JP3875605B2 - High strength steel with excellent cold workability and delayed fracture resistance - Google Patents

Description

【００７０】
【表２】

【００７１】
【表３】

【００７２】
【表４】

【００７３】
【表５】

【００７４】
【表６】

【００７５】
【表７】

【００７６】
［表１及び表４について］
試験Ｎｏ．１で使用した鋼Ａは従来のＳＣＭ４３５鋼に相当する。このＮｏ．１の試験結果より明らかなように、ＳＣＭ４３５鋼はＭｏ，Ｔｉ，Ｖの含有量が低く、耐遅れ破壊特性が悪い。また試験Ｎｏ．２で使用した鋼Ｂは従来のＳ４５Ｃ鋼に相当しており、前記ＳＣＭ４３５鋼と対比するとＣｒ含有量が少ない点で最も大きく相違する。そして試験Ｎｏ．１とＮｏ．２の対比より明らかなように、Ｍｏ、Ｔｉ、Ｖが少ない場合、Ｃｒを少なくしても冷間加工性は悪化しない。
【００７７】
試験Ｎｏ．３〜７は、Ｍｏ、Ｔｉ、Ｖのうち２種を複合添加した例である。すなわち試験Ｎｏ．３ではＭｏ、Ｖを添加した鋼Ｃを使用しているが、Ｔｉが少ないために高負荷応力下や高温下での耐遅れ破壊特性が不十分である。試験Ｎｏ．４ではＭｏ、Ｔｉを添加した鋼Ｄを使用しているが、Ｖが少ないために耐遅れ破壊特性が不十分である。試験Ｎｏ．５〜７ではＴｉとＶを添加した鋼Ｅ〜Ｇを使用しているが、Ｍｏが少ないために引張強さが高くならない。
【００７８】
試験Ｎｏ．８の鋼Ｈでは、Ｍｏ、Ｔｉ、Ｖの三成分を複合添加している。しかしＴｉ量がＶ量に比べて少なすぎ、Ｔｉ／Ｖが小さすぎるために、高負荷応力下や高温下での耐遅れ破壊特性が不十分である。
【００７９】
試験Ｎｏ．９〜１０で使用する鋼Ｉ，Ｊでは、Ｍｏ、Ｔｉ、Ｖの三成分を複合添加し、Ｔｉ／Ｖの値を適切に調節している。しかしＣｒを削減していないため、高負荷応力下での耐遅れ破壊特性が不十分である。
【００８０】
試験Ｎｏ．１１〜１２で使用する鋼Ｋ〜Ｍでは、Ｍｏ、Ｔｉ、Ｖの三成分を複合添加し、Ｔｉ／Ｖの値を適切に設定し、さらにはＣｒを削減している。そのため耐遅れ破壊特性に優れる。ところがＣｒを過度に削減しているため、冷間加工性が悪い。
【００８１】
［表２及び表５について］
試験Ｎｏ．１４〜２５では、Ｍｏ、Ｔｉ、Ｖの三成分を複合添加し、Ｔｉ／Ｖの範囲を適切に設定し、さらにはＣｒの削減量を適切な範囲に設定した鋼Ｎ〜Ｙを使用している。このような鋼Ｎ〜Ｙは、高負荷応力下や高温下であっても耐遅れ破壊特性に優れ、しかも冷間加工性にも優れる。
【００８２】
［表３及び表６について］
ただし、試験Ｎｏ．２６〜２８より明らかなように、Ｍｏ、Ｔｉ、Ｖなどを過剰に添加し過ぎると、冷間加工性が悪くなり、さらには耐遅れ破壊特性も悪くなる場合がある。そのためＭｏ、Ｔｉ、Ｖは適量を添加する必要がある。
【００８３】
また試験Ｎｏ．２９〜３０より明らかなように、Ｃが不足すると引張強さが確保できず（試験Ｎｏ．２９）、Ｃが過剰になると冷間加工性が低下すると共に耐遅れ破壊特性も悪化する（試験Ｎｏ．３０）。そのためＣ量も適切な範囲から選択する必要がある。
【００８４】
［表７について］
なお耐遅れ破壊特性及び冷間加工性にも優れる前記鋼Ｎ〜Ｙを使用する場合であっても、焼入れ・焼戻し条件は適切に設定する必要がある。すなわち試験Ｎｏ．３２より明らかなように焼入れ温度が低すぎるとＭｏ、Ｔｉ、Ｖなどの析出硬化型合金元素を十分に固溶させることができないため、引張強さが低下する。試験Ｎｏ．３３より明らかなように焼入れ温度が高すぎると、結晶粒が粗大化するため、耐遅れ破壊特性が低下する。また試験Ｎｏ．３４より明らかなように焼戻し温度が低すぎるとＭｏ，Ｔｉ，Ｖなどの微細炭化物が析出しないため、遅れ破壊特性が低下する。これらに対して、試験Ｎｏ．１４及び３１は、焼入れ温度及び焼戻し温度が適切であるため、耐遅れ破壊特性及び冷間加工性に優れている。
【００８５】
［図３について］
図３は上記試験Ｎｏ．１〜３４の全ての結果に基づき、鋼成分［Ｃ×（Ｍｏ＋４Ｔｉ＋２Ｖ）］と変形抵抗との関係をプロットしたものである。この図３より明らかなように、Ｍｏ、Ｔｉ、Ｖ量が少ない場合、より具体的にはＣ×（Ｍｏ＋４Ｔｉ＋２Ｖ）が約０〜０．３程度の場合には、Ｃｒ量が少ない場合（図中の×印及び破線参照）とＣｒ量が多い場合（図中の○印及び実線参照）とで、変形抵抗（冷間加工性）に大きな違いはない。ところがＭｏ、Ｔｉ、Ｖ量が多くなると、より具体的にはＣ×（Ｍｏ＋４Ｔｉ＋２Ｖ）が約０．４％以上になると、Ｃｒ量が少ない場合（図中の×印及び破線参照）は、Ｃｒ量が多い場合（図中の○印及び実線参照）に比べて冷間加工性が著しく悪くなる。
【００８６】
また前記図３より明らかなようにＣ×（Ｍｏ＋４Ｔｉ＋２Ｖ）が大きくなり過ぎると、Ｃｒ量が適切であっても冷間加工性が悪くなる。
【００８７】
【発明の効果】
本発明の高強度鋼（冷間加工用高強度鋼など）は、化学成分が所定の範囲に制御されているため、冷間加工性に優れており、さらには熱負荷や応力負荷が加わるような過酷な環境ですら耐遅れ破壊特性に優れている。
【００８８】
本発明の高強度鋼部品は、前記高強度鋼から製造されているため冷間加工時に金型工具寿命を劣化させることなく歩留まりよく得ることができ、しかも耐遅れ破壊特性にも優れている。
【００８９】
また前記高強度鋼及び高強度鋼部品は、Ｃ量を高めに設定して強度を高めた場合であっても、冷間加工性と耐遅れ破壊特性の両方に優れている。
【図面の簡単な説明】
【図１】図１は実験例の引張試験片を示す概略平面図である。
【図２】図２は実験例の遅れ破壊試験片を示す概略平面図である。
【図３】図３は鋼成分［Ｃ×（Ｍｏ＋４Ｔｉ＋２Ｖ）］と変形抵抗との関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to high-strength steel and high-strength steel parts useful for producing wire-drawn parts such as high-strength bolts, high-strength springs, high-strength PC steel wires, and high-strength reinforcing bars, and particularly has a tensile strength of 1300 N. / Mm²Even with the above, the present invention relates to high-strength steel and high-strength steel parts that are excellent in cold workability and delayed fracture resistance.
[0002]
[Prior art]
Conventionally, 10-12T (about 980-1372 N / mm²Grade) steel for high-strength bolts: SCr430 steel (C: 0.27 to 0.43%, Si: 0.15 to 0.35%, Mn: 0.55 to 0.90%, P: 0.030% or less, S: 0.030% or less, Cr: 0.85 to 1.25%), SCM435 steel (C: 0.32 to 0.39%, Si: 0.15 to 0.35%) Mn: 0.55 to 0.90%, P: 0.030% or less, S: 0.030% or less, Cr: 0.85 to 1.25%, Mo: 0.15 to 0.35%) SCM440 steel (C: 0.42-0.49%, Si: 0.15-0.35%, Mn: 0.55-0.90%, P: 0.030% or less, S: 0.030 % Or less, Cr: 0.85 to 1.25%, Mo: 0.15 to 0.35%) or the like (hereinafter referred to as the above-mentioned SCr430 steel, SCM4) 5 steel, and SCM440 steel and referred to as SCr430 steel, etc.). However, in general, high-strength bolts have a tensile strength of 1200 N / mm.²If it exceeds the limit, delayed fracture tends to occur, so there are restrictions on its use. Various methods have been proposed to improve delayed fracture characteristics.
[0003]
For example, in JP-A-60-114551, C: 0.30 to 0.50%, Si: less than 0.15%, Mn: 0.10 to 0.40%, P: 0.015% or less, S: 0.010% or less, Cr: 0.50 to 4.50%, Mo: 0.10 to 0.70% or less, and Si + Mn + 10 (P + S): 0.45% or less Propose to control. Compared with the above-mentioned SCr430 steel, etc., the greatest feature is that it has low Si and low Mn. This publication states that Si and Mn are harmful to delayed fracture resistance.
[0004]
In JP-A-2-267243, C: 0.18 to 0.35%, Si: 0.50 to 1.50%, Mn: 0.20 to 0.60%, Cr: It is proposed to control to 1.50 to 3.50% and Al: 0.008 to 0.070%. Compared with the above-mentioned SCr430 steel and the like, the greatest feature is that the amount of Si and Cr is large. In this publication, the limit of diffusible hydrogen that does not lead to delayed fracture can be increased by increasing Si and Cr.
[0005]
In JP-A-3-243745, in steel for machine structure, C: 0.35 to 0.50%, Si: 0.20% or less, Mn: 0.35% or less, P: 0.012% or less, S: 0.01% or less, Ni: 1.0 to 3.0%, Cr: 0.25% or less, Mo: 0.40 to 1.5%, V: 0.05 to 0.50%, Nb : 0.005 to 0.20%, and Al: 0.005 to 0.10%. Compared with the above-mentioned SCr430 steel, etc., it is characterized by low Mn, low Cr, high Mo, and the addition of Ni, Nb, V and the like. In this publication, by reducing Mn and Cr, grain boundary segregation can be remarkably reduced and grain boundaries can be strengthened, so that delayed fracture resistance can be improved, and Ni, Mo, V and Nb are added in combination. According to the report, fine graining can be promoted and grain boundary segregation can be reduced, so that delayed fracture resistance can be improved.
[0006]
However, delayed fracture occurs in a non-corrosive environment and in a corrosive environment, and it is difficult to identify the true cause because various factors are involved in a complex manner. In other words, as described above, reduction of grain boundary segregation and involvement of various elements have been pointed out as factors affecting delayed fracture characteristics, but tempering temperature, structure, material hardness, etc. have also been pointed out. Therefore, it is difficult to establish a true delayed fracture prevention means, and various methods are merely employed by trial and error in order to improve the delayed fracture resistance of various steel parts. For example, Japanese Patent Laid-Open No. 60-114551 recommends low Si, whereas Japanese Patent Laid-Open No. 2-267243 recommends high Si, which is contradictory. The above Japanese Patent Laid-Open No. 2-267243 recommends a high Cr content, whereas the Japanese Patent Laid-Open No. 3-243745 recommends a low Cr content, which conflicts.
[0007]
In addition, the inventors of the present invention have been disclosed in Materials and Processes, Vol. 14, 2001, 1308 [CAMPS-ISI Vol. 14 (2001) -1308], V can trap a large amount of diffusible hydrogen, but hydrogen trapping action is weak and hydrogen is released in a heat load environment. However, Ti carbonitride is a V carbonitride due to heat load. Retraps the hydrogen released from the trap and suppresses delayed fracture. Therefore, the combined addition of V-Ti improves delayed fracture resistance and further improves delayed fracture resistance in environments where thermal loads are applied. Announcing to do.
[0008]
[Problems to be solved by the invention]
An object of the present invention is to improve delayed fracture resistance and maintain cold workability even in an environment in which a heat load or stress load is applied to high strength steel and high strength steel parts.
[0009]
Another object of the present invention is to provide a high-strength steel part that is obtained with a good yield without deteriorating the tool life during cold working and has excellent delayed fracture resistance.
[0010]
Another object of the present invention is to provide high-strength steel and high-strength steel parts that are excellent in both delayed fracture resistance and cold workability even if the strength is further increased as compared with conventional steel.
[0011]
[Means for Solving the Problems]
In the process of earnestly researching to solve the above problems, the present inventors have made SCr445 steel (C: 0.42) having a larger amount of C than the above-mentioned SCr430 steel in order to achieve higher strength. To 0.49%, Si: 0.15 to 0.35%, Mn: 0.55 to 0.90%, P: 0.030% or less, S: 0.030% or less, Cr: 0.85 to We decided to proceed with development based on 1.25%), and first focused on the combined addition technology of Mo and V. When Mo and V are added in combination, the delayed fracture resistance can be improved as pointed out by the above-mentioned JP-A-3-243745. However, only the combined addition of Mo and V results in insufficient delayed fracture resistance when a thermal load or stress load is applied.
[0012]
Therefore, Ti was also added. This is because, when Ti is added in addition to V, it is expected that the delayed fracture resistance can be improved even in an environment where a thermal load is applied as described in the materials and processes. However, the SCr445 steel contains Cr at a level of about 1%, similar to the SCr430 steel and the like. For this reason, it has been difficult to sufficiently enhance the delayed fracture resistance.
[0013]
Therefore, since JP-A-3-243745 describes that the resistance to delayed fracture is improved when the Cr content is lowered, the present inventors also expect the improvement of the delayed fracture resistance in a thermal load environment. Attempts were also made to reduce Cr. However, the present inventors have unexpectedly found that when Cr is reduced to a level of 0.25% or less as recommended by JP-A-3-243745, cold workability is unexpectedly deteriorated. That is, even if Cr is simply reduced from the conventional SCr430 steel or the like or SCr445 steel, the cold workability is hardly changed, but the aim is to improve the delayed fracture resistance under a heat load environment. It has been discovered that cold workability deteriorates when Cr is reduced after adding more than a predetermined amount of Ti and Ti. Normally, medium or high carbon steel such as SCr430 steel and SCr445 steel is improved in workability by spheroidizing annealing prior to cold working, but after adding Mo, V, and Ti in combination. If too much Cr is reduced, large spherical carbides are not generated during the spheroidizing annealing process, and a large amount of fine high-hardness carbides derived from Mo, V, Ti, etc. remain, and cold workability deteriorates. There was a problem of doing.
[0014]
Further, when the relationship between Mo, V, Ti, and Cr and cold workability is further studied in detail, it is defined by the formula [C] × ([Mo] + 4 × [Ti] + 2 × [V]). Has been found to have a great influence on cold workability. That is, as a result of adding Mo, V, and Ti, when [C] × ([Mo] + 4 × [Ti] + 2 × [V]) is 0.4% or more, Cr is excessively reduced. It has been found that the cold workability is significantly reduced. When the amount of Cr reduction is controlled within an appropriate range (for example, about 0.27 to 0.5%), the delayed fracture resistance under a thermal load environment is sufficiently enhanced while preventing a decrease in cold workability. In addition, it has been found that the delayed fracture resistance can be sufficiently enhanced even in a stress load environment as in the case of a heat load environment. Moreover, when the Cr reduction amount is controlled within an appropriate range, there is an excellent correlation between the formula [C] × ([Mo] + 4 × [Ti] + 2 × [V]) and cold workability. It is found that if the value of [C] × ([Mo] + 4 × [Ti] + 2 × [V]) is 0.85% or less, cold workability can be reliably maintained. Completed the invention.
[0015]
That is, the high-strength steel excellent in cold workability and delayed fracture resistance according to the present invention is mass% (hereinafter the same),
C: 0.35-0.55%,
Mo: 0.50 to 1.50%,
V: 0.10 to 0.5%,
Ti: contains 0.010 to 0.15%,
Each component satisfies the following formulas (1) and (2),
Furthermore, it has a gist in that Cr is controlled in the range of 0.27 to 0.5%.
[0016]
[Ti] / [V] ≧ 0.1 (1)
0.4 ≦ [C] × ([Mo] + 4 × [Ti] + 2 × [V]) ≦ 0.85 (2)
[In the formula, [Ti], [V], [C], and [Mo] are Ti amount (% by mass), V amount (% by mass), C amount (% by mass), and Mo amount in steel ( Mass%)]
The steel desirably contains various additive elements as long as the cold workability and delayed fracture resistance are not impaired. Examples of additive elements include Ni: 1% or less (not including 0%), Cu: 2% or less (not including 0%), Zr: 0.5% or less (not including 0%), W: 0.5% or less (not including 0%), Nb: 0.5% or less (not including 0%), B: 0.003% or less (not including 0%), and the like.
[0017]
The basic additive elements and impurities of the steel are, for example, as follows.
[0018]
Mn: 0.1 to 0.8%
Si: 0.2% or less
Al: 0.5% or less
P: 0.02% or less
S: 0.02% or less
N: 0.02% or less
The steel is processed in the order of hot rolling and spheroidizing annealing, then cold worked into a predetermined shape, the cold worked steel is quenched at a heating temperature of 890-960 ° C., and then tempered at a heating temperature of 570 ° C. or higher. By processing, high strength steel parts having excellent delayed fracture resistance can be produced.
[0019]
The high-strength steel parts have the characteristics shown in the following 1) to 4).
[0020]
1) Tensile strength: 1300 N / mm²more than
2) Austenite grain size number: 9 or more
3) A 10 mm diameter round bar-shaped test piece having an annular V notch (V notch angle 60 °, depth 2 mm, notch bottom radius 0.03 mm) is processed by cutting, and this test piece is hydrochloric acid aqueous solution (HCl concentration: 15 mass) %) For 30 minutes, washed with water and dried, and then 1500 N / mm in air.²Will not break even if stress is continuously applied for 100 hours
4) Processing into a test piece having the same shape as in 3) above, and immersing the annular V-notch portion of this test piece in distilled water at a temperature of 80 ° C., 1500 N / mm²Will not break even if stress is continuously applied for 100 hours
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail. First, the reasons for limiting chemical components in the present invention will be described.
[0022]
C: 0.35 to 0.55% (meaning mass%, hereinafter the same)
C is an element necessary for ensuring the hardenability and strength of steel. In particular, the steel of the present invention contains precipitation hardening type elements such as Mo, V, and Ti as described later, and further, these elements are solid-solved by high-temperature quenching, and the above-described Mo, V, Ti, etc. by high-temperature tempering. Is deposited. In order to ensure a predetermined tensile strength even at such high temperature tempering, C needs to be 0.35% or more, preferably 0.40% or more, more preferably 0.42% or more. On the other hand, if C is excessive, the toughness of the steel is deteriorated, so that the delayed fracture characteristics are lowered, and further, the cold workability is also deteriorated. Therefore, C is 0.55% or less, preferably 0.50% or less, more preferably 0.48% or less.
[0023]
Mo: 0.50 to 1.50%
Mo is a hardenability improving element and is a precipitation hardening type element, so it is useful for securing strength. Furthermore, it has a grain boundary strengthening effect and is useful in improving delayed fracture resistance. In order to exhibit these effects, Mo is 0.50% or more, preferably 0.6% or more, and more preferably 0.8% or more. On the other hand, when Mo becomes excessive, cold workability will fall. Therefore, Mo is made 1.50% or less, preferably 1.2% or less, more preferably 1.0% or less.
[0024]
V: 0.10 to 0.5%
Ti: 0.010 to 0.15%
[Ti] / [V] ≧ 0.1
V is useful in that fine VC can be efficiently generated by high-temperature quenching and high-temperature tempering, and the strength can be improved by precipitation hardening. Furthermore, it has the effect of trapping hydrogen. However, the hydrogen trapping capability of VC is relatively weak. When stress or heat is applied, the trapped hydrogen is released, and conversely, the amount of diffusible hydrogen increases, causing a delayed fracture characteristic to deteriorate. On the other hand, Ti is also a precipitation hardening type element, and its carbide has a strong trapping ability and has a property of not releasing hydrogen even when a high stress or heat is applied. That is, the addition of V alone does not provide sufficient hydrogen trapping capability, and Ti addition is essential to make up for it. By adding V and Ti in combination, V-Ti composite carbides, V carbides, and Ti carbides can be formed, and carbides can be efficiently precipitated and a carbide group having a strong trapping ability can be formed.
[0025]
The addition amount of V can be set from the viewpoints of precipitation hardening action, hydrogen trap amount, and the like, and can be selected from a range of 0.10% or more, preferably 0.12% or more. The amount of Ti added can also be set from the viewpoints of precipitation hardening action, hydrogen trapping ability, etc., and can be selected from the range of 0.010% or more, preferably 0.03% or more, and more preferably 0.05% or more.
[0026]
It is useful to set the ratio of V and Ti from the viewpoint of hydrogen trap ability when stress or heat is applied. In the present invention, from various experimental results, the ratio of V and Ti satisfies the following formula (1), preferably the following formula (1a), and more preferably the following formula (1b).
[0027]
[Ti] / [V] ≧ 0.1 (1)
[Ti] / [V] ≧ 0.2 (1a)
[Ti] / [V] ≧ 0.3 (1b)
[Wherein, [Ti] and [V] respectively represent Ti amount (mass%) and V amount (mass%) in steel]
On the other hand, when V or Ti is excessively added, cold workability is lowered. Furthermore, the V and Ti giant carbides produced during the melting of the steel do not sufficiently dissolve during the quenching heating, thereby degrading the toughness of the steel. For this reason, the delayed fracture resistance may be lowered. V is 0.5% or less, preferably 0.4% or less, and more preferably 0.3% or less. Ti is 0.15% or less, preferably 0.10% or less, more preferably 0.08% or less.
[0028]
Cr: 0.27 to 0.5%
The object of the present invention is to improve delayed fracture resistance. For this purpose, Mo, Ti and V are added in combination, but this is not sufficient. For example, conventionally, 10-12T (about 980-1372 N / mm²Grade) SCr430 steel, SCM435 steel, SCM440 steel, etc. are used as steel for high-strength bolts of the class, and even if these SCr steels are combined with Mo, Ti and V, the improvement in delayed fracture resistance is insufficient. It is. About 1% of Cr was added to the SCr steel for the purpose of improving hardenability and ensuring high strength, but this Cr deteriorated the delayed fracture resistance. This is because when about 1% of Cr is added, the steel material corrodes in a pit shape, and this pit-shaped portion becomes a stress-concentrated portion and the delayed fracture resistance is deteriorated. Accordingly, when the Cr addition amount is reduced, pit-like corrosion is reduced and the steel surface is uniformly corroded, so that the stress concentration portion is reduced and delayed fracture resistance is improved. In the present invention, since Mo, Ti, and V are added in combination, the grain boundary is strengthened and the trapping ability of diffusible hydrogen is excellent. Therefore, even if Cr is reduced in order to improve delayed fracture resistance, the amount of reduction of Cr may be relatively small. Cr is 0.5% or less, preferably 0.4% or less, and more preferably 0.35% or less.
[0029]
On the other hand, if Cr is excessively reduced, cold workability is reduced in the combined addition system of Mo, Ti and V. Generally, in medium or high carbon steels such as the present invention, workability is improved by spheroidizing annealing prior to cold working, but Cr is reduced by adding Mo, V, and Ti in combination. If the amount is too large, large spherical carbides are not generated during the spheroidizing annealing treatment, and a large amount of fine high-hardness carbides derived from Mo, V, Ti, etc. remain, and cold workability deteriorates. In order to ensure cold workability after spheroidizing annealing, Cr is 0.27% or more, preferably 0.28% or more, more preferably 0.29% or more, and particularly 0.30% or more.
[0030]
0.4 ≦ [C] × ([Mo] + 4 × [Ti] + 2 × [V]) ≦ 0.85
The present inventors have found that cold workability is lowered when Cr is excessively reduced in a composite additive steel of Mo, Ti and V. On the other hand, when there is little addition amount of Mo, Ti, and V, there is no correlation between Cr amount and cold workability. In view of this, an investigation was made as to the extent to which the amounts of addition of Mo, Ti, and V need to be strictly controlled in order to ensure cold workability. As a result, when the composite addition amount X defined by the following formula (2a) is 0.4% or more, particularly when it is 0.5% or more (for example, 0.6% or more), Cr is excessively reduced. Then, it was found that the cold workability is remarkably lowered.
[0031]
Compound addition amount X = [C] × ([Mo] + 4 × [Ti] + 2 × [V]) (2a)
[In the formula, [C], [Mo], [Ti], and [V] are C amount (mass%), Mo amount (mass%), Ti amount (mass%), and V amount (in the steel, respectively). Mass%)]
Furthermore, the present inventors have also found that the composite addition amount X has an extremely excellent correlation with the cold workability of the composite addition steel of Mo, Ti and V. Therefore, in order to ensure good cold workability of the steel of the present invention, it is also important to define the upper limit of the composite addition amount X. The upper limit of the composite addition amount X is about 0.85, preferably about 0.80, and more preferably about 0.75.
[0032]
The steel of the present invention often contains basic additive elements (Mn, Si, Al, etc.) in addition to the essential components (C, Mo, V, Ti, Cr). The amount is not particularly limited as long as it does not adversely affect cold workability and delayed fracture resistance to such an extent that the effects of the present invention are lost. Preferred amounts of basic additive elements are as shown below.
[0033]
Mn: 0.1 to 0.8%
Mn is a hardenability improving element and is useful for achieving high strength. The amount of Mn is, for example, 0.1% or more, preferably 0.3% or more, more preferably 0.35% or more, and particularly 0.40% or more. On the other hand, when Mn is excessive, the cold workability is liable to be lowered, and further segregated at the grain boundary to lower the grain boundary strength, and the delayed fracture resistance is liable to be lowered. The amount of Mn is, for example, 0.8% or less, preferably 0.6% or less, more preferably 0.55% or less, and particularly about 0.50% or less.
[0034]
Si: 0.2% or less
Since Si is added as a deoxidizer, it remains in the steel, but as the remaining amount of Si increases, cold workability tends to decrease, and further, grain boundary oxidation occurs during heat treatment such as quenching. This promotes the resistance to delayed fracture and tends to decrease. The amount of Si is, for example, 0.2% or less, preferably 0.1% or less, more preferably 0.07% or less, particularly 0.05% or less (for example, 0.03% or less). Actually, Si never becomes 0%.
[0035]
Al: 0.5% or less
Since Al is added as a deoxidizer, it remains in the steel, and since it is an element that can also be expected to improve corrosion resistance due to rust densification, it may be actively left. Although the lower limit of the amount of Al is not particularly limited, it is practically over 0% when considering use as a deoxidizer, and from the viewpoint of exerting an effect of improving corrosion resistance, for example, 0.01% or more, preferably Is 0.02% or more, more preferably about 0.03% or more. On the other hand, as the remaining amount of Al increases, the amount of oxide inclusions increases, and the delayed fracture resistance tends to deteriorate. The amount of Al is, for example, 0.5% or less, preferably 0.3% or less, more preferably 0.1% or less, and particularly 0.05% or less.
[0036]
It is desirable that at least one of the basic additive elements (Mn, Si, Al) satisfies the above range, and it is desirable that two or more, particularly all, satisfy the above range.
[0037]
In the steel of the present invention, the balance other than the essential elements and basic additive elements is Fe and inevitable impurities (for example, P, S, N, O, etc.), and other elements (Cu, Ni, etc.) as necessary. Corrosion resistance improving elements; fine carbonitride-forming elements such as Zr, W and Nb; B etc.) may be added. The amounts of the inevitable impurities and other elements are not particularly limited as long as the effects of the present invention are not lost, but are preferably in the following ranges.
[0038]
[Inevitable impurities]
P: 0.02% or less
P is an element remaining in the steel as an impurity, and tends to cause segregation at the grain boundary to deteriorate the delayed fracture resistance. The amount of P is, for example, 0.02% or less, preferably 0.015% or less, more preferably 0.010% or less, and particularly 0.005% or less. P is never 0%.
[0039]
S: 0.02% or less
S is also an element remaining in the steel as an impurity, and forms MnS to become a stress concentration portion, so that the delayed fracture resistance is easily lowered. The amount of S is, for example, 0.02% or less, preferably 0.015% or less, more preferably 0.010% or less, and particularly 0.005% or less. S is never 0%.
[0040]
N: 0.02% or less
Since N is harmful to delayed fracture characteristics, it is desirable to reduce N. The N amount is, for example, 0.02% or less, preferably 0.015% or less, more preferably 0.010% or less, and particularly 0.007% or less (for example, 0.005% or less). N is never 0%.
[0041]
It is desirable that at least one of the impurities (P, S, N) satisfies the above range, and it is desirable that two or more, particularly all, satisfy the above range.
[0042]
[Corrosion resistance improving element]
Ni: 1% or less (excluding 0%)
Ni has an effect of enhancing the toughness and hardenability of the steel, and is desirably added positively in order to improve the corrosion resistance and suppress the hydrogen penetration. When Ni is positively added (that is, when it exceeds 0%), it is preferably at least 0.1%, more preferably at least 0.2%, particularly at least 0.3%. On the other hand, even if Ni is added excessively, the effect is saturated and not only the cost is increased, but also cold workability is lowered. Therefore, when Ni is positively added, the addition amount is 1% or less, preferably 0.6% or less, more preferably 0.55% or less, and particularly 0.50% or less.
[0043]
Cu: 2% or less (excluding 0%)
Since Cu is useful for increasing the corrosion resistance of steel and suppressing the intrusion of hydrogen that adversely affects delayed fracture resistance, it is desirable to add it positively. When Cu is positively added (that is, when it is more than 0%), it is preferably 0.15% or more, more preferably 0.3% or more. On the other hand, when Cu is added excessively, not only the effect is saturated, but also the toughness of the steel is lowered. Therefore, when Cu is positively added, the addition amount is 2% or less, preferably 1% or less, more preferably 0.7% or less, and particularly 0.6% or less.
[0044]
The said corrosion-resistant element (Ni, Cu) can be added individually or in combination of 2 types.
[0045]
[Fine carbonitride-forming elements]
Zr: 0.5% or less (excluding 0%)
W: 0.5% or less (excluding 0%)
Nb: 0.5% or less (excluding 0%)
Zr, W, and Nb, like Ti, form fine carbonitrides and contribute to the improvement of delayed fracture characteristics, so it is desirable to add them positively. Furthermore, nitrides and carbides of these elements are effective for making crystal grains finer. When Zr, W, and / or Nb is positively added (ie, more than 0%), the addition amount of these elements is preferably 0.03% or more, more preferably 0.05% or more. And On the other hand, if Zr, W, and / or Nb is added excessively, delayed fracture resistance and toughness are impaired. Therefore, when Zr, W, and / or Nb is positively added, the amount added is 0.5% or less, preferably 0.3% or less, more preferably 0.1% or less.
[0046]
Zr, W, and Nb can be added singly or in combination of two or more.
[0047]
[B]
B: 0.003% or less (excluding 0%)
Since B is useful for improving hardenability, toughness after quenching and tempering, fatigue characteristics, and the like, it is desirable to add B actively. When B is positively added (that is, when it exceeds 0%), it is preferably 0.0005% or more, more preferably 0.0010% or more. On the other hand, if B is added excessively, the toughness is inhibited. Therefore, when B is positively added, the addition amount is 0.003% or less, preferably 0.0025% or less, more preferably about 0.0020% or less.
[0048]
Since the steel having the chemical component is excellent in cold workability after spheroidizing annealing, it can be easily processed into various forms. For example, after processing in the order of hot working (hot rolling, etc.) and spheroidizing annealing, it can be processed into various parts by cold working (cold drawing, cold forging, etc.). Since the steel of the present invention is excellent in cold workability, it can be processed with a high yield without deteriorating the tool life during cold working. Note that either one of the cold wire drawing and the cold forging may be employed, or both may be combined. Further, it is desirable to perform a coating treatment (pickling, lubricating coating treatment, etc.) between spheroidizing annealing and cold working.
[0049]
A high-strength steel part can be manufactured by quenching and tempering the processed product. Although the steel having the above chemical components is potentially excellent in high strength and delayed fracture resistance, various high strength steel parts are manufactured so as to exhibit these high strength and delayed fracture resistance. For this purpose, it is necessary to appropriately set the quenching condition and the tempering condition.
[0050]
That is, since the steel contains precipitation hardening elements such as Mo, Ti, and V, it is necessary to solidify the precipitation hardening elements during quenching and to precipitate as fine carbides during tempering. The fine carbides of Mo, Ti, and V are useful not only for improving strength but also for improving delayed fracture resistance. Further, in order to achieve a predetermined delayed fracture resistance, it is necessary to prevent the crystal grains from becoming coarse during quenching.
[0051]
Specifically, the heating temperature at the time of quenching is desirably 890 to 960 ° C. If the quenching temperature is too low, the precipitation hardening type element is not sufficiently dissolved in the steel, and even if tempering is performed, a sufficient amount of precipitation of carbide cannot be secured, and the tensile strength is lowered. Furthermore, since the pre-quenching structure is a spheroidized structure, the spheroidized carbide remains undissolved when heating is insufficient, and the tensile strength also decreases from this point. On the other hand, if the quenching temperature is too high, the crystal grains become coarse and the delayed fracture characteristics deteriorate. The preferable heating temperature is about 900 ° C. or higher (eg, 910 ° C. or higher, particularly 920 ° C. or higher) and 950 ° C. or lower (eg, 945 ° C. or lower, particularly 940 ° C. or lower).
[0052]
The heating temperature during tempering is desirably 570 ° C. or higher. If the tempering temperature is too low, the precipitation hardening type element does not sufficiently precipitate as fine carbides, and the tensile strength does not increase. Particularly, if the fine carbides of Mo, Ti, and V are not sufficiently precipitated, the delayed fracture resistance is also deteriorated. Preferred tempering temperatures are about 580 ° C. or higher (particularly 600 ° C. or higher) and 650 ° C. or lower (particularly 625 ° C. or lower).
[0053]
The other conditions for quenching and tempering can be appropriately set in consideration of the heating temperature and the characteristics of the precipitation hardening element, and can be selected from the following ranges, for example.
[0054]
[Hardening conditions]
Holding time after heating: 10 minutes or more (for example, 20 minutes or more), 1 hour or less (for example, 40 minutes or less)
Cooling condition: oil cooling or water cooling
[Tempering conditions]
Holding time after heating: 30 minutes or more (for example, 70 minutes or more), 3 hours or less (for example, 2 hours or less)
Cooling condition: oil cooling or water cooling
The high-strength steel parts obtained as described above are not only excellent in tensile strength, but also austenite crystal grains are not coarsened, and are also resistant to delayed fracture under high load stress and high temperature. Are better. The tensile strength, crystal grain number, and delayed fracture resistance are, for example, as follows.
[0055]
[Tensile strength]
1300 N / mm²Or more, preferably 1400-1700 N / mm²Degree, more preferably 1500-1700 N / mm²Degree, especially 1600-1700 N / mm²degree.
[0056]
[Austenite grain size number]
Austenite grain size number: 9 or more, preferably about 10-15, more preferably about 11-15.
[0057]
[Delayed fracture resistance under high load stress]
By cutting the cold-worked product, a round bar-shaped test piece having a diameter of 10 mm having an annular V-notch (V notch angle 60 °, depth 2 mm, notch bottom radius 0.03 mm) is manufactured. Immerse in an aqueous solution (HCl concentration: 15% by mass) for 30 minutes, wash with water and dry, then 1500 N / mm in air²Even if the stress is continuously applied for 100 hours, it does not break.
[0058]
[Delayed fracture resistance at high temperatures]
The cold-worked product is cut into a round bar-shaped test piece having the same shape as described above, and the annular V-notch portion of the test piece is 1500 N / mm while being immersed in distilled water at a temperature of 80 ° C.²Even if the stress is continuously applied for 100 hours, it does not break.
[0059]
According to the high strength steel part of the present invention, it has excellent delayed fracture resistance despite its high strength. Therefore, for example, a bolt can be used in a wide range without being restricted in use.
[0060]
【Example】
EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and both are included in the technical scope of the present invention.
[0061]
Experimental example 1
A wire rod (diameter: 12 mm) was manufactured by hot working test steels having chemical components (mass%) shown in Tables 1 to 3 below. The wire was heated to a temperature of 740 ° C. and held for 5 hours, and then gradually cooled to a temperature of 650 ° C. (cooling rate = about 10 ° C./hr), and then allowed to cool to the atmosphere for spheroidizing annealing. The obtained wire was cut and cut to produce a compression test piece having a diameter of 10 mm and a height of 15 mm, and the cold workability was evaluated under the following conditions.
[0062]
[Cold workability]
The compression test piece was compressed while restraining both end faces thereof, and the stress (deformation resistance) necessary to obtain a rolling reduction of 70% was measured. The smaller the deformation resistance, the better the cold workability.
[0063]
The spheroidizing annealing wire was quenched (retention time after heating: 30 minutes, cooling condition: oil cooling) and tempering (retention time after heating: 90 minutes, cooling condition: water cooling) under the conditions shown in Tables 4 to 7 below. )did. The austenite grain size number of the obtained tempered steel was measured according to JISG 0551 “Austenite grain size test method for steel”.
[0064]
By cutting and cutting the tempered steel, a tensile test piece shown in FIG. 1 and a delayed fracture test piece shown in FIG. 2 (annular V having a V notch angle of 60 °, a depth of 2 mm, and a notch bottom radius of 0.03 mm). A round bar-shaped test piece having a notch and a diameter of 10 mm was manufactured. The tensile strength was measured using the tensile test piece, and the delayed fracture resistance was examined using the delayed fracture test piece as follows.
[0065]
[Delayed fracture resistance under low load stress]
The delayed fracture test piece was immersed in an aqueous hydrochloric acid solution (HCl concentration: 15% by mass) for 30 minutes, washed with water and dried. 1000 N / mm²A stress of about 50% of the notch tensile strength was applied in the atmosphere, and the presence or absence of fracture after 100 hours was observed.
[0066]
[Delayed fracture resistance under high load stress]
Load stress is 1500 N / mm²Except for (about 75% of the notch tensile strength), the same as in the case of “delayed fracture resistance under normal stress” described above.
[0067]
[Delayed fracture resistance under high temperature and high load stress]
1500 N / mm while the annular V notch of the delayed fracture specimen is immersed in distilled water at a temperature of 80 ° C.²And the presence or absence of fracture after 100 hours was observed.
[0068]
The results of the above tests are shown in Tables 4 to 7 and FIG.
[0069]
[Table 1]

[0070]
[Table 2]

[0071]
[Table 3]

[0072]
[Table 4]

[0073]
[Table 5]

[0074]
[Table 6]

[0075]
[Table 7]

[0076]
[About Table 1 and Table 4]
Test No. Steel A used in No. 1 corresponds to conventional SCM435 steel. This No. As is clear from the test result of No. 1, SCM435 steel has a low content of Mo, Ti and V and has poor delayed fracture resistance. In addition, Test No. Steel B used in No. 2 corresponds to the conventional S45C steel, and is the most different from the SCM435 steel in that the Cr content is small. And test no. 1 and No. As is clear from the comparison of 2, when Mo, Ti, and V are small, cold workability does not deteriorate even if Cr is decreased.
[0077]
Test No. 3 to 7 are examples in which two of Mo, Ti, and V are added in combination. That is, test no. No. 3 uses steel C to which Mo and V are added. However, since Ti is small, the delayed fracture resistance under high load stress and high temperature is insufficient. Test No. In No. 4, steel D to which Mo and Ti are added is used, but since V is small, delayed fracture resistance is insufficient. Test No. In Nos. 5 to 7, steels E to G to which Ti and V are added are used. However, since Mo is small, tensile strength does not increase.
[0078]
Test No. In Steel No. 8, three components of Mo, Ti, and V are added in combination. However, since the Ti amount is too small compared to the V amount and Ti / V is too small, the delayed fracture resistance under high load stress and high temperature is insufficient.
[0079]
Test No. In steels I and J used in 9 to 10, the three components of Mo, Ti, and V are added in combination, and the value of Ti / V is appropriately adjusted. However, since Cr is not reduced, delayed fracture resistance under high load stress is insufficient.
[0080]
Test No. In steels K to M used in 11 to 12, three components of Mo, Ti, and V are added in combination, the value of Ti / V is appropriately set, and Cr is further reduced. Therefore, it has excellent delayed fracture resistance. However, since Cr is excessively reduced, cold workability is poor.
[0081]
[About Table 2 and Table 5]
Test No. In Nos. 14 to 25, steels N to Y in which the three components of Mo, Ti, and V are added in combination, the Ti / V range is set appropriately, and the Cr reduction amount is set to an appropriate range are used. Yes. Such steels N to Y are excellent in delayed fracture resistance even under high load stress and high temperature, and are also excellent in cold workability.
[0082]
[About Table 3 and Table 6]
However, test no. As is clear from 26 to 28, if Mo, Ti, V, etc. are added excessively, the cold workability is deteriorated and the delayed fracture resistance is also sometimes deteriorated. Therefore, it is necessary to add appropriate amounts of Mo, Ti, and V.
[0083]
In addition, Test No. As apparent from 29 to 30, when C is insufficient, tensile strength cannot be secured (Test No. 29), and when C is excessive, cold workability is deteriorated and delayed fracture resistance is also deteriorated (Test No.). .30). Therefore, it is necessary to select the C amount from an appropriate range.
[0084]
[About Table 7]
Even when the steels N to Y having excellent delayed fracture resistance and cold workability are used, it is necessary to appropriately set the quenching and tempering conditions. That is, test no. As apparent from FIG. 32, when the quenching temperature is too low, precipitation hardening type alloy elements such as Mo, Ti, and V cannot be sufficiently dissolved, so that the tensile strength decreases. Test No. As is clear from FIG. 33, when the quenching temperature is too high, the crystal grains become coarse, so that the delayed fracture resistance is deteriorated. In addition, Test No. As is clear from FIG. 34, if the tempering temperature is too low, fine carbides such as Mo, Ti, and V do not precipitate, so that the delayed fracture characteristics deteriorate. On the other hand, test no. Nos. 14 and 31 are excellent in delayed fracture resistance and cold workability because the quenching temperature and tempering temperature are appropriate.
[0085]
[About Figure 3]
FIG. Based on all the results of 1 to 34, the relationship between the steel component [C × (Mo + 4Ti + 2V)] and the deformation resistance is plotted. As apparent from FIG. 3, when the amount of Mo, Ti, V is small, more specifically, when C × (Mo + 4Ti + 2V) is about 0 to 0.3, the amount of Cr is small (in the figure). There is no significant difference in deformation resistance (cold workability) between the case where the amount of Cr is large (see the circle and solid line in the figure). However, when the amount of Mo, Ti, and V increases, more specifically, when C × (Mo + 4Ti + 2V) is about 0.4% or more, when the amount of Cr is small (see the x and broken lines in the figure), the amount of Cr Compared to the case where there are many (see the ○ mark and solid line in the figure), the cold workability is remarkably deteriorated.
[0086]
In addition, as apparent from FIG. 3, when C × (Mo + 4Ti + 2V) becomes too large, cold workability deteriorates even if the Cr amount is appropriate.
[0087]
【The invention's effect】
The high-strength steel of the present invention (high-strength steel for cold working, etc.) is excellent in cold workability because the chemical composition is controlled within a predetermined range, and moreover, heat load and stress load are added. Even in harsh environments, it has excellent delayed fracture resistance.
[0088]
Since the high-strength steel part of the present invention is manufactured from the high-strength steel, it can be obtained with a high yield without deteriorating the tool life during cold working, and is also excellent in delayed fracture resistance.
[0089]
Further, the high-strength steel and the high-strength steel parts are excellent in both cold workability and delayed fracture resistance even when the C content is set high and the strength is increased.
[Brief description of the drawings]
FIG. 1 is a schematic plan view showing a tensile test piece of an experimental example.
FIG. 2 is a schematic plan view showing a delayed fracture test piece of an experimental example.
FIG. 3 is a graph showing a relationship between a steel component [C × (Mo + 4Ti + 2V)] and deformation resistance.

Claims (5)

Ingredient composition is mass% (hereinafter the same),
C: 0.35-0.55%,
Si: 0.2% or less,
Mn: 0.1 to 0.8%
Al: 0.5% or less,
Mo: 0.50 to 1.50%,
V: 0.10 to 0.5%,
Ti: satisfies the requirement of 0.010 to 0.15%,
C, Mo, V, and Ti further satisfy the following formulas (1) and (2),
Cr is controlled in the range of 0.27 to 0.5%,
High balance steel with excellent cold workability and delayed fracture resistance, with the balance being Fe and inevitable impurities.
[Ti] / [V] ≧ 0.1 (1)
0.4 ≦ [C] × ([Mo] + 4 × [Ti] + 2 × [V]) ≦ 0.85 (2)
[In the formula, [Ti], [V], [C] and [Mo] are Ti amount (% by mass), V amount (% by mass), C amount (% by mass), and Mo amount in steel ( Mass%)] Furthermore, as other ingredients,
Ni: 1% or less (not including 0%), and Cu: 2% or less (not including 0%)
The high strength steel according to claim 1, comprising at least one selected from the group consisting of: Furthermore, as other ingredients,
Zr: 0.5% or less (excluding 0%),
W: 0.5% or less (not including 0%), and Nb: 0.5% or less (not including 0%)
The high-strength steel according to claim 1 or 2, comprising at least one selected from the group consisting of: Furthermore, as other components B: 0.003% or less (excluding 0%)
The high strength steel in any one of Claims 1-3 containing. P: 0.02% or less,
S: 0.02% or less,
The high-strength steel according to any one of claims 1 to 4, which satisfies at least one requirement selected from N: 0.02% or less.

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