Method of making a sheet or strip of zircaloy with good formability and the strips obtained

A method of making a strip of ZIRCALOY 2 or 4 is disclosed wherein an ingot is worked, roughly shaped into a billet then quenched from the beta range, hot rolled in alpha range, annealed and cold rolled to 0.3 to 0.9 mm. The O and C, in ppm, are selected to satisfy the formula: O.sub.2 <1200-0.75 x C (R) so that a T texture is obtained systematically for thicknesses of at least 0.8 mm. The disclosure also concerns the strips obtained. The method can be applied to obtaining strips of excellent formability for the production of components for nuclear water reactors.

BACKGROUND OF THE INVENTION 
The subject of the invention is a method of making a strip of zircaloy 2 or 
4 with good formability. 
In the publication by CHARQUET D., ALHERITIERE, E., and BLANC, TG., 
"Cold-Rolled and Annealed Textures of Zircaloy-4 Thin Strips", Zirconium 
in the Nuclear Industry: Seventh International Symposium ASTM STP 936 
R.B., Adamson and L.F.P. Van Swan, Eds., American Society for Testing of 
Materials, Philadelphia, 1987, pages 663-672, the authors state that the 
texture of zircaloy strips has a great effect on their mechanical 
properties and formability. 
The T texture of crystal orientation is similar to that of pure zirconium; 
in it the base poles (0002) are typically disoriented by 20.degree. to 
40.degree. towards the transverse direction, while the [1120] is parallel 
to the rolling direction. The T texture has a better breaking load, creep 
strength and deformability (bending or stretch forming) than so-called C 
textures (centred isotrope) or L textures (base poles swung towards the 
rolling direction). The effect of cold rolling and annealing on texture is 
also discussed in this document; a return to a T structure is no longer 
possible once an L or C structure has been obtained. 
An initial T structure is obtained by hot rolling in the alpha range, but 
the preservation of a T structure through cold rolling and annealing 
cycles is uncertain. 
Applicants have sought to define the conditions which will ensure that the 
T structure is preserved, so that the formability of the corresponding 
strips of ZIRCALOY 4 or ZIRCALOY 2 can be improved systematically. 
It will be recalled that the composition of these two alloys is given in 
the ASTM B 352-79 specifications; ZIRCALOY 4 and ZIRCALOY 2 correspond 
respectively to grades R 60804 and R 60802. 
SUMMARY OF THE INVENTION 
The subject of the invention is a method of making a strip of ZIRCALOY 2 or 
4 with good formability in which--in a manner known from the 
above-mentioned publication--an ingot is produced and hot worked into a 
billet, typically by forging; the billet is heated to beta and water 
quenched, then hot rolled in the alpha range and annealed in the alpha 
range. It is then cold rolled with intermediate annealing operations, to a 
selected thickness from 0.3 to 0.9 mm. According to the invention, 
production of the ingot is carried out such that the carbon (C) and oxygen 
(O.sub.2) content obtained thereby satisfy the formula: 
(R) O.sub.2 &lt;1200-0.75xC and preferably (R') O.sub.2)21 1150-0.75xC, the O 
and C content being expressed in ppm. The strips thus obtained have T 
textures systematically in the case of thicknesses of 0.8 mm and over. In 
cases where the final thickness has to be smaller, the three recommended 
preferred measures should be applied, singly or combined generally: 
(a) hot rolling the billet at the top of the alpha range with rolling 
starting at a temperature of from 730.degree. to 795.degree. C.; 
moderating the annealing operations to keep the texture obtained by the 
following means: 
(b) carrying out the annealing operation which follows hot rolling at a 
temperature below 640.degree. C.; 
(c) carrying out each intermediate annealing operation between cold rolling 
either at from 600.degree. to 640.degree. C. for 3 and 4 hours, or at from 
650.degree. to 700.degree. C., for 1 to 5 minutes, or with any other 
(temperature, time) pair which gives an annealing result (hardness, 
recrystallisation) equivalent to either intermediate annealing operation. 
To obtain a strip 0.6 mm thick, it is preferable to carry out three cold 
rolling/annealing cycles; thus there are two intermediate annealing 
operations. For a thickness of 0.4 mm, 3 or 4 cold rolling/annealing 
cycles are carried out, so there are 2 or 3 intermediate annealing 
operations. 
The solution to formula (R) surprising enables a T texture to be obtained 
in a strip at least 0.8 mm thick, and this is obtained whether the strip 
is work hardened or in an annealed state. It also enables the T texture to 
e preserved with smaller thicknesses, if the clearly defined methods (a), 
(b), and(c) are followed. In the case of ZIRCALOY 4, maintenance of the T 
texture goes together with the presence and maintenance of precipitates 
based on (Fe, Cr), which also give an improvement in the resistance to 
uniform corrosion in water of PWR reactors. 
The range defined by (R) is normally limited by the maximum C content 
defined by ASTMB specification 352, namely 270 ppm. Obtaining the minimum 
mechanical properties required for some types of strip for the production 
of grids holding nuclear fuel elements (see U.S. Pat. No. 4,717,427 and EP 
246986=U.S. Pat. No. 4,881,992: E.sub.0.2 at 315.degree. C..gtoreq.250 
MPa, breaking load 315.degree. C..gtoreq.280 MPa), and the wish to avoid 
Zr carbide precipitation leads (sic) to the following complementary 
preferred limitations: 
EQU C.ltoreq.180 ppm and O.sub.2 .gtoreq.600 ppm, 
which respectively enhance ductility and increase mechanical strength at 
315.degree. C., taking into account the texture produced by formula (R). 
If the best possible formability is to be obtained, it is advisable for 
the selection of (C) and (O.sub.2) content according to the invention to 
be combined with the cold rolling conditions already described by 
Applicants: 
either, in accordance with FR 2 575 764, carrying out the last rolling 
process with deformation of from 30 to 55% and the final annealing 
operation at from 490.degree. to 580.degree. C. for 1 to 10 minutes, 
thereby obtaining incipient recrystallisation, which typically involves 
0.5 to 5% of the volume of the strip; 
or, in accordance with EP 246986=U.S. Pat. Nos. 4,775,428 and 4,881,992 
(mentioned above), and as an alternative form of method (c), with cold 
rolling processes and intermediate annealing operations adjusted so as to 
have intermediate states where the alloy is just recrystallised with very 
fine grains: carrying out cold rolling with at least two intermediate 
annealing operations and a final annealing operation, each of the two 
intermediate ones being from 0.5 to 10 minutes at from 650.degree. to 
750.degree. C., the amount of deformation between annealing operations 
being from 20 to 55% before the penultimate one, from 30 to 55% between 
the last two intermediate ones and between the last intermediate one and 
the final one, the final annealing operation being from 1.5 to 7 minutes 
at from 590.degree. to 630.degree. C. and producing partial 
recrystallisation of the strip obtained, involving 20 to 40% of its 
volume. 
In these cold rolling methods each amount of deformation is calculated by 
the formula: 
(l-e/E).times.100 where e and E are respectively the thicknesses after and 
before rolling. 
It will be seen from the tests that, when the 0 content (ppm) is increased 
above (1150-0.75xC), L textures are obtained, or T textures which are 
easily degraded by cold rolling. When the 0 content is only increased thus 
by less than 200 to 250 ppm relative to (1150-0.75xC), the T texture can 
still be obtained provided that the transformation conditions are 
selected, whereas beyond that increase only L texture is possible. 
It should be noted, of the products obtained according to the invention, 
two grades of strip are particularly important: 
A. Annealed strips of ZIRCALOY 2 or 4 from 0.3 to 0.9 mm thick, which 
satisfy the mechanical properties at 288.degree. C. (550.degree. F.) laid 
down by ASTM specification 352: 
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breaking load in longitudinal direction (L) 
186 MPa 
breaking load in transverse direction (T) 
179 MPa 
and elastic limit at 0.2% (YS) in direction (L) 
103 MPa 
and elastic limit at 0.2% (YS) in direction (T) 
120 MPa 
having a T texture with 0.sub.2 .gtoreq. 700 ppm and 0.sub.2 &lt; 1150 - 
0.75xC.; 
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B. Strips of annealed ZIRCALOY 2 or 4 with incomplete recrystallisation or 
"restored" strips, again from 0.3 to 0.9 mm thick, satisfying the 
following minimum mechanical properties: 
E.sub.0.2 at 315.degree. C.:250 MPa, breaking load R in direction (L) 310 
MPa, and also having a T texture with O.sub.2 .gtoreq.700 ppm and O.sub.2 
&lt;1150-0.75xC. 
The amount of recrystallisation is then 0.5 to 40% of the volume. 
The essential advantage of the invention is that it systematically gives 
strips of ZIRCALOY 4 or ZIRCALOY 2 with the required level of mechanical 
properties and excellent formability. Formability can typically be 
assessed by testing their suitability for drawing operations, for example 
by the ERICHSEN tests. The improvement in formability is accompanied by an 
improvement in resistance to uniform corrosion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The following examples concern sheets or strips of ZIRCALOY 4, obtained 
from a plurality of casting operations, the numbers of which are given in 
Table 1. 
The graph in FIG. 1 shows the content pairs (C,O) of the various sheets or 
strips in Table 1, with the straight lines forming the limits of formulas 
(R) and (R') shown respectively as R and R'. 
FIG. 2 represents a T texture and FIG. 3 an L texture; these two diagrams 
are taken from the publication quoted t the beginning of this 
specification. 
The texture in FIG. 2, described as a "T texture", has two base poles 
(0002) 1 and 2 which are disoriented by 20.degree. to 40.degree. in the 
transverse direction TD. There are two curves of relative maxima 3 and 4. 
The so-called "L texture" in FIG. 3 is very different: the two base poles 5 
and 6 are disoriented by 10.degree. to 20.degree. in the rolling direction 
LD, while the relative maxima 7 and 8 surround the poles 5 and 6 and are 
closer to them. 
In all cases the ingot is roughly shaped hot into a billet, the billet is 
quenched from beta range then hot rolled in the alpha range to a thickness 
of o4 to 6 mm, and the rolled ingot or rough rolled strip is annealed at 
630.degree. C. 
As indicated in the above-mentioned publication, recrystallisation of a 
cold rolled material with a T texture does not markedly change the 
orientation of the base poles (002). Conversely, when an L texture or a 
centred texture has been obtained it is not possible to return to a T 
texture, for example trough annealing. 
Samples 1 to 6 and 8 in Table 1, which have an O and C content complying 
with formula (R) and with preferred formula (R'), have a T texture at a 
thickness of 0.8 mm, either in the annealed state (Samples 1 to 5 and 8) 
or in the work hardened state (Sample 6): as indicated above, the T 
texture is that of all the samples both in the work hardened and annealed 
state. 
Sample 7 (0.4 mm thick) illustrates the fact that with further cold rolling 
the texture may change to an L texture. The change is very evident in this 
case. 
In the case of Sample 10, which is examined at a thickness of 2 mm in the 
annealed state and a thickness of 1.2 mm in the work hardened state, the 
change in texture with rolling is premature in view of the strip 
thicknesses used for spacing grids of nuclear fuel elements (0.3 to 0.9 mm 
thick). 
Samples 9 and 11 to 15 show that, in the case of thicknesses from 0.4 mm to 
1.5 mm, an L texture is observed, the corresponding (C,N) pairs being 
located above the limit line R (FIG. 2). 
APPLICATIONS 
The invention makes it possible to obtain sheets and strips of ZIRCALOY 4 
or ZIRCALOY 2 with excellent formability, for producing components for use 
in nuclear water reactors of the PWR or BWR type, for example spacing 
grids or casings. 
TABLE 1 
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SAMPLE 
CASTING 
O C TYPE OF 
THICKNESS 
STATE AND METHOD 
REF. NO. (ppm) 
(ppm) 
TEXTURE 
(mm) OF OBTAINING IT 
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1 30 790 
57 T 0.8 Annealed (2 mm) 
2 48 930 
49 T 0.8 " 
3 978 1000 
155 T 0.8 " 
4 85 1025 
102 T 0.8 " 
5 82 1060 
80 T 0.8 " 
6 33 1090 
35 T 0.8 Work hardened (2 mm) 
7 " " " ? 0.4 " 
8 58 1095 
60 T 0.8 Annealed (2 mm) 
9 86 1210 
101 L 0.6 Partially annealed 
(0.5 to 5% 
recrystallised) 
10 87 1320 
150 T/L 2/1.2 Annealed/work 
hardened 
11 79 1285 
90 L 1.0 Work hardened (2 mm) 
12 25 1300 
120 " 0.8 Partially annealed 
12 bis 
" " " L " Work hardened (2 mm) 
13 10 1350 
96 L 1.5 Work hardened (2 mm) 
14 17 1430 
114 L 1.2 Work hardened (2 mm) 
15 678 1490 
77 L 0.4 Work hardened (2 mm) 
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