JP3565428B2 - Steel for machine structure - Google Patents
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- JP3565428B2 JP3565428B2 JP2000399075A JP2000399075A JP3565428B2 JP 3565428 B2 JP3565428 B2 JP 3565428B2 JP 2000399075 A JP2000399075 A JP 2000399075A JP 2000399075 A JP2000399075 A JP 2000399075A JP 3565428 B2 JP3565428 B2 JP 3565428B2
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Description
【0001】
【発明の属する技術分野】
本発明は、機械構造用鋼材に関する。詳しくは、自動車の等速ジョイントやシャフトを初めとした各種の高周波焼入れされる部品の素材として好適な機械構造用鋼材に関する。更に詳しくは、高周波焼入れ性、冷間加工性、被削性及び耐疲労特性に優れ、高周波焼入れされる部品の素材として好適な機械構造用鋼材に関する。
【0002】
【従来の技術】
動力機械の出力アップや燃費向上のための軽量化を目的に、動力伝達部品の高強度化が求められている。なかでも、自動車の等速ジョイントやシャフトに対する高強度化の要求が大きい。
【0003】
自動車の等速ジョイントやシャフトなどは、鍛造後の寸法精度が高く切削量の低減が可能な冷間鍛造で成形加工された後、表面硬化処理としての高周波焼入れが施されて製造されることが多い。したがって、素材となる鋼材には良好な高周波焼入れ性が要求される。なお、近年、一層のコスト低減及び製造工程の簡便化のために、良好な高周波焼入れ性を有するとともに、より優れた被削性と冷間加工性とを有する鋼材に対する要求が大きくなっている。又、上記自動車の等速ジョイントやシャフトなどには繰り返し応力が作用するため、耐疲労特性が一層優れた鋼材に対する要求も大きくなっている。
【0004】
高周波焼入性を確保しながら被削性と冷間加工性のいずれか一方を高める技術は従来数多く報告されているが、被削性と冷間加工性の双方を高めることに関しては必ずしも配慮されていない。
例えば、特開平9−287054号公報に「冷間鍛造―高周波焼入れ用鋼」として、又、特開平9−287055号公報に「冷間加工性の優れた高周波焼入れ用鋼」として、冷間加工性と高周波焼入れ性とを兼備できる鋼が開示されている。しかし、上記の各公報で提案された鋼の場合には被削性に関しては全く配慮されていない。
【0005】
鋼にPb、Te、Bi、Caなどの快削元素(被削性改善元素)を添加すれば被削性が高まることはよく知られている。しかし、特開平7−97656号公報の「冷間鍛造用鋼」として開示された鋼のように、単に上記快削元素を添加しただけでは冷間加工性の低下が生ずることがある。
【0006】
特開平7−242989号公報に「冷間鍛造性、被削性及びねじり疲労強度に優れた高周波焼入用鋼」として提案された鋼では、被削性向上のためにB窒化物を利用しているが、焼入れ性との両立のためにCr、Ni、Moなどの合金元素を多量に添加する必要があり、このため冷間加工時の変形抵抗が大きくなって、十分な冷間加工性が確保できない場合がある。
【0007】
特開平11−1743号公報には「被削性に優れた高強度高靱性調質鋼材」として、Ti硫化物のサイズと量を制御することで被削性を向上させる技術が提案されている。しかし、鋼を冷間加工する場合には硫化物だけではなく酸化物、窒化物などの非金属介在物(以下、単に介在物という)が冷間加工性の低下を引き起こすことがあるため、単にTi硫化物のサイズと量を規定するだけでは、被削性と冷間加工性の両立が図れない場合がある。
一方、耐疲労特性を高める技術が、例えば、特開平8−283910号公報に提案されている。上記公報に記載の技術は、フェライトの組織分率と粒径を規制して疲労強度の向上を図ろうとするものである。
【0008】
しかしながら、機械構造部品の表面付近に大型の介在物が存在すると、これを起点に疲労き裂が発生して破壊に到るので、フェライトの組織と粒径を規制しても必ずしも疲労強度の向上に繋がらない場合もある。
【0009】
介在物を低減して耐疲労特性を高める技術が、例えば、特開平8−283910号公報に開示されている。上記公報で提案された技術は、Al2O3(酸化物)とTiN(窒化物)を極力少なくすることにより、疲労特性の向上を図ったものである。
【0010】
しかし、自動車の等速ジョイントやシャフトなど実際の部品に加わる応力は、圧延方向や鍛錬軸に垂直に作用することが多いので、酸化物や窒化物に比べて圧延方向や鍛錬軸方向に展伸している粗大なMnSが疲労破壊の起点となる。すなわち、自動車の等速ジョイントやシャフトなどにおいては、酸化物や窒化物よりもMnSが疲労強度低下の原因となるので、疲労強度向上のためには、酸化物と窒化物だけではなく、MnSを含む全ての介在物を微細にコントロールする必要がある。
【0011】
硫化物系の介在物、なかでもMnSを微細化する技術として、既に述べた特開平11−1743号公報が挙げられるが、上記公報においては全介在物についての検討がなされているわけではないので、圧延方向や鍛錬軸に垂直な応力が作用する場合の疲労強度を高めるのには必ずしも十分とはいえない。
【0012】
【発明が解決しようとする課題】
本発明は、上記現状に鑑みなされたもので、その目的は、高周波焼入れ性、冷間加工性及び被削性に優れ、しかも、耐疲労特性、なかでも圧延方向や鍛錬軸に垂直な応力が負荷された場合の耐疲労特性にも優れ、高周波焼入れされる部品の素材として好適な機械構造用鋼材を提供することである。
【0013】
【課題を解決するための手段】
本発明の要旨は、下記(1)及び(2)に示す機械構造用鋼材にある。
【0014】
(1)化学組成が質量%で、C:0.30〜0.60%、Si:0.01〜0.40%、Mn:0.10〜0.60%、S:0.005〜0.05%、Ti:0.05〜0.2%、B:0.0005〜0.005%、Nb:0〜0.05%、Cu:0〜0.20%、Ni:0〜0.20%、Cr:0〜0.20%、Mo:0〜0.20%、Al:0.01%以下を含有し、残部はFe及び不純物からなり、不純物中のNは0.008%以下、O(酸素)は0.0025%以下、且つ、下記 (1)式で表されるfn1の値が0以上であり、更に、長手方向縦断面における介在物の長径をL(μm)、短径をW(μm)として極値統計処理によって予測される累積分布関数が99%時の下記 (2)式で表される介在物の最大等価円直径fn2が30μm以下である機械構造用鋼材。
fn1=Ti(%)−3S(%)−3.4N(%)・・・(1)
fn2=(πLW/4)0.5 ・・・(2)
(2)化学組成が質量%で、C:0.30〜0.60%、Si:0.01〜0.40%、Mn:0.10〜0.60%、S:0.005〜0.05%、Ti:0.2%以下、Zr:0.2%以下で、且つ、Ti(%)+Zr(%):0.05〜0.2%、B:0.0005〜0.005%、Nb:0〜0.05%、V:0〜0.3%、Cu:0〜0.20%、Ni:0〜0.20%、Cr:0〜0.20%、Mo:0〜0.20%、Al:0.01%以下を含有し、残部はFe及び不純物からなり、不純物中のNは0.008%以下、O(酸素)は0.0025%以下、且つ、下記 (3)式で表されるfn3の値が0以上であり、更に、長手方向縦断面における非金属介在物の長径をL(μm)、短径をW(μm)として極値統計処理によって予測される累積分布関数が99%時の下記 (2)式で表される非金属介在物の最大等価円直径fn2が30μm以下である機械構造用鋼材。
fn3=Ti(%)+Zr(%)−3S(%)−3.4N(%)・・・(3)
fn2=(πLW/4)0.5 ・・・(2)
なお、本発明でいう鋼材の「長手方向縦断面」(以下、「L断面」という)とは、鋼材の圧延方向又は鍛錬軸に平行に、その中心線を通って切断した面をいう。
又、L断面において、極値統計処理によって予測される累積分布関数が99%時の前記 (2)式で表される介在物の最大等価円直径fn2は次のようにして求めたものを指す。
【0015】
▲1▼鋼材から採取した試験片のL断面を鏡面研磨した後、その研磨面を被検面とし、光学顕微鏡の倍率を400倍として、JIS G 0555に規定された「鋼の非金属介在物の顕微鏡試験方法」中の「5.点算法による顕微鏡試験方法」に則って50視野測定し、個々の介在物の長径をL(μm)、短径をW(μm)として、各視野における(πLW/4)0.5 の値が最大になるものを求める。
【0016】
▲2▼上記▲1▼で求めた50の(πLW/4)0.5 の値を小さいものから順に並べ直してそれぞれ(πLW/4)0.5 j(ここで、j=1〜50)とし、それぞれのjについて累積分布関数Fj =100(j/51)(%)を計算する。
【0017】
▲3▼基準化変数yj =−loge (−loge (j/51) )を縦軸に、横軸に(πLW/4)0.5 jを取ったグラフを書き、最小自乗法によって近似直線を求める。
【0018】
▲4▼上記▲3▼で求めた直線から、累積分布関数Fj が99%となる時(すなわち、基準化変数yj ≒4.6となる時)の(πLW/4)0.5 jの値を読みとり、これを最大等価円直径fn2=(πLW/4)0.5 とする。
以下、上記の(1)、(2)に記載のものをそれぞれ(1)の発明、(2)の発明という。
【0019】
本発明者らは、前記した課題を解決するために種々検討を行い、下記の知見を得た。
【0020】
(a)L断面において、全介在物の極値統計処理によって予測される累積分布関数が99%時の前記 (2)式で表される介在物の最大等価円直径fn2が30μm以下であれば、介在物起点による冷間加工性の低下が防止できるとともに、圧延方向や鍛錬軸に垂直な応力が作用する場合の疲労強度の低下を防止することができる。
そこで更に検討を続けた結果、下記の事項が明らかになった。
(b)上記(a)における介在物の最大等価円直径fn2を30μm以下とするには、第1に粗大なMnSが生成することを防止すればよい。
【0021】
(c)Alの含有量を質量%で0.01%以下に抑え、Mnよりも安定して硫化物を形成するTiやZrを添加し、更に、前記 (1)式で表されるfn1の値や前記 (3)式で表されるfn3の値を0以上とすれば、Ti硫化物やZr硫化物の形成によって粗大なMnSの生成が防止でき、しかも、生成するTi硫化物やZr硫化物は微細である。なお、本発明でいうTi硫化物やZr硫化物には、TiやZrの単なる硫化物だけではなく「炭硫化物」をも含む。
【0022】
(d)Alは鋼中で酸化物となって容易にクラスターを形成するが、Al含有量を質量%で0.01%以下にするとともに適正量のTiやZrを添加することで、酸化物クラスターの形成を防止することもできる。
(e)Nの含有量を質量%で0.008%以下とすれば、粗大なTiNやZrNの形成を防止することができる。
【0023】
本発明は、上記の知見に基づいて完成されたものである。
【0024】
【発明の実施の形態】
以下、本発明の各要件について詳しく説明する。なお、各元素の含有量の「%」表示は「質量%」を意味する。
(A)鋼材の化学組成
C:
Cは、高周波焼入れ性に影響を及ぼし、焼入れ硬化層の硬さと深さを確保するために必要な元素で、そのためには0.30%以上含有させることが必要である。しかし、Cを0.60%を超えて含有させると冷間加工性及び靱性が損なわれるし、被削性が低下する。更に焼割れの発生を招くこともある。したがって、Cの含有量を0.30〜0.60%とした。なお、Cの含有量は0.35〜0.55%とすることが好ましい。
【0025】
Si:
Siは、鋼の脱酸及び強化に有効な元素である。しかし、その含有量が0.01%未満では添加効果に乏しい。一方、0.40%を超えると冷間加工時の変形抵抗が大きくなって冷間加工性の低下を招いたり被削性が低下することがある。したがって、Siの含有量を0.01〜0.40%とした。なお、Siの含有量は0.01〜0.30%とすることが好ましい。
【0026】
Mn:
Mnは、脱酸作用を有し、又、焼入れ性を高めて疲労強度を向上させる作用がある。しかし、その含有量が0.10%未満では前記効果が得難い。一方、Mn含有量が0.60%を超えると、冷間加工時の変形抵抗が大きくなって冷間加工性が低下したり被削性が低下することがある。したがって、Mnの含有量を0.10〜0.60%とした。なお、Mnの含有量は0.10〜0.45%とすることが好ましい。
【0027】
S:
Sは、Ti、ZrやMnと結合して硫化物を形成し、鋼の被削性を高める作用を有する。しかし、その含有量が0.005%未満では前記の効果が得られない。一方、Sを多量に含有させると硫化物系介在物の生成量が多くなりすぎて冷間加工性が低下し、特に、S含有量が0.05%を超えると冷間加工性の低下が著しくなる。したがって、Sの含有量を0.005〜0.05%とした。なお、Sの含有量は0.005〜0.03%とすることが好ましい。
【0028】
Ti、Zr:
Ti、Zrは本発明において粗大なMnSの生成を防止するための重要な合金元素である。又、Ti、Zrには鋼を脱酸、脱窒する作用や、高周波焼入れ時の結晶粒粗大化を防止する作用がある。
【0029】
Tiを単独で添加する場合、その含有量が0.05%以上の場合に前記の効果が確実に得られる。更に、TiがSと結合したTi硫化物によって被削性が向上する。なお既に述べたように、本発明でいうTi硫化物には、Tiの単なる硫化物だけではなく「炭硫化物」をも含む。しかし、Tiを0.2%を超えて多量に含有させても、Ti硫化物の形成による粗大MnSの生成防止効果が飽和するし、鋼の脱酸、脱窒作用なども飽和するのでコストが嵩んでしまう。したがって、(1)の発明にあってはTiの含有量を0.05〜0.2%とした。なお、(1)の発明の場合には、Tiの含有量は0.05〜0.15%とすることが好ましい。
【0030】
一方、上記の効果は、TiとZrの含有量に関し、Ti(%)+Zr(%)の値が0.05%以上の場合にも確実に得られる。しかし、Ti(%)+Zr(%)の値で0.2%を超えるTiとZrを含有させても、Ti硫化物やZr硫化物の形成による粗大MnSの生成防止効果が飽和するし、鋼の脱酸、脱窒作用や、高周波焼入れ時の結晶粒粗大化防止作用が飽和するのでコストが嵩んでしまう。なお、Ti(%)+Zr(%)の値で0.05〜0.2%でありさえすればよいので、必ずしもTiとZrとを複合して含有させる必要はない。Zrを添加しない場合は前記した(1)の発明になり、この場合はTiを0.2%を超えて含有させると疲労強度の劣化が著しくなる場合がある。Tiを添加しない、つまりZrを単独で添加する場合に、Zrを0.2%を超えて多量に含有させても、Zr硫化物の形成による粗大MnSの生成防止効果が飽和するし、鋼の脱酸、脱窒作用なども飽和するのでコストが嵩んでしまう。したがって、(2)の発明にあってはTiとZrの含有量をいずれも0.2%以下で、且つ、Ti(%)+Zr(%)の値を0.05〜0.2%とした。なお、(2)の発明の場合、Ti(%)+Zr(%)の値は0.05〜0.15%とすることが好ましい。なお既に述べたように、本発明でいうZr硫化物には、Zrの単なる硫化物だけではなく「炭硫化物」をも含む。
【0031】
B:
Bは、冷間加工性を阻害することなく焼入れ性を高める作用がある。しかし、その含有量が0.0005%未満では添加効果に乏しい。一方、0.005%を超えて含有させても前記の効果は飽和しコストが嵩むばかりである。したがって、Bの含有量を0.0005〜0.005%とした。なお、Bの含有量は0.0005〜0.004%とすることが好ましい。
Nb:
Nbは添加しなくてもよい。添加すれば、高周波焼入れ性を高める作用があり、更に、高周波焼入れ時の結晶粒粗大化を防止する作用も有する。この効果を確実に得るには、Nbは0.005%以上の含有量とすることが好ましい。しかし、その含有量が0.05%を超えると、粗大な未固溶炭窒化物が生じて冷間加工性の劣化を招くことがある。したがって、Nbの含有量を0〜0.05%とした。
V:
Vは添加しなくてもよい。添加すれば、微細な炭・窒化物を形成して疲労強度を高める作用がある。この効果を確実に得るには、Vは0.05%以上の含有量とすることが好ましい。しかし、その含有量が0.3%を超えると、冷間加工性の劣化を招くことがある。したがって、Vの含有量を0〜0.3%とした。
【0032】
Cu:
Cuは添加しなくてもよい。添加すれば焼入れ性を高める効果を有する。この効果を確実に得るには、Cuは0.02%以上の含有量とすることが好ましいが、その反面においてCuは、変形抵抗を高めて冷間加工性を劣化させ、特に、Cuの含有量が0.20%を超えると冷間加工性の劣化が著しくなることがある。したがって、Cuの含有量を0〜0.20%とした。
【0033】
Ni:
Niは添加しなくてもよい。添加すれば焼入れ性を高める効果を有する。この効果を確実に得るには、Niは0.02%以上の含有量とすることが好ましいが、その反面においてNiは、変形抵抗を高めて冷間加工性を劣化させ、更に、球状化焼鈍後のスケール除去を困難にし、特に、Niの含有量が0.20%を超えると冷間加工性の劣化、スケール除去性の低下が著しくなることがある。したがって、Niの含有量を0〜0.20%とした。
【0034】
Cr:
Crは添加しなくてもよい。添加すれば焼入れ性を高める効果を有する。この効果を確実に得るには、Crは0.02%以上の含有量とすることが好ましいが、その反面においてCrは、変形抵抗を高めて冷間加工性を劣化させ、更に、球状化焼鈍後のスケール除去を困難にし、特に、Crの含有量が0.20%を超えると冷間加工性の劣化、スケール除去性の低下が著しくなることがある。したがって、Crの含有量を0〜0.20%とした。
【0035】
Mo:
Moは添加しなくてもよい。添加すれば焼入れ性を高める効果を有する。この効果を確実に得るには、Moは0.02%以上の含有量とすることが好ましいが、その反面においてMoは、変形抵抗を高めて冷間加工性を劣化させ、更に、球状化焼鈍後のスケール除去を困難にし、特に、Moの含有量が0.20%を超えると冷間加工性の劣化、スケール除去性の低下が著しくなることがある。したがって、Moの含有量を0〜0.20%とした。
【0036】
なお、焼入れ性を高める目的でCu、Ni、Cr、Moを添加する場合には、これらの元素の1種以上を添加すればよい。
【0037】
Al:
Alは添加しなくてもよい。脱酸などの目的からAlを添加する場合でも、本発明においては、Alの含有量を0.01%以下に抑えることが極めて重要である。すなわち、Alの含有量を0.01%以下に抑え、Ti、Zr、Sを既に述べた範囲の含有量とし、更に、前記 (1)式で表されるfn1の値、前記 (3)式で表されるfn3の値を0以上にすることで、Ti硫化物、Zr硫化物は微細になり、粗大なMnSの生成が防止されるし、酸化物クラスターの形成も防止されるからである。Alの含有量が0.01%を超えると、fn2の値が30μmを超えることがあるため、たとえ他の条件が満たされても疲労強度と冷間加工性が著しく低下する場合がある。したがって、Alの含有量を0.01%以下とした。なお、Al含有量の上限は0.008%とすることが好ましい。
【0038】
本発明においては、不純物元素としてのN及びO(酸素)の含有量を下記のとおりに制限する。
【0039】
N:
Nは、変形抵抗を高めて冷間加工性を低下させ、又、Ti、Zrと結合して粗大なTiN、ZrNが生成すると疲労強度が低下する。特に、Nの含有量が0.008%を超えると、fn1あるいはfn3の値が0以上であっても、fn2の値が30μmを超えることがあるため、冷間加工性と疲労強度の低下が著しくなる場合がある。したがって、不純物元素としてのNの含有量は0.008%以下とした。なお、不純物元素としてのNの含有量は0.006%以下とすることが好ましい。
【0040】
O(酸素):
O(酸素)は酸化物を形成して鋼中に存在し、疲労強度や冷間加工性を低下させる。特に、Oの含有量が0.0025%を超えると、fn1あるいはfn3の値が0以上であっても、fn2の値が30μmを超えることがあるため、疲労強度の低下や冷間加工性の低下が著しくなる場合がある。したがって、不純物元素としてのOの含有量は0.0025%以下とした。なお、不純物元素としてのOの含有量は0.0015%以下とすることが好ましい。
なお、N及びO(酸素)以外の不純物元素のうちPの含有量は0.03%以下とすることが好ましい。
【0041】
fn1、fn3:
(1)の発明において、Ti、S、Alを既に述べた範囲の含有量とし、更に、前記 (1)式で表されるfn1の値を0以上にすることで、Ti硫化物が微細になり、粗大なMnSの生成が防止され、更に、酸化物クラスターの形成も防止できるので、良好な被削性と冷間加工性とを確保することができ、又、耐疲労特性の低下を防止することができる。fn1の値が0未満の場合、粗大なMnSの生成を抑制することができず、したがって、L断面において、前記 (2)式で表される介在物の最大等価円直径fn2を30μm以下にすることができないので、大幅な疲労強度の低下が生じる。更に冷間加工性も低下してしまう。このため、(1)の発明におけるfn1の値を0以上と規定した。なお、既に述べた範囲のTi、S、Al、N及びOの含有量の下で、Ti硫化物の微細化、粗大なMnSの生成防止及び酸化物クラスターの形成防止の各作用を安定して確保し、一層良好な被削性と冷間加工性を確保するとともに、耐疲労特性の低下を防止するためには、fn1の値は0.01以上であることが好ましい。
(2)の発明において、Ti、Zr、S、Alを既に述べた範囲の含有量とし、更に、前記 (3)式で表されるfn3の値を0以上にすることで、Ti硫化物、Zr硫化物が微細になり、粗大なMnSの生成が防止され、更に、酸化物クラスターの形成も防止できるので、良好な被削性と冷間加工性とを確保することができ、又、耐疲労特性の低下を防止することができる。fn3の値が0未満の場合、粗大なMnSの生成を抑制することができず、したがって、L断面において、前記 (2)式で表される介在物の最大等価円直径fn2を30μm以下にすることができないので、大幅な疲労強度の低下が生じる。更に冷間加工性も低下してしまう。このため、(2)の発明におけるfn3の値を0以上と規定した。なお、既に述べた範囲のTi、Zr、S、Al、N及びOの含有量の下で、Ti硫化物やZr硫化物の微細化、粗大なMnSの生成防止及び酸化物クラスターの形成防止の各作用を安定して確保し、一層良好な被削性と冷間加工性を確保するとともに、耐疲労特性の低下を防止するためには、fn3の値は0.01以上であることが好ましい。
(B)介在物
前記(A)項に記載した化学成分に加えて、硫化物、酸化物、窒化物など全介在物のサイズを制御することではじめて、機械構造用鋼材に良好な高周波焼入れ性、冷間加工性と被削性とを具備させることができ、更に、良好な耐疲労特性、なかでも圧延方向や鍛錬軸に垂直な応力が負荷された場合における良好な耐疲労特性を付与することができる。
すなわち、L断面において、全介在物の極値統計処理によって予測される累積分布関数が99%時の前記 (2)式で表される介在物の最大等価円直径fn2が30μm以下の場合に、介在物起点による冷間加工性の低下が防止され、加えて、圧延方向や鍛錬軸に垂直な応力が作用する場合の疲労強度の低下が防止される。
【0042】
L断面において、前記 (2)式で表される介在物の最大等価円直径fn2が30μmを超える場合には、介在物起点の破壊が生じて冷間加工性(特に限界据え込み率)が低下するし、疲労強度も大きく低下してしまう。
【0043】
したがって、L断面において、全介在物の極値統計処理によって予測される累積分布関数が99%時の前記 (2)式で表される介在物の最大等価円直径fn2を30μm以下と規定した。前記 (2)式で表される介在物の最大等価円直径fn2は20μm以下であることが好ましい。
【0044】
なお、前記(A)項で述べた化学成分範囲、及び (1)式で表されるfn1の値や(3)式で表されるfn3の値に関する規定を満たしても、酸化物などの生成によって、前記した介在物の最大等価円直径fn2が30μmを超える場合があるため、例えば、転炉溶製後、Siなど脱酸作用を有する元素で充分に脱酸し、その後通常の方法で炉外精錬を行い、その処理の末期にTiやZrを添加する製鋼法を採用することが望ましい。
【0045】
以下、実施例により本発明を説明する。
【0046】
【実施例】
(実施例1)
表1〜3に示す化学組成を有する鋼を試験溶解炉を用いて溶製した。表1〜3では前記fn1とfn3は (3)式で計算した値をfnとして記載した。
【0047】
表1〜3における鋼1〜18及び鋼35は成分が本発明で規定する条件を満たす鋼、表2、表3における鋼19〜34及び鋼36〜38は成分のいずれかが本発明で規定する条件から外れた比較例の鋼である。比較例の鋼のうち鋼36〜38はCの含有量がそれぞれ0.40、0.50、0.58%のJIS機械構造用炭素鋼、すなわち、JIS G 4051に記載のS40C、S50C、S58Cに相当する鋼である。なお、鋼1〜38におけるPの含有量はすべて0.03%以下であった。
【0048】
鋼27と鋼35を除いた上記の鋼は、真空溶解炉を用いて溶製し、Al、Si等で脱酸した後、Ti、Zrの少なくとも1種以上を添加して製造した。一方、鋼27と鋼35は、大気溶解炉を用いて溶製し、Al、Siで脱酸した後、Ti、Zrの少なくとも1種以上を添加して製造した。
【0049】
【表1】
【表2】
【表3】
次いで、これらの鋼を1250℃に加熱してから1000℃で仕上げる熱間鍛造を行って、直径60mmの丸棒を作製した。
【0050】
上記のようにして得た直径60mmの丸棒は845℃に加熱して2時間保持した後、大気中で放冷した。
【0051】
上記の処理を施した直径60mmの丸棒からJIS G 0555の図3に則って介在物測定用の試験片を切り出し、既に述べた方法によって前記 (2)式で表される介在物の最大等価円直径fn2の値を求めた。
【0052】
すなわち、▲1▼鋼材から採取した試験片のL断面を鏡面研磨した後、その研磨面を被検面とし、光学顕微鏡の倍率を400倍として、JIS G 0555に規定された「鋼の非金属介在物の顕微鏡試験方法」中の「5.点算法による顕微鏡試験方法」に則って50視野測定し、個々の介在物の長径をL(μm)、短径をW(μm)として、各視野における(πLW/4)0.5 の値が最大になるものを求め、次に、▲2▼上記▲1▼で求めた50の(πLW/4)0.5 の値を小さいものから順に並べ直してそれぞれ(πLW/4)0.5 j(ここで、j=1〜50)とし、それぞれのjについて累積分布関数Fj =100(j/51)(%)を計算した。更に、▲3▼基準化変数yj =−loge (−loge (j/51) )を縦軸に、横軸に(πLW/4)0.5 jを取ったグラフを書き、最小自乗法によって近似直線を求め、最後に、▲4▼上記▲3▼で求めた直線から、累積分布関数Fj が99%となる時(すなわち、基準化変数yj ≒4.6となる時)の(πLW/4)0.5 jの値を読みとり、これを最大等価円直径fn2=(πLW/4)0.5 とした。
【0053】
最大等価円直径fn2を求めた一例として、図1に、鋼4と鋼28の場合を示す。
【0054】
冷間加工性の調査は次のようにして行った。すなわち、前記の熱処理を施した直径60mmの丸棒の「R/2部(Rは丸棒の半径、すなわち30mm)」から直径30mmの丸棒を切り出し、通常の方法で球状化焼鈍を施した後この丸棒から直径が15mmで長さが22.5mmの冷間加工用試験片を作製した。次いで、500t高速プレス機を用いて通常の方法で冷間拘束型据え込み試験を行い、割れが発生する限界の据え込み率を測定して冷間加工性の調査を行った。
【0055】
なお、据え込み率が75%まで、各条件ごとに5回の据え込み試験を行い、3個以上に割れが発生する最小の据え込み率を限界据え込み率として評価した。更に、全ての鋼の限界据え込み率以下である60%据え込み率の場合の変形抵抗を測定した。
【0056】
旋削試験による被削性の調査も行った。すなわち、前記の熱処理を施した直径60mmの丸棒を、超硬工具ST20を用いて、乾式、切削速度100m/分、送り速度0.25mm/rev、切り込み2mmの条件で切削し、逃げ面磨耗量VBが0.2mmまでの切削時間を測定した。
【0057】
表4に、前記の各試験結果をまとめて示す。
【0058】
【表4】
表4から、鋼の化学成分及び介在物の最大等価円直径fn2が本発明で規定する条件を満たす本発明例の試験番号1〜18の場合、JIS G 4051に記載の同等のC含有量を有する機械構造用炭素鋼に相当する鋼を素材とする場合に比べて、据え込み時の変形抵抗は10%以上低く、変形能としての割れが発生する限界据え込み率は75%以上であるうえに、工具寿命も長いことがわかる。
【0059】
これに対して、比較例の試験番号19〜35の場合、(イ)JIS G 4051に記載の同等のC含有量を有する機械構造用炭素鋼に相当する鋼に比べて、据え込み時の変形抵抗が10%以上低下する、(ロ)限界据え込み率が75%以上、の少なくともいずれかを満たしておらず、工具寿命も短い。
【0060】
(実施例2)
前記実施例1で熱処理した直径60mmの丸棒のうち、鋼1〜18、鋼22〜27、鋼32、鋼33、鋼35〜38を素材鋼とするものを用いて平滑小野式回転曲げ疲労試験片(平行部の直径が6mmで長さが25mm)を作製し、耐疲労特性を調査した。
【0061】
図2に、各丸棒からのサンプル切り出し方法を示す。図2に示すように、各丸棒についてその鍛錬軸に平行な方向(以下、「L方向」という)の「R/2部」及び鍛錬軸に垂直な方向(以下、「T方向」という)の中心部からサンプルを採取した。なお、図3に示すように、T方向から採取した各サンプルはその両端を電子ビーム溶接して接合し、L方向から採取したサンプルとともに所定の平滑小野式回転曲げ疲労試験片の寸法に仕上げた。
【0062】
上記のようにして得た平滑小野式回転曲げ疲労試験片に、高周波焼入れ深さが1.5mmになる条件で高周波焼入れし、次いで、180℃で焼戻し処理を行った。なお、上記の高周波焼入れ深さが1.5mmになる条件は、各素材鋼に対して予備試験を行って求めておき、この条件で高周波焼入れしたものである。
【0063】
小野式回転曲げ疲労試験は室温大気中で行ない、各供試材の107 回の疲労強度(疲労限度)を測定した。
表5に、疲労試験結果を示す。なお、表5には前記実施例1で求めた最大等価円直径fn2の値も併せて示した。
【0064】
【表5】
表5から、鋼の化学成分及び介在物の最大等価円直径fn2が本発明で規定する条件を満たす本発明例の試験番号39〜56の場合、T方向でも900MPaを超える疲労強度を有しており、しかも、L方向の疲労強度に対するT方向の疲労強度の低下は小さい。したがって、鍛錬軸に垂直な応力が作用する場合の疲労強度が低下しにくいことが明らかである。
【0065】
これに対して、比較例の試験番号57〜68の場合、L方向の疲労強度に対するT方向の疲労強度の低下が著しい。試験番号64の場合、鋼35の化学成分は本発明で規定する条件を満たすものの、fn2の値が本発明で規定する条件から外れるため、前記本発明例の試験番号39〜56の場合に比べて、L方向の疲労強度に対するT方向の疲労強度の低下が著しい。
【0066】
【発明の効果】
本発明の機械構造用鋼材は、高周波焼入れ性、冷間加工性、被削性に優れ、更に、疲労強度、なかでもT方向の疲労強度に優れ、圧延方向や鍛錬軸に垂直な応力が負荷された場合の耐疲労特性の低下が防止できるので、自動車の等速ジョイントやシャフトを初めとした各種の高周波焼入れされる部品の素材として利用することができる。
【図面の簡単な説明】
【図1】実施例の鋼材における最大等価円直径fn2の一例を示す図である。
【図2】鍛錬軸に平行な方向及び鍛錬軸に垂直な方向から平滑小野式回転曲げ疲労試験片のサンプルを切り出す方法を説明する図である。
【図3】鍛錬軸に垂直な方向から切り出したサンプルの両端を電子ビーム溶接して接合し、所定の平滑小野式回転曲げ疲労試験片に仕上げたことを説明する図である。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to steel materials for machine structures. More specifically, the present invention relates to a steel material for machine structure suitable as a material for various induction hardened parts such as constant velocity joints and shafts of automobiles. More specifically, the present invention relates to a steel material for a machine structure which is excellent in induction hardening properties, cold workability, machinability and fatigue resistance, and is suitable as a material for components to be induction hardened.
[0002]
[Prior art]
For the purpose of increasing the output of a power machine and reducing its weight for improving fuel efficiency, it is required to increase the strength of power transmission components. Above all, there is a great demand for high strength of constant velocity joints and shafts of automobiles.
[0003]
CVJs and shafts of automobiles are manufactured by cold forging, which has high dimensional accuracy after forging and can reduce the amount of cutting, and then is subjected to induction hardening as a surface hardening treatment. Many. Therefore, good induction hardenability is required for the steel material. In recent years, in order to further reduce costs and simplify the manufacturing process, there has been an increasing demand for steel materials having good induction hardenability and more excellent machinability and cold workability. Further, since a stress is repeatedly applied to a constant velocity joint, a shaft and the like of the automobile, a demand for a steel material having more excellent fatigue resistance is increasing.
[0004]
Many techniques have been reported to enhance either machinability or cold workability while ensuring induction hardening. However, it has not always been considered to enhance both machinability and cold workability. Not.
For example, Japanese Patent Application Laid-Open No. 9-287054 discloses "cold forging-induction hardening steel" and Japanese Patent Application Laid-Open No. 9-287055 discloses "induction hardening steel having excellent cold workability". There is disclosed a steel capable of having both heat resistance and induction hardening property. However, in the case of the steels proposed in the above publications, no consideration is given to the machinability.
[0005]
It is well known that the addition of free-cutting elements (elements for improving machinability) such as Pb, Te, Bi and Ca to steel increases machinability. However, as in the steel disclosed as “steel for cold forging” in JP-A-7-97656, the cold workability may be deteriorated by simply adding the above free-cutting element.
[0006]
In a steel proposed in Japanese Patent Application Laid-Open No. Hei 7-24289 as "an induction hardening steel excellent in cold forgeability, machinability and torsional fatigue strength", a B nitride is used to improve machinability. However, it is necessary to add a large amount of alloying elements such as Cr, Ni, and Mo for compatibility with hardenability, so that deformation resistance at the time of cold working is increased, and sufficient cold workability is obtained. May not be secured.
[0007]
Japanese Patent Application Laid-Open No. H11-1743 proposes a technology for improving machinability by controlling the size and amount of Ti sulfide as "high-strength, high-toughness tempered steel having excellent machinability". . However, when cold-working steel, not only sulfides but also non-metallic inclusions such as oxides and nitrides (hereinafter simply referred to as inclusions) may cause a reduction in cold-workability. Simply defining the size and amount of Ti sulfide may not achieve both machinability and cold workability.
On the other hand, a technique for improving the fatigue resistance has been proposed in, for example, Japanese Patent Application Laid-Open No. 8-283910. The technique described in the above publication is intended to improve the fatigue strength by regulating the structure fraction and grain size of ferrite.
[0008]
However, if there are large inclusions near the surface of the mechanical structural component, fatigue cracks will be generated starting from this, leading to fracture. Therefore, even if the ferrite structure and grain size are regulated, the fatigue strength is not necessarily improved. May not be connected.
[0009]
A technique for increasing the fatigue resistance by reducing inclusions is disclosed in, for example, Japanese Patent Application Laid-Open No. 8-283910. The technology proposed in the above publication is Al2O3(Oxide) and TiN (nitride) are reduced as much as possible to improve fatigue characteristics.
[0010]
However, since the stress applied to actual parts such as constant velocity joints and shafts of automobiles often acts perpendicular to the rolling direction and the wrought axis, the stress extends in the rolling direction and the wrought axis direction compared to oxides and nitrides. Coarse MnS is a starting point of fatigue fracture. That is, in constant velocity joints and shafts of automobiles, MnS causes a decrease in fatigue strength more than oxides and nitrides. Therefore, in order to improve fatigue strength, not only oxides and nitrides but also MnS is used. It is necessary to finely control all the inclusions including them.
[0011]
As a technique for refining sulfide-based inclusions, especially MnS, Japanese Patent Application Laid-Open No. H11-1743 has already been mentioned. However, since the above publications do not consider all inclusions, However, it is not necessarily sufficient to increase the fatigue strength when a stress perpendicular to the rolling direction or the wrought axis acts.
[0012]
[Problems to be solved by the invention]
The present invention has been made in view of the above-mentioned situation, and its object is to achieve excellent induction hardening properties, excellent cold workability and machinability, and furthermore, fatigue resistance properties, in particular, stress perpendicular to the rolling direction and the forging axis. An object of the present invention is to provide a steel material for machine structural use which is excellent in fatigue resistance characteristics when subjected to a load and is suitable as a material for a component subjected to induction hardening.
[0013]
[Means for Solving the Problems]
The gist of the present invention resides in steel materials for machine structures shown in the following (1) and (2).
[0014]
(1) Chemical composition in mass%, C: 0.30 to 0.60%, Si: 0.01 to 0.40%, Mn: 0.10 to 0.60%, S: 0.005 to 0 0.05%, Ti: 0.05-0.2%, B: 0.0005-0.005%, Nb: 0-0.05%, Cu: 0-0.20%, Ni: 0-0. 20%, Cr: 0 to 0.20%, Mo: 0 to 0.20%, Al: 0.01% or less, with the balance being Fe and impurities, N in the impurities being 0.008% or less , O (oxygen) is 0.0025% or less, and the value of fn1 represented by the following formula (1) is 0 or more, and the major axis of the inclusion in the longitudinal longitudinal section is L (μm), The maximum equivalent circular diameter fn2 of the inclusion represented by the following equation (2) when the cumulative distribution function predicted by the extreme value statistical processing is 99% with the diameter being W (μm) is 0μm or less is mechanical structural steel.
fn1 = Ti (%)-3S (%)-3.4N (%) (1)
fn2 = (πLW / 4)0.5 ... (2)
(2) Chemical composition in mass%, C: 0.30 to 0.60%, Si: 0.01 to 0.40%, Mn: 0.10 to 0.60%, S: 0.005 to 0 0.05%, Ti: 0.2% or less, Zr: 0.2% or less, and Ti (%) + Zr (%): 0.05 to 0.2%, B: 0.0005 to 0.005 %, Nb: 0 to 0.05%, V: 0 to 0.3%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Cr: 0 to 0.20%, Mo: 0 0.20%, Al: 0.01% or less, the balance is composed of Fe and impurities, N in the impurities is 0.008% or less, O (oxygen) is 0.0025% or less, and The value of fn3 represented by the equation (3) is equal to or greater than 0, and the major axis of the nonmetallic inclusion in the longitudinal cross section is L (μm), and the minor axis is W (μm), and extreme value statistical processing is performed. Mechanical structural steel maximum equivalent circular diameter fn2 cumulative distribution function nonmetallic inclusions represented by the following equation (2) when 99% is 30μm or less to be predicted.
fn3 = Ti (%) + Zr (%)-3S (%)-3.4N (%) (3)
fn2 = (πLW / 4)0.5 ... (2)
The “longitudinal longitudinal section” (hereinafter, referred to as “L section”) of a steel material as referred to in the present invention refers to a surface cut through a center line of the steel material in a direction parallel to a rolling direction or a forging axis.
In the L-section, when the cumulative distribution function predicted by the extreme value statistical processing is 99%, the maximum equivalent circular diameter fn2 of the inclusion represented by the above equation (2) indicates the value obtained as follows. .
[0015]
{Circle around (1)} After mirror-polishing the L cross section of a test piece taken from a steel material, the polished surface was used as a test surface, and the magnification of an optical microscope was set to 400 times, and “non-metallic inclusions of steel” specified in JIS G 0555 were used. 50 field measurements in accordance with “5. Microscope test method by point calculation” in “Microscope test method”, and the major axis of each inclusion is L (μm) and the minor axis is W (μm). πLW / 4)0.5 Find the one that maximizes the value of
[0016]
(2) 50 (πLW / 4) obtained in (1) above0.5 Are rearranged in ascending order, and are respectively (πLW / 4)0.5 j(Where j = 1 to 50), and for each j, the cumulative distribution function Fj = 100 (j / 51) (%) is calculated.
[0017]
(3) Normalized variable yj = -Loge (-Loge (J / 51)) on the vertical axis and (πLW / 4) on the horizontal axis.0.5 jIs drawn, and an approximate straight line is obtained by the least squares method.
[0018]
(4) From the straight line obtained in (3), the cumulative distribution function Fj Is 99% (ie, the scaling variable yj (ΠLW / 4) when 時 4.60.5 jAnd read it as the maximum equivalent circular diameter fn2 = (πLW / 4)0.5 And
Hereinafter, those described in the above (1) and (2) are referred to as the invention of (1) and the invention of (2), respectively.
[0019]
The present inventors have conducted various studies in order to solve the above-mentioned problems, and have obtained the following findings.
[0020]
(A) In the L section, if the maximum equivalent circular diameter fn2 of the inclusion represented by the above formula (2) is 30 μm or less when the cumulative distribution function predicted by the extreme value statistical processing of all the inclusions is 99%, In addition, it is possible to prevent a decrease in cold workability due to inclusion starting points, and to prevent a decrease in fatigue strength when a stress perpendicular to the rolling direction or the wrought axis acts.
Therefore, as a result of further study, the following matters became clear.
(B) In order to set the maximum equivalent circular diameter fn2 of the inclusion in the above (a) to 30 μm or less, first, formation of coarse MnS may be prevented.
[0021]
(C) The content of Al is suppressed to 0.01% or less by mass%, Ti or Zr which forms sulfide more stably than Mn is added, and further, fn1 represented by the formula (1) is added. When the value or the value of fn3 represented by the formula (3) is set to 0 or more, coarse MnS can be prevented from being formed due to the formation of Ti sulfide or Zr sulfide, and the generated Ti sulfide or Zr sulfide can be prevented. Things are fine. The Ti sulfide and Zr sulfide referred to in the present invention include not only mere sulfides of Ti and Zr but also "carbosulfides".
[0022]
(D) Al readily forms a cluster as an oxide in the steel. However, when the Al content is reduced to 0.01% or less by mass% and an appropriate amount of Ti or Zr is added, the oxide is formed. Cluster formation can also be prevented.
(E) When the content of N is 0.008% or less by mass%, formation of coarse TiN or ZrN can be prevented.
[0023]
The present invention has been completed based on the above findings.
[0024]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" of the content of each element means "% by mass".
(A) Chemical composition of steel
C:
C affects the induction hardening property and is an element necessary for securing the hardness and depth of the hardened hardened layer. For this purpose, C must be contained in an amount of 0.30% or more. However, when C is contained in excess of 0.60%, cold workability and toughness are impaired, and machinability is reduced. Further, the occurrence of burn cracking may be caused. Therefore, the content of C is set to 0.30 to 0.60%. Note that the content of C is preferably set to 0.35 to 0.55%.
[0025]
Si:
Si is an element effective for deoxidizing and strengthening steel. However, if the content is less than 0.01%, the effect of addition is poor. On the other hand, if it exceeds 0.40%, the deformation resistance during cold working becomes large, which may cause a reduction in cold workability or a decrease in machinability. Therefore, the content of Si is set to 0.01 to 0.40%. The content of Si is preferably set to 0.01 to 0.30%.
[0026]
Mn:
Mn has a deoxidizing effect and also has an effect of improving hardenability and improving fatigue strength. However, if the content is less than 0.10%, it is difficult to obtain the above effect. On the other hand, if the Mn content exceeds 0.60%, the deformation resistance during cold working increases, and the cold workability or machinability may decrease. Therefore, the content of Mn is set to 0.10 to 0.60%. The Mn content is preferably set to 0.10 to 0.45%.
[0027]
S:
S combines with Ti, Zr and Mn to form a sulfide, and has an effect of enhancing the machinability of steel. However, if the content is less than 0.005%, the above effects cannot be obtained. On the other hand, when a large amount of S is contained, the amount of sulfide-based inclusions becomes too large and the cold workability decreases, and in particular, when the S content exceeds 0.05%, the cold workability decreases. Become significant. Therefore, the content of S is set to 0.005 to 0.05%. Note that the content of S is preferably set to 0.005 to 0.03%.
[0028]
Ti, Zr:
Ti and Zr are important alloying elements for preventing the formation of coarse MnS in the present invention. Further, Ti and Zr have an action of deoxidizing and denitrifying steel and an action of preventing crystal grains from being coarsened during induction hardening.
[0029]
In the case of adding Ti alone, the above-mentioned effect can be surely obtained when the content is 0.05% or more. Further, machinability is improved by Ti sulfide in which Ti is combined with S. As described above, the Ti sulfide in the present invention includes not only a simple sulfide of Ti but also a “carbosulfide”. However, even if a large amount of Ti is contained in excess of 0.2%, the effect of preventing the formation of coarse MnS due to the formation of Ti sulfide is saturated, and the deoxidizing and denitrifying effects of steel are also saturated. It will be bulky. Therefore, in the invention of (1), the content of Ti is set to 0.05 to 0.2%. In the case of the invention (1), the content of Ti is preferably set to 0.05 to 0.15%.
[0030]
On the other hand, the above effect can be reliably obtained even when the value of Ti (%) + Zr (%) is 0.05% or more with respect to the contents of Ti and Zr. However, even when Ti and Zr containing more than 0.2% of Ti (%) + Zr (%) are included, the effect of preventing the formation of coarse MnS due to the formation of Ti sulfide or Zr sulfide is saturated, and However, the deoxidation and denitrification of the metal and the effect of preventing crystal grain coarsening during induction hardening are saturated, so that the cost increases. Note that the value of Ti (%) + Zr (%) only needs to be 0.05 to 0.2%, so that it is not always necessary to include Ti and Zr in a composite manner. In the case where Zr is not added, the invention of the above-mentioned (1) is obtained. In this case, if Ti is contained in excess of 0.2%, the fatigue strength may be significantly deteriorated. When Ti is not added, that is, when Zr is added alone, even if Zr is contained in a large amount exceeding 0.2%, the effect of preventing the formation of coarse MnS due to the formation of Zr sulfide is saturated, and Since the deoxidizing and denitrifying effects are also saturated, the cost increases. Therefore, in the invention of (2), the contents of Ti and Zr are both 0.2% or less, and the value of Ti (%) + Zr (%) is 0.05 to 0.2%. . In the case of the invention (2), the value of Ti (%) + Zr (%) is preferably set to 0.05 to 0.15%. As described above, the Zr sulfide according to the present invention includes not only a simple sulfide of Zr but also a “carbosulfide”.
[0031]
B:
B has the effect of increasing hardenability without impairing cold workability. However, if the content is less than 0.0005%, the effect of addition is poor. On the other hand, if the content exceeds 0.005%, the above effect is saturated and the cost is increased. Therefore, the content of B is set to 0.0005 to 0.005%. Note that the content of B is preferably set to 0.0005 to 0.004%.
Nb:
Nb may not be added. If added, it has the effect of increasing the induction hardening properties, and also has the effect of preventing the crystal grains from becoming coarse during induction hardening. In order to surely obtain this effect, the content of Nb is preferably set to 0.005% or more. However, if the content exceeds 0.05%, coarse undissolved carbonitrides are generated, which may cause deterioration in cold workability. Therefore, the content of Nb was set to 0 to 0.05%.
V:
V need not be added. If added, it has the effect of forming fine carbon and nitride to increase fatigue strength. In order to surely obtain this effect, it is preferable that the content of V is 0.05% or more. However, if the content exceeds 0.3%, the cold workability may deteriorate. Therefore, the content of V is set to 0 to 0.3%.
[0032]
Cu:
Cu need not be added. If added, it has the effect of increasing the hardenability. To ensure this effect, the content of Cu is preferably set to 0.02% or more. On the other hand, Cu increases the deformation resistance and degrades the cold workability. If the amount exceeds 0.20%, the cold workability may significantly deteriorate. Therefore, the content of Cu is set to 0 to 0.20%.
[0033]
Ni:
Ni may not be added. If added, it has the effect of increasing the hardenability. To ensure this effect, it is preferable that the content of Ni be 0.02% or more, but on the other hand, Ni increases the deformation resistance and deteriorates the cold workability. The subsequent removal of the scale becomes difficult. In particular, if the Ni content exceeds 0.20%, the cold workability may deteriorate and the scale removal may decrease significantly. Therefore, the content of Ni is set to 0 to 0.20%.
[0034]
Cr:
Cr may not be added. If added, it has the effect of increasing the hardenability. To ensure this effect, it is preferable that the content of Cr be 0.02% or more. On the other hand, Cr increases the deformation resistance and deteriorates the cold workability. The subsequent removal of the scale becomes difficult. In particular, when the content of Cr exceeds 0.20%, the cold workability may deteriorate and the scale removal may decrease significantly. Therefore, the content of Cr is set to 0 to 0.20%.
[0035]
Mo:
Mo may not be added. If added, it has the effect of increasing the hardenability. To ensure this effect, it is preferable that the content of Mo be 0.02% or more. On the other hand, Mo increases the deformation resistance and deteriorates the cold workability. It is difficult to remove the scale later, and in particular, when the content of Mo exceeds 0.20%, the cold workability may be deteriorated and the scale may be significantly deteriorated. Therefore, the content of Mo is set to 0 to 0.20%.
[0036]
When Cu, Ni, Cr, or Mo is added for the purpose of improving the hardenability, one or more of these elements may be added.
[0037]
Al:
Al does not have to be added. Even when Al is added for the purpose of deoxidation or the like, in the present invention, it is extremely important to suppress the Al content to 0.01% or less. That is, the content of Al is suppressed to 0.01% or less, the content of Ti, Zr, and S is set in the range described above. Further, the value of fn1 represented by the formula (1) and the value of the formula (3) By setting the value of fn3 represented by 0 to 0 or more, Ti sulfide and Zr sulfide become finer, formation of coarse MnS is prevented, and formation of oxide clusters is also prevented. . If the Al content exceeds 0.01%, the value of fn2 may exceed 30 μm, so that even if other conditions are satisfied, the fatigue strength and cold workability may be significantly reduced. Therefore, the content of Al is set to 0.01% or less. The upper limit of the Al content is preferably set to 0.008%.
[0038]
In the present invention, the contents of N and O (oxygen) as impurity elements are limited as follows.
[0039]
N:
N increases the deformation resistance and lowers the cold workability, and when combined with Ti and Zr to form coarse TiN and ZrN, the fatigue strength decreases. In particular, when the content of N exceeds 0.008%, even if the value of fn1 or fn3 is 0 or more, the value of fn2 may exceed 30 μm. May be significant. Therefore, the content of N as an impurity element is set to 0.008% or less. Note that the content of N as an impurity element is preferably set to 0.006% or less.
[0040]
O (oxygen):
O (oxygen) forms an oxide and exists in the steel, and reduces fatigue strength and cold workability. In particular, when the content of O exceeds 0.0025%, the value of fn2 may exceed 30 μm even if the value of fn1 or fn3 is 0 or more, so that the fatigue strength is reduced and the cold workability is reduced. The decrease may be significant. Therefore, the content of O as an impurity element is set to 0.0025% or less. Note that the content of O as an impurity element is preferably set to 0.0015% or less.
The content of P among the impurity elements other than N and O (oxygen) is preferably set to 0.03% or less.
[0041]
fn1, fn3:
In the invention of (1), the content of Ti, S, and Al is set in the range described above, and the value of fn1 represented by the formula (1) is set to 0 or more, so that Ti sulfide can be finely divided. The formation of coarse MnS can be prevented, and the formation of oxide clusters can be prevented, so that good machinability and cold workability can be ensured, and deterioration of fatigue resistance characteristics is prevented. can do. When the value of fn1 is less than 0, generation of coarse MnS cannot be suppressed, and therefore, in the L section, the maximum equivalent circular diameter fn2 of the inclusion represented by the formula (2) is set to 30 μm or less. In this case, a significant reduction in fatigue strength occurs. Further, the cold workability also decreases. For this reason, the value of fn1 in the invention of (1) is defined as 0 or more. In addition, under the contents of Ti, S, Al, N, and O in the ranges described above, the respective actions of refining Ti sulfide, preventing formation of coarse MnS, and preventing formation of oxide clusters are stably performed. In order to ensure sufficient machinability and cold workability, and to prevent a decrease in fatigue resistance, the value of fn1 is preferably 0.01 or more.
In the invention of (2), the content of Ti, Zr, S, and Al is set in the above-mentioned range, and the value of fn3 represented by the formula (3) is set to 0 or more, whereby Ti sulfide, Since the Zr sulfide becomes fine, the formation of coarse MnS is prevented, and the formation of oxide clusters can also be prevented, good machinability and cold workability can be ensured. It is possible to prevent a decrease in fatigue characteristics. When the value of fn3 is less than 0, the formation of coarse MnS cannot be suppressed, and therefore, in the L section, the maximum equivalent circular diameter fn2 of the inclusion represented by the formula (2) is set to 30 μm or less. In this case, a significant reduction in fatigue strength occurs. Further, the cold workability also decreases. For this reason, the value of fn3 in the invention of (2) is defined as 0 or more. In addition, under the contents of Ti, Zr, S, Al, N, and O in the above-mentioned ranges, it is possible to reduce the size of Ti sulfide or Zr sulfide, prevent formation of coarse MnS, and prevent formation of oxide clusters. The value of fn3 is preferably 0.01 or more, in order to ensure each action stably, to ensure better machinability and cold workability, and to prevent a decrease in fatigue resistance. .
(B) Inclusion
Only by controlling the size of all inclusions such as sulfides, oxides and nitrides in addition to the chemical components described in the above item (A), good induction hardenability and cold workability of steel for machine structural use And machinability, as well as good fatigue resistance characteristics, especially good fatigue resistance characteristics when a stress perpendicular to the rolling direction or the wrought axis is applied.
That is, in the L section, when the maximum equivalent circular diameter fn2 of the inclusion represented by the formula (2) when the cumulative distribution function predicted by the extreme value statistical processing of all the inclusions is 99% is 30 μm or less, A decrease in cold workability due to inclusion starting points is prevented, and in addition, a decrease in fatigue strength when a stress perpendicular to the rolling direction or the wrought axis acts is prevented.
[0042]
In the L section, when the maximum equivalent circular diameter fn2 of the inclusion represented by the above formula (2) exceeds 30 μm, the starting point of the inclusion is broken, and the cold workability (particularly the limit upsetting ratio) is reduced. In addition, the fatigue strength is greatly reduced.
[0043]
Therefore, in the L section, the maximum equivalent circular diameter fn2 of the inclusion represented by the above formula (2) when the cumulative distribution function predicted by the extreme value statistical processing of all the inclusions is 99% is specified to be 30 μm or less. It is preferable that the maximum equivalent circular diameter fn2 of the inclusion represented by the formula (2) is 20 μm or less.
[0044]
Even if the chemical composition range described in the above item (A) and the value of fn1 represented by the formula (1) and the value of fn3 represented by the formula (3) are satisfied, formation of oxides and the like may occur. Since the maximum equivalent circular diameter fn2 of the above-mentioned inclusions may exceed 30 μm, for example, after melting the converter, it is sufficiently deoxidized with an element having a deoxidizing effect such as Si, and then the furnace is subjected to a normal method. It is desirable to employ a steelmaking method in which external refining is performed and Ti or Zr is added at the end of the processing.
[0045]
Hereinafter, the present invention will be described with reference to examples.
[0046]
【Example】
(Example 1)
Steels having the chemical compositions shown in Tables 1 to 3 were melted using a test melting furnace. In Tables 1 to 3, fn1 and fn3 are described as values calculated by the formula (3) as fn.
[0047]
[0048]
The above-mentioned steels except steel 27 and steel 35 were produced by melting using a vacuum melting furnace, deoxidizing with Al, Si or the like, and then adding at least one of Ti and Zr. On the other hand, steels 27 and 35 were produced by melting using an air melting furnace, deoxidizing with Al and Si, and then adding at least one or more of Ti and Zr.
[0049]
[Table 1]
[Table 2]
[Table 3]
Next, these steels were heated to 1250 ° C. and then subjected to hot forging to finish at 1000 ° C. to produce round bars having a diameter of 60 mm.
[0050]
The round bar having a diameter of 60 mm obtained as described above was heated to 845 ° C., kept for 2 hours, and then allowed to cool in the air.
[0051]
A test piece for measuring inclusions was cut out from the round bar having a diameter of 60 mm subjected to the above processing in accordance with FIG. 3 of JIS G 0555, and the maximum equivalent of the inclusion represented by the formula (2) was determined by the method described above. The value of the circle diameter fn2 was determined.
[0052]
That is, (1) After mirror-polishing the L section of a test piece taken from a steel material, the polished surface is used as a test surface, and the magnification of an optical microscope is set to 400 times, the "non-metallic steel" specified in JIS G 0555. Measurement of 50 visual fields in accordance with “5. Microscopic test method by dot calculation” in “Microscopic test method of inclusions”, and setting the major axis of each inclusion to L (μm) and the minor axis to W (μm), At (πLW / 4)0.5 Is obtained, and then (2) 50 (πLW / 4) obtained in the above (1) are obtained.0.5 Are rearranged in ascending order, and are respectively (πLW / 4)0.5 j(Where j = 1 to 50), and for each j, the cumulative distribution function Fj = 100 (j / 51) (%) was calculated. Further, (3) the standardized variable yj = -Loge (-Loge (J / 51)) on the vertical axis and (πLW / 4) on the horizontal axis.0.5 j, An approximate straight line is obtained by the least square method, and finally, the cumulative distribution function F is calculated from the straight line obtained in (4) above (3).j Is 99% (ie, the scaling variable yj (ΠLW / 4) when 時 4.60.5 jAnd read it as the maximum equivalent circular diameter fn2 = (πLW / 4)0.5 And
[0053]
As an example of obtaining the maximum equivalent circular diameter fn2, FIG. 1 shows the case of
[0054]
The cold workability was investigated as follows. That is, a round bar having a diameter of 30 mm was cut out from “R / 2 part (R is the radius of the round bar, that is, 30 mm)” of a round bar having a diameter of 60 mm that had been subjected to the heat treatment, and subjected to spheroidizing annealing by a normal method. Thereafter, a test piece for cold working having a diameter of 15 mm and a length of 22.5 mm was prepared from the round bar. Next, a cold restraint type upsetting test was performed by a normal method using a 500 t high-speed press machine, and the limit upsetting rate at which cracking occurred was measured to investigate cold workability.
[0055]
In addition, five upsetting tests were performed for each condition until the upsetting ratio was 75%, and the minimum upsetting ratio at which three or more cracks occurred was evaluated as the limit upsetting ratio. Furthermore, the deformation resistance at a 60% upsetting ratio which is equal to or less than the limit upsetting ratio of all steels was measured.
[0056]
Investigation of machinability by turning test was also conducted. That is, the heat-treated round bar having a diameter of 60 mm is cut using a carbide tool ST20 under the conditions of a dry process, a cutting speed of 100 m / min, a feed speed of 0.25 mm / rev, and a cutting depth of 2 mm, and flank wear. The cutting time until the amount VB was 0.2 mm was measured.
[0057]
Table 4 summarizes the results of the above tests.
[0058]
[Table 4]
From Table 4, in the case of Test Nos. 1 to 18 of the present invention examples in which the chemical composition of steel and the maximum equivalent circular diameter fn2 of inclusions satisfy the conditions specified in the present invention, the equivalent C content described in JIS G 4051 is Deformation resistance at the time of upsetting is 10% or more lower than at the time of using steel equivalent to carbon steel for machine structural use as a material, and the critical upsetting rate at which cracking as deformability occurs is 75% or more. In addition, it can be seen that the tool life is long.
[0059]
On the other hand, in the case of the test numbers 19 to 35 of the comparative example, (a) the deformation at the time of upsetting compared to the steel corresponding to the carbon steel for machine structural use having the same C content described in JIS G 4051. The resistance falls by 10% or more, and (b) the limit upsetting rate does not satisfy at least one of 75% or more, and the tool life is short.
[0060]
(Example 2)
Among the round bars having a diameter of 60 mm heat-treated in Example 1, steels 1 to 18, steels 22 to 27, steel 32, steel 33, and steels 35 to 38 were used as the material steels. A test piece (parallel portion having a diameter of 6 mm and a length of 25 mm) was prepared and its fatigue resistance was investigated.
[0061]
FIG. 2 shows a method of cutting out a sample from each round bar. As shown in FIG. 2, for each round bar, the “R / 2 part” in a direction parallel to the training axis (hereinafter referred to as “L direction”) and a direction perpendicular to the training axis (hereinafter referred to as “T direction”). A sample was taken from the center of the sample. As shown in FIG. 3, each sample taken from the T direction was joined by electron beam welding at both ends thereof, and finished together with the sample taken from the L direction to the dimensions of a predetermined smooth Ono-type rotating bending fatigue test piece. .
[0062]
The smooth Ono-type rotary bending fatigue test piece obtained as described above was subjected to induction hardening under the condition that the induction hardening depth became 1.5 mm, and then tempered at 180 ° C. The conditions under which the induction hardening depth becomes 1.5 mm are obtained by conducting a preliminary test on each material steel and then induction hardening under these conditions.
[0063]
The Ono-type rotating bending fatigue test was performed in the air at room temperature.7 The fatigue strength (fatigue limit) of each time was measured.
Table 5 shows the results of the fatigue test. Table 5 also shows the value of the maximum equivalent circular diameter fn2 obtained in Example 1 above.
[0064]
[Table 5]
From Table 5, in the case of the test numbers 39 to 56 of the examples of the present invention in which the maximum equivalent circular diameter fn2 of the chemical composition of the steel and the inclusions satisfying the conditions specified in the present invention, even in the T direction, the fatigue strength exceeds 900 MPa. In addition, the decrease in the fatigue strength in the T direction relative to the fatigue strength in the L direction is small. Therefore, it is clear that the fatigue strength when the stress perpendicular to the wrought axis acts is hard to decrease.
[0065]
On the other hand, in the case of the test numbers 57 to 68 of the comparative examples, the fatigue strength in the T direction is significantly reduced with respect to the fatigue strength in the L direction. In the case of Test No. 64, although the chemical composition of the steel 35 satisfies the conditions specified in the present invention, the value of fn2 deviates from the condition specified in the present invention. Therefore, the fatigue strength in the T direction is significantly reduced with respect to the fatigue strength in the L direction.
[0066]
【The invention's effect】
The steel material for machine structural use of the present invention has excellent induction hardening properties, cold workability, and machinability, and further, has excellent fatigue strength, especially fatigue strength in the T direction, and is subject to stress perpendicular to the rolling direction and the forging axis. Therefore, it can be used as a material for various induction hardened parts such as constant velocity joints and shafts of automobiles.
[Brief description of the drawings]
FIG. 1 is a view showing an example of a maximum equivalent circular diameter fn2 of a steel material according to an embodiment.
FIG. 2 is a diagram illustrating a method of cutting out a sample of a smooth Ono-type rotating bending fatigue test piece from a direction parallel to the wrought axis and a direction perpendicular to the wrought axis.
FIG. 3 is a diagram illustrating that both ends of a sample cut out from a direction perpendicular to a wrought axis are joined by electron beam welding to finish a predetermined smooth Ono-type rotating bending fatigue test piece.
Claims (2)
fn1=Ti(%)−3S(%)−3.4N(%)・・・(1)
fn2=(πLW/4)0.5 ・・・(2)Chemical composition in mass%, C: 0.30 to 0.60%, Si: 0.01 to 0.40%, Mn: 0.10 to 0.60%, S: 0.005 to 0.05% , Ti: 0.05 to 0.2%, B: 0.0005 to 0.005%, Nb: 0 to 0.05%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Cr: 0 to 0.20%, Mo: 0 to 0.20%, Al: 0.01% or less, the balance being Fe and impurities, N in the impurities is 0.008% or less, and O ( Oxygen) is 0.0025% or less, and the value of fn1 represented by the following formula (1) is 0 or more. Further, the major axis of the nonmetallic inclusion in the longitudinal longitudinal section is L (μm), the minor axis Is the maximum equivalent circular diameter fn of the nonmetallic inclusion represented by the following equation (2) when the cumulative distribution function predicted by the extreme value statistical processing is 99%, where Mechanical structural steel but is 30μm or less.
fn1 = Ti (%)-3S (%)-3.4N (%) (1)
fn2 = (πLW / 4) 0.5 (2)
fn3=Ti(%)+Zr(%)−3S(%)−3.4N(%)・・・(3)
fn2=(πLW/4)0.5 ・・・(2)Chemical composition in mass%, C: 0.30 to 0.60%, Si: 0.01 to 0.40%, Mn: 0.10 to 0.60%, S: 0.005 to 0.05% , Ti: 0.2% or less, Zr: 0.2% or less, and Ti (%) + Zr (%): 0.05 to 0.2%, B: 0.0005 to 0.005%, Nb : 0 to 0.05%, V: 0 to 0.3%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, Cr: 0 to 0.20%, Mo: 0 to 0. 20%, Al: 0.01% or less, with the balance being Fe and impurities, N in the impurities is 0.008% or less, O (oxygen) is 0.0025% or less, and the following (3) The value of fn3 represented by the formula is 0 or more, and furthermore, is predicted by extreme value statistical processing with the major axis of the nonmetallic inclusion in the longitudinal cross section being L (μm) and the minor axis being W (μm). Mechanical structural steel maximum equivalent circular diameter fn2 cumulative distribution function nonmetallic inclusions represented by the following equation (2) when 99% is 30μm or less to be.
fn3 = Ti (%) + Zr (%)-3S (%)-3.4N (%) (3)
fn2 = (πLW / 4) 0.5 (2)
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