Abstract:
Methods and apparatus enabling simultaneous electrolytic tinplating of bottom and top surfaces of continuous-strip flat-rolled steel substrate while moving in the direction of its length, are disclosed. Both surfaces are plated in a first cell of a multi-cell horizontally-oriented tinplating line. The stannous ion plating source and the source of electrical power, for each surface, differ. Tin pellets for an upper surface anode are confined in a solution-permeable material confined, within an electrically-conductive lining. Dissolution of surface iron from the steel strip is substantially eliminated, improving the stannous ion plating solution quality, eliminating requirements for harmful additives, and increasing the variety of electrolytically tinplated continuous-strip steel products and processes.

Description:
RELATED APPLICATIONS 
     This application claims the priority benefit of co-owned and copending U.S. patent application Ser. No. 60/102,645 filed Oct. 1, 1998 entitled ELECTROLYTIC TINPLATING OF STEEL SUBSTRATE, and is a continuation-in-part of: co-owned U.S. patent application Ser. No. 08/445,530 filed May 23, 1995, entitled APPARATUS FOR CONTINUOUS FLAT-ROLLED STEEL STRIP CLEANING AND FINISHING OPERATIONS (now U.S. Pat. No. 5,599,395), and co-owned U.S. patent application Ser. No. 09/076,979, filed May 13, 1998, entitled ELECTROLYTIC PLATING OF STEEL SUBSTRATE (now U.S. Pat. No. 5,928,487). 
    
    
     INTRODUCTION 
     This invention relates to new electrolytic tinplating processes, apparatus and product; and, more particularly, is concerned with continuous-strip acidic-electrolyte tinplating lines utilizing horizontally-oriented plating cells which eliminate chemical dissolution of surface iron from steel substrate into stannous-ion acidic plating solutions. 
     OBJECTS OF THE INVENTION 
     A major object of the invention is to enable continued commercial usage of horizontally-oriented electrolytic tinplating lines which use acidic stannous-ion plating solutions. Eliminating dissolution of iron from an exposed surface of continuous-strip steel is important in achieving that object. 
     An added object is elimination of environmental concern, previously associated with horizontally-oriented continuous-line tinplating practice, which resulted from utilizing cyanide compounds to precipitate surface iron dissolved from steel substrate in prior horizontally-oriented acidic stannous-ion plating cells. 
     A specific object of the invention is eliminating loss of stannous plating ions, primarily due to surface iron dissolution, from halogen-bath or methylsulfonic acid tinplating solutions during horizontally-oriented tinplating operations. 
     The above, and other contributions and advantages of the invention are described in greater detail with reference to the accompanying drawings in which like reference numbers are used for like parts, whenever possible, in the various drawings briefly described below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general arrangement box diagram presentation for describing the relationship of a new combination of process steps and apparatus of the invention for carrying out acidic stannous ion tinplating operations during passage of continuous-strip steel, through a plurality of horizontally-oriented plating cells, free of surface dissolution of iron; 
     FIG. 2 is a diagrammatic elevational view, partially in cross section, of a single-tier horizontally-oriented tinplating line made possible by the invention; 
     FIG. 3 is an enlarged schematic cross-sectional view of a portion of FIG. 2, for describing a horizontally-oriented entry cell of the invention; 
     FIG. 4 is a detailed perspective view, in cross section, for describing features of a soluble second-anode structure of the invention; 
     FIG. 5 is a detailed cross-sectional view of a portion of FIG. 4, indicated by the circle labeled “FIG.  5 ,” for describing second-anode power supply features of the invention; 
     FIG. 6 is a cross-sectional view of such second-anode structure of the invention, taken along a plane indicated by interrupted line  6 — 6  of FIG. 4, for describing selected materials of the invention, and their function; 
     FIG. 7 is a perspective view of a second-anode support structure of the invention; 
     FIG. 8 is a schematic top plan view of a graphite liner structure of the invention for interfitting within the support structure of FIG. 7; 
     FIG. 9 is an enlarged cross-sectional view of a portion of a new electrolytically tinplated flat-rolled steel product of the invention made possible by the FIG. 2 single-tier embodiment of the invention; 
     FIG. 10 is a diagrammatic elevational view, partially in cross section, of a two-tier horizontally-oriented electrolytic tinplating line of the invention, and 
     FIG. 11 is and enlarged cross-sectional view of a new electrolytically tinplated flat-rolled steel product of the FIG. 10 embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Continuous-strip steel is selectively processed to desired thickness gauge and to establish desired substrate characteristics for intended use of an electrolytically tinplated product. Such processing includes cold rolling gauge reduction, an annealing-type heat treatment, temper rolling to establish desired substrate temper, and side-trimming for uniform width dimension. 
     Preparation of steel strip surface for electrolytic plating preferably removes and separates iron fines and other surface contaminants resulting largely from cold reduction operation. In-line surface cleansing operations are carried out at station  20  (FIG. 1) on continuous-strip steel  21  from coil  22 . Surface scrubbing, with separation of such contaminants from the cleansing solution, is described in more detail in copending and co-owned U.S. patent application Ser. No. 08/445,530 entitled APPARATUS FOR CONTINUOUS FLAT-ROLLED STEEL STRIP CLEANSING AND FINISHING OPERATION (now U.S. Pat. No. 5,599,395), which is included herein by reference. 
     Following such cleansing and rinsing of the strip at rinse station  24 , surface oxide is removed at station  26 , preferably by acid pickling. Surface rinsing at station  28  is carried out prior to further processing. 
     The horizontally-oriented plating line of the invention presents an entry cell which simultaneously plates both top surface and bottom surface of continuous-strip horizontally-oriented steel substrate, with each such surface being plated from a separate and distinct soluble tin anode. 
     In practice of the invention, surface-prepared continuous-strip steel substrate  30  enters a horizontally-oriented electrolytic plating line  32  solely through entry cell  34 ; both top and bottom surfaces of such horizontally-oriented substrate are plated simultaneously in entry cell  34 . 
     Plating control in such entry cell, as described later herein, prevents surface dissolution of iron during passage through such entry cell, and through subsequent individual horizontally-oriented plating cells indicated by station  35  of FIG.  1 . 
     Removal, at stations  20  to  28 , of continuous-strip surface iron fines and iron oxides (as described above) prior to electrolytic plating, and other steps described later, help to diminish iron contamination of a stannous ion acidic plating solution. However, the overwhelmingly predominant source of iron contamination in prior horizontally-oriented halogen-bath tinplating practice has been found to originate from dissolution of iron from the top surface of the steel strip which, previously, was not plated in the first tier of a two-tier electrolytic plating line, as previously practiced in such horizontally-oriented tinplating of both bottom and top surfaces of a steel strip. 
     Only the bottom surface of the strip was previously tinplated as the strip traveled, with such bottom surface submersed at plating solution surface level, through a plurality of horizontally-oriented cells in a first tier of a two-tier line (see “The Technology of Tinplate,” by W. E. Hoare, et al. ©1965, St. Marten&#39;s Press New York, N.Y. 10010, pages 239-244.) which is included herein by reference. However, iron, from the opposed “top” surface of the steel substrate, dissolved in the acidic plating solution, as it traveled is such first tier. 
     Acidic halogen-bath (approximate pH of 3.5) plating solution previously predominated in such two-tier horizontally-oriented lines. However, methylsulfonic acid plating solution (approximate pH of 1.0) is finding increased usage and, due to its pH, increases the potential for such undesirable dissolution of surface iron. 
     The presence of iron in both plating solutions promotes the conversion of bivalent stannous plating ions to quadrivalent stannic ions as dissolved ferric ions are converted to ferrous ions. The increase in stannic ions decreases the stannous ions available for tinplating, and the loss of plating efficiency is significant (see Bethlehem Steel U.S. Pat. No. 4,073,701 which is included herein by reference). Cyanide compounds have been used in order to precipitate such dissolved iron. The precipitate is referred to as “prussian blue” and is a matter of increasing environmental concern which threatens the existence of otherwise advantageous horizontally-oriented tin mill practice. 
     The present invention functions to electrolytically tinplate both surfaces of the steel substrate simultaneously in an entry cell which is the sole access of substrate for plating in a horizontally-oriented plating line, thus eliminating dissolution of surface iron due to exposure of the substrate surface to such acidic plating solution. 
     The invention includes both single-tier and two-tier plating line embodiments for purposes of producing new tin mill product, as described later herein. The simultaneous plating of both surfaces of the steel substrate upon entry to a horizontally-oriented plating line, made possible by the invention, is carried out in the entry cell of such single-tier embodiment, and in the entry cell of the first tier of a two-tier embodiment of the invention. 
     An additional factor decreasing plating efficiency is the introduction of oxygen from the atmosphere into the plating bath by strip movement and bath agitation. Such presence of oxygen results in the oxidation of bivalent tin ions to quadrivalent tin ions and a resulting decrease in plating efficiency. To reduce such oxidation, a suitable antioxidant is incorporated with the bath. Such antioxidants are available from Atotech USA, Inc., 100 Harvard Avenue, Cleveland, Ohio 44109. 
     FIG. 2 shows a single-tier embodiment and FIG. 3 shows an enlarged cross-sectional view of entry cell  34 . The direction of travel for strip  30  is indicated by arrow  37  in both figures. 
     An early step of the invention elevates the surface level of the plating solution of entry cell  34  so as to enable plating of the top surface simultaneously with plating of the bottom surface of the horizontally-travelling strip. 
     A combination of strip entry and strip exit structures in such entry cell enables at least partial submersion of a soluble second-anode. Strip entry structure  38  (FIG. 3) provides a vertical dam with a pliable curtain for entry of strip  30 . Strip exit structure  39  provides interfitting blocks, one at each edge of a pair of entry cell exit rolls  40 ; such strip exit structure substantially blocks run-around of plating solution which is otherwise held back by exit rolls  40 . 
     A raised surface level  41  for the plating solution is thus made possible in entry cell  34 . Any slight escape of plating solution, at strip entry structure  38  or strip exit structure  39 , is recovered at overflow station  42  and  43 , respectively, for recycled usage. Circulation of plating solution includes return into entry cell  34 , through conduit  44 , which maintains the desired raised surface level  41 . 
     Prior horizontally-oriented plating lines were limited to two-tier lines and tin-plating in the first tier was limited to the bottom surface of the strip, which is in confronting relationship with submerged anode bars, such as  45  and  46  of FIG.  3 . Simultaneously tinplating both surfaces of a steel substrate in a single tier, as described herein, was not available prior to the present invention. 
     In a single tier, or on the first tier of a two-tier embodiment, ions for bottom surface plating are made available from soluble anode bars which extend, in uniformly spaced relationship from that bottom surface, across the full width of the strip, as described in U.S. Pat. No. 3,445,371 ANODE STRUCTURE FOR CONTINUOUS STRIP ELECTROPLATING (which is included herein by reference). 
     The present invention controls entry cell solution level as described in relation to FIG. 3 to enable submerging a soluble second-anode below the new elevated plating solution level  41 . Such second-anode enables electrolytically tinplating the top surface of the steel substrate simultaneously with plating of the bottom surface using the submerged anode bars, (such as  45 ,  46 , of FIG.  3 ). 
     Provisions are made for sufficient plating thickness, and continuity of top surface plating, to provide a protective barrier layer, which prevents access of the acidic plating solution to cause dissolution of surface iron, throughout the remainder of the initial tier of a horizontally-oriented plating line. 
     Such protective barrier layer is sustained as a continues layer during travel throughout plural plating cell tier  35  (FIG.  1 ); that is, such barrier layer, tinplated from the second-anode, is maintained within a preselected coating weight range, notwithstanding chemical action of acidic plating solution on such protective barrier layer. 
     In preferred practice, provision is made for separately controlling top surface plating in such entry cell so as to prevent dissolution of iron from both surfaces in the event of interruption of plating operations in which continuous-strip is suspended in plating solution. 
     Another objective is to provide for sufficient second-anode plating remaining on such top surface, after passage through such initial tier, so as to provide a coating weight in the range of about 0.02 to about 0.05 #/bb tin; such selection enables production of a new single-tier tin mill product, described in more detail later herein. 
     The soluble tin second-anode means for top surface plating, as taught herein, is provided with a power supply which is independent of the power supply for plating the bottom surface of the strip in plating cells of the initial tier of the horizontally-oriented plating line. 
     In order to provide for independent and sustained supply of stannous ions for plating such top surface, the invention provides pelletized tin, generally of spherical configuration (see FIGS.  4 - 6 ), and provides for control of plating solution level to enable such pelletized tin to be at least partially submerged in such plating solution in such entry cell. One or more tin pellet support structure units provide uniform spacing of such second-anode soluble tin pellets, from such remaining top surface of the continuous-strip which is opposed to the bottom surface plated from such submerged anode bars. 
     Preferably, plating practice with the soluble second-anode structure of the invention utilizes a plurality of individual soluble anode tin pellet units, in at least the entry cell, of a horizontally-oriented tinplating line. One of the objectives in providing a plurality of such second-anode structures is to decrease the size and weight of each, so as to decrease the support required for each. Individual units of the soluble-anode structure are preferably confined to the entry cell for a coating line and supplied from a single power bus; however, possible addition of top-surface second-anode structures to additional horizontally-oriented cells should not be excluded. Disposition of such plurality of individual second-anode structure units is best depicted in FIG. 3; details of each second-anode and support structure unit are disclosed in the detailed views of FIGS. 4-6; and construction and assembly are explained in more detail in relation to FIGS. 7-8. 
     In the specific embodiment of the entry cell of FIG. 3, three individual soluble-anode units are utilized, with power supplied to each from single bus bar  48 . A length dimension on an elongated second-anode unit, such as  49 , is shown by the cross-sectional view of FIG.  4 . The length of the tin pellet containing portion is indicated by  52  and extends transversely across the full width of the continuous-strip. Further details of electrically-conductive anode materials and a conducting chamber for establishing bus bar  53  conduction at a longitudinal end of second-anode structure  49  can be seen, and are described in relation to FIG.  5 . The cross-sectional view of FIG. 6 is in a plane (indicated by interrupted line  6 — 6  of FIG. 4) perpendicularly transverse to the elongated dimension of second-anode structure  49 . 
     In FIGS. 4 and 6, it can be seen that substantially spherical tin pellets  55  can be introduced and replenished in the pellet containing portion though upper access opening  57  (indicated in FIG.  6 ). 
     The plating solution level established in entry cell  34  enables electrolytic submersion of second-anode units  49 ,  50 , and  51  which are located sequentially in the direction of strip travel (FIG.  3 ). Each provides a source of stannous plating ions extending across the full width of the strip, as described in relation to FIG.  4 . Each tin pellet containing portion presents a source of stannous ions in substantially parallel confronting relationship with the top surface of the continuous-strip. Power-supply bus bar  53  is located at a longitudinal end (shown in detail in FIG. 5) of each second-anode unit. 
     Referring to FIGS. 4-8, a substantially-rigid heavy-gauge woven-wire support structure  58  functions to contain each such unit. Two sidewalls, two end walls and a bottom wall are provided with open upper access  57  for introduction of the tin pellets. Sufficient strength and rigidity to support tin pellets  55 , without distortion of the unit, is provided. In a preferred embodiment, heavy-duty woven-wire mesh (approximately 0.2″ diameter steel wire) is used. Material for construction is selected to provide desired strength while maximizing openness (about {fraction (1/2+L )}″) of the woven-wire for access of the plating solution to the tin pellets, after electrically insulating such support structures. In FIG. 7, such woven-wire mesh support structure  58  is shown, for clarity, in a cross-sectional perspective view absent other anode structure components. Such cross section corresponds to that of FIG.  4 . For additional strength, woven-wire mesh can be reinforced with framework  59 , primarily located at edges and corners, which can also be used to support the unit in the entry cell. In a preferred embodiment, steel rod is used for such reinforcement. 
     The woven-wire and reinforcing members of support structure  58  are coated with an electrically insulating material; and can, for example, be dip-coated with a plastisol (PVC) to provide a continuous coating on the woven-wire and framework, while maintaining desired mesh openness. Such insulation material is also selected to be inert to the acidic plating solution. 
     An electrically conductive graphite liner  60  (shown in plan view in FIG. 8) is fitted within the woven-wire support structure  58 . Such graphite liner includes peripheral wall  61  along the sides and ends, dividing wall  62  and bottom wall  63 . Graphite peripheral wall  61  (in part) and dividing wall  62  define the tin pellet containing portion and graphite peripheral wall  61  (in part), dividing wall  62  and bottom wall  63 , (FIG. 5) define the electrically-conductive chamber  64  circumscribing bus bar  53  at a longitudinal end of the graphite liner. Graphite peripheral wall  61 , dividing wall  62  and bottom wall  63  (FIG. 5) are assembled so as to provide conductivity between their contacting surfaces, to form an integral electrically-conductive graphite-lined structure for containing tin pellets  55  and electrically conductive chamber  64  for bus bar  53  connection. 
     During fabrication of the second-anode, electrical contact is established between electrically-conductive chamber  64  (FIG. 5) and bus bar  53  by pouring molten metal  66  (preferably tin) around bus bar  53  (preferably copper) to fill the compartment, and allowing the poured metal to solidify. 
     Dimension  52  (FIG. 4) of the elongated pellet containing portion of the graphite liner does not include the dimension of electrically-conductive chamber  64 . Dimension  52  is selected to be at least equal to the width of the widest continuous-strip steel to be plated by such unit such that the top surface of the strip is confronted, edge to edge, by the tin pellet containing portion of the graphite lined second-anode. 
     Such pellets are circumscribed by electrically-conductive graphite liner peripheral wall  61  and dividing wall  62  (FIG.  8 ). Graphite liner dividing wall  62  separates the pellet containing portion from conducting chamber  64 . Support structure  58  extends beyond the longitudinal edges of plating cell  34  in order to be supported by additional structures. 
     In preparing for tinplating operations, tin pellets are supplied to substantially fill the volume defined by the graphite walls (FIG.  4 ). The pellets, as supplied, are initially substantially uniform in size and selected to be in the range of about one-half to one inch in diameter and of substantially spherical configuration. It should be noted in FIGS. 4 and 6 that the pellets start diminishing in size below plating solution surface level  41 . As the tin pellets go into solution and decrease in volume, replacement pellets are periodically added to maintain a pellet depth of at least to, but preferably above, the plating solution surface level  41  shown in FIGS. 3-6. 
     Peripherally-located tin pellets make electrical contact at points of contact with the inner surfaces of the graphite liner; and, in turn, electrical conduction extends from pellet to pellet throughout the pellets within the graphite liner. 
     The outer surfaces of graphite liner walls  61 , opposed to the pellet contacting surfaces, are coated with electrically insulating material  70  (such as PVC) to provide added protection against an electrical short circuit between the graphite liner and metal woven-wire support structure  58 ; or, with the continuous-strip substrate which is polarized to be cathodic in relation to the tin pellets of the graphite liner. Referring to the enlarged cross-sectional view of FIGS. 5 and 6, it should be noted that in addition to the outer vertical surfaces of the sidewalls and endwalls, the bottom horizontal surface of graphite bottom wall  63 , is covered by electrical insulation  70  (such as PVC). The vertical dimension of the longitudinal graphite walls making up peripheral wall  61  is selected to be less than the full height of pellet support structure  58  (as seen in FIG. 6) such that tin pellets spread horizontally under the graphite walls, and, in that way confront the steel substrate with a maximum soluble anode area dimension for each anode unit. That is, the full width of each source of tin within each second-anode unit confronts the strip for plating. 
     In addition to woven-wire support structure  58 , which is provided with open mesh characteristics, support for diminishing size pellets having a size less than such open mesh is augmented by additional support means. As best seen in FIGS. 5 and 6, in a preferred embodiment a pair of support layers  72  and  74  are used. They include electrically-non-conductive mesh  72 , comprising a material such as polypropylene, presenting a mesh opening size of about one-eighth to one-quarter inch and electrically-nonconductive, solution-penetrable liner material  74  located outboard of mesh  72  (FIGS. 5,  6 ). 
     Such liner material  74  confines tin, which has diminished to particulate size, to within the anode structure. These precautions are taken to prevent tin particulate contact with, and damage to, the tinplated surface. 
     Such solid particulate between the strip plated surface and the support structure, or passing between the electrical contact rolls, could damage the tinplating or cause and indentation on the strip surface. Solution penetrable “anode bag cloth” for such liner material allows free penetration of plating solution and free migration of tin ions but restricts solid particulate from exiting into the plating solution. 
     Such support layers  72 ,  74  in addition to extending along the bottom wall, as described, are extended upward along the side walls and end walls, so as to facilitate attachment at an upper edge of woven-wire structure  58 , to eliminate any possibility of particulate exiting the unit near bottom corner portions of the graphite liner structure. In a preferred embodiment, support liners  72  and  74  are shaped to be fitted over the graphite liner. In a preferred embodiment, mesh material  72  is fitted over graphite liner  60  first, followed by anode cloth liner material  74 . The graphite liner is thus encased prior to inserting it into woven-wire support structure  58 . Although the sequence of support liners in the preferred embodiment is as described above, reversal of the order of the layers is not to be excluded. In still a third embodiment (not shown), such anode cloth material is outboard of the woven-wire support structure. 
     The invention provides for side-by-side placement of a plurality of such second-anode units ( 49 ,  50 ,  51  of FIG. 3) in the direction of substrate travel. Sufficient anode surface area is provided, in confronting relationship with the strip top surface, for electrolytically tinplating a barrier layer of sufficient thickness and continuity to prevent dissolution of iron from such top surface of the strip, during travel of the strip through a first, or single, tier of a horizontally-oriented plating line in which the bottom surface is being finish-surface tinplated using submerged tin anode bars. And, in another embodiment, the top surface plating is selected to provide about 0.02 to about 0.05 #/bb tin for processing as a new tin mill product. 
     Referring to FIG. 9, such new tin mill product  76  includes steel substrate  77 , barrier layer tinplate  78  (about 0.02 to about 0.05 #/bb), and finish-surface tinplate  79 , which can be selected for finish surface coating in a range of about 0.125 #/bb to about 0.75 #/bb. 
     The single-tier plating line of FIG. 2, and the first tier of a two-tier horizontally oriented plating line shown in FIG. 10, can be identical. Each will include an entry cell, as shown in more detail in FIG. 3, for carrying out the functions of the second-anode as described in relation to FIGS. 4-6. FIG. 10 depicts a two-tier plating system and FIG. 11 depicts a new tin mill product of such system. In FIGS. 10 and 11, continuous-strip steel  80 , which has been simultaneously plated on both planar surfaces in first tier  35  in order to provide finish surface tinplating  81  and barrier layer tinplating  82 , is directed to second tier  83  for supplemental finish-surface plating  84  (FIG. 11) to form tin mill product  85  of FIG.  11 . Supplemental finish-surface tinplating  84  is added to barrier layer  82  to provide a desired total plating weight of about 0.125 #/bb to 0.75 #/bb of plated surface. 
     Referring to FIGS. 1,  2 , and  10 , after plating in a single-tier, or in a two-tier horizontally-oriented plating line, the tinplated substrate is rinsed at plating solution recovery station  86 . Flow-brightening can be selected at station  87  with barrier layer at least partially alloyed, or a matte finish, as plated, is available by passage along line  88 ; tin-iron alloy on each surface can also be provided by passage through heating structure  89 . Tinplate surface can be delivered for surface passivation at station  91  and prepared for storage at station  92  and coiling as coil  93 . In FIG. 1, such passivating treatment can be bypassed by delivery along line  94 . 
     Tin mill product can be provided as follows: 
     (1) With an as-plated matte-finish tinplate on both surfaces of the products of FIGS. 9 and 11; 
     (2) with the barrier-layer alloyed with the steel substrate; 
     (3) with the tin-iron alloy on both plated surfaces of the substrate; 
     (4) flow-brightened surface on the heavy plated surface and barrier lay alloy on the remaining surface, and 
     (5) flow-brightened surface on each planar surface of the tin mill product. 
     For extended storage purposes, prior to additional in-line or sheet processing, any of the tinplated surfaces can be coated with a chrome oxide surface layer applied by either chemical dip treatment or electrolytic deposition at station  91  of FIGS. 1,  2  and  10 . 
     Chemical treatment of the product is followed by rinsing and drying, followed by an electrostatically applied light coiling lube coating at station  92 , prior to forming coil  93 . 
     While specific materials, processing and fabrication steps have been set forth for purposes of describing embodiments of the invention, various modifications can be made in the light of the above teachings without departing from applicants novel contributions; therefore in determining the scope of the present invention, reference should be made to the claims.