STEEL FOR MACHINE STRUCTURE EXHIBITING EXCELLENT
MACHINABILITY
Technical Field
[0001]
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 July 14, 2010; Japanese Patent Application No.
2010-160108 filed in Japan on July 14, 2010; Japanese Patent Application No.
2010-160141 filed in Japan on July 14, 2010; and Japanese Patent Application No.
2010-160140 filed in Japan on July 14, 2010, the disclosures of which are incorporated
herein by reference in their entirety.
Background Art
[0002]
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.
[0003]
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.
[0004]
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).
[0005]
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 urn or less.
[0006]
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 x 105 cycles or more with a rotary bending stress of 280 MPa.
[0007]
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
[0008]
Patent Document 1: Japanese Unexamined Patent Application, First Publication
r Mi i j £
Is
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
[0009]
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.
[0010]
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
[0011]
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:
[0012]
2>
*
(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 x [C] + 30.6 x [Si] + 42.8 x [Mn] -14.7 x [Al] < 177 - (1)
0.72 < [C] + 1/7 x [Si] + l/5x[Mn] < 1.539 - (2)
[0013]
(2) The steel for a machine structure according to (1) above, in which the steel
further satisfies the following Expression (3).
113 - 135x[C] - 27x[Mn] < 13 - (3)
[0014]
(3) The steel for a machine structure according to (1) above, in which the steel
further satisfies the following Expression (4).
55 < 33 + 31x[C] +4.5x[Si] + 1.5x[Mn] < 72.45 - (4)
[0015]
(4) The steel for a machine structure according to (2) above, in which the steel
further satisfies the following Expression (4).
55 < 33 + 31x[C] + 4.5x[Si] + 1.5x[Mn] < 72.45 - (4)
[0016]
(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] + 1.8x[Mn] < 6.24 - (5)
[0017]
(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%.
[0018]
4
(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 < 214x[C] + 30.6x[Si] + 42.8x[Mn] + 23.8x[Cr] - 14.7x[Al] < 177 -
(6)
0.72 < [C] + l/7x[Si] + l/5x[Mn] + l/9x[Cr] < 1.627 - (7)
113 - 135x[C] - 27x[Mn] - 18x[Cr] < 13 - (8)
55 < 33 + 31x[C] + 4.5x[Si] + 1.5x[Mn] + 2.4x[Cr] < 74.37 - (9)
[0019]
(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%.
[0020]
(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%.
[0021]
(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%.
[0022]
-5/32"
*
(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
[0023]
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
[0024]
FIG. 1 is a diagram illustrating a relationship between a hardness (Hv), and a
carbon equivalent Ceq (= [C] + 1/7 x [Si] + 1/5 x [Mn]) and a carbon equivalent Ceq (=
[C] + 1/7 x [Si] + 1/5 x [Mn] + 1/9 x [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 [um]
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 a fraction
(%), an index A(= 113 -135 x [C] - 27 x [Mn]), and an indexA(= 113 - 135 x [C] - 27
x [Mn]-18x[Cr]).
FIG 5 is a diagram illustrating a relationship between tempered hardness (Hv)
at 300°C, and an index RT (33 + 31 x [C] + 4.5 x [Si] + 1.5 x [Mn]) and an index RT
4UJXZc
*
(33 + 31 x [C] + 4.5 x [Si] + 1.5 x [Mn] + 2.4 x [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
[0025]
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.
[0026]
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).
[0027]
139.38 < 214 x [C] + 30.6 x [Si] + 42.8 x [Mn] -14.7 x [Al] < 177 - (1)
0.72 < [C] + 1/7 x [Si] + 1/5 x [Mn] < 1.539 - (2)
[0028]
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 x [C] - 27 x [Mn] < 13 - (3)
55 < 33 + 31 x [C] + 4.5 x [Si] + 1.5 x [Mn] < 72.45 - (4)
*
1.5 < [Si] + 1.8 x [Mn] < 6.24 - (5)
Each of the expressions above will be described later.
[0029]
First, the reason for limiting components for the steel according to the present
invention will be described. Hereinbelow, the unit % represents a mass %.
[0030]
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%.
[0031]
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%.
[0032]
•8-Z-32
8
*
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%.
[0033]
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%.
[0034]
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 AI2O3 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 Fe304 film or NiO film provided on the surface of
the tool.
[0035]
In the case where the amount of Al is less than 0.1% or less, the amount of
AI2O3 generated is small, and the AI2O3 film is not formed on the surface of the tool.
1
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%.
[0036]
N: 0.001% to 0.02%
N is an element that forms A1N, 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%.
[0037]
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%.
[0038]
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
10
*
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.
[0039]
In the steel according to the present invention, [C], [Si], [Mn], and [Al] satisfy
the following Expressions (1) and (2).
[0040]
First, Expression (1) will be described.
[0041]
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.
(al) Relationship between hardness and carbon equivalent that has a large effect on
the hardness
(al) Relationship between hardness and machinability affected by the hardness
[0042]
The carbon equivalent Ceq is defined as Ceq = [C] + 1/7 x [Si] + 1/5 x [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 x [Si] + 1/5 x [Mn] + 1/9 x [Cr].
[0043]
The relationship between the hardness and the carbon equivalent was examined
in the following manner.
[0044]
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
.11 / 32
*
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 1/4 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 (al).
Hardness (Hv) = 214 x Ceq + 49 - (al)
[0045]
For the relationship between the hardness and the machinability, the amount of
Al that generates AI2O3 to form an AI2O3 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.
[0046]
Square test pieces with a size of 40 x 40 x 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 Dec.2002 P78." Table 1 shows test
conditions.
[0047]
[Table 1]
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[0048]
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 urn
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 um or less under the same condition.
[0049]
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 x [Al] + 226 in the case of a steel containing Al in the
range of over 0.1% to 1.0%.
[0050]
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 x [Al] + 226 - (a2)
[0051]
From Expression (al) and Expression (a2), the following expression can be
obtained.
214 x Ceq + 49 < 14.7 x [Al] + 226 - (1")
Substituting the expression of the carbon equivalent in the above-described
expression yields the following expression.
214 x [C] + 30.6 x [Si] + 42.8 x [Mn] -14.7 x [Al] < 177 - (1')
[0052]
43/32'
+
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
[0053]
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 x [C] + 30.6 x [Si] + 42.8 x [Mn] + 23.8 x [Cr] - 14.7 x [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 x [Si] + 1/5 x [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 x [Si] + 1/5 x [Mn] + 1/9 x [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
14/32
#
Expression (1') and Expression (5') can be determined as follows:
214 x 0.72 - 14.7 x i.o = 139.38 - (1'")
In other words, Expression (1) and Expression (6) can be expressed as follows:
139.38 < 214 x [C] + 30.6 x [Si] + 42.8 x [Mn] -14.7 x [Al] < 177 - (1)
139.38 < 214 x [C] + 30.6 x [Si] + 42.8 x [Mn] + 23.8 x [Cr] - 14.7 x [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.
[0054]
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.
[0055]
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 a 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.
[0056]
After being subjected to a condition of 900°C x 1 hour, the steel bar was
•15 / 32'
\6
air-cooled. Test pieces having a large diameter portion 26cp 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.
[0057]
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.
[0058]
FIG. 4 illustrates a relationship between a pro-eutectoid a fraction (%), and an
index A(= 113 - 135 x [C] - 27 x [Mn]) and an index A (= 113 - 135 x [C] - 27 x [Mn] -
18 x [Cr]).
[0059]
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 x [C] - 27
x [Mn] — (3') is given, and in the case where the steel contains Cr, index A = 113 - 135
x [C] - 27 x [Mn] - 18 x [Cr] - (8') is given.
[0060]
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 a fraction. Further,
effects of the amounts of C, Mn, and Cr contained in the steel material on the
pro-eutectoid a fraction were obtained through a multiple regression analysis to
16/32
1?
calculate the coefficients in the index A. Note that the pro-eutectoid a 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 x 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.
[0061]
From FIG. 4, it can be understood that the pro-eutectoid a fraction (%) linearly
increases with the increase in the value of the index A.
[0062]
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 a
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 x [C] - 27 x [Mn] < 13 - (3)
[0063]
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.
[0064]
In the case where the steel contains Cr, the following Expression (7) can be
obtained by measuring the pro-eutectoid a fraction in the steel material containing Cr:
0.01% to 0.8% in a similar manner to that described above.
113 - 135 x [C] - 27 x [Mn] - 18 x [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
.17/32-
IS
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.
[0065]
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 x[C]+4.5 x [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 x [C] + 4.5 x [Si] + 1.5 x [Mn] + 2.4 x [Cr] - (9')
[0066]
-18/32-
11
m
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.
[0067]
After being subjected to a condition of 900°C x 1 hour, the steel bar was
air-cooled. Test pieces having a large diameter portion 26(p 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 x 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.
[0068]
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.
[0069]
Through a roller pitting test, the present inventors confirmed that the fatigue
strength is excellent if the tempered hardness (Hv) at 300°C is 610Hv or more.
[0070]
Roller pitting test pieces having a large diameter portion (test portion) 26(p
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 x 90 minutes. Then, a grip portion was subjected to a finishing process to
increase the accuracy of the test.
%
[0071]
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.
[0072]
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.
[0073]
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 610Hv
or more. From FIG. 5, it can be understood that, in order to obtain the tempered
hardness at 300°C of 610Hv 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 x[C]+4.5x[Si] + 1.5 x [Mn] < 72.45 - (4)
55 < 33 + 31 x [C] + 4.5 x [Si] + 1.5 x [Mn] + 2.4 x [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.
2A
+
[0074]
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.
[0075]
1.5 < [Si] + 1.8 x [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 x [Mn] > 1.5. Although it is not
necessary to particularly set the upper limit of [Si] + 1.8 x [Mn], the upper limit of [Si]
+ 1.8 x [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.
[0076]
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 %.
[0077]
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 y
2-2.
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%.
[0078]
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.
[0079]
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%.
[0080]
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
32/32-
of Ni and Cu contained is set to be not more than 5.0%.
[0081]
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.
[0082]
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.
[0083]
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
23/32-
more than 0.2%.
[0084]
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%.
[0085]
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.
[0086]
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%.
[0087]
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%.
[0088]
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-AkOa-SiCh-based oxide to form oxide having a low melting point. The resulting
oxide adheres, as be lag, 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.
[0089]
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%.
[0090]
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.
•25/32'
IX
•
*
Examples
[0091]
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.
[0092]
[Examples]
Steels having components shown in Table 2 and Table 3 were produced through
melting, and were rolled to form a steel bar with 65(p. 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.
[0093]
After being subjected to a condition of 900°C x 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 (urn) 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 (urn) of the tool was obtained such that the test piece with
36/32
»
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.
[0094]
[Table 2]
[Table 3]
[Table 4]
[Table 5]
[Table 6]
[0095]
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.
[0096]
The amount of Al added was not sufficient for the steels of Number 98 and
Number 99, and hence, no AI2O3 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.
[0097]
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
2-?
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tool.
[0098]
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.
[0099]
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.
[0100]
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
[0101]
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.
-28/32 •
3v

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: 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, wherein
an amount of C: [C], an amount of Si: [Si], an amount of Mn: [Mn], and an
amount of Al: [Al] satisfy following Expression (1) and Expression (2).
139.38 < 214 X [C] + 30.6 x [Si] + 42.8 x [Mn] -14.7 x [Al] < 177 - (1)
0.72 < [C] + 1/7 X [Si] + 1/5 X [Mn] < 1.539 - (2)
2. The steel for a machine structure according to Claim 1, wherein
the steel further satisfies following Expression (3).
113 - 135 X [C] - 27 X [Mn] < 13 - (3)
3. The steel for a machine structure according to Claim 1, wherein
the steel further satisfies following Expression (4).
55 < 33 + 31 X [C] + 4.5 x [Si] + 1.5 x [Mn] < 72.45 - (4)
4. The steel for a machine structure according to Claim 2, wherein
the steel further satisfies following Expression (4).
55 < 33 + 31 x [C] + 4.5 x [Si] + 1.5 x [Mn] < 72.45 - (4)
3^
5. The steel for a machine structure according to any one of Claims 1 to 4,
wherein
the steel further satisfies following Expression (5).
1.5 < [Si] + 1.8 X [Mn] < 6.24 - (5)
6. The steel for a machine structure according to any one of Claims 1 to 4,
wherein
the steel further comprises B: 0.0001% to 0.015% in mass %.
7. The steel for a machine structure according to any one of Claims 1 to 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%>, following Expression
(6) is used in place of Expression (1), following Expression (7) is used in place of
Expression (2), following Expression (8) is used in place of Expression (3), and
following Expression (9) is used in place of Expression (4).
139.38 < 214 X [C] + 30.6 x [Si] + 42.8 x [Mn] + 23.8 x [Cr] -14.7 x [Al] <
177 - (6)
0.72 < [C] + 1/7 x [Si] + 1/5 x [Mn] + 1/9 x [Cr] < 1.627 - (7)
113 - 135 X [C] - 27 X [Mn] - 18 x [Cr] < 13 - (8)
55 < 33 + 31 X [C] + 4.5 x [Si] + 1.5 x [Mn] + 2.4 x [Cr] < 74.37 - (9)
8. The steel for a machine structure according to any one of Claims 1 to 4,
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%.
1C
9. The steel for a machine structure according to any one of Claims 1 to 4,
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 any one of Claims 1 to 4,
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 any one of Claims 1 to 4,
wherein
the steel fiirther 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: 0.0000If/o to
0.005%, and Sr: 0.00001% to 0.005%.