JP2006328513A - Induction-hardened steel component with axial part having excellent static strength and fatigue property - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an induction-hardened steel component with an axial part having excellent static strength and excellent fatigue properties. <P>SOLUTION: The induction-hardened steel component with an axial part having excellent static strength and fatigue properties has a composition satisfying, by mass, 0.30 to 0.50% C, 0.5 to 2.0% Si, 0.5 to 2.0% Mn, 0.10 to 1.0% Cr, ≤0.1% (excluding 0%) Al, 20 to 200 ppm N, and the balance iron with inevitable impurities, and in which the ratio between the depth (t) of the effective hardened layer and the minimum radius (r) of the axial part, (t/r) is ≥0.75. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an induction-hardened steel part having a shaft-like portion having high static strength and excellent fatigue characteristics. The induction-hardened steel part of the present invention is widely used in a wide range of industrial fields such as the automobile field, the construction machine field or the industrial machine field. However, in the following, it is applied to a power transmission part for automobiles as a representative example. The explanation will be made by taking the case as an example.

  Shaft steel parts such as drive shafts, which are power transmission parts of automobiles, are required to exhibit excellent fatigue characteristics as well as high static strength in order to cope with higher engine output. The steel parts are generally obtained by induction hardening. This is because induction hardening can effectively achieve surface hardening and compressive residual stress, and is useful for improving fatigue strength and static strength. In addition, induction hardening has the advantage of being environmentally friendly, in addition to its low processing cost and extremely short processing time, so that it can be processed efficiently.

  As a technique for increasing the fatigue strength of steel parts by adopting such induction hardening, for example, Patent Document 1 proposes a technique for improving the fatigue strength by applying an induction hardening process and applying compressive residual stress. Has been. However, as in Patent Document 1, it is very difficult to control the depth of the hardened layer by subjecting the teeth to induction hardening.

Patent Document 2 relates to an induction-quenched shaft part. The torsional fatigue characteristics are controlled by controlling the depth ratio of the hardened layer after induction hardening of the part to 0.3 to 0.7 and further performing shot peening. It has been proposed that it can be improved. Patent Document 3 is also a technique related to induction-hardened components, and it has been proposed that the torsional strength can be improved by setting the depth ratio of the hardened layer after induction hardening to 0.3 to 0.8. In these technologies, the upper limit of the hardened layer depth after induction quenching is specified in order to give compressive residual stress for the purpose of ensuring fatigue properties, but it is difficult to increase the static strength together with the fatigue properties. Conceivable.
Japanese Patent Publication No. 5-58055 Japanese Patent Publication No.7-90379 Japanese Patent No. 2916069

  This invention is made | formed in view of the said situation, The objective is to provide the induction-hardened steel component which has a shaft-shaped part which exhibits high static strength and the outstanding fatigue characteristic collectively.

Induction hardened steel parts having a shaft-like part according to the present invention are:
C: 0.30 to 0.50% (meaning mass%, the same shall apply hereinafter)
Si: 0.5 to 2.0%,
Mn: 0.5 to 2.0%
Cr: 0.10 to 1.0%,
Al: 0.1% or less (excluding 0%),
N: 20 to 200 ppm
And the balance consists of iron and inevitable impurities,
It is characterized in that the ratio (t / r) between the effective hardened layer depth (t) and the minimum radius (r) of the shaft-like portion is 0.75 or more.

The induction-hardened steel parts are further elements,
(A) One or more selected from the group consisting of V: 0.2% or less (excluding 0%), Ni: 0.1-2.0%, and Mo: 0.1-1.0% (B) Ti: 0.005-0.05% and B: 5-30 ppm
(C) S: 0.030-0.20%, Ca: 0.0005-0.005% and / or Mg: 0.0001-0.0020%
May be included.

  In addition, the induction-hardened steel part of the present invention is preferably one having a surface layer compressive residual stress of 400 MPa or more because of excellent fatigue characteristics.

  ADVANTAGE OF THE INVENTION According to this invention, the induction-hardened steel component which has a shaft-shaped part optimal as a power transmission component for motor vehicles with high static strength and excellent fatigue characteristics is realizable.

  From the viewpoint of improving the static strength of the induction-hardened component having the shaft-shaped portion, it is effective to increase the hardened layer depth ratio and the surface hardness. However, if the depth ratio of the hardened layer is remarkably increased or the surface hardness is increased in this way, there is a problem that the fatigue strength decreases due to a decrease in compressive residual stress and an increase in notch sensitivity associated with an increase in hardness. is there. Therefore, the present inventor has intensively studied to improve fatigue characteristics in a state where the hardened layer depth ratio is increased in order to ensure high static strength. As a result, it has been found that it is very effective to positively contain Si in steel parts having a relatively large hardened layer depth. Hereinafter, the present invention will be described in detail.

  First, in the present invention, in order to achieve a high static strength, the ratio (t) between the effective hardened layer depth (t) of an induction-hardened steel part having a shaft part and the minimum radius (r) of the part of the shaft part (t) / R) is 0.75 or more (preferably 0.80 or more). This is because securing the effective hardened layer depth in this way is most effective for securing high static strength.

  Then, by increasing the t / r, the compressive residual stress is lowered, and as a result, the fatigue characteristics that deteriorate as a result are improved by making Si actively contained and making it higher than conventional steel parts. I found it very important. The reason why the fatigue strength can be increased by containing a relatively large amount of Si as described above is that the fatigue softening at the crack tip during fatigue crack propagation is suppressed by Si (work hardening occurs at the crack tip). This is thought to be because, as a result of increased crack propagation resistance, both intragranular fracture and intergranular fracture are less likely to occur, and the fatigue crack propagation life is prolonged.

  In order to sufficiently exhibit the above effects, the Si amount needs to be 0.5% or more. Preferably it is 0.6% or more, More preferably, it is 0.7% or more. However, if Si is excessive, decarburization is promoted and machinability is lowered, so the content is suppressed to 2.0% or less (preferably 1.5% or less).

  From the viewpoint of suppressing the decrease in compressive residual stress, the t / r is preferably 0.90 or less.

  In order to improve the static strength and fatigue characteristics of the induction-hardened steel part having the shaft-shaped portion as described above, the present invention particularly improves the effective hardened layer depth (t) and the minimum radius (r of the shaft-shaped portion). ) And the amount of Si in steel and the amount of Si in the steel are controlled, but other components need to be controlled as follows in order to reliably exhibit the effects. .

<C: 0.30 to 0.50%>
C is an element indispensable for increasing the fatigue strength after induction hardening. In order to exhibit this effect, it is necessary to contain C 0.30% or more, preferably 0.40% or more. However, if C is excessive, quench cracking occurs during quenching or notch sensitivity increases, and the fatigue strength decreases. Therefore, it is suppressed to 0.50% or less (preferably 0.48% or less).

<Mn: 0.5 to 2.0%>
Mn is an element that improves the hardenability of the steel material and needs to be 0.5% or more (preferably 0.6% or more). However, if Mn is contained excessively, a bainite structure is formed, which causes a decrease in machinability and an increase in cracking susceptibility of the hardened portion, so it is suppressed to 2.0% or less (preferably 1.5% or less).

<Cr: 0.10 to 1.0%>
Cr is an element that improves the hardenability of the steel material. In the present invention, Cr is contained in an amount of 0.10% or more (preferably 0.4% or more). However, if Cr is excessively contained, a bainite structure is formed and machinability is lowered, so the content is suppressed to 1.0% or less (preferably 0.8% or less).

<Al: 0.1% or less (excluding 0%)>
Al is an element effective for deoxidation and crystal grain refinement, and in order to exert these effects, it is preferable to contain 0.01% or more. However, even if Al is contained excessively, the effect of refining the crystal grains is saturated, and rather, the inclusions formed by combining with oxygen increase, resulting in an adverse effect that machinability deteriorates. % Or less (preferably 0.05% or less).

<N: 20 to 200 ppm>
N is an element useful for refining crystal grains by combining with Al or Ti described later to suppress the growth of austenite crystal grains after induction hardening. In order to exert such an effect, the N amount is set to 20 ppm or more (more preferably 50 ppm or more). However, if the amount of N is excessive, surface cracks during continuous casting become significant, so the amount is suppressed to 200 ppm or less. Preferably it is 150 ppm or less.

  The contained elements specified in the present invention are as described above, and the balance is iron and inevitable impurities. As the inevitable impurities, P brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. and O of 0.0020% or less. Etc. can be allowed to be mixed. P causes segregation at the grain boundary to lower the grain boundary strength and causes embrittlement. Therefore, it is recommended to suppress it to 0.015% or less (preferably 0.010% or less).

  Furthermore, it is also possible to positively contain the following elements as long as the effects of the present invention are not adversely affected.

<V: 0.2% or less (excluding 0%),
Ni: 0.1-2.0% and Mo: 0.1-1.0%
One or more selected from the group consisting of>
V is an element effective for ensuring the hardness and fatigue strength of the non-induction-hardened portion of the steel part, and in order to exert this effect, it is preferably contained in an amount of 0.1% or more. However, since the effect is saturated even if V is excessive, it is preferable to suppress it to 0.2% or less (more preferably 0.18% or less).

  Ni is an element useful for increasing the hardenability of the steel material, and is preferably 0.1% or more, more preferably 0.2% or more in order to exert the effect. However, if Ni is excessive, the machinability deteriorates, so it is preferable to keep it at 2.0% or less. More preferably, it is 1.5% or less.

  Mo is also an element useful for increasing the hardenability of the steel material. In order to exhibit this effect, it is preferable to contain 0.1% or more, and more preferably 0.2% or more. However, if Mo is excessive, the machinability deteriorates, so it is preferable to keep it at 1.0% or less. More preferably, it is 0.7% or less.

<Ti: 0.005 to 0.05%>
Ti is an element that binds to these elements because of its strong affinity with N and O, and keeps B, which will be described later, in a solid solution state, and effectively exhibits the effect of improving the hardenability of B and the effect of strengthening grain boundaries. is there. Moreover, it is effective for refinement | miniaturization of a crystal grain, and in order to exhibit these effects, it is preferable to make Ti amount 0.005% or more. More preferably, it is 0.01% or more. However, if Ti is excessive, machinability is hindered, so 0.05% is preferable as the upper limit. More preferably, it is 0.03% or less.

<B: 5 to 30 ppm>
B has the effect of increasing the hardenability in a small amount and also has the effect of increasing the grain boundary strength. In order to exert these effects, the B content is preferably 5 ppm or more. More preferably, it is 10 ppm or more. However, even if B is excessive, the above effect is only saturated, so the B content is preferably 30 ppm or less. More preferably, it is 25 ppm or less.

<S: 0.030 to 0.20%;
Ca: 0.0005 to 0.005% and / or Mg: 0.0001 to 0.0020%>
S is an element effective for securing machinability by forming MnS. Further, by adding Ca and / or Mg, stretching of MnS is suppressed, and machinability and anisotropy are improved. .

  In order to effectively exhibit the above effects, it is preferable that S is 0.030% or more (more preferably 0.040% or more). When Ca is contained, it is 0.0005% or more (more preferably 0.0010% or more), and when Mg is contained, it is 0.0001% or more (more preferably 0.0005% or more). Is good.

  However, if S is excessive, hot working cracks and the like are likely to occur, so S is preferably suppressed to 0.20% or less (more preferably 0.15% or less). Further, even if Ca and Mg become excessive, the effect is only saturated, so Ca is 0.005% or less (more preferably 0.003% or less), and Mg is 0.0020% or less (more Preferably it is 0.0015% or less.

  In order to further increase the fatigue strength of the steel part of the present invention, the surface compressive residual stress of the steel part is preferably 400 MPa or more, more preferably 500 MPa or more. Thus, in order to increase the surface compressive residual stress of the part, it is effective to perform shot peening after induction hardening. The present invention is not limited to the conditions for the shot peening, and general conditions can be adopted. However, as described above, the shot peening can be performed under the following conditions in order to make the surface layer compressive residual stress efficiently 400 MPa or more. Recommended.

・ Shot hardness: HRC50 or more ・ Shot particle size: φ0.3-0.8mm
・ Projection speed: 50 m / s or more The conditions other than the above are not particularly limited, and after melting and casting under general production conditions, hot forging, warm forging, and cold forging are performed, and further, cutting is appropriately performed. Thereafter, the steel part of the present invention can be obtained by induction hardening so that t / r is 0.75 or more as described above.

  The induction-hardened steel part of the present invention is not limited to its shape, but if applied to a part having a shaft-shaped portion having a minimum diameter of 8 to 40 mm, the effect of the present invention can be effectively exhibited. The induction-hardened steel parts of the present invention are used in a wide range of industrial fields such as the automobile field, construction machine field or industrial machine field, and are particularly suitable as power transmission parts for automobiles having shaft-shaped parts such as drive shafts and transmissions. is there.

  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. It is also possible to implement, and they are all included in the technical scope of the present invention.

  Conventional melting method for steels having the chemical components shown in Table 1 (the amount of P is 0.007% for A5 in Table 1, 0.025% for B8, and 0.015 to 0.025% for others). According to the above, after melting / casting, hot forging and normalizing were performed, and then cutting was performed to prepare a specimen for fatigue test having a parallel portion of φ8 mm. After subjecting the obtained specimen for fatigue test to induction hardening under the following conditions, the effective hardened layer depth (t) was measured as described in JISG 0559, and the effective hardened layer depth (t) and the parts The ratio (t / r) to the minimum radius (r; 4 mm in this example) was determined.

  Moreover, the residual stress of the surface layer of the test piece after induction hardening was measured with the X-ray residual stress measuring apparatus (In addition, about A7 of Table 1, the residual stress was measured after the following shot peening).

<Induction hardening conditions>
After heating under the following conditions, it was cooled with water and then tempered at 200 ° C. for 60 minutes.

・ Frequency: 40 kHz
・ Voltage: 7.5kV
・ Current: 2.5A
・ Secondary current: 7.0A
・ Work coil moving speed: 6mm / sec
About A7 of Table 1, after the said induction hardening, the shot peening process was performed on the following conditions.

<Shot peening (S / P) processing conditions>
-Shot grain hardness: HRC61-64
・ Shot particle size: φ0.6mm
In addition, a tensile test was performed to measure the tensile strength (static strength), and a fatigue test was performed under the following conditions.

<Fatigue test>
A tensile compression load [stress ratio R (minimum stress / maximum stress) = − 1] was applied to the test piece, and the maximum stress that achieves a life of 1 × 10 ⁶ times was defined as fatigue strength.

  These results are also shown in Table 1.

  It can be considered as follows from Table 1 (note that the following A1 to B9 indicate symbols in Table 1). Since A1 to A8 satisfy the requirements defined in the present invention, the static strength is high and the fatigue characteristics are also excellent.

  On the other hand, B1 to B9 did not satisfy any of the conditions defined in the present invention, so the static strength was small or the fatigue characteristics were inferior. Specifically, B1 and B2 have a small hardened layer depth ratio (t / r), so that fatigue characteristics can be secured, but the static strength is insufficient. B3 and B4 are examples in which the amount of Si is insufficient, and the fatigue characteristics are high to some extent, but the static strength is small because they are not within the range defined by the hardened layer depth ratio.

  B5 has a low static strength because of a small depth ratio of the hardened layer, and is inferior in fatigue characteristics because the amount of Si is insufficient. B6 and B7 are inferior in fatigue characteristics because of the insufficient amount of Si. B8 is inferior in fatigue characteristics due to insufficient amount of Si. B9 has a low static strength because the depth ratio (t / r) of the hardened layer is small.

It is a graph which shows the relationship between hardened layer depth ratio (t / r) and fatigue strength.

Claims (5)

C: 0.30 to 0.50% (meaning mass%, the same shall apply hereinafter)
Si: 0.5 to 2.0%,
Mn: 0.5 to 2.0%
Cr: 0.10 to 1.0%,
Al: 0.1% or less (excluding 0%),
N: 20 to 200 ppm
And the balance consists of iron and inevitable impurities,
An induction-hardened steel part having a shaft-shaped part,
A ratio of the effective hardened layer depth (t) to the minimum radius (r) of the shaft-shaped portion (t / r) is 0.75 or more, and the shaft-shaped portion having excellent static strength and fatigue characteristics Induction hardened steel parts. Furthermore,
V: 0.2% or less (excluding 0%),
Ni: 0.1-2.0% and Mo: 0.1-1.0%
The induction-hardened steel part according to claim 1, comprising at least one selected from the group consisting of: Furthermore,
Ti: 0.005 to 0.05%, and B: 5 to 30 ppm
The induction-hardened steel part according to claim 1 or 2, comprising: Furthermore,
S: 0.030 to 0.20%,
The induction-hardened steel part according to any one of claims 1 to 3, comprising Ca: 0.0005 to 0.005% and / or Mg: 0.0001 to 0.0020%.   The induction-hardened steel part according to any one of claims 1 to 4, wherein the surface layer compressive residual stress is 400 MPa or more.

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