JP2014109048A - Steel for machine structural use excellent in cold forgeability and toughness - Google Patents
Steel for machine structural use excellent in cold forgeability and toughness Download PDFInfo
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 29
- 239000010959 steel Substances 0.000 title claims abstract description 29
- 239000012535 impurity Substances 0.000 claims abstract description 3
- 229910000746 Structural steel Inorganic materials 0.000 claims description 2
- 238000005097 cold rolling Methods 0.000 abstract 1
- 238000000137 annealing Methods 0.000 description 12
- 238000010273 cold forging Methods 0.000 description 12
- 238000010438 heat treatment Methods 0.000 description 10
- 230000006698 induction Effects 0.000 description 9
- 238000012360 testing method Methods 0.000 description 9
- 230000007423 decrease Effects 0.000 description 7
- 230000000694 effects Effects 0.000 description 7
- 239000013078 crystal Substances 0.000 description 6
- 238000011282 treatment Methods 0.000 description 6
- 238000004519 manufacturing process Methods 0.000 description 5
- 239000000203 mixture Substances 0.000 description 5
- 238000005496 tempering Methods 0.000 description 5
- 230000006835 compression Effects 0.000 description 4
- 238000007906 compression Methods 0.000 description 4
- 238000005242 forging Methods 0.000 description 4
- 238000005098 hot rolling Methods 0.000 description 4
- 239000000463 material Substances 0.000 description 4
- 229910000859 α-Fe Inorganic materials 0.000 description 4
- 150000001247 metal acetylides Chemical class 0.000 description 3
- 238000000034 method Methods 0.000 description 3
- 238000010791 quenching Methods 0.000 description 3
- 230000000171 quenching effect Effects 0.000 description 3
- 230000002411 adverse Effects 0.000 description 2
- 238000005336 cracking Methods 0.000 description 2
- 238000005520 cutting process Methods 0.000 description 2
- 230000000593 degrading effect Effects 0.000 description 2
- 238000011156 evaluation Methods 0.000 description 2
- 239000002994 raw material Substances 0.000 description 2
- 229910018072 Al 2 O 3 Inorganic materials 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
- 229910001566 austenite Inorganic materials 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 238000001816 cooling Methods 0.000 description 1
- 238000011161 development Methods 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 238000009863 impact test Methods 0.000 description 1
- 238000005457 optimization Methods 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 230000001376 precipitating effect Effects 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 230000001737 promoting effect Effects 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 238000005096 rolling process Methods 0.000 description 1
- 229920006395 saturated elastomer Polymers 0.000 description 1
- 239000006104 solid solution Substances 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- 239000002344 surface layer Substances 0.000 description 1
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Abstract
Description
本発明は、建産機や自動車、さらには各種産業機械等の部品の素材に供して好適な、冷間鍛造性および靱性に優れた機械構造用鋼に関するものである。 The present invention relates to steel for machine structures that is suitable for use as a raw material for parts such as construction machinery, automobiles, and various industrial machines, and has excellent cold forgeability and toughness.
一般に、機械構造部品は、切削加工や鍛造加工等によって所定の形状に加工された後、焼入れ・焼戻し処理や高周波焼入れ等の表面硬化処理を施す場合が多い。特に、鍛造加工、中でも冷間鍛造は、加工後の寸法精度がよく、また切削加工に比べて生産性が高いことから、機械構造部品の製造に広く用いられている。このように冷間鍛造に供される機械構造用鋼は、高い焼入れ性が要求されることから、JISに規定されたSCM440等を素材として、軟化焼鈍処理により冷間鍛造性を改善したものが使用されるのが通例である。 In general, machine structural parts are often processed into a predetermined shape by cutting, forging, or the like, and then subjected to surface hardening treatment such as quenching / tempering treatment or induction hardening. In particular, forging, especially cold forging, is widely used in the manufacture of machine structural parts because it has good dimensional accuracy after processing and has higher productivity than cutting. In this way, steel for machine structural use that is subjected to cold forging is required to have high hardenability, so the one with improved cold forgeability by softening annealing treatment using SCM440 specified in JIS as a material. It is customary to use it.
この軟化焼鈍処理は、厳しい条件下で冷間鍛造を行う場合に不可欠であるものの、一方で、鍛造の程度によっては熱間圧延ままで使用可能な冷間鍛造用鋼に対する要求も強いものがある。
また、冷間鍛造により複雑な形状を有する機械部品を製造する場合は、数回に分けて冷間鍛造を行うのが一般的であるが、製造工程の省略のために、より変形能の高い冷間鍛造用鋼が望まれている。
かような冷間鍛造用鋼について特許文献１には、Si，Mnの低減に加え、焼入れ性を確保するための高Cr化と圧延条件の適正化によってフェライト分率を低減する、圧延ままで冷間鍛造性に優れる機械構造用強靭鋼の製造方法が提案されている。
This soft annealing treatment is indispensable when cold forging is performed under severe conditions. However, depending on the degree of forging, there is a strong demand for cold forging steel that can be used in hot rolling. .
Moreover, when manufacturing a machine part having a complicated shape by cold forging, it is common to perform cold forging in several steps, but because the manufacturing process is omitted, the deformability is higher. Cold forging steel is desired.
Regarding such cold forging steel, in Patent Document 1, in addition to reducing Si and Mn, the ferrite fraction is reduced by increasing the Cr to ensure hardenability and optimizing the rolling conditions. A method for producing a tough steel for machine structure having excellent cold forgeability has been proposed.
さらに、鋼材コストの低減のために、SCM440と同等の特性を有するMoフリー鋼として、特許文献２には、Moフリーによる焼入れ性の低下を、Mnの増加並びに、TiおよびＢの添加にて補うことが提案されている。
Further, as a Mo-free steel having the same characteristics as SCM440 in order to reduce the steel material cost,
しかしながら、特許文献１に記載された方法は、焼入れ性の低下を2.0％以上のCr添加にて補っているため、原料に要するコストの上昇を招き、またこの方法で得られる冷間鍛造用鋼は、軟化焼鈍を施した場合でも十分な冷間鍛造性が得られるとは限らないところに問題を残していた。 However, since the method described in Patent Document 1 compensates for the decrease in hardenability by adding 2.0% or more of Cr, it causes an increase in the cost required for the raw material, and the steel for cold forging obtained by this method. However, a problem remains in that sufficient cold forgeability is not always obtained even when soft annealing is performed.
また、特許文献２に記載された方法では、得られる冷間鍛造用鋼について、Ｂにより焼入れ性の向上をはかっているが、Ｂの焼き入れ性は不安定であり、この効果を安定化させるためには、過剰のTi，Ｂ添加が必要なことから、逆に疲労強度の低下や、鋳片または圧延材の表面性状の低下を招き、結果として、手入れ負荷の増大による製造コストの増加を余儀なくされる。
Moreover, in the method described in
本発明は、上記の問題を有利に解決するものであって、熱間圧延ままおよび球状化焼鈍などの軟化焼鈍後のいずれにおいても、冷間鍛造性に優れかつ高靱性を有し、しかもMoを含有しないために安価である、機械構造用鋼を提供しようとするものである。 The present invention advantageously solves the above-mentioned problems, and is excellent in cold forgeability and high toughness both in hot rolling and after soft annealing such as spheroidizing annealing, and Mo Therefore, it is intended to provide a steel for machine structure, which is inexpensive because it does not contain.
さて、発明者らは、上記の課題を解決すべく鋭意研究を重ねた結果、以下に述べる知見を得た。
すなわち、熱間圧延ままで冷間鍛造性を向上させるには、単純な強度の低下を図るのではなく、逆にフェライト分率の低減が有効であり、そのためには、Ｃ、MnおよびCrの含有量の適正化が有効である。加えて、破壊の起点となる微小クラックの進展を抑制することが有効であり、そのためにはNbの適量添加が重要である。
また、球状化焼鈍後の冷間鍛造性を向上させるには、炭化物の球状化率を高めることが有効であり、そのためには、Ｃ，Si，MnおよびCr量のバランスを厳密に制御する必要がある。
本発明は、上記の知見に立脚するものである。
As a result of intensive studies to solve the above problems, the inventors have obtained the following knowledge.
That is, in order to improve the cold forgeability with hot rolling, it is effective to reduce the ferrite fraction instead of simply reducing the strength. For that purpose, the content of C, Mn and Cr is effective. Optimization of the content is effective. In addition, it is effective to suppress the development of microcracks that are the starting point of fracture, and for that purpose, the addition of an appropriate amount of Nb is important.
In order to improve the cold forgeability after spheroidizing annealing, it is effective to increase the spheroidizing rate of carbides. To that end, it is necessary to strictly control the balance of the amounts of C, Si, Mn and Cr. There is.
The present invention is based on the above findings.
すなわち、本発明の要旨構成は次のとおりである。
１．質量％で、
Ｃ：0.30〜0.55％、
Si：0.03〜0.30％、
Mn：0.50〜l.50％、
Ｐ：0.018％以下、
Ｓ：0.02％以下、
Al：0.010〜0.060％、
Cr：1.2〜2.0％および
Nb：0.005〜0.06％
を、下記(l)式を満足する範囲で含有し、残部はFeおよび不可避的不純物の組成からなることを特徴とする冷間鍛造性と靱性に優れた機械構造用鋼。
記
［％Ｃ］−［％Si］/２＋［％Mn］/５＋２［％Cr］≧3.2 ・・・ (1)
That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.30 to 0.55%,
Si: 0.03-0.30%,
Mn: 0.50 ~ l.50%,
P: 0.018% or less,
S: 0.02% or less,
Al: 0.010 to 0.060%,
Cr: 1.2-2.0% and
Nb: 0.005-0.06%
Is contained in a range that satisfies the following formula (l), and the balance is composed of Fe and inevitable impurities, and is a steel for machine structure excellent in cold forgeability and toughness.
[% C]-[% Si] / 2 + [% Mn] / 5 + 2 [% Cr] ≧ 3.2 (1)
２．さらに、質量％で、
Ti：0.1％以下および
Ｖ：0.1％以下
のうちから選んだ１種または２種を含有することを特徴とする前記１に記載の冷間鍛造性と靱性に優れた機械構造用鋼。
2. Furthermore, in mass%,
2. The steel for machine structure having excellent cold forgeability and toughness according to the above 1, characterized by containing one or two selected from Ti: 0.1% or less and V: 0.1% or less.
３．さらに、質量％で、
Cu：0.30％以下および
Ni：0.30％以下
のうちから選んだ１種または２種を含有することを特徴とする前記１または２に記載の冷間鍛造性と靱性に優れた機械構造用鋼。
3. Furthermore, in mass%,
Cu: 0.30% or less and
Ni: Machine structural steel excellent in cold forgeability and toughness as described in 1 or 2 above, comprising one or two selected from 0.30% or less.
本発明によれば、熱間圧延ままあるいは軟化焼鈍後のいずれにおいても、冷間鍛造性が優れ、焼入れ性にも優れた高靱性の機械構造用鋼を、安価かつ安定して得ることができる。 According to the present invention, it is possible to obtain a high-toughness steel for machine structural use that is excellent in cold forgeability and excellent in hardenability, either hot-rolled or after soft annealing, at low cost and stably. .
以下、本発明を具体的に説明する。
まず、本発明において、鋼の成分組成を前記の範囲に限定した理由について説明する。なお、成分に関する「％」表示は、特に断らない限り質量％を意味するものとする。
Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of steel is limited to the above range in the present invention will be described. In addition, unless otherwise indicated, "%" display regarding a component shall mean the mass%.
Ｃ：0.30〜0.55％
Ｃは、冷間鍛造後の焼入れ・焼戻し処理または各種表面硬化熱処理によって所望の表面硬さを確保するために必要な元素であるが、含有量が0.30％未満では必要な硬さが得られず、一方0.55％を超えると冷間鍛造後の熱処理で焼割れが発生し易くなるため、Ｃ量は0.30〜0.55％の範囲に限定した。
C: 0.30 ~ 0.55%
C is an element necessary for ensuring the desired surface hardness by quenching / tempering treatment after cold forging or various surface hardening heat treatments, but if the content is less than 0.30%, the necessary hardness cannot be obtained. On the other hand, if it exceeds 0.55%, it becomes easy for cracking to occur in the heat treatment after cold forging, so the C content is limited to a range of 0.30 to 0.55%.
Si：0.03〜0.30％
Siは、脱酸剤として有効であるため、少なくとも0.03％は添加するものとした。しかしながら、Siは、フェライトに固溶して変形抵抗を高め、冷間鍛造性を劣化させる作用があるため、上限を0.30％とした。好ましくは0.05〜0.25％の範囲である。
Si: 0.03-0.30%
Since Si is effective as a deoxidizer, at least 0.03% was added. However, since Si has the effect of dissolving in ferrite and increasing the deformation resistance and degrading the cold forgeability, the upper limit was made 0.30%. Preferably it is 0.05 to 0.25% of range.
Mn：0.50〜l.50％
Mnは、焼入れ性に有効な元素であるので、少なくとも0.50％は添加するものとした。これを下回ると、フェライト分率が増加し、熱間鍛造ままで優れた冷間鍛造性が得られない。しかしながら、Mnは、Siと同様に、変形抵抗を高め冷間鍛造性を劣化させる作用があるため、上限を1.50％とした。好ましくは0.60〜1.40％の範囲である。
Mn: 0.50 ~ l.50%
Since Mn is an element effective for hardenability, at least 0.50% was added. Below this, the ferrite fraction increases, and excellent cold forgeability cannot be obtained as hot forging. However, Mn, like Si, has the effect of increasing deformation resistance and degrading cold forgeability, so the upper limit was made 1.50%. Preferably it is 0.60 to 1.40% of range.
Ｐ：0.018％以下
Ｐは、結晶粒界に偏析して靭性を低下させるため、その混入は極力低減する方が望ましいが、0.018％までは許容される。好ましくは0.016％以下である。なお、含有量を０％とすることは工業的には難しく、0.001％程度で不可避的に混入するのが一般的である。
P: 0.018% or less P is segregated at the grain boundaries to reduce toughness. Therefore, it is desirable to reduce the amount of P as much as possible, but 0.018% is acceptable. Preferably it is 0.016% or less. In addition, it is difficult industrially to make content 0%, and it is common to inevitably mix at about 0.001%.
Ｓ：0.02％以下
Ｓは、変形能に悪影響を与えるMnＳを形成するために少ないほどよいが、0.02％以下であれば実用上問題は生じないことから、0.02％以下とした。なお、含有量を０％とすることは工業的には難しく、0.001％程度で不可避的に混入するのが一般的である。
S: 0.02% or less The S content is preferably as small as possible to form MnS that adversely affects the deformability. However, if it is 0.02% or less, there is no practical problem, so 0.02% or less. In addition, it is difficult industrially to make content 0%, and it is common to inevitably mix at about 0.001%.
Al：0.010〜0.060％
Alは、脱酸剤として有用であり、少なくとも0.010％の添加が必要である。また、AlはＮと結合してAlＮを形成し、オーステナイト結晶粒の微細化に寄与する元素である。そのためには、0.010％以上の添加を必要とする。しかしながら、含有量が0.060％を超えると疲労強度に対して有害なAl2O3介在物の生成を助長するため、Al量は0.010〜0.060％の範囲に限定した。
Al: 0.010-0.060%
Al is useful as a deoxidizer and requires at least 0.010% addition. Al is an element that combines with N to form AlN and contributes to the refinement of austenite crystal grains. For that purpose, addition of 0.010% or more is required. However, when the content exceeds 0.060%, the production of Al 2 O 3 inclusions harmful to fatigue strength is promoted, so the Al content is limited to a range of 0.010 to 0.060%.
Cr：1.2〜2.0％
Crは、焼入れ性のみならず、焼戻し軟化抵抗性の向上に寄与し、さらには炭化物の球状化促進にも有用な元素であるが、含有量が1.2％に満たないとその添加効果に乏しい。一方、2.0％を超えるとこれらの効果は飽和し、むしろ固溶強化が高まり十分な加工性が得られなくなる。よって、Cr量は1.2〜2.0％の範囲に限定した。好ましくは1.35〜1.9％の範囲である。
Cr: 1.2-2.0%
Cr is an element that contributes not only to hardenability but also to improved resistance to temper softening and is also useful for promoting the spheroidization of carbides. However, if its content is less than 1.2%, its addition effect is poor. On the other hand, if it exceeds 2.0%, these effects are saturated, rather solid solution strengthening increases and sufficient workability cannot be obtained. Therefore, the Cr content is limited to the range of 1.2 to 2.0%. Preferably it is 1.35 to 1.9% of range.
Nb：0.005〜0.06％
Nbは、微細な炭窒化物を析出させることにより、結晶粒界をピン止めし、結果として、結晶粒を著しく微細化させて、冷間鍛造性と靱性を高めるために有効な元素である。しかしながら、含有量が0.06％を超えると、粗大なNb炭窒化物が析出しやすくなり、ピン止め能力が低下し結晶粒が粗大化してしまう。一方、0.005％未満のNb添加では、微細な炭窒化物の析出量が少なくなり、結晶粒の微細化効果に乏しくなるため、Nb量は0.005〜0.06％の範囲に限定した。好ましくは、0.01〜0.05％の範囲である。
Nb: 0.005-0.06%
Nb is an element effective for pinning crystal grain boundaries by precipitating fine carbonitrides, and as a result, refining the crystal grains significantly to improve cold forgeability and toughness. However, if the content exceeds 0.06%, coarse Nb carbonitride tends to precipitate, pinning ability decreases, and crystal grains become coarse. On the other hand, when Nb is added in an amount of less than 0.005%, the amount of fine carbonitride deposited decreases and the effect of crystal grain refinement becomes poor, so the Nb content is limited to a range of 0.005 to 0.06%. Preferably, it is 0.01 to 0.05% of range.
以上、本発明の基本成分の適正組成範囲について説明したが、本発明では、各々の元素が単に上記の範囲を満足するだけでは不十分であり、Ｃ，Si，MnおよびCrについては、次式(1)の関係を満足させることが重要である。
［％Ｃ］−［％Si］/２＋［％Mn］/５＋２［％Cr］≧3.2 ・・・ (1)
As mentioned above, the proper composition range of the basic component of the present invention has been described. However, in the present invention, it is not sufficient that each element simply satisfies the above range. For C, Si, Mn and Cr, It is important to satisfy the relationship (1).
[% C]-[% Si] / 2 + [% Mn] / 5 + 2 [% Cr] ≧ 3.2 (1)
すなわち、上掲(1)式は、熱間圧延ままの強度や炭化物の球状化の容易さの指標となるものであり、この値が3.2に満たないと、後述の実施例に示すように、球状化熱処理後の炭化物球状化率が低下し、また、高周波焼入れ後の表面硬さも不足となり、本発明で所期した良好な冷間鍛造性や焼入れ性が得られない。 That is, the above formula (1) is an index of the strength as it is hot-rolled and the ease of spheroidizing the carbide, and if this value is less than 3.2, as shown in the examples below, The carbide spheroidization rate after the spheroidizing heat treatment decreases, and the surface hardness after induction hardening becomes insufficient, and the good cold forgeability and hardenability expected in the present invention cannot be obtained.
以上、本発明の基本成分について説明したが、本発明では、より顕著に靱性を高めるために、必要に応じて、以下に述べる元素を適宜含有させることができる。 The basic components of the present invention have been described above. However, in the present invention, the elements described below can be appropriately contained as necessary in order to significantly increase toughness.
Ti：0.1％以下
Ｖ：0.1％以下
TiおよびＶは、ともに結晶粒の微細化に有効な元素であり、靱性を高めることが可能であるが、過剰に添加した場合は、粗大析出物が疲労強度に悪影響を及ぼすため、添加量は0.1％以下の範囲にする。なお、靭性を高める効果を発現させるためには、いずれの元素についても0.005％以上添加することが好ましい。より好ましくは、0.01〜0.05％の範囲である。
Ti: 0.1% or less V: 0.1% or less
Both Ti and V are effective elements for crystal grain refinement and can increase toughness. However, when added excessively, coarse precipitates adversely affect fatigue strength. The range is 0.1% or less. In order to develop the effect of increasing toughness, it is preferable to add 0.005% or more of any element. More preferably, it is 0.01 to 0.05% of range.
また、冷間鍛造後に行う焼入れ・焼戻し処理における焼入れ性を高めるために、必要に応じて、以下に示す元素を適宜含有させることができる。
Cu：0.30％以下
Cuは、焼入れ性の向上に有効な元素であり、そのためには、0.05％以上で添加することが推奨される。一方、多量に添加すると鋼材の表面性状の劣化や合金コストの増加を招くことから、0.30％以下で添加することが好ましい。
Moreover, in order to improve the hardenability in the quenching / tempering treatment performed after cold forging, the following elements can be appropriately contained as required.
Cu: 0.30% or less
Cu is an element effective for improving hardenability, and for that purpose, it is recommended to add 0.05% or more. On the other hand, if added in a large amount, the surface properties of the steel material are deteriorated and the alloy cost is increased. Therefore, it is preferable to add it at 0.30% or less.
Ni：0.30％以下
Niは、焼入れ性や靭性の向上に有効な元素であり、そのためには、0.05％以上で添加することが推奨される。しかしながら、高価であるため、上限を0.30％とすることが好ましい。
Ni: 0.30% or less
Ni is an element effective for improving hardenability and toughness, and for that purpose, it is recommended to add 0.05% or more. However, since it is expensive, the upper limit is preferably set to 0.30%.
表１に示す成分組成になる鋼を溶製し、連続鋳造により鋳片とした。ついで、鋳片を熱間圧延により60mmφの棒鋼とした後、得られた各棒鋼について、冷間鍛造性、靱性、球状化熱処理性および高周波焼入れ性について調査した。その結果を、表２に示す。 Steel having the composition shown in Table 1 was melted and cast into a slab by continuous casting. Next, the slab was made into a steel bar of 60 mmφ by hot rolling, and each of the obtained steel bars was examined for cold forgeability, toughness, spheroidizing heat treatment property and induction hardenability. The results are shown in Table 2.
なお、各特性の評価方法は次のとおりである。
（１）冷間鍛造性
冷間鍛造性は、限界据え込み率および変形抵抗の２項目にて評価した。
すなわち、圧延ままの棒鋼（直径Ｄ）の表面から径方向に1／4Ｄの深さ位置（以下、1／4Ｄ位置という）から、試験片を採取した。試験片形状は、図１に示すように、直径：14mm、高さ：21mmの円柱形であり、その高さ方向を棒鋼の長さ方向とし、上下面に拘束溝および側面にＶ溝を有する切欠き付円柱試験片である。
ここで、図１において、(a)は上面図、(b)は側面図、(c)はＶ字状の溝の詳細寸法を示す図であり、番号１でＶ字状の溝を、また番号２で被圧縮面（上下面）を示す。
In addition, the evaluation method of each characteristic is as follows.
(1) Cold forgeability Cold forgeability was evaluated by two items, the limit upsetting rate and the deformation resistance.
That is, a test piece was sampled from a depth position of 1 / 4D in the radial direction from the surface of the rolled steel bar (diameter D) (hereinafter referred to as 1 / 4D position). As shown in FIG. 1, the shape of the test piece is a cylindrical shape having a diameter of 14 mm and a height of 21 mm. It is a notched cylindrical specimen.
Here, in FIG. 1, (a) is a top view, (b) is a side view, (c) is a diagram showing the detailed dimensions of the V-shaped groove, and the number 1 indicates the V-shaped groove,
冷間鍛造性の評価は、この試験片の上下面を拘束した状態で被圧縮面２に圧縮荷重を加えて圧縮試験を行い、変形能と変形抵抗を測定した。変形能は、Ｖ溝１の溝底から割れが発生するまでの最大圧縮率（限界圧縮率と呼ぶ）で評価し、また変形抵抗は圧縮率：30％のときの変形荷重（30％変形抵抗と呼ぶ）で評価した。
限界圧縮率が50％以上、変形抵抗値が239MPa以下であれば、冷間鍛造性に優れていると判定した。
In the evaluation of cold forgeability, a compressive test was performed by applying a compressive load to the
When the critical compression ratio was 50% or more and the deformation resistance value was 239 MPa or less, it was determined that the cold forgeability was excellent.
（２）靱性
靱性は、シャルピー衝撃試験(JIS Ｚ2242)にて評価した。
すなわち、圧延ままの棒鋼を、850℃に加熱後60℃の油中にて焼入れ、ついで500℃にて１ｈの焼戻し処理を行い空冷した。空冷後の棒鋼の1／4Ｄ位置から、試験片を採取した。試験片形状は長さ55mmであり、一辺が10mmの正方形断面を有し、長さの中心位置に深さ２mm及び半径１mmのノッチを有する。試験は20℃で実施した。衝撃吸収エネルギーが50J以上であれば、靱性に優れていると判定した。
(2) Toughness Toughness was evaluated by Charpy impact test (JIS Z2242).
That is, the rolled steel bar was heated to 850 ° C., quenched in oil at 60 ° C., then tempered at 500 ° C. for 1 h and air-cooled. A specimen was taken from the 1 / 4D position of the steel bar after air cooling. The specimen has a length of 55 mm, a square cross section with a side of 10 mm, and a notch with a depth of 2 mm and a radius of 1 mm at the center of the length. The test was conducted at 20 ° C. If the impact absorption energy was 50 J or more, it was determined that the toughness was excellent.
（３）球状化焼鈍性
球状化熱処理性は、球状化熱処理後の炭化物の球状化率、限界据え込み率および変形抵抗の３項目で評価した。
上記（１）と同様にして、圧延ままの棒鋼に、図２に示す条件で球状化焼鈍を行ったのち、図１に示す形状の試験片を1／4Ｄ位置から採取した。球状化率は、試験片のＣ断面（棒鋼のＣ断面）を走査型電子顕微鏡（SEM）にて5000倍で観察して求めた。観察された炭化物のうちの炭化物のアスペクト比（長径／短径）が２以下のものの割合を球状化率とした。この割合が51％以上であれば、球状化熱処理性に優れていると判定した。
また、球状化熱処理後の限界据え込み率が55％以上、変形抵抗値が204MPa以下であれば、冷間鍛造性は良好であると判定した。
(3) Spheroidizing annealing property The spheroidizing heat treatment property was evaluated by three items of the spheroidizing rate, the limit upsetting rate, and the deformation resistance of the carbide after the spheroidizing heat treatment.
In the same manner as in (1) above, the rolled steel bar was subjected to spheroidizing annealing under the conditions shown in FIG. 2, and then a test piece having the shape shown in FIG. 1 was taken from the 1 / 4D position. The spheroidization ratio was determined by observing the C cross section of the test piece (C cross section of the steel bar) at 5000 times with a scanning electron microscope (SEM). Of the observed carbides, the ratio of the carbide having an aspect ratio (major axis / minor axis) of 2 or less was defined as the spheroidization ratio. If this ratio was 51% or more, it was determined that the spheroidizing heat treatment property was excellent.
Further, if the limit upsetting rate after spheroidizing heat treatment was 55% or more and the deformation resistance value was 204 MPa or less, it was determined that the cold forgeability was good.
（４）高周波焼入れ性
高周波焼入れ性は、直径：30mmの試験片に高周波焼入れ・焼戻しを実施してから、表面硬さ（表層より２mmの位置）を測定することにより行った。表層硬さがHV640以上であれば、高周波焼入れ性に優れていると判定した。なお、高周波焼入れは、加熱温度920℃とし、この加熱温度に到達後即焼入れを行った。焼戻しは、170℃で20分保持することで行った。
(4) Induction hardenability Induction hardening was performed by subjecting a test piece having a diameter of 30 mm to induction hardening and tempering, and then measuring the surface hardness (
表２に示したとおり、本発明に従い得られた発明例はいずれも、熱間圧延ままで優れた冷間鍛造性を有しているのは勿論のこと、靱性にも優れており、さらには、球状化焼鈍後も優れた冷間鍛造性を有し、また高周波焼入れ性にも優れていた。 As shown in Table 2, all the inventive examples obtained according to the present invention are excellent in toughness as well as having excellent cold forgeability as hot rolled, Furthermore, it had excellent cold forgeability after spheroidizing annealing and was also excellent in induction hardenability.
本発明によれば、熱間圧延ままあるいは球状化焼鈍後のいずれにおいても、冷間鍛造性が優れ、焼入れ性にも優れた高靱性の機械構造用鋼を、安価かつ安定して得ることができ、工業上、極めて有用である。 According to the present invention, it is possible to stably and inexpensively obtain a steel having high toughness for mechanical structure that is excellent in cold forgeability and excellent in hardenability either as hot rolled or after spheroidizing annealing. It is very useful industrially.
１ Ｖ字状の溝
２ 被圧縮面
1 V-shaped
Claims (3)
Ｃ：0.30〜0.55％、
Si：0.03〜0.30％、
Mn：0.50〜l.50％、
Ｐ：0.018％以下、
Ｓ：0.02％以下、
Al：0.010〜0.060％、
Cr：1.2〜2.0％および
Nb：0.005〜0.06％
を、下記(1)式を満足する範囲で含有し、残部はFeおよび不可避的不純物の組成からなることを特徴とする冷間鍛造性および靱性に優れた機械構造用鋼。
記
［％Ｃ］−［％Si］／２＋［％Mn］／５＋２［％Cr］≧3.2 ・・・ (1) % By mass
C: 0.30 to 0.55%,
Si: 0.03-0.30%,
Mn: 0.50 ~ l.50%,
P: 0.018% or less,
S: 0.02% or less,
Al: 0.010 to 0.060%,
Cr: 1.2-2.0% and
Nb: 0.005-0.06%
In a range satisfying the following formula (1), the balance being composed of Fe and inevitable impurities, a machine structural steel excellent in cold forgeability and toughness.
[% C]-[% Si] / 2 + [% Mn] / 5 + 2 [% Cr] ≧ 3.2 (1)
Ti：0.1％以下および
Ｖ：0.1％以下
のうちから選んだ１種または２種を含有することを特徴とする請求項１に記載の冷間鍛造性と靱性に優れた機械構造用鋼。 Furthermore, in mass%,
The steel for machine structure excellent in cold forgeability and toughness according to claim 1, characterized in that it contains one or two selected from Ti: 0.1% or less and V: 0.1% or less.
Cu：0.30％以下および
Ni：0.30％以下
のうちから選んだ１種または２種を含有することを特徴とする請求項１または２に記載の冷間鍛造性と靱性に優れた機械構造用鋼。 Furthermore, in mass%,
Cu: 0.30% or less and
Ni: One or two selected from 0.30% or less is contained, and the steel for machine structure excellent in cold forgeability and toughness according to claim 1 or 2.
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| JPS4963615A (en) * | 1972-10-23 | 1974-06-20 | ||
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