JP3604035B2 - Method for producing maraging steel - Google Patents
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Description
【０００１】
【発明の属する技術分野】
本発明は、マルエージング鋼の製造方法に関するものである。
【０００２】
【従来の技術】
マルエージング鋼は、２０００ＭＰａ前後の非常に高い引張強さをもつため、高強度が要求される部材、例えば、ロケット用部品、遠心分離機部品、航空機部品、自動車エンジンの無段変速機用部品、金型、等種々の用途に使用されている。
その代表的な組成には、質量％で１８％Ｎｉ−８％Ｃｏ−５％Ｍｏ−０．４５％Ｔｉ−０．１％Ａｌ−ｂａｌ．Ｆｅが挙げられる。そして、マルエージング鋼は、強化元素として、Ｍｏ、Ｔｉ、を適量含んでおり、時効処理を行うことによって、Ｎｉ３Ｍｏ、Ｎｉ３Ｔｉ 、Ｆｅ２Ｍｏ等の金属間化合物を析出させて高強度を得ることのできる鋼である。
【０００３】
しかし、マルエージング鋼は、非常に高い引張強度が得られる一方、疲労強度に関しては必ずしも高くない。この疲労強度を劣化させる最大の要因に、ＴｉＮやＴｉＣＮ等といった窒化物や炭窒化物の非金属介在物があり、この非金属介在物が鋼中で大きく成長してしまうと、介在物を起点として疲労破壊を生じることになる。
そのため、一般的に鋼中に存在する非金属介在物を少なくするために、真空アーク再溶解（以下、ＶＡＲと記す）法が用いられている。
【０００４】
【発明が解決しようとする課題】
このVAR法で製造されるマルエージング鋼は、均質(成分偏析が少ない)でしかも、非金属介在物の量が少なくなると言った利点を有するものである。
しかしながら、VAR法で製造するマルエージング鋼にも、比較的大きなTiNやTiCN等の窒化物や炭窒化物の非金属介在物が残留し、残留した大きな非金属介在物は、VAR後に行う熱間鍛造、熱処理、熱間圧延、冷間圧延を行った後の素材中にもそのまま残留し、残留する大きな非金属介在物を起点とした疲労破壊を生じる原因となっていた。
本発明の目的は、VARを行って得られるマルエージング鋼中に残留する、TiNやTiCN等をはじめとする非金属介在物の大きさを最長で14.5μm以下にできるマルエージング鋼の製造方法を提供することである。
【０００５】
【発明が解決しようとする課題】
上述したように、VARを行うことによって、均質で非金属介在物の量を低減できるという利点がある。本発明者等は、この利点を損なうことなく、窒化物や炭窒化物の非金属介在物の大きさを小さくする製造条件について鋭意検討を行った。
この検討を行うに際して、例えば、ロケット用部品、遠心分離機部品、航空機部品、自動車エンジンの無段変速機用部品、金型、等種々の用途の使用に耐え得る窒化物や炭窒化物等の非金属介在物の大きさとして、最長で14.5μｍ以下であれば、非金属介在物に起因する不良が低減若しくは抑制できるものと判断した。
そして、上記の最終製品に近い状態で最長で14.5μｍ以下の非金属介在物に調整する方法を溶解工程から順に検討を試みた。
その結果、最も効果的な方法としてVARに用いる電極中に存在する窒化物や炭窒化物の非金属介在物の大きさを特定の大きさ以下に制御した電極を用いることで、最終製品に近い状態で最長で14.5μｍ以下に調整できることを知見し、本発明に到達した。
【０００６】
即ち本発明は、真空アーク再溶解を用いるマルエージング鋼の製造において、真空アーク再溶解に用いる電極は鋳型比 1.5 以上で製造し、且つ電極中に存在する窒化物や炭窒化物の非金属介在物の大きさを、最長で10μm以下に調整したマルエージング鋼の製造方法である。
更に好ましくは、マルエージング鋼の化学組成が質量%で、C:0.01%以下、Ni:8.0〜22.0%、Co:5.0〜20.0%、Mo:2.0〜9.0%、Ti:2.0%以下、Al:1.7%以下、O:0.0015%以下、残部は実質的にFeからなるマルエージング鋼の製造方法である。
【０００７】
【発明の実施の形態】
本発明の最大の特徴は、ＶＡＲに用いる電極中の窒化物や炭窒化物の非金属介在物を特定の大きさ以下に制御してＶＡＲを行うことによって、均質で非金属介在物の量を低減できるという利点を活かしながら、更にＴｉＮやＴｉＣＮ等の窒化物や炭窒化物の非金属介在物の大きさを小さく制御できたことにある。
以下に、本発明を詳しく説明する。
【０００８】
本発明者らは、ＶＡＲ用の電極鋼塊およびＶＡＲ後の鋼塊の窒化物や炭窒化物に関する研究から、以下の知見を得た。
マルエージング鋼は窒素や炭素の親和力が大きいＴｉを含有していることから、ＶＡＲ用の電極鋼塊製造段階でＴｉＮやＴｉＣＮ等の窒化物や炭窒化物の非金属介在物が存在する。これらの非金属介在物は、ＶＡＲの再溶解時に一部はＴｉＮ→Ｔｉ＋ＮやＴｉＣＮ→Ｔｉ＋Ｃ＋Ｎの反応により溶鋼中へ溶解し、溶存窒素や溶存炭素が増加する。また一部は完全には溶解せずにＴｉＮやＴｉＣＮ等の窒化物や炭窒化物の非金属介在物の状態で溶鋼プール内に浮遊する。
溶鋼プールは凝固殻への抜熱により逐次凝固が進行していくが、凝固前面付近では溶鋼温度が低下し、溶鋼中に溶存している窒素や炭素は上述の未固溶のＴｉＮやＴｉＣＮ表面上に晶出し成長していく。
【０００９】
また、ＶＡＲでは水冷Ｃｕモールドと接する溶鋼プール表面の外周部にはシェルフと称する凝固殻が成長する。このシェルフには浮上分離したＴｉＮやＴｉＣＮ等の窒化物や炭窒化物の非金属介在物が付着しているが、再溶解の進行による湯面の上昇により、シェルフの一部が溶解する。
このときシェルフに捕捉されていたＴｉＮやＴｉＣＮ等の窒化物や炭窒化物の非金属介在物は溶鋼プール内を浮遊する。これらの浮遊ＴｉＮはＴｉＣＮは上述同様に溶鋼温度低下に伴い成長する。
【００１０】
以上のようにVAR再溶解時に溶鋼プール内を浮遊するTiNやTiCN等の窒化物や炭窒化物の存在により、TiNやTiCN等の窒化物や炭窒化物が大きくなることがわかる。
従ってVAR鋼塊内のTiNやTiCN等の窒化物や炭窒化物を微細にするには、再溶解時の溶鋼プール内を浮遊するTiNやTiCN等の窒化物や炭窒化物を小さくする方法をとることが必要である。
すなわち、本発明者らは電極中の窒化物や炭窒化物はVARにより粗大化することを見出し、電極中の窒化物や炭窒化物の大きさを最長で10μm以下に制御した電極でVARすることで製品の窒化物や炭窒化物の大きさを最長で14.5μm以下にできることを見出した。
そして、VARプロセスは電極先端部の連続的で短時間の昇温工程で電極先端部の溶解が進行するために、電極の窒化物や炭窒化物が10μm以上の大きさでは、未溶解の窒化物や炭窒化物が大きくなるため、VAR後の鋼塊の窒化物や炭窒化物が14.5μmを超えることになる知見を得た。
【００１１】
電極の窒化物や炭窒化物を微細にする方法として、電極鋼塊製造時の凝固速度を高めること、電極鋼塊の窒素濃度を下げることなどがあげられる。
凝固速度を高めるには、鋳型比を例えば 1.5 以上に大きくすることを必須とし、電極鋼塊の径を例えば800mm以下程度、好ましくは550mm以下程度に小さくすること、鋳造後鋳型の衝風冷却や散水冷却、水冷鋳型の採用、Cuのような高熱伝導率の鋳型採用等で、抜熱速度を高める方法がある。また、鋳造温度を合金の液相線温度直上とし，凝固時間を短縮する方法も有効である。
また、窒素濃度を下げる方法としては、窒素含有量の低い原料を使用すること、真空誘導溶解炉での溶解精錬を高真空下で行うことで平衡窒素濃度を下げること、真空精錬時に溶鋼中にアルゴンガス等の不活性ガスを吹き込むこと等の方法がある。
以上の方法の組合せで電極中の窒化物や炭窒化物の大きさを10μm以下に制御することでVAR後の鋼塊の窒化物や炭窒化物の大きさを14.5μm以下にできる。
なお、大気圧下、加圧下または減圧下でESRを実施した鋼塊をVAR用の電極とすることがあるが、この場合、ESR後の鋼塊で窒化物や炭窒化物の大きさを10μm以下に制御されていることが必要である。
【００１２】
次に、本発明の好ましい組成範囲の限定理由について述べる。
Ｃは炭化物を形成し、金属間化合物の析出量を減少させて疲労強度を低下させるため本発明ではＣの上限を０．０１％以下とした。
Ｎｉは靱性の高い母相組織を形成させるためには不可欠の元素であるが、８．０％未満では靱性が劣化する。一方、２０％を越えるとオーステナイトが安定化し、マルテンサイト組織を形成し難くなることから、Ｎｉは８．０〜２０．０％とした。
【００１３】
Ｃｏは、マトリックスであるマルテンサイト組織を安定性に大きく影響することなく、Ｍｏの固溶度を低下させることによってＭｏが微細な金属間化合物を形成して析出するのを促進することによって析出強化に寄与するが、その含有量が５．０％未満では必ずしも十分効果が得られず、また２０．０％を越えると脆化する傾向がみられることから、Ｃｏの含有量は５．０〜２０．０％にした。
Ｍｏは時効処理により、微細な金属間化合物を形成し、マトリックスに析出することによって強化に寄与する元素であるが、その含有量が２．０％未満の場合その効果が少なく、また９．０％を越えて含有すると延性、靱性を劣化させるＦｅ、Ｍｏを主要元素とする粗大析出物を形成しやすくなるため、Ｍｏの含有量を２．０〜９．０％とした
【００１４】
Ｔｉは、Ｍｏと同様に時効処理により微細な金属間化合物を形成し、析出することによって強化に寄与する元素であるが、その含有量が２．０％を越えて含有させると延性、靱性が劣化する。また、Ｍｏで十分硬さが得られている場合は無添加でも良いため、Ｔｉの含有量を２．０％以下とした。
Ａｌは、時効析出した強化に寄与するだけでなく、脱酸作用を持っているが、１．７％を越えて含有させると靱性が劣化することから、その含有量を１．７％以下とした。
【００１５】
Ｏは酸化物系非金属介在物を形成するため、０．００１５％以下に制限する。Ｏが０．００１５％を超えて含有すると疲労強度が著しく低下するため、その含有量を０．００１５％以下にした。
【００１６】
なお、本発明ではこれら規定する元素以外は実質的にFeとしているが、例えばBは、結晶粒を微細化するのに有効な元素でるため、靱性が劣化させない程度の0.01%以下の範囲で含有させても良い。
窒素は窒化物や炭窒化物の形成元素であるため、少なくすることが好ましいが、疲労特性としては窒化物や炭窒化物の大きさが14.5μm以下にできれば、窒素の上限について特に規定はない。
また、不可避的に含有する不純物元素のSi、Mnは脆化をもたらす粗大な金属間化合物の析出を促進して延性、靭性を低下させたり、非金属介在物を形成して疲労強度を低下させるので、Si、Mn共に0.1%以下に、望ましくは0.05%以下とすれば良く、また、P、Sも粒界脆化させたり、非金属介在物を形成して疲労強度を低下させるので、0.01%以下とすると良い。
【００１７】
【実施例】
以下、実施例として更に詳しく本発明を説明する。
表１に示す化学組成Ｎｏ１に対して、同一材質の鋳型φ３００ｍｍ、φ４５０ｍｍおよびφ６５０ｍｍを用いて電極鋼塊を造り、これらの電極鋼塊を据込み鍛造、鍛伸、旋削によりＶＡＲ用の消耗電極（φ４３０ｍｍ）を各二本用意した。
電極製造時の鋳型比は２．５ とし、鋳造後鋳型の衝風冷却によって，凝固速度を高めた。なお、原料は窒素含有量が１５ｐｐｍといった窒素含有量の低い原料を用いた。
また化学組成Ｎｏ２に対して、同一径４５０ｍｍの砂型鋳型、鋳鉄鋳型、Ｃｕ鋳型を用いて電極鋼塊を造り、旋削でφ４３０に寸法を合わせＶＡＲ用消耗電極を各二本用意した。なお、原料は窒素含有量が５０ｐｐｍとし，真空溶解炉の底部よりＡｒガスを１５ ｌ／分吹き込むことにより窒素濃度を低減させた。
【００１８】
同一条件で製造した各二本の消耗電極中の一本は電極中の窒化物や炭窒化物の非金属介在物の大きさを調査するために、電極の両端（上側、下側）および真中の位置で横断面の試験片を採取した。各横断面試験片の中央部からダライ粉５ｇを採取し、硝酸溶液で溶解後、フィルターでろ過を行いフィルター上の窒化物や炭窒化物からなる残渣をＳＥＭで観察を行い最大の窒化物または炭窒化物の大きさを測定した。
この時、非金属介在物の大きさは、非金属介在物に外接する円の直径で評価し、この外接する円の直径を非金属介在物の最長の長さと定義する。観察された非金属介在物のうち、最も大きいものを最大最長とし、表２に示した。
【００１９】
【表１】
【表２】
次に、この電極を用いて電流を７０００Ａとして、ＶＡＲを行い径が５００ｍｍの鋼塊を作製した。なお、この時の真空度は１．０Ｐａであった。
そして、ＶＡＲ鋼塊で１２５０℃×２０時間のソーキングを行い、次いで熱間鍛造を行い熱間鍛造品とした。
次に、これら材料に熱間圧延、８２０℃×１時間の固溶化処理、冷間圧延、８２０℃×１時間の固溶化処理と４８０℃×５時間の時効処理を行い、マルエージング鋼の帯鋼を作製した。
【００２２】
得られたマルエージング鋼の帯鋼の両端（電極上側、下側に対応）および１／２長さ位置（電極真中に対応）の３つの位置の幅中央部より介在物評価用の試験片５ｇを採取し、同様に硝酸溶液でＴｉＮ、ＴｉＣＮの非金属介在物を抽出し、ＳＥＭ観察で大きさを測定した。ＴｉＮ、ＴｉＣＮの非金属介在物の最大最長を表３に示した。
【００２３】
【表３】
表３より、VAR電極中の非金属介在物の最大最長が10μm以下に制御した本発明方法では、板状まで加工した帯鋼においても非金属介在物の最大最長が14.5μm以下になっていることが分かる。
一方、VAR電極中での非金属介在物の大きさが10μmを超える比較材では非金属介在物のサイズが大きくなり、板材では14.5μmを超える粗大な非金属介在物の存在が認められたことから、この非金属介在物を起点とした疲労破壊が起こる可能性が大きい結果となった。
【００２５】
【発明の効果】
以上のような結果から、本発明の製造方法を適用すると、ＴｉＮやＴｉＣＮ等の非金属介在物の大きさが小さく、優れた疲労強度を有するマルエージング鋼を製造することが出来る。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for producing maraging steel.
[0002]
[Prior art]
Since maraging steel has a very high tensile strength of about 2000 MPa, members requiring high strength, for example, parts for rockets, parts for centrifuges, parts for aircraft, parts for continuously variable transmissions of automobile engines, It is used for various purposes such as molds.
The typical composition includes 18% Ni-8% Co-5% Mo-0.45% Ti-0.1% Al-bal. Fe. The maraging steel contains appropriate amounts of Mo and Ti as strengthening elements, and is subjected to aging treatment to precipitate intermetallic compounds such as Ni 3 Mo, Ni 3 Ti, and Fe 2 Mo, thereby achieving high strength. It is steel that can obtain.
[0003]
However, while maraging steel can provide a very high tensile strength, the fatigue strength is not always high. The biggest factor that deteriorates the fatigue strength is non-metallic inclusions such as nitrides and carbonitrides, such as TiN and TiCN. If these non-metallic inclusions grow large in steel, they start from the inclusions. Fatigue fracture.
Therefore, a vacuum arc remelting (hereinafter, referred to as VAR) method is generally used to reduce nonmetallic inclusions present in steel.
[0004]
[Problems to be solved by the invention]
The maraging steel manufactured by the VAR method has the advantage of being homogeneous (less component segregation) and having a reduced amount of nonmetallic inclusions.
However, relatively large nonmetallic inclusions such as nitrides and carbonitrides such as TiN and TiCN also remain in the maraging steel manufactured by the VAR method, and the large nonmetallic inclusions that remain remain after the VAR. It remains in the raw material after forging, heat treatment, hot rolling and cold rolling, and causes fatigue fracture starting from the remaining large nonmetallic inclusions.
An object of the present invention is to provide a method for producing a maraging steel that can reduce the size of non-metallic inclusions such as TiN and TiCN remaining at most 14.5 μm or less in a maraging steel obtained by performing VAR. To provide.
[0005]
[Problems to be solved by the invention]
As described above, performing VAR has the advantage of reducing the amount of homogeneous and non-metallic inclusions. The present inventors have intensively studied manufacturing conditions for reducing the size of nonmetallic inclusions such as nitrides and carbonitrides without impairing this advantage.
In conducting this study, for example, parts such as rocket parts, centrifuge parts, aircraft parts, parts for continuously variable transmissions of automobile engines, molds, molds, etc. It was determined that if the size of the non-metallic inclusions was 14.5 μm or less at the longest, defects caused by the non-metallic inclusions could be reduced or suppressed.
Then, a method of adjusting a non-metallic inclusion having a maximum length of 14.5 μm or less in a state close to the final product was examined in order from the melting step.
As a result, the most effective method is to use an electrode in which the size of non-metallic inclusions such as nitrides and carbonitrides in the electrode used for VAR is controlled to a specific size or less, so that it is close to the final product The inventors have found that the length can be adjusted to 14.5 μm or less in the maximum state, and have reached the present invention.
[0006]
That is, the present invention relates to a method of manufacturing a maraging steel using vacuum arc remelting, in which an electrode used for vacuum arc remelting is manufactured at a mold ratio of 1.5 or more, and non-metallic inclusions of nitride and carbonitride present in the electrode. This is a method for producing a maraging steel in which the size of the object is adjusted to a maximum of 10 μm or less.
More preferably, the chemical composition of the maraging steel is% by mass, C: 0.01% or less, Ni: 8.0 to 22.0%, Co: 5.0 to 20.0%, Mo: 2.0 to 9.0%, Ti: 2.0% or less, Al: This is a method for producing a maraging steel substantially consisting of 1.7% or less, O: 0.0015% or less, and the balance being substantially Fe.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
The greatest feature of the present invention is that the VAR is performed by controlling the non-metallic inclusions such as nitrides and carbonitrides in the electrode used for the VAR to a specific size or less, thereby reducing the amount of the homogeneous and non-metallic inclusions. The advantage is that the size of non-metallic inclusions such as nitrides and carbonitrides, such as TiN and TiCN, can be controlled to be small while taking advantage of the advantage that they can be reduced.
Hereinafter, the present invention will be described in detail.
[0008]
The present inventors have obtained the following knowledge from studies on nitrides and carbonitrides of VAR electrode steel ingots and post-VAR steel ingots.
Since maraging steel contains Ti having a high affinity for nitrogen and carbon, nonmetallic inclusions such as nitrides and carbonitrides, such as TiN and TiCN, are present in the step of producing an electrode steel ingot for VAR. These nonmetallic inclusions are partially dissolved in the molten steel by the reaction of TiN → Ti + N or TiCN → Ti + C + N when the VAR is redissolved, and the dissolved nitrogen and dissolved carbon increase. Further, a part thereof is not completely dissolved and floats in the molten steel pool in a state of nonmetallic inclusions such as nitrides and carbonitrides such as TiN and TiCN.
In the molten steel pool, solidification proceeds gradually due to heat removal to the solidified shell, but the temperature of the molten steel decreases near the solidification front, and the nitrogen and carbon dissolved in the molten steel are removed from the undissolved TiN and TiCN surfaces described above. Crystallizes and grows on top.
[0009]
In VAR, a solidified shell called a shelf grows on the outer peripheral portion of the surface of the molten steel pool in contact with the water-cooled Cu mold. Non-metallic inclusions such as nitrides and carbonitrides, such as TiN and TiCN, separated by flotation adhere to the shelf, but a part of the shelf is melted due to a rise in the molten metal level due to the progress of remelting.
At this time, non-metallic inclusions such as nitrides and carbonitrides such as TiN and TiCN trapped in the shelf float in the molten steel pool. These floating TiNs grow as the temperature of the molten steel decreases as in the case of TiCN as described above.
[0010]
As described above, it can be seen that the presence of nitrides and carbonitrides such as TiN and TiCN floating in the molten steel pool during VAR remelting increases nitrides and carbonitrides such as TiN and TiCN.
Therefore, in order to make nitrides and carbonitrides such as TiN and TiCN in the VAR ingot fine, a method of reducing nitrides and carbonitrides such as TiN and TiCN floating in the molten steel pool during remelting is required. It is necessary to take.
That is, the present inventors have found that nitrides and carbonitrides in electrodes are coarsened by VAR, and VAR is performed on electrodes in which the size of nitrides and carbonitrides in electrodes is controlled to 10 μm or less at the longest. It has been found that the size of nitrides and carbonitrides in products can be reduced to 14.5 μm or less at the longest.
In the VAR process, the melting of the electrode tip progresses in a continuous and short-time heating process at the electrode tip.If the electrode nitride or carbonitride has a size of 10 μm or more, undissolved nitride It was found that the nitride and carbonitride of the ingot after VAR exceeded 14.5 μm due to the large size of the carbide and carbonitride.
[0011]
Examples of a method for reducing the nitride or carbonitride of the electrode include increasing the solidification rate during production of the electrode ingot and lowering the nitrogen concentration of the electrode ingot.
To increase the rate of solidification, an essential to increase the template ratio for example 1.5 or higher, the degree of the diameter of the electrode steel ingot example 800mm or less, preferably to be small as below 550 mm, blast cooling Casting after mold and watering cooling, the adoption of a water-cooled mold, with mold adoption of high thermal conductivity, such as Cu, is a heat extraction rate is high Mel method. It is also effective to reduce the solidification time by setting the casting temperature to just above the liquidus temperature of the alloy.
In addition, as a method of lowering the nitrogen concentration, using a raw material having a low nitrogen content, lowering the equilibrium nitrogen concentration by performing melting and refining in a vacuum induction melting furnace under high vacuum, There is a method of blowing an inert gas such as an argon gas.
By controlling the size of the nitride or carbonitride in the electrode to 10 μm or less by a combination of the above methods, the size of the nitride or carbonitride of the ingot after VAR can be reduced to 14.5 μm or less.
In addition, the steel ingot subjected to ESR under atmospheric pressure, under pressure or under reduced pressure may be used as an electrode for VAR.In this case, the size of the nitride or carbonitride in the steel ingot after ESR is 10 μm. It must be controlled as follows.
[0012]
Next, the reasons for limiting the preferred composition range of the present invention will be described.
In the present invention, the upper limit of C is set to 0.01% or less in the present invention because C forms carbide and reduces the precipitation amount of the intermetallic compound to lower the fatigue strength.
Ni is an indispensable element for forming a matrix structure having high toughness, but if it is less than 8.0%, toughness is deteriorated. On the other hand, if it exceeds 20%, austenite is stabilized, and it becomes difficult to form a martensite structure. Therefore, Ni is set to 8.0 to 20.0%.
[0013]
Co reduces precipitation of Mo by reducing the solid solubility of Mo without greatly affecting the martensitic structure as a matrix, thereby promoting the formation of Mo to form fine intermetallic compounds and the precipitation strengthening. However, if the content is less than 5.0%, a sufficient effect is not necessarily obtained, and if the content exceeds 20.0%, embrittlement tends to be observed, so that the Co content is 5.0 to 5.0. 20.0%.
Mo is an element that forms a fine intermetallic compound by aging treatment and contributes to strengthening by precipitating in a matrix. When its content is less than 2.0%, its effect is small, and it is 9.0. %, It is easy to form coarse precipitates containing Fe and Mo as main elements, which deteriorate ductility and toughness. Therefore, the content of Mo is set to 2.0 to 9.0%.
Ti is an element that contributes to strengthening by forming and precipitating a fine intermetallic compound by aging treatment like Mo, but when its content exceeds 2.0%, ductility and toughness are reduced. to degrade. Further, when sufficient hardness is obtained with Mo, the addition of Ti may be omitted.
Al not only contributes to aging-precipitated strengthening but also has a deoxidizing effect. However, if it is contained in excess of 1.7%, toughness is deteriorated, so that the content is limited to 1.7% or less. did.
[0015]
O forms an oxide-based nonmetallic inclusion, and is therefore limited to 0.0015% or less. If O is contained in excess of 0.0015%, the fatigue strength is significantly reduced, so the content was made 0.0015% or less.
[0016]
In the present invention, elements other than these specified elements are substantially Fe, but for example, B is an element effective for refining crystal grains, so that B is contained in a range of 0.01% or less to the extent that toughness is not deteriorated. You may let it.
Nitrogen is a forming element of nitride and carbonitride, so it is preferable to reduce it.However, as long as the size of nitride and carbonitride can be reduced to 14.5 μm or less, there is no particular upper limit for nitrogen. .
In addition, inevitably contained impurity elements Si and Mn promote the precipitation of coarse intermetallic compounds that cause embrittlement and reduce ductility and toughness, and reduce fatigue strength by forming nonmetallic inclusions. Therefore, both Si and Mn should be 0.1% or less, desirably 0.05% or less.Also, P and S may also cause grain boundary embrittlement or non-metallic inclusions to reduce fatigue strength. % Or less is recommended.
[0017]
【Example】
Hereinafter, the present invention will be described in more detail by way of examples.
With respect to the chemical composition No. 1 shown in Table 1, electrode steel ingots were produced using molds of the same material φ300 mm, φ450 mm, and φ650 mm, and these electrode steel ingots were subjected to upsetting, forging, elongation, and turning to obtain VAR consumable electrodes ( (φ430 mm) were prepared.
The mold ratio at the time of electrode production was 2.5, and the solidification rate was increased by blast cooling of the mold after casting. The raw material used was a raw material having a low nitrogen content such as a nitrogen content of 15 ppm.
For the chemical composition No. 2, an electrode steel ingot was produced using a sand mold, a cast iron mold, and a Cu mold having the same diameter of 450 mm, and the dimensions were adjusted to φ430 by turning to prepare two consumable electrodes for VAR. The raw material had a nitrogen content of 50 ppm, and the nitrogen concentration was reduced by blowing Ar gas at a rate of 15 l / min from the bottom of the vacuum melting furnace.
[0018]
One of the two consumable electrodes manufactured under the same conditions was used to investigate the size of non-metallic inclusions such as nitrides and carbonitrides in the electrodes. The test piece of the cross section was taken at the position of. 5 g of Dalai powder was collected from the center of each cross-section test piece, dissolved in a nitric acid solution, filtered through a filter, and the residue consisting of nitrides and carbonitrides on the filter was observed with a SEM. The size of the carbonitride was measured.
At this time, the size of the nonmetallic inclusion is evaluated by the diameter of a circle circumscribing the nonmetallic inclusion, and the diameter of the circumscribed circle is defined as the longest length of the nonmetallic inclusion. Among the observed nonmetallic inclusions, the largest was the largest and the largest was shown in Table 2.
[0019]
[Table 1]
[Table 2]
Next, VAR was performed using this electrode at a current of 7000 A to produce a steel ingot having a diameter of 500 mm. The degree of vacuum at this time was 1.0 Pa.
Then, VAR steel ingot was soaked at 1250 ° C. for 20 hours, and then hot forged to obtain a hot forged product.
Next, these materials were subjected to hot rolling, solution treatment at 820 ° C. × 1 hour, cold rolling, solution treatment at 820 ° C. × 1 hour, and aging treatment at 480 ° C. × 5 hours to obtain a maraging steel strip. Steel was made.
[0022]
5 g of a test piece for inclusion evaluation from the center of the width of the obtained maraging steel strip at both ends (corresponding to the upper and lower sides of the electrode) and at one-half length position (corresponding to the center of the electrode). Was collected, and similarly non-metallic inclusions of TiN and TiCN were extracted with a nitric acid solution, and the size was measured by SEM observation. Table 3 shows the maximum length of the nonmetallic inclusions of TiN and TiCN.
[0023]
[Table 3]
According to Table 3, in the method of the present invention in which the maximum length of the non-metallic inclusions in the VAR electrode was controlled to 10 μm or less, the maximum length of the non-metallic inclusions was 14.5 μm or less even in a steel strip processed to a plate shape. You can see that.
On the other hand, the size of nonmetallic inclusions in the VAR electrode with nonmetallic inclusions larger than 10 μm increased in the comparative material, and the presence of coarse nonmetallic inclusions in the plate material exceeding 14.5 μm was observed. Therefore, the result that the possibility of fatigue fracture starting from the non-metallic inclusions is large was obtained.
[0025]
【The invention's effect】
From the above results, when the manufacturing method of the present invention is applied, a maraging steel having a small size of nonmetallic inclusions such as TiN and TiCN and having excellent fatigue strength can be manufactured.
Claims (2)
残部は実質的にFeからなることを特徴とするマルエージング鋼の製造方法。The chemical composition of the maraging steel according to claim 1 is mass%, C: 0.01% or less, Ni: 8.0 to 22.0%, Co: 5.0 to 20.0%, Mo: 2.0 to 9.0%, Ti: 2.0% or less, Al: 1.7% or less, O: 0.0015% or less,
A method for producing a maraging steel, wherein the balance substantially consists of Fe.
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