is 0.70% or less and 0.50% or less. In consideration of the solid solubility limit of N, it may be more preferable to contain Mn exceeding 0.4%. Therefore, 0.40%<Mn ≦ 1.0%It was.
[0016]
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, it is an element effective in improving stress corrosion cracking resistance.0%If it exceeds 1, the hot workability deteriorates and the hot deformation resistance increases. 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.
[0017]
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
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
[0018]
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
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 in improving general corrosion resistance, but in order to obtain the effect, it is necessary to contain 0.01% or more. 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
[0019]
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 (mass%)), The reason is that when Cr + 3.3Mo + 20N is less than 38, the mass ratio of CaO + MgO in the oxide-based inclusions in steel, which is the main component of the present invention, is clear from the results of
Of CaO + MgO in oxide inclusions in steelmass20% 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 each component element content (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
[0020]
[Examples and Comparative Examples]
Next, this invention is demonstrated based on the Example shown below. Here, various component steels shown in
[0021]
[Table 2]
[0022]
[Table 3]
[0023]
【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 result 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 change with time of 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 (5)
Cr+3.3Mo+20N≧38 (1)
(式中Cr、Mo、Nは各成分の含有量(質量%)を示す)
Si+Al−100(Ca+Mg)≧0 (2)
(式中Si、Al、Ca、Mgは各成分の含有量(質量%)を示す)By mass%, C: 0.05% or less, Si: 1.0% or less, 0.4 0% <Mn ≦ 1.0 %, P: 0.040% or less, S: 0.003% or less, 15 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 An austenitic stainless steel having a ratio of 20% or less and satisfying the following formulas (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)
Cr+3.3Mo+20N≧38 (1)
(式中Cr、Mo、Nは各成分の含有量(質量%)を示す)
Si+Al−100(Ca+Mg)≧0 (2)
(式中Si、Al、Ca、Mgは各成分の含有量(質量%)を示す)By mass%, C: 0.05% or less, Si: 1.0% or less, 0.4 0% <Mn ≦ 1.0 %, P: 0.040% or less, S: 0.003% or less, 15 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 An austenitic 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)
Priority Applications (4)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP2002020854A JP4325141B2 (en) | 2002-01-30 | 2002-01-30 | Stainless steel for use in environments containing organic acids and salt |
PCT/JP2002/004581 WO2003044237A1 (en) | 2001-11-22 | 2002-05-10 | Stainless steel for use under circumstance where organic acid and saline are present |
CNB028232623A CN1303241C (en) | 2001-11-22 | 2002-05-10 | Stainless steel for use under circumstance where organic acid and saline are present |
US10/493,639 US20050016636A1 (en) | 2001-11-22 | 2002-05-10 | Stainless steel for use under circumstance where organic acid and saline are present |
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
JP2002020854A JP4325141B2 (en) | 2002-01-30 | 2002-01-30 | Stainless steel for use in environments containing organic acids and salt |
Publications (2)
Publication Number | Publication Date |
---|---|
JP2003221653A JP2003221653A (en) | 2003-08-08 |
JP4325141B2 true JP4325141B2 (en) | 2009-09-02 |
Family
ID=27744239
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
JP2002020854A Expired - Lifetime JP4325141B2 (en) | 2001-11-22 | 2002-01-30 | Stainless steel for use in environments containing organic acids and salt |
Country Status (1)
Country | Link |
---|---|
JP (1) | JP4325141B2 (en) |
Families Citing this family (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JP2008095268A (en) * | 2006-09-13 | 2008-04-24 | Toray Ind Inc | Melt-spinning apparatus and melt-spinning method for thermoplastic resin |
US10519532B2 (en) | 2013-12-12 | 2019-12-31 | Jfe Steel Corporation | Steel material having excellent alcohol-induced pitting corrosion resistance and alcohol-induced SCC resistance |
-
2002
- 2002-01-30 JP JP2002020854A patent/JP4325141B2/en not_active Expired - Lifetime
Also Published As
Publication number | Publication date |
---|---|
JP2003221653A (en) | 2003-08-08 |
Similar Documents
Publication | Publication Date | Title |
---|---|---|
JP2500162B2 (en) | High strength duplex stainless steel with excellent corrosion resistance | |
WO2012018074A1 (en) | Ferritic stainless steel | |
JP2011190524A (en) | Ferritic stainless steel having excellent oxidation resistance, secondary processing brittleness resistance and weld zone toughness | |
JP5324149B2 (en) | Corrosion resistant austenitic stainless steel | |
JP4657862B2 (en) | Duplex stainless steel for equipment using hypochlorite | |
JP4312408B2 (en) | Corrosion resistant austenitic alloy | |
JP4080729B2 (en) | Stainless steel for food plant | |
JP6750082B1 (en) | Fe-Ni-Cr-Mo-Cu alloy with excellent corrosion resistance | |
JP4325141B2 (en) | Stainless steel for use in environments containing organic acids and salt | |
US20050016636A1 (en) | Stainless steel for use under circumstance where organic acid and saline are present | |
JP3501573B2 (en) | Ferritic stainless steel pipe excellent in secondary work crack resistance and method for producing the same | |
TWI721924B (en) | Austenitic Iron-Fertilizer Iron Series Two-phase Stainless Steel Plate | |
CA1306123C (en) | Exterior protective member made of austenitic stainless steel fora sheathing heater element | |
JPH0775790B2 (en) | Duplex stainless steel welding wire with excellent resistance to concentrated sulfuric acid corrosion | |
JP3266247B2 (en) | Duplex stainless steel with excellent hot workability | |
JP2756545B2 (en) | Austenitic stainless steel with excellent corrosion resistance in hot water | |
KR102672051B1 (en) | Austenitic-ferritic duplex staninless steel sheet | |
JP3890223B2 (en) | Austenitic stainless steel | |
JP3276303B2 (en) | Ferritic stainless steel with excellent rust resistance that does not easily cause initial rust | |
JP2834982B2 (en) | Alloy with excellent hot workability and corrosion resistance in high temperature water | |
JP2004143576A (en) | Low nickel austenitic stainless steel | |
JPH02115346A (en) | Ferritic stainless steel having excellent corrosion resistance in high concentrated halide | |
JP2833385B2 (en) | Corrosion resistant austenitic Fe-based alloy | |
JPH09241809A (en) | Chromium-containing ferritic iron-base alloy excellent in corrosion resistance | |
JP3294282B2 (en) | Austenitic stainless steel with excellent sulfuric acid corrosion resistance and workability |
Legal Events
Date | Code | Title | Description |
---|---|---|---|
A621 | Written request for application examination |
Free format text: JAPANESE INTERMEDIATE CODE: A621 Effective date: 20031212 |
|
A131 | Notification of reasons for refusal |
Free format text: JAPANESE INTERMEDIATE CODE: A131 Effective date: 20070116 |
|
A521 | Request for written amendment filed |
Free format text: JAPANESE INTERMEDIATE CODE: A523 Effective date: 20070319 |
|
A02 | Decision of refusal |
Free format text: JAPANESE INTERMEDIATE CODE: A02 Effective date: 20070717 |
|
A521 | Request for written amendment filed |
Free format text: JAPANESE INTERMEDIATE CODE: A523 Effective date: 20070913 |
|
A911 | Transfer to examiner for re-examination before appeal (zenchi) |
Free format text: JAPANESE INTERMEDIATE CODE: A911 Effective date: 20071212 |
|
A912 | Re-examination (zenchi) completed and case transferred to appeal board |
Free format text: JAPANESE INTERMEDIATE CODE: A912 Effective date: 20080201 |
|
A521 | Request for written amendment filed |
Free format text: JAPANESE INTERMEDIATE CODE: A523 Effective date: 20090422 |
|
A01 | Written decision to grant a patent or to grant a registration (utility model) |
Free format text: JAPANESE INTERMEDIATE CODE: A01 |
|
A61 | First payment of annual fees (during grant procedure) |
Free format text: JAPANESE INTERMEDIATE CODE: A61 Effective date: 20090601 |
|
FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20120619 Year of fee payment: 3 |
|
R150 | Certificate of patent or registration of utility model |
Free format text: JAPANESE INTERMEDIATE CODE: R150 Ref document number: 4325141 Country of ref document: JP Free format text: JAPANESE INTERMEDIATE CODE: R150 |
|
FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20120619 Year of fee payment: 3 |
|
FPAY | Renewal fee payment (event date is renewal date of database) |
Free format text: PAYMENT UNTIL: 20130619 Year of fee payment: 4 |
|
R250 | Receipt of annual fees |
Free format text: JAPANESE INTERMEDIATE CODE: R250 |
|
R250 | Receipt of annual fees |
Free format text: JAPANESE INTERMEDIATE CODE: R250 |
|
R250 | Receipt of annual fees |
Free format text: JAPANESE INTERMEDIATE CODE: R250 |
|
R250 | Receipt of annual fees |
Free format text: JAPANESE INTERMEDIATE CODE: R250 |
|
R250 | Receipt of annual fees |
Free format text: JAPANESE INTERMEDIATE CODE: R250 |
|
R250 | Receipt of annual fees |
Free format text: JAPANESE INTERMEDIATE CODE: R250 |
|
R250 | Receipt of annual fees |
Free format text: JAPANESE INTERMEDIATE CODE: R250 |
|
R250 | Receipt of annual fees |
Free format text: JAPANESE INTERMEDIATE CODE: R250 |
|
R250 | Receipt of annual fees |
Free format text: JAPANESE INTERMEDIATE CODE: R250 |
|
R250 | Receipt of annual fees |
Free format text: JAPANESE INTERMEDIATE CODE: R250 |
|
EXPY | Cancellation because of completion of term |