U.S. patent application number 12/097313 was filed with the patent office on 2008-12-18 for steel for springs, process of manufacture for spring using this steel, and spring made from such steel.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Kazuhisa Kawata, Jacques Languillaume, Julie Mougin, Nao Yoshihara.
Application Number | 20080308195 12/097313 |
Document ID | / |
Family ID | 36933407 |
Filed Date | 2008-12-18 |
United States Patent
Application |
20080308195 |
Kind Code |
A1 |
Yoshihara; Nao ; et
al. |
December 18, 2008 |
Steel For Springs, Process Of Manufacture For Spring Using This
Steel, And Spring Made From Such Steel
Abstract
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% V=0.003-0.8%
Cu=0.10-1% Ti=0.020-0.2% Nb=traces-0.2% AI=0.002-0.050%
P=traces-0.015% S=traces-0.015% O=traces-0.0020% N=0.0020-0.0110%
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 %]+0.12 [Si %]+0.17 [Mn %]-0.1
[Ni %]+0.13 [Cr %]-0.24 [V %] is between 0.80 and 1.00%, and whose
hardness, after quenching and tempering, is greater than or equal
to 55 HRC.
Inventors: |
Yoshihara; Nao; (Hyogo,
JP) ; Kawata; Kazuhisa; (Hyogo, JP) ; Mougin;
Julie; (Pontcharra, FR) ; Languillaume; Jacques;
(Arvillard, FR) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi
JP
ASCOMETAL
Courbevoie
FR
|
Family ID: |
36933407 |
Appl. No.: |
12/097313 |
Filed: |
December 11, 2006 |
PCT Filed: |
December 11, 2006 |
PCT NO: |
PCT/FR2006/002700 |
371 Date: |
July 18, 2008 |
Current U.S.
Class: |
148/547 ;
148/332 |
Current CPC
Class: |
C22C 38/50 20130101;
C22C 38/06 20130101; C21D 8/06 20130101; C22C 38/42 20130101; C22C
38/44 20130101; C22C 38/48 20130101; C22C 38/02 20130101; C22C
38/46 20130101; C21D 2211/004 20130101; C22C 38/04 20130101; C21D
9/02 20130101 |
Class at
Publication: |
148/547 ;
148/332 |
International
Class: |
C21D 9/02 20060101
C21D009/02; C22C 38/20 20060101 C22C038/20; C22C 38/22 20060101
C22C038/22; C22C 38/34 20060101 C22C038/34; C22C 38/40 20060101
C22C038/40; C22C 38/42 20060101 C22C038/42 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 15, 2005 |
FR |
05 12775 |
Claims
1: A spring steel composition characterized by high fatigue
resistance in air and in corrosive conditions and high resistance
to cyclic sag, said composition comprising 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% 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% O=traces-0.0020%
N=0.0020-0.0110% the balance being iron, and impurities resulting
from the steel making process, wherein the carbon equivalent (Ceq)
content calculated according to the 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%, and whose hardness, after quenching and tempering, is
greater than or equal to 55 HRC.
2: The sing steel according to claim 1, characterized in that the
maximum size of titanium nitrides or carbonitrides observed at
1.5.+-.0.5 mm of the surface area of a bar, a wire rod, a slug or a
spring over 100 mm.sup.2 of the surface area of the section is less
than or equal to 20 .mu.m, said size being the square root of the
surface area of the inclusions considered as squares.
3: The spring steel according to claim 1, characterized in that its
composition comprises: 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% V=0.003-0.5%
Cu=0.10-0.90% Ti=0.020-0.15% Nb=traces-0.15% AI=0.002-0.050%
P=traces-0.010% S=traces-0.010% O=traces-0.0020% N=0.0020-0.0110%
the balance being iron and impurities resulting from the steel
making process.
4: A manufacturing process for a spring steel with high fatigue
resistance in air and in corrosive conditions and high resistance
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 for transformation into springs, characterized in
that: the steel is the type according to claim 1: the blooms,
billets or ingots after they become solid have a minimum mean
cooling rate of 0.3.degree. C./s between 1450-1300.degree. C.; said
blooms, billets or ingots are rolled between 1200-800.degree. 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.degree. C., followed by a water quench, a polymer quench
or an oil quench, and by tempering at 300-550.degree. C., so as to
deliver steel with a hardness greater than or equal to 55 HRC.
5: A spring, characterized in that it is made of a steel according
to claim 1.
6: A spring according to claim 5, characterized in that it is made
of a steel comprising 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% 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% O=traces-0.0020% N=0.0020-0.0110%
the balance being iron, and impurities resulting from the steel
making process, wherein the carbon equivalent (Ceq) content
calculated according to the 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%,
and whose hardness, after quenching and tempering, is greater than
or equal to 55 HRC, wherein said steel is made in a converter or an
electric furnace, its composition is adjusted and is cast into
blooms or continuous flow billets or ingots that are left to cool
to room temperature and are rolled into bars, wire rods or slugs
for transformation into springs, characterized in that: the blooms,
billets or ingots after they become solid have a minimum means
cooling rate of 0.3.degree. C./s between 1450-1300.degree. C.; said
blooms, billets or ingots are rolled between 1200-800.degree. 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.degree. C., followed by a water quench, a polymer quench
or an oil quench, and by tempering at 300-550.degree. C., so as to
deliver steel with a hardness greater than or equal to 55 HRC.
Description
[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-A-2740476 and
JP-3474373B 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 .mu.m in
diameter, and to be very numerous (10,000 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 50 HRC 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 53.5
HRC. 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 55 HRC, a
high fatigue resistance in air and fatigue resistance in corrosive
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:
[0007] C=0.45-0.70%
[0008] Si=1.65-2.50%
[0009] Mn=0.20-0.75%
[0010] Cr=0.60-2%
[0011] Ni=0.15-1%
[0012] Mo=traces-1%
[0013] V=0.003-0.8%
[0014] Cu=0.10-1%
[0015] Ti=0.020-0.2%
[0016] Nb=traces-0.2%
[0017] AI=0.002-0.050%
[0018] P=traces-0.015%
[0019] S=traces-0.015%
[0020] O=traces-0.0020%
[0021] N=0.0020-0.0110%
[0022] 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 %]+0.12 [Si %]+0.17 [Mn %]-0.1 [Ni %]+0.13 [Cr %]-0.24 [V
%]
[0023] is between 0.80 and 1.00%, and whose hardness, after
quenching and tempering, is greater than or equal to 55 HRC.
[0024] 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
rod, a slug or a spring over 100 mm.sup.2 of the surface area of
the section is preferably less than or equal to 20 .mu.m, which
size being the square root of the surface area of the inclusions
considered as squares.
[0025] Preferably, the composition of the steel is:
[0026] C=0.45-0.65%
[0027] Si=1.65-2.20%
[0028] Mn=0.20-0.65%
[0029] Cr=0.80-1.7%
[0030] Ni=0.15-0.80%
[0031] Mo=traces-0.80%
[0032] V=0.003-0.5%
[0033] Cu=0.10-0.90%
[0034] E=0.020-0.15%
[0035] Nb=traces-0.15%
[0036] Al=0.002-0.050%
[0037] P=traces-0.010%
[0038] S=traces-0.010%
[0039] O=traces-0.0020%
[0040] N=0.0020-0.0110%
[0041] The balance being iron and impurities resulting from the
steel making process.
[0042] 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 resistance 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:
[0043] the steel is of the previous type;
[0044] after they become solid the blooms, billets or ingots have a
minimum mean cooling rate of 0.3.degree. C./s between
1450-1300.degree. C.;
[0045] the blooms, billets or ingots are rolled between
1200-800.degree. C. in one or two reheating and rolling cycles;
[0046] and bars, wire rods or slugs, or springs made from these,
are austenitized between 850-1000.degree. C., followed by a water
quench, a polymer quench or an oil quench, and by tempering at
300-550.degree. C., so as to deliver steel with hardness greater
than or equal to 55 HRC.
[0047] A further object of the invention is springs made from such
steel, and springs made of steel obtained by the previous
process.
[0048] In an unexpected way, the inventors realized that a steel
with the characteristics of the previously cited inclusion
composition and morphology ensured, after steelmaking, casting,
rolling, quenching and tempering done in specific conditions, a
hardness greater than 55 HRC, 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.
[0049] The invention will be better understood upon reading the
description that follows, given in reference to the following
appended figures:
[0050] FIG. 1 which shows the results of hardness and cyclic sag
tests for steels according to the invention and reference
steels;
[0051] FIG. 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;
[0052] FIG. 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
[0053] FIG. 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.
[0054] The steel composition according to the invention must meet
the following conditions.
[0055] 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 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%.
[0056] 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%, preferably 2.2%, 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 corrosive 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%.
[0057] 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 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 Ceq 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%.
[0058] The chromium content must be between 0.60% 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%, and
preferably at least 0.80%. 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%.
[0059] 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,
hard 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 as needed. As a non-oxidizable element, nickel improves
fatigue resistance in corrosive conditions. To ensure that these
effects are significant, the nickel content must not be lower than
0.15%. In contrast, above 1%, preferably 0.80%, nickel can lead to
overly high residual austenite content, whose presence is very
harmful to fatigue resistance in corrosive conditions. Furthermore,
high nickel level significantly increase the cost of the steel. For
all these reasons the nickel content must not exceed 1%, preferably
0.80%.
[0060] 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%, preferably
0.80%, 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%.
[0061] 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
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 steel unnecessarily.
[0062] The copper content must be between 0.10% and 1%. Copper is
an element 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. Form 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 0.10%. 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%.
[0063] The titanium content must be between 0.020% and 0.2%.
Titanium is added to form, 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. According, 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 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 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 and
hardness in the steel according to the invention. For these reasons
the titanium content must not exceed 0.2, preferably 0.15%.
[0064] 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 ample). 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%.
[0065] The aluminum content must be between 0.002% and 0.050%.
Aluminum can be added to complete steel deoxidation and to obtain
the lowest possible oxygen contents, certainly less than 0.0020% 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 inclusions 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%.
[0066] 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
intergranular 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
steel tensile strength 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%.
[0067] 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% 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.
[0068] 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 inclusions 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%.
[0069] The nitrogen content must be between 0.0020% 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 .mu.m, 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 inclusions that were located precisely in the cited area
of the surface of the springs, when these inclusions were
present.
[0070] To estimate the size 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.
[0071] A manufacturing process for springs according to the
invention will now be described.
[0072] 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, nitrogen, 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.degree. C./s or higher between 1450-1300.degree. 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 .mu.m. The location and size of these
titanium precipitates will be discussed hereinafter.
[0073] 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.degree. 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 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.degree. 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.
[0074] 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.degree. C. that
deliver the minimum targeted hardness of 55 HRC.
[0075] Since nitrides and carbonitrides are very hard, their size
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.
[0076] The invention delivers spring steels that can combine high
hardness and tensile strength 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 susceptibility to concentrations of stresses 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 route of the steel.
[0077] The invention is now illustrated using examples and
reference examples. Table 1 shows steel compositions according to
the invention and reference steels. The carbon equivalent Ceq is
given by the following formula:
Ceq=[C]+0.12 [Si]+0.17 [Mn]-0.1 [Ni]+0.13 [Cr]-0.24 [V]
[0078] where [C], [Si], [Mn], [Ni], [Cr] and [V] represent the
content of each element in weight percent.
TABLE-US-00001 TABLE 1 Chemical compositions of the tested steels
(in %) C Si Mn Ni Cr V Ti Cu Mo Nb P S Al N O Ceq Steel of the 0.48
1.82 0.21 0.15 1.48 0.204 0.072 0.20 0.02 0 0.006 0.006 0.034
0.0051 0.0007 0.86 invention 1 Steel of the 0.58 1.79 0.22 0.15
0.98 0.216 0.073 0.20 0.03 0 0.006 0.008 0.032 0.0051 0.0007 0.89
invention 2 Steel of the 0.59 1.80 0.22 0.15 0.99 0.212 0.025 0.20
0.03 0.022 0.007 0.008 0.032 0.0066 0.0008 0.91 invention 3 Steel
of the 0.48 2.10 0.21 0.70 1.50 0.152 0.069 0.51 0.03 0 0.005 0.005
0.032 0.0042 0.0008 0.86 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 0.60 1.73 0.88 0.08 0.20 0.154 0.002
0.19 0.03 0.020 0.010 0.019 0.002 0.0084 0.0010 0.94 steel 1
Reference 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 0.002 0.19 0.02 0 0.011 0.013 0.003 0.0062 0.0010
0.82 steel 3
[0079] Table 2 shows the hardness values obtained for steels
according to the invention and reference steels as a function of
the quenching temperature that was used.
TABLE-US-00002 TABLE 2 Hardness and tensile strength as a function
of the tempering temperature Quenching HRC Quenching HRC
temperature hard- temperature hard- (.degree. C.) ness (.degree.
C.) ness 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
[0080] Table 3 shows the maximum size 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.
[0081] The maximum size 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 mm.sup.2
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 size 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 root 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 mm.sup.2 of each section. The steel cast conforms
to the invention when the maximum size of the abovementioned
inclusions observed on 100 mm.sup.2 at 1.5 mm.+-.0.5 mm under the
surface is less than 20 .mu.m. The corresponding results obtained
on steels according to the invention and reference steels are given
in table 3.
[0082] As regards the reference tests 1 and 3, their titanium
content is practically nil and no nitrides and carbonitrides are
observed.
TABLE-US-00003 TABLE 3 Maximum size of the largest titanium nitride
or carbonitride inclusions at 1.5 mm from the surface of the
samples Size of the largest nitride or carbonitride observed on Ti
(%) 100 mm.sup.2 (.mu.m) 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 --
[0083] 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.
[0084] 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 included 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 cycles to fracture was counted.
The tests were stopped after 2.10.sup.6 cycles if the samples had
not broken.
[0085] 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 included rough
machining, austenitization, oil quenching, tempering, grinding and
shot-peening. These samples were tested for fatigue in corrosive
conditions, i.e. 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:
[0086] one stage being a wet stage, with spraying of a 5% NaCl
solution for 5 minutes at 35.degree. C.;
[0087] one stage being a dry stage without spraying, for 30 minutes
at a temperature of 35.degree. C.
[0088] The number of cycles to fracture was considered to be the
fatigue life in corrosive conditions.
[0089] 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.
[0090] 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 .mu.m precision. A
preload was applied so as to simulate spring presetting, this
presetting being a compression stress of 2200 MPa.
[0091] 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 height, as a percentage of the initial height, was
lower.
[0092] 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.
TABLE-US-00004 TABLE 4 Results of fatigue, fatigue in corrosive
conditions and sag tests Fatigue life- time in corrosive Tensile
Fatigue life- conditions HRC strength time (number (number of Sag
hardness (MPa) of cycles) cycles) (%) 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 56.7 2131 514200 96672 0.03 steel 1 Reference
53.8 1898 217815 241011 0.10 steel 2 Reference 55.6 2062 301524
150875 0.075 steel 3
[0093] From these tables, we see that the various reference steels
are unsatisfactory, in particular for the following masons.
[0094] Reference steel 1, in particular, has sulfur content that is
too high for good 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
air.
[0095] 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.
[0096] Reference steel 3, in particular, has silicon content that
is too low for good sag resistance and also good fatigue resistance
in air.
[0097] Sag resistance is higher for the steels of the invention
than for reference steels, as FIG. 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 ease
of the reference steels (reference steel 1).
[0098] 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 FIG. 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 inclusions appearing. As table 3
shows, the largest inclusions found in steels according to the
invention do not exceed 14.1 .mu.m, where inclusions larger than 20
.mu.m 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 55 HRC or higher, see FIG. 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 stresses that we can 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.
[0099] 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 55 HRC as FIG. 4 shows.
[0100] 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.
* * * * *