US 3355265 A
Description (OCR text may contain errors)
United States Patent I 3,355,265 METHOD OF PRODUCING DUCTILE COATED STEEL AND NOVEL PRODUCT Robert M. Hudson, Churchill Borough, Allegheny County, and Andrew Lesney, Frazer Township, Allegheny County, Pa., assignors to United States Steel Corporation, a corporation of Delaware No Drawing. Filed Apr. 16, 1965, Ser. No. 448,875 14 Claims. (Cl. 29-1835) ABSTRACT OF THE DISCLOSURE A method of producing ductile coated steel product having a coating of, for example, chromium, copper, nickel and titanium, which comprises cold reducing a metallurgically bonded coated steel strip and annealing the coated steel strip at a temperature below the Ac; temperature to completely recrystallize the strip without causing substantial alloying of the steel and coating metal. Also disclosed herein is a fully annealed coated steel foil 0.004-inch thick or less having a protective coating of, for example, one of the aforementioned coating metals, metallurgically bonded thereto but not completely alloyed therewith.
This invention relates to a method for producing ductile coated steel product and to a novel product. More particularly, the invention is directed to a method of producing steel having a protective metal coating and which is soft and has excellent folding endurance. The method is pain ticularly suitable to the production of ductile coated steel in very light gauges and, in particular, ductile coated steel foil.
There is currently a need for steel product in light gauges which contains a coating of protective metal. Uncoated steel which is available with good ductility possesses unsatisfactory properties for many purposes with respect to corrosion resistance. To improve its surface properties, steel has been coated with a variety of materials, the more common being tin, zinc and lead. The recent innovation of steel foil, i.e. steel which is cold reduced to a thickness of less than 0.004 inch, has held out the promise of a significantly improved packaging material replacing such materials as aluminum, plastics and paper. Plastic and paper do not provide adequate resistance to penetration of vapors and moisture and, along with aluminum, have a low tearing strength and puncture resistance. In contrast, the high strength and excellent chemical, biological and radiological resistance of steel foil renders it well suited for use as a packaging material. Coated steel foil would add improved surface properties to the already impressive list of steel foil advantages.
In conventional manufacture of coated steel products such as tin plate, galvanized sheet and plate, etc., uncoated steel, referred to as black plate is cold reduced on a cold-reduction mill generally before, but sometimes after coating. The cold reduction increases the strength of the steel but also increases the steels hardness and stiffness, which of course, means that the steel has a low' ductility. Cold reducing steel to foil gauge, i.e. less than 0.004 inch, greatly lowers ductility and thefoil may be unsuitable for many uses, particularly in the packaging field where fonmability is very important. Thus, the poor ductility severely limits the use of the coated steel product by restricting its ability to be shaped or fabricated and the full potential of coated steel in light gauges carinot be realized.
Ordinarily, cold-reduced steel can be softened by annealing; however, coated steel products cannot be conventionally annealed without undue reduction in strength and loss of the protection olfered by the coating because of alloying between the steel base and the coating. These difiicul-ties are particularly troublesome where light-gauge steel such as foil is involved. Coated steel foil will usually have a very thin metal coating, and such a thin layer may readily alloy with the steel base or, in some instances, may react with carbon in the steel to form carbides of a generally brittle nature which make shaping without cracking virtually impossible. The present invention provides a method for producing coated steel in a ductile condition which has excellent folding endurance and, therefore, in light gauges is very suitable for packaging applications.
According to the present invention, a coated-steel product is cold reduced to desired thickness and then annealed without causing substantial alloying of the steel and coating. The annealing is performed at a temperature below the critical (A0 temperature to avoid transformation of the steel microstructure and complete alloying with the coating. The A0 temperature is the temperature at which on heating, the body-centered cubic structure, ferrite, changes to face-centered cubic, austenite. For steels having above 0.0Q5 carbon, this temperature is 1333 F. Metal coatings useful in our method are those which have a melting point sufiiciently high to permit complete recrystallization of the steel during annealing without completely alloying with the steel. Such metals suited for use with the invention include chromium, copper, nickel and titanium which have melting points that permit annealing of the steel without complete alloying with it. I
Although some alloying is desirable to insure a chemical bond, i.e. metallurigical bond, between the coating and the steel base, substantial alloying of the steel and the coating should be avoided in order to :maintain the integrity of the coating and the improved properties and corrosion resistance which it provides. A metallurgical bond is much stronger than a mere mechanical bond and is less likely to peel or flake otf during cold reduction; however, as pointed out above, complete alloying of the coating and base is to be avoided to preserve the integrity of the coating and its value as a protective surface for the steel. If the coating completely alloys with the steel, the resulting alloy will not possess the protective characteristics of the unalloyed metal coating. The invention provides a method for making a coated steel product in which the integrity of the coating is maintained but which results in a completely recrystallized steel base which is ductile and possesses a high folding endurance.
Some metals useful as protective coatings for steel undergo chemical reaction with uncombined carbon in steels to form carbides. The carbides are generally very brittle and when present in sufiicient quantity make it impossible to satisfactorily shape the steel. Furthermore, the carbides do not otter the same surface appearance or degree of protection to the steel base. When coating materials are used which react with uncombined carbon, it is necessary to employ either steels which have been decarburized or carbon stabilized steels. It has been found that in the case of plain carbon, decarburized steels, the carbon content should be brought to a level below 0.007 percent (by weight) to provide a satisfactory steel base without undue carbon reaction with the coating. Decarburization is, however, fairly time consuming and for this reason carbon stabilized steels may be preferred. The term carbon stabilized steels as used herein refers to steels in which a known carbon stabilizer such as titanium, vanadium, etc. has been added which lowers the level of uncombined carbon to 0.007 percent or less. In such steels, the stabilizer is present according to known relationships in sufiicient quantity so that the carbon content is at the desired level. Thus, for example, titanium may be used at a ratio of four parts titanium to one part carbon to lower the uncombined carbon content. However, not all potentially useful coating metals undergo reaction with carbon to form undesirable carbides. Some metals, such as nickel, may be used without danger of brittle carbide formation and would not require the aforementioned carbon restrictions.
Although, as pointed out above, a number of metal coatings are suitable in practicing the invention, chromium coated products are currently of particular interest to industry, and accordingly, specific examples herein will be directed to this embodiment. In the preferred practice of the invention, decarburized steels are used. In decarburizing according to the preferred practice, plain carbon cold rolled steel is decarburized as an opened coil in a decarburizing and reducing atmosphere typically containing hydrogen, water vapor and inert gas. The decarburization is generally conducted at a temperature in the range of 1250 F. to 1350 F. for a time sufficient to lower the carbon content to the desired level. Decarburization can be performed in any suitable reducing atmosphere. Where hydrogen and water vapor are used, as in the preferred embodiment, the hydrogen need not be as pure as is necessary in annealing chromized steel to preserve a characteristic bright appearance. The metal coating, e.g. chromium, may be advantageously applied by circulating chromium-halogen-containing gases through the spaces between the wraps of the open coil according to known chromizing techniques. After coating, e.g. chromizing, the coated steel coil can be cold rolled to foil gauge. The coated steel foil is then annealed, preferably in a high-purity hydrogen atmosphere, either by box annealing, i.e. batch annealing, or continuous annealing to completely recrystallize the steel but without causing substantial alloying of the steel and coating.
Because of the short time periods involved, continuous annealing is less arduous and the conditions employed are less critical. For example, the hydrogen atmosphere need not have as high a degree of purity as that required for box annealing where much longer time is involved. Conventional continuous annealing lines may be used and the upper portion of the 1050 F. to 1300 F. range is more useful with continuous annealing than the batch annealing. Satisfactory product has been produced by continuous annealing coated steel foil at l200' F. to 1300 F. for up to 40 seconds; the preferred conditions being 125 F. for 20 to 30 seconds. Because of difficulties associated with handling foil gauges in a continuous annealing operation (e.g. strip breakage) box annealing offers some advantages over the continuous treatment.
For a particularly outstanding surface appearance of chromium coated foil, it is necessary to employ controlled annealing conditions and an atmosphere containing very high-purity hydrogen. It has been found that the atmosphere should contain hydrogen of a purity attainable by diffusion through a heated (450 C.) palladium-silver alloy membrane. Prior to heating, the system should be carefully purged to insure that the gas purity in the annealing chamber will be adequate at the elevated temperatures. It has been further found that initial reduction of the gas pressure to about 5 mm. is helpful. During heating, occluded oxidizing gases such as water vapor and oxygen will evolve from the coil and it is advantageous to preheat the coil to about 600 F. to 900 F. and to maintain this temperature for at least four hours before further heating. The chromized steel foil should be wrapped on a stainless steel rather than a carbon steel sleeve to minimize the quantity of evolved impurity gases from this source. The high-purity hydrogen gasfiow rate used during box annealing should be as high as practicable and if possible to a maximum of 200 cubic feet per hour for a single coil-stand base. After this preheating stage, the coil is then heated to an annealing temperature for recrystallization in the range of about 1050 F. to about 1300" F. for a time sufficient to completely recrystallize the steel, but insufiicient to cause any substantial alloying of the coating and the steel. The preferred temperature for box annealing is in the range of l050 F. to 1150 F. Rapid cooling to a temperature at which no gas-metal reactions occur, e.g. about 400 F. is desirable after which hydrogen may be purged from the system with nitrogen or other suitable gas prior to uncovering the coil.
As a typical example of the preferred embodiment of the invention, a coil of plain-carbon cold rolled steel of about 0.0359 inch thick and with a carbon content of 0.10 percent was decarburized by open-coil annealing in an atmosphere of about 9 percent hydrogen and the balance nitrogen at a temperature of about 1275 F. In this operation, the carbon content was reduced to about 0.006 percent. The opened coil was then coated with chromium by vapor deposition to provide a one mil (0.001 inch) thick coating. The coated steel was then cold reduced about 97 percent to a foil gauge of approximately 0.0010 inch. The foil was then cleaned according to the preferred practice to remove the rolling lubricant which was used. A portion of the foil was continuous annealed at a temperature of 1250 F.; another portion was annealed in coil form using the aforementioned box annealing techniques. Upon micrographic examination, both specimens were shown to possess a smooth substantially uniform chromium coating metallurgically bonded to the steel base and a uniform surface without jagged raised portions indicative of cracking or flaking.
Table I below shows the properties of coated steel foil produced according to the procedure described herein. For contrast, the properties of the coated foil are compared with aluminum foil properties.
AzChromized steel foil continuous annealed.
Bzcliromized steel foil box annealed.
C=Alun1inum foil box annealed.
It is apparent that various changes and modifications may be made without departing from the invention. For example, various coating techniques may be employed to apply a protective metal covering to a steel base. For drastic cold reduction, as is performed in the production of steel foil, the coating should be metallurgically bonded to the steel base to minimize or prevent peeling, cracking or flaking of the coating during cold reduction. A preferred annealing temperature range of 1050 F. to 1300 F. has been presented herein; however, it should be understood that lower or slightly higher temperatures may be employed as long as the cold reduced steel is completely recrystallized to lower its hardness and increase its ductility and complete alloying between the coating and steel base is avoided.
1. A method for producing a ductile coated steel foil comprising cold reducing steel having a metallurgically bonded coating of a metal from the group consisting of chromium, copper, nickel, and titanium and annealing said cold reduced, coated steel at a temperature below the (Ac temperature for a time suflicient to completely recrystallize said steel but insuflicient to cause substantial alloying of said coating and said steel at the coating metal surface.
2. A method of producing ductile coated steel foil comprising applying a protective metal coating from the group consisting of chromium, copper, nickel and titanium to a steel from the group consisting of plain carbon steel decarburized to a carbon level of less than about 0.007 percent and carbon stabilized steel having an uncombined carbon content of not greater than 0.007%, cold rolling said coated steel to foil gauge and annealing said cold rolled, coated steel at a temperature below the (Ac temperature for a time suflicient to completely recrystallize said steel but insufiicient to cause substantial alloying of said coating and said steel at th coating metal surface.
3. A method according to claim 2 wherein said coated steel foil is annealed at a temperature in the range of about 1050 F. to about 1300 F.
4. A method of producing ductile steel foil having a protective metal coating comprising applying a coating of chromium to steel strip, said steel being selected from the group consisting of plain carbon steel decarburized to a carbon level of less than about 0.007 percent and carbon stabilized steel, cold rolling said coated steel strip to foil gauge and annealing said coated steel foil at a temperature in the range of about 1050 F. to about 1300 F. for a time sufiicient to completely recrystallize said steel but insuflicient to cause substantial alloying of said steel and said chromium metal coating at the surface of the chromium coating.
5. A method according to claim 4 wherein said coated steel foil is box annealed at a temperature of about 1050 F. to about 1200 F., for up to about five hours.
6. A method according to claim 5 wherein said box annealing is conducted in an atmosphere containing highpurity hydrogen.
7. A method according to claim 4 wherein said coated steel foil is continuously annealed at a temperature of about 1200 F. to about 1300 F. for up to about 40 seconds.
8. A method for producing ductile steel foil having a protective metal coating comprising decarburizing an open coil of plain carbon steel in a reducing atmosphere to a carbon level of less than about 0.007 percent, vapor coating a protective layer of chromium onto said decarburized steel, cold rolling said chromium coated steel to foil gauge and subsequently annealing said coated steel foil in an atmosphere containing high-purity hydrogen and inert gas at a temperature in the range of about 1050 F. to about 1300 F. for a time sufiicient to completely recrystallize said steel but insufficient to cause substantial alloying of said steel and said chromium coating at the surface of the chromium coating.
9. A method according to claim 8 wherein said steel is decarburized in an atmosphere containing water vapor, hydrogen and inert gas.
10. A method for producing ductile steel foil having a protective metal coating comprising heating said foil to decarburizing temperature in a reducing atmosphere to decarburize said steel to a carbon level of less than about 0.007 percent, coating a protective layer of chromium onto said decarburized steel, cold rolling said chromium coated steel to foil gauge, heating the cold rolled coated steel foil to about 600 F. to 900 F. in a high-purity hydrogen atmosphere to evolve oxidizing gases from the coil, subsequently annealing said coated steel foil in said high-purity hydrogen atmosphere at a temperature in the range of about 1050 F. to about 1300 F. for a time suflicient to completely recrystallize said steel but insufiicient to cause substantial alloying of said steel and said chromium coating at the surface of the chromium coating.
11. A method according to claim 10 wherein, after annealing, the steel is rapidly cooled to a temperature below which gas-metal reactions will not occur and then the reducing atmosphere is purged from the system with inert gas.
12. A method for producing ductile steel foil having a protective metal coating comprising heating said foil to decarburizing temperature in a reducing atmosphere to decarburize said steel to a carbon level of less than 0.007 percent, vapor coating a protective layer of chromium onto said decarburized steel, cold rolling said chromium coated steel to foil gauge, heating the cold rolled coated steel foil at about 600 F. to 900 F. in a high-purity hydrogen atmosphere to evolve oxidizing gases from the coil, subsequently annealing said coated steel foil in said high-purity hydrogen atmosphere at a temperature in the range of about 1050 F. to about 1150 F. for a time suflicient to completely recrystallize said steel but insuflicient to cause substantial alloying of said steel and said chromium coating at the surface of the chromium coating and cooling said annealed coil in said high-purity hydrogen atmosphere to a temperature below about 400 F. before withdrawing said coil from said high-purity hydrogen atmosphere.
13. A steel foil product comprising a fully annealed, cold rolled steel base less than 0.004 inch thick having a protective metal coating from the group consisting of chromium, copper, nickel and titanium metallurgically bonded thereto at the interface but not completely alloyed therewith at the surface of said coating.
14. A steel foil product according to claim 13 wherein said steel has a protective coating of chromium.
References Cited UNITED STATES PATENTS 3,058,856 10/1962 Miller 148-16 3,095,361 6/1963 Stone 20429 3,123,493 3/1964 Brick 117-50 3,214,820 11/1965 Smith et al. 2918 3,244,565 4/1966 Mayer et al. 148-42 3,281,262 10/1966 Brick 117-50 3,285,790 11/1966 Lockwood 14812.1
DAVID L. RECK, Primary Examiner. HYLAND BIZOT, Examiner.
H. F. SAITO, Assistant Examiner.