Wire conductors for automobiles

An electric wire conductor for use in automobiles made by twisting together a plurality of strands. Each of the strands has a surface layer made of copper or a copper alloy and a core made of steel containing 0.05-0.85 percent of carbon and other elements such as Si, Mn, Ni, Cr, Mo, Nb, V, B, Be, Al, or Ti and P, S and Cu. Each strand has a tensile strength of 80-160 kgf/mm.sup.2 before being twisted and a conductivity of 25% IACS or more. The conductor has a total sectional area of 0.05-0.30 mm.sup.2, a breaking load of more than 6 kgf, and an elongation at break not less than 2%.

This invention relates to a lightweight electric wire conductor for 
automobiles. 
As an electric wire conductor used for wiring in automobiles, wires made by 
twisting wires made of annealed copper (under JIS C 3102) or those plated 
with tin have heretofore been used. The wires are then covered with an 
insulating material such as vinyl chloride, crosslinked vinyl or 
crosslinked polyethylene. 
Modern cars have an increasing number of control circuits to achieve high 
performance and as a result the number of wiring points is increasing. 
This has led to an ever increasing demand for lighter wires while 
maintaining high reliability. Thus, the above-described conventional wire 
conductors are rapidly losing popularity. 
Although most of electric wires for control circuits have a permissible 
current of 1 ampere or less since they are used merely to pass signal 
currents, it was heretofore necessary to use wires having a larger 
diameter than electrically necessary in order to assure their mechanical 
strength. 
As one solution for achieving lightness in weight of such wires, 
consideration was given to the use of aluminum (including its alloy; all 
references to aluminum should be so understood hereinafter) as a material 
for the conductors. Also, wires made of copper alloy containing 0.3-0.9 
percent of tin and ones made of phosphor bronze containing 4-8 percent of 
tin have been developed (Japanese Patent Examined Publications 60-30043 
and 61-29133) and are now in actual use. 
Also, a wire conductor having a tensile strength of 90-140 kgf/mm.sup.2 and 
a load at break of 6 kgf or more has been developed. 
Since aluminum is poor in strength, the wires made of aluminum have to have 
an increased outer diameter or have to consist of an increased number of 
strands to be twisted together in order to assure a sufficient strength. 
This will lead to increase in the amount of insulating material used and 
in the wiring space, which will in turn result in the increased cost and 
make it more difficult to decrease the weight of the wires. 
The wiring in an automobile requires the use of a great number of 
terminals. This poses such problems as electrical corrosion at the 
terminals and deterioration of solderability. 
On the other hand, the wire conductors disclosed in the abovementioned 
publications show an increased strength due to the addition of tin to 
copper, which in turn makes it possible to reduce the sectional area of 
the conductor twisted together. But even with these wires the minimum 
value of the sectional area is 0.15-0.3 mm.sup.2. If lowered to the 
currently required level of 0.05-0.15 mm.sup.2, the strength will be 
insufficient. Even if strength is sufficient, the electrical resistance 
will be too large because the conductivity will be less than 20 percent 
IACS. 
Further, though the conductor wires having a tensile strength of 90-140 
kgf/mm.sup.2 and a load at break of 6 kgf or more have a satisfactory 
static strength, they are liable to break when subjected to impact tension 
during manufacture or mounting on a vehicle. 
It is an object of the present invention to provide an electric wire 
conductor for use in an automobile which is lighter in weight and 
reliable. 
The wire conductor according to this invention is made by twisting a 
plurality of element conductors having the following structure and 
composition after thermal treatment such as tempering or annealing so that 
their tensile strength will be 80-160 kgf/mm.sup.2 and the elongation at 
break E be 2% or more. The conductor after twisting has a sectional area D 
of 0.05-0.3 mm.sup.2, a load at break T of 6 kgf or more and an elongation 
at break E of 2% or more. 
The element wires used are composite wires having a conductivity of 25% 
IACS or more and having a surface layer made of copper or its alloy and a 
steel core containing 0.15-0.85% of carbon, one or more of the elements 
selected from the group consisting of Si, Mn, Ni, Cr, Mo, Nb, V, B, Be, Al 
and Ti in the amount of 0.05-0.3% Si, 0.3-1.9% Mn, 0.5-5.0% Ni, 0.2-2.0% 
Cr, 0.1-1.0% Mo. 0.01-0.2% Nb, 0.01-1.0% V, 0.001-0.006% B, 0.1-1.0% Be, 
0.02-1.0% Al, 0.02-1.0% Ti, a total of 0.05% or less of P and S as 
unavoidable impurities, and 0.3% or less of copper. Percent is all by 
weight. Carbon is added to the element wires to improve annealing 
properties and to increase the mechanical strength, Si, Mn and Al for 
deoxidation and to improve the annealing properties, Ni, Cr, B, Be and Mo 
to improve annealing properties and to prevent embrittlement due to 
tempering, and Nb, V and Ti to increase deposition. 
The composite element wires may be subjected to heat treatment after wire 
drawing and twisting. Also, they may be drawn after heat treatment or 
annealing to the extent that the t, T and E values clear required levels. 
The term heat treatment herein used refers to ordinary hardening or 
tempering or a continuous cooling transformation treatment such as 
austempering or martempering (FIG. 3). 
FIG. 1 shows the section of the wire conductor according to this invention 
in which the conductor 1 is made by twisting seven strands 2 each having a 
diameter d. In this figure, numeral 3 designates a steel wire as a core of 
each strand 2. A surface layer 4 is of oxygen-free copper covering the 
core 3. 
Supposing that the sectional area of the conductor is the same, it is 
desirable to use as many strands as possible to assure a good flexibility 
of the conductor. But it is troublesome to set a large number of fine 
strands on a twisting machine. Thus, the number of strands used should be 
2-37, preferably 7-19. 
The content of oxygen-free copper or copper alloy put on the outer 
periphery of the core of the strand should be 20-80 percent by weight. 
The conductivity of the strand should not exceed 80 percent IACS. 
By using a composite material having a covering of copper (or its alloy) as 
an element conductor or strand, the conductivity required (25 percent IACS 
or more) and good solderability are achieved by the covering. 
Also, since a steel wire containing 0.20-0.75 percent of carbon is used as 
the core, the conductor has a higher tensile breaking load T, a higher 
terminal housing retainability and a higher flexibility than conventional 
conductors. This makes it possible to reduce the sectional area and the 
weight of the conductor after twisting. 
According to this invention, the tensile strength t of each strand should 
be within 80-160 kg/mm.sup.2. This is because if less than 80 kg/mm.sup.2, 
the load at break of the conductor will be 6 kg or less, if the conductor 
is made up of seven strands and the total sectional area D is 0.1 
mm.sup.2. Such a wire will be more liable to breakage and cannot retain a 
terminal with a sufficient force. On the other hand, if more than 160 
kgf/mm.sup.2, it is impossible to achieve a 2% elongation E because the 
wire is as drawn. Considering the terminal retaining force, the tensile 
strength t should be preferably 90-140 kg/mm.sup.2. 
The elongation E at break of each strand or the twisted wire should be 2% 
or more. If less than 2%, the wire is liable to breakage due to impact 
tension during manufacture (when connecting to terminals or covering with 
vinyl) or when mounted on a car. The preferred E value should preferably 
be 3% or more. 
The conductivity of each strand should be 25 percent IACS or more. This 
value was obtained by calculating the permissible current from the 
electrical resistance of the conductor composed of strands having their 
surface layer of oxygen-free copper or copper alloy. Supposing that the 
lowest permissible current is 1 ampere, the conductivity should be 25 
percent or more, preferably 30-40 percent IACS or more. In order to 
maintain the required tensile strength by use of the composite material, 
the conductivity should not exceed 80 percent IACS. If larger than that, 
the tensile strength will be sacrificed. 
The total sectional area D of the conductor after twisting should be 
0.05-0.30 mm.sup.2. If more than 0.30 mm.sup.2, the required strength can 
be obtained even with a conventional conductor, but it is impossible to 
achieve decrease in weight. On the other hand, if less than 0.05 mm.sup.2, 
the conductor will be liable to deform by tensile force provided the 
conductor has a T value of 5 kg or less and is composed of seven strands 
having a diameter of 0.08 mm. More preferably, the D value should be 
0.07-0.20 mm.sup.2. 
With a conventional annealed copper wire, the lower limit of the total 
sectional area D is 0.5 mm.sup.2 in view of its mechanical properties. In 
case of a copper wire containing tin (0.3-0.9 percent), the lower limit of 
the D value is ordinarily 0.2 mm.sup.2. In contrast, according to the 
present invention, even if the D value is around 0.1 mm.sup.2, the 
strength equivalent to that of a conventional wire having a D value of 0.3 
mm.sup.2 can be expected. This will permit a considerable reduction in 
weight of the conductor (for example, if D is 0.1 mm.sup.2, the weight 
will be 60 percent less than the 0.3 mm.sup.2 structure. 
The content of carbon in the steel wire as the core of each strand should 
be 0.15-0.85 percent. If less than 0.15%, it is difficult to achieve both 
a tensile strength t of 60 kgf/mm.sup.2 or more and an elongation E of 2% 
or more, for any copper content. If more than 0.85%, the steel wire will 
be too hard to achieve sufficiently high tensile strength t and elongation 
e simultaneously and be difficult to thin. 
One or more of the following elements should be added. Each of the elements 
should have the following content for the reasons set forth below. 
Si: It should be 0.05% to 0.3%. If less than 0.05%, the effect of 
deoxidation will be insufficient. This leads to increase oxide content in 
the steel and poor elongation, poor hardening properties and insufficient 
strength. If more than 0.3%, the wire will be embrittled so much that the 
elongation will be lower than 2%. Also, due to embrittlement, the 
elongation properties will be poor. 
Mn: It should be 0.3% to 1.9%. If less than 0.3%, the effect of deoxidation 
and hardening property will be insufficient for the same reasons as for 
Si. Also, the improvement in the corrosion resistance, which is one of the 
secondary effects, cannot be expected so much. If more than 1.9%, the 
elongation will be insufficient and the elongation properties will worsen 
as in the case of Si. 
Ni: It should be 0.5 to 5.0%. If less than 0.5%, no improvement in the 
hardening properties can be expected. The improvement in corrosion 
resistance will not be expected. If more than 5%, the hardening properties 
will not be so high as to justify the increase in cost. 
Cr: It should be 0.2 to 2.0%. If less than 0.2%, the hardening properties 
will not be satisfactory and the strength of the wire will not be 
sufficient. The corrosion resistance will be bad, too. If more than 2.0%, 
the strength and elongation properties will not improve so much as to 
justify the increase in cost for heat treatment. 
B: It should be 0.001 to 0.006%. If less than 0.001, the hardening 
properties as well as strength will not improve. If more than 0.006%, the 
elongation will be poor due to embrittlement. This will make the thinning 
of wire difficult. 
Be: It should be 0.1 to 1.0%. If less than 0.1%, the hardening properties 
will not improve. If more than 1.0%, the wire will be embrittled. 
Mo: It should be 0.1 to 1%. If less than 0.1%, the hardening properties 
will not be good and embrittlement due to tempering cannot be prevented 
effectively. If more than 1%, the temperature for the steel to be 
transformed into austenite (Ms temperature) will rise excessively. This 
will increase the time for transformation so as not to justify the 
increased cost as compared with the effect achieved. 
Nb: It should be 0.01% to 0.2%. If less than 0.01%, it is difficult to 
harden the wire by the deposition of carbides or to reduce the crystal 
grain size. If more than 0.2%, the wire will be hardened excessively by 
the deposition of carbides. This will incur cost increase as in case of 
Mo. 
V: It should be 0.01% to 1.0%. If not, there will be the same problems as 
with Nb. 
Ti: It should be 0.02% to 1.0%. If less than 0.02%, the effect of improving 
hardening properties will be insufficient. If more than 1.0%, the 
hardening properties will worsen. This may make it difficult to obtain a 
thin wire by drawing. 
Al: It should be 0.02 to 1.0%. If less than 0.02%, the effect of 
deoxidation when dissolved will be insufficient. If more than 1.0%, oxides 
such as Al.sub.2 O.sub.3 will develop, causing such trouble as wire 
breakage during wire drawing. 
P and S: These elements should be contained as impurities after dissolution 
in the amount of 0.05% or less. 
Cu: This element improves corrosion resistance if contained in trace 
amounts. But if more than 0.3%, the wire may develop cracks during hot 
processing or it may crack or be broken during drawing for thinning after 
heat treatment. 
According to this invention, the weight of the electric wire conductor can 
be reduced remarkably while keeping the mechanical properties such as 
terminal housing retaining force, tensile breaking load and flexing 
resistance, electrical properties and solderability at satisfactory 
levels. This prevents increases in the weight of wires and space for 
wiring with increase in the wiring points, thereby reducing the amount of 
insulating material used and thus the cost. 
Further, as is well known, by the addition of Cr, Mn, Ni or Cu, the 
corrosion resistance of the core can be improved effectively.

As core materials, different kinds of steel rods having a diameter of 8.0 
mm and different contents of carbon and other elements as shown in Table 1 
were prepared. As the covering copper tubes, tubes made of oxygen-free 
copper (under JIS 3510) (hereinafter referred to as OFC tubes) were 
prepared. These covering copper tubes were straight tubes having an 
external diameter of 16 mm and an internal diameter of 12 mm. 
Next, in order to make composite strands from these materials, the steel 
rods were inserted into the OFC tubes while dry-polishing (shot blast 
polishing) their surfaces. The resulting materials were drawn by a die to 
reduce the diameter to about 10 mm. The copper composite materials thus 
obtained showed a conductivity of about 30% for specimen No. 1, about 
38-40% for specimen Nos. 2-9 and 11-16 and 30% specimen No. 10. 
These materials were subjected to repeated drawings and softenings to 
reduce the diameter to 0.13 mm. As the final softening step, specimen Nos. 
1, 3-8, 10, 13, 14 and 16 were subjected to water quenching starting from 
890.degree. C. and tempering at 200.degree. C., speciment No. 2 was 
subjected to continuous cooling transformation treatment starting from 
890.degree. C., and specimen No. 9 was subjected to water quenching from 
890.degree. C., tempering at 400.degree. C. and light wire drawing. 
Specimen Nos. 11-17 were prepared for comparison purposes, of which 
specimen No. 11 was drawn to a high degree and its elongation at break was 
less than 2%. 
No. 12 contained carbon in the amount not within the range defined by the 
present invention. It was drawn to a high degree and its elongation at 
break was less than 2%. 
No. 13 also contained carbon in the amount not within the range defined by 
the present invention. Its tensile strength was less than 160 kgf/mm.sup.2 
even after subjecting it to final heat treatment. 
No. 14 contained carbon in the amount within the range defined in the 
present invention but the contents of other additives were not within the 
range defined by the present invention. Thus, its tensile strength was 
less than 60 kgf/mm.sup.2. 
In No. 15, the content of carbon and the contents of Si, Mn, Cr, Nb, Ni and 
V were all within the range defined in the present invention. But the 
degree of drawing was high. Its elongation at break was less than 2%. 
In No. 16, the content of carbon was within the range defined by the 
present invention but those of Si, Mn, Cr, Nb, Ni, Al and Mo were not. 
Thus, wire breakage happened frequently during wire drawing and while 
assembling a harness due to the existence of oxides (inclusions) in the 
core. 
The tensile strength t and conductivity of the thus obtained strands are 
shown in Table 1. 
Thereafter, seven strands of each specimen were twisted together to form 
wire conductors having a total sectional area D of 0.08-0.1 mm.sup.2. They 
were then covered with vinyl chloride to a thickness of 0.2 mm for use as 
electric wires in automobiles. 
Various characteristics of these wire conductors are shown in Table 4 
together with those of conventional and comparative conductors. 
For electric wires for automobiles, the terminal housing retaining force is 
an important property for high reliability of the connecting portions to 
terminals. To evaluate this property, after connecting each conductor to a 
terminal by compressed bonding, it was pulled by a tension tester to 
measure the load when it comes out of the connecting portion (or when it 
is broken). Such retaining force should be 7 kg or more, preferably 10 kg 
or more. 
Also, the tensile breaking load should preferably be about 10 kg or more as 
far as the elongation of the conductor is 3% or more. 
Also, the electric wire should have a flexing resistance high enough not to 
get broken when bent repeatedly near the terminal. To measure the flexing 
resistance, an electric wire 5 having a covering was held by a jig 6 shown 
in FIG. 2 and bent right and left by an angle of 90 degrees in each 
direction with the load W of 500 g put on one end thereof. The flexing 
resistance was given in terms of the number of reciprocating motions of 
the wire done without being broken. 
As for the solderability, after immersing the specimens in white rosin 
flux, they were immersed in eutectic solder kept at 230.degree. C. for 2 
seconds and the area ratio of the surface wet with molten solder to the 
entire immersed surface area was measured. A good mark was given for 90% 
or more and a bad mark was given for less than 90 %. 
As is apparent from the data in the Tables, comparing the electric wires 
according to the present invention with the conventional wires, the 
conductors having a total sectional area of 0.3 mm.sup.2 (specimen No. 19) 
weigh 4.5 g/m whereas the conductors having a total sectional area of 0.1 
mm.sup.2 (specimen Nos. 1-4) weigh 1.4-1.5 g/m. In other words, the weight 
reduced by about 3.0 g/m or 70 percent. As for the strength, the wires 
according to the present invention were substantially the same as the 
conventional wires. 
TABLE 1 
__________________________________________________________________________ 
Specimen 
Material for Composition of core (wt %) 
No. conductor C Si Mn P S Cu Cr B Al Nb Ni Mo V 
__________________________________________________________________________ 
Present invention 
1 OFC-clad 0.3% C steel 
0.3 
0.27 
1.86 
0.015 
0.008 
0.01 
0.91 
0.002 
-- -- -- -- -- 
(hardened and tempered) 
2 OFC-clad 0.3% C steel 
" " " " " " " " -- -- -- -- -- 
(continuous cooled, 
transformed) 
3 OFC-clad 0.4% C steel 
0.45 
0.22 
0.7 
0.013 
0.009 
-- 1.05 
-- -- -- -- -- -- 
(hardened and tempered) 
4 OFC-clad 0.25% C steel 
0.23 
" 0.7 
" " 0.01 
0.98 
-- -- -- -- 0.40 
-- 
(hardened and tempered) 
5 OFC-clad 0.3% C steel 
0.3 
" 0.5 
" " -- 0.90 
-- -- -- 2.7 
-- -- 
(hardened and tempered) 
6 OFC-clad 0.4% C steel 
0.45 
0.22 
0.8 
" " -- 0.91 
-- -- -- 1.8 
0.18 
-- 
(hardened and tempered) 
7 OFC-clad 0.2% C steel 
0.22 
0.21 
0.8 
0.015 
0.008 
0.01 
0.91 
0.003 
-- -- 0.8 
0.50 
0.07 
(hardened and tempered) 
8 OFC-clad 0.42% C steel 
0.44 
0.21 
0.62 
0.016 
0.015 
0.02 
-- -- -- -- -- -- -- 
(hardened and tempered) 
9 OFC-clad 0.25% C steel 
0.25 
0.26 
1.20 
0.012 
0.01 
0.01 
-- -- 0.03 
0.07 
-- -- -- 
(15% drawn after 
hardened and tempered) 
10 OFC-clad 0.6% C steel 
0.6 
0.21 
0.47 
0.012 
0.004 
0.005 
-- -- -- -- -- -- -- 
(hardened and tempered) 
Comparative example 
11 OFC-clad 0.42% C steel 
0.44 
0.21 
0.62 
0.016 
0.015 
0.02 
-- -- 
-- -- -- -- -- 
(drawn) 
12 OFC-clad 0.1% C steel 
0.12 
0.25 
0.9 
0.015 
0.009 
0.02 
-- -- -- -- -- -- -- 
(drawn) 
13 OFC-clad 0.1% C steel 
0.12 
0.25 
1.3 
0.015 
0.015 
0.02 
0.91 
0.002 
-- -- -- 0.50 
-- 
(hardened and tempered) 
14 OFC-clad 0.20% C steel 
0.24 
0.03 
0.1 
0.015 
0.015 
0.02 
0.11 
-- 0.01 
-- 0.2 
0.05 
-- 
(hardened and tempered) 
15 OFC-clad 0.2% C steel 
0.24 
0.28 
1.85 
0.013 
0.008 
0.01 
1.01 
-- -- 0.1 
0.8 
-- 0.09 
(drawn) 
16 OFC-clad 0.42% C steel 
0.44 
0.03 
0.2 
0.04 
0.02 
0.02 
0.10 
-- 0.01 
-- -- 0.05 
-- 
(hardened and tempered) 
17 Aluminium (drawn) 
-- 
Prior art 
18 Tough pitch soft copper 
-- 
19 CU-0.62% Sn alloy 
-- 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
Element conductor's 
characteristics 
Conductor 
Structure 
Conduc- 
Tensile 
Elon- 
Conductor 
Specimen 
outer dia. 
of tivity 
strength 
gation 
weight 
No. (mm) conductor 
(IACS %) 
(kgf/mm.sup.2) 
(%) (g/m) 
__________________________________________________________________________ 
Present invention 
1 0.4 7/0.13.phi. 
30 120 7.5 0.84 
2 " " 38 110 8.0 0.85 
3 " " " 120 5.0 0.85 
4 " " " 108 8.0 0.85 
5 " " " 109 7.5 0.85 
6 " " " 115 7.5 0.85 
7 " " " 108 8.5 0.85 
8 " " 40 115 5.5 0.85 
9 " " 40 110 45 0.85 
10 " " 60 108 4.8 0.86 
Comparative example 
11 " " 40 120 1.0 0.85 
12 " " 40 90 0.8 0.85 
13 " " 40 53 10 0.85 
14 " " 40 55 10 0.85 
15 " " 38 125 0.8 0.85 
16 " " 40 85 3.3 0.85 
17 0.96 7/0.32.phi. 
63 23 2.5 1.5 
Prior art 
18 0.78 7/0.26.phi. 
100 28 20 3.4 
19 0.6 7/0.2.phi. 
60 55 18 2.0 
__________________________________________________________________________ 
Wire Terminal 
Flexing 
weight 
Conductor 
housing 
resist- Wire broken 
after 
tensile 
holding 
ance when 
Specimen 
covering 
break load 
force 
(number 
Solder- 
assembling 
No. (g/m) 
(kgf) (kgf) 
of times) 
ability 
harness? 
__________________________________________________________________________ 
Present invention 
1 1.4 11.5 10.8 8850 Good 
No 
2 1.4 11.0 10.4 8860 " " 
3 " 11.3 10.9 8250 " " 
4 " 10.5 10.0 8440 " " 
5 " 10.6 10.1 8350 " " 
6 " 11.0 10.6 8710 " " 
7 " 10.9 10.2 8050 " " 
8 " 11.0 10.6 8700 " " 
9 " 11.0 10.6 7750 " " 
10 " 10.7 10.0 6800 " " 
Comparative example 
11 " 11.5 10.9 2150 " Yes 
12 " 8.6 8.2 1030 " Yes 
13 " 5.0 4.3 2800 " Yes 
14 " 5.1 4.5 2050 " Yes 
15 " 11.9 11.0 1050 " Yes 
16 " 8.1 7.1 610 " Yes, 
while drawing 
17 5.0 4.2 4.0 3000 Bad Yes 
Prior art 
18 5.0 10.6 10.0 7200 Good 
No 
19 4.5 11.9 11.0 7700 Good 
No 
__________________________________________________________________________