WO2004063411A1

WO2004063411A1 - Iron-nickel alloy with low coefficient of thermal expansion for making shade masks

Info

Publication number: WO2004063411A1
Application number: PCT/FR2003/003785
Authority: WO
Inventors: Fabien Gaben; Sylvain Witzke; Olena Danylova
Original assignee: Imphy Alloys
Priority date: 2002-12-20
Filing date: 2003-12-18
Publication date: 2004-07-29
Also published as: EP1581664A1; AU2003299338A1; FR2849061A1; FR2849061B1; CN100354446C; CN1742107A; US20060171840A1

Abstract

The invention concerns an alloy whereof the chemical composition comprises, by weight: 35 % =Ni = 37 %, 0.001 % = C = 0.05, % Mn = 0.10 %, Si = 0.15 %, Co = 0.5 %, S < 0.002 %, P < 0.006 %, B = 0.0005 %, Al+Mo+Cu+Cr = 0.15 % 0.015 % = 2(V+Ti)+Nb + Zr + Ta + Hf = 0.2 %, 0.0025 % = N+O = 0.015 % optionally calcium and/or magnesium with total content ranging between 0.0001 and 0.005 %, the rest consisting of iron and unavoidable impurities resulting from preparation, and a method for making a strip of said alloy.

Description

IRON-NICKEL ALLOY WITH VERY LOW THERMAL EXPANSION COEFFICIENT FOR THE MANUFACTURE OF SHADOW MASKS

The present invention relates to an alloy based on iron and nickel with a very low coefficient of expansion, which can in particular be used for the manufacture of shadow masks for cathode-ray tubes for color display. In order to avoid local deformation by thermal expansion of the shadow masks for cathode-ray tubes for color display, it is desirable to use for their manufacture an alloy having a coefficient of thermal expansion as low as possible. Thus, for example, an FeNi alloy containing approximately 36% nickel and approximately 0.3% manganese, well known under the name Invar, is used. Such an alloy has a coefficient of thermal expansion between 20 ° C and 100 ° C of the order of 1x10 ^"6 / K.

However, this coefficient of expansion is still too high for certain applications, such as application to flat screens, and it has been proposed to use an FeNi alloy of which a few% of nickel is replaced by cobalt. This alloy has the advantage of having a coefficient of thermal expansion of the order of 0.4 × ¹⁰ −6 ^" / K, which leads to a gain of 60%, but it has the disadvantage of containing cobalt. Indeed , shadow masks are metallic sheets pierced with very fine holes obtained by chemical etching, and cobalt causes annoying pollution in chemical etching baths. In addition, cobalt is a very expensive element and it is desirable to reduce the its content as much as possible.

Also, it has been proposed to use an FeNi alloy, with a low residual and cobalt content, containing in particular less than 0.1% of manganese. This alloy has the advantage on the one hand of containing little or no cobalt, and on the other hand of having a coefficient of thermal expansion of the order of 0.8x10 ^"6 / K, lower than that of l 'classic FeNi (Invar) alloy, however, the coefficient of expansion is still too high, especially for large or shallow flat screens. In addition, it is desirable to use thinner masks in order to reduce their manufacturing cost and to improve the quality and accuracy of the images. However, the alloys of the prior art do not have sufficient mechanical characteristics to allow the thickness of the masks to be reduced while retaining a resistance of the masks to deformations which may appear during the various stages of transport and handling.

The object of the present invention is to remedy the drawbacks of alloys of the prior art by proposing an alloy which can be used in particular for the manufacture of shadow masks, containing little or no cobalt, the coefficient of thermal expansion of which is lower than that of the known FeNi alloys having an elastic limit on the annealed state maintained or even improved.

To this end, the first object of the invention is an alloy whose chemical composition comprises, by weight:

35% <Ni <37%

0.001% <C <0.05%

Mn <0.10%

If <0.15% Co <0.5%

S <0.002%

P <0.006%

B <0.0005%

Al + Mo + Cu + Cr <0 _. 15% 0.015% <2 (V + Ti) + Nb + Zr + Ta + Hf <0.2%

0.0025% <N + O <0.015% optionally calcium and / or magnesium in a total content between 0.0001 and 0.005%, the rest being made up of iron and inevitable impurities resulting from the production. In a preferred embodiment, the alloy also has a niobium content of less than 0.1%, or even less than 0.07%.

In another preferred embodiment, the alloy has a carbon content greater than 0.0035%. In another preferred embodiment, the alloy has a grain size less than 10, or even less than 9 (according to G ASTM E 112).

In another preferred embodiment, the alloy has a coefficient of thermal expansion between 20 ° C and 100 ° C less than 0.70x10 ^"6 / K, and preferably less than 0.65x10 ^{" 6} / K. In all cases, the coefficient of expansion obtained is less than 0.75x10 ^"6 / K.

In another preferred embodiment, the alloy has a conventional yield strength of 0.2% in the annealed state greater than 250 MPa, preferably greater than 280 MPa, and more particularly preferably, greater than 300 MPa, even at 310 MPa. In another preferred embodiment, the niobium and carbon contents of the alloy composition are such that:

Nb x C ≤ O.01. This embodiment makes it possible to improve the elastic limit of the grade in the annealed state by the formation of carbides of size less than one micron. In another preferred embodiment, the titanium, niobium and nitrogen contents of the alloy composition are such that:

Ti x N <0.00006 Nb x N <0.001 This embodiment makes it possible to avoid the presence of too large a quantity of titanium and / or niobium nitrides, which have a size of the order of a few hundred nanometer or even a few microns, and which pose a problem during the fabrication by etching of shadow masks.

In another embodiment, the alloy contains precipitates based on titanium, and / or niobium, and / or vanadium, and / or tantalum, and / or zirconium and / or hafnium, the average size is less than or equal to 100 nm, preferably less than or equal to 70 nm and more particularly less than 50 nm. The second object of the invention is a method of manufacturing an alloy strip according to the invention comprising the steps according to which:

- A semi-finished product of this alloy is hot rolled after reheating to a temperature higher than 850 ° C and lower than 1350 ° C, so that the rolling temperature is higher than the re-solution temperature of the precipitates based titanium and / or niobium, and / or vanadium and or zirconium, and or tantalum and or hafnium and the end of rolling temperature is lower than the temperature at which precipitation of said precipitates begins, in order to get a hot strip,

- The hot strip is cold rolled in one or more passes, to obtain a cold strip with possibly one or more intermediate anneals between two passes.

In a first preferred embodiment, the temperature of the intermediate annealing (s) carried out during the cold rolling is lower than the temperature of redissolving said precipitates.

In a second preferred embodiment, the temperature of the intermediate annealing (s) carried out during the cold rolling is higher than the temperature of redissolving said precipitates. These two different embodiments allow to play on the formation of the precipitates and on the grain size. By way of non-limiting indication, a grain size greater than 7 is generally obtained for the first embodiment, while grain sizes less than 7.5 are generally obtained for the second embodiment. In another preferred embodiment, the end temperature of hot rolling is less than or equal to 850 ° C., which makes it possible to obtain finer grains.

A third object of the invention is the use of the alloy described above for the manufacture of shadow masks for cathode ray tubes for color display, for the manufacture of shadow masks stretched in the vertical or horizontal direction for flat screen televisions, for the manufacture of shadow mask support frames, for the manufacture of cryogenic storage containers, but also for the manufacture of grids electron guns, thanks to its very good mechanical cutting ability.

The invention is based on the fact that the inventors have found in a new and surprising manner that the precipitation of compounds formed from titanium, and / or niobium, and / or vanadium, and / or zirconium, and / or tantalum, and / or hafnium, on the one hand, and carbon, oxygen and / or nitrogen on the other hand, leads to a significant reduction in the coefficient of expansion when the alloy has a low content of Si and Mn. The precise analysis of the compounds formed is delicate, but there are in particular carbides, nitrides, carbonitrides, oxides and / or oxynitrides of the metals mentioned above.

Without wishing to be bound by a theory, the inventors believe that this effect could be due to the fact that these various compounds have for the most part a crystal structure of cubic type, and form precipitates whose size is generally of the order of several tens. nanometers when formed in solid phase. These small precipitates precipitate in the matrix and not at the grain boundaries, as is conventionally the case.

This effect on the coefficient of expansion of the alloy is in particular visible in FIG. 1, which represents the variations of this coefficient between 20 and 100 ° C., depending on the sum of the oxygen and nitrogen contents, for an alloy whose composition comprises titanium at contents of between 0.01 and 0.05%, less than 5 ppm of boron, less than 5 ppm of sulfur and no aluminum. The same effect is obtained with an alloy containing niobium as a total or partial replacement for titanium, within the limits imposed by claim 1.

The alloy according to the invention contains, in% by weight:

- from 35% to 37% of nickel, and preferably between 35.5% and 36.5%, in order to obtain a low coefficient of thermal expansion between 20 ° C and 100 ° C,

- from 0.001% to 0.05% carbon to form fine precipitates of carbides. The formation of nanometric-sized carbide precipitates has the effect of reducing the coefficient of expansion and improving the mechanical properties of the product. We limit its content to 0.05% to avoid the formation of large inclusions of insoluble carbides. It is preferred that the carbon content is greater than 0.0035% in order to have a volume fraction of carbon sufficient to obtain improved mechanical characteristics. It is also preferred to keep the carbon content below

0.010%, or even less than 0.007% to further limit the size of the carbides formed.

- less than 0.1% of manganese, because this element increases the coefficient of expansion of the alloy and must be limited, - less than 0.15% of silicon, because this element increases the coefficient of expansion of the alloy and must be limited,

- less than 0.5% cobalt, in order not to pollute the chemical etching baths of shadow masks,

- possibly from 0.0001 to 0.005% of at least one element taken from calcium and magnesium in order to trap the sulfur which still exists as an impurity and thus ensure good aptitude for hot deformation,

- optionally sulfur in a content of less than 0.002% so as not to deteriorate the ability to heat transform the alloy, - possibly phosphorus in a content of less than 0.006% so as not to deteriorate the ability to process hot alloy,

- optionally boron in a content of less than 0.0005%, and preferably equal to 0%: in fact, the inventors have found that in the presence of boron the thermal expansion coefficients increased significantly,

- optionally aluminum, molybdenum, copper or chromium in a total content of less than 0.15%, since these elements increase the coefficient of thermal expansion of the alloy.

- titanium, vanadium, niobium, tantalum, zirconium and / or hafnium in amounts such as the sum 2 (V + Ti) + Nb + Ta +

Zr + Hf is between 0.015% and 0.2%, in order to be able to form precipitates based on these elements, these precipitates preferably having an average size less than 100 nm, and so preferred less than 70 nm, and more particularly preferably less than 50 nm. It is further preferred that the niobium content is less than 0.1%, or even 0.07%, in order to further reduce the coefficient of expansion as well as the size of the precipitates. - oxygen and / or nitrogen in quantities such that the sum of their contents is between 0.0025% and 0.015%, because the inventors have found in a new way that the presence of oxygen and / or nitrogen in these contents in the alloy makes it possible to lower the coefficient of expansion when it is associated with the presence of titanium, and / or niobium and / or vanadium, and / or tantalum, and / or zirconium and or hafnium.

The sum of these contents is limited to 0.015% to avoid the formation of large oxides or nitrides. - The rest of the composition consists of iron and impurities resulting from the production.

The alloy can be produced for example in an arc furnace with a phase of refining with AOD or VOD converters; it can also be prepared in a vacuum induction oven. This preparation must be carried out so as to obtain the desired residual contents. The alloy is then cast in the form of a semi-finished product such as an ingot, a billet or a reflow electrode. It can also be poured directly in the form of a thin slab or a thin strip with a thickness of less than 15 mm, and preferably with a thickness of between 8 and 12 mm.

When the alloy is cast in the form of a reflow electrode, it is remelted under an electrically conductive slag in order to obtain better homogeneity of the chemical composition and of the solidification structure.

The semi-finished product, or the thin strip obtained by direct casting, is then hot rolled at a temperature above 850 ° C, and preferably above 1150 ° C, but below 1350 ° C to obtain a hot strip d thickness generally between 2 mm and 6 mm, and preferably between 3 and 5 mm, which is cold rolled in one or more passes with optionally annealing above 800 ° C. The strip heating temperature applied between the hot rolling steps or cold rolling, may be chosen so that the precipitates of oxides, carbides or nitrides can be optionally redissolved. Rapid cooling may also be applied to maintain in solid solution in the alloy these elements liable to form precipitates. Equilibrium precipitation treatments can then be carried out by maintaining at temperatures between 750 ° C and 1200 ° C (but preferably below 1050 ° C).

The invention will now be described in a more precise but nonlimiting manner and illustrated by examples.

testing

By way of example, the alloys identified 1 to 16 according to the invention and 17 to 23 have been prepared by way of comparison, the composition of which is described in Table 1 below. The chemical compositions and the coefficients of expansion α between 20 and 100 ° C, were measured on test specimens taken from the hot-rolled strips. Each of these test pieces was annealed for 30 minutes at 950 ° C., and cooled in ambient air before carrying out the measurements of coefficient of thermal expansion. The results of the tests are collated in Table 2, in which the coefficient of expansion α is expressed in 10 ^"6 / K.

The etching tests were carried out on cold-rolled products from experimental castings, partially coated with photosensitive resin. The engravings were carried out at 60 ° C. with a solution of FeCl ₃ having a density of 45.5 ° Be. The quality of the engravings was assessed by measures of regularity of the cut out contours, as well as by the presence of defects linked to the presence of particles.

-3 CD ω

_Q co

I-

Table 2

* ¹ Values measured after a heat treatment of 15 minutes at 850 ° C.

* ² B: engravings deemed good - P: presence of defects linked to the presence of particles.

* ³ Grain size measured according to G ASTM E 112, plus or minus 0.5 units.

In view of this table, it can be seen that all the strips according to the invention have a coefficient of expansion less than 0.70 × ¹⁰ −6 ^" / K and even less than 0.65 × ¹⁰ −6 ^" / K in most cases. On the other hand, the bands given by way of comparison have expansion coefficients substantially greater than 0.70 × ¹⁰ −6 ^"6 / K.

Counter examples 17 and 18 show the harmful effect of boron on the coefficient of expansion. Counterexamples 19 and 21 show the harmful influence of sulfur on the coefficient of expansion. These counterexamples also show the importance of the oxygen and nitrogen contents on the coefficient of expansion.

Counterexample 20, which corresponds to the conventional low manganese FeNi alloy, gives the reference showing the advantages of the invention. In fact, in the absence of compounds allowing the formation of precipitates in the solid phase, the expansion coefficients measured are greater.

Counterexample 21 shows the harmful effect of sulfur on the coefficient of expansion.

Counterexamples 22 and 23 show the importance of the nitrogen and oxygen contents on the expansion coefficient.

The alloy according to the invention can also be used for the manufacture of support frames for shadow masks. This alloy exhibits good behavior in chemical etching linked to the low controlled presence of residuals of type C, S, N in solid solution, and by its small amounts of inclusions of micrometric sizes.

Claims

1. Alloy whose chemical composition includes, by weight:

35% <Ni ≤ 37% 0.001% <C <0.05%

Mn <0 _. 10%

If <0.15%

Co <0.5%

S <0.002% P <0.006%

B <0.0005%

Al + Mo + Cu + Cr <0.15%

0.015% <2 (V + Ti) + Nb + Zr + Ta + Hf <0.2%

0.0025% <N + O <0.015% optionally calcium and / or magnesium in a total content between 0.0001 and 0.005%, the rest being made up of iron and inevitable impurities resulting from the production.

2. Alloy according to claim 1, further characterized in that its niobium content is less than 0.1%.

3. An alloy according to claim 1 or 2, further characterized in that its carbon content is greater than 0.0035%.

4. Alloy according to any one of claims 1 to 3, characterized in that its grain size is less than 10 (according to G ASTM E112).

5. Alloy according to any one of claims 1 to 4, characterized in that it has a coefficient of thermal expansion between 20 ° C and 100 ° C less than 0.7x10 ^"6 / K.

6. Alloy according to any one of claims 1 to 5, characterized in that its conventional yield strength at 0.2% in the annealed condition is greater than 280 MPa.

7. An alloy according to claim 6, further characterized in that its conventional yield strength at 0.2% in the annealed condition is greater than 300 MPa.

8. Alloy according to any one of claims 1 to 7, further characterized in that the niobium and carbon contents are such that:

Nb x C <0.01

9. Alloy according to any one of claims 1 to 8, further characterized in that the titanium, niobium and nitrogen contents of the alloy composition are such that:

Ti x N <0.00006 Nb x N ≤ 0 _. 001

10. Alloy according to any one of claims 1 to 9, characterized in that it contains precipitates based on titanium, and / or niobium, and / or vanadium, and / or tantalum, and or zirconium and or hafnium, the average size of which is less than or equal to 100 nm.

11. Method for manufacturing an alloy strip according to any one of claims 1 to 10, comprising the steps according to which:

- A semi-finished product of said alloy is hot rolled after reheating to a temperature higher than 850 ° C and lower than 1350 ° C, so that the rolling temperature is higher than the re-solution temperature of the precipitates based on titanium and / or niobium, and / or vanadium and / or zirconium, and / or tantalum and / or hafnium and the end of rolling temperature is lower than the temperature at which precipitation of said precipitates begins, so to get a hot strip,

12. Method according to claim 11, characterized in that the temperature of the intermediate annealing (s) carried out during the cold rolling is lower than the temperature of redissolving said precipitates.

13. Method according to claim 11, characterized in that the temperature of the intermediate annealing (s) carried out during the cold rolling is higher than the temperature of redissolving said precipitates.

14. The method of claim 11 or 12, characterized in that the end temperature of hot rolling is less than or equal to 850 ° C.

15. Use of an alloy according to any one of claims 1 to 10, for the manufacture of shadow masks for cathode ray tubes for color display.

16. Use of an alloy according to any one of claims 1 to 10 for the manufacture of cryogenic storage containers.

17. Use of an alloy according to any one of claims 1 to 10, for the manufacture of grids of electron guns.

18. Use of an alloy according to any one of claims 1 to 10, for the manufacture of shadow masks stretched in the vertical or horizontal direction for flat screen televisions.

19. Use of an alloy according to any one of claims 1 to 10, for the manufacture of support frames for shadow masks.