JP2004083972A

JP2004083972A - Ferritic stainless steel cold rolled, annealed material having excellent secondary workability, and production method therefor

Info

Publication number: JP2004083972A
Application number: JP2002245082A
Authority: JP
Inventors: Yasutoshi Hideshima; 秀嶋　保利; Hiroki Tomimura; 冨村　宏紀; Naoto Hiramatsu; 平松　直人
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 2002-08-26
Filing date: 2002-08-26
Publication date: 2004-03-18
Anticipated expiration: 2022-08-26
Also published as: JP3886864B2

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel cold rolled, annealed material in which secondary working embrittlement is suppressed even when subjected to secondary working under severe conditions, and which can be worked into a satisfactory product shape. <P>SOLUTION: The ferritic stainless steel cold rolled, annealed material comprises ≤0.015% C, ≤0.5% Si, ≤0.5% Mn, ≤0.050% P, ≤0.01% S, 10.0 to 23.0% Cr, ≤0.10% Al, ≤0.020% N, 0.10 to 0.25% Ti, 0.15 to 0.35% Nb and 0.0005 to 0.0035% B, and in which the mass ratio of Nb/Ti is ≥0.9, and the minimum Lankford value r<SB>min</SB>in all the directions is ≥1.8. The ferritic stainless steel cold rolled, annealed material is produced by subjecting a hot rolled steel strip to annealing so as to be heated in the temperature range of 700 to 950°C for ≤1 hr, subjecting the hot rolled steel strip to intermediate cold rolling, thereafter performing process annealing so as to be heated in the temperature range of (a recrystallization temperature-100°C) to a recrystallization completion temperature for ≤1 min, and performing finish cold rolling at a rolling ratio of ≥80%. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【産業上の利用分野】
本発明は、過酷な加工条件下で製品形状に成形される耐二次加工脆性に優れたフェライト系ステンレス鋼冷延焼鈍材及びその製造方法に関する。
【０００２】
【従来技術及び問題点】
フェライト系ステンレス鋼は、良好な耐食性を呈し、高価なＮｉを必要としないことからオーステナイト系ステンレス鋼に比較して経済的に有利である。このような特徴を活用し、各種耐久消費財を始めとして広範な分野で使用されている。用途の多様化，機器の高性能化等に応じてステンレス鋼板をプレス成形加工する際の加工条件が過酷になってきており、従来よりも一段と加工性に優れたフェライト系ステンレス鋼鋼板の要求が強くなってきている。
【０００３】
フェライト系ステンレス鋼に含まれるＣ，Ｎ量を低減し、Ｔｉ，Ｎｂ等の炭窒化物形成元素を添加すると、フェライト系ステンレス鋼鋼板の成形性が向上する。たとえば、ＪＩＳ　Ｇ４３０５は、Ｃｒ：１６．００〜１８．００質量％，Ｃ：０．０３質量％以下，Ｓｉ：０．７５質量％以下，Ｍｎ：１．００質量％以下，Ｐ：０．０４０質量％以下，Ｓ：０．３０質量％以下，Ｔｉ又はＮｂ：０．１０〜１．００質量％を含むフェライト系ステンレス鋼ＳＵＳ４３０ＬＸを規定している。しかし、Ｃ，Ｎ量を低減し、Ｔｉ，Ｎｂ等の炭窒化物形成元素を添加すると、プレス加工後に二次加工脆化に起因する縦割れが発生しやすくなる。
【０００４】
Ｔｉ，Ｎｂ添加高純度フェライト系ステンレス鋼鋼板の耐二次加工脆性を改善するため、Ｔｉ，Ｂの併用添加（特公平２−７３９１号公報）が知られているが、Ｂの過剰添加に伴い伸びやランクフォード値が低下する弊害が顕在化し、異方性，伸び等を含めた総合的な材質設計が未解決のままである。Ｂｅ添加（特開平８−２６０１０８号公報）も知られているが、高価なＢｅを消費するため鋼材コストが上昇する。Ｍｇ添加により耐二次加工脆性を改善する（特開２００１−３１４４号公報）ことも検討されているが、Ｍｇ添加で低下する清浄度に起因する他の問題が派生しやすい。
【０００５】
また、従来では高々２．３程度の絞り比による試験で耐二次加工脆性を調査しているに過ぎない。しかし、最近のプレス加工の進歩に対応し、多段絞りによる絞り比が５を超える超深絞りも採用され始めている。プレス加工中又はプレス後に十分な耐二次加工脆性をもつフェライト系ステンレス鋼を従来技術では製造できないことから、縦割れの発生がなく過酷なプレス加工に耐える材質を得るため、従来よりも耐二次加工脆性が格段に優れたフェライト系ステンレス鋼鋼板が要求されている。
【０００６】
【課題を解決するための手段】
本発明は、このような要求に応えるべく案出されたものであり、合金設計に加えて素材段階での加工性を規制することにより、プレス加工中及びプレス加工後に十分な耐二次加工脆性が確保され、過酷な二次加工を受けても加工割れの発生がなく良好な製品形状に加工されるフェライト系ステンレス鋼冷延焼鈍材を提供することを目的とする。
【０００７】
本発明のフェライト系ステンレス鋼冷延焼鈍材は、その目的を達成するため、Ｃ：０．０１５質量％以下，Ｓｉ：０．５質量％以下，Ｍｎ：０．５質量％以下，Ｐ：０．０５０質量％以下，Ｓ：０．０１質量％以下，Ｃｒ：１０．０〜２３．０質量％，Ａｌ：０．１０質量％以下，Ｎ：０．０２０質量％以下，Ｔｉ：０．１０〜０．２５質量％，Ｎｂ：０．１５〜０．３５質量％，Ｂ：０．０００５〜０．００３５質量％を含み、残部が実質的にＦｅの組成をもち、Ｎｂ／Ｔｉの質量比が０．９以上で、面内全方向に沿って測定したランクフォード値ｒの最小値ｒ_ｍｉｎが１．８以上であることを特徴とする。
【０００８】
フェライト系ステンレス鋼は、更にＮｉ：０．５質量％以下，Ｍｏ：３．０質量％以下，Ｃｕ：０．３質量％以下，Ｖ：０．３質量％以下，Ｚｒ：０．３質量％以下の１種又は２種以上を含むことができる。この場合、Ｎｂ／Ｔｉの質量比を１．０以上にすることが好ましい。
このフェライト系ステンレス鋼冷延焼鈍材は、所定組成をもつステンレス鋼スラブを熱延鋼帯とし、７００〜９５０℃の温度域で１時間以下加熱する焼鈍を熱延鋼帯に施し、中間冷間圧延後に（再結晶温度−１００℃）〜再結晶完了温度の温度域に１分以下加熱する中間焼鈍を施し、圧延率８０％以上で仕上げ冷間圧延することにより製造される。
【０００９】
【作用】
絞り比が比較的小さい従来の二次加工性の評価条件では加工割れ等の欠陥が発生しないとされる材料であっても、絞り比の増加に応じて二次加工割れ等の加工欠陥が発生する。そこで、本発明者等は、フェライト系ステンレス鋼冷延焼鈍材を二次加工した際に生じる加工欠陥を成分・組成，素材特性等、種々の面から検討した。その結果、冷延焼鈍材の面内全方向に沿って測定したランクフォード値ｒの最小値ｒ_ｍｉｎを１．８以上に規制すると共に、特定組成の合金設計を採用するとき、フェライト系ステンレス鋼冷延焼鈍材の耐二次加工脆性が改善されることを見出した。
【００１０】
最小ランクフォード値ｒ_ｍｉｎが二次加工脆化に及ぼす影響は、次のように推察される。
圧延方向から圧延方向に直交する方向までの９０度の範囲にわたる各方向に沿って測定したランクフォード値ｒは、当該方向に関するフェライト系ステンレス鋼冷延焼鈍材の加工性を示す。フェライト系ステンレス鋼冷延焼鈍材は、通常、圧延方向，直交方向に沿って加工性が良いが、斜め方向に沿った加工性は低くなっている。加工性が面内方向に関して変動すると、絞り比の大きな深絞り加工等では材料の流動が面内方向にばらつき、加工歪みが特定方向に蓄積される。歪の蓄積量が閾値を超えると、当該特定方向に関して二次加工脆化、ひいては加工割れが発生する。
【００１１】
面内方向で異なる加工性が二次加工脆化に与える悪影響は、全方向に沿って測定したランクフォード値ｒの最小値ｒ_ｍｉｎを大きくすることにより抑制できる。最小ランクフォード値ｒ_ｍｉｎが大きくなると、面内方向に加工性が多少変動しても、加工性に劣る方向と加工性の良好な方向との間に生じる材料の流動変動が加工性の良好な方向に沿った塑性流動で吸収される。その結果、特定方向に関する加工歪みが二次加工脆化を引き起こすまで蓄積されない。したがって、加工性に劣る方向に関しても二次加工割れ等の加工欠陥が生じることなく、目標形状への二次加工が可能となる。
【００１２】
本発明者等による調査・研究の結果、最小ランクフォード値ｒ_ｍｉｎを１．８以上に調整することにより、加工性の面内異方性が耐二次加工脆性に悪影響を及ぼさない程度に抑えられることが判った。後述の実施例で具体的に示されているように、最小ランクフォード値ｒ_ｍｉｎ≧１．８を満足させることによって初めて過酷な条件下で二次加工された鋼材に発生しがちな二次加工割れを防止できる。これに対し、同じ成分・組成をもつフェライト系ステンレス鋼冷延焼鈍材であっても、最小ランクフォード値ｒ_ｍｉｎ≧１．８が満足されないと二次加工割れが発生しがちになった。
【００１３】
最小ランクフォード値ｒ_ｍｉｎはステンレス鋼板のステンレス鋼板の集合組織に依存しており、ｒ_ｍｉｎ≧１．８と最小ランクフォード値ｒ_ｍｉｎが高いステンレス鋼板の集合組織は異方性を大きくする。すなわち、最小ランクフォード値ｒ_ｍｉｎを相対的に下げる方位である［２１１］等の集合組織の発達が小さく、全方位にわたってランクフォード値ｒを向上させる［１１１］集合組織が高度に発達している。このような集合組織を形成するためには、低温の熱延板焼鈍によってＴｉ，Ｎｂの炭化物，窒化物やＮｂのラーベス相及びこれらの複合物を析出させることが好適である。中間焼鈍時にピンニング作用を呈する析出物で［２１１］等の特定の結晶方位をもつ再結晶の成長を抑制し、仕上げ圧延率を十分に配分することによって目標とする集合組織が作り込まれる。
【００１４】
次いで、本発明で採用した合金設計を説明する。
Ｃ：０．０１５質量％以下
炭化物を形成する合金成分であり、最小ランクフォード値ｒ_ｍｉｎを下げる特定の結晶方位をもつ再結晶粒の成長を抑制するピンニング作用を呈し、中間焼鈍時に異方性を大きくする。この作用は、０．００３質量％のＣで効果的になる。しかし、０．０１５質量％を超える過剰量のＣを添加すると、鋼材の強度を上昇させ、延性を低下させる。
Ｓｉ：０．５質量％以下
製鋼段階で脱酸剤として添加される成分であるが、固溶強化能が大きく、０．５質量％を超える過剰量のＳｉが含まれると鋼材が硬質化し延性が低下する。
【００１５】
Ｍｎ：０．５質量％以下
Ｓを析出固定させ、熱間加工性に有効な合金成分である。しかし、０．５質量％を超える過剰量のＭｎが含まれると、Ｍｎ系ヒュームの発生等によって製造性が低下する。
Ｐ：０．０５０質量％以下
熱間加工性に有害な成分であるが、０．０５０質量％以下に規制することによりＰの悪影響が抑えられる。
Ｓ：０．０１質量％以下
結晶粒界に偏析しやすく、粒界脆化等の欠陥を引き起こす成分である。Ｓの悪影響は、０．０１質量％以下にＳ含有量を規制することにより抑制される。
【００１６】
Ｃｒ：１０．０〜２３．０質量％
ステンレス鋼に必要な耐食性を確保する上で必須の合金成分であり、Ｃｒ：１０．０質量％以上でＣｒの添加効果が顕著になる。しかし、２３．０質量％を超える過剰量のＣｒ添加は、靭性，加工性を低下させる。
Ａｌ：０．１０質量％以下
製鋼段階で脱酸剤として添加される成分であるが、０．１０質量％を超える過剰量のＡｌを添加すると非金属介在物が増加し，靭性低下や表面欠陥の原因となる。
【００１７】
Ｎ：０．０２０質量％以下
窒化物となって、中間焼鈍時に特定の結晶方位をもつ再結晶粒の成長を抑制するピンニング作用を呈し、０．００５質量％以上のＮ含有で顕著になる。しかし、０．０２０質量％を超える過剰量のＮを添加すると、延性が低下する。
Ｔｉ：０．１０〜０．２５質量％
Ｃ，Ｎを固定し、加工性，耐食性の向上に有効な合金成分である。Ｔｉの添加効果は、０．１０質量％以上のＴｉ添加でみられるが、０．２５質量％を超える過剰量のＴｉ添加は鋼材コストの上昇を招き、表面欠陥の原因であるＴｉ系介在物が増加する。
【００１８】
Ｎｂ：０．１５〜０．３５質量％
Ｔｉと同様にＣ，Ｎを固定し、加工性，耐食性の向上に有効な合金成分である。熱延焼鈍材にＮｂ系炭化物，Ｆｅ_２Ｎｂ等として析出する。このような効果は、０．１５質量％以上のＮｂ添加で顕著になる。しかし、０．３５質量％を超える過剰量のＮｂを添加すると、必要量以上のＮｂ系化合物が析出し、再結晶温度を上げることになる。
Ｎｂ含有量は、Ｔｉ含有量との関連でＮｂ／Ｔｉ質量比が０．９以上となるように定められる。Ｎｂ／Ｔｉ≧０．９は、最小ランクフォード値ｒ_ｍｉｎの増加に有効なＮｂ系介在物の作用を効果的に発現させる上で重要な要因である。Ｎｂ／Ｔｉ＜０．９では、全方位のランクフォード値ｒの最小値ｒ_ｍｉｎが１．８を超えることができず、要求する耐二次加工脆性が得られない。なお、Ｎｉ，Ｍｏ，Ｖ，Ｚｒ等の任意成分を添加した系では、Ｎｂ／Ｔｉ質量比の下限を１．０に設定することが好ましい。
【００１９】
Ｂ：０．０００５〜０．００３５質量％
耐二次加工脆性の改善に有効な成分であり、０．０００５質量％以上でＢの添加効果が顕著になる。しかし、０．００３５質量％を超える過剰量のＢ添加は、熱間加工性，溶接性等を低下させる原因となる。
Ｎｉ：０．５質量％以下
必要に応じて添加される合金成分であり、熱延板の靭性改善に寄与すると共に、過酷な腐食環境に曝されるようとでは高耐食性にも有効である。しかし、高価な元素のため鋼材コストを上昇させ、鋼材を硬質化することにもなるので、Ｎｉ含有量の上限を０．５質量％に設定した。
【００２０】
Ｍｏ：３．０質量％以下
必要に応じて添加される合金成分であり、耐食性の改善に有効であるが、３．０質量％を超える過剰量のＭｎ添加は熱間加工性を低下させる。
Ｃｕ：０．３質量％以下
溶製段階でスクラップ等の溶解減量から混入してくる不純物であり、過剰量のＣｕが含まれると熱間加工性，耐食性が劣化するので、Ｃｕ含有量の上限を０．３質量％に設定することが好ましい。
Ｖ，Ｚｒ：０．３質量％以下
Ｖは固溶Ｃを炭化物として析出させ、Ｚｒは鋼中のＯを酸化物として捕捉することにより、何れも加工性の改善に有効な成分である。しかし、過剰添加は製造性を低下させるので、共に上限を０．３質量％以下に設定する。
【００２１】
以上に掲げた成分の他に、スクラップ等の溶解原料から混入してくるＣａ，Ｍｇ，Ｃｏ等を耐二次加工脆性に悪影響がない程度に含むことができる。
所定組成に調整されたフェライト系ステンレス鋼は、溶製後に鋳造され、熱間圧延，中間焼鈍を伴う冷間圧延，仕上げ冷間圧延，仕上げ焼鈍を経て冷延焼鈍材とされる。
熱延段階では、熱延板を比較的低温で焼鈍することにより、Ｔｉ，Ｎｂの炭化物，窒化物，Ｎｂのラーベス相，及びこれらの複合物の析出を促進させる。析出物は、後続する中間焼鈍段階で生じる再結晶をピンニングし、特定結晶方位をもつ再結晶粒の成長を抑制する。また、仕上げ圧延率の十分な配分と相俟って、最小ランクフォード値ｒ_ｍｉｎを改善する。析出物を目標状態で析出させるためには、７００℃以上で熱延板を焼鈍することが必要である。しかし、焼鈍温度が９５０℃を超えると、或いは焼鈍時間が１時間を超えると、析出物が粗大に成長しやすくなる。
【００２２】
中間焼鈍段階では、フェライト粒が再結晶するが、熱延板焼鈍で生じた析出物のピンニング作用によって微細な再結晶組織となる。焼鈍温度は再結晶組織の粗大化を抑制するため比較的低温に設定されるが、冷延鋼帯の歪取り，軟質化を狙って再結晶完了温度直下に定めることが好ましい。なお、再結晶完了温度から１００℃低い温度までの温度域では、再結晶化していない圧延組織が若干残るものの微細な再結晶組織が得られるため、中間焼鈍の下限温度を（再結晶完了温度−１００℃）に設定する。通常の連続焼鈍ラインを想定し、１分以下の短時間熱処理を採用すると、再結晶粒の成長が抑制される。
【００２３】
中間焼鈍された鋼帯は、圧延率８０％以上で仕上げ冷間圧延される。仕上げ冷間圧延時の高い圧延率は、中間焼鈍で生成した微細な再結晶組織との相互作用によって最小ランクフォード値ｒ_ｍｉｎ、ひいては耐二次加工脆性を改善する。因みに、仕上げ圧延率が８０％に満たないと、ランクフォード値ｒの面内異方性が悪化し、最小ランクフォード値ｒ_ｍｉｎも低位に推移する。
鋼帯のランクフォード値ｒは、通常、圧延方向（Ｌ方向），圧延方向に直交する方向（Ｔ方向），圧延方向に４５度傾斜した方向（Ｄ方向）の三方向に沿って測定し、平均ｒ値，異方性の指標Δｒを求めている。また、Ｌ，Ｔ，Ｄの三方向の中での最小値で便宜的に最小ランクフォード値ｒ_ｍｉｎ（便宜）を表している。
【００２４】

【００２５】
求められた平均ｒ値，異方性Δｒ，最小ｒ値ｒ_ｍｉｎ（便宜）は、耐二次加工脆性との間に明確な関連性がないと従来から扱われてきた。ところが、Ｌ方向からＴ方向まで９０度の範囲を５度刻みに設定して各方向に沿ってランクフォード値ｒを測定し、最小の測定値を最小ランクフォード値ｒ_ｍｉｎとし、最小ランクフォード値ｒ_ｍｉｎと耐二次加工脆性との関係をみると明確な関係が成立していることを見出した。換言すると、冷延焼鈍材の面内全方向に関する最小ランクフォード値ｒ_ｍｉｎは耐二次加工脆性に大きく影響しており、なかでも絞り比が５に達する超深絞り加工では二次加工割れ防止に重要な因子であることが判った。
耐二次加工脆性に及ぼす最小ランクフォード値ｒ_ｍｉｎの影響は、多段絞り等の高度加工時に肉厚減少や歪分布が最小ランクフォード値ｒ_ｍｉｎの方向で大きく変動し、最小ランクフォード値ｒ_ｍｉｎを１．８以上に調整することにより肉厚減少や歪分布の変動が抑制されることに起因するものと推察される。
【００２６】
【実施例１：基礎実験】
Ｃ：０．００７質量％，Ｓｉ：０．２０質量％，Ｍｎ：０．２０質量％，Ｐ：０．０３０質量％，Ｓ：０．０００５質量％，Ｃｒ：１６．５２質量％，Ａｌ：０．０４質量％，Ｎ：０．０１１質量％，Ｎｂ：０．２４質量％，Ｔｉ：０．１７質量％，Ｂ：０．００１５質量％を含むステンレス鋼を実験用溶解炉で溶製した。ステンレス鋼から得た鋳片を板厚５ｍｍに熱間圧延し、表１に示す製造条件下で板厚０．５ｍｍの冷延焼鈍材を製造した。
【００２７】

【００２８】
得られた各冷延焼鈍材からＪＩＳ　１３Ｂ号試験片を切り出し、ランクフォード値ｒを測定すると共に、二次加工試験に供した。
ランクフォード値ｒの測定では、圧延方向から圧延方向に直交する方向の９０度の範囲を５度刻みで設定した各方向に沿ってランクフォード値ｒを測定し、最も低い測定値を最小ランクフォード値ｒ_ｍｉｎとして求めた。
二次加工試験では、多段絞りにより絞り比５で直径１５ｍｍのカップを作製した。カップの耳部を切断し、−１０℃に保持し、カップの頂点に頂角５度の円錐ポンチを被せ、カップ頭部に１ｋｇの重錘を高さ１０ｃｍから落下させた。重錘の落下により拡管方向への衝撃歪を加えた後、カップ側壁部を観察して脆性割れの有無を調査した。同じ冷延焼鈍材から切り出された５個の試験片について重錘を落下させ、全ての試験片で割れが発生しなかった場合を○，一個でも割れが発生した場合を×として耐二次加工脆性を評価した。
【００２９】
表２の調査結果にみられるように、最小ランクフォード値ｒ_ｍｉｎ≧１．８を満足する製造条件Ａで製造された冷延焼鈍材は、重錘の落下衝撃で脆性割れが発生せず、耐二次加工脆性が改善されていた。他方、製造条件Ｂ，Ｃで製造された冷延焼鈍材では、最小ランクフォード値ｒ_ｍｉｎが１．８未満であり、重錘の落下衝撃で脆性割れが発生していた。
【００３０】
最小ランクフォード値ｒ_ｍｉｎと脆性割れの発生有無との関係から、絞り比５と非常に過酷な条件下で加工された後の耐二次加工脆性が最小ランクフォード値ｒ_ｍｉｎで評価できることが判る。すなわち、最小ランクフォード値ｒ_ｍｉｎ≧１．８では、ランクフォード値ｒの面内異方性に起因する蓄積歪による影響が小さく、二次加工脆化が抑制されていることが推察される。当該推察は、脆性割れの発生個所が最小ランクフォード値ｒ_ｍｉｎの方向とほぼ一致していることによっても支持される。全方位のランクフォード値ｒの最小値である最小ランクフォード値ｒ_ｍｉｎが耐二次加工脆性に大きく影響していることをベースに、適切な合金設計を組み合わせることにより、耐二次加工脆性に優れたフェライト系ステンレス鋼冷延焼鈍材が製造されることが判る。
【００３１】

【００３２】
【実施例２：実試験】
表３の組成をもつフェライト系ステンレス鋼を溶製し、鋳造後、板厚５ｍｍに熱間圧延した。各熱延板に表４の条件下で冷間圧延，焼鈍を施し、板厚０．５ｍｍの冷延焼鈍材を製造した。なお、中間焼鈍，仕上げ焼鈍の時間は、何れの場合も６０秒に設定した。
【００３３】

【００３４】

【００３５】
製造された各冷延焼鈍材からＪＩＳ　１３Ｂ号試験片を切り出し、実施例１と同様に最小ランクフォード値ｒ_ｍｉｎ及び耐二次加工脆性を調査した。
表５の調査結果にみられるように、合金設計，製造条件共に本発明で既定した条件を満足する冷延焼鈍材では、最小ランクフォード値ｒ_ｍｉｎが１．８を超えており、従来法で製造した冷延焼鈍材（たとえばＡ１）に比較すると耐二次加工脆性が優れていた。
【００３６】
Ａ１〜Ａ３，Ｂ１，Ｃ１の冷延焼鈍材は、本発明で規定した組成条件を満足するものの、製造条件が本発明で規定した条件を外れるため、最小ランクフォード値ｒ_ｍｉｎが１．８を超えておらず、耐二次加工脆性が劣っていた。本発明で規定した製造条件で製造された冷延焼鈍材でも、組成が本発明で規定した条件を満足しない場合、Ｅ〜Ｇにみられるように最小ランクフォード値ｒ_ｍｉｎが１．８を超えておらず、耐二次加工脆性が劣っていた。
この対比から、合金設計，製造条件共に耐二次加工脆性の向上に重要であることが判る。
【００３７】

【００３８】
【発明の効果】
以上に説明したように、合金設計に併せて、熱延板焼鈍，中間焼鈍，仕上げ冷間圧延等の製造条件を適正に管理することにより、面内方向全方位に沿ったランクフォード値ｒの最小値ｒ_ｍｉｎが１．８以上となり、過酷な条件下で二次加工されても二次加工脆化に起因する割れの発生がなく、良好な製品形状に成形できるフェライト系ステンレス鋼冷延焼鈍材が製造される。このフェライト系ステンレス鋼冷延焼鈍材は、優れた耐二次加工脆性及び耐食性を活用し、シンク，各種器物，コンロ用バーナ等の家庭用機器の部品、燃料等のタンク，給油管，パイプ，モータケース，カバー，センサー，インジェクタ，サーモスタットバルブ，ベアリングシール材，フランジ等、広範な分野で使用される。
【図面の簡単な説明】
【図１】冷延焼鈍材の面内全方位に沿って測定したランクフォード値が測定方位で変わることを示したグラフ[0001]
[Industrial applications]
The present invention relates to a ferritic stainless steel cold-rolled annealed material which is formed into a product shape under severe processing conditions and has excellent secondary work brittleness resistance, and a method for producing the same.
[0002]
[Prior art and problems]
Ferritic stainless steel has good corrosion resistance and does not require expensive Ni, and is therefore economically advantageous as compared with austenitic stainless steel. Utilizing such characteristics, it is used in a wide range of fields including various durable consumer goods. Due to the diversification of applications and the high performance of equipment, the processing conditions for press forming stainless steel sheets have become severe, and the demand for ferritic stainless steel sheets that are even more workable than before has been increasing. It is getting stronger.
[0003]
When the amounts of C and N contained in the ferritic stainless steel are reduced and carbonitride forming elements such as Ti and Nb are added, the formability of the ferritic stainless steel sheet is improved. For example, according to JIS G4305, Cr: 16.0 to 18.00% by mass, C: 0.03% by mass or less, Si: 0.75% by mass or less, Mn: 1.00% by mass or less, P: 0.040% A ferritic stainless steel SUS430LX containing less than 0.3% by mass, less than 0.30% by mass of S, and 0.10 to 1.00% by mass of Ti or Nb is specified. However, when the amounts of C and N are reduced and a carbonitride forming element such as Ti or Nb is added, longitudinal cracking due to secondary working embrittlement tends to occur after press working.
[0004]
In order to improve the resistance to secondary working brittleness of a high-purity ferritic stainless steel sheet containing Ti and Nb, the combined use of Ti and B (Japanese Patent Publication No. 2-7391) is known. The adverse effects of the reduction in elongation and Rankford value become apparent, and the overall material design including anisotropy and elongation remains unsolved. The addition of Be (Japanese Patent Application Laid-Open No. 8-260108) is also known, but the consumption of expensive Be increases the steel material cost. Although it has been considered to improve the secondary working brittleness resistance by adding Mg (Japanese Patent Application Laid-Open No. 2001-3144), other problems due to the reduced cleanliness due to the addition of Mg are likely to be caused.
[0005]
Conventionally, the secondary working embrittlement resistance is only investigated by a test using a draw ratio of at most about 2.3. However, in response to recent advances in press working, ultra-deep drawing with a drawing ratio of more than 5 using multi-stage drawing has begun to be adopted. Ferrite stainless steel with sufficient secondary work brittleness cannot be manufactured by conventional technology during or after press working.Therefore, to obtain a material that does not generate vertical cracks and can withstand severe press working, There is a demand for a ferritic stainless steel sheet having extremely excellent secondary working brittleness.
[0006]
[Means for Solving the Problems]
The present invention has been devised to meet such a demand, and by controlling the workability at the material stage in addition to the alloy design, sufficient secondary work brittleness resistance during and after press working. It is an object of the present invention to provide a ferritic stainless steel cold-rolled annealed material which can be processed into a good product shape without generation of processing cracks even when subjected to severe secondary processing.
[0007]
In order to achieve the object, the ferritic stainless steel cold-rolled annealed material of the present invention has C: 0.015% by mass or less, Si: 0.5% by mass or less, Mn: 0.5% by mass or less, and P: 0. 0.050% by mass or less, S: 0.01% by mass or less, Cr: 10.0 to 23.0% by mass, Al: 0.10% by mass or less, N: 0.020% by mass or less, Ti: 0.10% 0.25% by mass, Nb: 0.15 to 0.35% by mass, B: 0.0005 to 0.0035% by mass, and the balance substantially has the composition of Fe, and the mass ratio of Nb / Ti Is 0.9 or more, and the minimum value r _min of the Rankford value r measured along all directions in the plane is 1.8 or more.
[0008]
Ferritic stainless steel further contains Ni: 0.5% by mass or less, Mo: 3.0% by mass or less, Cu: 0.3% by mass or less, V: 0.3% by mass or less, Zr: 0.3% by mass. One or more of the following may be included. In this case, the mass ratio of Nb / Ti is preferably set to 1.0 or more.
This ferritic stainless steel cold-rolled annealed material is formed by turning a stainless steel slab having a predetermined composition into a hot-rolled steel strip, and performing annealing in a temperature range of 700 to 950 ° C. for 1 hour or less on the hot-rolled steel strip. It is manufactured by performing intermediate annealing in a temperature range from (recrystallization temperature −100 ° C.) to a recrystallization completion temperature after rolling, for 1 minute or less, and performing finish cold rolling at a rolling reduction of 80% or more.
[0009]
[Action]
Even if it is assumed that defects such as processing cracks do not occur under the conventional secondary workability evaluation conditions with a relatively small drawing ratio, processing defects such as secondary processing cracks occur as the drawing ratio increases. I do. Therefore, the present inventors have studied the processing defects generated when the cold-rolled annealed ferritic stainless steel is subjected to secondary processing from various aspects such as components, compositions, and material properties. As a result, the minimum value r _min of the Rankford value r measured along all in-plane directions of the cold-rolled annealed material is restricted to 1.8 or more, and when an alloy design of a specific composition is adopted, the ferritic stainless steel It has been found that the resistance to secondary working brittleness of the cold-rolled annealed material is improved.
[0010]
The effect of the minimum Rankford value r _min on the secondary work embrittlement is presumed as follows.
The Rankford value r measured along each direction over a range of 90 degrees from the rolling direction to the direction perpendicular to the rolling direction indicates the workability of the cold-rolled annealed ferritic stainless steel in that direction. Ferritic stainless steel cold-rolled annealed materials usually have good workability in the rolling direction and the orthogonal direction, but have low workability in the oblique direction. If the workability fluctuates in the in-plane direction, the flow of the material fluctuates in the in-plane direction in deep drawing with a large drawing ratio, and the processing strain is accumulated in a specific direction. If the amount of strain accumulation exceeds the threshold value, secondary working embrittlement in the specific direction and work cracking occur.
[0011]
The adverse effect on workability embrittlement due to different workability in the in-plane direction can be suppressed by increasing the minimum value r _min of the Rankford value r measured along all directions. When the minimum Rankford value r _min is large, even if the workability slightly fluctuates in the in-plane direction, the flow fluctuation of the material generated between the direction in which the workability is inferior and the direction in which the workability is good is improved. Absorbed by plastic flow along the direction. As a result, processing strain in a specific direction is not accumulated until secondary working embrittlement is caused. Therefore, it is possible to perform secondary processing to a target shape without causing processing defects such as secondary processing cracks even in a direction in which the processability is inferior.
[0012]
As a result of investigation and research by the present inventors, by adjusting the minimum Rankford value r _min to 1.8 or more, the in-plane anisotropy of the workability is suppressed to the extent that the secondary work brittleness resistance is not adversely affected. It turned out to be. As specifically shown in the examples described later, secondary processing that tends to occur in steel that has been secondary processed under severe conditions only by satisfying the minimum Rankford value r _min ≧ 1.8. Cracks can be prevented. On the other hand, even in the case of ferritic stainless steel cold-rolled annealed materials having the same components and compositions, secondary working cracks tend to occur unless the minimum Rankford value r _min ≧ 1.8 is satisfied.
[0013]
Minimum Lankford value r _min depends on the texture of stainless steel stainless steel plate, the texture of r _min ≧ 1.8 and a minimum Lankford value r _min is high stainless steel sheet to increase the anisotropy. That is, the texture such as [211], which is an orientation that relatively lowers the minimum Rankford value r _min , is small, and the [111] texture, which improves the Rankford value r in all directions, is highly developed. . In order to form such a texture, it is preferable to precipitate carbides and nitrides of Ti and Nb, Laves phases of Nb, and their composites by low-temperature hot-rolled sheet annealing. A target texture is produced by suppressing the growth of recrystallization having a specific crystal orientation such as [211] by a precipitate exhibiting a pinning action during intermediate annealing and by sufficiently distributing the finish rolling ratio.
[0014]
Next, the alloy design employed in the present invention will be described.
C: an alloy component that forms carbides of 0.015% by mass or less, exhibits a pinning effect of suppressing the growth of recrystallized grains having a specific crystal orientation that lowers the minimum Rankford value r _min, and is anisotropic during intermediate annealing To increase. This effect is effective at 0.003% by mass of C. However, when an excessive amount of C exceeding 0.015% by mass is added, the strength of the steel material is increased, and the ductility is reduced.
Si: 0.5% by mass or less Si is a component added as a deoxidizing agent in the steelmaking stage, but has a large solid solution strengthening ability. If an excessive amount of Si exceeding 0.5% by mass is contained, the steel material becomes hard and ductility increases. Decreases.
[0015]
Mn: 0.5 mass% or less S is an alloy component that precipitates and fixes S and is effective for hot workability. However, when an excessive amount of Mn exceeding 0.5% by mass is included, productivity is reduced due to generation of Mn-based fumes and the like.
P: 0.050% by mass or less A component harmful to hot workability, but by regulating the content to 0.050% by mass or less, the adverse effect of P can be suppressed.
S: 0.01% by mass or less Se is a component that is easily segregated at crystal grain boundaries and causes defects such as grain boundary embrittlement. The adverse effect of S is suppressed by regulating the S content to 0.01% by mass or less.
[0016]
Cr: 10.0 to 23.0% by mass
It is an essential alloy component for ensuring the corrosion resistance required for stainless steel. The effect of adding Cr becomes remarkable when Cr is 10.0% by mass or more. However, the addition of an excessive amount of Cr exceeding 23.0% by mass lowers toughness and workability.
Al: 0.10% by mass or less Al is a component added as a deoxidizing agent in the steelmaking stage. However, if an excessive amount of Al exceeding 0.10% by mass is added, nonmetallic inclusions increase, and toughness decreases and surface defects are reduced. Cause.
[0017]
N: becomes 0.020 mass% or less of nitride, exhibits a pinning effect of suppressing the growth of recrystallized grains having a specific crystal orientation during intermediate annealing, and becomes remarkable when N content is 0.005 mass% or more. However, when an excessive amount of N exceeding 0.020% by mass is added, ductility decreases.
Ti: 0.10 to 0.25 mass%
It is an alloy component that fixes C and N and is effective for improving workability and corrosion resistance. The effect of the addition of Ti can be seen with the addition of 0.10% by mass or more of Ti. However, the excessive addition of Ti exceeding 0.25% by mass causes an increase in steel material cost, and Ti-based inclusions causing surface defects. Increase.
[0018]
Nb: 0.15 to 0.35% by mass
Similar to Ti, it is an alloy component that fixes C and N and is effective for improving workability and corrosion resistance. Precipitates as Nb-based carbide, Fe ₂ Nb, etc. on the hot-rolled annealed material. Such effects become remarkable when 0.15% by mass or more of Nb is added. However, when an excessive amount of Nb exceeding 0.35% by mass is added, an Nb-based compound in a necessary amount or more is precipitated, and the recrystallization temperature is increased.
The Nb content is determined so that the Nb / Ti mass ratio becomes 0.9 or more in relation to the Ti content. Nb / Ti ≧ 0.9 is an important factor for effectively exhibiting the action of Nb-based inclusions effective for increasing the minimum Rankford value r _min . When Nb / Ti <0.9, the minimum value r _min of the Rankford value r in all directions cannot exceed 1.8, and the required secondary work embrittlement resistance cannot be obtained. In a system to which arbitrary components such as Ni, Mo, V, and Zr are added, the lower limit of the Nb / Ti mass ratio is preferably set to 1.0.
[0019]
B: 0.0005 to 0.0035% by mass
It is a component effective for improving the secondary work brittleness resistance, and the effect of adding B becomes remarkable at 0.0005% by mass or more. However, excessive addition of B exceeding 0.0035% by mass causes deterioration of hot workability, weldability, and the like.
Ni: 0.5% by mass or less Ni is an alloy component added as necessary, and contributes to improvement of the toughness of the hot-rolled sheet, and is also effective for high corrosion resistance when exposed to a severe corrosive environment. However, since the expensive material increases the cost of the steel material and hardens the steel material, the upper limit of the Ni content is set to 0.5% by mass.
[0020]
Mo: 3.0% by mass or less Mo is an alloy component added as needed and is effective in improving corrosion resistance. However, excessive addition of Mn exceeding 3.0% by mass lowers hot workability.
Cu: 0.3% by mass or less Cu is an impurity that is mixed in from the melting loss of scrap and the like at the melting stage. If an excessive amount of Cu is contained, hot workability and corrosion resistance are deteriorated. Is preferably set to 0.3% by mass.
V, Zr: 0.3% by mass or less V precipitates solid solution C as carbide, and Zr is a component effective for improving workability by trapping O in steel as an oxide. However, since excessive addition lowers the productivity, the upper limit is set to 0.3% by mass or less.
[0021]
In addition to the components listed above, Ca, Mg, Co, etc., which are mixed in from a raw material such as scrap, may be contained to such an extent that the secondary working embrittlement resistance is not adversely affected.
The ferritic stainless steel adjusted to a predetermined composition is cast after smelting and is subjected to hot rolling, cold rolling with intermediate annealing, finish cold rolling, and finish annealing to form a cold-rolled annealed material.
In the hot rolling step, the precipitation of carbides and nitrides of Ti and Nb, the Laves phase of Nb, and a composite thereof is promoted by annealing the hot rolled sheet at a relatively low temperature. The precipitates pin the recrystallization that occurs in the subsequent intermediate annealing step, and suppress the growth of recrystallized grains having a specific crystal orientation. Also, the minimum Rankford value r _min is improved in combination with a sufficient distribution of the finish rolling ratio. In order to precipitate the precipitate in a target state, it is necessary to anneal the hot-rolled sheet at 700 ° C. or higher. However, if the annealing temperature exceeds 950 ° C. or if the annealing time exceeds 1 hour, the precipitates are likely to grow coarsely.
[0022]
In the intermediate annealing stage, the ferrite grains are recrystallized, but a fine recrystallized structure is formed by the pinning action of the precipitate generated by the hot-rolled sheet annealing. The annealing temperature is set to a relatively low temperature in order to suppress the coarsening of the recrystallization structure, but it is preferable to set the annealing temperature immediately below the recrystallization completion temperature in order to remove the strain and soften the cold-rolled steel strip. In the temperature range from the recrystallization completion temperature to a temperature lower by 100 ° C., a fine recrystallization structure is obtained although a rolled structure that has not been recrystallized slightly remains. Therefore, the lower limit temperature of the intermediate annealing is set to (recrystallization completion temperature− 100 ° C). Assuming a normal continuous annealing line and employing a short-time heat treatment of 1 minute or less, the growth of recrystallized grains is suppressed.
[0023]
The intermediately annealed steel strip is subjected to finish cold rolling at a rolling reduction of 80% or more. The high rolling reduction during finish cold rolling improves the minimum Rankford value r _{min and} , consequently, the resistance to secondary working embrittlement by interaction with the fine recrystallized structure generated by the intermediate annealing. Incidentally, when the finish rolling reduction is less than 80%, the in-plane anisotropy of the Rankford value r deteriorates, and the minimum Rankford value _rmin also changes to a low level.
The Rankford value r of the steel strip is usually measured along three directions: a rolling direction (L direction), a direction perpendicular to the rolling direction (T direction), and a direction inclined 45 degrees to the rolling direction (D direction). The average r value and the index Δr of anisotropy are obtained. The minimum value in the three directions L, T, and D conveniently represents the minimum Rankford value r _min (for convenience).
[0024]

[0025]
The obtained average r value, anisotropy Δr, and minimum r value r _min (for convenience) have conventionally been treated as having no clear relationship with the secondary work embrittlement resistance. However, the range of 90 degrees from the L direction to the T direction is set in increments of 5 degrees, and the Rankford value r is measured along each direction. The minimum measured value is defined as the minimum Rankford value _rmin , and the minimum Rankford value is determined. Looking at the relationship between r _min and the resistance to secondary working embrittlement, it was found that a clear relationship was established. In other words, the minimum Rankford value r _min in all directions in the plane of the cold-rolled annealed material has a great effect on the secondary work brittleness resistance. Was an important factor.
Effect of minimum Lankford value r _min on secondary work embrittlement resistance is thickness reduction and distortion distribution varies greatly in the direction of the minimum Lankford value r _min during deep processing of such multistage aperture, the minimum Lankford value r _min Is adjusted to 1.8 or more, it is presumed to be caused by suppression of wall thickness reduction and fluctuation of strain distribution.
[0026]
[Example 1: Basic experiment]
C: 0.007% by mass, Si: 0.20% by mass, Mn: 0.20% by mass, P: 0.030% by mass, S: 0.0005% by mass, Cr: 16.52% by mass, Al: A stainless steel containing 0.04% by mass, N: 0.011% by mass, Nb: 0.24% by mass, Ti: 0.17% by mass, and B: 0.0015% by mass was melted in a laboratory melting furnace. . A slab obtained from stainless steel was hot-rolled to a thickness of 5 mm to produce a cold-rolled annealed material having a thickness of 0.5 mm under the production conditions shown in Table 1.
[0027]

[0028]
A JIS No. 13B test piece was cut out from each obtained cold-rolled annealed material, and a Rankford value r was measured and subjected to a secondary processing test.
In the measurement of the Rankford value r, the Rankford value r is measured along a direction set in a direction of 90 degrees from the rolling direction to a direction perpendicular to the rolling direction at intervals of 5 degrees, and the lowest measured value is the minimum Rankford value. The value was determined as r _min .
In the secondary processing test, a cup having a drawing ratio of 5 and a diameter of 15 mm was produced by multistage drawing. The ears of the cup were cut off, kept at -10 ° C, a conical punch with a vertex angle of 5 ° was put on the top of the cup, and a 1 kg weight was dropped on the cup head from a height of 10 cm. After applying an impact strain in the pipe expansion direction by dropping the weight, the side wall of the cup was observed to check for brittle cracks. Weights were dropped on five test pieces cut out from the same cold-rolled annealed material, and when no cracks occurred in all the test pieces, ○ was given. The brittleness was evaluated.
[0029]
As can be seen from the survey results in Table 2, the cold-rolled annealed material manufactured under the manufacturing condition A satisfying the minimum Rankford value r _min ≧ 1.8 does not generate brittle cracks due to the drop impact of the weight, The secondary work brittleness resistance was improved. On the other hand, in the cold-rolled annealed materials manufactured under the manufacturing conditions B and C, the minimum Rankford value r _min was less than 1.8, and brittle cracks were generated by the drop impact of the weight.
[0030]
The relationship between the minimum Lankford value r _min and occurrence or non-occurrence of brittle cracks, resistance to secondary working brittleness after being processed in a very harsh conditions and the aperture ratio 5 it can be seen that can be evaluated by a minimum Lankford value r _min . That is, when the minimum Rankford value r _min ≧ 1.8, the influence of the accumulated strain due to the in-plane anisotropy of the Rankford value r is small, and it is inferred that the secondary working embrittlement is suppressed. This speculation is supported by the fact that the location of the brittle crack occurrence substantially coincides with the direction of the minimum Rankford value r _min . Based on the fact that the minimum Rankford value r _min, which is the minimum value of the omnidirectional Rankford value r, greatly affects the resistance to secondary processing embrittlement, by combining an appropriate alloy design, It can be seen that an excellent ferritic stainless steel cold-rolled annealed material is manufactured.
[0031]

[0032]
[Example 2: Actual test]
A ferritic stainless steel having the composition shown in Table 3 was melted, cast, and hot-rolled to a thickness of 5 mm. Each hot-rolled sheet was subjected to cold rolling and annealing under the conditions shown in Table 4 to produce a cold-rolled annealed material having a sheet thickness of 0.5 mm. The time of the intermediate annealing and the finish annealing was set to 60 seconds in each case.
[0033]

[0034]

[0035]
A JIS No. 13B test piece was cut out from each of the produced cold-rolled annealed materials, and the minimum Rankford value r _min and the resistance to secondary working embrittlement were examined in the same manner as in Example 1.
As can be seen from the investigation results in Table 5, in the case of a cold-rolled annealed material satisfying the conditions specified in the present invention in both the alloy design and the manufacturing conditions, the minimum Rankford value r _min exceeds 1.8, and the conventional method is used. Compared with the manufactured cold rolled annealed material (for example, A1), the secondary work brittleness resistance was excellent.
[0036]
Although the cold-rolled annealed materials of A1 to A3, B1, and C1 satisfy the composition conditions specified in the present invention, the manufacturing conditions are out of the conditions specified in the present invention, and therefore, the minimum Rankford value r _min is 1.8. It did not exceed, and the secondary work brittleness resistance was inferior. Even in the case of the cold-rolled annealed material manufactured under the manufacturing conditions specified in the present invention, when the composition does not satisfy the conditions specified in the present invention, the minimum Rankford value r _min exceeds 1.8 as seen in EG. And the secondary work brittleness resistance was inferior.
From this comparison, it is understood that both the alloy design and the manufacturing conditions are important for improving the resistance to secondary working brittleness.
[0037]

[0038]
【The invention's effect】
As described above, by appropriately controlling the manufacturing conditions such as hot-rolled sheet annealing, intermediate annealing, and finish cold rolling in conjunction with the alloy design, the Rankford value r along all directions in the in-plane direction can be obtained. The minimum value r _min is 1.8 or more, and even if the secondary processing is performed under severe conditions, there is no occurrence of cracks due to secondary processing embrittlement, and the ferritic stainless steel cold-rolled annealing can be formed into a good product shape. The material is manufactured. This ferritic stainless steel cold rolled annealed material makes use of excellent secondary processing brittleness resistance and corrosion resistance, and is used for household equipment parts such as sinks, various objects, stove burners, fuel tanks, oil supply pipes, pipes, Used in a wide range of fields such as motor cases, covers, sensors, injectors, thermostat valves, bearing seal materials, flanges, etc.
[Brief description of the drawings]
FIG. 1 is a graph showing that the Rankford value measured along all in-plane directions of a cold-rolled annealed material changes with the measured direction.

Claims

C: 0.015% by mass or less, Si: 0.5% by mass or less, Mn: 0.5% by mass or less, P: 0.050% by mass or less, S: 0.01% by mass or less, Cr: 10.0 23.0% by mass, Al: 0.10% by mass or less, N: 0.020% by mass or less, Ti: 0.10 to 0.25% by mass, Nb: 0.15 to 0.35% by mass, B : 0.0005 to 0.0035% by mass, with the balance having a substantially Fe composition, a mass ratio of Nb / Ti of 0.9 or more, and a Rankford value r measured along all in-plane directions. A cold-rolled annealed ferritic stainless steel excellent in secondary workability, characterized in that a minimum value r _min of 1.8 is not less than 1.8.

Further, one or two of Ni: 0.5% by mass or less, Mo: 3.0% by mass or less, Cu: 0.3% by mass or less, V: 0.3% by mass or less, and Zr: 0.3% by mass or less. 2. The cold-rolled annealed ferritic stainless steel according to claim 1, comprising at least one species and having a mass ratio of Nb / Ti of 1.0 or more.

A stainless steel slab having the composition according to claim 1 or 2 is used as a hot-rolled steel strip, and the hot-rolled steel strip is annealed at a temperature of 700 to 950 ° C. for 1 hour or less, and after intermediate cold rolling (recrystallization). Production of a ferritic stainless steel cold-rolled annealed material, which is subjected to intermediate annealing in a temperature range from a temperature of -100 ° C) to a recrystallization completion temperature for 1 minute or less and finish cold rolling at a rolling reduction of 80% or more. Method.