DESCRIPTION
CASE HARDENING STEEL WIRE
The present invention relates to a case hardening steel wire.
BACKGROUND ART
[0002]
From the viewpoint of improving bending fatigue strength and surface fatigue
strength (pitting strength), automotive parts, especially parts used for a transmission such as
a gear or a shaft, are generally produced by performing a case hardening treatment such as
carburizing and quenching followed by tempering.
[0003]
In general, "carburizing and quenching" is a treatment in which, using a low-carbon
"case hardening steel" as a starting material steel (base metal steel), C is intruded and
diffused in the austenitic zone at a high temperature of Ac3 point or higher, and thereafter
the steel is quenched.
[0004]
In recent years there are demands for motor vehicles to have a lighter weight and
higher torque. Consequently, it is necessary for the carburized parts such as the
aforementioned gear to have a higher bending fatigue strength and higher pitting strength
than before. Note that, in the present description, hereinafter an explanation may be given
in which a "carburized part" is represented by a "gear".
[0005]
When large amounts of alloying elements such as Ni, Cr and Mo are contained in a
case hardening steel, a high bending fatigue strength and a high pitting strength can be
secured for the gear. In particular, both ofNi and Mo are important elements that increase
the depth of a carburized layer and the hardness of a core part (base metal), and are also
elements that improve the temper softening resistance. Moreover, because Ni and Mo are
both non-oxidizing elements, both Ni and Mo also have an effect of improving the
hardenability of a carburized layer without increasing the depth of an inter granular oxidation
formed on the surface during gas carburization.
[0006]
Therefore, as a "case hardening steel" which serves as a starting material for a gear,
a "nickel-chrome-molybdenum steel" such as SNCM 220H or a "chrome-molybdenum steel"
such as SCM 420H specified in JIS G 4052 (2008) are often used.
[0007]
However, increasing the amount of alloying elements leads to a problem of
increased component costs. In particular, in consideration of the situation with respect to
the steep rise in the cost ofNi and Mo in recent years, there is a demand to keep the content
ofNi and Mo as low as possible and thereby decrease the component cost.
[0008]
For example, "case hardening steel" and "steel for a gear that is excellent in surface
fatigue strength" are proposed in Patent Document 1 and Patent Document 2, respectively.
[0009]
Specifically, the "case hardening steel" disclosed in Patent Document 1 is a case
hardening steel for a mechanical structure containing, by mass, AI: 0.02 to 0.06%, N: 0.015
to 0.03% and Nb: 0.01 to 0.08% and satisfying the ranges expressed by the following
formulas:
N(%) ~ -0.2xNb(%)+0.028
Al(%) ~ 2.0x{N(%)-0.15xNb (%)},
and which is further restricted to 0 s 15 ppm and S S 0.015%.
[0010)
Further, in Patent Document 2, "steel for a gear that is excellent in surface fatigue
strength" is disclosed that contains, by mass%, C: 0.10 to 0.30%, Si: 0.35% or less, Mn:
0.8% or less and Cr: 1.5 to 2.5%, and further contains, as necessary, one or more elements
selected from Ni: 3.0% or less, Mo: 1.0% or less and Cu: 1.0% or less, the balance being Fe
and unavoidable impurities, and in which a Si+Cr amount is in a range of 1.8 to 2.8% and
that also has a cas.e hardened layer formed by quenching and tempering after carbo-nitriding,
and in which a C+N amount from the surface to a depth of 0.1 mm is in a range of 1.0 to
2.0%.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENTS
[0011]
Patent Document 1: JP58-45354A
Patent Document 2: JP9-296250A
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
[0012]
Although the technique disclosed in Patent Document 1 has the technical idea of
ensuring grain-coarsening resistance during case hardening treatment by adjusting the
amount of AI, Nb and N in the steel, and also ensuring cold workability by actively restricting
an upper limit of S and 0, no consideration is given to the manner of precipitation.
Therefore, according to the technique disclosed in Patent Document 1, a high pitting strength
can not necessarily always be ensured for parts such as a gear and a shaft.
[0013]
Although the technique disclosed in Patent Document 2 has the technical idea of
providing a steel that is excellent in surface fatigue strength by containing an Si+Cr amount
in a range of 1.8 to 2.8% and also having a case hardened layer formed by quenching and
tempering after carbo-nitriding and containing a C+N amount from the surface to a depth of
0.1 mm that is in a range of 1.0 to 2.0%, no consideration is given to cold workability.
Therefore, both a high pitting strength and cold workability can not necessarily always be
ensured for parts such as a gear and a shaft.
[0014]
An objective of the present invention is to provide a case hardening steel wire which
is favorable for use as a starting material for carburized parts such as a gear or a shaft, and
for which the component cost is low and which is excellent in bending fatigue strength,
pitting strength, cold workability and grain-coarsening resistance. More specifically, an
objective of the present invention is to provide a case hardening steel wire that has favorable
workability when performing cold rolling such as cold forging, and even in a case where Ni
and Mo that are expensive elements are not contained or where the content ofNi and Mo is
decreased as much as possible, can ensure bending fatigue strength, pitting strength and
grain-coarsening resistance of the same level as or higher than in a case of using the "chromemolybdenum
steel" SCM 420H specified in JIS G 4052 (2008) as a starting material steel
for a carburized part.
[0015]
The term "steel wire" as used in the present description refers to a steel material that
is rolled or forged into a bar shape during hot processing, that is, the term "steel wire" refers
to a material obtained by spheroidizing annealing of a "steel bar", a "wire rod" or a "bar-incoil",
and thereafter finishing by performing wire drawing or cold drawing or the like as cold
processing. Shapes of the steel wire include a so-called "bar shape" in addition to wire that
is wound in a coil shape.
MEANS FOR SOLVING THE PROBLEMS
[0016]
The present inventors conducted various studies to solve the aforementioned
problem. As a result, first, the present inventors obtained the findings described hereunder
in (a) to (d).
[0017]
(a) In order to ensure a high bending fatigue strength and a high pitting strength
without, as much as possible, containing Ni and Mo, the chemical composition of the steel
has to be made a chemical composition that is capable of suppressing a decrease in
hardenability that arises due to the reduction in the Ni and Mo content.
[0018]
(b) Since a decrease in bending fatigue strength occurs due to the formation of
coarse MnS, it is necessary to suppress the formation of coarse MnS to ensure a high bending
.k
5
fatigue strength.
[0019]
(c) Coarse MnS becomes a starting point of cracking during cold forging.
Therefore, it is necessary to make the amount of coarse MnS as low as possible in order to
suppress the cracking during cold forging.
[0020]
(d) In order to reduce the amount of coarse MnS as much as possible, not only the
respective contents of Mn and S have to be controlled, but also the content balance between
Mn and S has to be optimized. Specifically, the formation of coarse MnS can be suppressed
by controlling Fnl represented by the formula [Fnl = Mn/S], in which the symbol of an
element in the formula represents the content by mass percent ofthat element, to [15 ~ Fnl
~ 150]. Therefore, in order to suppress cracking during cold forging while good cold
forgeability is ensured and also to ensure a high bending fatigue strength, the respective
contents ofMn and Shave to be controlled, and also these contents have to satisfy the above
relational expression.
[0021]
Therefore, the present inventors conducted further various kinds of studies
regarding steel in which the hardenability is ensured so as to offset a decrease in Ni and Mo
content, and the respective contents ofMn and Sand the balance therebetween are optimized
to suppress the formation of coarse MnS. As a result, the fmdings of the following (e) to
(i) were obtained.
[0022]
(e) A high bending fatigue strength and a high pitting strength cannot be ensured
merely by suppressing the formation of coarse MnS. In addition to suppressing formation
of coarse MnS, it is also necessary to decrease the depth of carburized abnormal structures,
that is, the depth of an intergranular oxidation and a non-martensitic structure.
[0023]
(f) The depths of the intergranular oxidation and the non-martensitic structure,
which are the carburized abnormal structures, can be decreased by optimizing the content
balance of oxidizing elements, especially Cr, Si and Mn. Specifically, the depth of the
carburized abnormal structures can be decreased by controlling Fn2 that is represented by
the formula [Fn2=Cr/(Si+2Mn)], in which the symbol of an element in the formula
represents the content by mass percent of that element, to [0.75 ~ Fn2 ~ 1.40], whereby a
high bending fatigue strength and a high pitting strength can be ensured.
[0024]
(g) In the case of a chemical composition in which the contents of Ni and Mo are
suppressed as much as possible, it is necessary to optimize the content balance between Si
and Cr to ensure high temperature strength after carburizing and quenching, that is, temper
softening resistance. Specifically, with respect to Fn3 that is represented by the formula
[Fn3 = SixCr] when the symbol of an element in the formula represents the content by mass
percent of that element, by controlling Fn3 to a range represented by [0.30 ~ Fn3 ~ 0.65],
high temper softening resistance can be ensured and a high pitting strength is obtained.
[0025]
(h) In order to ensure a high bending fatigue strength and a high pitting strength,
large-sized hard inclusions of type B and type D as measured in conformity with method A
of ASTM-E45-13, that is, inclusions with a large thickness among the inclusions of type B
that consist mainly of Ah03-based inclusions and the inclusions of type D that consist
mainly of TiN-based inclusions have to be suppressed. This is because the large-sized hard
inclusions of type B and type D described above become starting points of fatigue fracture.
[0026]
(i) In order to suppress the formation of the aforementioned large-sized hard
inclusions of type B and type D, the contents of especially Ti and 0 (oxygen) among the
impurities have to be controlled to 0.005% or less and 0.0020% or less, respectively.
Further, in order to suppress the formation of the large-sized hard inclusions of type B and
typeD, it is desirable that the steel be melted in a vacuum furnace, or in a case where the
steel is melted in a converter, secondary refining be repeated or electromagnetic stirring be
performed during continuous casting.
[0027]
Furthermore, in addition to (c) and (d) described above, the present inventors
conducted studies for further enhancing the cold workability and for also ensuring graincoarsening
resistance during carburizing and quenching. As a result, the findings of the
following G) and (k) were obtained.
[0028]
G) In cold rolling, and especially cold forging, after subjecting a hot working
material to spheroidizing annealing, in many cases wire drawing, cold drawing or the like is
further performed. Accordingly, to ensure favorable cold workability, it is necessary to
lower as much as possible the hardness of the steel wire after the aforementioned wire
drawing, cold drawing or the like. Specifically, a high level of cold workability is obtained
if the hardness at a position that is 50~ from the surface of the steel wire is 250 or less in
HV.
[0029]
(k) In order to suppress grain coarsening during carburizing and quenching, coarse
precipitates that are not effective for pinning have to be reduced as much as possible, and a
large amount of fine precipitates that serve as pinning particles have to be distributed.
Specifically, grain coarsening during carburizing and quenching can be suppressed if, at a
position at a depth equivalent to one-half of the radius from the surface of the steel wire, a
total number of AI precipitates and Nb precipitates having a circle-equivalent diameter of
1 00 nm or more is 1 00 or less per 1 00 11m2
, and a total number of AI precipitates and Nb
precipitates having a circle-equivalent diameter in a range of 5 nm or more to less than 100
llJl?. is 1 00 or more per 25 J.Lm2.
[0030]
The present invention has been completed based on the fmdings described above,
and the gist thereof is a case hardening steel wire described hereunder.
[0031]
(1) A case hardening steel wire having a chemical composition consisting, by
mass%, of
C: 0.10 to 0.24%,
Si: 0.16 to 0.35%,
Mn: 0.40 to 1.00%,
S: 0.005 to 0.050%,
Cr: 1.65 to 1.90%,
Al: 0.015 to 0.060%,
Nb: 0.005 to 0.060%,
N: 0.0130 to 0.0250%,
Cu: 0 to 0.20%,
Ni: 0 to 0.20%,
V: 0 to 0.20%,
Ca: 0 to 0.0050%, and
a balance: Fe and impurities, wherein:
values ofFn1, Fn2 and Fn3 represented by Formula (i), Formula (ii), and Formula
(iii) hereunder satisfy 15 ~ Fnl :5 150, 0. 75 ~ Fn2 ~ 1.40, and 0.30 ~ Fn3 :5 0.65, respectively;
the contents ofP, Ti, Mo and 0 in the impurities are P: 0.020% or less, Ti: 0.005%
or less, Mo: 0.03% or less and 0: 0.0020% or less, respectively;
a hardness at a position 50 1-llil from a surface is 250 or less in HV; and
at a position at a depth equivalent to one-half of a radius from the surface, a total
number of Al precipitates and Nb precipitates having a circle-equivalent diameter of 1 00 run
or more is 100 or less per 100 ~2 , and a tot~ number of AI precipitates and Nb precipitates
having a circle-equivalent diameter in a range of 5 nm or more to less than 100 nm is 100 or
more per 25 J.l.m2
;
Fn 1 = Mn/S (i)
Fn2 = Cr/(Si+2Mn) (ii)
Fn3 = SixCr (iii)
where, a symbol of an element in the formulas above represents a content by mass
percent of the element.
[0032]
(2) The case hardening steel wire according to (1), wherein the chemical
composition contains, by mass%, one or more elements selected from:
[0033]
Cu: 0.05 to 0.20%, and
Ni: 0.05 to 0.20%.
(3) The case hardening steel wire according to (1) or (2), wherein the chemical
composition contains, by mass%,
V: 0.05 to 0.20%.
[0034]
(4) The case hardening steel wire according to any one of (1) to (3), wherein the
chemical composition contains, by mass%,
Ca: 0.0003 to 0.0050%.
ADVANTAGEOUS EFFECTS OF THE INVENTION
[0035]
The case hardening steel wire of the present invention is excellent in cold
workability and has a low component cost. Furthermore, a carburized part that employs
this case hardening steel wire as a starting material has a bending fatigue strength, a pitting
strength and grain-coarsening resistance of the same level as or higher than a carburized part
that uses the 11chrome-molybdenurn steel11 SCM 420H specified in JIS G 4052 (2008) as a
starting material. Therefore, the case hardening steel wire of the present invention is
suitably used as a starting material of a carburized part such as a gear or a shaft which is
required to have high bending fatigue strength and high wear resistance in order to reduce
weight and increase torque.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036]
[Figure 1] Figure 1 is a figure illustrating a rough shape of a notched Ono type rotating
bending fatigue test specimen used in the Examples. The measurement unit in the figure is
ummu.
[Figure 2] Figure 2 is a figure illustrating a rough shape of a roller-pitting small roller
specimen used in the Examples. The measurement unit in the figure is 11tnm11
•
[Figure 3] Figure 3 is a figure illustrating the shape of a test specimen used for cold
workability evaluation that is used in the Examples. The measurement unit in the figure is
ummu.
.%
\0
[Figure 4] Figure 4 is a figure illustrating the shape of a test specimen used for graincoarsening
resistance evaluation in the Examples. The measurement unit in the figure is
"mm".
[Figure 5] Figure 5 is a figure illustrating a rough shape of a roller-pitting large roller
specimen that is used in the Examples. In Figure 5, (a) is a front view when a roller-pitting
large roller specimen having a rough shape is divided in half along a center line, and (b) is a
cross-sectional view along the center line. The measurement unit in the figure is "mm".
[Figure 6] Figure 6 is a figure illustrating a heat pattern of "carburizing and quenchingtempering"
performed on the test specimens shown in Figure 1 and Figure 2 in the Examples.
[Figure 7] Figure 7 is a figure illustrating a heat pattern of "carburizing and quenchingtempering"
performed on the test specimen shown in Figure 5 in the Examples.
[Figure 8] Figure 8 is a figure illustrating the finished shape of a notched Ono type rotating
bending fatigue test specimen used in the Examples. The measurement unit in the figure is
"mm".
[Figure 9] Figure 9 is a figure illustrating the finished shape of a roller-pitting small roller
specimen used in the Examples. The measurement unit in the figure is "mm".
[Figure 1 OJ Figure 10 is a figure illustrating the finished shape of a roller-pitting large roller
specimen used in the Examples. In Figure 10, (a) is a front view when a roller-pitting large
roller ~pecimen is divided in half along a center line, and (b) is a cross-sectional view along
the center line. The measurement unit in the figure is·"mm".
MODE FOR CARRYING OUT THE INVENTION
[0037]
Hereunder, requirements for the present invention are described in detail. The
symbol "%" for the content of each element means "percent by mass", in the following
description.
[0038]
(A) Chemical Composition:
C: 0.10 to 0.24%
C is an essential element for securing the strength of a carburized part such as a
,.18"
\ gear or a shaft, and therefore a content of 0.10% or more of C is necessary. However, if
the content of C is too high, the hardness increases, and this leads to a decrease in the
machinability. In particular, when the C content is more than 0.24%, the decrease in
machinability caused by the increase in hardness becomes noticeable. Therefore, the
content ofC is 0.10 to 0.24%. The content ofC is preferably 0.13% or more, and preferably
0.23% or less.
[0039]
Si: 0.16 to 0.35%
Si has a hardenability improving function and a deoxidizing function. Also, Si
has resistance to temper-softening, and has an effect of preventing surface softening in a
situation in which the sliding surface of the gear or the like is exposed to a high temperature.
In order to obtain these effects, 0.16% or more of Si has to be contained. However, since
Si is an oxidizing element, if the content thereof increases, Si is selectively oxidized by a
minute amount of H20 or C02 contained in a carburizing gas, and Si oxides are formed on
the steel surface, and consequently the depths of the intergranular oxidation and the nonmartensitic
structure that are carburized abnormal structures, increase. The increase in
depth of the carburized abnormal structures leads to a decrease in bending fatigue strength
and pitting strength. Further, if the Si content increases, not only is the temper-softening
resisting effect saturated, but the carburizing property is also hindered, and the machinability
also decreases. In particular, if the Si content is more than 0.35%, due to the increase in
depth of the carburized abnormal structures and the decrease in surface hardness caused by
hindrance of the carburizing property, a decrease in the bending fatigue strength and the
pitting strength becomes noticeable, and a decrease in the machinability is also noticeable.
Therefore, the content of Si is 0.16 to 0.35%. The content of Si is preferably 0.18% or
more, and preferably 0.30% or less.
[0040]
Note that, when the content ofSi is within the aforementioned range, it is necessary
that Fn2 and Fn3 that are represented by Formula (ii) and Formula (iii) described above also
satisfy the formulas 0.75 ~ Fn2 ~ 1.40 and 0.30 ~ Fn3 ~ 0.65.
[0041]
Mn: 0.40 to 1.00%
Mn has a hardenability improving function and a deoxidizing function. Mn also
has an effect of suppressing temper-softening. In order to obtain these effects, an Mn
content of0.40% or more is necessary. However, as the Mn content increases, the hardness
increases, which leads to a decrease in the machinability. In particular, when the Mn
content is more than 1.00%, the decrease in machinability caused by the increase in hardness
becomes noticeable. Moreover, since, similarly to Si, Mn is an oxidizing element, when
the content thereof increases, Mn oxides are formed on the steel surface, an~ therefore the
depths of the intergranular oxidation and the non-martensitic structure, which are the
carburized abnormal structures, increase. The increase in depth of the carburized abnormal
structures leads to a decrease in bending fatigue strength and pitting strength. In particular,
when the Mn content is more than 1.00%, the decrease in bending fatigue strength and pitting
strength caused by the increase in depth of the carburized abnormal structures becomes
noticeable. Therefore, the content ofMn is 0.40 to 1.00%. The Mn content is preferably
0.50% or more, and preferably 0.95% or less.
[0042]
Note that, when the content of Mn is within the aforementioned range, it is
necessary that Fnl and Fn2 that are represented by Formula (i) and Formula (ii) described
above also satisfy the formulas 15 ::c:;; Fn1 ~ 150 and 0.75 ~ Fn2 ~ 1.40.
[0043]
S: 0.005 to 0.050%
S combines with Mn to form MnS, and has a function of improving the
machinability. In order to obtain the effect of improving the machinability, it is necessary
for the S content to be 0.005% or more. However, if the S content is more than 0.050%,
coarse MnS is formed and the cold forgeability and bending fatigue strength decrease.
Therefore, the content of S is 0.005 to 0.050%. The content of S is preferably 0.010% or
more, and is preferably 0.040% or less.
[0044]
Note that, when the content of S is within the aforementioned range, it is necessary
that Fnl that is represented by Formula (i) described above also satisfies the formula 15 ~
Fnl ~ 150.
[0045]
Cr: 1.65 to 1.90%
Cr has an effect of improving the hardenability. Cr has resistance to tempersoftening,
and also has an effect of preventing surface softening in a situation in which the
sliding surface of a carburized part such as a gear or a shaft is exposed to a high temperature.
In order to obtain these effects, it is necessary for the Cr content to be 1.65% or more.
However, as the Cr content increases, the hardness increases, which leads to a decrease in
the machinability. In particular, if the Cr content is more than 1.90%, the decrease in
machinability caused by the increase in hardness becomes noticeable. Moreover, since,
similarly to Si and Mn, Cr is an oxidizing element, when the content thereof increases, Cr
oxides are formed on the steel surface, and therefore the depths of the intergranular oxidation
and the non-martensitic structure, which are the carburized abnonnal structures, increase.
The increase in depth of the carburized abnormal structures leads to a decrease in bending
fatigue strength and pitting strength. In particular, when the Cr content is more than 1.90%,
the decrease in bending fatigue strength and pitting strength caused by the increase in depth
of the carburized abnormal structures becomes noticeable. Therefore, the content of Cr is
1.65 to 1.90%. The content of Cr is preferably 1.85% or less.
[0046]
Note that, when the content ofCr is within the aforementioned range, it is necessary
that Fn2 and Fn3 that are represented by Formula (ii) and Formula (iii) described above also
satisfy the formulas 0.75 ~ Fn2 ~ 1.40 and 0.30$ Fn3 ~ 0.65.
[0047]
AI: 0.015 to 0.060%
AI has a deoxidizing function. Further, Al combines with N to form AlN and
refine the crystal grains, and thus also has a function of strengthening the steel. However,
if the content of AI is less than 0.015%, it is difficult to obtain the aforementioned effects.
On the other hand, if the AI content is excessively high, hard and coarse Ah03 is formed,
which leads to a decrease in the machinability. Furthermore, the bending fatigue strength
also decreases. In particular, when the AI content is more than 0.060%, the machinability
and bending fatigue strength noticeably decrease. Therefore, the content of AI is 0.015 to
0.060%. The AI content is preferably 0.020% or more, and also is preferably 0.055% or
less.
[0048]
Nb: 0.005 to 0.060%
Nb combines with C and N to form fine carbides, nitrides or carbo-nitrides and
thereby refines crystal grains, and thus has an effect of improving bending fatigue strength
and pitting strength. In order to obtain this effect, the Nb content of 0.005% or more is
necessary. However, if the Nb content is excessively high, it leads to a decrease in the hot
ductility. In particular, if the content thereof is more than 0.060%, a decrease in the hot
ductility becomes noticeable, and surface flaws are liable to arise during hot working such
as hot rolling and hot forging. Accordingly, the content ofNb is set within a range ofO.OOS
to 0.060%. The content ofNb is preferably 0.015% or more, and is also preferably 0.050%
or less.
[0049]
N: 0.0130 to 0.0250%
N refines crystal grains by forming nitrides, and therefore has an effect ofimproving
the bending fatigue strength. In order to obtain this effect, 0.0130% or more ofN has to be
contained. However, if the content of N is excessively high, coarse nitrides are formed,
and this leads to a decrease in toughness. In particular, when the N content is more than
0.0250%, a decrease in the toughness and a decrease in the cold forgeability become
noticeable. Further, large-sized inclusions of type D are liable to be formed, and the
bending fatigue strength and pitting strength decrease. Therefore, the content of N is set
within a range of0.0130 to 0.0250%. The content ofN is preferably 0.0200% or less.
[0050)
The case hardening steel wire according to the present invention has a chemical
composition that contains the elements from C to N that are described above, with the
balance of Fe and impurities, and also satisfies conditions regarding Fnl, Fn2 and Fn3 that
are described later, and in which the contents ofP, Ti, Mo and 0 (oxygen) in the impurities
are restricted to ranges described later.
[0051]
The term "impurities" in "Fe and impurities" as the balance refers to components
that become mixed into the chemical composition due to various factors during the
production process such as use of a raw material such as ore or scrap when a steel material
is produced on an industrial scale.
[0052]
Fnl: 15 to 150
Even if the contents of Mn and S are within the ranges described in the foregoing,
if coarse MnS is formed, a decrease in the bending fatigue strength occurs. Accordingly,
the formation of coarse MnS has to be suppressed to ensure a high bending fatigue strength.
Moreover, since the coarse MnS also becomes a starting point of cracking during cold
forging, coarse MnS has to be minimized as much as possible in order to suppress cracking
during cold forging. Therefore, the balance between the contents ofMn and S is important,
and Fnl that is represented by Formula (i) has to be within a fixed range.
Fnl = Mn/S ... (i)
Where, the symbol of an element in the above formula represents the content by
mass percent of the element.
[0053]
When Fnl is less than 15, the content of S becomes excessively high, and the
formation of coarse MnS is unavoidable. On the other hand, when Fnl is more than 150,
the content of Mn becomes excessively high, and coarse MnS is formed in a central
segregation portion. Therefore, in both such cases, the bending fatigue strength is
decreased, and moreover, it is difficult to avoid cracking during cold forging. Therefore,
Fnl is set to a value such that 15 ~ Fn1 ~ 150. Preferably, Fnl is 30 or more, and is also
preferably 100 or less.
[0054]
Fn2: 0.75 to 1.40
In order to provide a high bending fatigue strength and a high pitting strength
without, as much as possible, containing Ni and Mo, it is necessary to decrease the depths
of the intergranular oxidation and the non-martensitic structure, which are the carburized
abnormal structures, while ensuring the hardenability. For this purpose, among the
oxidizing elements, in particular the contents of Cr, Si and Mn are set within the ranges
described above, and additionally, it is necessary for Fn2 that is represented by the following
Formula (ii) as the content balance among these elements to be within a range of 0. 7 5 to
1.40.
Fn2 = Cr/(Si+2Mn)... (ii)
Where, the symbol of an element in the above formula represents the content by
mass percent of the element.
[0055]
When Fn2 is less than 0.75 or when Fn2 is more than 1.40, the depth of the
carburized abnormal structures increases, and consequently the bending fatigue strength and
the pitting strength decreases. Therefore, Fn2 is set so as to be a value such that 0.75 ~ Fn2
~ 1.40. Preferably Fn2 is 0.80 or more, and preferably is also 1.30 or less.
[0056]
Fn3: 0.30 to 0.65
In order to provide a high pitting strength without, as much as possible, containing
Ni and Mo, it is necessary to increase the high temperature strength, that is, the temper
softening resistance. Specifically, among the elements that increase the temper softening
resistance, in particular the contents of Si and Cr are set within the ranges described above,
and additionally, it is necessary for Fn3 that is represented by the following Formula (iii) to
be within a range of 0.30 to 0.65.
Fn3 = SixCr (iii)
Where, the symbol of an element in the above formula represents the content by
mass percent of the element.
[0057)
When Fn3 is less than 0.30, the temper softening resistance is low and a desired
pitting strength is not obtained. On the other hand, when Fn3 is greater than 0.65, a
decrease in the bending fatigue strength and the pitting strength that is caused by an increase
in the depth of the carburized abnormal structures becomes noticeable. Therefore, Fn3 is
set to a value that satisfies the formula 0.30 ~ Fn3 ~ 0.65. Preferably Fn3 is 0.60 or less.
[0058]
In addition, in the present invention, it is necessary to restrict the contents of P, Ti,
Mo and 0 among the impurities toP: 0.020% or less, Ti: 0.005% or less, Mo: 0.03% or less
and 0: 0.0020% or less, respectively.
[0059]
This restriction of the contents of P, Ti, Mo and 0 is described hereunder.
[0060]
P: 0.020% or less
P is an impurity that is contained in a steel, and segregates at crystal grain
boundaries and embrittles the steel. In particular, when the content of P is more than
0.020%, the degree of embrittlement is noticeable. Therefore, the content of P is 0.020%
or less. The content of P is preferably 0.015% or less.
[0061]
Ti: 0.005% or less
Ti has a high affinity toN, and therefore combines with Nina steel to form TiN
which is a non-metallic inclusion of typeD that is hard and coarse and which thus decreases
the bending fatigue strength and pitting strength and also decreases the machinability.
Therefore, the content ofTi is 0.005% or less.
[0062]
Mo: 0.03% or less
Mo is an expensive element and, when contained, leads to an increase in the
component cost. Further, the mechanical properties become unstable in a case where the
content ofMo in the impurities fluctuates. Therefore, the content ofMo is 0.03% or less.
[0063]
0: 0.0020% or less
0 (oxygen) combines with Si, AI, and the like in a steel to form oxides. Among
these oxides, especially Ah03 that is a non-metallic inclusion of type B is hard, and thus
decreases the machinability and also leads to a decrease in the bending fatigue strength and
pitting strength. Therefore, the content of 0 is 0.0020% or less. The content of 0 is
preferably 0.0015% or less.
[0064]
The case hardening steel wire according to the present invention may also contain
one or more elements selected from elements described in the following <1> to <3>.
<1> Cu: 0 to 0.20% and Ni: 0 to 0.20%
<2> V: 0 to 0.20%
<3> Ca: 0 to 0.0050%
[0065]
Hereunder, the operational advantages of the aforementioned optional elements and
the reasons for restricting the contents thereof are described.
[0066]
<1> Cu: 0 to 0.20% and Ni: 0 to 0.20%
Cu and Ni each have a function of enhancing the hardenability. Therefore, when
it is desired to obtain a higher level of hardenability, Cu and Ni may be contained within the
ranges described hereunder.
[0067]
Cu: 0 to 0.20%
Cu has a function of enhancing the hardenability, and therefore Cu may be
contained to further improve the hardenability. However, Cu is an expensive element, and
also leads to a decrease in the hot workability when the content thereof is high. In particular,
when the content of Cu is more than 0.20%, a decrease in the hot workability becomes
noticeable. Therefore, when contained, the content of Cu is 0.20% or less.
[0068]
On the other hand, in order to stably obtain the aforementioned effect of Cu, the
content of Cu is preferably 0.05% or more, and more preferably 0.07% or more.
[0069]
Ni: 0 to 0.20%
Ni has a function of enhancing the hardenability. Further, Ni has a function of
improving the toughness, and because of being a non-oxidizing element, Ni can also
strengthen the steel surface without increasing the depth of the intergranular oxidation during
carburization. Therefore, Ni may be contained to obtain these effects. However, Ni is an
expensive element, and therefore excessive addition thereof leads to a rise in the component
cost. In particular, when the content ofNi is more than 0.20%, the cost rises significantly.
Therefore, when contained, the content ofNi is 0.20% or less.
[0070]
On the other hand, in order to stably obtain the aforementioned effect of Ni, the
content ofNi is preferably 0.05% or more, and more preferably 0.07% or more.
[0071]
The total amount of the aforementioned Cu and Ni in a case where these two types
of elements are contained compositely is preferably 0.30% or less.
[0072]
<2> V: 0 to 0.20%
V combines with C and N to form fine carbides, nitrides and carbo-nitrides and
refine the crystal grains, and has an effect of enhancing the bending fatigue strength and
pitting strength. Therefore, V may be contained to obtain these effects. However, if the
content of V is excessively high, it will lead to a decrease in hot ductility. In particular, if
the content of V is more than 0.20%, a decrease in the hot ductility will be noticeable, and
surface flaws are liable to arise during hot working such as hot rolling and hot forging.
Therefore, when contained, the content of V is 0.20% or less. The content of V is
preferably 0.10% or less.
[0073)
On the other hand, in order to stably obtain the aforementioned effects of V, the
content ofV is preferably 0.05% or more, and more preferably 0.07% or more.
[0074]
<3> Ca: 0 to 0.0050%
Ca has a function of improving machinability. Therefore, Ca may be contained to
improve the machinability. However, excessive addition of Ca leads to a rise in the
component cost. In particular, when the content of Ca is more than 0.0050%, the
advantageous effect of improving machinability is saturated, and hence only the cost
increases and economic efficiency is deteriorated. Furthermore, when the content of Ca is
more than 0.0050%, Ca forms coarse oxides and leads to a decrease in the bending fatigue
strength and pitt~g strength. Therefore, when contained, the content ofCa is 0.0050% or
less. The content of Ca is preferably 0.0030% or less.
[0075]
On the other hand, in order to stably obtain the aforementioned effect of Ca, the
content of Ca is preferably 0.0003% or more, and more preferably 0.0005% or more.
[0076]
(B) Hardness at position 50 J.UD. from surface:
In a case where the hardness of a steel wire that is a cold rolling starting material is
high, cold-rolling cracking that is caused by surface defects cannot be suppressed during
cold rolling, and especially during cold forging. However, in a case where the hardness of
a steel wire is low, specifically, in a case where the hardness at a position that is 50 J.l.m froll].
the surface of the steel wire is 250 or less in HV, a high cold workability is obtained, and
hence cracking can be prevented during cold rolling of a part that requires intense working.
[0077]
For example, after melting the case hardening steel having the chemical
composition described in the foregoing section (A), a cast piece or ingot obtained by casting
the molten steel is heated for 30 to 1200 min at a temperature of 1150 to 1350°C and
thereafter subjected to blooming to form a slab. Next, in order to adjust the hardness after
hot rolling to a lower hardness, after heating the aforementioned slab for 15 to 120 min at a
temperature of 1150°C or less, preferably a temperature in a range of 900 to 1150°C, hot
rolling is performed at a finishing temperature of 800°C or more, and after the end of the hot
rolling, cooling is performed under conditions of a temperature range of 800 to 500°C and a
cooling rate of 1.0°C/s or less, preferably 0.1 to 1.0°C/s, to produce a steel bar or a wire rod
or the like. After the resultant steel bar or wire rod or the like is subjected to spheroidizing
annealing, by finishing by performing wire drawing or cold drawing or the like as cold
processing with a reduction of area of 5% or more, preferably 5 to 15%, the hardness at a
position that is 50 ~ from the surface of the steel wire can be controlled so as to be within
the aforementioned range.
[0078]
The hardness at a position that is 50 J.UD. from the surface is more preferably 230 or
less in HV. When produced as described above, a lower limit of the hardness at a position
that is 50 JliD. from the surface of the case hardening steel wire according to the present
invention is about 130 in HV.
(0079]
Further, when producing the case hardening steel wire according to the present
invention, preferably the case hardening steel having the chemical composition described in
section (A) is melted in a vacuum furnace, or in a case where the steel is melted in a converter,
secondary refining is repeated or electromagnetic stirring is performed during continuous
casting.
[0080]
A spheroidizing annealing method when producing the case hardening steel wire
according to the present invention is not particularly limited and, for example, a suitable
method may be selected from a prolonged heating method, a repeated heating and cooling
method, a slow cooling method, and an isothennal transfonnation method. In addition, a
method of wire drawing or cold drawing or the like as cold processing is also not particularly
limited, and it is sufficient to employ a common method.
[0081]
(C) Precipitates at position at a depth equivalent to 1/2 of radius from surface:
In the case hardening steel wire according to the present invention,. at a position at
a depth equivalent to one-half of the radius from the surface, a total number of Al precipitates
and Nb precipitates having a circle-equivalent diameter of 100 nm or more has to be 100 or
less per 100 J..lm2, and a total number of AI precipitates and Nb precipitates having a circleequivalent
diameter that is 5 nm or more and less than 1 00 nm has to be 1 00 or more per 25
J..lm2.
[0082]
In order to suppress grain coarsening during carburizing and quenching, coarse
precipitates that are not effective for pinning have to be reduced as much as possible, and a
large amount of fine precipitates that serve as pinning particles have to be distributed.
[0083]
If there is a large amount of coarse precipitates which have a small effect in tenns
of pinning crystal grain boundaries, specifically, precipitates having a circle-equivalent
diameter of 1 00 run or more, the number of fine precipitates that have a pinning effect,
specifically, precipitates having a circle-equivalent diameter of 5 nm or more and less than
100 run will decrease, and furthermore the coarse precipitates will adversely affect the pitting
strength. In particular, at a position at a depth equivalent to one-half of the radius from the
surface, as the aforementioned coarse precipitates, if the total number of AI precipitates and
Nb precipitates having a circle-equivalent diameter of 100 nm or more exceeds 100 per 1 00
J.LII12
, not only will it not be possible to secure a pinning effect, but a decrease will occur in
the pitting strength.
[0084]
The total number of the aforementioned Al precipitates and Nb precipitates having
a circle-equivalent diameter of 100 nm or more is preferably 80 or less per 100 !J.It12
, and a
total number of 0 is extremely preferable.
[0085]
Even when precipitates are fine precipitates, ifthe circle-equivalent diameter of the
precipitates is less than 5 run it is difficult to obtain an effect as pinning particles because
the precipitates are very small. Therefore, as well as restricting the number coarse
precipitates, by also distributing a large number of precipitates that have a circle-equivalent
diameter that is 5 nm or more and is less than 100 nm that are fine precipitates that serve as
pinning particles, grain coarsening during carburizing and quenching can be suppressed and
a favorable pitting strength is also obtained. In particular, in a case where, at a position at
a depth equivalent to one-half of the radius from the surface, a total number of AI precipitates
and Nb precipitates having a circle-equivalent diameter that is 5 nm or more and is less than
100 nm as the aforementioned fine precipitates is 100 or more per 25 !J.It12, a pinning effect
can be exerted and grain coarsening during carburizing and quenching can be suppressed,
and furthermore, a favorable pitting strength can be secured.
[0086]
The circle-equivalent diameter of the AI precipitates and Nb precipitates as fine
precipitates at the aforementioned position at a depth equivalent to one-half of the radius
from the surface is preferably 10 nm or more, and is also preferably 80 nm or less.
[0087]
Further, the total number of AI precipitates and Nb precipitates having a circleequivalent
diameter that is 5 nm or more and is less than 100 run at the aforementioned
position is preferably 110 or more per 25 llm2
, and the higher that the total number of A1
precipitates and Nb precipitates is, the better.
[0088]
In the present invention, the tenn "Al precipitates" refers specifically to "AIN", and
the term "Nb precipitates" refers specifically to "NbC, NbN and Nb(CN)".
[0089]
The precipitates at the aforementioned position at a depth equivalent to one-half of
the radius from the surface of the case hardening steel wire according to the present invention
can be obtained using, for example, a steel bar or a wire rod or the like produced by the
production method described in the foregoing section (B), that is, by melting a case
hardening steel having the chemical composition described in section (A), subjecting a cast
piece or ingot obtained by casting of the molten steel to blooming to form a slab, subjecting
the slab to bot rolling, and adjusting the cooling conditions after the end of hot rolling, and
then performing spberoidizing annealing of the resulting steel bar or wire rod or the like and
thereafter further performing wire drawing or cold drawing or the like as cold processing.
This is because AI and Nb dissolve in steel when heated for 30 to 1200 min at a temperature
of 1150 to 1350°C when performing the blooming mentioned in section (B), and further, it
is difficult for Ostwald growth of precipitates to occur during the hot rolling of the slab
mentioned in section (B).
[0090]
Hereunder, the present invention is described in further detail by way of examples.
EXAMPLES
[0091]
Steels 1 to 22 having the chemical compositions given in Table 1 were melted by
using a converter or a vacuum furnace to prepare a cast piece or ingots.
[0092]
Specifically, for steels 2, 6, 9, 11 and 18, each steel was melted by using a 70-ton
converter, and after component adjustment was made by perfonning secondary refining
twice, the steel was continuously cast to prepare a cast piece. During continuous casting,
inclusions were caused to float and sufficiently removed by controlling the electromagnetic
stirring.
[0093]
For steel!, 3 to 5, 7, 8, 10, 12 to 17 and 19 to 22, after the steels had been melted
by using a 150-kg vacuum furnace, casting was performed to prepare ingots.
[0094]
Steels 1 to 10 and 18 to 20 in Table 1 were each a steel whose chemical composition
was within the range defined in the present invention.
[0095]
On the other hand, steels 12 and 13 were steels in which although the contents of
the individual elements were within the ranges defined in the present invention, Fn 1 deviated
from the condition defined in the present invention. Further, steels 14 and 15 were steels
in which although the contents of the individual elements were within the ranges defmed in
the present invention, Fn2 deviated from the condition defined in the present invention.
Furthermore, steels 16 and 17 were steels in V{hich although the contents of the individual
elements were within the ranges defined in the present invention, Fn3 deviated from the
condition defined in the present invention. In addition, steels 21 and 22 were steels in
which the content of at least one element deviated from the conditions defined in the present
invention.
[0096]
Steel 11 was a steel corresponding to SCM 420H specified in JIS G 4052 (2008),
and was a steel that deviated from the conditions defmed in the present invention.
[0097]
[Table 1]
'"d
'"1
~
.g 0
0
~
\0
00
(1)
.......,
0..
§' .~....
a g.
Table 1
Steel
Chemical comoosition lin mass%. balance: Fe and imj)urities_l
c Si Mn p s Cr Al Nb Ti N Mo 0 others Fn1 Fn2 Fn3
1 0.21 0.22 0.83 0.008 0.022 1.66 0.031 0.035 0.002 0.0185 <0.01 0.0018 - 38 0.88 0.37
("J '"1
Ill (1)
en CIJ
r+ '"d
2 0.21 0.27 0.79 0.013 O.Dl5 1.85 0.028 0.044 0.002 0.0190 <0.01 0.0010 - 53 1.00 0.50
3 0.22 0.18 0.90 0.012 0.013 1.73 0.035 0.024 0.001 0.0135 <0.01 0.0007 - 69 0.87 0.31
"tj (1) ..... ("J
(1) r+
("J r+ g 0
~
CIJ
r+
(1)
(1)
~ -CIJ
'"d
.!'->
'"1 ~0'.
0
0 ,__
4 0.12 0.28 0.94 0.011 0.007 1.90 0.023 0.010 0.002 0.0170 0.03 0.0011 - 134 0.88 0.53
5 0.15 0.34 0.82 0.015 0.016 1.78 0.028 0.032 0.001 0.0230 0.02 0.0017 Cu:O.l6, Ni:O.l4 51 0.90 0.61
6 0.20 0.24 0.73 0.014 0.011 1.81 O.D25 0.047 0.002 0.0190 <0.01 0.0009 - 66 1.06 0.43
7 0.18 0.19 0.93 0.010 0.046 1.88 0.026 0.028 0.001 0.0180 0.03 0.0012 Cu:0.07, Ni:0.07, V:0.14 20 0.92 0.36
8 0.24 0.31 0.51 0.011 0.014 1.85 0.042 0.037 0.001 0.0150 <0.01 0.0008 - 36 1.39 0.57
9 0.19 0.24 0.75 0.019 0.037 1.80 0.034 0.050 0.002 0.0190 <0.01 0.0018 - 20 1.03 0.43
10 0.20 0.27 0.86 0.011 0.010 1.79 0.032 0.042 0.002 0.0195 <0.01 0.0013 Ca:0.00l4 86 0.90 0.48
(1)
C/:1
,__
en § (1)
CIJ p..
11 0.20 0.25 0.80 0.013 0.018 *1.10 0.034 • - 0.001 0.0150 *0.22 0.0010 - 44 • 0.59 • 0.28
12 0.21 0.24 0.53 0.013 0.045 1.71 0.034 0.024 0.002 0.0150 <0.01 0.0012 - • 12 1.32 0.41
~ ~
p..
(1)
,__
en ~00 B. CIJ
0" r+
(1)
13 0.14 0.27 0.93 0.009 0.005 1.80 0.028 0.034 0.001 0.0190 <0.01 0.0010 - • 186 0.85 0.49
14 0.20 0.33 0.98 0.014 0.013 1.68 0.035 0.037 0.001 0.0145 <0.01 0.0014 - 75 • 0.73 0.55
15 0.19 0.17 0.52 0.011 0.018 1.88 0.033 0.031 0.001 0.0185 <0.01 0.0009 - 29 * 1.55 0.32
(1) g.
0.. s· cr'
~
~
::T
16 0.22 0.17 0.54 0.010 0.022 1.67 0.029 0.046 0.002 0.0195 <0.01 0.0011 - 25 1.34 * 0.28
17 0.21 0.35 0.68 0.012 0.013 1.89 0.028 0.041 0.001 0.0180 <0.01 0.0012 - 52 1.11 * 0.66
18 0.24 0.29 0.88 0.012 0.010 1.81 0.026 0.038 0.002 0.0220 <0.01 0.0010 - 88 0.88 0.52
.~.... .t~.:.l. .
0
~
OQ
s· Ill
0.
OQ .....
,.,._..._., s .......,
$P.
§ ~
19 0.17 0.19 0.82 0.009 0.013 1.77 0.034 0.029 0.002 0.0190 0.03 0.0016 Ni:0.07 63 0.97 0.34
20 0.20 0.27 0.89 0.013 0.014 1.85 0.033 0.033 0.001 0.0180 <0.01 0.0013 - 64 0.90 0.50
21 0.19 0.18 *1.20 0.012 *0.067 *2.03 0.031 0.036 0.001 0.0170 <0.01 *0.0040 - 18 0.79 0.37
22 0.21 0.23 0.89 0.009 0.015 1.82 0.033 0.040 *0.006 0.0165 <0.01 0.0014 - 59 0.91 0.42
Fn1=Mn/S, Fn2=Cr/(Si+2Mn), Fn3=SixCr
* indicates that conditions do not satisfy those defined bv the present invention.
,.0........, 0
N
......,
......., t...l
VI
~ ~
::T ~
~~ ~
(1)
with respect to steel 9, a bar-in-coil having a diameter of 35 nun was prepared from the cast
piece by the processes described in [ 1] and [2].
[0099]
With respect to steels 1, 3 to 5, 7, 8, 10, 12 to 17 and 19 to 22, steel bars having a
diameter of 35 mm were prepared from ingots by the processes described in the following
[2] and [3].
[0100]
[ 1] Blooming:
After being held at 1250°C for the time period described in Table 2, each cast piece
was subjected to blooming to thereby produce a 160 mm-square billet.
[0101]
[2] Hot Working:
Surface defects of the respective 160 rom-square billets produced by the blooming
were removed with a grinder, and after being held at 1100°C for 50 min, with respect to the
steels 2, 6, 11 and 18, the billets were hot-rolled to produce steel bars having a diameter of
35 nun, while with respect to the steel 9, the billet was hot-rolled to produce a bar-in-coil
having a diameter of 35 mm. The rolling finishing temperature and cooling rate in a
temperature range of 800 to 500°C for the aforementioned steel bars and bar-in-coil were as
shown in Table 2.
[0102]
With regard to the other steels, the respective ingots were held at a temperature and
for a time period described in Table 2, and thereafter subjected to hot forging to produce
steel bars having a diameter of 45 mm. The forging fmishing temperature and cooling rate
in a temperature range of 800 to 500°C for the aforementioned steel bars were as shown in
Table 2.
[0103]
[3] Peeling Treatment:
The respective steel bars having a diameter of 45 mm were subjected to peeling
treatment and fmished into steel bars with a diameter of 35 mm.
[0104]
Subsequently, steel wire having a diameter of33 mm was prepared by the processes
described in the following [4] and [5] based on the steel bar and bar-in-coil having a diameter
of 35 mm obtained as described in the foregoing.
[0105]
[ 4] Spheroidizing Annealing:
The steel bars and bar-in-coil having a diameter of 35 mm were subjected to
spheroidizing annealing in which the steel bars and bar-in-coil were held for 10 hat 760°C,
slowly cooled over 9 h to 650°C, and thereafter allowed to cool.
[0106]
[ 5] Cold Drawing:
After spheroidizing annealing, the steel bars and bar-in-coil were pickled, subjected
to cold drawing as cold processing using a lubricant, and finished into steel wire having a
so-called "bar shape" with a diameter of 33 mm. The reduction of area at this time was
11.1%.
[0107]
Each steel wire having a diameter of 33 mm obtained as described above was
transversely cut, that is, cut perpendicularly to the longitudinal direction, subsequently
embedded in a resin so that the cut surface was a surface to be examined, and thereafter the
surface was polished into a mirror fmish to prepare a test specimen for measuring hardness.
[0108]
Further, each steel wire having a diameter of 33 mm was longitudinally cut at a
position at a depth equivalent to one-half of the radius from the surface, that is, was cut
parallel to the longitudinal direction, and an extraction replica sample was prepared by a
common method from the cut surface.
[0109]
In addition, various kinds of test specimens were prepared by the processes
described in the following [ 6] to [8] from the remainder of the respective steel wires having
a diameter of 33 mm obtained as described in the foregoing. Further, a test specimen
described in [6] was cut out from a separately prepared steel bar of SCM 420H specified in
JIS G 4052 (2008) having a diameter of 140 mm.
[0110]
[6] Machining (Rough Working or Finish Working):
From a central portion of each of the 33 mm-diameter steel wires, a notched Ono
type rotating bending fatigue test specimen having a rough shape that is shown in Figure 1,
a roller-pitting small roller specimen having a rough shape that is shown in Figure 2, a test
specimen for evaluating cold workability that is shown in Figure 3, and a test specimen for
evaluating grain-coarsening resistance that is shown in Figure 4 were cut out in parallel with
the rolling direction or the forging axis.
[0111]
Further, a roller-pitting large roller specimen having a rough shape that is shown in
Figure 5 was cut out from a steel bar having a diameter of 140 mrn that was produced by
subjecting a cast piece of the SCM 420H specified in JIS G 4052 (2008) that was melted
using a converter to blooming, holding the steel bar at 920°C for 2 h, and then normalizing
the steel bar by allowing to cool in atmospheric air.
[0112]
Note that the unit of the dimensions for all of the cut-out test specimens shown in
Figures 1 to 5 is "mm", and three kinds of inverted triangular symbols in the figures are
"finish symbols" indicating surface roughness that are described in Explanation Table 1 of
JIS B 0601 (1982).
[0113]
Further, the character "G" attached to a finish symbol is an abbreviation of a
working method indicating "grinding" that is defined in JIS B 0122 (1978).
[0114]
The remaining part of each ofthe 33 mm-diameter steel wires was water-quenched,
and thereafter was used for non-metallic inclusion examination. The details of the
examination method will be described later.
[0115]
[7] Carburizing and Quenching-Tempering:
The notched Ono type rotating bending fatigue test specimens and roller-pitting
small roller specimens having a rough shape that had been cut out in the above [6] were
subjected to "carburizing and quenching-tempering" using the heat pattern shown in Figure
6. Further, the roller-pitting large roller specimen having a rough shape that had been cut
out in [6] was subjected to "carburizing and quenching-tempering" using the heat pattern
shown in Figure 7.
[0116]
The aforementioned notched Ono type rotating bending fatigue test specimens and
roller-pitting small roller specimens were subjected to the aforementioned treatment in a
hung state in which a wire was passed through a hole formed for hanging.
[0117]
The characters 11Cp11 in Figure 6 and Figure 7 represent carbon potential. Further,
the term 11 130°C oil quenching11 refers to quenching in an oil having an oil temperature of
130° C, and furthermore the characters "AC" represents air cooling.
[0118]
The oil quenching was performed by putting the test specimen into a stirred
quenching oil so that quenching was performed uniformly.
[0119]
[8] Machining (Finish Working of Material Subjected to Carburizing and
Quenching-Tempering):
Each of the aforementioned test specimens that were subjected to carburizing and
quenching-tempering treatment were subjected to finish working to prepare the notched Ono
type rotating bending fatigue test specimen shown in Figure 8, the roller-pitting small roller
specimen shown in Figure 9, and the roller-pitting large roller specimen shown in Figure 10.
[0120]
Note that the unit ofthe dimensions for each of the aforementioned test specimens
shown in Figures 8 to 10 is 11nun11
, and two kinds of inverted triangular symbols in each of
the aforementioned figures are 11finish symbols" indicating surface roughness that are
described in Explanation Table 1 of JIS B 0601 (1982).
[0121]
Further, the character "G" attached to a finish symbol is an abbreviation of a
working method indicating "grinding11 that is defined in liS B 0122 (1978).
[0122]
Further, the "- (swung dash)" symbol in Figure 8 is a "waveform symbol" that
indicates a base metal, that is, a surface as it is after undergoing the carburizing and
quenching-tempering treatment of the above [7].
[0123)
For each of steels 1 to 22, measurement of hardness at a position 50 J.UD from the
surface, examination of precipitates at a position at a depth equivalent to one-half of the
radius from the surface, examination of cold workability by means of a cold compression
test specimen, examination of grain-coarsening resistance and examination of non-metallic
inclusions were conducted. Further, examination of surface hardness, examination of core
hardness, examination of effective hardened layer depth, examination of intergranular
oxidation depth, examination of depth of non-martensitic structure, examination of bending
fatigue strength through the Ono type rotating bending fatigue test, and examination of
pitting strength through the roller pitting test after "carburizing and quenching-tempering",
respectively, were performed.
[0124]
The contents of each of the aforementioned examinations are described in detail
below.
[0125]
<<1>> Measurement of Hardness at Position 50 ).liD from Surface:
The HV (Vickers hardness) at 10 locations that were positioned 50 ).liD from the
surface of the aforementioned test specimen for hardness measurement that was mirror
polished was measured in conformity with the "Vickers hardness test-Test method"
described in JIS Z 2244 (2009) using a Vickers hardness testing machine with the test force
being 0.98 N, and a value obtained by arithmetically averaging the hardness values at the 10
locations was taken as the hardness at a position 50 11m from the surface.
[0126)
<<2>> Examination of Precipitates at Position at a Distance Equivalent to One-half
of the Radius from the Surface
The aforementioned extraction replica samples that were prepared by a common
method were observed using a transmission electron microscope (hereunder, referred to as
"TEM") equipped with an energy dispersive X-ray spectroscope (hereunder, referred to as
"EDX"), and the total precipitation density was examined with regard to AlN as A1
precipitates and NbC, NbN and Nb(CN) as Nb precipitates.
[0127)
Specifically, the contained amount and shape of A1 and Nb precipitates were
checked based on element analysis by means of the EDX, 20 visual fields were observed at
a TEM magnification of 30000, the areas of Al precipitates and Nb precipitates were
calculated by image analysis, the calculated areas were each converted to an area of a circle
and the circle-equivalent diameters were determined.
(0128]
Next, the total number of AI precipitates and Nb precipitates having a circleequivalent
diameter of 100 nm or more was counted, and the obtained number was converted
to a number per 100 J.Ul12 area. Similarly, the total number of Al precipitates and Nb
precipitates having a circle-equivalent diameter of 5 nm or more and less than 100 nm was
counted, and the obtained number was converted to a number per 25 J..Lm2 area.
[0129]
<<3>> Examination of Cold Workability
Test specimens shown in Figure 3 that were prepared in the manner described in
[6] above were compressed at normal temperature by a hydraulic press taking the
longitudinal direction as the height, and were compressed until cracking occurred at the outer
circumference. For each of the steels, a compression test using a hydraulic press was
performed five times, and cracking was observed with a magnifying glass. A compression
ratio when cracking was recognized in three or more test specimens among the five test
specimens was defined as the critical compression ratio, and in a case where the critical
compression ratio was of the same level as or higher than that of steel 11 as steel
corresponding to SCM 420H specified in JIS G 4052 (2008), the steel was evaluated as being
excellent in cold workability, and this cold workability was defined as the target.
[0130]
<<4>> Examination of Grain-coarsening Resistance Property:
Test specimens shown in Figure 4 that were prepared in the manner described in
[6] above were compressed at normal temperature to 70% of the height by a hydraulic press,
taking the longitudinal direction as the height. Thereafter, to simulate heating during
carburization, after each test specimen was heated at 950°C and held for 5 h, waterquenching
was performed, and each test specimen was longitudinally cut so as to include
the central axis, that is, was cut in parallel with the height direction, and the test specimen
was embedded in a resin so that the cut surface was the surface to be examined, and the
surface was polished into a mirror finish. Next, the surface of each test specimen was
etched with an aqueous solution saturated with picric acid to which a surfactant was added,
10 visual fields were randomly observed using an optical microscope at a magnification of
100, the grain size number specified in JIS G 0551 (2013) was examined, and the values of
the 10 visual fields were arithmetically averaged to thereby determine the austenite grain
size number of each test specimen. In a case where the austenite grain size number was of
the same level as or higher than that of steel 11 as steel corresponding to SCM 420H
specified in JIS G 4052 (2008), the steel was evaluated as being excellent in grain-coarsening
resistance, and this grain-coarsening resistance was defined as the target.
[0131]
<<5>> Examination ofNon-metallic Inclusions:
The remainder of each steel wire having a diameter of 33 mm was held at 920°C
for 30 min, and thereafter was water-quenched. After being water-quenched, the steel wire
was embedded in a resin so that a face that was cut in parallel with the longitudinal direction
of the steel wire along the center line thereof was a surface to be examined, and the face was
polished into a mirror finish.
[0132]
Next, in conformity with method A of ASTM-E45-13, inclusions having a large
thickness among the non-metallic inclusions of type B and type D, specifically, inclusions
having a thickness greater than 4 j.Ul1 and not more than 12 )..LID and inclusions having a
thickness greater than 8 )..LID and not more than 13 )..I.ID, respectively, were measured, and the
class of each inclusion was judged.
[0133]
In the following description, the non-metallic inclusions of type B and type D that
have a large thickness are referred to as "BH" and "DH", respectively.
[0134]
<<6>> Examination of Surface Hardness and Core Hardness
The notched Ono type rotating bending fatigue test specimen having a rough shape
that was subjected to carburizing and quenching-tempering treatment as described in the
above [7] was used for this examination, with the notch portion thereof having a diameter of
8 mm being transversely cut, and the test specimen then embedded in a resin so that the cut
surface was a surface to be examined. Thereafter, the surface was polished into a mirror
surface finish, and the surface hardness and the core hardness were examined using a micro
Vickers hardness tester.
[0135]
Specifically, in conformity with "Vickers hardness test-Test method" described in
JI-S Z 2244 (2009), the HV was measured at ten arbitrary points at a position at a depth of
0.03 mm from the surface of the test specimen using a micro Vickers hardness tester, with
the test force being 0.98 N. The measurement values were arithmetically averaged, and the
surface hardness was evaluated.
[0136]
Likewise, in conformity with aforementioned specification of JIS, the HV was
measured at ten arbitrary points in the core part, which is a portion of base metal not affected
by carburization, by using a micro Vickers hardness tester with the test force of 2.94 N.
The measurement values were arithmetically averaged, and the core hardness was evaluated.
[0137]
With respect to the roller-pitting small roller specimen having a rough shape that
was subjected to carburizing and quenching-tempering treatment as described in the above
[7] also, a region having a diameter of 26.2 mm was transversely cut, and the surface
hardness and core hardness were measured by the same method as that use for the notched
Ono type rotating bending fatigue test specimen.
[0138]
Note that, with respect to the roller-pitting small roller specimen having a rough
shape that was subjected to carburizing and quenching-tempering treatment as described in
the above [7], in the case where the test specimen was further subjected to treatment in which
it was tempered at 300°C for 1 h using a vacuum furnace and thereafter water-cooled, the
surface hardness was measured by the same method as described above.
[0139]
<<7>> Examination of Effective Hardened Layer Depth:
Examination of the effective hardened layer depth was performed using the resinembedded
test specimens of the notched Ono type rotating bending fatigue test specimens
and the roller-pitting small roller specimens that had a rough shape that were used for the
examination of surface hardness and core hardness in the above <<6>> after merely being
subjected to carburizing and quenching-tempering treatment in the above [7].
[0140]
Specifically, similarly to the case of the examination of surface hardness in the
above <<6>>, in conformity with "Vickers hardness test-Test method" described in JIS Z
2244 (2009), the HV was measured in a direction· from the mirror-finished surface of the test
specimen toward the center using a micro Vickers hardness tester with the test force being
2.94 N. The depth from the surface in the case where the HV was 550 was measured. The
minimum value of measurement values obtained from 10 arbitrary locations was taken as
the effective hardened layer depth.
[0141]
<<8>> Examination of Intergranular Oxidation Depth and Non-Martensitic ·
Structure Depth:
The intergranular oxidation depth and the non-martensitic structure depth were
examined using the resin-embedded test specimens of the notched Ono type rotating bending
fatigue test specimen that had a rough shape used in the above <<6>> and <<7>>.
[0142]
Specifically, the test specimen embedded in a resin was polished again, and the
surface part of the test specimen was observed as it was in a mirror finished state and without
being etched, in 10 arbitrary visual fields under an optical microscope at a magnification of
1000. Oxidized structures observed along the grain boundary in the surface part were
defined as the intergranular oxidation, and the depths of these structures were arithmetically
averaged, and the intergranular oxidation depth was evaluated.
[0143]
Further, the same test specimen was etched with nita! for 0.2 to 2 s, and the surface
part of the test specimen was observed in 10 arbitrary visual fields under an optical
microscope at a magnification of 1000. Portions in which the degree of etching was more
remarkable than that of the periphery in the surface part were defined as the non-martensitic
structures, and the depths of these structures were arithmetically averaged, and the nonmartensitic
structure depth was evaluated.
[0144]
<<9>> Examination of Bending Fatigue Strength Through Ono Type Rotating
Bending Fatigue Test:
An Ono type rotating bending fatigue test was conducted under the following test
conditions using the notched Ono type rotating bending fatigue test specimen that was
finished in the above [8].
The bending fatigue strength was evaluated based on the maximum strength at the
time when the test specimen did not rupture when the number of repetitions was 107
•
[0145]
-Temperature: room temperature
- Atmosphere: in atmospheric air
-Number of rotations: 3000 rpm
[0146]
In a case where the bending fatigue strength was of the same level as or higher than
that ofsteelll as steel corresponding to SCM 420H specified in JIS G 4052 (2008), the steel
was evaluated as being excellent in bending fatigue strength, and this bending fatigue
strength was defmed as the target.
[0147]
<<10>> Examination of Pitting Strength Through Roller Pitting Test:
A roller pitting test was conducted under the following test conditions using the
roller-pitting small roller specimen and roller-pitting large roller specimen that were finished
in the above [8]. That is, the roller-pitting small roller specimen and roller-pitting large
roller specimen were rotated in a contacting state, and lubricating oil was sprayed onto the
contact portion under the conditions described below. A largest strength at which pitting
of a width of 1 mm or more did not occur on the surface of the roller-pitting small roller
specimen during a number of repetitions of 107 was evaluated as the pitting strength. In a
case where the pitting strength was of the same level as or higher than that of steel 11 as
steel corresponding to SCM 420H specified in JIS G 4052 (2008), the steel was evaluated
as being excellent in pitting strength, and this pitting strength was defmed as the target.
[0148]
- Slip factor: 40%
- Number of rotations of roller-pitting small roller specimen: 1500 rpm
- Lubrication: Automatic transmission lubricating oil having an oil temperature of
1 oooc was squirted at a contact portion between a roller-pitting small roller specimen and a
roller-pitting large roller specimen at a rate of 2.0 liters per minute
[0149]
Where, the term "slip factor" refers to a value calculated by the following formula
in which "V1" represents the tangential velocity of the roller-pitting small roller specimen
surface and "V2" represents the tangential velocity of the roller-pitting large roller specimen
surface.
{(V2.:V1)/Vl }xlOO
[0150]
The results of the respective examinations described above are shown collectively
in Table 2 and Table 3.
[0151]
[Table 2]
Table2
Conditions of
Rolling or Hardness at
Precipitates at position at a depth
Cold Worlmbility
Non-metallic
heating ond holding of Cooling nlc in cquivalcnl'to 112 ofrRdius frum surface Austenite grain Inclusions
Test Cllst picte or ingot
forging
o temperature
position SO [critical
Steel size number finishing IJlll &tun Pn:cipitalc.~ having Pnx:ipilalll!l having compression
No. Heating Holding temperature
nmgc uf 800 to
surface o cin:le-equivalent a cirel&-equivalatt mtio)
aficrht:llling BH DH
temperature time ("C)
SOO"C ("Cis)
[HV] diamoh:r of<: I 00 nm diameter ur;::s nm 811d (%)
at9SO"C>
I I l2SO 120 1000 0.17 1113 73 IJS 69.2 10.2 0.0 0.0
2 2 1250 960 liDO 0.40 192 56 176 68.0 10.8 0.0 0.0
3 3 1250 70 1000 0.60 214 83 122 67.9 10.2 0.0 0.0
4 4 1250 1140 LDOO 0.11 171 65 183 74.4 11.3 0.0 0.0
Inventive 5 5 1250 930 1000 0.13 1711 74 174 72.1 10.7 0.0 0.0
E.'iiUIIplc 6 6 1250 100 840 0.22 183 62 129 69.7 10.0 0.0 0.0
7 7 1250 240 1000 0.15 180 41 137 71.5 10.0 0.0 0.0
8 8 1250 900 1000 0.71 237 67 168 66.2 11.0 0.0 0.0
9 9 1250 50 900 0.27 184 80 104 67.9 9.7 0.0 0.0
10 10 1250 1070 1000 0.14 179 74 177 703 11 .7 0.0 0.0
11 •1) 1250 720 1010 0.28 184 67 163 S 6S.O s 9.6 0.0 0.0
~ ~ 12 •12 1250 1100 1000 0.43 19S 53 177 !I 61.8 11.2 0.0 0.0
13 *13 1250 1010 1000 0.11 176 76 176 II S8.4 10.5 0.0 0.0
14 *14 1250 420 1000 0.16 181 59 142 70.2 10.1 0.0 0.0
IS •t5 12SO 320 1000 0.12 177 55 137 69.7 Ill-~ 0.0 0.0
Compurntive 16 •16 12SO ss 1000 0.42 194 72 lOR 68.8 9.7 0.0 0.0
Example 17 •17 1250 60 1000 0.45 213 69 114 67.9 10.4 0.0 0.0
18 18 1250 110 1030 1.05 • 254 68 129 # 63.5 9.9 0.0 0.0
19 19 13SS 1SOO 1000 0.35 185 • 114 • 76 71.9 II 1.0 0.0 0.0
20 20 1100 28 1000 0.15 180 92 • 89 70.4 II 2.0 0.0 0.0
21 •z1 1250 90 1000 0.36 186 71 125 II 63.8 10.1 2S 0.0
22 •22 1250 840 1000 037 IIIII 89 164 68.8 10.7 0.0 l.S
Column Ml'ra.~pilalo=~ ol pumli1m lllu d"Pih llljUiva).,.,llu 1/2 ufradiuK frum 110rfuc.:" indica!"" Inial pnocipitalilm den•ily uf AlN as AI prccipilaiCK snd NbC, NhN and Nh(CN) 8ll Nh pnx:ipilalcA.
Values in colunut ''NOIHnelallic Inclusions" indic:ale the class judged by measuring inclusions having a large dtickness having a lhickness >4 IJln and ~12 pm and inclusions having a thickness >8 Jill! and :SJ3 jUII,
11111ong lhe uon-metollic inclusions of typo B nnd typeD respectively, in conformity \\;lh method A of ASTM-E4S-13
• indicates that conditions do not satisfY those dofi1ted by lhe present invention.
S indica leo cvolunliun critcriu, nnd # indicaleM U1sllhc resulL• dn nnl reach lh" large!.
~ ~
,.....,
0.. ...
Vo w
Table 3
Examinalion of Bending Falii,'IIC Stn:nglh
1'hrough Ono Typo Rolllting Bending Fatigue Test
Test Effevtivo Non- Steel lnlcrgrnnular No. SurfaCil Cur" Hardened Marlcn.•ilic Oxidation
Hnnlness Hnrdnr:ss Layer
Depth
Slnlcture
[HV] [HV] Depth
(11111)
Depth
(nun) (Jllll)
I I 712 309 1.03 5.7 10.8
2 2 708 316 1.02 6.8 11.9
3 3 705 298 1.07 6.1 113
4 4 713 322 0.99 5.0 10.0
Inventive 5 5 717 306 1.08 5.1 10.5
Example 6 6 708 322 1.03 6J 11.7
7 7 702 314 0.92 4.9 10.2
8 8 705 307 0.99 73 12.4
9 9 714 291 1.04 6.4 11.4
10 10 711 310 1.02 5.1 JO.l
11 *11 702 304 1.03 8.9 153
12 •t2 704 293 0.98 6.2 113
13 •13 711 314 1.08 53 103
14 •14 716 320 0.95 15.2 27.6
15 •t5 709 299 1.06 163 28.9
Comparutive 16 *16 707 316 1.07 10.2 15.4
Example 17 *17 714 318 1.07 14.7 213
18 18 705 294 1.01 5.1 10.2
19 19 716 303 1.00 7 .4 12.4
20 20 715 312 1.04 6.4 11.6
21 *21 706 307 1.01 12.8 19.7
22 •22 708 296 0.98 6.0 11.3
• indicates Uta! conditions do not satisfY those defined by lhe present invention.
$ indicates evalunlion criteria, and # indica~_ lhnt the results do not reach the target.
E!aninalion of Pilling Sln::nglh
Through RoUer Pitting Test
Surfncc:
Surface Cure Hanln""'•
Hardness Hnnlncss llftc:r tempering
[HV] IHVJ al 300"C
[HV]
742 295 670
741 301 662
736 287 662
74H 305 664
751 293 671
739 303 664
734 300 654
738 295 664
748 279 673
745 296 667
7.23 298 650
737 278 669
744 299 664
734 304 661
728 280 657
725 302 642
745 305 676
737 280 6S7
746 283 672
744 298 663
740 294 665
739 285 660
Elfevtive
Bending
Fatigue
Hanlt:ncd Stn:ngth
Layer (MPa)
Depth
(mm)
0.72 S20
0.71 SIS
0.7S SJO
0.69 S20
0.78 520
0.71 515
0.68 530
0.69 510
0.72 515
0.71 510
0.73 s soo
0.69 II 490
0.76 # 480
0.68 " 480
0.73 II 470
0.74 510
0.75 II 490
0.71 50()
0.70 510
0.72 510
0.71 II 480
0.69 II 470
Pitting
Strang1h
(MPa)
2050
2100
2100
2150
2200
2100
2200
2150
2200
2050
s 2000
2000
2000
II 1950
II 1950
II 1850
II 1900
2100
II 1900
II 1850
II 1850
II 1900
~
-~ CD
w
'--'
,.......,
.0.. ..
lJt
tv
'--'
Based on Table 2 and Table 3 it is apparent that, in the case of test Nos. 1 to 10 that
satisfy the conditions defmed in the present invention, steels 1 to 10 have favorable cold
workability and, furthermore, even though steels 1 to 10 did not contain Ni and Mo or had
an extremely small content ofNi and Mo, an austenite grain size number, a bending fatigue
strength and a pitting strength were obtained that were of the same level as or higher than
the corresponding values in the case of test No. 11 which was conducted using steel 11 that
corresponds to the 11chrome-molybdenum steel11 SCM 420H specified in JIS G 4052 (2008).
Thus, it is apparent that steels 1 to 10 have excellent grain-coarsening resistance and it is
possible to secure a high bending fatigue strength and a high pitting strength.
[0154]
In contrast, in the case of test Nos. 12 to 22 of comparative examples that deviated
from the conditions defined in the present invention, at least one characteristic among cold
workability, grain-coarsening resistance, bending fatigue strength and pitting strength was
poor.
[0155]
In the case of test No. 12, because Fn1, that is, [Mn/S] of steel 12 was lower than
the range defined in the present invention, the cold workability was poor. Further, the
bending fatigue strength was 490 MPa, which is low in comparison to test No. ll.
[0156]
In the case oftest No. 13, because Fn1, that is, [Mn/S] of steel 13 was higher than
the range defmed in the present invention, the cold workability was poor. Further, the
bending fatigue strength was 480 MPa, which is low in comparison to test No. 11.
[0157]
In the case oftest No. 14, because Fn2, that is, [Cr/(Si+2Mn)] of steel14 was lower
than the range defined in the present invention, the bending fatigue strength was 480 MPa,
which is low in comparison to test No. 11. Further, the pitting strength was 1950 MPa,
which is also low in comparison to test No. 11.
[0158]
In the case of test No. 15, because Fn2, that is, [Cr/(Si+2Mn)] of steel 15 was higher
than the range defined in the present invention, the bending fatigue strength was 470 MPa,
which is low in comparison to test No. 11. Further, the pitting strength was 1950 MPa,
which is also low in comparison to test No. 11.
[0159]
In the case of test No. 16, because Fn3, that is, [SixCr] of steel16 was lower than
the range defined in the present invention, the pitting strength was 1850 MPa, which is low
in comparison to test No. 11.
[0160]
In the case oftest No. 17, because Fn3, that is, [SixCr] of steel 17 was higher than
the range defined in the present invention, the bending fatigue strength was 490 MPa, which
is low in comparison to test No. 11. Further, the pitting strength was 1900 MPa, which is
also low in comparison to test No. 11.
[0161]
In the case oftest No. 18, although the chemical composition of steel 18 was within
the range defined by the present invention, the hardness at a position 50 ).LID from the surface
was a HV of 254, which is higher than the hardness defmed in the present invention, and
hence the cold workability was poor.
[0162]
In the case of test No. 19, although the chemical composition ofsteel19 was within
the range defined in the present invention, at a position at a depth equivalent to one-half of
the radius from the surface, the total number of AI precipitates and Nb precipitates having a
circle-equivalent diameter of I 00 nm or more was higher than the range defined in the
present invention, and furthermore, the total number of AI precipitates and Nb precipitates
having a circle-equivalent diameter of 5 run or more and less than 100 nm was lower than
the range defined in the present invention. Consequently, grain-coarsening resistance and
pitting strength were poor compared to test No. 11.
[0163]
In the case of test No. 20, although the chemical composition of steel 20 was within
the range defined in the present invention, at a position at a depth equivalent to one-half of
the radius from the surface, the total number of AI precipitates and Nb precipitates having a
circle-equivalent diameter of 5 nm or more and less than 100 nm was lower than the range
defined in the present invention. Consequently, grain-coarsening resistance and pitting
strength were poor compared to test No. 11.
[0164]
In the case of test No. 21, the contents ofMn, Sand 0 in steel21 were higher than
the values defined in the present invention, and the Cr content was also higher than the value
defined in the present invention. Consequently, the cold workability was poor.
Furthermore, large-sized hard inclusions of type B of class 2.5 were observed, and the
bending fatigue strength was 480 MPa and the pitting strength was 1850 MPa, which were
low in comparison to test No. 11.
[0165]
In the case of test No. 22, the Ti content ofsteel22 was higher than the value defined
in the present invention. Consequently, large-sized hard inclusions of type D of class 1.5
were observed, and the bending fatigue strength was 470 MPa and the pitting strength was
1900 MPa, which were low in comparison to test No. 11.
INDUSTRIAL APPLICABILITY
[0166]
The case hardening steel wire of the present invention is excellent in cold
workability and has a low component cost. Furthermore, a carburized part that employs
this case hardening steel wire as a starting material has a bending fatigue strength, a pitting
strength and grain-coarsening resistance of the same level as or higher than a carburized part
that uses the "chrome-molybdenum steel" SCM 420H specified in JIS G 4052 (2008) as a
starting material. Therefore, the case hardening steel wire of the present invention is
suitably used as a starting material of a carburized part such as a gear or a shaft which is
required to have high bending fatigue strength and high wear resistance to reduce weight
and increase torque.

We claim:
1. A case hardening steel wire having a chemical composition consisting, by
mass%, of
C: 0.10 to 0.24%,
Si: 0.16 to 0.35%,
Mn: 0.40 to 1.00%,
S: 0.005 to 0.050%,
Cr: 1.65 to 1.90%,
Al: 0.015 to 0.060%,
Nb: 0.005 to 0.060%,
N: 0.0130 to 0.0250%,
Cu: 0 to 0.20%,
Ni: 0 to 0.20%,
V: 0 to 0.20%,
Ca: 0 to 0.0050%, and
a balance: Fe and impurities, wherein:
values of Fn 1, Fn2 and Fn3 represented by Formula (i), Formula (ii), and Formula
(iii) hereunder satisfy 15 ::s; Fnl ::s; 150, 0. 75 ::s; Fn2 ::s; 1.40, and 0.30 ::s; Fn3 ::s; 0.65, respectively;
the contents ofP, Ti, Mo and 0 in the impurities are P: 0.020% or less, Ti: 0.005%
or less, Mo: 0.03% or less and 0: 0.0020% or less, respectively;
a hardness at a position 50~ from a surface is 250 or less in HV; and
at a position at a depth equivalent to one-half of a radius from the surface, a total
number of AI precipitates and Nb precipitates having a circle-equivalent diameter of 100 run
or more is 100 or less per 100 j..t.m2
, and a total number of AI precipitates and Nb precipitates
having a circle-equivalent diameter in a range of 5 run or more to less than 100 run is 100 or
more per 25 ~m2 ;
Fnl = Mn/S (i)
Fn2 = Cr/(Si+2Mn) (ii)
Fn3 = SixCr (iii)
where, a symbol of an element in the formulas above represents a content by mass
percent of the element.
2. The case hardening steel wire according to claim 1, wherein:
the chemical composition contains, by mass%, one or more elements selected from:
Cu: 0.05 to 0.20%, and
Ni: 0.05 to 0.20%.
3. The case hardening steel wire according to claim 1 or 2, wherein:
the chemical composition contains, by mass%,
V: 0.05 to 0.20%.
4. The case hardening steel wire according to any one of claims 1 to 3, wherein:
the chemical composition contains, by mass%,
Ca: 0.0003 to 0.0050%.