DESCRTPTION
TITLE OF INVENTION
SPRING STEEL AND METHOD FOR PRODUCING THE SAME
TECHNICAL FIELD
[0001]
The present invention relates to a spring steel and a method for producing the
same.
BACKGROT]ND ART
[0002]
Spring steels are used in automobiles or-machines in general. When a spring
steel is used for an automobile suspension spring, for example, the spring steel must
have high fatigue strength. Recently, there has been a need for automobiles having
reduced weight and higher po\¡/er output for improved fuel economy. Accordingly,
spring steels that are used for engines or suspensionq are required to have even
higher fatigue strength.
[0003]
Steel products may contain oxide inclusions typified by alumina. Coarse
oxide inclusions decrease fatigue strength.
[0004]
The alumina forms when the molten steel is deoxidized in the refining step.
Ladles or the like often contain alumina refractory materials. For this reason,
alumina may form in the molten steel not only in the case of Al deoxidation but also
when deoxidation is carried out with an element other than Al (e.g., Si or Mn).
Alumina in the molten steel tends to agglomerate and form clusters. In other words,
alumina tends to be coarse.
[000s]
Techniques for refining oxide inclusions typified by alumina are disclosed in
Japanese Patent Application Publication No. 05-311225 (PaIent Literature 1),
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Japanese Patent Application Publication No. 2009-263704 (Patent Literature 2),
Japanese Patent Application Publication No. 09-263820 (Patent Literature 3), and
Japanese Patent Application Publication No. 11-279695 (Patent Literature 4).
[0006]
Patent Literature 1 discloses the following. A Mg alloy is added to the
molten steel. As a result, the alumina is reduced and instead spinel (MgO'AlzO:) or
MgO is formed. Consequently, coarsening of the alumina due to agglomeration of
the alumina is inhibited.
[0007]
However, the production method of Patent Literature 1 poses the possibility
of nozzle clogging in a continuous casting machine. In such a case, coarse
inclusions are more likely to become entrapped in the molten steel. This results in
reduced fatigue strength ofthe steel.
[0008]
Patent Literature 2 discloses the following. The average chemical
composition of SiOz-AlzO:-CaO oxides at a longitudinal cross-section of the steel
wire rod is controlled to. be SiOz: 30 to 60%o,A1z0::,1 to 3}%o,and CaO: l0 to 50%o
so that the melting point of the oxides is not more than 1400"C. Furthermore, 0.1 to
10% of BzOs is included in the oxides. As a result, the oxide inclusions are finely
dispersed.
[000e]
However, although BzO¡ is effective for the above oxides, it sometimes
cannot inhibit alumina clustering sufficiently. In such a case, the fatigue strength
decreases.
[0010]
Patent Literature 3 discloses the following. In the method of producing an
Al-killed steel, an alloy made of two or more selected from the group consisting of
Ca, Mg, and rare earth metal (REM) and Al is added to the molten steel for
deoxidation.
[001] j
tù
However, in some cases, addition of the above alloy to a spring steel does not
cause refinement of oxide inclusions. In such cases, the fatigue strength of the
spring steel decreases.
[0012]
Patent Literature 4 discloses the following. The bearing steel wire rod
includes equal to or less than 0.010% of REM (0.003% in the example) so that
inclusions can be spheroidized.
[0013]
However, in some cases, addition of the above content of REM to a spring
steel does not cause refinement of oxide inclusions. In such cases, the fatigue
strength ofthe spring steel decreases.
t00141
Furthermore, suspension springs have the role of absorbing vibrations of the
vehicle body caused by irregularities of the road surface on which it is traveling.
Accordingly, suspension springs must have not only fatigue strength but also high
toughness.
[0015]
Methods for producing a spring include hot forming and cold forming. In
cold forming, coiling is performed by cold operation to produce springs.
Accordingly, spring steels must have high ductility for cold operation.
CITATION LIST
PATENT LITERATURE
[0016]
Patent Literature 1: Japanese Patent Application Publication No. 05-31 1225
Patent Literature 2: Japanese Patent Application Publication No. 2009-263704
Patent Literature 3: Japanese Patent Application Publication No. 09-263820
Patent Literature 4: Japanese Patent Application Publication No. l1-279695
SUMMARY OF INVENTION
[0017]
An object of the present invention is to provide a spring steel that exhibits
excellent fatigue strength, toughness, and ductility.
[0018]
A spring steel according to the present embodiment has a chemical
composition consisting of, in masso/o, C: 0.4 to 0.7Yo, Si: 1.1 to 3.0%o, Mn: 0.3 to
l.5o/o,P: equal to or less than 0.030/o, S: equal to or less than 0.05%, Al: 0.01 to
O.\so/o,rare earth metal: 0.0001 to 0.002o/o,N: equal to or less than 0.015%, O: equal
to or less than 0.0030Yo,Ti:0.02 to 0.7o/o, Ca:0 to 0.0030%, Cr: 0 to 2.0%o, Mo: 0 to
1.0ol0, W: 0 to 1.00lo, V: 0 to 0.l0yo, Nb: 0 to less than 0.050o/o, Ni: 0 to 3.5%o, Cu: 0
to 0.5%o, and B: 0 to 0.0050%, with the balance being Fe and impurities. In the
spring steel, the number of oxide inclusions having an equivalent circular diameter of
equal to or greater than 5 pm is equal to or less than 0.2/mm2, the oxide inclusions
each being one of an Al-based oxide, a complex oxide containing REM, O and 41,
and a complex oxysulfide containing REM, O, S, and Al. Furthermore, a maximum
value among equivalent circular diameters of the oxide inclusions is equal to or less
than 40 ¡rm.
[001e]
The spring steel according to the present embodiment exhibits excellent
fatigue strength, toughness, and ductility
BRIEF DESCRIPTION OF DRAWINGS
[0020]
[FIG. 1] FIG. 1 is an SEM image of a complex oxysulf,rde containing Al, O (oxygen),
REM (Ce in this embodiment), and S in a spring steel of the present embodiment.
IFIG. 2] FIG. 2 is a transverse cross-sectional view of a semi-fmished product for
illustrating a method for measuring the cooling rate of the semi-finished product in a
casting step.
[FIG. 3A] FIG. 3A is a side view of an ultrasonic fatigue test specimen.
[FIG. 3B] FIG. 3B is a schematic diagram illustrating a location for cutting a rough
test specimen that serves as a material for the ultrasonic fatigue test specimen
illustrated in FIG. 3A.
DESCRIPTION OF EMBODIMENTS
[0021]
A spring steel according to the present embodiment has a chemical
composition consisting of, in masso/o, C: 0.4 to 0.7%o, Si: 1.1 To 3.0o/o, Mn: 0.3 to
7.5o/o,P: equal to or less fhan 0.03%o, S: equal to or less than 0.05%, Al: 0.01 to
0.05yo, rare earth metal: 0.0001 To 0.002o/o, N: equal to or less than 0.015%, O: equal
to or less than 0.0030%o, Ti:0.02 to 0.7o/o, Ca:0 to 0.0030%, Cr: 0 to 2.0%o, Mo: 0 to
1.0%, W: 0 to 1.00lo, V: 0 to 0.70yo, Nb: 0 to less than 0.0500Á, Ni: 0 to 3.5%o,Cl:0
to 0.5o/o, and B: 0 to 0.0050%, with the balance being Fe and impurities. In the
spring steel, the number of oxide inclusions having an equivalent circular diameter of
equal to or greater than 5 ¡rm is equal to or less than}.2/mm2, the oxide inclusions
each being one of an Al-based oxide, a complex oxide containing REM, O and Al,
and a complex oxysulfide containing REM, O, S, and 41. Furthermore, a maximum
value among equivalent circular diameters of ihe oxide inclusions is equal to or less
than 40 p.m.
100221
In the spring steel according to the present embodiment, the oxide inclusions,
each of which is one of an Al-based oxide, a complex oxide (inclusion containing
REM and containing Al and O), and a complex oxysulf,rde (inclusion containing
REM and containing Al, O, and S), are finely dispersed. As a result, the spring
steel has high fatigue strength. Furthermore, the spring steel of the present
embodiment includes Ti and therefore has high toughness. As a result, the spring
steel according to the present embodiment exhibits excellent ductility.
[0023]
The chemical composition of the above spring steel may include Ca: 0.0001
to 0.0030%. The chemical cornposition of the above spring steel may include one
or more selected from the group consisting of, Cr: 0.05 to 2.0%o, Mo: 0.05 to 1.0%o,
W: 0.05 to 7.0o/o, V: 0.05 to 0.70%o, Nb: 0.002 to less than 0.050%, Ni: 0.1 fo 3.5%o,
Cu: 0.1 To 0.5o/o, and B: 0.0003 to 0.0050%.
î00241
A method for producing the spring steel of the present embodiment includes
the steps of: refining molten steel having the above chemical composition; producing
a semi-fmished product using the reflrned molten steel by a continuous casting
process; and hot working the semi-finished product. The step of refining molten
steel includes: a step of deoxidizing the molten steel using Al during ladle refining;
and a step of deoxidizing the molten steel using REM for at least 5 minutes after the
deoxidation with Al. The step of producing a semi-f,mished product includes: a step
of stirring the molten steel within a mold to swirl the molten steel in a horizontal
direction at a flow velocity of 0.1 m/min or faster; and a step of cooling the semifinished
product being cast at a cooling rate of 1 to 1O0'C/min.
[0025]
In the refining step, Al deoxidation and REM deoxidation are performed in
this order during the ladle refining with the REM deoxidation being performed for at
least 5 minutes. Then, in the continuous casting step, swirling is performed at the
aforementioned flow velocity and cooling is pêrformed at the aforementioned
cooling rate. With this production method, it is possible to produce a spring steel
that satisfies the number of coarse oxide inclusions and the maximum value among
equivalent circular diameters of the coarse oxide inclusions mentioned above.
[0026] 1
The spring steel of the present embodiment will be described in detail below.
In the contents of the elements, "oTo" means "o/oby mass".
100271
[Chemical Composition]
The chemical composition of the spring steel according to the present
embodiment includes the following elements.
[0028]
C: 0.4 to 0.7%o
Carbon (C) increases the strength of the steel. If the C content is too low,
this advantageous effect cannot be produced. On the other hand, ifthe C content is
too high, pro-eutectoid cementites will form excessívely in the cooling process after
hot rolling. In such a case, the workability for wire drawing of the steel decreases.
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Accordingly, the C content ranges from 0.4 Io 0.7o/o. The lower limit of the C
content is preferably greater than 0.4%o, more preferably 0.45o/o, and even more
preferably 0.5%. The upper limit of the C content is preferably less than 0.7ol0,
more preferably 0.65%o, and even more preferably 0.6%o.
[002e]
Si: 1.1 to3.0%o
Silicon (Si) increases the hardenability of the steel and increases the fatigue
strength ofthe steel. In addition, Si increases sag resistance. Ifthe Si content is
too low, these advantageous effects cannot be produced. On the other hand, if the
Si content is too high, the ductility of ferrite in pearlite will decrease. In addition, if
the Si content is too high, decarbonization will be promoted in the processes of
rolling, quenching, and tempering, resulting in a decrease in the strength of the steel.
Accordingly, the Si content ranges from 1 .1 ro 3 .0%o. The lower limit of the Si
content is preferably greater than 1 .7%o, more preferably I.2%o, and even more
preferably I.3%o. The upper limit of the Si co¡rtent is preferably less than 3.0o%,
more preferably 2.5%o, and even more preferably 2.0%o.
[0030]
Mn: 0.3 Io 7.5%o
Manganese (Mn) deoxidizes the steel. In addition, Mn increases the strength
of the steel. If the Mn content is too low, these advantageous effects cannot be
produced. On the other hand, if the Mn content is too high, segregation will occur.
In the segregation portion, micromartensite will form. The micromartensite will be
a factor that causes flaws in the rolling process. Furthermore, the micromartensite
decreases the workability for wire drawing of the steel. Accordingly, the Mn
content ranges from 0.3 to 1.5%o. The lower limit of the Mn content is preferably
greater lhan0.3%o, more preferably 0.4o/o, and even more preferably 0.5o/o. The
upper limit of the Mn content is preferably less than 7 .5o/o, more preferably I .4o/o,
and even more preferably 1.2%o.
[0031]
P: equal to or less than 0.03%
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Phosphorus (P) is an impuríty. P segregates at the grain boundaries, which
results in a decrease in the fatigue strength of the steel. Accordingly, the P content
is preferably as low as possible. The P content is equal to or less than 0.03%. The
upper limit of the P content is preferably less than 0.03yo, and more preferably 0.02%.
[0032]
S: equal to or less than 0.05%
Sulfur (S) is an impurity. S forms coarse MnS, which results in a decrease in
the fatigue strength of the steel. Accordingly, the S content is preferably as low as
possible. The s content is equal to or less than 0.05%. The upper limit of the s
content is preferably less than O.05yo, more preferably 0.03%o, and even more
preferably 0.0I%.
[0033]
Al:0.01 to 0.05%
Aluminum (Al) deoxidizes the steel. In addition, Al adjusts the grains of the
steel. If the A1 content is too low, these advantageous effects cannot be produced.
On the other hand, if the Al content is too high, the above advantageous effects will
reach saturation. In addition, if the Al content is too high, large amounts of alumina
will remain. Accordingly, the Al content ranges from 0.01 to 0.05%. The lower
limit of the Al content is preferably greater than 0.01%. The upper limit of the Al
content is preferably less than a.05yo, and more preferably 0.035%. The Al content
as referred to in this specification means the content of the so-called total Al.
[0034]
REM: 0.0001 to O.002%o
Rare earth metal (REM) desulfurizes and deoxidizes the steel. In addition,
REM bonds with Al-based oxides to refine oxide inclusions. This is described
below.
[003s]
In this specification, the oxide inclusions are one or more of Al-based oxides
typified by alumina, complex oxides, and complex oxysulfides. The Al-based
oxide, complex oxide, and complex oxysulfide are defined as follows.
[0036]
The Al-based oxide includes at least 30% of O (oxygen) and at least 5% of Al.
The Al-based oxide may further include at least one or more deoxidizing elements
such as Mn, si, ca, and Mg. The REM content in the Al-based oxide is less than
t%.
[0037]
The complex oxide includes at least 30%o of o (oxygen), at least 5o/o of Al,
and at least 7%o of REM. The complex oxide may firrther include at least one or
more deoxidizing elements such as Mn, Si, Ca, and Mg.
[0038]
The complex oxysulfide includes at least 30% of O (oxygÐ, at least 5%o of
Al, at least 1% of REM, and S. The complex oxysulfide may further include at
least one or more deoxidizing elements such as Mn, Si, Ca, and Mg.
[003el
The REM reacts with Al-based oxides in the steel to form complex oxides.
The complex oxides may further react with S to form complex oxysulfides. Thus,
the REM transforms Al-based oxides into complex oxides or complex oxysulfides.
This inhibits the Al-based oxides from agglomerating in the molten steel to form
clusters, thereby making it possible to disperse f,ine oxide inclusions in the steel.
100401
FIG. 1 is an SEM image illustrating an example of a comprex oxysulf,rde in
the spring steel of the present embodiment. The equivalent circular diameter of the
complex oxysulfide in FIG. 1 is less than 5 pm. The chemical composition of the
complex oxysulfide in FIG. 1 includes 64.4% of o (oxygen), r8.4%oof Al, 5.5% of
}l4n,4.6%o of S, and 3.8% of Ce (REM).
[0041]
The complex oxides and complex oxysulflrdes, which are represented by FIG.
l, have equivalent circular diameters of about 1 to 5 pm and therefore are fine. In
addition, neither the complex oxides nor complex oxysulfides are extended to
become coarse or form clusters. Thus, neither the complex oxides nor complex
oxysulfides are likely to act as initiation points for fatigue fracture. As a result, the
fatigue strength of the spring steel increases.
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100421
The spring steel of the present embodiment preferably includes at least the
complex oxysulflrdes of all the oxide inclusions. In this case, S is immobilized in
the complex oxysulfides. As a result, precipitation of Mns is inhibited and
precipitation of TiS at the grain boundaries is also inhibited. consequently, the
ductility of the spring steel increases.
[0043]
If the REM content ís too low, these advantageous effects cannot be produced.
on the other hand, if the REM content is too high, the inclusions containing r{EM
may clog the nozzle in continuous casting. Even in the case where the inclusions
containing REM do not clog thenozzle, the coarse inclusions containing REM are
included in the steel, which results in a decrease in the fatigue strength of the steel.
Accordingly, the REM content ranges from 0.0001 ro 0.002o/o. The lower limit of
the REM content is preferably greater than 0.0001%, more preferably 0.0002o/o, and.
even more preferably greater than 0.0003%. 'The upper limit of the REM content is
preferably less than 0t.002%o,more preferably 0.0015%, still more preferably
0.0010%, and even more preferably 0.0005%.
[0044]
The REM as referred to in this specification is a generic term for lanthanides
from lanthanum (La) with atomic number 57 through lutetium (Lu) with atomic
number 71, scandium (Sc) with atomic number 21, andyttrium (Ð with atomic
number 39.
[004s]
N: equal to or less than 0.015%
Nitrogen (N) is an impurity. N forms nitrides, which results in a decrease in
the fatigue strength of the steel. In addition, N causes strain aging, which results in
a decrease in the ductility and toughness of the steel. Accordingly, the N content is
preferably as low as possible. The N content is equal to or less than 0.015%. The
upper limit of the N content is preferably less than 0.015yo, more preferably 0.010%,
still more preferably 0.008%, and even more preferably 0.006%.
[0046]
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O: equal to or less than 0.0030%
Oxygen (O) is an impurity. O forms Al-based oxides, complex oxides, and
complex oxysulfides. If the O content is too high, large amounts of coarse Al-based
oxides will fonn, which will shorten the fatigue lifetime of the steel. Accordingly,
the O content is equal to or less than 0.0030%. The upper limit of the O content is
preferably less than 0.0030%, more preferably 0.0020 Vo, andeven more preferably
0.0015%. The O content as referred to in this specification is the so-called total
oxygen amount (T. O).
[0047]
Ti: 0.02 To 0.1%o
Titanium (Ti) forrns f,ine Ti carbides and Ti carbonitrides in the austenite
temperature range above the A¡ temperature. During heating for quenching, the Ti
carbides and Ti carbonitrides exert the pinning effect on the austenite grains to refine
the grains and make them uniform. Thus, Ti, increases the toughness of the steel.
[0048]
In general, when Ti is included, Ti carbides and Ti carbonitrides form and
further TiS precipitates at the grain boundaries. TiS decreases the ductility of steel
similarly to MnS.
too4gl i
However, as described above, in the spring steel of the present embodiment, S
bonds with REM to form complex oxysulfides. As a result, S does not segregate at
the grain boundaries and therefore neither TiS nor MnS are likely to form. Thus, in
the present embodiment, the contained Ti increases the toughness and also provides
high ductility. If the Ti content is too low, these advantageous effects cannot be
produced.
[0050]
On the other hand, if the Ti content is too high, coarse TiN will form. TiN
tends to be a fracture initiation point and also be a hydrogen trapping site. As a
result, the fatigue strength of the steel will decrease. Accordingly, the Ti content
ranges from 0.02 To 0.Io/o. The lower limit of the Ti content is preferably greater
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than0.02%o, and more preferably 0.04%. The upper limit of the Ti content is
preferably less than 0.lo%, more preferably 0.08%, and even more preferably 0.06%.
[0051]
The balance of the chemical composition of the spring steel according to the
present embodiment is Fe and impurities. The irnpurities herein refer to impurities
that find their way into the steel from ores and scrap as raw materials or from the
production environment, for example, when a steel product is industrially produced
and which are allowed within a range that does not adversely affect the advantageous
effects of the spring steel of the present embodiment.
[0052]
The chemical composition of the spring steel according to the present
embodiment may further include Ca in place of part of Fe.
[0053]
Ca: 0 to 0,0030%
Calcium (Ca) is an optional element anä may not be included. When Ca is
included, the Ca desulfurizes the steel. On the other hand, if the Ca content is too
high, coarse,low melting point Al-Ca-O oxides will form. In addition, if the Ca
content is too high, complex oxysulfides will absorb Ca. Complex oxysulfides that
have absorbed Ca tend to become coarse. Such coarse oxides tend to be fracture
initiation points for steels. Accordingly, the Ca content ranges from 0 to 0.0030%.
The lower limit of the Ca content is preferably not less than 0.0001%, more
preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the Ca
content is preferably less than 0.0030%, more preferably 0.0020o/o, and even more
preferably 0.0015%.
[0054]
The chemical composition of the spring steel according to the present
embodiment may further include, in place of part of Fe, one or more selected from
the group consisting of, Cr, Mo, W, V, Nb, Ni, Cu, and B. All of these elements
increase the strength of the steel.
[005s]
Cr:-0 to 2.0%o
13
Chromium (Cr) is an optional element and may not be included. When
included, the Cr increases the strength of the steel. In addition, Cr increases the
hardenability of the steel and increases the fatigue strength of the steel. In addition,
Cr increases the temper softening resistance. On the other hand, if the Cr content is
too high, the hardness of the steel íncreases excessively, which results in a decrease
in ductility. Accordingly, the Cr content ranges from 0 to 2.0o/o. The lower limit
of the Cr content is preferably 0.05%. When the temper softening resistance is to
be increased, the lower limit of the Cr content is preferably 0.5010, and more
preferably 0.7%o. The upper limit of the Cr content is preferably less than 2.0ol0.
'When
the spring steel product is to be produced through cold coiling, the upper limit
of the Cr content is more preferably 1.5%o.
[00s6]
Mo: 0Io I.jVo
Molybdenum (Mo) is an optional element and may not be included. When
included, the Mo increases the hardenability ofithe steel and increases the strength of
the steel. In addition, Mo increases the temper softening resistance of the steel. In
addition, Mo forms fine carbides to refine the grains. Mo carbides precipitate at
lower temperatures than vanadium carbides. Thus, Mo is effective in ref,rning the
grains of high strength spring steels, which are tempèred at low temperatures.
[0057]
On the other hand, if the Mo content is too high, a supercooled structure tends
to form in the cooling process after hot rolling. supercooled structures can be a
cause of season cracking or cracking during working. Accordingly, the Mo content
ranges from 0 to 7.0o/o. The lower limit of the Mo content is preferably 0.050%, and
more preferably 0.10%. The upper limit of the Mo content is preferably less than
1.00lo, more preferably 0.75o/o, and even more preferably 0.50%o.
[0058]
W: 0 to 1.0%
Tungsten (W) is an optional element and may not be included. 'When
included, the W increases the hardenability of the steel and increases the strength of
the steel similarly to Mo. In addition, W increases the temper softening resistance
14
of the steel. On the other hand, if the W content is too high, a supercooled structure
will form as with Mo. Accordingly, the'W content ranges from 0 Io 7.0o/o. When
high temper softening resistance is to be obtained, the lower limit of the W content is
preferably 0.05yo, and more preferably 0.1%. The upper lirnit of the W content is
preferably less than 1.00lo, more preferably 0.7 syo,and even more preferably 0.50%.
[005e]
V: 0 to 0.70%
Vanadium (V) is an optional element and may not be included. When
included, the V forms fine nitrides, carbides, and carbonitrides. These precipitates
increase the temper softening resistance of the steel and the strength of the steel. kr
addition, these precipitates refine the grains. On the other hand, if the V content is
too high, the V nitrides, V carbides, and V carbonitrides will not dissolve sufficiently
when heated for quenching. Undissolved V nitrides, V carbides, and V
carbonitrides become coarse and remain in the steel, which results in a decrease in
the ductility and fatigue strength of the steel. .In addition, if the V content is too
high, a supercooled structure will form. Accordingly, the V content ranges from 0
Io 0.70o/o. The lower limit of the V content is preferably 0.05yo, more preferably
0.06yo, and even more preferably 0.08%. The upper limit of the V content is
preferably less than O.T\yqmore preferably 0.50o/o,'still more preferably 0.30%o, and
most preferably the upper limit is 0.25%o.
[0060]
Nb: 0 to less than 0.050%
Niobium (Nb) is an optional element and may not be included. When
included, similarly to V, the Nb forms nitrides, carbides, and carbonitrides, which
increases the strength and temper softening resistance of the steel and refines the
grains. On the other hand, if the Nb content is too high, the ductility of the steel
will decrease. Accordingly, the Nb content ranges from 0 to less than 0.050%.
The lower limit of the Nb content is preferably 0.002yo, more preferably 0.005%, and
even more preferably 0.008%. When springs are to be produced through cold
coiling, the upper limit of the Nb content is preferably less than 0.030o/o, and more
preferably less than 0.020%.
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[0061]
Ni: 0 to 3.5%
Nickel (Ni) is an optional element and may not be included. 'When
included,
the Ni increases the strength and hardenability of the steel similarly to Mo. In
addition, when Cu is included, the Ni forms an alloy phase with the Cu to inhibit the
decrease in hot workability of the steel. On the other hand, if the Ni content is too
high, the amount of retained austenite will increase excessively, which results in a
decrease in the strength of the steel after quenching. In addition, the retained
austenite will transform into martensite in use to cause swelling. As a rèsult, the
dimensional accuracy of the product decreases. Accordingly, the Ni content ranges
from 0 Io 3 .5o/o. The lower limit of the Ni content is preferably 0. 1
o/o, more
preferably 0.2%o, and even more preferably 0.3%. The upper limit of the Ni content
is preferably less than 3.5o/o, more preferably 2.5Vo, and even more preferably i.0%.
When Cu is included, the Ni content is preferably not less than the Cu content.
[0062] i
Cu: 0 to 0.5%
Copper (Cu) is an optional element and may not be included. When
included, the Cu increases the hardenability of the steel and increases the strength of
the steel. In addition, Cu increases the corrosion résistance of the steel and inhibits
decarburization of the steel. On the other hand, if the Cu content is too high, the hot
workability decreases. In such a case, flaws tend to occur in the production
processes such as casting, rolling, and forging. Accordingly, the Cu content ranges
from 0 Ío 0.5%o. The lower limit of the Cu content is preferably 0.1olo, and more
preferably 0.2%o. The upper limit of the Cu content is preferably less than 0.57o,
more preferably 0.4Yo, and even more preferably 0.3o/o.
[0063]
B: 0 to 0.0050%
Boron (B) is an optional element and may not be included. When included,
the B increases the hardenability of the steel and increases the strength of the steel.
[0064]
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In addition, B is held in solid solution in the steel to segregate at the grain
boundaries. The solute B inhibits grain boundary segregation of grain boundary
embrittling elements such as P, N, and S. Thus, B strengthens grain boundaries.
In the spring steel of the present embodiment, S segregation at grain boundaries is
significantly inhibited when B is included together with Ti and REM. As a result,
the fatigue strength and toughness of the steel increase.
[0065]
On the other hand, if the B content is too high, a supercooled structure such as
martensite or bainite will form. Accordingly, the B content ranges from 0 to
0.0050%. The lower limit of the B content is preferably not less than 0.0003%,
more preferably 0.0005%o, and even more preferably 0.0008%. The upper limit of
the B content is preferably less than 0.0050%o, more preferably 0.0030%, and even
more prefer ably 0.0020%.
[0066]
fMicrostructure] i
[Number TN of Coarse Oxide Úrclusions]
ln the spring steel having the above-described chemical composition, the
number TN of oxide inclusions having an equivalent circular diameter of equal to or
greater than 5 pm is equal to or less than 0.21mrfi, tire oxide inclusions each being
one of an Al-based oxide, a complex oxide, and a complex oxysulfide.
[0067]
The equivalent ci¡cular diameter refers to the diameter of a circle determined
to have the same area as the area of each of the oxide inclusions (Al-based oxides,
complex oxides, and complex oxysulfides). Hereinafter, oxide inclusions having an
equivalent circular diameter of equal to or greater than 5 pm are designated as
"coarse oxide inclusions". The number TN of the coarse oxide inclusions may be
determined in the following manner.
[0068]
A rod-shaped or line-shaped spring steel is cut along the axial direction. The
cross section is mirror polished. Selective Potentiostatic Etching by Electrolytic
Dissolution (SPEED method) is performed on the polished cross section. On the
I7
etched cross section, five fields are freely selected which are rectangular regions with
a2 mm width in a radial direction and a 5 mm length in an axial direction, with a
location R/2 deep from the surface of the spring steel (R is the radius of the spring
steel) being the center.
[006e]
Using a scanning electron microscope (SEM) equipped with an energy
dispersive X-ray microanalyzer (EDX), the fields are each observed at a
magnification of 2000x and images of the fields are acquired. Inclusions in the
fields are identif,red. Using the EDX, the chemical composition (Al content, O
content, REM content, S content, etc. in the inclusion) of each of the identified
inclusions is analyzed. Based on the analysis results, oxide inclusions (Al-based
oxides, complex oxides, and complex oxysulfides) are identified among the
inclusions.
f00701
The equivalent circular diameters of the identified oxide inclusions (Al-based
oxides, complex oxides, and complex oxysulfides) are determined by image
processing to identiff oxide inclusions having an equivalent circular diameter of
equal to or greater than 5 pm (coarse oxide inclusions).
t00711
The total number of the coarse oxide inclusions in the five fields is
determined and the number TN (number/mm2) of the coarse oxide inclusions is
determined by the following formula.
TN : Total number of coarse oxide inclusions in f,rve f,relds/Total area of five
fields
100721
In the spring steel of the present embodiment, the number TN of coarse oxide
inclusions is not greater ïhan}.2lmnf . The appropriate amount of REM contained
under appropriate production conditions transforms Al-based oxides into fine
complex oxides or complex oxysulfides. This results in achieving the low number
TN. Consequently, high fatigue strength is obtained.
[0073]
..:
':
'l_
l8
:'.;
Ë"i
11
iìi
i.l i
ti
,.1
g
i:{
fMaximum Value Dmax among Equivalent Circular Diameters of Oxide Inclusions]
Furthermore, in the spring steel of the present embodiment, the maximum
value Dmax among equivalent circular diameters of the oxide inclusions is equal to
or less than 40 pm.
100741
The maximum value Dmax is determined in the following manner. When
measuring the number TN described above, the equivalent circular diameters of the
oxide inclusions in the five fields are determined. The maximum value among the
determined equivalent circular diameters is designated as the maximum value Dmax
among equivalent circular diameters of the oxide inclusions.
[007s]
In the spring steel of the present embodiment, the maximum value Dmax is
not greater than 40 pm. The appropriate amount of REM contained therein
transforms Al-based oxídes into fine complex oxides or complex oxysulflrdes to
thereby achieve the low maximum value Dmai. Consequently, high fatigue
strength is obtained.
[0076]
fProduction Method]
An exemplary method for producing the above spring steel is described. The
method for producing the spring steel of the present embodiment includes: a step of
refining molten steel (refining process); a step of producing a semi-finished product
using the refined molten steel by a continuous casting process (casting process); a
step of hot working the semi-f,inished product to produce the spring steel (hot
working process).
100771
[Reflrning Process]
In the refining process, molten steel is refined. First, molten steel is
subjected to ladle refining. Any known ladle ref,ming may be employed as the ladle
reflrning. Examples of ladle refining include a vacuum degassing process using RH
(Ruhrstahl-Heraeus).
[0078]
19
While ladle refining is being performed, Al is introduced into the molten steel
to Al-deoxidizelhe molten steel. Preferably, the O content (total oxygen amount)
in the molten steel after Al deoxidation is not greater than 0.0030%.
[007e]
After the Al deoxidation, REM is introduced into the molten steel to perform
deoxidation by REM deoxidation for at least 5 minutes.
[0080]
After the REM deoxidation, ladle refining including a vacuum degassing
process may further be performed. With the refining step described above, molten
steel having the above chemical composition is produced.
[0081]
I¡a the refining process described above, the REM deoxidation is performed
after the Al deoxidation for at least 5 minutes. This results in transformation of the
Al-based oxides into complex oxides or complex oxysulfides and refinement thereof.
Consequently, coarsening (clustering) of Al-bàsed oxides as in the conventional art is
inhibited.
[0082]
If the REM deoxidation lasts for less than 5 minutes, the transformation of Albased
oxides into complex oxides or complex oxysuifides will be insufficient.
Consequently, the number TN will exceed 0.2lmm2 and/or the maximum value
Dmax among equivalent circular diameters of the oxide inclusions will exceed 40
pm.
[0083]
In addition, if deoxidation is carried out with an element other than Al before
the REM deoxidation, the transformation of Al-based oxides into complex oxides or
complex oxysulfides will be insufficient. Consequently, the number TN will
exceed 0.2lmm2 and,/or the maximum value Dmax among equivalent circular
diameters of the oxide inclusions will exceed 40 pm.
[0084]
For the REM deoxidation, for example, a misch metal (mixture of REM's)
may be used. In such a case, a lump-like rnisch metal may be added to the molten
20
steel. At the last stage of the refining, a Ca-Si alloy, CaO-CaFz flux, or another
substance may be added to the molten steel to carry out desulfurization.
[0085]
[Casting Process]
Using the ladle-refined molten steel, a semi-finished product is produced by a
continuous casting process.
[0086]
Even after the ladle refining, the REM and Al-based oxides react with each
other in the molten steel to form complex oxysulf,rdes and complex oxides.
Therefore, by swirling the molten steel within the mold, the reaction between REM
and Al-based oxides can be facilitated.
[0087]
Accordingly, in the casting process, the molten steel within the mold is stined
and swirled in the horizontal direction at a flow velocity of 0.1 m/min or faster.
This promotes the reaction between REM andìAl-based oxides to form complex
oxides and complex oxysulfides. As a result, the number TN of coarse oxide
inclusions is not greater than 0.2/m# and the maximum value Dmax of the oxide
inclusions is not greater than 40 pm. On the other hand, if the flow velocity is less
than 0.1 m./min, the reaction between REM and Al-6ased oxides is less likely to be
promoted. Consequently, the number TN will exceed 0.2lmr* and/or the
maximum value Dmax will exceed 40 ¡rm. Stirring of the molten steel is carried
out by electromagnetic stirring, for example.
[0088]
In addition, the cooling rate RC of the semi-finished product being cast affects
the coarsening of oxide inclusions. In the present embodiment, the cooling rate RC
ranges from I to 1O0'C/min. The cooling rate refers to a rate of cooling from the
liquidus temperature to the solidus temperature at a location T/4 deep (T is the
thickness of the semi-finished product) from the upper or lower surface of the semif,
mished product. If the cooling rate is too low, the coarsening of oxide inclusions is
more likely to occur. Thus, if the cooling rate RC is less than 1"C/min, the number
ì
21
TN of coarse oxide inclusions will exceed 0.2lmm2 and/or the maximum value Dmax
among equivalent circular diameters of the oxide inclusions will exceed 40 pm.
[008e]
on the other hand, if the cooling rate RC is greater than 1O0'c/min, coarse
oxide inclusions will be trapped in the steel before floating during casting.
Consequently, the number TN of coarse oxide inclusions will exceed 0.2lmm2 and/or
the maximum value Dmax among equivalent circular diameters of the oxide
inclusions will exceed 40 pm.
[00e0]
When the cooling rate RC ranges from 1 to 1OO"C/min, the number TN of
coarse oxide inclusions is not greater than 0.2/mm2 and the maximum value Dmax
among equivalent circular diameters of the oxide inclusions is not greater than 40 pm.
[00e1]
The cooling rate may be determined in,the following manner. FIG. 2
illustrates a transverse cross section (cross secfion perpendicular to the axial direction
of the semi-finished product) of the cast semi-finished product. Refening to FIG. 2,
in the transverse cross section of the semi-finished product, any point P that is T/4
deep from the upper or lower surface of the semi-finished product at the time of
casting is selected. T is the thickness (mm) of the semi-finished product. In the
solidifred structure at point P, the secondary dendrite arm spacing À (¡.rm) in the
thickness T direction is measured. Specifically, the secondary dendrite arm spacing
in the thickness T direction is measured at 10 locations and the average of the
measurements is designated as the spacing À.
[00e2]
The determined spacing À is substituted into Formula (1) to determine the
cooling rate RC ("C/min).
RC : 0,1770'¡-(t/o+r) (1)
[00e3]
The lower limit of the cooling rate RC is preferably 5"C/min. The upper
limit of the cooling rate RC is preferably less than 60"C/min and more preferably
fi
22
less than 30"C/min. Under the production conditions described above, the semifinished
product is produced.
[00e4]
[Hot'Working Process]
The produced semi-finished product is subjected to hot working to produce a
wire rod. For example, the semi=finished product is subjected to billeting to
produce a billet. The billet is subjected to hot rolling to produce a wire rod. Using
the production method described above, the wire rod is produced.
[00e5]
When springs are produced using the wire rod, either a hot forming process or
a cold forming process may be used. The hot forming process may be implemented
as follows, for example. The wire rod is subjected to wire drawing to obtain a
spring steel wire. The spring steel wire is heated to above the A¡ temperature.
The heated spring steel wire (austenite structure) is wound around a mandrel to be
formed into a coil (spring). The formed spring is subjected to quenching and
tempering to adjust the strength of the spring. The quenching temperature ranges
from 850 to 950oC, for example, with oil cooling being performed. The tempering
temperature ranges fuom 420 to 500"C, for example. Using the steps described
above, springs are produced.
[00e6]
The cold forming process is implemented as follows. The wire rod is
subjected to wire drawing to obtain a spring steel wire. The spring steel wire is
subjected to quenching and tempering to produce a strength-adjusted steel wire.
The quenching temperature ranges from 850 to 950oC, for example, and the
tempering temperature ranges from 420 to 500"C, for example. Cold coil forming
is carried out using a cold coiling machine to produce springs.
[00e7]
The spring steel according to the present embodiment has excellent fatigue
strength as well as excellent toughness and ductility. Thus, even when a cold
forming process is employed to form springs, plastic deformation of the spring steel
is readily accornplished without breaking off during forming.
I
23
'f
:'F I*
..1,
-1
ã
$:ì
EXAMPLES
[00e8]
Ladle refining was carried out to produce molten steels having chemical
compositions shown in Tables I and2.
[00ee]
24
:.! '
i$
t4:
iì,..
'd' :
.a:
:qi I
':{ !{. á--
i::
.ì
..:
[Table 1]
TABLEI
Test No.
Chemical cornposition (in masso/o, balance is Fe and irnpurities)
C Si Mn P S T.AI REM T.N T.O Ti
0.56 1.65 1.07 0.006 0.005 0.022 0.0004 0.0069 0.0008 0.047
2 0.46 2.16 0.88 0.009 0.006 0.017 0.0004 0.0044 0.00r2 0.033
J 0.48 r.64 0.74 0.008 0.006 0.019 0.0005 0.00s7 0.00r 2 0.048
4 0.56 2.23 0.88 0.008 0.005 0.02s 0.0002 0.0063 0.0015 0.0s9
5 0.56 2.07 0.91 0.009 0.007 0.02s 0.0002 0.006r 0.0008 0.062
6 0.54 t.49 0.87 0.010 0.003 0.025 0.000r 0.0069 0.0015 0.05 r
7 0.57 2.28 1.02 0.011 0.004 0.024 0.0006 0.0076 0.0006 0.058
8 0.5'7 r.92 1.00 0.008 0.004 0.02s 0.0009 0.0078 0.0013 0.078
9 0.56 1.83 1.09 0.011 0.010 0.029 0.0006 0.0041 0.0009 0.076
0 0.s4 2.r0 0.68 0.006 0.00s 0.030 0.0007 0.0051 0.0012 0.022
0.56 1.68 1.00 0.012 0.00s 0.023 0.000s 0.0080 0.00 I 0.044
2 0.s6 t.47 0.'75 0.012 0.004 0.029 0.0006 0.0042 0.0009 0.034
3 0.57 2.12 0.96 0.01r 0.010 0.026 0.0008 0.0066 0.001r 0.052
4 0.56 1.75 0.87 0.009 0.010 0.037 0.0004 0.006s 0.0013 0.023
5 0.s6 2.46 1.05 0.012 0.006 0.030 0.0002 0.0045 0.0012 0.042
6 0.s8 2.00 0.68 0.006 0.006 0.036 0.0008 0.0073 0.0009 0.069
7 0.s6 1.62 1.03 0.007 0.004 0.019 0.0003 0.00s6 0.0009 0.039
8 0.56 2.21 1.09 0.011 0.008 0.032 0.0002 0.0071 0.00r3 0.054
9 0.s5 2.09 1.13 0.005 0.009 0.038 0.0003 0.0076 0.0009 0.048
20 0.53 2.27 0.92 0.006 0.009 0.033 0.0006 0.0064 0.0014 0.026
21 0.s6 2.26 0.92 0.010 0.00s ,0.024 0.0006 0.0043 0.0008 0.033
22 0.56 2.11 1.08 0.007 0.008 0.037 0.0005 0.0077 0.00r4 0.074
23 0.55 l.5l 0.80 0.009 0.009 0.024 0.0002 0.0060 0.00 2 0.064
24 0.55 2.13 0.73 0.006 0.004 0.033 0.0005 0.0067 0.0006 0.040
25 0.53 2.t4 4.92 0.008 0.007 0.038 0.0008 0.0060 0.0014 0.040
26 0.57 2.08 0.67 0.0r 1 0.003 0.028 0.0002 0.0043 0.0010 0.038
27 0.53 41 0.78 0.006 0.006 0.031 0.0002 0.0044 0.0006 0.045
28 0.55 .86 1.00 0.008 0.007 0.021 0.0003 0.0066 0.0014 0.076
29 0.55 71 0.84 0.009 0.009 0.034 0.0004 0.0070 0.0008 0.03s
30 0.s4 3l 1.06 0.007 0.003 0.026 0.0004 0.0042 0.0009 0.030
3l 0.s7 2.07 0.66 0.008 0.008 0.032 0.0007 0.0059 0.00 4 0.023
32 0.s8 1.88 0.95 0.007 0.007 0.039 0.000s 0.0075 0.0012 0.044
33 0.53 2.25 0.69 0.009 0.007 0.039 0.0055 0.0006
34 0.46 1.69 0.68 0.009 0.009 0.022 0.0008 0.00s4 0.0033 0.034
35 0.57 2.28 1.05 0.007 0.007 0.040 0.0004 0.0053 0.0009 0.058
36 0.46 50 0.70 0.007 0.007 0.0r9 0.0004 0.0070 0.0013 0.044
37 0.58 45 0.'79 0.007 0.007 0.031 0.0260 0.0077 0.0007 0.027
38 0.49 .67 0.84 0.00s 0.007 0.027 0.0048 0.0074 0.0014 0.035
39 0.44 .60 0.ó8 0.006 0.008 0.034 0.00006 0.0075 0.00 2 0.060
40 0.48 53 0.75 0.01I 0.008 0.028 0.0006 0.0120 0.0006 0.170
41 0.55 96 0.73 0.009 0.007 0.025 0.0016 0.0043 0.00 2 0.1 89
42 0.55 .49 0.79 0.012 0.010 0.024 0.00r4 0.0079 0.00 J 0.026
43 0.57 94 0.70 0.009 0.003 0.030 0.0003 0.00s0 0.00 0 0.052
44 0.53 .89 0.1s 0.008 0.009 0.023 0.0046 0.00 0 0.048
45 0.56 74 0.77 0.007 0.010 0.029 0.00s5 0.00 0 0.002
46 0.54 18 0.75 0.007 0.009 0.027 0.0045 0.00 0 0.02s
47 0.58 .64 0.79 0.006 0.008 0.030 0.0008 0.0077 0.00 7 0.003
25
[0100]
[Table 2]
26
TABLE2
Test No
Chemical composition (continuation of Table 1, in mass%. balance is Fe and impurities)
Ca Cr Mo w Nb Ni Cu B
I 0.60
2 0.70
J t.20
4 0.62
5 0.61 0.0029
6 0.63 0.0019
7 0.72 0.0030
8 0.81 0.08 0.24 0.0010
9 0.71 0.14 0.0008
0 0.12 0.05 0.12 0.00r 3
I 1.00
2 o.13
3 0.96
4 0.78
5 0.63
6 0.68
7 0.15
8
9 0.0008
20 0.90 0.22
2t 0.0010 0.87
22 0.61 0.20
23 0.40 0.24
24 0.68 0.029
25 0.15 0.20 0.21
26 0.89 0.23 0.022
27 0.70 0. 18 0.16
28 1.61
29 0.61 0.22 1.57 0.21
30 1.60 0.23
31 0.0010 0.72 0.22
32 0.0008 0.90
33 0.9s
34 o.61
35 0.95
36 0.84
3t 0.73
38 0.60
39 0.67
40 0.82
4l 0.63 0.25 0.019
42 0.72
43 0.95
44 0.19
45 0.78
46 0.85 0.0021
4'7 0.82
':
:it
!t
[0101]
27
[Table 3]
TABLE 3
Test No.
Ladle
refining
Order of
addition
Circulation time
with finally added
deoxidizer (min)
6
Swirling flow
velocity
(m/min)
0.2
RC
('Clrnin)
C Al-+REM 20
2 C Al-+REM 6 0.2 29
3 C Al+REM 6 0.2 21
4 c A1+REM 6 0.25 21
5 C Al-+REM 6 0.2s 23
6 C Al-+REM 6 0.2 t9
'l C Al+REM 8 0. l5 22
8 C Al-+REM 8 0.35 22
9 C Al-+REM 8 0.3 l3
0 C AI+REM 8 0.2 t2
I c Al+REM 8 0.2 16
2 C A1-+REM 8 0.2 18
-t C Al-+REM t0 0.25 25
4 C AI-+REM l0 0.2 23
5 C AI-+REM l0 0.2 2l
6 C A1-+REM 6 0.2 15
7 C Al-+REM 8 0.2 27
8 C A1-+REM 8 0.2 t3
9 C Al+REM 8t 0.2 22
20 c Al+REM 8 0.2 t7
2t c Al+REM 8 0.2 t4
22 C Al+REM 8 0.2 z7
23 C Al-+REM 8 0.2 14
24 C Al-+REM 8 0.2 t4
25 C Al-+REM 8 0.2 29
26 C Al-+REM 8 0.2 12
27 C AI-+REM 8 0.2 l0
28 C Al-+REM 8 0.2 t4
29 C AI-+REM 8 0.2 24
30 c A1-+REM 8 0.2 l4
31 C Al+REM 8 0.2 l1
32 C Al-+REM 8 0.2 27
JJ c AI 6 0.2 29
34 NC AI-+REM 6 0.2 23
35 C Al-+REM ) 0.2 I'7
36 C A1-+REM 6 0.05 l8
5t C Al-+REM 6 0.3 2Q
38 C Al-+REM 6 0.2 12
39 C AI-+REM J 0.2 t9
40 C AI-+REM 6 0.2 30
41 C REM+Al õ 0.2 26
42 C Al-+REM-+Ca 6 0.2 t10
43 C Al-+REM-+Ca 6 0.2 0.06
44 c AI 6 0.2 t4
45 C AI 6 0.2 t7
28
[0102]
The molten steels of Tests Nos. I To 47 shown in Tables I and 2 were
subjected to refining under the conditions shown in Table 3. Specifically, in Tests
Nos. I to 33 and 35 to 47,ladle refining was first performed on the molten steels.
On the other hand, for the molten steel of Test No. 34, ladle refining was not
performed. In the "Ladle refining" column in Table 3, "C" indicates that ladle
rehning was performed on the molten steel of the corresponding test number and
"NC" indicates that ladle ref,ining was not performed. The ladle refining was
performed under the same conditions for all numbers of tests.
[0103]
Specifically, in the ladle refining, the molten steels were circulated for 10
minutes using an RH apparatus. After the ladle refming was carried out,
deoxidation was performed. The "Order of alidition" column in Table 3 shows
deoxidizers used and the order of addition of the deoxidizers. "41-)REM"
indicates that after deoxidation was performed by addition of Al, frrther deoxidation
was performed by addition of REM. "Al" indicates that only Al deoxidation was
performed without performing deoxidation with another deoxidizer (e.g., REM).
"REM-+AI" indicates that REM deoxidation was performed and then Al deoxidation
was performed. "AI-+REM+Ca" indicates that Al deoxidation was performed and
then REM deoxidation was performed and finally Ca deoxidation was performed.
Metal Al was used for the Al deoxidation, a misch metal was used for the REM
deoxidation, and a Ca-Si alloy and a flux of CaO:CaFz : 50:50 (mass ratio) were
used for the Ca deoxidation. The circulation time in Table 3 is a circulation time
after the final deoxidizer was added, i.e., the time of deoxidation with the finally
added deoxidizer. When the finally added deoxidizer is REM, the time of the REM
deoxidation is indicated.
[0104]
É.,,
29
ln the cases in which REM deoxidation was performed, the circulation times
(times of deoxidation) after addition of REM were as shown in Table 3. By the
steps described above, the molten steels of Tests Nos. I to 47 were produced.
[010s]
Using the produced molten steels, blooms (semi-f,rnished products) having a
transverse cross section of 300 mm x 300 mm were produced by a continuous
casting process. At that time, the molten steels within the mold were stirred by
electromagnetic stirring. The velocities (m/min) of the swirling flows of the molten
steels within the mold in the horizontal direction during stining were as shown in
Table 3. Using one of the produced blooms of each test number, the cooling rate
RC ('Clmin) of the blooms of each test number was determined in the abovedescribed
manner. The determined cooling rates RC are shown in Table 3.
[0106]
The blooms were heated to 1200 to 1250"C. The heated blooms were
subjected to billeting to produce billets having'a transverse cross section of 160 mm
x 160 mm. The billets were heated to 1100'C or more. After the heating, wire
rods (spring steels) having a diameter of 15 mm were produced.
[0107]
fEvaluation Test]
fPreparation of Ultrasonic Fatigue Test Specimens]
For each test number, the ultrasonic fatigue test specimen illustrated in FIG.
3A was prepared in the following manner. The numerical values in FIG. 3A
indicate dimensions (in mm) at respective locations. "rp3" indicates that the
diameter is 3 mm.
[0108]
FIG. 3B is a view of a transverse cross section (cross section perpendicular to
the axis of the wire rod) of the wire rod 10 having a diameter of l5 mm. The
broken line in FIG. 3B indicates the location where a rough test specimen 11 (a test
specimen 1 mm larger than the shape illustrated in FIG. 3A) for the ultrasonic fatigue
test specimen is cut. The longitudinal direction of the rough test specimen 11 was
the longitudinal dilection of the wire rod 10. The rough test specimen 11 was cut at
q
t'-
:.':
')
il:
.:L,
.1 .ì.
30
the cutting location illustrated in FIG. 3B so that the load bearing portion of the
ultrasonic fatigue test specimen does not include the centerline segregation of the
wire rod.
[010e]
The rough test specimens cut from the wire rods of the respective test
numbers were subjected to quenching and tempering to adjust the Vickers hardnesses
(HV) of the rough test specimens to 500 to 540. For all numbers of tests, the
quenching temperature was 900oC and the holding time therefor was 20 minutes.
For the test numbers in which the C content is greater than 0.50%, the tempering
temperature was 430"C and the holding time therefor was 20 minutes. For the test
numbers in which the C content is not greater than 0.50%, the tempering temperature
was 41OoC and the holding time therefor was 20 minutes.
[0110]
After being heat treated as describedäbou", the rough test specimens were
given substantially the same properties as thosè of coiled springs. Thus, these
rough test specimens were used for evaluation of the perfoÍnance of the spring.
[0111]
After the heat treatment, the rough test specimens \ryere subjected to a
finishing process to prepare a plurality of the ultrasoiric fatigue test specimens having
the dimensions illustrated in FIG. 3A for each test number.
[0] l2]
[Measurement of Number TN of Coarse Oxide Inclusions and Maximum Value
Dmaxl
The prepared ultrasonic fatigue test specimens were each cut along the axial
direction so as to form a cross section containing the central axis. The cross section
of each ultrasonic fatigue test specimen was mirror polished. Selective
Potentiostatic Etching by Electrolytic Dissolution (SPEED method) was performed
on the polished cross section. In the cross section subjected to the SPEED method,
5 fields in the portion of 10 mm in diameter were freely selected. Each f,reld was
rectangular having a width of 2 mm in a radial direction and a length of 5 mm in an
E
3l
axial direction, with its center being located at a depth W2 from the surface of the
ultrasonic fatigue test specimen (R is the radius, 5 mm in this example).
[0] l3]
Each field was observed using a scanning electron microscope (SEM)
equipped with an energy dispersive X-ray microanalyzer (EDX). The observation
was carried out at a magnification of 1000x. Inclusions in the fields were identified.
Then, the chemical compositions of the identif,red inclusions were analyzedusing the
EDX to identify Al-based oxides, REM-containing complex oxides, and REMcontaining
complex oxysulfides. Furthermore, the equivalent circular diameter of
each of the identified inclusions was determined by image analysis. Based on the
results of analynngthe chemical compositions of the inclusions and the equivalent
circular diameters of the inclusions, the numbers TN of coarse oxide inclusions and
the maximum values Dmax of the oxide inclusions were determined.
[0114]
lUltrasonic Fatigue Testl
An ultrasonic fatigue test was conducted using the prepared ultrasonic fatigue
test specimens. The testing system used was an ultrasonic fatigue testing system,
USF-2000, manufacturedby SHIMADZU CORPORATION. The frequency was
set to 20 kHz and the test stress was set to 850 MPa fo 1000 MPa. Six test
specimens were used for each test number to carry out the ultrasonic fatigue test.
The maximum load at which resonarce of equal to or greater than 107 cycles is
possible is designated as the fatigue strength (MPa) of the test number.
[0] l5]
[Vickers Hardness Test]
A Vickers hardness test in accordance with JIS Z 2244 was conducted using
the prepared ultrasonic fatigue test specimens. The test force was set to 10 kgf :
98.07 N. The hardness was measured at three freely selected points in the portion
of 10 mm in diameter in each ultrasonic fatigue test specimen and the average value
of the measurements was designated as the Víckers hardness (HV) of the test number.
[0116]
[Charp¡z Impact Test]
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Rough test specjmens having a square transverse cross section of 11 mm x 1l
mm were prepared from the wire rods of the respective test numbers. The rough
test specimens were subjected to quenching and tempering under the same conditions
as those for the ultrasonic fatigue test specimens. Thereafter, they were subjected to
a finishing process to prepare JIS No. 4 test specimens. In the frnishing process, a
U-notch was formed. The depth of the U notch was 2 mm. A Charpy impact test
in accordance with IIS Z 2242 was conducted using the prepared test specimens.
The test temperature'\¡/as room temperature (25"C).
[0117]
fTensile Test]
From the wire rods of all test numbers, rough test specimens 1 mm larger than
the shape of a round bar test specimen having a flat portion of 6 mm in diameter
(corresponding to the No. 144 test specimen specifred in JIS Z 2207) were prepared.
The rough test specimens were subjected to quenching and tempering under the same
conditions as those for the ultrasonic fatigue test specimens. Thereafter, they were
subjected to a finishing process to prepare round bar test specimens. In accordance
with JIS Z 2247, a tensile test was conducted at room temperature (25"C) to
determine the elongation at break (%) and the reduction narea(%o).
t0l 181
[Test Results]
The test results are shown in Table 4.
[01 l e]
[Table 4]
TABLE 4
.:l
Test
No.
Casting
results
Main
inclusions
TN
(number/mm2)
Dmax
(pm)
Fatigue
strength
(MPa)
Hardness
(HV)
Charpy
(x l0al/m2)
Elongation
(%)
Reduction
in area
(o/"I S
REM-AIo-
s 0.0s2 JJ 957 532 58.5 10.1 57.7
2 S
REM-AIo-
s 0.032 40 9s4 5t7 s6.8 t0.7 s9.4
J S
REM-AIo-
s 0.031 38 9'71 531 62.9 10.2 s3.8
4 S
REM-AIo-
s 0.087 34 978 518 49.5 11.2 54.6
5 S
REM-AIo-
s 0.037 32 9s8 538 63.7 I 1.5 55.2
JJ
6 S
REM-AIo-
s 0.075 26 955 523 74.2 11.0 56.1
7 S
REM-AIo-
s 0.063 32 958 534 64.0 10.8 60.4
8 S
REM-AIo-
s 0.076 36 9't8 537 71.6 12.0 s6.3
9 S
REM-AIo-
s 0.021 27 974 516 69.4 10.7 55.4
l0 S
REM-AI.
o-s 0.083 39 961 514 66.6 12.8 61.0
1l S
REM-AIo-
s 0.030 31 951 515 60.2 l1.3 53.3
12 S
REM-AIo-
s 0.065 3l 961 521 60.9 I1.6 53.5
l3 S
REM-AIo-
s 0.065 30 97s 519 60.8 10.8 53.8
t4 S
REM-AIo-
s 0.074 32 956 517 s9.8 I 1.3 52.1
15 S
REM-AIo-
s 0.049 26 968 535 58.6 10.2 59.',l
16 S
REM-AIo-
s 0.044 26 970 525 s0.2 t2.0 s9.6
l7 S
REM-AIo-
s 0.086 35 964 535 50.9 10.7 53.2
18 S
REM-AIo-
s 0.037 30 972 522 58.6 r0.8 53.6
I9 S
REM-AIo-
s 0.070 32 955 533 55.6 t1.l 53.2
20 S
REM-AIo-
s 0.087 39 952 511 58.3 1 1.3 52.3
2t S
REM-AI.
o-s 0.070 26 970 s39 62.r 10.7 58. I
22 S
REM-AI.
o-s 0.038 31 957 521 56.5 10.5 53.4
23 S
REM-AIo-
s 0.040 3l 952 512 50.8 l 1.5 53.2
24 S
REM-AI- ' o-s 0.073 39 973 s32 60.s 10.9 59.3
25 S
REM-AIo-
s 0.053 27 978 522 55. I 9.8 55.4
26 S
REM-AIo-
s 0.068 26 974 535 49.5 10.3 54.1
27 S
REM-AIo-
s 0.027 28 963 539 53.4 lt.2 55.0
28 S
REM-AIo-
s 0.045 32 977 s29 63.6 10.9 56.9
29 S
REM-AIo-
s 0.038 33 9s2 526 53.7 11.8 53.8
30 S
REM-AIo-
s 0.081 35 979 534 OJ.J 9.7 53.3
3l S
REM-AIo-
s 0.022 39 971 s29 50;Ì 10.4 s8. l
32 S
REM-AIo-
s 0.041 36 976 510 54.1 l1.l 54.6
JJ S Al-o u.255 45 89s 540 38.6 7.8 44.3
34
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F.
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:
[0120]
In Table 4, in the "Casting results" column, "S" means that casting was
accomplished without causing nozzle clogging. "F" means that the nozzlebecame
clogged during casting. The "Main inclusions" column lists oxide inclusions that
had an area fraction of not less than 5% n the five fields in the SEM observation.
"REM-Al-o-s" refers to complex oxysulfides. "Al-o" refers to Al-based oxides.
rrMnsrr refers to Mns. In Tests Nos. 1 lo 32 and34To47, complex oxides having an
area fraction of less lhan 5o/o were also present in the steels.
[0121]
Referring to Table 4, in Tests Nos. 1 to 32, the chemical compositions were
appropriate. Furthermore, in all of them, the number TN of coarse oxide inclusions
was not greater than 0.2/mm2 and the maximum value Dmax among equivalent
circular diameters of the oxide inclusions was not greater than 40 pm. As a result,
the fatigue strengths of Tests Nos. 1 to 32 were all high at 950 MPa or greater.
,{i
i{
34 S
Al-o,
REM-AIo-
s
0.32 46 891 514 60.1 11.4 s4.6
35 S
Al-o,
REM-AIo-
s
0.1 r 47 896 535 62.s l 1.5 55.7
36 S Ar-o 0.2s l9 920 5lt 49.1 10.7 57.3
37 F
38 S
REM-AIo-
s 0.356 36 916 539 58.s t0.7 53.1
39 S Al-o 0.400 JJ 892 519 48.3 8.9 48.2
40 S
REM-AIo-
s 0.044 30 902 s39 62.4 10.5 55.6
41 S
Al-o,
REM-AIo-
s
0.250 37 906 514 60.s r 1.9 59.3
42 S
REM-AIo-
s 0.452 48 910 532 58.7 10.1 53.4
43 S
Al-
O,REM.
At-o-s
0.489 52 891 520 55.1 10.3 59.9
44 S Al-o 0.221 49 871 s29 56.5 10.0 s8.7
45 S
Al-o,
MnS 0.322 54 9il 523 40.5 8.2 45.7
46 S Al-o 0.3t2 44 909 532 55.4 10.6 s4.6
47 S
REM-AIo-
s 0.083 30 r 959 s24 39.8 9.2 48.s
35
14122)
Furthermore, the chemical compositions of rests Nos. 5 to 10 included B.
As a result, they had high Charpy impact values and exhibited excellent toughness
compared with Tests Nos. 1 to 4 and ll Io 32.
l0l23l
On the other hand, in Test No. 33, the chemical composition did not include
REM. As a result, neither complex oxides nor complex oxysulf,rdes formed, and the
number TN of coarse oxide inclusions exceeded 0.2lmm2 and further the maximum
value Dmax of the oxide inclusions exceeded 40 pm. consequently, the fatigue
strength was low at less than 950 MPa. Furthennore, in Test No. 33, the chemical
composition did not include Ti. As a result, the Charpy impact value was less than
40 x 104 JlrÊ and,the toughness was low. Furthermore, the elongation at break was
less than 9.5%o and the reduction in area was less Than 50o/o.
L0r24l
ln Test No. 34, the O content was too high. As a result, the number TN was
too high and the maximum value Dmax was too great. consequently, the fatigue
strength was low at less than 950 MPa.
[012s]
In Test No. 35, the chemical composition *uJ appropriate. However, the
circulation time in REM deoxidation was too short. As a result, the maximum
value Dmax exceeded 40 ¡rm. Consequently, the fatigue strength was low at less
than 950 MPa.
[0126]
In Test No. 36, the chemical composition was appropriate. However,
electromagnetic stirring within the mold was insufficient and the flow velocity within
the mold was less than 0.1 m/min. As a result, the number TN was too high.
Consequently, the fatigue strength was low at less than 950 MPa.
l0t27l
In Test No. 37, the REM content was excessively high. As a result, nozzle
clogging occurred during continuous casting and therefore a semi-frrished product
could not be produced.
:
36
[0i28]
In Test No. 38, the REM content was too high. As a result, coarse oxide
inclusions in the steel increased, resulting in the excessively high number TN.
Consequently, the fatigue strength was low at less than 950 MPa.
l0tzel
úr Test No. 39, the REM content was too low. As a result, neither complex
oxides nor complex oxysulfides formed and therefore Al-based oxides became
coarse, resulting in the excessively high number TN. Consequently, the fatigue
strength was low at less than 950 MPa. ln addition, the too low REM content
resulted in the low elongation at break of less than 9 .5o/o and the low reduction in
area of less Than 50%o. It is considered that the too low REM content caused
formation of TiS at the grain boundaries resulting in the decreased ductility.
[0130]
In Tests Nos. 40 and 41, the Ti contentwas too high. Consequently, the
fatigue strength was low at less than 950 MPa.' It is considered that coarse TiN had
formed and this resulted in the decreased fatigue strength.
I0l3ll
In Test No. 42, the chemical composition was appropriate but the cooling rate
RC during continuous casting was too fast. As a result, the number TN was too
high and the maximum value Dmax was too great. Consequently, the fatigue
strength was low at less than 950 MPa.
[0132]
In Test No. 43, the chemical composition was appropriate but the cooling rate
RC was too slow. As a result, the number TN was too high and the maximum value
Dmax was too great. Consequently, the fatigue strength was low at less than 950
MPa.
[0133]
In Tests Nos. 44 to 46, none of the chemical compositions included REM.
As a result, the number TN was too high and the maximum value Dmax was too
great. Consequently, the fatigue strength was low at less than 950 MPa.
[0134]
37
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In addition, in Test No. 45, the Ti content in the chemical composition was
too low. As a result, the Charpy impact value was approximately 40 x 104 J/m2 and
the toughness was low. Furthermore, the elongation at break was less than9.5%o
and the reduction in area was less than 50%o.
[013s]
ln Test No. 47, the Ti content in the chemical composition was too low. As
a result, the Charpy impact value was less than 40 x 104 J/m2 andthe toughness was
low. Furthermore, the elongation at break was less hhan9.5%o and the reduction in
area was less than 50%.
[0136]
In the foregoing specification, an embodiment of the present invention has
been described. However, it is to be understood that the above embodiment is
merely an illustrative example by which the present invention is implemented.
Thus, the present invention is not limited to the above embodiment, and
modifications of the above embodiment may be made appropriately without
departing from the spirit and scope of the invention.

We claim:
1. A spring steel having a chemical cornposition consisting of,
in masso/0,
C: 0.4 to 0.1%o,
Si: 1.1 to3.0%o,
Mn: 0.3 to 1.5%o,
P: equal to or less tlian 0.030/0,
S: equal to or less than 0.05olo,
Al: 0.01 to 0.050/o,
rare earth metal: 0.0001 fo 0.002o/o,
N: equal to or less than 0.015%,
O: equal to or less than 0.0030%,
Ti: 0.02 to 0.1%o,
Ì
Ca: 0 to 0.0030%,
Cr: 0 to 2.0o/o,
Mo: 0 to l.}Yo,
W: 0 to 1.0%,
!
V: 0 to 0.ljyo,
Nb: 0 to less than 0.050o/o,
Ni: 0 to 3.5%,
Cu: 0 to 0.5%o, and
B: 0 to 0.0050%, with the balance being Fe and impurities,
wherein a number of oxide inclusions having an equivalent circular diarneter
of equal to or greater than 5 ¡rm is equal to or less than 0.2/mm2, the oxide inclusions
each being one of an Al-based oxide, a complex oxide containing REM, O and 41,
ancl a complex oxysulf,rde containing REM, O, S, and Al, and
wherein a rnaxirnum value among equivalent circular diameters of the oxide
inclusions is equal to or less than 40 prn.
2. The spring steel according to claim l,
39
'tt
?-;
.*i :
ta .- ;
:T
t:
iÈ':r
R'
wherein the chemical composition includes Ca: 0.0001 to 0.0030%.
3. The spring steel according to claim I or 2,
wherein the chemical composition includes one or more selected from the
group consisting of,
Cr: 0.05 to 2.0o/o,
Mo: 0.05 to I.0o/o,
W: 0.05 to 1.0%o,
V: 0.05 to 0.70%o,
Nb: 0.002 to less than 0.0507o,
Ni: 0.1 to 3.5%o,
Cu: 0.1 to 0.50%, and
0.0003 to 0.0050%.
I 4. A method for producing a spring steel, the rnethod comprising the steps of:
refining molten steel having the chemical composition according to any one
of claims I to 3;
producing a semi-finished product from the refined molten steel by a
continuous casting process; and t
hot working the semi-finished product,
wherein the step of refining the molten steel includes the steps ofi
performing ladle refining on the molten steel;
deoxidizing the molten steel using Al subsequent to the ladle refining; and
deoxidizing the molten steel using REM for at least 5 minutes after the
deoxidation with Al, and
wherein the step of producing the semi-finished product includes the steps of:
stining the molten steel within a rnold to swirl the molten steel in a
horizontal direction at a flow velocity of 0.1 m/min or faster; and
cooling the semi-finished product being cast at a cooling rate of I to
100'C/rnin.