JP2004225082A - High strength low permeability austenitic stainless steel sheet, method of producing the same, and method of producing washer for bolt fastening - Google Patents
High strength low permeability austenitic stainless steel sheet, method of producing the same, and method of producing washer for bolt fastening Download PDFInfo
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【0001】
【発明の属する技術分野】
本発明は、高強度で低透磁率のオーステナイト系ステンレス鋼およびそれを使用するボルト締結用座金に関するものである。さらに詳しくは、本発明は、リニアーモーターカー等に使用される電磁コイルの締結用座金に使用される工業的に製造可能な高強度低透磁率のオーステナイト系ステンレス鋼薄板およびそれから加工製造される座金に関するものである。
【0002】
【従来の技術】
ボルトやねじの締結に使われる座金用材料には、高窒素の高強度オーステナイト系ステンレス鋼SUS304Nが用いられることが多い。リニアモーターカー用に使用される電磁コイルの締結用座金や皿バネにもSUS304Nの使用が検討されている。コイルは熱膨張したり、振動を受けるため適度な軸力で緩まない締結が必要であり、座金の果たす役割は極めて重要である。
【0003】
さらに、電磁コイル用に使用される締結材は磁性を嫌うため、低透磁率であることが機能上必要不可欠である。
【0004】
高強度でかつ非磁性のオーステナイト系ステンレス鋼に関する先行技術としては、特許文献1、特許文献2,特許文献3および特許文献4があるが、これらは高Mn鋼あるいはS添加快削鋼に関するものである。しかしながら、本発明が課題とする18Cr−8Ni鋼ベースの窒素添加高強度ステンレス鋼の低透磁率化に関する先行技術の開示はない。
【0005】
【特許文献1】
特開昭62−294130号公報
【特許文献2】
特開昭63−38558号公報
【特許文献3】
特開平03−17245号公報
【特許文献4】
特開平08−269639号公報
【0006】
【発明が解決しようとする課題】
18Cr−8Ni鋼ベースの窒素添加ステンレス鋼は高強度を要求されることから、熱処理後に冷間圧延を行ったままで焼鈍を行わずに製品としており、一般的にハード材と呼ばれている。SUS304のようなオーステナイト系ステンレス鋼では、冷間圧延率を上げると高強度化する一方、冷間圧延により加工誘起マルテンサイトが生成するために(マルテンサイトは磁性を有する結晶構造)、磁性を帯びるようになる。マルテンサイト量に対応して透磁率が大きくなるので、高強度化と低透磁率化は相反する関係にあり、両者を両立することが困難である。
【0007】
そこで、本発明の目的は、このような問題を解決し、電磁コイル用締結用座金や皿バネなどに適した低透磁率と高強度を同時に満たした18Cr−8Ni鋼ベースの高窒素オーステナイト系ステンレス鋼およびこれからなる座金を提供するものである。
【0008】
【課題を解決するための手段】
本発明は、この目的のために、成分、冷間圧延条件を検討した結果完成したものであり、冷間圧延でのマルテンサイト生成量を少なくするためには、成分バランスをオーステナイト安定系にすることがこのような目的に合致することを見出したものである。その要旨とするところは以下のとうりである。
【0009】
すなわち、本発明の目的は、下記(1)〜(5)に記載のステンレス鋼薄板、およびその製造方法、並びにこれを用いた用途であるところのボルト締結用座金により達成されるものである。
(1)質量%でC:0.02〜0.08%、Si:0.2〜1.0%、Mn:0.2〜2.5%、Cr:17.0〜22.0%、Ni:7.0〜9.0%、Nb:0.05〜0.15%、N:0.12〜0.30%を含有し、P:0.040%以下、S:0.030%以下であり、残部がFeおよび不可避的不純物よりなり、式(1)のMd30Nが−80℃以下、板厚が2〜6mm、透磁率が1.2以下、0.2%耐力が900N/mm2以上、引張強さが1100N/mm2以上である高強度低透磁率オーステナイト系ステンレス鋼板。
Md30N(℃)=551−462(C+N)−9.2Si−8.1Mn−13.7Cr−29(Ni+Cu)−18.5Mo−68Nb・・・式(1)
式(1)で、C、N、Si、Mn、Cr、Ni、Cu、Mo、Nbは、各成分含有量(質量%)を表す。
(2)さらにBを5〜40ppm含有する(1)に記載の高強度低透磁率オーステナイト系ステンレス鋼板。
(3)冷間圧延コイルを焼鈍および酸洗し、続いて圧延率が30〜40%の冷間圧延を行って板厚2〜6mmの鋼板を製造する上記高強度低透磁率オーステナイト系ステンレス鋼板の製造方法。
(4)熱間圧延コイルを焼鈍および酸洗し、圧延率が20%以下の第一冷間圧延し、続いてキズ取り研削し、続いてキズ取り研削後に第二冷間圧延を行い、第一冷間圧延と第二冷間圧延との合計の圧延率が30〜40%の冷間圧延を行って板厚2〜6mmの鋼板を製造する上記高強度低透磁率オーステナイト系ステンレス鋼板の製造方法。
(5)上記高強度低透磁率オーステナイト系ステンレス鋼板を用いてボルト締結用座金を成形後450〜550℃の温度域で窒化処理焼鈍を施す高強度低透磁率オーステナイト系ステンレスボルト締結用座金の製法方法。
【0010】
【発明の実施の形態】
本発明が対象とするオーステナイト系ステンレス鋼板(ハード材)の成分範囲は、オーステナイト安定度によって規制されるものである。
【0011】
条件式(1)は、オーステナイト安定度を示し、加工誘起マルテンサイト量に関係する。本発明のオーステナイト系ステンレス鋼は式(1)に示すMd30Nが−80℃以下の場合、冷間圧延後でも低透磁率を維持する。
Md30N(℃)=551−462(C+N)−9.2Si−8.1Mn−13.7Cr−29(Ni+Cu)−18.5Mo−68Nb ・・・式(1)
【0012】
条件式(1)のMd30Nの値が−80℃を越えると加工誘起マルテンサイト生成により透磁率が1.2を越え低透磁率を維持できないため、上限を−80℃とした。
【0013】
Md30Nに及ぼす成分の影響は式(1)の係数から判断され、侵入型元素のC,Nが極めて大きく、ついで置換型元素ではNb>Ni、Cu>Mo>Cr>Si>Mnの順である。添加量が多くなるほどMd30Nは低い値となる。
【0014】
本発明が対象とするステンレス鋼の各成分範囲の限定理由は次の通りである。
【0015】
C:Cはオーステナイト安定化元素であり、Md30Nを下げて冷間加工によるマルテンサイト変態を抑制する有効な元素である。また一方では冷間加工後の高強度化にも有効な元素でもあり強度確保に0.02質量%以上必要であるが、多すぎるとCr炭化物が析出し、耐食性を損なうため、上限を0.08質量%とした。好ましい範囲は、0.03〜0.06質量%である。
【0016】
N:Nも強力なオーステナイト安定化元素であり、Md30Nを下げて冷間加工によるマルテンサイト変態を有効に抑制する。同時に、侵入型の固溶強化およびNb(C,N)析出による結晶粒の微細化、析出強化作用を有し、Cとともにマトリックスの主要強化元素であり、耐食性改善にも寄与する元素である。これらの効果を得るために0.12質量%以上含有させる必要がある。しかしながら、0.30質量%を越えて多量に含有しすぎると熱間加工性を損なうので、0.30質量%以下とした。
【0017】
Nb:Nbは残存CをNbCとして固定し、耐食性を改善し、またNb(C,N)析出による結晶粒微細化および強度を改善する元素である。その効果を発揮するには0.05質量%以上の添加が必要であるので、下限を0.05質量%とした。一方、0.15質量%を越えると動的再結晶が遅れ、熱間加工性が低下するので、上限を0.15質量%とした。
【0018】
Ni:Niはオーステナイト系ステンレス鋼の基本成分の一つである。加工性、耐食性に有効な元素であり、7.0質量%以上添加する。しかしながら、9.0質量%を越えて添加してもこれらの効果は飽和に達するので、7.0〜9.0質量%の範囲とした。望ましくは7.5質量%以上8.5質量%以下である。
【0019】
Cr:Crはステンレス鋼の基本成分であり、優れた耐食性を与える。また窒素の固溶度を著しく高めて窒素添加によるマトリックスの強化に役立つ。これらの効果は17.0質量%以上で顕著になるので下限を17.0質量%とした。一方、22.0質量%を越えると加工性が劣化するので、上限を22.0質量%とした。好ましい範囲は18.0〜20.0質量%である。
【0020】
Si:Siは脱酸に0.2質量%以上必要である。しかし、1.0質量%を越えると加工性を損なうと同時に熱間圧延時のデスケーリングが困難になるため、上限を1.0質量%とした。好ましい範囲は、0.4〜0.9質量%である。
【0021】
Mn:Mnはオーステナイト安定化元素として有効であると同時に、Sを固定して熱間加工性を向上させるために添加される。しかしながら、含有量が0.2質量%に満たないとその効果に乏しく、一方2.5質量%を越えると熱間圧延時のデスケーリングが困難になるので、0.2〜2.5質量%の範囲とした。
【0022】
Cu、Mo:これら元素は請求項に規制はないが、ステンレス鋼の製造において不可避的に0.5質量%以下混入するものである。置換型元素としては、Md30Nに及ぼす影響が大きい元素である。
【0023】
P:Pは0.040質量%を越えて含有すると熱間加工性、耐食性が著しく劣化するので、0.040%質量以下とした。
【0024】
S:Sは、0.030質量%を越えて含有すると熱間加工性、耐食性が著しく劣化するので、0.030質量%以下とした。
【0025】
B:Bは、必要に応じて添加する。Nを多量に含有する場合の熱間加工性を向上させるための重要な元素であり、熱間加工温度条件に関わりなく効果を発揮する。Bは粒界偏析することでSやPなどの不純物元素の粒界偏析を抑制し粒界脆化を抑止するが、過剰に添加すると硼化物が生成し熱間加工性を劣化させる。その効果は5〜40ppmで顕著であるので、本発明においては、その含有量を5〜40ppmとした。
【0026】
本発明のオーステナイト系ステンレス鋼においては、板厚は2〜6mmである。ボルト締結用座金の強度部材としては、2mm以下では強度不足であり、6mm以上では座金への加工が困難になる。従って、板厚を2〜6mmとした。
【0027】
ボルト締結用座金の強度部材としての必要特性として、透磁率が1.2以下が必要である。電磁コイルへの磁性の影響を検討した結果、ステンレス鋼板の透磁率1.2超では磁性の影響が顕在化し、着磁性を帯びるようになる。また強度特性としては、0.2%耐力が900/mm2以上、引張強さが1100N/mm2以上が必要である。これ以下では、使用中にバネ特性の低下を招き、ボルト締結材としての機能を消失する。
【0028】
バネ特性で最も重要な特性としては疲労強度がある。疲労強度は引張強度との間にはほぼ正比例の関係がある。したがって、ボルト締結用座金の疲労強度を向上するためには、引張強度を向上することが有効な手段である。
【0029】
本発明に於ける高強度非磁性オーステナイト系ステンレス鋼板のハード材の製造方法は、以下に述べる冷間圧延コイルの冷間圧延法で製造してもよいし、熱間圧延コイルの冷間圧延法で製造してもよい。
【0030】
冷間圧延コイルの冷間圧延法の製造方法は以下のようである。まず、冷間圧延コイルを焼鈍および酸洗後、冷間圧延を実施して板厚2〜6mmの鋼板(ハード材)を製造する。ここで後から行う冷間圧延は30〜40%の冷間圧延率で実施する。比較的薄手の材料に適用される。
【0031】
熱間圧延コイルの冷間圧延法で製造する場合は以下のようである。熱間圧延コイルの焼鈍および酸洗後、第一冷間圧延を実施、続いてキズ取り研削の後さらに第二冷間圧延を実施して板厚2〜6mmの鋼板(ハード材)を製造する。熱間圧延コイルの焼鈍および酸洗後の第一冷間圧延の圧延率は20%以下で実施する。キズ取り研削のために熱間圧延コイルの形状を整え平滑化するためである。キズ取り研削の後の第二冷間圧延は2回の冷間圧延のトータル圧延率が30〜40%の冷間圧延率で実施する。本冷間圧延法は2回の冷間圧延の中間にキズ取り研削工程が入り、一旦冷間圧延を中断するプロセスである。特に板厚が2mm以上の厚手ハード材の場合、板厚を確保する必要上、冷間圧延コイルの焼鈍および酸洗後に30〜40%の冷間圧延率の冷間圧延を実施することができない場合があるので、熱間圧延コイルの焼鈍および酸洗の後に冷間圧延を実施してハード材を製造するものである。
【0032】
冷間圧延コイルの冷間圧延法および熱間圧延コイルの冷間圧延法ともに、焼鈍後に行う1回又は2回の冷間圧延のトータル圧延率は30〜40%である。本発明のオーステナイト系ステンレス鋼はハード材のため、焼鈍後冷間圧延を実施し高強度化する。30%未満では強度不足を生じ、40%超では加工誘起マルテンサイト生成により磁性を帯び低透磁率を維持できなくなるため、下限を30%、上限を40%とした。
【0033】
本発明の高強度低透磁率オーステナイト系ステンレス鋼板からなるボルト締結用座金は、ボルト締結用座金を成形後450〜550℃の温度域で窒化処理焼鈍を施す。この熱処理により、歪み時効現象を活用してステンレス鋼を高強度化する。450℃以下では歪み時効の効果がなく、550℃以上では、加工歪みが転位の再配列により消失していくので、高強度が維持できない。歪み時効現象を活用して高強度化すると、ボルト締結用座金の疲労強度が向上し、バネ特性が向上する。
【0034】
【実施例】
以下、実施例で本発明を具体的に説明する。
【0035】
(実施例1)
表1に示す化学組成を有する合金35kgを真空溶解炉で溶製・鋳造し、熱間圧延により板厚6mmの圧延板とし、再結晶を目的とした熱間圧延板焼鈍(1100℃で30秒)を実施し、ショットブラスト処理および酸洗により脱スケールし、冷間圧延を実施した。冷間圧延は、冷間圧延開始温度を10℃以下の一定温度に保持した後に実施した。合計冷間圧延率は10〜45%の範囲で変化させ、1回の冷間圧延、又は2回の冷間圧延(第一冷間圧延と第二冷間圧延)を実施した。前者は冷間圧延コイルの冷間圧延法、後者は熱間圧延コイルの冷間圧延法のシミュレーションに相当するものである。
【0036】
合計冷間圧延率35%の結果を表2に示す。その一部の例では、1回のみの冷間圧延を圧延率35%で行った(表2の冷間圧延−1)。他の例では、圧延率10%および20%の第一冷間圧延した(表2の冷間圧延−1)後、キズ取り研削を実施し、続いて再度10℃以下の一定温度に保持し、圧延率15、25%の第二冷間圧延(表2の冷間圧延−2)を実施し、合計圧延率35%とした。本発明例は、透磁率、0.2%耐力、引張強さだけでなく熱間加工性および耐食性ともに良好である。なお、合計冷間圧延率が10〜45%の結果のうち、合計冷間率35%以外のデータは表2に示していない。合計冷間圧延率が30〜40%では、表2の結果と同様の結果である。
【0037】
なお 評価試験は下記の方法で実施した。
【0038】
成分は熱間圧延板から試験片をサンプリングして成分分析を行った。
【0039】
Md30Nは化学成分から式(1)によって計算して表示した。
【0040】
板厚;目標製品板厚をmmで表示した。
【0041】
透磁率は冷間圧延仕上げ材(ハード材)を用いて、米国セバン社低透磁率(ローミュー)計で実施した。表中の○×は透磁率が1.2以下を〇、1.2超を×で表示した。
【0042】
0.2%耐力および引張強さは冷間圧延仕上げ材(ハード材)からJISZ 2201の13B号試験片を作成し、JIS Z 2241の試験方法でインストロン型引張試験機を用いて試験した。L方向(圧延方向に平行)のデータを測定した。表中の〇×は0.2%耐力、引張強さそれぞれ900、1100N/mm2以上を〇、900、1100N/mm2未満を×で示した。
【0043】
熱間加工性は鋳造したインゴット表層下5mm研削の後試験片平行部90mm、8mmΦの試験片を作成し、引張試験を実施して、絞り値で評価した。引張試験は1250℃で30秒保定の後、20℃/秒で冷却し、1000℃で30秒保定の後、2cm/秒の歪み速度で実施した。評価は破断後の絞り値を測定し60%以上を〇60%未満を×で表中に示した。
【0044】
耐食性の評価は冷間圧延仕上げ材(ハード材)を鋭敏化熱処理(1100℃×10min水冷→650℃×2hr.空冷)を施した後、顕微鏡観察用に加工後バフ研磨したサンプルに10%蓚酸溶液にてエッチングを施し、鋭敏化(粒界腐食)の発生有無を顕微鏡で判定した。粒界腐食なしの場合を○、発生した場合を×で示した。
【0045】
【表1】
【0046】
【表2】
【0047】
図1は透磁率に及ぼすMd30Nと冷間圧延率の関係である。本発明範囲のMd30Nが−80℃以下では、冷間圧延率40%以下で透磁率は1.2以下であり低透磁率であるが、45%で透磁率1.2を越えるようになる。本発明範囲外のMd30Nが−80℃超では、冷間圧延率30%未満の範囲でも透磁率が1.2以上になる。
【0048】
図2に透磁率とマルテンサイト量との関係を示す。マルテンサイト量の増加に伴い透磁率が上昇している。マルテンサイト量の測定はフェライトメーターで実施した。
【0049】
図3に合計冷間圧延率30%材の引張試験の結果を示す。窒素量増に伴い0.2%耐力、引張強さともに増加している。比較例のサンプル8、9および10は0.2%耐力と引張強さの少なくとも一つが本発明範囲を外れている。
【0050】
図4に冷間圧延率と透磁率、0.2%耐力および引張強さの関係を示す。図4に示す例は、表1のサンプル番号1の試料を用いた結果であり、Mn30Nは−81.6℃である。冷間圧延率25%以下では、0.2%耐力が900N/mm2以下、引張強さが1100N/mm2以下となり、冷間圧延率45%以上では透磁率が1.2以上となる。したがって、冷間圧延率が30〜40%の場合に、透磁率1.2以下と0.2%耐力900N/mm2以上、引張強さ1100N/mm2以上を同時に満たす。
【0051】
図5に透磁率に及ぼすMd30Nと冷間圧延率(合計冷延圧下率35%)の関係を示す。本発明範囲のMd30Nが−80℃以下では、1.2以下の低透磁率が達成されるが、本発明範囲外のMd30Nが−80℃超では透磁率が非常に高くなる。本発明例は、透磁率、0.2%耐力、引張強さだけでなく熱間加工性および耐食性ともに良好である。
【0052】
図6に本発明例及び比較例を透磁率と強度(0.2%耐力、引張強さ)のマップ上に示す。実施例1と2の結果を合わせて示す。Md30N≦−80℃かつ冷間圧延率30〜40%の本発明例(図中●)は、透磁率≦1.5、0.2%耐力≦900N/mm2、引張強さ≦1030N/mm2を実現している。一方、Md30N≦−80℃と冷間圧延率30〜40%の条件から外れる比較例(図中○)は、透磁率≦1.5、0.2%耐力≦900N/mm2、引張強さ≦1030N/mm2を達成することができなかった。
【0053】
(実施例2)
表3に示す化学組成を有する鋼を実機溶製し、熱間圧延により板厚6.2mmの圧延板としたのち、熱間圧延コイルの焼鈍および酸洗後、冷間圧延、キズ取り研削、冷間圧延、冷間圧延コイルの焼鈍および酸洗し、3.8mmの冷間圧延鋼板とし、さらに冷間圧延の製造工程を経て板厚2.1〜2.9mmのハード材を製造した。
【0054】
評価結果を表4に示す。本発明例では、1.2以下の低透磁率が達成され、他の特性も良好であるが、比較例では透磁率1.2以下と0.2%耐力900N/mm2以上、引張強さ1100N/mm2以上のいずれか一つが達成されない。
【0055】
【表3】
【0056】
【表4】
【0057】
(実施例3)
表3に示す化学組成を有する鋼を実機溶製し、熱間圧延により板厚6.2mmの圧延板としたのち、熱間圧延コイルの焼鈍および酸洗後、圧下率10%あるいは20%の第一冷間圧延、キズ取り研削、第二冷間圧延の製造工程を経て3.4〜4.7mmの厚手ハード材を製造した。
【0058】
評価結果を表5に示す。本発明例では、1.2以下の低透磁率が達成され、他の特性も良好であるが、比較例では透磁率1.2以下と0.2%耐力900N/mm2以上、引張強さ1100N/mm2以上のいずれか一つが達成されない。
【0059】
【表5】
【0060】
(実施例4)
実施例2および3のハード材を外形45mmφ×内径20mmφに打ち抜き加工してボルト締結用座金を成型後、バレル研磨の後、表6および7に示す窒化処理焼鈍を実施して座金を製造した。表6および7に評価結果を示す。本発明例の窒化処理焼鈍の座金の透磁率は1.2以下であり、ボルト座金特性も良好であった。ボルト座金特性は107回の疲労強度で評価した。550N/mm2以上を○、550N/mm2未満を×で示した。
【0061】
【表6】
【0062】
【表7】
【0063】
【発明の効果】
本発明の高窒素オーステナイト系ステンレス鋼は、板厚の大小にかかわらず低透磁率と高強度を同時に実現するものであり、電磁コイル等の磁性を嫌うボルトの座金、皿ばね等に使用可能な極めて優れたステンレス鋼であり、その工業的価値は著しく大なるものである。
【図面の簡単な説明】
【図1】冷間圧延実験(合計冷間圧延率10〜45%)におけるMd30Nと透磁率との関係を示す図。
【図2】透磁率とマルテンサイト量との関係を示す図。
【図3】N量と冷間圧延率30%鋼板の0.2%耐力と引張強さの関係を示す図。
【図4】冷間圧延率と透磁率、0.2%耐力および引張強さの関係を示す図。
【図5】冷間圧延実験(合計冷間圧延率35%)における透磁率とMd30Nの関係を示す図。
【図6】0.2%耐力および引張強さと透磁率の関係を示す図。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-strength, low-permeability austenitic stainless steel and a washer for bolt fastening using the same. More particularly, the present invention relates to an industrially manufacturable high-strength low-permeability austenitic stainless steel sheet used for a washer for fastening an electromagnetic coil used in a linear motor car or the like, and a washer processed and manufactured therefrom. It is about.
[0002]
[Prior art]
A high-nitrogen, high-strength austenitic stainless steel SUS304N is often used as a washer material used for fastening bolts and screws. The use of SUS304N is also being studied for fastening washers and disc springs for electromagnetic coils used for linear motor cars. Since the coil undergoes thermal expansion and vibrations, it is necessary to fasten the coil with an appropriate axial force so that the coil is not loosened.
[0003]
Furthermore, since the fastening material used for the electromagnetic coil dislikes magnetism, it is indispensable for its function to have a low magnetic permeability.
[0004]
Prior art relating to high-strength and non-magnetic austenitic stainless steel includes Patent Document 1, Patent Document 2,
[0005]
[Patent Document 1]
JP-A-62-294130 [Patent Document 2]
JP-A-63-38558 [Patent Document 3]
JP 03-17245 A [Patent Document 4]
Japanese Patent Application Laid-Open No. 08-269639
[Problems to be solved by the invention]
Since 18Cr-8Ni steel-based nitrogen-added stainless steel is required to have high strength, it is used as a product without annealing while being subjected to cold rolling after heat treatment, and is generally called a hard material. In austenitic stainless steel such as SUS304, while increasing the cold-rolling ratio increases the strength, on the other hand, cold-rolling forms work-induced martensite (the martensite is a crystalline structure having magnetism), so it is magnetic. Become like Since the magnetic permeability increases in accordance with the amount of martensite, high strength and low magnetic permeability are in a contradictory relationship, and it is difficult to achieve both.
[0007]
Accordingly, an object of the present invention is to solve such a problem and to simultaneously provide a low magnetic permeability and a high strength suitable for a fastening washer for an electromagnetic coil, a disc spring, and the like, and a high nitrogen austenitic stainless steel based on 18Cr-8Ni steel. The present invention provides steel and a washer made of the same.
[0008]
[Means for Solving the Problems]
The present invention has been completed for this purpose as a result of examining the components and cold rolling conditions.In order to reduce the amount of martensite generated in cold rolling, the component balance is changed to an austenitic stable system. Has been found to meet such a purpose. The summary is as follows.
[0009]
That is, the object of the present invention is achieved by a stainless steel thin plate described in the following (1) to (5), a method for producing the same, and a washer for bolt fastening as an application using the same.
(1) In mass%, C: 0.02 to 0.08%, Si: 0.2 to 1.0%, Mn: 0.2 to 2.5%, Cr: 17.0 to 22.0%, Ni: 7.0 to 9.0%, Nb: 0.05 to 0.15%, N: 0.12 to 0.30%, P: 0.040% or less, S: 0.030% The balance is Fe and unavoidable impurities, Md30N of the formula (1) is -80 ° C or less, the plate thickness is 2 to 6 mm, the magnetic permeability is 1.2 or less, and the 0.2% proof stress is 900 N / mm. A high-strength, low-permeability austenitic stainless steel sheet having a tensile strength of 2 or more and a tensile strength of 1100 N / mm 2 or more.
Md30N (° C) = 551-462 (C + N) -9.2Si-8.1Mn-13.7Cr-29 (Ni + Cu) -18.5Mo-68Nb Formula (1)
In the formula (1), C, N, Si, Mn, Cr, Ni, Cu, Mo, and Nb represent respective component contents (% by mass).
(2) The high-strength, low-permeability austenitic stainless steel sheet according to (1), further containing 5 to 40 ppm of B.
(3) The above-mentioned high-strength low-permeability austenitic stainless steel sheet in which a cold-rolled coil is annealed and pickled, and then cold-rolled at a rolling ratio of 30 to 40% to produce a steel plate having a thickness of 2 to 6 mm. Manufacturing method.
(4) The hot-rolled coil is annealed and pickled, first cold-rolled at a rolling ratio of 20% or less, subsequently flaw-removed and ground, and subsequently flaw-removed and then subjected to second cold rolling. Manufacture of the high-strength low-magnetic-permeability austenitic stainless steel sheet in which a cold rolling of a total of 30-40% of the first cold rolling and the second cold rolling is performed to produce a steel sheet having a thickness of 2 to 6 mm. Method.
(5) A method of manufacturing a high-strength low-magnetic-permeability austenitic stainless steel bolt-fastening washer in which after forming a bolt-fastening washer using the above-mentioned high-strength low-magnetic-permeability austenitic stainless steel plate, nitriding treatment is performed in a temperature range of 450 to 550 ° C. Method.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
The component range of the austenitic stainless steel sheet (hard material) targeted by the present invention is regulated by austenite stability.
[0011]
Conditional expression (1) indicates austenite stability and relates to the amount of work-induced martensite. When the Md30N shown in the formula (1) is equal to or lower than -80 ° C, the austenitic stainless steel of the present invention maintains a low magnetic permeability even after cold rolling.
Md30N (° C.) = 551-462 (C + N) -9.2Si-8.1Mn-13.7Cr-29 (Ni + Cu) -18.5Mo-68Nb Formula (1)
[0012]
If the value of Md30N in the conditional expression (1) exceeds -80 ° C, the magnetic permeability exceeds 1.2 due to the formation of work-induced martensite, and a low magnetic permeability cannot be maintained.
[0013]
The influence of the component on Md30N is determined from the coefficient of the equation (1), and C and N of the interstitial elements are extremely large, and then Nb> Ni, Cu>Mo>Cr>Si> Mn in the case of the substitution elements. . As the amount of addition increases, Md30N becomes a lower value.
[0014]
The reasons for limiting the range of each component of the stainless steel targeted by the present invention are as follows.
[0015]
C: C is an austenite stabilizing element, and is an effective element that lowers Md30N and suppresses martensitic transformation due to cold working. On the other hand, it is also an element effective for increasing the strength after cold working and is required to secure 0.02% by mass or more for securing the strength. However, if it is too large, Cr carbide precipitates and the corrosion resistance is impaired. 08 mass%. A preferred range is from 0.03 to 0.06% by mass.
[0016]
N: N is also a strong austenite stabilizing element and lowers Md30N to effectively suppress martensitic transformation by cold working. At the same time, it has an effect of interstitial solid solution strengthening and refinement of crystal grains and precipitation strengthening by Nb (C, N) precipitation, and is an element that is a main strengthening element of the matrix together with C and also contributes to improvement of corrosion resistance. In order to obtain these effects, it is necessary to contain 0.12% by mass or more. However, if the content exceeds 0.30% by mass, the hot workability is impaired. Therefore, the content is set to 0.30% by mass or less.
[0017]
Nb: Nb is an element that fixes residual C as NbC, improves corrosion resistance, and refines crystal grains and improves strength by Nb (C, N) precipitation. Since 0.05% by mass or more must be added to exhibit the effect, the lower limit is set to 0.05% by mass. On the other hand, if it exceeds 0.15% by mass, the dynamic recrystallization is delayed and the hot workability decreases, so the upper limit was made 0.15% by mass.
[0018]
Ni: Ni is one of the basic components of the austenitic stainless steel. It is an element effective for workability and corrosion resistance, and is added in an amount of 7.0% by mass or more. However, these effects reach saturation even if added in excess of 9.0% by mass, so that the range was 7.0 to 9.0% by mass. Desirably, the content is 7.5% by mass or more and 8.5% by mass or less.
[0019]
Cr: Cr is a basic component of stainless steel and provides excellent corrosion resistance. In addition, the solid solubility of nitrogen is remarkably increased to help strengthen the matrix by adding nitrogen. Since these effects become remarkable at 17.0% by mass or more, the lower limit is set to 17.0% by mass. On the other hand, if it exceeds 22.0% by mass, the workability deteriorates, so the upper limit was set to 22.0% by mass. A preferred range is 18.0 to 20.0% by mass.
[0020]
Si: Si needs to be 0.2% by mass or more for deoxidation. However, if the content exceeds 1.0% by mass, workability is impaired, and at the same time, descaling during hot rolling becomes difficult. Therefore, the upper limit is set to 1.0% by mass. A preferred range is 0.4 to 0.9% by mass.
[0021]
Mn: Mn is effective as an austenite stabilizing element, and is added to fix S and improve hot workability. However, when the content is less than 0.2% by mass, the effect is poor. On the other hand, when the content exceeds 2.5% by mass, descaling during hot rolling becomes difficult. Range.
[0022]
Cu, Mo: These elements are not restricted in the claims, but are inevitably mixed in 0.5% by mass or less in the production of stainless steel. The substitutional element is an element having a large effect on Md30N.
[0023]
P: If P is contained in excess of 0.040% by mass, hot workability and corrosion resistance are significantly deteriorated.
[0024]
S: If S is contained in excess of 0.030% by mass, hot workability and corrosion resistance are significantly deteriorated.
[0025]
B: B is added as needed. It is an important element for improving hot workability when a large amount of N is contained, and exerts an effect irrespective of hot working temperature conditions. B suppresses grain boundary segregation of impurity elements such as S and P by suppressing grain boundary segregation and suppresses grain boundary embrittlement. However, if added excessively, boride is generated to deteriorate hot workability. Since the effect is remarkable at 5 to 40 ppm, the content is set to 5 to 40 ppm in the present invention.
[0026]
In the austenitic stainless steel of the present invention, the plate thickness is 2 to 6 mm. If the strength member of the bolt fastening washer is less than 2 mm, the strength is insufficient, and if it is more than 6 mm, processing into the washer becomes difficult. Therefore, the plate thickness was set to 2 to 6 mm.
[0027]
As a necessary characteristic of the bolt fastening washer as a strength member, a magnetic permeability of 1.2 or less is required. As a result of examining the effect of magnetism on the electromagnetic coil, when the magnetic permeability of the stainless steel sheet exceeds 1.2, the effect of magnetism becomes apparent, and the stainless steel sheet becomes magnetized. Further, as the strength characteristics, a 0.2% proof stress of 900 / mm 2 or more and a tensile strength of 1100 N / mm 2 or more are required. Below this, the spring characteristics are reduced during use, and the function as a bolt fastening material is lost.
[0028]
The most important spring property is fatigue strength. Fatigue strength is almost directly proportional to tensile strength. Therefore, in order to improve the fatigue strength of the bolt fastening washer, it is effective to improve the tensile strength.
[0029]
The method for producing a hard material of a high-strength nonmagnetic austenitic stainless steel sheet according to the present invention may be produced by a cold rolling method of a cold-rolled coil described below, or a cold rolling method of a hot-rolled coil. May be manufactured.
[0030]
The manufacturing method of the cold rolling method of the cold rolled coil is as follows. First, a cold-rolled coil is annealed and pickled, and then cold-rolled to produce a steel plate (hard material) having a thickness of 2 to 6 mm. Here, the cold rolling performed later is performed at a cold rolling reduction of 30 to 40%. Applies to relatively thin materials.
[0031]
The case where the hot-rolled coil is manufactured by the cold rolling method is as follows. After annealing and pickling of the hot-rolled coil, the first cold rolling is performed, followed by flaw removal grinding and then the second cold rolling to produce a steel plate (hard material) having a thickness of 2 to 6 mm. . The first cold rolling after the annealing and pickling of the hot-rolled coil is performed at a rolling reduction of 20% or less. This is because the shape of the hot-rolled coil is adjusted and smoothed for flaw removal grinding. The second cold rolling after flaw removal grinding is performed at a cold rolling rate of a total rolling rate of two cold rollings of 30 to 40%. This cold rolling method is a process in which a flaw removal grinding step is inserted in the middle of two cold rollings, and the cold rolling is temporarily interrupted. In particular, in the case of a thick hard material having a thickness of 2 mm or more, it is not possible to perform cold rolling at a cold rolling reduction of 30 to 40% after annealing and pickling of the cold-rolled coil because it is necessary to secure the thickness. In some cases, a hard material is manufactured by performing cold rolling after annealing and pickling of the hot-rolled coil.
[0032]
In both the cold rolling method of the cold rolling coil and the cold rolling method of the hot rolling coil, the total rolling reduction of one or two cold rollings performed after annealing is 30 to 40%. Since the austenitic stainless steel of the present invention is a hard material, it is subjected to cold rolling after annealing to increase the strength. If it is less than 30%, the strength will be insufficient, and if it exceeds 40%, it will become magnetic due to the formation of work-induced martensite and it will not be possible to maintain a low magnetic permeability.
[0033]
The bolt fastening washer made of the high-strength, low-permeability austenitic stainless steel sheet of the present invention is subjected to nitriding annealing in a temperature range of 450 to 550 ° C. after forming the bolt fastening washer. By this heat treatment, the strength of the stainless steel is increased by utilizing the strain aging phenomenon. At 450 ° C. or lower, there is no effect of strain aging, and at 550 ° C. or higher, high strength cannot be maintained because the processing strain disappears due to rearrangement of dislocations. When the strength is increased by utilizing the strain aging phenomenon, the fatigue strength of the bolt fastening washer is improved, and the spring characteristics are improved.
[0034]
【Example】
Hereinafter, the present invention will be described specifically with reference to Examples.
[0035]
(Example 1)
35 kg of an alloy having the chemical composition shown in Table 1 was melted and cast in a vacuum melting furnace, hot-rolled into a rolled plate having a thickness of 6 mm, and hot-rolled plate annealing for recrystallization (1100 ° C. for 30 seconds) ), Descaling by shot blasting and pickling, and cold rolling. The cold rolling was performed after maintaining the cold rolling start temperature at a constant temperature of 10 ° C. or less. The total cold rolling rate was changed in the range of 10 to 45%, and one cold rolling or two cold rollings (first cold rolling and second cold rolling) were performed. The former corresponds to a simulation of a cold rolling method of a cold-rolled coil, and the latter corresponds to a simulation of a cold rolling method of a hot-rolled coil.
[0036]
Table 2 shows the results of the total cold rolling reduction of 35%. In some of the examples, only one cold rolling was performed at a rolling reduction of 35% (Cold rolling-1 in Table 2). In another example, after the first cold rolling at a rolling reduction of 10% and 20% (cold rolling-1 in Table 2), flaw removal grinding was performed, and then the temperature was again maintained at a constant temperature of 10 ° C or less. The second cold rolling (cold rolling-2 in Table 2) was performed at a rolling rate of 15 and 25% to obtain a total rolling rate of 35%. The examples of the present invention have good hot workability and corrosion resistance as well as magnetic permeability, 0.2% proof stress, and tensile strength. Table 2 does not show data other than the result of the total cold rolling reduction of 35% among the results of the total cold rolling reduction of 10 to 45%. When the total cold rolling reduction is 30 to 40%, the results are the same as the results in Table 2.
[0037]
The evaluation test was performed by the following method.
[0038]
The components were analyzed by sampling a test piece from a hot-rolled plate.
[0039]
Md30N was calculated from the chemical components according to equation (1) and displayed.
[0040]
Plate thickness: The target product plate thickness was expressed in mm.
[0041]
The magnetic permeability was measured using a cold-rolled finished material (hard material) with a low magnetic permeability (Lo-mu) meter from Severn, USA. In the table, × indicates that the magnetic permeability is 1.2 or less, and Δ indicates that the magnetic permeability exceeds 1.2.
[0042]
For the 0.2% proof stress and tensile strength, a JISZ2201 No. 13B test piece was prepared from a cold-rolled finished material (hard material) and tested using an Instron-type tensile tester according to the test method of JISZ2241. Data in the L direction (parallel to the rolling direction) was measured. In the table, 〇 indicates 0.2% proof stress and tensile strength of 900 and 1100 N / mm 2 or more, and 〇 indicates 900% and less than 1100 N / mm 2 , respectively.
[0043]
The hot workability was evaluated by forming a test piece having a parallel portion of 90 mm and a diameter of 8 mm after grinding 5 mm below the surface layer of the cast ingot, performing a tensile test, and evaluating the reduction value. The tensile test was held at 1250 ° C. for 30 seconds, cooled at 20 ° C./second, held at 1000 ° C. for 30 seconds, and performed at a strain rate of 2 cm / second. For evaluation, the aperture value after breaking was measured, and 60% or more and Δ less than 60% were indicated in the table by x.
[0044]
The corrosion resistance was evaluated by subjecting a cold-rolled finished material (hard material) to a sensitizing heat treatment (1100 ° C. × 10 min water cooling → 650 ° C. × 2 hr. Air cooling), followed by processing for microscopy and buffing the sample to 10% oxalic acid. Etching was performed with the solution, and the occurrence of sensitization (intergranular corrosion) was determined with a microscope. The case where there was no intergranular corrosion was indicated by 、, and the case where it occurred was indicated by x.
[0045]
[Table 1]
[0046]
[Table 2]
[0047]
FIG. 1 shows the relationship between Md30N and the cold rolling rate that affect the magnetic permeability. When Md30N in the range of the present invention is −80 ° C. or lower, the permeability is 1.2 or less at a cold rolling rate of 40% or less, which is a low magnetic permeability. However, the magnetic permeability exceeds 1.2 at 45%. When Md30N outside the range of the present invention exceeds -80 ° C, the magnetic permeability becomes 1.2 or more even in a range where the cold rolling reduction is less than 30%.
[0048]
FIG. 2 shows the relationship between the magnetic permeability and the amount of martensite. The magnetic permeability increases with an increase in the amount of martensite. The amount of martensite was measured with a ferrite meter.
[0049]
FIG. 3 shows the results of a tensile test of a 30% total cold-rolled material. Both the 0.2% proof stress and the tensile strength increased as the amount of nitrogen increased.
[0050]
FIG. 4 shows the relationship between the cold rolling ratio and the magnetic permeability, the 0.2% proof stress, and the tensile strength. The example shown in FIG. 4 is a result using the sample of sample number 1 in Table 1, and Mn30N is −81.6 ° C. When the cold rolling reduction is 25% or less, the 0.2% proof stress is 900 N / mm 2 or less, and the tensile strength is 1100 N / mm 2 or less, and when the cold rolling reduction is 45% or more, the magnetic permeability is 1.2 or more. Therefore, when the cold rolling ratio is 30 to 40%, the magnetic permeability is 1.2 or less, the 0.2% proof stress is 900 N / mm 2 or more, and the tensile strength is 1100 N / mm 2 or more.
[0051]
FIG. 5 shows the relationship between Md30N and the cold rolling reduction (total cold rolling reduction of 35%) affecting the magnetic permeability. When Md30N in the range of the present invention is -80 ° C or lower, a low magnetic permeability of 1.2 or less is achieved, but when Md30N outside the range of the present invention exceeds -80 ° C, the magnetic permeability is extremely high. The examples of the present invention have good hot workability and corrosion resistance as well as magnetic permeability, 0.2% proof stress, and tensile strength.
[0052]
FIG. 6 shows the magnetic permeability and strength (0.2% proof stress, tensile strength) of the present invention example and the comparative example on a map. The results of Examples 1 and 2 are shown together. Examples of the present invention (Md30N ≦ −80 ° C. and
[0053]
(Example 2)
A steel having the chemical composition shown in Table 3 was melted in a real machine, and hot-rolled into a rolled sheet having a thickness of 6.2 mm. Then, after annealing and pickling of a hot-rolled coil, cold rolling, flaw removal grinding, Cold rolling, annealing of the cold-rolled coil and pickling were performed to obtain a 3.8 mm cold-rolled steel sheet, and further, through a cold rolling manufacturing process, a hard material having a sheet thickness of 2.1 to 2.9 mm was manufactured.
[0054]
Table 4 shows the evaluation results. In the example of the present invention, a low magnetic permeability of 1.2 or less is achieved and other characteristics are also good. However, in the comparative example, the magnetic permeability is 1.2 or less, the 0.2% proof stress 900 N / mm 2 or more, and the tensile strength. Any one of 1100 N / mm 2 or more is not achieved.
[0055]
[Table 3]
[0056]
[Table 4]
[0057]
(Example 3)
A steel having the chemical composition shown in Table 3 was melted in an actual machine and hot-rolled into a 6.2 mm-thick rolled sheet. After annealing and pickling of the hot-rolled coil, the rolling reduction was 10% or 20%. A thick hard material having a thickness of 3.4 to 4.7 mm was manufactured through manufacturing processes of first cold rolling, flaw removal grinding, and second cold rolling.
[0058]
Table 5 shows the evaluation results. In the present invention embodiment are achieved 1.2 or less low permeability, although other properties are also satisfactory, in the comparative example permeability than 1.2 and 0.2% proof stress 900 N / mm 2 or more, the tensile strength Any one of 1100 N / mm 2 or more is not achieved.
[0059]
[Table 5]
[0060]
(Example 4)
The hard materials of Examples 2 and 3 were punched into an outer diameter of 45 mmφ and an inner diameter of 20 mmφ to form washers for bolt fastening, and after barrel polishing, nitriding treatment shown in Tables 6 and 7 was performed to manufacture washers. Tables 6 and 7 show the evaluation results. The magnetic permeability of the washer after nitriding treatment according to the present invention example was 1.2 or less, and the bolt washer characteristics were also good. Bolt washer properties were evaluated in the fatigue strength of 10 7 times. ○: 550 N / mm 2 or more is indicated by X, and less than 550 N / mm 2 is indicated by X.
[0061]
[Table 6]
[0062]
[Table 7]
[0063]
【The invention's effect】
The high-nitrogen austenitic stainless steel of the present invention achieves low magnetic permeability and high strength simultaneously regardless of the thickness of the plate, and can be used for a washer of a bolt that dislikes magnetism such as an electromagnetic coil, a disc spring, and the like. It is a very good stainless steel and its industrial value is remarkably great.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between Md30N and magnetic permeability in a cold rolling experiment (total cold rolling reduction: 10 to 45%).
FIG. 2 is a diagram showing a relationship between magnetic permeability and martensite amount.
FIG. 3 is a graph showing the relationship between N content, 0.2% proof stress of a 30% cold-rolled steel sheet, and tensile strength.
FIG. 4 is a diagram showing a relationship between a cold rolling ratio and a magnetic permeability, a 0.2% proof stress, and a tensile strength.
FIG. 5 is a diagram showing a relationship between magnetic permeability and Md30N in a cold rolling experiment (total cold rolling reduction: 35%).
FIG. 6 is a graph showing the relationship between 0.2% proof stress and tensile strength and magnetic permeability.
Claims (5)
C:0.02〜0.08%、
Si:0.2〜1.0%、
Mn:0.2〜2.5%、
Cr:17.0〜22.0%、
Ni:7.0〜9.0%、
Nb:0.05〜0.15%、
N:0.12〜0.30%を含有し、
P:0.040%以下、
S:0.030%以下であり、
残部がFeおよび不可避的不純物よりなり、式(1)のMd30Nが−80℃以下、板厚が2〜6mm、透磁率が1.2以下、0.2%耐力が900N/mm2以上、引張強さが1100N/mm2以上であることを特徴とする高強度低透磁率オーステナイト系ステンレス鋼板。
Md30N(℃)=551−462(C+N)−9.2Si−8.1Mn−13.7Cr−29(Ni+Cu)−18.5Mo−68Nb ・・・式(1)In mass%,
C: 0.02 to 0.08%,
Si: 0.2-1.0%,
Mn: 0.2-2.5%,
Cr: 17.0 to 22.0%,
Ni: 7.0 to 9.0%,
Nb: 0.05-0.15%,
N: 0.12-0.30%,
P: 0.040% or less,
S: 0.030% or less,
The balance consists of Fe and unavoidable impurities, Md30N of the formula (1) is −80 ° C. or less, plate thickness is 2 to 6 mm, magnetic permeability is 1.2 or less, 0.2% proof stress is 900 N / mm 2 or more, tensile A high-strength, low-permeability austenitic stainless steel sheet having a strength of 1100 N / mm 2 or more.
Md30N (° C.) = 551-462 (C + N) -9.2Si-8.1Mn-13.7Cr-29 (Ni + Cu) -18.5Mo-68Nb Formula (1)
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