[Technical Field of the Invention]
[OOOl]
The present invention relates to steel for spring which is used as suspension
device of automobile and the like, and to a method of manufacturing the sanie.
Particularly, the present invention relates to spring steel in which generation
of a REM inclusion is controlled to remove a bad effect of a harmful inclusion such as
alumina, TiN or MnS, and which has fatigue resistance, and to a method of
manufacturing the same.
[Related Art]
[0002]
Spring steel is used as a suspension springs for suspension device of
automobile or the like, and high fatigue resistance is required to the spring steel.
Particularly, demands for reducing the weight and improving the output of the
automobile become higlier so as to reduce the amount of exhaust gas and improve fuel
consumption in recent years, and high stress design of suspension springs which are
used for an engine or a suspeiision or the like has been desired.
[0003]
Therefore, the spring steel is intended to increase strength and reduce wire
diameter, and it is expected that load stress is increasing more and more.
Accordingly, the spring steel having high-performance in which fatigue
strength is Inore improved and settling resistance is more excellent has been required.
[0004]
One of the reasons that fatiguc resistance and settling resistance of the spring
steel are deteriorated is due to coarse ~nclusions( hereinafter, these are called inclusion)
such as alumina and TiN of non-metallic hard inclusion or MnS, which are contained
in the steel.
These inclusions easily become the origin in which stress is concentrated.
[OOOS]
In addition, when a coating on a surface of a suspension spring is peeled off
and then the exposed surface of the material is corroded, the fatigue strength of the
suspension spring may be deteriorated due to the irruption of hydrogen into the steel
from the moisture which is adhered to the exposed surface of the material.
In this case, the inclusions act as a hydrogen trap site, and then hydrogen is
easily concentrated in the steel.
Therefore, an influence by inclusion itself and an intluence by hydrogen are
superimposed with each other. As a result, it causes the deterioration of fatigue
strength.
[0006]
From this viewpoint, it is needed that alumina, MnS and TiN which are
contained in the steel are reduced as possible in order to improve the fatigue resistance
and settling resistance of the spring steel.
[OO07]
Since dissolved oxygen in a large amount is included in molten steel refined
by a converter or a vacuum processing vessel, this excessive oxygen is deoxidized by
Al with a strong affinity with oxygen.
In addition, a ladle and the like are constructed by an alumina-based
refractory in many cases.
Accordingly, even in a case of deoxidation by Si or Mn, not by Al, alulnina
that is the refractory is dissociated due to a reaction between molten steel and the
refractory, and then, alumina is eluted a? Al in molten steel.
Therefore, the eluted Al is re-oxidized and alumina is generated in the molten
steel.
[OOOS]
An alumina inclusion in the molten steel aggregates and integrates with each
other, and can be easily clustered.
The clustered alumina inclusion remains in the products and brings an adverse
effect on the fatigue strength.
[0009]
Accordingly, in addition to the reduction of products obtained by deoxidation,
reduction of inclusion and improvement of cleanliness are performed by a combination
of (I) prevention of re-oxidation due to deaeration, slag reforming and the like, and (2)
reduction of a mixed-in oxide-based inclusion caused by slag-cutting through the
application of a secondav refining apparatus such as a RH degasser and a powder
blowing apparatus in order lo reduce and remove the alumina inclusion.
[OOl 01
On the other hand, as disclosed in Patent Document 1, as a technique for
refining an aluminum-based inclusion and removing the adverse effect, the method of
reforming aluminum into spinel (A1203.Mg0) or MgO by adding Mg alloy to the
molten steel is kuown.
According to this method, coarsening of alumina due to agglutination can be
prevented, and it is possible to avoid adverse effects of alumina for the steel quality.
[OOll]
However, in this method, softening the steel during hot roiling or friability of
inclusions during drawing is not sufficient due to a crystalline phase in an oxide-based
inclusion.
Therefore, miniaturization of inclusions is insufficient.
[OOI 21
Patent Document 2, in addition to controlling an average composition of the
Si02-Al203-Ca0-based oxide having the thickness 2pm or more in the longitudinal
section of the longitudinal direction of steel wire rod to be SOz: 30 to 60%, AIzO,: to
30% and CaO: 10 to 50%, and to controlling the melting point of the composite oxide
to be 1400°C or lower, preferably to be 1350°C or lower, discloses'that the oxide-based
inclusion is dispersed finely by further including B203: 0.1 to 10% in the oxides,
thereby remarkably improving the drawability and fatigue strength.
[0013]
However, the addition of B203 is effective for suppressing crystallization of a
Ca0-Al203-Si02-MgzO-based oxide, but it cannot be said that the addition of Bz03 is
useful for limiting or detoxifying TIN, MnS or alumina cluster which becomes a place
where fatigue accumulates as a fracture initiation point in the spring steel.
100141
In addition, with regard to manufacturing Al-killed steel that contains 0.005%
by mass or more of acid-soluble Al, an alloy composed of two or more kinds of
elements selected from Ca, Mg, and E M , and Al is added to the molten steel.
Therefore, a method of manufacturing alumina cluster free Al-killed steel through
adjusting the amount ofAlzO3 in a generated inclusion to a range of 30 to 85 mass% is
known.
[00 151
For example, as disclosed in Patent Docunient 3, in a case of adding mM, an
inclusion with a low melting point is formed by adding two or more ltinds of elements
selected from REM, Mg, and Ca so as to prevent generation of an alumina cluster
Although this technique is effective at preventing sliver flaws, it is difficult to
make the size of the inclusion small to a level that is demanded for the spring steel.
The reason is thxt inclusions with a low melting point aggregates and
integrates with each other, and thus the inclusion tends to be relatively coarsened,
when the inclusions with a low melting point is used.
[0016]
Since the addition of REM of more than 0.010 mass% maltes inclusion
increase, rather than fatigue life is deteriorated. For example, as disclosed in Patent
Document 4, it is known that it is necessary for limiting the addition of REM to 0.010
mass% or less.
However, Patent Document 4 does not disclose mechanism of this
phenomenon, composition and state of inclusion.
[00 1 71
In addition, wl~cna n inclusion made of a sulfide such as MnS is stretched by a
process such as rolling, it may become a place where fatigue accumulates as a fracture
initiation point, and deteriorate the fatigue resistance of the steel.
Accordingly, to improve the fatigue resistance, it is necessary to limit the
sulfide which stretches.
In addition, as a method of preventing generation of a sulfide, a method in
which Ca is added for desnlfurization is known.
However, an AI-Ca-0 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
fracture mitiation point.
[OO 1 81
In addition, since TiN is very hard, and clystalizes or precipitates in steel in a
sharp shape, TiN becomes a place where fatigue accumulates and a fracture initiation
point, and thus, an influence on the fatigue resistance is great.
[00 1 91
For example, as disclosed in Patent Document 5, when the amount of Ti
exceeds 0.001 mass%, the fat~guere sistance 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 Si-alloy, and thus it is difficult to
avoid mixing-in of Ti as an impurity.
In addition, it is necessary not to contain N in a molten steel, hut this results in
an increase in the costs of steel-making, and is not realistic.
[Prior Art Document]
[Patent Document]
[0020]
[Patent Documcnt I] Japanese Unexamined Patent Application, First
Publication No. H05-3 11225
[Patent Document 21 Japanese Unexamined Patent Application, First
Publication No. 2009-263704
[Patent Document 31 Japanese Unexamined Patent Application, First
Publication No. H09-263820
[Patent Document 41 Japanese Unexamined Patent Application, First
Publication No. H11-279695
[Patent Document 51 Japanese Unexamined Patent Application, First
Publication No. 2004-277777
[Disclosure ofthe invention]
[Problems to be Solved by the Invention]
[0021]
An object of the invention is to provide spring steel w~thex cellent fatigue
resistance by detoxifying alumina, TiN and MnS which deteriorates fatigue resistance
of the spring steel and a method of manufacturing the same.
[Means for Solving the Probiem]
[0022]
The gist of the invention is as follows.
[0023]
(1) According to a first aspect of the invention, a spring steel includes as a
chemical composition, by mass%: C: 0.4% to less than 0.9%, Si: 1.0% to 3.0%, Mn:
0.1% to 2.0%,Al: 0.01% to 0.05%, E M : 0.0001% to 0.005%, T.0: 0.0001% to
0.003%, Ti: less than 0.005%, N: 0.015% or less, P: 0.03% or less, S: 0.03% or less,
Cr: 0% to 2.096, Cu: 0% to 0.5%, Ni: 0% to 3.5%, Mo: 0% to 1.0%, W: 0% to 1.0%, B:
0% to 0.005%, V: 0% to 0.7%, Nb: 0% to 0.05%, Ca: 0% to 0.0020%, and the balance
consists of Fe and impurities. The spring steel includes a composite inclusion having
a maximum diameter of 2 pm or more that TiN is adhered to an inclusion containing
REM, 0 and Al, in which a number of the composite inclusion is 0.004 pieces/mm2 to
10 pieces/mm2, the maximum diameter of the composite inclusion is 40pm or less.
The sum of the number density of an alumina cluster having the maximum diameter of
10 pn or more, MnS having a maximum diameter of 10 pin or more and TiN having a
maximum diameter of 1 pm or inore is I0 pieces/mm2 or less.
(2) The spring steel according to ( I ) further includes as the chemical
composition, one or more liinds of elements selected from the group consisting of, by
mass%; Cr: 0.05% to 2.0%, CLI: 0.1% to 0.5%, Ni: 0.1% to 3.5%, Mo: 0.05% to 1.0%,
W: 0.05% to 1.0%, B: 0.0005% to 0.005%, V: 0 05% to 0 7%, Nh: 0.005% to 0.05%
and Ca: 0.0001% to 0.0020%.
(3) According to a second aspect of the invention, a method of manufacturing
the spring steel according to (I), the method includes; a process of performing a
deoxidation by using A1 and then performing a deoxidation by using REM for 5
minutes or longer when a molten steel having the chemical composition according to
(1) is refined in a ladle with vacuum degassing, a process of performing a circulation
of the molten steel in a mold in a horizontal direction at 0.1 mlminute or faster when
the molten steel is cast in the mold, and a process of performing a soaking treatment in
which a cast piece obtained by casting is held at a temperature region of 1200°C to
1250°C for 60 seconds or longer and then blooming the cast piece.
(4) According to a third aspect of the invention, a spring includes the spring
steel according to (1).
[Effects of the invention]
[0024]
According to the aspects of the invention, in spring steel, an alumina is
reformed into a REM-Al-0 inclusion, and thus it is possible to prevent coarsening the
alumina. In addition, S is fixed as a REM-AI-0-S inclusion, and thus and thus it is
possible to litnit generation of coarse MnS. Furthermore, TiN is adhered to the REMAI-
0 inclusion or the REM-Al-0-S inclusion to form a composite inclusion, thereby
reducing a number density of harmful TiN that is independently precipitated without
adhesion to the inclusion. Accordingly, it is possible to provide spring steel with
excellent fatigue resistance.
[Brief Description of the Drawings]
[0025]
FIG. 1 is a view showing an example of a composite inclusion observed in a
spring steel according to the invention that TiN is compositely precipitated to a REMAI-
0 inclusion.
[Embodiments of the Invention]
[0026]
The present inventors have performed a thorough experiment and have made
a thorough investigation to solve the problems in the related art.
As a result, the present inventors have obtained the following findings by
adjusting the amount of REM in the spring steel and by controlling deoxidation
process and a method of manufacturing the spring in order to suppress and control a
form of harmful inclusion in the spring steel. When an alumina is reformed into an
oxide containing REM, 0 and Al (hereinafter that may be cited "REM-AI-O"), it is
possible lo prevent coarsening of an oxide. When S is f ~ eads an oxysulfide
containing REM, 0, S and A1 (hereinafter that may be cited "REM-AI-0-S"), it is
possible to limit generation of coarse MnS. Furthermore, when TiN is conjugated to
the REM-AI-0 inclusion or the REM-AI-0-S inclusion, it is possible to reduce the
number density of harmful TiN.
[0027]
Hereinafter, spring steel and a method of manufacturing the same according to
an embodiment of the invention made on the basis of the above-described findings will
be described in detail.
[0028]
First, a chemical con~positiono f thc spring steel according to this embodiment
and the reason why the chemical composition is li~nitedw ill be described.
In addition, %relating to the amount of each of the following elements
represents mass%.
[0029]
C: 0.4% or more and less than 0.9%
C is an effective element to secure strength.
However, when the amount of C is less than 0.4%, it is difficult to give a high
strength to a final spring product.
On the other hand, when the amount of C is 0.9% or more, proeutectoid
cementite is generated excessively in the cooling process after hot rolling, and thus,
workability is remarkably deteriorated.
[0030]
Therefore, the amount of C is set to 0.4% to less than 0.9%.
The amount of C is preferably 0.45% or more, and is more preferably 0.5% or
more.
In addition, the amount ofC is preferably 0.7% or less, and is more preferably
0.6% or less.
[003 11
Si: 1 .O% to 3.0%
Si is an element that increases hardenability and improves fatigue life, it is
necessary for the steel to contain 1.0% or more of Si.
On the other hand, when the amount of Si exceeds 3.0%, the ductility of the
ferrite phase in the pearlite is deteriorated.
[0032]
Si has a function of improving settling resistance that is important in a spring
However, when the amount of Si exceeds 3.0%, the effect is saturated and the cost is
not effective. In addition, decarhurization is promoted.
Accordingly, the amount of Si is set to 1 .O% to 3.0%.
The amount of Si is preferably 1.2% or more, and is more preferably 1.3% or
more.
In addition, the amount of Si is preferably 2.0% or less, and is more
preferably 1.9% or less.
[0033]
Mn: 0. I % to 2.0%
Mn is an element effective for deoxidation and ensuring the strength, when
the amount thereof is less than 0.1%, the effect is not exhibited.
On the other hand, when the amount of Mn exceeds 2.0%, segregation easily
occurs and micro-martensite is generated in the segregated portion. Therefore, the
workability and fatigue resistance are deteriorated.
Accordingly, the amount of MI] is set to 0.1% to 2.0%.
The amount of Mn is preferably 0.2% or more and is more preferably 0.3% or
more.
In addition, the amount of Mn is preferably 1.5% or less, and is more
preferably 1.4% or less.
[0034]
REM: 0.0001% to 0.005%
REM is a strong desulfurizing and deoxidizing element, and plays a very
important role in the spring steel according to this ernbodime~il.
Here, REM is a general term of a total of 17 elements including 15 elements
from lanthanum (atomic number: 57) to lutetium (atomic number: 71), and scandium
(atomic number: 21), and yttrium (atomic number: 39).
[0035]
First, REM reacts with alumina in the steel to separate 0 of alumina, thereby
generating the EM-AI-0 inclusion. Next, REM produces a REM-AI-0-S inclusion
by absorbing S in steel.
[0036]
Functions of REM in the spring steel according to this embodiment are as
follows. REM reforms alumina into REM-Al-0 containing REM, 0, and Al, thereby
preventing coarsening of an oxide.
REM fixes S through formation of REM-AI-0-S containing Al, REM, 0, and
S, and limits generation of coarse MnS.
In addition, TIN is compositely generated using the REM-AI-0 or the REMAI-
0-S as a nucleus site, thereby forming an approximately spherical composite
inclusion having a main stmcture of REM-AI-0-(TiN) or REM-AI-0-S-(TiN). The
amount of precipitated TiN which is independently precipitated and has a hard and
sharp square shape is deteriorated.
[0037]
Here, (TiN) represents TiN adhering to a surface of the REM-AI-0 or the
REM-Al-0-S and forms a composite.
[0038]
The composite inclusion having a main structure of REM-AI-0-(TiN) or
REM-AI-0-S-(TiN) is different from TiN precipitate that is independently precipitated.
For example, as shown in FIG. 1, since the composite inclusion has an approximately
spherical shape, it is difficult for stress to concentrate around the composite inclusions.
In addition, the composite inclusion of REM-AJ-0-(TiN) or REM-AI-O-S(
TiN) has a diameter of 1 to 5 pm, and is not stretched and coarsened, or clustered.
Therefore, since the composite inclusion does not become a fracture initiation
point, the composite inclusion is not a harmless inclusion.
[0039]
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 avalue obtained by dividing the major axis of the inclusion by the minor axis of
the inclusion is 3 or less.
In addition, the reason why TiN is compositely precipitated is assumed to be
because a crystal lattice structure of TiN is similar to a crystal lattice structure of REMA1-
0 or REM-Al-0-S at many points.
[0040]
In addition, Ti is not contained in the REM-AI-0 or in the REM-Al-0-S of the
spring steel according to this embodiment as an oxide.
This is considered to be because T.0 (total oxygen amount) in the spring steel
according to this embodiment is low, and the amount of a Ti oxide generated is very
small.
In addition, Ti is not contained in the inclusions as an oxide, and thus the
crystal lattice structure of the REM-Al-0 or the REM-A1-0-S and the crystal lattice
structure of TIN become similar to each other.
[0041]
Furthermore, REM has a function of preventing coarsening of an alumina
cluster by reforming the alumina into the REM-AI-0 by limiting aggregation and
integration of the alumina.
100421
To express the effect, the steel must contain a predetermined amount or more
of REM so that it is necessary to reformthe alumina into REM-AI-0.
In addition, it is necessary for the molten steel to contain a constant amount or
more of REM based on the amount of S so that S is fixed by forming REM-A1-0-S
inclusions.
[0043]
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, the effect of REM that is contained in steel is insufiicient.
Accordingly, the amount of REM is set to 0.0001% or more, preferably
0.0002% or more, more preferably 0.001% or more, and still more preferably 0.002%
or more.
On the other hand, when the amount of REM is 0.005% or more, it is easy to
contaminate a coarse inclusion into a product by falling off an unstable deposit from a
refractory. Therefore, the fatigue strength of the product is deteriorated.
Accordingly, the amount of REM is set to 0.005% or less, preferably 0.004%
or less, and more preferably 0.003% or less.
[0044]
Al: 0.01% to 0.05%
Al is a deoxidizing element that reduces the total oxygen amount, and is an
element that can be used to adjust the grain size of steel. Therefore, it is necessary for
the steel to contain 0.01% or more, and is preferably 0.02% or more ofAl.
However, when the amount ofAl exceeds 0.05%, the effect of adjusting the
grain size is saturated and a large number of alumina is remained. Therefore, that is
not preferable.
[0045]
T.O (total oxygen amount): 0.003% or less
0 is an impurity element which is removed from steel by deoxidation, b~tt
some will always remain. 0 generates a composite inclusion having a main structure
of REM-A1-0-(TIN) or REM-AI-0-S-(TiN).
However, when the T.O becomes large, especially when the amount of 0
exceeds 0.003%, a large amount of an oxide such as alumina generates, and thus 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%.
[0046]
In the spring steel according to this embodiment, it is necessary to limit Ti, N,
P, and S, which are impurities, as follows.
[0047]
Ti: less than 0.005%
Ti is an impurity which is contaminated from Si-alloy and forms coarse
inclusions such as TiN having an angular shape.
The coarsc inclusion tends to become a fracture initiation point and to act as a
hydrogen trapping site, and thus, deteriorates fatigue resistance.
Therefore, it is very important to limit the generation of the coarse inclusion
having an angular shape.
[0048]
In the spring steel according to this embodiment, generation of isolated TiN
which is harmful can be prevented, by compounding TIN with REM-AI-0 or REM-AI-
0-S.
As a result from the experimental stud~est,h e amount of Ti is limited to less
than 0.005% so as to prevent the generation of isolated TIN.
The amount of Ti is preferably 0.003% or less.
The amount of Ti includes 0%, but it is industrially difficult to stably reduce
Ti. Therefore, the industrial lower limit of the amount of Ti is 0.0005%.
[0049]
N: 0.015% or less
N is an impurity and forms a nitride and deteriorates the fatigue resistance.
In addition, ductility and touglmess are deteriorated due to strain aging.
When the amount of N exceeds 0.015%, a harmful result becomes significant,
and thus, the amount of N is limited to 0.015% or less, is preferably 0.010% or less,
and is more preferably 0.008% or less.
The amount of N includes 094, but it is industrially difficult to stably reduce N.
Therefore, the industrial lower limit of the amount of N is 0.002%.
[OOSO]
P: 0.03% or less
P is an impurity and segregates at a grain boundary, and thus, decreases the
fatigue life.
When the amount of P exceeds 0.03%, a decrease in the fatigue life becomes
significant. Accordingly, the amount of P is limited to 0.03% or less, and is
preferably 0.02% or less.
The amount of P includes 0%, but it is industrially difficult to stably reduce P.
Therefore, the industrial lower limit of the amount of P is 0.001%.
coos 11
S: 0.03% or less
S is an impurity and forms a sulfide.
When the amount of S exceeds 0.03%, S forms coarse MnS and decreases the
fatigue life. Accordingly, the amount of S is limited to 0.03% or less, and is
prefcrable 0.01% or less.
The amount of S inclndes 0%, but it is industrially difficult to stably reduce S.
Therefore, the industrial lower limit of the amount of S is 0.001%.
[0052]
The above-described components are included as a basic chemical
composition of the spriug steel according to this embodiment, and the balance consists
of Fe and impurities.
In addition, "impurities" in the "the balance consists of Fe and impurities"
represents ore or scrap as a raw material when steel is industrially manufactured, or a
material that is mixed in due to the manufacturing environment and the 11lte.
In addition to the above-described elements, the following elements may be
selectively contained. Hereinafter, a selective element will be described.
[0053]
The spring steel according to this embodiment may contain one or more kind
of 2.0% or less of Cr, 0.5% or less of Cu, 3.5% or less of Ni, 1 .O% or less oi'Mo, 1.0%
or less of W, and 0.005% or less of B.
[0054]
Cr: 2.0% or less
Cr is an effective element that increases the strength, and increases the
hardenability and improves the fatigue life.
In a case where the hardenability and temper softening resistance are needed
and 0.05% or more of Cr is contained, it is possible to stably express this effect.
Especially, to obtain excellent leruper softening resistance, it is necessary for
the steel to contain 0.5% or more of Cr, and is preferably 0.7% or more of Cr.
[0055]
However, when the amount of Cr exceeds 2.0%, the hardness of the steel is
increased, and thus the cold workability decreases. Accordingly, the amount of Cr is
set to 2.0% or less.
Specially, in the case of cold-coiling, the amount of Cr is preferably 1.5% or
more so as to improve the stability in the cold- coiling.
[0056]
Cu: 0.50% or less
Cu has an influence on the hardenability, moreover, is an element which
effects corrosion resistance and limits decarburization.
When the amount of Cu is 0.1% or more, and is preferably 0.2% or more, the
effect of limiting decarburization and corrosion is expressed.
[0057]
However, when the amount of Cu is large, hot-ductility is deteriorated, and
thus, cracks and flaws are occurred in the manufacturing process of casting, rolling or
forging. Therefore, thc amount of Cu is 0.5% or less, and is preferably 0.3% or less.
The deterioration in the hot-ductility due to Cu, as described below, can be
relieved by containing Ni. Then, when the amount of Cu i the anlount ofNi, the
deterioration in the hot-ductility can be suppressed and thus high quality can be
maintained.
lo0581
Ni: 3.5% or less
Ni is an element that improves the strength and the hardenability of steel.
When the amount of Ni is 0.1% or more, the effect is expressed.
[0059]
Ni has an influence 011 the amount of retained austenite after quenching too
When the amount of Ni exceeds 3 5%, the amount of the retained austenite becomes
large, and thus, there is a case in which the performance of the spring is insufficient
due to retention softness after quenching.
Accordingly, when the amount ofNi exceeds 3.5%, and thus, instability of the
materials for product is led and the amount of Ni is set to 3.5% or less.
[0060]
In addition, Ni is an expensive element, and is preferably limited from the
view point of manufacturing cost.
From the view point of thc retained austenite and the hardenability, the
amount ofNi is preferably 2.5% or less, and is more preferablj 1.0% or less.
[006 I]
When Cu is contained in the steel, Ni has an effect for suppressing the adverse
effect due to Cu.
That is, Cu is an element that deteriorates the hot-ductility in the steel, and
thus, cracks and flaws are sometimes occurred in the hot-rolling or hot-forgit~g.
100621
However, when Ni is contained, Ni forms an alloy phase with Cu and hotductility
is limited.
In the case where Cu is mixed in the steel, the anlount ofNi is preferably
0.1% or more, and is more preferably 0.2% or more.
In addition, the amount of Cu 5 the amount ofNi is preferable in the
relationship with Cu.
[0063]
Mo: 1.0% or less
Mo is an effective element for improving the hardenability and the temper
softening resistance.
Specially, to improve the temper softening resistance, the amount of Mo is set
to 0.05% or more. Mo is an element that forms Mo-based carbide in the steel.
[0064]
The temperature in which Mo-based carbide is precipitated is lower than Vbased
carbide thereof. Then, it is effective element for the spring steel having highstrength
tempered in the relatively low temperature.
When the amount of Mo is 0.05% or more and this effect is expressed. The
amount of Mo is preferably 0.1% or more.
[0065]
On the other hand, when the amount of Mo exceeds 1.0%, it is easy to form
supercooling structure during cooling in the heat treatment before working or hotrolling.
The amount of Mo is set to 1.0% or less, preferably 0.75% or less so as to
suppress the generation of the supercooling structure that causes delayed cracks or
cracks during worlting.
[0066]
In addition, when it is focused on ensuring production stability by limiting
variation in quality during manufacturing the spring, the amount of Mo is preferably
0.5% or less.
Furthermore, the amount of Mo is preferably 0.3% or less so as to stabilize
shape accuracy by precisely controlling temperature variation - transfortnation strain
during cooling.
100671
W: 1.0% or less
As with Mo, W is an effective element for improving the hardenability and the
temper softening resistance and is an element that precipitates as carbide in the steel.
Specially, the amount of W is set to 0.05% or more, is preferably 0.1% or
more so as to improve the temper softening resistance.
[006S]
On the other hand, when the amount of W exceeds 1.0%, it is easy to form
supercooling structure during cooling in the heat treatment before working or hotrolling.
The amount of W is set to 1.0% or less, preferably 0.75% or less so as to limit
tlse generation of the supercooling structure that causes delayed cracks or cracks during
working.
[0069]
B: 0.005% or less
B is an element for improving the hardenability of the steel by adding the
small amount of 9.
In addition, in a case where a base metal IS high carbon material, B forms
boron-iron carbide in the cooling process after hot-rolling and increases growth rate of
ferrite, and thus, promotes softening the steel.
[0070]
Furthermore, when 0.0005% or more ofB 1s contained in the steel, B
suppresses the segregation of P by segregating at grain boundary of austenite, and thus,
B contributes to an improvement in the fatigue resistance and impact strength due to
strengthening grain boundary.
[0071]
However, when the amount of B exceeds 0.005%, the effect is saturated.
Then it is easy to form supercooling structure such as martensite or bainite during
manufacturing such as casting, hot-rolling and forging, and thus, manufacturability of
product and impact strength may be deteriorated. Therefore, the amount of B is set to
0.005% or less, and more preferably 0.003% or less.
[0072]
'The spring steel according to this embodiment may contain one or more kind
of 0.7% or less of V and 0.05% or less ofNb, by mass%.
[0073]
V: 0.7% or less
V is an element that is coupled to C and N in steel to form a nitride, a carbide
or a carbonitride. Usually, V becomes a minute nitride, a minute carbide or a minute
carbonitride of V having a circle equivalent diameter of less than 0.2 pm, and thus, it is
effective for improving the temper softening resistance, raising the yield point and
refining prior austenite.
[0074]
When V is sufficiently precipitated in the steel by tempering, hardness and
tensile strength can be improved, and thus, V is set to a selected element that is
contained as necessary.
To attain these effects, the amount of V is set to 0.05% or more, preferably
0.06% or more.
[0075]
On the other hand, when the a~nounot f V exceeds 0.7%, carbide and
carbonitrides is not sufficiently soluted in the heating before quenching and remain as
coarse spherical carbide, that is, undissolved carbides. Therefore, since the
worltability and the fatigue resistance are deteriorated, the amount of V is set to 0.7%
or less.
[0076]
When V is contained excessively, since it is easy to form a supercooling
structure that causes craclts or breaking before working, it is preferable that the amount
of V is 0.5% or less.
When it focuses on ensuring production stability by suppressing variation in
quality during manufacturing the spring, the amount of V is preferably 0.3% or less.
[0077]
In addition, since V is an element that has large influence on the generation of
the retained austenite, it is necessary to precisely control the anlou~lto f V.
Accordingly, in a case where other elements that improve the hardenability
are contained, for example, one or more kinds of Mn, Ni, Mo and W is contained, and
the mount of V is preferably 0.25% or less.
[0078]
Nb: less than 0.05%
Nb is an element that is coupled to C and N in steel to form a nitride, a
carbide or a carbonitride.
Compared to a case where Nb is not contained in the steel, even the amount of
Nb is small, so it is very effective for limiting the generation of coarse grain.
These effects are expressed when the amount of Nb is set to 0.005% or more.
[0079]
However, Nb is an element that deteriorates the hot-ductility. When Nb is
contained excessively, Nb causes cracks during casting, rolling and forging, and thus,
manufacturab~lityis much deteriorated.
Therefore, the amount of Nb is set to 0.05% or less
Furthermore, in a caye where it focuscs on the workability such as the cold
coilability, the amount ofNb is less than 0.03%, and is preferably less than 0.02%.
LOOSO]
The spring steel according to this embodiment may contain 0.0020% or less
of Ca, by mass%.
Ca: 0.0020% or less
Ca has a strong desulfurizing effect and is effective for limiting the generation
of MnS. Accordingly, 0.0001% or more of Ca may be contained for the purpose of
desulfnrization.
However, Ca is absorbed into REM-AI-0 inclusion or REM-AI-0-Sinclusion
in the steel and forms REM-Ca-Al-0-S or REM-Ca-AI-0-S.
[OOSZ]
Compared to REM-Al-0 and REM-AI-0-S, REM-Ca-AI-0 and REM-Ca-AI-
0-S tends to increase thc size thereof, in the case where the oxide in which thc amount
of oxygen is large is the main inclusion in the inclusions. Furthermore, since REMCa-
AI-0 and REM-Ca-AI-0-S deteriorates the ability in which TiN is compositely
precipitated, from the view point of removing the adverse effect, the amount of Ca is
preferably small.
The reason is assumed that REM-Ca-AI-0 and REM-Ca-A1-0-S is inferior to
REM-AI-0 and REM-AI-0-S with respect to the similarity in the crystal lattice
structure with TiN.
[0083]
In addition, when the amount of Ca exceeds 0.0020% in the steel, many Al-
Ca-0 oxides having a low melting point are generated and become coarse inclusions
due to stretching by rolling. Therefore, the place coarse inclusions become places
where fatigue accumulates or fractures start.
Accordingly, Ca is a selected element and the amount of Ca is set to 0.0001%
to 0.0020%.
[0084]
Next, influenccs on the fatigue life due to the inclusions will be described as
follows.
The inventors obtained the findings as below through the experimental studies.
(1) As shown in FIG I , since 0.004 pieces/mm2 or more of the composite
inclusions having a maximum diameter of 2 pm that TiN is adhered to the inclusions
containing REM, 0 and Al, or the inclusions containing REM, 0, S and Al, are
contained, the generation of isolated TiN that is independently precipitated is limited,
and thus, the fatigue life can be improved.
(2) However, when the composite inclusions having a circle equivalent
diameter of more than 10 pm are observed, even the composite inclusions tend to
deteriorate the fatigue strength.
(3) In addition, when the total of isolated inclusions (a), (b) and (c) that is
separated from the above composite inclusions and has a negative effect, which is
equivalent to each other, on the fatigue life is 10 pieces/mm2 or less, the excellent
fatigue life can be obtained.
(a) MnS having a maximum diameter of 10 pm or more (Stretched MnS)
(b) Alun~inac luster having a maximum diameter of 10 pm or more
(c) TiN having a maximum diameter of 1 pm or more (isolated TIN)
Since alumina is reformed into REM-AI-0 in the spring steel according to this
embodiment, the generation of alumina cluster which is harmful for fatigue resistance
is limited.
In addition, since S is fixed as REM-AI-0-S, the generation of MnS that is
stretched and deteriorates the fatigue resistance, and the like
[OOSS]
Furthermore, for example, as shown in FIG. 1, since TiN is conjugated to
REM-AI-0-S and an approximately spherical composite inclusion having a main
structure of REM-Al-0-S-(TiN) is formed, the generation of TiN that is independently
precipitated and has an adverse effect on the fatigue life is limited.
[0087]
As a result, the total number density of (a) MnS having a maximum diameter
of 10 pm or more (Stretched MnS), (b) Alumina cluster having a maximum diameter of
10 pm or more and (c) TiN having a maximum diameter of 1 pm or more (isolated
TiN) is limited to be 10 pieces/mm2 or less. Therefore, the fatigue life can be
improved.
[OOSS]
Amethod of manufacturing the spring steel according to this embodiment will
be described.
[0089]
When molten steel for the spring steel according to this embodiment is refined,
a sequence of adding a deoxidizing agent and the deoxidation time are important.
In this manufacturing method, first, deoxidation is performed by using Al and
T.0 (total oxygen amount) is set to 0.003% or less.
Then, deoxidation is performed for 5 minutes or longer by using REM, and
then ladlc refining including vacuum degassing is performed.
[0090]
Prior to deoxidation with REM, when deoxidation is performed by using an
element other than Al, it is difficult to stably reduce an amount of oxygen. In addit~on,
after deoxidizing by using Al, deoxidation is performed by using REM, and the
composite inclusions that TiN is adhered to REM-AI-0 or REM-Al-0-S tends to be
generated.
In addition, when deoxidation time is shorter than 5 minutes after adding
REM, alumina cannot be sufficiently reformed.
In this manufacturing method, the deoxidizing agent is added in the above
order and REM-AI-0 inclusion is generated, and thus, the generation of harmful
alumina is limited.
[009l]
For the REM added, a misch metal (alloy composed of a plurality of rareearth
metals) and the like may be used, and for example, an aggregated misch metal
may be added to moltell steel.
In addition, at the end of the refining, Ca-Si alloy or flux such as CaO-CaF2
can be added to approximately perform desulfurization by Ca.
[0092]
The specific gravity of REM-Al-0 or REM-AI-0-S generated by deoxidation
in the molten steel that refined by ladle is 6 and is close to a specific gravity of 7 of
steel, and thus floating and separation are less likely to occur.
Thercfore, when pouring molten steel into a mold, the EM-A1-0 or REMAI-
0-S penetrates up to a deep position of unsolidified layer of a cast piece due to a
downward flow, and thus REM-Al-0 or EM-AI-0-S tends to segregate at the central
portion of the cast piece.
[0093]
When EM-A1-0 or REM-Al-0-S segregates at the central portion of the cast
piece, REM-AI-0 or REM-AI-0-S is deficient in a surface layer portion of the cast
piece. Therefore, it is difficult to generate a composite inclusion by adhering TiN to
the FEM-AI-0 or REM-AI-0-S. Accordingly, a detoxifying effect of TiN is
weakened at a surface layer portion of a product.
Accordingly, in this manufacturing method, to prevent segregation of the
REM-AI-0 and REM-AI-0-S, molten steel is stirred and circulated in the mold in a
horizontal direction to realize uniform dispersion of the inclusions.
[0094]
The circulation of the molten steel inside the mold is performed at a flow rate
of 0.1 miminute or faster so as to realize further uniform dispersion of REM-Al-0 and
REM-Al-0-S in this manufacturing method.
When the circulation speed inside the mold is slower than 0.1 miminute,
REM-A1-0 and REM-AI-0-S are less likely to be uniformly dispersed.
As stirring means, for example, an electromagnetic force and the like may be
applied.
[0095]
Next, soaking treatment is performed to the cast steel, and then, blooming is
performed.
The cast piece is held at a temperature region of 1250°C to 1200°C for 60
seconds or more to obtain the above-described composite inclusion in the soaking
treatment.
[OOY 61
This temperature region is a temperature region at which a composite
precipitation of TiN with respect to REM-Al-0 and REM-AI-0-S are started. TiNis
allowed to sufficiently grow at the surface of REM-Al-0 and REM-Al-0-S in this
temperature region. To limit the generation of isolated TiN that is independently
precipitated, it is necessary to be hold the cast piece at a temperature region of 1250°C
to 1200°C for 60 seconds or more.
The present Inventors obtained the knowledge through experimental studies.
[OOY 71
In addition, typically, when the cast piece is heated at a temperature region of
1250°C to 1200°C, TiN is solid-soluted.
However, in the spring steel according to this embodiment, the amount of C is
0.4% to 0.Y%, and is high. Many cementite are existed in the spring steel and
solubility of N in the cementite is low, and thus, it is assumed that TiN is precipitated
and grows at the surface of REM-Al-0 and EM-M-0-S.
[OOY 81
Two kinds of hot forming method and cold forming method are uscd as
forming method of the spring.
In the hot forming method, after the wire rod is manufactured by blooming
and wire rolling, the steel wire is manufactured by small wire drawing so as to adjust
the roundness. Then, after the steel wire is heated and hot-formed into the spring
shape at 900°C to 1050°C, the strength is adjusted by quenching at 850°C to Y50°C
and by tempering at 420°C to 500°C in the heat treatment.
[OOYY]
On the other hand, in the cold forming method, after the wire rod is
manuhctured by blooming and wire rolling, the steel wire is manufactured by small
wire drawing so as to adjust the roundness. Before the steel wire is formed into the
spring shape, the steel wire is heated and the strength of the steel wire is adjusted by
quenching at 850°C to 950°C and by tempering at 420°C to 500°C in the heat
treatment. Then, the steel wire is formed into the spring shape in room temperature.
[OlOO]
Thereafter, shot peening is performed as necessary. In addition, it is
subjected to plating or resin coating on the surface of the steel wire, and products are
manufactured.
Example
[OlOl]
Next, examples of the invention will be described, but conditions in the
examples are conditional examples that are employed to confirm applicability and an
effect of the invention and the invention is not limited to the conditional examples.
The invention can employ various conditions as long as the object of the
invention is achieved without departing from the gist of the invention.
[0102]
During the vacuum degassing in the ladle refining, refining was performed
under conditions shown in Table 1 by using metal Al, a misch metal, Ca-Si alloy and a
flux of CaO:CaF2=50:50 (mass ratio) to obtain molten steel having a chemical
composition shown in Table 2 and Table 3. The molten steel was cast to a 300 mm
square cast piece by using a continuous casting apparatus.
[0 1 031
At that time, circulation inside a mold was performed by electromagnetic
agitation under conditions shown in Table 1 , thereby manufacturing a bloom.
The bloom was heated at 1200°C to 1250°C for a time as shown in Table 1
and blooming was performed to manufacture a billet, and billet having a size of
160mm x 160mm was manufactured. The billet was reheated at llOO°C, and steel
bar having a diameter of 15mm was obtained by bar-rolling
Furthermore, quenching at 900°C for 20 minutes and tempering heat
treatment at 450°C for 20 minutes were performed to the sample cut from the bar steel
and water cooling was performed, and thus, hardness of wire rod was adjusted 480 HV
to 520 HV by Vickers hardness.
[0 1041
Thereafter, No. 1 test specimen (total length; 80mm, grip length; 20mm, grip
diameter Do=12mm, parallel portion diameter d = 6mm, parallel portion
length=lOmm) for Method of Rotating Bending Fatigue Testing of Metals of JIS
22274 (1978) was fabricated by finish machining.
In addition, electrolytic charging was performed in the an aqueous solution of 3%NaC1
+ 0.3%ammonium thiocyanate as the test specimen being a cathode, thereby, 0.2 to 0.5
ppm of the hydrogen was included in the steel.
[0105]
After charging, hydrogen was filled in the test specimen by performing Zncoating.
Then, rotating bending fatigue test was performed to the test specimen using
Ono-type rotating bending fatigue testing machine by applying both pretend stress
repeated stress according to JIS 22273 (1978), and load stress at the fatigue limit up to
5 x lo5 was evaluated.
[01061
In addition, with regard to the above-described sample, 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 performed with respect to
inclusions in steel in a range of 2 mm width in a radial direction which centers around
a depth of the half of a radius from a surface, and a length of 5 mm in a rolling
direction, a composition of the inclusion was analyzed using EDX, and inclusions in
10 mm2 of the sample were counted to measure the number density.
[0107]
[Table 11
[0108]
[Table 21
[O 1091
[Table 31
loll01
The results were shown in Table 4.
The oxide inclusions of examples Nos. 1 to 28, as shown in FIG .I, were
reformed into the composite inclusion that TiN was adhered to REM-AI-0 or REM-A1-
0-S and alumina cluster having a maximum diameter of 10pn1 or more was trot
included. The total number of MnS having a maximum diameter of 1 Opm or more
and TiN having a maximum diameter of lpm or more, as shown in Table 4, was 10
pieces/mm2 or less.
[Olll]
In addition, in the examples Nos. 1 to 28, fatigue strength obtained by rotating
bending fatigue test was higher several tens of MPa than that of comparative examples
Nos. 1 to 7, and ihus, it is seen that excellent fatigue resistance were obtained.
[01121
In the comparative example 1, since A1 was only added and REM was not
added in the steel, there were many alumina clusters, MnS and TiN in the steel.
In the comparative example 2, since the amount of REM was small, there
were many alumina clusters, MnS and TiN in the steel.
In the comparative example 3, since the amount of S was large, there were
many MnS in the steel.
In the comparative example 4, since the reflux time after adding REM was
shorter, there were many alumina clusters, MnS and TIN in the steel.
In the comparative example 5, since circulation flow rate of molten steel
inside mold was slower, there were many TiN at the surface portion due to segregation
of REM-Al-0 or REM-Al-0-S at near the center portion of the cast piece.
In the comparative example 6, since holding time at 1250°C to 1200°C is
shorter, there were many TiN in the steel.
In the comparative example 7, since the amount of REM was large, the
maximum diameter of the composite inclusion to which TiN was adhered became
larger.
[0113]
In the comparative examples described above, the fatigue strengths of the
products were poor due to the influence of the inclusions.
[0114]
[Table 41
[Industrial Applicability]
[0115]
According to the invention, the alumina is reformed into the REM-AI-0 and it
is possible to prevent coarsening oxide, in addition, S is fixed as REM-AI-0-S and it is
possible to limit coarsening MnS, fi~rthermoreT, iN is conjugated to REM-AI-0-S
inclusion and the number density of isolated TIN that is independently precipitated can
be reduced. Therefore, it is possible to provide spring steel with excellent fatigue
resistance. Accordingly, it can be said that the industrial applicability of the invention
is high.
[Brief Description ofthe Reference Symbols]
[0116]
B: TiN THAT IS COMPOSITELY PRECIPITATED AT SURFACE OF
REM-AI-0-S
C: PRO-EUTECTOID CEMENTITE

LAIMS
What is claimed is:
[Claim I]
A spring steel comprising as a chemical composition, by mass%:
C: 0.4% to less than 0.9%;
Si: l.O%to 3.0%;
Mn: 0.1% to 2.0%;
Al: 0.01% to 0.05%;
REM: 0.0001% to 0.005%;
T.0: 0.0001% to 0.003%;
Ti: less than 0.005%;
N: 0.015% or less;
P: 0.03% or less;
S: 0.03% or less;
Cr: 0% to 2.0%;
Cu: 0% to 0.5%;
Ni: 0% to 3.5%);
Mo: 0% to 1.0%;
W: 0% to 1.0%;
B: 0% to 0.005%;
V: 0% lo 0.7%;
Nb: 0% to 0.05%;
Ca: 0% to 0.0020%; and
the balance collsisling of Fe and impurities, wherein;
the spring steel includes a composite inclusion having a maximum diameter of
2 pm or more that TiN is adhered to an inclusion containing REM, 0 and Al;
a number of the composite inclusion is 0.004 pieces/~nmt2o 10 pieces/mmz,
and a maximum diameter of the composite inclusion is 40pm or less; and
a sum of the number density of an alumina cluster having the maximum
diameter of 10 pm or more, MnS having the maximum diameter of 10 pm or more and
TiN having the maximum diameter of 1 pm or more is 10 pieces/mm2 or less.
[Claim 21
The spring steel according to Claim 1, further comprising as the chemical
composition, one or more kinds of elements selected from the group consisting of, by
mass%:
Cr: 0.05% lo 2.0%;
Cu: 0.1% to 0.5%;
Ni: 0.1% to 3.5%;
Mo: 0.05% to 1.0%;
W: 0.05% to 1.0%;
B: 0.0005% to 0.005%;
V: 0.05% 0.7%;
Nb: 0.005% 0.05%; and
Ca: 0.0001% 0.0020%.
[Claim 31
A method of manufacturing the spring steel according lo Claim 1, the method
comprising;
a process of performing a deoxidation by using Al and then performing a
~
deoxidation by using REM for 5 lnilli~teso r longer whell'a molten steel havihg the
1 chemical coruposition according to Claitn 1 is refined in a ladle with vacLluln
I
I a process of perforining a circulatioll of the lnoltell steel in a mold ill a
i
horizontal direction at 0.1 tnirninute or fasler when the molten steel is cast in the mold, 1 and
6 .
1 a process of performi~~ag s oalti~lgtr eatment in which a cast piece obtained by
casllng is held at a temperatule reglon of 1200°C to 1250°C for 60 seconds or longel
and then bloolnlng thc cast piece
4 A spring co~nprisillgth e spring steel according to Clailh