DESCRIPTION
SPRING STEEL AND SPRING
5 TECHNICAL FIELD
[ 0 0 0 1 ]
The present invention relates to a spring steel used
for valve springs, clutch springs, and suspension springs
employed in automobiles and, particularly, to a high
10 strength spring steel and a spring, which suppress
softening due to a heat treatment, such as a quenching
and tempering treatment or a nitriding treatment, after
wire drawing or coiling, and also suppress the formation
of hard inclusions of Si02 and the generation of
decarburization, which may cause deterioration of fatigue
characteristics of the spring.
BACKGROUND ART
[0002]
Due to trends toward weight reduction and higher
performance of automobiles, springs have been
strengthened and high strength steels having a tensile
strength of more than 1,600 MPa after a heat treatment
have been applied to springs. In recent years, steels
having a tensile strength of more than 1,900 MPa have
also been used.
[0003]
Methods for producing a coil spring using a steel
include a hot coiling process comprising heating the
steel to the austenite region for coiling, and then
quenching and tempering the steel; and a cold coiling
process comprising cold coiling a high strength steel
wire made of the steel quenched and tempered in advance.
In both cases, the fundamental strength of the spring is
determined by quenching and tempering. Therefore,
composition design considering characteristics after
quenching and tempering is important for a spring steel.
For example, like Patent Documents 1 to 3, for the
purpose of strengthening, a large amount of C is
basically added and also alloying elements such as V and
5 Mo are added to thereby improve hardenability and temper
softening resistance.
[0005]
Like Patent Document 4, surface hardening due to a
nitriding treatment is effective so as to further
10 strengthen a spring. Usually, the nitriding treatment is
applied after coiling of the spring. Since this
treatment is performed by heating at 400 to 600°C, a
surface of the spring is hardened, while a core portion
is softened and spring performances such as fatigue
15 characteristics may conversely deteriorate if the core
portion does not have a sufficient softening resistance.
Therefore, alloying elements capable of imparting temper
softening resistance are commonly added.
[0006]
20 However, even if alloying elements capable of
imparting temper softening resistance are added, the
fatigue strength is not increased by enhancing temper
softening resistance so as to increase the strength if
hard inclusions such as Si02 exist in the steel, or a
25 decarburized layer exists on a surface layer.
[0007]
For example, in Patent Document 5, a slag
composition in a molten steel treatment is controlled in
an appropriate range to thereby enhance ductility of
30 inclusions which may cause a decrease in fatigue
strength, and also inclusions are refined by hot rolling
to thereby increase the fatigue strength.
[OOOS]
For example, in Patent Document 6, heating
conditions before hot rolling and cooling conditions
after rolling are appropriately controlled, and scales on
a surface are removed before hot rolling to thereby
suppress the formation of a decarburized layer. It has
recently been required for springs used in automobiles to
have more increased strength. The fact is, however, that
conventional steels for high strength spring cannot
5 satisfy such requirements.
[0009]
In addition, Patent Document 7 describes a hot
rolled wire rod usable as a raw material of drawn wire
products such as a spring steel, which is excellent in
wire drawability and also can suppress wire breakage even
in the case of heavy drawing from a thick wire rod.
Patent Document 8 discloses a steel wire for cold formed
spring, which is excellent in cold cuttability and
fatigue characteristics.
Prior Art Documents
Patent Documents
[OOlO]
Patent Document 1: JP 57-32353 A
Patent Document 2: JP 1-83644 A
Patent Document 3: JP 2-57637 A
Patent Document 4: JP 2004-315968 A
Patent Document 5: JP 61-136612 A
Patent Document 6: JP 2003-268483 A
Patent Document 7: JP 2007-231347 A
Patent Document 8: JP 2007-169688 A
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[ 0 0 11 ]
Regarding spring steels, many patent documents
already exist and these patent documents describe a very
broad range of steel compositions. However, there has
never been known a steel with all requisites for
suppression of the formation of hard inclusions of Si02
and the generation of decarburization, which may cause
deterioration of fatigue characteristics of the spring,
and suppression of softening of a spring core portion due
to a tempering or nitriding treatment.
[0012]
An object of the present invention is to provide a
5 high strength spring steel, which is finally formed into
a high strength spring, by making use of, at a maximum,
the temper softening resistance effect due to alloying
elements to thereby increase the strength of a spring
core portion while suppressing the formation of hard
10 inclusions of Si02 and the generation of decarburization,
which may cause deterioration of fatigue characteristics
of the spring.
[0013]
It is also an object of the present invention to
15 provide a high strength spring which is produced by using
the spring steel of the present invention.
Means for Solving the Problems
[0014]
20 The present inventors have studied a component
composition which is excellent in temper softening
resistance while suppressing the formation of Si02
inclusions formed during the spring production process,
and the generation of decarburization, and thus created
an optimum component composition of a high strength
spring steel to obtain the following findings (A) to (E).
[ 00151
(A) Hardness after a tempering or nitriding
treatment is important as characteristics of a steel for
high strength spring. Particularly, although the
nitriding treatment is a heat treatment which is
effective to improve the hardness of a surface, a core
portion is tempered at high temperature over a long
period of time during the nitriding treatment and thus
undergoes drastic softening. The inventors have
introduced the following indicator H so as to
quantitatively evaluate a relation between tempering
hardness of the core portion of the steel after the
nitriding treatment and the component composition of the
steel to thereby find an effective amount of temper
softening resistance in each element:
5 H = 33.6[C] + lO.O[Si] + 5.95[Mn] + 11.1[Cr] +
21.9[Mo] + 34.O[V] + 90.0,
wherein [C], [Si], [Mn], [Cr], [Mol, and [Vl represent
the contents (% by mass) of the respective elements in
the steel.
[0016]
(B) It has been found that, in particular, the
addition of Si and Cr in a large amount of more than 3.0%
imparts significant temper softening resistance.
[0017]
(C) However, the addition of a large amount of Si
causes the formation of a large amount of hard inclusions
Si02, thus failing to refine inclusions by hot rolling.
Therefore, since a spring is obtained while remaining
coarse inclusions in the steel, the steel undergoes
fatigue fracture at these inclusions as starting points,
leading to drastic deterioration of fatigue
characteristics. However, it has been found that, even
if Si is added in an amount more than that of Si
contained in a conventional spring steel, it is possible
to maintain fatigue characteristics by suppressing the
formation of Si02 through adjustment of an amount of Mn
which forms oxides, like Si. In order to suppress
fatigue fracture at hard inclusion of Si02 as starting
points, the C-value defined by the following equation (b)
is adjusted to 3.25 or less:
C = [Si] /[Mn] (b)
wherein [Si] and [Mn] represent the contents ( % by mass)
of the respective elements in the steel.
[0018]
(D) Not only the increased amount of Si02 formed but
also a large amount of Si added causes drastic
decarburization to occur on a surface layer, leading to
drastic deterioration of fatigue characteristics. The
amount of decarburization is also significantly
influenced by the heating temperature and, therefore,
when a large amount of Si is added, the heating
5 temperature is preferably low so as to suppress
decarburization. In a steel material having V or Mo
added, which forms alloy carbide, the heating temperature
becomes higher than that of a V or Mo-free steel material
so as to sufficiently obtain the effect of temper
10 softening resistance due to V and Mo, resulting in
increased amount of decarburization. Accordingly, in the
steel material containing a large amount of Si, when a
comparison is made between fatigue characteristics of
steel materials having the same tempering hardness, the
15 steel material having V or Mo added is excellent in
fatigue characteristics as compared with the V or Mo-free
steel material, due to an influence of the amount of
decarburization. In the case of the V or Mo-free steel
material, in order to obtain the fatigue strength of 800
20 MPa or more, the tempering hardness is adjusted to 550 or
more in terms of Vickers hardness, and to do so, the Hvalue
defined by the following equation (a) is adjusted
to 160 or more. In the case of the steel material having
V or Mo added, in order to obtain a fatigue strength of
25 the same level or more, a Vickers hardness is adjusted to
605 or more, and to do so, the H-value defined by the
following equation (c) is adjusted to 173 or more:
H = 33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] +
90.0 (a), and
30 H = 33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] +
21.9[Mo] + 34.O[V] + 90.0 (c)
wherein [C], [Si], [Mn], [Cr], [Mo], and [V] represent
the contents (% by mass) of the respective elements in
the steel.
35 [0019]
(E) When a large amount of Cr is added, it is
necessary to make the heating temperature higher so as to
sufficiently solutionize Fe-based carbides in order to
stabilize the carbides. When Cr is added in a large
amount of more than 3.5%, drastic decarburization occurs
on a surface layer, leading to drastic deterioration of
5 fatigue characteristics.
[0020]
The present invention has been completed based on
the above findings, and the gists thereof are as follow.
[0021]
10 (1) A spring steel characterized by including, in % by
mass:
C: 0.50 to 0.70%,
Si: 1.00 to 5.00%,
Mn: 0.30 to 2.00%,
15 P: 0.0002 to 0.0500%,
S: 0.0002 to 0.0500%,
Cr: 0.10 to 3.50%,
Al: 0.0005 to 0.0500%, and
N: 0.0020 to 0.0100%,
with the balance being Fe and inevitable impurities,
wherein the H-value defined by the following equation (a)
is 160 or more, and the C-value defined by the following
equation (b) is 3.25 or less:
H = 33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] + 90.0 (a),
and
C = [Sil/[Mnl (b),
wherein [C], [Si], [Mn], and [Cr] represent the contents
( % by mass) of the respective elements in the steel.
[0022]
(2) The spring steel according to the above (I),
characterized by further including, in % by mass, one or
more of:
Mo: 0.01 to 1.00%, and
V: 0.01 to 0.20%,
wherein the H-value defined by the following equation (c)
in place of the formula (a) is 173 or more:
H = 33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] + 21.9[Mo] +
34.0[V] + 90.0 (c),
wherein [C], [Si], [Mn], [Cr], [Mo], and [V] represent
the contents ( % by mass) of the respective elements in
the steel.
5 COO231
(3) The spring steel according to the above (1) or (2),
characterized by further including, in % by mass:
Nb: 0.001 to 0.200%.
[0024]
(4) The spring steel according to any one of the above
(1) to (3), characterized by further including, in % by
mass, one or more of:
Ca: 0.0002 to 0.0100%,
Mg: 0.0002 to 0.0100%, and
Zr: 0.0005 to 0.1000%.
[0025]
(5) A spring produced by subjecting a steel material to a
heat treatment, such as a quenching and tempering
treatment or a nitriding treatment, after wire drawing,
characterized in that the steel material includes, in %
by mass:
C: 0.50 to 0.70%,
Si: 1.00 to 5.00%,
Mn: 0.30 to 2.00%,
P: 0.0002 to 0.0500%,
S: 0.0002 to 0.0500%,
Cr: 0.10 to 3.50%,
Al: 0.0005 to 0.0500%, and
N: 0.0020 to 0.0100%,
with the balance being Fe and inevitable impurities,
wherein the spring has an H-value defined by the
following equation (a) of 160 or more, and a C-value
defined by the following equation (b) of 3.25 or less:
H = 33.6[C] + lO.O[Si] + 5.95[Mnl + ll.l[CrI + 90.0 (a),
and
C = [Sil/[Mnl (b),
wherein [C], [Si], [Mn], and [Cr] represent the contents
(8 by mass) of the respective elements in the steel.
100261
(6) The spring according to the above (5), characterized
in that the steel material further includes, in % by
5 mass, one or more of:
Mo: 0.01 to 1.00%, and
V: 0.01 to 0.20%,
wherein an H-value defined by the following equation (c)
in place of the equation (a) is 173 or more:
10 H = 33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] + 21.9[Mo] +
34.0[V] + 90.0 (c)
wherein [Cl, [Si], [Mn], [Cr], [Mo], and [V] represent
the contents (% by mass) of the respective elements in
the steel.
15 [0027]
(7) The spring according to the above (5) or (6),
characterized in that the steel material further
includes, in % by mass:
Nb: 0.001 to 0.200%.
20 [0028]
(8) The spring according to any one of the above (5) to
(7), characterized in that the steel material further
includes, in % by mass, one or more of:
Ca: 0.0002 to 0.0100%,
25 Mg: 0.0002 to 0.0100%, and
Zr: 0.0005 to 0.1000%.
[0029]
According to the present invention, there is also
provided a method for evaluating a fatigue strength of a
30 spring, using the above H-value and C-value. The gists
of the method for evaluating the fatigue strength of a
spring are as follows.
[0030]
(a) A method for evaluating the fatigue strength of
35 a spring, characterized by evaluating the fatigue
strength of a spring produced by subjecting a steel
material including, in % by mass:
C: 0.50 to 0.70%,
Si: 1.00 to 5.00%,
Mn: 0.30 to 2.00%,
P: 0.0002 to 0.0500%,
S: 0.0002 to 0.0500%,
Cr: 0.10 to 3.50%,
Al: 0.0005 to 0.0500%, and
N: 0.0020 to 0.0100%,
with the balance being Fe and inevitable impurities, to a
heat treatment, such as a quenching and tempering
treatment or a nitriding treatment, after wire drawing,
by use of an H-value defined by the following equation
(a) and a C-value defined by the following equation (b):
H = 33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] + 90.0 (a),
and
C = [Sil /[Mnl (b),
wherein [C], [Si], [Mn], and [Cr] represent the contents
(% by mass) of the respective elements in the steel.
[0031]
(b) The method for evaluating the fatigue strength
of a spring according to the above (a), characterized in
that the steel material further includes, in % by mass,
one or more of:
Mo: 0.01 to 1.00%, and
V: 0.01 to 0.20%,
wherein the H-value defined by the following equation (c)
in place of the formula (a) is 173 or more:
H = 33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] + 21.9[Mo] +
34.0[V] + 90.0 (c)
wherein [C], [Si], [Mn], [Cr], [Mo], and [V] represent
the contents ( % by mass) of the respective elements in
the steel.
COO321
(c) The method for evaluating the fatigue strength
of a spring according to the above (a) or (b),
characterized in that the steel material further
includes, in % by mass:
Nb: 0.001 to 0.200%.
[0033]
(d) The method for evaluating the fatigue strength
of a spring according to any one of the above (a) to (c),
5 characterized in that the steel material further
includes, in % by mass, one or more of:
Ca: 0.0002 to 0.0100%,
Mg: 0.0002 to 0.0100%, and
Zr: 0.0005 to 0.1000%.
[0034]
According to the present invention, there is also
provided a method for producing a high strength spring,
which satisfies definitions based on the above H-value
and C-value. The gists of the method for producing a
high strength spring are as follows.
[0035]
(e) A method for producing a high strength spring,
characterized by subjecting a steel material including,
in % by mass:
C: 0.50 to 0.70%,
Si: 1.00 to 5.00%,
Mn: 0.30 to 2.00%,
P: 0.0002 to 0.0500%,
S: 0.0002 to 0.0500%,
Cr: 0.10 to 3.50%,
Al: 0.0005 to 0.0500%, and
N: 0.0020 to 0.0100%,
with the balance being Fe and inevitable impurities, to a
heat treatment, such as a quenching and tempering
treatment or a nitriding treatment, after wire drawing to
produce a spring, to thereby adjust the following H-value
defined by the following equation (b) to 160 or more, and
the C-value defined by the following equation (b) to 3.25
or less:
H = 33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] + 90.0 (a),
and
C = [Sil /[Mnl (b),
wherein [C], [Si], [Mn], and [Cr] represent the contents
( % by mass) of the respective elements in the steel.
[0036]
(f) The method for producing a high strength spring
5 according to the above (e), characterized in that the
steel material further includes, in % by mass, one or
more of:
Mo: 0.01 to 1.00%, and
V: 0.01 to 0.20%,
10 and wherein the H-value defined by the following equation
(c) in place of the formula (a) is 173 or more:
H = 33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] + 21.9[Mo] +
34.0[V] + 90.0 (c)
wherein [C], [Si], [Mn], [Cr], [Mo], and [V] represent
15 the contents (% by mass) of the respective elements in
the steel.
[0037]
(g) The method for producing a high strength spring
according to the above (e) or (f), characterized in that
20 the steel material further includes, in % by mass:
Nb: 0.001 to 0.200%.
[0038]
(h) The method for producing a high strength spring
according to any one of the above (e) to (g),
25 characterized in that the steel material further
includes, in % by mass, one or more of:
Ca: 0.0002 to 0.0100%,
Mg: 0.0002 to 0.0100%, and
Zr: 0.0005 to 0.1000%.
30
Effects of the Invention
COO391
The steel of the present invention enables the
production of a high strength spring by reducing elements
35 capable of lowering the spring strength and also making
use of, at a maximum, the temper softening resistance
effect due to alloying elements, and therefore exerts
industrially remarkable high effects.
BRIEF DESCRIPTION OF THE DRAWINGS
[0040]
5 Fig. 1 shows a relation between the indicator H (=
33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] + 90.0) of a V
and Mo-free steel material and the indicator H (= 33.6[C]
+ lO.O[Si] + 5.95[Mn] + ll.l[Cr] + 21.9[Mo] + 34.O[V] +
90.0) of a material having V or Mo added, and the
10 tempering hardnesses.
Fig. 2 shows a relation between the tempering
hardness (HV) and the fatigue strength (MPa) .
Fig. 3 shows a relation between the indicator C (=
[Si] / [Mn] ) and the fatigue strength (MPa) .
15
MODE FOR CARRYING OUT THE INVENTION
[0041]
The present invention will be described in detail
below.
20 First, the above equations (a) and (c) defined in
the present invention will be described.
[0042]
Among characteristics of a steel for high strength
spring, a hardness after a tempering or nitriding
25 treatment is important. Particularly, the nitriding
treatment is a heat treatment which is effective to
improve the hardness of a surface, while a core portion
is tempered at a high temperature over a long period of
time during the nitriding treatment and thus undergoes
30 drastic softening. The inventors have experimentally
introduced the following indicators H so as to
quantitatively evaluate a relation between tempering
hardness of the core portion of the steel after the
nitriding treatment and the component composition of the
steel:
COO431
In the case of a steel free from Mo and V:
Indicator H = 33.6 [C] + 10.0 [Si] + 5.95 [Mn] + 11.1 [Cr] +
90.0 (a) , and
in the case of a steel containing Mo and/or V:
Indicator H = 33.6 [C] + 10.0 [Si] + 5.95 [Mn] + 11.1 [Cr]
+ 21.9[Mo] + 34.O[V] + 90.0 (c).
COO441
The indicator H is an indicator for additively
evaluating the degree of an influence of [C], [Si], [Mnl,
[Cr], [Mo], and [V] on the tempering hardness of a core
portion of the steel after a nitriding treatment by
weighing the degree of an influence of each element. C,
Si, Mn, Cr, Mo, and V are main temper softening
resistance-improving elements.
[0045]
Using, as the subjects of the test, various steel
materials including C in an amount of 0.50 to 0.70% (in %
by mass, the same shall apply hereinafter), Si in an
amount of 1.00 to 5.00%, Mn in an amount of 0.20 to
2.00%, P in an amount of 0.001 to 0.0500%, S in an amount
of 0.001 to 0.0500%, Cr in an amount of 0.10 to 4.00%, A1
in an amount of 0.001 to 0.010%, N in an amount of 0.0030
to 0.0060%, Mo in an amount of 0.01 to 1.00%, and V in an
amount of 0.01 to 0.30%, with the balance being Fe and
inevitable impurities, the steel materials satisfying the
above equation (b), each steel material was subjected to
a heat treatment in which the tempering hardness of the
core portion after the nitriding treatment had been
reproduced. A specimen was heated to a temperature at
which alloy carbides or Fe-based carbides can be
solutionized, quenched in oil at 60°C, and then subjected
to a tempering treatment at 450°C for 120 minutes.
Subsequently, the specimen was cut in a cross section of
a diameter direction and embedded in a resin and, after
polishing a surface layer, Vickers hardness (HV) was
measured. The results are shown in Fig. 1.
[0046]
As is apparent from Fig. 1, there is an extremely
satisfactory correlation between the indicator H and the
tempering hardness of the core portion after the
nitriding treatment.
100471
5 Next, a relation between the tempering hardness and
the fatigue characteristics was evaluated. The fatigue
test used was a Nakamura-type rotating-bending fatigue
test. A specimen was subjected to the test after scales
at a surface layer due to heat treatment were removed.
10 As a result, a maximum load stress, at which 10 samples
exhibited the number of load cycles of not smaller than
lo7 with a probability of 50% or more, was regarded as the
fatigue strength. The results are shown in Fig. 2. The
fatigue strength of a steel material SWOSC-V (JIS) often
15 used for springs was also determined, and shown in the
drawing. Since the fatigue strength of SWOSC-V is 660
MPa, target fatigue strength was set at 800 MPa or more
which has been increased by about 20% therefrom.
[ 0 0 4 8 1
20 As is apparent from Fig. 2, it is necessary to
secure a tempering hardness of 550 HV or more for a steel
containing neither Mo nor V, while it is necessary to
secure a tempering hardness of 650 HV or more for a steel
containing Mo or V, so that a fatigue strength of 800 MPa
25 or more is secured. As is apparent from Fig. 1, it is
necessary to maintain the indicator H at 160 or more so
as to secure a tempering hardness of 550 HV or more,
while it is necessary to maintain the indicator H at 173
or more so as to secure a tempering hardness of HV 605 or
30 more. Accordingly, the indicator H was defined to be 160
or more for a steel containing neither Mo nor V, while
the indicator H was defined to be 173 or more for a steel
containing Mo and/or V. The indicator H is preferably
163 or more, and more preferably 165 or more, for a steel
35 containing no V, while it is preferably 175 or more, and
more preferably 177 or more, for a steel containing Mo
and V.
The above equation (b) will now be described.
[0050]
As is apparent from the above equations (a) and (c),
5 Si is an element which significantly contributes to
temper softening resistance, and a large amount of Si is
added to steel materials for springs to thereby increase
the strengths of the springs. However, hard inclusion
Si02 formed in a large amount in the steel provides
10 starting points of fatigue fracture to thereby cause
deterioration of fatigue characteristics. It depends on
a balance of amounts of easily oxidizable elements added
whether or not Si02 is formed in a large amount. The
present inventors have studied a relation between the
15 ratio of an amount of Si to that of Mn and the fatigue
characteristics.
[0051]
Using, as the subjects of the test, various steel
materials including C in an amount of 0.60% (in % by
20 mass, the same shall apply hereinafter), Si in an amount
of 2.0 to 4.0%, Mn in an amount of 0.40 to 1.40%, P in an
amount of 0.005 to 0.050%, S in an amount of 0.001 to
0.050%, Cr in an amount of 2.5%, A1 in an amount of 0.001
to 0.010%, and N in an amount of 0.0030 to 0.0050%, with
25 the balance being Fe and inevitable impurities, the steel
material satisfying the above equation (a), each steel
material was subjected to a heat treatment in which the
tempering hardness of the core portion after the
nitriding treatment had been reproduced. A specimen was
30 heated to a temperature at which alloy carbides or Febased
carbides can be solutionized, quenched in oil at
60°C, and then subjected to a tempering treatment at 450°C
for 120 minutes. In order to eliminate an influence of
decarburization, a region of 0.5 mrn from the surface was
35 ground off. A Nakamura-type rotating-bending fatigue
test was used for evaluation of fatigue characteristics.
The results are shown in Fig. 3.
[0052]
As is apparent from Fig. 3, the fatigue strengths
are 800 MPa or more when a ratio of an amount of Si to
that of Mn (hereinafter referred to as [Si] / [Mn] ) is 3.25
or less, while the fatigue strengths are low and 800 MPa
or less when [Si] / [Mn] is more than 3.25. Starting
points of fatigue fracture of these low fatigue strength
materials were observed and, as a result, hard inclusions
composed mainly of Si02 served as starting points. Thus,
it is considered that a decrease in fatigue strength is
mainly caused by Si02. Accordingly, a ratio [Si] / [Mn] of
the indicator C was defined to be 3.25 or less. As is
apparent from Fig. 3, when the content of Si is constant,
although the fatigue strength will not so much change at
[Si] / [Mn] of 3.25 or less, there can be seen a tendency
that, in particular when the content of Si increases, the
fatigue strength increases as [Si]/[Mn] decreases.
Therefore, the ratio [Si] / [Mn] is preferably 3.00 or
less, and more preferably 2.80 or less.
[0053]
As mentioned above, the steel of the present
invention has excellent characteristics as a high
strength spring steel, by defining the component
composition using the equation (a) or (c) and the
equation (b) .
The reasons for restricting the contents of
respective elements in the steel of the present invention
will now be described. Percentages of components are
denoted by mass.
[0054]
C: 0.50 to 0.70%
C is an important element which governs the strength
of a steel. In order to obtain sufficient strength, the
lower limit thereof is to be 0.50%. As compared with
other alloying elements, alloying cost is inexpensive and
it is possible to reduce alloying cost of a steel
material if a large amount of C can be added. However,
since hot ductility drastically deteriorates when a large
amount of C is added, the upper limit thereof is to be
0.70%. It is preferably 0.67% or less, and more
preferably 0.65% or less.
5 [0055]
Si: 1.00 to 5.00%
Si is an element required to secure the strength and
hardness of a spring, and the lower limit thereof is to
be 1.00% so as to obtain sufficient strength.
10 Furthermore, Si is an important element which
significantly contributes to temper softening resistance,
and the addition of Si leads to strengthening of the
spring. Therefore, the lower limit of Si is preferably
2.50%, more preferably 2.70%, and most preferably 3.00%.
15 On the other hand, when a large amount of Si is added,
not only the strength of the steel increases, but also
significant embrittlement thereof occurs. Therefore, the
upper limit is to be 5.00%.
[0056]
20 Mn: 0.30 to 2.00%
Mn is very often used since it fixes S in the steel
as MnS and enhances hardenability to obtain an enough
hardness after a heat treatment. Furthermore, in the
present invention, it is an important element which
25 governs whether or not Si02 is formed. Even in the case
of adding a large amount of Si, use of appropriate
amounts of Si and Mn in the steel enables prevention of
deterioration of fatigue characteristics. In order to
obtain such effects, the content of Mn is adjusted to
30 0.30% or more. On the other hand, when Mn is added in an
amount of more than 2.00%, the hardness of the base
material increases, leading to embrittlement. Therefore,
the upper limit is to be 2.00%.
[0057]
35 P: 0.0002 to 0.0500%
Since P is usually contained in a steel as an
inevitable impurity in an amount of 0.0002% or more, the
lower limit thereof is to be 0.0002%. Even if P is
added, P segregates at prior austenite grain boundaries
to cause drastic embrittlement, and therefore, the upper
limit thereof is to be 0.0500%. It is preferably 0.0300%
5 or less, more preferably 0.0200% or less, and still more
preferably 0.0150% or less.
[0058]
Like PI S is usually contained in a steel as an
10 inevitable impurity in an amount of 0.0002% or more, and
embrittles the steel when it exists in the steel. In the
case of S, although influences thereof are reduced as
small as possible by Mn, MnS is in the form of an
inclusion, leading to deterioration of fatigue
15 characteristics. Particularly, in a high strength steel,
fracture may occur from a trace amount of MnS and the
content of S is desirably reduced as much as possible.
Therefore, the upper limit thereof is to be 0.0500%.
Accordingly, the content of S is adjusted in a range from
20 0.0002 to 0.0500%. The upper limit is preferably
0.0300%, more preferably 0.0200%, and still more
preferably 0.0150%.
[0059]
Cr: 0.10 to 3.50%
25 Cr is an important element which significantly
contributes to temper softening resistance, and the
addition thereof leads to strengthening of a spring. In
order to obtain this effect, the amount of Cr added is to
be 0.10% or more. However, Cr is solid-soluted in Fe-
30 based carbides to thereby stabilize them. Accordingly,
in order to obtain the effect of temper softening
resistance, a heating temperature must be drastically
raised. In this case, when Cr is added in an amount of
more than 3.50%, drastic decarburization occurs, leading
35 to a decrease in fatigue strength. Therefore, the upper
limit of an amount of Cr is to be 3.50%.
[0060]
Al: 0.0005 to 0.0500%
Since A1 is usually contained in a steel as an
inevitable impurity in an amount of 0.0005% or more, the
lower limit thereof is to be 0.0005%. Even if A1 is
added, A1 forms oxides such as A1203, which provide
starting points of fatigue fracture to thereby cause
deterioration of fatigue characteristics of a spring.
Therefore, the upper limit thereof is to be 0.0500% but
is desirably reduced as much as possible. Preferably, it
is 0.0100% or less.
[0061]
N: 0.0020 to 0.0100%
N is combined with various alloying elements, such
as V and Nb, to form nitrides to thereby suppress
austenite grain growth, and exerts an influence on
properties of a steel and a spring. In order to obtain
these effects, the lower limit of the content of N is to
be 0.0020%. On the other hand, when the content of N
increases, hot ductility of the steel drastically
deteriorate to cause a problem of generation of marks
during hot rolling of a raw steel bar. Therefore, the
upper limit is to be 0.0100%.
[0062]
At least one of Mo: 0.01 to 1.00% and V: 0.01 to
0.30%
Mo and V are important elements which significantly
contribute to temper softening resistance, and the
addition of at least one of them leads to strengthening
of a spring. In order to obtain this effect, the amount
of each element added must be 0.01% or more. On the
other hand, when a large amount is added, a heating
temperature must be raised so as to obtain the effect of
temper softening resistance, and the amount of
decarburization generated as the heating temperature
increases, leading to a decrease in fatigue strength.
Accordingly, the upper limit of Mo is to be 1.00% and the
upper limit of V is to be 0.30%. V is combined with N to
form a nitride, which contributes, as pinning particles,
to the refinement of austenite grain.
[0063]
Nb: 0.001 to 0.200%
5 Like V, Nb is combined with N to form nitrides,
which contributes, as pinning particles, to the
refinement of austenite grain. In order to obtain this
effect, the amount of Nb added is to be 0.001% or more.
On the other hand, when the amount is more than 0.200%,
10 not only the effect is saturated, but also hot ductility
of the steel drastically deteriorates to cause a problem
of generation of marks during hot rolling of a raw steel
bar. Therefore, the upper limit is to be 0.200%.
[0064]
15 One or more of Ca: 0.0002 to 0.0100%, Mg: 0.0002 to
0.0100%, and Zr: 0.0005 to 0.1000%
Ca, Mg, and Zr form oxides, which serve as
crystallization nuclei for Mn sulfide and have an effect
on uniform fine dispersion of the Mn sulfide. In order
20 to exhibit this effect, the lower limits of Ca and Mg are
to be 0.0002%, and the lower limit of Zr is to be
0.0005%. On the other hand, when the amounts of Ca and
Mg are more than 0.0100%, and the amount of Zr is more
than 0.1000%, hard inclusions such as oxides and sulfides
25 thereof are formed in large amounts, leading to
deterioration of fatigue characteristics of the steel.
Accordingly, the upper limits of Ca and Mg are to be
0.0100%, while the upper limit of Zr is to be 0.1000%.
[0065]
30 A significant feature in the spring steel of the
present invention is that the content of Si is higher
than that of a conventional spring steel. Si is an
important element which is required to secure the
strength of a spring and significantly contributes to
35 temper softening resistance of the steel, leading to
strengthening of the spring. However, it was previously
not easy to realize a spring steel having a large amount
of Si added of, for example, more than 2.5%. This is
because hard inclusion Si02 exists in the steel. Fatigue
characteristics have hitherto been improved by
controlling the slag composition in a molten steel
5 treatment in an appropriate range to thereby form
inclusions having high ductility rather than hard
inclusions such as Si02, and refining the formed
inclusions by hot rolling. Since an increase in an
amount of Si contained in the steel leads to a rise in
crystallization temperature of Si02, there was a need to
make the heating temperature before rolling higher than
the crystallization temperature so as to suppress the
formation of Si02. However, the increase in heating
temperature before rolling causes drastic deterioration
of hot ductility, and thus the upper limit of the amount
of Si was hitherto 2.5% so as to attain the heating
temperature which enables production while suppressing
the formation of Si02.
[0066]
In the present invention, as the result of the
finding that the adjustment of a ratio of the contents of
Si and Mn, which are liable to form oxides, enables
control of the crystallization temperature of Si02, the Cvalue
defined by [Si] / [Mn] is adjusted to 3.25 or less,
based on the knowledge described previously, so as to
suppress the occurrence of fatigue fracture at the hard
inclusion Si02 serving as starting points thereof. In the
present invention, as the result of the adjustment of the
C-value, it is possible to realize a spring steel with a
high Si content. In the spring steel with a high Si
content, a rise in heating temperature leads to an
increase in the amount of decarburization, resulting in
deterioration of fatigue characteristics. In particular,
for V and Mo contained in a spring steel, there is a need
to raise the heating temperature depending on the amounts
thereof so as to exhibit the effect of temper softening
resistance. Therefore, it has been found that no
addition of V and Mo, or a decrease in the amounts
thereof contributes to realization of a spring steel with
a high Si content. Based on the synergistic effect of
the combination of these, the present invention provides
a high strength spring steel, which will be finally
formed into a high strength spring, by making use of, at
a maximum, the temper softening resistance effect due to
alloying elements while suppressing the formation of hard
inclusion of Si02 that causes deterioration of fatigue
characteristics of the spring to thereby increase the
strength of the spring core portion, regardless of its
higher Si content than that of a conventional spring
steel.
[0067]
According to the present invention, there is also
provided a spring produced using the spring steel of the
invention.
[0068]
The spring of the invention is produced by
subjecting a steel material with the composition defined
in the invention, namely, a steel material including, in
% by mass:
C: 0.50 to 0.708,
Si: 1.00 to 5.00%,
Mn: 0.30 to 2.00%,
P: 0.0002 to 0.0500%,
S: 0.0002 to 0.0500%,
Cr: 0.10 to 3.50%,
Al: 0.0005 to 0.0500%, and
N: 0.0020 to 0.0100%,
with the balance being Fe and inevitable impurities, to a
heat treatment, such as a quenching and tempering
treatment or a nitriding treatment, after wire drawing.
The spring of the invention is characterized in that the
H-value defined by the following equation (a) is 160 or
more, and the C-value defined by the following equation
(b) is 3.25 or less:
H = 33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] + 90.0 (a).
and
C = [Sil/ [Mnl (b) ,
wherein [C], [Si], [Mn], and [Cr] represent the contents
5 ( % by mass) of the respective elements in the steel.
[0069]
The steel material can further include, in % by
mass, one or more of:
Mo: 0.01 to 1.00%, and
10 V: 0.01 to 0.20%.
The spring obtained using this steel material is
characterized in that the H-value defined by the
following equation (c) in place of the above formula (a)
is 173 or more:
15 H = 33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] + 21.9[Mo] +
34.0[V] + 90.0 (c),
wherein [C], [Si], [Mnl, [Crl, [Mol, and [Vl represent
the contents (8 by mass) of the respective elements in
the steel.
20 [ 0 0 7 0 ]
The steel material may further include, in % by
mass:
Nb: 0.001 to 0.200%.
[ 0 0 7 11
25 The steel material can further include, in % by
mass, one or more of:
Ca: 0.0002 to 0.0100%,
Mg: 0.0002 to 0.0100%, and
Zr: 0.0005 to 0.1000%.
30 [0072]
The invention will now be described in detail below
by way of Examples. These Examples are used for the
purpose of describing technical meanings and effects of
the invention rather than limiting the scope of the
35 invention.
EXAMPLES
Steels composed of chemical components shown in
Table 1 were melted in a vacuum melting furnace and then
hot-rolled to obtain 6 mrn diameter steel wire rods. The
composition of the steel of the Conventional Example
corresponded to the composition of Si-Cr steel SWOSC-V
for valve spring of JIS G 3561. The steel wire rods were
subjected to a heat treatment in which the tempering
hardness of the core portion after the nitriding
treatment was reproduced. Specifically, the steel wire
rods were heated, depending on their steel compositions,
to a temperature at which alloy carbides or Fe-based
carbides can be solutionized, selected from 850 to 1,150°C
as shown in Table 2, quenched in oil at 60°C, and then
subjected to a tempering treatment at 450°C for 120
minutes.
[0074]
In order to measure the tempering hardnesses of the
heat-treated materials, a cross section of a diameter
direction was cut from each specimen and, after polishing
the cross section, Vickers hardness (300 gf) was measured
at the position of 2 mrn away from the surface layer. The
total decarburized depth was measured by the method using
a microscope defined in JIS G 0558. The total
decarburized depth of 0 (zero) means that total
decarburization could not be confirmed by the microscope.
[0075]
The fatigue test used was a Nakamura-type rotatingbending
fatigue test. A specimen was subjected to the
30 test after scales at a surface layer due to the heat
treatment were removed. A maximum load stress, at which
10 samples exhibited the number of load cycles of not
smaller than 10' with a probability of 50% or more, was
regarded as the fatigue strength.
35 [0076]
In Examples Nos. 1 to 11 of the present invention,
wherein the steel compositions are within the defined
range and both the indicators H and indicators C are also
within the defined ranges, the specimens excel in fatigue
characteristics and have fatigue strength of 800 MPa or
more.
100771
On the other hand, in Comparative Example No. 12,
although both the indicators H and C are in the defined
ranges, drastic decarburization occurred because of the
high content of Cr, leading to the low fatigue strength.
In Comparative Example No. 13, although both the
indicators H and C are also in the defined ranges, the
content of A1 is high and the fatigue strength is low.
Observation of the fractured specimens among samples
subjected to the fatigue test for the starting points of
fatigue fracture revealed inclusions composed mainly of
A1203 in all specimens. In Comparative Examples Nos. 14
and No. 15, although the steel compositions are in the
defined ranges, the fatigue strengths are low and 800 MPa
or less since the indicator H is out of the defined
range. In Comparative Examples Nos. 16 and 17, although
the steel compositions are also in the defined ranges,
the fatigue strengths are also low and 800 MPa or less
since the indicators C are out of the defined range.
Observation of the fractured specimens among samples
subjected to the fatigue test for the starting points of
fatigue fracture revealed inclusions composed mainly of
Si02 in all specimens.
[0078]
As is apparent from the above, the examples
satisfying all conditions defined in the present
invention are excellent in fatigue characteristics as
compared with the Comparative Examples and Conventional
Example.
[0079]
Table 1
NTest Classification
Examples of
the invention
Comparative
Table 2
Test
Classification Indicator H Indicator C
No.
Examples of
the invention
( Comparative
Examples
16 5.30
Conventional
l8 1 Examole 1 134 1 2.14
Solution
treatment
temperature
("C
950
Tempering Total Fatigue
hardness decarburized strength
(HV) depth (mm) (MPa)

1. A spring steel characterized by comprising, in
% by mass:
C: 0.50 to 0.70%,
Si: 1.00 to 5.00%,
Mn: 0.30 to 2.00%,
P: 0.0002 to 0.0500%,
S: 0.0002 to 0.0500%,
Cr: 0.10 to 3.50%,
Al: 0.0005 to 0.0500%, and
N: 0.0020 to 0.0100%,
with the balance being Fe and inevitable impurities,
wherein an H-value defined by the following equation (a)
is 160 or more, and a C-value defined by the following
equation (b) is 3.25 or less:
H = 33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] + 90.0
(a), and
C = [Sil /[Mnl (b),
wherein [C], [Si], [Mn], and [Cr] represent contents
( % by mass) of the respective elements in the steel.
2. The spring steel according to claim 1,
characterized by further comprising, in % by mass, one or
more of:
Mo: 0.01 to 1.00%, and
V: 0.01 to 0.30%,
wherein an H-value defined by the following equation
(c) in place of the formula (a) is 173 or more:
H = 33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] +
21.9[Mo] + 34.O[V] + 90.0 (c),
wherein [C], [Si], [Mn], [Cr], [Mo], and [V]
represent contents ( % by mass) of the respective elements
in the steel.
3. The spring steel according to claim 1 or 2,
characterized by further comprising, in % by mass:
Nb: 0.001 to 0.200%.
4. The spring steel according to claim 1 or 2,
characterized by further comprising, in % by mass, one or
more of:
Ca: 0.0002 to 0.0100%,
Mg: 0.0002 to 0.0100%, and
Zr: 0.0005 to 0.1000%.
5. A spring produced by subjecting a steel
material to a heat treatment after wire drawing,
characterized in that the steel material comprises, in %
by mass:
C: 0.50 to 0.70%,
Si: 1.00 to 5.00%,
Mn: 0.30 to 2.00%,
P: 0.0002 to 0.0500%,
S: 0.0002 to 0.0500%,
Cr: 0.10 to 3.50%,
Al: 0.0005 to 0.0500%, and
N: 0.0020 to 0.0100%,
with the balance being Fe and inevitable impurities,
wherein the spring has an H-value defined by the
following equation (a) of 160 or more, and a C-value
defined by the following equation (b) of 3.25 or less:
H = 33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] + 90.0
(a), and
C = [Sil /[Mnl (b),
wherein [C], [Si], [Mn], and [Cr] represent contents
( % by mass) of the respective elements in the steel.
6. The spring according to claim 5, characterized
in that the steel material further comprises, in % by
mass, one or more of:
Mo: 0.01 to 1.00%, and
V: 0.01 to 0.20%,
wherein an H-value defined by the following equation
(c) in place of the equation (a) is 173 or more:
H = 33.6[C] + lO.O[Si] + 5.95[Mn] + ll.l[Cr] +
21.9[Mo] + 34.O[V] + 90.0 (c)
wherein [C], [Si], [Mn], [Cr], [Mo], and [V]
represent contents ( % by mass) of the respective elements
in the steel.
Dated
7. The spring
characterized in that the steel material further
comprises, in % by mass:
Nb: 0.001 to 0.200%.
8. The spring according to claim 5 or 6,
characterized in that the steel material further
comprises, in % by mass, one or more of:
Ca: 0.0002 to 0.0100%,
Mg: 0.0002 to 0.0100%, and
Zr: 0.0005 to 0.1000%.
this 23/01/2014
( S ~ A TPIA HUJA)
OF REMFRY & SAGAR
ATTORNEY FOR THE APPLICANTS