JP4420176B2 - Method for producing martensitic stainless steel with excellent corrosion resistance and toughness - Google Patents
Method for producing martensitic stainless steel with excellent corrosion resistance and toughness Download PDFInfo
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- JP4420176B2 JP4420176B2 JP2003092081A JP2003092081A JP4420176B2 JP 4420176 B2 JP4420176 B2 JP 4420176B2 JP 2003092081 A JP2003092081 A JP 2003092081A JP 2003092081 A JP2003092081 A JP 2003092081A JP 4420176 B2 JP4420176 B2 JP 4420176B2
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- 238000005260 corrosion Methods 0.000 title claims description 38
- 230000007797 corrosion Effects 0.000 title claims description 38
- 229910001105 martensitic stainless steel Inorganic materials 0.000 title claims description 13
- 238000004519 manufacturing process Methods 0.000 title claims description 10
- 238000010791 quenching Methods 0.000 claims description 35
- 230000000171 quenching effect Effects 0.000 claims description 35
- 238000005496 tempering Methods 0.000 claims description 29
- 229910000831 Steel Inorganic materials 0.000 claims description 27
- 239000010959 steel Substances 0.000 claims description 27
- 239000012298 atmosphere Substances 0.000 claims description 16
- 229910052739 hydrogen Inorganic materials 0.000 claims description 9
- 239000012299 nitrogen atmosphere Substances 0.000 claims description 8
- 239000001257 hydrogen Substances 0.000 claims description 7
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 claims description 6
- 239000000203 mixture Substances 0.000 claims description 6
- 239000012535 impurity Substances 0.000 claims description 3
- 239000000463 material Substances 0.000 description 28
- 238000005452 bending Methods 0.000 description 10
- 239000002344 surface layer Substances 0.000 description 10
- 238000000034 method Methods 0.000 description 7
- 238000005121 nitriding Methods 0.000 description 5
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 4
- 239000007789 gas Substances 0.000 description 4
- 238000010438 heat treatment Methods 0.000 description 4
- 229910052757 nitrogen Inorganic materials 0.000 description 4
- 229910052760 oxygen Inorganic materials 0.000 description 4
- 239000000047 product Substances 0.000 description 4
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- 238000011156 evaluation Methods 0.000 description 3
- 150000001247 metal acetylides Chemical class 0.000 description 3
- 238000010301 surface-oxidation reaction Methods 0.000 description 3
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Chemical compound O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- 230000015572 biosynthetic process Effects 0.000 description 2
- 238000005097 cold rolling Methods 0.000 description 2
- 230000002950 deficient Effects 0.000 description 2
- 230000006866 deterioration Effects 0.000 description 2
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- 150000004767 nitrides Chemical class 0.000 description 2
- 230000003647 oxidation Effects 0.000 description 2
- 238000007254 oxidation reaction Methods 0.000 description 2
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- 241000251730 Chondrichthyes Species 0.000 description 1
- 229910017060 Fe Cr Inorganic materials 0.000 description 1
- 229910002544 Fe-Cr Inorganic materials 0.000 description 1
- 238000005299 abrasion Methods 0.000 description 1
- 238000000137 annealing Methods 0.000 description 1
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- UPHIPHFJVNKLMR-UHFFFAOYSA-N chromium iron Chemical compound [Cr].[Fe] UPHIPHFJVNKLMR-UHFFFAOYSA-N 0.000 description 1
- 239000010960 cold rolled steel Substances 0.000 description 1
- 238000004040 coloring Methods 0.000 description 1
- 239000002131 composite material Substances 0.000 description 1
- 238000007796 conventional method Methods 0.000 description 1
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- 150000002431 hydrogen Chemical class 0.000 description 1
- 229910000765 intermetallic Inorganic materials 0.000 description 1
- JEIPFZHSYJVQDO-UHFFFAOYSA-N iron(III) oxide Inorganic materials O=[Fe]O[Fe]=O JEIPFZHSYJVQDO-UHFFFAOYSA-N 0.000 description 1
- 229910000734 martensite Inorganic materials 0.000 description 1
- 238000002844 melting Methods 0.000 description 1
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Description
【０００１】
【産業上の利用分野】
本発明は、フラットヘルド，ドロッパー，筬羽，変形筬等の耐摩耗性品や刃物，ノズル，弁座，バルブ等に用いられる耐食性，靭性に優れたマルテンサイト系ステンレス鋼の製造方法に関する。
【０００２】
【従来の技術】
フラットヘルド，ドロッパー，筬羽等の耐摩耗鋼としてはＳＵＳ４２０Ｊ２、あるいは特開２０００−１９２１９８号公報で提案されているような、Ｔｉ，Ｎｂの炭化物により耐アブレッシブ摩耗性を向上させた鋼が使用されている。また、刃物，ノズル，弁座，バルブ等にはＳＵＳ４２０Ｊ２が用いられている。
これらの鋼板には、強度と靭性を付与するために、高温に加熱して焼入れし、引き続きＡｃ1点以下の温度での焼戻し行う熱処理が施されている。通常、時間を要する加熱や焼戻しは、表面酸化による着色を防止するため、水素雰囲気あるいは７５％Ｈ2‐２５％Ｎ2ガスであるＡＸガス雰囲気の下で行われている。また、特開平１０−１３０７９１号公報では、焼入れ時の加熱雰囲気を微量の酸素を含む窒素雰囲気やＡＸガス雰囲気とすることも提案されている。
【０００３】
【発明が解決しようとする課題】
表面酸化を抑える雰囲気下で焼入れ処理や焼戻し処理を施しても、その途中で表面変質を起こすことがある。このため、焼入れ−焼戻しの処理が施された鋼板で、耐食性や靭性が低下することがある。この結果、フラットヘルドや筬羽等の製品に加工された後の品質が一定に保てず、歩留まりの低下や製造コストの増大につながる等、さまざまな問題が発生することになる。特に、フラットヘルドの耐食性が低下すると機織中に赤錆が発生し、織物を汚染するという問題を発生させることになる。
本発明は、このような問題を解消すべく案出されたものであり、焼入れ−焼戻し処理時にあっても表面変質を抑え、その結果、耐食性や靭性に優れたマルテンサイト系ステンレス鋼を安定して製造する方法を提供することを目的とする。
【０００４】
【課題を解決するための手段】
本発明の耐食性，靭性に優れたマルテンサイト系ステンレス鋼の製造方法は、その目的を達成するため、Ｃ：０．０５〜１．２０質量％，Ｃｒ：８．０〜１８．０質量％，Ｓｉ：１．０質量％以下，Ｍｎ：１．０質量％以下を含み、さらに必要に応じてＴｉ，Ｎｂを単独であるいは合計量で０．０５〜１．０質量％含み、残部がＦｅおよび不可避不純物からなる組成をもつ鋼板を、下記（１）式，（２）式を満たす露点ＤＰ（℃）および温度Ｔ（℃）の条件の水素窒素雰囲気中、または下記（２）式，（３）式を満たす露点ＤＰ（℃）および温度Ｔ（℃）の条件の水素雰囲気中で焼入れ処理を施した後、Ａｃ１点以下の温度で焼戻し処理を施すことを特徴とする。
０．００９５Ｔ−１６８≦ＤＰ≦０．１０５Ｔ−１３２ ・・・（１）
９５０≦Ｔ≦１２００ ・・・（２）
ＤＰ≦０．１０５Ｔ−１３２ ・・・（３）
【０００５】
【本発明の実施の態様】
本発明者等は、焼入れ時に耐食性や靭性を低下させるような表面変質を引き起こす原因について鋭意検討を重ねてきた。その結果、焼入れ時の温度、雰囲気条件の違いにより、表面酸化や表面窒化が起こり、それにより、耐食性や靭性の低下につながることを見出した。
まず、この現象を見出した予備試験について説明する。
Ｆｅ−０．３％Ｃ−０．５％Ｓｉ−０．５％Ｍｎ−１３％Ｃｒ−０．３１％Ｎｂを基本成分とするマルテンサイト系ステンレス鋼の０．５ｍｍ厚の冷延鋼板を焼入れ−焼戻し処理した試験材を、焼入れ条件を種々変えて実験室的に作製し、各試験材の表面状態や耐食性及び靭性の関係を調査した。なお、焼入れ処理は７５％Ｈ2‐２５％Ｎ2のＡＸガス雰囲気１０４０℃で、露点を変えて行った。焼戻しは窒素雰囲気中４００℃で行った。
【０００６】
試験材（ａ）は、露点−２０℃で焼入れしたもので、焼戻し後着色していた。試験材（ｂ）は、露点−５０℃で焼入れしたもので、焼戻し後も着色されていなかった。試験材（ｃ）は、露点−７５℃で焼入れしたもので、焼戻し後も着色されていなかった。
これらの試験材について、キャス試験による耐食性の評価と、図１に示すようなＵ曲げ破断試験によるＵ曲げ破断距離の測定により靭性の評価を行った。なお、Ｕ曲げ破断距離が小さいほど靭性は良好であると評価される。
ＪＩＳ Ｈ８５０２−７に準拠した２４時間のキャス試験によると、試験材（ａ）と試験材（ｃ）は発錆が著しく、耐食性が低下していた。一方、試験材（ｂ）の発錆はごく僅かであった。
【０００７】
Ｕ曲げ破断試験後の破断距離を測定すると、試験材（ａ）と（ｂ）のＵ曲げ破断距離はごく僅かであったのに対して、試験材（ｃ）のＵ曲げ破断距離は大きかった。
その結果をまとめて図２に示す。
図２の結果から、焼入れ−焼戻し処理後には、試験材（ａ）のみが着色していること、試験材（ｂ）の発錆はごく僅かであるのに対して試験材（ａ），（ｃ）は著しく発錆した耐食性が低下していること、および試験材（ｃ）のみ靭性が著しく低下していることがわかる。
【０００８】
焼入れ−焼戻し処理後の各試験材の表面状態を知るために、試験材表層部をＧＤＳ（グロー放電発光分析装置）で分析した。その結果を図３に示す。
表面着色がなかった試験材（ｂ），（ｃ）の表層にはＳｉ，Ｍｎ，Ｏの濃化が認められるのに対して、着色していた試験材（ａ）の表層にはＳｉ，Ｍｎ，ＯのみならずＣｒの著しい濃化が認められた。
一般にＦｅ−Ｃｒ系鋼板の表層にＣｒの酸化物が生成している場合には、スケール直下にＣｒ欠乏層が形成され、耐食性が低下することが知られている。したがって、試験材（ａ）の耐食性低下は表層にＣｒ酸化物が生成したことに起因していると推察される。
【０００９】
一方、図３からは、試験材（ａ），（ｂ）に比べて試験材（ｃ）の表層のＮの濃化が大きく、Ｓｉ，Ｍｎ，Ｏの濃化が小さいこともわかる。この試験材（ｃ）をさらに詳細に調査した結果、焼入れ−焼戻し処理後にＮ量が増加していること、焼入れ−焼戻し処理後には表層部付近にＣｒ窒化物が生成していることがわかった。これらの結果から、試験材（ｃ）では、焼入れ−焼戻し処理時の窒化により表層部でＣｒ窒化物が生成し、その周辺部にＣｒ欠乏層が形成されたために耐食性が低下したものと推測される。
また、試験材（ｃ）のみが著しく窒化した原因として、表層のＳｉ，Ｍｎ，Ｏの濃化が十分ではなかったため、焼入れ−焼戻し処理時の窒化を抑制できなかったものと推察される。
【００１０】
各試験材について、Ｎ含有量を分析して焼入れ−焼戻し処理により増加したΔＮ量とＵ曲げ破断距離との関係も調査した。その結果を図４に示す。
ΔＮ量の増加に伴いＵ曲げ破断距離が増加し、ΔＮ量が多かった試験材（ｃ）のみ靭性が不良であることがわかる。この靭性低下は、窒化に伴う表面層の硬化に起因していると推察される。
以上の知見をまとめると、焼入れ−焼戻し処理時にＣｒの酸化が起こると耐食性が低下し、窒化が起こると耐食性および靭性が低下することになる。
【００１１】
本発明者等は、マルテンサイト系ステンレス鋼の耐食性、靭性が低下する要因となるＣｒの酸化や窒化は、焼入れの際の温度，雰囲気，露点等の条件に依存すると推測した。そして、成分組成を規定したマルテンサイト系ステンレス鋼について、種々の鋼を種々の条件下で焼入れし、その後焼戻し処理を施した試料の耐食性および靭性を調査する試験を繰り返すことにより、下記（１）〜（４）式を満たす温度Ｔ（℃），露点ＤＰ（℃），雰囲気で焼入れを行うと、耐食性および靭性に優れたものが得られることを見出した。
なお、（１）式および（３）式は、上記繰返し行った多数の試験結果をグラフ上に表示し、耐食性および靭性の点で満足できるものと満足できないものとを区分する線を引き、その線を近似式で書き表した実験式である（図５参照）。
【００１２】
すなわち、水素窒素雰囲気の場合には、
０．００９５Ｔ−１６８≦ＤＰ≦０．１０５Ｔ−１３２ ・・・（１）
９５０≦Ｔ≦１２００ ・・・（２）
水素雰囲気の場合には、
ＤＰ≦０．１０５Ｔ−１３２ ・・・（３）
９５０≦Ｔ≦１２００ ・・・（４）
を満足する範囲である。
なお、ここで、水素窒素雰囲気あるいは水素雰囲気におけるＤＰ≦０．１０５Ｔ−１３２は、Ｃｒ酸化物が生成しない範囲であり、水素窒素雰囲気における０．００９５Ｔ−１６８≦ＤＰは、窒化が抑制される範囲である。
露点とは、「雰囲気中で物体を冷却していって、物体の表面に露ができはじめる温度。この時の水蒸気分圧は飽和水蒸気圧に等しい」（岩波理化学辞典）に定義されている。露点が高いとは、水蒸気圧が高いこと、すなわちＣｒなどの元素が酸化しやすいことを意味する。したがって、温度との関係で露点を低くするとＣｒ酸化物が生成されがたく、耐食性を維持できることになる。
水素雰囲気の場合には、著しい窒化は起こらないので低露点側の規制はない。
【００１３】
また、焼入れ温度については、焼入れの処理の目的が鋼を高温のγ域から冷却することにより強度の高いマルテンサイト組織を得ることにあるので、９５０℃以上にする必要がある。しかし、１２００℃以上では、鋼板の高温強度が低くなり、板が破断する可能性がある。このため、焼入れ温度の上限は１２００℃とする。
焼戻し処理は、焼入れ処理によって固溶した炭素を炭化物として析出させるとともに基地組織を回復させ靭性を付与することを目的としているため、温度はＡｃ1点以下であればよい。
【００１４】
次に、本発明焼入れ−焼戻し処理が施される鋼板の成分について説明する。
Ｃ：０．０５〜１．２０質量％
Ｃは、焼入れ処理により基地中に固溶させて高い強度をえるためには、０．０５％以上含有させる必要がある。しかし、１．２０％を超えると、巨大な共晶炭化物が多量に析出して熱間加工性を低下させる。したがって、Ｃ量は０．０５〜１．２０％とする。
Ｃｒ：８．０〜１８．０質量％
Ｃｒは、耐食性を付与するために必要な元素である。しかし、本発明に係る鋼材が用いられる環境を考慮すると８．０％以上の添加が必要である。しかし、過剰な添加は製造コストの増大を招くので上限は１８．０％とした。
【００１５】
Ｓｉ：１．０質量％以下，Ｍｎ：１．０質量％以下
Ｓｉ，Ｍｎは、溶製時に脱酸剤として添加される成分である。しかし、過剰の添加はその硬化を飽和させるばかりでなく、製造コストの増大を招く。したがって、上限は１．０％に設定した。
Ｔｉ，Ｎｂ：単独あるいは合計量で０．０５〜１ . ０質量％
ＴｉあるいはＮｂの炭化物は極めて硬質である。このためこれらの炭化物を鋼の基地中に分散させることにより耐食性が著しく向上する。したがって、耐摩耗性が要求されるような用途に用いる鋼に対しては、Ｔｉ，Ｎｂを添加することが必要である。ただし、炭化物の分散量が少ないと、その効果が低いためＴｉ単独添加，Ｎｂ単独添加あるいはＴｉ，Ｎｂ複合添加で０．０５以上とした。しかし、Ｔｉ，Ｎｂの過剰添加は、金属間化合物の生成による靭性の低下を招くことになるので、上限は１．０％とした。
【００１６】
【実施例】
表１に示す各種鋼を常法に従って溶製し、スラブに鋳造した。溶体化処理後、スラブを板厚５．０ｍｍまで熱間圧延した。熱延板に７８０℃×９時間の熱処理を施し、炉冷した。その後、酸洗，冷間圧延，中間焼鈍後、冷間圧延で板厚０．３ｍｍの鋼板を得た。表１に示す鋼は、いずれも本発明で規定される範囲の化学組成を有するものである。
【００１７】
その後、表２に示す条件で焼入れ処理を施し、窒素雰囲気４００℃で焼戻し処理を行った後、耐食性，靭性およびΔＮ量を測定・評価した。
なお、表２中、Ａ，Ｂ，Ｃは本発明の範囲内の条件であるが、Ｄ，Ｅ，Ｆは本発明の条件を外れるものである。
【００１９】
耐食性はＪＩＳ Ｈ８５０２−７に準拠したキャス試験を実施し、２４時間後に目視観察にて発錆が軽微なものを耐食性良好○、発錆が著しいものを耐食性不良×と評価した。
靭性は、図１で示した態様のＵ曲げ破断試験を行い、Ｕ曲げ破断距離Ｄが１ｍｍ以下のものを靭性良好○，それを超えるものを靭性不良×と判断した。
ΔＮ量は、焼入れ−焼戻し処理後の窒素Ｎ量と処理前のＮ量の差から求めた。なお、Ｓ１，Ｓ２，Ｓ３鋼の焼入れ前のＮ量は、それぞれ０．０３０％，０．０２８％，０．０３１％である。
それぞれの評価結果を併せて表３に示す。
【００２１】
表３の結果からもわかるように、本発明で規定された成分組成を有するＳ１，Ｓ２，Ｓ３鋼を、本発明で規定する焼入れ条件を満たす工程Ａ，Ｂ，Ｃで処理した鋼板については、耐食性，靭性ともに良好であった。
これに対して、Ｓ１，Ｓ２，Ｓ３鋼をＤあるいはＦ工程で処理した鋼板は、靭性は良好であるが、焼入れ処理時の露点が高いためにＣｒ酸化物が生成し、耐食性に劣っていた。また、Ｓ１，Ｓ２，Ｓ３鋼をＥ工程で処理した鋼板は、焼入れ処理時の露点が低いため窒化が起こり、耐食性，靭性ともに低下していた。
【００２３】
【発明の効果】
以上に説明したように、本発明により、マルテンサイト系ステンレス鋼に焼入れ−焼戻し処理を施す際に、温度，露点および雰囲気を互いに所定の関係になるように調整した条件の下で焼入れ処理を行うと、焼入れ時のＣｒの酸化や窒化が抑制され、それに起因する耐食性や靭性の低下がないマルテンサイト系ステンレス鋼を安定して製造することができる。
これにより、フラットヘルド，ドロッパー，筬羽，変形筬等の耐摩耗性品や刃物，ノズル，弁座，バルブ等に用いられる耐食性，靭性に優れたマルテンサイト系ステンレス鋼を低コストで提供することが可能となる。
【図面の簡単な説明】
【図１】 靭性の評価方法であるＵ曲げ試験方法を説明する図
【図２】 焼入れ−焼戻し処理後の鋼板を、耐食性，靭性の観点から分類した鋼板の外観，耐食性試験後の外観、および靭性評価結果を示した図
【図３】 焼入れ−焼戻し処理後のＧＤＳ分析結果を示した図
【図４】 焼入れ−焼戻し処理前後の窒素量の差ΔＮ量とＵ曲げ破断距離Ｄとの関係を示した図
【図５】 本発明の、雰囲気，温度，露点の関係からなる焼入れ条件範囲を示す図[0001]
[Industrial application fields]
The present invention relates to a method for producing martensitic stainless steel having excellent corrosion resistance and toughness, which is used for wear-resistant products such as flat healds, droppers, wings and deformed ridges, blades, nozzles, valve seats, valves and the like.
[0002]
[Prior art]
As the wear-resistant steel such as flat heald, dropper, wing, etc., SUS420J2 or steel with improved abrasion resistance by Ti and Nb carbides as proposed in Japanese Patent Laid-Open No. 2000-192198 is used. ing. Moreover, SUS420J2 is used for a cutter, a nozzle, a valve seat, a valve, and the like.
In order to impart strength and toughness, these steel sheets are subjected to a heat treatment in which they are quenched by heating to a high temperature and subsequently tempering at a temperature of Ac 1 point or less. Usually, time-consuming heating and tempering are performed in a hydrogen atmosphere or an AX gas atmosphere of 75% H 2 -25% N 2 gas in order to prevent coloring due to surface oxidation. Japanese Patent Application Laid-Open No. 10-130791 proposes that the heating atmosphere during quenching is a nitrogen atmosphere containing a trace amount of oxygen or an AX gas atmosphere.
[0003]
[Problems to be solved by the invention]
Even if quenching or tempering is performed in an atmosphere that suppresses surface oxidation, surface alteration may occur during the process. For this reason, corrosion resistance and toughness may deteriorate in a steel plate that has been subjected to quenching and tempering treatment. As a result, the quality after being processed into products such as flat healds and shark feathers cannot be kept constant, resulting in various problems such as a decrease in yield and an increase in manufacturing cost. In particular, when the corrosion resistance of the flat heald is lowered, red rust is generated in the weaving machine, which causes a problem of contaminating the fabric.
The present invention has been devised to solve such problems and suppresses surface alteration even during quenching and tempering treatment, and as a result, martensitic stainless steel excellent in corrosion resistance and toughness is stabilized. It is an object of the present invention to provide a manufacturing method.
[0004]
[Means for Solving the Problems]
The method for producing martensitic stainless steel excellent in corrosion resistance and toughness according to the present invention achieves the object, C: 0.05-1.20% by mass, Cr: 8.0-18.0% by mass, Si: 1.0% by mass or less, Mn: 1.0% by mass or less, and if necessary, Ti or Nb alone or a total amount of 0.05 to 1.0% by mass, with the balance being Fe And a steel plate having a composition composed of inevitable impurities in a hydrogen-nitrogen atmosphere under conditions of dew point DP (° C.) and temperature T (° C.) satisfying the following equations (1) and (2), or the following equations (2), ( 3) A quenching treatment is performed in a hydrogen atmosphere under conditions of a dew point DP (° C.) and a temperature T (° C.) satisfying the formula, and then a tempering treatment is performed at a temperature of Ac 1 point or less.
0.0095T-168 ≦ DP ≦ 0.105T-132 (1)
950 ≦ T ≦ 1200 (2)
DP ≦ 0.105T-132 (3)
[0005]
[Embodiments of the present invention]
The inventors of the present invention have made extensive studies on the cause of surface alteration that reduces corrosion resistance and toughness during quenching. As a result, it has been found that surface oxidation and surface nitridation occur due to differences in quenching temperature and atmospheric conditions, thereby leading to deterioration of corrosion resistance and toughness.
First, a preliminary test that found this phenomenon will be described.
Quenched 0.5 mm thick cold-rolled steel sheet of martensitic stainless steel containing Fe-0.3% C-0.5% Si-0.5% Mn-13% Cr-0.31% Nb as basic components -The tempered test materials were produced in a laboratory under various quenching conditions, and the relationship between the surface condition, corrosion resistance and toughness of each test material was investigated. The quenching treatment was performed at 1040 ° C. in an AX gas atmosphere of 75% H 2 -25% N 2 and changing the dew point. Tempering was performed at 400 ° C. in a nitrogen atmosphere.
[0006]
The test material (a) was quenched at a dew point of −20 ° C. and was colored after tempering. The test material (b) was quenched at a dew point of −50 ° C. and was not colored after tempering. The test material (c) was quenched at a dew point of −75 ° C. and was not colored even after tempering.
These test materials were evaluated for toughness by evaluating the corrosion resistance by a cast test and measuring the U-bending fracture distance by a U-bending fracture test as shown in FIG. In addition, it is evaluated that toughness is so favorable that U bending break distance is small.
According to a 24-hour cast test in accordance with JIS H8502-7, the test material (a) and the test material (c) were significantly rusted and the corrosion resistance was reduced. On the other hand, the rusting of the test material (b) was very slight.
[0007]
When the fracture distance after the U-bending fracture test was measured, the U-bending fracture distance of the test materials (a) and (b) was very small, whereas the U-bending fracture distance of the test material (c) was large. .
The results are summarized in FIG.
From the results of FIG. 2, after the quenching and tempering treatment, only the test material (a) is colored, and the test material (b) has very little rusting, whereas the test materials (a), ( It can be seen that c) shows significantly reduced corrosion resistance due to rusting, and only the test material (c) has significantly reduced toughness.
[0008]
In order to know the surface state of each test material after quenching and tempering treatment, the surface layer portion of the test material was analyzed by GDS (glow discharge light emission analyzer). The result is shown in FIG.
Concentration of Si, Mn, and O was observed on the surface layers of the test materials (b) and (c) that were not surface-colored, whereas Si and Mn were observed on the surface layer of the colored test material (a). , O as well as a significant concentration of Cr was observed.
In general, when an oxide of Cr is formed on the surface layer of an Fe—Cr steel plate, it is known that a Cr-deficient layer is formed immediately below the scale and the corrosion resistance is lowered. Therefore, it is surmised that the decrease in corrosion resistance of the test material (a) is caused by the formation of Cr oxide on the surface layer.
[0009]
On the other hand, FIG. 3 also shows that the concentration of N in the surface layer of the test material (c) is large and the concentration of Si, Mn, and O is small compared to the test materials (a) and (b). As a result of investigating this test material (c) in more detail, it was found that the amount of N increased after quenching and tempering, and that Cr nitride was generated near the surface layer after quenching and tempering. . From these results, it is presumed that in the test material (c), Cr nitride was formed in the surface layer part by nitriding during the quenching and tempering treatment, and the Cr deficient layer was formed in the peripheral part, so that the corrosion resistance was lowered. The
Further, it is presumed that the reason why only the test material (c) was significantly nitrided was that the concentration of Si, Mn, and O in the surface layer was not sufficient, so that nitriding during the quenching-tempering treatment could not be suppressed.
[0010]
For each test material, the N content was analyzed, and the relationship between the ΔN amount increased by the quenching-tempering treatment and the U-bending fracture distance was also investigated. The result is shown in FIG.
It can be seen that the U-bending fracture distance increases with the increase in the amount of ΔN, and only the test material (c) having a large amount of ΔN has poor toughness. This decrease in toughness is assumed to be due to the hardening of the surface layer accompanying nitriding.
To summarize the above findings, the corrosion resistance is reduced when Cr is oxidized during the quenching and tempering treatment, and the corrosion resistance and toughness are reduced when nitriding occurs.
[0011]
The present inventors have speculated that the oxidation and nitridation of Cr, which cause the corrosion resistance and toughness of martensitic stainless steel to decrease, depend on conditions such as temperature, atmosphere, and dew point during quenching. And about the martensitic stainless steel which prescribed | regulated the component composition, by repeating the test which investigates the corrosion resistance and toughness of the sample which hardened various steel on various conditions and gave the tempering treatment after that, (1) It has been found that when quenching is performed at a temperature T (° C.), a dew point DP (° C.), and an atmosphere satisfying the formula (4), a product excellent in corrosion resistance and toughness can be obtained.
In addition, the formulas (1) and (3) are displayed on the graph with the results of the repeated tests described above, and a line is drawn to distinguish what is satisfactory and not satisfactory in terms of corrosion resistance and toughness. It is an empirical formula in which lines are expressed by approximate formulas (see FIG. 5).
[0012]
That is, in the case of a hydrogen nitrogen atmosphere,
0.0095T-168 ≦ DP ≦ 0.105T-132 (1)
950 ≦ T ≦ 1200 (2)
For a hydrogen atmosphere,
DP ≦ 0.105T-132 (3)
950 ≦ T ≦ 1200 (4)
It is the range which satisfies.
Here, DP ≦ 0.105T-132 in a hydrogen-nitrogen atmosphere or a hydrogen atmosphere is a range in which no Cr oxide is generated, and 0.0095T-168 ≦ DP in a hydrogen-nitrogen atmosphere is a range in which nitridation is suppressed. It is.
The dew point is defined as “the temperature at which an object is cooled in the atmosphere and the surface of the object begins to be dewed. The water vapor partial pressure at this time is equal to the saturated water vapor pressure” (Iwanami Rikagaku Dictionary). A high dew point means that the water vapor pressure is high, that is, elements such as Cr are easily oxidized. Therefore, if the dew point is lowered in relation to the temperature, the Cr oxide is hardly generated and the corrosion resistance can be maintained.
In the case of a hydrogen atmosphere, no significant nitridation occurs, so there is no restriction on the low dew point side.
[0013]
Further, the quenching temperature needs to be 950 ° C. or higher because the purpose of the quenching process is to obtain a martensite structure having high strength by cooling the steel from a high temperature γ region. However, if it is 1200 degreeC or more, the high temperature strength of a steel plate will become low and a plate may fracture | rupture. For this reason, the upper limit of quenching temperature shall be 1200 degreeC.
The purpose of the tempering treatment is to precipitate carbon dissolved in the quenching treatment as carbides and to recover the base structure and impart toughness. Therefore, the temperature may be at most Ac 1 point.
[0014]
Next, the components of the steel sheet that is subjected to the quenching and tempering treatment of the present invention will be described.
C: 0.05 to 1.20% by mass
C is required to be contained in an amount of 0.05% or more in order to obtain a high strength by being dissolved in the matrix by quenching. However, if it exceeds 1.20%, a huge amount of eutectic carbide precipitates and the hot workability is lowered. Therefore, the C content is 0.05 to 1.20%.
Cr: 8.0-18.0 mass%
Cr is an element necessary for imparting corrosion resistance. However, considering the environment in which the steel material according to the present invention is used, an addition of 8.0% or more is necessary. However, excessive addition causes an increase in production cost, so the upper limit was made 18.0%.
[0015]
Si: 1.0% by mass or less, Mn: 1.0% by mass or less Si and Mn are components added as a deoxidizer during melting. However, excessive addition not only saturates the cure but also increases production costs. Therefore, the upper limit was set to 1.0%.
Ti, Nb:. Either alone or in a total amount from 0.05 to 1 0 wt%
Ti or Nb carbide is extremely hard. For this reason, the corrosion resistance is remarkably improved by dispersing these carbides in the steel matrix. Therefore, it is necessary to add Ti and Nb to steel used for applications where wear resistance is required. However, since the effect is low when the amount of carbide dispersion is small, the addition of Ti alone, Nb alone, or Ti and Nb composite addition is set to 0.05 or more. However, excessive addition of Ti and Nb leads to a decrease in toughness due to the formation of intermetallic compounds, so the upper limit was made 1.0%.
[0016]
【Example】
Various steels shown in Table 1 were melted in accordance with conventional methods and cast into slabs. After the solution treatment, the slab was hot rolled to a thickness of 5.0 mm. The hot-rolled sheet was heat-treated at 780 ° C. for 9 hours and cooled in the furnace. Then, after pickling, cold rolling and intermediate annealing, a steel plate having a thickness of 0.3 mm was obtained by cold rolling. All the steels shown in Table 1 have a chemical composition in the range defined by the present invention.
[0017]
Thereafter, a quenching treatment was performed under the conditions shown in Table 2, and after a tempering treatment at 400 ° C. in a nitrogen atmosphere, the corrosion resistance, toughness, and ΔN amount were measured and evaluated.
In Table 2, A, B, and C are conditions within the scope of the present invention, but D, E, and F are outside the conditions of the present invention.
[0019]
Corrosion resistance was evaluated by carrying out a cast test in accordance with JIS H8502-7. After 24 hours, a sample with slight rusting was evaluated as good with good corrosion resistance and a sample with significant rusting was evaluated as poor corrosion resistance.
As for toughness, the U-bending fracture test of the embodiment shown in FIG.
The ΔN amount was determined from the difference between the nitrogen N amount after the quenching and tempering treatment and the N amount before the treatment. In addition, N amount before hardening of S1, S2, S3 steel is 0.030%, 0.028%, and 0.031%, respectively.
The evaluation results are shown in Table 3.
[0021]
As can be seen from the results in Table 3, with respect to the steel sheets processed in steps A, B, and C that satisfy the quenching conditions defined in the present invention, S1, S2, and S3 steels having the component composition defined in the present invention, Both corrosion resistance and toughness were good.
On the other hand, the steel sheets obtained by treating the S1, S2, and S3 steels in the D or F process have good toughness, but because the dew point at the time of the quenching process is high, Cr oxide is generated and the corrosion resistance is poor. . In addition, the steel sheets obtained by treating S1, S2, and S3 steels in the E step had low dew points during the quenching process, so that nitriding occurred and both corrosion resistance and toughness were reduced.
[0023]
【The invention's effect】
As described above, according to the present invention, when martensitic stainless steel is subjected to a quenching-tempering process, the quenching process is performed under conditions in which the temperature, dew point, and atmosphere are adjusted to have a predetermined relationship with each other. And the oxidation and nitridation of Cr at the time of quenching are suppressed, and the martensitic stainless steel without the deterioration of corrosion resistance and toughness resulting therefrom can be stably produced.
This provides low-cost martensitic stainless steel with excellent corrosion resistance and toughness used for wear-resistant products such as flat healds, droppers, wings, and deformed knives, blades, nozzles, valve seats, valves, etc. Is possible.
[Brief description of the drawings]
FIG. 1 is a diagram for explaining a U-bending test method, which is a toughness evaluation method. FIG. 2 shows the appearance of steel plates classified from the viewpoint of corrosion resistance and toughness after quenching and tempering, and the appearance after corrosion resistance test. Fig. 3 shows the results of toughness evaluation. Fig. 3 shows the results of GDS analysis after quenching and tempering. Fig. 4 shows the relationship between the difference in nitrogen content before and after quenching and tempering. Figure [Figure 5] A diagram showing the quenching condition range of the present invention, which includes the relationship between atmosphere, temperature, and dew point.
Claims (3)
０．００９５Ｔ−１６８≦ＤＰ≦０．１０５Ｔ−１３２ ・・・（１）
９５０≦Ｔ≦１２００ ・・・（２）C: 0.05 to 1.20 mass%, Cr: 8.0 to 18.0 mass%, Si: 1.0 mass% or less, Mn: 1.0 mass% or less, the balance being Fe and inevitable A steel sheet having a composition composed of impurities is subjected to a quenching treatment in a hydrogen-nitrogen atmosphere under conditions of dew point DP (° C.) and temperature T (° C.) satisfying the following formulas (1) and (2), and then Ac 1 point A method for producing martensitic stainless steel having excellent corrosion resistance and toughness, characterized by performing tempering at the following temperatures.
0.0095T-168 ≦ DP ≦ 0.105T-132 (1)
950 ≦ T ≦ 1200 (2)
ＤＰ≦０．１０５Ｔ−１３２ ・・・（３）
９５０≦Ｔ≦１２００ ・・・（４）C: 0.05 to 1.20 mass%, Cr: 8.0 to 18.0 mass%, Si: 1.0 mass% or less, Mn: 1.0 mass% or less, the balance being Fe and inevitable A steel sheet having a composition composed of impurities is quenched in a hydrogen atmosphere under conditions of dew point DP (° C.) and temperature T (° C.) satisfying the following formulas (3) and (4), and then Ac is 1 point or less. A method for producing martensitic stainless steel with excellent corrosion resistance and toughness, characterized by performing tempering treatment at a temperature of 5 ° C.
DP ≦ 0.105T-132 (3)
950 ≦ T ≦ 1200 (4)
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| JP6232227B2 (en) * | 2013-08-12 | 2017-11-15 | 株式会社吉野工作所 | Method for manufacturing thin-walled members made of stainless steel |
| JP6228020B2 (en) * | 2014-01-15 | 2017-11-08 | 株式会社吉野工作所 | Transport cart and method for manufacturing the same |
| KR101834996B1 (en) | 2016-10-19 | 2018-03-06 | 주식회사 포스코 | High hardness martensitic stainless steel with excellent hardenability and method of manufacturing the same |
| CN110359006A (en) * | 2019-07-18 | 2019-10-22 | 中国科学院金属研究所 | A method of improving the resistance to liquid metal corrosion of heat resisting steel |
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| R250 | Receipt of annual fees |
Free format text: JAPANESE INTERMEDIATE CODE: R250 |
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| R250 | Receipt of annual fees |
Free format text: JAPANESE INTERMEDIATE CODE: R250 |
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