【発明の詳細な説明】[Detailed description of the invention]
本発明は冷間鍛造性を向上せしめた機械構造用
炭素鋼に係り、冷間鍛造性の優れた強靭鋼を提供
しようとするものである。
一般に強靭鋼は使用時の耐摩耗性、耐転動疲労
性より0.4％以上の高炭素鋼を焼入、焼もどし処
理し、表面硬さを高くして使用される。ところが
このような高炭素の材料は球状化焼鈍処理（以下
S.A.と略す）を行つても変形抵抗（加工に要する
力の大きさで示す）が高い。それゆえ従来は冷間
鍛造が困難で熱間鍛造にて成形されていた。しか
し熱間鍛造では寸法精度が悪く、その後に切削工
程が必要である。この切削工程は多くの工数を要
し誠に煩雑であるので寸法精製がよく切削工程の
大巾削減ができる冷間鍛造化が望まれる所であ
る。
本発明者らは、これらの点に鑑み種々検討した
結果、変形抵抗にはSi、Mn、Ｐ、Ｎ、Ｏの方が
Ｃよりむしろ悪影響をもたらしていることをつき
とめ、これに対処するには低Ｐ、低Ｎ、低Ｏを必
須とし、Si及びMnの上限をおさえ、それによる
焼入性をＣ、Crで補い、変形抵抗を著しく改良
した鋼材を開発することができた。
本発明はこの知見によりなされたものでその要
旨とするところは重量％で、C0.40〜0.65％、
Si0.01〜0.05％、Mn0.20〜0.40％、S0.02〜0.03
％、Al0.01超0.04％以下、Cr0.3〜0.8％を含有し、
且つP0.01％以下、N0.05％以下、O0.005％以下
に制限し、残部はFeと不可避不純物から構成さ
れることを特徴とする冷間鍛造用高靭鋼にある。
以下に本発明について詳細に説明する。先ず本
発明において対象とする鋼は重量％でC0.40〜
0.65％を含有する機械構造用炭素鋼であつて特に
Ｃを限定したのはＣが焼入硬さを決定する重要な
合金元素だからであり、0.40％未満では十分な表
面硬さが得られないので0.40％以上とした。また
0.65％超では焼入時の焼割れが発生しやすくなる
ので、上限を0.65％とした。
Siは製鋼時の脱酸元素として必要であるため従
来は0.2〜0.3％含まれていた。しかし、SiはS.A.
時の強度上昇が大きく、特に高変形時の変形抵抗
を大きく増大させる。従つて変形抵抗の十分な低
下のために上限を0.05％とし、過度に少ないと脱
酸不足によるＢ系介在物が増加し冷間鍛造時の割
れを生じることがあるので若干のSiの存在は必要
であり、従つてその下限を0.01％とした。
次にMnは不純物としてのＳを固定し、熱間圧
延時の表面疵発生防止上必要であるが、その量は
0.4％で十分である。従来鋼では比較的安価で焼
入性確保に重宝な元素として0.7〜0.8％添加され
ていた。しかし、MnはSi同様高変形時の変形抵
抗下に大きな阻害元素となるので、かゝる見地か
らMn量の上限を0.4％とした。なおＭ量の上限を
0.4％としたことによる焼入性の不足分は後述の
Crで補うものである。一方、Mnは熱間圧延時の
割れ発生原因となる不純物ＳをMnSとして固
定・無害化するため少量の添加は必要であり、従
つて0.2％を下限とした。
Ｓは加工硬化を助長するため低い方が望ましい
が、ある程度の被削性を確保するためにはＳの存
在が必要となるのでその範囲を0.02〜0.03％とし
た。0.02％未満では充分な被削性が得られず、
0.03％超では被削性は満足できるが加工硬化を起
こすようになる。
またAlは脱酸に有効であるのみでなく、Ｎを
固定してAlNとなつて結晶粒細粒化の役目を果
たす有効な合金元素であるため下限を0.01％超と
し、一方0.04％を超えると脱酸時に生成する
Al2O3が冷間鍛造時の割れの原因となるため上限
を0.04％とした。
次にCr焼入性の不足分を補うために添加する
ものであり、S.A.時の強度をあまり上昇しないで
焼入性を高くするもつとも優れた元素である。変
形抵抗低下のためSi及びMnの上限をおさえ、そ
れによる焼入性の不足をCr添加で補うためには
0.3％以上が必要である。しかしその量が多いと
固溶硬化を生じS.A.時の強度を上昇するので上限
を0.8％とした。
次に本発明においてはＰ、Ｎ、Ｏを夫々低減せ
しめることを特徴の一つとするものであり、その
理由は次の通りである。
Ｐ、Ｎは鋼中に不純物として含まれるが、冷間
鍛造時の温度上昇に伴ない時効硬化させて変形抵
抗上昇の原因となるので夫々0.01％以下、0.005
％以下とした。
ＯはAl、Siなどと結びついて酸化物となり、
冷間鍛造時の割れの原因となるので上限を0.005
％とした。
次に本発明の効果を実施例に基づいてさらに具
体的に説明する。
表に示す化学成分の鋼材を用いて、球状化焼鈍
処理を行ない圧縮試験時の荷重を調査すると共
に、30φの試験材の表面を高周波焼入、焼もどし
試験を行い、表面から３mmの位置の硬さを測定し
その結果もあわせて示した。
The present invention relates to a carbon steel for mechanical structures with improved cold forgeability, and an object thereof is to provide a strong steel with excellent cold forgeability. In general, high-strength steel is used by quenching and tempering high-carbon steel that has a wear resistance and rolling fatigue resistance of 0.4% or more during use to increase its surface hardness. However, such high-carbon materials undergo spheroidizing annealing treatment (hereinafter referred to as
Even when machining is performed (abbreviated as SA), the deformation resistance (indicated by the amount of force required for machining) is high. Therefore, in the past, cold forging was difficult and hot forging was used. However, hot forging has poor dimensional accuracy and requires a subsequent cutting process. Since this cutting process requires a large number of man-hours and is very complicated, cold forging is desired because it allows for better dimensional refinement and greatly reduces the cutting process. As a result of various studies in view of these points, the present inventors found that Si, Mn, P, N, and O have a more negative effect on deformation resistance than C. By making low P, low N, and low O essential, by suppressing the upper limits of Si and Mn, and by supplementing the resulting hardenability with C and Cr, we were able to develop a steel material with significantly improved deformation resistance. The present invention was made based on this knowledge, and its gist is that the weight percentage is C0.40-0.65%,
Si0.01~0.05%, Mn0.20~0.40%, S0.02~0.03
%, Al over 0.01 and 0.04% or less, Cr 0.3 to 0.8%,
In addition, the high toughness steel for cold forging is limited to P0.01% or less, N0.05% or less, and O0.005% or less, with the remainder being composed of Fe and unavoidable impurities. The present invention will be explained in detail below. First of all, the steel targeted in the present invention has a C0.40~ by weight%.
The reason why carbon steel for machine structures containing 0.65% is specifically limited to C is because C is an important alloying element that determines quenching hardness, and if it is less than 0.40%, sufficient surface hardness cannot be obtained. Therefore, it was set at 0.40% or more. Also
If it exceeds 0.65%, quenching cracks are likely to occur during quenching, so the upper limit was set at 0.65%. Since Si is necessary as a deoxidizing element during steel manufacturing, it has conventionally been contained in an amount of 0.2 to 0.3%. However, Si is SA
The strength increase is large at times, and particularly the deformation resistance at high deformations is greatly increased. Therefore, in order to sufficiently reduce the deformation resistance, the upper limit is set at 0.05%, and if the amount is too low, B-based inclusions will increase due to insufficient deoxidation, which may cause cracks during cold forging, so the presence of some Si should be avoided. Therefore, the lower limit was set at 0.01%. Next, Mn fixes S as an impurity and is necessary to prevent surface defects during hot rolling, but the amount is
0.4% is sufficient. In conventional steel, 0.7 to 0.8% of C was added as a relatively inexpensive and useful element for ensuring hardenability. However, like Si, Mn becomes a large inhibiting element in the deformation resistance during high deformation, so from this point of view, the upper limit of the Mn content was set at 0.4%. Note that the upper limit of M amount is
The lack of hardenability due to the setting of 0.4% will be explained later.
It is supplemented by Cr. On the other hand, it is necessary to add a small amount of Mn to fix and render harmless the impurity S, which causes cracking during hot rolling, as MnS, and therefore 0.2% was set as the lower limit. A low S content is desirable because it promotes work hardening, but the presence of S is necessary to ensure a certain degree of machinability, so the range was set to 0.02 to 0.03%. If it is less than 0.02%, sufficient machinability cannot be obtained,
If it exceeds 0.03%, machinability will be satisfactory, but work hardening will occur. In addition, Al is not only effective in deoxidizing, but also is an effective alloying element that fixes N and becomes AlN, thereby refining crystal grains. is produced during deoxidation.
Since Al 2 O 3 causes cracks during cold forging, the upper limit was set at 0.04%. Next, Cr is added to compensate for the lack of hardenability, and is an excellent element that increases hardenability without increasing the strength during SA. In order to suppress the upper limit of Si and Mn to reduce deformation resistance, and to compensate for the resulting lack of hardenability by adding Cr,
0.3% or more is required. However, if the amount is too large, solid solution hardening occurs and the strength during SA increases, so the upper limit was set at 0.8%. Next, one of the features of the present invention is to reduce each of P, N, and O, and the reason for this is as follows. P and N are contained as impurities in steel, but they age harden as the temperature rises during cold forging and cause an increase in deformation resistance, so they should be less than 0.01% and 0.005% respectively.
% or less. O combines with Al, Si, etc. to form oxides,
The upper limit is set at 0.005 as it may cause cracking during cold forging.
%. Next, the effects of the present invention will be explained in more detail based on examples. Using steel materials with the chemical composition shown in the table, we performed spheroidizing annealing treatment and investigated the load during compression tests, and also conducted induction hardening and tempering tests on the surface of the 30φ test material, The hardness was measured and the results are also shown.
【表】【table】
【表】
位置の硬さ
表のNo.１〜８の比較材でありNo.９〜No.13は本発
明鋼である。先ず比較鋼においてNo.１はS30Cの
規格材であり、圧縮試験時の荷重は60tと比較的
低いが、高周波焼入・焼もどし後の硬さは450で、
一般に耐摩耗性、耐転動疲労性より目標レベルと
される550より低く、焼入性不足となつている。
No.２はS45Cの規格材であり高周波焼入・焼もど
し後の硬さは620と高いが圧縮試験時の荷重が69t
で高すぎる。
No.３はSiとMnを通常レベルより下げ圧縮試験
時の荷重低下を狙つたものであり50tと低いが、
高周波焼入・焼もどし後の硬さが420と低く焼入
性不足となつている。No.４はＳを高くした快削鋼
で、工具寿命は50cmと他より長いが圧縮試験時の
荷重が73tと高くなつている。No.５およびNo.６は
S55C材であり、高周波焼入・焼もどし後の硬さ
は690660と高く焼入性を向上しているが圧縮試験
時の荷重79t、75tと非常に高くなつている。さら
にNo.６の場合Ｓが0.001と低いため工具寿命は15
cmと短くなつている。
No.７はMnを低くし、圧縮試験時の荷重の低下
を狙つたものであるがSiは通常レベルでＯが
105ppmと高いため、その荷重は77tであつて、高
すぎる。No.８はさらにＣを高めて焼入性を上げた
もので高周波焼入・焼もどし後の硬さは730と高
くなるが、圧縮試験時の荷重も83tとさらに高ま
る。
これに対してNo.９〜No.13の本発明鋼は、いずれ
も高周波焼入・焼もどし後の硬さが目標レベル
550を十分に満足し、しかも圧縮試験時の荷重が
60t以下であり、工具寿命が快削鋼に近い性能を
示すものである。
上記のごとく本発明鋼は焼入・焼もどし後の強
度を十分満たしながら冷間鍛造特性（変形抵抗）
を向上させ、被削性を十分満足した鋼を提供する
もので工業上非常に有用である。[Table] Position Hardness Nos. 1 to 8 in the table are comparative materials, and Nos. 9 to 13 are the steels of the present invention. First of all, the No. 1 comparison steel is the S30C standard material, which has a relatively low load of 60t during compression testing, but has a hardness of 450 after induction hardening and tempering.
In general, wear resistance and rolling fatigue resistance are lower than the target level of 550, and hardenability is insufficient.
No. 2 is a standard material of S45C and has a high hardness of 620 after induction hardening and tempering, but the load during compression test is 69t.
It's too expensive. No. 3 aims to reduce the load during compression tests by lowering Si and Mn below normal levels, and is low at 50t.
The hardness after induction hardening and tempering is as low as 420, resulting in insufficient hardenability. No. 4 is a free-cutting steel with a high S value, and the tool life is longer than the others at 50cm, but the load during the compression test is high at 73t. No.5 and No.6 are
It is an S55C material, and the hardness after induction hardening and tempering is 690,660, which improves hardenability, but the compression test loads are extremely high at 79t and 75t. Furthermore, in the case of No. 6, S is as low as 0.001, so the tool life is 15
It has become shorter to cm. No. 7 has a low Mn content and aims to reduce the load during compression tests, but the Si content is at the normal level and the O content is low.
Since it is high at 105ppm, the load is 77t, which is too high. No. 8 has even higher C content to improve hardenability, and the hardness after induction hardening and tempering is as high as 730, but the load during the compression test is even higher at 83 tons. On the other hand, steels of the present invention No. 9 to No. 13 all have hardness at the target level after induction hardening and tempering.
550, and the load during the compression test was
It has a tool life of 60 tons or less, and exhibits performance similar to that of free-cutting steel. As mentioned above, the steel of the present invention satisfies the strength after quenching and tempering and has cold forging properties (deformation resistance).
It provides steel with improved machinability and is extremely useful industrially.