Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2001-02-23
2003-02-25
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C427S376200, C427S376500, C427S406000, C427S419600, C427S433000, C428S630000, C428S659000, C428S469000, C428S471000, C428S332000, C428S433000, C428S926000
Reexamination Certificate
active
06524725
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to steel surfaces with a zinc or zinc-alloy coating comprising a vitreous enamel layer and a process for enamelling of zinc or zinc-alloy precoated steel surfaces.
2. Description of the Related Art
Steel surfaces in particular in the form of sheet steel are a unique material for countless applications. It can be cut and shaped by bending, folding, deep drawing and punching. It has a high mechanical strength and it can easily be joined together by welding. Since iron oxidizes easily, corrosion protective surfaces in particular made of a number of metallic, organic and inorganic coatings are known. For example the coating of steel with a thin layer of zinc or zinc-alloys is well-known in the art. Furthermore, it is well-known in the art that vitreous enamel has very special properties which differ in many aspects from other coatings.
Vitreous enamel is a long established finish for metal. Vitreous enamel is an entirely inorganic finish related to the glass family. It mainly is based on quartz, boron oxide, aluminium oxide, (earth) alkaline oxides and fluorine, which are crosslinked to a three-dimensional network. According to the requested application, a variety of vitreous enamels differing widely in chemical composition, are used in practice.
Vitreous enamelled products are widely used in different applications for example in washing machines, sanitary ware such as bath tubes, shower bases, cooking ranges, domestic appliances as well as internal and outside construction materials. The vitreous enamel is applied to the substrate surface after an intensive pretreatment to insure and improve the adherence of the enamel. In order to improve the adherence of the enamel, customarily a very complicated pretreatment as well as a high firing temperature is required. The pretreatment causes a serious problem of large amounts of waste liquids. When steel sheet is used for enamelling, the procedure for direct white enamelling can be described as following: rinsing, degreasing, rinsing, pickling, rinsing, nickling, rinsing, passivating, rinsing, applying the enamel and firing typically at a temperature in the range of 800 to 900° C. By this firing an adherence providing intermetallic layer of the Fe—Ni—Ti-type is formed at the interface between steel and enamel whereby Ti comes from the used frits and Ni is the customarily used material for the pretreatment. The procedure for a two coat one fire enamelling can be described as following: rinsing, degreasing, rinsing, pickling, rinsing, passivating, rinsing, applying ground enamel layer, applying top enamel layer, firing at a temperature between typically 800 and 900° C. By this firing the adherence is caused by reactions between the Ni-, Co-, Cu-, Mn- and/or Sb-ions of the ground enamel and the iron/iron ions coming from the steel.
JP 95-176857 A corresponding to JP 09003660 discloses enamelling of a zinc-alloyed surface without pretreatment. A steel sheet is provided with the layer of alloyed Zn on the surface by alloying just after the dipping in the Zn-bath. With respect to the enamel frit a firing temperature of up to 600° C. is disclosed. However, this reference discloses that firing at a temperature above 550° C. shows that the Fe-content of the intermediate phase is in excess of the optimum range, so that the formability of the substrate is extremely lowered. As a consequence, the adherence of the enamel is low. The reference does not discuss any specific kind of steel to be used.
JP 60-169571 A discloses an enamel coated steel plate obtained by forming an enamel coating on an aluminum zinc-alloy coating formed on plates. The coated steel plate is used for facing materials, flat plates, and embossed plates in applications of sidings, sash, interiors, and roofings. Low costs, high corrosion, wear, chemical, and weather resistances are provided along with good ceramic image, and high dimensional stability. The Al-Zn plated coating may contain La, Ce, Mg as an additive to improve wettability to the steel, coating layer and qualities with contents about 0.01 to 0.5% of the plating layer. Due to the low melting point of aluminum the firing requires a temperature of less than 660° C.
JP 52-105921 discloses enamelling steel plates having firmly adhered coatings with pretreatment including electrodeposition of zinc. The steel plate containing more than 0.03% C is preliminary washed with acid and then treated with Ni. It is then electroplated with Zn in electrolytic solution, and coated with a titanium white enamel, dried and burned. The Zn is plated to a thickness of 0.1 to 1 &mgr;m. The steel plate has firmly adhered enamel coating and formation of foams is avoided. U.S. Pat. No. 2,056,399 discloses a coating process and coated articles. The reference discloses the application to the article of an intermediate layer of a surfacing material having the desired tenacity imparting, and the desired rust resisting characteristics. The material is applied through the use of a metal which is capable of producing a thin, spongy, metal like layer, zinc being preferably used on account of its commercially availability at low cost.
GB-A-818,264 discloses enamelled ferrous articles. The first step of the process consists in a heat treatment of a zinc or cadmium coated steel sheet or article in order to allow the zinc or cadmium to diffuse into the metal. That layer is provided with the frit which may be of any of the general composition recommended for enamelling aluminum.
U.S. Pat. No. 3,849,175 corresponding to DE-A-21 03 368 and U.S. Pat. No. 3,935,088 corresponding to DE-A-20 45 265 disclose a method for coating steel parts with enamel. The method relates to the direct application of enamel to steel parts consisting of coating the surface of the steel parts with a zinc layer of between 0.3 to 5 g/m
2
before the enamel coating is applied over the zinc layer in order to suppress defects which occur in the surface of the steel parts and provide a long-lasting enamel surface. Taking into account that a surface layer of 7.13 g/m
2
relates to 1 &mgr;m, a very thin zinc layer is provided. U.S. Pat. No. 3,935,088, column 2, line 27 ff. discloses that the coating of metal should not be too thick, for otherwise the enamel adhesion is poor.
Thus, it is the object of the invention to enamel a zinc or zinc-alloy coated steel surface directly with an enamel by avoiding either the application of a pretreatment with Ni or by avoiding the application of a ground coat as a kind of adhesive. Furthermore, no pickling step is required, whereby the selected vitreous enamel composition should have a firing temperature as high as possible.
SUMMARY OF THE INVENTION
Thus, in a first embodiment of the invention the above mentioned objective is attained by a steel surface with a zinc or zinc-alloy coating having a thickness in the range of 1 to 30 &mgr;m and in contact thereto comprising a vitreous enamel layer being essentially free of adherence providing oxides selected from the group of Co, Ni, Cu, Sb or Mn.
Thus, the present invention in particular avoids the use of an adherence providing nickel layer on the steel surface and the use of adherence providing oxides in the composition of the vitreous enamel.
As distinct from the enamels used for enamelling aluminium and aluminised steel sheet, whose chemical resistance is poor compared to the conventional steel enamels, the vitreous enamels for enamelling a zinc or zinc-alloy coated steel substrate can reach the same chemical resistance (alkaline or acid) as do the conventional steel enamels.
DESCRIPTION OF THE PREFERRED EMBODIMENT
In a preferred embodiment of the present invention the appropriate metal substrate has to be selected carefully. The inventors have found that certain qualities of steel are particularly suited for direct enameling according to the present invention. Mainly cold rolled steel sheet with carbon contents of less than 0.08% by weight are preferably used according to the present invention. In particular, preferred are
Lips Koen
Schlegel Christian
Enamels and Ceramic Coatings International C.V.
Koehler Robert R.
Venable
Wells Ashley J.
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