JP4292375B2 - Non-heat treated steel for cracking connecting rod - Google Patents

Description

【００７７】
【表２】

【００７８】
次いで、これらの本発明例の鋼及び比較例の鋼を通常の方法によって鋼片とした後、鋼１〜１６及び鋼２１については鍛造のための加熱温度を１２５０℃、鍛造仕上げ温度を１０５０℃として直径２０ｍｍの丸棒に熱間鍛造し、鍛造終了後は８００〜６００℃における冷却速度を１１０℃／分として冷却し、６００℃を下回る温度域は通常の大気中放冷とした。
【００７９】
一方、鋼１７及び鋼１８については鍛造のための加熱温度を１３００℃、鍛造仕上げ温度を１２００℃として直径２０ｍｍの丸棒に熱間鍛造し、鍛造終了後は８００〜６００℃における冷却速度を１１０℃／分として冷却し、６００℃を下回る温度域は通常の大気中放冷とした。また、鋼１９については鍛造のための加熱温度を１１５０℃、鍛造仕上げ温度を１０００℃として直径２０ｍｍの丸棒に熱間鍛造し、鍛造終了後は８００〜６００℃における冷却速度を１１０℃／分として冷却し、６００℃を下回る温度域は通常の大気中放冷とした。
【００８０】
なお、比較例の鋼のうちＪＩＳのＳ４８Ｃに相当する鋼である鋼２０については、１２５０℃に加熱後通常の方法で熱間鍛造して得た直径２０ｍｍの丸棒に、１１００℃に加熱して油焼入れ後５００℃で焼戻しする調質処理を施した。
【００８１】
このようにして得た直径が２０ｍｍの丸棒から各種の試験片を採取して、ミクロ組織、ビッカース硬さ（以下、Ｈｖ硬さという）、引張特性、耐疲労特性、衝撃特性及び被削性を調査した。
【００８２】
すなわち、前記した各丸棒から鍛錬軸に垂直な面を観察面とするミクロ試験片を切り出し、鏡面研磨してナイタル腐食した後、倍率を４００倍とした光学顕微鏡で観察して、ミクロ組織の判定を行った。
【００８３】
上記のようにして観察した組織がフェライト・パーライトであったものについて、更に、通常の方法で画像解析し、フェライト・パーライト組織におけるフェライトの割合を測定した。組織がフェライト・パーライトであったものについては、フェライトのＨｖ硬さ（αＨｖ）を０．０９８０７Ｎの試験力で測定した。
【００８４】
また、各丸棒から鍛錬軸に垂直な面を試験面とする試験片を切り出して鏡面研磨した後、全硬さとしてのＨｖ硬さ（ＴＨｖ）を９８．０７Ｎの試験力で測定した。
【００８５】
引張特性は、各丸棒から平行部の直径が７ｍｍのＪＩＳ１４Ａ号引張試験片を切り出し、通常の方法により室温で引張試験を行い、ＴＳ（引張強さ）を測定した。
【００８６】
耐疲労特性は、各丸棒から平行部の直径が８ｍｍの小野式回転曲げ疲労試験片を切り出し、通常の方法により室温で回転曲げ疲労試験を行い、疲労強度（σｗ）を測定した。このσｗと前記のＴＳとから耐久比（σｗ／ＴＳ）を求めた。なお、調質処理したＪＩＳのＳ４８Ｃ相当鋼である鋼２０を用いた試験番号２０の耐久比である「０．４７９」を基準性能とし、これ以上の耐久比が得られた場合に耐疲労特性が良好と判断した。
【００８７】
衝撃特性は、各丸棒からJIS Z 2202(1998)に記載の幅１０ｍｍのＶノッチ試験片を切り出し、通常の方法により室温でシャルピー衝撃試験を行い、衝撃値を測定した。この幅１０ｍｍのＶノッチ試験片を用いた室温でのシャルピー衝撃値（ｖＥ_RT）は、クラッキング性を評価する１つの指標となり得るもので、その値が小さいほどクラッキング性が良好といえる。このため、欧州で既に実用化されているクラッキングコンロッド用鋼に相当する鋼である鋼２１を用いた試験番号２１の衝撃値である「７．０Ｊ／ｃｍ² 」を基準性能とし、これ以下の衝撃値が得られた場合に衝撃特性が低く、したがって、クラッキング性が良好と判断した。
【００８８】
被削性は、丸棒から１０ｍｍ×１０ｍｍの断面を有する板状試験片を切り出し、深さ１０ｍｍの貫通孔をドリルで穿孔し、孔を１００個あけた後のドリルのコーナー摩耗量（ドリル最外周部の摩耗量）を測定して評価した。なお、欧州で既に実用化されているクラッキングコンロッド用鋼に相当する鋼である鋼２１を用いて穿孔試験した試験番号２１の場合の上記コーナー摩耗量を基準性能とし、その３０％未満のコーナー摩耗量の場合に被削性が良好と判断した。
【００８９】
穿孔試験条件は次に示すとおりである。
【００９０】
ドリル：Ｐ２０超硬の直径８ｍｍのストレートシャンクドリル、
回転数：３０００ｒｐｍ、
送り：０．１５ｍｍ／ｒｅｖ、
潤滑：水溶性潤滑剤。
【００９１】
なお、調質処理したＪＩＳのＳ４８Ｃ相当鋼である鋼２０の丸棒については被削性試験を行わなかった。
【００９２】
表３に上記の各試験結果をまとめて示す。表３におけるαＨｖ及びＴＨｖはそれぞれ、ビッカース硬さでのフェライトの硬さ及び全硬さを表し、また、ｖＥ_RTが幅１０ｍｍのＶノッチ試験片を用いた室温でのシャルピー衝撃値を表すことは既に述べたとおりである。被削性の欄における「◎」、「○」、「△」及び「×」はそれぞれ、コーナー摩耗量が鋼２１を穿孔試験した場合のコーナー摩耗量の１０％未満、１０％以上３０％未満、３０〜１００％及び１００％を超えることを示す。
【００９３】
なお、試験番号１３においては、フェライト面積率が小さいため０．０９８０７Ｎの試験力では圧子がパーライトにもかかってフェライト単独の硬さを測定することができなかった。
【００９４】
【表３】

【００９５】
表３から明らかなように、本発明で定める化学組成、組織、フェライトの硬さ及びフェライトの硬さと全硬さの比を有する試験番号１〜１１の場合は、いずれも目標とする被削性、クラッキング性及び耐疲労特性が得られている。すなわち、ドリルのコーナー摩耗量は、鋼２１を用いて穿孔試験した試験番号２１の場合の摩耗量の３０％未満（マークで「◎」又は「○」）であり、被削性は極めて良好である。クラッキング性を評価する１つの指標となり得るｖＥ_RTは、同じく鋼２１を用いた試験番号２１の場合のｖＥ_RTである７．０Ｊ／ｃｍ² を下回り、したがって、クラッキング性は良好である。更に、耐疲労特性は、ＪＩＳのＳ４８Ｃ相当鋼である鋼２０を調質処理した試験番号２０の場合の耐久比である０．４７９を上回り、耐疲労特性も良好である。
【００９６】
上記試験番号のうちでも、試験番号４及び試験番号６〜１１の被削性はなお一段と良好なことが明らかである。
【００９７】
これに対して、試験番号１２〜１６の場合は、少なくとも化学組成が本発明で規定する条件から外れているため、被削性、クラッキング性及び耐疲労特性のいずれか１つ以上において目標に達していない。
【００９８】
ＪＩＳのＳ４８Ｃ相当鋼である鋼２０を調質処理した試験番号２０の場合は、ｖＥ_RTが鋼２１を用いた試験番号２１の場合のｖＥ_RTである７．０Ｊ／ｃｍ² を上回り、少なくともクラッキング性に劣る。
【００９９】
また、欧州で既に実用化されているクラッキングコンロッド用鋼に相当する鋼である鋼２１を用いた試験番号２１の場合は、上記ＪＩＳのＳ４８Ｃ相当鋼である鋼２０を調質処理した試験番号２０の場合の耐久比である０．４７９を下回り、耐疲労特性に劣る。
【０１００】
一方、試験番号１７及び試験番号１８の場合は、鋼１７及び鋼１８は本発明で定める化学組成を有するものの、組織が本発明で規定する条件から外れている。このため、試験番号１７の場合は、被削性に劣るし、耐久比が低くて耐疲労特性も低い。また、試験番号１８の場合は、被削性、クラッキング性及び耐疲労特性のいずれの特性にも劣っている。
【０１０１】
試験番号１９の場合、鋼１９は本発明で定める化学組成を有するものの、Ｔｉ及びＶによる十分なフェライトの強化がなされておらず、ビッカース硬さでのフェライトの硬さ（αＨｖ）及びフェライトの硬さと全硬さの比（αＨｖ／ＴＨｖ）が本発明で規定する条件から外れている。このためｖＥ_RTが７．０Ｊ／ｃｍ² を上回り、クラッキング性に劣る。
【０１０２】
【発明の効果】
本発明のクラッキングコンロッド用非調質鋼は、破断分割性及び耐疲労特性に優れているとともに極めて良好な被削性を有しているので、自動車エンジンなどのクラッキングコンロッド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】コンロッドの詳細を示す図である。
【符号の説明】
１：コンロッド、
２：コンロッド本体、
３：コンロッドキャップ、
４：ボルト、
５：大端部[0001]
BACKGROUND OF THE INVENTION
  The present inventionFor cracking connecting rodFor non-heat treated steel, in detail, fracture splitting property, fatigue resistance and machinability are excellent as fracture splitting property and fatigue resistance, and machinability is very good, and it is suitable as a material for connecting rods such as automobile engines. ExcellentFor cracking connecting rodIt relates to non-tempered steel.
[0002]
[Prior art]
A connecting rod (hereinafter referred to as a connecting rod) such as an automobile engine is an engine component that connects a piston and a crankshaft, and plays a role of transmitting explosive force to a drive shaft. For this reason, the connecting rod is required to have a high fatigue limit (hereinafter also referred to as fatigue strength, represented by the symbol σw). In particular, with the recent increase in engine output, fatigue strength required for connecting rods is increasing. Moreover, since the bolt hole of a connecting rod and the part connected with a piston and a crankshaft are cut, the material requires good machinability.
[0003]
If the carbon steel for mechanical structure such as S48C specified in JIS is subjected to so-called tempering treatment of quenching and tempering, it will stably have a large fatigue limit ratio (the value obtained by dividing the fatigue limit σw by the tensile strength TS). In the following, it is also referred to as the durability ratio, which is represented by the symbol of σw / TS). For this reason, the conventional connecting rod has been manufactured by tempering carbon steel for mechanical structures such as S48C.
[0004]
However, reflecting the recent severe economic situation, the movement of reducing the manufacturing cost of various automobile parts has become active, and this movement is no exception in the connecting rod which is an engine part. For this reason, there is a great demand for a connecting rod that does not undergo quenching-tempering tempering, which is expensive to manufacture, that is, non-tempering and can obtain the same durability ratio as when the carbon steel for mechanical structure is tempered. It has begun to be adopted in some car models.
[0005]
Although the connecting rod is shown in FIG. 1, the conventional connecting rod 1 is subjected to bolt hole processing and finish shaping by cutting on the connecting rod body 2 and the connecting rod cap 3 that have been hot forged in another process. It is necessary to go through a process of connecting and assembling a crankshaft having a complicated shape with the bolt 4. Therefore, even though non-tempering can be achieved, it is not as satisfactory in terms of cost reduction as the “cracking connecting rod” described below.
[0006]
That is, recently, in order to further reduce manufacturing costs in addition to non-tempered steel, both the connecting rod body 2 and the connecting rod cap 3 are integrally formed by hot forging, and then the connecting rod body at the large end 5. So-called “cracking connecting rods” that are cracked (divided) into 2 and connecting rod caps 3 are being studied.
[0007]
For the above cracking, a method is applied in which a jig is inserted into a hole (for example, the N portion in FIG. 1) of the large end portion 5 which is a portion to be divided of the integrally molded material, and a fracture is caused by applying stress. .
[0008]
If the cracked connecting rod 1 (that is, the cracking connecting rod) is a brittle fracture surface with a smooth fracture surface when cracked, when connecting it to the crankshaft, the connecting rod body 2 and the connecting rod cap 3 are cranked. After sandwiching the shaft, it is only necessary to match the fracture surfaces and connect the connecting rod body 2 and the connecting rod cap 3 with the bolts 4.
[0009]
Therefore, if the cracking connecting rod has a brittle fracture surface with a smooth fracture surface, cutting of the mating surface, which is the part sandwiching the crankshaft, is unnecessary, and the manufacturing cost can be reduced, and the cracking connecting rod can be connected at the fracture surface. Therefore, the fastening rigidity, that is, the strength is excellent.
[0010]
As for the above-mentioned cracking connecting rod, a material made of non-heat treated steel containing about 0.7% by mass of C (carbon) disclosed in Patent Document 1 has already been put into practical use in Europe. However, the connecting rods put into practical use in Europe are inferior in durability ratio to conventional connecting rods tempered with carbon steel for machine structural use, and are not always able to meet the demands of the industry in terms of fatigue resistance.
[0011]
Therefore, it has a fracture splitting property (hereinafter referred to as cracking property) equal to or higher than that practically used in Europe, and has at least the same durability ratio as a tempered connecting rod in a non-tempered state, In addition, there is a growing demand for steel for connecting rods with extremely good machinability.
[0012]
In Patent Documents 2 to 15, “low ductility non-tempered steel” with improved cracking property by controlling the chemical composition of steel, or improved cracking property and machinability by controlling the chemical composition and carbon sulfide. In addition, “low ductility non-tempered steel” is disclosed. However, none of the non-heat treated steels proposed in Patent Documents 2 to 15 has been considered for fatigue resistance.
[0013]
In Patent Document 16, in a low carbon region having a C content of 0.2% to less than 0.35% by weight, the content of Mn is reduced, and ferrite having high ductility as a high V composition is further embrittled. In addition, “steel for machine structure excellent in break separation property and durability” in which break separation property is improved by dispersing coarse TiC particles is disclosed. However, the steel for machine structural use proposed in this publication simply contains S in an amount of 0.01 to 0.2% by weight in order to improve machinability. It does not satisfy the required machinability.
[0014]
[Patent Document 1]
US Pat. No. 5,135,587
[Patent Document 2]
JP-A-9-3589
[Patent Document 3]
JP-A-9-31594
[Patent Document 4]
Japanese Patent Laid-Open No. 9-111412
[Patent Document 5]
JP-A-9-176785
[Patent Document 6]
JP-A-9-176786
[Patent Document 7]
JP-A-9-176787
[Patent Document 8]
Japanese Patent Laid-Open No. 11-50184
[Patent Document 9]
Japanese Patent Laid-Open No. 11-199967
[Patent Document 10]
Japanese Patent Laid-Open No. 11-199968
[Patent Document 11]
JP-A-11-236643
[Patent Document 12]
JP-A-11-286746
[Patent Document 13]
JP-A-11-286750
[Patent Document 14]
JP-A-11-302778
[Patent Document 15]
JP 2000-345298 A
[Patent Document 16]
JP 11-315340 A
[0015]
[Problems to be solved by the invention]
  The present invention has been made in view of the above situation, and its purpose is excellent in fracture splitting and fatigue resistance and extremely good machinability, and is suitable as a material for connecting rods such as automobile engines.For cracking connecting rodIt is to provide non-tempered steel.
[0016]
[Means for Solving the Problems]
The gist of the present invention resides in the non-heat treated steel shown in the following (1) to (8).
[0017]
  (1) By mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: more than 0.130% and 0 Up to 200%, V: 0.10 to 0.50%, Ti: Over 0.10%0.30%, Al: 0.002 to 0.100% and N: 0.002 to 0.020%, the balance is a chemical composition composed of Fe and impurities, and the structure has a ferrite ratio of 40% or more -It is a pearlite structure, and further, the hardness of ferrite is 250 or more in terms of Vickers hardness, and the ratio of the hardness of ferrite to the total hardness is 0.80 or more.For cracking connecting rodNon-tempered steel.
[0018]
  (2) By mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: over 0.130% and 0 Up to 200%, V: 0.10 to 0.50%, Ti: Over 0.10%0.30%: Al: 0.002 to 0.100%, N: 0.002 to 0.020% and P: 0.150% or less, with the balance being a chemical composition composed of Fe and impurities, the structure being ferrite The ferrite-pearlite structure having a ratio of 40% or more is characterized in that the ferrite hardness is 250 or more in terms of Vickers hardness and the ratio of the ferrite hardness to the total hardness is 0.80 or more. DoFor cracking connecting rodNon-tempered steel.
[0019]
  (3) By mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: more than 0.130% and 0 Up to 200%, V: 0.10 to 0.50%, Ti: Over 0.10%0.30%: Al: 0.002 to 0.100%, N: 0.002 to 0.020% and Cr: 0.50% or less, with the balance being a chemical composition composed of Fe and impurities, the structure being ferrite The ferrite-pearlite structure having a ratio of 40% or more is characterized in that the ferrite hardness is 250 or more in terms of Vickers hardness and the ratio of the ferrite hardness to the total hardness is 0.80 or more. DoFor cracking connecting rodNon-tempered steel.
[0020]
  (4) By mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: over 0.130% and 0 Up to 200%, V: 0.10 to 0.50%, Ti: Over 0.10%0.30%, Al: 0.002 to 0.100% and N: 0.002 to 0.020%, Pb: 0.30% or less, Te: 0.30% or less, Ca: 0.010% 1 or more selected from the following and Bi: 0.30% or less, the balance is a chemical composition composed of Fe and impurities, the structure is a ferrite pearlite structure in which the proportion of ferrite is 40% or more, The ferrite hardness is 250 or more in terms of Vickers hardness, and the ratio of the hardness of the ferrite to the total hardness is 0.80 or more.For cracking connecting rodNon-tempered steel.
[0021]
  (5) By mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: over 0.130% and 0 Up to 200%, V: 0.10 to 0.50%, Ti: Over 0.10%0.30%: Al: 0.002 to 0.100%, N: 0.002 to 0.020%, P: 0.150% or less and Cr: 0.50% or less, with the balance being Fe and impurities And a ferrite pearlite structure in which the ratio of ferrite is 40% or more, the ferrite hardness is 250 or more in terms of Vickers hardness, and the ratio of ferrite hardness to total hardness is 0. 80 or moreFor cracking connecting rodNon-tempered steel.
[0022]
  (6) By mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: more than 0.130% and 0 Up to 200%, V: 0.10 to 0.50%, Ti: Over 0.10%0.30%: Al: 0.002 to 0.100%, N: 0.002 to 0.020% and P: 0.150% or less, Pb: 0.30% or less, Te: 0.30% Hereinafter, it contains one or more selected from Ca: 0.010% or less and Bi: 0.30% or less, with the balance being a chemical composition composed of Fe and impurities, and the proportion of ferrite in the structure is 40% or more. It is a ferrite-pearlite structure, and further, the hardness of the ferrite is 250 or more in terms of Vickers hardness, and the ratio of the hardness of the ferrite to the total hardness is 0.80 or more.For cracking connecting rodNon-tempered steel.
[0023]
  (7) By mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: more than 0.130% and 0 Up to 200%, V: 0.10 to 0.50%, Ti: 0.10% over0.30%, Al: 0.002 to 0.100%, N: 0.002 to 0.020% and Cr:0.50% Or less, Pb: 0.30% or less, Te: 0.30% or less, Ca: 0.010% or less, and Bi: 0.30% or less. Ferrite and pearlite structure with a chemical composition composed of Fe and impurities, and the ratio of ferrite is 40% or more. Further, the hardness of the ferrite is 250 or more in terms of Vickers hardness, and the hardness of the ferrite and the total hardness. The ratio is 0.80 or moreFor cracking connecting rodNon-tempered steel.
[0024]
  (8) By mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: more than 0.130% and 0 Up to 200%, V: 0.10 to 0.50%, Ti: Over 0.10%0.30%: Al: 0.002 to 0.100%, N: 0.002 to 0.020%, P: 0.150% or less and Cr: 0.50% or less, and Pb: 0.30% Hereinafter, it contains at least one selected from Te: 0.30% or less, Ca: 0.010% or less, and Bi: 0.30% or less, with the balance being a chemical composition comprising Fe and impurities, and the structure being ferrite. The ferrite-pearlite structure having a ratio of 40% or more is characterized in that the ferrite hardness is 250 or more in terms of Vickers hardness and the ratio of the ferrite hardness to the total hardness is 0.80 or more. DoFor cracking connecting rodNon-tempered steel.
[0025]
Here, the ferrite-pearlite structure refers to a mixed structure of ferrite and pearlite. Each phase described above can be confirmed by observation using an optical microscope or an electron microscope.
[0026]
The Vickers hardness as the hardness of ferrite refers to a value measured with a test force of 0.09807N, and the Vickers hardness as a total hardness refers to a value measured with a test force of 98.07N.
[0027]
Moreover, total hardness refers to the average value when four places are measured at random.
[0028]
In addition, the ferrite referred to in the present invention does not include ferrite that forms pearlite together with cementite.
[0029]
  Hereinafter, the above (1) to (8)For cracking connecting rodNon-tempered steel (Hereinafter, it is also simply referred to as “non-tempered steel”. )These inventions are referred to as (1) to (8), respectively.
[0030]
DETAILED DESCRIPTION OF THE INVENTION
The present inventors have made various studies in order to achieve the above-described object, and obtained the following findings (a) to (f).
[0031]
(B) The structure of steel when both cracking and machinability are good is a ferrite pearlite structure.
[0032]
(B) Ferrite hardness in the ferrite-pearlite structure affects cracking properties, machinability and fatigue resistance.
[0033]
(C) Increasing the proportion of ferrite in the ferrite-pearlite structure increases the durability ratio and improves fatigue resistance.
[0034]
(D) When the ratio of the ferrite hardness to the total hardness in the ferrite / pearlite structure is large, the cracking property is good, the durability ratio is large, the fatigue resistance is excellent, and the machinability is also good.
[0035]
(E) By adding a proper amount of Ti and V in combination, ferrite can be significantly strengthened compared to conventional non-heat treated steel to which V is added alone. Moreover, the strengthening of ferrite by the combined addition of Ti and V reduces the difference between the total hardness and the hardness of the ferrite in the ferrite-pearlite structure, so the ratio of the hardness of the ferrite and the total hardness is increased, and cracking occurs. , Machinability and fatigue resistance are all improved.
[0036]
(F) If S is added in a large amount, and particularly S containing more than 0.130% by mass%, the machinability is greatly improved.
[0037]
The present inventions (1) to (8) have been completed based on the above findings.
[0038]
Hereinafter, each requirement of the present invention will be described in detail. In addition, "%" display of the content of each element means "mass%".
(A) Chemical composition of steel
C: 0.20 to 0.40%,
C has the effect | action which raises the intensity | strength of steel and an effect is acquired by making it contain 0.20% or more. However, if its content exceeds 0.40%, the proportion of ferrite in the ferrite-pearlite structure decreases, and a high durability ratio cannot be obtained, resulting in a decrease in fatigue resistance. Therefore, the content of C is set to 0.20 to 0.40%. The C content is preferably 0.20 to 0.35%.
[0039]
Si: 0.05 to 1.50%
Si is effective for deoxidizing steel and has an effect of increasing the strength of the steel by solid solution strengthening. However, if the content is less than 0.05%, the effect of addition is poor. On the other hand, even if Si is contained exceeding 1.50%, the above effect is saturated and the cost is increased. Moreover, the hot workability of steel is reduced. Therefore, the Si content is set to 0.05 to 1.50%. The Si content is preferably 0.15 to 0.90%.
[0040]
Mn: 0.30 to 2.00%
Mn has a deoxidizing action of steel and has an action of improving hardenability and improving the strength of steel. In order to obtain these effects, the Mn content needs to be 0.30% or more. However, if the Mn content exceeds 2.00%, the hot workability of the steel is lowered. Further, the hardenability becomes too high to form a bainite structure, resulting in a decrease in cracking properties and machinability. Therefore, the content of Mn is set to 0.30 to 2.00%. Note that the Mn content is preferably 0.30 to 1.40%, and more preferably 0.60 to 1.20%.
[0041]
S: Over 0.130% to 0.200%
S has the effect of enhancing the machinability of steel by forming a sulfide together with Mn and Ti. In particular, when the content exceeds 0.130%, machinability is greatly improved. However, when the content is excessive, the hot workability of the steel is deteriorated. In particular, when the content exceeds 0.200%, the hot workability is significantly reduced. Therefore, the content of S exceeds 0.130% and reaches 0.200%.
[0042]
V: 0.10 to 0.50%
V is an important element in the present invention. That is, V precipitates as a carbonitride in ferrite and has the effect of improving strength, and also has the effect of increasing cracking properties by adding it in combination with Ti described later. In order to obtain these effects, the V content needs to be 0.10% or more. However, even if V is contained in excess of 0.50%, the above-described effect is hardly increased, and the cost is extremely increased. Therefore, the content of V is set to 0.10 to 0.50%. The V content is preferably 0.10 to 0.35%, more preferably 0.15 to 0.35%.
[0043]
  Ti: over 0.10%0.30%Until
  Ti is an important element in the present invention. That is, Ti, like V, precipitates as a carbonitride in the ferrite to increase the strength, and further has a function of significantly strengthening the ferrite when added in combination with V. This strengthening of ferrite reduces the difference between the total hardness and the hardness of ferrite in the ferrite and pearlite structure, so that good cracking properties can be secured.Further, since strengthening ferrite leads to suppression of fatigue cracks, it has a large durability ratio. Can also be secured. Ti also has the effect of improving the machinability by forming sulfides. In order to obtain the above-described effects remarkably, it is necessary to contain Ti exceeding 0.10%. However, the Ti content isGet higherAnd causes a decrease in hot workability. Therefore, the Ti content exceeds 0.10%.0.30Up to%It was.
[0044]
Al: 0.002 to 0.100%
Al is an element effective as a deoxidizer for steel. One of the features of the present invention is the combined addition of Ti and V as described above, but Ti forms an oxide because of its strong deoxidizing power, and the ratio of forming carbonitride is relatively reduced. End up. Therefore, the effect | action which Ti strengthens a ferrite with V reduces. In addition, there is a concern that the yield of Ti may be reduced to increase the manufacturing cost. The reason for adding Al is to deoxidize the steel with Al to stabilize the deoxidation, and at the same time, to secure Ti that strengthens the ferrite by forming carbonitrides and to prevent a decrease in the yield of Ti. Also serves. The Al content necessary for that is 0.002% or more, but the effect is saturated at 0.100%. Therefore, the Al content is set to 0.002 to 0.100%. The Al content is preferably 0.002 to 0.050%.
[0045]
In addition, in order to stabilize the deoxidation by deoxidizing the steel with the above-mentioned Al, while ensuring the action of Ti strengthening the ferrite together with V, and to prevent the decrease in the yield of Ti, for example, with Al Ti should be added after sufficient deoxidation, that is, the order of addition should be Al and Ti.
[0046]
N: 0.002 to 0.020%
N contributes to strengthening of steel by forming carbonitride with V and Ti. In order to acquire this effect, N needs to contain 0.002% or more. However, the above effect is saturated even if N is contained in excess of 0.020%. Therefore, the N content is set to 0.002 to 0.020%.
[0047]
The non-heat treated steel according to the invention of (1) is a steel having the chemical composition of the above elements C to N, the balance being Fe and impurities.
[0048]
The non-tempered steel according to the invention of (2) contains P: 0.150% or less in place of part of Fe of the steel of the invention of (1) for the purpose of further improving cracking properties. Steel having a controlled chemical composition.
[0049]
The above-mentioned P segregates at the grain boundaries, embrittles the steel, and has a function of further improving cracking properties, so it may be contained within the range described below.
[0050]
P: 0.150% or less
When P is added, it segregates at the grain boundaries and embrittles the steel, so it is effective for making the fracture surface during cracking of the connecting rod a smooth brittle fracture surface. In order to reliably obtain this effect, it is desirable that the P content is 0.04% or more. However, when P is added excessively, the hot workability of the steel is lowered. In particular, when the content exceeds 0.150%, the hot workability is significantly lowered. Therefore, when adding P, the content is preferably 0.150% or less. Note that the P content is more preferably 0.100% or less.
[0051]
The non-heat treated steel according to the invention of (3) contains Cr: 0.50% or less in place of part of Fe of the steel of the invention of (1) for the purpose of increasing strength. Steel with chemical composition.
[0052]
Since the above Cr has the effect of improving the hardenability of the steel and increasing the strength, it may be contained within the range described below.
[0053]
Cr: 0.50% or less
When added, Cr has the effect of improving the hardenability of the steel and increasing the strength. In order to reliably obtain this effect, the Cr content is desirably 0.02% or more. However, when the content exceeds 0.50%, a bainite structure is formed, and cracking properties and machinability are deteriorated. Therefore, when adding Cr, the content is preferably 0.50% or less. It is more preferable that the Cr content is 0.30% or less.
[0054]
The non-heat treated steel according to the invention of (4) is replaced with a part of Fe of the steel of the invention of (1) for the purpose of further improving machinability, and Pb: 0.30% or less, Steel having a chemical composition containing at least one selected from Te: 0.30% or less, Ca: 0.010% or less, and Bi: 0.30% or less.
[0055]
Since any of the elements from Pb to Bi has an effect of enhancing the machinability of the steel, the elements from Pb to Bi may be contained alone or in the range described below. The above may be combined and contained.
[0056]
Pb: 0.30% or less
Pb has the effect | action which improves the machinability of steel. In order to reliably obtain this effect, the Pb content is preferably 0.02% or more. However, when the content exceeds 0.30%, the hot workability is lowered. Therefore, when Pb is added, the content is preferably 0.30% or less.
[0057]
Te: 0.30% or less
Te has the effect | action which improves the machinability of steel. In order to reliably obtain this effect, Te is preferably contained in a content of 0.002% or more. However, when the content exceeds 0.30%, the hot workability is lowered. Therefore, the Te content when added is preferably 0.30% or less.
[0058]
Ca: 0.010% or less
Ca has the effect | action which improves the machinability of steel. In order to reliably obtain this effect, the Ca content is preferably 0.0005% or more. However, when the content exceeds 0.010%, the hot workability is lowered. Therefore, when adding Ca, the content is preferably 0.010% or less.
[0059]
Bi: 0.30% or less
Bi has the effect | action which improves the machinability of steel. In order to reliably obtain this effect, it is preferable that Bi is contained in an amount of 0.03% or more. However, when the content exceeds 0.30%, the hot workability is lowered. Therefore, the Bi content when added is preferably 0.30% or less.
[0060]
The non-heat treated steel according to the invention of (5) is replaced with P: 0 instead of a part of Fe of the steel of the invention of (1) for the purpose of further improving cracking property and strength. Steel having a chemical composition containing 150% or less and Cr: 0.50% or less.
[0061]
The non-heat treated steel according to the invention of (6) is replaced with a part of Fe of the steel of the invention of (1) described above for the purpose of further improving the cracking property and further improving the machinability, In addition to P: 0.150% or less, Pb: 0.30% or less, Te: 0.30% or less, Ca: 0.010% or less, and Bi: 0.30% or less are included. Steel having a different chemical composition.
[0062]
The non-heat treated steel according to the invention of (7) is Cr instead of a part of Fe of the steel of the invention of (1) described above for the purpose of increasing strength and further improving machinability. : 0.50% or less, Pb: 0.30% or less, Te: 0.30% or less, Ca: 0.010% or less, and Bi: 0.30% or less are included. Steel with chemical composition.
[0063]
The non-heat treated steel according to the invention of (8) is one of Fe of the steel of the invention of (1) described above for the purpose of further improving cracking property, enhancing strength and further improving machinability. Instead of P: 0.150% or less and Cr: 0.50% or less, Pb: 0.30% or less, Te: 0.30% or less, Ca: 0.010% or less, and Bi: 0.00. It is a steel having a chemical composition containing one or more selected from 30% or less.
(B) Non-tempered steel structure
The structure of the non-heat treated steel according to the present invention needs to be a ferrite pearlite structure having a ferrite ratio of 40% or more.
[0064]
First, the ferrite / pearlite structure is used to improve both cracking and machinability. The cracking property of the structure containing martensite, bainite and austenite is low, and in addition, the machinability is inferior to that of the ferrite / pearlite structure. Here, as already described, the “ferrite / pearlite structure” refers to a mixed structure of ferrite and pearlite.
[0065]
In the ferrite-pearlite structure, by setting the ratio of ferrite strengthened by solid solution and / or precipitation to 40% or more, the durability ratio can be increased and the fatigue resistance can be enhanced.
[0066]
Therefore, in the non-heat treated steel according to the present invention, the structure is a ferrite pearlite structure having a ferrite ratio of 40% or more. The upper limit of the ferrite ratio in the ferrite / pearlite structure is about 90%.
[0067]
Here, it is known that the volume ratio of a certain phase is equal to the area ratio. Therefore, the ratio of the ferrite to the structure is, for example, a normal two-dimensional evaluation method, that is, an optical microscope or an electron microscope. What is necessary is just to determine from the ratio of the ferrite calculated by observation using the
(C) Ferrite hardness and the ratio of ferrite hardness to total hardness
Regarding the hardness of the non-heat treated steel according to the present invention, the hardness of the ferrite must be 250 or more in terms of Vickers hardness, and the ratio of the hardness of the ferrite to the total hardness must be 0.80 or more.
[0068]
That is, the hardness of ferrite in the ferrite-pearlite structure affects cracking properties, machinability and fatigue resistance, and it is preferable that the hardness of ferrite is Hv 250 or more. Cracking properties, machinability and fatigue resistance properties It is a condition to ensure. Further, the difference between the total hardness and the hardness of the ferrite in the ferrite-pearlite structure should be reduced, in other words, the ratio of the hardness of the ferrite to the total hardness should be increased. In particular, the above ratio should be increased to 0.80 or more. By doing so, it is possible to ensure good cracking properties, a large durability ratio (and therefore good fatigue resistance) and good machinability.
[0069]
Therefore, in the non-heat treated steel according to the present invention, the ferrite hardness in the ferrite / pearlite structure is 250 or more in terms of Vickers hardness, and the ratio of the ferrite hardness to the total hardness is 0.80 or more.
[0070]
Here, the Vickers hardness as the hardness of the ferrite means a value measured with a test force of 0.09807N, and the Vickers hardness as a total hardness means a value measured with a test force of 98.07N. Moreover, as already stated, it indicates the average value when the total hardness is measured at four points at random.
[0071]
Here, as already described, the ferrite in the present invention does not contain ferrite that forms pearlite together with cementite.
[0072]
The steel having the chemical composition described in the item (A) is intended to stabilize the deoxidation by deoxidizing the steel with Al, and at the same time, ensure the action of Ti strengthening ferrite together with V. In order to prevent the yield from being lowered, for example, Ti is added after sufficiently deoxidizing with Al, that is, the ingot is made into an ingot or steel piece after melting in the order of Al and Ti. And then, after the steel ingots and steel slabs are left, or after being subjected to hot rolling and / or forging by a usual method, for example, the heating temperature for forging is 1200 to 1350 ° C., forging An integrated body in which the connecting rod body 2 and the connecting rod cap 3 are connected by hot forging and cooling with a finishing temperature exceeding 800 ° C. to 1300 ° C. and a cooling rate at 800 to 600 ° C. after forging at 100 to 150 ° C./min. Then, after the bolt hole is machined, it is cracked to the connecting rod body 2 and the connecting rod cap 3 at the large end portion 5. Next, the divided connecting rod body 2 and connecting rod cap 3 are assembled by being connected to the crankshaft by bolts 4.
[0073]
In addition, the temperature and cooling rate in the above-mentioned hot forging conditions are values on the surface of the steel slab or the integral molded material, and cooling after cooling the temperature range of 800 to 600 ° C. as described above is particularly limited. It is not a thing.
[0074]
Hereinafter, the present invention will be described in more detail with reference to examples.
[0075]
【Example】
Steel having the chemical composition shown in Table 1 and Table 2 was melted using a vacuum melting furnace to form a steel ingot. Steels 1 to 18 were sufficiently deoxidized with Al and then added with Ti (that is, Al and Ti were added in the order of Al and Ti) to be melted and then cast. On the other hand, for steel 19, Al was added at the end of the component adjustment (that is, the order of addition was Ti and Al), and was cast immediately thereafter. In Tables 1 and 2, Steels 1 to 11 and Steels 17 to 19 are steels according to the present invention within the range defined by the present invention in chemical composition, and Steels 12 to 16, Steel 20 and Steel 21 are any of the components. Is a steel of a comparative example that deviates from the content range defined in the present invention. Of the comparative steels, steel 20 and steel 21 are steel corresponding to JIS S48C and steel corresponding to cracking connecting rod steel disclosed in Patent Document 1 and already put into practical use in Europe.
[0076]
[Table 1]

[0077]
[Table 2]

[0078]
Then, after making the steel of these examples of the present invention and the steel of the comparative example into a steel slab by a usual method, the heating temperature for forging is 1250 ° C. and the forging finishing temperature is 1050 ° C. As described above, hot forging was performed on a round bar having a diameter of 20 mm, and after completion of the forging, cooling was performed at a cooling rate of 800 to 600 ° C. at 110 ° C./min, and a temperature range below 600 ° C. was allowed to cool in the normal atmosphere.
[0079]
On the other hand, the steel 17 and the steel 18 were hot forged into a round bar having a diameter of 20 mm at a heating temperature for forging of 1300 ° C. and a forging finishing temperature of 1200 ° C. After the forging was completed, the cooling rate at 800 to 600 ° C. was 110. Cooling was performed at a rate of ° C./min, and the temperature range below 600 ° C. was allowed to cool in the normal atmosphere. Steel 19 was hot forged into a round bar with a diameter of 20 mm at a heating temperature for forging of 1150 ° C. and a forging finishing temperature of 1000 ° C. After the forging was completed, the cooling rate at 800 to 600 ° C. was 110 ° C./min. The temperature range below 600 ° C. was set as normal air cooling.
[0080]
Of the comparative steels, steel 20 corresponding to JIS S48C was heated to 1100 ° C. on a round bar with a diameter of 20 mm obtained by heating to 1250 ° C. and hot forging by a normal method. After the oil quenching, a tempering treatment was performed by tempering at 500 ° C.
[0081]
Various test pieces were collected from the round bar having a diameter of 20 mm thus obtained, and the microstructure, Vickers hardness (hereinafter referred to as Hv hardness), tensile characteristics, fatigue resistance characteristics, impact characteristics, and machinability. investigated.
[0082]
That is, a micro test piece having a surface perpendicular to the forging axis as an observation surface was cut out from each of the above-described round bars, mirror-polished and subjected to nitral corrosion, and then observed with an optical microscope with a magnification of 400 times. Judgment was made.
[0083]
The structure observed as described above was ferrite / pearlite, and image analysis was further performed by a usual method to measure the ratio of ferrite in the ferrite / pearlite structure. In the case where the structure was ferrite pearlite, the Hv hardness (αHv) of the ferrite was measured with a test force of 0.09807N.
[0084]
Moreover, after cutting out the test piece which made a surface perpendicular | vertical to a forge axis | shaft a test surface from each round bar and carrying out mirror surface polishing, Hv hardness (THv) as a total hardness was measured with the test force of 98.07N.
[0085]
As for the tensile properties, a JIS No. 14A tensile test piece having a parallel part diameter of 7 mm was cut out from each round bar, a tensile test was performed at room temperature by a normal method, and TS (tensile strength) was measured.
[0086]
For fatigue resistance, an Ono-type rotating bending fatigue test piece having a parallel part diameter of 8 mm was cut out from each round bar, and a rotating bending fatigue test was performed at room temperature by a normal method to measure fatigue strength (σw). The durability ratio (σw / TS) was determined from this σw and the TS. In addition, when the tempered steel of JIS S48C equivalent steel 20, which is the durability ratio of test number 20 using “0.479”, is the standard performance, and a durability ratio higher than this is obtained, fatigue resistance characteristics are obtained. Was judged to be good.
[0087]
For impact characteristics, a V-notch test piece having a width of 10 mm described in JIS Z 2202 (1998) was cut out from each round bar, and a Charpy impact test was performed at room temperature by an ordinary method to measure the impact value. Charpy impact value (vE) at room temperature using a V-notch specimen having a width of 10 mm._RT) Can be one index for evaluating the cracking property, and the smaller the value, the better the cracking property. For this reason, the impact value of test number 21 using steel 21, which is a steel corresponding to cracking connecting rod steel already put to practical use in Europe, is 7.0 J / cm.² ”As the standard performance, and impact values lower than this were obtained, the impact characteristics were low, and therefore the cracking properties were judged to be good.
[0088]
The machinability is determined by cutting a plate-shaped test piece having a cross section of 10 mm × 10 mm from a round bar, drilling a through hole with a depth of 10 mm with a drill, and drilling 100 corners of the drill after drilling 100 holes. The amount of wear on the outer periphery was measured and evaluated. The corner wear amount in the case of test number 21 in which a drilling test was performed using steel 21, which is a steel equivalent to cracking connecting rod steel already in practical use in Europe, was used as a standard performance, and corner wear of less than 30% thereof. In the case of quantity, it was judged that machinability was good.
[0089]
The drilling test conditions are as follows.
[0090]
Drill: 8mm diameter straight shank drill with P20 carbide,
Rotational speed: 3000rpm,
Feed: 0.15mm / rev,
Lubrication: Water-soluble lubricant.
[0091]
In addition, the machinability test was not performed on the round bar of steel 20 which was tempered JIS S48C equivalent steel.
[0092]
Table 3 summarizes the above test results. ΑHv and THv in Table 3 represent the hardness and total hardness of the ferrite at Vickers hardness, respectively, and vE_RTRepresents a Charpy impact value at room temperature using a V-notch specimen having a width of 10 mm, as already described. “◎”, “○”, “△”, and “×” in the machinability column indicate that the corner wear amount is less than 10% and less than 10% to less than 30% when the steel 21 is subjected to a piercing test. , 30 to 100% and exceeding 100%.
[0093]
In Test No. 13, since the ferrite area ratio was small, the hardness of the ferrite alone could not be measured with the indenter applied to the pearlite with the test force of 0.09807N.
[0094]
[Table 3]

[0095]
As is apparent from Table 3, in the case of test numbers 1 to 11 having the chemical composition, structure, hardness of ferrite and ratio of ferrite hardness to total hardness defined in the present invention, all of the target machinability Cracking properties and fatigue resistance properties are obtained. That is, the corner wear amount of the drill is less than 30% of the wear amount in the case of test No. 21 drilled using steel 21 (marked “◎” or “又 は”), and the machinability is very good. is there. VE can be an index to evaluate cracking_RTIs the vE in the case of test number 21 using the same steel 21_RT7.0 J / cm² Therefore, the cracking property is good. Furthermore, the fatigue resistance exceeds 0.479 which is the durability ratio in the case of test number 20 in which the steel 20 which is JIS S48C equivalent steel is tempered, and the fatigue resistance is also good.
[0096]
Among the above test numbers, it is clear that the machinability of test numbers 4 and 6 to 11 is still better.
[0097]
On the other hand, in the case of test numbers 12 to 16, since the chemical composition is at least out of the conditions defined in the present invention, the target is reached in any one or more of machinability, cracking properties and fatigue resistance properties. Not.
[0098]
In the case of test number 20 in which steel 20 which is JIS S48C equivalent steel is tempered,_RTIs the test number 21 using steel 21_RT7.0 J / cm² And at least inferior in cracking properties.
[0099]
Further, in the case of test number 21 using steel 21, which is steel equivalent to cracking connecting rod steel already put to practical use in Europe, test number 20 obtained by tempering steel 20 which is steel equivalent to S48C of the above JIS. The durability ratio is lower than 0.479, which is inferior in fatigue resistance.
[0100]
On the other hand, in the case of the test number 17 and the test number 18, although the steel 17 and the steel 18 have the chemical composition defined in the present invention, the structure is out of the conditions defined in the present invention. For this reason, in the case of test number 17, the machinability is inferior, the durability ratio is low, and the fatigue resistance is also low. Moreover, in the case of the test number 18, it is inferior to all the characteristics of machinability, cracking property, and fatigue resistance.
[0101]
In the case of the test number 19, although the steel 19 has the chemical composition defined in the present invention, the ferrite is not sufficiently strengthened by Ti and V, and the ferrite hardness (αHv) at the Vickers hardness and the hardness of the ferrite are not obtained. And the total hardness ratio (αHv / THv) deviate from the conditions defined in the present invention. For this reason, vE_RTIs 7.0 J / cm² The cracking property is inferior.
[0102]
【The invention's effect】
  Of the present inventionFor cracking connecting rodNon-tempered steel is excellent in fracture splitting property and fatigue resistance and has very good machinability, and can therefore be used as a material for cracking connecting rods such as automobile engines.
[Brief description of the drawings]
FIG. 1 is a view showing details of a connecting rod.
[Explanation of symbols]
1: Connecting rod,
2: Connecting rod body,
3: Connecting rod cap,
4: Bolt,
5: Large end

Claims (8)

In mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: more than 0.130% and 0.200% V: 0.10 to 0.50%, Ti: more than 0.10% to 0.30 %, Al: 0.002 to 0.100% and N: 0.002 to 0.020% And the balance is a chemical composition composed of Fe and impurities, the structure is a ferrite pearlite structure in which the proportion of ferrite is 40% or more, and the hardness of the ferrite is 250 or more in terms of Vickers hardness, Non-tempered steel for cracking connecting rods , characterized in that the ratio of total hardness is 0.80 or more. In mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: more than 0.130% and 0.200% V: 0.10 to 0.50%, Ti: more than 0.10% to 0.30 %, Al: 0.002 to 0.100%, N: 0.002 to 0.020% and P: 0.150% or less, the balance is a chemical composition composed of Fe and impurities, the structure is a ferrite pearlite structure in which the proportion of ferrite is 40% or more, and the ferrite hardness is Vickers hardness Non-tempered steel for cracking connecting rods , characterized in that the ratio of the hardness of the ferrite to the total hardness is not less than 250 and not less than 0.80. In mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: more than 0.130% and 0.200% V: 0.10 to 0.50%, Ti: more than 0.10% to 0.30 %, Al: 0.002 to 0.100%, N: 0.002 to 0.020% and Cr: 0.50% or less, the balance is a chemical composition composed of Fe and impurities, and the structure is a ferrite pearlite structure in which the proportion of ferrite is 40% or more, and the hardness of the ferrite is Vickers hardness Non-tempered steel for cracking connecting rods , characterized in that the ratio of the hardness of the ferrite to the total hardness is not less than 250 and not less than 0.80. In mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: more than 0.130% and 0.200% V: 0.10 to 0.50%, Ti: more than 0.10% to 0.30 %, Al: 0.002 to 0.100% and N: 0.002 to 0.020% And at least one selected from Pb: 0.30% or less, Te: 0.30% or less, Ca: 0.010% or less, and Bi: 0.30% or less, with the balance being Fe and impurities A ferrite / pearlite structure in which the ratio of ferrite is 40% or more, the ferrite hardness is 250 or more in terms of Vickers hardness, and the ratio of ferrite hardness to total hardness is 0 .Non-tempered steel for cracking connecting rods characterized by being 80 or more. In mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: more than 0.130% and 0.200% V: 0.10 to 0.50%, Ti: more than 0.10% to 0.30 %, Al: 0.002 to 0.100%, N: 0.002 to 0.020%, P: 0.150% or less and Cr: 0.50% or less, with the balance being a chemical composition comprising Fe and impurities, the structure being a ferrite pearlite structure with a ferrite ratio of 40% or more, and ferrite A non-tempered steel for cracking connecting rods having a Vickers hardness of 250 or more and a ratio of ferrite hardness to total hardness of 0.80 or more. In mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: more than 0.130% and 0.200% V: 0.10 to 0.50%, Ti: more than 0.10% to 0.30 %, Al: 0.002 to 0.100%, N: 0.002 to 0.020% and P: 0.150% or less, Pb: 0.30% or less, Te: 0.30% or less, Ca: 0.010% or less and Bi: 0.30% or less And the balance is a chemical composition composed of Fe and impurities, and the structure is a ferrite / pearlite structure in which the ratio of ferrite is 40% or more. Further, the hardness of the ferrite is 250 or more in terms of Vickers hardness, and the hardness of the ferrite Cracking characterized by the ratio of the total hardness to 0.80 or more Non-tempered steel for connecting rods . In mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: more than 0.130% and 0.200% V: 0.10 to 0.50%, Ti: more than 0.10% to 0.30 %, Al: 0.002 to 0.100%, N: 0.002 to 0.020% and One or more selected from Cr: 0.50 % or less, Pb: 0.30% or less, Te: 0.30% or less, Ca: 0.010% or less, and Bi: 0.30% or less And the balance is a chemical composition composed of Fe and impurities, and the structure is a ferrite / pearlite structure in which the ratio of ferrite is 40% or more. Further, the hardness of the ferrite is 250 or more in terms of Vickers hardness, and the hardness of the ferrite it is to crackin the ratio of total hardness, characterized in that at least 0.80 Non-heat treated steel for the connecting rod. In mass%, C: 0.20 to 0.40%, Si: 0.05 to 1.50%, Mn: 0.30 to 2.00%, S: more than 0.130% and 0.200% V: 0.10 to 0.50%, Ti: more than 0.10% to 0.30 %, Al: 0.002 to 0.100%, N: 0.002 to 0.020%, P: 0.150% or less and Cr: 0.50% or less, Pb: 0.30% or less, Te: 0.30% or less, Ca: 0.010% or less, and Bi: 0.30% or less And the balance is a chemical composition comprising Fe and impurities, the structure is a ferrite pearlite structure in which the proportion of ferrite is 40% or more, and the hardness of the ferrite is Vickers hardness 250 or more, and the ratio of the hardness of the ferrite to the total hardness is 0.80 or more. Characterized non-heat treated steel for cracking connecting rod .

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