JP2004107704A - Method for manufacturing boron-containing ferritic stainless steel strip - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a boron-containing hot-rolled ferritic stainless steel strip suitable for a stock of a polymer electrolyte fuel cell separator component. <P>SOLUTION: Ferritic stainless steel containing, by mass, ≥ 0.1% B and 15-32% Cr is heated to the temperature of ≤ 1,230°C, hot rolling is completed with the finish thickness t to satisfy the relationship (1) t ≤ 5-2B (%), and at the finish temperature TF to satisfy the relationshipes (2) TF ≥ 750 if B content is 0.1-0.25%, and (3) TF ≥ 280 × B(%) + 680, the unit of the finish thickness t is mm, and the finish temperature of hot rolling is °C. <P>COPYRIGHT: (C)2004,JPO

Description

【００２１】
各鋼塊から、５０ｍｍ厚さ×１４０ｍｍ幅×８５ｍｍ長さの熱間圧延用素材を加工し、３スタンドの熱間圧延機が約１ｍの間隔で配置されたタンデム圧延が模擬できる実験用設備を用いて熱間圧延を行った。
【００２２】
なお、熱間圧延試験においては、ロールの回転数とパス間の待ち時間の調整によって仕上げ温度ＴＦを変化させた。
【００２３】
図１に、一例として、２２％Ｃｒ−２％Ｍｏ系でＢ量がそれぞれ０．１０％、０．６３％及び１．０４％の鋼９、鋼１３及び鋼１９の熱間圧延試験結果を示す。
【００２４】
図１から、Ｂ量が多いほど、又、仕上げ温度ＴＦが低いほど熱間圧延で生じる最大耳割れ長さが増加することが明らかになった。
【００２５】
図２に、各鋼塊から採取した試験片を高温引張試験して絞りを測定した結果の一例を示す。なお、この図は、２０％Ｃｒ−２％Ｍｏ系でＢ量がそれぞれ０．１２％と１．４９％の鋼２と鋼６及び２２％Ｃｒ−２％Ｍｏ系でＢ量が０．６０％の鋼２０についてのものである。
【００２６】
図２から、引張試験温度が１１５０℃よりも高い温度域では温度上昇にともない或る温度で絞りが低下しはじめ、１２３０℃になるといずれの鋼種も絞りが零となることが明らかである。この絞りが零となる温度はＢ量によらずほぼ一定であった。したがって、本発明が対象とする含硼素フェライト系ステンレス鋼の鋼塊の加熱温度は、Ｂ含有量にかかわらず１２３０℃以下とする必要があることが判明した。
【００２７】
上記の熱間圧延試験から、熱間圧延の加熱温度を１２３０℃以下とし、Ｂ量に応じて前記式▲２▼又は式▲３▼を満たす仕上げ温度ＴＦ（℃）以上で熱間圧延を仕上げることによって、含硼素フェライト系ステンレス鋼熱延鋼板の耳割れ長さを５ｍｍ以下にするという所期の目標が達成できることが明らかになった。
更に、耳割れの発生をなくするには仕上げ圧延温度を、Ｂ含有量が０．１〜０．２５％の場合には８００℃以上、又、Ｂ含有量が０．２５％以上の場合には「２８０×Ｂ（％）＋７３０」℃以上とすることが好ましいことが判明した。
【００２８】
次いで、熱延鋼板の通板性の評価に関して行った検討内容について述べる。
【００２９】
熱延鋼板の冷間での通板を模擬するため、熱延ままの耳部を付けた２５ｍｍ幅×２００ｍｍ長さの短冊試験片をシャーリングにより採取し、通板中に破断が生じるか否かを曲げ試験によって評価した。
【００３０】
すなわち、シャーリングにより採取した２５ｍｍ幅×２００ｍｍ長さの各種板厚の短冊試験片を用いて下記の曲げ試験を実施した。なお、熱延鋼板の板厚は２．６ｍｍを基本条件とし、４．０ｍｍ、３．２ｍｍ、２．０ｍｍについての試験も一部の鋼種について実施した。ここで、上記「熱延ままの耳部」には鋼のＢ含有量と熱間圧延仕上げ温度ＴＦの大小によって長さ０〜１５ｍｍの耳割れが発生していた。なお、耳割れ長さが０ｍｍとは耳割れが生じていなかったことを意味する。
【００３１】
上記の長さ０〜１５ｍｍの耳割れを有する２５ｍｍ幅×２００ｍｍ長さの短冊試験片に対して、「５０ｍｍＲの９０度曲げを実施→元のフラット状態に伸ばす→逆方向に５０ｍｍＲの９０度曲げを実施→再度元のフラット状態に伸ばす」というサイクルでの繰り返し曲げを１０回繰り返し、曲げ割れによる試験片の破断有無を評価した。
【００３２】
繰返し曲げ試験の結果、熱延ままの耳部又はシャーリング端面から曲げ割れが発生し、Ｂ含有量が多く且つ板厚が厚いもので破断が生じた。
【００３３】
上記の曲げ試験から、熱延鋼板の曲げ性確保、つまり冷間での通板性確保のためには、熱間圧延仕上げ板厚ｔが前記▲１▼式を満たす必要のあることが明らかとなった。
【００３４】
表２に、熱間圧延仕上げ板厚ｔ（ｍｍ）とＢ含有量が熱延鋼板の曲げ性に及ぼす影響の一例として、鋼１〜５について調査した場合の結果を整理して示す。なお、表中の「○」及び「×」印はそれぞれ、前記繰返し曲げを１０回行った後に破断がなかったもの及び破断したものを示している。
【００３５】
【表２】

【００３６】
又、図３に、熱間圧延仕上げ温度ＴＦ（℃）とＢ含有量が熱延鋼板の曲げ性に及ぼす影響の一例として、鋼６、鋼９、鋼１０、鋼１３及び鋼２０において圧延長さが約１．５ｍで板厚が２．６ｍｍの熱延鋼板の圧延定常部から採取した曲げ試験片を用いた場合の結果を整理して示す。なお図中の「○」及び「×」印はそれぞれ、前記繰返し曲げを１０回行った後に破断がなかったもの及び破断したものを示している。
【００３７】
以上述べた熱間圧延試験、高温引張試験及び繰り返し曲げ試験の結果から、熱間圧延での耳割れ長さを５ｍｍ以下に抑制できる仕上げ温度ＴＦ（℃）とＢ含有量の領域に加えて、熱延鋼板の仕上げ板厚を規定することで曲げ性、つまり冷間での通板性が確保できる含硼素フェライト系ステンレス鋼の熱延鋼帯を得ることができ、その熱延鋼帯は冷延工程で脆性破断するおそれが少なく、冷延鋼板製造用素材として適切な特性を有するので、量産設備での製造が可能であるとの結論に達した。
【００３８】
本発明は、上記の検討内容に基づいて完成されたものである。
【００３９】
【発明の実施の形態】
以下、本発明の各要件について詳しく説明する。なお、既に述べたように各元素の含有量の「％」表示は「質量％」を意味する。
（Ａ）化学組成
本発明においては、対象とする含硼素フェライト系ステンレス鋼帯に対する固体高分子型燃料電池セパレータ部品の素材としての特性付与という意味合いを重視し、そのステンレス鋼の化学成分としてＢ量、Ｃｒ量のみを下記の範囲に限定する。
【００４０】
Ｂ：０．１％以上
導電性を有する金属介在物としてのＭ_２Ｂ　型硼化物の体積率はＢ量に依存し、通電部品としての特性を確保するには最低０．１％のＢ含有が必要である。Ｂの含有量は多ければ多いほど上記特性にとって好ましいので、その上限値は特に限定するものではない。しかし、加熱温度の上限が１２３０℃であることから、熱間圧延の仕上げ温度ＴＦの上限も１２３０℃となり、したがって、計算上のＢ含有量の上限は約１．９６％となる。但し、現状の熱延技術では、Ｂ含有量の上限として１．５％程度が製造可能な範囲と思われる。以上の理由から、Ｂの含有量を０．１％以上とした。
【００４１】
Ｃｒ：１５〜３２％
Ｃｒはステンレス鋼の耐食性を確保するための基本元素であり、燃料電池環境での耐食性を付与するためには最低限１５％の含有量が必要である。耐食性を十分なものとするには、「Ｃｒ（％）−２．５Ｂ（％）」（Ｍｏを含有する場合には「Ｃｒ（％）＋３Ｍｏ（％）−２．５Ｂ（％）」）で計算される値が１８％以上になるようにＣｒを含有させることが好ましい。Ｃｒは含有量が多いほど耐食性を向上させるものの、その一方で脆化を促進する元素であるし、鋼のコストの上昇も招く。特に３２％を超えて含有させるとコスト上昇が著しくなる。したがって、Ｃｒの含有量を１５〜３２％とした。
【００４２】
本発明が対象とする含硼素フェライト系ステンレス鋼のＢ及びＣｒ以外の他の化学成分の組成に関しては特別な限定を加える必要はない。固体高分子型燃料電池セパレータ部品の素材として要求される特性の付与が可能であるような成分範囲でありさえすれば良い。
【００４３】
具体的には、例えば、ＢとＣｒ以外の元素として、Ｃ：０．１５％以下、Ｓｉ：０〜１．５％、Ｍｎ：０〜１．５％、Ｐ：０．０４％以下、Ｓ：０．０１％以下、Ａｌ：０〜６％、Ｎ：０．１０％以下、Ｎｉ：０〜５％、Ｍｏ：０〜７％、Ｃｕ：０〜１％、Ｔｉ：０〜２５×｛Ｃ（％）＋Ｎ（％）｝、Ｎｂ：０〜２５×｛Ｃ（％）＋Ｎ（％）｝、Ｖ：０〜２％、Ｃａ：０〜０．０１％、Ｍｇ：０〜０．０１％、Ｏ（酸素）：０．０１％以下を含有し、残部はＦｅと不純物からなるものであればよい。
【００４４】
なお、ＢとＣｒ以外の上記した元素については、Ｃ：０．０１５％以下、Ｓｉ：０〜０．６％、Ｍｎ：０〜１．０％、Ｐ：０．０４％以下、Ｓ：０．０１％以下、Ａｌ：０．０２〜０．２％、Ｎ：０．０１５％以下、Ｎｉ：０〜４％、Ｍｏ：０〜４％、Ｃｕ：０〜１％、Ｔｉ：０〜０．３％、Ｎｂ：０〜０．５％、Ｖ：０〜０．５％、Ｃａ：０〜０．００３％、Ｍｇ：０〜０．００３％、Ｏ（酸素）：０．００６％以下の含有量とすることが好ましく、Ｃ：０．０１５％以下、Ｓｉ：０．０１〜０．６％、Ｍｎ：０．０１〜１．０％、Ｐ：０．０４％以下、Ｓ：０．０１％以下、Ａｌ：０．０２〜０．２％、Ｎ：０．０１５％以下、Ｎｉ：０．０１〜４％、Ｍｏ：０．０１〜４％、Ｃｕ：０．０１〜１％、Ｔｉ：０．００１〜０．３％、Ｎｂ：０．００１〜０．５％、Ｖ：０．０１〜０．５％、Ｃａ：０．０００１〜０．００３％、Ｍｇ：０．０００１〜０．００３％、Ｏ（酸素）：０．００６％以下の含有量とすることが一層好ましい。
【００４５】
ここで、Ｍｏ、Ｃｕ、Ｎｉ及びＶはステンレス鋼の耐食性を高める作用を有するので耐食性を一層高めたい場合に添加してもよい。
【００４６】
ＮｂはＣとＮを固定して耐食性を改善するため、耐食性を一層高めたい場合に添加してもよい。
【００４７】
Ｓｉ、Ｍｎ及びＡｌは脱酸作用を有するため、脱酸のために添加しても良い。
【００４８】
Ｔｉは凝固組織を微細化する作用を有し熱間加工性を改善するため、熱間加工性を一層高めたい場合に添加してもよい。
【００４９】
Ｃａ及びＭｇは凝固組織を改善する作用を有するので、凝固組織を一層改善したい場合に添加してもよい。
【００５０】
なお、Ｃ、Ｎ、Ｐ及びＯ（酸素）は鋼の靱性を低下させ、Ｓは熱間加工性を低下させるので、これらの元素は不純物としてその含有量をできるだけ低くするのがよい。
【００５１】
本発明が対象とする含硼素フェライト系ステンレス鋼は、通常の方法で溶製して鋳造し、鋼塊にすればよい。なお、インゴット造塊による鋳造も可能であるが、製造コストの観点から連続鋳造するのがよい。
【００５２】
含硼素フェライト系ステンレス鋼は固相線温度が低く、凝固割れ感受性の高い鋼種であるため、連続鋳造にするに際しては、ブレークアウトが起こらないよう溶鋼加熱温度ΔＴを小さくして鋳造するのがよい。
【００５３】
鋳造後の鋼塊は表面欠陥を含む場合もあるためグラインダ−による手入れを行うことが望ましい。但し、鋼塊の衝撃による延性−脆性遷移温度が２５０℃前後にあるため手入れ中の温度低下により２００℃以下に鋼塊温度が低下すると熱応力により鋼塊折損のおそれがある。このためグラインダーによる手入れ中に温度低下して２００℃以下に冷えるおそれがある場合は手入れを省略もしくは中断し、加熱炉に装入するのがよい。
（Ｂ）熱間圧延
（Ｂ−１）加熱温度
得られた鋼塊は熱間圧延を行うために加熱するが、この際の加熱温度は１２３０℃以下にする必要がある。
【００５４】
すなわち、１２３０℃を超えて加熱するとＭ_２Ｂ　型硼化物が分解し、低融点の液相を形成して液膜脆性が生じるため熱間加工性が劣化し、熱間圧延時に表面割れ及び耳割れが発生する。したがって、熱間圧延の加熱温度を１２３０℃以下とした。なお、液膜脆性をより確実に回避するためには熱間圧延の加熱温度を１２１０℃以下とするのがよい。
【００５５】
上記熱間圧延の加熱温度は１０００℃以上とするのがよい。加熱温度が１０００℃に満たないと圧延荷重が高くなりすぎ、所望の板厚まで圧延できない場合があるからである。
【００５６】
（Ｂ−２）仕上げ温度ＴＦ（℃）
本発明が対象とする含硼素フェライト系ステンレス鋼は、既に述べたように、仕上げ温度ＴＦの低下及びＢ含有量の増加とともに単調に熱間加工性が低下する。このため耳割れを小さくするために高温で仕上げることが必要である。
【００５７】
耳割れの大きさはＢ含有量及び、仕上げ温度ＴＦによって決まるが、上記のように熱間圧延の加熱温度を１２３０℃以下とし、前記式▲２▼又は式▲３▼を満たす仕上げ温度ＴＦで熱間圧延を終了すれば、耳割れの長さを目標とする５ｍｍ以下にすることができる。したがって、熱間圧延の仕上げ圧延温度ＴＦを、Ｂ含有量に応じて式▲２▼又は式▲３▼を満たすように、つまり、Ｂ含有量が０．１〜０．２５％の場合には７５０℃以上、又、Ｂ含有量が０．２５％以上の場合には「２８０×Ｂ（％）＋６８０」℃以上と規定した。
【００５８】
なお、熱間圧延の仕上げ圧延温度ＴＦは、Ｂ含有量が０．１〜０．２５％の場合には８００℃以上、又、Ｂ含有量が０．２５％以上の場合には「２８０×Ｂ（％）＋７３０」℃以上とすることが好ましい。
【００５９】
（Ｂ−３）仕上げ板厚ｔ（ｍｍ）
本発明が対象とする含硼素フェライト系ステンレス鋼は、上記した加熱温度と仕上げ温度で熱間圧延されるが、その際仕上げ板厚ｔ（ｍｍ）は前記式▲１▼を満たす必要がある。すなわち、仕上げ板厚ｔが「５−２Ｂ（％）」ｍｍを超えると、熱延鋼板に十分な曲げ性、つまり冷間での通板性が確保されず、その熱延鋼板をコイル状に巻いた鋼帯は冷延工程で脆性破断する場合がある。したがって、熱間圧延による仕上げ板厚ｔが式▲１▼を満たすように規定した。
【００６０】
なお、仕上げ板厚ｔは小さいほど好ましいが、薄肉熱延では仕上げ温度が低下しやすく前記（Ｂ−２）項で規定した仕上げ温度ＴＦを確保することが困難となり、例えば、保熱のための設備が必要になるし、エネルギーコストも嵩む。このため、通常は仕上げ板厚ｔを１．５ｍｍを超えるようにするのがよい。
【００６１】
熱間圧延を施された鋼板は、コイル状に巻き取られて鋼帯となる。この巻き取り温度は特に規定しなくてもよい。しかし、前記（Ｂ−１）〜（Ｂ−３）項の熱間圧延条件と低温巻き取りを組み合わせることが一層の脆性破断対策となるので、巻き取り温度は低くする方がよく、ホットラン水冷の能力が十分にあれば４５０℃以下で巻き取りを行ってもよい。
【００６２】
冷延鋼帯は上記のようにして得た熱延鋼帯からに製造されるが、その製造方法は特に規定しなくてもよい。
【００６３】
なお、板厚が１ｍｍを下回ると脆性破壊はほとんど問題にならないので、冷延と焼鈍を繰り返すことで、いわゆる「箔」の板厚まで冷間圧延することが可能であるが、熱延鋼帯からの最初に冷間圧延する際の冷延率を過度に高く採ると破断の危険性が高くなるので、前記の冷延率は５０％以下とするのがよい。２回目以降の冷間圧延ではより高い冷延率の採用が可能である。
【００６４】
なお、冷延率とは被加工材の冷延前の厚さをｔ_０、冷延後の厚さをｔ_１とした場合、｛（ｔ_０−ｔ_１）／ｔ_０　｝×１００（％）で表される値をいう。
【００６５】
本発明に係る熱延鋼帯を厚さ０．２ｍｍ以下にまで冷延すれば、固体高分子型燃料電池セパレーター部品用として軽量、且つ機能的な素材が得られる。
【００６６】
【実施例】
前記表１に示す化学組成を有する鋼２２について、量産ステンレス鋼板製造設備を用いて製造実験を行った。
【００６７】
すなわち、２２％Ｃｒ−２％Ｍｏ−０．６％Ｂ系の含硼素フェライト系ステンレス鋼である鋼２２の溶鋼７０トンをＡＯＤとＶＯＤ設備を用いて脱炭・脱窒精錬し、２００ｍｍ厚さ×８００ｍｍ幅の鋳型で連続鋳造した。
【００６８】
得られた約６ｍ長さの連続鋳造鋳片８本を保温カバー台車内で徐冷し、その表面温度が２５０℃にまで低下する前に均熱温度を１１９０℃に設定した加熱炉に装入した。装入温度を２５０℃以上とした理由はスラブの脆性割れを防止するためである。なお、加熱炉へ装入するまでの間に鋳片の熱間での表面手入れは実施しなかった。
【００６９】
上記のようにして加熱した８本の鋳片をそれぞれ３．６ｍｍ、２．８ｍｍ及び２．０ｍｍの厚さで、３００〜６００ｍの長さに熱間圧延し、ホットラン水冷した後、６５０〜５５０℃の温度で巻き取りを行った。なお、仕上げスタンド出側及びコイル幅中央で測定する熱間圧延の仕上げ温度ＴＦはコイル長手方向及びコイル毎の変動値として、９３０〜９８０℃の範囲にあった。
【００７０】
上記のようにして巻き取った一部の熱延コイルについては、いわゆる「４７５℃脆性」による脆化を抑制するために、巻き取り後３０分以内に水槽中に浸漬して冷却した。他の熱延コイルでは水槽浸漬を省略した。
【００７１】
このようにして得た熱延鋼板の通板性を確認するために、コイル展開ラインで外周から内周にかけて通板した。なお、通板試験は先後端の非定常部１〜５巻き程度を切り下げ、その後、定常部の片面当たり１５ｍｍのトリム処理を実施し、トリム部の割れを調査した。
【００７２】
破断に到るような割れ等は一切発生せず、問題なく通板が行え、本発明方法の効果が実証できた。
【００７３】
【発明の効果】
本発明の方法によれば、熱延後に施される種々の工程で脆性的な破断を生じることの少ない耳割れ長さが５ｍｍ以下となる熱延鋼帯が得られ、これによって固体高分子型燃料電池セパレータ部品の素材として好適な含硼素フェライト系ステンレス冷延鋼帯を安定且つ安価に供給することができるので、産業上の効果は極めて大きい。
【図面の簡単な説明】
【図１】２２％Ｃｒ−２％Ｍｏ系でＢ量がそれぞれ０．１０％、０．６３％及び１．０４％の鋼９、１３及び２０についての熱間圧延試験結果を示す図である。
【図２】２０％Ｃｒ−２％Ｍｏ系でＢ量がそれぞれ０．１２％と１．４９％の鋼２と鋼６及び２２％Ｃｒ−２％（１．８％？）Ｍｏ系でＢ量が０．６０％の鋼２０の各鋼塊から採取した試験片を高温引張試験して絞りを測定した結果を示す図である。
【図３】鋼６、鋼９、鋼１０、鋼１３及び鋼２０において圧延長さが約１．５ｍで板厚が２．６ｍｍの熱延鋼板の圧延定常部から採取した曲げ試験片を用いた場合の、熱間圧延仕上げ温度ＴＦ（℃）とＢ含有量が熱延鋼板の曲げ性に及ぼす影響を示す図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing a boron-containing ferritic stainless steel strip, and more particularly to a method for producing a boron-containing ferrite stainless steel hot-rolled steel strip suitable as a material for a polymer electrolyte fuel cell separator component.
[0002]
[Prior art]
Normal steel materials are ductile at temperatures above room temperature, and there is no need to worry about brittle fracture in producing steel sheets. However, among stainless steels, ferritic stainless steels, particularly ferritic stainless steels containing a large amount of Cr, may exhibit brittle properties, and among them, M ₂ B type as a conductive metal inclusion. It is no exaggeration to say that boron-containing ferritic stainless steel containing boride is the most brittle material among steel materials.
[0003]
Note that the boron-containing ferritic stainless steel containing M ₂ B-type boride as the metal inclusion having conductivity described in Patent Document 1 by the present inventors has a low contact electric resistance and a low battery electric environment. This is a material proposed as one of the raw materials for a polymer electrolyte fuel cell separator component having a small change with time.
[0004]
In order to use the above boron-containing ferritic stainless steel as a material for fuel cell separator parts, it is necessary to provide a cold-rolled thin steel sheet of the stainless steel stably and inexpensively. It is extremely important to consider brittle fracture in hot-rolled coils.
[0005]
It is known that even in brittle steel materials, if the thickness (plate thickness) is reduced, the ductile-brittle transition temperature necessarily decreases and brittle fracture does not occur at room temperature. In other words, brittle steel materials are generally susceptible to brittle fracture when their thickness is large. Since the thickest steel material used in the cold rolling process is a hot-rolled steel strip, as an effective measure to prevent brittle fracture of ferritic stainless steel, a method of reducing the thickness is considered. In fact, in the case of ferritic stainless steel having good hot workability, it is easy to reduce the wall thickness, which is a sufficient measure against brittle fracture.
[0006]
As a technique for lowering the ductile-brittle transition temperature to prevent brittle fracture, a “low-temperature winding” technique is also well known. For example, in order to improve the toughness of the hot-rolled steel strip of high Cr ferritic stainless steel, the hot-rolled steel strip is water-cooled after finish rolling, and the winding temperature is controlled to 450 ° C. or lower. This “low temperature winding” is to avoid the so-called “475 ° C. brittleness” by controlling the internal quality of the steel material even if the thickness is about 4 to 9 mm, and to make the material not brittle. Patent Literature 2 discloses, as an improved technique of the above technique, an impact value at 20 ° C. of 50 J / cm by setting the winding temperature T to “t × T ≦ 3600” according to the thickness t of the hot-rolled steel strip. A technique for increasing the toughness to ^two or more is disclosed.
[0007]
Boron-containing ferritic stainless steel, unlike ordinary ferritic stainless steel, cannot be said to have good hot workability, and ear cracks may occur when hot rolling is performed. Once the edge crack occurs, a notch for brittle fracture is given, and it becomes difficult to perform annealing, pickling and cold rolling of the hot-rolled steel sheet. In particular, when the edge crack of the hot-rolled material exceeds 5 mm, brittle fracture is likely to occur during cold rolling. Therefore, simply reducing the thickness of the hot-rolled steel strip of boron-containing ferritic stainless steel cannot be a measure against brittle fracture.
[0008]
In other words, in the case of a steel material that tends to have ear cracks in the hot rolling process, reducing the wall thickness promotes the generation of defects such as ear cracks and surface cracks, and depending on the degree, the production in the next step, the cold rolling process Can be difficult. This is because defects such as edge cracks and surface cracks act as notches for brittle fracture. In the process of welding, inspecting, rewinding, continuous annealing and pickling, slitting, trimming, and cold rolling the steel sheet called the leader material to the hot-rolled steel strip, the steel starting from some defects Strip breakage occurs, and trim scraps are brittle and cannot be wound continuously.Therefore, the continuous production line is stopped, and trim scraps are continued to be wound up by manual operator intervention. May occur.
[0009]
Furthermore, since boron-containing ferritic stainless steel cannot be said to have good workability unlike ordinary ferritic stainless steel, simply controlling the winding temperature to 450 ° C. or lower may cause breakage. For this reason, it is also difficult to prevent brittle fracture by a simple “low temperature winding” technique.
[0010]
[Patent Document 1]
JP 2001-32056 A [Patent Document 2]
JP-A-8-199237
[Problems to be solved by the invention]
The present invention has been made in view of the above situation, and its purpose is to supply a boron-containing ferrite-based stainless steel cold-rolled steel strip suitable as a material for polymer electrolyte fuel cell separator parts in a stable and inexpensive manner. An object of the present invention is to provide a method for producing a hot-rolled steel strip which is less likely to cause brittle fracture in various steps performed after hot-rolling.
[0012]
[Means for Solving the Problems]
The gist of the present invention resides in a method for producing a boron-containing ferrite stainless steel strip described below.
[0013]
That is, "After heating a ferritic stainless steel containing 0.1% or more of B and 15 to 32% of Cr by mass% to a temperature of 1230 ° C. or less, a finished plate satisfying the following formula (1): Production of a boron-containing ferritic stainless steel hot rolled steel strip characterized in that hot rolling is completed at a thickness t and a finishing temperature TF satisfying the following equation (2) or (3) and then wound as a coil. Method: t ≦ 5-2B (%) (1), when B content is 0.1 to 0.25%: TF ≧ 750 (2), B content is 0.25% In the above case: TF ≧ 280 × B (%) + 680... (3) ”.
[0014]
The unit of the finished plate thickness t is mm, and the unit of the hot rolling finishing temperature TF is ° C.
[0015]
The present inventors have conducted various studies in order to achieve the above-described object, and assuming that a method of reducing the thickness of the hot-rolled steel strip is adopted, even if it is a brittle steel type, It has been concluded that the method of the present invention can overcome poor hot workability and can be manufactured in mass production equipment.
[0016]
That is, the present inventors first cast a boron-containing ferritic stainless steel to a thickness of 50 to 100 mm on a laboratory scale, and repeatedly performed hot rolling test and hot workability evaluation on the ingot by a hot rolling test and a high temperature tensile test. Was. Further, a repeated bending test was performed to evaluate the passability of the hot-rolled steel sheet in the next step.
[0017]
First, the contents of the study conducted on the evaluation of hot workability will be described.
[0018]
In terms of mass% (hereinafter, “%” of the content of each element means “mass%”), 17.22 to 25.23% of Cr, 0 to 2.96% of Mo, and 0.0002 to For a ferritic stainless steel containing 1.49% B, a hot rolling test was performed at a plate thickness of 2 to 5 mm and a finishing temperature of 650 to 1080 ° C., and the maximum edge crack length generated at this time was evaluated. did.
[0019]
Table 1 shows the chemical composition of the test steel. Note that steels 1 to 21 are cast materials on a laboratory scale. Steels 1 to 18 having an ingot thickness of 50 mm are small ingots, and steels 19 to 21 having a thickness of 100 mm are continuous cast slabs cast by a continuous casting facility on a laboratory scale. The steel 22 is a continuous cast slab of an actual machine scale to be described later.
[0020]
[Table 1]

[0021]
From each steel ingot, a 50 mm thickness x 140 mm width x 85 mm length hot rolling material is processed, and an experimental facility capable of simulating tandem rolling in which three hot rolling mills are arranged at intervals of about 1 m. And hot-rolled.
[0022]
In the hot rolling test, the finishing temperature TF was changed by adjusting the number of roll rotations and the waiting time between passes.
[0023]
FIG. 1 shows, as an example, the results of a hot rolling test of Steel 9, Steel 13 and Steel 19 with a B content of 0.10%, 0.63% and 1.04% in a 22% Cr-2% Mo system, respectively. Show.
[0024]
From FIG. 1, it was found that the larger the B content and the lower the finishing temperature TF, the longer the maximum edge crack length generated by hot rolling.
[0025]
FIG. 2 shows an example of a result obtained by subjecting a test piece taken from each steel ingot to a high-temperature tensile test and measuring the drawing. In addition, this figure shows that the B content is 0.12% and 1.49% for steel 2 and steel 6, respectively, and the B content is 0.60 for 22% Cr-2% Mo based on 20% Cr-2% Mo system. % Steel 20.
[0026]
From FIG. 2, it is clear that in a temperature range where the tensile test temperature is higher than 1150 ° C., the drawing starts to decrease at a certain temperature with a rise in temperature, and when the temperature reaches 1230 ° C., the drawing becomes zero for all steel types. The temperature at which the throttle became zero was almost constant regardless of the B amount. Therefore, it was found that the heating temperature of the ingot of the boron-containing ferrite stainless steel targeted by the present invention needs to be 1230 ° C. or less regardless of the B content.
[0027]
From the above hot rolling test, the heating temperature of the hot rolling is set to 1230 ° C. or less, and the hot rolling is finished at a finishing temperature TF (° C.) or more that satisfies the above formula (2) or (3) depending on the amount of B. As a result, it became clear that the intended target of reducing the edge crack length of the hot-rolled boron-containing ferritic stainless steel sheet to 5 mm or less can be achieved.
Furthermore, to eliminate the occurrence of edge cracks, the finish rolling temperature is set to 800 ° C. or more when the B content is 0.1 to 0.25%, and to the finish rolling temperature when the B content is 0.25% or more. Has been found to be preferably at least “280 × B (%) + 730” ° C.
[0028]
Next, the contents of the study on the evaluation of the hot-rolled steel sheet's threadability will be described.
[0029]
In order to simulate the cold rolling of hot-rolled steel sheets, strip specimens of 25 mm width x 200 mm length with hot-rolled ears were sampled by shearing, and whether or not breakage occurred during the rolling. Was evaluated by a bending test.
[0030]
That is, the following bending test was carried out using strip test pieces of various plate thicknesses of 25 mm width × 200 mm length collected by shearing. In addition, the test | inspection about 4.0 mm, 3.2 mm, and 2.0 mm was implemented about some steel grades about the board thickness of a hot-rolled steel sheet as a basic condition of 2.6 mm. Here, in the above "hot-rolled ears", ear cracks having a length of 0 to 15 mm were generated due to the B content of the steel and the magnitude of the hot-rolling finishing temperature TF. In addition, the ear crack length of 0 mm means that the ear crack did not occur.
[0031]
For the strip test specimen of 25 mm width x 200 mm length having the above-mentioned ear cracks of 0 to 15 mm length, perform "90-degree bending of 50 mmR → stretch to the original flat state → 90-degree bending of 50 mmR in the opposite direction. The test was repeated 10 times in a cycle of “Perform the above-mentioned process and stretch again to the original flat state” ten times, and the presence or absence of breakage of the test piece due to bending cracks was evaluated.
[0032]
As a result of the repeated bending test, bending cracks were generated from the as-heated ears or the sheared end faces, and breakage occurred due to the large B content and the large thickness.
[0033]
From the above bending test, it is clear that in order to ensure the bendability of the hot-rolled steel sheet, that is, to ensure the cold-passing property, the hot-rolled finished sheet thickness t needs to satisfy the above formula (1). became.
[0034]
Table 2 summarizes the results of investigations on steels 1 to 5 as an example of the effects of the hot-rolled finished sheet thickness t (mm) and the B content on the bendability of a hot-rolled steel sheet. In the table, “O” and “X” indicate the case where there was no break and the case where there was no break after the above-mentioned repeated bending was performed 10 times, respectively.
[0035]
[Table 2]

[0036]
FIG. 3 shows an example of the effects of the hot-rolling finishing temperature TF (° C.) and the B content on the bendability of a hot-rolled steel sheet, in which the steel sheet 6, steel 9, steel 10, steel 13 and steel 20 are rolled. The results of using a bending test piece taken from a steady portion of a hot-rolled steel sheet having a thickness of about 1.5 m and a sheet thickness of 2.6 mm are summarized and shown. In the figures, “O” and “X” indicate the case where there was no break and the case where the break occurred after the above-mentioned repeated bending was performed 10 times.
[0037]
From the results of the hot rolling test, the high-temperature tensile test, and the repeated bending test described above, in addition to the finishing temperature TF (° C.) and the B content region in which the edge crack length in hot rolling can be suppressed to 5 mm or less, By defining the finish thickness of the hot-rolled steel sheet, it is possible to obtain a hot-rolled steel strip of boron-containing ferritic stainless steel capable of securing the bending property, that is, the passability in a cold state, and the hot-rolled steel strip has a It has been concluded that there is little risk of brittle rupture in the rolling process and that it has suitable properties as a material for manufacturing cold-rolled steel sheets, so that it can be manufactured with mass production equipment.
[0038]
The present invention has been completed based on the above examination contents.
[0039]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, each requirement of the present invention will be described in detail. As described above, “%” of the content of each element means “% by mass”.
(A) Chemical Composition In the present invention, emphasis is placed on the property of imparting properties as a material of a polymer electrolyte fuel cell separator component to a target boron-containing ferritic stainless steel strip, and B content is defined as a chemical component of the stainless steel. , Cr amount is limited to the following range.
[0040]
B: The volume fraction of M ₂ B-type boride as a metal inclusion having a conductivity of 0.1% or more depends on the amount of B, and at least 0.1% of B is contained in order to secure the characteristics as a current-carrying part. is necessary. Since the higher the B content, the better the above properties, the upper limit is not particularly limited. However, since the upper limit of the heating temperature is 1230 ° C., the upper limit of the finishing temperature TF of the hot rolling is also 1230 ° C., and therefore, the calculated upper limit of the B content is about 1.96%. However, with the current hot rolling technology, it is considered that the upper limit of the B content is about 1.5%, which is a range that can be manufactured. For the above reasons, the content of B is set to 0.1% or more.
[0041]
Cr: 15 to 32%
Cr is a basic element for ensuring the corrosion resistance of stainless steel, and a minimum content of 15% is required to impart corrosion resistance in a fuel cell environment. In order to ensure sufficient corrosion resistance, “Cr (%) − 2.5 B (%)” (or “Cr (%) + 3 Mo (%) − 2.5 B (%)” when Mo is contained) is used. It is preferable to include Cr so that the calculated value is 18% or more. The higher the content of Cr, the higher the corrosion resistance, but on the other hand, it is an element that promotes embrittlement and causes an increase in steel cost. In particular, when the content exceeds 32%, the cost increases significantly. Therefore, the content of Cr is set to 15 to 32%.
[0042]
There is no need to add a special limitation on the composition of chemical components other than B and Cr of the boron-containing ferritic stainless steel targeted by the present invention. The component range only needs to be such that the characteristics required as a material of the polymer electrolyte fuel cell separator component can be imparted.
[0043]
Specifically, for example, as elements other than B and Cr, C: 0.15% or less, Si: 0 to 1.5%, Mn: 0 to 1.5%, P: 0.04% or less, S: : 0.01% or less, Al: 0 to 6%, N: 0.10% or less, Ni: 0 to 5%, Mo: 0 to 7%, Cu: 0 to 1%, Ti: 0 to 25 × ｛ C (%) + N (%)}, Nb: 0 to 25 × {C (%) + N (%)}, V: 0 to 2%, Ca: 0 to 0.01%, Mg: 0 to 0.01 %, O (oxygen): 0.01% or less, with the balance being Fe and impurities.
[0044]
For the above elements other than B and Cr, C: 0.015% or less, Si: 0 to 0.6%, Mn: 0 to 1.0%, P: 0.04% or less, S: 0 0.01% or less, Al: 0.02 to 0.2%, N: 0.015% or less, Ni: 0 to 4%, Mo: 0 to 4%, Cu: 0 to 1%, Ti: 0 to 0% 0.3%, Nb: 0 to 0.5%, V: 0 to 0.5%, Ca: 0 to 0.003%, Mg: 0 to 0.003%, O (oxygen): 0.006% or less , C: 0.015% or less, Si: 0.01 to 0.6%, Mn: 0.01 to 1.0%, P: 0.04% or less, S: 0 0.01% or less, Al: 0.02 to 0.2%, N: 0.015% or less, Ni: 0.01 to 4%, Mo: 0.01 to 4%, Cu: 0.01 to 1% , Ti: 0.001-0.3%, Nb 0.001 to 0.5%, V: 0.01 to 0.5%, Ca: 0.0001 to 0.003%, Mg: 0.0001 to 0.003%, O (oxygen): 0.006 % Is more preferable.
[0045]
Here, Mo, Cu, Ni and V have an effect of improving the corrosion resistance of stainless steel, and may be added when it is desired to further increase the corrosion resistance.
[0046]
Nb fixes C and N to improve corrosion resistance, and may be added when it is desired to further increase the corrosion resistance.
[0047]
Since Si, Mn and Al have a deoxidizing effect, they may be added for deoxidizing.
[0048]
Ti has a function of refining a solidified structure and improves hot workability, and may be added when it is desired to further enhance hot workability.
[0049]
Since Ca and Mg have an effect of improving the solidified structure, they may be added when it is desired to further improve the solidified structure.
[0050]
Since C, N, P and O (oxygen) reduce the toughness of steel and S lowers the hot workability, the content of these elements as impurities should be as low as possible.
[0051]
The boron-containing ferritic stainless steel targeted by the present invention may be melted and cast by a usual method to form a steel ingot. Although casting by ingot ingot is possible, continuous casting is preferable from the viewpoint of manufacturing cost.
[0052]
Since boron-containing ferritic stainless steel is a steel type having a low solidus temperature and a high susceptibility to solidification cracking, it is preferable to cast the molten steel at a low heating temperature ΔT so that breakout does not occur during continuous casting. .
[0053]
Since the ingot after casting may include surface defects, it is desirable to perform maintenance using a grinder. However, since the ductile-brittle transition temperature due to the impact of the steel ingot is about 250 ° C., if the temperature of the steel ingot falls to 200 ° C. or less due to the temperature drop during maintenance, there is a possibility that the steel ingot may break due to thermal stress. For this reason, when there is a possibility that the temperature may drop to 200 ° C. or less during the maintenance by the grinder, the maintenance may be omitted or interrupted, and then the heating furnace may be charged.
(B) Hot rolling (B-1) Heating temperature The obtained steel ingot is heated to perform hot rolling, and the heating temperature at this time needs to be 1230 ° C or lower.
[0054]
That is, when heated above 1230 ° C., the M ₂ B-type boride is decomposed, a liquid phase having a low melting point is formed, and liquid film embrittlement occurs, thereby deteriorating hot workability. Cracks occur. Therefore, the heating temperature of the hot rolling was set to 1230 ° C. or less. In order to more reliably avoid liquid film brittleness, the heating temperature of hot rolling is preferably set to 1210 ° C. or less.
[0055]
The heating temperature of the hot rolling is preferably 1000 ° C. or higher. If the heating temperature is lower than 1000 ° C., the rolling load becomes too high, and it may not be possible to roll to a desired thickness.
[0056]
(B-2) Finishing temperature TF (° C)
As described above, the hot workability of the boron-containing ferritic stainless steel to which the present invention is applied monotonously decreases as the finishing temperature TF decreases and the B content increases. Therefore, it is necessary to finish at a high temperature in order to reduce ear cracks.
[0057]
The size of the edge crack is determined by the B content and the finishing temperature TF. As described above, the heating temperature of the hot rolling is set to 1230 ° C. or less, and the finishing temperature TF satisfying the above formula (2) or (3) is obtained. When the hot rolling is completed, the length of the edge crack can be reduced to the target value of 5 mm or less. Therefore, the finishing rolling temperature TF of the hot rolling is adjusted so as to satisfy the formula (2) or the formula (3) according to the B content, that is, when the B content is 0.1 to 0.25%, When the B content was 750 ° C. or more and the B content was 0.25% or more, it was defined as “280 × B (%) + 680” ° C. or more.
[0058]
The finish rolling temperature TF of the hot rolling is 800 ° C. or more when the B content is 0.1 to 0.25%, and “280 × when the B content is 0.25% or more. B (%) + 730 ”° C. or higher.
[0059]
(B-3) Finished plate thickness t (mm)
The boron-containing ferritic stainless steel to which the present invention is applied is hot-rolled at the above-mentioned heating temperature and finishing temperature, and at that time, the finished plate thickness t (mm) needs to satisfy the above formula (1). That is, if the finished sheet thickness t exceeds “5-2B (%)” mm, sufficient bendability of the hot-rolled steel sheet, that is, cold sheet passing property is not ensured, and the hot-rolled steel sheet is formed into a coil shape. The wound steel strip may break brittlely in the cold rolling process. Therefore, the thickness t of the finished plate by hot rolling is defined to satisfy the formula (1).
[0060]
Although the finished plate thickness t is preferably as small as possible, the finishing temperature tends to decrease in thin hot rolling, and it becomes difficult to secure the finishing temperature TF defined in the above section (B-2). Equipment is required and energy costs increase. For this reason, it is usually good to make the finished plate thickness t exceed 1.5 mm.
[0061]
The hot-rolled steel sheet is wound into a coil to form a steel strip. This winding temperature need not be specified. However, the combination of the hot rolling conditions and the low temperature winding described in the above items (B-1) to (B-3) is a further measure against brittle fracture. The winding may be performed at 450 ° C. or lower if the capacity is sufficient.
[0062]
The cold-rolled steel strip is manufactured from the hot-rolled steel strip obtained as described above, but the manufacturing method is not particularly limited.
[0063]
When the thickness is less than 1 mm, the brittle fracture is hardly a problem. Therefore, it is possible to cold-roll to a so-called “foil” thickness by repeating cold rolling and annealing. If the cold rolling reduction during the first cold rolling is excessively high, the risk of breakage increases. Therefore, the above cold rolling reduction is preferably 50% or less. In the second and subsequent cold rolling, a higher cold rolling reduction can be employed.
[0064]
The cold rolling ratio is defined as {(t ₀ −t ₁ ) / t ₀ } × 100 (%, where t _{0 is} the thickness of the workpiece before cold rolling and t ₁ is the thickness after cold rolling. ).
[0065]
When the hot-rolled steel strip according to the present invention is cold-rolled to a thickness of 0.2 mm or less, a lightweight and functional material for a polymer electrolyte fuel cell separator component can be obtained.
[0066]
【Example】
A production experiment was conducted on steel 22 having the chemical composition shown in Table 1 using mass production stainless steel sheet production equipment.
[0067]
That is, 70 tons of molten steel of steel 22, which is a boron-containing ferritic stainless steel of 22% Cr-2% Mo-0.6% B system, is decarburized and denitrified using AOD and VOD equipment, and has a thickness of 200 mm. Continuous casting was performed using a mold having a width of 800 mm.
[0068]
Eight continuous cast slabs having a length of about 6 m obtained were gradually cooled in a heat insulating cover cart, and charged into a heating furnace having a soaking temperature set at 1190 ° C before the surface temperature decreased to 250 ° C. did. The reason for setting the charging temperature to 250 ° C. or higher is to prevent brittle cracking of the slab. In addition, hot surface maintenance of the slab was not performed until it was charged into the heating furnace.
[0069]
The eight slabs heated as described above are hot-rolled to a length of 300 to 600 m with a thickness of 3.6 mm, 2.8 mm and 2.0 mm, respectively, and after hot-run water cooling, 650 to 550 Winding was performed at a temperature of ° C. In addition, the finishing temperature TF of hot rolling measured on the exit side of the finishing stand and the center of the coil width was in a range of 930 to 980 ° C. as a variation value in the coil longitudinal direction and for each coil.
[0070]
Some of the hot rolled coils wound as described above were immersed in a water bath and cooled within 30 minutes after winding in order to suppress embrittlement due to so-called “475 ° C. embrittlement”. In other hot-rolled coils, immersion in the water tank was omitted.
[0071]
In order to confirm the passing property of the hot-rolled steel sheet thus obtained, the hot-rolled steel sheet was passed from the outer periphery to the inner periphery on a coil deployment line. In the passing test, about 1 to 5 turns of the unsteady portion at the front and rear ends were cut down, and thereafter, a trim treatment of 15 mm was performed on one side of the steady portion to check for cracks in the trim portion.
[0072]
No cracking or the like that would lead to breakage occurred at all, the threading could be performed without any problem, and the effect of the method of the present invention was proved.
[0073]
【The invention's effect】
According to the method of the present invention, a hot-rolled steel strip having an edge crack length of 5 mm or less, which does not cause brittle breakage in various steps performed after hot rolling, is obtained. Since a boron-containing ferritic stainless steel cold-rolled steel strip suitable as a material for a fuel cell separator component can be supplied stably and inexpensively, the industrial effect is extremely large.
[Brief description of the drawings]
FIG. 1 is a view showing the results of hot rolling tests on steels 9, 13 and 20 with a B content of 0.10%, 0.63% and 1.04% in a 22% Cr-2% Mo system, respectively. .
FIG. 2 shows steels 2 and 6 having a B content of 0.12% and 1.49% in a 20% Cr-2% Mo system, respectively, and B in a 22% Cr-2% (1.8%?) Mo system. It is a figure which shows the result of having measured the drawing by performing the high temperature tensile test of the test piece extract | collected from each steel ingot of the steel 20 of 0.60%.
FIG. 3 is a drawing of a bending test piece taken from a steady portion of a hot-rolled steel sheet having a rolling extension of about 1.5 m and a thickness of 2.6 mm in steel 6, steel 9, steel 10, steel 13 and steel 20. It is a figure which shows the influence which the hot-rolling finishing temperature TF (degreeC) and B content give to the bending property of a hot-rolled steel plate in case it was.

Claims (1)

After heating a ferritic stainless steel containing 0.1% or more of B and 15 to 32% of Cr by mass% to a temperature of 1230 ° C. or less, a finished plate thickness t satisfying the following formula (1): A method for producing a hot rolled boron-containing ferritic stainless steel strip, comprising finishing hot rolling at a finishing temperature TF satisfying the following formula (2) or formula (3), and then winding as a coil.
t ≦ 5-2B (%) (1)
When the B content is 0.1 to 0.25%: TF ≧ 750 (2)
When the B content is 0.25% or more: TF ≧ 280 × B (%) + 680 (3)
Here, the unit of the finished plate thickness t is mm, and the unit of the finished temperature TF of hot rolling is ° C.

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