FIELD OF THE INVENTION
The present invention generally relates to a method of improving the zinc coatbaility of Advanced High Strength Steels (AHSS) during hot dip galvanizing. More particularly, the invention relates to application of an intermediate copper layer in different annealing conditions to reduce the uncoated spots on the surface of AHSS during galvanizing.
BACKGROUND OF THE INVENTION
Advanced high strength steels such as dual phase (DP), interstitial free (IF), transformation induced plasticity (TRIP) etc. steels are alloyed with Mn, Cr, Al, Si and Ti among other elements. The complex chemical composition of the steel leads to difficulties during annealing and hot dip galvanizing of these steels. Although the annealing atmosphere is reducing to the Fe substrate, the oxidation potential is large enough to oxidize other elements in steel. Some of these alloying elements have a high affinity towards oxygen and are known to diffuse to the steel surface and form oxides. The oxides formed are very stable and cannot be reduced under standard industrial annealing atmospheres. The galvanized coating quality depends on the surface condition of the steel and the presence of surface oxides adversely affects the zinc wettability of the surface and leads to uncoated spots.
A typical industrial annealing cycle includes heating to about 800 °C in 5 % H2 -N2 atmosphere at dew point - 40 °C, holding for 60s at this temperature and subsequent cooling.
A number of different ways have been explored to resolve the problem of zinc wetting, as has been reported by Oren and Goodwin in Galvatech 2004 [1].

However, none of them involved applying a thin copper layer on AHSS surface by dipping and annealing the AHSS in various annealing conditions, such as reducing, successive oxidizing-reducing and oxidizing, with a purpose of shielding the alloying elements from segregating to the surface.
Gomersall (U.S Patent 4,285,995) [2] had applied copper coating to steel strips for a different purpose of achieving a zinc-iron alloyed coating without having to heat the steel again after hot dip galvanizing. At least one surface of the steel was covered with copper and then the steel strip was annealed in a conventional reducing atmosphere prior to galvanizing. With a very thin hot-dip zinc coating (thickness less than 0.1mil) post galvanizing heating step was no longer required for inducing iron-zinc alloying on the mentioned side. The objective here was to encourage the diffusion of copper into the iron surface during annealing which would promote the formation of iron-zinc alloy even when the strip is cooled to ambient temperature after galvanizing.
In some other related work, Gondek (US Patent 3957086) [3] applied successive thin layers of copper and nickel on steel tubing by electroplating process with an aim to improve its corrosion resistance property. The steel tubes were further subjected to terne coating. The multilayer coating exhibited more corrosion resistance than the sum of corrosion resistance of each layer when used alone.
Other known techniques deal with zinc recirculation systems involving impingement of liquid zinc with the steel sheet in the snout area, as described in patent by Patil and Sippola (US Patent 6,177,140 B1) [4]. The approach is based on the fact that the galvanizability of substrates is strongly influenced by its impingement and interaction with liquid zinc in the snout area. Some pretreatment techniques have been also tried which involved electroplating with Fe, Cu or Ni and have reportedly given better wetting property, although at a

higher cost. Choi et al [5] have reported the use of iron and copper by electroplating as an intermediate layer on AHSS before galvanizing which resulted in an effective suppression of formation of Si and Mn oxides on the surface of high strength steels.
JP 5320849 proposes a steel sheet containing, by weight %, 0.001 to 0.1% C, 0.05 to 1.0% Si, 0.005 to 2.0% Mn, <=0.15% P, and 0.02 to 0.5% Ti, satisfying Si(%)/5+Mn(%)/10 <=Ti(%) and the balance Fe with inevitable impurities is used as the base material and is coated with the galvanizing layers. The alloy layer composed of Fe and Zn may be provided between the base material and the galvanizing layers. As a result, the galvanizing of the high-strength steel sheet is executed without subjecting the steel sheet to a pretreatment of Fe or Ni electroplating.
KR 20020047582 proposes a hot dip galvanizing process for high strength steel sheets with excellent surface property. The method includes the steps of preparing a pre-treatment solution containing 10-25% of sulfuric acid or chloric acid, 0.5-2% of hydrogen peroxide; immersing a high strength steel sheet containing one or more constituents selected from Si less than 2 wt.%, Mn less than 3 wt.%, and Cr less than 3 wt.% in the pre-treatment solution(ambient temperature to 30°C) for 2-7 seconds before annealing the high strength steel sheet to form ferrous oxide; annealing and hot dip galvanizing under conventional conditions.
KR 20090120758 proposes a manufacturing method of an advanced high strength molten galvanizing steel plate to improve plating adhesion drastically, and to minimize surface tension lowering of a plating layer. The method includes the following steps of: supporting steel sheets on a hot-dip galvanizing plating bath; injecting a phosphoric acid aqueous solution on the steel sheets; and

performing a thermal process of the steel sheets. The concentration of the phosphoric-based liquid is 1~5wt%. The temperature of the steel plate is 430~450°C. The phosphoric-based liquid includes phosphoric acid ammonium or sodium phosphate.
SU 1303623 teaches a method for producing thin high-strength steel strip with iron-zinc alloy, comprising the steps of cleaning the strip surface and coating it with zinc after hot rolling and picking. The coated strip is then cold-rolled by applying an overall reduction of 65-90%. The annealing of the zinc-coated strip takes place in batch operating units at 450-530 deg. C during 5-10h, or in a continuous furnace at
550-580 °C. within 1-3 mins. The cleaning of the hot-rolled strip involves thermochemical degreasing in a nonoxidizing atmos. at 350-500 deg. C, and is followed by continuous hot galvanizing by electroplating Zn. The use of the hot roller strip for Zn coating offers a smaller surface for treatment compared with the surface of the same wt. of strip after cold reduction. It also eliminates a secondary cold rolling and secondary heat treatment. The mutual diffusion of Fe and Zn gives rise to the formation of the iron-zinc cpd. in the outer layer of the strip which exhibits enhanced paint take-up power and weldability.
JP 4268057 teaches a high strength galvanized steel sheet and a high strength galvannealed steel sheet consisting of 0.05 to 0.18% C, 0.5 to 1.5% Si, 0.7 to 1.5% Mn, <=0.02% P, <=0.005% S, 0.0005 to 0.005% Ca and 0.01 to 0.1% Al, and in which the structural rate of cementite having >=0.1 mum radius equivalent to a circle is regulated <=0.1%. Furthermore, as for their manufacturing method, hot rolling conditions, heating temperature in a

continuous galvanizing line and, according to necessary, pretreatment and / or its heating atmosphere before continuous galvanizing are prescribed.
JP 7252622 proposes a high strength galvanized steel sheet excellent in plating quality by applying ferrous precoating to a cold rolled steel strip containing Si, Mn, P, and Nb, applying annealing, and subjecting the steel strip to hot-dip galvanizing and alloying treatment. According to the invention, a steel, having a composition consisting of 0.05-0.3% C, <=1.0% Si, 0.5-3.5% Mn, <=0.1% P, <=0.1% S, 0.01-0.3% Nb, <=100ppm N, and the balance essentially Fe, is used. The steel is hot-rolled at a temperature not lower than the Ar3 transformation point, coiled, and then cold-rolled, Ferrous precoating is applied to the resulting cold rolled steel strip by electroplating, etc. Then, in a continuous hot-dip galvanizing line, the steel strip is heated and held at a temperature in the recrystallization temperature region for 10-300sec to undergo annealing treatment, followed by hot-dip galvanizing. Successively, alloying treatment is done at 450-600 deg. C. It is preferable to regulate the coating weight of the ferrous precoating to >= about 0.5g/m<2> per side.
The resulting galvannealed steel sheet can be suitably used for various purposes requiring high strength and high corrosion resistance.
ЈP6207259 teaches a process to produce galvanized high tensile strength steel sheet. According to this invention, a high tensile strength steel sheet is provided as the base material containing >=1.0wt.% the total of one or more kinds among Mn, Si and Cr as the strengthening elements after treating the surface of this high tensile strength steel sheet with a solution containing an organic acid, the ordinary annealing and galvanizing treatments are executed. The solution containing the organic acid, contains the organic acid generating unsoluble iron

salt as oxalic acid, malonic acid, maleic acid etc., and further, may contain the saturated iron salt of the organic acid and / or H2O2.
JP3069351 discloses a delustered precoated steel plate having excellent workability, flawing resistance and corrosion resistance. According to this invention an undercoating layer and a top coating layer consisting of delustering paints are formed onto the surface of a zinc group plate steel place through a chemical treatment layer composed of a phosphate or a chromate in a precoated steel plate. The under coating layer is formed of paint containing a resin containing bisphenol A in the resin structure and having weight average molecular weight of 1000 or 60000 at the rate of 10 wt.% or more per paint-resin solid matter and blocked isocyanate and /(or) an amino resin at the rate of 10 or 40 wt.% per the paint-resin solid matter. A film is liable to be cracked and peeled on fabrication when the molecular weight of the resin containing bisphenol A is less than 1000, and working coatability is deteriorated when it exceeds 60000.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose a method to improve zinc coatability of Advanced High Strength Steels (AHSS), which eliminates disadvantages of prior art.
A further object of the invention is to propose a method to improve zinc coatability of Advanced High Strength Steels (AHSS), which reduces the un-coated spots on the surface of the AHSS during hot dip galvanizing.
SUMMARY OF THE INVENTION
The present invention proposes application of a thin intermediate copper layer

prior to annealing which suppress the formation of oxides on the AHSS surface during annealing in all annealing conditions such as conventional reducing, successive oxidizing-reducing as well as with high oxidation potential, and thus improves zinc coatability.
According to the invention, a thin intermediate layer of copper coating is applied on both surface of an AHSS sample by dipping it in an acidic copper sulphate solution at around room temperature after the steel sample is properly cleaned by successive steps of alkali cleaning and acid pickling. The copper coated AHSS steel is then annealed and hot dip galvanized to give a uniform zinc coating over the surface without any uncoated spots. Different annealing atmospheres were used, such as, typical reducing, successive oxidizing-reducing and those with such high oxidation potential which are normally considered unsuitable for hot dip galvanizing.
Hot dip galvanizing was carried out by conventional process. The galvanized surface thus obtained in all cases was better than those when no intermediate metallic layer was used. Coating adhesion in all cases was found to be good.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Shows a dual phase (DP) steel sample hot dip galvanized without the use of an intermediate copper layer.
Figures 2, 3 and 4 - Show galvanized dual phase (DP) steel samples with an intermediate copper coating, annealed in a reducing atmosphere and a successive oxidizing-reducing atmosphere and in high oxidation potential atmosphere respectively, the encircled spots marked 1 and 2 on the sample surface being the marks of GDOES analysis.

Figure 5 demonstrates a strong adhesion of zinc coating on a copper coated dual phase steel sample, bent by 180°.
DETAILED DESCRIPTION OF THE INVENTION
Copper coating has been used in the inventive process as an intermediate metallic layer on AHSS surface to shield the alloying elements within the substrate from migrating to the surface and forming oxides during annealing stage prior to hot dip galvanizing. The AHSS steel samples are first degreased in acetone solution and alkali cleaned in a solution of sodium hydroxide and an appropriate percentage of a surfactant. This is followed by acid pickling in a solution of hydrochloric acid and inhibitors for an optimized period of time. The cleaned samples are then rinsed and dried and then coated with copper.
Cleaning of the samples is a critical step which ensures a uniform copper coating coverage. Copper coating is applied by dipping the steel in copper sulphate solution at room temperature for 5-50 seconds. This is again followed by rinsing and drying. The thickness of copper coating obtained is in the submicron range and is found to be sufficient to shield the segregation of alloying additions.
The copper coated samples are then annealed and galvanized in a hot dipping galvanizing simulator. The surface of the galvanized AHSS is found to have a defect free uniform zinc coating. Different parameters of annealing atmosphere, such as dew point and gaseous atmosphere are varied to evaluate their impact on galvanizing of copper coated AHSS samples. Dew point is varied to different points in the range - 60 °C to + 20 °C. The gaseous atmospheres used are a mixture of H2 and N2 gases (referred to as HNX hereon) with varying percentage

of H2 in the range 0-25%. Different combinations of dew point and gaseous composition led to a varying degree of oxidation potential of the environment.
The chemical composition versus thickness profile of the galvanized samples showed that the copper coated samples led to a better shielding of the alloying elements from diffusing to the surface and hence led to a uniform galvanized surface. Manganese diffusion was shielded only partially but the other alloying elements of AHSS including the most troublesome silicon were shielded effectively. As was found from the final galvanized surface manganese oxides did not interfere much with zinc wetting property of the steel, which has also been corroborated by other investigations. Microstructure of the galvanized region indicated a uniform coverage of zinc coating and a sufficient coating thickness. It was also found that as the oxidation potential of the environment increased the tendency of oxidation of alloying elements shifted from external (at the surface) to internal (within the substrate). The combination of an intermediate copper coating and an internal oxidation of the alloying additions resulted in the most effective galvanized surface of AHSS. The galvanized surface showed a marked reduction in bare spots, irrespective of the reduction step that is carried out conventionally after internal oxidation.
However, a following reducing step showed more consistency in producing galvanized surface free from uncoated spots. The 180° -bend tests showed that the zinc layer adhesion to the substrate was very good despite the presence of an intermediate copper layer and no zinc peel off was encountered during the tests.
EXAMPLES
Different materials were evaluated for being used as an intermediate layer out of

which copper was found to be most suitable in terms of ease of application, properties achieved and economic feasibility. The copper sulphate solution that was used had 5-100 gm of copper sulphate powder and 10-100 ml of sulphuric acid in a litre of water. The minimum amount of time which was required to completely cover the steel surface was used as the dipping time in copper sulphate solution, so that the processing line need not be slowed down due to this additional step. The time required for imparting a uniform coverage of copper was less than 10 seconds.
The acid pickling solution and time were selected in such a way that the AHSS surface is completely free of all oxides in order that the reaction between copper sulphate and iron takes place uniformly. The pickling time was kept in a range of 10-150 seconds.
The annealing atmosphere was varied to evaluate the effect of dew point and the gaseous atmosphere over the galvanized surface. Dew points were varied to different point in the range - 60 °C to + 20 °C and the gaseous atmosphere was also changed by varying the percentage of hydrogen in the furnace in the range 0-25%. Different annealing cycles were used which included a typical reducing atmosphere and a successive oxidizing-reducing atmosphere and those with very high oxidation potential. The oxidation potential was varied with the help of different combinations of dew point and gaseous mixture present in the furnace and their effects were observed on the final galvanized surface.

REFERENCES :
1. E.C Oren, F.E. Goodwin, Galvatech '04 Conference Proceedings, 2004,pp.737-749
2. U.S Patent 4,285,995, "Process for increasing alloying rate of galvanized coating on steel", David W. Gomersall, 1981
3. U.S Patent 3,957,086, "Corrosion resistant tubing", Stanley F. Gondek, 1976
4. U.S Patent 6,177,140 B1, "Method for galvanizing and galvannealing employing a bath of zinc and aluminium", R.S. Patil, P. Sippola, 2001
5. Y. Choi, W. Beom, C. Park, D. Pai, M. Hong, Metallurgical and Materials Transaction A, 41A, 2010, pp. 3379-3385

WE CLAIM
1. A method to improve zinc coatability of Advanced High Strength Steels
(AHSS) comprising the steps of:
- cleaning in a first step the AHSS - sample by dipping into a solution of sodium hydroxide including a percentage of surfactant;
- a second - step cleaning of the AHSS sample in a solution of hydrochloric acid including inhibitors for an optimized period of time;
- rinsing and drying the cleaned sample;
- dipping the cleaned and dried steel sample in copper sulphate solution at room temperature for a period of 5-50 seconds, the thickness of the copper coating maintained in submicron range sufficient to shield segregation of alloying addition;
- rinsing and drying the copper coated steel sample;
- annealing the copper-coated dry sample under varying annealing atmosphere including annealing cycles with dew point between - 60°C and + 20°C, and under HNX atmosphere wherein H2 range in the furnace being 0 - 25% ; and
- galvanizing the coated and annealed AHSS sample in hot-dip galvanizing bath.
2. The method as claimed in claim 1, wherein the copper sulphate solution

comprises 5 - 100 gms of copper sulphate powder and 10 - 100 ml of sulphuric acid diluted in a litre of water.
3. The method as claimed in claim 1, wherein the second step cleaning time as optimized includes 10 - 150 seconds.
4. The method as claimed in claim 1, wherein the copper coating shields the alloyed additions of the AHSS from migrating to the steel surface, prevents formation of surface oxides during annealing and results in an improved galvanized surface.
5. The method as claimed in claim 1, wherein different annealing conditions may comprise of a typical reducing annealing cycle, or a successive oxidizing-reducing annealing cycle, or an annealing cycle with very high oxidizing potential; with dew points in the range -60 °C to +20 °C.
6. The method as claimed in any of the preceding claims, wherein adhesion of the zinc coating obtained over the copper coated AHSS surface is strong and eliminates peeling off during bending.
7. A method to improve zinc coatability of Advanced High Strength Steels (AHSS), as substantially described and illustrated herein with reference to the accompanying drawings.

ABSTRACT

The present invention relates to a method to improve zinc coatability of Advanced High Strength Steels (AHSS) comprising cleaning in a first step the AHSS - sample by dipping into a solution of sodium hydroxide including a percentage of surfactant; a second - step cleaning of the AHSS sample in a solution of hydrochloric acid including inhibitors for an optimized period of time; rinsing and drying the cleaned sample; dipping the cleaned and dried steel sample in copper sulphate solution at room temperature for a period of 5-50 seconds, the thickness of the copper coating maintained in submicron range sufficient to shield segregation of alloying addition; rinsing and drying the copper coated steel sample; annealing the copper-coated dry sample under varying annealing atmosphere including annealing cycles with dew point between - 60°C and + 20°C, and under HNX atmosphere wherein H2 range in the furnace being 0 - 25%; and galvanizing the coated and annealed AHSS sample in hot-dip galvanizing bath. Show galvanized dual phase steel samples with an intermediate copper coating, annealed in a reducing atmosphere and a successive oxidizing-reducing atmosphere and in high oxidation potential atmosphere respectively, the encircled spots marked 1 and 2 on the sample surface being the marks of GDOES analysis.