[Document Type] Specification
[Title of the Invention] STEEL FOR INDUCTION HARDENING WITH
EXCELLENT FATIGUE PROPERTIES
[Technical Field of the Inventiotl]
[OOOl]
The present inve~ltionr elates to steel for induction hardening in which a nonmetal
inclusion is finely dispersed, and which is with excelle~lfta tigue properties, and
more particularlj: to steel for induction hardening in \vhich generation of a REM
inclusion is controlled for removing a bad effect of a har~iifi~inlc lusion such as TiN
and MnS, and \vIlicli has satisfactory fatigue properties.
Priority is clainled on Japanese Patent ApplicationNo. 2012-232141, filed on
October 19,2012, the conlent of which is ir~corporatedh erein by referelice.
[Related Art]
[0002]
Steel for induction harde~~ii~s iugse d as a rolling bearing such as a "ball
bearirig" and a "roller bearing" wliich are used in various kinds of iiidlistrial machines,
vehicles, and the like, and a rolling member such as a gear. hi addition, receatly, steel
for induction hardening is also used in bearings or sliding members in electro~lic
equipnient that drives a hard disk used in a hard disk drive n~liichis a magnetic
recording medium, household electric appliances or instruments, n~edicale quipment,
and the like.
[0003]
The steel for induction hardening that is used in the rolling member or the
sliding member is demanded to have excellent fatigue properties. However, when
inclusions are contained in the steel for induction hardening, an increase in the number
of inclusions and an increase in the size of i~lclusio~hlas ve an adverse effect 011 fatigue
life. Accordingly, in order to improve the fatigue properties, it is necessary to make
the inclusions as small as possible and to decrease the number thereof
[0004]
As inclusions contained in the steel for inductio~hl ardenitlg, itlclusions made
of an oxide such as alumitla (A1203), a sulfide SLIC~aI s lnallganese sulfide (MnS), and a
nitride such as tita~liu~nlilt ride (TiN) are knowtl.
[OOOS]
An alaluali~lum-basedi nclusion 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 affinity with oxygen. In addition, a
ladle and the like are constntcted by an alumina-based rekactory it1 many cases.
Accordingly, during deoxidation, alumina is eluted as A1 in molten steel due to a
reaction between nlolten steel and the refractory, and is re-oxidized to an aluminabased
inclusion.
[0006]
Accorditlgly, reduction and ret~~ovoafl the alumitla-based inclusion are
performed by a combination of (1) prevention of re-oxidation due to deaeration, slag
reforming and the like, and (2) reduction of a mixed-in oxide-based i~~clusiocnau sed
by slag-cutting tllrough the application of a secorldary refining apparatus such as a RH
degasser and a powder blowing apparatus.
[0007]
In addition, with regard to a method of maaofacturing Al-killed steel that
contains 0.005% by mass or more of acid-soluble Al, an alloy conlposed of two or
111ore kinds of elements selected from Ca, Mg, and REM, and AI is added to the molten
steel. Therefore, a niethod of manufacturi~~algu mina cluster free Al-killed steel
throogh adjusting the amount of All03 in a generated inclusion to a range of 30% to
85% by mass is known.
[0008]
For example, as disclosed in Patent Document 1, a method, in which two or
nlore kinds of elen~entss elected fro111 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. However, in
this method, it is difficult to make the size of the inclusion snlall to a level that is
denlanded for the steel for induction hardening. The reason is that inclusions wit11 a
low melting point are aggregated and integrated, and tllus the inclusion tends to be
relatively coarsened.
[0009]
REM is an element that spheroidizes an inclusion and i~uprovesf atigue
properties. REM is added to nlolten steel as necessary, but when REM is excessively
added, the nunber of inclusions increases, and thus a fatigue life that is one of the
fatigue properties deteriorates. For example, as described in Patent Document 2, it is
also known that it is necessary to set the amount of REM to 0.010% by mass or less in
order to not decrease the fatigue life. I-Io\vevel; Patent Document 2 does not disclose
a tnechanisnl for decreasing the fatigue life and a state that the inclosion exists.
[OOIO]
In addition, \when a1 inclusion made of a sulfide such as MnS is stretched by a
process such as forging, it nlay become a place wvllere fatigue accumulates as a starting
point of fracture, and deteriorate the fatigue properties of the steel. Accordingly, to
improve the fatigue properties, it is necessary to control the nu~nbero f the sulfide
inclusions and the size thereof.
[OOll]
011 tlie other hand, REM is coupled to oxygeli to form an oxide, and is
coupled to sulfur to fomi 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, and thus REM has at1 adverse effect on the fatigue
properties. To prevent this adverse effect, it is necessary to control tlie size of tlie
it~clusions.
[0012]
To co~itrolth e size of the inclusions, it is necessary to add REM in an amount
appropriate for the amount of oxygeii iii tlie steel. Before adding an appropriate
amount of REM to tlie steel, it is preferable to reduce the auiount of oxygen present in
the steel. In addition, sulfide inclusio~isin the steel are one type of inclusion that
decreases fatigue life of tlie steel for i~iductiol~iair deniug, and thus it is preferable to
prevent tlie generation of coarse sulfides, and ill particular MIIS. For this reason, it is
preferable that the anloullt of sulfur in the steel be reduced, and then that an
appropriate aoiount of REM be added to tlie steel for tlie amount of sulfur preseut in
order to generate an oxysulfide, thus, generation of MnS can be suppressed. That is,
it is preferable to add an aliiount of REM appropriate for the aliiounts of both oxygen
and sulfur. However, this technical idea is not disclosed in Patent Document 2 or the
like.
[0013]
In addition, as a method of preventing generation of a sulfide, a method in
\vliicli Ca is added for desulfitrization is known. However, although the addition of
Ca is effective for preventing the generation of sulfide, it is not effective at preveriti~ig
the generation of TiN, whicli is a nitride.
[0014]
As shown in FIG. 2, TiN is very hard, and crystalizes or precipitates in steel in
a sharp shape. According to this, TiN becomes a place where fatigue accutnulates
source as a starting point of fracture, and has an adverse effect on the fatigue properties.
For example, as disclosed in Patent Document 3, ~vhenth e amount of Ti exceeds
0.001% by mass, the fatigue properties deteriorate. As a countermeasure thereof, it is
in~portanto adjust the anlount of Ti to 0.001% by Illass or less, but Ti is also contained
in 11ot metal or slag, and tllus it is difficult to avoid mixing-in of Ti as an impurity.
Accordingly, it is difficult to stably reduce Ti to a desired level.
[0015]
Accordingly, it is necessary to reduce the amount of Ti and N or to remove
them in a niolten steel. However, this results in an increase in the costs of steelmaking,
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 ~vl~efraeti gue accumulates as a starting point of fractures.
[Prior Art Document]
[Patent Docunient]
[0016]
[Patent Docitment 11 Japanese Unexa~nined Patent Application, First
Publication No. H09-263820
[Patent Document 21 Japanese Unexanlined Patent Application, First
Publication No. HI 1-279695
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. 2004-277777
[Disclosure of the Inve~ltio~l]
[Problems to be Solved by the Invetition]
[OOl I]
The i~lve~ltiohtals been made in consideration of the proble~llsin the related
art, and an object thereof is to provide steel for induction hardening with excellent
fatigue properties by detoxifying TiN, an Al-0-based inclusion, Al-Ca-0-based
inclusion, and MIIS wvhich tend to be where fatigue accumolates as a starting point of
fractures.
[Means for Solving the Problem]
[0018]
The gist of tlie invention is as follows.
[OOl 91
(I) According to a first aspect of the invention, a steel for induction hardening
i~lcludesa s a chemical composition, by mass%: C: 0.45% to 0.85%, Si: 0.01% to
0.80%, Mt1: 0.1% to 1.5%,Al: 0.01% to 0.05%, REM: 0.0001% to 0.050%, 0 :
0.0001% to 0.0030%, Ti: less than 0.005%, N: 0.015% or less, P: 0.03% or less, S:
0.01% or less, and the balance consists of Fe and itnpurities. The steel for induction
hardening includes a composite inclusio~iw hich is at1 inclosion containing REM, 0 , S,
and N, to which TiN is adhered. The sum of the number density of TiN having a
maximun~ diameter of 1 Fun or more which independently exists without adhesion to
the inclusion, and tlie number density of MIIS having a maximum diameter of 10 pm or
more, is 5 pieces/mm2 or less.
[0020]
(2) According to a second aspect of the invention, a steel for itidoction
hardening inclndes as a che~llicalc omposition, by mass%; C: 0.45% to 0.85%, Si:
0.01% to 0.80%, MI]: 0.1% to l.S%,Al: 0.01% to 0.05%, Ca: 0.0050% or less, REM:
0.0001% to 0.050%, 0: 0.0001% to 0.0030%, 'I'i: less than 0.005%, N: 0.015% or less,
P: 0.03% or less, S: 0.01% or less, and the balance consists of Fe and impurities. The
steel for induction hardening includes a composite inclusion w11icI1 is an inclusio~l
containing REM, Ca, 0 , S, and Al, to which TiN is adhered. The sum of the nntnber
density of TIN having a maximu~nd iameter of 1 {on or more which independently
exists \vithout adhesion to the incli~sion,a nd the iiomber density of MtlS having a
maximum diameter of 10 Lum or more, is 5 pieces/n~mo~r l ess.
[0021]
(3) The steel for induction hardening according to (1) or (2) fi~rtherin cludes
as the che~nicalc omposition, one or Inore kinds of elenients selected fro111 the group
consisting of, by mass%; Cr: 2.0% or less, V: 0.70% or less, Mo: 1.00% or less, W:
1.00% or less, Ni: 3.50% or less, Cu: 0.50% or less, Nb: less than 0.050%, and B:
0.0050% or less.
[Effects of the Invention]
[0022]
According to the aspects of the invention, an Al-0-based inclosion is
refonned into a REM-Al-0-based inclusion, or an AI-Ca-0-based inclusion is
reformed into a REM-Ca-Al-0-based inclusion, and thus it is possible to prevent
stretching or coarsening of the oxide-based inclusion. In addition, S is fixed to tlte
REM-Al-0-based inclusion or the REM-Ca-Al-0-based inclusion to for111 a REM-AI-
0-S-based inclusio~or~ a REM-Ca-Al-0-S-based inclusion, and thus it is possible to
suppress generation of coarse M S . In addition, TiN is adhered to the REM-AI-O-Sbased
inclusion or the REM-Ca-Al-0-S-based inclusion to foml a co~npositein clusion,
thereby reducing a number density of TiN that independently exists without adhesion
to the inclusion. Accordingly, it is possible to provide steel for induction hardening
with excellent fatigue properties, particularly with excelle~f~att igue life.
[Brief Description of tlie Dra~vings]
[0023]
FIG. 1 is a view sl~owinga form of an inclusion (composite inclusion) in
which REM-Al-0-S-based inclusion and TiN fonlls a composite.
FIG. 2 is a view showing a generation aspect of coarse MnS and TiN having
an angt~lars hape.
FIG. 3 is a view showing tlie shape of a fatigue specimen.
[Embodinlents of the hlventioti]
[0024]
'Flie present inventors have perfornied a thorough experiment and liave made
a thorough irivestigation to solve the problems in the related art. As a result, tlie
present inventors liave obtained the following findings by adjusting tlie amount of
REM in tlie steel and by adding tlie aiiount of Ca to tlie steel correspond to the amount
of REM, arid by coritrolling a deoxidation process.
(1) When an Al-0-based inclusion, which is an oxide, is refornied into a
REM-Al-0-based inclusion, or an Al-Ca-0-based inclusion, which is an oxide, is
refornied into a REM-Ca-Al-0-based inclusion, it is possible to prevent stretching or
coarsening of an oxide-based inclusion.
(2) When S is fixed to the REM-Al-0-based inclusion that is an oxide or the
REM-Ca-Al-0-based inclusion tliat is an oxide for being reformed into a REM-AI-OS-
based inclusion that is an oxysulfide or a REM-Ca-Al-0-S-based inclusion that is an
oxysulfide, it is possible to suppress generation of coarse MnS.
(3) When TiN is adhered to the REM-Al-0-S-based inclosion tliat is an
oxysulfide or the REM-Ca-Al-0-S-based inclusion that is an oxysulfide, it is possible
to reduce the number density of single TiN that independently exists without adhesion.
[0025]
Hereinafter, steel for induction hardening and a method of manufacturing the
same according to an en~bodimenot f the invention made on the basis of the abovedescribed
findings will be described in detail.
[0026]
First, a cl~emicacl o~npositiono f the steel for induction hardening according to
this embodi~nea~i~dt t he reason wvlly the che~nicacl o~npositionis limited will be
described. In addition, %relating to the amount of each of the following elements
represents mass%.
[0027]
C: 0.45% to 0.85%
C is an element that secures hardness by induction hardening and improves a
fatigue life. To secure strength and hardness by induction hardening, it is necessaly
for the steel to contain 0.45% or more of C. However, \w~hen the amount of C exceeds
0.85%, hardness is excessively increased, and thus the tool service life during cutting
decreases and C becomes a cause of a quenching crack. Accordingly, the amount of
C is set to 0.45% to 0.85%, is preferably set to more than 0.45% and 0.85% or less, and
is more preferably set to 0.50% to 0.80%.
[0028]
Si: 0.01% to 0.80%
Si is an element that increases hardenability and improves fatigue life. To
attain this effect, it is necessary for the steel to contain 0.01% or more of Si. I-Iowever,
\vhen the amount of Si exceeds 0.80%, the effect that the hardenability is improved is
saturated and l~ardnesso f a base metal is increased. Therefore, the tool service life
during cutting decreases. Accordingly, the amount of Si is set to 0.01% to 0.80%, and
is preferably 0.07% to 0.65%.
[0029]
MI]: 0.1% to 1.5%
Mn is an element that increases the strength by increasing the hardenability,
and improves fatigue life. To attaiu this effect, it is necessary for the steel to contain
0.1% or more of MI]. However, wi~l~ethne amount of Mn exceeds 1.5%, the effect that
the l~ardenabilityis in~l~roveisd s aturated and hardness of the base metal is increased.
Therefore, a tool service life during cutting decreases. In addition, when the amount
of Mn exceeds IS%, hardness of the base metal increases, and thus MI] becon~esa
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%.
[0030]
Al: 0.01% to 0.05%
A1 is a deoxidizing element that reduces the total oxygen a~lloun(t 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 ofAl.
[003 11
However, when the amount of A1 is large, ,41203 becomes more stable than the
REM-Al-0-based inclusions or the REM-Ca-Al-0-based inclusions which are oxidebased
inclusions, or the REM-Al-0-S-based inclusio~ol r the REM-Ca-Al-0-S-based
inclusion which are oxysulfide-based inclusions, and thus it is considered that it is
difficult to reform A1203 into REM-Al-0-based inclusions or REM-Ca-Al-0-based
inclosions which are oxide-based inclusion, or into the REM-Al-0-S-based inclusions
or REM-Ca-Al-0-S-based inclusions xx~hich are oxysulfide-based inclusions.
Accordingly, the amount ofAl is set to 0.05% or less.
[0032]
REM: 0.0001% to 0.050%
REM is a strong desulfurizing and deoxidizing element, and plays a very
important role in the steel for induction hardening according to this emboditnent.
Here, REM is a general tenn of a total of 17 elements including 15 elements from
lanthanum wit11 an atomic nuniber of 57 to lutetiu~nw ith an atomic number of 71,
scandiu~nx vitli a1 atonlic number of 21, and yttrium with an atomic number of 39.
[0033]
First, REM reacts \vithAl203 in the steel to separate 0 of A1203, thereby
generating the REM-Al-0-based inclusion that is an oxide-based inclusion. Then, in
a case where Ca is added to the steel, REM reacts with Ca to generate the REM-Ca-AI-
0-based inclusions that is an oxide-based inclusion. In addition, the above-described
oxide attracts S in the steel to generate REM-Al-0-S-based inclusions that is an
oxysulfide-based inclusion containing REM, 0, S, and Al. In addition, in a case
where an oxide containing Ca exists, a REM-Ca-Al-0-S-based inclusion that is an
oxysulfide-based inclusion containing REM, Ca, 0, S, and Al is generated. hl
addition, in the REM-Ca-Al-0-S-based inclusions that is an oxysulfide-based inclusion,
Ca does not exist as CaS independently from the oxysulfide, but forms a solid solution
in the REM-Ca-Al-0-S-based inclusions.
[0034]
Functions of REM in the steel for induction hardening according to this
embodiment are as follows. REM reforn~s,4 1203 into REM-AI-0-based inclusions
containing REM, 0, and Al, thereby preventing coarsening of at1 oxide. In a case
xvl~ere Ca is added to the steel, REM reforms A1203 into the REM-Ca-Al-0-based
inclusions, thereby preventing coarsening of an oxide. In addition, REM fixes S
through formation of REM-Al-0-S-based inclusions containing Al, REM, 0, and S, or
REM-Ca-Al-0-S-based inclusions containing Al, REM, Ca, 0, and S, and suppresses
generation of coarse MIIS. In addition, REM generates TiN using the REM-AI-O-Sbased
inclusions or the REM-Ca-Al-0-S-based inclusions as a nucleus, thereby
forming an approximately spherical composite inclusion having a main structure of
REM-Al-0-S-(TiN) or REM-Ca-AI-0-S-(TiN).
[0035]
For example, as sho\vn in FIG. 1, the approximately spherical composite
inclusion has a form to \vl~iclTi iN adheres. 111 addition, it can be seen that the
approxinlatelp spherical composite inclusio~h~asv e a volume much larger than that of
TiN. in addition, an amonnt of precipitation of TiN, which independently exists
without adhesion to the REM-Al-0-S-based inclusions or the REM:Ca-Al-0-S-based
inclusions and which is hard and has a sharp angular shape, is reduced. Here, (TiN)
represents that TIN adheres to a surface of the REM-Al-0-S-based inclusions or the
REM-Ca-Al-0-S-based inclusions and fonns a composite.
[0036]
For example, as shown in FIG. 1, a composite inclusion, which has a maill
structnre of REM-AI-0-S-(TiN) or REM-Ca-AI-0-S-(TiN), has a height of surface
unevenness of 0.5 pm or less and an approximately splierical shape. Accordingly,
this co~ilpositein clusion is a l~ar~nleisnsc lusion that does not become a starting point
of fracture. 111 addition, the reason why TiN precipitates to the surface of REM-AI-OS
or REM-Ca-AI-0-S is assnmed to be as follows. Acrystal lattice structure of TiN
is similar to a crystal lattice structure of REM-A1-0-S or REM-Ca-AI-0-S, that is, TIN
and REM-Al-0-S or REM-Ca-AI-0-S have a crystal struchlre matching properly.
I-Iereinafter, REM-AI-0-S-(TiN) or REM-Ca-AI-0-S-(TiN) may be referred to as a
con~positein clusion, and the REM-Al-0-S-based inclusion or the REM-Ca-AI-0-Sbased
inclusion may be referred to as an oxysulfide-based inclusion in some cases.
[0037]
In addition, Ti is not contained in the REM-Al-0-S-based inclusions or in the
REM-Ca-Al-0-S-based inclusions of the steel for inductiorl hardening according to
this embodi~ne~alst an oxide. This is coasidered to be because the amount of C in the
steel for i~lductioh~al rdening accordirlg to this elllbodillle~lti s 0.45% to 0.85% aud
high, the oxygeu level during deoxidation is low, and the alnolltlt of a Ti oxide
generated is veqr small. In addition, Ti is not co~ltai~leitd1 the REM-Al-0-S-based
inclusions or the REM-Ca-Al-0-S-based inclusions as an oxide, and tllus the c~ystal
lattice structure of the REM-Al-0-S-based inclusions or the REM-Ca-Al-0-S-based
inclusions and the crystal lattice structure of TiN becolne silnilar to each other.
[0038]
In addition, REM has a futlctiotl of preve~ltiligs tretching or coarsening of an
oxide such as an AI-0-based inclusion or an Al-Ca-0-based inclusion by reforming the
Al-0-based inclusion or the Al-Ca-0-based illclusion into the REM-AI-0-S-based
inclusion or the REM-Ca-Al-0-S-based i~~clusi~ov~hli clhi ave a high melting point.
In addition, in a case where Ca is included, Ca is included in the steel that REM is
contained, and thus CaS which is Ca-based sulfide, a Ca-Mn-S-based inclusion and the
like do not exist.
[0039]
To attain the effect, the steel must contain a constant atnount or more of REM
based on the total oxygen amount (T.O amount). In a case where the molten steel
does not contain a predeter~nined amount or more of REM, AI-0 or Al-Ca-0, which
are not refor~ned into REM-Al-0-S-based inclusions or REM-Ca-Al-0-S-based
itlclusions, remain. Therefore, this case is not preferable. In addition, it is tlecessary
for the molten steel to contain a co~lstanat nlount or more of REM based on the atnou~lt
of S. In a case where the molten steel does not contain a constant amount or more of
REM, it is difficult to fix S by for~ningR EM-AI-0-S-based inclusio~~ors REM-Ca-AI-
0-S-based inclusions, and thus coarse MnS is generated. Therefore, this case is not
preferable.
[0040]
In addition, it is necessary for the steel to contain a constant atnount or more
of the REM-Al-0-S-based inclusion or the REM-Ca-Al-0-S-based inclusion. In a
case where the number of the EM-Al-0-S-based inclusions or the EM-Ca-Al-0-Sbased
inclusions is small, generation of a REM-Al-0-S-(TiN)-based co~nposite
inclusion or a REM-Ca-Al-0-S-(TiN)-based composite inclusion beco~nesin sufficient,
aud tlltts this case is not preferable.
[0041]
The present inventors have made an examination from the above-described
viewpoint, and they have experimentally found that when the steel contains less than
0.0001% of REM, an effect by REM that is contained in steel is insufficient.
Accordingly, the lo\ver li~niot f the anlount of REM is set to 0.0001%, preferably
0.0003% or inore, more preferably 0.0010% or inore, and still more preferably
0.0020% or inore. However, wllen the alnoutlt of REM exceeds 0.050%, the cost
increases, and clogging of a cast nozzle tends to occur. Therefore, the n~anufactureo f
steel is hindered. Accordingly, the upper litnit of the amount of REM is set to
0.050%, is preferably set to 0.035%, and is Inore preferably set to 0.020%.
0 is an element which is renloved fro111 steel by deoxidation, but 0 is
necessaly to generate a composite inclusion having a rnain stnlcttire of REM-A1-0-S-
(TiN) or REM-Ca-AI-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 tnore of 0. Ho\veve~; when the
amootit of 0 exceeds 0.0030%, a large amount of an oxide such as A1203 remains, and
tl1~1tsh e fatigue life decreases. Accordingly, the upper limit of the amount of 0 is set
to 0.0030%. In addition, the atnount of 0 is preferably 0.0003% to 0.0025%.
[0043]
Ca: 0.0050% or less
Ca [nay be contained in steel as necessaq1. The steel contains Ca that is
coupled to REM and 0 to forn~a composite inclusion having a tnail~st ructure of REMCa-
AI-0-S-(TiN). Therefore, it is preferable that the steel contain 0.0005% or nlore
of Ca and Illore preferably contain 0.0010% or more of Ca. However, wvhen the
amount of Ca exceeds 0.0050%, a large amount of coarse CaO is generated, and thus
the fatigue life decreases. Accordingly, the upper litnit thereof is set to 0.0050%. In
addition, the anloutit of Ca is preferably 0.0045% or less.
[0044]
The above-described components are included as a basic chemical
con~positiono f the steel for induction hardening according to this embodiment, and the
balance consists of Fe and itnpurities. In addition, "impurities" in the "the balance
consists of Fe and itnpurities" represents ore or scrap as a raw material when steel is
indr~striallytn anufactared, or a material that is unavoidably nixed in due to the
manufach~ringe nvironment and the like. Howeve]; in the steel for induction
liardening according to this entbodinient, it is necessary to li~i~Tiit, N, P, and S, which
are impurities, as follows.
[0045]
Ti: less than 0.005%
Ti is an impurity. When Ti exists in steel, i~tclusionss uch as Tic, TiN, and
TiS are generated. - The inclusions deteriorate tlie fatigue properties. Accordingly,
the a~ilounot f Ti is less than 0.005%, and is preferably 0.0045% or less.
[0046]
Particularly, for example, TiN is generated in an angular shape as sliown in
FIG. 2. The TiN having an angi~lars hape beconies a starting point of frach~re.
Accordingly, TiN is forn~eda composite xvith REM-Al-0-S or REM-Ca-Al-0-S. The
lower limit of tlie amount of Ti includes 0%, but it is uidustrially difticult to realize 0%.
[0047]
111 addition, in the steel for i~lductionh ardening according to this embodinlent,
even tliough the steel contains more tlian 0.001% of Ti that is upper linlit of an alnount
of Ti in the related art, wvlien a steel for induction hardening contains less than 0.005%
of Ti as a impurity, TiN fonns a composite inclusion with REM-AI-0-S or REM-Ca-
AI-0-S, and thus the fatigue properties do not deteriorate. Accordingly, it is possible
to stably nianufactore steel for induction hardening with excelle~ifta tigue properties.
100481
N: 0.01 5% or less
N is an i~npurity. When N exists 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 harnifi~lr esult, such as
deterioration in the fatigue properties, the ductility, attd the tougltness, becomes
significant. Accordingly, the upper lin~iot f the alnount of N is 0.015%. The an~ount
of N is preferably 0.005% or less. The lo\ver litnit of the amount of N includes 0%,
but it is industrially difficult to realize 0%.
[0049]
P:,0.03% or less
P is an impurity. When P exists in steel, P segregates at a grain boutida~ya nd
decreases the fatigue life. When the amount of P exceeds 0.03%, a decrease in the
fatigue life becomes significant. Accordingly, the upper lin~iot f the alnout~ot f P is
0.03%. The amount of P is preferably 0.02% or less. The lower limit of the amount
of P includes 0%, but it is industrially difficult to realize 0%.
[0050]
S: 0.01% or less
S is an itnpurity. When S exists in steel, S forms a sulfide. When the
amount of S exceeds 0.01%, for example, as sl~o~vinn F IG. 2, S is coupled to Mn to
form coarse MnS, and decreases the fatigue life. Accordingly, the upper limit of the
amount of S is 0.01%. The amount of S is preferably 0.0085% or less. It is
industrially difficult to set the lower limit of the amount of S to 0%.
1005 I]
In addition to the above-described elements, the following elen~entsm ay be
selectively contained. Hereinafter, a selective element will be described.
[0052]
The steel for induction hardening according to this embodiment may contain
at least one of 2.0% or less of Cr, 0.70% or less of V, 1.00% or less of Mo, 1.00% or
less of W, 3.50% or less of Ni, 0.50% or less of Cu, less than 0.050% of Nb, and
0.0050% or less of B.
[0053]
Cr: 2.0% or less
Cr is an elenlent that increases tlie hardenability and improves the fatigue life.
To attain this effect, it is preferable for the steel to contain 0.05% or more of Cr.
However, \\then the amount of Cr exceeds 2.0%, the effect that the hardenability is
inlproved is saturated and hardness of tlie base metal is increased, and thus the tool
service life during cutting decreases. In addition, Cr becomes a cause of a quenching
crack. Accordingly, tlie upper limit of the anlou~lot f Cr is set to 2.0%, and tlie
a~llou~olft Cr is preferably set to 0.5% to 1.6%.
[0054]
V: 0.70% or less
V is an elenlent that is coupled to C and N in steel to for111 a carbide, a nitride,
or a carbonitride, and contributes to precipitation strengthening of steel. To stably
attain this effect, it is preferable that the steel contain 0.05% or more of V, and more
preferably 0.1% or nlore of V. Howevel; \vhen 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 an~ounot f V is preferably set to 0.50% or less.
[0055]
Mo: 1.00% or less
Mo is an element that is coupled to C in steel to for111 a carbide and
contributes to an itnprovetllent in strength of steel due to precipitation strengthening.
To stably attain this effect, it is preferable that the steel contain 0.05% or more of Mo,
and more preferably 0.1% or nlore of Mo. Ho~vever, wllen the alnount 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 amount of Mo is preferably 0.75% or less.
[0056]
W: 1.00% or less
W is an eletnetlt that fortns a hard phase and contributes to an improvement in
the fatigue properties. To stably attain this effect, it is preferable that steel contain
0.05% or more of W, and more preferably contains 0.1% or more of W. However,
when the amount of W exceeds 1.00%, the macl~inabilityo f the steel decreases.
Accordingly, the upper limit of the atnount of W is set to 1.00%. The amount of W is
preferably 0.75% or less.
[0057]
Ni: 3.50% or less
Ni is an element that increases corrosion resistance and contributes to at1
improveme~ltin the fatigue life. To stably attain this effect, it is preferable that the
steel contain 0.10% or more of Ni, and more preferably 0.50% or more of Ni.
However, ~vllenth e amount of Ni exceeds 3.50%, inachinability of steel decreases.
Accordingly, the upper limit of the amount of Ni is set to 3.50%. The atnount ofNi is
preferably 3.00% or less.
[005S]
Cu: 0.50% or less
Cn is an elenlent that contributes to an improvement in the fatigue properties
due to a strengthening of the base metal. To stably attain this effect, it is preferable
that the steel contain 0.10% or more of Cu, and more preferably 0.20% or more of Cu.
,However, nrl~enth e amount of Co exceeds 0.50%, cracks are generated during hot
working. Accordingly, the upper limit of the amount of Co is set to 0.50%. The
atnount of Co is preferably 0.35% or less.
[0059]
Nb: less than 0.050%
Nb 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, it is preferable
that the steel contain 0.005% or more ofNb and Inore preferably 0.010% or more of
Nb. However, when the amount of Nb is 0.050% or more, the effect by contaitling
Nb becomes saturated. Accordingly, the amount of Nb is set to less than 0.050%.
The amount ofNb is preferably 0.030% or less.
[0060]
B: 0.0050% or less
B is an element that contributes to an inlprovernent in the fatigue properties
and strength due to grain boundary strengthening. To stably attain this effect, it is
preferable that the steel contain 0.0005% or more of B, and more preferably 0.0010%
or 111ore of B. However, \\then the amount of B exceeds 0.0050%, the effect by
containing B becomes saturated. Accordingly, the upper linlit of the a~nonnot f B is
set to 0.0050%. The anlount of B is preferably 0.0035% or less.
[0061]
In the steel for induction hardening according to this embodiment, S is fixed
as the REM-Al-0-S-based inclusion or the REM-Ca-Al-0-S-based inclusion.
Accordingly, generation of MnS, which is stretched to 10 pm or Inore and hinders the
fatigne properties, is suppressed. Typically, in a case where MnS exists in steel, as
shown in FIG. 2, MnS is stretched by rolling. However, in the steel for inductio~l
hardening according to this embodiment, REM fixes S to generate the REM-A1-0-Sbased
inclusion or the REM-Ca-Al-0-S-based inclusion. These oxysulfides are hard,
and thus even when being subjected to rolling, the size thereof does not vary. In
addition, S is consunled as the REM-Al-0-S-based inclt~siono r the REM-Ca-AI-0-Sbased
inclusion, and thus MIIS is not generated or the antount thereof generated is
reduced. In addition, in the steel for induction hardening according to this
enlbodiment, as shown in FIG. 1, TiN adheres to the REM-Al-0-S-based inclusion or
the REM-Ca-Al-0-S-based i~lclusiona, nd thus a11 approximately spherical co~nposite
inclusion having a main structure of REM-AI-0-S-(TiN) or REM-Ca-AI-0-S-(TiN) is
fornled. . . .. . .
[0062]
Here, for exanlple, as shown in FIG. 1, the "approximately spherical shape"
represents a shape in n~l~ica hm axinlum height of surface unevenness is 0.5 pm or less,
and a value obtained by dividing the major axis of the inclosion by the minor axis of
the inclusion, that is, a1 aspect ratio is 3 or less.
[0063]
For example, as shown in FIG. 2, hard TIN, which does not adhere to REMA1-
0-S or REM-Ca-AI-0-S and independently exists in steel, has a maximu~nd iameter
of 1 pm or more and has an angular shape. Therefore, TiN, which does not adhere to
REM-Al-0-S or REM-Ca-Al-0-S and independently exists in steel, becomes a starting
point of fracture, and thus TiN has an adverse effect on the fatigue life. However, in
the steel for induction hardening according to this embodiment, TiN adheres to REMAl-
0-S or REM-Ca-Al-0-S, and constitutes the approximately spherical composite
inclusion having a main stnrcture of REM-AI-0-S-(TiN) or REM-Ca-Al-0-S-(TiN),
and thus the above-described adverse effect due to the shape of TiN that does not form
the composite inclusion is not generated.
[0064]
hl addition, in the steel for induction hardening according to this embodiment,
to improve the fatigue life, it is necessary to suppress the amount of "MnS having a
maxi~numd iameter of 10 [ I I ~or more" iuld "TiN having a maximu~nd iameter of 1 p n
or more" generated, which have an adverse effect on the fatigue life, to a total of 5
pieceshntn2 or less on the basis of a nutnber density. In addition, it is preferable that
the amount of "MnS having a nlaxinlu~nd iameter of 10 pm or more" and "TiN having
a rnaximum diameter of 1 pm or more" generated be as small as possible. The
amount thereof generated is preferably 4 pieces/mm2 or less, and is more preferably 3
pieces/n~rno~r less.
[0065]
A preferred method of manufactt~riogth e steel for induction hardening
according to this embodi~nen\t\ (ill be described.
[0066]
In the method of manufacturi~lgth e steel for induction hardening according to
this embodiment, a sequence of adding a deoxidizing agent is important during
refilling of molten steel. In this manufactt~ringm ethod, first, deoxidatioa is
perfornled by using Al. Then, deoxidation is perfor~nedf or 5 minutes or longer by
using REM, and then ladle refining including vacuunl degassing is perfomled.
Alternatively, after deoxidation using REM, Ca is added as necessary, and then the
ladle refining including the vacuum degassing is perfornled.
[0067]
Prior to deoxidation with REM, when deoxidation is performed by using an
element other than Al, it is dificolt to stably reduce an amount of oxygen. Therefore,
in this manufacturing method, the deoxidizing agent is added in the order ofAl and
REM, or in the order of AI, REM, and Ca. As a result, the REM-Al-0-based
inclusion that is an oxide-based inclusion or the REM-Ca-Al-0-based inclusion that is
an oxide-based inclusion is generated. Accordingly, generation of the Al-0-based
inclusions or the Al-Ca-0-based inclusions, which are har~nfi~isl, p revented. I11
addition, for the REM added, a misch metal (alloy composed of a plurality of rareearth
tnetals) and the like may be used, and for example, an aggregated misch metal
may be added to molte~sl teel at the end of the refining. At this time, a flux such as
CaO-CaFz is added to approximately perform desnlfitrization and refining of an
inclusion by Ca.
[0068]
Deosidation with REM is performgd for 5 minutes or longer. When a
deoxidation time is shorter than 5 minutes, reforming of the Al-0-based inclusions or
the Al-Ca-0-based inclusions, which are generated once, does not progress, and as a
result, it is difficult to reduce an~ounot f the Al-0-based itlclusions or the alnoullt of the
Al-Ca-0-based inclusions. In addition, when deoxidation is performed by using an
element other than Al firstly, it is difficolt to reduce the amount of oxygen. In
addition, even in a case where Ca is added to nlolten steel by adding a flux thereto, it is
necessary to perform deoxidatio~wl it11 REM for 5 minutes or longer.
[0069]
In a case where Ca is added as necessary for deoxidation, when Ca is added
prior to REM, the nr~ntbero f Al-Ca-0-based i~lclusio~wlsh ich tend to be stretched at a
low inelting point are generated. As a result, even \vl~enR EM is added after rnany
numbers of Al-Ca-0-based inclusions are generated, it is difficult to refor111 a
composition of the inclusions. Accordingly, in a case where Ca is added, it is
necessary to add Ca after REM is added.
[0070]
As described above, in this manufacturing method, since S is fixed by the
REM-Al-0-S-based inclusion that is an oxysulfide-based inclusion or the REM-Ca-AI0-
S-based inclusion that is an oxysulfide-based inclusion, generation of coase MnS is
suppressed. In addition, since tlie REM-Al-0-S-based inclusion that is an oxysulfide
or the REM-Ca-Al-0-S-based inclusion that is an oxysulfide form a composite with
TiN, the number of TiN, whicl~d oes not adhere to tlie EM-Al-0-S-based inclusio~~
that is an oxysulfide or the REM-Ca-Al-0-S-based inclusion that is an oxysulfide and
independently precipitate, decreases. Accordingly, the fatigne properties of the steel
for induction hardening are improved.
[007 11
Ho\vever, particularly, in a case where the steel for induction hardeaing
according to this emboditnent is used in a bearing, it is ideal that the amount of MIIS
generated and the anlount of TiN that independently exists generated are small, but it is
not necessaql that no MnS or TiN exist at all. In addition, MnS independently
crystallizes in many cases using an oxide as a nucleus. Accordingly, an oxide map be
fonnd at the inside such as the central portion of MnS in many cases. The MnS is
distinguished from the EM-Al-0-S-based inclusion that is an oxysulfide or the REMCa-
Al-0-S-based inclosion that is an oxysulfide.
[0072]
To reliably improve the fatigue properties demanded for the steel for induction
hardening, it is necessaqr for the REM-Al-0-S-based inclusion that is an oxysulfidebased
inclusion or the REM-Ca-Al-0-S-based i~~clusiothna t is an oxysulfide-based
inclnsion, and the atnount of MnS and TiN that independently exist generated satis*
the following conditions. . Specifically, it is necessary for the sum of the number
density of MnS having a ~naxi~nudtnia meter of 10 Lrnl or more and the number density
of TiN having a maxi~nnm diameter of 1 pm or more to be set to a total of 5 pieces or
less per observation surface of 1 mmz.
[0073]
As described above, MnS is stretched by rolling. When a repetitive stress is
applied to the stretched MnS, the stretched MnS 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 maxi~nutnd iameter of 10 inn or more,
have an adverse effect on the fatigne life, and thus the maxi~numd iameter of MnS does
not have the opper limit thereof. In addition, although TiN is not stretched by rolling
as such as MIIS, the angular shape thereof beco~nesa starting point of fracture.
Coarse TIN has an adverse effect on the fatigue life similar to MIIS. All TiN having a
maximum diameter of 1 pm or more have an adverse effect on fatigue life.
[0074]
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, that is, when a nomber density exceeds 5
pieces/m~n~th,e fatigue properties of the steel for induction I~ardeningd eteriorate.
Particularly, in a case where the steel for induction hardening according to this
e~nbodinlentis used in a bearing, MnS and TiN greatly deteriorate the fatigue
properties. Accordingly, it is preferable that the sum of the number of MnS and the
number of TiN per observation surface of 1 mn12 be 5 pieces or less. More preferably
tile sum of the number of MIS and the number of TiN per observation surface of 1
IIIIII~is set to 4 pieces or less, that is, the nu~nberd ensity is set to 4 pieceslmm2 or less.
Still Inore preferably, the sum of the nu~nbero f MnS and the number of TiN per
observation surface of 1 IIIIII~is set to 3 pieces or less, that is, the number density is set
to 3 pieces/tnm2 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 observation surface of 1 mm2.
[0075]
In addition, to reliably itnprove the fatigue properties, it is preferable that the
nlnnber fraction of a cotnposition inclusion to wliich TiN adheres with respect to the
total inclusions be 50% or more. The angular shape of TiN, which independently
exists mithout adhesion to an inclusion, becomes a stattitlg point of fracture. In
addition, in a manner similar to MnS, TiN which is coarsened without adhesion to an
inclusion has an adverse effect on fatigue life. Particularly, when the nunlber fraction
of a conlposite inclusion, to \vI~icTl~iN adheres, with respect to the total inclusions is
less than SO%, coarse TiN greatly deteriorate the fatigue properties. Accordingly, the
number fraction of tlie cotnposite inclusions, to wliich TiN adheres, wit11 respect to tile
number of total inclusions is preferably 50% or more.
[0076]
As described above, the atnount of the Al-0-based inclusion and the AI-Ca-0-
based inclusion of an oxide such as ,41203, which is a hannfi~el lement having an
adverse effect on the fatigue properties of tlie steel for induction hardening, is reduced
because the Al-0-based inclusions and tlle Al-Ca-0-based inclusions are mainly
reformed into REM-Al-0-based inclusions or the REM-Ca-Al-0-based inclusions,
which are oxide-based inclusion, due to an addition effect of REM. In addition, MnS
that fornl liarnifiil inclusiot~sis refornied into REM-Al-0-S-based inclusions or REMCa-
Al-0-S-based inclusiot~sw, vl~icha re oxysulfide-based inclusions, and t1111s the
anlount of MnS generated is limited. Particularly, the amount of MnS generated is
snppressed due to Ca.
[0077]
III addition, TiN that is a liarn~fulin clusion preferentially crystallizes or
precipitates to a surface of the REM-Al-0-S-based inclusion that is an oxysulfidebased
inclusion or the REM-Ca-Al-0-S-based inclusion that is an oxysulfide-based
inclusion. As described above, generation of MnS or TiN, which are har~nfiil, is
suppressed due to the addition of REM or Ca, and tlins it is possible to obtain steel for
induction hardening with excellent fatigue properties.
[0078]
The specific gravity of the REM-Al-0-S-based inclusions or the REM-Ca-AI-
0-S-based inclusions, ~\hiclia re oxysulfide-based inclusions, is 6 and is close to a
specific gravity of 7 of steel, and thus floating and separation are less likely to occur.
I11 addition, when pouring 11iolte11 steel into a mold, the oxysulfides penetrate op to a
deep position of unsolidified layer of a cast piece due to a do\\ln\vard flow, and thus the
oxysulfides tend to segregate at tlie central portion of tlie cast piece. When the
oxysulfides segregate at the central portion of the cast piece, the oxysnlfides are
deficient in a surface layer portion of the cast piece. Therefore, it is difficult to
generate a co~npositein clusion by adhering TiN to the surface of the oxysulfides.
Accordingly, a detoxifying effect of TiN is weakened at a surface layer poltion of a
product.
[0079]
Accordingly, in this manufacturing method, to prevent segregation of the
REM-Al-0-S-based inclusions or the REM-Ca-Al-0-S-based iticlusions, whic11 are
oxysulfides, molten steel is circr~lated in the nlold in a horizontal direction to realize
uniform dispersion of the inclusions. The circulation of the nlolten steel inside the
mold is preferably perfomled at a flow rate of 0.1 m/~ninoteo r faster so as to realize
further unifornl dispersion of the oxjrsulfide-based inclusions. When the circulation
speed inside the mold is slower than 0.1 nlhinute, the oxysulfide-based inclusions are
less likely to be nnifomily dispersed. Accordingly, the molten steel nlay be stirred to
realize unifonn dispersion of the oxysolfide-based inclusions. As stirring means, for
example, an electrolnagnetic force and the like may be applied.
[OOSO]
Next, the cast piece after casting is held at a temperature region of 1200°C to
1250°C for 60 seconds to 60 ~uinutesto obtain the above-described composite
inclusion. This temperature region is a temperature region at which a conlposite
precipitation effect of TiN with respect to the REM-Al-0-S-based inclusions or the
REM-Ca-Al-0-S-based inclusions, which are oxpsulfide-based inclusion, is large.
Holding at this tenlperature region for 60 seconds or Inore is a preferable condition at
n41ich TIN is allowed to sufficiently grow at tile surface of the REM-Al-0-S-based
inclusion or the REM-Ca-Al-0-S-based inclusion which are oxysulfides. However,
even when the steel is held at this temnperature region for 60 niinutes or more, it is
difficult to grow up to a size of TiN Inore than the required size of TiN and thus a
holding time is preferably 60 minutes or less. As described above, in order to fort11 a
conlposite with the REM-Al-0-S-based inclusions or the REM-Ca-Al-0-S-based
inclusions and to suppress generation of TiN that is independently generated without
adhesion to these inclusions, it is preferable to hold the cast piece after casting at a
temperature region of 1200°C to 1250°C for 60 seconds to 60 nlinutes.
[OOSll
In addition, typicallj: the cast piece after casting contains TiN that have
crystallized already, and Ti and N that fornl a solid solution and promote growth of TiN
during a cooling process to room temperature. When the cast piece is heated at a
temperature region of 1200°C to 1250°C, Ti and N which for111 a solid solution are
dispersed to a position, at whicli TiN crystallizes and grows already as a nucleus, and
grows as TiN at the position. In the invention, TiN crystallizes or precipitates using
the REM-N-0-S-based inclusions or the REM-Ca-Al-0-S-based inclusions as a
nucleus. Accordingly, when heating is perfornled at a temperature region of 1200°C
to 1250°C, it is considered that Ti and N which form a solid solution in steel can be
dispersed and grow as TiN. In this manner, dispersion of TiN is promoted, and thus it
is possible to suppress generation of coarse TiN that independently exists.
[0082]
In this maliufactnring method, the cast piece after casting is heated to a
heating temperature and is held at a temperature region of 1200°C to 1250°C for 60
seco~idsto 60 minutes, and then hot-rolling or hot-forging is perfonned to manufactnre
the steel for i~lductionh ardening. I11 addition, cutting into a allape close to a filial
shape is perfomled, and ind~~ctiohnar dening is performed to make the Vickers
hardness of the surface be 600 Hv or more.
A rolling menlber or a sliding inemnber, wl~ichu se the steel for induction
hardening of the invention, is excellent in the fatigue properties. In addition, the rolling
member or the sliding 111e111ber is typically finished to a final product by using means
capable of performing hig11-hardness and high-accuracy processing such as grinding as
necessaly.
Examples
[0083]
Next, exan~pleso f the invention will be described, but conditioris in the
examples are conditional examples that are e~nployedt o confirm applicability and an
effect of the invention arid the invention is not limited to the conditional examples.
.The invention can employ various conditions as longas the object of the invention is
achieved without departing from the gist of the invention.
[0084]
During thevacuum degassing in the ladle refining, refining was perfomled
under conditiolls sltown in Table 1 by usitlg metal AI, a misclt metal, and a flux of
CaO:CaF2=50:50 (mass ratio), and a Ca-Si alloy as necessagl to obtain molten steel
having a cl~emicalc otnposition shown in Table 2A and Table 2B, or Table 4A and
Table 4B. The molten steel was casted to a 300 mm square cast piece by using a
cotltinoous casting apparatus. At that time, circulation inside a mold was perfom~ed
by electrornagtletic agitation under conditions sllo~vain Table 1, thereby castittg a cast
piece.
[OOSS]
The cast piece, wvhich \vas ladle-refined and casted under the conditions
shown in Table 1, was heated and held under conditions shown in Table 1, was hotforged
into a c~rlindricarl od \\it11 4 of 50 mm, and was finally subjected to griiiding
into I$ of 10 mm. A plurality of cylindrical rods with of 10 111111, \vI~ichw ere
composed of a raw nlaterial for test specimens, was prepared fro111 the same kind of
steel. One of tl~ecy lindrical rods was provided for chemical compositioll analysis
and inclusion analysis.
[0086]
In addition, with regard to the remaining final cylindrical rods with I$ of 10
inm anlong the plurality of cylilldrical rods that were manufactured, for supply to a
fatigue test for confir~nationo f suitability for the rolling ~ne~nboerr t he sliding ~~leniber
\vItich are used after performing induction hardening, and tempering, a raw material,
~vllichis larger than a shape of the fatigue specimen by approxitnately 0.3 mm,w as cut
from the cylindrical rods with I$ of 10 mm, and,induction hardening was performed in
order for a load application portion to uniformly have the same hardness of 600 Hv or
tnore as that of a coating material for bearings. Then, tempering was perfomled at
180°C, and was finished by grinding and polishing to become a fatigue specimen
I~aviaga shape shown in FIG. 3. With regard to partial fatigue specimens, samples for
measurement of Vickers hardness were collected fro111 the load application portion.
[0087]
With regard to the above-described sample for chemical composition analysis
and inclusio~ia nalysis, a cross-section in a stretching direction thereof was mirrorpolished,
and was processed with selective potentiostatic etching by an electrolytic
dissolution method (SPEED method). Then, measure~nenwt ith a scanning electron
~nicroscopew as perfornled ~vithre spect to inclusions in steel in a range of 2 111111w idth
in a radial direction \vhich centers around a depth of the half of a radius fiotn a surface,
that is, a depth of 2.5 mm from the surface, and a length of 5 mm in a rolling direction,
a composition of the itlclusion was analyzed using EDX, and i~lclusionsin 10 mm2 of
the sample were counted to measure a number density. In addition, the fatigue life
was measured \vitI~r espect to the fatigue specimen by applying a repetitive stress by
using an lrltrasonic fatigue test, and the number of cycles at which 10% of the
evaluation sample was fractured was evaluated as fatigue properties Llo by using
Weibull statistics. The fatigue test was perfornled by using an ultrasonic fatigue
tester (USF-2000, manufactured by Sl~imadzuC orporation). As test conditions, a test
frequency was set to 20 kHz, a stress ratio (R) was set to -1, and an actual load
amplitude was set to 1000 MPa. In addition, a 180°C tempering Vickers hardness test
was perfonned in accorda~lccw ith JIS Z 2244.
[OOSS]
Table 1 sllows manufacturing conditions including steel refining conditions,
casting conditions, heating and holding conditions after casting in the examples.
Manufacturing conditions A, E, F, J, K, L, M, N, and 0 pertain to manufacturing
conditions according to the present examples. Manufacturing conditions B, C, D, I, P,
and Q are manufacturing conditions in a case where the manufacturing conditions are
not preferable and do not pertain to the present exan~ples.
[0089]
Anlong the heating and holding conditio~~slslo wn in Table 1, in the
nlanufacturitlg condition B, a holding time was lower than a preferable range. In the
manufacturing condition C, a holding temperature was lower than a preferable range.
In the nlan~~facturincogn dition D, the holding temperature was higher than the
preferable range. In addition, \\it11 regard to the manufacturing co~iditioI~, ia
deoxidizing time after adding REM among ladle refining co~~ditiownsa s lower than the
preferable range. In addition, wit11 regard to the manufacturing condition P and the
manofactoring cot~ditionQ , a sequence of adding REM was not preferable in a
deoxidizing process. The above-described mannfactoring conditions B, C, D, I, P,
and Q are employed it1 steel numbers 52, 62, 63, 56, 57, and 58, respectively, in Table
4A, Table 4B, Table 5A, and Table 5B. 111 any steel number, a chemical co~nposition
is included in a range of the invention as described in Table 4A and Table 4B.
However, as described in Table SAand Table 5B, the llutilber fraction of a colilposite
inclusion, lo which TiN is adhered, with respect to total inclusions was less than 50%,
the number density of MIIS having a maximum diameter of 10 11n1 and TiN having a
maxinlu~nd iatneter of 1 pm or Illore which independently existed was excessive and
exceeded the range of the invention, and t1u1s the fatigue properties Llo in a case of
perfor~llingin duction hardening were inferior to those of the present examples.
[0090]
With regard to a steel number 55 in which REM was excessively included, as
sho\\~nin Table 5A and Table 5B, the manufacturing condition A was intended to be
employed, but a casting nozzle was clogged, and thus casting was inlpossible.
Therefore, the residne of steel that renlained in a casting nozzle or a tundish was
collected and a chemical composition was analyzed. The results are shown in Table
4A aild Table 4B as a composition of co~tiparalives teel. As a resuli, with regard to
the steel number 55, it was proved that the amount of REM was more excessive than
the range of the invention.
[0091] .
As shown in Table 4A, steel number 54 contained less REM than is contained
in a steel of the invention, and tllus as showw~n in Table SA, an effect by adding REM
sobstantially disappeared, and an Al-Ca-0-based precipitation increased. In the steel
numbers 52, 54, 56, 57, 58, 62, and 63, the number fraction of a composite inclusion,
to which TiN adhered, with respect to the total inclusions was less than 50%, and the
nambcr dcnsity of MnS having a niaximu~nd iameter of 10 {uu and TiN haviug a
maximum diameter of 1 pni or more which independently existed was excessive and
exceeded the range of the invention, and thus the fatigue properties Llo were inferior to
those of the present examples.
[0092]
In steel numbers 60 and 61 showvn in Table 4A, the amount of Ca was
excessive, and precipitation ofAl-Ca-0 and the like increased in the respective steel
numbers as shown in Table 5A and Table 5B, and thus the balance of inclusion
generation collapsed. Therefore, the ni~n~bferar ction of a con~positein clusion, to
which TiN adhered, with respect to the total inclusions was less than 50%, and a
number density of MIS having a ~naxinlundi iameter of 10 pm and TiN having a
maximum diameter of 1 { I I o~r more which independently existed was excessive and
exceeded the range of the invention, and thus the fatigue properties Llo were inferior to
those of the present examples.
[0093]
In steel numbers 53 and 59, as slio\vn in Table 4A, Ti or S was more than the
range of the invention, and thus a number of TiN, MnS, and the like were generated.
As a result, the balance of inclusion generation collapsed. Therefore, the sun1 of the
number density of TiN having a niaximum diameter of 1 pm or more which
independently existed withoot adllesion to an inclusion, and the nutnber density of
MnS having a maximom diameter of 10 pal or more was 5 pieces/nlm2 or more. In
addition, as shown in Table 5Aand Table 5B, the number fraction of composite
inclusions, to which TiN adhered, with respect to the total inclusions mas less than
50%, and tl~usth e fatigue properties LIOw ere inferior to those of the present examples.
In addition, in a steel nulnber 70 ~vhichc ontained more P than is contained in a steel of
the invention, as shown in Table 5A and Table SB, the nutnber fraction of composite
inclusions, to which TiN adhered, wit11 respect to total inclusions was 50% or more.
However, P segregated at a grain boundary, and thus the fatigue properties LI" were
lower thai those of the present examples.
[0094]
As sho\vn in Table 4A, steel nuniber 65 contained more C, wi~iche ssentially
plays a role in precipitation strengthening, than is contained in a steel of the invention.
In addition, as shown in Table 4A, steel number 67 contained more Si, which is
necessary for securing l~ardenabilityt, han is contained in a steel of the invention. In
addition, shown in Table 4A, 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 numbers 65,67, and 69, as shown in Table 5A, a quenching crack was
generated during induction hardening, and thus evaluation other than a chen~ical
con~positiona nalysis was stopped.
[0095]
As sho\\fn in Table 4A, steel number 64 contains more C than is contained in a
steel of the invention. In addition, as shown in Table 4A, steel number 66 contains
less Si than is contained in a steel of the invention. In addition, steel nu~llber6 8
contained less Mn than is contained in a steel of the invention. In these steel numbers,
as shown in Table 5A and Table 5B, the number fraction of a composite inclusion, to
which TIN adhered, with respect to the total inclusion was secured. However, the
fatigue properties Llo and the 180°C tempering Vickers hardness \irere inferior to those
of the present examples.
[0096]
Cr is at1 elenlent that increases hardenability. However, as showvn in Table
4B, steel nunlber 71 contained more Cr than is contained in a steel of the invention,
and tlu~sa s shown in Table 5A, a quenching crack was generated. Therefore,
evaluation with respect to the steel nunlber 71 \iras stopped.
[0097]
As sho\\~nin Table 4A, steel number 72 contains less Al than is contained in a
steel of the invention. On the other hand, as shown in Table 4A, steel number 73
contained Illore AI than is contained in a steel of the invention. As shown in Table 4A,
steel number 74 contained nlore N than is contailled in a steel of the invention. As
shown in Table 4A, steel number 75 contained less O than is contained in'a steel of the
invention. On the other hand, as shown in Table 4A, steel number 76 contains more
0 than is contained in a steel of the invention. Accordingly, in these steel numbers,
as shown in Table 5A and Table 5B, the number fractions of composite inclusions, to
which TiN adhered, with respect to the total inclusion was less than 50%, and the
ntul~berd ensities of MnS having a rnaui~nu~dnia meter of 10 [un and TiN having a
~naxi~ntndnia tneter of 1 111n or inore which independently existed were excessive and
were greater than in a steel of the invention, and thus the fatigue properties LIO were
inferior to those of the present examples.
[0098]
As shown in Table 4B, with regard to a steel number 78 wllicl~c ontained a
greater amount of Mo than is contained in a steel of tile invention, a steel number 79
which contained a greater aalount of W than is contained in a steel of the invention, a
steel number 81 ~ v l ~ iccoln~ta ins a greater amount of Cu than is contai~ledin a steel of
the invention, a steel ntnnber 82 which contained a greater a~nounot f Nb than is
contained in a steel of the invention, and a steel number 83 \vllich contains a greater
amount of B than is contained in a steel of the i~ivention,a crack occurred during
processing into a cylindrical rod shape, and thus evaluation other thal cchemical
composition analysis was stopped.
[0099]
The present examples are shown as steel numbers 5 to 48 and 51 in Table 2A,
Table 2B, Table 3A, and Table 3B. From Table 3A and Table 3B, it could be seen that
in the present examples, the sum of a number density of TiN having a maximtml
diameter of 1 {un or more which independently existed without adhesion to an
inclusion, and a nu~nberd ensity of MnS havi~lga ~naxitnundl iameter of 10 {lm or
more was 5 pieces/mm2 or less in all of the steel numbers. In addition, it could be
seen that the number fraction of a conlposite inclusion, to which TiN adhered, with
respect to all inclusions was secured to a value of 50% or more. 111 addition, in the
present examples subjected to induction hardening, and 180°C tempering, the fatigue
properties Llo evaluated by a repetitive stress were lo7 cycles or more, and were
superior to those of steel numbers of comparative examples out of range of the
invention. In addition, it can be seen that it1 tlie present examples, the 180°C
tempering Vickers hardness is 600 Hv or more, and is suitable for a rolling member or
a sliding ~llelllbee
[OlOO]
[Table 11
[OlOl]
[Table 2A]
[O 1021
[Table 2B]
[0 1031
[Table 3A]
[0104]
[Table 3B]
[OlOS]
[Table 4A]
[0106]
[Table 4B]
[0107]
[Table 5A]
[0108]
[Table 5B]
[Illdustrial Applicability]
[0 1091
According to the it~ventiont,h e Al-0-based inclusiotl is refortned into the
REM-Al-0-S-based itlclusion, or the Al-Ca-0-based inclusion is reformed into the
REM-Ca-Al-0-S-based inclusion, and thus it is possible to prcvent stretching or
coarsening of an oxide-based inclusion. In addition, TiN is forn~eda composite wit11
the REM-Al-0-S-based inclusion or the REM-Ca-AI-0-S-based inclusion, and thus it
is possible to reduce a number density of TiN ~vhichin dependently exists without
adhesion to the composite inclusion. In addition, S is fixed and tl~usg eneration of
coarse MnS can be suppressed, and thus it is possible to provide steel for induction
hardening with excellent fatigue properties. Accordingly, it can be said that the
industrial applicability of the invention is high.
[Brief Description of the Reference Sy~nbols]
[OllO]
A: REM-Ca-AI-0-S-BASED TNCLUSION
B: TiN
C: PRO-EUTECTOID CEMENTITE
D: MnS
oz 1 OZZ 1 082 1 2'0 I 0
OZ 1
01 1
09 1 ozz 1 082 1 2'0 9 r
OZ 1
022 1
OZZ 1
1 091 1 0211 0821 1 E'O 1 9 1 3NISSV33a+e3+W3tl+lV 1 3 1
OZZ 1
08
082 1
082 1
OZ 1 3NISSV33a+W3tl+lV
082 1
OZZ L
(puogas)
3W I1
9N I alOH
2'0 1 9
2'0
OZZ 1 2'0 Zl N
SC'O
082 1
(30)
3dnlVd3dW31
SN I alOH
W3H-3NISSV33a-lV
9
082 1
SNOIlIaN03 9NIalOH
aNV SNIIMH
d
8
TO
(30)
3dnlVtl3d81
SNIlV3H
*xnN+W3tltlV
T 218VL
SNOIlIaN03 9N IlSV3
0
3NISSV33a-W3tl-lV
C
(axnu ! u/u)
aiow ~ ~ I S1N33I1s
N311OW JO 31Vtl
Mold NOIlVlfl3dI3
W
SNO I 1 I ON03 SN I N I43d 3laVl
3NISSV33a+W3tl+lV
( a3~Wn1u1! ~ )
N O I ~ V ~ I X O ~ ~
w3d
nz
g?3
I
SS330dd SNIssvS3a W ~ R ~ V A
dO 'SS330dd x n l j 'SS330tld
NO 1lya1~03~a 3 8' ~ ~ 3 3 0 ~ d
N O I L V ~ I X OI;V~d ~o 33~3no3s
S% 05
s! 3
25
l----J---lA---L-- I
T L L X X X X O ~ I L Z X X X X X X X ~ ~ ~ ~ ~ X T T T ~ T T T T T T T T T T T T T T T T
MANUFACTURING
CONDITION CODt
TABLE 2B
TABLE 3A -
u w $S
30
t-z
00 ZE
30
Z Z
40 z u -
-F
F -
F -
F -
F
-F
F -
F -
F
-F
-F
-F
-F
F -
-F
-F
-F
F -
F -
F -
-K
K -
K -
K -
-K
K -
K -
K -
K
-K
-K
K -
K -
-N
-J
' M -
L -
-0
-K
K -
K -
-K
J -
-J
F
STATE OF MOST
ABUNDANT
INCLUSIONS WITH RESPECT TO TOTAL
INCLUSIONS Pi)
TABLE 3B
TABLE 4A
* STEEL NUMBER 55 WAS INTENDED TO BE MANUFACTURED UNDER THE CONDITION A.
BUT CASTING WAS IMPOSSIBLE DUE TO CLOGGING OF A CASTING NOZZLE
TABLE 4B
* STEEL NUMBER 55 WAS INTENDED TO BE MANUFACTURED UNDER THE CONDITION A,
BUT CASTING WAS IMPOSSIBLE DUE TO CLOGGING OF A CASTING NOZZLE
TABLE 5A
STATE OF MOST
ABUNDANT
INCLUSIONS
AI-Ca-0
---
OCCURRENCE OF NOZZLE CLOGGING
REM-Ca-AI-0-S
MnS
CaO, Al-Ca-0
CaO
OCCURRENCE OF QUENCHING CRACK
OCCURRENCEOFQUENCHINGCRACK
REM-Ca-AI-0-S-(TiN)
OCCURRENCEOFQUENCHINGCRACK
OCCURRENCEOFQUENCHINGCRACK
TiN, REM-Ca-AI-0-S-(TiN)
A1203, REM-Ca-AI-0-S-(TiN)
OCCURRENCE OF CRACK DURING PROCESSING
OCCURRENCE OF CRACK DURING PROCESSING
OCCURRENCE OF CRACK DURING PROCESSING
OCCURRENCE OF CRACK DURING PROCESSING
OCCURRENCE OF CRACK DURING PROCESSING
46
NUMBER FRACTION OF
COMPOSITI: INCLUSION
TO WI (ICI I TIN ADHERES
Wl l H RtSl'tC1 1 0 TOTAL
INCLUSIONS (OV)
TABLE 5B
6.34 3.7 706.5 COMPARATIVE EXAMPLE
- - - COMPARATIVE EXAMPLE
12.46 4.1 739.0 COMPARATIVE EXAMPLE
d z
LLI w
fl t o
52 t-
O0 5
40
ZO
57
58
59
60
6 1 1 A 1 -7.93
1 6 6 1 F I 0.10 1 7.8 1 580.0 1 COMPARATIVE EXAMPLE)
SUM OF NUMBER DENSITY OF
TiN HAVING MAXIMUM DIAMETER
OF 1 fl m OR MORE WHICH
INDEPENDENTLY EXISTS WITHOUT
ADHESION TO INCLUSION AND
NUMBER DENSITY OF MnS HAVING
MAXIMUM DIAMETER OF 100 m
OR MORE (PIECES/mm2)
1
Q
F
F
62
63
64
65
1 6 7 1 F I - I - I - ICOMPARATIVE EXAMPLE)
722.3
PROPERTIES
Li0(x106)
(CYCLES)
-9.14
-9.56
7.10
11.50
COMPARATIVE EXAMPLE
C
D
F
F
- - I* 0.10
COMPARATIVE EXAMPLE
697.6 /COMPARATIVE EXAMPLE
0.10
180°C
TEMPERING
VICKERS
HARDNESS
(H,)
6.2
7.5
8.3
8.5
8.02
11.02
0.1 0
-
REMARK
7.7
72
73
74
722.6
734.6
660.0
660.0
5.0
5.1
8.0
-
75
COMPAFATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPAWIVE EXAMPLE
COMPARATIVE EXAMPLE
585.0
F
F
F
6.4
8 3 1 F I
1 - - - I COMPARATIVE EXAMPLE
..
720.0
720.0
590.0
-
COMPARATIVE EXAMPLE
F
79
81
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
-- 10.50
-9.40
-8.56
697.6
7.58 ----
COMPARATIVE EXAMPLE
-
F
F
6.3
6.5
6.3
- I COMPARATIVE EXAMPLE
6.3
J -
-
697.6
697.6
697.6
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
697.6
-
-
COMPARATIVE EXAMPLE
-
-
COMPARATIVE EXAMPLE
COMPARATIVE EXAMPLE
[Document Type] CLAIMS
1. A steel for induction hardening, comprising as a cliemical composition, by mass%:
C: 0.45% to 0.85%;
Si: 0.01% to 0.80%;
Mn: 0.1% to 1.5%;
Al: 0.01% to 0.05%;
REM: 0.0001% to 0.050%;
0 : 0.0001% to 0.0030%;
Ti: less than 0.005%;
N: 0.015% or less;
P: 0.03% or less;
S: 0.01% or less; and
the balance consisting of Fe and impurities,
wherein tlie steel for induction hardening includes a co~npositein clusion
which is an inclusion containing REM, 0, S, and Al, to wvl~ichT iN is adhered, and
the sun1 of a number density of TiN having a maximum diameter of 1 pm or
more which independently exists without adhesion to tlie inclusion, and a number
density of MnS having a maximum diatileter of 10 ptn or more is 5 pieces/ni~n2o r less.
2. A steel for induction hardening, conlprising as a chemical composition, by mass%:
C: 0.45% to 0.85%;
Si: 0.01% to 0.80%;
Mn: 0.1% to 1.5%;
Al: 0.01% to 0.05%;
Ca: 0.0050% or less;
REM: 0.0001% to 0.050%;
0 : 0.0001% to 0.0030%;
Ti: less than 0.005%;
N: 0.015% or less;
P: 0.03% or less;
S: 0.01% or less; and
the balance consisting of Fe and impurities,
wherein the steel for induction hardening includes a conlposite inclusion
which is an inclusion containing REM, Ca, 0, S, and Al, to \vllich TiN is adhered, and
the sum of a nutnber density of TiN having a maximum diameter of 1 pm or
more ~vllicliin dependently exists \vitl~outa dhesion to the inclusion, and a number
density of MIIS having a maximn~nd iameter of 10 pm or more is 5 pieces/mm2 or less.
3. The steel for induction hardening according to Claim 1 or 2, further comprising as
the chemical composition, one-or more kinds of elements selected fiom the group
consisting of, by mass%:
Cr: 2.0% or less;
V 0.70% or less;
Mo: 1.00% or less;
W: 1.00% or less;
Ni: 3.50% or less;
Cu: 0.50% or less;
Nb: less than 0.050%; and
B: 0.0050% or less.