DESCRIPTION
METHOD FOR PRODUCING STEEL COMPONENT
TECHM[CAL FIELD
~00011
The present invention relates to a method for producing a steel component.
Particularly, the present invention relates to a method for producing a power
transmitting component made of steel, which is excellent in machinability in
cutting process, or in cold forgeability during net-shape forming, and is also
excellent in high-cycle and low-cycle bending fatigue strengths and pitting
resistant strength, after carburizing-quenching or carbonitriding-quenching.
BACKGROUND ART
[00021
Generally, a "case hardening treatment" such as induction quenching,
carburizing-quenching, carbonitriding-quenching, and the like is applied to steel
components of automobiles and industrial machinery, in particular, for gears,
pulleys, shafts and the like made of steel which are used as power transmitting
components.
[00031
Among those described above, the "induction quenching1' is a quenching
in which steel components are rapid heated into a high-temperature austenite
region with a temperature of the Aca points or higher, and thereafter cooled.
The induction quenching has an advantage in that the adjustment of case depth
is relatively easy. In order to obtain a necessary surface hardness, case depth,
and core hardness, it is general to use, as the material to be treated, a medium
carbon steel such as S46C specified in JIS G 4061 (2009) and SCr440 specified
in JIS G 4063 (2008).
roo041
However, since a medium carbon steel has a higher material hardness
than that of a low carbon steel, it is worse in machinability in cutting process as
well as in cold forgeability during net-shape forming. In addition, in the case
of induction quenching, a problem exists in that an induction heating coil needs
to be made for each component.
[0006]
For that reason, the carburizing-quenching or carbonitriding quenching
has become more frequently used for the case hardening treatment of the steel
components mentioned above.
[OOOS]
When the carburizing-quenching or carbonitriding quenching is used as
the case hardening treatment, the eteel components mentioned above are
produced, for example, by the following method.
[00071
(1) A rolled steel bar or wire rod made of steel for machine structure is
prepared. As the steel for machine structure, a steel which has a lower C
content than that of a steel used for induction quenching, such as SCr420,
SCM420, and SNCM420 etc. specified in JIS G 4063 (20081, is used.
~00081
(2) The prepared rolled steel bar or wire rod is hot forged to be roughly
formed into an intermediate product.
[00091
(3) The roughly formed intermediate product of (2) described above is
subjected to cutting after being subjected to normalizing treatment as needed.
(3') The roughly formed intermediate produce of (2) described above ie
.subjected to net-shape forming by cold forging after being subjected to
normalizing treatment as needed.
l00101
(4) The intermediate product which has been subjected to cutting or netshape
forming is subjected to carburizing-quenching or carbonitridingquenching
as the case hardening treatment and further to tempering at a
temperature not more than 200°C as needed, to obtain the steel components
described above.
[Oolll
(6) Shotpeening andlor surface grinding may be further performed after
the case hardening or after the tempering of (4) described above to obtain the
steel components.
[00121
In recent years, to improve the fuel economy of automobiles and industrial
machines, or to realize higher output power of engines, reduction in weight and
size of steel components has been promoted. However, as a result of such
reduction in weight and size, load applied to the steel components tends to
increase. For that reason, improvements in the bending fatigue strengths in a
high-cycle region, as well as improvements in bending fatigue strength in a lowcycle
region and pitting resistant strength is demanded for the steel components.
LOO 131
To be specSc, for example, in the case of a gear for automobiles, at a gear
tooth root, higher bending fatigue strength in a high-cycle region of a number of
load repetitions of about 1.0~107is demanded in respect of suppressing a tooth
root breakage, and also higher bending fatigue strength in a low-cycle region of
a number of load repetitions of about 1.0 x 106 is demanded in respect of
suppressing a tooth root breakage at a large load applied at the start of driving.
Further, at a tooth face, higher pitting resistant strength is demanded in respect
of suppressing noises during meshing of gears and suppressing a breakage of
tooth starting at a portion of exfoliation.
roo14
Hereafter, the bending fatigue strength in a high-cycle region as described
above is referred to as "high-cycle bending fatigue strength", and the bending
fatigue strength in a low-cycle region is referred to as "low-cycle bending fatigue
strength".
[OO16 1
To meet such demands, a technique of performing a case hardening
treatment by carburizing-quenching or carbonitridmg-quenching using a steel
containing a large amount of alloying elements compared with the steel for
machine structure specified in JIS G 4063 (2008) described above, and a
technique of further performing shotpeening after case hardening treatment
have been proposed.
[OOI~I
Patent Document 1 (JP6-306672A) discloses a "gear" made of a material
which contains elements such as Si: not more than 0.1%, Ni: 0.4 to 0.6%, Mo: 0.6
to 1.0%, and Nb: 0.02 to 0.6% with the balance being Fe, and has a carburized
abnormal layer of not more than 6 p and a grain size of No. 9 or larger.
[0017]
Patent Document 2 (JP64-31927A) discloses a "method for producing a
heat treated steel component" in which a steel consisting of C: 0.10 to 0.40%, Si:
not less than 0.06% and less than O.16%, Mn: 0.30% to 1.00%, Cr: 0.90% to 1.20%,
Mo: more than 0.30% and not more than 0.60%, the balance being Fe, is
subjected to carburizing-quenching or carbonitriding-quenching, and
subsequent$ to shotpeening.
[oolsl
Patent Document 3 (JP60-21359A) discloses a "steel for gear to be used
with carburking treatment" which contains elements such as Cr: 0.40 to 1.60%
and Si: not more than 0.10% and further contains, as needed, one or more kinde
of Ni: not more than 2.60%, Mo: not more than 0.40%, and Nb: 0.006 to 0.0255,
with the balance being substantially Fe.
[0019]
Patent Document 4 (JP~-242994A) discloses a "steel for gear having
excellent tooth face strength" which contains elements such as Si: not more than
1.0% and Cr: 1.50 to 5.0% with the balance being Fe and impurities, and a
"method fbr producing a gear having excellent tooth face strength" in which a
case hardening treatment by carburizingquenching and tempering or
carbonitriding-quenching and tempering ia performed using the aforementioned
steel for gear, or shotpeening is further performed, as needed, after the case
hardening treatment.
[00201
Patent Document 5 (JP7-126803A) discloses a "steel for carburized gear"
which contains elements such as Si: 0.35 to 3.056, Cr: 0.3 to 5.0%, and V: 0.06 to
0.6% and further contains, as needed, one or more kinds of Ni: not more than
3.0%, Mo: not more than 1.0%, and Nb: not more than 0.1%, with the balance
being Fe and inevitable impurities.
LIST OF PRIOR ART DOCUMENT(S)
[00211
[Patent Document 11 JP6-306572A
[Patent Document 21 JP64-31927A
patent Document 31 JP60-21359A
[Patent Document 41 JP7-2429944
[Patent Document 51 JP7-126803A
DISCLOSURE OF THE INVENTION
PROBLEMS TO BE SOLVED BY THE INVENTION
100221
In general, since the hardness of steel increase8 as the content of alloying
elements increases, its machinability deteriorates and also its cold forgeability
during net-shape forming deteriorates, accordingly. Thie is also the case with
steels disclosed in Patent Documents 1 to 6 having a large content of alloying
elements.
lo0231
Therefore, when any of the steels proposed in Patent Documents 1 to 6 is
used, it is necessary, in the production of a steel component, to add a process of
softening heat treatment before cutting or net-shape forming to avoid the
deterioration of machinability in cutting process, or the deterioration of cold
forgeability during net-shape forming. This will cause an increase of process
steps reducing the productivity as well as increasing the production cost of the
component.
LO024
Further, in view of steep rises in the prices of alloying elements in recent
years, there is growing demand for reducing the material cost particularly by
decreasing the contents of Ni and Mo.
[00261
The technique proposed in Patent Document 1 requires that Ni: 0.4 to
0.6%, Mo: 0.6 to 1.0%, and Nb: 0.02 to 0.6% be contained as essential elements.
For this reason, it cannot sufficiently meet the above mentioned demand for
reducing the material cost.
LO0261
In the technique proposed in Patent Document 2, shotpeening is
performed at an appropriate condition. As a result of this, the technique is able
to achieve improvements in high-cycle bending fatigue strength as shown in
Table 3 thereof. However, it lacks consideration of low-cycle bending fatigue.
Therefore, the technique cannot meet the demand for improving low-cycle
fatigue strength to cope with the reduction in weight and size of a steel
component.
[0027l
According to the technique proposed in Patent Documents 3 to 6, it is
possible to achieve excellent bending fatigue strength and surface fatigue
strength by carburizing-quenching or carbonitridingquenching. However, it is
difEcult, as described above, to avoid deterioration of machinability or
deterioration of cold forgeability during net-shape forming. That is, it is not
possible to combine those mutually contradictory properties: bending and
surface fatigue strengths, and the machinability or cold forgeability, at a high
level.
[00281
The present invention has been made in view of the above described
circumstances and its object is to provide a method for producing a steel
component, particularly a method for producing a power transmitting
component made of steel such as a gear, a pulley, and a shaft, wherein the
method can reduce the contents of expensive alloying elements to suppress the
material cost, while allowing the steel component to have excellent high-cycle
bending fatigue strength, low-cycle bending fatigue strength, and pitting
resistant strength, and the method can ensure a sufficient machinability in the
cutting process or a sufficient cold forgeability in the net-shape forming process
when producing the component, and further can suppress the variation of heat
treatment distortion at the time of carburizing-quenching or cabonitriding
quenching.
MEANS FOR SOLVING THE PROBLEMS
[00291
The present inventors have continued investigations and research to solve
the above described problems. As a result, they have obtained the following
findinge (a) to (n).
[0030]
(a) To ensure excellent machinability during cutting or excellent cold
forgeability during net-shape forming, it is necessary that the microstructure
before cutting or net-shape forming, that is, the microstructure after hot forging,
or the microstructure after normalizing when the normalking treatment is
performed after hot forging, is a stable mixed structure of ferrite and pearlite.
When the microstructure is mixed with bainite, the hardness of the
microstructure increases and thereby cutting resistance increases, resulting in
deterioration of machinability, and also the deformation resistance increaees,
resulting in deterioration of cold forgeability.
[00311
(b) As a result of a crack being initiated from the outer layer of the
component, the low-cycle bending fatigue strength deteriorates. For that
reason, to increase the low-cycle bending fatigue strength, it is important to
perform two things; one is to suppress distortion which initiates a crack during
loading, and the other is to increase critical strength at which a crack is initiated
in the outer layer of the component (hereafter, referred to as "crack-initiation
strength"). To suppress distortion to initiate a crack during loading, it is
effective to increase the core hardness of the component (the hardness of the
base metal of the component).
100321
(c) To increase the core hardness of the component, it is effective to
increase the contents of Mo andlor Ni. However, when the contents of those
elements are too high, since bainite is likely to be mixed in the microstructure
after hot forging, or the microstnxcture after normalizing when normalizing
treatment is performed after hot forging, the machinability and the cold
forgeability deteriorate. For that reason, it is difficult to combine the low-cycle
bending fatigue strength and the machinability or the cold forgeability at a high
level. That is, it is important to increase the core hardness by other means.
loo331
(d) To increase the crack-initiation strength in the outer layer of the
component, first it is important to reduce intergranular oxidation depth. To
this end, although reducing the Si content is effective, that is not suEcient and
the ratio of the contents of Cr and Mn, that is, the range of "Cr/MnN needs to be
controlled.
Lo034
(el To increase the crack-initiation strength in the outer layer of the
component, it is also necessary to reduce impurities present at grain boundaries.
The "impurities present at grain boundaries" refer to, for example, P which
segregates at grain boundaries, cementite formed at grain boundaries during
carburizing-quenching or carbonitriding-quenching.
[0035]
(0 The carburizing-quenching or carbonitriding-quenching is generally
performed by using oil having a temperature of 100 to 150°C. However, when
oil of the aforementioned temperature is used, the component may be subjected
to self-tempering during quenching treatment so that film-like cementite is
produced at grain boundaries. Since the presence of such film-like cementite
causes deterioration of grain boundary strength, the low-cycle bending fatigue
strength of the component will not be improved. Furthermore, if the steel
having been subjected to self-tempering is tempered at a temperature of about
200°C, since P becomes likely to segregate at interface between cementite and
grain boundary, further deteriorating the grain boundary strength, the lowcycle
bending fatigue strength of the steel will not be improved. That is, to
increase the low-cycle bending fatigue strength, it is important that the steel is
rapidly cooled so as not to be subjected to self-tempering during carburizingquenching
or carbonitFiding-quenching. If the steel is allowed to rapidly cool
during carburking-quenching or carbonitriding-quenching, it is also possible to
increase the core hardness even without increasing the contents of Mo and Ni
so much.
[0036l
(g) As in the case of the low-cycle bending fatigue strength, the high-cycle
bending fatigue strength is improved by increasing the crack-initiation strength
in the outer layer of the component. Therefore, to increase the high-cycle
bending fatigue strength, it is necessary to reduce the intergranular oxidation
depth. That is, in addition to reducing the Si content as described above, it is
necessary to control the ratio of the contents of Cr and Mn, that is, the range of
"Cr/Mnn.
Lo0371
(h) It is also possible to improve the high-cycle bending fatigue strength
by increasing the outer layer hardness of the component. Further, increasing
the outer layer hardness of the component also leads to increasing pitting
resistant strength.
[00381
(i) To increase the outer layer hardness of the component, it is preferable
to reduce slack quenching structure in the vicinity of grain boundary oxides in
the outer layer of the component, which are generated at the time of carburizingquenching
or carbonitriding-quenching.
[00391
(j) The slack quenching structure, which is generated in the vicinity of the
grain boundary oxides in the outer layer portion of the component, is pearlite
and/or bainite. It is when the steel is slowly cooled around 500 to 600°C that
pearlite is produced during cooling of carburizing-quenching or carbonitriding
quenching. For that reason, it is possible to suppress the production of pearlite
by rapidly cooling the steel through the aforementioned temperature range.
[00401
(k) At the time of quenching, the component is cooled through each of the
stages of "h boiling heat transfer", "nucleate boiling heat transfer", and
"convection heat transfer". Among the aforementioned three stages, one
having highest cooling power is the stage of "nucleate boiling heat transfer".
For that reason, by making the stage of "nucleate boiling heat transfer" cover a
wide temperature range, it is possible to increase the cooling rate of the outer
layer portion of the component.
[00411
(1) To increase the temperature range of the stage of "nucleate boiling heat
transfer", it is important to lower the starting temperature of the "convection
heat transfer" stage, and to this end, it is necessary to lower the kinetic viscosity
of the quenching oil. Since lowering the starting temperature of the
"convection heat transfer" stage makes it possible to increase the cooling rate
around the Ms point which is the temperature at which rnartensitic
transformation starts, it is possible to suppress the production of bainite.
LO0421
(m) Concerned issues when the kinetic viscosity of the quenching oil is
changed are the amount of variatzon of heat treatment distortion cawed by
quenching. However, as an example is shown in an Example to be described
below, even when quenching oil having a low kinetic viscosity and a low starting
temperature of the "convection heat transfer" stage is used, the amount of
variation of heat treatment distortion caused by quenching is about the same
level as when conventional quenching oil which is to be used at a temperature
of about 100 to 160°C and which has a high kinetic viscosity is used. Further,
when quenching oil which has a low kinetic viscosity and a low starting
temperature of the "convection heat transfer" stage is used, even if Mo is not
contained so much and Ni is not contained in the steel, it is possible to suppress
the production of slack quenching structure in the vicinity of grain boundary
oxides in the outer layer portion of the component.
LO0431
(n) In addition to controlling the specific range of the content of each
constituent element of the steel, by controlling the ratio of the contents of Cr
and Mn as described in term (dl, that is, the range of "Cr/MnMto be in a specific
range, it is possible to simultaneously achieve the following three items:
111 suppressing the material cost,
[21 allowing the steel component to have excellent high-cycle bending
fatigue strength, low-cycle bending fatigue strength, and pitting resistant
strength, and
[31 ensuring sufficient machinability in the cutting process or suiKcient
cold forgeability in net-shape forming process, when producing the component.
roo441
The present invention has been completed based on the above described
findings, and its gist lies in the methods for producing a steel component shown
below.
roo461
(1) A method for producing a steel component by performing the
treatments of the following sequentially steps 1 and 2 to a steel material
containing, by mass%,
C: 0.16 to 0.26%,
Si: 0.01 to 0.10%,
Mn: 0.60 to 0.80%,
S: 0.003 to 0.030%,
Cr: 0.80 to 1.20%,
Mo: 0.30 to 0.46%,
Al: 0.016 to 0.060%, and
N: 0.010 to 0.026%, wherein
fn represented by Formula (1) shown below is 1.3 to 2.4, the balance being
Fe and impurities, and contents of P and 0 among the impurities are
P: not more than 0.010%, and
0: not more than 0.0020%:
step 1: a treatment which holds the steel material at a temperature of 850
to 1000°C in a carburizing atmosphere or carbonitriding atmosphere, and
step 2: a treatment which quenches the carburized or carbonitrided steel
material, by using quenching oil having a temperature of 40 to 80°C and a
kinetic viscosity of 20 to 25 mmz/s at 40°C;
fn = CrfMn ... (3.)
where, Cr and Mn in Formula (1) represent the content of each element
in mass%.
lo0461
(2) The method for producing a steel component according to the above (11,
wherein
the step 1 includes a holding process at a temperature of 860 to 1000°C in
a carburizing atmosphere or carbonitriding atmosphere followed by a holding
process at a temperature of 800 to 900°C in a carburizing atmosphere or
carbonitriding atmosphere.
100471
(3) The method for producing a steel component according to the above (1)
or (21, wherein
the steel material contains, in mass%, Nb: not more than 0.08% in lieu of
a part of Fe.
Lo0481
It is noted that the "impurities" in the "Fe and impurities" as the balance
refer to those which are mixed &om ores and scraps as the raw material or from
production environments when the steel material is industrially produced.
[00491
The holding process at a temperature of 800 to 900°C in a carburizing
atmosphere or carbonitriding atmosphere following the holding process at a
temperature of 860 to 1000°C in a carburizing atmosphere or carbonitriding
atmosphere refers to a so-called "heating treatment for quenching".
ADVANTAGEOUS EFFECT@) OF THE INVENTION
[00601
According to the present invention, it is possible to obtain a steel
component which has excellent high-cycle bending fatigue strength, low-cycle
bending fatigue strength, and pitting resistant strength despite small contents
of expensive alloying elements, and furthermore can ensure sufficient
machinability in the cutting process or sufficient cold forgeability in the netshape
forming process at the time of production, and in which the variation of
heat treatment distortion at the time of carburizing-quenching or
carbonitridingquenching is suppressed.
BRtIEF DESCRIPTION OF THE DRAWING@)
lo06 11
Figure 1 is a diagram showing the shape of a specimen for measuring the
amount of variation of heat treatment distortion, which is used in Examples.
Where, the unit of the dimensions in the figure is "mm".
Figure 2 is a diagram to illustrate the positions where the specimens for
measuring the amount of variation of heat treatment distortion in Figure 1 are
disposed in a basket in carburizing-quenching performed in Examples.
Numerals "l", "2", and "3" represent "left-upper-rear", "centert1, and "rightlower-
hntl' positions, respectively.
Figure 3 is a diagram showkg a heat pattern of "carburizing-quenching
and temperingn which is applied to various test specimens used in Examples.
In the figure, 119300C"",8 30°C", and "180°C" refer to "carburking temperature1',
"heating temperature for quenching1', and "tempering temperaturen,
respectively. "Cpn represents "carbon potential". "OQ" represents "oil
quenching". Atmospheric cooling is exploited EDr the cooling after tempering,
and is designated as "AC" in the figure.
Figure 4 is a diagram showing the shape of an Ono-type rotating bending
fatigue test specimen used for evaluating high-cycle bending fatigue strength in
Examples. Where, the unit of the dimensions in the figure is "mrn".
Figure 6 is a diagram showing the shape of a 4-point bending fatigue test
specimen which is used for evaluating the low-cycle bending fatigue strength in
Examples. The unit of the dimensions in the figure is "mm".
Figure 6 is a diagram showing the shape of a roller-pitting small roller
specimen used in Examples.
Figure 7 is a diagram showing the shape of a roller-pitting large roller
specimen used in Examples.
Figure 8 is a diagram to illustrate "4-point bending fatigue test" for
evaluating the low-cycle bending fatigue strength conducted in Examples.
Figure 9 is a diagram to illustrate a roller pitting test conducted in
Examples.
MODE FOR CARRYING OUT THE INVENTION
f00621
Hereafter, each requirement of the present invention will be described in
detail. It is noted that "Om" of the content of each element means "mass%".
[00631
(A) Chemical composition of steel material:
C: 0.15 to 0.26%
C (carbon) ie an essential element to ensure the core strength (the
strength of the base metal) of a steel component subjected to carburizingquenching
or carbonitriding quenching, and its content needs to be not less than
0.16%. However, if the content of C increases to exceed 0.26%, increase in the
amount of deformation (heat treatment distortion) of the component when
subjected to carburizing-quenching or carbonitriding quenching becomes
pronounced. Therefore, the content of C is specified to be 0.15 to 0.25%. The
lower limit of the content of C is preferably 0.16%, and the upper limit thereof
is preferably 0.23%.
[0054]
Si: 0.01 to 0.10%
Si (silicon) increases the intergranular oxidation depth when performing
carburizing treatment or carbonitriding treatment. Particularly, if its content
exceeds 0.10%, the intergranular oxidation depth significantly increases and
thereby the bending fatigue strength si@cantly deteriorates, making it
impossible to achieve the object of the present invention. However, at the time
of mass production, it is diEicult to control the content of Si to be less than 0.01%.
Therefore, the content of Si is specified to be 0.01 to 0.10%. The upper limit of
the content of Si is preferably 0.09%. It is noted that considering the
production cost at the time of mass production, the lower limit of Si content is
preferably 0.03%.
[0066]
Mn: 0.50 to 0.80%
Mn (manganese), which has a signiscant effect of increasing the
hardenability, is an essential element to ensure the core strength of the
component when subjected to carburizingquenching or carbonitriding
quenching, and its content needs to.be not less than 0.60%. However, if the
content of Mn increases to exceed 0.80%, not only its effect is saturated, but also
machinability in cutting process as well as cold fbrgeability during net-shape
forming remarkably deteriorates. Therefore, the content of Mn is specdied to
be 0.60 to 0.80%. The lower limit of the content of Mn is preferably 0.55%, and
the upper limit thereof is preferably 0.75%.
roo661
S: 0.003 to 0.030%
S (s*) combines with M. to form MnS, thereby improving
machinability. However, when its content is less than 0.003%, the
aforementioned effect is not likely to be achieved. On the other hand, when the
content of S increases, coarse MnS tends to be produced thereby deteriorating
low-cycle bending fatigue strength, and particularly, if the content of S exceeds
0.030%, the deterioration of the low-cycle bending fatigue strength becomes
pronounced. Therefore, the content of S is specified to be 0.003 to 0.030%.
The lower limit of the content of S is preferably 0.005%, and the upper limit
thereof is preferably 0.020%.
I00671
Cr: 0.80 to 1.20%
Cr (chromium) ia an element which has a significant effect of increasing
the hardenability and the tempering softening resistance and is effective to
improve the bending fatigue strength and the pitting resistant strength.
However, when the content of Cr is less than 0.80%, the above described effect
is not sacient, and it is not possible to achieve excellent bending fatigue
strength and pitting resistant strength, which are an object of the present
invention. On the other hand, if the content of Cr exceeds 1.20%, bainite
becomes to be easily produced in the microstructure before cutting or the
microstructure before net-shape fbrming, that is, the microstructure after hot
forging, or the microstructure after normalizing when normalizing treatment is
performed after hot forging. For this reason, increase in the material hardness
lea& to increase in the cutting resistance, thereby deteriorating machinability,
and ale0 deformation resistance increases, thereby deteriorating cold
forgeability. Therefore, the content of Cr is specified to be 0.80 to 1.20%. The
lower limit of the content of Cr is preferably 0.90%, and the upper limit thereof
is preferably 1.10%.
[0068]
Mo: 0.30 to 0.46%
Mo (molybdenum) is an element which has a significant effect of
increasing the hardenability and the tempering softening resistance and is
effective to improve the high-cycle bending fatigue strength, the low-cycle
bending fatigue strength, and the pitting resistant strength. However, when
the content of Mo is less than 0.30%, since the production of pearlite in the
vicinity of grain boundary oxides cannot be suppressed, it is not possible to
obtain excellent high-cycle bending fatigue strength and pitting resistant
strength which are an object of the present invention. On the other hand, when
the content of Mo exceeds 0.46%, bainite becomes to be easily produced in the
microstructure before cutting or net-shape forming, that is, the microstructure
after hot forging, or the microstructure after normalizing when normalizing
treatment is performed a h r hot forging. For that reason, increase in the
material hardness leads to increase in cutting resistance, thereby deteriorating
machinability, and also leads to deformation resistance, thereby deteriorating
cold forgeability. Therefore, the content of Mo is specified to be 0.30 to 0.46%.
The lower limit of the content of Mo is preferably 0.31%, and the upper limit
thereof is preferably 0.40%.
[00691
Al: 0.016 to 0.060%
Al (aluminum) has a deoxidizing action. Further, Al is an element which
combines with N to form AIN and is effective in preventing the coarsening of
austenite grains during carburizing or carbonitriding. However, when the
content of Al is less than 0.016%, it is not possible to stably prevent the
coarsening of austenite grains, and when the coarsening occurs, the high-cycle
, bending fatigue strength and the low-cycle bending fatigue strength will be
deteriorated. On the other hand, if the content ofAl exceeds 0.060%, it becomes
easy to form coarse oxides, thereby deteriorating the bending fatigue strength.
For that reason, the content of Al is specified to be 0.016 to 0.060%. The lower
limit of the content of Al is preferably 0.016%, and the upper limit thereof is
preferably 0.040%.
I00601
N: 0.010 to 0.026%
N (nitrogen) combines with A1 to form AlN, and also combines with Nb to
form NbN. The above AlN and NbN have an effect of preventing the coarsening
of austenite grains during carburizing or carbonitridmg. However, when the
content of N is less than 0.010%, since the formation of the nitrides becomes
unstable, it is not possible to stably prevent the coarsening of austenite grains.
On the other hand, when the content of N exceeds 0.026%, stable production
becomes m c u l t in mass production in the steel making process. For that
reason, the content of N is specified to be 0.010 to 0.026%. The lower limit of
the content of N is preferably 0.011%, and the upper limit thereof is preferably
0.022%.
1006 11
fn: 1.3 to 2.4
The steel material used in the production method of the present invention
must have a fn of 1.3 to 2.4, where fn is represented as:
h = Cr/Mn ... (1)
where, C and Mn in Formula (1) mean the content of each element in
mass%.
[0062l
To reduce the intergranular oxidation depth, although it is effective as
described above to reduce the content of Si, that is not sufficient, and it becomes
possible to utilize such effect by controlling fn. which is the ratio between the
contents of Cr and Mn, to be within a specilic range. When fn is less than 1.3,
since the intergranular oxidation caused by Mn becomes pronounced, it is not
possible to suBiciently achieve the effect of reducing the intergranular oxidation
depth by reducing the content of Si. On the other hand, even if fn increases to
more than 2.4, the above described effect of decreasing the intergranular
oxidation depth will be saturated.
[0063]
The lower limit of fh is preferably 1.4 within the range of the contents of
Cr and Mn described so far, and the upper limit thereof is preferably 2.2.
lo0641
One chemical composition of the steel material to be used in a production
method of the present invention contains the above described elements, with the
balance being Fe and impurities, in which the contents of P and 0 are P: not
more than 0.010% and 0: not more than 0.0020%.
100661
It is noted that the "impurities" in the "Fe and impurities" as the balance
refer to for example P which is mixed from ores as the raw material, for example
Cu and Ni which are mixed &om scraps, or for example 0 (oxygen) which is
mixed h m the production environments when the steel material is industrially
produced.
[00661
However, it is necessary to limit P and 0 in the impurities in a
particularly strict manner, which will be described below.
100671
P: not more than 0.010%
Since P (phosphorus) is an element which ia likely to segregate at grain
boundaries thereby embrittling the grain boundaries, when the content thereof
exceeds 0.0 lo%, it deteriorates the low-cycle bending fatigue strength.
Therefore, the content of P in the impurities is spesed to be not more than
0.010%. The content of P in the impurities is preferably not more than 0.008%.
[0068l
0: not more than 0.0020%
Since 0 (oxygen) is likely to combine with Al to form hard oxide-base
inclusions, thereby deteriorating the bending fatigue strength. Particularly,
when the content of 0 exceeds 0.0020%, the deterioration of the bending fatigue
strength becomes pronounced. Therefore, the content of 0 in the impurities is
specified to be not more than 0.0020%. It is noted that although the content of
0 in the impurities is preferably as low as possible, considering the cost in the
steel making process, the lower limit thereof will be around 0.0010%.
[0069]
Another chemical composition of the steel material to be used in a
production method of the present invention contains Nb in lieu of part of Fe
described above. Hereafter, advantageous effects of Nb which is an optional
element and the reason to limit the content thereof will be described.
[OO~O]
Nb: not more than 0.08%
Nb (niobium), which is likely to combine with C and N to form NbC, NbN,
and Nb(C, N), is an element which is effective to supplement the above described
effect of AN to prevent the coarsening of austenite grains during carburizing or
carbonitriding. For that reason, Nb may be contained. However, when the
content of Nb increases to exceed 0.08%, the effect of preventing the coarsening
of austenite graina will rather deteriorate. Therefore, the upper limit of the
amount of Nb, when it is contained, is speczed to be not more than 0.08%. The
amount of Nb, when it is contained, is preferably not more than 0.06%.
roo711 r
On the other hand, to stably obtain the above described effect of Nb, the
content of Nb is preferably not less than 0.01%, and ia further preferably not
less than 0.02%.
100721
(B) Production condition of steel component:
A method for producing a steel component according to the present
invention is characterized by performing the treatments of the following
sequentially steps 1 and 2 to the steel material having the chemical composition
according to term (A) described above:
100731
Step 1: a treatment which holds the steel material at a temperature of 860
to 1000°C in a carburizing atmosphere or carbonitriding atmosphere.
lo0741
Step 2: a treatment which quenches the carburized or carbonitrided steel
material, by using quenching oil having a temperature of 40 to 80°C and a
kinetic viscosity of 20 to 26 mrn2ts at 40°C.
LO0761
The steel component relating to the present invention is specifically
produced by, for example, the following process.
LO0761
First, a steel satisfying the chemical composition according to term (A) is
melted and is subjected to casting, billeting, and so on, and thereafter the steel
is subjected to hot rolling as the ihal process to produce a hot rolled steel bar or
wire rod.
[00771
Next, the hot rolled steel bar or wire rod is hot forged to be roughly formed
into an intermediate product having a predetermined shape.
lo0781
The roughly formed intermediate product, or an intermediate product
which has been subjected to normalizing treatment after the rough forming is
subjected to cutting to be formed into a predetermined component shape.
Alternatively, the roughly formed intermediate product is subjected to
net-shape forming by cold forging, in place of the cutting, after being subjected
to normalizing treatment as needed.
[00791
Carburizing-quenching or carbonitriding-quenching is performed on the
intermediate product after the cutting or the net-shape forming in the order of
step 1 and step 2.
roo801
Tempering may be conducted after the carburizing-quenching or
carbonitriding-quenching is performed.
[00811
Moreover, shotpeening andlor s h c e grinding may be conducted after
the carburizingquenching or carbonitriding-quenching is performed.
100821
Shotpeening and/or surface grinding may be performed after further
conducting tempering after the carburizing-quenching or carbonitridingquenching.
[00831
It is noted that there is no need of particularly limiting conditions of the
processes h m the melting of steel till the production of the hot rolled steel bar
or wire rod.
100841
Provided that the hot rolled steel bar or wire rod satisfies the chemical
composition according to term 0, the hot rolled steel bar or wire rod may be
produced at commonly used conditions for all the processes such as the melting,
casting, billeting, and hot rolling of the steel.
[0086]
Moreover, there is also no need of particularly limiting the condition of
the process for obtaining an intermediate product which has been roughly
formed by hot forging a hot rolled steel bar or wire rod.
[0086l
The material to be hot forged (the hot rolled steel bar or wire rod) may be
roughly fbrmed into an intermediate product having a predetermined shape at
commody used conditions for all the processes such as the temperature to heat
it, the reduction rate of forging, the finishing temperature of forging, the cooling
condition after forging, and the like.
100871
Further, there is also no need of particularly limiting the condition at
which normalizing treatment is performed on the roughly formed intermediate
product, and the normalizing treatment may be performed in a commonly used
manner.
[00881
There is also no need of particularly limiting the condition at which the
intermediate product which has been roughly formed, or the intermediate
product which has been further subjected to normalizing treatment after being
roughly formed, is cut into a predetermined component shape, and the cutting
may be performed in a commonly used manner.
[00891
Moreover, there is also no need of particularly limiting the condition at
which the intermediate product which has been roughly formed, or the
intermediate product which has been further subjected to normalizing
treatment after being roughly formed, is subjected to cold fbrging to be net-shape
formed into a predetermined component shape, and the net-shape forming may
be performed in a commonly used manner.
[00901
It is necessary that the treatments of step 1 and step 2 be sequentially
performed on the intermediate product which has been subjected to cutting, or
to net-shape forming.
[00911
It is noted that the step 1 may include a holding process at a temperature
of 860 to 1000°C in a carburizing atmosphere or carhonitriding atmosphere
followed by a holding process at a temperature of 800 to 900°C in a carburiring
atmosphere or carbonitriding atmosphere.
[00921
After the carburizing-quenching or carbonitridingquenching is
performed, tempering may be further performed, as needed, at a temperature of
100 to 200°C. Provided the temperature is 100 to 200°C, there is no need of
particularly limiting other conditions fbr the tempering, and tempering may be
performed in a commonly used manner.
[00931
Further, shotpeening andlor surface grinding may be performed after the
carburizing-quenching or cmbonitriding-quenching is performed, or the
tempering is pedormed.
[00941
There is no need of particularly limiting the condition of the shotpeening,
and the shotpeening may be performed in a commonly used manner. Similarly,
there is no need of particularly limiting the condition of surface grinding, and
the surface grinding may be performed in a commonly used manner.
[00961
Hereafter, the conditions for the carburizing-quenching or carbonitriding
quenching of steps 1 and 2, which are to be sequentially performed on the
intermediate product which has been subjected to cutting or net-shape forming
will be described in detail.
[0096]
When performing carburizing or carbonitriding, holding the intermediate
product in a carburizing atmosphere or carbonitriding atmosphere having a
temperature of more than 1000°C is likely to cause coarsening of crystal grains
leading to deterioration of the strength after carburizing-quenching or
carbonitriding-quenching. On the other hand, holding the intermediate
product in a carburizing atmosphere or carbonitriding atmosphere having a
temperature of less than 860°C makes it difficult to obtain a sufficient case
depth after the carburizing-quenching or carboaitriding-quenching. Therefore,
it is determined that, h t , the treatment of step 1, that i*, carburizing or
carbonitriding is performed on the intermediate product while being held in a
carbuFizing atmosphere or carbonitriding atmosphere having a temperature of
860 to 1000°C. The lower Limit of the holding temperature is preferably 900°C,
and the upper limit thereof is preferably 980°C.
[00971
Although the holding time at the temperature of the carburizing
atmosphere or carbonitriding atmosphere depends on the required case depth,
it may be, for example, about 2 to 16 hours.
[00981
There is no need of particularly limiting the carbon potential in the
carburizing atmoephere, and it may be appropriately determined from
viewpoints of a targeted surface carbon concentration, an effective case depth,
efficient operation and the like.
[00991
For the "carburizing", "gas carburizing" may be applied in which
carburizing is performed by using an atmosphere obtained by adding gas which
is referred to "enriching gas", such as butane and propane, to so-called "RX gas"
which is endothermic and is a gaseous mixture of CO, Ha, and N2 obtained by
mixing and modifying hydrocarbon gas such as butane and propane with air.
In this case, the carbon potential can be controlled in most cases by the amount
of addition of enriching gas.
[01001
Similarly, there is also no need of particularly limiting the carbon
potential and nitrogen potential in the carbonitriding atmosphere, and they may
be appropriately determined from viewpoints of a targeted surface carbon
concentration, a surface nitrogen concentration, an effective case depth, efficient
operation, and the like.
[0101]
For "carbonitriding", "gas carbonitriding" can be applied in which
carbonitriding is performed by using an atmosphere which is obtained by adding
ammonia to t.he carburizing gas described above. In this case, the carbon
potential and nitrogen potential can be controlled by the adding amount of the
enriching gas and the ammonia gas, respectively.
[01021
Moreover, performing the holding process in a carburizing atmosphere or
carbonitriding atmosphere having a temperature of 800 to 900°C ae the heating
treatment for quenching, following the holding process in a carburizing
atmosphere or carbonitriding atmosphere having a temperature of 850 to
1000°C makes it possible to perform stable carburizing or carbonitriding at a
reduced amount of heat treatment .distortion. The lower limit of the above
described heat treatment temperature for quenching is preferably 830°C, and
the upper limit thereof is preferably 88O0C.
r01031
When performing the above described heating treatment for quenching,
the holding time in the carburizing atmosphere or carbonitriding atmosphere
having a temperature of 800 to 900°C may be, for example, around 0.5 to 2 hours.
10 1041
There is no need of particularly limiting the carbon potential in the
carburizing atmosphere when performing heating treatment for quenching, and
it may be appropriately determined from viewpoints of a targeted surface carbon
concentration, an effective case depth, and efficient operation, etc.
[0106]
Similarly, there is also no need of particularly limiting the carbon
potential and the nitrogen potential in the carbonitriding atmosphere when
performing heating treatment for quenchmg, and they may be appropriately
determined from viewpoints of a targeted surface carbon concentration, an
effective case depth, and efficient operation, etc.
[0106]
Next, the reason why quenching of the intermediate product which has
been subjected to carburizing or carbonitriding is performed by using quenching
oil having a temperature of 40 to 80°C and a kinetic viscosity of 20 to 26 mm2Ie
at 40°C is described.
[01071
As already described above, when carburizing-quenching or
carbonitriding-quenching is performed using oil having a temperature of around
100 to 160°C, the intermediate product may be subjected to sewtempering
during quenching treatment so that film-like cementite is produced at grain
boundaries. For this reason, the low-cycle bending fatigue strength will not be
improved. Therefore, it is necessary that the intermediate product is rapidly
cooled so as not to be subjected to self-tempering during carburizing-quenching
or carbonitriding-quenching. Accordingly, although it is necessary to use a
quenching oil of low temperature, in the case of a quenching oil having a
temperature of less than 40°C, the kinetic viscosity becomes excessively large,
and the flowability of the quenching oil at the time of quenching decreases,
resulting in a problem that the convective stirring effect of oil decreases and
difference in cooling power occurs between upper and lower positions of the oil
bath, thua leading to increase in the variation of heat treatment distortion. On
the other hand, in the case of quenching oil having a temperature of more than
80°C, it is not possible to stably suppress the film-like cementite from being
produced at grain boundaries due to self-tempering. Further, in this case,
since the degradation of the additive to the quenching oil is likely to proceed,
the kinetic viscosity of the oil is likely to increase as it is repeatedly used,
resulting in increase in the variation of heat treatment distortion, and also, since
the volatilization amount of quenching oil increases, the frequency to replace
the quenching oil increases.
[oioal
Therefore, when performing carburizing-quenching or carbonitriding
quenching, it is necessary to use quenching oil having a temperature of 40 to
80°C. The upper limit of the temperature of the quenching oil is preferably
60°C.
r01091
By using quenching oil having a low kinetic viscosity upon quenching, it
becomes possible to lower the starting temperature of the "convection heat
transfer" stage and thus expand the temperature range of the "nucleate boiling
heat transfer" stage, thereby increasing the hardness of the outer layer of the
component. When the kinetic viscosity at 40°C is not more than 26 mm2/s, the
above described effect can be stably achieved. Furthermore, since decreasing
the kinetic viscosity at 40°C causes the convection of the oil to be more likely to
occur even .in the stage of "convection heat tranefer", it is inferred that the
cooling rate in the vicinity of the Ma point wiU become faster.
[01101
On the other hand, when the kinetic viscosity at 40°C is less than 20
mm2/s, since the oil becomes likely to be degraded due to the repetition of
quenching, it becomes necessary to frequently replace the oil, thereby increasing
costs. Therefore, the kinetic viscosity of quenching oil is speaed to be 20 to
26 rnrnzls at 40°C.
roll11
It is noted that in the case of conventional quenching oil which has been
used at a temperature of around 100 to 150°C, the kinetic viscosity is as large
as not less than 100 mm2ls at a temperature as low as 40°C. Since, for that
reason, the flowability is low at the oil temperature of 40 to 80°C and the
convective stirring effect of oil ia small, difference in cooling power occurs
between upper and lower positions of the oil bath, thus leading to increase in
the variation of heat treatment distortion.
Lo1 121
The lower limit of the kinetic viscosity of the quenching oil at 40°C is
preferably 21 mm2/s, and the upper limit thereof is preferably 24'mm2/s.
101131
Hereafter, although the present invention will be described in more detail
by way of Examples, the present invention will not be limited to those Examples.
EXAMPLE@)
[0114l
(Example 1)
Each of Steels "a" to "en having chemical compositions shown in Table 1
was melted in an amount of 180 kg in a vacuum furnace, and thereafter was
cast into a mold having a diameter of 210 mm to obtain an ingot, the ingot
thereafter being cooled to the room temperature.
[01161
It is noted that Steel "d" in Table 1 was a steel which had a chemical
composition within the range specsed in the present invention.
[01161
On the other hand, Steels "a" to "c" and Steel "e" are steels of Comparative
Examples whose chemical compositions deviated from the condition speczed in
the present invention. Among those, Steel "e" was a high-Ni and high-Mo steel.
[01171
Table 1
S h l
a
Chemical composition Lase%) Balance: Fe and impurities
C
0.18
Si
0.33
Mn P
*1.40
S
*0.011 0.018
Ni 1 Cr I Mo
- 1 *0.66 1 -
A1
0.033
N I O fh
0.0122 ( 0.0015 *0.5
[01181
Each ingot of Steels "a" to "el' described above was subjected to hot forging
after being held at 1250°C for 120 min to be f i s h e d into a steel bar having a
diameter of 70 mm at a temperature of not less than 1000°C, and thereafter was
allowed to cool to the room temperature in the atmosphere.
roll91
Each steel bar described above was subjected to "normalizing", in which
it was held at 926OC for 60 min and thereafter allowed to cool to the room
temperature in the atmosphere.
[01201
After the normalizing, 60 specimens for measuring the amount of
variation of heat treatment distortion, which had the shape shown in Figure 1,
b
c
d
e
were fabricated h m the central portion of steel bar for each steel. It i~ noted
h=cMh
nw 4 indicates deviation h m the condition a p e a d in the preeent invention.
0.20
0.18
0.19
0.20
0.38
0.28
0.06
0.06
that specimens were taken in each steel such that the axial direction of the steel
bar coincided with the axial direction of specimen.
[01211
By using those specimens, the amount of variation of quenching distortion
*0.81
0.79
0.60
0.60
at the time of carburizing-quenching was evaluated.
r01221
To be specific, for each steel, five specimens, for which the spacing of A
*0.015
*0.016
0.007
0.010
portion in Figure 1 was measured by a micrometer in advance before
carburizing-quenching, were disposed in each of three positions: left-upper-rear,
center, and right-lower-front positions, of a heat treatment basket as shown in
Figure 2, and were subjected to carburizing-quenching by a batch type gas
0.018
0.018
0.012
0.010
-
- -
*0.60
1.11
1.10
0.90
*0,68
*-
*0.18
0.30
*0.66
0.028
0.030
0.028
0.026
0.0144
0.0160
0.0144
0.0161
0.0010
0.0010
0.0018
*0.0021
1.4
1.4
1.6
1.3
carburizing furnace. After quenching, tempering was performed. It is noted
that numerals "ln, 1121a1n, d 1t31i1n Figure 2 represent the above described 'leftupper-
reart1, ltcenterO, and "right-lower-front" positions of disposition in the
baeket, respectively.
101231
Figure 3 shows a heat pattern of "carburizing-quenching and temperi&I.
It is noted that "Cp" represents "carbon potential1' in Figure 3. Moreover, "OQ
represents "oil quenching", which was performed by using oils of conditions [il
..
to bvl shown in Table 2. Atmospheric cooling was exploited for the cooling after
tempering, and is designated as "AC" in the figure.
[0124]
Table 2
[01261
After quenching was performed by using oil of each condition and
subsequently tempering was performed, the spacing of A portion was measured
by a micrometer to determine the difference in the spacing of A portion between
before and after "carburizing-quenching and tempering". It was assumed that
the amount of distortion be the average value of the differences in the spacing
of A portion for each five (a total of 16) specimens which were disposed in "1" to
"3" positions in the heat treatment basket. Moreover, the standard deviation
of the difference in the spacing of A portion for the 15 specimens described above
was calculated to evaluate the variation of quenching distortion.
Condition of
quenching oil
hl
lid
[iii]
hd 1 *28 60
Xinetic viscosity at 40°C
( m d s )
23
*300
23
"*" mark indicates deviation from the condition specified in the
present invention.
Oil temperature
(OC)
60
*I20
*I20
[01261
Moreover, one specimen was arbitrarily selected for each condition out of
the specimens for which the spacing of portion A was measured by a micrometer
after " c a r b q u e g and tempering", and a longitudinal section of B
portion in Figure 1 of the specimen was mirror-ground to thereafter observe an
outer layer portion on the outer peripheral side of the section of B portion at a
magnification of 1000 times by using an optical microscope; and the section was
f'urther etched with Nital to observe the outer layer portion at a magnEcation
of 3000 times by using a scanning electron microscope (hereafter, referred to as
'ISEM").
[O 1271
In the observation at a magnihcation of 1000 time by using the optical
microscope, an inter granular oxidation depth was measured. Moreover, in the
observation at a magnification of 3000 times, the presence or absence of slack
quenching structure in &he microstructure particularly in the vicinity of grain
boundary oxides was investigated.
Lo1281
Table 3 organizes and shows the test results described above. It is noted
that in Table 3, the "amount of distortion" refers to the average value of the
differences in the spacing of "A" portion for each five specimens of 1 to 3 positions,
that is, 16 specimens in total. Moreover, the "intergranular oxidation depth"
refers to a maximum intergranular oxidation depth among those observed in B
portion.
[01291
Table 3
[01301
From Table 3, it is clear that although there was no recognizable
difference in the variation of quenching distortion at A portion of specimen
among conditions [il to bvl of quenching oil, there are some differences in the
observation results of B portion of specimen by using SEM.
[0131]
Ted
number
1
2
3
4
6
6
7
8
9
10
11
12
13
14
16
16
17
18
19
20
First, regarding slack quenching structure, the results were as follows.
[O 1321
In Steel "e" having high contents of Ni and Mo, no slack quenching
structure was recognized in conditions [il, hi], and hv] of quenching oil.
Moreover, in Steel "d", slack quenching structure was not recognized only in the
case of condition [i] of quenching oil. On the other hand, in Steels "a" to "c",
"*" mark indicate8 deviation h m the condition epecified in the present invention.
I
*a
fb
*c
d
*e
*a
*b
*c
d
*e
*a
*b
*c
d
*e
*a
*b
*c
d
*e
Conditionof
quenchingd
W
[ivl
Amomtof
*n~nm)
0.41
0.43
0.43
0.43
0.47
0.33
0.36
0.36
0.36
0.41
0.34
0.36
0.36
0.38
0.39
0.41
0.42
0.42
0.43
0.46
Variation of
quenching
bdard
deviation)
0.010
0.012
0.012
0.011
0.012
0.010
0.011
0.013
0.011
0.012
0.012
0.012
0.013
0.012
0.014
0.012
0.012
0.014
0.013
0.014
midatiDndaptb
17.8
11.1
11.7
4.7
16.3
18.2
11.8
12.6
6.1
16.2
18.0
11.6
12.2
4.9
16.8
18.6
12.2
12.2
6.5
16.8
Presence or
abeence of slack
etructure
Present
Present
Present
Abeent
Absent
Preeent
Preeent
Reeent
Preeent
Preeent
Present
Preeent
Preaent
Present
Absent
Prasent
Preeent
Preeent
Preeent
Absent
some slack quenching stzucture was recognized in all of conditions bl to Evl of
quenching oil.
lo1331
Next, regarding the intergranular oxidation depth, it was as small as 4.7
to 6.6 pm regardless the condition of quenching oil in the case of Steel "d" whose
chemical composition was within the range specified in the present invention.
In contrast to this, in the case of Steels "a" to "c" and Steel "en whose chemical
compositions deviated fiom the condition specified in the present invention, the
intergranular oxidation depth was as large aa not less than 11.1 pm, and
therefore wae worse than in the case of Steel "d".
[0134]
That is, in the case of Steels "a" to "c" whose chemical compositions
deviated from the condition apedied in the present invention, even in the case
of [il in which the quenching oil satisfied the condition specified in the present
invention, it was not possible to suppress slack quenching structure, and further
the intergranular oxidation depth was large.
[01361
In contrast to this, in the case of Steel "d" whose chemical composition was
within the range specified in the present invention, by performing quenching by
using oil li] which satisfied the condition specified in the present invention, it
was possible to suppress the production of slack quenching structure as in Steel
"en which was a high-Ni and high-Mo steel, and further to keep the variation of
heat treatment distortion at the same level as in conventional art.
Furthermore, in the case of Steel "e", the intergranular oxidation depth was
large, and therefore worse than in the case of Steel "dm.
lo1361
(Example 2)
Each of Steels 1 to 31 having chemical compositions shown in Tables 4
and 6 was melted in an amount of 180 kg in a vacuum furnace, and thereafter
was cast into a mold having a diameter of 210 mm to obtain an ingot, the ingot
thereafter being cooled to the room temperature.
r01371
It is noted that Steels 1 to 13 in Table 4 were steels each of which had a
chemical composition within the range specified in the present invention.
[0138]
On the other hand, Steels 14 to 31 in Tables 4 and 6 were steels of
Comparative Examples whose chemical compositions deviated from the
condition apedied in the present invention. Among those steeh of
Comparative Examples, Steel 16 was a steel corresponding to SCr420 speciiied
in JIS G 4063 (2008).

[01411
Each ingot of Steele 1 to 31 described above was subjected to hot forging
after being held at 1260°C for 120 m . to be finiahed into a steel bar having a
diameter of 36 mm at a temperature of not less than 1000°C, and thereafter was
allowed to cool to the mom temperature in the atmosphere.
CO 1421
Each of the above described steel bars was subjected to 'lnormalizingl', in
which it was held at 926OC for 60 min and thereafter allowed to cool to the room
temperature in the atmosphere.
[O 1431
From a central portion of each of thus obtained steel bars having a
diameter of 35 mm after normalization, round bars were cut out which
respectively had diameters of 10 mm, 20 mm, and 30 mm, and each had a length
of 100 mm.
[0144]
The round bars which were cut out to respectively have diameters of 10
mm, 20 mm, and 30 mm, and each has a length of 100 mm were subjected to
"normalizing", in which they were held at 926OC for 60 min and thereafter
allowed to cool to the room temperature in the atmosphere. Moreover, during
this cooling, the cooling rate from 800°C to 600aC was 0.8 to 1.2°Clsec.
101461
A cross section at a position of a length of 60 mm of each round bar after
normalization, which had a diameter of 10 mm, 20 mm, or 30 mm, was mirrorground,
and Vickers hardness was measured at four points at positions 2.6 mm,
6 mm, and 7.6 mm &om the center by a Vickers hardness meter according to
"Vickers hardness test - test method" described in JIS Z 2244 (2009). Where,
the test force was 9.8N.
101461
It waa assumed that an average value of Vickers hardness (hereafter,
referred to I1HV1) of each four points described above be the HV after
normalization in the round bar having reepective diameters. Furthermore, in
each of the cases where the diameter was 10 mm, 20 mm, and 30 mm, when HV
was not more than 192, evaluation was made that the hardness was low, and
the machinability and cold forgeability were excellent.
[O 1471
Moreover, fir each steel, 16 Ono-type rotating bending fatigue test
specimens having the shape shown in Figure 4, four 4point bending fatigue test
specimens having the shape shown in Figure 6, and eight rollerpitting small
roller specimens having the shape shown in Figure 6 were cut out &om the
central portion of a steel bar having a diameter of 36 mm after normalization,
each specimen being in parallel with the axial direction.
[01481
Each specimen described above was subjected to "carburizing-quenching
and tempering1 in the heat pattern shown in Figure 3. Moreover, quenching
was performed by using any of the oils of conditions bl to hvl shown in Table 2.
[O 1491
Each test specimen fabricated as described above was subjected to an Onotype
rotating bending fatigue test, a 4point bending fatigue test, a roller pitting
test, and an observation of the outer layer portion by using SEM.
[0160]
Further, in test symbol S in Table 6 shown below, after notch portions of
the Ono-type rotating bending fatigue test specimen and the 4point bending
fatigue test specimen, which had been subjected to l1carburizing-quenching and
tempering" were ground, and also after a 26 mm diameter portion of the roller
pitting small roller specimen was ground, the ground portions were subjected to
shotpeening at conditions of an arc height: 0.4 mmA and a coverage: 300%, using
steel balls having a diameter of 0.6 mm and an HV of 800, and thereafter were
subjected to the Ono-type rotating bending fatigue test, the 4point bending
fatigue test, the roller pitting test, and the observation of outer layer portion by
using SEM.
[0161]
Moreover, in Test symbol T in Table 6, shotpeening was performed at
conditions of an arc height: 0.4 mmA and a coverage: 300% on the notch portions
of the Ono-type rotating bending fatigue test specimen and the 4point bending
fatigue test specimen, and the 26 mm diameter portion of the roller pitting small
roller specimen, which had been subjected to "carburizing-quenching and
tempering", by using steel balls having a diameter of 0.6 mm and an HV of 800,
and thereafter the notch portions of the Ono-type ktating bending ihtigue test
specimen and the 4point bending fatigue teet specimen, and the 26 mm
diameter portion of the roller pitting small roller specimen were ground to be
subjected to the Ono-type rotating bending fatigue test, the 4point bending
fatigue test, the roller pitting test, and the observation of outer layer portion by
using SEM.
[01621
Moreover, a roller pitting large roller specimen having the shape shown
in Figure 7 and to be used for the roller pitting test was fabricated using
SCM420H specified in JIS G 4062 (2008) by a commonly used production process,
that is, processes of "normalizing, test specimen processing, eutectoid
carburizing by a gas carburizing furnace, low temperature tempering, and
grinding".
[01631
Hereafter, contents of each test will be described in detail.
[0164]
Observation of outer layer portion using SEM:
After the cross section of the notch portion of the Ono-type rotating
bending fatigue test specimen was mirror-ground, the outer layer portion was
observed at a magnXcation of 1000 times using an optical microscope to
measure the intergranular oxidation depth. Further, the cross section was
etched with Nital and the outer layer portion was observed at a magnification
of 3000 timers by us-mg SEM to investigate the presence or absence of alack
quenching structue in the vicinity of grain boundary oxides.
[0165]
Further, it was aimed that no slack quenching structure is present in the
outer layer portion.
101661
Ono-type rotating bending fatigue test:
An Ono-type rotating bending fatigue test was conducted at below
described test conditions by using eight Ono-type rotating bending fatigue test
specimens to evaluate the "high-cycle bending fatigue strength" baaed on a
maximum strength at which rupture does not occur at a number of repetitions
of 1.0~107.
[0167]
The test results were normalized by assuming that the rotating bending
fatigue strength of Test symbol V in Table 6, which used Steel 16 corresponding
to SCr420 specified in JIS G 4053 (2008), at a number of repetitions of 1.0~107
be 100. Then, when a specimen exhibited a rotating bending fatigue strength
of more than 1.15 times the evaluation criterion, that is, more than 116, the
specimen was estimated to have excellent high-cycle bending fatigue strength.
[01581
- Temperature: room temperature
- Atmosphere: the air
- Number of revolutions: 3000 rpm
[Ol69]
4point bending fatigue test:
A &point bending fatigue test was conducted by the method shown in
Figure 8 by using two Qpoint bending fatigue test specimens. The number of
repetitions at which the specimen waa ruptured wae investigated at test load:
the maximum load is 10 kN and the minimum load is 1 kN, and at a repetition
eequency of 20 Hz, to evaluate the "low-cycle bending fhtigue strength".
[01601
The test resulta were normalized by assuming that the rupture Life of Test
symbol V, which used Steel 16 corresponding to SCr420 described above, be 100.
Then, when a specimen exhibited a rupture life of not less than two times the
evaluation criterion, that is, not less than 200, the specimen was estimated to
have excellent low-cycle fatigue strength.
[01611
Roller pitting test:
The roller pitting test was conducted by combining a roller pitting small
roller specimen having the shape shown in Figure 6 with a roller pitting large
roller having the shape shown in Figure 7 as shown in Figure 9 at the following
conditions.
Lo1621
The number of tests for the roller pitting test was six, and an S-N diagram
in which the ordinate represents the interfacial pressure and the abscissa
represents the number of repetitions before the occurrence of pitting was created,
and it was assumed that the "pitting resistant strength" be the largest of
interfacial pressures at which no pitting occurred before a number of repetitions
of 2.0~107.
LO1631
Furthermore, it was assumed that pitting had occurred when the largest
of surface damages which had occurred on the surface of the test portion of the
roller pitting small roller had an area of not less than 1 mm2.
Lo1661
The test results were normalized by assuming that the pitting resistant
strength of Test symbol V, which used Steel 16 corresponding to SCr420
described above, be 100. Then, when a specimen exhibited an pitting resistant
strength of not less than 1.20 times the evaluation criterion, that is, not less
than 120, the specimen was estimated to have excellent pitting resistant
strength.
[0166]
- Slip rate: -40%
- Interfacial pressure: 1600 to 3000 MPa
- Number of revolutions of small roller specimen: 1500 rpm
- Circumferential speed: Circumferential speed of large roller specimen
V1: 2.86 m/s
Circumferential speed of small roller specimen
V2: 2.04 m/s
- Lubricant oil: Kind: automatic transmission oil
Oil temperature: 90°C
Amount of oil: 1.0 L1mi.n
101661
Moreover, lubrication was conducted by ejecting the above described
lubricant oil to the contact portion between the roller pitting smaller roller
specimen and the roller pitting large roller specimen.
LO1671
The above described "slip rate" refers to a value calculated by the
following formula.
((V2 - Vl)N2) x 100
ro1sa1
Each test result described above is summarized and shown in Table 6. It
is noted that in the "HV after normalization" column of Table 6, only the largest
value of measured HV values which were measured on round bars having
diameters of 10 mm, 20 mm, and 30 mm for each Test symbol is shown.
[01691
Table 6
Teat
m b o
1
A
B
C
D
E
F
C)
H
I
J
K
L
M
N
0
P
Q
R
9
T
U
V
W
X
Y
Z
M
AB
AC
AD
AE
AF
A0
AH
A1
AJ
AL
"*" mark
"W mark
7
7
7
10
12
1
2
3
4
6
6
7
8
9
10
11
12
13
7
7
*I4
*16
*l6
*I7
*18
*19
*20
*21
*22
*23
*24
*26
*26
"27
*28
*29
*30
*31
indicates
Presence
or
absence
of*
que*
g
atmcttue
#Resent
#Re=&
#Preeent
#Preeent
#Preeent
Absent
Abeent
Absent
Absent
Absent
Absent
Abeent
Absent
&sent
Absent
Abeent
Absent
Absent
Absent
#Present
#Present
#Preeent
Absent
Absent
#Present
Absent
Absent
Abeent
#Present
Absent
Abeent
Absent
Absent
Absent
Absent
Absent
Abeent
in the
indicates that
of
quenchin
g oil
*[ii]
*[iiil
*bl
*[id
*Ed
rd
[il
Id
bl
[il
li]
W
rd
[il
Ci]
[il
rd
[i]
ril
li]
li]
ril
rd
ril
[i]
W
W
li]
[i]
[il
r.i.l
[.I
li]
ril
[il
ril
li]
li]
from the
criterion,
HVaftar
n-lliaatio
n
144
144
144
184
188
160
167
164
178
148
166
144
144
188
184
186
186
188
144
144
160
144
178
173
144
#210
168
162
#218
160
#266
148
183
190
165
162
#220
#211
deviation
indicates evaluation
s
8 e $
m
A
W
a B tJ
reached.
Intergranula
r oxidation w'ho
6.8
7.2
7.1
4.1
4.6
6.6
4.6
49
4.0
6.0
3.0
6.4
6.3
2.8
3.4
6.2
7.6
4.1
17.8
10.1
11.8
10.7
2.1
10.6
3.3
6.9
4.2
3.2
3.7
10.6
3.2
2.2
2.6
6.9
7.1
6.6
condition epecified
and 'Wn mark
wh- LOWcpcle
cycle Pitting
b.dg
.h.w
h
#I08
#lo3
#lo8
#I08
#I08
116
116
118
118
116
118
118
120
120
120
116
116
116
140
140
#86
%I00
#I03
116
116
#lo8
#I10
116
116
#lo8
116
#I10
#90
#90
#90
116
120
116
present
ita farget
fa*e
s~~
h
#I60
#I40
#I60
#I80
#18b
220
210
240
200
210
220
220
220
220
230
200
200
220
260
260
#80
BlOO
#I01
#I46
#I65
#I20
#I10
200
200
#I60
220
#I60
#I20
200
#I20
#I20
200
#I60
invention.
had not
nd.t.n
t
strength
#I16
#I16
#I16
#I16
#I16
120
120
120
120
120
120
120
120
120
u0
120
120
120
120
130
#SO
Z100
#I10
120
120
132
130
#I10
120
#I16
120
#I16
120
120
120
120
120
120
been
From Table 6, it is clear that in the case of Teat symbols of Inventive
Examples which satisfied the conditions specified in the present invention, all
of the targeted hardness, microstructure of outer layer portion, high-cycle
bending fiztigue strength, low-cycle bending fatigue strength, and pitting
resistant fatigue strength had been achieved.
1017 11
In contrast to this, in the case of Test symbols of Comparative Examples
which deviated &om the conditions specified in the present invention, one or
more properties of the targeted hardness, microstructure of outer layer portion,
high-cycle bending fatigue strength, low-cycle bending fatigue strength, and
pitting resistant fatigue strength had not been achieved.
Lo1721
In the cases of Test symbols A to E, although the chemical compositions
of used Steels 7, 10, and 12 were within the range specaed in the present
invention, the conditions of quenching oil deviated from the condition specified
in the present invention, so that the micFostrudure of outer layer portion, the
high-cycle bending fatigue strength, the low-cycle bending fatigue strength, and
the pitting resistant fatigue strength had not reached their respective targets.
[01731
Moreover, in the cases of Test symbols U to Z and Test aymbols of AA to
AL, although the conditions of quenching oil were within the range specified in
the preeent invention, all of the chemical compositions of used Steels 14 to 31
deviated from the conditions specified in the present invention. For that
reason, one or more properties of the hardness, the microstructure of outer layer
portion, the high-cycle bending fatigue strength, the low-cycle bending fatigue
strength, and the pitting resistant fatigue strength had not reached their
respective targets.
INDUSTRIAL APPLICABILITY
According to the present invention, it is poeaible to obtain a ateel
component which has excellent high-cycle bending fatigue strength, low-cycle
bending fatigue strength, and pitting resistant strength despite small contents
of expensive alloying elements, and furthermore can ensure eufficient
machinability in the cutting proceee or s a c i e n t cold forgeability in the netshape
forming process at the time of production, and in which the variation of
heat treatment distortion at the time of carburizing-quenching or
carbonitriding-quenching ie suppressed.
We claim:
1. A method for producing a steel component by performing the
treatments of the following sequentially steps 1 and 2 to a steel
material containing, by mass%,
C: 0.15 to 0.25%,
sk 0.01 to 0.10%,
Mu: 0.50 to 0.80%,
S: 0.003 to 0.030%,
Cr: 0.80 to 1.20%,
Mo: 0.30 to 0.46%,
Al: 0.015 to 0.050%, and
N: 0.010 to 0.025%, wherein
fn represented by Formula (1) shown below is 1.3 to 2.4, the
balance being F;! and impurities, and contents of P and 0 among the
impuritiee are
P: not more than 0.010%, and
0: not more than 0.0020%:
step 1: a treatment which holds the steel material at a
temperature of 850 to 1000°C in a carburizing atmosphere or
carbonitriding atmosphere, and
step 2: a treatment which quenches the carburized or
carbonitrided steel material, by using quenching oil having a
temperature of 40 to 80°C and a kinetic viscosity of 20 to 25 mm2ts at
40°C;
fn = CrMn ... (1)
where, Cr and Mn in Formula (1) represent the content of each
element in mass%.
2. The method for producing a steel component according to claim 1,
wherein the step 1 includes a holding process at a temperature of 860
to 1000°C in a carburizing atmosphere or carbonitriding atmosphere
Mowed by a holding process at a temperature of 800 to 900°C in a
carburizing atmosphere or carbonitriding atmosphere.
3. The method for producing a steel component according to claims 1 or
2, wherein the steel material contains, in mass%, Nb: not more than
0.08% in lieu of a part of Fe.