U.S. patent application number 10/588287 was filed with the patent office on 2008-11-06 for spring steel wire.
Invention is credited to Yoshiro Fujino, Nozomu Kawabe, Teruyuki Murai, Takayuki Shiwaku, Norihito Yamao.
Application Number | 20080271824 10/588287 |
Document ID | / |
Family ID | 34835907 |
Filed Date | 2008-11-06 |
United States Patent
Application |
20080271824 |
Kind Code |
A1 |
Fujino; Yoshiro ; et
al. |
November 6, 2008 |
Spring Steel Wire
Abstract
The present invention provides a spring steel wire which has a
tempered martensitic structure brought about by
quenching-tempering. The present spring steel wire has a 40% or
higher reduction of area and a 1,000 MPa or higher shear yield
stress after subjected to heat treatment for at least hours at a
temperature ranging from 420.degree. C. to 480.degree. C. The
present steel wire preferably constitutes, based on mass %, C:
0.50-0.75%, Si: 1.80-2.70%, Mn: 0.1-0.7%, Cr: 0.70-1.50%, Co:
0.02-1.00%, and remnants consisting of Fe and impurities, or
constitutes, based on mass %, C: 0.50-0.75%, Si: 1.80-2.70%, Mn:
over 0.7-1.50%, Cr: 0.70-1.50%, and remnants consisting of Fe and
impurities.
Inventors: |
Fujino; Yoshiro; (Hyogo,
JP) ; Kawabe; Nozomu; (Hyogo, JP) ; Murai;
Teruyuki; (Hyogo, JP) ; Yamao; Norihito;
(Hyogo, JP) ; Shiwaku; Takayuki; (Hyogo,
JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Family ID: |
34835907 |
Appl. No.: |
10/588287 |
Filed: |
February 4, 2005 |
PCT Filed: |
February 4, 2005 |
PCT NO: |
PCT/JP05/01703 |
371 Date: |
August 4, 2006 |
Current U.S.
Class: |
148/598 ;
148/328 |
Current CPC
Class: |
C21D 2211/008 20130101;
C21D 9/525 20130101; C21D 1/18 20130101; C21D 1/20 20130101; C22C
38/30 20130101; C21D 8/065 20130101; C21D 1/25 20130101; C21D 9/02
20130101 |
Class at
Publication: |
148/598 ;
148/328 |
International
Class: |
C21D 9/52 20060101
C21D009/52; C21D 8/06 20060101 C21D008/06; C22C 38/02 20060101
C22C038/02; C22C 38/30 20060101 C22C038/30; C22C 38/34 20060101
C22C038/34 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 4, 2004 |
JP |
2004-027891 |
Claims
1. A spring steel wire having a tempered martensitic structure
brought about by quenching-tempering, the spring steel wire
comprising: a 40% or higher reduction of area; and a 1,000 Mpa or
higher shear yield stress after subjected to heat treatment for at
least 2 hours at a temperature ranging from 420.degree. C. to
480.degree. C.
2. The spring steel wire according to claim 1 consisting of, based
on mass %, C: 0.50-0.75%, Si: 1.80-2.70%, Mn: 0.1-0.7%, Cr:
0.70-1.50%, Co: 0.02-1.0%, and remnants consisting of Fe and
impurities.
3. The spring steel wire according to claim 1 consisting of, based
on mass %, C: 0.50-0.75%, Si: 1.80-2.70%, Mn: over 0.7-1.5%, Cr:
0.70-1.50%, and remnants consisting of Fe and impurities.
4. The spring steel wire according to claim 1 consisting of, based
on mass %; C: 0.50-0.75%, Si: 1.80-2.70%, Mn: over 0.7-1.5%, Cr:
0.70-1.50%; at least one element of Ni: 0.1-1.0% and Co:
0.02-1.00%; and remnants consisting of Fe and impurities.
5. The spring steel wire according to claim 1 consisting of, based
on mass %; C: 0.50-0.75%, Si: 1.80-2.70%, Mn: 0.1-0.7%, Cr:
0.70-1.50%, Co: 0.02-1.00%; at least one element selected from the
group of 5 elements consisting of V: 0.05-0.50%, Mo: 0.05-0.50%, W:
0.05-0.15%, Nb: 0.05-0.15% and Ti: 0.01-0.20; and remnants
consisting of Fe and impurities.
6. The spring steel wire according to claim 1 consisting of, based
on mass %; C: 0.50-0.75%, Si: 1.80-2.70%, Mn: over 0.7-1.5%, Cr:
0.70-1.50%; at least one element selected from the group of 5
elements consisting of V: 0.05-0.50%, Mo: 0.05-0.50%, W:
0.05-0.15%, Nb: 0.05-0.15% and Ti: 0.01-0.20%; and remnants
consisting of Fe and impurities.
7. The spring steel wire according to claim 1 consisting of, based
on mass %, C: 0.50-0.75%, Si: 1.80-2.70%, Mn: over 0.7-1.5%, Cr:
0.70-1.50%, at least one element of Ni: 0.1-1.0% and Co:
0.02-1.00%, at least one element selected from the group of 5
elements consisting of V: 0.05-0.50%, Mo: 0.05-0.50%, W:
0.05-0.15%, Nb: 0.05-0.15% and Ti: 0.01-0.20%, and remnants
consisting of Fe and impurities.
8. The spring steel wire according to claim 1 comprising austenite
grains (prior austenite grains) which have an average grain size in
the range of 3.0-7.0 .mu.m.
9. A spring manufactured from the spring steel wire according to
claim 1.
10. A spring manufactured from the spring steel wire according to
claim 8.
11. A method of manufacturing a spring steel wire, comprising the
steps of: patenting a steel consisting of chemical compositions
given below; drawing the thus patented steel into a steel wire; and
subjecting the resultant steel wire to quenching-tempering; wherein
said patenting process comprises: an austenization step in which
the steel is heated at 900-1,050.degree. C. for 60 to 180 seconds;
and an isothermal transformation step in which the thus austenized
steel is heated at 600-750.degree. C. for 20 to 100 seconds;
Chemical compositions (based on mass %): C: 0.50-0.75%, Si:
1.80-2.70%, Mn: 0.1-0.7%, Cr: 0.70-1.50%, Co: 0.02-1.00%, and
remnants consisting of Fe and impurities.
12. A method of manufacturing a spring steel wire, comprising the
steps of: patenting a steel consisting of chemical compositions
given below; drawing the thus patented steel into a steel wire; and
subjecting the resultant steel wire to quenching-tempering; wherein
said patenting process comprises: an austenization step in which
the steel is heated at 900-1,050.degree. C. for 60 to 180 seconds;
and an isothermal transformation step in which the thus austenized
steel is heated at 600-750.degree. C. for 20 to 100 seconds;
Chemical compositions (based on mass %): C: 0.50-0.75%, Si:
1.80-2.70%, Mn: over 0.7-1.5%, Cr: 0.70-1.50%, and remnants
consisting of Fe and impurities.
13. A method of manufacturing a spring steel wire, comprising the
steps of: patenting a steel consisting of chemical compositions
given below; drawing the thus patented steel into a steel wire; and
subjecting the resultant steel wire to quenching-tempering; wherein
said patenting process comprises: an austenization step in which
the steel is heated at 900-1,050.degree. C. for 60 to 180 seconds;
and an isothermal transformation step in which the thus austenized
steel is heated at 600-750.degree. C. for 20 to 100 seconds;
Chemical compositions (based on mass %): C: 0.50-0.75%, Si:
1.80-2.70%, Mn: over 0.7-1.5%, Cr: 0.70-1.50%, at least one element
of Ni: 0.1-1.0% and Co: 0.02-1.00%, and remnants consisting of Fe
and impurities.
Description
RELATED APPLICATION
[0001] This application is a national phase of PCT/JP2005/001703
filed on Feb. 4, 2005, which claims priority from Japanese
Application No. 2004-027891 filed on Feb. 4, 2004, the disclosures
of which Applications are incorporated by reference herein. The
benefit of the filing and priority dates of the International and
Japanese Applications is respectfully requested.
FIELD OF THE INVENTION
[0002] The present invention relates to a spring steel wire having
a tempered martensitic structure brought about by
quenching-tempering, to a method of manufacturing the spring steel
wire in a well-suited efficient manner, and to a spring
manufactured from the steel wire. More particularly, the present
invention relates to a high toughness spring steel wire having a
high strength with excellent fatigue properties that is
advantageously applicable to engine valve springs or those springs
used for transmission interior parts, etc. of automobiles.
BACKGROUND ART
[0003] In recent years, as momentum toward low fuel consumption
increases in automobiles, the industry has made continued efforts
to achieve a further reduction in size and weight of automobile
parts, including parts of their engines and transmissions. In
connection with this, springs including engine valve springs,
springs for transmission parts, etc. have come to be exposed to
increasingly severer stress environments year after year, and thus
spring materials used therefor are also required to be provided
with much more improved fatigue properties accordingly. Heretofore,
to manufacture those engine valve springs or springs for
transmission parts as described above, it has been known to use
silicon-based oil tempered steel wires such as, for example, those
described in the patent documents 1-3 listed below.
[0004] [Patent document 1] Japanese Patent Publication No.
2842579
[0005] [Patent document 2] Japanese Provisional Patent Publication
No. 2002-194496
[0006] [Patent document 3] Japanese Patent Publication No.
3045795
DISCLOSURE OF THE INVENTION
Problem to be Solved by the Invention
[0007] However, springs such as engine valve springs or springs for
transmission parts have been increasingly required to have better
mechanical or physical properties in recent years, so that further
improvement has come to be demanded in spring steel wires and
springs worked from the steel wire. Especially, it is desired that
such spring steel wires and springs manufactured therefrom be
provided with fatigue properties and toughness in better balance
than ever.
[0008] On the other hand, as improvement in fatigue strength
(fatigue limit) is requested recently, springs worked from steel
wires are typically subjected to heat treatment (nitriding
treatment) at elevated temperatures (specifically, around
420-480.degree. C.).
[0009] The patent document 1 discloses a technique that aims at
improving the toughness of a steel wire by providing it with a C
(carbon) content ranging from 0.3% to 0.5% by weight. However,
since a steel wire with a carbon content as low as less than 0.50%
by weight will have a reduced thermal resistance, if a spring
worked from such a low carbon content steel wire is subjected to
nitriding treatment at elevated temperatures as described above,
the resultant spring will have a reduced fatigue strength, so that
it may undergo internal breakage when put into practical use.
[0010] The patent document 2 discloses a technique that aims at
improving the fatigue strength of a steel wire by achieving a fine
structure having an average grain size of 1.0-7.0 micrometers as
austenite after quenching. However, if the quenching temperature is
lowered to make the austenite grain size smaller, there will remain
undissolved carbide, which may lower the toughness of the resultant
steel wire. Further, with such reduction in toughness, the steel
wire will become more susceptible to breakage while being worked
into spring and consequently the mass productivity of the spring
therefrom will be adversely affected thereby.
[0011] The patent document 3 discloses a technique that aims at
improving a steel wire in its workability into spring by
decarbonizing its surface purposely during the oil tempering so as
to reduce the surface hardness, but this prior art technique is
inadequate for the mass production of such a steel wire or spring
because it is practically difficult to obtain a uniform
decarburized layer in the surface of the steel wire. Moreover, the
oxygen concentration must be well controlled when heating the steel
wire (during the oil tempering), thus adding to the cost
accordingly.
[0012] Further, in any of the technologies disclosed in the
above-cited prior art documents, the proof stress of the material
(spring) to a stress exerted inside in its torsional direction,
i.e., the shear yield stress of the spring is not examined
subsequent to the nitriding treatment to which the spring is
subjected after worked from the steel wire.
[0013] Accordingly, a principal object of the present invention is
to provide a high strength spring steel wire which is excellent not
only in fatigue strength but also in toughness. Also, it is another
object of the present invention to provide a spring manufactured
from the above-described steel wire and a suitable method to
manufacture the spring steel wire.
Means for Solving Problem
[0014] With the aforementioned objects in view, the present
invention provides a spring steel wire, in which its reduction of
area after quenching-tempering and its shear yield stress after
subjected to heat treatment comparable to nitriding treatment
following the above quenching-tempering are limited to specific
ranges, respectively.
[0015] That is, the present invention provides a spring steel wire
which has a tempered martensitic structure brought about by
quenching-tempering. The present spring steel wire is characterized
by a 40% or higher reduction of area and by a 1,000 MPa or higher
shear yield stress after subjected to heat treatment for at least 2
hours at a temperature ranging from 420.degree. C. to 480.degree.
C.
[0016] According to the present invention, the spring steel wire
preferably comprises any one of the following chemical formulations
1 through 6:
[0017] 1. Based on mass %; C: 0.50-0.75%, Si: 1.80-2.70%, Mn:
0.1-0.7%, Cr: 0.70-1.50%, Co: 0.02-1.00%, and remnants consisting
of Fe and impurities
[0018] 2. Based on mass %; C: 0.50-0.75%, Si: 1.80-2.70%, Mn: over
0.7-1.5%, Cr: 0.70-1.50%, and remnants consisting of Fe and
impurities
[0019] 3. Based on mass %; C: 0.50-0.75%, Si: 1.80-2.70%, Mn: over
0.7-1.5%, Cr: 0.70-1.50%, at least one element of Ni: 0.1-1.0% and
Co: 0.02-1.00%, and remnants consisting of Fe and impurities
[0020] 4. Based on mass %; C: 0.50-0.75%, Si: 1.80-2.70%, Mn:
0.1-0.7%, Cr: 0.70-1.50%, Co: 0.02-1.00%, at least one element
selected from the group of 5 elements consisting of V: 0.05-0.50%,
Mo: 0.05-0.50%, W: 0.05-0.15%, Nb: 0.05-0.15% and Ti: 0.01-0.20%,
and remnants consisting of Fe and impurities
[0021] 5. Based on mass %; C: 0.50-0.75%, Si: 1.80-2.70%, Mn: over
0.7-1.5%, Cr: 0.70-1.50%, at least one element selected from the
group of 5 elements consisting of V: 0.05-0.50%, Mo: 0.05-0.50%, W:
0.05-0.15%, Nb: 0.05-0.15% and Ti: 0.01-0.20%, and remnant
consisting of Fe and impurities
[0022] 6. Based on mass %; C: 0.50-0.75%, Si: 1.80-2.70%, Mn: over
0.7-1.5%, Cr: 0.70-1.50%, at least one element of Ni: 0.1-1.0% and
Co: 0.02-1.00%, at least one element selected from the group of 5
elements consisting of V: 0.05-0.50%, Mo: 0.05-0.50%, W:
0.05-0.15%, Nb: 0.05-0.15% and Ti: 0.01-0.20%, and remnant
consisting of Fe and impurities
[0023] The present invention also provides a method of
manufacturing the above-described spring steel wire in a well
suited manner therefor, as will be described herein below. More
specifically, the method of manufacturing the spring steel wire
according to the present invention comprises patenting a steel
having any one of the chemical formulations (A) through (C) given
below, drawing the patented steel into a steel wire, and subjecting
the resultant steel wire to quenching-tempering. The
above-mentioned patenting process comprises an austenization step
in which the steel is heated at 900-1,050.degree. C. for 60 to 180
seconds, and an isothermal transformation step in which the thus
austenized steel is heated at 600-750.degree. C. for 20 to 100
seconds.
[0024] (A) Based on mass %; C: 0.50-0.75%, Si: 1.80-2.70%, Mn:
0.1-0.7%, Cr: 0.70-1.50%, Co: 0.02-1.00%, and remnants consisting
of Fe and impurities
[0025] (B) Based on mass %; C: 0.50-0.75%, Si: 1.80-2.70%, Mn: over
0.7-1.5%, Cr: 0.70-1.50%, and remnants consisting of Fe and
impurities
[0026] (C) Based on mass %; C: 0.50-0.75%, Si: 1.80-2.70%, Mn: over
0.7-1.5%, Cr: 0.70-1.50%, at least one element of Ni: 0.1-1.0% and
Co: 0.02-1.00%, and remnants consisting of Fe and impurities
[0027] In addition to those compositions in any one of the chemical
formulations (A) through (C) given above, the steel may contain,
based on mass %, at least one element selected from the group of 5
elements consisting of V: 0.05-0.50%, Mo: 0.05-0.50%, W:
0.05-0.15%, Nb: 0.05-0.15%, and Ti: 0.01-0.20%.
[0028] Hereinafter, the present invention will be described in
detail.
(Improving the Fatigue Properties)
[0029] The improvement of a spring in its fatigue properties may
preferably be addressed in terms of the suppression of its fatigue
breakage. When a spring is operated repeatedly, a repetitive stress
is exerted on the spring not only in the tensile and compression
directions but also in the shear direction simultaneously. Thus,
with the repetitive stress applied externally, the spring undergoes
a repetitive slip deformation (plastic deformation) locally or
intensively and creates projections and depressions in the surface
region to induce cracks leading to breakage, namely resulting in
fatigue breakage. Therefore, for suppressing the fatigue breakage
of the spring, it will be effective to suppress such a local or
concentrated plastic deformation. Heretofore, in order to suppress
such a plastic deformation, the steel wire is typically subjected
to heat treatment such as nitriding treatment after worked into
spring to increase its surface hardness and thereby to increase its
fatigue limit. However, nowadays, when springs have come to be used
under conditions where a large stress is applied thereto, mere an
increase in fatigue limit of the springs may sometimes be
insufficient to allow their practical use, because such springs
tend to undergo permanent set in fatigue. This may be accounted by
the fact that even if a high hardness nitrided layer at the spring
surface formed by the above-mentioned heat treatment such as
nitriding treatment or like does not undergo permanent set in
fatigue, such a large stress can reduce the strength of the inner
part of the spring so as to put springs into permanent set in
fatigue. Therefore, for springs, it is desired that springs are
improved not only in their fatigue limit but in their torsional
proof stress, i.e., shear yield stress itself, in addition to
having a high strength. Under these circumstances, the inventors
have studied above-described subject from various aspects to find
out that an adequate torsional proof stress provided inside the
material (i.e., spring) after the above-mentioned heat treatment
such as nitriding treatment or like is substantially effective for
meeting these requirements. More specifically, it turned out that
the fatigue properties of a spring can be improved, if the spring
has a 1,000 MPa or higher shear yield stress after the
above-mentioned heat treatment such as nitriding treatment or like.
Based on these findings, the present invention provides a spring
steel wire having a shear yield stress limited to a specific range
of 1,000 MPa or higher after subjected to particular heat treatment
following the quenching-tempering.
[0030] (High Toughness)
However high the strength of a steel wire may be, it will undergo
in-process breakage when the steel wire is worked into a spring if
toughness of the steel wire is insufficient. Consequently the mass
productivity of the spring will be hampered. Further, as the
toughness of a steel wire used as material for a spring decreases,
the fatigue properties of the spring will also decrease. Under
these circumstances, the inventors have studied this problem from
various aspects to find out that providing the steel wire with a
40% or higher reduction of area after quenching-tempering is
effective for the prevention of in-process breakage of the steel
wire when worked into spring and thus leads to excellent mass
productivity of the spring. Based on these findings, the present
invention provides a spring steel wire having a reduction of area
limited to a specific range of 40% or higher. With a reduction of
area lower than 40%, the steel wire tends to undergo in-process
breakage when worked into spring and its mass productivity could be
substantially compromised thereby. In this regard, the reduction of
area may decrease a little when subjecting the steel wire to such
particular heat treatment comparable to nitriding treatment that is
accomplished at a temperature ranging from 420.degree. C. to
480.degree. C. for at least 2 hours following the
quenching-tempering as described previously. However, if the steel
wire has a 40% or higher reduction of area after
quenching-tempering as described above, it can maintain a 35% or
higher reduction of area even after the above-described heat
treatment, and a spring manufactured from this steel wire can have
a high fatigue properties.
[0031] Thus, according to the present invention, the reduction of
area of a spring steel wire and its shear yield stress after
subjected to heat treatment comparable to nitriding treatment
following the above quenching-tempering are limited to specific
ranges, respectively, to provide the spring steel wire and the
spring manufactured from the steel wire with a high fatigue
strength and high toughness in adequate balance.
[0032] In order to provide such a spring steel wire and a spring
that are excellent both in fatigue properties and in toughness as
described above, the present invention specifically limits the
present steel wire to predetermined optimal chemical compositions
and optimal manufacturing conditions, especially patenting
conditions.
<Chemical Compositions>
[0033] First, while the fatigue limit of a spring can be improved
by increasing the surface hardness of the spring by subjecting it
to the heat treatment such as nitriding treatment or like after it
is worked from a steel wire, an internal hardness of the spring
decreases by the heat treatment to sometimes cause the spring to
undergo internal breakage in use. Thus, according to the present
invention, the steel wire to be worked into a spring contains
carbon (C) and silicon (Si) in a quantity (in mass %) falling in a
predetermined range in order to improve the thermal resistance of a
matrix of the steel wire. Besides, the steel wire contains a
predetermined quantity of chromium (Cr) in order to produce carbide
in the structure of the steel wire when it is tempered and to
thereby increase the softening resistance of the steel wire. In
addition to this predetermined Cr content, the steel wire may
contain also a predetermined quantity of molybdenum (Mo), vanadium
(V), niobium (Nb), Tungsten (W), or titanium (Ti) to effectively
increase the softening resistance. Then the inventors have found
out that, for improving the shear yield stresses of the steel wire
and the spring manufactured therefrom of the present invention, it
is effective to provide the steel wire with a 0.02-1.00 mass %
cobalt (Co) content or a rather excess manganese (Mn) content (over
0.7 to 1.5 mass %). Thus, the steel wire of the present invention
has Mn and Co contents limited to specific ranges, respectively.
The ranges of these contents and the grounds for such limitation
will be described in detail herein later.
[0034] <Manufacturing Conditions>
The spring steel wire of the present invention is obtained by
subjecting a steel having the above-described chemical compositions
to the following processes in sequence: steel ingot
making.fwdarw.hot forging.fwdarw.hot
rolling.fwdarw.patenting.fwdarw.wire
drawing.fwdarw.quenching.fwdarw.tempering
[0035] (Patenting Conditions)
According to the present invention, a steel rod is subjected,
before wire drawing, to patenting under particular conditions to
fully austenitize the structure of the steel to thereby dissolve
the undissolved carbide and to obtain a homogeneous pearlitic
structure through an appropriate isothermal transformation
following the austenitization. Insufficient austenitization may
cause the reduction of toughness and shear yield stress of the
resultant steel wire. Then, for fully austenitizing the steel, it
is preferred to heat the steel rod at a temperature of
900-1,050.degree. C. for 60 to 180 seconds. If the heating
temperature is lower than 900.degree. C., or if the heating
temperature falls in the range of 900-1,050.degree. C. but the
heating time is shorter than 60 seconds, sufficient austenitization
will not be achieved and undissolved carbide will remain. However,
if the heating temperature is higher than 1,050.degree. C., or if
the heating temperature falls in the range of 900-1,050.degree. C.
but the heating time is longer than 180 seconds, austenite grains
will become coarse, thus tending to produce martensite during the
succeeding transformation, so that the drawability of the steel rod
will not be secured during the wire drawing process.
[0036] For the isothermal transformation of the steel following the
austenitization, it is preferred to heat the steel rod at
600-750.degree. C. for 20 to 100 seconds. If the heating
temperature is higher than 750.degree. C., or if the heating
temperature falls in the 600-750.degree. C. range but the heating
time is longer than 100 seconds, cementite spheroidizes in the
structure of the steel, which may degrade the drawability of the
steel rod. On the other hand, if the heating temperature is lower
than 600.degree. C., or if the heating temperature falls in the
600-750.degree. C. range but the heating time is shorter than 20
seconds, the transformation to pearlite will not be completed and
martensite will be produced to thereby degrade the drawability.
[0037] (Quenching and Tempering)
If the steel wire obtained by drawing the steel rod which is
subjected to patenting as above is then subjected to quenching at
too low a temperature, undissolved carbide will remain in the
structure of the steel wire, which acts to reduce the toughness of
the steel wire. On the contrary, if the quenching temperature is
too high, the austenite grains will grow to larger sizes and
consequently the fatigue limits of the steel wire and the spring
manufactured therefrom will be reduced. Thus, it is preferred that
the quenching temperature be higher than 850.degree. C. but lower
than 1,050.degree. C.
[0038] <Structure>
According to the present invention, the spring steel wire has a
tempered martensitic structure. Moreover, if the austenite grains
(prior austenite grains) of the steel wire are rendered fine as
observed after subjected to the quenching-tempering, such a steel
wire and the spring manufactured from the steel wire will become
hard to undergo a slip deformation locally or intensively even when
a repetitive stress is applied thereto. That is to say, since the
shear yield stress of the steel wire or spring can be improved by
rendering fine the austenite grains (prior austenite grains), this
consequently contributes to improved fatigue properties of the
steel wire or spring.
[0039] Specifically, it is preferred that the average grain size of
the austenite grains (prior austenite grains) fall in the range of
3.0-7.0 micrometers. The average grain size can be changed by
varying the temperature for patenting the steel rod. More
specifically, if the austenitization during patenting is effected
at a lower temperature, the grain size will tend to become smaller,
while if this austenitizing temperature is increased, the grain
size tends to increase. With an average grain size smaller than 3.0
micrometers, undissolved carbide will remain due to the lower
austenitizing temperature and tend to reduce the toughness of the
steel wire. Meanwhile, if the average grain size is larger than 7.0
micrometers, it is difficult to improve the fatigue limit of the
steel wire or the spring manufactured therefrom. Now it is to be
noted that the average grain size herein is given in measurements
taken on steel wires after drawing and then subjected to
quenching-tempering.
[0040] Hereinafter, the description will be made on the grounds on
which the elements are selected and their contents are limited to
specific ranges according to the present invention. In the
description to follow, numerical values accompanying the individual
elements are all given in mass %.
[0041] C: 0.50-0.75
Carbon (C) is an important element which determines the strength of
steel, and since a carbon content lower than 0.50 mass % of the
total steel will not allow a resulting steel wire to have a
sufficient strength, while a carbon content exceeding 0.75 mass %
will result in reduced toughness, it is preferred that the carbon
content ranges from 0.50 mass % to 0.75 mass %.
[0042] Si: 1.80-2.70
Silicon (Si) is used as a deoxidizer when melting and smelting a
raw steel. Moreover, Si is solid-dissolved in steel's ferrite to
improve the thermal resistance of the steel and has the effect of
preventing the hardness reduction inside the steel wire (spring)
due to heat treatment such as strain relief annealing or nitriding
treatment to which the spring is subjected after worked from the
steel wire. It is preferred that the steel have a Si content
ranging from 1.80 mass % to 2.70 mass %, because the 1.80 mass % or
higher Si content is required to maintain an adequate thermal
resistance but the toughness will decrease if the Si content
exceeds 2.70 mass %.
[0043] Mn: 0.1-1.5
Like Si, manganese (Mn) is used as a deoxidizer when melting and
smelting a raw steel. Therefore, it is preferred that the Mn
content required for such a deoxidizer has a lower limit of 0.1
mass %. Moreover, Mn has the effect of improving the hardenability
of the steel wire to thereby increase its strength and improve the
shear yield stress of the steel wire and the spring manufactured
therefrom. However, since an Mn content higher than 1.5 mass % of
the total steel tends to produce martensite in the steel during the
patenting process and thus wire breakage may be caused thereby in
the drawing process, the Mn content preferably has an upper limit
of 1.5 mass %. Particularly, in cases where the steel contains
cobalt (Co) to be described herein below, the Mn content may fall
in a rather lower range of 0.1-0.7 mass %, while it is preferred
for a formulation without Co content that the Mn content fall in a
rather higher range of over 0.7 to 1.5 mass %. A formulation having
a rather higher Mn content may contain also Co.
[0044] Cr: 0.70-1.50
Since chromium (Cr) acts to improve the hardenability and thus the
softening resistance of the steel, it is effective for preventing
the spring worked from the steel wire from softening when subjected
to heat treatment such as tempering and nitriding treatment. Since
a Cr content lower than 0.70 mass % of the total steel will not
work to provide a sufficient effect of preventing the softening,
preferably the Cr content is 0.70 mass % or higher, while a Cr
content exceeding 1.50 mass % will tend to produce martensite
during the patenting process to thus cause wire breakage in the
drawing process and further to reduce the toughness of the patented
(oil-tempered) steel. Therefore, the Cr content preferably falls in
the range of 0.70 to 1.50 mass %.
[0045] Co: 0.02-1.00
A small quantity of cobalt (Co) added to a steel acts to improve
the shear yield stress of the resultant steel wire and the spring
worked from the steel wire. Also, Co is effective for improving the
thermal resistance of the steel wire and for the softening
prevention of the spring worked from the steel wire and subjected
to the tempering and nitriding treatment. Further, Co does not act
to reduce the toughness of the steel wire, so long as its content
is low. A Co content lower than 0.02 mass % is hard to contribute
to any improved shear yield stress for the steel wire or the spring
as described above or to any improved thermal resistance for the
steel wire. Also, even if the Co content exceeds 1.00 mass %, no
significant improvement in effect can be observed over cases with a
1.00 mass % or lower Co content but it just adds to the
manufacturing cost of the steel wire or spring. Accordingly, it is
preferred that the Co content fall in the range of 0.02 mass % to
1.00 mass %. In addition, where the steel contains Co, Mn content
of the steel may fall in a rather low range of 0.1-0.7 mass %, as
described above.
[0046] Ni: 0.1-1.0
Nickel (Ni) contained in the steel has the effect of improving the
corrosion resistance and toughness of the resultant steel wire. An
Ni content lower than 0.1 mass % is hard to contribute to any
improved properties of the steel wire as mentioned above, and even
if the Ni content exceeds 1.0 mass %, no further improvement in the
toughness of the resultant steel wire cannot be achieved, but it
just adds to its manufacturing cost. Thus, the Ni content
preferably ranges from 0.1 mass % to 1.0 mass %.
[0047] Mo, V: 0.05-0.50
[0048] W, Nb: 0.05-0.15
These elements act to produce carbide in the structure of a steel
wire when it is tempered and have the effect of tending to increase
the softening resistance of the steel wire. If the content of each
of molybdenum (Mo), vanadium (V), tungsten (W) or niobium (Nb) is
lower than 0.05 mass % of the total steel, the above-described
effect will be hard to achieve. Meanwhile, if the Mo content
exceeds 0.50 mass %, if the V content exceeds 0.50 mass %, if the W
content exceeds 0.15 mass %, or if the Nb content exceeds 0.15 mass
%, the resultant steel wire tends to have reduced toughness in
either case.
[0049] Ti: 0.01-0.20
Titanium (Ti) acts to produce carbide when the steel wire is
tempered and has the effect of tending to increase a softening
resistance of the steel wire. A Ti content lower than 0.01 mass %
will not yield the above-mentioned effect, while a Ti content
higher than 0.20 mass % will produce a high-melting point
non-metallic inclusion TiO in the structure of the steel wire,
tending to reduce the toughness of the steel wire. Thus, the Ti
content preferably ranges from 0.01 mass % to 0.20 mass %.
[0050] The spring steel wire of the present invention may have any
cross-sectional shape as cut by a plane perpendicular to the
longitudinal direction (drawing direction) of the steel wire,
including a typical circular shape and other special or peculiar
cross-sectional shapes such as an ellipse, a trapezoid, a square, a
rectangle, and so on.
[0051] The spring of the present invention may be provided by
subjecting the above-described spring steel wire to any known
spring forming process such as coiling. Especially, it is to be
noted here that by subjecting the spring worked from the present
spring steel wire to heat treatment such as nitriding treatment or
like, the resultant spring can have an improved surface hardness
and thus an excellent fatigue limit.
BEST MODE FOR CARRYING OUT THE INVENTION
[0052] Preferred embodiments of the present invention are
demonstrated hereinafter. A steel of each formulation containing
chemical elements given in Table 1 with remnants consisting of Fe
and impurities were melted in a vacuum melting furnace to prepare
an ingot and then the resultant ingot was worked through hot
forging and hot rolling into a wire rod of 6.5 mm.phi.. Then, the
wire rod was subjected to patenting
(austenitizing.fwdarw.isothermal transformation), shaving,
annealing, and drawing processes in sequence to obtain a steel wire
of 3.0 mm.phi.. The patenting conditions are shown in Table 2. In
this typical embodiment, the respective 6.5 mm.phi. wire rods were
patented under several varied patenting conditions, including
austenitizing conditions under which the wire rods were heated at
varied temperatures for varied retention times, and conditions for
isothermal transformation under which the wire rods were heated
also at varied temperatures for varied retention times subsequently
to the austenization, as shown in Table 2.
TABLE-US-00001 TABLE 1 Formulation Chemical composition (mass %)
samples C Si Mn Cr Co Ni Others A 0.45 2.2 0.5 0.9 0.3 -- -- B 0.78
2.0 0.6 0.8 -- -- -- C 0.68 1.6 0.5 1.0 -- -- -- D 0.63 2.8 0.6 0.9
-- -- -- E 0.61 2.2 1.7 1.0 -- 0.3 -- F 0.60 2.2 0.6 0.5 -- -- -- G
0.64 2.3 0.5 1.7 -- -- -- H 0.62 2.1 0.5 1.1 -- -- -- I 0.64 2.2
0.6 1.2 -- -- V: 0.6 J 0.63 2.1 0.5 1.1 -- -- Ti: 0.3 K 0.55 2.4
0.5 1.3 0.2 -- -- L 0.72 2.3 0.55 1.2 0.5 -- -- M 0.63 1.9 1.2 1.4
-- 0.3 -- N 0.62 2.5 0.2 0.9 0.3 -- -- O 0.64 2.3 0.8 1.1 0.4 -- --
P 0.65 2.2 0.9 0.9 0.3 0.5 -- Q 0.65 2.0 0.4 1.0 0.3 -- V: 0.15 R
0.60 2.3 1.0 0.8 -- -- Mo: 0.20 S 0.63 2.1 0.9 1.1 0.4 0.3 Ti:
0.10
TABLE-US-00002 TABLE 2 Patenting conditions Isothermal
Austenization transformation Heating Retention Heating Retention
temperature time temperature time Conditions (.degree. C.) (sec)
(.degree. C.) (sec) I 920 120 630 80 II 980 60 700 30 III 880 120
650 50 IV 950 190 650 50 V 950 50 650 50 VI 1,070 60 650 50 VII 920
120 580 50 VIII 920 120 650 15 IX 920 120 650 120 X 920 120 780
50
[0053] The resultant steel wires (3.0 mm.phi.) were then subjected
to quenching-tempering. For the quenching, the conditions shown in
Table 3 were used, while the tempering was carried out using a
heating temperature of 450-530.degree. C. for all wires. The
reduction of area (RA) and the average grain sizes (average .gamma.
grain size) of austenite grains (prior austenite grains) were
measured on the respective quench-tempered wires. The results are
shown in Table 3. Further, the wire quenching temperature was
varied to change the average grain size of austenite grains (prior
austenite grains). The average grain size of austenite grains was
determined based on the intercept method subject to JIS G 0552.
[0054] Further, the shear yield stress and the fatigue properties
(fatigue limit) were measured on those steel wires which were
subjected, after the quenching-tempering, to heat treatment
(420.degree. C. for 2 hours or 480.degree. C. for 2 hours)
comparable to nitriding treatment. The results are shown also in
Table 3. The shear yield stress of the steel wires which were
heat-treated as above was determined from torque-.theta. curves
obtained through twisting tests on samples of 100 d in length (d:
sample diameter). The fatigue limit was evaluated based on a
Nakamura-type rotating bending fatigue test.
TABLE-US-00003 TABLE 3 Average Shear Shear Quenching .gamma. grain
yield yield Fatigue temperature size RA stress stress limit No.
Samples Conditions (.degree. C.) (.mu.m) (%) 420.degree. C. .times.
2 hr 480.degree. C. .times. 2 hr (MPa) 1 A I 920 4.5 45 985 892 715
2 B II 930 4.8 35 955 864 705 3 C I 920 4.3 48 938 821 730 4 D I
950 5.4 37 941 823 735 5 E II -- -- -- -- -- -- 6 F II 940 5.0 42
923 815 720 7 G II -- -- -- -- -- -- 8 H I 930 4.4 45 921 810 705 9
H I 850 2.8 31 928 815 715 10 H I 1,050 8.9 50 925 810 710 11 I II
920 3.8 29 925 835 695 12 J I 910 3.5 41 930 830 705 13 K I 930 4.3
46 1,098 1,021 850 14 L II 910 3.2 43 1,130 1,043 865 15 M II 940
5.2 48 1,178 1,098 875 16 N I 1,020 6.5 44 1,084 1,015 855 17 O I
980 6.2 45 1,195 1,078 875 18 P II 950 5.2 48 1,168 1,054 880 19 Q
II 930 3.5 45 1,121 1,038 865 20 R I 920 3.4 47 1,154 1,069 870 21
S I 940 4.4 46 1,211 1,113 895
[0055] As shown in Table 3, it is understood that the steel wires
of samples No. 13 through 21 having a 40% or higher reduction of
area (RA) and a 1,000 MPa or higher shear yield stress after the
heat treatment comparable to nitriding treatment all have a high
fatigue limit. Moreover, since the steel wires of these samples
have a high shear yield stress, it is considered that these steel
wires will be excellent in their permanent set properties. Thus, it
is understood that the spring steel wire of the present invention
is provided with high toughness while having excellent fatigue
properties.
[0056] On the other hand, the samples No. 1-4, 6, and 8 having a
low shear yield stress after the heat treatment comparable to
nitriding treatment turned out to have a low fatigue limit.
Especially, the samples No. 2 and 4 had also an inferior toughness
with a low reduction of area. Further, the steel wires of the
samples No. 5 and 7 underwent martensite generation in their wire
rod structures during patenting and then frequent wire breakage in
the succeeding shaving step, and thus the experiment was forced to
stop continuing. For the sample No. 11, since it had a higher V
content of the total steel in addition to its low shear yield
stress after the heat treatment, it had a lowered reduction of area
of the steel wire to thus reduce its fatigue limit. For the sample
No. 12, since it had a higher Ti content in addition to its low
shear yield stress after the heat treatment, it underwent a
reduction in fatigue limit owing to breakage caused by Ti-based
inclusions.
[0057] For the sample No. 9, since it had a smaller average grain
size of the austenite grains (prior austenite grains) in addition
to its low shear yield stress after the heat treatment, it showed
also a low reduction of area. On the other hand, the sample No. 10
showed a reduction in fatigue limit, because it had a large average
grain size of the austenite grains (prior austenite grains) in
addition to its low shear yield stress after the heat
treatment.
[0058] In the same manner as the above-described embodiment, a
steel having the chemical compositions of the sample K of Table 1
was worked to prepare a wire rod of 6.5 mm.phi., and the resultant
wire rod was then worked into a steel wire of 3.0 mm.phi. likewise
as above. In this case, the patenting conditions employed were
varied as shown in Table 2. The wire thus obtained was subjected to
quenching-tempering (quenching temperature: 940.degree. C.,
tempering temperature: 450-530.degree. C.), and the reduction of
area (RA) of the resultant wire and its average grain size of the
austenite grains (prior austenite grains) were measured. The
results are shown in Table 4. Further, the shear yield stress and
the fatigue properties (fatigue limit) were measured on those steel
wires which were subjected, after the quenching-tempering, to heat
treatment (420.degree. C. for 2 hours or 480.degree. C. for 2
hours) comparable to nitriding treatment. The results are shown
together with the temperature conditions in Table 4. Measurement of
the physical properties was carried out like the preceding
examples.
TABLE-US-00004 TABLE 4 Average Shear Shear Quenching .gamma. grain
yield yield Fatigue temperature size RA stress stress limit No.
Samples Conditions (.degree. C.) (.mu.m) (%) 420.degree. C. .times.
2 hr 480.degree. C. .times. 2 hr (MPa) 22 K I 940 4.5 45 1,098
1,021 865 23 K II 940 4.5 46 1,083 1,015 860 24 K III 940 4.4 37
930 824 730 25 K IV -- -- -- -- -- -- 26 K V 940 4.3 36 934 829 728
27 K VI -- -- -- -- -- -- 28 K VII -- -- -- -- -- -- 29 K VIII --
-- -- -- -- -- 30 K IX 940 4.6 35 932 823 731 31 K X 940 4.7 36 925
815 734
[0059] As shown in Table 4, it is understood that the samples No.
22 and 23 which were patented under particular conditions
(austenitization: 900-1,050.degree. C. for 60 to 180 seconds,
isothermal transformation: 600-750.degree. C. for 20 to 100
seconds) both had a high fatigue limit.
[0060] However, since the samples No. 25, and 27-29 underwent
martensite generation in their wire rod structures during patenting
and then frequent wire breakage in the drawing step, the experiment
was forced to stop continuing. In the samples No. 24 and 26, since
there remained undissolved carbide, the wires each had a lowered
reduction of area and a reduced fatigue limit. Moreover, the
samples No. 24 and 26 each had also a low shear yield stress. The
samples No. 30 and 31 underwent cementite spheroidization in their
wire rod structures so that there remained undissolved carbide,
which resulted in reduced reduction of area and lower shear yield
stress of each steel wire.
INDUSTRIAL APPLICABILITY
[0061] Since the spring steel wire of the present invention is
excellent both in fatigue properties and in toughness, it is best
suited as a material for springs that are used for parts requiring
an adequate fatigue strength.
* * * * *