Classifications

• • 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
• • C—CHEMISTRY; METALLURGY
  • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
  • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
  • C09D5/00—Coating compositions, e.g. paints, varnishes or lacquers, characterised by their physical nature or the effects produced; Filling pastes
  • C09D5/08—Anti-corrosive paints
  • C09D5/082—Anti-corrosive paints characterised by the anti-corrosive pigment
  • C09D5/084—Inorganic compounds
• • 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
  • C21D1/68—Temporary coatings or embedding materials applied before or during heat treatment
• • 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
  • C21D1/68—Temporary coatings or embedding materials applied before or during heat treatment
  • C21D1/70—Temporary coatings or embedding materials applied before or during heat treatment while heating or quenching
• • 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
  • C21D1/68—Temporary coatings or embedding materials applied before or during heat treatment
  • C21D1/72—Temporary coatings or embedding materials applied before or during heat treatment during chemical change of surfaces
• • 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
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • 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
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
• • 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
  • C23C2/04—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
• • 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
  • C23C28/00—Coating for obtaining at least two superposed coatings either by methods not provided for in a single one of groups C23C2/00 - C23C26/00 or by combinations of methods provided for in subclasses C23C and C25C or C25D

Definitions

• the invention relates to a coated hot-formable steel strip, sheet or blank suitable for hot-forming and to a method for manufacturing the same.
• the invention also relates to a method for hot-forming the coated hot-formable steel strip, sheet or blank and to the hot-formed product thus produced.
• Hot-forming is a process that is commonly used to produce high-strength, lightweight body parts for motor vehicles.
• the process generally comprises the steps of cutting a blank from a steel strip or sheet, heating the blank to above the Acl temperature of the steel material being processed, hot-pressing the heated blank to form an automotive body part and hardening the part.
• uncoated steel is susceptible to high temperature oxidation, which is a corrosion process where iron in the steel reacts with atmospheric oxygen at elevated temperatures.
• high temperature oxidation is a corrosion process where iron in the steel reacts with atmospheric oxygen at elevated temperatures.
• the presence of iron oxides at the steel surface reduces the weldability of the hardened part and also adversely affects the aesthetic appearance of the hardened part after paint baking.
• the steel material may be heated in a controlled atmosphere and/or provided with a metallic coating.
• Al-Si metallic coatings to inhibit high temperature oxidation is known from EP0971044. Although Al-Si coatings protect the steel from high temperature oxidation, they do not provide cathodic corrosion protection and are difficult to paint.
• Zinc or zinc alloy coatings can also be used to protect steel from oxidation during hot-forming. Such coatings are known from EP1 143029. However, due to the relatively low melting point of zinc, such a method suffers from the disadvantage that zinc is lost during the heat treatment and/or hot-pressing due to evaporation. Consequently the degree of passivation and cathodic corrosion protection afforded to the steel is reduced. A further disadvantage of using zinc based coatings is that a thin layer of zinc oxide forms at elevated temperatures, which reduces the weldability of the zinc coated steel. An additional process step is therefore required to remove the zinc oxide layer before welding can occur.
• a hot-formable steel strip, sheet or blank provided with a metallic coating, which enables improved retention of the coating during hot-forming.
• Another object of the present invention is to provide a hot-formable steel strip, sheet or blank that affords improved passivation corrosion protection and cathodic corrosion protection to the steel.
• a further object of the invention is to provide a coated steel strip, sheet or blank suitable for hot forming that is economical to produce. It is also an object of the invention to a provide hot- formable steel strip, sheet or blank that exhibits improved weldability after hot-forming.
• Such 'steel making slag' includes blast furnace slag, blast oxygen furnace slag, ladle furnace slag and electric arc furnace slag
• Blast furnace (BF) slag is produced as a by-product during the manufacture of pig iron.
• BF slag may be recycled externally, for instance as an aggregate in cements or as a fertiliser in agriculture, BF slag is often not re-used by the steel manufacturer. In this respect BF slag may not be re-introduced into the sinter plant or into the blast furnace without further treatment due to the presence of phosphorous, which forms phosphate precipitates that are detrimental to the steel thus produced.
• Blast oxygen furnace slag is produced as a by-product of the basic oxygen steel (BOS) making process in which molten pig iron from a blast furnace is charged into a basic oxygen furnace (BOF) together with scrap metal, fluxes, alloys and high purity oxygen.
• Oxygen is used primarily for the decarburisation and conversion of molten pig iron to liquid steel, while alloys are added to tailor the properties of the steel itself.
• Fluxes such as burnt lime or dolomite are provided to form slag, the purpose of which is to absorb impurities in the steel and those introduced during the steel making process.
• BOF slag prevents or at least limits steel manufacturers from recycling BOF slag themselves.
• the presence of phosphorous in BOF slag prevents it from being re-introduced into the steel making process.
• BOF slag The relatively high CaO content in BOF slag also makes BOF slag less preferred as material for use in the manufacture of concrete. This is primarily due to the increased risk of cracks forming in the concrete caused by the hydration reaction of CaO to calcium hydroxide ((CaOH 2 ).
• BOF slag contains Cr(lll) (Cr 2 0 3 ) compounds, which when heated to above 1400 ° C, for instance in a cement kiln, oxidise to form hexavalent chromium compounds that are hazardous to human health,
• a hot-formable steel for use in hot forming comprising a hot-formable steel strip, sheet or blank, a metal or metal alloy coating formed on the hot-formable steel strip, sheet or blank and a metallurgical slag based coating formed on the metal or metal alloy coating, wherein the metal or metal alloy coating comprises zinc and/or aluminium and the metallurgical slag based coating comprises a metallurgical slag and a polymeric material.
• the metallurgical slag based coating adheres very well to the steel strip, sheet or blank, which ensures that the coating does not delaminate prior to hot-forming.
• the inventors also observed that the extent of high temperature oxidation can be reduced by providing a steel strip, sheet or blank with the metallurgical slag based coating prior to subjecting the coated steel to a high temperature heat treatment. This has been attributed to the formation of a layer on the metal or metal alloy coating, that is formed from the metallurgical slag during a heat treatment up to 1200 " C. This layer also acts as physical barrier that reduces zinc evaporation when the metallic coating is zinc or a zinc alloy. Consequently the corrosion protective properties of the zinc or zinc alloy coating after the heat treatment are also improved.
• the hot- formable steel strip, sheet or blank comprising the metal or metal alloy coating and the layer formed from the metallurgical slag exhibited an electrical resistivity below 1 mOhm.
• the hot-formable steel strip, sheet or blank of the invention is therefore very suitable for welding.
• the polymeric material is water soluble and comprises polyimides, preferably polyamideimide.
• polystyrene resin polystyrene resin
• LCP liquid crystal polymers
• PES polyarylether sulphones
• PEI polyetherimides
• PSU polysulphones
• PEEK polyetherketones
• FP fluoropolymers
• PAR polyacrylates
• PPA polyphthalamides
• the thermal degradation temperature of the polymeric material is at least 250 C, preferably at least 400 ° C.
• the cured polymeric material provides flexibility to the metallurgical slag based coating and ensures that the coating adheres to the steel strip, sheet or blank prior to the heat treatment.
• the polymeric material also acts as a physical barrier that prevents or at least reduces abrasion of the underlying metal or metal alloy coating.
• Below the thermal degradation temperature of the polymeric material both the polymeric material and the metallurgical slag protect the coated steel from oxidation at lower temperatures.
• the metallurgical slag protects the coated steel from high temperature oxidation.
• the dry film thickness of the metallurgical slag based coating is up to 30 ⁇ , preferably between 2 and 10 ⁇ .
• a dry film thickness of up to 30 ⁇ is also preferred since above this thickness the metallurgical slag based coating may delaminate.
• a dry film thickness between 2 and 10 m is most preferred since such coatings are robust and zinc evaporation is kept to a minimum.
• the metallurgical slag comprises basic oxygen furnace (BOF) slag, blast furnace (BF) slag or a mixture thereof.
• BOF basic oxygen furnace
• BF blast furnace
• layers formed from BOF and/or BF slag are capable of retarding oxidation and retaining the metal or metal alloy at the steel surface.
• CaO is particularly effective as a high temperature oxidation protective barrier.
• EAF slag The composition and properties of EAF slag are similar to BOF slag, although EAF slag generally has a lower content of free magnesium and calcium oxides.
• Ladle slag contains more aluminium oxide and less iron oxide compared to BOF slag.
• Such steel making slags comprise free oxides and mixed oxide compounds formed from oxide anions and two or more elemental cations.
• BOF slag comprises both CaO and calcium silicate, with calcium silicate being formed from the reaction between CaO and silica (Si0 2 ). Such compounds are only formed at high temperatures such as those used in blast oxygen, blast, electric arc and ladle furnaces.
• the metallurgical slag comprises one or more oxide particles of Ca, Al, Si, Fe. Mg, Mn, Ti, P, V, Cr, Na, K and S.
• the metallurgical slag comprises one or more of CaO, Al 2 0 3 , Si0 2 , FeO, Fe 2 0 3 , MgO, MnO, Mn 3 0 4 , Ti0 2 , P 2 0 5 , V 2 0 5 , Cr0 2 , Na 2 0, K 2 0 and S0 3 .
• metallurgical slag based coatings comprising one or more of the above oxides are very suitable for retarding oxidation and preserving the metal or metal alloy coating during hot-forming.
• oxides of at least Al, Si and Mn preferentially react with the metal or metal coating during the heat treatment to form a layer on the metal or metal alloy coating capable of providing passive corrosion protection and protection against high temperature oxidation.
• the metallurgical slag comprises a mixed oxide compound formed from oxide anions and at least two elemental cations selected from Ca, Al, Si, Fe. Mg, Mn, Ti, P, V, Cr, Na, K and S.
• Such mixed oxide compounds contribute to retarding oxidation during hot-forming and to providing passivation corrosion protection to the underlying hot-formable steel substrate.
• Examples of such mixed oxide compounds include (Fei. x Mg x )0, Ca 2 Fe 2 0 5i Ca 2 Si0 4 as well as those disclosed in Table 1.
• the metallurgical slag comprises 20 to 75 wt% of calcium oxides.
• the content of calcium oxides is within the aforementioned range the extent of high temperature oxidation at the steel surface is greatly reduced.
• the layer formed from such a metallurgical slag composition afforded very good passive corrosion protection to the underlying substrate.
• the metallurgical slag comprises:
• the metallurgical slag comprises:
• the above oxides can all contribute to improving passivating properties of the layer formed from the metallurgical slag after hot-forming.
• the wt% of the oxides (Ca, Al, Si, Fe, Mg, Mn, Ti, P. V, Cr and Na) mentioned hereinabove takes into account both the wt% of the free oxide and the wt% of the oxides comprised in a mixed oxide compound.
• the above weight percentages were determined using wavelength dispersive X-ray fluorescence (WDXRF).
• the oxide particles have a size dimension between 0: 1 and 30 ⁇ , preferably between 0.1 and 10 ⁇ and more preferably between 0.1 and 1 ⁇ . It was found that the surface quality of the metallurgical slag based coating and the coated hot-formable steel strip, sheet or blank after the heat treatment can be improved by providing a metallurgical slag containing oxide nanoparticles.
• the metal or metal alloy coating is a galvannealed coating containing zinc and Fe up to 70 weight %, preferably containing Fe up to 40 weight %, more preferably containing Fe up to 20 weight %, still more preferably containing Fe up to 10 weight %.
• the inventors found that both zinc and iron were homogeneously distributed on the surface of the steel and that the metallurgical slag based coating exhibited good adherence to the galvannealed coating. After the heat treatment, the surface of the galvannealed coating was covered with a layer formed from the metallurgical slag. The inventors found that the surface coverage of the layer could be increased by reducing the iron content in the galvannealed coating.
• the hot formable steel is a boron steel, preferably having the composition in weight percent:
• Such steel types are generally known and used for hot forming purposes.
• the second aspect of the invention relates to a method for manufacturing a hot-formable steel according to the first aspect of the invention, which comprises the steps of:
• composition which composition comprises water, a polymeric material and a metallurgical slag
• the coating composition comprises
• metallurgical slag 1-30 wt% metallurgical slag, 0.1 -10 wt% polymeric material and water.
• a composition can be produced at low cost.
• the inventors found that if the metallurgical slag content is greater than 30 wt% then the composition is difficult to process and the cured coating is brittle once formed.
• the inventors also found that a composition comprising more than 10 wt% of the polymeric material is less suitable for retaining the metal or metal alloy coating during hot-forming.
• the coating composition comprises 5-30 wt% metallurgical slag.
• the step of providing the coating composition comprises the consecutive steps of providing the metallurgical slag in water to form a metallurgical slag solution and then adding the polymeric material to the metallurgical slag solution.
• metallurgical slag solutions having a high solids content may be obtained.
• Coatings obtained from such solutions exhibit increased 'compactness' relative to coatings obtained from solutions having a lower solids content.
• the compactness of the coating may be increased by providing a metallurgical slag comprising nanometer sized particles.
• the coated steel strip, sheet or blank is heated using electromagnetic radiation, preferably near infrared radiation.
• electromagnetic radiation preferably near infrared radiation.
• infrared radiation means that the coating composition may be cured in 5-10 seconds. This is much faster than curing by conventional curing means.
• the coated steel strip, sheet or blank is heated, at least in part, using residual heat from the hot-dip galvanised or hot-dip aluminised steel strip, sheet or blank.
• residual heat from the hot-dip galvanising or hot-dip aluminising process allows cost savings to be made since the amount of energy that is required to cure the coating composition, either by conventional or electromagnetic means, is reduced.
• the third aspect of the invention relates to a method for manufacturing a hot- formed product, which comprises the steps of providing the hot-formable steel strip, sheet or blank according to the first aspect of the invention, heating the hot-formable steel strip, sheet or blank to the Ac1 -temperature or above, hot-forming the hot- formable steel strip, sheet or blank to form the hot-formed product and quenching the hot-formed product.
• This is the usual hot forming process, which is now performed using the hot-formable steel strip, sheet or blank of the invention.
• the metallurgical slag based coating protects the metal or metal alloy coated steel strip, sheet or blank from abrasion and corrosion if the steel, strip, sheet or blank is stored prior to hot-forming.
• the polymeric material thermally degrades leaving behind a layer formed from the metallurgical slag. This layer prevents or at least reduces high temperature oxidation at the steel surface and prevents or at least reduces evaporation of the metal or metal alloy coating.
• the hot-formable steel strip, sheet or blank according to the first aspect of the invention is pre-formed.
• the inventors found that the metallurgical slag based coating provides very good abrasion protection during pre-forming. Moreover, the metallurgical slag provides active corrosion protection in case the metal or metal alloy coating is damaged during pre-forming or during handling of the preformed strip, sheet or blank.
• the fourth aspect of the invention relates to a hot-formed product produced according to the method of the third aspect of the invention wherein the hot-formed product comprises a layer formed from a metallurgical slag on the metal or metal alloy coating.
• This layer covers the surface of the metal or metal alloy and comprises loosely adherent oxides from the metallurgical slag and reaction products obtained from the reaction between a metal from the metal or metal alloy coating and an oxide from the metallurgical slag.
• Such reaction products are stable and provide passive corrosion protection, whereas the loosely adherent oxides provide temporary active corrosion protection e.g. during storage of the hot-formed product.
• the loosely adherent oxides may be easily removed by brushing, rinsing or by air jet cleaning. Sand blasting may also be used but is not necessary and is less preferred since damage to the metal or metal alloy coating may occur.
• the layer formed from the metallurgical slag comprises oxides of aluminium, silicon and manganese. These oxides, particularly Al 2 0 3 , form an inert barrier on the metal or metal alloy coating. Preferably Al 2 0 3 covers at least 50% of the metal or metal alloy coating surface.
• Figure 1 shows the results of potentiodynamic experiments conducted to determine the corrosion protective properties of boron coated steels after a heat treatment at 900 ° C for 5 minutes.
• the results relate to a galvannealed boron steel strip (that has not undergone the heat treatment) (A), a galvannealed boron steel strip provided with the metallurgical slag based coating of the present invention (B), a commercially available Al-Si (Alusi ® ) coated boron steel strip (C) and a galvanised boron steel strip provided with the metallurgical slag based coating of the present invention (D).
• Example 1 Preparation of a metallurgical slag based coating solution
• a Metallurgical slag (4 g) obtained from a ladle furnace is added to a vessel containing water (100 mL) to form a metallurgical slag solution.
• the metallurgical slag contained in weight %: CaO (54.03), Al 2 0 3 (33.20), Si0 2 (4.78), MgO (3.72) FeO
• a second metallurgical slag based coating solution was prepared by adding a BOF slag (4 g) to a vessel containing water (100 mL).
• the BOF slag contained in weight % CaO (42.61 ), Al 2 0 3 (1.34), Si0 2 (13.59), MgO (7.70) FeO (26.88), MnO
• X-ray diffraction was used to determine the presence and structure of crystalline compounds in the metallurgical slag.
• XRD was also used to determine the presence and amount of amorphous phases in the metallurgical slag.
• XRD patterns were recorded in the range of 10 to 130 0 (2 ⁇ ) in reflection mode using a fully automated Panalytical Xpert MPD diffractometer (CoK a -radiation) equipped with a position sensitive detector. The step size was 0.02 °, time per step was 1 s. Quantitative determination of phase proportions was performed by Rietveld analysis. The refinement was done on the assumption of pure phases. Unit cell parameters, background coefficients, preferred orientations, profile parameters and phase proportions were refined using the TOPAS software package for Rietveld refinement.
• the metallurgical slag based coating solution is then applied on a hot-dip galvanised boron steel substrate by dipping or by spraying; thereafter the applied coating solution is cured using near infrared radiation at 260 ° C for 5-10 seconds. After curing, the dry film thickness of the coating was 6 ⁇ .
• the boron steel substrate contains in weight % C (0.21), Si (0.192), Mn (1.189), Ni (0.022), Cr (0.25), Al (0.044), P (0.013), Ti (0.035), N (62 ppm), S (0.006 ppm) and B (31 ppm).
• the metallurgical slag used in this example was a BOF slag containing in weight % CaO (42.61 ), Al 2 0 3 (1.34), Si0 2 (13.59), MgO (7.70), FeO (26.88), MnO (4.24), Na 2 0 (0.20), Ti0 2 (0.92), V 2 0 5 (0.71), P 2 0 5 (1.56) and Cr 2 O 3 (0.24), as determined by WDXRF.
• FEG-SEM field emission gun scanning electron microscope
• the SEM images also show the presence of a thin layer on the galvanised coating after the heat treatment.
• This layer covers a large proportion of the galvanised surface and contains a high content of aluminium. It is understood that this layer mainly consists of a compound formed between zinc and/or other alloying elements in the galvanised coating and Al 2 0 3 from the metallurgical slag. The inventors believe that this layer acts as an isolating layer which prevents corrosive electrolytes from reaching the galvanised coating and the steel substrate surface. This layer does not dissolve in water.
• the SEM images also show the presence of silicon and manganese in the layer, which suggests that during the heat treatment a reaction involving Si0 2 and MnO with zinc and/or other alloying elements in the galvanised takes place.
• Si0 2 and MnO also provide active corrosion protection because these oxides can dissociate, which increases the pH and further reduces the rate of corrosion.
• Potentiodynamic experiments were performed on coated boron steel substrates to assess the corrosion protective properties of the coatings following a heat treatment at 900 ° C for 5 minutes. The experiments were carried out using a Solartron Model 1280C Potentiostat/Galvanostat. The potentiodynamic scan was performed using 0.6M sodium chloride (NaCI) solution as a conductive medium and the polarization was applied at a scanning rate of 0.5mV/sec delineating the anodic and cathodic branches.
• NaCI sodium chloride
• the coated boron steel substrates under investigation include a galvannealed boron steel substrate having an average coating thickness of 10 ⁇ , the coating comprising zinc and 10% iron (A), a galvannealed boron steel substrate provided with the metallurgical slag based coating of the invention (B), a commercially available Al-Si coated boron steel from Arcelor Mittal (USIBOR®), the Al-Si coating having an average thickness of 40 pm and comprising 90% aluminium and 10% silicon (Alusi®) (C) and a galvanised boron steel provided with the metallurgical slag based coating of the invention, the galvanised coating having an average coating thickness of 20 ⁇ (D).
• the layer formed from the metallurgical slag was brushed to remove any loosely adherent oxides from the galvanised surface.
• the potentiodynamic results show that the coated boron steel substrate of the invention (D) exhibits superior corrosion protection properties relative to the galvannealed boron steel substrate and the Al-Si coated boron steel substrate. This has been attributed to the isolating layer that is formed from the metallurgical slag providing passive corrosion protection and the retained galvanised coating providing cathodic corrosion protection. In contrast, the Al-Si coating only affords passive corrosion protection to the steel substrate, whereas the galvannealed coating only affords cathodic corrosion protection.
• the metallurgical slag based coating on the galvannealed boron steel strip B
• superior corrosion protection properties were obtained relative to the galvannealed boron steel strip (A). Similar results were obtained for galvannealed and galvanised boron steel substrates that were provided with ladle furnace slag based coatings,
• the low ohm meter has a resolution of 1 milli-ohm and its copper wires were soldered directly into the copper electrodes to avoid any potential resistance contribution from the setup.
• the copper electrode surfaces in contact with the testing samples were ground on 4000 grit silicone carbide paper before use, while the reverse sides were covered with insulating tape.

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• 2013
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