Note: Descriptions are shown in the official language in which they were submitted.
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LOWLY DECARBURIZABLE SPRING STEEL
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a spring steel for
use in the production of springs, and more particularly
to a spring steel having a remarkably low
decarburizability during the hot rolling thereof (rolling
of a bar stock or a plate material) as well as during
heating and heat treatment thereof for hot forming and
working thereof.
2. Description of the Prior Art
In general, flat springs, coil springs and torsion
bars are produced through quench hardening and heat
treatment such as tempering of a hot-rolled material. If
the surface of a steel material is decarburized in the
foregoing production process, the fatigue strength
thereof is notably lowered to present a problem that the
resulting product cannot exhibit the properties of a
spring.
Accordingly, a method wherein a coating layer of a
decarburization-proofing agent is formed on the surface
of a steel material, followed by heat treatment thereof,
a method wherein a steel material is heat-treated in an
atmosphere, and the like have heretofore been adopted as
decarburization-preventive technologies.
A method wherein a variety of elements) is added to
a steel material to lower the decarburizability thereof
has also been adopted. For example, Japanese Patent
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Laid-Open No. 170,542/1986 discloses Cu and Ni as such
additive elements, Japanese Patent Laid-Open No.
274,058/1987 discloses Sb as such an additive element,
and Japanese Patent Laid-Open No. 223,148/1988 discloses
Pb, Bi and Sn as such additive elements.
The conventional decarburization-preventive
technologies involve the following problems, because of
which an increase in cost cannot be avoided.
a. The method wherein a coating layer of a
decarburization-proofing agent is formed on the
surface of a steel material is time-consuming
because of the time for application and it
requires equipment therefor.
b. The method wherein a steel material is heat-
treated in an atmosphere is costly in connection
with the formation of an atmospheric gas and it
involves the use of an atmospheric heating
furnace, which is expensive as compared with a
common furnace .
SUMMARY OF THE INVENTION
According to the present invention, the combination
and amounts of Ni, Cu and S as trace components as are
usually contained in a spring steel in the production
thereof are controlled to lower the decarburizability
thereof, whereby a spring steel can be provided
inexpensively as compared with those produced according
to conventional methods.
The present invention provides a lowly
decarburizable spring steel comprising, on a weight
basis, 0.40 to 0.700 of C, 0.15 to 2.500 of Si, 0.40 to
1.20$ of Mn, 0.005 to 0.100% of Al, 0.005 to 0.0500 of S,
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at least one of 0.05 to 2.50$ of Ni and 0.05 to 1.00$ of
Cu and, optionally at least one of 0.20 to 1.50$ of Cr,
0.05 to 1.00$ of Mo, 0.01 to 0.50$ of V, 0.010 to 0.300$
of Nb and 0.0005 to 0.0050$ of B, and the balance
consisting of Fe and unavoidable impurities.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The reasons for specifying the components and the
contents thereof are as follows.
C: C is an element which is effective in enhancing
the strength of steel. When the C content is lower than
0.40$, the strength required of a spring cannot be
secured. When it exceeds 0.70k the resulting spring
becomes too brittle. Thus, it is set in the range of
0.40 to 0.70$.
Si: Si is an element which can function as a
deoxidizer during the melt production of steel and is
effective in improving the strength of steel through the
solid solution thereof in ferrite. When the Si content
is lower than 0.15$, a sufficient deoxidation function of
Si and the strength required of a spring cannot be
secured. When it exceeds 2.500, the toughness of steel
deteriorates. Thus, it is set in the range of 0.15 to
2.50$.
Mn: Mn is an element which is effective in
improving the hardenability of steel, for the purpose of
which the Mn content must be at least 0.400. When it
exceeds 1.200, however, the toughness of steel
deteriorates. Thus, it is set in the range of 0.40 to
1.200.
A1: A1 is an element which is a deoxidizer for
steel and is required in order to control the size of
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austenite crystal grains. When the A1 content is lower
than 0.00y, the crystal grains cannot be made fine. On
the other hand, when it exceeds 0.100$, the castability
of molten steel at the time of solidification thereof is
liable to lower. Thus, it is set in the range of 0.005
to 0.100.
S, Ni, Cu: S, Ni and Cu are concentrated under a
scale layer during the course of heating the spring steel
in a heating furnace to prevent C from escaping through
the surface of the steel, and hence are elements which
are effective in lowering the decarburizability thereof.
Further, three kinds of combinations of elements among
these three elements, i.e., S + Ni, S + Cu, and S + Ni +
Cu, when added, are especially effective in lowering the
decarburizability of steel. The S content must be at
least 0.005$ for securing the foregoing effect. When it
exceeds 0.050k however, the toughness of the steel is
lowered. Thus, it is set in the range of 0.005 to
0.050$. The Ni content must be at least 0.05 for
securing the foregoing effect. Ni is also an element
which is effective in improving the hardenability of
steel. When the Ni content exceeds 2.50k however, the
amount of residual austenite in a spring after the quench
hardening and tempering thereof is increased to adversely
affect the fatigue strength of the spring. Thus, it is
set in the range of 0.05 to 2.50. The Cu content must
be at least 0.05 for securing the foregoing effect.
When it exceeds 1.00o, however, the hot workability of
steel is lowered. Thus, it is set in the range of 0.05
to 1.00$.
A description will now be made of the reasons for
specifying the contents of Cr, Mo, V, Nb and B as
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components which improve the properties of a spring when
added in combination with the foregoing elements.
Cr: Cr is an element which is effective in
enhancing the strength of steel. When the Cr content is
lower than 0.20k the foregoing effect cannot
satisfactorily be expected. When it exceeds 1.50$, the
toughness of steel deteriorates. Thus, it is set in the
range of 0.20 to 1.50$.
Mo: Mo is an element which ensures a hardenability
of steel and enhances the strength and toughness of
steel. When the Mo content is lower than 0.05k the
foregoing effects cannot satisfactorily be expected.
When it exceeds 1.00k coarse carbide grains are liable
to precipitate to deteriorate the properties of the
resulting spring. Thus, it is set in the range of 0.05
to 1.00.
V: V is an element which enhances the strength of
steel. When the V content is lower than 0.01k the
foregoing effect cannot satisfactorily be expected. When
it exceeds 0.50k the amount of carbide undissolved in
austenite is increased to deteriorate the properties of
the resulting spring. Thus, it is set in the range of
0.01 to 0.50.
Nb: Nb is an element which makes crystal grains
fine and makes fine carbide grains precipitate to enhance
the strength and toughness of steel. When the Nb content
is lower than O.Olo, the foregoing effect cannot
satisfactorily be expected. On the other hand, when it
exceeds 0.300, the amount of carbide undissolved in
austenite is increased to deteriorate the properties of
the resulting spring. Thus, it is set in the range of
0.01 to 0.300.
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B: B is an element which improves the
hardenability of steel. When the B content is lower than
0.0005$, the foregoing effect cannot satisfactorily be
expected. On the other hand, when it exceeds 0.0050k
the foregoing effect is saturated. Thus, it is set in
the range of 0.0005 to 0.0050.
Examples
Steel samples having respective compositions as
shown in tables 1 and 2 were kept at 900~C for 30
minutes, quench-hardened, and then tempered at 455~C for
1 hour to measure the depth of the resulting decarburized
ferrite layer and the depth of the resulting entire
decarburized layer according to a JIS microscopic method.
The results are shown in Tables 3 and 4.
Table 1
Sample Ch emicalComponent (wt.~)
\o. C Si Mn A1 S Ni Cu Cr Mo V Nb B Fe
1 0.60 2.00 0.90 0.007 0.007 0.08 - - - - - -
balance _
2 U.58 2.45 0.86 U.028 0.030 - 0.06- - - - -
balance .
3 0.40 1.87 l.15 0.095 0.029 2.48 0.05- - - - -
balance
4 0.68 0.16 0.43 0.025 0.048 U.26 0.95- - - - -
balance
0.62 2.03 0.88 0.018 0.006 0.05 0.05- - - - - balance
6 0.57 0.22 0.83 0.023 0.005 - 0.070.82 - - - -
balance
7 0.5l 0.25 0.84 0.Q24 0.007 0.06 - 0.91 - 0.18 - -
balance
~
8 0.61 0.27 0.95 0.02i 0.014 0.12 0.150.98 - - - 0.00l0
balance c
,
9 ~0~.52 1.45 0.7Z 0.028 0.018 0.l4 0.090.77 - - - -
balance i
0.58 0.23 0.98 0.031 0.0l0 0.1l 0.090.84 0.30 - - - balance
11 0.51 0.22 0.78 0.Q26 0.0l1 0.05 0.080.96 - 0.l5 0.048 -
balance
.
12 0.56 1.5Z 0.45 0.020 0.006 0.06 0.070.81 0.10 0.20 0.028
balance
13 0.40 1.51 U.75 0.03U 0.0l5 2.45 0.050.98 0.78 0a10 0.0l3 -
balance
14 0.68 2.43 1.17 0.029 0.047 0.06 0.100.23 0.07 0.03 0.255 -
balance
0.55 0.17 0.82 0.098 0.007 0.10 0.09l.45 - - - 0.0045 balance
.-
16 0.56 1.52 0.42 0.020 0.009 0.08 0.090.90 0.95 - - -
balance
17 0.57 1.58 0.45 0.006 0.008 0.06 0.980.78 0.05 0.47 - -
balance
18 0.61 2.U5 U.92 0.U23 U.U02 0.08 0.07- -. - - -
balance
19 0.59 2.23 O.ss o.018 0.0l0 0.03 0.02- - - - -
balance
0.63 2.17 0.91 0.027 0.00l 0.09 - - - - - - balance
21 0.59 2.08 U.85 0.021 0.002 - 0.10- - - - -
balance
saTdumxg an-r~.as~dmo~ : LZ-8T 'sod aTdur~s
uoz~uanul ~uasa~a ~o saTdu~axg : LT-T ' sod aTdumg
a~.o~
aa~TE4 Z00'0 b0'0 0Z'0 I1'0 L6'0 0'0 b0'0 L00'0 S0'0 L'0 6b'T bS'0 LZ
d~
i
ao~lEq - tr0'06I'0 - b0'T SI'0 0T'0 I00'0 T0'0 06'0 8Z'0 ZS'0 9Z
d~ ~~T~q - 8Z0'00Z'0 0T'0 S8'0 60'0 - Z00'0
0Z0'0Sb'0 8S'1 S5'0 SZ
ao~t~q - 00'0 IZ'0 II'0 96'0 -~ 80'0 00'0 6Z0'0LG'0 ZS'I S'0 bZ
~~ElE4 - bZ0'06I'0 OI'0 Z8'0 Z0'0 0'0 Z00'0..Z0'0 Lb'0 TS'T 9S'0 Z
aauETeq- - - - - Z0'0 Z0'0 b00'0 t0'0 L8'0 OZ'Z 9S0 ZZ
a3 H MI A oY~I s~ n~ tI~I S td ~ iS ~
0
' tvi
(off ald~S
~~)
luaaod~uo~
~ay~
Z aTaas
~~.6~5'~9
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Table 3
Sample Depth of Depth of Whole
No. Dccarburized FerriteDecarburized Note
Layer (min) Layer
(mm)
1 0 0.03
2 0 0.03
3 0 0.00
4 0 0.00
0 0.03
6 0 0.02
7 0 0.02
8 0 0.00
9 0 0.0l Ex. of Present
Invention
0 0.00
11 0 0.01
12 0 0.02
13 0 0.00
14 0 0.02
0 0.01
16 0 0.01
17 0 0.00
18 0.03 0.07
19 0.02 0.06
Comp. Ex.
0.04 0.08
21 0.04 0.08
~.~64~'~9
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Table 4
Depth of Depth of Whoie
Sample Decarburized Decarburized Note
No. Ferrite Layer Layer (mm)
(mm)
22 0.04 0.09
23 0.04 0.07
24 0.03 0.05 Comp.
25 0.04 0.07 Ex.
26 - 0.01 0.04
27 0.01 0.04
l
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The results shown in Tables 3 and 4 demonstrate that
decarburization of ferrite in steel did not occur in any
of the Examples of the present invention, whereas
decarburization of ferrite in steel occurred in the
Comparative Examples, and that the depth of the entire
decarburized layer in steel was smaller in the Examples
of the present invention than in the Comparative
Examples.
It is also demonstrated that the steel samples of
the Examples of the present invention wherein components
were controlled in the range of the composition specified
in the claims, particularly with addition of S and Ni, S
and Cu, or S, Ni and Cu in combination, could very
effectively prevent decarburization of the ferrite and
decrease the depth of the entire decarburized layer.
The spring steel of the present invention can be
remarkably lowered in decarburizability during hot
working and heat treatment thereof without the need of
any decarburization-proofing agents and any specific heat
treatment equipment. Accordingly, it has a very
excellent effect that the decarburization thereof can be
remarkably decreased with a low cost when it is applied
to a coil spring, a flat spring or a torsfon bar.
