JPH1192868A - Steel for cold forging excellent in crystal grain-coarsening preventability and delayed fracture resistance and its production - Google Patents
Steel for cold forging excellent in crystal grain-coarsening preventability and delayed fracture resistance and its productionInfo
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- JPH1192868A JPH1192868A JP15367498A JP15367498A JPH1192868A JP H1192868 A JPH1192868 A JP H1192868A JP 15367498 A JP15367498 A JP 15367498A JP 15367498 A JP15367498 A JP 15367498A JP H1192868 A JPH1192868 A JP H1192868A
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Abstract
Description
【0001】[0001]
【発明の属する技術分野】本発明は、結晶粒粗大化防止
特性と耐遅れ破壊特性に優れた冷間鍛造用鋼とその製造
方法に関するものである。BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for cold forging which is excellent in crystal grain coarsening prevention characteristics and delayed fracture resistance characteristics and a method for producing the same.
【0002】[0002]
【従来の技術】冷間鍛造(転造も含む)は製品の表面
肌、寸法精度が良く、熱間鍛造に比べて製造コストが低
く、歩留まりも良好であるためボルト、ギア部品、シャ
フトをはじめとする多くの分野に適用されている。これ
らの部品の冷間鍛造は、例えばJIS G 4051、
JIS G 4052、JIS G 4104、JIS
G 4105、JIS G 4106などに規定されて
いる中炭素の機械構造用炭素鋼、合金鋼を使用し、例え
ば熱間圧延−焼鈍−冷間鍛造−焼入れ−焼戻しのように
冷間鍛造前に焼鈍、あるいは球状化焼鈍工程を付加する
工程が一般的である。これは上記のような中炭素の炭素
鋼、合金鋼は圧延ままの硬度が高く、ボルト等の部品成
形時の冷間鍛造工具の消耗が著しくコスト高となった
り、素材の延性が不足しているため部品の成形時に割れ
を生じたり、等といった製造上の問題があるためであ
る。2. Description of the Related Art Cold forging (including rolling) has good surface texture and dimensional accuracy of products, has lower manufacturing costs than hot forging, and has a good yield. And has been applied to many fields. Cold forging of these parts is performed, for example, according to JIS G 4051,
JIS G 4052, JIS G 4104, JIS
G 4105, JIS G 4106, etc., using carbon steel and alloy steel for machine structural use of medium carbon, for example, before cold forging such as hot rolling-annealing-cold forging-quenching-tempering. Or a step of adding a spheroidizing annealing step. This is because medium-carbon carbon steel and alloy steel as described above have high hardness as rolled, and the consumption of cold forging tools when forming parts such as bolts becomes extremely expensive and the ductility of the material is insufficient. This is because there are manufacturing problems such as cracks occurring during the molding of parts.
【0003】しかし、焼鈍にはエネルギー費、人件費、
設備費など多大なコストがかかるため、この工程を省略
しうる素材およびプロセスが求められてきた。そこで鋼
材のC量、合金元素量を低減することによって熱間圧延
ままの硬度を低減し、延性を向上して焼鈍工程を省略
し、Cr、Mo等の合金元素量の低減による焼入れ性の
低下を微量のBを添加することによって補う、いわゆる
低炭素ボロン鋼が、例えば特開平5−339676、特
公平5−63524、特開昭61−253347のよう
に数多く提案されている。Bは微量の添加で焼入れ性を
向上できるが、鋼中に固溶Nが存在するとBNが生成
し、Bの持つ焼入れ性向上効果は失われてしまうため、
Tiを添加して鋼中NをTiNの形で固定し、BNの生
成を抑制することが一般に行われている。[0003] However, annealing requires energy cost, labor cost,
Since a large cost such as equipment cost is required, a material and a process that can omit this step have been demanded. Therefore, by reducing the amount of C and alloying elements in the steel material, the hardness during hot rolling is reduced, the ductility is improved, the annealing step is omitted, and the hardenability is reduced by reducing the amount of alloying elements such as Cr and Mo. Are supplemented by adding a small amount of B, so-called low-carbon boron steels have been proposed, for example, in Japanese Patent Application Laid-Open Nos. 5-339676, 5-63524 and 61-253347. B can improve the hardenability by adding a small amount, but if solid solution N is present in steel, BN is generated and the hardenability improving effect of B is lost.
It is common practice to add Ti to fix N in steel in the form of TiN and to suppress the formation of BN.
【0004】部品の高強度化のニーズが強くなるに従
い、上記のような低炭素ボロン鋼をより高強度の部品に
適用する試みがなされている。しかし低炭素ボロン鋼は
C量、合金元素量を低減しているため、引張強さが10
00MPa以上となるように熱処理を行うと遅れ破壊特
性が低下するという問題がある。高強度を得るために低
温の焼き戻しを行うと遅れ破壊強度が低下することが知
られているが、高温の焼き戻しでも高い強度を得て、遅
れ破壊強度を実用上問題ないレベルとするために、C量
の添加量を増加したり、SCR、SCMなどの合金鋼を
使用すると、素材の強度が増加し、焼鈍が省略できなく
なる。焼鈍を省略できる低炭素ボロン鋼は経済的だが、
高強度を得るためには焼き戻しの温度を低くせざるを得
ず、その結果遅れ破壊強度が低下し、実用上の問題とな
るため、高強度部品への適用は難しかった。[0004] As the need for increasing the strength of parts has increased, attempts have been made to apply the above low-carbon boron steel to parts having higher strength. However, since low carbon boron steel has a reduced C content and alloying element content, it has a tensile strength of 10%.
When the heat treatment is performed so as to be at least 00 MPa, there is a problem that the delayed fracture characteristic is deteriorated. It is known that when low-temperature tempering is performed to obtain high strength, the delayed fracture strength decreases.However, in order to obtain high strength even at high-temperature tempering and to reduce the delayed fracture strength to a level at which there is no practical problem. If the amount of C added is increased or alloy steels such as SCR and SCM are used, the strength of the material increases and annealing cannot be omitted. Low-carbon boron steel, which can eliminate annealing, is economical,
In order to obtain high strength, the tempering temperature must be lowered, and as a result, the delayed fracture strength is reduced, which is a practical problem. Therefore, application to high strength parts has been difficult.
【0005】高強度部品へのボロン鋼適用の要求に応え
るため、不純物の量を低減し、遅れ破壊特性を合金鋼と
同等程度とした鋼が、例えば特開平8−60245等の
ように提案されている。しかし上記のようなボロン鋼
は、切削肌の試験片での評価では合金鋼よりも優れた遅
れ破壊特性を示すが、実際の製造ラインで部品を作成
し、熱処理肌の状態で遅れ破壊特性の評価を行うと、ボ
ロン鋼の部品は合金鋼よりも遅れ破壊特性が悪くなると
いう問題が見出されている。したがって、上記のような
技術で部品の高強度化に対応するには限界がある。[0005] In order to meet the demand for the application of boron steel to high-strength parts, a steel in which the amount of impurities is reduced and the delayed fracture characteristics are made comparable to that of an alloy steel has been proposed, for example, as disclosed in Japanese Patent Application Laid-Open No. H8-60245. ing. However, boron steel as described above shows a delayed fracture characteristic superior to that of alloy steel in the evaluation of a test piece of a cutting surface, but a part is created on an actual production line, and the delayed fracture characteristic Upon evaluation, it has been found that boron steel components have worse delayed fracture properties than alloy steel. Therefore, there is a limit in responding to the enhancement of the strength of the component by the above-described technology.
【0006】また、上記の問題に加え、ボロン鋼は焼鈍
材に比べて焼入れ加熱時に特定のオーステナイト結晶粒
が異常に粗大化しやすくなるという問題がある。結晶粒
の粗大化が発生した部品は、焼入れ歪みによる寸法精度
の劣化、衝撃値、疲労寿命の低下、特に高強度部品にお
いて遅れ破壊特性の低下を招くため、ボロン鋼を高強度
部品に適用するには結晶粒の粗大化を抑制し、かつ結晶
粒を微細化しなければならない。この結晶粒の粗大化を
抑制するには結晶粒界の移動をピン止めする粒子を多
量、微細に分散させることが有効である。In addition to the above-mentioned problems, boron steel has a problem that specific austenite crystal grains tend to become abnormally large during quenching and heating as compared with annealed material. For parts with coarsened crystal grains, deterioration of dimensional accuracy due to quenching strain, reduction of impact value and fatigue life, especially of high-strength parts, cause a decrease in delayed fracture characteristics, so boron steel is applied to high-strength parts. Therefore, it is necessary to suppress the coarsening of the crystal grains and to make the crystal grains fine. In order to suppress the coarsening of the crystal grains, it is effective to disperse a large amount and fine particles for pinning the movement of the crystal grain boundaries.
【0007】上記のようなボロン鋼の結晶粒粗大化を防
止するための技術が提案されている。例えば、特開昭6
1−217553はTiとNの量を0.02<Ti−
3.42NとすることによってTiCを生成し、結晶粒
界をピン止めすることを目的としている。しかし、成分
を規定しただけではTiCを微細に分散させることはで
きず、結晶粒の粗大化を防止できない。また例えば、特
公昭63−64495は0.0035%以下の極低Nと
し、Ti量をN量に対して過剰とした成分を低温加熱圧
延を行うことによって結晶粒粗大化を防止することを目
的としている。しかし、焼入れ加熱前のTiC、Ti
(CN)の析出状態を最適化しない限り結晶粒の粗大化
を防止できない。[0007] Techniques for preventing the above-described coarsening of crystal grains of boron steel have been proposed. For example, JP
1-217553 indicates that the content of Ti and N is 0.02 <Ti−
The purpose is to generate TiC by setting it to 3.42 N and to pin the crystal grain boundaries. However, only by defining the components, TiC cannot be finely dispersed, and coarsening of crystal grains cannot be prevented. For example, Japanese Patent Publication No. 63-64495 has an object to prevent the crystal grain from becoming coarse by performing low-temperature heat rolling of a component having an extremely low N of 0.0035% or less and an excessive amount of Ti relative to the amount of N. And However, TiC, Ti before quenching heating
Unless the precipitation state of (CN) is optimized, coarsening of crystal grains cannot be prevented.
【0008】また、例えは特開昭52−114545は
素材段階でTiCを固溶させ、焼入れ加熱時に初めてT
iCを微細析出させることを目的としている。しかし焼
入れ加熱時にピン止め粒子を析出させる場合、TiCの
析出量は焼入れ加熱、または浸炭加熱時の加熱速度の影
響を受けるためピン止め効果の発現が不安定であり、同
じ素材を用いても部品のサイズや熱処理炉を変えただけ
で粗大化防止特性が劣化する可能性が高いため、実工程
での品質の安定性の点で課題を残している。For example, Japanese Patent Application Laid-Open No. Sho 52-114545 discloses a method in which TiC is dissolved in a raw material phase and T
The purpose is to finely precipitate iC. However, when pinning particles are precipitated during quenching heating, the amount of TiC deposited is affected by the heating speed during quenching heating or carburizing heating, so the expression of the pinning effect is unstable. Since there is a high possibility that the coarsening prevention characteristics are deteriorated only by changing the size and the heat treatment furnace, there remains a problem in terms of the stability of the quality in the actual process.
【0009】[0009]
【発明が解決しようとする課題】上記のような開示され
た方法では、冷間鍛造前の焼鈍、あるいは球状化焼鈍工
程を省略し、かつ高強度に熱処理を行った際の実部品で
の遅れ破壊特性を合金鋼と同等以上にすることができな
い。本発明はこのような問題を解決して、結晶粒粗大化
防止特性と耐遅れ破壊特性に優れた冷間鍛造用鋼とその
製造方法を提供するものである。In the disclosed method as described above, the annealing before the cold forging or the spheroidizing annealing step is omitted, and the delay in actual parts when the heat treatment is performed with high strength. Fracture characteristics cannot be equal to or higher than alloy steel. The present invention solves such a problem and provides a steel for cold forging which is excellent in the crystal grain coarsening prevention property and the delayed fracture resistance property, and a method for producing the same.
【0010】[0010]
【課題を解決するための手段】本発明者らは、上記目的
を達成するために実部品の熱処理肌での遅れ破壊特性に
及ぼす諸因子の影響について鋭意調査し、 (ア)実部品での遅れ特性には、表面の性状が大きく影
響していること、すなわち、熱処理スケールが付着した
実ボルト(熱処理肌)と、表面層を切削・研削等によっ
て除去した試験片(切削肌)とでは同一条件で遅れ破壊
試験を行っても特性が大きく異なり、熱処理スケールが
付着した実部品の方が遅れ破壊特性が劣ること。 (イ)熱処理肌での遅れ破壊特性を改善するにはCrを
ある最適範囲で添加し、部品の熱処理時に生成されるス
ケールをCrの濃化した緻密なスケールとし、耐食性を
増すことでスケールおよびスケールの内側の鋼表面層が
腐食される過程で発生する水素量を低減させ、遅れ破壊
特性を向上することができること。 (ウ)ボロン鋼を引張強さ1000MPa以上のボルト
等の高強度部品に適用する場合には、遅れ破壊特性の向
上のためP、S量を一定量以下に制限することが必要な
こと、および結晶粒の粗大化を防止することが必要なこ
と。 (エ)結晶粒の粗大化を防止するにはピン止め粒子とし
て微細なTiC、Ti(CN)、NbC、Nb(C
N)、(Nb、Ti)(CN)粒子が有効であり、結晶
粒粗大化特性とこれらの析出物のサイズおよび分散状態
(析出粒子数)には極めて密接な関係があること、析出
物のピン止め効果を安定して発揮させるには、焼入れ加
熱前に一定量以上のTiC、Ti(CN)、NbC、N
b(CN)、(Nb、Ti)(CN)のうち1種以上の
粒子をあらかじめ微細析出させておくことが必要なこと
を見出し、本発明に至った。Means for Solving the Problems In order to achieve the above object, the present inventors have conducted intensive studies on the effects of various factors on the delayed fracture characteristics of heat-treated skin of real parts. The properties of the surface greatly affect the lag characteristics, that is, the actual bolts with heat-treated scale (heat-treated skin) are the same as the test pieces (cut skin) with the surface layer removed by cutting or grinding. Even if a delayed fracture test is performed under the same conditions, the characteristics are significantly different, and the actual component to which the heat treatment scale is attached is inferior in delayed fracture characteristics. (A) In order to improve delayed fracture characteristics on heat-treated skin, Cr is added in a certain optimum range, and the scale generated during heat treatment of the component is a dense scale in which Cr is concentrated, and the scale and the corrosion resistance are increased. The ability to reduce the amount of hydrogen generated in the course of corrosion of the steel surface layer inside the scale and improve delayed fracture characteristics. (C) When boron steel is applied to high-strength parts such as bolts having a tensile strength of 1000 MPa or more, it is necessary to limit the amounts of P and S to a certain amount or less in order to improve delayed fracture characteristics, and It is necessary to prevent coarsening of crystal grains. (D) To prevent coarsening of crystal grains, fine TiC, Ti (CN), NbC, Nb (C
N) and (Nb, Ti) (CN) particles are effective, and there is a very close relationship between the crystal grain coarsening characteristics and the size and dispersion state (the number of precipitated particles) of these precipitates. To stably exhibit the pinning effect, a certain amount or more of TiC, Ti (CN), NbC, N
The present inventors have found that it is necessary to previously precipitate one or more particles of b (CN) and (Nb, Ti) (CN) finely, and have reached the present invention.
【0011】本発明の特性は、(1)C:0.10〜
0.40%、Si:0.15%以下、Mn:0.30〜
1.00%にすることにより焼入れ、焼戻し後の部品の
強度を確保し、P:0.015%以下(0%を含む)、
S:0.015%以下(0%を含む)に制限することに
よって遅れ破壊特性を改善し、B:0.0003〜0.
0050%に制限することによって焼入れ性を確保し、
Cr:0.50〜1.20%にすることによって熱処理
肌での遅れ破壊特性を改善し、実部品になった時の遅れ
破壊特性を顕著に改善することができる。さらに、N:
0.0100%以下(0%を含む)に制限し、Ti:
0.020〜0.100%にすることによってTiC、
Ti(CN)を生成し、結晶粒の粗大化を防止するため
のピン止め粒子として利用することができる。マトリッ
クス中に直径0.2μm以下のTiC、Ti(CN)の
うち1種または2種の粒子の総個数を20個/100μ
m2 以上有することによってピン止め効果を最大限に発
揮させ、焼入れ加熱時の結晶粒の粗大化を防止するとと
もに旧オーステナイト結晶粒を微細化することができる
冷間鍛造用鋼である。The characteristics of the present invention are as follows: (1) C: 0.10
0.40%, Si: 0.15% or less, Mn: 0.30%
By setting it to 1.00%, the strength of the part after quenching and tempering is secured, and P: 0.015% or less (including 0%);
S: Improves delayed fracture characteristics by limiting the content to 0.015% or less (including 0%), and B: 0.0003-0.
By limiting to 0050%, hardenability is secured,
By setting the Cr content to 0.50 to 1.20%, the delayed fracture characteristic on the heat-treated skin can be improved, and the delayed fracture characteristic when a real part is obtained can be significantly improved. Further, N:
0.0100% or less (including 0%), Ti:
By making the content 0.020 to 0.100%, TiC,
It produces Ti (CN) and can be used as pinning particles for preventing crystal grains from becoming coarse. The total number of particles of one or two of TiC and Ti (CN) having a diameter of 0.2 μm or less in the matrix is 20 particles / 100 μm.
is maximize the pinning effect by having m 2 or more, a cold forging steel for the austenite grain can be miniaturized while preventing grain coarsening during heating for quenching.
【0012】(2)また本発明の他の特徴は、上記成分
に加えて、Nb:0.003〜0.100%を含有し、
マトリックス中に直径0.2μm以下のTiC、Ti
(CN)、NbC、Nb(CN)、(Nb、Ti)(C
N)のうち1種以上の粒子の総個数を20個/100μ
m2 以上有することによって結晶粒の粗大化を防止する
ことができる冷間鍛造用鋼である。(2) Another feature of the present invention is that, in addition to the above components, Nb: 0.003 to 0.100% is contained,
TiC, Ti having a diameter of 0.2 μm or less in the matrix
(CN), NbC, Nb (CN), (Nb, Ti) (C
N) The total number of one or more particles is 20 particles / 100 μ
a cold forging steel capable of preventing the coarsening of crystal grains by having m 2 or more.
【0013】(3)また本発明の他の特徴は、上記
(1)または(2)の成分に加えて、V:0.05〜
0.30%、Zr:0.003〜0.100%のうち1
種または2種を含有することによって旧オーステナイト
結晶粒をさらに微細化することができ、マトリックス中
に直径0.2μm以下のTiC、Ti(CN)、Nb
C、Nb(CN)、(Nb、Ti)(CN)のうち1種
以上の粒子の総個数を20個/100μm2 以上有する
ことによって結晶粒の粗大化を防止することができる冷
間鍛造用鋼である。(3) Another feature of the present invention is that, in addition to the component (1) or (2), V: 0.05 to
0.30%, Zr: 1 out of 0.003 to 0.100%
By containing one or two species, the prior austenite crystal grains can be further refined, and TiC, Ti (CN), Nb having a diameter of 0.2 μm or less are contained in the matrix.
C, Nb (CN), (Nb, Ti) (CN) For cold forging, wherein the total number of at least one type of particles is at least 20/100 μm 2, whereby coarsening of crystal grains can be prevented. It is steel.
【0014】(4)また本発明の他の特徴は、上記
(1)、(2)、(3)の成分よりなる鋼を1050℃
以上に加熱してTiC、Ti(CN)、NbC、Nb
(CN)、(Nb、Ti)(CN)を一旦マトリックス
中に固溶させ、線材または棒鋼に熱間圧延した後、60
0℃以下の温度まで冷却するに際して2℃/s以下の冷
却速度で徐冷して軟質化するとともに、マトリックス中
に直径0.2μm以下の微細なTiC、Ti(CN)、
NbC、Nb(CN)、(Nb、Ti)(CN)のうち
1種以上の粒子の総個数を20個/100μm2 以上分
散した鋼とする冷間鍛造用鋼の製造方法である。(4) Another feature of the present invention is that a steel comprising the above components (1), (2) and (3) is heated at 1050 ° C.
By heating as above, TiC, Ti (CN), NbC, Nb
(CN), (Nb, Ti) (CN) are once dissolved in a matrix and hot-rolled into a wire or a steel bar.
When cooled to a temperature of 0 ° C. or less, the material is gradually cooled at a cooling rate of 2 ° C./s or less to be softened, and fine TiC, Ti (CN) having a diameter of 0.2 μm or less in a matrix.
This is a method for producing a steel for cold forging, in which the total number of one or more particles of NbC, Nb (CN), and (Nb, Ti) (CN) is dispersed into 20/100 μm 2 or more.
【0015】[0015]
【発明の実施の形態】以下、本発明について詳細に説明
する。まず、成分の限定理由について説明する。Cは鋼
に必要な強度を与えるのに有効な元素であるが、0.1
0%未満では必要な引張強さを確保することができず、
0.40%を超えると冷間鍛造性が低下し、冷間鍛造前
の焼鈍、あるいは球状化焼鈍工程を省略することができ
ない。また、部品の延性、靱性が劣化し、さらには遅れ
破壊特性も劣化する傾向があるので、0.10〜0.4
0%の範囲内にする必要がある。好適範囲は0.20〜
0.30%である。BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. First, the reasons for limiting the components will be described. C is an element effective in giving the necessary strength to steel, but 0.1
If it is less than 0%, the required tensile strength cannot be secured,
If it exceeds 0.40%, the cold forgeability decreases, and the annealing before the cold forging or the spheroidizing annealing step cannot be omitted. In addition, the ductility and toughness of the parts tend to deteriorate, and the delayed fracture characteristics also tend to deteriorate.
It must be within the range of 0%. The preferred range is 0.20
0.30%.
【0016】Siは鋼の脱酸に有効な元素であるととも
に、鋼に必要な強度、焼入れ性を与え、焼戻し軟化抵抗
を向上するのに有効な元素であるが、0.15%を超え
ると靱性、延性が劣化し、硬さの上昇を招き冷間鍛造性
が劣化するので、0.15%以下の範囲内にする必要が
ある。好適範囲は0.10%以下である。Si is an element effective for deoxidizing steel, and is an element effective for imparting necessary strength and hardenability to steel and improving temper softening resistance. Since toughness and ductility are deteriorated, the hardness is increased, and the cold forgeability is deteriorated, it is necessary to be within a range of 0.15% or less. A preferred range is 0.10% or less.
【0017】Mnは鋼の脱酸に有効な元素であるととも
に、鋼に必要な強度、焼入れ性を与えるのに有効な元素
であるが、0.30%未満では効果は不十分であり、
1.00%を超えると硬さの上昇を招き冷間鍛造性が劣
化するので、0.30%〜1.00%の範囲にする必要
がある。好適範囲は0.40〜0.70%である。Mn is an element effective for deoxidizing steel and is an element effective for imparting necessary strength and hardenability to steel. However, if it is less than 0.30%, the effect is insufficient.
If it exceeds 1.00%, the hardness will increase and the cold forgeability will deteriorate, so it is necessary to be in the range of 0.30% to 1.00%. The preferred range is 0.40 to 0.70%.
【0018】Pは冷間鍛造時の変形抵抗を高め、靱性を
劣化させる元素であるため、冷間鍛造性が劣化する。ま
た、焼入れ、焼戻し後の部品の結晶粒界を脆化させるこ
とによって遅れ破壊特性を劣化させるのでできるだけ低
減することが望ましい。したがって、その含有量を0.
015%以下に制限する必要がある。好適範囲は0.0
10%以下である。P is an element that increases the deformation resistance during cold forging and deteriorates toughness, so that cold forgeability is deteriorated. Further, since the grain boundary of the quenched and tempered component is embrittled to degrade the delayed fracture characteristics, it is desirable to reduce as much as possible. Therefore, its content is reduced to 0.1.
It is necessary to limit to 015% or less. The preferred range is 0.0
10% or less.
【0019】Sは冷間鍛造時に割れを生じやすくする元
素であるため、冷間鍛造性が劣化する。また、Pと同様
に焼入れ、焼戻し後の部品の結晶粒界を脆化させること
によって遅れ破壊特性を劣化させるのでできるだけ低減
することが望ましい。したがって、その含有量を0.0
15%以下に制限する必要がある。好適範囲は0.01
0%以下である。Since S is an element that easily causes cracking during cold forging, the cold forgeability deteriorates. Further, similarly to P, the delayed fracture characteristics are deteriorated by embrittlement of the crystal grain boundaries of the parts after quenching and tempering, so that it is desirable to reduce as much as possible. Therefore, the content is 0.0
It must be limited to 15% or less. The preferred range is 0.01
0% or less.
【0020】Crは鋼に強度、焼入れ性を与え、焼戻し
軟化抵抗を向上するのに有効な元素であるとともに特に
熱処理肌における遅れ破壊特性を顕著に改善する元素で
ある。Crは熱処理時に生成されるスケールをCrの濃
化した緻密なスケールとし、耐食性を増すことでスケー
ルが腐食される過程で発生する水素量を低減させ、遅れ
破壊特性を向上する効果がある。引張強さ1350MP
a近傍に調質したときの遅れ破壊特性に及ぼすCr量の
影響を図1に示す。[0020] Cr is an element that imparts strength and hardenability to steel and is effective in improving temper softening resistance, and is also an element that significantly improves delayed fracture characteristics, particularly in heat-treated skin. Cr has the effect of improving the delayed fracture characteristics by reducing the amount of hydrogen generated in the course of corrosion of the scale by increasing the corrosion resistance by increasing the scale formed during the heat treatment to a dense scale in which Cr is concentrated. 1350MP tensile strength
FIG. 1 shows the effect of the amount of Cr on the delayed fracture characteristics when tempering near a.
【0021】図1は0.1N HCl中での試験結果で
あるが、1%H2 SO4 中でもほぼ同様の傾向を示す。
図1から明らかなように、熱処理肌における遅れ破壊特
性にはCr量の影響が大きく、0.50%未満では充分
な遅れ破壊特性の向上効果を得ることができず、1.2
0%を超えて添加すると硬さの上昇を招き冷間鍛造性が
劣化するだけではなく、熱処理時に発生する表層の粒界
酸化の発生を助長し、遅れ破壊特性がかえって劣化す
る。この傾向は部品強度が高くなるほど顕著に現れる。
したがって、Crの添加量は、0.50〜1.20%の
範囲内にする必要がある。好適範囲は0.60〜0.9
0%である。FIG. 1 shows the test results in 0.1N HCl, but shows almost the same tendency in 1% H 2 SO 4 .
As is clear from FIG. 1, the amount of Cr has a great influence on the delayed fracture characteristics on the heat-treated skin, and if it is less than 0.50%, a sufficient effect of improving the delayed fracture characteristics cannot be obtained.
Addition of more than 0% not only increases the hardness and deteriorates the cold forgeability, but also promotes the generation of grain boundary oxidation of the surface layer generated during the heat treatment, and rather deteriorates the delayed fracture characteristics. This tendency appears more remarkably as the component strength increases.
Therefore, the addition amount of Cr needs to be in the range of 0.50 to 1.20%. The preferred range is 0.60 to 0.9
0%.
【0022】Bは微量の添加で鋼に焼入れ性を与えるの
に有効な元素であるが、0.0003%未満ではその効
果は不十分であり、0.0050%を超えると効果は飽
和するので、0.0003〜0.0050%の範囲内に
する必要がある。好適範囲は0.0010〜0.003
0%である。B is an element effective for imparting hardenability to steel with a small amount of addition, but if its content is less than 0.0003%, its effect is insufficient, and if it exceeds 0.0050%, its effect is saturated. , 0.0003 to 0.0050%. The preferred range is 0.0010 to 0.003
0%.
【0023】NはBと結び付いてBNを生成し、Bの持
つ焼入れ性向上効果を低下させるため、本発明のような
B添加鋼では有害である。また、鋼中のTiと結び付く
とピン止めにほとんど寄与しない粗大なTiNを生成
し、Tiを含有する炭窒化物となりうるTi量が減少
し、微細な析出物の量が減少するため、できるだけ低減
することが望ましい。したがって、その含有量をできる
だけ低く抑えることが結晶粒の粗大化抑制のポイントで
あり、また後述のようにN量が少なければTiの添加量
も少なくて済む。しかし実際の製造工程でNを完全に除
くことは難しいため、その範囲を0.0100%以下と
定めた。好適範囲は0.0050%以下である。N is harmful to B-added steel as in the present invention because N combines with B to form BN and reduces the effect of B on improving the hardenability. In addition, when combined with Ti in steel, coarse TiN that hardly contributes to pinning is generated, the amount of Ti that can be carbonitride containing Ti is reduced, and the amount of fine precipitates is reduced. It is desirable to do. Therefore, keeping the content as low as possible is the point of suppressing the coarsening of the crystal grains, and as will be described later, the smaller the amount of N, the smaller the amount of Ti added. However, since it is difficult to completely remove N in an actual manufacturing process, the range is set to 0.0100% or less. A preferred range is 0.0050% or less.
【0024】Tiは鋼中のC、Nと結び付いてTiC、
Ti(CN)を形成し、結晶粒の微細化、および結晶粒
の粗大化抑制に有効な元素である。また、Bとともに添
加した場合、鋼中の固溶NをTiN、Ti(CN)の形
で固定することによってBNの生成を抑制し、Bによる
焼入れ性向上効果を得るのに有効な元素であるが、0.
020%未満では効果は不十分であり、0.100%を
超えるとその効果は飽和するのみならず硬さの上昇を招
き冷間鍛造性が劣化するので、0.020〜0.100
%の範囲内にする必要がある。好適範囲は0.025〜
0.50%である。もちろん、鋼中の固溶Nを全てTi
Nの形で固定するためには、N量に応じてTi量も増加
させる必要があるし、結晶粒界のピン止めに有効な微細
なTiC、Ti(CN)を十分な量確保するためにも、
N量に応じてTi量も増加させる必要がある。少なくと
も、3.4N%を超えるTiの添加が必要である。Ti combines with C and N in steel to form TiC,
It is an element that forms Ti (CN) and is effective in refining crystal grains and suppressing coarsening of crystal grains. Further, when added together with B, it is an effective element for suppressing the generation of BN by fixing solid solution N in the steel in the form of TiN and Ti (CN), and obtaining the effect of improving the hardenability by B. Is 0.
If it is less than 020%, the effect is insufficient. If it exceeds 0.100%, the effect is not only saturated, but also the hardness is increased and the cold forgeability is deteriorated.
% Must be within the range. The preferred range is 0.025 to
0.50%. Of course, all the solute N in steel
In order to fix in the form of N, it is necessary to increase the amount of Ti according to the amount of N. In order to secure a sufficient amount of fine TiC and Ti (CN) effective for pinning the crystal grain boundaries. Also,
It is necessary to increase the Ti amount according to the N amount. At least the addition of Ti exceeding 3.4 N% is necessary.
【0025】Nbは鋼中のC、Nと結び付いてNbC、
Nb(CN)、(Nb、Ti)(CN)を形成し、結晶
粒の微細化、および結晶粒の粗大化抑制に有効な元素で
ある。NbはTiとともに添加された場合、そのほとん
どが安定な(Nb、Ti)(CN)を形成し、安定した
ピン止め効果を得ることができるが、0.003%未満
では効果は不十分であり、0.100%を超えるとその
効果は飽和するのみならず硬さの上昇を招き冷間鍛造性
が劣化するので、0.003〜0.100%の範囲内に
する必要がある。好適範囲は0.005〜0.030%
である。Nb is combined with C and N in steel to form NbC,
Nb (CN) and (Nb, Ti) (CN) are formed and are effective elements for refining crystal grains and suppressing coarsening of crystal grains. When Nb is added together with Ti, most of them form stable (Nb, Ti) (CN) and can obtain a stable pinning effect, but if less than 0.003%, the effect is insufficient. If it exceeds 0.100%, the effect is not only saturated, but also the hardness is increased, and the cold forgeability is deteriorated. Therefore, it is necessary to be within the range of 0.003 to 0.100%. The preferred range is 0.005 to 0.030%
It is.
【0026】Vは鋼中のC、Nと結び付いてVC、VN
を形成し、結晶粒の微細化に有効な元素であるが、0.
05%未満では効果は不十分であり、0.30%を超え
るとその効果は飽和するのみならず硬さの上昇を招き冷
間鍛造性が劣化するので、0.05〜0.30%の範囲
内にする必要がある。好適範囲は0.10〜0.20%
である。V is combined with C and N in the steel to form VC, VN
And is an element effective for refining crystal grains.
If it is less than 05%, the effect is insufficient. If it exceeds 0.30%, the effect not only saturates, but also increases the hardness and deteriorates the cold forgeability. Must be within range. The preferred range is 0.10 to 0.20%
It is.
【0027】Zrは鋼中のC、Nと結び付いてZrC、
ZrNを形成し、結晶粒の微細化に有効な元素である
が、0.003%未満では効果は不十分であり、0.1
00%を超えるとその効果は飽和するのみならず硬さの
上昇を招き冷間鍛造性が劣化するので、0.003〜
0.100%の範囲内にする必要がある。好適範囲は
0.005〜0.030%である。Zr combines with C and N in steel to form ZrC,
It is an element that forms ZrN and is effective in refining crystal grains. However, if the content is less than 0.003%, the effect is insufficient.
If it exceeds 00%, the effect is not only saturated, but also the hardness is increased and the cold forgeability is deteriorated.
It must be within the range of 0.100%. The preferred range is 0.005 to 0.030%.
【0028】なお、V,Zrは本発明において必須の元
素ではないが、結晶粒の微細化の目的のため必要に応じ
添加することができる。Although V and Zr are not essential elements in the present invention, they can be added as needed for the purpose of making crystal grains fine.
【0029】本発明はAl添加量を規定していないが、
鋼の脱酸に有効な元素であるため、通常脱酸に使用され
るAl量を含有してもよい。通常のAl含有量は0.0
10〜0.050%程度である。但し、Alに代わる元
素(Si、Mn、Ti、Zr等)を脱酸剤として用いる
場合は必ずしもAlを添加しなくとも良い。Although the present invention does not specify the amount of Al added,
Since it is an element effective for deoxidizing steel, it may contain an Al amount usually used for deoxidizing. Normal Al content is 0.0
It is about 10 to 0.050%. However, when an element (such as Si, Mn, Ti, or Zr) instead of Al is used as the deoxidizing agent, it is not always necessary to add Al.
【0030】次にマトリックス中のTiC、Ti(C
N)、NbC、Nb(CN)、(Nb、Ti)(CN)
の分散状態について説明する。結晶粒の粗大化を抑制す
るには結晶粒界をピン止めする粒子を多量、微細に分散
させることが有効であり、粒子の直径が小さいほど、ま
た量が多いほどピン止め粒子の数が増加するため好まし
い。微細TiC、Ti(CN)と結晶粒粗大化温度との
関係を図2に示す。なお、NbC、Nb(CN)、(N
b、Ti)(CN)についても同様の効果があり、図2
の関係に従う。Next, TiC, Ti (C
N), NbC, Nb (CN), (Nb, Ti) (CN)
Will be described. In order to suppress the coarsening of the crystal grains, it is effective to disperse a large amount and finely of the particles that pin the crystal grain boundaries, and the number of the pinned particles increases as the diameter of the particles decreases or the amount increases. Is preferred. FIG. 2 shows the relationship between the fine TiC and Ti (CN) and the crystal grain coarsening temperature. Note that NbC, Nb (CN), (N
b, Ti) (CN) has the same effect.
Follow the relationship.
【0031】図2から明らかなように、結晶粒粗大化特
性と微細な析出粒子数には極めて密接な関連があり、マ
トリックス中に直径0.2μm以下のTiC、Ti(C
N)、NbC、Nb(CN)、(Nb、Ti)(CN)
のうち1種以上の粒子の総個数を20個/100μm2
以上分散させると実用上の焼入れ加熱、あるいは浸炭加
熱温度域において結晶粒の粗大化が生じず、優れた結晶
粒粗大化防止特性が得られるため、マトリックス中に直
径0.2μm以下のTiC、Ti(CN)、NbC、N
b(CN)、(Nb、Ti)(CN)のうち1種以上の
粒子の総個数が20個/100μm2 以上分散している
ことが必要である。As is evident from FIG. 2, there is a very close relationship between the coarsening characteristics of the crystal grains and the number of fine precipitate particles, and TiC and Ti (C
N), NbC, Nb (CN), (Nb, Ti) (CN)
The total number of one or more particles is 20 particles / 100 μm 2
When dispersed, the crystal grains are not coarsened in a practical quenching heating or carburizing heating temperature range, and excellent crystal grain coarsening prevention properties are obtained. Therefore, TiC or Ti having a diameter of 0.2 μm or less is contained in the matrix. (CN), NbC, N
It is necessary that the total number of at least one kind of particles of b (CN), (Nb, Ti) (CN) is dispersed at least 20 particles / 100 μm 2 .
【0032】次に製造条件について説明する。上記の本
発明成分からなる鋼を、転炉、電気炉等の通常の方法に
よって溶製し、成分調整を行い、鋳造工程、必要に応じ
て分塊圧延工程を経て圧延素材とする。なお、分塊圧延
工程の前に鋳片を1200〜1350℃程度の温度に数
時間保定する均熱拡散処理を行うと、P等の不純物元素
の偏析が軽減され、実部品での遅れ破壊特性が更に向上
するだけでなく、鋳造工程で析出する粗大な析出物が一
旦溶体化でき、次工程で析出物のマトリックスへの固溶
が容易になるため、この処理を行うとさらに特性が得ら
れる。Next, the manufacturing conditions will be described. The steel comprising the above-described component of the present invention is melted by a usual method such as a converter or an electric furnace, the components are adjusted, and the material is subjected to a casting process and, if necessary, a slab rolling process to obtain a rolled material. In addition, if the slab is subjected to soaking diffusion treatment in which the slab is kept at a temperature of about 1200 to 1350 ° C. for several hours before the slab rolling step, segregation of impurity elements such as P is reduced, and delayed fracture characteristics in actual parts are reduced. In addition to the further improvement, coarse precipitates precipitated in the casting step can be once solutionized, and the precipitates can be easily dissolved in the matrix in the next step, so that further characteristics can be obtained by performing this treatment. .
【0033】次に、圧延素材を1050℃以上の温度で
加熱する。加熱条件は、1050℃未満ではTiC、T
i(CN)、NbC、Nb(CN)、(Nb、Ti)
(CN)を一旦マトリックス中に固溶させることができ
ず、熱間圧延後にTiC、Ti(CN)、NbC、Nb
(CN)、(Nb、Ti)(CN)のうち1種以上の粒
子を微細析出した鋼とすることができない。また固溶さ
せることができなかった粗大なTiC、Ti(CN)、
NbC、Nb(CN)、(Nb、Ti)(CN)は、多
量に存在すると部品の延性を劣化させ、遅れ破壊特性に
も悪影響を及ぼす。更に、粗大な析出物が多く残存して
いると圧延後の冷却時に析出核として働き、さらに粗大
に成長するため、ピン止め粒子をマトリックス中に微細
に分散させることが困難になる。従って、加熱温度はで
きるだけ高温にすることが望ましい。好適範囲は115
0℃以上である。Next, the rolled material is heated at a temperature of 1050 ° C. or higher. The heating conditions are as follows: TiC, T
i (CN), NbC, Nb (CN), (Nb, Ti)
(CN) cannot be dissolved in the matrix once, and after hot rolling, TiC, Ti (CN), NbC, Nb
It is not possible to use a steel in which one or more particles of (CN) and (Nb, Ti) (CN) are finely precipitated. Coarse TiC, Ti (CN) that could not be dissolved,
NbC, Nb (CN), and (Nb, Ti) (CN), when present in large amounts, degrade the ductility of the component and adversely affect delayed fracture characteristics. Furthermore, if many coarse precipitates remain, they act as precipitation nuclei during cooling after rolling and grow more coarsely, making it difficult to finely disperse the pinned particles in the matrix. Therefore, it is desirable that the heating temperature be as high as possible. The preferred range is 115
0 ° C. or higher.
【0034】次に、1050℃以上に加熱した圧延素材
を線材または棒鋼形状に熱間圧延した後、600℃以下
の温度まで冷却するに際して2℃/s以下の冷却速度で
徐冷する。冷却条件は、2℃/sを超えるとTiC、T
i(CN)、NbC、Nb(CN)、(Nb、Ti)
(CN)の析出温度域を短時間しか通過させることがで
きず、析出量が不十分となり、ピン止め粒子として有効
なTiC、Ti(CN)、NbC、Nb(CN)、(N
b、Ti)(CN)を多量・微細析出した鋼とすること
ができない。また、冷却速度が大きいと圧延材の硬さが
上昇し、冷間鍛造性が劣化するため、冷却速度はできる
だけ小さくするのが望ましい。好適範囲は1℃/s以下
である。なお、熱間圧延後にさらに低い温度域(500
℃以下)まで2℃/sの冷却速度で徐冷するのが好まし
い。低い温度域まで徐冷すると圧延材がさらに軟質化
し、冷間鍛造性が向上する。Next, after the rolled material heated to 1050 ° C. or more is hot-rolled into a wire or a steel bar, the material is gradually cooled at a cooling rate of 2 ° C./s or less when cooled to a temperature of 600 ° C. or less. When the cooling condition exceeds 2 ° C./s, TiC, T
i (CN), NbC, Nb (CN), (Nb, Ti)
(C) can only pass through the precipitation temperature range for a short time, the amount of precipitation becomes insufficient, and TiC, Ti (CN), NbC, Nb (CN), (N
b, Ti) (CN) cannot be made into steel with a large amount and fine precipitation. Further, if the cooling rate is high, the hardness of the rolled material increases, and the cold forgeability deteriorates. Therefore, it is desirable to reduce the cooling rate as much as possible. A preferred range is 1 ° C./s or less. In addition, after hot rolling, a lower temperature range (500
(Lower than or equal to ° C) at a cooling rate of 2 ° C / s. When gradually cooled to a low temperature range, the rolled material is further softened, and the cold forgeability is improved.
【0035】[0035]
【実施例】以下に、実施例により本発明をさらに説明す
る。表1に示す組成を有する転炉溶製鋼を連続鋳造し、
必要に応じて均熱拡散処理工程、分塊圧延工程を経て1
62mm角の圧延素材とした。続いて圧延素材を105
0℃以上の温度で加熱し、直径5〜50mmの棒鋼、線
材に熱間圧延した。一部は比較のために加熱温度を10
50℃以下とした。次に、圧延ラインの後方に設けた保
温カバーを使用し、徐冷を行った。一部は比較のために
徐冷を行わなかった。The present invention will be further described below with reference to examples. Continuously cast converter steel having the composition shown in Table 1,
If necessary, pass through the soaking diffusion process
A 62 mm square rolled material was used. Next, rolled material was added to 105
It was heated at a temperature of 0 ° C. or more, and hot-rolled into a steel bar or a wire having a diameter of 5 to 50 mm. Some have a heating temperature of 10 for comparison.
The temperature was set to 50 ° C. or less. Next, slow cooling was performed using a heat retaining cover provided behind the rolling line. Some did not cool slowly for comparison.
【0036】[0036]
【表1】 [Table 1]
【0037】ピン止め粒子として有効なTiC、Ti
(CN)、NbC、Nb(CN)、(Nb、Ti)(C
N)の分散状態は、棒鋼、線材のマトリックス中に存在
する析出物を抽出レプリカ法によって採取し、透過型電
子顕微鏡で観察することによって測定した。観察方法は
15000倍で20視野程度観察し、1視野中の直径
0.2μm以下のTiC、Ti(CN)、NbC、Nb
(CN)、(Nb、Ti)(CN)の総数を数え、10
0μm2 あたりの数に換算した。TiC, Ti effective as pinning particles
(CN), NbC, Nb (CN), (Nb, Ti) (C
The dispersion state of N) was measured by extracting precipitates present in the matrix of the steel bar and the wire rod by the extraction replica method and observing them with a transmission electron microscope. The observation method is to observe about 20 visual fields at 15000 magnifications, and TiC, Ti (CN), NbC, Nb having a diameter of 0.2 μm or less in one visual field.
The total number of (CN), (Nb, Ti) (CN) was counted, and 10
It was converted to a number per 0 μm 2 .
【0038】上記の工程で製造した線材または棒鋼の結
晶粒粗大化温度を測定した。圧延材に減面率70%の冷
間引き抜き加工を行った後、840〜1200℃に30
分間加熱−水焼入れした。その後、切断面に研磨−腐食
を行い、旧オーステナイト粒径を観察して粗粒発生温度
(結晶粒粗大化温度)を求めた。[0038] The crystal grain coarsening temperature of the wire rod or the steel bar manufactured in the above process was measured. After subjecting the rolled material to cold drawing with a surface reduction rate of 70%, the temperature is reduced to 840 to 1200 ° C. by 30%.
Heat-water quenched for minutes. Thereafter, the cut surface was polished and corroded, and the old austenite grain size was observed to determine the coarse grain generation temperature (crystal grain coarsening temperature).
【0039】ボルト等の実部品の焼入れ工程ではAc3〜
900℃の温度域で行われることが多いため、粗粒発生
温度が900℃未満のものは結晶粒粗大化特性に劣ると
判定した。なお、旧オーステナイト粒度の測定はJIS
G 0551に準じて行い、400倍で10視野程度
観察し、粒度番号5番以下の粗粒が1つでも存在すれば
粗粒発生と判定した。In the quenching process of actual parts such as bolts, A c3 ~
Since it is often carried out in a temperature range of 900 ° C., those having a coarse grain generation temperature of less than 900 ° C. were judged to be inferior in crystal grain coarsening characteristics. The measurement of the prior austenite grain size is based on JIS
The measurement was performed according to G 0551, and the observation was performed at a magnification of about 400 for about 10 visual fields.
【0040】次にこれらの材料の遅れ破壊特性を調査す
るため、70%の冷間引き抜き加工を行った材料を環状
Vノッチ付きの遅れ破壊試験片に加工した。その後90
0℃×30分−焼入れ、その後焼戻しを行い、引張強さ
1350MPa級に調質し、実部品の表面肌に近い、熱
処理肌の遅れ破壊試験片を製作した。この遅れ破壊試験
片を0.1N HCl中に浸漬し、負荷応力を変化させ
て破断までの時間を測定した。試験時間は最大200時
間とし、200時間破断しない最大の負荷応力を測定し
た。200時間破断しない最大の負荷応力を大気中での
破断応力で割った値を「遅れ破壊強度比」と定義し、遅
れ破壊特性の指標とした。Next, in order to investigate the delayed fracture characteristics of these materials, a material subjected to cold drawing of 70% was processed into a delayed fracture test piece having an annular V notch. Then 90
0 ° C. × 30 minutes-quenching, followed by tempering, tempering to a tensile strength of 1350 MPa class, and production of a delayed fracture test piece of heat-treated skin close to the surface skin of the actual part. The delayed fracture test piece was immersed in 0.1N HCl, and the time until fracture was measured while changing the applied stress. The test time was a maximum of 200 hours, and the maximum applied stress that did not break for 200 hours was measured. The value obtained by dividing the maximum applied stress that does not break for 200 hours by the breaking stress in the atmosphere was defined as “delayed fracture strength ratio” and used as an index of delayed fracture characteristics.
【0041】現在、引張強さが1000〜1400MP
a級の部品に多く使われているSCM435の遅れ破壊
強度比が0.5程度であることから、遅れ破壊強度比が
0.5未満のものは遅れ破壊特性に劣ると判断した。他
方で、遅れ破壊試験に供した試験片の結晶粒度を調査し
た。整粒の場合はマトリックスの平均粒度を測定し、混
粒の場合、あるいは粗大粒がある場合は観察視野中の最
大粒の粒度番号も測定した。なお、旧オーステナイト粒
度の測定は上記の結晶粒粗大化温度の調査と同じ方法で
行った。At present, the tensile strength is 1000-1400MP
Since the delayed fracture strength ratio of SCM435, which is often used for a-class parts, is about 0.5, it was determined that the delayed fracture strength ratio of less than 0.5 was inferior to the delayed fracture characteristic. On the other hand, the grain size of the test piece subjected to the delayed fracture test was examined. In the case of sizing, the average particle size of the matrix was measured, and in the case of mixed or coarse particles, the particle size number of the largest particle in the observation visual field was also measured. The measurement of the prior austenite grain size was performed in the same manner as in the above-described investigation of the crystal grain coarsening temperature.
【0042】[0042]
【表2】 [Table 2]
【0043】これらの各種試験結果を表2、表3、表4
に示す。表2の記号N、記号OはTiまたはN量が本発
明の範囲から外れているため、微細TiC、Ti(C
N)、NbC、Nb(CN)、(Nb、Ti)(CN)
の析出数が不足し、結晶粒粗大化特性が劣化している。
記号V、記号X、記号Yは圧延加熱温度が低いためTi
C、Ti(CN)、NbC、Nb(CN)、(Nb、T
i)(CN)を一旦マトリックス中に固溶させることが
できず、熱間圧延後の冷却時に析出物を微細析出した鋼
とすることができないため、結晶粒粗大化特性が劣化し
ている。Tables 2, 3 and 4 show the results of these various tests.
Shown in Symbols N and O in Table 2 indicate that the amount of Ti or N is out of the range of the present invention.
N), NbC, Nb (CN), (Nb, Ti) (CN)
Is insufficient, and the crystal grain coarsening characteristics are deteriorated.
The symbols V, X, and Y are Ti
C, Ti (CN), NbC, Nb (CN), (Nb, T
i) Since (CN) cannot be once dissolved in the matrix and cannot be made into steel in which precipitates are finely precipitated upon cooling after hot rolling, the crystal grain coarsening characteristics are deteriorated.
【0044】また、記号W、記号Zは圧延後の冷却速度
が大きすぎるため、微細析出物の析出量が不十分とな
り、結晶粒粗大化特性が劣化している。Further, the symbols W and Z indicate that the cooling rate after rolling is too high, so that the amount of fine precipitates becomes insufficient and the crystal grain coarsening characteristics are deteriorated.
【0045】[0045]
【表3】 [Table 3]
【0046】[0046]
【表4】 [Table 4]
【0047】表3は表2の圧延材を1350MPa程度
に、表4は1200MPa程度に調質したときの遅れ破
壊特性を示したものである。表3の記号P、記号Q、記
号TはCr添加量が本発明の範囲から外れているため、
遅れ破壊特性が劣化している。記号R、記号SはP量ま
たはS量が本発明の範囲から外れているため、遅れ破壊
特性が劣化している。なお、結晶粒粗大化特性に劣るも
の(記号N、O、V、W、X、Y、Z)は遅れ破壊試験
片に粗大粒が発生しているため、遅れ破壊特性も劣化し
ている。表4は引張強さが1200MPa程度であるた
め、表3よりも遅れ破壊特性が向上している。なお、表
1の成分番号21、表2、表3の記号Uは現在多く使用
されている焼鈍を省略できない合金鋼の例である。これ
らの表から明らかなように、本発明で規定する条件を全
て満たすものは比較例に比べて結晶粒粗大化防止特性お
よび耐遅れ破壊特性ともに優れた特性を示している。Table 3 shows the delayed fracture characteristics when the rolled material in Table 2 was tempered to about 1350 MPa and Table 4 was tempered to about 1200 MPa. Symbols P, Q, and T in Table 3 indicate that the amount of Cr added is out of the range of the present invention.
The delayed fracture characteristics have deteriorated. In the symbols R and S, since the P amount or the S amount is out of the range of the present invention, the delayed fracture characteristics are deteriorated. Incidentally, those having poor crystal grain coarsening properties (symbols N, O, V, W, X, Y, and Z) have deteriorated delayed fracture properties because coarse grains are generated in the delayed fracture test specimen. In Table 4, since the tensile strength is about 1200 MPa, the delayed fracture characteristics are improved as compared with Table 3. The component number 21 in Table 1 and the symbol U in Tables 2 and 3 are examples of alloy steels that are currently frequently used and for which annealing cannot be omitted. As is clear from these tables, those satisfying all of the conditions specified in the present invention show superior properties in both the grain coarsening prevention property and the delayed fracture resistance property as compared with the comparative example.
【0048】[0048]
【発明の効果】本発明の冷間鍛造用鋼およびその製造方
法を用いれば、冷間鍛造前の焼鈍を省略でき、かつ熱処
理時の結晶粒粗大化による焼入れ歪みによる寸法精度の
劣化、衝撃値、疲労強度の低下が従来より少なく、しか
も熱処理肌で使用される実部品での遅れ破壊特性に特に
優れたボルト、ギア部品、シャフト等の素材を提供する
ことができる。According to the steel for cold forging and the method for producing the same of the present invention, annealing before cold forging can be omitted, and dimensional accuracy deterioration and impact value due to quenching distortion due to coarsening of crystal grains during heat treatment. Further, it is possible to provide a material such as a bolt, a gear component, and a shaft, which has less decrease in fatigue strength than the conventional one and has particularly excellent delayed fracture characteristics in an actual component used on a heat-treated skin.
【図1】熱処理肌での遅れ破壊特性に及ぼすCr量の影
響について解析した一例を示す図FIG. 1 is a diagram showing an example of analyzing the effect of the amount of Cr on delayed fracture characteristics on heat-treated skin
【図2】焼入れ加熱前の鋼のマトリックス中の微細Ti
C、Ti(CN)の総個数と結晶粗大化温度の関係につ
いて解析した一例を示す図FIG. 2 Fine Ti in matrix of steel before quenching and heating
The figure which shows an example which analyzed the relationship between the total number of C and Ti (CN) and the crystal coarsening temperature.
───────────────────────────────────────────────────── フロントページの続き (72)発明者 蟹澤 秀雄 北海道室蘭市仲町12番地 新日本製鐵株式 会社室蘭製鐵所内 (72)発明者 村上 敦 埼玉県和光市中央一丁目4番1号 株式会 社本田技術研究所内 (72)発明者 石田 正雄 埼玉県和光市中央一丁目4番1号 株式会 社本田技術研究所内 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hideo Kanisawa 12 Nakamachi, Muroran-shi, Hokkaido Inside Nippon Steel Corporation Muroran Works (72) Inventor Atsushi Murakami 1-4-1 Chuo, Wako-shi, Saitama Stock Company Inside the Honda R & D Center (72) Inventor Masao Ishida 1-4-1 Chuo, Wako-shi, Saitama Pref.
Claims (4)
なり、かつ鋼のマトリックス中に直径0.2μm以下の
TiC、Ti(CN)のうち1種または2種の粒子の総
個数を20個/100μm2 以上有することを特徴とす
る結晶粒粗大化防止特性と耐遅れ破壊特性に優れた冷間
鍛造用鋼。C .: 0.10 to 0.40%, Si: 0.15% or less, Mn: 0.30 to 1.00%, Cr: 0.50 to 1.20% by weight%, B: 0.0003 to 0.0050%, Ti: 0.020 to 0.100%, P: 0.015% or less (including 0%), S: 0.015% or less (0% , N: 0.0100% or less (including 0%), and the balance is Fe and unavoidable impurities, and TiC, Ti (CN) having a diameter of 0.2 μm or less in a steel matrix. ), Wherein the total number of one or two kinds of particles is at least 20 particles / 100 μm 2 or more.
なり、かつ鋼のマトリックス中に直径0.2μm以下の
TiC、Ti(CN)、NbC、Nb(CN)、(N
b、Ti)(CN)のうち1種以上の粒子の総個数を2
0個/100μm2以上有することを特徴とする結晶粒
粗大化防止特性と耐遅れ破壊特性に優れた冷間鍛造用
鋼。2. In% by weight, C: 0.10 to 0.40%, Si: 0.15% or less, Mn: 0.30 to 1.00%, Cr: 0.50% to 1.20% , B: 0.0003 to 0.0050%, Ti: 0.020 to 0.100%, Nb: 0.003 to 0.100%, and P: 0.015% or less ( S: 0.015% or less (including 0%), N: 0.0100% or less (including 0%), and the balance consists of Fe and unavoidable impurities, and TiC, Ti (CN), NbC, Nb (CN), (N
b, Ti) (CN), the total number of one or more particles is 2
A cold forging steel excellent in the crystal grain coarsening prevention characteristic and the delayed fracture resistance characteristic characterized by having 0/100 μm 2 or more.
可避的不純物よりなり、かつ鋼のマトリックス中に直径
0.2μm以下のTiC、Ti(CN)、NbC、Nb
(CN)、(Nb、Ti)(CN)のうち1種以上の粒
子の総個数を20個/100μm2 以上有することを特
徴とする結晶粒粗大化防止特性と耐遅れ破壊特性に優れ
た冷間鍛造用鋼。3. In addition to the steel component according to claim 1 or 2, further contains one or two of V: 0.05 to 0.30%, Zr: 0.003 to 0.100%, and the balance Is composed of Fe, unavoidable impurities, and TiC, Ti (CN), NbC, Nb having a diameter of 0.2 μm or less in a steel matrix.
(CN), (Nb, Ti), (CN), wherein the total number of one or more particles is at least 20 particles / 100 μm 2 or more. Steel for forging.
050℃以上に加熱して線材または棒鋼に熱間圧延した
後、600℃以下の温度まで冷却するに際して、2℃/
s以下の冷却速度で徐冷し、マトリックス中に直径0.
2μm以下のTiC、Ti(CN)、NbC、Nb(C
N)、(Nb、Ti)(CN)のうち1種以上の粒子の
総個数を20個/100μm2 以上分散した鋼とするこ
とを特徴とする結晶粒粗大化防止特性と耐遅れ破壊特性
に優れた冷間鍛造用鋼の製造方法。4. The steel component according to claim 1 or 2 or 3,
After being heated to 050 ° C or higher and hot-rolled to a wire or steel bar, when cooled to a temperature of 600 ° C or lower, 2 ° C /
Slow cooling at a cooling rate of not more than s
TiC, Ti (CN), NbC, Nb (C
N), a steel in which the total number of one or more particles of (Nb, Ti) and (CN) is dispersed at least 20 particles / 100 μm 2 , characterized in that the crystal grain coarsening prevention property and the delayed fracture resistance are improved. Excellent cold forging steel manufacturing method.
Priority Applications (2)
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JP15367498A JP3490293B2 (en) | 1997-07-23 | 1998-05-20 | Cold forging steel excellent in crystal grain coarsening prevention property and delayed fracture resistance, and its manufacturing method |
US09/314,733 US6261388B1 (en) | 1998-05-20 | 1999-05-18 | Cold forging steel having improved resistance to grain coarsening and delayed fracture and process for producing same |
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JP9-211251 | 1997-07-23 | ||
JP21125197 | 1997-07-23 | ||
JP15367498A JP3490293B2 (en) | 1997-07-23 | 1998-05-20 | Cold forging steel excellent in crystal grain coarsening prevention property and delayed fracture resistance, and its manufacturing method |
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JP2008081841A (en) * | 2006-08-28 | 2008-04-10 | Kobe Steel Ltd | Case hardening steel having excellent cold forgeability and crystal grain coarsening prevention property, and machine part obtained therefrom |
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