Steel for machine structure exhibiting excellent machinability

Information

  • Patent Grant
  • 9139894
  • Patent Number
    9,139,894
  • Date Filed
    Monday, July 11, 2011
    12 years ago
  • Date Issued
    Tuesday, September 22, 2015
    8 years ago
Abstract
This steel for a machine structure contains, in mass %: C: 0.40% to less than 0.75%; Si: 0.01% to 3.0%; Mn: 0.1% to 1.8%; S: 0.001% to 0.1%; Al: more than 0.1% and not more than 1.0%; N: 0.001% to 0.02%; and P: limited to not more than 0.05%, with a balance including Fe and inevitable impurities, in which the steel satisfies 139.38≦214×[C]+30.6×[Si]+42.8×[Mn]−14.7×[Al]≦177 and 0.72≦[C]+1/7×[Si]+1/5×[Mn]<1.539.
Description
TECHNICAL FIELD

The present invention relates to a steel for a machine structure, and in particular, to a steel for a machine structure exhibiting excellent machinability, which can be used for manufacturing a high-strength automobile part.


The present application claims priority based on Japanese Patent Application No. 2010-160136 filed in Japan on Jul. 14, 2010; Japanese Patent Application No. 2010-160108 filed in Japan on Jul. 14, 2010; Japanese Patent Application No. 2010-160141 filed in Japan on Jul. 14, 2010; and Japanese Patent Application No. 2010-160140 filed in Japan on Jul. 14, 2010, the disclosures of which are incorporated herein by reference in their entirety.


BACKGROUND ART

Conventionally, machinability-improving elements such as S, Pb, and Bi are added to improve the machinability of steels. However, adding Pb and Bi reduces the strength of the steels while little affecting the forgeability. Note that the amount of Pb used has been decreasing from the viewpoint of environmental protection.


S forms MnS (soft inclusion for cutting work) to improve the machinability. However, MnS has particles larger than those of Pb and the like, and thus, stress is more likely to concentrate on MnS. Further, in the case where MnS is drawn through forging and rolling, anisotropy occurs in the steel structure, significantly reducing the strength in a specific direction. As described above, adding the machinability-improving elements leads to a reduction in the strength, and thus, it is difficult to obtain both the strength and the machinability only by adjusting the components.


To deal with these problems, studies have been made to obtain a desired strength using high-frequency hardening, and several steels for high-frequency hardening have been proposed (see Patent Documents 1 to 5).


For example, Patent Document 4 proposes a steel material exhibiting excellent machinability and fatigue characteristics after the high-frequency hardening. This steel material contains predetermined components, and has a base structure including a ferrite and a pearlite (total of both is 90 vol % or more). Further, the maximum thickness of the ferrite in the steel material is 30 μm or less.


Patent Document 5 proposes a high-frequency-hardened steel for a pinion exhibiting excellent machinability. This steel contains predetermined components, and has an average aspect ratio of inclusions including MnS of 10 or less. This steel is subjected to a high-frequency thermal treatment to make the center portion of the steel become hard, thereby obtaining bending fatigue characteristics of bending fatigue life: 1.0×105 cycles or more with a rotary bending stress of 280 MPa.


In recent years, there has been an increasing demand for automobile parts having higher machining accuracy and improved fatigue strength. However, conventional steels for a machine structure cannot satisfy this demand.


RELATED ART DOCUMENTS
Patent Documents

Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2002-146473


Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2007-131871


Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2007-107020


Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2006-28598


Patent Document 5: Japanese Unexamined Patent Application, First. Publication No. 2007-16271


DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention

Conventional steels for a machine structure used by applying the high-frequency hardening have a problem in that the steel for a machine structure contains a large amount of C (usually, 0.4 mass % or more) to obtain the surface hardness after the high-frequency hardening, which results in high hardness but low machinability.


In view of the facts described above, an object of the present invention is to solve the problem described above by optimizing the components in the steel, and provide a steel for a machine structure exhibiting excellent machinability.


Means for Solving the Problems

The present invention has been made on the basis of the findings described above, and the main points of the present invention are as follows:


(1) A steel for a machine structure including, in mass %: C: 0.40% to less than 0.75%; Si: 0.01% to 3.0%; Mn: 0.1% to 1.8%; S: 0.001% to 0.1%; Al: more than 0.1% and not more than 1.0%; N: 0.001% to 0.02%; and P: limited to not more than 0.05%, with a balance including Fe and inevitable impurities, in which the amount of C: [C], the amount of Si: [Si], the amount of Mn: [Mn], and the amount of Al: [Al] satisfy following Expression (1) and Expression (2).

139.38≦214×[C]+30.6×[Si]+42.8×[Mn]−14.7×[Al]≦177  (1)
0.72≦[C]+1/7×[Si]+1/5×[Mn]<1.539  (2)


(2) The steel for a machine structure according to (1) above, in which the steel further satisfies the following Expression (3).

113−135×[C]−27×[Mn]≦13  (3)


(3) The steel for a machine structure according to (1) above, in which the steel further satisfies the following Expression (4).

55≦33+31×[C]+4.5×[Si]+1.5×[Mn]<72.45  (4)


(4) The steel for a machine structure according to (2) above, in which the steel further satisfies the following Expression (4).

55≦33+31×[C]+4.5×[Si]+1.5×[Mn]<72.45  (4)


(5) The steel for a machine structure according to any one of (1) to (4) above, in which the steel further satisfies the following Expression (5).

1.5<[Si]+1.8×[Mn]<6.24  (5)


(6) The steel for a machine structure according to any one of (1) to (5) above, in which the steel further includes, in mass %, B: 0.0001% to 0.015%.


(7) The steel for a machine structure according to any one of (1) to (6) above, in which

    • the steel further includes, in mass %, one or more elements of Cr: 0.01% to 0.8%, Mo: 0.001% to 1.0%, Ni: 0.001% to 5.0%, and Cu: 0.001% to 5.0%, and
    • in the case where the steel includes Cr: 0.01% to 0.8%, the following Expression (6) is used in place of Expression (1), the following Expression (7) is used in place of Expression (2), the following Expression (8) is used in place of Expression (3), and the following Expression (9) is used in place of Expression (4).

      139.38≦214×[C]+30.6×[Si]+42.8×[Mn]+23.8×[Cr]−14.7×[Al]≦177  (6)
      0.72≦[C]+1/7×[Si]+1/5×[Mn]+1/9×[Cr]<1.627  (7)
      113−135×[C]−27×[Mn]−18×[Cr]≦13  (8)
      55≦33+31×[C]+4.5×[Si]+1.5×[Mn]+2.4×[Cr]<74.37  (9)


(8) The steel for a machine structure according to any one of (1) to (7) above, in which the steel further includes, in mass %, one or more elements of Ca: 0.0001% to 0.02%, Mg: 0.0001% to 0.02%, Zr: 0.0001% to 0.02%, and Rem: 0.0001% to 0.02%.


(9) The steel for a machine structure according to any one of (1) to (8) above, in which the steel further includes, in mass %, one or more elements of Ti: 0.005% to 0.5%, Nb: 0.0005% to 0.5%, W: 0.0005% to 0.5%, V: 0.0005% to 0.5%, Ta: 0.0001% to 0.2%, and Hf: 0.0001% to 0.2%.


(10) The steel for a machine structure according to any one of (1) to (9) above, in which the steel further includes, in mass %, one or more elements of Sb: 0.0001% to 0.015%, Sn: 0.0005% to 2.0%, Zn: 0.0005% to 0.5%, Te: 0.0003% to 0.2%, Se: 0.0003% to 0.2%, Bi: 0.001% to 0.5%, and Pb: 0.001% to 0.5%.


(11) The steel for a machine structure according to any one of (1) to (10) above, in which the steel further includes, in mass %, one or more elements of Li: 0.00001% to 0.005%, Na: 0.00001% to 0.005%, K: 0.00001% to 0.005%, Ba: 0.00001% to 0.005%, and Sr: 0.00001% to 0.005%.


Effects of the Invention

According to the present invention, it is possible to provide a steel for a machine structure exhibiting excellent machinability, which can be used for manufacturing a high-strength gear exhibiting improved fatigue strength.





BRIEF DESCRIPTION OF THE DRAWINGS


FIG. 1 is a diagram illustrating a relationship between a hardness (Hv), and a carbon equivalent Ceq (=[C]+1/7×[Si]+1/5×[Mn]) and a carbon equivalent Ceq (=[C]+1/7×[Si]+1/5×[Mn]+1/9×[Cr]).



FIG. 2 is a diagram illustrating a relationship between the amount of Al contained in a steel, the hardness (Hv) of the steel, and machinability (face wear [μm] after cutting 3 m).



FIG. 3A illustrates a microstructure that has a large ferrite area and is not preferable as a steel for a machine structure.



FIG. 3B illustrates a microstructure that has a small ferrite area and is preferable as a steel for a machine structure.



FIG. 4 is a diagram illustrating a relationship between a pro-eutectoid α fraction (%), an index A (=113−135×[C]−27×[Mn]), and an index A (=113−135×[C]−27×[Mn]−18×[Cr]).



FIG. 5 is a diagram illustrating a relationship between tempered hardness (Hv) at 300° C., and an index RT (33+31×[C]+4.5×[Si]+1.5×[Mn]) and an index RT (33+31×[C]+4.5×[Si]+1.5×[Mn]+2.4×[Cr]).



FIG. 6 is a diagram illustrating a relationship between tempered hardness (Hv) at 300° C. and a fatigue limit (MPa) at 107 cycles.





EMBODIMENTS OF THE INVENTION

Hereinbelow, as an embodiment of the present invention, a detailed description will be made of a steel for a machine structure that can be used for manufacturing high-strength automobile parts and exhibits excellent machinability. The steel for a machine structure according to the present invention can be suitably used as a steel for high-frequency hardening.


The steel for a machine structure according to the present invention (hereinafter, also referred to as “steel according to the present invention”) contains, by mass %, C: 0.40% to less than 0.75%, Si: 0.01% to 3.0%, Mn: 0.1% to 1.8%, S: 0.001% to 0.1%, Al: over 0.1% to 1.0%, N: 0.001% to 0.02%, and P: limited to 0.05%, with a balance including Fe and inevitable impurities.


Further, in the steel according to the present invention, the amount of C: [C], the amount of Si: [Si], the amount of Mn: [Mn], and the amount of Al: [Al] satisfy the following Expression (1) and Expression (2).

139.38≦214×[C]+30.6×[Si]+42.8×[Mn]−14.7×[Al]≦177  (1)
0.72≦[C]+1/7×[Si]+1/5×[Mn]<1.539  (2)


Further, in the steel according to the present invention, it is preferable that [C], [Si], [Mn], and [Al] satisfy one or more of the following Expressions (3), (4), and (5).

113−135×[C]−27×[Mn]≦13  (3)
55≦33+31×[C]+4.5×[Si]+1.5×[Mn]<72.45  (4)
1.5<[Si]+1.8×[Mn]<6.24  (5)


Each of the expressions above will be described later.


First, the reason for limiting components for the steel according to the present invention will be described. Hereinbelow, the unit % represents a mass %.


C: 0.40% to Less than 0.75%


C is an element added to obtain the strength of the steel and the surface hardness after the high-frequency hardening. In the case where the amount of C added is less than 0.40%, the above-described effect cannot be obtained. On the other hand, in the case where the amount of C added is 0.75% or more, the toughness of the steel reduces, which possibly leads to season cracking of the rolled material. Thus, the amount of C is set to be not less than 0.40% and less than 0.75%. In order to obtain the effect obtained by addition of C in a stable manner, it is preferable to set the amount of C in the range of 0.45% to 0.73%, it is more preferable to set the amount of C in the range of 0.48% to 0.70%, and it is much more preferable to set the amount of C in the range of 0.50% to 0.61%.


Si: 0.01% to 3.0%


Si is an element that contributes to deoxidation during the steel making process, and also contributes to improving the strength of the steel. In the case where the amount of Si added is less than 0.01%, the desired effect cannot be obtained. On the other hand, in the case where the amount of Si added exceeds 3.0%, the toughness and the ductility of the steel deteriorate. Further, hard inclusions are generated, reducing the machinability of the steel. Thus, the amount of Si is set to be in the range of 0.01% to 3.0%. Preferably, the amount of Si is set to be in the range of 0.05% to 2.5%. More preferably, the amount of Si is set to be in the range of 0.1% to 1.5%.


Mn: 0.1% to 1.8%


Like Si, Mn is an element that contributes to improving the strength of the steel. In the case where the amount of Mn is less than 0.1%, the effect of the addition of Mn cannot be obtained. On the other hand, in the case where the amount of Mn exceeds 1.8%, bainite or insular martensite appears, and workability deteriorates. Thus, the amount of Mn is set to be in the range of 0.1% to 1.8%. It is preferable to set the amount of Mn in the range of 0.2% to 1.0%. It is more preferable to set the amount of Mn in the range of 0.4% to 0.8%.


S: 0.001% to 0.1%


S is an element that contributes to improving the machinability. In the case where the amount of S is less than 0.001%, the minimum required machinability for the steel cannot be obtained. On the other hand, in the case where the amount of S exceeds 0.1%, the toughness and the fatigue strength of the steel deteriorate. Thus, the amount of S is set to be in the range of 0.001% to 0.1%. It is preferable to set the amount of S in the range of 0.005% to 0.07%. It is more preferable to set the amount of S in the range of 0.01% to 0.05%.


Al: Over 0.1% to 1.0%


Al is an element that improves the machinability. A solute Al reacts with oxygen during cutting work to form a film of Al2O3 on the surface of the tool. This film prevents the tool from wearing. This film is formed in a manner such that the solute Al in the steel reacts with oxygen existing in the atmosphere, oxygen in the cutting oil, or oxygen existing in the Fe3O4 film or NiO film provided on the surface of the tool.


In the case where the amount of Al is less than 0.1% or less, the amount of Al2O3 generated is small, and the Al2O3 film is not formed on the surface of the tool. On the other hand, in the case where the amount of Al exceeds 1.0%, an A3 point (transformation point at which a phase is transformed from a ferrite into an austenite) becomes high, and the phase transformation does not occur with the high-frequency hardening. Thus, the amount of Al is set to be in the range of over 0.1% to 1.0%. It is preferable to set the amount of Al in the range of 0.12% to 0.8%. It is more preferable to set the amount of Al in the range of 0.14% to 0.4%.


N: 0.001% to 0.02%


N is an element that forms AIN, and contributes to preventing the crystal grain from coarsening. In the case where the amount of N is less than 0.001%, the effect of addition of N cannot be obtained. On the other hand, in the case where the amount of N exceeds 0.02%, hot shortness occurs at the time of rolling. Thus, the amount of N is set to be in the range of 0.001% to 0.02%. It is preferable to set the amount of N in the range of 0.002% to 0.012%. It is more preferable to set the amount of N in the range of 0.004% to 0.008%.


P: 0.05% or Less


The amount of P added may be set to 0%, or may be set to more than 0%. In the case where the appropriate amount of P is added, P contributes to improving the machinability of the steel. In the case where the amount of P exceeds 0.05%, the hardness of the steel excessively increases, which reduces the workability. Thus, the amount of P is set to 0.05% or less. From the viewpoint of machinability, it is preferable to set the amount of P to 0.005% or more. It is more preferable to set the amount of P in the range of 0.008% to 0.02%.


In addition to the elements described above, the steel according to the present invention may only contain iron and inevitable impurities. Further, the steel according to the present invention may contain other elements as selective components within the amount in which the characteristics of the steel according to the present invention are not impaired. Note that the selective components will be described later.


In the steel according to the present invention, [C], [Si], [Mn], and [Al] satisfy the following Expressions (1) and (2).


First, Expression (1) will be described.


The steel for high-frequency hardening contains a large amount of C (usually, 0.4 mass % or more) in order to obtain the surface hardness after high-frequency hardening. Thus, the hardness of the steel for high-frequency hardening is high, deteriorating the machinability. To solve this problem, the present inventors examined the following two types of relationships in terms of the hardness of the steel for high-frequency hardening.


(a1) Relationship between hardness and carbon equivalent that has a large effect on the hardness


(a2) Relationship between hardness and machinability affected by the hardness


The carbon equivalent Ceq is defined as Ceq=[C]+1/7×[Si]+1/5×[Mn], by focusing on the effect of C, Si, and Mn on the hardness. In the case where Cr is contained as the selective element, the carbon equivalent Ceq is defined as Ceq=[C]+1/7×[Si]+1/5×[Mn]+1/9×[Cr].


The relationship between the hardness and the carbon equivalent was examined in the following manner.


Plural hot rolling steel bars having a diameter of 65 mm were prepared, in which the hot rolling steel bars contain C: 0.45% to less than 0.75%, Si: 0.05% to 2.00%, Mn: 0.25% to 1.8%, S: 0.005% to 0.1%, P: 0.05% or less, N: 0.0030% to 0.0100%, and Al: over 0.03% to 1.0%, and in the case where the hot rolling steel bars contain Cr, further contain Cr: 0.01% to 0.8%. Further, the hot rolling steel bars satisfy 0.65≦Ceq≦1.02. After being hot rolled, the steel materials were kept at 900° C. for one hour, then, were air-cooled, and were cut by a cross-section in the direction of diameter. These obtained test pieces were embedded in resins, the resins were polished, and then, Vickers hardness was measured for the polished resins at a position located at ¼ of the diameter. FIG. 1 shows the results of the measurement. From FIG. 1, it can be understood that the Ceq and the hardness (Hv) have a relationship according to the following Expression (a1).

Hardness(Hv)=214×Ceq+49  (a1)


For the relationship between the hardness and the machinability, the amount of Al that generates Al2O3 to form an Al2O3 film on the surface of the tool was changed in the range of 0.03% to 1.0%, and examination was made in the following manner.


Square test pieces with a size of 40×40×250 mm were cut out from the steel bars, and cutting tests were performed to the test pieces using a fly tool as a simulation for hobbing of a gear. Note that a cutter used in a hobbing process at the time of manufacturing products includes plural cutting teeth. On the other hand, the fly tool is a cutter only having one hobbing tooth. It has been confirmed that cutting results obtained by the fly tool and those obtained by the cutter including plural cutting teeth exhibit a favorable relationship. Thus, the fly tool is used in a test in lieu of the hob cutter. The test method for the fly tool cutting is described in detail, for example, in “TOYOTA Technical Review Vol. 52 No. 2 December 2002 P78.” Table 1 shows test conditions.


[Table 1]


After the test pieces were cut for three minutes, the maximum face-wear depth (crater wear depth) of the tool was measured with a contact-type roughness meter. FIG. 2 shows the results of the measurement. The minimum wear amount was about 75 μm in the case where JIS-SCr420 (Al: 0.03%) used for a carburized gear is cut under this condition. Thus, the machinability is considered to be favorable if the amount of wear of the test samples is 75 μm or less under the same condition.


As illustrated in FIG. 2, there is a broken point in the relationship between the hardness and the machinability. Once the hardness reaches the broken point, the machinability sharply drops. The present inventors made a study by focusing on the existence of this broken point, and found that the broken point can be expressed by a mathematical expression of 14.7×[Al]+226 in the case of a steel containing Al in the range of over 0.1% to 1.0%.


In other words, the present inventors found that significantly excellent machinability can be obtained if the hardness (Hv) of the steel and the amount of Al [Al] (mass %) in the steel satisfy the following Expression (a2). This is the fundamental finding of the present invention.

Hardness(Hv)≦14.7×[Al]+226  (a2)


From Expression (a1) and Expression (a2), the following expression can be obtained.

214×Ceq+49≦14.7×[Al]+226  (1″)


Substituting the expression of the carbon equivalent in the above-described expression yields the following expression.

214×[C]+30.6×[Si]+42.8×[Mn]−14.7×[Al]≦177  (1′)


The above-described Expression (1′) means that, in the steel for high-frequency hardening having the large amount of C (normally, 0.4 mass % or more), the desired hardness and machinability can be obtained by associating [C], [Si], [Mn], and [Al] with each other. Thus, with the steel for a machine structure according to the present invention, the problem of high hardness and less machinability can be solved by forming the composition of the steel so as to satisfy the above-described Expression (1′).


In the case where the steel contains Cr, the following expression can be obtained in a similar manner to Expression (1″) and Expression (1′).

214×[C]+30.6×[Si]+42.8×[Mn]+23.8×[Cr]−14.7×[Al]≦177  (6′)


In the case where the steel for a machine structure is used for high-strength machine parts, the steel needs to have hardness of about 200 Hv or more, and hence, Ceq needs to be 0.72 or more from FIG. 1. In other words, the steel according to the present invention needs to have components that also satisfy the following Expression (2) and/or Expression (7).


In other words, in the case where the steel does not contain Cr, the steel also needs to satisfy the following Expression (2).

0.72≦[C]+1/7×[Si]+1/5×[Mn]<1.539  (2)


In the case where the steel contains Cr, the steel also needs to satisfy the following Expression (7).

0.72≦[C]+1/7×[Si]+1/5×[Mn]+1/9×[Cr]<1.627  (7)


The value of Ceq of Expression (2) and Expression (7) is set preferably to 0.74 or more, more preferably to 0.76 or more, yet more preferably to 0.79 or more, yet more preferably to 0.82 or more. The upper limit of Ceq is determined on the basis of the upper limits of C, Si, Mn, and Cr.


It should be noted that, since the lower limits of Expression (2) and Ceq of Expression (2) are 0.72, and the upper limit of [Al] is 1.0%, the lower limits of Expression (1′) and Expression (5′) can be determined as follows:

214×0.72−14.7×1.0=139.38  (1′″)

In other words, Expression (1) and Expression (6) can be expressed as follows:

139.38≦214×[C]+30.6×[Si]+42.8×[Mn]−14.7×[Al]≦177  (1)
139.38≦214×[C]+30.6×[Si]+42.8×[Mn]+23.8×[Cr]−14.7×[Al]≦177  (6)


The upper limits of Expression (1) and Expression (6) are preferably set to 163 or less, more preferably to 155 or less.


In the steel according to the present invention, it is preferable that [C], [Si], [Mn], and [Al] satisfy either one of or both of the above-described Expression (3) and Expression (4).


Next, Expression (3) will be described.


The steel for a machine structure used by applying conventional high-frequency hardening has a problem in that, since high-frequency hardening is applied as the heat-hardening to a gear in an accelerated manner within a short period of time, the hardness varies depending on positions or sufficient hardness after the hardening cannot be obtained. To solve these problems, the present inventors examined a relationship between an index A related to C and Mn that have an effect on the microstructure of the steel and a pro-eutectoid α fraction that has an effect on the hardenability of the steel. Since the high frequency heating is applied within a short period of time, C atoms do not disperse over the entire ferrite portions at the time of high frequency heating if the pro-eutectoid area is large. This leads to generation of a martensite having a lower hardness, possibly causing hardness variation or insufficient hardness of the steel. The examination above was made in the following manner.


After being subjected to a condition of 900° C.×1 hour, the steel bar was air-cooled. Test pieces having a large diameter portion 26φ were cut out from the steel bar. The cut-out test pieces were cut by a cross-section in the diameter direction, and were embedded into resins. The surfaces of the resins were polished, and were subjected to etching with a nital solution. Then, microstructures on the surfaces were observed with an optical microscope. FIG. 3A and FIG. 3B show an example of the observation results.


In FIG. 3A and FIG. 3B, white areas are ferrite, and black areas are pearlite. In other words, FIG. 3A illustrates a microstructure that has large ferrite areas and is not favorable as the steel for a machine structure, and FIG. 3B illustrates a microstructure that has a small ferrite area and is favorable as the steel for a machine structure.



FIG. 4 illustrates a relationship between a pro-eutectoid α fraction (%), and an index A (=113−135×[C]−27×[Mn]) and an index A (=113−135×[C]−27×[Mn]−18×[Cr]).


The present inventors define, as the following expressions, the index A used for taking the effect of C, Mn and Cr on the microstructure of the steel into consideration.


In the case where the steel does not contain Cr, index A=113−135×[C]−27×[Mn] . . . (3′) is given, and in the case where the steel contains Cr, index A=113−135×[C]−27×[Mn]−18×[Cr] . . . (8′) is given.


Coefficients included in these expressions were obtained in the following manner. Various steel materials containing C: 0.40% to less than 0.75%, Mn: 0.1% to 1.8%, and Cr: 0.01% to 0.8% were prepared, and microstructures of the steel materials were observed in the following method to obtain the pro-eutectoid α fraction. Further, effects of the amounts of C, Mn, and Cr contained in the steel material on the pro-eutectoid α fraction were obtained through a multiple regression analysis to calculate the coefficients in the index A. Note that the pro-eutectoid α fraction was obtained in a manner such that 20 views were photographed with an optical microscope with a magnification of 400 power (view with a size of about 0.32 mm×0.24 mm), areas of ferrite portions were measured through an image analysis, and a ratio of the areas of the ferrite portions relative to the entire area photographed was calculated.


From FIG. 4, it can be understood that the pro-eutectoid α fraction (%) linearly increases with the increase in the value of the index A.


In order to obtain favorable hardenability with the accelerated high-frequency hardening applied within a short period of time, it is preferable to set the pro-eutectoid α fraction to 13% or less. To this end, it is preferable to set the index A to 13 or less. Thus, the following Expression (3), which associates [C] and [Mn] with each other, can be obtained.

113−135×[C]−27×[Mn]≦13  (3)


In other words, by setting [C] and [Mn] in the steel for high-frequency hardening so as to satisfy Expression (3) described above, it is possible to reduce the variation in hardness after the hardening and prevent insufficient hardness after the hardening.


In the case where the steel contains Cr, the following Expression (7) can be obtained by measuring the pro-eutectoid α fraction in the steel material containing Cr: 0.01% to 0.8% in a similar manner to that described above.

113−135×[C]−27×[Mn]−18×[Cr]≦13  (8)


In other words, by setting [C], [Mn], and [Cr] contained in the steel for high-frequency hardening so as to satisfy the above-described Expression (8), it is possible to reduce the variation in hardness after the hardening and prevent insufficient hardness after the hardening.


It should be noted that the left-hand side of each of Expression (3) and Expression (8) is preferably 11 or less, more preferably 9 or less. If the left-hand side of Expression (3) and Expression (8) is 3.75 or less, the pro-eutectoid ferrite does not exist.


Although it is not necessary to set the lower limit value for the left-hand side of Expression (3) and Expression (8), the theoretical lower limit value calculated from the component range of each element is −51.25.


The above-described Expression (4) will be described. The steel for a machine structure used by applying conventional high-frequency hardening has the following problem. More specifically, in many cases, parts subjected to the conventional high-frequency hardening have the surface layer containing C in the range of 0.4% to 0.6 mass %, and exhibit lower fatigue strength, as compared with the carburized part having the surface layer containing C of about 0.8%. Thus, the present inventors made a study to solve this problem in the following manner.


In terms of characteristics of the steel for high-frequency hardening, the tempered hardness after the hardening is important to improve the pitting fatigue strength of the part. The present inventors introduced the following index RT to quantitatively evaluate the tempered hardness after the high-frequency hardening so as to associate the tempered hardness with the components of the steel.


In the case where the steel does not contain Cr, the RT is defined by the following Expression (4′).

RT=33+31×[C]+4.5×[Si]+1.5×[Mn]  (4′)


In the case where the steel contains Cr, the RT is defined by the following Expression (9′).

RT=33+31×[C]+4.5×[Si]+1.5×[Mn]+2.4×[Cr]  (9′)


The index RT is an index that additively evaluates how much [C], [Si], [Mn], and [Cr] have an effect on the tempered hardness after the hardening, by putting a weight to the degree of influence that each of the elements has. Note that C, Si, Mn, and Cr are primary elements that increase the hardness of the steel.


After being subjected to a condition of 900° C.×1 hour, the steel bar was air-cooled. Test pieces having a large diameter portion 26φ were cut out from the steel bar. The large diameter portion was subjected to the high-frequency hardening so that the depth of the effective case-hardening layer was 1.5 mm, and then, was subjected to a tempering process under the condition of 300° C.×90 minutes. After this, the large diameter portion was cut by a cross-section in the diameter direction, and was embedded into a resin. Then, after the surface layer of the resin was polished, Vickers hardness (Hv) was measured at a position of 0.05 mm from the surface layer. FIG. 5 shows the results.


From FIG. 5, it can be determined that the index RT and the tempered hardness (Hv) at 300° C. exhibit a significantly favorable correlation, and the hardness of 610 Hv or more can be obtained if the RT is more than or equal to 55.


Through a roller pitting test, the present inventors confirmed that the fatigue strength is excellent if the tempered hardness (Hv) at 300° C. is 610 Hv or more.


Roller pitting test pieces having a large diameter portion (test portion) 26φ produced from the steel bar were subjected to high-frequency hardening so that the large diameter portion includes the effective case-hardening layer having a depth of 1.5 mm. Further, the roller pitting test pieces were subjected to a tempering process of 160° C.×90 minutes. Then, a grip portion was subjected to a finishing process to increase the accuracy of the test.


The roller pitting test was performed under the conditions in which a large roller was formed by an SCM 420 carburized roller with crowning of 150R; the number of rotations was 2000 rpm; a transmission oil was used as a lubricating oil; an oil temperature was 80° C.; a slip ratio was −40%; and the maximum cycle was 10,000,000. On the basis of the test, an S—N curve was created to obtain a fatigue limit (MPa, roller pitting fatigue strength). FIG. 6 shows the results of the test.


For comparison purposes, the fatigue limit at 107 cycles was obtained for JIS-SCr420, which is widely used for carburized gears, and is shown in the drawing. The fatigue limit at 107 cycles of JIS-SCr420 was 2600 MPa. The target value of the fatigue limit (roller pitting fatigue strength) of the steel according to the present invention was set to 3200 MPa or more, which is about 20% higher than that of JIS-SCr420.


From FIG. 6, it can be understood that, in order to obtain the fatigue limit of 3200 MPa or more, it is necessary to obtain the tempered hardness at 300° C. of 610 Hv or more. From FIG. 5, it can be understood that, in order to obtain the tempered hardness at 300° C. of 610 Hv or more, it is necessary to keep the index RT of 55 or more. In other words, by setting the index RT≧55, it is possible to obtain the favorable fatigue strength.


It should be noted that the upper limit of the index RT can be determined depending on the upper limit of C, Si, Mn, and Cr. In other words, Expression (4) and Expression (9) are given as below.

55≦33+31×[C]+4.5×[Si]+1.5×[Mn]<72.45  (4)
55≦33+31×[C]+4.5×[Si]+1.5×[Mn]+2.4×[Cr]<74.37  (9)


In order to obtain much higher strength, it is preferable to set the RT to be not less than 57, and it is more preferable to set the RT to be not less than 59.


As described above, according to the steel of the present invention, the problems are solved by defining the components therein using Expressions (1), (2), (3), and (4) described above. Thus, the steel according to the present invention exhibits excellent characteristics as a steel for a high-frequency hardening for use in a high-strengthened part.

1.5<[Si]+1.8×[Mn]<6.24  (5)


Si and Mn are elements that are dissolved in solid solution in ferrite, and strengthen ferrite. For the steel for a machine structure required to have a high strength, it is preferable to enhance the strength of ferrite in order to prevent the steel material from breaking from ferrite having a soft structure in the steel. To this end, it is preferable that Si and Mn in total satisfy [Si]+1.8×[Mn]>1.5. Although it is not necessary to particularly set the upper limit of [Si]+1.8×[Mn], the upper limit of [Si]+1.8×[Mn] is determined to be 6.24 or less on the basis of the upper limit of the amount of each of Si and Mn added.


As described above, the steel according to the present invention may contain other elements as selective components within the amount in which the excellent characteristics of the steel according to the present invention are not impaired. Addition of the selected elements is not essential in terms of solving the problems of the present invention. Below, a description will be made of addition of the selected elements and reasons for limitation of selected elements. Note that the unit “%” means mass %.


B: 0.0001% to 0.015%


B is an element that imparts hardenability to the steel, and enhances the strength of the grain boundary. With the small amount of B added, B segregates in a γ grain boundary to enhance the hardenability, and suppress breakage of the grain boundary in the surface layer during the high-frequency hardening. To obtain these effects, it may be possible to add B of not less than 0.0001%. In the case where the amount of B added exceeds 0.015%, the material becomes brittle. Thus, the amount of B added is set in the range of 0.0001% to 0.015%. The amount of B added is set preferably in the range of 0.0005% to 0.010%, more preferably in the range of 0.001% to 0.003%.


One or More Elements of Cr: 0.01% to 0.8%, Mo: 0.001% to 1.0%, Ni: 0.001% to 5.0%, and Cu: 0.001% to 5.0%


Cr, Mo, Ni, and Cu are elements that enhance the strength of the steel. To obtain this effect, it may be possible to add Cr of not less than 0.01%, Mo of not less than 0.001%, Ni of not less than 0.001%, and/or Cu of not less than 0.001% within the amount in which addition of these elements does not impair the excellent characteristics of the steel according to the present invention.


In the case where the amount of Cr exceeds 0.8%, the hardenability excessively improves. This leads to generation of bainite or insular martensite, deteriorating the workability. Thus, the amount of Cr contained is set to be not more than 0.8%, preferably to be not more than 0.4%. In the case where Mo exceeds 1.0%, the hardenability excessively improves as in Cr. This leads to generation of bainite or insular martensite, deteriorating the workability. The amount of Mo is set to be not more than 1.0%, preferably to be not more than 0.5%, more preferably to be not more than 0.2%, yet more preferably to be less than 0.05%.


In the case where the amount of Ni and Cu exceeds 5.0%, the hardenability excessively improves as with Cr and Mo. This leads to generation of bainite or insular martensite, deteriorating the workability. Thus, the upper limit of the amount of each of Ni and Cu contained is set to be not more than 5.0%.


One or More Elements of Ca: 0.0001% to 0.02%, Mg: 0.0001% to 0.02%, Zr: 0.0001% to 0.02%, and Rem: 0.0001% to 0.02%


Ca, Mg, Zr, and a rare earth metal (Rem) are elements that control the formation of MnS in the steel, and contribute to improving the mechanical properties of the steel. To obtain these effects, it may be possible to add each of Ca, Mg, Zr, and Rem of not less than 0.0001% within the amount in which addition of Ca, Mg, Zr, and Rem does not impair the excellent characteristics of the steel according to the present invention. In the case where the amount of each of Ca, Mg, Zr, and Rem exceeds 0.02%, oxides coarsen, and the fatigue strength deteriorates. Thus, the amount of each of Ca, Mg, Zr, and Rem is set to be not more than 0.02%. Note that Rem represents a rare earth metal element, and includes one or more elements selected from Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu.


One or More Elements of Ti: 0.005% to 0.5%, Nb: 0.0005% to 0.5%, W: 0.0005% to 0.5%, V: 0.0005% to 0.5%, Ta: 0.0001% to 0.2%, and Hf: 0.0001% to 0.2%


Ti, Nb, Ta, and Hf suppress the undesirable growth of the crystal grain, and contribute to homogenization of the structure. To obtain these effects, it may be possible to add Ti of not less than 0.005%, Nb of not less than 0.0005%, Ta of not less than 0.0001%, and Hf of not less than 0.0001% within the amount in which addition of Ti, Nb, Ta, and Hf does not impair the excellent characteristics of the steel according to the present invention.


On the other hand, in the case where the amount of each of Ti and Nb exceeds 0.5% or the amount of each of Ta and Hf exceeds 0.2%, hard carbides are generated, which leads to deterioration in the machinability. Thus, the amount of each of Ti and Nb is set to be not more than 0.5%, and the amount of each of Ta and Hf is set to be not more than 0.2%.


W and V form fine carbides, nitrides, and/or carbonitrides with C and/or N, preventing the crystal grain from coarsening and contributing to homogenization of the structure of the steel. To obtain these effects, it may be possible to add W of not less than 0.0005% and/or V of not less than 0.0005% within the amount in which addition of these elements does not impair the excellent characteristics of the steel according to the present invention. If either one of W and V exceeds 0.5%, hard carbides are generated, which leads to deterioration in the machinability. Thus, the amounts of W and V are each set to be not more than 0.5%.


One or More Elements of Sb: 0.0001% to 0.015%, Sn: 0.0005% to 2.0%, Zn: 0.0005% to 0.5%, Te: 0.0003% to 0.2%, Se: 0.0003% to 0.2%, Bi: 0.001% to 0.5%, and Pb: 0.001% to 0.5%


Sb, Te, Se, Bi, and Pb are elements that enhance the machinability. To obtain this effect, it may be possible to add Sb of not less than 0.0001%, Te of not less than 0.0003%, Se of not less than 0.0003%, Bi of not less than 0.001%, and/or Pb of not less than 0.001% within the amount in which addition of these elements does not impair the excellent characteristics of the steel according to the present invention.


In the case where Sb exceeds 0.015%, Te exceeds 0.2%, Se exceeds 0.2%, Bi exceeds 0.5%, or Pb exceeds 0.5%, hot shortness occurs, causing damage or making rolling operations difficult. Thus, the amount of Sb is set to be not more than 0.015%, the amount of Te is set to be not more than 0.2%, the amount of Se is set to be not more than 0.2%, the amount of Bi is set to be not more than 0.5%, and the amount of Pb is set to be not more than 0.5%.


Sn and Zn are elements that make ferrite brittle to prolong the tool life, and improve the surface roughness. To obtain these effects, it may be possible to add each of Sn and Zn of not less than 0.0005% within the amount in which addition of these elements does not impair the excellent characteristics of the steel according to the present invention. In the case where Sn exceeds 2.0% or Zn exceeds 0.5%, it is difficult to produce the steel. Thus, the amount of Sn is set to be not more than 2.0%, and the amount of Zn is set to be not more than 0.5%.


One or More Elements of Li: 0.00001% to 0.005%, Na: 0.00001% to 0.005%, K: 0.00001% to 0.005%, Ba: 0.00001% to 0.005%, and Sr: 0.00001% to 0.005%


Li, Na, K, Ba, and/or Sr each form oxide, and the formed oxide is captured by CaO—Al2O3—SiO2-based oxide to form oxide having a low melting point. The resulting oxide adheres, as belag, to the surface of a tool used at the time of cutting operations, thereby improving the machinability. To obtain this effect, it may be possible to add each of these elements of not less than 0.00001% within the amount in which addition of these elements does not impair the excellent characteristics of the steel according to the present invention.


In the case where the amount of each of these elements exceeds 0.005%, refractories retaining the molten steel may melt and be damaged. Thus, the amounts of Li, Na, K, Ba, and Sr are each set to be not more than 0.005%.


The steel according to the present invention has components as described above, with a balance including Fe and inevitable impurities. Note that, although inevitable impurities such as As and Co may intrude into the steel depending on raw materials, tools, production equipment or other factors, intrusion of these inevitable impurities is allowable provided that the amount of the inevitable impurities intruded falls within the amount in which the intrusion of the inevitable impurities does not impair the excellent characteristics of the steel according to the present invention.


EXAMPLES

Next, Examples of the present invention will be described. Conditions used in Examples are merely examples of the conditions, and are employed to demonstrate feasibility and effects of the present invention. The present invention is not limited to these examples of conditions. It may be possible to employ various conditions that do not depart from the scope of the present invention and can achieve the object of the present invention.


Examples

Steels having components shown in Table 2 and Table 3 were produced through melting, and were rolled to form a steel bar with 65φ. Tables 4 to 6 show values of Expression (1), values of Expression (2), values of Expression (3), values of Expression (4), and values of Expression (5) in connection with steels of Number 1 to Number 105. For the steel containing Cr, values of Expression (6), values of Expression (7), values of Expression (8), and values of Expression (9) are shown. Steels of Number 1 to Number 94 correspond to examples of the present invention, and steels of Number 95 to Number 105 correspond to comparative examples.


After being subjected to a condition of 900° C.×1 hour, the steel bars were air-cooled, and test pieces were cut out from the steel bar. Before hardening, measurement was made of hardness (Hv), the amount of wear (m) of the tool, and pro-eutectoid ferrite fraction (%). The results of measurement are shown in Table 4 to Table 6. Further, after the hardening, measurement was made of the tempered hardness (Hv) at 300° C. of the hardened layer and the roller pitting fatigue strength (fatigue limit, MPa). The results of the measurement are shown in Table 4 to Table 6. Note that the amount of wear (μm) of the tool was obtained such that the test piece with 3 m was cut, and the maximum crater wear depth of the tool was measured with a contact-type roughness meter. In Table 2 to Table 6, underlines are applied for components or expressions that do not satisfy the conditions of the present invention.

  • [Table 2]
  • [Table 3]
  • [Table 4]
  • [Table 5]
  • [Table 6]


The steels of Number 95 and Number 96 did not satisfy Expression (1) or Expression (6), and hence, the amount of wear of the tool was large.


The steel of Number 97 did not satisfy Expression (2), and hence, the hardness was low. Thus, the steel of Number 97 could not be used as the steel for a machine structure for use in the high-strength machine part.


The amount of Al added was not sufficient for the steels of Number 98 and Number 99, and hence, no Al2O3 film was formed on the surface of the tool, which resulted in the increase in the amount of wear of the tool.


The excessive amount of Al was added to the steel of Number 100, which resulted in the increase in the A3 point. Thus, the high-frequency hardening could not be applied.


The amount of Mn added to the steel of Number 101 was excessive, which resulted in deterioration in the workability, and an increase in the amount of wear of the tool.


The amount of Cr added to the steel of Number 102 was excessive, which resulted in deterioration in the workability, and an increase in the amount of wear of the tool.


The amount of N added to the steel of Number 103 was excessive. Hence, hot shortness occurred in the steel bar during rolling, and hence, this steel could not be used for production.


The amount of C added was not sufficient for the steel of Number 104, which resulted in insufficient surface hardness after the high-frequency hardening.


The amount of C added to the steel of Number 105 was excessive, which resulted in the occurrence of season cracking.


The steels of Number 1 to Number 94 had components that satisfy Expression (1), Expression (2), Expression (6), and Expression (7). Thus, these steels had the desired characteristics


INDUSTRIAL APPLICABILITY

As described above, according to the present invention, it is possible to provide a steel for a machine structure that exhibits excellent machinability and can be used for a high-strength part exhibiting excellent fatigue characteristics. Thus, the present invention is highly applicable in the machine-manufacturing industry.











TABLE 1







Conditions for cutting work
Speed
150 m/min  



Feed
  0.12 mm/rev



Amount of shift
1.8 mm



Cutting depth
4.5 mm



Oil for cutting work
Not applied (dry)


Tool
Type
Fly tool (φ90)



Material
High-speed steel (made by Nachi-Fujikoshi)



Coating
TiAlN (no cutting face)








Equipment
Holizontal machining center

















TABLE 2








Components (mass %)



















Number
C
Si
Mn
P
S
Al
N
B
Cr
Mo
Ni
Cu





1
0.65
0.05
0.40
0.009
0.017
0.130
0.0050







2
0.55
0.85
0.25
0.015
0.015
0.110
0.0050

0.11





3
0.60
0.65
0.20
0.013
0.018
0.145
0.0043







4
0.40
0.45
1.45
0.010
0.014
0.115
0.0045
0.0040






5
0.45
1.20
0.55
0.014
0.020
0.200
0.0055
0.0011






6
0.50
0.25
1.00
0.019
0.023
0.560
0.0060







7
0.47
0.50
0.91
0.009
0.023
0.150
0.0040

0.04





8
0.63
0.05
0.73
0.010
0.072
0.149
0.0110







9
0.65
0.10
0.73
0.007
0.011
0.150
0.0016
0.0020

0.25




10
0.65
0.11
0.72
0.013
0.018
0.145
0.0043
0.0019

0.04




11
0.64
0.03
0.10
0.015
0.016
0.110
0.0100

0.70





12
0.61
0.05
0.75
0.003
0.023
0.150
0.0055







13
0.62
0.01
0.11
0.004
0.032
0.160
0.0042

0.79





14
0.52
0.05
1.25
0.007
0.080
0.152
0.0160







15
0.41
0.60
1.77
0.010
0.015
0.350
0.0050
0.0005






16
0.51
0.60
1.25
0.006
0.035
0.430
0.0016

0.06





17
0.40
0.01
1.71
0.015
0.010
0.130
0.0045
0.0093

0.11




18
0.50
0.05
1.30
0.006
0.020
0.150
0.0090



1.00



19
0.51
0.05
1.25
0.003
0.014
0.150
0.0011




0.10


20
0.43
0.05
1.72
0.003
0.025
0.170
0.0050







21
0.60
0.05
0.88
0.005
0.006
0.150
0.0110

0.02





22
0.60
0.10
0.86
0.008
0.070
0.151
0.0050
0.0028






23
0.50
0.10
1.52
0.008
0.006
0.150
0.0080
0.0004






24
0.53
0.05
1.30
0.040
0.009
0.125
0.0045

0.03





25
0.56
0.05
1.10
0.015
0.015
0.300
0.0050

0.11





26
0.47
0.05
1.80
0.011
0.020
0.150
0.0040







27
0.45
0.30
1.50
0.016
0.055
0.970
0.0110
0.0140
0.40





28
0.55
0.60
1.20
0.033
0.005
0.790
0.0063
0.0110
0.01





29
0.59
0.10
1.08
0.010
0.030
0.115
0.0016







30
0.60
0.30
0.95
0.018
0.090
0.250
0.0057

0.03





31
0.40
0.01
1.18
0.009
0.031
0.150
0.0035

0.79





32
0.65
0.45
0.40
0.013
0.025
0.125
0.0047
0.0019

0.24




33
0.62
0.67
0.22
0.008
0.070
0.150
0.0050







34
0.71
0.01
0.10
0.020
0.019
0.130
0.0049



0.05



35
0.55
1.05
0.20
0.033
0.005
0.480
0.0063

0.01

0.50
0.60


36
0.64
1.09
0.20
0.018
0.028
0.250
0.0180


0.03




37
0.65
0.95
0.22
0.003
0.023
0.150
0.0055


0.03
0.10
0.05


38
0.66
0.73
0.36
0.010
0.072
0.155
0.0110







39
0.70
0.65
0.20
0.014
0.020
0.193
0.0055


0.90
0.03
0.07


40
0.56
0.90
0.40
0.015
0.015
0.110
0.0050

0.11
0.40
1.00
0.10


41
0.45
2.10
0.25
0.006
0.020
0.150
0.0090







42
0.60
0.77
0.43
0.018
0.090
0.250
0.0057

0.03

0.73



43
0.41
2.31
0.40
0.013
0.025
0.125
0.0047


0.02

0.05


44
0.49
1.78
0.39
0.008
0.006
0.150
0.0080







45
0.53
1.05
0.78
0.007
0.080
0.150
0.0160




0.06


46
0.57
0.80
0.76
0.008
0.019
0.150
0.0110







47
0.61
0.73
0.60
0.005
0.006
0.150
0.0110

0.02





48
0.47
1.45
0.77
0.010
0.014
0.115
0.0045




0.02


49
0.50
1.39
0.22
0.046
0.009
0.150
0.0045

0.03





50
0.50
1.40
0.25
0.013
0.018
0.145
0.0043












Components (mass %)


















Number
Ca
Mg
Zr
Rem
Ti
Nb
W
V
Ta
Hf
Others





1













2













3













4













5













6













7













8













9













10













11













12













13













14













15













16













17













18













19













20
0.0012












21

0.0020











22


0.0020










23



0.0010









24




0.02 








25





0.02 







26













27






0.13






28







0.05





29








0.10




30









0.10



31










Sn: 0.09


32










Te: 0.001


33













34













35













36













37













38













39













40













41
0.0020
0.0023
0.0015
0.0001









42




0.008








43





0.005







44













45

0.0035

0.0004
0.02 

0.15






46





0.01 

0.23





47
0.0025

0.0007



0.43

0.06




48


0.0100

0.005

0.01

0.01

Li: 0.0001


49













50

















TABLE 3








Component (mass %)



















Number
C
Si
Mn
P
S
Al
N
B
Cr
Mo
Ni
Cu





 51
0.40
1.69
0.10
0.011
0.010
0.120
0.0070

0.79





 52
0.57
0.88
0.38
0.013
0.040
0.150
0.0120







 53
0.48
1.73
0.22
0.016
0.051
0.970
0.0110

0.40





 54
0.55
1.02
0.50
0.019
0.023
0.600
0.0060







 55
0.40
2.10
0.10
0.003
0.023
0.110
0.0050







 56
0.44
2.45
0.21
0.010
0.015
0.350
0.0050







 57
0.52
1.74
0.38
0.006
0.035
0.430
0.0016

0.06





 58
0.56
1.36
0.40
0.013
0.040
0.150
0.0120

0.01





 59
0.61
1.01
0.37
0.006
0.050
0.150
0.0048







 60
0.45
2.97
0.10
0.011
0.020
0.990
0.0040







 61
0.65
0.25
0.50
0.007
0.011
0.150
0.0016
0.0020






 62
0.65
0.59
0.50
0.018
0.028
0.250
0.0180







 63
0.73
0.60
0.10
0.010
0.019
0.150
0.0039







 64
0.69
0.57
0.25
0.006
0.035
0.405
0.0016

0.06





 65
0.67
0.21
0.49
0.035
0.070
0.150
0.0050







 66
0.71
0.29
0.25
0.002
0.060
0.150
0.0016







 67
0.69
0.40
0.40
0.003
0.014
0.150
0.0011







 68
0.73
0.39
0.20
0.008
0.019
0.150
0.0110







 69
0.73
0.04
0.25
0.009
0.070
0.141
0.0037







 70
0.74
0.05
0.20
0.010
0.030
0.150
0.0016



0.03



 71
0.72
0.10
0.40
0.013
0.040
0.150
0.0120

0.01





 72
0.73
0.08
0.38
0.008
0.058
0.150
0.0046


0.01
4.50
0.02


 73
0.70
0.05
0.49
0.009
0.017
0.130
0.0050


0.12
0.01
0.03


 74
0.74
0.07
0.22
0.009
0.017
0.130
0.0050


0.04
1.00
3.00


 75
0.64
0.44
0.57
0.016
0.050
0.970
0.0110

0.37





 76
0.70
0.30
0.45
0.007
0.080
0.700
0.0160







 77
0.72
0.10
0.43
0.009
0.052
0.150
0.0064
0.0015


1.00
1.50


 78
0.74
0.10
0.34
0.010
0.021
0.200
0.0110







 79
0.66
0.29
0.66
0.010
0.014
0.115
0.0045


0.02




 80
0.72
0.01
0.11
0.011
0.020
0.128
0.0039


0.01
0.25



 81
0.73
0.05
0.24
0.019
0.023
0.780
0.0060


0.03
0.004



 82
0.74
0.05
0.20
0.002
0.090
0.880
0.0016

0.02





 83
0.73
0.05
0.78
0.010
0.019
0.980
0.0039







 84
0.74
0.29
0.25
0.033
0.005
0.795
0.0063
0.0019
0.01





 85
0.60
0.59
0.75
0.003
0.025
0.170
0.0050







 86
0.64
0.57
0.54
0.010
0.007
0.150
0.0030







 87
0.59
0.61
0.75
0.005
0.006
0.125
0.0110

0.02





 88
0.58
0.65
0.74
0.015
0.015
0.110
0.0050

0.11





 89
0.61
0.44
0.80
0.045
0.005
0.160
0.0020



1.30



 90
0.60
0.48
0.84
0.008
0.070
0.150
0.0050



0.80
1.00


 91
0.60
0.48
0.83
0.014
0.020
0.200
0.0055







 92
0.65
0.15
0.79
0.008
0.006
0.150
0.0080







 93
0.65
0.45
0.61
0.018
0.090
0.250
0.0057

0.03





 94
0.64
0.21
0.83
0.010
0.072
0.150
0.0110







 95
0.65
1.30
0.87
0.015
0.030
0.900
0.0042







 96
0.55
0.70
1.20
0.020
0.020
0.150
0.0060

0.10





 97
0.44
0.50
0.90
0.010
0.012
0.150
0.0080







 98
0.55
0.71
0.40
0.014
0.020

0.030

0.0050







 99
0.57
0.25
0.62
0.005
0.016

0.090

0.0110







100
0.55
1.00
0.40
0.015
0.010

1.200

0.0040
0.0010






101
0.41
0.30

2.14

0.012
0.050
0.800
0.0055
0.0018






102
0.51
0.23
0.78
0.008
0.080
0.270
0.0113


0.90






103
0.45
0.09
1.78
0.007
0.050
0.121

0.0210

0.0020






104

0.35

1.00
1.50
0.008
0.070
0.130
0.0100







105

0.75

0.10
0.25
0.010
0.040
0.015
0.0040












Component (mass %)


















Number
Ca
Mg
Zr
Rem
Ti
Nb
W
V
Ta
Hf
Others





51













52













53













54













55













56













57













58













59













60













61













62













63













64










Pb: 0.05


65










Bi: 0.l


66










Te: 0.001


67










Ba: 0.0001


68
0.0008












69

0.0008











70


0.0003










71













72



0.0009









73
0.0004

0.0003










74

0.0012

0.0006









75
0.0013


0.0011









76

0.0018
0.0021










77



0.0048
0.05

0.10


0.01



78




0.30
0.02
0.05
0.46
0.15
0.0006



79
0.0013

0.0020

0.13


0.12

0.16
Sn: 0.1, Zn: 0.011


80

0.0110
0.0050
0.0110

0.04


0.0007
0.05
Te: 0.001, Bi: 0.07


81
0.0020
0.0150



0.07
0.003

0.003
0.003
Ba: 0.0001, Na: 0.0001


82



0.0152









83













84

0.0005




0.0006






85


0.0147




0.07


Sb: 0.005


86










Se: 0.0001, Pb: 0.07


87




0.11
0.0008




K: 0.0001


88
0.0001



0.42


0.0008


Sr: 0.0001, Na: 0.0001


89








0.10




90

0.0001







0.10



91
0.0150






0.03





92





0.10







93







0.005





94
0.0100




0.41







95













96













97













98













99













100













101













102













103













104













105

















TABLE 4








Expression














In the case where steel does not contain Cr








(1) 139.38 ≦ 214 × [C] + 30.6 × [Si] +

In the case where steel does not contain Cr

In the case where steel does not contain Cr




42.8 [Mn] − 14.7 × [Al] ≦ 177

(2) 0.72 ≦ [C] + 1/7 [Si] + 1/5 [Mn] <
In the case where steel does not contain Cr
(4) 55 ≦ 33 + 31 × [C] + 4.5 × [Si] + 1.5 ×




In the case where steel contains Cr

1.539
(3) 113 − 135 × [C] − 27 × [Mn] ≦ 13
[Mn] < 72.45




(6) 139.38 ≦ 214 × [C] + 30.6 × [Si] +

In the case where steel contains Cr
In the case where steel contains Cr
In the case where steel contains Cr
(5) 1.5 <



42.8 × [Mn] + 23.8 [Cr] − 14.7 × [Al] ≦

(7) 0.72 ≦ [C] + 1/7 [Si] + 1/5 [Mn] + 1/9
(8) 113 − 135 × [C] − 27 × [Mn] − 18 ×
(9) 55 ≦ 33 + 31 × [C] + 4.5 × [Si] + 1.5 ×
[Si] + 1.8


Number
177

[Cr] < 1.627
[Cr] ≦ 13
[Mn] + 2.4 × [Cr] < 74.37
[Mn] < 6.24





1
155.84

0.737

14.45


53.98


0.770



2
155.41

0.734

30.02


54.51


1.300



3
154.72

0.733

26.60


54.83


1.010



4
159.74

0.754

19.85


49.60

3.060


5
153.62

0.731

37.40


53.18

2.190


6
149.22

0.736

18.50


51.13

2.050


7
153.58

0.728

24.26


51.28

2.138


8
165.40

0.783
8.24

53.85


1.364



9
171.20

0.810
5.54

54.70


1.414



10
171.15

0.810
5.81

54.73


1.406



11
157.20

0.742
11.30

54.81


0.210



12
161.97

0.767
10.40

53.26


1.400



13
153.07

0.726
12.79

54.17


0.208



14
164.08

0.777
9.05

51.22

2.300


15
176.71

0.850
9.86

51.07

3.786


16
176.11

0.852
9.32

53.53

2.850


17
157.18

0.743
12.83

48.01

3.088


18
161.97

0.767
10.40

50.68

2.390


19
161.97

0.767
10.40

50.91

2.300


20
164.67

0.781
8.51

49.14

3.146


21
165.87

0.785
7.88

53.19

1.634


22
166.05

0.786
8.78

53.34

1.648


23
172.91

0.818
4.46

51.23

2.836


24
169.47

0.800
5.81

51.68

2.390


25
166.66

0.799
5.72

52.50

2.030


26
176.95

0.837
0.95

50.50

3.290


27
164.94

0.837
4.55

51.51

3.000


28
176.05

0.877
6.17

54.57

2.760


29
173.85

0.820
4.19

53.36

2.044


30
175.28

0.836
5.81

54.45

2.010


31
153.01

0.725
12.92

49.11

2.134


32
168.15

0.794

14.45

55.78

1.170



33
160.39

0.760

23.36

55.57

1.066



34
153.55

0.726

15.13

55.05

0.190



35
151.57

0.741

33.17

55.10

1.410

















Characteristics before hardening
Characteristics after hardening



















Pro-
Tempered







Amount
eutectoid
hardness (Hv)
Roller pitting





Hardness
of wear of
ferrite
at 300° C. of
fatigue strength




Number
(HV)
tool (μm)
fraction (%)
hardened layer
(MPa)
Remarks






1
207
27
14.6
595
3150
Example of the present invention



2
206
27
35.7
604
3150
Example of the present invention



3
206
26
31.1
608
3150
Example of the present invention



4
210
29
21.9
516
2800
Example of the present invention



5
206
26
45.8
580
3100
Example of the present invention



6
206
23
20.1
543
2950
Example of the present invention



7
205
26
27.9
546
2950
Example of the present invention



8
217
27
6.1
592
3150
Example of the present invention



9
222
40
2.4
608
3150
Example of the present invention



10
222
36
2.8
608
3150
Example of the present invention



11
208
28
10.3
609
3150
Example of the present invention



12
213
31
9.0
582
3100
Example of the present invention



13
204
25
12.3
598
3150
Example of the present invention



14
215
26
7.2
545
2950
Example of the present invention



15
231
39
8.3
542
2900
Example of the present invention



16
231
38
7.6
587
3100
Example of the present invention



17
208
30
12.3
487
2700
Example of the present invention



18
213
31
9.0
535
2900
Example of the present invention



19
213
31
9.0
539
2900
Example of the present invention



20
216
32
6.5
507
2800
Example of the present invention



21
217
33
5.6
580
3100
Example of the present invention



22
217
27
6.8
583
3100
Example of the present invention



23
224
37
1.0
545
2950
Example of the present invention



24
220
35
2.8
553
2950
Example of the present invention



25
220
33
2.7
568
3050
Example of the present invention



26
228
33
0.0
532
2900
Example of the present invention



27
228
28
1.1
550
2950
Example of the present invention



28
237
38
3.3
605
3150
Example of the present invention



29
225
38
0.6
583
3100
Example of the present invention



30
228
32
2.8
603
3150
Example of the present invention



31
204
25
12.5
507
2800
Example of the present invention



32
219
38
14.6
627
3300
Example of the present invention



33
212
24
26.7
623
3250
Example of the present invention



34
204
26
15.5
614
3250
Example of the present invention



35
208
24
40.0
615
3250
Example of the present invention

















TABLE 5








Expression













In the case where steel does not contain Cr







(1) 139.38 ≦ 214 × [C] + 30.6 × [Si] +
In the case where steel does not contain Cr

In the case where steel does not contain Cr




42.8 × [Mn] − 14.7 × [Al] ≦ 177
(2) 0.72 ≦ [C] + 1/7 [Si] + 1/5 [Mn] <
In the case where steel does not contain Cr
(4) 55 ≦ 33 + 31 × [C] + 4.5 × [Si] +




In the case where steel contains Cr
1.539
(3) 113 − 135 × [C] − 27 × [Mn] ≦ 13
1.5 × [Mn] < 72.45




(6) 139.38 ≦ 214 × [C] + 30.6 × [Si] +
In the case where steel contains Cr
In the case where steel contains Cr
In the case where steel contains Cr




42.8 × [Mn] + 23.8 [Cr] − 14.7 × [Al] ≦
(7) 0.72 ≦ [C] + 1/7 [Si] + 1/5 [Mn] + 1/9
(8) 113 − 135 × [C] − 27 × [Mn] − 18 ×
(9) 55 ≦ 33 + 31 × [C] + 4.5 × [Si] + 1.5 ×
(5) 1.5 < [Si] + 1.8


Number
177
[Cr] < 1.627
[Cr] ≦ 13
[Mn] + 2.4 x [Cr] < 74.37
[Mn] < 6.24





36
175.20
0.836

21.20

58.05

1.450



37
175.38
0.830

19.31

57.76

1.346



38
176.71
0.836

14.18

57.29

1.378



39
175.41
0.833

13.10

57.93

1.010



40
165.50
0.781

24.62

55.27
1.620


41
169.06
0.800

45.50

56.78
2.550


42
167.41
0.799

19.85

55.78
1.544


43
173.71
0.820

46.85

56.71
3.030


44
173.82
0.822

36.32

56.79
2.482


45
176.73
0.836

20.39

55.33
2.454


46
176.78
0.836

15.53

55.41
2.168


47
176.83
0.837

14.09

56.14
1.810


48
176.22
0.831

28.76

55.25
2.836


49
157.46
0.746

39.02

55.16
1.786


50
158.41
0.750

38.75

55.18
1.850


51
158.63
0.749

42.08

55.05
1.870


52
162.97
0.772

25.79

55.20
1.564


53
160.34
0.816

35.06

56.96
2.126


54
161.49
0.796

25.25

55.39
1.920


55
152.52
0.720

56.30

55.00
2.280


56
172.97
0.832

47.93

57.98
2.828


57
175.90
0.851

31.46

57.66
2.424


58
176.61
0.835

26.42

57.10
2.080


59
175.08
0.828

20.66

57.01
1.676


60
176.91
0.894

49.55

60.47
3.150


61
165.95
0.786
11.75
55.03

1.150



62
174.88
0.834
11.75
56.56

1.490



63
176.66
0.836
11.75
58.48

0.780



64
171.28
0.828
12.02
57.47

1.020



65
168.57
0.798
9.32
55.45

1.092



66
169.31
0.801
10.40
56.69

0.740



67
174.82
0.827
9.05
56.79

1.120



68
174.51
0.826
9.05
57.69

0.750



69
166.07
0.786
7.70
56.19

0.490



70
166.25
0.787
7.70
56.47

0.410

















Characteristics before hardening
Characteristics after hardening



















Pro-
Tempered







Amount
eutectoid
hardness (Hv)
Roller pitting





Hardness
of wear of
ferrite
at 300° C. of
fatigue strength




Number
(HV)
tool (μm)
fraction (%)
hardened layer
(MPa)
Remarks






36
228
38
23.7
668
3450
Example of the present invention



37
227
38
21.2
663
3450
Example of the present invention



38
228
33
14.2
654
3400
Example of the present invention



39
227
44
12.7
666
3450
Example of the present invention



40
216
37
28.4
618
3250
Example of the present invention



41
220
35
56.8
645
3350
Example of the present invention



42
220
28
21.9
627
3300
Example of the present invention



43
224
37
58.6
644
3350
Example of the present invention



44
225
37
44.3
645
3350
Example of the present invention



45
228
33
22.6
619
3250
Example of the present invention



46
228
39
16.0
620
3250
Example of the present invention



47
228
39
14.1
634
3300
Example of the present invention



48
227
39
34.0
618
3250
Example of the present invention



49
209
28
48.0
616
3250
Example of the present invention



50
210
29
47.6
616
3250
Example of the present invention



51
209
29
52.1
614
3250
Example of the present invention



52
214
31
30.0
617
3250
Example of the present invention



53
224
25
42.6
648
3350
Example of the present invention



54
219
30
29.2
620
3250
Example of the present invention



55
203
25
71.5
613
3200
Example of the present invention



56
227
37
60.1
667
3450
Example of the present invention



57
231
38
37.7
661
3400
Example of the present invention



58
228
39
30.8
651
3400
Example of the present invention



59
226
35
23.0
649
3350
Example of the present invention



60
240
33
62.3
711
3650
Example of the present invention



61
217
33
10.9
613
3200
Example of the present invention



62
228
38
10.9
641
3350
Example of the present invention



63
228
33
10.9
676
3500
Example of the present invention



64
226
36
11.2
658
3400
Example of the present invention



65
220
28
7.6
621
3250
Example of the present invention



66
221
32
9.0
643
3350
Example of the present invention



67
226
38
7.2
645
3350
Example of the present invention



68
226
38
7.2
661
3450
Example of the present invention



69
217
27
5.4
634
3300
Example of the present invention



70
217
33
5.4
639
3350
Example of the present invention


















TABLE 6








Expression















In the case where steel does not contain Cr








(1) 139.38 ≦ 214 × [C] + 30.6 × [Si] +
In the case where steel does not

In the case where steel does not





42.8 × [Mn] − 14.7 × [Al] ≦ 177
contain Cr (2) 0.72 ≦ [C] + 1/7
In the case where steel does not contain Cr
contain Cr (4) 55 ≦ 33 + 31 × [C] +





In the case where steel contains Cr
[Si] + 1/5 [Mn] < 1.539
(3) 113 − 135 × [C] - 27 × [Mn] ≦ 13
4.5 × [Si] + 1.5 × [Mn] < 72.45

Characteristics



(6) 139.38 ≦ 214 × [C] + 30.6 × [Si] +
In the case where steel contains Cr
In the case where steel contains Cr
In the case where steel contains Cr

before hardening



42.8 × [Mn] + 23.8 [Cr] − 14.7 × [Al] ≦
(7) 0.72 ≦ [C] + 1/7 [Si] + 1/5 [Mn] +
(8) 113 − 135 × [C] − 27 × [Mn] − 18 ×
(9) 55 ≦ 33 + 31 × [C] + 4.5 × [Si] +
(5) 1.5 < [Si] + 1.8
Hardness


Number
177
1/9 [Cr] < 1.627
[Cr] ≦ 13
1.5 × [Mn] + 2.4 × [Cr] < 74.37
[Mn] < 6.24
(HV)





71
172.29
0.815
4.82
56.39

0.820

223


72
172.73
0.817
4.19
56.56

0.764

224


73
170.39
0.805
5.27
55.66

0.932

221


74
168.01
0.794
7.16
56.59

0.466

219


75
169.37
0.858
4.55
56.56

1.466

233


76
167.95
0.833
6.35
56.73

1.110

227


77
173.34
0.820
4.19
56.42

0.874

225


78
173.03
0.822
3.92
56.90

0.712

225


79
176.67
0.833
6.08
55.76

1.478

227


80
157.21
0.743
12.83
55.53

0.208

208


81
156.56
0.785
7.97
56.22

0.482

217


82
155.99
0.789
7.34
56.51

0.410

218


83
176.73
0.893
−6.61
57.03

1.454

240


84
166.49
0.833
6.17
57.64

0.740

227


85
176.06
0.834
11.75
55.38
1.940
228


86
175.31
0.829
12.02
56.22
1.542
226


87
175.66
0.829
12.74
55.21
1.960
226


88
176.68
0.833
12.74
55.28
1.982
227


89
175.89
0.833
9.05
55.09
1.880
227


90
176.84
0.837
9.32
55.02
1.992
228


91
175.67
0.835
9.59
55.01
1.974
228


92
175.30
0.829
3.92
55.01
1.572
226


93
176.02
0.840
8.24
56.16
1.548
229


94
176.71
0.836
4.19
55.03
1.704
228


95

202.89

1.010
1.76
60.31
2.866
265


96

190.66

0.901
4.55
55.24
2.860
242


97
145.78

0.691


29.30


50.24

2.120
197


98
156.11
0.731

27.95


53.85


1.430

206


99
154.84
0.730

19.31


52.73


1.366

205


100
147.78
0.773

27.95

55.15
1.720
214


101
176.75
0.881
−0.13

50.27

4.152
238


102
167.01
0.799
6.89

53.18

1.634
220


103
173.46
0.819
4.19

50.03

3.294



104
167.79
0.793

25.25


50.60

3.700
219


105
174.04
0.814
5.00
57.08

0.550

















Characteristics before hardening
Characteristics after hardening

















Pro-
Tempered






Amount
eutectoid
hardness (Hv)
Roller pitting





of wear of
ferrite
at 300° C. of
fatigue strength




Number
tool (μm)
fraction (%)
hardened layer
(MPa)
Remarks






71
37
1.5
638
3350
Example of the present invention



72
34
0.6
641
3350
Example of the present invention



73
38
2.1
625
3250
Example of the present invention



74
34
4.6
642
3350
Example of the present invention



75
31
1.1
641
3350
Example of the present invention



76
27
3.5
644
3350
Example of the present invention



77
34
0.6
638
3350
Example of the present invention



78
37
0.2
647
3350
Example of the present invention



79
39
3.2
627
3300
Example of the present invention



80
28
12.3
623
3250
Example of the present invention



81
27
5.7
635
3300
Example of the present invention



82
20
4.9
640
3350
Example of the present invention



83
33
0.0
649
3400
Example of the present invention



84
32
3.3
661
3400
Example of the present invention



85
39
10.9
620
3250
Example of the present invention



86
38
11.2
635
3300
Example of the present invention



87
39
12.2
617
3250
Example of the present invention



88
39
12.2
618
3250
Example of the present invention



89
39
7.2
615
3250
Example of the present invention



90
33
7.6
613
3200
Example of the present invention



91
39
7.9
613
3200
Example of the present invention



92
38
0.2
613
3200
Example of the present invention



93
33
6.1
634
3300
Example of the present invention



94
33
0.6
614
3200
Example of the present invention



95
180
0.0
708
3600
Comparative Example



96
95
1.1
617
3250
Comparative Example



97
21
34.7
527
2850
Comparative Example



98
100
32.9
592
3150
Comparative Example



99
90
21.2
572
3050
Comparative Example













100
22
32.9
High-frequency hardening
Comparative Example






could not be applied















101
85
0.0
528
2850
Comparative Example



102
79
4.3
580
3100
Comparative Example











103
Hot shortening appeared during rolling
Comparative Example













104
34
29.2
Lack of surface hardness
Comparative Example











105
Seasoning crack occurred in rolled material
Comparative Example








Claims
  • 1. A steel for a machine structure, the steel comprising, in mass %: C: 0.40% to less than 0.75%;Si: 0.01% to 3.0%;Mn: 072% to 1.8%;S: 0.001% to 0.1%;Al: more than 0.1% and not more than 1.0%;N: 0.001% to 0.02%; andP: limited to not more than 0.05%,with a balance including Fe and inevitable impurities, whereinan amount of C: [C], an amount of Si: [Si], an amount of Mn: [Mn], and an amount of Al: [Al] satisfy the following Expression (1) and Expression (2): 139.38≦214×[C]+30.6×[Si]+42.8×[Mn]−14.7×[Al]≦177  (1)0.72≦[C]+1/7×[Si]+1/5×[Mn]<1.539  (2).
  • 2. The steel for a machine structure according to claim 1, wherein the steel further satisfies the following Expression (3): 113−135×[C]−27×[Mn]≦13  (3).
  • 3. The steel for a machine structure according to claim 1, wherein the steel further satisfies the following Expression (4): 55≦33+31×[C]+4.5×[Si]+1.5×[Mn]<72.45  (4).
  • 4. The steel for a machine structure according to claim 2, wherein the steel further satisfies the following Expression (4): 55≦33+31×[C]+4.5×[Si]+1.5×[Mn]<72.45  (4).
  • 5. The steel for a machine structure according to claim 1, wherein the steel further satisfies the following Expression (5): 1.5<[Si]+1.8×[Mn]<6.24  (5).
  • 6. The steel for a machine structure according to claim 1, wherein the steel further comprises B: 0.0001% to 0.015% in mass %.
  • 7. The steel for a machine structure according to claim 4, wherein the steel further comprises, in mass %, one or more elements of Cr: 0.01% to 0.8%, Mo: 0.001% to 1.0%, Ni: 0.001% to 5.0%, and Cu: 0.001% to 5.0%, and in a case where the steel comprises Cr: 0.01% to 0.8%, the following Expression (6) is satisfied instead of Expression (1), the following Expression (7) is satisfied instead of Expression (2), the following Expression (8) is satisfied instead of Expression (3), and the following Expression (9) is satisfied instead of Expression (4): 139.38≦214×[C]+30.6×[Si]+42.8×[Mn]+23.8×[Cr]−14.7×[Al]≦177  (6)0.72≦[C]+1/7×[Si]+1/5×[Mn]+1/9×[Cr]<1.627  (7)113−135×[C]−27×[Mn]−18×[Cr]≦13  (8)55≦33+31×[C]+4.5×[Si]+1.5×[Mn]+2.4×[Cr]<74.37  (9).
  • 8. The steel for a machine structure according to claim 1, wherein the steel further comprises, in mass %, one or more elements of Ca: 0.0001% to 0.02%, Mg: 0.0001% to 0.02%, Zr: 0.0001% to 0.02%, and Rem: 0.0001% to 0.02%.
  • 9. The steel for a machine structure according to claim 1, wherein the steel further comprises, in mass %, one or more elements of Ti: 0.005% to 0.5%, Nb: 0.0005% to 0.5%, W: 0.0005% to 0.5%, V: 0.0005% to 0.5%, Ta: 0.0001% to 0.2%, and Hf: 0.0001% to 0.2%.
  • 10. The steel for a machine structure according to claim 1, wherein the steel further comprises, in mass %, one or more elements of Sb: 0.0001% to 0.015%, Sn: 0.0005% to 2.0%, Zn: 0.0005% to 0.5%, Te: 0.0003% to 0.2%, Se: 0.0003% to 0.2%, Bi: 0.001% to 0.5%, and Pb: 0.001% to 0.5%.
  • 11. The steel for a machine structure according to claim 1, wherein the steel further comprises, in mass %, one or more elements of Li: 0.00001% to 0.005%, Na: 0.00001% to 0.005%, K: 0.00001% to 0.005%, Ba: 0.00001% to 0.005%, and Sr: 0.00001% to 0.005%.
  • 12. The steel for a machine structure according to claim 2, wherein the steel further satisfies the following Expression (5): 1.5<[Si]+1.8×[Mn]<6.24  (5).
  • 13. The steel for a machine structure according to claim 3, wherein the steel further satisfies the following Expression (5): 1.5<[Si]+1.8×[Mn]<6.24  (5).
  • 14. The steel for a machine structure according to claim 4, wherein the steel further satisfies the following Expression (5): 1.5<[Si]+1.8×[Mn]<6.24  (5).
  • 15. The steel for a machine structure according to claim 2, wherein the steel further comprises B: 0.0001% to 0.015% in mass %.
  • 16. The steel for a machine structure according to claim 3, wherein the steel further comprises B: 0.0001% to 0.015% in mass %.
  • 17. The steel for a machine structure according to claim 4, wherein the steel further comprises B: 0.0001% to 0.015% in mass %.
  • 18. A steel for a machine structure, the steel comprising, in mass %: C: 0.40% to less than 0.75%;Si: 0.01% to 3.0%;Mn: 0.72 to 1.8%;S: 0.001% to 0.1%;Al: more than 0.1% and not more than 1.0%;N: 0.001% to 0.02%;at least one element selected from the group consisting of B: 0.0001% to 0.015%, Cr: 0.01% to 0.8%, Mo: 0.001% to 1.0%, Ni: 0.001% to 5.0%, Cu: 0.001% to 5.0%, Ca: 0.0001% to 0.02%, Mg: 0.0001% to 0.02%, Zr: 0.0001% to 0.02%, Rem: 0.0001% to 0.02%, Ti: 0.005% to 0.5%, Nb: 0.0005% to 0.5%, W: 0.0005% to 0.5%, V: 0.0005% to 0.5%, Ta: 0.0001% to 0.2%, Hf: 0.0001% to 0.2%, Sb: 0.0001% to 0.015%, Sn: 0.0005% to 2.0%, Zn: 0.0005% to 0.5%, Te: 0.0003% to 0.2%, Se: 0.0003% to 0.2%, Bi: 0.001% to 0.5%, Pb: 0.001% to 0.5%, Li: 0.00001% to 0.005%, Na: 0.00001% to 0.005%, K: 0.00001% to 0.005%, Ba: 0.00001% to 0.005%, and Sr: 0.00001% to 0.005%; andP: limited to not more than 0.05%,with a balance including Fe and inevitable impurities, whereinan amount of C: [C], an amount of Si: [Si], an amount of Mn: [Mn], and an amount of Al: [Al] satisfy the following Expression (1) and Expression (2): 139.38≦214×[C]+30.6×[Si]+42.8×[Mn]−14.7×[Al]≦177  (1)0.72≦[C]+1/7×[Si]+1/5×[Mn]<1.539  (2), andin a case where the steel comprises Cr: 0.01% to 0.8%, the following Expression (6) is satisfied instead of Expression (1), and the following Expression (7) is satisfied instead of Expression (2): 139.38≦214×[C]+30.6×[Si]+42.8×[Mn]+23.8×[Cr]−14.7×[Al]≦177  (6)0.72≦[C]+1/7×[Si]+1/5×[Mn]+1/9×[Cr]<1.627  (7).
  • 19. The steel for a machine structure according to claim 18, wherein the steel further satisfies the following Expression (3): 113−135×[C]−27×[Mn]≦13  (3), andin a case where the steel comprises Cr: 0.01% to 0.8%, the following Expression (8) is satisfied instead of Expression (3): 113−135×[C]−27×[Mn]−18×[Cr]≦13  (8).
  • 20. The steel for a machine structure according to claim 18, wherein the steel further satisfies the following Expression (4): 55≦33+31×[C]+4.5×[Si]+1.5×[Mn]<72.45  (4), andin a case where the steel comprises Cr: 0.01% to 0.8%, the following Expression (9) is satisfied instead of Expression (4): 55≦33+31×[C]+4.5×[Si]+1.5×[Mn]+2.4×[Cr]<74.37  (9).
  • 21. The steel for a machine structure according to claim 19, wherein the steel further satisfies the following Expression (4): 55≦33+31×[C]+4.5×[Si]+1.5×[Mn]<72.45  (4), andin a case where the steel comprises Cr: 0.01% to 0.8%, the following Expression (9) is satisfied instead of Expression (4): 55≦33+31×[C]+4.5×[Si]+1.5×[Mn]+2.4×[Cr]<74.37  (9).
  • 22. The steel for a machine structure according to claim 18, wherein the steel further satisfies the following Expression (5): 1.5<[Si]+1.8×[Mn]<6.24  (5).
  • 23. The steel for a machine structure according to claim 19, wherein the steel further satisfies the following Expression (5): 1.5<[Si]+1.8×[Mn]<6.24  (5).
  • 24. The steel for a machine structure according to claim 20, wherein the steel further satisfies the following Expression (20): 1.5<[Si]+1.8×[Mn]<6.24  (5).
  • 25. The steel for a machine structure according to claim 21, wherein the steel further satisfies the following Expression (5): 1.5<[Si]+1.8×[Mn]<6.24  (5).
Priority Claims (4)
Number Date Country Kind
2010-160108 Jul 2010 JP national
2010-160136 Jul 2010 JP national
2010-160140 Jul 2010 JP national
2010-160141 Jul 2010 JP national
PCT Information
Filing Document Filing Date Country Kind 371c Date
PCT/JP2011/065782 7/11/2011 WO 00 12/21/2012
Publishing Document Publishing Date Country Kind
WO2012/008405 1/19/2012 WO A
US Referenced Citations (4)
Number Name Date Kind
20020185201 Kitano Dec 2002 A1
20090311125 Miyanishi et al. Dec 2009 A1
20110002807 Saitoh et al. Jan 2011 A1
20110229363 Sakamoto et al. Sep 2011 A1
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Non-Patent Literature Citations (5)
Entry
Machine-English translation of Japanese patent 09-217143, Yasumoto Satoshi et al., Aug. 19, 1997.
International Search Report, dated Sep. 6, 2011, issued in PCT/JP2011/065782.
Written Opinion of the International Searching Authority, dated Sep. 6, 2011, issued in PCT/JP2011/065782.
Chinese Office Action and Search Report, dated May 16, 2014, issued in Chinese Application No. 201180033837.0, with an English translation.
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Related Publications (1)
Number Date Country
20130101457 A1 Apr 2013 US