Powder Metal and/or Refractory Coated Ferrous Metal

As a specific illustration of producing a cured powder metal-resin coated steel strip by the process of the present invention, a strip of cold rolled aluminum killed low carbon steel having a carbon content of 0.05 wt. %, a thickness of about 2 mm. and a width of about 1050 mm. was continuously immersed in an alkaline cleaning bath while travelling at a line speed of about 0.60 meters per second. The alkaline cleaning bath contained about 15 mg per liter (2 oz. per gal.) "Parker 356 Cleaner" heated to a temperature of between 60.degree. C. -77.degree. C. (150.degree. F.-170.degree. F.). The strip leaving the alkaline cleaning bath was then continuously wet brushed with a Scotch-Brite brush operating at 75% of full load capacity. The stip was again passed through an alkaline cleaning bath of identical composition and temperature and thereafter spray rinsed with water at a temperature of 16.degree. C.-27.degree. C. (60.degree. F.-80.degree. F.), and dried with an air knife at 60.degree. C. (140.degree. F.). In some runs the strip was contacted with a 3% aqueous solution of H.sub.2 SO.sub.4 at about 47.degree. C. (115.degree. F.) before the final rinse.

The clean strip was roll coated in a conventional manner with a fluid coating composition prepared by dispersing aluminum metal powder (Reynolds-511) having an average particle size of about 9-13 microns uniformly in a thermoplastic resin binder composition in amounts which provide after curing a dry film composed of about 67 wt. percent (about 48 volume percent) aluminum metal powder and about 33 wt. percent (about 52 volume percent) thermoplastic resin. The thermoplastic resin binder composition was formed of ethyl methacrylate copolymer (Acryloid B-72, a product of the Rohm & Haas Company), containing Cellosolve acetate solvent in a ratio of 40 grams resin to about 170 ml.(6 oz.) solvent to provide a coating composition having a viscosity of 0.57 Pa.S (43 seconds #4 Zahn) and having 38.2 percent volume total solids. The ethyl methacrylate copolymer resin had a Tukon hardness value of about 10-11, and a Glass Transition temperature of about 40.degree. C. The solvent was admixed to adjust the viscosity of the composition to provide the desired coating thickness for the roll coating line and the ratio of metal powder to resin was selected to provide the desired concentration of metal in the coating.

The foregoing powdered aluminum-resin binder fluid coating composition was applied to both surfaces of the strip by conventional roll coating procedure at a first coating station and the coating cured in the first heating oven where the strip was heated at a temperature ranging between about 177.degree. C. and 279.degree. C. (350.degree. F. and 535.degree. F.), and preferably to a peak metal temperature between 207.degree. C. to 212.degree. C. (405.degree. F. to 415.degree. F.) as measured at the exit end of the heating oven to effect complete removal of the volatile Cellosolve acetate organic solvent from the coating and set the resin but without removing or degrading the resin binder and providing a cured dry coating having a thickness of about 30.5 .mu.m (1.2 mils). Where the coated strip is to be immediately heat treated in-line to form an alloyed surface coating, as hereinafter described, the resin coated strip need not be heated to a temperature of 207.degree. C. but only heated sufficiently to substantially remove the solvent (i.e. as low as about 150.degree. C.). Thereafter, during this run the viscosity of the coating system was reduced by adding to the coating bath an amount of Cellosolve acetate solvent which provided in the above manner coated material having a dry coating thickness of about 20.0 .mu.m (0.8 mils). Similar coating solutions were prepared which provided coatings having a dry coating thickness from about 12.7 .mu.m (0.5 mil) to 50.8 .mu.m (2.0 mil). The stability of the fluid powdered aluminum coating systems was excellent, since the coating compositions were used over an 8 hour period without any separation problems occurring. During the foregoing runs no problems were noted in applying the aluminum coating to the surfaces of the steel strip by the roll coating procedure.

As a further example of producing a cured powder metal-resin coated low titanium alloy low carbon steel, a low-carbon aluminum killed full hard steel strip containing about 0.03 wt. % carbon and about 0.4 wt. % metallic titanium with 0.09 wt. % aluminum and the usual amounts of manganese, sulfur, phosphorous, silicon and incidental impurities was cleaned and roll coated in the following manner. Prior to the run, the resin binder (R&H Acryloid B-72) and the solvent (Cellosolve acetate) were mixed in the ratio of 195 g of binder to 1 dm.sup.3 of solvent (1.625 lbs of binder to 1 gal of solvent, and 110 dm.sup.3 (29 gal) of the binder/solvent solution were put in a 208 dm.sup.3 (55 gal) drum along with 154 kg (340 lb) of aluminum metal powder (Reynolds Aluminum R-400). The binder/solvent solution and aluminum powder were thoroughly mixed in the drum before pumping the dispersion into the roll coating pans. The viscosity of the dispersion was adjusted with Cellosolve acetate to 0.26-0.30 Pa S (21-24 seconds #4 Zahn). After adjustment, the wet film coating mixture contained 42.0% total volume solids. The preferred dry film coating which is formed after removal of the solvent contains 90.1 wt. % aluminum powder and 9.9 wt. % resin binder (i.e. 90:10 ratio) and has a dry film thickness of 22.9 .mu.m (0.9 mils). Before roll coating the steel strip was cleaned using the following sequence: (1) Alkaline spraying with a 1.21 kg/m.sup.3 (2.5 oz/gal) solution of "Parker No. 338" cleaner at 65.5.degree. C. (150.degree. F.); (2) Wet brushing; (3) Alkaline spraying with a 0.997 kg/m.sup.3 (2.0 oz/gal) solution of "Parker No. 338" cleaner at 65.5"C. (150.degree. F.); (4) Pickling with a 3% aqueous H.sub.2 SO.sup.4 solution at 47.7.degree. C. (118.degree. F.); (6) Rinse spraying with water at 60.degree. C. (140.degree. F.); and (7) Drying.

The aluminum powder coating was applied by roll coating two sides of the strip at the finish coater station and then drying and curing the coating. Temperature tapes, contact pyrometer and Ircon infrared instrumentation were used to control the oven temperature to obtain a peak metal curing temperature suitable for processing and acceptable coating adhesion. Depending on line conditions, peak metal temperatures between 193.3.degree. C. and 210.degree. C. (380.degree. F.-410.degree. F.) and preferably at about 207.degree. C. were maintained during the processing as measured by the Ircon infrared non-contact temperature sensing device using an emissivity setting of 0.73.

During setup for the run, it was noted that coating transfer occurred at the first support contact roll on the bottom side of the strip after the curing oven. This was caused by inadequate cooling time prior to contact with the roll. Therefore, the coating on the strip bottom side was limited to a maximum thickness of 38.1-40.6 .mu.m (1.5-1.6 mils) wet film thickness which provided a 25.4 .mu.m (1.0 mil) dry film thickness. Water sprays were applied to the bottom side of the strip to eliminate coating transfer to the contact roll after the curing time. Dry coatings between about 12.7 .mu.m (0.5 mil) and 38.1 .mu.m (1.5 mil) were produced in the foregoing run.

A powder metal-resin coated steel strip having a composition similar to a "Zincrometal" coating (i.e. a zincchromate product of the Diamond Shamrock Corporation of Cleveland, Ohio) was also produced by roll coating as described herein using as the powder material a mixture of metallic zinc powder and chromium oxides (CrO.sub.3 /Cr.sub.2 O.sub.3) powder which was mixed with Acryloid B-72 thermoplastic resin in a proportion of about 86 wt. percent of the zinc-chromate powder mixture to about 14 wt. percent of the thermoplastic resin, such as Acryloid B-72.

A ferrous metal strip can also be provided with a multiple powdered metal coating comprising a prime or undercoating of one powdered metallic material, such as zinc, and a finish or surface coating of second powdered metallic material, such as aluminum. The ferrous metal strip is cleaned as described herein or in any conventional manner and provided with the first coating comprising a suspension of powdered metallic zinc in a thermoplastic resin, such as ethyl methacrylate copolymer, having the above specified properties in a ratio of about 70 wt. % (about 50 volume percent) powdered zinc and about 30 wt. % (about 50 volume percent) resin on a dry basis, and containing sufficient Cellosolve acetate solvent to reduce the viscosity sufficiently to permit forming a uniform fluid suspension of the powdered zinc, and applying the fluid suspension to an endless low carbon cold rolled steel strip by roll coating to provide a dry coating having a thickness of about 15.2 .mu.m (0.6 mils) after heating at a peak temperature between 207.degree. C.-212.degree. C. (405.degree. F.-415.degree. F.) to remove the solvent and effect curing. The powdered zinc coating is smooth, non-porous ductile and provides good salt fog resistance. A powdered aluminum metal coating is applied directly to the dry zinc coating in the same manner as previously described for applying a powder aluminum coating followed by heating at a peak temperature of between 177.degree. C. and 279.degree. C. (350.degree. F. and 525.degree. F.) to remove the solvent and set or cure the resin in the outer coating. The coatings can be temper rolled to provide a smooth impervious surface having good oxidation resistance.

An improved protective refractory coating is also provided on a clean surface of a steel panel or strip by the process of the present invention using a fluid suspension comprising a dispersion of finely divided glass frit, Acryloid B-72 resin, and Cellosolve acetate solvent so as to provide after removal of the solvent a thin dry coating having a thickness between about 15.2 .mu.m (0.6 mil) and about 30.5 .mu.m (1.2 mil). By roll coating a first coating a steel strip using a resin to glass frit ratio of about 35 parts by wt. of the resin to 65 parts by wt. glass frit followed by applying a conventional white enamel top coating and firing at a temperature of 1460.degree. F. for 4 minutes a composite refractory coating is formed which passes the Olsen Button Test with a rating of about 10, where "O" represents 100% coating removed after taping and "10" represents no coating removed after taping. The glass frit containing the following ingredients is prepared by heating the following several specified ingredients or their equivalents to a temperature of about 1010.degree. C. (1850.degree. F.) for about 5 minutes, quenching in water, and grinding to a very fine powder:

The glass frit can also be mixed with metallic aluminum powder or other powdered protective metal in a ratio of about 35 parts by wt. glass frit to about 65 parts by wt. metallic powder, and a suspension thereof in Acryloid B-72 resin applied to a steel panel in the same manner as described herein to provide a formable cermet coating.

In the further embodiment of the present invention where the cured powder-resin coatings must be subjected to deep drawing or frictional deformation during fabrication the adherence of the resin-powder metal coatings is greatly improved by applying a very thin surface coating of a dry film lubricant to the cured thermoplastic resin-powder coatings (i.e. after removing the solvent but before removal of the resin binder). The dry film lubricant is preferably formed by applying a lubricant composition over the cured resin coating in the form of an ultra thin film of a lubricant material dispersed in a volatilizable liquid which when removed leaves a thin dry surface lubricating film. The General Motors (GM) Double Draw Test is commonly used to evaluate the ability of protective metal coatings on a ferrous metal strip to withstand substantial surface abrasion applied by a deforming tool without causing the coating to separate from the steel base.

In the following Table I the effectiveness of applying the herein disclosed dry film lubricant coating is demonstrated by comparing the percentage of the powder coating removed when a steel strip having the indicated coating thereon is subjected to the General Motors Double Draw Test when the dry film used is: (1) Molybdenum disulfide, (2) Graphite, (3) a slushing oil, and (4) no lubricant coating on several powder coatings. The data give the average results:

Although titanium is the essential alloying element to be added to the low carbon steel where good room temperature formability and oxidation resistance at temperatures above about 677.degree. C. (1250.degree. F.) is required, it is also within the scope of the invention to add small amounts of other metallic alloying elements, such as molybdenum or columbium, to improve the physical properties of the base steel. However, the secondary alloying elements should not impair the oxidation resistance of the steel, and the amount of secondary alloying elements preferably should not exceed about 2 percent by weight of the steel. Thus, the base steel of the present invention is in every instance a low alloy low carbon steel.

In the specification and claims the term "powder aluminum" or "aluminum coating", where the context does not require otherwise, designates finely divided metallic aluminum and metallic aluminum containing minor amounts of other metals, such as between about 5 to 12 wt. % silicon, which do not significantly reduce the oxidation resistance characteristic of an aluminum base coating. Also, the term "ferrous metal" as used in the specification and claims designates a low carbon steel and low alloy steel wherein the added alloying element or elements which are not normally present in a low carbon steel do not exceed a total of about 2 wt. % of the steel.

The present invention comprises a process for providing metal strips having a ferrous metal base with improved protective resin-powder coatings which are useful in the cured resin state and which can be further processed or when heated to an elevated temperature in use provides a ductile, impervious protective metal coating, such as a metallic sinter coating or a solid solution iron-alumium diffusion alloy coating, and is uniquely adapted for producing in an economical manner with conventional equipment a wide range of coated strip material using relatively inexpensive material which can be used in place of more expensive coated steel and alloy sheet material.