U.S. patent application number 13/574175 was filed with the patent office on 2012-11-22 for drawn heat treated steel wire for high strength spring use and pre-drawn steel wire for high strength spring use.
This patent application is currently assigned to NIPPON STEEL CORPORATION. Invention is credited to Tetsushi Chida, Masayuki Hashimura.
Application Number | 20120291927 13/574175 |
Document ID | / |
Family ID | 45441342 |
Filed Date | 2012-11-22 |
United States Patent
Application |
20120291927 |
Kind Code |
A1 |
Hashimura; Masayuki ; et
al. |
November 22, 2012 |
DRAWN HEAT TREATED STEEL WIRE FOR HIGH STRENGTH SPRING USE AND
PRE-DRAWN STEEL WIRE FOR HIGH STRENGTH SPRING USE
Abstract
Drawn heat treated steel wire for high strength spring use is
provided containing, by mass %, C: 0.67% to less than 0.9%, Si: 2.0
to 3.5%, Mn: 0.5 to 1.2%, Cr: 1.3 to 2.5%, N: 0.003 to 0.007%, and
Al: 0.0005% to 0.003%, having Si and Cr satisfying the following
formula: 0.3%.ltoreq.Si-Cr.ltoreq.1.2%, and having a balance of
iron and unavoidable impurities, having as impurities, P: 0.025% or
less and S: 0.025% or less, furthermore having a circle equivalent
diameter of undissolved spherical carbides of less than 0.2 .mu.m,
further having, as a metal structure, at least residual austenite
in a volume rate of over 6% to 15%, having a prior austenite grain
size number of #10 or more, and having a circle equivalent diameter
of undissolved spherical carbides of less than 0.2 .mu.m.
Inventors: |
Hashimura; Masayuki;
(Chiyoda-ku, JP) ; Chida; Tetsushi; (Chiyoda-ku,
JP) |
Assignee: |
NIPPON STEEL CORPORATION
Tokyo
JP
|
Family ID: |
45441342 |
Appl. No.: |
13/574175 |
Filed: |
July 5, 2011 |
PCT Filed: |
July 5, 2011 |
PCT NO: |
PCT/JP2011/065749 |
371 Date: |
July 19, 2012 |
Current U.S.
Class: |
148/580 ;
148/333; 148/334; 420/104; 420/110; 420/84; 72/200 |
Current CPC
Class: |
C21D 9/02 20130101; C22C
38/34 20130101; C22C 38/00 20130101; C21D 6/008 20130101; C21D
2211/008 20130101; C22C 38/22 20130101; C21D 8/065 20130101; C22C
38/002 20130101; C22C 38/38 20130101; C22C 38/001 20130101; C22C
38/04 20130101; C22C 38/26 20130101; C22C 38/06 20130101; C22C
38/28 20130101; C22C 38/24 20130101 |
Class at
Publication: |
148/580 ;
420/104; 420/84; 420/110; 148/333; 148/334; 72/200 |
International
Class: |
C21D 9/02 20060101
C21D009/02; C22C 38/24 20060101 C22C038/24; B21B 1/16 20060101
B21B001/16; C22C 38/28 20060101 C22C038/28; C22C 38/34 20060101
C22C038/34; C22C 38/22 20060101 C22C038/22; C22C 38/26 20060101
C22C038/26 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 6, 2010 |
JP |
2010-154030 |
Claims
1. Pre-drawn steel wire for high strength spring use characterized
by containing, by mass %, C: 0.67% to less than 0.9%, Si: 2.0 to
3.5%, Mn: 0.5 to 1.2%, Cr: 1.3 to 2.5%, N: 0.003 to 0.007%, and Al:
0.0005% to 0.003%, having Si and Cr satisfying the following
formula: 0.3%.ltoreq.Si-Cr.ltoreq.1.2%, having a balance of iron
and unavoidable impurities, having P and S as impurities comprising
P: 0.025% or less and S: 0.025% or less, and, furthermore, having a
circle equivalent diameter of undissolved spherical carbides of
less than 0.2 .mu.m.
2. Pre-drawn steel wire for high strength spring use as set forth
in claim 1 characterized by, further, containing, by mass %, one or
more of V: 0.03 to 0.10%, Nb: 0.015% or less Mo: 0.05 to 0.30%, W:
0.05 to 0.30% Mg: 0.002% or less, Ca: 0.002% or less, and Zr:
0.003% or less, when containing V satisfying
1.4%.ltoreq.Cr+V.ltoreq.2.6% and 0.70%.ltoreq.Mn+V.ltoreq.1.3%,
and, when containing Mo and W, satisfying
0.05%.ltoreq.Mo+W.ltoreq.0.5%.
3. Drawn heat treated steel wire for high strength spring use
characterized by containing, by mass %, C: 0.67% to less than 0.9%,
Si: 2.0 to 3.5%, Mn: 0.5 to 1.2%, Cr: 1.3 to 2.5%, N: 0.003 to
0.007%, and Al: 0.0005% to 0.003%, having Si and Cr satisfying the
following formula: 0.3%.ltoreq.Si-Cr.ltoreq.1.2%, and having a
balance of iron and unavoidable impurities, having P and S as
impurities comprising P: 0.025% or less and S: 0.025% or less,
furthermore, having a metal structure comprised of at least
residual austenite in a volume rate of over 6% to 15%, having prior
austenite grain size number of #10 or more, and having a circle
equivalent diameter of undissolved spherical carbides of less than
0.2 .mu.m.
4. Drawn heat treated steel wire for high strength spring use as
set forth in claim 3 characterized by, further, containing, by mass
%, one or more of V: 0.03 to 0.10%, Nb: 0.015% or less Mo: 0.05 to
0.30%, W: 0.05 to 0.30% Mg: 0.002% or less, Ca: 0.002% or less, and
Zr: 0.003% or less, when containing V satisfying
1.4%.ltoreq.Cr+V.ltoreq.2.6% and 0.70%.ltoreq.Mn+V.ltoreq.1.3%,
and, when containing Mo and W, satisfying
0.05%.ltoreq.Mo+W.ltoreq.0.5%.
5. Drawn heat treated steel wire for high strength spring use as
set forth in claim 3 characterized in that said drawn heat treated
steel wire for high strength spring use has a tensile strength of
2100 to 2400 MPa.
6. Drawn heat treated steel wire for high strength spring use as
set forth in claim 5 characterized in that said drawn heat treated
steel wire for high strength spring use has a yield strength of
1600 to 1980 MPa.
7. Drawn heat treated steel wire for high strength spring use as
set forth in claim 3 characterized in that said drawn heat treated
steel wire for high strength spring use has a surface Vicker's
hardness of HV750 or more and an internal Vicker's hardness of
HV570 or more aftersoft nitriding of keeping at 500.degree. C. for
1 hour.
8. A method of production of pre-drawn steel wire for high strength
spring use characterized by taking a bloom containing, by mass %,
C: 0.67% to less than 0.9%, Si: 2.0 to 3.5%, Mn: 0.5 to 1.2%, Cr:
1.3 to 2.5%, N: 0.003 to 0.007%, and Al: 0.0005% to 0.003%, having
Si and Cr satisfying the following formula:
0.3%.ltoreq.Si-Cr.ltoreq.1.2%, having a balance of iron and
unavoidable impurities, having P and S as impurities comprising P:
0.025% or less and S: 0.025% or less, heating the bloom to
1250.degree. C. or more, then hot rolling the bloom to produce a
billet and heating the billet to 1200.degree. C. or more, then hot
rolling to produce pre-drawn steel wire.
9. A method of production of pre-drawn steel wire for high strength
spring use as set forth in claim 8 characterized by the bloom
further, containing, by mass %, one or more of V: 0.03 to 0.10%,
Nb: 0.015% or less Mo: 0.05 to 0.30%, W: 0.05 to 0.30% Mg: 0.002%
or less, Ca: 0.002% or less, and Zr: 0.003% or less, when
containing V satisfying 1.4%.ltoreq.Cr+V.ltoreq.2.6% and
0.70%.ltoreq.Mn+V.ltoreq.1.3%, and, when containing Mo and W,
satisfying 0.05%.ltoreq.Mo+W.ltoreq.0.5%.
10. A method of production of pre-drawn steel wire for high
strength spring use characterized by further heating pre-drawn
steel wire as set forth in claim 8 to 900.degree. C. or more, then
patenting it at 600.degree. C. or less.
11. A method of production of heat treated steel wire for high
strength spring use characterized by drawing said pre-drawn steel
wire which was produced by the method of production of pre-drawn
steel wire as set forth in claim 8, heating it at a heating rate of
10.degree. C./sec or more up to an A.sub.3 point, holding it at a
temperature of the A.sub.3 point or more for 1 minute to 5 minutes,
then cooling it at a cooling rate of 50.degree. C./sec or more down
to 100.degree. C. or less.
12. A method of production of heat treated steel wire for high
strength spring use characterized by drawing said pre-drawn steel
wire which was produced by the method of production of pre-drawn
steel wire as set forth in claim 10, heating it at a heating rate
of 10.degree. C./sec or more up to an A.sub.3 point, holding it at
a temperature of the A.sub.3 point or more for 1 minute to 5
minutes, then cooling it at a cooling rate of 50.degree. C./sec or
more down to 100.degree. C. or less.
13. A method of production of heat treated steel wire for high
strength spring use as set forth in claim 11 characterized by
further holding and tempering it at 400 to 500.degree. C. for 15
minutes or less.
14. A method of production of heat treated steel wire for high
strength spring use as set forth in claim 12 characterized by
further holding and tempering it at 400 to 500.degree. C. for 15
minutes or less.
15. Drawn heat treated steel wire for high strength spring use as
set forth in claim 4 characterized in that said drawn heat treated
steel wire for high strength spring use has a tensile strength of
2100 to 2400 MPa.
16. Drawn heat treated steel wire for high strength spring use as
set forth in claim 4 characterized in that said drawn heat treated
steel wire for high strength spring use has a surface Vicker's
hardness of HV750 or more and an internal Vicker's hardness of
HV570 or more aftersoft nitriding of keeping at 500.degree. C. for
1 hour.
17. A method of production of pre-drawn steel wire for high
strength spring use characterized by further heating pre-drawn
steel wire as set forth in claims 9 to 900.degree. C. or more, then
patenting it at 600.degree. C. or less.
18. A method of production of heat treated steel wire for high
strength spring use characterized by drawing said pre-drawn steel
wire which was produced by the method of production of pre-drawn
steel wire as set forth in claim 9, heating it at a heating rate of
10.degree. C./sec or more up to an A.sub.3 point, holding it at a
temperature of the A.sub.3 point or more for 1 minute to 5 minutes,
then cooling it at a cooling rate of 50.degree. C./sec or more down
to 100.degree. C. or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to drawn heat treated steel
wire for high strength spring use which can be used as a material
for high strength springs produced by cold coiling and to pre-drawn
steel wire.
BACKGROUND ART
[0002] The springs which are used for automobile engines, clutches,
etc. are being required to offer more advanced performance and
higher durability in order to deal with the trend toward lighter
weights and higher performances of automobiles. For this reason,
their materials, that is, drawn heat treated steel wire for high
strength spring use, are also being required to offer high material
strength. In general, when producing such small sized, high
strength springs, the material of the drawn heat treated steel wire
for high strength spring use is quenched and tempered to impart
higher material strength in the drawn heat treated steel wire for
high strength spring use, then is cold coiled to obtain a coil
spring shape. Furthermore, stress-relief annealing or other heat
treatment and nitriding are performed to obtain a finished coil
spring. For this reason, drawn heat treated steel wire for high
strength spring use is required to have not only high strength, but
also to have a high enough workability that it will not break at
the cold coiling and to suppress softening due to the annealing,
nitriding, and other heat treatment performed after coiling, that
is, to have temper softening resistance.
[0003] A spring is required to have fatigue characteristics, so
drawn heat treated steel wire for high strength spring use is used
as a material and further nitrided or shot peened to raise the
hardness of the surface layer of the spring. The durability of a
spring includes fatigue characteristics and a sag property. The
fatigue characteristics are affected by the surface layer hardness.
The sag property (property of the spring ending up plastically
deforming in the load direction during use) is greatly affected by
not only the surface layer hardness, but also the hardness of the
base material of the spring. For this reason, in steel wire for
high strength spring use, the surface layer hardness after
nitriding and the temper softening resistance at the inside where
nitrogen is not introduced by nitriding are important.
[0004] Furthermore, when producing a spring by cold coiling, when
producing the material of the drawn heat treated steel wire for
high strength spring use, oil tempering, induction hardening
treatment, etc. where rapid heating and rapid cooling are possible
may be used.
[0005] For this reason, the drawn heat treated steel wire for high
strength spring use can be reduced in prior austenite grain size,
so a spring with excellent fracture characteristics can be
obtained. However, if drawn heat treated steel wire for high
strength spring use becomes higher in strength, in cold coiling,
breakage may occur and the spring shape may not be able to be
formed.
[0006] To deal with this problem, some of the inventors proposed
drawn heat treated steel wire for high strength spring use obtained
by controlling the carbides, making the prior austenite finer, and
achieving both strength and cold coiling ability (PLT 1).
Furthermore, they proposed drawn heat treated steel wire for high
strength spring use obtained by controlling the residual austenite
and carbides, refining the prior austenite, and achieving both
strength and cold coiling ability (PLT 2 to PLT 4). In particular,
the starting points of fracture caused by the formation of coarse
oxides and carbides are suppressed and the distribution of fine
carbides of cementite required for securing strength is made
uniform so as to suppress deterioration of the fatigue
characteristics and workability of the drawn heat treated steel
wire for high strength spring use.
[0007] PLT 2 focuses on the fact that the region of sparse
spherical carbides with a circle equivalent diameter of 2 .mu.m or
more in the region of a sparse distribution of fine spherical
carbides (in particular, cementite) affects the dynamic
characteristics and defines that region.
[0008] PLT 3 and PLT 4 take note of the effect of precipitation of
fine carbides due to the addition of the alloy element V and limits
the nitrogen (N) content to suppress undissolved spherical
carbides. That is, they utilize the effect of precipitation of
carbides, nitrides, and carbonitrides of V to enable utilization
for hardening the steel wire at the tempering temperature or
hardening the surface layer in nitriding. Furthermore, there is
also an effect on suppressing coarsening of the austenite grain
size due to the formation of precipitates. The effect of addition
of V is remarkable. However, undissolved carbides or nitride easily
form, so even if suppressing the nitrogen (N), the control of
precipitation has to be performed precisely.
[0009] Therefore, PLT 4 quantitatively compares the undissolved
spherical carbides and the precipitated carbides and defines the
amounts so as to obtain as much precipitated V carbides, which are
effective for the final spring performance, as possible.
Specifically, it proposes to weigh the residue of V carbides in the
electrolytic solution at a constant potential and compare this with
the amount of V which passes through the filter (amount of
precipitated V).
CITATION LIST
Patent Literature
[0010] PLT 1: Japanese Patent Publication (A) No. 2002-180198
[0011] PLT 2: Japanese Patent Publication (A) No. 2006-183137
[0012] PLT 3: Japanese Patent Publication (A) No. 2006-342400
[0013] PLT 4: International Publication WO2007/114491
SUMMARY OF INVENTION
Technical Problem
[0014] In recent years, to raise the durability of high strength
springs, surface hardening by nitriding has become a general
practice. Furthermore, increasing the nitrided depth and shortening
the nitriding time by raising the treatment temperature is being
studied. For this reason, drawn heat treated steel wire for high
strength spring use is being required to be further improved in
temper softening resistance.
[0015] That is, a further better cold coiling ability than even
with conventional drawn heat treated steel wire for high strength
spring use, excellent temper softening resistance even after being
held at 500.degree. C. for 1 hour, internal softening kept to a
minimum, and greater hardness of the surfacemost layer are being
sought.
[0016] The above conventional drawn heat treated steel wire for
high strength spring use secures a certain extent of uniform
dispersion of fine carbides for improving the fatigue
characteristics and workability. However, to improve the temper
softening resistance, further uniform dispersion is necessary. In
particular, the addition of V proposed in PLT 3 and PLT 4 does
indeed have the effect of hardening the steel wire at the tempering
temperature, hardening the surface layer in nitriding, and refining
the austenite. However, on the other hand, control of the nitrogen
(N) content is not easy. As a result, coarse carbides, nitrides,
and carbonitrides are precipitated and cause degradation in the
fatigue strength.
[0017] PLT 3 adds Nb and Ti with the aim of the effect of trapping
excess nitrogen (N). However, even if doing this, control to a
suitable amount of N content is still not easy.
[0018] PLT 4 samples the residue of undissolved spherical carbides
obtained as a result and compares it with the dissolved carbides.
Therefore, it does not proactively control uniform dispersion of
fine carbides.
[0019] Due to the above, the present invention has as its object to
keep to a minimum the addition of V and other alloy elements, that
is, without precisely controlling the N content, develop drawn heat
treated steel wire for high strength spring use which has excellent
yield strength and hardness and excellent workability and which has
superior surface layer hardness and internal hardness even after
nitriding.
[0020] Further, as described in PLT 3 and PLT 4, to obtain
excellent yield strength and hardness and excellent workability,
the size of the undissolved spherical carbides in the steel should
be small. The effective size is preferably 0.1 .mu.m or less. If
over 1 .mu.m, the contribution to strength and workability is lost
and the deformation characteristics are just degraded. For this
reason, the density of presence of undissolved spherical carbides
with a circle equivalent diameter of 0.2 .mu.m or more becomes an
important indicator. Therefore, the present invention has as its
object the development of steel wire for high strength spring use
not allowing the presence of undissolved spherical carbides with a
circle equivalent diameter of 0.2 .mu.m or more.
Solution to Problem
[0021] The inventors engaged in intensive research to solve the
above problems and as a result obtained the following
discoveries:
[0022] (a) It was discovered that by strictly controlling the
contents of C, Si, Mn, and Cr in the steel wire to suppress the
formation of spherical carbides and by utilizing the residual
austenite, even without adding alloy elements such as V, the drawn
heat treated steel wire for high strength spring use is improved in
strength and cold coiling ability compared with the
conventional.
[0023] (b) It was also discovered that by adding both Cr and Si in
the steel wire in suitable amounts, the formation of undissolved
spherical carbides and the softening in annealing or nitriding
after coiling are suppressed, and, furthermore, greater hardness of
the nitrided layer can be achieved.
[0024] That is, for increasing the strength in the fatigue
characteristics, addition of Cr is effective, but Cr is an element
which easily leaves behind undissolved spherical carbides which
would have a detrimental effect on the cold coiling ability. For
this reason, the amount of addition had to be restricted. The
inventors also took note of Si which suppresses the growth of
undissolved spherical carbides and the formation of cementite. They
discovered that if adding Si and together increasing the amount of
addition of Cr, the drawn heat treated steel wire can be increased
in strength. Quantitatively, it is sufficient to add large amounts
of both Si and Cr and, as the relationship between them, control
the difference in amount of addition of Si and the amount of
addition of Cr, that is, (Si-Cr) %.
[0025] (c) Further, it was discovered that by heating the bloom to
1250.degree. C. or more, it is possible to make Cr and other alloy
elements in the steel material uniformly disperse and suppress the
formation of coarse undissolved spherical carbides and,
furthermore, make fine carbides uniformly disperse.
[0026] Undissolved spherical carbides are present in the steel
material just after casting and become causes of not only poor
coiling ability, but also breakage in rolling and drawing. For this
reason, to prevent a detrimental effect in the steps of blooming
after casting, wire rod rollingwire rod, patenting, quenching, and
drawing, it is effective to raise the heating temperature in each
step and constantly suppress undissolved spherical carbides.
[0027] (d) Furthermore, it was discovered that the addition of V
has a detrimental effect on the mechanical properties and fatigue
strength of steel wire for spring use.
[0028] That is, from just after casting to being worked into a
spring, a steel material is repeatedly heated. Usually, the
undissolved spherical carbides are mainly cementite (Fe.sub.3C).
However, by repeating the heating, the undissolved spherical
carbides often include Cr, V, etc. It is learned that not only are
Cr, V, and other alloy elements wastefully consumed, but there is
also a possibility of degrading the mechanical characteristics
after nitriding (surface hardness, internal hardness, etc.)
[0029] Further, as explained above, with the addition of V, control
of the nitrogen (N) content is not easy. As a result, coarse
carbides, nitrides, and carbonitrides precipitate and become causes
of degradation in fatigue strength.
[0030] From these facts, the inventors discovered that by not
adding V, or an extremely small amount even though added, and
further, as explained above, controlling the amount of Cr in
balance with the amount of Si, it is possible to suppress
coarsening of the undissolved spherical carbides.
[0031] Here, "undissolved spherical carbides" means undissolved
carbides with a ratio of the maximum size (long size) and minimum
size (short size) (aspect ratio) of 2 or less. Actually, "carbides"
and "spherical carbides" are also undissolved. Here, in the sense
of emphasis, while synonymous, these respectively are also called
"undissolved carbides" and "undissolved spherical carbides".
[0032] The present invention was made based on these discoveries.
The gist of the invention is as follows:
[0033] (1) Pre-drawn steel wire for high strength spring use
characterized by containing, by mass %,
C: 0.67% or greater and less than 0.9%,
Si: 2.0 to 3.5%,
Mn: 0.5 to 1.2%,
Cr: 1.3 to 2.5%,
N: 0.003 to 0.007%, and
Al: 0.0005% to 0.003%,
[0034] having Si and Cr satisfying the following formula:
0.3%.ltoreq.Si-Cr.ltoreq.1.2%,
having a balance of iron and unavoidable impurities, having P and S
as impurities comprising P: 0.025% or less and S: 0.025% or less,
and, furthermore, having a circle equivalent diameter of
undissolved spherical carbides of less than 0.2 .mu.m.
[0035] (2) Pre-drawn steel wire for high strength spring use as set
forth in (1) characterized by, further, containing, by mass %, one
or more of
V: 0.03 to 0.10%,
[0036] Nb: 0.015% or less
Mo: 0.05 to 0.30%,
W: 0.05 to 0.30%
[0037] Mg: 0.002% or less, Ca: 0.002% or less, and Zr: 0.003% or
less, when containing V satisfying 1.4%.ltoreq.Cr+V.ltoreq.2.6% and
0.70%.ltoreq.Mn+V.ltoreq.1.3%, and, when containing Mo and W,
satisfying 0.05%.ltoreq.Mo+W.ltoreq.0.5%. (3) Drawn heat treated
steel wire for high strength spring use characterized by
containing, by mass %, C: 0.67% or greater and less than 0.9%,
Si: 2.0 to 3.5%,
Mn: 0.5 to 1.2%,
Cr: 1.3 to 2.5%,
N: 0.003 to 0.007%, and
Al: 0.0005% to 0.003%,
[0038] having Si and Cr satisfying the following formula:
0.3%.ltoreq.Si-Cr.ltoreq.1.2%, and
having a balance of iron and unavoidable impurities, having P and S
as impurities comprising P: 0.025% or less and S: 0.025% or less,
furthermore, having a metal structure comprised of at least
residual austenite in a volume rate of over 6% to 15%, having prior
austenite grain size number of #10 or more, and having a circle
equivalent diameter of undissolved spherical carbides of less than
0.2 .mu.m. (4) Drawn heat treated steel wire for high strength
spring use as set forth in (3) characterized by, further,
containing, by mass %, one or more of
V: 0.03 to 0.10%,
[0039] Nb: 0.015% or less
Mo: 0.05 to 0.30%,
W: 0.05 to 0.30%
[0040] Mg: 0.002% or less, Ca: 0.002% or less, and Zr: 0.003% or
less, when containing V satisfying 1.4%.ltoreq.Cr+V.ltoreq.2.6% and
0.70%.ltoreq.Mn+V.ltoreq.1.3%, and, when containing Mo and W,
satisfying 0.05%.ltoreq.Mo+W.ltoreq.0.5%. (5) Drawn heat treated
steel wire for high strength spring use as set forth in (3) or (4)
characterized in that said drawn heat treated steel wire for high
strength spring use has a tensile strength of 2100 to 2400 MPa. (6)
Drawn heat treated steel wire for high strength spring use as set
forth in any one of (3) to (5) characterized in that said drawn
heat treated steel wire for high strength spring use has a yield
stress of 1600 to 1980 MPa. (7) Drawn heat treated steel wire for
high strength spring use as set forth in any one of (3) to (6)
characterized said drawn heat treated steel wire for high strength
spring use has a a surface Vicker's hardness of HV750 or more and
an internal Vicker's hardness of HV570 or more aftersoft nitriding
of keeping at 500.degree. C. for 1 hour. (8) A method of production
of pre-drawn steel wire for high strength spring use characterized
by taking a bloom containing, by mass %, C: 0.67% or greater and
less than 0.9%,
Si: 2.0 to 3.5%,
Mn: 0.5 to 1.2%,
Cr: 1.3 to 2.5%,
N: 0.003 to 0.007%, and
Al: 0.0005% to 0.003%,
[0041] having Si and Cr satisfying the following formula:
0.3%.ltoreq.Si-Cr.ltoreq.1.2%,
having a balance of iron and unavoidable impurities, having P and S
as impurities comprising P: 0.025% or less and S: 0.025% or less,
heating the bloom to 1250.degree. C. or more, then hot rolling the
bloom to produce a billet and heating the billet to 1200.degree. C.
or more, then hot rolling to produce pre-drawn steel wire. (9) A
method of production of pre-drawn steel wire for high strength
spring use as set forth in (8) characterized by the bloom further,
containing, by mass %, one or more of
V: 0.03 to 0.10%,
[0042] Nb: 0.015% or less
Mo: 0.05 to 0.30%,
W: 0.05 to 0.30%
[0043] Mg: 0.002% or less, Ca: 0.002% or less, and Zr: 0.003% or
less, when containing V satisfying 1.4%.ltoreq.Cr+V.ltoreq.2.6% and
0.70%.ltoreq.Mn+V.ltoreq.1.3%, and, when containing Mo and W,
satisfying 0.05%.ltoreq.Mo+W.ltoreq.0.5%. (10) A method of
production of pre-drawn steel wire for high strength spring use
characterized by further heating pre-drawn steel wire as set forth
in (8) or (9) to 900.degree. C. or more, then patenting it at
600.degree. C. or less. (11) A method of production of heat treated
steel wire for high strength spring use characterized by drawing
said pre-drawn steel wire which was produced by the method of
production of pre-drawn steel wire as set forth in any one of (8)
to (10), heating by a heating rate of 10.degree. C./sec or more up
to an A.sub.3 point, holding at a temperature of the A.sub.3 point
or more for 1 minute to 5 minutes, then cooling by a cooling rate
of 50.degree. C./sec or more down to 100.degree. C. or less. (12) A
method of production of heat treated steel wire for high strength
spring use as set forth in (11) characterized by further holding
and tempering it at 400 to 500.degree. C. for 15 minutes or
less.
Advantageous Effects of Invention
[0044] According to the present invention, in particular, due to
the excellent cold coiling ability and temper softening resistance,
even with nitriding at 500.degree. C. for 1 hour, drawn heat
treated steel wire for high strength spring use with a high surface
layer hardness and internal hardness and, furthermore, high
strength spring excellent in durability can be provided. The
contribution in industry is extremely great.
BRIEF DESCRIPTION OF DRAWINGS
[0045] FIG. 1 is a micrograph of the metal structure showing one
example of spherical carbides in the drawn heat treated steel wire
for high strength spring use of the present invention. At the tips
of the arrows in the figure, undissolved spherical carbides are
observed.
[0046] FIG. 2 is a view showing the shape of a punch for providing
a notch in a test piece.
[0047] FIG. 3 is a view showing the step of providing a notch in a
test piece.
[0048] FIG. 4 is a view showing an outline of a notch bending
test.
[0049] FIG. 5 is a view showing a method of measurement of a notch
bending angle.
DESCRIPTION OF EMBODIMENTS
[0050] In general, a wire rod for a spring is produced as follows:
Of course, production of springs is not limited to this here
described process. This is described as just one example.
[0051] A bloom made of steel containing predetermined chemical
compositions is rolled to obtain a billet. Next, the billet is
rolled to produce a predetermined diameter of steel wire. The steel
wire which is produced at this stage is called the "pre-drawn steel
wire".
[0052] The steel wire which is produced after rolling is patented
and drawn to obtain further finer steel wire, then the working
stress at the surface layer is removed and subsequent cold coiling
workability is obtained by heat treatment (quenching and
tempering). The steel wire which is produced at this stage is
called the "drawn heat treated steel wire".
[0053] Next, the spring is worked by cold coiling and is improved
in strength and surface hardness by nitriding. In this way, a
"spring" is produced as a final product.
[0054] First, the chemical compositions of the drawn heat treated
steel wire for high strength spring use of the present invention
and its material, that is, pre-drawn steel wire for high strength
spring use, will be explained. Here, the "%" in the chemical
compositions means mass % except when otherwise indicated.
[0055] C: 0.67% to less than 0.9%
[0056] C is an important element which has a great effect on the
strength of the steel material and contributes to the formation of
residual austenite as well. In the present invention, to obtain
sufficient strength, the lower limit of the amount of C is made
0.67% or more. To raise the strength, the amount of C is preferably
made 0.70% or more, more preferably 0.75% or more.
[0057] On the other hand, if the amount of C becomes 0.9% or more,
excessive coprecipitation results, a large amount of coarse
cementite is precipitated, and the toughness remarkably falls.
Further, if the amount of C is excessive, coarse spherical carbides
are formed and the coiling ability is impaired. Therefore, the
upper limit of the amount of C is made less than 0.9%. From the
viewpoint of suppressing the formation of spherical carbides, the
upper limit of the amount of C is preferably 0.85%, more preferably
0.80%.
[0058] Si: 2.0 to 3.5%
[0059] Si is an important element for improving the temper
softening resistance of the steel and the sag property of the
spring. To obtain these effects, 2.0% or more has to be added.
Further, Si is effective for spheroidization and refinement of the
cementite. To suppress the formation of coarse spherical carbides,
2.1% or more of Si is preferably added. To raise the internal
hardness after nitriding and other treatment for making the surface
layer harder, 2.2% or more of Si is more preferably added.
Furthermore, from the balance with Cr, Si is more preferably made
2.3% or more. Si is sometimes made 3.0% or more.
[0060] On the other hand, if excessively adding Si, the steel wire
hardens and becomes brittle, so the upper limit of the amount of Si
is made 3.5% or less. From the viewpoint of the prevention of
embrittlement, the upper limit is preferably made 3.4%, more
preferably 3.3% or less.
[0061] Mn: 0.5 to 1.2%
[0062] Mn is an element which is important for raising the
quenchability and stably securing the amount of residual austenite.
In the present invention, to raise the yield strength of the steel
wire and secure the residual austenite, Mn has to be added in 0.5%
or more, more preferably 0.65% or more, still more preferably 0.70%
or more.
[0063] On the other hand, if excessively adding Mn, the residual
austenite increases. In working, work-induced martensite is formed
and the cold coiling ability is impaired. To prevent embrittlement
due to excessive addition of Mn, the upper limit of the amount of
Mn is made 1.2% or less, preferably 1.1% or less, more preferably
1.0% or less.
[0064] Cr: 1.3 to 2.5%
[0065] Cr is an element which is effective for improving the
quenchability and temper softening resistance. To obtain these
effects, 1.3% or more of Cr has to be added. When performing the
nitriding, it is possible to make the hardened layer obtained by
nitriding deeper by the addition of Cr. Therefore, when imparting
hardening by nitriding and softening resistance at the nitriding
temperature, over 1.5% of Cr is preferably added. More preferably,
1.7% or more should be added.
[0066] On the other hand, if the amount of Cr is excessive, the
production cost becomes higher. Not only this, dissolution of the
carbides is impaired, undissolved spherical carbides become
greater, and the coiling ability is impaired, so the upper limit of
the amount of Cr is made 2.5% or less. Further, if the amount of Cr
is large, to suppress formation of coarse cementites, the amount of
Cr is preferably suppressed to 2% or less. Furthermore, to obtain
both strength and coiling ability, the upper limit of the amount of
Cr is preferably made 1.8% or less.
[0067] N: 0.003 to 0.007%
[0068] N is an element, in the present invention, which forms
nitrides with Al etc. included as impurities in the steel. To
utilize the fine nitrides and refine the prior austenite, 0.003% or
more of N has to be included. On the other hand, if the amount of N
is excessive, the nitrides coarsen and the cold coiling ability and
fatigue characteristics fall. Therefore, the upper limit of the
amount of N is made 0.007% or less. Further, if considering the
ease of heat treatment etc., the amount of N is preferably 0.005%
or less.
[0069] P: 0.025% or less
[0070] P is an impurity. It causes the steel to harden, forms
segregation, and causes embrittlement, so the upper limit of the
amount of P is made 0.025% or less. Further, the P which segregates
at the prior austenite grain boundaries causes the toughness and
delayed fracture resistance etc. to fall, so the upper limit of the
amount of P is preferably made 0.015% or less. Furthermore, when
the yield strength of the steel wire will exceed 2150 MPa, the
amount of P is preferably limited to less than 0.010%.
[0071] S: 0.025% or less
[0072] S is also an impurity. If present in steel, it causes the
steel to embrittle, so the upper limit of the amount of S is made
0.025% or less. To suppress the effect of S, addition of Mn is
effective. However, MnS is an inclusion. In particular in high
strength steel, MnS sometimes becomes starting points of fracture.
Therefore, to suppress the occurrence of fracture, the upper limit
of the amount of S is preferably made 0.015% or less. Furthermore,
when the yield strength of the drawn heat treated steel wire for
high strength spring use will exceed 2150 MPa, the amount of S is
preferably limited to less than 0.01%.
[0073] Al: 0.0005 to 0.003%
[0074] Al is a deoxidizing element. It affects the formation of
oxides. If forming hard oxides, the fatigue durability falls. In
particular, in high strength springs, if excessively adding Al, the
fatigue strength fluctuates and the stability is impaired. In the
drawn heat treated steel wire for high strength spring use of the
present invention, if the amount of Al exceeds 0.003%, the rate of
occurrence of fracture due to inclusions becomes greater, so the
amount of Al is limited to 0.003% or less. The upper limit value of
the amount of Al is preferably 0.0028%, more preferably
0.0025%.
[0075] On the other hand, if the amount of Al becomes less than
0.0005%, silica-based hard oxides are easily formed. For this
reason, the amount of Al is made 0.0005% or more. The lower limit
of the amount of Al is preferably 0.0007%, more preferably 0.0008%,
further preferably 0.001% or more.
[0076] Next, the point of the present invention, that is, the
relationship between Si and Cr, will be explained. It is already
known that Si and Cr are both important for increasing the strength
of spring steel. However, excessive addition causes problems.
0.3%.ltoreq.Si-Cr.ltoreq.1.2%
[0077] If the amount of Si exceeds the prescribed amount, the
embrittlement becomes extreme and the workability in coiling is
impaired. Not only that, decarburization in the intermediate
processes becomes remarkable. For this reason, in the final product
of the spring, the surface layer hardness becomes lower and the
durability falls. Further, decarburized parts are randomly formed,
so the stability of the strength of the spring product is impaired.
When the amount of Si is smaller than the prescribed amount, the
strength falls. Furthermore, the sag property is insufficient. This
appears in the hardness after nitriding as well. Sufficient
hardness cannot be secured both at the surface layer and
inside.
[0078] However, the relationship between Si and Cr through the
cementite in the steel is important. That is, Si is an element
which destabilizes cementite. If adding a large amount of Cr or
other element which stabilizes cementite, in heating, there is the
effect of promoting the formation of a solid solution by the
cementite. Therefore, regardless of adding a large amount of Cr, if
the amount of addition of Si is small, the amount of undissolved
spherical carbides becomes greater and the workability is
remarkably reduced. The inventors discovered that it is possible to
use the difference between the Si content (mass %) and Cr content
(mass %) in the steel, that is, the Si-Cr amount, as a yardstick.
That is, when the value of Si-Cr is smaller than 0.3%, the amount
of Cr becomes relatively large and undissolved spherical carbides
easily remain. On the other hand, if over 1.2%, Si becomes
relatively excessive and easily causes embrittlement,
decarburization, or other problems. Therefore, the value of Si-Cr
should be made 0.3 to 1.2%.
[0079] From the viewpoint of suppressing the formation of carbides,
a larger amount of Si-Cr enables the undissolved carbides to be
suppressed, but industrially, if the Si is too great, the depth of
the hardened layer formed by the nitriding will easily become
shallow. For this reason, if considering the behavior of
undissolved spherical carbides and the hardened layer formed by
nitriding, preferably Si-Cr.ltoreq.0.9%, more preferably
Si--Cr.ltoreq.0.75%. Further, from the viewpoint of relatively
reducing the amount of Cr and reducing the residual presence of
undissolved spherical carbides, the lower limit is preferably
0.35.ltoreq.Si-Cr, more preferably 0.4.ltoreq.Si-Cr.
[0080] Next, the selectively added chemical compositions will be
explained.
[0081] V: 0.03 to 0.10%
[0082] V is an element which forms nitrides, carbides, and
carbonitrides. Fine V nitrides, carbides, and carbonitrides with a
circle equivalent diameter of less than 0.2 .mu.m are effective for
refinement of the prior austenite. Further, they may also be
utilized for hardening the surface layer by nitriding. However, on
the other hand, undissolved carbides and nitrides are easily
formed, so even if suppressing the nitrogen (N), it is necessary to
precisely control the precipitation.
[0083] For this reason, in the present invention, V is not
deliberately added.
[0084] To obtain such an effect of addition of V, a fine amount can
be added. To obtain these effects, V should be added in 0.03% or
more, preferably 0.035% or more, more preferably 0.04% or more.
[0085] On the other hand, if adding over 0.10% of V, coarse
spherical carbides are formed and the cold coiling ability and
spring fatigue characteristics are impaired. Therefore, the V
content should be made 0.1% or less. Further, by the addition of V,
before drawing, a supercooled structure causing cracks and breakage
in drawing easily is formed. For this reason, the upper limit of
the amount of V is preferably made 0.09% or less, more preferably
0.08% or less, most preferably 0.05% or less. In particular, in the
case of adding a fine amount of Nb, the amount of addition of V is
preferably made 0.05% or less. Further, V is an element which
greatly affects the formation of residual austenite in the same way
as Mn, so the amount of V has to be precisely controlled together
with the amount of Mn.
[0086] Nb: 0.015% or less
[0087] Nb is an element which forms nitrides, carbides, and
carbonitrides in steel. These precipitates are sometimes used for
control of the austenite grain size etc. However, simultaneously,
excessive addition reduces the ductility when hot and results in
easier cracking in rolling or hot forging. For this reason,
excessive addition must be avoided.
[0088] Nb is added for the purpose of controlling the amount of N.
The precipitates are not directly used for controlling the quality.
Valve springs and other springs are produced by quenching,
tempering, then cold coiling, but at that time, the dissolved
nitrogen obstructs color deformation and reduces the limit strain.
For this reason, the coiling ability is impaired. Therefore, by
adding Nb and forming nitrides at a high temperature, there is the
effect that the dissolved nitrogen in the steel in the steel matrix
is lowered and the cold workability is improved.
[0089] Further, the addition of a fine amount of Nb is also
effective for suppressing V and other undissolved spherical
carbides mixed in as unavoidable impurities. V is an element which
is effective for improving the temper softening resistance in
nitriding and the surfacemost layer hardness. However, if the
amount of addition becomes greater, even in the patenting,
quenching, and other heating for obtaining an austenite phase for
producing drawn heat treated steel wire for high strength spring
use, V nitrides, V carbides, and V carbonitrides often are not
sufficiently dissolved. The undissolved spherical carbides of V
grow from cores of the V-based nitrides formed at the time of
normal high temperature. As a result, undissolved spherical
carbides remain and the coiling ability is impaired. For this
reason, when suppressing the undissolved spherical carbides, it is
necessary to suppress the amount of addition of V. In the present
invention, V was not made an essential element.
[0090] As opposed to this, Nb forms nitrides at a higher
temperature than V. For this reason, in the steelmaking process,
addition of Nb suppresses the formation of V nitrides. That is, Nb
forms nitrides in the high temperature region where V dissolves and
does not form nitride. Furthermore, at the high temperature where V
nitrides are formed, Nb consumes nitrogen, so V nitrides become
harder to form even when cooled. For this reason, the addition of a
fine amount of Nb is particularly effective for suppressing
undissolved spherical carbides and securing coiling ability when
adding a large amount of V.
[0091] If the amount of addition of Nb is over 0.015%, the hot
ductility is impaired and the occurrence of defects and other
problems in rolling becomes easy. For this reason, the amount of
addition is made 0.015% or less, preferably 0.010% or less, more
preferably 0.005% or less, most preferably less than 0.001%.
[0092] On the other hand, the effect of Nb in controlling the
amount of N in spring steel appears starting from 0.0005%, so when
adding Nb, 0.0005% or more is preferably added. Further, when
adding V etc., addition of a fine amount of Nb is more effective. A
range of 0.003 to 0.012% is preferable. Furthermore, a range of
0.005 to 0.009% is more preferable. The effect is obtained even at
0.005 to 0.001%.
1.4%.ltoreq.Cr+V.ltoreq.2.6%
[0093] In the present invention, V is not deliberately added.
However, as explained above, addition of a fine amount of V has an
effect on the refinement of the prior austenite and formation of
residual austenite. By precisely controlling the sum of the amounts
of addition of Cr and V with respect to V, it is possible to raise
the strength to make the surface layer hardness after nitriding and
the internal hardness suitable for high strength springs.
[0094] Cr and V are both elements which prevent softening upon the
heating by the annealing or nitriding etc. performed after the
spring coiling, that is, impart so-called temper softening
resistance. In particular, nitriding causes nitrides to precipitate
at the nitrided part of the surface layer to thereby improve the
surface hardness and increase the nitriding effect. Further, even
at the inside where nitriding does not spread, decomposition of the
carbides is suppressed. Further, there is the effect of suppressing
softening by precipitation of carbides. On the other hand, both are
elements which facilitate the formation of undissolved spherical
carbides. Cr dissolves in the cementite to increase the stability
so in the heating steps for dissolving the cementite (heating at
time of patenting and heating at time of quenching), suppresses the
dissolution of the cementite, often remains as undissolved
spherical carbides. Further, V also has a dissolution temperature
of the precipitates higher than the A3 point of steel, so easily
remains as undissolved spherical carbides.
[0095] If the total of the contents of Cr and V, that is, Cr+V, is
less than 1.4%, the surface layer hardness of the high strength
spring falls below HV750 and the internal hardness falls below
HV570. For this reason, Cr+V is preferably 1.4% or more.
Furthermore, 1.5% or more is preferable. On the other hand,
excessive addition of Cr+V of over 2.6% leaves behind large amounts
of undissolved spherical carbides, so the coiling ability is
impaired. Therefore, 2.6% is made the upper limit. Further, the
Cr+V is preferably 2% or less, more preferably 1.8% or less.
0.7%.ltoreq.Mn+V.ltoreq.1.3%
[0096] Mn and V are elements which improve the quenchability and
also have a large effect on the formation of residual austenite. If
Mn is larger than the prescribed amount, a large amount of residual
austenite remains. Therefore, the sum of both Mn and the V which is
included as an unavoidable impurity has a direct effect on the
austenite behavior. If these exceed their prescribed amounts, the
amount of residual austenite increases. Not only is the workability
affected, but also the yield strength is greatly affected.
Sufficiently durability cannot be secured.
[0097] For this reason, in the present invention, the total of the
contents of Mn and V, that is, Mn+V, is made 0.7 to 1.3%. To secure
a volume rate of over 6% of residual austenite, the lower limit of
Mn+V has to be made 0.7% or more.
[0098] As a result, transformation induced plasticity causes the
ductility to be improved and enables the cold coiling ability to be
secured. On the other hand, to make the residual austenite a volume
rate of 15% or less, the upper limit value of Mn+V has to be made
1.3% or less. Due to this, the formation of work-induced martensite
due to strike marks in cold coiling is suppressed and local
embrittlement can be prevented.
[0099] Mo: 0.05 to 0.30%
[0100] Mo is an element which improves the quenchability. Further,
it is also extremely effective for improving the temper softening
resistance. In the present invention, in particular, to further
improve the temper softening resistance, 0.05% or more of Mo can be
added. Further, Mo is an element which forms Mo-based carbides in
the steel. The temperature at which the Mo-based carbides
precipitate is lower than carbides of V etc. For this reason,
addition of a suitable amount of No is also effective for
suppressing coarsening of carbides. Addition of 0.10% or more of Mo
is preferable. On the other hand, if the amount of addition of Mo
is over 0.30%, a supercooled structure easily forms in hot rolling,
the patenting before drawing, etc. Therefore, to suppress the
formation of a supercooled structure causing cracking or wire
breakage in drawing, the upper limit of the amount of Mo is made
0.30% or less, preferably 0.25% or less. Further, if the amount of
Mo is large, in the patenting, the time until the end of the
pearlite transformation becomes longer, so the amount of Mo is
preferably made 0.20% or less. Furthermore, to shorten the
patenting time and stably end the pearlite transformation, 0.15% or
less is preferable.
[0101] W: 0.05 to 0.30%
[0102] W, like Mo, is an element which is effective for improvement
of the quenchability and temper softening resistance and is an
element which precipitates in the steel as carbides. In the present
invention, in particular, to improve the temper softening
resistance, 0.05% or more of W is added.
[0103] On the other hand, if excessively adding W, a supercooled
structure is formed which causes cracking or wire breakage in
drawing, so the amount of W has to be made 0.30% or less.
[0104] Furthermore, if considering the ease of heat treatment etc.,
the amount of W is preferably 0.1 to 0.2%, more preferably 0.13 to
0.18%.
[0105] 0.05%.ltoreq.Mo+W.ltoreq.0.5%
[0106] Mo and W are elements which are effective for improvement of
the temper softening resistance. If adding the two combined, the
growth of carbides is suppressed and the temper softening
resistance can be remarkably improved compared with addition of Mo
and W alone. In particular, to improve the temper softening
resistance in heating to 500.degree. C., Mo+W has to be made 0.05%
or more, preferably 0.15% or more.
[0107] On the other hand, if Mo+W is over 0.5%, in hot rolling,
patenting before drawing, etc., a so-called supercooled structure
of martensite, bainite, etc. is formed. To suppress the formation
of a supercooled structure causing cracks or wire breakage in
drawing, the upper limit of Mo+W is made 0.5% or less, preferably
0.35% or less.
[0108] Next, Mg, Ca, and Zr will be explained.
[0109] Mg: 0.002% or less
[0110] Mg forms oxides in molten steel higher in temperature than
the MnS forming temperature. At the time of formation of MnS, it is
already present in the molten steel. Therefore, it can be used as a
nuclei for precipitation of MnS. Due to this, the distribution of
the MnS can be controlled. Further, in number distribution as well,
Mg-based oxides are more finely dispersed in the molten steel
compared with the Si- and Al-based oxides which are often seen in
conventional steel, so MnS formed around cores of Mg-based oxides
are finely dispersed in the steel. Therefore, even with the same S
content, depending on the presence of Mg, the MnS distribution
differs. Adding these makes the MnS grain size finer. By making the
MnS finely disperse, it is possible to render the action as a
starting point of fatigue of MnS harmless. The effect is
sufficiently obtained even in fine amounts. Preferably Mg 0.0002%
or more, more preferably 0.0005% or more, should be added.
[0111] However, with addition of over 0.001%, it is difficult for
the Mg to remain in the molten steel, there is an effect on the
oxide composition, and the rate of appearance of oxides as
initiation sites of fatigue becomes higher, so 0.002% is made the
upper limit. Therefore, the upper limit of the amount of addition
of Mg was made 0.002%, preferably 0.0015% or less. Furthermore, in
the case of spring steel, compared with other steel for structural
use, the amount of addition of S is suppressed, so if considering
the yield etc., 0.001% or less is preferable. Further, when used
for a high strength valve spring, the inclusion susceptibility is
high, so Mg has the effect of improving the corrosion resistance
and resistance to delayed fracture preventing rolling cracks due to
the effect of the distribution of MnS etc. Addition of as much as
possible is preferable, so control of the amount of addition in the
extremely narrow range of 0.0002 to 0.001% is preferable.
[0112] Ca: 0.002% or less
[0113] Ca is an oxide- and sulfide-forming element. In spring
steel, it makes the MnS spherical to thereby suppress the length of
MnS serving as initiation sites of fatigue and other fracture and
render it harmless. The effect is similar to Mg. Addition of
0.0002% or more is preferable. Further, even if over 0.002% is
added, not only is the yield poor, but also oxides and CaS and
other sulfides are formed and trouble in production and degradation
in spring fatigue durability characteristics are caused, so the
amount was made 0.002% or less. Regarding the amount of addition,
when used for a high strength valve spring, the inclusion
susceptibility is high, so the amount is preferably 0.0015% or
less, more preferably 0.001% or less.
[0114] Zr: 0.003% or less
[0115] Zr is an oxide-, sulfide-, and nitride-forming element. In
spring steel, the oxides are finely dispersed, so like with Mg,
form nuclei for precipitation of MnS and can make the MnS finely
disperse. Due to this, it is possible to improve the fatigue
durability and, further, increase the ductility to thereby improve
the coiling ability. 0.0002% or more is preferably added. Further,
even if over 0.003% is added, not only is the yield poor, but
oxides and ZrN, ZrS, and other nitrides and sulfides are formed and
trouble in production or degradation in the spring fatigue
durability characteristics is caused, so the amount is made 0.003%
or less. The amount of addition is preferably 0.0025% or less.
Furthermore, when used for high strength valve spring, there is
also the effect that the coiling ability is improved by the control
of sulfides, so addition is preferred, but to minimize the effects
on the dimensions of inclusions, suppression to 0.0015% or less is
preferable.
[0116] Note that, the above optionally added chemical compositions,
if contained in fine amounts, do not impair the effects of the
steel wire comprised of the basic chemical compositions of the
present invention.
[0117] Next, the metal structure of the steel wire for high
strength spring use of the present invention will be explained.
[0118] Undissolved Spherical Carbides
[0119] Undissolved spherical carbides perform the important role of
securing strength in steel wire for high strength spring use. On
the other hand, the presence of undissolved spherical carbides
causes the coiling ability to deteriorate. Further, coarse carbides
cause the fatigue characteristics to degrade as well. Therefore,
suppressing undissolved spherical carbides in coiling and causing
uniform dispersion of fine carbides after the final nitriding are
essential for solving the problem of the present invention.
[0120] The steel wire for high strength spring use of the present
invention has a long size of the undissolved spherical carbides of
0.2 .mu.m or less, that is, is suppressed in coarsening. The
undissolved spherical carbides are already present after wire rod
rolling (that is, the pre-drawn steel wire).
[0121] The undissolved spherical carbides are hard to go into
solid-solution in the subsequent heat treatment (patenting,
generation of working heat in drawing, and quenching and tempering,
for instance). Rather, they sometimes grow in these heat treatment
steps and coarsen. That is, the undissolved spherical carbides in
the pre-drawn steel wire sometimes act as nuclei for coarsening of
themselves.
[0122] For this reason, to restrict the coarsened undissolved
spherical carbides of the steel wire after heat treatment (heat
treated steel wire), it is important to reduce as much as possible
the undissolved spherical carbides which are present in the
pre-drawn steel wire. Due to the above, the definition regarding
the "undissolved spherical carbides" has important meaning in not
only the pre-drawn steel wire for high strength spring use
according to the present invention, but also the drawn heat treated
steel wire for high strength spring use.
[0123] The steel wire for high strength spring use of the present
invention is increased in strength by having C added, having Mn and
Cr added and, further having Mo, W, and other so-called alloy
elements added. When adding large amounts of C and, in particular,
Cr and other alloy elements which form nitrides, carbides, and
carbonitrides, spherical cementite carbides and alloy-based
carbides easily remain in the steel. Spherical cementite carbides
and alloy-based carbides are undissolved spherical carbides which
do not dissolve in the steel in heating in the hot rolling.
[0124] Note that, in the present invention, spherical alloy-based
carbides and spherical cementite carbides will be referred to all
together as spherical carbides. In the steel, there are pin-shaped
carbides corresponding to the pin-shaped structure of tempered
martensite, but these pin-shaped carbides are not included in the
spherical carbides of the present invention. The pin-shaped
carbides are not present right after quenching and precipitate in
the process of tempering. The tempered martensite structure is a
structure suitable for achieving both strength and toughness and
workability. Being pin-shaped is, in a certain sense, the ideal
form in carbides.
[0125] Strictly speaking, if carbides with an aspect ratio of 2 or
more (pin-shaped carbides) also coarsen, the workability may be
impaired. However, in actuality, pin-shaped carbides become coarse
when the tempering temperature is high or the holding time in
tempering is extremely long. The effect on performance is to make
the strength and hardness insufficient. Problems arise in different
areas than with undissolved spherical carbides. In the 2100 MPa or
so strength steel wire covered by the present invention, coarse
pin-shaped carbides are not formed. Therefore, in the present
invention, pin-shaped carbides are not covered. As explained above,
the normally precipitated carbides are undissolved, but in the
present invention, the term "undissolved" added to the top. This
just stresses the undissolved nature. In the present invention,
"undissolved spherical carbides" and "spherical carbides" are
synonymous.
[0126] The undissolved spherical carbides can be observed under a
scanning electron microscope (SEM) by polishing a sample obtained
from pre-drawn steel wire or drawn heat treated steel wire for high
strength spring use to a mirror finish and etching it by picral or
electrolytically etching it. Further, they can be observed by the
replica method under a transmission type electron microscope
(TEM).
[0127] FIG. 1 shows an example of a structural photograph of a
sample after electrolytic etching as observed under an SEM. In the
structural photograph of FIG. 1, the steel is observed to have two
types of structures of the matrix, that is, pin-shaped structures
and spherical structures. Among these, the pin-shaped structures
are tempered martensite formed by quenching and tempering. On the
other hand, the spherical structures are carbides 1 made spherical
by not dissolving into the steel due to the heating of the hot
rolling and by being made spherical by quenching and tempering by
oil tempering or induction hardening treatment (undissolved
spherical carbides). Spherical carbides can be observed at the
front end of the arrow in FIG. 1.
[0128] Circle Equivalent Diameter of Undissolved Spherical Carbides
of Less Than 0.2 .mu.m
[0129] In the present invention, the undissolved spherical carbides
affect the characteristics of the drawn heat treated steel wire for
high strength spring use, so are controlled in size as follows:
Note that, in the present invention, compared with the prior art,
further finer spherical carbides are defined for achieving both
higher performance and workability. Spherical carbides with a
circle equivalent diameter of less than 0.2 .mu.m are extremely
effective for securing the strength and temper softening resistance
of the steel.
[0130] On the other hand, spherical carbides with a circle
equivalent diameter of 0.2 .mu.m or more do not contribute to
improvement of the strength or temper softening resistance and
degrade the cold coiling ability. For this reason, the present
invention is characterized by not allowing the formation of
spherical carbides with a circle equivalent diameter of 0.2 .mu.m
or more.
[0131] The pre-drawn steel wire and drawn heat treated steel wire
of the present invention is characterized in that the undissolved
spherical carbides have a circle equivalent diameter of less than
0.2 .mu.m. For this reason, it is possible to secure strength while
securing workability as well.
[0132] As explained above, the pre-drawn steel wire has to be then
patented, drawn and heated, quenched and tempered, or otherwise
heat treated, so the undissolved spherical carbides may grow and
coarsen. For this reason, the circle equivalent diameter of the
undissolved spherical carbides in the pre-drawn steel wire is
preferably made smaller than 0.2 .mu.m.
[0133] From the results of experiments of the inventors, the circle
equivalent diameter of undissolved carbides of the pre-drawn steel
wire is confirmed to be able to be reduced to 0.18 .mu.m or less.
Further, it is also confirmed that if making the billet heating
temperature 1250.degree. C. or more, the diameter can be made 0.15
.mu.m or less.
[0134] Here, the methods of measuring the circle equivalent
diameter and density of presence of spherical carbides will be
explained. A sample which is taken from steel wire for high
strength spring use is polished and electrolytically etched. Note
that, the observed location is randomly selected near the center of
the radius of the heat treated wire rod (steel wire), that is, the
so-called "1/2R part", so as to enable elimination of special
conditions such as decarburization and center segregation. Further,
the measurement area is 300 .mu.m.sup.2 or more. In electrolytic
etching, the surface of the sample is corroded by electrolytic
action in an electrolytic solution (a mixture of acetyl acetone 10
mass %, tetramethyl ammonium chloride 1 mass %, and a balance of
methyl alcohol) using the sample as the anode and platinum as the
cathode using a current generator with a lower potential. The
potential becomes constant at a potential suitable for the sample
in the range of -50 to -200 mV vs SCE. For the steel wire of the
present invention, it is preferable that it become constant at -100
mV vs SCE.
[0135] The amount of power run can be found by the total surface
area of the sample.times.0.133 [c/cm.sup.2]. Note that, when
embedding the sample in a resin, not only the polished surface, but
also the area of the sample surface in the resin are added to the
total surface area of the sample. The power starts to be run, then
the sample is held for 10 seconds, then the power is stopped and
the sample is cleaned.
[0136] After that, the sample is observed under an SEM and a
structural photograph of the spherical carbides is taken. Under the
SEM, the structures which appear relatively white and which have a
ratio (aspect ratio) of the maximum size (long size) and minimum
size (short size) of 2 or less are the spherical carbides. The
magnification of the photograph taken under the SEM is X1000 or
more, while X5000 to X20000 is preferable. For the measurement
locations, 10 fields were randomly selected from locations at a
depth of about 0.5 to 1 mm from the surface of the wire rod while
avoiding the center segregation parts. The thus captured SEM
structural photographs were processed by image processing to
measure the minimum size (short size) and maximum size (long size)
of the spherical carbides seen in the measured fields and calculate
the circle equivalent diameter. The circle equivalent diameter is
the diameter when calculating the area of an undissolved carbide in
a field by image processing and converting it to a circle of the
same area. Further, it is also possible to measure the density of
presence of spherical carbides with a circle equivalent diameter of
0.2 .mu.m or more seen in the measurement field.
[0137] Metal Structure of Pre-Drawn Steel Wire For High Strength
Spring Use and Drawn Heat Treated Steel Wire
[0138] The metal structure of the drawn heat treated steel wire for
high strength spring use according to the present invention is
comprised of, by volume rate, over 6% to 15% of residual austenite
and a balance of tempered martensite. Fine inclusions are allowed.
The "fine inclusion" are oxides and sulfides. The oxides are
deoxidation products of Al and Si etc., while the sulfides
correspond to MnS, CaS, etc. Further, the balance of the tempered
martensite structure also includes undissolved spherical carbides
in fine amounts.
[0139] The prior austenite grain size number in the structure is
#10 or more, while the circle equivalent diameter of the spherical
carbides is less than 0.2 .mu.m.
[0140] Further, in the metal structure of the pre-drawn steel wire
for high strength spring use according to the present invention,
the pearlite structure accounts for 90% or more, preferably 95% or
more, more preferably 98% or more. A substantially 100% pearlite
structure is ideal.
[0141] Prior Austenite Grain Size Number: #10 or more
[0142] The drawn heat treated steel wire for high strength spring
use of the present invention is mainly comprised of tempered
martensite in structure. The prior austenite grain size has a great
effect on the characteristics. That is, if refining the grain size
of the prior austenite, due to the effect of grain refinement, the
fatigue characteristics and the coiling ability are improved. In
the present invention, to obtain sufficient fatigue characteristics
and coiling ability, the prior austenite grain size number is made
#10.
[0143] Refining the prior austenite is particularly effective for
improving the characteristics of the drawn heat treated steel wire
for high strength spring use. The prior austenite grain size number
is preferably made #11, more preferably #12. To refine the grain
size of prior austenite, it is effective to lower the heating
temperature of the quenching. Note that, the "prior austenite grain
size number" is based on JIS G 0551. If actually performing the
quenching by lowering the heating temperature and shortening the
time, the prior austenite grain size can be refined, but
unreasonable low temperature, short time treatment not only
increases the undissolved spherical carbides, but also sometimes
results in insufficient austenite transformation itself and
two-phase quenching. Conversely, sometimes the coiling ability and
the fatigue characteristics are lowered. For this reason usually
#13.5 is the upper limit.
[0144] Residual Austenite: Over 6% to 15% (volume rate)
[0145] The microstructure at the drawn heat treated steel wire for
high strength spring use after quenching and tempering is comprised
of tempered martensite, residual austenite, and a slight volume
fraction of inclusions (here, precipitates also expressed included
in inclusions). Residual austenite is effective for improving the
cold coiling ability. In the present invention, to secure the cold
coiling ability, the volume rate of the residual austenite is made
over 6%, preferably 7% or more, more preferably 8% or more.
[0146] On the other hand, if the residual austenite exceeds a
volume rate of 15%, the martensite which is formed due to the
work-induced transformation causes the cold coiling characteristics
to drop. Therefore, the volume rate of the residual austenite is
made 15% or less, preferably 14% or less, more preferably 12% or
less.
[0147] The volume rate of the residual austenite can be found by
the X-ray diffraction method and the magnetic measurement method.
Among these, the magnetic measurement method enables simple
measurement of the volume rate of the residual austenite, so is the
preferable measurement method. Here, the volume rate is measured,
but the obtained figures are the same as the area rate.
[0148] Note that, residual austenite is softer than tempered
martensite, so reduces the yield strength. Further, the
transformation induced plasticity is used to improve the ductility,
so this remarkably contributes to improvement of the cold coiling
ability. On the other hand, residual austenite often remains at the
segregated parts, prior austenite grain boundaries, and near
regions sandwiched by the sag grains, so the martensite which is
formed by the work-induced transformation (work-induced martensite)
becomes starting points of fracture. Further, if the residual
austenite increases, the tempered martensite falls relatively.
[0149] For this reason, in the past, the drop in the strength and
cold coiling ability due to the residual austenite had been
considered an issue. However, in high strength steel wire of over
2000 MPa, the amounts of addition of C, Si, Mn, Cr, etc. become
greater, so for improvement of the cold coiling ability,
utilization of the residual austenite is extremely effective.
Further, recently, high precision spring working technology has
made it possible to suppress the deterioration of the coiling
characteristics even if high hardness parts are locally formed due
to the work-induced martensite formed in shaping the spring.
[0150] Next, the mechanical properties of the drawn heat treated
steel wire for high strength spring use of the present invention
will be explained.
[0151] To reduce the size and lighten the weight of a spring, it is
effective to make it higher in strength. Further, a spring is
required to have a superior fatigue strength. In the present
invention, a high strength spring is produced by bending the
material of the drawn heat treated steel wire for high strength
spring use to a desired shape, then nitriding, shot peening, or
otherwise hardening the surface. In the nitriding, the spring is
heated to 500.degree. C. or so, so the spring is sometimes softened
more than the material of the drawn heat treated steel wire for
high strength spring use.
[0152] Therefore, to raise the strength of the spring and improve
the fatigue characteristics, it is necessary to secure the yield
strength of the material of the drawn heat treated steel wire for
high strength spring use. Further, in order for the drawn heat
treated steel wire for high strength spring use to be worked into
the desired shape of a spring, cold coiling ability is demanded, so
the upper limit of the yield strength has to be limited.
[0153] Yield Strength: 2100 to 2400 MPa
[0154] If the drawn heat treated steel wire for high strength
spring use is high in yield strength, it is possible to improve the
fatigue characteristics and sag property of the spring hardened at
the surface by nitriding etc. In the present invention, to improve
the fatigue characteristics and sag property of the spring, the
yield strength of the drawn heat treated steel wire for high
strength spring use is made 2100 MPa or more.
[0155] Further, the higher the drawn heat treated steel wire for
high strength spring use in yield strength, the better the spring
in fatigue characteristics, so the drawn heat treated steel wire
for high strength spring use has a yield strength of preferably
2200 MPa or more, more preferably 2250 MPa or more.
[0156] On the other hand, if the drawn heat treated steel wire for
high strength spring use is too high in yield strength, the cold
coiling ability falls, so the yield strength is made 2400 MPa or
less.
[0157] Yield strength (if yield strength cannot be seen, 0.2% proof
stress): 1600 to 1980 MPa
[0158] In the present invention, the yield strength or yield point
of the drawn heat treated steel wire for high strength spring use
means the top yield strength when a yield point is seen at the
stress-strain curve in a single-axis tensile test and the 0.2%
proof stress when no yield point is seen. To secure the strength or
sag resistance of the spring, which elastically deformed by
repeated stress, raising the yield strength is preferable. To raise
the yield strength of the spring, raising the yield strength of the
material, that is, the drawn heat treated steel wire for high
strength spring use, is preferable.
[0159] On the other hand, if the drawn heat treated steel wire for
high strength spring use becomes high in yield strength, the cold
coiling ability is sometimes impaired. Therefore, the drawn heat
treated steel wire for high strength spring use preferably has a
yield strength of 1600 MPa or more for securing the strength and
sag property of the spring.
[0160] To impart further higher durability, 1700 MPa or more is
preferable.
[0161] On the other hand, if the yield strength exceeds 1980 MPa,
the cold coiling ability is sometimes impaired, so the yield
strength is preferably made 1980 MPa or less. Note that to raise
the yield strength of the drawn heat treated steel wire for high
strength spring use of the material having the same yield strength
right after short time quenching and tempering, it is preferable to
lower the volume the volume rate of the residual austenite.
[0162] Vicker's hardness after nitriding by holding at 500.degree.
C. for 1 hour: Surface layer hardness HV.gtoreq.750, internal
hardness HV.gtoreq.570
[0163] A high strength spring is improved in surface layer hardness
in nitriding, but the inside softens. For example, in gas soft
nitriding at 500.degree. C., if the conventional heating
temperature becomes 500.degree. C., it was difficult to suppress
softening at the inside of the drawn heat treated steel wire for
high strength spring use. The drawn heat treated steel wire for
high strength spring use of the present invention is excellent in
temper softening resistance and enables fatigue characteristics and
the sag property of the spring after heating at 500.degree. C. to
be secured. In the present invention, the surface layer hardness
and the internal hardness after gas soft nitriding are defined.
[0164] The surface layer hardness is made a micro Vicker's hardness
at the depth of 50 to 100 .mu.m from the surface layer of 750 or
more. If less than 750, the surface layer hardness becomes
insufficient and the fatigue durability also becomes inferior, so
residual stress after shot peening cannot be sufficiently imparted.
Preferably, the surface layer hardness is 780 or more.
[0165] On the other hand, in internal hardness, the Vicker's
hardness is sometimes measured when, in quenching, the temperature
of the surface layer of the steel wire is higher than the inside,
so measuring this at a position of 500 .mu.m depth from the surface
is preferable. To secure the spring fatigue characteristics and sag
property, the Vicker's hardness after heat treatment holding the
wire at 500.degree. C. for 1 hour should be 570 or more.
Furthermore, 575 or more is preferable.
[0166] Note that, the upper limit of the Vicker's hardness after
holding at 500.degree. C. for 1 hour for heat treatment is not
particularly defined, but to ensure that the Vicker's hardness
before the heat treatment is not exceeded, usually it is made 783
or less.
[0167] Furthermore, when using the drawn heat treated steel wire
for high strength spring use of the present invention as the
material for production of high strength springs, the surface layer
is hardened by shot peening, nitriding, etc. On the other hand, the
Vicker's hardness at a position of 500 .mu.m depth from the surface
of the high strength spring (internal hardness) is affected by the
heating in nitriding. Therefore, when actually producing a spring,
the internal hardness will fluctuate depending on the temperature
of the nitriding.
[0168] Note that, when using the drawn heat treated steel wire for
high strength spring use of the present invention as the material
for production of high strength springs, it is cold coiled and
nitrided. For this reason, the residual austenite at a position of
500 .mu.m depth from the surface of the high strength springs falls
somewhat compared with the material of the drawn heat treated steel
wire for high strength spring use.
[0169] However, the chemical compositions, spherical carbides, and
prior austenite crystal grain size are believed to be little
affected by the cold coiling and nitriding. Therefore, the chemical
compositions, spherical carbides, and prior austenite crystal grain
size of the high strength steel made using the drawn heat treated
steel wire for high strength spring use of the present invention as
a material are the same extent as the chemical compositions,
spherical carbides, and prior austenite crystal grain size of the
drawn heat treated steel wire for high strength spring use of the
present invention.
[0170] Next, the method of production of the drawn heat treated
steel wire for high strength spring use of the present invention
will be explained.
[0171] A steel bloom adjusted to predetermined chemical
compositions was rolled to produce a steel billet reduced in size.
Further, the billet was heated, then hot rolled to obtain pre-drawn
steel wire for high strength spring use. This pre-drawn steel wire
for high strength spring use was patented, the shaped and,
furthermore, was annealed for softening the hard layer. It was then
drawn, quenched, and tempered to produce drawn heat treated steel
wire for high strength spring use. The "patenting" is heat
treatment for making the structure of the steel wire after hot
rolling ferrite and pearlite and is performed for softening the
steel wire before drawing. After drawing, oil tempering, induction
hardening treatment, and other quenching and tempering are
performed to adjust the steel wire in structure and
characteristics.
[0172] In the method of production of pre-drawn steel wire for high
strength spring use of the present invention, the process of
preventing coarsening of the spherical carbides is important.
[0173] In particular, when containing high C and high Cr like in
the present invention, it is extremely important to sufficiently
heat the bloom or billet before rolling in that state and ease
precipitation inside the steel and to dissolve the internal coarse
carbides (alloy carbides and cementite) and make the material
uniform. To prevent the formation of coarse spherical carbides, the
coarse carbides which are formed at the bloom or billet must be
made to dissolve in the steel. Furthermore, causing uniform
dispersion in the steel is necessary. For this reason, raising the
heating temperature is preferable.
[0174] Therefore, first, the bloom or billet after casting is made
a heating temperature of 1250.degree. C. or more. Due to this, it
is possible to make the undissolved spherical carbides sufficiently
dissolve. For this reason, in the heating of the subsequent
rolling, patenting, and quenching, the heating temperature and the
heating time are insufficient, so undissolved spherical carbides
easily remain, but to enable sufficient dissolution from the start,
the dimensions of the undissolved spherical carbides can be
controlled to less than 0.2 .mu.m. The bloom heating temperature
should be 1270.degree. C. or more.
[0175] Next, the billet which is produced by rolling the bloom is
further hot rolled (wire rod is rolled) to produce pre-drawn steel
wire for high strength spring use. At this time, the heating
temperature of the billet is made 1200.degree. C. or more.
Preferably, the heating temperature of the billet should be made
1250.degree. C. or more.
[0176] After extracting the steel from the heating furnace, the
temperature falls and precipitates grow. For this reason, after
extraction from the heating furnace, the hot rolling is preferably
completed within 5 minutes. By the above heating of the bloom and
billet, the coarse carbides in the steel are uniformly dispersed
and dissolved and can uniformly finely precipitate in the later
precipitation.
[0177] Note that, when rolling a bloom into steel wire without
going through a billet, the heating temperature before rolling of
the bloom should be made 1250.degree. C. or more, more preferably
1270.degree. C. or more.
[0178] In the above way, to suppress coarsening of the undissolved
carbides of the steel wire after heat treatment, even if greatly
reducing the undissolved carbides which are present before drawing
(that is, after wire rod rolling) and if for example undissolved
carbides had been present, it is necessary to make the size finer
to prevent easy coarsening.
[0179] Therefore, in the rolling step of heating before drawing, it
is important to make the bloom heating temperature and the billets
heating temperature sufficiently high for the carbides to dissolve.
Due to this, the size of the undissolved spherical carbides can be
kept small. The rolling of the spring steel is completed in several
minutes from extraction of the billet from the heating furnace to a
size of material before drawing of about .phi.10 mm. For this
reason, it is important to heat to 1200.degree. C. or more where
the effect of the billet heating temperature is the largest.
1250.degree. C. or more is more preferable. 1270.degree. C. or more
is more preferable.
[0180] After rolling, the wire is taken up in a coil and air cooled
at that time as general practice. For this reason, usually the
microstructure of the pre-drawn steel wire (steel wire after
rolling of wire rod) is comprised of ferrite and pearlite or
pearlite with a high pearlite structure fraction since the amount
of C is high. Undissolved spherical carbides are present in the
base material.
[0181] The undissolved spherical carbides can be observed by
observing a polished and etched detection sample by an SEM. The
undissolved carbides can be clearly differentiated from the
lamellar cementite contained in the pearlite structure of the base
material since they are spherical. Of course, the magnitude may
also be measured.
[0182] Due to the above step, a pre-drawn steel wire for spring use
(rolled wire rod) is obtained.
[0183] After hot rolling, the pre-drawn steel wire for spring use
is patented. The heating temperature of this patenting may be made
900.degree. C. or more to promote dissolution of the carbides. A
high temperature of 930.degree. C. or more is more preferable.
Further, 950.degree. C. or more is preferable. After that, the wire
may be patented at 600.degree. C. or less. In the pre-drawn steel
wire for spring use according to the present invention, the method
of patenting and drawing is not limited. If a general patenting and
drawing method for steel wire, the same treatment as usual may be
performed.
[0184] When drawing by the wire diameter and precision required is
omitted, the patenting step before the drawing may be omitted. In
this case, by making the heating temperature in the later explained
quenching high (for example, 970.degree. C. or more), dissolution
of the undissolved spherical carbides is promoted.
[0185] The quenching after the drawing is performed by heating to
temperature of the A.sub.3 point or more. To promote the
dissolution of carbides, it is preferable to raise the heating
temperature of the quenching. In the quenching, to suppress the
growth of carbides, the heating rate is preferably made 10.degree.
C./sec or more and the holding time at the temperature of the
A.sub.3 point or more is preferably made 1 minute to 5 minutes. To
suppress grain growth of the austenite, it is preferable to shorten
the holding time. To promote the quenching and martensite
transformation, the cooling rate is preferably made 50.degree.
C./sec to 100.degree. C.
[0186] The coolant in the quenching process is preferably made
100.degree. C. or less, more preferably a low temperature of
80.degree. C. or less, but in the present invention, to precisely
control the amount of residual austenite, the coolant temperature
is made 40.degree. C. or more. The coolant is not particularly
limited so long as being an oil, a water soluble quenching agent,
water, or other coolant which enables quenching. Further, the
cooling time may be shortened like with oil tempering and induction
hardening treatment. It is preferable to avoid extending the
holding time at a low temperature for greatly reducing the residual
austenite and lowering the coolant temperature to 30.degree. C. or
less. That is, the quenching is preferably ended within 5
minutes.
[0187] After quenching, tempering is performed. The tempering
suppresses the growth of carbides, so it is preferable to make the
heating rate 10.degree. C./sec or more and make the holding time 15
minutes or less. The holding time fluctuates due to the chemical
compositions and the targeted strength, but the material is usually
held at 400 to 500.degree. C.
[0188] The pre-drawn steel wire for high strength spring use is
cold coiled to work it to the desired spring shape, is relieved of
stress, and is nitrided and shot peened to produce the spring.
[0189] The cold coiled steel wire is reheated by stress-relieving
annealing, nitriding, etc. At this time, the inside is softened, so
the performance of the spring falls. In particular, in the present
invention, even if performing the nitriding at a high temperature
of about 500.degree. C., sufficient hardness is maintained. As a
result, if using the pre-drawn steel wire for high strength spring
use of the present invention as a material, it is possible to make
the micro Vicker's hardness at a depth of 500 .mu.m from the
surface layer of high strength springs HV575 or more. Note that,
the micro Vicker's hardness is measured at a depth of 500 .mu.m
from the surface layer of the spring so as to evaluate the Vicker's
hardness of the base material not affected by nitriding and shot
peening for hardening.
EXAMPLES
[0190] Steels having the chemical compositions shown in Tables 1-1
to 1-4 were smelted in a 10 kg vacuum melting furnace and cast to
obtained blooms or billets. These vacuum melted materials were hot
forged up to .phi.8 mm. After that, the materials hot forged up to
.phi.8 mm were heated at 1270.degree. C..times.4 hr. Further, part
of the samples were refined in a 250 ton converter, continuously
cast to prepare blooms, then heated at 1270.degree. C..times.4 hr
or more, then made into cross-section 160 mm.times.160 mm billets.
Furthermore, these were rolled to obtained .phi.8 mm rolled wire
rods. The heating temperature of the billets before rolling was
made 1200.degree. C. or more.
[0191] A diameter 8 mm pre-drawn steel wire (rolled wire rod) is
preferably made an easily drawn structure by patenting it before
drawing. The heating temperature at the patenting is preferably
900.degree. C. or more so that the carbides etc. sufficiently
dissolve. The patenting is performed by heating at 930.degree. C.,
then charging the sample into a 600.degree. C. flowing bed. After
patenting, the wire is drawn to obtain a diameter 4 mm drawn wire
rod. In this way, by heating the bloom at a high temperature, then
making the temperature in the rolling process, patenting, and
quenching as high as possible, it is possible to suppress growth of
undissolved spherical carbides and keep the dimensions down to 0.2
.mu.m or less.
[0192] To adjust the yield strength of the patented and drawn steel
wire, the wire was quenched and tempered to produce pre-drawn steel
wire for spring use. Note that, a sample which broke in the drawing
was not quenched and tempered. The quenching and tempering were
performed by heating the drawn steel wire by a 10.degree. C./sec or
more heating rate at 950.degree. C. or 1100.degree. C. (temperature
of A.sub.3 point or more), holding at the peak heating temperature
for 4 minutes to 5 minutes, then placing the steel in a room
temperature water tank so that the cooling rate became 50.degree.
C./sec or more and cooling down to 100.degree. C. or less.
[0193] As the results of evaluation, the state of wire breakage,
prior austenite grain size number, residual austenite amount (vol
%), circle equivalent diameter and density of presence of carbides,
yield strength, 0.2% proof stress, notch bending angle, average
fatigue strength, and Vicker's hardness after gas soft nitriding
are shown.
[0194] The target values to be passed were made as follows with
reference to conventional steel wire for high strength spring
use.
[0195] Prior austenite grain size number: 10 degrees or more
[0196] Residual austenite amount (vol %): 20% or less
[0197] Circle equivalent diameter of spherical carbides: 0.2 .mu.m
or less
[0198] Yield strength: 2100 MPa or more
[0199] 0.2% proof stress: 1800 MPa or more
[0200] Yield ratio: 75% to 95%
[0201] Notch bending angle: 28 degrees or more
[0202] Average fatigue strength (Nakamura type rotating bending
strength): 900 MPa or more
[0203] Internal hardness by Vicker's hardness after gas nitriding:
590 Hv or more
[0204] Nitrided layer hardness by Vicker's hardness after gas
nitriding: 750 Hv or more
[0205] Note that, in the steel wire according to the present
invention, the strength and workability (coiling ability) both have
to be achieved, so if the yield ratio is too high, the workability
deteriorates. Therefore, the upper limit of the yield ratio is
preferably 90%, more preferably 88% or less.
[0206] A sample was taken from the obtained drawn heat treated
steel wire for spring use, evaluated for prior austenite grain
size, volume rate of residual austenite, and carbides, then was
subjected to a tensile test, notch bending test, and micro Vicker's
hardness test. Note that, the fatigue characteristics were
evaluated by treatment simulating production of a spring (below,
referred to as "spring production and treatment") including gas
nitriding simulating nitriding performed on the spring after
working (500.degree. C., 60 minutes), shot peening (diameter of cut
wire 0.6 mm, 20 minutes), and low temperature stress-relieving
treatment (180.degree. C., 20 minutes).
[0207] The prior austenite grain size number was measured based on
JIS G 0551. The circle equivalent diameter and density of presence
of the carbides were measured by using an electrolytically etched
sample, obtaining a SEM structural photograph, and analyzing the
image. Further, the volume rate of the residual austenite was
measured by the magnetic measurement method.
[0208] The fatigue test is a Nakamura type rotating bending fatigue
test (fatigue test bending by two-point supported weight and
turning by motor to apply compressive and tensile stress to surface
of wire). The maximum load force of 10 samples showing a lifetime
of 10.sup.7 cycles or more by a probability of 50% or more was made
the average fatigue strength. The notch bending test is a test for
evaluating the cold coiling ability and is performed as
follows.
[0209] A punch 2 with an angle of the tip shown in FIG. 2 of
120.degree. was used to provide a groove (notch) of a maximum depth
of 30 .mu.m in the test piece. Note that, as shown in FIG. 3, the
notch 4 was provided at a right angle to the longitudinal direction
at the center of the test piece 3 in the longitudinal direction.
Next, as shown in FIG. 4, from the opposite side of the notch 4, a
pusher 5 was used to apply a load P of a maximum tensile stress
through a load-use fixture 6 and the test piece was deformed by
three-point bending. Note that, the radius of curvature r of the
tip of the load-use fixture 6 was made 4.0 mm, while the difference
L between supports was made L=2r+3D. Here, D is the diameter of the
test piece.
[0210] The bending deformation continued to be applied until the
notch part fractured. The bending angle at the time of fracture
(notch bending angle) was measured as shown in FIG. 5. Note that,
when the test piece was split, the fractured parts were placed
together to measure the notch bending angle .theta.. In the present
invention, a sample with a notch bending angle of 28.degree. or
more is judged to be excellent in cold coiling ability.
[0211] The micro Vicker's hardness after nitriding was evaluated
using the depth of 500 .mu.m or more from the surface layer as the
internal hardness was defining the micro Vicker's hardness of a
depth of 50 .mu.m from the surface layer as the "nitrided layer
hardness". The measurement weight was 10 g.
[0212] The results of these tests are shown in Tables 1-5 to 1-8.
Note that, in Tables 1-5 to 1-8, the metal structure is comprised
of residual austenite (.gamma.) plus tempered martensite and slight
inclusions. Further, the balance of the chemical compositions was
iron and unavoidable impurities.
[0213] The pre-drawn steel wire (steel wire after rolling wire rod)
was evaluated only by the circle equivalent diameter of the
undissolved spherical carbides. This is because since this is
before heat treatment, even if measuring the mechanical properties
or the austenite grain size etc., there is not much meaning to the
figures.
[0214] Examples 1 to 47 of the present invention all have the
indicator of the cold coiling ability, that is, the notch bending
angle, of a good 28.degree. or more and have an excellent indicator
of the spring durability, that is, the Nakamura type rotating
bending fatigue strength (hereinafter simply referred to as the
"fatigue durability") and an excellent indicator of the sag
property and temper softening resistance, that is, the nitrided
layer hardness.
[0215] Comparative Examples 48 and 49 are examples where the amount
of addition of C is outside the range of the claims. If C is over
the prescribed amount (Comparative Example 48), the undissolved
spherical carbides become greater and the indicator of the cold
coiling ability, the notch bending angle, is low. On the other
hand, if C is smaller than the prescribed amount (Comparative
Example 49), a sufficient yield strength cannot be secured. In
particular, the internal hardness after nitriding becomes lower and
the spring fatigue durability (Nakamura type rotating bending
fatigue strength) and the sag property (internal hardness after
nitriding).
[0216] Comparative Examples 50 and 51 are examples where the amount
of addition of Si is outside the range of the claims. If Si exceeds
the prescribed amount, the matrix is embrittled and the workability
is impaired, that is, the notch bending angle is low. On the other
hand, if Si is smaller than the prescribed amount, the quenching
and tempering characteristics deteriorate, so sufficient strength
cannot be secured after heating by nitriding. In particular, the
internal hardness after nitriding and the nitrided layer hardness
become low.
[0217] Comparative Examples 52 and 53 are examples where the amount
of addition of Mn is outside the range of the claims. If Mn is over
the prescribed range, the residual austenite becomes greater, the
yield strength falls, and the fatigue durability (Nakamura type
rotating bending fatigue strength) is inferior. On the other hand,
when Mn is smaller than the prescribed amount, the residual
austenite falls too much and the workability deteriorates, so the
notch bending angle falls.
[0218] Comparative Examples 54 and 55 are examples where the amount
of addition of Cr is outside the range of the claims. If the Cr is
over the prescribed range, cementite stabilizes and even in the
high temperature heating of the bloom or billet, quenching and
tempering, etc., undissolved carbides increase and the spring
workability is greatly reduced. For this reason, the notch bending
angle falls. On the other hand, if Cr is smaller than the
prescribed amount, the steel ends up softening in the heat
treatment in the nitriding etc. and the so-called temper softening
resistance otherwise becomes insufficiently so the nitrided layer
hardness falls.
[0219] Comparative Examples 56, 57, and 58 are examples where the
amounts of addition of Mo, W, and Mo+W are over the ranges of the
claims. If Mo and W exceed the prescribed amounts, in rolling and
cooling and after patenting and other heat treatment, a supercooled
structure of martensite, bainite, etc. forms, the wire breaks in
the conveyance or drawing process, and the measurement test cannot
be performed.
[0220] Comparative Example 59 is an example of excessive addition
of V. V is an element which forms carbides in the steel. Excessive
addition causes undissolved carbides to form around the V, the
workability to deteriorate, and the notch bending angle to
fall.
[0221] Comparative Examples 60 and 61 are cases where the amount of
N is excessive compared with the range of the claims. This
excessive N raises the temperature of formation of nitrides and
carbonitrides of V, Nb, etc. and causes coarsening of carbides and
other precipitates using these as nuclei. Further, when used for
repeated heating such as in the present invention, the nitrides,
carbonitride, and carbides are incompletely dissolved and a large
amount of coarse undissolved spherical carbides remain. As a
result, the workability is impaired. This is an example where the
notch bending angle falls.
[0222] Comparative Examples 62 and 63 are examples where the amount
of addition of Nb is outside the range of the claims. If Nb exceeds
the prescribed amount, the hot ductility is remarkably impaired,
numerous surface flaws occur at the rolled material, wire breakage
occurs during drawing, and a measurement test could not be run.
[0223] Comparative Examples 64 is the case where the sum of the
amounts of addition of Mn and V is more than the range explained in
the present invention. The amount of residual austenite in the
steel wire becomes greater than the prescribed value. In the notch
bending test, the notch part hardens due to the stress-induced
transformation and the workability falls. This is an example where
the notch bending angle falls. While repeating ourselves, V is not
added in the present invention, but sometimes V is included as an
unavoidable impurity, so this is a limitation for rendering the V
harmless.
[0224] Comparative Examples 65 is the case where the sum of the
amounts of addition of Mn and V is lower than the range explained
in the present invention. The amount of residual austenite is
smaller than the optimum range, so the workability, that is, the
notch bending angle, falls.
[0225] Comparative Example 66 is the case where the sum of the
amounts of addition of Cr and V is greater than the scope explained
in the present invention. The undissolved spherical carbides
excessively remain and the workability, that is, the notch bending
angle, falls.
[0226] Comparative Example 67 is the case where the sum of the
amounts of addition of Cr and V is less than the range explained in
the present invention. The workability is excellent, but the
internal hardness after nitriding and the nitrided layer hardness
are insufficient and the spring performance is not sufficient.
[0227] Comparative Examples 68 to 70 are cases where the difference
between the amount of Si and the amount of Cr ([Si %]-[Cr %]) is
off from the scope of the claims and the amount of Cr is greater
than the amount of Si. If Cr is excessive with respect to the
amount of Si, undissolved spherical carbides remain and the
workability is degraded, that is, that is, the notch bending angle
falls.
[0228] Similarly, Comparative Examples 71 and 72 are the case where
the difference of the amount of Si and the amount of Cr ([Si %]-[Cr
%]) is larger than the upper limit of the range of the claims. Si
is very excessive compared with the amount of Cr. In these cases,
the surface layer decarburized layer of the rolled material greatly
grows and cannot be sufficiently removed by a slight amount of
surface layer shaving. For this reason, the fatigue durability
(Nakamura type rotating bending fatigue strength) was inferior.
[0229] Comparative Examples 73 and 74 are respectively the
Invention Example 1 and Invention Example 23 where the steel is
rolled at the billet heating temperature 1100.degree. C. At the
start of the rolling, undissolved spherical carbides remain. The
effects finally remain, so the workability is degraded, that is,
the notch bending angle falls.
[0230] Invention Examples 101 to 109 are examples of the pre-drawn
steel wires of Invention Examples 1 to 5 and 20 to 23. Comparative
Examples 110 and 111 are the Invention Examples 101 and 106 where
the billet heating temperature is made 1100.degree. C.
[0231] The pre-drawn steel wire is evaluated, so only the maximum
circle equivalent diameter of the undissolved spherical carbides is
evaluated. If the billet heating temperature is high, it is learned
that the circle equivalent diameter of the undissolved spherical
carbides becomes smaller.
TABLE-US-00001 TABLE 1-1 Chemical compositions (mass %) Ex. C Si Mn
P S Cr Al N V Nb 1 Inv. ex. 0.78 2.48 0.68 0.0076 0.0045 1.57
0.0022 0.0031 -- -- 2 Inv. ex. 0.77 2.41 0.68 0.0034 0.0047 2.05
0.0011 0.0042 -- -- 3 Inv. ex. 0.68 2.38 0.87 0.0045 0.0061 1.53
0.0013 0.0033 -- -- 4 Inv. ex. 0.88 2.50 0.87 0.0063 0.0071 1.71
0.0018 0.0035 -- -- 5 Inv. ex. 0.78 2.11 0.84 0.0057 0.0039 1.50
0.0010 0.0031 -- -- 6 Inv. ex. 0.72 2.62 0.62 0.0054 0.0031 1.96
0.0022 0.0038 -- -- 7 Inv. ex. 0.72 2.67 0.58 0.0037 0.0035 1.51
0.0015 0.0032 -- -- 8 Inv. ex. 0.76 2.28 1.02 0.0067 0.0069 1.82
0.0028 0.0031 -- -- 9 Inv. ex. 0.73 2.23 0.73 0.0054 0.0077 1.53
0.0015 0.0058 -- -- 10 Inv. ex. 0.77 2.35 0.80 0.0041 0.0047 1.52
0.0019 0.0032 -- -- 11 Inv. ex. 0.77 2.59 0.83 0.0060 0.0074 1.53
0.0012 0.0063 -- 0.008 12 Inv. ex. 0.75 2.52 0.68 0.0054 0.0056
1.67 0.0013 0.0039 0.06 -- 13 Inv. ex. 0.75 2.33 0.83 0.0066 0.0060
2.00 0.0030 0.0037 0.09 0.001 14 Inv. ex. 0.72 2.41 0.86 0.0040
0.0068 1.77 0.0013 0.0033 -- -- 15 Inv. ex. 0.77 2.57 0.75 0.0078
0.0075 1.91 0.0018 0.0043 -- -- 16 Inv. ex. 0.72 2.42 0.84 0.0044
0.0053 1.90 0.0019 0.0056 -- -- 17 Inv. ex. 0.76 2.53 0.67 0.0061
0.0076 1.65 0.0028 0.0039 -- -- 18 Inv. ex. 0.73 2.46 0.66 0.0051
0.0060 1.60 0.0023 0.0035 -- -- 19 Inv. ex. 0.73 2.34 0.75 0.0039
0.0071 1.97 0.0028 0.0032 -- -- 20 Inv. ex. 0.73 2.35 0.70 0.0046
0.0033 1.91 0.0016 0.0034 0.10 0.008 21 Inv. ex. 0.77 2.46 0.71
0.0031 0.0054 1.95 0.0013 0.0038 0.03 0.003 22 Inv. ex. 0.76 2.35
0.64 0.0051 0.0072 1.72 0.0016 0.0044 0.07 0.007 23 Inv. ex. 0.73
2.36 0.73 0.0033 0.0073 1.80 0.0019 0.0033 0.08 0.006 24 Inv. ex.
0.76 3.20 0.72 0.0076 0.0038 2.10 0.0010 0.0037 0.06 -- Chemical
compositions (mass %) Ex. Mo W Mg Zr Ca Mn + V Cr + V Si--Cr Mo + W
1 Inv. ex. -- -- -- -- -- -- -- 0.90 -- 2 Inv. ex. -- -- -- -- --
-- -- 0.35 -- 3 Inv. ex. -- -- -- -- -- -- -- 0.85 -- 4 Inv. ex. --
-- -- -- -- -- -- 0.79 -- 5 Inv. ex. -- -- -- -- -- -- -- 0.61 -- 6
Inv. ex. -- -- -- -- -- -- -- 0.66 -- 7 Inv. ex. -- -- -- -- -- --
-- 1.16 -- 8 Inv. ex. -- -- -- -- -- -- -- 0.46 -- 9 Inv. ex. -- --
-- -- -- -- -- 0.70 -- 10 Inv. ex. -- -- -- -- -- -- -- 0.83 -- 11
Inv. ex. -- -- -- -- -- -- -- 1.07 -- 12 Inv. ex. -- -- -- -- --
0.74 1.74 0.84 -- 13 Inv. ex. -- -- -- -- -- 0.93 2.10 0.32 -- 14
Inv. ex. 0.12 -- -- -- -- -- -- 0.64 0.12 15 Inv. ex. -- 0.15 -- --
-- -- -- 0.66 0.15 16 Inv. ex. 0.12 0.16 -- -- -- -- -- 0.52 0.27
17 Inv. ex. -- -- 0.0005 -- -- -- -- 0.87 -- 18 Inv. ex. -- -- --
0.0002 -- -- -- 0.86 -- 19 Inv. ex. -- -- -- -- 0.0011 -- -- 0.37
-- 20 Inv. ex. 0.12 0.17 0.0002 0.0003 0.0011 0.79 2.00 0.44 0.28
21 Inv. ex. 0.15 0.16 0.0003 0.0002 0.0003 0.74 1.98 0.51 0.31 22
Inv. ex. 0.11 0.17 0.0005 0.0003 0.0006 0.71 1.79 0.64 0.28 23 Inv.
ex. 0.11 0.15 0.0004 0.0001 0.0012 0.81 1.88 0.56 0.27 24 Inv. ex.
0.10 -- -- -- -- 0.77 2.16 1.10 0.10
TABLE-US-00002 TABLE 1-2 Chemical compositions (mass %) Ex. C Si Mn
P S Cr Al N V Nb 25 Inv. ex. 0.73 2.05 0.80 0.0062 0.0052 1.71
0.0011 0.0033 0.08 0.009 26 Inv. ex. 0.76 2.52 0.71 0.0069 0.0080
1.69 0.0024 0.0039 0.05 -- 27 Inv. ex. 0.75 2.30 0.82 0.0074 0.0055
1.73 0.0018 0.0038 0.08 0.010 28 Inv. ex. 0.73 2.24 1.20 0.0052
0.0074 1.52 0.0027 0.0033 0.09 -- 29 Inv. ex. 0.73 2.44 0.74 0.0076
0.0046 1.73 0.0024 0.0032 0.05 0.010 30 Inv. ex. 0.74 2.24 0.76
0.0052 0.0076 1.41 0.0016 0.0030 0.05 0.005 31 Inv. ex. 0.75 2.28
0.89 0.0065 0.0043 1.70 0.0011 0.0043 0.03 0.010 32 Inv. ex. 0.74
2.23 0.89 0.0043 0.0049 1.50 0.0022 0.0032 0.04 -- 33 Inv. ex. 0.77
2.86 0.77 0.0057 0.0057 2.48 0.0015 0.0040 0.09 0.005 34 Inv. ex.
0.77 2.29 0.79 0.0055 0.0072 1.85 0.0016 0.0032 0.09 0.004 35 Inv.
ex. 0.73 2.52 0.76 0.0078 0.0067 2.00 0.0022 0.0036 0.09 0.009 36
Inv. ex. 0.77 2.31 0.89 0.0046 0.0080 1.91 0.0021 0.0032 0.06 0.000
37 Inv. ex. 0.73 2.53 0.69 0.0046 0.0060 2.01 0.0022 0.0042 --
0.005 38 Inv. ex. 0.76 2.35 0.81 0.0034 0.0036 1.80 0.0016 0.0040
0.04 0.006 39 Inv. ex. 0.74 2.38 0.72 0.0031 0.0064 1.82 0.0010
0.0043 0.07 0.009 40 Inv. ex. 0.75 2.32 0.72 0.0034 0.0038 1.56
0.0017 0.0033 0.09 0.002 41 Inv. ex. 0.76 2.37 0.69 0.0056 0.0067
1.63 0.0028 0.0041 0.04 -- 42 Inv. ex. 0.76 2.48 0.73 0.0050 0.0035
1.97 0.0016 0.0034 0.09 -- 43 Inv. ex. 0.72 2.26 0.68 0.0053 0.0043
1.51 0.0027 0.0032 0.05 -- 44 Inv. ex. 0.76 2.38 0.86 0.0077 0.0039
1.61 0.0020 0.0052 -- -- 45 Inv. ex. 0.77 2.23 0.76 0.0060 0.0061
1.69 0.0029 0.0053 0.05 0.006 46 Inv. ex. 0.76 2.35 0.86 0.0060
0.0067 1.87 0.0025 0.0035 0.05 0.010 47 Inv. ex. 0.71 2.28 0.77
0.0050 0.0058 1.88 0.0030 0.0033 0.07 0.003 Chemical compositions
(mass %) Ex. Mo W Mg Zr Ca Mn + V Cr + V Si--Cr Mo + W 25 Inv. ex.
0.15 0.16 -- -- -- 0.88 1.79 0.34 0.31 26 Inv. ex. 0.28 -- 0.0002
0.0003 0.0015 0.76 1.74 0.84 0.28 27 Inv. ex. 0.13 0.15 0.0004
0.0001 0.0005 0.91 1.82 0.56 0.28 28 Inv. ex. 0.11 -- 0.0003 0.0001
0.0011 1.29 1.62 0.72 0.11 29 Inv. ex. 0.12 0.15 0.0001 0.0003
0.0013 0.79 1.78 0.71 0.27 30 Inv. ex. 0.11 0.16 0.0003 0.0001
0.0011 0.81 1.46 0.83 0.27 31 Inv. ex. 0.14 0.16 0.0004 0.0002
0.0012 0.93 1.73 0.58 0.31 32 Inv. ex. 0.11 -- 0.0001 0.0001 0.0002
0.93 1.54 0.73 0.11 33 Inv. ex. 0.11 0.16 0.0002 0.0002 0.0005 0.86
2.57 0.38 0.27 34 Inv. ex. 0.23 0.15 0.0004 0.0003 0.0006 0.88 1.93
0.45 0.38 35 Inv. ex. 0.18 0.28 0.0005 0.0001 0.0005 0.84 2.09 0.52
0.46 36 Inv. ex. 0.13 0.15 0.0001 0.0002 0.0006 0.95 1.97 0.40 0.28
37 Inv. ex. 0.14 0.17 0.0005 0.0002 0.0006 -- -- 0.52 0.31 38 Inv.
ex. 0.12 0.09 0.0004 0.0001 0.0009 0.85 1.84 0.55 0.21 39 Inv. ex.
0.11 0.28 0.0002 0.0002 0.0007 0.80 1.89 0.55 0.39 40 Inv. ex. 0.13
0.15 0.0004 0.0002 0.0012 0.81 1.65 0.76 0.28 41 Inv. ex. 0.14 0.17
0.0002 0.0002 0.0008 0.73 1.67 0.74 0.30 42 Inv. ex. 0.10 0.16 --
-- -- 0.82 2.06 0.51 0.26 43 Inv. ex. 0.14 0.14 0.0002 0.0003
0.0002 0.73 1.56 0.75 0.29 44 Inv. ex. 0.14 0.15 -- -- -- -- --
0.77 0.28 45 Inv. ex. 0.11 0.15 -- -- -- 0.81 1.74 0.54 0.26 46
Inv. ex. 0.10 0.15 0.0005 0.0003 0.0010 0.90 1.92 0.48 0.25 47 Inv.
ex. 0.10 0.17 0.0005 0.0003 0.0006 0.84 1.95 0.41 0.27
TABLE-US-00003 TABLE 1-3 Chemical compositions (mass %) Ex. C Si Mn
P S Cr Al N V Nb 48 Comp. ex. 0.95 2.47 0.61 0.0033 0.0078 1.54
0.0022 0.0043 0.09 0.006 49 Comp. ex. 0.58 2.59 0.62 0.0079 0.0057
1.81 0.0021 0.0066 0.09 0.005 50 Comp. ex. 0.71 3.80 0.85 0.0049
0.0035 2.03 0.0017 0.0032 0.05 0.002 51 Comp. ex. 0.73 1.86 0.83
0.0034 0.0060 1.32 0.0013 0.0031 0.09 0.004 52 Comp. ex. 0.72 2.46
1.54 0.0076 0.0066 2.02 0.0024 0.0043 0.06 0.008 53 Comp. ex. 0.72
2.22 0.21 0.0057 0.0064 1.58 0.0024 0.0036 0.09 0.005 54 Comp. ex.
0.73 3.13 0.71 0.0048 0.0030 2.72 0.0014 0.0030 0.04 0.007 55 Comp.
ex. 0.78 2.21 0.68 0.0051 0.0041 1.02 0.0013 0.0031 0.09 0.011 56
Comp. ex. 0.75 2.42 0.74 0.0034 0.0067 1.78 0.0017 0.0033 0.09
0.005 57 Comp. ex. 0.73 2.45 0.81 0.0046 0.0061 1.75 0.0027 0.0046
0.11 0.010 58 Comp. ex. 0.73 2.30 0.64 0.0044 0.0034 1.80 0.0011
0.0037 -- 0.001 59 Comp. ex. 0.74 2.23 0.80 0.0060 0.0046 1.72
0.0026 0.0032 0.46 0.007 60 Comp. ex. 0.77 2.43 0.83 0.0062 0.0072
1.96 0.0025 0.0076 0.08 -- 61 Comp. ex. 0.75 2.26 0.78 0.0069
0.0058 1.99 0.0013 0.0085 0.07 0.010 62 Comp. ex. 0.73 2.50 0.62
0.0049 0.0062 1.73 0.0020 0.0053 0.05 0.035 63 Comp. ex. 0.72 2.21
0.72 0.0074 0.0051 1.86 0.0026 0.0036 -- 0.024 64 Comp. ex. 0.74
2.50 1.18 0.0053 0.0056 2.02 0.0022 0.0042 0.09 0.006 65 Comp. ex.
0.74 2.52 0.51 0.0064 0.0046 1.73 0.0019 0.0039 0.06 0.003 66 Comp.
ex. 0.75 2.76 0.72 0.0048 0.0058 2.45 0.0014 0.0032 0.09 0.004 67
Comp. ex. 0.77 2.20 0.79 0.0059 0.0059 1.31 0.0016 0.0035 0.03
0.002 68 Comp. ex. 0.76 2.12 0.66 0.0068 0.0075 2.31 0.0026 0.0033
0.09 -- 69 Comp. ex. 0.74 2.10 0.88 0.0056 0.0056 2.23 0.0023
0.0043 0.06 0.005 70 Comp. ex. 0.78 2.23 0.73 0.0067 0.0075 2.41
0.0023 0.0048 0.11 0.000 71 Comp. ex. 0.71 3.12 0.76 0.0068 0.0044
1.54 0.0021 0.0035 0.07 0.005 72 Comp. ex. 0.76 3.45 0.66 0.0071
0.0052 1.66 0.0017 0.0036 0.04 0.005 73 Comp. ex. 0.78 2.48 0.68
0.0076 0.0045 1.57 0.0022 0.0031 -- -- 74 Comp. ex. 0.73 2.36 0.73
0.0033 0.0073 1.80 0.0019 0.0033 0.08 0.006 Chemical compositions
(mass %) Ex. Mo W Mg Zr Ca Mn + V Cr + V Si--Cr Mo + W 48 Comp. ex.
0.12 0.14 0.0004 0.0001 0.0013 0.70 1.63 0.93 0.26 49 Comp. ex.
0.15 0.15 0.0003 0.0001 0.0014 0.71 1.90 0.77 0.29 50 Comp. ex.
0.14 0.16 0.0002 0.0003 0.0008 0.90 2.09 1.77 0.31 51 Comp. ex.
0.14 0.14 0.0004 0.0003 0.0005 0.92 1.41 0.54 0.29 52 Comp. ex.
0.14 0.15 0.0003 0.0003 0.0014 1.60 2.08 0.44 0.29 53 Comp. ex.
0.15 0.17 0.0001 0.0002 0.0011 0.30 1.67 0.64 0.31 54 Comp. ex.
0.14 0.15 0.0003 0.0002 0.0011 0.75 2.76 0.41 0.29 55 Comp. ex.
0.14 0.17 0.0002 0.0001 0.0012 0.77 1.11 1.19 0.31 56 Comp. ex.
0.42 0.07 0.0004 0.0003 0.0005 0.83 1.87 0.63 0.49 57 Comp. ex.
0.12 0.50 0.0003 0.0001 0.0008 0.92 1.86 0.70 0.62 58 Comp. ex.
0.26 0.27 0.0005 0.0002 0.0007 -- -- 0.50 0.53 59 Comp. ex. 0.11
0.16 0.0004 0.0001 0.0015 1.26 2.18 0.51 0.27 60 Comp. ex. 0.15
0.17 0.0002 0.0001 0.0004 0.91 2.04 0.47 0.32 61 Comp. ex. 0.10
0.16 0.0003 0.0002 0.0005 0.85 2.06 0.27 0.27 62 Comp. ex. 0.13
0.17 -- -- -- 0.67 1.78 0.77 0.29 63 Comp. ex. 0.12 0.17 0.0002
0.0001 0.0012 -- -- 0.34 0.29 64 Comp. ex. 0.10 0.16 -- -- -- 1.27
2.11 0.48 0.26 65 Comp. ex. 0.12 0.16 0.0004 0.0002 0.0007 0.57
1.79 0.79 0.28 66 Comp. ex. 0.11 0.15 0.0002 0.0002 0.0015 0.81
2.54 0.31 0.26 67 Comp. ex. 0.14 0.16 0.0003 0.0001 0.0011 0.82
1.34 0.89 0.30 68 Comp. ex. 0.15 0.16 0.0003 0.0001 0.0013 0.75
2.40 -0.19 0.31 69 Comp. ex. 0.14 0.16 -- -- -- 0.94 2.29 -0.13
0.30 70 Comp. ex. 0.12 0.17 0.0004 0.0003 0.0013 0.84 2.52 -0.18
0.28 71 Comp. ex. 0.12 0.15 -- -- -- 0.84 1.61 1.58 0.26 72 Comp.
ex. 0.11 0.16 0.0004 0.0002 0.0002 0.70 1.70 1.79 0.26 73 Comp. ex.
-- -- -- -- -- -- -- 0.90 -- 74 Comp. ex. 0.11 0.15 0.0004 0.0001
0.0012 0.81 1.88 0.56 0.27
TABLE-US-00004 TABLE 1-4 Chemical compositions (mass %) Ex. C Si Mn
P S Cr Al N V Nb 101 Inv. ex. 0.78 2.48 0.68 0.0076 0.0045 1.57
0.0022 0.0031 -- -- 102 Inv. ex. 0.77 2.41 0.68 0.0034 0.0047 2.05
0.0011 0.0042 -- -- 103 Inv. ex. 0.68 2.38 0.87 0.0045 0.0061 1.53
0.0013 0.0033 -- -- 104 Inv. ex. 0.88 2.50 0.87 0.0063 0.0071 1.71
0.0018 0.0035 -- -- 105 Inv. ex. 0.78 2.11 0.84 0.0057 0.0039 1.50
0.0010 0.0031 -- -- 106 Inv. ex. 0.73 2.35 0.70 0.0046 0.0033 1.91
0.0016 0.0034 0.10 0.008 107 Inv. ex. 0.77 2.46 0.71 0.0031 0.0054
1.95 0.0013 0.0038 0.03 0.003 108 Inv. ex. 0.76 2.35 0.64 0.0051
0.0072 1.72 0.0016 0.0044 0.07 0.007 109 Inv. ex. 0.73 2.36 0.73
0.0033 0.0073 1.80 0.0019 0.0033 0.08 0.006 110 Comp. ex. 0.78 2.48
0.68 0.0076 0.0045 1.57 0.0022 0.0031 -- -- 111 Comp. ex. 0.73 2.36
0.73 0.0033 0.0073 1.80 0.0019 0.0033 0.08 0.006 Chemical
compositions (mass %) Ex. Mo W Mg Zr Ca Mn + V Cr + V Si--Cr Mo + W
101 Inv. ex. -- -- -- -- -- -- -- 0.90 -- 102 Inv. ex. -- -- -- --
-- -- -- 0.35 -- 103 Inv. ex. -- -- -- -- -- -- -- 0.85 -- 104 Inv.
ex. -- -- -- -- -- -- -- 0.79 -- 105 Inv. ex. -- -- -- -- -- -- --
0.61 -- 106 Inv. ex. 0.12 0.17 0.0002 0.0003 0.0011 0.79 2.00 0.44
0.28 107 Inv. ex. 0.15 0.16 0.0003 0.0002 0.0003 0.74 1.98 0.51
0.31 108 Inv. ex. 0.11 0.17 0.0005 0.0003 0.0006 0.71 1.79 0.64
0.28 109 Inv. ex. 0.11 0.15 0.0004 0.0001 0.0012 0.81 1.88 0.56
0.27 110 Comp. ex. -- -- -- -- -- -- -- 0.90 -- 111 Comp. ex. 0.11
0.15 0.0004 0.0001 0.0012 0.81 1.88 0.56 0.27
TABLE-US-00005 TABLE 1-5 Billet Patenting Quenching Wire break etc.
Max. Prior heating temp. heating temp. heating temp. [good = no
spherical carbide austenite grain Residual austenite Ex. (.degree.
C.) (.degree. C.) (.degree. C.) abnormality] diameter .mu.m size
(.gamma.#) (vol %) 1 Inv. ex. 1250 930 950 good 0.06 10 9 2 Inv.
ex. 1250 930 950 good 0.13 12 9 3 Inv. ex. 1250 930 950 good 0.06
11 8 4 Inv. ex. 1250 930 950 good 0.09 12 13 5 Inv. ex. 1250 930
950 good 0.13 13 11 6 Inv. ex. 1250 930 950 good 0.10 12 10 7 Inv.
ex. 1200 930 950 good 0.08 10 10 8 Inv. ex. 1250 930 950 good 0.02
10 6 9 Inv. ex. 1250 930 950 good 0.07 13 11 10 Inv. ex. 1200 930
1010 good 0.11 13 9 11 Inv. ex. 1250 930 1010 good 0.13 10 10 12
Inv. ex. 1250 930 1010 good 0.10 12 7 13 Inv. ex. 1250 930 950 good
0.10 13 13 14 Inv. ex. 1250 930 950 good 0.07 13 10 15 Inv. ex.
1250 930 950 good 0.10 11 12 16 Inv. ex. 1250 930 950 good 0.01 12
9 17 Inv. ex. 1250 930 950 good 0.12 11 13 18 Inv. ex. 1250 930 950
good 0.09 12 9 19 Inv. ex. 1250 930 950 good 0.11 10 7 20 Inv. ex.
1250 930 950 good 0.09 10 8 21 Inv. ex. 1250 930 950 good 0.04 12
12 22 Inv. ex. 1250 -- 970 good 0.04 10 12 23 Inv. ex. 1250 930 950
good 0.06 12 11 24 Inv. ex. 1200 930 950 good 0.10 11 11 Notch
Internal Hardness of Tensile bending deg. Nakamura type rotary
hardness after nitrided Ex. strength (MPa) 0.2% proof stress Yield
ratio (%) (deg.) bending (MPa) nitriding (HV) layer (HV) 1 Inv. ex.
2214 1887 85 36 927 597 788 2 Inv. ex. 2288 1939 85 39 918 638 787
3 Inv. ex. 2383 1945 82 37 924 599 819 4 Inv. ex. 2217 1811 82 39
915 621 799 5 Inv. ex. 2305 1844 80 36 918 627 817 6 Inv. ex. 2241
1899 85 39 918 623 816 7 Inv. ex. 2243 1978 88 39 911 618 793 8
Inv. ex. 2301 1811 79 38 914 600 789 9 Inv. ex. 2293 1880 82 35 928
607 806 10 Inv. ex. 2263 1801 80 37 912 612 795 11 Inv. ex. 2283
1939 85 37 929 608 808 12 Inv. ex. 2330 1844 79 38 913 616 780 13
Inv. ex. 2169 1904 88 39 925 627 819 14 Inv. ex. 2346 1854 79 35
919 612 808 15 Inv. ex. 2189 1917 88 36 924 599 804 16 Inv. ex.
2285 1822 80 38 916 625 805 17 Inv. ex. 2248 1950 87 40 929 595 787
18 Inv. ex. 2193 1904 87 38 925 616 804 19 Inv. ex. 2180 1949 89 36
928 598 786 20 Inv. ex. 2374 1891 80 39 924 612 817 21 Inv. ex.
2368 1846 78 36 911 638 806 22 Inv. ex. 2254 1853 82 40 914 629 795
23 Inv. ex. 2311 1852 80 39 923 625 783 24 Inv. ex. 2250 1956 87 35
911 616 793
TABLE-US-00006 TABLE 1-6 Billet Patenting Quenching Wire break etc.
Max. Prior heating temp. heating temp. heating temp. [good = no
spherical carbide austenite grain Residual austenite Ex. (.degree.
C.) (.degree. C.) (.degree. C.) abnormality] diameter .mu.m size
(.gamma.#) (vol %) 25 Inv. ex. 1250 930 950 good 0.10 10 8 26 Inv.
ex. 1250 930 950 good 0.12 11 13 27 Inv. ex. 1250 930 950 good 0.11
11 12 28 Inv. ex. 1200 930 950 good 0.05 11 13 29 Inv. ex. 1250 930
950 good 0.09 13 9 30 Inv. ex. 1250 930 950 good 0.04 11 7 31 Inv.
ex. 1250 930 950 good 0.07 12 10 32 Inv. ex. 1250 930 950 good 0.01
11 12 33 Inv. ex. 1200 930 950 good 0.02 10 10 34 Inv. ex. 1250 930
950 good 0.03 11 11 35 Inv. ex. 1250 930 950 good 0.01 12 9 36 Inv.
ex. 1250 930 950 good 0.05 12 13 37 Inv. ex. 1250 -- 970 good 0.01
10 9 38 Inv. ex. 1250 930 950 good 0.07 11 6 39 Inv. ex. 1250 930
950 good 0.15 12 6 40 Inv. ex. 1250 930 950 good 0.06 13 8 41 Inv.
ex. 1250 930 950 good 0.01 10 8 42 Inv. ex. 1250 930 950 good 0.06
13 6 43 Inv. ex. 1250 930 950 good 0.08 12 12 44 Inv. ex. 1250 930
950 good 0.06 12 8 45 Inv. ex. 1250 930 950 good 0.09 12 6 46 Inv.
ex. 1250 930 950 good 0.13 12 12 47 Inv. ex. 1250 930 950 good 0.03
11 7 Notch Internal Hardness of Tensile bending deg. Nakamura type
rotary hardness after nitrided Ex. strength (MPa) 0.2% proof stress
Yield ratio (%) (deg.) bending (MPa) nitriding (HV) layer (HV) 25
Inv. ex. 2357 1953 83 39 916 619 792 26 Inv. ex. 2195 1865 85 35
930 628 794 27 Inv. ex. 2226 1872 84 38 922 592 795 28 Inv. ex.
2181 1854 85 38 910 631 784 29 Inv. ex. 2277 1890 83 37 916 635 786
30 Inv. ex. 2330 1938 83 37 922 593 798 31 Inv. ex. 2305 1867 81 38
921 630 791 32 Inv. ex. 2216 1880 85 35 919 592 813 33 Inv. ex.
2285 1929 84 39 911 597 797 34 Inv. ex. 2329 1944 83 39 916 601 794
35 Inv. ex. 2310 1824 79 35 911 609 793 36 Inv. ex. 2153 1951 91 37
927 615 810 37 Inv. ex. 2220 1904 86 35 919 606 816 38 Inv. ex.
2178 1971 91 38 921 605 803 39 Inv. ex. 2268 1820 80 39 920 590 784
40 Inv. ex. 2302 1860 81 39 919 604 814 41 Inv. ex. 2190 1896 87 37
924 611 806 42 Inv. ex. 2218 1893 85 37 927 639 787 43 Inv. ex.
2382 1949 82 40 929 624 819 44 Inv. ex. 2269 1869 82 35 918 618 806
45 Inv. ex. 2155 1880 87 37 919 627 782 46 Inv. ex. 2314 1964 85 38
912 639 813 47 Inv. ex. 2220 1892 85 37 919 610 798
TABLE-US-00007 TABLE 1-7 Billet Patenting Quenching Wire break etc.
Max. Prior heating temp. heating temp. heating temp. [good = no
spherical carbide austenite grain Residual austenite Ex. (.degree.
C.) (.degree. C.) (.degree. C.) abnormality] diameter (.mu.m) size
(.gamma.#) (vol %) 48 Comp. ex. 1250 930 950 good 0.31 12 9 49
Comp. ex. 1250 930 950 good 0.02 13 7 50 Comp. ex. 1250 930 950
good 0.04 13 8 51 Comp. ex. 1250 930 950 good 0.12 11 7 52 Comp.
ex. 1250 930 950 good 0.08 12 21 53 Comp. ex. 1250 930 950 good
0.10 11 2 54 Comp. ex. 1250 930 950 good 0.26 13 10 55 Comp. ex.
1250 930 950 good 0.03 10 7 56 Comp. ex. 1250 930 950 wire Break --
-- -- 57 Comp. ex. 1250 930 950 wire break -- -- -- 58 Comp. ex.
1250 930 950 wire break -- -- -- 59 Comp. ex. 1250 930 950 good
0.42 12 7 60 Comp. ex. 1250 930 950 good 0.25 12 8 61 Comp. ex.
1250 930 950 good 0.13 12 6 62 Comp. ex. 1250 930 950 wire break --
-- -- 63 Comp. ex. 1250 930 950 wire break -- -- -- 64 Comp. ex.
1250 930 950 good 0.03 11 17 65 Comp. ex. 1250 930 950 good 0.11 11
3 66 Comp. ex. 1250 930 950 good 0.33 11 10 67 Comp. ex. 1250 930
950 good 0.03 10 9 68 Comp. ex. 1250 930 950 good 0.28 12 11 69
Comp. ex. 1250 930 950 good 0.22 13 6 70 Comp. ex. 1250 930 950
good 0.26 12 8 71 Comp. ex. 1250 930 950 Decarburized -- 12 9 72
Comp. ex. 1250 930 950 Decarburized -- 10 12 73 Comp. ex. 1100 930
950 good 0.26 10 9 74 Comp. ex. 1100 930 950 good 0.28 10 6 Notch
Internal Hardness of Tensile bending deg. Nakamura type rotary
hardness after nitrided Ex. strength (MPa) 0.2% proof stress Yield
ratio (%) (deg.) bending (MPa) nitriding (HV) layer (HV) 48 Comp.
ex. 2320 1857 80 24 915 617 807 49 Comp. ex. 2002 1806 90 39 812
554 813 50 Comp. ex. 2338 1841 79 23 926 612 812 51 Comp. ex. 2236
1954 87 36 915 568 728 52 Comp. ex. 2318 1680 75 37 820 612 816 53
Comp. ex. 2395 1971 82 25 923 637 801 54 Comp. ex. 2363 1909 81 23
920 629 795 55 Comp. ex. 2382 1840 77 39 919 608 731 56 Comp. ex.
-- -- -- -- -- -- -- 57 Comp. ex. -- -- -- -- -- -- -- 58 Comp. ex.
-- -- -- -- -- -- -- 59 Comp. ex. 2227 1885 85 21 929 635 815 60
Comp. ex. 2250 1881 84 17 914 640 816 61 Comp. ex. 2384 1830 77 17
916 606 817 62 Comp. ex. -- -- -- -- -- -- -- 63 Comp. ex. -- -- --
-- -- -- -- 64 Comp. ex. 2233 1560 70 21 786 561 783 65 Comp. ex.
2232 1860 83 24 911 606 815 66 Comp. ex. 2311 1838 80 21 916 613
783 67 Comp. ex. 2276 1977 87 43 789 561 732 68 Comp. ex. 2297 1934
84 24 923 612 809 69 Comp. ex. 2243 1838 82 22 914 629 813 70 Comp.
ex. 2329 1813 78 21 922 596 787 71 Comp. ex. 2359 1898 80 43 774
609 780 72 Comp. ex. 2371 1830 77 43 781 631 797 73 Comp. ex. 2114
1827 86 25 911 591 793 74 Comp. ex. 2251 1822 81 22 910 602 751
TABLE-US-00008 TABLE 1-8 Billet Patenting Quenching Wire break etc.
Max. Prior heating temp. heating temp. heating temp. [good = no
spherical carbide austenite grain Residual austenite Ex. (.degree.
C.) (.degree. C.) (.degree. C.) abnormality] diameter .mu.m size
(.gamma.#) (vol %) 101 Inv. ex. 1250 -- -- -- 0.03 -- -- 102 Inv.
ex. 1200 -- -- -- 0.15 -- -- 103 Inv. ex. 1250 -- -- -- 0.08 -- --
104 Inv. ex. 1250 -- -- -- 0.04 -- -- 105 Inv. ex. 1250 -- -- --
0.09 -- -- 106 Inv. ex. 1250 -- -- -- 0.03 -- -- 107 Inv. ex. 1250
-- -- -- 0.02 -- -- 108 Inv. ex. 1250 -- -- -- 0.03 -- -- 109 Inv.
ex. 1200 -- -- -- 0.14 -- -- 110 Comp. 1100 -- -- -- 0.21 -- -- ex.
111 Comp. 1100 -- -- -- 0.22 -- -- ex. Notch Internal Hardness of
Tensile bending deg. Nakamura type rotary hardness after nitrided
Ex. strength (MPa) 0.2% proof stress Yield ratio (%) (deg.) bending
(MPa) nitriding (HV) layer (HV) 101 Inv. ex. -- -- -- -- -- -- --
102 Inv. ex. -- -- -- -- -- -- -- 103 Inv. ex. -- -- -- -- -- -- --
104 Inv. ex. -- -- -- -- -- -- -- 105 Inv. ex. -- -- -- -- -- -- --
106 Inv. ex. -- -- -- -- -- -- -- 107 Inv. ex. -- -- -- -- -- -- --
108 Inv. ex. -- -- -- -- -- -- -- 109 Inv. ex. -- -- -- -- -- -- --
110 Comp. -- -- -- -- -- -- -- ex. 111 Comp. -- -- -- -- -- -- --
ex.
INDUSTRIAL APPLICABILITY
[0232] The present invention can be utilized for the production of
steel wire for high strength spring use. The high strength spring
material can be utilized in many industrial fields starting from
the automotive industry.
REFERENCE SIGNS LIST
[0233] 1 spherical carbides [0234] 2 punch [0235] 3 test piece
[0236] 4 notch [0237] 5 pusher [0238] 6 load use fixture [0239] P
load [0240] L distance between supports [0241] .theta. notch
bending angle
* * * * *