US20150247230A1 - Method of producing metal-coated steel strip - Google Patents

Method of producing metal-coated steel strip Download PDF

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Publication number
US20150247230A1
US20150247230A1 US14/436,524 US201314436524A US2015247230A1 US 20150247230 A1 US20150247230 A1 US 20150247230A1 US 201314436524 A US201314436524 A US 201314436524A US 2015247230 A1 US2015247230 A1 US 2015247230A1
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cooling
alloy
water
strip
method defined
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Aaron Kiffer Neufeld
Wayne Andrew Renshaw
Geoff Tapsell
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BlueScope Steel Ltd
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BlueScope Steel Ltd
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Priority claimed from AU2012904524A external-priority patent/AU2012904524A0/en
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Assigned to BLUESCOPE STEEL LIMITED reassignment BLUESCOPE STEEL LIMITED ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: NEUFELD, AARON KIFFER, RENSHAW, WAYNE ANDREW, TAPSELL, Geoff
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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
    • C23C2/04—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
    • C23C2/12—Aluminium or alloys based thereon
    • 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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C18/00—Alloys based on zinc
    • 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
    • C23C2/04—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
    • C23C2/26—After-treatment
    • 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
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/12—All metal or with adjacent metals
    • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
    • Y10T428/12771—Transition metal-base component
    • Y10T428/12861—Group VIII or IB metal-base component
    • Y10T428/12951—Fe-base component
    • Y10T428/12972—Containing 0.01-1.7% carbon [i.e., steel]

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 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.
  • the Al—Zn—Si—Mg alloy of the present invention comprises the following ranges in % by weight of the elements Al, Zn, Si, and Mg:
  • the Al—Zn—Si—Mg alloy of the present invention comprises the following ranges in % by weight of the elements Al, Zn, Si, and Mg:
  • 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, Sr, Cr, and V.
  • 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 partixdla dissolution of the metal strip into the coating during the coating process.
  • the metal-coated strip may be painted, for example with a polymeric paint, on one or both surfaces of the strip.
  • the metal-coated strip may be sold as an end product itself or may have a paint coating applied to one or both surfaces and be sold as a painted end product.
  • 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.
  • Mg when Mg is included in a 55% Al—Zn alloy coating, 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 research and development work that is relevant to the present invention included a series of plant trials on metal coating lines of the applicant to investigate the viability of forming Al—Zn—Si—Mg alloy coatings on steel strip on these metal coating lines.
  • the plant trials found that Al—Zn—Si—Mg alloy coatings are far more reactive with quench water used to cool metal alloy coatings on strip after coated strip leaves molten alloy baths in the metal coating lines than conventional Al—Zn coatings.
  • 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.
  • the applicant found that pH control of cooling water and to a lesser extent cooling water temperature control made it possible to reduce the extent of precipitate formation and allowed the cooling water heat exchangers to perform in a practical manner. More particularly, the applicant found that the precipitate problem could be addressed by suppressing the alkalinity of cooling water via pH control of cooling water and to a lesser extent cooling water temperature control (operating at low temperatures) to thereby reduce the corrosiveness of the cooling water towards Al—Zn—Si—Mg alloy coatings.
  • 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 8.
  • the cooling step may include controlling the pH of cooling water to be less than 7.
  • 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 60° C.
  • the cooling step may include controlling cooling water temperature to be less than 55° C.
  • the cooling step may include controlling cooling water temperature to be less than 50° 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 water quench step.
  • 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 more than 0.3% by weight Mg.
  • the Al—Zn—Si—Mg alloy may include more than 1.0% by weight Mg.
  • the Al—Zn—Si—Mg alloy may include more than 1.3% by weight Mg.
  • the Al—Zn—Si—Mg alloy may comprise more than 1.5% by weight Mg.
  • the Al—Zn—Si—Mg alloy may include less than 3% by weight Mg.
  • the Al—Zn—Si—Mg alloy may include more than 2.5% by weight Mg.
  • the Al—Zn—Si—Mg alloy may include more than 1.2% by weight Si.
  • the Al—Zn—Si—Mg alloy may include less than 2.5% by weight Si.
  • 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 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.
  • the Al—Zn—Si—Mg alloy used to form the coating of the Al—Zn—Mg—Si alloy coated steel strip may include the following ranges in % by weight of the elements Al, Zn, Si, and Mg:
  • 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: 30 to 60%, Si: 0.3 to 3%, Mg: 0.3 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 plant trials carried out by the applicant comprised initial plant trials on Al—Zn—Si—Mg alloys in two groups of alloy compositions 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.
  • 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 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 pH and the concentration of the dosing acid were 1.6 and 53.6 g/L H 3 PO 4 respectively. During the trial the dosing acid consumption was quite low, approximately 17 L/day, or less than about 1 L/day of concentrated phosphoric acid (85 wt %). Quench tank dosing proved effective at controlling white precipitate formation and preventing quench heat exchanger blockage. Another outcome of pH dosing was that the pH probe did not foul.
  • the set point temperature for the quench tank sprays was lowered 50° C. to 35° C., and pH dosing was discontinued.
  • the quench tank was flushed with water to remove residual salts from the pH control trial. This change caused wet strip conditions further downstream but it also showed that temperature is an important variable for quench tank control.
  • the quench tank temperature was typically 15° C. higher than the spray temperature.
  • the quench tank temperature was 48-50° C. rather than the 65-70° C. typical of normal MCL 1 quenching conditions.
  • 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.
  • both aluminium and calcium seem to follow the same trend ( FIG. 2 ).
  • Lower quench tank temperature and pH dosing lowered the level of these ions in the quench water, with the calcium levels dropping substantially.
  • the level of Al in the quench water is considerably higher for Al—Zn—Si—Mg alloy coatings than Al—Zn alloy coatings (typical Al—Zn concentrations in quench water are 4-20 mg/L).
  • the impact of pH control on magnesium concentration is shown in FIG. 3 . It increased considerably during the 4 day test period. Increased magnesium levels are also evident for cooler quench tank conditions. Zinc levels also increased during pH control and for the coldest quench tank trial (35° C.) but was still at low levels overall.
  • 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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US14/436,524 2012-10-17 2013-10-17 Method of producing metal-coated steel strip Abandoned US20150247230A1 (en)

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AU2012904524A AU2012904524A0 (en) 2012-10-17 Method of Producing Metal-Coated Steel Strip
AU2012904524 2012-10-17
PCT/AU2013/001198 WO2014059476A1 (en) 2012-10-17 2013-10-17 Method of producing metal-coated steel strip

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ES2865732T3 (es) 2021-10-15
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US20190085439A1 (en) 2019-03-21
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CN104736738A (zh) 2015-06-24
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