FIELD OF THE INVENTION
The present disclosure generally relates to a field of material science and metallurgy. Particularly, but not exclusively the present disclosure relates to a coated steel. Further, embodiments of the present disclosure disclose coated steel and a method for coating the steel, which offers high mechanical resistance after thermal treatment.
BACKGROUND OF THE DISCLOSURE
Reduction of the vehicle weight to improve the fuel efficiency without compromising the passenger safety is a burning topic of research for last two decades. Hot forming process is one of the great solutions to achieve very high strength as high as 1500-1900 MPa steels [1]. The hot formed steels are used for side impact beams, bumpers, A and B pillars, roof rails, cross members, and tunnels etc. in automotive body. In hot stamping process the steel blanks are heated to austenitizing temperature to make it fully austenite. The blanks are then transferred to forming press where they are formed and subsequently quenched to achieve high strength by generating fully martensitic microstructure in the steel.
The typical heat treatment schedule for direct hot stamping process is heating the article up to 900oC to 950oC and isothermal holding for prolonged time such as 300s. Then the article is brought to 850oC and stamped. Just after stamping the article is quenched to form fully martensitic microstructure in the steel [1, 2]. The temperature drop from 950oC to 850oC is during the robotic transfer of the blank from furnace to the forming press.
In the hot stamping process, the steels are usually heated in a furnace for austenitization in normal atmospheric condition. However, the surface of the uncoated steel blanks get oxidized at such high temperature even decarburisation takes place from the surface of the steel. This problem has been addressed by applying a coating on the steel surface which will be able to prevent surface oxidation of steel and also will be helpful to retain the strength of the steel after hot stamping operation [3, 4, 5]. Moreover the coating provides corrosion resistance during service life. So, suitable coating system for hot stamping steels is essential. Zinc coatings that are the mostly used coating to protect steel owing to its sacrificial protection. However, the zinc based coatings are not suitable for direct hot forming due to its low melting temperature. During the direct hot stamping process, the blank temperature goes up to the austenitization temperature i.e. above 900oC and often kept as 950oC. The melting

point of pure zinc is 420oC and the boiling point is 907oC. At such high temperature there is considerable diffusion of iron from steel substrate in to the coating. This results in formation of different Fe-Zn phases during the hot stamping heat treatment. Zinc as well as the Fe-Zn phases is liquid at such elevated temperature and if formed beyond 782oC the material fails due to Liquid Metal Induced Embrittlement (LMIE) [6,7]. If the material is deformed at 700oC then the material is not subjected to LMIE whereas at deformation temperature of 850oC the material failed at only 8-10% elongation [3, 8]. The zinc based coatings are therefore not suitable for direct hot stamping process and they are only being used for indirect hot stamping process.
The most widely used coating for press hardened steels are the aluminium silicon coatings commercially known as USIBOR [9]. The coating process is by hot dipping and the bath composition is from 9% to 10% silicon, from 2% to 3.5% iron, the remainder being aluminium [9,5]. The dipping temperature is typically about 675oC. The coating microstructure has primary Al–Si eutectic matrix. A 5 μm thick Fe2SiAl7 inhibition layer is present at the coating/steel interface. Additionally, a thin layer of Fe2Al5 and FeAl3 with a thickness of less than 1mmcan be observed between the Fe2SiAl7 layer and substrate steel. The intermetallic phases are solid at the hot stamping heat treatment temperature eliminating the chance of LMIE and it can withstand the elongation of 40% at 700-850oC without failure of the material [10]. However, the Fe-Al intermetallics are brittle and forms cracks during forming which causes flacking of the coating from the substrate [1,10]. Due to the flacking of the coating the exposed substrate gets oxidised during the stamping process.
Aluminium silicon coating cannot offer any cathodic corrosion protection to the steel substrate [5]. However, Al-Si coating does not affect the mechanical property of the substrate blank during the direct hot stamping treatment. However, there are major drawbacks of the product in terms of brittle interface of coating as well as absence of sacrificial corrosion protection.
There are other types of coatings being tried out recently as a potential replacement of zinc coatings and Al-Si coatings. One such type of coating is the galvalume coating [11] by dipping the substrate in 55 wt.% Al–Zn bath, with 1.6 wt.% Si, and at a bath temperature of

680°C. There are other types of coatings being researched like, dual Layer Zn–Al coating, Zn–Al–Mg post-process coating etc [5]. The Zn-Al-Mg post process coating is done on the hot stamped parts. There is another system being explored for press hardenable steels which is deposition of different phases on steel substrate by electrodeposition. It was reported to deposit Ni-Zn phase having composition of 11wt-% Ni, 0.6wt-% Fe and balance zinc [12]. This corresponds to Ni-Zn gamma (γ) phase with higher melting point (880oC) than the Fe-Zn phases (782oC). It is claimed that the final microstructure is composed of iron based solid solution as well as Ni-Zn gamma phase that do not cause the LMIE [13,14]. Similar approach was reported [15]where the coating is described as a combined coating having first plating layer containing 60% by mass or more of Ni and the remainder consisting of Zn and second plating layer containing 10 to 25% by mass of Ni and the remainder consisting of Zn. The Ni-Zn phases are able to withstand the LMIE. However electrodeposition of Ni-Zn phases on bare steel can be a cause of hydrogen embrittlement [16]. Furthermore the iron concentration in the coating becomes largest with very few Ni-Zn islands that will reduce the sacrificial nature of the coating.
SUMMARY OF THE DISCLOSURE
Drawbacks of the conventional coated steel have been overcome through the present invention. Additionally, the present invention has been found to provide additional advantages over the conventional process. Other embodiments and different aspects of the present invention are discussed in detail and are considered as a part of the claimed discloser.
In one non-limiting embodiment of the present disclosure, a coated steel is disclosed. The coated steel comprises a steel substrate and a nickel-zinc-aluminium coating on the steel substrate which has been produced through electroplating followed by hot dip galvanizing in an aluminium containing bath. The present coating forms an iron rich solid solution layer of iron-nickel on the steel substrate, a nickel rich layer on the iron-nickel solid solution layer, a layer of nickel-zinc solid solution nickel rich layer, a nickel-zinc gamma layer on the nickel-zinc solid solution layer, a nickel-zinc delta layer on the nickel-zinc gamma layer and an overlay zinc layer on the nickel-zinc delta layer which is having dispersed precipitates of Ni-Zn-Al phases.

In an embodiment, the steel substrate is coated with nickel followed by hot dipping in zinc-aluminium bath comprising aluminium from about 0.1% by weight to about 1% by weight.
In an embodiment, the nickel-zinc-Alumnium coating comprises of iron of about 25% to about 50% by weight, zinc of about 15% to about 40% by weight, and nickel of about 20% to about 40% by weight, and Al of about 0.01% to about 2% by weight and thickness of the nickel-zinc coating ranges from 20µm to 50µm.
In an embodiment, the iron-nickel solid solution layer comprises iron of about 20% to about 95% by weight, zinc of about 0.1 % to about 1% by weight and nickel of about 4 % to about 80% by weight, and thickness of the iron-nickel solid solution layer ranges from 0.5µm to 2µm.
In an embodiment, the iron rich iron-nickel solid solution layer is formed of Body Centered Cubic crystals and phase fraction of iron-nickel solid solution is about 2-4% of total coating.
In an embodiment, the nickel layer (2a) comprises iron of about 2% to about 20% by weight, zinc of about 0.5% to about 10% by weight and nickel of about 85 to about 98% by weight, and thickness of the nickel layer (2a) ranges from 1µm to 5µm.
In an embodiment, the nickel layer (2a) is formed of Face Centered Cubic crystal, and has a hardness of about 6.71 GPa.
In an embodiment, the nickel-zinc solid solution layer (2b) comprises iron of up to 20% by weight, zinc of about 0.5% to about 25% by weight and nickel of about 35% to about 100% by weight, and thickness of the nickel-zinc solid solution layer (2b) ranges from 0.5µm to 1µm.
In an embodiment, the nickel-zinc solid solution layer (2b) is formed of Face Centered Cubic crystals.
In an embodiment, the nickel-zinc gamma layer (3) comprises iron up to 0.5% by weight, zinc of about 70% to about 85% by weight and nickel of about 15% to about 30% by weight, and thickness of the nickel-zinc gamma layer (3) ranges from 6µm to 10µm.

In an embodiment, the nickel-zinc gamma layer (3) is formed of cubic structure crystals, and, the nickel-zinc gamma layer has a hardness of about 4.83 GPa.
In an embodiment, the nickel-zinc delta layer (4) comprises iron up to 0.5 % by weight, zinc of about 88% to about 90% by weight and nickel of about 10% to about 12% by weight, and thickness of the nickel-zinc delta layer ranges from 0µm to 10µm.
In an embodiment, nickel-zinc delta layer(4) is formed of monoclinic crystals, and phase fraction of nickel-zinc delta layer about 20% to about 25% of the total coating, and wherein the nickel-zinc delta layer has a hardness of about 2.82 GPa.
In an embodiment, wherein the overlay zinc layer (5) comprises iron of about 0.1% to about 1% by weight and zinc of about 80% to about 100% by weight and aluminium of about 0.1% to about 1% by weight and remainder of the composition includes oxygen by weight, and thickness of the overlay zinc layer ranges from 2µm to 25µm. wherein the overlay zinc layer (5) Phase fraction of the overlay zinc layer comprises of zinc, Ni-Zn-Al precipitates and zinc-oxide. The overlay zinc layer (5) volume fraction comprises 5-60% of NI-ZN-AL precipitates.
In an embodiment, the Ni-Zn-Al phase comprises nickel about 20% to about 50% by weight, zinc about 10% to 50 % by weight, and aluminium about 10% to about 40% by weight. In an embodiment, the overlay zinc layer (5) is formed of hexagonal closed pack crystals.
In an embodiment, the steel substrate is boron steel, comprising:
Carbon from about 0.2 % to about 0.25 % by weight;
Manganese from about 1.15 % to about 1.4 % by weight;
Sulphur less than 0.01 % by weight;
Phosphorus less than 0.05 % by weight;
Silicon from about 0.2 % to about 0.35 % by weight;
Aluminium less than 0.1 % by weight;
Copper less than 0.05 % by weight;
Chromium from about 0.15 % to about 0.35 % by weight;
Nickel less than 0.1 % by weight;
Molybdenum less than 0.01 % by weight;

Vanadium less than 0.01 % by weight;
Niobium less than 0.01 % by weight;
Titanium from about 0.02 % to about 0.05 % by weight;
Nitrogen less than 50ppm;
Boron from about 0.002 % to about 0.005 % by weight; and
wherein the balance being iron optionally along with incidental elements of
the alloy.
In an embodiment, the steel substrate is formed of Body Centered Cubic crystals, and phase fraction of iron in the steel substrate comprises of pearlite structure of about 22% to about 26% and remainder being ferrite structure, and wherein the steel substrate has a mechanical resistance of about 2.4 GPa.
In another non-limiting embodiment of the disclosure, a hot worked coated steel is disclosed. The hot worked coated steel comprises a steel substrate and a nickel-zinc-aluminium coating on the steel substrate. The nickel-zinc-aluminium coating forms an iron-nickel-zinc solid solution layer on the steel substrate, an upper coating layer on the iron-nickel-zinc-aluminium solid solution layer and an oxide layer on the upper coating layer.
In an embodiment, the iron-nickel-zinc-Aluminium coating comprises iron of about 25% to about 50% by weight, zinc of about 15% to about 40% by weight, and nickel of about 20% to about 40% by weight, and Al of about 0.01% to about 2% by weight, and the thickness of the iron-nickel-zinc-Al coating ranges from 20µm to 70µm.
In an embodiment, the iron-nickel-zinc solid solution layer comprises iron of about 30% to about 70% by weight, zinc of about 10% to about 30% by weight and nickel of about 10% to about 35% by weight and thickness of the iron-nickel solid solution layer ranges from 2µm to 15µm.
In an embodiment, iron-nickel-zinc solid solution layer is formed of Face Centered Cubic crystals, and phase fraction of the iron-nickel solid solution layer comprises of about 5% to about 20% of the total coating.

In an embodiment, the upper coating layer of iron-nickel-zinc-aluminum comprises iron of about 10% to about 50% by weight, zinc of about 10% to about 50% by weight, nickel of about 20% to about 50% by weight, Al of about 0.1% to about 2% by weight, and oxygen of about 0.1% to about 5% by weight and thickness of the upper coating layer ranges from 15µm to 55µm.
In an embodiment, wherein the upper coating layer is formed of Face Centered Cubic crystals, and phase fraction of the upper coating layer comprises of about 75% to about 90% of the total coating.
In an embodiment, the oxide layer comprises iron of about 0.01% to about 1% by weight, zinc of about 75% to about 80% by weight, aluminium of about 0.01% to about 0.05% by weight and rest oxygen and thickness of the oxide layer ranges from 0.1µm to 3µm.
In an embodiment, the steel substrate is boron steel, comprising:
Carbon from about 0.2 % to about 0.25 % by weight;
Manganese from about 1.15 % to about 1.4 % by weight;
Sulphur less than 0.01 % by weight;
Phosphorus less than 0.05 % by weight;
Silicon from about 0.2 % to about 0.35 % by weight;
Aluminium less than 0.1 % by weight;
Copper less than 0.05 % by weight;
Chromium from about 0.15 % to about 0.35 % by weight;
Nickel less than 0.1 % by weight;
Molybdenum less than 0.01 % by weight;
Vanadium less than 0.01 % by weight;
Niobium less than 0.01 % by weight;
Titanium from about 0.02 % to about 0.05 % by weight;
Nitrogen less than 50ppm;
Boron from about 0.002 % to about 0.005 % by weight; and
wherein the balance being iron optionally along with incidental elements of
the alloy.

In an embodiment, the steel substrate is formed of Body Centered Tetragonal crystals, and phase fraction of iron in the steel substrate comprises of martensite structure of about 95% to about 100%, and wherein the steel substrate has a mechanical resistance of about 4.31 GPa.
In yet another non-limiting embodiment of the disclosure, a method for coating a steel substrate is disclosed. The method comprising acts of cleaning the steel substrate, electroplating the steel substrate with nickel in a nickel bath, at a temperature ranging from about 700C to about 900C fluxing and drying the substrate and applying a coat of zinc onto the steel substrate through hot dipping in molten Zn-Al bath.
In an embodiment, to remove oil remnants the steel substrate is cleaned by a caustic solution at a temperature ranging from about 500C to about 700C for a time ranging from 2 minutes to 5 minutes and thereafter rinsing the steel substrate in water to clean carry overs of the caustic solution. Then steel substrate is pickled in an acidic solution at temperature ranging from about 600C to about 700C for a time ranging from about 1 minute to about 5 minutes, to remove surface oxides. The steel substrate is then rinsed in a solution to clean carry overs of the acidic solution.
In an embodiment, the nickel coating is done using a nickel bath consisting 150-200 g/l of NiSO4, 30-40g/l of NiCl2, 5-8 g/l of H3BO3, and the bath is maintained at a pH ranging from 2 to 7.For the electroplating of the steel substrate a plate of nickel is used as anode and the steel substrate as cathode. The electroplating is carried out by maintaining a current of 2-5mA/cm2 and a voltage of 0.5 -1 V respectively, for 1-30 minutes.
In an embodiment, Fluxing of the nickel coated steel substrate is done in a 20% by volume NH4Cl-80% by volume Zn4Cl solution. The fluxing is done at room temperature for 30 seconds. Thereafter the fluxed substrate is dried in hot air at a temperature up to 100oC.
In an embodiment, the coat of zinc is applied by dipping the steel substrate (1a) in a molten zinc solution at a temperature ranging from about 4500C to about 4700C and for a time ranging from about 2 seconds to about 10 seconds.

It is to be understood that the aspects and embodiments of the disclosure described above may be used in any combination with each other. Several of the aspects and embodiments may be combined together to form a further embodiment of the disclosure.
The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The novel features and characteristics of the disclosure are set forth in the appended description. The disclosure itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying figures. One or more embodiments are now described, by way of example only, with reference to the accompanying figures wherein like reference numerals represent like elements and in which:
Figure 1 illustrates optical micrograph of initial microstructure of steel substrate, in accordance with one embodiment of the present disclosure.
Figure 2 illustrates flow chart of the process used for coating a steel substrate, in accordance with an embodiment of the present disclosure.
Figure 3a illustrates graphical representation elemental analysis through depth of the coating on the steel substrates before hot stamping process for coating acquired from 0.2 wt.% Al-Zn solution, in accordance with an embodiment of the present disclosure.
Figure 3b illustrates graphical representation elemental analysis through depth of the coating on the steel substrates before hot stamping process for coating acquired from 0.9 wt.% Al-Zn solution, in accordance with an embodiment of the present disclosure.
Figure 4a illustrates cross-sectional elemental view of the coated boron steel in low Al containing Zn bath, in accordance with an embodiment of the present disclosure.

Figure 4b illustrates cross-sectional elemental view of coated boron steel in low Al containing Zn bath, in accordance with an embodiment of the present disclosure.
Figure 5a illustrates cross-sectional elemental view of the coated boron steel in high Al containing Zn bath, in accordance with an embodiment of the present disclosure.
Figure 5b illustrates cross-sectional elemental view of the coated boron steel in high Al
containing Zn bath, in accordance with an embodiment of the present disclosure.
Figure 6 illustrates graphical representation of heat treatment schedule followed for hot
stamping process of the coated steel, in accordance with an embodiment of the present
disclosure.
Figure 7 illustrates optical micrograph of microstructure of the coated steel substrate after heat treatment process, in accordance with an embodiment of the present disclosure.
Figure 8 illustrates (a) micrograph and elemental analysis plots of (b) iron (c) oxygen (d) Ni (e) zinc and (f) aluminium of final microstructure of the coating on the steel substrate acquired by hot dipping in zinc bath consisting low aluminium, in accordance with an embodiment of the present disclosure.
Figure 9 illustrates (a) micrograph and elemental analysis plots of (b) iron (c) oxygen (d) Ni (e) zinc and (f) aluminium of final microstructure of the coating on the steel substrate acquired by hot dipping in zinc bath consisting high aluminium, in accordance with an embodiment of the present disclosure.
Figure 10 illustrates graphical representation of stress-strain curve of a tensile test carried on coated steel till fracture, in accordance with an embodiment of the present disclosure.
The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.

DETAILED DESCRIPTION OF THE DISCLOSURE
The foregoing has broadly outlined the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and advantages of the disclosure will be described hereinafter which form the subject of the description of the disclosure. It should also be realized by those skilled in the art that such equivalent methods do not depart from the scope of the disclosure. The novel features which are believed to be characteristic of the disclosure, as to method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure.
In the present document, the word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.
While the disclosure is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the disclosure.
The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a method that comprises a list of acts does not include only those acts but may include other acts not expressly listed or inherent to such method. In other words, one or more acts in a method proceeded by “comprises… a” does not, without more constraints, preclude the existence of other acts or additional acts in the method.
The present disclosure provides coated steel which offers high mechanical strength after thermal treatment. Direct and indirect hot stamping process can be applied on the coated steel, without deteriorating mechanical properties like strength or ductility of steel. The

coated steel comprises a steel substrate and a nickel-zinc-aluminium coating on the steel substrate. In an embodiment, the steel substrate is a boron steel comprising carbon, manganese, sulphur, phosphorus, silicon, aluminium, copper, chromium, nickel, molybdenum, vanadium, niobium, titanium, nitrogen and boron in predetermined quantities with the balance quantity being iron along with incidental elements of the alloy.
A method for applying a Ni-Zn-Al coating on the steel substrate also has been discussed in the present disclosure. The method comprises acts of cleaning of steel substrates, and then electroplating nickel onto cleaned steel substrate in a nickel bath which is maintained at a predetermined temperature. The electroplated steel substrate is dipped into fluxed solution and heated to a temperature of 100oC for drying. Lastly, a coat of zinc-aluminium is applied by dipping the heated steel substrate into a zinc-aluminium solution, thereby coating a nickel-zinc-aluminium layer on the steel substrate.
The nickel-zinc-aluminium coating applied on the steel substrate forms an iron-nickel solid solution layer, a nickel layer, a nickel-zinc solid solution layer, nickel-zinc gamma layer, a nickel-zinc delta layer and an overlay zinc layer with dispersed Ni-Zn-Al phases on the steel substrate. In an embodiment, the nickel-zinc-aluminium coating on the steel substrate comprises of iron, zinc, nickel, aluminium and oxygen in predetermined quantities and is of predetermined thickness as per requirement. In an embodiment, each of the layers in the nickel-zinc-aluminium coating comprises of iron, zinc, nickel, aluminium and oxygen in predetermined quantities and is of predetermined thickness.
The coated steel after hot press forming forms hot worked coated steel. The hot worked coated steel comprises steel substrate and a nickel-zinc-aluminium coating on the steel substrate. In an embodiment, the steel substrate is a boron steel comprising carbon, manganese, sulphur, phosphorus, silicon, aluminium, copper, chromium, nickel, molybdenum, vanadium, niobium, titanium, nitrogen and boron in predetermined quantities with the balance quantity being iron along with incidental elements of the alloy. After hot forming, the nickel-zinc-aluminium coating on the steel substrate forms an iron-nickel solid solution at the interface of the substrate and coating, an upper coating later and an oxide layer on the steel substrate.

The formation of a nickel-zinc layer on the coated steel surface before and after heat treatment process provides the strength as well as ductility to the coating. Additionally, the nickel-zinc layer provides required cathodic protection to the steel surface from corrosion during use.
In the following detailed description of the embodiments of the disclosure, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the disclosure may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the disclosure, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present disclosure. The following description is, therefore, not to be taken in a limiting sense.
Figure 1 is an exemplary embodiment of the present disclosure which illustrates optical micrograph of an initial microstructure of steel substrate (1a). In an embodiment, the steel substrate (1a) is a boron steel with an initial microstructure of pearlite (P) [illustrated with smaller grain boundaries] and ferrite (F) [illustrated with larger grain boundaries]. The steel substrate (1) comprises a composition of alloys as shown in table 1 below.

C Mn S P Si Al Cu Cr Ni Mo V Nb Ti N B
0.2-0.25 1.15-1.4 <0.01 <0.05 0.2-0.35 <0.1 <0.05 0.15-0.35 <0.1 <0.01 <0.01 <0.01 0.02-0.05 50ppm 0.002-0.005
Table. 1 the balance being iron optionally along with incidental elements of the alloy.
Boron in steel enhances the formation martensite in the steel and increases the hardenability of steel. Presence of boron in the steel substrate enhances the formation of martensite (1) during hot stamping process. In an embodiment, the steel substrate (1) is formed of Body Centered Cubic crystals, and phase fraction of iron in the steel substrate (1) comprises of pearlite structure of about 22% to about 26% and remainder being ferrite structure. The steel substrate (1) having the composition of the alloys shown in table 1 may have a mechanical strength of about 2.4 GPa.

Referring now to Figure 2, which is an exemplary embodiment of the present disclosure illustrating flow chart of a method for coating the steel substrate (1) having the composition of the alloys shown in table 1. In step 101, the steel substrate (1) for example steel sheet is subjected for cleaning to remove oil contaminants, grease residue, corrosion products or any other foreign entities deposited on the surface of the steel substrate (1). In the cleaning process, the steel substrate (1) is degreased with a caustic solution at a temperature ranging from about 500C to about 700C, The degreasing time typically maintained from 2 minutes to 5 minutes. Degreasing of the steel substrate (1) removes oil and grease from the surface of the steel substrate (1). To clean carry overs of the caustic solution on the surface of the steel substrate (1) the steel substrate (1) is then rinsed in water. The steel substrate (1) is then pickled in an acidic solution for time ranging from about 1 minute to about 5 minutes at a temperature from about 600C to about 700C. Acid pickling of the steel substrate (1) removes corrosion products from the steel surface. The steel substrate (1) is then rinsed in a solution to clean carry overs of the acidic solution. In an embodiment, the solution for cleaning carry overs of the acidic solution is water.
In step 102, the cleaned steel substrate is electroplated with nickel, in a nickel bath. The nickel bath includes NiSO4 of about 150 g/l to about 200 g/l, NiCl2 of about 30 g/l to about 40.9 g/l and H3BO3 of about 5 g/l to about 8 g/l. The nickel bath is maintained at a pH ranging from about 2 to about 7. A plate of nickel is used as anode and the steel substrate (1) as cathode for electroplating nickel on steel substrate (1). A current from about 2 mA/cm2 to about 5mA/cm2 and a voltage of 0.5V to about 1V respectively is maintained for the nickel plating on the steel substrate (1). The electroplating time typically ranges from 1 minute to about 30 minutes. This configuration of the circuit will enable the anodic nickel particles to get deposited on the cathodic steel substrate (1) surface. Nickel is a very good barrier to hydrogen diffusion. So, Electroplated nickel deposition on the steel substrate (1) will act as a potential barrier to hydrogen into the steel substrate (1). and will improve the hydrogen embrittlement.
In step 103, the electroplated steel substrate is dipped into flux solution having ZnCl2 about 80% by volume and NH4Cl about 20% by volume and rest is water. The fluxed steel substrate (1) is then dried at 100oC The fluxing operation of steel substrate (1) prohibit oxygen to

diffuse into steel substrate (1). This inherently eliminates possibility of oxygen concentration in subsequent steps of the process of coating the steel substrate (1).
In step 104, the heated steel substrate is dipped in molten zinc baths containing different aluminium concentrations maintained at a temperature of 4600C. The nickel coated steel substrate is dipped for about 2 seconds to about 10 seconds, for applying a coat of zinc. The molten zinc solution comprised of aluminium of different concentrations from about 0.1% to about 1% by weight. The metallic zinc and aluminium adhere to the steel substrate (1) by formation of different phases, when the steel substrate (1) is immersed in the molten zinc-aluminium solution. The excess zinc adhered to the surface, may be wiped out using air/nitrogen wipers. Thus, the process from steps 101-104 provides a steel substrate (1) coated with a nickel-zinc-aluminium coating of predetermined thickness. In an embodiment, the nickel-zinc-aluminium coating applied on the steel substrate (1) by a method steps 101-104 comprises iron of about 20% to about 30% by weight, zinc of about 50% to about 70% by weight, and nickel of about 10% to about 20% by weight, and aluminium of about 0.1 to about 1% by weight, and thickness of the nickel-zinc coating ranges from 20µm to 50µm. Referring to Figure 3a which illustrates graphical elemental profile through depth of the coating on the steel substrate (1) acquired through 0.2 wt.% Al-Zn solution. In an embodiment the elemental analysis of the coating on the steel substrate (1) is obtained by Wavelength Dispersive Spectroscopic (WDS) and Electron Dispersive Spectroscopic technique (EDS). As shown in Figure 3a the nickel-zinc-aluminium coating on steel substrate (1) carried by step 101-104 when galvanised in 0.2 wt.% Al-Zn solution results in different layers of different phases and interfaces between them. In an embodiment, the layers formed on the steel surface include (1) iron-nickel solid solution on the surface of the steel substrate (1). A nickel rich layer (2) is formed on the iron-nickel solid solution layer. On the nickel layer (2) a nickel-zinc solid solution layer is formed, followed by a nickel-zinc gamma phase and a nickel-zinc delta phase layers. The, overlay zinc coating is formed as a top coating layer. In the overlay zinc layer Ni-Zn-Al phases has precipitated dispersedly. Each of the layers and interfaces are formed with a predetermined thickness, composition, crystal structure and microstructure.

Referring to Figure 3b which illustrates graphical elemental profile through depth of the coating on the steel substrate (1) acquired through 0.9 wt.% Al-Zn solution. In an embodiment the elemental analysis of the coating on the steel substrate (1) is obtained by Wavelength Dispersive Spectroscopic (WDS) and Electron Dispersive Spectroscopic technique (EDS). As shown in Figure 3b the nickel-zinc-aluminium coating on steel substrate (1) carried by step 101-104 when galvanised in 0.9 wt.% Al-Zn solution results in different layers of different phases and interfaces between them. In an embodiment, the layers formed on the steel surface include (1) iron-nickel solid solution on the surface of the steel substrate (1). A nickel rich layer (2) is formed on the iron-nickel solid solution layer. On the nickel layer (2) a nickel-zinc solid solution layer is formed, followed by a nickel-zinc gamma phase. Unlike of phase formed in the coating acquired from low Al (i.e. 0.2 wt.%) containing Zn bath Ni-Zn delta phase is absent in the coating acquired in high Al (i.e. 0.9 wt.%) containing Zn solution. The, overlay zinc coating is formed as a top coating layer. In the overlay zinc layer Ni-Zn-Al phases has precipitated dispersedly. However, the volume fraction of the Ni-Zn-Al phase is very high compared to the coating acquired in low Al (i.e. 0.2 wt.%) containing Zn solution. Each of the layers and interfaces are formed with a predetermined thickness, composition, crystal structure and microstructure.
In an embodiment, the iron-nickel solid solution layer comprises iron of about 20% to about 95% by weight, zinc of about 0.1 % to about 1% by weight and nickel of about 4 % to about 80% by weight. The thickness of the iron-nickel solid solution layer ranges from 0.5µm to 2µm. The iron-nickel solid solution layer has a body centre cubic crystal structure, and the iron-nickel solid solution layer comprises of about 2% to about 4% of the total coating.
In an embodiment, the nickel layer (2) shown as Ni rich layer in FIG. 3a and Fig 3b is formed on the iron-nickel solid solution. The nickel rich layer comprises iron of about 2% to about 20% by weight, zinc of about 0.5% to about 23% by weight and nickel of about 75 to about 90% by weight. The nickel rich layer (2) has a layer thickness ranging from 1µm to 3µm. The nickel layer (2) has hardness of about 6.71 GPa. The nickel layer (2) has face centre cubic crystal structure, and phase fraction of the nickel layer (2) comprises of about 4% to about 6% of the total coating.

In an embodiment, the nickel-zinc solid solution layer shown as Ni (Zn) layer in FIG. 3a and FIG 3b is formed on the Nickel rich layer. The nickel-zinc solid solution layer comprises iron of up to 20% by weight, zinc of about 0.5% to about 25% by weight and nickel of about 35% to about 100% by weight. The thickness of the nickel-zinc solid solution layer ranges from 0.5µm to 1µm. The nickel-zinc solid solution layer has face centre cubic crystal structure, and phase fraction of the nickel-zinc solid solution layer comprises of about 2% to about 4% of the total coating.
In an embodiment, the nickel-zinc gamma layer (3) shown as gamma layer in FIG 3a and FIG 3b is formed on the nickel-zinc solid solution layer comprises iron of up to 0.5% by weight, zinc of about 70% to about 85% by weight and nickel of about 15% to about 30% by weight. The thickness of the nickel-zinc gamma layer (3) ranges from 6µm to 10µm. The nickel-zinc gamma layer (3) has a mechanical resistance of about 4.83 GPa. Further, the nickel-zinc gamma layer (3) has of cubic crystal structure, and phase fraction of the nickel-zinc gamma layer (3) comprises of 10% to about 30% of the total coating.
In an embodiment, the nickel-zinc delta layer (4) shown as delta layer in Fig 3a is formed on the nickel-zinc gamma layer (3). The nickel-zinc delta layer (4) comprises iron up to 0.5 % by weight, zinc of about 88% to about 90% by weight and nickel of about 0% to about 12% by weight, and thickness of the nickel-zinc delta layer (4) ranges from 0µm to 10µm. The nickel-zinc delta layer (4) has hardness of about 2.82 GPa. The nickel-zinc delta layer (4) has monoclinic crystal structure, and phase fraction of nickel-zinc delta layer (4) comprises of about 0% to about 25% of the total coating.
In an embodiment, the overlay zinc layer (5) shown as overlay zinc in Fig. 3a and 3b, is formed on the nickel-zinc delta layer (4). The overlay zinc layer (5) comprises iron of about 0.1% to about 1% by weight and zinc of about 80% to about 100% by weight and aluminium of about 0.1% to about 1% by weight and remainder of the composition includes oxygen by weight remainder of the composition includes oxygen by weight, and thickness of the overlay zinc layer (5) ranges from 2µm to 25µm. Dispersed Ni-Zn-Al precipitates are present in the matrix of overlay zinc. The phase fraction of the Ni-Zn-Al depends on the aluminium bath concentration of the Al (i.e. 0.1 to 1% by weight)-Zn solution. The overlay zinc matrix layer

(5) is formed of hexagonal closed pack crystals, and phase fraction of the overlay zinc layer (5) comprises of zinc, Ni-Zn-Al precipitates and and zinc-oxide.
In the exemplary embodiment illustrated in Fig 3a, the overlay zinc layer (5) extends up to 7 µm the nickel-zinc delta layer (4) and the nickel-zinc gamma layer (3) extends up to 16 µm, the nickel-zinc solid solution layer extends up to 17 µm, the nickel layer (2) extends up to 19 µm and the iron-nickel solid solution layer extends up to 20 µm.
Whereas, in the exemplary embodiment illustrated in Fig 3b the overlay zinc (4) extends up to 17 µm the nickel-zinc delta layer is not present as an effect of high aluminium concentration of 0.9% by weight in the zinc coating solution. The gamma layer (3) extends up to 38 µm, the nickel–zinc solid solution layer extends up to 39 µm, the nickel layer extends up to 41 µm and the iron-nickel solid solution layer extends up to 42 µm.
The aforementioned characteristics of each layer of the coating provide the necessary protection to high temperature oxidation/decarburisation of the steel. The coating also provides the necessary strength and ductility for the resultant coated steel (CS).
The coated steel (CS) may be subjected for hot forming to form the steel to a required shapes and dimensions. Now referring to, Figure 7 which illustrates an exemplary heat treatment schedule for the coated steel (CS). In an embodiment, the heat treatment is hot stamping process. The coated steel (CS) is heated at a rate of 100C/s up to 9500C, and is maintained at this temperature for about 300 seconds. Then, the coated steel (CS) is cooled at a rate of 300C/s up to 8500C and is maintained at this temperature for 3 seconds. The coated steel (CS) is then subjected to a strain in a forming press at a rate of 0.5/s up to 40% of strain of the coated steel (CS). The strained coated steel (CS) is then quenched to room temperature to obtain hot worked coated steel. Due to the hot stamping process, the microstructure of the steel substrate (1) fully converts into martensite structure (M) [as shown in figure 8]. Conversion of microstructure of the steel substrate (1) from pearlite and ferrite to martensite after hot stamping, will significantly improve the strength of the steel substrate (1). In an embodiment, the strength of the steel substrate (1) is in terms of mechanical strength or hardness.

Subsequently, as the coating on the steel substrate (1) is also subjected to the hot stamping process, there will be changes in the microstructure of the coating. During heating of the coated steel (CS), evaporation of overlay zinc layer (5 in Fig 5/4 in Fig 6) takes place due to its boiling point of 907oC. Also, during heating of the coated steel (CS) the overlay zinc layer (5 in Fig 5/4 in Fig 6) oxidises. Thus, due to evaporation and oxidation of the overlay zinc layer (5 in Fig 5/4 in Fig 6), the amount of zinc in the final microstructure varies, based on the thickness of the overlay zinc layer (5 in Fig 5/4 in Fig 6). However the rate of evaporation of overlay also depends on the presence of aluminium. Presence of aluminium forms an impervious layer of aluminium oxide which retards the zinc evaporation. Hence, the higher the aluminium in the overlay zinc less evaporation will take place. So, coatings with high aluminium will result in higher zinc in the final microstructure after the hot stamping process. The presence of different Ni-Zn phases in the initial microstructure also influences the final microstructure after stamping process. Thus, the final coating thickness depends on initial nickel coating thickness, galvanising time, initial phase fraction of Ni-Zn delta, Ni-Zn gamma and Ni-Zn-Al and thickness of overall zinc layer.
An example of variation of final microstructure of the coating on the steel substrate (1) with initial 7µm (5 in Fig 5) and 17µm (4 in Fig 6) of overlay zinc layer is illustrated in figures 9 and 10 respectively. As illustrated, in both the cases a continuous interfacial layer rich in nickel and iron is present in the coating. However, amount of zinc is higher in the coating with high aluminium concentration in initial microstructure acquired from hot dipping in 0.9% by weight Al-Zn solution [as shown in figure 10]. Whereas, coating acquired from hot dipping in 0.2% by weight Al-Zn solution showed high amount of iron and nickel with less zinc in the final microstructure [as shown in figure 9]. In both the cases due to the presence of a ductile diffused interfacial layer of iron nad nickel cracks will be arrested when subjected to tensile forces. Thus, the mechanical properties of the steel are retained, even after coating of the steel substrate (1). At the same time due to the presence of high melting phases of iron-nickel and nickel-zinc in the coating, the possibility of LMIE of zinc is prevented. Exemplary Experimental results:
Test specification of the hot tensile test were conducted for the coated steel (CS) is given in the Table 2. Stress-strain behaviour till fracture for hot tensile test of the coated steel (CS) is shown in Figure 11.

Coate
d Steel
sampl
e Heatin g rate (oC/s) Peak
temp.
(oC) Holdin
g time
(s) Interme
diate
cooling
rate
(oC/s) Deformat
ion temp.
(oC) Soaking
time
before
forming
(s) Strain rate of 20ormi ng (s-1) Engg.
Strain
(%)
Test 1 10 950 300 30 850 3 0.5 Till fracture
Test 2 10 950 300 30 850 3 0.5 40
Table 2 Referring to Figure 11, it is evident that the coating was able to retain the mechanical strength of the bare steel till fracture. It could be said that the elastic strength, yield strength, ultimate tensile strength and fracture toughness are same as bare steel. This is the effect of the ductile interface and the high melting phase formation during hot stamping process.
Advantages:
The present discloser provides superior properties as compared to conventional coated steel.
The present discloser provides a mean to form phases with high melting temperatures at the
time of hot dipping process.
The present discloser provides a mean to provide high amount of zinc in the final
microstructure without deteriorate, which is beneficial to provide cathodic protection to
substrate.
The discloser provides coated steel, which retains the mechanical property of the bare steel at
the time hot stamping process.
Equivalents
With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.
REFERRAL NUMERALS:

Referral numerals Description
101 Cleaning step
102 Electrodeposition step
103 Fluxing and heating step
104 Zinc coating step
1 Steel substrate
2 Nickel layer
3 Nickel-Zinc gamma layer
4 Nickel-Zinc delta layer
5 Overlay Zinc layer
6 Interstitial free steel
B Bare steel
CS Coated steel
F Ferrite microstructure in steel substrate
P Pearlite microstructure in steel
M Martensite microstructure in hot worked steel

WE CLAIM:
1. A coated steel, comprising:
a steel substrate (1),
a nickel-zinc-Alumium coating on the steel substrate (1a), wherein the nickel-zinc-Aluminium coating forms:
an iron-nickel solid solution layer (1b) on the steel substrate (1a); a nickel layer (2a) on the iron-nickel solid solution layer; a nickel-zinc solid solution layer on the nickel layer (2b); a nickel-zinc gamma layer (3) on the nickel-zinc solid solution layer; a nickel-zinc delta layer (4) on the nickel-zinc gamma layer (3); and
an overlay zinc layer (5) with on the nickel-zinc-Aluminium precipitate ranging from 5 to 60 percent volume fraction on delta layer (4).
2. The coated steel as claimed in claim 1, wherein the nickel-zinc-Alumnium coating
comprises of iron of about 25% to about 50% by weight, zinc of about 15% to about
40% by weight, and nickel of about 20% to about 40% by weight, and Al of about
0.01% to about 2% by weight.
3. The coated steel as claimed in claim 2, wherein the nickel-zinc-Alumnium coating thickness ranges from 20µm to 50µm.
4. The coated steel as claimed in claim 1, wherein the iron-nickel solid solution layer (1b) comprises iron of about 20% to about 95% by weight, zinc of about 0.1 % to about 1% by weight and nickel of about 4 % to about 80% by weight, and thickness of the iron-nickel solid solution layer ranges from 0.5µm to 2µm.

5. The coated steel as claimed in claim 4, wherein the iron rich iron-nickel solid solution layer (1b) is formed of Body Centered Cubic crystals and phase fraction of iron-nickel solid solution is about 2-4% of total coating.
6. The coated steel as claimed in claim 1, wherein the nickel layer (2a) comprises iron of about 2% to about 20% by weight, zinc of about 0.5% to about 10% by weight and nickel of about 85 to about 98% by weight, and thickness of the nickel layer (2a) ranges from 1µm to 5µm.
7. The coated steel as claimed in claim 6, wherein the nickel layer (2a) is formed of Face Centered Cubic crystal, and has a hardness of about 6.71 GPa.
8. The coated steel as claimed in claim 1, wherein the nickel-zinc solid solution layer (2b) comprises iron of up to 20% by weight, zinc of about 0.5% to about 25% by weight and nickel of about 35% to about 100% by weight, and thickness of the nickel-zinc solid solution layer (2b) ranges from 0.5µm to 1µm.
9. The coated steel as claimed in claim 8, wherein the nickel-zinc solid solution layer (2b) is formed of Face Centered Cubic crystals.
10. The coated steel as claimed in claim 1, wherein the nickel-zinc gamma layer (3) comprises iron up to 0.5% by weight, zinc of about 70% to about 85% by weight and nickel of about 15% to about 30% by weight, and thickness of the nickel-zinc gamma layer (3) ranges from 6µm to 10µm.
11. The coated steel as claimed in claim 1, the nickel-zinc gamma layer (3) is formed of cubic structure crystals, and, the nickel-zinc gamma layer has a hardness of about 4.83 GPa.
12. The coated steel as claimed in claim 1, wherein the nickel-zinc delta layer (4) comprises iron up to 0.5 % by weight, zinc of about 88% to about 90% by weight and nickel of about 10% to about 12% by weight, and thickness of the nickel-zinc delta layer ranges from 0µm to 10µm.

13. The coated steel as claimed in claim 1, wherein nickel-zinc delta layer(4) is formed of monoclinic crystals, and phase fraction of nickel-zinc delta layer about 20% to about 25% of the total coating, and wherein the nickel-zinc delta layer has a hardness of about 2.82 GPa.
14. The coated steel as claimed in claim 1, wherein the overlay zinc layer (5) comprises iron of about 0.1% to about 1% by weight and zinc of about 80% to about 100% by weight and aluminium of about 0.1% to about 1% by weight and remainder of the composition includes oxygen by weight, and thickness of the overlay zinc layer ranges from 2µm to 25µm.
15. The coated steel as claimed in claim 14, wherein the overlay zinc layer (5) Phase fraction of the overlay zinc layer comprises of zinc, Ni-Zn-Al precipitates and zinc-oxide.
16. The coated steel as claimed in claim 15, wherein the overlay zinc layer (5) volume fraction comprises 5-60% of NI-ZN-AL precipitates.
17. The coated steel as claimed in claim 14, wherein the Ni-Zn-Al phase comprises nickel about 20% to about 50% by weight, zinc about 10% to 50 % by weight, and aluminium about 10% to about 40% by weight.
18. The coated steel as claimed in claim 14, wherein the overlay zinc layer (5) is formed of hexagonal closed pack crystals.
19. The coated steel as claimed in claim 1, wherein the steel substrate (1a), is a boron steel, comprising:
Carbon from about 0.2 % to about 0.25 % by weight;
Manganese from about 1.15 % to about 1.4 % by weight;
Sulphur less than 0.01 % by weight;
Phosphorus less than 0.05 % by weight;
Silicon from about 0.2 % to about 0.35 % by weight;
Aluminium less than 0.1 % by weight;
Copper less than 0.05 % by weight;
Chromium from about 0.15 % to about 0.35 % by weight;

Nickel less than 0.1 % by weight;
Molybdenum less than 0.01 % by weight;
Vanadium less than 0.01 % by weight;
Niobium less than 0.01 % by weight;
Titanium from about 0.02 % to about 0.05 % by weight;
Nitrogen less than 50ppm;
Boron from about 0.002 % to about 0.005 % by weight; and wherein the
balance being iron optionally along with incidental elements of the alloy.
20. The coated steel as claimed in claim 15, wherein the steel substrate (1) is formed of body centered cubic crystals, and phase fraction of iron in the steel substrate (1) comprises of pearlite structure of about 22% to about 26% and remainder being ferrite structure, and wherein the steel substrate (1) has a mechanical resistance of about 2.4 GPa.
21. A hot worked coated steel, comprising:
a steel substrate ,
a nickel-zinc-Aluminium coating on the steel substrate, wherein the nickel-zinc-Aluminium coating forms:
an iron-nickel-zinc solid solution layer on the steel substrate; an upper coating layer of iron-nickel-zinc-aluminum; and an oxide layer on the upper coating layer.
22. The hot worked coated steel of claim 21, wherein the steel substrate is formed of Body Centered Cubic crystals, and phase fraction of iron in the steel substrate comprises of pearlite structure of about 22% to about 26% and remainder being ferrite structure, and wherein the steel substrate has a mechanical resistance of about 2.4 GPa.
23. The hot worked coated steel of claim 21, wherein the nickel-zinc coating forms an iron-nickel-zinc solid solution layer on the steel substrate, an upper coating layer of

iron-nickel-zinc-aluminium solid solution layer and an oxide layer on the upper coating layer of of iron-nickel-zinc-aluminium solid solution layer.
24. The hot worked coated steel of claim 21, wherein the iron-nickel-zinc-Aluminium coating comprises iron of about 25% to about 50% by weight, zinc of about 15% to about 40% by weight, and nickel of about 20% to about 40% by weight, and Al of about 0.01% to about 2% by weight, and the thickness of the iron-nickel-zinc-Al coating ranges from 20µm to 70µm.
25. The hot worked coated steel of claim 23, wherein the iron-nickel-zinc solid solution layer comprises iron of about 30% to about 70% by weight, zinc of about 10% to about 30% by weight and nickel of about 10% to about 35% by weight and thickness of the iron-nickel solid solution layer ranges from 2µm to 15µm.
26. The hot worked coated steel of claim 23, wherein the iron-nickel-zinc solid solution layer is formed of Face Centered Cubic crystals, and phase fraction of the iron-nickel solid solution layer comprises of about 5% to about 20% of the total coating.
27. The hot worked coated steel of claim 21, wherein the upper coating layer of iron-nickel-zinc-aluminum comprises iron of about 10% to about 50% by weight, zinc of about 10% to about 50% by weight, nickel of about 20% to about 50% by weight, Al of about 0.1% to about 2% by weight, and oxygen of about 0.1% to about 5% by weight and thickness of the upper coating layer ranges from 15µm to 55µm.
28. The hot worked coated steel of claim 21, wherein the upper coating layer is formed of Face Centered Cubic crystals, and phase fraction of the upper coating layer comprises of about 75% to about 90% of the total coating.
29. The hot worked coated steel of claim 21, wherein the oxide layer comprises iron of about 0.01% to about 1% by weight, zinc of about 75% to about 80% by weight, aluminium of about 0.01% to about 0.05% by weight and rest oxygen and thickness of the oxide layer ranges from 0.1µm to 3µm.

30. A method for coating a steel substrate (1a), the method comprising acts of:
cleaning the steel substrate (1a);
electroplating the steel substrate (1a) with nickel in a nickel bath, at a temperature
ranging from about 700C to about 900C;
fluxing and drying the steel substrate (1a) up to 100oC temperature; and
applying a coat of zinc onto the steel substrate (1a) through hot dip coating.
31. The method as claimed in claim 30, wherein the cleaning the steel substrate (1)
comprises acts of:
washing the steel substrate (1a) by a caustic solution at a temperature ranging
from about 500C to about 700C for a time ranging from 2 minutes to 5 minutes to
remove oil remnants;
rinsing the steel substrate (1a) in water to clean carry overs of the caustic solution;
pickling the steel substrate (1a) in an acidic solution at temperature ranging from
about 600C to about 700C for a time ranging from about 1 minute to about 5
minutes, to remove corrosion; and
rinsing the steel substrate (1a) in a solution to clean carry overs of the acidic
solution.
32. The method as claimed in claim 30, wherein, the nickel bath includes 150-200 g/l of
NiSO4, 30-40 g/l of NiCl2, 5-8 g/l of H3BO3, and the bath is maintained at a pH
ranging from 2 to 7; and the electroplating of the steel substrate (1a) is carried out by
configuring a plate of nickel as anode and the steel substrate (1a) as cathode, and the
electroplating is carried out by maintaining a current of 2-5mA/cm2 and a voltage of
0.5 -1 V respectively, for 1-30 minutes.
33. The method as claimed in claim 30, wherein, Fluxing of the nickel coated steel substrate is done in a 20% by volume NH4Cl-80% by volume Zn4Cl solution. The fluxing is done at room temperature for 30 seconds. Thereafter the fluxed substrate is dried in hot air at a temperature up to 100oC.

ABSTRACT
TITLE: “A COATED STEEL AND A METHOD OF COATING A STEEL
SUBSTRATE”
The invention relates to a steel substrate with nickel-zinc-aluminium coating has been disclosed in the present disclosure. The nickel-zinc-aluminium coating on the steel substrate comprises an iron-nickel solid solution layer on steel substrate, a nickel layer on iron-nickel solid solution layer, a nickel-zinc solid solution layer on nickel layer, a nickel-zinc gamma layer on nickel-zinc solid solution layer, a nickel-zinc delta layer on the nickel-zinc gamma layer and an overlay zinc layer on nickel-zinc delta layer with nickel-zinc-aluminium dispersed precipitates. The coating is found to protect the substrate from oxidation at the time of heating process in atmospheric condition at the time of hot stamping process. During the heat treatment process the coating forms a ductile interface layer of iron-nickel solid solution on the substrate and an upper coating layer on iron-nickel-zinc solid solution layer and an oxide layer on upper coating layer, which resist the brittle failure of the substrate at the time of forming process at high temperatures. The coating showed similar mechanical strength of bare steel at high temperature deformation process.