BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to a high strength spring and a method of manufacturing
the same, and a steel for a high strength spring and a method of manufacturing the
same.
2. Description of the Related Art
[0002] High strength springs are used for automobiles and the like. As the high strength
spring has high strength, the high strength spring can be formed by a thin wire, and
can contribute to lightening of an automobile, and also improve fuel consumption of
the automobile. However, when the strength of the spring is increased, fatigue strength,
hydrogen embrittlement resistance, delayed fracture resistance and the like under
corrosive environment are lowered.
[0003] Thus, a steel for a spring disclosed in Patent Document 1 is configured to capture
hydrogens entering the steel from external environment by a hydrogen trap site made
of a precipitate containing V and the like to suppress diffusion of the hydrogen in
the steel.
[0004] JP2016125119 A relates to a high strength hollow seamless steel pipe for springs, and more particularly
to a seamless steel pipe suitable for manufacturing a hollow suspension made of steel
and the like used in automobiles and the like.
[Patent Document]
[0005] [Patent Document 1] Japanese Laid-open Patent Publication No.
2001-288539
[0006] In order to ensure hydrogen embrittlement resistance, it is effective to increase
the number of precipitates that function as hydrogen trap sites. The precipitates
contain V and the like.
[0007] However, there is a problem that the number of precipitates is not increased and
coarse precipitates are formed just by increasing the content of an element such as
V.
[0008] Further, in order to obtain high strength, it is effective to increase the content
of C. However, if the content of C is too much, corrosion durability is lowered.
[0009] In order to obtain high strength with a small content of C, a tempering process at
low temperature is effective. However, if a content of N is too much, low temperature
temper brittleness is generated. As a result, as toughness is lowered, delayed fracture
resistance is also lowered.
SUMMARY OF THE INVENTION
[0010] The present invention is made in light of the above problems, and mainly provides
a high strength spring which has good hydrogen embrittlement resistance, corrosion
durability and delayed fracture resistance.
[0011] According to an embodiment, there is provided a high strength spring containing,
by mass%, C: 0.40 to 0.50%, Si: 1.00 to 3.00%, Mn: 0.30 to 1.20%, Ni: 0.05 to 0.50%,
Cr: 0.35 to 1.50%, Mo: 0.03 to 0.50%, Cu: 0.05 to 0.50%, Al: 0.005 to 0.100%, V: 0.05
to 0.50%, Nb: 0.005 to 0.150%, N: 0.0100 to 0.0200%, P: limited to be less than or
equal to 0.015%, S: limited to be less than or equal to 0.010%, and the balance of
Fe and inevitable impurities, wherein a Nb-compound including at least one of Nb-carbide,
Nb-nitride and Nb-carbonitride is included, and wherein a V-compound including at
least one of V-carbide and V-carbonitride that is precipitated around the Nb-compound
is included, wherein the number of the complex precipitates, formed by the Nb-compound
and the V-compound precipitated around the Nb-compound, per unit is greater than or
equal to 100/mm
2 and less than or equal to 100 000/mm
2.
[0012] According to the invention, a high strength spring and a steel for a high strength
spring are provided which have good hydrogen embrittlement resistance, corrosion durability
and delayed fracture resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013]
Fig. 1-(a) to Fig. 1-(e) are SEM images of a part of a cross-section of a steel after
a tempering process in example 1;
Fig. 2-(a) to Fig. 2-(e) are SEM images of another part of the cross-section of the
steel after the tempering process in example 1;
Fig. 3 is a view illustrating results of a rotating bending fatigue test of example
1 and comparative example 1; and
Fig. 4 is a view illustrating results of a durability test of example 3 and comparative
example 2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0014] Hereinafter, embodiments of the present invention are described.
[0015] A high strength spring is used for, for example, a suspension spring of an automobile.
Here, "high strength" means that its tensile strength is greater than or equal to
1800 MPa. The shape of a test piece used in a measurement of tensile strength is based
on the shape of a No. 4 test piece described in Japan Industrial Standard (JIS Z2241).
[0016] The high strength spring may be a coil spring. The coil spring is manufactured by
hot spring forming, cold spring forming or the like. According to the hot spring forming,
after a wire is hot formed into a coil shape, a quenching process and a tempering
process are performed. Further, according to the cold spring forming, after performing
a quenching process and a tempering process on a wire, the wire is cold formed into
a coil shape.
[0017] Here, although a coil spring is exemplified as the high strength spring in this embodiment,
the high strength spring may be a leaf spring or the like. The embodiment of the high
strength spring is not specifically limited. Further, the purpose for the high strength
spring to be used is not limited to a suspension device of an automobile as well.
[0018] The high strength spring is made of a steel for a high strength spring. The steel
for a high strength spring is obtained by performing a quenching process and a tempering
process, and has a martensitic structure obtained by the quenching process. Before
the quenching process, a pearlite structure is dominant, an austenite structure is
dominant at quenching temperature, and the martensitic structure is dominant after
the quenching process.
[0019] As long as the quenching process and the tempering process are performed on the steel
for a high strength spring, its shape is not specifically limited. For example, for
the hot spring forming, the steel for a high strength spring may have a shape of a
spring (a coil shape, for example). Meanwhile, for the cold spring forming, the steel
for a high strength spring may have the shape of the spring, or a shape (a stick shape,
for example) before being shaped into the shape of the spring.
[0020] The steel for a high strength spring contains, by mass%, C: 0.40 to 0.50%, Si: 1.00
to 3.00%, Mn: 0.30 to 1.20%, Ni: 0.05 to 0.50%, Cr: 0.35 to 1.50%, Mo: 0.03 to 0.50%,
Cu: 0.05 to 0.50%, Al: 0.005 to 0.100%, V: 0.05 to 0.50%, Nb: 0.005 to 0.150% and
N: 0.0100 to 0.0200%, wherein P is limited to be less than or equal to 0.015% and
S is limited to be less than or equal to 0.010%, and contains the balance of Fe and
inevitable impurities. Hereinafter, each component is described. For the description
of each component, "%" means mass%.
[0021] C is an element effective for increasing strength of the steel. The content of C
is 0.40 to 0.50%. When the content of C is less than 0.40%, strength necessary for
a spring cannot be obtained. Meanwhile, when the content of C exceeds 0.50%, corrosion
durability is lowered.
[0022] Si is an element effective for improving strength of the steel by being solid-dissolved
in ferrite. The content of Si is 1.00 to 3.00%. When the content of Si is less than
1.00%, strength necessary for a spring cannot be obtained. Meanwhile, when the content
of Si exceeds 3.00%, when the spring is hot formed, decarbonizing at a surface easily
occurs and durability of the spring is lowered.
[0023] Mn is an element effective for improving hardenability of the steel. The content
of Mn is 0.30 to 1.20%. When the content of Mn is less than 0.30%, an effect of improving
the hardenability cannot be sufficiently obtained. Meanwhile, when the content of
Mn exceeds 1.20%, toughness is deteriorated.
[0024] Ni is an element necessary for increasing corrosion durability of the steel. The
content of Ni is 0.05 to 0.50%. When the content of Ni is less than 0.05%, an expected
effect of increasing the corrosion durability of the steel cannot be sufficiently
obtained. As Ni is expensive, an upper limit of the content of Ni is 0.50%. Cr is
an element effective for increasing strength of the steel. The content of Cr is 0.35
to 1.50%. When the content of Cr is less than 0.35%, an expected effect of increasing
the strength of the steel cannot be sufficiently obtained. Meanwhile, when the content
of Cr exceeds 1.50%, toughness is easily deteriorated.
[0025] Mo is an element that ensures hardenability of the steel, and increases strength
and toughness of the steel. The content of Mo is 0.03 to 0.50%. When the content of
Mo is less than 0.03%, an expected effect of adding Mo cannot be sufficiently obtained.
Meanwhile, when the content of Mo exceeds 0.50%, the effect of adding Mo is saturated.
[0026] Cu is a component that increases corrosion durability. The content of Cu is 0.05
to 0.50%. When the content of Cu is less than 0.05%, an effect of increasing the corrosion
durability cannot be sufficiently obtained. Meanwhile, when the content of Cu exceeds
0.50%, cracking and the like may occur during hot rolling.
[0027] Al is an element necessary as a deoxidizer of the steel and for adjusting an austenite
grain size. The content of Al is 0.005 to 0.100%. When the content of Al is less than
0.005%, the crystal grain cannot be finely formed. Meanwhile, when the content of
Al exceeds 0.100%, castability is easily lowered.
[0028] V is an element effective for increasing strength of the steel, and suppressing hydrogen
embrittlement. The content of V is 0.05 to 0.50%. When the content of V is less than
0.05%, an expected effect of adding V cannot be sufficiently obtained. Meanwhile,
when the content of V exceeds 0.50%, carbide that does not dissolve in austenite increases,
and spring characteristics are deteriorated.
[0029] Nb is an element that increases strength and toughness of the steel by finely forming
a crystal grain and precipitating fine carbide. Further, Nb is an element that contributes
to fine dispersion of a V-compound including at least one of V-carbide and V-carbonitride
(hereinafter, simply referred to as a "V-compound"), and increases hydrogen embrittlement
resistance. The content of Nb is 0.005 to 0.150%. When the content of Nb is less than
0.005%, an expected effect of adding Nb cannot be sufficiently obtained. Meanwhile,
when the content of Nb exceeds 0.150%, carbide that does not dissolve in austenite
increases, and spring characteristics are deteriorated.
[0030] N is an element that forms A1N or NbN by bonding with Al or Nb, and has an effect
in making austenite grain size fine. With this fine structure, toughness is improved.
The content of N is 0.0100 to 0.0200%. When the content of N is greater than or equal
to 0.0100%, a sufficient effect of improving toughness can be obtained. Meanwhile,
if N is excessively added, bubbles may be generated at a surface of a steel ingot
in solidification, or castability of the steel may be deteriorated, an upper limit
of the content of N is 0.0200%.
[0031] P becomes a factor to lower an impact value by being precipitated at an austenite
grain boundary to embrittle the grain boundary. In order to suppress this problem,
the content of P is limited to be less than or equal to 0.015%.
[0032] S exists as an inclusion of MnS in the steel, and becomes a factor to lower fatigue
life and corrosion durability. The inclusion means something that is already formed
when the steel is molten. In order to decrease the inclusion, the content of S is
limited to be less than or equal to 0.010%, preferably, less than or equal to 0.005%.
[0033] In order to finely disperse the V-compound as the hydrogen trap site, the steel for
a high strength spring is manufactured by having the V-compound solid-dissolved in
iron at the quenching temperature, and thereafter, precipitating the V-compound around
the Nb-compound that is finely dispersed in the steel. Thus, the steel for a high
strength spring includes the Nb-compound and the V-compound precipitated around the
Nb-compound. As long as the V-compound is precipitated to be adjacent to the Nb-compound,
the V-compound may not completely surround a periphery of the Nb-compound or may completely
surround a periphery of the Nb-compound. In the steel for a high strength spring,
the Nb-compound may exist inside the V-compound.
[0034] The Nb-compound is a precipitate that is precipitated in iron while a molten steel
is being solidified. The Nb-compound includes at least one of Nb-nitride, Nb-carbide
and Nb-carbonitride. The Nb-compound is finely dispersed in the steel before the quenching
process, is not solid-dissolved in iron at the quenching temperature, and becomes
a starting point of precipitation of the V-compound in quenching from the quenching
temperature or in the tempering process. As the starting point of the precipitation
of the V-compound, Nb-nitride that is more finely dispersed is preferably used compared
with Nb-carbide and Nb-carbonitride.
[0035] As the V-compound exists in the steel as a coarse precipitate before the quenching
process, the V-compound is solid-dissolved in iron at the quenching temperature, and
thereafter, is precipitated from the Nb-compound as the starting point. As the Nb-compound
is finely dispersed, the V-compound that is precipitated from the Nb-compound as the
starting point can be finely dispersed. By finely forming the V-compound, the number
of the V-compounds can be increased, and the steel for a high strength spring which
has good hydrogen embrittlement resistance can be obtained.
[0036] The quenching temperature is set to be greater than or equal to 950 °C and less than
or equal to 1000 °C in order for the V-compound to be solid-dissolved in iron at the
quenching temperature. Such quenching temperature is higher than dissolution temperature
at which the V-compound is solid-dissolved in iron, and when the content of V is less
than or equal to 0.50% as described above, the V-compound is completely solid-dissolved
in iron according to the calculation of a solubility product. As the quenching temperature
is high temperature, in order to suppress the crystal grain to be coarse, appropriate
amounts of Nb, Al, N and the like are added. With this, lowering of toughness can
be suppressed, and lowering of delayed fracture resistance can be suppressed as well.
Therefore, the steel for a high strength spring which has good delayed fracture resistance
can be obtained.
[0037] A complex precipitate is formed by the Nb-compound and the V-compound precipitated
around the Nb-compound. An average grain size of the complex precipitate may be greater
than or equal to 0.01 µm and less than or equal to 1 µm. Further, the number of the
complex precipitates per unit may be greater than or equal to 100 / mm
2 and less than or equal to 100000 / mm
2. The average grain size and the number per unit are measured using a SEM (Scanning
Electron Microscope), for example. The average grain size is obtained by measuring
each equivalent area diameter (diameter) of 100 complex precipitates, and calculating
an average value of the measured values. The number per unit is obtained by measuring
the number of the complex precipitates those exist at a region whose total area is
15 mm
2, and dividing the number by the total area.
[0038] In the steel for a high strength spring, in order to suppress lowering of corrosion
durability, the content of C is limited to be less than or equal to 0.5%. Further,
in order to ensure strength of the steel within a range where the content of C is
less than or equal to 0.5%, the tempering temperature is limited to be less than 390
°C. Therefore, the steel for a high strength spring which has good corrosion durability
and high strength can be obtained. Here, in order to obtain a sufficient effect of
improving toughness by the tempering process, a lower limit of the tempering temperature
is set to be 250 °C, more preferably, to be 300 °C.
[0039] In order to sufficiently finely disperse nitride, the steel for a high strength spring
includes 0.0100 to 0.0200% of N. In order to suppress low temperature temper brittleness
due to N, the steel for a high strength spring contains appropriate amounts of Nb
and Al, and N is detoxified by precipitating NbN and AlN instead of N. With this,
lowering of toughness can be suppressed, and lowering of delayed fracture resistance
can be suppressed as well. Thus, the steel for a high strength spring which has good
delayed fracture resistance can be obtained.
[Examples]
[0040] Hereinafter, specific examples, comparative examples and the like are described.
(Example 1)
[0041] In example 1, a quenching process and a tempering process were performed on a steel
having a composition as follows, and a rotating bending fatigue test piece and a hydrogen
embrittlement test piece were manufactured by machining.
[0042] As the steel, a steel containing, by mass%, C: 0.44%, Si: 1.75%, Mn: 0.45%, Ni: 0.25%,
Cr: 0.75%, Mo: 0.08%, Cu: 0.35%, Al: 0.023%, V: 0.25%, Nb: 0.020%, N: 0.0130%, P:
limited to be less than or equal to 0.010%, S: limited to be less than or equal to
0.003%, and the balance of Fe and inevitable impurities, was used.
[0043] The quenching temperature was 950 °C, and its retention time was 30 minutes. Oil
cooling was used to cool the steel from the quenching temperature.
[0044] The tempering temperature was 360 °C, and its retention time was 1 hour. Air cooling
was used to cool the steel from the tempering temperature.
[0045] Vickers hardness of the steel after the tempering process was 590 Hv.
[0046] Further, the obtained steel was observed by an electron microscope. Fig. 1-(a) to
Fig. 1-(e) are SEM images of a part of a cross-section of the steel after the tempering
process in example 1, and Fig. 2-(a) to Fig. 2-(e) are SEM images of another part
of the cross-section of the steel after the tempering process in example 1. Fig. 1-(a)
and Fig. 2-(a) are backscattered electron images, Fig. 1-(b) and Fig. 2-(b) are characteristic
X ray maps of Nb, Fig. 1-(c) and Fig. 2-(c) are characteristic X ray maps of N, Fig.
1-(d) and Fig. 2-(d) are characteristic X ray maps of V, and Fig. 1-(e) and Fig. 2-(e)
are characteristic X ray maps of C. Here, in the backscattered electron images of
Fig. 1-(a) and Fig. 2-(a), white portions indicate the Nb-compound, and black portions
around the white portions indicate the V-compound. In the characteristic X ray maps
of each element of Fig. 1-(b) to Fig. 1-(e) and Fig. 2-(b) to Fig. 2-(e), brightness
of color indicates the amount of the element, and the brighter (more white) the color
is, the greater the content of the element. As the backscattered electron images of
Fig. 1-(a) and Fig. 2-(a) are images of reflected electrons of electron beam that
rebound near the cross-section of the steel, those images express the size of observed
surfaces as they are. Meanwhile, the characteristic X ray maps of Fig. 1-(b) to Fig.
1-(e) and Fig. 2-(b) to Fig. 2-(e) are images of characteristic X rays generated when
the electron beam enters the steel from the cross-section of the steel. Further, a
threshold value is provided for intensity of the characteristic X ray to be detected.
Thus, the images of the characteristic X ray maps are different from the size that
is observed at the observed surface.
[0047] As is apparent from the backscattered electron image of Fig. 1-(a), it was observed
that a portion (black portion) in which concentration of V is higher than that at
its periphery exists in the steel of example 1, and a portion (white portion) exists
in the black portion in which concentration of Nb is higher than that at outside of
the black portion. Further, from the characteristic X ray maps of Fig. 1-(b) to Fig.
1-(e), it was observed that a portion in which concentrations of N and C are high
exists in each of the black portion and the white portion in Fig. 1-(a), and a portion
in which concentration of N is high and a portion in which concentration of C is high
at least overlap. Thus, it can be said that in the steel of example 1, at least V-carbonitride
was precipitated such that to surround at least Nb-carbonitride after the tempering
process.
[0048] As is apparent from the backscattered electron image of Fig. 2-(a), at the other
part of the steel of example 1, it was observed that a portion (black portion) in
which concentration of V is higher than that at its periphery exists in the steel
of example 1, and a portion (white portion) exists in the black portion in which concentration
of Nb is higher than that at outside of the black portion. Further, from the characteristic
X ray maps of Fig. 2-(b) to Fig. 2-(e), it was observed that a portion in which concentrations
of N and C are high exists in each of the black portion and the white portion in Fig.
2-(a), and a portion in which concentration of N is high and a portion in which concentration
of C is high at least overlap. Thus, it can be said that in the steel of example 1,
at least V-carbonitride was precipitated such that to surround at least Nb-carbonitride
after the tempering process after the tempering process.
[0049] Thus, it was confirmed that the V-compound was precipitated such that to surround
the Nb-compound after the tempering process in the steel of example 1.
[0050] The shape of the test piece was based on the shape of a No. 1 test piece described
in Japan Industrial Standard (JIS Z2274). The test piece has a constriction portion
called a parallel part at a center portion of a round bar.
[0051] For the rotating bending fatigue test piece, the diameter of both end parts was 15
mm, the diameter of the parallel part was 8 mm, and the length of the parallel part
was 20 mm.
[0052] For the hydrogen embrittlement test piece, the diameter of both end parts was 10
mm, the diameter of the parallel part was 4 mm, and the length of the parallel part
was 15 mm.
(Comparative example 1)
[0053] In comparative example 1, a quenching process and a tempering process were performed
on a steel having a composition as follows, and a rotating bending fatigue test piece
and a hydrogen embrittlement test piece were manufactured by machining.
[0054] As the steel, a steel containing, by mass%, C: 0.52%, Si: 1.50%, Mn: 0.45%, Ni: 0.26%,
Cr: 0.80%, Mo: 0.09%, Cu: 0.12%, Al: 0.023%, V: 0.16%, Nb: 0.025%, N: 0.0120%, P:
0.010%, S: 0.009%, and the balance of Fe and inevitable impurities, was used.
[0055] The quenching temperature was 900 °C, and its retention time was 30 minutes. Oil
cooling was used to cool the steel from the quenching temperature.
[0056] The tempering temperature was 420 °C, and its retention time was 1 hour. Air cooling
was used to cool the steel from the tempering temperature.
[0057] Vickers hardness of the steel after the tempering process was 570 Hv.
[0058] The shapes of the test pieces were the same as those of the test pieces of example
1.
(Rotating bending fatigue test)
[0059] In a rotating bending fatigue test, sine-wave stress was loaded on the test piece
by rotating the test piece, to which a certain bending moment was applied, at 3000
rpm, and the repeated times until the test piece was broken were counted.
[0060] Fig. 3 illustrates results of the rotating bending fatigue test of example 1 and
comparative example 1. In Fig. 3, a solid line illustrates the result of the rotating
bending fatigue test of example 1, and a broken line illustrates the result of the
rotating bending fatigue test of comparative example 1.
[0061] As is apparent from Fig. 3, it was confirmed that the steel of example 1 had good
bending fatigue strength compared with the steel of comparative example 1.
(Hydrogen embrittlement test)
[0062] In a hydrogen embrittlement test, maximum stress by which the test piece was not
broken was measured, by soaking the parallel part of the test piece in electrolyte,
charging hydrogens generated by an electric field of the electrolyte to the test piece
for 48 hours, and thereafter, applying a load on the test piece while the parallel
part was soaked in the electrolyte. As the electrolyte, aqueous solution containing
5% of ammonium thiocyanate at 50 °C was used. As a tester to apply the load on the
test piece, a lever-operated constant load tester was used. The test period for confirming
the maximum stress by which the test piece was not broken (hereinafter, referred to
as "non-breaking stress") was 96 hours. This hydrogen embrittlement test was also
a corrosion durability test and a delayed fracture resistance test, and the aqueous
solution containing 5% of ammonium thiocyanate functioned as the electrolyte and also
corrosion solution.
[0063] The non-breaking stress of the test piece of example 1 was 325 MPa, while the non-breaking
stress of the test piece of comparative example 1 was 240 MPa. Thus, it was confirmed
that the steel of example 1 had good hydrogen embrittlement resistance, corrosion
durability and delayed fracture resistance compared with the steel of comparative
example 1.
[0064] After the hydrogen embrittlement test, a diffusible hydrogen amount of the test piece
was measured. The test piece was heated to increase temperature of the test piece
at constant speed, the amount of hydrogen discharged from the test piece was continuously
measured by a gas chromatography method, and the diffusible hydrogen amount was obtained
from its profile.
[0065] The hydrogen discharged at temperature less than 300 °C is diffusible hydrogen, and
the hydrogen discharged at temperature greater than or equal to 300 °C is non-diffusible
hydrogen. Discharging of the diffusible hydrogen is almost finished before the temperature
of the test piece reaches 220 °C, and when the temperature of the test piece exceeds
400 °C, the non-diffusible hydrogen is started to be discharged. The hydrogen captured
at the hydrogen trap site is not discharged at the temperature less than 300 °C.
[0066] The diffusible hydrogen amount of the test piece of example 1 was 0.36 mass ppm,
while the diffusible hydrogen amount of the test piece of comparative example 1 was
1.87 mass ppm. Thus, it was confirmed that the steel of example 1 had more hydrogen
trap sites and had good hydrogen embrittlement resistance compared with the steel
of comparative example 1.
(Example 2)
[0067] In example 2, a quenching process and a tempering process were performed on a steel
having a composition same as that of the steel of example 1, and a tensile strength
test piece was manufactured by machining to conduct a tensile test.
[0068] The quenching temperature was 950 °C, and its retention time was 30 minutes. Oil
cooling was used to cool the steel from the quenching temperature.
[0069] The tempering temperature was 380 °C or 350 °C, and its retention time was 1 hour.
Air cooling was used to cool the steel from the tempering temperature.
[0070] The shape of the tensile test piece was based on the shape of a No. 4 test piece
described in Japan Industrial Standard (JIS Z2241).
[0071] In the tensile test, tensile strength, 0.2% yield strength, elongation after fracture,
reduction of area and the like were measured.
[0072] The tempering temperature, results of the tensile test and Vickers hardness are illustrated
in Table 1.
(Table 1)
TEMPERING TEMPERATURE [°C] |
TENSILE STRENGTH [MPa] |
0.2% YIELD STRENGTH [MPa] |
BREAKING ELONGATION [%] |
DRAWING [%] |
HARDNESS [Hv] |
380 |
1973 |
1765 |
14 |
50 |
584 |
350 |
2055 |
1827 |
14 |
49 |
601 |
[0073] As is apparent from Table 1, it was confirmed that the steel of example 2 had high
strength.
(Example 3)
[0074] In example 3, a steel having a composition same as that of the steel of example 1
and example 2 was hot formed into a coil shape. Thereafter, a quenching process, a
tempering process, shot peening and setting were performed on the obtained component
to manufacture a coil spring. Thereafter, a durability test of the obtained coil spring
was conducted. The quenching temperature was 990 °C, and its retention time was 20
minutes. Oil cooling was used to cool the coil spring from the quenching temperature.
The tempering temperature was 360 °C, and its retention time was 1 hour. Air cooling
was used to cool the coil spring from the tempering temperature. Vickers hardness
of the coil spring after the tempering process was 580 Hv.
(Comparative example 2)
[0075] In comparative example 2, a steel having a composition same as that of the steel
of comparative example 1 was hot formed into a coil shape similarly as example 3,
and a component having a shape same as that of example 3 was obtained. Thereafter,
a quenching process, a tempering process, shot peening and setting were performed
on the obtained component, and a coil spring having the same shape as that of example
3 was manufactured. Thereafter, a durability test of the obtained coil spring was
conducted. The quenching temperature was 940 °C, and its retention time was 20 minutes.
Oil cooling was used to cool the coil spring from the quenching temperature. The tempering
temperature was 420 °C, and its retention time was 1 hour. Air cooling was used to
cool the coil spring from the tempering temperature. Vickers hardness of the coil
spring after the tempering process was 560 Hv.
(Durability test)
[0076] In the durability test, stress was repeatedly loaded to the coil spring by various
stress amplitudes while setting the average stress to be 735 MPa, and the repeated
times until the coil spring was broken were counted. Here, in example 3, the stress
amplitudes were 735 MPa ± 620 MPa (maximum stress: 1355 MPa, minimum stress: 115 MPa)
and 735 MPa ± 550 MPa (maximum stress: 1285 MPa, minimum stress: 185 MPa). In comparative
example 2, the stress amplitudes were 735 MPa ± 525 MPa (maximum stress: 1260 MPa,
minimum stress: 210 MPa) and 735 MPa ± 500 MPa(maximum stress: 1235 MPa, minimum stress:
235 MPa).
[0077] Fig. 4 illustrates results of the durability test of example 3 and comparative example
2. In Fig. 4, a solid line illustrates the result of the durability test of example
3, and a broken line illustrates the result of the durability test of comparative
example 2. As is apparent from Fig. 4, it was confirmed that the coil spring of example
3 had good durability compared with the coil spring of comparative example 2.
1. A high strength spring containing, by mass%, C: 0.40 to 0.50%, Si: 1.00 to 3.00%,
Mn: 0.30 to 1.20%, Ni: 0.05 to 0.50%, Cr: 0.35 to 1.50%, Mo: 0.03 to 0.50%, Cu: 0.05
to 0.50%, Al: 0.005 to 0.100%, V: 0.05 to 0.50%, Nb: 0.005 to 0.150%, N: 0.0100 to
0.0200%, P: limited to be less than or equal to 0.015%, S: limited to be less than
or equal to 0.010%, and the balance of Fe and inevitable impurities,
wherein a Nb-compound including at least one of Nb-carbide, Nb-nitride and Nb-carbonitride
is included, and
wherein a V-compound including at least one of V-carbide and V-carbonitride that is
precipitated around the Nb-compound is included, and wherein the number of the complex
precipitates, formed by the Nb-compound and the V-compound precipitated around the
Nb-compound, per unit is greater than or equal to 100/mm2 and less than or equal to 100 000/mm2.
2. A method of manufacturing a high strength spring, the method comprising:
performing a quenching process in which quenching temperature is greater than or equal
to 950 °C and less than or equal to 1000 °C, and a tempering process in which tempering
temperature is greater than or equal to 250 °C and less than 390 °C, on a steel containing,
by mass%, C: 0.40 to 0.50%, Si: 1.00 to 3.00%, Mn: 0.30 to 1.20%, Ni: 0.05 to 0.50%,
Cr: 0.35 to 1.50%, Mo: 0.03 to 0.50%, Cu: 0.05 to 0.50%, Al: 0.005 to 0.100%, V: 0.05
to 0.50%, Nb: 0.005 to 0.150%, N: 0.0100 to 0.0200%, P: limited to be less than or
equal to 0.015%, S: limited to be less than or equal to 0.010%, and the balance of
Fe and inevitable impurities,
wherein a V-compound including at least one of V-carbide and V-carbonitride is solid-dissolved
in Fe at the quenching temperature, and thereafter, the V-compound is precipitated
around a Nb-compound including at least one of Nb-carbide, Nb-nitride and Nb-carbonitride,
and
and wherein the number of the complex precipitates, formed by the Nb-compound and
the V-compound precipitated around the Nb-compound, per unit is greater than or equal
to 100/mm
2 and less than or equal to 100 000/mm
2.
3. A steel for a high strength spring, containing, by mass%, C: 0.40 to 0.50%, Si: 1.00
to 3.00%, Mn: 0.30 to 1.20%, Ni: 0.05 to 0.50%, Cr: 0.35 to 1.50%, Mo: 0.03 to 0.50%,
Cu: 0.05 to 0.50%, Al: 0.005 to 0.100%, V: 0.05 to 0.50%, Nb: 0.005 to 0.150%, N:
0.0100 to 0.0200%, P: limited to be less than or equal to 0.015%, S: limited to be
less than or equal to 0.010%, and the balance of Fe and inevitable impurities,
wherein a Nb-compound including at least one of Nb-carbide, Nb-nitride and Nb-carbonitride
is included, and
wherein a V-compound including at least one of V-carbide and V-carbonitride that is
precipitated around the Nb-compound is included, and wherein the number of the complex
precipitates, formed by the Nb-compound and the V-compound precipitated around the
Nb-compound, per unit is greater than or equal to 100/mm2 and less than or equal to 100 000/mm2.
4. A method of manufacturing a steel for a high strength spring, the method comprising:
performing a quenching process in which quenching temperature is greater than or equal
to 950 °C and less than or equal to 1000 °C, and a tempering process in which tempering
temperature is greater than or equal to 250 °C and less than 390 °C, on a steel containing,
by mass%, C: 0.40 to 0.50%, Si: 1.00 to 3.00%, Mn: 0.30 to 1.20%, Ni: 0.05 to 0.50%,
Cr: 0.35 to 1.50%, Mo: 0.03 to 0.50%, Cu: 0.05 to 0.50%, Al: 0.005 to 0.100%, V: 0.05
to 0.50%, Nb: 0.005 to 0.150%, N: 0.0100 to 0.0200%, P: limited to be less than or
equal to 0.015%, S: limited to be less than or equal to 0.010%, and the balance of
Fe and inevitable impurities,
wherein a V-compound including at least one of V-carbide and V-carbonitride is solid-dissolved
in Fe at the quenching temperature, and thereafter, the V-compound is precipitated
around a Nb-compound including at least one of Nb-carbide, Nb-nitride and Nb-carbonitride,
and wherein the number of the complex precipitates, formed by the Nb-compound and
the V-compound precipitated around the Nb-compound, per unit is greater than or equal
to 100/mm2 and less than or equal to 100 000/mm2.
1. Hochfeste Feder, Folgendes, in Massen-%, enthaltend: C: 0,40 bis 0,50%, Si: 1,00 bis
3,00%, Mn: 0,30 bis 1,20%, Ni: 0,05 bis 0,50%, Cr: 0,35 bis 1,50%, Mo: 0,03 bis 0,50%,
Cu: 0,05 bis 0,50%, Al: 0,005 bis 0,100%, V: 0,05 bis 0,50%, Nb: 0,005 bis 0,150%,
N: 0,0100 bis 0,0200%, P: begrenzt auf weniger als oder gleich 0,015%, S: begrenzt
auf weniger als oder gleich 0,010%, und den Rest Fe und unvermeidbare Verunreinigungen,
wobei eine Nb-Verbindung mit mindestens einem von Nb-Carbid, Nb-Nitrid und Nb-Carbonitrid
eingeschlossen ist, und
wobei eine V-Verbindung mit mindestens einem von V-Carbid und V-Carbonitrid eingeschlossen
ist, die um die Nb-Verbindung herum ausgefällt ist, und wobei die Anzahl der komplexen
Ausfällungen, gebildet aus der Nb-Verbindung und der V-Verbindung, die um die Nb-Verbindung
herum ausgefällt ist, pro Einheit größer als oder gleich 100/mm2 und kleiner als oder gleich 100.000/mm2 ist.
2. Verfahren zur Herstellung einer hochfesten Feder, wobei das Verfahren Folgendes umfasst:
Durchführung eines Abschreckhärtungsprozesses, bei dem die Abschrecktemperatur höher
als oder gleich 950°C und geringer als oder gleich 1000°C ist, und eines Anlassprozesses,
bei dem die Anlasstemperatur höher als oder gleich 250°C und geringer als 390°C ist,
an einem Stahl, Folgendes, in Massen-%, enthaltend: C: 0,40 bis 0,50%, Si: 1,00 bis
3,00%, Mn: 0,30 bis 1,20%, Ni: 0,05 bis 0,50%, Cr: 0,35 bis 1,50%, Mo: 0,03 bis 0,50%,
Cu: 0,05 bis 0,50%, Al: 0,005 bis 0,100%, V: 0,05 bis 0,50%, Nb: 0,005 bis 0,150%,
N: 0,0100 bis 0,0200%, P: begrenzt auf weniger als oder gleich 0,015%, S: begrenzt
auf weniger als oder gleich 0,010%, und den Rest Fe und unvermeidbare Verunreinigungen,
wobei eine V-Verbindung mit mindestens einem von V-Carbid und V-Carbonitrid bei der
Abschrecktemperatur fest im Eisen gelöst ist, und anschließend die V-Verbindung um
eine Nb-Verbindung mit mindestens einem von Nb-Carbid, Nb-Nitrid und Nb-Carbonitrid
herum ausgefällt wird, und
wobei die Anzahl der komplexen Ausfällungen, gebildet aus der Nb-Verbindung und der
V-Verbindung, die um die Nb-Verbindung herum ausgefällt ist, pro Einheit größer als
oder gleich 100/mm2 und kleiner als oder gleich 100.000/mm2 ist.
3. Stahl für eine hochfeste Feder, Folgendes, in Massen-%, enthaltend: C: 0,40 bis 0,50%,
Si: 1,00 bis 3,00%, Mn: 0,30 bis 1,20%, Ni: 0,05 bis 0,50%, Cr: 0,35 bis 1,50%, Mo:
0,03 bis 0,50%, Cu: 0,05 bis 0,50%, Al: 0,005 bis 0,100%, V: 0,05 bis 0,50%, Nb: 0,005
bis 0,150%, N: 0,0100 bis 0,0200%, P: begrenzt auf weniger als oder gleich 0,015%,
S: begrenzt auf weniger als oder gleich 0,010%, und den Rest Fe und unvermeidbare
Verunreinigungen,
wobei eine Nb-Verbindung mit mindestens einem von Nb-Carbid, Nb-Nitrid und Nb-Carbonitrid
eingeschlossen ist, und
wobei eine V-Verbindung mit mindestens einem von V-Carbid und V-Carbonitrid eingeschlossen
ist, die um die Nb-Verbindung herum ausgefällt ist, und wobei die Anzahl der komplexen
Ausfällungen, gebildet aus der Nb-Verbindung und der V-Verbindung, die um die Nb-Verbindung
herum ausgefällt ist, pro Einheit größer als oder gleich 100/mm2 und kleiner als oder gleich 100.000/mm2 ist.
4. Verfahren zur Herstellung eines Stahls für eine hochfeste Feder, wobei das Verfahren
Folgendes umfasst:
Durchführung eines Abschreckhärtungsprozesses, bei dem die Abschrecktemperatur höher
als oder gleich 950°C und geringer als oder gleich 1000°C ist, und eines Anlassprozesses,
bei dem die Anlasstemperatur höher als oder gleich 250°C und geringer als 390°C ist,
an einem Stahl, Folgendes, in Massen-%, enthaltend: C: 0,40 bis 0,50%, Si: 1,00 bis
3,00%, Mn: 0,30 bis 1,20%, Ni: 0,05 bis 0,50%, Cr: 0,35 bis 1,50%, Mo: 0,03 bis 0,50%,
Cu: 0, 05 bis 0,50%, Al: 0, 005 bis 0,100%, V: 0, 05 bis 0,50%, Nb: 0,005 bis 0,150%,
N: 0,0100 bis 0,0200%, P: begrenzt auf weniger als oder gleich 0,015%, S: begrenzt
auf weniger als oder gleich 0,010%, und den Rest Fe und unvermeidbare Verunreinigungen,
wobei eine V-Verbindung mit mindestens einem von V-Carbid und V-Carbonitrid bei der
Abschrecktemperatur fest im Eisen gelöst ist, und anschließend die V-Verbindung um
eine Nb-Verbindung mit mindestens einem von Nb-Carbid, Nb-Nitrid und Nb-Carbonitrid
herum ausgefällt wird, und wobei die Anzahl der komplexen Ausfällungen, gebildet aus
der Nb-Verbindung und der V-Verbindung, die um die Nb-Verbindung herum ausgefällt
ist, pro Einheit größer als oder gleich 100/mm2 und kleiner als oder gleich 100.000/mm2 ist.
1. Ressort à haute résistance contenant, en % en masse, C: 0,40 à 0,50%, Si: 1,00 à 3,00%,
Mn: 0,30 à 1,20%, Ni: 0, 05 à 0,50%, Cr: 0,35 à 1,50%, Mo: 0, 03 à 0,50%, Cu: 0, 05
à 0,50%, Al:0,005 à 0,100%, V: 0, 05 à 0,50%, Nb: 0,005 à 0,150%, N : 0,0100 à 0,0200%,
P : limité à 0,015% ou moins, S: limité à 0,010% ou moins, le reste étant du Fe et
des impuretés inévitables,
dans lequel un composé à base de Nb contenant au moins l'un parmi le carbure de Nb,
le nitrure de Nb et le carbonitrure de Nb est inclus, et
dans lequel un composé à base de V contenant au moins l'un parmi le carbure de V et
le carbonitrure de V qui est précipité autour du composé à base de Nb est inclus,
et dans lequel le nombre des précipités complexes, formés par le composé à base de
Nb et le composé à base de V précipité autour du composé à base de Nb, par unité est
supérieur ou égal à 100/mm2 et inférieur ou égal à 100 000/mm2.
2. Procédé de fabrication d'un ressort à haute résistance, le procédé comprenant :
la mise en œuvre d'un procédé de trempe dans lequel la température de trempe est supérieure
ou égale à 950 °C et inférieure ou égale à 1000 C, et d'un procédé dans lequel la
température de revenu est supérieure ou égale à 250 C et inférieure à 390C, sur un
acier contenant, en % en masse, C: 0,40 à 0,50%, Si: 1,00 à 3,00%, Mn: 0,30 à 1,20%,
Ni: 0,05 à 0,50%, Cr: 0,35 à 1,50%, Mo: 0,03 à 0,50%, Cu: 0,05 à 0,50%, Al: 0,005
à 0,100%, V: 0,05 à 0,50%, Nb: 0,005 à 0,150%, N: 0,0100 à 0,0200%, P: limité à 0,015%
ou moins, S: limité à 0,010% ou moins, le reste étant du Fe et des impuretés inévitables,
dans lequel un composé à base de V contenant au moins l'un parmi le carbure de V et
le carbonitrure de V est dissous en solution solide dans du Fe à la température de
trempe, et ensuite le composé à base de V est précipité autour d'un composé du Nb
contenant au moins l'un parmi le carbure de Nb, le nitrure de Nb et le carbonitrure
de Nb, et
dans lequel le nombre des précipités complexes, formés par le composé à base de Nb
et le composé à base de du V précipité autour du composé du Nb, par unité est supérieur
ou égal à 100/mm2 et inférieur ou égal à 100 000/mm2.
3. Acier pour ressort à haute résistance contenant, en % en masse, C: 0,40 à 0,50 %,
Si: 1,00 à 3,00%, Mn: 0,30 à 1,20%, Ni: 0,05 à 0,50%, Cr: 0,35 à 1,50%, Mo: 0,03 à
0,50%, Cu: 0, 05 à 0,50%, Al: 0, 005 à 0,100%, V: 0,05 à 0,50%, Nb : 0,005 à 0,150%,
N: 0,0100 à 0,0200%, P: limité à 0,015% ou moins, S: limité à 0,010% ou moins, le
reste étant du Fe et des impuretés inévitables,
dans lequel un composé du Nb contenant au moins l'un parmi le carbure de Nb, le nitrure
de Nb et le carbonitrure de Nb est inclus, et
dans lequel un composé du V contenant au moins l'un parmi le carbure de V et le carbonitrure
de V qui est précipité autour du composé du Nb est inclus, et dans lequel le nombre
des précipités complexes, formés par le composé du Nb et le composé du V précipité
autour du composé du Nb, par unité est supérieur ou égal à 100/mm2 et inférieur ou égal à 100 000/mm2.
4. Procédé de fabrication d'un acier pour ressort à haute résistance, le procédé comprenant
:
la mise en œuvre d'un procédé de trempe dans lequel la température de trempe est supérieure
ou égale à 950 °C et inférieure ou égale à 1000 C, et d'un procédé de revenu dans
lequel la température de revenu est supérieure ou égale à 250 C et inférieure à 390
C, sur un acier contenant, en % en masse, C: 0,40 à 0,50%, Si: 1,00 à 3,00%, Mn: 0,30
à 1,20%, Ni: 0, 05 à 0,50%, Cr: 0,35 à 1,50%, Mo: 0, 03 à 0,50%, Cu: 0, 05 à 0,50%,
Al: 0,005 à 0,100%, V: 0,05 à 0,50%, Nb: 0,005 à 0,150%, N: 0,0100 à 0,0200%, P: limité
à 0,015% ou moins, S: limité à 0,010% ou moins, le reste étant du Fe et des impuretés
inévitables,
dans lequel un composé à base de V contenant au moins l'un parmi le carbure de V et
le carbonitrure de V est dissous en solution solide dans du Fe à la température de
trempe, et ensuite le composé à base de V est précipité autour d'un composé à base
de Nb contenant au moins l'un parmi le carbure de Nb, le nitrure de Nb et le carbonitrure
de Nb, et
dans lequel le nombre des précipités complexes, formés par le composé à base de Nb
et le composé à base de V précipité autour du composé à base de Nb, par unité est
supérieur ou égal à 100/mm2 et inférieur ou égal à 100 000/mm2.