U.S. patent application number 14/932370 was filed with the patent office on 2016-12-22 for bearing steel having improved fatigue durability and method of manufacturing the same.
The applicant listed for this patent is Hyundai Motor Company, Korea Institute of Science and Technology. Invention is credited to Sung-Chul Cha, Seung-Hyun Hong, Woo-Sang Jung, Young-Sang Ko, Jin-Yoo Suh.
Application Number | 20160369370 14/932370 |
Document ID | / |
Family ID | 57466871 |
Filed Date | 2016-12-22 |
United States Patent
Application |
20160369370 |
Kind Code |
A1 |
Cha; Sung-Chul ; et
al. |
December 22, 2016 |
BEARING STEEL HAVING IMPROVED FATIGUE DURABILITY AND METHOD OF
MANUFACTURING THE SAME
Abstract
The present invention relates to a steel composition for bearing
having improved fatigue durability and a method of manufacturing
the same. The steel composition comprises: an amount of about 0.08
to 1.0 wt % of carbon (C); an amount of about 0.9 to 1.6 wt % of
silicon (Si); an amount greater than 0 wt % and of about 0.03 wt %
or less of phosphorus (P); an amount greater than 0 wt % and of
about 0.01 wt % or less of sulfur (S); an amount of about 0.01 to
0.1 wt % of copper (Cu); an amount of about 0.01 to 0.06 wt % of
aluminum (Al); an amount greater than 0 wt % and of about 0.006 wt
% or less of nitrogen (N); an amount greater than 0 wt % and of
about 0.001 wt % or less of oxygen (O); one or more selected from
the group consisting of: an amount of about 0.5 to 1.00 wt % of
manganese (Mn), an amount of about 0.1 to 0.6 wt % of nickel (Ni),
an amount of about 1.4 to 1.55 wt % of chromium (Cr), an amount of
about 0.2 to 0.5 wt % of molybdenum (Mo), and an amount greater
than 0 wt % and of about 0.4 wt % or less of vanadium (V); and iron
(Fe) constituting the balance of the weight of the steel
composition, all wt % based on the total weight of the alloy steel
composition
Inventors: |
Cha; Sung-Chul; (Seoul,
KR) ; Hong; Seung-Hyun; (Seoul, KR) ; Ko;
Young-Sang; (Gunpo, KR) ; Jung; Woo-Sang;
(Seoul, KR) ; Suh; Jin-Yoo; (Seoul, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hyundai Motor Company
Korea Institute of Science and Technology |
Seoul
Seoul |
|
KR
KR |
|
|
Family ID: |
57466871 |
Appl. No.: |
14/932370 |
Filed: |
November 4, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/06 20130101;
C22C 38/44 20130101; C21D 8/065 20130101; C22C 38/001 20130101;
C21D 6/005 20130101; C22C 38/46 20130101; C21D 6/008 20130101; C22C
38/42 20130101; C22C 38/04 20130101; C21D 9/525 20130101; C21D 1/56
20130101; C22C 38/002 20130101; C21D 6/004 20130101; C21D 1/32
20130101; C22C 38/02 20130101 |
International
Class: |
C21D 9/52 20060101
C21D009/52; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/06 20060101 C22C038/06; C21D 1/32 20060101
C21D001/32; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C21D 8/06 20060101 C21D008/06; C21D 6/00 20060101
C21D006/00; C21D 1/56 20060101 C21D001/56; C22C 38/46 20060101
C22C038/46; C22C 38/04 20060101 C22C038/04 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 22, 2015 |
KR |
10-2015-0088341 |
Claims
1. A steel composition for bearing, comprising: an amount of about
0.8 to 1.0 wt % of carbon (C); an amount of about 0.9 to 1.6 wt %
of silicon (Si); an amount greater than 0 wt % and of about 0.03 wt
% or less of phosphorus (P); an amount greater than 0 wt % and of
about 0.01 wt % or less of sulfur (S); an amount of about 0.01 to
0.1 wt % of copper (Cu); an amount of about 0.01 to 0.06 wt % of
aluminum (Al); an amount greater than 0 wt % and of about 0.006 wt
% or less of nitrogen (N); an amount greater than 0 wt % and of
about 0.001 wt % or less of oxygen (O); and iron (Fe) constituting
the balance of the weight of the steel composition, all wt % based
on the total weight of the steel composition.
2. The steel composition of claim 1, further comprising: an amount
of about 0.5 to 1.00 wt % of manganese (Mn), based on the total
weight of the steel composition.
3. The steel composition of claim 1, further comprising: an amount
of about 0.1 to 0.6 wt % of nickel (Ni), based on the total weight
of the steel composition.
4. The steel composition of claim 1, further comprising: an amount
of about 1.4 to 1.55 wt % of chromium (Cr), based on the total
weight of the steel composition.
5. The steel composition of claim 1, further comprising: an amount
of about 0.2 to 0.5 wt % of molybdenum (Mo), based on the total
weight of the steel composition.
6. The steel composition of claim 1, further comprising: an amount
greater than 0 wt % and of about 0.4 wt % or less of vanadium (V),
based on the total weight of the steel composition.
7. The steel composition of claim 1, further comprising: one or
more selected from the group consisting of: an amount of about 0.5
to 1.00 wt % of manganese (Mn), an amount of about 0.1 to 0.6 wt %
of nickel (Ni), an amount of about 1.4 to 1.55 wt % of chromium
(Cr), an amount of about 0.2 to 0.5 wt % of molybdenum (Mo), and an
amount greater than 0 wt % and of about 0.4 wt % or less of
vanadium (V), all wt % based on the total weight of the steel
composition.
8. The steel composition of claim 1, consisting essentially of: an
amount of about 0.08 to 1.0 wt % of carbon (C); an amount of about
0.9 to 1.6 wt % of silicon (Si); an amount greater than 0 wt % and
of about 0.03 wt % or less of phosphorus (P); an amount greater
than 0 wt % and of about 0.01 wt % or less of sulfur (S); an amount
of about 0.01 to 0.1 wt % of copper (Cu); an amount of about 0.01
to 0.06 wt % of aluminum (Al); an amount greater than 0 wt % and of
about 0.006 wt % or less of nitrogen (N); an amount greater than 0
wt % and of about 0.001 wt % or less of oxygen (O); and iron (Fe)
constituting the balance of the weight of the steel composition,
all wt % based on the total weight of the steel composition.
9. The steel composition of claim 1, consisting essentially of: an
amount of about 0.08 to 1.0 wt % of carbon (C); an amount of about
0.9 to 1.6 wt % of silicon (Si); an amount greater than 0 wt % and
of about 0.03 wt % or less of phosphorus (P); an amount greater
than 0 wt % and of about 0.01 wt % or less of sulfur (S); an amount
of about 0.01 to 0.1 wt % of copper (Cu); an amount of about 0.01
to 0.06 wt % of aluminum (Al); an amount greater than 0 wt % and of
about 0.006 wt % or less of nitrogen (N); an amount greater than 0
wt % and of about 0.001 wt % or less of oxygen (O); one or more
selected from the group consisting of: an amount of about 0.5 to
1.00 wt % of manganese (Mn), an amount of about 0.1 to 0.6 wt % of
nickel (Ni), an amount of about 1.4 to 1.55 wt % of chromium (Cr),
an amount of about 0.2 to 0.5 wt % of molybdenum (Mo), and an
amount greater than 0 wt % and of about 0.4 wt % or less of
vanadium (V); and iron (Fe) constituting the balance of the weight
of the steel composition, all wt % based on the total weight of the
steel composition.
10. A method of manufacturing a bearing steel, the method
comprising: manufacturing a wire rod comprising an alloy steel
composition; heat-treating the wire rod for primary spheroidizing;
wire drawing the heat-treated wire rod; secondary heat-treating the
wire-drawn wire rod for secondary spheroidizing; forging the
secondary heat-treated wire rod; quenching the forged wire rod; and
tempering the quenched wire rod, wherein the alloy steel
composition comprises: an amount of about 0.08 to 1.0 wt % of
carbon (C); an amount of about 0.9 to 1.6 wt % of silicon (Si); an
amount greater than 0 wt % and of about 0.03 wt % or less of
phosphorus (P); an amount greater than 0 wt % and of about 0.01 wt
% or less of sulfur (S); an amount of about 0.01 to 0.1 wt % of
copper (Cu); an amount of about 0.01 to 0.06 wt % of aluminum (Al);
an amount greater than 0 wt % and of about 0.006 wt % or less of
nitrogen (N); an amount greater than 0 wt % and of about 0.001 wt %
or less of oxygen (O); and iron (Fe) constituting the balance of
the weight of the steel composition, all wt % based on the total
weight of the alloy steel composition.
11. The method of claim 10, wherein the alloy steel composition
further comprises one or more selected from the group consisting
of: an amount of about 0.5 to 1.00 wt % of manganese (Mn), an
amount of about 0.1 to 0.6 wt % of nickel (Ni), an amount of about
1.4 to 1.55 wt % of chromium (Cr), an amount of about 0.2 to 0.5 wt
% of molybdenum (Mo), and an amount greater than 0 wt % and of
about 0.4 wt % or less of vanadium (V), all wt % based on the total
weight of the alloy steel composition.
12. The method of claim 10, wherein the heat-treating for the
primary spheroidizing is performed at a temperature of about 720 to
850.degree. C. for about 4 to 8 hours.
13. The method of claim 10, wherein the secondary heat-treating for
the secondary spheroidizing is performed at a temperature of about
720 to 850.degree. C. for about 4 to 8 hours.
14. The method of claim 10, wherein the quenching is performed at a
temperature of about 840 to 860.degree. C. for about 0.5 to 2
hours.
15. The method of claim 10, wherein the tempering is performed at a
temperature of about 150 to 190.degree. C. for about 0.5 to 2
hours.
16. The method of claim 10, wherein the bearing steel contains a
complex carbide.
17. The method of claim 16, wherein the complex carbide comprises
one or more selected from the group consisting of M.sub.3C,
M.sub.7C.sub.3, and M.sub.23C.sub.6 carbides, and MC carbides,
wherein M is a metal or a transition metal.
18. The method of claim 16, wherein the M of the M.sub.3C,
M.sub.7C.sub.3, and M.sub.23C.sub.6 carbides is one or more
selected from the group consisting of chromium (Cr), iron (Fe), and
manganese (Mn) and the M of the MC carbide is one or more selected
from the group consisting of vanadium (V) and molybdenum (Mo).
19. A vehicle part that comprises a steel composition of claim
1.
20. The vehicle part of claim 19, wherein the vehicle part is a
bearing of an engine or a transmission.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority under 35 U.S.C. .sctn.119
to Korean Patent Application No. 10-2015-0088341, filed on Jun. 22,
2015, in the Korean Intellectual Property Office, the disclosure of
which is incorporated herein by reference in its entirety.
TECHNICAL FIELD
[0002] The present invention relates to a steel composition for a
bearing having improved fatigue durability, and a method of
manufacturing the same. In particular, the steel composition may
contain a spheroidized complex carbide to provide substantially
improved hardness, strength, and toughness, and improved fatigue
strength and fatigue life.
BACKGROUND
[0003] Recently, an environmental problem has been on the rise
around the globe, and thus a method of reducing fuel in accordance
with such problems which encompasses all industries has been
sought. In particular, for reducing fuel consumption a vehicle
industrial field has proposed improving fuel efficiency of a
vehicle engine and reducing a weight of vehicles. By reducing the
weight of vehicles, for example, fuel efficiency of the vehicle can
be increased. However, when reducing the weight of vehicles, there
occurs a problem in that strength and durability required in
vehicles are not satisfied. Therefore, it is the greatest goal of
the vehicle industry to solve this problem.
[0004] Therefore, under the environmentally-friendly trend, in the
vehicle industry, various environmentally-friendly vehicles have
been developed with an object of reducing a discharge amount of
carbon dioxide to 95 g/km that is 27% of a current discharge amount
thereof by 2021 based on European regulations. Further, vehicle
makers have developed a technology to downsize and improve fuel
economy in order to meet 54.5 mpg (23.2 km/l) which is a regulation
value of corporate average fuel economy (CAFE) in the USA by
2025.
[0005] Generally, with increase in number of parts or an increase
in weight of additional parts, a weight of a material for those
parts should be reduced. In this case, as a weight reduction
method, a heat-treating technology for implementing high strength
of the material or hardening a material surface has been frequently
used. Further, as shapes of parts being complicated, precise
joining, low distortion welding, and low distortion heat-treating
technologies have been used. In addition to this, as a technology
for reducing noise, a technology of reducing distortion caused by
heat treating, and a technology for reducing noise and removing
dust have been used.
[0006] Particularly, a high performance and high efficiency
technology of engines and transmissions for maximizing fuel economy
of vehicles has been developed, and this technology may include an
increase in the number of gears, a novel concept driveaway device,
high efficiency of a two-pump system, a fusion hybrid technology,
technologies relating to an automatic/manual fusion transmission
and a hybrid transmission, and the like.
[0007] An alloy steel used in this technology relating to the
engines and the transmissions has been used in parts of the
engines, carriers of the manual or automatic transmissions, annulus
gears, gears, shafts, synchronizer hubs, or the like, and such an
alloy steel used in the engine may correspond to about 32 to 40%
based on the weight of the engine and the alloy steel used in the
transmission may correspond about 58 to 62 wt % based on the weight
of the transmission. Particularly, as the materials of the
transmission parts, for example, the gear or the shaft, development
of high strengthening and high durability materials has been
continuously required with demand for a weight reduction and
downsizing. However, a technology relating to downsizing the parts,
reducing sizes of the parts, or improving fuel efficiency has
problems. For instance, when a load applied to the parts of the
engines is increased, a quality of the parts may be reduced and a
durability life may be reduced due to burning, friction, abrasion,
and the like. Moreover, due to an increase in severity of parts and
a lack of durability of a material, surface damage may occur, and
when the alloy steel is used without a lubricant, a surface
temperature may be increased, and thus hardness may be reduced at
high temperatures or under an environment requiring a lot of
rotation. Therefore, durability of the bearing steel in the related
art may require to be reinforced.
[0008] Generally, the gears of the transmission of the vehicle are
parts performing a role of directly transferring engine power to a
differential system and effectively transferring rotation or power
between two or more shafts, such that engine power is attuned to a
driving state of the vehicle. In addition, the gears of the
transmission receive bending stress and contact stress
simultaneously. In the gears, when durability of the material is
insufficient, fatigue failure (tooth breakage) due to a lack of
bending fatigue strength and fatigue damage (pitting) due to a lack
of contact fatigue strength may frequently occur. Therefore, in the
gears, physical properties such as high hardness, strength,
toughness, fatigue strength, and a fatigue life are required.
[0009] In the related arts, a conventional bearing steel, such as
SUJ2, including iron (Fe) as a main component, 1.00 wt % of carbon
(C), 0.27 wt % of silicon (Si), 0.38 wt % of manganese (Mn), 0.012
wt % of phosphorus (P), 0.005 wt % of sulfur (S), 1.46 wt % of
copper (Cu), 0.05 wt % of nickel (Ni), 1.46 wt % of chromium (Cr),
0.02 wt % of molybdenum (Mo), 0.017 wt % of aluminum (Al), 0.0035
wt % of nitrogen (N), and 0.0006 wt % of oxygen (O) has been
typically used. However, this steel has a problem in terms of
durability and thus has a problem in that surface damage (flaking)
and pin abrasion of a pinion shaft are severe.
[0010] Therefore, the present inventors have tried to develop a
steel composition for bearing and the bearing steel having improved
physical properties such as hardness, strength, toughness, fatigue
strength, and a fatigue life, and a method of manufacturing the
same.
SUMMARY OF THE INVENTION
[0011] In preferred aspects, the present invention provides a steel
composition for bearing and a method of manufacturing the same. The
steel composition may comprise iron (Fe) as a main component,
carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur
(S), copper (Cu), nickel (Ni), chromium (Cr), molybdenum (Mo),
aluminum (Al), nitrogen (N), oxygen (O), and vanadium (V) to
improve physical properties such as hardness, strength, and
toughness and thus improve durability, fatigue strength, and a
fatigue life.
[0012] An exemplary embodiment of the present invention provides a
steel composition for bearing that comprises: an amount of about
0.08 to 1.0 wt % of carbon (C), an amount of about 0.9 to 1.6 wt %
of silicon (Si), an amount greater than 0 wt % and of about 0.03 wt
% or less of phosphorus (P), an amount greater than 0 wt % and of
about 0.01 wt % or less of sulfur (S), an amount of about 0.01 to
0.1 wt % of copper (Cu), an amount of about 0.01 to 0.06 wt % of
aluminum (Al), an amount greater than 0 wt % and of about 0.006 wt
% or less of nitrogen (N), an amount greater than 0 wt % and of
about 0.001 wt % or less of oxygen (O); and iron (Fe) being the
balance of the weight of the steel composition, all wt % based on a
total weight of the steel composition.
[0013] It would be understood that all the weight % (wt %) referred
to herein is based on the total weight of the steel or alloy steel
composition, unless otherwise indicated.
[0014] Additionally, the steel composition of the present invention
may further comprise manganese (Mn), and preferably, a content of
manganese (Mn) may be an amount of about 0.5 to 1.00 wt %.
[0015] The steel composition of the present invention may further
comprise nickel (Ni), and preferably, a content of nickel (Ni) may
be an amount of about 0.1 to 0.6 wt %.
[0016] The steel composition of the present invention may further
comprise chromium (Cr), and preferably, a content of chromium (Cr)
may be an amount of about 1.4 to 1.55 wt %.
[0017] The steel composition of the present invention may further
comprise molybdenum (Mo), and preferably, a content of molybdenum
(Mo) may be an amount of about 0.2 to 0.5 wt %.
[0018] The steel composition of the present invention may further
comprise vanadium (V), and preferably, a content of vanadium (V)
may be an amount greater than 0 wt % and of about 0.4 wt % or
less.
[0019] Alternatively, the steel composition of the present
invention may further include one or more of selected from the
group consisting of manganese (Mn), nickel (Ni), chromium (Cr),
molybdenum (Mo), or vanadium (V), and preferably, a content of
manganese (Mn) may be an amount of about 0.5 to 1.00 wt %, a
content of nickel (Ni) may be an amount of about 0.1 to 0.6 wt %, a
content of chromium (Cr) may be an amount of about 1.4 to 1.55 wt
%, a content of molybdenum (Mo) may be an amount of about 0.2 to
0.5 wt %, and a content of vanadium (V) may be an amount greater
than 0 wt % and of about 0.4 wt % or less.
[0020] Also provided is the steel composition or steel alloy
composition of the invention that may consist of, essentially
consist of, or consist essentially of the components above. For
example, the steel composition for bearing may consist of,
essentially consist of, or consist essentially of: an amount of
about 0.08 to 1.0 wt % of carbon (C); an amount of about 0.9 to 1.6
wt % of silicon (Si); an amount greater than 0 wt % and of about
0.03 wt % or less of phosphorus (P); an amount greater than 0 wt %
and of about 0.01 wt % or less of sulfur (S); an amount of about
0.01 to 0.1 wt % of copper (Cu); an amount of about 0.01 to 0.06 wt
% of aluminum (Al); an amount greater than 0 wt % and of about
0.006 wt % or less of nitrogen (N); an amount greater than 0 wt %
and of about 0.001 wt % or less of oxygen (O); and iron (Fe)
constituting the balance of the weight of the steel composition,
all wt % based on the total weight of the alloy steel composition.
Further, the steel composition for bearing may consist of,
essentially consist of, or consist essentially of: an amount of
about 0.08 to 1.0 wt % of carbon (C); an amount of about 0.9 to 1.6
wt % of silicon (Si); an amount greater than 0 wt % and of about
0.03 wt % or less of phosphorus (P); an amount greater than 0 wt %
and of about 0.01 wt % or less of sulfur (S); an amount of about
0.01 to 0.1 wt % of copper (Cu); an amount of about 0.01 to 0.06 wt
% of aluminum (Al); an amount greater than 0 wt % and of about
0.006 wt % or less of nitrogen (N); an amount greater than 0 wt %
and of about 0.001 wt % or less of oxygen (O); one or more selected
from the group consisting of: an amount of about 0.5 to 1.00 wt %
of manganese (Mn), an amount of about 0.1 to 0.6 wt % of nickel
(Ni), an amount of about 1.4 to 1.55 wt % of chromium (Cr), an
amount of about 0.2 to 0.5 wt % of molybdenum (Mo), and an amount
greater than 0 wt % and of about 0.4 wt % or less of vanadium (V);
and iron (Fe) constituting the balance of the weight of the steel
composition, all wt % based on the total weight of the alloy steel
composition.
[0021] Another exemplary embodiment of the present invention
provides a method of manufacturing a bearing steel. The method may
comprise: manufacturing a wire rod comprising an alloy steel
composition; heat-treating the wire rod for primary spheroidizing;
wire drawing the heat-treated wire rod; secondary heat-treating the
wire-drawn wire rod for secondary spheroidizing; forging the
secondary heat-treated wire rod; quenching the forged wire rod; and
tempering the quenched wire rod.
[0022] In particular, the alloy steel composition may comprise: an
amount of about 0.08 to 1.0 wt % of carbon (C); an amount of about
0.9 to 1.6 wt % of silicon (Si); an amount greater than 0 wt % and
of about 0.03 wt % or less of phosphorus (P); an amount greater
than 0 wt % and of about 0.01 wt % or less of sulfur (S); an amount
of about 0.01 to 0.1 wt % of copper (Cu); an amount of about 0.01
to 0.06 wt % of aluminum (Al); an amount greater than 0 wt % and of
about 0.006 wt % or less of nitrogen (N); an amount greater than 0
wt % and of about 0.001 wt % or less of oxygen (O); and iron (Fe)
constituting the balance of the weight of the alloy steel
composition, all wt % based on the total weight of the alloy steel
composition. Further, the alloy steel may further comprise one or
more selected from the group consisting of: an amount of about 0.5
to 1.00 wt % of manganese (Mn), an amount of about 0.1 to 0.6 wt %
of nickel (Ni), an amount of about 1.4 to 1.55 wt % of chromium
(Cr), an amount of about 0.2 to 0.5 wt % of molybdenum (Mo), and an
amount greater than 0 wt % and of about 0.4 wt % or less of
vanadium (V).
[0023] The term "spheroidizing" or "spheroidize", as used herein,
refers to a heat-treating process, particularly used for iron based
alloy steel or a composition thereof. Particularly, the
spheroidizing may refer to a heat-treat process that changes the
shape or crystalline shape of carbons of carbide or carbide complex
contained in the iron based steel into, for example, globular form,
spheroid, or elliptical form, to provide desirable physical
properties, such as mechanical strength, high temperature
resistance, ductility, machinability and the like. During the
spheroidizing, the temperature may be increased up to an iron based
alloy steel
[0024] The heat-treating for the primary spheroidizing may be
performed at a temperature of about 720 to 850.degree. C. for about
4 to 8 hours.
[0025] The secondary heat-treating for the secondary spheroidizing
may be performed at a temperature of about 720 to 850.degree. C.
for about 4 to 8 hours.
[0026] The quenching may be performed at a temperature of about 840
to 860.degree. C. for about 0.5 to 2 hours.
[0027] The tempering may be performed at a temperature of about 150
to 190.degree. C. for about 0.5 to 2 hours.
[0028] Preferably, thus manufactured bearing steel may contain a
carbide complex which may include one or more selected from the
group consisting of M.sub.3C, M.sub.7C.sub.3, and M.sub.23C.sub.6
carbides, and MC carbides.
[0029] The term "carbide complex", as used herein, refers to a
compound comprising at least carbon and other elements that is less
electronegative to be positive or partially positive when combined
with carbon. The carbide complex may be suitably formed with at
least carbon and metal, and the metal may be an alkali metal, an
alkali earth metal or a transition metal, a post-transition metal,
a lanthanide, or an actinide, without limitation. In particular,
the M of the M.sub.3C, M.sub.7C.sub.3, and M.sub.23C.sub.6 carbides
may be one or more selected from the group consisting of chromium
(Cr), iron (Fe), and manganese (Mn) and the M of the MC carbide may
be one or more selected from the group consisting of vanadium (V)
and molybdenum (Mo).
[0030] The bearing or bearing steel of the steel composition
comprising iron (Fe) as a main component, carbon (C), silicon (Si),
manganese (Mn), phosphorus (P), sulfur (S), copper (Cu), nickel
(Ni), chromium (Cr), molybdenum (Mo), aluminum (Al), nitrogen (N),
oxygen (O), and vanadium (V) may contain the complex carbide such
as Me.sub.3C, Me.sub.7C.sub.3, Me.sub.23C.sub.6 (Me: Cr, Fe, Mn)
that is finely formed through adjustment of alloy components of the
bearing steel and control of a process condition. As such, physical
properties of the bearing steel, such as hardness, strength, and
toughness of a bearing and thus improve durability, fatigue
strength, and a fatigue life, may be substantially improved.
Moreover, high strengthening of the bearing steel may be obtained,
and thus through a thickness reduction thereof, a weight reduction
of about 20%, and the like, may secure the degree of freedom in
design of a vehicle and reduce manufacturing costs.
[0031] Further provided is a vehicle part that can be manufactured
by the bearing steel of the alloy composition as described above.
For example, a transmission and an engine for a vehicle can be
manufactured by using the bearing comprising the steel or alloy
steel composition as described above, such that it is possible to
improve durability of the vehicle and reduce weight of the vehicle,
thereby improving fuel efficiency and prevent environmental
pollution.
[0032] Other aspects of the invention are disclosed infra.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] FIG. 1 shows a flowchart of an exemplary method of
manufacturing a bearing steel according to an exemplary embodiment
of the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0034] Hereinafter, preferable exemplary embodiments of the present
invention will be described in detail. Prior to this, terms or
words used in the present specification and claims should not be
interpreted as being limited to typical or dictionary meanings, but
should be interpreted as having meanings and concepts which comply
with the technical spirit of the present invention, based on the
principle that an inventor can appropriately define the concept of
the term to describe his/her own invention in the best manner.
Accordingly, the embodiment described in the present specification
is just the most preferred embodiment of the present invention but
does not represent all technical spirits of the present invention.
Therefore, it should be understood that there are various
equivalents and modifications replacing the embodiments at the time
of filing of the present application.
[0035] It would be understood that the terms "comprises" and/or
"comprising," when used in this specification, specify the presence
of stated features, integers, steps, operations, elements, and/or
components, but do not preclude the presence or addition of one or
more other features, integers, steps, operations, elements,
components, and/or groups thereof. As used herein, the term
"and/or" includes any and all combinations of one or more of the
associated listed items.
[0036] Unless specifically stated or obvious from context, as used
herein, the term "about" is understood as within a range of normal
tolerance in the art, for example within 2 standard deviations of
the mean. "About" can be understood as within 10%, 9%, 8%, 7%, 6%,
5%, 4%, 3%, 2%, 1%, 0.5%, 0.1%, 0.05%, or 0.01% of the stated
value. Unless otherwise clear from the context, all numerical
values provided herein are modified by the term "about."
[0037] It would be also understood that the term "vehicle" or
"vehicular" or other similar term as used herein is inclusive of
motor vehicles in general such as passenger automobiles including
sports utility vehicles (SUV), buses, trucks, various commercial
vehicles, watercraft including a variety of boats and ships,
aircraft, and the like, and includes hybrid vehicles, electric
vehicles, plug-in hybrid electric vehicles, hydrogen-powered
vehicles and other alternative fuel vehicles (e.g. fuels derived
from resources other than petroleum). As referred to herein, a
hybrid vehicle is a vehicle that has two or more sources of power,
for example both gasoline-powered and electric-powered
vehicles.
[0038] Hereinafter, the present invention will be described in
detail. The present invention relates to a bearing steel having
improved fatigue durability and a method of manufacturing the same,
and in one aspect, the present invention relates to a bearing steel
having improved fatigue durability.
[0039] The steel composition for bearing may have improved
durability. The steel composition may comprise: an amount greater
than 0 wt % and of about 0.03 wt % or less of phosphorus (P), an
amount greater than 0 wt % and of about 0.01 wt % or less of sulfur
(S), an amount of about 0.01 to 0.1 wt % of copper (Cu), an amount
of about 0.01 to 0.06 wt % of aluminum (Al), an amount greater than
0 wt % and of about 0.006 wt % or less of nitrogen (N), an amount
greater than 0 wt % and of about 0.001 wt % of oxygen (O), and iron
(Fe) constituting the balance of the weight of the steel
composition. All the weight % (wt %) are based on the total weight
of the steel composition.
[0040] In particular, according to a necessity of the invention,
the steel composition for bearing may suitably comprise one or more
of an amount of about 0.8 to 1.0 wt % of carbon (C), an amount of
about 0.9 to 1.6 wt % of silicon (Si), an amount of about 0.5 to
1.00 wt % of manganese (Mn), an amount of about 0.1 to 0.6 wt % of
nickel (Ni), an amount of about 1.4 to 1.55 wt % of chromium (Cr),
an amount of about 0.2 to 0.5 wt % of molybdenum (Mo), and an
amount greater than 0 wt % and of about 0.4 wt % or less of
vanadium (V).
[0041] Hereinafter, each component of the steel composition and
content thereof will be described in detail
[0042] (1) Carbon (C)
[0043] Carbon (C), as used herein, may be important to secure
strength of the bearing steel, and stabilize residual
austenite.
[0044] Preferably, the content of carbon (C) may be an amount of
about 0.8 to 1.0 wt % based on the total weight of the alloy steel
composition. When the content of carbon (C) is less than about 0.8
wt %, strength of the steel used as bearing steel may not be
sufficiently obtained and a reduction in fatigue strength and the
like may be caused. On the other hand, when the content of carbon
(C) is greater than about 1.0 wt %, an undissolved large carbide
may remain in the steel, and thus fatigue strength, a durability
life, and the like may be reduced and processability before
quenching and the like may be reduced.
[0045] (2) Silicon (Si)
[0046] Silicon (Si), as used herein, may serve as a deoxidizer, and
suppress formation of a pin hole of the alloy steel, thereby
increasing strength of the alloy steel by a solid-solution
strengthening effect as being solid-solved in a matrix, and
increasing activity of carbon (C) and the like.
[0047] Preferably, the content of silicon (Si) may be an amount of
about 0.9 to 1.6 wt % based on the total weight of the alloy steel
composition. When the content of silicon (Si) is less than about
0.9 wt %, oxide by oxygen may not be sufficiently removed and
remain in the alloy steel, and thus strength of the alloy steel may
be reduced and a sufficient solid-solution strengthening effect may
not be obtained. When the content of silicon (Si) is greater than
about 2.0 wt %, decarbonization may occur by an interpenetration
reaction in a tissue, such as a site competition reaction with
carbon (C) by the excessive content of silicon (Si), and
processability may be rapidly reduced due to an increase in
hardness before quenching.
[0048] (3) Manganese (Mn)
[0049] Manganese (Mn), as used herein, may improve a quenching
property of the alloy steel and improve the strength of the alloy
steel and the like.
[0050] Preferably, the content of manganese (Mn) may be an amount
of about 0.5 to 1.0 wt %. When the content of manganese (Mn) is
less than about 0.5 wt %, an improvement effect of the quenching
property of the alloy steel may be reduced. On the other hand, when
the content of manganese (Mn) is greater than about 1.0 wt %,
processability before quenching may be reduced and MnS reducing
center segregation and a fatigue life may be precipitated.
[0051] (4) Nickel (Ni)
[0052] Nickel (Ni), as used herein, may micronize crystal grains of
the alloy steel and may be solid-solved in austenite and ferrite to
strengthen a matrix. Moreover, nickel may improve toughness to an
impact at low temperatures and a hardening ability, and reduce a
temperature of an A1 transformation point to expand austenite.
Further, nickel may increase activity of carbon.
[0053] Preferably, the content of nickel (Ni) may be an amount of
about 0.1 to 0.6 wt %. When the content of nickel (Ni) is less than
about 0.1 wt %, an effect of micronization of the crystal grains
and improvement effect such as solid-solution strengthening and
matrix strengthening may not be sufficiently obtained. On the other
hand, when the content of nickel (Ni) is greater than about 0.6 wt
%, red shortness and like may be caused in the alloy steel.
[0054] (5) Chromium (Cr)
[0055] Chromium (Cr), as used herein, may improve a quenching
property of the alloy steel, provide hardenability, and
simultaneously, micronize a tissue of the alloy steel and
spheroidize the tissue by heat-treating. Further, chromium may
harden a lamella in cementite.
[0056] Preferably, the content of chromium (Cr) may be an amount of
about 1.5 to 3.0 wt %. When the content of chromium (Cr) is less
than about 1.5 wt %, the quenching property and hardenability may
be limited and sufficient micronization and spheroidizing of the
tissue may not be obtained. On the other hand, when the content of
chromium (Cr) is greater than about 3.0 wt %, an increase effect of
the content may not be sufficient, and thus manufacturing costs may
be increased.
[0057] (6) Molybdenum (Mo)
[0058] Molybdenum (Mo), as used herein, increase a quenching
property of the alloy steel thereby improving hardenability,
toughness, and the like of the alloy steel after tempering and
providing brittleness resistance. Further, molybdenum may reduce
activity of carbon.
[0059] Preferably, the content of molybdenum (Mo) may be an amount
of about 0.2 to 0.5 wt %. When the content of molybdenum (Mo) is
less than about 0.2 wt %, hardenability and toughness of the alloy
steel, and the like may not be sufficiently secured. On the other
hand, when the content of molybdenum (Mo) is greater than about 0.5
wt %, processability (machinability) and productivity of the alloy
steel, and the like may be reduced and an increase effect of the
content may not be sufficient and thus manufacturing costs may be
increased.
[0060] (7) Vanadium (V)
[0061] Vanadium (V), as used herein, may form precipitates such as
carbides, strengthen a matrix tissue and thus improve strength and
wear resistance through a precipitation strengthening effect. In
addition, vanadium may reduce activity of carbon, and further, at
the same cooling rate, strength of the alloy steel may be increased
by addition of vanadium.
[0062] Preferably, the content of vanadium (V) may be an amount
greater than 0 wt % and of about 0.4 wt % or less. When the content
of vanadium (V) is greater than about 0.4 wt %, toughness and
hardness of the alloy steel and the like may be reduced.
[0063] (8) Aluminum (Al)
[0064] Aluminum (Al), as used herein, may be an element serving as
a strong deoxidizer and serving to improve cleanliness of the alloy
steel and be reacted with nitrogen (N) in the alloy steel to form
nitride and thus micronize the crystal grains.
[0065] Preferably, the content of aluminum (Al) may be an amount of
about 0.01 to 0.06 wt %. When the content of aluminum (Al) is less
than about 0.01 wt %, sufficient effects relating to the
deoxidizer, cleanliness, and micronization of the crystal grains
may not be obtained. On the other hand, when the content of
aluminum (Al) is greater than about 0.06 wt %, coarse oxide
inclusions and the like may be formed to reduce a fatigue life of
the steel and the like.
[0066] (9) Nitrogen (N)
[0067] Nitrogen (N), as used herein, may stabilize austenite,
micronizing crystal grains, and improve tensile strength, yield
strength, and elongation of the alloy steel and the like. However,
an impurity or AlN (aluminum nitride) may be formed to reduce a
durability life when excessive amount of nitrogen is included.
[0068] Preferably, the content of nitrogen (N) may be greater than
0 wt % and of about 0.006 wt % or less. When the content of
nitrogen (N) is greater than 0.006 wt %, brittleness may be caused
and the durability life and the like may be reduced.
[0069] (10) Oxygen (O)
[0070] Oxygen (O), as used herein, may increase generation of the
impurity of the alloy steel to reduce cleanliness and degrade the
alloy steel through contact fatigue.
[0071] Preferably, the content of oxygen (O) may be an amount of
about 0.001 wt % or less. When the content of oxygen (O) is greater
than 0.001 wt %, the impurity of the alloy steel may be increased
to degrade the alloy steel due to contact fatigue.
[0072] (11) Phosphorus (P)
[0073] Phosphorus (P), as used herein, may induce crystal grain
boundary segregation to reduce toughness of the alloy steel.
[0074] Preferably, the content of phosphorus (P) may an amount
greater than 0 wt % and of about 0.03 wt % or less. When the
content of phosphorus (P) is greater than about 0.03 wt %,
toughness of the alloy steel may be reduced.
[0075] (12) Sulfur (S)
[0076] Sulfur (S), as used herein, may increase machinability of
the alloy steel to facilitate processing, and also reduce toughness
of the alloy steel due to grain boundary segregation and reduce a
fatigue life of the alloy steel by being reacted with manganese
(Mn) to form MnS.
[0077] Preferably, the content of sulfur (S) may be an amount
greater than 0 wt % and of about 0.01 wt % or less. When the
content of sulfur (S) is greater than about 0.01 wt %, toughness of
the alloy steel may be reduced thereby reducing a fatigue life of
the steel.
[0078] (13) Copper (Cu)
[0079] Copper (Cu), as used herein, may improve hardenability of
the alloy steel and the like.
[0080] Preferably, the content of copper (Cu) may be an amount of
about 0.01 to 0.1 wt %. When the content of copper (Cu) is less
than about 0.01 wt %, sufficient hardenability improvement effect
may not be obtained. Meanwhile, when the content of copper (Cu) is
greater than about 0.1 wt %, since a solid-solution limitation may
be exceeded, a strength improvement effect of the steel may be
saturated, and thus manufacturing costs may be increased and red
shortness may be caused.
[0081] Since the steel composition for bearing including the
aforementioned components may have superior hardness, strength,
toughness, fatigue strength, and a fatigue life. Accordingly, the
steel composition may be applied to vehicle parts and the like. For
example, the steel composition for bearing may be applied to
automatic or manual transmissions and the like of vehicles, and
among the transmission parts, the bearing steel may be applied to
carriers, annulus gears, gears, shafts, synchronizer hubs, or the
like.
[0082] Hereinafter, in another aspect, the present invention
relates to a method of manufacturing a bearing steel having
improved fatigue durability.
[0083] The bearing steel having improved fatigue durability
according to the present invention may be suitably manufactured by
a person with ordinary skill in the art with reference to a
publicly known technology.
[0084] The method of manufacturing a bearing steel having improved
fatigue durability according to the present invention may
comprise:
[0085] For example, as shown in FIG. 1, the method may comprise:
mixing components of the alloy steel for a bearing (S10);
heat-treating the alloy steel for primary spheroidizing at a
temperature of about 720 to 850.degree. C. for about 4 to 8 hours
(S20); wire drawing the heat-treated alloy steel (S30); secondary
heat-treating the wire-drawn alloy steel for second spheroidizing
at a temperature of about 720 to 850.degree. C. for about 4 to 8
hours (S40); forging the secondary heat-treated alloy steel (S50);
quenching the forged alloy steel at a temperature of about 840 to
860.degree. C. for about 0.5 to 2 hours (S60); and tempering the
quenched alloy steel at a temperature of about 150 to 190.degree.
C. for about 0.5 to 2 hours (S70).
[0086] In the method of manufacturing the bearing steel, the
complex carbide may be formed and spheroidized in the steel.
Particularly, the complex carbide may include one or more selected
from the group consisting of M.sub.3C, M.sub.7C.sub.3, and
M.sub.23C.sub.6 carbides, and MC carbides that are precipitates. M
may be a metal or a transition metal without limitation.
[0087] Preferably, M of the M.sub.3C and M.sub.7C.sub.3 carbides,
and the M.sub.23C.sub.6 carbides may be one or more selected from
the group consisting of chromium (Cr), iron (Fe), and manganese
(Mn), and M of the MC carbide may be one or more selected from the
group consisting of vanadium (V) and molybdenum (Mo). When the
aforementioned complex carbides are formed, strength and hardness
of the bearing steel, and the like may be improved, and a
durability life and the like may be prolonged.
[0088] The method may comprise manufacturing a wire rod using the
alloy steel for bearing, as described above. For example, the wire
rod comprising the alloy component as described above may be
manufactured by adding and mixing one or more components selected
from carbon (C), silicon (Si), manganese (Mn), nickel (Ni),
chromium (Cr), molybdenum (Mo), or vanadium (V) to iron (Fe) as the
main component, phosphorus (P), sulfur (S), copper (Cu), aluminum
(Al), nitrogen (N), and oxygen (O). Thus manufactured wire rod may
be further heat treated for primary spheroidizing, for example, at
a temperature of about 720 to 850.degree. C. for about 4 to 8 hours
(S20); wire drawn (S30); secondary heat-treated for second
spheroidizing, for example, at a temperature of about 720 to
850.degree. C. for about 4 to 8 hours (S40); forged (S50);
quenched, for example, at a temperature of about 840 to 860.degree.
C. for about 0.5 to 2 hours (S60); and tempered, for example, at a
temperature of about 150 to 190.degree. C. for about 0.5 to 2 hours
(S70).
[0089] The quenching of the manufacturing method may be performed
at a temperature of about a temperature of about 840 to 860.degree.
C. for about 0.5 to 2 hours, and the tempering may be performed at
a temperature of about 150 to 190.degree. C. for about 0.5 to 2
hours.
[0090] When the quenching temperature is less than about
840.degree. C. or the quenching time is less than about 0.5 hours,
a rapidly cooled tissue may be formed nonuniformly thereby causing
material deviation. On the other hand, when the quenching
temperature is greater than about 860.degree. C. or the quenching
time is greater than about 2 hours, the spheroidized complex
carbide formed by the primary and secondary spheroidizing
heat-treating may be dissolved.
[0091] When the tempering temperature is less than about
150.degree. C. or the tempering time is less than about 0.5 hours,
physical properties such as toughness of the bearing steel may not
be secured. On the other hand, when the tempering temperature is
greater than about 190.degree. C. or the tempering time is greater
than about 2 hours, hardness of the bearing steel and the like may
be rapidly reduced, and thus it may be difficult to improve a
durability life.
[0092] Meanwhile, when temperatures in the primary and secondary
spheroidizing heat-treating of the manufacturing method are each
less than about 720.degree. C. or the spheroidizing heat-treating
time is less than about 4 hours, a lot of spheroidizing time of the
complex carbide may be required, and thus manufacturing costs may
be rapidly increased. On the other hand, when temperatures in the
primary and secondary spheroidizing heat-treating are greater than
about 850.degree. C., since the formed complex carbide is
dissolved, a possibility of forming a lamella-type complex carbide
instead of a spherical complex carbide during a cooling process may
be significantly increased. Further, when times in the primary and
secondary spheroidizing heat-treating are greater than about 8
hours, a spheroidizing rate of the complex carbide may be slowed
thereby rapidly increasing manufacturing costs.
Example
[0093] Hereinafter, the present invention will be described in more
detail through the Examples. These Examples are only for
illustrating the present invention, and it will be obvious to those
skilled in the art that the scope of the present invention is not
interpreted to be limited by these Examples.
[0094] In order to compare physical properties of the bearing steel
having improved fatigue durability according to exemplary
embodiments of the present invention, Comparative Examples and
Examples having the components as described in the following Table
1 were manufactured. The conditions of the primary and secondary
spheroidizing heat-treating temperatures, the quenching temperature
and time, and the tempering temperature and time applied are
described in the following Table 2.
TABLE-US-00001 TABLE 1 (Unit: wt %) C Si Mn P S Cu Ni Cr Mo Al N O
V Comparative 1.00 0.27 0.38 0.012 0.005 0.05 0.05 1.46 0.02 0.017
0.0035 0.0006 -- Example 1 Comparative 0.75 0.85 0.63 0.012 0.004
0.049 0.52 1.55 0.24 0.024 0.0049 0.0004 0.26 Example 2 Comparative
0.95 1.02 0.62 0.010 0.005 0.047 0.62 1.42 0.23 0.023 0.0044 0.0003
0.43 Example 3 Comparative 0.99 1.53 0.60 0.011 0.004 0.048 0.19
1.32 0.54 0.026 0.0052 0.0004 0.47 Example 4 Comparative 0.92 1.72
0.70 0.014 0.004 0.049 0.44 1.47 0.048 0.017 0.0049 0.0003 --
Example 5 Comparative 0.91 1.05 0.72 0.013 0.005 0.047 0.08 1.48
0.49 0.018 0.0042 0.0004 0.42 Example 6 Comparative 0.88 1.45 0.73
0.011 0.004 0.048 0.72 1.13 0.62 0.015 0.0049 0.0005 -- Example 7
Comparative 1.05 0.79 0.69 0.007 0.006 0.049 0.51 1.43 0.15 0.018
0.0052 0.0004 0.16 Example 8 Comparative 0.83 1.24 0.68 0.012 0.005
0.047 0.66 1.45 0.13 0.015 0.0042 0.0005 0.43 Example 9 Comparative
0.81 1.57 0.89 0.014 0.004 0.042 0.18 1.59 0.35 0.018 0.0047 0.0003
0.46 Example 10 Example 1 0.81 1.04 0.65 0.010 0.003 0.052 0.13
1.52 0.21 0.025 0.0048 0.0006 -- Example 2 0.91 1.52 0.73 0.009
0.004 0.051 0.58 1.43 0.43 0.017 0.0053 0.0005 0.39 Example 3 0.98
0.92 0.83 0.011 0.005 0.048 0.35 1.49 0.48 0.011 0.0049 0.0003
0.32
[0095] Table 1 shows the constitutional components and the contents
of Comparative Examples 1 to 10 according to the bearing steel in
the related art and the constitutional components and the contents
of Examples 1 to 3 according to the present invention.
TABLE-US-00002 TABLE 2 Temperature in Temperature in primary
secondary Quenching Tempering spheroidizing spheroidizing
temperature temperature Classi- heat-treating heat-treating
(.degree. C.)/ (.degree. C.)/ fication (.degree. C.) (.degree. C.)
time (h) time (h) Condition 800 720 850/1 150/1
[0096] Table 2 shows, among the manufacturing conditions of
Comparative Examples 1 to 10 and Examples 1 to 3 having the
constitutional components and the contents of Table 1, temperatures
in the primary and secondary spheroidizing heat-treating, the
quenching temperature and time, and the tempering temperature and
time. Herein, all of Comparative Examples 1 to 10 and Examples 1 to
3 satisfied temperatures in the primary and secondary spheroidizing
heat-treating, the quenching temperature and time, and the
tempering temperature and time according to the present
invention.
TABLE-US-00003 TABLE 3 Rotation number of rotation bending fatigue
tester under surface pressure Comparison of Hardness at of 6.2 GPa
durability lives room Hardness at 150.degree. C. (based on
temperature at 300.degree. C. (L10 life, Comparative Classification
(HV) (HV) times) Example 1) Comparative 720 698 8,400,000 100%
Example 1 Comparative 764 725 9,320,000 111% Example 2 Comparative
763 723 9,290,000 111% Example 3 Comparative 768 715 9,410,000 112%
Example 4 Comparative 751 717 8,620,000 103% Example 5 Comparative
765 724 8,690,000 103% Example 6 Comparative 741 718 8,750,000 104%
Example 7 Comparative 759 712 9,540,000 114% Example 8 Comparative
773 726 9,140,000 109% Example 9 Comparative 748 718 9,390,000 112%
Example 10 Example 1 842 829 18,579,000 221% Example 2 843 831
18,347,000 218% Example 3 847 834 18,482,000 220%
[0097] Table 3 shows hardness at room temperature, hardness at
300.degree. C., the rotation number of the rotation bending fatigue
tester to the L10 life at 150.degree. C. when surface pressure is
6.2 GPa, and the durability life considering after Comparative
Examples 1 to 10 and Examples 1 to 3 having the constitutional
components and the contents of Table 1 were manufactured according
to the condition of Table 2.
[0098] Hardness at room temperature, hardness at 300.degree. C.,
and hardness at 150.degree. C. when surface pressure was 6.2 GPa
were measured at 300 gf according to the KS B 0811 measurement
method by using the Micro Vickers Hardness tester. For the rotation
number of the rotation bending fatigue tester, the L10 life was
measured according to the KS B ISO 1143 measurement method under
the condition of the maximum flection moment of about 20 kgfm, the
rotation number of about 200 to 3000 RPM, the maximum load of about
100 kg or less, and electric power of three phases, 220 V, and 7 kW
by using the standard line diameter of the diameter of about 4 mm
through the rotation bending fatigue tester. The L10 life is the
durability life of the specimen, and means the total rotation
number of the rotation bending fatigue tester until about 10% of
the specimen is damaged.
[0099] Accordingly, reviewing hardness at room temperature (about
25.degree. C.), it can be confirmed from Table 3 that in Examples 1
to 3, as compared to Comparative Examples 1 to 10, hardness at room
temperature was improved by 8.9% to 17.5%. Additionally, reviewing
hardness at 300.degree. C., it can be confirmed from Table 3 that
in Examples 1 to 3, as compared to Comparative Examples 1 to 10,
hardness at room temperature was improved by 14.4% to 19.4%.
[0100] It can be confirmed that when surface pressure was 6.2 GPa,
of the rotation number of the rotation bending fatigue tester to
the L10 life at 150.degree. C., the average value of Examples 1 to
3 was 18,469,333 and was about two times higher than 9,055,000
which was the average value of Comparative Examples 1 to 10. That
is, it can be confirmed from Table 3 that the bearing steels of the
present invention were improved by 192.3% to 221.1% as compared to
the related art.
[0101] In order to compare the durability lives of Comparative
Examples 1 to 10 and Examples 1 to 3 based on the rotation number
of the rotation bending fatigue tester, rotation number of the
rotation bending fatigue tester of Comparative Example 1,
8,400,000, was set as the basis of the durability life of 100%. As
compared to the rotation number of the rotation bending fatigue
tester of Comparative Example 1 as the basis, the difference
showing the degree of increase or decrease in rotation number of
the rotation bending fatigue tester of Comparative Examples 2 to 10
and Examples 1 to 3 was represented as the percentage. That is, the
percentage for comparing the durability lives of Comparative
Examples 1 to 10 and Examples 1 to 3 is a value representing the
degree of relative increase and decrease of the rotation numbers of
the rotation bending fatigue tester of the residual Comparative
Examples 2 to 10 and Examples 1 to 3 based on Comparative Example
1.
[0102] Herein, through comparison of the durability lives of the
Comparative Examples and the Examples, it could be seen from Table
3 that like the rotation number of the rotation bending fatigue
tester, the durability life of Examples 1 to 3 was about two times
greater than the durability life of Comparative Examples 1 to
10.
[0103] As described above, in order to check the reason why
hardness and the durability life of the Examples were better than
those of the Comparative Examples, the types and vol % of the
complex carbides included in Comparative Example 1 and Examples 1
to 3 are described in the following Table 4.
TABLE-US-00004 TABLE 4 Classification Me.sub.3C Me.sub.7C.sub.3 VC
+ NbC MoC Comparative Example 1 12.7 -- -- 0.02 Example 1 8.14 --
-- 0.23 Example 2 7.58 -- 1.06 0.31 Example 3 9.19 -- -- 1.19 Unit:
vol % Me: One or more selected from the group consisting of
chromium (Cr), iron (Fe), and manganese (Mn)
[0104] Table 4 shows the contents of the complex carbides included
in Comparative Example 1 and Examples 1 to 3. As shown in Table 4,
the complex carbide of Comparative Example 1 mainly included
M.sub.3C with a small amount of MoC, but Examples 1 to 3 relatively
uniformly included VC and NbC as well as M.sub.3C and MoC. This
difference in constitution of the complex carbides may be
considered as one of reasons why the Examples have hardness and the
durability life that are better than those of the Comparative
Examples.
[0105] Therefore, it could be experimentally confirmed that
Examples 1 to 3 satisfying the components and the content range
according to the present invention and manufactured through the
heat-treating process according to the present invention included
various complex carbides and the like, and thus had strength and
the durability life that were better than those of Comparative
Examples 1 to 10.
[0106] As described above, the present invention has been described
in relation to specific embodiments of the present invention, but
the embodiments are only illustration and the present invention is
not limited thereto. Embodiments described may be changed or
modified by those skilled in the art to which the present invention
pertains without departing from the scope of the present invention,
and various alterations and modifications are possible within the
technical spirit of the present invention and the equivalent scope
of the claims which will be described below.
* * * * *