U.S. patent application number 15/110351 was filed with the patent office on 2016-11-17 for bearing part, steel for bearing part and method for producing thereof.
This patent application is currently assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION. The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Junichi KODAMA, Yutaka NEISHI, Masashi SAKAMOTO.
Application Number | 20160333437 15/110351 |
Document ID | / |
Family ID | 53524009 |
Filed Date | 2016-11-17 |
United States Patent
Application |
20160333437 |
Kind Code |
A1 |
SAKAMOTO; Masashi ; et
al. |
November 17, 2016 |
BEARING PART, STEEL FOR BEARING PART AND METHOD FOR PRODUCING
THEREOF
Abstract
A bearing part according the present invention includes: C:
0.95% to 1.10%; Si: 0.10% to 0.70%; Mn: 0.20% to 1.20%; Cr: 0.90%
to 1.60%; Al: 0.010% to 0.100%; N: 0.003% to 0.030%; P: 0.025% or
less; S: 0.025% or less; O: 0.0010% or less; optionally one or more
of the group consisting of Mo: 0.25% or less, B: 0.0050% or less,
Cu: 1.0% or less, Ni: 3.0% or less, and Ca: 0.0015% or less; and a
remainder including Fe and impurities; wherein a metallographic
structure includes a retained austenite, a spherical cementite and
a martensite; in which an amount of the retained austenite is 18%
to 25%, by volume %; an average grain size of a prior-austenite is
6.0 .mu.m or less; an average grain size of the spherical cementite
is 0.45 .mu.m or less; and a number density of the spherical
cementite is 0.45.times.10.sup.6 mm.sup.-2 or more in the
metallographic structure.
Inventors: |
SAKAMOTO; Masashi;
(Kamaishi-shi, JP) ; KODAMA; Junichi;
(Kamaishi-shi, JP) ; NEISHI; Yutaka; (Osaka,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
NIPPON STEEL & SUMITOMO METAL
CORPORATION
Tokyo
JP
|
Family ID: |
53524009 |
Appl. No.: |
15/110351 |
Filed: |
January 9, 2015 |
PCT Filed: |
January 9, 2015 |
PCT NO: |
PCT/JP2015/050528 |
371 Date: |
July 7, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B22D 23/00 20130101;
F16C 33/30 20130101; C22C 38/06 20130101; C21D 2211/004 20130101;
C22C 38/54 20130101; C21D 2211/003 20130101; C22C 38/02 20130101;
C22C 38/40 20130101; C21D 9/36 20130101; C22C 38/18 20130101; C22C
38/22 20130101; C22C 38/32 20130101; F16C 2204/66 20130101; C22C
38/20 20130101; F16C 2223/02 20130101; C21D 1/18 20130101; C21D
1/32 20130101; C22C 38/44 20130101; C21D 2211/008 20130101; C22C
38/002 20130101; F16C 33/62 20130101; C22C 38/04 20130101; C22C
38/00 20130101; C22C 38/001 20130101; F16C 2204/72 20130101; F16C
33/64 20130101; F16C 2223/10 20130101; C22C 38/08 20130101; C21D
8/065 20130101; C21D 9/38 20130101; C21D 9/40 20130101; F16C
2204/70 20130101; F16C 2220/44 20130101; C21D 2211/005 20130101;
C21D 2211/001 20130101 |
International
Class: |
C21D 9/36 20060101
C21D009/36; C22C 38/54 20060101 C22C038/54; C22C 38/44 20060101
C22C038/44; C22C 38/32 20060101 C22C038/32; C22C 38/22 20060101
C22C038/22; C22C 38/20 20060101 C22C038/20; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00; B22D 23/00 20060101
B22D023/00; C21D 8/06 20060101 C21D008/06; C21D 1/32 20060101
C21D001/32; C21D 1/18 20060101 C21D001/18; F16C 33/30 20060101
F16C033/30 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 10, 2014 |
JP |
2014-003338 |
Apr 16, 2014 |
JP |
2014-084952 |
Claims
1. A bearing part comprising, as a chemical composition, by mass %,
C: 0.95% to 1.10%; Si: 0.10% to 0.70%; Mn: 0.20% to 1.20%; Cr:
0.90% to 1.60%; Al: 0.010% to 0.100%; N: 0.003% to 0.030%; P:
0.025% or less; S: 0.025% or less; O: 0.0010% or less; and
optionally one or more selected from the group consisting of: Mo:
0.25% or less, B: 0.0050% or less, Cu: 1.0% or less, Ni: 3.0% or
less, and Ca: 0.0015% or less; and a remainder including Fe and
impurities; wherein a metallographic structure includes a retained
austenite, a spherical cementite and a martensite; wherein an
amount of the retained austenite is 18% to 25%, by volume %;
wherein an average grain size of a prior-austenite is 6.0 .mu.m or
less; wherein an average grain size of the spherical cementite is
0.45 .mu.m or less; and wherein a number density of the spherical
cementite is 0.45.times.10.sup.6 mm.sup.-2 or more in the
metallographic structure.
2. The bearing part according to claim 1 comprising, as the
chemical composition, by mass %, one or more of Mo: 0.01% to 0.25%,
B: 0.0001% to 0.0050%, Cu: 0.1% to 1.0%, Ni: 0.05% to 3.0%, and Ca:
0.0003% to 0.0015%.
3. A steel for a bearing part comprising, as a chemical
composition, by mass %, C: 0.95% to 1.10%; Si: 0.10% to 0.70%; Mn:
0.20% to 1.20%; Cr: 0.90% to 1.60%; Al: 0.010% to 0.100%; N: 0.003%
to 0.030%; S: 0.025% or less; P: 0.025% or less; O: 0.0010% or
less; optionally one or more selected from the group consisting of:
Mo: 0.25% or less, B: 0.0050% or less, Cu: 1.0% or less, Ni: 3.0%
or less, and Ca: 0.0015% or less; and a remainder including Fe and
impurities; wherein a metallographic structure includes a spherical
cementite and a ferrite; wherein a number density of the spherical
cementite having a grain size of 0.5 .mu.m to 3.0 .mu.m is
2.0.times.10.sup.6 mm.sup.-2 or more in the metallographic
structure.
4. The steel for the bearing part according to claim 3 comprising,
as the chemical composition, by mass %, one or more of Mo: 0.01% to
0.25%, B: 0.0001% to 0.0050%, Cu: 0.1% to 1.0%, Ni: 0.05% to 3.0%,
and Ca: 0.0003% to 0.0015%.
5. A method for producing a steel for a bearing part comprising, a
casting process for obtaining a steel piece having a chemical
composition according to claim 3; a heating process of heating the
steel piece to 900.degree. C. to 1300.degree. C.; a hot rolling
process for obtaining a hot rolled wire rod by subjecting the steel
piece to hot rolling at a finish rolling temperature of 850.degree.
C. or less after the heating process; a winding process of winding
the hot rolled wire rod after the hot rolling process at a winding
temperature of 800.degree. C. or less; a cooling process for making
a metallographic structure of the hot rolled wire rod a pearlite by
cooling the hot rolled wire rod to 600.degree. C. at a cooling rate
of 3.0.degree. C./second or less after the winding process; a wire
drawing process of subjecting the hot rolled wire rod to wire
drawing in which a total reduction of an area is 50% or more after
the cooling process; and a spheroidizing annealing process for
obtaining the steel for the bearing part by performing a
spheroidizing annealing in which the hot rolled wire rod after the
wire drawing process is held at a temperature of 650.degree. C. to
the lower of 750.degree. C. and A.sub.1-5.degree. C. for 0.5 hours
to 5 hours; wherein, A.sub.1 represents a temperature at which
A.sub.1 transformation starts, and is a predicted value which is
calculated by a following equation 1 based on the chemical
composition, and, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [Al] and
[B] in the equation 1 represent, by mass %, the content of C, the
content of Si, the content of Mn, the content of Cu, the content of
Ni, the content of Cr, the content of Mo, the content of Al and the
content of B, respectively, in the hot rolled wire rod,
A.sub.1=750.8-26.6.times.[C]+17.6.times.[Si]-11.6.times.[Mn]-22.9.times.[-
Cu]-23.0.times.[Ni]+24.1.times.[Cr]+22.5.times.[Mo]-169.4.times.[Al]-894.7-
.times.[B] (Equation 1).
6. A method for producing a bearing part comprising, a casting
process for obtaining a steel piece having a chemical composition
according to claim 1; a heating process of heating the steel piece
to 900.degree. C. to 1300.degree. C.; a hot rolling process for
obtaining a hot rolled wire rod by subjecting the steel piece to
hot rolling at a finish rolling temperature of 850.degree. C. or
less after the heating process; a winding process of winding the
hot rolled wire rod after the hot rolling process at a winding
temperature of 800.degree. C. or less; a cooling process for making
a metallographic structure of the hot rolled wire rod a pearlite by
cooling the hot rolled wire rod to 600.degree. C. at a cooling rate
of 3.0.degree. C./second or less after the winding process; a wire
drawing process of subjecting the hot rolled wire rod to wire
drawing in which a total reduction of an area is 50% or more after
the cooling process; a spheroidizing annealing process for
obtaining a steel for a bearing part by performing spheroidizing
annealing in which the hot rolled wire rod after the wire drawing
process is held at a temperature of 650.degree. C. to the lower of
750.degree. C. and A.sub.1-5.degree. C. for 0.5 hours to 5 hours; a
forming process of rough forming the steel for the bearing part
after the spheroidizing annealing process; a quenching process of
performing quenching by heating the steel for the bearing part
after the forming process to 800.degree. C. to 890.degree. C.; a
tempering process of subjecting the steel for the bearing part to
tempering at 250.degree. C. or less after the quenching process;
and a finish machining process for obtaining the bearing part by
subjecting the steel for the bearing part to finish machining after
the tempering process; wherein, A.sub.1 represents a temperature at
which A.sub.1 transformation starts, and is a predicted value which
is calculated by a following equation 2 based on the chemical
composition, and, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [Al] and
[B] in the equation 2 represent, by mass %, the content of C, the
content of Si, the content of Mn, the content of Cu, the content of
Ni, the content of Cr, the content of Mo, the content of Al and the
content of B, respectively, in the hot rolled wire rod,
A.sub.1=750.8-26.6.times.[C]+17.6.times.[Si]-11.6.times.[Mn]-22.9.times.[-
Cu]-23.0.times.[Ni]+24.1.times.[Cr]+22.5.times.[Mo]-169.4.times.[Al]-894.7-
.times.[B] (Equation 2).
7. A method for producing a steel for a bearing part comprising, a
casting process for obtaining a steel piece having a chemical
composition according to claim 4; a heating process of heating the
steel piece to 900.degree. C. to 1300.degree. C.; a hot rolling
process for obtaining a hot rolled wire rod by subjecting the steel
piece to hot rolling at a finish rolling temperature of 850.degree.
C. or less after the heating process; a winding process of winding
the hot rolled wire rod after the hot rolling process at a winding
temperature of 800.degree. C. or less; a cooling process for making
a metallographic structure of the hot rolled wire rod a pearlite by
cooling the hot rolled wire rod to 600.degree. C. at a cooling rate
of 3.0.degree. C./second or less after the winding process; a wire
drawing process of subjecting the hot rolled wire rod to wire
drawing in which a total reduction of an area is 50% or more after
the cooling process; and a spheroidizing annealing process for
obtaining the steel for the bearing part by performing a
spheroidizing annealing in which the hot rolled wire rod after the
wire drawing process is held at a temperature of 650.degree. C. to
the lower of 750.degree. C. and A.sub.1-5.degree. C. for 0.5 hours
to 5 hours; wherein, A.sub.1 represents a temperature at which
A.sub.1 transformation starts, and is a predicted value which is
calculated by a following equation 1 based on the chemical
composition, and, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [Al] and
[B] in the equation 1 represent, by mass %, the content of C, the
content of Si, the content of Mn, the content of Cu, the content of
Ni, the content of Cr, the content of Mo, the content of Al and the
content of B, respectively, in the hot rolled wire rod,
A.sub.1=750.8-26.6.times.[C]+17.6.times.[Si]-11.6.times.[Mn]-22.9.times.[-
Cu]-23.0.times.[Ni]+24.1.times.[Cr]+22.5.times.[Mo]-169.4.times.[Al]-894.7-
.times.[B] (Equation 1).
8. A method for producing a bearing part comprising, a casting
process for obtaining a steel piece having a chemical composition
according to claim 2; a heating process of heating the steel piece
to 900.degree. C. to 1300.degree. C.; a hot rolling process for
obtaining a hot rolled wire rod by subjecting the steel piece to
hot rolling at a finish rolling temperature of 850.degree. C. or
less after the heating process; a winding process of winding the
hot rolled wire rod after the hot rolling process at a winding
temperature of 800.degree. C. or less; a cooling process for making
a metallographic structure of the hot rolled wire rod a pearlite by
cooling the hot rolled wire rod to 600.degree. C. at a cooling rate
of 3.0.degree. C./second or less after the winding process; a wire
drawing process of subjecting the hot rolled wire rod to wire
drawing in which a total reduction of an area is 50% or more after
the cooling process; a spheroidizing annealing process for
obtaining a steel for a bearing part by performing spheroidizing
annealing in which the hot rolled wire rod after the wire drawing
process is held at a temperature of 650.degree. C. to the lower of
750.degree. C. and A.sub.1-5.degree. C. for 0.5 hours to 5 hours; a
forming process of rough forming the steel for the bearing part
after the spheroidizing annealing process; a quenching process of
performing quenching by heating the steel for the bearing part
after the forming process to 800.degree. C. to 890.degree. C.; a
tempering process of subjecting the steel for the bearing part to
tempering at 250.degree. C. or less after the quenching process;
and a finish machining process for obtaining the bearing part by
subjecting the steel for the bearing part to finish machining after
the tempering process; wherein, A.sub.1 represents a temperature at
which A.sub.1 transformation starts, and is a predicted value which
is calculated by a following equation 2 based on the chemical
composition, and, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [Al] and
[B] in the equation 2 represent, by mass %, the content of C, the
content of Si, the content of Mn, the content of Cu, the content of
Ni, the content of Cr, the content of Mo, the content of Al and the
content of B, respectively, in the hot rolled wire rod,
A.sub.1=750.8-26.6.times.[C]+17.6.times.[Si]-11.6.times.[Mn]-22.9.times.[-
Cu]-23.0.times.[Ni]+24.1.times.[Cr]+22.5.times.[Mo]-169.4.times.[Al]-894.7-
.times.[B] (Equation 2).
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to a bearing part such as
needle bearings, roller bearings and ball bearings, steel for a
bearing part which is a material for the bearing part, and a method
for producing thereof.
[0002] Priority is claimed on Japanese Patent Application No.
2014-3338, filed on Jan. 10, 2014, and Japanese Patent Application
No. 2014-84952, filed on Apr. 16, 2014, the contents of which are
incorporated herein by reference.
RELATED ART
[0003] Bearing parts such as needle bearings, roller bearings and
ball bearings are continually used under a situation where a
foreign material such as burr or abrasion powder is mixed into
lubricating oil, that is, are continually used even in contaminated
environments. Therefore, it is important to improve rolling contact
fatigue life of a bearing part in contaminated environments. In
order to improve the rolling contact fatigue life of the bearing
part in contaminated environments, it is known that an increase in
retained austenite is effective. Accordingly, steel for the bearing
part is subjected to a surface treatment such as a carburizing or
nitriding.
[0004] However, there are some problems that not only a surface
treatment such as carburizing or nitriding for the steel for the
bearing parts is high cost, but also a variation in qualities of a
bearing part occurs due to the influence of variations in the
treatment atmosphere. For example, Patent Document 1 discloses a
bearing part including large amount of retained austenite
manufactured by quenching and tempering and without carburizing or
a nitriding.
[0005] A bearing part disclosed in the Patent Document 1 secures
the amount of retained austenite by lowering the martensite start
temperature (Ms point) by including C, Mn and Ni, or Mo into the
steel. However, when a content of Mn, which is added to the steel,
is increased in order to secure the amount of retained austenite,
the hardenability of the steel is raised. As a result, a
supercooled structure such as martensite is generated during
cooling after hot rolling, and the workability, ductility and
toughness of hot rolled wire rod is deteriorated.
[0006] In addition, the Patent Document 2 discloses a method of
generating retained austenite while suppressing grain coarsening by
using spherical cementite. However, in the method disclosed in
Patent Document 2, spheroidizing at high temperature is performed
for long period of time. As a result, C is solid-soluted into an
austenite phase and the number density of spherical cementite is
insufficient. Furthermore, the grain size of austenite is coarsened
and an improvement effect of rolling contact fatigue life cannot be
obtained sufficiently.
[0007] Since the treatment time of spheroidizing annealing is long,
when the number of times of the annealing is increased, it is known
that production efficiency is deteriorated by increasing a
manufacturing cost. To solve this problem, for example, the Patent
Document 3 discloses a high-carbon steel rolled wire rod for
bearing parts that was invented by some of the present inventors
and the high-carbon steel rolled wire rod for bearing parts is
capable of wire drawing without spheroidizing annealing.
[0008] In addition, conventionally, spheroidizing annealing is
performed before or after the wire drawing. Among the spheroidizing
annealing, Patent Document 4 discloses a method in which the rolled
wire rod is subjected to the wire drawing after the hot rolling is
completed and the spheroidizing annealing is performed so that the
spheroidizing annealing before the wire drawing can be omitted.
[0009] However, the method disclosed in the Patent Document 4 is
not intended to shorten the treatment time of the spheroidizing
annealing.
PRIOR ART DOCUMENT
Patent Document
[0010] [Patent Document 1] Japanese Unexamined Patent Application,
First Publication No. 2004-124215
[0011] [Patent Document 2] Japanese Unexamined Patent Application,
First Publication No. 2007-077432
[0012] [Patent Document 3] PCT International Publication WO
2013/108828
[0013] [Patent Document 4] Japanese Unexamined Patent Application,
First Publication No. 2004-100016
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0014] However, as in Patent Document 1, when the content of Mn in
the steel is increased, it is difficult to process the hot rolled
wire rod by omitting spheroidizing annealing, and it is necessary
to perform spheroidizing several times so as to obtain the bearing
part for the foregoing reasons. In addition, the present inventors
found that it is necessary to control the microstructure through
the use of wire drawing and quenching where the quenching
temperature is controlled, in order to manufacture a bearing part
having an excellent rolling contact fatigue life in contaminated
environments using the material disclosed in Patent Document 3.
[0015] The present invention has been made in view of such
circumstances, and the aim of the present invention is to provide a
bearing part having an excellent rolling contact fatigue life in
environment including contaminated environments, steel for a
bearing part which is a material for the bearing part, and a method
for manufacturing them, in which the number of spheroidizing
annealing is only one and which takes a short amount of time.
Means for Solving the Problem
[0016] The present inventors found that the average grain size of
prior-austenite of a bearing part can be refined by subjecting
steel including Cr in which the metallographic structure consists
of pearlite to spheroidizing annealing at a lower temperature than
usual after wire drawing. Furthermore, the present inventors found
that the amount of retained austenite can be secured by refining
the average grain size of the prior-austenite. Then, the present
inventors found that the rolling contact fatigue life of the
bearing part can be improved not only under normal conditions, but
also in contaminated environments by securing the amount of
retained austenite.
[0017] The summary of the present invention is as follows.
[0018] (1) A bearing part according to one aspect of the present
invention includes, as a chemical composition, by mass %: C: 0.95%
to 1.10%; Si: 0.10% to 0.70%; Mn: 0.20% to 1.20%; Cr: 0.90% to
1.60%; Al: 0.010% to 0.100%; N: 0.003% to 0.030%; P: 0.025% or
less; S: 0.025% or less; O: 0.0010% or less; optionally one or more
of the group consisting of: Mo: 0.25% or less, B: 0.0050% or less,
Cu: 1.0% or less, Ni: 3.0% or less, and Ca: 0.0015% or less; and a
remainder including Fe and impurities; a metallographic structure
includes a retained austenite, a spherical cementite and a
martensite; an amount of the retained austenite is 18% to 25%, by
volume %; an average grain size of a prior-austenite is 6.0 .mu.m
or less; an average grain size of the spherical cementite is 0.45
.mu.m or less; and a number density of the spherical cementite is
0.45.times.10.sup.6 mm.sup.-2 or more in the metallographic
structure.
[0019] (2) The bearing part according to (1) may include, as the
chemical composition, by mass %, one or more of Mo: 0.01% to 0.25%,
B: 0.0001% to 0.0050%, Cu: 0.1% to 1.0%, Ni: 1.0% to 3.0%, and Ca:
0.0001% to 0.0015%.
[0020] (3) A steel for a bearing part according to one aspect of
the present invention includes, as a chemical composition, by mass
%: C: 0.95% to 1.10%; Si: 0.10% to 0.70%; Mn: 0.20% to 1.20%; Cr:
0.90% to 1.60%; Al: 0.010% to 0.100%; N: 0.003% to 0.030%; S:
0.025% or less; P: 0.025% or less; O: 0.0010% or less; and
optionally one or more of the group consisting of; Mo: 0.25% or
less, B: 0.0050% or less, Cu: 1.0% or less, Ni: 3.0% or less, and
Ca: 0.0015% or less; and a remainder including Fe an impurities; a
metallographic structure includes a spherical cementite and a
ferrite; a number density of the spherical cementite having a grain
size of 0.5 .mu.m to 3.0 .mu.m is 2.0.times.10.sup.6 mm.sup.-2 or
more in the metallographic structure.
[0021] (4) The steel for the bearing part according to (3) may
include, as the chemical composition, by mass %, one or more of Mo:
0.01% to 0.25%, B: 0.0001% to 0.0050%, Cu: 0.1% to 1.0%, Ni: 1.0%
to 3.0%, and Ca: 0.0001% to 0.0015%.
[0022] (5) A method for producing a steel for a bearing part
according to one aspect of the present invention includes: a
casting process for obtaining a steel piece having a chemical
composition according to (3) or (4); a heating process of heating
the steel piece to 900.degree. C. to 1300.degree. C.; a hot rolling
process for obtaining a hot rolled wire rod by subjecting the steel
piece to hot rolling at a finish rolling temperature of 850.degree.
C. or less after the heating process; a winding process of winding
the hot rolled wire rod after the hot rolling process at a winding
temperature of 800.degree. C. or less; a cooling process for making
a microstructure of the hot rolled wire rod a pearlite by cooling
the hot rolled wire rod to 600.degree. C. at a cooling rate of
3.0.degree. C./second or less after the winding process; a wire
drawing process of subjecting the hot rolled wire rod to wire
drawing in which a total reduction of an area is 50% or more after
the cooling process; and a spheroidizing annealing process for
obtaining the steel for the bearing part by performing
spheroidizing annealing in which the hot rolled wire rod after the
wire drawing process is held at a temperature of 650.degree. C. to
the lower of 750.degree. C. and A.sub.1-5.degree. C. for 0.5 hours
to 5 hours; wherein, A.sub.1 represents a temperature at which
A.sub.1 transformation starts, and is a predicted value which is
calculated by a following equation 1 based on the chemical
composition, and, [C], [Si] [Mn], [Cu], [Ni], [Cr], [Mo], [Al] and
[B] in the equation 1 represent, by mass %, the content of C, the
content of Si, the content of Mn, the content of Cu, the content of
Ni, the content of Cr, the content of Mo, the content of Al and the
content of B, respectively, in the hot rolled wire rod.
A.sub.1=750.8-26.6.times.[C]+17.6.times.[Si]-11.6.times.[Mn]-22.9.times.-
[Cu]-23.0.times.[Ni]+24.1.times.[Cr]+22.5.times.[Mo]-169.4.times.[Al]-894.-
7.times.[B] (Equation 1)
[0023] (6) A method for producing a bearing part according to one
aspect of the present invention includes: a casting process for
obtaining a steel piece having a chemical composition according to
(1) or (2); a heating process of heating the steel piece to
900.degree. C. to 1300.degree. C.; a hot rolling process for
obtaining a hot rolled wire rod by subjecting the steel piece to
hot rolling at a finish rolling temperature of 850.degree. C. or
less after the heating process; a winding process of winding the
hot rolled wire rod after the hot rolling process at a winding
temperature of 800.degree. C. or less; a cooling process for making
a microstructure of the hot rolled wire rod a pearlite by cooling
the hot rolled wire rod to 600.degree. C. at a cooling rate of
3.0.degree. C./second or less after the winding process; a wire
drawing process of subjecting the hot rolled wire rod to wire
drawing in which a total reduction of an area is 50% or more after
the cooling process; a spheroidizing annealing process for
obtaining a steel for a bearing part by performing spheroidizing
annealing in which the hot rolled wire rod after the wire drawing
process is held at a temperature of 650.degree. C. to the lower of
750.degree. C. and A.sub.1-5.degree. C. for 0.5 hours to 5 hours; a
forming process of rough forming the steel for the bearing part
after the spheroidizing annealing process; a quenching process of
performing quenching by heating the steel for the bearing part
after the forming process to 800.degree. C. to 890.degree. C.; a
tempering process of subjecting the steel for the bearing part to
tempering at 250.degree. C. or less after the quenching process;
and a finish machining process for obtaining the bearing part by
subjecting the steel for the bearing part to finish machining after
the tempering process; wherein, A.sub.1 represents a temperature at
which A.sub.1 transformation starts, and is a predicted value which
is calculated by a following equation 2 based on the chemical
composition; and, [C], [Si], [Mn], [Cu], [Ni] [Cr] [Mo], [Al] and
[B] in the equation 2 represent, by mass %, the content of C, the
content of Si, the content of Mn, the content of Cu, the content of
Ni, the content of Cr, the content of Mo, the content of Al and the
content of B, respectively, in the hot rolled wire rod.
A.sub.1=750.8-26.6.times.[C]+17.6.times.[Si]-11.6.times.[Mn]-22.9.times.-
[Cu]-23.0.times.[Ni]+24.1.times.[Cr]+22.5.times.[Mo]-169.4.times.[Al]-894.-
7.times.[B] (Equation 2)
Effects of the Invention
[0024] According to the above aspects of the present invention, a
bearing part having an excellent rolling contact fatigue life can
be obtained by controlling the average grain size of
prior-austenite, the amount of retained austenite and the number
density of spherical cementite and by performing spheroidizing
annealing for a short amount of time and once, not only under the
normal conditions, but also in contaminated environments, without
performing carburizing, nitriding and spheroidizing annealing for a
long period of time. Furthermore, steel for a bearing part which is
a material for a bearing part can be obtained by controlling the
metallographic structure. In addition, according to the above
aspects of the present invention, manufacture thereof is possible.
Therefore, when the bearing parts according to the above aspects
are applied to a vehicle or industrial machinery, it is possible to
extend the life of the machine and reduce the manufacturing cost.
That is, contribution of the present invention to the industry is
extremely significant.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a view showing a metallographic structure of a
bearing part.
[0026] FIG. 2 is a view showing a metallographic structure of steel
for a bearing part.
[0027] FIG. 3 is a view showing a relationship between the number
density of spherical cementite in steel for a bearing part and the
average grain size of prior-austenite of a bearing part.
[0028] FIG. 4 is a view showing a relationship between the average
grain size of prior-austenite and the amount of retained austenite
of a bearing part.
[0029] FIG. 5 is a view showing a relationship between the amount
of retained austenite and the rolling contact fatigue life of a
bearing part in a contaminated environment.
[0030] FIG. 6 is a view showing a relationship between the number
density of spherical cementite in a bearing part and the rolling
contact fatigue life of a bearing part.
EMBODIMENTS OF THE INVENTION
[0031] Increasing the amount of retained austenite, controlling the
size of spherical cementite and controlling the number density of
spherical cementite are effective for improving the rolling contact
fatigue life in contaminated environments. The present inventors
investigated the appropriate amount of retained austenite and the
manufacturing conditions for controlling the amount of retained
austenite and found the following. The amount of retained austenite
(volume %) can be determined by, for example, calculating the ratio
of the diffraction intensity of retained austenite .gamma. (220) to
the diffraction intensity of martensite .alpha. (211) measured by
X-ray diffraction. The amount of retained austenite (the amount of
V.gamma.) can be determined by, for example, obtaining the X-ray
diffraction peaks of the microstructures of the steel sheet using
an X-ray diffraction measuring apparatus (RAD-RU300 made by Rigaku
Denki), in which a Co target is used and the target output is set
to 40 kV-200 mA, and calculating the theoretical intensity ratio by
Rietveld analyses.
[0032] Along with increasing the amount of retained austenite, the
rolling contact fatigue life can be improved even in contaminated
environments. In order to stably obtain this effect, the essential
amount of retained austenite is 18% or more, by volume %. On the
other hand, when the amount of retained austenite is more than 25%
by volume %, the hardness is reduced, the normal rolling contact
fatigue strength of the bearing part is reduced, and aging of
dimensions become larger. As a result, the functional usage of the
bearing part is reduced. Therefore, in order to improve the rolling
contact fatigue life of the bearing part in contaminated
environments, it is necessary to control the amount of retained
austenite to be 18% to 25%, by volume %.
[0033] During quenching, it is necessary to stabilize the austenite
phase in order to increase the amount of retained austenite. In
addition, lowering the martensite start temperature (Ms point) is
an effective way of doing so. The Ms point is affected by
solid-soluted amount of the elements such as C, Si and Mn in the
austenite phase, particularly, is greatly affected by an amount of
solid-soluted C in the austenite phase. However, when the heating
temperature during the quenching is increased in order to increase
the amount of solid-soluted C, the average grain size of
prior-austenite coarsens. Furthermore, the amount of solid-soluted
C in martensite is increased after the quenching. Therefore, the
rolling contact fatigue life and the toughness of the bearing part
are reduced.
[0034] Then, the present inventors focused their investigation on
stabilization of the austenite phase due to grain refinement. As a
result, the present inventors found the following.
[0035] Firstly, processing strain is introduced by wire drawing a
hot rolled wire rod having pearlite structure (pearlite steel).
Next, the hot rolled wire rod is subjected to spheroidizing
annealing at a lower than conventional temperature after the wire
drawing. It was found that spherical cementite can be finely
dispersed due to this spheroidizing annealing. Then, it was found
that when the spherical cementite is finely dispersed, the average
grain size of prior-austenite after quenching of steel for a
bearing part, which had been subjected to the spheroidizing
annealing, can be refined.
[0036] In addition, the present inventors found that the amount of
retained austenite can be controlled to 18% to 25% by setting the
average grain size of the prior-austenite of the bearing part to
6.0 .mu.m or less.
[0037] In order to reduce the average grain size of the
prior-austenite, it is preferable to control the total reduction of
area during the wire drawing and the heating temperature of the
spheroidizing annealing. In addition, after the spherical cementite
is finely precipitated due to these controls, it is preferable to
perform the quenching. Specifically, after the hot rolled wire rod
(pearlite steel) is subjected to wire drawing with a total
reduction of area of 50% or more, the spheroidizing annealing is
performed at a temperature of 650.degree. C. to the lower of
750.degree. C. and A.sub.1-5.degree. C. Then, the hot rolled wire
rod is cooled to 400.degree. C. or less at a cooling rate of
0.1.degree. C./s or more. Next, the hot rolled wire rod is heated
to 800.degree. C. to 890.degree. C., and the quenching is
performed. As a result, the average grain size of the
prior-austenite can be suppressed to 6.0 .mu.m or less and the
amount of retained austenite can be controlled to be 18% to
25%.
[0038] In addition, the average grain size of the prior-austenite
can be obtained by the following method. Firstly, in the center of
a longitudinal direction of a bearing part, a C cross section
perpendicular to the longitudinal direction is polished and
corroded, and thus, a grain boundary of the prior-austenite
appears. Secondary, a range within a radius of 3 mm from a center
of the C cross section is set as the center portion, and
photographs are taken of the center portion using an optical
microscope at 400 times magnification. Then, the captured images
are measured by a counting method defined in JIS G 0551. In
addition, four visual fields are measured in each sample, and the
average value of grain sizes of the prior-austenite in the obtained
four visual fields is set as the average grain size of the
prior-austenite.
[0039] When the total reduction of area during the wire drawing is
less than 50%, the spherical cementite is not sufficiently
spheroidized during the spheroidizing annealing, and the number
density of spherical cementite having a grain size of 0.50 .mu.m to
3.0 .mu.m in the metallographic structure of the steel for the
bearing part is decreased. As a result, the average grain size of
the prior-austenite of the bearing part after the quenching is
increased, and there are cases where the average grain size of
prior-austenite is more than 6.0 .mu.m.
[0040] FIG. 3 shows a relationship between the number density of
spherical cementite having a grain size of 0.50 .mu.m to 3.00 .mu.m
in steel for a bearing part after spheroidizing annealing and the
average grain size of prior-austenite of a bearing part which is
obtained through subsequent quenching and tempering.
[0041] As shown in FIG. 3, when the number density of spherical
cementite (grain size is 0.5 .mu.m to 3.0 .mu.m) in the steel for
the bearing part was 2.0.times.10.sup.6 mm.sup.-2 or more, the
average grain size of prior-austenite of a bearing part after
quenching and tempering is refined to be 6.0 .mu.m or less. In this
way, there is a correlation between the amount of spherical
cementite having a prescribed size in the steel for the bearing
part and the average grain size of prior-austenite of the bearing
part.
[0042] In addition, FIG. 4 shows the relationship between the
average grain size of prior-austenite of a bearing part and the
amount of retained austenite of a bearing part. As shown in FIG. 4,
when the average grain size of the prior-austenite of the bearing
part is 6.0 .mu.m or less, the amount of retained austenite of the
bearing part is 18% or more, by volume %. On the other hand, when
the average grain size of the prior-austenite of the bearing part
is more than 6.0 .mu.m, the amount of retained austenite of the
bearing part is reduced to less than 18%, by volume %.
[0043] FIG. 5 shows the relationship between the amount of retained
austenite in a bearing part and the rolling contact fatigue life of
a bearing part in a contaminated environment. As shown in FIG. 5,
when the amount of retained austenite in the bearing part is 18% or
more, by volume %, the rolling contact fatigue life of the bearing
part in a contaminated environment is good. On the other hand, when
the amount of retained austenite in the bearing part is less than
18%, by volume %, the rolling contact fatigue life of the bearing
part in a contaminated environment is reduced.
[0044] Next, the present inventors investigated the method for
increasing the number density of spherical cementite having an
average grain size of 0.45 .mu.m or less in the bearing part after
the quenching.
[0045] When spherical cementite, which is fine and hard, is
dispersed in the steel, the spherical cementite contributes to
strengthening of the bearing part. Therefore, when the number
density of spherical cementite having the prescribed size is
increased, the rolling contact fatigue life or an impact property
of the bearing part is improved. In their investigations, the
present inventors found that it is important that the average grain
size of spherical cementite of the bearing part be set to 0.45
.mu.m or less and the number density of spherical cementite be set
to 0.45.times.10.sup.6 mm.sup.-2 or more. When the number density
of spherical cementite is less than 0.45.times.10.sup.6 mm.sup.-2,
the rolling contact fatigue life of the bearing part is decreased.
The number density of spherical cementite is preferably
0.5.times.10.sup.6 mm.sup.-2 or more.
[0046] In addition, when the average grain size of the spherical
cementite of the bearing part is more than 0.45 .mu.m, generation
and extension of fatigue crack is promoted. Therefore, it is
necessary that the average grain size of spherical cementite of the
bearing part is 0.45 .mu.m or less. On the other hand, in normal
operating conditions, it is difficult to reduce the average grain
size of spherical cementite to less than 0.10 .mu.m.
[0047] Since the spherical cementite is conventionally
solid-soluted into an austenite phase which is a base metal, when
the steel is heated to A.sub.1 point or more during quenching, the
number density of spherical cementite of the bearing part is
lowered than the number density of spherical cementite in the steel
for the bearing part which is a material for the bearing part. When
the heating temperature of the quenching is set to a lower
temperature, the number density of spherical cementite in the
bearing part is increased; however, the amount of retained
austenite in the bearing part is reduced. Therefore, the rolling
contact fatigue life of the bearing part in the contaminated
environment is reduced.
[0048] However, the present inventors found that when the number
density of spherical cementite having a grain size of 0.5 .mu.m to
3.0 .mu.m in steel for a bearing part before quenching is increased
to 2.0.times.10.sup.6 mm.sup.-2 or more, the number density of
spherical cementite having an average grain size of 0.45 .mu.m or
less can be secured for 0.45.times.10.sup.6 mm.sup.-2 or more in a
bearing part after the quenching.
[0049] FIG. 6 shows a relationship between the number density of
spherical cementite having the prescribed size in a bearing part
and the rolling contact fatigue property of a bearing part. As
shown in FIG. 6, when the number density of spherical cementite
having an average grain size of 0.45 .mu.m or less is
0.45.times.10.sup.6 mm.sup.-2 or more, the rolling contact fatigue
life of the bearing part is good.
[0050] The number density of spherical cementite having the
prescribed size can be obtained by the following method. In the
center of the longitudinal direction of steel for a bearing part
and a bearing part, the bearing part is cut with a cross section
perpendicular to the longitudinal direction. A cut C cross section
is mirror-polished; the observation is performed at the center
portion of the C cross section using a scanning electron microscope
(SEM) with 5000 times magnification; and photographs are taken at
ten visual fields. Then, the number of spherical cementite having
the prescribed size is measured at each visual field and the number
is divided by the visual field area, therefore, the number density
of spherical cementite having the prescribed size can be obtained.
In addition, the center portion of the C cross section means a
circle region which is 3 mm of radius from the center point of the
C cross section, and the observation field is 0.02 mm.sup.2.
[0051] A metallographic structure of a bearing part according to
the present embodiment will be described. The metallographic
structure of the bearing part according to the present embodiment
is retained austenite, spherical cementite and martensite. FIG. 1
shows a SEM image of the microstructure of the bearing part
according to the present embodiment. The SEM image of FIG. 1 shows
the microstructure where the spherical cementite 2 precipitates in
the martensite 1. Because retained austenite cannot be observed
with SEM, retained austenite can be determined by the ratio of the
diffraction intensity of martensite to the diffraction intensity of
retained austenite with X diffraction method (XRD).
[0052] In the metallographic structure of the bearing part
according to the present embodiment, the amount of retained
austenite is 18% to 25% by volume %. The sum of spherical cementite
and martensite is preferably 75% to 82% by volume %, which is
obtained by subtracting the amount of retained austenite from the
total volume.
[0053] A metallographic structure of steel for the bearing part
according to the present embodiment will be described. The
metallographic structure of the steel for the bearing part
according to the present embodiment includes spherical cementite
and ferrite. However, in order to obtain the rolling contact
fatigue life and the hardness of the bearing part, it is preferable
that microstructures other than spherical cementite and ferrite are
not included in the steel for the bearing part. FIG. 2 shows a SEM
image of the metallographic structure of the steel for the bearing
part after the spheroidizing annealing according to the present
embodiment. The SEM image of FIG. 2 shows the microstructure where
the spherical cementite 5 precipitates in the ferrite 4.
[0054] Hereinafter, regarding the chemical composition of the base
elements of the bearing part, and the steel for the bearing part
according to the present embodiment, the numerical limitation range
and the reasons for the limitation will be described. Here, "%" in
the following description represents "mass %".
[0055] C: 0.95% to 1.10%
[0056] C (Carbon) is an element for enhancing the strength of a
bearing part. When the content of C is less than 0.95%, the
strength and the rolling contact fatigue life of the bearing part
in the contaminated environment cannot be improved. On the other
hand, when the content of C is more than 1.10%, carbide becomes
coarse and the amount of retained austenite is excessive. As a
result, not only is the hardness of the bearing part lowered, but
also secular changes of dimensions (aging deteriorations) become
larger. Therefore, the content of C is set to 0.95% to 1.10%. In
order to more reliably obtain the effect for improving the rolling
contact fatigue life, the content of C is preferably 0.96% to
1.05%. More preferably, the content of C is 0.97% to 1.03%.
[0057] Si: 0.10% to 0.70%
[0058] Si (Silicon) is an element for enhancing strength and
functions as a deoxidizer. When the content of Si is less than
0.10%, these effects cannot be obtained. On the other hand, when
the content of Si is more than 0.70%, SiO.sub.2-based inclusion
generates in steel, and thus, the rolling contact fatigue life of
the bearing part is reduced. Therefore, the content of Si is set to
0.10% to 0.70%. In order to more reliably not lower the rolling
contact fatigue life, the content of Si is preferably 0.12% to
0.56%. More preferably, the content of Si is 0.15% to 0.50%.
[0059] Mn: 0.20% to 1.20%
[0060] Mn (Manganese) is an element which functions as a deoxidizer
and as a desulfurizer. Furthermore, Mn is an element useful for
securing the hardenability of the steel and the amount of retained
austenite. When the content of Mn is less than 0.20%, deoxidation
is insufficient and oxide is generated. As a result, the rolling
contact fatigue life of the bearing part is reduced. On the other
hand, when the content of Mn is more than 1.20%, supercooled
structure such as martensite is generated during cooling after hot
rolling, and thus voids occur during wire drawing. Furthermore,
when the content of Mn is more than 1.20%, the amount of retained
austenite is excessive, and thus, the hardness of the bearing part
is reduced. Therefore, the content of Mn is set to 0.20% to 1.20%.
In order to more reliably promote deoxidization and not to lower
the rolling contact fatigue life, the content of Mn is preferably
0.21% to 1.15%. More preferably, the content of Mn is 0.25% to
1.00%.
[0061] Cr: 0.90% to 1.60%
[0062] Cr (Chromium) is an element for improving the hardenability
of the steel. Furthermore, Cr is an extremely effective element for
promoting spheroidizing of carbide and for increasing the amount of
carbide. When the content of Cr is less than 0.90%, the amount of
solid-soluted C into austenite is increased, and excessive retained
austenite is generated in the bearing part. On the other hand, when
the content of Cr is more than 1.60%, dissolution of carbide is
limited during quenching, and thus, the amount of retained
austenite is reduced or the hardenability of the bearing part is
reduced. Therefore, the content of Cr is set to 0.90% to 1.60%. In
order to more reliably improve the rolling contact fatigue life of
the bearing part, the content of Cr is preferably 0.91% to 1.55%.
More preferably, the content of Cr is 1.10% to 1.50%. Most
preferably, the content of Cr is 1.30% to 1.50%.
[0063] Al: 0.010% to 0.100%
[0064] Al (Aluminum) is an element which functions as a deoxidizer.
When the content of Al is less than 0.010%, deoxidation becomes
insufficient and oxide precipitates. As a result, the rolling
contact fatigue life of the bearing part is reduced. On the other
hand, when the content of Al is more than 0.100%, AlO-based
inclusion are generated. As a result, the wire drawability of the
steel for the bearing part is reduced or the rolling contact
fatigue life of the bearing part is reduced. Therefore, the content
of Al is set to 0.010% to 0.100%. In order to more reliably not
lower the rolling contact fatigue life, the content of Al is
preferably 0.015% to 0.078%. More preferably, the content of Al is
0.018% to 0.050%.
[0065] N: 0.003% to 0.030%
[0066] N forms nitride with Al or B. These nitrides function as
pinning particles, and thus, grains are refined. Therefore, N
(Nitrogen) is an element which suppresses grain coarsening. When
the content of N is less than 0.003%, this effect cannot be
obtained. On the other hand, when the content of N is more than
0.030%, coarse inclusions are generated, and thus, the rolling
contact fatigue life is reduced. Therefore, the content of N is set
to 0.003% to 0.030%. In order to more reliably not lower the
rolling contact fatigue life, the content of N is preferably 0.005%
to 0.029%. More preferably, the content of N is 0.009% to
0.020%.
[0067] P: 0.025% or Less
[0068] P (Phosphorus) is an impurity that is unavoidably included
in steel. When the content of P is more than 0.025%, P segregates
in an austenite grain boundary and embrittles an austenite grain
boundary. As a result, the rolling contact fatigue life of the
bearing part is reduced. Therefore, it is necessary for the content
of P to be limited to 0.025% or less. In order to more reliably not
lower the rolling contact fatigue life, the content of P may be
limited to 0.020% or less, and more preferably limited to 0.015% or
less. In addition, since the content of P is preferably as small as
possible, a case where the content of P is 0% is contained into the
above limited range. However, controlling the content of P to be 0%
is not technically easy. Therefore, from the viewpoint of
steelmaking cost, the lower limit of the content of P may be set to
0.001%. Considering normal operating conditions, the content of P
is preferably 0.004% to 0.012%.
[0069] S: 0.025% or Less
[0070] S (Sulfur) is an impurity that is unavoidably included in
steel. When the content of S is more than 0.025%, coarse MnS forms,
and thus, the rolling contact fatigue life of the bearing part is
lowered. Therefore, it is necessary that the content of S is
limited to 0.025% or less. In order to more reliably not lower the
rolling contact fatigue life, the content of S may be limited to
0.020% or less, and preferably limited to 0.015% or less. Since the
content of S is desirably as small as possible, and thus, a case
where the content of S is 0% is contained into the above limited
range. However, controlling the content of S to be 0% is not
technically easy. Therefore, from the viewpoint of steelmaking
cost, the lower limit of the content of S may be set to 0.001%.
Considering the normal operating conditions, the content of S is
preferably 0.003% to 0.011%.
[0071] O: 0.0010% or Less
[0072] O (Oxygen) is an impurity that is unavoidably included in
steel. When the content of O is more than 0.0010%, oxide inclusion
forms, and thus, the rolling contact fatigue life of the bearing
part is lowered. Therefore, it is necessary that the content of O
is limited to 0.0010% or less. Since the content of O is desirably
as small as possible, and thus, a case where the content of O is 0%
is contained into the above limited range. However, controlling the
content of O to be 0% is not technically easy. Therefore, from the
viewpoint of steelmaking cost, the lower limit of the content of O
may be set to 0.0001%. Considering the normal operating conditions,
the content of O is preferably 0.0005% to 0.0010%.
[0073] In addition to the base elements and impurity elements
mentioned above, the bearing part according to the present
embodiment may optionally include at least one of the group
consisting of Mo, B, Cu, Ni and Ca. In this case, one or more of
Mo, B, Cu and Ni for improving the hardenability, and Ca for
refining the inclusion can be selected. Regarding these chemical
elements, since there is no need to add them to the steel for the
bearing part and the bearing part, the lower limits of these
chemical elements are 0% and are not limited.
[0074] Hereinafter, the preferred range of the selective elements
and the reasons for limiting will be described. Here, "%" in the
following description represents "mass %".
[0075] Mo: 0.25% or Less
[0076] Mo is an element for improving the hardenability. In
addition, Mo has effects for improving grain boundary strength of
the steel after quenching is performed and for enhancing the
toughness of the steel. If it is desirable to more reliably secure
the hardenability and toughness, the content of Mo is preferably
set to 0.01% or more. However, when the content of Mo is more than
0.25%, these effects are saturated. Therefore, the content of Mo is
preferably 0.01% to 0.25%. More preferably, the content of Mo is
0.01% to 0.23%. Even more preferably, the content of Mo is 0.10% to
0.23%.
[0077] B: 0.0050% or Less
[0078] B is an element for improving the hardenability, even if the
content of B is small. In addition, B also has effects for
suppressing segregation of P or S at the austenite grain boundary
during quenching. If it is desirable to obtain these effects, the
content of B is preferably set to 0.0001% or more. However, when
the content of B is more than 0.0050%, these effects are saturated.
Therefore, the content of B is preferably 0.0001% to 0.0050%. More
preferably, the content of B is 0.0003% to 0.0050%. Even more
preferably, the content of B is 0.0005% to 0.0025%. Most
preferably, the content of B is 0.0010% to 0.0025%.
[0079] Cu: 1.0% or Less
[0080] Cu is an element for improving the hardenability. If it is
desirable to more reliably secure the hardenability, the content of
Cu is preferably set to 0.05% or more. However, when the content of
Cu is more than 1.0%, this effect is saturated, furthermore, hot
workability is deteriorated. Therefore, the content of Cu is
preferably 0.05% to 1.0%. More preferably, the content of Cu is
0.10% to 0.50%. Even more preferably, the content of Cu is 0.19% to
0.31%.
[0081] Ni: 3.0% or Less
[0082] Ni is an element for improving the hardenability. In
addition, Ni has an effect of improving the toughness of the steel
after quenching is performed. If it is desirable to more reliably
secure the hardenability and toughness, the content of Ni is
preferably set to 0.05% or more. However, when the content of Ni is
more than 3.0%, this effect is saturated. Therefore, the content of
Ni is preferably 0.05% to 3.0%. More preferably, the content of Ni
is 0.10% to 1.5%. Even more preferably, the content of Ni is 0.21%
to 1.2%. Most preferably, the content of Ni is 0.21% to 1.0%.
[0083] Ca: 0.0015% or Less
[0084] Ca is an element that is solid-soluted into sulfide and
forms CaS, and thus, refines the sulfide. If it is desirable to
more improve the rolling contact fatigue life by refining the
sulfide, the content of Ca is preferably set to 0.0003% or more.
However, when the content of Ca is more than 0.0015%, this effect
is saturated. Furthermore, since oxide inclusion coarsens, and it
causes a lowering of the rolling contact fatigue life. Therefore,
the content of Ca is preferably 0.0003% to 0.0015%. More
preferably, the content of Ca is 0.0003% to 0.0011%. Even more
preferably, the content of Ca is 0.0005% to 0.0011%.
[0085] The bearing part and the steel for the bearing part
according to the present embodiment include the above described
components, and the remainder of the chemical composition consists
of substantially Fe and unavoidable impurities.
[0086] A metallographic structure of a bearing part according to
the present embodiment will be described.
[0087] The metallographic structure of the bearing part according
to the present embodiment is retained austenite, spherical
cementite and martensite.
[0088] Among these microstructures, the amount of retained
austenite is 18% to 25%, by volume %. In order to improve the
rolling contact fatigue life even in a contaminated environment, it
is necessary that the amount of retained austenite 18% to 25%, the
average grain size of spherical cementite be 0.45 .mu.m or less,
and the number density of spherical cementite be
0.45.times.10.sup.6 mm.sup.-2 or more. The number density of
spherical cementite is preferably 0.5.times.10.sup.6 mm.sup.-2 or
more. In addition, the upper limit of the number density of
spherical cementite is not particularly limited; however, the upper
limit of the number density of spherical cementite is preferably
1.0.times.10.sup.6 mm.sup.-2 in view of considering the constraints
on production or securing the rolling contact fatigue life. In
addition, although the average grain size of spherical cementite is
reduced excessively, the effect for improving the rolling contact
fatigue life is small, and it is difficult to produce them.
Therefore, the average grain size of the spherical cementite of the
bearing part is preferably 0.25 .mu.m or more. That is, the
preferred average grain size of spherical cementite of the bearing
part according to the present embodiment is 0.25 .mu.m to 0.45
.mu.m.
[0089] In addition, the average grain size of spherical cementite
can be obtained by the following method. Firstly, the bearing part
is cut on the cross section (C cross section) perpendicular to the
longitudinal direction in a center of a longitudinal direction of
the bearing part. A range within a radius of 3 mm from a center of
the C cross section is set to a center portion, and photographs are
taken at the center portion using a SEM with 2000 times
magnification. Next, the spherical cementite among the captured
images is copied using the tracing sheet or the like; and the grain
size of cementite is measured by image analyzing the tracing sheet.
In addition, four visual fields are measured in each sample, and an
average value of grain sizes of spherical cementite in the obtained
four visual fields is set to the average grain size of spherical
cementite.
[0090] Furthermore, in order to secure the amount of retained
austenite, the average grain size of prior-austenite of the bearing
part according to the present embodiment is 6.0 .mu.m or less. When
the average grain size of prior-austenite is more than 6.0 .mu.m,
the necessary amount of retained austenite cannot be obtained. On
the other hand, if the average grain size of prior-austenite of the
bearing part is refined to less than 3.0 .mu.m or less, the
manufacturing load is increased. Therefore, the average grain size
of prior-austenite of the bearing part is preferably 3.0 .mu.m or
more. That is, the average grain size of prior-austenite of the
bearing part according to the present embodiment is preferably 3.0
.mu.m to 6.0 .mu.m.
[0091] Next, a metallographic structure of steel for a bearing part
which is a material for a bearing part according to the present
embodiment will be described.
[0092] The metallographic structure of the steel for the bearing
part according to the present embodiment includes spherical
cementite and ferrite. Among these structures, the number density
of spherical cementite having a grain size of 0.5 .mu.m to 3.0
.mu.m is 2.0.times.10.sup.6 mm.sup.-2 or more. When the number
density of spherical cementite having a prescribed size in the
steel for the bearing part is less than 2.0.times.10.sup.6
mm.sup.-2, the spherical cementite of the bearing part after
quenching and tempering is reduced and the rolling contact fatigue
life of the bearing part is lowered. In addition, the upper limit
of the number density of spherical cementite is not particularly
limited; however, the upper limit of the number density of
spherical cementite is preferably 5.0.times.10.sup.6 mm.sup.-2 in
view of considering the constraints on production or securing the
rolling contact fatigue life.
[0093] Next, a microstructure of hot rolled wire rod which is a
material for steel for a bearing part will be described.
[0094] The hot rolled wire rod has the same chemical composition as
the bearing part. Then, it is preferable that the hot rolled wire
rod has a metallographic structure consisting of pearlite and 5% or
less of proeutectoid cementite by area ratio. When the hot rolled
wire rod has supercooled structure such as martensite in the
metallographic structure, it cannot be uniformly deformed during
wire drawing. As a result, it may cause breaking wire. Therefore,
it is preferable that the hot rolled wire rod does not include
martensite and mainly include pearlite as the microstructure.
[0095] In addition, the size of the pearlite block has a very
strong correlation with ductility. In other words, the wire
drawability can be improved by refining the pearlite. Therefore, it
is preferable that the average grain size of the pearlite block
(circle equivalent diameter) be 15 .mu.m or less. When the average
grain size of the pearlite block is more than 15 .mu.m, an effect
of improving the wire drawability may not be obtained. On the other
hand, it may be industrially difficult to control the grain size of
the pearlite block to 1 .mu.m or less. Therefore, the grain size of
the pearlite block is preferably 1 .mu.m to 15 .mu.m. More
preferably, the grain size of the pearlite block is 1 .mu.m to 10
.mu.m.
[0096] The average grain size of the pearlite block (circle
equivalent diameter) can be measured with an electron backscatter
diffraction apparatus (EBSD).
[0097] The proeutectoid cementite generally has little plastic
deformability. Therefore, the proeutectoid cementite is divided by
wire drawing, and it results in formation of voids. However, when
the area ratio of the proeutectoid cementite is low and the
thickness of the proeutectoid cementite is small, wire drawability
is not inhibited. Accordingly, it is preferable that the
proeutectoid cementite be 5% or less by area ratio and the
thickness of the proeutectoid cementite be 1.0 .mu.m or less. More
preferably, the proeutectoid cementite is 3% or less by area ratio
and the thickness of the proeutectoid cementite is 0.8 .mu.m or
less.
[0098] The area ratio of the proeutectoid cementite and the
thickness of the proeutectoid cementite can be measured by
observation with SEM.
[0099] When the above described chemical composition and
metallographic structure are satisfied, a bearing part having an
excellent rolling contact fatigue life can be obtained even in a
contaminated environment. In order to obtain the above-described
bearing part, the bearing part may be produced by the manufacturing
method, which will be described later.
[0100] Next, it will be described the preferred method for
producing a bearing part and steel for a bearing part according to
the present embodiment.
[0101] A bearing part according to the present embodiment can be
manufactured as follows.
[0102] In addition, the method for producing a bearing part, steel
for a bearing part which is a material for a bearing part and the
method for producing a hot rolled wire rod which is a material for
steel for a bearing part as described below are examples to obtain
a bearing part according to the present invention; and the present
invention is not limited to the following procedures and methods.
As long as the method can realize the configuration of the present
invention, it is possible to adopt any methods.
[0103] Normal manufacturing conditions can be employed as a method
for producing a hot rolled wire rod which is a material for steel
for a bearing part.
[0104] For example, steel having the chemical composition adjusted
in the usual manner is melted and casted, and then, the steel is
subjected to soaking treatment and blooming as needed, to form a
billet. Next, the obtained billet is heated and is subjected to hot
rolling. Then, after the hot rolled steel is annularly wound, the
wound steel is cooled.
[0105] The hot rolled wire rod which is the material for the steel
for the bearing part according to the present embodiment can be
manufactured through the above processes.
[0106] In casting process, the method for casting is not
particularly limited, and a vacuum casting, a continuous casting or
the like may be used.
[0107] In addition, the soaking treatment (soaking diffusion
treatment), to which cast piece after the casting process is
subjected as needed, is a heat treatment for reducing segregation
which the casting or the like causes. The steel piece obtained
through these processes is commonly referred to as a billet.
[0108] Furthermore, a heating temperature during the soaking
treatment is preferably 1100.degree. C. to 1200.degree. C. In
addition, a holding time of the soaking treatment is preferably 10
hours to 20 hours.
[0109] Next, the billet is heated in the heating process before hot
rolling. The heating temperature of the billet is preferably set to
900.degree. C. to 1300.degree. C.
[0110] Then, the above billet is subjected to the hot rolling as
the hot rolling process. In the hot rolling process, a finish
rolling temperature is preferably set to 850.degree. C. or
less.
[0111] When the finish rolling temperature is 850.degree. C. or
less, the proeutectoid cementite is dispersed and precipitated. As
a result, the thickness of the proeutectoid cementite can be
reduced. In addition, when the finish rolling temperature is
850.degree. C. or less, a nucleation site of pearlite is increased
during a transformation. As a result, the pearlite block can be
refined. More preferably, the finish rolling temperature is
800.degree. C. or less. On the other hand, when the finish rolling
temperature is less than 650.degree. C., there is a case where
pearlite block cannot be refined. Therefore, the finish rolling
temperature is preferably 650.degree. C. or more.
[0112] A temperature of the billet during the hot rolling can be
measured with radiation thermometer.
[0113] Steel, which has passed through the hot rolling process and
is a material for steel for a bearing part, that is, steel after
the finish rolling is commonly referred to as the hot rolled wire
rod.
[0114] After the hot rolling process is finished, that is, after
the finish rolling, the hot rolled wire rod is annularly wound at a
temperature of 800.degree. C. or less. This process is commonly
referred to as the winding process.
[0115] In the winding process, when the winding temperature is
high, there is a case where austenite grain grows and the pearlite
block coarsens. Therefore, the winding temperature is preferably
800.degree. C. or less. More preferably, the winding temperature is
770.degree. C. or less. On the other hand, when the winding
temperature is less than 650.degree. C., there is a case where the
wire breakage occurs. Therefore, the winding temperature is
preferably 650.degree. C. or more.
[0116] In addition, after the hot rolling process is finished,
there may be a cooling process before the winding, in which the
cooling is performed as needed.
[0117] After the winding process, the rolled wire rod is cooled to
600.degree. C. This process is commonly referred to as the cooling
process.
[0118] The cooling rate to 600.degree. C. is preferably set to
0.5.degree. C./s to 3.0.degree. C./s.
[0119] After the rolled wire rod is wound, when the wound wire rod
is cooled to 600.degree. C., transformation to the pearlite is
completed. There is a case where the cooling rate after winding
influences the transformation to pearlite from austenite.
Therefore, in order to suppress the precipitation of the
supercooled structure such as martensite or bainite, the cooling
rate after winding is preferably 3.0.degree. C./s or less. More
preferably, the cooling rate after winding is 2.3.degree. C./s or
less. On the other hand, there is a case where the cooling rate
after winding also influences the precipitation of proeutectoid
cementite. Therefore, in order to suppress the excessive
precipitation of proeutectoid cementite or coarsening of
proeutectoid cementite, the cooling rate after winding is
preferably 0.5.degree. C./s or more. More preferably, the cooling
rate after winding is 0.8.degree. C./s or more.
[0120] Although a conventional method for producing a bearing part
has a spheroidizing annealing process before wire drawing, steel
for a bearing part according to the present embodiment does not
have the spheroidizing annealing process before the wire drawing.
That is, the hot rolled wire rod obtained through the above
processes is not subjected to the spheroidizing annealing and is
subjected to the wire drawing in which the total reduction of area
is 50% or more. Then, after the wire drawing, the spheroidizing
annealing is performed at a temperature of 650.degree. C. to the
lower of 750.degree. C. and A.sub.1-5.degree. C.; thereby, the
steel for the bearing part is manufactured.
[0121] Then, after the obtained steel for the bearing part is
formed, the bearing part is obtained by performing the quenching
and the tempering.
[0122] When the hot rolled wire rod which is a material for the
steel for the bearing part is subjected to the wire drawing in
which the total reduction of area is 50% or more, spheroidizing of
the cementite is promoted during the spheroidizing annealing
because of introduced strain. Therefore, the cementite is
spheroidized at a low temperature in a short time; the average
grain size of spherical cementite of the steel for the bearing part
can be refined and the number density of spherical cementite in the
steel for the bearing part can be larger. Since the steel for the
bearing part manufactured in this manner has a sufficient amount of
spherical cementite, the average grain size of austenite can be
refined during quenching. Then, when the average grain size of
austenite is refined, the amount of retained austenite of the
bearing part can be secured and the grain size of the
prior-austenite of the bearing part can be refined.
[0123] When the total reduction of area is less than 50%, a
prescribed amount of retained austenite cannot be secured in the
steel for the bearing part. In addition, there is a case where the
prior-austenite of the bearing part cannot be refined due to
insufficient spheroidizing of cementite. On the other hand, when
the total reduction of area is more than 97%, there is a concern
that the wire breakage occurs during the wire drawing. Therefore,
the total reduction of area is preferably set to 50% to 97%.
[0124] A heating temperature of the spheroidizing annealing after
the wire drawing is set to less than 750.degree. C. and to
A.sub.1-5.degree. C. or less in order to increase the number
density of spherical cementite in the steel for the bearing part.
In addition, the number of times of the spheroidizing annealing is
set to one.
[0125] When the spheroidizing annealing is performed at a high
temperature that is 750.degree. C. or more or more than
A.sub.1-5.degree. C., the number density of spherical cementite in
the steel for the bearing part is lowered. Furthermore, since
transformation to austenite is started in the steel for the bearing
part, there is a case where changes in wire diameter become larger.
Therefore, it is preferably heated at a temperature of 750.degree.
C. or A.sub.1-5.degree. C., whichever is less.
[0126] On the other hand, when the heating temperature of the
spheroidizing annealing after the wire drawing is less than
650.degree. C., the cementite is spheroidized in the steel for the
bearing part insufficiently, and thus, the cementite remains as the
pearlite. Therefore, since grain size of austenite is coarsened
during quenching and the hardness is increased, and there is a
concern that workability of the bearing part is reduced.
Accordingly, the heating temperature of the spheroidizing annealing
after the wire drawing is preferably 650.degree. C. or more.
[0127] That is, the heating temperature of the spheroidizing
annealing is a temperature of 650.degree. C. to the lower of
750.degree. C. and A.sub.1-5.degree. C.
[0128] In addition, A.sub.1 means a temperature at which A.sub.1
transformation starts and unit thereof is Celsius degree (.degree.
C.). Furthermore, A.sub.1 can be calculated simply by the following
equation 1 based on the chemical components.
[0129] Here, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [Al] and [B]
in the equation represent, by mass %, the content of C, the content
of Si the content of Mn, the content of Cu, the content of Ni, the
content of Cr, the content of Mo, the content of Al and the content
of B in the hot rolled wire rod, respectively.
A.sub.1=750.8-26.6.times.[C]+17.6.times.[Si]-11.6.times.[Mn]-22.9.times.-
[Cu]-23.0.times.[Ni]+24.1.times.[Cr]+22.5.times.[Mo]-169.4.times.[Al]-894.-
7.times.[B] (Equation 1)
[0130] In addition, it is held at the above temperature for 0.5
hours to 5 hours in the spheroidizing annealing. When the holding
time is less than 0.5 hours, spheroidizing is insufficient. When
the holding time is more than 5 hours, there is a case where the
number density of prescribed spherical cementite is reduced.
[0131] The steel for the bearing part after the spheroidizing
annealing is formed (rough formed), and then the quenching is
performed. The heating temperature of quenching in the quenching
after the spheroidizing annealing is preferably set to 820.degree.
C. or more so that a certain amount of cementite can be
solid-soluted into the austenite. When the heating temperature of
the quenching is less than 820.degree. C., the cementite is not
solid-soluted into the austenite sufficiently, and thus, there is a
case where hardness of the bearing part is reduced. On the other
hand, when the heating temperature of the quenching is more than
890.degree. C., there is a concern that the average grain size of
the prior-austenite in the bearing part coarsens. Therefore, the
heating temperature of the quenching is preferably set to
820.degree. C. to 890.degree. C.
[0132] In addition, a holding time in the quenching is preferably
0.5 hours to 2 hours. When the holding time is less than 0.5 hours,
the cementite is not solid-soluted into the austenite sufficiently.
When the holding time is more than 2 hours, the cementite is
resolved and C is solid-soluted into the austenite excessively. As
a result, the retained austenite of the bearing part may be
increased, or the average grain size of the prior-austenite of the
bearing part may be coarsened.
[0133] A tempering temperature in the tempering is preferably
150.degree. C. or more in order to secure the toughness and adjust
the hardness. When the tempering temperature is less than
150.degree. C., there is a case where the toughness of the bearing
part cannot be secured. On the other hand, when the tempering
temperature is more than 250.degree. C., the hardness of the
bearing part is lowered, and there is a concern that the rolling
contact fatigue life is reduced. Therefore, the tempering
temperature is preferably set to 150.degree. C. to 250.degree.
C.
[0134] In addition, a holding time in the tempering is preferably
0.5 hours to 3 hours. When the holding time is less than 0.5 hours,
there is a case where the toughness of the bearing part is not
secured. Even when the tempering is performed for more than 3
hours, properties are not changed, and thus, only productivity is
reduced.
[0135] The finish machining is subjected to the steel for the
bearing part after the tempering; therefore the bearing part can be
obtained.
[0136] As described above, when the preferred manufacturing method
of the present invention is employed, the metallographic structure
of the bearing part and the metallographic structure of the steel
for the bearing part can be within the scope of the present
invention.
EXAMPLES
[0137] Hereinafter, effects of the bearing part according to the
present embodiment will be more specifically described by using
examples of the bearing part of the present invention. Here,
conditions of Examples are merely examples of conditions employed
to check the operability and effects of the present invention, and
the present invention is not limited to the following examples of
conditions. The present invention is also possible to put into
practice after appropriate modifications or variations within the
scope adaptable to the gist without departing from the gist of the
present invention as long as the object of the present invention
can be accomplished. Accordingly, the present invention can employ
various conditions, and all of these conditions are contained in
the technical features of the present invention.
[0138] By subjecting the wire rods or bars having components shown
in Table 1-1 and Table 1-2 to heat treatment or hot forging, hot
rolled wire rods having microstructure shown in Table 2-1 and Table
2-2 were obtained. Next, the hot rolled wire rods were subjected to
cold wire drawing until diameters were .phi. 10 mm. Then, the hot
rolled wire rods obtained by the wire drawing were cut (into
lengths of 10 mm) so that the length in the longitudinal direction
was 10 mm. Then, the hot rolled wire rods obtained by cutting were
subjected to the spheroidizing annealing at spheroidizing
temperature as shown in Table 2-1 and Table 2-2 for 0.5 hours to 3
hours, and thereby they were adjusted to the microstructures
including spherical cementite and ferrite. As a result, steels for
bearing parts were obtained. The steels for bearing parts were
formed into sphere types of .phi. 9.5 mm. Next, after the quenching
and the tempering were performed, the steels for bearing parts were
subjected to the finish machining; thereby manufacturing the
bearing parts. The quenching was performed at the quenching
temperature as shown in Table 2-1 and Table 2-2, in which the
holding time of the quenching was 60 minutes, and the cooling was
oil cooling. Next, the tempering was performed at the tempering
temperature as shown in Table 2-1 and Table 2-2, and the holding
time of the tempering was 90 minutes. The structures of the hot
rolled wire rods, the reduction of area during the wire drawing,
the temperatures of the quenching and the tempering are shown in
Table 2.
[0139] The microstructures of the hot rolled wire rods, the
microstructures of the steels for bearing parts and the bearing
parts were evaluated using an optical microscope. Then, the average
grain size of prior-austenite (m) of the bearing parts was measured
using a SEM. In addition, the measurements of the average grain
size of spherical cementite (.mu.m) of the steels for bearing parts
and the bearing parts, and the measurements of the number density
of spherical cementite (mm.sup.-2) in the steels for bearing parts
and the bearing parts were carried out using a SEM. In addition,
the measurement of the amount of retained austenite (volume %) of
the bearing parts was carried out using X-ray diffraction
method.
[0140] The measurement of the rolling contact fatigue life of the
bearing parts was carried out using a radial type fatigue testing
machine.
[0141] Normal rolling contact fatigue life of the bearing parts was
tested in an environment including only lubricating oil.
Furthermore, rolling contact fatigue life of the bearing parts was
measured in a contaminated environment where 1 g of an iron powder
having a hardness of 750 Hv to 800 Hv and a particle size of 100
.mu.m to 180 .mu.m was mixed into 1 L of lubricating oil. Then,
life of the cumulative failure probability of 10% was obtained
using the measured rolling contact fatigue life with Weibull
statistical analysis. The rolling contact fatigue life in a normal
and contaminated environment was based on the life of B1, which has
the same component as SUJ2, manufactured in the current production
method, and it was expressed by the ratio to the life of B1, in
Table 2-1 and Table 2-2.
[0142] [Table 1-1]
[0143] [Table 1-2]
[0144] [Table 2-1]
[0145] [Table 2-2]
[0146] The evaluation results of the microstructures of the
materials, the producing method, the microstructures of the bearing
parts and the rolling contact fatigue life are shown in Table 2-1
and Table 2-2. A1 to A19 are within the appropriate range of the
present invention in the Table 1-1 and Table 2-1; the ratio of the
normal rolling contact fatigue life is 1.5 times or more of the B1,
the rolling contact fatigue life in a contaminated environment is 2
times or more of the B1, and thus, A1 to A19 have an excellent
rolling contact fatigue life.
[0147] All of the metallographic structures of the steels for the
bearing part consisted of spherical cementite and ferrite in
examples of the present invention. In addition, all of the
metallographic structures of the bearing parts consisted of
spherical cementite and martensite.
[0148] On the other hand, A20 to A38, B1 and B2 are Comparative
Examples. Since Comparative Examples of A20 to A38, B1 and B2
failed to satisfy one or both of the chemical composition and
microstructures of the bearing parts, which are defined by the
present invention, the rolling contact fatigue life was equivalent
to a conventional bearing part, or inferior to a conventional
bearing part.
[0149] The chemical compositions of A20 to A31 were out of the
scope of the present invention. Since the content of C was small in
A20, the number density of spherical cementite in the steel for the
bearing part was lowered. Therefore, since the average grain size
of austenite was coarsened during quenching and the amount of
retained austenite was insufficient, the rolling contact fatigue
life of the bearing part in the contaminated environment was
lowered. In addition, since the content of C was small, the number
density of the spherical cementite in the bearing part was also
insufficient. When the content of C was small, the strength during
quenching was also low, and thus, the rolling contact fatigue life
even under the normal conditions was lowered. On the other hand,
the content of C was excessive in A21. Therefore, since the amount
of retained austenite and the average grain size of the spherical
cementite were excessive, the normal rolling contact fatigue life
was not improved.
[0150] The content of Mn was excessive in A23. Therefore, since
cracks occurred during wire drawing due to martensite of the steel
for the bearing part, both the normal rolling contact fatigue life
and the rolling contact fatigue life in the contaminated
environment were lowered in the bearing part. Furthermore, since
wire drawability was lowered, enough amount of wire drawing could
not be secured. Therefore, the number density of the spherical
cementite during the spheroidizing annealing was lowered.
Accordingly, since sufficient pinning effect could not be ensured,
the average grain size of austenite was coarsened during the
quenching. In addition, since the spherical cementite of the steel
for the bearing part was few, the number density of the spherical
cementite in the bearing part after quenching was insufficient. In
addition, since the content of Mn was excessive, the Ms point was
reduced, and the amount of retained austenite was excessive.
Therefore, even though martensite was not included in the
microstructure and cracks did not occur, the rolling contact
fatigue life was reduced. Regarding A24, the content of Mn was
small and the amount of retained austenite was insufficient.
Therefore, the rolling contact fatigue life of the bearing part in
the contaminated environment was not improved. Regarding A25, since
the content of Cr was small, the cementite was easily solid-soluted
during the quenching, and the number density of the spherical
cementite was insufficient. Furthermore, the amount of retained
austenite was excessive, and the rolling contact fatigue life of
the bearing part was lowered. Regarding A27, since the content of
Cr was large and the cementite was stabilized, the cementite was
not solid-soluted during the quenching, and the retained austenite
was insufficient. Therefore, the rolling contact fatigue life of
the bearing part in the contaminated environment was not
improved.
[0151] The content of Si was excessive in A22, the content of Al
was excessive in A26 and the content of O was excessive in A30. Due
to inclusions, both the normal rolling contact fatigue life and the
rolling contact fatigue life in the contaminated environment were
inferior to those of the conventional, in A22, A26 and A30. Since
the content of S was excessive in A28, both the normal rolling
contact fatigue life and the rolling contact fatigue life in the
contaminated environment were inferior to those of the conventional
due to sulfide. Since the content of N was excessive in A31, both
the normal rolling contact fatigue life and the rolling contact
fatigue life in the contaminated environment were inferior to those
of the conventional due to nitride. Since the content of P was
excessive in A29, grain boundary was embrittled. Therefore, both
the normal rolling contact fatigue life and the rolling contact
fatigue life in the contaminated environment were lowered in
A29.
[0152] Although the chemical compositions of A32 to A38 were within
the scope of the present invention, the microstructures of the
bearing part were out of the scope of the present invention.
Regarding A32 and A33, since the total reduction of area of the
wire drawing is low, part of the pearlite structure was not
spheroidized during the spheroidizing annealing and the number
density of the spherical cementite in the steel for the bearing
part was decreased. Therefore, since the average grain size of
austenite was coarsened during the quenching, the amount of
retained austenite was insufficient. Then, the number density of
the spherical cementite in the bearing part was also decreased, and
the rolling contact fatigue life in the contaminated environment
was insufficient.
[0153] Regarding A34, since the heating temperature during the
spheroidizing annealing was low, part of the pearlite structure was
not spheroidized during the spheroidizing annealing and the number
density of the spherical cementite in the steel for the bearing
part was lowered. Therefore, since the average grain size of
austenite was coarsened during the quenching, the amount of
retained austenite of the bearing part was decreased. Furthermore,
the number density of the spherical cementite in the bearing part
was also decreased, and the rolling contact fatigue life in the
contaminated environment was insufficient. Regarding A35, since the
temperature during the spheroidizing annealing was high, the number
density of the spherical cementite in the steel for the bearing
part was decreased. As a result, since the average grain size of
austenite was coarsened during the quenching, the amount of
retained austenite was decreased. Furthermore, since the number
density of the spherical cementite in the bearing part was
decreased, the rolling contact fatigue life in the contaminated
environment was insufficient.
[0154] Regarding A36, since the heating temperature during
quenching was low, the amount of solid-soluted C was decreased and
the amount of retained austenite was insufficient. Therefore, the
rolling contact fatigue life in the contaminated environment was
insufficient. Regarding A37, since the heating temperature of
quenching was high and the cementite was excessively solid-soluted,
the amount of retained austenite was excessive and the number
density of the spherical cementite was also decreased. Therefore,
normal rolling contact fatigue life was decreased.
[0155] 131 and 132 are the current materials in which the
spheroidizing annealing was performed before the wire drawing. In
the B1, the number density of the spherical cementite in the steel
for the bearing part was small. Therefore, the average grain size
of austenite was coarsened during the quenching and the amount of
retained austenite was decreased. As a result, the number density
of the spherical cementite in the bearing part was lowered. Then,
in the B1, the rolling contact fatigue life in the contaminated
environment was low. In addition, since the content of Mn is
increased in B2, the amount of retained austenite was increased
compared with the B1. However, since the number density of the
spherical cementite in the bearing part was small, the normal
rolling contact fatigue property was insufficient.
INDUSTRIAL APPLICABILITY
[0156] According to the above aspects of the present invention, a
bearing part can be obtained by suppressing the content of Mn and
by shortening spheroidizing annealing in order to secure good wire
drawability. As a result, since the bearing part having an
excellent rolling contact fatigue life in an environment including
a contaminated environment can be obtained, the present invention
is highly applicable to industries.
BRIEF DESCRIPTION OF THE REFERENCE SYMBOLS
[0157] 1 Martensite [0158] 2 Spherical cementite [0159] 4 Ferrite
[0160] 5 Spherical cementite
TABLE-US-00001 [0160] TABLE 1-1 CHEMICAL COMPOSITION (mass %) No. C
Si Mn Cr P S Al N O Mo B Cu Ni Ca A.sub.1 REMARKS A1 1.01 0.25 0.35
1.41 0.007 0.005 0.018 0.005 0.0009 755 EXAMPLE A2 1.05 0.15 0.30
1.40 0.008 0.004 0.020 0.012 0.0010 0.0003 752 EXAMPLE A3 1.00 0.20
0.50 1.10 0.008 0.005 0.015 0.013 0.0008 0.03 0.21 742 EXAMPLE A4
0.97 0.12 0.21 0.91 0.010 0.009 0.078 0.012 0.0005 0.05 0.0001 0.50
723 EXAMPLE A5 1.05 0.54 1.15 1.55 0.008 0.005 0.032 0.018 0.0007
0.01 751 EXAMPLE A6 0.98 0.15 0.99 1.50 0.006 0.011 0.025 0.011
0.0006 0.0002 0.0006 748 EXAMPLE A7 1.00 0.56 0.25 1.41 0.004 0.005
0.023 0.014 0.0006 0.23 766 EXAMPLE A8 1.01 0.24 0.28 1.38 0.011
0.008 0.019 0.029 0.0008 0.0021 0.31 750 EXAMPLE A9 0.99 0.26 0.34
1.40 0.007 0.008 0.021 0.012 0.0009 0.19 751 EXAMPLE A10 1.00 0.26
0.37 1.41 0.007 0.009 0.018 0.014 0.0008 1.20 728 EXAMPLE A11 1.03
0.25 0.35 1.44 0.012 0.010 0.019 0.013 0.0007 0.0011 755 EXAMPLE
A12 0.96 0.21 0.33 1.36 0.009 0.006 0.019 0.015 0.0008 755 EXAMPLE
A13 1.00 0.25 0.34 1.41 0.007 0.003 0.019 0.009 0.0007 755 EXAMPLE
A14 1.00 0.25 0.34 1.41 0.007 0.003 0.019 0.009 0.0007 755 EXAMPLE
A15 1.00 0.25 0.34 1.41 0.007 0.003 0.019 0.009 0.0007 755 EXAMPLE
A16 1.00 0.25 0.34 1.41 0.007 0.003 0.019 0.009 0.0007 755 EXAMPLE
A17 1.00 0.25 0.34 1.41 0.007 0.003 0.019 0.009 0.0007 755 EXAMPLE
A18 1.00 0.25 0.34 1.41 0.007 0.003 0.019 0.009 0.0007 755 EXAMPLE
A19 1.00 0.25 0.34 1.41 0.007 0.003 0.019 0.009 0.0007 755
EXAMPLE
TABLE-US-00002 TABLE 1-2 CHEMICAL COMPOSITION (mass %) No. C Si Mn
Cr P S Al N O Mo B Cu Ni Ca A.sub.1 REMARKS A20 0.91 0.25 0.35 1.40
0.005 0.005 0.011 0.012 0.0006 761 COMPARATIVE EXAMPLE A21 1.19
0.25 0.28 1.43 0.006 0.006 0.021 0.011 0.0005 751 COMPARATIVE
EXAMPLE A22 1.06 0.83 0.29 1.35 0.008 0.005 0.011 0.013 0.0008 0.05
766 COMPARATIVE EXAMPLE A23 0.96 0.18 1.56 1.44 0.007 0.005 0.030
0.012 0.0006 0.0002 0.0005 740 COMPARATIVE EXAMPLE A24 0.99 0.25
0.06 1.45 0.007 0.005 0.029 0.014 0.0010 0.0001 758 COMPARATIVE
EXAMPLE A25 1.05 0.35 0.35 0.80 0.008 0.004 0.021 0.013 0.0008 741
COMPARATIVE EXAMPLE A26 1.05 0.25 0.36 1.46 0.006 0.006 0.190 0.012
0.0005 726 COMPARATIVE EXAMPLE A27 1.05 0.50 0.23 1.63 0.011 0.008
0.016 0.011 0.0005 766 COMPARATIVE EXAMPLE A28 1.00 0.28 0.34 1.40
0.006 0.031 0.050 0.014 0.0007 0.21 755 COMPARATIVE EXAMPLE A29
1.00 0.27 0.35 1.41 0.029 0.007 0.051 0.014 0.0007 750 COMPARATIVE
EXAMPLE A30 1.02 0.25 0.35 1.39 0.008 0.010 0.008 0.011 0.0012
0.0002 756 COMPARATIVE EXAMPLE A31 1.01 0.24 0.34 1.41 0.009 0.009
0.016 0.041 0.0009 755 COMPARATIVE EXAMPLE A32 1.00 0.24 0.34 1.41
0.008 0.005 0.021 0.010 0.0007 755 COMPARATIVE EXAMPLE A33 1.00
0.24 0.34 1.41 0.008 0.005 0.021 0.010 0.0007 755 COMPARATIVE
EXAMPLE A34 1.00 0.24 0.34 1.41 0.008 0.005 0.021 0.010 0.0007 755
COMPARATIVE EXAMPLE A35 1.00 0.24 0.34 1.41 0.008 0.005 0.021 0.010
0.0007 755 COMPARATIVE EXAMPLE A36 1.00 0.24 0.34 1.41 0.008 0.005
0.021 0.010 0.0007 755 COMPARATIVE EXAMPLE A37 1.00 0.24 0.34 1.41
0.008 0.005 0.021 0.010 0.0007 755 COMPARATIVE EXAMPLE B1 1.01 0.25
0.35 1.41 0.007 0.005 0.018 0.005 0.0009 755 COMPARATIVE EXAMPLE B2
1.04 0.25 1.01 0.91 0.007 0.005 0.018 0.005 0.0009 0.20 0.0020 1.00
0.0015 718 COMPARATIVE EXAMPLE
TABLE-US-00003 TABLE 24 BEARING PART RATIO OF METHOD FOR STEEL FOR
AV- ROLLING PRODUCING BEARING PART BEARING PART AV- ERAGE CONTACT
HOT TOTAL NUMBER ERAGE GRAIN NUMBER FATIGUE ROLL- RE- DENSITY GRAIN
AMOUNT SIZE DENSITY RATIO LIFE ED DUCTION OF SIZE OF OF OF OF IN
WIRE OF AREA SPHERO- QUENCH- TEMPER- SPHER- OF RE- SPHER- SPHER-
ROLLING CONTAM- ROD DURING IDIZING ING ING ICAL PRIOR- TAINED ICAL
ICAL CONTACT INATED MICRO WIRE TEMPER- TEMPER- TEMPER- MICRO CE-
MICRO AUS- AUS- CE- CE- FATIGUE ENVI- STRUC- DRAWING ATURE ATURE
ATURE STRUC- MENTITE STRUC- TENITE TENITE MENTITE MENTITE LIFE
RONMENT No. TURE (%) (.degree. C.) (.degree. C.) (.degree. C.) TURE
(.times.10.sup.6 mm.sup.-2) TURE (.mu.m) (%) (.mu.m)
(.times.10.sup.6 mm.sup.-2) (L.sub.10/B1.sub.10)
(L.sub.10/B1.sub.10) REMARKS A1 P + .theta. 75 700 850 170
SPHERICAL 2.34 RETAINED .gamma. + 5.6 18.5 0.40 0.47 1.56 2.2
EXAMPLE .theta. + .alpha. SPHERICAL .theta. + M A2 P + .theta. 75
680 850 170 SPHERICAL 2.87 RETAINED .gamma. + 4.5 19.4 0.41 0.51
1.60 2.2 EXAMPLE .theta. + .alpha. SPHERICAL .theta. + M A3 P +
.theta. 75 680 850 170 SPHERICAL 2.65 RETAINED .gamma. + 5.1 20.1
0.39 0.48 1.68 2.1 EXAMPLE .theta. + .alpha. SPHERICAL .theta. + M
A4 P + .theta. 75 680 850 170 SPHERICAL 2.24 RETAINED .gamma. + 5.5
18.2 0.35 0.46 1.48 2.0 EXAMPLE .theta. + .alpha. SPHERICAL .theta.
+ M A5 P + .theta. 75 680 850 170 SPHERICAL 2.93 RETAINED .gamma. +
4.3 24.1 0.44 0.56 1.62 2.6 EXAMPLE .theta. + .alpha. SPHERICAL
.theta. + M A6 P + .theta. 75 680 850 170 SPHERICAL 2.89 RETAINED
.gamma. + 4.3 22.4 0.41 0.50 1.62 2.5 EXAMPLE .theta. + .alpha.
SPHERICAL .theta. + M A7 P + .theta. 75 680 850 170 SPHERICAL 2.66
RETAINED .gamma. + 4.7 23.9 0.41 0.48 1.60 2.3 EXAMPLE .theta. +
.alpha. SPHERICAL .theta. + M A8 P + .theta. 75 680 850 170
SPHERICAL 2.73 RETAINED .gamma. + 4.6 19.8 0.42 0.52 1.72 2.2
EXAMPLE .theta. + .alpha. SPHERICAL .theta. + M A9 P + .theta. 75
680 850 170 SPHERICAL 2.65 RETAINED .gamma. + 4.5 20.2 0.40 0.49
1.62 2.3 EXAMPLE .theta. + .alpha. SPHERICAL .theta. + M A10 P +
.theta. 75 680 850 170 SPHERICAL 2.67 RETAINED .gamma. + 5.2 24.3
0.39 0.49 1.68 2.4 EXAMPLE .theta. + .alpha. SPHERICAL .theta. + M
A11 P + .theta. 75 680 850 170 SPHERICAL 2.76 RETAINED .gamma. +
4.5 19.5 0.41 0.52 1.65 2.3 EXAMPLE .theta. + .alpha. SPHERICAL
.theta. + M A12 P + .theta. 75 680 830 170 SPHERICAL 2.57 RETAINED
.gamma. + 4.9 20.9 0.38 0.48 1.58 2.4 EXAMPLE .theta. + .alpha.
SPHERICAL .theta. + M A13 P + .theta. 61 680 850 170 SPHERICAL 2.36
RETAINED .gamma. + 5.3 18.7 0.41 0.51 1.62 2.1 EXAMPLE .theta. +
.alpha. SPHERICAL .theta. + M A14 P + .theta. 75 680 850 170
SPHERICAL 2.96 RETAINED .gamma. + 4.8 20.6 0.39 0.49 1.58 2.2
EXAMPLE .theta. + .alpha. SPHERICAL .theta. + M A15 P + .theta. 82
680 850 170 SPHERICAL 2.87 RETAINED .gamma. + 4.5 21.5 0.43 0.48
1.50 2.2 EXAMPLE .theta. + .alpha. SPHERICAL .theta. + M A16 P +
.theta. 75 650 850 170 SPHERICAL 3.24 RETAINED .gamma. + 4.3 22.7
0.37 0.47 1.58 2.5 EXAMPLE .theta. + .alpha. SPHERICAL .theta. + M
A17 P + .theta. 75 720 850 170 SPHERICAL 2.13 RETAINED .gamma. +
5.8 19.0 0.41 0.52 1.60 2.3 EXAMPLE .theta. + .alpha. SPHERICAL
.theta. + M A18 P + .theta. 75 680 810 170 SPHERICAL 2.25 RETAINED
.gamma. + 4.1 19.3 0.44 0.54 1.66 2.1 EXAMPLE .theta. + .alpha.
SPHERICAL .theta. + M A19 P + .theta. 75 680 870 170 SPHERICAL 2.09
RETAINED .gamma. + 5.8 22.6 0.38 0.46 1.46 2.5 EXAMPLE .theta. +
.alpha. SPHERICAL .theta. + M
TABLE-US-00004 TABLE 2-2 BEARING PART RATIO OF METHOD FOR STEEL FOR
ROLLING AV- ROLLING PRODUCING BEARING PART BEARING PART AV- ERAGE
CONTACT HOT TOTAL NUMBER ERAGE GRAIN NUMBER FATIGUE ROLL- RE-
DENSITY GRAIN AMOUNT SIZE DENSITY RATIO LIFE ED DUCTION OF SIZE OF
OF OF OF IN WIRE OF AREA SPHERO- QUENCH- TEMPER- SPHER- OF RE-
SPHER- SPHER- ROLLING CONTAM- ROD DURING IDIZING ING ING ICAL
PRIOR- TAINED ICAL ICAL CONTACT INATED MICRO WIRE TEMPER- TEMPER-
TEMPER- MICRO CE- MICRO AUS- AUS- CE- CE- FATIGUE ENVI- STRUC-
DRAWING ATURE ATURE ATURE STRUC- MENTITE STRUC- TENITE TENITE
MENTITE MENTITE LIFE RONMENT No. TURE (%) (.degree. C.) (.degree.
C.) (.degree. C.) TURE (.times.10.sup.6 mm.sup.-2) TURE (.mu.m) (%)
(.mu.m) (.times.10.sup.6 mm.sup.-2) (L.sub.10/B1.sub.10)
(L.sub.10/B1.sub.10) REMARKS A20 P + .theta. 75 720 850 170
SPHERICAL 1.68 RETAINED .gamma. + 6.4 13.4 0.37 0.35 0.14 0.8
COMPARATIVE .theta. + .alpha. SPHERICAL .theta. + M EXAMPLE A21 P +
.theta. 75 680 850 170 SPHERICAL 3.02 RETAINED .gamma. + 4.9 25.8
0.49 0.56 0.96 2.2 COMPARATIVE .theta. + .alpha. SPHERICAL .theta.
+ M EXAMPLE A22 P + .theta. 75 680 850 170 SPHERICAL 2.78 RETAINED
.gamma. + 5.0 18.4 0.40 0.49 0.04 0.4 COMPARATIVE .theta. + .alpha.
SPHERICAL .theta. + M EXAMPLE A23 P + .theta. + M 44 680 850 170
SPHERICAL 2.14 RETAINED .gamma. + 7.5 26.6 0.36 0.41 0.02 0.5
COMPARATIVE .theta. + .alpha. + M SPHERICAL .theta. + M EXAMPLE A24
P + .theta. 75 680 850 170 SPHERICAL 2.84 RETAINED .gamma. + 5.5
12.5 0.41 0.48 1.28 1.1 COMPARATIVE .theta. + .alpha. SPHERICAL
.theta. + M EXAMPLE A25 P + .theta. 75 680 850 170 SPHERICAL 3.84
RETAINED .gamma. + 6.3 25.9 0.31 0.24 0.72 2.5 COMPARATIVE .theta.
+ .alpha. SPHERICAL .theta. + M EXAMPLE A26 P + .theta. 75 680 870
170 SPHERICAL 2.78 RETAINED .gamma. + 5.1 18.8 0.40 0.51 0.02 0.2
COMPARATIVE .theta. + .alpha. SPHERICAL .theta. + M EXAMPLE A27 P +
.theta. 75 680 870 170 SPHERICAL 2.15 RETAINED .gamma. + 4.8 6.7
0.48 0.61 1.42 1.2 COMPARATIVE .theta. + .alpha. SPHERICAL .theta.
+ M EXAMPLE A28 P + .theta. 75 680 870 170 SPHERICAL 2.81 RETAINED
.gamma. + 5.4 19.4 0.41 0.49 0.04 0.4 COMPARATIVE .theta. + .alpha.
SPHERICAL .theta. + M EXAMPLE A29 P + .theta. 75 680 870 170
SPHERICAL 2.79 RETAINED .gamma. + 4.9 18.9 0.39 0.51 0.06 0.8
COMPARATIVE .theta. + .alpha. SPHERICAL .theta. + M EXAMPLE A30 P +
.theta. 75 680 870 170 SPHERICAL 2.76 RETAINED .gamma. + 4.9 19.1
0.42 0.48 0.08 1.1 COMPARATIVE .theta. + .alpha. SPHERICAL .theta.
+ M EXAMPLE A31 P + .theta. 75 680 870 170 SPHERICAL 2.76 RETAINED
.gamma. + 5.7 19.3 0.38 0.48 0.02 0.7 COMPARATIVE .theta. + .alpha.
SPHERICAL .theta. + M EXAMPLE A32 P + .theta. 20 680 850 170
SPHERICAL 1.24 RETAINED .gamma. + 8.3 12.3 0.37 0.33 1.22 1.6
COMPARATIVE .theta. + .alpha. SPHERICAL .theta. + M EXAMPLE A33 P +
.theta. 46 720 850 170 SPHERICAL 1.76 RETAINED .gamma. + 7.1 15.1
0.38 0.37 1.26 1.7 COMPARATIVE .theta. + .alpha. SPHERICAL .theta.
+ M EXAMPLE A34 P + .theta. 75 630 850 170 SPHERICAL 1.67 RETAINED
.gamma. + 7.5 15.4 0.41 0.31 1.30 1.9 COMPARATIVE .theta. + .alpha.
SPHERICAL .theta. + M EXAMPLE A35 P + .theta. 75 760 850 170
SPHERICAL 1.73 RETAINED .gamma. + 7.4 15.5 0.47 0.42 1.36 1.8
COMPARATIVE .theta. + .alpha. SPHERICAL .theta. + M EXAMPLE A36 P +
.theta. 75 680 780 170 SPHERICAL 2.87 RETAINED .gamma. + 5.8 10.8
0.38 0.72 1.24 1.4 COMPARATIVE .theta. + .alpha. SPHERICAL .theta.
+ M EXAMPLE A37 P + .theta. 75 680 900 170 SPHERICAL 2.79 RETAINED
.gamma. + 7.1 26.5 0.50 0.33 0.82 2.2 COMPARATIVE .theta. + .alpha.
SPHERICAL .theta. + M EXAMPLE B1 SPHER- 61 680 870 170 SPHERICAL
1.1 RETAINED .gamma. + 10.5 4.7 0.56 0.28 1.00 1.0 COMPARATIVE ICAL
.theta. + .alpha. SPHERICAL .theta. + M EXAMPLE .theta. + .alpha.
B2 SPHER- 61 680 830 170 SPHERICAL 1.05 RETAINED .gamma. + 7.4 18.5
0.49 0.25 1.06 2.1 COMPARATIVE ICAL .theta. + .alpha. SPHERICAL
.theta. + M EXAMPLE .theta. + .alpha.
* * * * *