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
  • C23C2/04—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

Definitions

• the invention relates to a steel strip provided with a hot dip galvanized zinc alloy coating layer and to a process for hot dip galvanising a steel strip with a zinc alloy coating layer, in which the coating of the steel strip is carried out in a bath of molten zinc alloy.
• European patent 0 594 520 mentions the use of 1 to 3.5 weight % magnesium and 0.5 to 1.5 % aluminium, together with the addition of silicon to a percentage of 0.0010 to 0.0060 in weight %.
• the silicon has been added in such a small quantity to improve the quality of the zinc coating, which had been found to comprise zones where no zinc had been present (bare spots).
• the patent mentions a zinc coated steel in which the coating has the composition 2.55 weight % magnesium, 0.93 weight % aluminium, 60 ppm silicon, rest zinc and inevitable impurities.
• a steel strip provided with a hot dip galvanized zinc alloy coating layer, characterized in that the zinc alloy consists of:
• the magnesium level has been limited to a maximum of 2.3 weight %.
• a minimum of 0.3 weight % magnesium is necessary to have a sufficient high corrosion resistance; magnesium additions improve the corrosion resistance of the coated strip.
• the magnesium level of 0.3 - 2.3 weight % is high enough to obtain a corrosion protection against red rust that is far higher than the corrosion protection of conventional galvanized strip.
• Aluminium has been added to reduce dross formation on the bath. In combination with magnesium it also improves the corrosion resistance of the coated strip. Aluminium moreover improves the formability of the coated strip material, meaning that the adhesion of the coating on the strip is good when the strip is for instance bended. Since increased aluminium levels will deteriorate the weldability, the aluminium level has been limited to a maximum of 2.3 weight %.
• An optional element that could be added in a small amount, less than 0.2 weight %, could be Pb or Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr or Bi.
• Pb, Sn, Bi and Sb are usually added to form spangles.
• a further advantage of the zinc alloy coated steel strip according to the invention is that the galling behaviour is better than the galling behaviour of conventional galvanized strip material.
• the steel strip has been provided with a hot dip galvanized zinc alloy coating layer in which the zinc alloy contains 1.6 - 2.3 weight % magnesium and 1.6 - 2.3 weight % aluminium.
• the zinc alloy contains 1.6 - 2.3 weight % magnesium and 1.6 - 2.3 weight % aluminium.
• the steel strip has been provided with a hot dip galvanized zinc alloy coating layer in which the zinc alloy contains 0.6 - 1.3 weight % aluminium and/or 0.3 - 1.3 weight % magnesium.
• the zinc alloy contains 0.6 - 1.3 weight % aluminium and/or 0.3 - 1.3 weight % magnesium.
• magnesium at levels between 0.3 and 1.3 weight % improves the corrosion resistance considerably.
• more than 0.5 weight % of aluminium has to be added to prevent that more oxidic dross is formed on the bath than for conventional baths; dross can lead to defects in the coating.
• the coatings with these amounts of magnesium and aluminium are optimal for applications with high demands on surface quality and improved corrosion resistance.
• the zinc alloy contains 0.8 - 1.2 weight % aluminium and/or 0.8 - 1.2 weight % magnesium. These amounts of magnesium and aluminium are optimal to provide a coating with both a high corrosion resistance, an excellent surface quality, an excellent formability, and a good weldability at limited extra costs as compared to conventional hot dipped galvanising.
• the steel strip has been provided with a hot dip galvanized zinc alloy coating layer in which the amount of aluminium in weight % is the same as the amount of magnesium in weight % plus or minus a maximum of 0.3 weight %. It has been found that the dross formed on the bath is suppressed to a considerable level when the amount of aluminium equals or almost equals the amount of magnesium.
• the steel strip has been provided with a hot dip galvanized zinc alloy coating layer in which the zinc alloy coating layer has a thickness of 3 - 12 ⁇ m.
• Thin coatings are of course less expensive, but they are also better formable and weldable than thick coatings.
• the coating thicknesses according to the invention provide enough corrosion protection, even for building and construction purposes, where conventionally hot dipped galvanised coating have a thickness of about 20 ⁇ m.
• the coating layer according to the invention can have a thickness of 3 - 10 ⁇ m.
• the invention also relates to a process for hot dip galvanising a steel strip with a zinc alloy coating layer, in which the coating of the steel strip is carried out in a bath of molten zinc alloy, wherein the zinc alloy consists of:
• the zinc alloy bath contains 1.5 - 2.3 weight % magnesium and 1.5 - 2.3 weight % aluminium, as discussed above for the steel strip.
• the zinc alloy bath contains 0.6 - 1.3 weight % aluminium and/or 0.3 -1.3 weight % magnesium, as discussed above.
• the zinc alloy bath contains 0.7 - 1.2 weight % aluminium and/or 0.7 - 1.2 weight % magnesium, as discussed above.
• the temperature of the bath of molten zinc is kept between 380° C and 550° C, preferably between 420° C and 480° C.
• the melting point of pure zinc is 419° C, and with 3.2% Al and 3.3% Mg the melting temperature is about 337° C, so 380° C is a reasonable lower limit to avoid local solidification.
• a lower limit of 440° C is absolutely safe to avoid any solidification.
• Increasing the zinc bath temperature increases the zinc evaporation and leads to dust formation in the galvanising line, giving rise to surface defects.
• the upper limit should thus be reasonably low, for which 550° C is fair, and preferably 480° C as a technically possible upper limit.
• the temperature of the steel strip before entering the bath of molten zinc alloy is between 380° C and 850° C, more preferably between the temperature of the bath of molten zinc alloy and 25° C above the bath temperature.
• the temperature of the steel strip should not be lower than the melting point of the zinc alloy to avoid local solidification of the zinc bath. High steel strip temperatures will lead to higher evaporation of the zinc, resulting in dust formation. High steel strip temperatures can also heat up the zinc bath, requiring continuous cooling of the zinc in the bath, which is expensive. For these reasons a temperature of the steel strip just above the bath temperature is preferred.
• Table 1 composition of bath and coating Ref# Bath Bath Coating Coating Coating Coating Coating Al% Mg% g/m2 Al% Mg% Fe% 1 0,2 0,5 99 0,4 0,5 2 0,8 0,9 1,0 0,8 0,11 3 1,0 0,9 1,1 0,9 0,18 4 1,0 1,0 1,2 1,0 0,14 5 1,9 1,0 2,0 0,9 0,07 6 1,1 1,1 42 1,3 0,9 0,29 7 1,2 1,2 1,4 1,2 0,15 8 1,5 1,5 1,6 1,4 0,14 9 0,9 1,6 1,1 1,6 0,26 10 1,7 1,7 1,9 1,7 0,10 11 2,5 2,0 2,5 1,8 0,05 12 1,0 2,1 77 1,2 1,8 0,13 13 1,0 2,1 39 1,2 1,8 0,21 14 2,1 2,1 2,2 2,1 0,15 15 1,0 2,5 1,1 2,8 0,06 Table 2: corrosion resistance of flat panel Ref# Bath Bath Coating Corrosion flat panel Al% Mg% thickness ( ⁇ m) 1 0,2 0,0 10
• the steel used for the experiments is an ultra low carbon steel having the composition (all in weight %): 0.001 C, 0.105 Mn, 0.005 P, 0.004 S, 0.005 Si, 0.028 Al, 0.025 Alzo, 0.0027 N, 0.018 Nb and 0.014 Ti, the remainder being unavoidable impurities and Fe.
• the steel panels have been made from cold rolled steel and have a size of 12 by 20 cm and a thickness of 0.7 mm. After degreasing they have been subjected to the following treatment:
• step 1 and 2 In some experiments the atmosphere in step 1 and 2 has been changed to 85% N 2 and 15% H 2 , but no effect on the coating quality has been observed.
• a Fischer Dualscope according to ISO 2178 has been used to determine the coating thickness at each side of the panel, using the average value of nine points.
• the corrosion resistance has been measured using the salt spray test (ASTM-B 117) to get an idea of the corrosion resistance under severe, high chloride containing, wet conditions, which represents some critical corrosive automotive as well as building microclimates.
• the test has been performed in a corrosion cabinet wherein the temperature is maintained at 35°C, while a water mist containing 5%NaCl solution is continuously sprayed over the samples mounted into racks under an angle of 75°.
• the side of the sample to be evaluated for its corrosion behaviour is directed towards the salt spray mist.
• the edges of the samples are taped off to prevent possible, early red rusting at the edges disturbing proper corrosion evaluation at the surface.
• First red rust is the main criterion for the corrosion resistance of the product.
• Reference product is conventional hot dip galvanized steel with a 10 ⁇ m zinc coating thickness.
• Table 3 shows the corrosion resistance of deformed panels. Deformation has been done by an Erichsen 8 mm cup. As can be seen, the corrosion resistance here depends to a large extend on the coating thickness of the zinc alloy layer. However, it is clear that a higher amount of the alloy elements Al and Mg results in a better corrosion resistance of the zinc alloy layer.
• Table 4 shows the galling performance of the hot dip galvanised steel. All coatings for which the bath contained approximately 1 weight % Al and Mg and more show an excellent galling performance.
• the galling performance has been measured using the linear friction test (LFT) method. This method uses severe conditions to accelerate galling. The method uses one flat tool and one round tool to develop a highpressure contact with the sample surface. The tool material used was in accordance with DIN 1.3343.
• strips of 50mm width and 300mm length were pulled at a speed of 0.33mm/s between the set of tools (one flat, one round) pushed together with a force of 5kN.
• the strips were drawn through the tools ten times along a testing distance of 55mm. After each stroke the tools were released and the strips returned to the original starting position in preparation for the next stroke. All tests were conducted at 20°C and 50% humidity.
• Table 5 shows the surface quality and formability of a number of panels.
• the surface quality has been measured by visual inspection of the panels on bare spots, irregularities sticking from the surface (usually caused by dross) and the general appearance or homogeneity of gloss over the panel. As follows from the table, the surface quality is good between approximately 0.5 weight % Al and Mg and 2.1 weight % Al and Mg. With higher amounts of aluminium, the amount of dross in the bath increases, resulting in a lower surface quality.
• the formability of the coating has been measured by visual inspection on cracks in the coating after a full bend (0T) of the panel. With higher amounts of magnesium the formability appears to decrease.
• Table 6 shows that the dross formation is less than for a conventional zinc bath when the amount of Al and Mg is between approximately 0.5 and 2.1 weight %.
• the dross formation has been judged quantatively as compared to the amount of foam and adhering dross measured for four bath compositions: Zn + 0.2 % Al, Zn + 1% Al + 1% Mg, Zn + 1% Al + 2% Mg and Zn + 1% Al + 3% Mg.
• argon gas has been bubbled for 2.5 hours through the liquid zinc alloy in a vessel to break up the oxide film layer on the surface. After this, the foam on the surface is removed and weighed.
• Table 7 shows that only a few weldability tests have been performed. The weldability appears not to be influenced by the amount of Al and Mg in the zinc bath.
• a weld growth curve has been made by making welds with increasing welding current with electrodes of 4.6 mm in diameter and a force of 2 kN. The welding range is the difference in current just before splashing and the current to achieve a minimum plug diameter of 3.5 ⁇ t , with t the steel thickness.
• Table 7 shows that 0.5% and 1% Mg and Al-alloyed coated steel have a similar welding range as regular galvanized steel (Ref# 1).
• Table 8 shows that the influence of the temperature of the bath and the temperature of the strip when it enters the bath is minimal. A temperature of 410° C or 460° C of the bath appears to make no difference, and the same holds for a strip entry temperature of 420° C or 475° C.
• coatings and the coating method can also be used for strip having a composition different from that used for the above experiments.

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Citations (9)

• 2004
  • 2004-07-28 EP EP04077168A patent/EP1621645A1/fr not_active Withdrawn

Patent Citations (9)

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