JP4213855B2 - Case-hardening steel and case-hardening parts with excellent torsional fatigue properties - Google Patents

Description

【００５６】
【表２】

【００５７】
【表３】

【００６１】
【発明の効果】
本発明の捩り疲労特性に優れた肌焼用鋼ならびに肌焼き部品を用いれば、各種シャフト部品として優れた捩り疲労特性を有する製品を得ることができる。本発明鋼と本発明部品を用いることによって、各種シャフト類の捩り疲労強度の向上が可能になり、自動車の高出力化や軽量化が可能になる。以上のように、本発明による産業上の効果は極めて顕著なるものがある。
【図面の簡単な説明】
【図１】 捩り疲労試験における時間強度とＭｎＳのアスペクト比の関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a case hardening steel, and more particularly, to a case hardening steel excellent in torsional fatigue characteristics suitable as a material for a shaft part of an automobile transmission.
[0002]
[Prior art]
The shaft parts and CVJ parts of automobile transmissions are usually made of a medium carbon alloy steel for machine structural use as defined in JIS G 4052, JIS G 4104, JIS G 4105, JIS G 4106, etc. Forging (including rolling)-manufactured in a process of carburizing and quenching after being processed into a predetermined shape by cutting. These various shafts have a strong tendency to increase in strength in accordance with the recent increase in output of automobile engines or compliance with environmental regulations. The main required characteristic of these parts is torsional fatigue characteristics.
[0003]
As a prior art for increasing the strength of carburized materials, many techniques for improving bending fatigue strength are recognized. For example, in Japanese Patent Laid-Open No. 9-176784, S: 0.003 to 0.070% or other steel material having a specific composition and parallel to the axis in a longitudinal section passing through the axis of a linear or rod-shaped rolled material. And a diameter composed of an oxide system and a sulfide system existing in a test area of 100 mm ² including an imaginary line separated from the axis by 1/4 · D (D represents the diameter of the rolled material) as a center line. Fatigue properties, characterized in that there are no more than 20 composite inclusions of 10 μm or more and 50 or more sulfide inclusions having a diameter of 3 μm or more and less than 10 μm present in the same test area as above A steel for case hardening with excellent machinability is shown. The present invention is characterized in that the number of oxide inclusions and sulfide inclusions is defined in order to improve the bending fatigue strength of the longitudinal and transverse eyes. In the present invention, B: 0.0003 to 0.005%, Ca: 0.0005 to 0.01%, Te: 0.1% or less, Zr: 0.1% or less are included as selective elements. I can do it. The aim of the addition of Ca, Te and Zr of the present invention is to improve the anisotropy by spheroidizing MnS and to improve the machinability without deteriorating toughness and bending fatigue properties. The present invention focuses only on bending fatigue characteristics as fatigue characteristics, and makes no mention of torsional fatigue characteristics. Bending fatigue is a phenomenon in which a crack is generated and propagated in a cross section perpendicular to the axial direction due to tensile stress at or near the surface, leading to fracture. In contrast, torsional fatigue, which is taken up in the present invention, causes a crack to occur in a plane parallel to the axial direction due to shear stress on the surface or in the vicinity of the surface, and then propagates on a plane that forms 45 degrees with the axial direction. It is a phenomenon. That is, the torsional fatigue failure and the bending fatigue failure are different in the acting stress that causes the failure, the cross-section where the crack occurs, and the failure mode. From the above, the description relating to the bending fatigue characteristics in Japanese Patent Application Laid-Open No. 9-176784 does not give any suggestion regarding the torsional fatigue strength taken up in the present invention.
[0004]
Next, the present invention is characterized by boron addition. However, boron steel tends to cause a phenomenon in which some austenite crystal grains become coarse during carburizing heating. Therefore, several techniques for preventing the generation of coarse grains during carburizing heating of case-hardened boron steel have been proposed. For example, Japanese Patent Application Laid-Open No. Sho 61-217553 has an object of producing TiC by pinning the grain boundaries by setting the amounts of Ti and N to 0.02 <Ti-3.42N. However, the ability of the steel to suppress coarse grains is unstable, and the reality is that the generation of coarse grains during carburization cannot be suppressed depending on the steel manufacturing process. Moreover, since the steel adds a large amount of Ti with respect to the N amount, a large amount of TiC is generated, and therefore, cracks and scratches are likely to occur during the manufacture of the steel material, and it is hard and cold worked in the state of the material. It has disadvantages such as poor performance.
[0005]
Japanese Patent Laid-Open No. 63-103052 discloses a case hardening steel for cold forging with a reduced amount of Si and Mn, and an N amount of 0.008% or less and an Nb of 0.01 to 0.20. It is shown. However, the steel also has an unstable ability to suppress coarse grains, and depending on the manufacturing process of the steel material, the generation of coarse grains may or may not be possible. The reality is that it cannot be suppressed. Further, as apparent from the examples, the steel has N content in the range of 0.005 to 0.008 except for one steel type. Even at this level of N content, as described later, the grain coarsening characteristics It has an adverse effect. In addition, one steel type of the embodiment of the present invention has a low N content of 0.002%, but Nb is added in a large amount of 0.05%, so that a large amount of NbC is produced, and therefore the state of the material. It is hard and cold workability is not good.
[0006]
As described above, there are some prior arts for the case-hardened boron steel, although there are problems regarding the prevention of coarse grains. However, these prior arts make no mention of torsional fatigue properties. That is, prior art examined from the viewpoint of improving torsional fatigue characteristics is not recognized for carburized shaft parts.
[0007]
[Problems to be solved by the invention]
Although the technology for improving the torsional fatigue characteristics of the case such as shafts has not been studied so far, the present invention clarifies the technology for improving the torsional fatigue characteristics of the case-hardened parts, and the torsional fatigue characteristics The steel for case hardening excellent in the is provided.
[0008]
[Means for Solving the Problems]
The present inventor has solved the above problems using the following means.
[0009]
That is, in mass%,
C: 0.1-0.4%
Si: 0.01-1.2%
Mn: 0.2 to 0.65%,
S: 0.005 to 0.15%,
Cr: 0.5 to 1.6%,
B: 0.0005 to 0.006%,
Al: 0.015-0.1%
In addition,
Te: 0.0005 to 0.02%,
Ca: 0.0005 to 0.02%,
Zr: 0.0003 to 0.01%
Mg: 0.001 to 0.035%,
Y: 0.001 to 0.1%
Rare earth elements: 0.001 to 0.15%
One or more of these, or
Ti: 0.05% or less, or further
Nb: 0.05% or less,
V: 0.4% or less,
Containing one or two of, or
Mo: 1% or less,
Ni: containing one or two of 2.5% or less,
P: 0.025% or less,
N: 0.007% or less,
O: Each is limited to 0.0025% or less,
The balance consists of iron and inevitable impurities,
And, the structural fraction of bainite is limited to 15% or less, an excellent hardening steel in torsional fatigue characteristics, wherein the ferrite grain size is No. 8 or higher.
[0010]
A fifth aspect of the present invention is a case-hardened part having the components of the first to fourth aspects and having an aspect ratio of MnS of 10 or less and excellent in torsional fatigue characteristics.
[0011]
By using the steel of the present invention, excellent torsional fatigue characteristics can be obtained after carburizing.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
In the manufacture of carburized shaft components, the present inventors have intensively investigated to realize excellent torsional fatigue characteristics after carburizing and quenching, and have clarified the following points.
[0013]
(1) Torsional fatigue failure of carburized and quenched members occurs in the following process.
A. A crack is generated on the surface or a plane parallel to the axial direction at the boundary between the hardened layer and the core.
B. The crack propagates in the plane parallel to the axial direction. This is hereinafter referred to as mode III destruction.
C. After mode III fracture, a brittle fracture occurs with a grain boundary crack at a plane of 45 degrees in the axial direction, and a final fracture occurs. This is hereinafter referred to as mode I destruction.
[0014]
(2) Torsional fatigue crack generation and initial propagation occur in a plane parallel to the axial direction. At this time, if elongated MnS exists in the axial direction, crack generation and initial propagation are promoted. Therefore, by granulating and refining MnS, crack generation and initial propagation are suppressed, and the torsional fatigue strength is dramatically improved. Addition of Te, Ca, Zr, Mg, Y and rare earth elements is effective for preventing the formation of elongated MnS, granulating and refining MnS. The addition of a large amount of these elements is inappropriate because it causes formation of nitrides and oxides such as coarse ZrN and inhibits cold workability. Granulation of MnS by adding these elements is also effective for preventing cracks during induction hardening. As described in the prior art, Japanese Patent Application Laid-Open No. Hei 9-176784 discloses Ca, Te, and Aim for the purpose of improving the anisotropy by spheroidizing MnS and improving the machinability. The addition of Zr is described. However, the aim of granulating MnS by adding Ca, Te, Zr is to improve anisotropy and improve machinability without degrading toughness and bending fatigue properties. On the other hand, in the present invention, the improvement in torsional fatigue characteristics is clearly different between the two. In addition, although the invention refers to bending fatigue, it does not refer to any torsional fatigue characteristics, and as described above, in torsional fatigue fracture and bending fatigue fracture, acting stress that causes fracture, Since both the cross-section where the crack occurs and the form of fracture differ greatly, Japanese Patent Application Laid-Open No. 9-176784 does not include any information that suggests the above technical idea of the present invention.
[0015]
(3) Next, when a bainite structure is mixed in the raw material stage, after carburizing and quenching, coarse grains are originally generated or mixed in a portion of the bainite structure, and hardness unevenness occurs in the vicinity thereof. This region exists in a band shape parallel to the axial direction. For this reason, generation of mode III torsional fatigue cracks and initial propagation are promoted in this region of hardness unevenness caused by the bainite structure. For the above reasons, in order to improve the torsional fatigue characteristics after carburizing, it is necessary to regulate the bainite fraction at the material stage.
[0016]
(4) after carburizing and refining the ferrite grain size of the material even tissue uniformly miniaturized, hardness unevenness decreases, the occurrence of torsional fatigue crack mode III, the initial propagation Ru is suppressed.
[0017]
(5) Next, in order to suppress the brittle fracture mode I accompanied by intergranular cracking at a 45 degree surface in the axial direction described in the column of the torsional fatigue fracture process "C." Boundary strengthening is effective.
(1) Add B as an essential element. B is due to the effect of expelling the grain boundary segregation P from the grain boundaries.
(2) Reduction of the amount of P and O which are segregation elements at grain boundaries.
(3) Suppression of coarse grains during carburization by regulating the bainite structure fraction of the material and refinement of the austenite grain structure during carburization by refinement of the ferrite structure of the material.
(4) When coarse grains are likely to be generated in the cold forging-carburizing process, in order to prevent the formation of coarse grains, Ti and Nb are added and Ti (CN) and Nb (CN) are finely dispersed. It is effective.
(5) In order to further improve torsional fatigue strength, it is effective to refine grain boundary carbides by increasing Si.
[0018]
(6) Since many of the parts targeted by the present invention are manufactured by cold working such as cutting or cold forging, ensuring cold workability is also an important issue. It is effective to add B as an essential element in order to suppress the improvement in hardness at the material stage and improve the hardenability. In order to make B effective for hardenability, it is necessary to reduce N. In the present invention, the amount of N is reduced to 0.0070% or less.
[0019]
The present invention has been made based on the above new findings.
[0020]
Hereinafter, the present invention will be described in detail.
[0021]
First, the reasons for limiting the components will be described.
[0022]
C is an effective element for imparting the necessary strength to steel, but if it is less than 0.10%, the required tensile strength cannot be ensured, and if it exceeds 0.4%, it becomes hard and cold work is performed. The core portion toughness after carburizing deteriorates as well as the properties deteriorate, so it is necessary to set the content within the range of 0.1 to 0.4%.
[0023]
Si is an element effective for deoxidation of steel and is an element effective for imparting necessary strength and hardenability to steel and improving temper softening resistance. However, if it is less than 0.01%, the effect is ineffective. It is enough. On the other hand, if it exceeds 1.2%, the hardness is increased and the cold forgeability is deteriorated. For the above reasons, the content needs to be in the range of 0.01 to 1.2%. The preferred range when the cold workability is emphasized is 0.01 to 0.5%, and the preferred range when the cold workability is particularly important is 0.01 to 0.15%. Further, the preferred range when the torsional fatigue characteristics are emphasized is more than 0.35 to 1.2%, and particularly when increasing the strength is desired, addition in the range of 0.5 to 1.2% is desirable.
[0024]
Mn is an element effective for imparting the hardenability and strength necessary for steel, but if it is less than 0.2%, the effect is insufficient, and if it exceeds 0.65%, the effect is not only saturated, Since the increase in hardness is caused and the cold forgeability is deteriorated, it is necessary to be within the range of 0.2% to 0.65%. A preferable range is 0.3 to 0.65%.
[0025]
S forms MnS in the steel and is added for the purpose of improving machinability. However, if it is less than 0.005%, its effect is insufficient. On the other hand, if it exceeds 0.15%, the effect is saturated, and rather, grain boundary segregation occurs, leading to grain boundary embrittlement. For these reasons, the S content needs to be in the range of 0.005 to 0.15%. The preferred range is 0.005 to 0.04%.
[0026]
Cr is an element effective for imparting strength and hardenability to steel, but if it is less than 0.5%, its effect is insufficient, and if it exceeds 1.6%, it will cause an increase in hardness and cold. Forgeability deteriorates. For the above reasons, the content needs to be in the range of 0.5 to 1.6%. The preferred range is 0.7 to 1.5%, and in the case of particularly aiming for high hardenability, addition in the range of 1.0 to 1.5% is desirable.
[0027]
B is added for the following three points. (1) In steel bar / wire rolling, boron iron carbide is generated in the cooling process after rolling, thereby increasing the growth rate of ferrite and promoting softening during rolling. (2) When carburizing and quenching, impart hardenability to the steel. (3) Improve the fatigue strength and impact strength of carburized parts by improving the grain boundary strength of the carburized material. When the amount is less than 0.0005%, the above effect is insufficient. When the amount exceeds 0.006%, the effect is saturated, so the content must be within the range of 0.0005 to 0.006%. There is. The preferred range is 0.002 to 0.004%.
[0028]
Al is useful as a deoxidizer and is useful for securing solid solution B by fixing solid solution N present in steel as AlN. However, if the amount of Al is too large, Al ₂ O ₃ will be generated excessively, increasing internal defects and degrading the cold workability. Therefore, in the present invention, it was made 0.0015 to 0.1%. In addition, when Ti is not added and has an action of fixing solute N, Al is preferably 0.04 to 0.1%.
[0029]
Next, in this invention, 1 type (s) or 2 or more types are contained as an essential element among Te, Ca, Zr, Y, Mg, and rare earth elements. Each of these elements generates an oxide, and this oxide serves as a nucleus for forming MnS, and MnS is compositionally modified such as (Mn, Ca) S or (Mn, Mg) S. This improves the stretchability of these sulfides during hot rolling and finely disperses the granular MnS, thereby improving the torsional fatigue characteristics after induction hardening. Such effects are Te: less than 0.0005%, Ca: less than 0.0005%, Zr: less than 0.0003%, Mg: less than 0.001%, Y: less than 0.001%, rare earth element: 0 Addition of less than 0.001% is insufficient. On the other hand, Te: more than 0.02%, Ca: more than 0.02%, Zr: more than 0.01%, Mg: more than 0.035%, Y: more than 0.1%, rare earth elements: more than 0.15% The above effects are saturated, and excessive addition of these produces rather coarse oxides such as CaO and MgO and their clusters, and hard precipitates such as ZrN, which are cold workable. It causes deterioration. For these reasons, these contents are set to Te: 0.0005 to 0.02%, Ca: 0.0005 to 0.02%, Zr: 0.0003 to 0.01%, Mg: 0.001 to 0 0.035%, Y: 0.001 to 0.1%, and rare earth elements: 0.001 to 0.15%. In addition, the rare earth element as used in the field of this invention refers to the element of atomic number 57-71.
[0030]
Since P is an element that increases deformation resistance during cold forging and deteriorates toughness, cold forgeability deteriorates. Further, since the fatigue strength is deteriorated by embrittlement of the grain boundaries of the parts after quenching and tempering, it is desirable to reduce them as much as possible. Therefore, it is necessary to limit the content to 0.025% or less. The preferred range is 0.015% or less.
[0031]
N is preferably limited as much as possible for the following reasons. B is added for the purpose of improving hardenability and strengthening grain boundaries as described above. However, since the effect of these B is manifested in the state of solid solution B in the steel, the amount of N is reduced to reduce BN. It is essential to suppress the generation of. In addition, in Ti-added steel and Nb-added steel, when N is combined with Ti in the steel, coarse TiN that hardly contributes to grain control is generated, which is NbC, NbC-based Nb (CN), TiC, TiC-based Ti. It becomes a precipitation site of (CN), inhibits fine precipitation of these Ti carbonitrides and Nb carbonitrides, and promotes the formation of coarse particles. The above adverse effect is particularly noticeable when the N content exceeds 0.007%. For the above reasons, the content needs to be 0.007% or less.
[0032]
O forms oxide inclusions such as Al ₂ O _{3 in} the steel. When a large amount of oxide inclusions are present in steel, in Ti-added steel and Nb-added steel, it becomes a precipitation site for Nb precipitates and Ti precipitates, and Nb precipitates and Ti precipitates during hot working The matter precipitates coarsely, and it becomes impossible to suppress the coarsening of crystal grains during carburizing. When the O content exceeds 0.0025%, such an adverse effect becomes remarkable, so the content needs to be limited to 0.0025% or less. The preferred range is 0.002% or less.
[0033]
The above are the basic components of steel targeted by the present invention. In the second claim of the present invention, by adding Ti, N is fixed as TiN by Ti, and N is rendered harmless. . Ti is an element having a deoxidizing action. However, if Ti is added in excess of 0.1%, precipitation hardening due to TiC becomes remarkable, and cold workability is remarkably deteriorated. For this reason, Ti: 0.1% or less was included as necessary. The preferred range when emphasizing cold workability is 0.05% or less.
[0034]
Next, in the third aspect of the present invention, one or two of Nb and V are contained.
[0035]
Nb combines with C and N in steel during carburizing heating to form Nb (CN), and is an element effective for suppressing coarsening of crystal grains. However, if it exceeds 0.05%, the hardness of the material becomes hard and the cold workability deteriorates, and it is difficult to form a solution during heating of the steel bar and wire rod. For the above reasons, the content needs to be 0.05% or less. The preferred range is 0.03% or less.
[0036]
V is added for the same effect as Nb. However, if it exceeds 0.4%, the hardness of the material becomes hard and the cold workability deteriorates, and it becomes difficult to form a solution during heating of the steel bar and wire rod. For the above reasons, the content needs to be 0.4% or less. The preferred range is 0.3% or less.
[0037]
Next, the fourth aspect of the present invention contains one or two of Mo and Ni.
[0038]
Mo is an element effective for imparting strength and hardenability to steel, but if added over 1%, the hardness increases and cold workability deteriorates. For the above reasons, the content needs to be in the range of 1% or less.
[0039]
Ni is also an effective element for imparting strength and hardenability to steel, but if added over 2.5%, the hardness is increased and cold forgeability is deteriorated. For the above reasons, the content needs to be in the range of 2.5% or less.
[0040]
Next, in the present invention, the structure fraction of bainite after hot working is limited to 15% or less. The reason for this limitation will be described below.
[0041]
If a bainite structure is mixed in the steel material after hot working, it causes coarse grains during carburizing heating. When the bainite structure fraction exceeds 15%, the generation of coarse grains becomes particularly noticeable. In addition, suppression of bainite contamination is also desirable from the viewpoint of improving cold workability. For the above reasons, it is necessary to limit the structural fraction of bainite after hot working to 15% or less. The preferred range is 10% or less.
[0042]
Next, in the present invention, the ferrite crystal grain size number is 8 or more. The reason for this limitation will be described below. If the ferrite grains after hot working are coarser than # 8, the ductility of the hot work material is deteriorated, and the cold workability and cold forgeability are deteriorated. In addition, the grains after carburization become coarse, causing hardness unevenness, and mode III cracks are easily generated, the grain boundary strength is reduced, mode I fracture is easily caused, and the torsional fatigue characteristics are deteriorated. For these reasons, the ferrite grain size number needs to be 8 or more.
[0045]
Next, the invention of claim 5 is an invention relating to a case that is excellent in torsional fatigue characteristics. It has the said component of Claims 1-4, and the aspect ratio of MnS restrict | limits to 10 or less. FIG. 1 shows the results of investigating the relationship between the aspect ratio of MnS and the time strength in torsional fatigue of the case-hardened shaft component. When the aspect ratio of MnS exceeds 10, the torsional fatigue characteristics are significantly deteriorated. For the above reasons, the aspect ratio of MnS was limited to 10 or less.
[0046]
In the present invention, the size of the slab, the cooling rate during solidification, and the ingot rolling conditions are not particularly limited, and any conditions may be used as long as the requirements of the present invention are satisfied. In addition, the steel of the present invention is not only a process for forming an as-rolled steel bar into parts by cold forging, but also a warm / hot forging process when passing through an annealing process or warm / hot forging before cold forging. It can also be applied to the case where it is formed into a part by the cutting process.
[0047]
【Example】
Hereinafter, the effects of the present invention will be described more specifically by way of examples.
[0048]
Example 1
Steel having the composition shown in Table 1 was melted. Here, it is a method for analyzing Zr in steel. After the sample was processed in the same manner as in Appendix 3 of JIS G 1237-1997, the amount of Zr in steel was measured by ICP (inductively coupled plasma) in the same manner as the analysis of Nb in steel. (Emission spectroscopy). However, the sample used for the measurement in the examples of the present invention was 2 g, and the calibration curve in ICP was set to be suitable for a very small amount of Zr. That is, the Zr standard solution was diluted so that the Zr concentration was 1 to 200 ppm to prepare solutions having different Zr concentrations, and the calibration curve was prepared by measuring the Zr amount. In addition, about the common method regarding these ICP, it is based on JISK0116-1995 (general rules of emission spectroscopic analysis method) and JIS Z 8002-1991 (general rules of tolerance of analysis and test).
[0049]
After forming a 162 mm square rolled material, steel bars having a diameter of 34 to 42 mm were manufactured by hot rolling. For cooling after hot rolling, some materials were air-cooled, and some materials were cooled at a slower cooling rate than air-cooling using a heat insulating cover installed on the cooling floor.
[0050]
The structure of the steel bar after rolling was observed to determine the structure fraction of bainite and the ferrite grain size.
[0051]
Further, the Vickers hardness of the rolled steel bar was measured. Furthermore, an upsetting test piece was prepared from the rolled steel bar, and the cold deformation resistance and the limit upsetting rate were obtained as indicators of cold workability. The cold deformation resistance was represented by the deformation resistance at an equivalent strain of 1.0.
[0052]
Further, static torsional test pieces and torsional fatigue test pieces having a parallel part diameter of 20 mm were collected from the rolled material. This test piece was carburized and quenched under conditions of 930 ° C. × 5 hours, and then tempered under conditions of 170 ° C. × 1 hour. Thereafter, a static torsion test and a torsional fatigue test were performed. The torsional fatigue characteristics were evaluated by time strength at 1 × 10 ⁵ cycles. Moreover, the aspect ratio of MnS was calculated | required using the image analysis apparatus in the cross section of the longitudinal direction of a torsion test piece. These survey results are shown in Tables 2 and 3.
[0053]
Comparative Example 25 is a characteristic of JIS SCr420 equivalent steel, and Comparative Example 26 is a characteristic of JIS SCM420 equivalent steel. Comparative examples 27 and 28 are characteristics of boron steel. In each of these comparative examples, the aspect ratio of MnS exceeds the range specified in the present invention. When comparing the inventive example and the comparative example, the torsional fatigue strength of the inventive example is significantly superior to that of the comparative example.
[0054]
Next, Comparative Example 29 is a case where after rolling, annealing was subsequently performed in a furnace at 650 ° C., and the ferrite crystal grain size was below the range specified in the present invention. Comparative Examples 30 and 31 are cases where accelerated cooling by water cooling is subsequently performed after rolling, and the bainite structure fraction exceeds the range specified in the present invention. Comparative Examples 29 to 31 are all inferior in torsional fatigue characteristics to the inventive examples.
[0055]
[Table 1]

[0056]
[Table 2]

[0057]
[Table 3]

[0061]
【The invention's effect】
If the case hardening steel and case hardening parts excellent in torsional fatigue characteristics of the present invention are used, products having excellent torsional fatigue characteristics as various shaft parts can be obtained. By using the steel of the present invention and the parts of the present invention, the torsional fatigue strength of various shafts can be improved, and the output and weight of the automobile can be increased. As described above, the industrial effects of the present invention are extremely remarkable.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between time strength and MnS aspect ratio in a torsional fatigue test.

Claims (5)

% By mass
C: 0.1-0.4%
Si: 0.01-1.2%
Mn: 0.2 to 0.65%,
S: 0.005 to 0.15%,
Cr: 0.5 to 1.6%,
B: 0.0005 to 0.006%,
Al: 0.015-0.1%
In addition,
Te: 0.0005 to 0.02%,
Ca: 0.0005 to 0.02%,
Zr: 0.0003 to 0.01%
Mg: 0.001 to 0.035%,
Y: 0.001 to 0.1%
Rare earth elements: 0.001 to 0.15%
Containing one or more of them,
P: 0.025% or less,
N: 0.007% or less,
O: Each is limited to 0.0025% or less,
The balance consists of iron and inevitable impurities,
Moreover, a steel for case hardening excellent in torsional fatigue characteristics, characterized in that the structure fraction of bainite is limited to 15% or less and the ferrite crystal grain size is No. 8 or more.   The steel for case hardening having excellent torsional fatigue characteristics according to claim 1, further comprising, by mass%, Ti: 0.1% or less.   Furthermore, it is excellent in the torsional fatigue property of Claim 1 or Claim 2 containing 1 type or 2 types in Nb: 0.05% or less V: 0.4% or less by the mass% Case-hardening steel.   The torsional fatigue property according to any one of claims 1 to 3, further comprising one or two of Mo: 1% or less and Ni: 2.5% or less in mass%. Excellent for case hardening steel.   A case-hardened part having excellent torsional fatigue characteristics, comprising the component according to any one of claims 1 to 4 and having an aspect ratio of MnS of 10 or less.

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