JP2006307273A - Case hardening steel having excellent crystal grain coarsening resistance and cold workability and in which softening can be obviated, and method for producing the same - Google Patents

Case hardening steel having excellent crystal grain coarsening resistance and cold workability and in which softening can be obviated, and method for producing the same Download PDF

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JP2006307273A
JP2006307273A JP2005130190A JP2005130190A JP2006307273A JP 2006307273 A JP2006307273 A JP 2006307273A JP 2005130190 A JP2005130190 A JP 2005130190A JP 2005130190 A JP2005130190 A JP 2005130190A JP 2006307273 A JP2006307273 A JP 2006307273A
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steel
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JP4464864B2 (en
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Toshio Murakami
俊夫 村上
Hitoshi Hatano
等 畑野
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Kobe Steel Ltd
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<P>PROBLEM TO BE SOLVED: To provide a case hardening steel which exhibits excellent crystal grain coarsening resistance even when subjected to carburizing at a temperature higher than that in the conventional example so as to be case hardened such as carburized and carbonitrided as the stock for machine parts requiring carburizing depth and excellent workability and which also has excellent cold workability without being subjected to softening annealing. <P>SOLUTION: The case hardening steel having excellent crystal grain coarsening resistance and cold workability and in which softening can be obviated is composed of a rolled steel in which the contents of C, Si, Mn or the like are specified, and further, the contents of N, Al, Nb and Ti are specified. The case hardening steel is characterized in that the average value of the Vickers hardness in the cross-section is ≤180, and also, the maximum value of the standard deviation in the vickers hardness is ≤5. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

本発明は自動車などの輸送機器や、建設機械その他の産業機械などにおいて、肌焼き処理して使用される機械部品用の素材となる肌焼用鋼に関し、特に、軸受やCVT用プーリー、シャフト類、歯車、軸付き歯車などの素材として肌焼き処理して使用する際に、結晶粒粗大化防止特性に優れると共に、冷間加工性が良好で加工前の軟化焼鈍を省略することのできる肌焼用鋼とその製法に関するものである。   TECHNICAL FIELD The present invention relates to a case hardening steel used as a material for machine parts used by carrying out case hardening processing in transportation equipment such as automobiles, construction machinery and other industrial machines, and in particular, bearings, pulleys for CVT, and shafts. When used as a material for gears, gears with shafts, etc., it is excellent in crystal grain coarsening prevention properties, has good cold workability and can eliminate soft annealing before processing. Steel and its manufacturing method.

自動車、建設機械、その他の各種産業機械用として用いられる機械部品において、特に高強度が要求される部品には、従来から浸炭、窒化および浸炭窒化などの表面硬化熱処理(肌焼き処理)が行なわれている。これらの用途には、通常、SCr、SCM、SNCMなどの如きJIS規格で定められた肌焼用鋼を使用し、鍛造・切削等の機械加工により所望の部品形状に成形した後、浸炭、浸炭窒化などの表面硬化熱処理を施し、その後、研磨などの仕上工程を経て製造される。   In machine parts used for automobiles, construction machinery, and other various industrial machines, surface hardening heat treatment (case hardening) such as carburizing, nitriding, and carbonitriding has been conventionally performed for parts that require particularly high strength. ing. For these applications, the case-hardening steels such as SCr, SCM, SNCM, etc. are usually used, and after forming into the desired part shape by machining such as forging and cutting, carburizing and carburizing. A surface hardening heat treatment such as nitriding is performed, and then a finishing process such as polishing is performed.

近年、上記の様な機械部品についても製造原価の低減、リードタイムの短縮などが望まれており、肌焼き処理を高温化することによって熱処理時間を短縮することが行なわれている。しかし、肌焼き処理温度を高めると、素材のオーステナイト(γ)結晶粒が粗大化し、熱処理歪量が増大するという問題が生じてくる。   In recent years, it has been desired to reduce the manufacturing cost and the lead time for the mechanical parts as described above, and the heat treatment time has been shortened by increasing the case baking temperature. However, when the skin baking temperature is increased, the austenite (γ) crystal grains of the raw material become coarse and the amount of heat treatment strain increases.

そこで、肌焼用鋼の結晶粒粗大化防止特性(以下、耐結晶粒粗大化特性ということがある)を改善したものとして、TiやNbを添加した肌焼き鋼が提案されている(特許文献1〜3)。これらは、鋼中に0.1〜0.2質量%程度のTiやNbを添加することによって遊離窒素(free−N)を固定し、且つTiやNbの炭化物やそれらを含む複合炭化物、窒化物などを微細に析出させることで、肌焼き処理のための加熱時のγ結晶粒の粗大化を抑制するものである。   Then, the case hardening steel which added Ti and Nb is proposed as what improved the grain coarsening prevention characteristic (henceforth a crystal grain coarsening resistance characteristic) of steel for case hardening (patent document). 1-3). These materials fix free nitrogen (free-N) by adding about 0.1 to 0.2% by mass of Ti or Nb in steel, and Ti or Nb carbides, composite carbides containing them, nitriding By precipitating an object or the like finely, the coarsening of the γ crystal grains during heating for the skin baking treatment is suppressed.

一方、肌焼用鋼においては、部品形状に成形する際に冷間加工が行なわれるため、冷間加工性も重要な要求特性となる。また鋼材を冷間加工する際には、冷間加工性を高めるため加工前に球状化焼鈍などの軟化焼鈍を行うことが多いが、軟化焼鈍には設備費や人件費、エネルギー費などを含めて多大なコストがかかるので、この様な軟化焼鈍をせずとも加工できる様な冷間加工性に優れた鋼材が望まれる。   On the other hand, in case hardening steel, cold workability is an important required characteristic because cold work is performed when forming into a part shape. Also, when cold working steel, soft annealing such as spheroidizing annealing is often performed before processing to improve cold workability, but softening annealing includes equipment costs, labor costs, energy costs, etc. Therefore, a steel material excellent in cold workability that can be processed without such soft annealing is desired.

この様なことから、TiやNbが添加された肌焼用鋼においても、軟化焼鈍をせずとも冷間加工できる様に加工性を改善した鋼材が開発されている(特許文献4〜9など)。これらの発明では、主として鋼中の冷間加工性に影響を及ぼす成分の種類や含有率を適正に調整することで、冷間加工性を改善している。また上記特許文献には、更なる冷間加工性改善策として、熱間圧延後の冷却速度を適正に制御する方法や、熱延材の金属組織を制御する方法なども開示されている。   For this reason, even in case hardening steel to which Ti or Nb is added, steel materials having improved workability have been developed so that cold work can be performed without softening annealing (Patent Documents 4 to 9, etc.) ). In these inventions, the cold workability is improved by appropriately adjusting the types and contents of components mainly affecting the cold workability in steel. The above-mentioned patent documents also disclose a method for appropriately controlling the cooling rate after hot rolling, a method for controlling the metal structure of the hot rolled material, and the like as further measures for improving the cold workability.

しかしこれら従来の肌焼用鋼では、形状が複雑であったり強加工を受けたりする部品に適用した場合、軟化焼鈍なしでの冷間加工性が必ずしも十分とは言えず、更なる改善が望まれる。
特開平10−81938号公報 特開平10−130720号公報 特開2001−303174号公報 特開昭63−4042号公報 特開平6−299241号公報 特開平10−130777号公報 特開平11−43737号公報 特開平9−78184号公報 特開2003−89818号公報
However, these conventional case-hardening steels cannot be said to have sufficient cold workability without soft annealing when applied to parts that have complex shapes or undergo strong processing, and further improvements are desired. It is.
Japanese Patent Laid-Open No. 10-81938 JP-A-10-130720 JP 2001-303174 A JP-A 63-4042 JP-A-6-299241 JP-A-10-130777 Japanese Patent Laid-Open No. 11-43737 JP-A-9-78184 JP 2003-89818 A

本発明は上記の様な事情に着目してなされたものであって、その目的は、前掲の従来技術に開示された肌焼用鋼の特性を更に改善し、特に、冷間加工性を一段と高めて軟化焼鈍の省略を可能にすると共に、肌焼きのための熱処理による結晶粒の粗大化を一段と抑制し、物理的特性や寸法精度の良好な肌焼部品を与える肌焼用鋼を提供し、更にはその様な特性を備えた肌焼用鋼を確実に得ることのできる製法を提供することにある。   The present invention has been made paying attention to the circumstances as described above, and its purpose is to further improve the characteristics of the case hardening steel disclosed in the above-mentioned prior art, and in particular, to further improve the cold workability. Providing a case-hardening steel that makes it possible to omit softening annealing and further suppresses the coarsening of crystal grains due to heat treatment for skin hardening, giving a case-hardened part with good physical properties and dimensional accuracy. Furthermore, another object of the present invention is to provide a production method capable of reliably obtaining a case-hardening steel having such characteristics.

上記課題を解決することのできた本発明に係る耐結晶粒粗大化特性と冷間加工性に優れ、軟化焼鈍の省略を可能にした肌焼用鋼は、質量%で、
C:0.05〜0.30%、
Si:0.05〜2.0%、
Mn:0.2〜2.0%、
S:0.002〜0.2%、
N:0.003〜0.030%、
Al:0.01〜0.12%、
Nb:0.01〜0.20%、
Ti:0.005〜0.12%、
を含み、残部が実質的にFeよりなる鋼からなり、横断面内におけるビッカース硬さの平均値が180以下で、且つビッカース硬さの標準偏差の最大値が5以下であるところに特徴を有している。
The steel for case hardening, which is excellent in crystal grain coarsening characteristics and cold workability according to the present invention, which has been able to solve the above-mentioned problems, and which can omit the softening annealing, is in mass%,
C: 0.05 to 0.30%
Si: 0.05-2.0%,
Mn: 0.2 to 2.0%,
S: 0.002 to 0.2%,
N: 0.003-0.030%,
Al: 0.01 to 0.12%,
Nb: 0.01-0.20%,
Ti: 0.005 to 0.12%,
And the balance is substantially made of steel, the average value of Vickers hardness in the cross section being 180 or less, and the maximum standard deviation of Vickers hardness being 5 or less. is doing.

本発明に係る上記肌焼用鋼において、横断面内における金属組織の主体がフェライト+パーライトで、これらの組織が80%以上を占めるものは、上記硬さの標準偏差がより低く抑えられたものになるので好ましい。   In the case-hardening steel according to the present invention, the main component of the metal structure in the cross section is ferrite + pearlite, and these structures occupy 80% or more, the standard deviation of the hardness is suppressed lower This is preferable.

また本発明の上記鋼には、前掲の必須元素に加えて、求められる特性に応じて下記1)〜4)に示す群から選ばれる1種以上の元素を含有させることも有効である。   In addition to the above-mentioned essential elements, it is also effective for the steel of the present invention to contain one or more elements selected from the groups shown in the following 1) to 4) according to the required properties.

1)Cu:3.0%以下(0%を含まない)、Ni:3.0%以下(0%を含まない)、Cr:2.0%以下(0%を含まない)、Mo:2.0%以下(0%を含まない)よりなる群から選択される少なくとも1種、
2)B:0.0005〜0.010%、
3)V:0.3%以下(0%を含まない)および/またはZr:0.3%以下(0%
を含まない)、
4)REM:0.03%以下(0%を含まない)、Ca:0.03%以下(0%を含まない)、Mg:0.03%以下(0%を含まない)、Pb:0.3%以下(0%を含まない)、Bi:0.3%以下(0%を含まない)、Te:0.3%以下(0%を含まない)、Se:0.3%以下(0%を含まない)、Sn:0.3%以下(0%を含まない)よりなる群から選ばれる少なくとも1種。
1) Cu: 3.0% or less (not including 0%), Ni: 3.0% or less (not including 0%), Cr: 2.0% or less (not including 0%), Mo: 2 At least one selected from the group consisting of 0.0% or less (excluding 0%),
2) B: 0.0005 to 0.010%,
3) V: 0.3% or less (excluding 0%) and / or Zr: 0.3% or less (0%
Not included),
4) REM: 0.03% or less (not including 0%), Ca: 0.03% or less (not including 0%), Mg: 0.03% or less (not including 0%), Pb: 0 .3% or less (excluding 0%), Bi: 0.3% or less (not including 0%), Te: 0.3% or less (not including 0%), Se: 0.3% or less ( 0% is not included), Sn: at least one selected from the group consisting of 0.3% or less (not including 0%).

また本発明の製法は、上述した特性を備えた肌焼用鋼を工業的に安定して確実に製造することのできる方法として位置付けられるもので、上記成分組成の要件を満たす鋼を、1250℃以上の温度で均熱し、そのまま直接、若しくは圧延してからAr1変態点以下の温度まで冷却した後、1050〜1200℃に再加熱し、そのまま直接、若しくは圧延してからAr1変態点以下の温度まで冷却する処理を1回以上行ない、次いで850〜1000℃に再加熱してから圧延し、最終圧延温度を700〜850℃とするところに特徴を有している。 Further, the production method of the present invention is positioned as a method capable of industrially and reliably producing a case-hardening steel having the above-described characteristics, and a steel satisfying the above-mentioned component composition requirements at 1250 ° C. After soaking at the above temperature, directly or after rolling and cooling to a temperature below the Ar 1 transformation point, reheated to 1050 to 1200 ° C., directly or after rolling and below the Ar 1 transformation point. It is characterized in that the treatment to cool to the temperature is performed once or more, then reheated to 850 to 1000 ° C. and then rolled, and the final rolling temperature is set to 700 to 850 ° C.

本発明によれば、鋼の化学成分を特定する他、特に、横断面内におけるビッカース硬さの平均値を特定するとともに、該硬さの標準偏差を極力抑え、より好ましくは鋼横断面の金属組織をフェライト+パーライト主体の組織とすることによって、軟化焼鈍をせずとも複雑形状への加工や強加工に耐える優れた冷間加工性を有すると共に、表面硬化処理のための肌焼き熱処理による耐結晶粒粗大化特性に優れ、機械的特性と寸法精度に優れた肌焼部品を与える肌焼用鋼を提供できる。   According to the present invention, in addition to specifying the chemical composition of steel, in particular, the average value of Vickers hardness in the cross section is specified, and the standard deviation of the hardness is suppressed as much as possible, more preferably the metal of the steel cross section By making the structure mainly composed of ferrite + pearlite, it has excellent cold workability to withstand complex processing and strong processing without softening annealing, and it has resistance to heat treatment by surface hardening heat treatment for surface hardening treatment. It is possible to provide a case-hardening steel that provides a case-hardening part having excellent crystal grain coarsening properties and excellent mechanical properties and dimensional accuracy.

本発明者らは前述した様な従来技術の下で、耐結晶粒粗大化特性と冷間加工性を更に改善すべく、それらの性能に影響を及ぼす熱間圧延材の成分組成や物理的特性、結晶構造などを主体にして研究を重ねてきた。その結果、上記の様に、鋼の成分組成を特定する他、熱延材横断面におけるビッカース硬さの平均値を低めに抑えると共に、該硬さの標準偏差を少なくし、或いは更に金属組織を適正化してやれば、軟化焼鈍を省略した場合でも優れた冷間加工性を有すると共に、肌焼き処理のための加熱による耐結晶粒粗大化特性においても良好な肌焼用鋼が得られることを知り、上記本発明に想到した。   Under the prior art as described above, the present inventors further improve the grain coarsening resistance and cold workability, and the composition and physical properties of hot rolled materials that affect their performance. Research has been conducted mainly on crystal structures. As a result, as described above, in addition to specifying the component composition of steel, the average value of Vickers hardness in the cross section of the hot rolled material is kept low, the standard deviation of the hardness is reduced, or the metal structure is further reduced. It is known that if it is optimized, it has excellent cold workability even when softening annealing is omitted, and it can obtain a good case-hardening steel in terms of the grain coarsening resistance property due to heating for the case-hardening treatment. The present invention has been conceived.

以下、本発明において鋼の化学成分を定めた理由を明らかにし、引き続いて、鋼断面内のビッカース硬さの平均値や硬さの標準偏差、更には金属組織を定めた理由を明確にしていく。   Hereinafter, the reason for determining the chemical composition of steel in the present invention will be clarified, and subsequently, the average value of Vickers hardness in the steel cross section, the standard deviation of hardness, and further the reason for determining the metal structure will be clarified. .

まず、鋼の化学成分を定めた理由を説明する。   First, the reason for determining the chemical composition of steel will be described.

C:0.05〜0.30%;
Cは機械部品として必要な芯部硬さを確保する上で重要な元素であり、0.05%未満では硬さ不足により機械部品としての静的強度が不足気味となる。しかしC量が多過ぎると、硬くなり過ぎて芯部の靭性が低下すると共に冷間加工性も悪くなるので、0.30%以下に抑える必要がある。より好ましいC含量は、0.10%以上、0.25%以下である。
C: 0.05-0.30%;
C is an important element for securing the core hardness necessary for a machine part. If it is less than 0.05%, the static strength as a machine part becomes insufficient due to insufficient hardness. However, if the amount of C is too large, it becomes too hard and the toughness of the core part is lowered and the cold workability is also deteriorated, so it is necessary to keep it to 0.30% or less. A more preferable C content is 0.10% or more and 0.25% or less.

Si:0.05〜2.0%;
Siは脱酸剤として作用し、酸化物系介在物量を低減して内部品質を高める作用を有すると共に、焼戻し処理時の硬さ低下を抑えて肌焼き部品の表層硬さを確保するのに有効な元素であり、0.05%以上の添加を必要とする。しかし、Si量が多過ぎると、素材が硬くなり過ぎて冷間加工性が劣化するばかりでなく、浸炭処理時の粒界酸化層の形成が助長されて機械的特性にも悪影響が現われてくるので、これらの障害を抑えるため2.0%を上限と定めた。より好ましいSi含量は、0.10%以上、1.5%以下である。
Si: 0.05-2.0%;
Si acts as a deoxidizer, has the effect of reducing the amount of oxide inclusions and improving internal quality, and is effective in ensuring the surface hardness of case-hardened parts by suppressing the decrease in hardness during tempering. Element, and requires addition of 0.05% or more. However, if the amount of Si is too large, the material becomes too hard and the cold workability deteriorates, and the formation of a grain boundary oxide layer during the carburizing process is promoted, and the mechanical properties are adversely affected. Therefore, 2.0% is set as the upper limit in order to suppress these obstacles. A more preferable Si content is 0.10% or more and 1.5% or less.

Mn:0.2〜2.0%;
Mnは脱酸剤として作用し、酸化物系介在物量を低減して鋼材の内部品質を高める作用を有すると共に、浸炭焼入れ時の焼入性を著しく高める作用を有しており、こうした作用を有効に発揮させるには0.2%以上含有させる必要がある。しかし多過ぎると、冷間加工時の変形抵抗が増大して加工性が低下するばかりか、浸炭時の粒界酸化層の形成を助長して機械的特性にも悪影響を及ぼす様になるので、上限を2.0%とする。Mnのより好ましい含有量は0.4%以上、1.8%以下である。
Mn: 0.2-2.0%;
Mn acts as a deoxidizer, has the effect of reducing the amount of oxide inclusions and improving the internal quality of the steel, and also has the effect of significantly increasing the hardenability during carburizing and quenching. Therefore, it is necessary to contain 0.2% or more. However, if it is too much, not only the deformation resistance during cold working increases and the workability decreases, but also the formation of grain boundary oxide layer during carburizing is promoted, and the mechanical properties are adversely affected. The upper limit is 2.0%. A more preferable content of Mn is 0.4% or more and 1.8% or less.

S:0.002〜0.2%;
Sは、MnやTiなどと結合してMnS介在物やTiS介在物などを形成し、部品の衝撃特性に悪影響を及ぼすので、なるべく少なく抑えるのが好ましく、衝撃特性が求められる本発明では上限を0.2%と定めた。しかし反面Sは、切削性を高める作用を有しており、切削性が強く求められる場合は適量含有させるのがよく、0.002%程度以上は含有させるのがよい。通常の機械構造用鋼では0.01%程度以上、0.07%程度以下が好ましい。
S: 0.002 to 0.2%;
S combines with Mn, Ti, etc. to form MnS inclusions, TiS inclusions, etc., and adversely affects the impact characteristics of the parts. Therefore, it is preferable to suppress as much as possible. In the present invention where impact characteristics are required, the upper limit is set. Set to 0.2%. However, S, on the other hand, has an effect of improving the machinability, and when the machinability is strongly required, it should be contained in an appropriate amount, and more preferably about 0.002% or more. In normal steel for machine structural use, it is preferably about 0.01% or more and about 0.07% or less.

N:0.003〜0.030%;
Nは、Al,Ti,Nbと結合して窒化物や炭窒化物を形成し、浸炭加熱時におけるγ結晶粒の成長を抑制する作用を有しており、この作用を有効に発揮させるには0.003%以上含有させねばならず、好ましくは0.005%以上含有させるのがよい。しかしN量が多過ぎると、加工性や衝撃特性に悪影響を及ぼす様になるので、多くとも0.030%以下、より好ましくは0.025%以下、更に好ましくは0.020%以下に抑えるのがよい。
N: 0.003-0.030%;
N combines with Al, Ti, Nb to form nitrides and carbonitrides, and has the effect of suppressing the growth of γ crystal grains during carburizing heating. It must be contained by 0.003% or more, preferably 0.005% or more. However, if the amount of N is too large, the workability and impact properties will be adversely affected, so at most 0.030% or less, more preferably 0.025% or less, and even more preferably 0.020% or less. Is good.

Al:0.01〜0.12%;
Alは鋼材組織の結晶粒の調整に有効な元素である。即ちAlは、鋼中のNと結合して窒化物を生成するが、この窒化物は熱処理時におけるγ結晶粒の成長を抑制する作用を発揮するのである。しかも、Alを後述するNbやTiと複合添加すると、Al系の単独析出物よりも安定なAl窒化物とTi炭窒化物との複合析出物や、Al窒化物とNb炭窒化物との複合析出物、或いはAl窒化物とNb−Ti複合炭窒化物との複合析出物を形成し、高温浸炭時の耐結晶粒粗大化特性を高める。これらの作用を有効に発揮させるには、0.01%以上含有させる必要がある。しかしAl含量が多過ぎると、硬質で粗大な非金属介在物(Al23)が生成して衝撃特性を劣化させるので、0.12%を上限と定めた。Alのより好ましい含有量は0.015%以上、更に好ましくは0.02%以上で、0.10%以下、更に好ましくは0.07%以下である。
Al: 0.01 to 0.12%;
Al is an element effective for adjusting the crystal grains of the steel structure. That is, Al combines with N in the steel to form a nitride, which exhibits the action of suppressing the growth of γ crystal grains during heat treatment. In addition, when Al is added in combination with Nb or Ti, which will be described later, a composite precipitate of Al nitride and Ti carbonitride that is more stable than an Al-based single precipitate, or a composite of Al nitride and Nb carbonitride A precipitate or a composite precipitate of Al nitride and Nb—Ti composite carbonitride is formed, and the grain coarsening resistance characteristics during high-temperature carburization are enhanced. In order to exhibit these effects effectively, it is necessary to contain 0.01% or more. However, if the Al content is too high, hard and coarse non-metallic inclusions (Al 2 O 3 ) are generated and impact properties are deteriorated, so the upper limit was set to 0.12%. A more preferable content of Al is 0.015% or more, more preferably 0.02% or more, and 0.10% or less, and further preferably 0.07% or less.

Nb:0.01〜0.20%;
Nbは本発明において特に重要な役割を果たす元素であり、鋼中のNおよびCと結合して窒化物や炭化物もしくは炭窒化物を形成し、浸炭加熱時における結晶粒粗大化の抑制に寄与する元素であり、0.01%未満では、高温で安定な窒化物や炭化物、もしくは炭窒化物が生成しないため、耐結晶粒粗大化特性が不足気味となる。しかもNbは、AlやTiと複合添加することで、Nbを含む単独析出物よりも安定なAl窒化物とNb炭窒化物の複合析出物やNb−Ti複合炭窒化物、あるいはAl窒化物とNb−Ti複合炭窒化物の複合析出物を形成し、高温浸炭時の耐結晶粒粗大化特性の向上に寄与する。
Nb: 0.01-0.20%;
Nb is an element that plays a particularly important role in the present invention, and combines with N and C in steel to form nitrides, carbides or carbonitrides, and contributes to suppression of grain coarsening during carburizing heating. If it is less than 0.01%, nitrides, carbides, or carbonitrides that are stable at high temperatures are not generated, so that the crystal grain coarsening resistance is insufficient. In addition, Nb is combined with Al and Ti, so that a stable precipitate of Al nitride and Nb carbonitride, Nb-Ti composite carbonitride, or Al nitride is more stable than a single precipitate containing Nb. A composite precipitate of Nb—Ti composite carbonitride is formed, which contributes to the improvement of crystal grain coarsening characteristics during high-temperature carburization.

しかし、Nb含量が多過ぎるとNbを含む粗大な析出物が生成してオストワルド粒成長を加速し、耐結晶粒粗大化特性を逆に劣化させるので、0.20%以下に抑えるべきである。Nbのより好ましい含有率は0.02%以上、更に好ましくは0.03%以上で、0.15%以下、更に好ましくは0.10%以下である。   However, if the Nb content is too large, coarse precipitates containing Nb are generated to accelerate the Ostwald grain growth and reversely deteriorate the crystal grain coarsening characteristics. Therefore, it should be suppressed to 0.20% or less. A more preferable content of Nb is 0.02% or more, more preferably 0.03% or more, 0.15% or less, and further preferably 0.10% or less.

Ti:0.005〜0.12%;
Tiも本発明において重要な役割を果たす元素である。すなわち、鋼中のTiはNおよびCと結びついて炭化物、窒化物、炭窒化物を形成し、高温浸炭時のγ結晶粒の粗大化を抑制する。また、AlやNbと複合添加することで、Tiを含む単独析出物よりも安定なAl窒化物とTi炭窒化物の複合析出物やNb−Ti複合炭窒化物、あるいは、Al窒化物とNb−Ti複合炭窒化物の複合析出物を形成し、耐結晶粒粗大化特性の向上に寄与する。Ti含量が0.005%未満では、析出するTi炭窒化物や他元素との複合炭窒化物の数が不十分となり、満足のいく耐結晶粒粗大化特性が得られない。しかし反面、Ti含量が0.12%を超えて過度に多くなると、粗大なNb−Ti炭窒化物が生成してオストワルド成長を促進し、耐結晶粒粗大化特性を却って劣化させる。Tiのより好ましい含有量は、0.008%以上で、0.10%以下、より好ましくは0.05%以下である。
Ti: 0.005 to 0.12%;
Ti is also an element that plays an important role in the present invention. That is, Ti in steel is combined with N and C to form carbides, nitrides, carbonitrides, and suppresses coarsening of γ crystal grains during high-temperature carburization. In addition, by compound addition with Al or Nb, a composite precipitate of Al nitride and Ti carbonitride, Nb-Ti composite carbonitride, or Al nitride and Nb is more stable than a single precipitate containing Ti. Forms composite precipitates of -Ti composite carbonitrides and contributes to improvement of the resistance to grain coarsening. When the Ti content is less than 0.005%, the number of precipitated Ti carbonitrides and composite carbonitrides with other elements becomes insufficient, and satisfactory crystal grain coarsening resistance characteristics cannot be obtained. On the other hand, if the Ti content exceeds 0.12% and becomes excessively large, coarse Nb—Ti carbonitrides are generated to promote Ostwald growth and deteriorate the anti-crystal grain coarsening characteristics. The more preferable content of Ti is 0.008% or more, 0.10% or less, and more preferably 0.05% or less.

本発明で用いる鋼の必須構成元素は以上の通りであり、残部は実質的にFeである。「実質的に」とは不可避的に混入してくる元素、例えばP(リン)やO(酸素)などの不可避不純物量の混入を許容するという意味であり、それらが含まれることによる障害を極力抑えるには、Pは0.03以下、Oは0.003%以下に抑えるのがよい。   The essential constituent elements of the steel used in the present invention are as described above, and the balance is substantially Fe. “Substantially” means to allow the entry of unavoidable elements such as P (phosphorus) and O (oxygen), which are inevitably mixed. In order to suppress it, it is preferable to suppress P to 0.03 or less and O to 0.003% or less.

ちなみに、Pは結晶粒界に偏析して部品の衝撃特性や冷間加工性を低下させるので、極力少なく抑えるべきであり、多くとも0.03%以下、より好ましくは0.010%以下に抑えるのがよい。またO(酸素)は鋼材の強度特性を低下させるので、0.003%以下、より好ましくは0.001%以下に抑えるのがよい。   By the way, P segregates at the grain boundaries and lowers the impact characteristics and cold workability of the parts. Therefore, it should be suppressed as much as possible, at most 0.03% or less, more preferably 0.010% or less. It is good. Further, O (oxygen) lowers the strength characteristics of the steel material, so 0.003% or less, more preferably 0.001% or less is preferable.

また本発明で用いる鋼材には、上記必須元素に加えて、所望に応じて更なる付加的特性を与えるため、下記の様な選択元素を含有させることも有効であり、必要に応じてそれらの元素を添加したものも本発明の技術的範囲に含まれる。   In addition to the above essential elements, the steel material used in the present invention is also effective to contain the following selective elements in order to give further additional characteristics as desired. What added the element is also contained in the technical scope of this invention.

Cu:3.0%以下(0%を含まない)、Ni:3.0%以下(0%を含まない)、Cr:2.0%以下(0%を含まない)、Mo:2.0%以下(0%を含まない)よりなる群から選択される少なくとも1種;
Cu,Ni,Cr,Moは、何れも焼入れ性の向上に寄与するという点では同効元素であり、且つこれらのうちCuは耐食性の向上にも寄与する。またNi,Moは鋼材の靭性向上にも寄与し、Crは浸炭硬化性を高める作用も有している。しかし、それら各元素の効果は各々上記上限値付近で飽和するので、それ以上の添加は不経済であるばかりでなく、過剰量のCrは靭性に悪影響を及ぼし、Moは靭性や冷間加工性に悪影響を及ぼすので、上限値を超える添加は避けるべきである。
Cu: 3.0% or less (not including 0%), Ni: 3.0% or less (not including 0%), Cr: 2.0% or less (not including 0%), Mo: 2.0 % Or less (excluding 0%), at least one selected from the group consisting of:
Cu, Ni, Cr, and Mo are all effective elements in that they all contribute to improvement in hardenability, and among these, Cu also contributes to improvement in corrosion resistance. Ni and Mo also contribute to improving the toughness of the steel material, and Cr also has the effect of increasing the carburizing hardenability. However, since the effect of each element is saturated near the upper limit, adding more than that is not economical, excessive amount of Cr adversely affects toughness, Mo is toughness and cold workability Addition exceeding the upper limit value should be avoided.

また、これらの元素のうち特にCuは、単独で添加すると鋼材の熱間加工性を劣化させる傾向があるが、Cuと共に適量のNiを併用すると、こうしたCu添加による弊害も回避できるので好ましい。   Of these elements, Cu, in particular, tends to deteriorate the hot workability of the steel material when it is added alone. However, it is preferable to use an appropriate amount of Ni together with Cu because the adverse effects of such addition of Cu can be avoided.

B:0.0005〜0.010%;
Bは微量で鋼材の焼入性を大幅に高める作用を有しており、しかも結晶粒界を強化して衝撃特性を高める作用も有している。こうした作用は0.0005%以上添加することで有効に発揮される。しかし、それらの効果は約0.010%で飽和し、またB量が多過ぎると、B窒化物が生成し易くなって冷間加工性に顕著な悪影響を及ぼすので、多くとも0.010%以下に抑えるべきである。より好ましいB含量は0.0007%以上、0.0050%以下である。
B: 0.0005 to 0.010%;
B has the effect of significantly increasing the hardenability of the steel material in a small amount, and also has the effect of enhancing the impact characteristics by strengthening the grain boundaries. Such an effect is effectively exhibited by adding 0.0005% or more. However, these effects saturate at about 0.010%, and if the amount of B is too large, B nitride is easily formed and significantly affects cold workability, so at most 0.010% Should be kept below. A more preferable B content is 0.0007% or more and 0.0050% or less.

V:0.3%以下(0%を含まない)および/またはZr:0.3%以下(0%を含
まない);
V,Zrは、何れも炭化物や窒化物からなる析出物を形成してγ結晶粒の粗大化を抑える作用を有しているが、多過ぎると上記析出物量が多くなり過ぎて成形加工性に悪影響を及ぼす様になるので、夫々0.3%以下に抑えるべきである。
V: 0.3% or less (not including 0%) and / or Zr: 0.3% or less (not including 0%);
V and Zr both have the effect of suppressing the coarsening of the γ crystal grains by forming precipitates made of carbides and nitrides, but if the amount is too large, the amount of the precipitates will increase so much that the moldability will be improved. Each of them should be kept at 0.3% or less because it will have an adverse effect.

REM:0.03%以下(0%を含まない)、Ca:0.03%以下(0%を含まない)、Mg:0.03%以下(0%を含まない)、Pb:0.3%以下(0%を含まない)、Bi:0.3%以下(0%を含まない)、Te:0.3%以下(0%を含まない)、Se:0.3%以下(0%を含まない)、Sn:0.3%以下(0%を含まない)よりなる群から選ばれる少なくとも1種;
これらの元素は、何れも鋼材の被削性向上に有効に作用するが、多過ぎると靭性を著しく劣化させるので、添加するにしても夫々上限値以下に抑えるべきである。
REM: 0.03% or less (not including 0%), Ca: 0.03% or less (not including 0%), Mg: 0.03% or less (not including 0%), Pb: 0.3 % Or less (excluding 0%), Bi: 0.3% or less (not including 0%), Te: 0.3% or less (not including 0%), Se: 0.3% or less (0% At least one selected from the group consisting of Sn: 0.3% or less (not including 0%);
Any of these elements effectively works to improve the machinability of the steel material, but if it is too much, the toughness is remarkably deteriorated, so even if it is added, it should be kept below the upper limit value.

本発明では、上述した鋼成分の制限に加えて、圧延材としての重要な物理的特性として、横断面内におけるビッカース硬さの平均値が180以下で、該硬さの標準偏差の最大値が5以下であることを必須の要件とする。即ち本発明者らが、上記成分組成の要件を満たす圧延鋼材について、その冷間加工性と熱処理時の耐結晶粒粗大化特性に及ぼす影響について様々の角度から研究を進めたところ、上記の様に、供試鋼材の横断面内におけるビッカース硬さの平均値と該硬さの標準偏差がそれらの特性に顕著な影響を及ぼし、該ビッカース硬さの平均値が180以下、より好ましくは170以下で、且つ硬さの標準偏差の最大値が5以下であるものは、軟化焼鈍を省略した場合でも安定して優れた冷間加工性を有すると共に、肌焼きのための熱処理時における耐結晶粒粗大化特性においても優れた性能を示すことが確認された。   In the present invention, in addition to the restriction of the steel components described above, as an important physical characteristic as a rolled material, the average value of Vickers hardness in the cross section is 180 or less, and the maximum value of the standard deviation of the hardness is An essential requirement is 5 or less. That is, the present inventors have conducted research from various angles on the cold workability and the effect on the grain coarsening resistance property during heat treatment of rolled steel that satisfies the above-mentioned component composition requirements. In addition, the average value of the Vickers hardness and the standard deviation of the hardness in the cross section of the test steel material have a significant effect on their properties, and the average value of the Vickers hardness is 180 or less, more preferably 170 or less. And having a maximum hardness standard deviation of 5 or less has stable and excellent cold workability even when softening annealing is omitted, and is resistant to crystal grains during heat treatment for skin hardening. It was confirmed that excellent performance was exhibited in the coarsening characteristics.

この様な傾向が得られる理論的な理由は、現在のところ未だ明確にされていないが、次の様なことが考えられる。即ち、ビッカース硬さの平均値が相対的に低いということは、相対的に軟質で変形し易いことを意味しており、この値を所定値以下に抑えることで冷間加工性の向上が図られる。具体的にはその値を180以下、より好ましくは170以下に抑えることで、軟化焼鈍なしでも優れた冷間加工性を得ることが可能となる。但し、たとえビッカース硬さの平均値が180以下であっても、該硬さの標準偏差が大きくなると、局部的に変形能の小さな領域が存在することとなってその領域が破壊の起点となり、鋼全体としての冷間加工性が低下してくる。そこで、硬さバラツキに起因する加工性劣化を抑えて軟化焼鈍なしでも優れた冷間加工性を確保するには、硬さの標準偏差の最大値を5以下に抑えることが必要であり、より好ましくは該最大値を4以下に抑えるのがよい。   The theoretical reason why such a tendency can be obtained has not been clarified yet, but the following can be considered. That is, a relatively low average value of Vickers hardness means that it is relatively soft and easily deformed, and by suppressing this value to a predetermined value or less, it is possible to improve cold workability. It is done. Specifically, by suppressing the value to 180 or less, more preferably 170 or less, it is possible to obtain excellent cold workability without softening annealing. However, even if the average value of the Vickers hardness is 180 or less, when the standard deviation of the hardness is large, there is a locally small region of deformability, and that region becomes a starting point of destruction, The cold workability of the steel as a whole decreases. Therefore, in order to suppress workability deterioration due to hardness variation and ensure excellent cold workability without softening annealing, it is necessary to suppress the maximum standard deviation of hardness to 5 or less. Preferably, the maximum value is suppressed to 4 or less.

更に、こうしたビッカース硬さの平均値と当該の標準偏差の最大値に与える圧延鋼材の金属組織についても検討を進めた結果、断面金属組織中に占めるフェライトとパーライトのトータル面積率が高いものほど上記ビッカース硬さの平均値は相対的に低い値で安定すると共に、当該硬さの標準偏差の最大値は小さくなり、該トータル面積率が少なくとも80%、好ましくは90%以上、更に好ましくは95%以上であるものは、加工後の軟化焼鈍を省略し得るほどに優れた冷間加工性を示すことが確認された。   Furthermore, as a result of studying the metal structure of the rolled steel material to give the average value of such Vickers hardness and the maximum value of the standard deviation, the higher the total area ratio of ferrite and pearlite in the cross-sectional metal structure, the above The average value of Vickers hardness is stabilized at a relatively low value, and the maximum value of the standard deviation of the hardness becomes small, and the total area ratio is at least 80%, preferably 90% or more, more preferably 95%. Those described above were confirmed to exhibit excellent cold workability to such an extent that softening annealing after processing could be omitted.

ちなみに、フェライト+パーライトのトータル面積率が大きいということは、それ以外の組織、例えばベイナイトやマルテンサイトなどが少ないことを意味しており、金属組織が全体的に均質であることから、ビッカース硬さが全体的に略均等で硬さの標準偏差が小さくなるものと思われる。   By the way, the fact that the total area ratio of ferrite + pearlite is large means that there are few other structures such as bainite and martensite, and since the metal structure is entirely homogeneous, Vickers hardness Is generally uniform and the standard deviation of hardness is considered to be small.

上記の様に本発明によれば、鋼の成分組成を特定すると共に、当該鋼断面のビッカース硬さの平均値と硬さの標準偏差の最大値を低く抑え、好ましくは更に、金属組織をフェライト+パーライトの総和で80%以上を確保することによって、軟化焼鈍を省略した場合でも優れた冷間加工性を確保しつつ、肌焼き処理のための加熱による耐結晶粒粗大化特性に優れ、強度特性と寸法精度の良好な肌焼き部品を与える肌焼用鋼を提供できる。   As described above, according to the present invention, the component composition of the steel is specified, and the average value of the Vickers hardness and the maximum standard deviation of the hardness of the steel cross section are suppressed to a low value. + By securing 80% or more of the total pearlite, excellent cold workability is ensured even when softening annealing is omitted, and it has excellent resistance to crystal grain coarsening due to heating for case hardening, and strength. It is possible to provide steel for case hardening that provides case hardening parts with good characteristics and dimensional accuracy.

次に、上記の様な特性を備えた肌焼用鋼を得るには、前述した化学成分の要件を満たす鋼材を1250℃以上の温度で均熱し、そのまま直接、若しくは圧延してからAr1変態点以下の温度まで冷却した後、1050〜1200℃に再加熱し、そのまま直接、若しくは圧延してからAr1変態点以下の温度まで冷却する処理を1回以上行ない、次いで850〜1000℃に再加熱してから圧延し、最終圧延温度を700〜850℃に制御することが極めて有効である。 Next, in order to obtain a case-hardening steel having the above-described characteristics, a steel material satisfying the above-mentioned chemical composition requirements is soaked at a temperature of 1250 ° C. or higher and directly or rolled, and then Ar 1 transformation is performed. After cooling to a temperature below the point, reheat to 1050 to 1200 ° C., directly or directly after rolling and cool to a temperature below the Ar 1 transformation point, and then reheat to 850 to 1000 ° C. It is very effective to roll after heating and to control the final rolling temperature to 700 to 850 ° C.

1250℃以上で均熱した後、Ar1変態点以下まで冷却するのは、加熱時に粗大化したオーステナイト(γ)をフェライトに変態させ、その後の加熱でオーステナイトに逆変態させた時のγ結晶粒を微細化すると共に、Ti含有物を析出させて次工程の再加熱でTi析出物の大きさを適度に調整できる様にするためである。 After soaking at 1250 ° C. or higher, cooling to below the Ar 1 transformation point is because γ crystal grains when austenite (γ) coarsened during heating is transformed into ferrite and then transformed back to austenite by heating. This is because the Ti-containing material is precipitated and the size of the Ti precipitate can be appropriately adjusted by reheating in the next step.

上記1250℃以上の温度での均熱後に、一旦Ar1変態点以下の温度まで冷却するのは、加熱時に粗大化したオーステナイトをフェライトに変態させ、その後の圧延前の加熱によってオーステナイトに逆変態させ、γ結晶粒を微細化すると共に、フェライト変態時にNb−Ti含有析出物を析出させるためであり、その為には、均熱後Ar1変態点以下の温度にまで冷却することが必須となる。 After soaking at a temperature of 1250 ° C. or higher, the cooling to a temperature below the Ar 1 transformation point is performed by transforming austenite coarsened during heating into ferrite and then reversely transforming to austenite by heating before rolling. In order to refine the γ crystal grains and to precipitate Nb—Ti-containing precipitates at the time of ferrite transformation, it is essential to cool to a temperature below the Ar 1 transformation point after soaking. .

その後、再び1050〜1200℃の温度域で再均熱するのは、オーステナイト中に適度な大きさのNB−Ti含有析出物を析出させ、肌焼処理のための加熱時におけるγ結晶粒の粗粒化を抑制すると共に、その後の圧延前の加熱時のNbやTiの固溶量を低減させて圧延後の冷却過程で析出するNb−Ti含有析出物による析出強化を抑えることによって、圧延鋼材の平均硬さと硬さの標準偏差を低く抑えるためである。ちなみに、再均熱温度が1200℃を超えると、Nb−Ti含有析出物の固溶量が増大し、また1050℃未満ではTi含有析出物が適当な大きさに成長しないので、いずれの場合も再均熱の目的が果たせなくなる。   After that, reheating in the temperature range of 1050 to 1200 ° C. is because the NB—Ti-containing precipitates having an appropriate size are precipitated in austenite, and the γ crystal grains are coarsened during heating for the skin baking treatment. Rolled steel by suppressing graining and suppressing precipitation strengthening due to Nb-Ti-containing precipitates that precipitate in the cooling process after rolling by reducing the solid solution amount of Nb and Ti during heating before rolling. This is to keep the average hardness and standard deviation of hardness low. Incidentally, when the re-soaking temperature exceeds 1200 ° C, the solid solution amount of the Nb-Ti-containing precipitate increases, and when it is less than 1050 ° C, the Ti-containing precipitate does not grow to an appropriate size. The purpose of re-heating is not fulfilled.

従って、この再均熱とAr1変態点以下の温度まで冷却を1回以上行ない、もしくはその間に圧延を行うと、圧延材断面の硬さの標準偏差が更に小さくなると共に、冷間加工性は一段と高められることになる。 Therefore, if the reheating is performed at least once to the temperature below the Ar 1 transformation point, or if rolling is performed during that time, the standard deviation of the hardness of the cross section of the rolled material is further reduced, and the cold workability is It will be further enhanced.

その後、850〜1000℃に再加熱してから圧延し、最終圧延温度は700〜850℃の範囲内となる様に制御する。圧延前の再加熱温度を850℃以上に定めたのは、850℃未満では圧延中の変形抵抗が大き過ぎて圧延機にかかる負荷が過大となるからである。また再加熱温度を1000℃以下に抑えるのは、圧延後のγ結晶粒を微細化し、圧延材の金属組織を微細フェライト+微細パーライト主体の組織とすることによって冷間加工性を高めるためである。圧延前のより好ましい再加熱温度は950℃以下である。   Then, after reheating to 850-1000 degreeC, it rolls and it controls so that the final rolling temperature may be in the range of 700-850 degreeC. The reason why the reheating temperature before rolling is set to 850 ° C. or more is that if it is less than 850 ° C., the deformation resistance during rolling is too large and the load on the rolling mill becomes excessive. Moreover, the reason why the reheating temperature is suppressed to 1000 ° C. or less is to improve cold workability by refining the γ crystal grains after rolling and making the metal structure of the rolled material mainly composed of fine ferrite and fine pearlite. . A more preferable reheating temperature before rolling is 950 ° C. or less.

また、再加熱後に行われる圧延時の最終圧延温度が700℃未満では、圧延工程中にフェライトの析出が起こって変形抵抗が更に高まり、圧延負荷が大きくなって実操業にそぐわなくなる。逆に最終圧延温度が850℃を超えると、圧延後のγ結晶粒が粗大化し、冷間加工性に好適な微細フェライト+微細パーライト組織が得られ難くなる。   Moreover, if the final rolling temperature at the time of rolling performed after reheating is less than 700 ° C., precipitation of ferrite occurs during the rolling process, the deformation resistance further increases, and the rolling load becomes large, making it unsuitable for actual operation. On the other hand, if the final rolling temperature exceeds 850 ° C., the γ crystal grains after rolling become coarse, and it becomes difficult to obtain a fine ferrite + fine pearlite structure suitable for cold workability.

その他の製造条件は特に限定されず、公知の条件範囲の中から適宜最適の条件を選択して適用すればよい。   Other manufacturing conditions are not particularly limited, and an optimum condition may be appropriately selected and applied from a known condition range.

以下、実施例を挙げて本発明の構成および作用効果をより具体的に説明するが、本発明はもとより下記実施例によって制限を受けるものではなく、前・後記の趣旨に適合し得る範囲で適当に変更を加えて実施することも可能であり、それらは何れも本発明の技術的範囲に含まれる。   Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and is suitable as long as it can meet the purpose described above and below. It is also possible to carry out the invention with modifications, and these are all included in the technical scope of the present invention.

実施例1
表1,2に示す化学組成の鋼材を小型溶製炉で溶製し、鋳造、均熱ののち熱間鍛造を行なって一辺が155mm角の鋼片を得た。この鋼片を使用し、表3,4に示す如く1300℃または1200℃で60分間均熱してから室温まで空冷した。次いで、各均熱材の一部を1000〜1250℃の範囲の各温度に再均熱してから室温まで空冷した。その後、再均熱材を同表に示す如く870℃から1100℃の範囲の各温度に加熱し、同表に示す最終圧延温度で圧延することによって、直径30mmの棒鋼を得た。
Example 1
Steel materials having chemical compositions shown in Tables 1 and 2 were melted in a small smelting furnace, and after casting and soaking, hot forging was performed to obtain a steel piece having a side of 155 mm square. This steel slab was used, soaked at 1300 ° C. or 1200 ° C. for 60 minutes as shown in Tables 3 and 4, and then cooled to room temperature. Next, a part of each soaking material was reheated to each temperature in the range of 1000 to 1250 ° C. and then air-cooled to room temperature. Thereafter, the reheat soaking material was heated to each temperature in the range of 870 ° C. to 1100 ° C. as shown in the same table, and rolled at the final rolling temperature shown in the same table to obtain a steel bar having a diameter of 30 mm.

得られた各圧延棒鋼の横断面を観察できるサンプルを切り出し、鏡面状に研磨した後、腐食液「エタノール+3%ナイタール」で処理した後、図1に示す如く、表面から深さ1mm位置、D/8位置(Dは棒鋼の直径を表す)、D/4位置、3D/8位置から任意に各4箇所を選んで合計16箇所を光学顕微鏡により倍率400倍で観察し、ポイントカウンティング法によってフェライト(F)+パーライト(P)面積率を求めた。なお残部組織は全てベイナイトであった。また上記と同じ横断面位置のビッカース硬さを各3断面で測定し、その平均値と硬さの標準偏差の最大値を求めた。尚、ビッカース硬さの測定は荷重10kgで行なった。   A sample that can observe the cross section of each rolled steel bar was cut out, polished to a mirror surface, treated with a corrosive solution “ethanol + 3% nital”, and then, as shown in FIG. / 8 position (D represents the diameter of steel bar), D / 4 position, 3D / 8 position each arbitrarily selected 4 locations, a total of 16 locations were observed with an optical microscope at 400 times magnification, and the point counting method was used for ferrite (F) + Perlite (P) area ratio was determined. All the remaining structures were bainite. Moreover, the Vickers hardness of the same cross-sectional position as the above was measured in each of three cross sections, and the average value and the maximum value of the standard deviation of the hardness were obtained. The Vickers hardness was measured with a load of 10 kg.

各供試材の耐結晶粒粗大化特性は、各供試棒鋼について、圧下率70%で冷間鍛造した後、1000℃で3時間加熱した後のγ結晶粒度をJIS G 0551に定めるγ結晶粒度試験方法に則って測定し、結晶粒度番号で5番以下の粗大粒の面積率によって評価した。5%を超えるもの:不良(×)、5%以下のもの:良好(○)。   The grain coarsening resistance characteristics of each test material are as follows: γ crystal grain size defined in JIS G 0551 for each test bar steel after cold forging at a reduction rate of 70% and heating at 1000 ° C. for 3 hours It measured according to the particle size test method, and evaluated by the area ratio of coarse particles having a crystal particle size number of 5 or less. More than 5%: Defect (x), 5% or less: Good (◯).

また冷間加工性は、各熱延材に「770℃×5時間加熱後、15℃/sで冷却する」球状化焼鈍を施した後、直径27.5mmに引抜き加工した各供試材から、図2に示す如く長さ41.3mmのノッチ付き試験片を作製し、それぞれ5個の端面完全拘束試験を行い、圧下率50%に圧下したときに割れが発生した試験片の数によって評価した。◎:割れなし、○:割れ1個、×:割れ2個以上。   The cold workability of each hot-rolled material was determined from each sample material that had been subjected to spheroidizing annealing that was “heated at 770 ° C. × 5 hours and then cooled at 15 ° C./s” and then drawn to a diameter of 27.5 mm As shown in FIG. 2, test pieces with notches having a length of 41.3 mm were prepared, 5 end face complete restraint tests were performed, and evaluation was made based on the number of test pieces that had cracks when reduced to a reduction ratio of 50%. did. A: No crack, B: One crack, X: Two or more cracks.

結果を表3,4に示す。   The results are shown in Tables 3 and 4.

表1〜4より次の様に考えることができる。   From Tables 1 to 4, the following can be considered.

No.1〜8,15〜40は、本発明の規定要件を全て満たす実施例であり、耐結晶粒粗大化特性と冷間加工性のいずれも良好で、総合判定で良好の結果が得られている。なおNo.9,10は、再均熱温度が規定範囲を外れるため、ビッカース硬さの平均値が180を超えると共に同硬さの標準偏差の最大値が5を超えており、成形性(冷鍛性)が悪い。No.11は、最初の均熱温度が低過ぎるため耐結晶粒粗大化特性が悪く、またNo.12は最初の均熱温度が低く、硬さの標準偏差の最大値が規定値を超え、しかもフェライト+パーライト組織分率も相対的に低いため、耐結晶粒粗大化特性が悪く且つ冷鍛性も悪い。   No. Examples 1 to 8 and 15 to 40 are examples that satisfy all of the prescribed requirements of the present invention, and both the grain coarsening resistance characteristics and the cold workability are good, and good results are obtained by comprehensive judgment. . No. Nos. 9 and 10 have a re-soaking temperature outside the specified range, so the average value of Vickers hardness exceeds 180 and the maximum standard deviation of the same hardness exceeds 5, and formability (cold forging) Is bad. No. No. 11 has poor crystal grain coarsening characteristics because the initial soaking temperature is too low. No. 12 has a low initial soaking temperature, the maximum standard deviation of hardness exceeds the specified value, and the ferrite + pearlite structure fraction is also relatively low. Is also bad.

No.13は、圧延加熱温度が高過ぎるため、硬さの標準偏差の最大値が規定値を超え、しかもフェライト+パーライト組織分率も低いため、耐結晶粒粗大化特性は良好であるが、冷鍛性が悪い。No.14は、最終圧延温度が高過ぎるため、硬さの標準偏差の最大値が規定値を超え、しかもフェライト+パーライト組織分率も低いため、耐結晶粒粗大化特性は良好であるが、冷鍛性が悪い。   No. No. 13, because the rolling heating temperature is too high, the maximum value of the standard deviation of hardness exceeds the specified value, and the ferrite + pearlite structure fraction is also low, so the crystal grain coarsening characteristics are good, but cold forging The nature is bad. No. No. 14, because the final rolling temperature is too high, the maximum value of the standard deviation of hardness exceeds the specified value, and the ferrite + pearlite structure fraction is also low. The nature is bad.

またNo.41〜51は、用いた鋼材の化学成分が規定要件の何れかを欠く比較例であり、ビッカース硬さの平均値、および硬さの標準偏差の最大値の何れか1方が規定要件を外れているため、耐結晶粒粗大化特性と冷鍛性の何れかが不良で本発明の目的を達成できていない。   No. 41 to 51 are comparative examples in which the chemical composition of the steel used lacks any of the prescribed requirements, and either one of the average value of the Vickers hardness and the maximum value of the standard deviation of the hardness is outside the prescribed requirements. Therefore, either the grain coarsening resistance property or the cold forgeability is poor and the object of the present invention cannot be achieved.

圧延後の棒鋼断面の金属組織とビッカース硬さの測定位置を示す説明図である。It is explanatory drawing which shows the measurement structure of the metal structure and Vickers hardness of the steel bar cross section after rolling. 実験で採用した冷間加工性評価用の試験片を示す図である。It is a figure which shows the test piece for cold workability evaluation employ | adopted in experiment.

Claims (7)

質量%で、
C:0.05〜0.30%、
Si:0.05〜2.0%、
Mn:0.2〜2.0%、
S:0.002〜0.2%、
N:0.003〜0.030%、
Al:0.01〜0.12%、
Nb:0.01〜0.20%、
Ti:0.005〜0.12%、
を含み、残部は実質的にFeよりなる鋼からなり、横断面内におけるビッカース硬さの平均値が180以下で、且つビッカース硬さの標準偏差の最大値が5以下であることを特徴とする、耐結晶粒粗大化特性と冷間加工性に優れた軟化焼鈍の省略可能な肌焼用鋼。
% By mass
C: 0.05 to 0.30%
Si: 0.05-2.0%,
Mn: 0.2 to 2.0%,
S: 0.002 to 0.2%,
N: 0.003-0.030%,
Al: 0.01 to 0.12%,
Nb: 0.01-0.20%,
Ti: 0.005 to 0.12%,
The balance is made of steel substantially consisting of Fe, the average value of Vickers hardness in the cross section is 180 or less, and the maximum value of the standard deviation of Vickers hardness is 5 or less This is a steel for skin hardening that can be omitted for soft annealing, with excellent grain coarsening resistance and cold workability.
横断面内における金属組織の80%以上が、フェライト+パーライトである請求項1に記載の肌焼用鋼。   The steel for case hardening according to claim 1, wherein 80% or more of the metal structure in the cross section is ferrite + pearlite. 鋼が、更に他の元素として、Cu:3.0%以下(0%を含まない)、Ni:3.0%以下(0%を含まない)、Cr:2.0%以下(0%を含まない)、Mo:2.0%以下(0%を含まない)よりなる群から選択される少なくとも1種の元素を含むものである請求項1または2に記載の肌焼用鋼。   Still other elements of steel are Cu: 3.0% or less (excluding 0%), Ni: 3.0% or less (excluding 0%), Cr: 2.0% or less (0% The steel for skin hardening according to claim 1 or 2, which contains at least one element selected from the group consisting of Mo: 2.0% or less (not including 0%). 鋼が、更に他の元素として、B:0.0005〜0.010%を含むものである請求項1〜3のいずれかに記載の肌焼用鋼。   The steel for case hardening according to any one of claims 1 to 3, wherein the steel further contains B: 0.0005 to 0.010% as another element. 鋼が、更に他の元素として、V:0.3%以下(0%を含まない)および/またはZr:0.3%以下(0%を含まない)を含むものである請求項1〜4のいずれかに記載の肌焼用鋼。   The steel according to any one of claims 1 to 4, wherein the steel further contains V: 0.3% or less (not including 0%) and / or Zr: 0.3% or less (not including 0%) as other elements. Crazing steel for crab. 鋼が、更に他の元素として、REM:0.03%以下(0%を含まない)、Ca:0.03%以下(0%を含まない)、Mg:0.03%以下(0%を含まない)、Pb:0.3%以下(0%を含まない)、Bi:0.3%以下(0%を含まない)、Te:0.3%以下(0%を含まない)、Se:0.3%以下(0%を含まない)、Sn:0.3%以下(0%を含まない)よりなる群から選ばれる少なくとも1種の元素を含むものである請求項1〜5のいずれかに記載の肌焼用鋼。   Still other elements of steel are REM: 0.03% or less (excluding 0%), Ca: 0.03% or less (not including 0%), Mg: 0.03% or less (0% Not included), Pb: 0.3% or less (not including 0%), Bi: 0.3% or less (not including 0%), Te: 0.3% or less (not including 0%), Se Any one element selected from the group consisting of: 0.3% or less (not including 0%), Sn: 0.3% or less (not including 0%) The steel for case hardening described in 1. 前記請求項1〜6のいずれかに記載された成分組成の要件を満たす鋼を、1250℃以上の温度で均熱し、そのまま直接、若しくは圧延してからAr1変態点以下の温度まで冷却した後、1050〜1200℃に再加熱し、そのまま直接、若しくは圧延してからAr1変態点以下の温度まで冷却する処理を1回以上行ない、次いで850〜1000℃に再加熱してから圧延し、最終圧延温度を700〜850℃とすることを特徴とする、耐結晶粒粗大化特性と冷間加工性に優れた軟化焼鈍の省略可能な肌焼用鋼の製法。 After steel that satisfies the requirements of the component composition described in any one of claims 1 to 6 is soaked at a temperature of 1250 ° C. or higher, and directly cooled or cooled to a temperature below the Ar 1 transformation point after rolling. , Reheated to 1050 to 1200 ° C., directly or directly after rolling and then cooled to a temperature below the Ar 1 transformation point, then reheated to 850 to 1000 ° C. and then rolled, A method for producing a case-hardening steel capable of omitting softening annealing, which is excellent in crystal grain coarsening characteristics and cold workability, characterized in that the rolling temperature is 700 to 850 ° C.
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