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US20160168658A1 - Method of producing metal-coated steel strip - Google Patents

Method of producing metal-coated steel strip Download PDF

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Publication number
US20160168658A1
US20160168658A1 US14/436,536 US201314436536A US2016168658A1 US 20160168658 A1 US20160168658 A1 US 20160168658A1 US 201314436536 A US201314436536 A US 201314436536A US 2016168658 A1 US2016168658 A1 US 2016168658A1
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Prior art keywords
alloy
cooling
water
method defined
strip
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US14/436,536
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English (en)
Inventor
Wayne Andrew Renshaw
Aaron Kiffer Neufeld
Ross McDowall Smith
Geoff Tapsell
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BlueScope Steel Ltd
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BlueScope Steel Ltd
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Priority claimed from AU2012904523A external-priority patent/AU2012904523A0/en
Application filed by BlueScope Steel Ltd filed Critical BlueScope Steel Ltd
Publication of US20160168658A1 publication Critical patent/US20160168658A1/en
Assigned to BLUESCOPE STEEL LIMITED reassignment BLUESCOPE STEEL LIMITED ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: RENSHAW, WAYNE ANDREW, SMITH, ROSS MCDOWALL, NEUFELD, AARON KIFFER, TAPSELL, Geoff
Abandoned legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
    • C21D1/60—Aqueous agents
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C18/00—Alloys based on zinc
    • C22C18/04—Alloys based on zinc with aluminium as the next major constituent
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C21/00—Alloys based on aluminium
    • C22C21/10—Alloys based on aluminium with zinc as the next major constituent
    • 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
    • C23C2/0034—Details related to elements immersed in bath
    • C23C2/00342—Moving elements, e.g. pumps or mixers
    • C23C2/00344—Means for moving substrates, e.g. immersed rollers or immersed bearings
    • 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
    • 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
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • 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
    • C23C2/16—Removing excess of molten coatings; Controlling or regulating the coating thickness using fluids under pressure, e.g. air knives
    • C23C2/18—Removing excess of molten coatings from elongated material
    • C23C2/20—Strips; Plates
    • 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
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • 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
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • C23C2/29—Cooling or quenching
    • 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
    • C23C2/40—Plates; Strips

Definitions

  • the present invention relates to the production of metal strip, typically steel strip, which has a coating of a corrosion-resistant metal alloy that contains aluminium-zinc-silicon-magnesium as the main elements in the alloy, and is hereinafter referred to as an “Al—Zn—Si—Mg alloy” on this basis.
  • the present invention relates particularly, although by no means exclusively, to the production of strip, typically steel strip, which has a coating of a corrosion-resistant metal alloy that contains aluminium-zinc-silicon-magnesium as the main elements in the alloy in the following ranges in % by weight:
  • the Al—Zn—Si—Mg alloy may contain other elements that are present in the alloy as deliberate alloying additions or as unavoidable impurities.
  • the phrase “Al—Zn—Si—Mg alloy” is understood herein to cover alloys that contain such other elements as deliberate alloying additions or as unavoidable impurities.
  • the other elements may include by way of example any one or more of Fe, Ti, Cu, Ni, Co, Ca, Mn, Be, Sr, Ca, Cr, and V.
  • the present invention relates to a hot-dip metal coating method of forming a coating of an Al—Zn—Si—Mg alloy on a strip that includes dipping uncoated strip into a bath of molten Al—Zn—Si—Mg alloy and forming a coating of the alloy on the strip.
  • composition of the as-solidified coating of the Al—Zn—Si—Mg alloy may be different to an extent to the composition of the Al—Zn—Si—Mg alloy used to form the coating due to factors such as partial dissolution of the metal strip into the coating during the coating process.
  • the profiled sheets are usually manufactured by cold forming painted, metal alloy coated strip. Typically, the profiled sheets are manufactured by roll-forming the painted strip.
  • Another Al—Zn—Si—Mg alloy coating that is described in the patent literature although not commercially available in Australia is formed from an alloy that contains in % by weight: Al: 2 to 19%, Si: 0.01 to 2%, Mg: 1 to 10%, balance Zn and unavoidable impurities.
  • the alloy coating is described and claimed in Australian patent 758643 entitled “Plated steel product, plated steel sheet and precoated steel sheet having excellent resistance to corrosion” in the name of Nippon Steel Corporation.
  • Mg when Mg is included in Al—Zn coating compositions, Mg brings about certain beneficial effects on product performance, such as improved cut-edge protection.
  • the applicant has carried out extensive research and development work in relation to Al—Zn—Si—Mg alloy coatings on strip such as steel strip.
  • the present invention is the result of part of this research and development work.
  • the applicant found that there was greater dissolution of Al—Zn—Si—Mg alloy coatings into quench water than was the case with conventional Al—Zn coatings and the dissolution resulted in precipitates in quench water that caused a rapid deterioration of cooling water circuit heat exchangers and caused undesirable coatings to form on cooling water storage tank surfaces in the quench water circuits in the metal coating lines.
  • the precipitation problem is a potentially serious maintenance issue.
  • a method of forming a coating of an Al—Zn—Si—Mg alloy on a steel strip to form an Al—Zn—Mg—Si coated steel strip including the steps of dipping steel strip into a bath of molten Al—Zn—Si—Mg alloy and forming a coating of the alloy on exposed surfaces of the steel strip and cooling the coated strip with cooling water, with the cooling step including controlling the pH of cooling water to be in a range of pH 5-9.
  • the cooling step may include controlling the pH of cooling water to be less than 7.5.
  • the cooling step may include controlling the pH of cooling water to be greater than 5.5.
  • the cooling step may include controlling the pH of cooling water to be greater than 6.
  • the cooling step may include controlling the temperature of cooling water to be in a range of 25-80° C.
  • the cooling step may include controlling the temperature of cooling water to be less than 70° C.
  • the cooling step may include controlling cooling water temperature to be less than 45° C.
  • the cooling step may include controlling cooling water temperature to be greater than 30° C.
  • the cooling step may include controlling cooling water temperature to be greater than 35° C.
  • the cooling step may include controlling cooling water temperature to be greater than 40° C.
  • the cooling step may include controlling the pH by adding acid to the cooling water.
  • the cooling step may include controlling the pH by adding acid and other salts, buffers, wetting agents, surfactants, coupling agents, etc.
  • the acid may be any suitable acid such as phosphoric acid and nitric acid by way of example.
  • the cooling step may be a closed loop in which water is circulated through a circuit that supplies water to the coated strip and collects and cools water and returns the cooled water for cooling the coated strip.
  • the closed loop may include a water storage tank, a spray system for supplying water to the coated strip from the tank, and a heat exchanger for cooling water after it has been sprayed onto the strip.
  • the cooling step may be an open loop in which cooling water is not recycled in the cooling step.
  • the cooling step may include controlling the operating conditions to cool the coated strip to a temperature range of 28-55° C.
  • the cooling step may include controlling the operating conditions to cool the coated strip to a temperature range of 30-50° C.
  • the method may include other steps including any one or more of the steps of pre-treating strip to clean the strip before the hot dip coating step, controlling the thickness of the coated strip immediately after the coating step, rolling the coated strip, treating the coated strip with a passivation solution, and coiling the coated strip.
  • the Al—Zn—Si—Mg alloy may include the following ranges in % by weight:
  • the Al—Zn—Si—Mg alloy may include the following ranges in % by weight of the elements Al, Zn, Si, and Mg:
  • the Al—Zn—Si—Mg alloy coating may contain other elements that are present as deliberate alloying additions or as unavoidable impurities.
  • the other elements may include by way of example any one or more of Fe, Sr, Cr, and V.
  • the other elements may include Ca for dross control in molten coating baths.
  • the steel may be a low carbon steel.
  • the present invention also provides an Al—Zn—Mg—Si alloy coated steel strip produced by the above-described method.
  • FIG. 1 is a schematic drawing of one embodiment of a continuous metal coating line for forming an Al—Zn—Si—Mg alloy coating on steel strip in accordance with the method of the present invention
  • FIG. 2 is a graph of the Al and Ca concentrations in cooling water used during the course of a plant trial carried out by the applicant.
  • FIG. 3 is a graph of the Mg and Zn concentrations in cooling water used during the course of the plant trial carried out by the applicant.
  • coils of cold rolled low carbon steel strip are uncoiled at an uncoiling station 1 and successive uncoiled lengths of strip are welded end to end by a welder 2 and form a continuous length of strip.
  • the strip is then passed successively through an accumulator 3 , a strip cleaning section 4 and a furnace assembly 5 .
  • the furnace assembly 5 includes a preheater, a preheat reducing furnace, and a reducing furnace.
  • the strip is heat treated in the furnace assembly 5 by careful control of process variables including: (i) the temperature profile in the furnaces, (ii) the reducing gas concentration in the furnaces, (iii) the gas flow rate through the furnaces, and (iv) strip residence time in the furnaces (i.e. line speed).
  • the process variables in the furnace assembly 5 are controlled so that there is removal of iron oxide residues from the surface of the strip and removal of residual oils and iron fines from the surface of the strip.
  • the heat treated strip is then passed via an outlet snout downwardly into and through a molten bath containing an Al—Zn—Si—Mg alloy held in a coating pot 6 and is coated with Al—Zn—Si—Mg alloy.
  • the Al—Zn—Si—Mg alloy in the coating pot 6 comprises in % by weight: Zn: 2 to 19%, Si: 0.01 to 2%, Mg: 1 to 10%, and balance Al and unavoidable impurities.
  • the coating pot 6 may also contain Ca for dross control in the molten bath.
  • the Al—Zn—Si—Mg alloy is maintained molten in the coating pot at a selected temperature by use of heating inductors (not shown).
  • the strip passes around a sink roll and is taken upwardly out of the bath.
  • the line speed is selected to provide a selected immersion time of strip in the coating bath. Both surfaces of the strip are coated with the Al—Zn—Si—Mg alloy as it passes through the bath.
  • the strip After leaving the coating bath 6 the strip passes vertically through a gas wiping station (not shown) at which its coated surfaces are subjected to jets of wiping gas to control the thickness of the coating.
  • the exposed surfaces of the Al—Zn—Si—Mg alloy coating oxidise as the coated strip moves through the gas wiping station and a native oxide layer forms on the exposed surfaces of the coating.
  • the native oxide is the first oxide to form on the surface of the metal alloy coating, with its chemical make-up being intrinsically dependent on the composition of the metal alloy coating, including Mg oxide, Al oxide, and a small amount of oxides of other elements of the Al—Zn—Si—Mg alloy coating.
  • the coated strip is then passed through a cooling section 7 and is subjected to forced cooling by means of a water quench step.
  • the forced cooling may include a forced air cooling step (not shown) before the water quench step.
  • the water quench step is, by way of example, a closed loop in which water sprayed onto coated strip is collected and then cooled for re-use to cool coated strip.
  • the cooling section 7 includes a coated strip cooling chamber 7 a , a spray system 7 b that sprays water onto the surface of the coated strip as it moves through the cooling chamber 7 a , a water quench tank 7 c for storing water that is collected from the cooling chamber 7 b , and a heat exchanger 7 d for cooling water from the water quench tank 7 c before transferring the water to the spray system 7 b.
  • the pH of the cooling water supplied to the spray system 7 b is controlled to be in a range of pH 5-9, typically in a range of 5-8, more typically in a range of 5.5-7.5 and (b) the temperature of the cooling water supplied to the spray system is controlled to be in a relatively low temperature range of 30-50° C. Both control steps (a) and (b) minimise dissolution of the Al—Zn—Si—Mg alloy coating on the coated strip.
  • the pH and temperature control may be achieved, by way of example, by using a pH probe and a temperature sensor in an overflow tank of the water quench tank 7 c and supplying data from the probe/sensor to a PLC and calculating required acid additions to maintain the pH at predetermined set points for pH and the water temperature, with any acid additions and temperature adjustments being made so that the water in the water quench tank 7 c is controlled to the set points for pH and temperature.
  • This is not the only possible option for achieving pH and temperature control.
  • the pH, temperature, and chemical control may also be achieved by way of example, by using a once through water cooling system where the quench water is not recirculated and the input water has pH and temperature properties as described above.
  • the cooled, coated strip is then passed through a rolling section 8 that conditions the surface of the coated strip.
  • This section may include one or more of skin pass and tension leveling operations.
  • the conditioned strip is then passed through a passivation section 10 and coated with a passivation solution to provide the strip with a degree of resistance to wet storage and early dulling.
  • the coated strip is thereafter coiled at a coiling station 11 .
  • the Springhill metal coating lines are similar in general terms to the line shown in FIG. 1 and include a closed loop quench step on each of the three lines (MCL 1 , MCL 2 , and MCL 3 ). Each closed loop processes a relatively small volume (approx 5000 L) of water. The cooling water is cooled by dedicated heat exchangers on each line.
  • the white precipitates formed on cooling system equipment surfaces and covered an initial layer of grey material.
  • the grey layer was found to contain Al(OH) 3 and Al 2 O 3 .3H 2 O from previous line operations using conventional Al—Zn alloys.
  • the white precipitates were found to contain Mg 4 Al 2 (OH) 14 .3H 2 O and Al 2 O 3 .3H 2 O.
  • These magnesium/aluminium oxy/hydroxides also contained magnesium carbonate compounds.
  • the applicant carried out x-ray photoelectron spectroscopy (XPS) depth profiling analysis to assess the condition of the surfaces of coatings of the Al—Zn—Si—Mg alloys used in the above-described initial plant trials.
  • the XPS depth profiling analysis was carried out on steel panels produced on the Hot Dip Process Simulator (HDPS) at the research facilities of the applicant.
  • the HDPS is a state-of-the-art unit purpose-built to the specifications of the applicant by Iwatani International Corp (Europe) GmbH.
  • the HDPS unit comprises a molten metal pot furnace, an infrared heating furnace, gas wiping nozzles, de-drossing mechanisms, gas mixing and dewpoint management functions, and computerized automatic control systems.
  • the HDPS unit is capable of simulating a typical hot dip cycle on a conventional metal coating line.
  • the XPS depth profiling analysis identified thin oxide layers on the Al—Zn—Si—Mg alloy coated steel panels. The oxide layers consisted primarily of aluminium and magnesium oxides.
  • the HDPS has gas cooling but no water quench, and thus the oxide layers are representative of oxides forming on the surface of the molten coatings at elevated temperatures after coated strip emerges form a coating bath.
  • Group (a) alloys include the following ranges in % by weight of the elements Al, Zn, Si, and Mg: Al: 2 to 19%, Si: 0.01 to 2%, Mg: 1 to 10%, and balance Zn and unavoidable impurities.
  • Group (b) alloys include the following ranges in % by weight of the elements Al, Zn, Si, and Mg: Al: 30 to 60%, Si: 0.3 to 3%, Mg: 0.3 to 10%, and balance Zn and unavoidable impurities.
  • the plant trials carried out by the applicant comprised the above-described initial plant trials on Al—Zn—Si—Mg alloys that identified the precipitate problem in the first instance and later more extensive plant trials that confirmed the precipitate problem and evaluated several options to minimise the problem.
  • the later plant trials on the MCL 1 line were carried out by hot dip coating steel strip with the following alloys in coating baths: (a) a known Al—Zn alloy (hereinafter referred to as “AZ”) and (b) an Al—Zn—Si—Mg alloy (hereinafter referred to as “AM”) having the following compositions, in wt. %:
  • AZ Al—Zn alloy
  • AM Al—Zn—Si—Mg alloy
  • the first week of the plant trials on the MLC1 line was run with the AZ (Al—Zn) alloy and produced standard Zincalume (Registered Trade Mark) coated strip.
  • the line was run in accordance with established operating conditions. In terms of the water cooling step on the line, the quench water was at a temperature of 50-60° C. upstream of the water sprays. There was no pH control of the quench water. Under these conditions the quench water became saturated with aluminium and the pH increased to around 8.5 (at 60° C.).
  • a trial to control quench tank pH using phosphoric acid ran for 4 days.
  • the control system was set to allow a pre-determined [OH ⁇ ] ion value of 1.0 ⁇ 10 ⁇ 6 mol/L.
  • Table 2 provides the values of the pH set point for different water quench tank temperatures to maintain a set pH.
  • the periods 1 - 4 represent pH control (1), low temperature control (35° C.) (2), quench tank set point at 50° C. (3), and quench tank set point at 40° C., respectively.
  • the quench water pH control and temperature control component of the research and development work reported in the specification focuses on a group (b) alloy composition of 53Al-43Zn-2Mg-1.5Si—0.45Fe-incidental impurities. There are no specific results on quench water pH control and temperature control for the group (a) composition range reported in the specification. Nevertheless, the body of research and development work carried out by the applicant in relation to Al—Zn—Si—Mg alloy coatings on steel strip, across the group (a) and group (b) alloy composition ranges, including laboratory work and metallurgical modelling work, indicates that the quench water pH control and temperature control requirements for group (b) alloy compositions also apply to group (a) alloy compositions.
  • the present invention is not so limited and extends to any suitable water cooling system, such as dunk or immersion tanks.

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WO2017017485A1 (fr) 2015-07-30 2017-02-02 Arcelormittal Procédé de fabrication d'une pièce phosphatable à partir d'une tôle d'acier revêtue d'un revêtement métallique à base d'aluminium
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AU2020230266A1 (en) 2020-10-01
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