FIELD OF THE INVENTION
This present invention generally relates to a process for improving the zinc wettability of Advanced High Strength Steels (AHSS) during hot dip galvanizing. More particularly, the invention relates to a process to improve galvanisability of advanced high strength steel (AHSS). The invention further relates to an organic pre-treatment chemical applicable prior to hot-dip galvanizing to reduce the bare or uncoated spots on the surface of galvanized AHSS.
BACKGROUND OF THE INVENTION
Auto industry is one of the highest consumers of flat steel products worldwide. With the increase in cost of fuel and stringent safety rules, it is necessary for the auto industry to improve the overall efficiency of their vehicles in a cost effective manner to make them more affordable and safe. The drive to improve fuel efficiency without compromising on safety, led to the ambitious project of Ultra-Light Steel Auto Body-Advanced Vehicle concept (ULSAB-AVC) within the steel industry throughout the world. Eventually, steel industries came up with a solution, by developing high strength (HSS) and advanced high strength steel grades (AHSS) with high formability.

From the different types of High Strength Steels, DP steels comprise about 74% by weight of the AVC body structure and 58% of the closure panels. The high utilization of DP steels for structural applications is because of their superior energy absorbing capacity coupled with improved formability at a given strength level. But the main challenge that still requires a solution is the requirement of defect-free hot-dip galvanizability of TRIP, DP and UHS Steels, even though the electrogalvanizing route has been successfully used for galvanizing some of the above mentioned steel grades.
The promising performance of steel is always achieved by well-designed heat treatment routines and appropriate addition of alloy elements, such as C, Si, Mn, Cr, or Al. Though these elements are favorable for the strength and ductility, they are adverse to the continuous hot-dip galvanizing because of their preferential oxidation over iron during annealing process, which is a selective oxidation. The oxides cover the steel surface, resulting in poor wetting of the zinc.
Different approaches have been used in prior art to solve the wettability problems of these steels. Broadly these approaches can be classified as;
A. Change in chemistry of steel sheet and galvanizing bath
B. Change in process parameters i.e. dew point, annealing cycle, composition
of reducing gases, etc.

C. Pre-treatment of steel sheet
i. Inorganic Pre-Treatment
ii. Organic pre-treatment
Dual phase steels show galvanizing problems mainly due to oxide layer formation. B. Schumacher, A Hennion and co-workers have successfully tried to precoat the steel sheet with Fe, Cu or Ni to get good wettability. The problem with such approach has been the cost of the process and there could be problems with coating adhesion upon forming. As per US patent 5677005 to Makoto Isobe and coworkers, a method involving change in parameters and pre-treatment of steel sheet was used to minimize bare spots during galvanizing. This prior patent proves effective in minimizing the bare spots but is too expensive for industrial implementation. The invention involved heating the steel strip to recrystallization annealing temperature (850oC for 20 sec) cooling the sample, removing the surface layer concentrated with Mn, Si, Cr oxides by pickling and polishing, and subjecting the steel strip again to 850oC for 20 sec in reducing atmosphere, and subsequently dipping it. The effect of this invention on the concentration of surface oxides is clearly visible by GDS results.
Researchers have also successfully galvanized high tensile strength steel by pretreatment of sheet with Ni and Fe using electroplating [JP-A 194156/1990]. The technique has drawbacks like setting up an electroplating plant, complexity of process increases with increase in number of steps and low productivity. One

more method used by researchers was to clad the steel sheet with steel having lower content of Si, Mn and Cr [JP-A 199363/1991]. Here also the complicacy of process increases and lowers the productivity with additional cost.
To reduce the body weight of modern cars, advanced high strength steel, such as Dual Phase steel and TRIP-aided steel has been developed. When adapted to automotives, lower thickness of these steel sheets can be considered without losing their mechanical performance. The higher performance is always achieved by application of well-designed heat treatment routines and appropriate addition of alloy elements, such as C, Si, Mn, Cr, or Al. Though these elements are favorable for strength and ductility, they are adverse to the process of continuous hot-dip galvanizing because of their preferential oxidation over iron during annealing process. This is called selective oxidation. The oxides cover the steel surface, resulting in poor wetting of the zinc.
A typical industrial annealing cycle includes heating of the product to about 800°C in 5 % H2 - N2 atmosphere at dew point - 40°C, holding for about 60s at this temperature and subsequent cooling. However, as has been reported by Oren and Goodwin in Galvatech 2004, a number of different ways have been adopted to resolve the problem of zinc wetting. The processing parameters of the steel mill, such as hot mill processing (high coiling temperatures and slow cooling) can be used to get a favorable distribution of oxides for improved

galvanizability. Pre-annealing the sheets and removing the surface oxides by pickling or mechanical approaches have also been reported. Use of different alloying elements such as molybdenum has been tried by some producers to produce unexposed quality hot dip galvanised steels. Surface modification involving nitriding and carburizing have also been attempted. One of the more popular approaches includes modifying the annealing atmospheres. An alternate oxidizing and reducing atmosphere is employed in the furnace to achieve an improved surface condition. The oxidation potential in the direct fired furnace (DFF) is raised to such levels that there is a shift from external oxidation to internal oxidation and oxygen diffuses into and oxidizes the alloying elements within the substrate. It is followed by reduction in the heating zone which reduces iron oxides and frees the surface of all oxides resulting in enhanced galvanizability. Other techniques deal with zinc recirculation systems involving impingement of liquid zinc with the steel sheet in the snout area, as described in patents by Sippola and Patil. 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. Different approaches have been used to solve the wettability problem of AHSS.
Researchers across the world have tried different pretreatment methods to produce defect free hot dipped or electro-galvanized products for high strength and advanced high strength steels. Mainly the pre-treatments involve metallic

coat or surface modification using acids or salts to enhance the chances of complete coverage during galvanizing. Recently the concept of selective oxidation is used to produce high and ultra-high strength galvanized steel sheets. JP2010174287A describes the use of 0.01 mass% of carboxylic acid as a washing liquid for steel sheet prior to galvanizing, thereby creating a layer to avoid formation of bare spots. WO2010055145 discloses a method of acid pickling using sulphuric acid with AC current to have a clean surface of high strength steel suitable for electro-galvanizing, while the Japanese patent JP5320849 mentions about getting good galvanizing by controlling the steel chemistry, mainly Si and Mn content and having a pretreatment of Fe-Zn layers. The cost of production cannot be justified in this case and limiting the Si and Mn content can lead to limitations in strength and elongation values for high strength steel grades. The concept of selective oxidation is used in patent KR20020047582 to form ferrous oxide using a pre-treatment solution of sulphuric / chloric acid and hydrogen peroxide followed by conventional annealing and galvanizing. The concept is good by it requires an additional setup prior to annealing furnace and also has there are always chances of over pickling to be addressed by stringent control of pretreatment bath conditions and dipping time. KR2009120758 reported the use of phosphoric acid ammonium or sodium phosphate with a thermal treatment, just prior to hot dipping to get higher adhesion and reduced surface tension values during galvanizing. A conventional fluxing approach was

discussed in patent FR2827615A1 using 300 to 350 g/l of zinc chloride and 100 to 150 g/l of ammonium chloride at 38 degrees followed by hot dip galvanizing in bath containing Al and nearly 1% Bi.
US5759629 describes a method of preventing corrosion of a metal sheet, comprising the steps of: providing a metal sheet selected from the group consisting of coated steel sheet, aluminum sheet and aluminum alloy sheet; and applying a solution containing at least one hydrolyzed vinyl silane to the metal sheet. A coating of paint or other polymer may be applied directly on top of the vinyl silane coating.
US2005145303 describes a process for chemical pretreatment, before an organic coating, of composite metal structures that contain aluminum or aluminum alloy portions together with zinc or zinc alloy portions, and steel, galvanized steel and/or alloy-galvanized steel portions, such that the phosphate level and the operating temperature of the zinc phosphate solution is reduced, and the nitrite accelerator level of the solution is increased. The process of the present invention reduces the sludge produced by the process, the temperature at which the process operates, and the need for monitoring the process. Furthermore, the pretreatment process of the invention forms a conversion layer on all exposed metal surfaces that is suitable as a corrosion-preventing paint substrate, especially before a cathodic electro-dipcoating.

US2008305341 teaches a process for coating metallic surfaces with an anti-corrosive composition that contains a conductive polymer and is a dispersion that contains the at least one conductive polymer mainly or entirely in particulate form, as well as a binder system. The conductive polymer is at least one polymer based on polyphenylene, polyfuran, polyimidazole, polyphenanthrene, polypyrrole, polythiophene and/or polythiophenylene charged with anti-corrosive mobile anions. Alternatively, the metallic surfaces can be first coated with a dispersion based on conductive polymers in particulate form, then coated with a composition which contains a binder system.
US2005244459 teaches a coating composition and related method for use in applying a bioactive agent to a surface in a manner that will permit the bioactive agent to be released from the coating in vivo. The composition is particularly applicable for coating the surface of implantable medical device, such as a stent or catheter, in order to permit the device to release bioactive agent to the surrounding tissue over time. The composition includes a plurality of compatible polymers having different properties that can permit them to be combined together to provide an optimal combination of such properties as durability, biocompatibility, and release kinetics.

US2008022886 teaches Coating compositions that include corrosion resisting particles such that the coating composition can exhibit corrosion resistance properties. The invention further teaches substrates at least partially coated with a coating deposited from such a composition and multi-component composite coatings, wherein at least one coating later is deposited from such a coating composition. Methods and apparatus for making ultrafine solid particles are also disclosed.
Although there has been a substantial research work done by changing the steel chemistry and process parameters to overcome wettability problem but no work is reported with organic pretreatment chemicals.
SUMMARY OF THE INVENTION
According to the invention, a thin intermediate organic coating layer is proposed for application on AHSS surface by one of a dipping or spraying method after the steel sheet is cleaned by a successive steps of alkali cleaning and acid pickling. The thin organic coated AHSS steel is then annealed and hot dip galvanized to produce a uniform zinc coating over the surface without any uncoated spots. Two different annealing parameters are applied, such as, a typical reducing and an oxidizing-reducing atmosphere, and an improved galvanized surface is obtained in both without the use of any intermediate organic layer. Coating adhesion in all cases are found to be good.

According to the invention, various organic pre-treatment chemicals are applied on steel sheet to improve wettability at low cost. Different silane and mercapto ethanol group have been used to form a pre-coat in order to improve the Zinc wettability.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a process to improve galvanisability of advanced high strength steel (AHSS).
Another object of the invention is to propose a process to improve galvanisability of advanced high strength steel (AHSS), which uses an organic pre-treatment step of AHSS before galvanizing.
A still another object of this invention is to propose a process to improve galvanisability of advanced high strength steel (AHSS), which provides a uniform zinc coating with no bare or uncoated spot or area on the surface of advanced high strength steel.
A further object of the invention is to propose an organic pre-treatment chemical to improve wettability at low cost.
These and other objects of the invention is to develop water based eco friendly pretreatment chemical for AHSS.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig. 1: shows pretreated DP steel sample galvanized without being annealed
Fig.2: shows the pretreated DP steel sample galvanized after annealing
Fig. 3: shows a Cross section of the pretreated DP steel sample galvanized without annealing
Fig.4: shows a Cross section of the pretreated DP steel sample galvanized after annealing
Table 1: Elemental analysis for figure 3
Table 2: Elemental analysis for figure 4
DETAILED DESCRIPTION OF THE INVENTION
Different types of silane compound like Tetraethyl orthosilicate, Methyl Triethoxy silane, gama propyl trimethoxy silane , methyl hydrogen polysiloxane and mercapto containing additive were used at different concentration ( 0.05 to 2 % aqueous solution). The steel sheets coupons were dipped in the solution for 10-15 seconds followed by hot air drying for 20-30 seconds. DP 590 material was selected as base study material.
Since DP contains more amounts of Mn, Al and some Si which are easily prone to oxidation, experiments were performed with different silane and mercapto containing additive at different concentration and dipping time. The 1.0 %

sample turned totally blue during a 30 sec. annealing treatment even in full Nitrogen atmosphere while 0.5% sample was found to be better and was galvanized. The sample exhibits very good galvanizability and Zn showed good adhesion. This result prompted to vary the annealing time and nitrogen atmosphere conditions during annealing, transition period from furnace to cooling box and nitrogen content of the cooling zone.
These samples were annealed for 10 seconds to avoid bluing of steel and also during the transition from furnace to cooling box; any air exposure was avoided by keeping an extra nitrogen supply. The as coated samples in 0.5 and 1.0 % chemical concentration are shown in Fig.1 & 2, while the galvanized samples are shown in Fig. 3.
Concept of the invention
The pretreatment chemical contains one active sulfur group (-SH). During the pretreatment process sulfur tries to get adsorbed on the steel surface and form a very thin layer of coating. SH groups are known to be adsorbed to metals with which they have a strong affinity to form a mercaptide compound (R-S-M) by reacting with the metal ions
R-SH + ne ► R-S-M + nH
This reaction is reversible and its equilibrium is greatly displaced to the right. Mercapto has an extremely strong affinity for transition metals. During heating and annealing process the active sulfur try to diffuse in to the steel sheet and

forming metals compounds such as MnS etc. The process suppresses surface segregation of Mn and shuts off the diffusion passage of Si to the surface of steel sheet owing to the existence of sulfur-segregated layer. The selective oxidation of Mn and Si is reduced by the presence of mercapto group which enhances the galvanizability of AHSS.
Example
The trials were carried out with DP 590 steel samples. The initial experiment was done without giving annealing treatment. The idea was to avoid oxidation of Mn, Si and Al to get the best possible effect of chemical pre-treatment on galvanizing. The results for cross section are reported in Fig. 3 and Table 1. The interface adhesion is again quite good. Extra nitrogen supply helped us to avoid these problems. The cross section of galvanized samples for this experiment is given in Fig. 4 and Table 2. The results are quite good compared to previous one.

WE CLAIM
1. A process for improving the zinc wettability of Advanced High Strength Steel
(AHSS), comprising the steps of:
- cleaning an AHSS by successive steps of alkali cleaning and acid pickling;
- applying an intermediate chemical pretreatment layer on the cleaned surface of the AHSS;
- allowing the coated AHSS to undergo an annealing step for about 10 seconds under a full nitrogen environment by supplying nitrogen gas; and
- hot-dip galvanizing of the annealed AHSS,
wherein the intermediate pretreatment layer constitutes a solution comprising a silane compound selected from a group consisting of Tetraethyl orthosilicate, Methyl Triethoxy silane, gamma propyl trimethoxy silane, methyl hydrogen polysiloxane or mercapto containing group, which is mixed with deionised water as additive with 0.05 to 2% concentration, and in that the cleaned AHSS is dipped (or spray) in the solution for about 10-15 seconds followed by hot air drying for about 20-30 seconds.
2. The process as claimed in claim 1, wherein the silane additive consists of a
single chemical or combination of Tetraethyl orthosilicate, Methyl Triethoxy
silane, gamma propyl trimethoxy silane, methyl hydrogen silicon.

3. The process as claimed in claim 1, wherein the mercapto additive consist of a single or combination of 2 amino thiophenol or thiophenol or 2-benzimidazolethiol or 2-mercapto ethanol.
4. The process as claimed in claim 1, wherein the pretreated layer is enabled to shield alloying additions of the AHSS from migrating to the steel surface and prevents formation of surface oxides thus resulting in an improved zinc wetting and galvanizing of the AHSS.

ABSTRACT

The invention relates to a process for improving the zinc wettability of Advanced High Strength Steels (AHSS), comprising the steps of cleaning an AHSS by successive steps of alkali cleaning and acid pickling; applying an intermediate chemical pretreatment layer on the cleaned surface of the AHSS; allowing the coated AHSS to undergo an annealing step for about 10 seconds under a full nitrogen environment by supplying nitrogen gas; and hot-dip galvanizing of the annealed AHSS, wherein the intermediate pretreatment layer constitutes a solution comprising a silane compound selected from a group consisting of Tetraethyl orthosilicate, Methyl Triethoxy silane, gamma propyl trimethoxy silane, methyl hydrogen polysiloxane, Mercapto derivatives, which is mixed with deionised water as additive with 0.05 to 2% aqueous concentration, and in that the cleaned AHSS is dipped (or spray) in the solution for about 10-15 seconds followed by hot air drying for about 20-30 seconds.