JP2005060721A - Steel material superior in delayed fracture resistance and fatigue characteristic, and manufacturing method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stably provide a steel material having a combination of high torsional fatigue strength and satisfactory delayed fracture resistance. <P>SOLUTION: The steel material has a composition comprising, by mass%, 0.35-0.7% C, 0.30-1.1% Si, 0.2-2.0% Mn, 0.005-0.25% Al, 0.05-0.6% Mo, 0.0003-0.006% B, 0.06 mass% or less S, 0.02% or less P, 0.2% or less Cr, one or two elements selected from 0.06-0.10% Ti and 0.25-0.50% V, and the balance Fe with unavoidable impurities; has a base metal structure containing at least a bainite structure and a martensite structure in an amount of 10 vol.% or more in total; and has a former austenite grain size of 5 μm or smaller in an induction-hardened layer through the whole thickness direction of the hardened layer. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

【００５７】
【表２】

【００５８】
表２から明らかなように、本発明で規定した成分組成範囲を満足し、かつ本発明の高周波焼入れ条件を満たす条件で製造した鋼材はいずれも、硬化層の旧オーステナイト粒径が全厚にわたって５μｍ 以下を満たしており、その結果良好な耐遅れ破壊特性と ７００ ＭＰａ以上の高いねじり疲労強度とを得ることができた。
【００５９】
なお、表２中のＮｏ．１と２あるいはＮｏ． ４と５を比較すると、焼入れ回数を１回から２回に増やすことで、硬化層の粒径が微細化し、耐遅れ破壊特性およびねじり疲労強度がさらに上昇することが分かる。
【００６０】
また、Ｎｏ．８， Ｎｏ．３５， Ｎｏ．３６を比較すると、焼入れ回数を１回から２回に増やした場合において、２回目の焼入れ深さの方が浅い場合（Ｎｏ．３５）には、１回しか施さなかった場合よりも耐遅れ破壊特性およびねじり疲労強度はむしろ低下するのに対し、２回目の焼入れ深さを深くした場合（Ｎｏ．３６）には、１回しか施さなかった場合に比べて耐遅れ破壊特性およびねじり疲労強度は大幅に向上することが分かる。Ｎｏ．３６ では、硬化層厚方向で、表面から硬化層厚の４／５ 位置で最も旧オーステナイト粒径が大きく４．２ μｍ であったが、表層近傍（表面から硬化層厚の１／５ 位置）では旧オーステナイト粒径は２．８ μｍ であり、表層の粒径が微細化していることが、疲労強度、耐遅れ破壊特性の向上に寄与したものと考えられる。
【００６１】
これに対し、Ｎｏ． ９は、加工後の冷却速度が小さいため、ベイナイトとマルテンサイトの合計組織分率が１０％未満となっており、その結果、硬化層粒径が粗大となり、耐遅れ破壊特性およびねじり疲労強度が低い。
Ｎｏ．２２ は、硬化層粒径は微細であるものの、Ｃ含有量が本発明の範囲より高いため、粒界強度の低下を招き、そのため耐遅れ破壊特性およびねじり疲労強度が劣っている。
Ｎｏ．２３， ２４， ２５ は、それぞれＣ， Ｓｉ， Ｍｏの含有量が本発明の適正範囲よりも低いため、硬化層粒径が粗大となり、耐遅れ破壊特性およびねじり疲労強度が劣っている。
Ｎｏ．２６ はＢ含有量が低く、またＮｏ．２７ はＭｎ含有量が、Ｎｏ．２８ はＳおよびＰ含有量が、Ｎｏ．２９ はＣｒ含有量が、それぞれ本発明の適正範囲を超えているため、いずれも粒界強度の低下を招き、耐遅れ破壊特性およびねじり疲労強度が劣っている。
Ｎｏ．３０ は、Ｔｉ及びＶ含有量が本発明の適正範囲を超えているため、耐遅れ破壊特性およびねじり疲労強度が劣っており、逆にＮｏ．３３ はＴｉ及びＶ含有量が低いため、耐遅れ破壊特性が劣っている。
Ｎｏ．３１ は、高周波焼入れ時の加熱温度が高すぎるため硬化層の粒径が粗大となり、一方Ｎｏ．３２ は、高周波焼入れ時の加熱温度が低すぎるため硬化層が形成されず、いずれも耐遅れ破壊特性およびねじり疲労強度に劣っている。
Ｎｏ．３４ は、Ｓｉ量が本発明の下限に満たない０．２８ｍａｓｓ％の場合であるが、この例のように、Ｓｉ量が本発明の下限をわずかでも下回る場合には、硬化層全厚にわたって１２μｍ 以下の粒径を得ることができず、その結果、耐遅れ破壊特性およびねじり疲労強度に劣っている。
【００６２】
【発明の効果】
かくして、本発明によれば、耐遅れ破壊特性および高いねじり疲労強度を有する鋼材を安定して得ることができ、その結果、自動車用部材の軽量化の要求に対し偉功を奏する。[0001]
BACKGROUND OF THE INVENTION
The present invention has excellent delayed fracture resistance and post-quenching fatigue characteristics suitable for application to automobile drive shafts and constant velocity joints, as well as hub bearings, crankshafts, etc., which have a hardened layer by induction hardening on the surface. The present invention relates to a steel material as a raw material, a steel material excellent in fatigue characteristics after being actually quenched, and a method for producing the same.
[0002]
[Prior art]
Conventionally, mechanical structural members such as automobile drive shafts and constant velocity joints are processed by hot forging, hot cutting, further cutting, cold forging, etc. into a predetermined shape and induction hardening. By tempering, it is common to ensure torsional fatigue strength, which is an important characteristic as a machine structural member.
On the other hand, in recent years, there is a strong demand for weight reduction of automobile members due to environmental problems, and further improvement in torsional fatigue strength of automobile members is required from this viewpoint.
[0003]
In order to improve the torsional fatigue strength, for example, it is conceivable to increase the quenching depth by induction quenching. However, even if the quenching depth is increased, the fatigue strength is saturated at a certain depth.
Further, improvement of the grain boundary strength is also effective for improving the torsional fatigue strength. From this viewpoint, a technique for refining the prior austenite grain size by dispersing TiC has been proposed (for example, see Patent Document 1). )
[0004]
In the technique described in Patent Document 1 described above, since fine TiC is dispersed in a large amount during induction hardening heating, the prior austenite grain size is refined, so that TiC is solutionized before quenching. It is necessary to adopt a process of heating to 1100 ° C. or higher in the hot rolling process. Therefore, there is a problem that it is necessary to increase the heating temperature during hot rolling, resulting in poor productivity.
Further, even with the technique disclosed in the above-mentioned Patent Document 1, there is still a problem in that it cannot sufficiently meet the recent demand for torsional fatigue strength.
[0005]
Furthermore, in Patent Document 2, the ratio (CD / R) of the hardened layer depth CD and the radius R of the induction-hardened shaft component is limited to 0.3 to 0.7, and then this CD / R and induction hardening are performed. It is defined by the austenite grain size γf from the subsequent surface to 1 mm, the average Vickers hardness Hf up to (CD / R) = 0.1 as it is induction-hardened, and the average Vickers hardness Hc at the axial center after induction hardening. There has been proposed a shaft component for a mechanical structure in which the torsional fatigue strength is improved by controlling the value A within a predetermined range in accordance with the C amount.
However, in this component, since the prior austenite grain size over the entire thickness of the hardened hardened layer is not taken into consideration, it is still impossible to sufficiently meet the recent demand for torsional fatigue strength.
[0006]
By the way, as a solution to the above-mentioned problems, the inventors previously optimized the chemical composition, structure, quenching conditions of steel, and the prior austenite grain size over the entire thickness of the hardened layer after quenching. A steel material capable of obtaining fatigue strength was developed and proposed as Patent Document 3.
This patent document 3 was developed based on the novel knowledge as described below.
[0007]
(1) The torsional fatigue strength is significantly improved by quenching the steel adjusted to an appropriate chemical composition and setting the prior austenite grain size over the entire thickness of the quenched and hardened layer to 12 μm or less. Specifically, with regard to chemical composition, especially by adding Si and Mo within an appropriate range, the number of austenite nucleation sites during induction hardening increases, and the growth of austenite grains is suppressed. The grain size of the quenched and hardened layer is effectively refined, and as a result, the torsional fatigue strength is significantly improved. In particular, by adding 0.30 mass% or more of Si, a hardened layer having a particle size of 12 μm or less can be obtained over the entire thickness of the hardened layer after induction hardening.
[0008]
(2) When the structure of the base material, that is, the structure before quenching, is a structure in which the bainite structure and / or the martensite structure is contained in a specific fraction, the bainite structure or the martensite structure is compared with the ferrite-pearlite structure. Since the carbide is a finely dispersed structure, the area of the ferrite / carbide interface, which is the nucleation site of austenite, increases during quenching heating, and the generated austenite becomes finer. As a result, the grain size of the quenched and hardened layer becomes fine, which improves the grain boundary strength and increases the torsional fatigue strength.
[0009]
(3) As described above, by using a steel material having a chemical composition and a structure adjusted, and appropriately controlling induction hardening conditions (heating temperature, time, number of times of quenching), the hardened layer particle size is remarkably refined, Grain boundary strength is improved. Specifically, by setting the heating temperature to 800 to 1000 ° C. and the heating time to 5 seconds or less, fine particles having a particle size of 12 μm or less can be stably obtained over the entire thickness of the cured layer. Furthermore, by repeating the quenching process under the above conditions twice or more, a finer hardened layer particle size can be obtained as compared with one quenching.
[0010]
[Patent Document 1]
JP 2000-154819 A (Claims, paragraph [0008])
[Patent Document 2]
JP-A-8-53714 (Claims)
[Patent Document 3]
Japanese Patent Application No. 2003-9349 (Claims)
[0011]
[Problems to be solved by the invention]
The present invention is a further improvement of the technique of the above-mentioned Patent Document 3, and aims to propose a steel material having good delayed fracture resistance in addition to excellent torsional fatigue strength, together with its advantageous manufacturing method. To do.
[0012]
Here, delayed fracture resistance means that when steel products are used in the presence of stress, hydrogen in the steel or hydrogen that has penetrated from the usage environment diffuses and accumulates in stress-concentrated parts such as grain boundaries. It is the resistance to the phenomenon that weakens the grain boundary bonding force and causes the grain boundary fracture over time. Therefore, delayed fracture resistance is required for members that are used in the atmosphere, particularly in environments where corrosion progresses, and that are further stressed, that is, products such as drive shafts and constant velocity joints.
[0013]
[Means for Solving the Problems]
That is, the gist configuration of the present invention is as follows.
1. C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and Cr: 0.2 mass% or less, and Ti: 0.06-0.10 mass% and V: 0.25-0.50 mass%
1 or 2 selected from the above, the balance is Fe and inevitable impurities, the base material structure has a bainite structure and / or martensite structure, and these bainite structure and martensite A steel material excellent in delayed fracture resistance and fatigue characteristics after induction hardening, characterized by having a total structure fraction of 10% or more.
[0014]
2. Said 1 WHEREIN: The said steel materials are further Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Delayed fracture resistance and fatigue after induction hardening, characterized in that the composition contains one or more selected from Co: 1.0 mass% or less and Nb: 0.1 mass% or less Steel material with excellent characteristics.
[0015]
3. C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and Cr: 0.2 mass% or less, and Ti: 0.06 to 0.10 mass% and V: 0.25 to 0.50 mass%
1 or 2 selected from the above, the balance is Fe and inevitable impurities, the base material structure has a bainite structure and / or martensite structure, and these bainite structure and martensite The delayed fracture resistance and fatigue characteristics are characterized in that the total structure fraction of the structure is 10% or more, and the prior austenite grain size of the hardened layer after induction hardening is 5 μm or less over the entire thickness of the hardened layer. Excellent steel material.
[0016]
4). 3. A steel material excellent in delayed fracture resistance and fatigue characteristics, wherein the thickness of the hardened layer after induction hardening is 2 mm or more in 3 above.
[0017]
5). In the above 3 or 4, the steel material is further Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less and Nb: 0.1 mass% or less, a composition containing one or more selected from the above, and excellent in delayed fracture resistance and fatigue properties Steel material.
[0018]
6). C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and Cr: 0.2 mass% or less, and Ti: 0.06 to 0.10 mass% and V: 0.25 to 0.50 mass%
A steel material containing one or two selected from the above, with the balance being Fe and an inevitable impurity composition, is cooled at a rate of 0.2 ° C./s or higher after hot working. A method for producing a steel material having excellent delayed fracture resistance and fatigue properties after induction hardening.
[0019]
7). In said 6, the said steel materials are further Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Delayed fracture resistance and fatigue after induction hardening, characterized in that the composition contains one or more selected from Co: 1.0 mass% or less and Nb: 0.1 mass% or less Steel material with excellent characteristics.
[0020]
8). C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and Cr: 0.2 mass% or less, and Ti: 0.06-0.10 mass% and V: 0.25-0.50 mass%
Steel material containing one or two selected from the above, with the balance being Fe and inevitable impurities composition, after hot working, cooled at a rate of 0.2 ° C / s or higher and then quenched A method for producing a steel material excellent in delayed fracture resistance and fatigue properties, characterized by performing induction hardening under conditions of 800 to 1000 ° C.
[0021]
9. In the above 8, the steel material is further Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less and Nb: 0.1 mass% or less, a composition containing one or more selected from the above, and excellent in delayed fracture resistance and fatigue properties Steel manufacturing method.
[0022]
10. In the above 8 or 9, after the cooling, induction hardening is repeated a plurality of times, and the heating temperature at the final induction hardening is set to 800 to 1000 ° C. Production method.
[0023]
11. 10. The method for producing a steel material excellent in delayed fracture resistance and fatigue characteristics, characterized in that the heating temperature during induction hardening is set to 800 to 1000 ° C. for all of the plurality of induction hardenings.
[0024]
12 Any one of the above 8 to 11, wherein the heating time in the heating temperature range is 5 seconds or less per induction hardening, and the method for producing a steel material excellent in delayed fracture resistance and fatigue characteristics .
[0025]
13. The method for producing a steel material excellent in delayed fracture resistance and fatigue characteristics, wherein the thickness of the hardened layer on the steel material surface by induction hardening is 2 mm or more in any one of 8 to 12 above.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
The present invention will be specifically described below.
First, the reason why the steel material and the component composition of the steel material are limited to the above range in the present invention will be described.
C: 0.35-0.7 mass%
C is an element having the greatest influence on hardenability, and increases the hardness and depth of the hardened hardened layer and effectively contributes to the improvement of torsional strength. However, if the content is less than 0.35 mass%, in order to ensure the required torsional strength, the quench hardening depth must be drastically increased, and the occurrence of quenching cracks becomes noticeable, Since it becomes difficult to produce a bainite structure, 0.35 mass% or more is added. On the other hand, when the content exceeds 0.7 mass%, the grain boundary strength is lowered, and accordingly, the torsional fatigue strength is lowered, and the machinability, cold forgeability and fire cracking resistance are also lowered. For this reason, C was limited to the range of 0.35-0.7 mass%. Preferably it is the range of 0.4-0.6 mass%.
[0027]
Si: 0.30 to 1.1 mass%
Si has the effect of increasing the number of nucleation sites of austenite during quenching heating, suppressing grain growth of austenite, and reducing the grain size of the quenched hardened layer. Moreover, carbide | carbonized_material production | generation is suppressed and the fall of the grain boundary strength by carbide | carbonized_material is suppressed. Furthermore, it is an element useful for the generation of a bainite structure, and these improve the torsional fatigue strength.
Thus, Si is a very important element in the present invention, and it is essential to contain 0.30 mass% or more. This is because if the Si amount is less than 0.30 mass%, fine particles having a particle size of 12 μm or less cannot be obtained over the entire thickness of the cured layer, regardless of how the manufacturing conditions and quenching conditions are adjusted. . However, if the amount of Si exceeds 1.1 mass%, the hardness increases due to the solid solution hardening of the ferrite, leading to a decrease in machinability and cold forgeability. Therefore, Si was limited to the range of 0.30 to 1.1 mass%. Preferably it is the range of 0.40-1.0 mass%.
[0028]
Mn: 0.2 to 2.0 mass%
Mn is an essential component for improving the hardenability and ensuring the hardening depth during quenching, so it is actively added. However, if the content is less than 0.2 mass%, the effect of addition is poor. It was set to 0.2 mass% or more. Preferably it is 0.3 mass% or more. On the other hand, if it exceeds 2.0 mass%, the retained austenite after quenching increases, and on the contrary, the surface hardness decreases, and as a result, the torsional fatigue strength decreases. Therefore, Mn is set to 2.0 mass% or less. It should be noted that if Mn is contained in a large amount, the base material is hardened, which may be disadvantageous in machinability. Therefore, the content is preferably set to 1.2 mass% or less. More preferably, it is 1.0 mass% or less.
[0029]
Al: 0.005 to 0.25 mass%
Al is an element effective for deoxidation. Moreover, it is an element useful also in refine | miniaturizing the particle size of a hardening hardening layer by suppressing the austenite grain growth at the time of quenching heating. However, if the content is less than 0.005 mass%, the effect of addition is poor. On the other hand, if the content exceeds 0.25 mass%, the effect is saturated, and rather, a disadvantage that causes an increase in the component cost occurs. Was limited to the range of 0.005 to 0.25 mass%. Preferably it is the range of 0.05-0.10 mass%.
[0030]
Mo: 0.05-0.6 mass%
Mo promotes the formation of a bainite structure, thereby minimizing the austenite grain size during quenching and heating and reducing the grain size of the quenched hardened layer. Moreover, it has the effect | action which refines | miniaturizes the particle size of a hardening hardening layer by suppressing the grain growth of austenite at the time of quenching heating. Furthermore, since it is an element useful for improving hardenability, it is used for adjusting hardenability. In addition, Mo is an element that suppresses the formation of carbides and effectively prevents a decrease in grain boundary strength due to carbides.
Thus, Mo is a very important element in the present invention. If the content is less than 0.05 mass%, the particle size over the entire thickness of the hardened layer can be adjusted no matter how the manufacturing conditions and quenching conditions are adjusted. Cannot be made into fine grains having a size of 12 μm or less. However, if the content exceeds 0.6 mass%, the hardness of the rolled material is remarkably increased, resulting in a decrease in workability. Therefore, Mo is limited to the range of 0.05 to 0.6 mass%. Preferably it is the range of 0.1-0.6 mass%.
[0031]
B: 0.0003 to 0.006 mass%
B has an effect of promoting the formation of a bainite structure or a martensite structure. B also has the effect of improving the torsional strength by improving the hardenability by adding a small amount and increasing the quenching depth during quenching. Further, B preferentially segregates at the grain boundary, reduces the concentration of P segregating at the grain boundary, improves the grain boundary strength, and thus has an effect of improving torsional fatigue strength.
For this reason, in the present invention, B is positively added. However, when the content is less than 0.0003 mass%, the effect of addition is poor, while when the content exceeds 0.006 mass%, the effect is saturated, Rather, in order to increase the component cost, B is limited to the range of 0.0003 to 0.006 mass%. Preferably it is the range of 0.0005-0.004 mass%.
[0032]
S: 0.06 mass% or less S is a useful element that forms MnS in steel and improves the machinability, but if it exceeds 0.06 mass%, it segregates at the grain boundary and decreases the grain boundary strength. Therefore, S is limited to 0.06 mass% or less. Preferably it is 0.04 mass% or less.
[0033]
P: 0.020 mass% or less P segregates at the grain boundary of austenite and reduces the torsional fatigue strength by lowering the grain boundary strength. In addition, there is a harmful effect that promotes burning cracks. Therefore, it is desirable to reduce the P content as much as possible, but 0.020 mass% is acceptable.
[0034]
Cr: 0.2 mass% or less Cr stabilizes carbides to promote the formation of residual carbides, lowers the grain boundary strength, and deteriorates torsional fatigue strength. Therefore, it is desirable to reduce the Cr content as much as possible, but it is acceptable up to 0.2 mass%. Preferably it is 0.05 mass% or less.
[0035]
In the present invention, in addition, one or two selected from Ti and V are contained as components for improving delayed fracture resistance. That is, delayed fracture occurs when hydrogen that has entered from the steel or the environment diffuses and accumulates in the stress-concentrated portion to lower the grain boundary strength. Therefore, suppressing the diffusion of hydrogen in the steel is extremely effective for improving the delayed fracture resistance. Since Ti and V carbonitrides can serve as trap sites for hydrogen, either one or two of Ti and V are added.
[0036]
Ti: 0.06-0.10 mass%
Ti is an effective component for suppressing the diffusion of hydrogen in steel by forming carbonitride, and for that purpose, addition of 0.06 mass% or more is necessary. However, if added in excess, the carbonitride becomes coarse and the effect on delayed fracture resistance becomes small, so 0.10 mass% is made the upper limit.
[0037]
V: 0.25 to 0.50 mass%
V is an effective component for suppressing the diffusion of hydrogen in steel by forming carbonitride, and for that purpose, addition of 0.25 mass% or more is necessary. However, if added in excess, the carbonitride becomes coarse and the effect on delayed fracture resistance becomes small, so 0.50 mass% is made the upper limit.
[0038]
The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
Cu: 1.0 mass% or less Cu is effective in improving hardenability, and is solid-solved in ferrite, and this solid solution strengthening improves torsional fatigue strength. Further, by suppressing the formation of carbides, the decrease in grain boundary strength due to carbides is suppressed, and the torsional fatigue strength is improved. However, if the content exceeds 1.0 mass%, cracking occurs during hot working, so 1.0 mass% or less is added. In addition, Preferably it is 0.5 mass% or less.
[0039]
Ni: 3.5 mass% or less Ni is an element that improves hardenability, and is used when adjusting hardenability. Moreover, it is an element which suppresses the production | generation of a carbide | carbonized_material and suppresses the fall of the grain boundary strength by a carbide | carbonized_material, and improves torsional fatigue strength. However, Ni is an extremely expensive element, and if added in excess of 3.5 mass%, the cost of the steel material increases, so the addition is made 3.5 mass% or less. In addition, since the effect of improving hardenability and the effect of suppressing the decrease in grain boundary strength are small when added at less than 0.05 mass%, it is desirable to add 0.05 mass% or more. Preferably it is 0.1-1.0 mass%.
[0040]
Co: 1.0 mass% or less Co is an element that suppresses the formation of carbides, suppresses a decrease in grain boundary strength due to carbides, and improves torsional strength and torsional fatigue strength. However, Co is an extremely expensive element, and if added in excess of 1.0 mass%, the cost of the steel material increases, so the addition is made 1.0 mass% or less. In addition, since addition of less than 0.01 mass% has a small effect of suppressing the decrease in grain boundary strength, it is desirable to add 0.01 mass% or more. Preferably it is 0.02-0.5 mass%.
[0041]
Nb: 0.1 mass% or less Nb not only has an effect of improving hardenability, but also combines with C and N in steel and acts as a precipitation strengthening element. It is also an element that improves tempering softening resistance, and torsional fatigue strength is improved by these effects. However, even if the content exceeds 0.1 mass%, the effect is saturated, so 0.1 mass% is the upper limit. It should be noted that addition of 0.005 mass% or more is desirable because addition of less than 0.005% has little effect of improving precipitation strengthening action and tempering softening resistance. Preferably it is 0.01-0.05 mass%.
[0042]
Mg ≦ 0.02 mass%, Hf ≦ 0.1 mass%
Both Mg and Hf are not only deoxidizing elements, but also serve as a stress concentration source and have an effect of improving delayed fracture resistance, so that they can be added as necessary. However, if added excessively, the effect is saturated and the component cost is increased. More preferably, the ranges are Mg ≦ 0.01% and Hf ≦ 0.05%.
[0043]
The preferred component composition range has been described above, but in the present invention, it is not sufficient to limit the component composition to the above range, and adjustment of the matrix structure is also important.
That is, in the present invention, the structure of the base material, that is, the structure before quenching (corresponding to the structure other than the hardened layer after induction hardening) has a bainite structure and / or a martensite structure, and these bainite structure and martensite structure. The total tissue fraction of the site organization needs to be 10% or more in volume fraction (vol%). The reason for this is that the bainite structure or martensite structure is a structure in which carbides are finely dispersed compared to the ferrite-pearlite structure, so the area of the ferrite / carbide interface, which is an austenite nucleation site, is increased during quenching heating. This is because the generated austenite is refined, and thus contributes effectively to refine the grain size of the quenched and hardened layer. And grain boundary intensity | strength rises and the torsional fatigue strength improves by refinement | miniaturization of the particle size of a hardening hardening layer.
Here, it is more preferable that the total structure fraction of the bainite structure and the martensite structure is 20 vol% or more.
[0044]
The remaining structure other than the bainite structure or martensite structure may be ferrite, pearlite, or the like, and is not particularly defined.
Also, regarding the refinement of the grain size of the hardened layer after quenching, the martensite structure has the same effect as the bainite structure, but from an industrial point of view, the bainite structure is better than the martensite structure. Since the addition amount of the alloy element is small and it can be produced at a low cooling rate, it is advantageous in production.
[0045]
In the present invention, it is also important to adjust the prior austenite particle size of the hardened layer after induction hardening. That is, regarding the hardened layer after induction hardening, the prior austenite grain size needs to be 5 μm or less over the entire thickness. This is because if the grain size over the entire thickness of the hardened hardening layer exceeds 5 μm, sufficient resistance to delayed fracture cannot be obtained, and satisfactory delayed fracture resistance cannot be expected to be improved. In addition, Preferably it is 3 micrometers or less.
[0046]
Here, the measurement of the prior austenite grain size over the entire thickness of the hardened hardened layer is performed as follows.
In the steel material of the present invention after induction hardening, the outermost surface of the steel material at the induction-hardened portion has a martensite structure of 100% in area ratio. Then, as it goes from the surface to the inside, a region of 100% martensite structure continues at a certain depth, but the area ratio of the martensite structure decreases rapidly from a certain depth.
In the present invention, the depth region from the steel material surface until the area ratio of the martensite structure is reduced to 98% is defined as the hardened layer in the induction-quenched portion.
And about this hardened layer, the average prior austenite particle size in each position of 1/5 position, 1/2 position, and 4/5 position of hardened layer thickness is measured from the surface, and all the average prior austenite particle diameters are 5 micrometers or less. In this case, it is assumed that the prior austenite grain size over the entire thickness of the quenched and hardened layer is 5 μm or less. The average prior austenite particle size was measured from 400 times (1 field area: 0.25 mm × 0.225 mm) to 1000 times (1 field area: 0.10 mm × 0.09 mm) by an optical microscope. 5 fields of view are observed for each position, and the average particle diameter is measured by an image analyzer.
[0047]
Furthermore, in the present invention, the thickness of the hardened layer by induction hardening is preferably 2 mm or more. This is because when the desired properties depend only on the structure near the extreme surface layer such as the rolling fatigue life, a moderate effect can be obtained even if the thickness of the hardened layer is about 1 mm. This is because when the fatigue strength is a problem, the hardened layer thickness is preferably as thick as possible. Therefore, a more preferable hardened layer thickness is 2.5 mm or more, and more preferably 3 mm or more.
[0048]
Next, the manufacturing conditions of the present invention will be described.
The steel material adjusted to a predetermined component composition is subjected to steel bar rolling or hot forging, and then subjected to cold rolling or cold forging as necessary, followed by cutting and induction hardening to obtain a product.
In the present invention, the base material structure has the above-described bainite structure and / or martensite structure, and the total structure fraction of these bainite structure and martensite structure is 10 vol% or more. The material steel before quenching needs to be cooled at a rate of 0.2 ° C./s or higher after being processed into a predetermined shape by hot working such as rolling and forging. This is because when the cooling rate is less than 0.2 ° C./s, it becomes difficult to obtain a bainite or martensite structure, and the total structure fraction of these structures may not reach 10 vol%. is there. The preferable range of the cooling rate after hot working is 0.3 to 30 ° C./s.
In addition, it is preferable to perform a hot working in the temperature range over 900 degreeC-1150 degreeC. If the temperature is 900 ° C. or lower, the necessary bainite structure and / or martensite structure cannot be obtained. On the other hand, if it exceeds 1150 ° C., the heating cost increases, which is economically disadvantageous.
[0049]
Next, in the present invention, induction hardening is performed in order to obtain the above-described cured layer, and the heating temperature range during this induction hardening needs to be 800 to 1000 ° C. This is because when the heating temperature is less than 800 ° C., the austenite structure is not sufficiently generated, and as a result of insufficient generation of the hardened layer structure described above, sufficient torsional fatigue strength cannot be ensured. When the heating temperature exceeds 1000 ° C., the growth of austenite grains is promoted and becomes coarse, and the grain size of the hardened layer becomes coarse, so that the torsional fatigue strength is also lowered. A more preferable heating temperature range is 800 to 950 ° C.
[0050]
When the above-described induction hardening is repeated a plurality of times, at least the final induction hardening may be performed at a heating temperature of 800 to 1000 ° C. Here, when induction hardening is repeated a plurality of times, it is most desirable to set the heating temperature to 800 to 1000 ° C. for all induction hardening. Further, by performing repeated quenching twice or more, a finer cured layer particle size can be obtained as compared with the single quenching.
When induction hardening is repeated a plurality of times, it is preferable that at least the final quenching depth by induction hardening be equal to or greater than the previous quenching depth by induction hardening. This is because the crystal grain size of the hardened layer is most strongly affected by the last induction hardening, so if the hardening depth by the last induction hardening is smaller than the previous hardening depth by the induction hardening, This is because the average crystal grain size over the entire thickness is rather increased and the torsional fatigue strength tends to decrease.
[0051]
Further, in the present invention, the induction hardening is preferably performed with the heating time in the above heating temperature range being 5 seconds or less. This is because when the heating time is set to 5 seconds or less, the austenite grain growth can be further suppressed as compared with the case where the heating time exceeds 5 seconds, and a very fine hardened layer particle size can be obtained. . A more preferable heating time is 3 seconds or less.
[0052]
【Example】
Steel materials having the composition shown in Table 1 were melted by a converter and cast into continuous slabs. The slab size was 300 × 400 mm. The slab was rolled into a 150 mm square billet through a breakdown process, and then rolled into a steel bar having a diameter of 24 to 60 mm. The finishing temperature of rolling was over 900 ° C. as a suitable temperature from the viewpoint of bainite or martensite structure formation. Cooling after hot rolling was performed under the conditions shown in Table 2.
[0053]
Next, a delayed fracture resistant test piece with a parallel part 6 mmφ and a V-shaped annular notch (notch bottom diameter 3 mmφ, notch bottom R = 0.05 mm) was prepared from this bar, and further, the parallel part: 20 mmφ, stress A torsional test piece having a notch with a concentration factor α = 1.5 was produced. Next, both test pieces were quenched under the conditions shown in Table 2 using an induction hardening apparatus having a frequency of 15 kHz, and then tempered under conditions of 170 ° C. × 30 minutes using a heating furnace, Thereafter, a delayed fracture resistance test and a torsional fatigue test were performed using each test piece.
[0054]
Here, in the delayed fracture resistance test, a test piece was immersed in 10% hydrochloric acid for 1 hour to allow hydrogen to penetrate, and then a test was performed in which a constant tensile load was applied in the atmosphere. Then, the stress at which the rupture time becomes 100 hours is obtained, and this is compared with the stress value at which the rupture time is similarly obtained without immersing a test piece prepared using the same steel material in hydrochloric acid (100 hours). Delayed fracture strength ratio). The obtained results are also shown in Table 2.
[0055]
In addition, the torsional fatigue test is performed using a torsional fatigue testing machine with a maximum torque of 4900 N · m (= 500 kgf · m 2), changing the stress conditions with both swings, and a stress that gives a life of 1 × 10 ⁵ times. Was evaluated as fatigue strength.
The obtained results are also shown in Table 2.
In addition, for the torsional fatigue test piece produced under the same conditions, the average hardened layer particle size (old austenite particle size) obtained over the base material structure of the steel material, the hardened layer thickness after quenching, and the entire thickness of the hardened layer was measured using an optical microscope. And measured.
Table 2 also shows these results.
Here, as described above, the hardened layer thickness was from the steel surface to the depth at which the area ratio of the martensite structure was reduced to 98%. Moreover, about what performed induction hardening several times, the hardening layer thickness after each hardening was measured. Further, for the hardened layer particle size, the average prior austenite particle size at each of the 1/5 position, 1/2 position and 4/5 position of the hardened layer thickness from the surface was measured, and the maximum value thereof was shown. In addition, about what performed induction hardening several times, the average prior-austenite particle size after final hardening was measured.
[0056]
[Table 1]

[0057]
[Table 2]

[0058]
As apparent from Table 2, all the steel materials manufactured under the conditions satisfying the component composition range defined in the present invention and satisfying the induction hardening conditions of the present invention have an austenite grain size of the hardened layer of 5 μm over the entire thickness. As a result, good delayed fracture resistance and high torsional fatigue strength of 700 MPa or more could be obtained.
[0059]
In Table 2, No. 1 and 2 or no. Comparing 4 and 5, it can be seen that by increasing the number of times of quenching from one to two, the particle size of the hardened layer becomes finer, and the delayed fracture resistance and torsional fatigue strength further increase.
[0060]
No. 8, no. 35, no. 36, when the number of quenching was increased from 1 to 2 and the second quenching depth was shallower (No. 35), the delayed fracture resistance than the case where the quenching was performed only once While the properties and torsional fatigue strength are rather reduced, the delayed fracture resistance and torsional fatigue strength are higher when the second quenching depth is deeper (No. 36) than when only once. It turns out that it improves significantly. No. 36, the oldest austenite grain size was 4.2 μm at the 4/5 position of the hardened layer thickness from the surface in the hardened layer thickness direction, but in the vicinity of the surface layer (1/5 position of the hardened layer thickness from the surface). Thus, the prior austenite grain size is 2.8 μm, and the finer grain size of the surface layer is considered to have contributed to the improvement of fatigue strength and delayed fracture resistance.
[0061]
In contrast, no. No. 9 has a low cooling rate after processing, so the total structural fraction of bainite and martensite is less than 10%. As a result, the hardened layer particle size becomes coarse, delayed fracture resistance, and torsional fatigue strength. Low.
No. No. 22, although the hardened layer particle size is fine, since the C content is higher than the range of the present invention, the grain boundary strength is lowered, and therefore the delayed fracture resistance and torsional fatigue strength are inferior.
No. In Nos. 23, 24 and 25, the content of C, Si and Mo is lower than the appropriate range of the present invention, so the hardened layer particle size is coarse and the delayed fracture resistance and torsional fatigue strength are inferior.
No. No. 26 has a low B content. No. 27 has a Mn content of No. 27. No. 28 has S and P contents of No. 28. No. 29 has a Cr content exceeding the appropriate range of the present invention, so that both of them cause a decrease in grain boundary strength and are inferior in delayed fracture resistance and torsional fatigue strength.
No. No. 30 is inferior in delayed fracture resistance and torsional fatigue strength because the Ti and V contents exceed the proper range of the present invention. Since No. 33 has low Ti and V contents, the delayed fracture resistance is inferior.
No. In No. 31, the particle size of the hardened layer becomes coarse because the heating temperature during induction hardening is too high. No. 32 does not form a hardened layer because the heating temperature during induction hardening is too low, and both are inferior in delayed fracture resistance and torsional fatigue strength.
No. 34 is a case where the amount of Si is 0.28 mass%, which is less than the lower limit of the present invention. However, as in this example, when the Si amount is slightly below the lower limit of the present invention, 12 μm over the entire thickness of the hardened layer. The following particle sizes cannot be obtained, and as a result, the delayed fracture resistance and torsional fatigue strength are poor.
[0062]
【The invention's effect】
Thus, according to the present invention, it is possible to stably obtain a steel material having delayed fracture resistance and high torsional fatigue strength. As a result, the present invention is excellent for the demand for weight reduction of automobile members.

Claims (13)

C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and Cr: 0.2 mass% or less, and Ti: 0.06-0.10 mass% and V: 0.25-0.50 mass%
1 or 2 selected from the above, the balance is Fe and inevitable impurities, the base material structure has a bainite structure and / or martensite structure, and these bainite structure and martensite A steel material excellent in delayed fracture resistance and fatigue characteristics after induction hardening, characterized in that the total structural fraction of the structure is 10% or more. In Claim 1, the said steel materials are further Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Delayed fracture resistance and fatigue after induction hardening, characterized in that the composition contains one or more selected from Co: 1.0 mass% or less and Nb: 0.1 mass% or less Steel material with excellent characteristics. C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and Cr: 0.2 mass% or less, and Ti: 0.06 to 0.10 mass% and V: 0.25 to 0.50 mass%
1 or 2 selected from the above, the balance is Fe and inevitable impurities, the base material structure has a bainite structure and / or martensite structure, and these bainite structure and martensite The delayed fracture resistance and fatigue characteristics are characterized in that the total structure fraction of the structure is 10% or more, and the prior austenite grain size of the hardened layer after induction hardening is 5 μm or less over the entire thickness of the hardened layer. Excellent steel material. The steel material excellent in delayed fracture resistance and fatigue characteristics according to claim 3, wherein the thickness of the hardened layer after induction hardening is 2 mm or more. In Claim 3 or 4, the said steel materials are further Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less and Nb: 0.1 mass% or less, a composition containing one or more selected from the above, and excellent in delayed fracture resistance and fatigue properties Steel material. C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and Cr: 0.2 mass% or less, and Ti: 0.06 to 0.10 mass% and V: 0.25 to 0.50 mass%
A steel material containing one or two selected from the above, with the balance being Fe and an inevitable impurity composition, is cooled at a rate of 0.2 ° C./s or higher after hot working. A method for producing a steel material having excellent delayed fracture resistance and fatigue properties after induction hardening. In Claim 6, the said steel materials are further Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Delayed fracture resistance and fatigue after induction hardening, characterized in that the composition contains one or more selected from Co: 1.0 mass% or less and Nb: 0.1 mass% or less Steel material with excellent characteristics. C: 0.35-0.7 mass%,
Si: 0.30 to 1.1 mass%,
Mn: 0.2 to 2.0 mass%,
Al: 0.005 to 0.25 mass%,
Mo: 0.05-0.6 mass%,
B: 0.0003 to 0.006 mass%,
S: 0.06 mass% or less,
P: 0.02 mass% or less and Cr: 0.2 mass% or less, and Ti: 0.06-0.10 mass% and V: 0.25-0.50 mass%
Steel material containing one or two selected from the above, with the balance being Fe and inevitable impurities composition, after hot working, cooled at a rate of 0.2 ° C / s or higher and then quenched A method for producing a steel material excellent in delayed fracture resistance and fatigue properties, characterized by performing induction hardening under conditions of 800 to 1000 ° C. In Claim 8, the steel material is further Cu: 1.0 mass% or less,
Ni: 3.5 mass% or less,
Co: 1.0 mass% or less and Nb: 0.1 mass% or less, a composition containing one or more selected from the above, and excellent in delayed fracture resistance and fatigue properties Steel manufacturing method. The steel material excellent in delayed fracture resistance and fatigue characteristics according to claim 8 or 9, wherein after the cooling, induction hardening is repeated a plurality of times, and the heating temperature at the final induction hardening is set to 800 to 1000 ° C. Manufacturing method. The method for producing a steel material excellent in delayed fracture resistance and fatigue characteristics according to claim 10, wherein the heating temperature during induction hardening is set to 800 to 1000 ° C. for all of the plurality of induction hardenings. The production of a steel material excellent in delayed fracture resistance and fatigue characteristics according to any one of claims 8 to 11, wherein the heating time in the heating temperature range is 5 seconds or less per induction hardening. Method. The method for producing a steel material having excellent delayed fracture resistance and fatigue properties according to any one of claims 8 to 12, wherein the thickness of the hardened layer on the surface of the steel material by induction hardening is 2 mm or more.