US3726705A - Process for galvanizing a ferrous metal article - Google Patents

Process for galvanizing a ferrous metal article Download PDF

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US3726705A
US3726705A US00158225A US3726705DA US3726705A US 3726705 A US3726705 A US 3726705A US 00158225 A US00158225 A US 00158225A US 3726705D A US3726705D A US 3726705DA US 3726705 A US3726705 A US 3726705A
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coating
strip
ferrous metal
galvanizing
hot
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W Carter
A Preban
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Inland Steel Co
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Inland Steel Co
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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/003—Apparatus
    • C23C2/0038—Apparatus characterised by the pre-treatment chambers located immediately upstream of the bath or occurring locally before the dipping process
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0222—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating in a reactive atmosphere, e.g. oxidising or reducing atmosphere
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/024—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by cleaning or etching
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/026—Deposition of sublayers, e.g. adhesion layers or pre-applied alloying elements or corrosion protection

Definitions

  • the non-ferrous metals found most useful for surface coating of the steel strip are nickel, copper, and combinations of nickel and copper.
  • the non-ferrous metal coating is preferably applied to the strip by plating either electrolytically or by nonelectrolytic displacement to provide a coating having a weight ranging between about 10 and 100 mg./ft.
  • the present invention relates to a process of hot-dip galvanizing ferrous metal articles, including hot-dip coating a steel sheet with zinc and with alloys of zinc contain ing aluminum, magnesium, or aluminum and magnesium and the like alloying metals which are suitable for forming a protective zinc base coating.
  • a clean ferrous metal sheet free of scale is oxidized to provide a thin uniform iron oxide surface film which is free of all oxidizable impurities. Since the oxide surface is not coated by a hot-dip galvanizing bath when immersed therein, however, the sheet must be passed through a reducing atmosphere heated to an elevated temperature to reduce the iron oxide film to metallic iron. And, while the substantially pure iron surface is protected against reoxidadon, the strip is immersed in a hot-dip galvanizing bath to provide an adherent galvanized coating.
  • Sendzimir hot-dip galvanizing process forms an adherent uniform protective metal coating on a ferrous metal strip and the surface thereof is readily oxidized when exposed to an oxidizing atmosphere
  • Heating a steel strip to a temperature ranging between about F. and F. is well above the temperature which causes a substantial change in the physical properties of the steel where the strip has been provided with special physical properties before the strip is galvanized in a Sendizimir-type galvanizing process.
  • a steel sheet is provided with a martensitic structure before being coated in a Sendzimir-type galvanizing process, the sheet will have practically none of the desired martensitic structure after passing through the reduction step wherein the strip is heated to a temperature of about 1500 F. or above. It is preferable not to heat the martensitic strip substantially above 900 F. during the galvanizing process, if the martensitic structure is to be largely retained.
  • a clean scale-free ferrous metal article such as a steel strip or sheet
  • a thin metallic surface coating of a non-ferrous metal which readily forms an oxide which is reducible to the metallic form at a temperature substantially below 1500 F., and preferably below a maximum temperature of 1050 F.
  • a marked reduction or complete loss of the special physical properties of a ferrous metal strip such as Where the strip has been treated to provide full hard mechanical properties, results when the strip is heated to a temperature range between 1500 F. and 1800 F., in the regular Sendzimir-type galvanizing process.
  • the non-ferrous metals whose oxide has the foregoing properties and which are suitable for surface coating a steel strip in accordance with the present invention include nickel, copper, silver and cobalt.
  • Nickel or copper and combinations thereof have been found most suitable from a cost standpoint, and because the coatings form oxides which are reducible to the metallic form at a relatively low temperature (i.e., below 900 F.) and at a temperature close to the melting point of the galvanizing bath.
  • thin oxide coatings of nickel, copper and combinations thereof are sufficiently ductile to remain on the strip during the bending of the strips while passing through a continuous coating line.
  • non-ferrous metals which otherwise would be suitable but which have a boiling point below the temperature of the galvanizing bath or a boiling'point below the reduction-heat treating temperature required for reducing the oxide of the thin surface coating of non-ferrous metal to the metallic form are not preferred for use in accordance with the present invention.
  • the non-ferrous metal surface coating can be formed by any suitable means or process and can be extremely thin without impairing the usefulness in the present invention.
  • the non-ferrous metal surface coating on the ferrous metal can have a coating weight ranging between about and 100 rng./ft. and preferably between about 25 and 50 mg./ft.
  • the ferrous metal article having a surface coating of a suitable nonferrous metal is oxidized to form a surface oxide film by exposing to oxidizing conditions by any suitable procedure while maintaining the temperature of the article as low as possible and below about 1050" F. and preferably below about 900 F. Thereafter, and immediately before hot-dip galvanizing, the article having the oxidized surface film of non-ferrous metal is passed through a reducing zone which heats the article to a maximum temperature of about 1050" F., and preferably below about 900 F., to effect complete reduction of the oxide film to the metallic state.
  • the article is cooled, if necessary, to just above the galvanizing bath temperature and the ferrous metal article is passed through a hot-dip galvanizing bath of any type to provide a uniform protective hot-dip gal- I vanized surface coating thereon.
  • the hot-dip galvanizing process of the present invention is preferably carried out continuously on a continuous coating line of the type shown in the flow sheet in the accompanying drawing, wherein a ferrous metal sheet material, such as a steel strip 10, is continuously supplied to a cleaning apparatus 11 as an endless strip.
  • the cleaning apparatus 11 the sheet 10 is preferably first cleaned in an alkaline cleaning bath of a type conventionally used to remove surface contamination and prepare the ferrous metal strip for coating. Thereafter, the strip 10 is passed through a hot water rinse tank 12 and treated in a conventional acid pickling bath 13 which can comprise dilute hydrochloric or sulfuric acid to remove surface rust and scale before coating.
  • the strip 10 free of scale, surface oxides, and surface contamination is passed through a cold water rinse bath 14 and is in condition to receive a surface coating of a non-ferrous metal of the type previously described herein and which forms an oxide film reducible to the metallic state at a temperature below about 1050 F. and preferably below 900 F. to avoid causing drastic changes in the physical properties of the ferrous metal base material having the special physical properties.
  • the steel strip 10 is preferably provided with the thin surface coating of a suitable non-ferrous metal by continuously passing the strip 10 through a metal plating bath 15.
  • the non-ferrous metal applied as a thin coating to the strip 10 is nickel or copper or a combination of both nickel and copper in any proportion and is preferably applied electrolytically or by means of a chemical dip in a preferred coating weight ranging between about 25 mg./ft. and 50 mg./ft. by coordinating the length of time the strip is immersed in the plating bath with the current density or bath composition and the like plating variables.
  • the dwell time of the strip in the plating bath can range between about 4 and 15 seconds.
  • the thickness or weight of the metal surface coating is preferably such that the coating is not so thick that it is brittle or appreciably interferes with the formation of a diffusion layer between the ferrous metal base material and the hot-dip coating material.
  • Acid nickel plating baths are preferred for electroplating a thin nickel film, and a typical bath for nickel electroplating has the following composition:
  • An acid copper plating bath can be used, but instead of electroplating a copper coating, a special electroless copper plating bath is used with a copper sulfate to sulfuric acid having a weight ratio of about 1 to 10 and immersion time and condtions regulated to prevent forming excessively thick and uneven coatings of metallic copper.
  • a suitable acid copper electroless plating bath developed for the present process has the following composition:
  • Alkaline copper plating solutions such as those disclosed in any metal plating handbook can be used to provide a suitable thin electroplated metallic copper coating which when treated in accordance with the present invention results in forming a high quality firmly adherent hot-dip coating.
  • the strip 10 having the thin non-ferrous metal metallic coating on both surfaces is passed through a heating and oxidizing zone 18 to form a thin film of oxide on the surface of the non-ferrous metal coating.
  • the oxidizing zone 18 can be of any type which will oxidize the surface of the non-ferrous metal coating, such as an oxidizing chamber which contains an atmosphere which is oxidizing to the metal coating and heated to a temperature sufiicient to oxidize the metal surface coating.
  • the temperature within the chamber or zone 18, of course, can be as high as about 1200 F. or even higher and as low as about 250 F., depending on the dwell time of the strip 10 in the zone 18 and the nature of the nonferrous metal coating.
  • the oxidizing zone 18 can also comprise a flame directed onto the surface of the coated strip 10 just before the strip enters the reducing zone 19.
  • the thickness of the oxidized film preferably is such that the oxide film does not exceed that which can be completely reduced to the metallic state while being passed continuously through the reducing zone 19.
  • the oxide film can be thin and consists essentially of surface oxidation of the non-ferrous metal coating.
  • the sheet 10 immediately before immersion in a hot-dip coating bath 20 is passed through a reducing zone 19 capable of reducing the oxide film of nonferrous metal which preferably comprises a chamber containing a heated reducing non-oxidizing atmosphere which heats the sheet to a temperature not exceeding about 1050 F. and preferably below about 900 F. (i.e. between about 660 F. and 880 F.) to effect complete reduction of the oxide film to the metallic form, thereby forming a film which is readily coated by the molten hot-dip galvanizing bath.
  • a reducing zone 19 capable of reducing the oxide film of nonferrous metal which preferably comprises a chamber containing a heated reducing non-oxidizing atmosphere which heats the sheet to a temperature not exceeding about 1050 F. and preferably below about 900 F. (i.e. between about 660 F. and 880 F.) to effect complete reduction of the oxide film to the metallic form, thereby forming a film which is readily coated by the molten hot-dip galvanizing bath.
  • the reducing atmosphere can be formed of hydrogen in diluted form (i.e., 20% hydrogen-80% nitrogen) or disassociated ammonia or the like, with care being taken to maintain the dew point sufficiently low to prevent the atmosphere in the reducing zone 19 becoming oxidizing to the metal surface coating.
  • the strip may require up to 3-4 minutes to pass through the reduction-heat treating chamber.
  • the strip 10 is passed through the hot-dip galvanizing bath 20 to form a firmly adherent hot-dip galvanized coating over the entire surface of the strip 10.
  • Tests were conducted on an experimental continuous coating line which closely approximates actual continuous galvanizing coating line conditions and in which a continuous strip of the material as indicated in the Table I was hot-dip coated by passing the strip through each of the steps of the present process, including applying nickel or copper in coating weights of about 10-100 mg./ft. and immersing in a conventional hot-dip zinc coating bath travelling at the line speed indicated in Table I and during which the strip was heated to the maximum temperature, as indicated in Table I. Control runs were also made under identical conditions but without application of a thin coating of nickel or copper. The resulting galvanized product was examined for completeness and uniformity of zinc coverage and adherence of the zinc coating. The following Table I shows the results observed:
  • Table I shows that excellent zinc coverage is provided When a ferrous metal strip has a nickel or copper coating applied thereto and heated to a peak temperature above 550 F. and below about 900 F. during a Sendzimir-type galvanizing process.
  • steel strips which have been box-annealed prior to hot-dip galvanizing can be reduction-heat treated in the galvanizing process at a temperature sufliciently low to maintain the very soft properties of box-annealing Without endangering proper coverage by the molten zinc.
  • steels which have full-hard properties produced by tandem rolling can also be hot-dip galvanized to provide excellent zinc coverage without heating above the temperature which causes a substantial change in the crystal structure thereof resulting in a serious loss of the full-hard or annealed properties of the steel.
  • high strength steels such as martensitic steel strips can be effectively continuously hot-dip galvanized at a maximum strip temperature range of about 850 F.-900 F so that the high strength mechanical properties of the steel are substantially retained after hot-dip galvanizing.
  • the process of the present invention in addition to making it possible to hot-dip galvanize ferrous metal sheets which have been treated to provide special physical or mechanical properties not possessed by conventional galvanizing steel sheet material without destroying the special physical or mechanical properties, also significantly reduces the expense of heating and cooling the sheet in a Sendzimir-type galvanizing line. And, since an excellent zinc base coating can be hot-dip applied in accordance with the present invention when heating the strip in the oxide reducing zone to a temperature of about 660 F., still further savings in heat and time would be achieved, if a hot-dip galvanizing bath having a melting point below 850 F.-875 F. were used.
  • a process of hot-dip galvanizing a ferrous metal article which comprises: applying on a surface of a ferrous metal article to be hot-dip galvanized an adherent surface coating of a non-ferrous metal which forms an oxide when heated in an oxidizing atmosphere to a temperature below about 1050 F. and whose oxide is reducible in a reduction-heating zone at a temperature below about l050 F., oxidizing said coating of non-ferrous metal to form a surface oxide film without heating said article above about 1050 F., heating said article having said surface oxide film of non-ferrous metal in a reductionheating zone containing a reducing non-oxidizing atmosphere to a temperature below about 1050 F. to effect complete reduction of said surface oxide film to a metallic form, and immersing said article in a hot-dip galvanizing bath while said non-ferrous metal is maintained in said metallic form to provide an adherent galvanized coating on said ferrous metal article.

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Abstract

IN ASENDZIMIR-TYPE PROCESS FOR CONTINUOUSLY HOT-DIP GALVANIZING A STEEL TRIP PROVIDED WITH SPECIAL PHYSICAL PROPERTIES BEFORE GALVANIZING WHICH AVOIDS DESTRUCTION OF THE SPECIAL PHYSICAL PROPERTIES DURING THE GALVANIZING PROCESS, A STEEL STRIP IS PLACED WITH A THIN SURFACE CATING OF A NON-FERROUS METAL WHICH FORMS AN OXIDE AND WHOSE OXIDE ISREDUCIBLE AT A TEMPERATURE BELOW 10500*F., AND PREFERABLY BELOW ABOUT 900*F., AND THE COATING OF NONFERROUS METAL ISOXIDIZED IN AN OXIDIZING ATMOSPHERE TO FORM AN OXIDE FILM, THE OXIDE FILM IS REDUCED TO THE METALIC FORM BY HEATING THE STRIP IN A REDUCING ATMOSPHERE TO A MAXIMUM TEMPERATURE OF 1050*F. AND PREFERABLY NO HIGHER THAN ABOUT 900*F., AND THEN THE STRIP IS IMMERSED IN A HOT-DIP GALVANIZING BATH WHILE THE NON-FERROUS METAL ISMAINTAINED IN THE METALLIC FORM. THE NON-FERROUS METALS FOUND MOST USEFUL FOR SURFACE COATING OF THE STEEL STRIP ARE NICKEL, COPPER, AND COMBINATIONS OF NICKEL AND COPPER. THE NON-FERROUS METAL COATING IS PREFERABLY APPLIED TO THE STRIP BY PLATING EITHER ELECTROLYTICALLY OR BY NONELECTROLYTIC DISPLACEMENT TO PROVIDE A COATING HAVING A WEIGHT RANGING BETWEEN ABOUT 10 AND 1000 MG./FT.2 WITH APREFERRED COATING WEIGHT OF BETWEEN ABOUT 25 AND 50 MG.0FT.2.

Description

United States Patent @ftice Filed June so, 1971, Ser. No. 158,225
1m. (:1. B4411 1/34; C23c 1/02 U.S. Cl. 117-51 Claims ABSTRACT OF THE DISCLOSURE In a Sendzimir-type process for continuously hot-dip galvanizing a steel strip provided with special physical properties before galvanizing which avoids destruction of the special physical properties during the galvanizing process, a steel strip is placed with a thin surface coating of a non-ferrous metal which forms an oxide and whose oxide is reducible at a temperature below 1050 F., and preferably below about 900 F., and the coating of nonferrous metal is oxidized in an oxidizing atmosphere to form an oxide film, the oxide film is reduced to the metallic form by heating the strip in a reducing atmosphere to a maximum temperature of 1050 F. and preferably no higher than about 900 F., and then the strip is immersed in a hot-dip galvanizing bath while the non-ferrous metal is maintained in the metallic form. The non-ferrous metals found most useful for surface coating of the steel strip are nickel, copper, and combinations of nickel and copper. The non-ferrous metal coating is preferably applied to the strip by plating either electrolytically or by nonelectrolytic displacement to provide a coating having a weight ranging between about 10 and 100 mg./ft. with a preferred coating weight of between about and 50 mgr/fif The present invention relates to a process of hot-dip galvanizing ferrous metal articles, including hot-dip coating a steel sheet with zinc and with alloys of zinc contain ing aluminum, magnesium, or aluminum and magnesium and the like alloying metals which are suitable for forming a protective zinc base coating.
In galvanizing a ferrous metal strip or sheet material in accordance with a Sendzimir-type hot-dip coating process which eliminates the use of fluxes, a clean ferrous metal sheet free of scale is oxidized to provide a thin uniform iron oxide surface film which is free of all oxidizable impurities. Since the oxide surface is not coated by a hot-dip galvanizing bath when immersed therein, however, the sheet must be passed through a reducing atmosphere heated to an elevated temperature to reduce the iron oxide film to metallic iron. And, while the substantially pure iron surface is protected against reoxidadon, the strip is immersed in a hot-dip galvanizing bath to provide an adherent galvanized coating.
While the foregoing Sendzimir hot-dip galvanizing process forms an adherent uniform protective metal coating on a ferrous metal strip and the surface thereof is readily oxidized when exposed to an oxidizing atmosphere, it is necessary to subject the ferrous metal strip having the iron oxide film on the surface thereof to a reduction-heat treatment at an elevated temperature of between about 1500 F. and 1800 F. and generally to about 1700" F. in a reducing non-oxidizing atmosphere so that the iron 3,726,703 Patented Apr. 10, 1973 oxide surface film will be reduced to metallic iron in order to provide a surface to which the zinc will adhere. It is also advisable to cool the strip while maintaining the strip in a reducing non-oxidizing atmosphere to just above the temperature of the hot-dip galvanizing bath, which is generally at a temperature of about 860 F., before immersing the strip in the galvanizing bath.
Heating a steel strip to a temperature ranging between about F. and F., however, is well above the temperature which causes a substantial change in the physical properties of the steel where the strip has been provided with special physical properties before the strip is galvanized in a Sendizimir-type galvanizing process. For example, if a steel sheet is provided with a martensitic structure before being coated in a Sendzimir-type galvanizing process, the sheet will have practically none of the desired martensitic structure after passing through the reduction step wherein the strip is heated to a temperature of about 1500 F. or above. It is preferable not to heat the martensitic strip substantially above 900 F. during the galvanizing process, if the martensitic structure is to be largely retained.
It is therefore the object of the present invention to provide an improved Sendzimir-type hot-dip galvanizing process which makes it unnecessary to heat a ferrous metal strip appreciably above the temperature of a hot-dip galvanizing bath when preparing the surface of the strip for coating in a hot-dip galvanizing bath.
It is another object of the present invention to provide an improved Sendzimir-type hot-dip process for galvanizing a steel strip having a martensitic structure which greatly reduces loss of martensitic structure in the strip during the galvanizing process.
It is a further object of the present invention to provide an improved Sendzimir-type hot-dip process for galvanizing a steel strip having a full hard crystalline structure which avoids significantly changing the physical properties of the strip during the galvanizing process.
It is still another object of the present invention to provide an improved Sendzimir-type hot-dip galvanizing process which eliminates the expense and time required to heat the ferrous metal strip substantially above the temperature of a galvanizing bath and then cool the strip down to about about the temperature of the galvanizing bath when preparing a steel strip for the hot-dip galvanizing bath.
It is a further object of the invention to provide an improved Sendzimir-type galvanizing process which can be operated more economically and at a faster line speed.
Other objects of the present invention will be apparent from the detailed description and claims to follow when read in conjunction with the accompanying drawing comprising a process flow chart showing the steps of the process.
In achieving the foregoing objects in accordance with the present invention a clean scale-free ferrous metal article, such as a steel strip or sheet, is provided with a thin metallic surface coating of a non-ferrous metal which readily forms an oxide which is reducible to the metallic form at a temperature substantially below 1500 F., and preferably below a maximum temperature of 1050 F., since a marked reduction or complete loss of the special physical properties of a ferrous metal strip, such as Where the strip has been treated to provide full hard mechanical properties, results when the strip is heated to a temperature range between 1500 F. and 1800 F., in the regular Sendzimir-type galvanizing process. The non-ferrous metals whose oxide has the foregoing properties and which are suitable for surface coating a steel strip in accordance with the present invention include nickel, copper, silver and cobalt. Nickel or copper and combinations thereof have been found most suitable from a cost standpoint, and because the coatings form oxides which are reducible to the metallic form at a relatively low temperature (i.e., below 900 F.) and at a temperature close to the melting point of the galvanizing bath. In addition thin oxide coatings of nickel, copper and combinations thereof are sufficiently ductile to remain on the strip during the bending of the strips while passing through a continuous coating line. Other non-ferrous metals which otherwise would be suitable but which have a boiling point below the temperature of the galvanizing bath or a boiling'point below the reduction-heat treating temperature required for reducing the oxide of the thin surface coating of non-ferrous metal to the metallic form are not preferred for use in accordance with the present invention. The non-ferrous metal surface coating can be formed by any suitable means or process and can be extremely thin without impairing the usefulness in the present invention. The non-ferrous metal surface coating on the ferrous metal can have a coating weight ranging between about and 100 rng./ft. and preferably between about 25 and 50 mg./ft.
In accordance with the present process the ferrous metal article having a surface coating of a suitable nonferrous metal is oxidized to form a surface oxide film by exposing to oxidizing conditions by any suitable procedure while maintaining the temperature of the article as low as possible and below about 1050" F. and preferably below about 900 F. Thereafter, and immediately before hot-dip galvanizing, the article having the oxidized surface film of non-ferrous metal is passed through a reducing zone which heats the article to a maximum temperature of about 1050" F., and preferably below about 900 F., to effect complete reduction of the oxide film to the metallic state. Thereafter, and while the surface coating of the non-ferrous metal remains in the metallic state under the protection of a non-oxidizing or reducing atmosphere, the article is cooled, if necessary, to just above the galvanizing bath temperature and the ferrous metal article is passed through a hot-dip galvanizing bath of any type to provide a uniform protective hot-dip gal- I vanized surface coating thereon.
The hot-dip galvanizing process of the present invention is preferably carried out continuously on a continuous coating line of the type shown in the flow sheet in the accompanying drawing, wherein a ferrous metal sheet material, such as a steel strip 10, is continuously supplied to a cleaning apparatus 11 as an endless strip. In the cleaning apparatus 11 the sheet 10 is preferably first cleaned in an alkaline cleaning bath of a type conventionally used to remove surface contamination and prepare the ferrous metal strip for coating. Thereafter, the strip 10 is passed through a hot water rinse tank 12 and treated in a conventional acid pickling bath 13 which can comprise dilute hydrochloric or sulfuric acid to remove surface rust and scale before coating. The strip 10 free of scale, surface oxides, and surface contamination is passed through a cold water rinse bath 14 and is in condition to receive a surface coating of a non-ferrous metal of the type previously described herein and which forms an oxide film reducible to the metallic state at a temperature below about 1050 F. and preferably below 900 F. to avoid causing drastic changes in the physical properties of the ferrous metal base material having the special physical properties.
The steel strip 10 is preferably provided with the thin surface coating of a suitable non-ferrous metal by continuously passing the strip 10 through a metal plating bath 15. In the preferred embodiments of the present invention the non-ferrous metal applied as a thin coating to the strip 10 is nickel or copper or a combination of both nickel and copper in any proportion and is preferably applied electrolytically or by means of a chemical dip in a preferred coating weight ranging between about 25 mg./ft. and 50 mg./ft. by coordinating the length of time the strip is immersed in the plating bath with the current density or bath composition and the like plating variables. In a continuous nickel or copper plating line the dwell time of the strip in the plating bath can range between about 4 and 15 seconds. The thickness or weight of the metal surface coating is preferably such that the coating is not so thick that it is brittle or appreciably interferes with the formation of a diffusion layer between the ferrous metal base material and the hot-dip coating material.
Acid nickel plating baths are preferred for electroplating a thin nickel film, and a typical bath for nickel electroplating has the following composition:
Nickel sulfate oz./gal 44 Nickel chloride .oz./gal 6 Boric acid oz./ga1 5 pH l.54.5.
Bath temperature F -14O Current density amp./ft. 20-100 An acid copper plating bath can be used, but instead of electroplating a copper coating, a special electroless copper plating bath is used with a copper sulfate to sulfuric acid having a weight ratio of about 1 to 10 and immersion time and condtions regulated to prevent forming excessively thick and uneven coatings of metallic copper. A suitable acid copper electroless plating bath developed for the present process has the following composition:
Copper sulfate-1 oz./ gal. Sulfuric acidl0 mL/gal. Bath temperatureRoom temperature.
Alkaline copper plating solutions such as those disclosed in any metal plating handbook can be used to provide a suitable thin electroplated metallic copper coating which when treated in accordance with the present invention results in forming a high quality firmly adherent hot-dip coating.
Following hot water rinsing in the bath 16 and air drying the chamber 17 the strip 10 having the thin non-ferrous metal metallic coating on both surfaces is passed through a heating and oxidizing zone 18 to form a thin film of oxide on the surface of the non-ferrous metal coating. The oxidizing zone 18 can be of any type which will oxidize the surface of the non-ferrous metal coating, such as an oxidizing chamber which contains an atmosphere which is oxidizing to the metal coating and heated to a temperature sufiicient to oxidize the metal surface coating. The temperature within the chamber or zone 18, of course, can be as high as about 1200 F. or even higher and as low as about 250 F., depending on the dwell time of the strip 10 in the zone 18 and the nature of the nonferrous metal coating. The oxidizing zone 18 can also comprise a flame directed onto the surface of the coated strip 10 just before the strip enters the reducing zone 19. The thickness of the oxidized film preferably is such that the oxide film does not exceed that which can be completely reduced to the metallic state while being passed continuously through the reducing zone 19. The oxide film can be thin and consists essentially of surface oxidation of the non-ferrous metal coating.
Following surface oxidation, the sheet 10 immediately before immersion in a hot-dip coating bath 20 is passed through a reducing zone 19 capable of reducing the oxide film of nonferrous metal which preferably comprises a chamber containing a heated reducing non-oxidizing atmosphere which heats the sheet to a temperature not exceeding about 1050 F. and preferably below about 900 F. (i.e. between about 660 F. and 880 F.) to effect complete reduction of the oxide film to the metallic form, thereby forming a film which is readily coated by the molten hot-dip galvanizing bath. The reducing atmosphere can be formed of hydrogen in diluted form (i.e., 20% hydrogen-80% nitrogen) or disassociated ammonia or the like, with care being taken to maintain the dew point sufficiently low to prevent the atmosphere in the reducing zone 19 becoming oxidizing to the metal surface coating. In a Sendzimir-type continuous galvanizing line the strip may require up to 3-4 minutes to pass through the reduction-heat treating chamber.
While the surface coating remains in the fully reduced form under a protective reducing non-oxidizing atmosphere, and while the strip is at a temperature preferably only slightly above the temperature of the hot-dip galvanizing bath 20 (i.e., at a temperature of about 860 F.), the strip 10 is passed through the hot-dip galvanizing bath 20 to form a firmly adherent hot-dip galvanized coating over the entire surface of the strip 10.
Tests were conducted on an experimental continuous coating line which closely approximates actual continuous galvanizing coating line conditions and in which a continuous strip of the material as indicated in the Table I was hot-dip coated by passing the strip through each of the steps of the present process, including applying nickel or copper in coating weights of about 10-100 mg./ft. and immersing in a conventional hot-dip zinc coating bath travelling at the line speed indicated in Table I and during which the strip was heated to the maximum temperature, as indicated in Table I. Control runs were also made under identical conditions but without application of a thin coating of nickel or copper. The resulting galvanized product was examined for completeness and uniformity of zinc coverage and adherence of the zinc coating. The following Table I shows the results observed:
The foregoing Table I shows that excellent zinc coverage is provided When a ferrous metal strip has a nickel or copper coating applied thereto and heated to a peak temperature above 550 F. and below about 900 F. during a Sendzimir-type galvanizing process.
It will be evident that with the present process steel strips which have been box-annealed prior to hot-dip galvanizing can be reduction-heat treated in the galvanizing process at a temperature sufliciently low to maintain the very soft properties of box-annealing Without endangering proper coverage by the molten zinc. Also, steels which have full-hard properties produced by tandem rolling can also be hot-dip galvanized to provide excellent zinc coverage without heating above the temperature which causes a substantial change in the crystal structure thereof resulting in a serious loss of the full-hard or annealed properties of the steel. In like manner, high strength steels, such as martensitic steel strips can be effectively continuously hot-dip galvanized at a maximum strip temperature range of about 850 F.-900 F so that the high strength mechanical properties of the steel are substantially retained after hot-dip galvanizing.
The process of the present invention, in addition to making it possible to hot-dip galvanize ferrous metal sheets which have been treated to provide special physical or mechanical properties not possessed by conventional galvanizing steel sheet material without destroying the special physical or mechanical properties, also significantly reduces the expense of heating and cooling the sheet in a Sendzimir-type galvanizing line. And, since an excellent zinc base coating can be hot-dip applied in accordance with the present invention when heating the strip in the oxide reducing zone to a temperature of about 660 F., still further savings in heat and time would be achieved, if a hot-dip galvanizing bath having a melting point below 850 F.-875 F. were used.
We claim:
1. A process of hot-dip galvanizing a ferrous metal article which comprises: applying on a surface of a ferrous metal article to be hot-dip galvanized an adherent surface coating of a non-ferrous metal which forms an oxide when heated in an oxidizing atmosphere to a temperature below about 1050 F. and whose oxide is reducible in a reduction-heating zone at a temperature below about l050 F., oxidizing said coating of non-ferrous metal to form a surface oxide film without heating said article above about 1050 F., heating said article having said surface oxide film of non-ferrous metal in a reductionheating zone containing a reducing non-oxidizing atmosphere to a temperature below about 1050 F. to effect complete reduction of said surface oxide film to a metallic form, and immersing said article in a hot-dip galvanizing bath while said non-ferrous metal is maintained in said metallic form to provide an adherent galvanized coating on said ferrous metal article.
2. A process of hot-dip galvanizing as in claim 1, wherein said article is an endless steel sheet which has been treated to provide special physical properties not possessed by conventional galvanizing steel sheet material, and wherein said strip is heated to a temperature which does not exceed about 900 F. during said oxidizing and metal is selected from the group consisting of nickel,
copper and a combination of nickel and copper in any proportion.
4. A process as in claim 2, wherein said coating of non-ferrous metal has a weight between about 10 mg./ft. and mg./ft.
5. A process as in claim 3, wherein said coating of nonferrous metal has a coating weight between about 25 and 50 mg./ft.
6. A process of hot-dip galvanizing as in claim 3, wherein said sheet having said surface oxide film of nonferrous metal thereon is heated in said reducing nonoxidizing atmosphere at a temperature between about 660 F. and 880 F.
7. A process as in claim 1, wherein said metal article is an endless steel strip which prior to applying said coating of non-ferrous metal thereto is treated to increase the hardness thereof above that of conventional galvanizing steel.
8. A process as in claim 7, wherein said endless strip of sheet steel is provided with martensitic structure before application of said coating of non-ferrous metal, and wherein the maximum temperature to which said strip is heated during the said process is about 880 F., whereby the galvanized martensitic strip retains a major proportion of said martensitic structure.
9. A process as in claim 1, wherein said metal article is an endless steel strip which prior to applying said coating of non-ferrous metal thereto is annealed to provide full soft properties therein.
10. A process as in claim 1, wherein a metallic copper coating is applied to said article by immersing said article in an electroless copper plating bath for a period of between about 4 seconds and 15 seconds, with said copper plating bath containing copper sulfate and sulphuric acid in an approximate weight ratio of 1 to 10.
References Cited UNITED STATES PATENTS ALFRED L. LEAVITT, Primary Examiner I. A. BELL, Assistant Examiner US. Cl. X.R.
11750, 71 M, 130 R, 131, 114 A
US00158225A 1971-06-30 1971-06-30 Process for galvanizing a ferrous metal article Expired - Lifetime US3726705A (en)

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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4053663A (en) * 1972-08-09 1977-10-11 Bethlehem Steel Corporation Method of treating ferrous strand for coating with aluminum-zinc alloys
DE3201475A1 (en) * 1981-05-22 1982-12-09 Hermann Huster GmbH & Co, 5800 Hagen METHOD FOR FIRE GALVINATING METAL WORKPIECES
US5846675A (en) * 1997-02-21 1998-12-08 Samsung Display Devices Co., Ltd. Current collector for lithium ion batteries
EP1109627A1 (en) * 1998-06-09 2001-06-27 International Lead Zinc Research Organization, Inc. Manufacturing process for noncontinuous galvanization with zinc-aluminum alloys over metallic manufactured products

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4053663A (en) * 1972-08-09 1977-10-11 Bethlehem Steel Corporation Method of treating ferrous strand for coating with aluminum-zinc alloys
DE3201475A1 (en) * 1981-05-22 1982-12-09 Hermann Huster GmbH & Co, 5800 Hagen METHOD FOR FIRE GALVINATING METAL WORKPIECES
US5846675A (en) * 1997-02-21 1998-12-08 Samsung Display Devices Co., Ltd. Current collector for lithium ion batteries
EP1109627A1 (en) * 1998-06-09 2001-06-27 International Lead Zinc Research Organization, Inc. Manufacturing process for noncontinuous galvanization with zinc-aluminum alloys over metallic manufactured products
EP1109627A4 (en) * 1998-06-09 2001-09-12 Internat Lead Zinc Res Manufacturing process for noncontinuous galvanization with zinc-aluminum alloys over metallic manufactured products

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