Patent Application
20180209021
This invention relates to a steel sheet provided with a coating comprising a layer of zinc optionally covered by paint, which is intended in particular for the fabrication of automobile parts, although it is not limited to that application.
Galvanized coatings comprising essentially zinc are conventionally used for the effective protection they provide against corrosion, whether in the automotive sector or in the construction industry, for example.
In the following text, a zinc coating means a coating of pure zinc, potentially including the unavoidable impurities acquired during production and present in trace quantities.
Coils of steel coated in this manner can sometimes remain in storage warehouses for several months, and their surface must not be altered by the appearance of surface corrosion before they are shaped by the final user. In particular, no incipient corrosion may appear, whatever the storage environment, even if the steel is exposed to the sun and/or to a moist, humid or even saline environment. A layer of protective oil can also be applied to the surface of the steel coil to provide temporary protection in the event of storage in a moist or humid environment
The methods most frequently used to deposit a zinc coating on the surface of a steel sheet are galvanizing and electrogalvanizing. However, these conventional methods do not make it possible to coat grades of steel that contain high levels of oxidizable elements such as Si, Mn, Al, P, Cr or B, which has led to the development of new coating methods, and in particular vacuum deposition technologies such as jet vapor deposition (JVD).
Nevertheless, these vacuum coatings do not provide the same level of temporary protection as conventional coatings, even after application of a layer of protective oil.
An objection of the present invention is therefore to eliminate the disadvantages of steels coated using methods of the prior art by making available a steel sheet coated with zinc by vacuum deposition that offers good temporary protection against corrosion.
The present invention provides a steel sheet having good temporary protection against corrosion. The steel sheet has a coating which includes at least one layer of zinc, obtained by a jet vapor deposition method in which a ratio between the pressure inside the deposition chamber Pchamber and the pressure inside the zinc ejection chamber Peject is between 2·10−3 and 5.5·10−2.
The sheet can also have the following characteristics, considered individually or in combination:
The present invention further provides a method for the fabrication of a coated sheet. The method includes the steps of coating the sheet by a sonic vapor jet of zinc via an ejection chamber 7 maintained at a pressure Peject and contained inside a deposition chamber maintained at a pressure Pchamber, the ratio of the pressures Pchamber to Peject being between 2·10−3 and 5.5·10−2.
The method can also have the following characteristics, considered individually or in combination:
Additional characteristics and advantages of the invention are described in greater detail below.
To illustrate the invention, tests have been conducted and will be described by way of non-restricting examples, in particular with reference to the accompanying figures, in which:
The sheet coated according to the invention first comprises a steel substrate, preferably hot-rolled then cold-rolled so that it can be used for the fabrication of automobile body parts. The invention is not limited to this field, however, and can be used for any steel part regardless of its intended final use.
The steel substrate can in particular be one of the following grades of a VHS (Very High Strength steel, generally between 450 and 900 MPa) or UHS (Ultra High Strength, generally greater than 900 MPa) steel, which contain high levels of oxidizable elements:
The steel sheet can optionally be coated with one or more additional layers in addition to the zinc layer in a manner appropriate to the desired properties of the final product. The zinc layer will preferably be the top layer of the coating.
A method for the fabrication of the steel sheet according to the invention is illustrated more particularly in
Located facing the surface of the strip to be coated is an ejection chamber 7 equipped with a slot 8, the upper part of the slot 8 being located at a distance d from the surface of the strip to be coated of for example, between 20 and 60 mm. This chamber 7 is mounted on an evaporation crucible 4 that contains the liquid zinc 9 to be deposited on the surface of the steel strip 3. The evaporation crucible 4 is advantageously equipped with an induction-heating device 5 that makes possible the formation of the vapor. The vapor then escapes from the crucible via a conduit 10 that conducts it to the ejection chamber 7 and the slot 8, which is preferably calibrated to form a jet directed toward the surface of the substrate to be coated. The presence of the slot 8 allows for the regulation of the mass flow of vapor, at a constant sonic speed along the slot (sonic throat) that has the advantage of achieving a uniform deposit. Reference to this technology is made below, using the acronym “JVD” (for Jet Vapor Deposition). Additional information on this technology is presented in patent EP07447056.
In another embodiment not illustrated, the crucible and the ejection chamber are one and the same part, comprising a slot directed toward the surface of the substrate to be coated. In this embodiment, the vapor created by heating the zinc rises directly toward the slot and forms a jet directed toward the surface of the substrate to be coated.
The pressure Pchamber in the deposition chamber 2 and the pressure Peject in the ejection chamber 7 are maintained so that the ratio Pchamber to Peject is between 2·10−3 and 5.5·10−2.
The pressure in the deposition chamber 2 is optionally maintained at a pressure between 6·10−2 mbar and 2·10−1 mbar to improve the surface appearance of the coating.
A layer of oil is then applied to the surface of the sheet thus coated to provide temporary protection when the sheet is stored in a moist, humid and/or saline environment before delivery or the transformation into the final product.
The sheet 3, which may or may not have been subjected to a skin-pass step, can then be cut and shaped, for example by stamping, bending or shaping, to form a part that can then be painted to form a paint film on the coating.
The invention will now be explained below on the basis of tests performed by way of non-restricting examples.
The test performed is also called a “humidotherm test” and is conducted in a controlled-climate chamber as specified by DIN EN ISO 6270-2
The temporary protection against corrosion can be assessed on flat panels simulating a sheet during storage or transport, or on deformed panels representing a metal part (e.g. a vehicle door) stamped at one site and transported to another.
Each panel to be tested is de-greased, and then coated on both faces with an appropriate oil by means of an oil-spray device as specified by ISO 6270-2. The panels are oiled using Fuchs Anticorit RP 4107s, Fuchs 3802/395 or Zeller PL61 at 1.2 g/m2 when the test is performed for a German automaker, or using Quaker Ferrocoat N 6130 at 1.2 g/m2±0.3 g/m2 when the test is performed for a French automaker. The panels are left unassembled and unpainted.
A series of 24-hour aging cycles is then applied to the panels, each cycle including:
8 hours at 40° C.±3° C. and approximately 100% relative humidity (climate-controlled chamber closed, including during the heating), then
16 hours between 18° C. and 28° C. and at ambient humidity (climate-controlled chamber open or ventilated, including during cooling).
At the end of the series of cycles, the modification of the surface appearance of the panels is assessed in terms of percentage of the surface that has been altered.
The proportion of modification of the surface appearance must be less than 10% after 10 cycles for the specimens treated with the Quaker oil or after 15 cycles for the samples treated with one of the Fuchs oils or the Zeller oil.
Four series of cold-rolled IF steel sheet specimens, type DC06, of the type marketed by ArcelorMittal were produced, which comprise a zinc coating 7.5 μm thick. This coating was deposited using a jet vapor-deposition method.
For each of the specimens, the coating was applied with a different pressure ratio between the pressure in the deposition chamber Pchamber and the pressure in the ejection chamber Peject. The distance d between the upper part of the slot 8 of the ejection chamber and the surface of the strip to be coated is identical and equal to approximately 35 mm, and the pressure inside the ejection chamber 7 is approximately 3.4 mbar. The specimens coated as described above are then covered with protective oil and then subjected to a certain number of cycles.
The tests are carried out for 10 or 15 cycles, depending on the type of oil applied, but are interrupted before the end of the 10 or 15 cycles if the entire surface has been degraded.
They are then subjected to the surface corrosion tests, the results are presented in table 2, with the ratio between the pressure Pchamber in the deposition chamber and the pressure Peject in the ejection chamber being:
It has been found that the sheets coated according to the invention exhibit better surface corrosion resistance than a sheet that has not been coated according to the invention.
This is a divisional of U.S. patent application Ser. No. 14/908,014 filed Jan. 27, 2016, which is a national stage of international PCT/M2013/001681 filed Aug. 1, 2013, the entire disclosures of which are hereby incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14908014 | Jan 2016 | US |
| Child | 15934638 | US |
