US3104981A - Hot dip metal coating method - Google Patents

Hot dip metal coating method Download PDF

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US3104981A
US3104981A US71009A US7100960A US3104981A US 3104981 A US3104981 A US 3104981A US 71009 A US71009 A US 71009A US 7100960 A US7100960 A US 7100960A US 3104981 A US3104981 A US 3104981A
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coating
strip
molten
bead
draping
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US71009A
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John T Mayhew
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National Steel Corp
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National Steel Corp
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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
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/38—Wires; Tubes
    • 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
    • C23C2/14—Removing excess of molten coatings; Controlling or regulating the coating thickness

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  • FIGZ T. MAYHEW HOT DIP METAL COATING nmaon Filed Nov. 22, 1960 FIG] FIGZ
  • the invention is concerned generally with hot-dip coating and more particularly with a method for producing an even coating on heavy coated flat rolled products.
  • Uneven galvanize coating has other disadvantages in addition to its appearance.
  • the draping actually removes coating from the product so that the valleys formed are below coating specifications. Correction of draping has long been a problem in the hot-dip galvanizing industry.
  • All previously known solutions employed a process known as puddling. In this process, streams of compressed air were blown vertically upward onto the surface of the flat rolled product in a direction opposite to the direction of drape. If the air streams were properly positioned and at frequent enough intervals, a fair job could be done in minimizing the draping effect. Actually only the heaviest draping was smoothed over and the coating was moved by the air blasts so that the problem of uneven coating of the product still remained.
  • FIG. 1 is a side view of hot-dip coating apparatus
  • FIG. 2 is a frontal view of the apparatus of FIG. 1;
  • FIG. 3 is an enlarged view of a portion of hot-dip coating apparatus.
  • FIG. 4 is an enlarged view of a portion of coating apparatus employing the invention.
  • a flat rolled product in hot-dip galvanizing a flat rolled product is introduced into a molten bath 11 through a controlled atmosphere chute 12.
  • the flat rolled product is at a temperature above that of the molten bath and maintains the bath molten.
  • the bath is normally zinc base with aluminum additions to suppress zinc-iron a1- loying.
  • Coating control is effected by coating rolls -15 and 16; with the level of the molten bath 11, the pressure between the rolls 15 and 16, and the number of circumferential grooves present on the rolls determining the coating thickness.
  • the coated product 17 travels upwardly into cooling towers (not shown). Normally the coating begins to solidify at about 8 to 15 feet above the nip of 3,164,981 Patented Sept. 24, 1963 "ice the coating rolls, the distance varying with the temperature employed, the gauge of the base metal being coated, and the coating thickness. Before solidification some galvanized coatings above one ounce per square foot of product, and in all cases where galvanized coatings above about 1.5 ounces per square foot of product are applied, the draping eifect described earlier takes place.
  • the invention includes the discovery of causes for the aforedescribed draping of molten coatings in hot-dip coating practice.
  • molten metal accumulates above the nip 21 of the rolls 15 and 16 in crevices 22 and 23, existing on either side of the product between the coated flat rolled product 17 and the adjacent coating roll.
  • the metal in the crevices '22 and 23 takes on a rotary motion because of the movement of the coated product 17 and the coating rolls 15 and 16 so that beads of metal 26 and 27 are formed in each crevice.
  • the bead of metal in each crevice due to the relative motion between it and the coated product 17 picks up additional metal from the coated product and grows with a snowballing efiect.
  • This pick up of metal from the surface of coated product is believed to be caused by the surface tension between the molten coating metal on coated product 17 and metal in the crevices 22 and 23 respectively.
  • the cohesive force of a bead of metal is. overcome and a heavy blob of metal is deposited on the product. As the product 17 travels upwardly the heavy blobs of metal start to drape and pull further coating from the product above the drape.
  • the build-up of metal in the crevices 22 and 23 and the deposition of such metal on the coated product 17 is prevented.
  • This buildup and deposition of; metal is prevented by blowing a gas downwardly into the crevices between the coated product 17 and the coating rolls 15 and 16.
  • the gas reduces the surface tension between the beads of metal 26 and 27 and the coating on the prodnot 17 allowing the metal to flatten in the crevice as shown in FIG. 4.
  • a gas is introduced into the crevices through the pipes 311 and 31 and flows with a high volume and low pressure.
  • the gas after contact in the crevices flows outwardly toward the lateral edges of the coated product 17 and the coating rolls 15 and 16.
  • Suitable gases include compressed air, or inert gases such as nitrogen. It is preferred that the gas be hot, either heated or in the form of a flame to prevent chilling of the coating rolls 15 and 16. Therefore, when heated gas is used a temperature approaching that of the coating bath 11 is employed to prevent chilling of galvanizing material on the rolls and the fouling thereof. When coke oven gas, blast furnace gas, or their equivalent is used a normal yellow, low-temperature flame is satisfactory as undue heating of the rolls or coating material is not desired. It is emphasized that having heat present in the gas'prevents chilling of the:coating rolls.
  • a method for preventing such draping of the molten coating comprising the steps of applying a galvanize coating to the continuous strip steel by passing the strip upwardly through a molten metal bath,
  • controlling the weight of the galvanize coating to be at least one and one-half ounces'per square foot by passing the coated strip between coating control rolls located so as to deliver the strip upwardly from the molten bath, the exiting strip and the coating control rolls forming a crevice located on a face of the strip and containing a bead ofmolten metal, and preventing draping of the molten metal coating on the strip after passage of the strip through the Coating control rolls without changing the coating weight by contacting the surface of the head of molten metal with an ignited combustible gas along substantially the entire length of the bead, with the direction and velocity of the burning gas being such that the gas exerts substantially no force on the major portion of the length of the bead.

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  • Chemical & Material Sciences (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Coating With Molten Metal (AREA)

Description

p 1953 .1. T. MAYHEW HOT DIP METAL COATING nmaon Filed Nov. 22, 1960 FIG] FIGZ
I m d INVENTOR.
JOHN T MAYHEW ATTORNEY United States Patent 3,104,981 HGT DIP METAL COATING METHGD John T. Mayhew, Toronto, Ohio, assignor to National Steel Corporation, a corporation of Delaware Filed Nov. 22, 1960, Ser- No. 71,009 2 Claims. (iii. 117-46) The invention is concerned generally with hot-dip coating and more particularly with a method for producing an even coating on heavy coated flat rolled products.
In the hot-dip coating of flat rolled products, such as hot-dip galvanizing, considerable difiiculty has been experienced with draping of the coating when a heavy coating is applied. For example, in the continuous hotdip galvanizing wherein a coating of about 1.5 ounces per square foot of the product is applied, the molten metal will run at frequent intervals along the surface of the product. Visually the running or draping of the molten galvanize coating has the same appearance as that occurring when a too heavy brush stroke of wet paint is applied to a wall. This draping of the molten coating appears frequently, usuflly at less than two foot intervals longitudinally and less than 1 foot intervals laterally along the surface of the product. In solidifying, the run or drape presents an uneven, unattractive surface.
Uneven galvanize coating has other disadvantages in addition to its appearance. The draping actually removes coating from the product so that the valleys formed are below coating specifications. Correction of draping has long been a problem in the hot-dip galvanizing industry. All previously known solutions employed a process known as puddling. In this process, streams of compressed air were blown vertically upward onto the surface of the flat rolled product in a direction opposite to the direction of drape. If the air streams were properly positioned and at frequent enough intervals, a fair job could be done in minimizing the draping effect. Actually only the heaviest draping was smoothed over and the coating was moved by the air blasts so that the problem of uneven coating of the product still remained. Poor puddling resulted in splattering of the metal and wasted product. Puddling, even at its highest efiiciency, is not adequate to the job as it is merely a remedial process rather than a preventative process. A significant part of the invention was the discovery and analysis of the source of the problem; a novel preventative process resulted therefrom.
In explaining the invention reference will be had to the accompanying drawings in which:
FIG. 1 is a side view of hot-dip coating apparatus;
FIG. 2 is a frontal view of the apparatus of FIG. 1;
FIG. 3 is an enlarged view of a portion of hot-dip coating apparatus; and
FIG. 4 is an enlarged view of a portion of coating apparatus employing the invention.
Referring to FIG. 1, in hot-dip galvanizing a flat rolled product is introduced into a molten bath 11 through a controlled atmosphere chute 12. The flat rolled product is at a temperature above that of the molten bath and maintains the bath molten. The bath is normally zinc base with aluminum additions to suppress zinc-iron a1- loying.
The product travels downwardly around submerged roll 14- and then upwardly toward exit means which also function as coating control means. Coating control is effected by coating rolls -15 and 16; with the level of the molten bath 11, the pressure between the rolls 15 and 16, and the number of circumferential grooves present on the rolls determining the coating thickness.
After exit from the molten bath through the coating rolls 15 and 16 the coated product 17 travels upwardly into cooling towers (not shown). Normally the coating begins to solidify at about 8 to 15 feet above the nip of 3,164,981 Patented Sept. 24, 1963 "ice the coating rolls, the distance varying with the temperature employed, the gauge of the base metal being coated, and the coating thickness. Before solidification some galvanized coatings above one ounce per square foot of product, and in all cases where galvanized coatings above about 1.5 ounces per square foot of product are applied, the draping eifect described earlier takes place.
The invention includes the discovery of causes for the aforedescribed draping of molten coatings in hot-dip coating practice. As the coated flat rolled product 17 moves upwardly through the coating rolls 15 and 16, molten metal accumulates above the nip 21 of the rolls 15 and 16 in crevices 22 and 23, existing on either side of the product between the coated flat rolled product 17 and the adjacent coating roll. The metal in the crevices '22 and 23 takes on a rotary motion because of the movement of the coated product 17 and the coating rolls 15 and 16 so that beads of metal 26 and 27 are formed in each crevice. The bead of metal in each crevice due to the relative motion between it and the coated product 17 picks up additional metal from the coated product and grows with a snowballing efiect. This pick up of metal from the surface of coated product is believed to be caused by the surface tension between the molten coating metal on coated product 17 and metal in the crevices 22 and 23 respectively. At periodic intervals, as metal accumulates along crevices 22 and 23 the cohesive force of a bead of metal is. overcome and a heavy blob of metal is deposited on the product. As the product 17 travels upwardly the heavy blobs of metal start to drape and pull further coating from the product above the drape.
In accordance with the invention the build-up of metal in the crevices 22 and 23 and the deposition of such metal on the coated product 17 is prevented. This buildup and deposition of; metal is prevented by blowing a gas downwardly into the crevices between the coated product 17 and the coating rolls 15 and 16. By properly selecting the direction of flow of gas into the crevices 22 and 23 the draping of molten coating is eliminated entirely. It is believed the gas reduces the surface tension between the beads of metal 26 and 27 and the coating on the prodnot 17 allowing the metal to flatten in the crevice as shown in FIG. 4.
When coke oven gas is blown into the crevices it is ignited by the temperatures existing around the coating pot. It has been observed, however, that the draping effect is discontinued immediately upon the flow of the gas, prior to the ignition thereof, so that a flame is not necessary to the operation of the invention.
Referring to FIGS. 2 and 4 a gas is introduced into the crevices through the pipes 311 and 31 and flows with a high volume and low pressure. The gas after contact in the crevices flows outwardly toward the lateral edges of the coated product 17 and the coating rolls 15 and 16. By the use of a high volume-low pressure gas splattering of the metal is avoided, and the coating remains evenly distributed on the coated flat rolled product -17.
Suitable gases include compressed air, or inert gases such as nitrogen. It is preferred that the gas be hot, either heated or in the form of a flame to prevent chilling of the coating rolls 15 and 16. Therefore, when heated gas is used a temperature approaching that of the coating bath 11 is employed to prevent chilling of galvanizing material on the rolls and the fouling thereof. When coke oven gas, blast furnace gas, or their equivalent is used a normal yellow, low-temperature flame is satisfactory as undue heating of the rolls or coating material is not desired. It is emphasized that having heat present in the gas'prevents chilling of the:coating rolls.
Under normal circumstances a single gas outlet located 7 centrally of the flat rolled product is sufficient. It is important that the direction of gas flow be substantially 3 ertically downward onto the molten metal in the crevices 22 and 23; such a path has the efifect of flattening the molten metal in the crevices as shown in FIG. 4. Normally this flow path of the gas will be substantially parallel to the path of the exiting flat rolled product 1'7.
In a specific embodiment of the invention employing coke oven gas the following values were measured:
Gas pressure, in pipe lbs./in. 2-3 Gas pressure, between pipe and crevice lb./in. Gas volume ft. /hr 306-500 Temperature, thermo-couple after 35 minutes in thefiame F 1000 Summarizing the invention, the cause of molten coating drape in hot-dip, heavy coating of fiat rolled product has been discovered and analyzed and a method devised for preventing draping in which a gas is blown downwardly into a crevice between the exiting fiat rolled product and the coating control means along a path which is substantially parallel to the path of the exiting flat rolled product.
What is claimed is:
1. In continuous strip-steel hot-dip metal coating wherein a heavy galvanize coating weight is applied such that draping of the molten galvanize coating on the strip would normally occur, a method for preventing such draping of the molten coating comprising the steps of applying a galvanize coating to the continuous strip steel by passing the strip upwardly through a molten metal bath,
controlling the weight of the galvanize coating to be at least one and one-half ounces'per square foot by passing the coated strip between coating control rolls located so as to deliver the strip upwardly from the molten bath, the exiting strip and the coating control rolls forming a crevice located on a face of the strip and containing a bead ofmolten metal, and preventing draping of the molten metal coating on the strip after passage of the strip through the Coating control rolls without changing the coating weight by contacting the surface of the head of molten metal with an ignited combustible gas along substantially the entire length of the bead, with the direction and velocity of the burning gas being such that the gas exerts substantially no force on the major portion of the length of the bead.
2. In continuous strip-steel hot-dip metal coating wherein a heavy galvanize coating weight is applied such that draping of the molten galvanize coating on the strip would normally occur, a method for preventing such the *ing of the molten coating comprising the steps of:
applying a galvanize coating to the continuous strip steel by passing the strip upwardly through a molten metal bath, 7 V controlling the weight of the galvanize coating to be at least one and one-half ounces per square foot by passing the coated strip between coating control rolls located so as to deliver the strip upwardly from the molten bath, the exiting strip and the coating control rolls forming a crevice located on a face of the strip and containing a bead of molten metal, and I preventing draping of the molten metal coating on the strip after passage of the strip through the coating control rolls Without changing the-coating weight by contacting the surface of the bead of molten metal with an ignited combustible gas along substantially the entire length of the bead, with the direction of how of the burning gas beingsubstantially parallel to the length of the bead along the major portion of the length of the bead whereby the burning gas exerts substantially no force on the major portion of the length of the bead.
References Cited in the file of this patent UNITED STATES PATENTS Whitley et al .July 18, 1961

Claims (1)

1. IN CONTINUOUS STRIP-STEEL HOT-DIP METAL COATING WHEREIN A HEAVY GALVANIZED COATING WEIGHT IS APPLIED SUCH THAT DRAPING OF THE MOLTEN GALVANIZE COATING ON THE STRIP WOULD NORMALLY OCCUR, A METHOD FOR PREVENTING SUCH DRAPING OF THE MOLTEN COATING COMPRISING THE STEPS OF: APPLYING A GALVANIZE COATING TO THE CONTINUOUS STRIP STEEL BY PASSING THE STRIP UPWARDLY THROUGH A MOLTEN METAL BATH, CONTROLLING THE WEIGHT OF THE GALVANIZE COATING TO BE AT LEAST ONE AND ONE-HALF OUNCES PER SQUARE FOOT BY PASSING THE COATED STRIP BETWEEN COATING CONTROL ROLLS LOCATED SO AS TO DELIVER THE STRIP UPWARDLY FROM THE MOLTEN BATH, THE EXITING STRIP AND THE COATING CONTROL ROLLS FORMING A CERVICE LOCATED ON A FACE OF THE STRIP AND CONTAINING A BEAD OF MOLTEN METAL, AND PREVENTING DRAPING OF THE MOLTEN METAL COATING ON THE STRIP AFTER PASSAGE OF THE STRIP THROUGH THE COATING CONTROL ROLLS WITHOUT CHANGING THE COATING WEIGHT BY CONTACTING THE SURFACE OF THE BEAD OF MOLTEN METAL WITH AN IGNITED COMBUSTIBLE GAS ALONG SUBSTANTIALLY THE ENTIRE LENGTH OF THE BEAD, WITH THE DIRECTION AND VELOCITY OF THE BURNING GAS BEING SUCH THAT THE GAS EXERTS SUBSTANTIALLY NO FORCE ON THE MAJOR PORTION OF THE LENGTH OF THE BEAD.
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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3369923A (en) * 1964-12-14 1968-02-20 Bethlehem Steel Corp Method of producing heavy coatings by continuous galvanizing

Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2034348A (en) * 1930-09-03 1936-03-17 Lytle Clark Kenneth Nonspangled galvanized sheet
US2708171A (en) * 1952-07-08 1955-05-10 United States Steel Corp Method of controlling coating thickness in continuous galvanizing
US2894856A (en) * 1956-08-31 1959-07-14 Inland Steel Co Apparatus for and method of controlling the coating thickness in continuous galvanizing
US2992941A (en) * 1958-05-07 1961-07-18 Armco Steel Corp Exit machine for coating apparatus and method of controlling coating thickness

Patent Citations (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2034348A (en) * 1930-09-03 1936-03-17 Lytle Clark Kenneth Nonspangled galvanized sheet
US2708171A (en) * 1952-07-08 1955-05-10 United States Steel Corp Method of controlling coating thickness in continuous galvanizing
US2894856A (en) * 1956-08-31 1959-07-14 Inland Steel Co Apparatus for and method of controlling the coating thickness in continuous galvanizing
US2992941A (en) * 1958-05-07 1961-07-18 Armco Steel Corp Exit machine for coating apparatus and method of controlling coating thickness

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3369923A (en) * 1964-12-14 1968-02-20 Bethlehem Steel Corp Method of producing heavy coatings by continuous galvanizing

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