Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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AGE-HARDENABLE COPPER ALLOY
BACKGROUND OF THE INVENTION
Field _of the Invention
The invention relates to an age-hardenable copper alloy as material for
producing blocks for the side dams of continuous strip-casting installations.
Description of Related Art
The worldwide aim, especially in the steel and copper industries, to cast
semifinished material to as close to the final dimensions as possible, in
order to
save hot forming and/or cold forming steps, led even before .1970 to the
development of the so-called Hazelettstrip-casting installations, in which the
molten metal solidifies in the gap between two bands guided in parallel. The
side dams, in, for example, the strip-casting installation known from U.S.
Patent
No. 3,865,176, are made of metallic mold blocks or darn blocks having a T
groove, which are lined up on a fle.xible continuous band such as one made of
steel, and which move in the longitudinal direction, synchronously with the
casting bands. In this context, the dam blocks bound the casting mold cavity
formed by the casting bands. .- ,-== -
Also !mown, from EP 0 974 413 Al are dam block chains for continuous
strip-casting, formed by blocks having a slot and key. The advantage of these
further developed mold blocks having a slot and key comes about due to a more
exact alignment, and guidance of the blocks during the casting process, and
leads to an improvement of the surface quality of the cast strip. In order to
prevent premature wear of the side edges of the blocks by plastic deformation
and the formation of cracks, a suitable material has to have great hardness
and
strength, a fine-grained microstructure and a good long-term resistance to
softening. In order to remove the heat of solidification from the liquid
molten
material, a high thermal conductivity of the mold block material is
additionally
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required.
Finally, optimum fatigue behavior of the material is of the most decisive
significance, which will ensure that, after leaving the casting segment, the
thermal stresses appearing during the cooling off of the blocks do not lead to
cracking of the blocks at the corners of the T groove incorporated for the
accommodation of the steel band. In this context, particularly great thermal
stresses are to be expected in dam blocks having the design using slot and
key,
on account of the unfavorable geometry and mass distribution.
If such cracks caused by thermal shock appear, the respective mold block will
fall out of the dam block chain of the continuous strip-casting machine after
even a short period, molten metal being able to run uncontrollably from the
casting mold cavity and to damage parts of the installation. For the purpose
of
exchanging the damaged mokLblocks, the entire strip-casting installation has
to
be stopped and the casting process has to be interrupted.
A testing method has proven itself for checking the tendency to crack, in
which
the mold blocks are submitted to heat treatment for two hours at 500 C and
are subsequently quenched in water at 20 to 25 C. Even if this thermal shock
test is repeated several times, in the case of a suitable material no cracks
may
appear in the T groove surface.
EP 0 346 645 Bi describes an age-hardenable copper-based alloy which is
made of 1.6 to 2.4 % nickel, 0.5 to 0.8 % silicon, 0.01 to 0.2 % zirconium,
optionally up to 0.4 % chromium and/or up to 0.2 % iron, the remainder being
copper including production-caused impurities. This known copper alloy
basically satisfies the prerequisites for a long service life, if used as the
material
for producing standard mold blocks for the side darns of continuous
,
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strip-casting installations. The following combination of properties is even
for
this copper alloy:
Rm at 20 C: 635 to 6601VIPa
Rm at 500 C: 286 to 372 MPa
Brinell hardness: 185 to 191 HB (corresponds to about 195 to 210
HV)
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Conductivity: 4L4 to 42.4 % IACS.
No cracks are to appear during the thermal shock test. One advantage
compared to beryllium-containing copper-based alloys is the possibility of
being
able to regrind the mold blocks manually, since no beryllium is contained in
the
grinding dust. The reprocessing of used dam blocks having slot and key is
considerably more costly and, as a rule, requires machine (wet) cleaning of
the T
groove and the casting surfaces (e.g. in closed chambers), whereby the
liberation
of grinding dusts is prevented, Thus, using beryllium-containing alloys would
basically be possible under these circumstances.
A dam block made of the CuNiSiZr alloy described in EP 0 346 645 81, however,
disadvantageously tends to premature wear of the side edges and casting
surfaces, at very high mechanical and thermal stresses in the casting
operation
of a continuous strip-casting installation. As the results of investigations
have
shown, this wear may be attributed to a material softening of the casting
edges
and surfaces to a value below 160 FN. Furthermore, the thermal shock
resistance of the known CuNiSiZr alloy is not always sufficient for
effectively
preventing the formation of cracks in the T groove during casting use, when
the
alloy is used for a dam block having a slot and key.
It is an object of the invention to create an age-hardenable copper alloy as a
material for producing dam blocks for continuous strip-casting installations,
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especially in a slot and key embodiment, which is stable to changing
temperature
stresses even at high casting speeds, and which has a great resistance to wear
and
resistance to softening, as well as great resistance to crack formation in the
T groove.
Thus, according to an aspect of the present invention, there is provided an
age-
hardenable copper alloy suitable as a material for producing block for the
side dams
of strip-casting installations, comprising: 1.2 to 2.7 weight % cobalt, which
may be
partially substituted with nickel, 0.3 to 0.7 weight % beryllium, 0.01 to 0.5
weight
% zirconium, optionally 0.005 to 0.2 weight % magnesium, optionally 0.005 to
0.2
weight % iron, optionally up to a maximum of 0.15 weight % of at least one
element
selected from the group consisting of niobium, tantalum, vanadium, hafnium,
chromium, manganese, titanium and cerium, and a balance of copper, in which
the =
alloy has a grain size between 30 and 90tim, ascertained according to ASTM E
112.
DETAILED DESCRIPTION OF THE INVENTION
By using a copper-based alloy made of 1.2 to 2.7 wt.% cobalt, 0.3 to 0.7 wt.%
beryllium, 0.01 to 0.5 wt.% zirconium, optionally 0.005 to 0.2 wt.% magnesium
and/or iron and of the remainder copper, including production-caused
impurities and
the usual processing additives, on the one hand, a sufficient age-
hardenability of the
material for achieving great strength, hardness and conductivity may be
ensured. On
the other hand, only a relatively slight cold working of up to a maximum of
40% is
required for establishing a fine-grained microstructure having a sufficient
plasticity.
Because of the deliberately graduated zirconium content, both the fatigue
resistance
and the heat resistance properties are improved.
A further improvement of the mechanical properties of the dam blocks,
especially an increase in tensile strength, may advantageously be achieved by
having the copper alloy contain 1.8 to 2.4 wt.% cobalt, 0.45 to 0.65 wt.%
beryllium, 0.15 to 0.3 wt.% zirconium, up to 0.05 wt.% magnesium and/or up
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to 0.1 wt. % iron.
The invention permits that, in the copper alloy, up to 80% of the cobalt
content
may be replaced by nickel.
Further improvements of the mechanical properties of a dam block may be
achieved if the copper alloy contains up to a maximum of 0.15 wt.% of at least
one element of the group including niobium, tantalum, vanadium, hafnium,
chromium, manganese, titanium and cerium. Usual deoxidants such as boron,
lithium, potassium and phosphorus may also be added up to a maximum of
0.03 vit.%, without negatively influencing the mechanical properties of the
copper alloy of the present invention.
According to another specific embodiment, a part of the zirconium content may
be replaced by up to 0.15 wt.% by at least one element of the goup including
cerium, hafnium, niobium, tantalum, vanadium, chromium, manganese and
titanium.
Advantageously, the blocks for the side darns of double strip-casting
installations are produced using the age-hardenable copper alloy, by the
method steps: casting, hot forming, cold forming up to, 40 %, solution
treating
at a temperature in the range of 850 to 970 C, as well as a 0.5 to 16-hour
age-hardening treatment at 400 to 550 C.
As a particular advantage, the copper alloy may be cold formed by 5 to 30 %
after hot forming. A cold forming degree of 10 to 15 % lying within this range
is
particularly preferred in this context.
It is especially advantageous if the dam blocks in the age-hardened condition
have a tensile strength of at least 650 Mpa, particularly 700 to 900 Mpa, a
Vickers hardness of at least 210 MV, particularly 230 to 280 MV, an electrical
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conductivity of at least 40 % 1ACS, particularly 45 to 50 A) IACS, a hot
tensile
strength at 500 C of at least 400 Mpa, particularly of at least 450 Mpa, a
minimum hardness of 160 MV after 500-hour ageing at 500 C, and a maximum
grain size according to ASTM E 112 of 0.5 mm.
Particularly preferred are dam blocks made of the copper alloy, if, in the
age-hardened condition they have a grain size, ascertained according to ASTM E
112, between 30 and 90 pm.
In a preferred embodiment, the copper alloy, after the hot forming of the cast
blank, is cold formed up Ito 40%, is then solution treated at a temperature
lying
in the temperature range of 850 to 970*C, and is subsequently submitted to a
0.5 to 16-hour age-hardening treatment at 400 to 550*C. In a surprisingly
simple way one may succeed in getting rid of the bad recrystallization
behavior
observed in the known CuCoBe alloys during the hot forming and solution
treatment. The bad recrystallization behavior, in the production of mold
blocks
made of CuCoBe alloys, in the hot formed, solution treated and age-hardened
condition leads to a coarse microstructure, that is not acceptable for the
application purpose, having grain sizes up to more than 1 min. However, if,
between the hot forming and the solution treatment, the material is submitted
to cold forming up to a maximum of 40 %, preferably up to a maximum of 15 %,
this additional processing step leads to a considerably more fine-grained
microstructure. Relevant investigative series have confirmed that materials
for
mold blocks for the side dams of strip-casting machines, which are cold formed
below the recrystAllizAtion temperature, and are subsequently solution
treated,
have a clearly finer microstructure at grain sizes below 0.5 mm, while higher
degrees of cold =forming, above approximately 40 %, lead during subsequent
solution treatment to a coarsening of the grain by secondary
recrysta11i7ation, at
grain sizes above 1 mm.
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.'EXAMPLES
The invention is explained below in greater detail, with the aid of exemplary
embodiments. The advantages of the copper alloys are demonstrated using õ
three alloys according to the invention (A, B and C) and three alloys for
reference (D, E and F). The composition of the copper alloys in wt.% is given
below in Table 1.
Table 1.
Alloy Co (%) Ni (%) Be (%) Zr (%) Si (%) Cr (%) Cu N
A 2.1 0.54 0,18
Remainder
B 2.2 0.56 0.24
Remainder
1.3 1.0 0.48 0.15 - Remainder
2.0 -= 0.16 0.62 0.34
Remainder
2.1 - 0.55 - - Remainder
1.0 1.1 0.62 - - - Remainder
In the case of the composition of alloy D, a known CuNiSi-based alloy is
involved, whereas E and F are normalized CuCo2Be or CuCoNiBe materials.
All the copper alloys were smelted in an induction crucible oven and were cast
by the continuous strip-casting method to round billets having a diameter of
280 mm. The round billets of exemplary alloy A, B and C were extruded on an
extrusion press at a temperature above 900 C to flat bar of a dimension 79 x
59 mm, and subsequently were drawn, at a loss in cross section of 12 %, to a
dimension of 75 x 55 mm. The blocks of the reference alloys D, E and F were
extruded at the same temperature, directly to the dimension 75 x 55 mm, and
were not submitted to additional cold forming. The CuCoBe and CuCoNiBe
materials were subsequently solution-treated at 900 to 950 C and were
age-hardened at a temperature range between 450 and. $50 C for 0.5 to 16
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hours.
The CuNiSi-based alloy was solution-treated at 800 to 850 C and age-hardened
under the same conditions. In the age-hardened condition, the tensile strength
Rm, the Vickers hardness HVIO, the electrical conductivity (as substitute
quantity for the heat conductivity), the gain size according to ASTM El12, the
heat resistance Rm at 500 C and the resistance to softening via Vickers
hardness measurement (liV10) after ageing at 500 C after 500 hours were
ascertained,
The thermal shock behavior was finally tested on mold blocks (1) of dimensions
70 x 50 x 40 mm and mold blocks (2), having slot and key, of dimensions 70 x
50 x 47 mm. For this, the mold blocks were first annealed for two hours at 500
C and then quenched in water at 20 to 25 C. The T groove of the blocks were
=then searched for cracks with the naked eye and a microscope at 10-fold
magnification.
All the test results are summarized in Table 2 below.
Table 2
Alloy Tensile Vickers Conductivity Grain Tensile
Vickers Hardness Behavior Alter
Strength Hardness (eketrical) % Size Strength 10 After Aget4 at
Theroloshock Test
2dPa liv10 IACS (500 C) 500 =C over SOO Block
(1) Block(2)
MF's Ii
A 801 254 50 . 30-90 523 173 crack-free
crack-free
B 804 24$ 51.5 45-90 464 175 crack-
free crack-free
C 812 255 49.5 45-90 485 167 crack-free
crack-free
1) 652 205 43 45-90 387 118 crack-free
cracked
E 786 260 50.5 10 5000 423 ISO
cracked cracked
P 807 248 49.5 to 3000 434 252
cracked cracked
In the mold blocks classified as "cracked", the extension of determined cracks
in
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the groove was 2 to 5 mm, and in individual cases, the length of the crack was
up to 10 mm. One may see from the reference that, as compared to materials E
and F, only copper alloys A, B and C, and produced using slight cold working,
at optimum properties, have a surprisingly uniform and fine-grained
microstructure, and have the necessary resistance to the formation of cracks
when used as mold block having slot and key. Even when used in a usual mold
block, the copper alloys in accordance with the invention have a clearly
better
resistance to softening compared to the known CuNiSi alloys D, and a
somewhat better resistance to softening when compared to alloys E and F.
Therefore, the copper alloy in accordance with the invention is eminently
suitable as the material for producing all mold blocks, that are submitted to
typically changing temperature stress during the casting process, for the side
dams of strip-casting instAl tions. These are both the mold blocks used up to
the present and the mold blocks embodied with slot and key as in
EP 0 974 413 A1.
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