[Title of the Document] Specification
[Title of the Invention] CASE HARDENING STEEL WITH EXCELLENT FATIGUE
PROPERTIES
[Technical Field]
The present invention relates to case hardening steel in which a non-metal
inclusion is finely dispersed, and which is with excellent fatigue properties, and more
particularly, to case hardening steel in which generation of a REM inclusion is controlled
for removing a bad effect of a harmful inclusion such as TiN and MnS, and which has
10 satisfactory fatigue properties.
[Background Art]
[0002]
Case hardening steel is used as a rolling bearing such as a "ball bearing" and a
"roller bearing" which are used in various kinds of industrial machines, vehicles, and the
15 like, and a rolling member such as a gear. In addition, recently, case hardening steel is
also used in bearings or sliding members in electronic equipment that drives a hard disk
used in a hard disk drive which is a magnetic recording medium, household electric
appliances or instruments, medical equipment, and the like.
[0003]
20 The case hardening steel that is used in the rolling member or the sliding member
is demanded to have excellent fatigue properties, and when inclusions are contained in the
case hardening steel, an increase in the number of inclusions and an increase in the size of
inclusions have an adverse effect on fatigue life. Accordingly, in order to improve the
fatigue properties, it is necessary to make the inclusions as small as possible and to
25 decrease the number thereof.
I P O DEkHL i Q 1 - 6 4 - R E l H 5 15:59
2
' / j
. ----
[0004]
As inclusions contained in the case hardening steel, inclusions made of an oxide
such as alumina (A1203), a sulfide such as manganese sulfide (MnS), and a nitride such as
titanium nitride (TiN) are known.
5 [0005]
An aluminum-based inclusion is generated when dissolved oxygen that remains
in a large amount in molten steel refined by a converter or a vacuum processing vessel is
bonded to A1 with a strong affinty with oxygen, and a ladle and the like are constructed
by an alumina-based refractory in many cases. Therefore, during deoxidation, alumina is
10 eluted as A1 in molten steel due to a reaction between molten steel and the refractory and
is re-oxidized to an alumina-based inclusion.
[0006]
Reduction and removal of the alumina-based inclusion are performed by a
combination of prevention of re-oxidation due to deaeration, slag reforming and the like,
15 and reduction of a mixed-in oxide-based inclusion caused by slag-cutting through the
application of a secondary refining apparatus such as a RH degasser and a powder blowing
apparatus.
[0007]
With regard to a method of manufacturing Al-killed steel that contains 0.005%
20 by mass or more of acid-soluble Al, an alloy composed of two or more kinds of elements
selected from Ca, Mg, and REM, and Al is added to the molten steel. Therefore, a
method of manufacturing alumina cluster free Al-killed steel through adjusting the amount
of A2O3 in a generated inclusion to a range of 30% to 85% by mass is knowi
[0008]
2 5 For example, as disclosed in Patent Document 1, a method, in which two or
I P O DELHH Of-64-2015 % 5 : 5 9
3
more kinds of elements selected from REM, Mg, and Ca are added to molten steel to form
-an inclusion with a low melting point so as to prevent generation of an alumina cluster, is
known. This method is effective at preventing sliver flaws, but in this method, it is
difficult to make the size of the incl1,ision small to a level that is demanded for the case
5 hardening steel. The reason is that inclusions with a low melting point are aggregated
and integrated. Therefore, the inclusion tends to be relatively coarsened.
[0009]
REM is an element that spheroidizes an inclusion and improves fatigue
properties. REM is added as necessary, but when 0.010 mass% of REM is added, the
10 number of inclusions increases. Therefore, a fatigue life that is one of the fatigue .
properties deteriorates. For example, as described in Patent Document 2, it is necessary
to set the amount of REM to 0.010% by mass or less. However, Patent Document 2 does
not disclose a mechanism for decreasing the fatigue life. Also, state of existence of
inclusions is not disclosed in Patent Document 2.
15 [OOlO]
When an inclusion made of a sulfide such as MnS is stretched by a process such
as forging, it may become a place where fatigue accumulates as a starting point of fracture,
and the fatigue properties of the steel deteriorates. Accordingly, to improve the fatigue
properties, it is necessary to control the number of the sulfide inclusions and the size
20 thereof.
[OOll]
REM is coupled to oxygen to form an oxide, and is coupled to sulfur to form a
sulfide. In addition, when the amount of REM is greater than the amount of REM that is
coupled to oxygen, a sulfide is generated and the size of the inclusions increases.
25 Therefore, REM has an adverse effect on the fatigue properties. Accordingly, thus it is
BEkHE 0 l - Q 4 - L 8 B S 15159
necessary to control the size of the inclusions.
[OO 1 21
It is necessary to prevent of coarse sulfides by controlling the amount of s u l k
with removing excessive amount of REM by balancing an additional amount of REM and
5 the amount of oxygen in order to control the size of the inclusions. However, this
technical idea is not disclosed in Patent Document 2 or the like.
[00 131
In addition, as a method of preventing generation of a sulfide, a method in
which Ca is added for desulfiu-ization is known. However, it is not effective at
10 preventing the generation of TiN.
[00 141
TIN is very hard, and crystalizes or precipitates in steel in a sharp shape and
TiN becomes a place where fatigue accumulates source as a starting point of fracture, and
has an adverse effect on the fatigue properties. For example, as disclosed in Patent
15 Document 3, when the amount of Ti exceeds 0.001% by mass, the fatigue properties
deteriorate. As a countermeasure thereof, it is important to adjust the amount of Ti to
0.001% by mass or less, but Ti is also contained in hot metal or slag. Therefore, it is
difficult to stably reduce Ti to a desired level.
[00 151
20 Accordingly, it is necessary to reduce the amount of Ti and N or to remove
them in a molten steel; however, this results in an increase in the costs of steel-making,
and is not preferable. In addition, an Al-Ca-0-based inclusion that is formed due to
addition of Ca has a problem in that it tends to be stretched, and tends to be a place where
fatigue accumulates as a starting point of fractures.
25 [Prior Art Document]
I F 6 EELHI 01-04-2015 15 159
[Patent Document]
[00 1 ti]
[Patent Document 11 Japanese Unexamined Patent Application, First
Publication No. H09-263820
5 [Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. H11-279695
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. 2004-277777
[Disclosure of the Invention]
10 [Problems to be Solved by the Invention]
[00 171
The invention has been made in consideration of the problems in the related art,
and an object thereof is to provide case hardening steel with excellent fatigue properties by
detoxifying TiN, an Al-0-based inclusion, Al-Ca-0-based inclusion, and MnS which tend
15 to be where fatigue accumulates as a starting point of fractures.
[Means for Solving the Problem]
[00 1 81
The present inventors have earnestly experimented and examined in order to
solve the above problems of prior art. As a result, they discovered the following.
20 [0019]
The present inventors have performed thorough experiments and have made
thorough investigations to solve the problems of the related art. As a result, the present
inventors have obtained the following findings by adjusting the amount of REM in the
steel and by adding an amount of Ca to the steel relative to the amount of REM, and by
25 controlling a deoxidation process.
I P Q DELMX Of - 8 4 - 8 8 1 5 151 5 9
6
(xl) When TiN is adhered to the REM-Al-0-S-based inclusion or the
EM-Ca-Al-0-S-based inclusion, it is possible Lo reduce h e~ iurrlbedr ensity of TiN
which does not adhere to any inclusion.
(x2) When S is fixed to the REM-Al-0-S-based inclusion or the REM-Ca-A1-0-
5 S-based inclusion, it is possible to suppress generation of coarse MnS.
(x3) When an Al-0-based inclusion is reformed into a REM-A1-0- S-based
inclusion, or an Al-Ca-0-based inclusion is reformed into a REM-Ca-Al-0- S-based
inclusion, it is possible to prevent stretching or coarsening of an oxide-based inclusion.
[0020]
10 The present invention has made on the basis of the above-described, the gist is
as follows.
[002 11
(1) A case hardening steel includes by mass%, C: 0.10 to 0.45%, Si: 0.01 to
0.80%, Mn: 0.1 to 1.5%, Cr: 0.1 to 2.0%, Al: 0.01 to 0.05%, REM: 0.0001 to 0.050% and
15 0: 0.0001 to 0.0030%, Ti: limited to less than 0.005%, N: limited to 0.015% or less, P:
limited to 0.03% or less and S: limited to 0.01% or less and the balance consists of Fe and
unavoidable impurities, includes a composite inclusion containing REM, 0 , S, and Al,
which includes a composite inclusion adhering to TiN.
[0022]
20 (2) A case hardening steel includes by mass%, C: 0.10 to 0.45%, Si: 0.01 to
0.80%, Mn: 0.1 to 1.5%, Cr: 0.1 to 2.0%, Al: 0.01 to 0.05%, Ca: 0.0005 to 0.0050%,
E M : 0.0001 to 0.050% and 0 : 0.0001 to 0.0030%, Ti: limited to less than 0.005%, N:
limited to 0.015% or less, P: limited to 0.03% or less and S: limited to 0.01% or less and
the balance consists of Fe and unavoidable impurities, includes a composite inclusion
25 containing E M , Ca, 0, S, and Al, which includes a composite inclusion adhering to TiN.
EPQ D E L H I O l - O 4 - h 8 H 5 f 5 : 59
[0023]
(3) The case hardening steel according to (1) or (2), fixher includes TiN in
which the sum of the number.density of TiN having a major diameter of 1 pm or more
which does not adhere to any inclusion, and the number density of MnS having a major
5 diameter of 10 pm or more, is 5 pieces/mm2 or less.
[0024]
(4) The case hardening steel according to any one of (1) to (3), further includes
one or more kinds of, by mass%, V: 0.05 to 0.70%, Mo: 0.05 to 1.00%, W: 0.05 to 1.00%,
Ni: 0.10 to 3.50%, Cu: 0.10 to 0.50%, Nb: 0.005 to less than 0.050%, and B: 0.0005 to
10 0.0050%.
[0025]
(5) A method of manufacturing a case hardening steel includes a first process of
deoxidation using Al, a second process of deoxidation using REM for 5 minutes or more,
a third process of ladle refining with vacuum degassing during ladle refining a molten
15 steel for case hardening steel which includes, by mass%, C: 0.10 to 0.45%, Si: 0.01 to
0.80%, Mn: 0.1 to 1.5%, Cr: 0.1 to 2.0%, Al: 0.01 to 0.05%, REM: 0.0001 to 0.050% and
0: 0.0001 to 0.0030%, Ti: limited to less than 0.005%, N: limited to 0.015% or less, P:
limited to 0.03% or less and S: limited to 0.01% or less and the balance consists of Fe and
unavoidable impurities, includes a composite inclusion containing REM, 0 , S, and Al,
20 which includes a composite inclusion adhering to TiN.
[0026]
(6) A method of manufacturing a case hardening steel includes a first process of
deoxidation using Al, a second process of deoxidation using REM for 5 minutes or more,
-- -
a third process of ladle refining with vacuum degassing by adding Ca during ladle refining
25 a molten steel for case hardening steel which includes, by mass%, C: 0.10 to 0.45%, Si:
DELHE 01-64-2015 15159
0.01 to 0.80%, Mn: 0.1 to 1.5%, Cr: 0.1 to 2.0%, Al: 0.01 to 0.05%, Ca: 0.0005 to
0.0050%, REM: 0.0005 to 0.050% and 0: 0.0001 to 0.0030%, Ti: limited to less than
0.005%, N: limited to 0.015% or less, P: limited to 0.03% or less and S: limited to 0.01%
or less and the balance consists of Fe and unavoidable impurities, includes a composite
5 inclusion containing REM, Ca, 0 , S, and Al, which includes a composite inclusion
adhering to TiN.
[0027]
(7) In the method of manufacturing a case hardening steel according to (5) or
(6), when the molten steel is casted in the mold, the circulation of the molten steel inside
10 the mold is performed at a flow rate of 0.1 m/m.inute in a horizontal direction.
[0028]
(8) In the method of manufacturing a case hardening steel according to any one
of (5) to (7), performing hot rolling or hot forging after holding the cast piece after casting
for 60 seconds or more at a temperature region of 1200 to 1250°C.
15 [0029]
(9) In the method of manufacturing a case hardening steel according to any one
of (5) to (8), the molten steel further includes one or more kinds of, by mass%, V: 0.05 to
0.70%, Mo: 0.05 to 1.00%, W: 0.05 to 1.00%, Ni: 0.10 to 3.50%, Cu: 0.10 to 0.50%, Nb:
0.005 to less than 0.050%, and B: 0.0005 to 0.0050%.
20 [Effects of the Invention]
[0030]
According to the invention, a number density of TiN which does not adhere to
any inclusion is reduced by adhering TiN to the REM-Al-0-S-based inclusion or the
REM-Ca-Al-0-S-based inclusion to form a composite inclusion, S is futed to suppress
25 generation of coarse MnS, and an Al-0-based inclusion is reformed into a
I P O DELH.1 Q B - Q 4 - 8 8 f 5 1 5 : 59
REM-Al-0-based inclusion, or an Al-Ca-0-based inclusion is reformed into a
REM-Ca-Al-0-based inclusion in order to prevent stretching or coarsening of the
oxide-based inclusion, accordingly, it is possible to provide case hardening steel with
excellent fatigue properties, particularly with excellent fatigue life.
5 [Brief Description of the Drawings]
[003 11
FIG. 1 is a view showing a form of an inclusion in which REM-Al-0-S-based
inclusion and TiN forms a composite.
FIG. 2 is a view showing a precipitation aspect of coarse MnS and TiN having
10 an angular shape.
[Embodiments of the Invention]
[0032]
The present invention will be described in detail as below.
[0033]
15 First, the reason why the chemical composition of the case hardening steel
according to the present invention (hereinafter, sometimes referred to as "the steel of the
present invention") is limited will be described. In addition, % relating to the chemical
composition represents mass%.
[0034]
20 Al: 0.01% to 0.05%
A1 is a deoxidizing element that reduces the total oxygen amount (T.O), and is an
element that can be used to adjust a grain size of steel. Therefore, it is necessary for the
steel to contain 0.01% or more of Al. However, when the amount of A1 exceeds 0.05%,
- --- -
an effect of adjustment of grain may saturate, moreover, it is impossible to change to
25 REM-oxide or REM-oxysulfide from A1203, an object of the present invention may not
=j=p(=j DELHI Of-Q4-2015 15: 5 9
10
I
achieve. Therefore, the upper limit is set to 0.05%.
When the amount of Al increases, A1203 is stabilized rather than REM-oxide or
REM-oxysulfide, it is thought that'it is impossible to change t.0 RRM-oxide or
5 REM-oxysulfide.
[0036]
REM: 0.0001% to 0.050%
REM is a strong desulfurizing and deoxidizing element, and plays a very
important role in the case hardening steel of the present invention. In addition, REM is a
10 general term of a total of 17 elements including 15 elements from lanthanum with an
atomic number of 57 to lutetium with an atomic number of 71, scandium with an atomic
number of 21, and yttrium with an atomic number of 39.
[003 71
First, REM reacts with A1203 in the steel to separate 0 of A1203, thereby
15 generating the EM-oxide. Then, in a case where Ca is added to the steel, REM reacts
with Ca, in addition, REM absorbs S in the steel, a composite inclusion ({Ca) means a
case that Ca is included, hereinafter, it may be described as [REM-{Ca)-A1-0-S])
including REM, {Ca), 0 , S and A1 is generated. In addition, in REM-{Ca)-A1-0-S, Ca
does not exist as as CaS (another phase), Ca is solid-soluted in REM-Ca-A1-0-S.
20 [0038]
Functions of REM in the steel of the present invention are as follows.
Generation of a coarse MnS is suppressed by stabilizing S due to a formation of composite
inclusion including Al, REM, {Ca), 0 and S. TiN is precipitated using the
REM-{Ca)-Al-0-S as a nucleus, thereby forming an approximately spherical composite
25 inclusion having a main structure of REM-{Ca)-A1-0-S-(TiN).
IPO ELH HI ~ & - ~ & - 2 O l1551 59
[0039]
As shown in FIG. 1, the spherical composite inclusion has a form to which TiN
adheres. In addition, it can be seen that the spherical composite inclusions have a
volume. much larger than that of TiN which seems to adhere to the inclusions. Then, a
5 solo amount of precipitation of TiN, which is hard and has a sharp angular shape, and AlN,
are reduced. Here, - (TiN) represents that TiN adheres to a surface of the
REM-{Ca) -Al-0-S.
[0040]
As shown in FIG. 1, a composite inclusion, which has a main structure of
10 REM-{Ca)-A1-0-S-(TiN), has a height of surface unevenness of 0.5 pm or less and an
approximately spherical shape. Therefore, this composite inclusion is a harmless
inclusion that does not become a starting point of fracture. In addition, the reason why
TiN precipitates to the surface of REM-{Ca)-A1-0-S is assumed to be as follows. A
crystal lattice structure of TiN is similar to a crystal lattice structure of REM-{Ca)-A1-0-S,
15 that is, TiN and REM- {Ca)-A1-0-S have a crystal structure matching property.
Hereinafter, composite inclusion like that may be referred to as a composite oxysulfide, or,
simply oxysulfide in some cases.
[0041]
In addition, Ti is not contained in the REM-{Ca)-A1-0-S in the steel of the
20 present invention as an oxide. This is considered to be because the amount of C in the
steel of the present invention is 0.10% to 0.45%, the oxygen level during deoxidation is
low, and the amount of a Ti oxide generated is very small. In addition, Ti is not
contained in the REM-{Ca)-Al-0-S as an oxide. Therefore, the crystal lattice structure
of the EM-{Ca)-Al=O-S and the crystal lattice structure of TiN become similar to each
25 other.
DEkHP 0 % - 0 4 - 2 0 1 5 15159
[0042]
In addition, REM has a function of preventing stretching or cornselling of an
oxide such as an Al-{Ca)-0 by reforming the Al- {Ca)-0 into the REM-{Ca) -A-0-S
which has a high melting point. In addition, in a case where Ca is added, Ca is added to
5 the steel after REM is contained. Therefore, such as CaS which is Ca-based sulfide or a
Ca-Mn-S-based inclusion does not be existed in a steel.
[0043]
To attain the effect of addition of REM, the steel must contain a constant amount
or more of REM based on an amount of A1203 that is the total oxygen amount (T.0
10 amount). In a case where a predetermined amount of REM is not contained in a steel, an
unreacted A1203 remains. Therefore, this case is not preferable. In addition, it is
necessary to be contained a constant amount or more of REM based on the amount of S.
In a case where a predetermined amount of REM is not contained in a steel, the fixation of
S is insufficient, and coarse MnS is generated. Therefore, this case is not preferable.
15 [0044]
In addition, it is necessary for the steel to contain a constant amount or more of
the REM-{Ca)-A1-0-S. In a case where the number of the REM-{Ca)-A1-0-S is small,
generation of a REM-{Ca)-Al-0-S-(TiN) becomes insufficient, and therefore, this case is
not preferable.
20 [0045]
From the above-described view point, when the amount of REM is less than
0.0001%, an effect of addition of REM is insufficient. Therefore, the lower limit of the
amount of REM is set to 0.0001%. The amount of REM is preferably set to 0.0003% or
more, more preferably set to 0.001 0% or more, and still more preferably set to 0.0020% or
25 more. However, when the amount of REM exceeds 0.050%, the cost increases, and
x$Q DEkME Ofl-Q4-ElfiSI15 I5159
clogging of a cast nozzle tends to occur. Therefore, it becomes difficult to manufacture a
steel. Accordingly, the upper limit of the mount of REM is set to 0.050%, is preferably
set to 0.035%, and is more preferably set to 0.020%.
[0046]
5 C: 0.10% to 0.45%
C is an element that secures hardness by carburizing and quenching and improves
a fatigue life. To secure strength and hardness by carburizing and quenching, it is
necessary for the steel to add 0.10% or more of C. However, when the amount of C
. exceeds 0.45%, hardness is excessively increased. Therefore, the tool service life during
10 cutting decreases and C becomes a cause of a quenching crack. Accordingly, the amount
of C is set to 0.10% to 0.45%, is preferably set to more than 0.15% and less than 0.40%,
and is more preferably set to 0.20% to 0.38%.
[0047]
Cr: 0.1% to 2.0%
15 Cr is an element that increases the hardenability and improves the fatigue life.
To attain this effect, it is necessary for the steel to contain 0.1% or more of Cr. However,
when the amount of Cr exceeds 2.0%, the effect that the hardenability is improved is
saturated and hardness of the base metal is increased. Therefore, the tool service life
during cutting decreases. In addition, Cr becomes a cause of quenching cracks.
20 Accordingly, the amount of Cr is set to 0.10% to 2.0%, and is preferably set,to 0.5% to
1.6%.
[0048]
Si: 0.01% to 0.80%
Si is an element that increases hardenability and improves fatigue life. To attain
25 this effect, it is necessary for the steel to contain 0.01% or more of Si. However, when
fPQ DEL[rk% 0 1 - 8 . 4 - E Q H 5 15: 59
the amount of Si exceeds 0.80%, the effect that the hardenability is improved is saturated
and hardness of a base metal is increased. Therefore, the tool service life during cutting
decreases. Accordingly, the amount of Si is set to 0.01% to 0.8% and is preferably
0.07% to 0.65%.
5 [0049]
Mn: 0.1% to 1.5%
Mn is an element that increases the strength by increasing the hardenability, and
improves fatigue life. To attain this effect, it is necessary for the steel to contain 0.1% or
more of Mn. However, when the amount of Mn exceeds 1.5%, the effect that the
10 hardenability is improved is saturated and hardness of the base metal is increased.
Therefore, a tool service life during cutting decreases and hardness of the base metal
increases, and Mn becomes a cause of a quenching crack. Accordingly, the amount of
Mn is set to 0.1% to 1.5%, and is preferably set to 0.2% to 1.15%.
[0050]
15 0 : 0.0001% to 0.0030%
0 is an element which is removed from steel by deoxidation, but 0 is necessary
to generate a composite inclusion having a main structure of REM-{Ca)-Al-0-S-(TiN).
To obtain an effect by 0 that is contained in steel, it is necessary for the steel to contain
0.0001% or more of 0 . However, when the amount of 0 exceeds 0.0030%, a large
20 amount of an oxide such as A1203 remains, and thereby the fatigue life decreases.
Accordingly, the upper limit of the amount of 0 is set to 0.0030%. In addition, the
amount of 0 is preferably 0.0003% to 0.0025%.
[005 11
Ca: 0.0005% to 0.0050%
2 5 Ca may be contained in steel as necessary for desulfurization and for
D L H L @ ~ - ~ 4 -f~5:~sea s
improvement of machinability. The steel contains Ca that is coupled to REM and 0 to
form a composite inclusion having a main structure of REM-Ca-Al-0-S-(TiN).
Therefore, it is preferable that the steel contain 0.0010% or more of Ca. However, when
the amount of Ca exceeds 0.0050%, a large amount of coarse CaO is generated.
5 Therefore, the fatigue life decreases. Accordingly, the upper limit thereof is set to
0.0050%. In addition, the upper limit of the amount of Ca is preferably 0.0045% or less.
[0052]
In the steel of the present invention, it is necessary to limit Ti, N, P, and S, which
are impurities, as follows.
10 [0053]
Ti: less than 0.005%
Ti is an impurity. When Ti is included in steel, inclusions such as Tic, TiN, and
TiS are generated. The inclusions deteriorate the fatigue properties. Accordingly, the
upper limit of the amount of Ti is set to less than 0.005%, and is preferably set to 0.0045%
. .
15 or less.
[0054]
Particularly, TiN is generated in an angular shape as shown in FIG. 2, and the
TiN having an angular shape becomes a starting point of fracture. Accordingly, TiN is
formed a composite with REM-Ca-Al-0-S. Therefore, it is possible to suppress an
20 independent generation of TiN which is harmful to the steel. The lower limit of the
amount of Ti includes 0%, but it is industrially difficult to realize 0%. At least, the
amount of Ti, which is unavoidably included in the steel, is 0.0003%.
[0055]
In addition, in the steel of the present invention, even though the steel contains
25 more than 0.001% of Ti that is upper limit of an amount of Ti in the related art, when the
steel contains less than 0.005% of Ti as a impurity. Therefore, the fatigue properties do
not deteriorate. Accordingly, it is possible to stably manufacture case hardening steel
with excellent fatigue properties.
[0056]
N: 0.015% or less
N is an impurity. When N is included in steel, N forms a nitride and deteriorates
the fatigue properties. In addition, ductility and toughness are deteriorated due to strain
aging. When an amount of N exceeds 0.01 5%, a h e 1 result, such as deterioration in
the fatigue properties, the ductility, and the toughness, becomes significant. Accordingly,
10 the upper limit of the amount of N is set to 0.015%. The amount of N is preferably
0.005% or less. The lower limit of the amount of N includes 0%, but it is industrially
difficult to realize 0%. At least, the amount of N, which is unavoidably included in the
steel, is 0.0005%.
[0057]
P: 0.03% or less
P is an impurity. When P is included in steel, P segregates at a grain boundary
and decreases the fatigue life. When the amount of P exceeds 0.03%, a decrease in the
fatigue life becomes significant. Accordingly, the upper limit of the amount of P is set to
0.03%. The amount of P is preferably set to 0.02% or less. The lower limit of the
20 amount of P includes 0%, but it is industrially difficult to realize 0%. At least, the
amount of P, which is unavoidably included in the steel, is 0.0005%.
[0058]
S: 0.01% or less
S is an-impurity: When S is included in steel, S forms a sulfide. When the
25 amount of S exceeds 0.01%, as shown in FIG. 2, S is coupled to Mn to form coarse MnS,
f.PO DELHT.. QP -04- ZBf 5 f 5 : 5 9
and decreases the fatigue life. Accordingly, the upper limit of the amount of S is set to
0.0 196. Thc amount of S is preferably set to 0.0085% or less. It is industrially difficult
to set the lower limit of the amount of S to 0%, and at least, the amount of S, which is
unavoidably included in the steel, is 0.0005%. Also, Si is an element which improves
5 machinability of the steel. In order to improve machinability of the steel, the amount of
S, which is necessary to add to the steel, is set to 0.005%.
[0059]
In addition to the above-described essential elements, the following elements may
be selectively contained.
10 [0060]
V: 0.05% to 0.70%
V is an element that is coupled to C and N in steel to form a carbide, a nitride, or
a carbonitride, and contributes to precipitation strengthening of steel. To stably attain
this effect, the steel contains 0.05% or more of V. The amount of V is preferably set to
15 0.1%. However, when the amount of V exceeds 0.70%, the effect by containing V
becomes saturated. Accordingly, the upper limit of the amount of V is set to 0.70%.
The amount of V is preferably 0.50% or less.
[006 11
Mo: 0.05% to 1.00%
20 Mo is an element that is coupled to C in steel to form a carbide and contributes to
an improvement in strength of steel due to precipitation strengthening. To stably attain
this effect, the steel contains 0.05% or more of Mo. The amount of Mo is preferably set
to 0.1%. However, when the amount of Mo exceeds 1.00%, the machinability of the
steel decreases-Accordingly, the upper limit of the amount of Mo is set to 1.00%. The
25 amount of Mo is preferably 0.75% or less.
HPO nELE$rP f . 3 1 - 0 4 - E 0 1 5 15:59
W: 0.05% to 1.00%
W is an element that forms a hard phase and contributes to an improvement in the
fatigue properties. To stably attain this effect, the steel conta.ins 0,05% or more of W,
5 The amount of W is preferably set to 0.1%. However, when the amount of W exceeds
1.00%, the machinability of the steel decreases. Accordingly, the upper limit of the
I amount of W is set to 1.00%. The amount of W is preferably 0.75% or less.
[0063]
Ni: 0.10% to 3.50%
10 Ni is an element that increases corrosion resistance and contributes to an
improvement in the fatigue life. To stably attain this effect, the steel contains 0.10% or
more of Ni. The amount of Ni is preferably set to 0.50%. However, when the amount
of Ni exceeds 3.50%, machinability of steel decreases. Accordingly, the upper limit of
the amount of Ni is set to 3.50%. The amount of Ni is preferably 3.00% or less.
15 [0064]
Cu: 0.10% to 0.50%
Cu is .an element that contributes to an improvement in the fatigue properties due
to a strengthening of the base metal. To stably attain this effect, the steel contains 0.10%
or more of Cu. The amount of Cu is preferably set to 0.20%. However, when the
20 amount of Cu exceeds 0.50%, cracks are generated during hot working. Accordingly, the
upper limit of the amount of Cu is set to 0.50%. The amount of Cu is preferably 0.35%
or less.
.-. -- - -
Nb: 0.005% to less than 0.050%
25 Nb is an element that contributes to an improvement in the fatigue properties due
I P Q DEkHZi: QI-O4-22Qs %f f 59
to a strengthening of the base metal. To stably attain this effect, the steel contains
0.005% or more of Nb. The anloult of Nb is preferably set to 0.010%. However, when
the amount of Nb is 0.050% or more, the effect by containing Nb becomes saturated.
Accordingly, the amount of Nb is set to less than 0.050%. The amount of Nb is
5 preferably 0.030% or less.
[0066]
B is an element that contributes to an improvement in the fatigue properties and
strength due to grain boundary strengthening. To stably attain this effect, the steel
10 contains 0.0005% or more of B. The amount of B is preferably set to 0.0010%.
However, when the amount of B exceeds 0.0050%, the effect by containing B becomes
saturated. Accordingly, the upper limit of the amount of B is set to 0.0050%. The
amount of B is preferably 0.0035% or less.
[0067]
15 In the steel of the present invention, S is fixed as the REM-{Ca)-A1-0-S, thus,
generation of MnS, which is stretched to 10 pm or more and hinders the fatigue properties,
is suppressed. In.addition, in the steel of the present invention, TiN adheres to the
REM- {Ca) -Al-0-S. Therefore, an approximately spherical composite inclusion having a
main structure of REM-{Ca) -4-0-S-(TiN) is formed.
20 [0068]
Here, for example, as shown in FIG. 1, the "approximately spherical shape"
represents a shape in which a maximum height of surface unevenness is 0.5 pm or less,
and a value obtained by dividing the major axis of the inclusion by the minor axis of the
inclusion, that is, an aspect ratio is 3 or less.
25 [0069]
I P O DELHP 61-84-2815 15:59
As shown in FIG. 2, hard TiN which does not adhere to any inclusion in steel, has
a major diameter of 1 pm or more and has an angular shape. Therefore, TiN which does
not adhere to any inclusion becomes a starting point of fracture. Therefore, TiN has an
adverse effect on the fatigue life. However, in the steel of the present invention, TiN
5 adheres to REM-{Ca)-A1-0-S and constitutes the approximately spherical composite
inclusion having a main structure of REM-{Ca)-A1-0-S-(TiN). Therefore, the
above-described adverse effect due to the shape of TiN is not generated.
[0070]
In addition, in the steel according of the present invention, to improve the fatigue
10 life, it is necessary to suppress the amount of "MnS having a major diameter of 10 ym or
more" and "TiN having a major diameter of 1 pm or more" generated, which have an
adverse effect on the fatigue life, to a total of 5 pieces/mm2 or less on the basis of a
number density. In addition, it is preferable that the amount of "MnS having a major
diameter of 10 pm or more" and "TiN having a major diameter of 1 pm or more"
15 generated be as small as possible. The amount thereof generated is preferably 4
pieces/mm2 or less, and is more preferably 3 pieces/mm2 or less.
[007 11
Precipitation number of TIN and MnS is measured by observing a cross-section
of a case hardening steel with a scanning electron microscope or optical microscope.
However, TiN having a major diameter of 0.1 pm or more was not detected. The reason
is considered that TiN has grown coarse by holding the slab after casting at temperature
region of 1250°C to 1200°C for 60 seconds or more in the manufacturing of the steel of
the present invention, as will be described later. TiN having a major diameter of 0.1 pm
or more was not detected, it is considered to be advantageous for that to the improvement
of the fatigue properties of the case hardening steel.
f P B DELHL E l l - 8 4 - Z B l f 15159
[0072]
A method of manufacturing the case hardening steel
In the method of manufacturing the case hardening steel of the present invention
(hereinafter, sometimes referred to as "a method of rnanufachlring of the present
5 invention"), a sequence of adding a deoxidizing agent is important during refining of
molten steel. In this manufacturing method of the present invention, first, deoxidation is
performed by using Al. Then, deoxidation is perfonned for 5 minutes or longer by using
REM, and then ladle refining including vacuum degassing is performed. Alternatively,
after deoxidation using REM, Ca is added as necessary, and then the ladle refining
10 including the vacuum degassing is performed.
[0073]
In this manufacturing method of the present invention, the deoxidizing agent is
added in the order of A1 and REM, or in the order of Al, REM, and Ca. As a result, the
REM-Al-0-based inclusion or the REM-Ca-Al-0-based inclusion (hereinafter, sometimes
15 referred to as "REM- {Ca} -A1-0") is generated. Accordingly, generation of the
Al-0-based inclusions or the Al-Ca-0-based inclusions, which are harmful, is prevented.
In addition, for the REM added, a misch metal and the like may be used, and an
aggregated misch metal is added to molten steel at the end of the refining. At this time, a
flux such as CaO-CaF2 is added to approximately perform desulfurization and refining of
20 an inclusion by Ca.
[0074]
Deoxidation with REM is performed for 5 minutes or longer. When a
deoxidation time is shorter than 5 minutes, and it is difficult to reduce the amount of the
- --
Al-{Ca}-0-based inclusions. In addition, when deoxidation is performed by using an
25 element other than A1 firstly, it causes a high cost in manufacturing. In addition, even in
fpa OELHI 6 1 - 0 4 - 2 8 1 5 1 5 : 5 9
a case where Ca is added to molten steel by adding a flux thereto, it is necessary to
perform deoxidation with REM for 5 minutes or longer.
[0075]
In a case where Ca is added as necessary for deoxidation, when Ca is nddcd
5 before adding REM to the steel, the number of Al-Ca-0-based inclusions which tend to be
stretched at a low melting point are generated. As a result, even when REM is added
after many Al-Ca-0-based inclusions are generated, it is difficult to reform a composition
of the inclusions. Accordingly, in a cqe where Ca is added, it is necessary to add Ca,
after adding REM to the steel.
10 [0076]
As described above, since S is fixed by the REM- {Ca) -Al-0-S-based inclusion,
generation of coarse MnS is suppressed. In addition, since the REM-(Ca) -Al-0-S-based
inclusion form a composite with TiN, the number of TiN, which precipitates in the
metallographic structure of the steel without adhering to any inclusion, decreases.
15 Accordingly, the fatigue properties of the steel are improved.
[0077]
However, particularly, in a case where the steel of the present invention is used in
a bearing, it is ideal that the amount of MnS, which is precipitated in the metallographic
structure of the steel, and the amount of TiN, which is precipitated in the metallographic
20 structure of the steel without adhering to any inclusion, are small, but it is not necessary
that no MnS or TiN at all are included. In addition, MnS independently crystallizes in
many cases using an oxide as a nucleus. Accordingly, an oxide may be found at the
inside such as the central portion of MnS in many cases. In this case, the MnS is existed
without adhering to thc REM-(Ca}-Al-0-S-based inclusion.
25 [0078]
To reliably improve the fatigue properties, which are demanded for the bearing,
the amount of MnS and TiN, which is precipitated in the metallographic structure of the
steel and is existed without adhering to the REM-Ca-Al-0-S-based inclusion, is needed to
satisfy the following conditions. Specifically, it is preferable for the sun of the number
5 density of MnS having a major diameter of 10 pm or more and the number density of TiN
having a major diameter of 1 pm or more, which is existed without adhering to the
REM-{Ca)-Al-0-S-based inclusion, to be set to a total of 5 pieces or less per observation
surface of 1 mm2.
10 When a repetitive stress is applied to the stretched MnS, the stretched MnS
having a major diameter of 10 pm or more becomes a starting point of fracture, and has an
adverse effect on the fatigue life. Accordingly, all MnS, which are stretched so as to
have a long diameter, that is, a major diameter of 10 pm or more, have an adverse effect
on the fatigue life. Therefore, the major diameter of MnS does not have the upper limit
15 thereof. In addition, TiN having a major diameter of 1 pm or more, which is existed
without adhering to the REM-{Ca)-Al-0-S-based inclusion, the angular shape thereof
becomes a starting point of fracture. TiN having the angular shape has an adverse effect
on the fatigue life similar to MnS. All TiN having a major diameter of 5 pm or more
have an adverse effect on fatigue life. Therefore, the upper limit of TiN is not limited.
20 [0080]
When the sum of the number of MnS and the number of TiN exceeds a total of 5
pieces per observation surface of 1 mm2, the fatigue properties of the bearing deteriorate.
Accordingly, it is preferable that the sum of the number of MnS and the number of TiN
per observation surface of 1 m2 be 5 pieces or less. More preferably, the sum of the
25 number of MnS and the number of TiN per observation surface of 1 mm2 is set to 4 pieces
DELWI 64-Q&-EQf5 I.5: 59
24
,-
or'less. Most preferably, the sum of the number of MnS and the number of TiN per
observation surface of 1 mm2 is set to 3 pieces or less. In addition, the lower limit of the
sum of the number of MnS and the number of TiN is more than 0.001 pieces per
ohsewation surface of 1 mm2.
5 [008 11
In addition, the bearing according to the embodiment of the present invention,
TiN having a major diameter of 5 pm or more was not detected by observing a
cross-section of the bearing with a scanning electron microscope. It is 'considered that
TiN has grown coarse on the oxysulfide and dispersed by heating and holding the slab
10 after casting at temperature region of 1250°C to 1200°C for 60 seconds or more and 60
minutes or less in the manufacturing of the steel of the present invention, as will be
described later. TiN having a major diameter of 5 pm or more was not detected.
Therefore, it is considered to be advantageous for the improvement of the fatigue
properties of the bearing.
15 [0082]
As described above, the amount of A1203 and the amount of Al-Ca-0-based
composite inclusion which are harmful elements having an adverse effect on the fatigue
properties of the bearing is reduced because the A1203 and the Al-{Ca)-0-based
inclusions are mainly reformed into REM-{Ca)-Al-0-S-based composite oxysulfide due
20 to an addition effect of REM. In addition, the precipitated amount of MnS that is
harmful inclusions is suppressed by an effect of desulfurization due to Ca and REM which
is included in REM- {Ca)-Al-0-S-based composite oxysulfide, in particular, by an effect
of desulfurization due to Ca.
2 5 In addition, TiN, which is a harmful inclusion, preferentially crystallizes or
I $ O DELHI @31-04-$OP5 15159
precipitates to a surface of the REM-{Ca)-Al-0-S-based composite oxysulfide.
'l'herefore, the precipitated amount of TIN that precipitates without adhering to the
REM-{Ca)-Al-0-S-based composite oxysulfide is reduced. As a result, it is possible to
obtain a bearing with excellent fatigue properties.
5 [0084]
The specific gravity of the REM-based inclusions in the molten steel which is
generated by deoxidation is 6 and is close to a specific gravity of 7 of steel. Therefore,
floating and separation are less likely to occur. In addition, when pouring molten steel
into a mold, the REM-based inclusions penetrate into a deep position of unsolidified layer
10 of a cast piece due to a downward flow. Therefore, the REM-based inclusions tend to
segregate at the central portion of the cast piece. When the REM-based inclusions
segregate at the central portion of the cast piece, the REM-based inclusion is deficient in a
surface layer portion of the cast piece. Therefore, it is difficult to generate a composite
inclusion by adhering TiN to REM-{Ca)-A1-0-S. Accordingly, a detoxifying effect of
15 TiN is weakened at a surface layer portion of a product.
[0085]
Accordingly, in this manufacturing method, to prevent'segregation of the REMbased
inclusions or adhesion to the nozzle, molten steel is circulated in the mold in a
horizontal direction to realize uniform dispersion of the inclusions. The circulation of the
20 molten steel inside the mold is preferably performed at a flow rate of 0.1 dminute or
faster so as to realize further uniform dispersion of the oxysulfide-based inclusions.
When the circulation speed inside the mold is slower than 0.1 dminute, the
oxysulfide-based inclusions are less likely to be uniformly dispersed. Accordingly, the
I
I life of nozzle and mold becomes shorter. Therefore, a production costs may be led to
25 increase.
I
I f P O DELHX QI-El4-2Ebl5 15; '59
[0086]
Accordingly, the molten steel may be stirred to realize uniform dispersion of the
oxysulfide-based inclusions. As stirring means, for example, an electromagnetic force
and the like map be applied,
[0087]
Next, the cast piece after casting is held at a temperature region of 1200°C to
1250°C for 60 seconds or more to obtain the above-described composite inclusion. This
temperature region is a temperature region at which a composite precipitation of TIN to
the REM-based inclusions is started.
I 10 [0088]
The inventors have found that it is necessary to hold the cast piece after casting at
a temperature region of 1200°C to 1250°C for 60 seconds or more in order to sufficiently
grow TiN at the surface of the REM-{Ca)-Al-0-S-based inclusion and to suppress a
generation of TiN which precipitates without adhering to an inclusion, through
15 experimentation.
[0089]
In addition, when the cast piece after casting is heated at a temperature region of
1200°C to 1250°C, TiN is solid-soluted generally. However, the steel of the present
invention includes 0.10% to 0.45% of C content which is higher than general steel for
20 bearing. Therefore, many cementite are existed in the steel of the present invention. A
solubility of N to cementite is 10,w. Therefore, it is considered that TiN grows by .
precipitating on the'surface of the REM-{Ca)-Al-0-S-based inclusion.
[0090]
The cast piece after casting is heated to a heating temperature and is held-2 a
25 temperature region of 1200°C to 1250°C for 60 seconds or more, and then hot-rolling or
1.PQ DELMI: 0.1 - 04--zQ'J 5 1 5 59'
hot-forging is performed to manufacture the case hardening steel. In addition, cutting
into a shape close to a final shape is performed, and carburizing and quenching are
performed to make the Vickers hardness of the surface be 700 Hv or more.
[009 11
5 This steel having high-hardness surface is preferable used for a rolling member or
a sliding member. In addition, it is common practice to complete a final product by
applying method such as grinding, which is capable of performing high-accuracy
processing using high-hardness tools, as necessary.
[Examples]
10 [0092]
Next, examples of the invention will be described, but conditions in the examples
are one of manufacturing method that is employed to confirm applicability and an effect of
the invention. Therefore, the invention is not limited to the examples. The invention
can employ various conditions as long as the object of the invention is achieved without
15 departing from the gist of the invention.
[0093]
(Example 1)
During the vacuum degassing in the ladle refining, refining was performed under
conditions shown in Table 1 by using metal Al, a misch metal, and a flux of
20 Ca0:CaF2=50:50 (mass ratio), and a Ca-Si alloy as necessary to obtain molten steel
having a chemical composition shown in Table 2. The molten steel was casted to a 300
mm square cast piece by using a continuous casting apparatus. At that time, circulation
inside a mold was performed by electromagnetic agitation under conditions shown in
Table 1, therebycasting a slab.
2 8
The cast piece, which was ladle-refined and casted under the conditions shown in
'l'able 1, was heated and held under conditions shown in Table 1, was hot-forged into a
cylindrical rod with + of 50 mm at 1 190°C, and was finally subjected to grinding into 4 of
10 mm. A plurality.of cylindrical rods with + of 10 mm, which were composed of a raw
5 material for test specimens, was prepared from the same kind of steel. One of the
cylindrical rods was provided for chemical composition analysis and inclusion analysis.
[0095]
In addition, with regard to the remaining final cylindrical rods with 4 of 10 mrn
among the plurality of cylindrical rods that were manufactured, for supply to a fatigue test
10 for confirmation of suitability for the rolling member or the sliding member which are
used after performing carburizing-quenching, and tempering. Also, after the specimen
was carburized and quenched, tempering was applied to the specimen at 180°C. With
regard to partial fatigue specimens which are carburized, quenched and tempered, samples
for measurement of Vickers hardness were collected from the load application portion.
15 [0096]
With regard to the above-described sample for chemical composition analysis and
inclusion analysis, a cross-section in a stretching direction thereof was mirror-polished,
and was processed with selective potentiostatic etching by an electrolytic dissolution
method (SPEED method). Then, measurement with a scanning electron microscope was
20 performed with respect to inclusions in steel, a composition of the inclusion was analyzed
using EDX, and inclusions in 10 mm2 of the sample were counted to measure a number
density. In addition, the fatigue life was measured with respect to the fatigue specimen
by applying a repetitive stress at the test condition of load amplitude being set to 1000
MPa by using an ultrasonic fatigue test, and the number of cycles at which 10% of the
25 evaluation sample was fractured was evaluated as fatigue properties Llo by using Weibull
IPO DELWL. ~ ~ - a d - r o15~ 1 sss
statistics.
Table 1 shows manufacturing conditions including steel refining conditions,
casting conditions, heating and holding conditions after casting in the examples.
5 Manufacturing conditions A, E, F, J, K, L, My N, and 0 pertain to manufacturing
conditions according to the present examples. Manufacturing conditions B, C, D, G, H, I,
P, Q and R are manufacturing conditions in a case where the manufacturing conditions are
not preferable and do not pertain to the present examples. The heating time and holding
time in the manufacturing condition B are out of the scope of the present invention. Also,
10 the holding temperature in the manufacturing conditions C and D is out of the scope of the
present invention.
[0098]
With regard to the manufacturing condition I, a deoxidizing time after adding
REM among ladle refining conditions was lower than the preferable range. In addition,
15 with regard to the manufacturing condition P, the vacuum degassing was performed before
charging REM, thus, a sequence of adding REM was not preferable in a deoxidizing
process. With regard to the manufacturing condition Q, Ca was charged into the steel as
a flux before charging REM. Therefore, a sequence of adding REM was not preferable
in a deoxidizing process. With regard to the manufacturing condition R, only A1 was
20 charged into the steel, but REM was not charged into the steel. The above-described
manufacturing conditions B, C, D, P, I, Q. and R are employed in Table 4, and the test
results is shown Table 5.
[0099]
.---
As shown in Table 1, the specimen that employed these manufacturing
25 conditions; the heating time and holding time in the manufacturing condition of B is lower
I P O DELMI OH-434-2015 15159
than the scope of the present invention. The holding temperatures in the manufacturing
conditions of C: and D are higher thm the scope of the presenl ill-velition. Also,
deoxidation time by REM in the manufacturing condition of I is lower than the scope of
the present invention. In addition, the sequence of deoxidation process in the
5 manufacturing condition of P and Q is different from the scope of the present invention;
These examples are referred as Steel No. 52,62,63, 56, 57 and 58 in Table 4 and 5. In
any steel number, a chemical composition is included in a range of the invention as
described in Table 4. However, as described in Table 5, the percentage of number
I fractions of REM-{Ca)-Al-0-S-based composite oxide, thus, oxysulfide to total inclusions
10 was lower than 50%, thus, the fatigue properties in a case of performing carburizing and
quenching were inferior to those of the present examples.
[O 1001
With regard to a steel number 55 in which REM was excessively added, as shown
in Table 4, the manufacturing condition A was intended to be employed, but a casting
15 nozzle was clogged. Therefore, casting was impossible. Therefore, the residue of steel
that remained in a casting nozzle or a tundish was collected and a chemical composition
was analyzed. The results are shown in Table 4as a composition of comparative steel.
As a result, with regard to the steel number 55, it was proved that the amount of REM was
greater than the range of the invention.
20 [OlOl]
As shown in Table 4, steel number 54 contained less REM than is contained in a
steel of the invention. In addition, steel number 85 employed the manufacturing
condition R, which did not charge REM into the steel. Therefore REM was not detected
-A- as a chemical composiTiEi in the steel. With regard to steel numbers 54 and 85, as
25 shown in Table 5, an effect by adding REM substantially disappeared, with regard to steel
I P O DELHX Q1-Q4-2Of 5 15: 59
number 54, Al-Ca-0-based precipitation increased, and with regard to steel number 85,
A1203 increased.
[O 1 021
Tn the steel numbers, the number fraction of an oxysulfidc with respect to tlie
5 total inclusions was less than 50%, and the number density of MnS having a major
diameter of 10 pm and TiN having a major diameter of 1 pm or more which did not be
adhered to an inclusion existed was excessive and exceeded the range of the invention.
Therefore, the fatigue properties were inferior to those of the present examples.
[0 1031
10 In steel numbers 60 and 61 shown in Table 4, the arhount of Ca was excessive,
and precipitation of A1-Ca-0 and the like increased in the respective steel numbers as
shown in Table 5. Therefore, the balance of inclusion generation collapsed, and the
number fraction of an oxysulfide with respect to the total inclusions was less than 50%.
[0 1041
15 With regard to flow rate of circulation of the molten steel inside the mold, as
shown in Table 1, the manufacturing conditions G and H employed a flow rate which is
slower than a flow rate of the present invention. However, with regard to steel numbers
1,2,3,4,49 and 50 which employed the above conditions, as shown in Table 3, the
chemical composition and the number fraction of an oxysulfide satisfy the scope of the
20 present invention. In addition, with regard to each steel number, as shown in Table 3, in
the case where carburizing and quenching are performed on the steel, fatigue properties
and hardness of the steel were superior to other examples. However, as a result of .
observing the presence area of inclusions, the density of inclusions was differed depending
on the observation area. In the case where a period of operation of nozzle and mold
25 decreases, it is preferable that flow rate of circulation of the molten steel inside the mold
I p Q D E L H I 01-64-2015 15: 5 9
be adjust in a range of the present invention.
[0 1051
In steel numbers 53 and 59, as shown in Table 4, the amount of Ti or S was over
the upper limits of the amount of Ti or S of the present invention. Therefore, a number
of TiN, MnS, and the like were generated. As a result, the balance of inclusion
generation collapsed. In addition, as shown in Table 5, the number fraction of an
oxysulfide with respect to the total inclusions was less than 50%. Therefore, the fatigue
properties were inferior to those of the present examples. In addition, in the steel number
69 which contained more P than is contained in a steel of the invention, as shown in Table
5, the number fraction of an oxysulfide with respect to total inclusions was secured.
However, P segregated at a grain boundary. heref fore, the fatigue properties were lower
than those of the present examples.
[0 1061
As shown in Table 4, steel number 65 contained more C, which essentially plays
15 a role in precipitation strengthening, than is contained in a steel of the invention. In .
addition, as shown in Table 4, steel number 67 contained more Si, which is necessary for
securing hardenability, than is contained in a steel of the invention. In addition, shown in
Table 4, steel number 69 contained more Mn, which is necessary for securing,
hardenability, than is contained in a steel of the invention. Accordingly, in the steel
20 numbers 65,67, and 69, as shown in Table 5, a quenching crack was generated during
carburizing and quenching. Therefore, evaluation other than a chemical composition
analysis was stopped.
[0 1071
As shown in Table 4, steel number 64 contains more C than is contained in the
25 steel of the invention. In addition, as shown in Table 4, steel number 66 contains less Si
, I P O DELHE 01-04-2015 1%: 5 8
than is contained in a steel of the invention. In addition, steel number 68 contained less
Mn than is contained in a steel of the invention. In these steel numbers, as shown in
Table 5, the number fraction of an oxysulfide with respect to the total inclusion was
secured. However, the fatigue properties and the 180°C tempering Vickers hardness
5 were inferior to those of the present examples.
[0 1 081
Cr is an element that increases hardenability. However, as shown in Table 4,
steel number 71 contained more Cr than is contained in a steel of the invention, and thus
as shown in Table 5, a quenching crack was generated. Therefore, evaluation with
10 respect to the steel number 71 was stopped. In addition, as shown in Table 4, steel
number 84 contained less Cr than is contained in a steel of the invention. Therefore,
hardenability was not secured, and, as shown in Table 5, in the steel number 84, the
fatigue properties and the 180°C tempering Vickers hardness were inferior to those of the
present examples.
15 [0109]
As shown in Table 4, steel number 72 contains less A1 than is contained in a steel
of the invention. On the other hand, as shown in Table 4, steel number 73 contained
more A1 than is contained in a steel of the invention. As shown in Table 4, steel number
74 contained more N than is contained in a steel of the invention. As shown in Table 4,
20 steel number 75 contained less 0 than is contained in a steel of the invention. On the
other hand, as shown in Table 4, steel number 76 contains more 0 than is contained in a
steel of the invention. Accordingly, in these steel numbers, as shown in Table 5, the
number fraction of an oxysulfide with respect to the total inclusion was less than 50%.
'l'herefbre, the fatigue property was inferior to those of the present examples.
25 [OllO]
In any of the steel numbers 77 or 80 which contained more V or Ni than is
contained in a steel of the invention, as shown in Table 4, the number fractions of an
oxysulfide with respect to the total inclusion was secured, but, in the case of containing V,
whose amount is over the upper limit of the amount of V of the present invention, the
5 fatigue properties were inferior to those of the present examples. In the case of
containing Ni, whose amount is over the upper limit of the amount of Ni of the present
invention, the 180°C tempering Vickers hardness was inferior to those of the present
examples.
[Ol 111
10 As shown in Table 4, with regard to any of a steel number 78,79, 81, 82 and 83,
which contained a greater amount of one of Mo, W, Cu, Nb, or B than is contained in a
steel of the invention, a crack occurred during processing into a cylindrical rod shape, and
thus evaluation other than chemical composition analysis was stopped.
[0112]
15 The present examples are shown as steel numbers 1 to 46 in Table 2 and Table 3.
From Table 3, it could be seen that in the present examples, the number fraction of an
oxysulfide with respect to all inclusions was secured to a value of 50% or more. In
addition, in the present examples subjected to carburizing and quenching, and 180°C
tempering, the fatigue properties Llo evaluated by a repetitive stress were 10' cycles or
20 more. In addition, it can be seen that in the present examples, the 180°C tempering
Vickers hardness is 600 Hv or more, and is suitable for a rolling member 6; a sliding
member.
[0113]
[Table 11
#I w 3 5
0 I Ladle refining conditions
I I
steel inside mold
(mlminute)
Sequence of
Manufacturing condition
temperature
("C)
Casting conditions Heating and holding conditions
Circulation flow rate of molten
Al deoxidation process,
REM deoxidation process,
Flux process, or
Heating
REM deoxidation
time (minute)
A
[Table 21
Holding temperature
("c)
I-lolding time
(second)
vacuum degassing process
Al-REM-Ca-degassing 6 0.2 1280

[Table 31
=%-
Sum of number density of TiN having major diameter of 1' 1 p n Steel grade I Manufacturing
Fatigue properties
Llo(x1 06)
(cycl'es)
15.6
180°C tempering
Vickcrs hardness
(Hv)
750.1
or more without adhering to inclusion and number dens:ty of
MnS having major diameter of 10pm or more (pieceslmm2)
2.15
State of oxysulfide
81 numbers I condition
Number fraction of an oxysulfide
with respect to total inclusions (%)
REM-Ca-AI-O-S-(TiN)
REM-Ca-AI-O-S-(TIN)
REM-Ca-AI-O-S-(TiN)
REM-Ca-AI-O-S-(TiN)
REM-Ca-AI-O-S-(TiN)
REM-Ca-AI-O-S-(TiN)
REM-AI-O-S-(TIN)
REM-AI-O-S-(TIN)
REM-AI-O-S-(TiN)
REM-AI-O-S-(TIN)
REM-AI-O-S-(TIN)
REM-AI-O-S-(TIN)
91.5
93.2
80.9
94.7
71.3
91.5
77.8
86.2
74.7
I I I I
75.2
89.3
85.9
REM-AI-O-S-(TIN)
REM-AI-O-S-(TiN)
REM-AI-O-S-(TiN)
REM-AI-O-S-(TIN)
REM-AI-O-S-(TIN)
REM-AI-O-S-(TiN)
REM-AI-O-S-(TiN)
REM-AI-O-S-(TiN)
REM-AI-O-S-(TiN)
REM-AI-O-S-(TIN)
0.03
0.07
0.07
0.07
0.07
0.04
0.06
0.04
0.09
I I I I
0.06
0.09
0.08
90.7
89.9
94.8
93.2
78.0
81.4
86.2
76.8
92.8
77.7
REM-AI-O-S-(TIN)
I I I I
18.0
17.1
17.5
18.4
16.7
16.4
17.1
17.6
19.3
REM-AI-O-S-(TIN)
17.5
16.6
19 1
0.02
0.04
0.04
0.06
73.4
752.0
756.1
752.1
751.1
719.9
71 8.2
7'19.6
737.1
n 5 . l
705. 1
n5.1
763.8
89.9
L
18 4
17 5
17.4
17.1
0.09
783.5
773.2
734.4
518.4
0.05
735.5
551.2
;56.8
i53.1
732.7
772.1
0.03 i 16.2
I
19.8
0.06
0.09
0.08
0.06
0.09
?32.1
2C.O
166
16.4
164
185
19.8
723.7

Manufacturing
condition
I Sum of number densitv of TIN I
Number fraction of an having major diameter of Ipm or
oxysulfide more without adhering to
State of oxysulfide
with respect to total inclusion and number density of LIO(X 1 06) Vickers hardness
inclusions (%) MnS having major diameter of (cycles)
I O p n or more (pieces/mm2)
REM-Ca-AI-0-S 33.4 8.03 4.9 758.4
Occurrence of nozzle clogging I I I I
MnS 42.0 7.10 8.3 660.0
CaO, AL-Ca-0 35.0 11.50 8.5 660.0
CaO I 33.0 I 7.93 I 5.6 1 722.3
Occurrence of quenching crack I I I I
Occurrence of quencing crack I I I I
Occurrence of quencing crack
U
0'
f=l'n
r
L
P-8
e.
P
!
q.
h.
I
p.' a
0"
hpr
bi
UI$
@
F
F
F
F
F
F
F
F
F
F
F
F
F
F
F
R
70
.
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
REM-Ca-AI-0-S-(TiN)
Occurrence of quencing crack
REM-Ca-0-S
AI2O1, AL-Ca-0
TIN, REM-Ca-AI-0-S-(TiN)
REM-Ca-AI-S
A1201, REM-Ca-AI-0-S-(TiN)
REM-Ca-AI-0-S-(TiN)
Occurrence of crack during processing
Occurrence of crack during processing
REM-Ca-AI-0-S-(TiN)
Occurrence of crack during processing
Occurrence of crack during processing
Occurrence of crack during processing
AL-Ca-0, REM-Ca-AI-0-S-(TiN)
A1203
72.6
36.0
37.0
33.0
35.0
36.0
72.6
72.6
68.2
15
0.10
10.50
9.40
8.56
-7.58
11.02
0.10
0.10
0.16
30
7.0
6.3
6.5
6.3
6.3
6.4
8.2
13.5
7.2
3.2
697.6
697.6
I
697.6
697.6
697.6
697.6
697.6
660.2
625.5
730
% \ F'
[Industrial' Applicability]
[0118]
As described above, according to the present invention, generation of a coarse
MnS is suppressed by stabilizing S and by reducing a number density of TiN which does
5 not be adhered to the composite inclusion, due to compound TiN with REM-Al-0-S-based
inclusion or REM-Ca-Al-0-S-based composite inclusion. In addition, the Al-0-based
inclusion is reformed into the REM-Al-0-S-based inclusion, or the Al-Ca-0-based
inclusion is reformed into the REM-Ca-Al-0-S-based inclusion. Therefore, it is possible
to prevent stretching or coarsening of an oxide-based inclusion, and it is possible to case
10 hardening steel with excellent fatigue properties. Accordingly, since the present
invention greatly contributes to a weight reduction of an automobile, industrial
applicability is high.
[Brief Description of the Reference Symbols]
[0119]
A: REM-Ca-A1-0-S
B: TiN
C: PRO-EUTECTOID CEMENTITE
D: MnS
[~ocurninTt ype] CLAIMS
[Clai~i1i 1
A case hardening steel comprising, by mass%;
C: 0.1 0 to 0.45%,
Si: 0.01 to 0.80%,
Mn: 0.1 to 1.5%,
Cr: 0.1 to 2.0%,
Al: 0.01 to 0.05%,
REM: 0.0001 to 0.050% and
0 : 0.0001 to 0.0030%,
Ti: limited to less than 0.005%,
N: limited to 0.015% or less,
P: limited to 0.03% or less and
S: limited to 0.0 1 % or less and
the balance consists of Fe and unavoidable impurities,
wherein the case hardening steel includes a composite inclusion containing
REM, 0 , S, and Al, which includes a composite inclusion to which TiN is adhered.
[Claim 21
A case hardening steel comprising, by mass%;
C: 0.10 to 0.45%,
Si: 0.01 to 0.80%,
Mn: 0.1 to 1.5%,
Cr: 0.1 to 2.0%,
A170701^t6 0:05%,
25 Ca: 0.0005 to 0.0050%,
I
REM: 0.0001 to 0.050% and
0 : 0.0001 to 0.0030%,
Ti: limited to less than 0.005%,
N: limited to 0.01 5% or less,
P: limited to 0.03% or less and
S: limited to 0.01% or less and
the balance consists of Fe and unavoidable impurities,
wherein the case hardening steel includes a composite inclusion containing
REM, Ca, 0 , S, and Al, which includes a composite inclusion to which TiN is adhered.
10 [Claim 31
The case hardening steel according to Claim 1 or 2, further comprising; TiN in
which the sum of the number density of TiN having a major diameter of 1 pm or more
which does not adhere the inclusion, and the number density of MnS having a major
diameter of 10 pm or more, is 5 pieces/rnrn2 or less.
15 [Claim 41
The case hardening steel according to any one of Claims 1 to 3, further
comprising; one or more kinds of, by mass%, V: 0.05 to 0.70%, Mo: 0.05 to 1.00%, W:
0.05 to 1.00%, Ni: 0.10 to 3.50%, Cu: 0.10 to 0.50%, Nb: 0.005 to less than 0.050%, and
B: 0.0005 to 0.0050%.
20 [Claim 51
A method of manufacturing a case hardening steel comprising;
a first process of deoxidation using Al,
a second process of deoxidation using REM for 5 minutes or more,
a third process of ladle refining with vacuum degassing during ladle refining a
25 molten steel for a case hardening steel which includes, by mass%, C: 0.10 to 0.45%, Si:
ZPO DEBHf 01-64-2015 15159
0.01 to 0.80%, Mn: 0.1 to 1.5%, Cr: 0.1 to 2.0%, Al: 0.01 to 0.05%, REM: 0.0001 to
0.050% and 0 : 0.0001 to 0.0030'%, Ti: limited to less than 0.005%, N: limited to C).OlS%
or less, P: limited to 0.03% or less and S: limited to 0.01% or less and the balance consists
of Fe and unavoidable impurities, wherein the case hardening steel includes a composite
5 inclusion containing REM, 0, S, and Al, which includes a composite inclusion to which
TiN is adhered.
[Claim 61
A method of manufacturing a case hardening steel comprising;
a first process of deoxidation using Al,
a second process of deoxidation using REM for 5 minutes or more,
a third process of ladle refining with vacuum degassing by adding Ca during
ladle refining a molten steel for a case hardening steel which includes, by mass%, C: 0.10
to 0.45%, Si: 0.01 to 0.80%, Mn: 0.1 to 1.5%, Cr: 0.1 to 2.0%, Cr: 0.1 to 2.0%, Al: 0.01 to
0.05%, Ca: 0.0005 to 0.0050%, REM: 0.0001 to 0.050% and 0: 0.0001 to 0.0030%, Ti:
15 limited to less than 0.005%, N: limited to 0.015% or less, P: limited to 0.03% or less and
S: limited to 0.01% or less and the balance consists of Fe and unavoidable impurities,
wherein the case hardening steel includes a composite inclusion containing REM, Ca, 0 , S,
and Al, which includes a composite inclusion to which TiN is adhered.
[Claim 71
20 The method of manufacturing the case hardening steel according to Claim 5 or 6,
when the molten steel is casted in the mold, a circulation of a molten steel inside the mold
is performed at a flow rate of 0.1 mlminute in a horizontal direction.
[Claim 81
The method of manufacturing the case hardening steel according to any one of
25 Claims 5 to 7, performing hot rolling or hot forging after holding the cast piece after
IPO DELHE 01 -64- 281 5 1 5 - 5 9
4 48
1 \/
casting for 60 seconds or more at a temperature region of 1200°C to 1250°C.
[Claim 91
! The method of manufacturing a case hardening steel according to any . one of
I Claims 5 to 8, the molten steel further includes one or more kinds of, by mass%, V: 0.05 to 0.70%, Mo: 0.05 to 1.00%, W: 0.05 to 1.005 to 1.00%, Ni: 0.10 to 3.5%, Cu: 0.10 to 0.50%,
Nb: 0.005 to less than 0.050%, and B: 0.0005 to 0.0050%.