Polyamide-imide coated substrate

Abstract

Steel substrate suitable for forming operations which includes a coating thereon wherein the coating includes a corrosion resistant layer of polyamide-imide having a dry film thickness between 1 and 10 μm.

Description

FILED OF THE INVENTION[0001]The present invention relates to a steel substrate comprising a corrosion protective coating and to a method for making the same. The present invention further relates to the use of the steel substrate in forming operations to form a part.BACKGROUND OF THE INVENTION[0002]Galvanized steel is used in applications where rust resistance is needed, for instance, in the automotive industry where the zinc layer reduces the onset of rust at exposed edges and surfaces.[0003]Additional coating layers are provided to make the galvanised steel substrate more aesthetically pleasing and to further protect the steel substrate from corrosion. In this respect galvanised steel substrates are first provided with a phosphate coating (2-5 μm) to enhance paint adhesion, by enhancing paint adhesion such phosphate coatings indirectly enhance corrosion resistance.[0004]The application of the phosphate coating comprises the steps of dissolving metal phosphate salts in a solution o...

AI Technical Summary

Benefits of technology

[0011]In a preferred embodiment of the invention the PAI layer has a thermal stability between −60° C. and 400° C. meaning that it will not crack or thermally degrade at reduced or elevated temperatures which may be required for certain forming operations.

[0012]In a preferred embodiment of the invention the thermal degradation temperature of the layer of PAI is at least 250° C., preferably at least 400° C.

[0013]In a preferred embodiment of the invention the layer of polyamide-imide has a dry film thickness between 3 and 5 μm. PAI layers having a thickness between 3 and 5 μm exhibit excellent performance in respect of barrier, adhesion and corrosion resistance properties that can be provided at reduced cost relative to other thicker coatings. Preferably the layer of polyamide-imide has a dry film thickness between 2 and 5 μm since further cost savings can be made by providing 2 μm thick PAI layers.

[0014]In a preferred embodiment of the invention the layer of polyamide-imide has a coefficient of friction between 0.13 and 0.2.5, preferably between 0.13 and 0.2 at a temperature between −10 and 120° C. Automotive manufactures specify that coefficient of friction CoF values should be between 0.13 and 2.5. The coated steel strips according to the invention comply with such standards even when no lubricant is applied.

[0015]In the automotive industry lubricant is typically required when forming galvanised steel strips, and in certain instances, it may also be necessary to provide the galvanised steel strips with texture to improve lubricant retention at the steel strip surface. However, providing texture can be detrimental to both the formability of the galvanised steel strip and the appearance of such formed and painted steel parts. Another consequence of providing texture is that the friction coefficient will increase relative to galvanised steel strips without texture. Advantageously the layer of polyamide-imide is lubricating, which means an additional step of providing a lubricant and / or texture during subsequent forming and / or stamping operations is not required.

[0016]In a preferred embodiment of the invention the difference in friction coefficient between the first tool pass and the last tool pass of the linear friction test is less than 0.2, preferably less than 0.1 and more preferably less than 0.05. A stable CoF, preferably between 0.13 and 0.25, is of importance to automotive manufacturers. A tool pass may be defined as the number of times a coated strip is drawn between a flat tool and a cylindrical tool of the linear friction test apparatus.

Problems solved by technology

This is not trivial and increases manufacturing costs.

The use of phosphoric acid also introduces environmental and safety issues related to the handling and disposal of such solutions.

The use of electrophoretic deposition to apply organic coatings increases the cost of manufacturing automotive parts due to the high voltages that are required to apply such coatings.

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