US3197329A

US3197329A - Method and means for forming uniform coatings

Info

Publication number: US3197329A
Application number: US227136A
Authority: US
Inventors: George O Maish
Original assignee: Bethlehem Steel Corp
Current assignee: Bethlehem Steel Corp
Priority date: 1962-10-01
Filing date: 1962-10-01
Publication date: 1965-07-27
Anticipated expiration: 1982-07-27

Description

July 27, 1965 e. o. MAISH 3,197,329

METHOD AND MEANS FOR FORMING UNIFORM COATINGS Filed Oct. 1, 1962 2 Sheets-Sheet 1 II II I II I I I l I )1 1: 32 :5 4 I O O p 4; lf}! I I l I George 0. Mo/Sb INVENTOR y 7, 1965 G- o. MAISH 3,197,329

METHOD AND MEANS FOR FORMING UNIFORM COATINGS Filed Oct. 1, 1962 2 Sheets-Sheet 2 INVENTOR George 0. Ma/Sb United States Patent() 3,197,329 METHOD AND MEANS FOR FGRNHNG UNIFQRM CQATINGS George 0. Maish, Bethlehem, Pa, assignor, by mesne assignments, to Bethlehem Steel Corporation, a corpora. tion of Delaware iled st. 1, 1962, 521'. No. 227,136

6 Claims. (Cl. 117-114) This invention relates to means for providing even distribution of coating material upon metal strip passing from a metallic coating bath. More particularly the present invention relates to means for the prevention of uneven longitudinal deposits of metallic coating materia upon metal strip coated in galvanizing or other molten coating baths and forthe control of the thickness or weight of such coating.

The development of high speed continuous galvanizing lines has been accompanied by a condition known to the industry as tiger stripingfor zebra striping. This term describes a longitudinal pattern of light and dark streaks on the coated strip, which detracts from the appearance of the strip. It is not merely a superficial matter of appearance. The streaks coincide witha variation in coating thickness which becomes more 'pronounced as line speed is increased. Some high speed lines have shown as much as a 50% variation in coating thickness between the stripes and the unstriped portions of the strip. Since the entire coating must meet the specified coating weight, the thicker portions represent, in effect, a waste of coating metal.

Careful investigation has shown that thestripes and coating variations are caused by a splitting of the liquid zinc meniscus which develops between the exit rolls and the strip just above the exit roll bite. At exit speeds above approximately 150 feet per minute the meniscus begins to develop a detectable jaggedness or uneven wave front transversely of the strip, i.e., the meniscus is split or interrupted by a number of relatively persistent, regularly spaced, narrow, upward cusps. Since at a particular speed the weight of coating is determined to a large extentby the height of the meniscus these local upward extensions of the meniscus result in streaks of heavier coating. This effect should be distinguished from edge build up, orthe build up of undesirably heavy deposits of coating metal along the edges of a coated strip, a phenomenon caused by excessive pumping of molten metal up through the roll bite adjacent the strip edges.

The present inventor has discovered that variations in the height of the meniscus may be effectively decreased, oreliminated completely, by positively rotating the exit rolls in a direction counter to the direction of movement of the strip through the exit rolls.

While the exact mechanism or action by which the reversely rotating exit rolls smooth out the meniscus is not completely understood it would appear that the rotation of the rolls drags the pool of hot metal in the nip or bite of the rolls downward in a kind of depressed menicus adjacent the rolls and that this depressed menicus adjacent the rolls stabilizes the meniscus adjacent the moving strip. p

The thickness of coating on the trip is controlled to. a large extent by the quantity of molten metal contained between the strip and the exit rolls and by the height of the meniscus on the strip as it leaves this pool. In prior apparatus the molten metal reaches this pool as a result of the pumping action of the rolls and dragging action of the strip. The quantity of metal in the pool has been controlled heretofore by raising or lowering the exit rolls in the bath and/ or by varying the speed of rotation of the rolls in relation to the speed of the strip being coated, a slower roll speed causing less pumping action. In the present invention substantially all 'the molten metal reaches the pool in the nip of the exit rolls by passing around the ouside of the rolls as they rotate. An arrangement of metering rolls associated with the reversely rotating exit rolls is therefore provided in order to regulate the amount of molten metal passing to the pool.

With the combination of reversely rotating exit rolls and adjacent metering rolls of the present invention, by varying the rotational speed of the exit rolls beyond the amount necessary to eliminate tiger striping a more precise control of the coating weight applied to the strip is also provided than has heretofore been possible with conventional forward rotating exit rolls.

It is an object of the present invention therefore to provide a method of preventing tiger striping of coated metal strip by positively rotating the exitrolls of a mol ten metal coating bath in a direction opposite to the movement of the strip past the rolls.

It is a further object of the present invention to provide an exit roll assembly for preventing tiger striping and for closely controlling the thickness of the coating.

While the present invention has been found particularly useful in preventing tiger striping in galvanized coatings the invention is also effective in other molten metal coating operations.

Other objects and advantages of the present invention will be made more evident by an examination of the accompanying figures and description in which like numerals designate like structures and in which:

FIGURE 1 is a side elevation of an exit roll assembly according to the present invention.

FIGURE 2 is a front elevation of approximately one half of the exit roll assembly shown in FIGURE 1 In the figures 10 indicates a portion of the side of a galvanizing pot. A sinker roll 12 is supported in bath 14 within pot 10 by supporting arms 16 Strip 18 passes downwardy around sinkerroll 12 and up between exit rolls 2%) and 22 partly. submerged below the surface 24 of galvanizing bath 14. Exit roll 20 is supported in bath 12 by back frame 26. Exit roll 22 is supported by front frame 28. As shown in FIGURE 2 front frame 28 is conventionally comprised of a main beam assembly 30 with supporting

end bracket assemblies

32 and 34.

Extension pieces

35 and 36 extend beyond end brackets 32 shownand 34 to engage the usual supporting structure not shown. It will be understood that back frame 26 is similar to front frame 28, both structures being conventional in the art of supporting exit rolls. Back frame 26 and ffont frame 28 are bolted together by threaded stud 37 during use to maintain alignment of the exit rolls.

Identical support plates 38 and 40 are secured in de pendent relationship from back frame 26.

Support plates

42 and 44 are pivoted on

pins

45 and 46 secured in dependent paired

plates

47 and 48 of front frame 28.

Expan'sion cylinders

49 and 50 which may be pneumatically or hydraulically operated are pivotally attached to

bracket extensions

52 and 53 of front frame 28. Piston rods 54 and 55 of

expansion cylinders

49 and 50 respectively are pivotally secured to extensions 56 and 57 of pivoted supporting

plates

42 and 44 by'clevises 58 and 59. An examination of FIGURE 1 will'show that activation of

expansion cylinders

49 and 50 will operate to

swing plates

42 and 44 about their

pivot'pins

45 and 46 toward or away from fixed support plates 38-and. 40'dependently secured to back frame 26.

Exit roll shafts 60 of exit roll 20 are supported upon hearing inserts 64 secured in lower extensions 66 of sup port plates 38 and 40. Shaft 60 of exit roll 20 is held against bearing block .64 by means of contact surface 68 of lever 70 pivoted at 72 and attached to lever .74 also pivoted at 72 in a manner such that rotation of adjustment screw 76 will bias contact surface 68 against or away 3 from shaft 60. Shaft 78 of exit roll 22 is likewise held against a bearing block 80 in extension 82 of front support plate 42 by means of levers 84 and 86 and adjustment screw 88.

A metering roll 90 is located adjacent exit roll 20 in a position such that it may regulate the amount of molten metal which will be carried around exit roll 20 from bath 14 to the small pool of molten metal 92 in the upper nip of

exit rolls

20 and 22 between exit roll 20 and strip 18. A second metering roll 94 is located adjacent exit roll 22 to meter the amount of metal which passes around roll 22 to the small molten pool 93 in the upper nip of the exit rolls between exit roll 22 and strip 18.

Metering rolls

90 and 94 may be grooved if desired to further control the amount of molten metal passing to the nip of the rolls.

Zinc pool 95 is established between exit roll 20 and metering roll 98 and zinc pool 96 is established between exit roll 22 and metering roll 94 by molten metal passing from the bath up through the bite between the respective rolls under the influence of the rotation of the rolls. The amount of molten metal reaching pools 95 and 96 is determined by the pressure maintained between the exit rolls and the metering rolls, by the rotational speeds of the rolls and by the groove pattern in the surface of the metering rolls if grooved rolls are used. The depth of pool 95 and 96 plus the rotational speeds of

exit rolls

20 and 22 determine how much molten metal is carried from pools 95 and 96 over the tops of

exit rolls

20 and 22 to the

molten zinc pools

92 and 93 maintained between the strip 18 and rolls 20 and 22 respectively, the depth of which

pools

92 and 93 in turn determined to a large extent the weight of coating deposited upon the strip.

Shaft 97 of metering roll 90 is supported upon a hearing block 98 mounted in extension 100 of support frame 38 and is maintained in contact with bearing block 98 through the agency of levers 102 and 104 and adjusting screw 106. Shaft 108 of metering roll 94 is likewise supported upon hard metal bearing block 110 mounted in extension 112 of pivoted supporting plate 42 and urged against bearing block 110 by the action of

levers

114 and 116 and adjusting screw 118.

Exit roll 20 is maintained in horizontal position through the action of lever 120 and adjusting screw 122 which press shaft 60 rearwardly against pillow block 124. Exit roll 22 likewise is maintained in position through the action of lever 126 and adjusting screw 128 which urge shaft 78 of roll 22 forwardly against pillow block 130.

Metering roll 90 is urged toward exit roll 20, in order to regulate the amount of molten metal which may pass, by the action of lever 132 pivoted at 134 and forced into contact with shaft 97 of metering roll 90 by means of threaded adjustment crank 136.

Metering roll 94 is likewise urged towards exit roll 22 through the agency of lever 138 pivoted at 140 and forced into contact with shaft 108 of meterining roll 94 by means of threaded adjustment crank 142. Threaded cranks 136 and 142 rather than the simpler adjustment screws used for the other positioning levers are used to adjust levers 132 and 138 respectively because more frequent adjustments in the horizontal position of the metering rolls are ordinarily to be expected.

The foregoing description of the lever systems for regulating the positioning of the roll shafts describes the roll positioning arrangement on only one side of the apparatus. A like system of levers and supporting structures will be provided on the opposite side of the apparatus to regulate the positioning of the other end of the rolls. a

Adjacent metering roll 94 is scraper bar .144 supported by

rods

146 and 148 fixed to U-bar 150 which is in turn pivotally attached to clevis

levers

152 and 153 pivotally secured to

bracket arms

154 and 155 by pins 156. The location of the pivotal pin connection between U-bar 150 and

clevis levers

152 and 153 may be changed in relation to roll 94 by turning adjustment screws 157 and 158 in or out to act upon clevis extension arms 159 and 160 to rotate clevis

levers

152 and 153 about pins 156. Scraper bar 144 hangs against roll 94 and it will be seen that varying the location of the pivotal connection between U-bar 150 and

levers

152 and 153 will bias the weight of scraper bar 144 more toward or away from the roll 94 to vary the pressure with which the scraper bears on the roll. A second scraper bar 161 is supported in the same manner by rod 162,'U-bar 163, clevis 164, bracket 165, clevis extension arm 166 and adjusting screw 167 at the rear of the apparatus to bear against metering roll 90.

Scraper bars 144 and 161 scrape off any dross deposits, which metering rolls 94 and respectively may drag from the .surface 24 of bath 14, and prevent such dross deposits from being carried around the bottom of the metering rolls to and up through the bites between the metering rolls and the respective adjacent exit rolls to pools 96 and from which the dross could be deleteriously conducted around the upper surfaces of exit rolls 22 or 20 to the strip coating pools in the bite between exit rolls 20 and 22. It will be readily recognized that meter rolls 90 and 94 not only act to meter the amount of molten metal reaching the strip being coated, but also effectively prevent dross from the surface of the bath from being carried around the outside of the contra-rotating exit rolls to the strip 18.

Preferably all the rolls are positively driven by couplings and shafts similar to coupling 168 and drive shaft 170, shown connected to roll 94 in FIGURE 2, and leading to appropriate drive motors, not shown, which will act to drive the rolls in directions as shown by arrows on FIGURE 1. If desired, only the two exit rolls 20 and 22 need be driven, allowing metering rolls 90 and 94 to be rotated by frictional contact with the exit rolls.

During operation strip 18 passes upwardly, as indicated by the arrows, between exit rolls 20 and 22 which rotate in directions as shown by arrows opposite to the direction of movement of the strip between the rolls. 7 The effect of the contra-rotating

rolls

20 and 22 is to reduce or eliminate the formation of uneven meniscus wave fronts across the surface of the strip so that the effective height of the meniscuses on the strip are substantially the same at all points and a uniform thickness of coating is deposited across the strip, effectively eliminating tiger striping. Ordinarily, the exit rolls will not be grooved. Smooth rolls may be used .or in some cases slightly roughened rolls.

Metering shoes may be used in place of the metering rolls if desired. Such shoes may take the form of solid contact members which extend across the length of the exit rolls 20 and 22 in a partly submerged position at the surface of the molten bath. The molten metal passing between the shoes and the surface of the exit rolls is metered by the shoes and undesirable dross from the surface of the bath is also prevented from being carried around the surface of the exit rolls to the nip of the rolls.

Theuse of metering rolls rather than metering shoes is much preferred as the metering rolls have been found to reduce the formation of deleterious zinc oxidation products and otherwise provide more positive control of the coating process.

Before beginning operation of the pot, rolls 20 and 22 will be locked into position by operating adjusting

screws

76 and 122, and 88 and 128 respectively for vertical and horizontal positioning of the

respective roll shafts

60 and 78 against their bearing and pillow blocks. The corresponding adjustment screws, on the opposite side of the apparatus, which are not shown, will be similarlyregulated to position the roll shafts on the other end of the rolls and correctly align the rolls.

Adjustment screws 106 and 118 will next be set to hold down the shafts 9'7 and 108 of metering rolls 90 and 94 and align them with the exit rolls. Corresponding adjustment screws at the opposite side of the machine will, of course, also be regulated appropriately for the same purpose. 7

Adjustment cranks 136 and 142 will be regulated to control the pressure between metering rolls 9t) and 94 and the adjustment exit rolls.

Cranks

136 and 142 may be adjusted at the intervals during the coating operation as may be necessary to maintain the correct depth of molten metal in the

pools

92 and 93 between the strip and the exit-rolls.

Scraper bars 144 and 162 may be appropriately adjusted by their regulating screws to place any desirable pressure on the metering rolls as may be determined by experience.

The clearance and pressure between exit rolls 20 and 22 is regulated by

expansion cylinders

49 and 50 which may be connected to an appropriate regulated pressure system.

Expansion cylinders

49 and 59 act to swing the roll assembly supported by front frame 28 including exit roll 22 and metering roll 94 toward or away from the roll assembly supported by back frame 26 including exit roll 20 and metering roll 90. In general, control of the coating weight is determined by the strip speed, the vertical position of the rolls relative to the bath surface, contact pressure between the rolls and the strip, the speed of the exit rolls, the pattern of grooves in the metering rolls, the speed of the metering rolls, and the contact pressure between the metering and exit rolls. The specific settings may be determined by experiment for each individual line.

In general I have found it preferable to establish a constant speed for the metering rolls and a generally constant pressure between the metering and exit rolls. This will largely determine the amount of molten metal conducted to pools 95 and 96. The amount of metal removed from pools 95 and 96 and transferred to

pools

92 and 93 between exit rolls 2t) and 22 respectively and strip 18, which pools 92 and 93 and their associated meniscuses determine to a large extent the weight of the coating applied to the strip, is then conveniently controlled or varied by regulating the rotational speed of exit rolls 20 and 22.

A rotational speed of exit rolls 20 and 22 suficient to even out the meniscuses adjacent the strip surface is first attained. The pressure between the metering rolls and the exit rolls and the speed of the metering rolls is next adjusted to provide the desired quantity of molten metal in the pools 95 and 96. The quantity of molten metal in

pools

92 and 93 may then be varied as desired by regulating the speeds of the exit rolls. More metal will be carried over by increasing the speed of rotation of the exit rolls. When speed control of the exit rolls proves insuflicient, the pressure between the metering rolls and exit rolls may be further adjusted by means of

cranks

136 and 142 to coarsely adjust the amount of metal reaching pools 95 and 96. The amount of

metal reaching pools

92 and 93 is then again finely controlled by regulating the speed of the exit rolls in the reverse direction. Very much closer control of the molten

metal reaching pools

92 and 93, and therefore much more precise control of the weight of coating applied to the strip, is thus provided than has heretofore been possible.

The exit rolls must be rotated at some minimum speed suflicient to even out the meniscus. It appears that this speed should be suflicient to give a slightly depressed meniscus against the roll. Greater speeds are not detrimental and normally the exit rolls will be variably rotated at greater speeds in order to precisely regulate the coating Weight applied to the strip. In a typical run using metering rolls having A inch V-grooves spaced /2" apart center to center, a surface speed of 94 feet per minute of the exit rolls with a strip speed of 220 feet per minute resulted in a 10 ounce galvanized coating per base box on the strip Without detectable tiger striping.

a a 6 The best speeds for particular baths and coating operations are best determined by experience.

It will be obvious that if it is only desired to control the coating applied to one side of a strip or other extended stock it is necessary only to apply the present invention to rolls on one side of the strip or stock.

I claim:

1. A method of coating metal strip comprising (a) positioning a pair of exit rolls in a molten metal bath with their axes approximately at the surface thereof,

(b) passing the strip in an upward direction from the bath through the nip of the rolls,

(c) driving the rolls in a direction opposite to the direction of movement of the strip to establish a pool of metal in the nip of the rolls, and

(d) maintaining the speed of the rolls sufficient to prevent the formation of cusps upon the molten metal meniscus adjacent the surface of the strip emerging from the pool of molten metal in the nip of the rolls.

2. A method of coating metal strip comprising (a) positioning a pair of exit rolls in a molten metal bath with their axes approximately at the surface thereof,

(0) driving the rolls in a direction opposite to the direction of movement of the strip to establish a pool of metal in the nip of the rolls,

(d) maintaining the speed of the rolls sufiicient to prevent the formation of cusps upon the molten metal meniscus adjacent the surface of the strip emerging from the pool of molten metal in the nip of the rolls, and

(e) providing metering means adjacent the surface of the exit rolls to control the amount of molten metal fed to the pool in the nip of the rolls.

3. A method of coating metal strip comprising (a) positioning a pair of exit rolls in a molten metal bath with their axes approximately at the surface thereof,

(c) driving the rolls in a direction opposite to the direction of movement of the strip to establish a pool of metal in the nip of the rolls,

(d) maintaining the speed of the rolls suflicient to prevent the formation of cusps upon the molten metal meniscus adjacent the surface of the strip emerging from the pool of molten metal in the nip of the rolls, and

(e) providing metering rolls adjacent the surface of the exit rolls to control the amount of molten metal fed to the pool in the nip of the exit rolls.

4. Apparatus for coating metal strip comprising:

(a) a bath of molten metal,

(b) a pair of exit rolls rotatably mounted in the molten bath with their axes approximately at the surface thereof,

(c) a pool of molten metal in the nip of the rolls, (d) means to pass the strip in an upward direction from the bath through the nip of the rolls, and V (e) means to drive the rolls in a direction opposite to the direction of movement of the strip to maintain the pool of molten metal in the nip of the rolls and at a speed suflicient to prevent the formation of cusps upon the molten metal meniscus adjacent the surface of the strip emerging from the pool of molten metal in the nip of the rolls.

5. Apparatus for coating metal strip comprising:

(a) a bath of molten metal,

(c) a pool of molten metal in the nip of the rolls,

((1) means to pass the strip in an upward direction from the bath through the nip of the rolls,

(e) means to drive the rolls in a direction opposite to the direction of movement of the strip to maintain the pool of molten metal in the nip of the rolls and at a speed sufiicient to prevent the formation of cusps upon the molten metal meniscus adjacent the surface of the strip emerging from the pool of molten metal in the nip of the rolls, and

(f) metering means adjacent the surface of the exit rolls to control the amount of molten metal fed to the pool in the nip of the rolls.

6. Apparatus for coating metal strip comprising:

(a) a bath of molten metal,

(b) a pair of exit rolls rotatably mounted partially submerged in the molten bath,

(0) means to pass a strip to be coated in an upward direction from the moten bath through the nip of the exit rolls,

(d) means to drive the exit rolls in a reverse direction with respect to the strip travel to maintain a pool of molten metal in the nip of the rolls and at a speed sufllcient to prevent the formation of cusps upon a molten metal meniscus adjacent the moving strip surface, and

(e) metering rolls adjacent the surface of the exit rolls to control the amount of molten metal fed to the pool in the nip of the rolls.

References Cited by the Examiner UNITED STATES PATENTS 171,687 1/76 Morewood et al. 118-206 1,608,855 11/26 Harris 118-117 RECHARD D. NEVIUS, Primary Examiner.

WILLIAM D. MARTIN, Examiner.

Claims

1. A METHOD OF COATING METAL STRIP COMPRISING (A) POSITIONING A PAIR OF EXIT ROLLS IN A MOLTEN METAL BATH WITH THEIR AXES APPROXIMATELY AT THE SURFACE THEREOF, (B) PASSING THE STRIP IN AN UPWARD DIRECTION FROM THE BATH THROUGH THE NIP OF THE ROLLS, (C) DRIVING THE ROLLS IN A DIRECTION OPPOSITE TO THE DIRECTION OF MOVEMENT OF THE STRIP TO ESTABLISH A POOL OF METAL IN THE NIP OF THE ROLLS, AND (D) MAINTAINING THE SPEED OF THE ROLLS SUFFICIENT TO PREVENT THE FORMATION OF CUSPS UPON THE MOLTEN METAL MENISCUS ADJACENT THE SURFACE OF THE STRIP EMERGING FROM THE POOL OF MOLTEN METAL IN THE NIP OF THE ROLLS.