U.S. patent application number 10/547303 was filed with the patent office on 2006-07-20 for steel having finely dispersed inclusions.
This patent application is currently assigned to Nippon Steel Corporation. Invention is credited to Hiroshi Hirata, Kohichi Isobe.
Application Number | 20060157162 10/547303 |
Document ID | / |
Family ID | 32984592 |
Filed Date | 2006-07-20 |
United States Patent
Application |
20060157162 |
Kind Code |
A1 |
Hirata; Hiroshi ; et
al. |
July 20, 2006 |
Steel having finely dispersed inclusions
Abstract
An object of the invention is to provide a steel having good
fatigue life characteristics and acoustic characteristics by
eliminating the harmful effects of oxide based inclusions. The
object is attained by the following steel. A steel having at least
a part comprising: REM of which amount meets formula (1); and
REM-containing inclusions of which number meets formula (2);
wherein the concentration of Al20.sub.3 in the REM-containing
inclusions is 30 mass % or less (including 0%); Formula (1);
-30<REM(ppm)-(T.O(ppm).times.280/48)<50, Formula (2); number
of REM-containing inclusions/total number of inclusions >0.8,
wherein the inclusions of Formula (2) having an equivalent diameter
of 1 .mu.m or more.
Inventors: |
Hirata; Hiroshi; (Hokkaido,
JP) ; Isobe; Kohichi; (Hokkaido, JP) |
Correspondence
Address: |
BIRCH STEWART KOLASCH & BIRCH
PO BOX 747
FALLS CHURCH
VA
22040-0747
US
|
Assignee: |
Nippon Steel Corporation
6-3, Otemachi 2-chome, Chiyoda-ku
Tokyo
JP
|
Family ID: |
32984592 |
Appl. No.: |
10/547303 |
Filed: |
March 11, 2004 |
PCT Filed: |
March 11, 2004 |
PCT NO: |
PCT/JP04/03251 |
371 Date: |
August 30, 2005 |
Current U.S.
Class: |
148/331 |
Current CPC
Class: |
C22C 38/005
20130101 |
Class at
Publication: |
148/331 |
International
Class: |
C22C 38/00 20060101
C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 13, 2003 |
JP |
2003-068541 |
Claims
1. A steel having at least a part comprising: rare earth metal
(REM) in an amount meeting formula (1); and REM-containing
inclusions in a number meeting formula (2); wherein a concentration
of Al.sub.2O.sub.3 in the REM-containing inclusions is 0-30 mass %;
-30<REM(ppm)-(T.O.(ppm).times.280/48)<50 formula (1) number
of REM-containing inclusions/total number of inclusions>0.8
formula (2) wherein the inclusions in formula (2) have an
equivalent diameter of 1 .mu.m or more.
2. The steel according to claim 1, wherein REM is at least one
selected from the group consisting of Ce, La, Nd and Pr.
3. The steel according to claim 2, wherein REM comprises at least
Ce, La, Nd and Pr.
4. The steel according to claim 1 or 2, wherein the steel meets at
least one of following four conditions: a) content of Al 0-0.05
mass %; b) content of total oxygen (T.O.)=0.005 mass %; c) content
of S=0.003 mass %; and d) content of Ti=0.001 mass %.
5. The steel according to claim 4, wherein the steel has a content
of Al 0-0.05 mass %.
6. The steel according to claim 4, wherein the steel has a content
of total oxygen (T.O.)=0.005 mass %.
7. The steel according to claim 4, wherein the steel has a content
of S=0.003 mass %.
8. The steel according to claim 4, wherein the steel has a content
of Ti=0.001 mass %.
9. The steel according to claim 1 or 2, wherein the steel has a
rolling fatigue life (L.sub.10) which is greater than 3.2 times the
L.sub.10 of essentially the same steel which has been prepared
without added REM.
10. The steel according to claim 9, wherein the steel has a rolling
fatigue life (L.sub.10) which is at least 7.6 times greater than
the L.sub.10 of essentially the same steel which has been prepared
without added REM.
11. The steel according to claim 9, wherein the steel has a rolling
fatigue life (L.sub.10) which is greater than 3.2 up to and
including 9.2 times the L.sub.10 of essentially the same steel
which has been prepared without added REM.
12. The steel according to claim 1, wherein the steel has no added
calcium or magnesium.
13. The steel according to claim 4, wherein the steel is a bearing
steel.
14. The steel according to claim 13, wherein the bearing steel is
shaped for use in a hard disk drive or audiovisual equipment.
15. A method for manufacturing a steel: said method comprising the
steps of: preparing a molten steel, adding rare earth metal (REM)
and oxygen to the molten steel, casting the molten steel into said
steel; wherein the steel has at least a part comprising REM in an
amount meeting formula (1); and REM-containing inclusions in a
number meeting formula (2); wherein a concentration of
Al.sub.2O.sub.3 in the REM-containing inclusions is 0-30 mass %;
-30<REM(ppm)-(T.O.(ppm).times.280/48)<50 formula (1) number
of REM-containing inclusions/total number of inclusions>0.8
formula (2) wherein the inclusions in formula (2) have an
equivalent diameter of 1 .mu.m or more.
16. The method according to claim 15, wherein the REM is added
during RH vacuum degassing treatment of the molten steel.
17. The method according to claim 16, wherein the aluminum is added
to the molten steel prior to addition of REM.
18. The method according to claim 16, wherein air is essentially
completely sealed after RH treatment.
Description
BACKGROUND OF THE INVENTION
[0001] The present application claims priority to Japanese
Application 2003-068541, filed in Japan on Mar. 13, 2003 and which
is herein incorporated by reference in its entirety.
FIELD OF THE INVENTION
[0002] This invention relates to a steel having finely dispersed
oxidized inclusions and oxysulfide inclusions, and particularly to
a steel having good fatigue life characteristic and acoustic
characteristics obtained by eliminating a harmful effect of oxide
based inclusions. The inclusions refer to an oxide inclusion, a
sulfide inclusion, an oxysulfide inclusion, a nitride inclusion and
a combination thereof. The steel refers to a molten steel, a cast
slab, cast bloom, cast billet and other half finished products and
final steel products and is not limited in use.
DESCRIPTION OF THE RELATED ART
[0003] Recently it is required that the quality for steel products
is more strict and diversified. A steel with a high degree of
cleanliness having inclusions which are made harmless is
demanded.
[0004] Molten steel refined in a converter or in a vacuum treating
vessel contains a large quantity of dissolved oxygen. This excess
of oxygen is usually reacted with aluminum which has strong
affinity for oxygen to form alumina (Al.sub.2O.sub.3) inclusions.
Even though Si or Mn is added to react with the oxygen, Al is
present in the molten steel because the ladles used are mostly made
of Al.sub.2O.sub.3 based refractory materials with which molten
steel reacts to reduce and dissociate Al.sub.2O.sub.3. The
dissociated Al is eluted into the molten steel and then re-oxidized
to form Al.sub.2O.sub.3 again.
[0005] These Al.sub.2O.sub.3 based inclusions are hard and
coagulate to form coarse Al.sub.2O.sub.3 clusters, which cause: (a)
snapping of wires such as tire cords; (b) an impairment in the
rolling fatigue characteristics in steel bars such as in bearing
steel; and (c) a crack in the can during the can-forming process
such as drawn and ironing (DI) can made of thin steel sheet.
Reducing the size of the inclusion as much as possible has been
demanded, in particular, to improve the fatigue life of bearing
steels.
[0006] Products incorporating bearing steel parts include electric
appliances such as VCR or CD player, general equipment such as
measuring instruments or medical instruments, office automation
equipment, and electronic devices such as hard disk drives. In
these products the bearing steel is used under a light load,
however, the sound or vibration generated from moving parts made of
bearing steel is required to be as low as possible. Among others,
miniature parts made of bearing steel for hard disk drives have
strict requirements to make no noise and no vibration. The main
cause for the generation of sound and/or vibration is likely to be
hard inclusions such as Ti carbonitride or Al.sub.2O.sub.3 based
inclusions which are at the surface of the bearing.
[0007] It is important to re-form Al.sub.2O.sub.3 based inclusions
and to make them harmless by reducing their size (fining), in
addition to reducing the total amount of Al.sub.2O.sub.3 based
inclusions of hard non-metal to upgrade the cleanliness. The
inclusions can be made harmless by conforming the inclusions
existing at the initial stage of the process following
deoxidization treatment of the refined molten steel, in
composition, shape and size, into harmless products.
[0008] In order to reduce and eliminate Al.sub.2O.sub.3 base
inclusions attempts have been made to: (1) inhibit re-oxidation of
Al by slag reformation and/or shutting off the air along with
reducing mixed-in non-oxide inclusions by slag-cutting, and (2)
reducing deoxidation products by applying a secondary refining
apparatus such as a RH-type vacuum degassing device or powder
injection device.
[0009] Conventionally, Al.sub.2O.sub.3 based inclusions are made
harmless by inhibiting the coarsening of Al.sub.2O.sub.3 through
coagulation by reforming Al.sub.2O.sub.3 into spinel (MgO
Al.sub.2O.sub.3) or MgO by adding Mg alloy to molten steel. (For
example, see JP H05-311225 A). With regard to the manufacturing of
Al killed steel containing 0.005% or more of Al by mass, a method
for manufacturing Al killed steel without clusters is known wherein
the alloy consisting of Al and more than two ingredients selected
from Ca, Mg and REM (rare earth metal) are added into molten steel
and the content of Al.sub.2O.sub.3 in the resulting inclusions is
limited to 30-85 mass %. (For example, see JP H09-263820 A).
[0010] However, the above-mentioned methods for eliminating
Al.sub.2O.sub.3 based inclusions are not sufficient to meet the
high quality level now in demand. Also, with respect to the
specific method for making inclusions finer by adding Mg and/or
REM, the following problems are known. In decreasing the size of
the inclusions by using Mg, the Mg which is added easily vaporizes
(boils away) since the temperature of molten steel to which Mg is
added is higher than the boiling point of metal Mg (1070.degree.
C.). Even if Mg is added in the form of a mixture with Si, Al or
Fe--Si, then the Mg activity still remains 1. Therefore, the
vaporization problem remains the same as with Mg alone. In the case
of using alloys such as Al--Mg or Si--Mg, the Mg loss due to
vaporization can be reduced to some extent. However, after being
dissolved into the molten steel, it is difficult to prevent Mg from
vaporizing as in the case of when Mg alone added, which leads to
low yields.
[0011] Mg vaporizes more quickly when it is added in a vacuum as
compared to when it is added under normal pressure. Therefore, the
Mg addition has to be made under normal pressure which is set up
after vacuum refining has been performed.
[0012] Further as shown in Table 1, a spinel formed in a reforming
step is not as hard as the initial Al.sub.20.sub.3, but it is still
hard. Therefore, the requirements of low vibration and the acoustic
characteristics for small bearing steel products still can not be
sufficiently met. TABLE-US-00001 TABLE 1 Micro-hardness (Hv)
Al.sub.2O.sub.3 3000.about.4000 TiC 2640.about.3100 MgO
Al.sub.2O.sub.3 2100.about.2400 TiN 1800.about.2300 SiO.sub.2 1600
REAlO.sub.3 1100 MgO 1000 RE.sub.2O.sub.2S 500 RE.sub.2S.sub.3 450
MnO SiO.sub.2 750
[0013] Conventional technology for fining (size-reducing)
inclusions by adding REM has the following problems. In the case of
REM addition disclosed in JP H09-263820 A, the addition of more
than two ingredients selected from Ca, Mg and REM for avoiding the
formation of the Al.sub.2O.sub.3 clusters, results in the formation
of a compound inclusion having a low melting point. This may be
helpful to prevent sliver defects but it can not reduce the size of
the inclusion to the level required for a bearing steel. This is
because low melting point inclusions usually coagulate to became
coarse.
[0014] Further, as for the fining method using conventional REM
addition, it is known that REM can be added to control the shape of
inclusions, since REM is capable of making the shape of the
inclusions spherical which provides better fatigue life. An amount
of the REM to be added should be 0.010 mass t or less, since the
REM addition of more than 0.010 mass % increases the amount of
inclusions which lowers the fatigue life. (See JP H11-279695 A).
However there is no analysis and no suggestion about the mechanism
and the state of existing composition of inclusions.
[0015] REM is an element which makes a bond with oxygen (O) to form
an REM oxide and also tends to form a sulfide by forming a bond
with sulfur. Therefore, it is thought that if there is more REM
present than is necessary to react with all of the O, then the
excess REM will form a sulfide which provides a harmful effect to
the fatigue life characteristics by increasing the size of the
inclusions. It is important to strictly control the composition of
the inclusions by adjusting the amount of REM added in order to
control the inclusion size. In other words, formation of coarse
sulfide should be prevented by balancing the amount of REM addition
with the content of O to avoid an excess amount of REM. The sulfide
also should be fined (reduced in size) as the sulfide also
influences the fatigue life characteristics. As mentioned above,
the content of S should be low to further reduce the size of the
inclusion structures, since REM easily forms a sulfide. These
technical ideas are not disclosed in JP H11-279695 A.
[0016] JP 2000-45048 A discloses a bearing steel containing Ti of 7
ppm or less and 0 of 7 ppm or less for reducing the amount of Ti
based inclusions and the size thereof so as to improve acoustic
characteristics of small bearing steel products which are used for
electric appliances such as a VCR's or CD players, general
equipment such as measuring instruments or medical instruments,
office automation equipment, and electronic devices such as hard
disk drives. JP H08-312651 A discloses a rolling bearing steel
containing no residual austenite (0%) in the carbonitrided layer
and having a surface hardness of HRC57 or more which is made by
tempering at high temperatures of 350.degree. C. or more after
quenching or carbonitriding. However, further improvement in the
acoustic characteristics can not be expected even if an absolute
amount of oxide based inclusions is reduced by lowering T.O. (total
oxygen), since oxide based inclusions are hard because of the
Al.sub.2O.sub.3. Al.sub.2O.sub.3 can be reformed into a fine spinel
by Mg addition, which is not as hard as Al.sub.2O.sub.3 as shown in
Table 1, but is still hard and does not provide adequate
improvement. To the contrary, REM based inclusions are soft as
shown in Table 1. Therefore, it is useful for improvement in the
acoustic characteristics to reform hard Al.sub.2O.sub.3 based
inclusions into soft REM based inclusions by adding REM.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] A more particular description of various aspects of the
embodiments of the invention illustrated in the appended drawings
will now be rendered. Understanding that such drawings depict only
exemplary embodiments of the invention, and are not therefore to be
considered limiting of the scope of the invention in any way,
various features of such exemplary embodiments will be described
and explained with additional specificity and detail through the
use of the accompanying drawings in which:
[0018] FIG. 1 shows an influence of a value of (REM (ppm)-T.O
(ppm).times.280/48) on size of inclusion;
[0019] FIG. 2(A) shows stable regions for an oxide of Ce,
oxysulfide of Ce and sulfide of Ce, and
[0020] FIG. 2(B) shows stable regions for an oxide of La,
oxysulfide of La and sulfide of La;
[0021] FIG. 3 shows an influence of an REM source on the size of
the inclusions; and
[0022] FIG. 4 shows an influence of [S] on the size of the
inclusions.
SUMMARY OF THE INVENTION
[0023] An object of the invention is to provide a steel having good
fatigue life characteristics and good acoustic/vibrational
characteristics by fining and dispersing oxide based and oxysulfide
based inclusions. This steel is referred to as
inclusion-fined-dispersed steel.
[0024] The term "REM-containing oxysulfide" is defined as follows.
REM forms oxides as mentioned above and also is capable of easily
forming sulfides. Therefore if S exists, it is possible that REM
couples with both S and O to form an REM oxysulfide, the
stoichiometric composition of which is represented as
RE.sub.2O.sub.2S, where RE represents REM.
[0025] After experiments and examinations were made, conditions for
attaining the object were obtained with respect to the REM addition
amount, inclusion composition and steel ingredients, which enable
the formation of fine inclusions. A brief summary is as
follows.
[0026] Item (1): A steel having at least a part comprising:
[0027] REM of which amount meets formula (1); and REM-containing
inclusions of which number meets formula (2); wherein the
concentration of Al.sub.2O.sub.3 included in the REM-containing
inclusions is up to 30 mass %:
-30<REM(ppm)-(T.O(ppm).times.280/48)<50; Formula (1) number
of REM-containing inclusions/total number of inclusions>0.8;
Formula (2)
[0028] wherein the inclusions of Formula (2) have an equivalent
diameter of 1 .mu.m or more.
[0029] Item (2): A steel described in Item (1) above, wherein REM
is at least one selected from the group consisting of Ce, La, Nd
and Pr.
[0030] Item (3): A steel described in Items (1) or (2) above,
wherein the steel meets at least one of following four
conditions:
[0031] content of Al=0.05 mass %;
[0032] content of T.O.=0.005 mass %;
[0033] content of S=0.003 mass %; and
[0034] content of Ti=0.001 mass %,
wherein the steel has improved fatigue life and/or acoustic
characteristics by fining inclusions using the above described
conditions.
[0035] Item (4): A steel described in Item (3) above, wherein the
steel is a bearing steel.
[0036] Item (5): A steel described in Item (4) above, wherein the
steel is used for a small bearing steel component for hard disk or
audiovisual equipment.
[0037] In Item (1), the steel is defined as having "at least a
part" which meets the requirements of Formulas (1) and (2). It is
generally known that in the manufacture of steel, the composition
of the surface region(s) can differ from the composition of the
core region(s) of the steel. By indicating that the inventive steel
has at least a part which meets the requirements of Formulas (1)
and (2), it is envisioned that the inventive steel may have
region(s) that do not satisfy both Formulas (1) and (2), but must
have at least one region which satisfies both Formulas (1) and
(2).
DETAILED DESCRIPTION OF THE INVENTION
[0038] Ingredients and compositions of steel of certain
non-limiting embodiments of the invention are explained below. The
concentration of each ingredient is represented by mass
percent.
Content of Al
[0039] As described above, a steel of the invention is made in a
process wherein inclusions such as oxides are converted from
Al.sub.2O.sub.3 into REM oxide or REM oxysulfide by adding REM. Al
is not an essential element to all types of steel, but is a useful
ingredient for adjusting the crystal grain size and as a
deoxidizing element for reducing T.O. However, after the Al content
reaches 0.05%, not only is there no additional affect on crystal
grain size, but the Al.sub.2O.sub.3 can not be converted into REM
oxide or REM oxysulfide, which makes it difficult to attain an
object of the invention. Therefore, an upper limit of Al content
should be 0.05%. Applicant theorizes that Al.sub.2O.sub.3 is in a
more stable thermodynamic state than REM oxide or REM oxysulfide
when the concentration of Al is high. Therefore, the REM oxide or
REM oxysulfide is not formed.
Content of T.O. (Total Oxygen)
[0040] As used herein, the term "Content of T.O." is substantially
the same as the amount of dissolved oxygen which forms an oxide
(mainly Al.sub.2O.sub.3) in the steel. Thus, the higher the T.O.
content becomes, the more Al.sub.2O.sub.3 to be reformed in the
steel becomes. It was found that when the T.O. content exceeds
0.0050%, the Al.sub.2O.sub.3 amount becomes too high to allow for
conversion of the entire Al.sub.2O.sub.3 amount into REM oxide or
REM oxysulfide by adding REM, i.e., some amount of Al.sub.2O.sub.3
is left in the steel. Therefore, the T.O. content is 0.0050% or
less.
[0041] REM is a strong deoxidizing element which is added to react
with Al.sub.2O.sub.3 in the steel to form REM oxide by reacting
with the oxygen in the Al.sub.2O.sub.3. Consequently, if an
appropriate amount of REM in proportion to the amount of
Al.sub.2O.sub.3, i.e., content of T.O. is not added, unreacted
Al.sub.2O.sub.3 is left. Further examination on this matter brought
out that there is a relationship between the REM content and T.O.
content as shown in the following formula (1). Formula
(1)=-30<REM(ppm)-(T.O(ppm).times.280/48)<50
[0042] This formula (1) is explained based on FIG. 1. FIG. 1 shows
how the inclusion is influenced by a value of (REM
(ppm)-(T.O.(ppm).times.280/48)) by using dmax, 30000 (.mu.m) at
0.005% of S content (hereinafter [S]) and 0.002% of [S].
[0043] The vertical axis, "dmax, 30000 (.mu.m)" represents the
maximum size of inclusions existing in the area of 30000 mm.sup.2
which is estimated by extreme value statistics (see Beretta et al.,
Metallurgical and Materials Transactions B. vol. 32B, pp. 517-523,
2001, herein incorporated by reference in its entirety). The
procedure for the estimation by extreme value statistics is as
follows:
i) Sixteen (16) samples of 10.times.10 mM (100 mm.sup.2) steel are
prepared for microscopic examination;
ii) Maximum size inclusion of each of 16 samples is specified and
the size is measured; and
iii) Maximum size existing in 30000 mm.sup.2 is estimated from 16
maximum size data by using the extreme value statistics
processing.
[0044] First, two kinds of molten bearing steel containing 8 ppm
T.O. of which [S] is 0.005% and 0.002% respectively are prepared,
then misch metal is added as REM. The behavior of the inclusion's
being fined is examined. How the inclusion size is affected by the
amount of added REM is estimated. Samples for microscopic
examination are taken from a steel ingot, the maximum size of the
inclusion is estimated by the extreme value statistics and the
relationship between the maximum size and the amount of added REM
is examined. In FIG. 1, the solid line represents the case of [S]
0.005% and the dashed line represents the case of [S] 0.002%.
[0045] The resulting inclusions are stably fined in the range where
the value of (REM(ppm)-(T.O.(ppm).times.280/48)) is between -30 and
50.
[0046] In the case of [S] 0.002%, the inclusion becomes finer.
Also, the compositions of inclusions satisfy formula (2) which is
later described and an amount of Al.sub.2O.sub.3 is 30 mass % or
less. That is, it is possible to prevent Al.sub.2O.sub.3 from
remaining unreacted and to convert the oxide to REM oxide which is
intended, by keeping the value of
(REM(ppm)-(T.O.(ppm).times.280/48)) from -30 to 50 as defined in
formula (1). If the addition is made so that the value
(REM(ppm)-(T.O.(ppm).times.280/48)) exceeds 50, formation of
sulfide is promoted and coarse sulfide is formed, which lowers the
fatigue life. If the addition is made so that the value
(REM(ppm)-(T.O.(ppm).times.280/48)) stays less than -30, the
formation of REM oxide or REM oxysulfide is too low to attain the
object of the invention.
Coefficient of T.O. (ppm) in Formula (1), i.e., 280/48
[0047] The main ingredients of misch metal are Ce, La, Nd and Pr.
The atomic mass of Ce is 140 and that of 0 is 16. It is assumed
that the REM oxide formed is RE.sub.2O 3, therefore the coefficient
"280/48" is used to maintain the (stoichiometric) balance of REM to
O content.
Reason for Limiting Percentage (Ratio) of the Number of Oxide Based
and Oxysulfide Based Inclusions of which Grain Diameters are 1
.mu.m or More.
[0048] In the process of steel refining, the steel has unavoidable
inclusions other than REM oxide based and REM oxysulfide based
inclusions. For example, in the case where a great deal of high
oxidation degree slag flows out from a decarburizing refining
converter. If the oxidation degree is not lowered in a secondary
refining process, the molten steel will re-oxidize with the slag,
which results in increased Al.sub.2O.sub.3 based inclusions. Also,
if air sealing is incomplete after the REM source has been added,
there will be no REM remaining to dissolve into the molten steel,
since all the REM has been used. Therefore, Al.sub.2O.sub.3 based
inclusions increase because of re-oxidization by air. Further, if a
slag basicity is high at the secondary refining process, Ca is fed
from the slag via an equilibrium reaction between the slag and the
molten steel. This causes formation of CaO-rich inclusions which
can not be reformed by REM. In the case where the inclusion fining
effect by REM addition can not be expected as in the case mentioned
above, eliminating the re-oxidation completely is important. Thus,
it is found that if the number of inclusions (comprising sulfides
and nitrides) other than REM-containing inclusions is less than 20%
of the total inclusions, i.e., formula (2) described below is met,
a high percentage of inclusions can be fined and stably dispersed
so that fatigue life is further improved. Formula (2)=Number of
REM-containing inclusions/total number of inclusions>0.8
[0049] The following is the method for checking the number of
inclusions.
[0050] The measurement is made using an analytical instrument
consisting of a combination of a X-ray micro-analyzer and computer
using the following steps:
i) Designation of measurement area of steel sample, one area of
viewing field is defined as 0.5 mm.times.0.5 mm and five areas of
the viewing field per sample are taken;
ii) Electron beam irradiation,
beam diameter is 0.5 .mu.m and the beam is irradiated 1000 times in
the "X" direction and 1000 times in the "Y" direction for each of
the five viewing fields to make an elemental analysis;
iii) Identification of inclusion,
Information from the elemental analysis by electron beam
irradiation was processed by computer to identify the
inclusions;
iv) Identification of REM-containing inclusion,
Information from the elemental analysis by electron beam
irradiation was processed by computer, and if an inclusion
contained an REM, the inclusion was identified as an REM-containing
inclusion, and the composition was quantitatively estimated;
and
v) Identification of the number,
Equivalent diameter of the inclusion grains of above iii) and iv)
were calculated to check the size of the inclusions and the number
of inclusions of above iii) and iv) existing in five 0.5
mm.times.0.5 mm viewing fields were counted.
[0051] The reason for limiting the amount of Al.sub.2O.sub.3 in the
inclusions to 30% or less is as follows. It was found that if more
than 30% Al.sub.2O.sub.3 is present, the inclusion becomes harder
compared to REM oxide and REM oxysulfide when less than 30%
Al.sub.2O.sub.3 is used. This brings negative effects to fatigue
life and acoustic characteristics. In view of this, the upper limit
of the Al.sub.2O.sub.3 amount in the inclusion is 30%. Preferably,
Al.sub.2O.sub.3 is less than 30 mass % and more preferably
Al.sub.2O.sub.3 is 0 to about 29 mass % in the REM containing
inclusions.
Using Misch Metal as the REM Source.
[0052] REM represents rare earth metal (rare earth element) which
is a generic term for 17 elements belonging to group III in the
periodic table, i.e., Sc (atomic number 21), Y (39) and lanthanoid
(57-71) of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb
and Lu. Although they have been lumped together as having the same
behavior in the literature, actually each of them has a different
propensity for forming an oxide, oxysulfide and sulfide. For
example, as shown in FIG. 2, La and Ce have different types of
compounds, respectively, so as to be stable even in the same O or S
concentrations.
[0053] It was found that addition of misch metal, of which the main
ingredients are Ce, La, Nd and Pr (summation of the 4 elements
accounts for more than 98%), can fine the inclusions more stably
than the addition of a single element. (see FIG. 3)
[0054] High S content promotes the formation of REM sulfide. Even
if REM sulfide is not formed, S exists more than the amount in the
stoichiometric composition of REM oxysulfide requires, which forms
coarse inclusions. In view of this, as shown in FIG. 4, low content
of S is preferable. For example, coarse sulfide is not formed and
good quality of material is obtained in the case of 0.003% or less
of S content. When the content of Ti exceeds 0.001%, the formation
of hard TiN increases drastically, which has a negative influence
on fatigue life, acoustic characteristics and vibration
characteristics. Therefore, the content of Ti should be 0.001% or
less.
[0055] In addition to the ingredients used in the inventive steel
product, which include at least one of following; Al content of
0.05% or less (includes 0%), T.O. content of 0.005% or less, S
content of 0.003% or less or Ti content of 0.001% or less; it is
envisioned that at least one of the following reinforcement
ingredients can be added specially for bearing steel or small
bearing steel products, i.e., Si of 0.01-0.4% of content, Mn of
0.1-0.5% of content and Cr of 0.01-1.5% of content.
C Concentration is not Particularly Limited in the Invention.
[0056] In the present invention, the carbon concentration is not
particularly limited. However, it is preferred that the C
concentration ranges from 0.005 to 1.2% because if the C
concentration exceeds 1.2%, the REM will form a carbide with C,
which lowers the efficiency of reforming Al.sub.2O.sub.3, and if
the C concentration is less than 0.005%, the amount of initially
existing Al.sub.2O.sub.3 is large, which lowers the efficiency of
the reforming Al.sub.2O.sub.3.
[0057] The manufacturing method for the steel of the invention is
not limited to a specific method. The base molten steel can be
prepared in a blast furnace with a converter or simply with an
electric furnace. Other ingredients can be added to the base molten
steel according to need. The method and apparatus for the addition
are not limited to specific ones. The addition of ingredients can
be made by free falling, and the ingredients can be blown into the
mixture using inert gas or by some other method. Furthermore, the
method where the steel ingot is formed from molten steel and the
ingot is rolled are not precluded.
[0058] The REM source is added after vacuum refining, such as RH.
For example, misch metal (in block), stored in a hopper located
outside on top of the vacuum chamber, is added to the surface of
molten steel in the vacuum chamber at the final stage of
Ruhrstahl-Hausen (RH) treatment.
EXAMPLES
Example 1
[0059] Pig iron discharged from blast furnace had dephosphorization
and desulfurization treatment, then 270 tons of the pig iron was
transported into the converter for oxygen blowing, and the base
molten steel for bearing steel which has prescribed contents of C,
P and S was prepared. Al, Si, Mn and Cr were added to the prepared
base molten steel while the base molten steel was discharged into a
ladle and having Ladle Furnace (LF) and RH vacuum degassing
treatment. At LF treatment, a high oxidation degree slag discharged
from the converter was reduced to a lower content of iron oxide and
MnO, and CaO was added to increase the CaO/SiO.sub.2 ratio, by
which ingredients causing re-oxidization was reduced. The content
of Al.sub.2O.sub.3 was adjusted to obtain a slag composition which
has a high capability of absorbing inclusions. At the RH treatment,
dehydrogenation and elimination of inclusions were made.
Furthermore, a predetermined amount of REM stored in a hopper
located outside on top of the RH vacuum chamber was added in a late
stage of the RH treatment step. Misch metal shown in Table 2 was
used as REM, having an average particle size of 35-45 mm.
TABLE-US-00002 TABLE 2 Composition Of Misch Metal (Mass %) La Ce Pr
Nd 30.7 52.2 4.3 10.9
[0060] A steel bloom is manufactured from the molten steel prepared
above by a continuous casting process. The steel bloom is rolled to
form a steel bar of bearing steel (diameter 65 mm.phi.) of which
chemical composition is shown in Table 3. TABLE-US-00003 TABLE 3
Example 1 and Comparative Example 1 REM Percentage addition of the
amount: number: dmax, Rolling Chemical Composition Of Bearing
Steel.sup.c formula formula 30000 fatigue C Si Mn Al S Ti T.O. REM
(1) (2) (.mu.m) life.sup.a Samples 1 1.00 0.24 0.40 0.02 0.006
0.0008 0.0006 0.0036 1 0.85 12.3 7.8 of 2 1.02 0.23 0.40 0.02 0.006
0.0008 0.0006 0.0075 40 0.92 12.5 7.6 Example 3 0.99 0.24 0.41 0.02
0.006 0.0008 0.0006 0.0010 -25 0.82 11.9 8.5 1 4 1.01 0.22 0.40
0.001 0.006 0.0007 0.0010 0.0061 2.7 0.87 12.1 7.7 5 1.00 0.23 0.40
0.02 0.002 0.0009 0.0006 0.0038 3 0.86 10.5 9.2 Compara- 1 1.00
0.23 0.41 0.02 0.006 0.0008 0.0006 0 -35 0.0 18.3 1.0 tive 2 1.01
0.24 0.42 0.02 0.006 0.0009 0.0005 0.0003 -32 0.82 16.5 1.8
Samples.sup.b 3 1.00 0.24 0.40 0.02 0.006 0.0009 0.0006 0.0102 67
0.95 17.4 1.4 of 4 1.02 0.22 0.41 0.02 0.006 0.0008 0.0006 0.0037 2
0.72 17.7 3.2 Example 5 1.00 0.23 0.42 0.02 0.006 0.0007 0.0006
0.0045 10 0.93 18.5 1.5 .sup.aRolling fatigue test results are
based on Comparative Example 1 wherein Comparative Example 1 has a
value set at 1.0. .sup.bComparative Samples prepared in the
procedure of Comparative Example 1 .sup.cCr: 1.40 .about. 1.44%, P:
0.0010 .about. 0.0015%
[0061] With respect to inclusions in the steel product (steel bar),
REM-containing inclusions accounted for a large percentage of
inclusions which were very fine in size. The maximum inclusion size
in a 30000 mm 2 area was estimated by using extreme value
statistics (base area: 100 mm.sup.2, n=16, estimation area: 30000
mm.sup.2) and the results indicate a good size was obtained as
shown in Table 3. Also the rolling fatigue test (see "Development
of High Temperature, Long Life Bearing Steel (STJ2)", Hiromasa
Tanaka et al., Technical Review, No. 68, May 2000, pages 51-57,
which is herein incorporated by reference in its entirety) gave
good results as shown in Table 3. The concentration of the
components of REM shown in Table 3 for each corresponding sample is
described in Table 4. TABLE-US-00004 TABLE 4 Al.sub.2O.sub.3
content in REM- Content Of REM Composition containing La Ce Pr Nd
inclusions Samples 1 0.0011 0.0020 0.0001 0.0004 0.about.18 2
0.0025 0.0038 0.0004 0.0008 0.about.10 3 0.0003 0.0005 0.0001
0.0001 0.about.29 4 0.0017 0.0035 0.0003 0.0006 0.about.12 5 0.0012
0.0018 0.0003 0.0005 0.about.20 Comparative 1 0 0 0 0 -- Samples 2
0.0001 0.0002 0 0 0.about.50 3 0.0032 0.0051 0.0007 0.0012
0.about.10 4 0.0012 0.0019 0.0002 0.0004 0.about.25 5 0.0014 0.0024
0.0002 0.0005 0.about.75
Comparative Example 1
[0062] A bearing steel shown in Table 3 was manufactured in the
same way as in example 1. However, in the comparative example 1,
the following conditions are adopted: the REM was not added at the
final stage in the RH treatment step as in Example 1; the REM
addition was made using the same technique as in example 1, but the
added amount of REM was either more than the upper limit of proper
REM amount or less than lower limit; and the air was not completely
sealed after the RH process was applied to increase the percentage
of REM-containing inclusions to be outside the proper range defined
by the invention. Results of the inclusion size and rolling fatigue
test of the bearing steel are shown in Table 3 as Comparative
Examples, which are inferior to the results of Example 1.
Example 2
[0063] A bearing steel described in Table 5 was manufactured in the
same way as in Example 1. TABLE-US-00005 TABLE 5 REM Percentage
addition of the Vibra- amount: number: dmax, Acoustic tional
Chemical composition of bearing steel.sup.b formula formula 30000
character- character- C Si Mn Al S Ti T.O. REM (1) (2) (.mu.m)
istic istic Samples 1 1.01 0.23 0.41 0.02 0.003 0.0005 0.0006
0.0033 -2 0.87 11.8 .circleincircle. .circleincircle. of 2 1.00
0.24 0.41 0.02 0.002 0.0005 0.0006 0.0077 42 0.93 12.3
.circleincircle. .circleincircle. Example 3 1.00 0.24 0.40 0.02
0.003 0.0006 0.0006 0.0012 -23 0.85 12.1 .circleincircle.
.circleincircle. 2 4 1.02 0.23 0.42 0.001 0.002 0.0004 0.0010
0.0042 7 0.90 11.5 .circleincircle. .largecircle. 5 0.99 0.24 0.41
0.02 0.006 0.0005 0.0006 0.0037 2 0.85 12.4 .largecircle.
.largecircle. Compara- 1 1.02 0.22 0.40 0.02 0.003 0.0005 0.0006 0
-35 0.0 19.2 .DELTA. .DELTA. tive 2 0.99 0.24 0.42 0.02 0.003
0.0005 0.0005 0.0003 -32 0.81 17.5 .DELTA. .DELTA. Samples.sup.a 3
1.00 0.22 0.41 0.02 0.003 0.0004 0.0006 0.0110 75 0.95 17.3 .DELTA.
.DELTA. 4 1.01 0.23 0.41 0.02 0.002 0.0005 0.0006 0.0040 5 0.65
16.3 .DELTA. .DELTA. 5 1.00 0.24 0.42 0.02 0.003 0.0015 0.0005
0.0042 7 0.85 12.2 X X 6 1.01 0.23 0.40 0.02 0.003 0.0004 0.0006
0.0045 10 0.95 16.8 X X .sup.aComparative Samples prepared using
the method of Comparative Example 2 .sup.bCr: 1.40 .about. 1.44%,
P: 0.0010 .about. 0.0015%
[0064] The number of REM-containing inclusions accounted for large
percentage of the number of inclusions in the bearing steel product
and the size was very fine. With respect to rolled steel bar
(diameter 65 mm.phi.), the maximum inclusion size in a 30000
mm.sup.2 area was estimated by using extreme value statistics (base
area: 100 mm.sup.2, n=16, estimation area: 30000 mm.sup.2 and the
result indicates that a good size was obtained as shown in Table 5.
The steel product was rolled into wire rod (10 mm.phi.) and then
made into a miniature bearing steel. The acoustic and vibrational
characteristics were then examined and found to be good. The REM
composition shown in Table 5 is described in Table 6.
TABLE-US-00006 TABLE 6 Al.sub.2O.sub.3 content in REM- Content Of
REM Composition containing La Ce Pr Nd inclusions Samples 1 0.0009
0.0017 0.0002 0.0005 0 .about. 18 2 0.0024 0.0042 0.0003 0.0008 0
.about. 11 3 0.0003 0.0006 0.0001 0.0002 0 .about. 29 4 0.0013
0.0023 0.0002 0.0004 0 .about. 21 5 0.0011 0.0018 0.0003 0.0005 0
.about. 17 Comparative 1 0 0 0 0 -- Samples 2 0.0001 0.0002 0 0 0
.about. 45 3 0.0035 0.0057 0.0005 0.0012 0 .about. 13 4 0.0012
0.0021 0.0002 0.0005 0 .about. 16 5 0.0013 0.0022 0.0003 0.0004 0
.about. 20 6 0.0014 0.0023 0.0003 0.0005 0 .about. 70
Comparative Example 2
[0065] A bearing steel shown in Table 5 was manufactured in the
same way as in Example 2. However, in the Comparative Example 2,
the following conditions are adopted: the REM is not added at the
final stage in the RH treatment step; the REM addition was made
using the same adding method as in Example 1, but the addition
amount of REM was either more than the upper limit of the inventive
REM amount or less than the lower limit; the air was not completely
sealed after the RH process was applied to increase the percentage
of REM-containing inclusions to be outside the proper range defined
by the invention; and Ti was added so that the content of Ti
exceeds the concentration range of the invention. The results of
inclusion size and acoustic/vibrational characteristics of the
bearing steel are shown in Table 5 as Comparative Samples, which
are inferior to the results of Example 2.
[0066] Thus, it is found that this invention relates to a steel
having finely dispersed REM based oxide and REM oxysulfide
inclusions, and can provide a steel having good fatigue life
characteristics and good acoustic/vibrational characteristics by
eliminating the harmful effect of oxide based inclusions.
* * * * *