JP3952338B2

JP3952338B2 - Food storage tank

Info

Publication number: JP3952338B2
Application number: JP09824498A
Authority: JP
Inventors: 俊郎足立; 和歌大原田; 敏郎名越
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 1998-03-27
Filing date: 1998-03-27
Publication date: 2007-08-01
Anticipated expiration: 2018-03-27
Also published as: JPH11279716A

Description

【０００１】
【発明の属する技術分野】
本発明は、清涼飲料水や酒、ビールなどのアルコール飲料水など液体の食品を貯蔵、運搬する容器に係わり、鋼材からの金属イオンの溶出が極めて小さく、上記食品の金属イオンによる汚染や変質がなく安全性の高いステンレス鋼製食品貯蔵タンクに関する。
【０００２】
【従来の技術】
従来、食品や飲料水の製造および保貯蔵容器の材料は、ＳＵＳ３０４やＳＵＳ３１６などのオーステナイト系ステンレス鋼が主であった。しかし、食品、とりわけ清涼飲料水の中には有機酸を多量に含み酸性を示すものがあり、ステンレス鋼からの金属イオンの溶出により長期間の保管ができない、紙コップの自動販売機に利用できないなどの問題があった。また、日本酒はごく微量のＦｅイオンの溶出であっても酒が汚染され黄変したり変質をきたすため、ステンレス鋼製容器タンクが使用できない等の問題がある。
【０００３】
上記の理由により、これら食品の保管容器用材料としては、やむなくＦＲＰや琺瑯製容器が使用されてきたが、先の阪神大震災で壊滅的な損壊が生じ、耐震性容器タンクの要求が高まってきた。この条件を満たすものとして琺瑯等の被覆を施さず、無垢で使用できる金属製容器タンクが求められている。
無垢での使用を前提とした金属製容器タンク用素材としてはチタンが挙げられるが、コストの著しい上昇は不可避で、さらに省資源の観点からも普及には問題がある。このような背景のもとでステンレス鋼製食品貯蔵タンクの特性改善が大きな課題となった。
【０００４】
【発明が解決しようとする課題】
ステンレス鋼製食品貯蔵タンクは溶接により組立てる一体型として施工される。従って、ステンレス鋼素材においては、貯蔵する食品、飲料水に対する耐食性はもちろんのこと、いわゆる不動態皮膜を通してわずかに溶出する金属イオンの低減が要求されるとともに、容器タンクの製造において適切な溶接方法もしくは溶接部の処理方法の提供が重要である。
したがって本発明の目的は、食品、飲料水に対する耐食性と金属イオンの耐溶出性に優れるステンレス鋼製食品貯蔵タンクを提供することにある。
【０００５】
【課題を解決するための手段】
本発明者らは、食品、飲料水に対して金属イオンの溶出が極めて少なく食品の貯蔵に適するタンク素材としてのステンレス鋼、ならびに溶接部においても前記の特性が阻害されない溶接方法および溶接部の処理方法について詳細な検討を行ってきた。
その結果、Ｃｒ量を高め、適量のＮｂ、ＴｉおよびＡｌを添加した含Ｍｏフェライト系ステンレス鋼において、鋼板の焼鈍・酸洗工程でふっ酸と硝酸の混酸を用いた酸洗仕上げをすることで、金属イオンの溶出が著しく低減すること、また溶接施工では、溶接ワイヤーに極低Ｃ量で素材と同レベルの耐食性を有するフェライト系あるいはオーステナイト系ステンレス鋼ワイヤーを用いて粒界腐食を抑制するとともに、溶接酸化スケールの研磨除去、さらにはふっ酸と硝酸を含む処理剤を用いて化学処理することが、溶接部からの金属イオン溶出の抑制に対して有効であることを見出した。
【０００６】
本発明はこの知見に基づき完成したものであり、その要旨とするところは、タンク素材に、質量％で、Ｃ：０.０３％以下、Ｓｉ：０.１〜０.６％、Ｍｎ：０.４％以下、Ｐ：０.０４％以下、Ｓ：０.００３％以下、Ｃｒ：１６〜３５％、Ｍｏ：０.８〜４.０％およびＮ：０.０３％以下を含み、場合によってはＣｕ：０.３〜１.５％を含有し、さらにＮｂ：０.１〜０.６％、Ｔｉ：０.０５〜０.５％およびＡｌ：０.３％以下の１種もしくは２種以上を含み、かつこれら成分の間にＮｂ＋Ｔｉ≧７（Ｃ＋Ｎ）＋０.１５の関係が成立し、残部は鉄および不可避的不純物からなるフェライト系ステンレス鋼で、その焼鈍、酸洗工程で硝酸とふっ酸からなる混酸中で酸洗仕上げしたものを用い、溶接接合ではＣ量が０.０２％以下でタンク素材と同レベルの耐食性を有するフェライト系もしくはオーステナイト系ステンレス鋼のワイヤーを用い、溶接後バフ研磨にて酸化スケールを除去、あるいはさらにふっ酸と硝酸からなる処理剤を用いることを特長とする食品貯蔵タンクである。
【０００７】
【作用】
以下、タンク用鋼組成の各成分の作用とその含有量の限定理由について説明する。
Ｃ，Ｎは鋼中に不可避的に含まれる元素である。これらを低減すると軟質になり加工性が向上する。また、本発明では溶接による鋭敏化の回避にＮｂとＴｉを添加するが、Ｃ，Ｎ量が多いとＮｂ、Ｔｉの添加量も増え、加工性や溶接性が損なわれるので低い方が好ましく、Ｃ≦０.０３％、Ｎ≦０.０３％、望ましくはＣ≦０.０２％、Ｎ≦０.０２％とし、Ｃ＋Ｎ≦０.０３５％とする。
【０００８】
Ｓｉは鋼の脱酸剤としてのほか、溶接時の溶け込み性向上に有効な元素であり、その効果を発現させるためには０.１％以上の添加が必要である。しかし、０.６％を越えると材料が硬質になり、加工性の低下や溶接部の靱性低下の原因となる。このため、Ｓｉ量は０.１〜０.６％とした。
【０００９】
Ｍｎは鋼中に不可避的に存在するＳと結合し、化学的に不安定な硫化物であるＭｎＳを形成し耐食性を低下させることのほか、ＭｎＳの溶出により食品の汚染を引き起こす。また、鋼中に固溶するＭｎも耐食性を阻害するので低い方が好ましく上限を０.４％とする。
【００１０】
Ｐは不純物として通常のステンレス鋼に含まれる量であればとくに特性上問題となることはない。したがって、通常のステンレス鋼に許容される量として、上限を０.０４％とする。
【００１１】
Ｓは鋼の耐食性に悪影響をおよぼすことのほか、イオンとして溶出すると食品の汚染を引き起こすため低い方が好ましく、上限を０.００３％とする。
【００１２】
Ｃｒはステンレス鋼の不動態皮膜を構成し、Ｍｏとともに鋼の耐食性を向上させるとともに不動態皮膜を通して溶出する金属イオンの低減に対して重要な元素である。塩化物イオンを含む中性あるいは酸性の水溶液環境における孔食や隙間腐食、ならびに不動態皮膜の強化に対し、１６％以上、望ましくは２０％を越えて添加することが必要である。しかし、３５％を越えると材料が硬質となり容器タンクへの加工が困難となるので、Ｃｒ量は１６〜３５％とする。
【００１３】
ＭｏはＣｒとともに鋼の耐食性向上に対して不可欠な元素であり、その効果はＣｒ量が増すにつれ大きくなる。本発明のＣｒ量レベルにおいては０.８％以上の添加でその効果が認められる。しかし、４％を超える添加は徒に鋼を硬質にし、さらに溶接時の溶け込み性を低下させるため容器タンクの生産性が低下する。このため、Ｍｏ量は０.８〜４.０％とする。
【００１４】
Ｃｕは、塩化物環境における耐局部腐食性や耐酸性を改善する。とくにＭｏ量の少ない場合にＣｕの添加は効果的で、０.３％以上の添加が必要である。しかし、１.５％を越えて添加してもその作用は飽和し、また鋼の溶接性を低下させるので添加量の上限を１.５％とする。
【００１５】
Ｔｉは鋼中のＣ，Ｎを固定して粒界腐食を防止する。ＣおよびＮの固定に必要な量は後述の限定式から計算される。また、ＴｉはＳを固定して化学的に安定な硫化物を形成し、ＭｎＳなどの化学的に不安定な非金属介在物の溶出による食品や飲料水の汚染を防ぐとともに、一定量の固溶Ｔｉは鋼の活性溶解を抑制し、酸性の飲料水に対する耐食性を改善する作用を有する。
さらに、ＮｂおよびＡｌとの複合添加を行うことで、ステンレス鋼板製造時の焼鈍後に行われる酸洗にふっ酸と硝酸の混酸を用いた酸洗の過程で強固な不動態化皮膜を作り、耐食性の改善とともに不動態皮膜を介して金属イオンが溶出するのを抑制する有用な元素である。しかしＴｉの含有量が多すぎると、クラスター状の介在物を生成し鋼の表面疵の原因となる。耐食性と耐溶出性の面からＴｉの下限は０.０５％とし、表面性状の面から上限を０.５％とした。
【００１６】
ＮｂはＴｉとともに本発明鋼のＣ量レベルのフェライト系ステンレス鋼で問題となる粒界腐食を防止するのに有用な元素である。ＣおよびＮの固定に必要な量は後述の限定式から計算されるが、０.１％未満では効果がなく、０.６％を越えて添加すると溶接部の高温割れ性や靱性を阻害するので、Ｎｂの範囲は０.１〜０.６％とした。
【００１７】
Ａｌは脱酸剤として効果的な元素であるが、本発明を構成する上で重要な元素である。すなわち、ＮｂおよびＴｉとの複合添加において鋼の焼鈍後のふっ酸と硝酸による酸洗時に良好な不動態皮膜を形成し、耐溶出性と耐食性の改善が著しい。したがって、一定レベルの耐食性を目標とした場合、Ｍｏの添加量を低めることができ、軟質でより良好な加工性が得られ、コストの上昇を最小限に抑えることができる利点もある。Ａｌ量が０.０１％未満ではその効果が得られず、また０.３％を越えて添加すると溶接性を阻害する。したがって、Ａｌ量の範囲は０.０１〜０.３％とする。
【００１８】
以上の各成分の含有量限定に加え、本発明においてはＣ、Ｎ、ＮｂおよびＴｉの各成分間において次の限定式、
Ｎｂ＋Ｔｉ≧７（Ｃ＋Ｎ）＋０.１５
を設ける。これは溶接部において耐粒界腐食性を確保するために必要な固定元素の量を求めるための指標である。Ｎｂ＋Ｔｉ量が７（Ｃ＋Ｎ）＋０.１５％以下では溶接部が鋭敏化し、粒界腐食が生じるとともに金属イオンの溶出が著しくなる。
【００１９】
【発明の実施の形態】
食品・飲料水の貯蔵タンクに要求される特性は、貯蔵する食品、飲料水に対する耐食性に加えて、金属イオンの溶出による貯蔵品の汚染や変質が少ないことである。
以下に本発明の作用効果を具体的に示す。表１は試験に供したステンレス鋼の化学成分、酸洗条件および表面仕上げを示す。Ｎｏ.１〜３鋼は比較例、Ｎｏ.４〜５鋼は本発明例である。Ｎｏ.１鋼はＳＵＳ３０４、Ｎｏ.２鋼はＳＵＳ４４４で、Ｎｏ.３鋼は成分は本発明の範囲にあるが、酸洗条件が異なる鋼である。
【００２０】
【表１】

【００２１】
耐溶出性の評価は、ｐＨ４の酢酸水溶液に４０℃で３０日間浸漬し、試験後に試験液のイオン分析にて行なった。板厚１.５ｍｍの冷延鋼板から、３０×５０ｍｍのクーポンを切り出し、端面を＃１０００まで湿式研磨を施し試験片とした。また、溶接試験片としてＮｏ.４鋼について、上記クーポン長手方向中央に、Ｃ量が０.０１％のＳＵＳ４４４系と３０８Ｌ系の溶接ワイヤーを用いてそれぞれＴＩＧ溶接を行い、バフ研磨で溶接酸化スケールを除去し、さらにふっ酸と硝酸を含むスケール除去剤で化学処理したものを作成した。
容量８００ｍｌの円筒形ガラス製容器に試験液を５００ｍｌ量り取り、液の中位に試験片を１枚吊り、恒温水槽を用いて浸漬試験を行なった。試験液のイオン分析はイオンクロマトグラフィーによった。
【００２２】
表２に浸漬試験結果を示す。本試験ではいずれの鋼も孔食などの腐食は生じなかったが、比較に用いた鋼、ならびに３０８Ｌ系ワイヤーを用いたＮｏ.４鋼のＴＩＧ溶接試験片を浸漬した試験液に着色が認められた。Ｎｏ.１鋼のＳＵＳ３０４は２ｐｐｍを超えるＦｅ２＋イオンと０.５ｐｐｍを超えるＣｒ３＋イオンが検出されたのに対して、比較例を含めたフェライト系鋼のそれはいずれも０.５ｐｐｍ以下であり、耐溶出性に関しては、明らかにフェライト系鋼の方がオーステナイト系鋼より優れる。
Ｎｏ.２鋼とＮｏ.４鋼の比較からＣｒ量を高めることが効果的であることがわかる。次にＮｏ.３鋼とＮｏ.４鋼の２Ｂ仕上げ材の比較から、鋼の成分が本件発明の範囲であっても仕上げの酸洗条件が本件発明で規定する条件と異なれば、良好な耐溶出性が得られないことがわかる。また、溶接試験片では、３０８Ｌ系のワイヤーを用いると溶接部からの溶出が増加することがわかる。なお、本件発明例はすべて、Ｆｅ２＋イオンが０.０１ｐｐｍ、Ｃｒ３＋イオンは０.０１ｐｐｍ以下と優れた耐溶出性を示した。
このように、良好な耐溶出性は、ＴｉとＡｌをＮｂに複合して添加した鋼をふっ酸と硝酸の混酸仕上げにすることによって得られるが、この理由としては、ふっ酸と硝酸の混酸中での酸洗により不動態皮膜中のＣｒ濃度が高くなることに関連すると考えられる。また、溶接ワイヤーを用いる溶接では、本発明に規定する鋼に比べて耐食性レベルの劣る溶接ワイヤーを用いると溶接部において良好な耐溶出性が得られない。
【００２３】
【表２】

【００２４】
溶接部の耐溶出性の評価はアノード分極曲線の測定に拠った。試験は、第１図に示す塗布型電極を用い、３０℃の０.１％乳酸に５０ｐｐｍのＣｌ−イオンを添加した試験液において、Ａｒ脱気を行ないながら、自然電位から２０ｍＶ／ｍｉｎの電位走査速度で、１．２Ｖ（飽和甘こう電極基準）まで測定を行なった。供試材はＮｏ.４鋼を用い、Ｃ量が０.０１％のＳＵＳ４４４系の溶接ワイヤーを用い、Ａｒバックガスシールドを施してＴＩＧ溶接を行なった。
試験片は、溶接のまま、溶接後バフ研磨を施し溶接時の酸化スケールを除去したもの、さらに５％のふっ酸と１５％の硝酸を含む溶接スケール除去剤を３０分間塗布して不動態皮膜の強化を施したものを用いた。
【００２５】
図２に測定結果を示す。溶接のまま（ａ）では、酸化スケール部の溶出に伴う電流のピークが認められ、いったんは電流が減少するが不動態化することなく孔食が生じ再び電流が増加した。バフ研磨を施し、溶接時の酸化スケールを除去したもの（ｂ）は、溶接の影響は取り除かれており、２Ｂ仕上げの母材とほぼ同レベルの不動態維持電流を示し、孔食の発生も認められなかった。ただし、分極曲線の全電位域で微少な電流のひげが認められた。一方、バフ研磨後ふっ酸と硝酸からなるスケール除去剤を塗布して不動態皮膜の強化処理をしたもの（ｂ）では電流のひげは殆どなく溶接の影響は殆ど取り除かれていることを示した。
【００２６】
【実施例】
以下に、実施例を挙げて本発明の作用効果を具体的に示す。表１に示した板厚１.５ｍｍのＮｏ.３鋼およびＮｏ.４鋼を用い、０.０５ｍ３の半タンクを作成した。タンクの溶接は胴部はＴＩＧ突き合わせ溶接を、下鏡板と胴のＴＩＧ溶接はＣ量が０.０１％の４４４系の溶接ワイヤーを用いた。Ｎｏ.３を用いたタンクでは、溶接部はすべてバフ研磨で溶接酸化スケールを除去し、その後ふっ酸と硝酸からなる酸化スケール除去剤を塗布して化学処理を行なった。Ｎｏ.４鋼では、溶接部をバフ研磨したものと、上記スケール除去剤による化学処理を施したものをそれぞれ１缶体づつ作成した。
【００２７】
試験は、半タンクに日本酒を８分目入れタンク上部をビニルにて密閉し、室温２０〜２５℃の恒温室にて３ヶ月の貯蔵試験を行なった。３ヶ月後開封し、酒の黄変の有無およびＦｅ２＋イオン濃度を調べた。Ｎｏ.３鋼のタンクでは酒の黄変が認められるとともに、０.１ｐｐｍのＦｅ２＋イオンの増加が見られた。一方、Ｎｏ.４鋼のタンクでは溶接部のバフ仕上げタンクおよびバフ後、化学処理を行なったタンクともに酒の黄変は認められず、鉄イオン濃度の増加も各々、０.０２、０.０１ｐｐｍであった。酒に対するＦｅ２＋イオンの許容量は０.１ｐｐｍ以下といわれていることから、本件発明は酒の貯蔵タンクとしても十分な特性を有することがわかった。
【００２８】
【発明の効果】
以上のように、本発明によれば中性あるいは酸性の清涼飲料水や酒などのアルコール飲料水に対して金属イオンの溶出に伴う汚染の少ないステンレス鋼製容器タンクが得られる。本発明によれば高価なチタンを用いることなく、特殊な溶接技術も必要としない利点を有する。また、使用するステンレス鋼のコストも従来の鋼に比べて若干のコストアップにとどまり比較的安価に製造することが可能となる。本発明は、上記食品の貯蔵タンクのほか他のあらゆる食品の製造機器、貯蔵タンク等に適用できる。
（以下余白）
【図面の簡単な説明】
【図１】アノード分極曲線の測定に用いた試料電極を示した図。
【図２】アノード分極曲線の測定結果を示した図。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a container for storing and transporting liquid foods such as soft drinks, alcoholic beverages such as liquor, beer, etc., and the elution of metal ions from steel is extremely small, and contamination and alteration of the foods by metal ions The present invention relates to a highly safe stainless steel food storage tank.
[0002]
[Prior art]
Conventionally, the production of food and drinking water and the material for storage containers have been mainly austenitic stainless steels such as SUS304 and SUS316. However, some foods, especially soft drinks, contain a large amount of organic acid and show acidity, and they cannot be stored for a long time due to elution of metal ions from stainless steel. There were problems such as. In addition, sake has a problem that even if a very small amount of Fe ion is eluted, the sake is contaminated and yellowed or deteriorated, so that a stainless steel container tank cannot be used.
[0003]
For the above reasons, FRP and smoked containers have been used as materials for food storage containers. However, catastrophic damage occurred in the previous Great Hanshin Earthquake, and the demand for earthquake-resistant container tanks has increased. . In order to satisfy this condition, a metal container tank that can be used without being covered with a cover such as a bag is required.
Titanium is an example of a metal container tank material that is assumed to be used purely, but a significant increase in cost is unavoidable, and there is a problem in the spread from the viewpoint of resource saving. Under such circumstances, improvement of the characteristics of stainless steel food storage tanks has become a major issue.
[0004]
[Problems to be solved by the invention]
Stainless steel food storage tanks are constructed as a single piece assembled by welding. Therefore, in the stainless steel material, not only the corrosion resistance to stored foods and drinking water but also the reduction of metal ions that are slightly eluted through the so-called passive film is required, and an appropriate welding method or It is important to provide a method for treating welds.
Accordingly, an object of the present invention is to provide a stainless steel food storage tank excellent in corrosion resistance to food and drinking water and resistance to elution of metal ions.
[0005]
[Means for Solving the Problems]
The inventors of the present invention have developed a stainless steel as a tank material that is very suitable for storing foods with very little metal ion elution from food and drinking water, and a welding method and a treatment of the welded part in which the above properties are not hindered even in the welded part. A detailed study of the method has been conducted.
As a result, by increasing the amount of Cr and Mo-containing ferritic stainless steel to which appropriate amounts of Nb, Ti and Al are added, by pickling finishing using a mixed acid of hydrofluoric acid and nitric acid in the annealing and pickling process of the steel sheet In addition, the elution of metal ions is remarkably reduced, and in welding work, the ferrite wire or austenitic stainless steel wire that has the same level of corrosion resistance as the material is used for the welding wire to suppress intergranular corrosion. It has been found that polishing removal of the weld oxide scale and chemical treatment using a treatment agent containing hydrofluoric acid and nitric acid are effective for suppressing metal ion elution from the weld.
[0006]
The present invention has been completed based on this finding, and the gist of the present invention is that the tank material is, in mass%, C: 0.03% or less, Si: 0.1-0.6%, Mn: 0. 0.4% or less, P: 0.04% or less, S: 0.003% or less, Cr: 16-35%, Mo: 0.8-4.0% and N: 0.03% or less Depending on the content of Cu: 0.3 to 1.5%, Nb: 0.1 to 0.6%, Ti: 0.05 to 0.5%, and Al: 0.3% or less, The relationship of Nb + Ti ≧ 7 (C + N) +0.15 is established between these components, and the balance is ferritic stainless steel made of iron and unavoidable impurities. In a welded joint, the C content is 0.02% or less, which is the same level as the tank material. Using a wire of ferritic or austenitic stainless steel having a consumable, a food storage tank that features the use of a treating agent comprising the oxide scale removal, or from further hydrofluoric acid and nitric acid by welding after buffing.
[0007]
[Action]
Hereinafter, the effect | action of each component of the steel composition for tanks and the reason for limitation of the content are demonstrated.
C and N are elements inevitably contained in the steel. If these are reduced, it will become soft and workability will improve. Further, in the present invention, Nb and Ti are added to avoid sensitization by welding, but if the amount of C and N is large, the amount of addition of Nb and Ti increases, and workability and weldability are impaired, so the lower one is preferable. C ≦ 0.03%, N ≦ 0.03%, preferably C ≦ 0.02%, N ≦ 0.02%, and C + N ≦ 0.035%.
[0008]
Si is an element effective not only as a deoxidizer for steel but also for improving the penetration at the time of welding, and 0.1% or more of addition is necessary to achieve the effect. However, if it exceeds 0.6%, the material becomes hard, which causes deterioration of workability and toughness of the welded portion. For this reason, the Si amount is set to 0.1 to 0.6%.
[0009]
Mn combines with S inevitably present in the steel to form MnS, which is a chemically unstable sulfide, lowering the corrosion resistance and causing food contamination by elution of MnS. Moreover, since Mn which dissolves in steel also inhibits corrosion resistance, the lower one is preferable and the upper limit is set to 0.4%.
[0010]
If P is an amount contained in ordinary stainless steel as an impurity, there is no problem in terms of characteristics. Therefore, the upper limit is set to 0.04% as the amount allowed for normal stainless steel.
[0011]
In addition to adversely affecting the corrosion resistance of the steel, S is preferable because it causes contamination of food when eluted as ions, and the upper limit is set to 0.003%.
[0012]
Cr constitutes a passive film of stainless steel, and together with Mo, is an important element for improving the corrosion resistance of steel and reducing metal ions eluted through the passive film. For pitting corrosion and crevice corrosion in a neutral or acidic aqueous solution environment containing chloride ions, and strengthening of the passive film, it is necessary to add over 16%, preferably over 20%. However, if it exceeds 35%, the material becomes hard and it becomes difficult to process into a container tank, so the Cr amount is 16 to 35%.
[0013]
Mo is an element indispensable for improving the corrosion resistance of steel together with Cr, and its effect increases as the Cr content increases. In the Cr level of the present invention, the effect is recognized with addition of 0.8% or more. However, the addition exceeding 4% naturally makes the steel hard, and further reduces the penetration during welding, so the productivity of the container tank decreases. For this reason, the Mo amount is set to 0.8 to 4.0%.
[0014]
Cu improves local corrosion resistance and acid resistance in chloride environments. In particular, when the amount of Mo is small, addition of Cu is effective, and addition of 0.3% or more is necessary. However, even if added over 1.5%, the effect is saturated and the weldability of the steel is lowered, so the upper limit of the amount added is 1.5%.
[0015]
Ti fixes C and N in steel and prevents intergranular corrosion. The amount required for fixing C and N is calculated from the limiting formula described below. Ti also fixes S to form chemically stable sulfides, preventing contamination of food and drinking water due to elution of chemically unstable non-metallic inclusions such as MnS, and a certain amount of solids. Molten Ti has the effect | action which suppresses the active melt | dissolution of steel and improves the corrosion resistance with respect to acidic drinking water.
Furthermore, by adding Nb and Al in combination, a strong passivation film is formed in the pickling process using a mixed acid of hydrofluoric acid and nitric acid for the pickling performed after annealing at the time of stainless steel plate manufacturing, and the corrosion resistance It is a useful element that suppresses the elution of metal ions through the passive film as well as improvement of the above. However, when there is too much content of Ti, a cluster-like inclusion will be produced and it will cause the surface flaw of steel. In terms of corrosion resistance and elution resistance, the lower limit of Ti is 0.05%, and the upper limit is 0.5% in terms of surface properties.
[0016]
Nb, together with Ti, is an element useful for preventing intergranular corrosion, which is a problem in ferritic stainless steel of the C level of the steel of the present invention. The amount required for fixing C and N is calculated from the following limiting formula, but if it is less than 0.1%, there is no effect, and if added over 0.6%, the hot cracking property and toughness of the welded part are hindered. Therefore, the range of Nb is set to 0.1 to 0.6%.
[0017]
Al is an effective element as a deoxidizer, but is an important element in constituting the present invention. That is, in the combined addition of Nb and Ti, a good passive film is formed at the time of pickling with hydrofluoric acid and nitric acid after annealing of the steel, and the improvement of dissolution resistance and corrosion resistance is remarkable. Therefore, when a certain level of corrosion resistance is targeted, there is an advantage that the amount of Mo added can be reduced, soft and better workability can be obtained, and cost increase can be minimized. If the Al content is less than 0.01%, the effect cannot be obtained. If the Al content exceeds 0.3%, the weldability is impaired. Therefore, the range of Al content is set to 0.01 to 0.3%.
[0018]
In addition to the above-mentioned content limitation of each component, in the present invention, the following limiting formulas among the components of C, N, Nb and Ti are as follows:
Nb + Ti ≧ 7 (C + N) +0.15
Is provided. This is an index for obtaining the amount of fixed elements necessary for ensuring intergranular corrosion resistance in the weld zone. When the amount of Nb + Ti is 7 (C + N) + 0.15% or less, the weld becomes sensitized, causing intergranular corrosion and elution of metal ions.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
The characteristics required of the storage tank for food and drinking water are not only the corrosion resistance to the food and drinking water to be stored, but also the contamination and alteration of the stored product due to elution of metal ions.
The function and effect of the present invention are specifically shown below. Table 1 shows the chemical composition, pickling conditions and surface finish of the stainless steel subjected to the test. No. 1 to 3 steels are comparative examples, and No. 4 to 5 steels are examples of the present invention. The No. 1 steel is SUS304, the No. 2 steel is SUS444, and the No. 3 steel is a steel in which the components are within the scope of the present invention, but the pickling conditions are different.
[0020]
[Table 1]

[0021]
The elution resistance was evaluated by immersing in an aqueous acetic acid solution having a pH of 4 at 40 ° C. for 30 days and performing ion analysis of the test solution after the test. A 30 × 50 mm coupon was cut out from a cold-rolled steel plate having a thickness of 1.5 mm, and the end surface was wet-polished to # 1000 to obtain a test piece. In addition, for No. 4 steel as a weld test piece, TIG welding was performed at the center of the coupon in the longitudinal direction of the coupon using SUS444 and 308L welding wires with 0.01% C content, and the weld oxide scale was buffed. Was further removed, and further chemically treated with a scale remover containing hydrofluoric acid and nitric acid.
500 ml of the test solution was weighed out in a cylindrical glass container having a capacity of 800 ml, and one test piece was hung in the middle of the solution, and the immersion test was performed using a constant temperature water bath. Ion analysis of the test solution was performed by ion chromatography.
[0022]
Table 2 shows the results of the immersion test. In this test, no corrosion such as pitting corrosion occurred in any of the steels, but coloring was observed in the test solution in which the steel used for comparison and the No. 4 steel TIG welded test piece using 308L wire were immersed. It was. In SUS304 of No. 1 steel, Fe2 + ions exceeding 2 ppm and Cr3 + ions exceeding 0.5 ppm were detected, whereas in ferritic steels including comparative examples, both were 0.5 ppm or less and were resistant to elution. Regarding fertility, ferritic steel is clearly superior to austenitic steel.
From the comparison of No. 2 steel and No. 4 steel, it can be seen that increasing the Cr content is effective. Next, from the comparison of No. 3 steel and No. 4 steel 2B finishing materials, even if the steel components are within the scope of the present invention, if the finishing pickling conditions are different from the conditions specified in the present invention, good resistance It can be seen that elution is not obtained. It can also be seen that, in the weld specimen, elution from the welded portion increases when a 308L wire is used. All of the inventive examples showed excellent elution resistance with Fe2 + ions of 0.01 ppm and Cr3 + ions of 0.01 ppm or less.
Thus, good elution resistance can be obtained by using a mixed acid finish of hydrofluoric acid and nitric acid for steel to which Ti and Al are added in combination with Nb. The reason for this is the mixed acid of hydrofluoric acid and nitric acid. This is considered to be related to an increase in Cr concentration in the passive film due to pickling in the inside. Further, in welding using a welding wire, if a welding wire having an inferior corrosion resistance level compared to the steel defined in the present invention is used, good elution resistance cannot be obtained at the welded portion.
[0023]
[Table 2]

[0024]
The evaluation of the elution resistance of the weld was based on the measurement of the anodic polarization curve. In the test, a coating electrode shown in FIG. 1 was used, and a potential of 20 mV / min from the natural potential was obtained while performing Ar deaeration in a test solution in which 50 ppm of Cl- ion was added to 30% 0.1% lactic acid. The measurement was carried out at a scanning speed up to 1.2 V (saturated ginger electrode reference). The test material was No. 4 steel, SUS444-based welding wire with 0.01% C content was used, and TIG welding was performed with an Ar back gas shield.
The test piece was welded and buffed after welding to remove oxide scale during welding, and then a weld scale remover containing 5% hydrofluoric acid and 15% nitric acid was applied for 30 minutes to passivate the film. The one with the strengthening of was used.
[0025]
FIG. 2 shows the measurement results. In the case of welding (a), a peak of current accompanying elution of the oxide scale portion was observed, and once the current decreased, pitting corrosion occurred without passivation, and the current increased again. Buffing and removing the oxide scale during welding (b) eliminates the influence of welding, shows a passive maintenance current at the same level as the 2B finished base metal, and causes pitting corrosion. I was not able to admit. However, a slight current whisker was observed in the entire potential region of the polarization curve. On the other hand, after the buffing, the descaling agent consisting of hydrofluoric acid and nitric acid was applied to strengthen the passive film (b), indicating that there was almost no electric current whisker and the effect of welding was almost eliminated. .
[0026]
【Example】
Below, an example is given and the operation effect of the present invention is shown concretely. Using a No. 3 steel and a No. 4 steel with a plate thickness of 1.5 mm shown in Table 1, a half tank of 0.05 m3 was prepared. For the tank welding, the body was TIG butt welded, and for the TIG welding of the lower end plate and the body, a 444 series welding wire having a C content of 0.01% was used. In the tank using No. 3, the welded oxide scale was removed from all the welds by buffing, and then chemical treatment was performed by applying an oxide scale remover composed of hydrofluoric acid and nitric acid. For No. 4 steel, one can was prepared for each of the welded parts buffed and one subjected to chemical treatment with the scale remover.
[0027]
In the test, sake was placed in a half tank for 8 minutes, and the upper part of the tank was sealed with vinyl, and a storage test was conducted for 3 months in a constant temperature room at room temperature of 20 to 25 ° C. After 3 months, the bottle was opened and the presence or absence of yellowing of the sake and the Fe2 + ion concentration were examined. In the No. 3 steel tank, yellowing of liquor was observed, and an increase of 0.1 ppm of Fe 2+ ions was observed. On the other hand, in the No. 4 steel tank, no yellowing of liquor was observed in both the buffed tank of the welded part and the tank subjected to chemical treatment after buffing, and the increase in iron ion concentration was 0.02 and 0.01 ppm, respectively. Met. Since the allowable amount of Fe 2+ ions with respect to sake is said to be 0.1 ppm or less, it was found that the present invention has sufficient characteristics as a storage tank for sake.
[0028]
【The invention's effect】
As described above , according to the present invention, it is possible to obtain a stainless steel container tank that is less contaminated with elution of metal ions in neutral or acidic soft drinks or alcoholic drinks such as sake. According to the present invention, there is an advantage that no special welding technique is required without using expensive titanium. In addition, the cost of the stainless steel to be used is only slightly increased compared to the conventional steel and can be manufactured at a relatively low cost. The present invention can be applied to all other food production equipment, storage tanks and the like in addition to the food storage tank.
(The following margin)
[Brief description of the drawings]
FIG. 1 is a diagram showing a sample electrode used for measurement of an anodic polarization curve.
FIG. 2 is a graph showing measurement results of an anodic polarization curve.

Claims

A material mass% of the tank, C: 0.03% or less, Si: 0.1~0.6%, Mn: 0.6% or less, P: 0.04% or less, S:. 0 003% or less , Cr: 16~35%, Mo: .. 0 8~4 0% and N: contains 0.03% or less, further Nb: 0.1~0.6%, Ti: 0.05~0.5 % and Al:. 0.01 to 0 3% of including one or two or more kinds, and the relationship Nb + Ti ≧ 7 (C + N) +0.15 is satisfied between these components, the balance being iron and unavoidable impurities A ferritic stainless steel comprising a stainless steel plate that is pickled and finished with a mixed acid composed of hydrofluoric acid and nitric acid in a final pickling tank in an annealing and pickling process.

% By mass of tank material, C: 0.03% or less, Si: 0.1-0.6%, Mn: 0.6% or less, P: 0.04% or less, S: 0.003% or less , Cr: 16 to 35% Mo: includes 0.03% or less, Cu:: 0.8 to 4.0% and N.. 0 3 to 1 and containing 5%, further Nb: 0.1 to One or more of 0.6%, Ti: 0.05-0.5% and Al: 0.01-0.3% are included, and Nb + Ti ≧ 7 (C + N) +0.1 between these components. 15 is established, and the balance is ferritic stainless steel made of iron and unavoidable impurities, and is a stainless steel plate that has been pickled with a mixed acid of hydrofluoric acid and nitric acid in the final pickling tank of the annealing and pickling process. A food storage tank characterized by its use.

In the assembly of the tank according to claim 1 or 2, the welding wire is welded using a ferritic or austenitic stainless steel wire having a C content of 0.02% or less and having the same level of corrosion resistance as the tank material. A food storage tank featuring:

The food storage tank according to claim 3, wherein the welded bead portion and the weld heat affected zone are removed from the oxide scale generated during welding by buffing or the like.

5. A food storage tank, wherein the weld bead portion and the weld heat affected zone from which the weld oxide scale has been removed by the method according to claim 4 are pickled with a scale remover comprising hydrofluoric acid and nitric acid.