Coating apparatus – Gas or vapor deposition – Running length work
Reexamination Certificate
2000-10-26
2002-09-24
Bueker, Richard (Department: 1763)
Coating apparatus
Gas or vapor deposition
Running length work
C118S720000, C118S726000
Reexamination Certificate
active
06454861
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to steel sheets provided with a coating comprising a main layer of zinc-chromium alloy, the predominant phase of which is &dgr; or &zgr;.
The invention also relates to the plant and the process for obtaining steel sheets coated with a coating of this type of alloy.
BACKGROUND OF THE INVENTION
Document EP 0 607 452 (KAWASAKI) describes the crystalline phases that can be obtained by depositing monolayer coatings of Zn—Cr alloys by electrodeposition and the main properties associated with these phases; depending on the respective proportions of zinc and chromium and depending on the electrodeposition conditions, the following main alloy phases may therefore be distinguished:
&eegr;, having a hexagonal structure identical to that of pure zinc, in which the chromium is in solid solution in small proportions;
&dgr;, also having a hexagonal structure, the cell parameter a of which is greater than that of the &eegr; structure, the cell parameter c of which is less than that of the &eegr; structure;
&Ggr;, having a body-centred cubic structure, the cell parameter of which is greater than that of pure chromium.
Document JP 08-013192 A (KAWASAKI) teaches that annealing a layer of zinc-chromium alloy, the phase of which is &dgr;, transforms the structure of this layer; under treatment conditions which vary depending on the chromium content, the annealing time and the annealing temperature (130° C. to 200° C.), what is obtained is:
a &zgr; phase, having a monoclinic structure.
FIG. 5
shows the diagram, established by the Applicant, of the amalgamation of the trials for the identification and thermostability of the phases of the Zn—Cr alloys as a function of the chromium content (%) in the particular case of coatings deposited under vacuum by PVD (Physical Vapour Deposition).
On page 9, lines 57-58 of document EP 0 607 452, it is mentioned that, below a 5% chromium weight content in the alloy, the &Ggr; phase will not form in the coating and that, above 30% chromium, the coating obtained does not exhibit good adhesion to the steel.
This information would therefore dissuade someone from using high chromium contents, greater than 30%, especially at the interface between the steel and the coating, in order to avoid impairing the adhesion.
According to that disclosure, in which a layer of zinc-chromium alloy adheres poorly to the steel if the interface between this layer and the substrate is too rich in chromium, it has actually been found that the layers of zinc-chromium graded alloy, produced under vacuum, having a chromium concentration of approximately 15% at the interface and of approximately 5% at the surface, posed problems of adhesion to the steel substrate; by bending a steel sheet coated with such a layer through 180°, a partial debonding of the alloy layer is in fact observed.
The object of the invention is to remedy this drawback.
SUMMARY OF THE INVENTION
For this purpose, the subject of the invention is a steel sheet provided with a coating comprising a main layer of zinc-chromium alloy, the predominant phase of which has a &dgr; and/or &zgr; structure, characterized in that the said coating also includes a subjacent adhesion layer made of zinc-chromium alloy, sandwiched between the steel of the sheet and the said main layer, which has:
a &Ggr;-type body-centred cubic crystal structure;
a chromium weight content high enough to obtain the said &Ggr; structure;
an at least partial epitaxial junction with the said steel, manifested by the presence of incomplete rings in an electron diffraction pattern of the said sublayer, produced on sections made near the interface with the steel and parallel to this interface.
The invention may also have one or more of the following features:
the chromium weight content in the said subjacent layer is between 30 and 70%;
the thickness of the said subjacent layer is between 0.01 &mgr;m and 1 &mgr;m;
the thickness of the said main layer is greater than 1 &mgr;m; this is because the roughness of the substrate is often of the order of 1 &mgr;m and sometimes greater; in order for the coating to provide effective protection against corrosion and good resistance to red rust, it is important that the coating be sufficiently covering and that its thickness be greater than the roughness of the substrate;
the said main layer is a layer with a chromium concentration gradient; preferably, the variation in chromium concentration through the thickness of the said main layer is greater than or equal to 10% by weight;
the chromium concentration in the said main layer may be higher near the surface than near the steel;
preferably, in order to obtain both effective protection against corrosion and good phosphatizability, the chromium concentration in the said main layer is higher near the steel than near the surface;
the predominant phase of the main layer has &zgr; structure;
the main layer of the coating may only partially have a &zgr; structure;
in particular, if the coating is exposed to X-radiation at a grazing angle of incidence of about 3°, the said radiation emanating from an X-ray tube with a cobalt anticathode supplied with 30 kV and outputting 30 mA, emitting a line of wavelength Ka=0.179026 nm, and if the diffraction of this radiation is analysed using a rear monochromator and a scintillation detector, the height of the (131) diffraction line of the &zgr;-ZnCr phase is between approximately 10% and 100% of the height of the (0002) line of the &dgr;-ZnCr phase.
The subject of the invention is also a process for manufacturing a sheet according to the invention from a sheet to be coated, characterized in that it comprises a step in which the said coating is applied by vacuum deposition to the surface of the said sheet to be coated.
The invention may also have one or more of the following features:
the said coating is applied by vacuum evaporation and/or sublimation of zinc and chromium;
the said sheet to be coated is heated during the deposition to a temperature of between approximately 170° C. and 230° C.;
immediately before the deposition step, the said surface is cleaned and/or brightened suitably for obtaining the said at least partial epitaxial junction, preferably by inert-gas ion bombardment.
The invention also relates to the plants for obtaining, continuously, coatings of alloys, essentially comprising two metal elements, on a sheet, and more particularly coatings of Zn—Cr alloy, by a process for the vacuum deposition of these elements, in which the said sheet is made to run continuously, in succession, past a source of the first alloy metal element and then past a source of the second metal alloy element.
The word “source” in this text may denote an evaporation source or a sublimation source which may be heated by electron bombardment, by conduction (resistance or induction heating), by radiation or by plasma.
In such a process, when the flow rate of vapour emitted by the metal source is low (<1 g/min.cm
2
), the metal vapour pressure gradient above the source is low and the evaporated or sublimed metal atoms consequently undergo very few collisions. It may therefore be considered that the great majority of atoms propagate between the source and the substrate in a straight line.
On the other hand, if the flow rate of vapour emitted by the metal source is high (>1 g/min.cm
2
), the metal vapour pressure gradient above the source is high and a great majority of the metal atoms which leave the source will undergo several collisions before reaching the substrate. The dispersion of the metal element over the substrate will accordingly be greater the higher the vapour flow rate.
It is known that, by placing a screen between the source and the substrate seen from this source at a given solid angle, some of the vapour emitted in this solid angle is stopped by this screen and no longer condenses on the substrate. If the vapour flow rate is low and if the residual pressure in the deposition chamber is low, between 10
−4
and 10
−1
Pa, it is practically all of the vapour which will be stop
Chaleix Daniel
Choquet Patrick
Lamande Alain
Olier Christophe
Scott Colin
Bueker Richard
Usinor
Young & Thompson
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