U.S. patent application number 16/613765 was filed with the patent office on 2020-02-27 for steel and part.
This patent application is currently assigned to NIPPON STEEL CORPORATION. The applicant listed for this patent is NIPPON STEEL CORPORATION. Invention is credited to Daisuke HIRAKAMI, Hideki IMATAKA, Yutaka NEISHI, Kosuke TANAKA, Tomohiro YAMASHITA.
Application Number | 20200063246 16/613765 |
Document ID | / |
Family ID | 64274214 |
Filed Date | 2020-02-27 |
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United States Patent
Application |
20200063246 |
Kind Code |
A1 |
YAMASHITA; Tomohiro ; et
al. |
February 27, 2020 |
STEEL AND PART
Abstract
Steel improved in all of the hardenability, toughness,
surface-originated micropitting life, and bending fatigue strength
and a part manufactured using such steel are provided having
predetermined constituents having an Fn1 defined by the following
formula (1) of 0.20 to 0.65 and having an Fn2 defined by the
following formula (2) of 0.50 to 1.00:
Fn1=4.2.times.[Cr]/(7.0.times.[Si]+16.0.times.[Mn]) (1) [Element]:
mass % of element Fn2=A1/A2 (2) A1: total area (.mu.m.sup.2) of
sulfide-based inclusions containing 1.0 mol % or more of Ca with
respect to the total number of moles in the sulfides and having a
circle equivalent diameter of 1.0 .mu.m or more in observation
regions of a total area of 4.0 mm.sup.2 A2: total area
(.mu.m.sup.2) of sulfide-based inclusions having a circle
equivalent diameter of 1.0 .mu.m or more in observation regions of
a total area of 4.0 mm.sup.2
Inventors: |
YAMASHITA; Tomohiro; (Tokyo,
JP) ; HIRAKAMI; Daisuke; (Tokyo, JP) ; NEISHI;
Yutaka; (Tokyo, JP) ; TANAKA; Kosuke; (Tokyo,
JP) ; IMATAKA; Hideki; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
NIPPON STEEL CORPORATION
Tokyo
JP
|
Family ID: |
64274214 |
Appl. No.: |
16/613765 |
Filed: |
May 15, 2018 |
PCT Filed: |
May 15, 2018 |
PCT NO: |
PCT/JP2018/018799 |
371 Date: |
November 14, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/20 20130101;
C22C 38/001 20130101; C22C 38/32 20130101; C22C 38/00 20130101;
C22C 38/44 20130101; C22C 38/04 20130101; C22C 38/002 20130101;
C22C 38/06 20130101; C22C 38/22 20130101; C22C 38/26 20130101; C22C
38/54 20130101; C22C 38/28 20130101; C22C 38/02 20130101 |
International
Class: |
C22C 38/44 20060101
C22C038/44; C22C 38/22 20060101 C22C038/22; C22C 38/20 20060101
C22C038/20; C22C 38/32 20060101 C22C038/32; C22C 38/28 20060101
C22C038/28; C22C 38/26 20060101 C22C038/26; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 15, 2017 |
JP |
2017-096281 |
Claims
1. A steel having a chemical composition comprising, by mass %, C:
0.10 to 0.30%, Si: 0.01 to 0.25%, Mn: 0.20 to 1.50%, P: 0.001 to
0.015%, S: 0.001 to 0.010%, Cr: 0.50 to 2.00%, Mo: 0.10 to 0.50%,
Al: 0.005 to 0.100%, Ca: 0.0002 to 0.0010%, N: 0.005 to 0.025%, O:
0.0015% or less, Cu: 0 to 0.20%, Ni: 0 to 0.20%, B: 0 to 0.005%,
Nb: 0 to 0.05%, Ti: 0 to 0.10%, and a balance of Fe and impurities,
wherein Fn1 defined by the following formula (1) is 0.20 to 0.65,
and Fn2 defined by the following formula (2) is 0.50 to 1.00:
Fn1=4.2.times.[Cr]/(7.0.times.[Si]+16.0.times.[Mn]) (1) [Element]:
mass % of element Fn2=A1/A2 (2) A1: total area (.mu.m.sup.2) of
sulfide-based inclusions containing 1.0 mol % or more of Ca with
respect to the total number of moles in the sulfides and having a
circle equivalent diameter of 1.0 .mu.m or more in observation
regions of a total area of 4.0 mm.sup.2 A2: total area
(.mu.m.sup.2) of sulfide-based inclusions having a circle
equivalent diameter of 1.0 .mu.m or more in observation regions of
a total area of 4.0 mm.sup.2.
2. The steel according to claim 1 wherein the chemical composition
includes, by mass %, at least one of Cu: 0.20% or less, Ni: 0.20%
or less, and B: 0.005% or less.
3. The steel according to claim 1 wherein the chemical composition
includes, by mass %, at least one of Nb: 0.05% or less and Ti:
0.10% or less.
4. The steel according to claim 1, wherein the steel is a steel
rod.
5. A part excellent in surface-originated micropitting life and
bending fatigue strength, a chemical composition at a region of a
depth of 500 .mu.m or more from the surface of the part comprising,
by mass %, C: 0.10 to 0.30%, Si: 0.01 to 0.25%, Mn: 0.20 to 1.50%,
P: 0.001 to 0.015%, S: 0.001 to 0.010%, Cr: 0.50 to 2.00%, Mo: 0.10
to 0.50%, Al: 0.005 to 0.100%, Ca: 0.0002 to 0.0010%, N: 0.005 to
0.025%, O: 0.0015%, Cu: 0 to 0.20%, Ni: 0 to 0.20%, B: 0 to 0.005%,
Nb: 0 to 0.05%, Ti: 0 to 0.10%, and a balance of Fe and impurities,
wherein Fn1 defined by the following formula (1) is 0.20 to 0.65,
and Fn2 defined by the following formula (2) is 0.50 to 1.00,
Fn1=4.2.times.[Cr]/(7.0.times.[Si]+16.0.times.[Mn]) (1) [Element]:
mass % of element Fn2=A1/A2 (2) A1: total area (.mu.m.sup.2) of
sulfide-based inclusions containing 1.0 mol % or more of Ca with
respect to the total number of moles in the sulfides and having a
circle equivalent diameter of 1.0 .mu.m or more in observation
regions of a total area of 4.0 mm.sup.2 A2: total area
(.mu.m.sup.2) of sulfide-based inclusions having a circle
equivalent diameter of 1.0 .mu.m or more in observation regions of
a total area of 4.0 mm.sup.2.
6. The part according to claim 5, wherein the chemical composition
includes at least one of, by mass %, Cu: 0.20% or less, Ni: 0.20%
or less, and B: 0.005% or less.
7. The part according to claim 5, wherein the chemical composition
includes at least one of, by mass %, Nb: 0.05% or less and Ti:
0.10% or less.
8. The part according to claim 5 wherein at a center part, an
absorption energy vE20 is 43 J/cm.sup.2 or more.
9. The steel according to claim 2 wherein the chemical composition
includes, by mass %, at least one of Nb: 0.05% or less and Ti:
0.10% or less.
10. The steel according to claim 2, wherein the steel is a steel
rod.
11. The steel according to claim 3, wherein the steel is a steel
rod.
12. The steel according to claim 9, wherein the steel is a steel
rod.
13. The part according to claim 6, wherein the chemical composition
includes at least one of, by mass %, Nb: 0.05% or less and Ti:
0.10% or less.
14. The part according to claim 6 wherein at a center part, an
absorption energy vE20 is 43 J/cm.sup.2 or more.
15. The part according to claim 7 wherein at a center part, an
absorption energy vE20 is 43 J/cm.sup.2 or more.
16. The part according to claim 13 wherein at a center part, an
absorption energy vE20 is 43 J/cm.sup.2 or more.
Description
FIELD
[0001] The present invention relates to steel improved in
hardenability, toughness, surface-originated micropitting life, and
bending fatigue strength and to a part produced using such
steel.
BACKGROUND
[0002] A bearing or other part for machine structure use or a
constant velocity joint, hub unit, or other auto part is repeatedly
subjected to high contact pressure, so an excellent rolling fatigue
characteristic is sought, but in recent years, along with the
improved fuel efficiencies of automobiles and higher output of
engines demanded, there have been much greater calls for making the
above parts lighter in weight, smaller in size, and more able to
handle high stress loads. The usage environment of the above parts
has become harsher.
[0003] To meet with such demands, in materials for bearing parts,
in general, attempts have been made to reduce as much as possible
the amounts of nonmetallic inclusions such as Al.sub.2O.sub.3 which
cause micropitting of rolling parts (below, sometimes referred to
as simply "inclusions") so as to improve the rolling fatigue
life.
[0004] However, recent advances in steelmaking technology have
enabled oxides to be rendered smaller in size. As a result, the
sulfides become relatively larger in size. With measures using only
oxides as a parameter, sometimes the variation in rolling fatigue
life becomes greater. For this reason, recently, attempts have been
made to control the composition and morphology of inclusions so as
to improve the rolling fatigue life.
[0005] For example, PTL 1 discloses steel for carburized bearing
use which has a chemical composition comprising, by mass %, C: 0.1%
to less than 0.4%, Si: 0.02 to 1.3%, Mn: 0.2 to 2.0%, P: 0.05% or
less, S: less than 0.010%, Cr: 0.50 to 2.00%, Al: 0.01 to 0.10%,
Ca: 0.0003 to 0.0030%, O: 0.0030% or less, and N: 0.002 to 0.030%
and a balance of Fe and impurities and which has
0.75.ltoreq.Ca/O.ltoreq.2.0 and Ca/O.ltoreq.1250S-5.8.
[0006] On the other hand, a bearing is subjected to repeated
bending stress, so bending fatigue strength is also sought.
Recently, to raise the bending fatigue strength of bearings, it has
been attempted to inhibit the formation of a grain boundary oxide
layer.
[0007] For example, PTL 2 discloses a steel material for carburized
part or carbonitrided part use which has a chemical composition
comprising, by mass %, C: 0.1 to 0.3%, Si: 0.01 to 0.25%, Mn: 0.2
to 1.5%, S: 0.003 to 0.05%, Cr: 0.5 to 2.0%, Mo: 0.1 to 0.8%, Al:
0.01 to 0.05%, and N: 0.008 to 0.025% and a balance of Fe and
impurities, which has impurities including Ti of 0.005% or less, O
(oxygen) of 0.002% or less, and a total of P and Sn of 0.030% or
less, and which has, in the steel material cross-section, a minimum
value of A=(1+0.681 Si)(I+3.066Mn+0.329Mn.sup.2) (1+2.07Cr
1+3.14Mo) of 13 or more and a maximum length of a group of
inclusions excluding sulfides in a cross-sectional area of 1500
mm.sup.2 of 30 .mu.m or less.
[0008] Further, PTL 3 discloses case hardened steel which has a
chemical composition comprising, by mass %, C: 0.15 to 0.30%, Si:
0.02 to 1.0%, Mn: 0.30 to 1.0%, S: 0.030% or less, Cr: 1.80 to
3.0%, Al: 0.010 to 0.050%, and N: 0.0100 to 0.0250%, having
contents of Si, Mn, Cr, and S having values of fn1 and fn2
expressed by formula (1) Mn/S and formula (2) Cr/(Si+2Mn)
satisfying respectively 30.ltoreq.fn1.ltoreq.150 and
0.75.ltoreq.fn2.ltoreq.1.1, and having a balance of Fe and
impurities and which has impurities including P, Ti, and O (oxygen)
of respectively P: 0.020% or less, Ti: less than 0.005%, and O:
0.0015% or less.
[0009] Furthermore, PTL 4 discloses a method for smelting a steel
material for carburized bearing steel use which has a chemical
composition comprising, by mass %, C: 0.05 to 0.30%, Si: 0.05 to
1.0%, Mn: 0.10 to 2.0%, P: 0.050% or less, S: 0.008% or less, Cr:
0.4 to 2.0%, Al: 0.010 to 0.050%, N: 0.010 to 0.025%, and O:
0.0015% or less and having a balance of Fe and impurities, which
smelting method comprises performing ladle refining treatment in
the order of step 1: flux injection treatment, step 2: slag
refining treatment, and step 3: molten steel reflux treatment so as
to control the sulfide-based inclusions so that the average
composition of S-containing compounds forming the sulfide-based
inclusions becomes CaS: 1.0% or more, MgS: 0 to 20%, and a total of
the three constituents of CaS, MgS, and MnS of 95% or more.
[0010] In addition, PTL 5 discloses a steel material for carburized
bearing steel use containing specific amounts of C, Si, Mn, P, S,
Al, Cr, N, and O, having a balance comprised of Fe and impurities,
having a projected AREA.sub.max of oxides and sulfides in 30,000
mm.sup.2 calculated using extreme value statistical processing when
measuring the maximum oxide size and maximum sulfide size in a 100
mm.sup.2 longitudinal direction vertical cross-section for 30
locations of 50 .mu.m or less and 60 .mu.m or less, having an
average aspect ratio of the maximum oxides and maximum sulfides
measured at the 30 locations of 5.0 or less, having contents in the
average compositions of the maximum oxides at the 30 locations of
CaO: 2.0 to 20%, MgO: 0 to 20%, and SiO.sub.2: 0 to 10% and having
a balance of Al.sub.2O.sub.3, comprised of any of the oxides of
specific two to four element systems, and having contents in the
average compositions of the maximum sulfides at the 30 locations of
single element system sulfides of CaS: 100% or specific two element
system or three element system sulfides of CaS.gtoreq.1.0%, MgS: 0
to 20%, and a balance of MnS.
CITATIONS LIST
Patent Literature
[0011] [PTL 1] Japanese Unexamined Patent Publication No.
2015-129335 [0012] [PTL 2] Japanese Patent No. 4243852 [0013] [PTL
3] Japanese Patent No. 5163242 [0014] [PTL 4] Japanese Unexamined
Patent Publication No. 2014-5520 [0015] [PTL 5] Japanese Unexamined
Patent Publication No. 2013-147689
SUMMARY
Technical Problem
[0016] The steel for carburized bearing use disclosed in PTL 1 may
fall in bending fatigue strength if the grain boundary oxide layer
is formed thick. The steel material for carburized part or
carbonitrided part use disclosed in PTL 2 and the case hardened
steel disclosed in PTL 3 may not be given excellent rolling fatigue
life if there are elongated coarse sulfides present. Therefore, in
the arts disclosed in PTLs 1 to 3, not all of the characteristics
of the hardenability, toughness, surface-originated micropitting
life, and bending fatigue strength can be stably realized.
[0017] Further, in the art disclosed in PTLs 4 and 5, there is the
possibility that not all of the hardenability, toughness,
surface-originated micropitting life, and bending fatigue strength
can be stably realized.
[0018] The present invention was made in consideration of the above
state of the prior art and has as its object the provision of steel
improved in all of the hardenability, toughness, surface-originated
micropitting life, and bending fatigue strength and a part produced
using such steel.
Solution to Problem
[0019] In general, rolling fatigue is the phenomenon where
inclusions present in a steel material are subjected to repeated
load, stress concentration causes cracks to form, and then repeated
load causes the cracks to gradually advance finally leading to
micropitting.
[0020] The present inventors engaged in various studies so as to
solve the above problem. As a result, they obtained the findings of
the following (a) and (b).
[0021] (a) By controlling the composition of sulfides, specifically
by controlling the composition so as, for example, to add Ca into
the molten steel so that the sulfides include (Mn, Ca)S, it is
possible to disperse and reduce in size the coarse sulfides acting
as the source of stress concentration in rolling fatigue.
[0022] (b) By rectifying the balance of the amounts of oxidizing
elements, in particular Cr, Si, and Mn, it is possible to reduce
the thickness of an abnormally carburized layer such as a grain
boundary oxide layer and slack quenched layer and as a result
possible to secure a high bending fatigue strength.
[0023] The present invention was made based on the above findings
(a) and (b) and has as its gist the following:
[0024] [1] Steel which has a chemical composition comprising, by
mass %,
[0025] C: 0.10 to 0.30%,
[0026] Si: 0.01 to 0.25%,
[0027] Mn: 0.20 to 1.50%,
[0028] P: 0.001 to 0.015%,
[0029] S: 0.001 to 0.010%,
[0030] Cr: 0.50 to 2.00%,
[0031] Mo: 0.10 to 0.50%,
[0032] Al: 0.005 to 0.100%,
[0033] Ca: 0.0002 to 0.0010%,
[0034] N: 0.005 to 0.025%,
[0035] O: 0.0015% or less,
[0036] Cu: 0 to 0.20%,
[0037] Ni: 0 to 0.20%,
[0038] B: 0 to 0.005%,
[0039] Nb: 0 to 0.05%.
[0040] Ti: 0 to 0.10%, and
[0041] a balance of Fe and impurities, wherein
[0042] Fn1 defined by the following formula (1) is 0.20 to 0.65,
and
[0043] Fn2 defined by the following formula (2) is 0.50 to
1.00:
Fn1=4.2.times.[Cr]/(7.0.times.[Si]+16.0.times.[Mn]) (1)
[0044] [Element]: mass % of element
Fn2=A1/A2 (2)
[0045] A1: total area (.mu.m.sup.2) of sulfide-based inclusions
containing 1.0 mol % or more of Ca with respect to the total number
of moles in the sulfides and having a circle equivalent diameter of
1.0 .mu.m or more in observation regions of a total area of 4.0
mm.sup.2
[0046] A2: total area (.mu.m.sup.2) of sulfide-based inclusions
having a circle equivalent diameter of 1.0 .mu.m or more in
observation regions of a total area of 4.0 mm.sup.2
[0047] [2] The steel according to [1] wherein the chemical
composition includes, by mass %, at least one of Cu: 0.20% or less,
Ni: 0.20% or less, and B: 0.005% or less.
[0048] [3] The steel according to [1] or [2] wherein the chemical
composition includes, by mass %, at least one of Nb: 0.05% or less
and Ti: 0.10% or less.
[0049] [4] The steel according to any one of [1] to [3], wherein
the steel is a steel rod.
[0050] [5] A part which is excellent in surface-originated
micropitting life and bending fatigue strength and which, at a
region of a depth of 500 .mu.m or more from the surface, has a
chemical composition comprising, by mass %,
[0051] C: 0.10 to 0.30%,
[0052] Si: 0.01 to 0.25%,
[0053] Mn: 0.20 to 1.50%,
[0054] P: 0.001 to 0.015%,
[0055] S: 0.001 to 0.010%,
[0056] Cr: 0.50 to 2.00%,
[0057] Mo: 0.10 to 0.50%,
[0058] Al: 0.005 to 0.100%,
[0059] Ca: 0.0002 to 0.0010%,
[0060] N: 0.005 to 0.025%,
[0061] O: 0.0015%,
[0062] Cu: 0 to 0.20%,
[0063] Ni: 0 to 0.20%,
[0064] B: 0 to 0.005%,
[0065] Nb: 0 to 0.05%,
[0066] Ti: 0 to 0.10%, and
[0067] a balance of Fe and impurities, wherein
[0068] Fn1 defined by the following formula (1) is 0.20 to 0.65,
and
[0069] Fn2 defined by the following formula (2) is 0.50 to
1.00,
Fn1=4.2.times.[Cr]/(7.0.times.[Si]+16.0.times.[Mn]) (1)
[0070] [Element]: mass % of element
Fn2=A1/A2 (2)
[0071] A1: total area (.mu.m.sup.2) of sulfide-based inclusions
containing 1.0 mol % or more of Ca with respect to the total number
of moles in the sulfides and having a circle equivalent diameter of
1.0 .mu.m or more in observation regions of a total area of 4.0
mm.sup.2
[0072] A2: total area (.mu.m.sup.2) of sulfide-based inclusions
having a circle equivalent diameter of 1.0 .mu.m or more in
observation regions of a total area of 4.0 mm.sup.2
[0073] [6] The part according to [5] wherein the chemical
composition includes, by mass %, at least one of Cu: 0.20% or less,
Ni: 0.20% or less, and B: 0.005% or less.
[0074] [7] The part according to [5] or [6] wherein the chemical
composition includes, by mass %, at least one of Nb: 0.05% or less
and Ti: 0.10% or less.
[0075] [8] The part according to any one of [5] to [7] wherein at a
center part, an absorption energy vE20 is 43 J/cm.sup.2 or
more.
Advantageous Effects of Invention
[0076] In the steel according to the present invention, a
predetermined chemical composition is realized, a balance of Cr,
Si, and Mn is made suitable, and in sulfide-based inclusions with a
circle equivalent diameter of a predetermined value, a ratio of the
sulfide-based inclusions with a ratio of Ca moles in the sulfides
of a predetermined value is made suitable. For this reason, in the
steel according to the present invention, all of the hardenability,
toughness, surface-originated micropitting life, and bending
fatigue strength can be improved.
BRIEF DESCRIPTION OF DRAWINGS
[0077] FIG. 1 is a view schematically showing one example of a
distribution of brightness of an SEM image in an observation
region.
[0078] FIG. 2 is a view schematically showing one example of an SEM
image in an observation region.
[0079] FIG. 3 is a view showing a relationship between a
temperature and time of quenching and tempering.
DESCRIPTION OF EMBODIMENTS
[0080] Below, the findings of the inventors leading up to the
present invention and an embodiment of the steel according to the
present invention, method of producing the same, and method of
producing a part (present embodiment) will be explained in detail.
Note that, below, the "%/" of the contents of the elements mean
"mass %".
Findings of Present Inventors
[0081] The present inventors engaged in intensive studies for
providing steel improved in all of hardenability, toughness,
surface-originated micropitting life, and bending fatigue strength
and a part produced using such steel. That is, the present
inventors investigated and studied the effects of the chemical
composition of steel, in particular the effects of Si, Mn, Cr, and
Ca, on the surface-originated micropitting life and bending fatigue
strength of a carburized part after a carburization process. As a
result, the present inventors obtained the following findings
regarding the bending fatigue strength, surface-originated
micropitting life, hardenability, and toughness.
[0082] (a) Regarding Bending Fatigue Strength
[0083] In steel for carburized bearing use, to secure a high
bending fatigue strength, it is necessary to reduce the thicknesses
of abnormally carburized layers such as the grain boundary oxide
layer and slack quenched layer, but by suitably setting the balance
of amounts of the oxidizing elements, in particular Si, Mn, and Cr
among them, it is possible to reduce the thicknesses of abnormally
carburized layers such as the grain boundary oxide layer and slack
quenched layer.
[0084] Specifically, if Fn1 defined by the following formula (1) is
0.20 to 0.65, it is possible to reduce the thicknesses of the grain
boundary oxide layer and slack quenched layer:
Fn1=4.2.times.[Cr]/(7.0.times.[Si]+16.0.times.[Mn]) (1)
[0085] [Element]: mass % of elements
[0086] Fn1: 0.20 to 0.65
[0087] If Fn1 is less than 0.20, the abnormally carburized layers
become thicker and it becomes difficult to secure a high bending
fatigue strength, so Fn1 is made 0.20 or more. Preferably, it is
made 0.25, more preferably 0.30 or more. On the other hand, if Fn1
exceeds 0.65, similarly the abnormally carburized layers become
thicker and it becomes difficult to secure a high bending fatigue
strength, so Fn1 is made 0.65 or less. Preferably it is made 0.60,
more preferably 0.55 or less.
[0088] (b) Regarding Surface-Originated Micropitting Life
[0089] Sulfide-based inclusions usually easily deform at a high
temperature, so easily deform and elongate at the time of hot
working. The elongated sulfide-based inclusions become starting
points of fatigue in the usage environment of carburized bearing
parts whereby the surface-originated micropitting life becomes
shorter. For this reason, to extend the surface-originated
micropitting life, it is effective to raise the deformation
resistance of the sulfide-based inclusions at a high
temperature.
[0090] That is, if raising the deformation resistance of the
sulfide-based inclusions at a high temperature, the sulfide-based
inclusions become harder to elongate at the time of hot working and
maintain their spherical shapes, so it becomes harder for
sulfide-based inclusions to become starting points of fatigue.
[0091] Compared with sulfides not containing Ca, sulfides
containing Ca are larger in deformation resistance. For this
reason, if making Ca form a solid solution in sulfide-based
inclusions, that is, if replacing the Mn in MnS with Ca, the result
is that the deformation resistance at a high temperature becomes
higher. The sulfide with the Mn in MnS replaced by Ca is made
(Mn,Ca)S. Specifically, by making the oxygen concentration strongly
drop and in that state performing secondary refining to make the
sulfide inclusions become mainly (Mn,Ca)S, it is possible to
include 1.0 mol % or more of Ca with respect to the total number of
moles in the sulfides.
[0092] In this way, sulfide-based inclusions containing Ca in solid
solution can be maintained spherical in shape even after hot
working, so the aspect ratio (long axis/short axis of sulfide-based
inclusions) is small. Specifically, sulfide-based inclusions
containing 1.0 mol % or more of Ca with respect to the total number
of moles in the sulfides are smaller in aspect ratio after hot
working compared with sulfide-based inclusions only containing less
than 1.0 mol % of Ca with respect to the total number of moles in
the sulfides. Ninety percent has an aspect ratio of 3 or less. Note
that, as a result of experiments, it is learned that the upper
limit value of Ca with respect to the total number of moles in the
sulfides is 50 mol %.
[0093] The present inventors discovered, based on the above
findings, that if Fn2 defined by the following formula (2) in the
steel for carburized bearing use is 0.50 to 1.00, the sulfide-based
inclusions in steel for carburized bearing use become higher in
deformation resistance at the time of hot working the sulfide-based
inclusions and the surface-originated micropitting life of the
carburized bearing part is extended.
Fn2=A1/A2 (2)
[0094] A1: total area (.mu.m.sup.2) of sulfide-based inclusions
containing 1.0 mol % or more of Ca with respect to the total number
of moles in the sulfides and having a circle equivalent diameter of
1.0 .mu.m or more in observation regions of a total area of 4.0
mm.sup.2
[0095] A2: total area (.mu.m.sup.2) of sulfide-based inclusions
having a circle equivalent diameter of 1.0 .mu.m or more in
observation regions of a total area of 4.0 mm.sup.2
Fn2(=A1/A2):0.50 to 1.00
[0096] Fn2 is a parameter relating to the aspect ratio of the
sulfide-based inclusions in the hot worked steel for carburized
bearing use. If Fn2 is 0.50 or less, at the time of hot working,
the sulfide-based inclusions are elongated and the aspect ratio of
the sulfide-based inclusions after hot working becomes greater.
[0097] If the aspect ratio of the sulfide-based inclusions after
hot working becomes greater, in the environment of use of a
carburized bearing part after carburization, the sulfide-based
inclusions become starting points of fatigue and the
surface-originated micropitting life becomes shorter, so Fn2 is
made 0.50 or more. Preferably it is made 0.55 or more, more
preferably 0.60 or more. The upper limit of Fn2 is, from its
definition, 1.00.
[0098] (c) Regarding Hardenability and Toughness
[0099] In the past, in steel for carburized bearing use, it was
difficult to improve the bending fatigue strength or the
surface-originated micropitting life while maintaining the
hardenability or toughness. If improving the bending fatigue
strength or the surface-originated micropitting life, there was the
problem that the hardenability or toughness fell.
[0100] The present inventors discovered that the steel according to
the present embodiment satisfying a predetermined chemical
composition and formula (1) and formula (2) improved the bending
fatigue strength or the surface-originated micropitting life while
also being excellent in hardenability and toughness.
[0101] Being excellent in hardenability means a hardness of HRC
after hardening at 500 .mu.m or less from the surface of the part
being 22 or more.
[0102] Being excellent in toughness means an absorption energy vE20
at the center part of 43 J/cm.sup.2 or more.
[0103] Steel
[0104] Chemical composition
[0105] Essential elements
[0106] C: 0.10 to 0.30%
[0107] C is an element raising the hardenability of steel and
raising the strength and toughness of the core part of the steel
material after hardening. Further, C is an element acting to extend
the surface-originated micropitting life of the carburized bearing
part after carburization.
[0108] If C is less than 0.10%, the effect of addition is not
sufficiently obtained, so C is made 0.10% or more. Preferably it is
made 0.13% or more, more preferably 0.15% or more. On the other
hand, if C exceeds 0.30%, the toughness falls, so C is made 0.30%
or less. Preferably it is made 0.29% or less, more preferably 0.28%
or less, still more preferably 0.25% or less.
[0109] Si: 0.01 to 0.25%
[0110] Si is an element functioning as a deoxidant and also
contributing to improvement of the hardenability. Further, Si is an
element acting to raise the temper softening resistance and keep
the steel from softening at a high temperature. However, Si is an
oxidizing element. If the amount increases, it is selectively
oxidized by the trace amounts of H.sub.2O and/or CO.sub.2 in the
carburizing gas, whereby the abnormally carburized layers of the
grain boundary oxide layer and slack quenched layer become thicker
and the bending fatigue strength falls.
[0111] If Si is less than 0.01%, the effect of addition is not
sufficiently obtained, so Si is made 0.01% or more. Preferably, it
is made 0.03% or more, more preferably 0.06% or more. On the other
hand, if Si exceeds 0.25%, the abnormally carburized layers of the
grain boundary oxide layer and slack quenched layer become thicker
and the bending fatigue strength fall, so Si is made 0.25% or less.
Preferably it is made 0.20% or less, more preferably 0.15% or
less.
[0112] Mn: 0.20 to 1.50%
[0113] Mn is an element which functions as a deoxidant and also
contributes to the improvement of the hardenability. Mn, like Si,
is an oxidizing element. If the amount increases, it is selectively
oxidized by the trace amounts of H.sub.2O and/or CO.sub.2 in the
carburizing gas, whereby the abnormally carburized layers of the
grain boundary oxide layer and slack quenched layer become thicker
and the bending fatigue strength falls.
[0114] If Mn is less than 0.20%, the effect of addition is not
sufficiently obtained, so Mn is made 0.20% or more. Preferably it
is made 0.30% or more, more preferably 0.40% or more. On the other
hand, if Mn exceeds 1.50%, the hardness rises and the
machineability remarkably falls and also the abnormally carburized
layers become thicker and the bending fatigue strength remarkably
falls, so Mn is made 1.50% or less. Preferably it is made 1.48% or
less, more preferably 1.30% or less, still more preferably 1.10% or
less.
[0115] P: 0.001 to 0.015%
[0116] P is an impurity element. It is an element which segregates
at the crystal grain boundaries and impairs the toughness of the
steel and the surface-originated micropitting life of a carburized
bearing part.
[0117] If P exceeds 0.015%, the toughness of the steel and the
surface-originated micropitting life of the carburized bearing part
remarkably fall, so P is made 0.015% or less. Preferably, it is
0.013% or less, more preferably 0.010% or less. A smaller amount of
P is preferable, but if lowering it to less than 0.001%, the
manufacturing costs rise, so P is made 0.001% or more. Preferably,
it is 0.003% or more.
[0118] S: 0.001 to 0.010%
[0119] S is an impurity element. It is an element which forms
sulfides and impairs the toughness and the cold forgeability of the
steel and which impairs the surface-originated micropitting life of
the carburized bearing part.
[0120] If S exceeds 0.010%, the toughness of the steel and the cold
forgeability remarkably fall and the surface-originated
micropitting life of the carburized bearing part remarkably falls,
so S is made 0.010% or less. Preferably it is made 0.008% or less,
more preferably 0.005% or less. S is preferably low, but if
lowering it to less than 0.001%, the manufacturing costs rise, so S
is made 0.001% or more. Preferably, it is 0.002% or more, more
preferably 0.003 or more, still more preferably 0.005% or more.
[0121] Cr: 0.50 to 2.00%
[0122] Cr is an element which acts to raise the hardenability and
also raise the temper softening resistance and keep the steel from
softening at a high temperature. However, Cr, like Si and Mn, is an
oxidizing element. If the amount increases, it is selectively
oxidized by the trace amounts of H.sub.2O and/or CO.sub.2 in the
carburizing gas, the abnormally carburized layers of the grain
boundary oxide layer and slack quenched layer become thicker, and
the bending fatigue strength falls.
[0123] If Cr is less than 0.50%, the effect of addition is not
sufficiently obtained, so Cr is made 0.50% or more. Preferably it
is made 0.70% or more, more preferably 0.90% or more. On the other
hand, if Cr exceeds 2.00%, the hardness rises and the machinability
remarkably falls and, also, the abnormally carburized layers become
thicker and the bending fatigue strength remarkably falls, so Cr is
made 2.00% or less. Preferably it is made 1.98% or less, more
preferably 1.80% or less, still more preferably 1.60% or less.
[0124] Mo: 0.10 to 0.50%
[0125] Mo is an element which raises the hardenability, improves
the surface hardness, hardened layer depth, and core hardness after
carburized hardening, and contributes to securing the strength of
the carburized part. Further, Mo is a nonoxidizing element, so is
an element acting to increase the strength and toughness of the
steel surface and raise the bending fatigue strength without
increase the thickness of the grain boundary oxide layer at the
time of carburization.
[0126] If Mo is less than 0.10%, the effect of addition is not
sufficiently obtained, so Mo is made 0.10% or more. Preferably it
is made 0.20% or more, more preferably 0.30% or more. On the other
hand, if Mo exceeds 0.50%, the hardness rises and the
machineability remarkably falls. Furthermore, the
surface-originated micropitting life of the carburized bearing part
falls. Further, the manufacturing cost also rises, so Mo is made
0.50% or less. Preferably, it is made 0.48% or less, more
preferably 0.45% or less.
[0127] Al: 0.005 to 0.100%
[0128] Al is an element acting to deoxidize steel. If A1 is less
than 0.005%, the effect of addition cannot be sufficiently
obtained, so A1 is made 0.005% or more. Preferably it is 0.010% or
more, more preferably 0.015% or more. On the other hand, if A1
exceeds 0.100%, coarse oxides are formed and the surface-originated
micropitting life of the carburized bearing part becomes shorter,
so A1 is made 0.100% or less. Preferably, it is made 0.070% or
less, more preferably 0.050% or less.
[0129] Ca: 0.0002 to 0.0010%
[0130] Ca is an element which forms a solid solution in
sulfide-based inclusions to act to make the sulfide-based
inclusions spheroidal. Further, Ca is an element which raises the
deformation resistance of sulfide-based inclusions at a high
temperature, inhibits elongation of sulfide-based inclusions to
maintain the spherical form at the time of hot working, and extends
the surface-originated micropitting life of the carburized bearing
part.
[0131] If Ca is less than 0.0002%, the effect of addition is not
sufficiently obtained, so Ca is made 0.0002% or more. Preferably it
is made 0.0003% or more, more preferably 0.0004% or more. On the
other hand, if Ca exceeds 0.0010%, coarse oxides are formed and the
surface-originated micropitting life of the carburized bearing part
becomes shorter, so Ca is made 0.0010% or less. Preferably it is
made 0.0009% or less, more preferably 0.0008% or less.
[0132] N: 0.005 to 0.025%
[0133] N is an element bonding with A1, Nb, and/or Ti to form AlN,
NbN, and/or TiN effective for refining the crystal grains and
contributing to improvement of the bending fatigue strength.
[0134] If N is less than 0.005%, the effect of addition is not
sufficiently obtained, so N is made 0.005% or more. Preferably it
is made 0.010% or more, more preferably 0.012% or more. On the
other hand, if N exceeds 0.025%, coarse nitrides are formed and the
toughness and bending fatigue strength fall, so N is made 0.025% or
less. Preferably it is made 0.022% or less, more preferably 0.020%
or less.
[0135] O (oxygen): 0.0015% or Less
[0136] O (oxygen) is an element which forms oxides and impairs
strength and which impairs the bending fatigue strength and the
surface-originated micropitting life of a carburized bearing
part.
[0137] If O (oxygen) exceeds 0.0015%, the strength and the bending
fatigue strength and the surface-originated micropitting life of
the carburized bearing part fall, so O (oxygen) is made 0.0015% or
less. Preferably, it is made 0.0013% or less, more preferably
0.0010% or less. Less O (oxygen) is preferable, but if decreasing O
(oxygen) to 0.0001% or less, the manufacturing costs greatly rise,
so for practical steel, 0.0001% is the substantive lower limit.
[0138] Optional Elements
[0139] In the present embodiment, the chemical composition of the
steel may contain, in addition to the above elements, to improve
the characteristics of the steel, furthermore, by mass %, at least
one type of element among the (a) group elements of Cu: 0.20% or
less, Ni: 0.20% or less, and B: 0.005% or less and at least one
type of element among the (b) group elements of Nb: 0.05% or less,
and Ti: 0.10%.
[0140] (a) Group Elements
[0141] Cu: 0.20% or Less
[0142] Cu is an element acting to raise the hardenability. If Cu
exceeds 0.20%, the hot workability falls and the steel cost rises,
so Cu is preferably made 0.20% or less. More preferably it is made
0.16% or less. From the viewpoint of reliably obtaining the effect
of addition of Cu, Cu is preferably made 0.05% or more. More
preferably it is made 0.10% or more.
[0143] Ni: 0.20% or Less
[0144] Ni is an element which improves the hardenability and also
contributes to the improvement of the toughness. Further, Ni is a
nonoxidizing element. It is an element which acts to strengthen and
toughen the steel surface without causing the grain boundary oxide
layer to increase in thickness at the time of carburization.
[0145] If Ni exceeds 0.20%, the effect of addition becomes
saturated and, further, the steel cost rises, so Ni is preferably
made 0.20% or less. More preferably it is made 0.16% or less. From
the viewpoint of reliably obtaining the effect of addition of Ni,
Ni is preferably made 0.05% or more. More preferably it is made
0.10% or more.
[0146] B: 0.005% or Less
[0147] B is an element which acts to raise the hardenability and
which also acts to keep P and S from segregating at the austenite
grain boundaries at the time of quenching. If B exceeds 0.005%, BN
forms and the toughness of the steel falls, so B is preferably made
0.005% or less. More preferably it is made 0.003% or less. From the
viewpoint of reliably obtaining the effect of addition of B, B is
preferably made 0.0003% or more. More preferably it is made 0.0005%
or more.
[0148] (b) Group Elements
[0149] Nb: 0.05% or Less
[0150] Nb is an element bonding with C and/or N to form fine
carbides, nitrides, and/or carbonitrides to refine the crystal
grains and contributing to improvement of the bending fatigue
strength.
[0151] If Nb exceeds 0.05%, the hot ductility remarkably falls and
defects easily form at the steel surface at the time of hot rolling
or hot forging and also the toughness of the steel falls, so Nb is
preferably made 0.05% or less. More preferably it is made 0.02% or
less. From the viewpoint of reliably obtaining the effect of
addition of Nb, Nb is preferably made 0.005% or more. More
preferably it is made 0.008% or more.
[0152] Ti: 0.10% or Less
[0153] Ti is an element which forms fine carbides etc. to refine
the crystal grains and contributes to improvement of the strength
of steel. If Ti exceeds 0.10%, the toughness of the steel and
bending fatigue strength fall, so Ti is preferably made 0.10% or
less. More preferably, it is made 0.08% or less. From the viewpoint
of reliably obtaining the effect of addition of Ti, Ti is
preferably made 0.005% or more. More preferably it is made 0.010%
or more.
[0154] Balance
[0155] In the chemical composition of the steel according to the
present embodiment, the balance is Fe and impurities. Here,
"impurities" are elements unavoidably being mixed in from the steel
raw materials (ore, scrap, etc.) and/or in the steelmaking process
and elements allowed in a range not obstructing the characteristics
of the steel according to the present embodiment. Specifically, Sb,
Sn, W, Co, As, Mg, Pb, Bi, and H may be mentioned. Note that, Sb,
Sn, W, Co, As, Mg, Pb, Bi, and H respectively may be allowed to be
included respectively in 0.010%, 0.10%, 0.50%, 0.50%, 0.005%,
0.005%, 0.10%, 0.10%, and 0.0010% in realizing the effects of the
present application.
[0156] Next, the Fn1 defined by the following formula (1) for the
chemical composition of the steel according to the present
embodiment and the Fn2 defined by the following formula (2) for the
sulfide-based inclusions of the steel according to the present
embodiment will be explained in detail.
[0157] Note that, in this Description, the sulfide-based inclusions
may include MnS, (Mn,Ca)S, CaS, and FeS. The amount of FeS present
is very small. In calculation, FeS is considered.
[0158] Fn1: 0.20 to 0.65
[0159] In the chemical composition of the steel according to the
present embodiment, Fn1 defined by the following formula (1) is
made 0.20 to 0.65.
Fn1=4.2.times.[Cr]/(7.0.times.[Si]+16.0.times.[Mn]) (1)
[0160] Note that, in the parentheses in formula (1), the mass % of
the element is introduced.
[0161] Fn1 is a parameter relating to the thickness of the
abnormally carburized layers. If Fn1 is less than 0.20 (amount of
Si is excessively large), the grain boundary oxide layer etc.
becomes thicker. Further, if Fn1 exceeds 0.65 (amount of Cr is
excessively large), the trace amounts of H.sub.2O and/or CO.sub.2
in the carburizing gas cause the Cr to be selectively oxidized. For
this reason, in each of these cases, the thicknesses of the
abnormally carburized layers increase and the bending fatigue
strength falls, so Fn1 is 0.20 or more and Fn1 is 0.65 or less. Fn1
is preferably 0.25 or more, more preferably 0.3 or more. Fn1 is
preferably 0.60 or less, more preferably 0.55 or less.
[0162] Fn2: 0.50 to 1.00
[0163] Regarding the sulfide-based inclusions of the steel of the
present invention, Fn2 defined by the following formula (2) is made
0.50 to 1.00:
Fn2=A1/A2 (2)
[0164] A1: total area (.mu.m.sup.2) of sulfide-based inclusions
containing 1.0 mol % or more of Ca with respect to the total number
of moles in the sulfides and having a circle equivalent diameter of
1.0 .mu.m or more in observation regions of a total area of 4.0
mm.sup.2
[0165] A2: total area (.mu.m.sup.2) of sulfide-based inclusions
having a circle equivalent diameter of 1.0 .mu.m or more in
observation regions of a total area of 4.0 mm.sup.2
[0166] Fn2 is a parameter relating to the aspect ratio of the
sulfide-based inclusions after hot working. If Fn2 is less than
0.50, the ratio of the sulfide-based inclusions with large aspect
ratios becomes greater.
[0167] Sulfide-based inclusions with large aspect ratios become
starting points for fatigue in the environment of use of carburized
bearing parts after carburizing and impair the surface-originated
micropitting life, so to decrease the ratio of the sulfide-based
inclusions with large aspect ratios, Fn2 is 0.5 or more. Fn2 is
preferably 0.55 or more, more preferably 0.60 or more. Fn2, from
its definition, is 1.00 or less.
[0168] Fn2 is found by the following method. A region of 1/10d-
7/16d from the surface of the cross-section parallel to the rolling
direction including the diameter of the rod shaped or wire shaped
steel was made the region for observation. Here, the diameter of
the steel is shown by "d".
[0169] The cross-section parallel to the rolling direction, the
region for observation, is polished by diamond to a mirror finish
to obtain the surface for examination. The sulfide-based inclusions
of the surface for examination are identified by a SEM (scanning
electron microscope). Specifically, 100 locations of any
observation regions in the surface for examination are selected by
a magnification of 500.times.. That is, an observation region means
any region of the surface for examination obtained by polishing the
regions for observation to a mirror finish and observed by a
magnification of 500.times.. The total area of the observation
regions is made at least 4.0 mm.sup.2. The total area of the
observation regions may also exceed 4.0 mm.sup.2. Note that, it is
enough that the surface for examination be prepared so that the
total area of the observation regions is at least 4.0 mm.sup.2. The
size of the surface for examination itself is not particularly
specified.
[0170] In each observation region, the sulfide-based inclusions are
identified based on the contrast of the backscattered electron
images observed by SEM. In the backscattered electron images, the
observation regions are shown by grayscale images. The contrasts of
the Fe base material, sulfide-based inclusions, and oxide-based
inclusions in the backscattered electron images respectively
differ.
[0171] The numerical range of the brightness showing sulfide-based
inclusions (several levels) is determined in advance by SEM and EDS
(energy dispersive type X-ray microanalyzer). Below, the numerical
range determined in advance as brightness showing sulfide-based
inclusions will be referred to as the "reference range". In an
observation region, the region with a brightness within the
reference range is determined. Below, a region with a brightness
within the reference range will be referred to as a "sulfide
region".
[0172] FIG. 1 schematically shows one example of the brightness
distribution of an SEM image in an observation region. In FIG. 1,
the ordinate shows the area ratio (%) in the observation region
while the abscissa shows the brightness. In FIG. 1, the region R1
shows the region of oxide-based inclusions, the region R2 shows the
region of sulfide-based inclusions, and the region R3 shows the
region of the Fe base material.
[0173] B1 to B2 in FIG. 1 are defined as reference ranges of
brightness. Regions of the reference ranges B1 to B2 are selected
from the observation region. FIG. 2 schematically shows one example
of an SEM image in an observation region. In FIG. 2, the sulfide
regions X1 to X4 are regions having brightnesses of the reference
ranges B1 to B2. The regions correspond to regions of sulfide-based
inclusions.
[0174] In FIG. 2, the regions Z1 to Z3 in the inclusions Y1 to Y3
correspond to regions of oxide-based inclusions. That is, the
inclusions Y1 to Y3 are composite inclusions comprised of
sulfide-based inclusions and oxide-based inclusions.
[0175] Next, the circle equivalent diameters of the identified
sulfide regions X1 to X4 are calculated. A "circle equivalent
diameter" is the diameter of the circle in the case of converting
the area of a sulfide region into a circle having the same area.
When calculating the circle equivalent diameters of the sulfide
regions X1 to X4, the calculations are performed excluding the
areas of the oxide-based inclusions (regions of Z1 to Z3 in FIG. 2)
present in the respective sulfide regions. In the 100 locations of
observation regions (total area of 4.0 mm.sup.2), the total area
(.mu.m.sup.2) of the sulfide regions with calculated circle
equivalent diameters of 1.0 .mu.m or more is defined as A2.
[0176] Next, the total area A1 of the sulfide-based inclusions
containing 1.0 mol % or more of Ca with respect to the total number
of moles in the sulfides and having a circle equivalent diameter of
1.0 .mu.m or more is found by the following method. In the above
100 locations of observation regions (total area 4.0 mm.sup.2), the
regions of sulfides with circle equivalent diameters of 1.0 .mu.m
or more are quantitatively analyzed by EDS. In the quantitatively
analyzed sulfide regions, the regions of sulfide-based inclusions
containing 1.0 mol % or more of Ca with respect to the total number
of moles in the sulfides are identified.
[0177] When quantitatively analyzing the Ca in sulfide-based
inclusions by EDS, the semiquantitative analysis method is used. In
the observation regions, not only are individual sulfide-based
inclusions present, but also, as explained above, composite
inclusions containing sulfide-based inclusions and oxide-based
inclusions are present.
[0178] Assume the case where the sulfide regions identified by the
SEM image are sulfide-based inclusions of composite inclusions. In
this case, even if firing electrons at the EDS apparatus aiming at
the sulfide-based inclusions, sometimes not only the sulfide-based
inclusions, but also the oxide-based inclusions adjoining the
sulfide-based inclusions are hit by the incident electron beam.
[0179] In such a case, the results of analysis include analysis
values of not only the sulfide-based inclusions, but also the
oxide-based inclusions. The oxide-based inclusions may be Ca
oxides. To avoid this problem, the semiquantitative measurement
method is employed. The semiquantitative measurement method is as
follows: The contents shown below are mol %.
[0180] Compare the S content and Mn content in sulfide-based
inclusions measured by EDS quantitative analysis. With EDS
quantitative analysis, for each inclusion, the measurement is
conducted in a region where the inclusion as a whole fits. A 5 kV
voltage and 20 nm beam diameter were used for analysis by a pitch
of 100 nm.
[0181] (i) Case where S Content is Mn Content or Less
[0182] S is stronger in bonding force with Mn compared with Ca, so
the S in the analyzed sulfide regions is formed as MnS. Ca is not
included. That is, (Ca,Mn)S is not present. The area of the sulfide
regions analyzed is not included in the A1 of the formula (2).
[0183] The Mn of the differential value obtained by subtracting
from the Mn content the S content (below, [Mn]*) is calculated
assuming inclusion in the oxide-based inclusions.
[Mn]*=Mn content-S content formula (A)
[0184] (ii) Case where S Content Exceeds Mn Content
[0185] When the Ca content is greater than the [S]* amount of the
following formula (B), the Ca corresponding to [S]* is calculated
as included in the sulfide regions as Ca as (Ca,Mn)S. The [Ca]*
amount of the following formula (C) forms oxides as CaO.
Accordingly, [Ca]* is excluded from the number of moles of the
analyzed sulfur regions.
[0186] When the Ca content is less than the [S]* amount of the
following formula (B), the S of the [S]* amount bonds with the Fe
whereby FeS is formed. In this case, the Ca content is included in
the sulfide regions as (Ca,Mn)S.
[S]*=S content-Mn content formula (B)
[Ca]*=Ca content-[S]* formula (C)
[0187] The above semiquantitative measurement method is used to
identify the Ca content in sulfide regions with a circle equivalent
diameter of 1.0 .mu.m or more. Further, the total area
(.mu.m.sup.2) of the sulfide regions containing 1.0 mol % or more
of Ca with respect to the total number of moles in the sulfides and
having a circle equivalent diameter of 1.0 .mu.m or more is found.
The found total area is defined as A1. When calculating A1 as well,
the area of the oxide-based inclusions present in the sulfide
regions (regions of Z1 to Z3 in FIG. 2) is excluded from the
calculation.
[0188] The total area A1 and the total area A1 calculated by the
above method are used to find Fn2.
[0189] The steel of the present invention is steel for carburized
bearing use. Normally, steel rod or wire rod is used as the steel
for carburized bearing use. The diameter of steel rods which are
generally in circulation is 16 mm to 200 mm, while the diameter of
wire rods is 4 mm to 20 mm. The steel of the embodiment of the
present invention may be defined as steel rods with a diameter of
16 mm to 200 mm or wire rods with a diameter of 4 mm to 20 mm.
[0190] Method of Production of Steel
[0191] Next, one example of the method of production for producing
the steel of the present invention will be explained.
[0192] Molten steel having the above chemical composition and
satisfying the above formula (1) is continuous cast to form a cast
slab. Ca is added by wire to the molten steel after addition of A1
and before insertion into the tundish. By adding Ca after adding
A1, coarse Ca oxides become harder to form. By addition by wire to
the molten steel before insertion into the tundish, it is possible
to decrease the amount of coarse (Mn, Ca)S precipitating in the
molten steel. Due to the presence of Ca forming a solid solution by
supersaturation, fine (Mn,Ca)S more easily precipitates at the time
of solidification and it becomes possible to satisfy the above
formula (2). Note that, fine CaO and CaS may also be formed before
fine (Mn,Ca)S. The molten steel may also be rendered into ingots by
the ingot casting method.
[0193] The cast slab or ingot is hot worked to produce a steel
slab. For example, blooming is used to form the cast slab or ingot
into a steel slab. The steel slab is hot worked to produce a steel
rod or wire rod or other steel material for carburized bearing use.
The hot working may be hot rolling or may be hot forging. The steel
material for carburized bearing use which is produced may, as
needed, be normalized or spherodizing annealed. Due to the above
process, steel for carburized bearing use is produced.
[0194] Method of Production of Part
[0195] One example of the method of production of a part (for
example, carburized bearing) using the steel of the present
embodiment is as follows. That is, first, the steel according to
the present embodiment is worked into a predetermined shape to
produce an intermediate part. The method of working it is for
example machining such as cutting.
[0196] Next, the intermediate part is carburized. The carburizing
process may be performed under known conditions. The quenching
conditions and tempering conditions in the carburizing treatment
are suitably adjusted by known methods to suitably adjust the
surface hardness of the part, surface concentration of C, etc.
[0197] Due to the above processes, it is possible to produce a
(carburized bearing) part. The part produced by known carburization
using the steel according to the present embodiment is excellent in
hardenability, toughness, surface-originated micropitting life, and
bending fatigue strength.
[0198] The part obtained by the method of production of the part
according to the present embodiment has a thickness of its
carburized layer of 0.5 to 2.0 mm. By the thickness of the
carburized layer being 0.5 mm or more, the surface-originated
micropitting life can be improved. On the other hand, to render it
2.0 mm or more, the carburizing time would end up becoming longer
and the cost would rise. The thickness of the carburized layer is
preferably 0.5 to 2.0 mm.
[0199] The thus obtained part has an absorption energy vE20 at its
center part of 43 J/cm.sup.2 or more and has an excellent
toughness.
[0200] The shape of the part differs depending on the type of the
part, so it is difficult to uniformly define the center part from
the shape of the part. Therefore, the center part is defined with
respect to the material before shaping to the part and after
carburization. The "center part" means, in a material before
shaping into a part and after carburization, a range of T to 3/5T
from the surface in the cross-section parallel to the rolling
direction. Here, "T" means the thickness of the material. Note
that, the above center part can be found when analyzing a part.
EXAMPLES
[0201] Next, examples of the present invention will be explained,
but the conditions in the examples just show single illustrations
of conditions employed for confirming the workability and effects
of the present invention. The present invention is not limited to
these single illustrations. The present invention can employ
various conditions so long as not departing from the gist of the
present invention and achieving the object of the present
invention.
Example 1
[0202] Preparation of Steel Rods
[0203] Molten steels having the chemical compositions shown in
Table 1 were produced in a 300 kg vacuum melting furnace and cast
into ingots. The ingots were heated at 1150.degree. C. for 30
minutes, then were hot forged to give finishing temperatures of
950.degree. C. or more to thereby produce diameter 60 mm steel
rods.
TABLE-US-00001 TABLE 1 Chemical composition (mass %), balance: Fe
and impurities Class Steel C Si Mn P S Cr Mo Al Ca N Inv. 1 0.10
0.10 0.82 0.008 0.008 1.80 0.38 0.030 0.0009 0.0059 ex. 2 0.30 0.08
0.30 0.009 0.007 0.68 0.45 0.035 0.0008 0.0067 3 0.25 0.01 1.25
0.008 0.003 1.71 0.26 0.025 0.0007 0.0070 4 0.21 0.25 0.64 0.010
0.004 1.46 0.37 0.026 0.0007 0.0095 5 0.17 0.20 0.20 0.012 0.007
0.71 0.28 0.029 0.0007 0.0075 6 0.12 0.05 1.50 0.011 0.004 1.69
0.18 0.037 0.0005 0.0060 7 0.23 0.07 0.51 0.010 0.008 0.50 0.24
0.029 0.0005 0.0095 8 0.22 0.19 0.81 0.009 0.005 2.00 0.21 0.017
0.0008 0.0089 9 0.22 0.24 0.68 0.007 0.005 1.32 0.10 0.035 0.0006
0.0080 10 0.11 0.02 0.42 0.007 0.010 0.86 0.50 0.029 0.0003 0.0096
11 0.23 0.08 0.78 0.008 0.004 1.02 0.37 0.028 0.0008 0.0058 12 0.24
0.09 0.86 0.009 0.004 1.06 0.36 0.034 0.0007 0.0110 13 0.21 0.10
0.87 0.009 0.003 1.08 0.38 0.037 0.0006 0.0094 14 0.23 0.20 0.86
0.008 0.005 1.07 0.36 0.034 0.0004 0.0058 15 0.17 0.05 0.69 0.011
0.008 0.61 0.41 0.078 0.0006 0.0076 16 0.22 0.04 0.73 0.007 0.004
1.04 0.45 0.025 0.0007 0.0105 17 0.29 0.24 1.48 0.009 0.002 1.98
0.49 0.097 0.0009 0.0248 Comp. 18 0.08 0.10 0.86 0.008 0.010 0.94
0.37 0.032 0.0008 0.0113 ex. 19 0.34 0.09 0.74 0.009 0.005 1.03
0.27 0.028 0.0008 0.0126 20 0.15 0.31 0.68 0.008 0.009 1.12 0.12
0.033 0.0007 0.0091 21 0.23 0.21 0.17 0.007 0.006 0.64 0.25 0.054
0.0008 0.0074 22 0.18 0.19 1.59 0.010 0.005 1.26 0.43 0.024 0.0009
0.0153 23 0.19 0.15 0.32 0.008 0.004 0.45 0.25 0.033 0.0007 0.0127
24 0.23 0.05 1.28 0.009 0.008 2.12 0.32 0.030 0.0008 0.0136 25 0.17
0.21 1.20 0.004 0.009 1.32 0.07 0.057 0.0008 0.0092 26 0.13 0.12
0.45 0.007 0.006 0.53 0.56 0.029 0.0010 0.0096 27 0.21 0.03 1.38
0.009 0.005 1.45 0.11 0.023 0.0009 0.0068 28 0.24 0.07 0.59 0.009
0.004 1.30 0.19 0.035 0.0001 0.0095 29 0.18 0.33 1.11 0.005 0.014
1.32 0.34 0.036 0.0001 0.0132 30 0.20 0.19 0.78 0.008 0.004 0.51
0.28 0.031 0.0008 0.0086 31 0.16 0.03 0.74 0.009 0.010 1.98 0.12
0.014 0.0009 0.0760 32 0.23 0.20 0.86 0.008 0.005 1.07 0.36 0.034
0.0004 0.0058 Chemical composition (mass %), balance: Fe and
impurities Class Steel O Cu Ni B Nb Ti Fn1 Fn2 Inv. 1 0.0010 0.55
0.65 ex. 2 0.0009 0.53 0.63 3 0.0009 0.36 0.58 4 0.0008 0.51 0.59 5
0.0008 0.65 0.56 6 0.0008 0.29 0.55 7 0.0009 0.24 0.51 8 0.0010
0.59 0.76 9 0.0007 0.44 0.59 10 0.0009 0.53 0.52 11 0.0007 0.10
0.33 0.67 12 0.0008 0.18 0.31 0.65 13 0.0008 0.0005 0.31 0.62 14
0.0009 0.005 0.30 0.53 15 0.0080 0.050 0.22 0.70 16 0.0008 0.008
0.37 0.64 17 0.0014 0.33 0.66 Comp. 18 0.0009 0.27 0.63 ex. 19
0.0008 0.35 0.65 20 0.0008 0.36 0.61 21 0.0012 0.64 0.68 22 0.0009
0.20 0.69 23 0.0010 0.31 0.60 24 0.0008 0.43 0.65 25 0.0012 0.27
0.70 26 0.0009 0.28 0.73 27 0.0007 0.550 0.27 0.71 28 0.0008 0.008
0.55 0.49 29 0.0008 0.28 0.47 30 0.0009 0.16 0.63 31 0.0012 0.69
0.65 32 0.0009 0.30 0.67
[0204] Parts of diameter 60 mm steel rods were cut and the cut
steel rods were hot forged to manufacture diameter 30 mm steel
rods. These steel rods were held at 1250.degree. C. for 12 hours,
then allowed to cool down to room temperature and furthermore were
heated and held at 925.degree. C..times.1 hour, then were allowed
to cool down to room temperature.
[0205] Various Evaluations Using Steel Rods
[0206] Normalized steel rods (diameter 60 mm and diameter 30 mm)
were used, as shown below, to perform inclusion evaluation tests,
hardenability evaluation tests, toughness evaluation tests,
surface-originated micropitting life evaluation tests, and rotating
bending fatigue strength evaluation tests.
[0207] Inclusion Evaluation Test
[0208] The inclusion evaluation test was performed by the following
method. From diameter 30 mm steel rods, positions of 3.00 to 13.12
mm from the top surfaces of the observed surfaces parallel to the
rolling directions of the steel rods were observed. The observed
surfaces parallel to the rolling direction were polished by diamond
to mirror finishes. The sulfide-based inclusions of the observed
surfaces after polishing to a mirror finish were identified by the
above method and Fn2 (=A1/A2) of the different test numbers were
found. The results relating to the Fn2 are shown together with the
results of calculation of Fn1 in Table 2.
[0209] Hardenability Evaluation Test
[0210] The hardenability evaluation test was performed by the
following method. From diameter 30 mm steel rods, flanged diameter
25 mm, length 100 mm Jominy test pieces were prepared by machining.
The test pieces of the different test numbers were subjected to
Jominy tests based on JIS G 0561 (2011). Note that, the quenching
temperature was made 950.degree. C. and the Steel Rods 1 to 32 were
treated over 6 hours.
[0211] After each test, the hardness J.sub.11 at a position of 11
mm from the water cooling end was measured and the measured
hardness J.sub.11 was used to evaluate the hardenability. The
hardness test was performed using a tip radius 0.2 mm and tip angle
120 degree diamond conical indenter for measurement under 150 kgf
conditions. If the hardness J.sub.11 was a Rockwell hardness HRC of
22 or more, it was judged that the hardenability was high ("G
(good)" in Table 2). If the hardness J.sub.11 was a Rockwell
hardness HRC of less than 22, it was judged that the hardenability
was low ("P (poor)" in Table 2). The results are shown together in
Table 2.
[0212] Toughness Evaluation Test
[0213] The toughness evaluation test was performed by the next
method. A diameter 30 mm steel rod was quenched and tempered by the
heat pattern shown in FIG. 3. Specifically, a diameter 30 mm steel
rod was held at 900.degree. C. for 4 hours, then was oil quenched
("OQ" in FIG. 3). The oil quenched steel rod was further held at
180.degree. C. for 2 hours for tempering, then was air cooled ("AC"
in FIG. 3).
[0214] From the above steel rod quenched and tempered, Charpy test
pieces having V-notches were prepared so that the centers of the
V-notch side surfaces in the width direction became the 1/8D'
positions. The Charpy test pieces of the different test numbers
were subjected to Charpy impact tests based on JIS Z 2242 (2009) at
room temperature. Here, D' shows the diameter of the steel rod
quenched and tempered.
[0215] The absorption energy obtained by each test was divided by
the original cross-sectional area of the notched part
(cross-sectional area of notched part of test piece before the
test) to find the impact value vE.sub.20 (J/cm.sup.2). If the
impact value vE.sub.20 is 43 J/cm.sup.2 or more, it was judged that
the toughness was high ("G (good)" in Table 2). If the impact value
vE.sub.20 is less than 43 J/cm.sup.2, it was judged that the
toughness was low ("P (poor)" in Table 2). The results are shown
together in Table 2.
[0216] Surface-Originated Micropitting Life Evaluation Test
[0217] The surface-originated micropitting life evaluation test was
performed by the following method. From a diameter 60 mm steel rod,
diameter 60 mm, thickness 5.5 mm disk shaped crude test piece were
prepared. The thicknesses of the crude test pieces (5.5 mm)
correspond to the longitudinal directions of the steel rods.
[0218] The crude test pieces of the different number test were
treated for carburization in a gas atmosphere with a carbon
equivalent of 0.8 mass % at 950.degree. C. for 6 hours (carburizing
condition A) or in a gas atmosphere with a carbon equivalent of 0.8
mass % at 950.degree. C. for 3 hours (carburizing condition B) and
quenched in 60.degree. C. oil, immediately tempered at 150.degree.
C. for 1.5 hours, then allowed to cool to prepare test pieces
simulating carburized bearing parts. Next, the surfaces of the
prepared test pieces were made to engage in sliding motion in a
state containing free abrasives (polishing agent) and the rolling
contact surfaces were slightly machined while polishing by lapping
to prepare rolling fatigue test pieces.
[0219] A thrust type rolling fatigue tester was used to perform the
rolling fatigue tests. The maximum contact pressure at the time of
the tests was made 5.0 GPa and the speed of repetition was made
1800 cpm (cycles per minute). The lubrication oil used at the time
of the tests contained gas atomized powder as foreign matter. The
gas atomized powder was converted to fine powder by gas atomization
using high speed steel with a Vickers' hardness of 750 Hv to
prepare powder classified into granularities of 100 to 180 .mu.m.
The amount of gas atomized powder mixed in was made 0.02% with
respect to the lubrication oil. The Vickers' hardness was the
average value of any five points with a measurement load of 10 kgf.
As the steel balls used at the time of the tests, the quenched and
tempered material of SUJ2 prescribed in JIS G 4805 (2008) was
used.
[0220] The results of the rolling fatigue test were plotted on
weibull probability paper. The L10 lifetime, showing a 10%
probability of breakage, was defined as the "surface-originated
micropitting life". In a harsh usage environment (present test) of
entry of foreign matter, if the L10 lifetime is 7.0.times.10.sup.5
or more, it was judged that the surface-originated micropitting
life was excellent ("G (good)" in Table 2). If the L10 lifetime was
less than 7.0.times.10.sup.5, the surface-originated micropitting
life was judged to be short ("P (poor)" in Table 2). The results
are shown together in Table 2.
[0221] Rotating Bending Fatigue Strength Evaluation Test
[0222] The rotating bending fatigue strength evaluation test was
performed by the following method. From diameter 30 mm steel rods,
Ono type rotating bending fatigue test pieces with diameters and
lengths of parallel parts of respectively 8 mm and 25 mm and radii
of shoulder parts of 12 mm were prepared. The longitudinal
directions of the Ono type rotating bending fatigue test pieces
correspond to the longitudinal directions of the steel rods.
[0223] The Ono type rotating bending fatigue test pieces of the
different number tests were treated for carburization in a gas
atmosphere with a carbon equivalent of 0.8 mass % at 950.degree. C.
for 6 hours (carburizing condition A) or in a gas atmosphere with a
carbon equivalent of 0.8 mass % at 950.degree. C. for 3 hours
(carburizing condition B) and quenched in 60.degree. C. oil,
immediately tempered at 150.degree. C. for 1.5 hours, then allowed
to cool to prepare test pieces simulating carburized bearing
parts.
[0224] The number of test rods in the Ono type rotating bending
fatigue test was made seven each. The usual method was used to
conduct the test at ordinary temperature in the atmosphere. The
highest stress in rods which did not break even up to
1.0.times.10.sup.7 repetitions was deemed the "rotating bending
fatigue strength". If the rotating bending fatigue strength is 800
MPa or more, it is judged that the test piece is excellent in
bending fatigue strength ("G (good)" in Table 2). If the rotating
bending fatigue strength is less than 800 MPa, it is judged that
the bending fatigue strength is inferior ("P (poor)" in Table 2).
The results are shown together in Table 2.
[0225] Further, Steel Rods 1 to 17 which passed all of the tests in
the above test results (hardenability evaluation test, toughness
evaluation test, surface-originated micropitting life evaluation
test, and rotating bending fatigue strength evaluation test) were
evaluated overall as "G (good)", while Steel Rods 18 to 32 which
failed in at least one of the test results were evaluated overall
as "P (poor)". The results are shown together in Table 2.
TABLE-US-00002 TABLE 2 Surface-originated Bending Hardenability
Toughness micropitting life fatigue strength Test Carburizing
J.sub.11 vE20 Eval- L10 Eval- Overall Class no. Steel Fn1 Fn2
condition (HRC) Evaluation (J/cm2) uation (.times.10.sup.5 cycles)
Evaluation (MPa) uation evaluation Inv. 1 1 0.55 0.65 A 22 G 78.6 G
39.0 G 860 G G ex. 2 2 0.53 0.63 A 46 G 43.6 G 28.0 G 840 G G 3 3
0.36 0.58 A 40 G 54.6 G 10.0 G 850 G G 4 4 0.51 0.59 A 35 G 63.8 G
12.0 G 860 G G 5 5 0.65 0.56 A 39 G 45.7 G 23.0 G 845 G G 6 6 0.29
0.55 A 25 G 75.4 G 18.0 G 840 G G 7 7 0.24 0.51 A 34 G 54.1 G 21.0
G 865 G G 8 8 0.59 0.76 A 31 G 64.2 G 29.0 G 850 G G 9 9 0.44 0.59
A 38 G 62.4 G 27.0 G 810 G G 10 10 0.53 0.52 A 24 G 77.4 G 33.0 G
850 G G 11 11 0.33 0.67 A 39 G 63.7 G 31.0 G 840 G G 12 12 0.31
0.65 A 40 G 60.8 G 30.0 G 840 G G 13 13 0.31 0.62 A 36 G 64.0 G
26.0 G 830 G G 14 14 0.30 0.53 A 38 G 64.2 G 29.0 G 840 G G 15 15
0.22 0.70 A 38 G 57.4 G 34.0 G 870 G G 16 16 0.37 0.64 A 38 G 60.5
G 32.0 G 850 G G 17 17 0.33 0.66 A 45 G 77.7 G 34.0 G 860 G G Comp.
18 18 0.27 0.63 A 17 P 83.7 G 32.0 G 770 P P ex. 19 19 0.35 0.65 A
52 G 29.6 P 25.0 G 850 G P 20 20 0.36 0.61 A 31 G 67.6 G 21.0 G 780
P P 21 21 0.64 0.68 A 21 P 55.0 G 8.0 G 780 P P 22 22 0.20 0.69 A
33 G 58.4 G 18.0 G 790 P P 23 23 0.31 0.60 A 35 G 56.8 G 5.8 P 770
P P 24 24 0.43 0.65 A 40 G 61.8 G 26.0 G 790 P P 25 25 0.27 0.70 A
41 G 48.6 G 5.4 P 750 P P 26 26 0.28 0.73 A 27 G 72.2 G 5.7 P 800 G
P 27 27 0.27 0.71 A 36 G 34.3 P 16.0 G 820 G P 28 28 0.55 0.49 A 41
G 62.1 G 6.1 P 810 G P 29 29 0.28 0.47 A 30 G 65.5 G 6.0 P 830 G P
30 30 0.16 0.63 A 36 G 66.6 G 11.0 G 790 P P 31 31 0.69 0.65 A 35 G
67.1 G 36.0 G 790 P P 32 32 0.30 0.67 B 38 G 64.0 G 2.6 P 780 P
P
[0226] As clear from Tables 1 and 2, it will be understood that in
Steel Rods 1 to 17 having the predetermined constituents of the
present application, having an Fn1 of 0.20 to 0.65, and having an
Fn2 of 0.50 to 1.00, good results were obtained for each of the
hardenability evaluation test, toughness evaluation test,
surface-originated micropitting life evaluation test, and rotating
bending fatigue strength evaluation test.
[0227] As opposed to this, it will be understood that in Steel Rods
18 to 31 not satisfying at least one of the predetermined
constituents of the present application and the predetermined Fn1
(0.20 to 0.65) and Fn2 (0.50 to 1.00) of the present application,
excellent results were not obtained for either of the hardenability
evaluation test, toughness evaluation test, surface-originated
micropitting life evaluation test, and rotating bending fatigue
strength evaluation test. Below, the results for the comparative
examples will be given together individually and specifically.
[0228] Regarding the Steel Rod 18, the C concentration is low and
the hardenability (J 1) is small, so the bending fatigue strength
becomes low.
[0229] Regarding the Steel Rod 19, the C concentration is high, so
the toughness becomes low.
[0230] Regarding the Steel Rod 20, the Si concentration is high, so
the bending fatigue strength becomes low.
[0231] Regarding the Steel Rod 21, the Mn concentration is low and
the hardenability (J 1) is small, so the bending fatigue strength
becomes low.
[0232] Regarding the Steel Rod 22, the Mn concentration is high, so
the bending fatigue strength becomes low.
[0233] Regarding the Steel Rod 23, the Cr concentration is low, so
both of the surface-originated micropitting life and bending
fatigue strength become low.
[0234] Regarding the Steel Rod 24, the Cr concentration is high, so
the bending fatigue strength becomes low.
[0235] Regarding the Steel Rod 25, the Mo concentration is low, so
both of the surface-originated micropitting life and bending
fatigue strength become low.
[0236] Regarding the Steel Rod 26, the Mo concentration is high, so
the surface-originated micropitting life becomes low.
[0237] Regarding the Steel Rod 27, the Nb concentration is high, so
the toughness becomes low.
[0238] Regarding the Steel Rods 28 and 29, the Ca concentration is
low and the Fn2 is low, so the surface-originated micropitting life
becomes low.
[0239] Regarding the Steel Rod 30, the Fn1 is low, so the bending
fatigue strength becomes low.
[0240] Regarding the Steel Rod 31, the Fn1 is high, so the bending
fatigue strength becomes low.
[0241] Regarding the Steel Rod 32, the constituents predetermined
in the present application are realized, the Fn1 is 0.20 to 0.65,
and the Fn2 is 0.50 to 1.00, but the carburization is insufficient,
so it is learned that the surface-originated micropitting life and
bending fatigue strength are not obtained.
* * * * *