TECHNICAL FIELD
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 resistance to liquid metal induced embrittlement after thermal treatment.
BACKGROUND OF THE DISCLOSURE
Reduction of Energy consumption by decreasing the vehicle weight is one of the highly researched areas in recent times. Many new steel chemistries as well as new process designs have been commercialised to address the issue. Hot forming process is one of the best solutions till date which can achieve as high as 1500 to 1900 MPa steels [1]. Side impact beams, bumpers, A and B pillars, roof rails, cross members, and tunnels etc. in automotive body are being manufactured by the hot formed steels. The process of hot stamping involves heating the steel above austenitizing (>= 900oC) temperature and holding for time such as 300s and then forming at high temperature and cooling simultaneously to form fully martensitic structure [1, 2].
The heating of the steel blanks in hot forming process is performed in normal atmosphere and it results in forming of oxides. This imposes an inherent difficulty to the manufacturer. When the steel is heated at such high temperatures in air they start to form high temperature oxide. Also, steel start losing carbon from the matrix which is called as decarburisation. Oxidation of these steels also hampers the martensitic transformation and eventually high strength is not achieved. To avoid such problems coating becomes one of the essential aspect of the process [3, 4, 5]. Coatings for this process should also be formable and corrosion resistance. Another aspect of these coating is that they should not form liquid phases at the interface of the steel and coating at the time of deformation. This is required because at the time of deformation the liquid phase may penetrate in the steel grain boundary and can result in liquid embrittlement of the steel.

Zinc coating such as galvanizing and galvannealing coatings are one of the most used coatings for steel for their high formability and corrosion resistance. However, these coatings find difficulties for hot stamping process. Zinc has melting temperature of 420oC. Hence, when steel coated with Zinc is heated at such a high temperature, it melts and forms a liquid phase. Also, at high temepratures, diffusion of iron takes place and iron and zinc combines together and form different intermetallic phase. Among different phases, only iron-zinc phase which has higher melting temperature than the austenetizing temperature is iron-zinc alpha solid solution. However, it takes very high time to form this phase which is not commercially viable. Therefore, galvanizing and galvannealing coatings are not suitable as hot stamping coating as they cause Liquid Metal Induced Embrittlement (LMIE) [6 - 8].
The other commercially available coating is Al-Si coating known as USIBOR [9]. These coatings have shown good viability as hot stamp coatings because of the formation of iron aluminium intermetallic phases at the coating steel interface [10]. However, these intermetallic phases are very hard and form cracks and peel off from the interface at the time of forming operation. Also, they are unable to provide any sacrificial corrosion protection to steel. However, they are able to resist the decarburisation of the steel surface and thus they are the most widely used coating for hot stamping process.
Another type of electrodeposited coating having Ni-Zn phase with composition of 11wt-% Ni, 0.6wt-% Fe and balance zinc is being researched [12]. The final microstructure of iron based solid solution has been reported for these coatings. It has been reported that these coatings are able to resist LMIE due to their high melting point [13, 14]. Multilayer coating of similar kind where 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 was reported [15]. These coating are reported to resist LMIE due to higher melting points of these Ni-Zn phases. However, electrodeposition of Ni-Zn phases on bare steel (B) can cause hydrogen embrittlement [16]. Furthermore, the presence of less zinc and high iron in these coatings reduce the sacrificial nature of the coating.

SUMMARY OF THE DISCLOSURE
The present invention has been found to overcome the drawbacks of the conventional coated steel. The present invention has also found to have improved properties with respect to 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-iron coating on the steel substrate. The coating has been produced through electroplating of Ni on steel substrate and followed by hot dip galvanizing in an aluminium containing bath. The present coating forms iron rich solid solution layer of iron-nickel on the steel substrate, a nickel rich solid solution layer on the iron-nickel solid solution layer, an aluminium-nickel-zinc-iron intermetallic layer on the nickel rich layer, and an overlay zinc-aluminium layer on the aluminium rich intermetallic layer which is having lamella of aluminium rich and zinc rich phases.
In an embodiment, the steel substrate is coated with nickel followed by hot dipping in zinc-aluminium bath comprising aluminium from about 2.0 % by weight to about 6.0 % by weight.
In an embodiment, the nickel-zinc-aluminium-iron coating comprises of iron of about 0.5 % to about 7.0% by weight, zinc of about 0.5% to about 98.0% by weight, and nickel of about 1.0% to about 95.0% by weight, and Al of about 0.5% to about 35.0% by weight and thickness of the coating ranges from 20µm to 50µm.
In an embodiment, the iron rich layer of iron nickel solid solution comprises iron of about 90% to about 99% by weight and nickel of about 0.01 % to about 1.5% by weight, and thickness of the iron-nickel solid solution layer ranges from 0.5 µm to 1.5 µm.
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 0.5-1.0% of total coating.

In an embodiment, the nickel rich layer (2) comprises iron of about 2% to about 10% by weight, zinc of about 0.5% to about 2% by weight and nickel of about 90 to about 97% by weight, and thickness of the nickel layer (2) ranges from 1 µm to 8 µm.
In an embodiment, the nickel rich layer (2) is formed of Face Centered Cubic crystal, and has a hardness of about 6.71 GPa.
In an embodiment, the aluminium-nickel-zinc-iron intermetallic layer (3) comprises iron from 0.3% to up to 1.0% by weight, zinc of about 15.0% to about 25.0% by weight, nickel of about 15.0% to about 30.0% by weight, and aluminium of about 30.0% to about 35.0% by weight and rest is oxygen. Thickness of the aluminium-nickel-zinc-iron intermetallic layer (3) ranges from 1 µm to 3 µm.
In an embodiment, wherein the overlay zinc-aluminium layer (4) comprises iron of about 0.1% to about 0.5% by weight, zinc of about 20% to about 95% by weight and aluminium of about 2.0% to about 35.0% by weight, nickel of about 0.5 to 1.5% by weight and remainder of the composition includes oxygen by weight. The overlay zinc has zinc rich phases and lamella of zinc rich and equiatomic aluminium-zinc phases.
The zinc rich phases comprises iron of about 0.3 to 0.5% by weight, zinc of about 25% to about 95% by weight and aluminium of about 2.0% to about 7.0% by weight, nickel of about 0.8 to 1.5% by weight and the rest is oxygen.
The equiatomic aluminium-zinc phase comprises iron of about 0.1 to 0.3% by weight, zinc of about 30% to about 40% by weight and aluminium of about 30% to about 40% by weight, nickel of about 0.1 to 0.8% by weight and the rest is oxygen.
Thickness of the overlay zinc layer ranges from 5 µm to 40 µm. wherein the overlay zinc layer (4) Phase fraction of the overlay zinc layer comprises of zinc rich phase, equiatomic aluminium-zinc phase and zinc-aluminium-oxide.

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 an iron-nickel solid solution layer on steel substrate and a nickel-zinc-aluminium-iron coating on the iron-nickel solid solution layer. The nickel-zinc-aluminium-iron coating forms an iron-nickel-zinc-aluminium layer on the steel substrate.
In an embodiment, the nickel-zinc-aluminium-iron coating on hot worked steel substrate comprises iron of about 15% to about 99% by weight, zinc of about 0.01% to about 20% by weight, and nickel of about 0.1% to about 65% by weight, and Al of about 0.01% to about

6% by weight, and the thickness of the iron-nickel-zinc-aluminium coating ranges from 20µm to 40µm.
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 hot worked 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 steel substrate is cleaned with degreasing solution and pickling solution. The steel substrate is then electroplated with nickel in a nickel bath at a temperature ranging from about 700C to about 900C. The electroplated steel substrate is then fluxed and dried and a zinc coating is applied through hot dipping in molten Zn-Al bath.

In an embodiment, the steel substrate is degreased in 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 on steel substrate. Thereafter the steel substrate is rinsed in water to remove eliminate the carryover of the caustic solution. Then steel substrate is pickled in 10% by volume H2SO4 acidic solution at temperature ranging from about 600C to about 700C for a time ranging from about 1 minute to about 5 minutes. Acid pickling ensures removal of surface oxide from the steel substrate. The steel substrate is again rinsed to eliminate carry overs of the acidic solution to the next process step.
In an embodiment, the steel substrate is nickel coated in a nickel bath comprising 150-200 g/l of NiSO4, 30-40g/l of NiCl2, 5-8 g/l of H3BO3. The pH of the bath is maintained at a range from 2 to 7. Nickel plate is used as anode and the steel substrate as cathode for the electroplating process. A current of 2-5mA/cm2 and a voltage of 0.5 -1 V is maintained for the electroplating process. The electroplating is done for 1-30 minutes.
In an embodiment, 20% by volume NH4Cl-80% by volume Zn4Cl solution has been used for the fluxing of the nickel coated steel substrate. The fluxing is done at room temperature for 30 seconds. A hot air drying process at 100oC has been used to dry the fluxed substrate.
In an embodiment, the zinc coating is applied by dipping the nickel coated steel substrate (1a) in a molten zinc-aluminium solution at a temperature ranging from about 4500C to about 4700C. The dipping time is 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 cross-sectional elemental view of the coated boron steel in 2-6 wt.% Al containing Zn bath, in accordance with an embodiment of the present disclosure.
Figure 3b illustrates cross-sectional view of the coated boron steel at high magnification in 5 wt.% Al containing Zn bath, in accordance with an embodiment of the present disclosure.
Figure 4a illustrates elemental analysis through depth of the coating on the steel substrates before hot stamping process for coating acquired from 2 wt.% Zn-Al solution, in accordance with an embodiment of the present disclosure.
Figure 4b illustrates elemental cross sectional elemental mapping of the coating on the steel substrates before hot stamping process for coating acquired from 5 wt.% Zn-Al solution, in accordance with an embodiment of the present disclosure.

Figure 5 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 6 illustrates optical micrograph of final microstructure of steel substrate after hot stamping process, in accordance with one embodiment of the present disclosure.
Figure 7 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 8 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-iron coating on the steel substrate. In an embodiment, the steel substrate comprising carbon, boron, manganese, sulphur, phosphorus, silicon, aluminium, copper, chromium, nickel, molybdenum, vanadium, niobium, titanium, nitrogen in predetermined quantities with the balance quantity being iron along with incidental elements of the alloy.
In the present disclosure, method of applying a Ni-Zn-Al-Fe coating on the steel substrate has been discussed. It comprises cleaning of steel substrates, electroplating nickel on the cleaned

steel substrate in a nickel bath. The electroplated steel substrate is dipped into fluxing solution to prevent oxidation and dried at 100oC to vaporise the water from flux. Thereafter the steel substrate is coated with zinc-aluminium by dipping the heated steel substrate into a zinc-aluminium solution. By the process a Ni-Zn-Al-Fe coating on the steel substrate has been achieved.
The nickel-zinc-aluminium-iron coating applied on the steel substrate forms an iron rich solid solution layer, a nickel rich solid solution layer, a nickel-zinc-aluminium-iron intermetallic layer, and an overlay zinc layer with zinc rich phases and lamella of zinc rich and equiatomic Zn-Al phases. In an embodiment, predetermined quantities of iron, zinc, nickel, aluminium and oxygen and thickness of the coating has been acquired. In an embodiment, predetermined quantities of iron, zinc, nickel, aluminium and oxygen are present in each of the layers in the nickel-zinc-aluminium-iron coating.
The hot worked coated steel is formed by hot press forming of the coated steel. The hot worked coated steel comprises steel substrate and a nickel-zinc-aluminium-iron coating on the steel substrate. In an embodiment, the steel substrate comprises carbon, boron, manganese, sulphur, phosphorus, silicon, aluminium, copper, chromium, nickel, molybdenum, vanadium, niobium, titanium, nitrogen in predetermined quantities with the balance quantity being iron along with incidental elements of the alloy. After hot forming, the nickel-zinc-aluminium-iron coating forms an iron-nickel solid solution layer at the interface of the substrate and an upper coating comprising nickel-zinc-aluminium-iron layer on the steel substrate.
The formation of a nickel-zinc-aluminium-iron layer on the coated steel surface before and after heat treatment process provides high temperature stability. Additionally, the nickel-zinc-aluminium-iron layer also provides required sacrificial corrosion protection to the steel 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 boron steel substrate (1a) has an initial microstructure of pearlite (P) and ferrite (F). The composition steel substrate (1a) is shown in table 1 below.

C
0.2-0.25 Mn
1.15-1.4 S
<0.01 P
<0.05 Si Al Cu
<0.05 Cr
0.15-0.35 Ni
<0.1 Mo
<0.01 V
<0.01 Nb
<0.01 Ti
0.02-0.05 N B

0.2-0.35 <0.1






50ppm 0.002-0.005
Table. 1 the balance being iron optionally along with incidental elements of the alloy.
Boron is used in steel to enhance the hardenability of steel. Boron in the steel substrate enhances the kinetics of martensite formation during hot stamping process. In an embodiment, the steel substrate (1a) have Body Centered Cubic crystals structure, and comprises of pearlite (a mixture of ferrite and cementite phase) of about 22% to about 26% and remainder being ferrite phase. The steel substrate (1a) has a mechanical strength of about 2.4 GPa having the composition as shown in table 1.
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 (1a) having the composition of the alloys shown in table 1. In step 101, the steel substrate (1a) for example steel sheet is subjected to cleaning to remove oil contaminants, grease residue, corrosion products or any other foreign entities deposited on the surface of the steel substrate (1a). In the cleaning process, the steel substrate (1a) is degreased with a caustic solution at a temperature ranging from about 500C to about 700C, the degreasing time typically varies from 2 minutes to 5 minutes. Degreasing of the steel substrate (1a) removes oil and grease from the surface of the steel substrate (1a). To clean carry overs of the caustic solution on the surface of the steel

substrate (1a) the steel substrate (1a) is then rinsed in water. The steel substrate (1a) 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 (1a) removes corrosion products from the steel surface. The steel substrate (1a) 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 (1a) as cathode for electroplating nickel on steel substrate (1a). 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 (1a). 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 (1a) surface. Nickel is a very good barrier to hydrogen diffusion. So, Electroplated nickel deposition on the steel substrate (1a) will act as a potential barrier to hydrogen into the steel substrate (1a). 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 (1a) is then dried at 100oC The fluxing operation of steel substrate (1a) prohibit oxygen to diffuse into steel substrate (1a). This inherently eliminates possibility of oxygen concentration in subsequent steps of the process of coating the steel substrate (1a).
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 2.0% to about 6.0% by weight. The metallic zinc and aluminium adhere to the steel substrate (1a) by formation of different phases, when the steel substrate (1a) 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 (1a) coated with a nickel-zinc-aluminium-iron coating of predetermined thickness. In an embodiment, the nickel-zinc-aluminium-iron coating applied on the steel substrate (1a) by a method steps 101-104 comprises iron of about 0.5 % to about 7.0% by weight, zinc of about 0.5% to about 98.0% by weight, and nickel of about 1.0% to about 95.0% by weight, and aluminium of about 0.5% to about 35.0% by weight and thickness of the coating ranges from 20µm to 50µm.
Referring to Figure 3a which illustrates cross-sectional elemental view of the coated boron steel in 2-6 wt.% Al containing Zn bath, in accordance with an embodiment of the present disclosure. As shown in Figure 3a the nickel-zinc-aluminium-iron coating on steel substrate (1a) carried by step 101-104 when galvanised in 5.0 wt.% Al-Zn solution results in different layers of different phases. In an embodiment, the layers formed on the steel surface include iron rich layer of iron-nickel solid solution (1b) on the surface of the steel substrate (1a). A nickel rich layer of nickel-iron solid solution (2) is formed on the iron-nickel solid solution layer. On the nickel rich layer (2) a nickel-zinc-aluminium-iron intermetallic layer (3) is formed. The, overlay zinc coating (4) is formed as a top coating layer over nickel-zinc-aluminium-iron intermetallic layer (3). In the overlay zinc layer (4) lamella of zinc rich phase and equiatomic Al-Ni phase has formed. Each of the layers and interfaces are formed with a predetermined thickness, composition, crystal structure and microstructure.
Referring to Figure 3b illustrates elemental point analysis of the coating obtained by Energy Dispersive Spectroscopic (EDS). The result shows compositions of different elements in the coating structure.
Referring to Figure 4a elemental profile through depth of the coating on the steel substrate (1a) acquired through 2.0 wt.% Al-Zn solution. In an embodiment the elemental analysis of the coating on the steel substrate (1a) is obtained by Energy Dispersive Spectroscopic (EDS). Similar coating structure as explained in Figure 3a was observed.

Referring to Figure 4b which illustrates elemental map of the coating on the steel substrate (1a) acquired through 5.0 wt.% Al-Zn solution. In an embodiment the elemental analysis of the coating on the steel substrate (1a) is obtained by Wavelength Dispersive Spectroscopic (WDS). As shown in Figure 4b the nickel-zinc-aluminium-iron coating on steel substrate (1a) carried by step 101-104 when galvanised in 5.0 wt.% Al-Zn solution results in different layers of different phases. In an embodiment, the layers formed on the steel surface include iron rich layer of iron-nickel solid solution (1b) on the surface of the steel substrate (1a). A nickel rich layer of nickel-iron solid solution (2) is formed on the iron-nickel solid solution layer (1b). On the nickel rich layer (2) a nickel-zinc-aluminium-iron intermetallic layer is formed (3). Overlay zinc coating (4) is formed as a top coating layer on over nickel-zinc-aluminium-iron intermetallic layer (3). In the overlay zinc layer (4) lamella of zinc rich phase and equiatomic Al-Ni phase has formed. However, the volume fraction of the equiatomic Al-Ni phase is very high compared to the coating acquired in 2.0 wt.% Al 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 rich layer of iron-nickel solid solution (1b) comprises iron of about 90% to about 99% by weight and nickel of about 0.01 % to about 1.5% by weight. The thickness of the iron-nickel solid solution layer ranges from 0.5µm to 1.5µm. The iron-nickel solid solution layer has a body centre cubic crystal structure, and the iron-nickel solid solution layer comprises of about 0.5% to about 1% of the total coating.
In an embodiment, the nickel rich layer of nickel-iron solid solution (2) shown as Ni rich layer in Figure 3a is formed on the iron rich layer of iron-nickel solid solution (1b). The nickel rich layer comprises iron of about 2% to about 10% by weight, zinc of about 0.5% to about 2% by weight and nickel of about 90 to about 97% by weight. The nickel rich layer (2) has a layer thickness ranging from 1µm to 8µm and has hardness of about 6.71 GPa. The nickel rich layer (2) comprises of face centre cubic crystal structure, and phase fraction of the nickel layer (2) comprises of about 5% to about 20% of the total coating.

In an embodiment, the aluminium-nickel-zinc-iron intermetallic layer (3) shown in Figure 3a is formed on the nickel-iron solid solution layer (2). The thickness of the aluminium-nickel-zinc-iron intermetallic layer (3) ranges from 1 µm to 3 µm. Phase fraction of the aluminium-nickel-zinc-iron intermetallic layer (3) comprises of 0.03% to about 0.1% of the total coating.
In an embodiment, the overlay zinc-aluminium layer (4) shown as overlay zinc in Fig. 4a, is formed on the aluminium-nickel-zinc-iron intermetallic layer comprising 25 to 30 Wt.% of oxygen, 30 to 35 wt.% of aluminium, 0.5 to 1 wt.% of iron, 15 to 30 wt.% of nickel and 15 to 25 wt.% of zinc. The overlay zinc-aluminium layer (4) comprises iron of about 0.1% to about 0.5% by weight, zinc of about 20% to about 95% by weight and aluminium of about 2.0% to about 35.0% by weight, nickel of about 0.5 to 1.5% by weight and remainder of the composition includes oxygen by weight, and thickness of the overlay zinc layer (4) ranges from 5 µm to 35 µm. The overlay zinc-aluminium has lamella of zinc rich and equiatomic aluminium-zinc phase. The zinc rich phases comprises iron of about 0.3 to 0.5% by weight, zinc of about 25% to about 95% by weight and aluminium of about 2.0% to about 7.0% by weight, nickel of about 0.8 to 1.5% by weight and the rest is oxygen. The equiatomic aluminium-zinc phase comprises iron of about 0.1 to 0.3% by weight, zinc of about 30% to about 40% by weight and aluminium of about 30% to about 40% by weight, nickel of about 0.1 to 0.8% by weight and the rest is oxygen.
In the exemplary embodiment illustrated in Fig 3a, the overlay zinc layer (4) extends up to 40 µm, aluminium-nickel-zinc-iron intermetallic layer (3) extends up to 43, the nickel rich layer of nickel-iron solid solution (2) extends up to 47 µm, the iron rich iron-nickel solid solution (1b) extends up to 48 µm. The thickness of the overlay zinc layer (4) depends on the Zn bath composition, bath temperature and withdraws speed.
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 achieve desired property of dimension and strength. Figure 5 illustrates an exemplary heat treatment schedule for the hot stamping process of coated steel (CS). 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. Presence of boron improves the hardenability of the steel. So, in the hot stamping process the steel substrate (1a) converts into full martensite structure (M) as shown in Figure 6. This microstructure of the steel substrate (1a) significantly improves the strength of the steel substrate (1a). In an embodiment, the strength of the steel substrate (1a) is in terms of mechanical strength or hardness.
Subsequently, because of the high temperature hot stamping process of the steel substrate (1a), microstructural changes of the coating is observed. Zn has a boiling point of 907oC. So, at high temperature Zn starts to evaporate from the coating and also forms oxides on the top of the coating. The amount of zinc in the final microstructure varies due to evaporation and oxidation of the overlay zinc layer. The remaining Zn in the coating depends on the initial thickness of the overlay zinc layer. Rate of evaporation of overlay Zn also depends on the presence of aluminium in the coating. Hence, less evaporation is expected for higher aluminium in the overlay zinc. So, coatings with high aluminium will result in higher zinc in the final microstructure after the hot stamping process. The final coating microstructure and thickness depends on initial nickel coating thickness, galvanising time, initial phase fraction and thickness of overall zinc layer.
An example of final microstructure of the coating on the steel substrate (1a) with initial 40 µm of overlay zinc layer is illustrated in Figures 7. A continuous interfacial iron rich layer of iron nickel solid solution is present in the coating. The iron rich layer of iron nickel solid solution comprises iron of about 98% to about 99% by weight, nickel of about 0.1% to about 0.5% by weight and thickness of the iron-nickel solid solution layer ranges from 1 µm to 3

µm. A of iron-nickel-zinc-aluminium layer is formed over the iron rich layer of iron-nickel solid solution. The iron-nickel-zinc-aluminium layer comprises iron of about 15% to about 70% by weight, zinc of about 10% to about 20% by weight, and nickel of about 20% to about 65% by weight, and Al of about 0.2% to about 6% by weight, and the thickness of the iron-nickel-zinc-aluminium coating ranges from 20 µm to 37 µm. Cracks generated during high temperature deformation are arrested at the interface of the steel and coating. This is due to the formation of high temperature melting phases which restricted the LMIE. Also formation of iron rich solid solution layer at the interface of the coating and steel substrate might have contributed in restricting the cracks at the interface due to its ductile nature.
Examples:
The steel samples were cleaned with alkali solution to remove oil and grease from the surface and pickled with acid to remove oxide from the substrate surface. The parameters are given in Table 2 below.

Table 2. Degreasing and Pickling parameters After surface preparation the samples were electroplated with pure Ni. Ni electroplating

Operation Composition Parameters Dipping time
Alkali Cleaning 20g/l sodium hydroxide solution (NaOH) Temperature: 70oC 30s
Pickling 8-10 volume% sulfuric acid (H2SO4) Temperature: 60oC 15s
parameters are listed in Table 3 below.

Operation Composition Parameters
Ni electroplating 300 gm/l NiSO4.6H2O 60 gm/l NiCl2.6H2O 40 gm/l Boric Acid pH 4 Dipping time: 10m Temperature: 55oC Current Density: 430 A/m2
Table 3. Ni electroplating parameters
The Ni electroplated samples were fluxed and dipped into Zn bath of different Al concentration. The galvanized samples were air cooled in normal condition. The Fuxing and galvanizing parameters are listed in Table 4 below.

Operation Composition Parameters
Fluxing 80-20 (wt.%) mixture of zinc chloride (ZnCl2) and ammonium chloride (NH4Cl) Temperature: 30 oC
Galvanizing Zinc bath composition (wt.%): 1. 2.0 to 6.0 wt.% Zn bath temperature: 460 oC Dipping time: 3 s
Table 4. Fuxing and galvanizing parameters
Specification of the hot tensile test were conducted for the coated steel (CS) is given in the Table 5 below. Test were also performed upto 40% strain and till fracture of the coated steel (CS) and bare steel (B) for comparison of the Stress-strain behaviour. Comparison of the stress-strain behaviour of the coated steel (CS) and the bare steel (B) is shown in Figure 8.

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 19ormi 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 5. Hot working test parameters
It is evident from Figure 8 that the coated steel (CS) shows similar stress-stain behaviour compared to bare steel (B). This implies that the coating was able to retain the mechanical strength of the bare steel (B) till fracture. It is safe to conclude that the coating is able retain the elastic strength, yield strength, ultimate tensile strength and fracture toughness of bare steel (B) in high temperature deformation process. This is possible because of the formation of the high melting ductile phases in the coating during hot stamping process.
Advantages:
The discloser provides mean to restrict the Ni dissolution to form low melting phase such as Ni-Zn delta phase at the time of hot dipping in zinc bath.

The present discloser provides the process to form coating phases with high melting temperature by hot dipping process.
The present discloser provides a mean to retain zinc, nickel and aluminium in final microstructure without deteriorating the forming property of the coated steel. The present discloser provide a mean to impart the sacrificial corrosion resistance in the coating for 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
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 (1a),
iron rich solid solution layer of iron-nickel on the steel substrate (1a),
a Nickel-Zinc-Aluminium-Iron (Ni-Zn-Al-Fe) coating on the steel substrate (1a),
wherein the Ni-Zn-Al-Fe coating forms:
iron rich solid solution layer (1b) of iron-nickel on the steel substrate (1a);
a nickel rich solid solution layer (2) on the iron-nickel solid solution layer
(1a);
an aluminium-nickel-zinc-iron intermetallic layer (3) on the nickel rich solid
solution layer (2);
and an overlay zinc-aluminium layer (4) on the aluminium-nickel-zinc-iron
intermetallic layer (3).
2. The coated steel as claimed in claim 1, wherein the Nickel-Zinc-Aluminium-Iron coating comprises of iron of about 0.5 % to about 7.0% by weight, zinc of about 0.5% to about 98.0% by weight, and nickel of about 1.0% to about 95.0% by weight, and Al of about 0.5% to about 35.0% by weight and thickness of the coating ranges from 20µm to 50µm.
3. The coated steel as claimed in claim 1, wherein the iron rich layer of iron nickel solid solution (1b) comprises iron of about 90% to about 99% by weight and nickel of about 0.01 % to about 1.5% by weight, and thickness of the iron-nickel solid solution layer ranges from 0.5 µm to 1.5 µm.
4. The coated steel as claimed in claim 1, wherein 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 0.5-1.0% of total coating.

5. The coated steel as claimed in claim 1, wherein the nickel rich layer (2) comprises iron of about 2% to about 10% by weight, zinc of about 0.5% to about 2% by weight and nickel of about 90 to about 97% by weight, and thickness of the nickel layer (2) ranges from 1 µm to 8 µm.
6. The coated steel as claimed in claim 1, wherein the nickel rich layer (2) is formed of Face Centered Cubic crystal, and has a hardness of about 6.71 GPa.
7. The coated steel as claimed in claim 1, wherein the aluminium-nickel-zinc-iron intermetallic layer (3) comprises iron from 0.3% to up to 1.0% by weight, zinc of about 15.0% to about 25.0% by weight, nickel of about 15.0% to about 30.0% by weight, and aluminium of about 30.0% to about 35.0% by weight and rest is oxygen.
8. The coated steel as claimed in claim 1, wherein thickness of the aluminium-nickel-zinc-iron intermetallic layer (3) ranges from 1 µm to 3 µm.
9. The coated steel as claimed in claim 1, wherein the overlay zinc layer (4) comprises
iron of about 0.1% to about 0.5% by weight, zinc of about 20% to about 95% by weight and aluminium of about 2.0% to about 35.0% by weight, nickel of about 0.5 to 1.5% by weight and remainder of the composition includes oxygen by weight.
10. 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.
11. The coated steel as claimed in claim 10, wherein the steel substrate (1a) is formed of
Body Centered Cubic crystals, and phase fraction of iron in the steel substrate (1a)
comprises of pearlite structure of about 22% to about 26% and remainder being
ferrite structure, and wherein the steel substrate (1a) has a mechanical resistance of about 2.4 GPa
12. A hot worked coated steel, comprising:
a steel substrate (1a),
a nickel-zinc-aluminium-iron coating on the steel substrate (1a), wherein the
nickel-zinc-aluminium-iron coating forms:
an iron-nickel solid solution layer on the steel substrate (1a); an upper coating layer iron-nickel-zinc-aluminium phase on the iron-nickel-zinc solid solution layer.
13. The hot worked coated steel as claimed in claim 12, wherein the iron-nickel-zinc-aluminium coating comprises iron of about 15% to about 99% by weight, zinc of about 0.01% to about 20% by weight, and nickel of about 0.1% to about 65% by weight, and Al of about 0.01% to about 6% by weight, and the thickness of the iron-nickel-zinc-aluminium coating ranges from 20µm to 40µm.

14 The hot worked coated steel as claimed in claim 12, wherein the iron-nickel solid solution layer comprises iron of about 98% to about 99% by weight, nickel of about 0.1% to about 0.5% by weight and thickness of the iron-nickel solid solution layer ranges from 1 µm to 3 µm.
15..The hot worked coated steel as claimed in claim 12, wherein the iron-nickel solid solution layer is formed of Body Centered Cubic crystals, and phase fraction of the iron-nickel solid solution layer comprises of about 1% to about 5% of the total coating.
16. The hot worked coated steel as claimed in claim 12, wherein the iron-nickel-zinc-
aluminium phase layer comprises iron of about 15% to about 70% by weight, zinc of
about 10% to about 20% by weight, and nickel of about 20% to about 65% by weight,
and Al of about 0.2% to about 6% by weight, and the thickness of the iron-nickel-
zinc-aluminium coating ranges from 20 µm to 37 µm.
17. The hot worked coated steel as claimed in claim 12, 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.
18. The hot worked coated steel as claimed in claim 12, the steel substrate (1a) 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 (1a) has a mechanical resistance of about 4.31 GPa.