JP2007332438A - Steel material for carburizing and quenching having excellent low cycle fatigue property and carburized and quenched component - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a steel material for carburizing and quenching having excellent low cycle fatigue properties. <P>SOLUTION: The steel material is characterized in that: a projected core hardness Hp-core=Hcore/(1-t/r) is ≥390 HV; the hardness at a position of 13 mm from the quenched edge in a Jominy test is ≥60×C<SP>0.5</SP>-5(HRC); and A=(Mo+0.227Ni+190B-0.087Si-17.2P-2.74V-7.18Cs-0.00955Hs+0.0344Nγ) and B=(t×(Hcore)<SP>2</SP>) satisfy the relation of A-0.00000293×B≥14; wherein, Hcore is a core hardness; (t) is an effective hardened layer depth; (r) is the radius of the failure zone or the half of the wall thickness of the failure zone; Cs is the carburization concentration (mass%) in the surface layer; Hs is a surface hardness (HV); and Nγ is the old austenite grain size of a carburized layer. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a carburized and quenched steel material having excellent low cycle fatigue characteristics and a carburized and quenched part, which is used for a part that is carburized in a surface layer portion.

  Gears, bearing parts, rolling parts, shafts, and constant velocity joint parts are usually made by cold forging (for example, medium-carbon alloy steels for machine structural use defined in JIS G 4052, JIS G 4104 to 4106, etc.) (Including rolling) or hot forging-cutting into a predetermined shape, followed by carburizing and quenching. The gears and parts described above may undergo fatigue failure due to repeated large impacts despite a small number of repetitions. For this reason, resistance to such fatigue fracture (hereinafter referred to as low cycle fatigue characteristics) is required.

As a technique for improving the low cycle fatigue characteristics, for example, there are a technique described in Patent Document 1 and a technique described in Patent Document 2.
The technique described in Patent Document 1 improves low cycle fatigue characteristics by setting the sum of plastic deformation resistance and grain boundary strength to a certain value or more. The plastic deformation resistance is calculated by an equation using the chemical component of the steel as a variable, and substantially increases as the core hardness increases. Further, when the grain boundary strength is high, the toughness becomes high. In order to maintain the machinability, an upper limit is set for the sum of the plastic deformation resistance and the grain boundary strength.

The technique described in Patent Document 2 improves the low cycle fatigue characteristics by setting the core hardness and the toughness of the carburized layer to be equal to or higher than a reference value. Each of the core hardness and the toughness of the carburized layer is organized by a function with the chemical component of the steel material as a variable.
JP-A-10-259450 (13th and 14th paragraphs, FIG. 1) JP 2004-238702 A (25th and 26th paragraphs)

According to the above-described conventional technique, the higher the toughness and core hardness of the hardened layer, the lower the low cycle fatigue characteristics. However, as a result of studies by the present inventors, it has been found that even if the core hardness is increased, the low cycle fatigue characteristics do not always exhibit a sufficient value.
The present invention has been made in consideration of the above-mentioned circumstances, and the object thereof is to provide a carburized and quenched steel material and a carburized and hardened component that are capable of stably improving the low cycle fatigue characteristics and excellent in the low cycle fatigue characteristics. Is to provide.

The gist of the present invention for solving the above problems is as follows.
(A) In mass%,
C: 0.1-0.4%
Si: 0.02-1.3%
Mn: 0.3 to 1.8%
S: 0.001 to 0.15%,
Al: 0.001 to 0.05%,
N: 0.003 to 0.020%,
P: 0.025% or less,
O: 0.0025% or less,
Cr: 1.8% or less,
Mo: 1.5% or less,
Ni: 3.5% or less,
B: 0.006% or less,
V: 0.5% or less,
Nb: 0.04% or less,
Ti: 0.2% or less,
1 type or 2 types or more, and the balance consists of iron and inevitable impurities,
A carburized and hardened steel material excellent in low cycle fatigue characteristics, characterized in that the projected core hardness Hp-core defined by the following formula (1) is HV390 or more.
Hp-core = Hcore / (1-t / r) (1)
Where Hcore: core hardness, t: effective hardened layer depth, r: half of the radius of the damaged part or the thickness of the damaged part.

(B) The ferrite + bainite fraction of the core structure is 50% or less and the balance is substantially martensite, and A defined by the following formula (2) and B defined by the following formula (3) are A-0.000002293 × B ≧ −14. The carburized and quenched steel material having excellent low cycle fatigue characteristics as described in (a) above.
A = Mo + 0.227Ni + 190B-0.087Si-17.2P-2.74V-7.18Cs-0.00955Hs + 0.0344Nγ (2)
Where Cs: carbon concentration (% by mass) of surface layer, Hs: surface hardness (HV), Nγ: former austenite grain size of carburized layer
B = t × (Hcore) ² (3)

(C) The hardness at a position of 13 mm from the quenching end in the Jomini test is 60 × C (mass%) ^0.5 −5 (HRC) or more as the hardenability of the steel material,
A defined by the following formula (2) and B defined by the following formula (3) have a relationship of A−0.00000293 × B ≧ −14, as described in the above (a) Carburized and hardened steel with excellent low cycle fatigue characteristics.
A = Mo + 0.227Ni + 190B-0.087Si-17.2P-2.74V-7.18Cs-0.00955Hs + 0.0344Nγ (2)
Where Cs: surface carburization concentration (% by mass), Hs: surface hardness (HV), Nγ: old austenite grain size of the carburized layer.
B = t × (Hcore) ² (3)

(D)% by mass,
C: 0.1-0.4%
Si: 0.02-1.3%
Mn: 0.3 to 1.8%
S: 0.001 to 0.15%,
Al: 0.001 to 0.05%,
N: 0.003 to 0.020%,
P: 0.025% or less,
O: 0.0025% or less,
Cr: 1.8% or less,
Mo: 1.5% or less,
Ni: 3.5% or less,
B: 0.006% or less,
V: 0.5% or less,
Nb: 0.04% or less,
Ti: 0.2% or less,
1 type or 2 types or more, and the balance consists of iron and inevitable impurities,
The hardness at a position 13 mm from the quenching end in the Jomini test is 60 × C ^0.5 -5 (HRC) or more,
Carburization excellent in low cycle fatigue characteristics, wherein A defined by the following formula (2) and B defined by the following formula (3) have a relationship of A-0.00000293 × B ≧ −14 Hardened steel.
A = Mo + 0.227Ni + 190B-0.087Si-17.2P-2.74V-7.18Cs-0.00955Hs + 0.0344Nγ (2)
However, Cs: Carburizing concentration of surface layer (mass%), Hs: Surface hardness (HV), Nγ: Old austenite grain size of carburized layer.
B = t × (Hcore) ² (3)

(E) The carburization concentration Cs of the surface layer is 0.5 to 0.8% by mass%, and excellent in low cycle fatigue characteristics as described in any one of the above (a) to (d) Carburized and hardened steel.
(F) The carburized and quenched steel material having excellent low cycle fatigue characteristics according to any one of the above (a) to (e), wherein the prior austenite grain size Nγ of the carburized layer is No. 8-15.
(G) The carburized and quenched steel material having excellent low cycle fatigue characteristics according to any one of the above (a) to (f), wherein the surface residual stress is −500 MPa or less.

  A carburized and quenched part using the carburized and quenched steel material according to any one of the above (a) to (g). The carburized and quenched part is, for example, a gear.

  According to the present invention, low cycle fatigue characteristics can be stably improved in carburized and quenched steel materials and carburized and quenched parts.

  The steel of the present invention is a carburized and hardened steel material used as gears, bearing parts, rolling parts, shafts, and constant velocity joint parts.

As a result of intensive studies, the present inventor has considered that low cycle fatigue failure of carburized and quenched steel material is caused by the following process.
(A) Distortion concentrates near the boundary between the carburized layer and the core, and microcracks are generated.
(B) The microcracks propagate to the carburized layer, and the carburized layer causes brittle fracture with intergranular cracking.
(C) Thereafter, the core portion is rapidly broken.

  First, means for suppressing the process (A) were studied. When the carburized layer depth is remarkably shallow, or when the core hardness is extremely low, strain concentration near the boundary between the carburized layer and the core and generation of microcracks occur very easily. For this reason, it is necessary to make each carburized layer depth and core part hardness more than a certain critical value.

FIG. 1 is a schematic diagram for explaining a fracture mechanism when a microcrack is generated. When the core hardness is increased from a to b in the figure, the fracture starting point does not change, but the fatigue strength increases. On the other hand, when the depth of the effective carburized layer depth, fracture origin fatigue strength to change from t ₁ to t ₂ is increased. Therefore, the projection core hardness Hp-core defined by the following formula (1) and FIG. 1 was defined as a new index that can describe both the carburized layer depth and the core hardness.
Hp-core = Hcore / (1-t / r) (1)
Where Hcore: core hardness, t: effective hardened layer depth (specified in JIS G 0557), r: half of the radius of the damaged part or the thickness of the damaged part. The damaged part is a so-called design critical section, and in the gear part, the part indicated by the arrow in FIG. 2 corresponds to the thickness of the damaged part. In a shaft-like component such as a shaft, the minimum diameter portion and the radius of the cross section where the stress concentration is maximum correspond to this.

  As a result of the study by the inventors, in the component system described later, if the projected core hardness Hp-core is less than HV390, microcracks are easily generated near the boundary between the carburized layer and the core during low cycle fatigue. However, it has been found that the occurrence of this microcrack can be delayed if it is HV390 or higher.

  Next, means for suppressing the process (B) were studied. It was considered that whether or not a microcrack generated near the boundary between the carburized layer and the core propagates to the carburized layer is determined by the toughness of the core, the toughness of the carburized layer, and the degree of triaxial stress at the tip of the microcrack.

The toughness of the core part depends on the structure of the core part. As a result of low hardenability, the toughness deteriorates significantly when the ferrite and bainite structure fraction exceeds 50% in the core. That is, in order to ensure excellent core toughness, it is necessary that the ferrite + bainite fraction of the core structure is 50% or less and the balance is substantially (substantially) martensite. In order to satisfy the above-described requirements, in the present invention, the hardenability of the steel material is determined according to the hardness at a position 13 mm from the quenching end in the Jomini test, according to the amount of C. 60 × (C) ^{1 /} be ² -5 (HRC) or higher has been found to be necessary. The above evaluation can be performed according to the provisions of JIS G 0561 by collecting a Jomini end-hardened test piece from the core.

  Thus, when the hardenability more than a specified value is ensured and the core portion is martensite, whether or not the microcracks propagate to the carburized layer depends on the toughness of the carburized layer and the triaxial stress degree at the tip of the microcrack. Determined.

Since the toughness of the carburized layer is determined by the carbon concentration (mass%) of the surface layer, surface hardness (HV), the prior austenite crystal grain size of the carburized layer and the chemical component of the core, an index defined by the following formula (2) A was introduced. The larger the index A, the better the toughness of the carburized layer.
A = Mo + 0.227Ni + 190B-0.087Si-17.2P-2.74V-7.18Cs-0.00955Hs + 0.0344Nγ (2)
Where Cs: surface carburization concentration (% by mass), Hs: surface hardness (HV), Nγ: old austenite grain size of the carburized layer.

On the other hand, the index B defined by the following formula (3), which is a function of the effective hardened layer depth (t) and the core hardness (Hcore), was introduced as an index representing the degree of triaxial stress at the tip of the microcrack. . The smaller the index B, the smaller the triaxial stress level at the tip of the microcrack.
B = t × (Hcore) ² (3)

FIG. 3 shows the relationship between the index B and the life in the low cycle fatigue test. The test piece has a cylindrical shape with a parallel part having a diameter of 9 mm, and has a semicircular arc notch in the center. The notch radius R = 1 and the notch bottom diameter is 7 mm. The test method is a load control four-point bending fatigue test. The maximum load stress is 1060 MPa, the stress ratio between the maximum load stress and the minimum load stress is 0.1, and the frequency is 5 Hz.
From this figure, it is understood that as the index B is smaller, brittle fracture of the carburized layer is suppressed and the low cycle fatigue strength is improved. That is, by the index B, the ease of brittle fracture of the carburized layer can be organized.

  And when the parameter | index A and the parameter | index B have the relationship of A-0.00000293 * B> =-14, it can suppress that a carburized layer carries out a brittle fracture. When aiming at a higher low cycle fatigue strength level, it is desirable to have a relationship of A−0.00000293 × B ≧ −13. Moreover, when aiming at a particularly high low cycle fatigue strength level, it is desirable to have a relationship of A−0.00000293 × B ≧ −12.

  Next, the reason which limited the component of this invention steel is demonstrated. In the following description, “%” means “% by mass”.

  C is an element effective for giving the steel the necessary strength. However, if it is less than 0.1%, the required tensile strength cannot be ensured, and if it exceeds 0.4%, it becomes hard and the core toughness after carburization decreases, and the cold workability also decreases. For this reason, it is necessary to make C into the range of 0.1 to 0.4%.

  Si is an element effective for deoxidation of steel, and is an element effective for imparting necessary strength and hardenability to the steel and further improving the temper softening resistance. However, if it is less than 0.02%, the effect is insufficient, and if it exceeds 1.3%, the hardness is increased and the cold forgeability is lowered. For this reason, it is necessary to make Si into the range of 0.02-1.3%. The preferable range of the steel material subjected to the cold working is 0.02 to 0.3%, but when the cold forgeability is particularly important, it is preferably within the range of 0.02 to 0.13%. On the other hand, Si is an element effective for increasing the grain boundary strength, and in a bearing part and a rolling part, it is an element effective for extending the life by suppressing the structural change and material change during the rolling fatigue process. Therefore, when aiming at high intensity | strength, the range of 0.2 to 1.3% is suitable. In particular, when the rolling fatigue strength is obtained at a high level, it is preferably in the range of 0.4 to 1.3%. In addition, the effect of suppressing the structural change and material change in the rolling fatigue process of bearing parts and rolling parts due to Si addition is when the retained austenite amount (residual γ amount) in the structure after carburizing is 30 to 40%. Especially great. Carburizing and nitriding treatment is effective for controlling the residual γ amount within this range. The carburizing and nitriding treatment involves nitriding in the diffusion treatment after carburizing. Here, it is preferable to adjust the nitriding conditions so that the surface nitrogen concentration is in the range of 0.2 to 0.6%.

  Mn is an element effective for deoxidation of steel and is an element effective for imparting the necessary strength and hardenability to the steel. However, if it is less than 0.3%, the effect is insufficient, and if it exceeds 1.8%, the effect is not only saturated, but also the hardness is increased and the cold forgeability is lowered. For this reason, it is necessary to make Mn into the range of 0.3-1.8%. The preferred range is 0.5-1.2%. In addition, when importance is attached to cold forgeability, it is preferable to set it as 0.5 to 0.75% of range.

  S forms MnS in the steel, thereby improving machinability. However, if the content is less than 0.001%, the effect is insufficient. If the content exceeds 0.15%, the effect is saturated while grain boundary segregation occurs, leading to grain boundary embrittlement. For this reason, S needs to be in the range of 0.001 to 0.15%. In addition, since MnS deteriorates the rolling fatigue life in bearing parts and rolling parts, it is necessary to reduce S as much as possible, and it is desirable to make it in the range of 0.001 to 0.01%.

  Al is added as a deoxidizing material. However, if it is less than 0.001%, the effect is insufficient, and if it exceeds 0.05%, AlN remains without forming a solution during rolling and heating, and becomes a precipitation site for Ti and Nb. Fine dispersion of these precipitates To promote coarsening of the crystal grains during carburizing. For this reason, it is necessary to make Al into the range of 0.001 to 0.05%.

  N combines with Al, V, Ti, Nb, etc. in the steel to form nitrides or carbonitrides, and suppresses coarsening of crystal grains. However, if it is less than 0.003%, the effect is insufficient, and if it exceeds 0.020%, the effect is saturated and cold workability is lowered. For this reason, it is necessary to make N into the range of 0.003-0.020%. In addition, in the case of B-added steel, or in the case of preventing coarsening of crystal grains mainly composed of TiC, N is preferably set in a range of 0.003 to 0.008%.

  P is an element that increases deformation resistance during cold forging and decreases toughness. Further, it is an element that embrittles crystal grain boundaries after quenching and tempering and reduces fatigue strength. For this reason, P needs to be 0.025% or less, preferably 0.015% or less.

  O is an element that causes grain boundary segregation to easily cause grain boundary embrittlement, and forms hard oxide inclusions in the steel to easily cause brittle fracture. For this reason, O needs to be 0.0025% or less.

  Further, the steel of the present invention contains one or more of Cr, Mo, Ni, B, V, Nb, and Ti.

  Cr is an element effective for imparting strength and hardenability to steel, and in bearing parts and rolling parts, increases residual γ after carburizing, and also causes structural changes and material deterioration during rolling fatigue. It is an element that is effective in extending the service life through suppression. However, if added over 1.8%, the hardness increases and cold forgeability decreases. For this reason, when adding Cr, it is necessary to make it 1.8% or less. It should be noted that the effect of suppressing the structural change and material deterioration in the rolling fatigue process of bearing parts and rolling parts due to the addition of Cr is particularly large when the residual γ content in the structure after carburizing is 30 to 40%. In order to control the residual γ amount within this range, it is effective to perform a carburizing and nitriding treatment so that the nitrogen concentration on the surface is in the range of 0.2 to 0.6%.

  Mo is also an element effective for improving the toughness of the carburized layer while giving strength and hardenability to the steel. Further, in bearing parts and rolling parts, it is an element effective in increasing the residual γ after carburizing and extending the life by suppressing the structural change and material deterioration in the rolling fatigue process. However, if added over 1.5%, the hardness is increased and the cold forgeability is lowered. For this reason, when adding Mo, it is necessary to make it 1.5% or less. A range of 0.02 to 0.5% is particularly suitable. It should be noted that the effect of suppressing the structural change and material deterioration in the rolling fatigue process of bearing parts and rolling parts due to the addition of Mo is particularly large when the amount of residual γ in the structure after carburizing is 30 to 40%. It is effective to perform the carburizing and nitriding treatment in the same manner as at the time so that the surface nitrogen concentration is in the range of 0.2 to 0.6%.

  Ni is also an element that gives strength and hardenability to the steel and is effective in improving the toughness of the carburized layer, but if added over 3.5%, the hardness is increased and cold forgeability is reduced. For this reason, when adding Ni, it is necessary to make it 3.5% or less. A range of 0.1 to 3.5%, more preferably 0.4 to 2.0% is preferable. The lower limit of the Ni content is preferably 0.1% or more, but is not limited to this.

V is also an element effective for imparting strength, hardenability and temper softening resistance to the steel, but if added over 0.5%, the hardness is increased and cold forgeability is lowered. For this reason, when adding V, it is necessary to make it 0.5% or less. In particular, a range of 0.03 to 0.5%, more preferably 0.07 to 0.2% is preferable.
B is also an element effective for imparting strength and hardenability to the steel, and has an effect of improving fatigue strength and impact strength as a carburized component by improving the grain boundary strength of the carburized material. However, if it exceeds 0.006%, the effect is saturated, and adverse effects such as impact strength deterioration may occur. For this reason, when adding B, it is necessary to make it 0.006% or less. The range of 0.0005 to 0.003% is particularly suitable.

  Nb combines with C and N in steel during carburizing heating to form Nb (CN), and is an effective element for suppressing the coarsening of crystal grains, but added over 0.04% As a result, the hardness is increased and the cold forgeability is lowered. For this reason, when adding Nb, it is necessary to make it 0.04% or less. Especially 0.03% or less is suitable. Moreover, the preferable range in the case where carburization is emphasized in addition to workability is 0.02% or less. Furthermore, the preferable range when carburizing property is particularly important is 0.01% or less. In addition, although it is preferable that content of Nb is 0.001% or more, it is not specifically limited to this.

  Ti produces fine TiC and TiCS in the steel, thereby making it possible to refine γ grains during carburization. Further, in the B-added steel, Ti is added for the purpose of preventing BN precipitation by forming TiN by combining with N in the steel, that is, ensuring solid solution B. However, if it exceeds 0.2%, the hardening of the steel due to the precipitation of TiC becomes remarkable and the cold workability is remarkably reduced, and the precipitates mainly composed of TiN increase and the rolling fatigue characteristics deteriorate. For this reason, it is necessary to make the addition amount of Ti 0.2% or less. The preferred range is 0.1% or less. In addition, when this invention steel is applied to a hot forging member, carbon and nitrogen which enter carburizing heating, and solid solution Ti react, and a lot of fine Ti (CN) precipitates in a carburized layer. For this reason, the rolling fatigue life is improved in bearing parts and rolling parts. In particular, when aiming at a high level of rolling fatigue life, it is effective to set the carbon potential at the time of carburizing to a high value in the range of 0.9 to 1.3% or to perform carburizing and nitriding treatment. .

  Moreover, it is preferable to adjust the addition amount of Ti according to the addition amount of Nb. For example, the preferable range of Ti + Nb is 0.04% or more and less than 0.17%. Particularly in high temperature carburizing and cold forging parts, the desirable range is from more than 0.091 to less than 0.17%.

  Next, a method for producing the steel of the present invention will be described. After adjusting the components of the molten steel in the steel making process, the molten steel is cast (for example, continuous casting) to produce a slab. Next, this slab is rolled, and further processed into a predetermined carburized part shape by performing forging, heat treatment and machining as necessary. Then, carburizing and quenching is performed. Further, tempering is performed as necessary.

  If necessary, peening may be performed after carburizing and tempering to improve toughness. In this case, it is preferable to perform the peening treatment so that the residual stress on the surface is −500 MPa or less. Further, the prior austenite grain size Nγ of the carburized layer is preferably No. 8-15.

  A plurality of types of carburized steel materials were formed so as to satisfy the conditions defined in the present invention, and the low cycle fatigue strength was measured. As a comparative example, a carburized steel material was formed using SCM420 defined in JIS, and the low cycle fatigue strength was measured. The test piece has a cylindrical shape with a parallel part having a diameter of 9 mm, and has a semicircular arc notch in the center. The notch radius R = 1 and the notch bottom diameter is 7 mm. The test method is a load control four-point bending fatigue test. The maximum load stress is 1060 MPa, the stress ratio between the maximum load stress and the minimum load stress is 0.1, and the frequency is 5 Hz.

  The results are shown in FIG. FIG. 4 is a graph in which an example and a comparative example of the present invention are plotted on a graph in which the X axis and the Y axis are the triaxial stress index B and the toughness index A of the carburized layer. The numerical value (low cycle fatigue strength) next to each point indicates the bending stress (MPa: calculated from the experimental result) when the load is broken at 5000 times.

The comparative example is A−0.00000293 × B <−14. In this case, the low cycle fatigue strength is 910 to 970 MPa, which does not indicate sufficient strength.
On the other hand, in the example satisfying A-0.00000293 × B ≧ −14, it was shown that the low cycle fatigue strength of the carburized steel material is 1000 MPa or more. Further, when A-0.00000293 × B ≧ −13 is satisfied, the low cycle fatigue strength of the carburized steel is 1100 MPa or more, and when A-0.00000293 × B ≧ −12 is satisfied, the low cycle of the carburized steel is satisfied. It was also shown that the fatigue strength was 1200 MPa or more.

  From the above, it was shown that the low cycle fatigue strength of the carburized steel material is sufficiently increased by satisfying A−0.00000293 × B ≧ −14.

Next, a steel material having the composition shown in Table 1 was melted and forged into a 40 mmφ bar steel by hot forging, and then subjected to a normalizing treatment. A test piece having a semicircular notch (notch bottom diameter: 7 mm) with a notch radius R = 1 at the center and a carburizing and tempering under various conditions is prepared from the above steel bar. The low cycle fatigue properties were evaluated after the treatment. The test method is a load control 4-point bending fatigue test, in which the stress ratio between the maximum load stress and the minimum load stress is 0.1, and the frequency is 5 Hz.
The results are shown in Table 2.

  In the present invention example, it is clear that the low cycle fatigue strength with a load of 5000 times shows a good characteristic of 1000 MPa or more. On the other hand, in Comparative Examples 18 to 21, the hardenability (jomini value) of the steel material is below the range specified in the present application, and the ferrite + bainite fraction of the core is higher than the range specified in the present application. The fatigue strength is less than 1000 MPa. In Comparative Examples 22 and 23, the C content is outside the range specified in the present application, so the low cycle fatigue strength of 5000 times is less than 1000 MPa. In Comparative Example 24, since the P content exceeds the range specified in the present application, the low cycle fatigue strength of 5000 times is less than 1000 MPa. In Comparative Examples 26 and 30, the projection core hardness is below the range of the present invention, so the 5000 low cycle fatigue strength is less than 1000 MPa. In Comparative Examples 25, 28 and 31, since the index of A-0.000002293B is below the range of the present invention, the low cycle fatigue strength of 5000 times is less than 1000 MPa. In Comparative Example 27, the γ particle size Nγ of the carburized layer is below the range of No. 8 to 15, so the low cycle fatigue strength of 5000 times is slightly higher than 1000 MPa. In Comparative Examples 25, 27, and 29, the carburization concentration Cs of the surface layer exceeds the specified range of 0.5 to 0.8% by mass%, so that the low cycle fatigue strength of 5000 times is less than 1000 MPa and slightly over 1000 MPa. Is a level.

  Next, about some test pieces, the Giotto peening process was performed on the conditions of arc height 0.5mmA. The results are shown in Table 3. It is clear that by providing shot peening, a further excellent low cycle fatigue strength can be obtained by setting the surface residual stress to −500 MPa or less.

The schematic diagram for demonstrating the fracture mechanism at the time of microcrack generation | occurrence | production. Schematic for demonstrating a damage site | part. The graph which shows the relationship between the parameter | index B and the lifetime in a low cycle fatigue test. The graph which plotted the Example and comparative example of this invention, after making each X-axis and Y-axis into the triaxial stress index B and the toughness index A of a carburized layer.

Claims (9)

% By mass
C: 0.1-0.4%
Si: 0.02-1.3%
Mn: 0.3 to 1.8%
S: 0.001 to 0.15%,
Al: 0.001 to 0.05%,
N: 0.003 to 0.020%,
P: 0.025% or less,
O: 0.0025% or less,
Cr: 1.8% or less,
Mo: 1.5% or less,
Ni: 3.5% or less,
B: 0.006% or less,
V: 0.5% or less,
Nb: 0.04% or less,
Ti: 0.2% or less,
1 type or 2 types or more, and the balance consists of iron and inevitable impurities,
A carburized and hardened steel material excellent in low cycle fatigue characteristics, characterized in that the projected core hardness Hp-core defined by the following formula (1) is HV390 or more.
Hp-core = Hcore / (1-t / r) (1)
Where Hcore: core hardness, t: effective hardened layer depth, r: half of the radius of the damaged part or the thickness of the damaged part. The fraction of ferrite + bainite in the core structure is 50% or less and the balance is substantially martensite, and A defined by the following formula (2) and B defined by the following formula (3) are A-0. The carburized and quenched steel material having excellent low cycle fatigue characteristics according to claim 1, wherein the carburized and quenched steel material has a relationship of 0.0000003 × B ≧ −14.
A = Mo + 0.227Ni + 190B-0.087Si-17.2P-2.74V-7.18Cs-0.00955Hs + 0.0344Nγ (2)
However, Cs: Carbon concentration (mass%) of surface layer, Hs: Surface hardness (HV), Nγ: Old austenite grain size of carburized layer.
B = t × (Hcore) ² (3) As the hardenability of the steel material, the hardness at a position of 13 mm from the quenching end in the Jomini test is 60 × C ^0.5 -5 (HRC) or more,
2. The low defined in claim 1, wherein A defined by the following formula (2) and B defined by the following formula (3) have a relationship of A−0.00000293 × B ≧ −14. Carburized and hardened steel with excellent cycle fatigue characteristics.
A = Mo + 0.227Ni + 190B-0.087Si-17.2P-2.74V-7.18Cs-0.00955Hs + 0.0344Nγ (2)
However, Cs: Carburizing concentration of surface layer (mass%), Hs: Surface hardness (HV), Nγ: Old austenite grain size of carburized layer.
B = t × (Hcore) ² (3) % By mass
C: 0.1-0.4%
Si: 0.02-1.3%
Mn: 0.3 to 1.8%
S: 0.001 to 0.15%,
Al: 0.001 to 0.05%,
N: 0.003 to 0.020%,
P: 0.025% or less,
O: 0.0025% or less,
Cr: 1.8% or less,
Mo: 1.5% or less,
Ni: 3.5% or less,
B: 0.006% or less,
V: 0.5% or less,
Nb: 0.04% or less,
Ti: 0.2% or less,
1 type or 2 types or more, and the balance consists of iron and inevitable impurities,
The hardness at a position 13 mm from the quenching end in the Jomini test is 60 × C ^0.5 -5 (HRC) or more,
Carburization excellent in low cycle fatigue characteristics, wherein A defined by the following formula (2) and B defined by the following formula (3) have a relationship of A-0.00000293 × B ≧ −14 Hardened steel.
A = Mo + 0.227Ni + 190B-0.087Si-17.2P-2.74V-7.18Cs-0.00955Hs + 0.0344Nγ (2)
However, Cs: Carburizing concentration of surface layer (mass%), Hs: Surface hardness (HV), Nγ: Old austenite grain size of carburized layer.
B = t × (Hcore) ² (3)
Where Hcore: core hardness, t: effective hardened layer depth.   The carburized and quenched steel material having excellent low cycle fatigue characteristics according to any one of claims 1 to 4, wherein the carburization concentration Cs of the surface layer is 0.5 to 0.8% by mass.   The carburized and quenched steel material having excellent low cycle fatigue characteristics according to any one of claims 1 to 5, wherein the prior austenite grain size Nγ of the carburized layer is No. 8-15.   The residual stress on the surface is -500 MPa or less, the carburized and quenched steel material excellent in low cycle fatigue characteristics according to any one of claims 1 to 6.   A carburized and hardened part using the carburized and hardened steel according to any one of claims 1 to 7. The carburized and hardened part according to claim 8, wherein the carburized and hardened part is a gear.