Note: Descriptions are shown in the official language in which they were submitted.
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STEEL FOR SPRINGS, PROCESS OF MANUFACTURE FOR SPRING USING
THIS STEEL, AND SPRING MADE FROM SUCH STEEL
[0001]
The invention relates to steel making, and more specifically, the
field of spring steel.
[0002]
Generally, as increasing fatigue stresses are applied to springs,
springs need continually increasing hardness and tensile strength.
Consequently, susceptibility to fractures that begin on defects, such as
inclusions or surface defects generated during spring manufacture,
increases, and fatigue resistance tends to become limited. Secondly,
springs used in highly corrosive environments, such as suspension springs,
must have at least equivalent and preferably better fatigue properties in
corrosive conditions since they use steels having higher hardness and
tensile strength. Accordingly, such springs tend to fracture at the defects,
immediately during the fatigue cycles in air, and more late during fatigue
cycles in a corrosive medium. In particular, for fatigue in corrosive
conditions, defects can begin in corrosion pits.
Furthermore, with
increasing applied stress, it is more difficult to improve the fatigue life in
corrosive conditions or to maintain it at an equivalent level, given the fact
that the effects of the concentration of stresses on the corrosion pits, on
the
surface defects of the springs that may be generated during spring coiling,
in other steps in the manufacturing process, or in non-metallic inclusions,
become more critical when spring hardness increases.
[0003] According to the prior art, documents FR 2740476 B1 published
1998-12-31 and JP 03474373 B2 published 2003-12-08 describe a spring
steel grade with good resistance to hydrogen embrittlement and good fatigue
resistance, in which inclusions of carbonitrosulfides containing at least one
of the elements titanium, niobium, zirconium, tantalium or hafnium are
controlled so as to have lower mean size, less than 5 m in diameter, and to
be very numerous (10,000
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or more on a cutting section).
[0004] However, this type of steels leads, after quenching and tempering
according to
the industrial spring manufacturing process, to a hardness level of only 50HRC
or a
little higher, corresponding to a tensile strength of 1700 MPa or a little
higher, but not
much over 1900 MPa, corresponding to a hardness of 53.5HRC. Because of this
moderate hardness level, this steel only has moderate sag resistance, steel
with a higher
tensile strength being needed to improve sag resistance. Accordingly, such
steel does
not ensure an excellent compromise between high resistance, which would be
above
2100 MPa, a hardness that would be higher than 55HRC, a high fatigue
resistance in
air and fatigue resistance in conosive conditions that is at least equivalent,
if not higher
than that needed for springs.
[0005] The purpose of the invention is to propose means to simultaneously
increase,
as compared to known steels, spring hardness and tensile strength, fatigue
properties
in air, making fatigue resistance in corrosive conditions at least equivalent,
if not higher,
increase spring sag resistance and to reduce susceptibility to surface defects
that can be
generated during spring coiling.
[0006] With this in mind the object of the invention is a spring steel with
high fatigue
resistance in air and in corrosive conditions and with high resistance to
cyclic sag,
having the composition in weight percent
C = 0.45 - 0.70%
Si = 1.65 - 2.50%
Mn = 0.20- 0.75%
Cr = 0.60 - 2%
Ni = 0.15 - 1%
Mo = traces - 1%
,
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V = 0.003 - 0.8%
Cu = 0.10 - 1%
Ti= 0.020 - 0.2%
Nb = traces - 0.2%
Al = 0.002 -0.050%
P= traces- 0.015%
S = traces- 0.015%
0= traces - 0.0020%
N = 0.0020 - 0.0110%
[0007] The balance being iron, and impurities resulting from the steel making
process,
where the carbon equivalent Ceq content calculated according to the formula:
Ceq% = [C%1+ 0.12 [SP/0] + 0.17 [Mn /0]- 0.1 [Ni%]+ 0.13 [Cr3/0] - 0.24 MN
is between 0.80 and 1.00%, and whose hardness, after quenching and tempering,
is
greater than or equal to 55HRC.
[0008] The maximum size of titanium nitrides or carbonitrides observed at 1.5
0.5
mm of the surface area of a bar, or a wire Pod, a slug or a spring over 100
mm2of the
surface area of the section is preferably less than or equal to 20 pm, which
size being
the square root of the surface area of the inclusions considered as squares.
Preferably, the composition of the steel is:
C = 0.45 -0.65%
Si = 1.65 - 2.20%
Mn = 0.20 - 0.65%
Cr = 0.80 - 1.7%
,
Ni = 0.15 - 0.80%
Mo = traces - 0.80%
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V= 0.003 - 0.5%
Cu = 0.10 - 0.90 /0
Ti = 0.020 - 0.15%
Nb = traces - 0.15%
Al = 0.002 - 0.050%
P= traces- 0.010%
S = traces-0.010%
0 = traces - 0.0020%
N = 0.0020 - 0.0110%
[0009] The balance being iron and impurities resulting from the steel making
process.
[0010] A further object of the invention is a manufacturing process for a
spring steel
with high fatigue resistance in air and in corrosive conditions and high
iesistance to
cyclic sag, according to which a liquid steel is made in a converter or an
electric furnace,
its composition is adjusted, it is cast into blooms or continuous flow billets
or ingots that
are left to cool to room temperature; that are rolled into bars, wire rods or
slugs and
transformed into springs, characterized in that:
- the steel is of the previous type;
- after they become solid the blooms, billets or ingots have a minimum mean
cooling
rate of 0.3 C/s between 1450-1300 C;
- the blooms, billets or ingots are rolled between 1200-800 C in one or two
reheating
and rolling cycles;
- and bars, wire rods or slugs, or springs made from these, are austenitized
between
850-1000 C, followed by a water quench, a polymer quench or an oil quench, and
by
tempering at 300-550 C, so as to deliver steel with hardness greater than or
equal to
55HRC.
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[0011] A further object of the invention is springs made from such steel,
and springs made of steel obtained by the previous process.
[0012] In an unexpected way, the inventors realized that a steel with the
characteristics of the previously cited inclusion composition and morphology
5 ensured, after steelmaking, casting, rolling, quenching and tempering done
in specific conditions, a hardness greater than 55HRC, while assuring
excellent compromise between high endurance level to fatigue in air and to
fatigue in corrosive conditions, high resistance to cyclic sag and low
sensitivity to surface defects arising during manufacture of the spring.
[0012a] In
one aspect, the present invention provides a spring steel
composition comprising iron and: in weight percent based on the total
weight of the steel
C = 0.45 ¨ 0.70%
Si = 1.65 ¨ 2.50%
Mn = 0.20 ¨ 0.75%
Cr = 0.60 ¨ 2%
Ni = 0.15¨ 1%
Mo = traces ¨ 1%
V = 0.003 ¨ 0.8%
Cu= 0.10 ¨ 1%
Ti = 0.020 ¨ 0.2%
Nb = traces ¨ 0.2%
Al = 0.002 ¨ 0.050%
P = traces ¨ 0.015%
S = traces ¨ 0.015%
0 = traces ¨ 0.0020%
N = 0.0020 ¨ 0.0110%,
wherein a carbon equivalent (Ceq) content of the steel calculated
according to the formula: Ceq /0 = [C%] + 0.12 [Si%] + 0.17 [Mn%] - 0.1 [Ni%]
+
0.13 [Cr%] - 0.24 [V%] is between 0.80 and 1.00%, the hardness of the steel,
after quenching and tempering, is greater than or equal to 55HRC, and the
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maximum size of titanium nitrides or carbonitrides observed in an area over
100 mm2 at 1.5 mm 0.5 mm below the surface of the steel is less than or
equal to 20 tun, said size being the square root of the surface area of the
inclusions considered as squares.
[00121)] In a further aspect, the present invention provides a spring steel
composition comprising iron and: in weight percent based on the total
weight of the steel
C = 0.45 ¨ 0.70%
Si = 1.65 ¨ 2.50%
Mn = 0.20 ¨ 0.75%
Cr = 0.60 ¨ 2%
Ni= 0.15-1%
Mo = traces ¨ 1%
V = 0.003 ¨ 0.8%
Cu= 0.10 ¨1%
Ti = 0.020 ¨ 0.2%
Nb = traces ¨ 0.2%
Al = 0.002 ¨ 0.050%
P = traces ¨ 0.015%
S = traces ¨ 0.015%
0 = traces ¨ 0.0020%
N = 0.0020 ¨ 0.0110%,
wherein a carbon equivalent (Ceq) content of the steel calculated
according to following formula: Ceq% = [C%] + 0.12 [Si%] + 0.17 [Mn%]-
0.1 [Ni%] + 0.13 [Cr%] ¨0.24 [V%] is between 0.80 and 1.00%, a hardness
of the steel, after quenching and tempering, is greater than or equal to
55HRC, and a maximum size of titanium nitrides or carbonitrides observed
in an area over 100 mm2 at 1.5 mm 0.5 mm below the surface of the steel
is less than or equal to 20 rn, said size being the square root of a surface
area of the inclusions considered as squares, and a fatigue life of the steel
is
1742967 cycles or more in a torsion-fatigue test in air at a shear stress of
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5b
856 494 MPa applied after shot-peening.
[0012c] In yet a further aspect, the spring steel in accordance with
the
present invention is made by a process comprising: preparing a liquid steel
in a converter or an electric furnace, adjusting a composition of the liquid
steel, casting the liquid steel into a bloom, billet, or ingot, cooling the
bloom,
billet, or ingot to room temperature and rolling the bloom, billet, or ingot
into
a bar, a wire, or a slug for transformation into a spring, wherein: the bloom,
billet, or ingot during cooling after solidification has a minimum mean
cooling rate of 0.3 C/ s between 1450-1300 C; the bloom, billet, or ingot is
rolled between 1200-800 C in one or two reheating and rolling cycles; and
the bar, wire rod, or slug made of the bloom, billet, or ingot, is
austenitized
between 850-1000 C, followed by a water quench, a polymer quench or an
oil quench, and by tempering at 300-550 C to provide a hardness value of
greater than or equal to 55 HRC.
[0013] The invention will be better understood upon reading the
description that follows, given in reference to the following appended
figures:
- Figure 1 which shows the results of hardness and cyclic sag tests for
steels according to the invention and reference steels;
- Figure 2 which shows the results of fatigue tests in air as a function of
steel hardness for steels according to the invention and reference steels;
- Figure 3 which shows the results of Charpy impact tests as a function of
the steel hardness for steels according to the invention and reference steels;
and
- Figure 4 which shows the results of fatigue tests in corrosive conditions
as a function of steel hardness for steels according to the invention and
reference steels.
[0014] The steel composition according to the invention must meet the
following conditions.
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[0015] The carbon content must be between 0.45% and 0.7%. After
quenching and tempering, carbon increases the tensile strength and
hardness of the steel. If the carbon content is less than 0.45%, in the
temperature range usually used to manufacture springs, no quenching and
tempering treatment leads to the high
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strength and hardness of the steel described in the invention. Secondly, if
the carbon
content exceeds 0.7% preferably 0.65%, coarse and very hard carbides, combined
with
chromium, molybdenum and vanadium, can remain undissolved during the
austenitization conducted before the quench, and can significantly affect
fatigue lifetime
in air, fatigue resistance in corrosive conditions and also toughness.
Consequently
carbon contents above 0.7% must be avoided. Preferably, it should not exceed
0.65%.
[0016] The silicon content is between 1.65% and 2.5%. Silicon is an important
element that ensures, through its presence in solid solution, high levels of
strength and
hardness, as well as high carbon equivalent values Ceq and sag resistance. To
have the
tensile strength and hardness values of the steel according to the invention,
the silicon
content must not be less than 1.65%. Furthermore, silicon contributes at least
partially
to steel deoxidation. If its content exceeds 2.5%, pieferably 2.2 /0, the
oxygen content of
the steel can be, by thermodynamic reaction, greater than 0.0020%, preferably
0.0025%. This involves formation of oxides of various compositions which are
harmful
to fatigue resistance in air. Furthermore, for silicon contents greater than
2.5%, the
various combined elements such as manganese, chromium or others can segregate
during solidification, after casting. This segregation is very harmful to
fatigue behavior
in air and to fatigue resistance in conosive conditions. Finally, for silicon
content greater
than 2.5%, decarburization at the surface of bars or wire rods for springs
becomes too
high for the in-service properties of the springs. This is why the silicon
content must not
exceed 2.5%, and preferably 2.2%.
[0017] The manganese content is between 0.20% and 0.75%. In combination with
residual sulfur at level of traces to 0.015%, the manganese content must be at
least ten
times higher than the sulfur content so as to avoid formation of iron sulfides
that are
extremely harmful to steel rolling. Consequently, a minimum manganese content
of
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0.20% is required. Furthermore, manganese contributes to solid solution
hardening
during the quenching of the steel as well as nickel, chromium, molybdenum and
vanadium, which delivers high tensile strength and hardness values and the
carbon
equivalent C,eq value of the steel described in the invention. Manganese
contents
greater than 0.75%, preferably 0.65%, in combination with silicon, can
segregate during
the solidification stage, after steel making and casting. These segregations
are harmful
to the in-service properties and to the homogeneity of the steel. This is why
the
manganese content must not exceed 0.75%, and preferably 0.65%.
[0018] The chromium content must be between 0.6(P/0 and 2%, and preferably
between 0.80% and 1.70%. Chromium is added to obtain, in solid solution after
austenitization, quenching and tempering, high values for tensile strength and
hardness, and to contribute to obtaining the carbon equivalent Ceq value, but
also to
increase fatigue resistance in corrosive conditions. To ensure these
properties the
chromium content must be at least 0.60 /0, and preferably at least 0.80 /0.
Above 2%,
preferably 1.7%, specific coarse, very hard chromium carbides, in combination
with
vanadium and molybdenum, can remain after the austenitization treatment that
precedes the quench. Such carbides greatly affect the fatigue resistance in
air. This is
why the chromium content must not exceed 2%.
[0019] The nickel content is between 0.15% and 1%. Nickel is added to increase
steel
hardenability, as well as tensile strength and hardness after quenching and
tempering.
Since it does not form carbides, nickel contributes to steel hardening, just
like
chromium, molybdenum and vanadium, without forming specific coarse, hanl
carbides which would not be dissolved during the austenitization that precedes
the
quench, and could be harmful to fatigue resistance in air. It also means that
the carbon
equivalent can be adjusted between 0.8% and 1% in the steel according to the
invention
= CA 02633153 2008-06-06
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as needed. As a non-oxidizable element, nickel improves fatigue resistance in
conosive
conditions. To ensure that these effects are significant, the nickel content
must not be
lower than 0.15%. In contrast, above 1%, preferably 0.8(P/o, nickel can lead
to overly
high residual austenite content, whose presence is very harmful to fatigue
resistance in
corrosive conditions. Furthermore, high nickel levels significantly increase
the cost of
the steel. For all these reasons the nickel content must not exceed 1%,
preferably
0.80%.
[0020] The molybdenum content must be between traces and 1%. As for chromium,
molybdenum increases steel hardenability, as well as strength. Furthermore, it
has low
oxidation potential. For these two reasons, molybdenum is favorable to fatigue
resistance in air and in corrosive conditions. But for contents above 1%,
ptierably
0.80 A, coarse, very hard molybdenum carbides can remain, optionally combined
with
vanadium and chromium, after the austenitization that precedes the quench.
These
particular carbides are very harmful for fatigue resistance in air. Finally,
adding more
than 1% molybdenum increases the cost of the steel unnecessarily. This is why
the
molybdenum content must not exceed 1%, and preferably 0.80%.
[0021] The vanadium content must be between 0.003% and 0.8%. Vanadium is an
element that increases hardenability, tensile strength and hardness after
quenching
and tempering Furthermore, in combination with nitrogen, vanadium forms a
large
number of fine submicroscopic vanadium or vanadium and titanium nitrides that
refine the grain and increase tensile strength and hardness levels, through
structural
hardening. To obtain formation of submicroscopic vanadium or vanadium and
titanium nitrides that refine the grain, vanadium must be present with a
minimum
content of 0.003%. But this element is expensive and it has to be kept at this
lower
limit if a compromise is sought between the cost of steel making and the grain
CA 02633153 2008-06-06
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refinement. Vanadium must not exceed 0.8% and, preferably, 0.5%, because
beyond
this value a precipitate of coarse, very hard vanadium-containing carbides, in
combination with chromium and molybdenum, can remain undissolved during the
austenitization that precedes the quench. This can be very unfavorable for
fatigue
resistance in air, for high values of strength and hardness in the steel
according to the
invention. Further, adding more than 0.8% vanadium increases the cost of the
stefl
unnecessarily.
[0022] The copper content must be between 0.10 /0 and 1%. Copper is an dement
that hardens steel when it is in solid solution after the quenching and
tempering
treatment Accordingly, it can be added along with other elements that
contribute in
increasing the strength and hardness of the steeL As it does not combine with
carbon,
it hardens the steel without forming coarse, hard carbides that harm fatigue
resistance
in air. From the electrochemical point of view, its passivation potential is
higher than
that of iron and, consequently, it favors steel fatigue resistance in
corrosive conditions.
To ensure that these effects are significant, the copper content must not be
lower than
0.103/0. In contrast, at contents of more than 1%, preferably 0.90%, copper
has a very
harmful influence on the behavior during hot rolling. This is why the copper
content
must not exceed 1%, and preferably 0.90%.
[0023] The titanium content must be between 0.020% and 0.2%. Titanium is added
to fourr, in combination with nitrogen, preferably also carbon and/or
vanadium, fine,
submicroscopic nitrides or carbonitrides that refine the austenitic grain
during the
austenitization that precedes the quench. Accordingly, it increases the
surface area of
the grain boundaries in the steel, thereby reducing the quantity of
unavoidable
impurities that segregate in the grain boundaries, such as phosphorus. Such
intergranular segregations would be very harmful to toughness and fatigue
resistance
CA 02633153 2008-06-06
in air if they are present at high concentrations per unit of surface area at
the grain
boundaries. Furthermore, combined with carbon and nitrogen, preferably with
vanadium and niobium, titanium leads to the formation of other fine nitrides
or
carbonitrides producing an irreversible trapping effect for some elements,
such as
5 hydrogen formed during corrosion reactions, and which can be extremely
harmful to
fatigue resistance in corrosive conditions. For good efficiency the titanium
content must
not be lower than 0.020%. In contrast, above 0.2%, preferably 0.15%, titanium
can
lead to the formation of coarse, hard carbonitrides that are very harmful to
fatigue
resistance in air. The latter effect is yet more harmful for high levels of
tensile strength
10 and hardness in the steel according to the invention. For these reasons
the titanium
content must not exceed 0.2%, preferably 0.15%.
[0024] The niobium content must be between traces and 0.2%. Niobium is added
to
form, in combination with carbon and nitrogen, extremely fine, submicroscopic
precipitates of nitrides and/or carbides and/or carbonitrides that refine the
austenitic
grain during the austenitization that precedes the quench, especially when the
aluminum content is low (0.002% for example). Accordingly, niobium increases
the
surface area of the grain boundaries in the steel, and contributes to the same
favorable
effect as titanium as regards embrittlement of grain boundaries by unavoidable
impurities such as phosphorus, whose effect is very harmful to toughness and
fatigue
resistance in corrosive conditions. Furthermore, extremely fine precipitates
of niobium
nitrides or carbonitrides contribute to steel hardening through structural
hardening.
However, the niobium content must not exceed 0.2%, preferably 0.15%, so that
the
nitrides or carbonitrides remain very fine, to ensure austenitic grain
refining and to
avoid cracks or splits forming during hot rolling. For these reasons the
niobium content
must not exceed 0.2%, preferably 0.15%.
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[0025] The aluminum content must be between 0.002% and 0.050 ,10. Aluminum
can be added to complete steel deoxidation and to obtain the lowest possible
oxygen
contents, certainly less than 0.0020 /0 in the steel according to the
invention.
Furthermore, in combination with nitrogen, aluminum contributes to refining
the grain
by forming submicroscopic nitrides. To ensure these two functions, the
aluminum
content must not be lower than 0.002%. In contrast, an aluminum content
exceeding
0.05% can lead to the presence of large, isolated indusions or to aluminates
that are
finer but hard and angular, in the form of long stringers that are harmful to
the fatigue
lifetime in air and to the cleanliness of the steel. This is why the aluminum
content
must not exceed 0.05%.
[0026] The phosphorus content must be between traces and 0.015%. Phosphorus is
an unavoidable impurity in steel. During a quenching and tempering treatment,
it
co-segregates with elements such as chromium or manganese in the former
austenitic
grain boundaries. The result is reduced cohesion in the grain boundaries and
intergrannlar embrittlement that is very harmful to fatigue resistance in air.
These
effects are even more harmful for the high tensile strengths and hardnesses
required in
steels according to the invention. With the aim of simultaneously obtaining
high spring
stefl tensile strtngth and hardness and good fatigue resistance in air and in
corrosive
conditions, the phosphorus content must be as low as possible and must not
exceed
0.015%, preferably 0.010%.
[0027] The sulfur content is between traces and 0.015%. Sulfur is an
unavoidable
impurity in steel. Its content must be as low as possible, between traces and
0.015%,
and preferably 0.010 A at most. Accordingly, we wish to avoid the presence of
sulfides
that are unfavorable to fatigue resistance in corrosive conditions and fatigue
resistance
in air, for high values of strength and hardness in the steel according to the
invention.
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[0028] The oxygen content must be between traces and 0.0020%. Oxygen is also
an
unavoidable impurity in steel. In combination with deoxidizing elements,
oxygen can
lead to isolated, coarse, very hard, angular inclusions appearing, or to
indusions that
are finer but in the form of long stringers which are very harmful to fatigue
resistance in
air. These effects are even more harmful at the high tensile strengths and
hardnesses of
the steels according to the invention. For these reasons, to ensure a good
compromise
between high tensile strength and hardness and high fatigue resistance in air
and in
corrosive conditions in the steel according to the invention, the oxygen
content must not
exceed 0.0020%.
[0029] The nitrogen content must be between 0.0020 /0 and 0.0110%. The
nitrogen
must be controlled in this range so as to form, in combination with titanium,
niobium,
aluminum or vanadium, a sufficient number of very fine submicroscopic
nitrides,
carbides or carbonitrides that refine the grain. Accordingly, to do so the
minimum
nitrogen content must be 0.0020%. Its content must not exceed 0.0110% so as to
avoid forming coarse, hard titanium nitrides or carbonitrides larger than 20
observed at 1.5 mm 0.5 mm from the surface of the bars or wire rods used to
manufacture the springs. This position is the place that is most critical as
regards the
fatigue loading of the springs. Indeed, such large nitrides or carbonitrides
are very
unfavorable to fatigue resistance in air for high strength and hardness values
for steels
according to the invention, given the fact that during the tests on fatigue in
air, these
springs fractured at the location of such large indusions that were located
precisely in
the cited area of the surface of the springs, when these inclusions were
present.
[0030] To estimate the si7P of the titanium nitrides and carbonitrides, we
consider the
inclusions as squares and we suggest that their size is equal to the square
root of their
surface area.
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13
[0031] A manufacturing process for springs according to the invention will now
be
described.
[0032] A non-limiting steel making process that conforms to the invention is
as
follows. Liquid steel is produced either in a converter, or in an electric
furnace, then
undergoes a ladle metallurgy treatment during which alloy elements are added
and
deoxidation is performed, and in general all secondary metallurgy operations
delivering
a steel having the composition according to the invention and avoiding
formation of
sulfide or "carbonitrosulfide" complexes of elements such as titanium and/or
niobium
and/or vanadium. To avoid formation of such coarse precipitates during steel
making
the inventors have discovered, in an unexpected way, that the contents of the
various
elements, in particular those of titanium, nitrown, vanadium and sulfur, must
be
carefully controlled in the previously cited limits. After the process that
has just been
described the steel is cast in the form of blooms or billets, or into ingots.
But to
completely avoid forming, or to avoid forming as much as possible, coarse
titanium
nitrides or carbonitrides during and after the solidification of these
products, we have
found that the mean cooling rate of these products (blooms, billets or ingots)
must be
controlled so as to be 0.3 C/s or higher between 1450-1300 C. When we operate
in
these conditions during the solidification and cooling stages, we observe in
an
unexpected way that the size of the coarsest titanium nitrides or
carbonitrides observed
on the springs is always less than 20 pm. The location and sizi- of these
titanium
precipitates will be discussed hereinafter.
[0033] When they have returned to room temperature, products having the
precise
composition according to the invention (blooms, billets or ingots) are next
reheated and
rolled between 1200-800 C into the form of wire rods or bars in a single or
double
heating and rolling process. So as to obtain the properties of the steel that
is specific to
CA 02633153 2008-06-06
14
the invention, the bars, rods, slugs, or even springs produced from these bars
or wire
rods, are next subjected to a water quench treatment, a polymer quench or an
oil
quench after austenitization in a temperature range from 850-1000 C, so as to
obtain a
fine austenitic grain where there are no grains coarser than 9 on the ASTM
grain size
scale. This quenching treatment is then followed by a tempering treatment
specifically
performed between 300-550 C, that delivers the high levels of tensile strength
and
hardness required for the steel, and avoids firstly a microstructure that
would lead to
embrittlement during tempering, and secondly, overly high residual austenite.
We
found that embrittlement during tempering and an overly high level of residual
austentite are extremely harmful to fatigue resistance in corrosive conditions
of the steel
according to the invention. In the case where the springs are manufactured
from bars
that have not been heat treated or from wire rods or slugs made from such
bars, the
abovementioned treatments (quenching and tempering) must be performed on the
springs themselves under the abovementioned conditions. In the case where the
springs are manufactured from using cold forming, these heat treatments can be
done
on the bars, wire rods or slugs made from these bars before manufacturing the
spring.
[0034] It is well known that the hardness of steel depends not only on its
composition,
but also on the quenching temperature that it was subjected to. It must be
understood
that for all the compositions of the invention, it is possible to find
quenching
temperatures in the industrial range of 300-550 C that deliver the minimum
targeted
hardness of 55HRC.
[0035] Since nitrides and carbonitrides are very hard, their sizi- as
previously defined
does not change at all during the steel transformation steps. Therefore it is
not
important whether it is measured on the intermediate product (bar, wire rod or
slug)
which will be used to manufacture the spring or on the spring itself
= CA 02633153 2008-06-06
[0036] The invention delivers spring stefls that can combine high hardness and
tensile stiength that are an improvement over the prior art, as well as
improved fatigue
properties in air and sag resistance, fatigue properties in corrosive
conditions at least
equivalent to those of known steels for this use, or even better, and lesser
susreptibility
5 to concentrations of stresces produced by surface defects that can form
during spring
manufacture, through addition of microalloyed elements, a reduction in
residual
elements and control of the analysis and production mute of the steel.
[0037] The invention is now Mush. ated using examples and reference
examples.
Table 1 shows steel compositions according to the invention and reference
steels. The
10 carbon equivalent Ceq is given by the following formula
Ceq = Eq + 0.12 [Si] + 0.17 [Mr]] 0.1 [Ni] +0.13 [Cr] - 0.24 [V]
where [C], [Si], [Mn], [Ni], [Cr] and [V] represent the content of each
element in weight
percent.
C Si Mn Ni Cr V Ti Cu Mo Nb P S Al N 0
Ceq
Steel of the
0.48 1.82 021 0.15 1.48 0204 0.072 02) 0.02 0 0.006 0.006 0,034 0.0051
0.0007 0.86
invention 1
Steel of the
0.58 1.79 022 0.15 0.98 0216 0.073 0.20 0.03 0 0.006 0.008 0o32 0.0051
0(1)27 0.89
invention 2
Steel of the 059 180 022 0.15 0.99 0212 0.025 0.20 0.03 0.022 0.007 0.(08
0.01) 003E6 0.0008 0.91
invention 3
Steel of the 0.48 2.10 021 0.70 1.50 0.152 0.069 0.51 003 0 0.005 0.005
0.032 00042 0.0008 0.85
invention 4
Steel of the
0.54 1.81 0.23 0.34 1.25 0.098 0.077 0.42 0.02 0 0.006 0.008 0.031 0.0041
0.0007 0.90
invention 5
Reference
OW 1.73 0.88 0.08 020 0.154 0.002 0.19 0.03 0.020 0.010 0.019 0.002 0.0084
0.0010 0.94
steel 1
Refelence
0.40 1.79 0.17 0.53 1.04 0.166 0.064 0.20 0.01 0 0.013 0.004 0.020 0.0034
0.0011 0.69
steel 2
Reference
0.48 1.45 0.89 0.11 0.47 0.136 0002 0.19 0.02 0 0.011 0.013 0o13 0.oc62
0.0010 0.82
steel 3
15 Table 1: Chemical compositions of the tested steels (in %)
CA 02633153 2008-06-06
16
[0038] Table 2 shows the hardness values obtained for steels acoording to the
invention and reference steels as a function of the quenching temperature that
was
used.
CA 02633153 2008-06-06
17
Quenching Quenching
HRC HRC
temperature temperature
hardness hardness
cc) cc)
Steel of the invention 1 350 56.9 400
55.3
Steel of the invention 2 350 58.5 400
57.1
Steel of the invention 3 350 59.0 400
57.2
Steel of the invention 4 350 56.7 400
55.6
Steel of the invention 5 350 57.6 400
55.8
Reference steel 1 350 57.9 400
55.1
Reference steel 2 350 54.2 400
52.5
Reference steel 3 350 54.8 400
51.3
Table 2: Hardness and tensile strength as a function of the tempering
temperature
[0039] Table 3 shows the MaXiMUM Si7P of the inclusions of titanium nitride or
carbonitrides observed at 1.5 mm from the surface of steels according to the
invention
and reference steels, as previously defined. We have also reported the
titanium
contents of the various steels.
p04oi The maximum sim of such titanium nitride or carbonitride inclusions is
determined as follows. On a section of bar or wire rod coming from a given
steel cast, a
surface area of 100 mm2 is examined at a point located 1.5 mm 0.5 mm below
the
surface of the bar or wire rod. After the observations, the si7e of the
titanium nitride or
carbonitride inclusion having the largest surface area is determined by
considering that
the inclusions are squares and that the size of each of these inclusions,
including the
inclusion having the largest surface area, is equal to the square mot of the
surface area
All the inclusions are observed on a section of bar or wire rod for springs,
and the
observations are performed on 100 mm2 of each section. The steel cast conforms
to the
invention when the maximum size of the abovementioned inclusions observed on
CA 02633153 2008-06-06
18
100 mm2 at 1.5 mm 0.5 mm under the surface is less than 20 pm. The
corresponding results obtained on steels according to the invention and
reference steels
are given in table 3.
[0041] As regards the reference tests 1 and 3, their titanium content is
practically nil
and no nitrides and carbonitrides are observed.
Size of the largest nitride or
Ti (%) carbonitride observed
on
100 mm2 (pm)
Steel of the invention 1 0.072 11.8
Steel of the invention 2 0.073 12.4
Steel of the invention 3 0.025 13
Steel of the invention 4 0.069 11.9
Steel of the invention 5 0.077 14.1
Reference steel 1 0.002
Reference steel 2
0.064 20.8
first exam)
Reference steel 2
0.064 29
(second exam)
Reference steel 3 0.002
Table 3: Maximum size of the largest titanium nitride or carbonitride
indusions at 1.5
mm from the surface of the samples
[0042] We did not measure the size of the inclusions with reference steels 1
and 3,
since their titanium content was low and did not conform to the invention: the
result
would not have been significant.
[0043] Samples for fatigue testing were taken from bars, and the final
diameter of the
samples was 11 mm. Preparation of the samples for fatigue testing induded
rough
machining, austenitization, oil quenching, tempering, grinding and shot-
peening.
These samples were torsion-fatigue tested in air. The shear stress applied was
856
494 MPa and the number of cydes to fracture was counted. The tests were
stopped
after 2.106 cycles if the samples had not broken.
CA 02633153 2008-06-06
19
[0044] Samples for fatigue testing in corrosive conditions were taken from
bars, and
the final diameter of the samples was 11 mm. Preparation of the samples for
fatigue
testing induded rough machining, austenitization, oil quenching, tempering,
grinding
and shot-peening. These samples were tested for fatigue in corrosive
conditions, Le.
corrosion was applied at the same time as a fatigue load. The fatigue load was
a shear
stress of 856 300 MPa. The corrosion applied was cyclic corrosion in two
alternating
stages:
- one stage being a wet stage, with spraying of a 5% NaC1 solution for 5
minutes at
35 C,
- one stage being a dry stage without spraying, for 30 minutes at a
temperature of
35 C.
[0045] The number of cycles to fracture was considered to be the fatigue life
in
corrosive conditions.
[0046] Sag resistance was determined using a cyclic compression test on
cylindrical
samples. The sample diameter was 7 mm and their height was 12 mm. They were
taken from steel bars.
[0047]
Preparation of the samples for sag testing included rough machining,
austenitizing, oil quenching tempering and final fine grinding. The height of
the sample
was measured precisely before starting the test by using a comparator having 1
pm
precision. A preload was applied so as to simulate spring presetting, this
presetting
being a compression stress of 2200 MPa.
[0048] Then the fatigue load cycle was applied. This stress was 1270 730
MPa. The
height loss in the sample was measured for a number of cycles, up to 1
million. At the
end of the test the total sag was determined by a precise measurement of the
remaining
height compared to the initial height, sag resistance being better when the
reduction in
CA 02633153 2008-06-06
height, as a percentage of the initial height, was lower.
[0049] The results of the fatigue tests, fatigue tests in corrosive conditions
and sag on
steels according to the invention and reference steels are given in table 4.
Fatigue life
time in
Fatigue life
Tensile
corrosive
HRC hardness time (number
strength (MPa) of cycles) conditions
(number of
cydes)
Steel of the
56.7 2129 1742967 192034 0.025
invention 1
Steel of the
56.4 2106 >2000000 138112 0.01
invention 2
Steel of the
56.5 2118 >2000000 135562 0.015
invention 3
Steel of the
56.9 2148 >2000000 202327 0.025
invention 4
Steel of the
57.0 2156 >2000000 139809 0.025
invention 5
Reference steel
56.7 2131 514200 96672 0.03
1
Refem ice steel
53.8 1898 217815 241011 0.10
2
Reference steel
55.6 2062 301524 150875 0.075
3
5 Table 4: Results of fatigue, fatigue in corrosive conditions and sag
tests
[0050] From these tables, we see that the various reference steels are
unsatisfactory,
in particular for the following reasons.
[0051] Reference steel 1, in particular, has sulfur content that is too high
for good
10 compromise between fatigue resistance in air and the content for fatigue
in corrosive
conditions. Furthermore, its manganese content is too high, which leads to
segregations that are harmful for the homogeneity of the steel and fatigue
resistance in
CA 02633153 2008-06-06
21
air.
[0052] Reference steel 2 has too low carbon content and carbon equivalent to
ensure
high hardness. Its tensile strength is too low for good fatigue resistance in
air.
[0053] Reference steel 3, in particular, has silicon content that is too low
for good sag
resistance and also good fatigue resistance in air.
[0054] Sag resistance is higher for the steels of the invention than for
reference steels,
as Figure 1 shows, where it is clear that according to the abovementioned sag
measurements, the values for sag are at least 32% lower for the worst case of
the steels
of the invention (steel of the invention 1) as compared to the best case of
the reference
steels (refem ice steel 1).
[0055] The fatigue lifetime in air is clearly higher for the steels of the
invention as
compared to the reference steels This is due to the increased hardness, as
Figure 2
shows, but increased hardness is not enough. In fact, generally, steels with
high
hardness are more susceptible to defects, such as inclusions and surface
defects as the
hardness increases. Accordingly, steels according to the invention are less
susceptible
to defects, in particular to coarse inclusions such as titanium nitrides or
carbonitrides,
given that the invention prevents such large indusions appearing. As table 3
shows,
the largest inclusions found in steels according to the invention do not
exceed 14.1 pm,
where inclusions larger than 20 pm are found in reference steel 2.
Furthermore, lower
susceptibility to surface defects such as those that arise during spring
manufacture or
other operations when steels of the invention are used can be illustrated by
strength
tests performed on steels of the invention and reference steels having
undergone a heat
treatment and having hardness of 55HRC or higher, see figure 3. The values
measured
during Charpy impact tests on the steels of the invention (where the sample
notch
simulates a concentration of stresses like other concentrations of sliesses
that we can
CA 02633153 2008-06-06
22
find on surface defects produced during the manufacture of the spring or other
operations) are higher than those measured on the reference steels. This shows
that
the steels according to the invention are less susceptible to concentrations
of stresses on
defects than reference steels according to the prior art
[0056] We know that increasing hardness reduces fatigue resistance in
corrosive
conditions. Accordingly, it seems that steels according to the invention have
the
advantage that their fatigue resistance in corrosive conditions is higher than
that of
reference steels according to the prior art, and in particular hardness
greater than
55HRC as Figure 4 shows.
[0057] Accordingly, the invention delivers higher hardness with a good
compromise
between fatigue lifetime in air and sag resistance, which are greatly
increased, and
fatigue lifetime in corrosive conditions which is better than those of
reference steels
according to the prior art Furthermore, lesser susceptibility to possible
surface defects,
in particular those generated during spring manufacture or other operations,
is also
obtained.
